JPH11291398A - Manufacture of polyolefin-coated steel pipe and manufacturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent appearance defects from generating at the time of a cooling process in manufacturing of a polyolefin-coated steel pipe. SOLUTION: In a manufacturing method of a polyolefin-coated steel pipe, a melted polyolefin band 3 is coated on a steel pipe, and then, while fine particles of water are continuously sprayed on the coating surface, waterdrops adhered to the coating surface are blown off by spraying compressed air. Thereby, temperature of the coating surface can be effectively lowered, and appearance defects such as marks of waterdrops, marks generated by contacting with a conveying roll, or the like can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for producing a polyolefin-coated steel pipe.
The present invention relates to a method and an apparatus for manufacturing a polyolefin-coated steel pipe, which efficiently cools and solidifies a coated surface after coating to prevent poor appearance.

[0002]

2. Description of the Related Art Polyolefin-coated steel pipes, in which the outer surface of a steel pipe is coated with a polyolefin such as polyethylene or polypropylene, have excellent corrosion resistance, and are therefore frequently used in construction materials such as various pipes and steel pipe piles. Many methods are known as a method for coating the outer surface of a steel pipe with a polyolefin. When coating a relatively large-diameter steel pipe, the T-die method is used from the viewpoints of productivity, economy, and corrosion resistance. . With the T-die method, a molten polyolefin is extruded in a belt shape using a screw type extruder and a T-die, and spirally wound around a preheated steel pipe conveyed in the pipe axis direction while rotating on a turning skew roll. It is a coating method. T
In the case of coating by a die method, it is necessary to cool and solidify the coating surface after the coating is applied and before the steel pipe comes into contact with the next turning skew roll. As a cooling method, a method of applying a coolant such as water from the outer surface of the coating is generally used,
When the water droplets adhere to the uncured polyolefin coating, the resin in the water droplet adhesion portion shrinks, and the surface of the coating becomes uneven, resulting in poor appearance of the coating. Therefore, reduce the line speed to allow time for the temperature to drop due to cooling, or blow compressed air before water cooling to perform forced air cooling to cool the coating surface to a temperature below which water marks remain.
The outer surface is cooled after solidification.

When the thickness of the steel pipe or the thickness of the polyolefin coating layer is large, the coating is stretched in the pipe axis direction from the step portion of the coating surface generated by the end of the polyolefin strip, and the coating surface is extended. In some cases, a continuous spiral recess may occur. The phenomenon that a continuous spiral concave portion is generated on the coating surface is because a large shrinkage stress is generated as the coating is cooled, and a stress concentration occurs on a step portion of the coating surface.

As the line speed is increased in order to improve the productivity, the appearance defects such as water droplet marks and spiral concave portions are hindered. Therefore, as a countermeasure, it is necessary to increase the cooling capacity, especially the capacity of forced air cooling before water cooling, but a large investment is required to enlarge the air compressor and lower the temperature of the air, and the air has poor thermal conductivity Therefore, there is a problem that the effect is not so improved.

As means for preventing appearance defects due to pock-shaped water droplet marks, Japanese Patent Application Laid-Open No. Hei 6-143417 discloses a method in which a coated steel pipe is covered with a heat-resistant resin sheet immediately after plastic coating, and water cooling is performed from the outside in this state. A method is disclosed. However, this method requires a step of winding a heat-resistant resin sheet, and is not suitable when the coating speed is high. As means for preventing the appearance defect due to the spiral concave portion, JP-A-56-30821 discloses a method in which a nozzle is disposed at a partially overlapping portion and a coolant is sprayed preferentially to the partially overlapping portion. A method is disclosed. However, in this method, if air is used as the coolant, the air collides with the coating and then diffuses to cool the surrounding coating. In addition, when water is used as a coolant, poor appearance due to water droplet marks occurs.

[0006]

Therefore, in the conventional method for producing a polyolefin-coated steel pipe, it has been difficult to achieve both the two problems, that is, the prevention of poor appearance of the coated surface and the improvement of productivity by increasing the coating.

