Generation Mechanism of MgO and Al2O3 Inclusions in 51CrV4 Spring Steel Based on the Ion–Molecule Coexistence Theory

Abstract

The presence of MgO·Al2O3 inclusions in 51CrV4 spring steel is detrimental to the alloy’s castability and fatigue properties. To effectively suppress these inclusions during production, accretions were collected from the immersion nozzle, and the MgO·Al2O3 inclusions in the steel billet were investigated. The generation mechanism of the inclusions was evaluated based on the ion–molecule coexistence theory, and the mass action–concentration model of CaO–SiO2–Al2O3–MgO–FeO–MnO slag was developed. Industrial experiments showed that nozzle clogging was primarily caused by MgAl2O4 spinel inclusions, and the MgO·Al2O3 spinel inclusions in the steel billet were investigated by non-aqueous electrolysis. The model calculation results indicate that the Mg content increased with an increasing basicity, CaO/Al2O3 ratio, and Al content during the ladle furnace (LF) process. In contrast, the Mg content decreased with increasing CO pressure under Ruhstahl-Hausen vacuum degassing process (RH) conditions.

Evolution behaviour and modification mechanism of inclusions in NM500 wear-resistant steel with calcium treatment

“…These results can be reasonably explained by the MgO in slag and lining reduced by Al and C in liquid steel. The content of the dissolved Mg in steel increases, and Mg further reacts with Al 2 O 3 inclusions in liquid steel to form MgO·Al 2 O 3 ,[38] which is confirmed by the decline of Al content in steel from 0.0466mass% after alloying in Ar station to 0.0302mass% before calcium treatment.

Figure 10. (a) Number densities and (b) average sizes of the different types of inclusions at different stages in steelmaking process.

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“…In the refining process, with decreasing S content and increasing basicity of slag, the number density of MnS shows an obvious decreasing trend, and the average size decreases as a whole. After alloying in Ar station, Al block, Al-Ca ball and other auxiliary materials are added into the steel to form MgO•Al 2 O 3 , [38] which is confirmed by the decline of Al content in steel from 0.0466mass% after alloying in Ar station to 0.0302mass% before calcium treatment.…”

“…The number of CaO·MgO·Al 2 O 3 inclusions in steel increases significantly after calcium treatment. Many studies have shown that CaO·MgO·Al 2 O 3 inclusions are formed by calcium treatment of MgO·Al 2 O 3 [2,3,15,31,36,38], but there is no clear mechanism analysis for the different types of CaO·MgO·Al 2 O 3 inclusions in calcium treatment steel. Combined with thermodynamic and SEM-EDS analyses, the formation mechanisms for the different types of CaO·MgO·Al 2 O 3 inclusions are clarified as follows.…”