EP0011547B1 - Process and apparatus for producing a zinc plate or strip lightly coated on one or both sides, and product obtained by this process - Google Patents

Description

The present invention relates to the manufacture of stampable galvanized steel sheets or strips.

It has been sought for a long time to produce galvanized steel sheets or strips whose coating is very adherent, compact and hard, and in recent years there has been an increasing demand for thin galvanized sheets with poor coating for cold forming.

Although great strides have been made in recent years in the manufacture of one-sided galvanized steel sheet or strip and improvements have been made to the continuous treatment processes for zinc-based coatings aimed at improving the adhesion of the coating to steel, reducing the thickness of the zinc layer and improving the behavior of the product during successive operations of using the product, that is to say during cold forming , welding, surface treatment before painting, painting, and the like, we have not yet succeeded in obtaining a product which together has excellent corrosion resistance, excellent weldability, supporting a deep drawing while ensuring very good adhesion of the paint.

Prior art:

The continuous hot-dip galvanizing processes have been perfected by techniques which have improved adhesion, such as the use of a non-oxidizing preheating furnace, and have made it possible to reduce and regularize the thickness of the coating. zinc, for example by pneumatic wiping.

The products thus produced are characterized by a coating of pure zinc, the thickness of which has so far been able to be reduced below 3 to 5 μm, the total weight of the coating always being greater than 20 g / m ² and per side.

Such coatings are susceptible to galling in cold forming if lubrication is not abundant. They require special settings for spot or seam welding, more frequent electrode maintenance, and the extraction of zinc fumes at the welding stations. The surface treatments before painting must be adapted to the product, as well as the painting techniques and the products used if we want to obtain a surface appearance after painting as bright as for bare steel sheet, and correct adhesion of the paint.

These particular conditions of implementation have slowed the development of the use of galvanized sheet continuously in industries such as automobile construction or household appliances.

According to a complementary process used in particular to obtain galvanized products having differential coatings, as described in particular in French patents 1 279 624 and US 3 260 577, part of the zinc is removed at the outlet of the bath and the sheet or strip to combine all the remaining zinc with the iron.

The products thus treated have improved stamping properties, are more easily weldable than the starting galvanized sheet, and do not require any special treatment before painting.

The alloy coating formed by heat treatment is adherent to steel, hard, but has a tendency to foul tools.

Although the behavior of such products is good for most cold forming operations, a tendency to powder the coating is observed for areas that are severely deformed in shrinkage or subjected to very high pressures. Until now, it has not been possible to reduce the thickness of such coatings below 6 to 7 μm, or 40 g / m ² , and per side, and the zinc fumes formed must be sucked at the level of the welding station.

The corrosion resistance of this product, after phosphating and painting, is comparable to that of phosphated galvanized sheet painted in the best conditions, the adhesion of the paint tested by the impact test is poor due to the brittleness of the coating of underlying alloy, and for equal thickness, the gloss of the paint is less than for bare steel because of the high roughness of the coating of underlying alloy.

French Patent No. 2,351,187 describes a process for the manufacture of a strip of ferrous metal coated with zinc on one side, by hot-dipping, according to which the side of the strip is wrung by means of powerful jets of gas. metal to be removed from any zinc coating, until the zinc layer is lowered to around 10 g / m ² and the lean coating side is then heated in an oven to form a brittle intermetallic layer. The coating thus obtained can finally be completely removed by cold brushing, for example at the time of use.

The product thus obtained is a sheet or strip of steel galvanized on one side and completely free of free or alloyed zinc on its other side.

It therefore has both the disadvantages of zinc coating on the gaivanized side and unprotected steel on the side stripped of its coating.

French Patent No. 1,388,422 relates to a process of continuous galvanizing of steel strip in which almost all of the free zinc is removed by wiping from the surface of the strip by passing it almost immediately upon leaving it. of the galvanizing bath between wiping means, that is to say when the zinc is still liquid.

U.S. Patent No. 3,383,189 relates to a process to prevent the appearance of oxide on a galvanized strip in a zinc bath containing an aluminum additive.

