Investigations On Control Technology Of Low Temperature Toughness Of Medium-Chromium Ferritic Stainless Steel

With the development of national economy and people's living standards,the stainless steel has a rapid development in our country.The production of stainless steel requires a lot of nickel resources.Lacking of nickel resources and fluctuating of nickel prices lead to research and develop nickel-saving and high-performance ferritic stainless steel replacing parts of austenitic stainless steel.This will become a trend.Compared with Cr-Ni austenitic stainless steel,ferritic stainless steel has low thermal expansion coefficient,high heat conductivity coefficient and not sensitive to stress corrosion,but it has lower toughness and higher ductile-brittle transition temperature,even at room temperature above.For the key problem of poor toughness and high ductile-brittle transition temperature,three aspects including the alloy composition,Sn micro-alloying,solidification structure have been studied in order to further improve the toughness of medium-chromium ferrite stainless steel at low temperature in this paper.The major work and achievements are as follows:(1)The influence of alloy composition on the micro structure,texture and mechanical properties of medium-chromium ferritic stainless steel was clarified.Through the study of the comparative experiments of different contents of Mn,the steel with low content of Mn had smaller grain and higher uniformity in the hot-rolled and annealed steel;it had ?-fiber texture which distributed more concentrated and stable in the central layer of hot-rolled annealed sheet;it had less band-like clusters of<001>//ND and<111>//ND oriented grains with a more homogeneous distribution along the rolling direction in the center layer of hot-rolled annealed sheet;it had lower ductile-brittle transition temperature and better toughness at low temperature.(2)By adding a small amount of Sn in the steel,the effects of Sn element on the microstructure,texture,mechanical properties and resistance to pitting corrosion properties of medium-chromium ferritic stainless steel were investigated.Compared with the steel without Sn,the steel with Sn had smaller grain in hot-rolled and annealed,so the Sn might play a role in the grain refining;but it had greater deviation and lower strength of a-fiber texture components {115}<110>in the center layer of hot-rolled annealed sheet;it had obvious band-like clusters of<001>//ND and<111>//ND oriented grains with a more concentrated distribution along the rolling direction in the center layer of hot-rolled annealed sheet;it had some improvement of plastic,but had higher ductile-brittle transition temperature and poorer toughness at low temperature while with better resistance to pitting corrosion properties in the both surface and center layer.But compared with the traditional medium-chromium ferritic stainless steel with the same thickness,the ductile-brittle transition temperature of the ferritic stainless steel with Sn element was decreased,and the toughness was improved.(3)Water cooling mold method was proposed,which can improve the solidification structure and the segregation behavior of Sn element.But the gains of hot-rolled and annealed structure were relatively coarse,because of the less original thickness of the slab and the lower hot rolling reduction;it had strong a-fiber and y-fiber texture in the central layer of hot-rolled annealed sheets;it had obvious band-like clusters of<001>//ND and<111>//ND oriented grains with a more concentrated distribution along the rolling direction in the center layer of hot-rolled annealed sheet.Compared with the traditional medium-chromium ferritic stainless steel with the same thickness,the toughness of the steel used water cooling mold casting was improved,but the promotion space was restricted.