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Optimized rolling process thanks to precise slab measurement

Topographic measurement of aluminium slab reduces milling removal

At its Pietermaritzburg facility in South Africa, Hulamin Rolled Products Ltd produces a wide range  of aluminium sheet, plate and foil and is thus the best-known semis manufacturer in Africa. In 2015 the company invested in the slab measurement system described in this article and for it, awarded the contract to nokra in Germany.

Aluminium is an essential part of everyday life. Thanks to a large variety of alloys the mechanical properties of the material can be influenced over a wide range. The main argument for the use of aluminium is and remains its low specific weight, which makes it indispensable as a lightweight material. The largest quantities purchased go to the food, building and automotive industries, the latter of these being responsible in the past two years for remarkable investments in the aluminium industry, particularly in Europe. Measurement technology has also benefited from this. In fact, the manufacturing processes are characterized by ever-increasing levels of automation. In that respect it is no longer mainly the saving of labour that is decisive, but rather the comprehensive monitoring of production, with 100% inspection and the optimization of the complete process chain that is closely associated with this.

Accordingly it has been demonstrated that it is not enough to measure and protocol just the end-products of semis production; quality monitoring has to begin already from the first technological steps. After the alloys have been determined by melting and casting, the first shaping operation takes place by hot rolling and a first strip is produced from the cast slabs. These are preheated to the necessary deformation temperature in a furnace and rolled down in a reversing process in the hot-rolling line to the material thickness desired for subsequent cold processing. Depending on the end product required, hot rolling is followed by further processes such as cold rolling, annealing, levelling, coating and slitting.

Regardless of whether or not the rolled product undergoes a surface finish or cut up into strips or sheet, measurement technology always plays an important part in the processing. To be able to control the rolling process optimally, the geometry of the slabs must be known with precision. Likewise, however, surface defects, cracks and inclusions must also be detected, since during the further processing these can lead to rejects or make the finished semifabricate unusable. If they are not recognized, processing faults (strip cracking) can result, which then usually lead to damaged equipment, work stoppages and hence elevated costs. On the other hand it would be fatal to supply defective semis since these, for example if they are used in safety-relevant components in the final product, would fail and cause serious accidents often resulting in personal injury or even death.

For all the above reasons cast slabs are nowadays scalped before further processing. That is usually done by milling in one or more passes in order to enable an optimum process sequence. The background is the limited chip removal and in that context attention is paid to the hardness of the alloy. The milling programme is determined individually for every slab. At the same time, valuable process time, melting energy and disposal costs for dross are minimized. The finish-milled slab must have an exactly flat surface and must no longer contain any cracks or surface defects at all. For this, all of the oxide layers on the top and bottom sides are removed. The milling depth is governed by the original topography of the slab, which must also be removed in order to achieve the required flatness. Thus, the milling depth must always be set for each individual slab so that, if possible in a single pass, milling reaches down to just under the lowest point.


This is where modern measurement technology comes into its own. Before the milling process the geometry of each slab is measured exactly. For this, the slab is pushed through under a measurement gantry. The measurement gantry incorporates several laser scanners arranged next to one another, which ensure an uninterrupted height profile recording. For this one uses laser triangulation, a tried and tested measurement principle which makes use of the back-scatter of a perpendicularly projected laser beam which, in turn, is received by an obliquely positioned detector.

The oblique positioning of the detector is the prerequisite for a height change of the object being examined to produce position change of the reflected laser beam in the receiver. If the optical receiver is in the form of a semiconductor detector array, the position can be read out very exactly and converted to the desired height or distance information. If a point laser is used (and in that case one speaks of point triangulation or 1-D triangulation), one obtains just one distance value. But if the surface profile is to be recorded and measured, a large number of consecutive 1-D triangulations has to be combined. With a probe grid of the order of millimetres and widths up to a metre this could no longer be done with individual sensors in a technically effective and cost-efficient manner. Laser scanners generate a large number of adjacent individual measurements and can therefore record a height profile. This is often also referred to as profile sensoring or 2-D triangulation. Instead of producing many individual laser points, these are combined to form a laser line. Hence also the term line scanner or laser light section sensors.

