Method of monitoring a butt weld between metal strips.

The present invention relates to a method of controlling a weld end to end metal strips.

It should be noted at the outset that the term "metal band" is used generically in the present invention, to mean flat products, hot-rolled or cold, the thickness is less than 10 mm and is preferably between 0.2 mm and 5 mm. These products may be in the form of strips, usually wound coils, but also in the form of individual sheets of any size. They are preferably steel but may also be any other metal or metal alloy weldable.

Among the many applications that use the butt welding of metal bands, the following description will be specifically devoted to the assembly of steel strips in an endless tape for a continuous process.

At present, use is increasingly continuous process for the manufacture or treatment of metal strips, especially steel strips. By way of example, mention will be made rolling, pickling, annealing, and coating the strips. To allow the implementation of the continuous process, it is common practice to assemble the individual strips end to end by welding, to form a continuous strip of theoretically unlimited length. The quality of the bands joining welds plays a crucial role in the conduct of these ongoing processes. Indeed, it influences the resistance of the web to longitudinal tensile forces which ensure the progression thereof.

Currently, these welds are controlled rudimentarily industrially. This control often is a quick visual examination, possibly with a mechanical test, such as a percussion test or Erichsen test. This review is conducted by an operator and therefore presents the risks of error and uncertainty; it provides no information on the internal health of the weld. This results in the absence of an objective and reliable inspection of welds, there occurs web breaks that cause long shutdowns of production facilities. In addition, the lack of confidence in the quality of welds result in slower production lines or treatment.

It has already been proposed by the patent application BE-A-08900729, a method of controlling a weld of this type based on the use of ultrasonic pulses. This process does not completely eliminate the risk of error or uncertainty, particularly because of its sensitivity to the wave frequency used and how they spread in the weld.

The present invention aims to overcome the above drawbacks, using a thermographic inspection conducted warm assembly butt welds metal bands.

According to the present invention, a method of controlling a weld end to end metal strips, wherein the welding is carried out along a direction transverse to the longitudinal axis of the strips, is characterized in that after the completion said weld, one establishes a thermal chart of the said weld and of the heat affected zone in the vicinity of said weld and in that one verifies that the temperature of at least a majority of points of said thermal map is between a maximum value and a predetermined minimum value, specific to each of said points.

In principle, the thermal map of the weld may be made by any suitable method known per se in the art.

In the context of the present invention, it has proved interesting to measure, at predetermined times after the completion of welding, the temperature of a plurality of ranges of said weld and heat-affected in the vicinity of said weld area and to establish the thermal map of said weld and heat affected zone in the vicinity of said solder from the measured values ​​of the temperature of said plurality of ranges.

According to a particular embodiment, is carried out a transverse scanning of the weld seam, this transverse direction being related to the direction of the cord, in a plurality of points distributed along said cord and with as short delay as possible with respect to the welding said points, and measuring the temperature of a plurality of areas situated on the path of said scanning.

The extent of said measuring ranges can be modified and made as small as desired, depending on the resolution of the measuring apparatus, and the desired accuracy of the thermal map determined by this method.

Ideally the welding progresses along a straight line, perpendicular to the longitudinal axis of the strips; this straight line is basically materialized by the line joining two successive strips. Ideally still, the transverse temperature profile of the weld - That is to say, the profile in a direction perpendicular to the abovementioned connecting line - is symmetrical with respect to this connecting line and the locus of the maximum temperatures of all these profiles coincides with the abovementioned connecting line.

In practice, however, the conditions may be different from these ideal conditions, firstly because the bands to join may be some overlap and partly because the quality and regularity of the weld can be affected by various factors including geometrical, mechanical, electrical or thermal properties.

According to a particular feature of the process of the invention, it is determined, at successive points distributed along the weld seam, a plurality of transverse profiles of the welding temperature and of the heat affected zone in the vicinity of said weld, said comparing profiles measured at at least one reference temperature profile and a signal is produced indicating that the gap between at least one of said measured profiles and at least one of said reference profiles reaches or exceeds a predetermined value.

