| 摘要 |
PURPOSE:To obtain a steel bar which is soft in the surface layer part and increased in strength and toughness in the inner part and has superior machinability by an on-line processing by subjecting a steel billet containing specific amounts of Mn to hot rolling and to cooling under respectively controlled conditions. CONSTITUTION:A billet of a steel having a composition containing, by weight, 0.20-0.60% C, 0.10-1.00% Si, and 0.50-1.80% Mn is heated up to 850-1000 deg.C and hot-rolled, and rolling is completed at Ac1-850 deg.C. The steel is quenched so that the surface temp. of the steel after the final rolling is regulated to 50-500 deg.C and then the steel surface is recuperated to 400-650 deg.C by means of the internally retained heat of the steel, by which fine ferrite, granular carbide, and lamellar carbide are formed in the surface layer part. Owing to the above metallic structure, the soft surface layer part can be obtained. If necessary, internal strength can be increased by regulating the surface temp. to <400 deg.C by applying quenching again. |
