JP2003088903A - Method for manufacturing hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the service life of a press die from lowering by suppressing the rise in the temperature of the press die at press working for edging a hot slab and to suppress the upward warpage in the top end part of the slab in rough rolling after that when manufacturing a hot-rolled steel sheet. SOLUTION: Cooling of the working face of the press die for performing the press working for edging the hot slab does not performed when edging the top end part of the hot slab and performed at least when edging the rear end part of the hot slab and during standby till the succeeding hot slab reaches the press die. In the part expecting the top end and rear end parts, cooling is performed by deciding the starting position of cooling so that the surface temperature of the working face of the press die and/or the amount of the upward warpage at the horizontal rolling which is generated in the rough rolling are in the allowable range. Furthermore, the part of a rough bar equivalent to the part of the hot slab where is cooled at least at edging is heated by heating means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot rolled steel sheet in a hot rolling facility equipped with a width reduction pressing means for hot slabs.

[0002]

2. Description of the Related Art The slab supplied to the hot rolling equipment is usually produced by a continuous casting machine,
These slab widths are integrated into several sizes. Therefore, in hot rolling, a width reduction technique has been developed in which a hot slab is rolled down in the width direction to obtain a slab having a predetermined width. In particular, in recent years, since the width reduction amount can be made larger than that of the method using the conventional vertical rolling mill, the width reduction press working method in which the width reduction is performed using the press dies facing each other across the hot slab transfer line has become popular. ing.

In this width reduction pressing method, the total length of the hot slab is obtained by opening and closing the opposing pressing dies while sandwiching the hot slab transfer line while feeding the hot slab by a predetermined amount in the longitudinal direction. Are sequentially reduced to a predetermined width. At this time, the processed surface of the press die is in contact with a high temperature slab for a long time, so that the temperature rises remarkably, and heat cracks, rough skin, deformation, wear, plastic flow, etc. occur on the die processed surface. Therefore, it is generally performed by injecting cooling water to the working surface of the press die during the width reduction press working to suppress the temperature rise of the die. However, during width reduction press working, it is unavoidable that the sprayed cooling water is also applied to the hot slab because the press die working surface and the hot slab are in contact with each other or face each other at a very narrow interval. . Therefore, there is a problem that the temperature of the end portion in the width direction of the slab is lowered, and the product quality is deteriorated due to a defective material or the like.

A method for solving the above problems, that is, a method for suppressing the temperature rise of the press die while preventing the supercooling of the slab width end portion due to the large amount of cooling water applied to the hot slab. As disclosed in JP-A-63-5837, when cooling a width reduction press die of a hot slab,
During the width reduction press working, the working surface of the press die is gently cooled, and during the waiting until the hot slab to be processed next arrives at the press die, the work surface of the press die is strongly cooled. It shows how to do it.

Further, in Japanese Unexamined Patent Publication No. 10-180301, when the width reduction press die of the hot slab is cooled, the work surface of the press die is kept on standby until the hot slab reaches the press die. Cooling the press die at all during the width reduction press working, and cooling the hot slab while waiting until the hot slab reaches the press die, and the width of the front and rear ends of the hot slab A method is shown in which cooling is not performed during reduction, and cooling is performed except when the width of the front end portion and the rear end portion is reduced.

[0006]

By the way, the present inventors have found that, as a problem caused by the cooling water being applied to the hot slab, the cooling water is accumulated on the upper surface of the slab in addition to the above cooling of the slab width end portion. As a result, it was found that a temperature difference occurs between the upper and lower surfaces of the slab, and subsequent horizontal rolling by a rough rolling mill causes warpage of the rolled material. This warp amount does not cause a problem if it is up to about 50 mm, but if it exceeds 200 mm, for example, troubles such as not being caught in the next horizontal rolling mill occur.

FIG. 4 shows the hot slab after being subjected to the width reduction press work. When the width reduction work is performed, a raised portion called a dog bone is formed slightly in the center from both ends in the width direction. . Therefore, the cooling water flowing in the vicinity of the center in the width direction cannot flow down in the width direction and is accumulated on the upper surface of the slab, which lowers the temperature of the upper surface of the hot slab and causes warpage during horizontal rolling. However,
In the vicinity of the rear end of the slab, there is no problem because the cooling water flows down from the rear end as the slab advances. Also, near the slab tip, if there is a temperature difference between the upper and lower surfaces, a large upward warpage occurs during horizontal rolling, but in the subsequent parts, the material on the leading side of the rolling mill restrains the upward warpage. Moreover, a large warp is unlikely to occur.

