| 摘要 |
PURPOSE: A method is provided which manufactures a high strength high toughness steel product for welded structure by using bainite and fine grain ferrite as a matrix and finely and uniformly distributing plenty of TiN precipitates, thereby minimizing toughness of the matrix and weld heat-affected zone. CONSTITUTION: The method for manufacturing a high strength steel product for welded structure by nitriding treatment comprises the steps of manufacturing 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 and 0.005 wt.% or less of O; nitriding the slab so that N, Ti, B and Al satisfy the following relational expression as N of the steel slab is ranging from 0.008 to 0.03 wt.% by heating the slab in the temperature range of 1000 to 1250 deg.C for 60 to 180 minutes, 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<=7 and 6.5<=(Ti+2Al+4B)/N<=14; and hot rolling the slab to a reduction ratio of 40 % or more by cooling the heated slab to the temperature range of Ar3 to (Ar3+50 deg.C), hot rolling the slab to a reduction ratio of 50 % or more by cooling the hot rolled slab to the temperature range of (Ar1+30 deg.C) to (Ar1+80 deg.C) at a cooling rate of 5 to 15 deg.C/sec, and cooling the hot rolled slab to a temperature of bainite transformation finishing temperature±10 deg.C at a cooling rate of 5 to 20 deg.C/sec.
|
