HOT ROLLING DEVICE FOR STEEL STRIPS

Abstract

 A hot rolling device for steel strips capable of reducing the length of a building of the whole plant and producing superior-quality steel strips. The hot rolling device is characterized in that a molten lead tank for storing therein molten lead whose temperature is 600 °C or higher and allowing steel strips emerging from a hot rolling mill to pass therethrough is disposed at an outlet side of the hot rolling mill, that the molten lead tank is constituted by two or more molten lead chambers continuously connected to each other and each storing molten lead of a different temperature, and that the molten lead is divided into portions in the molten lead chambers at the lower part thereof by bulkheads which are lower than the bath surface of the molten lead, while kept communicating at the upper part thereof.

Description

[Technical Field]

[0001] The present invention relates to a hot strip rolling mill that is used for producing steel strip.

[Background Art]

[0002] In the hot rolling of steel strip, the temperature of a steel strip that is ejected from the final roll stand is, for example, 1200°C, and the coiling temperature of the steel strip is, for example, 600°C. In a conventional hot strip mill used for steel strip production, a roller table of, for example, 200 m long is provided for the cooling of steel strip. While steel strip at a temperature of 1200°C is traveling along the roller table, water spraying is performed onto the steel strip, and when its temperature has been reduced to 600°C, it is wound to form a coil. This water spraying is performed to control the cooling speed of the steel strip in order to give the steel strip a desired micro structure.

[0003] For a conventional water spray device, a large number of spray nozzles are arranged along a roller table and cooling water is sprayed through the nozzles onto the steel strip. The control of the cooling speed and the final cooling temperature is performed by adjusting the amount of the cooling water that is used. However, the spray nozzles tend to clog, and when some of them clog, the steel strip is cooled unevenly. The sprayed water forms, on the surface of the steel strip, a steam film that has low heat conductivity. Since this steam film is repeatedly formed and extinguished according to the speed at which the steel strip is traveling and the spraying speed of the cooling water, the thickness of this steam film fluctuates and is not constant.

[0004] Because of the above, the conventional process of using spray nozzles has the shortcomings that the cooling speed for the steel strip and the final cooling temperature may vary greatly. When the steel strip is cooled too strongly by increasing the sprayed water, the steel strip becomes more unevenly cooled, or it is cooled to an excessively low temperature and can not be wound into a coil. In order to avoid these, a slow cooling must be performed. However, it requires an extremely long roller table of, for example, 200 m. Further, when a soft, hot steel strip travels along the roller table, as the roller table is a hard steel structure, scrubbing defects caused by the roller table easily occur on the steel strip.

[0005] Molten lead is known as a quenching agent for steel that is used in a laboratory. However, there is no hot strip rolling mill for producing steel strip, that uses molten lead. Although lead patenting process of steel wire is widely known. However in this process, a lead tank is provided at the outlet side of a heating furnace and not at the outlet side of a roll stand. Further, the temperature of the molten lead is normally 500°C or lower, and does not exceed 600°C. And the material that is to be processed is steel wire, and is not a steel strip.

[0006] In the hot dip galvanizing of steel sheets by a lead-zinc process, a galvanizing pot is used wherein a layer of molten zinc is suspended on a layer of molten lead. By passing a steel sheet through the molten lead layer and then through the molten zinc layer, zinc plating of the steel sheet is performed. With this method, however, the molten lead layer is provided at the outlet side of a flux tank, and is not at the outlet side of a roll stand. The temperature of the molten lead is usually 500°C or below, and does not exceed 600°C. And the material that is to be processed is normally cold rolled steel strip, and is not a hot rolled steel strip.

[0007] Japanese Patent Laid Open Publication No. Sho 63-176435 shows an apparatus wherein a steel strip is suspended over an air chamber. The air chamber ejects air upward in order to provide an air cushion bearing layer between the steel strip and the air chamber, and the steel strip that is conveyed does not contact with a hard steel structure. With this method, no scrubbing defect occurs on the strip. In this method, however, gas is only used and molten metal is not used. Further, this apparatus conveys steel strip that has a room temperature and not conveys steel strip that has a temperature of higher than 600°C.

