BG65571B1 - Method of producing a zinc-coated steel strip - Google Patents

Abstract

The method finds application in electrodeposition for the manufacture of protective covering structures. It helps improve the corrosion resistance of the strip and raise its moulding capacity. According to the method, the steel strip (S) is subjected to a first coating by means of hot dipping in a zinc bath, cooling after the first coating, and subsequent second coating through dipping. Before the first dipping, continuous tempering is carried out of the steel strip (S), while the first coating is realized by guiding the strip (S) through a first bath (2) for a first dipping time. The bath (2) contains zinc melt with low aluminum content, a first base layer (G) being formed while the strip (S) passes therethrough. After that, cooling is carried out of the strip with the base layer (G), a cover layer (D) being deposited on the latter during the second dipping by guiding the strip (S) through a second bath (8) for a second dipping time. The second bath (8) contains second zinc melt with higher aluminum content than the first bath (2).

Description

Technical field

The invention relates to a method of manufacturing a zinc-coated steel strip for use in electroplating for the manufacture of protective covering structures.

BACKGROUND OF THE INVENTION

The steel strips are made with zinc coating to improve their corrosion resistance. Zinc is usually used to provide active and passive protection of the steel strip and acts as a protective anode. Further improvement of its corrosion resistance can be achieved by depositing eutectic Zn-Al alloys with components up to 5% A1 to the metal strip. This combines the protective properties of aluminum with those of zinc.

The so-called so-called a double immersion method in which the steel strip passes through a melt of zinc in a first step of zinc electroplating, whereby a thick solid layer of zinc containing iron and zinc is formed on the surface. The second zinc electroplating step is then cooled, in which the strip with the first layer of zinc passes through the zinc-aluminum melt. In this case, the zinc-iron layer from the first immersion acts as a flux, thereby improving the wetting of the steel strip (1).

By the known method, the coating of a steel strip with a corrosion-resistant layer is limited by the thickness of the deposited coating from the flowability of the metal melt. This is also due to the brittleness of the zinc layer, which forms on the surface, as it forms as a mixture with the iron from the steel strip. This often results in the peeling of the layer. Upon deposition of the second zinc layer, the brittle first layer formed is transformed into a zinc-aluminum structure eutectoid by partially absorbing the aluminum deposited together with the zinc in the second immersion step.

The coating thus obtained should have a uniform and homogeneous microstructure and should adhere firmly to the steel strip. This is not always practically possible, and often the steel strip thus produced has a low degree of corrosion resistance.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for the production of a zinc-coated steel strip in which the deposited coating has improved corrosion resistance and increased molding capacity.

The problem is solved by a method of producing a zinc-coated steel strip in which the steel strip is first coated by hot dip in a zinc bath. It is then followed by cooling after the first coating and the subsequent second coating by immersion. According to the invention, before the first immersion is carried out, the steel strip is continuously unfastened, and the first coating is made by passing the strip through a first immersion bath for a first time. The tub contains a zinc melt with a low aluminum content, forming one base layer. The core layer is then cooled, on which the coating layer is deposited during the second immersion by passing the strip for a second time through a second immersion bath, which bath is filled with a second zinc melt with a high aluminum content of the first one.

Both immersion times in both tubs are from 2 to 20 s.

The two immersion times in both tubs are from 3 to 10 s.

The zinc melt from the first tub contains aluminum in 0.005 wt. % (wt%) to 0.25 wt. % (wt%), preferably from 0.01 to 0.12 wt. % (wt%).

The second-bath zinc melt contains 3% aluminum by weight. % (wt%) up to 15 wt. % (wt%).

The second-bath zinc melt contains 4 wt% aluminum. % (wt%) to 6 wt. % (wt%).

The temperature of the zinc melt in the first immersion bath is 440 to 490 ° C.

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The temperature of the zinc melt in the second immersion bath is from 420 to 480 ° C.

The return temperature is from 400 to 800 ° C.

Repulsion is performed under shielded gas.

A protective layer is deposited on the coating layer.

The coated steel strip is cooled after immersion in the second tub.

The cooling rate is 4 ° C / s.

The advantage of the method is that it allows one to harden the base solid layer from the first immersion before applying the next coating layer. The resulting first layer is free from conversion to its microstructure, which has absorbed aluminum by diffusion. This results in the formation of coatings, for example, up to 80 micrometer thick, which improves the corrosion resistance of the tape and allows its use as a reliable protection even in aggressive environments.

