Study On Thermo-mechanical Properyies And Phase Transformation Of High Strength Automobile Steel And Hot Stamping Of Part

In order to achieve the energy saving of the car, the high-strength steel and the ultra-high strength steel are used for the manufacturing of the body structural parts of the car, which can ensure the high safety of the structure at the same time a significant reduction in the weight of the parts. The hot forming as a new blank forming process combining the forming process with in-die heat treatment overcome the difficulty caused by the "spring back" and maintain the accuracy of the geometric dimension of the part. However, this innovative blank forming process needs the further research. According to the research of the hot forming process principle, the effect of process parameters on the performance of hot stamping parts was understood, and the hot stamping equipments were developed.The high strength hot stamping steel WHT1300HF developed by Wuhan Iron and Steel (Group) Corporation was used as the experimental material. And the research were focused on the following aspects:(1) the thermal mechanical properties under the isothermal conditions at elevated temperatures;(2) the effect of deformation parameters (deformation temperatures, strain rates, deformation amounts, etc.) on the phase transformation and the microstructure morphology;(3) the influence of the one-step quenching and partitioning heat treatment on the residual austenite content and the mechanical properties at room temperature. Simultaneously, the B pillar of Dongfeng S30passenger car was manufactured by utilized the hot stamping experimental line which was developed by WISCO-HUST joint laboratory. The microstructure and mechanical properties of the hot stamped part were analyzed. Through the research, the following results and conclusions are obtained in this paper:(1) In the isothermal uniaxial tensile, increasing the deformation temperatures and the strain rates cause the significant reduction and raise in the level of flow stress and work hardening exponent of the steel. Based on the Arrhenius-type constitutive model, the constitutive equation of the high strength steel WHT1300HF was obtained by fitting the true stress-true strain curves of the isothermal tension. The calculation relative error of the constitutive equation, which can be used for the design of the hot stamped part and the finite element numerical simulation, is less than5.15%.(2) Increasing the isothermal deformation amount of the austenite is conducive to the defect nucleation of the ferrite, promotes the ferrite transformation of the deformed austenite; the deformation strengthening of the austenite lead to the increasing of the resistance of martensite transformation; causes the decreasing of the martensite phase transformation start temperature (MS), the increasing of the number of small-angle grain boundaries and the number of the small grains. Increasing the isothermal deformation (40%) rate of the austenite promotes the martensite transformation and the ferrite transformation; but, the volume fraction of martensite and the number of small-angle grain boundaries are reduced. And the number of the small grains is increased. Reducing the isothermal deformation temperature intensifies the deformation strengthening of the austenite and results in the decrease of the MS temperature. The volume fraction of martensite and the proportion of the small-angle grain boundaries are reduced. The increase of the number of the small grains and the amount of the ferrite are significantly.(3) Due to the low Si content (0.25wt%) of WHT1300HF, isothermal (250℃,300℃,350℃) carbon partitioning (30,60,90s) lead to the precipitating of the ε-carbide to in the martensite matrix. The volume fraction of retained austenite at room temperature is significantly low as a result of insignificant carbon enrichment in the untransformed austenite; but, the treatment has an effect on the product of strength and plasticity. The maximum value of the PSP is17.6GPa%achieved at300℃for60s.(4) A digitalized pilot hot stamping production line contains a developed electro-servo press, a movable rack numerically controlled heating furnace and a blank conveying device was developed. And the overall production line is controlled by a real-time and coordinated control strategy to assure the safety of the operation and the high productivity. The microstructure of the manufactured B pillar of Dongfeng S30is uniform. The average strength is1489MPa and the average elongation is6.87%. The mechanical properties and the accuracy meets requirement of the body in white.