EP2432910B2 - Method for hot-dip coating a flat steel product containing 2-35 wt% mn and flat steel product - Google Patents

Description

The invention relates to a method for hot-dip coating a flat steel product containing 6-35% by weight of Mn with zinc or a zinc alloy, and a flat steel product provided with a zinc or zinc alloy coating.

In modern automobile construction, use is increasingly being made of high-strength and ultra-high-strength steels. Typical alloying elements are manganese, chromium, silicon, aluminum and others, which form stable, non-reducible oxides on the surface with conventional recrystallizing annealing. These oxides can prevent reactive wetting with a molten zinc.

Steels with a high manganese content are particularly suitable for use in vehicle construction due to their favorable combination of properties, consisting of high strengths of up to 1,400 MPa on the one hand and extremely high elongations (uniform elongation up to 70% and elongation at break up to 90%) on the other , especially in the automotive industry. Steels with high Mn contents of 6% by weight to 30% by weight that are particularly suitable for this purpose are, for example, from DE 102 59 230 A1 , the DE 197 27 759 C2 or the DE 199 00 199 A1 known. Flat products made from the known steels exhibit isotropic deformation behavior with high strength and are also still ductile even at low temperatures.

However, these advantages are offset by the fact that high-manganese steels tend to pit and are difficult to passivate. In comparison to low-alloy steels, this great tendency towards locally limited but intensive corrosion when exposed to increased chloride ion concentrations makes it difficult to use steels belonging to the material group of high-alloy steel sheets, especially in body construction. In addition, steels with a high manganese content tend to surface corrosion, which also limits the spectrum of their use.

It has therefore been proposed to also provide flat steel products made from high-manganese steels with a metallic coating in a manner known per se, which protects the steel from corrosive attack. Practical attempts to provide steel strips with high manganese contents with a metallic protective layer by means of a cost-effective hot-dip coating process have produced unsatisfactory results, in addition to fundamental problems with wetting with the Zn melt, particularly with regard to the adhesion to the steel substrate required of the coating during cold deformation.

The reason for these poor adhesion properties was determined to be the thick oxide layer that develops during the annealing process that is essential for hot-dip coating. The sheet metal surfaces oxidized in this way can no longer be wetted with the coating metal with the required uniformity and completeness, so that the goal of comprehensive corrosion protection is not achieved.

The possibilities of improving wettability by applying an intermediate layer of Fe or Ni, which are known from the area of high-alloy steels with lower Mn contents, did not lead to the desired success in steel sheets with at least 6% by weight manganese.

In the DE 10 2005 008 410 B3 it has been proposed to apply an aluminum layer to a steel strip containing 6-30% by weight of Mn before the final annealing preceding the hot-dip coating. The aluminum adhering to the steel strip prevents its surface from oxidizing during the annealing of the steel strip prior to hot-melt coating. The aluminum layer then acts like an adhesion promoter so that the coating produced by the hot-melt coating adheres firmly and over the entire surface to the steel strip even if the steel strip itself offers unfavorable conditions for this due to its alloy. For this purpose, in the known method, the effect is used that diffusion of the iron of the steel strip into the aluminum layer occurs during the annealing treatment that necessarily precedes the molten coating. In the course of the annealing, a metallic coating, consisting essentially of Al and Fe, is formed on the steel strip, which is materially connected to the substrate formed by the steel strip.

Another method for coating a high-manganese steel strip containing 0.35-1.05% by weight C, 16-25% by weight Mn, the remainder iron and unavoidable impurities is known from WO 2006/042931 A1 known. According to this known method, the steel strip composed in this way is first cold-rolled and then recrystallized annealed in an atmosphere which has a reducing effect on iron. The annealing parameters are selected in such a way that an intermediate layer is formed on both sides of the steel strip, which essentially consists entirely of amorphous (FeMn) oxide, and an additional outer layer is formed, which consists of crystalline Mn oxide, with the thickness of the both layers is at least 0.5 µm. Hot-dip coating then no longer takes place. Rather, the Mn oxide layer in combination with the (FeMn) oxide layer should offer adequate protection against corrosion.

This is based on a similar principle in the WO 2006/042930 ( EP 1 805 341 B1 ) described method, according to which a layer of iron-manganese mixed oxides is first produced on the steel substrate with a high Mn content and then an outer layer consisting of Mn mixed oxides is produced on this layer by two successive annealing steps. The steel strip coated in this way is then fed into a melt bath. In addition to zinc, this melt strip also contains aluminum in an amount sufficient to completely coat the MnO layer and the (FeMn)O layer at least partially reduced. As a result, a layer structure should be achieved in which three FeMnZn layers and an outer Zn layer can be identified.

