JPH0751732A - Hot rolling method of steel sheet with superior surface property - Google Patents

Abstract

PURPOSE:To perform a hot rolling of a steel sheet having a superior surface property without the generation of scale defects on the surface, red scale and cooling irregularity in a controlled cooling. CONSTITUTION:At least one descaling is performed with a high pressure water having an impact pressure of 38kgf/cm<2> before the start of a roughing and also before the start of a finish rolling. In addition, in the finish rolling process 7, a hot rolling is performed with a pass schedule for a larger draft to give the limit value HS of the scale layer thickness such that the destruction of the scale layer is not caused in a roll bite at the time of rolling reduction. HS=a.gamma<2>+b.gamma+c, where HS: limit value of scale layer thickness; gamma: rolling reduction; a, b, c: coefficient to be determined by kinds of steel and rolling temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hot rolling of steel sheets, and more particularly to a hot rolling method for steel sheets having excellent surface properties.

[0002]

2. Description of the Related Art In a steel sheet produced by hot rolling, a part of the scale produced during heating and hot rolling remains on the surface of the steel sheet without being removed by descaling during hot rolling, resulting in scale defects. May become. In addition, a so-called red scale in which a part or the whole surface of the scale layer is finely powdered may occur.

Further, when controlled cooling is performed subsequent to hot rolling, uneven cooling is caused by uneven thickness of the scale layer in the plate surface of the steel plate (variation of temperature in the plate surface when control cooling is stopped).
May occur.

In order to solve the problems caused by the scale on the surface of the steel sheet, it has been customary to add about 100-
A descaling device using high-pressure water of 150 kgf / cm ² is installed, and rolling is performed while peeling and removing scale on the surface of the steel sheet.

[0005]

However, according to the conventional descaling method, even if the thick scale layer on the surface of the steel plate that causes scale flaws can be removed, the thin scale layer generated at the base steel interface can be completely removed. The current situation is that it has not been removed. Even if the thin scale layer at the base steel interface could be removed by descaling during hot rolling, a new scale layer is formed on the steel sheet surface in the subsequent hot rolling step.

The uneven thickness of the scale layer on the surface of the steel sheet that causes the generation of the red scale and the uneven cooling is due to the thin scale layer remaining at the interface of the base steel and the scale newly generated in the hot rolling process. The layers are of great influence and it is highly desirable to be able to control the production of these scale layers.

The present invention has been made to solve the above-mentioned problems, and it goes without saying that the generation of a scale layer in the surface of a steel sheet is controlled and rolling is performed to prevent the occurrence of scale defects on the surface of the steel sheet. In addition, it is an object of the present invention to provide a hot rolling method for a steel sheet having excellent surface properties that can prevent the occurrence of red scale and uneven cooling during controlled cooling.

[0008]

[Means for Solving the Problems] When hot rolling a steel sheet between a heating furnace and a rough rolling mill, and in a hot rolling line having high-pressure water descaling devices on the front and rear surfaces of the finish rolling mill, before the rough rolling is started, Before starting the finish rolling, the high pressure water descaling device was used to perform descaling at least once with high pressure water having a collision pressure of 38 kgf / cm ² or more. Further, in the finish rolling process, the reduction rate of each pass was Is a method for hot rolling a steel sheet having excellent surface properties, which is hot-rolled with a pass schedule that is larger than the rolling reduction that gives the limit value of the scale layer thickness at which the scale layer does not break in the roll bite.

Further, when the rolling reduction of each pass is restricted from the viewpoint of the rolling functional force and securing of the shape of the steel sheet, the thickness of the scale layer is reduced with respect to the rolling reduction of at least the final three passes in the finish rolling process. Hot rolling by controlling the thickness of the scale layer with high-pressure water having a collision pressure of 38 kgf / cm ² or more using the high-pressure water descaling device so that it will be below the limit value that does not cause fracture in the roll bite. The method for hot rolling a steel sheet having excellent surface properties according to claim 1.

