EP0387425A1

EP0387425A1 - Apparatus for the continuous production of small-diameter welded pipes

Info

Publication number: EP0387425A1
Application number: EP89201176A
Authority: EP
Inventors: Steno Marcegaglia
Original assignee: Marcegaglia SpA
Current assignee: Marcegaglia SpA
Priority date: 1989-03-12
Filing date: 1989-03-12
Publication date: 1990-09-19
Also published as: EP0419494A1; WO1990010509A1; IN171318B

Abstract

In the apparatus for the continuous production of small diameter welded pipes, a strip (13) of metal is unwound and advanced through a degreaser (17) and passed through a forming machine (19) so as to assume a tubular configuration. The longitudinal edges (20) arranged adjacent are then joined by a welding machine (23) and finished at a bead remover (24), then the pipe is cooled before being fed to a cold drawing unit (27,29,30) for reduction of its diameter. The drawn pipe is straightened, degreased, annealed in a reducing atmosphere and is subsequently cooled before it being passed through a coating bath. The pipe is then wound on a spool, tested for air-tightness, and then coiled in reels with compact turns.

Description

Technical Field

The present invention relates to an apparatus for the continuous production of small-diameter welded pipes, as used for instance, in refrigerating circuits, motor vehicles etc.

Background Art

In order to continuously produce welded pipes in a uniform manner it is necessary to solve several technical problems, related to the uninterrupted and synchronized operation of numerous complicated devices which constitute the production apparatusal, and to provide, in some regions of the apparatus, an efficient and rapid heating and cooling of the pipe being formed, to draw said pipe and coat it with a uniform and continuous layer of molten metal, and the like. The coating of the pipe furthermore entails overcoming a whole series of difficulties related, for example, to the need to eliminate the consequences of the "drop effect", i.e. the forming, by gravity, of a bead of coating metal along the lower portion of the pipe as it exits horizontally from the molten-metal bath.
In order to obtain an efficient and competitive apparatus it is furthermore necessary to avoid the use of devices and machines which are too complicated and difficult to realize, since they would be excessively onerous.

Disclosure of the Invention

The aim of the present invention is to provide an apparatus for the continuous production of pipes with a relatively small diameter, i.e. less than 12 mm, which obtains finished pipes starting from reels of strips of mild steel, copper, aluminum and alloys thereof, with a high hourly production rate.
Within the above cited aim, an object of the present invention is to provide an apparatus which operates in a completely automatic manner and with a continuous process, to allow it to obtain, starting from reeled strips, a finished pipe which is also coiled in reels, is annealed and is in its natural state (black pipe) or hot-galvanized or hot-aluminized.
This aim, this object and other objects, which will become apparent hereinafter are achieved, according to the invention, by an apparatus for the continuous production of welded small-diameter pipes, having a plurality of devices which are arranged in sequence and comprise a device for unwinding strips from reels, a splicing-welding machine for connecting the ends of two successive strips, a strip accumulator for feeding the apparatus during the splicing of two strips, a strip degreasing unit, a strip forming machine to impart a tubular configuration with opposite and adjacent edges to said strip, a welding machine for welding the edges of the tubular configuration to one another to obtain a continuous pipe with an outer welding bead, a bead removing unit adapted to continuously perform the hot removal of the outer welding bead, a unit for calibrating the bead-free pipe to a slightly smaller diameter, a unit for cooling the bead-free pipe, a drawing machine for cold-reducing the diameter of the pipe by approximately 2.5 times, a unit for degreasing the drawn pipe, a furnace for preheating the pipe and a unit for annealing the preheated pipe and soaking it in a reducing atmosphere, a unit for cooling the tube in a reducing atmosphere, a galvanizing unit, a water cooling unit, a device for winding the pipe in reels, and a unit for testing and coiling the tested reels.

