System and method for preventing scale defects during hot rolling

Abstract: A system and a method are disclosed for preventing scale defects during hot rolling by hot rolling equipment having a scale breaker (FSB) provided at an entry side of a finishing mill line (F) composed of a plurality of rolling mills (F1 to F7) arranged in tandem. A descaler (D) is provided between a first stage rolling mill (F1) and a second stage rolling mill (F2) of the finishing mill line (F). A cooler (C) for cooling a hot rolled steel plate 1 is provided between the second stage rolling mill (F2) and a third stage rolling mill (F3) of the finishing mill line (F). A control device (8) is provided for controlling the descaler (D) and the cooler (C) to be selectively driven such that neither of the descaler (D) and the cooler (C) is actuated, one of the descaler (D) and the cooler (C) is actuated, or both of the descaler (D) and the cooler (C) are actuated, according to rolling conditions. Thus, the hot rolled steel plate 1 is rolled, with its oxide film thickness at an entry side of the third stage rolling mill (F3) being restricted to not more than a limiting oxide film thickness of 5 µm. Consequently, scale defects are prevented, and overcooling of the hot rolled steel plate 1 is suppressed, to improve the quality of a product.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system and a method for preventing scale defects on a finish rolling line of hot rolling equipment by descaling or cooling a surface of a material to be rolled (hot rolled steel plate) to suppress the formation of scale (oxide film). 2. Description of the Related Art In hot finish rolling, oxide film may grow after scale removal at an entry side of a row of finishing mills. Depending on the thickness of the oxide film, scale defects occur in a hot rolled steel plate to decrease the yield of the product and deteriorate its surface quality. To suppress the growth of oxide film, it has been performed empirically to set the temperature of the steel plate surface, or to control the temperature of the steel plate surface at the entry side of the row of finishing mills. When the growth of oxide film was suppressed by any of these conventional methods, the hot rolled steel plate was overcooled, or occurrence of scale defects was not fully prevented. FIG. 8 is a view showing another conventional method for preventing scale defects. In this drawing, a hot rolled steel plate 1, as a material to be rolled, passes, while being rolled, between a first stage rolling mill F1 and a seventh stage rolling mill F7 from an entry side to a delivery side (from left to right in the drawing). At a stage forward of the first stage rolling mill F1, a scale breaker FSB is placed for removing oxide film of the hot rolled steel plate 1 rough rolled by a roughing mill (not shown). High pressure water from a header 2 of the scale breaker FSB removes oxide film on the surface of the hot rolled steel plate 1. At an entry side of each of the second stage rolling mill F2 and the third stage rolling mill F3, descaling devices 12, 13 are placed. These descaling devices 12, 13 jet spray water when the thickness of oxide film on the surface of the steel plate is more than 10 µm. After being so descaled, the steel plate is rolled. However, when the descaling devices are arranged between the rolling mills as in FIG. 8 to perform descaling for oxide film more than 10 µm thick, the thickness of oxide film at the entry side of the third stage rolling mill F3 may exceed 5 µm as shown in FIG. 9. Finish rolling performed at an oxide film thickness of more than 5 µm results in the occurrence of scale defects on the surface of the hot rolled steel plate 1, debasing the quality of the product. A thermometer 11 is provided at the entry side of the row of finishing mills so that the thickness of oxide film is predicted from the temperature of the steel plate detected, as well as the speed of the steel plate. Actually, the distance from the position of temperature detection to the descaling devices is so short that descaling control tends to be performed with some delay. In the JP-A-09262602 there is already disclosed a method for preventing scale defects during hot rolling according to the preamble of claim 1. It is an object of the invention to improve this method concretely. This object is attained by the characterizing portion of claim 1. Preferred embodiments of the invention are illustrated in claims 2 and 3. BRIEF DESCRIPTION OF THE DRAWINGS The above and other obj ects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: FIG. 1 is an explanation drawing of a system for preventing scale defects in a hot rolled steel plate, showing a first embodiment of the present invention; FIG. 2 is a