Further, in the finish rolling process, the limit value HS of the scale layer thickness at which the fracture does not occur in the roll bite at the time of reduction with respect to the reduction rate of each pass is HS = a · γ ² + b · γ + c : The limit value of scale layer thickness γ: Reduction ratio a, b, c: The hot rolling method for a steel sheet having excellent surface properties according to claim 1 or claim 2, which is obtained from a coefficient determined by steel type and rolling temperature.

[0011]

The structure of the hot rolling line according to the present invention is shown in FIG. The high-pressure water descaling device 1 is for descaling the steel plate 3 extracted from the heating furnace 5 immediately before the start of rough rolling to remove the primary scale of the steel plate surface generated in the heating step. The steel plate 3 from which the primary scale has been removed by the high-pressure water descaling device 1 is roughly rolled by the rough rolling mill 6 and then moved to the finish rolling step. Roughly rolled steel plate 4
Is descaled by the high-pressure water descaling device 2 and finish-rolled by the finish rolling mill 7. The high-pressure water descaling device 2 is for controlling the secondary scale layer formed on the surface of the steel plate 4 in the finish rolling process.

In the high-pressure water descaling apparatus 1, the present inventors have found that high-pressure water 8 from which primary scale is completely removed.
Experiments were conducted on the collision pressure of. As a result, as shown in FIG. 1, by performing descaling with an impact pressure of 38 kgf / cm ² or more, the primary scale generated on the surface of the steel sheet 3 when heated to the range of 1000 to 1200 ° C. was completely eliminated. Can be removed.

Similarly, the high pressure water descaling device 2 is
Descaling is performed on the steel plate 4 immediately before the start of finish rolling to completely remove the secondary scale generated on the surface of the steel plate in the rough rolling process, and the necessary collision pressure may be 38 kgf / cm ² or more. It has been confirmed by experiments. Therefore, the collision pressure of the high-pressure water descaling devices 1 and 2 is 38 kg.
It was set to f / cm ² or more.

Even if the scale on the surface of the steel sheet just before the start of finish rolling is completely removed by using the high-pressure water descaling devices 1 and 2, the secondary scale is generated again during the finish rolling. When the degree of generation of this secondary scale is different and the scale layer thickness in the steel sheet plate surface after finishing rolling is different, uneven cooling during controlled cooling is caused.
Further, when the thickness of the scale layer is increased during the finish rolling, the scale layer is broken by the rolling and fine powder scale, that is, red scale is generated.

In order to solve these problems, the thickness of the secondary scale layer produced in the finish rolling step should be small.
In addition, it is necessary to control so as to be uniform, and as a result of intensive studies by the present inventors, as a result of ensuring the uniformity of the scale layer thickness, descaling during the finishing rolling and descaling water are performed. The number of times of descaling should be reduced so as to eliminate the temperature unevenness of the steel plate surface due to the co-flow.
Further, the rolling behavior and the thickness of the scale layer during rolling greatly contribute to the fracture behavior of the scale layer due to rolling, which causes red scale, and as shown in FIG. It was found that the larger the thickness, the more easily the scale layer is broken by rolling.

FIG. 2 is an example of the results of experiments conducted by the present inventors. The slab of steel grade JIS SM490 is heated and rolled down at 900 ° C., and the fracture behavior of the primary scale layer formed on the slab surface is illustrated. It is clear, however, that if the scale layer thickness is increased, the reduction rate at the time of rolling reduction at which the scale layer breaks also increases. The circles in the figure indicate the values calculated by the slab method.

Based on this knowledge, the change in the scale layer thickness on the surface of the steel sheet in each pass of the finish rolling process is predicted, and the reduction ratio with respect to the thickness of the scale layer is the scale layer in the roll bite at the time of reduction. By setting the pass schedule so as to be larger than the rolling reduction that gives the limit value of the scale layer thickness at which the fracture does not occur, the scale layer is not destroyed and the red scale does not occur. In this case, descaling is unnecessary.

Further, when the rolling reduction of each pass is restricted from the viewpoint of the rolling functional force and securing of the shape of the steel sheet, the rolling reduction of the scale layer is performed with respect to the rolling reduction of each pass, particularly the rolling reduction of the final three passes. When the thickness is less than the limit value that does not cause damage in the roll bite, roll without descaling,
When the thickness exceeds the limit (thickness at which the scale layer breaks), the above-mentioned collision pressure is descaled to 38 kgf / cm ² or more to reduce the thickness of the scale layer, or the scale layer is completely removed and rolled. It is clear that doing so is effective in solving problems related to scale.

