EP0429147A2 - Process and device for applying a protective coating onto a steel tube - Google Patents

Abstract

The invention relates to a process for applying a protective layer which is resistant to chemical and mechanical attack to a steel pipe, the pipe material first being galvanised and chromated and then being provided with a plastic coating in a continuous procedure by means of electrostatic powder coating, baking or curing and then being cooled, and the electrostatic powder coating, baking or curing and cooling being carried out while the pipe material is transported without support in this zone. In order to provide an improved process for the application of a protective layer to a steel pipe, by means of which extremely flexible and compressible protective layers can be produced and with which alternatively individual pipes or a continuous pipe train can be coated, it is proposed to connect to one another, before entry into the treatment station for coating with plastic, the individual pipes provided on the outer surface with a zinc and chromate layer by the electrochemical method, and to drive the resulting pipe train through the plant at a speed adjusted to the pipe dimension and, after coating of the pipes with a powder coating known per se, to fuse on or bake the layer at a temperature in the range from 250 to 350 degrees Celsius and to carry out the directly subsequent curing at a temperature lower than the baking temperature and during a minimum residence time of 10 seconds, and finally to quench the pipe train with water in a known manner. <IMAGE>

Description

The invention relates to a method for applying a chemical and mechanical attack resistant protective layer on a steel tube according to the preamble of the main claim.

From DE-OS 26 16 292 a generic method for applying a protective layer on a pipe made of steel, in particular a band welded from a steel pipe has become known. In this process, steel strip is unwound from a reel, formed into a slotted tube in a roll stand and welded into a longitudinally welded tube in a welding system. The endless pipe string produced in this way is first cleaned and then heated to a temperature above the melting point of a metal bath, for example zinc bath, and hot-dip galvanized during the continuous passage through the bath. The pipe material is then quenched in a water bath in order to then go through a calibration and straightening station. After straightening, the pipe material can either be chromated or phosphated. If this treatment is provided, the metal treatment station is preceded by a rinsing and air drying station. Before entering the treatment station for the plastic coating, the pipe string is supported by a pair of rollers and at the outlet by a pull-off auxiliary device, the support width being 21-24 m. In the area of the plastic coating itself, the pipe string is pulled without support at a speed of at least 18 - 21 m / min. The treatment station, viewed in the direction of transport, first has an induction heating station, in which the tube material is preheated to a temperature in the range between 66 and 204 degrees Celsius, the preheating temperature being dependent on the powder composition used in the powder booth. The powder booth follows the heating station, in which the powder used is electrostatically applied to the pipe surface. Here, for. B. vinyl layer thicknesses of at least 0.1 mm and at most 0.15 mm applied. The powder booth is followed by a heating or heating device in which the powder coating is baked and / or cured. The temperature for this baking is in a range between 93 - 343 degrees Celsius. After baking, a cooling station, preferably with water quenching, is used to quickly lower the temperature, so that the outer coating is not adversely affected by contact with the peeling auxiliary device immediately downstream.

This already known method is not suitable for coating small steel pipes for use as a brake or injection line, since these have to be extremely flexible and compressible. Disadvantages of this method are the strongly fluctuating thickness of the zinc layer as a result of hot-dip galvanizing and the unavoidable damage to the zinc layer by the downstream calibration and straightening station. In addition, the system takes up a lot of space due to its long span and only allows the coating of an endless pipe string.

The object of the invention is to provide an improved method for applying a protective layer on a steel tube, with which extremely flexible and compressible protective layers can be produced and with which individual tubes as well as an endless tube string can be coated.

This object is achieved with the means specified in the characterizing part of claim 1. Advantageous further developments and a device suitable for carrying out the method are specified in the subclaims.

