| 摘要 |
PURPOSE:To obtain the titled steel bar having high toughness as cooled in the air after hot forging without carrying out quenching followed by tempering by providing a specified composition consisting of C, Si, Mn, Cr, Al and Fe. CONSTITUTION:This unnormalized steel bar for hot forging consists of, by weight, 0.05-0.15% C, 0.10-1.00% Si, 0.60-3.00% Mn, 2.20-5.90% Cr+Mn, 0.01-0.05% Al and the balance essentially Fe. The steel has necessary strength and high toughness such as about 70-90kg/mm.<2> tensile strength and >= about 5kg-m/cm<2> impact value as cooled in the air after hot forging. Among said alloying components, C is required to attain said strength, yet >0.15% C makes the strength excessively high and reduces the toughness. >1.00% Si makes the strength excessively high. Mn is required to control the strength and toughness, yet >3.00% Mn deteriorates the machinability. Cr is required to attain said strength and toughness in combination with Mn, and Al is a necessary element acting as a deoxidizer.
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