Study On Austenite Transformation,Microstructure And Properties Of High Strength Cold Rolled Dual-Phase Steel

High strength cold rolled dual-phase steel has become an important component of automobile manufacturing industry due to its excellent performance and outstanding contribution to automobile lightweight.However,some problems will appear in the actual manufacturing and using process.For example,the uneven distribution of microstructure will lead to the reduction of performance and cracking in the process of production.This paper selected the bainite as initial microstructure,which has the higher density of dislocations and the more diffuse distribution,to compare with the ferrite-pearlite.The influence of different initial microstructures on the homogeneity of microstructure after annealing in intercritical region was studied.In addition,although Si element plays an important role in the dual-phase steel,the addition of Si element will cause bad influence on surface quality and coating ability.Based on that,the microstructure evolution of silicon-free dual-phase steel was studied at the same time.The effect of ferrite recrystallization on austenite transformation kinetics was investigated on the basis of the study of the laws of austenite transformation kinetics.And this paper obtained different types of distribution of martensite by controlling the fraction of ferrite recrystallization and heating rate.On the basis of previous.research,the inffuence of the distribution of martensite on mechanical properties and formability of DP steels was investigated.The main conclusions are as follows:(1)The austenite transformation kinetics of experimental steels was obtained and the model of austenite transformation kinetics was regressed on the basis of JMAK equation,which provided the theoretical foundation for the subsequent process formulation.It was found that for silicone-containing DP steel,the speed of austenite growth is faster when initial microstructure is ferrite-pearlite with heating rate of 5?/s.As for silicone-free DP steel,at the same holding temperature,it can get more austenite in the same holding time.(2)By analyzing the curves of austenite transformation kinetics,the influence of ferrite recrystallization on the transformation kinetics of austenite was studied.When the ferrite is almost never recrystallized,the superheat of experimental steel is higher;the nucleation work of austenite is smaller;the incubation period is shorter and the austenite transformation kinetics is faster.(3)The effect of the initial microstructure and heating rate and ferrite recrystallization on distribution of martensite was studied.It was found that the dispersed distribution of martensite can be obtained by the initial microstructure of bainite.Combining with faster heating rate,the grains.can be refined.The banding distribution of martensite was extremely significant after incritical annealing under rapid-heating for the initial microstructure of ferrite-pearlite.When ferrite is fully recrystallized,the austenite nucleates and grows at the ferrite grain boundaries.In this case,the martensite has a chain-like distribution,and the banding structure is obviously improved.(4)The effect of distribution of martensite on mechanical properties and formability of DP steel was studied.It was found that the banding distribution of martensite in the microstructure can result in the degree of adaptability between strength and ductility lower and depress the formability of experimental steel.The dispersed distribution of martensite was benefit to obtain a better formability on the basis of high strength.When martensite shows a chain-like distribution feature,the degree of adaptability between strength and ductility is higher and formability of experimental steel is improved.