In the method according to the invention, the zinc and chromate layer is galvanically applied to the pipe material on the outer surface, the chromating being carried out without current. Thereafter, a powder coating known per se is sprayed dry on the pipe material without prior heating. This powder coating is, for example, an acidic polyester or an epoxy resin or a mixture of the two. To burn in the powder mixture, the steel tube is also inductively heated in a manner known per se after being sprayed on by means of a coil to a temperature in the range between 250 to 350 degrees Celsius, preferably to 280 degrees Celsius. Essential to the invention is the separate curing of the melted powder layer at a temperature of at least 190 degrees Celsius and a minimum dwell time of 10 seconds, which is lower than the baking temperature. The curing preferably takes place in two different temperature stages, the first stage immediately following the baking having a temperature level of 250 degrees Celsius and the second stage a temperature level of 200 degrees Celsius and the total dwell time in both stages being approximately 20 seconds. After hardening, the water is rapidly cooled in a known manner. It is so that the plastic layer cannot be damaged or injured at any point during the duration of the procedure known to transport the pipe material without support through the treatment station. Only after the quenching process with water is the layer stable enough, for example to arrange a corresponding support roller, which is preferably made of Teflon or a steel roller provided with a Teflon coating. In contrast to the known prior art (DE-OS 2616292), individual pipes or an endless pipe string can be coated. In the case of individual pipes, these are connected to one another in a watertight manner by means of a double cone before entering the treatment station, and the pipe string thus formed is pushed through the system by a pair of double driver rollers. According to a further feature of the invention, the pairs of rollers are set somewhat obliquely, so that the pipe string rotates slightly about its longitudinal axis during the passage through the coating system. This rotation is of particular importance in the area of the powder booth and the stoving station, since this smoothes the applied protective layer. For the same purpose of uniformizing the layer thickness, it is also proposed to apply an air flow counteracting the forces of the electrostatic field to the powder particles. For this purpose, an extraction tube is provided on the underside of the powder booth, which is provided with a bore. By moving a sleeve, the hole can be more or less closed and thus the air flow in the powder booth can be regulated via the proportion of false air drawn in.

The space required for the devices installed in line between the two support rollers and the throughput speed of the pipe string must be coordinated so that, on the one hand, the holding time required for the crosslinking and hardening as a function of the temperature is maintained, and on the other hand, the deflection of the steel pipe to a measure is limited, which guarantees a contact-free passage.

With an increase in the throughput speed in the sense of an increase in the performance of the system, the distance between the two support rollers had to be increased further, since otherwise the required holding time was not kept for the curing at the same temperature. However, an increase in the distance also means a greater deflection, so that there is a risk of contact with the wall of one of the devices. Any damage or injury to the still soft layer then forms a weak point with regard to the corrosive resistance of the coated steel pipe. Alternatively, the temperature for the hardening can also be raised to a certain extent in order to compensate for the shorter holding time which results at a higher throughput speed. For reasons of space requirements and an acceptable throughput, a support-free distance in the range of 3 to 4 m is selected for the dimension range of pipes with an outer diameter ≦ 36 mm.

The layers applied to the steel tube are in the range from 12 to 15µ for the zinc layer, about 5µ for the chromate layer and 25 to 100µ, preferably between 40 to 60µ for the plastic layer, so that there is a total layer thickness in the range from 42µ to 120µ results. On the one hand, these layer thicknesses are thick enough to have the required resistance to chemical and / or mechanical attack and, on the other hand, they are thinner in comparison to the known state of the art and therefore cheaper in view of the better mechanical strength of the protective layer during bending and upsetting. In addition, the applied layer thicknesses are in the usual tolerance range for the outer pipe diameters used here, so that there are no changes for the further processor with regard to the machine settings and holding devices.

In a further development of the concept of the invention, the zinc and chromating system can be linked to the plastic coating system, the connection of the individual pipes already taking place before the entry into the galvanizing system and it is necessary that the pipe string, for example, after leaving the chromating system and before entering the plastic coating system is dried with a hot air blower. In such a combined system, the throughput speed of the pipe string through the zinc bath determines the cycle time of the overall system, since the galvanic deposition of the prescribed thickness of the zinc layer is the time-determining factor. In the case of separately operating systems, it is provided that the pipes, which already have a zinc and chromate layer, are first degreased and then dried before entering the plastic coating system so that oil or grease residues still adhering to them are not carried into the coating system. For the melting or baking of the electrostatically applied powder mixture, an induction coil is used in a known manner as a heating device. Crosslinking and curing take place, for example, in an electrically heatable muffle furnace, which is followed by a system equipped with infrared radiators. In this way, the predetermined temperature level can be set independently of each other in each heating device. The quenching device can be a water shower in a simple manner. Depending on the mass of the pipes to be coated, it is possible that the heat required for baking the powder coating is stored in the pipe to such an extent that it is sufficient for the subsequent crosslinking and curing of the applied plastic. In such a case, both heating devices or only one heating device can be switched off.

