Article

# A simulation study of pearlite-to-austenite transformation kinetics in rapidly heated hot-rolled low carbon steel

Authors:
• Thapar Institute of Engineering and Technology, Patiala, Punjab, India
• Tata Steel Limited, India
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... Commercial software, viz., THERMO-CALC 3.0, JMAT-PRO 6.1, and DICTRA software were used to determine the annealing process parameters. THERMO-CALC software (THERMO-CALC 3.0; Thermo-Calc Software AB, Stockholm, Sweden) is extensively used [11,12,30] to generate equilibrium phase diagrams for predicting lower and upper critical temperatures (Ac 1 and Ac 3 ) of a given steel. Steel composition is the main input parameter to generate equilibrium phase diagram by THERMO-CALC. ...
... It is also reported that carbon content in austenite phase is maximum if austenite formation occurs just simultaneously with pearlite dissolution, i.e., if condition of just pearlite dissolution for the formation of austenite is met [16][17][18]. Hence, in the present work, simulations were conducted on DICTRA software (Diffusion Controlled TRAnsformations software; DICTRA: version 27; developed by Thermo-Calc Software AB, Stockholm, Sweden) to determine the soaking time period required at a given annealing temperature for the condition "just to completion" of pearlite dissolution into austenite, i.e., to determine the minimum soaking time required at a given annealing temperature for complete pearlite dissolution so that the final DP microstructure after annealing comprises of ferrite and martensite phases, with no residual pearlite [23,30]. For DICTRA simulation work, pearlite shape was considered as lamellar. ...
... For DICTRA simulation work, pearlite shape was considered as lamellar. Also, a linear system (one-dimensional geometry), with ferrite to cementite weight ratio as 8:1 was considered as per the work of Sharma et al. [30]. The result window of DICTRA simulation showing holding required at annealing temperatures of 750 C (for complete pearlite dissolution) is shown in Fig. 1(b). ...
Article
The authors simulated the industrially used continuous annealing conditions to process dual phase steels by using a custom designed annealing simulator. 67 % cold rolled steel sheets were subjected to different processing routes including the conventional continuous annealing line (CAL), inter-critical annealing (ICA), and thermal cycling (TC) to investigate effect of change in volume fraction, shape, and spatial distribution of martensite on tensile deformation characteristics of DP steels. Annealing parameters were derived using commercial software including Thermo-Calc, JMat-Pro, and DICTRA. Through selection of appropriate process parameters, the authors found out possibilities of significantly altering the volume fraction, morphology, and grain size distribution of martensite phase. These constituent variations showed a strong influence on tensile properties of DP steels. It was observed that thermal cycling route modified the martensite morphology from the typical lath type to in-grain globular/oblong type and significantly reduced the martensite grain size. This route improved the strength–ductility combination from 590 MPa–33% (obtained through CAL route) to 660 MPa–30%. Finally, the underlying mechanisms of crack initiation/void formation etc. in different DP microstructures were discussed.
... Ultra-high-strength steels are used in the automotive and aerospace industries for better fuel economy and passenger safety. 1 Marageing steels are a special class of carbonless ultra-high-strength martensitic steels with an excellent combination of high strength, toughness, weldability, superior corrosion resistance and minimum distortion during hardening. [2][3][4][5][6][7] Due to these combinations of properties, marageing steel is preferred as a structural material for critical applications such as automobile, aerospace, nuclear and defence industries. ...
... Crystallite size is determined using the Scherrer equation which is a volume weight quantity and is dependent upon b hkl . Scherrer equation is presented in equation (1). 25 In this, the peak broadening is attributed to the crystallite size. ...
