LU84732A1 - PROCESS AND DEVICE FOR IMPROVING THE QUALITY OF METAL COATINGS FORMED BY HOT DIP - Google Patents

Description

A Claiming the priority of a request 3L-3451 / EM / EG

r | patent filed in Belgium on April 7 Jl 19S2 under No 892.795_ r! he

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Patent of invention a "a" 3 "3Ba3aa" aaaaK "" 3iiaMtacajc3 "timo - 1 - C 2184/8204.

METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Non-profit association -Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Method and device for improving the quality of metallic coatings formed by hot immersion.

The present invention relates to a method for improving the quality of metallic coatings formed by immersing the substrate in a coating metal bath.

It applies to coatings made of metals such as lead, tin, zinc or aluminum, or of alloys of several

Ide of these metals, in particular of alloys of zinc - aluminum or zinc - lead - aluminum in which the aluminum content can reach 80% by weight.

Although it is also applicable to coatings formed on the most diverse steel substrates, such as strips, wires, tubes or parts of any shape, and deposited by methods

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continuous, semi-continuous or discontinuous, the invention will be described here in its application to the treatment of a zinc coating formed continuously on a steel strip.

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After having undergone well-known surface preparation treatments, the steel strip is immersed in the zinc bath for a determined period.

At the exit from the zinc bath, the coating present on the surface of the strip is subjected to a thickness adjustment operation, by means also known, for example air knives.

The coated strip is then subjected to natural cooling with ambient air. The cooling rate depends on several factors, such as the bath temperature, the strip temperature, the massiveness of both the strip and the coating? it is generally less than 10 ° C / s.

Due to the low cooling rates that can be achieved in this way, the zinc coating solidifies relatively slowly; it crystallizes in a relatively coarse structure, favored moreover by certain elements such as lead or tin, which corresponds to normal flowering.

It has already been proposed to apply more intense cooling when it is desired to refine the structure of the zinc coating. In particular, it has been recommended to spray certain substances by means of air jets, with a view to promoting the formation of numerous solidification germs and thus causing a refinement of the zinc grain. Among the substances used as inoculants, mention may be made of zinc dust or a mist consisting of water vapor and an aqueous solution of ammonium phosphate. However, these techniques do not make it possible to reach cooling rates of the coating greater than a few tens of degrees per second and therefore do not allow the flowering of the coating to be completely eliminated.

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However, it has been found that these coatings cooled rapidly with minimized flowering still have serious drawbacks concerning in particular the resistance to corrosion by salt spray, the adhesion of the paints and the corrosion resistance of the painted coatings.

The subject of the present invention is precisely a method making it possible to remedy these drawbacks by ensuring better homogeneity of the structure of the metal deposit, leading to an almost total elimination of flowering.

The process which is the subject of the present invention, in which a steel strip is passed through a coating metal bath at the outlet of which the said coating is subjected to a cooling operation, is essentially characterized in that said cooling is carried out by spraying the coating with water and / or water vapor, preferably in the form of a mist formed by a carrier gas such as air or nitrogen.

According to the present invention, it is advantageous to initiate cooling by spraying water and / or steam, when the coating metal is still in the liquid state? in other words, the temperature of the coating, at the start of spray cooling, must be at least equal to the solidification temperature of the coating metal.

According to the invention, the cooling speed of the metal coating is adjusted by adjusting the flow rate and / or the pressure of the liquid and / or gaseous fluids projected onto the coating.

A particular implementation of the method of the invention consists in carrying out the cooling in a single step, with one! j speed greater than 100 ° C / s.

4 - 4 - i i \ According to an advantageous implementation of the method of the invention, i i performs the cooling of the coating in at least two stages which correspond to different cooling rates.

In this regard, it is particularly advantageous to ensure relatively slow cooling, for example at a speed of less than 100 ° C / s during a first step corresponding to the solidification of the metal coating, followed by more intense cooling, at a speed greater than 100 ° C / s, when the coating is solidified.

This implementation makes it possible to avoid any deterioration of the non-solidified coating while ensuring, after solidification, a cooling rate leading to the obtaining of a homogeneous structure of the metal deposit.

It has been found advantageous, within the framework of this implementation of the method of the invention, to regulate the cooling rate separately in each step, by adjusting the flow rates and / or the pressures according to the characteristics, in particular of the nature , temperature and coating thickness.

According to the invention, it has also been found advantageous to subject the coating to a step of cooling in air, immediately after leaving the bath, before cooling by spraying water and / or steam.

