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A review on hot stamping
Abstract
The production of high strength steel components with desired properties by hot stamping (also called press hardening) requires a profound knowledge and control of the forming procedures. In this way, the final part properties become predictable and adjustable on the basis of the different process parameters and their interaction. In addition to parameters of conventional cold forming, thermal and microstructural parameters complicate the description of mechanical phenomena during hot stamping, which are essential for the explanation of all physical phenomena of this forming method.In this article, the state of the art in the thermal, mechanical, microstructural, and technological fields of hot stamping are reviewed. The investigations of all process sequences, from heating of the blank to hot stamping and subsequent further processes, are described. The survey of existing works has revealed several gaps in the fields of forming-dependent phase transformation, continuous flow behavior during the whole process, correlation between mechanical and geometrical part properties, and industrial application of some advanced processes. The review aims at providing an insight into the forming procedure backgrounds and shows the great potential for further investigations and innovation in the field of hot sheet metal forming.
... Pode-se afirmar que, tanto a agilidade com que a transferência para a ferramenta ocorre, quanto a agilidade da operação de estampagem, são determinantes para o sucesso do processo, uma vez que é necessário evitar a difusão do carbono, que será o responsável pela resistência mecânica da peça [2]. Após a estampagem, a peça é temperada na própria ferramenta, que é resfriada em água, até que toda a austenita se transforme em martensita [3,4]. A grande utilização de aços com adição de manganês e boro nos processos de EQ é decorrente da maior facilidade de obtenção de microestrutura completamente martensítica após o processo de têmpera. ...
... A grande utilização de aços com adição de manganês e boro nos processos de EQ é decorrente da maior facilidade de obtenção de microestrutura completamente martensítica após o processo de têmpera. Esses aços apresentam excelente temperabilidade sob baixas taxas de resfriamento [2,3,5]. A adição de manganês é essencial para melhorar a temperabilidade. ...
... Sua presença é o que torna o processo viável industrialmente. O boro tem maior influência na resistência, uma vez que retarda a transformação da austenita em microestruturas menos duras [3,4]. Antes do processo de EQ, a microestrutura do aço é composta de ferrita e perlita, além de carbonetos finamente dispersos na matriz, com valores de resistência em torno de 600 MPa. ...
... The application of hot stamping steels (HSS) has been an important strategy to realize automotive lightweighting and enhance crash safety over the past two decades [1][2][3][4][5][6]. The hot forming process (Fig. 1) resolves the contradiction between strength and formability by forming first and hardening later, attributed to the obviously improved thermal processing performance of the material at high temperatures. ...
... The hot forming process (Fig. 1) resolves the contradiction between strength and formability by forming first and hardening later, attributed to the obviously improved thermal processing performance of the material at high temperatures. In hot stamping, the HSS sheet is usually heated to about 930 °C in a furnace first, then soaked for a few minutes to reach full austenite state, and stamped and quenched simultaneously in dies to obtain a full martensite microstructure [1,3]. The subsequent paint-baking process tends to increase the toughness of the martensite while gently decreasing its strength. ...
... A key focus of this review is how to apply these materials to achieve carbon reduction. In addition, advancements in heating processes and corresponding equipment have improved heating efficiency and temperature uniformity control [1,[23][24][25][26], such as the innovative fast heating and partial heating process. For example, multi-stage hearth furnaces can regulate the heating temperature of HSS sheets by smoothly controlling the stage temperature, while conduction heating could heat HSS sheets up to 800 °C in 2 s [27]. ...
Hot stamping steels have become a crucial strategy for achieving lightweighting and enhancing crash safety in the automotive industry over the past two decades. However, the carbon emissions of the materials and their related stamping processes have been frequently overlooked. It is essential to consider these emissions during the design stage. Emerging materials and technologies in hot stamping pose challenges to the automotive industry's future development in carbon emission reduction. This review discusses the promising materials for future application and their special features, as well as the emerging manufacturing and part design processes that have extended the limit of application for new materials. Advanced heating processes and corresponding equipment have been proven to improve heating efficiency and control temperature uniformity. The material utilization and the overall performance of the components are improved by tailored blanks and an integrated part design approach. To achieve low-carbon-emission (LCE) hot stamping, it is necessary to systematically consider the steel grade, heating process, and part design, rather than solely focusing on reducing carbon emissions during the manufacturing process stage. This review aims to present the latest progress in steel grade, heating process, and part design of hot stamping in the automotive industry, providing solutions for LCE from a holistic perspective.
... The process of press hardening can be divided into two main variants: direct and indirect press hardening [14]. For both variants, steel sheets are first cut from a steel coil. ...
... As mentioned before, temperature-induced transformations of the iron atom lattice are utilised during press hardening to achieve the high strength of manufactured components. For materials used in industrial press hardening processes, temperatures of 900 °C to 950 °C with furnace dwell times of several minutes are common, and a cooling rate of more than 25 K/s is sufficient for forming a high-strength martensitic structure [11,14,24]. Low-alloy boronmanganese steels are typically applied for industrial press hardening [12]. ...
... The alloying elements boron and manganese positively influence the alloyed steel's hardenability and required cooling rates and increase the attainable material strength [12,24]. Of the available boron-manganese steels, the steel 22MnB5 is the most commonly used [11,12,14]. After hardening, this steel reaches a tensile strength of approximately 1500 MPa and an elongation at fracture of approximately 5% [25]. ...
Press hardening is an established process for producing high-strength, lightweight structural automotive parts, utilising roller hearth furnaces to heat metal sheets. It presents a non-negligible
source of greenhouse gas emissions in part production processes. This paper investigates the ecological and economic advantages of induction as an alternative heating method and identifies break-even scenarios. Therefore, specific energy demands for various furnace and induction heating operating states are determined and applied to different energy system transition scenarios. It can be shown that while induction heating can be more energy efficient than furnace heating, its ecological and economic advantages are highly dependent on regional electricity-to-gas price ratios and electric energy emission factors. For high good-mass flows (0.5 kg/s) and 80% production capacity utilisation, an electricity-to-gas price ratio lower than 1.24 and an emission factor lower than 250 g CO2 /kWh are required.
... It presents the surface temperature change, mechanical characteristics, and microstructure of boron steel that has undergone a high-frequency induction heating process. Surface temperature data were analyzed at different high-frequency induction heating forces (15,18,21,24, 27, and 30 kW) and distances from specimens (6, 9, 12, and 15 mm). Two phases, austenite and ferrite, were formed in the low-temperature region, and martensite was formed in the high-temperature region. ...
... The physical characteristics of steel materials are affected by the coil shape, high-frequency induction power, and cooling conditions [13]. Under high-frequency induction heating, the mechanical characteristics of the steel change due to the conduction of heat from the surface inside of the material [14,15]. ...
In the automobile industry, high-strength plates are increasingly used to reduce vehicle weight due to strict regulations on fuel efficiency and safety, and these plates achieve a tensile strength of 1500 MPa due to the hot-stamping process. Recently, research has been conducted to examine the flow behavior of materials according to the relationship between hot stamping time-temperature characteristics, coil shape, cooling method, and thermodynamic flow characteristics of quenching materials. In this study, a basic experiment in the form of a plate was conducted using an eddy current generated during high-frequency induction heating. It presents the surface temperature change, mechanical characteristics, and microstructure of boron steel that has undergone a high-frequency induction heating process. Surface temperature data were analyzed at different high-frequency induction heating forces (15, 18, 21, 24, 27, and 30 kW) and distances from specimens (6, 9, 12, and 15 mm). Two phases, austenite and ferrite, were formed in the low-temperature region, and martensite was formed in the high-temperature region. Mechanical properties and microstructures were also analyzed under different high-frequency induction heating coil conditions. The correlation between the high-frequency induction heating force and the specimen with the maximum tensile strength was investigated. Due to high-frequency induction heating, scale generation and surface decarbonization can be avoided. As a result of this experiment, 1500 MPa of the same tensile strength as the mechanical characteristics obtained in the existing heat treatment could be obtained.
... With the increased strength levels of AHSS, the cold stamping approach has encountered significant engineering challenges such as the control of springback and avoidance of edge cracking, etc [1e3]. Meanwhile, the hot stamping process route elegantly explores the phase transformations in steels, allowing the forming step in the soft austenitic state to ensure formability and achieving the in-use ultra-high strength via quenching for martensitic microstructure [4]. ...
