LINEAR MOTOR FOR PROPULSION OF HOT-ROLLED STRIP BETWEEN THE LAST MILL STAND AND THE WIND-UP REEL

Abstract: The present invention relates to the runout table between the last mill stand (1) and a wind-up reel (2) for a rolled strip (4) in a hot rolling mill, wherein the runout table, counting from the last mill stand, is divided into a transport zone (3), a cooling zone (6), and a final zone (7), wherein the strip along the runout table has a first propulsion device comprising the transport zone and consisting of a roll table with rollers (5) with driving motors of their own, and a second propulsion device comprising the whole of or parts of the cooling zone and the final zone and consisting of a linear motor with a stator (8), and wherein the strip constitutes the movable part of the linear motor.

Description: Linear motor for propulsion of hot-rolled strip between the last mill stand and the wind-up reel TECHNICAL FIELD A hot rolling mill comprises, between the last mill stand and the final wind-up reel, a runout table where the rolled strip is cooled down from the rolling temperature, 900- 850°C, to the final temperature, 550-600°C, at the reel. The runout table may amount to about 100 m. For propulsion of the hot-rolled strip the runout table according to the prior art consists of a roller table with a large number of rolls which are each driven by their own motor. The intention of the present invention is to replace parts of the roller table, counting from the wind-up reel and backwards towards the last mill stand, by a linear motor consisting of a multi-pole, multi-phase stator winding with an associated laminated steel core and wherein the strip functions as the "rotor" of the linear motor. BACKGROUND ART, THE PROBLEMS To be able to explain the inventive concept, it may be suitable, by way of introduction, to divide the runout table into three zones. The first zone which the strip enters starts directly after the last mill stand and will here ¬ inafter be referred to as a transport zone. This is a rela ¬ tively short zone. Then comes a zone where the strip is actively cooled by cooling agents of various kinds. This zone will consequently be referred to as a cooling zone. The third and last zone between the active cooling zone and the wind-up reel will hereinafter be referred to as a final zone. A certain amount of cooling of the strip because of the surrounding agents will also take place when the strip passes through the final zone. To place the invention in its proper context, some of the most important process engineering functions which the run- CONFIRMATION COPY out table is to fulfil, as well as the problems which may arise in this connection, will first be described. As mentioned above, the hot-rolled strip, while being propelled from the last mill stand to the wind-up reel, is to be cooled so its temperature is reduced by approximately 300°C. This cooling is for the most part performed by spraying with large quantities of water within the cooling zone. The water spraying may influence the strip both from below and from above. The optimum rolling speed of the rolling mill is determined, inter alia, by the thread speed that can be maintained along the runout table. In case of thin strips, there is a risk of the strips folding down between the rollers. When the speed is too high, there is a risk of the strip rising from the rollers and hence of losing traction force in the forward direction. Particularly sensitive is the region within the last part of the final zone where the head end of the strip has a tendency of lifting upwards. It is also important that the speed of the strip along the whole runout table be kept constant . When threading the strip, a pulling force is desired which maintains the strip stretched along the runout table until the leading end of the strip has reached the wind-up reel and there formed a few turns. During rolling, the propulsion device used, that is, the roll table according to the state of the art, is to trans¬ port the strip with a tension from the rolling mill to the wind-up reel. When the trailing end of the strip has left the last mill stand, the propulsion device is to retain the strip such that it lies flat on the roll table. It is also important that the propulsion device should maintain the rolled strip centred on the roller table. For the above reason, the propulsion device should exert both a pulling force and a centring force on the strip as well as hold up the strip by way of a lifting force. If the above process engineering conditions are not ful¬ filled, the strip normally turns into scrap with ensuing time expenditure for preparing new strip to be rolled. Several of the above-mentioned problems also arise when it is a question of cold rolling. A large number of patents and publications describing part-solutions to the above- mentioned problems exist within that field. Some of these will be described in the following. To ensure that the strip does not rise from the roll table, Japanese patent application No. 63-205892 (22) of 26 Feb. 1988 describes a device in which one or more of the rollers in the roll table are replaced by a special type of roller with built-in permanent magnets or electromagnets which exert an attractive force on the strip, preventing the free end from rising. The surface of these rollers is coated with a non-magnetic and non-conducting material. A similar prin¬ ciple is presented also in SU 1486199 Al. Using linear motors in connection with roll tables is also described in several