Aluminum-Free Steelmaking: Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with CaO-SiO2-CaF2-MgO Slag

Stephano P T Piva,Petrus Christiaan Pistorius

doi:10.3390/pr9081258

Abstract

In some applications, deep desulfurization and deoxidation of steels without the use of aluminum are required, using Si as a deoxidant instead, with double-saturated slags in the CaO-SiO2-CaF2-MgO system. This work studied the desulfurization and nonmetallic inclusion evolution for the system using an induction furnace and compared the results with FactSage kinetic simulations. Steel samples were taken from the steel melt and analyzed with ICP-MS and combustion analysis for chemistry, and SEM/EDS for nonmetallic inclusion quantity, size, and composition. The results indicate that the steel was deeply desulfurized, with a final sulfur partition coefficient of 580; MgO was reduced from the slag, yielding dissolved [Mg] that transformed liquid Mn–silicate inclusions into forsterite and MgO. Intentional reoxidation of the melt with oxidized electrolytic iron demonstrated a significant concentration of dissolved [Mg] in the steel, by the formation of additional forsterite and MgO upon reoxidation.

Highlights

IntroductionAlumina-containing nonmetallic inclusions are undesirable, whether because of their effect on steel castability (the tendency of solid alumina and spinel inclusions to clog nozzles) or on product quality (due to the hardness and high melting point of alumina-based inclusions)
For certain steel products, alumina-containing nonmetallic inclusions are undesirable, whether because of their effect on steel castability or on product quality
Dissolved magnesium remained in solution after this time or was replenished by Si-manganese silicates to form (MgO) reaction, causing the inclusion compositions to change over time as steel–slag–inclusion reactions approached equilibrium

Summary

Introduction

Alumina-containing nonmetallic inclusions are undesirable, whether because of their effect on steel castability (the tendency of solid alumina and spinel inclusions to clog nozzles) or on product quality (due to the hardness and high melting point of alumina-based inclusions). To avoid aluminum pickup and enable deep desulfurization, slags in the. CaO-SiO2 -CaF2 -MgO system are typically used in such cases: fluorspar (CaF2 ) significantly increases the solubility of CaO [2], while the absence of Al2 O3 avoids aluminum pickup. To obtain a low oxygen activity at the steel–slag interface, to enable deep desulfurization, the silica activity in the slag must be low when using Si as deoxidant. The basic nature of CaO-rich, CaF2 -bearing slags and the high solubility of CaO allow liquid slags with low silica activity to be used. The low silica activity (deeply reducing conditions) drives the reduction of other oxides such as MgO and CaO from the slag, yielding dissolved magnesium and calcium [Mg] and [Ca] in the steel. Reduction of MgO from slag, refractory, or both has been demonstrated for both Si-killed and Al-killed steel types [3,4,5]

Methods

Results

Discussion

Conclusion