CPC C21D 9/0081 (2013.01) [C21D 1/18 (2013.01); C21D 1/84 (2013.01); C21D 6/004 (2013.01); C21D 6/005 (2013.01); C21D 6/008 (2013.01); C21D 8/005 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/42 (2013.01); C22C 38/44 (2013.01); C22C 38/46 (2013.01); C22C 38/48 (2013.01); C22C 38/50 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] | 10 Claims |
1. A steel material comprising:
in weight %, carbon (C): 0.10 to 0.25%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, aluminum (Al): 0.005 to 0.1%, phosphorus (P): 0.010% or less, sulfur (S): 0.0015% or less, niobium (Nb): 0.001 to 0.03%, vanadium (V): 0.001 to 0.03%, titanium (Ti): 0.001 to 0.03%, chromium (Cr): 0.01 to 0.20%, molybdenum (Mo): 0.01 to 0.15%, copper (Cu): 0.01 to 0.50%, nickel (Ni): 0.05 to 0.50%, calcium (Ca): 0.0005 to 0.0040%, with a balance Fe and unavoidable impurities,
wherein a microstructure in a center in a range of t/4 to t/2 (where t indicates ae thickness of a steel plate) consists of 35 to 40% of ferrite and a remainder of bainite composite structure in area %, a packet size of the bainite is 10 μm or less, and a porosity of the center is 0.1 mm3/g or less,
a depth of a surface crack is 0.5 mm or less, and
a center section hardness is 200 HB or less.
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