| 摘要 |
PROBLEM TO BE SOLVED: To produce 60 kilo class directly quenched and tempered steel excellent in weldability and toughness after strain aging. SOLUTION: This directly quenched and tempered steel has a chemical composition satisfying Pcm<=0.20% and Ceq (WES)<=0.42% and a structure in which ferrite is filmily precipitated on austenitic grain boundaries and, preferably, has a structure in which the thickness of the filmily precipitated ferrite is controlled to 0.1 to 5μm, and the occupancy ratio of the old austenitic grain boundaries 15 controlled to >=20%, where the occupancy ratio of the grain boundaries (%)=ΣLn/the whole length of the old austenitic grain boundaries*100,ΣLn: the whole length of the filmy 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. |