... The press-hardened steels (PHS) are designed for the hot stamping process. They are martensitic grades of steels with the addition of mainly carbon (!0.2 wt%) for strength, and alloying with manganese and boron for hardenability [4,5]. The first PHS was patented in 1977 and continuous improvements have been made since then, including better compatibility with high-temperature processing via the introduction of an AleSi coating, a refined balance between strength and weldability, and the improved predictive capabilities for microstructure and mechanical properties of hot-stamped parts [5e8]. ...
... The physical characteristics of steel materials are affected by the coil shape, high-frequency induction power, and cooling conditions [5]. Under high-frequency induction heating, the mechanical characteristics of the steel change due to the conduction of heat from the surface inside of the material [6,7]. ...
... To analyze the microstructures of the parts fabricated by each high-frequency induction heating power, a scanning electron microscope (S-4800, Hitachi Corp., City, Japan) was used. Specimens were 6 prepared by processing the plates subjected to high-frequency induction heating into the size of 10 mm × 10 mm. The specimens were observed after polishing with 1-μm diamond paste. ...
: In the automotive industry, high-strength plates have been increasingly used to reduce the vehicle body weight due to stringent regulations on fuel economy and safety. Such plates achieve tensile strengths as high as 1.5 GPa owing to the hot-stamping process. However, these plates suffer from scale generation and surface decarburization because of the surface oxidation of the material. Recently, studies on materials subjected to high-frequency induction heating have reported strength improvement owing to the coil shape and cooling method. This study presents the surface temperature changes, mechanical properties, and microstructures of boron steels subjected to longitudinal heating, which uses eddy currents generated from high-frequency induction heating. The surface temperature data were analyzed under different high-frequency induction heating powers (15, 18, 21, 24, 27, and 30 kW) and distances from the specimen (6, 9, and 15 mm). The mechanical properties and microstructures were also analyzed in different high-frequency induction heating coil directions. The correlations between the high-frequency induction heating power and the distance from the specimen with the maximum tensile strength were determined.
... Currently, the most commonly hot stamped parts include A-pillar, B-pillar, bumper, roof rail, rocker rail and tunnel. (Karbasian and Tekkaya, 2010) Parts made from thick sheets or plates are increasingly being manufactured via near-net shaping processes such as forging instead of cutting processes due to the considerable reduction of production costs. Plate forging is a semi stamping-forging process that implements the productivity rates seen in stamping and the forming loads seen in forging. ...
... Figure 3: CCT diagram for 22MnB5 (Billur, 2017) A continuous cooling transformation (CCT) diagram as seen in Figure 3, can be used to determine which microstructure or mixture of microstructures is most likely to form with a given cooling rate. (Karbasian and Tekkaya, 2010) The red lines in Figure 3 indicate the starting points of the different phases. The black solid and dashed lines indicate possible cooling rates from the austenitic phase. ...
Sheet bulk metal forming or plate forging is an innovative hot stamping and forging process combination that allows the stamping of thick parts with complicated geometries in a single step with press hardening. Without a dedicated cooling system, the high process temperatures cause an eventual heating up of the die. This leads to reduction of the temperature gradient between the tool and part, hindering the in tool quenching effect and resulting in parts with reduced hardness and strength. Dies at elevated temperatures are also subject to accelerated wear and erosion. The aim of this thesis was to firstly analyse the cooling down of the hot forging die with conventionally manufactured cooling channels and then to investigate the possibility of implementing additively manufactured cooling channels. To that effect, a simulation methodology to model the effect of cooling channels in the hot forging die was developed. Firstly material characterization of the billet material 34MnB5 was carried out in a Gleeble machine using hot tensile tests and the resulting flow curves were implemented in a thermo-mechanical forming simulation in the software FORGE NxT. The simulation was used to analyse the temperature evolution within the die and billet during forming. The model was validated based on thermal and geometrical experimental measurements carried out on an industrial forging tool. Next, a tool equipped with conventional cooling channels was analysed in a coupled thermal-fluid simulation in LS-Dyna and the results showed that they were sufficient to maintain the forging die temperature between 100 and 150 °C. However, due to drilling artefacts, the main punch exhibited stress concen-trations close to the tool yield limit. The flexibility of additive manufacturing was uti-lized to design conformal cooling channels for the main punch by avoiding the critically stressed zones and their cooling effect analysed using thermal-fluid simulations. The conformal designs were able to cool the forging die by 30 °C more than the conventional cooling channels, while stress analysis revealed that they did not weaken the die. It was also concluded that due to the short contact times, the cooling channels in rapid hot forging dies cannot influence the temperature distribution of the part, rather only the die temperature can be influenced and controlled.
... Friction problems in hot forming tools can be overcome by surface treatments and deposition of advanced coatings [8,9]. At this point, the localised heat input and the rapid solidification of materials inherent to laser processing, place laser-directed energy deposition (L-DED) in an advantageous position compared to traditional coating technologies [10]. ...
Wear-driven tool failure is one of the main hurdles in the industry. This issue can be addressed through surface coating with ceramic-reinforced metal matrix composites. However, the maximum ceramic content is limited by cracking. In this work, the tribological behaviour of the functionally graded WC-ceramic-particle-reinforced Stellite 6 coatings is studied. To that end, the wear resistance at room temperature and 400 °C is investigated. Moreover, the tribological analysis is supported by crack sensitivity and hardness evaluation, which is of utmost importance in the processing of composite materials with ceramic-particle-reinforcement. Results indicate that functionally graded materials can be employed to increase the maximum admissible WC content, hence improving the tribological behaviour, most notably at high temperatures. Additionally, a shift from abrasive to oxidative wear is observed in high-temperature wear testing.
... The hot forming or press hardening process is essentially to heat the steel sheet above the austenitizing temperature and quench it in a precision die that also forms the sheet into shape. The microstructure of the hotformed parts consists of martensite and in some cases, a small amount of retained austenite [7]. ...
Press-hardened steel (PHS) undergoes severe high-temperature oxidation after press hardening, which significantly degrades surface quality and increases manufacturing cost. This work reports the mechanism by which a coating-free PHS (CF-PHS) with the simultaneous presence of Cr, Si, and Mn alloying elements achieves excellent high-temperature oxidation resistance. It is found that Si and Mn in the CF-PHS promote the formation of a continuous Cr2O3 layer with good protective effect, thereby improving high-temperature oxidation resistance, during the press hardening. Moreover, the continuous amorphous SiO2 layer formed at the oxide scale/matrix interface and the Mn-rich oxides that improve the compactness of the scale further enhance the oxidation resistance of the CF-PHS.
... This generation aims to close the gap between the previous first-generation AHSS and the expensive, complex, twinning-induced second-generation AHSS. New generations of press-hardening steels (PHS) are also being developed offering an excellent combination of high strength and good local ductility allowing for more complex component shapes to be formed without the springback sometimes present in the cold forming of high-strength steels [15]. ...
To promote the use of new high-strength materials in the automotive industry, the evaluation of crashworthiness is essential, both in terms of finite element (FE) analysis aswell as validation experiments. This work proposes an approach to address the crash performance through high-speed imaging combined with 3D digital image correlation (3D-DIC). By tracking the deformation of the component continuously, cracks can be identified and coupled to the load and intrusion history of the experiment. The so-called crash index (CI) and its decreasing rate (CIDR) can then be estimated using only one single (or a few) component, instead of a set of components with different levels of intrusion and crushing. Crash boxes were axially and dynamically compressed to evaluate the crashworthiness of TRIP-aided bainite ferrite steel and press-hardenable steel. A calibrated rate-dependent constitutive model, and a phenomenological damage model were used to simulate the crash box testing. The absorbed energy, the plastic deformation, and the CIDR were evaluated and compared to the experimentally counterparts. When applying the proposed method to evaluate the CIDR, a good agreement was found when using CI:s reported by other authors using large sets of crash boxes. The FE analyses showed a fairly good agreement with some underestimation in terms of energy absorptions. The crack formation was overestimated resulting in too high a predicted CIDR. It is concluded that the proposed method to evaluate the crashworthiness is promising. To improve the modelling accuracy, better prediction of the crack formation is needed and the introduction of the intrinsic material property, fracture toughness, is suggested for future investigations and model improvements.
... Consequently, UHSS components have emerged as an excellent balance between vehicle safety and lightweight [4]. The efficacy of ultra-high-strength, press-hardened steel (PHS), invented to mitigate inherent spring-back and limit formability problems of traditional cold stamping through specialized hot forming technology [5], has been widely used. For example, in the weight proportion of PHS in Volvo XC90 and Audi A3, PHS accounts for 30% and 26%, respectively. ...
