Study On Microstructure And Properties Of 22MnB5 Steel For Hot Stamping

In recent years,hot stamping steel is a newly developed high-strength steel for automobile with the strength achieving 1500～2000MPa after stamping.The alloy contents of 22 MnB5 hot stamping steel are high and its microstructure evolution is complex.It has significant heredity to the quenching structure and properties.Therefore,it is very important to investigate the relationship among the hot processing,microstructure and properties of22 MnB5 hot stamping steel.The dynamic CCT curve and dynamic TTT curve of the 22 MnB5 hot stamping steel were measured via the deformation phase change dilatometer.In order to obtain hot-rolling strips consisting of ferrite and pearlite,the temperature in the width direction of the strip should be higher than 585℃ according to the CCT curve.According to the dynamic TTT curve,the incubation period of ferrite decreased and then increased with increasing the isothermal temperature.The incubation period of supercooling austenite to ferrite transformation was shortest(0.6s)at “nose” temperature(640℃).Two solutions were proposed to solve the coil collapses.One was high temperature coiling to slow down the phase transformation rate after coiling.The other one was low temperature coiling to suppress the ferrite transformation after coiling.Slowing down cooling rate to 20℃/s could prevent the temperature decrease of the edge of the steel,decreasing the hardness of the edge of the steel in the width direction.It was proposed that the strips with uniform properties and easy forming were obtained when microstructure consists of homogeneous ferrite and cementite coiling at 650～750℃.Through the hot stamping process of 22 MnB5 steel was optimized.After hot stamping,simulated coating process(170℃+30min)and tempering,good mechanical properties of1.5mm thick sheets were achieved.The sheets exhibited the tensile strength of 1516 MPa,yield strength of 1153 MPa,elongation of 8.2%.The bending angle of sheets in vertical rolling direction was 90° and the bending angle parallel to rolling direction was 88°.Figure 72;Table 20;Reference 52...