Brief description of the drawings

Further characteristics and advantages of the present invention will become mainly apparent from the following detailed description of a preferred but not exclusive embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein:

figure 1 A- B- C- D- and E- are a schematic front elevational and plan view of an apparatus according to the invention for continuously producing welded pipes;
figure 2 is a front elevational view, with sectioned parts, of a galvanizing tank or crucible;
figure 3 is a top plan view of the tank or crucible of figure 1;
figure 4 is a side view of a die illustrated in figure 5; and
figure 5 is a partially sectional view of the die of figure 4.

Ways of carrying out the invention

The apparatus according to the present invention, illustrated in the above described figures, is formed by a strip unwinding spool 10 adapted to unwind a reel 12 of strip 13 (figure 1(A)). Said strip 13 is moved so as to unwind by a strip accumulator 14 arranged after a welding machine 15, to which the strip unwound from the spool is sent. The spool 10 can comprise, for example, a fixed base 11 which has a pivot or rotation seat with a vertical axis, about which a structure for supporting two idly rotatable reel-holder spools can rotate (double spool). Each rotatable spool has a clutch apparatus (not illustrated) which can be, for example, operated pneumatically and is adapted to ensure a constant and slight tension of the strip 13, and a braking device (which is also not illustrated and may also be pneumatically operated) to allow rapid locking of each spool when the respective reel 12 is emptied. The braking device is controlled by an adapted control device (not visible in the drawing) arranged on the strip accumulator 14. A double spool thus structured allows, once it has emptied one of its reels, to rapidly restore the feeding of the strip by unwinding the other reel (new strip). The initial end of the new strip is inserted in the welding or splicing machine 15, where its end is faced by shears (not shown), moved to abut in alignment against the terminal end of the strip of the preceding reel and then automatically welded thereto. A grinder, not illustrated, removes the burr from both sides of the strip 13 along the welding bead in order to eliminate any irregularity in planarity possibly created by the welding.
The replacement of a reel 12 and the welding of the ends of two successive strips requires a certain working time, so that the strip accumulator 14 must be provided if the continuous operation of the apparatus is to be maintained and not interrupted. The accumulator 14 therefore has the purpose of constituting a reserve of strip 13 to be fed to the apparatus while splicing is performed in the welding machine 15. The accumulator 14 usually comprises a group of motorized winders for inserting the strip 13 therein, two concentric and variable-diameter roller cages and a central output pulley 16 for the strip.
The strip 13 exiting from the accumulator 14 enters a degreasing unit 17 (figure 1(B)) which may be constituted by a tank which contains a suitable solvent washing liquid, a plurality of idly rotatable transmission rollers 18, and oil-scraping pads (not illustrated). As it passes through the degreasing unit 17, the strip 13 is first scraped to remove the protective oil or grease applied thereon by the strip's manufacturer and is then immersed in the solvent washing liquid. At the output of the degreaser unit 17, the strip 13 is clean enough to be machined, i.e. to pass from a planar configuration to a three-dimensional tubular one. For this purpose the strip 13 is fed to a forming machine 19 which comprises a plurality of forming rollers and more precisely a set of input strip-guiding rollers, a certain number of vertical heads with motorized rollers, for example six heads, and a plurality of free horizontal output rollers. The forming machine 19 operates at a constant speed which determines the basic or primary speed of the entire apparatus.
The operating temperature and pressure of the forming machine are important parameters for the good operation of the apparatus; such parameters must be chosen taking into account the nature and the characteristics of the strip 13, as well as the perfect alignment of the forming machine's components with the strip's direction of advancement.
At the output of the forming machine 19 the strip 13 has assumed the shape of a pipe 20, having for example an average diameter of approximately 11 mm., with its edges arranged mutually adjacent, and is fed immediately thereafter into a welding area where presser rollers are provided to compress the edges of the pipe 20 one against the other and where said edges are mutually welded in any appropriate manner. For example, a high-frequency generator 23 (for example at approximately 400 KHz) can be provided to raise the temperature of the edges of the pipe to be welded to a temperature of 1,200-1,300°C, if the pipe is constituted by mild steel, as required to forge-weld, forming an outer welding bead.
