JPH0890047A - Production of controlled cooling steel plate - Google Patents

Abstract

PURPOSE: To eliminate cooling irregularity at peripheries in a controlled cooling steel plate. CONSTITUTION: When a controlled cooling steel plate is produced by the hot rolling line which has a high pressure water descaling device between heating furnace and finish rolling mill, an edge rolling mill between heating furnace and roughing mill and a controlled cooling device backward finish rolling mill, the slab extracted from heating furnace is, before start of rolling, subjected to one or more times descaling at least with use of a high pressure water descaling device by high pressure water of >=3.0MPa collision pressure. Further, in rough rolling process after edge rolling, descaling of one or more times at least is executed with high pressure water of >=3.0MPa collision pressure, or in finish rolling process, descaling of one or more times at least including descaling, before finish rolling start, is executed with high pressure water of >=3.0MPa collision pressure and then hot rolling is executed, after completing hot rolling, controlled cooling is executed with use of the controlled cooling device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooling process for producing a controlled cooling steel sheet, in which a temperature deviation (cooling unevenness) on the surface of the steel sheet when the controlled cooling is stopped is generated particularly at the leading end or the widthwise end of the steel sheet. The present invention relates to a method for manufacturing a controlled cooling steel plate that prevents unevenness.

[0002]

2. Description of the Related Art In the process of manufacturing a controlled cooling steel sheet, unevenness of cooling may occur on the surface of the steel sheet due to temperature deviation when cooling is stopped in the online controlled cooling subsequent to hot rolling. It is reported in Material and Process Vol.4 (1991-1563) that one of the causes of this uneven cooling is due to non-uniform scale thickness.

Conventionally, as means for solving problems caused by scales such as prevention of scale defects, for example, the steel plate temperature at the time of descaling is limited, and the pump discharge pressure for descaling is increased to improve the peelability of the scale. The method of doing is proposed.

[0004]

However, even with these methods, the current situation is that the primary scale layer formed at the interface of the base metal cannot be completely removed even by these methods. Even if this scale layer can be removed by descaling during hot rolling, secondary scale is generated in the subsequent rolling process. These scales generated during heating and rolling cause non-uniformity of the scale thickness of the steel sheet surface.Therefore, the temperature deviation of the steel sheet surface after the controlled cooling stop becomes large, and so-called cooling unevenness occurs. .

When such cooling unevenness occurs at the front and rear ends of the steel sheet in the longitudinal direction and the end in the width direction, this portion becomes unacceptable for strength, and the cut-off amount of the cooling unevenness-caused portion due to this increases and the yield decreases. Or, a problem such as a defective shape occurs.

The present invention has been made in order to solve the above-mentioned problems, and in a hot rolling process, a high-pressure water descaling device is used to make the scale thickness of the steel sheet surface uniform, thereby providing a controlled cooling steel sheet. It is an object of the present invention to provide a method for producing a controlled cooling steel sheet for preventing the occurrence of cooling unevenness, and particularly to provide a method for producing a controlled cooling steel sheet for preventing the occurrence of cooling unevenness at the four peripheral portions of the steel sheet.

[0007]

The gist of the present invention is to have a high-pressure water descaling device between the heating furnace and the rough rolling mill, and between the rough rolling mill and the finishing rolling mill, and further between the heating furnace and the rough rolling mill. In producing a controlled cooling steel plate in a hot rolling line having a controlled cooling device at the rear of the finishing rolling mill, a steel strip extracted from a heating furnace is used with the high-pressure water descaling device before starting rolling. Descaling is performed at least once with high-pressure water with a collision pressure of 3.0 MPa or more, and the collision pressure is 3.0 MPa in the rough rolling process after edging rolling.
At least one descaling with the above high pressure water or at least one descaling including the descaling before the start of finishing rolling with the high pressure water with a collision pressure of 3.0 MPa or more in the finishing rolling process. Go hot rolling
It is a method for producing a controlled cooling steel sheet, in which controlled cooling is performed using the controlled cooling device after hot rolling is completed.

