EP0095402B1 - Method and alloy for the hot dip galvanizing of silicon steel, and galvanized articles - Google Patents

Abstract

Disclosed herein is an alloy for galvanizing objects of steel which have a concentration of silicon lower than 0.45% by weight. The alloy consists essentially of 0.5 to 1.5% by weight of lead, 0.005 to 0.2% by weight of germanium and the balance zinc. The alloy may further contain 0.001 to 0.05% by weight aluminum.

Description

The present invention relates to the galvanization of objects formed from steel, in particular of a type containing silicon at a concentration of up to 0.45% by weight. More specifically, the invention relates to a galvanizing process, galvanized objects formed by its implementation, as well as an alloy used for the implementation of this process.

The hot dip galvanizing of steels containing less than 0.04% by weight of silicon does not pose significant problems concerning the quality of the coating formed. In particular, the main properties of the coatings formed are a shiny appearance, good corrosion resistance, good adhesion to the substrate and a thickness of the order of 70 to 90 and up to 120 micrometers.

It has long been found that the galvanizing of steels containing more than 0.04% by weight of silicon posed problems. These steels are known in the art under the names of semi-quenched steels, containing up to about 0.1% of silicon, of quenched steels, the silicon content of which is between 0.1 and 0.2% , and steels with high silicon content.

When galvanizing these types of steels using the baths normally used for effervescent steels (less than 0.04% of silicon), it is noted that zinc coatings often have a gray appearance which is an index of the formation of intermetallic compounds giving a brittleness to the coating. Not only does it not have a shiny appearance, it also has poor corrosion resistance and poor adhesion to the substrate. Very often, the coatings formed have an excessive thickness, of several hundred micrometers.

Attempts have already been made to resolve these problems posed by the galvanization of steels containing silicon. For example, preheating of the objects to be galvanized was used in a bath of molten salts as well as high temperature galvanization in a ceramic crucible. The implementation of these methods is very expensive, and they do not give reproducible results. In addition, high temperature galvanization causes the formation of a very large quantity of ash.

French Patent No. 2,366,376 describes a galvanizing process suitable for treating steel having a low to medium silicon content, which can reach 0.2% by weight at most. This process requires, before the actual galvanization, a very careful treatment of the surfaces, including in particular an additional pickling operation with concentrated hydrochloric acid, in addition to the conventional processes. The alloy bath used for this galvanizing contains aluminum, in an amount up to 0.5% by weight, magnesium in an amount up to 0.1% by weight, lead in an amount between 0.005 and 2% in weight and tin in quantity up to 2% by weight.

This process, although it constitutes an important advance and that it is used on an industrial scale for the galvanization of semi-quenched and quenched silicon steels, however has certain drawbacks, in particular those related to the low fluidity of the bath. In particular, it often causes significant flow restraint and the filling of small holes formed in the parts. In addition, the drapes and drops that often form at the edge of the pieces make it necessary to take them back before marketing. Finally, this process requires the temperature to be fixed at a value at least equal to 450 ° C., which causes the formation of a large quantity of ash.

This process certainly allows the galvanization of steels containing up to 0.2% silicon with thicknesses greater than or equal to the standards in force, without having to maintain a narrow range of aluminum in the molten bath. It is not the same when we want to galvanize steels containing more than 0.2% silicon.

The invention relates to a galvanizing process which, unlike the above process, is suitable even for parts with a high silicon content of up to 0.45% by weight of silicon, in the presence of a small amount of germanium incorporated in the bath. The bath also advantageously comprises lead and possibly aluminum. It is found in fact that germanium has a significant influence on the reactions of the iron-zinc couple, in the presence or absence of aluminum. It is further noted that the combination of lead and germanium gives the galvanizing bath a very high fluidity and surface tension which allow the implementation of the galvanization at a lower temperature than that which is commonly used, in particular close to 440 ° C.

