HRP20020642A2 - 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 steel strip supplied with a zinc coating, wherein the strip is subjected to hot dipping for the application of the zinc coating in at least two stages. The invention also relates to a steel strip that is coated with zinc.

Steel strips are supplied with a zinc coating to increase their corrosion resistance for many applications. The advantage of using zinc as a coating with other possible coating materials is that zinc provides active and passive protection to the steel strip as a type of sacrificial anode. A further improvement in corrosion resistance could be achieved by applying eutectic Zn-Al alloys to the steel strip with aluminum content up to a total of 5%. In this way, the excellent protective properties of aluminum can be combined with the equally favorable properties of zinc.

The basic problem in coating a steel strip with a metal layer that protects against corrosion is that the thickness of the protective layer applied to the steel strip is, on the one hand, limited by the flow behavior of the molten metal. On the other hand, the reason for this limitation is that the zinc layer formed on the surface is brittle as a result of forming a compound with the iron in the steel strip, so that peeling occurs as a result.

The solution to this problem is to subject the steel strip to a two-stage coating process. This "double dipping" procedure is described, for example, in the article entitled "AGOSAL - a new type of improved corrosion protection for steel strip", published in the magazine "Stahl", vol. 2/1997, page 48-49. In this well-known process, the steel strip passes through the zinc melt in the first stage of applying the zinc coating by hot dipping, so that a thick hard layer of zinc containing iron and zinc is formed on the surface of the steel strip. In the second stage of hot-dip zinc coating, the coated strip passes through the ZnAl melt. In this case, the iron-zinc layer acts as a flux, which improves the adhesion of the steel strip. In this way, it is possible to apply a thick layer of ZnAl on a strip that has been previously coated with zinc.

The known procedure further assumes that during the application of the second layer of zinc, the brittle hard layer of zinc, which is on the steel strip after the process of the first application of the zinc coating by hot dipping, has turned into a eutectic Zn-Al structure by absorbing part of the aluminum, which was applied together with zinc in the second dipping process, so that after the second stage of applying the zinc coating by hot dipping, there is a thick coating on the strip, which consists of zinc and aluminum. This has a uniform, homogeneous microstructure and firmly adheres to the steel strip as a result of the intimate connection between the steel strip and the zinc coating produced in the first stage of hot dipping.

Despite the increase in thickness of the zinc-aluminum coating which can be produced by the double dipping process explained above, at least when considered theoretically, it has been found in practice that the corrosion resistance of suitably coated steel strips is greatly improved.

In addition to the previous state of the art, which was previously explained, the procedure for coating individual steel products with zinc in two stages is known from JP-A 11-117052. According to this process, each individual zinc product is subjected, piece by piece, to a flux pretreatment, using Zn-Al plating flux that does not contain NH4Cl, before being immersed in the first zinc plating bath, which has an Al content of 0.01 to 0.1%. A coating with reduced Fe-Zn alloy phase growth is produced in this first stage of zinc plating. Steel products that have this basic coating are then cooled with water, before being immersed in another immersion bath. This second bath has an Al content between 3% and 20% and/or a manganese content of 0.01 to 1.0%. Using a known process, a coating shell will be obtained that is not rough or flaky and provides good corrosion resistance.

The aim of the present invention is to propose, starting from the state of the art explained above, a process for coating steel strip, by which zinc-coated steel strip can be produced in large quantities and which provides improved protection against corrosion and at the same time equally good forming properties. Additionally, the goal is to produce a steel strip that has particularly good corrosion resistance with great formability.

This goal is achieved by a process for the production of a steel strip having a zinc coating, in which the steel strip is continuously subjected to the following working stages, which are carried out one after the other, and thus the strip is subjected to the process of applying a zinc coating by hot dip, which is carried out in at least two degrees:

- continuous heat treatment of the steel strip before entering the first immersion bath,

- coating the steel strip in the first stage of hot dipping to apply the base layer of zinc coating, whereby the steel strip passes during the first immersion time through the first dipping bath, which contains a zinc melt with a low aluminum content,

- cooling of the steel strip coated with the base layer i

- application of the covering layer to the steel strip, which is covered with the base layer, whereby in the second stage of hot dipping for applying the coating, the steel strip, which is covered with the base layer, passes during the second immersion time through a second immersion bath, which contains a second zinc melt which has a higher aluminum content than the first zinc melt.

