Dispersion of fine primary inclusions of MgO and ZrO 2 in Fe-10 mass pct Ni alloy and the solidification structure
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I. INTRODUCTION
THE high number of nonmetallic inclusions, which will not go into solution at elevated temperatures, may effectively hinder grain growth. Thus, the positive use of primary deoxidation products is considered to be one of the challenging techniques for improving the mechanical properties of steel, such as strength and toughness. In order to introduce fine primary inclusions into a liquid steel homogeneously, a complete understanding of the major controlling factors such as coalescence and separation of inclusions during cooling and subsequent solidification of steel is required. It was confirmed in our previous experiments[1] that the primary inclusions having the mean diameter of roughly 1 mm could be obtained by (1) using a strong deoxidant, (2) reducing the time when the particles were in a fluid or partly fluid state, and (3) choosing a proper solidification rate. These findings can be explained as follows. A strong deoxidant increases the degree of supersaturation for oxide precipitation, which leads to an increase of the inclusion number. The FeOcontaining particles have higher wettability, thus leading to the particle coalescence, but a strong deoxidant reduces the FeO content in the initial deoxidation product to a considerable degree.[1] Inclusion particles are not pushed out by the advancing liquid-solid interface, when the growth rate is above the critical value, thus resulting in the uniform distribution. In order to elucidate the influence of inclusion particles on grain size control and their correlation with the mechanical properties, knowledge of particle size distribution is of crucial importance rather than that of a mean value. However, the data on size distribution can be validated only by comparing the volume fraction of particles, which can be obtained from the contents of oxygen and/or an insoluble element as inclusions, with that estimated from the microscopic inclusion count. KIMIAKI SAKATA, Graduate Student, Department of Metallurgy, and HIDEAKI SUITO, Professor, Institute for Advanced Materials Processing, are with Tohoku University, Sendai 980-8577, Japan. Manuscript submitted February 17, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
It has been known that primary inclusions and dissolved elements influence the solidification micro- and macrostructures. Primary inclusions act as nuclei and thus increase the amount of globular crystals, depending on their chemical composition and crystallographic parameters. A dissolved element influences solidification microstructure through the degree of constitutional supercooling, the solid-liquid interfacial tension, and other aspects. Furthermore, oxide particles and dissolved elements, which are redistributed during solidification, have a strong influence on the kinetics of the subsequent solid-state transformation and precipitation kinetics. In this investigation, an Fe-10 mass pct Ni alloy containing the initial oxygen of 70 to 160 mass ppm was deoxidized with an Ni-15 mass pct Mg alloy or an Fe-50 mass pct Zr alloy at 1873 K. T
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