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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. [ABSTRACT FROM AUTHOR]

There is a certain contradiction between the formability and strength of auto parts. In this work, the whole-process processing technology of hot stamping soft steel was designed, and 500 MPa grade mild steel (500HS) with uniform microstructure was prepared. To take into account the strength and formability of hot stamping soft steel, here, based on laser welding technology, 500 MPa grade soft steel, and 1500 MPa grade hard steel are benignly composited, and by adjusting the laser welding speed and heat treatment process, the loss of mechanical properties caused by the weld seam is eliminated. A soft/hard composite steel for automobiles with excellent strength, ductility and formability is obtained. To maintain excellent deformation resistance and bonding force of the weld, the heat-affected zone of 500HS retains part of bainite and pearlite, which is beneficial to the strain compatibility and stress partitioning with the microstructure of the base metal, and the hardness is low. After heat treatment, which helps to transfer the stress concentration effect to the 500HS base metal with strong energy absorption capacity, so that the clad steel has excellent comprehensive mechanical properties. This process is developed based on existing industrialized equipment and has broad application prospects. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Automobile Technology & Material is the property of Automobile Technology & Material Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The surface of hot stamping steel is severely oxidized during heating, holding, and transfer from the heating furnace to the stamping die in the production of traditional automotive parts. Coating-free hot stamping steel with Cr and Si elements exhibits excellent oxidation resistance during hot stamping without the protection of a surface coating. This paper investigates the oxidation behavior of three types of hot stamping steel at 800–1200 °C. The results show that although Cr-Si hot stamping steel performs excellently short-term (≤7.5 min) for oxidation resistance, its long-term (≥15 min) or high-temperature (≥1100 °C) oxidation resistance is much lower than that of the conventional hot stamping steel 22MnB5, affecting the production and surface quality control of the new coating-free Cr-Si hot stamping steel. By analyzing the oxidation kinetics and characterizing the structure of oxide layers in hot stamping steel, it was found that the structural change in the Cr and Si element enrichment layer between the oxide scale and the substrate varied in oxidation performance at different temperatures. When the oxidation temperature was below 1000 °C, the solid Cr and Si enrichment layer acted as a barrier to prevent the diffusion of Fe ions. When the oxidation temperature exceeded 1100 °C, the molten Cr and Si enrichment layer effectively adapted to the substrate and avoided blistering. Meanwhile, Fe2SiO4 penetrated the Fe oxide layer along the grain boundary and became a rapidly diffusing channel of Fe ions, contributing to a significant increase in the oxidation rate. [ABSTRACT FROM AUTHOR]

Copyright of Automobile Technology & Material is the property of Automobile Technology & Material Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this paper, the effect of batch-annealing temperature on the oxidation of 22MnB5 steel during austenitizing was studied. When the batch-annealing temperature was decreased from 690 to 680 °C, the surface roughness of cold-rolled 22MnB5 steel decreased, which reduced the oxide layer thickness during the subsequent austenitizing process and had little effect on the mechanical properties in the cold-rolled and quenched state. This indicated that oxidation during austenitizing could be reduced by properly reducing the batch-annealing temperature without affecting the mechanical properties. [ABSTRACT FROM AUTHOR]

A novel 2000HS hot stamping steel was fabricated for automotive applications. Compared to the traditional 1500HS hot stamping steel, it showed an excellent combination of strength and plasticity, and the thickness of the oxide layer was dramatically reduced during the hot stamping process. The tensile experiment, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and x-ray diffraction (XRD) characterization methods were employed, to clarify the best comprehensive mechanical properties of 2000HS steel. Experimental results indicated that the microstructure of Z740-A950-T170 consisted of lath martensite, large M23C6 precipitates, small (Nb, V) C precipitates and retained austenite (4∼6vol.%). The obtained best comprehensive mechanical properties of Z740-A950-T170 sample of the tensile strength was 2160 MPa, the yield strength was up to 1318 MPa, and the total elongation reached at 12%. The addition of Mo element inhibited the coarsening and lengthening of the precipitation. Large M23C6 precipitates (101.2 μm) and small (Nb, V) C precipitates (17.5 μm) played the role of pinning the dislocation and increased the yield strength. Regardless of any process, retained austenite was kept at room temperature (4–6 vol.%), which improving the strength and maintaining the total elongation by transformation induced plasticity (TRIP) effect. In addition, the oxidization layer thickness of the invented steel (12.4 μm) was thinner than that of the 1500HS (23.6 μm) after hot forming at 950 °C for 5 min. The significant reduction of oxidation was due to the high percent of Cr and Si contents within the designed steel, the formation of dense Cr and Si at the bottom of the oxide layer, which improving the antioxidant properties for studied steel.

