JP2008290156A - Cooling system for hot-rolled steel strip and its cooling method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cooling system for hot-rolled steel strips with which strong cooling is performed by stably controlling the winding temperature of the steel strip in the cooling process of the steel strip in a run out table from the final finishing mill to a winding machine and its cooling method. <P>SOLUTION: Two cooling systems whose cooling capacities are different are provided in the run out table 3 behind the final finishing mill 2E, a system on the upstream side is a first cooling system 5 for performing strong cooling and the other system on the downstream side is a second cooling system 6 for performing soft cooling. Cooling is continuously performed by these cooling systems, the length of a cooling zone is adjusted in accordance with acceleration and deceleration of the rolling speed of the steel strip, the value of the temperature drop of the steel strip is controlled roughly by using a rapid cooling system on the proceeding stage and minutely by using a slow cooling system on the succeeding stage and the steel strip is cooled so that stop temperature is in a prescribed winding temperature. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cooling device for cooling a hot-rolled high-temperature steel strip and a cooling method thereof.

  Generally, a hot-rolled steel strip is obtained by heating a slab to a predetermined temperature in a heating furnace, rolling the heated slab to a predetermined thickness with a roughing mill to form a rough bar, and then forming the rough bar with a plurality of stands. In a continuous hot finish rolling mill, the steel strip has a predetermined thickness. And after cooling this hot-rolled steel strip in the cooling stand on the run-out table which comprises a cooling device, it manufactures by winding up with a winder.

In such a cooling device that continuously cools the rolled high-temperature steel strip, firstly, the plate-passability of the steel strip is considered.
For example, in order to cool the upper surface of the steel strip, a plurality of laminar cooling waters are injected linearly across the width direction of the roller table from the circular laminar cooling nozzle to the roller table for transporting the steel strip. . Therefore, although the steel strip is pushed from above by water pressure, the pulse line of the passing steel strip does not have to be pushed from above the roller table.

  On the other hand, as a method for cooling the lower surface of the steel strip, a spray nozzle is provided between the roller tables, and a method of injecting cooling water therefrom is generally used. Therefore, such a cooling mode forms intermittent cooling with respect to the hot-rolled steel strip, and so-called mild cooling is performed.

  However, in recent years, hot-rolled steel strips with a small crystal grain size have been demanded for their excellent workability and high strength even at low Cep, and rapid cooling (strong cooling) is required for that purpose. Yes.

Thus, when performing rapid cooling with respect to a hot-rolled steel strip, the conventional cooling device has the following problems.
In other words, in order to ensure the plate-passability until the steel strip tip is taken up by the winder, and the steel until the steel strip rear end passes through the final finish rolling mill and is taken up by the winder. In order to suppress flapping at the end of the strip, rolling is performed to reduce the rolling speed at the time of biting the end of the steel strip and at the rear end of the strip, and to accelerate and decelerate the rolling speed of the steady portion between them.

  For example, this rolling pattern is controlled to accelerate from 600 mpm to about 1200 mpm, and then to slow again to 600 mpm during end rolling. At this time, for example, when cooling is performed at a cooling temperature of 500 ° C./s, the steel strip has a temperature drop of 25 ° C. while proceeding for 1 second at 600 mpm while passing 0.1 second, and at 50 ° C. and 1200 mpm for 1 m. Yes, with the same cooling zone length, the amount of cooling is almost double in one steel strip.

  In particular, when realizing such a strong cooling state, the cooling rate does not change much even if the amount of cooling water is slightly changed, and the cooling rate is a function of the plate thickness only. In order to accurately control the cooling stop temperature while performing strong cooling, it is necessary to finely divide the cooling zone length and control the cooling zone length according to the conditions.

  However, with regard to the subdivision of the cooling zone, there is a limit to the division of the cooling device between the roller tables, which is at most about 50 cm. Therefore, the accuracy of the cooling stop temperature for strong cooling was only about 25 ° C. at the maximum according to the acceleration / deceleration rolling.

  The present invention has been made in consideration of the above circumstances, and the object of the present invention is to provide a stable cooling process for the steel strip from the final finish rolling mill to the winder. Therefore, an object of the present invention is to provide a cooling device for hot-rolled steel strip that controls the winding temperature of the steel strip and performs strong cooling, and a cooling method therefor.

