SURFACE TREATMENT OF METAL ARTICLES

Abstract: A camera (32) is arranged to map defects in the molten metal bath formed by scarfing using a scarfing torch (34) on a steel slab (37). The camera has a field of vision of a large area (36) of the slab (37) and records the defects as flashes of high intensity light the nature of which identifies the type of defect.

Description: SURFACE TREATMENT OF METAL ARTICLES This invention relates to the surface treatment of metal articles and is particularly concerned to provide an improved means of identifying and repairing faults at or near the surface of metal. The invention is particularly useful in the identification and removal of blemishes, cracks, tears and inclusions at or below the surface of articles of hot semi-finished continuous cast products. Typically such products are to be hot rolled into sheets or the like. Unfortunately the semi-finished products can have visible (or invisible) defects or weak points, in or immediately below the surface, e.g. fissures, tears, folds or the like. If these are carried into the rolled products rejects will arise and even damage of the surface of the roller cylinders can occur. It is thus usual to inspect the semi-finished products before proceeding with the rolling, to determine whether they can be rolled directly or whether they must be treated beforehand to move or repair the defects which they have, or even if they are of such low quality that they must be rejected and recycled. Because the semi-finished products are hot, an observer cannot examine them directly. There have been a number of proposals based on the principle of forming an image of the defects from a distance which will then serve as a guide for the repair of the surface usually by machine or hand operated scarfing. In US-A-5071705, for example, there is described a scarfing torch to remove blemishes from the surfaces of steel billets, ingots and the like in which the torch is fitted with a nozzle having a visual observation optical device extending co-axially inside a central gas feed pipe and situated a specific distance from the main orifice of the nozzle so that the bounds of its angle of view pass through the edges of the main orifice. It is one object of the present invention to provide a method and an apparatus in which the field of vision surveyed by an optical device associated with a gas torch can be greatly enhanced and to provide a method of removing surface defects from the metal article by a continuous process of observation, identification and removal. Accordingly, in one aspect the invention provides a method of detecting and removing defects from metal articles by scarfing, the method comprising the steps of: • directing an optical viewing device which is mounted adjacent a scarfing torch to view the hot surface of the metal article to be scarfed and to locate surface or subsurfaces defects and identify them by area and depth; • adjusting the position of the torch according to sensed data to scarf away the defects; and • recording the defects and the scarfed area. Preferably the scarfing torch supplies flame to overheat the surface of the metal to be scarfed so that the overheated metal produces flashes of high intensity light which are sensed by the optical device and are identified as defects to be scarfed. These are then photographed to record the defects. This step may be in the form of a first scarfing pass. Preferably the size of the flash is used to determine the size of the defect. Also, the shape of the flash of light is used to identify the nature of the defect. Specific shapes and their meaning is set out below. Most preferably the method includes the step of moving the torch along successive wide generally parallel passes. Typically the defect detection and elimination is performed on long, flat, metallic products at an ambient or elevated temperature produced either by ingot casting or a continuous casting apparatus. It is a feature of the invention that the method involves analysing the hot bath of molten metal using a camera following the scarfing operation to provide an image to be reviewed by an operator. It is preferred that the optical viewing device is used to visualise the entire surface and depth of the molten bath. The image can be used to give a real time picture which can be analysed by an operator or by comparator means. In practice, the metal article, typically a hot slab, is placed at a scarfing station and then scarfed. This creates a bath of molten metal as the surface metal is removed, and the optical viewing device tracks the scarfed surface and takes photographs of the high intensity flashes of light created by the defects which remain after the initial scarfing. A map of the surface of the slab has previously been created, and the photographs are compared to the map to establish where the defects remain. The flashes of the light recorded on the photographs identify the nature of the defects, so the operator can readily carry out a secondary scarfing for local treatment, i.e. removal of these. By the method of the invention defects can be identified and removed in one continuous operation which can proceed at a speed compatible with that of a continuous casting operation and, because of the external mounting of the optical device adjacent the torch, a wide surface area can be seen at any one time. This enables the operator to locate defects over a wide area and treat them readily. According to the invention in another aspect there is provided apparatus for the detection and elimination of defects on metal articles, the apparatus comprising a scarfing torch to melt a localised region of the surface of the metal, an optical viewing device mounted adjacent and preferably externally of the scarfing torch arranged to provide a field of vision embracing substantially the bath of molten metal and a computer controlled means to quantify and qualify the defects detected in the hot metal bath. Preferably the apparatus includes means to analyse the image provided by the optical device in the form of flashes of light, and to classify these as defects according to