Practical investigations have shown that even steel strips that have been pre-coated in such a complex manner do not have the adhesion to the steel substrate required for cold forming in practice. In addition, this is evident from the WO 2006/042930 known processes as not sufficiently reliable due to the reactions taking place in the molten bath, which in practice can hardly be controlled.

Finally is out of the DE 10 2006 039 307 B3 a method for hot-dip coating a steel substrate having a high Mn content is known, in which the ratio %H ₂ O/%H ₂ of the water content %H ₂ O to the hydrogen The %H ₂ content of the annealing atmosphere is adjusted as a function of the respective annealing temperature T _G in such a way that the ratio %H ₂ O/%H ₂ is less than or equal to 8*10 ^-15 *x T _G ^3.529 , with T denoting the annealing temperature is. This specification is based on the knowledge that by suitably setting the annealing atmosphere, namely its hydrogen content in relation to its dew point, a surface quality of the steel strip to be coated is achieved during annealing that ensures optimal adhesion of the metallic protective coating subsequently applied by hot-dip coating . The annealing atmosphere set in this way has a reducing effect on both the iron and the manganese in the steel strip. The aim is to avoid the formation of an oxide layer that would impair the adhesion of the fused coating to the high-manganese steel substrate.

Practical investigations have shown that flat steel products prepared according to the known method explained above have good wetting behavior and adhesion of the Zn coating which is sufficient for many applications. However, when flat steel products with a corresponding coating were deformed into components, it was found that the coating still came off and cracked at high degrees of deformation.

Furthermore, the methods known from the prior art, particularly when high process temperatures are used, can adversely affect the mechanical properties in the flat steel product. Furthermore, no more economical operation that meets ecological requirements is possible with the existing processes.

Against this background, the object of the invention was to provide a method that allows flat steel products containing high levels of Mn to be provided with a zinc coating that protects against corrosion and in which further improved adhesion of the coating on the steel substrate is ensured. In addition, a steel flat product was to be created in which the Zn coating formed from zinc or a zinc alloy adheres securely to the steel substrate even under high degrees of deformation.

With regard to the method, this object is achieved according to the invention in that the work steps specified in claim 1 are carried out during the hot-dip coating of a flat steel product having a high Mn content.

With regard to the product, the object specified above has also been achieved by a flat steel product which, according to the invention, has the features specified in claim 7 .

According to the invention, a flat steel product in the form of a steel strip or sheet steel is first provided in a continuous process sequence for the hot-dip coating of a flat steel product containing 6-35% by weight of Mn.

The coating procedure according to the invention is particularly suitable for steel strips that are highly alloyed in order to ensure high strength and good elongation properties.

Steel strips which are provided with a metallic protective coating by hot-dip coating in the manner according to the invention contain (in % by weight) C: ≦1.6%, Mn: 6-35%, Al: ≦10%, Ni: ≦10%, Cr: ≤ 10%, Si: ≤ 10%, Cu: ≤ 3%, Nb: ≤ 0.6%, Ti: ≤ 0.3%, V: ≤ 0.3%, P: ≤ 0.1%,
B: ≤ 0.01%, Mo: ≤ 0.3%, N: ≤ 1.0%, balance iron and unavoidable impurities.

According to the invention, the effects achieved by the invention are effective in the coating of high-alloy steel strips which contain manganese contents of at least 6% by weight. It is thus evident that a steel base material which (in % by weight) C: ≤ 1.00%, Mn: 20.0 - 30.0%, Al: ≤ 0.5%, Si: ≤ 0.5% , B: ≤ 0.01%, Ni: ≤ 3.0%, Cr: ≤ 10.0%, Cu: ≤ 3.0%, N: < 0.6%, Nb: < 0.3%, Ti : < 0.3%, V: < 0.3%, P: < 0.1%, the remainder containing iron and unavoidable impurities, can be coated particularly well with a coating that protects against corrosion.

The same applies if a steel is used as the base material which (in % by weight) C: ≤ 1.00%, Mn: 7.00 - 30.00%, Al: 1.00 - 10.00%, Si : > 2.50 - 8.00% (whereby the sum of Al content and Si content is > 3.50 - 12.00%), B: < 0.01%, Ni: < 8, 00%, Cu: <3.00%, N: <0.60%, Nb: <0.30%, Ti: <0.30%, V: <0.30%, P: <0.01% , balance iron and unavoidable impurities.

As with conventional hot-dip coating, both hot-rolled and cold-rolled steel strips can be coated in the manner according to the invention as flat steel products, with the method according to the invention being particularly effective in the processing of cold-rolled steel strip.

The flat products made available in this way are annealed in a work step b). The glow temperature Tg is 600 - 1100 °C, while the annealing time, over which the steel flat product is kept at the annealing temperature, is 10 - 240 s.

