Substituting heat HAZ toughness with excellent ultra-low carbon bainite steel and its manufacturing method

摘要

PURPOSE: A non-heat treated bainite steel plate is provided which is capable of improving toughness of heat affected zone when high heat input welding a bainite steel plate in which fine TiN precipitate is uniformly distributed, and a method for manufacturing the same is provided. CONSTITUTION: The method comprises a step of heating a slab comprising 0.001 to 0.03 wt.% of C, 0.01 to 0.5 wt.% of Si, 1.0 to 3.0 wt.% of Mn, 0.005 to 0.2 wt.% of Nb, 0.005 to 0.2 wt.% of Ti, 0.005 to 0.1 wt.% of Al, 0.0005 to 0.01 wt.% of B, 0.008 to 0.030 wt.% of N, 0.03 wt.% or less of P, 0.03 wt.% or less of S, 0.005 wt.% or less of O and a balance of Fe and other impurities and satisfying the following relations: 1.2=Ti/N=2.5, 2=N/B=20, 2.5=Al/N=7, 3=Nb/N=10 and 10=(Ti+2Al+4B+Nb)/N=20 in the temperature range of 1,000 to 1,250 deg.C for 60 to 180 minutes; a step of hot rolling the heated slab at a reduction ratio of 40% or more in the austenite non-recrystallization region; and a step of air cooling the hot rolled steel plate to an ordinary temperature or cooling the hot rolled steel plate to an ordinary temperature in a cooling rate of 10 deg.C/sec or less.