METHOD AND SYSTEM FOR SUPPRESSING FLUCTUATION OF WIDTH IN HOT ROLLED STRIP OR SHEET METAL

Abstract: A method and system for suppressing necking and hunting of a hot rolled strip in a hot rolling line includes holding of the strip temperature at the outlet of a finishing mill at a temperature immediately above a transformation temperature. Air cooling of the strip is performed from the transformation start point to the transformation end point. The transformation end point, rapid cooling by water cooling is performed thereafter.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a method and system for suppressing fluctuation of width in a hot rolled strip or sheet metal, in a hot mill line. More specifically, the invention relates to a technique for cooling hot rolled strip or sheet metal transferred from a finishing mill to a coiler with suppressing fluctuation of width. Description of the Background Art In general, hot rolled strip is transferred from a finishing mill to a coiler in a hot mill line. When the leading edge of the hot rolled strip reaches the coiler and is coiled by the coiler, an impulsive tension force may be exerted on the strip. This impulsive tension force is transmitted throughout the hot rolled strip between the finishing mill and the coiler. As is well known, such impulsive tension force may particularly subject to the portion of the strip at a position downstream of the finishing mill in several tens meter to serve as a force causing longitudinal expansion. Consequently, necking may occur at the portion where the impulsive tension force affects, to reduced the width of the strip. Namely, in the conventional hot mill line, the hot rolled strip from the finishing mill is transferred through a run-out table and cooling stage where a cooling device discharging cooling water toward the hot rolled strip is provided, to the coiler. A pair of pinch rollers are provided in the vicinity of the coiler for assisting coiling. In the usual layout of the hot mill line, the finishing mill and the coiler is distanced at about 150 meters. Along the path of the hot rolled strip between the finishing mill and the coiler, a thickness gauge, a shape monitor, a width gauge, a thermometer and so forth are arranged. These strip condition monitoring facilities are generally provided in the vicinity of the outlet of the finishing mill. In order to allow arrangement of these strip condition monitoring facilities. a distance about 10 meters has to be provided between the finishing mill to the inlet of the cooling stage. Therefore, the hot rolled mill from the finishing strip has to be transferred in uncooled condition for about 10 meters. On the other hand, in order to hold the coiling performance and configuration of the end of the coil in good condition, the coiler should be driven at a leading speed which is 1.1 to 1.3 times higher than the line speed of the strip. Due to this difference of the speed between the coiler and the strip, impulsive tension force may be generated at the beginning of coiling. This impulsive tension force causes local necking particularly at portion of the strip where deformation resistance is small. In the experience, it has been appreciated that the impulsive tension force particularly locally affects the configuration of the strip at the portion about 20 meters from the finishing mill to cause local necking. Once the leading end of the strip is coiled by the coiler, the coiler speed becomes synchronous with the line speed of the strip. At the portion of the strip following the portion where necking is occurred, hunting in the width to fluctuate the width of the strip occurs. Such hunting in the width is considered to be caused by temperature difference influenced by skid marks at the outlet of the finishing mill and/or by relationship between hot strength of the strip and a unit tension. In order to suppress necking and hunting set forth above, JP-A-59-10418 discloses a system including a looper or pinch rollers which is vertically movable between the finishing mill and the coiler. The looper and pinch roller are responsive to the tension force to be exerted on the hot rolled strip for providing extra length of strip in order to absorb the extra tension force and whereby regulate the tension force to be exerted on the strip. On the other hand, JP-A-56-56705 discloses a method for absorbing the impulsive tension force by means of pinch rollers. In the disclosure, the pinch rollers pinch the hot rolled strip, hold the strip until the coiler speed becomes synchronous with the line speed, and release pinching force after the tension is substantially regulated. Furthermore, JP-A-49-23751 proposes to provide wider width for the portion of the hot rolled strip, where the necking is possibly occurred. The extra width to be provided for the possible portion to cause necking, will be determined at a value corresponding to reduction magnitude of the width due to necking. In the alternative, JP-A-49-23751 also proposes a