Effects of Mg Addition on Inclusions Formation and Resultant Solidification Structure Changes of Ti-stabilized Ultra-pure Ferritic Stainless Steel

Abstract

The effects of Mg addition on the formation of nonmetallic inclusions and solidification structure of Ti-stabilized ultra-pure ferritic stainless steels were investigated by experimentally casting ingots with different compositions. Thermodynamic analyses on the formation of complex inclusions after adding Mg into steels were carried out combined with the scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) analysis. And the EDS analysis showed that in steel samples with Mg addition, a new spinel crystal phase combined with Al2O3-TiOx formed. It was also found that after Mg addition, the proportions of equiaxed grain zone of 409L, 4003, 439 and 443NT steels increased from 10.2%, 21.8%, 13.4% and 18.6% to 84.3%, 92.3%, 91.1% and 100.0%, respectively. Since the planar disregistry between spinel and TiN is 5.1%, spinel could promote the precipitation of TiN and increase the number density of TiN inclusions in steel melts. The mechanism of solidification structure refinement after adding Mg into steels supposed that the complex inclusions of spinel and TiN in high number density enhanced columnar-to-equiaxed transition, since the planer disregistry between δ phase and spinel is 1.4%.