JP6760002B2 - Steel plate cooling method - Google Patents

Description

The present invention relates to a method for cooling a steel sheet.

The hot-dip plated steel sheet is made by immersing a steel material that has undergone hot-rolling, cold-rolling, and annealing treatment (that is, the original plating plate) in a hot-dip plating tank and alloying the plating as necessary to form a hot-dip plating layer. It is manufactured by cooling after it is formed, but the product may have stretcher strains (wrinkled surface defects).

As a technique for suppressing the generation of stretcher strain when manufacturing such a hot-dip galvanized steel sheet, for example, there is a technique of using a specific mist cooling device as a device for cooling the steel sheet after forming a hot-dip galvanized steel sheet. (See Patent Document 1).

The generation of stretcher strain can be suppressed by using the mist cooling device described in Patent Document 1, but for example, when ultra-low carbon steel having a very low carbon content is used as the plating base plate, the stretcher strain can be used. A method that is particularly likely to occur and that can suppress the generation of stretcher strain is desired.

Here, the temperature of the steel sheet is lowered to, for example, about 50 to 300 ° C. by performing mist cooling or the like after forming the hot-dip plating layer. Since the equipment is inexpensive for cooling such a low temperature steel plate, water droplets are sprayed onto the steel plate by a spray water cooling method, specifically, a spray nozzle such as a nozzle in which a plurality of holes are arranged in the header pipe. Cooling method is used. When the spray water cooling method is used to cool a low-temperature steel sheet, the transition temperature that changes from the transition boiling region to the nucleate boiling region (hereinafter, may be simply referred to as the transition temperature) is low, so that the steel sheet is cooled unevenly. Large, stretcher strain is generated on the steel plate.

Further, as another cooling method, there is an immersion water cooling method in which a steel plate is immersed in a water tank. It is considered that the transition temperature is increased and the occurrence of stretcher strain can be suppressed by using the immersion water cooling method in cooling the steel sheet at a low temperature of 50 to 300 ° C.

Japanese Unexamined Patent Publication No. 2015-4080

However, the immersion water cooling method requires a large amount of water such as a water tank facility, which causes a problem that the facility becomes expensive. Further, by installing a pass line roll for transporting the steel sheet in the water tank, roll entrainment of dirt adhering to the pass line roll occurs, and the dirt adheres to the steel sheet to cause the surface of the steel sheet. There is also the problem that the quality may deteriorate.

The problem that stretcher strain is generated by such cooling and the problem that the immersion water cooling method is expensive and deteriorates the quality of the steel sheet surface are low temperatures in the production of hot-dip galvanized steel sheets. It exists not only in the cooling of the steel sheet of No. 1 but also in the cooling of other low temperature steel sheets, of course.

The present invention has been made in view of such circumstances, and provides a method for cooling a low-temperature steel sheet, which is inexpensive and can suppress the generation of stretcher strain without deteriorating the quality of the steel sheet surface. With the goal.

As a result of studies to solve the above problems, the present inventors have used a slit nozzle water cooling method and used a slit nozzle over the width direction of the steel sheet with respect to both sides of the steel sheet at 50 to 300 ° C. to be conveyed in the vertical direction. By injecting water, it is possible to use virtual water tank cooling (immersion water cooling method), improve cooling efficiency, and suppress stretcher slits caused by cooling unevenness in the width direction of the steel sheet. Knowing what can be done, the present invention has been completed.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A method for cooling a steel sheet that cools a steel sheet that is conveyed in the vertical direction at 50 to 300 ° C.
It is characterized in that water is jetted from at least two slit nozzles provided across the steel plate and having openings arranged in the width direction of the steel plate on both sides of the steel plate in the width direction of the steel plate. How to cool the steel sheet.

[2] The method for cooling a steel sheet according to [1], wherein the temperature of the steel sheet is adjusted to be equal to or lower than the transition temperature at which the temperature changes from the transition boiling region to the nucleate boiling region.

[3] The method for cooling a steel sheet according to [1] or [2], wherein the steel sheet is a steel sheet after undergoing hot dip galvanizing treatment, alloying treatment, water mist cooling, and air cooling in this order.

[4] The method for cooling a steel sheet according to any one of [1] to [3], wherein the steel sheet is conveyed downward in the vertical direction.