size and depth of defect and optionally by nature (crack or inclusion) and number. It is preferred that the apparatus includes means to provide instructions for positioning the torch, to create a map of the locations of the defects. It is much preferred that the torch has a generally circular front face having multiple flames enabling simple defect detection by a simple single movement or elimination by large passes and the torch includes means for instantaneous change from one operating processes to another. Preferably the apparatus has means to process both television type and industrial type images at a speed of 25 or 30 images per second, in colour or black and white. The optical device may be connected to a visual display unit remote from the hot metal article whereby an operator can observe the localised region of the surface and react to any visual defects shown by appropriate scarfing using the torch. The optical device may be a video camera or CCD camera (charge coupled device) to transmit images to a remote, operator-viewed screen or to a microprocessor-controlled automated system. The optical device may be in the form of an array of optical fibres connected to a remote control system. The optical device may be connected to computer-controlled means to interpret the signals received, identify defects and control movement of the scarfing torch to remove defects in direct response to the signals received. The apparatus may also include a video cassette recorder or the like for a permanent record. The optical device may be mounted, for example, on a frame beneath which the metal article to be treated is passed. (The torch may conveniently be mounted on the same frame although this is not essential). If desired, the optical device and the torch may be mounted to travel along their supporting frame so that they can be moved transversely across the metal surface passing beneath them. This option is particularly useful where semi-finished metal products are of considerable width, e.g. up to several metres. Alternatively, it may be necessary install a multiplicity of torches and optical devices on the frame spanning the metal surface. Preferably the camera operated vision system causes a permanent picture of the bath while the blow torch is operated. The image is optimised to visualise the useful zone in the field of vision of the camera in order to detect defects (in the bath, both for defects with lower and greater depth than that of the bath pattern). The parameterization of the camera shooting conditions is adapted to the types of defects which are expected to be found on the basis of the product development data analysis (shutter adapted to a micro-inclusions for example). The photograph device is automatically adjusted to the bath image for both cold and hot slabs at a temperature up to 1000°C and over. The image processing system preferably sets the reference point in relation to the bath for defect detection. records at any time the position of the point of reference in relation to the product to establish a map of the defect locations. selects automatically the "interesting" zones for defect detection on the bath image (classified as: shallow, fairly deep, very deep and located at the rear of the bath). The depth is actually linked to the measurement of the bath pattern. In one preferred embodiment of the system, 4 distinct zones have been isolated and analysed one by one. detects the flashes of light by contrast on the basis of the average level of grey thresholds for the zone in question. distinguishes flashes of light indicative of defects from flashes of light produced by dust particles coming into the camera's field of vision. counts the number of defects in the zone in question. measures the coordinates of each defect on the basis of two reference axes. records the shape and the surface of the defects for comparison with those previously encountered and recorded. compares the evolution of the flashes with a similar reference axis on all successive images where they appear in order to confirm the nature of defects. The image processing system processes both images recorded from the detection operation and images recorded from the defect elimination operation. The system can process simultaneously images taken on the basis of different filming parameters to both detect and eliminate gross and tiny defects. Embodiments of the invention are described by way of example only with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic representation in part section of a prior art apparatus for the surface inspection and treatment of hot semi-finished metal products; Figure 2 is a representation in perspective view corresponding to Figure 1 ; Figures 3 and 4 are views similar to Figures 1 and 2 respectively but of an apparatus according to the present invention; Figure 5 is a flow chart of representative steps; and Figure 6 is an elevation of one scarfing station; and Figure 7 is a series of sketches showing the method applied to different defects. In Figures 1 and 2 a scarfing torch 10 comprises an annular peripheral array of burner nozzles 11 surrounding a central orifice 12. Orifice 12 tapers from a narrow central position 13 to a wider central outlet 14 at the burner end of the torch and to a wider inlet 15 at the opposite end. A camera 16 is mounted in inlet 15 to have a field of vision passing through narrow central portion 13 and outlet 14 to form an elliptical vision region 17 on the surface of the hot metal product to be viewed. The camera screen may have a pixel density of the order of 750 x 500 pixels, or similar. As shown clearly in Figure 2, region 17 represents only a small proportion of area 18 of the metal surface that is melted by the array of burner nozzles 11. Thus, as can be seen, a fault zz within region 17 can be detected and treated but faults such as **, yy and xx within region 18 lie outside region 17. One device of the present invention, shown in Figures 3 and 4, is a torch 20 having an annular peripheral array of burner nozzles at its front wall and producing flames 21. A camera 26 is mounted externally of and spaced