WO2015000707A1

WO2015000707A1 - Method for coating flat steel products with a metallic protective layer, and flat steel products coated with a metallic protective layer

Info

Publication number: WO2015000707A1
Application number: PCT/EP2014/062879
Authority: WO
Inventors: Marc Blumenau; Dirk Czupryna; Christopher Gusek; Hans-Joachim Heiler; Fred Jindra; Peter Schmidt; Rudolf Schönenberg; Jennifer Schulz; Hans-Joachim KRAUTSCHIK
Original assignee: Outokumpu Nirosta Gmbh
Priority date: 2013-07-03
Filing date: 2014-06-18
Publication date: 2015-01-08
Also published as: EP2821520B1; ES2851199T3; EP2821520A1

Abstract

The invention makes it possible to produce a flat steel product in operationally reliable manner and on an industrial scale that is protected in particularly effective manner against corrosive attacks, process in continuous feed mode, in which the flat steel product is a) provided; b) pickled; c) rinsed with an aqueous medium; d) conveyed through a flux bath that consists of an aqueous solution containing, besides unavoidable contaminants created during production and the process, 210 – 250 g/l chloride ions, and additionally 140 – 160 g/l zinc ions, 5 – 12 g/l ammonium ions or 30 – 40 g/l potassium ions and optionally in each case also 0.5 - 1.5 g/l sodium ions or calcium ions, and not more than 1 g/l magnesium ions, and traces of ions of the elements Al, Fe, Mn, Mo, Ni, P, Si, Sr and Li, wherein the density of the flux bath is at least 1.25 g/cm³ and at most 1.45 g/cm³, e) dried and warmed up to a bath entry temperature of 100 – 230 °C, f) hot dip coated in a melt bath, and g) optionally thermal, chemical or mechanical post-treated.

Description

METHOD FOR COATING FLAT STEEL PRODUCTS WITH A METALLIC PROTECTIVE LAYER, AND FLAT STEEL PRODUCTS COATED WITH A METALLIC PROTECTIVE LAYER The invention relates to a method for coating flat steel products with a metallic protective covering that has a Zn or Al base, and to flat steel products that are coated with such a protective layer.

The "flat steel products" to be coated for the purposes of the invention are strips or sheets produced from steel by hot or cold rolling processes, as well as blanks and slabs obtained therefrom.

Flat steel products that are produced from steels which are susceptible to corrosion, and which are intended for use in an environment where there is increased risk of corrosion, are usually furnished with a metallic protective coating that protects the respective steel substrate from corrosive attacks. One method that has proven successful for applying such a coating is hot dip galvanising, in which, after a pretreatment step, the flat steel product is passed through a melt bath within a brief immersion period, so that when it leaves the melt bath a coating of defined thickness remains on the surface of the flat steel product. The thickness of the coating may be adjusted with the aid of suitable scraping devices, which the flat steel product moves past after emerging from the melt bath. The method is also known by the technical term "hot dip coating", and is used to obtain flat steel products that have a significantly longer product service life than flat steel products that have not been finished in this way.

There are very many different processes that are intended to ensure optimal adhesion of the coating and optimal usage characteristics of the flat steel product to which the coating has been applied with optimal economy. As one of these developments it may be mentioned the suggestion to significantly increase the corrosion protection offered by the coating in this manner by adding more Mg to a Zn melt bath. Examples of coating methods based on this suggestion are explained in EP 1857566 A1 , EP 2055799 A1 and EP 1693477 A1 . In all cases, the success of the coating of flat steel products is determined to a critical degree on the adequate activation of the surface to be coated. Moreover, it must be ensured that the surface in question is as free as possible of impurities and oxides that might impair the coating result when it is dipped in the respective melt bath.

