EP2432910B1 - 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 process for the hot dip coating of a zinc flat or a zinc alloy containing 2 to 35% by weight of Mn, and to a flat steel product provided with a zinc or zinc alloy coating.

Modern automotive engineering increasingly relies on high-strength and ultra-high-strength steels. Typical alloying elements are manganese, chromium, silicon, aluminum, and the like, which form stable non-reducible surface oxides upon conventional recrystallizing annealing. These oxides can prevent the reactive wetting with a molten zinc.

On the other hand, steels with high manganese contents are, due to their favorable combination of properties consisting of high strengths of up to 1,400 MPa on the one hand and extremely high strains (uniform strains of up to 70% and elongations at break of up to 90%), in principle particularly suitable for use in the field of vehicle construction , especially in the automotive industry. Specially suitable steels with high Mn contents for this application from 6 wt .-% to 30 wt .-%, for example, from DE 102 59 230 A1 , the DE 197 27 759 C2 or the DE 199 00 199 A1 known. Flat products produced from the known steels have an isotropic deformation behavior at high strengths and, moreover, are still ductile even at low temperatures.

These advantages, however, contrast with the fact that high manganese steels tend to pitting and are difficult to passivate. This compared to lower alloyed steels when exposed to elevated chloride ion concentrations great tendency to locally limited, but intense corrosion makes the use of the material group of high-alloy steel sheets belonging steels straight in the body construction difficult. In addition, high manganese steels tend to surface corrosion, which also limits the spectrum of their use.

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

The reason for these poor adhesion properties was found to be the thick oxide layer which sets in the annealing required for hot-dip coating. The thus oxidized sheet surfaces can no longer be wetted with the required uniformity and completeness with the coating metal, so that the goal of a nationwide corrosion protection is not achieved.

The known from the field of high-alloyed, but lower Mn-containing steels known ways of improving the wettability by applying an intermediate layer of Fe or Ni resulted in steel sheets with at least 6 wt .-% manganese not to the desired success.

In the DE 10 2005 008 410 B3 For example, it has been proposed to apply an aluminum layer to a 6-30 wt.% Mn-containing steel strip prior to the final annealing preceding the hot-dip coating. The aluminum adhering to the steel strip prevents the annealing of the steel strip which precedes the melt coating from oxidizing its surface. Subsequently, the aluminum layer in the manner of an adhesion promoter, that the coating produced by the melt coating adheres firmly and fully on the steel strip, even if the steel strip itself offers unfavorable conditions due to its alloy. For this purpose, in the known method, the effect is utilized that diffusion of the iron of the steel strip into the aluminum layer occurs during the annealing treatment necessary upstream of the melt coating. in the As a result of the annealing, a metallic, essentially Al and Fe, overlay, which is firmly bonded to the substrate formed by the steel strip, is produced on the steel strip.

Another method of coating a high manganese content steel strip containing 0.35-1.05 wt% C, 16-25 wt% Mn, balance iron and unavoidable impurities is known from US Pat WO 2006/042931 A1 known. According to this known method, the steel strip thus composed is first cold-rolled and then annealed recrystallizing in an atmosphere which is reducing with respect to iron. In this case, the annealing parameters are selected such that on both sides of the steel strip an intermediate layer is formed, which consists essentially completely of amorphous (FeMn) oxide, and additionally adjusts an outer layer consisting of crystalline Mn oxide, wherein the thickness of two layers is at least 0.5 microns. A hot-dip coating then no longer takes place. Rather, the Mn oxide layer in combination with the (FeMn) oxide layer should provide adequate corrosion protection.

On a similar principle that is based in the WO 2006/042930 ( EP 1 805 341 B1 ) described method, according to the two successive annealing steps first on the high Mn-containing steel substrate, a layer of iron-manganese mixed oxides and then on this layer an outer Mn-mixed oxide layer is produced. Subsequently, the thus coated steel strip is passed into a melt bath. This In addition to zinc, melt ribbon additionally contains aluminum in an amount sufficient to completely reduce the MnO layer and at least partially reduce the (FeMn) O layer. As a result, a layer structure is to be achieved in which three FeMnZn layers and one outer Zn layer can be identified.