technique to perform rapid cooling for the strip so as to provide sufficient deformation resistance to the strip for preventing the strip from causing deformation including the necking. In JP-A-59-10418 , since the extra length of the strip is provided through the looper or pinch rollers, tension force at the initial stage becomes insufficient to hold the coiled leading end portion of the strip in good configuration. Especially, when waving is caused in the strip, the length of the strip to be provided by the looper or pinch roller becomes too excessive to make it possible to establish the metal strip coil in the desired coil configuration. On the other hand, in case of the Japanese Patent First Publication 56-56705 , the pinch rollers should be provided pinching force substantially corresponding to the possible impulsive tension force. Therefore, relatively bulky construction of the pinch roller is required for increasing the facility cost. Furthermore, in order to drive such bulky construction of the pinch rollers, relatively large electric power should be consumed. In addition, in case of the thin strip which tends to cause waving extra length of the strip may be provided between the pinch rollers and the coiler to reduce the tension force to be exerted on the strip therebetween. In the worst case, the substantial waving of the strip may provide the extra length of the strip to lose tension to be exerted on the strip. Therefore, similarly to that discussed about the technique of JP-A-59-10418 , the tension force becomes insufficient to hold good coil configuration. In such case, in order to make the coil configuration in good shape, the mandrel of the coiler has to be accelerated again after the pinch rollers are released. By accelerating the mandrel, the impulsive tension force may be exerted on the strip to cause necking and/or hunting. In case of the JP-A-49-23751 , in order to satisfactorily and completely compensate the reduction of the strip width in necking, it is necessary to provide the extra width in the portion of 50 meters in length which corresponds to 7 to 8 meters of the sheet bar. On the other hand, the longitudinal region to cause necking is about 20 meters. Therefore, the extra width of the strip may be lend in as length of 30 meters. When the coil with the extra width portion is processed in the cold mill line for example, edge folding may occur at the portion where the extra width is maintained when edge portion control is performed. In order to avoid possibility of causing edge folding, slow-down line speed in the cold mill line becomes necessary. Consequently, the conventionally proposed systems are not satisfactory in suppressing necking and/or hunting of the strip width, at all. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a method and system for satisfactorily suppress necking and hunting in the strip. Basically, it is found that necking and hunting will occur at the portion of a strip, where hot strength is small. In occurrence of necking, reduction of the width of the strip width occurs through the portion where the hot strength is small. Therefore, when the region of the portion of the strip where the hot strength is small, is limited, reduction of the width due to expansion of the strip length is distributed through the limited region. As a result, magnitude of reduction at the region becomes substantial. In other words, when the region where the hot strength is small, extends in relatively long range, the reduction is distributed through relatively long range to make magnitude of reduction of the width in each section smaller. Based on the idea set forth above, the present invention includes holding of the strip temperature at the outlet of a finishing mill at a temperature immediately above a transformation temperature. Air cooling of the strip is performed from the transformation start point to the transformation end point. Rapid cooling by water cooling is performed thereafter. According to one aspect of the invention, a method for suppressing fluctuation of width of a hot rolled strip transferred through a path extending from a finishing mill to a coiler in a hot rolling line comprises the steps of: maintaining the temperature of the hot rolled strip at an outlet of a finishing roll at a temperature slightly above the Ar₃ transformation temperature; performing air cooling of the hot rolled strip while it travels through the path, until the temperature of the hot rolled strip drops below a transformation end point; and discharging liquid state cooling medium onto said strip after the temperature of the hot rolled strip drops below the transformation end point. According to a preferred embodiment of the invention, the above-mentioned method further comprises the steps of: providing a plurality of nozzles for discharging the liquid state cooling medium along the path in alignment; connecting the nozzles to a cooling medium source via flow control valves; maintaining the temperature of the hot rolled