SK286485B6 - Method for producing a steel strip which is provided with a zinc coating and zinc-coated steel strip - Google Patents

Abstract

The invention relates to a method for producing a steel strip (S) which is provided with a zinc coating. According to said method, the steel strip (S) continually passes through a strip galvanizing process in which it is subjected to hot dip galvanization in at least two steps. In the first hot dip galvanization step, the steel strip (S) is coated with a base layer (G), said steel strip being guided through a first dipping bath (2) consisting of a zinc melt with a low aluminium content for a first dipping time. After being coated with the base coat (G), the steel strip (S) is cooled. In a second hot deep galvanization step, a cover layer (D) is applied to the steel strip (S) coated with the base coat (G) by guiding the same through a second dipping bath (8) consisting of a zinc melt with a higher aluminium content than the first zinc melt, for a second dipping time.

Description

The invention relates to a process for the production of a zinc-coated steel strip in which the strip is subjected to at least two-stage hot-dip galvanizing.

BACKGROUND OF THE INVENTION

Steel strips are coated with a zinc coating to improve their corrosion resistance for various applications. The advantage of using zinc as a protective layer over other metals considered as a protective layer is that zinc as the anode to be destroyed provides active and passive protection of the steel strip. A further improvement in corrosion protection could be achieved by applying eutectic zinc-aluminum alloys containing up to 5% aluminum to the steel sheet. In this way, the excellent protective properties of aluminum can be combined with equally favorable properties of zinc.

The basic problem in applying a corrosion protective metal layer to steel strips is that the thickness of the protective layer applied to the steel strip is, on the one hand, limited due to the flowability of the metal melt. On the other hand, the reason for this limitation is that the zinc coating formed on the surface is brittle due to its fusion with the iron of the steel strip. This property deteriorates the adhesion of the coating to the steel strip at high coating thicknesses, so that peeling occurs.

The solution to this problem is that the steel strip is subjected to a two-stage coating process. This so-called "double dip" method is described, for example, in "AGOZAL - new improved corrosion protection for strip steel", published in "STAHL", issue no. 2/1997; In this known method, the steel strip in the first stage of hot dip galvanizing is passed through the zinc melt so that a thick layer of hard zinc containing zinc and iron is formed on the surface of the steel strip. In the second stage of hot dip galvanizing, the coated strip is passed through a zinc-aluminum melt. The zinc-iron-containing layer is intended to act as a fluxing agent in order to improve the wettability of the steel strip. In this way, it should be possible to apply a coarse layer comprising zinc and aluminum to a strip pre-coated with zinc.

This known method furthermore assumes that the brittle hard zinc layer present on the steel strip after the first hot dip galvanization changes into the eutectic structure of zinc and aluminum during the deposition of the second zinc layer due to the absorption of part of the aluminum applied together with the zinc during the second dip. so that after the second stage of hot-dip galvanizing, there is a thick coating of zinc and aluminum on the strip. This coating should have a uniform homogeneous structure and due to the close bond formed in the first stage of the hot dip coating between the steel strip and the zinc coating, it should adhere firmly to the steel strip.

Despite the increase in the thickness of the zinc and aluminum coating, which, at least in theory, can be obtained by the "double dip" explained in practice, it has been shown in practice that the corrosion resistance of the coated steel strips is improved.

In addition to the prior art explained, JP-A 11-117052 discloses a process for two-stage galvanizing steel product pieces. According to this method, these steel products are subjected to a pre-treatment by fluxing with a fluxing additive for zinc and aluminum coatings that does not contain NH ₄ Cl before being immersed in a first galvanizing bath containing 0.01 to 0.1% aluminum. In this first stage of galvanizing, a coating with a reduced increase in the alloy phase of iron and zinc is formed on the steel product. Then the steel products with this backing layer were cooled with water to then be immersed in a second dipping bath. This second bath contains 3% to 20% aluminum and / or 0.01 to 1.0% magnesium. In this known manner, a coating is obtained which is not abrasive or has no cracks and has good corrosion resistance.

