Electric resistance welded steel pipes or steel tubes with a tensile property TS of not less than 434 MPa which have electric resistance welded parts exhibiting both excellent HIC resistance and excellent low-temperature toughness, and methods for manufacturing such steel pipes or steel tubes.

A high frequency power supply system provides highly regulated power and frequency to a workpiece load where the highly regulated power and frequency can be independent of the workpiece load characteristics by inverter switching control and an inverter output impedance adjusting and frequency control network that can include precision variable reactors with a geometrically-shaped moveable insert core section and a stationary split-bus section with a complementary geometrically-shaped split bus section and split electric terminal bus section where the insert core section can be moved relative to the stationary split-bus section to vary the inductance of the variable reactors.

An electric resistance welded steel pipe or tube comprises: a base metal being a steel sheet having a specific chemical composition and an electric resistance weld portion having a bond width of 4010.sup.6 m or more and 12010.sup.6 m or less, wherein C.sub.0-C.sub.1 is 0.05 mass % or less, where C.sub.0-C.sub.1 is a difference between C.sub.1 in mass % which is a minimum C content of the electric resistance weld portion and Co in mass % which is a C content of the steel sheet, and a depth of a total decarburized layer in each of an inner surface layer and an outer surface layer of the electric resistance welded steel pipe or tube is 5010.sup.6 m or less.

A high strength electric resistance welded steel pipe has a yield strength of 450 MPa or more and excellent resistance to softening for a long period in an intermediate temperature range and a method of manufacturing the steel pipe are provided. The steel pipe has a chemical composition containing, by mass%, C: 0.026% or more and 0.084% or less, Si: 0.10% or more and 0.30% or less, Mn: 0.70% or more and 1.90% or less, Al: 0.01% or more and 0.10% or less, Nb: 0.001% or more and 0.070% or less, V: 0.001% or more and 0.065% or less, Ti: 0.001% or more and 0.033% or less, Ca: 0.0001% or more and 0.0035% or less, in which the condition that Pcm is 0.20 or less is satisfied.

An electric resistance welded pipe or tube comprises a weld portion in a pipe or tube longitudinal direction, wherein a maximum value of a wall thickness distribution of the weld portion is less than or equal to 1.05 times an average wall thickness of the electric resistance welded pipe or tube, a width of a weld metal in the weld portion in a circumferential direction is 1 m or more and 800 m or less over an entire thickness of the electric resistance welded pipe or tube, and a ratio of a maximum value to a minimum value (=maximum value/minimum value) of the width of the weld metal is 1.0 or more and 2.5 or less.

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.

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.