High-strength hot-rolled steel sheet and a manufacturing method thereof having excellent toughness

The invention provides a high strength hot rolled steel sheet which exhibits excellent toughness in spite of the tensile strength being increased to 780 MPa or above. The steel sheet includes, in terms of mass%, C : 0.04 to 0.12%, Si : 0.5 to 1.2%, Mn : 1.0 to 1.8%, P : not more than 0.03%, S : not more than 0.0030%, Al : 0.005 to 0.20%, N : not more than 0.005% and Ti : 0.03 to 0.13%, the balance being Fe and inevitable impurities, includes a microstructure containing a bainite phase at an area fraction exceeding 95%, the bainite phase having an average grain diameter of not more than 3 µm, has a difference ”Hv1 of not more than 50 between a Vickers hardness value at 50 µm from the surface and a Vickers hardness value at 1/4 of a sheet thickness, has a difference ”Hv2 of not more than 40 between the Vickers hardness value at 1/4 of the sheet thickness and a Vickers hardness value at 1/2 of the sheet thickness, the sheet thickness being not less than 4.0 mm and not more than 12 mm, and has a tensile strength of not less than 780 MPa.