Study On Microstructure And Properties Of Cyclic-annealing & Cold-rolling Low Carbon And Low Alloy Steel And Intercritical Annealing

With the rapid development of automobile industry,lightweight and high performance for safety have become the goals of automobile manufacturing industry.Steels are the most widely used structural materials in the automotive industry.Developing steels with higher performance with low cost is one of the important goals of the automotive industry and the steel industry at present.Conventional methods of increasing alloy elements to improving mechanical properties are also faced with disadvantages such as high cost and waste of non-renewable resources.considering resource saving and sustainable development,scientists put forward the concept of plainified materials and heterostructured materials（HSM）.The ferrite phase and martensite phase in low carbons are analogous to the soft and hard zones in HSM respectively,which is a natural heterostructure model.Moreover,DP steel generally has low alloying elements,which meats the requirement of plainified materials and HSM.Therefore,the development of DP steels have become a research focus in the automotive field,and it is expected to obtain advanced DP steels with lower cost and higher performance.In this paper,three kinds of low-carbon low-alloy steels with 0.07/0.1/0.2 wt.%carbon are selected.Firstly,the annealing-induced hardening（AIH）phenomenon of the cold-rolled（CR）martensite samples during low-temperature annealing is systematically studied to obtain the best AIH parameters.Secondly,an appropriate cyclic annealing&cold-rolling（Ann CR）processing route was designed to prepare low-carbon steel with nano-lamellar structure based on the AIH.The microstructure evolution and mechanical properties of low-carbon martensite during Ann CR were studied in detail.The strengthening mechanism of low-carbon steel with nano-lamellar structure was also discussed.Finally,intercritical annealing at different temperatures were carried out to tailor the grain size,morphology and proportion of the constituent phases,in order to improve the overall mechanical properties of the low-carbon steels.The strengthening and toughening mechanism of ultrafine-grain heterostructured dual-phase（UFG-HSDP）steels were explored by cyclic load-unload-reload（LUR）tensile tests.The main conclusions of this work are as follows:（1）Three kinds of low-carbon martensitic steels were treated by CR40%and subsequent low-temperature annealing.It was found that low-temperature annealing at 100～300°C would produce obvious AIH phenomenon,in which exists optimum AIH process.The main reasons for this phenomenon are as follows:first,martensitic transformation and cold deformation introduce high-density dislocation;then in the process of low-temperature annealing,motion of carbon atoms are promoted,which tend to move to the the dislocation and interact with the dislocation,hindering the dislocation movement and improving the flow stress.（2）A novel Ann CR processing route combined CR and reasonable heat treatment was designed based on the mechanism of AIH.The results show that the Ann CR process has high efficiency in grain refinement,and the nano-lamellar structures were obtained in the three kinds of quenched steels under high rolling reduction.The average martensitic lamellae of the steels with 0.07/0.1/0.2 wt.%carbon are refined to 65 nm,83 nm and 75.4 nm,respectively.Under the same rolling reduction,the steels processed by Ann CR show higher strength than that processed by CR.The ultimate tensile strength（UTS）of three kinds of nano-lamellar steels are increased to 1783 MPa,1823 MPa and 2163 MPa after Ann CR with high rolling reductions,which even breaks the 2GPa strengthening limit of low-carbon low-alloy steel.The refinement and strengthening mechanism can be concluded as:first,martensitic transformation and cold rolling introduce high-density dislocation;then the interaction between carbon atoms and dislocations is promoted when annealed at low temperature～300°C,which causes strong pinning effect and hinders dislocation movement,thus promoting dislocation accumulation and structural refinement.The martensitic transformation,high density of dislocation and extremely refined lamellar spacing contribute to the ultra-high strength of the nano steels produced by Ann CR.（3）UFG-HSDP steels were prepared by intercritical annealing of the three kinds of low carbon steels with nano-lamellar martensitic structure.With the increase of heat treatment temperature,the volume fraction of martensite increases,and the grain size of ferrite decreases slightly.The yield strength（YS）and ultimate tensile strength（UTS）increase with martensite content gradually,together with the decreasing uniform elongation（UE）.The DP₁820（0.07wt.%C）,DP₂820（0.1 wt.%C）and DP₃840（0.2 wt.%C）samples with higher martensite content in the three UFG-HSDP steels show the structure where the soft ferrite grains are roundly constraint by the F/M interfaces,which is satisfied with the proposed heterostructure.Compared with the quenched martensite samples,the UFG-HSDP steels show superior combination of strength and ductility.This indicates that the strategy of obtaining UFG-HSDP structure from nano-lamellar martensitic structure can effectively improve the comprehensive mechanical properties of low-carbon steel.（4）The excellent strength-ductility synergy of the HSDP steel is contributed to its proposed DP heterostructure feature.The experimental results show that the hetero deformation-induced（HDI）stress level increases with the increase of martensite volume fraction.HDI stress increases with strain,providing a dynamic hardening effect.The extra HDI hardening helps with improving the strength and sustaining ductility of HSDP steels.