Investigation On The Roles Of Rare Earth Ce In00Cr17Ultra-Purity Ferrite Stainless Steel

The contradiction between continuously increasing of quantity required of stainless steels and limited nickel resources has become more and more serious in recent years. As a result, great attention is now focused on resource-saving-type stainless steel. Ultra-pure ferrite stainless steel has advantages of low cost, excellent corrosion resistance and high temperature oxidization resistance, however, coarse ferrite grains, high notch sensitivity,475℃embrittlement and poor formability limit widespread application. There is abundant resource of rare earth (RE) in China. RE had been extensively applicated in steels. But up tp date the roles of RE in ultra-pure ferrite stainless steel is not very clear and some research results contradict each other. Particularly, because of low impurity content in clean steel, the roles of rare earth will change, and the ratio of RE to sulfur content obtained by prior research can not be used to instruct practical production. Therefore, systematically study on the roles of RE in ultra-pure ferrite stainless steel is very necessary. It is of theoretical significance for developing performance-excellent ultra-pure ferrite stainless steel by taking advantage of RE resource of China.In the present study,00Cr17ultra-pure ferrite stainless steels with different Ce contents were prepared. By using of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction device (XRD), thermo-simulating testing, tensile testing and micro-hardness testing etc., the effects of Ce on cast structure, hot ductility, recrystallization dynamics, grain growth dynamics, impact toughness, formability,475℃brittleness and high temperature oxidization behavior were investigated. The roles of Ce in00Cr17ultra-pure ferrite stainless steel were systematically analyzed and discussed. The major results are as follows:Ce refines the cast microstructure of00Cr17steel, reduces the formation of columnar crystals and promotes the formation of equiaxed grains. The size of grains decreases with increasing Ce content, which is attributed to that the finer oxides, sulfide and Ce-bearing intermetallics increase the nucleus number of nonuniform nucleation during solidification. Ce inhibits the precipitation of Cr23C6phase at grain boundaries in the middle-temperature zone, decrease the tendency of melted grain boundaries leading to the formation of crack at1400℃high-temperature brittle zone. At the same time, addition of Ce to00Cr17steel completely eliminates middle-temperature brittle zone in temperature range of900℃to1050℃and improves the hot ductility in the temperature zone from600℃to1350℃.Solute Ce in00Cr17steel generates the distortion of lattice, increases the storage energy of cold deformation, accordingly increases the driving force of recrystallization, reduces the static recrystallization temperature of00Cr17steel. Ce increases the activation energy of grain growth, and hinders the grain growth of00Cr17steel during annealing treatment. Ce retards dynamic recrystallization during hot deformation because Ce enriches at the grain boundary and enhances the resistance of grain boundary immigration. Based on the theory of hot working constitutive model, deformation activation energies Q in Re-F steel and Re-H steel were calculated to be335.7kJ·mol-1and324.6kJ·mol"1respectively. The flow constitutive equations of the experimental steels were established:Transversal-and longitudinal impact toughnesses increase17%and14%, respectively when adding0.02%Ce to the00Cr17steel. Addition0.08%Ce has no effect on the impact toughness. Addition of Ce has no effect on the impact toughness at low temperatures (-20℃and-40℃). Ce eliminates the yield plateform phenomenon of ferrite stainless steel, enhances the8, n and r value and improves the formability of00Cr17steel. Addition of Ce inhibits the precipitation of α’ phase and decreases embrittlement speed when aged at475℃for less than196h. But as increasing aging time, the impact toughnesses of00Cr17steels with and without Ce gradually decrease.Oxidation kinetics of the experimental steels follow parabolic regularity in the temperature range of700℃to1000℃. Oxidation rate of the steel with containing Ce is obviously lower than that of the steel without Ce at the temperature higher than1000℃. The higher Ce content, the more significant the effect of oxidation resistance. Oxide films of experimental steels are made up of single Cr2O3in the temperature region from700℃to1000℃, and Cr2O3and Fe2O3at1100℃. Addition of Ce can obviously slow down the formation of Fe2O3in oxide film and enhance the adhesiveness between oxide film and matrix.