EP0579642B1 - Galvanizing method and zinc alloy for use therein - Google Patents

Abstract

A method for hot-dip galvanizing a series of individual steel objects. When all the objects to be galvanized contain at most 0.4 % of silicon, a zinc bath containing at least 0.005 % of silicon is used. When some of the objects contain over 0.04 % of silicon, a zinc bath containing at least 0.005 % of silicon as well as either 0.02-0.06 % of aluminium and 0.002-0.1 % of Ca, or 0.05-0.12 % of nickel, is used. A zinc-based alloy for compensating the galvanizing bath's consumption is also provided. The method may be used for coating steel.

Description

The present invention relates to a method of galvanizing by dipping a series of individual steel objects, according to which a zinc bath containing silicon is used at a concentration which can reach saturation.

Such a process is known from document DE-A-3734203. This document essentially concerns the dip galvanizing of steel objects containing more than 0.02% (by weight) of silicon. According to this document, the galvanization of such a steel poses serious problems, when using the conventional galvanization process, that is to say when using a bath essentially consisting of zinc. In fact, zinc coatings are obtained in this case, which are both too thick and too fragile and which, in addition, appear grayed out. This is due to the fact that the layer made of iron-zinc alloys, which forms on the surface of the steel in contact with a conventional galvanizing bath, increases linearly as a function of time throughout the duration of the immersion, when the steel contains more than 0.02% silicon. This is not the case with steels containing less silicon, the growth rate here being proportional to the root of the immersion time.
Still according to this document, it has been observed that the linear growth of the layer of iron-zinc alloys on steels with more than 0.02% of silicon is due to the disturbance of the crystal lattice of the alloys constituting the layer, by silicon having diffused steel into the layer, this disturbance facilitating the diffusion of iron from the steel through the layer.
Consequently, it is proposed in this document to counter the diffusion of the silicon from the steel into the layer by increasing the activity of the silicon within the bath. For this purpose, a zinc bath containing silicon, preferably saturated with silicon, is used not only when all the objects to be galvanized have a silicon content of more than 0.02%, but also when at least has objects to be galvanized to such a silicon content, that is to say when it is not excluded that among the objects to be galvanized there are some whose silicon content does not exceed 0.02%, because it has It has been found that galvanizing in such a bath, steels whose silicon content does not exceed 0.02%, leads to very good results which are quite comparable to those obtained with these types of steel in a bath classic.

The Applicant has found that this known method has the drawback of insufficiently meeting the current needs of galvanizers.

The object of the present invention is to provide a method as defined above, which avoids this drawback.

• (a) ou bien on constitue ladite série de manière que tous les objets soient en un acier contenant tout au plus 0,02 % de silicium et le bain contient dans ce premier cas au moins 0,005 % de silicium et facultativement un ou plusieurs des éléments suivants: du plomb à une concentration pouvant aller jusqu'à la saturation, 0,001-0,015% d'aluminium et 0,002-0,1% de calcium, le reste étant du zinc et des impuretés inévitables;
• (b) ou bien on constitue ladite série de manière qu'au moins un des objets soit en un acier contenant plus de 0,02 % de silicium et le bain contient dans ce second cas au moins 0,005% de silicium ainsi que
  soit 0,02-0,06% d'aluminium et 0,002-0,1% de calcium et facultativement du plomb à une concentration pouvant aller jusqu'à la saturation,
  soit 0,05-0,12% de nickel et facultativement un ou plusieurs des éléments suivants : du plomb à une concentration pouvant aller jusqu'à la saturation, 0,001-0,015% d'aluminium et 0,002-0,1% de calcium,
  le reste étant du zinc et des impuretés inévitables.

