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Medium-Mn steel is an outstanding representative of the newly developed third-generation advanced automotive steels. In our previous experimental studies, industrial trial production of 0.1C–5Mn medium-Mn steel has been realized, anti-decarburization ability of warm-formed medium-Mn steel has been assessed compared with the conventional hot-formed 22MnB5 steel, and heterogeneous spot-welding performance evaluation has also been completed on the industrial production line. Previous results proved that the medium-Mn steel have a development trend of replacing hot-formed boron steel in the automotive industry. Besides, numerical simulation analysis on the forming process of an actual automotive part is also an important topic of the applicability research of medium-Mn steel. Therefore, in this paper, a warm-formed medium-Mn steel B-pillar part was investigated by simulation in order to elucidate the effects of process parameters. Firstly, mechanical property tests were carried out to obtain the material parameters for establishing an accurate finite element model of the B-pillar part. After that, the warm-forming process was simulated and the effectiveness of the simulation was validated by benchmarking with the experimental result. Finally, the deep drawing zone with maximum fracture risk was focused to analyze the sensitivity of process parameters including initial blank temperature (IBT), blank holding force (BHF), and forming velocity (FV). Research results show that the IBT has the greatest influence on the thickness of the deep drawing zone. The recommended IBT is between 450 and 550 °C, which is favorable to obtain higher total elongation without loss of strength. This study is helpful to provide a theoretical basis for the large-scale application of warm-forming medium-Mn steel.

The thermal processing parameters is very important to the hot rolling and forging process for producing grain refinement in lightweight high-manganese and aluminum steels. In this work, the high temperature deformation behaviors of a low-density steel of Fe30Mn11Al1C alloyed with 0.1Nb and 0.1V were studied by isothermal hot compression tests at temperatures of 850–1150 °C and strain rates between 0.01 s−1 and 10 s−1. It was found that the flow stress constitutive model could be effectively established by the Arrhenius based hyperbolic sine equation with an activation energy of about 389.1 kJ/mol. The thermal processing maps were developed based on the dynamic material model at different strains. It’s shown that the safe region for high temperatures in a very broad range of both deformation temperature and deformation strain and only a small unstable high deformation region, located at low temperatures lower than 950 °C. The deformation microstructures were found to be fully recrystallized microstructure in the safe deformation region and the grain size decreases along with decreasing temperature and increasing strain rate. Whereas the deformation microstructures is composed by grain refinement-recrystallized grains and a small fraction of non-recrystallized microstructure in the unstable deformation region, indicating that the deformation behaviors controlled by continuous dynamic recrystallization. The Hall Petch relationship between microhardness and the grain size of the high temperature deformed materials indicates that high strength low-density steel could be developed by a relative low temperature deformation and high strain rate.

In this study, ultra-high-carbon steels with 1.4% carbon content alloyed with three different aluminum contents, 2.0%, 4.0% and 6.0%, were studied on their tempering stability and temperature resistance. The results showed that the addition of Al significantly enhanced the tempering stability and temperature resistance of ultra-high-carbon steel. The addition of Al inhibited the transformation of ε-carbide to cementite, suppressed the transition of martensite to ferrite and thus, endowed ultra-high carbon steels to maintain very high hardness during tempering within a wide range of temperature up to 500 °C. The present work provides a useful basis on which to develop bearing steel materials with low density and high hardness.

The present work was attempted to evaluate the influence of reverted austenite (RA) fraction on hydrogen embrittlement (HE) of a hot rolled transformation-induced plasticity (TRIP)-aided medium Mn steel (0.1C-5Mn) by using electrochemical charging, slow strain rate tensile (SSRT) test and thermal desorption spectrometry (TDS) analysis. Different volume fractions of RA (~10–30 vol%) in the tested steel sheet were obtained by changing intercritical annealing time. The result of TDS analysis demonstrated that the charged hydrogen is primarily diffusible hydrogen corresponding to low-temperature hydrogen desorption peak and its concentration increases almost linearly with an increase in the volume fraction of RA. It was found that the intercritically annealed specimen exhibits an increasing susceptibility to HE with an increase in the volume fraction of RA primarily due to the increased RA transformation to martensite during tensile deformation. It is thus regarded that the TRIP effect is harmful to the HE resistance of TRIP-aided steels, and therefore both the mechanical stability and amount of RA have a strong influence on the HE behavior of TRIP-aided medium Mn steel.

