| 摘要 |
<p>A Cr containing steel pipe for linepipe having high strength of X65 to X80 grade excellent in toughness, corrosion resistance, resistance to sulfide stress cracking and resistance to IGSCC in a welded heat affected zone is provided. Specifically, the steel pipe has a chemical composition consisting of, by mass%, C: 0.001% to 0.015%, Si: 0.05% to 0.50%, Mn: 0.10% to 2.0%, Al: 0.001% to 0.10%, Cr: 13% or more and less than 15%, Ni: 2.0% to 5.0%, Mo: 1.5% to 3.5%, V: 0.001% to 0.20%, N: 0.015% or less, and the balance being Fe and inevitable impurities, under the condition that P 1 is 11.5 to 13.3 and that P 2 =(0.5Cr+5.0)-P 1 is 0 or more. With this chemical composition, since a microstructure in a welded heat affected zone, which is subjected to heating up to a temperature range for forming ferrite single phase of 1300°C or higher and to cooling when welding is performed, is formed such that 50% or more of prior-ferrite grain boundaries, in a ratio with respect to the total length of the prior-ferrite grain boundaries, is occupied by martensite phase, and since formation of Cr carbide depleted zones is suppressed, a pipe having significantly increased resistance to IGSCC in a welded heat affected zone can be obtained. Since it is not necessary to perform a post weld treatment, there is a merit of significantly decreasing construction period of welded steel pipe structures.</p> |