It is crucial for the invention that the annealing temperature Tg and annealing duration mentioned above has a reducing effect on iron oxide FeO present on the steel flat product and an oxidizing effect on the manganese contained in the steel substrate. For this purpose, the _{annealing atmosphere} contains _0.01 - 85 vol ₂ O/H ₂ ratio applies: $$\mathrm{8th}\times {10}^{-15\ast }{\mathrm{day}}^{\mathrm{3,529}}<{\mathrm{H}}_2\mathrm{O}/{\mathrm{H}}_2\le 0.957$$

According to the invention, the H ₂ O/H ₂ ratio is to be set such that it is greater than 8×10 ^-15 *Tg ^3.529 and at most 0.957, with Tg denoting the respective annealing temperature.

In typical applications, which aim in particular to produce a Mg-containing zinc alloy coating on the respective steel substrate in a one-stage annealing process according to the invention, the dew point of the atmosphere is in the range from -50°C to +60°C. At the same time, the annealing atmosphere in this case typically contains 0.1-85% by volume of H ₂ . A particularly economical mode of operation of the continuous furnace used for annealing according to the invention can be achieved by keeping the dew point of the atmosphere at -20.degree. C. to +20.degree.

As a result, a 20 - 400 nm thick Mn mixed oxide layer covering at least sections of the flat steel product is produced by annealing on the flat steel product before hot-dip coating. With regard to the adhesion of the Zn coating on the steel substrate, it is particularly favorable if the Mn mixed oxide layer essentially completely covers the surface of the flat steel product after annealing. In the context of the invention, the Mn mixed oxide layer is defined as MnO·Fe _metal . This means that metallic iron is present in this Mn mixed oxide layer and not oxidized iron, as is the case in the prior art.

According to the invention, an Mn mixed oxide layer is set in a targeted manner via at least one annealing stage, in that the annealing (step b)) is carried out under an atmosphere that reduces FeO and an atmosphere that oxidizes Mn.

Surprisingly, it has been shown that a flat steel product is obtained in this way, which ensures good wetting during the hot-dip coating that is subsequently carried out. Likewise, the layer of Mn mixed oxides produced according to the invention on the steel substrate forms an adhesion base to which the subsequently applied zinc layer surprisingly adheres particularly securely. In contrast to in the WO 2006/042930 According to the state of the art described, the Mn mixed oxide layer is largely retained during the hot-dip coating process, so that it also ensures the permanent cohesion of the Zn coating and steel substrate in the finished product. After the annealing step explained above, the annealed flat steel product is cooled to a bath entry temperature at which it enters the molten Zn bath. Typically, the bath entry temperature of the steel flat product is in the range of 310 - 710 °C.

The flat steel product, which has been cooled to the bath inlet temperature, is then passed through an iron-saturated, 420 - 520 °C hot Zn molten bath within an immersion time of 0.1 - 10 seconds, in particular 0.1 - 5 s, which consists of the main component zinc and zinc unavoidable impurities and 0.05-8% by weight Al and/up to 8% by weight Mg, in particular 0.05-5% by weight Al and up to 5% by weight Mg. In addition, in the molten bath optionally Si < 2%, Pb < 0.1%, Ti < 0.2%, Ni < 1%, Cu < 1%, Co < 0.3%, Mn < 0.5%, Cr < 0.2%, Sr < 0.5%, Fe < 3%, B < 0.1%, Bi < 0.1%, Cd < 0.1% present in order to determine certain properties of the coating in a manner known per se set.

The flat steel product obtained in this way, which is hot-dip coated with a Zn protective coating to protect against corrosion, is finally cooled, it being possible for the thickness of the coating to be adjusted in a manner known per se before cooling.

The Zn coating according to the invention necessarily contains Al contents of 0.05-8% by weight and additional contents of up to 8% by weight Mg, with the upper limit of the contents of both elements being typically limited to a maximum of 5% by weight in practice. % is limited.

A flat steel product according to the invention with an Mn content of 6-35% by weight and a Zn protective coating that protects against corrosion is accordingly characterized in that the Zn protective coating has a Mn mixed oxide layer that essentially covers and adheres to the flat steel product, in which metallic Iron is present, and has a Zn layer that shields the flat steel product and the Mn mixed oxide layer adhering to it from the environment.

A particularly good adhesion of the zinc layer on the steel substrate results when the Zn protective coating comprises an Fe(Mn) ₂ Al ₅ layer arranged between the Mn mixed oxide layer and the Zn layer. This occurs when there is a sufficient quantity of aluminum of 0.05 - 5% by weight Al in the molten bath. The Fe (Mn) ₂ Al ₅ layer forms a barrier layer through which the reduction of the Mn mixed oxide layer melt dipping is reliably prevented. Depending on the Al content, which is in particular between 0.05 and 0.15% by weight, the barrier layer can transform into FeZn phases, with the Mn oxide layer nevertheless being retained.