In the method disclosed in Japanese Patent Laid-Open No. 63-5837, the press die is gently cooled during the width reduction press. However, watering on the upper surface of the slab is avoided even if the cooling is not so strong. However, since the cooling water collects on the upper surface of the slab, the warp of the slab tip during horizontal rolling cannot be avoided.

On the other hand, in the method according to Japanese Unexamined Patent Publication No. 10-180301, since the press die is not cooled at least during the width reduction pressing of the slab front end and the rear end, the slab front end is subjected to horizontal rolling. The problem of warpage does not occur. However, this method does not cool the die at the rear end of the slab where the temperature of the die is the highest, so the cooling efficiency is poor and it is not a preferable method for extending the life of the die. .

The object of the present invention is to solve the above-mentioned problems of the prior art and suppress the temperature rise of the press die during the width reduction press working of the hot slab in the production of hot rolled steel sheet. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet, which can suppress the reduction of life and suppress the warp at the tip of the slab during horizontal rolling in the subsequent rough rolling.

[0011]

Means for Solving the Problems As a result of various studies on the cooling method of the press die during the width reduction press working of the hot slab, the present inventors found that the width reduction of the tip of the hot slab Without cooling the press die working surface, at least during the width reduction of the hot slab rear end and after the end of the width reduction of the hot slab until the hot slab to be processed reaches the press die. We have found a method of cooling the working surface of the press die during standby. According to this method, the cooling efficiency of the die can be ensured and the life of the die can be extended by cooling the die when the width of the rear end of the slab where the temperature of the die is highest is reduced. Cooling the mold when the mold temperature reaches the maximum temperature is very important for extending the mold life. Further, since the mold is not cooled when the width of the slab tip is reduced, the cooling water does not collect on the upper surface of the slab tip, and it is possible to prevent the warpage during the subsequent horizontal rolling.

Further, according to this method, at the position where the press die such as the rear end portion of the slab is cooled, the temperature drop at the end portion of the slab width, which is pointed out in JP-A-10-180301, etc., occurs. Although it will occur, a rough bar (hot slab is roughly rolled) between the rough rolling mill and the finish rolling mill.
By providing a heating means for heating, and heating at least the temperature lowering portion, the product quality such as the material can be secured and no problem occurs. Therefore, whether or not cooling is performed at a position other than the slab front and rear ends (longitudinal center) depends on the life of the press die (for example, the surface temperature of the die surface) and the occurrence of warpage during horizontal rolling in rough rolling. It may be determined as appropriate.

The method of manufacturing a hot-rolled steel sheet according to the present invention is based on the above findings and has the following features.

(1) In a method for producing a hot rolled steel sheet, in which a hot slab is subjected to width reduction pressing using a press die, rough rolling is performed into a rough bar, and finish rolling is performed into a hot rolled steel sheet. Cooling the working surface of the press die for performing the width reduction pressing of the hot slab is not performed when the width reduction of the tip end of the hot slab is performed, and at least when the width reduction of the rear end of the hot slab and the heat treatment are performed. A method for manufacturing a hot-rolled steel sheet, which is carried out after waiting for the width reduction of the hot slab to be performed while waiting until the hot slab to be processed next reaches the press die.

(2) When the width of the hot slab other than the front and rear ends of the hot slab is reduced, the press die working surface is cooled by increasing the surface temperature and / or width of the press die working surface which is increased by contact with the hot slab. The cooling start position after the tip of the hot slab is determined and carried out so that the amount of warpage during horizontal rolling that occurs in rough rolling that continues after reduction falls within an allowable range. The method for manufacturing a hot-rolled steel sheet according to.

(3) At least the portion of the rough bar corresponding to the portion of the hot slab that has been cooled when the width of the hot slab is reduced in width is heated by a heating means. The method for manufacturing a hot rolled steel sheet according to 2).