[0007]

Means for Solving the Problems The present inventors have intensively studied to solve the above problems. As a result, as shown in FIG. 1, the molten polyolefin extruded from the T-die (2) is placed on the outer surface of the preheated steel pipe (1) transferred in the pipe axis direction while rotating on the turning skew roll (4). In the method for producing a polyolefin-coated steel pipe in which a part of the strip (3) is spirally coated while being overlapped with each other and then cooled and solidified, the preheated steel pipe (1) is coated with the polyolefin strip (3). Thereafter, while continuously spraying fine particles of water on the surface of the coating using the spray nozzle (5), air is blown using the air nozzle (6) to blow off water droplets attached to the coating surface, thereby efficiently. Can lower the surface temperature of the coating,
It has been found that appearance defects such as water droplets and deformation due to contact with the transport roll can be prevented. As a result of further study, in order to prevent the generation of spiral concave portions, a spray nozzle (7) was selectively applied to the step (7) on the coating surface caused by the overlap of the ends of the polyolefin strip (3) immediately after coating. The present inventors have found that it is only necessary to blow air on the surface of the coating to blow off water droplets attached to the coating while spraying fine particles of water sprayed in step 5) to preferentially cool the stepped portion, thereby leading to the present invention. In addition, (9) in FIG. 1 is a rubber roller for pressing the polyolefin strips together.

The gist of the present invention is as follows.

(1) A part of a strip of molten polyolefin is spirally coated on the outer surface of a steel pipe which is conveyed in the pipe axis direction while rotating while being overlapped with each other, and then cooled from the outer surface of the coating. A method for producing a steel pipe, wherein compressed water is blown to blow off water droplets attached to the coating while spraying fine water particles on the coating surface in a cooling process.

(2) The method for producing a polyolefin-coated steel pipe according to the above (1), wherein, in the cooling step, water fine particles are intensively sprayed onto a step portion of the coating surface caused by the overlapping of the polyolefin strips.

(3) An apparatus for producing a polyolefin-coated steel pipe, comprising: a transfer device for transferring the steel pipe in the axial direction while rotating the pipe; a screw-type extruder for coating the polyolefin; and a T-die and a cooling device for cooling from the outside. The apparatus for producing a polyolefin-coated steel pipe, wherein the cooling device is provided with a water spray nozzle and an air nozzle that can be adjusted in position.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION A steel pipe (1) used in the present invention is:
First, a rust removal treatment such as a blast treatment or a degreasing / pickling treatment is performed. The steel pipe that has been subjected to rust removal treatment is subjected to 120-250 with a high-frequency induction heating device or the like in order to prevent molding defects due to rapid shrinkage of the polyolefin coating and to increase the adhesive strength of the coating.
Heat to ° C. It is to be noted that a base treatment layer such as a chromate film or an epoxy primer layer may be formed in advance on the surface of the steel pipe in accordance with a conventional method in order to obtain greater adhesion and better corrosion protection.

When the adhesive strength of the polyolefin layer covering the steel pipe is poor, an adhesive layer can be interposed between the steel pipe and the polyolefin layer. For example, in the case of coating with polyethylene, if a maleic anhydride-modified polyethylene obtained by graft-polymerizing maleic anhydride to polyethylene is used as an adhesive, good results can be obtained with excellent adhesion to a steel pipe and fusion to a polyethylene layer. As a method for forming the adhesive layer, a T-die method is preferable. In this case, the two-layer T
The adhesive layer and the polyolefin layer can be formed simultaneously by extruding the two layers integrally using a die so that the adhesive layer is on the lower layer and the polyolefin layer is on the upper layer.

Polyethylene and polypropylene are generally used for the polyolefin layer from the viewpoint of availability and ease of molding and durability. These polyolefins can be used in combination with a coloring pigment, an antioxidant, an ultraviolet absorber, a lubricant, a flame retardant, an antistatic agent and the like according to the application.

As shown in FIG. 1, the molten polyolefin is extruded in a belt shape using a screw type extruder and a T-die (2), and is rotated on a turning skew roll (4) in the tube axis direction. It is formed by winding around a preheated steel pipe (1) to be transferred. At this time, the polyolefin strip (3) is spirally coated while partially overlapping each other in order to perform coating without pinholes. Further, when the pressing is performed by the rubber roller for pressing (9) immediately after the coating, the fusion of the polyolefin strips can be promoted, and the step (7) on the coating surface caused by the end of the polyolefin strip can be reduced, thereby further overlapping. Since the entrainment of air into the portion can be prevented, an integrated good coating can be obtained. In addition, it is preferable that the distance between the pair of turning skew rolls (4) and the skew angle be variable so that the transfer device can cope with changes in the size and transfer pitch of the steel pipe.

Subsequently, in order to prevent the uncured coating from being deformed by contact with the turning skew roll (4), water fine particles are continuously sprayed on the coating surface to cool and solidify the coating surface. At this time, some of the sprayed water fine particles aggregate on the coating surface and adhere as water droplets. However, if the water droplets adhering to the coating surface are left alone, pock-like irregularities are formed due to local cooling of the coating. Therefore, air is continuously blown to the coating surface to blow off and remove water droplets attached to the coating surface.

