PURPOSE: To produce a high strength hot rolled steel sheet for working excellent in workability and fatigue properties and excellent in endurance strength while its expansion flanging properties are maintained by adding a specified amt. of Cu to a steel having a specified compsn. and executing hot rolling under specified conditions.
CONSTITUTION: A steel contg., by weight, 0.01 to 0.15% C, 0.5 to 2.0% Si, 0.7 to 2.0% Mn, 0.01 to 0.06% Al, ≤0.005% S and 0.4 to 1.5% Cu is subjected to hot rolling. At this time, the hot rolling is executed so as to regulate the hot rolling finishing temp. to the Ar3 point or above, and cooling is executed at the average cooling rate of 5 to 70°C/sec from the hot rolling finishing temp. to the temp. in the range of the Ar3 point to the Ar1 point. Next, it is subjected to air cooling or gradual cooling from the same temp. for 2 to 20sec, is thereafter cooled to the temp. range of 400 to 550°C at the average cooling rate of 20 to 70°C/sec and is coiled. Then, the steel sheet with a composite structure in which the structure after the hot rolling is constituted of three phases of ferrite, bainite and martensite, and 4 to 45% Vfb bainite area ratio, 1 to 15% Vfm martensite area ratio and Vfb/Vfm≥3 are satisfied can be produced.
MIYOSHI TETSUJI
YOKOI TOSHIO