In particular, said transverse temperature profiles are determined at predetermined time intervals, preferably regular. Similarly, these profiles are also determined with a constant delay as possible in relation to the execution of the solder to the respective points considered; this delay will itself be as short as possible to minimize cooling of the welding before measurement.

According to further features of the method of the invention, the comparison of said transverse temperature profiles may be to different key parameters of these profiles, such as:<ul><li>the maximum value of the temperature of said profiles, individually considered;</li><li>width at different temperature levels profiles;</li><li>the position of the point corresponding to the maximum value of the temperature of said profiles, individually considered;</li><li>the symmetry of the profiles relative to the point corresponding to the maximum value of the temperature of the respective profiles;</li><li>the deformation of the profiles with respect to at least one reference profile.</li></ul>

Another feature of the method of the invention consists in determining the longitudinal profile of the maximum temperatures along the weld, so that the position of the profile with respect to the weld, in particular with respect to the central axis of the measuring field of the weld.

According to an additional characteristic of the method of the invention, compares the said longitudinal profile of the maximum temperatures, respectively the position of the profile, to a longitudinal reference profile, respectively to a reference position of this longitudinal profile, and a signal is produced indicating that the difference between the determined longitudinal profile and the longitudinal reference profile, respectively between the determined position and the reference position of this longitudinal profile, reaches or exceeds a predetermined value.

The longitudinal reference profile of the maximum temperatures can in particular be a predetermined right ordinate, corresponding to a predetermined temperature level. This ordered may be determined by any appropriate means, especially by an expert system taking into account the nature and respective thicknesses of the strips to be assembled, as well as the welding conditions to be applied, the result of empirical knowledge industrial practice.

This longitudinal profile of the maximum temperatures can also be subjected to various treatments known per se, for example a smoothing.

In particular, the longitudinal profile of the maximum temperatures can be divided into several sections or partial profiles, which then undergo individual treatments mentioned above for the complete longitudinal profile. For example, each section can include a constant number of individual profiles can be determined for it the average maximum temperature, which will be considered as the maximum temperature of said section, which will determine a smoothed longitudinal profile of the maximum temperatures welding.

We will now describe in detail a particular implementation of the inventive method, with reference to the accompanying drawings, in which<ul><li>Fig. 1 schematically shows a welding machine strips end to end; the</li><li>Fig. 2 shows (a) the evolution of the transverse profiles junction temperature and (b) the thermal map of a weld; the</li><li>Fig. 3 represents an individual transverse profile at the fusion involving the maximum temperature; the</li><li>Fig. 4 shows an individual temperature profile after welding; the</li><li>Fig. 5 is a double diagram showing (a) changes in maximum temperatures along the weld and their average value, and (b) the coordinates of points corresponding to these maximum temperatures; the</li><li>Fig. 6 shows the effect of a misalignment of the ends of the strips on the profile of the maximum temperatures along the weld; and</li><li>Fig. 7 shows the influence of the cleanliness of the surface strips on the profile of the maximum temperatures along the weld.</li></ul>

The measurements were made using an infrared thermographic detection system, capable of ensuring a high scanning frequency and providing excellent spatial resolution.

The detection system was mounted on a tape welding machine of the known type illustrated in Figure 1. This is a welder wheels, installed in a continuous annealing line as it ensures the junction of successive bands by splicing of cold rolled steel coils. The thicknesses and widths of the assembled strips respectively ranged from 0.35 mm to 1.8 mm and from 600 mm to 1600 mm.

The welding machine shown schematically in Figure 1 substantially comprises a shear 1, disposed on the path of the strip, and a movable frame 2 transversely to the strip. The strip to be welded moves perpendicularly to the drawing plane. The movable frame 2 carries a first pair of rollers 3, which are planarizing rollers and a second pair of rollers 4, which are the welding rollers. The two rollers 3, 4 are higher vertically movable to take account of the thickness of the strips to be welded.

