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The control of oxide dispersion is one of the most important factors in the newly explored field of study "oxides metallurgy in steels," utilizing fine oxide particles as heterogeneous nucleation sites for various precipitates. The distribution of oxides in the solidification structure is greatly influenced by interactions between the oxides and the advancing solid/Iiquid interface as solidification progresses. In order to clarify the behavior of oxides in steels during solidification, unidirectional sol idification experiments was conducted. The distribution of oxides was quantitatively analyzed by CMA, and the experimental results were discussed in comparison with a mathematical calculation taking account of microsegregation and precipitation during solidification. In Ti-deoxidized steel. Ti.O. segregated in the interdendritic region considerably more than calculated. The discrepancy means, that considerable numbers of oxides were rejected to the interdendritic region by advancing solid/liquid interface during solidification. In Zr-deoxidized steel, on the other hand, ZrO. distributed relatively uniformly, as expected from calculation. The difference between both species of oxides regarding the rejection/entrapment phenomena is most likely due to interfacial energy between the liquid steel and oxide particles.

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Mechanism of surface crack formation of steel induced by residual copper (Cu) is investigated using a new technique of Greeble test. Two kinds of experiments were carried out, to clarify the effect of temperature and to understand the behavior of crack growth. Crack is caused by liquid Cu, which precipitates at steel-scale interface during oxidation. However, no crack formed at higher temperature. Micro analysis indicates that it is due to the formation of liquid scale above eutectic temperature of FeO-2FeO¥SiO2. Liquid Cu-precipitates are trapped in the liquid scale area, and they cannot penetrate into austenite grain boundaries. The fact that silicon addition reduces the crack formation also supports this mechanism. Deformation test with various strains reveals that there exist two stages in the behavior of crack growth. At the first stage, crack grows deeper, because liquid Cu penetrates into the boundary. The crack stops growing along the depth direction and opens its width in the second stage, because of the lack of liquid Cu. That means the amount of Cuprecipitates decides the crack depth.