[0008]

The present invention will be described in detail below. FIG. 2 shows an example of the cooling unevenness generated on the four peripheral portions of the steel sheet. Figure 2
50mm to 300mm from the longitudinal edge of the steel plate as shown in
The temperature after cooling stop is high in the area of mm and the area of about 20 mm to 100 mm from the edge of the steel plate width. The temperature at this time is
The center of the steel plate surface is 440 to 460 ℃, while the four edges are 450 to 460 ℃
At 520 ℃, the circumference is 10 to 80 ℃ higher than the center. When the temperature of the four peripheral portions after cooling is increased in this way, this portion becomes insufficient in strength and is rejected. As a result of various studies on this phenomenon, it was found that the cooling unevenness occurring in the four peripheral portions of the steel sheet was caused by the edging rolling in the hot rolling process.

FIG. 3 shows an example of a conventional hot rolling process.
The heated steel slab is extracted from the heating furnace, the primary scale is removed by a descaling device (Hydraulic Scale Breaker), and the steel sheet is sequentially rolled by a rough rolling mill and a finish rolling mill to finish a steel sheet. During this period, in the rough rolling mill and the finishing rolling mill, the secondary scales are removed by the descalers on the front and rear surfaces of the rolling mill. Further, in order to adjust the size and shape of the steel sheet, an edging rolling mill (vertical rolling mill) is used at least once in hot rolling. The rolled steel sheet is controlled and cooled by a controlled cooling device to improve its mechanical properties.

FIG. 4 is an explanatory view when an edging rolling mill is used in the hot rolling process. In FIG. 4, the steel slab is rotated 90 ° and the edging rolling mill is used in the edging process to pass the top (T) and bottom () of the steel sheet. Align the shape of B), and then tenter rolling with a rough rolling mill to secure the width dimension of the final steel plate. After that, the steel sheet is rotated 90 °, passed through an edging rolling machine again to adjust the shape of the side of the steel sheet, and then rolled in the order of rough rolling and finish rolling. During this period, the scales on the top and bottom of the steel sheet are peeled off when the steel sheet is first passed through the edging rolling machine, and the scales on the steel sheet side are peeled off when the steel sheet is passed through the edging rolling machine. As described above, when the steel sheet is passed through the edging rolling mill, a portion where the scale layer is thinner than other portions of the steel sheet is formed on the four peripheral portions of the steel sheet.

During edging rolling, 20 to 20
The scale peels off from the area of 30 mm to several tens of mm to several hundreds of tens of mm. This is due to the fact that the scale produced during the heating of the billet remains without being completely removed by descaling after extraction, or even if the scale is completely removed, the subsequent hot work The secondary scale generated in the rolling process is locally peeled during the edging rolling. Therefore, if rolling is performed up to finish rolling in this state, the scale thickness on the surface of the steel sheet becomes non-uniform, and uneven cooling occurs in the subsequent controlled cooling.

[0012] Scale thickness was investigated for a steel sheet rolled from a billet heated to 1150 ° C and having uneven cooling. As a result, at the high temperature end after cooling stop, the scale thickness at the 40 mm position from the steel plate width end is 8 to 12 μm, while that at the cold steel plate width center after cooling stop is
It was 20 to 35 μm. The temperature deviation at this time was about 60 ° C.

As reported in Materials and Processes Vol. 4 (1991-1563), if the scale thickness of the steel sheet surface is non-uniform, the cooling rate will be different due to the cooling acceleration effect with the increase of the scale thickness, resulting in uneven cooling. Is happening. Therefore, the present invention makes the scale thickness of the steel plate surface uniform and prevents the occurrence of uneven cooling of the controlled cooling steel plate.

In the present invention, in order to remove the primary scale on the surface of the billet extracted from the heating furnace, a high-pressure water descaling device is used before the start of rolling, and at least once with high-pressure water having a collision pressure of 3.0 MPa or more. Descaling of. As shown in Fig. 1, the reason why the collision pressure of the high-pressure water is set to 3.0 MPa or more is that if the collision pressure of the high-pressure water is 3.0 MPa or more, the heating temperature is
This is because most of the primary scale on the surface of the billet from 1000 ℃ to 1230 ℃ can be removed.

Further, in order to eliminate the unevenness of the scale thickness in the part where the scale is separated and the part where the scale is not present in the four circumferences in the edging rolling, in the rough rolling step after the edging rolling, the impact pressure is at least 1 with high pressure water of 3.0 MPa or more. Descaling more than one time, or collision pressure during finish rolling process
Descaling is performed at least once with high-pressure water of 3.0 MPa or more, including descaling before the start of finish rolling.
In this way, after the edging rolling, the collision pressure is descaled with high-pressure water of 3.0 MPa or more, the residual primary scale and the secondary scale produced during hot rolling are removed, and the steel sheet produced during the subsequent rolling. The secondary scale thickness on the surface becomes uniform, and uneven cooling does not occur in the subsequent controlled cooling.

