Investigation Of Competitive Mechanism During Partitioning And Deformation Coordination Mechanism Of I&QP Steel

The third generation advanced high-strength steel(AHSS)has the characteristics of the first and the second generation AHSSs,and has the advantages of conserving resources,reducing costs,reducing vehicle weight and improving safety in automotive applications.Therefore,researching and manufacturing the third generation AHSS is the development trend of current world.I&QP steel has been industrial production and many related researches have been done;however,many problems have still not been solved,e.g.,the mechanism of bainite transformation during partitioning,the effect of carbon partitioning and carbide precipitation on the bainite transformation,the segregation mechanism at different interfaces,deformation coordination mechanism among different phases,the effect of multiphases on the fatigue crack propagation,etc.In present work,related research has been carried out based on the above problems to promote the development of I&QP process in other areas,e.g.,reamer of extruder,hammer of crusher,wearable steel ball,wear-resisting liner used in giant ball-mill,etc.The details and main conclusions are summaried as follows:Firstly,the relationship between heat treatment process and mechanical properties was investigated by grey relation theory.The optimum I&QP process was selected through grey relational grade,and thus the steel meets the requirement of the third generation AHSS.The effect of heat treatment parameters on the mechanical properties was analyzed using grey relational analysis,and the results indicate that quenching temperature has the most effect on the mechanical properties,followed by partitioning temperature and partitioning time.The effect of location,grain size,morphology,carbon content and orientation on the stability of retained austenite(RA)was investigated by TEM and EBSD.Additionally,the investigation of nucleation mechanism during intercritical heating can provide the theory basis for the stability of RA.RA exists in three kinds of locations,including at grain boundaries,within ferrite and among martensite and bainitic ferrite laths.Furthermore,these locations determine the grain size,morphology,carbon content and orientation of RA,which further determines the stability of RA.The RA with various states can transform into martensite progressively during deformation,which can improve the ductility of the steel.The mechanism of bainite transformation was investigated by the way of thermodynamics and kinetics,and the results indicate that the transformation is controlled by diffusionless mechanism.And on this basis,the model of baintie transformation was built.Carbon partitioning has an important influence on bainite transformation,because the first supersaturated martensite can accelerate the carbon diffusion.Therefore,the bainite transformation can be divided into two parts:transformation quickly and slowly.Moreover,when partitioning temperature is 450 °C,the transformation rate of bainite decreases,because the driving force of transformation is reduced and RA decomposes into carbides and ferrite.Therefore,carbon partitioning and carbide precipitation can both have the competitive relationship with bainite transformation.The C and Mn segregation at different interfaces was investigated by 3D-APT.The results indicate that the segregaton can occur in various degree.And then the segregaton behavior was modeled by Dictra,and the results indicate that carbon segregation can promote the diffuse of Mn.The C and Mn enrichment at the interface between ferrite and RA within ferrite can improve the stability of RA.The interface of the RA cannot transform into martensite under certain strain,while the interior can.And the untransformed interface can act as mechanical buffer zones connecting soft ferrite with hard martensite,which contributes to plasticity accommodation.The deformation coordination mechanism was investigated using in-situ high-energy X-ray diffraction technique.And a modified constitutive model was established for describing the micromechanical behavior of multiphase steel.The results indicate that the stress partitioning,mainly occurring in elastic and elastoplastic regions,is activated by the;"stress feedback mechanism"" And the stress transfers from ferrite to retained austenite and hard phase.During deformation,TRIP effect first occurs in the retained austenite with low carbon content,followed by the ones with high carbon content,which is denominated"selective transformation mechanism".Austenite stress is higher than martensite stress at the end of uniform deformation,which is related to the high carbon content in retained austenite.The effect of different phases on the fatigue crack propagation was investigated.TRIP steel has the highest fatigue strength due to the dispersed and small bainite grains.The annealed martensitic laths and RA among the laths can prevent the propagation of fatigue crack in TAM steel effectively.The RA with various states of I&QP steel can increase crack growth path and thus retard fatigue crack initiation and growth.