The effects of heat treatment on the microstructure evolution was studied in regards to austenite nucleation and grain growth. It was found that the austenite nucleation and matrix recrystallization kinetics of samples annealed at 675 &deg, C for different times were revealed, implying a strong interaction between the ferrite matrix and austenite was revealed. The recrystallization of the matrix during annealing provided favorable conditions for austenite nucleation and growth, and the formation of austenite during this process reduced the matrix recrystallization kinetics, thus delaying the recrystallization process of the matrix around the austenite grains. The statistical results for the austenite grain size under different annealing temperatures indicated that the average grain size of the austenite slightly increases with increasing of the annealing temperature, but the austenite with the largest grain size grows faster at the same temperature. This difference is attributed to the strict Kurdjumov Sachs (KS) orientation relationship (OR) between the austenite grains and the matrix, because the growth of austenite with a strict KS OR with the matrix is often inhibited during annealing. In contrast, the austenite maintains a non-strict KS OR with the matrix and can grow preferentially with increasing annealing temperature and time.

Sheared edge cracking phenomenon is a key problem limiting the application of high-strength steels. Therefore, it is very necessary to explore the cracking causes to promote the solution of this problem. In this paper, the damage in the sheared edge of DP780 steel is investigated by microstructure characterization, micro/macro mechanical property evaluation, and numerical simulation. Microvoids, microcracks, and work hardening behavior are identified as damage factors affecting the sheared edge cracking. Two types of microvoids are found based on the position of microvoids. Microvoids formed at phase interfaces have the characteristics of small size (≤ 5 μm) and large number (276–340), while microvoids generated from inclusions hold large size (> 5 μm) and small number (6–18). The damage degree of microvoids is evaluated by number, size, and distribution. The result shows the damage of microvoids becomes more serious with the increasing shearing clearance. During shearing, microvoids are split to form microcracks. These microcracks become the cracking source in the subsequent process. Therefore, microcracks are the most crucial to the formability. Reducing microcracks is an effective method to avoid sheared edge cracking. Furthermore, the shearing process is simulated in ABAQUS. A VUSDFLD subroutine is developed for implementing the constitutive model of DP780 steel. The simulated results match the experiment results very well, which indicates the damage of DP780 steel during shearing can be predicted by simulations. The research of this paper is beneficial to understand the cause of sheared edge cracking and design an optimal shearing process.

Failure evolving from sheared edge is common in the application of advanced high strength steels (AHSSs) to automotive body. In this paper, the third-generation automotive medium-Mn (TAMM) steel, as a newly-developed AHSS, is sheared by a self-designed shear testing platform under different clearances. Microstructure, microhardness and tensile properties are tested and shearing procedure is simulated to analyze the effect of shearing clearance on macro/micro performance of sheared edge. With the increase of shearing clearance, more large-size, elongated, and widespread microvoids are formed to decrease the formability of sheared edge. Moreover, the surface and subsurface of sheared edge are hardened because of work hardening and phase transformation strengthening induced by the plastic deformation during shearing. The hardened depth and degree increase, and the hardness uniformity of the hardened layer becomes poor as the shearing clearance increases, which leads to the increase of possibility of cracking. Besides, tensile displacement in uniaxial tensile test is obviously affected by shearing clearance. Quantitative analysis results show that tensile displacement is higher and less sensitive to clearance change under a smaller clearance. Thus, 2%t–5%t (t means the steel thickness) is an advisable shearing clearance range for TAMM steel by considering the microstructure, hardened layer and tensile properties of sheared edge comprehensively. This paper is helpful to solve the cracking problem of sheared edge in industry and promote the industrial application of TAMM steel.

