| 摘要 |
Disclosed is a steel pipe which comprises the following components (by mass): C: 0.1-0.45%, Si: 0.3-3.5%, Mn: 0.5-5%, P: 0.03% or less, S: 0.01% or less, sol.Al: 0.01-0.8% (the content of sol.Al is less than 0.1% when the Si content is 1.5% or more), N: 0.05% or less and O: 0.01% or less, with the remainder being Fe and impurities; which has a tensile strength of 600 MPa or more; which has such a uniform elongation that the requirement represented by the formula (1) below is fulfilled; and which has excellent enlarging properties. The steel pipe can be produced by, for example, heating a steel pipe having the above-mentioned chemical composition to 700 to 790°C and subsequently cooling the heated steel pile forcibly to a temperature of 100°C or lower at such a cooling ability that the average cooling rate in the temperature range of 700 to 500°C is 100°C/min or more. u-el = 28-0.0075TS (1) [wherein u-el represents an uniform elongation (%); and TS represents a tensile strength (MPa).] |