The operation of such a welding machine is well known in the art. Suffice it to recall here briefly. The first strip solder is positioned in the shears 1, which pissed off the rear end of the strip to give it a straight edge. A second strip, which follows the first, is in turn sheared leading end thereof, and is positioned relative to the rear end of the first strip. The strips can be positioned end to end or have some overlap; their junction zone is normally located in the path of rollers 3, 4.

When the bands to be welded are in place, the movable frame 2 turns from left to right movement, in Figure 1, and the planarizing rollers 3, followed by the welding rollers 4 roll over the junction area of ​​the bands, over the entire width thereof, in a race to go wherein the leveling foregoing welding. At the end of this stroke to go, the movement of the movable frame 2 is reversed, and it performs a return stroke, during which the leveling following welding. At the end of the return stroke, the movable frame 2 is returned to its original position; the strips are welded to one another and the web thus obtained is sent to the continuous process that follows, until it is necessary to perform the next weld at the end of this strip.

During the race to move the movable frame 2, the planarizing rollers 3 have the effect of eliminating any excess thickness of the strips by rolling their overlapping ends. During the return stroke, the planarizing rollers realize a forging of the weld, which may disturb the level and the temperature distribution in the weld. The temperature of the cord after leveling can be sufficiently representative of the quality of the weld. It is therefore preferable to measure the temperature of the weld between the weld and the leveling during the return stroke of the frame 2. For this purpose, the thermography apparatus 5 will be placed between the planarizing rollers 3 and the pebble welding 4. Furthermore, a too close scan of the weld point can lead to an excessive sensitivity to disturbances which are not always detrimental to the quality of the weld, for example sparks. By against a furthest extent of the weld point can be insensitive to significant variations in temperature for welding, due to the rapid diffusion of heat in the weld and in the neighboring area and local cooling which results. To account for these conditions, it is preferable that the temperature measurements are carried out at less than 150 mm, and preferably approximately 100 mm, upstream of the welding point in the return stroke. This distance allows to take into account congestion welding rollers 4 and thermographic device 5. If necessary, the measuring point may be referred by interposing a reflective mirror between that point and the measuring apparatus.

The instantaneous field of the measurement apparatus 5 the detector is moved in a transverse scanning movement, known per se, in order to meet the transverse temperature profile of the weld and the heat affected zone in the vicinity thereof . Combined movement of the movable frame 2 along the welding, the cross-scan allows measurement of various successive transverse temperature profiles, which together form a thermal map of the weld and the heat affected zone in the vicinity thereof.

At any moment of the welding operation, it raises the temperature profile at the location of the weld; this profile corresponds to the maximum temperature at this time. It is indicated by the point A in the route of Figure 2. At the same time, in front of the welding point, for example at B, the strips are heated by conduction and the temperature begins to increase; behind the welding point, for example C, the heat generated by the welding propagates in the strips, the profile flattens and widens to continuously as one moves away from the point A. Figure 2a thus shows, in three dimensions, a thermal map of the weld and the heat affected zone in the vicinity thereof at the instant when welding is performed at the point A. an analogous thermal map can be set for any position occupied by the point A along the weld. The thermal map may most commonly be presented, as shown in Fig. 2b, by a succession of measured temperature values ​​gradually as welding, along the transverse profile (scanning field) and along the weld. Zones of the same representation in this Fig. 2b (or dotted hatching) are insulated between the limits indicated on the scale that accompanies the heat map; this scale indicates the minimum and maximum values ​​for each zone.

Figure 3 shows a transversal profile of temperature at a point A of Figure 2; the vertical axis indicates the temperature in ° C and the horizontal axis indicates the distance from the point of welding, on both sides thereof, transversely to the weld. In principle, the temperature distribution in the weld and in the heat affected zone in the vicinity thereof is symmetrical with respect to the maximum temperature prevailing at the spot weld; thereby, the profile is symmetrical with respect to the temperature axis.

