Study On The Phase Transformation Behavior And Retained Austenite Stability Of Hot Rolled TRIP780 Steel Simulated By CSP Process

TRIP(Transformation Induced Plasticity)steel has excellent strong plasticity and formability.It is a typical representative of advanced high-strength steel for automobile.It is widely used in automobile safety structural parts,which is conducive to the realization of automobile lightweight,energy conservation and emission reduction under the background of double carbon.At present,TRIP780 steel in foreign countries is mainly produced by cold rolling and heat treatment,which has high energy consumption and high cost.In contrast,CSP(compact strip production)has the advantages of compact process,low energy consumption and low cost.Some products can be"replaced by hot".However,there are few reports on the research of CSP hot rolled TRIP780 steel,and its transformation law and the transformation factors of residual austenite stabilization are not clear.Based on this,TRIP780 steel is taken as the research object in this paper.The transformation law of CSP hot rolled TRIP800 steel was systematically studied by using Gleeble 3500 thermal simulation test machine,and the influencing factors of retained austenite stability were discussed.The main results obtained are as follows:(1)Established the CCT curve of TRIP780 steel.Without deformation,the temperature range of ferrite,pearlite and bainite transformation and the initial transformation temperature of martensite(M_S)are 520～745℃,560～640℃,460～560℃and 440℃respectively.Deformation can promote the transformation of ferrite and pearlite,but inhibit the occurrence of martensitic transformation.The temperature range and M_S temperature of ferrite,pearlite and bainite transformation are 500～727℃,550～650℃,530～600℃and 365.1℃respectively.(2)Established the Isothermal transformation of Pro eutectoid ferrite curve of TRIP steel.The temperature of nasal tip is 700℃,and the time required to complete 50%ferrite transformation at this temperature is 7.8s.Clarified the law of bainite isothermal transformation of TRIP780 steel.The driving force of bainite transformation is the largest at 400℃.When isothermal at 350℃,the phase fraction of retained austenite is the largest,reaching 9.63%.(3)Illustrated the relative influence relationships among the factors affecting the stability of the mid-steady residual austenite in TRIP780 steel.As follows:Grain morphology>Grain carbon content>Grain location>Grain size.The stability of retained austenite is also affected by the stress state.When the internal stress state of the same grain is uneven,the residual austenite is prone to partial transformation,that is,martensitic transformation occurs first in the part of stress concentration.