High strength steel

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PROBLEM TO BE SOLVED : To provide a high nitrogen-containing high-strength steel which can be used for applications in steel structures such as a ship, a bridge, a building, a construction machine and the like without using a rare element. SOLUTION : The high-strength steel includes C, Si, Mn, P, S, Al, N, and the balance Fe with unavoidable impurities, has a tensile strength (TS) of 500 MPa or more, contains 0.0040-0.0300 wt.% N, has a separation index (SI value) of 0.10 or less in the V-notch Charpy impact test, and has a ductility-brittleness transition temperature of -50°C or lower. The high-strength steel has a chemical composition including, by mass%, 0.10-0.20% C, 0.10-0.50% Si, 0.60-1.60% Mn, 0.025% or less P, 0.015% or less S, 0.010-0.060% Al, 0.0040-0.0300% N, and the balance Fe with unavoidable impurities. A rolled steel sheet is obtained by rolling the steel material which has the same constitution as the high-strength steel described in any one of the invention 1 to the invention 3. COPYRIGHT : (C)2011, JPO&INPIT