In order to clearly show the advantages of the process of the invention, we examine below, by way of nonlimiting example, its application to coatings made either of a low alloyed metal or of a highly alloyed metal, or an alloy I of eutectic composition.

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In the context of these examples, the coatings were subjected to cooling in two stages, the border between the two stages being chosen slightly below the solidification temperature (T) of the alloy considered, for example at

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Ts - 20 ° C.

To illustrate the case of low-alloyed metals, zinc-containing coatings with different lead and aluminum contents, but all less than 1% for lead and less than 1%, were used; 0.4% for aluminum.

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Alloys with very low lead and aluminum contents have an essentially single-phase structure.

The first cooling step, from the temperature of the coating on leaving the bath to (Ts -20 ° C), was carried out with a speed of between 10 and 100 ° C / s. It caused a refinement of the solidification grains.

The second stage, from (T - 20 ° C) to approximately (Ts - 200 ° C) was carried out with a speed greater than 100 ° C / s. It has made it possible to obtain a more homogeneous single-phase structure, in particular with regard to surface segregation or at grain boundaries. In addition to an improved exterior appearance and increased grain fineness, favorable for painting, these coatings also provide better passivability and greater resistance to corrosion.

When the metals have higher contents of alloying elements, but remaining within the limits indicated above, they give rise to two-phase structures which, thanks to the cooling stages indicated above, are clearly finer and can even be supersaturated . In the case of lead, whose solubility in solid zinc is practically zero, the very rapid cooling according to the invention gives rise to the formation of lead globules of a size less than> 41 ^ less than 0.5 / m. This structure is much more advantageous! "6" especially with regard to the adhesion of many types of paints, as the previous structures comprising lead globules with a size of the order of 0.5 to 3 .mu.m. As regards aluminum, the rapid cooling according to the invention also ensures finer precipitation in the solid state of the aluminum-rich phase. This results in an improvement in the resistance of these coatings to generalized corrosion and to intergranular corrosion.

I Among the coatings with high contents of alloying elements, the behavior of zinc alloys containing more than 15% Al and especially more than 50% Al has been examined. It has been found that very rapid cooling according to the invention, applied in particular in the range of temperatures corresponding to transformations in the solid state, made it possible to control the proportion of phase rich in zinc and to reduce the differences in average composition between the phase rich in zinc and the phase rich in aluminum.

As regards the coating alloys whose composition is equal to or close to eutectic, in particular zinc alloys containing approximately 5% Al, two distinct rapid cooling sequences according to the invention are applied.

The first sequence consisted of a single step, extending from the temperature at the outlet of the bath up to (T - 200 ° C), at a cooling rate greater than 100 ° C / s.

The second sequence was subdivided into two stages, either from the temperature at the outlet of the bath to (T_ - 10 ° C) at a speed of between 20 ° C / s and 100 ° C / s, then from (T - 10 ° C) to äs (T - 200 ° C) at a speed greater than 100 ° C / s.

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These cooling sequences according to the invention, applied to coatings of eutectic composition, both led to | : '- 7 - «- a refinement of the grain size at solidification? - refinement of the structure of the eutectic phase; - a more homogeneous distribution of the other elements present, for other reasons, in the coating metal (Sb, Pb, Mg, Cu, mischmetall, ...)? - the removal, by supercooling, of the primary hypo - or hypereutectic phases, for coatings with an Al content; differs by about 10% from the eutectic content.

The homogenization of structures, and in particular the elimination of j hypo- or hypereutectic phases, by the method of the invention, ί j have significant advantages over coatings subjected to conventional cooling.

First, the resistance to corrosion by salt spray is increased by about 50%.

In addition, the adhesion of the paints after aging is greatly improved, in particular in the vicinity of the edges of the parts or near the scratches that the paint may undergo. In particular, the homogenization of the coating structure greatly reduces the tendency to form blisters in paints.

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The present invention also relates to a device intended to cool, according to the process which has just been described, a metal coating formed by immersion of a steel object in a bath of molten metal.

1 For this purpose, the device which is the subject of the present invention is essentially characterized in that it comprises at least one set of nozzles, preferably of small dimensions: and in that each set of nozzles is placed along the path followed by the object, immediately after it leaves the coating bath.

♦ *: * -8 -: ί: ί j, To ensure uniform cooling of the entire surface! interested, the device of the invention advantageously comprises at least 400 nozzles per square meter of cooled surface.

It has also been found advantageous, according to the invention, to tilt the nozzles at an angle greater than 45 °, preferably of the order of 60 ° relative to the direction of movement of the object, so as to give the jet of sprayed fluid a composition of movement oriented in the direction of movement of the object leaving the bath.