from torch 20 so as to have a field of vision 27 that is larger than and includes the complete area 28 of the metal surface that is melted by the array of burner nozzles. Thus all faults * * , xx, yy and zz now lie within the field of vision of the camera and can be treated appropriately by a scarfing operation. The method can include one sequence of steps shown in Figure 5. The method will typically nclude the steps but the sequence of the later ones may be changed. Because the field of vision is wide, and faults of all types can easily be seen, the operator can react quickly to repair the detected faults. The torch of the invention can thus cope with relatively fast moving hot articles travelling from a continuous caster to a hot rolling mill without disrupting the flow. The apparatus may include a computer and a monitor, and also a video cassette recorder so that a record can be kept of the faults detected and that they have been addressed. It is believed that this is the first time it has been possible to record such data in relation to moving hot articles at a steelworks. Figure 6 shows an elevation of one workstation. The operator sits in a cabin 30 mounted on a support 31 and arranged so that it can rotate. An articulated arm 32 extends forward from the cabin 30 and has a depending foot 33 to the end of which is pivotally mounted a scarfing torch 34. A camera 35 is mounted below the arm 32 and is arranged to view a substantial area 36 of a slab 37 mounted at an angle on a support not shown. In use the slab 37 is moved under operator control passed the torch 34 which accordingly scarfes a length of slab; the position of the slab is moved for each pass so that substantially the total surface area is scarfed. The camera 35 is arranged to view the bath of molten metal created by the scarf and following the torch; it takes photographs at the rate of 25 to 35 images per second. A surface which is free of defects will appear generally grey, but a defect will cause a high intensity light which will be recorded on the photograph. A map of the surface of the slab 37 has been pre-recorded, and the operator, or more typically a computer comperator, will overlay the photographs with the map. This provides the operator with a record of remaining defects and their character. The operator can then carry out a second pass to remove these. Figure 7 shows a sketch for the time when the photograph is taken of the bath following the scarfing pass for different defects. Figure 7A shows the events for the case where there is an inclusion more shallow than the bath; Figure 7B is for the case where there is an inclusion deeper than the bath. Figures 7C and 7D are the same for an L-shaped crack and Figures 7E and 7F for a T-shaped crack. The invention is not limited to the embodiment shown. The image may be in colour or black and white; successive images may be merged to provide one picture. The images may be interpreted using a neural network.

Claims: CLAIMS 1. A method of detecting and removing defects from metal articles by scarfing, the method comprising the steps of: • directing an optical viewing device which is mounted on a scarfing torch to view the hot surface of the metal article to be scarfed and to locate defects and identify them by area and depth; • adjusting the position of the torch according to sensed data to scarf away the defects; and • recording the defects and the scarfed area. 2. A method according to Claim 1 , wherein the scarfing torch supplies is used to scarf the surface and thereby flame to overheat the surface of the metal to be scarfed so that the overheated metal produces flashes of high intensity light which are tracked by the optical device and are identified as defects to be scarfed. 3. A method according to Claim 2, wherein the size of the flash is used to determine the size of the defect. 4. A method according to Claim 2 or 3, wherein the shape of the flash of light is used to identify the nature of the defect. 5. A method according to Claim 3 or 4, wherein including the stop of moving the torch successive wide generally parallel passes. 6. A method according to any preceding Claim, wherein the metal to be scarfed is at about 1000°C or higher. 7. A method according to Claim 6, wherein defect detection and elimination is performed on long, flat, metallic products at an ambient or elevated temperature produced either by an ingot casting or a continuous casting apparatus. 8. Apparatus for the detection and elimination of defects on metal articles, the apparatus comprising a scarfing torch to melt a localised region of the surface of the metal, an optical viewing device mounted externally on the scarfing torch arranged to provide a field of vision embracing substantially the bath of molten metal and a computer controlled means to quantify and qualify the defects detected in the hot metal bath. 9. Apparatus according to Claim 8, wherein including means to analyse the image provided by the optical device in the form of flashes of light, and to classify these as defects according to size and depth of defect and optionally by nature (crack or inclusion) and number. 10. Apparatus according to Claim 8 or 9, including means to provide create a map of the area to be scarfed. 11. Apparatus according to any of Claims 8 to 10, wherein the torch has a generally circular front face having multiple flames enabling simple defect detection by a simple single movement or elimination by large passes and the torch includes means for instantaneous change from one operating processes to another. 12. Apparatus according to any of Claims 8 to 11 , including means to process both television type and industrial type images at a speed of 25 or 30 images per second, in colour or black and white. 13. Apparatus according to any of Claims 8 to 12, including a monitor for simultaneous control by the operator, coupled to a video tape recorder to a document a data record of the defects detected.

Inventors: DETRONDE MICHEL, DETRONDE, MICHEL
Applicants: DETRONDE MICHEL, HECKETT MULTISERV PLC, DETRONDE, MICHEL
Application year: 1999
Main classification: B23K7
IPC codes: B23K7