The MnO layer and the Fe(Mn) ₂ Al ₅ layer of a coating produced and configured according to the invention thus ensure, even after hot-dip coating, that the outer Zn layer adheres firmly to the steel substrate with high degrees of deformation.

However, the presence of a Mn mixed oxide layer on the surface of the steel substrate according to the invention not only has a positive effect if the Fe(Mn) ₂ Al ₅ layer is also formed, but also if magnesium is used as an alternative or in addition to aluminum in the molten bath is present in effective levels. Even when a ZnMg coating layer is produced on the steel substrate, the MnO layer produced according to the invention ensures particularly good and uniform wetting of the steel flat product with optimum adhesion and minimized risk of cracking or flaking even with high degrees of deformation.

A particularly practical embodiment of the invention results in this connection when the following applies to the ratio of the Al content %Al and the Mg content %Mg: %Al/%Mg<1. In this embodiment of the invention, the Al content is therefore of the melt bath is always smaller than its Mg content. This has the advantage that the boundary layer formation aimed at according to the invention leads to an increase in the metallic iron in the mixed oxide layer even without a special sequence of annealing steps in the context of the method according to the invention. Magnesium is characterized by a higher reduction potential to MnO than aluminum. Therefore, when higher Mg contents are present in the melted layer, there is a forced dissolution of the MnO structure of the mixed oxide layer. Since the mixed oxide is dissolved to a greater extent, more metallic iron "Fe _metal " from the "depth" of the mixed oxide layer is effectively available at the mixed oxide layer/zinc bath reaction front, so that the covering Fe(Mn) ₂ Al ₅ boundary layer is particularly effective as an adhesion promoter can train. Accordingly, the MnO reduction by dissolved magnesium in situ contributes with a particularly high degree of effectiveness to the boundary layer formation that is aimed at according to the invention and ensures particularly good adhesion of the Zn coating.

The annealing step (work step b)) carried out in preparation for the hot-dip coating in the context of the process according to the invention can be carried out in one or more stages. If the annealing is carried out in one stage, different hydrogen contents are possible in the annealing atmosphere depending on the dew point. If the dew point is in the range from -50 °C to +20 °C, the annealing atmosphere can contain at least 0.01% by volume H ₂ , but less than 3% by volume H ₂ . If, on the other hand, a dew point of at least +20 °C up to and including + 60 °C is set, the hydrogen content should be in the range of 3% to 85% so that the atmosphere has a reducing effect on iron. Taking into account the other parameters to be taken into account during the implementation of the annealing step according to the invention, the reducing effect on the FeO that may be present and the oxidizing effect on the Mn present in the steel substrate are thus reliably achieved.

If, on the other hand, the flat steel product is to be annealed in two stages before entering the molten bath, an additional annealing step can be carried out before the annealing step (work step b) of claim 1) carried out according to the invention, in which the flat steel product is heated to an annealing temperature of 200 - 1100 °C is maintained for an annealing period of 0.1-60 s under an atmosphere oxidative for both Fe and Mn, containing 0.0001-5 vol.% H ₂ and optionally 200-5500 vol.ppm O ₂ and a has a dew point in the range of -60 °C to +60 °C. The annealing step according to the invention is then carried out at a dew point in the range from -50 °C to +20 °C in an atmosphere containing 0.01-85% hydrogen, taking into account the other parameters to be taken into account when carrying out the annealing step according to the invention, before the steel flat product into the melt bath.

Optimum adhesion properties of the Zn coating are achieved with a coating produced according to the invention when the thickness of the Mn mixed oxide layer obtained after annealing (step b)) is 40-400 nm, in particular up to 200 nm.

It also contributes to optimizing the deformation behavior of a flat steel product produced according to the invention if the flat steel product provided with the Mn mixed oxide layer is subjected to an overaging treatment before it enters the molten bath.

Fig. 1

ein mit einem Al-haltigen Zn-Überzug versehenes Stahlflachprodukt in einer schematischen Schnittdarstellung;

Fig. 2

einen Schrägschliff einer Probe eines mit einem Zn-Überzugs versehenen Stahlflachprodukts;

Fig. 3

ein mit einem ZnMg-Überzug versehenes Stahlflachprodukt in einer schematischen Schnittdarstellung;

Fig. 4

einen Schrägschliff einer Probe eines mit einem ZnMg-Überzugs versehenen Stahlflachprodukts.

The invention of exemplary embodiments is explained in more detail below. Show it:

1

a flat steel product provided with an Al-containing Zn coating in a schematic sectional view;

2

a bevel section of a sample of a Zn-coated steel flat product;

3

a flat steel product provided with a ZnMg coating in a schematic sectional view;

4

a bevel section of a sample of a ZnMg-coated steel flat product.

