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

Jialiu Lei,Hangyu Zhu,Dongnan Zhao,Zhengliang Xue

doi:10.3390/met9080830

Jialiu Lei, Hangyu Zhu + Show 2 more

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https://doi.org/10.3390/met9080830

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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.

Highlights

In recent years, the increasing demand for alloyed steels with excellent properties has led to efforts to enhance the castability, surface quality, toughness, and fatigue behaviors of alloyed steels [1,2,3,4].Due to the addition of Cr and V, 51CrV4 spring steel exhibits an excellent intensity, fatigue performance, and hardenability; it has attracted considerable attention for use in large damping springs, suspension springs, and flat springs [5,6]
During the production of 51CrV4 steel, a high-basicity slag is adopted, as it is conductive to the removal of inclusions in bulk steel, desulfurization, and reducing the total oxygen content, but results in the formation of detrimental MgO·Al2 O3 inclusions [7,8,9]
The Mg content balanced with lining and slag can reach up to 17 ppm and 5 ppm, respectively, based on the ion–molecule coexistence theory (IMCT)

Summary

Introduction

The increasing demand for alloyed steels with excellent properties has led to efforts to enhance the castability, surface quality, toughness, and fatigue behaviors of alloyed steels [1,2,3,4].Due to the addition of Cr and V, 51CrV4 spring steel exhibits an excellent intensity, fatigue performance, and hardenability; it has attracted considerable attention for use in large damping springs, suspension springs, and flat springs [5,6]. During the production of 51CrV4 steel, a high-basicity slag is adopted, as it is conductive to the removal of inclusions in bulk steel, desulfurization, and reducing the total oxygen content, but results in the formation of detrimental MgO·Al2 O3 inclusions [7,8,9]. These inclusions have a steady face-centered cubic structure with a high melting point and are hard and un-deformable [10]. The accretions on the SEN might desquamate and become entrapped in the solidified shell, resulting in serious decreases in the internal quality [13]

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