JPH07252538A - Method for cooling thin steel sheet - Google Patents

Abstract

PURPOSE:To prevent the insufficient flatness accompanied with cooling of a thin steel sheet by regulating spray water spouting angle and cooling water flow rate of a nozzle, at the time of using the flat spray nozzle as a forcedly cooling means for the thin steel sheet in a continuous line. CONSTITUTION:The baking finish thin steel sheet 1 is forcedly cooled with the spray cooling device after slowly cooling with a gas jet cooling device in the continuous line. The spray cooling device is constituted with the flat spray nozzles 4b fitted at plural positions in the longitudinal direction of a cooling water supplying header 4a extended in the width direction of the thin steel sheet 1. The spray water spouting angle theta of this nozzle 4b is made to be 30-60 deg. against the thin steel sheet 1 and the cooling water flow rate is set in a range so that the temp. gradient variable rate in the line direction satisfies the inequality. In the inequality, DELTAdT/dX is the temp. gradient variable rate in the line direction ( deg.C/m); A, constant; W, width of the steel sheet (m); alpha; coefficient of thermal linear expansion (1/ deg.C); E, modulus of longitudinal elasticity (kgf/mm<2>); sigmay; yield stress of the steel material at the temp. gradient variation point (kgf/mm<2>); sigmau; tension of the line (kgf/mm<2>).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cooling a thin steel plate suitable for cooling after baking of a thin baked coated steel plate such as a pre-coated steel plate for household appliances, and more particularly, to a flat plate accompanying cooling of the thin steel plate. The present invention relates to a cooling method for preventing inferiority.

[0002]

2. Description of the Related Art Conventionally, the main manufacturing process of baking coated steel sheet is painting-baking-cooling. Of these steps, a combination of gas jet cooling and water cooling is usually used for cooling after baking (see JP-A-62-93317, etc.). The advantage of gas jet cooling is that it can be carried out in the immediate vicinity of the baking furnace, and since it is slow cooling, the temperature change is gentle and flat defects are less likely to occur. However, since it takes time to cool to a predetermined temperature only with gas jet cooling, the line length is shortened in combination with water cooling.

FIG. 5 is a schematic view showing an example of a continuous line in which gas jet cooling and water cooling are combined, and FIG. 6 is a perspective view showing a spray cooling device in the continuous line.
The steel plate 1 exiting the baking furnace 2 is gently cooled by the gas jet cooling device 3 and then strongly cooled by the spray cooling device 4 from the upper surface side and the lower surface side. Reference numerals 5 and 6 are a guiding roll and a stabilizer roll, respectively.

As shown in FIG. 6, the spray cooling device 4 generally has a structure in which flat spray nozzles 4b are attached to a plurality of longitudinal cooling water supply headers 4a extending in the width direction of the steel plate 1, Since the spraying range is elongated in the strip width direction to make the cooling temperature distribution in the strip width direction uniform, it is effective in preventing the flatness of the steel sheet from being defective.

[0005]

However, in the case of thin-walled materials such as pre-coated steel sheets for household appliances, especially thin-walled and wide-width materials with a low yield point, the flatness can be obtained even by the combination of the gas jet cooling and the spray cooling. Defects cannot be completely prevented. As a countermeasure for this, conventionally, an operation to reduce the line speed has been empirically taken, but since the productivity cannot be improved, a cooling method capable of preventing the flatness defect of the thin steel sheet at a high speed without decreasing the line speed is available. Long-awaited.

In view of the above situation, the present invention proposes a method for cooling a thin steel sheet which can prevent a flatness defect of the steel sheet while maintaining a high line speed.

[0007]

In cooling a thin steel sheet, a decrease in line speed is an effective means for preventing a flatness defect unless productivity is a problem. However, the reason for this is not clear, and empirically means for reducing the line speed have been adopted. The present inventors have conducted the earnest research to clarify the reason why the reduction of the line speed is effective in preventing the poor flatness, and have obtained the following findings.

When the flat spray nozzle is used as the strong cooling means for the thin steel plate, as shown in FIG. 6, a region (hatched portion) where water temporarily sits on the surface of the steel plate 1 is formed. This phenomenon is generally called "water riding". In addition, the length L of the water riding area is called the wet length. The wetting length L is expressed as the sum of the wetting length L ₁ on the upstream side of the header 4a and the wetting length L ₂ on the downstream side, and changes depending on the line speed. That is, the faster the line speed, the shorter the upstream wetting length L ₁ becomes.