Press-hardened steel (PHS) with extremely high strength has wide applications in vehicle body manufacturing as an innovative lightweight material. However, the poor weldability of PHS results in poor weld toughness and a high risk of interfacial fracture, posing challenges to the resistance spot welding (RSW) process. Introducing an external magnetic field in the welding process to perform electromagnetic stirring (EMS), magnetically assisted RSW (MA-RSW) process has been proven an effective method to improve the weld toughness of high-strength steel, but it may increase the risk of expulsion. In this study, a new process called SPMA-RSW is developed to improve the weldability of PHS by combining MA-RSW and the stepped-current pulses (SP) technique, which can enlarge the weld lobe. Nugget appearance, microstructure, microhardness, and mechanical properties were systematically investigated by comparing traditional RSW, MA-RSW, SP-RSW, and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, finer the grain, and harden the nugget. This increased the lap-shear strength, energy absorption, and changed the fracture mode from brittle interfacial (IF) mode to ductile plug fracture (PF) mode at the same heat input. Then, a simple model was developed to reveal the mechanism of the effect of EMS on the fracture mode transition and was verified by experiment. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.
... Furthermore, these studies did not evaluate the ability of die covers to withstand the forming conditions occurring under a significant number of cycles. Earlier in the literature, the boron alloys particularly 22MnB5 have shown promising mechanical resistances 11,12 . Yu et al. 10 have tested 22MnB5 as mask manufactured by deep drawing and later subjected to 40 forging cycles. ...
Herein, the applicability and performance of 22MnB5 steel sheets as protective masks over hot forging dies has been analyzed. Two masks were obtained following two different approaches; by heat treatment of flat sheets in cooling conditions similar to the process of hot stamping and hot stamping on the axial geometry of a cylindrical part. In both processes the sheets were austenitized at 1100° C for 7 min and to obtain bainite microstructure, they were maintained at temperatures higher than 700°C. The flat and axial masks have been subjected to 100 forging cycles for each geometry and positioned on the lower surface of hot forging dies. Surface integrity has been analyzed from microhardness profiles, roughness tests, thickness measurements, optical and electron microscopy. Wear mechanisms have been observed in both masks which was more expressive in the axial mask. Abrasive wear and plastic deformation have been actively observed in both masks; however, they have shown firmness for application as masks.
... Press-hardened steel (PHS) has been widely adopted on automobile components, which enhances both lightweighting and crash resistance due to the high strength level [1][2][3][4]. AlSi coating is used to improve the surface quality of PHS after hot stamping, which effectively suppresses oxidation and decarburization [5][6][7]. Currently, AlSi coatings are a widely used technique for PHS in the global automotive industry. ...
... The manufacturing process of thermoformed parts includes both austenitizing heating and hot stamping, with resulting oxidation. Therefore, the deterioration of surface quality and the increase in the surface treatment process are problems that should be addressed when processing thermoformed parts [3,4]. Widely used Al-Si-coated hot-forming steel [5,6] has excellent oxidation resistance. ...
The surface of hot stamping steel is severely oxidized during heating, holding, and transfer from the heating furnace to the stamping die in the production of traditional automotive parts. Coating-free hot stamping steel with Cr and Si elements exhibits excellent oxidation resistance during hot stamping without the protection of a surface coating. This paper investigates the oxidation behavior of three types of hot stamping steel at 800–1200 °C. The results show that although Cr-Si hot stamping steel performs excellently short-term (≤7.5 min) for oxidation resistance, its long-term (≥15 min) or high-temperature (≥1100 °C) oxidation resistance is much lower than that of the conventional hot stamping steel 22MnB5, affecting the production and surface quality control of the new coating-free Cr-Si hot stamping steel. By analyzing the oxidation kinetics and characterizing the structure of oxide layers in hot stamping steel, it was found that the structural change in the Cr and Si element enrichment layer between the oxide scale and the substrate varied in oxidation performance at different temperatures. When the oxidation temperature was below 1000 °C, the solid Cr and Si enrichment layer acted as a barrier to prevent the diffusion of Fe ions. When the oxidation temperature exceeded 1100 °C, the molten Cr and Si enrichment layer effectively adapted to the substrate and avoided blistering. Meanwhile, Fe2SiO4 penetrated the Fe oxide layer along the grain boundary and became a rapidly diffusing channel of Fe ions, contributing to a significant increase in the oxidation rate.
... Press hardened steel (PHS) has been widely used in vehicles to realize both lightweight and crashworthiness requirements in recent years [3]. This material avoids inherent spring-back and limits formability problems of traditional cold stamping of ultra-high-strength steels (UHSS) due to specific hot forming technology [4]. ...
Press-hardened steel (PHS) with extremely high strength has wide applications in vehicle body manufacturing as an innovative lightweight material. However, the poor weldability of PHS results in low weld toughness and a high risk of interfacial fracture, posing significant challenges to the resistance spot welding (RSW) process. Magnetically assisted RSW (MA-RSW) process, which introduces an external magnetic field in the welding process to perform electromagnetic stirring, has been shown to effectively improve the weld toughness for high-strength steel, although it may increase the risk of expulsion. To further improve the weldability of PHS, this study proposes a new process called stepped-current pulses magnetically assisted resistance spot welding (SPMA-RSW), which combines the MA-RSW with the stepped-current pulses (SP) method to enlarge the weld lobe. The study systematically investigates nugget appearance, microstructure, microhardness, and mechanical properties by comparing traditional RSW, MA-RSW and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, refine the grain size and harden the nugget region of PHS welds, resulting in increased lap-shear strength and a transition from a brittle interfacial (IF) mode to a ductile button pullout fracture (BPF) mode at the same heat input. Specifically, the peak load and energy absorption were increased by 32.3% and 84.2%, respectively. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.
... The manufacturing of complex components and assemblies, such as vehicle bodies, requires additional punching operations after the actual forming process in order to be able to realize the final component geometry [1,2]. On the one hand, this includes the trimming of formed parts, and on the other hand, holes or forming contours are cut into the component to ensure further processing as well as the function of the component [3]. Therefore, a primary challenge is ensuring the required cut quality [4,5]. ...
Given the use of high-strength steels to achieve lightweight construction goals, conventional shear-cutting processes are reaching their limits. Therefore, so-called high-speed impact cutting (HSIC) is used to achieve the required cut surface qualities. A new machine concept consisting of linear motors and an impact mass is presented to investigate HSIC. It allows all relevant parameters to be flexibly adjusted and measured. The design and construction of the test bench, as well as the mechanism for coupling the impact mass, are described. To validate the theoretically determined process speeds, the cutting process was recorded with high-speed cameras, and HSIC with a mild deep-drawing steel sheet was performed. It was discovered that very good cutting edges could be produced, which showed a significantly lower hardening depth than slowly cut reference samples. In addition, HSIC was numerically modelled in LS-DYNA, and the calculated cutting edges were compared with the real ones. With the help of adaptive meshing, a very good agreement for the cutting edges could be achieved. The results show the great potential of using a linear motor in HSIC.
... Multiple heating methods have been implemented alone or in combinations for hotforming processes, including conduction-based hot plates [1,3], batch or continuous convection furnaces powered by gas or electricity [5,8], resistive heating [2,9], infrared heaters [10] as well as electromagnetic induction heating [6, [11][12][13][14][15]. The latter uses a high-frequency alternating current through a conducting coil to heat up the metal sheet through eddy currents and the Joule effect. ...
... 22MnB5 steel is an ordinary steel with an ultimate tensile strength of 1.5 GPa after hot stamping. During the hot stamping process, 22MnB5 steel is heated to 900~950 • C, held for 5-10 min, and then stamped and quenched in a closed and water-cooled mold to obtain full-martensite [4]. However, there are some problems in the hot stamping process, such as oxidation and decarburization on the surface of the steel. ...
To weaken the harm of Al–Si coating and increase the strength of welded joints, variable thicknesses of Ni foil (Ni, an austenitic formation element) were added to the lap laser welding Al–Si-coated 22MnB5 hot stamping steel/galvanized steel joints. The joints’ weld appearance, microstructure, and mechanical properties were explored. The weld altered from an X shape to a Y shape with an increased thickness of Ni foil. During welding, Al–Si coating was melted and diluted into the welding pool, forming δ-ferrite (a rich-Al phase with low toughness and strength) in the fusion zone (FZ) and fusion boundary (FB). This phase deteriorated the strength of the joints. After adding Ni, the amount and size of the δ-ferrite phase decreased. With a significant thickness of Ni foil, δ-ferrite disappeared. However, a new phase (fresh martensite (FM), which formed at low temperature and contained rich Ni) probably formed, except PM (previous martensite (PM), which formed at high temperature and contained little Ni or no Ni). The heat-affected zone (HAZ) on the side of 22MnB5 comprised a coarse martensite zone, refined martensite zone, martensite + ferrite zone, and tempered martensite zone from the FZ to the basic material. HAZ on the side of galvanized steel mainly contained ferrite and pearlite. After adding the Ni foil, the strength of the joint was greater than that without Ni. The maximum strength of the joint can be up to 679 MPa because of the disappearance of δ-ferrite. Meanwhile, the toughness of the joint increased. The fracture mode was from three mixed fractures (cleavage, quasi-cleavage, and dimple) to one fracture (dimple).
