Method of producing tubing

Abstract: The invention relates to steel tubing having a contoured pattern embossed on its external surface and is particularly concerned to provide such patterns on mild steel tubing. A method is provided, of making mild steel tubing(21) having a contoured pattern(11) on its external surface, in which flat mild steel strip(10) of width corresponding to the outside diameter of the desired tubing(21) is passed through embossing rolls which emboss a contoured pattern(11) on one surface of the strip, the embossed strip is passed through shaping rolls(13, 14) which shape the strip to tubular form(21) with the embossed pattern outermost, the opposed longitudinal edges(16, 17) of the tubular form are subjected to high frequency induction heating and the heated edges are press-forged together to form a longitudinally-extending welded joint(20).

Description: This invention relates to a method of making steel tubing according to the preamble of claim 1. Steel tubing is conventionally made in a continuous process in which cold flat steel strip of the desired width is passed through a series of rolls which shape the strip from flat to circular transverse cross-sectional form. The so-shaped form, with the longitudinal edges of the erstwhile strip now abutting, is passed to a welding station where the abutting edges are welded together to form closed tubing of the desired diameter. The tubing is then passed through finishing stations where external and, if desired, internal flash is removed, the tubing is sized, tested for soundness of weld and then cut to desired length. For certain end uses it may be desirable to pattern the surface of the tubing and one means of achieving this is to emboss a pattern onto the flat steel strip prior to its formation into tubing. Although it has been proposed to manufacture embossed stainless steel tubing by this means, it has not hitherto proved possible to form satisfactorily mild steel tubing with a contoured external surface. It will be appreciated that the forming processes for stainless steel and mild steel tubing differ significantly, particularly in the welding techniques used and, consequently, in the speed of the continuous process. Stainless steel is normally welded into tubular form by so-called tungsten inert gas welding or, alternatively, by so-called metal inert gas welding. Both techniques involve passing the to-be-welded tubing through a work station where it is subjected to an electric arc which melts the steel in the region of the desired joint. A metal filler wire is directed into the joint region in the so-called metal inert gas welding process. This welding technique restricts the throughput of the forming process to the order of about five metres per minute. Mild steel is welded into tubular form by heating the abutting edges of the tubing by high frequency induction heating means and then press-forging the abutting edges together to form the joint. This enables the throughput of the process to be of the order of 400 metres per minute. In US-A-3,858,785 it has been disclosed to manufacture heat exchanger tube having patterned outside and/or inside surfaces in which the pattern is applied to flat sheet so as not to penetrate the full thickness of the sheet, the sheet is then formed to tubular form and the abutting longitudinal edges are joined by high frequency welding. It will be further appreciated that when a contoured pattern has been embossed onto the surface of the flat steel strip, the welding together of the abutting longitudinal edges of the tubing is difficult to achieve satisfactorily as exact mating of the corresponding higher and lower regions of the opposed edges cannot be expected. Clearly this problem is more acute in the higher speed process of mild steel tubing manufacture. Accordingly, the present invention provides a method of making steel tubing having a contoured pattern on its exterior surface in which flat steel strip of width corresponding to the outside diameter of the desired tubing is passed through embossing rolls which emboss a contoured pattern on at least one surface of the strip, the embossed strip is passed through shaping rolls which shape the strip to tubular form with the embossed pattern outermost, the opposed longitudinal edges of the tubular form are subjected to high frequency induction heating and the heated edges are press-forged together to form a longitudinally-extending welded joint, wherein the steel is mild steel and the welded joint is discontinuous and stitch-welded. The tube is then passed to a finishing station where any metal "flash" from the welding process is removed or "scarfed" from the outside (and inside, if so desired) of the still hot tubing. The tubing can then be cooled, sized and cut to desired length. It may be found convenient in the manufacturing process to emboss simultaneously a pattern on both surfaces of the flat strip. By this means indentations on one surface can correspond to protrusions on the other. We have found, surprisingly, that a satisfactorily welded joint is achieved despite the above-mentioned problem of alignment of the highs and lows of the contoured pattern on the opposed longitudinal edges of the tubing in the relatively high speed mild steel process. Thus although the longitudinal weld is formed to be not completely continuous and is stitch-welded, the resulting joint is satisfactory and can withstand, for example, subsequent bending of the tubing into desired shapes. Thus the method of the invention provides a steel tubing having a contoured external surface and a longitudinal welded joint, the steel being mild steel and the joint being discontinuous and stitch-welded. The joint may be discontinuous, i.e. stitch-welded, along the entire length or randomly dispersed along the length between continuous welded lengths. By "mild steel" in the present invention is meant to embrace all steels falling within the cold-rolled and hot-rolled definitions of Euro-Norms 111-77 and 130-77. The contoured pattern of the tubing can have a dual-purpose. It may be primarily decorative or primarily to provide frictional, e.g. hand-gripping, properties but in many applications both functions will be of value. Specific end uses for the tubing of the invention are manifold and the following list is by no means exhaustive. Handrails of many types, e.g. for buses, trains, stairways, ferries and in theatres. Shop fittings. Furniture. Architectural fittings, e.g. decorative and functional fittings inside and outside buildings. Any desired patterns may be applied to the surface of the strip subject only to the embossing capabilities of the rolls on mild steel. The invention is further illustrated by way of example only by reference to the accompanying drawings in which: Figure 1 is a perspective view of a mild steel strip passing through forming rolls; and Figure 2 is a side view of a later stage in the forming process showing the formation of the tubing at the welding station. In Figure 1, a strip of mild steel which was originally flat and has had an embossed pattern applied to one surface is shown passing though pairs of forming rolls 13 and 14 at the stage of the tubing forming process where it is in semi-cylindrical form 15. The embossed pattern is not shown in this view. The tubing being formed is passing from rolls 13 to 14 and undergoing a conventional reduction in diameter. In Figure 2, the semi-cylindrical form from the stage of Figure 1 is shown being formed into the desired tubular shape. As shown, the longitudinal edges 16 and 17 of the semi-cylindrical form are brought gradually closer together as they pass through the high frequency induction heating station 18. Opposed longitudinal edges 16 and 17 are then welded together while hot between forging rolls 19 to form the desired tubular product with welded joint line 20. The embossed pattern is indicated at 11 on the exterior surface of the product.

Claims: A method of making steel tubing having a contoured pattern (11) on its external surface, in which flat steel strip of width corresponding to the outside diameter of the desired tubing is passed through embossing rolls which emboss a contoured pattern (11) on one surface of the strip, the embossed strip is passed through shaping rolls (13, 14) which shape the strip to tubular form with the embossed pattern outermost, and the opposed longitudinal edges (16, 17) of the tubular form are subjected to high frequency induction heating and the heated edges are press-forged together to form a longitudinally-extending welded joint (20), characterised in that the steel is mild steel and the welded joint (20) is discontinuous and stitch-welded. A method according to Claim 1, characterised in that the welded tubing is subjected to a scarfing operation while still hot to remove metal flash from the exterior of the tubing. A method according to Claim 1 or 2, characterised in that a pattern is embossed simultaneously on both surfaces of the flat steel strip. A method according to Claim 1, 2 or 3, characterised in that the stitch-welded joint is randomly dispersed along the welded joint (20) between continuous welded lengths.

Inventors: WINFIELD PHILIP WILLIAM, WINFIELD, PHILIP WILLIAM
Applicants: GLYNWED METALS PROCESSING LIMI, GLYNWED METALS PROCESSING LIMITED
Application year: 1992
Main classification: B21C37
IPC codes: B21C37