  The present invention has been made to solve such problems, and includes two cooling means having different cooling capacities between the final finish rolling mill and the winder, and the upstream side is a cooling system that performs strong cooling. The means and the downstream side are slow cooling means, and cooling is continuously performed by these cooling means. The length of the cooling zone is adjusted according to the acceleration / deceleration of the rolling speed of the steel strip. The temperature drop of the steel strip is roughly the rapid cooling of the former stage, and the slow cooling of the latter stage is finely performed. A cooling device for a hot-rolled steel strip that is cooled so that the stop temperature falls within the cutting temperature, and a cooling method therefor.

  By adopting the cooling device and the cooling method as described above, stable rapid cooling of the hot-rolled steel strip becomes possible, and the winding temperature is reliably controlled. As a result, the cooling temperature, which is important in terms of material, is not only uniform in the width direction and longitudinal direction of the steel strip, but also the winding temperature is constant in the width direction and longitudinal direction of the steel strip, and the crystal grain size is uniform. Hot-rolled steel strip is obtained, the product yield is increased, and the quality is stabilized.

According to the present invention, the following effects can be obtained.
(1) Cooling is possible under uniform cooling conditions from the front end to the rear end of the steel strip, and the winding is constant particularly in the longitudinal direction and the width direction of the steel strip, so that the quality of the steel strip is stabilized. Along with this, the cutting margin at the tip is reduced and the yield is high.
(2) It is possible to create steel strips with different cooling rates in one finishing rolling mill group, making it possible to produce steel strips with different strength ranges from slabs or rough bars of the same composition, integrating and stocking steel types It leads to reduction of slabs.
(3) By performing rapid cooling, it became possible to stably produce a hot-rolled steel strip having a fine crystal grain size.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 schematically shows a production facility for a hot-rolled steel strip, and FIG. 2 schematically shows a first cooling device constituting a first cooling means.

  The rough bar 1 rolled by the roughing mill is conveyed on a roller table and continuously rolled to a predetermined thickness by seven continuous finish rolling mills 2, and then the runout table 3 behind the final finish rolling mill 2E. Led to. Most of the run-out table 3 constitutes a cooling device. After being cooled here, the run-out table 3 is wound up by a winder 4 to form a hot rolled coil.

  A first cooling device 5 that is a first cooling means is disposed on the upstream side along the run-out table 3, and a second cooling device 6 that is a second cooling means is disposed on the downstream side. .

  The first cooling device 5 is provided from a position about 2 m behind the final finish rolling mill 2E to a position about 8 m, and is configured as described later.

  The second cooling device 6 is installed on the downstream side of the first cooling device 5 over about 70 m, and is a plurality of circular tube laminators arranged at a predetermined pitch on the upper surface side of the run-out table 3. It consists of a nozzle 7 and a plurality of commercially available spray nozzles 8 disposed between the roller table 12 for conveying the steel strip on the lower surface side.

  Further, finishes are provided between the final finish rolling mill 2 and the first cooling device 5, between the first cooling device 5 and the second cooling device 6, and behind the second cooling device 6. A steel plate thermometer 9, an intermediate thermometer 10, and a take-up thermometer 11 are installed.

  The first and second cooling devices 5 and 6 arranged along the run-out table 3 perform a rapid cooling process immediately after rolling with the first cooling device 5 on the steel types that require strong cooling, and then perform predetermined processing. The cooling process can be performed by the second cooling device 6 at the rear so as to be wound at the winding temperature of.

  Moreover, about the steel strip which does not require strong cooling, the operation | movement of the rapid cooling of the 1st cooling device 5 is stopped, and the cooling process only with the 2nd cooling device 6 which is a conventional slow cooling is made. It is possible to make steel strips as materials.

The first cooling device 5 is configured as described below in order to realize a rapid cooling process for the hot-rolled steel strip.
As shown in FIG. 2, in the arrangement space of the first cooling device 5, roller tables 12 for conveying a steel strip having a diameter of 350 mm are arranged at a pitch of about 800 mm in the longitudinal direction. That is, these roller tables 12 are located on the lower surface side of the steel strip. Between the roller tables 12, a lower surface cooling box 13 having a length of about 430 mm and a width of about 1860 mm is provided.

