| 摘要 |
PROBLEM TO BE SOLVED: To restrain the surface defect and the internal defect and to improve the yield of a cast slab while eliminating grind-cleaning and developing scrapping by specifying a specified ration or more of the whole cross section on a cast slab so as to form fine solidified structure. SOLUTION: An equi-axed crystal of the case slab satisfies the formula at >=60% of the whole cross sectional area in the cast slab (in the formula, X: distance (mm) from the surface of the cast slab, D: diameter (mm) of the equi-axed crystal at the position of X from the surface of the cast slab). It is desirable to fill the equi-axed crystal in the cast slab excellent in a rolling characteristic in the whole cross section of the cast slab and be a steel material having the max. equi-axed crystal diameter in the cast slab within 3 times of the average equi-axed crystal diameter. In this way, the growth of columnar crystal on the surface layer of the cast slab is restrained and the fine equi-axed crystal is formed in the inner part. The surface defect of crack, recessed flow, etc., generated from micro-segregating part as the starting point at the cooling time and at the solidifying time, can be restrained. Further, the micro-segregation at the boundary part is reduced and the internal crack caused by the shrinkage, the bulging of the cast slab, the straightening of bending-back, etc., and the internal defect of the center segregation, etc., caused by the cavity and the fluid of unsolidified steel, are prevented.
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