In addition, the individual layers of the coating can be combined with one another to provide a strong adhesion of the coating to the strip. This also protects the tape from peeling.

The presence of aluminum in the coating improves the durability of a steel strip that is free of oxidation and leads to good deformability.

By the method, steel strips with more than two layers can be produced by passing the strip through different dipping tubs. In this case, the entrainment is cooled after each immersion.

The method can be applied to cold or hot rolled steel, low carbon steel, micro alloy steel or high elastic steel.

Explanations of the annexed figures

An exemplary embodiment is shown in the accompanying drawings, where:

Figure 1 is a schematic section of a zinc electroplating installation by the method;

Figure 2 is a schematic cross-section through a steel strip with a two-layer zinc coating.

Examples of implementation and application of the invention

Steel strip S, which may for example be hot-rolled sheet metal, contains A1-deoxygenated low carbon steel is unwound by uncoiling (not shown), and degreased conventionally in a pretreatment device (not shown), and etched into dilute acid. The steel strip S is then continuously returned to the furnace 1 for continuous venting under a shielded gas at a temperature of, for example, 600 ° C.

The steel strip S thus prepared is transported at a speed between 10 and 50 m / min in the direction of transport F in the first bath 2 with the melt of the first hot dip.

The melt bath 2 of the hot dip device 3 is formed of a zinc melt having an aluminum content of, for example, 0.05 wt. % (wt%). The temperature of the melt bath 2 is 460 ° C. The time required to pass through the melt bath 2 is, for example, 10 s.

The steel strip S then passes through the belt assembly 4 with nozzles. In the belt assembly 4, the thickness dl of the base layer G formed by the zinc melt of the zinc melt bath 2 is adjusted, whereby an excess amount of still liquid melt adhering to the surface O of the steel strip S is blown out of the steel strip S by cutting nozzles.

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

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

The steel strip S then passes into a second melt bath 8 of the second hot dip unit 9.

The second melt bath 8 of the second hot dip unit 9 is formed

65571 Bl of zinc melt which, in addition to zinc, has an aluminum content, e.g.

4.5 wt. % (wt%). The temperature of the melt bath 8 is also 460 ° C. The immersion time required to pass the melt bath 8 is approximately 10 s.

The steel strip S then passes through the blowing device 10. In the blowing device 10, the thickness d2 of the coating layer D formed on the base layer G of the zinc melt of the bath 8 with the melt is adjusted, whereby the excess amount of still liquid melt adhering on the surface O 'of the base layer G is blown out from the surface O' of the base layer G by means of cutting nozzles.

The blowing device 10 is then followed in the transport 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 the covering device (not shown), the steel strip S is then provided with an additional protective layer to create a temporary surface protection.

FIG. 2 it can be seen that as a result a steel strip S is produced which is coated B, adhered firmly to it. This coating B, on the one hand, is formed by a base layer G, which contains Fe in addition to Zn and A1, as a consequence of the influence of the iron contained in the steel strip. The thickness dl of the base layer G is approximately 25% of the total thickness of coating B formed by the thickness dl of the base layer G and the thickness d2 of the cover layer D.

FIG. 2 shows that the base layer G has an "outgrowth" on the steel strip S and the melt of the cover layer D diffused into the base layer G during its stay in the second bath 8 with the melt, so that an internal mixture is formed between the base layer G and the covering layer G.

The following Tables Ι-III, respectively, give the results of tests that were performed using a hot dip simulator (examples in Tables Ι-III) or using a cover installation.

The tests, the results of which are given in Table I, were carried out using a laboratory-scale hot dip simulator. In this case, the aluminum content of the first zinc bath was adjusted to 0.1 to 0.2 wt. % (wt%). Such aluminum contents have been used as standard in zinc electroplating installations with single-stage, single zinc electroplating (see Examples 1 and 2 in Table II) and are required for good zinc adhesion.

Alternatively, aluminum contents of up to 5% for single zinc coating in zinc stripe installations of S band are also set. Example 4 in Table I shows the result obtained using a zinc melt containing 5% aluminum for single zinc coating .

