| 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:1.00% to 2.00%, 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%, Ti:0.005% to 0.050%, Ca:0.0005% to 0.0050% and the balance Fe with inevitable impurities, and a structure having an area ratio of a ferrite phase of 20% to 65%, the area ratio of bainitic ferrite phase of 20% to 65%, the area ratio of a martensitic phase of 1% to 5% at a position of 0.2 mm from a surface of the steel sheet in a sheet thickness direction, an average crystal particle diameter of the bainitic ferrite phase of 0.5 μm to 4.0 μm at a position of 25 μm from the surface of the steel sheet in the sheet thickness direction, the area ratio of the bainitic ferrite phase of 90% to 98%, the area ratio of the martensitic phase of 1% to 5% and the average crystal particle diameter of the martensitic phase of 0.5 μm to 4.0 μm and the area ratio of a retained austenitic phase of 1% to 5% at a sheet thickness center position of the steel sheet. |