As the spray nozzle (5) used for spraying fine water particles, a two-fluid nozzle for mixing and spraying water and air is preferable because fine water particles can be sprayed efficiently. For example, good results can be obtained by using an Atmax nozzle manufactured by Atmax or a two-fluid fine spray nozzle manufactured by Spraying Systems Japan. The amount of water to be sprayed and the number of spray nozzles can be arbitrarily adjusted according to the surface temperature and the coating speed of the coating. When a plurality of spray nozzles are installed, if the spray nozzles are installed along the circumference of the pipe, the cooling start temperature by the water fine particles becomes uniform, so that good results can be obtained. The fine particles of water sprayed by the spray nozzle preferably have a particle diameter of 0.5 mm or less. When the particle size of water is large, water droplets are scattered over a wide area and adhere to the surface of the coating.

The air blow nozzle (6) used to blow off and remove water droplets may be any nozzle as long as it can efficiently remove water droplets. Pattern nozzles are preferred. For example, good results can be obtained by using a plurality of flat nozzles manufactured by Silvent or blow-off nozzles manufactured by Spraying Systems Japan along the pipe circumferential direction. The installation angle, wind speed, air volume and number of nozzles of the air blow nozzle can be arbitrarily adjusted according to the size of the steel pipe and the amount of water droplets.

When the thickness of the steel pipe or the thickness of the polyolefin coating layer is large, and a stepped portion of the coating surface generated by the end of the polyolefin strip is used as a starting point, a continuous spiral concave portion is generated on the coating surface. Spiral recesses are generated if several of the spray nozzles (5) are installed so that water particles directly hit the step (7) on the coating surface and the step (7) is cooled preferentially. Can be prevented. In this case as well, water drops adhering to the coating are removed by air blowing in order to prevent poor appearance due to water drop marks.
The amount of water to be sprayed and the number of spray nozzles can be arbitrarily adjusted according to the surface temperature of the coating and the coating speed. If the transfer speed of the steel pipe is high and cooling is insufficient, a plurality of spray nozzles may be provided along the step. When the transfer speed and the transfer pitch of the steel pipe are constant, if the position of the spray nozzle is fixed so that the spray hits the step on the coating surface at the start of coating, the nozzle position adjustment during coating becomes unnecessary.

After water droplets adhering to the coating surface are removed by air blow, when the coating surface temperature falls to a temperature at which no water droplets remain even if water droplets adhere, water is discharged to the outer surface water cooling nozzle (8).
To cool the entire outer periphery to obtain a polyolefin-coated steel pipe.

Hereinafter, the present invention will be described in detail by way of an example in which a steel pipe is coated with polyethylene.

[0022]

Examples (Table 1, Nos. 1 to 4) A steel pipe (SGP250) whose outer surface was rust-removed by blasting
A × 5500 mm length × 6.6 mm thickness) was placed on a skew-turning type transfer device, and transferred at a speed of 0.5 m / min in the tube axis direction while rotating. It heated so that surface temperature might be set to 180 degreeC with the high frequency induction heating apparatus. A T-die (die lip) is formed by integrally forming two layers of modified polyethylene (modified polyethylene obtained by modifying a homopolymer of ethylene with maleic anhydride) and high-density polyethylene (comprising 2.5% by weight of carbon black, density 0.95) in this steel tube. 180m wide
m) and extruded into a strip. At this time, the conveyance pitch was adjusted so that the outer surface of the steel pipe was double-coated with the polyethylene band. Immediately after the coating, a roller made of silicone rubber was pressed against the surface of the coating and pressed. The thickness of the coating was 3.0 mm. The temperature of the coating surface immediately after the pressure bonding was 220 ° C., and the height of the spiral step formed on the coating surface due to the overlapping of the polyethylene strips was 0.1 mm after the pressure bonding.
3 mm. Then, in order to cool and solidify the coating surface, three two-fluid fine spray nozzles (manufactured by Spraying Systems Japan, model number B1 / 4JN + SU23) are arranged along the circumference of the pipe, and water fine particles are sprayed on the coating surface. Was. At this time, one fine spray nozzle was installed facing the step on the coating surface. The flow rate of water supplied to each spray nozzle was 0.5 L / min, the air pressure was 2 kg / cm ² , the distance from the nozzle to the coating was 50 mm, and the surface temperature of the coating immediately before spraying was 160 ° C. Furthermore, a flat nozzle manufactured by Silvent (model number SL-920)
A) Using 6 pieces, blow compressed air to the coating surface,
Water droplets adhering to the coating surface were blown off in the traveling direction of the steel pipe and removed. Air pressure supplied to each air nozzle is 4kg
/ Cm ² . The coating surface temperature after removing the water droplets was 90 ° C. Water was cooled over the entire outer periphery of the coating to obtain a polyethylene-coated steel pipe.

