Excellent steel and a method of manufacturing the same in the of crack initiation characteristics of the weld heat-affected zone and Haha-zai part.

摘要

PROBLEM TO BE SOLVED: To provide a steel material excellent in ductility crack generation resistance of a welding heat affected part and a base material part, and a manufacturing method therefor.;SOLUTION: A steel material contains, by mass%, C:0.02 to 0.2%, Si:0.01 to 0.5%, Mn:0.1 to 2.5%, P:0.05% or less, S:0.05% or less, Al:0.1% or less, N:0.01% or less and one or more kinds selected from Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca and REM if needed, and the balance Fe with inevitable impurities, and has structure at 1/4 position of a sheet thickness made from ferrite and a hard phase, and an area fraction of the hard phase of 50 to 90% and an average aspect ratio of the ferrite of 1.5 or more. A steel raw material having the above components is re-heated at 1000°C or higher again, applied to rolling with a rolling reduction at 900°C or higher of 50% or more and a rolling finishing temperature of Ar₃ point to Ar₃-50°C followed by initiation of water cooling at Ar₃-10°C to Ar₃-70°C, finishing water cooling at 500°C or lower and tempering.;COPYRIGHT: (C)2014,JPO&INPIT