[5] The shape of the opening of each slit nozzle is a rectangle having a long side of 1 m to 2 m and a short side of 1 mm to 5 mm.
Two slit nozzles are placed on both sides of the steel plate,
Flow rate of water discharged from the slit nozzle, the opening of the steel plate width direction length 5 × ¹⁰ -3 ^m per ^{2m 3 / s~10 × 10 -3 m} 3 / s, the outlet of the opening of the slit nozzle The method for cooling a steel sheet according to any one of [1] to [4], wherein the speed is 1.25 m / s to 2.50 m / s.

According to the present invention, a slit nozzle water cooling method is used to virtually inject water from both sides of a steel sheet at 50 to 300 ° C., which is conveyed in the vertical direction, over the width direction of the steel sheet. Water tank cooling (immersion water cooling method) can be used, cooling efficiency can be improved, and stretcher slits generated by cooling unevenness in the width direction of the steel sheet can be suppressed. The cooling method of the present invention is an inexpensive cooling method because it does not require a large amount of water such as a water tank facility unlike the immersion water cooling method and does not cause a problem that the facility becomes expensive. Further, since the water tank and the pass line roll are not required, there is no problem that the quality of the steel sheet surface is deteriorated due to the roll entrainment of dirt. Therefore, by using the method for cooling a steel sheet of the present invention, for example, a hot-dip galvanized steel sheet can be manufactured, which has less stretcher strain, is inexpensive, and does not deteriorate the quality of the surface of the steel sheet. Further, according to the present invention, the cooling efficiency is also improved, so that the improvement of the cooling efficiency also has the effect of improving the efficiency of the production line.

It is the schematic (FIG. 1 (a)) which shows the cooling method of the steel plate of this invention, and the cross-sectional perspective view (FIG. 1 (b)) of a slit nozzle. It is the schematic (FIG. 2 (a)) and the cross-sectional perspective view (FIG. 2 (b)) which show the spray water cooling system which cools a steel plate. It is a conceptual diagram which shows the difference of cooling efficiency by the difference of a water cooling system. It is a schematic diagram of the continuous hot-dip galvanizing equipment which manufactures an alloyed hot-dip galvanized steel sheet. It is a schematic diagram of the cooling equipment used in an Example and a comparative example.

Hereinafter, embodiments of the present invention will be described.

The method for cooling a steel sheet of the present invention is a method for cooling a steel sheet that cools a steel sheet at 50 to 300 ° C. that is conveyed in the vertical direction. Water is jetted from at least two slit nozzles provided on both sides of the steel sheet in the width direction of the steel sheet.

The method for cooling the steel sheet of the present invention will be described with reference to FIG. FIG. 1 is a schematic view (FIG. 1 (a)) showing a method for cooling a steel plate of the present invention and a cross-sectional perspective view (FIG. 1 (b)) of a slit nozzle. As shown in FIG. 1, the slit nozzle 10 is provided with an elongated rectangular opening (slit) 12 in the header tube 11. Then, the slit nozzle 10 is arranged so as to sandwich the steel plate of the steel plate 1 which is continuously conveyed downward in the vertical direction. At this time, the water injected from the slit nozzle 10 is arranged so that the opening 12 of the slit nozzle 10 is arranged along the width direction of the steel plate 1, that is, the opening 12 faces the entire width of the steel plate 1. The slit nozzle 10 is arranged so as to cover the entire width of the steel plate 1. In other words, the slit nozzle 10 has an opening 12 parallel to the width direction of the steel plate 1, and the longitudinal direction of the opening 12 of the slit nozzle 10 is the same as the width of the steel plate 1 or longer than the width of the steel plate 1. The slit nozzle 10 is arranged in the steel plate passing line through which the steel plate 1 passes so that the longitudinal direction of the opening 12 is the width direction of the steel plate 1.

Then, water is supplied to the slit nozzle 10 from a water supply pipe (not shown), and water is discharged from the opening 12 of the slit nozzle 10 toward the steel plate 1, so that both sides of the steel plate 1 are spread over the width direction of the steel plate 1. The steel sheet 1 is cooled by the water flow 13 uniformly hitting the steel sheet 1 and the water hitting the steel sheet 1 flowing down in the vertical direction due to gravity. Since the opening 12 is a slit, unlike the water droplets discharged from the spray nozzle, the water becomes, for example, in the shape of a plate and uniformly contacts and flows down in the width direction of the steel plate 1. Therefore, the steel plate 1 can be cooled uniformly.

