| 摘要 |
The first invention provides a steel product which, while minimizing an increase in hardness after forging to ensure machinability and cold workability, is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened part, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength. The highly machinable and high strength steel for induction hardening comprises, by mass, carbon (C): not less than 0.40% and less than 0.50%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use, the steel satisfying a carbon equivalent (C<SUB>eq</SUB>) requirement represented by 0.75<=C<SUB>eq</SUB><=0.90, and a machinability index (a value) requirement represented by a value <=0.62. The second invention provides a steel product that is excellent in cold workability after hot forging and is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened portion, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength. The steel for induction hardening, having improved cold workability, rolling fatigue life in its hardened part, and bending fatigue strength in its non-hardened part comprises, by mass, carbon (C): 0.40 to 0.60%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.00%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, the percentage ferrite area after hot forging of the steel being not less than 15%, not less than 30% of the ferrite being accounted for by ferrite having a major axis/minor axis ratio of not more than 5, the steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2): <?in-line-formulae description="In-line Formulae" end="lead"?>C<SUB>eq</SUB>=C %+( 1/7)Si %+(1/5)Mn %+( 1/9)Cr %-( 5/7)S %+V % (1) <?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>0.75<=C<SUB>eq</SUB><=0.90 (2) <?in-line-formulae description="In-line Formulae" end="tail"?> wherein C<SUB>eq </SUB>represents the equivalent of carbon.
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