TPT March 2008

T echnology U pdate

Consortium to co-develop latest strip edge preparation unit

A new edge preparation unit has been developed in order to overcome problems caused by coating materials on strip edges in areas where welding occurs. This equipment will be marketed and manufactured by a group of companies including AG Shepherd (Machinery) Ltd (UK), Universal Tube & Rollform (USA), and D & E Precision Engineering (UK). The equipment has been designed for use in the preparation of pre-coated steel strip prior to the strip being formed and high frequency welded into tube. The tube is then further processed for use as products such as exhaust pipe for motor cars and trucks (aluminised tube), and as electrical conduit, air coolers, polytunnel and in leisure markets (galvanised tube). Mild steel strip is normally produced from ingots and billets, and successively hot rolled then cold rolled into coils. These coils are normally 1,500mm to 2,000mm wide and vary in thickness from 0.5mm to 3mm (or more) depending on the application.

Some strip is then further processed by having a surface coating applied.

In the automotive industry, a coating of aluminium is applied (aluminised strip) for exhaust pipe manufacture, with strip thickness ranging from 1.2mm to 2mm. For use in the manufacture of electrical conduit, a zinc coating is used (galvanised strip), on strip thicknesses from 1.5mm to 3mm. Once the strip has been processed into wide sheet coil form, it has to be slit into narrow widths which have to be precise for presentation into the welded tube mill, in order to produce the required diameter of tube. The slitting of the wide sheet coil is performed by two opposing cutters that cut half way through the thickness of the material, which then shears producing the separation.

› The new machine is unique because it allows the strip to run freely

coatings will melt (along with any material already trapped on the edges of the strip). Unless this is forged out during the welding process, it will act as an impurity and precipitate failure of the tube during further processing (eg flaring to produce the flange for exhaust pipes). The new edge preparation unit has been designed to overcome this problem by removing the coating materials from the edges (top and bottom of the strip) in the areas where the welding process will take place. The unit has two pairs of cutters on each side so that, as the strip runs through the unit, both sides of the strip (top, bottom and edge) can be scraped to remove the residual impurities. Several types of unit have been designed where the cutters are rigidly held, in an attempt to constrain the strip passing through the unit. However, this was not entirely successful, as due to the release of locked-in stresses in the strip, it passes through the unit in a very convoluted form (at a throughput rolling speed of up to 500ft/ min). This system could lead to the thinning or scalloping of the edges of the material, or it missing altogether, making welding more difficult than easy. The new design differs in that instead of trying to constrain the strip using fixed cutters, it allows the strip to run freely. The cutters follow the convolutions of the strip, allowing a uniform amount of material to be consistently removed from the top, bottom and edges of the strip prior to it entering the tube mill. In the original design, this was undertaken by spring loading the cutters to apply the pressure. This worked satisfactorily for some two years, giving considerably improved material yields both on the tube mill and during the subsequent processing of the tube. Replacing the spring operation

This process inevitably propels some of the coating onto the edges of the slit material, and also releases locked-in stresses induced into the strip during its manufacture. It is this issue that creates the problem of producing a satisfactory quality of welded tube suitable for the further processing it has to sustain. Welded tube is produced by running the slit strip through a series of forming rolls to create the correct round profile at the point of rolling. It is at this point that the edges of the strip are pressed together between welding rolls. The process produces heat due to the high frequency induction process, raising the temp- erature of the metal to a point where a satisfactory forged joint can be produced (ie near to the melting point of the steel). Because both aluminium and zinc have a lower melting point than steel, these

fi The new strip edge preparation unit in operation on a mill

fi Using the new edge preparation unit, it is possible to solve problems caused by coating materials on strip edges

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M arch 2008

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