A cold-rolled steel strip has been produced in a known manner from a high-manganese steel having the composition given in Table 1. Table 1 C Mn P si v Al Cr Ti Nb 0.634 22.2 0.02 0.18 0.2 0.01 0.08 0.001 0.001 Rest iron and unavoidable impurities, data in % by weight

A first sample of the cold-rolled steel strip was then annealed in a single-stage annealing process.

For this purpose, the steel strip sample was heated at a heating rate of 10 K/s to an annealing temperature Tg of 800° C., at which the sample was then held for 30 seconds. The annealing took place under an annealing atmosphere which consisted of 5% by volume H ₂ and 95% by volume N ₂ and whose dew point was +25°C. The annealed steel strip was then cooled at a cooling rate of 20 K/s to a bath inlet temperature of 480° C., at which point it was initially subjected to an overaging treatment for 20 seconds. The overaging treatment took place under the unchanged annealing atmosphere. Without leaving the annealing atmosphere, the steel strip was then fed into a 460 °C hot, Fe-saturated molten zinc bath which, in addition to Zn, unavoidable impurities and Fe, also contained 0.23% by weight Al. After an immersion time of 2 seconds, the steel strip, which is now hot-dip coated, was removed from the molten bath and cooled to room temperature.

In a second test, a second sample of the cold-rolled steel strip composed according to Table 1 was annealed in a two-stage process in a process sequence that also ran continuously and was then hot-dip coated.

For this purpose, the steel strip was first heated to 600 °C at a heating rate of 10 K/s and held at this annealing temperature for 10 seconds. The annealing atmosphere contained 2000 ppm O ₂ and the remainder N ₂ . Her dew point was -30 °C.

Immediately afterwards, the steel strip was heated in a _second annealing step to an annealing temperature Tg of 800 °C, at which it was held for 30 seconds under an annealing atmosphere containing ₅ vol -30 °C. The steel strip was then cooled to 480° C. under the annealing atmosphere at a cooling temperature of approx. 20 K/s and subjected to an overaging treatment for 20 seconds. The steel strip was then fed with a bath inlet temperature of 480 °C into a 460 °C hot melt bath saturated with Fe, which in turn contained 0.23% by weight Al and other elements in ineffective traces of impurities and the remainder zinc. After an immersion time of 2 seconds, the finished hot-dip coated flat steel product was then removed from the melt bath and cooled to room temperature.

In 1 the structure of the coating Z thus obtained on the steel substrate S is shown schematically. Accordingly, on the steel substrate S there is a Mn _y O _x manganese mixed oxide layer M (M = MnO Fe) on which there is an Fe(Mn) ₂ Al ₅ intermediate layer F (F = MnO Fe (Mn) ₂ Al ₅ ) or with an Al content of maximum 0.15 wt. The thickness of the Mn mixed oxide layer M is 20-400 nm, while the thickness of the Fe(Mn) ₂ Al ₅ intermediate layer F is 10-200 nm. The total thickness of the coating layers M and F is accordingly 20 - 600 nm. The zinc layer Zn, on the other hand, is significantly thicker at 3 - 20 µm.

In 2 an oblique section of a sample produced in the manner described above is shown. The steel substrate S and the Mn _y O _x manganese mixed oxide layer M lying thereon with embedded metallic iron, the Fe(Mn) ₂ Al ₅ intermediate layer F lying on the mixed oxide layer M and the Zn layer lying on the intermediate layer F can be clearly seen .

To check the success of the procedure according to the invention, twenty additional tests 1-20 were carried out in which the molten bath contained 0.23% by weight Al in addition to Zn and unavoidable impurities. The degree of wetting and the zinc adhesion were examined visually on the samples obtained in this way. The notched-bar impact test according to SEP 1931 was used as the test principle. The test parameters and results of these tests are given in Table 2.

In addition, a further sixteen tests 21 - 36 were carried out, in which the molten bath contained 0.11% by weight Al in addition to Zn and unavoidable impurities. Compared to the barrier layer shown in the experiment explained above, which was designed as an Fe(Mn) ₂ Al ₅ layer, an FeMnZn barrier layer formed with this lower Al content of the melt bath. The degree of wetting and the zinc adhesion were also examined on the samples obtained in this way. The test parameters and results of these tests are given in Table 3.

On the basis of further samples of the high-manganese steel strip cold-rolled from the steel composed according to Table 1, the influence of the dew point of the respective annealing atmosphere on the coating result was examined. For this purpose, the samples were each subjected to an annealing process in which they were also heated to an annealing temperature Tg of 800 °C at a heating rate of 10 K/s. The sample was then held at this annealing temperature for 60 seconds. The annealing took place in each case under an annealing atmosphere that consisted of 5% by volume H ₂ and 95% by volume N ₂ , the respective dew point of the annealing atmosphere being varied between −55° C. and +45° C.

