| 摘要 |
A high strength steel is provided to maintain physical properties of the weldments and obtain excellent yield strength of 550 MPa grade even after the PWHT by solving a problem that physical properties such as strength and toughness of weldments of a conventional riser pipe steel are deteriorated after PWHT(Post Weld Heat Treatment), and a method for manufacturing the high strength steel is provided. A manufacturing method of a riser pipe steel for ensuring physical properties of PWHT comprises the steps of: reheating a steel slab comprising, by weight percent, 0.04 to 0.10% of C, 0.05 to 0.50% of Si, 1.2 to 2.0% of Mn, 0.01 to 0.05% of Al, 0.02% or less of P, 0.005% or less of S, 0.005 to 0.02% of Ti, 0.002 to 0.01% of N, 0.02 to 0.07% of Nb, 0.02 to 0.07% of V, 0.3% or less of Ni, 0.1 to 0.5% of Mo, 0.0005 to 0.004% of Ca and the balance of Fe and other unavoidable impure elements to a temperature range of 1100 to 1300 deg.C; rolling the reheated steel slab to an average reduction ratio per pass of at least 10% at 1000 deg.C or more; cooling the rolled steel slab to 950 deg.C or less in a cooling rate of 2 deg.C/sec or more; hot rolling the cooled steel slab to an accumulated reduction ratio of 70% or more in a temperature range of 950 to 800 deg.C; and water-cooling the hot rolled steel slab to a temperature range of 400 to 550 deg.C in a cooling rate of 5 deg.C/sec or more, and then air-cooling the water-cooled steel slab to room temperature. |