To this end, according to the invention

• (a) or else the said series is constituted so that all the objects are made of a steel containing at most 0.02% of silicon and the bath contains in this first case at least 0.005% of silicon and optionally one or more of the elements following: lead at a concentration up to saturation, 0.001-0.015% aluminum and 0.002-0.1% calcium, the rest being zinc and unavoidable impurities;
• (b) or else the said series is constituted so that at least one of the objects is made of a steel containing more than 0.02% of silicon and the bath contains in this second case at least 0.005% of silicon as well as
  either 0.02-0.06% aluminum and 0.002-0.1% calcium and optionally lead at a concentration which may go as far as saturation,
  either 0.05-0.12% of nickel and optionally one or more of the following elements: lead at a concentration which may go up to saturation, 0.001-0.015% aluminum and 0.002-0.1% calcium,
  the remainder being zinc and unavoidable impurities.

Indeed, with regard to the dip galvanization of steels containing at most 0.02% silicon, that is to say steels free of or with a low silicon content, the applicant has found, on the contrary of which is suggested by the aforementioned document DE-A-3734203, that the addition of silicon to a conventional galvanizing bath, that is to say a bath consisting essentially of zinc, leads to obtaining coatings, which are considerably thinner than "conventional" coatings, that is to say the coatings obtained with steels containing at most 0.02% silicon in a conventional galvanizing bath. Thus, for example, the coating formed on a silicon-free steel immersed for 5 minutes in a conventional bath at 450 ° C has a "conventional" thickness of 66 "m, while one obtains on the same steel and in the same conditions a coating with a thickness of only 37 »m after addition of 0.03% of silicon to the bath. Needless to say, zinc consumption decreases to the same extent as the thickness of the coating. The reduction in thickness is already substantial when 0.005% of silicon is added to the bath.
It should be noted here that a "conventional" thickness, that is to say a thickness of about 70 μm, has become less and less necessary in many fields of technology. For example, for many applications, automakers would settle for a coating thickness of only about 40 ”m. Part (a) of process of the present invention is therefore particularly advantageous when it is necessary to galvanize a series of steel objects without or with a low silicon content (up to 0.02%) and when one wants to obtain a coating thickness less than 1 'classic thickness, more particularly a thickness less than 50 »m.

With regard to the dip galvanizing of steels containing more than 0.02% of silicon, the applicant has found, also contrary to what is suggested by the document DE-A-3734203 cited above, that it is not sufficient to '' add silicon to the zinc bath to adequately solve the problem of excessive growth of coatings, but must be added for this purpose, in addition to 0.005% or more of silicon, or 0.02-0.06% of aluminum and 0.002-0.1% calcium, or 0.05-0.12% nickel.
The minimum levels of aluminum (0.02%) and nickel (0.05%) are required to obtain a substantial reduction in the thickness of the coating. The minimum silicon content (0.005%) is necessary on the one hand to reinforce the inhibitory effect of aluminum and nickel on the growth of the coating, in particular when the steel to be galvanized contains up to 0.20% of silicon, and on the other hand to avoid the formation in the bath of nickel-iron mattes. The minimum calcium content (0.002%) is necessary to obtain a coating which is free or practically free from defects in the continuity of the coating. It has proven difficult to avoid these defects, even in the presence of calcium, when the aluminum content is greater than 0.06%. A nickel content greater than 0.12% leads to the formation of interfering Ni-Zn compounds. Calcium added above 0.1% plays no role.
In view of the above, it is obvious that the galvanizing bath according to part (b) of the process of the present invention is particularly advantageous, not only when one has to galvanize a series of steel objects which all have a silicon content of more than 0.02%, but also when a mixed series must be galvanized, consisting partly of free or low silicon content objects (up to 0.02%) and partially of objects with higher content silicon, and therefore, when a series of steel objects whose composition is unknown is to be galvanized, which is generally the case in custom galvanizing.
It should be noted here that it is already known from document CN-A-85109366 to use a galvanizing bath consisting of 0.1% silicon, 0.2% aluminum, 0.1% nickel, 0.1% tin, the rest being zinc. It must be a bath saturated with silicon, because the solubility of silicon in zinc at 450 ° C, i.e. the temperature normally used in galvanizing, is only about 0.03 %. It is not specified for what type of steel this bath is intended. It is also not specified whether the bath is intended for continuous galvanizing or for galvanizing individual objects. Be that as it may, the Applicant has found that such a bath is in no way suitable for the galvanization of individual objects, this bath certainly leading to multiple defects in the continuity of the coating, whatever the silicon content of the objects to be galvanized.