Article
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The present investigation is aimed to reduce the cost of production of various components fabricated by using cold rolled C250 marageing steel in the aerospace applications like landing gears of aeroplane and spacecraft. As cast C250 marageing steel was forged and solution annealed as per the specification of the customer. Forged C250 steel was cold rolled from 15 to 300%. Microstructure analysis revealed that grains got refined and elongated in the direction of cold working. Cold drawing increases the hardness, tensile strength and yield strength. However, loss in ductility and impact strength was observed. Scanning electron microscope fractographs of tensile and impact samples showed elongation of the dimples in cold working direction. The increase in cold working reduces the size of dimples indicating the trans�formation of fracture nature from the ductile-to-brittle regime. At higher cold working, >170% led to intergranular tearing and resulted in a loss of impact and mechanical properties of the steel. Further, cryogenic treatment of cold rolled C250 steel delayed ageing causing increase in tensile and yield strength with a marginal loss in ductility. The cost estimation of C250 steel revealed a significant cost saving (Indian Rupees [INR] 7500/1000 kg) for components made from different diameters of cold rolled steel over the solution annealed steel
... Ultra-high-strength steels are used in the automotive and aerospace industries for better fuel economy and passenger safety. 1 Marageing steels are a special class of carbonless ultra-high-strength martensitic steels with an excellent combination of high strength, toughness, weldability, superior corrosion resistance and minimum distortion during hardening. [2][3][4][5][6][7] Due to these combinations of properties, marageing steel is preferred as a structural material for critical applications such as automobile, aerospace, nuclear and defence industries. ...
... Crystallite size is determined using the Scherrer equation which is a volume weight quantity and is dependent upon b hkl . Scherrer equation is presented in equation (1). 25 In this, the peak broadening is attributed to the crystallite size. ...
Preprint
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The present investigation is aimed to reduce the cost of production of various components fabricated by using cold rolled C250 marageing steel in the aerospace applications like landing gears of aeroplane and spacecraft. As cast C250 marageing steel was forged and solution annealed as per the specification of the customer. Forged C250 steel was cold rolled from 15 to 300%. Microstructure analysis revealed that grains got refined and elongated in the direction of cold working. Cold drawing increases the hardness, tensile strength and yield strength. However, loss in ductility and impact strength was observed. Scanning electron microscope fractographs of tensile and impact samples showed elongation of the dimples in cold working direction. The increase in cold working reduces the size of dimples indicating the transformation of fracture nature from the ductile-to-brittle regime. At higher cold working, >170% led to intergranular tearing and resulted in a loss of impact and mechanical properties of the steel. Further, cryogenic treatment of cold rolled C250 steel delayed ageing causing increase in tensile and yield strength with a marginal loss in ductility. The cost estimation of C250 steel revealed a significant cost saving (Indian Rupees [INR] 7500/1000 kg) for components made from different diameters of cold rolled steel over the solution annealed steel.
... And based on the assumption that austenite mainly forms on the cementite/martensite interface, a diffusion couple of martensite and cementite with austenite nucleating in between is used. In addition, there are some works focusing mainly on cementite dissolution instead of austenite formation, or the steel used has an initial microstructure of ferrite and cementite before IA [25][26][27][28][29][30]. At present, the austenite growth from asquenched martensite during IA needs to be further studied regardless of whether cementite exist or not. ...
Article
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The growth of austenite from as-quenched martensite during intercritical annealing of a medium-Mn steel was studied by numerical simulation and experimental verification. The thickening of the austenite lath was numerically simulated under the assumption of local equilibrium. According to the presence or absence of cementite and the nucleation position of austenite, three models are proposed: austenite forming on the martensitic lath boundaries (Setup 1), on martensitic lath boundaries with far-away cementite location (Setup 2), and on the cementite/martensite interface (Setup 3). Result shows that the growth kinetics of austenite under the three models are slightly different, mainly because cementite is thermodynamically unstable in the two-phase zone. In the Setup 2, the growth of cementite and austenite occurs simultaneously at the early stage, in which the austenite grows slowly compared to Setup 1, and when cementite begins to dissolve, the austenite grows faster. In the Setup 3, cementite grows first and then dissolves to provide solute carbon for austenite growth, leading to the fastest austenite growth. In addition, with larger initial size of martensite, both growth kinetics of austenite and cementite and the dissolution kinetics of cementite are slowed down. Combining with microstructure characterization with EPMA, TEM and dilatometric measurement, the Setup 2 is considered to be the optimal model in which austenite mainly grows at the martensitic lath boundaries rather than at the ferrite/cementite interface inside the laths. Moreover, in order to elucidate the influence of cementite dissolution on the growth of austenite, a round tube austenite forming on the cementite rod (Setup 4) was proposed. The growth of austenite is accompanied with the continuous carbon supply from the dissolution of cementite, which could increase the stability of reverse transformed austenite but has little effect on the increase of austenite volume fraction. Graphical abstract
... The martensitic phase is obtained by quenching the austenite from high temperature. The corner and edge of ferrite grains are the preferred sites for nucleation of austenite [35,36]. Owing to the restriction of surrounding martensite grains, the grain growth of ferrite was limited during the annealing. ...