The appearance of the coating after cooling was good, with no deformation of the coating due to contact with the turning skew roll, no trace of water droplets, no spiral recesses originating from the step, and the like.

[0024] Polyethylene-coated steel pipes having different coating speeds were produced in the same procedure as above while changing the conveying speed of the steel pipe to 0.1 m / min and 2.0 m / min. The appearance was good at all levels.

Further, a polyethylene-coated steel pipe was produced in the same procedure as above, except that the thickness of the coating was 5 mm. The appearance was good.

Comparative Example 1 (Table 1, Nos. 5 to 8) A polyethylene coating was applied in the same manner as in the example, and a roller made of silicone rubber was pressed against the coated surface and pressed. Then, water was cooled over the entire outer periphery of the coating to obtain a polyethylene-coated steel pipe.

Comparative Example 2 (Table 1, Nos. 9 to 12) A polyethylene coating was applied in the same manner as in the Example, and a roller made of silicone rubber was pressed against the coated surface and pressed. Then, in the same manner as in the example, three two-fluid fine spray nozzles were arranged along the circumference of the tube, and fine particles of water were sprayed on the coating surface. Water was cooled over the entire outer periphery of the coating to obtain a polyethylene-coated steel pipe.

Comparative Example 3 (Table 1, Nos. 13 to 16) A polyethylene coating was applied in the same manner as in the example, and a roller made of silicone rubber was pressed against the coated surface and pressed. Then, in the same manner as in the example, compressed air was blown onto the coating surface using six flat nozzles. Water was cooled over the entire outer periphery of the coating to obtain a polyethylene-coated steel pipe.

[0029]

[Table 1] Table 2 summarizes the presence or absence of poor appearance of the polyethylene-coated steel pipes obtained in Examples and Comparative Examples.

From Table 2, it can be seen that, in a method for producing a polyolefin-coated steel pipe in which a molten polyolefin is spirally coated on the outer surface of a steel pipe transferred in the pipe axis direction while rotating, the coating is cooled and solidified. After coating the surface with polyolefin, spraying compressed air while spraying water particles on the coating surface to blow off water droplets adhering to the coating, the appearance defects such as water droplet marks and deformation due to contact with the transport rolls are reduced. Spiral concaves can be prevented by preferentially cooling the steps by spraying fine particles of water sprayed by the spray nozzle onto the steps on the coating surface preferentially.

[0031]

[Table 2]

[0032]

According to the method for producing a polyolefin-coated steel pipe of the present invention, the coated surface can be cooled and solidified more efficiently than the conventional method, and the effect of preventing appearance defects is excellent. Therefore, the coating speed is increased to improve the productivity. I can do it. for that reason,
It is useful as a method for producing polyolefin-coated steel pipes such as various pipes and steel pipe piles.

[0033]

[Brief description of the drawings]

[0034]

FIG. 1 is a schematic view showing a method for producing a polyolefin-coated steel pipe of the present invention.

[0035]

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Preheated steel pipe 2 T die 3 Melted polyolefin strip 4 Turning skew roll 5 Spray nozzle 6 Air nozzle 7 Surface step generated by overlapping of polyolefin strip 8 External water cooling nozzle 9 Rubber roll for crimping

Claims (3)

[Claims] 1. A polyolefin-coated steel pipe, wherein a part of a molten polyolefin strip is spirally coated on the outer surface of a steel pipe which is conveyed in the pipe axial direction while rotating while being overlapped with each other, and then cooled from the outer surface of the coating. The method for producing a polyolefin-coated steel pipe according to claim 1, wherein compressed water is blown to blow off water droplets attached to the coating while spraying fine water particles on the coating surface in a cooling step. 2. The method for producing a polyolefin-coated steel pipe according to claim 1, wherein, during the cooling step, fine water particles are intensively sprayed onto a step portion of the coating surface caused by the overlapping of the polyolefin strips. 3. A polyolefin-coated steel pipe manufacturing apparatus comprising: a conveying device for rotating a steel tube in a pipe axial direction while rotating the steel tube; a screw-type extruder for applying a polyolefin coating; and a T-die and a cooling device for cooling from an outer surface. An apparatus for producing a polyolefin-coated steel pipe, wherein the cooling device is provided with a water spray nozzle and an air nozzle that can be adjusted in position.