The welding bead usually traps impurities of various kinds, such as the oxides generated during the welding process. The bead furthermore constitutes an irregularity on the surface of the pipe 20 which would give rise to considerable complications in the successive machining and treatment units of the apparatus, so it must be eliminated. For this purpose a bead removing unit 24 is provided at the output of the welding area and continuously removes the welding bead; said unit is formed by at least one tool, made for example of hard metal or ceramic, and by a plurality of guiding rollers which keep the pipe 20 stable during the bead removing operation. The removed bead 25 is automatically collected for example above the pipe 20. Said pipe now has a uniform outer surface which also allows the trouble-free passage of the pipe through the successive machining units of the apparatus.
After the bead removing unit 24 there is a plurality of motorized calibration rollers synchronized with the forming machine 19; said rollers are adapted to reduce the diameter of the bead-free pipe, for example from 11 mm to approximately 10.6 mm.
In order to avoid the forming of clots on the welded, bead-free, calibrated but still-hot joint, causing the jamming of the reduction dies in the drawing machine 27 after the bead removing unit 24, one or more series cooling tanks 26 are provided. A coolant, for example an oil based liquid emulsified in water, is sprinkled in said tank or tanks by means of a pump and filter unit 28. A heat exchanger, a settling tank for the discharged coolant, a magnetic filter for retaining the ferromagnetic particles and an air blower are furthermore advantageously provided; said blower is arranged at the output to dry the cooled pipe 20 as it leaves the last cooling tank 26 before entering the drawing machine 27.
Said drawing machine cold-reduces the welded pipe 20 e.g., from a diameter of approximately 10.6 mm to a diameter of 4.5 mm, and may comprise four motorized and independent steps through which the pipe 20 is forced to pass. Each step has a rotating and lubricated die 29 and a device with a loop and a movable arm (dandy roller) 30. The drawing machine 27 ends with a straightening unit 21 which neutralizes the curving trend of the hardened pipe 20 exiting from the steps of the drawing machine and can simply consist of two groups of appropriately aligned horizontal and vertical rollers.
The straightened pipe 20 (Figure 1 (C)) is coated with a thin layer or film of lubricating oil applied thereon during its passage through the reducing drawing machine 26; said film must be removed before said pipe enters the pre-heating furnace 32 and the annealing furnace 34 to avoid the forming of surface oxides. This is achieved by a degreasing unit 35 which may be constituted by a tank containing an appropriate solvent, for example a liquid hydrocarbon such as kerosene, in which the pipe 20 is immersed in countercurrent, by a solvent recirculation pump 36, by a solvent purifying filter and by a blower to dry the exiting pipe 20.
The furnace 32 for preheating the degreased pipe 20 may be constituted by a medium-frequency induction generator or by a gas furnace, for example powered by methane gas, having a plurality of adjustable burners with a slightly oxidizing flame, with multiple and mutually thermally independent sections; the number of said sections depends on the diameter of the pipe 20 and on its advancement speed. The preheating furnace 32 is intended to heat the pipe 20 to approximately 200°C in order to burn the oil/solvent residual which may have remained on the pipe 20 after the degreasing operation and to slightly oxidize the pipe to ensure that the surface of the pipe is adequately clean, this being an indispensable prerequisite for an adequate adhesion of the final zinc or aluminum coating to the pipe. A pyrometer (not shown) measures the temperature and drives an automatic apparatus for controlling the flame of the burners. The preheating action of the furnace 32 furthermore reduces the load of the successive annealing furnace 34.
In the furnace 34, the preheated pipe 20 arriving from the preheating furnace 32 is subject to an annealing (recrystallization) process caused by raising the temperature of the pipe for example to approximately 800°C by means of a high-frequency generator 37 in a reducing atmosphere formed for example by approximately 12% H2 and approximately 88% N2. A pyrometer (not shown) measures the temperature and drives an automatic apparatus for controlling some parameters, such as the supply of electric current to an induction coil through which the pipe passes, the advancement speed of the pipe and the like, also as a function of said pipe's diameter.
