Research On Microstructure,Property And Corrosion Resistance Of Sn-bearing Ferritic Stainless Steel

With the rapid development of our national economy,stainless steels are widely used not only in industry like aircraft industry,nuclear energy,petrochemical industry and shipbuilding industry,but also in civilian fields like transportation(cars and trains),kitchenware,and architectural decoration etc.Since 2001,China has become the world’s largest consumer and manufacturer of stainless steels.However,due to the extreme shortage of chromium-nickel resource in our country,prices of Cr-Ni austenitic stainless steels are very high and always greatly affected by the price fluctuations of chromium and nickel.It has become primary contradiction which restricts sustainable development of stainless steel industry.Therefore,it is a very urgent and important task to further research,develop and popularize Cr-Ni resource-saving and high-performance new type ferritic stainless steels,which are expected to replace expensive austenitic stainless steels to ensure the sustained and sound development of stainless steel industry.In recent years,resource-saving and Sn-bearing ferritic stainless steels have provided a new idea for stainless steel industry development in our country.In this paper,researches were carried out about effects of Sn on thermal deformation behavior,recrystallization behavior and texture types.Besides,it has also been evaluated that effects of Sn on both formability and surface quality after shaping.Finally,it is systematically studied that pitting and general corrosion behavior of Sn containing ferritic stainless steels against Cl-and H2SO4,respectively.Based on the original work,following main conclusions are drawn:(1)Effects of Sn on thermal deformation behavior and softening mechanism were analyzed.Solution element Sn in ferritic stainless steels plays a main role of strengthening and clearly improves flow stress during hot deformation.Thus,it is much easier to trigger dynamic recovery softening mechanism for Sn containing steels,which accumulates enough strain energy first during hot deformation.Besides,segregation of Sn tends to increase the activity of other alloying elements to diffusion movements,and to accelerate initial recrystallization softening in deformed substrate.However,with increasing strain rate or deformation temperature,effects of Sn on deformed ferritic stainless steel become weaker.Besides,element Sn never changes softening mechanism types of ferritic stainless steel.(2)Effects of Sn on formability of cold rolled and annealed ferritic stainless steel sheet at room temperature and relevant mechanism from a texture perspective were comparatively studied with blank ferritic stainless steel sheet.Both yield strength and tensile strength of the steel increase with increasing content of Sn due to its strengthening effect.The tensile strength is more sensitive to Sn content.Sn can effectively mitigate surface ridging defect of the steel,while slightly decrease its formability.After analyzing results of both micro and macro textures,segregation of Sn suppresses both nucleation and growth of {111} and {112} texture grains,while promotes nucleation and growth of {110} and {100} texture grains along deformed grain boundaries.More random oriented grains formed in annealed Sn containing ferritic stainless steel decrease texture gradient through-thickness,also promote homogeneous deformation capability along thickness direction.The texture feature in Sn bearing steel is in favour of alleviation ridging defect,but decrease its r value.Besides,within the range of Sn content in this paper,further increasing Sn content will not decrease the volume fraction of {111} texture grains.(3)Effects of high temperature hot-roll annealing process on microstructure,texture and formability of corresponding sheet are explored.Raising hot-roll annealing temperature with proper soaking time is in favour of improving homogeneity of structure and eliminating banded structure along thickness direction.In the end,subsequent cold-rolled and annealed sheet can obtain larger elongation and better surface quality for its homogeneous final structure.Besides,with high temperature hot-roll annealing process,volume fractions of both<001>//ND and<011>//ND oriented recrystallized grains in cold-rolled and annealed sheet increase,while that of<111>//ND oriented recrystallized grains slightly decreases.The changes in texture components finally lead to slight decreasing of r value and remarkable increasing of Ar value.Thus,it is feasible to obtain ferritic stainless steels with outstanding overall performance by modestly raising hot-roll annealing temperature and proper annealing soaking time.(4)Pitting resistance of Sn-bearing Cr17 ferritic stainless steel and relevant mechanism was systematically studied.Proper addition of Sn can improve the corrosion resistance of Cr17 ultra-purified ferritic stainless steel in chloride-containing solution.EIS plots show that Sn-containing steels with no more than 0.23wt%Sn had higher passive film resistance and faster growth rate of more compact passive film.XPS results show the presence of SnO2 in passive film on Sn-bearing ferritic stainless steels.Mott-Schottky results reveal that element Sn in ferritic stainless steels can improve firmness or stability of passive film probably through reducing the charge carrier density in passive film.Moreover,Sn2+ dissolved from substrate can inhibit the elementary dissolution reactions such as the formation process of absorbed intermediates(MOHad and MClad),thereby decreases the corrosion rate of matrix metal.Excessive addition of Sn(about 0.33wt%)in Cr17 ultra-purified ferritic stainless steel may deteriorate its corrosion resistance than good.Sn grain segregations,over increasing of hydrogen evolution overpotential and subsequent hydrolysis of Sn2+ ions oxidized from excessive Sn addition may accelerate the dissolution of steels,usually due to acidification,stabilization of new metastable pit nuclei,and further propagation of pits,especially in strong oxidizing and aggressive chloridion-rich medium.This intensively adverse effect can no longer be counteracted by beneficial role of Sn.The content of Sn should be controlled in the Cr17 ultra-purified ferritic stainless steel.The optimum content Sn is about 0.1 wt%-0.23wt%to obtain better corrosion resistance.(5)The corrosion type of studied ferritic stainless steel in 20wt%H2SO4 at 25 ℃turns out to be uniform corrosion,and the corrosion rate can be greatly decreased with Sn addition due to highly improved hydrogen overvoltage,which intensively suppresses the cathodic reactions by surface poisoning.Furthermore,with increasing of immersion time,the continually accumulating Sn2+,as a cation inhibitor,will inhibit the dissolution of Sn-bearing specimens to some extent.With the increasing of Sn content,the better corrosion resistance can be reached.