The steel in the invention has specified chemical composition with the CEN valve specified by the following formula (1) in the specified range; the microscopic constitution composition of the steel is 65-85 volume% multilateral ferrite phase, 5-20 volume% strip pearlite phase, 3-15 volume% granulose tempered martensite phase existed in grain boundary ferrite; and the average equivalent circle diameter of the multilateral ferrite phase is 10-40 [mu]m. CEN=[C]+A(c).{[Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}...(1); wherein, A(c)=0.75+0.25.tanh{20[C] -0.12}, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb] and [V] respectively represent content (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb and V. According to the composition, like suiting for cold-forming round steel pipe, although the steel plate is exerted on strong processing (bending processing, tensile processing, compressing processing etc.) and does not execute SR processing, the tensile ratio is stable below 85% and has excellent toughness and weldability to be high-tension steel with tensile strength above 490 MPa.