| 摘要 |
PURPOSE: A process for preparing Cr-Mo steel and Cr-Mo-B steel used for round steel by suppressing impurities exerting a bad influence on fatigue strength and performing vacuum degassing for a long time is provided, thereby producing steel for low temperature bolt and nut having improved low temperature impact characteristics. CONSTITUTION: The titled steel comprises C: 0.40 to 0.44wt.%, Si: 0.15 to 0.35wt.%, Mn: 0.75 to 0.90wt.%, P: 0.030wt.%, S: 0.030wt.%, Cu: 0.30wt.%, Ni: 0.20 to 0.25wt.%, Cr: 0.80 to 1.10wt.%, Mo: 0.20 to 0.25wt.%, Al: 0.037 to 0.020wt.%, N2: 100ppm or less and the balance of Fe and inevitable impurities. The steel is prepared by the process consisting of: preparing steel; hot-rolling at 1,050±25deg.C at finish rolling temperature of 925±25deg.C; oil-quenching after quenching heat treatment at 830 to 880deg.C; and air- or water-quenching after tempering heat treatment at 640 to 690deg.C.
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