The detectors in these scanners are now no longer sensor lines but sensor matrix arrays, since a large number of reflection points combined in a reflection line have to be recorded. Modern laser scanners are characterized by the use of high-resolution CMOS arrays similar to those nowadays in Smartphones. However, in this case these are specially designed versions which are considerably more expensive to buy because the technical demands are substantially stricter for measurement technology applications. The ability to be synchronized, resistance to oversteering, rapid read-out times, special dynamic segmentation and illumination control are only some of the requirements. The designs of such sensors are highly specialized and differ in respect of basis distance, scan width, measurement rate, resolution and many other features. Whereas for small geometries up to 200 mm there are many manufacturers of triangulation sensors, the relevant dimensions of over 1,000 mm can only be recorded by means of special systems. 

nokra has specialized in these particular applications. The company, located near Aachen in Germany, in not only a system manufacturer but also develops and produces its own triangulation sensors. It produces high-grade measurement and testing systems for use in manufacturing. Its systems measure and test geometrical characteristics such as length, width, thickness, profile, shape and position. Measurement systems from nokra are found in the steel, aluminium and automotive industries, in plant engineering and in the plastics industry and glass processing. Using its own sensors has the advantage that the measurement technology solution can be better matched to the customer’s wishes. Moreover, distance and accuracy which are indeed physically inseparably coupled with one another, can be varied better than by relying on the technical data of the product range of another manufacturer. For detecting the geometrical data of aluminium slabs, nokia developed and produced a special gantry with six laser scanners. In the most recent installation the system is being used by the South African aluminium producer Hulamin Rolled Products Ltd, the largest plant in the Hulamin group.


Slab measurement at Hulamin

At its Pietermaritzburg facility, Hulamin produces a wide range of aluminium sheet, plate, coils and foil for markets in Africa, Europe, North America and around the world. For this, the plant in Pietermaritzburg has a recycling plant, casthouses with ingot and continuous casting machines, a hot-rolling plant, several strip and foil cold-rolling mills and a range of finishing machines (coating lines, process lines, slitting and tension levelling machines). Besides the strip plants, Hulamin operates an efficient plate plant with a solution heat treatment furnace, saws, levelling machines and cut-to-length lines for shearing of plates to final customer requirements.

In 2015 Hulamin invested in the slab measurement system described here, for which it awarded the contract to nokia in Aachen, Germany. The laser measurement system consists of a measurement gantry measuring 800 x 5,600 x 3,051 mm (L x W x H) with its shortest side in the processing direction. The laser scanners are fixed in three groups in separate protective housings, each attached to the upper gantry frame, and divide the measurement zone of 2,450 mm in the lateral direction. In this case each individual sensor has a lateral scan range of 450 mm. To be able to detect the surface profile over the full width required with no gaps, the six laser scanners are arranged next to one another in such manner that their ranges slightly overlap.

This ensures that the next-in-sequence evaluation can be unambiguously matched to the height values so that a correspondingly continuous lattice network is reproduced over the whole of the slab. For better visualization, the height profile can be recorded in a false-colour representation. The system is designed such that the profile measurement relates to an average pass line a distance of 1,030 mm away from the measurement heads. The measurement range for the various profile heights is 300 mm. The measurement gantry is positioned immediately before the milling machine and has to withstand the environmental and operational conditions there, in order to detect the slab dimensions, shape and topography accurately and reliably day after day. It is capable of measuring slabs of length 1,800 to 6,000 mm and width up to 2,000 mm with precision.

In the background the milling machine can be seen, and in the foreground, in front of the nokra measuring unit, there is an old two-point thickness measurement unit with which the necessary milling depth was previously measured approximately. In each case Hulamin uses the unit to measure the first two metres of the slab. That is the distance between the measurement position and the milling head before the milling cutters ‘bite’. In these first two metres the slabs have the largest dimensional deviations, which are often also referred to as the ‘ingot foot’. It is important to recognize these deviations in order, with that information plus a reserve, to position (adjust) the milling head at the correct height. The shape deviations of the slabs can sometimes be visually recognized clearly already in the slab store. The optical measurement quantifies these deviations and so makes it possible to specify a substantially smaller reserve than before at the milling machine.

After milling, the slab is turned over and processed on the other side in the same way. Since it was commissioned in 2015 the system has been working accurately and reliably. The benefits brought by this equipment are summarized by Tim Hawkins, head of Process Engineering, as follows: measurement of the slab contour is important for the following aspects during processing:

  1. The slab casting team is thereby given final information about the slabs produced, so as to improve further the shape and uniformity of their ingots
  2. The measurement data are used to establish a reference value, which enables optimum machining of the slabs before rolling
  3. The milling process is improved by virtue of more effective machining, i.e. reduction of the milling depth
  4. The quantity of chips is reduced; these can only be re-used by melting to the extent of 90%, whereas the remaining 10% have to be disposed of as unusable waste (dross). This aspect results in a not inconsiderable saving of energy.

Thanks to the use of this measurement system Hulamin has been able to reduce the weight of the swarf by 20%. In the longer term it may become possible to reduce the scalper chips still more, so as to be able to roll even more precisely. Already today, in the first year of the system’s use, it has paid for itself. As the next step, Hulamin is already planning to equip the system with a software extension which can recognize and characterize particular defect categories on the slab surface. That information will be used by Hulamin to improve its casting process still further.

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