SUMMARY OF THE INVENTION

Based on the prior art explained, the object of the invention is to provide a method by which a steel strip with a zinc coating can be produced with greater efficiency, which provides improved corrosion protection while providing good ductility properties. The steel strip produced by the process according to the invention should have an extremely good corrosion resistance with a high forming capacity.

This object is achieved by a process for the production of a zinc-coated steel strip, in which the steel strip continuously passes through the following working stages, while subjecting it to at least two-stage hot-dip galvanizing by hot-dip galvanizing:

- coating the steel strip with a backing layer of the strip in the first stage of galvanizing by immersion in molten zinc, so that the steel strip is passed through a first low-zinc melt-soaking bath during the first soaking period,

- cooling the steel strip coated with the undercoat,

- applying a covering layer to the steel strip coated with the undercoat so that the steel strip coated with the undercoat in the second stage of hot dip galvanizing is fed into the molten zinc during the second soaking period through a second soaking bath consisting of a second zinc melt with higher aluminum content a first zinc melt according to the invention, the principle of which is that

- the steel strip is continuously annealed before entering the first soaking bath.

According to the invention, a coating comprising at least two layers is obtained in a different manner from that of the prior art for the coating of steel strips by immersion. This is achieved by first passing the steel strip through a first dipping bath in which it obtains a first zinc coating. Thereafter, the web is cooled so that the coating applied to the first immersion solidifies. The rigid backing sheet is then coated with a cover layer for the second galvanizing process. This forms a steel strip coating produced by diffusion with a backing layer. In contrast to the prior art, the underlying layer does not change to a structure corresponding to the cover layer, but the original structure of the underlying layer is absorbed by aluminum diffusion and is maintained as a separate layer. Additional cover layers may be applied to the first cover layer by passing the steel strip through further dipping baths. In this case, cooling of the strip is always carried out between the soaks as required.

Due to the layered structure of the coating, the coating produced according to the invention has an increased corrosion resistance. Thus, on the one hand, a coating thickness improved in comparison with the prior art, for example up to 80 μπι, can be obtained by the method according to the invention, which in practice has so far been achieved only by galvanizing the individual pieces. The increased thickness of the coating leads to an increase in the time during which the coating provides reliable protection even in an aggressive environment. On the other hand, the individual layers of the coating may be coordinated with each other to provide extremely strong adhesion of the coating to the steel strip. At the same time, by suitably matching the properties of the layers, the possibility of undersized migration of the coating in the event of damage that otherwise entails the risk of peeling of the coating at high layer thicknesses can be limited.

The aluminum-containing zinc melt coating exhibits high ductility while providing optimum corrosion protection. The backing layer is also malleable and exhibits an additionally increased corrosion resistance compared to the covering layer. Due to the common ductility of the two layers, excellent plasticity is ensured. On the other hand, optimum matching of the properties of the individual layers of the coating also ensures good and long-lasting corrosion protection.

As a result, the invention makes it possible to apply a zinc coating on the steel strip which provides improved corrosion protection in comparison with the prior art. For this purpose, the determined aluminum content of the zinc baths, the determined residence times of the steel strip in the soaking baths, the determined temperature of the soaking baths and the cooling between the soaking baths are selected.

By continuously annealing the steel strip according to the invention before the first stage of hot dip galvanizing, the strip is pretreated so that the zinc coating adheres optimally to its surface. This results in smoother surfaces. The strip produced according to the invention is therefore particularly well suited for the following shaping. Surprisingly, it has been found that by using annealing as a pretreatment, zinc coatings can also be applied to high-strength steels, which are difficult to coat due to their alloying. A further advantage of the pretreatment by annealing is that the success of the pretreatment is reliably reproducible. Finally, by continuously annealing treatment, high production rates and machining times are achieved, which are reduced compared to conventional processes. The annealing temperature is in particular 400 to 800 ° C. By continuously annealing under a shielding gas, oxidation of the strip surface is reliably prevented.

Practical testing of the process according to the invention has shown that good processing results are obtained when the soaking times last for 1 to 20 seconds. In this case, the quality of the coating is further increased with respect to the melting bath used and the bath temperature set so that the soaking times are in the range of 3 to 10 seconds.

Additional improvements in coating results can be achieved by optimizing the composition of the molten metal bath.

