Study On Ferrite Recrystallization,Austenite Formation And Propertis Of High Strength Cold Rolled Dual-Phase Steel

In recent years,safety,environmental protection and energy conservation became the major developing trend of automobile manufacturing industry thanks to the aggravation of pollution and energy shortage.For the consideration of safety requirement and weight reduction,dual-phase steel proved to be the ideal material of automobile manufacturing and thus widely applied in the automobile industry.Currently,in the production of high strength cold rolled dual phase steel,the ferrite-pearlite was made to be the typical initial microstructure,which was prone to result in the banding distribution of martensite after intercritical annealing especially in the circumstance of rapid heating and depressed the micro structure homogeneity so that the cracking in forming occurred during the practical process of high strength cold rolled steel.Therefore,the research object was selected as bainite by controlling the hot rolling process in order to eliminating the banding distribution of martensite after intercritical annealing caused by the initial microstructure of ferrite-pearlite.Moreover,the effect of initial microstructure and annealing parameters on mechanical properties and formablity was investigated on the basis of the study on ferrite recrystallization and austenite formation laws during intercritical annealing to provide theoretical basis for practical production of high strength cold rolled steel.The main contents and conclusions are as follows:1.The effect of the initial micro structure on the deformation resistance during cold rolling.It was found that the deformation resistance of banite was higher than ferritepearlite,and the difference in deformation resistance between initial microtructures decreases with the of cold rolling reduction increases2.The effect of initial microstructure and heating rate on ferrite recrystallization during annealing.It was found that recrystallization rate grows with the increase of the heating rate because of the recovery was hindered by the rapid heating so that there was still a large amount of storage energy to motivate the recrystallization.Besides,the ferrite recrystallization activation energy was lower and the drive force was higher for bainite,which reduced the time required of fully recrystallized.3.The effect of the initial microstructure and heating rate on austenite formation.It was found that martensite banding distribution was extremely significant after incritical annealing under rapid heating for the initial micro structure of ferrite-pearlite,which was attributed by the prior nucleation of austenite in the strip shape pearlite and hindered the nucleation on ferrite grain boundaries.Meanwhile,the initial micro structure of bainite obtained relatively better microstructure homogenity with the network distribution of martensite because of the nucleation on recrystallized ferrite grain boundaries.4.The effect of ferrite recrystallization on austenite nucleation sites.It was found that by step heating to increase the fraction of recrystallized ferrite before intercritical region could relieve the banding distribution of martensite caused by the nucleation sites concentration in strip pearlite for the initial micro structure of ferrite-pearlite.5.The effect of initial micro structure and annealing parameters on mechanical properties and formability.It was found that both yield and tensile strength grow as the temperature increasement in different heating rate.Meanwhile,the formability was strongly influenced by the initial microstructure and annealing parameters.The banding distribution of martensite in the microstructure after intercritical annealing under rapid heating in initial microstructure of ferrite-pearlite depressed the formability.Meanwhile,for the initial microstructure of bainite,thanks to the uniform distribution of austenite nucleation sites,the martensite homogeneously distributed after intercritical annealing,which was benefit to obtain a better formability on the basis of high strength,especially in the circumstance of rapid heating.