patents. However, in these cases it is not a question of liner motors for propelling the rolled strip. DE 31 06 897 Al, "Electromagnetische Verstellungs- einrichtung fur BlechbSnder" , describes a device with linear driving elements placed in the roll table for the purpose of maintaining the steel strip centred in the roll table. According to the patent specification, the desired centring is achieved with the aid of two driving elements placed side by side. To handle the considerable thermal and environmen- tal aspects, the windings must be protected both mechani¬ cally with, for example, slide bars, and thermally by cooling the region around the linear motors with liquid or gas. The same patent specification states that the desired centring may be obtained with the aid of two brake electro¬ magnets, one on either side of the strip. US 4,407,438 describes a roll table for hot-rolled strips comprising one or more linear driving elements located between two consecutive rollers intended to achieve a travelling force field on the front end of the strip. The field is adapted to have a force component at right angles to the direction of movement of the strip in order to adjust and center the strip on the roll table, all according to the patent specification; that is, it is not a question of a device for propelling hot-rolled strip but only for centralizing the strip on the runout table. When the rolled strip leaves the last mill stand and enters the transport zone, it has, as mentioned above, a tempera¬ ture of 850 - 900°C, that is, it is in an austenitic phase state and is thus practically to be regarded as a non- magnetic material. In spite of this, that part of the strip which is within the transport zone may serve as a "rotor" in a linear motor although the lifting force is small. When the strip enters the cooling zone, the strip material nearest the outer surfaces will first be cooled such that the tem- perature drops past the Curie point of the material, which lies at about 625°C for currently used materials, and these parts of the strip are transformed into a ferritic phase state. During the further passage of the strip within the cooling zone, the remaining cross section of the strip material gradually changes from an austenitic to a ferro¬ magnetic material. This also means that the strip material, largely from the beginning of the cooling zone and when passing through the final zone up to the wind-up reel, is to be regarded as a ferromagnetic material and may thus consti- tute the "rotor" in a linear motor, where both a lifting and a pulling force on the strip may be obtained. The production in a hot rolling mill may vary greatly. Normally, however, the thickness of the strip is between 1.5 and 16 mm and the width of the strip is between 0.6 and 1.5 m. The pulling force per surface unit is normally between 5 and 17 N/mm*-- and the transport speed is between 1.5 and 15 m/s. SUMMARY OF THE INVENTION The invention comprises replacing that part of a conven¬ tional roll table, which comprises the whole of or parts of the cooling zone and the final zone, by the stator part of a linear motor. It should thus be designed to be able to raise the strip from the base such that an "air gap" is formed between the thermal protection covering the stator of the linear motor and the strip, and to be able to provide a pulling force in the strip and hence achieve a speed of the strip which coincides with the speed of the strip within the transport zone, and to be able to centre the strip on the roll table, and finally, to give the leading end such a force component that, at the end of the final zone, that is, before entering the wind-up reel, it does not rise. Calculations made show that, by a suitable combination of mechanical dimensioning, current intensity and frequency for supplying the stator of the linear motor, it is possible to fulfil the above-mentioned requirements for the various strip dimensions and transport speeds which may occur in a hot rolling mill. DESCRIPTION OF THE PREFERRED EMBODIMENTS The accompanying figure shows the runout table in a hot rolling mill from the last mill stand 1 to the wind-up reel 2. Within the transport zone 3, the rolled strip 4 runs on a first propulsion device in the form of conventional rollers 5 which are each driven by a motor (not shown) . A second propulsion device within the whole of or parts of the cooling zone 6 and the final zone 7 in the form of a linear motor consisting of a stator 8 and the moving strip leads the strip to the wind-up reel. Between the thermal protec- tion 9 of the stator and the strip, the air gap of the linear motor is shown at 10.

Claims: CLAIM A runout table between the last mill stand (1) and a wind-up reel (2) for a rolled strip (4) in a hot rolling mill, wherein the runout table, counting from the last mill stand, is divided into a transport zone (3), a cooling zone (6), and a final zone (7) , characterized in that the strip along the runout table has a first propulsion device com¬ prising the transport zone and consisting of a roll table with rollers (5) with driving motors of their own, and a second propulsion device comprising the whole of or parts of the cooling zone and the final zone and consisting of a linear motor with a stator (8) , and wherein the strip constitutes the movable part of the linear motor.

Inventors: LOCKLEY ROBERT, LOCKLEY, ROBERT
Applicants: ASEA BROWN BOVERI, LOCKLEY ROBERT, ASEA BROWN BOVERI AB, LOCKLEY, ROBERT
Application year: 1995
Main classification: B21B39
IPC codes: B21B39