| 摘要 |
PROBLEM TO BE SOLVED: To improve surface flaws in the steel without deteriorating its machinability by allowing the contents of B and O to satisfy specified relation. SOLUTION: This steel contains, by mass, <=0.4% Bi, >=0.0005% B and >=0.003% O. Bi improves its machinability. B is effective for suppressing the surface flaws on the steel. O improves its finish face roughness by spheroidizing MnS. Furthermore, as to the contents of B and O, it is important that the relation of [B]2.[O]3<=8×10-10 is satisfied. [B] and [O] respectively denote the contents (mass %) of B and O. In the case, the contents of B and O increase, and the value of [B]2.[O]3 exceeds 8×10-10, the liq. phase of B oxide is largely formed, and surface flaws increase. The steel preferably contains <=0.2% C, 0.3 to 2.0% Mn, <=0.05% Si, <=0.05% Al, <=0.2% P and 0.08 to 0.5% S as fundamental components.
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