Effect Of Carbon On Microstructure And Mechanical Properties Of Hot Stamping Steels

Vehicle emissions and safety issues have become severer and severer at present. The hotstamping steel with strength of more than1500MPa shows great advantages to reduce weightand improve vehicle safety, and undoubtedly has a potential application in automotive bodyparts in the future. In order to research and develop a new high-performance and ultra-high-strength hot stamping steel with independent intellectual property rights, for meeting the needsof the domestic market, four kinds of hot stamping steels containing different carbon contentwere designed in this thesis. Basic research on the designed steels was conducted to providebasic data and design principles for the development of new hot stamping steels.The continuous cooling transformation of the tested steels, determined CCT curves of thetested steels and microstructures of tested steels at different cooling rates have been studied bythe thermal dilation method and thermal analysis method. It was found that: the austenitestability of the hot stamping steel was effectively improved, the ferrite precipitation temperature,the pearlite transformation temperature, the bainitic transformation temperature and themartensitic transformation temperature decreased as the carbon content increased. Moreover,the hardenability characteristic could be effectively improved by increasing the carbon content,resulting in the movement of CCT curve to right and down. For the hot stamping steel with acarbon content of0.35%, fully martensitic microstructure could be obtained when the coolingrate is20°C/s. Moreover, the Vickers hardness after continuous cooling transformation can alsobe improved as the carbon content increases.Finally, the mechanical property of the tested steels was measured. It was found that: thestrength of the two tested steels with lower carbon content in full martensitic microstructure waslower than1500MPa, while brittle fracture was found in the two tested steels with higher carboncontent in full martensitic microstructure during the mechanical testing. For the two steels withhigher carbon content, both high strength and good plasticity can be achieved simultaneously byobtaining the mixed martensite, bainite or martensite, bainite, ferrite microstructures instead offull martensitic microstructure.