More specifically, the invention relates to a method of galvanizing steel objects, of the type which comprises the treatment of the surface of the objects to be galvanized, consisting of normal operations for treating the surfaces before galvanization, in particular degreasing, pickling , rinsing and fluxing, then immersing them in a galvanizing bath; this process includes adjusting the composition of the bath so that it contains 0.005 to 0.2% by weight of germanium, and adjusting the temperature of the bath to a value between about 440 and 460 ° C, preferably between 440 and 450 ° C.

The composition of the bath is advantageously adjusted so that the bath contains 0.5 to 1.5% by weight of lead, and preferably also 0.001 to 0.05% by weight of aluminum.

In a particularly advantageous example, the composition of the galvanizing bath is adjusted so that this bath contains, in weight percentages, 0.03 to 0.15% germanium and 0.8 to 1.2% lead, advantageously in the presence from 0.001 to 0.01% aluminum.

This adjustment of the composition of the bath is preferably carried out by adding suitable quantities of at least one master alloy.

The aforementioned process is suitable not only for steels having medium or high silicon contents but also for steels containing very little, in particular effervescent steels.

It has also been found that the implementation of the galvanizing process according to the invention allows the formation of galvanized steel objects in which the germanium has a particular characteristic distribution.

More specifically, the invention also relates to articles of galvanized steel, of the type which comprises a body of effervescent steel, semi-calmed or calmed with silicon, forming a substrate, and a galvanized coating with a laminated structure comprising, from of the substrate, a layer of intermetallic compounds 2 of iron and zinc, and an outer layer 3 having substantially a constant composition; the layer of intermetallic compounds contains germanium, the concentration of which varies in the thickness of this layer, this concentration being maximum in layer 2.

Said body of silicon quenched steel, the high silicon content of which can reach 0.45% by weight, forms a substrate, the galvanized coating of which has a stratified structure in which 0.005 to 0.2% of germanium is dispersed practically throughout its thickness. .

The thickness of the coating formed on these galvanized steel objects is advantageously between 60 and 120 micrometers.

The invention also relates to an alloy intended for the implementation of the abovementioned process, that is to say for the dip galvanization of steel objects whose silicon content is less than 0.45%, this alloy containing, in addition to zinc and in weight percentages, 0.5 to 1.5% lead, 0.005 to 0.2% germanium and 0.001 to 0.05% aluminum. Preferably, this alloy contains, in weight percentages, 0.9 to 1.2% of lead, 0.03 to 0.15% of germanium and 0.001 to 0.01% of aluminum.

• - la figure 1 est une coupe schématique très agrandie d'une partie d'un objet galvanisé selon l'invention;
• - la figure 2 est un graphique représentant, en unités arbitraires, la variation de la concentration du germanium dans une couche du revêtement d'un objet galvanisé selon l'invention.

Other characteristics and advantages of the invention will emerge more clearly from the description of particular examples of implementation and from the detailed description which follows, given with reference to the appended drawing in which:

• - Figure 1 is a very enlarged schematic section of part of a galvanized object according to the invention;
• - Figure 2 is a graph showing, in arbitrary units, the variation of the concentration of germanium in a layer of the coating of a galvanized object according to the invention.

Figure 1 is a schematic section, at a magnification of about 1000, of a part of a galvanized object according to the invention. This object comprises a substrate 1 of steel, for example of a semi-calm or calm steel. This substrate carries a coating which comprises a layer 2 of intermetallic compounds and an external layer 3 (known by the specialist under the name of eta layer) whose surface has not been shown and which has a substantially uniform composition. Layer 2 is represented with two sublayers 4 (known by the specialist under the name of delta layer) and 5 (known by the specialist under the name of dzeta layer) with different crystallographic characteristics. In addition, especially for long immersion times, we note the presence of a thin additional sub-layer 6 (known by the specialist under the gamma layer) directly in contact with the steel substrate 1. The assembly of these sublayers 4, 5 and possibly 6 forms the "layer of intermetallic compounds".

FIG. 2 represents the variation of the concentration of germanium in the layer of intermetallic compounds represented in FIG. 1. The abscissae correspond to the distances measured from the sublayer 6. Curve 7 represents the variation of the concentration of germanium in in the case of a galvanized object for a short immersion time, for example of the order of one minute at 440 ° C. In this case, the maximum of the concentration is in the sublayer 4. When the structure represented in FIG. 1 is obtained after a longer immersion time, of the order of five minutes for example, layer 2 then has a greater thickness but substantially the same constitution, and the germanium distribution curve, identified by the reference 8 in FIG. 2, indicates a maximum concentration in the sublayer 5.