Practical investigations have shown that even such elaborately precoated steel strips in practice do not have the required for a cold deformation adhesion to the steel substrate. In addition, this turns out to be from the WO 2006/042930 known processes due to running in the melt bath, hardly controllable in practice reactions as not sufficiently reliable.

Finally, out of the DE 10 2006 039 307 B3 A method for hot dip coating a high Mn-containing steel substrate is known in which the ratio% H ₂ O /% H _{2 of} the water content% H ₂ O to the hydrogen to produce a substantially free of oxidic interlayers metallic protective layer on the steel strip. The content% H _{2 of} the annealing atmosphere is set as a function of the respective annealing temperature T _G such that the ratio% H ₂ O /% H _{2 is} less than or equal to 8 × 10 ^-15 × T _G ^3.529 , where T denotes the annealing temperature is. This specification is based on the finding that, by means of a suitable setting of the annealing atmosphere, namely its hydrogen content in relation to its dew point, a surface finish of the steel strip to be coated sets during the annealing, which ensures optimum adhesion of the subsequently hot-dip coating guaranteed applied metallic protective coating. The annealing atmosphere set in this way reduces both the iron and the manganese of the steel strip. The aim is to avoid the formation of an adhesion of the melt coating on the high manganese steel substrate impairing oxide layer.

Practical investigations have shown that, according to the above-described known method, flat steel products prepared have a good wetting behavior and, for many applications, sufficient adhesion of the Zn coating. However, during the deformation of correspondingly coated flat steel products into components, high levels of deformation still lead to detachment and cracking of the coating.

Furthermore, the methods known from the prior art, in particular when using high process temperatures, can adversely affect the mechanical properties in the flat steel product. Furthermore, with the existing processes, it is not possible to operate economically and meet the ecological requirements.

Against this background, the object of the invention to provide a method which allows to provide high levels of Mn-containing flat steel products with a corrosion-protective zinc coating, in which a further improved adhesion of the coating is ensured on the steel substrate. In addition, a flat steel product should be created in which as well under high degrees of deformation of each formed of zinc or a zinc alloy Zn coating securely adheres to the steel substrate.

With regard to the method, this object is achieved in that the hot dip coating of a high Mn contents containing flat steel product, the steps specified in claim 1 are completed.

With respect to the product, the above-mentioned object has also been achieved by a flat steel product which according to the invention has the features specified in claim 8.

According to the invention, a flat steel product in the form of a steel strip or sheet steel is first provided for hot dip coating a 2 to 35 wt% Mn-containing flat steel product in a continuous process.

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

Steel strips provided with a metallic protective coating by hot-dip coating according to the invention contain (in% by weight) C: ≦ 1.6%, Mn: 2 - 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.

The effects achieved by the invention in the coating of high-alloy steel strips which contain manganese contents of at least 6% by weight are particularly advantageous. Thus, it can be seen that a steel base material containing (in wt%) 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%, balance iron and unavoidable impurities, coat particularly well with a corrosion-protective coating.

The same applies if a steel is used as the base material, which (in wt .-%) C: ≤ 1.00%, Mn: 7.00 - 30.00%, Al: 1.00 - 10.00%, Si :> 2.50 - 8.00% (assuming that 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% , Rest contains iron and unavoidable impurities.

As with the usual 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 thus provided flat products are annealed in a working step b). The annealing temperature Tg is 600 - 1100 ° C, while the annealing time, over which the flat steel product is kept at the annealing temperature, 10 - 240 s.