According to this process, the surface of the strip is scanned to remove the unalloyed zinc, this operation being carried out at a temperature lower than that of the bath, but at which the zinc not alloyed with the support is completely liquid, which makes it possible to obtain on the strip a thin layer of iron-zinc alloy.

The products obtained by means of these known processes do not therefore have all of the above-mentioned properties combined and those of them which they exhibit have not always been found to be sufficient.

The object of the invention is therefore to provide a process for the manufacture of hot-dip galvanized steel sheet or strip, with an extremely weak coating on one or two faces, which has properties far superior to those of galvanized sheet or strip one or two faces obtained by the methods of the prior art.

The object of the invention is also to produce a hot-dip galvanized sheet metal sheet or strip having all of the above-mentioned properties combined and which can also be implemented by means of the techniques used for steel without requiring any modification of the equipment, products or manufacturing sequences.

The invention uses the well known fact that when zinc is melted, it has a temporary state of very high friability in a temperature range which immediately precedes its melting point. Conversely, passing from the liquid state to the solid state, the zinc crosses this same state of great friability soon after its complete solidification.

It relates to a process for the manufacture of hot-dip galvanized sheet steel sheet or strip, with extremely low coating on at least one face, in which the sheet or strip is passed continuously in a conventional galvanizing bath. hot soaked and of the type in which a portion of the liquid zinc is scraped or wiped off on the sheet or strip at the end of the bath, characterized in that the distance is determined as a function of environmental conditions or others, relative to the surface of the bath, at which the temperature of the zinc coating on the sheet or strip is in the range in which the zinc goes through a liquid-solid transient state which is characterized by its friability, and this point a mechanical action on at least one of the surfaces of the sheet or strip to completely tear off all of the unalloyed zinc, leaving only a thin layer of iron-zinc alloy.

Advantageously, said mechanical action is a brushing carried out by means of a cylindrical wire brush whose temperature is controlled.

The invention also relates to a product consisting of a steel sheet or strip obtained by the method defined above, characterized in that it comprises on said face, or on each of said brushed faces, a thin coating of iron-zinc alloy having a thickness between 0.3 µm and 4 µm, representing a quantity of alloy between 2 and 30 g _{/ m2.}

Preferably, this thickness is between 1 and 2 μm, representing an amount of alloy between 7 and 15 g / m ² .

The invention also relates to a device for implementing the above method, characterized in that it comprises a support located on the path of the strip above the surface of the bath and carrying mechanical means constituted by at least one rotary cylindrical metallic brush adjustable at controlled temperature, adapted to tear off the zinc on at least one face of the strip or of the sheet, and at least one opposing support roller in contact with the face or the opposite faces of the sheet or the strip, and means for independently adjusting the positions of the brush or brushes and of the roller (s) parallel and perpendicular to the path of the sheet or strip and relative to the surface of the bath.

• - possibilité de mise en oeuvre par pliage; emboutissage, étirage, sans aucun arrachement ou poudrage du revêtement ;
• - la tôle peut être soudée par points, à la molette, par des procédés MIG, MAG ou TIG ou soudobrasée sans modification des réglages utilisés pour l'acier doux. L'usure des électrodes est normale, et le dégagement de vapeur de zinc est très réduit ;
• - la tôle peut être traitée avant mise en peinture dans les mêmes conditions que l'acier doux et elle peut être peinte par toute technique utilisée pour l'acier doux, et en utilisant les mêmes peintures primaires, sealer et finition sans modification des conditions d'application (au trempé, par électrophorèse, cataphorèse ou anaphorèse au pistolet, électrostatique, etc...) ;
• - l'aspect de surface des tôles peintes est le même que celui obtenu avec de l'acier nu de même rugosité ;
• - parfaite adhérence au choc de la peinture dans tous les tests normalisés ;
• - la tenue à la corrosion de la tôle phosphatée peinte a été étudiée par l'essai au brouillard salin sur éprouvette entaillée en croix selon la norme NF x 41002. Elle peut être caractérisée par une absence totale de cloquage, rouille, ou décollement de la peinture à l'arrachement au scotch le long de l'entaille après 1 000 heures ;
• - la tenue à la corrosion d'ensembles formés de tôles suivant l'invention soudées avec l'acier nu, phosphatés et peints, n'est pas. altérée au niveau des jonctions ;
• - la tenue à la corrosion de la tôle phosphatée peinte n'est pas modifiée par un étirage ou emboutissage préalable au traitement de surface, tel qu'essai erischen profondeur 6 mm ou essai d'étirage sur jonc avec un taux de déformation de 40%.

