To clarify the effect of inclusions on the Charpy impact properties, the 2 mm V-notched Charpy properties of X60 – X80-grades steel were numerically simulated using the finite element method code ABAQUS. The yield strength and the tensile strength of the steel were 562 MPa and 644 MPa, respectively. The striker’s velocity and the temperature dependency of the stress-strain curve were taken into account. To estimate the effect of nonmetallic inclusions, a 200 μm long virtual inclusion with a 1 μm edge radius was situated at the maximum point of the stress triaxiality. Four types of micro crack initiation were determined: (a) ductile void generation in the matrix, (b) cleavage crack generation in the matrix, (c) void generation by inclusion fracture and (d) void generation by matrix-inclusion interface debonding. Without inclusions, a ductile micro void was generated when the striker stroke was 3.3 mm, independent of the temperature. With inclusions, an inclusion fracture occurred when the striker stroke was 0.6 mm at room temperature. The striker stroke decreased as the temperature decreased.

Based on the above numerical estimation results, electric resistance welded (ERW) Linepipe with high-quality weld seam MightySeam® has been developed. Controlling the morphology and distribution of oxides generated during the welding process by means of temperature and deformation distribution control is the key factor for improving the low-temperature toughness. The Charpy transition temperature of the developed ERW pipe was much lower than −45°C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −20 °C, the MightySeam® weld provides a fracture performance that is the same as UOE Double Submerged Arc Welded pipe. The pipe has been used in actual, highly demanding, severe environments.

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