| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a non-heat treated steel thick plate with a high tensile strength which has the high strength of≥570 MPa tensile strength, less in variation of quality among test piece extracted places in the steel material and among respective steel materials, has excellent low temperature toughness, and is suitable to a bridge or the like. SOLUTION: The steel having a composition in which the appropriate quantity of Nb and B is added in the extremely low carbon-base is reheated to 1,050 to 1,250°C, then is subjected to a first-rolling at 30-80% accumulated rolling reduction ratio in the austenitic recrystallizing temperature range defined in formula (1) ä1040-0.05(R<SB>X1</SB>-30)<SP>2</SP><T<1160-0.05(R<SB>X1</SB>-30)<SP>2</SP>} and a second-rolling at the accumulated rolling reduction ratio defined in formula (2) ä(80-R<SB>X1</SB>)/(120-R<SB>X1</SB>)<R<SB>X2</SB>/100<(92-R<SB>X1</SB>)/(100-R<SB>X1</SB>)} at the austenitic non-recrystallizing temperature zone of 700-950°C are performed, thereby obtaining a bainite single structure indipendently of a cooling rate by controlling the austenitic state before transforming. In the formula (1), R<SB>X1</SB>is the accumulated rolling reduction ratio (%) in the first-rolling and T is temperature (°C). In the formula (2), R<SB>X2</SB>is the accumulated rolling reduction ratio (%) in the second-rolling. COPYRIGHT: (C)2006,JPO&NCIPI
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