JPH09300004A - Method for rolling hot rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure finished temp. in the tip part of a steel strip, to prevent the generation of scale-like surface flaws and to manufacture a hot rolled steel strip of a homogeneous material at high yield at the time of rolling the hot rolled thin steel strip of <=1.2mm thick in a continuous hot finishing mill. SOLUTION: When the rough rolling of a slab heated to a prescribed temp. is executed with a roughing mill, next, the finish rolling of the rough rolled rough bar is executed with the finishing mill 4, the thickness of the rough bar is defined as within the range of 20-30mm, the rough bar having this thickness is heated so that the temp. of the rough bar on the inlet side of finishing is within the range 1000-1150 deg.C with a heating device 2 provided on the inlet side of the finishing mill 4, the finish rolling of the rough bar heated to this temp. is executed and rolling is made to complete at a stand on the upstream side of the final stand.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, in a continuous hot rolling mill, ensures a stable finishing temperature at the tip of a steel strip when rolling a hot-rolled thin steel strip having a plate thickness of 1.2 mm or less. The object is to provide a method for rolling a hot-rolled steel strip that can be performed.

[0002]

2. Description of the Related Art Generally, a hot rolled steel strip is obtained by heating a slab to a predetermined temperature in a heating furnace and roughly rolling the heated slab to a thickness of about 30 mm by a rough rolling machine to form a rough bar. The obtained rough bar is finish-rolled in a continuous finishing mill usually consisting of 7 stands to form a hot-rolled steel strip having a predetermined thickness, and the hot-rolled steel strip is cooled on a runout table,
It is manufactured by winding with a coiler.

In the process of manufacturing such a hot rolled steel strip,
A non-uniform temperature distribution occurs in the length direction of the hot-rolled steel strip. That is, the slab when it comes out of the heating furnace is at a high temperature of 1000 ° C. or higher, but as it is rough-rolled and finish-rolled, it is radiatively cooled due to the rolling time, water-cooled for scale removal, and rolled. The temperature decreases due to heat removal due to contact.

As means for compensating for the above-mentioned temperature drop,
Conventionally, a rough bar is accelerated and rolled in a finish rolling mill. The accelerated rolling in this finish rolling step is performed by attaching the tip of the steel strip exiting the finishing rolling mill to a coiler and winding it at high speed by the coiler. By such accelerated cooling, the amount of temperature decrease of the steel strip in the finish rolling step is reduced, and the finish temperature, which is an important factor in terms of material quality, can be secured.

However, since the tip of the steel strip leaving the finish rolling mill until the tip of the steel strip is attached to the coiler, the rolling speed is restricted from the viewpoint of running performance on the run-out table. It cannot be accelerated rolled, so the finishing temperature at the tip of the steel strip is lower than at other parts.

As a matter of course, the lowering of the finishing temperature at the tip of the steel strip described above becomes greater as the plate thickness of the steel strip becomes thinner. Therefore, it has become more difficult to secure the finishing temperature of the leading end of a thin steel strip, which has been remarkably increasing in production in recent years, which is a problem in yield.

[0007] Various studies have been made in the past regarding means for solving the above-mentioned problems and ensuring the finishing temperature at the tip of the steel strip in the hot rolling of a thin steel strip. For example, the following method is known. ing.

After connecting a plurality of rough bars to each other, the rough bars are passed through a finishing mill at high speed to change the running plate thickness from a thick material to a thin material, so that finish rolling is completely continuous ( Hereinafter referred to as Prior Art 1).

As disclosed in Japanese Patent Laid-Open No. 1-232109, in a normal batch mill, a tip end of a rough bar to be finish-rolled is heated by a heating device installed on the inlet side of a finish-rolling machine in a line. By doing so, the finishing temperature at the tip of the steel strip is compensated (hereinafter referred to as prior art 2).

As disclosed in Japanese Patent Laid-Open No. 2-165802, the thickness of the rough bar is set to less than 20 mm in order to improve the dimensional accuracy of the plate profile, the plate shape, the plate width, the plate thickness, and the like. A coil box installed between the rough rolling mill and the finish rolling mill or an online heating device compensates for the temperature drop of the rough bar (hereinafter referred to as Prior Art 3).

The finishing temperature is secured by increasing the thickness of the rough bar (hereinafter referred to as prior art 4).
The above-mentioned Prior Art 1 is applied to a completely continuous mill, and Prior Art 2 to 4 is applied to a batch mill.

[0012]

[Problems to be Solved by the Invention] For example, a finish plate thickness of 1.0
When the thin steel strip of about mm is manufactured by the above-mentioned prior art, the following problems occur.

