JP4525133B2 - Manufacturing method of hot-rolled steel strip - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel strip by cooling a steel strip after hot finish rolling with cooling water.

  Generally, a hot-rolled steel strip heats a slab to a predetermined temperature in a heating furnace, rolls the heated slab to a predetermined thickness in a roughing mill to form a rough bar, and then the rough bar is composed of a plurality of rolling stands. It is manufactured by finish rolling in a finishing mill to obtain a hot-rolled steel strip having a predetermined thickness, and this hot-rolled steel strip is cooled by a cooling device on a hot run table and then wound by a coiler.

  In such a cooling device on a hot run table that continuously cools a rolled high-temperature steel strip, firstly, the stripability 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 poured linearly across the width direction from a cylindrical laminar cooling nozzle directly above the roller table for transporting the steel strip. On the other hand, as a method for cooling the bottom 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, in such a cooling mode, the cooling of the upper and lower surfaces of the steel strip is not strictly symmetrical, and the cooling of the steel strip is intermittent especially on the upper surface side, and rapid cooling (for example, with a plate thickness of 3 mm) A cooling rate of 200 ° C./second or more) is almost impossible.

  However, in recent years, hot-rolled steel strips with fine crystal grain sizes have been demanded because of their excellent workability and high strength even at low carbon equivalents, which requires rapid cooling (strong cooling). ing.

  As a cooling device capable of realizing such rapid cooling, cooling is performed on the lower surface of the hot-rolled steel strip that is disposed between the conveying rolls constituting the hot run table disposed on the exit side of the finishing mill. Lower cooling means for supplying water and an upper position of the hot run table, which is arranged at a position facing the lower cooling means in the vertical direction, and supplies cooling water to the upper surface of the hot-rolled steel strip running on the hot run table And an upper cooling means that is disposed near the upper surface of the hot-rolled steel strip and along the width direction of the steel strip in order to suppress the outflow of the cooling water supplied from the upper cooling means in the longitudinal direction of the hot-rolled steel strip. There is known a cooling device for a hot-rolled steel strip provided with a water draining means (see, for example, Patent Document 1).

As a method of cooling a hot-rolled steel strip using such a cooling device, it is known to set the water density of cooling water to 2500 L / min · m ² or more (see, for example, Patent Document 2). ).

According to Patent Document 2, in order to cool the steel strip after hot finish rolling so as to eliminate temperature unevenness in the width direction, and to make the temperature in the width direction after cooling uniform, the heat flow rate according to the temperature drop It is only necessary to cool in a heat transfer characteristic in which the water content decreases, that is, in a nucleate boiling state, and for this purpose, cooling should be performed at a water density of 2500 L / min · m ² or more. Under such heat transfer characteristics, the high temperature portion at the start of cooling has a higher heat flux than the low temperature portion, so that it cools faster and can realize cooling in which the difference between the two is reduced.
JP 2001-246413 A JP 2003-191005 A

  Usually, in a cooling device for a hot-rolled steel strip as shown in Patent Document 1, a lower cooling means disposed between adjacent conveying rolls and an adjacent draining roll facing the lower cooling means in the vertical direction. The upper cooling means arranged in the above is handled as a cooling box as a cooling unit.

  In the cooling device as shown in Patent Document 2, the water density when using each cooling box is generally constant, and when the required cooling amount is large, all cooling boxes constituting the cooling device are used and the cooling amount is used. When there is little, the cooling amount was adjusted by reducing the number of cooling boxes to be used.

  Here, if the cooling device is a device capable of cooling at 300 ° C., for example, and configured with 15 cooling boxes, it can be cooled at 20 ° C. per cooling box. Each cooling box includes a plurality of cooling nozzles in the steel strip longitudinal direction and the steel strip width direction, and has a mechanism that allows all the cooling nozzles to spray and stop simultaneously.

  Further, of the 15 cooling boxes, at least one cooling box is referred to as a fine cooling box (hereinafter referred to as a fine box). For example, the cooling box includes five cooling nozzles in the longitudinal direction of the steel strip. If it is a mechanism capable of injecting and stopping cooling water, it is possible to perform fine cooling control at 4 ° C. for each cooling nozzle row.

  Thus, the cooling device often has a mechanism for finely controlling cooling by combining a fine box in addition to a cooling box.

  In normal production of a hot-rolled steel strip, a method called accelerated rolling is used in which the rolling speed is gradually increased from the front end of the steel strip toward the tail end. In accelerated rolling, in addition to ensuring the finish rolling temperature, low-speed rolling is desirable at the end of the steel strip to ensure sheeting, and high-speed rolling is desirable at the center of the steel strip to ensure productivity. Has been done. When performing this accelerated rolling, it is necessary to increase the cooling rate of the steel strip proportionally as the rolling speed of the steel strip is increased. The number of cooling boxes to be used and / or the cooling nozzle rows of the fine boxes to be used Increase the number. Here, injecting cooling water from a plurality of cooling nozzles arranged in the steel strip longitudinal direction and steel strip width direction constituting each cooling box, and injecting cooling water from each cooling nozzle row of the fine box It is said that the cooling box and the fine box are turned on, and stopping the injection without jetting the cooling water is called turning off the cooling box and the fine box.