In practice, it has been found to be advantageous if the first zinc melt contains 0.005 wt. % to 0.25 wt. % of aluminum, with particularly good results being obtained when the first zinc melt contains 0.01 to 0.12 wt. aluminum. When using a melt of this composition in the first melt-dip coating step, a zinc-iron alloy is formed on the steel strip as a backing layer, which adheres extremely firmly to the surface of the steel strip and forms a zinc coating layer.

In the second stage of hot-dip galvanizing, the undercoat formed in the first stage is then converted into a zinc-aluminum-iron alloy to form a zinc-aluminum cover layer.

The high-quality coating can be produced in such a way that the second zinc melt deposited in the second deposition step contains 3 wt. % to 15 wt. % aluminum, in particular 4 wt. % to 6 wt. aluminum. A melt with such a composition deposited on the backing layer forms a zinc-aluminum covering layer which is characterized by high ductility and corrosion resistance.

The temperature at which the aluminum-containing zinc melt is applied to the molten zinc in the first stage of galvanizing should be 440 to 490 ° C. While maintaining this temperature range, a backing layer having the desired layer thickness can be formed without difficulty. Optimal coating results can then be achieved during the second coating step when the temperature in the second dipping bath is 420 ° C to 480 ° C.

Following the zinc coating, an organic protective layer may be applied to the coated steel strip to provide additional surface protection against damage during storage or transport.

In addition, it is expedient in connection with the production of extremely high-grade steel strips when the coated steel strip is cooled after the second dipping bath. Practical experiments in this context have shown that particularly good results are obtained when the cooling rate after the second soaking bath is at least 4 ° C / s.

The method according to the invention makes it possible to apply at least a double-layer coating consisting of a base layer on the surface of a melt steel strip to cold or hot-rolled steel strips of calibrated low-carbon steel, micro-alloyed ULC-steel or high-strength and high-strength steel. % of the zinc having the first composition and at least one coating layer applied to the backing layer, which is formed from a zinc melt that differs from the composition of the first zinc melt. The composition of the zinc melts is suitably selected such that the backing layer is made of zinc, aluminum and iron and the top layer of zinc and aluminum, and, as mentioned above, the melt used to produce the backing layer may contain, in addition to zinc, 0.005 to 0.25 % of aluminum, and the melt for the coating may contain, in addition to zinc, 3.5-15% of aluminum.

The steel strip produced according to the invention is covered with a coating formed from at least one underlayer and one covering layer applied thereto. This at least two-layer structure produced according to the invention allows the individual layers of the coating to be aligned with each other in such a way that their properties are optimally complemented in the said manner in connection with the method according to the invention. In this case, particularly favorable combinations of properties are provided when the backing layer consists essentially of zinc, aluminum and iron and the cover layer consists essentially of zinc and aluminum.

Preferably, the aluminum content of the individual coating layers applied to the steel strip is selected such that the aluminum content of the coating is at least 4% by weight. This can be achieved, for example, on the basis of a backing layer having an aluminum content of at least 5% by weight. In addition, in view of the formability of the coated steel strip, it is expedient that the aluminum content of the cover layer is lower than that of the base layer. Thereafter, the steel strips having a coating formed according to the invention and an aluminum content in the region of the covering layer of at least 3% by weight have extremely good ductile properties while at the same time having high corrosion resistance.

The proportion of the thickness of the backing layer to the total thickness formed by the sum of the thicknesses of the backing layer and the covering layer should preferably be at least 10%. In this way, the base layer is reliably adhered to the steel strip. On the other hand, in this way a precondition for producing a sufficiently thick and well adhered cover layer is created. The steel strips according to the invention have an overall coating thickness of at least 20 [mu] m.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in more detail on the basis of a drawing illustrating an exemplary embodiment. The figures show schematically:

Fig. 1 shows a cut-out of a device for dip galvanizing steel strips;

Fig. 2 shows a cross-section of a steel strip with a two-layer zinc coating.

DETAILED DESCRIPTION OF THE INVENTION

The steel strip S, for example, a hot-rolled thin sheet of low carbon aluminum steels, is unwound from an unreeled unwinder and degreased in a conventional acid pretreater and sea in a dilute acid as usual. Then, the steel strip S is continuously annealed in a furnace 1 with continuous operation under a shielding gas at a temperature of, for example, 600 ° C.