According to the invention, which differs from the previously described state of the art, which refers to immersion for applying a coating to a steel strip, the coating, which includes at least two layers, is carried out by a special procedure. This is achieved by first passing the steel strip through the first immersion bath where the first zinc coating is provided. The strip is then cooled, so that the coating applied during the first immersion hardens. Then the strip, which is supplied with the first solid base layer, is covered with a cover layer using a second dip to apply the zinc coating. As a result of diffusion, this covering layer forms a coating of the steel strip, which is already covered by the base layer. Also unlike the prior art, this does not introduce any conversion of the base layer into a microstructure analogous to the cover layer, but the original microstructure of the base layer absorbs aluminum by diffusion and is retained as an independent layer. Further overlays may be applied to the first overlay by passing the steel strip through further immersion baths. In this case, the strip is cooled between each of the dipping processes, as necessary.

As a result of the layer-by-layer structure of the coating, the coating produced according to the invention has increased corrosion resistance. In this way, an improved coating thickness of, for example, up to 80 μm compared to the state of the art can be produced using the process according to the invention which in practice could so far only be achieved by zinc coating individual pieces. The increased thickness of the coating leads to an increase in durability during which the coating provides reliable protection even in aggressive environments. Furthermore, the individual layers of the coating can also be stacked with each other so as to ensure a particularly stable adhesion of the coating to the steel strip. At the same time, as a result of a suitable matching of the properties of the layers, any penetration of the coating layer can be limited in case of damage, which otherwise means the risk of peeling of the coating with larger layer thicknesses.

The covering layer made of zinc melt containing aluminum has high elasticity with optimal corrosion protection at the same time. The base layer, on the other hand, is also stretchable and shows an additional increased resistance to corrosion compared to the cover layer. As a result of the extensibility, which is common to both layers, an excellent possibility of deformation is ensured, on the one hand. On the other hand, the optimal matching of the properties of the individual layers gives the coating equally good and long-lasting protection against corrosion.

As a result, the invention makes it possible to provide a zinc-coated steel strip that provides improved corrosion protection compared to the prior art. For this reason, a specific aluminum content in the zinc bath was chosen, specific immersion times of the steel strip in the bath were associated, specific temperatures of the immersion baths were set, and cooling was carried out between individual immersion baths.

In addition to the continuous heat treatment of the steel strip according to the invention, before the first stage of hot dipping for applying the coating, the strip is, on the one hand, pre-treated so that the zinc coating is optimally accepted on its surface. Additionally, smooth surfaces are obtained. The tape produced according to the invention is therefore particularly well adapted to the subsequent deformations. Surprisingly, it has been found that by using heat treatment as a pre-treatment, even high tensile strength steels, which are difficult to coat due to their alloying, can be provided with a zinc coating. Another advantage of heat treatment pretreatment is that the success of the pretreatment can be reliably reproduced. Finally, since the heat treatment process is carried out continuously, high production speeds and shortening of process times can be achieved, compared to the conventional process. In this case, the heat treatment temperature is preferably 400° C to 800° C. Since the continuous heat treatment is carried out under a shielding gas, oxidation of the tape surface is reliably prevented.

Practical tests of the procedure according to the invention have shown that good results of the procedure are obtained if the immersion times are between 1 and 20 seconds. In this case, the quality of the coating can be further increased, taking into account the melt bath used in each case and the set temperature in each case, if the immersion times lie within the range of 3 to 10 seconds.

Additional improvements in coating results can be achieved by optimizing the composition of the melt bath. In practice, it has been found to be advantageous if the first zinc melt contains 0.005 - 0.25% by weight of aluminum, with particularly good results obtained if the first zinc melt contains 0.01 - 0.12% by weight of aluminum. When a melt having such a composition is used in the first stage of hot dip coating, a Zn-Fe alloy is formed as the first layer on the steel strip, which adheres particularly tightly to the surface of the steel strip and forms a zinc overlay. In the second stage of hot dip zinc plating, the base layer formed in the first stage is then converted into a Zn-Al-Fe alloy and a Zn-Al cover layer is formed. A high-quality coating can be formed if a second zinc melt is used in the second coating flow, which contains 3 wt.% - 15 wt.%, especially 4 wt.% to 6 wt.% aluminum. A melt with such a composition applied to the base layer creates a ZnAl cover layer, which is distinguished by high ductility and high corrosion resistance.