diagram showing a steel plate temperature and an oxide film thickness in the first embodiment; FIG. 3 is a graph showing a descaling pressure and the oxide film thickness in the first embodiment; FIG. 4 is a control flow chart for a descaler and a cooler in the first embodiment; FIG. 5 is a view showing the relation between the oxide film thickness and scale defect ratings in a third stage rolling mill F3 according to the first embodiment; FIG. 6 is an explanation drawing of a system for preventing scale defects during hot rolling, showing a second embodiment of the present invention; FIG. 7 is a diagram showing a steel plate temperature and an oxide film thickness in the second embodiment; FIG. 8 is an explanation drawing of a conventional system for preventing scale defects during hot rolling; and FIG. 9 is a diagram showing a steel plate temperature and an oxide film thickness in the conventional system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a system and a method for preventing scale defects during hot rolling according to the present invention will now be described in detail by reference to the accompanying drawings. [First Embodiment] A first embodiment of the present invention is described with reference to FIGS. 1 to 5. The same members as those in FIG. 8 explained in connection with the earlier technology are assigned the same reference numerals, and overlapping explanations are omitted. According to this embodiment, a descaler (scale removing device) D is placed between a first stage rolling mill F1 and a second stage rolling mill F2, and a cooler C is placed between the second stage rolling mill F2 and a third stage rolling mill F3. The descaler D and the cooler C are controlled to be capable of restricting the thickness of oxide film (scale thickness) to fall within allowable values. First of all, the relation between the oxide film thickness and scale defects is described with reference to FIG. 5. FIG. 5 shows test values with the third stage rolling mill F3. Δ, □ , and ○ represent the appearances of the surface of a steel plate in each of Test Examples (1), (2) and (3), which are expressed as scale defect ratings. Regardless of the magnitude of a reduction in thickness, Re (%), when the thickness of oxide film is more than 5 µm, the scale defect rating is 2 or 4.5, meaning "Minor defects" or "Defects", respectively. When the oxide film thickness is 5 µm or less, the scale defect rating is 0, meaning "No defects". In view of these findings, when the thickness of oxide film is more than 5 µm at the third stage rolling mill F3, scale defects occur. When the thickness of oxide film is restricted to 5 µm or less, a hot rolled steel plate free from scale defects is obtained. Based on the above test results in combination with actual machine tests and laboratory tests, the present invention has set a limiting oxide film thickness, at more than which scale defects occur during hot rolling, to be about 5 µm at an entry side of the third stage rolling mill F3, and performs descaling and water cooling of a hot rolled steel plate while maintaining the set thickness. In FIG. 1, a hot rolled steel plate (strip plate) 1, as a material to be rolled, passes, while being rolled, between the respective rolling mills of a finishing mill line F comprising the first stage rolling mill F1 to a seventh stage rolling mill F7, from an entry side to a delivery side (from left to right in the drawing). In each of the rolling mills F1 to F7, a pair of work rolls 6, 6 and a pair of backup rolls 5, 5 are arranged at upper and lower positions, with the hot rolled steel plate 1 being sandwiched between the work rolls 6 and 6. Between the first stage rolling mill F1 and the second stage rolling mill F2, a descaler (scale removing device) D is placed. The descaler D comprises headers 3, 3 for a jet medium arranged at upper and lower positions, with the hot rolled steel plate 1 being sandwiched between the headers 3 and 3. From a nozzle at the tip of the header 3, a jet medium can be jetted toward the hot rolled steel plate 1. Between the second stage rolling mill F2 and the third stage rolling mill F3, a cooler C for a steel plate surface is disposed, which comprises headers 4, 4 for cooling water arranged at upper and lower positions, with the hot rolled steel plate 1 being sandwiched between the headers 4 and 4. From a nozzle at the tip of the header 4, cooling water can be jetted toward the hot rolled steel plate 1. At an entry side of the first stage rolling mill F1, a scale breaker FSB is placed for removing scale of the hot rolled steel plate 1 that has been rough rolled. The scale breaker FSB comprises headers 2, 2 disposed at upper and lower positions, with the hot rolled steel plate 1 being sandwiched