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According to the invention, it is also advantageous to have, upstream of the sprinkler assemblies, at least one air knife oriented at about 45 ° in the direction of movement of the part.

According to an advantageous embodiment, the device of the invention comprises at least one deflector, disposed upstream I of the nozzle assemblies, preferably downstream of the air knives.

) | The set of air knives - deflectors avoids the flow of water from the nozzles to the metal bath. In addition, the deflectors prevent the air knives from disturbing the rest! cooling ensured by sprinkler assemblies.

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j j The best results have been obtained by spraying the r surface with a mist consisting of droplets! 1 of small diameter and with a moderate amount of movement.

h f This mist was formed, according to the invention, by means of nozzles of the type described in Belgian patent N ° 853,821 by the same applicant; the sprinklers used in this device! comprise an air channel with a diameter of the order of 1 mm, water channels with a diameter of less than 2 mm, and preferably / 1 of less than 1 mm; they are supplied at pressures respectively less than 1 bar for water and between 2 • i 4 * - 9 - “and 5 bars for air. The temperature of the water is chosen as a function of the pressure of the water so as to reach the cooling rate adapted to the type of coating to be cooled.

In addition to the metallurgical advantages described above, the invention also makes it possible to reduce the duration of the cooling, and consequently the size, in particular the height of the cooling installation, in particular in the installations.

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continuous coating ions.

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Claims (15)

1. Method for improving the quality of a metallic coating formed by hot immersion on a steel object, in which said coating is subjected to a cooling operation at the outlet of the coating metal bath, characterized in that l 'said cooling is carried out by spraying the coating with water and / or „water vapor, preferably in the form of a mist formed by a gaseous agent such as air or nitrogen. I 2. Method according to claim 1, characterized in that cooling is started by spraying water and / or steam, while the coating is at a temperature equal to or higher than its solidification temperature. i 3. Method according to either of claims 1 and 2, \ [characterized in that the water and / or water vapor contain i! in suspension one or more solid substances finely di-i aimed, intended to favor the formation of the germs of cris-! coating tallization. i- 'r f 4. Method according to either of claims 1 to 3, t Jr characterized in that one carries out the cooling of the coating- Itement in a single step, with a cooling rate at least equal to 100 ° C. / s. 5. Method according to either of claims 1 to 3, characterized in that the coating is cooled in at least two steps which preferably correspond to different cooling rates. _ 6. Method according to claim 5, characterized in that during the first step ensuring the solidification of the coating, one operates with a lower cooling rate ^ dd at 100 ° C / s, and in that during d 'at least one subsequent step, t J 1 - 11 -' s · Îon operates with a cooling rate greater than 100 ° C / s. 7. Method according to either of claims 1 to 6, characterized in that the cooling speed of the coating is adjusted, preferably separately in each step, by adjusting the flow rate, the pressure and / or the temperature of water and / or water vapor and / or gaseous agent sprayed onto the coating: - - 8. Method according to either of claims 1 to 7, characterized in that one carries out the spraying of water and / or steam by means of nozzles, preferably by spraying. 9. Method according to either of claims 1 to 8, I characterized in that the cooling by spraying water and / or steam is preceded by a step of cooling the coating in air immediately after getting out of the bath. 10. Cooling device for implementing the methods described in any one of claims 1 to 9, characterized in that it comprises at least one set of nozzles, preferably of small dimensions, and in that each set sprinkler is arranged along the path followed by the steel object immediately after it leaves the coating bath. 11. Device according to claim 10, characterized in that it comprises at least 400 nozzles per square meter of coated surface to be cooled. 12. Device according to either of claims 10 Iet 11, characterized in that the nozzles are inclined by an λ angle greater than 45 °, preferably of the order of 60 °, by vJ ^ relative to the direction of movement of the object, so that! "· Μ" -12-. .1 give the projected fluid jet a movement component | oriented in the direction of movement of the object.! 13. Device according to either of claims 10 to 12, characterized in that it comprises at least one deflector disposed upstream of the sprinkler assemblies. 14. Device according to either of claims 10 to 13, characterized in that it comprises at least one air knife, preferably arranged upstream of said deflectors and oriented at about 45 ° relative to the direction moving the object. 15. Device according to either of claims 10 to 14, characterized in that the nozzles each comprise a water and air supply system, in that the air channel has a diameter of the order 1 mm, and in that the water channels have a diameter of less than 2 mm, preferably less than 1 mm. Λ