| 摘要 |
PROBLEM TO BE SOLVED: To provide steel for machine structural use having excellent machinability, in which graphite can be precipitated without requiring graphite precipitation treatment by reheating which has been essential heretofore after hot working in attaining the improvement of its machinability by precipitating graphite, and whose strength is not deteriorated even by the graphitization treatment. SOLUTION: The steel has a composition containing, by mass, 0.1 to <0.8% C, 0.5 to 2.0% Si, 0.02 to 0.5% Mn, 0.0003 to 0.015% B, 0.005 to 0.1% Al and 0.0015 to 0.015% N, in which the contents of Cr and O as impurities are respectively suppressed to <=0.05% Cr and <=0.0020% O, and the balance Fe with inevitable impurities. Further, the amount of graphite to be precipitated in the steel is controlled to 0.03 to 0.60% by volume, and also, the ratio of fine grains with a grain size of <=3μm in the precipitated graphite grains is controlled to >=60% of the whole.
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