strip at an outlet of a finishing roll at a temperature slight above the Ar₃ transformation temperature; setting material data including Ar₃ transformation point; deriving the transformation end point, and whereby determining a switching position in the path to terminate air cooling and to start cooling by the liquid state cooling medium on the basis of the transformation end point; transferring the hot rolled strip through a path extending between the finishing mill to the coiler; and controlling the flow control valves in such a manner that the flow control valves associated with nozzles oriented upstream of the transformation end point are shut-off and the flow control valves associated with nozzles oriented downstream of the transformation end point are open. In a method and process set forth above, it is preferred to include a further step of deriving orientation of the switching position as a distance La from the outlet of the finishing mill. In the practical process, the switching point La is determined by an equation of: La = [{(ϑ F - ϑ T ) x γ x β x T + H T x γ x T}/{α A x ϑ T }] x 6 x 10⁻² x V (1) where ϑ F is the temperature of the hot rolled strip at the outlet of the finishing mill (°C); ϑ T is a temperature at the transformation end point of the strip (°C); ϑ is a representative temperature during cooling, for example a medium temperature between cooling start and cooling end (°C) ϑ s is a surrounding temperature (°C) γ is density of steel (Kg/m³); β is relative temperature (kcal/kg °C); T is a thickness of the strip (mm); H T is latent heat of transformer (kcal/kg); α A is a heat ttransfer coefficient in air cooling (Kcal/m² hr °C); and V is a line speed of the strip (m/min). In the alternative, the switching point can be detected by means of least one sensor for monitoring state of the hot rolled strip and detecting the hot rolled strip at transformation end point for switching cooling mode from the air cooling to cooling by the liquid state cooling medium. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a fragmentary illustration of the preferred embodiment of a section in a hot mill line transferring hot rolled strip from a finishing mill to a coiler; Fig. 2(a) and 2(b) are charts showing material strength and strip temperature in relation to the distance from the finishing mill; and Fig. 3(a) and 3(b) are charts showing variation of the strip width in the invention and prior art. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1 , the preferred embodiment of a hot mill line for implementing suppression of fluctuation of width of hot rolled strip 2 , according to the present invention, is particularly directed to a transfer section for transferring the hot rolled strip 2 from a finishing mill 1 to a coiler 6 . The transfer section includes an upstream side run-out table 3 U , a cooling device 4 , a downstream side run-out table 3 D and a pair of pinch rollers 5a and 5b . The hot rolled strip 2 is transferred through the transfer section. A plurality of transfer rollers 3a , 3b ... 3n are provided between the uptream and downstream run-out tables 3 U and 3 D . An X-ray thickness gauge 7 , a shape monitor 8 , strip width gauge 9 and a thermometer 10 are provided along the upstream run-out table 3 U . On the other hand, a thermometer 11 is provided along the downstream run-out table 3 D . In the preferred hot rolling process, the temperature ϑ F at the outlet of the finishing mill 1 is adjusted slightly above a transformation temperature Ar₃ of the strip. Therefore, transformation of the hot rolled strip occurs in the vicinity of the outlet of the finishing mill 1 . If the transformation start point is set at the position between stands of the mills in the hot mill line, material strength upon transformation rapidly drops. Therefore, the tension force to be exerted on the strip between the mill stands becomes excessive to cause rapture to generate semi-finished products. At the same time, rolling in γ + α dual phase region may cause substantial variation of the deformation resistance, i.e. material strength which may results in fluctuation of the thickness of the strip. On the other hand, if the transformation start point is set at a position close to the coiler, it becomes difficult to control cooling performance in relation to a desired coiling temperature. Furthermore, in order to set the transformation start point near the coiler, the temperature of the strip has to be maintained above the transformation temperature through relatively long transferring range. This naturally requires high heating temperature to degrade fuel consumption rate. Therefore, the preferred position of the transformation start point is in the vicinity of the outlet of the finishing mill as proposed. The shown embodiment of the system thus controls the temperature of the strip at the outlet of the finishing mill at the temperature slightly above the transformation temperature so that transformation start point is set in the vicinity of the