The Applicant has also found that the presence in the bath of at least 0.005% of silicon prevents the formation of bottom mattes (iron-zinc mattes), regardless of the silicon content of the steel to be galvanized, which makes the process of the invention even more attractive.

It is desirable that the bath contains in both cases at least 0.01% of silicon, on the one hand to obtain a considerable reduction in the coating thickness, more particularly on steels with 0-0.20% of silicon , and secondly to avoid the formation in the bottom mat bath.
It is also desirable that the bath contains in both cases lead at a concentration which can go as far as saturation, for example 0.1 to 1.2%, this in order to reduce the surface tension of the bath.
It is further desirable that the bath contains in the first case 0.001-0.015% aluminum and / or 0.002-0.1% calcium, this to protect the zinc against oxidation; otherwise a yellowish film forms on the surface of the bath, which dirty galvanized objects. For the same reason it is advantageous that the nickel bath used in the second case also contains 0.001-0.015% aluminum and / or 0.002-0.1% calcium. The preferential calcium content is 0.005-0.05%.

• (a) dans le premier cas 0,1-1,5 % de silicium et facultativement un ou plusieurs des éléments suivants : 0,1-1,2 % de plomb, 0,01-0,8 % d'aluminium et 0,02-1 % de calcium ; et
• (b) dans le second cas 0,1-1,5 % de silicium ainsi que
  soit 0,1-0,8 % d'aluminium et 0,02-1 % de calcium et facultativement 0,1-1,2 % de plomb,
  soit 0,05-0,12 % de nickel et facultativement un ou plusieurs des éléments suivants : 0,1-1,2 % de plomb, 0,01-0,8 % d'aluminium et 0,02-1 % de calcium.

L'alliage à base de zinc à mettre en oeuvre, de préférence sous la forme de lingots, peut contenir, suivant le cas ,
soit 0,1-1,5 % de Si et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si et 0,02-1 % de Ca ;
soit 0,1-1,5 % de Si, 0,01-0,8 % d'Al et 0,02-1 % de Ca ;
soit 0,1-1,5 % de Si, 0,01-0,8 % d'Al et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,02-1 % de Ca et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,01-0,8 % d'Al, 0,02-1 % de Ca et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,1-0,8 % d'Al et 0,02-1 % de Ca ;
soit 0,1-1,5 % de Si, 0,1-0,8 % d'Al, 0,02-1 % de Ca et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si et 0,05-0,12 % de Ni ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni et 0,01-0,8 % d'Al ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni, 0,01-0,8 % d'Al et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni et 0,02-1 % de Ca ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni, 0,02-1 % de Ca et 0,1-1,2 % de Pb ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni, 0,01-0,8 % d'Al et 0,02-1 % de Ca ;
soit 0,1-1,5 % de Si, 0,05-0,12 % de Ni, 0,01-0,8 % d'Al, 0,02-1 % de Ca et 0,1-1,2 % de Pb ;
le reste étant du zinc et des impuretés inévitables.
Il est évident qu'on peut substituer à l'alliage à base de zinc un équivalent sous la forme d'au moins un alliage-mère et de zinc ou sous la forme d'au moins un alliage-mère et d'un alliage moins chargé d'additifs que l'alliage à remplacer.
C'est ainsi, par exemple, qu'on pourrait utiliser comme équivalent de 100 kg d'alliage de zinc à 1 % de Si et 0,1 % d'Al (premier cas) :
   soit 10 kg d'alliage-mère à 10 % de Si et 1 % d'Al (préparé par la métallurgie des poudres) et 90 kg de Zn,
   soit 10 kg d'alliage-mère à 10 % de Si (ex métallurgie des poudres), 1 kg d'alliagemère à 10 % d'Al et 89 kg de Zn,
   soit 1 kg d'alliage-mère à 10 % d'Al et 99 kg d'alliage à 1,01 % de Si.It is obvious that the composition of the bath will change during the operations, the consumption rates, by oxidation and other reactions, of the components, zinc and additives, at the temperature of use (normally close to 450 ° C) and in the presence of flux (ZnCl₂ and NH₄Cl) being different, and practically all the higher as the metal is oxidizable. Additive deficits as a result of oxidation relate mainly to silicon and calcium as well as aluminum when calcium is absent.
However, the Applicant has found that it is possible to maintain the composition of the bath during the galvanizing operations by compensating for the consumption of bath by adding a zinc-based alloy containing to the bath.

