Method for Manufacturing Fine Grain Ferritic Steel Having superior HAZ Toughness

摘要

PURPOSE: A method for manufacturing fine grain ferritic steel having superior weld heat affected zone toughness is provided to improve properties of the base metal and toughness of the weld heat affected zone by finely forming TiN precipitate and Ti-Nb composite precipitate on base metal and increasing fine ferrite phase fraction during rolling. CONSTITUTION: The method comprises the steps of manufacturing an ordinary 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, 0.005 wt.% or less of N, 0.0003 to 0.01 wt.% of B, 0.005 to 0.1 wt.% of Nb, 0.03 wt.% or less of P, 0.003 to 0.05 wt.% of S, 0.005 wt.% or less of O and a balance of Fe and other impurities; nitriding the steel slab so that N, Ti, B, Al and Nb contents satisfy the following relational expressions as N content of the steel becomes 0.008 to 0.03 wt.% by heating the manufactured steel slab in the temperature range of 1,000 to 1,150 deg.C for 100 to 180 minutes: 1.2=Ti/N=2.5, 10=N/B=40, 2.5=Al/N=7, 0.3=Nb/N=9, and 7=(Ti+2Al+4B+Nb)/N=17; and cooling the nitrided steel slab at a cooling rate of 10 to 20 deg.C/sec after controlling size of austenite grains of the nitrided steel slab to 20 m or less so that the cooled steel slab is hot rolled to a rolling ratio of 50% or more in the austenite non-recrystallization temperature range of 880 to 930 deg.C, and cooling the rolled steel slab to an ordinary temperature at a cooling rate of 10 to 20 deg.C/sec after cooling the hot rolled steel slab to the temperature range of 740 to 780 deg.C at a cooling rate of 5 to 15 deg.C/sec and rolling the cooled steel slab to a hot rolling ratio of 40% or more.