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.

The surface of hot stamping steel is severely oxidized during heating, holding, and transfer from the heating furnace to the stamping die in the production of traditional automotive parts. Coating-free hot stamping steel with Cr and Si elements exhibits excellent oxidation resistance during hot stamping without the protection of a surface coating. This paper investigates the oxidation behavior of three types of hot stamping steel at 800–1200 °C. The results show that although Cr-Si hot stamping steel performs excellently short-term (≤7.5 min) for oxidation resistance, its long-term (≥15 min) or high-temperature (≥1100 °C) oxidation resistance is much lower than that of the conventional hot stamping steel 22MnB5, affecting the production and surface quality control of the new coating-free Cr-Si hot stamping steel. By analyzing the oxidation kinetics and characterizing the structure of oxide layers in hot stamping steel, it was found that the structural change in the Cr and Si element enrichment layer between the oxide scale and the substrate varied in oxidation performance at different temperatures. When the oxidation temperature was below 1000 °C, the solid Cr and Si enrichment layer acted as a barrier to prevent the diffusion of Fe ions. When the oxidation temperature exceeded 1100 °C, the molten Cr and Si enrichment layer effectively adapted to the substrate and avoided blistering. Meanwhile, Fe2SiO4 penetrated the Fe oxide layer along the grain boundary and became a rapidly diffusing channel of Fe ions, contributing to a significant increase in the oxidation rate.

Clarifying the influence of Nb and V microalloying on the ultra-high strength hot stamping steel (UHSHSS) and exploring appropriate process parameters are the basis for effectively regulating properties of the final product. In this study, the effects of different austenitizing temperatures and holding times on the phase transitions, grain sizes and mechanical properties of 22MnB5NbV with Nb and V alloyed are studied by using JMatPro thermodynamic calculations and experiments. By comparing with 22MnB5 without Nb and V alloyed, the effects of Nb and V elements on quenching microstructures, grain sizes and mechanical properties of UHSHSS are analyzed. The suitable austenitizing process parameters of 22MnB5NbV have been obtained. The results show that the grain size of Nb-V-alloyed UHSHSS grows with the increase in the austenitizing temperature and holding time. The 22MnB5NbV steel can be completely austenitized while the austenitizing temperatures ≥870 °C and holding time ≥3 min. Combined with the actual production process, the best austenitizing temperature and holding time are 930 °C and 3 min. Under these conditions, the average grain size is 7.7 μm, the tensile strength, elongation and strength-ductility product are 1570.8 MPa, 6.6% and 10.4 GPa·%, respectively. With the addition of Nb and V elements, the nanoscale precipitates lead to the refinement of the quenched structure and grain size, and the comprehensive properties of UHSHSS have been effectively promoted, in which the elongation and strong-plastic products are increased by ~0.6% and ~1.2 GPa·%, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the effect of batch-annealing temperature on the oxidation of 22MnB5 steel during austenitizing was studied. When the batch-annealing temperature was decreased from 690 to 680 °C, the surface roughness of cold-rolled 22MnB5 steel decreased, which reduced the oxide layer thickness during the subsequent austenitizing process and had little effect on the mechanical properties in the cold-rolled and quenched state. This indicated that oxidation during austenitizing could be reduced by properly reducing the batch-annealing temperature without affecting the mechanical properties.

