| 摘要 |
PURPOSE:To obtain a free-cutting steel improved in rolling fatigue strength of machine structural steel and having improved machinability and cold forgeability by providing a steel which has a prescribed composition and in which practically globular sulfides are dispersed and the area ratio of alumina cluster is specified to a prescribed value or below. CONSTITUTION:The free cutting steel contains, by weight, 0.08-0.6% C, <=1.0% Si, <=2.0% Mn, 0.04-0.4% S, <=0.1% Te (where %Te/%S=0.04), <=0.0030% O, <=0.020% N, <=2.0% Al and one or more kinds among <=0.5% V, <=0.5% Ti, <=0.5% Nb and <=0.5% Zr, in which, as essential requirements, practically globular sulfides are uniformly dispersed and the area ratio of alumina cluster is limited to <=0.5%. As to the above sulfides, it is necessary that the ones having a major axis to minor axis ratio of <=5 comprise >=80% of the ones with a length of >=10mu. Owing to the above characteristics, this free-cutting steel is suitable for use even in gears, bearings, movable joints, etc., and has further extended application range as compared with conventional machine structural free-cutting steels.
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