Cold or hot rolled steel strips with widths of more than 600 mm, also known as "wide strips" can be coated in particularly cost-effective manner with a hot dip coating process in continuous feed mode. In the processes of this kind that are used in practice, the flat steel products that are to be coated pass through each of the process steps of "cleaning", "surface activation with annealing gas" and "hot dip coating" individually, in sequence and without interruption. As is described in WO 2009/030823 A1 , for example, the surface is typically activated in a continuous furnace by means of a heterogeneous annealing gas-metal reaction in an annealing atmosphere containing H₂ and N₂ at temperatures above 700 °C. If hot rolled wide strips are to be covered with a hot dip coating, they can be descaled in a pickling facility before the annealing treatment. There, mordants containing a correspondingly aggressive acid, particularly hydrochloric acid or sulphuric acid, are used to dissolve any scale still remaining on the hot rolled wide strip.

In practice, it has been found that the coating methods used so effectively for wide strips cannot be applied cost-effectively to narrow strips with a width of 600 mm or less. Accordingly, wide strips are subsequently separated lengthwise to form narrow strips of defined width and are then passed through a pickling device parallel with each other so that any oxidised residue and other contaminants can be dissolved and removed. Then, the mordant residue is rinsed off the surface of the narrow strips and the narrow strips are conveyed through the melt bath. The technical term used for this process is also known as "fluxing", and it has the effect of cleaning the narrow strips further and at the same time activating the surface thereof before coating. The flux material also prevents the activated surface from being back-passivated before it is dipped in the coating bath. Afterwards, still parallel to each other, the narrow strips pass through a drying furnace in which the flux is dried until it adheres securely to the narrow strips, but does not burn off. The flux thus ensures that the narrow strips will be wetted thoroughly and evenly when they subsequently pass through the melt bath. The corrosion protection coating is usually applied in the form of a Zn covering. For this purpose, the Zn-based melt bath into which the narrow strips are transported, still parallel to each other, typically has a temperature of 470 °C. However, it is also possible to apply an Al-based covering to the narrow strips with the hot dip coating method. In this case, the melt bath is typically at a temperature of up to 700 °C. The respective surface coating may be followed by a chemical passivation step, to protect the protection coating itself from moisture (see the brochure entitled "Feuerverzinkter Bandstahl" [Hot dip galvanised strip steel], published by ThyssenKrupp Steel Europe AG, version of 201 1 , www.thyssenkrupp-steel-europe.com/tiny/cqJ/download.pdf). As was indicated in the introduction, the method explained above is normally used to coat flat steel products that are very vulnerable to corrosion.

Steels that contain a significant quantity of Cr and Ni in the alloy are notable for their particularly good chemical stability and high corrosion resistance. This product quality is due to the formation of a stable layer of chromium oxide, which effectively passivates the steel surface against external influences even at higher temperatures. Mo further supports this passivation. Consequently, steel qualities with a Cr content > 10.5 % by weight are also called rust, heat and acid-resistant (RHA) steels, or simply "stainless steels" for short. Ni as the alloy component in steel stabilises the austenitic microstructure in much the same way as Mn or N, for example, toward lower temperatures, which can be used in selective manner to improve the mechanical properties of the material. Fully austenitic steel qualities with > 8 % by weight Ni still have no brittle-ductile transition, that makes possible low temperature applications. Compared with steel qualities having a high Mn content in the alloy, fully austenitic Ni-alloy steels have a significantly better environmental impact applicability. Because of these excellent material properties, the range of possible uses for flat steel products made from Ni alloy steel is enormous in both high and low temperature applications, including among others automobile structures, chemical apparatus engineering, machine and plant construction, and for ornamental products. Even with these exceptional specific material properties, particularly in respect of harmful environmental influences, the application qualities of CrNi alloyed steels can be enhanced still further by adding a metallic coating. For this purpose too, the hot melt dipping processes of the kind explained in the preceding are widely used in practice.

Against the background of the existing state of the art as explained above, the object of the invention was to describe a method that would be operable inexpensively on an industrial scale and highly reproducible, and which would enable the economical production of flat steel products having enhanced protection against attack by corrosive substances.

This object was solved according to the invention by the fact that in the course of being coated flat steel products undergo the work steps described in claim 1 . Advantageous variations of the invention are described in the dependent claims, and in the following these will be explained in detail and together with the general inventive attitude.