... The finite element method has been applied to failure prediction of warm deep drawing of cylindrical cups [10]. The literature has extensively investigated the stamping technology applied, especially to steel sheets [11,12]. Pereiraet et al. [13] investigated friction and deformation-induced heating during stamping under room temperature conditions. ...
This article investigates the impact of temperature regimes, corresponding to various climatic zones, on the manufacturing process and operation of beverage can ends made of aluminum alloy AA5182. The production process of aluminum beverage can ends involves multiple steps, including melting, rolling, and stamping, where different temperatures can influence both the production process and the properties of the final product. Furthermore, the mechanical behavior of the final product is affected by the aging process of alloy AA5182, which progresses at varying rates depending on the temperature conditions during storage. The objective of this study is to simulate the production process under various climatic conditions using the finite element method (FEM) and experimentally investigate the dependence of the strength of alloy AA5182 can ends on storage time and temperature. The findings of the study reveal that, under temperature conditions corresponding to warmer climates, the punching force can be reduced by approximately 15% compared to production in colder climates. Additionally, the strength of the finished can exhibits a decrease of about 10% during a month of storage in a warm climate, while no significant decrease was observed in colder climates. These results hold practical significance for the beverage production industry, as the manufacturing and operation of beverage cans are localized in diverse climatic zones.
... Furthermore, these studies did not evaluate the ability of die covers to withstand the forming conditions occurring under a significant number of cycles. Earlier in the literature, the boron alloys particularly 22MnB5 have shown promising mechanical resistances [12]. Yu et al (2019) have tested 22MnB5 as mask manufactured by deep drawing and later subjected to 40 forging cycles. ...
... PHS is widely used for automobile structural component due to its impressive mechanical properties and no springback after forming [1,2]. Based on the application, PHS is mainly divided into anti-intrusion PHS and high energy-absorbing PHS [3][4][5]. ...
1000 MPa grade low-carbon martensite press hardening steels (PHS) are widely used in energy-absorbing domains of automotive parts, such as the bottom of a B-pillar. To prevent oxide scale formation during hot forming, this PHS is often required to be protected by an additional Al–Si coating. In addition, although the low carbon martensitic microstructure grants it excellent bending toughness, the ductility tends to be limited. In this study, a novel 1000 MPa grade ultrafine-grained (UFG) martensite–ferrite (F–M) dual-phase (DP) PHS with superior oxidation resistance was designed using tailored additions of Cr, Mn, and Si, and refining the initial microstructure. Only 0.55 ± 0.18 μm thick oxide film is formed in the designed steel during austenitizing heating and stamping, which is significantly lower than the 24.6 ± 3.1 μm thick oxide film formed in conventional 1000 MPa grade low-carbon martensite PHS under the identical condition. The superior oxidation resistance of designed steel can be attributed to the rapid formation of the protective Si-rich, Cr-rich, and Mn-rich oxide layers during annealing. Moreover, due to the presence of ferrite and ultrafine microstructure, the designed steel also shows a significant improvement in ductility from 8.5% to 16.8% without sacrificing strength and bending toughness compared with conventional 1000 MPa grade low-carbon martensite PHS.
... In metallurgical processes, the stamping stage [1] involves the installation of workpieces onto a mold, which poses significant risks to humans. Consequently, some researchers have explored the use of exoskeletons [2] to assist humans. ...
... Recently, hot stamping technique has been applied to boron steels for fabricating automotive components with ultrahigh tensile strength (~1500-2000 MPa) [8,9]. Hot stamping utilizes the good formability of austenite at high temperatures to solve the springback issue and achieves ultrahigh strength by forming a full lath martensite microstructure via die quenching of austenite [8,10]. ...
... Hot stamping is an advanced sheet metal forming technology developed for manufacturing ultra-high-strength steels (UHSS) in the automotive industry, giving light weight, good formability and negligible springback [1]. The process involves a full austenitisation heat treatment, followed by forming at elevated temperatures and in-die quenching to room temperature simultaneously. ...
Medium-Mn (MMn) steels with low austenitisation temperatures have recently attracted much research attention because of their suitability for hot stamping to save cost and improve productivity. However, research on their formability under conditions that mimic industrial hot stamping is still lacking. In this study, an innovative method for biaxial testing, which includes a cruciform specimen design, is used on a Gleeble 3800 simulator to obtain the forming limit curves (FLCs) and fracture forming limit curves (FFLCs) of a representative MMn (8Mn) steel. The localised necking strain and fracture strain are measured using a newly developed spatio-temporal method. The FLCs and FFLCs are constructed using strain limits in different loading states, e.g. uniaxial, plane strain and equi-biaxial stretching. The results show that the loading state conditions are acceptably simulated using the biaxial test system, and the FLCs and FFLCs are well-constructed, displaying a V shape. The data will contribute to the design of forming process. In addition, the forming strain limits of the MMn steel are much higher than those of a conventional boron steel (22MnB5) under the same hot stamping conditions, suggesting that the MMn steel has great potential and benefit as an alternative in hot stamping.KeywordsMedium Manganese SteelForming Limit CurveHot StampingBiaxial Tensile Test
... During a typical direct hot stamping process, a boron alloyed steel blank is first heated in a continuous roller hearth furnace up to the austenization temperature and held (referred to as soaking) in order to transform the as-received pearlite-ferrite microstructure into a fully austenitic state [5]. The heated blank is then transferred into a cold die and simultaneously formed and quenched within the die to ensure that the part transforms into a fully martensitic microstructure [6]. ...
With the growing demand for new energy vehicles, high-strength and lightweight hot-stamping steel production has become increasingly important. The present study investigated a fast dip tester featuring a sub-rapid solidification process to refine the as-cast microstructures and adjust the mechanical properties of the 22MnB5 hot-stamping steel. It was found that the main microstructure of the studied steel evolved from ferrite and pearlite to bainite and martensite as the cooling rate increased. Moreover, the grain size of the sub-rapid solidification sample was significantly refined with high-density dislocations in the substructure. Still, the sub-rapid solidification process had little effect on grain orientation and texture. In addition, compared with the slow-cooled sample, the tensile strength of the sub-rapid solidification sample increased, but the elongation decreased.
In this study, an alternative to the conventional cold-roll-forming process was developed. The new in-line incremental die forming process is a continuous manufacturing process that can provide rapid prototyping of automotive components. This new process combines the pressing and drawing operations. The feasibility of using ultra-high-strength steel (UHSS) sheets to manufacture real automotive components usually produced with the conventional cold-roll forming process was evaluated. Finite-element simulations incorporating the mechanical properties of plasticity and ductile fracture were conducted to validate the new process. The results demonstrated that the proposed forming process efficiently produced automotive components with consistent shapes and lengths, and the simulation outputs were in good agreement with the experimental data in terms of dimensional accuracy and quality. Additionally, sensitivity analyses of process parameters such as friction and die design provide valuable insights for optimizing the newly proposed formation process of UHSS sheets.
The lightweight of automotive structures have expanded the application areas of galvanized 22MnB5 steel. However, the susceptibility of galvanized 22MnB5 steel to liquid metal embrittlement (LME) during hot stamping processes poses challenges to its performance. In this study, a method combining pre‐cooling and tube hydro‐forging (THFG) was proposed to produce galvanized tubular hot stamping parts. The manufacturing process primarily involves the following steps: firstly, heating and maintaining the temperature of the galvanized tubular component for a certain duration. Subsequently, the heated tubular component to a different environment for pre‐cooling treatment. Lastly, conduct a tube hydro‐forging operation on the tubular material after pre‐cooling. The composition and morphology of the zinc coating, as well as the microstructure and mechanical properties of the 22MnB5 steel, were investigated using two pre‐cooling methods (air and boiling water) at different hydro‐forging temperatures. The results showed that when the initial hydro‐forging temperature was below 800°C, the pre‐cooling combined with the tube hydro‐forging process eliminated LME. With increasing pre‐cooling time, the oxidation reaction resulted in the formation of ZnO and Fe 2 O 3 on the coating, which harmed the corrosion resistance of the coating. Boiling water pre‐cooling exhibited a faster cooling rate, higher production efficiency, and stable mechanical properties, making it more suitable for industrial production. By comparing the composition, morphology, and mechanical properties of the coatings at hydro‐forging temperatures of 800°C and 500°C, it was concluded that the optimal forming temperature for boiling water pre‐cooling was 750‐800°C. The findings of this study provide practical and theoretical references for the production of galvanized 22MnB5 tubular hot stamping parts in industrial applications.
