Hot stamped parts with desirable properties in medium Mn TRIP steels
Abstract
With the increasing requirements of the automotive industry for reduced weight and safety improvement, high strength steel hot stamping parts have been increasingly applied in the automotive industry. But the process is along with higher equipment requirements and tool wear because of the high temperature.In this study, warm tensile deformation behavior of 6Mn, 6MnNb and 6MnNbMo steels was investigated, in conjunction with warm stamping to understand the role of Mn and Mo and evaluate the mechanical properties of Ushaped parts.The results show that the hotstamped6MnNbMo Ushaped parts at both 700 °C and 760 °C exhibited the desirable mechanical properties, e.g. the SDB (Strength Ductility Balance) values reached 23.6-36.8 GPa·%, which were more than double or triple times higher than those of hotstamped boron steels. The work hardening rate increased with addition of Nb and Mo, while it decreased with flow stress.As the stamping temperature decreased from 760 to 700 °C, the fractions of soft phase ferrite and retained austenite increased, resulting in the enhanced elongation with reducing tensile strength from 1600-1814 MPa to 1361-1467 MPa.The combined addition of Nb and Mo raised UTS with a slight decrease in El, and improved the distribution uniformity of mechanical properties, which is of great importance when applying medium Mn steels to the automotive structural components for ensuring safety of passengers.
... Hot stamping is a process in which a certain material is heated to the austenitizing temperature, then formed and quenched, and finally, a martensitic structure is obtained. Hot stamping technology is an advanced technology to fulfill automobile lightweight, characterized by lower forming resistance, better formability, higher dimensional accuracy and less springback [6,7]. The Materials 2023, 16, 576 2 of 14 tensile strength of the hot stamped parts of medium manganese steel can reach at least 1500 MPa, while their elongation after fracture only ranges from 5% to 10% [8], of which the strength and plastic product is far below the requirements of the third-generation advanced high-strength steel. ...
In order to improve the plasticity of hot stamping parts, this paper combines the heat treatment process with the plastic forming of sheet metal, and creatively proposes a new process of hot stamping-carbon partitioning-intercritical annealing. The mechanical properties and microstructure are characterized under the newly proposed process, the quenching-partition (QP) process, and the intercritical annealing (IA) process, respectively. The new process firstly undergoes incomplete austenitizing treatment at 610 °C, then carries out distribution treatment while stamping at 300 °C, and finally conducts annealing treatment in critical zone at 680 °C in two-phase zone. The results show that a multi-phase refined microstructure composed of lath martensite, retained austenite, fresh martensite, and carbides are obtained by the new process. Most of the retained austenite is shaped in the thin film due to martensitic shear, in which carbon and manganese elements diffuse from martensite to austenite by heat treatment, thus stabilizing the retained austenite. Retained austenite with a volume fraction of 33.7% is obtained in the new process. The retained austenite with higher content and better stability is completely consumed during the stretching process, which gives full play to discontinuous TRIP effects, thus delivering the elongation of 36.8% and the product of strength and elongation (PSE) reached as high as 43.6 GPa%.
... 7%) is greatly reduced by the complete formation of martensite, which is why these components are only suitable to a limited extent for crash-relevant anti-intrusion areas of the car body. For this reason, tailoring methods such as tailored tempering with a partially heatable die [6] or tailored furnace modules for precise local heat treatment [7], as well as the use of third-generation advanced high-strength steels (AHSS) [8][9][10][11], are proceeding in the focus of research and development activities in the area of press hardening [3,[12][13][14]. ...
It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.
... In shotblasted parts, not only rough surface but also distortion of thin parts is caused. Uncoating is suitable for development of new steel sheets such as 1800 MPa sheets [19], 2000 MPa sheets [20], and medium manganese sheets [21]. ...
Dual-frequency ultrasonic cleaning with a diluted phosphoric acid solution was developed to remove oxide scales on surfaces of hot-stamped parts from uncoated steel sheets, and conventional shot blasting processes are omitted. The removal of the oxide scale by ultrasonic cleaning is accelerated by the phosphoric acid solution and the dual frequency. The removing time for the phosphoric acid solution was shorter than that for a hydrochloric acid solution, and rust appearing for leaving after cleaning was prevented by generating an iron phosphate layer. In dual-frequency ultrasonic cleaning with the diluted phosphoric acid solution, the oxide scale was dissolved, and then the oxide scales were exfoliated from the thin scale and high-pressure portions. The removing time decreased with decreasing pH and oxide scale thickness and with increasing solution temperature. The surface roughness and distortion of an ultrasonic-cleaned hot-stamped part were smaller than those for shot blasting, and the weldability and paintability were similar. The oxide scale of a hot-stamped part having a nonuniform distribution of oxide scale thickness was successfully removed by dual-frequency ultrasonic cleaning with the diluted phosphoric acid solution.
