JPH09194934A - Cooling method for steel plate for uniformizing quality - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cooling method for a steel plate, capable of reducing the occurrence of uneven cooling due to temp. deviation and inferior degree of flatness, uniformizing the quality in the steel plate, and securing superior degree of flatness, by straightening a hot rolled steel plate before cooling. SOLUTION: At the time of producing a steel plate by using hot rolling equipment having a cooling device and a straightening device on the rear side of a rolling mill, the quality is uniformized by straightening a hot rolled steel plate by means of the straightening device into flat state and then cooling it by means of the cooling device, in this cooling method for steel plate. Further, in this steel plate cooling method for uniformizing quality, hot rolling completion temp. is regulated o >=800 deg.C and cooling is started from a temp. not lower than the Ar3 transformation point after 1-3min from the completion of hot rolling and stopped at a temp. not higher than the bainitic transformation completion temp., and, after further cooling, the steel plate is straightened again by means of the straightening device into flat state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of steel sheet manufacturing, and more particularly to the field of steel sheet manufacturing technology in which a cooling step is performed after hot rolling.

[0002]

2. Description of the Related Art A method for cooling a hot rolled steel sheet using a hot rolling equipment in which a cooling device and a straightening device are arranged in this order on the rear surface of a rolling mill is a cooling method aimed at improving the mechanical properties of the steel sheet. Also, in Japanese Patent Publication No. 63-27409, etc., and for the purpose of improving the flatness of the steel sheet after cooling, the cooling zone is divided into three, and the cooling water distribution in the width direction of the steel sheet is changed in the intermediate cooling zone to obtain a steel sheet after cooling. Japanese Patent Publication No. 4-50368 discloses a cooling method for ensuring the flatness of the. Further, in order to secure the flatness after cooling, a method of hot-rolling and rapidly cooling a steel sheet, then hot-straightening, allowing to cool, and further cold-straightening is disclosed in JP-A-60-115306. ing. In order to improve the mechanical properties of the steel sheet, all of these cooling methods and straightening methods are once cooled in a cooling device and then straightened in a straightening device.

[0003]

In the prior art, since the shape of the steel sheet after hot rolling does not always have a good flatness, if it is cooled by the cooling device as it is, uneven cooling occurs in the flat portion and the non-flat portion. However, the material in the steel plate becomes uneven. In addition, if a temperature deviation due to uneven heating during billet heating, cooling during rolling, descaling during finish rolling, etc. remains in the steel sheet, uneven cooling may occur due to temperature deviation, which also causes material variations. . Furthermore, the steel plate after cooling is
It hardens due to temperature drop and transformation, and it becomes difficult to correct it and it is difficult to secure good flatness.

The present invention has been made to solve the above problems. By straightening a steel sheet after hot rolling before cooling, temperature unevenness and uneven cooling due to poor flatness are reduced, and the steel sheet is It is an object of the present invention to provide a method for cooling a steel sheet that makes the inner material uniform and ensures good flatness.

[0005]

Means for Solving the Problems The gist of the present invention is to manufacture a steel sheet by using a hot rolling equipment in which a cooling device and a straightening device are arranged in this order on the rear surface of a rolling mill. After flattening the steel sheet with a straightening device and then cooling it with a cooling device, the steel plate is cooled by a cooling device to make the material uniform. After 1 to 3 minutes from the completion of hot rolling, cooling is started from a temperature above the Ar ₃ transformation point, cooling is stopped at a temperature below the bainite transformation completion temperature, and after cooling, the steel sheet is straightened again with a straightening device. Is a method for cooling a steel sheet that makes the temperature uniform.

By straightening the steel sheet after hot rolling before cooling, uneven cooling due to poor flatness can be reduced, the material in the steel sheet can be made uniform, and straightening is performed at a high temperature before cooling. The flatness is better. In the present invention, since the steel sheet is further cooled and then straightened again, the flatness of the steel sheet is further improved. The steel sheet cooling method of the present invention can be applied to all steel sheets manufactured using a cooling device.

The hot rolling completion temperature of the steel sheet is
It is necessary to set the temperature to 800 ° C or higher in consideration of the temperature decrease during straightening. This is because the cooling start temperature is set to a temperature not lower than the Ar ₃ transformation point. That is, the amount of temperature drop from hot rolling completion Ar ₃ transformation point to the start cooling, for example, a temperature obtained by adding the amount of temperature drop shown FIGS. 2 and hot rolling completion temperature. Figure 2 shows the relationship between the plate thickness at 950 ° C and the temperature drop per 2 minutes and 30 seconds.

