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 of the present invention includes: an image acquisition step in which, when an electric resistance welded steel pipe is welded, 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, are imaged to obtain a captured image; a determination information acquisition step in which the captured image is image-processed, and a molten length from the V-convergence point to the position where the molten steel starts to be discharged or a molten width of the welding portion or a combination thereof is acquired as a determination information; and a determining step in which it is determined whether a cold welding has occurred by comparing the determination information with a determination threshold value set in advance for each wall thickness of the electric resistance welded steel pipe.