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B-pillar structural part is the key component of lightweight automobile body structure. To possess good strength and plastic fit, B-pillar generally has gradient characteristics of mechanical properties along its length, and there is a characteristic transitional range from high strength and low elongation hard parts to low strength and high elongation soft parts. To detect the local mechanical properties, the destructive sampling method is conventionally used for evaluating the mechanical properties of the transition region of automobile B-pillar. It is time-consuming and impossible to directly conduct rapid non-destructive testing of parts. Hence, a micromagnetic automatic detection system for the B-pillar of the body structure is developed to realize the non-destructive evaluation of the mechanical properties of the transition region. The BP neural network model is established by the magnetic characteristic parameters and mechanical performance indicators obtained from the B-pillar. It helps to realize the quantitative characterization of the mechanical properties of the transition region of the B-pillar structural part of an automobile, which has important engineering application prospects.
Manganese-boron steels are widely used in hot stamping, resulting in 1500 MPa (PHS 1500) or even 2000 MPa (PHS 2000) tensile strength after the hot stamping process. These steels are most commonly used in industry in the form of AlSi-coated sheets. However, recent developments are being made to enable the application of uncoated steels, resulting in cost reduction, but bringing technological challenges to be overcome. The coefficient of friction (µ) is a fundamental parameter in sheet metal forming and is influenced by several factors, such as: surface roughness, working temperature, lubrication, contact pressure and sliding speed. In hot stamping, it is known that the temperature can have a great influence on µ values for coated plates, but there is little information about uncoated steels. This work aims to investigate the tribological behavior of uncoated PHS 2000 steel plates using the pin-on-disk test. Tests were carried out under three temperatures (800 °C, 600 °C and 400 °C) and contact pressures of 5 and 10 MPa. Results showed that the friction coefficient tends to reduce with increasing temperature at a pressure of 5 MPa, as result of oxide formation on the steel’s surface. However, at a pressure of 10 MPa, this reduction was not observed.
The production of sheet safety parts for car bodies is currently carried out using press hardening technology. This technology enables the creation of complex-shaped profiles even from high-strength steels without a significant spring-back effect. In combination with modern high-strength steels, which have retained austenite in their structure, it is possible to achieve high ultimate strength and ductility values. As a result, these parts are used in bodywork areas with high energy absorption requirements during impact. To achieve the required mechanical properties, suitable processing parameters must be selected. High-strength steel with 0.2%C, 3%Mn and 2%Al was used for the experiment. Press-hardening was carried out in a tool fixed in a hydraulic press and can be heated up to temperatures around 450 °C. Different tool temperatures of room temperature and 425 °C were tested and different holding times in the tool from 1 s to 600 s. After hardening in a tool at RT, the ultimate strength of about 1400 MPa with a ductility of 18 % was obtained. But quenching and holding in the preheated tool caused the ductility to increase to 28% with a drop in ultimate strength to 1050 MPa.
Press hardened steel (PHS) plays a key role in the manufacture of anti-collision structural components. The formation of δ-ferrite is a suffering issue for the laser welding of Al–Si coated PHS. Oscillating laser was used to weld Al–Si coated 1.5 GPa PHS and novel 2 GPa PHS, and the effect of laser offset on the microstructure and properties of the dissimilar welded joints was studied. The results showed that a perfect weld profile was achieved by laser offset welding (LOW), without any welding defects. The δ-ferrite formed in as-received welds of laser alignment welding (LAW) with high Al content (up to 2.9 wt.%), but it disappeared with the laser offset to 2 GPa PHS, and the maximum Al content in the segregation zone reduced to 1.2 wt.%. After post-welding heat treatment, the δ-ferrite was coarsened and the α-ferrite formed in the secondary Al-rich area for the high Ac3 temperature, but the α-ferrite was few and fine in LOW welds. The hardness in the LAW welds was lower than that in the LOW welds, due to the presence of δ-ferrite, as well as less carbon content and Ti and V alloying elements. The tensile strength (1561 MPa) and elongation (5.4%) with LOW were higher than those with LAW (1490 MPa, 3.1%), and the fracture occurred in the Al–Si coated PHS. It is indicated that adjusting the laser offset is effective to prevent the formation of δ-ferrite and is potential to avoid the formation of α-ferrite. It also provides a wide heat treatment temperature window for the dissimilar welds of 1.5 GPa PHS and novel 2 GPa PHS.
For the press hardening process an Al-Si coating is usually applied on the surface of typical boron-manganese steels (22MnB5), to form a diffusion layer for protection against scaling. However, the diffusion layer requires long heating times and is environmentally questionable. This paper describes a new approach to substitute the Al-Si coating by a stainless steel (X5CrNi18–10) to provide an environmentally friendly alternative, save heating energy and raise the production efficiency. For this, the hot roll bonding of 22MnB5 and two protective layers of X5CrNi18-10 to produce laminated composites are investigated by experiments and FE-simulations. The aim is to set up a computer-aided design of the hot roll bonding using a calibrated isothermal bond strength model and to validate it by hot roll bonding experiments. Firstly, the bonding properties of the material combination 22MnB5/X5CrNi18-10 are characterized at 1000 ℃ in a lab scale experiment. With those truncated cone tests an isothermal bond strength model is parametrized by bond strength measurements at different height reductions and stress states. Secondly, the contact parameters are calibrated by the simulations of the truncated cone tests. With the calibrated contact parameters, the experimental compression and de-bonding forces are reproduced by the simulations with a general deviation of 15%. Finally, the calibrated contact parameters and the bond strength model are implemented into the hot roll bonding process model of the first two rolling passes. According lab-scale roll bonding experiments are performed to validate the simulations. A comparison of the rolling forces as well as layer thicknesses after each pass show a good agreement between the simulations and experiments.KeywordsRoll bondingFE-simulationMultilayer
Studies on crash performance show that hot stamped components with homogeneous mechanical properties can reach their limits in highly stressed car body areas. To face this challenge, hot stamped components can be optimized by integrating ductile areas. A suitable technology for adjusting tailored properties is based on a special furnace chamber within a multi-chamber furnace in which a cooled aluminum mask protects the blank locally from incident heat radiation. Simultaneously, the mask absorbs the blank’s own radiation. In addition to the temperature and radiation properties, this heat exchange depends in particular on the orientation and position of the radiating surfaces in the furnace chamber. For this reason, the mask distance to the blank can influence the width and characteristics of the transition zone after partial hot stamping. In order to predict the correlation between hard and ductile area, the process of partial hot stamping with varying mask distances is modelled in LS-DYNA® and validated experimentally. The results indicate that it is feasible to simulate the influence of the mask distance on the resulting hard, soft and transition zone after partial hot stamping. Furthermore, the results demonstrate that transition zones on a partially hot stamping part can be designed more flexibly, thus extending the limits in terms of part complexity.KeywordsHot StampingHeat TreatmentTailored PropertiesFinite-Element Simulation
Multi-stage press hardening enables a stepwise control of the thermo-mechanical history during press hardening, thereby facilitating a flexible hardness tailoring. Further degrees of freedom arise from utilizing rapid austenitization by induction heating within the first process stage. To assess the potential of rapid austenitization and a subsequent multi-stage thermo-mechanical history for tailoring the hardness, near-process characterizations are carried out with 22MnB5 steel. By a variation of the austenitizing temperature and dwell time, when cooling at a rate of 30 K/s, a hardness between 310 and 448 HV was set. By inducing a strain of 0.1 after rapid austenitization, the hardness was reduced by 15% due to an accelerated ferrite and bainite transformation. However, it was observed that a second, subsequent induced strain can potentially increase the hardness by work hardening. Using the example of multi-stage press hardening with rapid austenitization implemented in a progressive die, the hardness of 22MnB5 sheet material is tailored. A hardness between 350 and 445 HV was set by varying the austenitization parameters and the strain induced with a controllable blank holder.KeywordsMulti-stage; Press hardeningTailoringRapid heating