With the increasing requirement of lightweight and safety of automotive industry, the third-generation automotive medium-Mn (TAMM) steel with large-fractioned metastable austenite has been newly developed in order to enhance the strength and guarantee the formability. In this paper, the effect of plastic deformation condition, such as strain direction and strain rate, on mechanical properties and microstructure of TAMM steel is firstly investigated. The results show that the TAMM steel holds isotropic characteristic and the effect of strain direction and strain rate on mechanical properties can be neglected. Furthermore, the two-step forming process of TAMM steel is studied step by step to understand the effect of pre-forming on final forming. On the one hand, the martensitic transformation and work hardening during pre-deformation increase the strength but decrease the uniformity of mechanical properties, which decreases the final formability. On the other hand, with the increase of pre-strain, the enhanced yield strength, lengthened yield platform and undulating work hardening rate during final forming increase the probability of the crack. The simulated and experimented results prove that one-step forming process is suitable for the TAMM steel part with complex shape because the TRIP effect during one-step forming contributes to high plasticity.

Third-generation advanced automotive medium-Mn steel, which can replace 22MnB5 steel, was newly developed to improve the lightweight and crashworthiness of automobile. Studies on the formability and simulation method of medium-Mn steel have just been initiated. In this study, finite element simulation models of square-cup deep drawing were established based on various material property experiments and validated by experiments. The effects of blank holder force (BHF), fillet radii of tools (die and punch) on the maximum drawing depth (MDD), thickness distribution of the formed products, and the microstructure before and after forming were investigated and compared with those on 22MnB5 steel. Results show that the MDD of the two steels decreased with increased BHF but increased with the fillet radius of punch; however, the fillet radius of die showed no significant effect on the MDD for both steels. Compared with hot-formed 22MnB5 steel, the martensitic transformation of the hot-formed medium-Mn steel is rarely influenced by the process parameters; thus, it holds the complete, fine-grained, and uniform martensitic microstructure. Moreover, the medium-Mn has better formability, lower initial blank temperature, and smaller impact of BHF and fillet radius of tools on the hot-formed product. Thus, a theoretical basis for the replacement of 22MnB5 steel by medium-Mn steel in hot forming process is provided.

The medium-Mn steel, which is an outstanding representative of third-generation advanced automotive steel, is newly developed. Thus, studies on the thermal forming of this steel have just been initiated. This study was performed primarily on the thermal formability of medium-Mn steel. The uniaxial tensile simulation model was established in LSDYNA based on various material property experiments, and the ductile fracture critical values were obtained by simulated and experimental force–deformation curves. Results show that the medium-Mn steel is an isotropic material, and the strain rate has no significant effect on its flow stress at elevated temperature. The predicted thermal forming limit diagram (TFLD) was achieved from the numerical simulation results and was verified by the TFLD experiments based on the Nakazima test. Results show that the formability of medium-Mn steel increases rapidly at the beginning and then decreases gradually as the temperature increase. The theoretical optimal temperature for the formability was approximately 400 °C, and the recommended initial forming temperature is approximately 450 °C in actual industrial application of the thermal forming medium-Mn steel by considering the influence of contact heat transfer and heat radiation.

The third-generation automotive medium-Mn steel (TAMM steel) was newly developed. The medium-Mn steel holds the advantages of fine and even microstructure, low austenizing temperature and wide quenching rate range, and thereby it is possible to contribute to excellent properties, high efficiency and low cost in warm/hot forming. However, research on the forming of this TAMM steel has just started. It is necessary to understand the relation between forming process and heat transfer, and further apply the heat transfer characteristics to numerical simulation. Therefore, this paper explored the interfacial heat transfer rules during warm forming the TAMM steel and determined the interfacial heat transfer coefficient (IHTC) using a self-designed experimental platform and Beck's inverse estimation method. The results indicate that the IHTC is not constant but varies nonlinearly during quenching. Moreover, the influences of several factors, including stamping pressure, stamping temperature and surface coating, on the IHTC were investigated. The IHTC increases with the increase of stamping pressure and stamping temperature. However, it decreases greatly with the increase of coating thickness. It is also found that the microstructure and mechanical properties are not sensitive to the coating and quenching rate, which is beneficial to the achievement of even mechanical properties of the stamped TAMM part, and helpful to the flexible design of quenching method and coating application in actual warm stamping process.