between the headers 2 and 2. From a nozzle at the tip of the header 2, high pressure water is jetted toward the hot rolled steel plate 1 to remove scale on the surface of the hot rolled steel plate 1. A radiation thermometer 7 is disposed near a delivery side of a roughing mill R which is placed on the hot rolling line at a location several tens of meters to several hundreds of meters upstream from the first stage rolling mill F1. A control device 8 receives, whenever necessary, information on the operating conditions and the temperature of the steel plate surface at the delivery side of the roughing mill R, and computes the thickness of oxide film by simulation. Control signals based on the results of computation are fed to the cooler C and the descaler D. With the use of the descaler D and the cooler C disposed as described above, descaling and cooling of the hot rolled steel plate are performed so that the scale thickness can be restricted to fall within the allowable range. To suppress the occurrence of oxide film by the foregoing hot rolling equipment for preventing scale defects, the following steps are taken: The hot rolled steel plate 1 rough rolled by the roughing mill R is fed from left to right in the drawing. High pressure water at a jet pressure of, say, 150 kgf/cm 2 is jetted from the nozzle at the tip of the header 2 of the scale breaker FSB toward the hot rolled steel plate 1 to remove scale on the surface of the hot rolled steel plate 1. Furthermore, the descaler D and the cooler C are actuated, where necessary, so as to restrict the oxide film thickness at the entry side of the third stage rolling mill F3 to the allowable value or less. During this process, the hot rolled steel plate 1 is rolled by the first stage rolling mill F1 to the seventh stage rolling mill F7 to prevent its scale defects. The actuating state of the descaler D and the cooler C will be described with reference to FIG. 4. At Step 1, the operating conditions [FSB operation pattern (width of high pressure water jet, heat transfer coefficient, etc.), percentage reduction in thickness, duration of passage of the hot rolled steel plate 1 between stands, type of roll (coefficient of friction between hot rolled steel plate and roll, etc.), atmospheric conditions (temperature, emissivity of hot rolled steel plate, etc.), type of steel] are read into the control device 8. Then, at Step 2, the surface temperature of the hot rolled steel plate 1 near the delivery side of the roughing mill R is taken into the control device 8 by means of the radiation temperature 7. Based on these data entered, the oxide film thickness at the entry side of the third stage rolling mill F3 when the descaler D and the cooler C are inactive is computed at Step 3. Then, at Step 4, if the computed oxide film thickness is not more than the limiting film thickness, operation is continued, without actuating the descaler D and the cooler C, at step P5. If the computed oxide film thickness is more than the limiting film thickness at Step 4, conditions including the actuation of the descaler D are incorporated into the aforementioned operating conditions, and the oxide film thickness at the entry side of the third stage rolling mill F3 is computed again. Then, at Step 7, if the computed oxide film thickness is not more than the limiting film thickness, operation is continued, with the descaler D being actuated, at step P8. If the computed oxide film thickness is more than the limiting film thickness at Step 7, conditions including the actuation of the descaler D and the cooler C are incorporated into the aforementioned operating conditions, and the oxide film thickness at the entry side of the third stage rolling mill F3 is computed again. Then, at Step 10, if the computed oxide film thickness is not more than the limiting film thickness, operation is continued, with the descaler D and the cooler C being actuated, at step P11. If the computed oxide film thickness is more than the limiting film thickness at Step 10, a judgment is made, at Step 12, that the current operation surpasses a normal operational state. Thus, the working ability of the descaler D and the cooler C is increased, and the recomputation at Step 9 is repeated to restrict the film thickness to the limiting film thickness or less. In this state, the descaler D and the cooler C are actuated, and operation is performed. The descaler D actuated in this manner allows the nozzle at the tip of the header 3 thereof to jet low pressure water at a jet pressure of, say, 70 kgf/cm 2 toward the hot rolled steel plate 1. Thus, even if oxide film on the hot rolled steel plate 1 rolled by the first stage rolling mill F1 grows because of recuperation (temperature recovery), the thickness of oxide film