Welded steel pipe for linepipe with high compressive strength and manufacturing method thereof

摘要

Provided are a steel pipe for a linepipe with high compressive strength and a heavy wall thickness, and a method of manufacturing the same. Lowering of yield stress caused by a Bauschinger effect can be suppressed by optimizing the metal microstructure of a steel plate without requiring particular forming conditions in forming the steel pipe, and without requiring heat treatment after pipe making. A steel pipe has the composition which contains by mass % 0.03 to 0.10% C, 0.30% or less Si, 1.00 to 2.00% Mn, 0.015% or less P, 0.003% or less S, 0.080% or less Al, 0.005 to 0.035% Nb, 0.005 to 0.020% Ti, and Fe and unavoidable impurities as a balance. The steel pipe has metal microstructure where a fraction of bainite is 60% or more, a fraction of rolled ferrite is 5% or less, and a fraction of M-A constituent (MA) is 3% or less.

主权项

1. A welded steel pipe for a linepipe having a composition which contains by mass % 0.03 to 0.10% C, 0.15% or less Si, 1.50 to 2.00% Mn, 0.015% or less P, 0.003% or less S, 0.080% or less Al, 0.005 to 0.035% Nb, 0.005 to 0.020% TI, and Fe and unavoidable impurities as a balance, wherein
C(%)—0.065Nb(%) is 0.025 or more, and a Ceq value is 0.36 or more, and the steel pipe has metal microstructure where an area fraction of bainite is 60% or more, an area fraction of rolled ferrite is 5% or less, an area fraction of M-A constituent (MA) is 0.6 to 3%, an average grain size of MA is 2μm or less, and an aspect ratio of MA is 5 or less, wherein the Ceq value is expressed by the formula:
Ceq=C(%)+Mn(%)/6+{Cr(%)+Mo(%)+V(%)}/5+{(Cu(%)+NI(%)}/15.