No separate zinc coating in aluminum tubs containing up to 5% is carried out in zinc coating installations for flux pretreatment, as defects in the coating are obtained in this case. Example 3 in Table 1 confirms this claim.

In addition to the aluminum contents, in Examples 5-7 of Table I, which are zinc coated in two steps according to the invention and are subjected to pre-treatment for repulsion, the times for immersion in the second melt bath are varied between 10 and 20 s. In some cases the process is performed with an intermediate flux and in some cases without such a flux. The cooling rates achieved during final cooling are varied between 4.5 ° C / s and 40 ° C / s. Examples 5 to 7 in Table I show that under these conditions it is possible to produce coated steel strips that have particularly good adhesive properties, high forming speed and good corrosion resistance.

The results given in Table II are also determined in tests that were performed using a hot dip simulator on a laboratory scale. In the series of tests shown in Table II, the aluminum content of the first zinc bath was reduced to 0.08 or 0.05 wt. % (wt%). For a single-step zinc coating, flux pretreatment and also aversion pre-treatment results in poor adhesion, as can be seen, for example, from the results of Examples 8 and 9.

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For the two-step zinc coating according to the invention, the immersion times for the second zinc coating are kept constant at 10 s. The intermediate treatments (with and without) and the cooling rates for the final cooling, which are set between 4.5 ° C / s and 40 ° C / s, have been changed again. Examples 10 to 14, given in Table II, show that the samples produced using the method according to the invention have coatings with particularly good adhesion and in some cases excellent longevity in the corrosion test.

The tests, the results of which are presented in Table III, were also obtained from tests performed using a hot dip simulator. In these test series, the aluminum content of the first zinc bath was further reduced compared to the test series explained previously. They are 0.01 wt. % (wt%). However, all the examples in Table III show good adhesion and very good corrosion resistance.

□ Prev Abr. First submersible bath Int. flux Second Submersible Bath Cooling by HS] Covering Rating Alumni, sauder. (wt%) Temp. 1 ° C] Time to sink (h} A1 Contents [wt%] Theme [V] Immersion Time [$] A] Contains cover B (wt.% ( Al Contains Cover Oen. Layer G | wt%] Al Contains Cover Layer D [wi.% 1 Layer, Fat ina gm Adhesion (Degree (1) Corrosion. Behavior. (W (2) Notice. (4) 1 flux 0.15 465 10 10 0.33 223 2 216 V 2 Response 0.20 465 3 10 0.52 19.8 1 240 V 3 flux - - - 5 460 10 10 4.2 - 25.6 3 24 (3) V 4 Response 5 460 3 10 4.9 - 20.5 1 480 V Well 5 Response 0.10 465 10 With 5 460 10 10 6,1 4,8 32,0 2 755 Well 6 Response 0.10 465 10 Without 5 460 20 4.5 5.4 8.2 3.5 29.0 2 500 Well 7 Response 0.20 463 10 without 5 460 20 40 5.5 7.1 6.2 27.5 1-2 480 Well

Table 1 (1): Coating adhesion determined by ball impact test, sheet steel test, sheet 1931 (grade 1.2 = good, 3.4 = weak) (2): Corrosion behavior determined by salt test spray, DIN 50021 SS;

the test time is given in the hours before the first red rust appears;

(3): Example showing defects in the coating;

(4): V = Reference Example; E = example according to the invention.

P. Prev Abr First submersible bath Int. flux Second submersible bath Cooling given [-C1 Covering Rating Aluminum content (wt%) Temp. | -C] Immersion Time [a] A1 Contents [wt%] Temp. HS | Immersion Time [a] A1 Contains cover. In [wt%] Al Contains Cover Osn. G layer [wt% | Al Contents Cover Cover. Layer D fwt%] Layer, Fat and Hg Adhesion (Degree (I) Corrosion. Behavior. [hl (2) Notice. (4) 8 Flux 0.05 465 10 10 0.12 25,8 240 V 9 Response 0.05 465 10 0.15 21.2 3 216 V 10 Response 0.05 465 10 Without 5 460 10 10 6.4 11.9 5.1 53,6 1-2 2040 Well 11 Open - something ο.όί 465 10 With 5 460 10 4.5 6.1 12.9 4.5 49,0 1-2 2040 Well 12 Open chic 0.08 465 10 Without 5 460 10 4.5 10 ib 5.1 35,0 2 820 Well 13 Opening 0.08 465 10 With 5 460 10 10 7,6 14.5 4.2 37.5 2 820 Well 14 Response 0.05 465 10 without 5 460 10 40 7.4 12,8 5.9 44.5 1-2 1680 Well

Table II (1): Coating adhesion determined by ball impact test, sheet steel test, sheet 1931 (grade 1, 2 = good, 3, 4 = weak) (2): Corrosion behavior determined by salt test spray, DIN 50021 SS; 5 the test time is given in the hours before the first red rust appears;

(3): Example showing defects in the coating;

(4): V = Reference Example; E = example according to the invention.

