In situ characterisation of MnS precipitation in high carbon steel

Yasuhiro Tanaka,Veena Sahajwalla,Farshid Pahlevani,Suk-Chun Moon,Rian Dippenaar

doi:10.1038/s41598-019-46450-y

Abstract

Manganese sulphide (MnS) is one of the major non-metallic inclusions in steel with huge impact on steel property. In the case of high carbon steel, due to higher sulphur content and its brittleness, controlling MnS formation is one of the main issues. MnS has a complicated precipitation mechanism during solidification in liquid and solid steel and at the interface with oxide inclusions. Higher sulphur content, lower melting point and different oxide inclusions in high carbon steel will cause MnS precipitation at different stages. In this study, different stages of MnS precipitation from liquid and/or solid in high carbon steel and at the interface with oxide inclusion were investigated comprehensively via two different types of High Temperature Confocal Scanning Laser Microscope (HTCSLM). Samples were analysed further using SEM-EDS for better understanding the pertaining mechanisms. MnS precipitation on the surface of liquid steel was observed in situ in a HTCSLM by the use of a concentric solidification technique. Additionally, formation of MnS following solidification and at the interfaces of oxide inclusions, was investigated in situ in a HTCSLM, which has a uniform temperature profile across the specimen. These comprehensive descriptions about different stages of MnS precipitation in high carbon steel have been conducted for the first time and provide crucial information for controlling MnS morphology in high carbon steel.

Highlights

Controlling inclusions during the refining processes of steel is an essential step for producing high quality steel
Takada et al.[4] have shown that the morphology of Manganese sulphide (MnS) depends on the sulphur content in the base steel, and that a huge MnS dendrites can precipitate from liquid steel when the sulphur content is so high that the composition falls within the co-existence area of MnS and liquid Fe as shown in the ternary Fe-Mn-S phase diagram
Sims and Dahle[7] categorized MnS morphologies into three groups; globular, or angular phases, which precipitated from liquid steel, and a rod-like phase, which crystallises in solid steel

Summary

Introduction

Controlling inclusions during the refining processes of steel is an essential step for producing high quality steel. Diederichs et al.[11,12] modified the model developed by Schwerdtfeger[9,10] by considering the concentrations of manganese and sulphur, as well as the dendrite arm spacing in medium-carbon steel Another important type of MnS forms at the interface with oxide inclusions. High carbon steels are mainly deoxidized by silicon, leading to the formation of non-alumina based, semi-liquid oxide inclusions, which are very different than the inclusions formed in Al-killed low carbon steel. The aim of the present study is to conduct a comprehensive study of the different stages of MnS precipitation/crystallisation from liquid/solid high carbon steel and at the interface with oxide particles. The outcomes of this study are likely to provide a valuable contribution to the production of high-performance high carbon steel

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Conclusion