The temperature at which the zinc melt, which contains a specified amount of aluminum, is applied in the first stage of hot-dip zinc coating should be 440-490 °C. In this temperature range, a base layer with the desired thickness can be produced by adhesion without any problems.

Optimum coating results can then be achieved during the second coating stage, if the temperature in the second hot dip bath is 420 °C - 480 °C.

After zinc coating, the coated steel strip can be supplied with an organic protective layer that provides additional surface protection against damage during storage or transportation.

It is also advisable, considering the production of particularly high-quality steel strips, to cool the coated steel strip after the second immersion in the bath. Practical tests have shown in this regard that particularly good results are obtained if the cooling rate during the cooling time after the second immersion in the bath is at least 4 °C/second.

The process according to the invention makes it possible to obtain a cold-rolled or hot-rolled steel strip made of quenched, low-carbon steel, microalloyed ULC steel or high tensile strength and ultra high tensile strength steel, with at least two coating layers, comprising a base layer, which is produced on the surface steel strip of zinc melt having a first composition, and at least one covering layer which is applied to the base layer, and which is produced from zinc melt, which has a composition different from the first zinc melt. The compositions of the zinc melts are appropriately chosen, so that the base layer is made of ZnAlFe, and the cover layer is made of ZnAl, whereby, as mentioned earlier, the melt used for the production of the base layer contains 0.005 - 0.25 % of aluminum as well as zinc, and the melt used for the production of the covering layer contains 3.5 - 15 % aluminum as well as zinc.

The above-mentioned objective, as far as the steel strip is concerned, is solved by producing a steel strip according to the process according to the invention and coating it with a coating containing at least one base layer and a cover layer placed on it. The at least two-layer structure, which is in accordance with the invention, allows the individual layers of the coating to stack with each other, so that their properties are optimally complementary in the manner previously explained in connection with the process in accordance with the invention. Particularly advantageous combinations of properties are obtained if the base layer mainly consists of Zn-Al-Fe, and the covering layer mainly consists of Zn-Al.

The aluminum content of the individual layers of the coating, which is applied to the steel strip, is preferably selected so that the aluminum content in the coating is at least 4% by weight. This can be achieved, for example, on the basis of a base layer that has an aluminum content of at least 5% by weight. Given that the forming properties of the coated steel strip are also basically suitable, if the aluminum content of the cover layer is lower than the aluminum content of the base layer. In this way, steel strips, which are coated with a coating made in accordance with the invention, and which have an aluminum content of at least 3% by weight in the area of the covering layer, possess particularly good shaping properties and at the same time high corrosion resistance.

The share of the thickness of the base layer in the total thickness, which is made up of the sum of the thicknesses of the base and covering layers, should preferably be at least 10%. In this way, firstly, the adhesion of the base layer to the steel strip is fully ensured. Secondly, in this way, the requirements for the production of a sufficiently thick and firmly attached covering layer are met. The steel strips according to the invention thus have a total coating thickness of at least 20 μm.

The process according to the invention is particularly adapted to the production of steel strips which are also according to the invention.

The invention is explained in detail in the following part with reference to the drawings showing examples of implementation, wherein:

Fig. 1 is a schematic section of a plant for hot-dip zinc coating on steel strip;

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

The steel strip S, which may be, for example, a hot-rolled metal sheet containing Al-tempered low-carbon steel, unwound by an unwinding device, not shown, and degreased in a conventional manner in a pretreatment device, also not shown, and etched in a dilute acid. The steel strip is then continuously heat-treated in furnace 1 with shielding gas at a temperature of, for example, 600 °C.

The steel strip thus prepared is transported at a speed of between 10 and 50 m/min in the transport direction F into the melt bath 2 of the first hot dip device 3.

The melt bath 2 of the hot dip device 3 is filled with a zinc melt having an aluminum content of, for example, 0.05% by weight. The temperature of the melt bath 2 is 460 °C. The time required to pass through the melt bath is, for example, 10 seconds.

After emerging from the melt bath 2, the steel strip S passes through a device with nozzles for removing excess molten melt 4. In said device with nozzles for removing excess molten melt 4, the thickness d1 of the base layer G, which is formed from the zinc melt from the bath, is adjusted with melt 2, the amount representing the excess, still liquid melt adhering to the surface O of the steel strip S being blown off the steel strip S by means of slotted nozzles.