• (a) in the first case 0.1-1.5% of silicon and optionally one or more of the following elements: 0.1-1.2% of lead, 0.01-0.8% of aluminum and 0 , 02-1% calcium; and
• (b) in the second case 0.1-1.5% of silicon as well as
  either 0.1-0.8% aluminum and 0.02-1% calcium and optionally 0.1-1.2% lead,
  or 0.05-0.12% of nickel and optionally one or more of the following elements: 0.1-1.2% of lead, 0.01-0.8% of aluminum and 0.02-1% of calcium.

The zinc-based alloy to be used, preferably in the form of ingots, may contain, depending on the case,
or 0.1-1.5% of Si and 0.1-1.2% of Pb;
or 0.1-1.5% of Si and 0.02-1% of Ca;
or 0.1-1.5% of Si, 0.01-0.8% of Al and 0.02-1% of Ca;
or 0.1-1.5% of Si, 0.01-0.8% of Al and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.02-1% of Ca and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.01-0.8% of Al, 0.02-1% of Ca and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.1-0.8% of Al and 0.02-1% of Ca;
or 0.1-1.5% of Si, 0.1-0.8% of Al, 0.02-1% of Ca and 0.1-1.2% of Pb;
or 0.1-1.5% of Si and 0.05-0.12% of Ni;
or 0.1-1.5% of Si, 0.05-0.12% of Ni and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.05-0.12% of Ni and 0.01-0.8% of Al;
or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.05-0.12% of Ni and 0.02-1% of Ca;
or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.02-1% of Ca and 0.1-1.2% of Pb;
or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al and 0.02-1% of Ca;
or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al, 0.02-1% of Ca and 0.1-1, 2% Pb;
the remainder being zinc and unavoidable impurities.
It is obvious that the zinc-based alloy may be substituted for an equivalent in the form of at least one master alloy and of zinc or in the form of at least one master alloy and of a less alloy loaded with additives as the alloy to be replaced.
Thus, for example, we could use as equivalent of 100 kg of zinc alloy with 1% Si and 0.1% Al (first case):
or 10 kg of master alloy with 10% Si and 1% Al (prepared by powder metallurgy) and 90 kg of Zn,
or 10 kg of master alloy with 10% Si (ex powder metallurgy), 1 kg of alloy with 10% Al and 89 kg of Zn,
or 1 kg of master alloy at 10% Al and 99 kg of alloy at 1.01% Si.

The zinc-based alloy defined above can be used for other applications than that described above.

In the above all percentages are by weight.

EXAMPLE 1

This example concerns the galvanization of a steel having the following composition, in% by weight: 0.050 C, 0.28 Mn, 0.012 Si, 0.009 S, 0.014 P, 0.020 Al, 0.020 Ni, 0.020 Cr and 0.025 Cu.
In a first test a zinc bath is used and in a second test a zinc bath with 0.029% Si. In both cases the temperature of the bath is 450 ° C and the immersion time 5 minutes.
In the first test, a coating with a thickness of 66 "m is obtained and in the second test, a coating with a thickness of only 39" m.