... Their properties are suitable for the 3-GEN AHSS, they exhibit high combinations of strength (1000-1500 MPa) and ductility (31-44%) at reasonable cost [12]. These steels have a complex microstructure consisting of a nano/ultra-fine grained austenite and ferrite/martensite, with Mn content around (3-10 %), and high level of retained austenite (30 volume %) which enhanced ductility due to the higher strain hardenability as a results of the transformation-induced plasticity (TRIP) or even the twinning-induced plasticity (TWIP) effects [13]. Beside of alloying with Mn, certain amount (1-3 wt %) of Silicon and/or Aluminum and (up to 0.2 wt %) low Carbon contents are also added in order to raise precipitation hardening in the martensite matrix and improve the austenite stability, respectively [14]. ...
The modern vehicles demand a better fuel economy, decrease in ozone harming substance outflows, and superior safety requirements led to new developments of steel grades with higher strength and good formability. Third generation of advanced high strength steels are the next stage for the automotive companies in steel sheets development. The principal concept of third generation of AHSS is to reap the mechanical properties benefits from first and second generation of AHSS at cost neither too high nor too low. This literature review summarizes the results achieved in a previous paper of the Third Generation of Advanced High Strength Sheet steels literature published by D. Krizan et al. Where we intend to focus on, the recent developments and future trends of the third generation of advanced high strength sheet steels (3-GEN AHSSs) including quenching and partitioning (Q&P), TRIP bainitic ferrite (TBF), medium manganese, density reduced TRIP (δ-TRIP) and nano steels for the modern automotive industry, with emphasis on their main characteristics, processing, and applications .
The effects of different quenching methods on the microstructure and mechanical properties of 30MnB5NbTi hot stamping steel are investigated, and the quenched microstructure is characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The mechanical properties are evaluated by uniaxial tensile test. The results reveal that, in comparison to die quenching, oil quenching, or air cooling, water‐quenched samples (tempered after water quenching, die quenching and oil quenching) exhibit the highest ultimate tensile strength (UTS) and yield strength (YS), reaching 2052 and 1422 MPa, respectively. Various quenching methods enable multi‐level strength control for the same steel grade, achieving a strength control range of ≈1000 MPa. The microstructure of samples quenched by water, die, and oil is fully martensite, and martensite exhibited block and lath morphology. With the increase of cooling rate, martensite laths become finer and the prior austenite size decreased. The microstructure of air‐cooled samples is martensite, ferrite, and retained austenite. The strengthening mechanisms of different quenched samples are calculated. The results show that dislocation strengthening and precipitation strengthening are the two dominant strengthening mechanisms in 30MnB5NbTi hot stamping steel. Therefore, the properties of hot stamping steel can be improved by changing the cooling rate by designing physical dies.
A comparative study was conducted to reveal the effects of microalloying with Ti + Nb + V on the hydrogen permeation and damage behaviors of the 22MnB5 hot stamping steel. For 22MnB5, the hydrogen-induced blisters preferentially originated from grain boundaries and further propagated into cracks, which can be greatly suppressed by adding Ti + Nb + V. This can be attributed to the microalloying-induced increasing of binding energy and irreversible traps including nano-sized carbides, fined grains, as well as low angle grain boundaries, as confirmed by high resolution transmission electron microscopy and electron back-scattered diffraction determinations. The hydrogen diffusion coefficient of 1.67 × 10⁻⁵ cm s⁻¹ was thus obtained for the Ti + Nb + V added microalloyed 22MnB5, which is approximately four times lower than that for 22MnB5. Furthermore, a reasonable model was proposed to illustrate the improvements of hydrogen diffusion and hydrogen damage in the hot stamping steels.