The steel sheet 1 to be cooled in the steel sheet cooling method of the present invention is a steel sheet that is conveyed in the vertical direction. The conveying direction may be downward or upward as long as it is in the vertical direction, but it is preferable that the steel sheet is conveyed downward in the vertical direction.

Further, the steel sheet 1 to be cooled in the steel sheet cooling method of the present invention is a steel sheet having a surface temperature of 50 to 300 ° C., preferably a steel sheet having a surface temperature of 200 to 280 ° C. Specifically, the steel sheet 1 to be cooled in the steel sheet cooling method of the present invention is, for example, a steel sheet after undergoing hot dip galvanizing treatment, alloying treatment, water mist cooling, and air cooling in this order.

Here, for cooling a steel sheet at a low temperature of 50 to 300 ° C., a spray water cooling method is often used because the equipment is usually inexpensive. The spray water cooling method will be described with reference to FIG. Figure 2 is. It is the schematic (FIG. 2 (a)) and the cross-sectional perspective view (FIG. 2 (b)) which show the spray water cooling system which cools a steel plate. As shown in FIG. 2, the spray nozzle 20 is provided with a plurality of holes 22 for injecting water into the header pipe 21. The spray nozzle 20 is arranged, for example, across the steel plate passing line of the steel plate 1 which is continuously conveyed downward in the vertical direction, and the water droplet 23 discharged from the spray nozzle 20 corresponds to the steel plate 1, that is, a portion of the steel plate 1. The steel sheet 1 is cooled by being exposed to water.

As described above, when the spray water cooling method is used for cooling the low-temperature steel sheet, the transition temperature (the temperature at which the transition (change) from the transition boiling region to the nucleate boiling region) is low, so that a stretcher strain is generated on the steel plate 1. ..

Further, it is considered that by cooling the steel sheet at 50 to 300 ° C. by the immersion water cooling method, the cooling efficiency is increased, the transition temperature is increased, and the generation of stretcher strain can be suppressed. The immersion water cooling method is a method of cooling a steel sheet by immersing the steel sheet in a water tank in which water is stored. However, if the immersion water cooling method is used, there is a problem that a large amount of water such as a water tank facility is required and the facility becomes expensive. In addition, in the immersion water cooling method, it is necessary to transport the steel sheet in the water tank, so that the roll of dirt is involved by installing the pass line roll in the water tank, and the quality of the steel sheet surface is deteriorated. The problem also arises.

However, in the present invention, as shown in FIG. 1, the steel plate is conveyed in the vertical direction, and the steel plate is provided so as to sandwich the steel plate, and the openings are arranged from at least two slit nozzles arranged in the width direction of the steel plate. , Water is jetted on both sides of the steel sheet over the width direction of the steel sheet. As a result, on both sides of the steel sheet, water is uniformly applied over the width direction of the steel sheet, and the water that is uniformly applied over the width direction flows downward due to gravity. Therefore, the immersion water cooling for immersing the steel sheet in the water tank. Cooling can be performed in the same manner as when cooled by the method, that is, virtual water tank cooling (immersion water cooling method) can be used. Therefore, the cooling efficiency is increased, the transition temperature is increased, and the stretcher strain generated by the cooling unevenness in the plate width direction of the steel sheet can be suppressed. From the viewpoint of ease of controlling water such as the amount of water used for virtual water tank cooling, the steel sheet is preferably conveyed downward.

If it is possible to use virtual water tank cooling (immersion water cooling method), the cooling efficiency is increased, the transition temperature is increased, and the stretcher strain generated by the cooling unevenness in the plate width direction of the steel sheet can be suppressed. Will be described with reference to FIG.

FIG. 3 is a conceptual diagram showing a difference in cooling efficiency due to a difference in the water cooling method. In FIG. 3, the horizontal axis represents the temperature of the steel sheet, which increases toward the right. Further, in FIG. 3, the vertical axis represents the cooling efficiency, which increases as it goes up. Then, in FIG. 3, the solid line shows the cooling efficiency in the spray water cooling, and the dotted line shows the cooling efficiency in the slit nozzle water cooling of the present invention.