After the heat treatment, the annealed steel strip was cooled at a cooling rate of 20 K/s to a bath inlet temperature of 480° C., as in the test series described above, at which point it was initially subjected to an overaging treatment for 20 seconds. The overaging treatment took place under the unchanged annealing atmosphere. Without leaving the annealing atmosphere, the steel strip was then fed into a 460 °C hot, Fe-saturated molten zinc bath which, in addition to Zn, unavoidable impurities and Fe, also contained a combination of 0.4% by weight Al and 1.0 wt% Mg or alone 0.14 wt%, 0.17 wt% or 0.23 wt% Al. After an immersion time of 2 seconds, the steel strip, which is now hot-dip coated, was removed from the molten bath and cooled to room temperature.

In 3 1 is a schematic representation of the structure of the ZnMg coating Z' thus obtained on the steel substrate S'. Accordingly, on the steel substrate S' there is a Mn _y O _x manganese mixed oxide layer M' (M = MnO Fe) on which there is an Fe(Mn) ₂ Al ₅ intermediate layer F (F = MnO Fe (Mn) ₂ Al ₅ ) or, with an Al content of at most 0.15% by weight, has formed an FeMnZn layer in the melt bath, which in turn is shielded from the environment by a ZnMg layer. The thickness of the Mn mixed oxide layer M' is 20-400 nm, while the thickness of the Fe(Mn) ₂ Al ₅ intermediate layer F' is 10-200 nm. The total thickness of the coating layers M' and F' is accordingly 20 - 600 nm. The zinc layer ZnMg, on the other hand, is significantly thicker at 3 - 20 µm.

In 4 an oblique section of a sample produced in the manner described above is shown. The steel substrate S' and the Mn _y O _x manganese mixed oxide layer M' lying thereon with embedded metallic iron, the Fe(Mn) ₂ Al ₅ intermediate layer F' lying on the mixed oxide layer M and the ZnMg lying on the intermediate layer F' are clear - layer to recognize.

In addition to the already mentioned variation of the dew points of the annealing atmosphere, the Al and Mg contents of the melt bath were varied in twenty-one tests 37-57 carried out to check the success of the procedure according to the invention. The degree of wetting and the zinc adhesion were examined visually on the samples obtained in this way. The notched-bar impact test according to SEP 1931 was also used here as the test principle. The test parameters and results of these tests are given in Table 4.

It has been shown that with the combined presence of Al and Mg and an adjustment of the dew point to the range from -50 °C to +60 °C, zinc-based coatings can also be reliably produced on high-manganese steel substrates in a single-stage annealing process.

For comparison, a further three samples V1-V3 and V4-V6 were obtained from a cold-rolled steel strip that consisted of an Al-TRIP steel VS1 and a steel strip that consisted of a likewise cold-rolled Si-TRIP steel VS2. The composition of the steels VS1 and VS2 are given in Table 5. Table 5 C Mn P si v Al Cr Ti Nb VS1 0.22 1.1 0.02 0.1 0.002 1.7 0.06 0.1 0.001 VS2 0.18 1.8 0.02 1.8 0.002 0 0.06 0.01 0.001 Rest iron and unavoidable impurities, data in % by weight

