| 摘要 |
PROBLEM TO BE SOLVED: To provide a bainitic non-heat-treated steel sheet having small acoustic anisotropy, superior toughness and a high tensile-strength of 570 MPa or higher. SOLUTION: The high-strength and high-toughness bainitic non-heat-treated steel sheet with the small acoustic anisotropy comprises 0.02-0.08% C (by mass%, hereafter the same), 0.05-0.50% Si, 1.0-2.5% Mn, 0.015% P or less (excluding 0%), 0.006% S or less (excluding 0%), 0.50-1.50% Cr, 0.005-0.030% Ti, 0.0030-0.0080% N, 0.005% Nb solid solution or less (excluding 0%), 0.0006 to 0.0021% B solid solution, and the balance iron with unavoidable impurities; includes a former austenitic crystal grain having an average aspect ratio (average grain size in main rolling direction/average grain size in sheet thickness direction) of more than 1.8 but 5.3 or less in a cross section in a sheet thickness direction parallel to a main rolling direction; and has a tensile strength of 570 MPa or higher. COPYRIGHT: (C)2007,JPO&INPIT
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