[Disclosure of Invention]

[0008] A conventional hot strip rolling mill which has a roller table of, for example, 200 m long requires an extremely long work shop and the facility costs are also high. It is one object of the present invention to provide a hot strip rolling mill that permits the reduction of the overall length of a work shop. Further, a conventional hot strip rolling mill that employs a water spray to cool the steel strip does not accurately control the cooling speed as well as the final temperature of the steel strip. It is another object of the present invention to provide a hot strip rolling mill that can accurately control the cooling speed and the final temperature of the steel strip. Still further, a conventional hot strip rolling mill, wherein soft, hot steel strip is conveyed being supported by a roller table of a hand steel structure, defects due to scrubbing tend to occur on the steel strip. It is still another object of the present invention to provide a hot strip rolling mill that can convey the soft, hot steel strip being supported by a fluid and prevent the occurrence of scrubbing defects on the steel strip.

[0009] The present invention discloses a hot strip rolling mill wherein provided at the outlet side of its roll stand a lead tank, which contains molten lead at a temperature of 600°C or higher, so that a rolled steel strip passes through it in contact with the molten lead. Fig. 1 is an explanatory cross-sectional view of a lead tank in the present invention. A lead tank 2 is provided at the outlet side of a roll stand 3. Steel strip 1 that is ejected from the final roll stand 3 is moved in the direction as indicated by an arrow 7, is cooled in contact with molten lead 6 in the lead tank 2, and is then wound by a winder 4 to form a coil 8.

[0010] When the temperature of the steel strip 1 is lower than 600°C at the time it reaches the winder 4, a spring-back force in coiling of the steel strip 1 is large and makes it difficult to wind the steel strip 1. According to the present invention, the temperature of the molten lead 6 is 600°C or higher, and the spring-back force in coiling of the steel strip 1 is small and makes it easy to wind the steel strip 1.

[0011] The melting point of lead is 330°C and the boiling point is 1740°C. On the other hand, the temperature of the steel strip 1 is in 600 - 1200°C from the time it is ejected from the final roll stand 3 until it is wound by the winder 4. Lead neither solidifies nor boils within this temperature of 600 - 1200°C, and remains in the melted state. The specific gravity of the steel strip 1 is about 7, while the specific gravity of the molten lead 6 is about 10.5. And a steel strip of 5 mm thickness, for example, floats on the molten lead 6 sinking 3.3 mm of its thickness into the molten lead 6 and projecting 1.7 mm of its thickness above the surface of the molten lead 6.

[0012] According to the present invention, therefore, the steel strip 1 can be conveyed in the direction as indicated by the arrow 7 while floats on the surface of the molten lead 6. More specifically, the soft, hot steel strip 1 ejected from the final roll stand 3 is supported by and is moved along a liquid bearing layer of the molten lead 6, so that, unlike in a conventional hot strip rolling mill, the steel strip does not contact with a hard steel structure, such as a roller table, and the occurrence of scrubbing defects can be prevented.

[0013] As the molten lead in the lead tank has an excellent heat conductive properties and as vapor film is not formed, the temperature of the steel strip 1 in the lead tank 2 quickly drops to the temperature of the molten lead 6. In this invention, therefore, the cooling speed of the steel strip 1 is high, and within a short period of time the temperature of the steel strip 1 is reduced to a temperature at which the steel strip 1 can be wound. The distance from the roll stand 3 to the winder 4 can accordingly be shortened and the overall length of a work shop can be reduced.