Article
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The strengthening and ductilization of steels are of great importance to weight reduction of vehicles. In this work, the strength-ductility synergy of dual-phase (DP) steels was obtained by properly tailoring the structural heterogeneity, including the distribution and fraction of constituent phases. It was demonstrated that heterogeneous structural DP steels with high volume fraction of martensite (from 64 to 83%) led to a good combination of strength and ductility. Compared with the cold-rolled sheets, the tensile strength and uniform elon-gation of heterogeneous DP steel increased by 700 MPa (from 1.06 to 1.76 GPa) and 2.7% (from 1.3 to 4%), respectively. The contributions from back stress and effective stress were analyzed by cyclic loading-unloading experiments. The underlying deformation mechanism was discussed based on the results of mechanical test and microstructure observation.
... After the holding time at the peak temperature, the undissolved carbides in both occasions act as nucleation sites, which will lead to an ultrafine final microstructure. The carbides support the creation of new phases, such as bainite, mixed with martensite laths during the quenching process as discussed by Caballero et al. [41] and Sharma et al. [43]. The local chemical composition of the material after the ultrafast process is a crucial issue, because the local chemical compositions could affect the rate and temperature, by which phase transformations occur. ...
Article
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Main target of the present work is to elucidate the effect of both initial microstructure and heating rate on phase transformations that occur during ultrafast processing. For this purpose, two initial microstructures, a ferritic-pearlitic and a soft-annealed microstructure were considered. We applied different heating rates (10 °C/s, 200 °C/s, 300 °C/s) up to the peak austenitization temperature, θ ≅ 900 °C. The evolving microstructure is analysed via SEM and EBSD, whereas the carbide dissolution and austenite formation is simulated with Thermocalc® and DICTRA software. Data obtained in this research proves that, when the heating rate increases, the carbide dissolution rate is disseminated. Compared to a conventional heating rate, where the local chemical composition homogenizes as a result of diffusion, rapid reheating leads to intense segregation of the substitutional atoms at the cementite/austenite interface and turns diffusion to a sluggish process. This fact, combined to the infinitesimal time for diffusion, forms an inhomogeneous carbon distribution along the microstructure. This inhomogeneity is further enhanced by the presence of increased carbides’ size present in the initial microstructure. Due to rapid heating, these carbides cannot be decomposed since the diffusion distance of alloying elements increases and the diffusion of alloying elements is impeded during ultrafast heating, thus, remain undissolved at peak austenitization temperature. Their presence and effect in heterogeneous ferrite nucleation restrict austenite grain growth. Consequently, fine austenite grains in conjunction with their chemical heterogeneity lead to the coexistence of fine martensite, bainite laths and undissolved carbides in the final microstructure after quenching.
... The migrating boundaries of ferrite appear to sweep the pearlite colonies leaving fine cementite particles in recrystallized ferrite grains. The re-distributed fine cementite is expected to influence the kinetics of austenite nucleation sites and growth during subsequent inter-critical annealing process [24,37]. Inverse pole figure (IPF) map very distinctly displayed partial recrystallization of the cold-deformed structure with an extensive recovery of ferrite grains, inferred by the presence of low angle boundaries shown in blue colour in the image quality (IQ) map in Fig. 5c. ...
Article
Effect of industrial continuous annealing conditions on 67% cold rolled steel intended for producing dual phase steel is reported in this study. Evolution of dual phase microstructure and bulk texture during two different industrial continuous annealing process conditions were analysed using scanning electron microscope and x-ray diffractometer. Annealed dual phase steels were tensile deformed under quasi-static and dynamic deformation conditions. Deformed specimens were evaluated for bulk texture changes with respect to tensile deformation strain rates. The study revealed the effect of variation in industrial continuous annealing process on microstructure evolution, especially with respect to spatial second hard phase distribution and bulk texture. Depending on the heating rates and peak annealing temperature the spatial distribution of martensite was found to change from predominant grain boundary type to “in grain” type. A variation in bulk texture intensity was also observed. The weak annealing texture of α- and γ- bcc texture fibres were found to improve dynamic tensile deformation characteristic of steel.