The tests carried out on such a steel strip obtained by means of this process and of this device and thus treated on its two faces showed that this product had the following properties:

• - possibility of implementation by folding; stamping, drawing, without any tearing or dusting of the coating;
• - the sheet can be spot welded, with a seam wheel, by MIG, MAG or TIG processes or braze-welded without modifying the settings used for mild steel. The wear of the electrodes is normal, and the evolution of zinc vapor is very reduced;
• - the sheet can be treated before painting under the same conditions as mild steel and it can be painted by any technique used for mild steel, and using the same primary paints, sealer and finishing without modifying the conditions d 'application (by soaking, by electrophoresis, cataphoresis or anaphoresis with a gun, electrostatic, etc ...);
• - The surface appearance of the painted sheets is the same as that obtained with bare steel of the same roughness;
• - perfect adhesion to paint shock in all standardized tests;
• - the corrosion resistance of the painted phosphated sheet was studied by the salt spray test on a test piece notched in cross according to standard NF x 41002. It can be characterized by a total absence of blistering, rust, or detachment of the tear off paint along the cut after 1000 hours;
• - The corrosion resistance of assemblies formed of sheets according to the invention welded with bare steel, phosphated and painted, is not. altered at the junctions;
• - the corrosion resistance of the phosphated sheet painted is not modified by stretching or stamping prior to surface treatment, such as erischen test depth 6 mm or stretching test on rod with a deformation rate of 40%.

• la Figure 1 est une vue schématique en élévation d'une installation de galvanisation en continu classique comportant un dispositif suivant l'invention, monté pour agir sur une seule face d'une tôle ou d'un feuillard ;
• la Figure 2 est une vue schématique partielle d'une variante d'un dispositif d'essorage ;
• la Figure 3 est une vue schématique analogue à celle de la Fig. 1 d'une variante d'agencement de la brosse et de deux rouleaux d'appui ;
• la Figure 4 est une vue en élévation de face d'un dispositif à brosse réglable suivant l'invention ;
• la Figure 5 est une vue en coupe suivant la ligne 5-5 de la Fig. 4 ;
• la Figure 6 est une vue schématique en élévation latérale et en coupe d'un dispositif agissant sur deux faces.

Other characteristics and advantages of the invention will become apparent during the description which follows, made with reference to the appended drawing, given solely by way of example and in which:

• Figure 1 is a schematic elevational view of a conventional continuous galvanizing installation comprising a device according to the invention, mounted to act on one side of a sheet or strip;
• Figure 2 is a partial schematic view of a variant of a wringing device;
• FIG. 3 is a schematic view similar to that of FIG. 1 of a variant arrangement of the brush and two support rollers;
• Figure 4 is a front elevational view of an adjustable brush device according to the invention;
• Figure 5 is a sectional view along line 5-5 of FIG. 4;
• Figure 6 is a schematic side elevational view in section of a device acting on two sides.

Referring to the drawing and more particularly to Figs. 1 to 3, the device according to the invention is shown in its application to a conventional installation for continuous hot-dip galvanizing and suitable for treating a single face of a sheet or strip, in order to obtain a product having on one of its faces a normal zinc coating layer. and on its other face a layer of iron-zinc alloy totally devoid of unalloyed zinc according to the invention.

The installation comprises, as known per se a tank 1 containing a galvanizing bath 2 which is continuously traversed by a steel strip 3 to be galvanized which passes in known manner around a roller 4 so that the galvanized strip 5 comes out of this bath from bottom to top, roughly vertically.

At the exit of the bath the galvanized strip 5 is first of all wrung out by means of air jets or other gas directed in opposite directions on the opposite faces of the strip by nozzles 6 which in known manner sweep the surface of the strip coated with molten zinc to remove excess zinc. As a variant, this spinning can be ensured by means of two cylinders 6 ′ in a manner also known, as shown in FIG. 2.