Here, the change of the secondary scale layer thickness in the finish rolling step has already been reported by the present inventors to the application number 1992.
The prediction model formula of the scale layer thickness proposed in Japanese Patent Application No. 255857 can be used for highly accurate prediction.
The prediction model formula of the scale layer thickness is as follows. t = [a · τ · exp [b / (T + 273)]] ^C where t: thickness of scale layer τ: air cooling time T: steel plate surface temperature a, b, c: constant

Further, the limit value of the scale layer thickness to the rolling reduction of each pass of each steel type (the limit value of the scale layer thickness for preventing the scale layer from breaking in the roll bite) is shown in FIG. The general formula that can be obtained in the same manner as the result of the deformation behavior analysis of the scale layer in the roll bite shown in FIG. 2 and gives the limit scale layer thickness is represented by the following formula with the rolling reduction as a parameter. HS = a · γ ² + b · γ + c ……………………………… (1) However, HS: Limit value of scale layer thickness γ: Reduction ratio a, b, c: Coefficient determined by steel type and rolling temperature

As described above, descaling is performed at least once with high-pressure water having a collision pressure of 38 kgf / cm ² or more immediately before the start of rough rolling and immediately before the start of finish rolling. Obtain the secondary scale layer thickness in the finish rolling process from the prediction model formula, and give this scale layer thickness a limit value that does not cause the scale layer to fracture in the roll bite during the reduction obtained from formula (1) above. Obtain the rolling reduction, and perform rolling at a rolling reduction larger than this rolling reduction,
Alternatively, when the rolling reduction of each pass is restricted from the viewpoint of the rolling functional force and securing of the shape of the steel sheet, in the finishing rolling process, at least the rolling reduction of the final three passes is calculated by the formula (1) above. The roll bite does not cause the scale layer to be destroyed.Descaling is performed to the thickness of the scale layer to control the thickness of the scale layer and perform rolling to prevent the occurrence of scale flaws on the surface of the steel sheet. It is possible to obtain a steel sheet having excellent surface properties that prevents the occurrence of uneven cooling during scale or controlled cooling.

[0022]

EXAMPLES Examples of the method of the present invention will be described below. Thickness 27
Collision pressure of descaling water immediately before the start of rough rolling and immediately before the start of finish rolling shown in Table 1 after heating a steel plate (steel plate) with a width of 0 mm, a width of 1500 mm and a length of 2500 mm to 1150 ° C, and a reduction ratio in the pass schedule of finish rolling. The sheet thickness of 25
mm, a plate width of 2000 mm, and a plate length of 20000 mm were hot-rolled. Regarding the steel sheet after the hot rolling was completed, a part of the surface state of the steel sheet after air cooling was observed, and a part of the steel sheet was forcibly cooled and uneven cooling after the forced cooling was observed. The results are also shown in Table 1. Incidentally, in the method of the present invention, by bringing the nozzle header of the descaling device using high-pressure water installed between the heating furnace and the rough rolling mill and over the entire surface of the finishing mill close to each other, it was conventionally 10 to 30 kgf / cm ² . The collision pressure was set to 38 kgf / cm ² or more.

No. 2 of the method of the present invention shown in Table 1. 1 and 3
The range of the reduction ratio in the pass schedule of finish rolling is the scale layer in the roll bite at the time of reduction determined from the above equation (1) based on the scale layer thickness obtained from the scale layer thickness prediction model equation. The rolling reduction is larger than the rolling reduction that gives the limit value of the scale layer thickness at which fracture does not occur (in this example,
20% or more), the scale layer does not break, and strong descaling with a collision pressure of 38 kgf / cm ² is not necessary.
Therefore, No. The steel plate surface of No. 1 was a tight black scale. In addition, No. In No. 3, controlled cooling was performed after the rolling was completed, but the uneven cooling was small because the scale layer was uniform.