outlet of the finishing mill. For controlling the strip temperature at the outlet of the finishing mill, a controller 13 is provided in the system. The controller 13 is connected to the thermometer 10 and the thickness gauge 7 to receive therefrom strip temperature indicative data ϑ F and thickness indicative data T and other gauges to receive various control parameters therefrom. The controller 13 is also connected to an operation unit 14 including a memory 15 containing data such as transformation end temperature ϑ T (°C), transformation latent heat H T (kcal/kg), heat transfer coefficient α A (kcal/m² hr °C) and so forth. These data, e.g. Ar₃ transformation temperature ϑ T , transformation caloric value H T , heat transmission rate α A and so forth are set in the memory 15 in relation to the kind of strip or sheet metal to be produced. The controller 13 is further connected to a detector 12 for monitoring the rotation speed of the rolls. The detector 12 produces the roller rotation speed indicative data and feeds the same to the controller 13 . The controller 13 processes the roller rotation speed indicative data to derive the line speed V (m/min) in terms of the diameter of the roll. On the other hand, the controller 13 further controls the cooling device in order to perform air cooling of the strip for a predetermined distance from the transformation start point. The distance between the outlet of the finishing mill and the transformation end point will be hereafter referred to as "air cooling range". The controller 13 derives the length La of the air cooling range on the basis of the transformation end temperature ϑ T and other input data. An arithmetic operation is performed by the controller 13 utilizing the following equation (1): La = [{(ϑ F - ϑ T ) x γ x β x T + H T x γ x T}/{α A x ϑ T }] x 6 x 10⁻² x V (1) where ϑ is a representative temperature during cooling, for example a medium temperature between cooling start and cooling end (°C) ϑ s is a surrounding temperature (°C) γ is density of steel (Kg/m³); β is relative temperature (kcal/kg °C); T is a thickness of the strip (mm); The cooling device 4 comprises a plurality of cooling water discharge nozzles 4₁ , 4₂ , 4₃ ... 4 n . These nozzles 4₁ , 4₂ , 4₃ ... 4 n are aligned along the path of the hot rolled strip for transferring the strip from the finishing mill 1 to the coiler 6 . Each of the discharge nozzles 4₁ , 4₂ , 4₃ ... 4 n is connected to a cooling water source 4a via a cooling water delivery piping 4b . Electromagnetic valves 16₁ , 16₂ , 16₃ ... 16 n are associated with respective discharge nozzles 41 , 42 , 43 ... 4n for establishing and blocking connection between the cooling water source 4a and the discharge nozzle. The electromagnetic valves 16₁ , 16₂ , 16₃ ... 16 n are, on the other hand, connected to a drive signal generator circuit 17 to be controlled the position between open position establishing connection between the cooling water source and the valve and close position blocking the connection. In order to control the valve positions of the electromagnetic valves 16₁ , 16₂ , 16₃ ... 16 n , the drive signal generator circuit 17 generates drive signals and selectively feeds the drive signals to the electromagnetic valves. Namely, based on the length of the air cooling range as derived through the arithmetic operation utilizing the aforementioned equation (1), the controller 13 derives the electromagnetic valves to be placed at the closed position and at the open position to selectively control the drive signals so that only electromagnetic valves to be operated to the open positions may be driven by the drive signals. By selectively feeding the drive signals to the electromagnetic valves, some of the electromagnetic valves located at the upstream side are held in closed position so as to block the cooling water. Therefore, the hot rolled strip is cooled by exposing to the air so as to maintain the temperature of the strip within a transformation range from the transformation start point to the transformation end point. Figs. 2(a) and 2(b) shows variation of the material strength and strip temperature at respective positions in the path of the hot rolled strip between the finishing mill and the coiler as cooled in the preferred process. As will be seen from Fig. 2(a) and 2(b) , by cooling the hot rolled strip transferred from the outlet of the finishing mill is at first cooled by air cooling up to the transformation end point E which is determined by the length La of the air cooling range in relation to the transformation start point S. As will be appreciated, by air cooling, drop of temperature of the hot rolled strip becomes rather slow to expand the transformation range. Therefore, when the impulsive tension force is exerted on the strip to cause expansion in longitudinal direction, reduction of the width of the strip may be distributed over relatively wide range, i.e. throughout the transformation range, to make the reduction magnitude at each section of the strip