The application of high strength steels (HSS) for automotive structural parts is an effective way to realize lightweight and enhance safety. Therefore, improvements in mechanical properties of HSS are needed. In the present study, the warm stamping process of the third generation automotive medium-Mn steel was discussed, the characteristics of martensitic transformation were investigated, as well as the microstructure and mechanical properties were analyzed, compared to the popular hot-stamped 22MnB5 steel in the automotive industry. The results are indicated as follows. Firstly, the quenching rate of the medium-Mn steel can be selected in a wide range based on its CCT curves, which is beneficial to the control of forming process. Secondly, the influence of stamping temperature and pressure on the Ms temperature of the medium-Mn steel is not obvious and can be neglected, which is favorable to the even distribution of martensitic microstructure and mechanical properties. Thirdly, the phenomenon of decarbonization is hardly found on the surface of the warm-stamped medium-Mn steel, and the ultra-fine-grained microstructure is found inside the medium-Mn steel after warm stamping. Besides, the warm-stamped medium-Mn steel holds the better comprehensive properties, such as a lower yield ratio, higher total elongation and higher tear toughness than the hot-stamped 22MnB5 steel. Furthermore, an actual warm-stamped B-pillar of medium-Mn steel is stamped and ultra-fine-grained martensitic microstructure is obtained. The mechanical properties are evenly distributed. As a result, this paper proves that the warm-stamped medium-Mn steel part can meet the requirements of lightweight and crash safety, and is promising for the industrial production of automotive structural parts.

The influence of tempering on the hydrogen embrittlement (HE) behavior of a hot-rolled and intercritically annealed medium-Mn steel (0.1C-5Mn) was investigated by slow strain rate tensile testing. It was observed that the absorbed diffusible hydrogen content remained almost unchanged with increasing tempering temperature. The susceptibility to HE evaluated by the relative total elongation loss is most significant for the as-annealed specimen, while tempering treatment can notably alleviate it. It is nearly immune to HE when the tempering temperature was increased up to 500 °C. The fraction of hydrogen-induced brittle fracture decreases with increasing tempering temperature. The enhanced resistance to HE of the tested steel is primarily ascribed to the microstructural evolution of retained austenite and cementite during the tempering process. It is thus proposed that the HE susceptibility of medium-Mn steel could be significantly reduced through suitable tempering treatment after intercritical annealing provided no notable loss of its superior strength-ductility balance.

Three-body impact abrasive wear behaviors of 20Si2Ni3 steels treated by Quenching–Partitioning–Tempering (Q–P–T) and quenching–tempering (Q–T) processes were investigated under four impact energies from 0.5 J to 3.5 J. The results indicated that wear mass losses increased and then decreased with the increasing impact energy and the peak value occurred under 2.5 J. The Q–P–T steel owned less wear mass loss than the Q–T steel. The wear characteristics of two steels were mainly shown as ploughing‐cutting wear, abrasive-embedded wear and strain fatigue wear. The Q–P–T sample suffered heavier ploughing‐cutting wear than the Q–T sample under a low impact energy, and underwent more strain fatigue wears under a high impact energy. During the wear tests, several hundred microns of hardened layers were formed and their corresponding thicknesses increased with the increasing impact energy. The Q–P–T steel obtained thicker hardened layer because of its high phase transformation strengthening and work hardening during the wears. As a result, the wear resistance of the Q–P–T steel was enhanced effectively, and thereby the excellent impact abrasive wear performance was achieved.

