| 摘要 |
PROBLEM TO BE SOLVED: To manufacture a high strength steel product excellent in fatigue crack propagation resistance. SOLUTION: In the high strength steel product excellent in fatigue crack propagation resistance, a ferritic structure of <=100 Vickers hardness is allowed to exist by 100-50% by area ratio in a bainitic structure with 200-500 Vickers hardness, or a martensitic structure, or a mixed structure of bainite and martensite. The steel product having such a structure can be manufactured by performing, in reheating-hardening-tempering treatment or in direct hardening-temperature treatment, heating up to a temperature ranging from 650 deg.C to the Ac1 transformation point after proper hardening and then performing tempering of 18,000-21,000 tempering parameter to recrystallize a hardened structure.
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