on the surface of the hot rolled steel plate 1 can be decreased. When the cooler C is actuated, cooling water in an amount_determined in consideration of recuperation (temperature recovery) on the steel plate surface is jetted from the nozzle at the tip of the header 4 of the cooler C toward the hot rolled steel plate 1 rolled by the second stage rolling mill F2 and heading for the third stage rolling mill F3. Thus, growth of oxide film is suppressed to decrease its thickness to the allowable value or less. Fig. 2 shows an example of the relation between the steel plate temperature and the oxide film thickness during the above-described hot rolling. When the descaler D and the cooler C of the present invention are actuated, the oxide film thickness at the entry side of the third stage rolling mill F3 is shown to be restricted to about 5 µm. This diagram also shows that the oxide film thickness at the entry side of the rolling mill F3 is restricted to about 5 µm, when the oxide film thickness after actuation of the descaler D is about 1.7 µm. These findings demonstrate that the jet pressure (descaling pressure) of the descaler D for making the oxide film thickness 1.7 µm may be a low pressure of about 70 kgf/cm 2 as indicated in the graph of FIG. 3. Thus, economical descaling can be achieved by low pressure jetting. Fig. 9 shows an example of the relation between the steel plate temperature and the oxide film thickness when the cooler C is not actuated. The oxide film thickness at the entry side of the third stage rolling mill F3 is shown to exceed about 5 µm. According to the present embodiment described above, the descaler D is provided between the first stage rolling mill F1 and the second stage rolling mill F2, and the cooler is provided between the second stage rolling mill F2 and the third stage rolling mill F3. The descaler D and the cooler C are actuated so that the oxide film thickness can be restricted to fall within the allowable range. In this state, rolling is carried out, with the oxide film thickness being restricted to the limiting oxide film thickness or less at the entry side of the third stage rolling mill F3. Consequently, scale defects of the hot rolled steel plate 1 can be prevented, and a drop in the plate temperature of the hot rolled steel plate 1 can be minimized. Since scale defects are absent, moreover, the quality of a hot rolled steel plate product can be improved, and its yield can be increased. [Second Embodiment] A second embodiment of the present invention is described with reference to FIG. 6. The same members as those in FIG. 1 explained in connection with the First Embodiment are assigned the same reference numerals, and overlapping explanations are omitted. According to this second embodiment, the cooler C placed between the second stage rolling mill F2 and the third stage rolling mill F3 in the First Embodiment is abolished. Instead, another descaler (scale removing device) D2 is disposed, and the other constitutions are the same as in the First Embodiment. In Fig. 6, a descaler (scale removing device) D1 is placed between a first stage rolling mill F1 and a second stage rolling mill F2. The descaler D comprises headers 3, 3 for a jet medium disposed above and below a hot rolled steel plate 1, with the hot rolled steel plate 1 being sandwiched between the headers 3 and 3. From a nozzle at the tip of the header 3, a jet medium can be jetted toward the hot rolled steel plate 1. Similarly, a descaler (scale removing device) D2 is placed between the second stage rolling mill F2 and the third stage rolling mill F3. The descaler D2 comprises headers 3, 3 for a jet medium disposed above and below the hot rolled steel plate 1, with the hot rolled steel plate 1 being sandwiched between the headers 3 and 3. From a nozzle at the tip of the header 3, a jet medium can be jetted toward the hot rolled steel plate 1. The descaler D1 and the descaler D2 are arranged as described above, and the oxide film thickness at an entry side of the third stage rolling mill F3 is computed from the steel plate surface temperature from a radiation thermometer 7 and the operating conditions, as in the First Embodiment. In descaling the hot rolled steel plate 1, the actuation of the descalers D1 and D2 is controlled such that this oxide film thickness can be restricted to the limiting oxide film thickness or less. The other constitutions are nearly the same as in FIG. 1 for the First Embodiment, and their explanations are omitted. To suppress the occurrence of oxide film by the foregoing hot rolling equipment for preventing scale defects, the following steps are taken: The hot rolled steel plate 1 rough rolled by a roughing mill R is fed from left to right in the drawing. High pressure water