The comparative samples C1-C6 were also heat-treated in the manner described above for the samples according to the invention before they were hot-dip coated in the melt bath. In addition to Zn and unavoidable impurities, the molten bath contained 0.4% by weight Al and 1% by weight Mg in each case. The degree of wetting and the zinc adhesion were also examined on samples C1-C6 coated in this way. The test parameters and results of these tests are listed in Table 6. It turns out that due to the lower manganese content of steels VS1 and VS2, no MnO structure forms in the mixed oxidation layer on the surface of the steel substrate. As a result, no covering Fe(Mn) ₂ layer is formed as an adhesion promoter. As a result, there is insufficient MnO reduction in the molten bath due to dissolved magnesium, so that in the case of the comparison samples, adequate wetting and accordingly also insufficient adhesion of the coating cannot be achieved. Table 2 Test no.: 1st glow level 2nd glow stage zinc wetting zinc adhesion According to the invention annealing temp. [°C] annealing time [s] O ₂ content [ppm] annealing temp. Tg [°C] annealing time [s] H ₂ content [%] dew point [°C] 1 single stage 800 60 5 -50 no no no 2 800 60 5 -30 no no no 3 800 60 5 -15 Strongly disturbed no no 4 800 60 5 -5 Strongly disturbed no no 5 800 60 5 5 Strongly disturbed no no 6 800 60 5 +15 Disturbed Restricted no 7 800 60 5 +25 Yes Yes no 8th 800 60 5 +45 Yes Yes no 9 500 10 2000 800 30 5 -30 imperfections Yes no 10 600 10 2000 800 60 5 -30 Yes Yes no 11 700 10 2000 800 30 5 -15 imperfections Yes no 12 800 10 2000 800 30 5 -15 imperfections Yes no 13 500 10 2500 800 30 5 -15 imperfections Yes no 14 600 10 2500 800 30 5 -30 Yes Yes no 15 700 10 2500 800 30 5 -30 Yes Yes no 16 800 10 2500 800 30 5 -30 Yes Yes no 17 500 6 2500 800 30 5 -30 imperfections Yes no 18 600 6 2500 800 30 5 -30 Yes Yes no 19 700 6 2500 800 30 5 -30 Yes Yes no annealing temp. [°C] annealing time [s] O ₂ content [ppm] annealing temp. Tg [°C] annealing time [s] H ₂ content [%] dew point [°C] 20 800 6 2500 800 30 5 -30 Yes Yes no Test no.: 1st glow level 2nd glow stage zinc wetting zinc adhesion According to the invention annealing temp. [°C] annealing time [s] O ₂ content [ppm] annealing temp. Tg [°C] glowda [s] H ₂ content [%] dew point [°C] 21 single stage 800 60 5 -50 no no no 22 800 60 5 -30 no no no 23 800 60 5 -15 severely disturbed no no 24 800 60 5 -5 severely disturbed no no 25 800 60 5 +5 severely disturbed no no 26 800 60 5 +15 Disturbed Restricted no 27 800 60 5 +25 Yes Yes no 28 800 60 5 +45 Yes Yes no 29 500 10 2000 800 30 5 -30 imperfections Yes no 30 600 10 2000 800 60 5 -30 Yes Yes no 31 700 10 2000 800 30 5 -15 imperfections Yes no 32 800 10 2000 800 30 5 -15 imperfections Yes no 33 500 10 2500 800 30 5 -15 imperfections Yes no 34 600 10 2500 800 30 5 -30 Yes Yes no 35 700 10 2500 800 30 5 -30 Yes Yes no annealing temp. [°C] annealing time [s] O ₂ content [ppm] annealing temp. Tg [°C] annealing time [s] H ₂ content [%] dew point [°C] 36 800 10 250 0 800 30 5 -30 Yes Yes no Test no.: glow melt pool zinc wetting zinc adhesion According to the invention annealing temp. Tg [°C] holding time [s] H ₂ content [%] dew point [°C] Mg content [% by weight] Al content [% by weight] 37 800 60 5 +5 1 0.4 Yes Yes Yes 38 800 60 5 +15 1 0.4 Yes Yes Yes 39 800 60 5 +25 1 0.4 Yes Yes Yes 40 800 60 5 +45 1 0.4 Yes Yes Yes 41 800 60 5 -50 - 0.14 no no no 42 800 60 5 -30 - 0.14 no no no 43 800 60 5 -15 - 0.14 no no no 44 800 60 5 -50 - 0.17 no no no 45 800 60 5 -30 - 0.17 no no no 46 800 60 5 -15 - 0.17 no no no 47 800 60 5 -50 - 0.23 no no no 48 800 60 5 -30 - 0.23 no no no 49 800 60 5 -15 - 0.23 no no no 50 800 60 5 -55 1 0.9 imperfections no no 51. 800 60 5 -30 1 0.9 Yes Yes Yes 52 800 60 5 -15 1 0.9 Yes Yes Yes 53 800 60 5 -5 1 0.9 Yes Yes Yes 54 800 60 5 -55 5 1 imperfections no no 55 800 60 5 -30 5 1 Yes Yes Yes 56 800 60 5 -15 5 1 Yes Yes Yes 57 800 60 5 -5 5 0.4 Yes Yes Yes Test no.: steel glow melt pool zinc wetting zinc adhesion According to the invention annealing temp. Tg [°C] holding time [s] H ₂ content [%] dew point [°C] Mg content [% by weight] Al content [% by weight] V1 VS1 800 60 5 -50 1 0.4 no no no v2 VS1 800 60 5 -30 1 0.4 no no no V3 VS1 800 60 5 -15 1 0.4 no no no V4 VS2 800 60 5 -50 1 0.4 no no no V5 VS2 800 60 5 -30 1 0.4 no no no V6 VS2 800 60 5 -15 1 0.4 no no no

Claims (11)

