| 摘要 |
PROBLEM TO BE SOLVED: To obtain a high tensile strength hot-dip galvanized steel sheet having sufficient strength-elongation balance and excellent fatigue characteristics required of a material for automotive parts. SOLUTION: A steel sheet used as a material has a composition which contains, by mass, 0.05 to 0.25% C,≤1.0% Si, 0.5 to 3.0% Mn and≤2.0% Al and in which the sum of Si and Al is made to be 0.4 to 2.0 mass%, and has a steel structure which is constituted of a structure containing ferrite as a main phase and a low temperature transformed phase such as bainite and martensite and in which bainite occupies≥80% of the low temperature transformed phase. The steel sheet is subjected to final annealing where it is held at 700 to 950°C for 5 to 120 sec for annealing and then cooled to≤500°C at a rate of≥5°C/sec, subjected to hot-dip galvanizing treatment or further to alloying treatment, and then cooled to 300°C at a rate of≥5°C/sec. By this method, the steel sheet which is constituted of a composite structure consisting of the ferrite as a main phase, retained austenite of≥3% by volume fraction, and the low temperature transformed phase and in which retained austenite having an aspect ratio ranging from 0.2 to 0.4 occupies≥70% of the above retained austenite can be obtained. COPYRIGHT: (C)2004,JPO&NCIPI
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