The steel sheet 1 is rapidly cooled immediately below the spray, but is cooled more gently than this in the region where water is riding. On the other hand, the present inventors have already found that a rapid temperature change in the line direction causes a flatness defect. Therefore, it is considered that when the line speed becomes faster and the wetting length L becomes shorter, a temperature gradient enough to cause a flatness defect occurs in the line direction. Therefore, it is presumed that when the line speed was slowed down, the wetting length L ₁ became long, and the temperature gradient in the line direction was reduced to a level at which flatness defects did not occur. Further, the decrease in the line speed results in a decrease in the steel plate temperature before the start of water cooling, which is also considered to have contributed to the relaxation of the temperature gradient in the line direction.

From the above, it is expected that if the wetting length L ₁ on the surface of the steel sheet does not become short even if the conveying speed (line speed) of the thin steel sheet is increased, the flatness of the steel sheet can be prevented.

From the above findings, the present inventors have found that the wetting length L ₁ on the upstream side does not occur within a certain range of the collision angle between the flat spray nozzle and the steel plate, and the line speed is kept high. Has developed a cooling method for thin steel sheets that can prevent defective flatness of the steel sheets.

That is, according to the present invention, in the method of gently cooling a thin steel sheet in a continuous line and then strongly cooling it, as the strong cooling means, the spray water spray angle is 30 degrees or more and 60 degrees or less with respect to the thin steel sheet. The gist of the present invention is to use the flat spray nozzle set to No. 1 and set the cooling water flow rate within the range in which the amount of temperature gradient change in the line direction satisfies the following expression (1). ΔdT / dX <(σy−σu) / A · W · α · E (1) Formula ΔdT / dX: Change in temperature gradient in the line direction (° C / m) A: Constant W: Steel plate width (m) α: coefficient of linear thermal expansion (1 / ° C) E: coefficient of longitudinal elasticity (kgf / mm ² ) σy: yield stress of steel material at temperature gradient change point (kgf / m
m ² ) σu: Line tension (kgf / mm ² ).

[0013]

In the present invention, as shown in FIG. 1, the spray angle of the flat spray nozzles 4b, which are arranged at intervals in the longitudinal direction of the cooling water supply header 4a, with respect to the steel plate 1, that is, the collision angle θ of the spray water with the steel plate is set. Set to 30 to 60 degrees. The reason is that the collision angle θ with the steel plate of the spray water is 60 degrees or less, as shown in FIG. 2 which shows the relationship between the collision angle θ with the steel plate of the spray water, the wet length L ₁ on the upstream side and the transport speed. This is because the wetting length L ₁ on the upstream side can be completely eliminated. The lower limit of the collision angle θ is determined by the angle at which the thickness (about 10 mm) of the spray water flow sprayed from the flat spray nozzle collides with the steel plate, and is preferably 30 degrees or more industrially.

Amount of change in temperature gradient in the line direction (ΔdT / d
X) is (T ₂ −T ₁ ) / L as shown in FIG.
Over _{(T 1} over _{T 0)} / _L 0 become. That is, in the spray cooling of thin steel plates, the temperature gradient in the line direction in the slow cooling region due to gas jet cooling or cooling that is performed in the preceding stage of spray cooling (rapid cooling) is The temperature gradient becomes gentler than the directional temperature gradient, and the temperature gradient changes there. This temperature gradient change amount (ΔdT /
dX) changes depending on the position, but the problem here is the amount of rapid temperature gradient change in the line direction, that is, the position immediately after the start of water cooling (quick cooling) from slow cooling in the figure. At the position immediately after the start of the rapid cooling, when the injection angle of the flat spray nozzle 4b with respect to the steel plate 1 is, for example, 90 degrees, FIG.
As shown in (B), a wetting length L ₁ is generated on the upstream side, and as a result, the portion is rapidly cooled and flatness failure occurs. On the other hand, when the spray angle of the flat spray nozzle 4b with respect to the steel plate 1 is set to 60 degrees or less, as shown in FIG.
Since the spray collision area is expanded and the wet length L ₁ on the upstream side is completely eliminated, and at the same time, the water amount density is decreased, the cooling capacity immediately after the start of the rapid cooling can be reduced.