Due to the demographic developments of an increasingly aging society, the number of employees retiring is also rising. Likewise, the fluctuation within the individual companies has increased drastically within the last few years. As a result, experience gained over many years is being lost. On the other hand, there is a strong trend towards an ever-increasing number of very complex manufacturing processes with a large number of process influencing variables, where the human monitoring capabilities are limited. In this work we study the complex process of hot stamping of metal blanks for car body parts with a demonstrator plant. The demonstrator plant consists of five main components, comprising a magazine for material feed, an industrial robot for transferring the raw material and the finished part, an annealing furnace, and a hydraulic press with the temperature controlled hot forming tool inside. All machines have open communication interfaces with which all sensor data can be accessed. The demonstrator plant is additionally outfitted with different sensors measuring crucial process parameters. We use temperature sensors in the punch and die and make use of capturing the structure-borne sound and mechanical vibrations during the process to closely monitor the process. This sensor data is analyzed and methods of machine learning are used to detect anomalies in the process data. The goal of this work is to develop a methodology for the systematic detection of anomalies in the hot forming process using machine learning methods within a demonstrator plant that are suitable for real time anomaly detection for a high level control system. We use unsupervised machine learning techniques and neural networks to detect anomalies with great accuracy on our test data.KeywordsHot stampingindustrial control systemsindustrial AIsmart manufacturing
Additive manufacturing of hot forming tools provides increased flexibility regarding geometry and proximity of the cooling channels to the tools’ surface. In this field, the application of Directed Energy Deposition has not been investigated extensively. The conventional manufacturing route of cooling channels in hot stamping tools comprises the segmentation of the tool and hole drilling. This leads to the disadvantages of possible leakages and low flexibility regarding the channels’ size and location. Thus, a novel combination of Directed Energy Deposition and ball burnishing for leveling the additively manufactured surfaces is established to manufacture hot stamping tools for the forming of hat profiles. In the additively manufactured punch, near-surface cooling channels are integrated after a load-adapted definition of channel geometry and path, both based on numerical and experimental analysis. On the blank holder side, an additively manufactured texture, targeting the control of the heat balance and material flow, is applied. With these tools, hot stamping tests with a hat profile (22MnB5) are conducted and compared with conventionally manufactured tools with drilled cooling channels. It is demonstrated that the near-surface cooling channels are able to decrease the punch temperature in the hot stamping process by up to 50% in the investigated configuration. The textured blank holder creates a more homogeneous temperature distribution over the hat profile during forming. As a result, part properties can be enhanced by a lowered sheet thickness reduction and a higher and evenly distributed hardness. Numerical simulations can be used to depict those effects during process planning.KeywordsHot stampingDirected Energy Deposition (DED)near-surface cooling channelstemperature control
Im Rahmen der Arbeit werden die wesentlichen Einflussgrößen in den unterschiedlichen Prozessschritten des Presshärtens sowie ihre Wechselwirkungen untereinander untersucht. Schwerpunkt der Arbeit ist einerseits die Bestimmung des Wärmeübergangs zwischen Platine und Werkzeug in unterschiedlichen Kontaktsituationen. Andererseits steht die Charakterisierung des Stahls 22MnB5 unter den prozessspezifischen Bedingungen im Vordergrund.
Im Rahmen der Dissertation werden Methoden zur prozessnahen Charakterisierung des Umform- sowie des Umwandlungsverhaltens des presshärtbaren Bor-Manganstahles 22Mn85 vorgestellt. Einen zentralen Schwerpunkt der Arbeit nimmt hierbei die Beschreibung und Modellierung des Fließverhaltens des Versuchswerkstoffes in Abhängigkeit der signifikanten Prozesseinflussgrößen ein.
Better formability, less forming force and satisfactory quality are the
most important characteristics of warm forming processes. However, the
material models for either cold forming or hot forming cannot be
directly adopted for the numerical simulation of warm forming processes.
Supplement and modification are necessary. Based on the Zener-Hollomon
formulation, additional terms are proposed in the presented work to
describe the softening effect observed during warm forming processes as
well as the strain hardening effect. The numerical simulation provides
detailed information about the history and distribution of both
deformation and temperature, the phase transformation can then also be
evaluated, provided the experimental data are available.
Considerable changes have occurred in metal forming in the last decade. A record of these changes can be found in keynote papers presented by the members of the Scientific Technical Committee—Forming, at the CIRP Annual General Meeting each year. The keynote papers are excellent references on important developments in metal forming and are used as a reference, globally. Not only is this paper a compendium of most of the keynotes presented, but from 2001 onward, it has updates on new information on five keynote subject areas. The authors of each keynote have written an update with new information that has developed since the writing of the keynote. The authors of each section are shown in order of presentation.
To improve the modelling of the behaviour of steel profiles in the forming and quenching process, the in- fluences of high-temperature plastic deformation and applied stress on the martensitic transformation were investigated in a B-bearing steel by dilatometric measurements and compression tests. The plastic defor- mation of austenite was found to enhance ferrite formation so significantly that the dilatation due to the low-temperature transformation decreases even at a cooling rate of 280°C/s. The presence of ferrite in the microstructure results in markedly lower hardness and flow stress than the completely martensitic mi- crostructure. Possibilities to avoid ferrite formation have been discussed. Stress applied during the marten- sitic transformation increases diametric dilatation by as much as 200% under axial compression, which seems to result from the preferred orientation of the martensite formed. However, subsequent to a high- temperature plastic deformation, the influence of applied stress remains much smaller.
At present, hot stamping represents an innovative manufacturing process for forming of high strength steels, implying a sheet
at austenite temperature being rapidly cooled down and formed into a die at the same time (quenching). This forming process
is used for the manufacturing of automobile structural components with a strength of up to 1,500 MPa, thus enabling extensive
cost savings and good crash performance. Better formability at elevated temperatures and lower springback are further advantages
of parts formed by hot stamping. The Finite Element Analysis is an essential precondition for a good process design including
all process parameters. This paper presents the finite element simulation of a hot stamping process by means of experimentally
calculated material data and describes a number of procedures for the simulation of hot stamping, aiming at a notable decrease
in computation time. For a faster calculation thermal and mechanical phenomena are decoupled in two simulation programs.
In the present paper, physically-based constitutive models are developed for bcc and fcc metals. First, the theoretical basis of rate- and temperature-dependent finitedeformation plasticity is examined at the dislocation scale, leading to specific constitutive models for both bcc and fcc polycrystals. Using the results obtained through some novel experimental techniques, and for illustration, constitutive parameters are obtained for commercially pure tantalum (bcc) and OFHC copper (fcc). Then constitutive relations are developed for single crystals (mesoscale), based on exact kinematics of crystollographic slip due to the dislocation motion, accompanyed by elastic lattice distortion. The transition from single (mesoscale) to poly crystal (macroscale) response requires homogenization and averaging theorems. Here, however, we shall confine our effort to the simple Taylor averaging, and obtain for comercially pure tantalum the overal stress-strain relations based on crystal plasticity. A detailed discussion of various averaging methods can be found in Nemat-Nasser (1998a,b). 101
Production of parts with high strength by means of hot-forming process has been receiving considerable attention lately. Application of aluminized steel with good hardenability for this process has been investigated*. Tensile strength over than 1,500MPa was obtained with steel containing around 0.2mass% C. Surface coated layer changed from Al-10% Si to Fe-Al(-Si) phases through the hot-forming process. The steel with Fe-Al(-Si) surface layer showed good paintability even without phosphatized treatment. After painting, they showed good corrosion resistance in JASO-CCT as galvanized steel did. The steel showed good spot weldability due to the stable surface layer at high temperature. (*license product within the frame of global strategic alliance with Arcelor (USIBOR1500)).
Direct and indirect hot stamping presently constitutes one of the most innovative forming technologies in the automotive industry through the combination of forming and hardening in one process step or line. Thus, structural components with strength up to 1600 MPa can be accomplished with the quench hardenable ultra-high strength steel 22MnB5. With respect to the numerical investigation of the feasibility of different parts the knowledge of various thermal and mechanical material characteristics determined under process relevant conditions are required. Within the scope of this paper different experimental methods will be introduced for the determination of material properties according to the typical time-temperature characteristics of the hot stamping process, as well as the modelling of it as input data for the FE analysis.