The accurate prediction of mechanical properties and phase microstructures is of great significance to the design and development of the ultra-high strength hot stamping steel (UHSHSS). Based on the prediction of JMatPro and the experimental measurement, an accurate modified prediction model of JMatPro for the tensile strength of UHSHSS was proposed in the study, considering the influence of austenitizing temperature. At the same time, the prediction model of phase transition in quenching process of UHSHSS was established. The tensile strength and phase transition models of UHSHSS under different conditions were studied using the prediction models. Results showed that the tensile strength of UHSHSS increased with the increase of C content and decreased with the increase of Si content. For every 0.01 wt% increase in C content, the tensile strength increased by around 30 MPa. For the hot stamping process used in this study, the carbon content of hot stamping steel should be more than 0.35 wt% to achieve 2000 MPa in tensile strength. The volume fraction of the residual austenite in UHSHSS increased with the increase of C and Si content. During the quenching process, the phase transition in UHSHSS was the fastest between 4 s and 8 s, and the average decomposition rate of the austenite reached 17.8% per second. The average volume fraction of residual austenite and martensite in UHSHSS was 3.3% and 96.7%, respectively, when the quenching die temperature was 150 °C. With the increase of the die temperature, the volume fraction of residual austenite in steel increased.

Thin gauge hot strip steel can be produced by thin slab continuous casting and rolling (TSCR) process without the aid of cold rolling. However, few attempts have been made to produce hot stamping steel by using TSCR technology. As given steel, the hot stamping steel produced by the TSCR process was used to study the influence of austenitizing parameters on the final microstructures and mechanical properties. A Gleeble3500 thermal simulator was employed to heat the samples to obtain different austenite microstructures. After quenching, the Vickers hardness examination along the thickness of the sample and tensile testing were conducted. The microstructure was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that on the condition of relatively lower temperature and shorter holding time, the sample exhibited better mechanical properties at the cost of microstructure uniformity along the thickness, although higher austenitizing temperature enhanced the carbide dissolution which would increase the effective carbon content in the martensite matrix. Lower heating temperature led to small prior austenite grain size and small blocks which contributed greatly to the mechanical properties. [ABSTRACT FROM AUTHOR]

The effect of initial rolling temperature on the hydrogen-induced delayed fracture resistance of Nb alloyed hot stamping steel was studied by the constant load test and the electrochemical hydrogen permeation method. When the initial rolling temperature drops from 1000℃ to 950℃,the hydrogen diffusion coefficient and the corrosion resistance of the steel decrease,but the hydrogen-induced delayed fracture resistance increases. TEM test shows that when the initial rolling temperature is 1000℃,the MX precipitated phase has an average size of 30 nm,and the Cr2C3 precipitated phase can be observed with an average size of 100 nm; when the initial rolling temperature is 950℃,the MX precipitated phase has an average size of 5 nm. The main reason leading to the improvement of delayed fracture resistance is the hydrogen trapping effect of the MX precipitated phase. The reason for the difference of precipitated phases is that a higher rolling start temperature will prompt the coarsening of the MX precipitated phase,which will suppress the precipitation of the Cr2C3 phase.

The effects of the microalloying element niobium (Nb) on the microstructure and mechanical properties of the hot stamped steel 38MnB5 were investigated. The impact of Nb addition on the microstructure was studied through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The experimental results indicated that the microstructures of the steel containing Nb were finer than those of the steel without Nb. Moreover, Nb mainly presented as a second-phase particle in 38MnB5 steel, and the particles included Nb carbonitrides. In addition, the tensile strength and elongation of the hot rolled and hot stamped steels were also measured, and they demonstrated that the appropriate addition of Nb was beneficial to the mechanical properties of 38MnB5. Under the same conditions, the tensile strength of 38MnB5Nb was higher than that of 38MnB5, which increased from 2011 to 2179 MPa. The yield strength also increased from 1316 to 1476 MPa, and the elongation increased from 5.92% to 6.64%. Overall, Nb had a positive effect on the performance of the hot stamped steel. [ABSTRACT FROM AUTHOR]