On the other hand, in the spray cooling of the thin steel sheet, the stress acting on the side edge portion of the thin steel sheet is expressed as the sum of the thermal stress generated from the temperature distribution of the thin steel sheet and the line tension σu.
When there is no temperature distribution in the width direction of the thin steel sheet, thermal stress is generated only by the temperature distribution in the line direction. In addition, the temperature gradient in the line direction in the slow cooling area due to gas jet cooling or cooling that is performed before the spray cooling (rapid cooling) is gentler than the temperature gradient in the line direction in the rapid cooling area due to the subsequent spray. , Where a temperature gradient change occurs. The change (ΔdT / dX) in the temperature gradient becomes the convex pattern shown in FIG. 4, that is, the negative pattern. When the change in the temperature gradient in the line direction becomes negative, tensile thermal stress in the line direction occurs at the side edge of the thin steel sheet. The reason why such a thermal stress in the line direction occurs due to the change in the temperature gradient in the line direction is that the thin steel sheet has free ends (side edges). This thermal stress changes the temperature gradient in the line direction (Δd
It is proportional to T / dX) and the width (W) of the thin steel sheet. If the thermal stress thus generated is smaller than the yield stress (σy) of the thin steel sheet, flat failure does not occur in the thin steel sheet. The relationship between the thermal stress and the yield stress (σy) is expressed by the above formula (1), and if the amount of cooling water is adjusted so as not to exceed the yield stress (σy), it is possible to prevent the flatness failure. Therefore, in the present invention, the cooling water flow rate of the flat spray nozzle 4b in which the spray injection angle is set to 60 degrees or less is set within a range in which the temperature gradient change amount in the line direction satisfies the above expression (1).

[0016]

【Example】

Example 1 In producing a pre-coated steel sheet for home appliances (yield stress σy = 18 kgf / mm ² ) having a plate thickness of 0.45 mm and a plate width of 787 mm, quenching after baking and gas jet cooling is shown in Table 1.
The results obtained under the conditions shown in Table 2 are shown in Table 2 when the spray injection angle was changed variously and in comparison with the conventional method in which the spray injection angle was 90 degrees.

As is clear from the results shown in Table 2, in the conventional method in which the spray injection angle is 90 degrees, the steel plate conveying speed is 30 m.
When the conveyance speed was 50 m / min, the flatness defect did not occur, whereas when the conveyance speed was 50 m / min, the flatness defect did not occur. Even when the spray injection angle is more than 60 degrees and less than 90 degrees, the flatness failure occurs when the transport speed is 50 m / min. On the other hand, the spray injection angle is 30 degrees ~
In the present invention of 60 degrees, no flatness defect occurred at the conveying speed of 50 m / min, and even if the conveying speed was increased to 100 m / min, there was no flatness defect.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above, according to the method of the present invention, even a thin material such as a pre-coated steel sheet for home appliances can prevent the flatness defect without depending on the decrease of the line speed. There is a great effect that the productivity can be improved by increasing the speed.

[Brief description of drawings]

FIG. 1 is a spray angle of a flat spray nozzle of the present invention with respect to a steel plate (collision angle of spray water with a steel plate) θ.
FIG.

FIG. 2 is a diagram showing the relationship between the collision angle θ of the spray water with the steel plate, the upstream wetting length, and the transport speed.

FIG. 3 is an explanatory diagram showing a change in temperature gradient in the line direction and a relationship between a spray angle and a wetting length of a flat spray nozzle immediately after the start of quenching, and (A) is an explanatory diagram of a temperature gradient change amount in the line direction; (B) is an explanatory view showing the wetting length when the spray angle of the flat spray nozzle to the steel plate is 90 degrees, and (C) shows the wetting length when the spray angle of the flat spray nozzle to the steel sheet is 60 degrees. It is a figure.

FIG. 4 is a schematic diagram showing a pattern of changes in temperature gradient.

FIG. 5 is a schematic view showing an example of a continuous line in which gas jet cooling and water cooling are combined in a cooling facility used for cooling a coated steel sheet.

FIG. 6 is an explanatory diagram of a water riding phenomenon caused by a flat spray nozzle in the above cooling facility.

[Explanation of reference numerals] 1 steel plate 2 baking furnace 3 gas jet cooling device 4 spray cooling device 4a cooling water supply header 4b flat spray nozzle

Claims (1)

[Claims] 1. A method of gently cooling a thin steel plate in a continuous line and then strongly cooling it, wherein as the strong cooling means, a flat water spray angle is set to be 30 degrees or more and 60 degrees or less with respect to the thin steel sheet. A method for cooling a thin steel sheet, which comprises using a spray nozzle and setting a cooling water flow rate within a range in which a temperature gradient change amount in a line direction satisfies the following expression. ΔdT / dX <(σy−σu) / A · W · α · E ΔdT / dX: Change in temperature gradient in the line direction (° C / m) A: Constant W: Steel sheet width (m) α: Thermal linear expansion coefficient (1 / ° C) E: Modulus of longitudinal elasticity (kgf / mm ² ) σy: Yield stress of steel material at temperature gradient change point (kgf / m)
m ² ) σu: Line tension (kgf / mm ² ).