It is critical to the invention that the above-mentioned annealing temperature Tg and annealing time under a FeO iron oxide present on the steel flat product be reducing and oxidizing with respect to the manganese contained in the steel substrate. For this purpose, the annealing atmosphere contains 0.01-85 vol .-% H ₂ , H ₂ O and the balance N ₂ and technically unavoidable impurities and has a lying between -70 ° C and +60 ° C dew point, wherein for the H ₂ O / H ₂ ratio applies:

8x10 ^-15 * Tg ^3.529 <H ₂ O / H ₂ ≤ 0.957

According to the invention, therefore, the ratio H ₂ O / H ₂ is set so that it is greater than 8x10 ^-15 * Tg ^3.529 and on the other hand at most equal to 0.957, where Tg the respective annealing temperature is designated.

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

As a result, a 20-400 nm thick, the steel flat product at least partially covering Mn mixed oxide layer is produced by a performed before the hot dip coating annealing on the flat steel product, wherein it is particularly favorable in view of the adhesion of the Zn coating on the steel substrate, if the Mn mixed oxide layer substantially completely covers the surface of the flat steel product after annealing. The Mn mixed oxide layer is defined in the context of the invention as MnO · Fe _metal . That is, metallic iron is present in this Mn mixed oxide layer and not, as in the prior art, oxidized iron.

Thus, according to the invention, an Mn mixed oxide layer is deliberately set via at least one annealing stage by carrying out the annealing (step b)) under a FeO-reducing and an Mn-oxidizing atmosphere.

Surprisingly, it has been found that in this way a flat steel product is obtained which ensures good wetting during the subsequent hot-dip coating. Likewise, the layer of Mn mixed oxides produced on the steel substrate according to the invention forms a primer on which the subsequently applied zinc layer surprisingly adheres particularly securely. Unlike in the WO 2006/042930 In this case, the Mn mixed oxide layer remains during the Hot dip coating process as far as possible, so that it ensures the permanent cohesion of 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 inlet temperature with which it enters the Zn melt bath. Typically, the bath inlet temperature of the flat steel product is in the range of 310-710 ° C.

Subsequently, the cooled to the Badeintrittstemperatur steel flat product within a dipping time of 0.1 - 10 seconds, in particular 0.1 - 5 s, passed through a saturated with iron, 420 - 520 ° C hot Zn melt bath consisting of the main component zinc and unavoidable impurities and 0.05 to 8 wt .-% Al and / or up to 8 wt .-% Mg, in particular 0.05 to 5 wt .-% Al and / or up to 5% wt .-% Mg exists , In addition, in the melt 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 to certain properties of the coating in a conventional manner adjust.

The thus obtained, with a corrosion-protective Zn protective coating hot-dip coated steel flat product is finally cooled, and before cooling, the thickness of the coating can be adjusted in a conventional manner.

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

A flat steel product according to the invention having a Mn content of 2 to 35% by weight and a Zn protective coating which protects against corrosion is accordingly characterized in that the Zn protective coating comprises an Mn mixed oxide layer essentially covering and adhering to the flat steel product, in the metallic one Iron is present, and has a Zn layer which shields the flat steel product and the Mn mixed oxide layer adhering to it against 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 arises when in the melt bath, a sufficient amount of aluminum from 0.05 to 5 wt .-% Al is present. The Fe (Mn) ₂ Al ₅ layer forms a barrier layer, by means of which the reduction of the Mn mixed oxide layer during hot dip is reliably prevented. Depending on the particular between 0.05 - 0.15 wt .-% lying Al content, the barrier layer can convert into FeZn phases, wherein the Mn oxide layer is still preserved.

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

However, the presence of an Mn mixed oxide layer on the surface of the steel substrate according to the invention has a positive effect not only when the Fe (Mn) ₂ Al ₅ layer is additionally formed, but also when magnesium is used in the molten bath alternatively or in addition to aluminum 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 flat steel product with at the same time optimal adhesion and minimized risk of cracking or spalling even at high degrees of deformation.

