Behavior of nonmetallic inclusions in front of the solid-liquid interface in low-carbon steels
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I. INTRODUCTION
AMONG various types of nonmetallic inclusions, oxide and sulphide inclusions have been thought harmful for common steels. However, there are some positive roles in these inclusions. Oxide inclusions act as the trapping sites of hydrogen atoms in enameled steel and prevent the coating from stripping off. Sulfide inclusions also improve the machinability of free-cutting steels. Furthermore, it has been well known in welding that the tiny oxide inclusions act as the nucleation sites for very fine acicular ferrite crystals in the bond metal, providing very good ductility.[1–5] In the rolled steel products too, many fine intragranular ferrite crystals nucleate at some oxide inclusions inside of austenite grains. This technique is based on the concept of “Oxide metallurgy,”[6] in which an important role is given to oxide inclusions as inoculants for the heterogeneous nucleation of the phase transformation and the precipitation. Therefore, the purpose of the present study is to understand the phenomena of oxide particles at the solid-liquid interface in order to disperse them during solidification. There have been many studies performed to date, but the majority of these dealt with aluminum alloys or other materials in low-temperature experiments.[7–15] The only exception is the work done for steel by Emi and his coworkers.[16,17] In the present article, a novel result of in situ observation carried out on steel by using a laser microscope will be reported in detail. II. EXPERIMENTS A confocal scanning laser microscope was used to carry out the experiment of an in situ observation of nonmetallic SEI KIMURA, Research Fellow dispatched from Kobe Steel Company, Kakogawa, Japan, Y. NABESHIMA, Postgraduate Student, K. NAKAJIMA, Associate Professor, and S. MIZOGUCHI, Professor, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980-8577, Japan. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
inclusions in front of the solid-liquid interface. The principle and the method of operation of the laser microscope have been described in detail elsewhere.[16,17] Two samples were used. Sample A is a normal low-carbon, aluminum-killed steel cut out from the continuously cast slab. Sample B is a low-carbon steel of high oxygen content specially made in the laboratory. The 100 g sample A was remelted under Ar gas flow in an alumina crucible in a 5 kW electric resistance furnace. Subsequently, the 5 g Fe-10 mass pct Mg pressed powder cake was added to the molten sample A, and the melt was cooled by switching off the power. When Mg metal was added to the melt, strong boiling took place, and the melt absorbed oxygen from ambient air entering from the top port. Sample B, of high oxygen content, was, thus, obtained. Chemical analysis was made twice for sample B, before and after the in situ observation experiment, in order to
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