Study On The Cold Formability Of Cu-bearing 304L Stainless Steel

Copper(Cu),as a common alloy element in stainless steel,is an essential trace element for human body.By adding proper amount of Cu into the traditional austenitic stainless steel and aging treatment to precipitate enough Cu-rich phases from the steel matrix,the Cu-bearing austenitic stainless steel displays excellent mechanical properties and antibacterial properties,so it has been widely used in the fields of medical equipment,food and kitchen hygiene.However,it was found that the cold formability of this kind of material is relatively poor during the processing procedure.Earing,wrinkling and even fracture is prone to occur when deep-drawing austenitic stainless steel plates were fabricated into sinks and other components.To solve this problem,this paper systematically studied the influence of copper addition and Cu precipitates on the microstructure,mechanical properties and cold formability of 304L-Cu austenitic stainless steel.On this basis,304L-Cu stainless steel with the best comprehensive performance was selected,and the influence of cold deformation and heat treatment on its microstructure and mechanical properties was studied.The main research contents and results in this paper are as follows:(1)Effect of Cu addition and Cu-rich precipitates on the microstructure and properties of Cu-bearing 304L austenitic stainless steelIn this study,the balanced solid solubility of copper in 304L stainless steel and the stacking fault energies(SFE)of Cu-bearing stainless steels were calculated,and their microstructure,mechanical properties and formability were characterized.The results showed that the balanced solid solubility of copper in 304L stainless steel is lg[Cu]?=2.295-2018/T.The addition of Cu increased the SFE of 304L stainless steel,the martensite transformation decreased while the twins content increased and the dominated deformation mechanism changed from transformation-induced plasticity(TRIP)to twinning-induced plasticity(TWIP)gradually.When the Cu content increased from 1.5%to 3.5%,the tensile strength and total elongation of 304L-Cu stainless steel decreased slightly,the work hardening rate also decreased,the work hardening exponent(n)decreased by 0.04 and the cupping value(IE)decreased by 0.2 approximately.The addition of Cu slightly reduced the mechanical properties and formability of 304L stainless steel owing to limited solid solution strengthening effect of Cu.Compared with the solution treated Cu-bearing 304L stainless steel,the Cu-rich precipitates reduced the SFE of Cu-bearing 304L stainless steel,the microstructure consisted of martensite and deformation twins and the deformation mechanism was a synergistic effect of TRIP,TWIP and the Cu-rich precipitates induced plasticity.Compared Cu-bearing 304L stainless steel before and after age treatment,the Cu-rich precipitates improved the tensile strength of aged 304L-Cu stainless steel by 50 MPa,the synergistic deformation of the Cu-rich precipitates improved the plasticity of 304L-Cu stainless steel from 52.1%to 63.5%,the declining tendency of work hardening rate was obviously slowed down obviously when the true strain e was more than 0.15,the work hardening exponent(n)increased by 0.03 and the IE value was almost equivalent.Therefore,the synergistic deformation of the Cu-rich precipitates significantly improved the mechanical properties and cold formability of 304L-Cu stainless steel.(2)Effect of cold deformation and heat treatment on the microstructure and properties of 304L-3.5Cu austenitic stainless steelThe 304L-3.5Cu austenitic stainless steel with excellent comprehensive properties was selected to study the microstructure,mechanical properties and texture evolution under different cold deformation after solution treatment and aging treatment.The results showed that with the increase of cold deformation reduction,the content of martensite in the solution and age treated 304L-3.5Cu stainless steel increased by 60%and 30%,the tensile strength increased by 650 MPa and 600 MPa,the Vickers hardness increased by 125 HV and 150 HV,and the elongation decreased from 73%to 7%and from 47%to 7%,respectively.With the increase of cold deformation reduction,the martensite content of solution and age treated 304L-3.5Cu stainless steel increased,leading to the increase of tensile strength and hardness and the decrease of plasticity.Under the same cold deformation reduction,the content of martensite in aged 304L-3.5Cu stainless steel was always higher than that of solution treated 304L-3.5Cu stainless steel,so its strength and hardness are good,and the plasticity is relatively poor.The improvement of work hardening rate in the high strain stage of age treated 304L-3.5Cu stainless steel was higher than that of solution treated 304L-3.5Cu stainless steel.So the Cu-rich precipitates played a limited role in the increase of plasticity in cold deformation process and played a significant role in the improvement in the work hardening rate.The cold deformation texture of solution treated 304L-3.5Cu stainless steel exhibited a mixed Brass-type and Copper-type texture.