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Before. Abr. First dip * bath Int. flux Second submersible bath Cool dane ha Covering Rating Aluminum, sr. (wL%) Temp. re] Delay Time khc [s] AI Contents Temp, Mr. Potap Time Tf] A1 Contains cover. In [WL%] Al Contains cover. Main SloO G [wt%] Al Contains cover. Layer D Layer, Fat and Hg Adhesion [Degree] (I) Corrosion. Behavior. (h> (2) Notice. (4) - 15 Repulsion 0.01 465 10 Without 5 460 10 10 5.8 12,1 5.5 60.2 1'2 2880 Well 16 Repulsion 0.01 465 10 without 5 460 ιύ 4.5 6.6 β, 2 4.5 58,0 1-2 2450 Well

Table III (1): Coating adhesion determined by ball impact test, sheet steel test, sheet 1931 (grade 1.2 = good, 3.4 = weak) (2): Corrosion behavior determined by salt test spray, DIN 50021 SS; 5 the test time is given in the hours before the first red rust appears;

(3): Example showing defects in the coating;

(4): V = Reference Example; E = example according to the invention.

List of marking symbols

Flux device

First melt bath

The first hot dip device

Nozzle cutting device

Cooling device

Flux device

Second melt bath

Second hot dip

Blower

Cooling device

In Coverage

D Cover layer

G Base layer dl Base layer thickness G d2 Cover layer thickness D dg Total coating thickness B O Surface of steel strip S O 'Surface of base layer G S Steel strip

Claims (13)

Claims A method for the production of a zinc coated steel strip in which the steel strip (S) is first coated by hot dip in a zinc bath, cooled after the first coating and the subsequent second dipped coating, characterized in that before the first immersion is carried out, the steel strip (S) is continually unfastened, and the first coating is carried out by passing the strip (S) through the first immersion bath (2) for a first time, the bath (2) having a zinc melt low in alumina and forming a base layer (G), then cooling the strip with the base layer (G) on which a coating layer (D) is deposited during the second immersion by passing the strip (S) for one a second time through a second immersion bath (8), which bath is filled with a second zinc melt having a higher aluminum content than the first (2). -5 Λ Method according to claim 1, characterized in that the two immersion times in the two tubs (2, 8) are from 2 to 20 s. A method according to claim 2, characterized in that the two immersion times 35 in both tubs (2, 8) are 3 to 10 s. A method according to claims 1 to 3, characterized in that the zinc melt from the first bath (2) contains aluminum in 0.005 wt. % (wt%) to 0.25 wt. % (wt%), for pre- 40 esteem from 0.01 to 0.12 wt. % (wt%). Method according to claims 1 to 3, characterized in that the zinc melt from the second bath (8) contains aluminum in 3 wt. % (wt%) up to 15 wt. % (wt%). 45 Method according to claim 5, characterized in that the zinc melt from the second bath (8) contains aluminum in 4 wt. % (wt%) to 6 wt. % (wt%). Method according to claims 1 and 2, characterized in that the temperature of zinc 65571 The melt in the first immersion bath (2) is 440 to 490 ° C. Method according to claims 1 to 3, characterized in that the temperature of the zinc melt in the second dipping bath (8) is from 420 to 480 ° C. A method according to claim 1, characterized in that the return temperature is from 400 to 800 ° C. A method according to claim 1, characterized in that the repulsion is carried out under a shielding gas. A method according to claim 1, characterized in that a protective layer is deposited on the cover layer (D). A method according to claim 1, characterized in that the coated steel linen 5 (S) is cooled after immersion in the second bath (8). A method according to claim 12, characterized in that the cooling rate is 4 ° C / s.