[0014] According to the present invention, the cooling speed of the steel strip 1 is high. However, the temperature of the steel strip 1 will not be lower than that of the molten lead 6. The molten lead 6 in the lead tank 2 can accurately be maintained at a desired set temperature, and the steel strip 1 is cooled to the temperature of the molten lead 6. And as there is little fluctuation in the temperature of the molten lead 6, there is also less fluctuation in the final temperature to which the steel strip 1 is cooled. With a conventional water spraying method, since the cooling water is at room temperature, it is difficult to adjust the volume of water that is required to cool a steel strip to a desired temperature, and the final temperature to which the steel strip 1 is cooled also varies, whereas according to the present invention, the steel strip 1 can be easily and steadily cooled to a desired temperature.

[0015] In this invention, a solidified lead layer is not formed on the steel strip 1. In a lead plated steel sheet that is available on the market, a layer of another metal is formed on the steel and the solidified lead layer is formed on the layer of another metal. Since the present invention does not comprise such a special means, a solidified lead layer is not formed and lead is not consumed as a plating layer. Further, as the temperature of the molten lead 6 is sufficiently higher than the melting point of lead, the lead has a sufficient flowability. And since the temperature of the steel strip 1 that is moved in the lead tank 2 is sufficiently higher than the melting point of lead, lead does not stick to the surface of the steel strip 1.

[0016] In the present invention, the steel strip 1 is conveyed while being supported by the liquid bearing layer that is consisted by the molten lead 6. At this time, the molten lead 6 that is in contact with the steel strip 1 travels easily with the steel strip 1. Thus, only a small force is required to move the steel strip 1.

[0017] In hot rolling, the leading end of the steel strip 1 that has ejected from the final roll stand 3 is pushed by the followed portion of the steel strip 1 that is ejected by a driving force of the final roll stand 3 until it reaches to the winder 4. At this time, when friction that interferes smooth movement of the leading end of the steel strip 1 is large, the speed of the leading end of the steel strip 1 is reduced relative to that of its followed portion, and as a consequence, the steel strip 1 tends to move upward in its leading end before it reaches to the winder 4, which arises an operational trouble.

[0018] Since in the present invention, the steel strip 1 is conveyed while it is floated on the liquid bearing layer of the molten lead 6, the interferes with the smooth movement of the leading end of the steel strip 1 is small. Further, as was previously described, as the distance from the roll stand 3 to the winder 4 is short in this invention, the frequency at which upward warp of the steel strip 1 occurs is reduced. And in addition, since the steel strip 1 is quickly cooled to a coiling temperature, and the steel strip becomes hard quuckly, which prevents upward warp of its leading end.

[0019] In the present invention, when a forced flow is given to the molten lead 6 in the direction indicted by the arrow 7, the movement of the leading end of the steel strip 1 is influenced by the flowing molten lead 6 and the leading end of the steel strip 1 is moved more smoothly. In addition, in the present invention, when the temperature of the molten lead 6 is set, for example, at 700°C, which is lower than the magnetic transformation temperature of steel and the leading end of the steel strip 1 is attracted to a magnet or an electromagnet (neither of them shown in Fig.) that travels in the direction indicated by the arrow 7, and is guided to the winder 4 at a desired speed, so that the occurrence of upward warp of the steel strip 1 can be prevented. Further, in the present invention, when the upper surface of a steel strip 1 moving in the lead tank 2 is push down by using a driving roll or a driving roller (neither of them shown in Fig.) and the entire steel strip 1 is immersed in the molten lead 6, a driving force that is exerted in the direction 7 is applied to the steel strip 1 by the driving roll or the driving roller. As a result, the steel strip 1 smoothly travels in the direction indicated by the arrow 7.

[0020] According to the present invention, the molten lead 6 must be cooled because it is heated by the steel strip 1. This cooling process may be performed by a heat exchanger that is provided inside the lead tank 2, or in each lead chamber, or it may be performed by permitting molten lead to circulate in a heat exchanger that is provided outside the lead tank 2. The temperature of the molten lead 6 can also be controlled by, for example, adding another molten lead of lower temperature in the lead tank 2, or in the individual lead chambers, or by adding solid lead pellets. With a conventional water spraying method, it is difficult to utilize the heat that is obtained from the steel strip 1 while it is cooled from a rolling temperature to a coiling temperature, because the heat is dissipated as, steam or discharged water which are low and uncertain temperature. On the other hand, with the present invention, since the heat can be taken out in high temperature vapor, for example, that is generated by a heat exchanger of a temperature of 600°C or higher, the heat can be utilized efficiently.

