Manufacturing method of a cold workability and low carburizing distortion characteristics superior hardened steel

摘要

PROBLEM TO BE SOLVED: To provide a case hardening steel excellent in cold workability, capable of stably inhibiting the formation of coase grains at the time of carburizing, and resultantly capable of preventing the occurrence of strain and warpage after carburizing and quenching, and its production. ;SOLUTION: A steel has a composition containing, by weight, 0.1-0.3% C, 0.1-0.15% Si, 0.2-0.65% Mn, 0.005-0.15% S, 0.4-1.25% Cr, 0.0005-0.005% B, 0.015-0.05% Al, 0.002-0.035% Nb, 0.005% N, and Ti in an amount of 0.015 to 3.4N+0.02% according to N content. In this steel, the amount of precipitation of AlN is limited to ≤0.005%; ≤0.2 μm diameter Nb precipitates, Ti precipitates, or precipitates with a compound composition of Nb and Ti are contained in a matrix by ≥20 pieces/100 μm² in total; the structure fraction of bainite is limited to ≤15%; and the score of ferrite banding of the cross-section is parallel to the heat rolling direction is 1 to 5. As to the method of production, heating temperature of the above steel and hot rolling finishing temperature are regulated to ≥1,150°C and 840 to 1,000°C, respectively, and, in succession to hot rolling, slow cooling is carried out through the temperature region from 800 to 500°C at a cooling rate ≤1°C/sec.;COPYRIGHT: (C)1999,JPO