A particularly practical embodiment of the invention results in this context when Al and Mg are present in the specified limits simultaneously in the melt bath and for the ratio of the Al content% Al and the Mg content% Mg is:% Al /% Mg <1. In this embodiment of the invention, therefore, the Al content of the melt bath is always smaller than its Mg content. This has the advantage that the boundary layer formation desired according to the invention leads to an increase of the metallic iron in the mixed oxide layer even without a special annealing step sequence in the context of the method according to the invention. Magnesium is characterized by a higher reduction potential on MnO than aluminum. Therefore, in the presence of higher Mg contents in the melt layer, forced dissolution of the MnO skeleton of the mixed oxide layer occurs. Because the mixed oxide dissolved more is, is effectively more metallic iron "Fe _metal " from the "depth" of the mixed oxide layer on the reaction front mixed oxide layer / zinc bath available, so that the opaque Fe (Mn) ₂ Al ₅ boundary layer can form a particularly effective adhesion promoter. Accordingly, the MnO reduction by dissolved magensium in-situ with particularly high efficiency contributes to the envisaged boundary layer formation, which ensures the particularly good adhesion of the Zn coating.

The annealing step (step b)) carried out for preparing the hot-dip coating in the context of the method according to the invention can be carried out in one or more stages. In the case that the annealing is carried out in one stage, different hydrogen contents in the annealing atmosphere are possible depending on the dew point. If the dew point is in the range of -70 ° C to + 20 ° C, the annealing atmosphere may 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 consideration during the performance of the annealing step according to the invention, the reducing effect with respect to the FeO which may be present and the oxidizing effect with respect to the Mn present in the steel substrate are thus reliably achieved.

If, on the other hand, the steel flat product is to be annealed in two stages before it enters the melt bath, the annealing step carried out according to the invention can be used for this purpose (Step b) of claim 1) preceded by an additional annealing step, wherein the steel flat product is kept at an annealing temperature of 200 - 1100 ° C for an annealing period of 0.1 - 60 s under an oxidative atmosphere for both Fe and Mn Containing 0.0001 - 5 vol .-% H ₂ and optionally 200 - 5500 vol. ppm O ₂ and having a dew point lying in the range of -60 ° C to +60 ° C. Subsequently, the annealing step according to the invention is then carried out at a dew point in the range of -70 ° C to +20 ° C in a 0.01 to 85% hydrogen atmosphere taking into account the other parameters to be taken into account during the performance of the annealing step according to the invention, before the flat steel product is passed into the melt bath.

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

It likewise contributes to optimizing the deformation behavior of a flat steel product produced according to the invention when the steel flat product provided with the Mn mixed oxide layer is subjected to an overaging treatment before it enters the melt 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 embodiments will be explained in more detail. Show it:

Fig. 1

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

Fig. 2

an oblique cut of a sample of a Zn-coated steel flat product;

Fig. 3

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

Fig. 4

an oblique cut of a sample of a ZnMg coated steel flat product.

From a high manganese steel with the composition given in Table 1, a cold-rolled steel strip has been produced in a known manner. 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 Balance iron and unavoidable impurities, data in% by weight

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

For this purpose, the steel strip sample is heated at a heating rate of 10 K / s to an annealing temperature Tg of 800 ° C. where the sample was then held for 30 seconds. The annealing was carried out under an annealing atmosphere, which consisted of 5 vol .-% H ₂ and 95 vol .-% of N ₂ and whose dew point was +25 ° C. Subsequently, the annealed steel strip was cooled at a cooling rate of 20 K / s to a bath inlet temperature of 480 ° C, where it was first 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 passed into a 460 ° C, saturated to Fe zinc melt bath, which in addition to Zn, unavoidable impurities and Fe additionally contained 0.23 wt .-% Al. After a dipping time of 2 seconds, the hot-dip-coated steel strip has been led out of the molten bath and cooled to room temperature.

