| abstract |
The carbon equivalent having a predetermined chemical composition and represented by a specific formula is within a predetermined numerical range, and the metallographic structure at a 1/4 position (a quarter thickness of the plate thickness direction) from the surface of the steel plate is Electron beam of all phases consisting of 10% to 75% of ferrite phase, 10% to 90% of bainite phase, 0% to 15% of pearlite phase, and 0% to 1% of martensite / austenite mixed phase in area fraction An oxide, a sulfide, and an acid having an average particle diameter (diameter) of at most 20 μm as measured by backscattering diffraction and contained in 1/4 part of a plate thickness and having an equivalent circle diameter (diameter) of at least 2 μm The total position of sulfides is 50 pieces / mm 2 or less, and 1⁄2 position in the thickness direction from the surface of the steel plate to the Mn concentration (Mn (1⁄4)) at 1⁄4 of the thickness The ratio (Mn (1/2) / Mn (1/4)) of the Mn concentration (Mn (1/2)) in Steel plate which is 0.90 to 1.80. |