1. Method for hot-dip coating a flat steel product, consisting of (in % wt.) C: ≤1.6 %, Mn: 6 - 35 %, Al: ≤ 10 %, Ni: ≤ 10 %, Cr: ≤10 %, Si: ≤ 10 %, Cu: ≤ 3 %, Nb: ≤ 0.6 %, Ti: ≤ 0.3 %, V: ≤ 0.3 %, P: ≤ 0.1 %, B: ≤ 0.01 %, Mo: ≤ 0.3 %, N: ≤ 1.0 % and the remainder iron and unavoidable impurities, with zinc or a zinc alloy, comprising the following production steps:

   a) providing the flat steel product;

   b) annealing the flat steel product

   - at an annealing temperature Tg of 600 - 1100 °C,

   - for an annealing duration of 10 - 240 s under an annealing atmosphere having a reducing effect in relation to the FeO present on the flat steel product and having an oxidising effect in relation to the Mn contained in the steel substrate, this annealing atmosphere containing 0.01 - 85 % vol. H₂, H₂O and N₂ and technically induced unavoidable impurities as the remainder and having a dew point which is between -50 °C and +60 °C, wherein for the H₂O/H₂ ratio the following applies: $$8\times {10}^{-15\ast }{\mathrm{Tg}}^{3.529}<{\mathrm{H}}_2\mathrm{O}/{\mathrm{H}}_2\le 0.957,$$

   

   - so that a 20 - 400 nm thick Mn mixed oxide layer is formed on the steel flat product, covering the steel flat product at least in sections;

   c) cooling the annealed flat steel product down to a bath entry temperature;

   d) passing the flat steel product, cooled down to the bath entry temperature, through a 420 - 520 °C hot Zn molten bath saturated with iron within a dipping time of 0.1 - 10 s, so that the flat steel product is hot-dip coated with a Zn protective coating which protects against corrosion, wherein the Zn molten bath consists of the main constituent zinc and unavoidable impurities, as well as 0.05 - 8 % wt. Al and up to 8 % wt. Mg, as well as optionally Si < 2 %, Pb < 0.1 %, Ti < 0.2 %, Ni < 1 %, Cu < 1 %, Co < 0.3 %, Mn < 0.5 %, Cr < 0.2 %, Sr < 0.5 %, Fe < 3 %, B < 0.1 %, Bi < 0.1 %, Cd < 0.1 %;

   e) cooling the flat steel product flowing out of the molten bath and provided with the Zn coating.
2. Method according to Claim 1, characterised in that the flat steel product is provided as a cold-rolled steel strip.
3. Method according to either of Claims 1 and 2, characterised in that upstream of the annealing (production step b)) an annealing step is inserted in which the flat steel product is held at an annealing temperature of 200 - 1100 °C for an annealing duration of 0.1 - 60 s under an atmosphere which is oxidative for Fe and Mn, contains 0.0001 - 5 % vol. H₂ and optionally 200 - 5500 vol. ppm O₂ and has a dew point in the range from -60 °C to +60 °C.
4. Method according to any one of the preceding claims, characterised in that the dipping time in the Zn molten bath is 0.1 - 5 s.
5. Method according to Claim 1, characterised in that the Al content is in each case less than the Mg content of the molten bath.
6. Method according to any one of the preceding claims, characterised in that the temperature of the flat steel product when it enters the molten bath is 360 - 710 °C.
7. Flat steel product having a steel substrate, which consists of (in % wt.) C: ≤ 1.6 %, Mn: 6 - 35 %, Al: ≤ 10 %, Ni: ≤ 10 %, Cr: ≤10 %, Si: ≤ 10 %, Cu: ≤ 3 %, Nb: ≤ 0.6 %, Ti: ≤ 0.3 %, V: ≤ 0.3 %, P: ≤ 0.1 %, B: ≤ 0.01 %, Mo: ≤ 0.3 %, N: ≤ 1.0 % and the remainder iron and unavoidable impurities, and a Zn protective coating which protects against corrosion and is formed from zinc or a zinc alloy, characterised in that the Zn protective coating has a Mn mixed oxide layer consisting of MnO Fe_metal, which essentially covers the flat steel product and adheres to the flat steel product, and has a Zn layer shielding the flat steel product and the MnO Fe_metal layer adhering to it from the environment.
8. Flat steel product according to Claim 7, characterised in that the Zn protective coating comprises an Fe(Mn)₂Al₅ layer arranged between the MnO Fe_metal layer and the Zn layer.
9. Flat steel product according to either of Claims 7 or 8, characterised in that the Zn protective coating comprises an FeMnZn layer which lies between the MnO Fe_metal layer and the Zn layer.
10. Flat steel product according to any one of Claims 7 to 9, characterised in that the Zn protective layer is formed as a ZnMg alloy coating.
11. Flat steel product according to any one of Claims 7 to 10, characterised in that it is produced according to the method according to any one of Claims 1 to 6.

Images