As shown in FIG. 3, there is a temperature range (transition boiling region) in which the cooling efficiency increases as the steel sheet is cooled and the temperature of the steel sheet decreases. Then, when the steel sheet is further cooled from the transition boiling region, there is a temperature region (nucleate boiling region) in which the cooling efficiency decreases as the temperature of the steel sheet decreases. The temperature at which the transition from the transition boiling region to the nucleate boiling region, that is, the peak top temperature in FIG. 3 is the transition temperature.

When the steel sheet at 50 to 300 ° C., which is the object of cooling in the present invention, is cooled by spray water cooling, the cooling is first performed in the transition boiling region.

In the spray water cooling method, the temperature drops extremely at the place where the water droplet hits the steel sheet and the water droplet hits. Then, in the transition boiling region, the lower the temperature, the higher the cooling efficiency. Therefore, the temperature at the location where the water droplets hit decreases remarkably, so that the steel sheet temperature deviation increases and the non-uniformity of the steel sheet temperature increases. After that, by cooling to the nucleate boiling region, the higher the temperature, the higher the cooling efficiency, so the temperature non-uniformity of the steel sheet becomes smaller, but the non-uniformity of the steel sheet temperature generated in the transition boiling region is large, so that the final The temperature of the obtained steel sheet becomes non-uniform. As a result, a stretcher strain is generated.

On the other hand, in the present invention, cooling is performed by the above-mentioned specific slit nozzle water cooling method. When the low temperature steel sheet is cooled according to the present invention in this way, as shown by the dotted line (curve) in FIG. 3, the behavior of the cooling efficiency shifts to the high temperature side as compared with the case of the spray nozzle, and the transition temperature becomes high. Therefore, the temperature of the steel sheet at the start of cooling by the method for cooling the steel sheet of the present invention can be set as the nucleate boiling region, and cooling in the transition boiling region can be prevented, so that the surface temperature of the steel sheet can be set. Can be uniform. Further, even if the steel sheet temperature at the start of cooling by the steel sheet cooling method of the present invention is equal to or higher than the transition temperature, the transition temperature can be immediately set because the transition temperature is high, that is, cooling in the transition boiling region. Since the period can be shortened, the surface temperature of the steel sheet can be made uniform. Therefore, the occurrence of stretcher strain can be suppressed. For example, in the case of ultra-low carbon steel having a C content of 0.0030% by mass or less, stretcher strain is particularly likely to occur, but by using the steel sheet cooling method of the present invention, the generation of stretcher strain is suppressed. It is possible to produce ultra-low carbon steel having excellent uniformity.

When the steel sheet is cooled by the immersion water cooling method, it is the same as the slit nozzle water cooling method used in the present invention in FIG. 3, and the transition temperature is high.

In the present invention, the transition temperature can be adjusted by the amount of water, the flow velocity of water, and the like, and by adjusting these, the temperature of the steel sheet can be adjusted to be equal to or lower than the transition temperature. Then, by adjusting the temperature of the steel sheet to be equal to or lower than the transition temperature, cooling can be performed in the nucleate boiling region. As a result, the surface temperature of the steel sheet becomes more uniform, and the generation of stretcher strain can be further suppressed.

Further, the method for cooling the steel sheet of the present invention is not an immersion water cooling method, so the amount of water is not large, and for example, it can be cooled with an amount of water equivalent to that of spray cooling, so that it is an inexpensive cooling method.

Further, since the method for cooling the steel sheet of the present invention is not the immersion water cooling method, it is said that the quality of the surface of the steel sheet is deteriorated due to the roll entrainment of dirt by installing the pass line roll in the water tank. Since no problem occurs, the quality of the steel sheet surface is also good.

Although FIG. 1 shows an example in which two slit nozzles 10 are provided on both sides of the steel plate, at least one slit nozzle 10 may be provided on both sides of the steel plate.

The shape of each slit nozzle is preferably a rectangle having an opening having a long side of 1 m to 2 m and a short side (slit width) of 1 mm to 5 mm.

Further, it is preferable that two slit nozzles are arranged on both sides of the steel plate.

