METHOD FOR MANUFACTURING BENT STEEL PIPE EXCELLENT IN STRENGTH AND LOW-TEMPERATURE TOUGHNESS

METHOD FOR MANUFACTURING BENT STEEL PIPE EXCELLENT IN STRENGTH AND LOW-TEMPERATURE TOUGHNESS A method for manufacturing a bent steel pipe excellent in strength and low-temperature toughness, which comprises: applying a bending treatment to a steel pipe made of a steel sheet comprising at least one layer while subjecting the steel pipe to a heat treatment under the following conditions: heating temperature : from 900 to 1,150.degree.C, holding period : up to 15 minutes, and cooling rate : from 5 to 100.degree.C/second; the at least one layer of the steel sheet consisting substantially of: carbon : from 0.002 to 0.050 wt.%, silicon : from 0.05 to 0.80 wt.%, manganese : from 0.80 to 2.20 wt.%, niobium : from 0.002 to 0.100 wt.%, aluminum : from 0.01 to 0.08 wt.%, nitrogen : from 0.002 to 0.010 wt.%, and the balance being iron and incidental impurities; or the at least one layer of the steel sheet further additionally containing, as a strength-improving constituent, at least one element selected from the group consisting of: copper : from 0.05 to 1.00 wt.%, nickel : from 0.05 to 3.00 wt.%, chromium : from 0.05 to 1.00 wt.%, molybdenum : from 0.03 to 0.80 wt.%, vanadium : from 0.01 to 0.10 wt.%, boron : from 0.0003 to 0.0030 wt.%, titanium : from 0.005 to 0.100 wt.%, and calcium : from 0.0002 to 0.0100 wt.%; thereby manufacturing a bent steel pipe having a high strength and a high low-temperature toughness.