Study On Fine Composition Regulation And Stabilization Of Retained Austenite In Low Carbon High Strength Steels

Restrictions on energy consumption,environmental issue and safety concern have motivated the development of steel with high strength,high ductility and high toughness.In order to overcome the conflict between strength and ductility/toughness in steel,retained austenite is introduced into microstructure through multiphase designing,and its transformation induced plasticity(TRIP)effect is utilized to balance the strength and ductility of steel.The key to improve the mechanical properties of steel by TRIP effect is to regulate the stability of retained austenite.In this paper,the stability of retained austenite is mainly regulated by changing the contents of carbon and alloying elements in austenite,so as to develop high-performance steels with high strength and high ductility.In a 0.2C-2Mn-1Ni-1Cu-1 Al steel,cementite precipitated in the matrix during heating and enriched with much Mn during heating and initial holding at annealing treatment,which resulted in the sluggish dissolution rate of cementite at 690?.It is not conducive to carbon enrichment in austenite.Long time annealing dissolved cementite completely,but the increase of austenite content led to the decrease of the average carbon content in austenite.In addition,the size of austenite gradually increased,which decreased austenite stability.While in a two-step intercritical heat treatment,the cementite formed during heating dissolved fast at a higher annealing temperature(740?)during the first step intercritical annealing.And almost all carbon was stored in the reversed austenite,which transformed into fresh martensite and retained austenite during cooling.The retained austenite was stable and absorbed the carbon from fresh martensite during the subsequent heating process,which inhibited the precipitation of cementite and increased the carbon content in retained austenite.In addition,the two-step intercritical heat treatment refined the size of the austenite.Both of them improved stability of austenite.In a 0.2C-2Mn-2Ni-1Cu-1Al steel,the retained austenite formed after first step annealing at 760? was mainly distributed in three regions:i)inside martensite lath,ii)between martensite and ferrite,iii)between ferrite plates.During second step annealing at 680?,it was found that the growth of retained austenite distributed inside martensite was first controlled by carbon diffusion in austenite and then replaced by diffusion of alloying elements,which led to a profile of Mn in austenite as low in the center and high at both interfaces.The retained austenite distributed between martensite and ferrite mainly grew into martensite,and its growth was first controlled by carbon diffusion in austenite,and then replaced by diffusion of alloying elements,which led to the distribution of Mn in austenite as high in one side and low in other side.The retained austenite between ferrite plates can't get alloying elements from surrounding matrix,so the content of Mn in austenite was almost unchanged.The newly nucleated reversed austenite at martensite/ferrite interface mainly grew into martensite and the growth of austenite was controlled by diffusion of alloying elements,which resulted in uniform distribution of Mn in austenite.A low-carbon Cu bearing medium-Mn steel was subjected to intercritical annealing(IA)and tempering(IAT)process.After tempering,the yield strength and uniform elongation of sample IA were increased from 659 MPa and 10%to 911 MPa and 20%,respectively.The microstructure of sample IA was comprised of ferrite,retained austenite,fresh martensite and Cu precipitates.Carbon and alloying elements further partitioned from fresh martensite to retained austenite during tempering,which significantly improved the stability of retained austenite,and then the ductility.In the low strain regime,a number of stacking faults were observed in the retained austenite in sample IAT.With the increase in tensile strain,retained austenite gradually transformed into ?-martensite and ?'-martensite.The stacking faults and ?-martensite were present as single orientation lamellar structure and ?'martensite was mainly nucleated at the austenite/ferrite interface.Nanoscale Cu particles further precipitated during tempering,which increased yield strength.Before tempering the yielding of retained austenite was caused by martensite transformation at a low stress because of poor stability of retained austenite,which was avoided by enhanced austenite stability through tempering.