In a second experiment, a second sample of the cold-rolled steel strip assembled according to Table 1 was annealed in a likewise continuous process in a two-stage process and 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 . Their dew point was -30 ° C.

Immediately following this, the steel strip is in a second annealing step to a 800 ° C amount annealing temperature Tg was heated at which it was kept for 30 seconds under a 5 vol .-% H ₂ , remainder N ₂ containing annealing atmosphere whose dew point was -30 ° C. Thereafter, the steel strip has been cooled under the annealing atmosphere with a cooling temperature of about 20 K / s to 480 ° C and subjected to an overaging treatment for 20 seconds. Thereafter, the steel strip was passed at a bath inlet temperature of 480 ° C in a 460 ° C hot, saturated to Fe melt bath, in turn, 0.23 wt .-% Al and other elements contained in inactive traces of contamination and the remainder zinc. After a dipping time of 2 seconds, the finished hot-dip coated flat steel product is then led out of the melt bath and cooled to room temperature.

In Fig. 1 schematically shows the structure of the coating Z thus obtained on the steel substrate S. Accordingly, on the steel substrate S is a Mn _y O _x manganese mixed oxide layer M (M = MnO · Fe), on which a Fe (Mn) ₂ Al ₅ intermediate layer F (F = MnO · Fe (Mn) ₂ Al ₅ ) or at Al contents of at most 0.15 wt .-% in the melt bath has formed a FeMnZn layer, which in turn is shielded from the environment by a Zn layer Zn (η phase). 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. In contrast, the zinc layer Zn is significantly thicker at 3-20 μm.

In Fig. 2 an oblique cut of a sample produced in the manner described above is reproduced. The steel substrate S and the Mn _y O _x manganese mixed oxide layer M with embedded metallic iron lying thereon are clearly visible, 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 ,

To test the success of the procedure according to the invention, twenty additional experiments 1-20 were carried out, in which the melt bath contained 0.23 wt.% Al in addition to Zn and unavoidable impurities. The wetting degree and the zinc adhesion were visually examined on the samples thus obtained. As a test principle, the notch impact test according to SEP 1931 has been used. The experimental parameters and results of these experiments are given in Table 2.

In addition, another sixteen experiments 21-36 were carried out, in which the melt bath in addition to Zn and unavoidable impurities contained 0.11 wt .-% Al. Compared with the barrier layer formed as Fe (Mn) ₂ Al ₅ layer shown in the above-explained experiment, an FeMnZn barrier layer appeared at this lower Al content of the melt bath. The wetting degree and the zinc adhesion were also investigated on the samples thus obtained. The experimental parameters and results of these experiments are given in Table 3.

Based on further samples of the high manganese content cold rolled from the composite steel according to Table 1 Steel bands, the influence of the dew point of the respective annealing atmosphere has been examined for the coating result. The samples were each subjected to an annealing process in which they were also heated at a heating rate of 10 K / s to an annealing temperature Tg of 800 ° C. At this annealing temperature, the sample has then been held for 60 seconds. The annealing was carried out under an annealing atmosphere, each consisting of 5 vol .-% H ₂ and 95 vol .-% of N ₂ , wherein the respective dew point of the annealing atmosphere between -55 ° C and +45 ° C has been varied.

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 above-described series of experiments, where it was first 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 passed into a 460 ° C, saturated to Fe zinc melt bath, in addition to Zn, unavoidable impurities and Fe additionally in combination 0.4 wt .-% Al and 1.0 Wt .-% Mg or alone 0.14 wt .-%, 0.17 wt .-% or 0.23 wt .-% Al contained. After a dipping time of 2 seconds, the hot-dip-coated steel strip has been led out of the molten bath and cooled to room temperature.

