| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet excellent in bendability and low temperature toughness and a manufacturing method therefor.SOLUTION: There is provided a high strength hot rolled steel sheet excellent in bendability and low temperature toughness by making a composition containing, by mass%, C:0.020% to 0.080%, Si:0.05% to 0.50%, Mn:1.20% to 2.20%, P:0.001% to 0.020%, S:0.0001% to 0.0050%, Al:0.005% to 0.050%, N:0.0010% to 0.0060%, Nb:0.040% to 0.080%, Mo:0.01% to 0.50%, Ti:0.005% to 0.050%, Cr:0.01% to 0.50%, Ca:0.0005% to 0.0050% and the balance Fe with inevitable impurities, and a structure having an area ratio of a tempered martensitic phase of 90% to 100% and an average crystal particle diameter of the tempered martensitic phase of 1.0 μm to 5.0 μm at a position of 1.5 mm from a surface of a steel sheet in a sheet thickness direction, a fluctuation amount of a Vickers hardness in a sheet width direction &Dgr;Hv of 50 or less, the area ratio of a bainitic phase of 90% to 98%, the area ratio of a martensitic phase of 1% to 5%, and the average crystal particle diameter of the martensitic phase of 0.5 μm to 5.0 μm and the area ratio of a retained austenitic phase of 1% to 5% at a sheet thickness center position of the steel sheet and a thickness of an iron oxide coating of the surface of the steel sheet of 0.1 μm to 10 μm. |