After leaving the bath, the galvanized strip 5 passes through a mechanical device according to the invention for removing one of the layers of unalloyed zinc which is mounted in an adjustable manner above the surface of the bath. This device comprises a circular metallic brush 7 which is arranged so as to rotate in contact with one of the faces of the strip 5.

The device also comprises an opposing cylinder 8 for supporting the brush adapted to be in contact with the opposite face of the strip 5, in line with the cylindrical brush 7 or offset with respect thereto.

The brush 7 is rotated in one direction, preferably in the direction opposite to the direction of travel of the strip, and the roller 8 which provides support for the brush 7 can be either mounted idly and be driven as a result of its contact with the strap, or be itself rotated in the appropriate direction.

It is essential that the action exerted by the cylindrical brush 7 be applied to the strip, according to the method of the invention, shortly after the solidification of the zinc on the strip and precisely when the zinc is in the state liquid-solid transient characterized by its friability, so as to be easily detached from the surface of the strip while avoiding fouling of the brush.

Of course, the distance from the galvanizing bath surface at which the zinc coating deposited on the strip begins to exhibit this state of friability, is a function of a number of variable factors. Consequently this distance can vary and to allow always to exert the desired mechanical action on the face of the strip in the appropriate temperature range of the latter, the brush 7 and the support roller 8 are mounted independently adjustable according to the path of the strip. It is thus possible to move the brush 7 and its opposing roller 8 along this path as shown for example in 7 ^* 8 'and 7 "8" in FIG. 1.

The effectiveness of the mechanical action of the brush 7 on the zinc deposited on the strip 5 depends in particular on the characteristics of this brush, that is to say on its diameter, its hardness and the arrangement and the nature of the metal wires forming the brush, as well as its speed of rotation and the pressure it exerts on the surface of the strip. The support roller 8 can have a smooth surface or a uniform but variable roughness or an engraved pattern as known per se in the art in order to give the face comprising the conventional galvanized coating an identical appearance to the surface of the roller in order to allow it is up to the user to easily differentiate the faces of the strip or of the sheet.

Fig. 3 shows a variant of the arrangement of the brush 17 in which two support rollers 18 are provided which are arranged on the other side of the strip on either side of the axis of the brush 17, the action on the strip is then partly a function of the tension thereof.

In this variant also the height of the device can be adjusted relative to the surface of the galvanizing bath as shown in 17 'and 18'. Furthermore, the relative positions of the brush 17 and the rollers 18 can also be adjusted as shown in 17a and 17b in FIG. 3.

There is shown in Figs. 4 and 5 an embodiment of a device for the implementation of the method according to the invention, for treating a single face of a sheet or strip.

The rotary metallic brush 7 is mounted on a shaft 9 swiveling in flanges 10 fixed themselves on a support 11 mounted adjustable in height. The shaft 9 is coupled to the output shaft 12 of a DC motor 13 making it possible to easily adjust the speed of rotation of the brush 7 .. The support 11 can be adjusted in height by any device suitable, for example with a screw as shown in 14. The brush can also be adjusted horizontally, perpendicular to the path of the strip by means of screw devices 30.

The support rollers 8 are mounted parallel so as to pivot in a support 15 which is adjustable both in height parallel to the path of the strip and perpendicular to it by means of two devices, for example with screws, 19 and 20 respectively. It is thus possible not only to move the axes of the rollers 8 relative to the axis. of rotation of the brush 7 but also adjust the pressure exerted by the device on the strip on either side of the latter, and thus also on the tension thereof.

Finally, means are provided for cooling the brush and the support roller or rollers by circulation of an appropriate cooling fluid by means of pipes 21, 22, respectively.

It is understood of course that other means than the brush 17 can be used to remove the unalloyed zinc from the surface of the sheet or strip, provided that these means are placed in the desired location relative to the galvanizing bath. to attack the zinc coating when it is in the right temperature place.

It is also understood that it is possible to provide along the path of the strip or sheet; downstream from brush 7 and roller 8, other brushes or suitable mechanical, chemical, or electrolytic devices. or others to ensure the elimination of residues as well as the finishing.

