Study On The Hot Stamping Q&P Process Of High Strength Automobile Steel

To reduce fuel and raw materials consumption, decrease the weight as well as increase safety of the vehicles and lower carbon gas emission, advanced high strength steels(AHSS) are increasingly used by the automotive industry. The traditional hot stamping is an effective technology of achieving automobiles components with ultra-high strength and excellent forming capabilities, whereas the ductility is only 7 %~8 %, leading to the low product of strength and elongation, which cannot accomplish the mechanical properties required for 3rd generation AHSS. The Q&P heat-treatment step is to get a controlled amount of martensite and retained austenite by cooling the steel palte which is fully austenitised to a certain quench temperature(QT) between martensitic transformation start temperature(Ms) and martensitic transformation final temperature(Mf), then reheating it to the partitioning temperature(PT ? QT) and holding for a time to enrich the austenite with carbon by partitioning of carbon from carbon-supersaturated martensite. After Q&P process, a lot of retained austenite(RA) enriched with carbon is beneficial to improve the ductility, whereas the tensile strength is sacrificed. Given the incrementally improvement on the ductility of light automotive parts without sacrificing its high strength, in this study, we propose the application of quenching and partitioning process to the hot stamping line aiming at further improving the product of tensile strength and ductility of ultra-high strength automobile steels then meeting the goal required for 3rd generation AHSS.The relationship between quenched temperature and the volume fraction of retained austenite for the 30 Cr Mn Si2 Nb steel was calculated by the means of empirical formula and CCE model in order to choose the optimum QT. The influence of partitioning temperature and partitioning time on the content of RA and mechanical properties was investigated by one-step and two-steps Q&P process on Gleeble-3500 thermodynamic simulator. A series of experiments of hot stamping forming parts based on Q&P process are conducted to investigate the microstructure evolution and mechanical properties of the steel.The results showed that:(1) Mechanical properties of 30 Cr Mn Si2 Nb ultra-high strength steel with excellent ductility and high product of tensile strength and ductility, such as 19.1 %, 26.92 GPa%(the content of RA was 12.43 %)were obtained through the one-step and two-steps Q&P tests on Gleeble-3500 thermodynamic simulator, which are higher than that through direct quenching process and conventional Q&T process.(2) The experiment scheme and tools during the hot stamping-Q&P process were designed based on the Gleeble-3500 thermodynamic simulation tests and CCE theory model. The regular of process parameter on the evolution of microstructure was discussed. The microstructure of studied steel treated by hot stamping-Q&P process was composed of lath-shaped martensite(about 200 nm ~300 nm) and flake-shaped carbon enriched retained austenite(about 200 nm). In addition, twinned martensite is also obtained in the martensite microstructure. The maximum amount of 9.38 % retained austenite was observed by X-ray diffraction. The measured carbon content was about 1.33 wt. pct. and the stability coefficient of RA is the maximum.(3) During hot stamping-Q&P process, the QT temperature and quenched cooling speed for the given process window were achieved by means of heating rods inside the die and heat dissipation rapidly transmitted through the tool itself. The temperature of blank and mold during the Q-P hot stamping process was collected by K type thermocouple, which can accurately control and monitor the temperature evolution of the process window. The product of tensile strength and elongation approach 23.03 GPa%, which is much better than the direct quenched parts(8.97 GPa%) and Q&T parts(9.71 GPa%). Therefore, the hot stamping-Q&P process is an effective method to improve the product of tensile strength and elongation(PSE) for the ultra-high strength automotive steel and will has wide prospect of automotive industrial applications.