Effect of Tempering Time on the Microstructure and Mechanical Properties of Ultra- high Strength Hot Stamping Steel after Quenching

摘要

A ultra-high strength hot stamping steel was quenched and tempered at different times.The effects of tempering time on the microstructure and mechanical properties were investigated by SEM,TEM,and tensile testing.The best mechanical properties were obtained after the steel being tempered at 200℃ for 200 s,which are the tensile strength of 1676 MPa,the total elongation of 7.3％,and the product of tensile strength and elongation more than 12.24 GPa·％.It can meet the requirement for automotive structural parts after hot stamping.The microstructure of tested steel tempered at 200 s is lath martensite and ε-carbides.The mechanical properties non-monotonically change with tempering time.In order to study the service performance of ultra-high strength hot stamping steel under impact,high speed tensile test was done.The strain-stress curves were measured using Zwick HTM 16020 high-speed material tensile mechine at 400 s-1.The results indicate that compared with the quasi static stretching,plasticity and other mechanical properties of the tested steel under high-speed tensile strength are obvious changes,and at high speed tensiler the tensile strength are greater than the quasi static.The better mechanical properties tested steel at strain rates 400 s-1 the tensile strength of 1863 Mpa,the total elongation of 14.6％,and the product of tensile strength and elongation more than 27.20GPa·％.The tested steel experimental steel tensile fracture shows the characteristics of clear plastic fracture.Shear lip area,radiation area and fiber area were studied from the macro and the characteristics of shaft toughening nest and spin toughening nest were studied from the micro.The shape and size of the nests changed with the increase of strain rate.It is thus clear that the change of high strain rate deformation mechanisms is the reason for strain rate sensitivity under macro mechanical properties under dynamic load of tested steel.