| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having fatigue characteristics and burring workability (hole expandability) and having tensile strength of >=540 MPa and to provide a production method for inexpensively and stably producing the steel sheet. SOLUTION: This steel sheet with a strain induced transformation type composite structure having excellent burring workability is composed of steel containing 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, <=0.1% P, <=0.01% S and 0.005 to 1% Al, and whose microstructure is composed of the composite one containing retained austenite of 5 to 25% by volume fraction, and the balance mainly ferrite and bainite, in which the value obtained by dividing the volume fraction of retained austenite by its average grain size is 3 to 12, and also, the value obtained by dividing the average value of the hardness of retained austenite by the average value of the hardness of ferrite is 1.5 to 7, and, in the method for producing the same steel sheet, steel having the above components is subjected to hot finish rolling so as to be finished at the Ar3 transformation point temperature to the Ar3 transformation point temperature +100 deg.C, is thereafter retained at the temperature range of the Ar1 transformation point temperature to the Ar3 transformation point temperature in 1 to 20 seconds and, is cooled at a cooling rate of >=20 deg.C/s and is coiled at a coiling temperature at the temperature range of >350 to <450 deg.C.
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