| 摘要 |
PROBLEM TO BE SOLVED: To produce thick 60 kilo class steel excellent in low temperature toughness and weldability and to provide a method for producing the same. SOLUTION: This 60 kilo class high tensile strength steel excellent in weldability an toughness satisfies, by mass, P cm<=0.2% and Ceq (WES)<=0.42%, preferably has components containing, by mass, 0.04 to 0.09% C, 0.1 to 0.5% Si, 1.2 to 1.8% Mn, 0.1 to 0.5% Cr, 0.01 to 0.05% Nb, 0.002 to 0.07% sol.Al and 0.001 to 0.005% N, also satisfying 0.50<=Si+3Cr<=1.25%, and the balance iron with inevitable impurities as a steel composition and has a structure essentially consisting of bainite with a bainitic lath width of 2μm or less, and in which filmy ferrite with a thickness of 0.1 to 5μm is precipitated on the old austenitic grain boundaries in an occupancy ratio of 20% or more.
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