DE3828911C1 - Process for the hot metallisation of piece goods - Google Patents

Abstract

For the hot metallisation of piece goods, a process is proposed in which the article of iron or steel is first galvanised in a normal zinc melt and then treated with a melt which can contain aluminium and/or magnesium in addition to zinc. During the process, the composition of the melt is continuously adjusted to that of a low-melting eutectic, so that a completely homogeneous metallising layer providing optimised corrosion protection is created, which layer has outstanding adhesion to the steel substrate and can be applied at low coating temperatures.

Description

The invention relates to a method for hot metal sizing of general cargo on which an iron or Steel substrate a superficial layer of a Alloy containing zinc and aluminum applied is reserved by the subject in a known manner delt, galvanized by immersion in a molten zinc and then into the melt of zinc and aluminum containing alloy is immersed.

In addition, the invention relates to a device to carry out the procedure.

It has been known for a long time, objects made of iron or to protect steel against corrosion by galvanizing. Zinc very quickly forms a relative in the atmosphere stable surface layer made of corrosion handle prevented. In addition, zinc acts as a so-called Sacrificial element, since it is in a violation of the surface layer due to its less noble quality than iron electrochemical behavior from the corrosive media is consumed before the steel is attacked.

It is known that the anti-corrosion effect of Zinc due to metallic additives such as aluminum or Aluminum and magnesium can be improved dramatically  can. It is therefore for continuous metallization of steel strip alloys have been developed, for. B. have between 5 and 55% aluminum, the Protective effect of aluminum roughly with increasing Aluminum content increases.

However, due to increasing aluminum contents zinc anodic sacrifice by passivation reduced, so that finally a remote protection effect the metallization layer for the steel base no longer exists and the steel, for example in one Violation of the protective layer, is attacked directly.

In addition, aluminum clearly has one over zinc higher price so that it is desirable that Corrosion protection properties of the metallization coats even with low aluminum contents optimize.

In contrast, piece galvanizing of steel parts must do the opposite Always chemical for continuous strip galvanizing Pretreatment to remove the surface oxides of the Steel. Then that will cleaned material by immersing it in a river telbad solution prepared for the galvanizing process. By the reaction of the flux with aluminum against zinc baths it is very difficult to get a good surface surface quality without zinc or bad to get galvanized spots. It was therefore, for. B. in DE-OS 21 46 376 or DE-OS 31 24 161 suggested the pretreated steel object too next immerse in a normal zinc melt and only in a second step with a zinc aluminum or to coat zinc aluminum magnesium alloy, the first layer of zinc applied, so to speak as a flux for the subsequent final coating  should work.

The process described changes during the Production the composition of the second bath constantly, so that the output selected by the galvanizer composition can be readjusted from time to time must in order to be able to produce a uniform product.

In the known zinc aluminum or zinc aluminum Magnesium alloys became a metallization temperature of about 600 ° C suggested. Because the galvanizing temperature in the zinc bath is approx. 450 ° C the known metallization processes and alloys a significant additional energy to heat the second bath needed. Such a high temperature also already leads to recrystallization processes in the steel structure, so it is feared that the technological-mechanical values of the steel according to the Have changed metallization. Beyond kicking massive warpage problems, for example with welded steel parts len on.

When using the known alloys arise in the cooling process after metallization according to the corresponding condition diagrams mixed crystals, which tend to be determined at the grain boundaries technical melts always available other alloy constituents, their solubility when solidifying is less to be eliminated. This doesn't just exist an inhomogeneous structure of the metallization layer many single crystals, but those at the grain boundaries deposited foreign metals lead to the dreaded Phenomenon of so-called grain boundary corrosion, which since has long been known and causes great corrosion damage can lead.

The invention has for its object a method  for the hot metallization of piece goods made of iron or steel and an apparatus for performing the method to create where the generated metallization layer even with low aluminum or aluminum Magnesium kept a homogeneous composition has optimized corrosion protection properties that has good adhesion properties to the surface and only comparatively low for the application Temperatures are required.

This problem is solved in that the together Settlement of the melt of the zinc aluminum alloy constantly around that of the low-melting eutecti is set.

At the eutectic point of multi-component systems a homogeneous melt with two solid phases in the thermody namic balance, d. that is, the solidification not in a temperature interval, but, similarly like a pure substance, with a precisely defined one Temperature takes place. No size is formed ren crystal units from the known Grain boundaries and inhomogeneity problems. Rather, the melt set in this way also solidifies a microscopic microbial structure, with the solid components in the finest distribution exist side by side and therefore an even one optimized corrosion protection effect, an excellent Appearance and good adhesion to the surface is guaranteed.

A is preferably used as the eutectic to be observed zinc-rich eutectic selected in addition to the mentioned homogeneous crystallization image the low Most melting point, so that the energy costs are minimized for heating the melt.  

