| 摘要 |
PROBLEM TO BE SOLVED: To omit a normalizing treatment after hot-forging by providing a producing method of a carburized steel having the same characteristics as the micro-structure, hardness and crystal grain size obtained after executing the normalizing treatment in the hot-forging state. SOLUTION: The steel composed of wt.% of 0.1-0.35% C, 0.05-0.5% Si, 0.2-2.9% Mn, 0.005-0.05% Al and further, 0.1-0.3% Ti and/or Nb and the balance Fe with inevitable impurities is heated at 900-1150 deg.C, and hot-worked at <=1,150 deg.C. Further, the heating temp. and the hot-forging temp. are regulated to <=1,150 deg.C, and the cooling is executed at 0.2-2 deg.C/sec cooling speed and thus, the structure after hot-forging is made to <=5% bainite structure and the balance ferrite-pearlite structure, and the hardness thereof is made to <=220 Hv, and when the carburizing treatment is executed at 950 deg.C, the austenite crystal grain size is regulated to No.8 or higher and the different crystal coarse grain size of No.3 or higher is regulated so as not to exceed 20%.
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