It is characteristic to note, according to the invention, that this maximum is found in one of the sublayers 4 and 5, in general close to their interface, but always in the layer of intermetallic compounds 2 and at a distance from the interfaces. on the one hand with the body 1 of steel and on the other hand with the outer layer 3 of substantially uniform composition.

In the case of steels with a high silicon content, for example between 0.2 and 0.45% by weight, a structure of the type shown in FIG. 1 is observed only when the immersion time is very short. After a few minutes of immersion, the germanium diffuses and has a much more regular distribution throughout the thickness of the under-layer 2. This is, moreover, much less well defined and the stratified structure represented on Figure 1, but a polyphase structure.

This behavior of germanium is original insofar as on the one hand the technical literature does not refer to it and on the other hand this behavior is distinguished from that of other elements of addition of zinc baths, and in particular of that of aluminum. It is known that this reduces the reactivity of zinc vis-à-vis silicon steels. In particular, it has been found that the aluminum additions modify the reaction kinetics, and in particular its order. On the contrary, germanium appears to have only a very limited effect on the order of the reaction kinetics.

It seems that the action of germanium, especially combined with lead, in the weight percentages indicated, is more an adsorption of germanium at the interface of iron and zinc, and a regularization of reactions on both sides of this interface. The germanium then diffuses on both sides of this interface, as the layer 2 of intermetallic compounds is formed on both sides; diffusion can ultimately cause a nearly uniform distribution of germanium when the immersion time is long enough, especially in the presence of a large amount of silicon which accelerates the reactions. However, it should be noted that the scope of the invention is in no way limited by this interpretation.

Whatever the interpretation of the behavior of germanium, especially in the presence of lead, we find that the alloy has a fluidity and a surface tension which are excellent so that the temperature of the bath can be maintained at 440 ° C only, then that the temperature usually required is 450 to 470 ° C. The dripping of the rooms poses hardly any problems thanks to the fluidity of the bath. The coatings have exceptional shine and have excellent adhesion to the substrate.

Kinetic studies of the formation of layers of iron-zinc alloys have shown that in a bath containing aluminum between 350 and 500 grams per tonne, the presence of germanium led to an increase in the kinetics of iron-zinc reaction for immersion times during galvanizing greater than five minutes, in particular on steels containing more than 0.2% of silicon.

In the case of a zinc containing 400 to 500 grams per tonne of aluminum, the thickness of zinc coatings on steels containing more than 0.2% is generally less than 70 micrometers and almost independent of the residence time in the zinc bath and commonly used galvanizing temperatures.

The presence of germanium in increasing quantity in the zinc bath containing 400 to 500 grams per ton of aluminum increases the thickness of the zinc coatings obtained on steels to more than 0.2% of silicon.

This addition of germanium therefore exerts a beneficial effect on the galvanization of steels with more than 0.2% of silicon in a bath of zinc or of alloy containing aluminum.

The surface tension and fluidity of this zinc containing lead and germanium allows a reduction in the galvanizing temperature of the order of ten degrees without altering the productivity of the galvanizer which, in the case of a conventional zinc without germanium is in generally limited to a temperature of 450 ° C when it galvanizes loads of significant weight compared to the volume of zinc contained in the tank.

The possibility of working at a lower temperature in the presence of germanium in a bath containing lead is favorable to the galvanization of steels containing silicon. Indeed, at 440 ° C, the kinetics of iron-zinc reaction are lower than those obtained at 450 ° C on steels containing silicon.

The combination of the elements Pb, Ge, AI in the alloy makes it possible to galvanize practically any type of steel at temperatures from 440 to 450 ° C approximately with thicknesses between 70 and 200 micrometers.

When galvanizing steels at 440 ° C, the aluminum content should be between 250 and 350 grams per tonne while at 450 ° C, aluminum should be between 400 and 500 grams per tonne.