In order to reduce the hardness of the material of the pipe 20, said pipe must be kept at its annealing temperature for a preset time sufficient to allow a uniform distribution of the temperature in the pipe's entire thickness. For this purpose, the pipe 20 is passed (see figure 1 (C)) in an environment at a constant temperature or soaking section 38 formed for example by a long thermally insulated large-diameter duct within which a reducing atmosphere is provided. The soaking section 38 is followed by a cooling section 39 from which it is separated by a thermal coupling 33. In the cooling section 39 the pipe 20 is cooled, i.e. it passes from the annealing temperature to a lower temperature, for example 460°C, if the pipe is to be galvanized, by means of the forced circulation of a reducing atmosphere formed for example by a mixture of H2 and N2.
For this purpose, the cooling section 39 is formed by a large-diameter duct 40 aligned with the duct of the soaking section and is surrounded by a jacket of cooling fluid, for example water. The reducing atmosphere is blown at high speed along the duct 40 by a multistage centrifugal blower 41. At its ends, the pipe 40 is connected to vertical extensions 42 of another duct 43 which is parallel to the duct 40 and acts as return and delivery duct for the reducing gas pumped by the multistage centrifugal blower 41 in order to ensure a closed-circuit circulation of said reducing gas.
The duct 43 is in turn surrounded or contains an exchanger apparatus provided with fins with a circulation of cooling water to cool the heated reducing gas arriving from the duct 40. A mixer 45 for the reducing atmosphere ensures a uniform composition of the gas mixture, while an O2 analyzer (not shown) stops the apparatus if the percentage of oxygen in the reducing atmosphere exceeds a preset threshold value.
At the output of the cooling section, the pipe 20 is ready to be either sent to a pipe coiling device (in the case of production of so-called "black pipe"), as will be explained hereafter, or to enter the galvanizing or aluminizing unit 45 to receive a protective coating.
The galvanizing unit 45 comprises one or two tanks or crucibles 46, each containing molten zinc or aluminum 47 kept at its melting temperature, for example at approximately 460°C in the case of zinc, i.e. at the same temperature as the incoming pipe 20. The second tank or crucible, if provided, acts as reserve and feeding storage.
As is more clearly visible in figures 3, 4 and 5, each tank 46 has an entry opening 48 and an exit opening 49 for the pipe 20. Said two openings 48 and 49 may be structurally identical, and each may comprise a respective conical bush or die 50 having a frustum-like outer configuration with an also conical inner cavity 51. Longitudinal recesses 52 are provided on the outer surface of the die or bush 50 and are connected to one another by means of a larger or terminal annular cavity 53 and a smaller one 54. The die 50 is accommodated in a sleeve 56 which has a frustum-shaped internal accommodation cavity. The sleeve 56 is in turn screwed for a good portion of its length within a bush 57 which is protrudingly fixed, for example welded, to the wall of the tank 46. The sleeve 56 has, in its portion protruding from the bush 57, a lateral opening 58 which leads inward at the terminal recess 53 of the die 50 and can be externally connected to a source (not shown) of pressurized gaseous fluid (for example constituted by nitrogen). Inside the bush 57, the sleeve 56 abuts against the coupling base of said bush and has a recess 59 in which a ceramic calibrating bush 60 is accommodated; said calibrating bush extends through the coupling base of the bush 57 and through the wall of the tank until it is in contact with the molten-metal bath 47. One end of the calibrating bush 60 (the end accommodated in the sleeve 56) is advantageously flanged and abuts against the coupling base of the supporting bush 57; its other end (the one in contact with the bath 47) has a flared and rounded inlet 61. The calibrating bush 60 is internally provided with a conical opening 62 which widens starting from a little after the inlet 61 towards the flanged head of said bush, i.e. toward the sleeve 56, and is connected to the end thereof; said bush is furthermore axially aligned with the inner cavity 51 of the die 50.
As can be seen from figures 2 and 5, the inner cavity 51 of the die 50 is not entirely frustum-shaped, but it has a slight widening 63 which extends approximately from a level corresponding to that of the outer annular recess 54 until it gradually disappears a little ahead of the point of the die 50 and therefore of the calibrating bush 60.
At its rear end, the die 50 is retained within the sleeve 56 by a bush or flanged cover 64 screwed on the free end of the sleeve 56; the inner opening 65 of said cover 64 is axially aligned with the die 50 and with the bush 60.