The steel tubes coated in this way can be bent without any problems with bending radii that can be in the range of 3 x Da or less without that damage to the protective layer occurs. They are also insensitive to treatment in automatic transfer systems, where they are cut, bent and flanged or widened at the ends. The main area of application for these steel pipes coated in this way is exposed hydraulic pneumatic lines in the dimensional range of 5 - 36 mm outer diameter and a wall thickness between 0.5 and 3.0 mm, preferably in the automotive sector. The plastic layers required for mechanical protection can be applied easily and evenly when using the method according to the invention.

• Figur 1 eine schematische Darstellung der für die Durchführung des Verfahrens verwendeten Vorrichtung
• Figur 2 einen Querschnitt durch die Pulverkabine
• Figur 3 einen Schnitt entlang der Linie A-A in Figur 2

The method according to the invention is explained in more detail using the following exemplary embodiment and using the drawing. Show it:

• Figure 1 is a schematic representation of the device used to carry out the method
• Figure 2 shows a cross section through the powder booth
• FIG. 3 shows a section along the line AA in FIG. 2

Steel pipes 10, for example measuring 8 x 1 mm, are electroplated with the appropriate pretreatment in systems not shown here with a 15μ thick zinc layer and then with an approximately 5μ thick chromate layer. In the case of a separate plastic coating system, the pipes 10 pretreated in this way are conveyed to a roller table 2 via a support grid 1 and the individual pipes 10 are connected to one another in the form of a double cone by means of plugs (not shown). The pipe string thus formed is first fed to a cleaning and drying system 3 via the roller table 2. This ensures that, after the galvanic coating with zinc and chrome, grease or oil residues as well as adhering dirt particles do not come through the intervening manipulations be introduced into the coating system. The tubes 10 are driven at a speed of, for example, 3 m per minute by means of a double driver 4, which is also the first support point. The roles of the double driver 4 can be set slightly obliquely, so that the tubing string is slightly rotated in the longitudinal direction. After the first support point, the pipes 10 first pass freely through the powder booth 5, where the powder coating is electrostatically blown onto the pipe surface in a manner known per se. To melt the powder coating, an induction coil 6 is connected directly downstream of the powder booth 5. In order to shield the electric field in the powder booth 5 from the magnetic field of the downstream induction coil 6, a steel sheet 12 provided with a bore is arranged between the powder booth 5 and the induction coil 6. In the induction coil 6, the tubes 10 are preferably heated to 280 degrees, so that the powder coating melts or burns in. The thickness of the applied plastic layer is at least 25µ, preferably 40-60µ. A heating device 7 is arranged for the crosslinking and curing of the plastic layer. This consists, for example, of an electrically heated muffle furnace and a system equipped with infrared radiators. When this heating device 7 passes through, the tubes 10 are kept in the muffle furnace at a temperature of approximately 250 degrees Celsius and in the infrared radiator region of approximately 200 degrees Celsius with a total cooling time of approximately 20 seconds. The water quenching 8 then follows at a certain distance from the heating device 7. The cooled plastic layer has then reached a hardness which allows the second support point to be arranged in the form of a double driver 9. The distance 11 from the first support point 4 to the second 9 is, for example, 3.5 m. This distance 11 is chosen so that for the smallest pipe to be coated, the resulting deflection reaches a value which means that the pipes 10 pass through the different ones Enables systems without contact.