  By turning on / off each of the cooling nozzles constituting each of the cooling boxes one cooling box at a time, the cooling can be controlled step by step by 20 ° C., and by turning on / off one row of the fine boxes, the cooling step can be performed by 4 ° C. Cooling can be controlled one by one. Also, for example, instead of turning on the 5th row from the 4th row of the fine box, the cooling amount can be increased smoothly by 4 ° C by simultaneously turning on the 1 cooling box and turning off the 4 rows of the fine box. Such switching between the cooling box and the fine box is called a swapping process.

However, when the cooling device shown in Patent Document 1 is used and the water density is set to 2500 L / min · m ² or more according to Patent Document 2 and this swapping process is performed, the switching timing of the cooling box and the fine box is shifted. In this case, there is a problem that the cooling amount error increases. Here, the switching timing is shifted because of an error in collecting the steel strip speed record and the tracking being shifted, and the operation time of the fine box valve and the cooling box valve being varied.

  Here, an example in which the cooling amount error increases when the switching timing is shifted when performing the swapping process with reference to FIGS. 4 to 6 will be described.

  4 to 6 show how the cooling amount changes due to a shift in the switching timing of the cooling box and the fine box when the cooling amount of the hot-rolled steel strip is increased from 16 ° C. during accelerated rolling.

  FIG. 4 shows a case where the switching works normally. FIG. 4A shows the cooling amount of 16 ° C. with the 4 cooling nozzle rows turned on in the fine box and turned off in the 1 cooling box. FIG. Since all the fine boxes are switched off at the same time when the cooling box is turned on, the cooling amount is 20 ° C., and FIG. 4C shows that one cooling box is turned on and one cooling nozzle row of the fine box is turned on next. The amount of cooling is 24 ° C.

  On the other hand, FIGS. 5 and 6 show cases where the switching timing is shifted. In the case of FIG. 5, FIG. 5 (a) is the same as FIG. 4 (a). Next, it should originally be FIG. 5 (b), but the switching timing is shifted as shown in FIG. 5 (b-1). In this case, the cooling amount becomes 36 ° C. because the one cooling box is turned on while the four cooling nozzle rows are still on in the fine box. In the case of FIG. 6, FIG. 6 (a) is the same as FIG. 4 (a). Next, it should be the same as FIG. 6 (b), but the switching timing is shifted as shown in FIG. 6 (b-1). In the fine box, the cooling amount is 0 ° C. because all the cooling nozzle rows are off and one cooling box is not turned on but remains off. That is, in FIGS. 5 and 6 in which the switching timing acts abnormally with respect to FIG. 4 when the switching timing works normally, the cooling amount error (actual cooling amount−target cooling amount) is + 16 ° C. and −20 ° C. became.

  When such a cooling amount error occurs, a variation in material occurs in the longitudinal direction of the steel strip, which causes a problem.

Accordingly, an object of the present invention is to provide a method for producing a hot-rolled steel strip that can suppress a cooling amount error during a swapping process when the steel strip is rapidly cooled with cooling water having a water density of 2500 L / min · m ² or more. It is to provide.

  The features of the present invention for achieving such an object are as follows.

( 1 ) When the steel strip after hot finish rolling running on the hot run table is continuously cooled using a cooling device composed of a plurality of cooling boxes, the total cooling of the cooling device during the maximum cooling of the steel strip. Calculate the water density when using each cooling box so that 80% or more of the number of boxes is used, and determine the water density to the larger of the calculated water density and 2500 L / min · m ^2. A method for producing a hot-rolled steel strip.

( 2 ) At least one of the plurality of cooling boxes of the cooling device is a fine cooling box whose interior is divided into a plurality of rows, and the amount of cooling is finely controlled by switching between the cooling box and the fine cooling box. The method for producing a hot-rolled steel strip according to (1), which is characterized in that

According to the present invention, when rapidly cooling the steel strip with cooling water having a water density of 2500 L / min · m ² or more, the cooling amount error during the swapping process can be suppressed over the entire length of the steel strip. Variations in materials can be suppressed, and product quality and yield can be improved.

  FIG. 1 is a side view showing an example of production equipment used for carrying out the method for producing a hot-rolled steel strip of the present invention, and FIG. 2 is a side view showing an embodiment of a cooling device 4.

