JP2003181519A - Method and device for cooling metal material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for cooling a metal material capable of artificially and easily controlling quenching points in the cooling process of the metal material such as a hot-rolled steel plate or the like. <P>SOLUTION: The device for cooling the metal material is composed of a transfer device 2 to transfer the steel plate 1, a plasma generator 3 to generate plasma which irradiates the surface of the steel plate 1, and a water cooling zone 4 to cool the steel plate 1 with water sprayed from a spray nozzle 4a. The steel plate 1 is irradiated with plasma generated from the plasma generator 3 for a period of 60 seconds during which the steel plate is transferred, and surface wettability is temporarily improved. Then, when water is sprayed through the spray nozzle 4a, an angle of contact with a droplet becomes smaller and the quenching point moves to a high temperature side to shorten a cooling time. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooling method and apparatus for heat treatment of a metallic material, for example, a cooling technique for a high temperature metallic material after hot rolling of a hot rolled steel sheet.

[0002]

2. Description of the Related Art A metal material can be improved in various properties by subjecting it to heat treatment to change its structure.
The heat treatment is carried out by various combinations of heating and cooling. In particular, in the case of steel materials, various heat treatments such as refining of crystal grains by normalizing, quench hardening, etc. are put into practical use. For example, in a hot-rolled steel sheet, the strength is significantly increased by rapid cooling after hot rolling.

Conventionally, in the cooling process of hot rolled steel sheet, mist cooling, spray cooling, pipe laminar cooling,
Various types of water cooling such as direct quench cooling and slit laminar cooling (also referred to as curtain wall cooling) are performed, and by controlling the cooling rate, steel sheets having metallurgically desirable properties are manufactured. The cooling rate is controlled mainly by changing the water amount density.

By the way, when a material having a relatively high temperature, such as a hot rolled steel sheet immediately after finish rolling, is cooled with a boiling type refrigerant such as water, film boiling type heat transfer is performed. In this film boiling type heat transfer, since the liquid is not directly in contact with the heat transfer surface and is continuously covered with the vapor film, the temperature drop rate of the material is small. On the other hand, nucleate boiling type heat transfer is performed in cooling a material having a relatively low temperature. In this nucleate boiling type heat transfer, since the liquid directly contacts the heat transfer surface, the temperature drop rate is large.

The coiling temperature of the hot-rolled steel sheet is controlled by controlling the above-mentioned cooling rate. However, in cooling a coiling material having a low coiling temperature, that is, a low-temperature coiling material, usually, film boiling occurs during cooling. The heat transfer state changes to nucleate boiling (the point where the film boiling state transitions to the nucleate boiling state is called the quench point). In the nucleate boiling type heat transfer, since the temperature drop rate of the material is large and unstable, it is difficult to control the cooling stop temperature, and the quality is deteriorated due to the deviation of the winding temperature. In addition, temperature unevenness becomes large, and defective shapes are likely to occur.

[0006]

As described above, various methods have been proposed for the controlled cooling of steel materials with water, but no slit laminar cooling and no innovative cooling method have been proposed. . In either case, the cooling rate is controlled by changing the water amount density according to the running speed of the hot-rolled steel sheet, but this method cannot control the temperature at the quench point.

However, in these water cooling processes, it is extremely important that the quenching point temperature becomes high. If the quenching point can be controlled artificially and easily, the cooling merit will be improved. unfathomable.

Therefore, it is an object of the present invention to provide a method and apparatus for cooling a metallic material such as a hot rolled steel sheet, in which the quench point in the water cooling process can be artificially and easily controlled.

[0009]

In the method for cooling a metallic material according to the present invention, the metallic material is irradiated with plasma and then cooled with water. When the surface of the metal material is irradiated with plasma, the low-activity molecular layer adhering to the metal surface, that is, the solid surface energy of the molecular layer is decomposed, and a highly active oxide layer, that is, an oxide layer with high surface energy is formed. Since it appears on the surface, the wettability is significantly improved temporarily.

