JPH0570526B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for cold drawing an externally plated metal tube, which is used, for example, to manufacture a resin pipe-inserted/externally plated metal tube. [Prior Art] When a metal pipe is used as a water pipe, it is customary to coat the inner surface of the metal pipe with a resin or the like in order to protect the tap water from contamination caused by corrosion of the metal pipe. On the other hand, when a metal water pipe is buried in concrete, it is considered preferable to plate the outside surface of the water pipe with a highly corrosion-resistant metal such as zinc in order to protect the water pipe from corrosion caused by residual moisture in the concrete. There is. Even when buried underground, external plating is effective in protecting the pipe surface from damage caused by construction. For this reason, for water pipes etc. buried in concrete or soil, resin pipes such as vinyl chloride pipes and polyethylene pipes are inserted inside the pipes, and the outer surface of the pipes is plated with zinc etc. Externally plated metal tubes are often used. The following method is known as a method for manufacturing a metal tube with a resin tube inserted inside and an outer surface plated. Each method will be explained for the case where the externally plated metal pipe is a galvanized steel pipe. Note that thick-walled plating by melting is usually employed as the plating. Figure 2 A and B show the thermal expansion lining method. In A, a resin pipe 2 is coated with adhesive on its outer surface and inserted into the inner surface of a steel pipe 1 in advance, and then heated in a heating furnace 3.
After being heated to 200 to 250°C, the steel pipe is pulled out to the right in the figure, and the resin pipe 2 inserted by the fixing plug 4 inside it is expanded outward and brought into contact with the inside of the steel pipe, forming a lining. be done. In addition, in B, the steel pipe 1 is
After inserting a resin pipe 2 coated with adhesive on the outer surface, steam 6 is introduced from a nozzle 5 provided at one end of the pipe.
is blown into the resin pipe 2 to heat it, and then its vapor pressure is increased to expand the resin pipe 2 and line the inner surface of the steel pipe. Figure 3 shows the warm drawing method. A steel pipe 1 with a resin pipe 2 inserted therein is heated by a high-frequency heating coil 8, and then drawn without lubrication with a die 7 having a die angle α of 25° to line the steel pipe with the resin pipe 2. Figure 4 shows the die drawing method (Japanese Patent Application Laid-open No. 1983-
No. 167118). The steel pipe 1 into which the resin pipe 2 has been inserted is cold and injected with lubricant 9, and is then cut by a die 11 (Fig. 5) having a straight part 10 with a die angle α of 15° or less and a length S of 2 to 5 mm. and then cold drawn. Figure 6 shows the roll diameter reduction method (Special Publication Act 1973-
No. 11030). Steel pipe 1 with resin pipe 2 inserted is roll 1
2, the resin pipe 2 is lined with the steel pipe 1. FIG. 7 shows the swaging (die forging) diameter reduction method. External galvanized steel pipe 1 with resin pipe 2 inserted inside
By being swaged by the rattle 13, the resin pipe 2 is lined with the steel pipe 1. [Problems to be solved by the invention] In the thermal expansion lining method, since the inserted resin pipe is expanded and then lined with the steel pipe, there is a risk that the adhesion between the resin layer and the steel pipe after cooling may be insufficient. It was hot.
