Optimization of Slag Chemistry Toward Inclusion Control for 28CrMo47 Drill Pipe Steel Based on Viscosity and Equilibration Studies

Abstract

During the refining of 28CrMo47 drill pipe steel, it is found that the product often has internal defects and cracks. The present work is initiated with a view to examine if these problems could be solved by optimizing the slag chemistry of the refining slag. Three series of slags, namely, slag A (57.09%CaO–15.74%SiO2–16.35%Al2O3–5.35%MgO, original slag in the steel plant), slag B (55%CaO–21%SiO2–12%Al2O3–7%MgO–5%CaF2), and slag C (51%CaO–17%SiO2–25%Al2O3–7%MgO) are designed and their viscosities are measured. The effect of Al2O3 content on viscosity of the slag C is also investigated as part of the slag optimization. Slag–steel equilibrium experiments are subsequently carried out in order to understand the effect of the slag chemistry, especially the variation in the Al2O3 content in the same on the inclusions in drill pipe steel. It is concluded that both increasing Al2O3 content of the slag used in the refining unit in the plant and addition of CaF2 could decrease effectively the slag viscosity. Inclusions in steel equilibrated with slag C1, corresponding to 51%CaO–7%MgO–25%Al2O3–17SiO2, consisted mainly of low melting point CaO–Al2O3 complex, which did not have any negative effect on the steel quality.