Behavior And Influence Factors Of Massive Transformation In Ultra-low Carbon Steels

The characteristics,influence factors,kinetics and mechanical property of massive transformation in ultra-low carbon steels?pure iron,IF steel and Cr-series ferrite stainless steel?were investigated by using thermodilatometry,hot compression tests,and in-situ observation of high-temperature metallography,respectively.The results of continuous cooling tests show that there are two kinds of morphology of massive ferrite.When the degree of supercooling is 20¹15?,the morphology of massive ferrite is polygonal and named here as F_p.When the supercooling degree is larger than about 200?,the massive ferrite was named here as F_S with abundant sub-structure.These two kinds of massive ferrite can grow across parent grain boundary without orientation relationship with parent phase.By the OM,TEM,and EBSD analysis,???transition of ferrite stainless steels can also be massive transformation at cooling rate between 0.5³0?/s.A coarser austenite grain,larger cooling rate,excessive component of niobium and deformation can promote the formation of F_S massive ferrite.The strength and hardness of steel can be enhanced for increasing the amount of F_S massive ferrite.In0.52Nb-IF steel,the product is completely F_S massive ferrite by quenching in water,thus the yield strength is 1.7 times larger than that at annealing states.The strengthen factor is mainly substructure with an increment of about 270MPa.The kinetics of massive transformation is affected by cooling rate,Nb content and deformation.For pure iron,the calculated velocity of interphase of F_p is 0.5×10^-5m/s.For 0.52Nb-IF steel,the velocity is 0.65⁶.2×10^-5m/s for a range cooling rates of3³0?/s.In-situ observation of???_m in 0.52Nb-IF steel shows that?_m nucleated at grain corner when the degree of supercooling is 18?.The migrating velocity is5¹0?m/s,which is dependent of super heating degree.For lack effect of solution for0.07Ti-IF steel,the migrating velocity of?_m can up to 30?m/s.Hot compression test shows that the deformation induce massive transformation?DIMT?occurs when deformation temperature is at the critical zone.The DIMT have a softening effect,i.e.the true stress-true strain curve is gradually decreasing when DIMT occurrs.The morphology of DIMT ferrite is strip band along the grain boundary,which is a great deference with the conventional deformation induce ferrite transformation?DIFT?.