In Fig. 3 schematically shows the structure of the thus obtained on the steel substrate S 'ZnMg coating Z' shown. Accordingly, on the steel substrate S 'is a Mn _y O _x manganese mixed oxide layer M' (M = MnO · Fe), on which a Fe (Mn) ₂ Al ₅ intermediate layer F (F = MnO · Fe (Mn) ₂ Al ₅ ) or at Al contents of at most 0.15 wt .-% in the melt bath has formed a FeMnZn layer, 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. In contrast, the zinc layer ZnMg is significantly thicker at 3-20 μm.

In Fig. 4 an oblique cut of a sample produced in the manner described above is reproduced. 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 clearly visible 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 have been varied at twenty-one trials 37-57 carried out to test the success of the method according to the invention. The wetting degree and the zinc adhesion were visually examined on the samples thus obtained. As a test principle, the notch impact test in accordance with SEP 1931 has also been used here. The experimental parameters and results of these experiments are given in Table 4.

It can be seen that when Al and Mg are combined and the dew point is set within the range of -50 ° C to +60 ° C, zinc-based coatings can be reliably produced on high-manganese steel substrates even in a single-stage annealing process.

For comparison, a further three samples each of V1-V3 and V4-V6 were obtained from a cold-rolled steel strip consisting of an Al-TRIP steel VS1 and a steel strip consisting of a likewise cold-rolled Si-TRIP steel VS2. The composition of 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 Balance iron and unavoidable impurities, data in% by weight

Also, the comparative samples V1-V6 were heat-treated in the manner described above for the samples according to the invention before being hot-dip coated in the melt bath. In addition to Zn and unavoidable impurities, the melt bath contained in each case 0.4% by weight of Al and 1% by weight of Mg. The degree of wetting and the zinc adhesion were likewise examined in each case on the samples V1 - V6 coated in this way. The experimental parameters and results of these experiments are listed in Table 6. It turns out that due to the lower manganese contents of the steels VS1 and VS2 do not form MnO structure in the mixed oxidation layer on the surface of the steel substrate. As a result, no opaque Fe (Mn) ₂ layer is formed as a primer. As a result, there is no sufficient MnO reduction by dissolved magnesium in the melt bath, so that in the comparative samples also no sufficient wetting and, accordingly, no sufficient adhesion of the coating can be achieved. Table 2 Experiment No .: 1st annealing stage 2nd annealing stage Zinc wetting zinc liability According to the invention Glühtemp. [° C] Annealing time [s] O ₂ content [ppm] Glühtemp. 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 Heavily disturbed No No 4 800 60 5 -5 Heavily disturbed No No 5 800 60 5 5 Heavily disturbed No No 6 800 60 5 +15 Disturbed Limited No 7 800 60 5 +25 Yes Yes Yes 8th 800 60 5 +45 Yes Yes Yes 9 500 10 2000 800 30 5 -30 impurity Yes Yes 10 600 10 2000 800 60 5 -30 Yes Yes Yes 11 700 10 2000 800 30 5 -15 impurity Yes Yes 12 800 10 2000 800 30 5 -15 impurity Yes Yes 13 500 10 2500 800 30 5 -15 impurity Yes Yes 14 600 10 2500 800 30 5 -30 Yes Yes Yes 15 700 10 2500 800 30 5 -30 Yes Yes Yes 16 800 10 2500 800 30 5 -30 Yes Yes Yes 17 500 6 2500 800 30 5 -30 impurity Yes Yes 18 600 6 2500 800 30 5 -30 Yes Yes Yes 19 700 6 2500 800 30 5 -30 Yes Yes Yes 20 800 6 2500 800 30 5 -30 Yes Yes Yes Experiment No .: 1st annealing stage 2nd annealing stage Zinc wetting zinc liability According to the invention Glühtemp. [° C] Annealing time [s] O ₂ content [ppm] Glühtemp. Tg [° C] Annealing time [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 Heavily disturbed No No 24 800 60 5 -5 Heavily disturbed No No 25 800 60 5 +5 Heavily disturbed No No 26 800 60 5 +15 Disturbed Limited No 27 800 60 5 +25 Yes Yes Yes 28 800 60 5 +45 Yes Yes Yes 29 500 10 2000 800 30 5 -30 impurity Yes Yes 30 600 10 2000 800 60 5 -30 Yes Yes Yes 31 700 10 2000 800 30 5 -15 impurity Yes Yes 32 800 10 2000 800 30 5 -15 impurity Yes Yes 33 500 10 2500 800 30 5 -15 impurity Yes Yes 34 600 10 2500 800 30 5 -30 Yes Yes Yes 35 700 10 2500 800 30 5 -30 Yes Yes Yes 36 800 10 2500 800 30 5 -30 Yes Yes Yes Experiment No .: annealing melt bath Zinc wetting zinc liability According to the invention Glühtemp. Tg [° C] Holding time [s] H ₂ content [%] Dew point [° C] Mg content [% by weight] Al content [% by weight] 37th 800 60 5 +5 1 0.4 Yes Yes Yes 38th 800 60 5 +15 1 0.4 Yes Yes Yes 39th 800 60 5 +25 1 0.4 Yes Yes Yes 40th 800 60 5 +45 1 0.4 Yes Yes Yes 41st 800 60 5 -50 - 0.14 No No No 42nd 800 60 5 -30 - 0.14 No No No 43rd 800 60 5 -15 - 0.14 No No No 44th 800 60 5 -50 - 0.17 No No No 45th 800 60 5 -30 - 0.17 No No No 46th 800 60 5 -15 - 0.17 No No No 47th 800 60 5 -50 - 0.23 No No No 48th 800 60 5 -30 - 0.23 No No No 49th 800 60 5 -15 - 0.23 No No No 50th 800 60 5 -55 1 0.9 impurity No No 51st 800 60 5 -30 1 0.9 Yes Yes Yes 52nd 800 60 5 -15 1 0.9 Yes Yes Yes 53rd 800 60 5 -5 1 0.9 Yes Yes Yes 54th 800 60 5 -55 5 1 impurity No No 55th 800 60 5 -30 5 1 Yes Yes Yes 56th 800 60 5 -15 5 1 Yes Yes Yes 57th 800 60 5 -5 5 0.4 Yes Yes Yes Experiment No .: stole annealing melt bath Zinc wetting zinc liability According to the invention Glühtemp. 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 (12)

