Study On Recrystallization-Phase Transformation And Comprehensive Properties Of High Strength Cold Rolled Dual Phase Steel

In recent years,the problem of environmental pollution and waste of resources has become increasingly prominent.The whole society has paid more and more attention to energy conservation,and the automobile industry has continued to develop in a lighter direction.Therefore,higher requirements are placed on the strength and overall performance of automotive steel.Cold-rolled dual-phase steel is the most widely used in high-strength automotive steels due to its good strong plasticity.In this paper,microalloyed cold-rolled dual-phase steel was studied.The micro structure evolution during continuous annealing and the influence of annealing parameters on mechanical properties were studied.The bake hardening properties of the prepared cold-rolled dualphase steel were studied.In this paper,microalloyed cold-rolled dual-phase steel was studied.The microstructure evolution during continuous annealing,the influence of annealing process parameters on mechanical properties,and the bake hardening properties of cold-rolled dual-phase steel were studied.The main work content and results are as follows:(1)For both initial micro structures,the cold rolling deformation resistance curve was obtained.The results show that the deformation resistance of the two initial micro structures is close due to the dual effects of tissue type and precipitation state.(2)For both initial micro structures,the recrystallization of ferrite at different heating rates was studied by means of nanoindentation and EBSD.The results show that,below the Ac1 temperature,as the heating rate increases,ferrite recrystallization is suppressed and delayed to above Ac1 temperature.Austenite transformation inhibits ferrite recrystallization,which is more obvious for the initial microstructure of pearlite-ferrite.(3)The kinetics of austenite transformation in continuous heating and isothermal processes was studied,and experimental data was fitted with JMAK model.The results show that under continuous heating conditions,the austenite volume fraction decreases as the heating rate increases,which is related to the nucleation position of austenite.Under the isothermal conditions,as the heating rate increases,the volume fraction of austenite increases.(4)The effects of annealing process on microstructure and properties were studied by continuous annealing experiments combined with mechanical properties analysis.The results show that after continuous annealing,the tensile strength of the experimental steel can reach about 1100MPa,and the yield ratio is about 0.5.At slow heating,the ferrite recrystallizes sufficiently,and the martensite in the final microstructure is uniformly and finely distributed,which leads to a high strong-ductility combination.(5)The bake hardening properties of cold rolled dual phase steels were investigated for two different initial microstructures.The results show that the BH value increases when the pre-strain is 0?2%,and decreases when it is 2?7%.As the baking temperature increases,the BH value gradually increases.The BH value increased rapidly at the beginning of baking,and it remained stable with the extension of the baking time.