It is provided a hot-rolled steel sheet as the material of an electric resistance welded steel pipe or tube with high strength and excellent workability and toughness, which is suitable for a machine structural steel pipe or tube used as a part of an automobile, construction machinery, or industrial machinery. The hot-rolled steel sheet has a predetermined chemical composition and has a steel microstructure at the sheet thickness center with a volume fraction of bainite of 90% or more and the balance containing one or more of ferrite, pearlite, martensite, and austenite. The above steel microstructure at the sheet thickness center further has an average grain size of 10.0 m or less, a volume fraction of crystal grains with a grain size of 40.0 m or more, and a number density of Ti-based inclusions with a long diameter length of 5.0 m or more of 20 pieces/mm.sup.2 or less.

This electric resistance-welded steel tube for a hollow stabilizer is an electric resistance-welded steel tube for a hollow stabilizer including a base material portion and a weld, in which the base material portion has predetermined chemical components, a wall thickness of the base material portion is 2.0 to 6.0 mm, an outer diameter of the electric resistance-welded steel tube is 10 to 40 mm, in a C direction cross section of the electric resistance-welded steel tube, a recessed bead cut is present in a region including the weld on an inner surface side of the electric resistance-welded steel tube, when an imaginary line is drawn from one opening edge to the other opening edge of the bead cut in a shortest distance, a maximum depth from the imaginary line to a bottom of the bead cut is 300 m or less, a maximum inclusion diameter that is included in the base material portion is 300 m or less, in the base material portion of the electric resistance-welded steel tube, a surface roughness of the inner surface side is 300 m or less in terms of a maximum profile valley depth Rv, and maximum hardness of the electric resistance-welded steel tube including the weld is 300 Hv or less.

Provided is a welding monitoring apparatus that monitors a welding state of a V-convergence region in which a strip-shaped metal sheet is converged in a V-shape, when the metal sheet is cylindrically formed while being conveyed, and both side edges of the metal sheet are heated and melted in a manner of being butted each other while being converged in the V-shape, such that an electric resistance welded steel pipe is manufactured. This welding monitoring apparatus includes an image capturing unit that captures images of a region including the V-convergence region in time series; and an image processing unit that extracts a welding point based on the images captured in time series and detects the presence or absence and a position of irregular arcing at the welding point or on an upstream side of the welding point.

A method of preparing a pipe-section for welding to another pipe-section to form a pipeline comprising a plurality of said pipe-sections, the method comprising at least the steps of: (i) providing a pipe-section having first and second pipe-ends; (ii) defining a first portion L1 of the longitudinal length of the pipe-section from the first pipe-end being in the range 3% to 40% of the overall length of the pipe-section; (iii) defining a second portion L2 of the longitudinal length of the pipe-section from the end of the first portion L1 towards the second pipe-end; (iv) heating at least the first portion L1 to at least a first temperature T1 of at least 500 C. for at least 2 minutes; (v) maintaining a second temperature T2 of the second portion L2 during step (iv) below the first temperature T1. The invention is able to reduce the strain capacity during reel-laying of a pipeline formed from a plurality of pipe sections formed by the invention.