[0021] The vapor pressure of lead at a temperature of, for example, 1000°C is lower than 0.1 KPa. When an air-tight cover is provided for the lead tank 2 in this invention, leakage of lead vapor or of lead oxide can be prevented more efficiently. Further, in the present invention, when a known means is employed to vibrate, or to blow a high temperature gas against the steel strip 1 as it exits the lead tank 2, the molten lead 6 adhered to the steel strip 1 is further reduced and the amount of consumption of the molten lead 6 is therefore so small that it can be ignored.

[Brief Description Of The Drawings]

[0022] Fig. 1 shows an explanatory example of an apparatus according to the present invention.

[0023] Fig. 2 shows another example of an apparatus according to the present invention.

[Best Mode For Carrying Out The Invention]

[0024] Fig. 2 shows another example of the lead tank 2 according to the present invention. The lead tank 2 is so constructed that two or more lead chambers, for example, 9-1, 9-2 and 9-3, which contain molten lead 6 of different temperatures, are linked together in the direction in which the rolled steel strip 1 is conveyed as indicated by the arrow 7. The bottom portion of the molten lead tank 2 is separated by one or more partition walls 10-1 and 10-2, which extend upward until their upper edges are near and below the surface level of the molten lead 6 in the lead tank 2, while at the surface of the molten lead 6, molten lead in each lead chambers 9-1, 9-2 and 9-3 is connected with each other. The lead tank 2 is provided at the outlet side of the roll stand 3 in order to receive the steel strip 1 that exits the roll stand 3 and is conveyed floating on the molten lead 6 and not contacting with the upper edges of partition wall.

[0025] In the lead tank 2 in Fig. 2, the steel strip 1 passes through the lead chambers 9-1, 9-2, and 9-3, which contain molten lead having temperatures of T₁, T₂, and T₃, respectively. The steel strip 1 is rapidly cooled to temperature T₁, from a rolling temperature of, for example, 1200°C, and remains at that temperature while it is in the lead chamber 9-1. Then, the steel strip 1 is quickly cooled or heated to temperature T₂ and remains at that temperature while it is in the lead chamber 9-2. Subsequently, the steel strip 1 is quickly cooled to temperature T₃, for example, and remains at that temperature while it is in the lead chamber 9-3. Finally, the steel strip 1 exits the lead chamber 9-3 and is wound into a coil by the winder 4.

[0026] Molten lead has an excellent heat conductive property. And by using the lead tank 2 shown in Fig. 2, a heat treatment to change the temperature of the steel strip 1 to T₁, then to T₂, and finally to T₃ can be accomplished. At this time, the cooling speed and the temperature of the steel strip 1 can be controlled with a high degree of accuracy.

[0027] As is shown in Fig. 2, the molten lead is separated by partition walls 10-1, 10-2. However, over the upper edges of partition walls, the molten lead in each of the lead chambers 9-1, 9-2 and 9-3, are connected each other. The steel strip 1 is conveyed floating on the connected layer of the molten steel not contacting with the upper edges of partition walls. The steel strip 1 that floats on the surface of the molten lead, therefore, advances smoothly in the direction indicated by the arrow 7, not being hindered by any obstacle, and is subjected to T₁, T₂, and T₃.

[0028] Although the molten lead 6 that is retained in each lead chambers 9-1, 9-2, and 9-3 may be slightly mixed since the molten lead of the lead chambers 9-1, 9-2 and 9-3 is connected each other over the partition walls. However, the molten lead in each lead chambers has the same chemical composition, and there is no trouble caused by mixing of components. In Fig. 2, if the thickness (L) of the partition walls 10-1 and 10-2 and the depth (d) of the molten lead above the partition walls are properly selected, the substantial mixing of molten lead can be prevented and the temperatures of the molten lead in the individual lead chambers 9-1, 9-2 and 9-3 can be effectively maintained at the temperatures T₁, T₂, and T₃, respectively. When the total volume of the lead retained in the lead tank 2 is increased or decreased, the depth (d) of the molten lead above the partition walls can be adjusted as desired.