The eutectic is included with zinc aluminum alloys about 4.8 weight percent aluminum with a smelt point of approx. 380 ° C. A zinc aluminum magnet Sium alloy exhibits a zinc-rich eutectic a composition of 93 percent by weight zinc, 3 Weight percent magnesium and 4 weight percent aluminum nium on. The melting point of this eutectic is included approx. 330 ° C.

By checking the melt with analytical or other known methods, e.g. B. the recording of cooling curves, the melt can constantly turn on monitors their eutectic composition and, if necessary, the missing component in alloy form or pure are given.

The automatic mixing of the melt Diffusion, boiler thermics, Braun molecular motion and the plunge and undress to metallize the objects can be mixed e.g. B. by an inductor or by blowing Purge gas to be intensified.

In the case of a zinc aluminum alloy in the second The melt pool is the first immersion in the Zinc bath formed iron zinc alloy layer on plunge into the eutectic melt in a eutectic Alloy converted, when pulling out the Object a thin, about 10 microns thick surfaces layer on top of this converted layer. Since both Layers a homogeneous eutectic composition exhibit, forms an overall uniform Metallization layer. By the described It is possible to use protective layer thicknesses to achieve between about 40 and about 100 microns, so that a Protection against corrosion lasting for decades is accomplished.

The one after the second metallization  Solidification can be caused by diffusion phenomena also because of the different steel together Settlement can occur, again compositions result from the eutectic point of the alloy lying away and again for the training of Mixed crystals would lead. It can therefore be advantageous be sticky, the metallized objects of a targeted Submit cooling treatment, taking it however, it is also possible for certain use cases through a heat treatment a conversion of the previous to achieve eutectic structure.

Becomes a zinc aluminum magnet as a eutectic alloy Corrosion protection be further improved, while the Metallization temperature can be reduced.

A device for performing the method consists preferably of two heatable boilers, by the first of which is the molten zinc and the second is the Contains alloy melt. Both boilers are preferred adjustable in terms of their temperature. At least that second boiler can be lined with ceramic.

With a ceramic lining, it is advantageous inductive heating of the boiler, for example in the form of a flanged inductor.

Through the two kettles arranged one behind the other it is of course also possible to have a normal one To carry out galvanizing, the second boiler being simple is run over.

In addition, it is with a suitable pretreatment also possible to drive over the first boiler and the metallization only in the second boiler  to take. This procedure can use very thin layers thicknesses of about 10 to 30 µm can be achieved, which in some cases, e.g. B. in the automotive industry Weight reasons are desirable.

By using the method and the corresponding device is the zinc coating therefore drove the opportunity open a variety of marketable metallization types and layer thicknesses to be able to generate.

In addition, it is also possible to face the dreaded Sandeline effect that occurs with steels containing silicon occurs and to excessively high layer thicknesses when Galvanizing leads to decrease in that the Temperature of the second melt raised so much is that at least part of the excessively thick The layer from the first galvanizing melted again and therefore normal layer thicknesses even with such steel sensitive to the sandeline effect can be.

Claims (8)

1. A process for the hot metallization of piece goods, in which a surface layer of a zinc and aluminum-containing alloy is applied to an iron or steel substrate by pretreating the object in a known manner, galvanizing it by immersion in a zinc melt and then in the melt the zinc and aluminum-containing alloy is immersed, characterized in that the composition of the melt of the zinc aluminum alloy is constantly adjusted to approximately that of the low-melting eutectic. 2. The method according to claim 1, characterized ge indicates that the setting of the Eutectic through a continuous or discounted continuous review of the composition of the Zinc aluminum alloy melt and / or one Checking the melt for eutectic condition and by adding that to achieve the eutectic Point of the necessary metal or melt Metal alloy takes place, the respective addition amount is determined by the review.   3. The method according to claim 1 and 2, characterized characterized in that the melt of the Zinc aluminum alloy with the help of mixing devices such as purging gas stones or inductors, is mixed. 4. The method according to any one of claims 1 to 3, characterized in that the metallized objects after metallization process subjected to a cooling or heat treatment will. 5. The method according to any one of claims 1 to 4, characterized in that the Alloy containing zinc and aluminum at least another light metal, preferably magnesium, contains and the eutectic composition the of a zinc-rich, low-melting eutectic of the multi-component system in question. 6. The method according to any one of claims 1 to 5, characterized in that the Temperature of the melt of the zinc aluminum or Zinc aluminum magnesium alloy to a value between between 350 ° C and 580 ° C. 7. Device for performing the method according to one of claims 1 to 6, characterized net by two heatable boilers, one of which the first the molten zinc and the second the Contains zinc aluminum smelt, both boilers Have means for temperature control and at least at least the second boiler is lined with ceramic. 8. The device according to claim 7, characterized characterized in that at least the two te boiler can be heated inductively.