CA2106763A1 - Galvanizing method and zinc alloy for use therein - Google Patents

Abstract

ABSTRACT

GALVANIZING METHOD AND ZINC ALLOY FOR USE THEREIN

A method for hot-dip galvanizing a series of individual steel objects. When all the objects to be galvanized contain at most 0.02% of silicon, a zinc bath containing at least 0.005% of silicon is used. When some of the objects contain over 0.02% 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

GALVANIZ~NG METHOD AND ZINC ALLOY FOR USE THEREIN

The present invention relates to a process for ho~ dip galvanizing a series of individual articles of steel, according to which a zinc bath is llsed which contains silicon 5 at a concentration ~hat may attain saturation.

Such a process is known from the document DE-A-3734203. This document concerns essentially the hot dip galvanizing of articles of steel containing rnore than 0.02 % (in weight) of silicon. According to said document, galvanizing such a steel 10 poses serious problems when the classic galvanizing process is used, i.e. when a bath is used that consists essentially of zinc. Indeed, the resulting zinc coatings are then both too thick and too brittle and in addition they have a grayish aspect. This is due to the fact that the iron-zinc alloy layer that forms on the surface of the steel when the latter is in contact with a classic galvanizing bath, grows linearly with the time during the entire 15 duration of the immersion, when the steel contains more than 0.02 % of silicon. l his is not the case with steels containing less silicon, as the growth rate is here proportional to the square root of the immersion time.
Still according to said document, it has been observed that the linear growth of the layer of iron-zinc alloys on steels containing more than 0.02 % of silicon is due to the pertur-20 bation of the crystal lattice of the alloys which form the layer, by silicon which hasdiffused from the steel into the layer, said perturbation facilitating the diffusion of the iron of the steel through said layer.
It is thus proposed in said document to impede the diffusion of the silicon of the steel into the layer by increasing the activiiy of the silicon in the bath.
25 To this purpose a zinc bath is used which contains silicon, preferably a zinc bath which is saturated with silicon, not only when all the articles to be galvanized have a silicon content wllich is higher than 0.02 %, but also when at least one of the articles to be galvanized has such a silicon content, i.e. when it is not excluded that a~nong the articles to be galvanized there are some the silicon content of which does not exceed 0.02 %, 30 because it has been found that the galvanizing in such a bath of steeis, the silicon content of which does not exceed 0.02 %, yields very good results which are entirely comparable with ;the results obtained with these types of steel in a cla~ssic bath.
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The applicant has found that this process presents the drawback of not answering35 suf~lciently to *e actual needs of the galvanizers.

The object of the present invention is to provide a process such as defined above, which avoids said drawback.
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To this purpose, according to the invention (a) either the series is constituted in such a way that all the articles are of a steel, the silicon content of which is loweF than or equal to 0.02 %, and tbe bath contains in S 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 bath contains in this second case at least 0.005 % of silicon as well as either 0.02 %-0.06% of aluminhlm and 0.002~.1 % of calcium and optionally lead at a concentration that may attain saturation, or 0.05~.12 % of nickel and optionally one or more of the following elements: lead at a conentration that may attain saturation, 0.001~.015 % of aluminium and 0.002-0.1 % of calcium, the rest being zinc and unavoidable impurities.

Indeed, with regard to the hot dip galvanizing of steels containing no more than0.02 % of silicon, i.e. steels free of silicon or with a low silicon content, the applicant has found that, contrary to what is suggested in the aforementioned document DE-A- :
3734203, the addition of silicon to a classic galvanizing bath, i.e. a bath essentially made of zinc, produces coatings which are markedly thinner than the "classic" coatings, i.e. the ~
coatings obtained with steels, the silicon content of which is not higher than 0.02%, in a classic galvanizing bath. Thus, for example, the coating formed on a steel iree of silicon dipped for 5 minutes in a classic bath at 450C, has a "classic" thickness of 66 ~m, whereas with the same steel and under the same conditions, a coating with a thickness of only 37~m is obtained after addition of 0.03 % of silicon to the bath. Needless to say that the zinc consumption decreases to the same extent as the thickness of the coating.
The reduction in thickness is already substantial with an addition of 0.005 % of silicon to the bath.
It should be noted here that a "classic" thickness, i.e. a thickness of about 70 ~m~ has in many technical fields become less and less necessary. For the car makers, for instance, a coating thickness of about 40 ~m would be sufflcient for many applications.
Consequently, part (a) of the process of the present invention is particularly interesting when a series of articles in steel without or with a low silicon content (up to 0.02 %) has to be galvanized and when a coating thickness inferior to the classic thickness is desired, especially a thickness which is inferior to S0 ,um.