The process of improving the wettability by the plasma irradiation will be described in detail. (1) Plasma irradiated on the metal surface decomposes the bonding chains of the surface molecular layer to temporarily generate an OH group which is easily compatible with the liquid. To do. (2) Remove impurities that reduce the surface energy of the metal surface.
(3) The surface is roughened by nano-order. These three synergistic effects improve the wettability of the metal material surface.

When the wettability of the metal material is improved, the contact angle with the liquid droplet becomes small, so that the quench point moves to the high temperature side,
Cooling time becomes shorter. Therefore, it is possible to control the quench point artificially and easily by controlling the irradiation time and the irradiation intensity of the plasma for irradiating the metal material, and to control the water cooling rate of the high temperature surface of the metal material. Further, in the cooling method of the present invention, since the thermal characteristics of the metal material are not changed by the plasma irradiation, only the wettability can be changed for the purpose of cooling control, and fine cooling control can be performed.

Here, the plasma irradiation time is preferably 60 seconds or more, more preferably 100 seconds or more,
More preferably, it is 120 seconds or more. The longer the plasma irradiation time, the smaller the contact angle, but if it is 60 seconds or longer, the contact angle of most metals such as copper, stainless steel, and aluminum can be 10 ° or less. Particularly, when the irradiation time is 100 seconds or more, the difference in contact angle due to the surface roughness of the metal material becomes small, and when the irradiation time is 100 seconds or less, the contact angle becomes smaller as the surface of the metal material becomes rougher. At 120 seconds or longer, the contact angle is not significantly affected by the difference in surface roughness.

The water cooling after the plasma irradiation is most preferably immediately after the plasma irradiation, but if the water cooling is performed within 5 minutes after the plasma irradiation, the effect of temporarily improving the wettability after the plasma irradiation is obtained. It is possible to perform fine cooling control without impairing the temperature.

The metallic material cooling apparatus of the present invention comprises an irradiation unit for irradiating the metallic material with plasma, a cooling unit provided in the subsequent stage of the irradiation unit for cooling the metallic material with water, and a control unit for controlling the irradiation of plasma. It is equipped with. Here, it is desirable that the control unit controls either or both of the plasma irradiation time and the irradiation intensity as described above.

According to the metal material cooling apparatus of the present invention, the control unit controls the irradiation of the plasma, so that the wettability of the surface of the metal material can be remarkably improved to perform fine cooling control. It is possible to easily obtain a metal material having desired characteristics.

[0016]

1 is a schematic configuration diagram of a cooling device for a hot rolled steel sheet as a metal material according to an embodiment of the present invention. The hot-rolled steel plate cooling device shown in FIG. 1 includes a carrier device 2 for carrying the steel plate 1, a plasma generator 3 for generating plasma for irradiating the surface of the steel plate 1, and a spray nozzle 4.
The water cooling zone 4 cools the steel plate 1 with water sprayed from a. The steel sheet 1 conveyed by the conveying device 2 is a hot-rolled steel sheet at high temperature immediately after finish rolling, and its surface temperature is 300 to 1000 ° C. The surface roughness is preferably 0.4 μm or more in terms of root mean square roughness (RMS) from the viewpoint of improving water wettability.

The steel plate 1 is irradiated with the plasma generated by the plasma generator 3 for 60 seconds while being conveyed by the conveying device 2. The plasma generated by the plasma generator 3 is plasma in the atmosphere due to arc discharge. In addition, the plasma generator 3 includes a control device that controls the irradiation time and the irradiation intensity, and in the present embodiment, controls the irradiation time so that the discharge on time and the discharge off time are each repeated for 8 ms. It should be noted that the discharge-on time is set so that the plasma particles can be evenly spread to every corner of the surface of the steel plate 1.
It is preferable to change between 4 and 8 ms.