The steel strip S thus prepared is conveyed at a speed of 10 to 50 m / min. in the transport direction F to the first dip bath 2 of the first dip coating device 3.

The first dip bath 2 of the dip coating device 3 is formed by a zinc melt having an aluminum content of, for example, 0.05% by weight. The temperature of the dipping bath 2 is 460 ° C. The time required for passing through the dipping bath 2 is, for example, approximately 10 seconds.

Following the outlet of the first soaking bath 2, the steel strip S passes through the nozzle scraper 4. In the nozzle scraper 4, the thickness d1 of the backing layer G formed by the zinc melt of the first soaking bath 2 is adjusted so that excess liquid still adheres to The surface of the steel strip S is blown out of the steel strip S by means of slotted nozzles.

The steel strip S is then passed through a cooling device 5 in which it is cooled to 250 ° C. During this cooling, the melt adheres to the surface O of the steel strip S.

The cooling device 5 is followed by a fluxing device 7 in which the steel strip S is subjected, if necessary, to further treatment with a wetting agent followed by drying.

Then the cooled steel strip S subjected to the dipping bath 8 of the second dip coating device 9 passes through the cooled and, if necessary, wetting treatment.

The second dip bath 8 of the dip coating device 9 is formed by a zinc melt, which in addition to zinc contains aluminum, for example, in an amount of 4.5% by weight. The temperature of the second soaking bath 8 is also 460 ° C. The time required for passing through the soaking bath 8 is, for example, approximately 10 seconds.

Following the outlet of the second soaking bath 8, the steel strip S passes through the blowing device 10. In the blowing device 10, the thickness d2 of the cover layer D formed by the zinc melt of the second soaking bath 8 on the backing layer G is adjusted by excess. adhering to the surface O 'of the backing layer G blows out of the surface O' of the backing layer G by means of slotted nozzles.

The blowing device 10 is followed in the conveying direction F by another cooling device 11 in which the coated steel strip S is cooled to room temperature at a cooling rate of 40 K / s.

In one coating device (not shown), an additional protective layer is then applied to the steel strip S to provide temporary surface protection.

FIG. As can be seen from FIG. 2, a steel strip S having a firmly adhering coating B is produced. This coating B is on the one hand formed by a backing layer G which, due to the influence of the iron present in the steel strip S, contains iron in addition to zinc and aluminum. The thickness d1 of the backing layer G is approximately 25% of the total thickness dg of the coating B formed by the thickness d1 of the backing layer G and the thickness d2 of the cover layer.

D.

In FIG. 2 clearly shows how the backing layer G has "grown" on the steel strip S and as the melt of the backing layer D has diffused into the backing layer G while staying in the second soaking bath 8 so that a close bond is formed between the backing layer G and the backing layer.

The following Tables I to III show the results of the experiments carried out with the immersion plating simulator.

The experiments, the results of which are shown in Table I, were carried out using a dip coating simulator. Rhesca on a laboratory scale. The aluminum contents were adjusted to 0.1 to 0.2% by weight in the first zinc bath. Such aluminum contents are normally used in strip galvanizing devices in single-stage galvanizing (see Examples 1 and 2 of Table I) and are necessary for good zinc adhesion.

Alternatively, the aluminum contents of up to 5% can also be set in single-pass galvanizing in strip galvanizing systems. Melt compositions used for this purpose are known on the market under the trade name " Galfan. Example 4 of Table I shows the result obtained by using a zinc melt containing 5% aluminum in a single galvanizing.

However, in flux pretreated strip galvanizing plants, no one-time zinc plating is performed in aluminum baths with up to 5%, since there are defects in the coating. Example 3 of Table I confirms this finding.

In addition to the aluminum content, the soaking times in the second soaking bath were varied from 10 to 20 seconds in Examples 5 to 7 of Table I, in which the zinc coating according to the invention was carried out in two stages and with a pretreatment. Work was done partly with interfluxing ("yes") and partly without it ("no"). The cooling rate achieved in the final cooling varied from 4.5 ° C / s to 40 ° C / s. Examples 5 to 7 of Table I show that, under these conditions, steel strips with coatings having extremely good adhesive properties, high forming properties and good corrosion resistance can be produced.

