EP3096067B1 - Method for treating the surface of a gas canister - Google Patents

Description

The invention relates to a method for surface treatment of a lightweight gas cylinder from a dual-phase steel, in which the gas cylinder is blasted to a minimum weigth of 30 microns, then powder coated with paint and finally heat treated in an oven at a temperature of 150 ° C to 250 ° C. becomes.

Liquid gas cylinders, in particular propane gas bottles are widely used for commercial and private purposes, especially for the operation of grill stations. These are portable, refillable, welded steel cylinders, which have to withstand a test pressure of 30 bar and, for example, have a diameter of 300 mm with a capacity of, for example, 12.5 to 27.2 I. Such gas cylinders are on as ownership and rental bottles the market, are often refillable and require regular maintenance and testing for safety of use.

Especially with smaller filling volumes, the weight of the gas bottle makes up a significant part of the total weight. This has a disadvantageous effect on the handling and transportability.

To reduce the dead weight of LPG cylinders, a number of measures have been taken. For example, the Valve protection collar switched to plastic. At the same time, the metal handles were integrated into the plastic collar. The majority of its own weight, however, results from the metal body of the gas cylinder itself.

Here, a new steel quality remedy, the so-called dual-phase steel. Dual-phase steel or DP steel are steels whose microstructure consists of a ferritic matrix in which a predominantly martensitic second phase is incorporated. The ferritic matrix is rather soft, the martensitic second phase with a content of, for example, about 20% brings about an increased tensile strength. DP steels are significantly stronger than conventional steels and thus allow the use of thinner sheets without loss of strength in various applications, for example in the automotive industry, but also in the production of gas cylinders.

Dual phase steels are available, for example, from the companies ThyssenKrupp and ArcelorMittal under the name DP 600. WO 2011/039463 A1 shows a steel double-phase or complex-phase gas cylinder comprising two half-shells connected by welding.

Conventional gas cylinders are made, for example, from P 310 sheet steel with a thickness of 2.3 mm. For the gas cylinders made of DP 600 steel, sheets of 1.6 mm can be used, which results in a weight saving of around 30%.

Gas cylinders are made from appropriately cut sheets by cold forming (deep drawing). The DP steel has the advantage of a very good cold workability with solidification and improvement of the mechanical properties that are important for the gas cylinder. In particular, the yield point is improved compared to the starting steel.

Gas cylinders must be protected against corrosion especially on their outer side and are therefore usually painted. The painting must be sufficient to withstand a standardized salt spray test over 480 hours (corrosion protection category C4). For gas cylinders made of DP steel, however, a higher stability is required in the salt spray test over 720 h. This is not with a simple coating, as used for standard gas bottles (polyester paint coating) not represented. As a rule, this requires a higher layer thickness of the coating or even a multi-layered structure.

The corrosion is also problematic insofar as the conventional paints adhere poorly to the very smooth DP steel surfaces. This situation must also be taken into account.

The object of the invention is therefore to provide a surface treatment process, can be permanently and corrosion resistant coated with the manufactured from dual-phase steel LPG bottles.

This object is achieved by a method according to claim 1. Advantageous embodiments will be apparent from the dependent claims. Powder coatings are used in particular for the coating of coatings.

In practice, it has been shown that the coating process with a final heat treatment leads to a further improved in terms of steel quality product. Measurements have shown that a heat treatment in the range of 150 to 250 ° C (object temperature) over a defined time further improves the mechanical strength of the related steel. In particular, in the case of the steel quality DP 600, a so-called bake hardening, ie a further increase in strength, takes place in this subsequent heat treatment.

The inventive method for surface treatment and compensation of lightweight gas cylinders from dual-phase steel includes in a first step, the roughening of the surface. In this case, a minimum roughness of 30 microns, preferably 40 microns must be achieved to allow a durable coating that meets the required salt spray test. For roughening in particular, a blasting method with coarse-grained steel particles (GK-rays) is used, which detects all coming into contact with the paint parts of the bottle.

After blasting the surface, at least one lacquer layer is applied, preferably at least two. The application of a primer and then a topcoat has proven successful. For the primer in particular an epoxy powder coating in question, which has a compared to polyester powder coating significantly improved adhesion. After application, the epoxy powder coating is first gelled at elevated temperature before the topcoat layer is then applied. The topcoat is preferably a polyester powder coating. The two paint layers are then subjected to a heat treatment at 150C to 200 ° C, wherein it is important that the object (the gas cylinder) reaches this temperature. Preferably, this thermal treatment takes place at a temperature of 170 ° C to 220 ° C for 15 to 30 minutes and in particular at about 180 ° C for 20 minutes. At this temperature setting, the solidification effect has also been shown for the steel jacket of the gas cylinder.

