| Tin-bearing ferritic stainless steel has excellent corrosion resistance, high resistance to stress corrosion crack and low cost. For industrial applications, it is necessary to solve the problems of the mechanical property worsening caused by segregation of tin in steel and investigate the mechanical property. The research on the metallurgical behavior, corrosion resistance, passivation film structure and the precipitation behavior of tin-bearing stainless steel is significant for the development of this new generation resource-saving stainless steel. In this paper, a resistance furnace and an induction furnace under argon protection were used to prepare tin-bearing ferritic stainless steel. The metallurgical behavior, microstructure, mechanical property and corrosion property were investigated as follow:In the smelting process of tin-bearing stainless steel, the degree of tin's oxidation is not serous and the yield of tin is about 80% to 97%. The tipical inclusions in steel are oxide and sulfide inclusions, as well as some tin-containing inclusions with 1?m in size. In the molten steel, as the melting point of MnS is close to the steelmaking temperature, it exits as liquid state and forms precipitation nucleation later than other inclusions as Al2O3 with high melting point. Then in the solidification process of steel, it is easy for tin to precipitate around the nucleation particles to form tin-surrounded MnS inclusions.In solution temperature, tin will dissolve in the ferrtitic stainless steel matrix, and if preserved at a certain temperature and cooled, it will precipitate from alpha phase again. In the nonequilibrium solidification process, the contents of tin in grain and boundary are so different. When the content of tin in grain is higher than its solubility, it will precipitate around the grain boundary or interface, breaking the intercrystalline binding and the grain boundary continuity. In this research, when the tin content is more than 0.29wt% in ferritic stainless steel, the enrichment of tin can be found by scanning analysis, confirming the intracrystalline enrichment of tin.Based on micro-segregation model, the solidification segregation ratio of tin is lower than P and S, but much higher than other elements. With the increasing of solid fraction, the segregation ratio of tin increases from 1.0 to 2.6, and the segregation ratio of Mn and Cr increases to 1.4 and 1.2 respectively in the solidification end. The initial solidification phases are BCC phase, carbides and sigma phase with the decreasing of temperature. And with the increasing of carbon content in steel, the solidification phase will be mixed biphase or FCC phase which causes rapid increasing of solidification segregation ratio of other solutes, such as P, S and Sn. It gives an explanation of ductility though phenomenon of steel in III brittle temperature range.The tensile strength and yield strength of tin-bearing stainless steel increase and the elongation decreases a little. The tensile strength of tin-bearing stainless steel is about 450 MPa, about 20 MPa more than SUS 430 stainless steel. It is because that the lattice distortion of tin in iron atoms increases the tensile strength, hardness, cold work-hardening rate of steel and the tensile strength of tin-bearing stainless steel is slightly higher than 430 steel at room temperature. With the rising of temperature, it increases the atomic vibration and the lattice parameter, decreases the disordering of atom arrangement caused by Sn atoms. When the temperature exceeds 800?, the tensile strength of tin-bearing steel and 430 steel are almost identical to each other. The fracture morphology of ferritic stainless steel is composed with a large number of dimples as well as some inclusions. Small inclusions can be seen in the dimples and there is no evidence of tin-containing precipitation in the fracture facets. It can be concluded that this steel has superior toughness in the temperature range of 400 to 800?. With the increasing of tin content in steel, r and Ar value increase, and r value reaches to the maximum when the tin content is 0.29wt%.With the increasing of the strain rate, the grain boundary strength is greater than intracrystalline strength, causing the change of fracture types in the high temperature deformation process. The fracture morphology is brittle fracture type under strain rate of 0.002/s at 800?. And when the strain rate increases to 0.02/s, the fracture is mainly ductile. If the strain rate reaches to 0.2/s, the fracture transferres from ductile fracture to transgranular fracture. It is confirmed that the strain rate changed the fracture types of tin-bearing stainless steel.The uniform corrosion resistance test was conducted in 40wt% H2SO4 solution and the corrosion quantity changed linearly with immersion time. The uniform corrosion resistance is improved in tin-bearing stainless steel especially when the tin content is 0.38wt%. From the polarization curves and electrochemical impedance spectroscopy of tin-bearing stainless steel, the steel of 0.27wt% tin has lower anodic density and higher potential than blank steel. With the increasing of temperature, Cl-, H+ concentration, the pitting corrosion resistance decreases as well as the polarization resistance. With the increasing of acidity of the solution, the corrosion current density rises and the corrosion potential reduces. By contrast, the polarization has a lower current density and much higher corrosion potential in alkaline solution. And the intergranular corrosion resistance of steel is the best when tin content is 0.18wt%. Overall, the polarization curves and weight loss measurement indicate that the corrosion resistance of stainless steel is improved by adding small amount of Sn. The existence of Sn in passivation film suppresses the anodic dissolution of steels during long exposure to the aggressive solution. The clear effect of Sn on formation of a protective passivation film is presented in the XPS analysis section.It can be seen from the XPS narrow scan that outer passivation film is consist of Cr2O3, CrOOH, Cr(OH)3, Fe(OH)3, yet the inner passivation is consist of Cr2O3, CrO3, FeO, a-Fe2O3, Fe3O4 and small amount of CrO42-, Fe2+ satellite, Fe3+ satellite. The spectrum peak of Sn is weak and formed by Sn and Sno2. The peak of Sn3d indicates that SnO2 exists in the outer film and Sn exists in the inner side to maintain the stability of passivation film. In the outer layer of passivation film, the existing of hydroxyl oxide indicates that metal gives priority to combine with environmental water enhancing the re-passivation property. When exposed to corrosive solution, the metal ions immigrate to the surface and combine with surrounding bound water forming hydroxyl oxide to prevent the further destruction. Tin in the surface and the oxide/metal interface can improve the stability of passivation film to hinder pitting corrosion. |
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