The pipe 20 arriving from the cooling section 40 passes horizontally through the bath of molten metal 47, which is kept at a slightly higher level than that of the calibrating bushes 60 and is immersed the molten zinc or aluminum. i.e. it is covered with molten metal. As it passes through the entry opening 48, the pipe 20 is affected by a conical jet of pressurized nitrogen gas fed at the conical opening 62 of the bush 60 through the interspace between the sleeve 56 and the bush or die 50, the recesses 54, 52 and 53 and the opening 58. Besides preventing the escape of molten metal through the bush 60, the pressurized reducing gas has the specific function of protecting the pipe 20 against undesirable oxidation phenomena which would compromise the good adhesion between the pipe and the coating metal.
At the exit opening 49, the conical jet of nitrogen gas created within the bush 60 has the very important function of completing the calibrating and laminating action of the bush 60 on the thickness and on the distribution of the coating metal around the pipe, so as to thus obtain a continuous, uniform and regular coating. Once it has affected the pipe 20 arriving from the bath 47, the nitrogen gas is discharged, also because it is entrained by said pipe 20, through the internal opening 51 of the die 50 and then through the bush or cover 64, and is subject to a first slight expansion in the widening 63 and to a greater expansion in the remaining portion of the die 50.
In addition to the calibrating-laminating or dosage action performed by the conical jet, the jet of nitrogen gas also applies an initial cooling effect on the coating at the outlet 49; said coating solidifies completely in a successive cooling tank indicated by 67.
As can be seen the structure of an entry opening 48 or of an exit opening 49 is such as to allow an easy and rapid assembly and disassembly of its components 50, 56, 57, 60 and 64 both for installation and for maintenance and/or replacement. For example, in order to replace the bush 60 the level of the bath 47 is lowered inside the tank 46, then it is sufficient to unscrew the bush 64 (which is provided, for this purpose, with outer faces for the use of an adapted key), unscrew the sleeve 56 (which also has outer faces to move it by means of a key) from the supporting bush 57, replace the worn bush with a new bush 60 and then screw the assembly back into place.
If a tank or crucible 44 with a storage function is provided, the molten metal can be transferred from said tank to the galvanizing tank 46 either manually or by means of a pump.
As previously mentioned, the galvanizing unit 45 can be bypassed for the production of so-called "black pipe".
In any case, the pipe 20 then passes into a water-filled cooling tank 67 and advances toward a transmission 68 (Figure 1(D)) directed toward a winder 69. Said winder provides an adequate traction of the pipe on the line and winds the finished pipe in reels; said winder may furthermore comprise a motorized drum 70 synchronically with the die, and some turns of the pipe 20 are wound thereon to ensure the necessary grip. The pipe 20 then falls downward in loose turns and winds onto a band 71 arranged on a rotatable platform 72 which rotates synchronically with the drum 70. An automatic cutting device (not illustrated) cuts the pipe once a reel 73 of the required dimensions has been obtained.
The reels 73 are finally transported in succession to a testing device 74 to reveal any leaks in the pipe due to welding or splicing defects. The device comprises a tank containing an aqueous solution of rust-inhibiting agents, in which the reels 73 to be tested are introduced. Once each reel 73 is immersed, it is pressurized with dry air, for example at a pressure of 50 kg/sg.cm. At such a pressure, any leaks even small ones, are immediately revealed by air bubbles which rise to the surface of the solution.
Suitable raising/lowering and movement means send the tested reels to a coiling device (not illustrated) preset to wind the reels of pipe in compact turns. The coiling device may be constituted by an unwinding spool, by a straightener with motorized rollers which are synchronized with the successive winding spool, a motorized winding spool fitted with a device for aligning and forming compact turns, and a tilting holder for unloading the coiled and possibly strapped reels.
The forming speed of the pipe through the entire apparatus may be comprised between approximately 250 m/min in the case of a pipe with a diameter of 4.5 mm and approximately 130 m/min if the pipe has a diameter of 10-12 mm. The gross average hourly production rate may be of approximately 1,000 kg/hour.
The described apparatus may naturally be assigned to the production of pipes in mild steel, copper, aluminum and alloys thereof. It is furthermore suitable, as mentioned, for aluminizing, instead of galvanizing, of pipes, wires, bars and the like by appropriately increasing the temperature of the crucible 46 which should operate, in such a case, at least at approximately 630°C.
The materials employed for the various components of the apparatus and their dimensions may be varied according to the requirements. The apparatus according to the invention as described above is susceptible to numerous modifications and variations within the protective scope defined by the content of the following claims.