... The composition, structure and temperature influence the austenite formation [7]. By studying the austenization kinetics of the low alloy steel during the spheroidizing process and the austenization process of martensite in Fe-C-M alloys, it was found that the diffusion of carbon may change through the introduction of alloying elements in steel [8,9]. ...
Article
The influence of manganese on the microstructure and kinetics of the austenization in the Fe-Mn-C ternary alloys, which contains four grades of Mn concentration, is studied under continuous heating. The transformation of the pearlite-austenite has been measured by differential scanning calorimetry (DSC) in temperatures that ranged from room temperature to 1373 K. Also, the microstructure of the as-cast and the completion of the DSC test is obtained using optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The analysis of the microstructure indicates that the lamellar spacing of the initial pearlite is refined from 0.34 μm to 0.19 μm. It also indicates that austenite grain coarsening occurs as Mn concentration increases. The comparison of experimental kinetic curves and the Johnson-Mehl-Avrami-Kolmogorov model (JMAK) shows that the start temperature of austenization is decreased from 1010.5 K to 999.28 K. and that the austenite formation rate is accelerated. Moreover, the activation energy of diffusion decreases from 192.88 kJ mol -1 to 178.47 kJ mol -1 , and the pre-exponential factor decreases from 5.32 × 10 -3 m 2 s -1 to 3.1 × 10 -3 m 2 s -1 . DOI: http://dx.doi.org/10.5755/j01.mech.23.2.18111
... Subsequent studies have confirmed the conclusions above [2,3], and have served as a basis for the development of diffusional models describing austenite formation. However, most of the available kinetic descriptions of austenite formation are based upon isothermal conditions [4,[8][9][10][11][12][13][14][15][16] and not on heating experiments. ...
Article
Hydrogen flakes and elemental segregation are the main causes of steel rejection. To eliminate hydrogen flaking, the present study focuses on the manufacture of AMS-4340 ultra-high-strength steel through an alternate route. AMS-4340 was prepared using three different processing routes. The primary processing route consisted of melting in an electric arc furnace, refining in a ladle refining furnace, and vacuum degassing. After primary processing, the heat processes (D1, D2, and D3) were cast into cylindrical electrodes. For secondary processing, electroslag remelting (ESR) was carried out on the primary heats to obtain four secondary heats: E1, E2, E3, and E4. Homogenization of ingots E1, E2, E3, and E4 was carried out at 1220°C for 14, 12, 12, and 30 h, respectively, followed by an antiflaking treatment at 680°C and air cooling. In addition, the semi-finished ESR ingot E4 was again homogenized at 1220°C for 6–8 h and a second antiflaking treatment was performed at 680°C for 130 h followed by air cooling. The chemical segregation of each heat was monitored through a spectroscopy technique. The least segregation was observed for heat E4. Macrostructure examination revealed the presence of hydrogen flakes in heats E1, E2, and E3, whereas no hydrogen flakes were observed in heat E4. Ultrasonic testing revealed no internal defects in heat E4, whereas internal defects were observed in the other heats. A grain size investigation revealed a finer grain size for E4 compared with those for the other heats. Steel produced in heat E4 also exhibited superior mechanical properties. Therefore, the processing route used for heat E4 can be used to manufacture an AMS-4340 ultra-high-strength steel with superior properties compared with those of AMS-4340 prepared by the other investigated routes.
