Effect Of Making Process On Microstructure And Mechanical Properties Of Hot Stamping Steel

The booming development of the automotive industry places higher demands on the high strengthening and weight reduction of automotive materials.The application of hot stamping steel solves the problems of forming difficulties,large springback,and serious mold wear during cold stamping.However,its manufacturing process is long and costly.The production of hot stamping steel by thin slab continuous casting and rolling process can eliminate the cold rolling process,shorten the process,improve efficiency,and save energy and reduce emissions.At present,the production of hot stamping steel by thin slab continuous casting and rolling process is still in the exploration stage,and has its own characteristics compared with the conventional rolling process.The materials used in this paper were hot-rolled 22MnB5 hot stamping steel produced by CSP(Compact Strip Production)process and the cold-rolled annealed sheet produced by conventional process.The difference between the given steels produced by two processes are studied.And the effect of making process on the microstructure and properties of 22MnB was studiedFirstly,the microstructure and mechanical properties of the hot stamping given steel produced by the two processes were analyzed.The results show that the hot stamping steel produced by the CSP process is mainly composed of strip-shaped pearlite with yield strength of 435-450 MPa and tensile strength of 639-646 MPa,elongation 18.3-19.7%;and the microstructure of hot stamping steel produced by the conventional process is mainly equiaxed ferrite and carbide,with yield strength 305-347 MPa,tensile strength 495-514 MPa,elongation 25.8-27%.The steel produced by CSP process has higher strength,lower elongation and the microstructure uniformity need further improvementThe microstructure and mechanical properties of hot stamping steel produced by two processes under different quenching parameters were studied.The discrepancies were found mainly at relatively lower quenching temperature and shorter holding time.The hot stamping steel produced by CSP process had a finer lath,higher strength and elongation,and better mechanical properties.However.the hardness value fluctuates greatly along the thickness direction.And undissolved cementite was found in the martensite matrix.As the temperature increases,the difference in microstructure and mechanical properties of the hot stamping steel produced by the two processes diminished.The fracture morphology was dominated by ductile fracture with a small amount of cleavage fractureThe austenizing process of two hot stamping steels was studied by means of rapid heating and rapid cooling.The results show that the austenitizing start temperature of the hot stamping steel produced by the CSP process is about 35 ? higher than that of the conventional process.The addition of Nb and Cr elements can increase the austenitizing temperature,suppress the growth of the original austenite grain growth,and improve the strength and plasticity of the quenched martensite.The austenizing temperature(850-900 ?)is suggested to be about 30 ? lower than the conventional process.And the holding time is about 5 min.A better comprehensive mechanical properties can be obtained.The austenite grain growth model of the hot stamping steels produced by two processes in different heating conditions was established.