Effects Of Si On M-γ Transformation And Properties Of Deformed AISI 301 Precision Strip

With the vigorous development of micro manufacturing and microelectronics technology,people have higher requirements for micro size metal foil and its forming process.Ultra thin precision strip steel is one of the best representatives.After large deformation,this material produces grain structure as small as submicron,which has special "size effect",so it has very excellent strength and hardness.However,the uneven nucleation of martensite induced by deformation and the high residual internal stress and dislocation density lead to the significant decline of its plasticity and corrosion resistance.In order to obtain materials with better synergistic effect of "strength plasticity corrosion resistance",this experiment decides to start from two aspects: one is to carry out short-time reverse annealing on the basis of large plastic deformation,refine the austenite structure,reduce the content of martensite and dislocation and reduce the internal stress through two mechanisms of martensite to austenite(M-γ)reverse transformation and austenite recrystallization,so as to improve the plasticity and corrosion resistance.On the other hand,starting from the composition,Si can promote the transformation of deformation induced martensite and reduce the stacking fault energy(SFE)of AISI 301,homogenize the dual phase structure of recrystallized austenite and deformation induced martensite after annealing,improve the plasticity of the sample,and then promote the improvement of mechanical properties.Combined with the above experimental ideas and production practice,this paper compares the microstructure of AISI 301 precision strip with different thickness before and after annealing,and tests its mechanical properties and corrosion resistance by potentiodynamic reactivation method and unidirectional static tensile test,so as to analyze the influence of fabric change on properties.Further change the Si content to compare the microstructure and phase distribution of annealed samples under different strains,and analyze the effect of Si alloying on the content and uniformity of M-γ dual phase structure under different strains.In the experiment of "effect of reverse annealing on Microstructure and properties",it is found that the thinning of cold rolled samples increases the difficulty of microstructure deformation and fine crystallization,and also leads to the increase of the heterogeneity of grain size and phase distribution in the microstructure.After annealing at 625 ℃,the microstructure of the sample remains similar to that of cold rolling;When the temperature rises to 675-725℃,the content of deformation structures such as shear band decreases,and the refined reverse transformed austenite and recrystallized austenite nucleate and grow near martensite.The average grain size remains at the submicron level and gradually homogenizes.The change of microstructure is also reflected in the change of properties: the cold rolled 0.02 mm ultra-thin sample obtained a high strength of 1.8 GPa,but also resulted in low elongation and low corrosion resistance of less than 2%.After reverse transformation annealing at 725℃,the elongation of the sample increased significantly to 16%,and the pitting corrosion resistance was also significantly improved compared with that of the cold rolled state.Although the tensile strength decreased to 1 GPa,it remained at a high level.Obviously,the finer microstructure with more uniform size has significantly improved the coordination of strength,plasticity and corrosion resistance of ultra-thin precision strip steel.Based on the reverse annealing specimen,the effect of deformation induced martensitic transformation of Si under different strain was studied.At 10-20%low strain,high Si samples obtained higher content of deformation induced martensite,which also made the distribution of M-γ dual phase structure more uniform;When the deformation increases to 30%,although the martensite content of low Si sample is close to that of high Si sample,the martensite in high Si sample is finer than that of high Si sample.At low strain,the martensite of low Si samples is mainly distributed in the large deformation region such as grain boundary,and the content in the crystal is very low;The increase of Si content makes the distribution of deformation structures such as shear band more uniform in the crystal and grain boundary,and the distribution of martensite in the structure more refined and uniform.This is mainly related to the reduction of SFE by Si.After annealing the two samples at 950℃/180 s,it is found that the high Si samples have more significant work hardening at low strain.This is related to the increase of strain hardening rate of austenitic stainless steel by Si.