SOFTENING HEAT TREATMENT METHOD OF STEEL MATERIAL EXCELLENT IN COLD FORGEABILITY AND CRYSTAL GRAIN COARSENING RESISTANCE

PROBLEM TO BE SOLVED: To provide a softening heat treatment method of a steel material having a bainite structure and nonetheless excellent in cold forgeability and crystal grain coarsening resistance.SOLUTION: The softening heat treatment method is provided for softening a steel material containing, by mass%, C:0.10 to 0.30%, Si:0.30% or less, Mn:0.40 to 1.20%, P:0.03% or less, S:0.03% or less, Cu:0.50% or less, Ni:0.50% or less, Cr:1.60% or less, Mo:0.10 to 0.50%, B:0.0005 to 0.0050%, N:0.002 to 0.020%, one or more kind of Al:0.01 to 0.05%, Nb:0.02 to 0.10% and Ti:0.01 to 0.05% and the balance Fe with inevitable impurities. In the softening heat treatment method, the steel material having a percentage of a bainite structure before the softening heat treatment of 10% or more is heated at (A3 transformation point+10°C)-(A3 transformation+200°C), and then cooled to (A1 transformation point °C)-(A1 transformation point-50°C), a cooling rate in the cooling being set to be 13°C/h or less.SELECTED DRAWING: Figure 1 【課題】ベイナイト組織を有しても、冷間鍛造性及び耐結晶粒粗大化特性に優れた鋼材の軟化熱処理方法を提供する。【解決手段】質量％で、C:0.10-0.30%,Si:0.30%以下,Mn:0.40-1.20%,P:0.03%以下,S:0.03%以下,Cu:0.50%以下,Ni:0.50%以下,Cr:1.60%以下,Mo:0.10-0.50%, B:0.0005-0.0050%,N:0.002-0.020%と、Al:0.01-0.05%,Nb:0.02-0.10%,Ti:0.01-0.05%の何れか1種又は2種以上とを含有し、残部がFe及び不可避不純物からなる鋼材を軟化させる軟化熱処理方法であって、軟化熱処理前のベイナイト組織の比率が10％以上の鋼材に対し、(A3変態点+10℃）-(A3変態点+200℃)で加熱した後、(A1変態点℃)-(A1変態点-50℃)までの冷却速度を13℃/h以下に設定する。【選択図】図１