The cooling device 4 used in the present embodiment is a device capable of injecting cooling water having a water density of 2500 L / min · m ² or more to the hot-rolled steel strip 1 finish-rolled by the finish rolling mill 9. The cooling device 4 includes a plurality of lower cooling means 3 for supplying cooling water to the lower surface of the hot-rolled steel strip 1 traveling on the hot run table, and a plurality of upper portions for supplying cooling water to the upper surface of the hot-rolled steel strip 1. The cooling means 2, the transport roll 6, and two pairs of pinch rolls 7 and 8 disposed on the entry side and the exit side of the cooling device 4 are provided.

The cooling device 4 has a maximum water density of 5000 L / min · m ² and is composed of nine cooling boxes 10 and one fine box 11, and can control a cooling amount of 20 ° C. for each cooling box. It is. The cooling box 10 and the fine box 11 are each composed of five rows of cooling nozzles in the longitudinal direction of the steel strip, and the five rows of fine nozzles of the fine box can be controlled to be turned on / off for each row. The amount of cooling at 4 ° C. can be controlled.

  The lower cooling means 3 is disposed between the conveying rolls 6 constituting the hot run table, and is provided with a header disposed along the hot rolled steel strip width direction and at an appropriate interval along the longitudinal direction of the header. It consists of a cooling nozzle.

  The upper cooling means 2 is composed of a header arranged along the width direction of the hot rolled steel strip and cooling nozzles provided at appropriate intervals along the longitudinal direction of the header, and is suitable for the hot run table longitudinal direction. The lower cooling means 3 is arranged in a vertical direction with a small interval. Moreover, it arrange | positions between the draining means 5 arrange | positioned along the steel strip width direction near the upper surface of a hot-rolled steel strip.

  The draining means 5 is preferably a draining roll. It is effective to suppress the platenability and wrinkle generation of hot-rolled steel strip.

  A conventional laminar cooling device is installed on the outlet side of the cooling device 4 in order to adjust the cooling. However, the conventional laminar cooling device is not necessarily installed.

  Next, the manufacturing method of the hot-rolled steel strip using the cooling device 4 used for this embodiment demonstrated above is demonstrated.

In FIG. 1, the steel strip 1 leaving the finishing mill 9 and running on the hot run table is continuously cooled by cooling water in the cooling device 4 with cooling water having a water density of 2500 L / min · m ² or more, and further if necessary. After being cooled by a laminar cooling device, it is wound around a coiler. Here, in the cooling in the cooling device 4, the water density when using each cooling box is a constant value of 2500 L / min · m ² or more over the entire length of the steel strip, and all of the cooling device has a maximum value when the steel strip is maximum cooled. The steel strip is cooled using 80% or more of the number of cooling boxes.

For example, when the required cooling amount at the time of maximum accelerated rolling, which is the maximum cooling of the steel strip 1, is 80 ° C., the maximum water density of 5000 L / min · m ² that the cooling device 4 conventionally has at the time of maximum accelerated rolling. Whereas four cooling boxes were used, in the present invention, the water density of one cooling box was reduced to half of 2500 L / min · m ² , the cooling amount per cooling box was 10 ° C., and eight coolings were performed during maximum accelerated rolling. Use a box. In this way, the water density of each cooling box is a constant value of 2500 L / min · m ² or more, and at the maximum cooling of the steel strip, the steel strip is used by using a cooling box of 80% or more of all cooling boxes of the cooling device. Cooling. By reducing the water density of each cooling box in half, when performing the swapping process using FIGS. 4 to 6 described above, if the switching timing is shifted, the cooling error (actual cooling amount−target cooling amount) is + 8 ° C. and −10 ° C., the cooling amount error is half that of the conventional one, and the variation in the material in the longitudinal direction of the steel strip can be suppressed.

  In the present invention, the water density when each cooling box is used is determined by the maximum cooling amount of the steel strip and the number of cooling boxes of the cooling device.

An example of this will be described with reference to the flowchart shown in FIG.
[1] In each part (T, M, B) of the steel strip, the required number of cooling boxes when the maximum water density of the cooling device is used is calculated from the target cooling amount and the plate speed. Here, T is the front end, M is the center, and B is the rear end.
[2] By selecting the maximum number of required cooling boxes for each part of the steel strip calculated in [1] above, the maximum number of cooling boxes used in the steel strip when the maximum water density is obtained is obtained.
[3] It is checked whether the maximum number of cooling boxes used in the case of the maximum water density obtained in [2] is 80% or more of the total number of cooling boxes of the cooling device.
If “YES”, the water density is determined.
If “NO”, the water density is calculated by the calculation shown in [4] below.
[4] Calculate the water density according to the following formula (1) so that the maximum number of cooling boxes used in the steel strip is 80% or more of the total number of cooling boxes of the cooling device when the steel strip is maximum cooled. To do.