small. Furthermore, by moderately cooling the strip, rapid change of the material strength can be suppressed to successfully prevent the strip from causing necking and hunting in the width. In order to confirm the effect of the preferred process and system according to the invention, experiments are performed. Followings are discussion about the experiments performed with regard to the preferred embodiments of the process and system for cooling the hot rolled strip. EXAMPLE 1 In the first experiment, hot rolling is performed for extra low carbon steel of 0.001C%. The temperature of the hot rolled strip at the outlet of the finishing mill was 890 °C. On the other hand, the temperature of strip at the coiler was 540 °C. The slab was hot rolled to obtain strip of 3.2 mm thick and 1468 mm width. The air cooling range La was set in a length of 75 m. After the transformation end point, water cooling was performed for rapid cooling. In order to compare with the foregoing inventive process, a comparative experiment was performed utilizing the same material and setting of the temperature at the outlet of the finishing mill and at the coiler. In the comparative experiment, air cooling was performed in a first 10m and subsequently water cooling was performed. The result of the invention and comparative example are shown in the appended table 1. As will be seen from the table 1, by the invention, the magnitude of necking was reduced to about 1/3 of the comparative example. Similarly, by the invention, the magnitude of hunting was reduced to about 1/5 of the comparative example. EXAMPLE 2 In the second experiment, hot rolling is performed for extra low carbon steel of 0.001C%. The temperature of the hot rolled strip at the outlet of the finishing mill was 890 °C. On the other hand, the temperature of strip at the coiler was 700 °C. The slab bar was hot rolled to obtain strip of 3.5 mm thick and 1524 mm width. The air cooling range La was set in a length of 94m. After the transformation end point, water cooling was performed for rapid cooling. Similarly to the foregoing first experiment, a comparative experiment was performed utilizing the same material and setting of the temperature at the outlet if the finishing mill and at the coiler. In the comparative experiment, air cooling was performed in a first 10m and subsequently water cooling was performed. The result of the invention and comparative example are shown in the appended table 2. From the table 2, substantial improvement in magnitude of necking and hunting was obtained. EXAMPLE 3 In the third experiment, hot rolling is performed for low carbon steel of 0.04C%. The temperature of the hot rolled strip at the outlet of the finishing mill was 820 °C. On the other hand, the temperature of strip at the coiler was 540 °C. The slab was hot rolled to obtain strip of 1.6 mm thick and 928 mm width. The air cooling range La was set in a length of 46m. After the transformation end point, water cooling was performed for rapid cooling. Similarly to the foregoing first and second experiments, a comparative experiment was performed utilizing the same material and setting of the temperature at the outlet if the finishing mill and at the coiler. In the comparative experiment, air cooling was performed in a first 10m and subsequently water cooling was performed. The result of the invention and comparative example are shown in the appended table 3. From the table 3, substantial improvement in magnitude of necking and hunting was obtained. As will be seen from the table 3, by the invention, the magnitude of necking was reduced to about 1/3 of the comparative example. Similarly, by the invention, the magnitude of hunting was reduced to about 1/2 of the comparative example. EXAMPLE 4 In the fourth experiment, hot rolling is performed for carbon steel of 0.36C%. The temperature of the hot rolled strip at the outlet of the finishing mill was 790 °C. On the other hand, the temperature of strip at the coiler was 540 °C. The slab bar was hot rolled to obtain strip of 1.6 mm thick and 918 mm width. The air cooling range La was set in a length of 46m. After the transformation end point, water cooling was performed for rapid cooling. Similarly to the foregoing first and second experiments, a comparative experiment was performed utilizing the same material and setting of the temperature at the outlet if the finishing mill and at the coiler. In the comparative experiment, air cooling was performed in a first 10m and subsequently water cooling was performed. The result of the invention and comparative example are shown in the appended table 4. From the table 4, not so substantial improvement in magnitude of necking and hunting was observed. This is occurred since the material strength drop in the transformation range in the carbon steel is not so substantial as that in the extra low carbon steel or low carbon steel. From these experiments set forth above, it was confirmed that the preferred process is particularly effective in the hot rolling process of extra low carbon steel and low carbon steel. It should be appreciated that though the shown embodiment arithmetically derives the transformation end point, it may be possible to employ a transformation ratio sensor in the path to detect the transformation end point for controlling the cooling device. Furthermore, though the shown embodiment uses water as a medium for rapid cooling of the strip, the cooling medium for rapid cooling is not limited to the water but can be replaced any appropriate coolant. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