Article
A quantitative phase field method of multi-component diffusion-controlled phase transformations coupled with the Kim–Kim–Suzuki model was applied to study the effect of initial particle size distribution (PSD) in 3D and space distribution in 2D on dissolution of α particles in Ti–6Al–4V alloy below β transus temperature in real time and length scale. The thermodynamic and mobility data were obtained from Thermo-Calc and DICTRA softwares, respectively. The results show that the volume fractions of α particles decay with time as: $$f = f_{\text{eq}} + (f_{0} - f_{\text{eq}} )\exp ( - Kt^{n} )$$ for four cases of PSD. The sequence of dissolution kinetics from fast to slow is: uniform PSD, normal PSD, lognormal PSD and bimodal PSD. The space distribution is found to be a major factor affecting the dissolution kinetics and the microstructures. When the distance of the particles is less than critical value, the dissolution rates reduce with the decrease in distance. The Al and V concentration fields around the particles appear more obvious soft impingement.
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Hardcover ▶ 89,99 € | £81.00 | \$119.00 ▶ *96,29 € (D) | 98,99 € (A) | CHF 101.50 eBook Available from your library or ▶ springer.com/shop The book covers all types of advanced high strength steels ranging from dual-phase, TRIP. Complex phase, martensitic, TWIP steels to third generation steels, including promising candidates as carbide free bainitic steels, med Mn and Quenching&Partitioning processed steels. The author presents fundamentals of physical metallurgy of key features of structure and relationship of structure constituents with mechanical properties as well as basics of processing AHSS starting from most important features of intercritical heat treatment, with focus on critical phase transformations and influence of alloying and microalloying. This book intends to summarize the existing knowledge to show how it can be utilized for optimization and adaption of steel composition, processing, and for additional improvement of steel properties that should be recommended to engineering personal of steel designers, producers and end users of AHSS as well as to students of colleges and Universities who deal with materials for auto industry.
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Due to their exceptional strength properties combined with good workability the Advanced High-Strength Steels (AHSS) are commonly used in automotive industry. Manufacturing of these steels is a complex process which requires precise control of technological parameters during thermo-mechanical treatment. Design of these processes can be significantly improved by the numerical models of phase transformations. Evaluation of predictive capabilities of models, as far as their applicability in simulation of thermal cycles thermal cycles for AHSS is considered, was the objective of the paper. Two models were considered. The former was upgrade of the JMAK equation while the latter was an upgrade of the Leblond model. The models can be applied to any AHSS though the examples quoted in the paper refer to the Dual Phase (DP) steel. Three series of experimental simulations were performed. The first included various thermal cycles going beyond limitations of the continuous annealing lines. The objective was to validate models behavior in more complex cooling conditions. The second set of tests included experimental simulations of the thermal cycle characteristic for the continuous annealing lines. Capability of the models to describe properly phase transformations in this process was evaluated. The third set included data from the industrial continuous annealing line. Validation and verification of models confirmed their good predictive capabilities. Since it does not require application of the additivity rule, the upgrade of the Leblond model was selected as the better one for simulation of industrial processes in AHSS production.
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The kinetics and microstructural evolution of austenite formation in a low carbon steel, with initial microstructure composed of ferrite and pearlite, were studied during continuous heating, by using dilatometric analysis and measurements of microstructural parameters. The formation of austenite was observed to occur in two stages: (a) pearlite dissolution and (b) ferrite to austenite transformation. The critical temperatures of austenite formation in continuous heating increase with increasing heating rate, with greater influence on the finishing temperature of austenite formation. For both the 1 °C/s and 0.1 °C/s heating rates, the formation rate of austenite reaches a maximum at approximately the finishing temperature of pearlite dissolution, and the formation rate of austenite as a function of the temperature is greater at the higher heating rate.
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The influence of carbon content in the range of 0.01–0.3 wt.% on microstructure, hardness and tensile property of sintered Fe-Cr-Mo steels was investigated. The sintered Fe–3.0 wt.%Cr–0.5 wt.%Mo–(0.1, 0.2, 0.3) wt.% C steels were prepared by using powder metallurgical process. After sintering, the specimens were rapidly cooled by nitrogen at the rate of 5.4 °C/s. It was found that in the sintered steels with a lower carbon content of 0.01 and 0.1 wt.%, the allotriomorphic ferrite and Widmanstӓtten ferrite formed at austenite grain boundaries and grew to occupy the whole prior austenite grains. With higher carbon contents of 0.2 and 0.3 wt.%, the microstructures consist of bainite, martensite and some retained austenite. These steels exhibited increases of hardness, tensile strength and elongation at break with increasing carbon content. Increase of strength is due to the transformations from austenite, formed during sintering, to hard bainite and martensite structures.