The following nonlimiting exemplary embodiments of the present invention are intended to enable specialists to easily determine the operating conditions which should be used in each particular case.

In these examples, 100 × 100 millimeter test pieces having a thickness of 3 to 5 millimeters are used, formed from three different grades of steel, steel A being of effervescent type, steel B being a steel quenched with silicon, and steel C being a steel with a high silicon content. More precisely, the designation of these steels is as follows:

All the test pieces undergo, before the actual galvanization, that is to say the immersion in the bath of molten zinc, a conventional surface treatment. This treatment first comprises a degreasing at 70 ° C in an aqueous solution at 50 grams per liter of NaOH and 50 grams per liter of Na ₂ C0 ₃ . The test pieces are then rinsed with running water at room temperature.

They then undergo pickling in 50% commercial hydrochloric acid, in the presence of a well-known inhibitor "Socospar" C 51, for 30 to 45 minutes. Then, the test pieces are rinsed with running water, at room temperature.

The next treatment operation is fluxing at 80 ° C, in a solution of 200 grams per liter of zinc chloride and 200 grams per liter of NH ₄ CI. The test pieces are then dried in an oven at 100 ° C and are ready to be used for galvanizing.

The galvanization is carried out by immersing the test pieces, for the times indicated, in a bath contained in a 50 kilogram crucible, with a capacity of one ton. The temperature of the galvanizing bath is indicated for each example and it is adjusted to plus or minus 2 ° C.

The composition of the galvanizing baths is indicated in the table below.

The bath used in Examples 1 and 2 only contains traces of germanium and therefore does not allow the implementation of the invention. It is a classic bath with composition Z 7, French standard A 53-101. This standard specifies in particular the following composition:

The following standards define qualities of zinc close to zinc Z 7:

The results of the examples are shown in the following table.

It is first noted that, in the conventional bath (examples 1 and 2) the coating has a suitable thickness and appearance only in the case of effervescent steels. In the case of steels containing silicon, the coating has a large thickness which continues to grow even after long periods of immersion, and it has a gray or mottled appearance. These coatings are much too thick when the immersion time is long.

On the contrary, all the coatings obtained in Examples 3, 4 and 5, that is to say made according to the invention, have a shiny appearance. Their thicknesses are suitable for conventional galvanizing coatings. However, in Example 4, the thicknesses obtained with steels B and C are excessive for the only immersion time tested. This too great thickness is attributed to a too high reactivity, due to the temperature of 450 ° C. which proves to be too high in the case of these test conditions. Consequently, it is preferable that the temperature of the bath be reduced to 440 ° C. Example 5 differs from Example 4 only in this reduction in temperature. It is then noted that, even in the case of calm steels and with a high silicon content, the coatings obtained have a thickness which is perfectly suitable under industrial operating conditions.

Example 6

Galvanizing a new steel containing 0.38% silicon at 450 ° C for an immersion time of 5 minutes in a bath containing 1% lead and variable aluminum and germanium contents gave the following results:

The same steel galvanized at 450 ° C for 10 minutes in a bath containing 1% lead and varying aluminum and germanium contents gave the following results:

• Enfin, l'aspect des objets galvanisés est excellent car non seulement le revêtement est brillant, mais encore ce dernier ne forme pas de gouttes ou de drapeaux le long des bords des objets.

Thus, these results show the main advantages of the invention. More specifically, the incorporation of germanium, advantageously in the presence of lead, allows the formation of completely satisfactory coatings and corresponding to the conditions set by the users, even in the case of steels having a high silicon content, up to 0 , 45%. Then, this coating is obtained without using a particularly elaborate treatment since it implements the only operations usually used for the galvanization of steels without silicon. Then, the bath temperature is advantageously reduced to 440 ° C only. Under these conditions, the quantity of ash formed is reduced and the yield of the bath is increased:

• Finally, the appearance of galvanized objects is excellent because not only is the coating shiny, but the latter also does not form drops or flags along the edges of the objects.