If only one lacquer layer is applied, the layer thickness is in particular 120 to 180 μm, for two lacquer layers 40 to 60 μm for the primer, 50 to 100 μm for the cover layer.

By the invention, a lightweight gas cylinder can be made from a dual phase steel having an anticorrosive coating as described above. In this anticorrosive coating is applied to a roughened surface with a minimum weigth of 30 microns, preferably 40 microns, at least one coating layer. The gas cylinder preferably has a primer and a cover layer, the primer consisting of an epoxy lacquer layer and the cover layer being a polyester lacquer layer. Due to the heat treatment of the powder-coated lightweight gas cylinder, there is a further increase in strength of the DP steel used for the gas cylinder.

The production of a gas cylinder made of DP steel is explained in more detail below.

From a starting steel of ArcelorMittal with a sheet thickness of 1.6 mm discs are cut, which are wetted with a conventional drawing oil. The blanks are cold formed with a stamp in a drawing ring to the calotte. After forming, the dome edge is made uniform and every second dome is additionally fed in at the edge. From the calottes arise upper and lower part of the gas cylinder, wherein the bead facilitates the positioning of the components to each other and the Schweißbadsicherung is used.

Subsequently, the dome for the upper part is transported in a hole tool and prepared for the determination of the valve device. At the same time, a fit for the plastic cap and for the neck ring is formed in the upper part. The drawing oil is washed down from the calotte.

After the preparation and cleaning, the welding is done. A threaded sleeve is welded on and internally equipped with a kronic thread to receive the valve. Possibly. becomes a steel collar and steel handles are welded on, as far as a plastic collar is used for valve safety. In this case, the handles can be integrated in the plastic collar.

Following the welding of the valve seat upper part, lower part and foot ring are joined together by welding. For the foot rings, conventional steel qualities can be used here. In the case of gas cylinders with plastic collars, the production data are stamped in the foot ring, otherwise indicated on the carrying handles or the steel collar.

To prepare the paint, the bottle blanks are blasted to a minimum roughness of about 40 microns. For the radiation, a GK process is used, which works with steel particles. Subsequently, the pressure resistance of the gas cylinder, preferably with air, is tested up to a pressure of 30 bar.

For the coating, a conventional powder coating method is used, in which initially an epoxide powder layer is applied to the gas cylinders. This powder layer is gelled in an oven at a relatively low temperature to improve the application and adhesion of a topcoat layer. For the topcoat, a polyester powder layer is applied. Both layers of paint are then crosslinked in an oven at a temperature of 180 ° C (object temperature) for about 20 minutes.

Due to the heat treatment process at 180 ° C, the DP 600 material undergoes a further increase in firmness via so-called bake hardening.

In the final assembly, the gas cylinders are completed with a plastic collar, a valve, the safety sticker, if necessary a screen imprint with customer logo and a print on the dead weight and date of manufacture.

It should be understood that in some products, this example method may be deviated from. For example, gas bottles with only one coat or more than 2 layers of paint are possible. In the event that only one coat of lacquer is needed, it is preferably an epoxy coating.

Claims (7)

1. Method for the surface treatment of a lightweight-design gas canister produced using a dual-phase steel, in which method the gas canister is blasted to a minimum roughness depth of 30 µm, subsequently powder-coated with lacquer, and finally heat-treated in an oven at a temperature of from 150°C to 250°C.
2. Method according to claim 1, characterised in that the minimum roughness depth is 40 µm.
3. Method according to either claim 1 or claim 2, characterised in that the roughness depth is generated by coarse-grained blasting.
4. Method according to any of the preceding claims, characterised by the application first of a primer coat and subsequently of a top-coat lacquer.
5. Method according to claim 4, characterised in that an epoxy powder lacquer is used for the primer coat, which lacquer is gelled at an increased temperature.
6. Method according to either claim 4 or 5, characterised in that a polyester powder lacquer is used for the top-coat lacquer, which lacquer is applied to the gelled epoxy powder lacquer.
7. Method according to any of the preceding claims, characterised in that the lacquer coating is cured at a temperature of from 170°C to 220°C, for 15 to 30 minutes.