Annealing Transformation And Its Effect On Microstructure And Mechanical Behavior Of Cold-rolled Austenitic Stainless Steel

In order to better understand the effect of the transformation during annealing on microstructure and mechanical behaviour,AISI 321H（1Cr18Ni9Ti）metastable austenitic stainless steel was investigated in the present work.The martensite-to-austenite（α′→γ）transformation induced expansion in rolling direction of heavily cold-rolled metastable austenitic stainless steel during heating was observed by using dilatometry.Herein,a mechanistic insight into the anomalous dilatometric behavior has been provided.The effect of final annealing temperature on the microstructure,tensile property and deformation mechanism was systematically studied.Besides,bimodal grain size distribution was obtained in the AISI 321H austenitic stainless steel,the generation mechanism and mechanical property were studied in the present work.It is known that the martensite-to-austenite（α′→γ）transformation in steels results in volumetric contractions due to higher atomic packing factor of austenite than martensite.However,we observed that theα′→γtransformation induced expansion in rolling direction of heavily cold-rolled metastable austenitic stainless steel during heating.Interestingly,the other two directions,transverse and normal directions,exhibited the commonly observed contraction phenomenon.Herein,a mechanistic insight into the anomalous dilatometric behavior by in-situ XRD,dilatometry and TEM characterization was provided.The AISI 321H metastable austenitic stainless steel is repetitively cold rolled and reversely annealed.The effect of the final annealing temperature on microstructure,tensile properties,work hardening behaviour and tensile deformation mechanism is investigated by electron backscattering diffraction,X-ray diffraction and transmission electron microscopy.It is observed that the austenite grains are significantly refined（².3-4.8μm）in the final annealing temperature range of 800°C-950°C,and large amount of M₂₃C₆ carbide particles were diffusely distributed in the annealed microstructure.The reversed austenite grains gradually grew as the final annealing temperature is increased from 800°C to 950°C,whereas rapidly growth has been observed in 1000°C-annealed samples,which can be ascribed to the gradual dissolution of M₂₃C₆ carbide particles with increasing the annealing temperature and almost complete dissolution at the annealing temperature of 1000°C.The strength of austenite steel has been improved significantly by repetitively cold rolling and annealing.The yield strength and ultimate tensile strength have shown an inverse relationship with the annealing temperature due to grain growth,whereas the elongation has shown a direct relationship due to the austenite stability caused by carbides dissolution.Long lasting work hardening behaviour was shown in the samples after repetitively cold rolling and annealing.Precipitates played an important role in tensile deformation mechanism of the steel,the dissolution of the precipitates gradually increased as the annealing temperature increased.The microstructural examination of the annealed samples with different tensile strains indicates that the formation of strain-inducedα′-martensite has been retarded,whereas the formation of deformation twins has been promoted by increasing the final annealing temperature.The tensile deformation mechanisms are dependent on the annealing temperature,including transformation from strain-inducedα′-martensite to a hybrid of strain-inducedα′-martensite transformation and deformation twining,and to deformation twining as the annealing temperature increased from 800°C to 1000°C.The AISI 321H austenitic stainless steel was subjected to 90%cold rolling and annealing at 1000°C for 30 min to achieve coarse（⁴0μm）and fine（⁷-8μm）bimodal grain size distributions.Through the analysis of 90%deformed microstructure,the source of the bimodal grain size distribution was determined.The coarse grains were generated from the recrystallization of the remained deformed austenite during cold deformation,and the fine grains were mainly composed of the reverse transformation of strain induced martensite.The effects of bimodal grain size distributions on the mechanical properties of AISI 321H austenitic stainless steel were investigated.Combined with the hardening rate curve,the microstructure evolution and phase analysis during the deformation process,the deformation twins were the dominated deformation mechanism in early stage（0.08<ε<0.25）.With the increase of true strain,the deformation induced martensitic transformation dominates the deformation mechanism of later stage（0.25<ε<0.72）.The volume fraction of martensite and the hardening rate increase with the increase of true strain.Through the scanning electron microscope observation,it was observed that the tensile crack surface of the sample consist of large amount of dimples,and the fracture type is ductile fracture.The comprehensive mechanical properties of bimodal grain size distribution microstructure is excellent,since the fine grains result in higher strength,yield and tensile strength,while coarse grains lead to higher work hardening ability and elongation.