at a jet pressure of, say, 150 kgf/cm 2 is jetted from a nozzle at the tip of a header 2 of a scale breaker FSB toward the hot rolled steel plate 1 to remove scale on the surface of the hot rolled steel plate 1. Furthermore, the descaler D1 and the descaler D2 are actuated, where necessary, so as to restrict the oxide film thickness at an entry side of the third stage rolling mill F3 to the limiting oxide film thickness or less. During this process, the hot rolled steel plate 1 is rolled by the first stage rolling mill F1 to a seventh stage rolling mill F7 to prevent its scale defects. The actuation of the descaler D1 and the descaler D2 is performed in nearly the same manner as in the First Embodiment. That is, based on the operating conditions and the surface temperature of the hot rolled steel plate 1 near the delivery side of the roughing mill R, a control device 8 computes the oxide film thickness at the entry side of the rolling mill F3 in a state in which the descaler D1 and the descaler D2 are inactive. If the computed oxide film thickness is not more than the limiting film thickness, operation is continued, without actuating the descaler D1 and the descaler D2. If the computed oxide film thickness is more than the limiting film thickness, conditions including the actuation of the descaler D1 are incorporated into the aforementioned operating conditions, and the oxide film thickness at the entry side of the rolling mill F3 is computed again. If the results of computation show the oxide film thickness to be not more than the limiting film thickness, operation is continued, with the descaler D1 being actuated. If the computed oxide film thickness is more than the limiting film thickness, conditions including the actuation of the descaler D1 and the descaler D2 are incorporated into the aforementioned operating conditions, and the oxide film thickness at the entry side of the third stage rolling mill F3 is computed again. If the results of this computation show the oxide film thickness to be not more than the limiting film thickness, operation is continued, with the descaler D1 and the descaler D2 being actuated. If the computed oxide film thickness is more than the limiting film thickness, a judgment is made that the current operation surpasses a normal operational state. Thus, the working ability of the descaler D1 and the descaler D2 is increased, and the above recomputation is repeated to restrict the film thickness to the limiting film thickness or less. In this state, the descaler D1 and the descaler D2 are actuated, and operation is performed. The descaler D1 actuated in this manner jets low pressure water at a jet pressure of, say, 70 kgf/cm 2 toward the hot rolled steel plate 1. Hence, even if oxide film on the hot rolled steel plate 1 rolled by the first stage rolling mill F1 grows because of recuperation (temperature recovery), the descaler D1 can decrease the thickness of oxide film on the surface of the hot rolled steel plate 1. The descaler D2, when actuated, jets low pressure water at a jet pressure of, say, 70 kgf/cm 2 toward the hot rolled steel plate 1. Hence, even if oxide film on the hot rolled steel plate 1 rolled by the second stage rolling mill F2 grows because of recuperation (temperature recovery), the descaler D2 can decrease the thickness of oxide film on the surface of the hot rolled steel plate 1. Fig. 7 is a diagram showing the relation between the steel plate temperature and the oxide film thickness in accordance with the above-described hot rolling method. When the descaler D1 and the descaler D2 of the present invention are actuated, the oxide film thickness at the entry side of the third stage rolling mill F3 is shown to be restricted to about 5 µm or less. This diagram also shows that oxide film is descaled to about 1.7 µm by actuation of the descaler D1 and the descaler D2, whereby the oxide film thickness at the entry side of the third stage rolling mill F3 is restricted to about 4.3 µm, a value less than the limiting oxide film thickness (about 5 µm). The jet pressure (descaling pressure) of the descaler D1 and the descaler D2 on this occasion may be a low pressure of about 70 kgf/cm 2 as in the First Embodiment. Thus, economical descaling can be achieved by low pressure jetting. According to the present embodiment described above, the oxide film thickness at the entry side of the third stage rolling mill F3 can be made smaller than the limiting oxide film thickness (about 5 µm) by actuating the descaler D1 and the descaler D2 with low pressure jets. Thus, scale defects of the hot rolled steel plate can be dissolved, and rolling of the hot rolled steel plate at a higher rolling temperature than in the First Embodiment can be performed without scale defects.

