| 摘要 |
PROBLEM TO BE SOLVED: To provide a means for improving strength/toughness of a tempered high-tensile-strength steel sheet, by restraining formation of coarse austenite through controllingγ' transformation occurring in hardening and by improving the hardenability, in a process of manufacturing the tempered high-tensile- strength steel sheet having tensile strength of higher than a 570-MPa grade, which is produced through re-heating, quenching and tempering. SOLUTION: When subjecting an appropriately processed steel having a chemical composition, to quenching and tempering steps consisting of quenching it once or more times at a quenching temperature between an AC<SB>3</SB>transformation point and 1,000°C, or conducting multiplex quenching as needed, and then tempering it at a tempering temperature between 450°C and an AC<SB>1</SB>transformation point, this manufacturing method is characterized by making a structure of the steel sheet before quenching have either (1) 10% or more of ferrite fraction, or (2) 20μm or less of an average grain size of former austenite, and thereby restraining generation of coarse austenitic grains through controllingγ' transformation, and improving hardenability, to obtain adequate strength/ toughness. COPYRIGHT: (C)2003,JPO
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