FR2827615A1 - Hot galvanization of steel sheet involves pretreatment with zinc and ammonium chloride to form an interface layer followed by galvanization in a bath of zinc containing aluminum and bismuth - Google Patents

Abstract

Method for hot galvanization of steel sheet involves pretreatment with zinc and ammonium chloride to form an interface layer followed by galvanization in a bath of zinc containing aluminum and bismuth. A method for the hot galvanization of steel sheet, notably assembled as vehicle structural components with a particular content of silicon and phosphorus, uses a pretreatment in which the sheets are plunged into a flux bath containing 300 to 350 g/liter of zinc chloride and 100 to 150 g/liter of ammonium chloride at a temperature of 38 deg C to provide an interface layer. After drying, the coated sheets are plunged into a galvanization bath containing 0.05 to 0.20 wt. % of aluminum at a temperature of 440 to 450 deg C. The galvanization bath also contains 1.0 to 1.5 wt. % of bismuth.

Description

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 The present invention relates to a process for hot-dip galvanizing steel sheets, in particular assembled sheets such as structural parts of motor vehicles, distinguished by a particular content of silicon and phosphorus.

 It is well known that, in all areas of industry such as, for example, the automobile, household appliances, the canning industry, etc., it is necessary to protect the iron, cast iron or made of steel against corrosion.

 Among the processes most commonly used to carry out this protection, mention may be made of galvanization which has been known for many years.

 This operation which consists in covering the parts to be protected with a protective layer based on zinc can be carried out either by dipping in a bath of molten zinc (or coating with a roller), or by electrozincing. The deposit, which must be of controlled thickness, may or may not be carried out on all the faces of the part. It consists of pure zinc or with the addition of elements such as, for example, nickel, tin, copper, lead, iron, cobalt or even aluminum.

 It should be noted in particular that the addition of aluminum improves the gloss of the galvanizing coatings, reduces the surface oxidation of zinc, improves the fluidity of the bath, and makes it possible to control the zinc / iron reaction which contributes to obtaining of thickness.

outre, avoir une épaisseur homogène en règle générale de l'ordre de 10 à 70 um. In fact, to give satisfaction, the galvanizing coatings must have a uniform, unblasted and shiny appearance and, in

in addition, having a homogeneous thickness as a rule of the order of 10 to 70 μm.

 Among the galvanizing baths containing aluminum, specialists have taken the habit of distinguishing: - baths with low aluminum content containing approximately 0.005% of aluminum, - baths for continuous galvanization containing approximately 0.2% aluminum, - so-called polygalva baths containing from 0.04% to 0.05% aluminum.

 However, if all the non-alloy steels and the malleable cast irons can undergo a treatment in a satisfactory manner in a galvanizing bath containing aluminum, it is not the same for certain alloy steels, in particular steel sheets having high contents in

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 silicon or phosphorus, sheets more commonly known under the name of sheets with high elastic limit: THLE. Indeed, in the case of such steels, the hot-dip galvanizing operation results in the production of coatings having a mottled and matt gray appearance which are unsatisfactory from the point of view of aesthetics; moreover, the presence in the steel to be treated of particular silicon and phosphorus contents tends to accelerate the growth of the zinc-iron alloy layers, from which it results that the coatings obtained are thicker and less resistant to shocks (risk of chipping under occasional shocks).

 It should in particular be noted that, in the case of silicon steels, the thickness of the galvanizing coating is not a linear function of the silicon content, as evidenced by the Sandelin curve shown in FIG. 1 which indicates variations in the thickness of a galvanizing layer deposited on a steel part as a function of the silicon content of this steel.

 This curve is characterized by a thickness peak designated under the name of Sandelin peak, the presence of which proves that the growth of the galvanizing layer is very rapid when approaching a silicon content equal to 0.1%.

 We can thus distinguish three classes of steels, namely hyposandelin with a silicon content of less than 0.1%, sandelin with a silicon content of around 0.1%, and hypersandelin with a silicon content. greater than 0.1%.

 However, the automobile industry, one of the essential requirements of which is corrosion resistance, tends more and more to use hypersandelin coupled with sandelin and hyposandelin in assemblies which must be capable of absorbing shocks.

 However, the thicknesses of the hot-dip galvanizing coatings of sandelin and hypersandelin steels are very high, which results in a significant increase in the weight of the parts, associated with a relative fragility of the galvanizing layers formed therefore with insufficient corrosion resistance.

mant le pic de cette courbe et en inversant celle-ci pour les hypersandelin". In accordance with document FR-2 776 672, a method has already been proposed which makes it possible to remedy these drawbacks to a certain extent by smoothing the Sandelin curve, that is to say by removing

mant the peak of this curve and inverting it for hypersandelin ".

