JP2891131B2 - Necking prevention method in hot rolling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for hot rolling steel.
The present invention relates to a method for preventing necking that occurs when a hot strip after final finish rolling is wound by a winder.

[0002]

2. Description of the Related Art In hot strip rolling of a hot strip mill or the like, when a rolled strip is wound by a winder, an excessive tension is generated in the strip when the end of the strip is wound around the winder. As a result, as shown in FIG. 9, a local change in cross-sectional dimension called necking occurs. Normally, in the winding process after the rolling of the steel material, the rotation speed of the winding core is set slightly higher than the entering speed of the steel material so that the winding can be tightly performed without any loosening or the like. As a result, as shown in FIG. 7, immediately after winding, the winding core speed is restrained by the wound steel material speed and is reduced to the steel sheet speed. As a result, excessive tension is applied to the steel material between the rolling mill and the winding machine. As shown in FIG. 8, a relatively small portion of the steel material having a low deformation resistance is deformed, and the width of the plate is locally reduced to cause necking.

Conventionally, various methods for preventing the necking have been proposed. For example, Japanese Unexamined Patent Publication No.
In Japanese Patent Publication No. 190713 (Prior Art 1), after exiting the finishing mill, a presser roll is set slightly behind the position where necking occurs and before the winder, and the presser roll is used by the presser roll. A method for preventing necking due to excessive winding tension by keeping the tension between the rolling mill and the finishing mill within an allowable value is disclosed. In Japanese Unexamined Patent Application Publication No. 59-78721 (Prior Art 2), in the winding step of hot strip rolling, the motor power supplied to the winding mandrel is changed from the start of winding to the completion of winding. A method for preventing necking in which the supply is controlled so as to be reduced by the amount of over tension acting on the steel strip by changing the angular velocity of the mandrel is described. Further, JP-A-62-6
In Japanese Patent No. 8617 (Prior Art 3), the position and amount of the width drop due to necking are predicted, and the width of the corresponding portion of the rough bar on the entrance side of the hot finishing mill is widened in advance to an amount suitable for the predicted width drop. A method for preventing necking is disclosed.

On the other hand, in Japanese Patent Application Laid-Open No. 63-207410 (Prior Art 4), the outlet temperature of the final finishing mill is maintained just above the _Ar3 transformation point of the hot-rolled steel strip, and from the transformation start point. A method for preventing necking in which the cooling up to the transformation end point is performed only by air cooling, then liquid cooled, and then wound around a coiler is disclosed.

[0005]

However, in the case of the first prior art, it is necessary to make a significant design change to the existing equipment, and during that time, the operation of the line has to be stopped. It is not preferable in terms of, for example. Further, since it is necessary to give the presser roll a force opposing the tension of the winding machine, there are problems such as an increase in equipment cost and an increase in power consumption.

In the case of the preceding example 2, the tension from the finish rolling mill to the winding machine usually has a great influence on the winding appearance and strip winding property in addition to necking.
In particular, if the tension is too low, these may be adversely affected.However, in this method, if the amount of decrease in the supply power is overestimated, or if the supply power becomes low due to a control error, etc. May induce the above-mentioned adverse effects.

Further, in the case of the preceding example 3, since the tracking accuracy of the necking position is generally not high, the necking occurrence position may not coincide with the strip width increasing portion, and this problem is solved. For this reason, if the range of the strip width increasing portion is excessively widened, a problem such as a decrease in yield is caused.

On the other hand, in the prior art 4, when the deformation of the steel material is caused by the tension from the exit side of the finish rolling mill to the winding machine, if the area where the composition of the strip is deformed is increased, the distortion is reduced as a whole, and the longitudinal direction is reduced. Although the change in cross-sectional dimension (shrinkage in width) due to the elongation of decreases, as shown in FIG. 10, the air-cooling area is set to the completion of the _Ar3 transformation despite the fact that the easily deformable area is large. As shown in the figure, the temperature range from the start of the _Ar3 transformation to the end of the transformation, that is, the air-cooling region is very narrow, and the effect is substantially small.

