| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet satisfying yield strength in a class of≥650 MPa and having low acoustic anisotropy, and to provide a production method where off-line heat treatment for the steel sheet is not required. SOLUTION: The steel sheet has a composition comprising 0.04 to 0.09% C, 0.01 to 0.6% Si, >2.00 to 3.00% Mn, and 0.0005 to 0.005% B, and comprising Nb and Ti so as to satisfy 0.045%≤Nb+2Ti≤0.090%, and in which A=(Nb+2Ti)×(C+N×12/14) satisfies 0.0025 to 0.0055, and has a structure composed of≥80% bainite. In the method for producing the steel sheet, the componential steel is heated at T(=6300/(1.9-Log(A))-273) to 1,300°C, and is subjected to rolling so as to be rough-rolled at≥1,020°C, thereafter to be rolled at a cumulative draft of≤15% in the range from >920 to <1,020°C, to be rolled at a cumulative draft of 20 to 50% in the range from 860 to 920°C, and to be rolled at a finishing temperature of 860°C, successively, accelerated cooling is started from≥800°C, and the cooling is stopped at 420 to 570°C. COPYRIGHT: (C)2007,JPO&INPIT
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