STEEL SHEET EXCELLENT IN BENDING WORKABILITY, IMPACT PROPERTY AND TENSILE PROPERTY, AND MANUFACTURING METHOD THEREOF

摘要

PURPOSE: A steel sheet having an excellent bendability, an impact property, and tensile property and a manufacturing method thereof are provided to provide the same with a high yield strength, tensile strength, base material toughness, bendability, and toughness after a bending process even if the same is thick. CONSTITUTION: A steel sheet having an excellent bendability, an impact property, and tensile property includes 0.02 to 0.05 mass% of C, 0.10 to 0.40 mass% of Si, 1.85 to 2.50 mass% of Mn, below 0.012 mass% of P, below 0.005 mass% of S, 0.020 to 0.050 mass% of Nb, 0.005 to 0.020 mass% of Ti, 0.0020 to 0.0060 mass% of N, 0.010 to 0.060 mass% of Al, and remnants Fe and inevitable impurities. A weld crack susceptibility composition (Pcm) in steel for the steel sheet is below 0.20%, and is defined as an equation (1) below. In the whole structure of the steel, an acicular ferrite fraction is above 70 area%, the average crystal grain size (circular diameter) of the whole structure is below 7 um, and a martensite-austenite constituent (MA) fraction is below 0.5 area%. The maximum value of Vickers hardness on the surface part of the steel sheet is below 220. The equation above is Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr /20 + Mo /15 + V/10 + 5 B. In the equation, C, Si, Mn, Cu, Ni, Cr, Mo, V, and B represent the contents (mass%) of corresponding elements. [Reference numerals] (AA) Acicular ferrite fraction (area %)