In the present study, tensile and springback properties of medium (3–7 wt%) Mn steels for warm stamping were investigated as functions of Mn content, austenitizing temperature (Taus), V-bending temperature (Tb) and time (tb). For comparison, the tensile and springback properties of 22MnB5 steel used for hot stamping were also evaluated. Whereas medium-Mn steels were air-cooled to room temperature after austenitizing at various Taus, the 22MnB5 steel was water-quenched. The 5Mn steel exhibited the highest tensile properties due to the martensitic matrix with inactive auto-tempering and negligible amount of retained austenite. The springback of medium-Mn specimens was increased by the addition of Mn at a given Tb when tb was 0 s. At the Tb of 300-500 °C, the fraction of martensitic transformation was raised by increasing the Mn content, resulting in high springback. At the Tb of 600 °C, the rise in transformation strain by the addition of Mn raised springback. When Tb was fixed to be 600 °C, the springback of medium-Mn specimens was increased by the addition of Mn due to the increases in transformation strain at the tb of 0 s and the fraction of martensitic transformation during air cooling at the tb of 2 s and over.
A novel ultra-high strength TRIP (transformation induced plasticity) steel, with ~1.5. GPa strength and good ductility of ~26% has been produced. The microstructure consists of ultrafine ferrite, and a large volume fraction of austenite. The flow stress was significantly increased by a reduction in the grain size, but the effect of strain rate on the flow stress was negligible. The formation of stress induced martensite was found to increase linearly with strain, and a reduction in the grain size correlated with an increase in the stress required to form the martensite.
The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.
Quenching and partitioning (Q&P) produces steel microstructures with martensite and austenite that exhibit promising property combinations for third generation advanced high strength steels. Understanding the kinetics of reactions that compete for the available carbon, such as carbide formation, is critical for alloying and processing design and achieving austenite enrichment and retention during Q&P. Mössbauer effect spectroscopy (MES) was used to characterize Q&P microstructures in a 0.38C-1.54Mn-1.48Si wt. pct. steel after quenching to 225 C and partitioning at 400 C for 10 or 300 s, with an emphasis on transition carbides. The recoilless fraction for η-carbide was calculated and a correction for saturation of the MES absorption spectrum was applied, making quantitative measurements of small amounts of η-carbide, including non-stoichiometric η-carbide, possible in Q&P microstructures. Complementary transmission electron microscopy confirmed the presence of η-carbides, and MES and x-ray diffraction were used to characterize the austenite. The amount of η-carbide formed during Q&P ranged from 1.4 to 2.4 at. pct., accounting for a substantial portion (~24 to 41 pct.) of the bulk carbon content of the steel. The amount (5.0 at. pct.) of η-carbide that formed after quenching and tempering (Q&T) at 400 C for 300 s was significantly greater than after partitioning at 400 C for 300 s (2.4 at. pct.), suggesting that carbon partitioning from martensite to austenite occurs in conjunction with η-carbide formation during Q&P in these specimens.
The microstructure and tensile behavior of low-density steels containing 5 mass% Al were investigated. An alloy obtained under a specific heat treatment condition showed deformation-induced martensitic transformation, which yielded excellent mechanical properties of a high tensile strength of >900 MPa and a high total elongation of >50%. The volume fraction and grain size of the austenite depended on the annealing temperature, which resulted in a transition from stable to metastable behavior of the austenite. The effects of solute content on grains and of austenite grain size on stability were discussed.
Ultrafine-grained duplex manganese-bearing steels fabricated by quenching and annealing demonstrated excellent combinations of tensile elongation of 31–44% and tensile strength of 1–1.5 GPa and a three-stage work-hardening behavior. Their enhanced mechanical properties and work-hardening behavior were explained by their dynamic composition due to the strain induced phase transformation from large-fractioned austenite (>30%). It was suggested that the austenite volume fraction and its mechanical stability is the key to understand the phase transformation induced deformation behavior.
A model is developed to describe the endpoint of carbon partitioning between quenched martensite and retained austenite, in the absence of carbide formation. The model assumes a stationary α/γ interface, and requires a uniform chemical potential for carbon, but not iron, in the two phases, leading to a metastable equilibrium condition identified here as “constrained paraequilibrium” or CPE. The model is explained with example calculations showing the characteristics of the constrained paraequilibrium condition, and applications are discussed with respect to new microstructures and processes, including a new “quenching and partitioning,” or Q&P process, to create mixtures of carbon-depleted martensite, and carbon-enriched retained austenite. Important new implications with respect to fundamental elements of the bainite transformation are also discussed.