  A total of 20 of these lower surface cooling boxes 13 are arranged along the longitudinal direction of the apparatus, and the first cooling apparatus 5 has a total cooling length of about 8600 mm. And the distance of this lower surface cooling box 13 end surface and the steel strip 14 lower surface is set to about 50 mm.

  On the other hand, on the upper surface side of the steel strip 14 in the first cooling device 5, an upper surface cooling box 15 which is set at a position facing the lower surface cooling box 13 and having exactly the same length and width dimension is provided with the lower surface cooling box 13. The same number is arranged.

  The installation height of the upper surface cooling box 15 can be adjusted so that the distance between the upper surface of the steel strip 14 and the end surface of the upper surface cooling box 15 is equal to the distance between the end surface of the lower surface cooling box 13 and the lower surface of the steel strip 14. .

  In addition, when the first cooling device 5 is not cooled, the upper surface cooling box 15 at a position corresponding to the tip rises to a position about 500 mm above the line in time with the tip of the steel strip passing. Evacuate from the steel strip. During normal cooling action on the steel strip, the distance between the upper and lower cooling boxes 15 and 13 is set to be the thickness of the steel strip +100 mm.

  In this embodiment, in order to further improve the responsiveness of cooling to the hot-rolled steel strip 14, the upper surface cooling box 15 on the downstream side is cooled from the upper surface cooling box 15 in conjunction with the passage of the tip end of the steel strip 14 during the cooling action. The cooling starts in descending order over the box 15.

  Steel plates having a thickness of 16 mm are used for the end faces of the upper and lower cooling boxes 15 and 13 facing the steel strip 14. In this steel plate, nozzle holes with a diameter of 3 mmφ are provided in a staggered manner at a pitch of about 30 mm × 30 mm in length and width.

  Further, in order to stabilize the plate passing property, a so-called squirrel guide 16 is provided between the lower surface cooling box 13 and the roller table 12 for the lower surface of the steel strip 14 and between the upper surface cooling boxes 15 for the upper surface of the steel strip 14. In particular, the steel strip 14 is devised so that the tip of the steel strip 14 is not caught in each gap.

  It can be explained from the following that when the hot-rolled steel strip 14 is rapidly cooled, that is, when it is strongly cooled, it becomes more difficult to control the cooling temperature for the hot-rolled steel strip 14.

In order to control the cooling rate (° C./s) representing the temperature drop per predetermined time, it is usually performed by changing the water flow density (water flow rate per m ² ). However, the rapid cooling referred to here has an extremely large water density, for example, 1000 L / min · m ² or more and is close to the limit, so the cooling rate is a function of only the thickness of the hot-rolled steel strip regardless of the amount of cooling water.

  Therefore, in such rapid cooling, the temperature drop amount is adjusted by changing the length of the cooling zone where the cooling water comes into contact with the steel strip, and the operation is performed so that the cooling end temperature becomes constant even when accelerated rolling is performed. Done.

  Here, since the length of the single upper and lower cooling boxes 15 and 13 is about 430 mm as described above, the inside of the cooling box is subdivided into a plurality of chambers along the longitudinal direction, and each is turned on independently. It is conceivable that the cooling zone length can be changed by enabling off control.

  However, because the volume of piping and valves required for control is limited due to the equipment, one cooling box, that is, the upper and lower cooling boxes 15 and 13 each having a length of about 430 mm, is the minimum cooling zone length control. Become a unit.

  As shown in Table 1, it can be seen that the temperature range that can be changed by on-off control of one control unit in this embodiment is determined by the cooling rate and the steel strip conveyance speed.

As the cooling rate increases, the range of the cooling end temperature that can be changed by on-off control of one control unit increases. That is, as the cooling rate increases, it is indicated that fine cooling end temperature control is impossible, and this temperature variation leads to variations in the material of the steel strip.

  Therefore, the second cooling device 6 that is another cooling device for accurately controlling the winding temperature as in the present invention is required. As described above, the second cooling device 6 that performs this slow cooling is installed on the downstream side of the first cooling device 5 over about 70 m, and the circular tube laminar nozzle 7 on the upper surface side of the hot-rolled steel strip 14. Then, cooling water is supplied from the spray nozzle 8 on the lower surface side to the steel strip 14 to perform a cooling action.