Calculated water density = (maximum number of cooling boxes used in the steel strip in case of maximum water density x maximum water density of cooling device) / (total number of cooling boxes of cooling device x 0.8) (1)
The coefficient “0.8” used in (1) can be appropriately selected within the range of 0.8 to 1.0.
[5] The calculated water density obtained by the above equation (1) is compared with 2500 L / min · m ² to determine the larger water density.

  In addition, although the above description has demonstrated the case where a fine box is used, this invention is not limited to manufacture of the hot-rolled steel strip using a fine box. Even in a cooling device that does not have a fine box and only has a cooling box, by reducing the water density, the change in the cooling amount accompanying the increase or decrease of the cooling box to be used can be reduced. There is an effect of suppressing variation.

  The steel strip after hot finish rolling running on a hot run table was continuously cooled using the cooling device shown in FIG. 1 to produce a hot-rolled steel strip.

The cooling device 4 in FIG. 1 has a maximum water density of 5000 L / min · m ² , and is composed of nine cooling boxes 10 and one fine box 11. Each cooling box has a cooling amount of 20 ° C. It can be controlled. The cooling box 10 and the fine box 11 are each composed of five rows of cooling nozzles in the longitudinal direction of the steel strip, and the five rows of fine nozzles of the fine box can be controlled to be turned on / off for each row. The amount of cooling at 4 ° C. could be controlled.

  As an example of the present invention, for a steel strip whose cooling amount to be cooled by the cooling device 4 is (60 ° C., 80 ° C., 100 ° C.) with respect to each part (T, M, B) of the steel strip, first, FIG. According to the flowchart shown in Fig. 5, the water density when using each cooling box was determined.

That is, in this case, the required number of cooling boxes in each part (T, M, B) of the steel strip when the maximum water density is 5000 L / min · m ² is (3, 4, 5), and the maximum use cooling box The number was determined to be 5. Since this is less than 80% of the total number of cooling boxes, the calculated water density when using 80% of the total number of cooling boxes of the cooling device at the time of maximum cooling is calculated according to equation (1). It was determined to be 3125 L / min · m ² . Then, the calculation amount of water density is greater than 2500L / min · m ^2, to determine the water flow rate at the time of the cooling box used 3125L / min · m ^2. The amount of cooling per cooling box at this time is 12.5 ° C., and the number of cooling boxes used at the time of maximum cooling (part B) is 8.

On the other hand, as a comparative example, the cooling amount to be cooled by the cooling device 4 was cooled with the water amount density at the time of using each cooling box set to the maximum water amount density of 5000 L / min · m ² with respect to the steel strip similar to the example of the present invention. . The amount of cooling per cooling box at this time is 20 ° C., and the number of cooling boxes used for maximum cooling (part B) is five.

  When the hot-rolled steel strip was manufactured by performing the above cooling, the temperature fluctuation value of the comparative example sometimes occurred about ± 20 ° C. in the longitudinal direction of the steel strip, whereas in the present invention example, ± The temperature could be reduced to about 12.5 ° C., and variations in material could be suppressed over the entire length of the steel strip.

The side view which shows an example of the manufacturing equipment with which it uses for implementation of the manufacturing method of the hot-rolled steel strip of this invention Side view showing an embodiment of a cooling device Flow chart showing an example of determining the water density when using each cooling box Explanatory drawing which shows the cooling amount change by the switching timing error at the time of increasing the cooling amount of a hot-rolled steel strip from 16 degreeC at the time of accelerated rolling (switching timing is normal) Explanatory drawing which shows the cooling amount change by the switching timing error at the time of increasing the cooling amount of a hot-rolled steel strip from 16 degreeC at the time of accelerated rolling (switching timing abnormality) Explanatory drawing which shows the cooling amount change by the switching timing error at the time of increasing the cooling amount of a hot-rolled steel strip from 16 degreeC at the time of accelerated rolling (switching timing abnormality)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hot-rolled steel strip 2 Upper cooling means 3 Lower cooling means 4 Cooling device 5 Draining roll 6 Transport roll 7 Pinch roll 8 Pinch roll 9 Finishing rolling mill 10 Cooling box 11 Fine box

Claims (2)

When the steel strip after hot finish rolling running on the hot run table is continuously cooled using a cooling device composed of a plurality of cooling boxes, the number of total cooling boxes of the cooling device at the time of maximum cooling of the steel strip is The water amount density at the time of using each cooling box is calculated so that 80% or more is used, and the water amount density is determined to be larger of the calculated water amount density and 2500 L / min · m ^2. Manufacturing method of hot-rolled steel strip.   At least one of a plurality of cooling boxes included in the cooling device is a fine cooling box whose interior is divided into a plurality of rows, and the amount of cooling is finely controlled by switching between the cooling box and the fine cooling box. The manufacturing method of the hot-rolled steel strip of Claim 1.

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