As described above, by descaling and hot rolling, the scale thickness on the surface of the steel sheet becomes uniform, and uneven cooling does not occur in the subsequent controlled cooling using the controlled cooling device.

[0017]

EXAMPLES Examples of the present invention will be described below.
A steel slab heated at 1150 ° C. in an air atmosphere was extracted from the heating furnace, descaled under the conditions shown in Table 1, rough-rolled for one pass, and then edging-rolled with an edging amount of 20 mm. After the edging rolling, descaling was performed under the conditions shown in Table 1 and hot rolling was performed in the order of rough rolling and finish rolling to finish a steel sheet having a thickness of 22 mm. After completion of hot rolling, controlled cooling was performed from 870 ° C to 450 ° C with a controlled cooling device. After the cooling was stopped, the temperatures of the central portion of the steel plate and the four peripheral portions were measured with an infrared radiation thermometer. Table 1 shows the temperature difference between the central part and the four peripheral parts.
Shown in

[0018]

[Table 1]

As shown in Table 1, in No. 4 of the present invention, after extracting the billet, high pressure water descaling with a collision pressure of 3.0 MPa was performed once to remove the primary scale, and then the same descaling was performed after edging rolling. Scaling is done once. No.5 and 6 are
After descaling once after extracting the billet, descaling was performed once before the start of finish rolling, and in No. 6, descaling was performed twice during finish rolling. Thus, in the hot rolling process after edging rolling, the collision pressure is 3.0M.
By performing high-pressure water descaling of Pa or more, non-uniformity of the scale thickness on the steel sheet surface due to scale peeling during edging rolling is eliminated. Therefore, uneven cooling due to controlled cooling does not occur, and the temperature difference between the central portion of the steel sheet and the four peripheral portions is extremely small, 0 ° C to 10 ° C.

On the other hand, in Comparative Examples Nos. 1 and 2, descaling was performed once after the billet extraction and after the edging rolling or before the start of the finish rolling as in the case of the present invention, but the collision pressure was 0. .
Since it is as small as 8 MPa, the scale thickness on the surface of the steel sheet is not uniform and uneven cooling occurs, and the temperature difference between the central portion of the steel sheet and the four peripheral portions is 43
Great at ℃ and 62 ℃. In No. 3, the high-pressure water descaling with a collision pressure of 3.0 MPa was performed once after the steel billet was extracted to remove the primary scale, but the descaling collision pressure after edging rolling was as low as 0.8 MPa. The secondary scale was not removed, the scale thickness of the steel sheet surface was not uniform, and uneven cooling occurred, and the temperature difference between the central portion of the steel sheet and the four peripheral portions was as large as 38 ° C.

[0021]

As is apparent from the above description,
According to the present invention, it is possible to prevent uneven cooling from occurring in the four peripheral portions of the controlled cooling steel sheet, so that insufficient strength due to uneven cooling is eliminated, yield is improved, and defective shapes are eliminated.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of high-pressure water collision pressure on the removal of primary scale.

FIG. 2 is a diagram showing an example of cooling unevenness generated on the four peripheral portions of a steel plate.

FIG. 3 is a diagram showing an example of a conventional hot rolling process.

FIG. 4 is an explanatory diagram when an edger is used in a hot rolling process.

Claims (1)

[Claims] 1. A high-pressure water descaling device is provided between the heating furnace and the rough rolling mill, and between the rough rolling mill and the finishing rolling mill, and an edging rolling mill is further provided between the heating furnace and the rough rolling mill, and the rear of the finishing rolling mill is provided. In producing a controlled cooling steel plate in a hot rolling line having a controlled cooling device, the steel billet extracted from the heating furnace is
Before rolling, the high-pressure water descaling device is used to perform descaling with high-pressure water having a collision pressure of 3.0 MPa or more at least once, and in the rough rolling process after edging rolling, the collision pressure is 3.0 MPa or more. Descaling with water at least once or using high-pressure water with a collision pressure of 3.0 MPa or more in the finishing rolling process to perform descaling at least once including the descaling before the start of finishing rolling. A method for producing a controlled cooling steel sheet, which comprises performing hot rolling and performing controlled cooling using the controlled cooling device after completion of hot rolling.