Therefore, a main object of the present invention is to control the cooling process from the exit side of the finishing mill to the winding machine without particularly requiring a significant equipment change, thereby effectively reducing necking. It is an object of the present invention to provide a method for preventing necking in hot rolling which can be prevented.

[0010]

According to a first aspect of the present invention which solves the above-mentioned problems, according to the present invention , the necking which occurs when a hot strip coming out of a final finishing mill is wound by a winder, In a method for preventing necking, which is controlled by controlling a cooling process by a cooling device disposed between a final finishing mill and a winder, a tension application method is used.
Cree in the case of a force 2kgf / mm ² was pulling 10 seconds
Cooling only by air cooling to the temperature range where the strain is less than 2%
Prevention of necking in hot rolling
Is the way.

According to a second aspect of the present invention, there is provided a final finishing mill.
When the hot strip that comes out is taken up by a winder
Necking that occurs, the final finish rolling mill and the winding machine
The cooling process is controlled by a cooling device
In necking prevention method for preventing the Rukoto, charcoal
Final finish for materials with elemental content C ≦ 0.01%
After rolling, the hot strip temperature should be at least 830 ° C
Heat characterized by cooling only by air cooling
This is a method for preventing necking in hot rolling.

According to a third aspect of the present invention, there is provided a final finishing mill.
When the hot strip that comes out is taken up by a winder
Necking that occurs, the final finish rolling mill and the winding machine
The cooling process is controlled by a cooling device
In necking prevention method for preventing the Rukoto, charcoal
Final finish for materials with elemental content C> 0.01%
After rolling, the hot strip temperature should be at least 800 ° C
Heat characterized by cooling only by air cooling
This is a method for preventing necking in hot rolling.

[0013]

According to the present invention, a tensile stress of 2 kgf / mm ² is 10
Creep strain is less than 2% when pulled for 2 seconds.
Cooling is performed only by air cooling to a certain temperature range . FIG. 4 shows, as an example of the material properties, the relationship between the creep strain and the material temperature when a specimen of [C] <0.01% is pulled at a tensile stress of 2 kgf / mm ² for 10 seconds. Between the temperatures C _{1 and} C ₂ at the start point and the end point of the _Ar3 transformation,
The creep strain is 4% or more, and it has been found that the deformation resistance is very low in this temperature range. Necking is caused by excessive tension between the finish rolling mill and the winder, and is caused by deformation of a portion having a relatively low deformation resistance in a section where tension is applied. By ensuring a large length of the region having a small deformation resistance, the width contraction can be dispersed, and the necking amount can be reduced. For example, the A _r3 transformation point but are up A _r3 transformation completion the cooling region in the case of the preceding example 4 described above, the necking particularly problematic ultra low carbon steel 880
At around ° C., the temperature range from the start to the end of the Ar ₃ transformation point is very narrow as shown in FIG. 10, and it is not possible to effectively prevent necking only by air cooling only in this temperature range.

Therefore, in the present invention, specifically, as shown in FIG. 4, the temperature range (T ₂ ) where the creep strain when the tensile stress is 2 kgf / mm ² for 10 seconds is less than 2% is less than 2%.
Cool down by air cooling to the following. The tensile stress is 2kgf / m
By the creep strain when pulled at m ² 10 seconds to perform air-cooled to the temperature range to be less than 2%, sufficient plastic deformation portion reduces the necking extent not interfering the (small portion of deformation resistance) length Can be assured.
Further, as another defining method, from the graph of the width necking amount-water cooling start temperature shown in FIG. 6, when a material having a carbon content C ≦ 0.01% is targeted for each material component, the actual operation is not considered. From the viewpoint, it is cooled by air cooling so that the width necking amount becomes about 3.5 mm or less, that is, after the final finish rolling, at least until the hot strip temperature becomes 830 ° C.

Further, when a material having a carbon content C> 0.01% is targeted, the width necking amount is set to about 2 mm or less from the viewpoint of practical operation, that is, at least after the final finish rolling, at least the hot necking is performed. Cool by air cooling until the strip temperature reaches 800 ° C.