Claims

1. Apparatus for the continuous production of small-diameter welded pipes, having a plurality of working units which are arranged in sequence and comprise a device for unwinding strips from reels of strip, a splicing-welding machine for connecting the ends of two successive strips, a strip accumulator for feeding the apparatus during the splicing of two strips, a strip degreasing unit, a strip forming machine for imparting thereto a tubular configuration with opposite and adjacent edges, a welding machine for welding the edges of the tubular configuration to one another to obtain a continuous pipe with an outer welding bead, a bead removing unit adapted to continuously perform the hot removal of the outer welding bead, a unit for cooling the bead-free pipe, a drawing machine to cold-reduce the diameter of the pipe by approximately 2.5 times, a degreasing unit for the drawn pipe, a furnace for preheating the pipe a unit for annealing the preheated pipe and soaking it in a reducing atmosphere, a pipe cooling unit, a device for winding the pipe in reels, and a unit for testing and coiling the tested reels.

2. Apparatus according to claim 1, characterized in that it comprises a straightening unit arranged between the die and the pipe degreasing unit and adapted to eliminate the hardened pipe's trend to curve after passing through the drawing machine.

3. Apparatus according to claim 1 or 2, characterized in that it comprises a gas cooling section followed by a horizontal galvanizing or aluminizing unit between the annealing unit and the unit for cooling the pipe in water.

4. Apparatus according to claim 3, characterized in that said gas cooling section comprises an internal duct for the passage of the pipe, an external duct encircling the inner duct so that an interspace is defined between said two ducts for the passage of cooling water, a heat exchanger connected to the internal duct, and a multistage centrifugal blower for the closed-circuit recirculation of a reducing atmosphere through the internal duct and the heat exchanger.

5. Apparatus according to claim 3 or 4, characterized in that the galvanizing or aluminizing unit comprises at least one tank or crucible of molten metal, each tank or crucible having an inlet and an outlet arranged aligned and opposite at a level below the free surface of the metal bath in the crucible, each comprising a calibrating bush arranged in the wall of the crucible which is in contact with the metal bath, a conical bush arranged aligned with the calibrating bush, a sleeve with a conical opening for supporting and accommodating said conical bush, a conical interspace between said sleeve and said bush, and means for feeding a pressurized fluid to said conical interspace.

6. Apparatus according to claim 5, characterized in that said conical bush has a plurality of outer longitudinal recesses and at least one annular recess for interconnecting the longitudinal recesses, said annular recess being connected to said means for feeding a pressurized fluid.

7. Apparatus according to claim 6, characterized in that said sleeve has a terminal recess for accommodating an end of the calibrating bush and in that it furthermore comprises a ring for supporting and partially accommodating said sleeve, said ring being fixed to, or rigidly associated with, the wall of the crucible around said calibrating bush.

8. Apparatus according to claim 7, characterized in that it comprises a bush or a flanged cover which is coaxial to said bush and to said calibrating bush and can be screwed on the sleeve to lock the conical bush in place within the sleeve.