The increasing demand for car body structures with optimised energy absorption capacity and the ability to maintain their structural integrity even under the highest dynamic load has stimulated the development of new thermo-mechanical process routes for the production of pressed and roll-formed sheet metal parts in order to combine both extreme formability and a highest level of strength for the final product. These process routes offer a high potential for further improvements in the field of strength-strain correlation and load adapted property distribution of the components, as well as an enhanced process productivity. A new type of thermo-mechanical tailored processing of sheets and profiles is presented, based on the adequate application of differential heating and cooling strategies. By the control of local microstructural effects, the components develop a property distribution adapted to complex load situations. New tooling concepts complement these developments in order to ensure high process efficiency and reliability.
During forming process, e.g. hot stamping of manganese-boron steels or deep drawing of stainless steels, phase transformation from austenite to other phases can occur. This phenomenon is induced by temperature or by plastic deformation. For a more realistic prediction of the resulting compo-nent properties, like residual stresses and distortion, by means of FEA it is essential to consider the complex effects of phase transformation in process simulation. In this paper two material models which consider these effects are presented. The first material model is for hot stamping and in-cludes phase transformation and its interaction with the stresses. The second model is a material model considering transformation induced martensite evolution of metastable austenitic steels. Both material models allow an adequate description of the specific material behavior in the men-tioned forming processes. Results in calculation show good correlation with experimental results.
This paper presents a Finite Element Simulation of hot stamping processes by means of experimentally calculated material and process data for the process design. The quality and the significance of the simulation results are strongly dependent on the accuracy of the thermal and mechanical parameters used in FE models. Therefore, the process and material parameters used in the FE model in this investigation were optimized with a statistical method and by means of an experimental analysis. The identification of interaction between the thermal and mechanical parameters was based on design of experiments. This statistical method ensured an efficient analysis of the process parameters consisting of heat transfer coefficients as well as thermal and mechanical characteristics of material. Insights from design of experiments need to be applied in order to enable an efficient FE modeling of hot stamping for process optimization and process design.
Hot stamping of quenchable high strength steels is a fairly new process. It is increasingly used for automotive applications. The first stage of this process consists in heating up a blank in a furnace in order to reach a stable austenitic state. This hot blank is then moved into a stamping press. It is simultaneously stamped and quenched by cold tools. The stamped part displays superior mechanical properties. Arcelor has been developing a numerical tool to optimise the feasibility and the design of hot stamped parts made of USIBOR 1500 P®. This objective has been challenging because of the complex thermo-mechanical and metallurgical interactions and their influence on formability. It required to combine experimental tests and numerical simulations. Experimental hot stamping tests were carried out to provide formability data and to support the development of a fracture criterion. These kinds of tests are very different from the standard cold stamping tests, and require a specific preliminary analysis. This is the reason why experimental tests and a methodology to deduce formability data were developed. A set-up was built for Nakazima hot stamping tests. The results of a sensitivity analysis are presented, together with a methodology which allows to determine critical strains value. Bragard's method was modified in order to obtain critical strain values from such Nakazima hot stamping tests. The experimental sensitivity analysis then allows determine these parameters as a function of process conditions (thickness, temperature, strain path). The necking predictions given by this method are compared and validated with other laboratory tests results and analyses of industrial parts.
Direct and indirect hot stamping presently constitutes one of the most innovative forming technologies in the automotive industry through the combination of forming and hardening in one process step or line. Thus, structural components with strength up to 1600 MPa can be accomplished with the quench hardenable ultra-high strength steel 22MnB5. With respect to the numerical investigation of the feasibility of different parts the knowledge of various thermal and mechanical material characteristics determined under process relevant conditions are required. Within the scope of this paper different experimental methods will be introduced for the determination of material properties according to the typical time-temperature characteristics of the hot stamping process, as well as the modelling of it as input data for the FE analysis.
Due to increased usage of high strength steels in scope of lightweight construction design in this context thermal assistant forming technologies like hot stamping of boron-manganese steels of the type 22MnB5 obtained more and more industrial relevance in the last five years. For a reliable process design the knowledge of the material properties in dependency of the process influencing characteristics and hereby mainly the temperature displays a base requirement. In the following publication an overview will be given about different experimental methodologies developed at the Chair of Manufacturing technology for being capable to investigate the material behavior according to the typical time-temperature profile of hot stamping. Furthermore, main experimental results acquired regarding the thermal, the mechanical and the tribological properties of 22MnB5 will be exemplarily shown as well.
The increasing demand for car body structures with optimised energy absorption capacity and the ability to maintain their structural integrity even under the highest dynamic load has stimulated the development of new thermo-mechanical process routes for the production of pressed and roll-formed sheet metal parts in order to combine both extreme formability and a highest level of strength for the final product. These process routes offer a high potential for further improvements in the field of strength-strain correlation and load adapted property distribution of the components, as well as an enhanced process productivity. A new type of thermo-mechanical tailored processing of sheets and profiles is presented, based on the adequate application of differential heating and cooling strategies. By the control of local microstructural effects, the components develop a property distribution adapted to complex load situations. New tooling concepts complement these developments in order to ensure high process efficiency and reliability.
For lightweight construction, besides the conventional lightweight materials like aluminium and magnesium, ultra high strength steels are being increasingly used in the automotive industry. For example the quenchenable ultra high strength steel 22MnB5 is applied for the production of complex crash relevant components. The processing of the boron micro-alloyed steel 22MnB5 is carried out in an innovative, non-isothermal hot sheet metal forming process, which combines forming and quenching in one process step. The process itself as well as the experimental determination of relevant process parameters like thermal and mechanical material properties with respect to a more reliable numerical process design will be introduced and explained.
The true stress-strain curves of polycrystalline aluminum, copper, and stainless steel are shown to be adequately represented by an exponential approach to a saturation stress over a significant range. This empirical law, which was first proposed by Voce, is expanded to describe the temperature and strain-rate dependence, and is put on a physical foundation in the framework of dislocation storage and dynamic recovery rates. The formalism can be applied to the steady-state limit of creep in the same range of temperatures and strain rates; the stress exponent of the creep rate must, as a consequence, be strongly temperature dependent, the activation energy weakly stress dependent. Near half melting temperature, where available work-hardening data and available creep data overlap, they match. Extrapolation of the proposed law to higher temperatures suggests that no new mechanisms may be necessary to describe high-temperature creep. A new differential equation for transient creep also follows from the empirical work-hardening law.
Within hot stamping of quenchenable steels the blank is heated up to austenitization temperature, transferred to the tool, formed rapidly and quenched in the cooled tool. Essential for the complete process is the heating of the blank which is currently carried out with roller hearth furnaces. As a consequence of rising energy costs new and more time and energy effective heating systems are needed. The paper presents investigations on induction heating as an alternative heating technology. Results concerning the process windows as well as material characteristics and grain structure will be presented and discussed.
Stresses are developed internally in metals when a change in density and strength arises from a phase transformation. It is shown that plastic flow, generally confined to the weaker phase, results from the accommodation of strain due to the transformation front. The form of the plastic flow is considered in terms of the extreme cases of yield and creep behaviour. It is deduced that, for a complete cycle both ways through the transformation temperature, the resultant deformation varies linearly with the applied stress (provided this is small), the fractional volume change on transformation and inversely as the flow stress of the weaker phase. The deformation is not zero in the absence of external stress except where the phase transformation front has random orientation and movement. The theoretically deduced relations are examined experimentally by observing the deformation of specimens with attached weights, giving small tensile stresses, while their temperature was cycled through a transformation point. Phase transformations were examined in a number of metals involving a variety of crystal structures: reasonable agreement between theory and experiment was obtained in all cases.
The use of quenched boron steel components is an economic way to achieve significant improvements in terms of weight saving and crash performance. The material and process knowledge on the hot stamping of boron steels (e.g. Arcelor's USIBOR 1500 P®) by the stampers needs to be extended and accurate simulation tools must be developed to support the growth of this forming technology. This paper simultaneously addresses the specific requirements of the hot stamping simulation and the current state of the art in this field. A specific approach is presented for the detection of the process limits within the simulation tool. A software chain has been set up with the target to decrease the computation times.
The paper presents the approaches followed by two labs - LFT at the University of Erlangen-Nuremberg (Germany) and DIMEG at University of Padua (Italy) - in evaluating formability limits of 22MnB5 sheets when processed under hot stamping conditions. Details about the two testing apparatuses and the testing procedures are outlined, and the results in terms of Forming Limit Curves FLC compared and critically commented.
