Microstructure And Mechanical Properties Of A New Press Hardened Steel With High Strength And Toughness

The demand for high-strength steel has grown with the continuous improvement of the requirements for vehicle safety,energy saving and emission reduction.Although the traditional press hardened steel,i.e.,22MnB5,is widely used,it is difficult to meet the increasing requirements of toughness and energy absorption property.Although steels,such as medium manganese steel and martensitic stainless steel,have excellent mechanical properties for hot stamping,they are costly.Therefore,a new type low-cost press hardened automotive steel is designsed to achieve satisfactory strength and toughness,which are superior to those of 22MnB5.In this paper,based on various microstructural characterization and mechanical property examination methods,the microstructure and properties of hot-rolled and cold-rolled experimental steel with different hot stamping processes are systematically studied,and the optimal hot stamping process window is obtained.It provides a theoretical basis for the further application of new press hardened steel.The main results of the paper are as follows:(1)In order to improve the hardenability,achieve fine-grain strengthening,introduce into residual austenite,and comprehensively considering the processability,weldability and cost,the experimental steel was designed.The phase transformation temperature and critical cooling rate of the experimental steel were determined using dilatometer.The results show that the temperature Ac3 of experimental steel is close to that of 22MnB5.The critical cooling rate of experimental steel is 15℃/s,which is much lower than that of 22MnB5 which is 30℃/s.(2)The microstructure and properties of the experimental steel after hot stamping were verified by a conventional austenitizing process(900℃×360s).The results show that the microstructure of experimental steel consists of martensite matrix,4.47%residual austenite and dispersed carbides.The martensite lath is fine.The prior austenite grain size is 5.02μm.Under the combined effects of fine-grain strengthening and TRIP-effect of residual austenite,the tensile performance,three-point bending performance and impact performance at-40℃and 20℃ of the experimental steel are better than those of 22MnB5.(3)Different austenitizing processes and cooling methods were designed to study the microstructure and properties of hot-rolled experiment steel.The elongation of the experimental steel increases and the bending performance deteriorates significantly when introducing ferrite by incomplete austenitization.The dissolution of carbides increases the thermal stability of residual austenite and it has little effect on the residual austenite fraction.As the cooling rate decreases,the residual austenite fraction and the austenite carbon content increase simultanceously.Therefore,the total elongation is significantly improved.(4)The microstructure and properties of cold-rolled experimental steel under different hot stamping processes were studied and the best process window was obtained.The elongation of cold rolled experimental steel after hot stamping increases first and then decreases with the increase of austenitization temperature.The optimum austenitization parameter of the experimental steel is 920℃×360s.The tensile strength of the steel is 1735MPa and the total elongation is 8.54%.With the decrease of cooling rate,the ratio of large angle grain boundary increases,and the plasticity and toughness increase.(5)The microstructure and properties of hot stamped parts of the experimental steel and 22MnB5 were compared.There is a large amount of ferrite in the position with large deformation in 22MnB5,while ferrite does not exist in the experimental steel.The maximum bending force and bending energy absorption of the experimental steel parts are obviously better than those of 22MnB5,and the bending property is obviously improved.