[0017]

1 to 3 show an embodiment of the present invention. FIG. 1 is an overall configuration diagram of a hot rolling facility, and FIG.
FIG. 3 is a plan view showing a width reduction working state of a hot slab by a width reduction press device, and FIG. 3 is an explanatory view of a press die cooling device. In FIG. 1, 1 is a hot slab width reduction press, 2 is a rough rolling mill, 4 is a finish rolling mill, and 5 is a coiler. Further, in FIGS. 2 and 3, 6 is a hot slab supplied to the width reduction press machine, and 7 is the width reduction press machine 1.
Press die, 8 is a press working surface of the press die, 9
Is a cooling water jet nozzle for cooling the processed surface of the press die.

The hot slab supplied to the hot rolling equipment is
First, the width reduction press device 1 performs width reduction to a predetermined width, and subsequently, the rough rolling device 2 and the finish rolling device 4 roll to a predetermined plate thickness, and then the coiler 5 winds it. As shown in FIG. 2, when the hot slab 6 is conveyed, the width reduction press device 1 causes the press die 7 to open and close in the direction of the arrow every time the hot slab is fed by a predetermined amount, Slab 6
In the width direction. For example, 400mm hot slab
When the pressing die is opened and closed with a stroke of 200 mm on each side for each cycle, the total length is 6 m
By repeating the above cycle about 15 times for the slab, the width of the entire slab can be reduced by about 350 mm at the maximum. In addition, the width reduction press device 1 includes
As shown in FIG. 3, a cooling water jet nozzle 9 is provided so as to face the processing surface 8 of the press die 7, and the cooling water is jetted to cool the processing surface 8 of the press die.

According to the present invention, the cooling water is jetted from the cooling water jet nozzle 9 during the waiting period from the end of the width reduction of the preceding hot slab to the start of the width reduction of the subsequent hot slab, and the press die is pressed. The processed surface 8 of 7 is cooled. And hot slab 6
When the front end of reaches the entrance side of the width reduction press device 1, the cooling water injection from the cooling water injection nozzle 9 is stopped. With the cooling water stopped, the tip end portion of the hot slab 6 is subjected to width reduction pressing by the opening / closing operation of the press die 7. here,
The purpose of stopping the cooling water is to prevent the jetted cooling water from accumulating on the upper surface of the slab and causing warpage during horizontal rolling performed by the rough rolling mill 2 as described above. Therefore, the length of the hot slab tip that should stop the cooling water is
When cooling water collects on the upper surface of the slab, it is set to the range where warpage occurs during the subsequent horizontal rolling. Specifically, since it depends on the width and thickness of the slab, it is sufficient to have a table value for each size. Generally, at least 1.0 m from the tip is required. For example, if the typical slab size is 230 mm in thickness and 1250 mm in width, 1.2 m from the tip is generally required. .

On the other hand, when the width of the rear end of the hot slab is reduced, the temperature of the press die 7 rises due to the width reduction of the hot slab. The cooling water is jetted toward. Cooling of the press die at the rear end of the hot slab where the temperature of this press die is the highest,
It is essential for extending the life of the mold. Normally, the cooling water is kept jetting regardless of the opening / closing operation of the press die 7, but the cooling water is jetted during the opening operation of the die, and the jetting of the cooling water is stopped during the closing operation of the die. May be used. However, even in that case, the press die working surface 8
And the hot slab are facing each other at a very narrow interval,
It is unavoidable that cooling water is applied to the upper surface of the slab. However, in the vicinity of the rear end of the slab, the cooling water splashed on the upper surface of the slab flows down from the rear end of the slab, and is difficult to collect on the upper surface of the slab. Therefore, if the injection of cooling water is started from a position where the cooling water applied to the upper surface of the slab flows down from the rear end of the slab, for example, a position about 1.5 m from the rear end, the problem of warpage does not occur during subsequent horizontal rolling.

Here, except for the front and rear ends of the hot slab (the central portion in the longitudinal direction), the cooling water may be stopped or the cooling water may be sprayed. That is, it is preferable to inject cooling water in order to extend the life of the mold, but especially near the tip, there may be a slight upward warp during horizontal rolling thereafter, and the temperature drop at the slab width end There is also a problem that occurs. Therefore,
The allowable temperature of the working surface of the press die may be determined, and cooling water may be jetted so as not to exceed the allowable temperature. For example, the temperature at which the high-temperature strength of the material used as the press die decreases (about 550 ° C. in the case of ductile) can be set as the allowable temperature. In addition, the allowable range of the amount of warpage during horizontal rolling (for example, not exceeding a height of 200 mm) is set, and the injection start position of the cooling water is determined so that the amount of warpage does not exceed that of the preceding material. Good.