The idea underlying the invention is that, instead of the annealing treatment usual in the prior art in hot dip coating to clean and activate the surfaces of the wide strips to be coated, it is carried out a flux treatment with subsequent drying of the flux that is applied to the flat steel product during the flux treatment process. In this context, the flux that is used according to the invention has been modified in such manner that optimum coating results are obtained for flat steel products consisting of steels with large strip width and a very large range of alloy compositions.

In order to achieve this, a method according to the invention for coating a flat steel product with a metal, Zn- or Al-based protective covering, comprises the following operation steps that are performed in continuous feed mode:

a) providing the flat steel product;

b) pickling the flat steel product to remove any scale adhering to the flat steel product and to activate the surface thereof;

c) removing any mordants still present on the pickled surface of the flat steel product after the pickling process by rinsing the flat steel product with an aqueous medium;

d) transporting the pickled and rinsed flat steel product through a flux bath that consists of an aqueous solution, containing, besides unavoidable contaminants created during production and the process, chloride ions and ions from at least one of the elements of the group "zinc, ammonium and potassium" and optionally also ions of the elements "Na, Ca and Mg", and also optionally traces of the elements "Al, Fe, Mn, Mo, Ni, P, Sr,

Si and Li", with the qualification that

the total concentration of chloride ions c(CI^") is at least 210 g/l and at most 250 g/l,

the total concentration of zinc ions c(Zn²⁺) is at least 140 g/l and at most 160 g/l,

the total concentration of ammonium ions c(NH₄ ⁺) is at least 5 g/l and at most 12 g/l,

the total concentration of potassium ions c(K⁺) is at least 30 g/l and at most 40 g/l,

- the total concentration of optionally present sodium ions c(Na⁺) is at least 0.5 g/l and at most 1 .5 g/l, the total concentration of optionally present calcium ions c(Ca ) is at least 0.5 g/l and at most 1 .5 g/l,

the total concentration of optionally present magnesium ions c(Mg⁺) is at most 1 g/l,

- the ions of the elements Al, Fe, Mn, Mo, Ni, P, Si, Sr and Li, present in trace quantities do not exceed 10 mg/l, and the density of the flux bath is at least 1 .25 g/cm³ and at most 1 .45 g/cm³,

wherein in particular no ions of the elements fluorine, tin, lead, indium, thallium, antimony, bismuth or boron are present in the flux bath;

e) drying the flat steel product after it emerges from the flux bath and heating the flat steel product to a bath entry temperature of 100 - 230 °C; f) hot dip coating the flat steel product with a metallic protective covering based on Zn or Al, in a melt bath having the bath entry temperature when the flat steel product enters it;

g) optional thermal, chemical or mechanical post-treatment of the flat steel product that has been hot dip coated with the protective covering. With the procedure according to the invention, it is then possible to provide flat steel products with metallic protective coverings under considerably better financial conditions than are possible with batch galvanising. Since an energy- intensive annealing step is dispensed with, considerable savings are realised in terms of energy and operating materials, and a cost-effective, pro- environmental hot dip coating method is made possible particularly for narrow strips having a width of 600 mm or less. Of course, several such narrow strips may also be transported in parallel through the method according to the invention. In this context, the process according to the invention is suitable for applying metallic coatings to substrates having either a Zn base or an Al base. Practical experiments have yielded good coating results if the flat steel product provided in operation step a) is produced for example from a structural steel that contains, besides iron and contaminants that are unavoidable impurities of production (in % by weight)

C: 0.001 - 0.7 %,

Mn: 0.10 - 2.0 %,

Al: 0.01 - 2.0 %,

and optionally one or more elements from the group of Si, P, S, Cr, Cu, Mo, N, Ni, Nb, Ti, V, Zr, B", with the qualification that the following values apply for the content of the optionally added elements:

Si: 0.001 - 2.0 %,

P: up to 0.055 %,

S: up to 0.055 %,

Cr: 0.01 - 2.0 %,

Cu: up to 0.6 %,

Mo: up to 0.2 %,

N: up to 0.030 %,

Ni: up to 2.1 %,

Nb: up to 0.2 %,

Ti: up to 0.2 %,

V: up to 0.2 %,

Zr: up to 0.2 %,

B: up to 0.0060 %. Moreover, the method according to the invention is also suitable for hot dip coating flat steel products that have a ferritic, austenitic, multiphase or duplex microstructure, and may be made from a stainless CrNi steel that, besides iron and unavoidable production-related impurities also contains (in % by weight):

C: 0.001 - 0.5 %,

Mn: 0.10 - 6.0 %,

Al: 0.01 - 2.0 %,

Cr: 5.0 - 30.0 %, Ni: 2.00 - 30.0 % and, optionally in each case, one or more of the elements from the group Si, Cu, Mo, N, Nb, Ti, V with the condition that the following values apply for the contents of optionally added elements:

Si: 0.001 - 2.0 %

Cu: up to 2.0 %,

Mo: up to 5.0 %,

N: up to 0.2 %,

Nb: up to 1 .0 %,

Ti: up to 1 .0 %,

V: up to 0.5 %. Each flat steel product processed according to the invention may be made available in the cold or hot rolled state either with or without a previously pickled surface. The advantages of the method according to the invention are realised particularly when processing unpickled hot strip, wherein the process according to the invention has proven to be particularly financially advantageous when processing narrow strip.

In operation step b), each flat steel product to be coated passes through a pickling tank, in which any scale still adhering to the surface is removed. The pickling process ideally lasts from 10 to 120 seconds. Besides cleaning, the surface of the flat steel product to be coated is also activated by the pickling process. The mordants used in this process may be fluids that are known per se for this application, based on an acid, particularly hydrochloric acid or sulphuric acid. In this context, it has proven particularly advantageous for the effectiveness of the pickling process if the mordant is at a temperature of 30 - 100 °C. With pickling temperatures and pickling times in the respectively defined ranges, an optimum cleaning effect is achieved without excessive pickling of the grain boundaries on the steel surface. In addition, if the temperature range defined according to the invention is observed, excessive evaporation loss is also avoided. This applies particularly if the maximum pickling temperature is limited to 70 °C. The concentration of Fe in the pickling tank should be between 5 and 130 g/l, so that this supports optimum effectiveness of the pickling process.

After the pickling, any mordant remaining on the flat steel product is removed by rinsing the flat steel product with an aqueous medium. The rinsing time optimally lasts 10 - 30 seconds, wherein the temperature of rinsing fluid is optimally 30 - 100 °C, and particularly 30 - 70 °C. If these rinsing temperature and time ranges are observed, effective removal of the residual acid is assured, so that pickling residues are effectively prevented from being carried into and contaminating the subsequent flux bath. If the specified upper limit for the rinsing temperature is exceeded, operating costs are increased due to the greater loss of rinsing agent due to evaporation. Therefore, the maximum rinsing temperature is advantageously 70 °C.

In the method according to the invention, operation step d) is particularly important, this being the step in which the flat steel product undergoes fluxing. The purpose of this treatment is to complete the activation of the surface of the flat steel product, which has already been activated during pickling, and to prevent back-passivation. For this purpose, the flat steel product is conveyed through a flux that is adjusted in keeping with the specifications of the invention as described above, by the addition of ammonium chloride, in such manner that good coating results are obtained with operational certainty.

Ammonium ions are contained in the flux bath described according to the invention in quantities from 5 - 12 g/l, chloride ions in quantities from 210 - 250 g/l, and zinc ions in quantities from 140 - 160 g/l, so that a ZnCI₂/NH₄CI salt mixture forms on the steel surface during drying. This forms HCI through thermal dissociation during the drying process, thereby ensuring activation not only against the steel surface but also against the surface slag in the coating bath. If the specified upper or lower limits are not complied with, activation will not be effective, or the flux medium will be burned off too intensely during drying. Particularly if the chloride ion quantity is low, the surface slag will not be reduced sufficiently. If the chloride and zinc ion content is too high, chlorine- hydroxozinc acids can be formed in the liquid flux which acids unnecessarily increase the quantity of iron dissolved in the flux vessel.