Claims (17)

1. Process for galvanising steel objects, of a type which comprises:

- surface treatment of the objects to be galvanised, consisting of normal surface treatment before galvanisation, then

- their immersion in a galvanising bath,

characterised in that it comprises

- adjustment of the composition of the bath in order that the latter contains 0.006 to 0.2% in weight of germanium, and adjustment of the temperature of the bath to a value of between approximately 440 and 460°C.

2. Process according to claim 1, characterised in that it comprises in addition adjustment of the composition of the bath in such a way that the bath contains 0.5 to 1.5% in weight of lead.

3. Process according to claim 2, characterised in that it comprises in addition adjustment of the composition of the bath in such a way that the bath contains 0.001 to 0.05% in weight of aluminium.

4. Process according to any one of the above claims, characterised in that the adjustment of the composition of the galvanising bath is effected in such a way that the latter contains, in weight percentages, 0.03 to 0.15% of germanium and 0.8 to 1.2% of lead and 0.001 to 0.05% in weight of aluminium.

5. Process according to claim 4, characterised in that it comprises in addition adjustment of the composition of the bath so that the latter contains 0.001 to 0.01% in weight of aluminium.

6. Process according to any one of the above claims, characterised in that the objects are constituted of a steel chosen from a group which comprises unkilled, semi-killed and killed steels and steels with a high silicon content which may reach 0.45% in weight.

7. Process according to any one of the above claims, characterised in that the adjustment of the temperature of the bath to a value of between approximately 440 and 450°C.

8. Process according to any one of the above claims, characterised in that the adjustment of the composition of the bath is effected by the introduction of suitable quantities of at least one master alloy.

9. Galvanised steel object, of a type which comprises:

- a body made of unkilled, semi-killed, silicon-killed steel or steel with a high silicon content, forming a substratum, and

- a galvanisation coating, having a laminated structure and comprising-apart from the substratum-a layer 2 of intermetallic alloys of iron and zinc and an external layer 3 having a uniform structure,

characterised in that the layer of intermetallic alloys contains germanium, the concentration of which varies throughout the thickness of this layer, this concentration being maximum in layer 2.

10. Galvanised steel object according to claim 9, characterised by the fact that the body of silicon-killed steel or steel with a high silicon content which may reach 0.45% in weight and by the fact that the galvanisation coating, having a laminated structure, contains 0.005 to 0.2% germanium dispersed virtually throughout its thickness.

11. Galvanised steel object, characterised in that it is prepared by the implementation of a process according to claims 1, 3 and 4, taken separately, and in that the thickness of the coating formed is between 70 and 200 micrometres.

12. Alloy intended for the hot-dip galvanisation of steel objects, the silicon content of which is below 0.45% in weight, characterised by the fact that it contains, in weight percentages, 0.5 to 1.5% of lead, 0.005 to 0.2% of germanium and optionally 0.001 to 0.05% of aluminium, the remainder of the alloy being constituted of a zinc corresponding to one of the standards selected from the group Z 7, HZ 2, UNE 37301, ASTM 86, JIS H 2107, UNIMET 111/025, DIN 1706 and RS 3436.

13. Alloy according to claim 12, characterised in that it contains, in weight percentages, 0.9 to 1.2% of lead and 0.03 to 0.15% of germanium.

14. Alloy according to one of claims 12 and 13, characterised in that it contains 0.001 to 0.01% of aluminium.

15. Use for hot-dip galvanisation of an alloy containing, in weight percentages, 0.5 to 1.5% of lead, 0.005 to 0.2% of germanium and 0.000 (sic) to 0.05% of aluminium, the remainder of the alloy being constituted of a zinc corresponding to one of the selected standards in the group Z 7, HZ 2, UNE 37301, ASTM 86, JIS H 2107, UNIMET 111/025, DIN 1706 and RS 3436.

16. Use for hot-dip galvanisation according to claim 15, characterised by the fact that the said alloy contains, in weight percentages, 0.9 to 1.2% of lead and 0.03 to 0.15% of germanium.

17. Use for hot-dip galvanisation according to claim 16, characterised by the fact that the said alloy contains 0.001 to 0.01% of aluminium.

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