CPC C21D 9/46 (2013.01) [C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0263 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/32 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] | 3 Claims |
1. A steel sheet having impact resistance, comprising, in wt %:
0.05 to 0.12% of C, 0.01 to 0.5% of Si, 0.8 to 2.0% of Mn, 0.01 to 0.1% of Al, 0.005 to 1.2% of Cr, 0.005 to 0.5% of Mo, 0.001 to 0.01% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.001 to 0.03% of Nb, 0.005 to 0.03% of Ti, 0.001 to 0.2% of V, 0.0003 to 0.003% of B, and a remainder of Fe and unavoidable impurities,
a microstructure comprises tempered martensite as a main structure, and a remainder of tempered bainite,
the number of one or more of carbides and nitrides having a diameter of 0.1 μm or more per circle observed in a 1 cm2 unit area is 1×103 or less,
the number of precipitates having a diameter of 50 nm or more including one or more of Ti, Nb, V, and Mo observed in a 1 cm2 unit area is 1×107 or less,
the steel has a difference in hardness between a t/2 position and a t/4 position to 30 Hv or less, based on the thickness (t),
the steel sheet has a yield strength of 900 MPa or more, and a Charpy V-Notched energy at −40° C. of 30 J or more, and
the contents of Mn, Cr and Mo satisfy the following Relational expression 1:
T=Mn/(Cr+Mo), 1.0≤T≤3.0 [Relational expression 1].
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