Further, the cooling start temperature depends on the target tensile strength and
It depends on the cooling conditions to get it. For that purpose, it is necessary to previously obtain the relationship between the tensile strength and the cooling start temperature as shown in FIG. 3 for each condition such as the component system, the plate thickness, and the cooling rate. Fig. 3 shows that steel pieces having the chemical composition shown in Table 1 are rolled to a plate thickness of 50 mm at 950 ° C, then air-cooled to the cooling start temperature, and 450 ° C from the cooling start temperature at a cooling rate of 10 ° C / s. It is a graph showing the relationship between the cooling start temperature and the tensile strength at the time of cooling to 0.

When the target tensile strength is determined, the cooling start temperature is determined from FIG. Therefore, the hot rolling completion temperature can be obtained by adding the cooling start temperature obtained from FIG. 3 to the cooling drop amount obtained from FIG. For example, when the plate thickness is 50 mm and the hot rolling completion temperature is 950 ° C. or higher, a tensile strength of 500 N / mm ² or higher can be secured. That is, from Fig. 2, the temperature drop of the plate thickness of 50 mm is 150 ° C, and from the hot rolling completion temperature of 950 ° C
The cooling start temperature is 800 ℃ minus 150 ℃. From Fig. 3, if the cooling start temperature is 800 ° C, the tensile strength is 500 N / mm ² or more. However, the Ar ₃ transformation point of the steel sheet having the chemical composition shown in Table 1 is 746 ° C, and the bainite transformation completion temperature is 550
° C. In the present invention, at the time of operation, the relationship between the cooling start temperature and the tensile strength in consideration of the amount of temperature drop from the completion of hot rolling to the start of cooling, the component system, the plate thickness, the cooling rate, etc. is obtained in advance. There is a need.

[0010]

[Table 1]

In the present invention, cooling is started 1 to 3 minutes after the completion of hot rolling. By delaying the time from the completion of hot rolling to the start of cooling, the temperature deviation of the steel sheet can be reduced by reheat, the temperature distribution of the steel sheet before cooling can be made as uniform as possible, uneven cooling can be eliminated, and a steel sheet of uniform material can be obtained. Obtainable. The reason why uneven cooling occurs due to temperature deviation before cooling is that, as shown in FIG. 4, the lower the temperature, the larger the heat transfer coefficient of the steel sheet and the greater the effect of heat removal. If there is a region, the lower the temperature is, the faster it is cooled, and uneven cooling occurs. Note that FIG. 4 is a diagram showing the relationship between the surface temperature of the steel sheet and the heat transfer coefficient.

Therefore, it is necessary to reduce the temperature deviation of the steel sheet by recuperating before cooling to obtain flatness after cooling and uniform material. Figure 5 shows rolling completion temperature 9
It is an example showing the relationship between the elapsed time from the completion of hot rolling of a steel plate finished at 98 ° C, a plate thickness of 50 mm, a plate width of 2650 mm, and a plate length of 9800 mm, and a temperature deviation due to recuperation of the steel plate. As shown in FIG. 5, the temperature deviation of the steel sheet due to the recuperation becomes smaller as the time becomes longer. If the time for recuperation is less than 1 minute, recuperation is insufficient, and the temperature deviation is still large, and if it exceeds 3 minutes, the temperature deviation inside the steel sheet becomes small, but the four peripheral portions are overcooled.
The temperature deviation between the inside and the four peripheral parts becomes large, and the uneven cooling becomes large. The temperature deviation of the steel sheet from the viewpoint of the uniformity of the material is preferably 30 ° C. or less, and therefore the time for recuperation is preferably 2 minutes or more and 3 minutes or less.

In order to obtain the strength of a steel sheet by transformation strengthening,
Cooling is started from a temperature above the Ar ₃ transformation point and stopped at a temperature below the temperature at which the bainite transformation is surely completed. As the Ar ₃ transformation point, the value obtained by the following formula can be used. Ar ₃ = 908-223.7C-30.5Si-34.4Mn + 438.5P-23.0Ni + 37.9V +
2 (100C + 6Ni-54)

Further, after cooling, the steel plate is straightened again by a straightening device, whereby a steel plate having excellent flatness can be obtained. Further, the cooled steel sheet and the steel sheet re-corrected after cooling do not impair the object of the present invention even if tempered as necessary.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an outline of the arrangement of hot rolling equipment. Reference numeral 1 denotes a finish rolling mill, which is arranged on the rear surface of the finish rolling mill 1 in the order of an online cooling device 2 and a straightening device 3 in the sheet passing direction. The rolled steel plate 4 is passed through the cooling device 2 without being cooled and is straightened by the straightening device 3
It is fed back to the front surface of the cooling device 2 and cooled by the cooling device 2.
After cooling, the path is further straightened by the straightening device 3.