In this way, by securing a long region having a large deformation resistance, necking can be uniformly dispersed in the region, and the amount of necking can be reduced.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments.
FIG. 1 is a schematic view of a cooling step portion from a final finish rolling mill to a winder in hot rolling. The hot strip 1 discharged from the final finishing mill 2 is cooled by the cooling device 4 and the cooling device 6 before being wound by the winder 9, cooled to a predetermined temperature, and then wound via the pinch roll 8. Machine 9
It is wound up. The cooling devices 4 and 6 are provided with a large number of injection nozzles (not shown), and each of the injection nozzles can be opened and closed by a control valve.

Further, thermometers 3, 5, and 7 are provided on the outlet side of the final finishing mill 2, the intermediate portion between the cooling devices 4 and 6, and the outlet side of the cooling device 6, respectively. The thermometer 3 is a thermometer for controlling the outlet temperature of the final rolling mill 2, and the thermometer 5 is a predetermined position (finish) for use as an index for determining a water cooling start position in the cooling devices 4 and 6. The thermometer is located at the position L from the rolling mill 2), and the thermometer 7 is a thermometer for managing the winding temperature in order to secure material characteristics.

The identification of the water cooling timing using the thermometer 5 as an index is as follows. First, what the temperature of the thermometer 5 reaches in accordance with the specified water cooling start temperature is determined by a heat transfer model calculation.
Next, the cooling device 4 is controlled using the observation value of the thermometer 5 as an index. The water cooling timing position may be identified by a heat transfer model calculation without providing the thermometer 5.

In general, the width of a hot strip mill is slightly widened so that the width required for a product (lower limit width) can be ensured even if width fluctuations such as necking occur. Will be aimed at. The value which determines the degree of the slightly widening is, for a material having a large necking amount such as [C] ≦ 0.01% content, the necking amount becomes the value. (The lower limit of the width) is rolled so as to be as wide as the necking amount so as not to fall below (the lower limit of the width).

The difference between the width actually manufactured and the lower limit width is a portion that is not used as a product and a portion that causes scrap loss. Therefore, in such a material, the loss of yield can be reduced by reducing the necking amount and setting the difference between the target width and the lower limit width to be small.

In the present invention, when the tensile stress is 2 kgf / mm ² ,
Creep strain of less than 2% when pulled for 10 seconds
Cool by air cooling to the temperature range where
When targeting materials with a content C ≤ 0.01%,
After the final rolling, at least the hot strip temperature is 83
It is cooled by air cooling until it reaches 0 ° C., and carbon content C> 0.
In the case of 01% material, after the final finish rolling,
At least until the hot strip temperature reaches 800 ° C
It is cooled by air cooling.

For example, as shown in FIG. 5, the control valve of the cooling device 4 is opened to actively perform water cooling to increase the amount of temperature drop to the thermometer 5 position.
When considering the cooling process of the former stage slow cooling in which the control valve of the cooling device is throttled and the temperature drop amount to the position of the thermometer 5 is small, the region where the steel material is relatively easily deformed (the temperature T
₁ through T ₂₎ difference occurs in the length of the steel passing through. That is, when the cooling process of the first stage strong cooling is adopted, the steel material length corresponding to this temperature region is shortened, and when the first stage slow cooling is employed, the steel material length corresponding to this temperature region is long. .
As a result, in the case of the former stage slow cooling in which the length corresponding to the region where the deformation resistance is reduced is long, the deformation occurs over a long distance, so the peak value of the local deformation is small, and the necking amount is small. Become smaller. Usually, the problem is a local peak value of the deformation, and if this peak value is small, it does not matter even if the range of the deformation is large. By adopting a mode of slow cooling in the first stage while securing a certain length, the peak value of deformation can be reduced and the necking amount can be reduced.