Claims: A method for preventing scale defects during hot rolling by hot rolling equipment having a scale breaker (FSB) provided at an entry side of a finishing mill line (F) composed of a plurality of rolling mills (F1 to F7) arranged in tandem, comprising the following steps: descaling by the material to be rolled by a first descaler (D, D1) provided between a first stage rolling mill (F1) and a second stage rolling mill (F2) of the finishing mill line (F); cooling the material to be rolled by a cooler (C) or descaling by a second descaler (D2), provided between the second stage rolling mill (F2) and a third stage rolling mill (F3) of the finishing mill line (F), for cooling or descaling a material (1) to be rolled; and controlling the descaler (D, D1) and the cooler (C) or the second descaler (D2) by a controlling device (8), selectively driveable in a state where neither of the descaler (D, D1) and the cooler (C) or the second descaler (D2) is actuated, in another state, where one of the descaler (D, D1) and the cooler (C) or the second descaler (D2) is actuated, or still another state, where both of the descaler (D, D1) and the cooler (C) or second descaler (D2) are actuated, according to rolling conditions, whereby the material (1) is rolled, with an oxide film thickness of the material (1) at an entry side of the third stage rolling mill (F3) being restricted to not more than a limiting oxide film thickness, characterized in that the control device (8) computes the oxide film thickness of the material (1) at the entry side of the third stage rolling mill (F3), and the control device (8) actuates the descaler (D, D1), another descaler (D2) or the cooler (C) for restricting the oxide film thickness of the material (1) to not more than the limiting oxide film thickness, the control device (8) computes the oxide film thickness of the material (1) at the entry side of the third stage rolling mill (F3) based on a temperature of the material (1) at a delivery side of a roughing mill (R), and if the computed oxide film thickness is not more than the limiting oxide film thickness, the control device (8) actuates neither of the descaler (D, D1) and the second descaler (D2) or the cooler (C); if the computed oxide film thickness is more than the limiting oxide film thickness, the control device (8) incorporates the descaler (D, D1) into operating conditions and computes the oxide film thickness, and if the computed oxide film thickness is not more than the limiting oxide film thickness, the control device (8) actuates only the descaler (D, D1); if the oxide film thickness computed after incorporating the descaler (D, D1) into the operating conditions is more than the limiting oxide film thickness, the control device (8) incorporates both the descaler (D, D1) and the second descaler (D2) or the cooler (C) into the operating conditions and computes the oxide film thickness, and if the computed oxide film thickness is not more than the limiting oxide film thickness, the control device (8) actuates both the descaler (D, D1) and the second descaler (D2) or the cooler (C) and if the oxide film thickness computed after incorporating both the descaler (D, D1) and the second descaler (D2) or the cooler (C) into the operating conditions is more than the limiting oxide film thickness, the control device (8) actuates both the descaler (D, D1) and the second descaler (D2) and the cooler (C) while increasing the ability of both the descaler (D, D1) and the second descaler (D2) or the cooler (C). The method for preventing scale defects during hot rolling as claimed in claim 1, wherein the limiting oxide film thickness is five µm. The method for preventing scale defects during hot rolling as claimed in claim 1, wherein each of the descalers (D, D1, D2) jets water at a low pressure of about 70 kgf/cm 2 .

Inventors: CHOI WOON YONG, FUKUMORI JUNSO, KAWAMIZU TSUTOMU, KAYA AKIRA, LEE JONG WON, MIN KYUNG ZOON, CHOI, WOON YONG, FUKUMORI, JUNSO, KAWAMIZU, TSUTOMU, KAYA, AKIRA, LEE, JONG WON, MIN, KYUNG ZOON, Choi, Woon Yong, Pohang Iron &amp; Steel Co. Ltd., Fukumori, Junso, Mitsubishi Heavy Ind. Ltd., Kawamizu, Tsutomu, Mitsubishi Heavy Ind. Ltd., Kaya, Akira, Mitsubishi Heavy Ind. Ltd., Lee, Jong Won, Pohang Iron &amp; Steel Co. Ltd., Min, Kyung Zoon, Pohang Iron &amp; Steel Co. Ltd.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 2000
Main classification: B21B45
IPC codes: B21B1, B21B45