In the water cooling zone 4, water at 20 to 50 ° C. is sprayed from the spray nozzle 4a at ² to 2000 L / m ² min. For water spraying, an airless mist of high pressure water of 0.3 to 10a or an air mix mist mixed with compressed air of 4 to 15a can be used.

In the apparatus for cooling a hot-rolled steel sheet having the above structure, the steel sheet 1 conveyed by the conveyor 2 is irradiated with plasma generated by the plasma generator 3. afterwards,
In the water cooling zone 4, the steel plate 1 is cooled by the water sprayed from the spray nozzle 4a.

On the surface of the steel sheet 1, a low-activity molecular layer adhering to the surface of the steel sheet 1 is decomposed by the plasma irradiated by the plasma generator 3, and a high-activity oxide layer appears on the surface. As a result, the wettability of the surface of the steel sheet 1 is remarkably improved temporarily. When water is sprayed from the spray nozzle 4a in the water cooling zone 4 on the steel sheet 1 having improved surface wettability in this way, the contact angle with the liquid droplets becomes small, so the quench point moves to the high temperature side, and the cooling time Becomes shorter.

Here, FIG. 2 is a diagram schematically showing a temperature history near the surface of the steel sheet during water cooling. As shown in FIG.
In the temperature histories (I) and (II), the range from the cooling start point A to the quench points B and C is the film boiling state. Quenching points B and C are points at which the film boiling state in which the cooling rate is relatively slow transitions to the nucleate boiling state in which the cooling rate is high. As shown in FIG. 2, when the quench point C moves to the high temperature side and reaches the quench point B, the temperature history changes from (II) to (I), so the cooling time becomes short.

In the cooling apparatus for a hot-rolled steel sheet according to this embodiment, the quench point is artificially and easily controlled by controlling the irradiation time and the irradiation intensity of the plasma applied to the steel sheet 1 by the control device provided in the plasma generator 3. However, it is possible to control the water cooling rate of the high temperature surface of the steel plate 1. Also,
At this time, since the thermal characteristics of the steel sheet 1 are not changed by the plasma irradiation, only the wettability can be changed for the purpose of cooling control, and fine cooling control can be performed.

[0023]

Example 1 In this example, the relationship between plasma irradiation time and contact angle will be described. FIG. 3 is a diagram showing the relationship between the plasma irradiation time and the contact angle on the surfaces of five types of emery papers # 600, # 1000, # 1500, # 2000 and mirror-finished steel sheets. The contact angle was measured immediately after stopping the plasma irradiation. As shown in FIG. 3, when the irradiation time is short, the contact angle increases as the surface becomes smoother, but when the irradiation time exceeds 100 seconds, the difference in contact angle due to roughness decreases.

FIG. 4 shows that the same five types of steel plate surfaces are each provided with four
It shows that the contact angle increases with the lapse of time after the plasma irradiation for 0 seconds, 60 seconds, and 120 seconds. The contact angle decreases in the order of irradiation time of 40 seconds, 60 seconds, and 120 seconds. The data of irradiation time 120 seconds connected by a line has a relatively small contact angle.
The one polished by 0 has a stable contact angle of about 10 ° within the elapsed time of 300 seconds.

FIG. 5 shows the emery paper # 600 only once at the beginning.
The result is obtained by heating the heat transfer surface polished in step 1, and repeatedly performing plasma irradiation for 120 seconds and dropping the droplet diameter of 2.16 mm. The temperature of the heat transfer surface was set to about 60 ° at which the evaporation rate was slow in order to measure the contact angle. The contact angle was about 10 ° during the 9th repetition, and the emery paper # 6
It was confirmed that the heat transfer surface, which was obtained by irradiating the surface polished with No. 00 with plasma for 120 seconds, could be used during the drop time of the droplet.