Potassium ions are present in the flux in quantities from 30 - 40 g/l, since the flux is stabilised against attack by or reaction with the Al of the coating batch by the addition of potassium ions. This reduces smoke formation when the steel strip is dipped in the coating bath, which in turn constitutes a positive effect with regard to environmental preservation and occupational safety for employees. If the content of potassium ions in the flux is too low, the described effect thereof is insufficient. On the other hand, if the content of potassium ions is too high, the activation effect of the flux may be too weak.

Optionally in each case, potassium ions in quantities of 0.5 - 1 .5 g/l, sodium ions in quantities of 0.5 - 1 .5 g/l and magnesium ions in quantities of < 1 g/l, may be present in the flux used according to the invention. Ca, Na and Mg reinforce the effect of the potassium ions, which are essential elements of the flux, and reduce the surface tension of the flux. This improves the wetting of the steel surface by the flux. The increase in the quantity of any of the contents in question above the respective upper limit defined therefor, does not further reduce smoke formation, but the activation effect of the flux may be weakened.

Ions of Fe, Mn, Al, Mo, Ni, P, Si, Sr, Li may be present in trace amounts as unavoidable contaminants, but the quantity of each should be less than 10 mg/l. Optimum results are achieved for the flux treatment if immersion, for which the section of the flat steel product to be dipped is in contact with the flux, lasts 10 - 120 sec. The effectiveness of the flux may be enhanced if the temperature thereof is in the range from 40 - 100 °C, particularly 40 - 70 °C. If these specified ranges for immersion duration and flux temperature are maintained, an optimal ultra-fine cleaning effect of the steel surface is achieved without excessive evaporation losses, and the grain boundaries on the steel surface are not exposed to undue attack.

The coating weight of the coating that is to be deposited on the steel substrate may be influenced by adjusting the flux density within a range of 1 .25 - 1 .45 g/cm³. If the density of the flux falls below the lower limit specified according to the invention, the medium will be too watery and the ultra-fine cleaning effect on the steel surface will be unsatisfactory. On the other hand, if the density of the flux exceeds the upper limit specified according to the invention, the fluxing action becomes too aggressive and the grain boundaries on the steel surface will be attacked too intensely.

For example, if Zn-based coverings with a coating weight of 400 - 600 g/m² are to be applied to the respective steel product, flux baths having a density from 1 .25 - 1 .35 g/cm³ have proven effective. On the other hand, if Zn-based coverings with a coating weight less than 400 g/m² are to be applied to the respective steel product, flux baths having a density from > 1 .35 - 1 .45 g/cm³ have proven favourable.

The particular effectiveness of the flux used according to the invention is also further enhanced if the pH value thereof is 4 - 4.5. However, if the upper limit specified for the pH value of the flux is exceeded, the medium will be too watery, and the ultra-fine cleaning effect on the steel surface will be unsatisfactory. On the other hand, if the pH value of the flux falls below the specified lower limit, the fluxing action again becomes too aggressive and the grain boundaries on the steel surface may be attacked too intensely.

In operation step e), the flat steel product emerging from the flux bath is dried and brought to the entry temperature, at which it enters the melt bath as the following step of the process. The minimum temperature should be high enough to dry of all of the flux remaining on the surface of the flat steel product when it exits the flux bath, to prevent the wetting process from being impaired during coating in the melt bath. At the same time, the drying temperature should not be too high, so that the flux is not burned off. Therefore, it has proven particularly favourable for drying purposes if the flat steel product is heated to a temperature in the range from 100 - 230 °C. The strip entry temperature should be maintained for at least 10 seconds while drying, in order to heat the flat steel product through sufficiently. The maximum permissible drying duration depends on the performance capability of the drying system used. In this regard, practical experiments have demonstrated that for the systems in common use at the moment a maximum duration of 30 seconds is reasonable.

