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.