Claims: A method for suppressing fluctuation of width of a hot rolled strip transferred through a path extending from a finishing mill to a coiler in a hot rolling line comprising the steps of: maintaining the temperature of said hot rolled strip at an outlet of a finishing roll at a temperature slightly above the Ar₃ transformation temperature; performing air cooling of said hot rolled strip while it travels through said path, until the temperature of said hot rolled strip drops below a transformation end point; and discharging liquid state cooling medium onto said strip after the temperature of the hot rolled strip drops below said transformation end point. A method as set forth in claim 1, which further comprises the steps of: monitoring the temperature of said hot rolled strip at the outlet of said finishing roll for generating an initial strip temperature data; setting material data including Ar₃ transformation point; arithmetically deriving said transformation end point; and determining a switching position in said path to terminate air cooling and to start cooling by said liquid state cooling medium on the basis of said transformation end point. A method as set forth in claim 2, wherein said switching position is determined as a distance La from the outlet of said finishing mill and derived from an equation of: La = [{(ϑ F - ϑ T ) x γ x β x T + H T x γ x T}/{α A x ϑ T }] x 6 x 10⁻² x V (1) where ϑ F is the temperature of the hot rolled strip at the outlet of the finishing mill (°C); ϑ T is a temperature at the transformation end point of the strip (°C); ϑ is a representative temperature during cooling, for example a medium temperature between cooling start and cooling end (°C) ϑ s is a surrounding temperature (°C) γ is density of steel (Kg/m³); β is relative temperature (kcal/kg °C); T is a thickness of the strip (mm); H T is latent heat of transformer (kcal/kg); α A is a heat ttransfer coefficient in air cooling (Kcal/m² hr °C); and V is a line speed of the strip (m/min). A method as set forth in claim 1, which further comprises a steps of: providing a plurality of nozzles for discharging said liquid state cooling medium along said path in alignment; connecting said nozzles to a cooling medium source via flow control valves; controlling said flow control valves in such a manner that the flow control valves associated with nozzles oriented upstream of said transformation end point are shut-off and the flow control valves associated with nozzles oriented downstream of said transformation end point are open. A method as set forth in claim 4, which further comprises the steps of: monitoring the temperature of said hot rolled strip at the outlet of said finishing roll for generating an initial strip temperature data; setting material data including Ar₃ transformation point; arithmetically deriving said transformation end point; and determining a switching position in said path to terminate air cooling and to start cooling by said liquid state cooling medium on the basis of said transformation end point. A method as set forth in claim 5, wherein said switching position is determined as a distance La from the outlet of said finishing mill and derived from an equation of: La = [{(ϑ F - ϑ T ) x γ x β x T + H T x γ x T}/{α A x ϑ T }] x 6 x 10⁻² x V (1) where ϑ F is the temperature of the hot rolled strip at the outlet of the finishing mill (°C); ϑ T is a temperature at the transformation end point of the strip (°C); ϑ is a representative temperature during cooling, for example a medium temperature between cooling start and cooling end (°C) ϑ s is a surrounding temperature (°C) γ is density of steel (Kg/m³); β is relative temperature (kcal/kg °C); T is a thickness of the strip (mm); H T is latent heat of transformer (kcal/kg); α A is a heat transfer coefficient in air cooling (Kcal/m² hr °C); and V is a line speed of the strip (m/min). A method as set forth in claim 1 , which further comprises a sensor for monitoring state of said hot rolled strip and detecting said hot rolled strip at transformation end point for switching cooling mode from said air cooling to cooling by said liquid state cooling medium.

Inventors: HISHINUMA, ITARU C/O KAWASAKI STEEL CORPORATION, KAN, MEGUMI C/O KAWASAKI STEEL CORPORATION, KOMAMI, YUJI C/O KAWASAKI STEEL CORPORATION, TAMAI, TOSHIYUKI C/O KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: B21B37
IPC codes: B21B15, B21B1, B21B37