LAUSR

Ridging is a typical surface defect of ferritic stainless steel (FSS) in the deformation process. Many efforts have been made to reduce the occurrence of ridging defect. Most studies conclude that the origin of ridging is strongly related to the columnar structure in the as-cast structure of FSS.1–8) Equiaxed grain structure of FSS is desired to be obtained after the solidification process. Therefore, a lot of processes, such as electromagnetic stirring (EMS) technology, low-superheat casting, have been applied to improve the equiaxed grain ratio of as-cast structure of FSS.9,10) However, low-superheat casting affects the smoothness of CC and the application of EMS increases the equipment and operating costs. Moreover, low-superheat casting and EMS have a limited effect in raising the equiaxed grain ratio of high-purity ferritic stainless steel. Therefore, it has been paid more and more attention to refine the solidification structure by means of heterogeneous nucleation.9–11) Bramfitt12) found that TiN shows good coherency of crystal lattice parameters with δ-Fe. The research of Villafuerte et al.13) showed that the equiaxed grain ratio increased and the size of the equiaxed grains decreased in the weld structure when Ti was added in FSS gas-tungsten arc (GTA) welds. Itoh et al.14) studies the effect of Ti, B, N, Zr and REM addition on the equiaxed ratio of SUS430 steel. However, in order to obtain a high equiaxed grain ratio, an Effect of Nb on the As-cast Structure and Compactness Degree of Ferritic Stainless Steel Dual Stabilized by Ti and Nb