When the heating equipment is installed in front of the finish rolling mill as in Prior Art 2 to heat the tip end of the rough bar to be finished rolled, in the case of a thin material, especially in the rear stand of the finish rolling mill row. Temperature drop is remarkable. Therefore, in order to meet the target finishing temperature, the coarse bar must be heated to a fairly high temperature, which in turn promotes coarse bar scale formation,
Introduces scale defects on the product. On the other hand, when installing heating equipment between the stands of the finishing rolling mill row,
Since the rolling speed of the material to be rolled is high, high-output heating equipment is required.

The prior art 3 is aimed at improving the accuracy of the product dimension, and assuring the finishing temperature of the front end of the steel strip, in particular, regarding the finishing side temperature and the surface texture of the product, especially the scale surface flaw countermeasures. Has not been examined at all. Also,
If the thickness of the rough bar is less than 20 mm, the temperature drop in the rough rolling step becomes extremely large, and in order to compensate for this temperature drop, an extremely high output on-line heating device is required as described above.

The method of increasing the thickness of the rough bar as in the prior art 4 is effective when the product plate thickness is thick, but when the product plate thickness is thin, the load and power of the finish rolling mill are increased. It is difficult to realize due to such restrictions.

Further, as in the prior art 1, in order to join a plurality of rough bars and perform the complete continuous finish rolling, a huge capital investment is required. Therefore, an object of the present invention is to solve the above-mentioned problems, and in a continuous hot rolling mill, when rolling a hot-rolled thin steel strip having a plate thickness of 1.2 mm or less, the occurrence of scale surface flaws due to overheating. It is an object of the present invention to provide a method capable of producing a hot-rolled steel strip of a uniform material with a high yield while preventing it and securing a finishing temperature at the tip of the steel strip.

[0017]

From the above-mentioned viewpoint, the present inventors have ensured a stable finishing temperature at the tip of a steel strip when rolling a hot-rolled thin steel strip having a plate thickness of 1.2 mm or less. We have conducted intensive research to develop a method that can be done.

Finishing outlet temperature (F
DT) can be secured by heating the rough bar with a heating device installed on the inlet side of the finishing rolling mill to raise the finishing inlet temperature (FIT) of the rough bar rolled by the finishing rolling mill. As mentioned above, from the viewpoint of preventing the occurrence of scale surface flaws, there is a limit to increasing the temperature on the finishing side. Considering the descaling ability on the entry side of the finishing rolling mill, the upper limit of the finishing entry temperature is 1150 ° C, and at this temperature, the finishing exit temperature at the tip of the thin steel strip should be kept at a desired value. I can't.

On the other hand, in a conventional mill without a heating device for a rough bar, reducing the plate thickness of the rough bar causes a decrease in the temperature on the finishing inlet side, and it is impossible to secure a predetermined temperature on the finishing outlet side. It had been.

Therefore, if heating of the coarse bar and thinning of the coarse bar plate thickness are combined and a coarse bar having a plate thickness of 20 to 30 mm is heated to a temperature of 1150 ° C. on the entry side of the finish rolling mill, the coarse bar is produced. It is possible to prevent the temperature drop on the entry side of the finish rolling mill due to thinning, and because the load on the finishing mill is reduced, it is possible to reduce the number of rolling stands required for the production of thin steel strip. It has been found that the finish outlet temperature at the tip of the thin steel strip can be maintained at a desired value.

The present invention has been made on the basis of the above findings, in which a slab heated to a predetermined temperature is roughly rolled by a rough rolling machine, and then a rough rolled rough bar is put into a plurality of five or more stands. In the rolling method for hot-rolled steel strip, the thickness of the rough bar to be finished-rolled is 20 to Within a range of 30 mm, the rough bar having the above-mentioned thickness is heated to 1000 to 11 on the finishing side of the rough bar by a heating device provided on the inlet side of the finishing rolling mill.
It is heated so as to be within a range of 50 ° C., and the rough bar heated to the temperature is finish-rolled through a continuous hot finish rolling mill composed of the plurality of stands, and is finished rolling more than the final stand of the finish rolling mill. The feature is that the rolling is finished at the upstream stand.

The method of the heating device is not particularly limited, but since the main purpose is to heat the end portion in the lengthwise direction of the rough bar, an induction heating method that is capable of rapid heating and has good control response is desirable. .

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The finish bar temperature (FIT) of a rough bar heated on the bar side of a finish rolling mill is 1000-1.
It should be in the range of 150 ° C. If the finishing inlet temperature of the rough bar is less than 1000 ° C, the target finishing outlet temperature (FDT), which is an important factor in terms of material quality, cannot be secured, and the rolling load of the finishing rolling mill becomes large. Occurs. On the other hand, when the temperature on the finishing side of the rough bar exceeds 1150 ° C., the growth of scale is promoted, and there is a problem that a scale surface flaw is likely to occur in the product.