Figure 4 shows a transversal profile of temperature, similar to that of Figure 3 but raised at a point C of Figure 2. It is noted firstly that the maximum value of the temperature of the profile is significantly lower than the profile of Figure 3, and secondly that the profile is flattened as a result of the propagation of heat that tends to equalize the temperatures.

In these Figures 3 and 4, are plotted in dashed lines, profiles deformed relative to the reference patterns (R) in solid outline.

In Figure 3, the deformed profile (a) is offset towards the left and it therefore has no longer a perfect symmetry with respect to the median axis of the weld. In addition, the width of the profile at a predetermined temperature, for example T₁, is amended in proportions which can cause unacceptable degradation of the quality of the weld. Similarly, the maximum temperature of the deformed profile (a) is smaller than the reference profile (R). Too low maximum temperature soldering point can result from setting or incorrect operation of the welding machine and lead to a poor quality weld.

In Figure 4, the profile in chain line (b) is substantially distorted relative to the reference profile, having lost symmetry and having a maximum temperature substantially lower than that of the reference profile (R). This kind of distortion may be a sign of uneven cooling which can compromise the quality of the weld. Profile (c), drawn in broken lines, remains symmetrical but is well below the reference profile (R), which can be indicative of a too rapid cooling of the weld.

Figure 5 relates to the temperature distribution along the length (L) of the weld. In Figure 5a, there is shown the evolution usually called longitudinal profile of the maximum temperatures measured in each measuring point along the weld bead, thus each position of the point A in Figure 2. The maximum temperature that was observed remains between about 900 ° C and 975 ° C, with an average temperature of 933 ° C. The higher values ​​observed at both ends of the weld are not significant, since they are due to the proximity of the edge of the strips, which disturbs the propagation of heat.

It is possible to check, by means of this longitudinal profile, the maximum temperature of the solder is, at any point or at least a sufficient number of points of the weld, between two limit values; in this regard, it should be noted that the most significant is the lower limit value, which may be insufficient to achieve a good weld.

Figure 5b shows the variation in the longitudinal direction of the strips, the position of the point where it has raised the maximum temperature of the solder, for each of the profiles corresponding to successive points A. The position of successive points is reported to the central axis of the measurement of the welding field. It is noted that in the illustrated example, the maximum temperature remains substantially centered on the axis along the weld. A marked deviation from this diagram can express poor positioning tapes or poor clamping strips in the welding machine.

The longitudinal profile of the maximum temperatures shown in Figure 5a still can detect various imperfections such as misalignment of the bands or insufficient cleaning of these bands, which jeopardize obtaining a good weld quality.

Figure 6 illustrates the detection of an alignment of the strips to the assembly fault. Part (a) of the figure shows, in exaggerated form, two bands (6, 7), the edges to be joined are crossed instead of butt joints or present a substantially constant recovery. Also indicates the track (8) of the weld bead deposited by transversely welding machine of Figure 1. The longitudinal profile of the maximum temperatures of this cord has two horizontal portions corresponding to the respective paths of the welding machine on both bands; these portions are separated by a clear hollow reflecting a maximum temperature drop in the zone of intersection of the edges of the two strips. The length of this hollow is an indication of the length of the crossing region of the edge of the strips dan s the junction zone; it varies according to the importance of misalignment of the bands.

The cleanliness of the strips to be joined can also be enjoyed through the longitudinal temperature profile. Figure 7 is an example. It represents a longitudinal profile of the maximum temperatures along a weld between two strips, properly positioned with respect to one another, but the left half was coated with grease while the right half was clean. The right side of the profile is steady, while the left side is strongly disturbed.

The method of the invention offers a reliable means of controlling the quality of a butt weld, or with a certain overlap, of two metal strips. It can also detect the causes of various defects observed in these welds, and to propose remedies to eliminate these defects. In particular, comparison of temperature profiles deformed with reference profiles led to the identification of anomalies of operation of the welding machine or the welding sequence and therefore remedied.