In operation step f), the flat steel product passes through the melt bath. Optimum coating results are then obtained if the exposure time, for which the respective section of the flat steel product, is in contact with the melt bath lasts from 1 - 120 seconds, particularly 1 - 60 seconds. In this process, Zn or Al coverings may be produced on the respective flat steel product via means of a correspondingly adjusted alloy in the melt bath. The Zn- or Al-based coverings in this context include:

Z coverings that are produced on the basis of a melt bath which, besides Zn and unavoidable contaminants created during production and the process, contains Al in contents of more than 0.10 % by weight and up to 0.3 % by weight, Si in contents of up to 0.2 % by weight, and Fe in contents of less than 0.5 % by weight,

ZA coverings that are produced on the basis of a melt bath which, besides Zn and unavoidable contaminants created during production and the process, contains Al in contents of more than 0.10 % by weight and up to 5 % by weight, Si in contents of up to 0.2 % by weight, and Fe in contents of less than 0.5 % by weight,

ZM coverings that are produced on the basis of a melt bath which, besides Zn and unavoidable contaminants created during production and the process, contains Al in contents of more than 0.10 % by weight and up to 8.0 % by weight, Mg in contents of 0.2 - 8.0 % by weight, Si in contents of less than 2.0 % by weight, Pb in contents of less than 0.1 % by weight, Ti in contents of less than 0.2 % by weight, Ni in contents of less than 1 % by weight, Cu in contents of less than 0.1 % by weight, Co in contents of less than 0.3 % by weight, Mn in contents of less than 0.05 % by weight, Cr in contents of less than 0.1 % by weight, Sr in contents of less than 0.5 % by weight, B in contents of less than 0.1 % by weight, Bi in contents of less than 0.1 % by weight, Cd in contents of less than 0.1 % by weight, and Fe in contents of less than 3.0 % by weight, wherein for the ratio %AI/%Mg of the Al content, the %AI to the Mg content %Mg, %AI/%Mg < 1 ,

ZF coverings that are produced on the basis of a melt bath which, besides Zn and unavoidable contaminants created during production and the process, contains Al in contents of more than 0.1 % by weight and up to 0.15 % by weight, Si in contents of up to 0.2 % by weight, and Fe in contents of less than 0.5 % by weight,

and

AS coverings that are produced on the basis of a melt bath which, besides Al and unavoidable contaminants created during production and the process, contains Si in contents from 1 -15 % by weight, and Fe in contents of 1 .0-5.0 % by weight.

Experiments have confirmed that flat steel products that are produced from stainless CrNi steels which fall within the parameters of the alloy specification described above, and that have been processed in accordance with the invention, can be coated with any of the coverings described above, and that flat steel products made from a structural steel of the type described above may be protected from corrosion particularly effectively with a ZM covering applied in a manner according to the invention. Flat steel products that are produced from stainless steel and are furnished with an AS covering in accordance with the invention are particularly well suited for high temperature applications. The AS coating lends such flat steel products protection against the formation of heat tinting.

If the melt bath is a Zn-based bath, the temperature in the melt bath is 430 - 700 °C, typically 430 - 530 °C, whereas a melt bath that is based on Al is typically at a temperature up to 780 °C, particularly 650 - 780 °C.

If the hot dip coated flat steel product is to undergo inline thermal postprocessing (galvanization) in order to create a Fe-Zn alloy covering, it has proven effective if the melt bath is adjusted such that a ZF coving is produced on the steel substrate.

Also optionally, like the galvanization treatment, the hot dip coated flat steel product obtained may undergo passivation by a corresponding chemical treatment or by rerolling to improve its dimensional stability and mechanical properties.