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The influence of carbon content in the range of 0.01–0.3 wt.% on microstructure, hardness and tensile property of sintered Fe-Cr-Mo steels was investigated. The sintered Fe–3.0 wt.%Cr–0.5 wt.%Mo–(0.1, 0.2, 0.3) wt.% C steels were prepared by using powder metallurgical process. After sintering, the specimens were rapidly cooled by nitrogen at the rate of 5.4 °C/s. It was found that in the sintered steels with a lower carbon content of 0.01 and 0.1 wt.%, the allotriomorphic ferrite and Widmanstӓtten ferrite formed at austenite grain boundaries and grew to occupy the whole prior austenite grains. With higher carbon contents of 0.2 and 0.3 wt.%, the microstructures consist of bainite, martensite and some retained austenite. These steels exhibited increases of hardness, tensile strength and elongation at break with increasing carbon content. Increase of strength is due to the transformations from austenite, formed during sintering, to hard bainite and martensite structures.
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The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension-uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching-uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.
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An investigation was made to determine the effect of heating rate on ferrite recrystallization and austenite formation of cold rolled dual-phase (DP) steel (0.1C-0.4Si-1.6Mn). Three heating rates, 5, 50 and 500 degrees C/s, and different annealing temperatures ranged from 650 degrees C to 950 degrees C were applied to study their effects on the progress of recrystallliztion and austenitization. It has been found that faster heating strongly influences the overlapping extent of the ferrite recrystallization and austenite formation process, the morphology of martensite is observed to shift from network structure to a chain shape under lower temperatures. Higher annealing temperatures minimize the transformation and microstructure difference caused by a difference in heating rate. Rapid heating with high temperature could improve the strength. The microstructural evolution mechanism under rapid heating conditions was also tentatively discussed. (c) 2013 Elsevier B.V. All rights reserved.
Article
A concise semi-analytical mixed-mode model is proposed to describe the ferrite-to-austenite transformation kinetics. The initial microstructure for the model consists of a ferrite matrix with supersaturated austenite grains. The carbon supersaturation of austenite grains resulted from the rapid dissolution of pearlite colonies present in the initial microstructure. A similar approach to the one used for describing the ferrite growth kinetics (Bos and Sietsma, 2007) was used, employing a sharp interface between the phases. In comparison to ferrite growth, the carbon concentration profile of the growing austenite contains an additional parameter, which is the carbon concentration at the centre of the grain. This extra parameter is accounted for by assuming a parabolic carbon profile in the austenite. A comparison with a numerical solution of the differential equations shows that the developed semi-analytical mixed-mode model gives a good description of the ferrite-to-austenite transformation, significantly more accurate over the entire course of the transformation than the classical interface-controlled and diffusion-controlled models. Significant deviations from the numerical solution, as a result of the parabolic approximation, are limited to the initial stage of the transformation.
Article
In the automotive industries, dual phase (DP) steels have increasingly used for various car body parts due to their excellent combination of high strength and good formability. The microstructure of DP steel basically consists of a matrix of ferrite embedded by martensitic islands. For the mechanical and fracture behaviors of the DP steel, effects of martensite phase fraction, morphology, and phase distribution play an important role. In this work, dual phase steel sheets with different martensite contents were produced by intercritical annealing process. Subsequently, a Finite Element (FE) based modeling using Representative Volume Elements (RVEs) approach was proposed for predicting overall stress–strain behavior of the investigated DP steels. Two dimensional RVE models were generated from micrographs of the DP steels on the microstructure level. For the individual single phases flow curves based on dislocation theory and local chemical composition were applied. Additionally, Geometrically Necessary Dislocations (GNDs), which accumulates at the phase boundaries due to the austenite–martensite transformation during quenching process, was taken into account. A local hardening effect due to these phase boundary dislocations was applied at the interface layer between ferrite and martensite. The calculated stress–strain responses of the DP steels were verified with experimental results determined from tensile tests. The micromechanics model could be then used to describe the local stress and strain evolution of the individual phases in the DP microstructures.