FIG. 6 shows the evaporation time measured by dropping droplets of four different sizes on the heat transfer surface polished with emery paper # 600. When the heat transfer surface temperature is low, the larger the droplet diameter, the longer the evaporation time, and when the temperature is high, the difference disappears. The wetting limit temperature at this time was about 122 ° C. regardless of the size of the droplet. The contact angle of this heat transfer surface is about 90
°. Here, the "wetting limit temperature" is the temperature immediately before transitioning to the film boiling state when gradually increasing the surface temperature while heating, and conversely, when cooling from high temperature, It refers to the temperature immediately before the transition to each boiling.

FIG. 7 shows the results of an experiment conducted in the same procedure as in FIG.
It was irradiated for a second. Compared to Fig. 6, in the low temperature range,
The larger the droplet diameter, the longer the evaporation time, but the difference between the evaporation times is smaller. The wetting limit temperature is about 144 ° C., which is a large difference from the heat transfer surface that is not irradiated with plasma. The high temperature at which the surface begins to get wet means that the quench point becomes high. That is, it was confirmed that the plasma irradiation can move the quench point to the higher temperature side and shorten the cooling time.

In order to confirm the appearance of evaporation, a droplet diameter of 2.16 mm and a heat transfer surface temperature of about 60 ° C. and a droplet diameter of 2.16 mm and a heat transfer surface temperature of 120 ° C. Comparing the images obtained by the high-speed camera for the contact angle of 10 °, it was confirmed that the contact angle was stable even at high temperature because the appearance did not change even if the temperature of the heat transfer surface changed.

(Embodiment 2) In this embodiment, the effect of plasma irradiation in immersion cooling will be described. FIG. 8 is a temperature history during immersion cooling with a pellet-shaped copper sample having a diameter of 30 mm and a thickness of 8 mm, and corresponds to FIG. FIG. 8 shows and compares two cooling curves with and without plasma irradiation on the surface of a copper sample polished with emery paper # 600. In each case, the copper sample was heated to 300 ° C. and then immersed in water at 100 ° C.

As shown in FIG. 8, when the plasma is irradiated, the quench point X moves to the high temperature side, and the cooling is completed in about 10 seconds shorter than the quench point Y when the plasma is not irradiated. There is. That is, the effect of plasma irradiation on cooling could be confirmed from FIG.

[0031]

According to the present invention, by irradiating a metal material with plasma and then cooling it with water, the wettability of the surface of the metal material can be remarkably improved and fine cooling control can be performed. It is possible to artificially and easily control the quench point of a metal material such as a hot-rolled steel sheet in a water cooling process to obtain a metal material having desired characteristics.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a cooling device for a hot rolled steel sheet as a metal material according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a temperature history near the surface of a steel plate during water cooling.

FIG. 3 is a diagram showing a relationship between a plasma irradiation time and a contact angle on a steel plate surface.

FIG. 4 is a diagram showing how the contact angle rises with the lapse of time after plasma irradiation.

FIG. 5 is a diagram showing a result of repeatedly performing plasma irradiation and dropping of droplets.

FIG. 6 is a diagram showing a result of measuring an evaporation time of a droplet that has dropped onto a heat transfer surface.

FIG. 7 is a diagram showing a result of irradiating plasma for 120 seconds immediately before dropping a droplet.

FIG. 8 is a diagram showing a temperature history during immersion cooling with a copper sample.

[Explanation of symbols]

1 steel plate 2 Conveyor 3 Plasma generator 4 water cooling zone 4a spray nozzle

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Claims (4)

[Claims] 1. A method for cooling a metal material, which comprises cooling the water with water after irradiating the metal material with plasma. 2. The method for cooling a metal material according to claim 1, wherein the plasma irradiation time is 60 seconds or more. 3. A metal material, comprising: an irradiation unit for irradiating the metal material with plasma; a cooling unit provided at a subsequent stage of the irradiation unit for cooling the metal material with water; and a control unit for controlling irradiation of the plasma. Cooling system. 4. The cooling device for a metallic material according to claim 3, wherein the control unit controls one or both of an irradiation time and an irradiation intensity of the plasma.