| 摘要 |
PROBLEM TO BE SOLVED: To provide steel types and its production method where, in a high-strength steel material having a high yield ratio, materials can be produced individually per strength level according to production conditions in an easy way even on the same steel type. SOLUTION: A steel having a chemical composition comprising, by mass, 0.05 to 0.45% C,≤1.0% Si, 0.2 to 3.0% Mn,≤0.1% P,≤0.02% S,≤0.2% Al and≤0.01% N, and the balance Fe with impurities is made into an austenite single phase state, is thereafter cooled to a temperature range below an Ms point at a cooling rate of the upper critical cooling rate or above, and is then subjected to tempering treatment under the conditions where the parameterλprescribed by the following formula reaches 11,000 to 16,500:λ=TäLog(t)+20}, wherein T is a tempering temperature (K), and (t) is a tempering time (h), and, the tempering treatment is preferably performed under the conditions whereλsatisfies the following inequality: (3,700×C-1.1×TS<SP>QT</SP>+1,760)/0.11≤λ≤(3,700×C-0.9×TS<SP>QT</SP>+1,760)/0.11; wherein, C is the C content (mass%) in the steel, and TS<SP>QT</SP>is its tensile strength (MPa). COPYRIGHT: (C)2008,JPO&INPIT
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