Abstract
Large sizes of columnar crystals and TiN particles have a great influence on the surface quality of ferritic stainless steel. In the present paper, this study proposed to obtain fine-grained equiaxed structures by Mg–Ti treatment. Through the experiment happened in resistance furnace with argon protection, the refining effect of Mg–Ti addition on the microstructure and TiN particles were investigated, and the refinement mechanism was discussed from interface coherence theory. It was found that due to adding Mg and Ti into molten ferritic stainless steel, the equiaxed crystal ratio increased from 37% to 50%, and the size of TiN particles reduced at the same time. The lattice matching characteristics of MgAl2O4/TiN and TiN/δ-Fe were investigated by FIB-HRTEM. According to Bramfitt’s equation, the lattice misfit for (400)MgAl2O4∥(200)TiN and (200)TiN∥(110)δ-Fe was 5.02% and 4.41%, respectively, which were all belong to the effective nucleation range. It could be considered that MgO and MgAl2O4 formed in the molten steel promoted TiN nucleation easier to precipitate out with large quantities in the liquid phase. The TiN particles with more uniform distribution significantly enhanced the heterogenous nucleation of ferritic phase during initial solidification process base on the good lattice fitting condition. Finally the equiaxed crystal ratio of δ-Fe phase increased dramatically.
Highlights
Ferritic stainless steel, with chromium content from 15% to 30%, has been widely used for its outstanding characteristics, such as lower thermal expansion coefficient, better resistance to stress corrosion cracking, and the problems that need to improve are present
With the addition of 0.05% Ti, the equiaxed crystal ratio increased from 15% to 37%, and the mean grain size decreased from 4.24 mm to 1.79 mm shown as Figure 3b
The grain refinement was related to the heterogeneous nucleation behavior of δ-Fe with TiN which is precipitated at the solidification front [16,17,18]
Summary
With chromium content from 15% to 30% (weight), has been widely used for its outstanding characteristics, such as lower thermal expansion coefficient, better resistance to stress corrosion cracking, and the problems that need to improve are present. Ridging is a kind of surface defect that occurs as metal products are fabricated. Ridging causes disfigurement of the surface and a grinding process is required for amelioration. Ridging is partly caused by the large columnar structures formed during solidification [1,2,3]. During continuous casting of ferritic stainless steel, the equiaxed ratio of slab is normally kept in 20–30%. In the factory production process, surface ridging defect is inhibited by stimulating recrystallization during hot rolling or repeated cycles of cold rolling and annealing
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