EXPERIMENTAL EXAMPLE 1 Using the cooling device of the hot strip 1 shown in FIG. 1, a cooling process of a steel sheet was performed on a material having a carbon content [C] ≦ 0.01% and a thickness of 3.6 mm. A test was performed on the difference in necking amount due to the difference in

The pattern A (the former stage strong cooling) having a large temperature drop to the position (L = 60 m) of the thermometer 5 and the thermometer 5
The pattern C having a small amount of temperature drop to the position (the former stage slow cooling) and the pattern B having an intermediate temperature drop between them
(Intermediate cooling) and four cases of Pattern D in which water cooling was started immediately after completion of the _Ar3 transformation were tested. Table 1 shows the temperature control patterns for the patterns A to D,
FIG. 2 shows a cooling process diagram of the patterns A to C among these.

[0026]

[Table 1] The test results are shown in the right column of Table 1. FIG. 3 shows the relationship between the width necking amount and the steel sheet temperature at the position of the thermometer 5, and FIG. 6 shows the relationship between the water cooling start temperature and the width necking amount.

As is clear from FIG. 3, the pattern A → B
→ It was found that the necking amount decreased in the order of C, that is, as the temperature at the start of water cooling decreased, and in the case of pattern C, the necking amount was reduced to about 比 べ compared with the case of pattern A. ing.

[Experimental Example 2] Next, a test was performed on a material having a carbon content [C] = 0.05% and a plate thickness of 2.0 mm for a difference in necking amount due to a difference in a cooling process of the steel plate. .

A pattern A 'having a large temperature drop to the thermometer 5 position, a pattern C' having a small temperature drop to the thermometer 5 position, and a pattern B 'having an intermediate temperature drop between them. Tests were performed on three cases. Table 2 shows detailed temperature control patterns for the patterns A ′ to C ′.

[0030]

[Table 2] In Table 2, although the overall necking amount is smaller than that of Experimental Example 1, the effect is not remarkable, but the necking amount decreases as the cooling pattern changes from A ′ to C ′, that is, as the air-cooled region becomes longer. It turns out that you are.

[0031]

As described in detail above, according to the present invention, necking can be achieved by controlling the cooling process from the exit side of the finishing mill to the winding machine without requiring a significant equipment change. Can be effectively prevented.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a cooling process from a finishing mill to a winding machine.

FIG. 2 is a cooling process diagram of patterns A to C in Experimental Example 1.

FIG. 3 is a test result diagram in Experimental Example 1.

FIG. 4 is a diagram showing the relationship between material temperature and creep strain.

FIG. 5 is an explanatory diagram of cooling control according to the present invention.

FIG. 6 is a relationship diagram between a water cooling start temperature and a width necking amount.

FIG. 7 is an explanatory diagram of an excessive tension generation mechanism.

FIG. 8 is an explanatory diagram of a necking occurrence mechanism.

FIG. 9 is a diagram showing a state where necking occurs.

FIG. 10 is a relationship diagram between a material temperature and a width reduction ratio.

[Explanation of symbols]

1: hot strip, 2: finishing mill, 3, 5, 7, ...
Thermometer, 4.6, cooling device, 9: winder

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) B21B 37/00, 45/00 B21C 47/00 C21D 9/52

Claims (3)

(57) [Claims] 1. The necking that occurs when a hot strip discharged from a final finishing mill is wound by a winder,
In the necking preventing method, in which the cooling process is controlled by a cooling device disposed between the final finishing mill and the winder, the necking is prevented by pulling at a tensile stress of 2 kgf / mm ² for 10 seconds.
Only air cooling to the temperature range where creep strain is less than 2%
Necking in hot rolling characterized by cooling
Prevention method. 2. Hot strip from the final finishing mill
Necking that occurs when the
A cooling device disposed between the final finishing mill and the winder;
To prevent the cooling process by controlling the cooling process.
In the anti-locking method, the final finishing was carried out for materials with carbon content C ≤ 0.01%.
After top rolling, at least hot strip temperature is 830 ° C
It is characterized by cooling only by air cooling until
A method for preventing necking in hot rolling. 3. A hot strip from a final finishing mill.
Necking that occurs when the
A cooling device disposed between the final finishing mill and the winder;
To prevent the cooling process by controlling the cooling process.
In the anti-locking method, the final finishing was carried out for materials with carbon content C> 0.01%.
After top rolling, at least hot strip temperature is 800 ° C
It is characterized by cooling only by air cooling until
A method for preventing necking in hot rolling.