MnS precipitation in association with manganese silicate inclusions in Si/Mn deoxidized steel
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INTRODUCTION
OWING to their beneficial effects on grain refinement by facilitating the nucleation of intragranular acicular ferrite (IGF), utilization of inclusions in steel has attracted much attention in recent years.[1–13] In particular, fine inclusions of oxides, sulfides, and nitrides, which are formed during solidification, have been reported to be especially effective for IGF nucleation. Inclusions comprised of Ti2O3,[14] TiN,[15] CuS,[16] and MnS[17,18] have been reported to be among the most effective nonmetallic nucleants of IGF. A number of different mechanisms have been suggested to explain how the nonmetallic inclusions facilitate the nucleation of IGF. The most persuasive suggestion is the existence of a zone around the inclusions where manganese is depleted, namely, a Mn-depleted zone, which acts as the nucleation site of intragranular ferrite in the austenite matrix. Wakoh et al.[19] proposed that the Mn-depleted zone formed around a manganese silicate (MnO-SiO2) inclusion by the nucleation of MnS on it and the subsequent diffusion of Mn in the steel matrix into the MnS nucleus. A recent study has indicated that a Mn-depleted zone also exists around Ti2O3 inclusions and can enhance the formation of IGF.[14] Some investigators also report the possibility of the formation of a Mn-depleted zone around cation-vacancy-type oxides, including Ti2O3.[10] In the present study, an attempt has been made to elucidate the mechanism(s) of the formation of a Mn-depleted zone by examining the precipitation behavior of MnS on MnOSiO2 oxide inclusions in Si/Mn deoxidized steel. In particular, the behavior of both MnS and the Mn-depleted zone was investigated under different cooling rates and isothermal
holding times, both of which simulated casting and reheating processes. II. EXPERIMENTAL Experimental work was carried out in order to understand the fundamental phenomena both of the formation of secondary inclusions of manganese silicate types and of MnS precipitation during the solidification and cooling of Mn/Si deoxidized molten steel. A semilevitation melting technique was employed to facilitate the removal of primary inclusions from the melt. A. Master Alloy Preparation Electrolytic iron (300 g) was melted in an induction furnace together with Fe2O3 powder (0.33 g) and FeS (0.05 g) powder in a magnesia crucible (40 mm in diameter and 150 mm in depth) under a purified argon gas atmosphere. After stabilization of the melt at 1600 ⬚C, ferrocarbon (6 mass pct C) was added to attain 0.1 mass pct C in the steel melt. Pure metallic manganese and silicon were then added. The melt was held for 30 seconds after deoxidation, after which a portion of the melt was sucked into fused silica tubes (5 mm in diameter) to make rod-shaped samples, which were used in the subsequent experiment of semi-levitation melting. The melt remaining in the crucible was cooled down to 1200 ⬚C at a predetermined rate of 40 ⬚C/min and then was allowed to cool in the crucible by switching the power off. The composition of the rod specimen is give
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