Moreover, the manufacturing cost is high due to low efficiency. The warm drawing method requires high-frequency heating immediately before the die, so the process is complicated and manufacturing costs are high. In the die drawing method, when cold drawing is performed by inserting a resin lining pipe into a galvanized steel pipe, if the outer galvanized layer is thick, there is a high risk that the galvanized layer will crack or peel off due to the die shape described above. It was. The equipment for the roll diameter reduction method is relatively inexpensive, but because it is a roll reduction method, the roundness of the resin-lined steel pipe is not very good. The swaging diameter reduction method has the disadvantage that the galvanized layer of the externally galvanized steel pipe tends to peel off. It is an object of the present invention to be applied to the production of metal tubes with resin inner tubes and outer surface plating, to be inexpensive, to prevent peeling of the plating layer, to have good roundness, and to improve the adhesion between the inner resin tube and the metal tube. An object of the present invention is to provide a method for cold drawing an externally plated metal tube, which can produce a product with good quality. [Means for solving the problem] Among the above-mentioned methods for manufacturing metal tubes with resin pipes inserted inside and externally plated, the only drawback of die drawing is the peeling and cracking of the plating layer.If this can be solved, the above objective can be achieved. achieved. In conventional cold drawing, the die shown in FIG. 5 has been used as described above. In this die, the approach part taper angle (both angles, hereinafter the same will be omitted) is 15 degrees or less, and the bearing part 10 is 2 to 5 mm.
It adopted a straight shape. When using the die for cold drawing of externally thick-walled galvanized steel pipes, peeling of the plating layer could not be avoided. The inventor of the present invention obtained the following findings as a result of experiments and research on prevention of peeling of the plating layer. During cold drawing, there is a difference between the elongation rate of the zinc-iron alloy layer with poor ductility, which occurs at the boundary between the galvanized layer and the iron ingot, and the elongation rate of the steel pipe. Cracks and peeling occur between the galvanized layer and the galvanized layer. In order to eliminate this cause, it is effective to reduce the taper angle of the approach portion of the die to reduce the difference in elongation between the steel pipe and the zinc-iron alloy layer immediately before contacting the die. This is due to the following reason. Figure 8 shows the diameter reduction of the externally galvanized steel pipe just before it contacts the die. The outer galvanized steel pipe 1 having the galvanized layer 15 on the outer surface of the steel pipe ingot 14 is being drawn into the die 11. If the diameter of the externally galvanized steel pipe before entering the die is D, and the diameter when it contacts the die is D', the difference is ΔD=D-D'.
ΔD/D indicates the diameter reduction ratio, and a large value means that there is a large difference in elongation between the steel pipe and the zinc-iron alloy layer before entering the die. The smaller the taper angle of the approach part of the die, the smaller the diameter reduction rate ΔD/Δ, which suppresses the difference in elongation between the steel pipe and the zinc-iron alloy layer, and prevents cracking and peeling of the galvanized layer of the galvanized pipe. was getting smaller. Figure 9 shows the die taper angle and diameter reduction rate ΔD/
Experimental values of D are shown. According to this figure, the diameter reduction rate is 0.6%.
If the following is desired, the die taper angle should be 10° or less. There are frictional forces between the externally galvanized tube and the die, which create shear stresses in the galvanized layer. This shear stress occurs when the die taper angle is small. FIG. 10 shows an example of the die taper angle and the drawing stress generated in the galvanized pipe during cold drawing. As the taper angle decreases, the pull-out stress increases, causing shear stress. FIG. 11 shows the state of peeling of the galvanized layer due to this shear stress. External galvanized layer 1
5 is drawn into the die 11 and drawn, a normal force P acts on the die surface, a friction force μp due to P acts, and the galvanized layer is sheared by μp. , resulting in plucking. This phenomenon occurred in the straight bearing part of conventional dies. Therefore, it has become clear that the absence of straight portions is effective in preventing tearing. As mentioned above, in cold drawing of thick-walled externally plated pipe by melting, the approach taper angle (both angles) is set at 2 to 10 degrees, and the bearing part is designed to eliminate the straight part and provide die strength. It has been found that by making the surface into an arcuate rotating surface, it is possible to prevent all peeling, cracking, and peeling. The present invention has been made based on the above findings, and involves cold drawing of an outer surface melt-plated metal tube having a plating layer on the outer surface of the metal tube and an alloy layer formed between the plating layer and the underlying metal. External plating, characterized in that the drawing die has a die hole approach part with a one-step taper, a taper angle (both angles) of 2° to 10°, and a bearing part with an arcuate rotating surface. The gist is the cold drawing method for metal tubes. [Function] In the method of the present invention, peeling and cracking of the outer surface plating layer is prevented. Here, if the approach part taper angle (both angles) is less than 2 degrees, shear stress due to friction between the externally plated tube and the die will increase, making peeling and cracking due to peeling of the externally plated layer more likely to occur. In addition, the length of the die becomes longer, making it unsuitable for actual work. Further, if the angle exceeds 10°, the difference in elongation between the steel pipe and the alloy layer formed below the plating layer becomes large during diameter reduction, making it easy for the outer plating layer to peel or crack. Therefore, the approach part taper angle (both angles) was set to 2 to 10 degrees. The method of the present invention can be applied to external hot-dip galvanized metal pipes that produce an alloy layer between the plating layer and the base metal, and is particularly applicable to thick-walled metal pipes (600 g/m ² in the case of external galvanized steel pipes).