After the rear end of the hot slab 6 has passed through the width reduction press device 1, the cooling water jet nozzle 9 continues while waiting until the hot slab to be processed next reaches the press die 7. Cooling water is jetted from to cool the working surface 8 of the press die.

Next, the hot slab 5 that has passed through the width reduction pressing device 1 is continuously conveyed to the rough rolling mill 2 and is roughly rolled. At this time, if the cooling water is collected on the upper surface near the slab tip portion, due to the temperature difference between the upper and lower surfaces of the slab, warping will occur during horizontal rolling, but in the present invention, the width reduction press of the slab tip portion is used. Since the cooling water at that time is stopped, no upward warpage occurs. Further, after the central portion in the longitudinal direction, the material on the leading side of the rolling mill restrains the warp in a direction to suppress the warp, so that the warp does not pose a problem even if the cooling water is accumulated.

The rough bar coming out of the rough rolling mill 2 is used as a finishing rolling mill 4
However, at the position corresponding to the slab position where the cooling water was sprayed during the width reduction press working such as the rear end part, the temperature of the width end part has dropped, so if it is a product with defective materials etc. It may cause a defect. Therefore,
As shown in FIG. 1, it is preferable to install a heating device 3 between the rough rolling mill 2 and the finish rolling mill 4 and heat the rough bar by the heating device 3. The heating device 3 may heat only the vicinity of the width end portion of the rough bar called an edge heater, but may heat the entire width direction of the rough bar. Further, regarding the position in the longitudinal direction of the rough bar to be heated, it is necessary to heat at least the position where the temperature drop occurs at the width end. As a method for specifying the position, a method of tracking the actual result of injection from the cooling water injection nozzle 9 at the time of width reduction of the hot slab to obtain a rough bar position where a temperature decrease occurs, and a heating device It is possible to use a method in which a thermometer for actually measuring the temperature of the width end of the rough bar is installed on the inlet side of 3, and the heating device 3 is controlled based on the temperature measurement result. The entire length of the rough bar may be heated, and in that case, it is more preferable to make the output of the heating device at the temperature lowering position larger than that at other positions.

Further, in FIG. 1, the heating device 3 is connected to the rough rolling mill 2
The same effect can be obtained even if it is installed between the width reduction press 1 and the rough rolling mill 2.

[0026]

EXAMPLES Hot-rolled steel sheets were manufactured using the hot rolling equipment and the width reduction press shown in FIGS. here,
As the heating device 3, an edge heater that induction-heats the width end of the rough bar is adopted. Further, a thermometer (not shown) is installed in the width reduction press device 1 so that the surface temperature of the press die working surface 8 can be measured in real time. The hot slab used has a thickness of 125 to 250 mm and a width of 600 to 1.
The length was 950 mm and the length was 3.5 to 9.6 m, and the surface temperature of the hot slab before the width reduction pressing was 1000 to 1300 ° C. The material of the press die 7 is ductile,
The average heat transfer coefficient is 1000 as the cooling condition of the die processing surface 8.
Approximately 4000 kcal / m ² · hr · ° C.

In Table 1, No. Examples 1 to 3 of the present invention in which the coarse bar was not heated, No. Nos. 4 to 6 are examples of the present invention in which a coarse bar is heated, No. Reference numerals 7 and 8 are comparative examples. In addition, the life of the press die, the warpage during rough rolling, and the material of the end portion of the product width were evaluated under each condition. Here, the life of the press die is 10 times the hot slab.
The rough surface condition of the die surface after the reduction of ten thousand tons was evaluated, and the warp was evaluated as the height of the warp generated during horizontal rolling by the rough rolling mill 2.

[0028]

[Table 1]

First, Comparative Example 7 in which cooling was carried out over the entire length of the slab.
In the case of the rough rolling, the warp of the tip portion occurred and the material of the end portion of the product width deteriorated. Further, in Comparative Example 8 in which no cooling was performed on the entire length of the slab, the roughening of the press die surface was serious, and the die life was a problem.