Also, the results shown in Table II were obtained in experiments carried out with a Rhesco dip coating simulator on a laboratory scale. In the series of experiments shown in Table II, the aluminum contents of the first dipping bath were reduced to 0.08% by weight and 0.05% by weight, respectively. This resulted in poor adhesion resulting from the single-stage galvanizing in both flux pretreatment and annealing pretreatment, as can be seen, for example, in the results of Examples 8 and 9.

In the two-stage galvanizing process according to the invention, the soaking times of the second galvanizing were determined to be constant for 10 seconds. On the other hand, the interflux treatment ("yes" and "no") and the cooling rate at the final cooling, which was set at 4.5 ° C / s to 40 ° C / s, were still changing. Examples 10 to 14 shown in Table II show that the samples produced by the process of the invention have coatings with an extremely good adhesion and a partially excellent durability in the corrosion resistance test.

The experiments, the results of which are shown in Table III, were also obtained in experiments carried out in the Rhesco Dip Coating Simulator. In this series of experiments, the aluminum contents in the first zinc bath were further reduced compared to the previous series of experiments. They represented 0.01 wt. However, all of the examples in Table III are characterized by good adhesion and very high resistance to co-corrosion.

No. pretreatment First dipping bath Medzifluxovanie Second soaking bath Cooling [° C / s] coating evaluation Contents Al [wt.%] Pulse. [° C] Dipping time [with] Al content [% by weight] Pulse. [° C] Dipping time 1S] Contents Al coating B 1% by weight.] Content Al backing layer G [wt%] Content AIcovering layer D [% by weight] Layer thickness [gm] Adhesion [degree] (D Corrosion behavior [h] (1) Note (4) 1 fusion 0.15 465 10 - - - - 10 0.33 - - 22.3 2 216 P 2 annealing 0.20 465 3 - - - - 10 0.52 - - 19.8 1 240 P 3 fusion - - - - 5 460 10 10 4.2 - - 25.6 3 24 (3) P 4 annealing - - - - 5 460 3 10 4.9 - - 20.5 1 480 P 5 annealing 0.10 465 10 Yes 5 460 10 10 6.3 8.1 4.8 32.0 2 700 in 6 annealing 0.10 465 10 not 5 460 20 4.5 5.4 8.2 3.5 29.0 2 500 in 7 annealing 0.20 465 10 not 5 460 20 40 5.5 7.1 6.2 27.5 1 -2 480 in

Table I (1): Adhesion of coating determined by impact hardness test, steel iron test foil 1931 (grade

1,2 = good, 3, 4 = bad) (2): corrosion behavior determined according to salt fog test, DIN 50021 SS, test time in h until first red rust is established;

(3): The sample showed defects in the coating;

(4): P = comparative sample, V = sample according to the invention.

no. pretreatment First dipping bath Medzifluxovanie Second soaking bath Cooling [° C / s] coating evaluation Al content [wt.%] Pulse. [° C] Dipping time [WITH] Al content [% by weight] Pulse. ( "C] Dipping time [with] Al content of coating B [wt.%] Content Al underlayer G [wt%] Content Allocation layer D [% by weight] Layer thickness [/ im] Adhesion [degree] (D Corrosion behavior [h] (2) Note (4) 8 fusion 0.05 465 10 - - - - 10 0.12 - - 25.6 4 240 P 9 annealing 0.05 465 3 - - - - 10 0.15 - - 21.2 3 216 P

No. pretreatment First dipping bath Medzifluxovanie Second soaking bath Cooling [° C / s] coating evaluation Al content [wt.%] Pulse. [° C] Dipping time [with] Contents Al [wt.%] Pulse. [° C] Dipping time [WITH] Al content of coating B [wt.%] Contents Alpine backing layer G [% by weight] Content Allocation layer D [% by weight] Layer thickness [gm] Adhesion [degree] (D Corrosion behavior [h] (2) Note (4) 10 annealing 0.05 465 10 not 5 460 10 10 6.4 11.9 5.1 53.5 1-2 2040 in 11 annealing 0.05 465 10 Yes 5 460 10 4.5 6.1 12.9 4.5 49.0 1 -2 2040 in 12 annealing 0.05 465 10 not 5 460 10 4.5 10 17.5 5.1 35.0 2 820 in 13 annealing 0.05 465 10 Yes 5 460 10 10 7.6 14.5 4.2 37.5 2 820 in 14 annealing 0.05 465 10 not 5 460 10 40 7.4 12.8 5.9 44.5 1-2 1680 in