It is thus possible, thanks to the method and device according to the invention, to directly manufacture a sheet or strip of galvanized steel on one side and the other side of which has only an extremely weak coating of iron-zinc alloy, the zinc torn from the surface can then be recovered.

It will be noted that in a high-speed galvanizing installation, suitable means can easily be provided for automatically adjusting the position of the device relative to the surface of the galvanizing bath so that the abrasive action of the brush 17 is always exerted. at the location of the strip where the zinc coating passes through its state of friability, these means being conventionally controlled by sensors sensitive to the various factors influencing the temperature of the coating.

These means being well known in the art do not form part of the invention and therefore will not be described.

There is shown schematically in FIG. 6 an example of a device according to the invention in its application to the manufacture of a product treated on its two faces, and it is consequently installed at the outlet of a conventional hot dip galvanizing installation, so that the galvanized strip 5 passes, after spinning in a conventional manner, between two circular metallic brushes 7, 7a, which are arranged so as to rotate in contact with the strip 5, on either side thereof.

The mounting of the brushes on either side of the strip being identical, only the arrangement of one of them will be described.

The brush 7 is mounted on a hollow shaft 9 swiveling in a support 10 and is vertically adjustable by means of a screw 14, placed at each end of the shaft 9.

This assembly is also mounted adjustable in the horizontal direction on the support 11, perpendicular to the plane of the strip 5 by means of a screw 30 making it possible to move the brush 7 by bringing it closer or away from the surface of the strip.

A support roller 8 is mounted above the shaft 9 of each brush and at a fixed adjustable distance from the latter and from the strip.

As in the previous example, this support roller 8 is intended to ensure, in conjunction with the opposing roller 8a forming part of the symmetrical device, the appropriate tension of the strip in contact with the brushes 7, 7a, without any transverse bending.

The numerous tests carried out by the Applicant have made it possible to determine that the characteristics of the brush or brushes, that is to say its diameter, the density of its bristles, the diameter and the length of the steel wires which compose it, their nature, their temperature, as well as the relative linear speed of the brush or brushes with respect to the sheet, and finally the distance separating them (that is to say the pressure exerted by the brush or brushes on the sheet) are so many critical factors contributing together to obtain a final product with characteristics such that it has optimal properties.

The properties of resistance to stretching, seizing, deep drawing, corrosion, etc. of the product according to the invention are ensured by a coating of compact iron-zinc alloy which does not have roughness visible, having a thickness of approximately 1.5 μm and composed of approximately 80% of phase 8 and of approximately 20% of phase g.

This alloy layer is obtained on one side or on both sides of the sheet or strip by means of the process according to the invention in which at least one cylindrical brush with bristles or steel wires having a diameter of 5 is used. at 50/100 mm and a length between 35 and 185 mm, the brush itself having a diameter between 200 and 500 mm, the distance between the brushed surface and the envelope of the brush from which extend the hairs being between 30 and 150 mm.

This or these brushes are rotated in the opposite direction of movement of the sheet or strip so that the relative speed brush / sheet is between 15 m / s and 80 m / s.

Finally the temperature of the strip, on the one hand, at the level of the brush or brushes, and the temperature of the bath, on the other hand, are chosen so that the zinc reaches the level of the brush or the brushes with a temperature slightly higher than its solidification temperature.

In a typical installation, the temperature of the branch at the level of the brush or brushes is between 410 and 450 ° C. and the temperature of the base of the bristles of the brush or brushes is maintained at 50 ° C. at most.

In other words, this amounts to adjusting the distance between the device and the outlet from the bath automatically by means of a mechanism known per se, controlled by thermal probes or other suitable sensors applying to this mechanism electrical control signals. .

Advantageously, the device comprises, in the vicinity of each brush a collector 35 connected to a blower 36 or other suitable suction device, by means of a filter 37 or the like in which the zinc particles are collected. solid torn off by brushes.

The function of the support rollers 8, 8a is to maintain the sheet or strip, on the one hand against any transverse bending and, on the other hand, at a fixed distance from the brushes. Their temperature is, therefore, low enough to avoid any sticking of zinc on their surface.

