Introduction

The worldwide production of semi-finished products of copper and copper alloys totals around 14 - 15 million tonnes per year, with annual growth rates of approx. 3%. Tubes make up roughly 10% of this figure. Important producer regions are Western Europe (36%), America (20%), Asia including Australia (27%). The remaining 17% are spread over the rest of the world.

Whereas in some Western European countries, the relevant standards require that installation and ACR tubes of copper are produced by seamless methods, the ASTM standards which are also employed in Asia also allow welded tubes. A further expansion of the use of welded tubes has been prevented to date by the high production costs for the cold rolled strip. For some time now, however, an increasing trend towards welded tubes has been observed due to the better surface quality and closer dimensional tolerances. This trend is taking place worldwide and has already taken over in the production of finned ACR tubes. Due to the high strip costs, installation tubes are almost exclusively produced in seamless processes. An acceptance by the market for welded water pipes can only be expected when significant competitive advantages in terms of product quality and lower production costs can be achieved with a new strip production process. The production of an inexpensive cold-rolled strip could thus revolutionise the copper tube market worldwide. First of all the welding of ACR tubes would become more attractive, and secondly the large volumes of industrial and installation tubes could be produced more cost-efficiently and in a higher quality.

The Copper Plants Division of Mannesmann Demag AG owns the Contirod process for continuous casting and in-line hot rolling of wire rod for wire production. This process is already well known as a result of its low production costs and high productivity. It is also possible to cast hot roll and cold roll flat material on this line for in-line production of narrow cold strip in the ideal widths for tube welding of 65 to approx. 150 mm (CONAS process). This report describes how a CONAS line is configured and goes on to illustrate the various possibilities of tube production from the CONAS strip. Comparisons are drawn with the production of seamless tubes.

CONAS Line Concept

The basis for this concept is the CONTIROD System (Figure 1) for the production of copper wire rod. 32 CONTIROD lines worldwide with capacities from 5 - 60 t/h have been commissioned to date.

Figure 1. CONTIROD System

The schematic layout of a CONTIROD line comprises a melting facility, a Hazelett twin belt caster, a hot rolling mill, a cooling and pickling line and a coiler (Figure 2).

Figure 2. The schematic layout of a CONTIROD line

With this concept, the rectangular profile cast in the Hazelett caster can be rolled out into flat material instead of round material. A corresponding casting cross-section merely has to be selected and the sizing and number of rolling stands for the desired final size have to be calculated. The CONAS line (Figure 3) therefore has a similar fundamental configuration to that of CONTIROD lines.

Figure 3. The CONAS line

A melting facility, for CONAS with an induction-heated holding furnace for better control of the melt, is followed by the Hazelett caster, the hot rolling mill, the cooling and pickling line, a cold rolling stand and 2 strip coilers to take up the continuously produced strip. The cold rolling stand integrated into the CONAS line ensures that the cold strip has some mechanical strength and is far less susceptible to mechanical damage than hot strip. Furthermore, the use of the cold rolling mill offers the advantage that the strip can be rolled inline to the ingoing thickness suitable for a mother tube in a welding machine.

CONAS lines can be built with capacities from 7 - 20 t/h. Although these capacities may appear to be very high for some users, this fact is put into perspective by two other factors so that it is economically viable to install a CONAS line even for smaller tube capacities. The first reason are the generally low production costs of this line. The second reason is its versatility, since a combined CONAS/CONTIROD line (Figure 4) can be used to produce both rounds and flats in a very wide range of sizes.

Figure 4. Combined CONAS/CONTIROD line

As the costs for wear parts, standardised for one tonne of copper, are almost constant and the costs for consumables tend to decrease with increasing capacities, higher capacities offer major benefits with respect to cost-effectiveness (Figure 5).

Tube Welding Mill Concept

New concepts for tube production can be developed on the basis of the CONAS process for the production of continuous cold strip. Starting with a cold strip, in this example 130 x 1.8 mm, two concepts are feasible (Figure 6).

Figure 6. Two concepts with CONAS process

Process Line A

The CONAS line produces a cold strip which is buffer-stored in coil form or can be transported directly to the welding machine. On the welding line, the strip is HF welded to produce a mothertube 39 mm x 1.8 mm. The welding machine has a double uncoiler, cross-welder, strip accumulator, strip forming section, welding table, sizing mill, cut-off device, vertical coiler and an overhead carrousel conveyor. The carrousel conveyor is required to suspend the coils in order to allow any remaining chips to be blown out of the tube. After this, the tubes are automatically placed into a basket which is connected via a basket transport system to the downline facilities, e.g. spinner blocks and finishing equipment. Downstreams of the spinner blocks are machines for production of bars or level wound coils (Figure 7). Depending on the specification, the tubes are then annealed before they are shipped.

Figure 8. CONAS process

Process line A is suitable for the production of both installation tubes and ACR tubes without or with conventional fins, as with the seamless tubes. This solution is particularly attractive where high investments for modernisations are planned in the area of the casthouse and extrusion shop. One major benefit here is the elimination of the eccentricity of the mother tube for the drawing line and the high coil weight of 1,000 - 1,500 kg (3,000 kg is theoretically possible).

Process Line B

The cold strip coming from the CONAS line is further rolled down to the necessary tube thickness for the welding process on a separate reversing cold rolling mill (Figure 8).

Figure 8. Cold rolling mill

On the basis of a strip width of 130 mm and a coil weight of approx. 3,000 kg, a rolling mill for 1.8 mm to 0.4 mm would have an annual capacity of 15,000 - 18,000 t. After rolling, the strip is either slit directly into the strips required for welding or, if an internally finned tube is to be produced, first annealed and then cut to the necessary strip widths, depending on the intended purpose (Figure 9). The welding line consisting of uncoiler, strip accumulator, 2 finning stations, forming section, welding table, sizing section and coiler welds a tube with the required final dimension.