Article
Due to its excellent strength and formability combinations, dual phase (DP) steels offer the potential to improve the vehicle crashworthiness performance without increasing car body weight and have been increasingly used in new vehicles. However, a new type of crack mode termed as shear fracture is accompanied with the stamping application of these high strength DP steel sheets. This paper presents a study of DP shear fracture through macroscopic identification and micro-level metallographical observation. By the cup drawing experiment to identify the limit drawing ratio (LDR) of three DP AHSS with strength level from 600 MPa to 1000 MPa, the study compared and categorized the macroscopic failure mode of these three types of materials. The metallographical observation using scanning electron microscopy (SEM) along the direction of crack was conducted for the DP steels to discover the micro-level propagation mechanism of the fracture and its relation with the martensite volume and distribution. Plasticity analysis and finite element method (FEM) simulation were provided to explain the observed behavior, which was further confirmed by the comparison between DP fracture and high strength low alloy (HSLA) fracture.
Article
The deformation characteristics of dual phase twinning-induced plasticity (TWIP) steel containing different ferrite volume fractions have been investigated through tensile testing method. The results show that the yield and ultimate tensile strengths are increased by ferrite volume fraction, while the ductility is marginally influenced. The former is attributed to the formation of DO3 ordered intermetallic compound inside the ferrite phase. Furthermore, the SEM examination of fracture surfaces reveals the pattern of brittle facets corresponding to the ferrite regions. These are surrounded by ductile dimples belonging to the austenite areas. Moreover, the signs of plane sliding in the austenite phase have been recognized, which demonstrates the domination of the ductile fracture in the austenite areas.
Article
In the present work, fast-heating annealing was performed on a cold-rolled Fe–0.07C–1.7Mn–0.429Si dual-phase steel. In contrast to commercial conventional continuous annealed steel, 6.6% higher ultimate tensile strength and 14.1% greater elongation were obtained. Transmission electron microscopy observations reveal the incomplete ferrite recrystallization, the formation of bainite and fine fiber-like martensite in fast-heating processed dual-phase steel. High-efficiency simplified fast-heating annealing process demonstrates great potential for large-scale production.
Article
The final fractions of austenite after the isothermal austenite-to-ferrite transformation and its reverse transformations (including the martensite-to-austenite and the pearlite + ferrite-to-austenite transformations) in the intercritical annealing region have been studied using full equilibrium (FE), paraequilibrium (PE), and local equilibrium (LE) calculations and experiments. The LE model predictions are in a very good agreement with the experimental results, while the FE and PE model predictions deviate significantly. It is also found that the LEP/LENP transition for the austenite-to-ferrite transformation deviates from those of its reverse transformations. The magnitude of deviation increases with the increasing Mn concentration.
Article
TRIP (Transformation Induced Plasticity) steels are under development for automotive applications that require high strength and excellent formability. Conventional TRIP steels consist of a multiphase microstructure comprised of a ferrite matrix with a dispersion of bainite and metastable retained austenite. The high ductility exhibited by these steels results from the transformation of the metastable retained austenite to martensite during straining. In conventional TRIP steel processing, the multiphase microstructure is obtained by controlled cooling from the alpha + gamma region to an isothermal holding temperature. During this holding, bainite forms and carbon is rejected out into the austenite, which lowers the Ms temperature and stabilizes the austenite to room temperature. In this research project, a fundamental study of a low-Si, Mo-Nb added cold rolled TRIP steel with and without Al additions was conducted. In this study, the recrystallization of cold-rolled ferrite, the formation of austenite during intercritical annealing and the characteristics of the decomposition of the intercritically annealed austenite by controlled cooling rates were systematically assessed. Of special interest were: (i) the effect of the initial hot band microstructure, (ii) the formation of epitaxial ferrite during cooling from the intercritical annealing temperature to the isothermal holding temperature, (iii) the influence of the intercritically annealed austenite on the formation of bainite during the isothermal holding temperature, and (iv) the influence of the processing variables on the type, amount, composition and stability of the retained austenite. During this research study, techniques such as OM, SEM, EBSD, TEM, XRD and Magnetometry were used to fully characterize the microstructures. Furthermore, a Gleeble 3500 unit at US Steel Laboratories was used for dilatometry studies and to simulate different CGL processing routes, from which specimens were obtained to evaluate the mechanical properties.