The thickness of the rough bar to be finish rolled is 20 to 30.
Should be in the mm range. If the thickness of the coarse bar is less than 20 mm, the desired finishing temperature cannot be secured, while if the thickness of the coarse bar exceeds 30 mm, the rolling load of the finish rolling mill becomes too large, and the run-out table is too large. The running of the steel strip becomes unstable and problems occur when winding the coiler.

The above-mentioned finishing-side temperature is 1000 to 11
Heated to be in the range of 50 ° C., thickness 20 to 3
Finish rolling of a 0 mm rough bar using a continuous hot finishing mill consisting of, for example, 7 stands requires finishing rolling at, for example, an F6 stand upstream of the final stand of the finishing mill. If the rolling is finished at the final stand, the target finishing temperature cannot be secured.

FIG. 1 is a schematic explanatory view of a continuous hot rolling line for carrying out the method of the present invention. As shown in FIG. 1, a rough rolling mill 1, a rough bar heating device 2, a continuous hot finish rolling mill 4 including 7 stands, a runout cooling device 6 and a coiler 8 are arranged in this order. Reference numeral 3 is a finishing inlet side thermometer provided on the inlet side of the finishing rolling mill, 5 is a finishing outlet side thermometer provided on the outlet side of the finishing rolling mill, and 7 is a winding thermometer.

The rough bar, that is, the material to be rolled 9 rolled by the rough rolling mill 1 is heated to a predetermined temperature by the heating device 2 installed on the inlet side of the finishing rolling mill 4. At that time, the temperature of the material 9 to be rolled measured by the thermometer 3 on the entry side of the finish rolling mill is set to the heating device 2
And is controlled so that the coarse bar 9 is heated to a uniform predetermined temperature in the length direction.

The material to be rolled 9 heated to a predetermined temperature in this way is a continuous hot finish rolling mill 4 consisting of 7 stands.
In the above, it is finish-rolled, cooled by the run-out cooling device 6, and then wound by the coiler 8.

As an example of the temperature change of the rolled material in the finish rolling mill, the temperature change of the tip end of the rolled material when a steel strip having a finished plate thickness of 3.0 mm is manufactured by a finish rolling mill composed of 7 stands is illustrated. 6 (Source: “Theory and practice of sheet rolling” p. 158, Iron and Steel Institute, 1984). FIG.
As is clear from the above, the temperature decrease (center and average temperature) at the leading end of the rolled material becomes larger in the latter stage, and this tendency becomes more remarkable as the finished plate thickness becomes thinner.

Therefore, to finish rolling before the final stand, that is, to reduce the number of passes of finish rolling,
This will significantly reduce the temperature drop of the material to be rolled in the finish rolling process. It should be noted that the reduction of the plate thickness of the rough bar can be easily performed by increasing the number of passes of rough rolling, and there is no problem in operation.

A rough bar having a plate thickness of 30 mm is heated in the heating device 2 so that the finishing inlet temperature (FIT) becomes 1000 ° C., 1100 ° C. and 1200 ° C., and then a finishing rolling mill comprising seven stands F1 to F7. According to 4, the plate thickness: 1.
Fig. 2 shows the temperature transition at the tip of the steel strip when finish rolling to a steel strip with 0 mm and a target finish outlet temperature (FDT): 800 ° C. As is clear from FIG. 2, the finish-side temperature is 1200
If the rough bar is heated so as to be equal to or lower than ℃, it is possible to secure the target finish outlet temperature of the front end of the steel strip. However, in the actual operation, when the finishing inlet temperature exceeds 1150 ° C., a scale flaw may occur, so the heating temperature of the coarse bar should be 1150 ° C. or less.

Next, in the heating device 2, the finishing bar temperature (FIT) of the rough bars having plate thicknesses of 20, 25 and 30 mm was set to 1
FIG. 3 shows a temperature transition at the tip of the steel strip when the steel strip having a thickness of 1 mm is finish-rolled by the finish rolling mill 4 after being heated to 100 ° C. As is clear from FIG. 3, when the plate thicknesses of the rough bars were 20 mm and 25 mm, the finish rolling could be completed at the F6 stand, which is one stage upstream from the final stand. On the other hand, when the plate thickness of the rough bar is 30 mm, the finish rolling cannot be finished at the F6 stand, and it becomes necessary to finish the finish rolling at the final F7 stand. It is not possible to secure the finishing outlet temperature.