Water discharged from the slit nozzle is preferably the flow rate of the opening in the steel plate width direction is the length 5 × ¹⁰ -3 ^m per ^{2m 3 / s~10 × 10 -3 m} 3 / s.

The water discharged from the slit nozzle preferably has an outlet speed (outflow speed) of the opening of the slit nozzle of 1.25 m / s to 2.50 m / s.

The material (component composition) of the steel sheet to be cooled of the present invention is not particularly limited, but particularly for steel sheets in which stretcher strain is likely to occur, for example, ultra-low carbon steel having a C content of 0.0030% by mass or less. On the other hand, it can be suitably cooled.

A hot-dip galvanized steel sheet can be produced by using the method for cooling a steel sheet of the present invention. An example of a method for manufacturing an alloyed hot-dip galvanized steel sheet (alloyed hot-dip galvanized steel sheet) to which the method for cooling a steel sheet of the present invention is applied will be described with reference to FIG. FIG. 4 is a schematic view of a continuous hot-dip galvanizing facility for manufacturing an alloyed hot-dip galvanized steel sheet. In FIG. 4, the arrow indicates the transport direction of the steel plate 1. As shown in FIG. 4, the continuous hot-dip galvanizing facility 50 includes a hot-dip galvanizing bath 51 in which hot-dip galvanizing is stored and wiping to remove excess plating from the surface of the steel sheet 1 from the upstream side in the transport direction of the steel sheet 1. The nozzle 52, the alloying treatment device 53 that heats the plating on the surface of the steel sheet 1 by induction heating or the like to form an alloy, and the heat retaining device 54 that keeps the steel sheet 1 having the alloyed plating on the surface at a predetermined temperature to stabilize the alloy. , A water mist cooling device 55 that blows water mist onto the steel sheet 1 to cool the steel sheet 1, an air cooling device 56 that blows air onto the steel sheet 1 to cool the steel sheet 1, and a water cooling device 57 that cools the steel sheet 1 using water. Has.

In such a continuous hot-dip galvanizing facility 50, the steel sheet 1 is conveyed in the direction of the arrow in FIG. 4, and the annealed steel sheet 1 is immersed in the hot-dip galvanizing bath 51 to perform hot-dip galvanizing treatment. .. Then, the steel sheet 1 pulled up from the hot-dip galvanizing bath 51 is removed from excess plating by a wiping nozzle 52 such as a gas wiping nozzle, and the amount of plating adhered is controlled. Next, the steel sheet 1 is heated and alloyed by the alloying treatment device 53, and the alloy is stabilized by the heat retaining device 54. After that, the water mist is injected onto the steel sheet 1 by the water mist cooling device 55 to be cooled. Subsequently, the top roll 71 changes the direction of the steel sheet 1 so that the transport direction is downward in the vertical direction, and then the air cooling device 56 blows air onto the steel sheet 1 to cool the steel sheet 1. By these coolings, the surface temperature of the steel plate 1 introduced into the water cooling device 57 in the subsequent stage is set to 50 to 300 ° C. Then, the alloyed hot-dip galvanized steel sheet can be manufactured by cooling with water in the water cooling device 57. When a steel sheet having a hot-dip galvanized layer that has not been alloyed is manufactured, the alloying treatment by the alloying treatment apparatus 53 and the stabilization of alloying by the heat retaining apparatus 54 are not performed.

The water cooling device 57 is a device to which the method for cooling the steel sheet of the present invention is applied, and a slit nozzle 10 for cooling the steel sheet of the present invention is provided on the upstream side in the vertical direction on both sides of the steel sheet passing line. Two are provided.

The steel plate introduced into such a water cooling device 57 is a steel sheet having a surface temperature of 50 to 300 ° C. by being cooled on the upstream side of the water cooling device 57 as described above. In the water cooling device 57, water is discharged from the slit nozzle 10 to the steel sheet 1 to cool the steel sheet 1 by the method for cooling the steel sheet of the present invention. Since the method for cooling the steel sheet of the present invention can suppress the generation of stretcher strain, the steel sheet 1 treated by the water treatment device 57 is a steel sheet having no or little stretcher strain.

