The steel pipe not only satisfies predetermined expression, but also regulates chemical composition, and satisfies following conditions (A)-(C): (A) the central portion for removing the surface portion 2mm from the front and the back face of the steel pipe has an average Vickers hardness from 230 to 310; (B) in microscopic constitution of steel pipe, bainitic ferrite structure is over 80% in area, martensitic phase is below 5% in area; (C) average Vickers hardness Hv of the surface portion 2mm from the front and the back face of the steel pipe is below 1.3 times of the average hardness Hv of the central portion. According to the construction, high strength and low yield ratio can be realized in the mean time, tractility can be ensured by reducing hardness on external surface of the steel pipe due to bending machining in steel pipe forming, cracking resistance in welding can be improved because of improved shock resistance for steel pipes with highest tensile strength at 780MPa for building reinforcement.