21~67~3 With regard to the hot dip galvanizing of steels containing more than 0.02 % of silicon, the applicant has found, also contrary to what is suggested in the aforementioned document DE-A-3734203, that it is not enough to add silicon to the zinc bath to resolve 5 conveniently the problem of the excessive growth of the coatings, but that it is necessary to add, besides 0.005 % or more silicon, either 0.02~.06 % of aluminium and 0.002-0.1 % of calcium or 0.05~.12 % of nickel.
The minimum contents of aluminium (0.02%) and nickel (O.OS %) are required for obtaining a substantial reduction of the thickness of the coating. The minimum content of 10 silicon (O.OOS %) is necessary on the one hand to enhance the inhibiting effect of the aluminium and nickel on the growing rate of the coating, especially when the steel to be galvanized contains up to 0.20 % of silicon, and on tne other hand to avoid the formation of nickel-iron mattes in the bath. The minimum content of calcium (0.002 %) is necessary to obtain a coating which is free or practically free of continuity defaults. It IS appearecl difficult to avoid such defaults, even in the presence of calcium, when the aluminium content is higher than 0.06 %. A nickel content which is higher than 0.12 %
leads to the formation of undesirable Ni-Zn intermetallic compounds. The calcium which is added above the content of 0.1 % has no effect.
Taking the foregoing into account, it is clear that the galvanizing bath according to part 20 (b) of the process of the present invention, is particularly interesting, not only when the series of steel articles to be galvanized is wholly constituted of articles the silicon content of which is higher than 0,02 %, but also when a mixed series has to be galvanized which is constituted partly of articles without or with a low silicon content (up to 0.02 %) and partly of articles with a higher silicon content and, consequently, when a series of steel 25 articles has to be galvanized the composition of wbich is unknown, which is often the case in customer galvanizing.
It should be noted hcre that the use of a gaivanizing bath composed of 0.1 % of silicon, 0.2 % of aluminium, 0.1 % of nickel, 0.1 % of tin, and the balance zinc, is already known from document CN-A-85109366. This must be a ba~h saturated with silicon, 30 because the solubility of silicon in zinc at 450 C, which is the temperature normally used in galvanization, only amounts to about 0.03 %. It is not specified for what kind of steel this bath is destined. Nor is it specified if the bath is destined ~or continuous galvanizing or for galvanizing individual articles. Anyhow, the applicant has found that such a bath is not suited for the galvanizing of individual articles, as it gives assuredly 35 rise to many continuity defaults in the coating, whatever may be the silicon content of the articles to begalvanized.

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2 ~ 7 ~ 3 The applicant has also found that the presence in the bath of at least O.OOS % of silicon prevents the formation of bottom mattes (iron-zinc mattes), whatever may be the silicon content of the steel to be galvanized, which makes the process of the invention still more attractive.

It is desirable that the bath should in both cases contain at least 0.01 % of silicon, on the one hand in order to obtain an important reduction of the thickness of the coating, more especially on steels containing 0-0.20 % of silicon and on Ihe other hand to avoid with certainty the formation of bottom mattes.
It is also desirable that the bath should in both cases contain lead at a concentration which may attain saturation, for example 0.1 to 1.2 %, with a view to decrease ehe surface tension of the bath.
It is further desirable that the bath in tlie first case should contain 0.001~.015 % of aluminium and/or 0.002~.1 % of calcium, in order to protect the zinc from oxidaeion;
otherwise a yellowish pellicle is formed on the sur&ce of the bath, which fouls the galvanized articles. For the same reason, it is advantageous that the nickeliferous bath used in the second case should also contain O.OOI~.OlS % of aluminium and/or 0.002- -0.1 % of calcium.
The preferred calcium content is O.OOS~.OS %.
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It is evident that the composition of the bath will change in the course of the -operations, because the rates of consumption, by oxidation and other reactions, of the components, zinc and additives, at the operating temperature (normally about 450C) and in the presence of flux (~nC12 and NH4CI) are different and praceically the higher as tL~e 2S metal is oxidable. The additive shortage due to oxidation concerrls mainly silicon and calcium and also aluminium in the absence of calcium.
Now, the applicant has found that it is possible to maintain the composition of the bath during the galvanizing operation by cornpensating the bath consumption through addition to the bath of a zinc base alloy containing: `
(a) in the ~Irst case O. I-I .S % of silicon and optiooally one or more of the foliowing elements: 0. !-2 .2 % of lead, 0.01~.8 % of aluminium and 0.02-1 % of calcium; :
and (b) in the second case O. I-l .S % 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~.12 % of nickel and optionally one or more of the following elements: 0.1- - ~
1.2 % of lead, 0.û1~.8 % of aluminium and 0.02-1 % of calcium. , :, :