The interfacial heat transfer coefficient (IHTC) between the steel blank and die directly affects temperature distribution during hot forming of advanced high-strength steel automobile parts and further affects martensite microstructure distribution and formability. In this paper, an experimental platform is established and Beck's nonlinear inverse estimation method is employed to solve the IHTC and investigate the rules of IHTC following contact pressure. This paper demonstrates that the IHTC increases with the increase in contact pressure. An actual hot-formed automotive B-pillar was obtained in the experiment. Results show that a high contact pressure corresponds to a high IHTC, which promotes fast blank cooling on the fillet corner area. By contrast, a low contact pressure results in low IHTC and cooling rate on the side wall area. Thus, a significant temperature difference induces asynchronous martensite transformation between the fillet corners and the side walls, where crack is more likely to occur. As a solution, local oil coating is used on fillet corners with high contact pressure before hot forming to improve the formability and avoid cracks. This finding indicates that local oil coating can effectively weaken the heat transfer performance, balance the temperature field in the different areas of the part, and consequently achieve better formability for the hot forming process.

A two-step quenching and partitioning (Q&P) treatment was applied to low-carbon alloy steels. The relation of initial martensite - retained austenite - fresh martensite and its effect on microstructure and mechanical properties were investigated by experiments. The results reveal that the volume fraction of retained austenite can reach the peak value of 17%, and the corresponding volume fractions of initial martensite and fresh martensite are 40% and 43%, respectively, when the tested steel is treated by initial quenching at 330°C, partitioning at 500°C for 60s and final quenching to room temperature. Moreover, the micromorphologies of austenite and martensite become finer with the increasing of initial martensite fraction. The elongation is the highest when the volume fractions of initial martensite and retained austenite are 70% and 11%, respectively, meanwhile, the yield strength increases and tensile strength decreases gradually with the increase of initial martensite fraction, which proves that the mechanical properties including elongation, yield strength and tensile strength are based on the comprehensive effect of the retained austenite fraction, the finer microstructure and austenite stability.

Discontinuously reinforced titanium matrix composites (DRTMCs), as one of the most important metal matrix composites (MMCs), are expected to exhibit high strength, elastic modulus, high-temperature endurability, wear resistance, isotropic property, and formability. Recent innovative research shows that tailoring the reinforcement network distribution totally differently from the conventional homogeneous distribution can not only improve the strengthening effect but also resolve the dilemma of DRTMCs with poor tensile ductility. Based on the network architecture, multiscale architecture, for example, two-scale network and laminate-network microstructure can further inspire superior strength, creep, and oxidation resistance at elevated temperatures. Herein, the most recent developments, which include the design, fabrication, microstructure, high-temperature performance, strengthening mechanisms, and future research opportunities for DRTMCs with multiscale architecture, are captured. In this regard, the service temperature can be increased by 200 °C, and the creep rupture time by 59-fold compared with those of conventional titanium alloys, which can meet the urgent demands of lightweight nickel-based structural materials and potentially replace nickel base superalloys at 600-800 °C to reduce weight by 45%. In fact, multiscale architecture design strategy will also favorably open a new era in the research of extensive metallic materials for improved performances.

The forming process of sheet metal is a complicated procedure relating to tensile and compressive plastic deformation. The in-plane tensile-compressive test is a main way to understand the plastic behavior under the complicated loading paths. In this paper, a new device is developed to realize in-plane tensile-compressive tests in order to study the plastic behavior during tension and compression of sheet metal. The newly-developed device has the advantages of high efficiency and low cost. It can support a reliable continuous, large-strain tensile-compressive test for more materials with higher strength based on any common universal testing machine. A set of friction-counteracting supporting mechanism including T-shaped supporting plates, Teflon sheets and upper/lower fixtures is designed to prevent buckling deformation of sheet metal during compression. Moreover, the tensile-compressive tests of DP780 steel are conducted by using the new device to obtain the tensile-compressive stress-strain curve reflecting the Bauschinger behavior. The constitutive model of DP780 steel is established and the optimal model parameters are achieved for the simulation analysis. As a result, the simulated tensile-compressive curves match the experimental curves very well. The newly-developed device is favorable to accurately describe the plastic behavior under the complicated loading paths during forming and contribute to precise simulation analysis.