In a subsequent adjustment area, not shown here, the pipe string is separated again, the plugs connecting the individual pipes 10 being removed and collected. If necessary, the ends are cut off and the finished coated tubes 10 are placed in a trough.

In Figure 2, the powder booth 5 is sketched in a cross section and in Figure 3 in a section along the line A-A. The spray head 13 for the supply of the powder coating is arranged at the top, while the pipe string 10 moves horizontally in the direction of the arrow 14 below. The lower part of the powder booth 5 is almost completely covered with an intermediate floor 15, only on the sides are slots 16, 16 ', 16 ˝, 16 ‴ provided. The intermediate floor 15 is connected via supports 17, 17 ', 17 über, 17 ‴ to the floor 18 of the powder booth. A suction pipe 19 is arranged below the intermediate floor 15 and is guided outwards on one side. In a section outside the powder booth 5, the suction pipe 19 has a transverse bore 20 which can be closed by a sleeve 21. The displaceability of the sleeve 21 is indicated by an arrow 22. The suction pipe 19 is connected in its rearward extension via a pipe 23 to a suction device, not shown here. Depending on the position of the sleeve 21, a more or less large part of the transverse bore 20 is released and correspondingly more or less false air is sucked in. In this way, the air flow generated by the suction in the powder booth 5 can be regulated in a simple manner depending on the strength of the electrostatic field.

Claims (11)

1. A method for applying a protective layer resistant to chemical and mechanical attack on a steel pipe, the pipe material first being galvanized and chromated and then provided continuously with a plastic coating by means of electrostatic powder coating, baking or curing and then cooled, the electrostatic Powder coating, baking or curing and cooling is carried out while the pipe material is being transported in this area without support,
characterized,
that the individual pipes, which are provided with a zinc and chromate layer on the outer surface by means of a galvanic process, are connected to one another before entering the treatment station for the plastic coating and the pipe string thus formed is driven through the system at a throughput speed which is matched to the pipe dimensions and after the coating of the tubes with a powder coating known per se, the layer is melted or baked at a temperature in the range from 250 to 350 degrees Celsius and the immediately subsequent curing takes place at a temperature which is lower than the baking temperature with a minimum dwell time of 10 seconds and finally in a known manner the pipe string is quenched with water. 2. The method according to claim 1,
characterized,
that the curing takes place at a temperature of greater than 190 degrees Celsius. 3. The method according to claims 1 and 2,
characterized,
that the curing takes place in two stages in terms of temperature, the first stage of curing immediately following baking having a temperature level of 250 degrees Celsius and the second stage having a temperature level of 200 degrees Celsius. 4. The method according to claim 1,
characterized,
that the individual steel pipes are connected to one another before entering the galvanizing plant and the pipe string is galvanized, chromated and provided with a plastic layer in a single pass, the pipe string being dried after leaving the zinc and chromating plant and before entering the plastic coating plant. 5. The method according to any one of the preceding claims,
characterized,
that the powder particles are subjected to an air flow counteracting the forces of the electrostatic field. 6. The method according to any one of the preceding claims,
characterized,
that the pipe string is rotated about its longitudinal axis during longitudinal transport through the plastic coating system. 7. Application of the method according to one of claims 1 to 6, for producing a protective layer on steel pipes with a diameter ≦ 36 mm for exposed hydraulic pneumatic lines, which have a uniform plastic layer thickness of at least 25µ and max. Have 100µ. 8. Device for performing the method according to claim 1,
characterized,
that seen in the direction of transport in the plastic coating system between the two supports (4,9) of the powder booth (5), the baking device (6) designed as a coil and then immediately followed by a controllable heating device (7) and finally the quenching device (8) and the plastic coating system a cleaning and drying system (3) is placed in front. 9. Device for performing the method according to claim 4,
characterized,
that a hot air blower is arranged before entering the plastic coating system. 10. The device according to claim 8,
characterized,
that the heating device (7) has an electrically heatable muffle furnace and a system equipped with infrared radiators. 11. The device according to claim 8,
characterized,
that between the powder booth (5) and the stoving device (6) is provided with a steel sheet provided with a bore (12).

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