Effect of Mg Addition on Solidification Structure of Low Carbon Steel

Abstract

Effect of Mg addition on the solidification structure of low carbon steel containing about 0.1 mass% carbon was investigated in order to improve internal quality of cast blooms of this steel by some experiments using small size ingot casting. Some kinds of thermodynamic analyses on the crystallization of oxides and nitrides were also conducted along with FE-SEM observation on inclusions formed in cast ingots.The results in this study were summarized as follows.Adding Mg to the steel allows the equi-axed crystallization to be facilitated. In particular, the promotion effect is significant in low-carbon steel containing approximately 0.2–0.25 mass% Ti which leads to crystallizations of TiN until the end of solidification. The facilitating effect of equi-axed crystallization due to Mg addition in the steel without Ti content was presumably attributable to the heterogeneous nucleation by MgO or MgO·Al2O3, however, the facilitating degree is less compared with that in the steel containing Ti. It is apparent that MgO is also effective for TiN crystallization as the agent resulting in the significant promotion of heterogeneous nucleation of δ-Fe and the equi-axed crystallization. Cooperative effect of MgO and TiN was confirmed in the low-carbon steel containing Ti by Mg addition. By Ca addition, the facilitating effect due to Mg addition is assumed to decrease to liquidize oxides and to reduce total oxygen, so that the crystallized amount of solid MgO or solid MgO·Al2O3 decreases. It was verified that planar disregistry is a useful index to evaluate the relative heterogeneous nucleation capacity.