Article
An investigation was made to determine the effect of water quenching process on microstructure and tensile properties of cold rolled dual-phase (DP) steel (0.06C–0.20Si–1.30Mn). Correlations of microstructure, tensile properties, and water quenching parameters were established. Analysis of optical and transmission electron microscopy shows that the microstructures are mainly composed of ferrite and martensite phases. The fraction of martensite covers from 20% to 68.36% in this study, which plays a key role on tensile properties. A small amount of retained austenite or bainite can also be found for different water quenching processes. It is observed that water quenching process can improve the DP steel grade of 450MPa up to 600MPa and still keep good ductility. Further, the ultimate tensile strength can even be improved above to 1000MPa under some water quenching processes.
Article
Now in its seventh edition, this accessible book provides readers with clear and concise discussions of key concepts while also incorporating familiar terminology. The author treats the important properties of the three primary types of materials (metals, ceramics, and polymers) and composites, as well as the relationships that exist between the structural elements of materials and their properties. Throughout, the emphasis is placed on mechanical behavior and failure, including techniques that are employed to improve performance.
Article
Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe–C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding 1-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour.
Article
Austenite growth in the intercritical annealing of ternary (Fe-C-Mn) and quaternary (Fe-C-Mn-Si) dual phase steels is studied. The growth process is modelled assuming local equilibrium at austenite/ferrite interfaces and diffusion control. An isothermal anneal starting with a ferrite-pearlite mixture is examined. Growth proceeds in four stages: (1) austenite nucleation from pearlite (which is not examined); (2) attainment of uniform carbon activity within each austenite particle; (3) attainment of uniform carbon activity over large distances in the material (e.g., 100 (mu)m); and (4) austenite growth controlled by diffusion of alloying element(s) in ferrite. The last stage is the only stage where significant alloying element partitioning occurs. This leads to the centre of each austenite particle retaining its initial alloying element concentration while the rim is enriched or depleted in alloying element depending on the value of the alloying element diffusion coefficient. This non-uniform concentration profile is associated with the end of austenite growth and results in a volume fraction greater than the equilibrium volume fraction. Full equilibration of the austenite does not occur in practical time periods. The preceding description applies to all cases treated. Material initially homogeneous with respect to alloying elements was examined: (1) for a planar geometry; (2) for a spherical geometry; and (3) with different size particles. A material initially non-uniform with respect to alloy element (e.g., having an initial sinusoidal fluctuation of alloying element) was examined for the case of: (1) a wavelength comparable with spacing between austenite particles; and (2) longer wavelengths. The assumption that local equilibrium established in short times (100 s) was verified with Scanning Transmission Electron Microscope (STEM) observations. STEM and microprobe analysis data also qualitatively verified predicted Mn concentration profiles for long (50 h) anneals. Volume fractions predicted were in reasonable agreement with observed and published volume fraction data.
Article
Dual phase (DP) steels having a microstructure consists of a ferrite matrix, in which particles of martensite are dispersed, have received a great deal of attention due to their useful combination of high strength, high work hardening rate and ductility. In the present work, a microstructure based micromechanical model is developed to capture the deformation behavior, plastic strain localization and plastic instability of DP 590 steel. A microstructure based approach by means of representative volume element (RVE) is employed for this purpose. Dislocation based model is implemented to predict the flow behavior of the single phases. Plastic strain localization which arises due to incompatible deformation between the hard martensite and soft ferrite phases is predicted for DP 590 steel. Different failure modes arise from plastic strain localization in DP 590 steel are investigated on the actual microstructure by finite element method.
Article
The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.
Method for Making a Multiphase Hot-Rolled Steel Strip
• X Cornet
• J C Herman
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The Kinetics of Austenite Formation during Continuous Heating of a Multi-Phase Steel
• Y.-B Cho
Y.-B. Cho, The Kinetics of Austenite Formation during Continuous Heating of a Multi-Phase Steel, The University of British Columbia, 2000.
Introduction to Physical Metallurgy, second ed. Tata McGraw Hill
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