[0029] When, in Fig. 2, the temperature of the molten lead in the lead chamber 9-1, for example, is set to a temperature equal to or higher than Ar₁ point of the steel, the precipitation of carbide and nitride of Al, Ti, or Nb, which are contained in steel, as well as the grain size of the steel can be controlled. Further, when the temperature of the molten lead in, for example, the lead chamber 9-2 is set to a temperature lower than Ar₁ point, a steel strip that has the grain size and the micro structure shown in the Isothermal Transformation Diagram and the TTT Diagram can be obtained. When the temperature of the molten lead in the lead chamber 9-3 is set high, the coiling temperature rises and the precipitation of carbide and nitride after it is wound can be adjusted. When the temperature is set low, the steel strip 1 having less scales can be obtained.

[0030] In the hot strip rolling mill of the present invention, another device can be provided at the inlet side or the outlet side of the lead tank 2. When, for example, a quenching device (not shown in Fig.) using excessive amount of cooling water is provided between the final roll stand 3 and the lead tank 2 in Fig. 1 in order to cool the steel strip 1, the steel strip 1 can be cooled within a short time, the overall length of the work shop can be reduced. But the steel strip 1 is excessively cooled and becomes difficult to wind in coil. However, the excessively cooled steel strip 1 can be rapidly heated to the desired temperature by passing it through the lead tank 2 of the present invention, and can be satisfactorily wound by the winder 4. Further, when a slow cooling furnace (not shown in Fig.) is provided between the lead tank 2 and the winder 4 in Fig. 1, for example, and slowly cools the steel strip 1, the steel strip 1 can acquire a micro structure that is obtained in the gradual cooling.

[Industrial Applicability]

[0031] The hot strip rolling mill according to the present invention can reduce the overall length of the work shop, and can save the facility costs for a factory installation. According to the present invention, fluctuations in the cooling speed and the coiling temperature of the steel strip can be reduced. And the steel strip of excellent quality can be obtained. Further, according to the present invention, since the steel strip can be prevented from contacting with a steel structure, and the scrubbing defects can be reduced. In addition, according to the present invention, the heat obtained in cooling the steel strip can be recycled efficiently.

List of Reference Numerals, Symbols, and Items

[0032] 

1:

steel strip.

2:

lead tank.

3:

roll stand.

4:

coiler (winder).

5:

pinch roll.

6:

molten lead bath.

7:

moving direction of steel strip.

8:

coil.

9 (9-1, 9-2, 9-3):

lead chamber.

10 (10-1, 10-2):

partition wall.

L:

thickness of partition wall.

d:

thickness of molten lead over the partition wall.

T₁:

temperature of molten lead in lead chamber 9-1.

T₂:

temperature of molten lead in lead chamber 9-2.

T₃:

temperature of molten lead in lead chamber 9-3.

Claims

1. A hot strip rolling mill used for producing steel strip, characterized in that it comprises:
   a lead tank being provided at an outlet side of its final roll stand and is contained molten lead of higher than 600°C, into which a steel strip being ejected from the final roll stand is passed through in contact with the said molten lead.

2. A hot strip rolling mill used for producing steel strip according to claim 1, characterized in that the lead tank is constructed by connecting 2 or more lead chambers in the rolling direction, each lead chamber contains molten lead of different temperature, the lead chamber is separated from the adjacent lead chamber by partition wall, the partition wall has a height not reach to the surface of the molten lead, the partition wall is separating molten lead in the lower portion of the molten lead, and is connecting together the molten lead in each lead chamber at the surface of the molten lead.

Drawing