| 摘要 |
PURPOSE: A method for manufacturing a steel plate for welded structure is provided which not only minimizes a toughness difference between a base metal and a weld heat-affected zone in the welding heat input ranging from medium heat input to ultra high heat input, but also accomplishes high strength of the base metal using TiN precipitate and Mg-Ti complex oxide. CONSTITUTION: The high strength steel plate for welded structure having TiN precipitate and Mg-Ti complex oxide comprises 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, 0.008 to 0.030 wt.% 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 and 0.001 to 0.005 wt.% of Mg satisfying 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<=7, 6.5<=(Ti+2Al+4B)/N<=14, 4<=Ti/O<= 10, 0.2<=Mg/O<=3 and 3<=(Ti+Mg)/O<=12, 30 to 80 wt.% of bainite, and a balance of ferrite which is 20 microns or less. The method for manufacturing the high strength steel plate for welded structure having TiN precipitate and Mg-Ti complex oxide comprises the processes of heating 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, 0.008 to 0.030 wt.% 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 and 0.001 to 0.005 wt.% of Mg satisfying 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<=7, 6.5<=(Ti+2Al+4B)/N<= 14, 4<=Ti/O<=10, 0.2<=Mg/O<=3 and 3<=(Ti+Mg)/O<=12 to the temperature range of 1100 to 1250 deg.C for 60 to 180 minutes; hot rolling the heated steel slab to a rolling ratio of 40% or more in the Austenite recrystallization zone; and cooling the hot rolled steel slab to the bainite transformation finishing temperature±10 deg.C in a cooling rate of 5 to 20 deg.C/sec.
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