Third-generation advanced automotive medium-Mn steel, which can replace 22MnB5 steel, was newly developed to improve the lightweight and crashworthiness of automotive. Studies on the formability and simulation method of medium-Mn steel have just been initiated. This study elucidated the effects of initial forming temperature (IFT) on formability, thickness distribution, macro mechanical property and micro performance of medium-Mn steel hot-formed products. Results show that the IFT intensely affects the formability and the thickness distribution of the deep drawing zone on the medium-Mn steel hot-formed part, and the recommended IFT range is between 400 °C and 500 °C.

The advanced high strength steels(AHSS) for auto manufacturing have been developed to the 3rd generation. The typical one is steel treated by Q-P-T process, which consists of the prior martensite, fresh martensite and retained austenite. In this paper, the non-uniform distributive formation of martensite and untransformed austenite during the one-step quenching treatment was observed by laser scanning confocal microscope. For the Q-P-T process, the non-uniform distributions of untransformed austenite, prior martensite and segmented austenitic grains are the main reasons to form the multi-scale martenite. Moreover, the fresh martensite possesses three highlights, such as microstructure of finer laths, same orientation in a blocky area considered as a “packet” and higher mechanical properties. The research result of this paper reveals the microstructure evolution of Q-P-T treated steel, and provides the accurate theoretical basis for improving the Q-P-T process and the mechanical properties.

Two-step quenching and partitioning treatment with hot stamping was applied to advanced high-strength steel (AHSS). The newly treated steel possesses a fine microstructure and typically curved micromorphology. The martensite start temperature of the newly treated steel is increased through the effect of plastic deformation on austenitic microstructure. However, the martensite volume fraction of this steel is deceased because of the enhanced stability of the untransformed austenite after plastic deformation. Consequently, the fraction of retained austenite is increased. The newly treated steel also shows excellent mechanical properties. The volume fraction of retained austenite reaches the highest value of 17.2% when hot stamping is performed at 750 °C. Hence, the steel displays favorable plasticity with an elongation of 14.5%. Moreover, the highest hardness value of 426 HV is obtained when hot stamping is performed at 650 °C. The newly developed process may be employed to develop a new generation of AHSSs.

The microstructural evolution of a cold-rolled steel 0.1C–5Mn was characterized by SEM and EBSD, and the α → γ phase transformation of the steel was determined by differential scanning calorimetry (DSC) upon heating. The retained austenite fraction and the dislocation density were examined by XRD. Based on the DSC data, the Kissinger׳s method was used to calculate the activation energy of the cold rolled steel. The activation energy of 363–824 kJ/mol suggested a sluggish phase transformation during the heating process and it showed an increase trend with the increase of thickness reduction which was affected by the density of the dislocation and the drag effect by the solute Mn. The stored energy (14–30 J/g) was associated with energy released from the α → γ transformation during the heating process. Based on the effects of temperature on the phase transformation behavior, the cold rolled medium-Mn steel with different thickness reductions were annealed at the onset phase transformation temperature with 5 min and 6 h, which results in a very good combination strength and ductility.

Non-isothermal tension and compression tests of 22MnB5 boron steel were carried out in this study. How different stress state influences the martensitic transformation of advanced high strength steel (AHSS) parts was analyzed. The analysis reveals that the martensitic transformation starting temperature ( M s ) changes with different stress states. Specifically, the M s temperature rises with increasing tensile stress, however, it rises first and then drops with increasing compressive stress. Moreover, a higher initial forming temperature leads to a higher M s temperature under the same stress. Simulation of an actual hot-formed AHSS B-pillar together with the microscopic metallography, hardness and martensitic content shows that in higher tensile stress dominated area, the martensitic content and hardness are usually higher than in other areas. Although the stress can promote the M s temperature, a lower cooling rate may lead to less martensite fraction.