Table II (1): Coating adhesion determined by impact hardness test, steel, iron test film 1931 (grade 1,2 = good, 3,4 = bad) (2): Corrosion behavior determined from salt fog test, DIN 50021

SS, the test time in h until the first red rust is established;

(3): The sample showed defects in the coating;

H): P = comparative sample, V = sample according to the invention.

no. pretreatment First dipping bath Medzifluxovanie Second soaking bath Cooling [° C / s] coating evaluation Al content [wt.%] Pulse. [° C] Dipping time [with] Al content [wt.%] Pulse. [° C] Dipping time [with] Contents Al coating B [% by weight.] Contents Alpine Underlay G [wt%] Al content of coating D [% by weight] Layer thickness M Adhesion [degree] (D behavior P Corrosion [h] (2) Note (4) 15 annealing 0.01 465 10 not 5 460 10 10 5.8 12.1 5.5 60.2 1 -2 2880 IN 16 annealing 0.01 465 10 not 5 460 10 4.5 6.6 13.2 4.5 58.0 1 -2 2450 in

Table III (1): Coating adhesion determined by impact hardness test, steel, iron, test film 1931 (grade 1,2 = good, 3, 4 = bad) (2): corrosion behavior determined from salt fog test, DIN 50021

SS, the test time in h until the first red rust is established;

(3): The sample showed defects in the coating;

(4): P = comparative sample, V = sample according to the invention.

Claims (13)

PATENT CLAIMS Method for producing a zinc-coated steel strip (S), in which the steel strip (S) continuously passes through the following working steps, while subjecting it to at least two-stage hot-dip galvanizing: - coating the steel strip (S) with a backing layer ( G) in a first stage of hot dip galvanizing by passing the steel strip through a first dipping bath (2) of a low-aluminum zinc melt during the first soaking period, - cooling the steel strip (S) coated with the undercoat (G), and - applying a coating (D) to the steel strip (S) coated with the undercoat (G) so that the steel strip (S) coated with the undercoat (G) in the second stage of hot dip galvanizing is passed through the second dipping period during the second soaking period a soaking bath (8) consisting of a second zinc melt with a higher aluminum content than the first zinc melt, characterized in that the steel strip (S) is continuously annealed before entering the first dipping bath. Method according to claim 1, characterized in that the soaking times last from 1 to 20 seconds. Method according to claim 2, characterized in that the soaking times are in the range of 3 to 10 seconds. Method according to one of the preceding claims, characterized in that the first zinc melt contains 0.005 wt. % to 0.25 wt.%, preferably 0.01 to 0.12 wt. aluminum. Method according to one of the preceding claims, characterized in that the second zinc melt contains 3 wt. % to 15 wt. aluminum. The process according to claim 5, wherein the second zinc melt comprises 4 wt. % to 6 wt. aluminum. characterized Method according to one of the preceding claims, the zinc vine in the first soaking bath (2) is 440 ° C to 490 ° C. Method according to one of the preceding claims, characterized in that the second dipping bath (8) is 420 ° C to 480 ° C. Method according to one of the preceding claims, characterized in that the charge is 400 to 800 ° C. Method according to one of the preceding claims, characterized in that the annealing takes place under a shielding gas. Method according to one of the preceding claims, characterized in s a by s a t ý m s a by s a by s a team. that the temperature inquires the temperature in that the vein temperature is continuous after poin For example, by dipping into molten zinc, a protective layer is applied to the cover layer (D) to protect the surface of the cover layer (D). Method according to one of the preceding claims, characterized in that the coated steel strip (S) is cooled after the second dipping bath (8). Method according to claim 12, characterized in that the cooling rate is at least 4 ° C / s.