Method for monitoring welding of electric resistance welded steel pipe, method for manufacturing electric resistance welded steel pipe, device for monitoring welding of electric resistance welded steel pipe, and device for manufacturing electric resistance welded steel pipe

A method for monitoring welding of an electric resistance welded steel pipe, wherein the welding includes using a heating unit to heat a portion of the pipe to melt a pair of facing butt end surfaces, the method comprising:

• an image acquisition step including capturing, via a camera, a captured image of the electric resistance welded steel pipe while welding the electric resistance steel pipe, the captured image including a V-convergence portion including an edge detection area and a V-convergence point, and a welding portion including a position where a molten steel starts to be discharged from an inside of a wall thickness and a bead;
• a determination information acquisition step including image-processing the captured image to obtain determination information, and acquiring, as part of the determination information, a molten length from the V-convergence point to the position where the molten steel starts to be discharged, a molten width of the welding portion, or both the molten length and the molten width;
• a determining step including comparing the determination information with a determination threshold value set in advance for each wall thickness of the electric resistance welded steel pipe, and determining whether cold welding has occurred, the cold welding being a welding state in which a full thickness of the welding portion in the wall thickness direction has not melted and a partially un-melted portion remains;
• and
• a controlling step including controlling, via a control device, the heating unit based on a result of the determining step.