The target finish outlet temperature (FDT) is 800 ° C., and the conditions of the method of the present invention (finish inlet temperature: 1100 ° C., rough bar thickness: 25 mm, finish steel strip thickness: 1.0 mm, 6 stand finish) ), The temperature on the finishing side of the steel strip tip is the same as the conventional method (finishing side temperature: 1100 ° C, rough bar thickness: 30 mm, finishing steel strip thickness: 1.0 mm, 7 stands). The results are shown in FIG. 4 in comparison with the case of finish rolling.

As is apparent from FIG. 4, in the case of the method of the present invention, the target finishing temperature is secured up to the tip of the steel strip, whereas in the case of the conventional method, the temperature at the tip of the steel strip is large. , The target finish temperature on the outlet side could not be secured.

[0035]

Next, the present invention will be described with reference to embodiments. C
A slab having a thickness of 220 mm and made of ordinary steel having a content of 0.05 wt.% Was roughly rolled into a rough bar having a plate thickness of 25 mm in a rough rolling mill 1 shown in FIG. 1 and provided on the entrance side of the finish rolling mill. In the heating device 2, the finishing inlet temperature (FIT) is 1100 ° C.
The tip of the rough bar was heated so that

Then, it was passed through a continuous finishing mill 7 consisting of 7 stands at a constant speed for finish rolling, and by the sixth stand finishing, an ultra-thin hot-rolled steel sheet having a sheet thickness of 0.1 mm was manufactured. The rolling speed of the front end of the steel strip before winding by the coiler 8 was 650 mpm.

FIG. 5 shows the change in the finish outlet temperature (FDT) in the length direction of the steel strip. As is clear from FIG. 5, the finish outlet temperature is 80 over the entire length including the tip of the steel strip.
It was possible to secure 0 ° C or higher. The surface properties of the steel strip were comparable to those of the 1.6 mm material that is usually manufactured.

[0038]

As described above, according to the method of the present invention, when a hot-rolled thin steel strip having a plate thickness of 1.2 mm or less is rolled in the continuous hot rolling mill, the tip of the steel strip is finished. The temperature can be stably ensured, the occurrence of scale-like surface defects due to overheating can be prevented, and a hot-rolled steel strip of a uniform material can be manufactured with a high yield, which is industrially useful.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a continuous hot rolling line for carrying out the method of the present invention.

FIG. 2 is a diagram showing a temperature transition of a tip end portion of a steel strip when finish rolling is performed from a rough bar having a strip thickness of 30 mm to a steel strip having a strip thickness of 1 mm.

FIG. 3 is a diagram showing a temperature transition of a tip end portion of a steel strip when finish rolling is performed on a steel strip having a strip thickness of 1 mm by changing the strip thickness of a rough bar.

FIG. 4 is a diagram showing a change in finishing temperature at the tip of the steel strip.

FIG. 5 is a diagram showing changes in finishing temperature in the length direction of the steel strip.

FIG. 6 is a diagram showing a temperature change of a tip end portion of a material to be rolled in a finish rolling mill including seven stands.

[Explanation of symbols]

 1 rough rolling mill 2 heating device 3 finishing rolling mill inlet side thermometer 4 finishing rolling mill 5 finishing rolling mill outlet side thermometer 6 runout cooling device 7 winding thermometer 8 coiler 9 rolled material

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sadakazu Masuda 1-2, Marunouchi, Chiyoda-ku, Tokyo Day Steel Pipe Co., Ltd. (72) Yutaka Mihara 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Inside the Steel Pipe Co., Ltd. (72) Inventor Masaaki Yamamoto 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Inside the Steel Pipe Co., Ltd. (72) Naomi Eda 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Inside Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A slab heated to a predetermined temperature is roughly rolled by a rough rolling mill, and then a rough rolled rough bar is finish rolled by a continuous hot finish rolling mill consisting of a plurality of five or more stands, and a plate is obtained. A method of rolling a hot-rolled steel strip, which produces a thin steel strip having a thickness of 1.2 mm or less, wherein the thickness of the rough bar to be finish rolled is within a range of 20 to 30 mm, and the rough bar having the thickness is A heating device provided on the entry side of the finishing rolling mill heats the rough bar so that the finishing entry temperature of the rough bar is in the range of 1000 to 1150 ° C., and the rough bars heated to the temperature are heated by the plurality of stands. A method for rolling a hot-rolled steel strip, comprising: finishing rolling by passing through a continuous hot finishing rolling mill consisting of 1) and terminating the rolling at a stand upstream of the final stand of the finishing rolling mill. 2. The method according to claim 1, wherein an induction heating device is used as a heating device provided on the entrance side of the finish rolling mill.