It is effective for metal pipes with outer surface melt-welded. Regarding the inner surface of the metal pipe, since hot-dip galvanized steel pipes used for water pipes and the like are usually plated on the inside and outside because the plating cost is rather low, it does not matter whether or not there is a plating layer on the inner surface. [Example] Next, an example will be described. Water pipe Galvanized steel pipe (JIS SGPW) (The average amount of galvanized coating is 600 g/ ^m2 or more, 550 g/m2 or more for one piece)
g/m ² or less) size 20A was cold drawn using the die of the present invention shown in FIG. 1 and the conventional die shown in FIG. Approach parts with taper angles of 2°, 4°, 8°, 10°, and 12° were attached to the tips of the dies, respectively. The total diameter reduction ratio was 5.6% (outer diameter of the raw tube - outer diameter of the product / outer diameter of the raw tube x 100% = 28.8 mm - 27.2 mm / 28.8 mm x 100%). Table 1 shows the results of investigating the incidence of plating peeling for the method of the present invention, the comparative method (die without straight section), and the conventional method (die with straight section).

〔Effect of the invention〕

As is clear from the above description, the method for cold drawing an externally plated metal tube of the present invention prevents peeling and cracking of the plated layer. Therefore, we have developed a product that can be applied to the production of resin pipe inserts and externally plated metal pipes, is inexpensive, has no peeling of the plating layer, has good roundness, and has good adhesion between the inner resin pipe and the metal pipe. Can be manufactured.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the die used in the present invention, Figure 2 A and B are schematic diagrams of the thermal expansion lining method, Figure 3 is a schematic diagram of the warm drawing method, and Figure 4 is the conventional die drawing method. Schematic diagram, Figure 5 is a cross-sectional view of the die shape of the conventional method, Figure 6 is a schematic diagram of the roll diameter reduction method, Figure 7 is a schematic diagram of the swaging diameter reduction method, and Figure 8 is a steel pipe in contact with the die. An explanatory diagram showing the diameter reduction rate just before the cutting, Figure 9 is a diagram of the diameter reduction rate ΔD/D x 100% and the tie taper angle, Figure 10 is a diagram of the drawing stress of the steel pipe depending on the die taper angle, FIG. 11 is a schematic diagram of peeling of the galvanized layer due to frictional force with the die. 1: steel pipe, 2: resin pipe, 3: heating furnace, 4: plug, 5: nozzle, 6: steam, 7: die, 8: high frequency heating coil, 9: lubricant, 10: straight part, 11: die, 11': holder, 12: roll, 13: pounded, 14: steel pipe metal, 15: galvanized layer, 16: die, 16': holder,
17: Chip, 18: Chip.

Claims (1)

[Claims] 1. In cold drawing of a metal tube with an outer surface melt-plated, which has a plating layer on the outer surface of the metal tube and an alloy layer is formed between the plating layer and the underlying metal, the die hole approach part is used as a drawing die. With one step taper,
A method for cold drawing an externally plated metal tube, characterized in that the taper angle (both angles) is 2° to 10°, and the bearing portion is an arc rotating surface.