To improve the low formability that HSS sheets exhibit at room temperature, innovative forming technologies like the hot stamping process are currently applied. In order to avoid scaling and decarburization during the heating step, metal sheets coated with a specially developed Al-Si coating are utilized. In the present work the coating characteristics in terms of morphology, surface roughness and tribological behaviour are investigated as function of heating temperature, holding time and cooling rate that are typical of hot stamping processes.
In order to increase the accuracy of numerical simulations of the hot stamping process, an accurate and robust constitutive model is crucial. During the process, a hot blank is inserted into a tool where it is continuously formed and cooled. For the steel grades often used for this purpose, the initially austenitized blank will decompose into different product phases depending on the cooling and mechanical history. As a consequence, the phase proportions change will affect both the thermal and mechanical properties of the continuously formed and cooled blank. A thermo-elastic–plastic constitutive model based on the von Mises yield criterion with associated plastic flow is implemented into the LS-Dyna finite element code. Models accounting for the austenite decomposition and transformation induced plasticity are included in the constitutive model.
The implemented model results are compared with experimental dilatation results with and without externally applied forces. Further, the calculated isothermal mechanical response during the formation of a new phase is compared with the corresponding experimental response for two different temperatures.
Oxidation in hot stamping of quenchable steel sheets heated in an electrical furnace was prevented by coating the sheets with oxidation preventive oil. A solid film is generated on the surface of the sheet by drying the coated sheet, and the film changes into a liquefied film having an oxidation barrier at elevated temperatures. Hot hat-shaped bending of the coated sheet was performed to examine the properties of the products. For the bent products, the oxidation preventive oil was effective, the shape accuracy was very high and the hardness increased to a level of 1.5GPa in tensile strength.
Tensile loading-unloading tests of high-strength steel sheets in an elevated temperature range are conducted using a 100kN servo-controlled hydraulic dynamic fatigue testing machine, aiming at clarifying the mechanism of the springback-free phenomenon occurring in warm sheet forming. From stress-strain curves obtained by the tests, it is found that the abrupt decrease in the springback of formed steel sheets at approximately 750K in isothermal v-bending tests is mainly caused by high-temperature transient creep deformation. Also, from the results obtained by the isothermal v-bending test, bending-unbending deformation observed at temperatures higher than 750K, as a result of high-temperature creep, was found to have a secondary effect in the springback-free phenomenon.
Within the framework of this paper two methodologies, which enable the determina- tion of the thermal as well as the frictional properties of metallic coating systems used as corrosion preventing layers for quenchenable high strength steels during hot stamping will be presented. For the determination of the thermal properties with the focus on the resulting heat transfer coefficient between workpiece and die in de- pendency of the particular properties of the coating system cooling experiments un- der process relevant conditions have been carried out. The procedure regarding the evaluation of tribological coating properties hereby characterizes a combined ex- perimental-analytical-numerical method for the evaluation of the friction coefficient µ based on Siebel's approach for the modeling of the maximum drawing force.
A thermal model based on explicit time integration is developed and implemented into the explicit finite element code DYNA3D to model simultaneous forming and quenching of thin-walled structures. A staggered approach is used for coupling the thermal and mechanical analysis, wherein each analysis is performed with different time step sizes. The implementation includes a thermal shell element with linear temperature approximation in the plane and quadratic in the thickness direction, and contact heat transfer. The material behaviour is described by a temperature-dependent elastic–plastic model with a non-linear isotropic hardening law. Transformation plasticity is included in the model. Examples are presented to validate and evaluate the proposed model. The model is evaluated by comparison with a one-sided forming and quenching experiment. Copyright © 2004 John Wiley & Sons, Ltd.
This paper presents an innovative experimental procedure, based on Nakazima test, for evaluating the formability limits in the hot stamping of high strength steel (HSS) which is capable of generating formability data suitable for an FE modelling of the process. The approach is based on two complementary tests which are aimed at evaluating the actual phase transformation kinetics for the material under deformation conditions and the combinations of microstructure, temperature and straining path that lead to localized necking or fracture.
The recent years have witnessed an increasing usage of high-strength steels as structural reinforcements and in energy-absorbing systems in automobile applications due to their favourable high-strength-to-weight ratios. Owing to poor formability, complex-shaped high-strength steel components are invariably produced through hot-metal forming. The high-strength steel sheets are in some instances used with an Al–Si-coating with a view to prevent scaling of components during hot-metal forming. However, friction and wear characteristics of Al–Si-coated high-strength steel during interaction with different tool steels have not yet been investigated. With this in view, friction and wear behaviours of different tool steels sliding against Al–Si-coated high-strength steel at elevated temperatures have been investigated by using a high-temperature version of the Optimol SRV reciprocating friction and wear tester at temperatures of 40, 400 and 800 °C. In these studies both temperature ramp tests with continuously increasing temperature from 40 to 800 °C and constant temperature tests at 40, 400 and 800 °C, have been conducted. The results have shown that both the friction and wear of tool steel/Al–Si-coated high-strength steel pairs are temperature dependent. Friction decreased with increasing temperature whereas wear of the tool steel increased with temperature. On the other hand, the Al–Si-coated high-strength steel showed significantly lower wear rates at 800 °C as compared to those at 40 and 400 °C. The Al–Si-coated surface undergoes some interesting morphological changes when exposed to elevated temperatures and these changes may affect the friction and wear characteristics. The mechanisms of these changes and their influence on the tribological process are unclear and further studies are necessary to fully explain these mechanisms.
In order to increase the accuracy of numerical simulations of the hot stamping process, reliable material data is crucial. Traditionally, the material is characterized by several isothermal compression or tension tests performed at elevated temperatures and different strain rates. The drawback of the traditional methods is the appearance of unwanted phases for some test temperatures and durations. Such an approach is also both time consuming and expensive. In the present work an alternative approach is proposed, which reduces unwanted phase changes and the number of experiments. The isothermal mechanical response is established through inverse modelling of simultaneous cooling and compression experiments. The estimated material parameters are validated by comparison with data from a separate forming experiment. The computed global response is shown to be in good agreement with the experiments.
During the past years hot-stamped components have gained considerable importance in the automotive industry. This is due to
the advantageous properties involved, such as good crash behavior and high strength. In the production of hot-stamped components
the blank is rapidly cooled by the tool. This exerts an important influence on the final properties and the process time.
The tool is actively cooled by cooling ducts through which a medium flows. This article will present a newly developed method
in which the design of the cooling ducts can be systematically optimized. A test tool is used to show the results of this
method. It is easy to realize an optimized tool for the hot stamping process.
A warm and hot stamping process of ultra high tensile strength steel sheets using resistance heating was developed to improve springback and formability. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance, the so-called Joule heat. Since the heating time up to 800°C is only 2 seconds, the resistance heating is rapid enough to synchronise with a press. The effects of the heating temperature on the springback and formability of ultra high tensile strength steel sheets were examined. The springback in hat-shaped bending of the high tensile strength steel sheets was eliminated by heating the sheet. In addition, the ultra sheet having a tensile strength of 980MPa was successfully drawn by the heating. The heating temperature is optimum around 600°C due to the small springback and oxidation and the increase in hardness.
Within the innovative hot forming process for sheet metals, called hot stamping, it is possible to combine forming and quenching in one process step. This affords the opportunity to manufacture components with complex geometric shapes, high strength and a minimum of springback which currently find applications as crash relevant components in the automotive industry. As standard material for hot stamping the quenchenable high strength steel 22MnB5 is commonly used. With regard to the numerical modeling of the process, the knowledge of thermal and thermo-mechanical properties of the material is required. To determine the thermo-mechanical material characteristics, the flow behavior of the steel 22MnB5 in the austenitic state has been investigated by conductive, hot tensile tests with a Gleeble 1500 system dependent on the time–temperature characteristic of the hot stamping process.
In the present paper, the effects of process parameters on phase transformations during non-isothermal deformations are described and discussed. Non-isothermal high temperature compressive deformations were conducted on 22MnB5 boron steel by using deformation dilatometry. Cylindrical samples were uniaxially deformed at different strain rates ranging from 0.05 to 1.0 s−1 to a maximum compressive strain of 50%. Qualitative and quantitative investigations were carried out using surface hardness mapping data as well as dilatation curves. It was observed that a higher initial deformation temperatures resulted in a higher martensite fraction of the microstructure, while a variation in the martensite start temperature was negligible. Another conclusion was that by applying larger amounts of strain as well as higher force levels, not only the martensite start temperature, but also the amount of martensite was reduced. Moreover, it was concluded that using surface hardness mapping technique and dilatometry experiments were very reliable methods to quantify and qualify the coexisting phases.