Research And Application Of Temperature-Controlled Gradient Phase Transformation Hardening Pattern For Ultra-High-Strength Steel

The growing concern for occupant safety leads to the new requirements of tailored components which improve the integrity of the vehicle during a crash while also improving the energy absorption and crash performance.These components are required to have stepwise or gradient distributed mechanical properties,that is to say the tailored properties.Overseas and domestic scholars have performed related researches,and produced hot stamped parts with stepwise properties using different approaches.However,these approaches cannot satisfy the increasingly requirements of designing complex hot stamped components in respects to process flexibility,adaptability and robustness.This work proposed a new approach of tailor hot forming technology with outstanding designability via gradient temperature controlled phase transformation on the basis of Hot Forming Die Quenching(HFDQ)(also hot stamping or press hardening).The objectives of this research were to design and implement a lab-sized flat tool with temperature control system used for tailor hot stamping of component with tailored properties.With the help of the designed temperature control system,controlled phase transformation of boron steel BR1500 HS were implemented to investigate the feasibility of producing hot stamped part with tailored mechanical properties.The main research contents and conclusions were as follows:(1)Designed a hot stamping die with gradient temperature distribution to perform numerical simulation of tailor hot stamping.The change of sheet metal temperature,die temperature,material phase composition with time and space were studied.(2)Developed automatic temperature-controlled gradient phase transformation testing platform draw on the lessons from the design and development of traditional hot stamping die.The influences of different gradient cooling conditions arise from different initial forming temperature,die temperature gradient and holding time on the performance of hot stamping parts were discussed.(3)Established the mapping relationship between the tailored gradient cooling conditions with the as-quenched multiphase microstructures,mechanical properties indicators including hardness,tensile strength and elongation at break.(4)These mapping relationships were applied to design the tailor hot stamping process of objective vehicle crash structures member,automobile B-pillar,tailored mechanical properties.The numerical simulation was also carried out to evaluate the feasibility of this approach.