EXAMPLE 2

This example concerns the galvanization of a steel having the following composition, in% by weight: 0.144 C, 0.920 Mn, 0.092 Si, 0.010 S, 0.014 P, 0.048 Al, 0.020 Ni, 0.020 Cr, 0.025 Cu.
In a first test a zinc bath with 0.029% Si is used, in a second test a zinc bath with 0.10% Ni and in a third test a zinc bath with 0.10% Ni and 0.029% of Si. In the three tests the bath temperature is 450 ° C and the immersion time 5 minutes.
In the first test we obtain a thickness of more than 200 »m, in the second test a thickness of 69» m and in the third test a thickness of only 50 »m.

Claims (6)

1. A process for hot dip galvanizing a series of individual articles of steel, according to which a zinc bath is used which contains silicon at a concentration that may attain saturation, characterized in that

   (a) either the series is constituted in such a way that all the articles are of a steel, the silicon content of which is lower than or equal to 0.02 %, and the zinc bath contains in this first case at least 0.005 % of silicon and optionally one or more of the following elements : lead at a concentration that may attain saturation, 0.001-0.015 % of aluminium and 0.002-0.1 % of calcium, the rest being zinc and unavoidable impurities ;

   (b) or the series is constituted in such a way that at least one of the articles is of a steel, the silicon content of which is higher than 0.02 %, and the zinc bath contains in this second case at least 0.005 % of silicon as well as
   either 0.02-0.06 % of aluminium and 0.002-0.1 % of calcium and optinally lead at a concentration that may attain saturation,
   or 0.05-0.12 % of nickel and optionally one or more of the following elements : lead at a concentration that may attain saturation, 0.001-0.015 % of aluminium and 0.002-0.1 % of calcium,
   the rest being zinc and unavoidable impurities,

   and the percents being expressed by weight.
2. A process according to claim 1, characterized in that the bath contains in both cases at least 0.01 % of silicon.
3. A process according to claim 1 or 2, characterized in that the bath contains 0.1-1.2 % of lead.
4. A process according to any of the claims 1-3, characterized in that the bath contains in the second case 0.02-0.06 % of aluminium and 0.005-0.05 % of calcium.
5. A process according to any of the claims 1-4, characterized in that the composition of the bath is maintained during the galvanizing operation by compensating the bath consumption through addition to the bath
   either of a zinc base alloy containing

   (a) in the first case 0.1-1.5 % of silicon and optionally one or more of the following elements : 0.1-1.2 % of lead, 0.01-0.8 % of aluminium and 0.02-1 % of calcium ; and

   (b) in the second case 0.1-1.5 % of silicon as well as
   either 0.1-0.8 % of aluminium and 0.02-1 % of calcium and optionally 0.1-1.2 % of lead,
   or 0.05-0.12 % of nickel and optionally one of more of the following elements : 0.1-1.2 % of lead, 0.01-0.8 % of aluminium and 0.02-1 % of calcium,

   or of an equivalent of said alloy in the form of at least one mother alloy and pure zinc or in the form of at least one mother alloy and a zinc base alloy containing less additives than said alloy.
6. An ingot of zinc base alloy, especially for use in the process according to claim 5, characterized in that it contains, in percent by weight,
   either 0.1-1.5 % of Si and 0.02-1 % of Ca ;
   or 0.1-1.5 % of Si, 0.01-0.8 % of Al and 0.02-1 % of Ca ;
   or 0.1-1.5 % of Si and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.01-0.8 % of Al and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.02-1 % of Ca and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.01-0.8 % of Al, 0.02-1 % of Ca and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.1-0.8 % of Al and 0.02-1 % of Ca ;
   or 0.1-1.5 % of Si, 0.1-0.8 % of Al, 0.02-1 % of Ca and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si and 0.05-0.12 % of Ni ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni and 0.01-0.8 % of Al ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni, 0.01-0.8 % of Al and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni and 0.02-1 % of Ca ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni, 0.02-1 % of Ca and 0.1-1.2 % of Pb ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni, 0.01-0.8 % of Al and 0.02-1 % of Ca ;
   or 0.1-1.5 % of Si, 0.05-0.12 % of Ni, 0.01-0.8 % of Al, 0.02-1 % of Ca and 0.1-1.2 % of Pb.
   the rest being zinc and unavoidable impurities.