| 摘要 |
PROBLEM TO BE SOLVED: To produce 60 kilo class direct-quenched and tempered steel excellent in weldability and toughness after strain aging. SOLUTION: This steel has a chemical compsn. satisfying Pcm <=0.20% and Ceq (WES)<=0.42% and a structure in which ferrite is precipitated into a film shape on old austenitic grain boundaries and preferably has a structure in which the thickness of ferrite precipitated into a film shape is 0.1 to 5μm, and the occupancy ratio of old austenitic grain boundaries is->=20%, where the grain boundary occupancy ratio (%)=ΣLn/the whole length of old austenitic grain boundaries*100,ΣLn: the whole length of film-shaped ferrite, Pcm=C+Mn/20+Si/30+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and Ceq (WES)=C+Mn/6+ Si/24+Ni/40+Cr/5+Mo/4+V/14.
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