| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for attaining an ultrafine-grained structure of 1-3μm average ferrite grain size over the whole thickness of steel and obtaining high-tensile steel combinedly having extremely excellent toughness, brittle crack arrest property, and fatigue characteristics of welded joint and having about >=490 MPa class tensile strength. SOLUTION: A manufacturing method wherein, as a basic requirement, the temperature of the steel during rolling stands within the range from (Ac1 transformation point - 50 deg.C) to (Ac3 transformation point - 50 deg.C) is applied to the steel having a microstructure with a prescribed component composition when the steel is heated to a temperature between 300 deg.C and rolling temperature at (0.1 to 50) deg.C/s temperature-rise rate and rolling is started in the course of temperature raise from the Ac1 transformation point to (Ac3 transformation point - 100 deg.C) or after holding at this temperature for 1-60 s and then rolling of 50-90% cumulative draft is applied. By this method, the ultrafine-grained structure of 1-3μm minimal in the degree of duplex grain can be obtained, and extremely excellent toughness, brittle crack arrest property, and fatigue characteristics at welded joint can be attained.
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