In FIG. 4, zinc is illustrated as hot dip galvanizing, but the plating is not limited to zinc, and examples of plating include Zn-based plating and Al-based plating. Examples of the Zn-based plating include general hot-dip galvanizing (GI), Zn-Ni-based plating, and Zn-Al-based plating. Further, as the Al-based plating, Al-Si-based plating (for example, Al-Si-based plating containing 10 to 20% by mass of Si) and the like can be exemplified. The hot-dip galvanized layer may be an alloyed alloyed hot-dip galvanized layer. Examples of the alloyed hot dip galvanized layer include an alloyed hot dip galvanized (GA) layer.

Hereinafter, examples will be used for further understanding of the present invention, but the examples do not limit the present invention in any way.

(Example 1 and Comparative Example 1)
FIG. 5 shows a schematic diagram of the cooling equipment used in Examples and Comparative Examples. As shown in FIG. 5, in the cooling facility 80, the slit nozzle 10 and the spray nozzle 20 are continuously installed in the transport direction of the steel plate 1. Further, two slit nozzles 10 and two spray nozzles 20 are installed on both sides of the steel plate 1. Then, the switching valve 82 provided in the water pipe 81 can switch between slit nozzle water cooling using the slit nozzle 10 and spray water cooling using the spray nozzle 20 during the passage of the steel plate 1.

Using such a cooling facility of FIG. 5, a steel plate 1 having a size of 0.75 mm in thickness × 1298 mm in width and a C content of 0.0020 mass% is conveyed downward in the vertical direction and cooled during the plate passing. Switching between slit nozzle water cooling (Example 1) and spray water cooling (Comparative Example 1) was carried out while keeping the surface temperature condition of the previous steel sheet constant. The slit nozzle 10 used in Example 1 has a rectangular shape with a long side of 2 m and a short side (slit width) of 2 mm, and the flow rate of water discharged from the slit nozzle is the opening in the width direction of the steel plate. The outlet speed (outflow rate) of the opening of the slit nozzle is 1.75 m / s at 7 × 10 ^-3 m ³ / s per 2 m of the length.

A cut plate of a steel plate after each cooling was collected and press-formed. As a result, no stretcher strain was observed in Example 1 in which the slit nozzle water cooling of the present invention was performed, but stretcher strain was generated in Comparative Example 1. Therefore, by the method for cooling the steel sheet of the present invention, the generation of stretcher strain could be suppressed with the same amount of water as in Comparative Example 1 using spray water cooling.

1 Steel plate 10 Slit nozzle 11, 21 Header tube 12 Opening (slit)
13 Water flow 20 Spray nozzle 22 Hole 23 Water droplet 50 Continuous hot-dip galvanizing equipment 51 Hot-dip galvanizing bath 52 Wiping nozzle 53 Alloying treatment device 54 Heat insulation device 55 Water mist cooling device 56 Air cooling device 57 Water cooling device 71 Top roll

Claims (3)

A cooling method for alloyed hot-dip galvanized steel sheets that cools steel sheets transported in the vertical direction at 50 to 300 ° C.
The steel sheet is a steel sheet after undergoing hot dip galvanizing treatment, alloying treatment, water mist cooling, and air cooling in this order.
Water is jetted from at least two slit nozzles provided so as to sandwich the steel plate and having openings arranged in the width direction of the steel plate on both sides of the steel plate in the width direction of the steel plate.
The steel plate is conveyed in the vertical direction and
A method for cooling an alloyed molten zinc-plated steel sheet, which comprises adjusting the temperature of the steel sheet at the time of starting cooling to be equal to or lower than the transition temperature at which the temperature changes from the transition boiling region to the nucleate boiling region. The method for cooling an alloyed hot-dip galvanized steel sheet according to claim 1 , wherein the steel sheet is conveyed downward in the vertical direction. The shape of the opening of each slit nozzle is a rectangle with a long side of 1 m to 2 m and a short side of 1 mm to 5 mm.
Two slit nozzles are placed on both sides of the steel plate,
Flow rate of water discharged from the slit nozzle, the opening of the steel plate width direction length 5 × ¹⁰ -3 ^m per ^{2m 3 / s~10 × 10 -3 m} 3 / s, the outlet of the opening of the slit nozzle The method for cooling an alloyed hot-dip zinc-plated steel sheet according to claim 1 or 2 , wherein the speed is 1.25 m / s to 2.50 m / s.

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