THICK, HIGH-STRENGTH, SOUR-RESISTANT LINE PIPE AND METHOD FOR PRODUCING SAME

摘要

A line pipe having a wall thickness of 20 mm or more and a tensile strength of 560 MPa or more and a production method therefor are provided. The base metal portion contains particular amounts of C, Si, Mn, P, S, Al, Nb, Ca, N, and O, one or more component selected from Cu, Ni, Cr, Mo, V, and Ti as optional components, and the balance being Fe and unavoidable impurities. The microstructure in the pipe thickness direction contains 90% or more bainite and 1% or less of MA in a region that extends from a position 2 mm from an inner surface to a position 2 mm from an outer surface. In a hardness distribution in the pipe thickness direction, the hardness in a region other than a center segregation area is 220 Hv10 or less and the hardness in the center segregation area is 250 Hv10 or less. The major axes of pores, inclusions, and inclusion clusters that are present in a portion that extends from, a position 1 mm from the inner surface to a 3/16 position of the tube thickness and in a portion that extends from a position 1 mm from the outer surface to a 13/16 position of the tube thickness in the tube thickness direction are 1.5 mm or less.

主权项

1. A heavy wall, high-strength line pipe for sour gas service, wherein a chemical composition of a steel pipe base metal portion contains, in terms of % by mass, C: 0.020 to 0.060%, Si: 0.50% or less, Mn: 0.80 to 1.50%, P: 0.008% or less, 5: 0.0015% or less, Al: 0.080% or less, Nb: 0.005 to 0.050%, Ca: 0.0010 to 0.0040%, N: 0.0080% or less, O: 0.0030% or less, and the balance being Fe and unavoidable impurities, Ceq expressed by equation (1) is 0.320 or more, PHIC expressed by equation (2) is 0.960 or less, ACRM expressed by equation (3) is 1.00 to 4.00, and PCA expressed by equation (4) is 4.00 or less;
a microstructure in a pipe thickness direction contains 90% or more bainite and 1% or less martensite-austenite constituent in a region that extends from a position 2 mm from an inner surface to a position 2 mm from an outer surface; in a hardness distribution in the pipe thickness direction, a hardness of a region other than a center segregation area is 220 Hv10 or less and a hardness of the center segregation area is 250 Hv10 or less; and major axes of pores, inclusions, and inclusion clusters that are present in a portion that extends from a position 1 mm from the inner surface to a 3/16 position of a pipe thickness and in a portion that extends from a position 1 mm from the outer surface to a 13/16 position of the pipe thickness in the pipe thickness direction are 1.5 mm or less:
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   equation (1)PHIC=4.46C+2.37Mn/6+(1.74Cu+1.7Ni)/5+(1.18Cr+1.95Mo+1.74V)/15+22.36P   equation (2)ACRM=(Ca−(1.230−0.000365))/(1.25S)   equation (3)PCA=10000CaS0.28   equation (4) where respective alloying elements in equations (1) to (4) represent their contents (% by mass) in the chemical composition.