The steel strip S then passes through the cooling device 5 where it is cooled to a temperature of 250 °C. During this cooling, the melt, which has adhered to the surface O of the steel strip S, hardens.

Behind the cooling device 5 there is a flux device 7 in which the steel strip, if necessary, goes through a further wetting process followed by drying.

The steel strip S, which has been cooled and, if necessary, passed through another wetting process, then enters the melt bath 8 of the second hot dip device 9.

The melt bath 8 of the hot dip device 9 is filled with zinc melt, which, in addition to zinc, also has an aluminum content of, for example, 4.5% by weight. The temperature of the bath with melt 8 is also 460 °C. The immersion time required to pass through the melt bath 8 is, for example, 10 seconds.

After emerging from the melt 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, which is placed on the base layer G, and obtained from the zinc melt from the melt bath 8, is adjusted. wherein the excess, still liquid melt adhering to the surface O' of the base layer G is blown away from the surface O' of the base layer G by means of a gapped nozzle.

Behind the blowing device 10 in the direction of movement F is 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 a coating device not shown, the steel strip is then given an additional protective layer which gives it a temporary surface protection.

From Figure 2 it can be seen that as a result a steel strip S is produced which is provided with a coating B which firmly adheres to it. This coating B is, on the one hand, obtained from the base layer G containing Fe and additionally Zn and Al, as a result of the influence of the iron contained in the steel strip. The thickness d1 of the base layer G is approximately 25% of the total thickness dg of the coating B created from the thickness d1 of the base layer G and the thickness d2 of the cover layer D.

From Figure 2 it can be clearly seen that the base layer G has "formed" on the steel strip S and that the melt of the cover layer D has diffused into the base layer G during the time spent in the second melt bath 8, so that an inner layer is formed between the base layer G and covering layer D.

The following Tables I - IV each give the results of tests conducted using a hot dip simulator (examples in Tables I - III) or using a strip coating plant (examples in Table IV).

The tests whose results are given in Table I were performed using the Rhesca hot dip simulator under laboratory conditions. In this case, the aluminum content in the first bath with zinc was between 0.1 and 0.2% by weight. Such aluminum contents are used as a standard in single-stage single zinc strip coating plants (see examples 1 and 2 in Table II) and are required for good zinc adhesion.

Alternatively, aluminum contents of up to 5% are also taken for single zinc coating in strip zinc coating plants. The composition of the melt intended for these needs is known under the production name "Galfan". Example 4 in Table I shows the results obtained using a zinc melt containing 5% aluminum in a single zinc coating.

In strip zinc plating plants with flux pretreatment, no single zinc plating is performed in baths containing up to 5% aluminum, since defects appear in this case. Example 3 in Table I confirms this statement.

In addition to the aluminum contents, in Examples 5-7 in Table I, which were zinc coated in two stages in accordance with the invention and underwent a preliminary heat treatment, the immersion times in the second melt bath varied between 10 and 20 seconds. In some cases the process was carried out "with" interphase flux, and in some cases "without" such flux. The cooling rate achieved during final cooling 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 steel strips with a coating that has particularly good adhesion properties, great formability and good corrosion resistance.

The results listed in Table II were also obtained from tests that were conducted using the Rhesca hot dip simulator under laboratory conditions. In the series of tests shown in Table II, the aluminum contents in the first zinc bath were reduced for each of them to 0.08 or 0.05% by weight. For single-step zinc plating, both with flux pretreatment and also with thermal pretreatment result in poor adhesion, as can be seen, for example, from the results of examples 8 and 9.

For the two-stage coating carried out in accordance with the invention, the immersion times for the second zinc coating were held constant at 10 seconds. However, they were sometimes different, for the procedure with interphase flux (“with” and “without”) and the cooling rates for the final walk, which were set between 4.5 °C/s and 40 °C/s. Examples 10 to 14 show in Table II that samples produced using a process in accordance with the invention have a coating with particularly good adhesion and, in some cases, excellent corrosion test life.

The tests whose results are presented in Table III were also obtained in tests conducted using the Rhesca hot dip simulator. In that series of tests, the aluminum contents in the first zinc bath were reduced even more compared to the series of tests previously explained. These were 0.01% by weight. Nevertheless, all the examples in Table III show good adhesion and very high corrosion resistance.

