| 摘要 |
PURPOSE: A method of manufacturing high strength steel plate having TiN precipitates and composite oxides of Mg-Ti by nitriding treatment for welded structures is provided to minimize toughness difference between base metal and weld heat affected zone. CONSTITUTION: The method for manufacturing the steel plate for welded structure comprises the processes of casting a steel slab comprising 0.03 to 0.17 wt.% of C, 0.01 to 0.5 wt.% of Si, 0.4 to 2.0 wt.% of Mn, 0.005 to 0.2 wt.% of Ti, 0.0005 to 0.1 wt.% of Al, 0.005 wt.% or less of N, 0.0003 to 0.01 wt.% of B, 0.001 to 0.2 wt.% of W, 0.03 wt.% or less of P, 0.03 wt.% or less of S, 0.002 to 0.03 wt.% of O, 0.001 to 0.005 wt.% of Mg, a balance of Fe and other inevitable impurities wherein the steel plate satisfies the follow inequalities: 0.2<=Mg/O<=3, 3<=(Ti+Mg)/O<=12, 4<=Ti/O<=10; heating the steel slab in the temperature range of 1100 to 1250°C for 60-180 minutes so that N content in the steel slab satisfies 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<=7, 6.5<=(Ti+2Al+4B)/N<= 14 within the nitrogen content range of 0.008 to 0.03%; hot rolling the heated steel slab at a reduction ratio of higher than 40% over austenite recrystallization zone; and cooling the hot rolled steel sheet to a bainite transformation finish temperature±10°C at a cooling rate of 5 to 20°C/sec.
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