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 According to this prior process, in a pretreatment step, the part to be treated is immersed in a pretreatment bath known as a fluxing bath containing between 300 and 350 g / l, preferably about 325 g / l of zinc chloride and between 100 and 150 g / l, preferably about 120 g / l of ammonium chloride at a temperature of the order of 38 C, so as to coat this part with an interface layer, then, after drying, the part is immersed thus coated in a galvanizing bath containing between 0.05% and 0.20%, preferably between 0.09% and 0.11% by weight of aluminum, at a temperature of 440 to 450 C.

 This process therefore consists in depositing, between the surface of the part to be treated and the galvanizing layer, an interface layer containing between 40 and 46% of zinc and between 54 and 60% of ammonium chloride. The presence of this interface layer has thus made it possible to obtain galvanizing coatings having a uniform thickness which are generally satisfactory from the point of view of adhesion and aesthetics.

 The properties of these galvanizing coatings could be improved by adding a proportion by weight of between 1.0% and 1.5% of lead to the galvanizing bath, which however has the disadvantage of having a marked toxic character.

 The hot-dip galvanizing coatings thus obtained generally have thicknesses of between 10 and 40 μm depending on the percentage of silicon contained in the steel to be treated, which may be insufficient for certain uses: in particular, automobile manufacturers recommend hot dip galvanizing coatings with a thickness of 30 pu minimum and 60-70 g maximum, the minimum thickness being the most critical for corrosion resistance.

 The object of the present invention is to remedy these drawbacks by proposing a hot-dip galvanizing method of the aforementioned type, that is to say comprising a pretreatment step in a fluxing bath based on zinc chloride and chloride d ammonium followed by a step of hot galvanizing in a galvanizing bath containing aluminum-making it possible to obtain galvanizing coatings having the advantages of those obtained by the implementation of the above-mentioned prior process from the point of view of uniformity, corrosion resistance and aesthetics, without having to use constituents that are toxic and harmful to the environment.

 According to the invention, this process is characterized in that the galvanizing bath contains between 1.0% and 1.5% by weight of bismuth.

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 The invention therefore schematically consists in replacing the lead contained in the hot-dip galvanizing bath in accordance with the aforementioned prior art by bismuth which surprisingly has the same behaviors as lead without having the ecological disadvantages, this in the same concentrations.

 According to a preferred characteristic of the invention, the galvanizing bath also contains between 0.1% and 1.0% by weight of manganese so as to obtain a coating having a uniform thickness approximately of between 30 and 70 mm.

 It has indeed been observed that, the addition of manganese makes it possible to significantly increase the thicknesses of the galvanizing coatings without altering the other properties of these coatings.

 According to the invention, the part to be treated must as a rule be immersed for approximately 30 seconds in the fluxing bath and then for approximately 2 to 6 minutes in the galvanizing bath.

 It is necessary to provide a drying step of 10 to 20 minutes at a minimum temperature of 120 C before immersing the parts leaving the fluxing bath in the hot-dip galvanizing bath.

 To verify the particularly advantageous nature of the process according to the invention compared to that described in the prior document FR-2 776 672, this process was applied to the galvanization of steel parts having high contents of silicon and phosphorus.

 FIG. 2 represents the variations as a function of the silicon content of these steels, of the thickness of the galvanizing coating obtained (in, μm) by the process in accordance with document FR-2 776 672 (curve 1) on the one hand and by the process according to the invention (curve II) on the other hand.

 This figure clearly shows that the thickness of the coating obtained in accordance with the prior art is limited to approximately 10 to cm while the process according to the invention makes it possible to obtain coatings of significantly greater thickness.

Claims (4)

 CLAIMS 1) Process for hot galvanizing steel sheets, in particular assembled sheets such as structural parts of motor vehicles distinguished by a particular silicon and phosphorus content by which in a pre-treatment step the part is immersed treat in a pretreatment bath known as a fluxing bath containing between 300 and 350 g / l preferably about 325 g / liter of zinc chloride and between 100 and 150 g / 1 preferably about 120 g / 1 of ammonium chloride to a temperature of the order of 38 ° C. so as to coat this part with an interface layer, then after drying, the part thus coated is immersed in a galvanizing bath containing preferably between 0.05% and 0.20% between 0.09% and 0.11% by weight of aluminum, at a temperature of 440 to 450 C, characterized in that the galvanizing bath contains between 1.0% and 1.5% by weight of bismuth.  ganèse so as to obtain a coating having a uniform thickness of approximately between 30 and 70 μm.

 2) Method according to claim 1, characterized in that the galvanizing bath contains between 0.1% and 1.0% by weight of man- 3) Method according to any one of claims 1 and 2, characterized in that the part to be treated is immersed in the fluxing bath for about 30 seconds.  4) Method according to any one of claims 1 to 3, characterized in that the part to be treated is immersed in the galvanizing bath for about 2 to 6 minutes.