| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile-strength hot-rolled steel sheet which has high strength, superior workability, and superior surface properties because of preventing an island scale from forming thereon. SOLUTION: The high-tensile-strength hot-rolled steel sheet has a chemical composition comprising 0.05-0.30% C, less than 0.5% Si, 0.5-3.0% Mn and 0.1-2.0% Al while controlling Si+Al to 0.5 to 2.0%, and the balance Fe with impurities; and has a structure containing an austenite of 5% or more and a polygonal ferrite of 60% or more by a volume fraction. The austenite has an average grain size of 2.0μm or smaller, and the polygonal ferrite has the average grain size of more than 1.0μm but 3.0μm or less. The austenite also contains 0.7 to 2.0% C. The steel sheet may further contain one or more elements selected from at least one group of the following groups (1) and (2): (1) 0.005-0.10% Nb, 0.005-0.20% Ti and 0.005-0.20% V; and (2) 0.0002-0.01% Ca, 0.002-0.10% Zr and 0.002-0.10% REM. COPYRIGHT: (C)2007,JPO&INPIT
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