| 摘要 |
PURPOSE: A fine grain type ferritic steel for welded structure having superior toughness in weld heat affected zone is provided to improve toughness of the weld heat affected zone by uniformly distributing fine TiN precipitate and Ti-Nb composite precipitate on base metal as increasing fine ferrite phase fraction, and a method for manufacturing the same are provided. CONSTITUTION: The method for manufacturing fine grain type ferritic steel for welded structure having superior toughness in weld heat affected zone comprises the processes of heating a steel slab comprising 0.02 to 0.18 wt.% of C, 0.05 to 0.5 wt.% of Si, 0.4 to 2.0 wt.% of Mn, 0.005 to 0.2 wt.% of Ti, 0.005 to 0.1 wt.% of Al, 0.008 to 0.030 wt.% of N, 0.0003 to 0.01 wt.% of B, 0.03 wt.% or less of P, 0.03 wt.% or less of S, 0.005 wt.% or less of O, 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<=7, 0.3<=Nb/N<=9, 7<=(Ti+2Al+4B+Nb)/N<=17 and a balance of Fe and other impurities in the temperature range of 1,000 to 1,150 deg.C for 60 to 180 minutes; cooling the heated steel slab to the austenite non-recrystallization temperature range of 880 to 930 deg.C at a cooling rate of 10 to 20 deg.C/sec by controlling size of austenite grains to 20 μm or less; rolling the cooled steel slab to a reduction ratio of 50% or more, and hot rolling the rolled steel sheet to a rolling ratio of 40% or more in the two-phase ferrite and austenite region temperature range of 740 to 780 deg.C at a cooling rate of 5 to 15 deg.C/sec; and cooling the hot rolled steel sheet to an ordinary temperature at a cooling rate of 5 to 20 deg.C/sec.
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