Study On Microstructure-properties Control And Deformation Behaviour Of Low Alloy Quenched And Partitioned Steel

To achieve the goals of both light weight and safety,the development of automobile-used advanced high strength steel with both high strength and high ductility is in need urgently.Under this background,Speer and his co-workers proposed a novel quenching and partitioning(Q&P)process,which resulted in a low-cost high-performance automobile-used steel composed of martensite and retained austenite,so-called Q&P steel.In recent years,numerous studies have been carried out on Q&P steel,however,most of them focused on the theoretical model optimization,process improvement and structure-property characteristics.Academe is lacking in studies on deformation mechanism,effectiveness of TRIP effect and strain partitioning behavior between soft phase and hard phase in Q&P steel.In current study,the process-structure-properties relationship was systematically investigated and the effect of the process parameters on austenite stabilization,retained austenite state and TRIP effect was also clarified.The element partitioning behavior and strain partitioning behavior in a ferrite-contained hot-rolled directly quenched and partitioned(HDQ&P)steel were also quantitatively investigated in this study.The main work and research results are as follows:(1)The microstructure-properties relations in a cold-rolled ultra-high strength Q&P steel were systematically studied.One-step Q&P proesses with different partitioning time were performed to a low carbon silicon-manganese alloyed steel.The effect of partitioning time on the microstructure and mechanical properties of the tested steel was analyzed in detail.The effects of martensite tempering behavior and TRIP effect on the yielding and work hardening behavior were discussed emphatically.The main factors controlling the work hardening rate of Q&P steel were also summarized.The results show that the martensite lath width increases and the martensite dislocation density decreases with the increase of the partitioning time,leading to the decrease of the ultimate tensile strength(UTS).The precipitation of carbides in the martensite laths when the partitioning time exceeds 100 s increases the yield strength and promotes the austenite decomposition.The work hardening behavior in the early stage of the deformation is mainly controlled by the interaction of high density dislocations in the martensite,while it is determined by TRIP effect in the later stage of the deformation.(2)A novel HDQ&P process was proposed by combining carbon partitioning theory with hot-rolling process.The differences in microstructure and properties between dynamically partitioned steel and isothermally partitioned steel were compared and investigated.The effect of the partitioning process on the martensite sub-structure state,retained austenite stability and TRIP effect effectiveness were analyzed emphatically.The austenite stabilization mechanism in the dynamical partitioning process was also discussed.The results show that the dynamically partitioned steel possesses narrower martensite laths,higher martensite dislocation density and finer carbides,leading to higher UTS.The inhomogeneous distribution of carbon in the retained austenite caused by the dynamical partitioning process promotes the occurrence of TRIP effect.The stabilization of retained austenite under the dynamical partitioning process is mainly attributed to the efficient carbon partitioning behavior in the 30-80 nm thick austenite layer near the martensite/austenite interface and the effective inhibition of carbide precipitation by the continuous cooling process.(3)The microstructures and mechanical properties of ferrite-contained HDQ&P steels were investigated systematically.The retained austenite state,TRIP effect and work hardening behavior of the steel were analyzed and compared with the hot-rolled TRIP steel with the same composition and similar ferrite fraction.The results show that the introduction of appropriate(about 30 vol.%)amount of ferrite can decrease the yield ratio of Q&P steel to the level of TRIP steel and maintain the UTS and elongation in the level of conventional ultra-high strength Q&P steel.The introduction of ferrite can promote the formation of blocky retained austenite.Compared with Q&P steel,the retained austenite in TRIP steel possesses larger size,more obvious blocky shape,lower carbon content and softer surrounding phases,resulting in lower stability.Thus,the retained autenite in TRIP steel is easier to perform TRIP effect.The softer matrix and the more effective TRIP effect account for the higher uniform elongation of the TRIP steel compared with the Q&P steel.(4)The effect of intercritical deformation on the phase components,austenite stabilization,TRIP effect and mechanical properties of the low carbon manganese-aluminum alloyed HDQ&P steel was systematically investigated.The effect of bainite introduction on the compatible deformation capability of soft/hard phases and TRIP effect effectiveness in the ferrite-contained Q&P steel was analyzed emphatically.The results show that the intercritical deformation cannot obviously promote the ferrite formation and the recrystallization of the deformed ferrite grains when the finish intercritical rolling temperature is above 800 ?.The diffusion channel effect of the sub-structures in the intercritically deformed ferrite and austenite and semi-diffusional characteristic of bainitic transformation jointly promote the carbon partitioning behavior between austenite and its surrounding phases.When the finish intercritical rolling temperature is below 800 ?,the intercritical deformation can observably promote the the ferrite formation and the recrystallization of the deformed ferrite grains.The introduction of a large amount of ferrite decreases the amount of austenite and promotes more carbon-enriched austenite transforming to martensite,leading to the decrease of retained austenite fraction in the final microstructure.The introduction of an appropriate amount of bainite can significantly improve the deformation compatibility of the phases,enhance the TRIP effect and thus improve the ductility of the Q&P steel.(5)The element partitioning behavior and strain partitioning behavior between the phases in the ferrite-contained Q&P steel were quantitatively investigated by means of atom probe tomography(APT)technology and digital image correlation(DIC)technology.The transformation sequence of different types of retained austenite with the increase of strain,the inhomogeneous carbon distribution of retained austenite,the effect of martensite morphology on strain partitioning and TRIP effect were analyzed emphatically.The results show that the inhomogeneity of strain distribution in the tensile sample existed throughout the tensile deformation procedure.The strain of the uniform elongation deformation stage was below 15%,the strain at the central position of the necking part was up to 67%when the tensile sample fractured.The relatively large retained austenite grains distributed near the ferrite/martensite interfaces can transform to martensite with the strain below 10%,while the relatively small retained austenite grains located in the martensitic regions has not transformed to martensite when the local strain is up to 36%.There exists obvious carbon concentration gradient on both sides of the martensite/austenite interfaces.The retained austenite grains with smaller size possess higher carbon content than the retained austenite grains with larger size.Promoting the polygonization of martensitic regions can effectively increase the local strain level in the martensitic regions,thus it is benefit to improve the TRIP effect of the retained austenite located in the martensitic regions.