1. Method for hot-dip coating a flat steel product, consisting of (in % wt.) C: ≤1.6 %, Mn: 2 - 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 -70 °C and +60 °C, wherein for the H₂O/H₂ ratio the following applies: $$8\mathrm{x}{10}^{-15}*{\mathrm{Tg}}^{3.529}<{\mathrm{H}}_2\mathrm{O}/{\mathrm{H}}_2\le 0.957,$$

   

   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 as0.05 - 8 % wt. Al and/or 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 any one of the preceding claims, characterised in that the Zn molten bath in each case contains both Al and Mg.
6. Method according to Claim 5, characterised in that the Al content is in each case less than the Mg content of the molten bath.
7. 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.
8. Flat steel product having a steel substrate, which consists of (in % wt.) C: ≤ 1.6 %, Mn: 2 - 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 an 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.
9. Flat steel product according to Claim 8, characterised in that the Zn protective coating comprises an Fe(Mn)₂Al₅ layer arranged between the MnO Fe_metal layer and the Zn layer.
10. Flat steel product according to either of Claims 8 or 9, characterised in that the Zn protective coating comprises an FeMnZn layer which lies between the MnO Fe_metal layer and the Zn layer.
11. Flat steel product according to any one of Claims 8 to 10, characterised in that the Zn protective layer is formed as a ZnMg alloy coating.
12. Flat steel product according to any one of Claims 8 to 11, characterised in that it is produced according to the method according to any one of Claims 1 to 7.

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