| 摘要 |
PURPOSE: To produce a non-heat treated steel excellent in fatigue strength by specifying the componental compsn. of a steel and specifying the heating temp. at the time of forging and the subsequent cooling rate. CONSTITUTION: A steel having a compsn. contg., by weight, 0.20 to 0.28% C, 0.05 to 0.50% Si, 0.5 to 1.5% Mn, 0.5 to 1.5% Cr, 0.05 to 0.30% V, 0.03 to 0.30% Pb and 0.4 to 0.20% S, furthermore satisfying each condition of 0.43×C%+0.49×Mn%+0.27×Cr%<=0.85 and 100×V%+20×Cr%--83×C %-17×S%-3×Mn%>=1.74, and the balance Fe with inevitable impurites is subjected to hot forging at 900 to 1100 deg.C. Next, it is cooled in the temp. range of 800 to 500 deg.C at 2 to 6 deg.C/sec average cooling rate to form its structure into a mixed one contg. ferrite and pearlite and to regulate its tensile strength to 70 to 94kgf/mm<2> . Thus, the non-heat treated steel having high strength and excellent fatigue strength and good in machinability can be obtd.
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