  The laminar water flow supplied from the circular tube laminar nozzle 7 is attached so as to collide and fall directly on the roller table 12 on the lower surface. The spray nozzle 8 on the lower surface does not cool the lower surface of the steel strip 14 upward from between the roller tables 12 and can adjust the amount of cooling water.

Below, the cooling process with respect to the hot-rolled steel strip 14 is demonstrated.
The upper surface cooling box 15 corresponding to the tip of the hot-rolled steel strip 14 unloaded from the final finish rolling mill 2E passes through the first cooling device 5 is lowered and corresponds to the lowered upper surface cooling box 15. Injection of cooling water from the lower surface cooling box 13 is started, and the steel strip is strongly cooled, which is rapid cooling.

  This is because if cooling water is jetted from the upper and lower surface cooling boxes 15 and 13 before the tip of the hot-rolled steel strip passes, the cooling water becomes resistance to passage with respect to the tip of the steel strip, and there is a risk of impeding the plate passing property of the tip. Because there is.

  Once the tip of the steel strip 14 has passed, the pass line of the steel strip 14 is constant due to the balance between the pressure of the cooling water injected from the upper surface cooling box 15 and the pressure of the cooling water injected from the lower surface cooling box 13. To be kept. Therefore, even if no tension is applied to the steel strip 14, the plateability of the steel strip 14 is stabilized, and uniform strong cooling of the steel strip 14 is performed.

  The steel strip 14 tip enters the first cooling device 5 and the cooling water is jetted from the upper and lower cooling boxes 15 and 13 corresponding to the tip. At this time, even though the upper cooling box 15 is kept in the raised position. Good. And even if the upper surface cooling box 15 is lowered at the stage where the plate-passability is stabilized, the plate-passability of the steel strip portion that has already passed and the steel strip portion that is about to pass through will not be adversely affected.

The distance between the upper and lower cooling boxes 15 and 13 and the steel strip 14 constituting the first cooling device 5 is set to 50 mm here, for the following reason.
That is, if the distance between the cooling means and the steel strip is further increased, the momentum of the cooling water is absorbed by the fluid (cooling water) existing between the steel strip and the cooling device, and weakens. On the contrary, if the distance between the cooling means and the steel strip is made closer, the momentum of the cooling water increases, so that the steel strip balances the surface pressure received from the cooling water injected from the upper surface and the surface pressure received from the lower surface. The effect of correcting and centering the vibration of the steel strip and the offset running through the position works.

  Therefore, if the diameter of the laminar flow nozzle hole is about 2 to 5 mm, this distance is preferably 30 to 100 mm. When the distance is 100 mm or more, the momentum of the cooling water flow is weakened and strong cooling becomes impossible. On the other hand, if it is too close to 30 mm or less, there is no place for the cooling water and it becomes difficult to obtain a good water flow. Therefore, rapid cooling becomes impossible, or the flow of cooling water is greatly different between the central portion and the end portion of the steel strip, resulting in uneven cooling.

  In the above equipment, an ultra-low carbon steel strip with a finishing plate width of 1500 mm and a finishing plate thickness of 3 mm is accelerated at a threading speed of 600 mpm and an acceleration rate of 10 mpm / s, accelerated to a maximum of 1200 mpm, then decelerated at 600 mpm. The rolling finish temperature of the steel strip when the rear end of the steel strip was removed was 920 ° C.

  When the steel strip 14 is accelerated, the cooling zone used in the first cooling device 5 is cooled simultaneously from the upper and lower surface cooling boxes 15 and 13 extending from the first to the downstream side on the final finish rolling mill 2E side to the eighth. To cool the vicinity of the tip of the steel strip 14.

  Then, the injection of cooling water is sequentially started in the upper and lower cooling boxes 15 and 13 so that the time for passing through the first cooling device 5 as the steel strip 14 accelerates, that is, the cooling time is the same throughout the steel strip. To do. When the maximum speed is 1200 mpm, cooling water is jetted with the upper and lower cooling boxes 15 and 13 extending from the first to the 16th as cooling zones. When the vehicle is decelerated, the cooling water injection is stopped sequentially from the upstream side.