21~671~3 The ~inc base alloy to be used, preferably in ~he form of ingots, may contain according to the case:

either 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~.8 % of Al and 0.02-1 % of Ca;
or 0.1-1.5 % of c,;, 0.01~.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;
10 or 0.1-1.~ % of Si, 0.01~.8 % of Al, 0.02-1 % of Ca and 0.1-1.2 % of Pb;
or O.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~.12 9~ of Ni;
or 0. I-i .5 % of Si, 0.05~.12 % of Ni and 0.1-1.2 5~ of Pb;
or 0.1-1.5 % of Si, O.OS-Q.12 % of Ni and 0.01-0.8 % of Al;
or 0.1-1.5 % of Si, 0.0',~.12 % of Ni, 0.01~.8 æ of Al and 0.1-1.2 % of Pb;
or 0.1-1.5 % of Si, 0.05~.12 % of Ni a~td 0.02-1 % of Ca;
or 0.1-1.5 % of Si, o.oc,~. 12 % of Ni, 0.02-1 % of Ca a~td 0.1-1.2 % of Pb;
or 0.1-1.5 % of Si, 0.05~.12 % of Ni, 0.01-0.8 % of Al and 0.02-1 % of Ca;
or 0.1-1.5 % of Si, O.OS{).12 % of Ni, 0.01~,.8 % of Al, 0.02-1 % of Ca and 0.1-1.2 % of Pb, the rest being zinc and unavoidable impurities.
It is clear that the ~inc base alloy can be replaced by an equivalent in the form of at least one mother alloy and zinc, or in tbe form of at least one mother alloy and an alloy 2~ containing less additives than ~he alloy to be replaced.
Thus, for exarnple, it is possible to replace 100 kg of zinc alloy containing I % of Si and 0.1 % Al (first case) by the following equivalent:
eitner 10 kg of mother alloy wit h IC % of Si and I % of Al (prepared by powder metallurgy) and 90 kg of Zn or IQ kg of mottter of alloy with 10 % of Si (from powder metallurgy)~ I kg of mother alloy with 10 % of A! and 89 kg of Zn, or I kg of mother alloy wit'n 10 % of Al and 99 kg of alloy with l.QI % of Si.

~he zhtc base alloy defined hereabove may be used for other applications as tne one described.
: ' ,' ' ' In the preceding all percentages are in weight.

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' 2~067~3 EXAMPLE I

This example relates to the galvanizing of a steel having the following composition in % -in 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 f~rst test use is made of a zinc bath and in a second test of a zinc bath with 0.029 %
of Si. In both cases the temperature of the bath is 450~C and the immersion time 5 minutes.
The coating obtained in the first test has a thickness of 66 ~m, while in the second test it has a thickness of only 39 ~m.

This example relates ~o the galvanizing of a steel having the following composition in %
in 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 and 0.02$ Cu.
In a first test use is made of a zinc bath with 0.029 % of Si, in a second test of a 7inc bath with 0.10 ~o of Ni and in z~ third test of a zinc bath with 0.10 % of Ni and 0.029 %
of Si. In these three cases the temperature of the bath is 450C and the immersion time S
minutes.
The coating obtained in the first test has a thickness of 200 ~m, for the second test it has a thickness of 69 ~m and for the third test a thickness of only 50 ~m.
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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.

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 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.