| 摘要 |
PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet for working, excellent in fatigue characteristic and suitable for use as a stock requiring coexistence of durability and workability, such as for automobile under carriage parts and road wheel, and its manufacturing method. SOLUTION: The hot rolled steel sheet for working, excellent in fatigue characteristic, is composed of a dual-phase steel sheet having ferrite containing, by mass, 0.03-0.20% C, 0.2-2.0% Cu, and 2-20 ppm B as a principal phase and also having martensite and bainite as a secondary phase. Moreover, the state of existence of Cu in the ferritic phase is a solid-solution and/or precipitated state where the size of grains constituted of Cu alone is regulated to <=2 nm. This steel sheet can be manufactured by finishing the hot finish rolling of a steel with the above composition in the temperature region not lower than the Ar3 transformation point, holding the resultant steel sheet for 1 to 10 sec in the temperature region between the Ar3 and the Ar1 transformation point, applying cooling at >=20 deg.C/s cooling rate, and then coiling the steel sheet at <=350 deg.C coiling temperature.
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