Investigations Of The Microstructure And Property Of The SUS301L Austenitic Stainless Steel Under Cold Deformation And Annealing Processes

Austenitic stainless steel is the most widely used series of stainless steels.Besides the excellent corrosion resistance,formability and heat resistance,it will undergo a strain induced ?'-martensite transformation under cold deformations,which enhances the material and make it to be an ideal train body material for the demand of lightweight?safety and environmental friendly.This thesis focuses on the cold deformation and subsequently annealing of SUS301L stainless steel,aiming to investigate the microstructure evolution and strengthen mechanism.The main contents of this thesis as follows:(1)The TEM microstructures of cold deformed material reveal that:After a cold deformation,the austenite matrix produces defects such as dislocations,stacking faults and stacking faults bundles.a large number of parallel and intersected shear bands are formed with the accumulated deformation.Strain-induced ?'-martensite nucleates at the intersections of these shear bands and even at a single shear band.The ?'-martensite dispersedly distributes on the austenitic matrix,which has a dispersive strengthening effect on the matrix.With the increasing of deformation,the ?'-martensite nucleates and grows continuously,and eventually connects together to form lath martensite,and replaces the austenite phase to become the matrix gradually.(2)The transformation of ?'-martensite in the cold rolling deformation modes is slower at the early stage of deformation.When the deformation amount exceeds 23%,the volume fraction of strain-induced ?'-martensite increases sharply to 90%,nearly be close to the saturation.In the uniaxial tensile deformation mode,the transformation speed of?'-martensite is faster than that of the cold rolling mode,the ?'-martensite volume fraction is up to 90%under 30%tension.(3)The strain hardening rate and the work hardening index n of the material show a parabolic shape with the increasing of deformation due to the strain induced?'-martensite.The maximum of the work hardening index is about 1.4.In addition,the strain-induced ?'-martensitic transformation delays the intersection of strain hardening rate-true strain curve and true stress-true strain curve,which enhances the anti-necking ability of the materials and improves the plasticity of the material.(4)A flexible annealing process is developed in the laboratory:LT grade annealing at 1050?1150?,holding for 60s;DLT grade annealing at 900?950?,holding for 30?120s;ST grade annealing at 800?900?,holding for 30?120s;HT grade annealing at 750?800?,holding for 30?120s;HT grade annealing at 700?750?,holding for 30?120s.Compared with the conventional process,the new process greatly improves the yield ratio and ductility of the cold rolled SUS301L stainless steel.