In the following, the invention will be explained in greater detail with reference to exemplary embodiments thereof. In the drawing:

Fig. 1 shows a system for hot dip coating including the work stations that are essential for carrying out the method according to the invention and those that are provided additionally for optional purposes;

Fig. 2 is a representation of a cross-section through a flat steel product with a ZM covering applied to the flat steel product in accordance with the inventive method;

Fig. 3 is a representation of a cross-section through a flat steel product with an AS covering applied to the flat steel product in accordance with the inventive method;

Fig. 4 is a representation of a cross-section through a flat steel product with a ZM covering applied to the flat steel product in accordance with the inventive method. System 1 for hot melt dipping a flat steel product P provided as a steel strip that has been hot rolled and wound into a coil C comprises, in a sequentially in-line configuration in conveying direction F, an unwinding station 2, a pickling station 3, a rinsing station 4, a flux station 5, a drying station 6, a hot melt dipping station 7 and cooling station 8, and a winding station 9.

In the unwinding station 2, the flat steel product P is unwound from the respective coil C and first passes through the pickling station 3 and the following rinsing station 4 before reaching the flux station 5. The flat steel product P that exits the flux station 5 passes through the drying station 6 and is then forwarded to the melt bath S in the melt dipping station 7. The flat steel product P leaving the melt bath S then passes through the cooling station 8, where it is cooled to room temperature before being wound into a coil again in the winding station 9. In the coating system 1 , 55 tests were carried out with flat steel products P delivered in the form of hot rolled strips, which had been produced from different steels, W1 , W2, W3, W4, W5, W6, W7. The compositions of the steels W1 - W7 are presented in Table 1 . The steels W1 - W4 are conventional structural steels, whereas the steels W5 - W7 are conventional stainless CrNi stainless steels.

In the pickling station 3, each of the flat steel products P to be processed passed through a pickling bath B with conventional hydrochloric acid base, which had been heated to a temperature TB and through which the respective section of the respective flat steel product P passed within a pickling time tB.

Then, the flat steel products P passed through a rinsing bath V filled with demineralised water in the rinsing facility 4, which had been heated up to a temperature TS, and through which the respective section of the respective flat steel product P passed within a rinsing period tS. Afterwards, the flat steel products P were conveyed through a flux bath X in the flux station 5, through which the respective section of the respective flat steel product P passed within a period tF, and which was at a temperature TF, had a pH value pH_F, and a density r-F. In this context, twelve different flux baths X compositions were used during the tests. The twelve compositions X1 - X12 of flux baths X are listed in table 2.

In the drying station 6, the flat steel products were dried and brought to the respective bath entry temperature TE.

Then, the flat steel products P passed through the respective melt bath S in the hot melt dipping station 7, which is maintained at a temperature Tbad.

Upon leaving the hot melt dipping station 7, the thickness of the hot dip coating applied to each flat steel product was adjusted in known manner by means of a scraper device, not shown here.

The operating parameters for each of a total of 55 experiments are summarised in tables 3a, 3b. Table 3a shows the experiments that were run according to the invention, which yielded a good, error-free coating result, wherein tables 3b, 3c summarise the experiments that returned faulty coating results.

For each of the 55 experiments, information is given for the respective steel W1 - W7 from which each processed flat steel product P was made, the mordant temperature TB, the pickling duration tB, the temperature TS of the rinsing agent and the rinsing time tS, the flux bath X1 - X12 in flux station 5 through which each flat steel product passed, the time tF taken for each flat steel product to pass through the respective flux bath X1 - X12, the respective flux bath temperature TF, the respective flux bath temperature TF, the respective pH value pH_F and the respective density r-F of the respective flux bath X1 - X12, the respective drying or bath entry temperature TE, the composition of the respective melt bath S, the temperature Tbad of the respect melt bath S, and the respective coating weight AG reached on one of the coated sides of flat steel product P. The exposure time in the drying station was 20 seconds in each case, and the time in the melt bath was 10 seconds in each case. The cross-sectional representation shown in Fig. 2 was taken from the stainless steel flat steel product according to the invention coated with a ZM covering in the test 40.

The cross-sectional representation shown in Fig. 3 was taken from the stainless steel flat steel product according to the invention coated with an AS covering in the test 51 .