The thermal deformation behavior and constitutive models of an Nb micro-alloyed 22MnB5 steel were investigated by conducting isothermal uniaxial tensile tests at the temperature range of 873-1223 K with strain rates of 0.1-10 s. The results indicated that the investigated steel showed typical work hardening and dynamic recovery behavior during hot deformation, and the flow stress decreased with a decrease in strain rate and/or an increase in temperature. On the basis of the experimental data, the modified Johnson-Cook (modified JC), modified Norton-Hoff (modified NH), and Arrhenius-type (AT) constitutive models were established for the subject steel. However, the flow stress values predicted by these three models revealed some remarkable deviations from the experimental values for certain experimental conditions. Therefore, a new combined modified Norton-Hoff and Arrhenius-type constitutive model (combined modified NH-AT model), which accurately reflected both the work hardening and dynamic recovery behavior of the subject steel, was developed by introducing the modified parameter k. Furthermore, the accuracy of these constitutive models was assessed by the correlation coefficient, the average absolute relative error, and the root mean square error, which indicated that the flow stress values computed by the combined modified NH-AT model were highly consistent with the experimental values ( R = 0.998, AARE = 1.63%, RMSE = 3.85 MPa). The result confirmed that the combined modified NH-AT model was suitable for the studied Nb micro-alloyed hot stamping steel. Additionally, the practicability of the new model was also verified using finite element simulations in ANSYS/LS-DYNA, and the results confirmed that the new model was practical and highly accurate. [ABSTRACT FROM AUTHOR]

Heat treatment simulations have been carried out to batch annealing process of 1700 MPa grade hot stamping steel. Effect of different annealing parameters on microstructure and mechanical property of steels before hot stamping has been investigated. The result showed that after annealing at 760°C for 60 minutes, the overall property of investigated steel was optima. [ABSTRACT FROM AUTHOR]

Martensite is a typical hierarchical microstructure, but which substructure plays a major role on fatigue crack growth behavior is still not clear. Therefore, this paper studied the microstructure and mechanical properties of 22MnB5 hot stamping steels, the influence of hierarchical martensitic microstructure on fatigue crack growth behavior was analyzed. Results show that higher hot stamping temperature results in larger size of prior austenite grain and substructure, the tensile strength decreased with increasing temperature, while the fatigue life and critical crack length were higher. It was concluded that block is the effective substructure to control the fatigue crack growth behavior of the steel. The fatigue fracture of stable-growing region was typical fatigue striations along with secondary cracks, crack closure was observed in rapid-growing region. • Higher hot stamping temperature results in lager substructure size. • Tensile strength decreased with stamping temperature, while fatigue life increased. • Block is the effective substructure to control the fatigue crack growth behavior. • Crack closure was observed in the fatigue fracture of rapid-growing region. [ABSTRACT FROM AUTHOR]

This study reports a novel 2000 MPa grade ultra-strong hot stamping steel, which contains lath martensite, nanometer-sized film-shaped retained austenite and precipitates. The yield strength significantly increases with increasing tempering temperature as the precipitation strengthening, grain refinement strengthening and reducing microstructure residual stress. Multi-strengthening mechanisms (i.e. grain refinement strengthening, precipitation strengthening and transformation-induced plasticity effect) work together to improve the strength and plasticity at the same time. The yield strength reached 1600 MPa, together with an ultimate tensile strength of 1939 MPa and total elongation of 9.14% when tempered at 573 K. Compared with commercial 22mnB5 steel, the tested steels have remarkable improvement in mechanical properties. [ABSTRACT FROM AUTHOR]

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. Image 1 • Hydrogen diffusivity of material-2 is four times lower than that of material-1. • Blisters originate from grain boundaries and further propagete into cracks. • Suppressed hydrogen-induced damage is obtained by adding Ti + Nb + V in 22MnB5. • A model is proposed to illustrate the suppression of hydrogen-induced damage. [ABSTRACT FROM AUTHOR]

The effects of the combined addition of Zn and Mg on the corrosion resistance of AlSi-based coating for automotive steel sheets were investigated using a variety of analytical and electrochemical techniques. The preferential dissolution of Mg and Zn from MgZn2/Mg2Si phases occurred on the AlSi-based coating that had been alloyed with a smaller portion of Zn and Mg, which contributed to the rapid surface coverage by corrosion products with a protective nature, reducing the corrosion current density. On the other hand, localized corrosion attacks caused by the selective dissolution of Mg were also observed in the AlSi-based coating with a smaller portion of Zn and Mg. Such alloying can also worsen its corrosion resistance when coated additionally with electrodeposited paint. The mechanistic reasons for these conflicting results are also discussed. [ABSTRACT FROM AUTHOR]