Swift cup stamping tests were carried out on blanks degreased with trichloride before stamping of a sheet treated according to the invention on its two faces and by way of reference on a bare steel sheet for deep drawing ES degreased in the same conditions. The ratio between the diameter of the blank and the diameter of the flat-bottomed bucket is 2.

In these exceptionally severe conditions, it is possible to adjust the pressure of blank holder to obtain correct stampings with the sheet according to the invention, without breakage or trace of seizure, with the trace of the folds appearing only inside. No limit has been found on the number of stampings that can be made under these conditions.

On the other hand, for bare ES steel, the first bucket has cracks and constrictions on seizure, and the following break systematically whatever the pressure of blank holder adopted, as a result of seizures at the entry radius of the matrix.

In these tests, there was no dusting or tearing of the zinc coating of the sheet tested.

Vickers microhardness tests carried out on the alloy have shown that the hardness of the layer is between 250 and 420 HV, depending on the stress state of the surface.

Finally, it will be noted that the zinc torn off by the brush or brushes is recovered in the collectors 35 in a noble form allowing its industrial reuse, which again constitutes an advantage of the method and of the device according to the invention.

Claims (11)

1. A process for producing a hot galvanized press-formable steel sheet or strip having an extremely light coating on at least one side, comprising continuously passing the sheet or strip in a conventional hot dipping galvanizing bath including scraping or wiping a part of the liquid zinc on the sheet or strip when it issues from the bath, characterized by the following steps : determining the distance from the surface of the bath at which the temperature of the zinc coating on the sheet or strip is in the range in which the zinc passes through a liquid-solid transitory state which is characterized by its friability, a little after its solidification, exerting at this point a mechanical action on at least one of the sides of the sheet or strip so as to completely remove the whole of the non-alloyed zinc. and leave only a thin coating of iron-zinc alloy.

₂. A process as claimed in claim 1, comprising the use of a cylindrical metal brush for exerting said mechanical action, the temperature at the base of the wires and bristles of said brush being maintained a little lower than the temperature of solidification of the zinc.

3. A process as claimed in claim 2, wherein the temperature of the base of the bristles of the brush is maintained, by means of the circulation of a cooling agent, so as to be at most of 50 °C.

4. A process as claimed in claim 3, wherein there is employed a brush comprising steel wires fixed to a hollow cylindrical case, said wires having a diameter of 5/100 to 50/100 mm and a length of between 35 and 185 mm, the total diameter of the brush being between 200 and 500 mm.

5. A process claimed in claim 4, wherein the brush or brushes are maintained relative to the sheet or strip in such manner that the distance between the base of the bristles and the brushed surface is between 30 and 150 mm.

6. A process as claimed in claim 5, wherein the brush of brushes are rotated in the direction opposed to the travel of the sheet or strip in such manner that the relative velocity between the brush and sheet or strip is between 15 meters per second and 80 meters per second.

7. A device for carrying out the process according to any one of the claims 1 to 6, said device comprising a support located in the path of the sheet or strip above the surface of the bath and carrying mechanical means comprising at least one rotary cylindrical. metal brush having a controlled temperature and adapted to tear away the zinc on at least one side of the sheet or strip, and at least one opposing support roller in contact with the side, or the- opposite sides, of the sheet or strip, and means for independently adjusting the positions of the brush or brushes and the roller or rollers in a direction parallel to and perpendicular to the path of the sheet or strip relative to the surface of the bath.

8. A product obtained by means of the process according to any one of the claims 1 to 6, said product comprising on said brushed side or on each of said brushed sides, a thin iron-zinc alloy coating having a thickness of between 0.3 µm and 4 µm, representing an amount of alloy of between 2 and: 30 g/m².

9. A product as claimed in claim 8, wherein said thickness is between 1 and 2 mm, representing an alloy thickness of between 7 and 15 g/m².

10. A product as claimed in claim 8 or 9, wherein said iron-zinc alloy coating comprises about 80 % of phase 8 and 20 % of phase g.

11. A device as claimed in claim 7, comprising collectors disposed in the vicinity of the brushes respectively and connected to suction and separating or filtering means for collecting the particles of solidified zinc torn away by the brushes.

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