[0024]

[Table 1]

No. 2 of the method of the present invention. 2 shows that the range of the reduction ratio in the pass schedule of finish rolling is based on the scale layer thickness obtained from the prediction model formula of the scale layer thickness (1) above.
Was calculated from the equation, smaller than a reduction ratio which gives the limit values of the scale layer thickness destruction of the scale layer does not occur in the roll bite at the time of reduction, impact pressure to start rolling immediately before the final second pass 38kgf / cm ² A strong descaling was performed to completely remove the scale layer. Therefore, red scale did not occur, and the steel plate surface was a tight black scale.

On the other hand, in Comparative Example No. In Nos. 4 and 5, since the collision pressure of the descaling water immediately before the start of rough rolling and immediately before the start of finish rolling was low, the primary and secondary scales were not completely removed, and the range of the reduction ratio in the pass schedule of finish rolling was Based on the scale layer thickness obtained from the prediction model equation of the scale layer thickness, obtained from the above equation (1),
The scale layer is not destroyed in the roll bite during rolling.Although it is smaller than the rolling reduction, which gives the limit value of the scale layer thickness, strong descaling is performed during the finish rolling to completely remove the scale layer. Not not. Therefore,
No. In No. 4, the scale layer was destroyed, and red scale was generated on the entire surface of the steel sheet. In addition, No. No. 5 was subjected to controlled cooling after completion of rolling, but the scale layer was not uniform, so that the cooling unevenness was large.

[0027]

As is clear from the above description, according to the present invention, it is possible to obtain a steel sheet which prevents generation of scale flaws on the surface of the steel sheet, as well as generation of red scale and uneven cooling in controlled cooling. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a collision pressure of high-pressure water of a high-pressure water descaling device and a state of a primary scale of a steel plate (steel piece) surface after descaling.

FIG. 2 is a diagram showing a relationship between a limit value of a scale layer thickness at which a scale layer is not broken in a roll bite during rolling and a rolling reduction.

FIG. 3 is a diagram showing a configuration of a hot rolling line according to the present invention.

[Explanation of symbols]

1 ... High-pressure water descaling device, 2 ... High-pressure water descaling device, 3 ... steel plate, 4 ... steel plate, 5 ... heating furnace, 6 ... rough rolling machine, 7 ... finishing rolling machine, 8 ... high pressure water.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Miyawaki 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kado Steel Works Kakogawa Works (72) Inventor Kaichi Obanya 1-kanazawa-machi, Kakogawa-shi, Hyogo Co., Ltd. Kadogawa Works, Kakogawa Works (72) Inventor Takaya Fujino 1 Kanazawa-machi, Kakogawa City, Hyogo Prefecture Kadogawa Works, Kakogawa Works

Claims (3)

[Claims] 1. When hot rolling a steel sheet between a heating furnace and a rough rolling mill, and in a hot rolling line having high pressure water descaling devices on the front and rear surfaces of the finish rolling mill, before starting rough rolling and before starting finish rolling. Descaling is performed at least once with high-pressure water having a collision pressure of 38 kgf / cm ² or more using the high-pressure water descaling device, and in the finishing rolling process, the reduction rate of each pass is within the roll bite at the time of reduction. A method for hot rolling a steel sheet having excellent surface properties, characterized in that hot rolling is performed with a pass schedule that is larger than a rolling reduction that gives a limit value of scale layer thickness at which scale layer failure does not occur. 2. The high-pressure water descaling device so that, in the finish rolling step, the thickness of the scale layer is not more than a limit value at which breakage does not occur in the roll bite at the time of rolling, with respect to the rolling reduction of at least the final 3 passes. Collision pressure is 38
The hot rolling method for a steel sheet having excellent surface properties according to claim 1, wherein the hot rolling is performed by controlling the thickness of the scale layer with high-pressure water of kgf / cm ² or more. 3. In the finish rolling step, the limit value HS of the scale layer thickness that does not cause damage in the roll bite during rolling is HS = a · γ ² + b · γ + c with respect to the rolling reduction of each pass. : The limit value of the scale layer thickness γ: Reduction ratio a, b, c: The hot rolling method for a steel sheet having excellent surface properties according to claim 1 or claim 2, which is obtained from a coefficient determined by steel type and rolling temperature.