In this embodiment, steel strips having the chemical composition shown in Table 1 are roughly rolled, and then finished by a finishing rolling mill 1 to have a plate thickness of 50 mm and a width of 50 mm.
A steel plate with a length of 2600 mm and a length of 10149 mm was finished. The hot rolling completion temperature is 1005 ° C in consideration of the temperature decrease due to conveyance and straightening. After that, the steel sheet was conveyed to the straightening device 3 and straightened at 963 ° C. As shown in FIG. 6, the deformation resistance when the steel sheet is straightened rises as the temperature decreases, so it is desirable to straighten the steel sheet before and after cooling at a temperature as high as possible.

After the straightening, the steel sheet was sent back to the front surface of the cooling device 2 and cooled by the cooling device 2. The time from the completion of finish rolling to the start of cooling is 2 minutes and 30 seconds, during which the temperature deviation in the steel sheet is sufficiently small. At this time, the cooling start temperature is 878 ° C and the cooling stop temperature is 440 ° C. After the cooling was stopped, the steel sheet was re-straightened by the straightening device 3. The flatness of the steel sheet after cooling and after re-straightening was good, and the wave height was
It was 5 mm or less.

After cooling and re-straightening, the steel sheet was tempered at 600 ° C. for 100 minutes, and then, for material inspection, test pieces were taken from the top portion, the center portion and the bottom portion of the steel sheet, and the tensile strength and 0 The Charpy impact absorbed energy at ℃ was measured. FIG. 7 shows the result. As is clear from FIG.
From the top portion to the bottom portion, no significant change in the material is observed, which indicates that the present invention is a method for cooling a steel sheet that makes the material uniform. Table 2 shows the manufacturing conditions and test results in the examples.

[0019]

[Table 2]

[0020]

As is apparent from the above description,
According to the present invention, after the hot rolling is completed, it is corrected before the start of cooling, and the flatness defect and the temperature deviation are reduced, so that the material in the steel sheet is uniform, and a steel sheet having good flatness is obtained. Obtainable.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of an arrangement of hot rolling equipment.

FIG. 2 is a diagram showing a relationship between a plate thickness at 950 ° C. and a temperature drop amount per 2 minutes and 30 seconds.

[Fig. 3] A steel sheet having the chemical composition shown in Table 1 is rolled at 950 ° C, and then air-cooled to a cooling start temperature to obtain 10 ° C / s.
FIG. 4 is a diagram showing the relationship between the cooling start temperature and the tensile strength when cooled to 450 ° C. at the cooling rate of.

FIG. 4 is a diagram showing a relationship between a surface temperature of a steel sheet and a heat transfer coefficient.

FIG. 5 is a diagram showing a relationship between an elapsed time from the completion of hot rolling and a temperature deviation due to reheat of a steel sheet.

FIG. 6 is a diagram showing a relationship between a straightening temperature and a deformation resistance.

FIG. 7 is a diagram showing the tensile strength from the top part to the bottom part of the steel sheet of the example and the distribution of Charpy impact absorbed energy at 0 ° C.

[Explanation of symbols]

1 ... Finishing rolling mill, 2 ... Cooling device, 3 ... Straightening device, 4 ...
steel sheet.

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[Procedure amendment]

[Submission date] February 16, 1996

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 5

[Correction method] Change

[Correction contents]

[Figure 5]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiaki Suga, 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kamido Steel Works, Ltd. Kakogawa Works (72) Inventor Mitsuaki Shibata, Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kamido Co., Ltd. Steel Works Kakogawa Steel Works (72) Inventor Kengo Abe 1 Kanazawa-machi, Kakogawa City, Hyogo Prefecture Kado Steel Works Kakogawa Steel Works

Claims (4)

[Claims] 1. When manufacturing a steel sheet using a hot rolling equipment in which a cooling device and a straightening device are arranged in this order on the rear surface of a rolling mill, the steel plate after hot rolling is flattened by the straightening device and then cooled. A method for cooling a steel sheet, wherein the material is uniformly cooled. 2. The method for cooling a steel sheet according to claim 1, wherein the cooled steel sheet is straightened again by a straightening device. 3. The material according to claim 1 or 2, wherein the steel sheet that has been hot-rolled at a temperature of 800 ° C. or higher has a cooling start temperature of Ar ₃ transformation point or higher and a cooling stop temperature of bainite transformation completion temperature or lower. Method for cooling steel plate to make uniform. 4. The method for cooling a steel sheet according to claim 1, 2 or 3, wherein cooling is started 1 to 3 minutes after the completion of hot rolling.