  When such cooling control was performed, the temperature of the steel strip 14 was 750 ° C. ± 20 ° C. when the first cooling device 5 was exited. Further, in the second cooling device 6, the amount of water and the cooling zone length to be used are increased or decreased according to the instruction value of the intermediate thermometer 10 provided between the first cooling device 5 and the second cooling device 6. The coiling temperature was controlled to be a constant 640 ° C.

  As a result, the steel strip 14 passed through the first and second cooling devices 5 and 6 stably from the front end to the rear end, and the winding temperature was 640 ± 10 ° C. An extremely fine hot-rolled steel strip having a crystal grain size of 15 μm from the front end to the rear end can be stably produced. Moreover, since the variation of the winding temperature is within 10 ° C., stable cooling was realized. When the cooling rate of the steel strip 14 is estimated from the actually measured value of each thermometer, the first cooling device 5 realizes rapid cooling at 500 ° C./s.

  As a second embodiment, a hot-rolled steel strip having a finished sheet thickness of 3 mm is rolled with the same rolling equipment as in the first embodiment, and normal cooling is performed only by the second cooling device 6 described above. Will be explained.

  A steel strip having a finished sheet thickness of 3 mm was accelerated at a threading speed of 600 mpm and an acceleration rate of 10 mpm / s, accelerated to a maximum of 1200 mpm, decelerated, and the rear end of the steel band was pulled through at 600 mpm. At this time, cooling was performed with the maximum amount of cooling water that allows stable passage only with the second cooling device 6.

  The cooling rate was 70 ° C./s, and the winding temperature was 640 ° C. ± 15 ° C. In this state, the crystal grain size of the hot-rolled steel strip 14 was rough, about 30 μm, as compared with the first embodiment.

  In any case, it is possible to make different steel strips of the same material from the same steel type by using the two cooling devices 5 and 6 separately.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above-described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram of the rolling equipment which shows one embodiment of this invention. The schematic block diagram of the 1st cooling device which makes rapid cooling of the embodiment.

Explanation of symbols

  2E ... Final finish rolling mill, 5 ... 1st cooling device, 6 ... 2nd cooling device, 9 ... Finished steel plate thermometer, 10 ... Intermediate thermometer, 11 ... Winding thermometer.

Claims (5)

A hot-rolled steel strip cooling device disposed between a final finish rolling mill and a winder in a hot-rolled steel strip manufacturing facility,
Located at the rear of the final finish rolling mill, the length of the cooling zone is adjusted according to the acceleration / deceleration of the rolling speed of the hot-rolled steel strip, and the hot-rolled steel strip is rapidly cooled at a water density of 1000 L / min · m ² or more. A first cooling means;
A second cooling means disposed behind the first cooling means for slowly cooling the hot-rolled steel strip;
An apparatus for cooling a hot-rolled steel strip.   The first cooling means includes a top surface cooling box having a flat surface with respect to the steel strip, symmetrical up and down, arranged close to the steel strip, each having nozzle holes for injecting cooling water, and a bottom surface 2. The cooling device for a hot-rolled steel strip according to claim 1, wherein the cooling device is a cooling box.   The temperature detection means for detecting the temperature of the cooled hot-rolled steel strip is disposed between the first cooling means and the second cooling means and behind the second cooling means. The cooling device for a hot-rolled steel strip according to any one of claims 1 and 2. Between the final finish rolling mill and the winder in the hot rolled steel strip manufacturing facility, behind the final finish rolling mill,
According to the acceleration / deceleration of the rolling speed of the hot-rolled steel strip, the cooling zone length is adjusted so that the temperature drop amount is constant in the steel strip, and the hot-rolled steel strip has a water density of 1000 L / min · m ² or more. A rapid cooling process for rapid cooling;
After this rapid cooling step, a slow cooling step of slowly cooling the hot-rolled steel strip so as to be wound at a predetermined winding temperature;
A method for cooling a hot-rolled steel strip.   The method of cooling a hot-rolled steel strip according to claim 4, wherein the slow cooling step controls the cooling zone length and / or cooling water injection conditions such as the amount of cooling water.

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