[image]

[image]

[image]

List of reference symbols

1 Flux device

2 Melt bath

3 Hot dip device

4 Device with a nozzle for removing excess molten coating

5 Cooling device

6 Flux device

8 Melt bath

9 Hot dip device

10 Blowing device

11 Cooling device

B Coating

D Overlay

G Base layer

d1 Thickness of the base layer G

d2 Thicker covering layer D

dg Total coating thickness B

O The surface of the steel strip S

O' The surface of the base layer G

S Steel strip

Claims (22)

1. A process for the production of a steel strip (S), which is provided with a zinc coating, characterized in that the steel strip (S) continuously passes through the following working processes, which follow one another, and is subjected to at least a two-stage application process hot dip zinc coating: - continuous heat treatment of the steel strip (S) before entering the first immersion bath, coating the steel strip (S), in the first stage of applying the zinc coating by hot dipping with the base layer (G), whereby the steel strip passes during the first immersion time through the first immersion bath (2), which contains a zinc melt that has a low aluminum content, - cooling of the steel strip (S) which is coated with the base layer G-i - application of the covering layer (D) on the steel strip (S) which is covered with the base layer (G), while in the second stage of coating by hot dipping the steel strip (S), which is covered with the base layer (G), the steel strip (S) passes in the second immersion time through the second immersion bath (8), which contains a second zinc melt that has a higher aluminum content than the first bath with zinc. 2. The method according to claim 1, characterized in that the immersion times are 1 to 20 seconds. 3. The method according to claim 2, characterized in that the immersion times are in the range of 3 to 10 seconds. 4. Process according to one of the previous patent claims, characterized in that the first zinc melt contains 0.005% by weight - 0.25% by weight, preferably 0.01 to 0.12% by weight of aluminum. 5. Process according to one of the previous patent claims, characterized in that the second zinc melt contains 3% by weight - 5% by weight of aluminum. 6. Process according to patent claim 5, characterized in that the second zinc melt contains 4% by weight - 6% by weight of aluminum. 7. The method according to one of the previous patent claims, characterized in that the temperature of the zinc melt in the first immersion bath (2) is 440 °C - 490 °C. 8. The method according to one of the previous patent claims, characterized in that the temperature in the second immersion bath (8) is 420 °C - 480 °C. 9. Process according to one of the previous patent claims, characterized in that the heat treatment temperature is between 400 and 800 °C. 10. The process in accordance with one of the previous patent claims, characterized in that the thermal treatment takes place continuously under a protective gas. 11. The method according to one of the previous patent claims, characterized by the fact that after applying the zinc coating, a protective layer is applied to the covering layer (D) by hot dipping, which protects the surface of the covering layer (D). 12. The method according to one of the preceding patent claims, characterized in that the coated steel strip (S) is cooled after the second immersion bath (8). 13. The method according to claim 12, characterized in that the cooling rate is at least 4 °C/s. 14. A steel sheet produced by a process according to one of claims 1 to 13, characterized in that it is coated with a coating (B) made of at least one base layer (G) and a cover layer (D), which is applied to it. 15. Steel sheet according to claim 14, characterized in that the base layer (G) essentially consists of Zn-Al-Fe, and the covering layer (D) essentially consists of Zn-Al. 16. Steel sheet according to one of patent claims 14 or 15, characterized in that the aluminum content of the coating (B) is at least 4% by weight. 17. Steel sheet in accordance with one of patent claims 14 to 16, characterized in that the aluminum content of the base layer (G) is at least 5% by weight. 18. Steel sheet in accordance with one of patent claims 14 to 17, characterized in that the aluminum content of the covering layer (D) is lower than the aluminum content of the base layer (G). 19. Steel sheet in accordance with claim 18, characterized in that the aluminum content of the covering layer (D) is at least 3% by weight. 20. Steel sheet in accordance with one of patent claims 14 to 19, characterized in that the proportion of the thickness (d1) of the base layer (G) in the total thickness (dg), which includes the sum of the thicknesses (d1, d2) of the base layer (G ) and covering layer (D), at least 10%. 21. A steel sheet according to one of claims 14 to 20, characterized in that the total thickness (dg) is at least 20 μm. 22. Steel sheet in accordance with one of patent claims 14 to 21, characterized in that it is produced by a process in accordance with one of patent claims 1 to 16.