On the other hand, in Example 1 of the present invention in which cooling is performed only at the rear end of the slab of 1.5 m, the life of the press die is slightly insufficient as compared with the other examples of the present invention, but the warpage during rough rolling is increased. None, it was good. Further, in Example 3 of the present invention in which the length of the hot slab tip portion at which cooling water should be stopped is set as a table value in Table 2 and cooling is performed after this position, the tip portion is slightly warped during rough rolling due to the slab. However, the life of the press die was good.

[0031]

[Table 2]

Further, in Example 2 of the present invention, the surface temperature of the working surface 8 of the press die during the width reduction press was measured, and when the temperature reached 500 ° C., cooling of the press die was started,
Cooling was continued until the rear end of the slab. The change in surface temperature of the press die working surface at that time is shown in FIG. For comparison, the case of Comparative Example 2 in which no cooling is performed is also shown.
About 800 when not cooled as in Comparative Example 2.
The mold surface temperature reaches up to ℃, the mold surface temperature is 5
By starting cooling when the temperature reaches 00 ° C, about 5
The temperature could be suppressed to 50 ° C or lower. As a result, good results were obtained for both die life and warpage during rough rolling.

However, No. 1 of the present invention example. Since Nos. 1 to 3 did not heat the coarse bar, deterioration of the material was sometimes observed at the width end of the product.

On the other hand, No. 1 of the present invention example. Nos. 4 to 6 are Nos. For each of Nos. 1 to 3, the rough bar is heated by the heating device 3 (edge heater).
The heating method is to track the coarse bar position corresponding to the slab position where the injection of the cooling water in the width reduction press device 1 is started, and start heating the edge heater when the position reaches the edge heater, Heating was continued to the rear end of the coarse bar. As a result, No. The material at the width end of the product was good under any of the conditions of 4 to 6. For other mold lives and warpage during rough rolling, the corresponding N
o. It was similar to the results of 1-3.

[0035]

As described above, according to the present invention,
When manufacturing the hot-rolled steel sheet, by appropriately cooling the press die processed surface of the width reduction pressing means, it is possible to suppress the die life reduction due to the temperature increase of the press die processed surface, and after that, It is possible to suppress warpage in rough rolling and stably manufacture a hot rolled steel sheet.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an example of a hot rolling facility used for implementing the present invention.

FIG. 2 is a plan view showing a width reduction working condition of a hot slab by a width reduction press device.

FIG. 3 is an explanatory view showing an example of cooling equipment for a width reduction press die.

FIG. 4 is a perspective view of a hot slab subjected to width reduction by a width reduction press device.

FIG. 5 is a graph showing changes in the surface temperature of the press die working surface of Example 2 of the present invention and Comparative Example 2 in Examples.

[Explanation of symbols]

1 Width reduction press 2 rough rolling mill 3 heating devices 4 Finishing mill 5 coilers 6 hot slabs 7 Width reduction press die 8 Processing surface of press die 9 Cooling water injection nozzle 10 dog bones 11 Cooling water collected on the upper surface of the slab

Continued front page    (72) Inventor Tatsuya Jinnai             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4E002 AB04 AD04 BD07 BD08 CA07

Claims (3)

[Claims] 1. A method for producing a hot-rolled steel sheet comprising subjecting a hot slab to width reduction pressing using a press die, rough rolling to a rough bar, and finish rolling to a hot rolled steel sheet,
Cooling the working surface of the press die for performing the width reduction press working of the hot slab is not performed at the time of the width reduction of the leading end of the hot slab, and at least when the width reduction of the rear end of the hot slab is performed. A method for manufacturing a hot-rolled steel sheet, which is carried out after waiting for the width reduction of a hot slab to be completed and before the hot slab to be processed next reaches the press die. 2. The cooling of the press die working surface during the width reduction other than the front and rear end portions of the hot slab is performed by increasing the surface temperature and / or the width reduction of the press die working surface which is increased by the contact with the hot slab. 2. The cooling start position after the tip of the hot slab is determined and carried out so that the amount of warpage during horizontal rolling that occurs in the rough rolling that follows is within an allowable range. Method for manufacturing hot rolled steel sheet. 3. The heating means is used to heat at least a portion of the coarse bar corresponding to a portion of the hot slab that has been cooled when the width of the hot slab is reduced. The method for manufacturing a hot-rolled steel sheet according to.