Introduction: In Duplex Stainless Steel (DSS), duplex refers to two phase microstructure. At present, it has been widely used in chemical industry, ocean project, and oil industry and so on. Compared with other types of stainless steel, DSS has a distinctive feature that the ferritic phase and austenitic phase took approximately equal share of the solid solution. Thus it has excellent intergranular corrosion resistance, welding performance and super plasticity etc. However the two phase microstructure at elevated temperature makes the production more difficult. DSS usually contains a higher content of N (about 0.20%), the solubility of N atom in δ-ferrite is only 0.1% which is in a saturated state. It would easily precipitate during solidification and the defects might occur under the surface. In addition, grain of δ-ferrite in casting microstructure is coarse, the segregation is severe, the formed columnar crystal zone is well developed, the two phase microstructure of α+γ is distributed unevenly along the slab cross section, hot-working property is poor, and edge crack appears easily. Therefore, improving the solubility of N in δ-ferrite and refining the grain to increase the equiaxed grain ratio, is the effective measure to improve the surface quality, thermoplasticity, low temperature strength and toughness of duplex stainless steel. It was
It was found by former researchers that element of Ti could improve the quality of stainless steel solidification microstructure. It was thought that, 1) TiN could be used as the effective nucleus of ferrite, the equiaxed grain structure could be refined obviously by adding 1%Ti and 0.05%N into 430 stainless steel; 2) for austenitic stainless steel, the effect of TiN refining equiaxed grain was not obvious; 3) for duplex stainless steel, TiN particle had an obvious effect of refining the columnar crystal. Single TiN particle did not show apparent effect of refining δ-ferrite of duplex stainless steel. If TiN attached to the oxides surface formed by elements of Al, Ca and Mg, there would be more obvious refining effect. The nucleus forming mechanism of TiN particle easily attached to the surface of Ti2O3 particles, i.e. only when δ-ferrite firstly precipitated from solidification structure, complex-nucleus with Ti could probably serve as heterogeneous effective nucleus.
Authors: Zongxu Pang, Rong Zhu, and Kailu Tu
Keywords: Duplex Stainless Steel (DSS); Spinel Solid Solution; TiN