The cross-sectional representation shown in Fig. 4 was taken from the structural steel flat steel product according to the invention coated with a ZM covering in the test 28.

A comparison of Figs. 2 and 4 shows that both method variants in which the respective flat steel product P was hot melt dipped with a ZM covering, resulted in practically identical coverings regardless of the material used. Thus, each covering Z has a top layer of Zn mixed crystals (η phase) with stamped ZnMg₂ phases between the Zn mixed crystals, and a Fe-Zn alloy layer, formed between flat steel product P and the top layer and consisting of Fe-Zn phases, via which the top layer is permanently adhesively bonded to the steel substrate formed by the flat steel product P.

In the AS covering A shown in Fig. 3, a top layer AS consisting of AISi phases is positioned on a Fe-AI-Si alloy layer of Fe-AI-Si, via which in this case the top layer AS is attached to the flat steel product P. Steel C Si Mn Al Cr Nb Mo Ti Ni

W1 0.08 0.06 0.50 0.050 0.12 0.003 0.05 0.023 0.12

W2 0.15 0.06 0.80 0.050 0.12 0.003 0.05 0.023 0.12

W3 0.055 0.04 0.30 0.060 0.060 0.004 0.020 0.004 0.090

W4 0.080 0.48 1 .25 0.070 0.15 0.050 0.040 0.008 0.15

W5 0.03 2.00 2.00 0.20 18.50 1 .00 2.50 1 .00 13.00

W6 0.15 2.00 2.00 0.20 19.00 1 .00 0.80 1 .00 9.50

W7 0.07 2.00 2.00 0.20 17.00 0.00 0.00 1 .00 10.50

Remainder iron and unavoidable contaminants, values in % by weight

Table 1

X11 250 150 12 35 1.2 1.1 0

X12 260^*) 150 13^*) 35 1.2 1.1 2^*)

Values in g/l, ^*) not according to invention

Table 2

Test Steel TB tB TS ts Flux TF tF pH- r F TE Melt bath [% by weight] TBad AG

ra [s] ra [s] ra [s] F [g/cm³] ra Base Al Mg Si Fe ra [g/m²] metal

3 W1 50 30 50 15 X3 60 30 4 1.29 180 Zn 0.8 0.9 - - 480 400

4 W1 50 40 50 20 X4 60 40 4.3 1.41 180 Zn 0.8 0.9 - - 480 100

10 W2 50 30 50 15 X10 60 30 4.4 1.32 180 Zn 0.8 0.9 - - 465 500

11 W2 60 30 50 15 X11 60 30 4.3 1.28 180 Zn 0.8 0.9 - - 465 500

15 W2 50 30 50 15 X3 60 30 4 1.29 180 Zn 0.8 0.9 - - 465 400

16 W3 50 30 50 15 X4 60 30 4.3 1.41 180 Zn 0.8 0.9 - - 465 100

18 W3 50 30 50 15 X6 60 30 4.2 1.28 180 Zn 0.8 0.9 - - 465 550

22 W3 50 30 50 15 X10 60 30 4.4 1.32 180 Zn 0.8 0.9 - - 465 400

23 W4 50 30 50 15 X11 60 30 4.3 1.28 180 Zn 0.8 0.9 - - 465 400

27 W4 50 30 50 15 X3 60 30 4 1.29 180 Zn 0.6 0.9 - - 465 400

28 W4 50 30 50 15 X4 60 30 4.3 1.41 180 Zn 0.2 0.9 - - 455 100

30 W5 50 30 50 15 X6 60 30 4.2 1.28 180 Zn 1.5 4.8 - - 455 450

33 W5 50 30 50 15 X9 60 30 4.3 1.32 180 Zn 0.2 0.9 - - 455 450

34 W4 50 30 50 15 X10 60 30 4.4 1.32 180 Zn 0.2 0.9 - - 455 450

35 W4 50 30 50 15 X11 60 30 4.3 1.28 180 Zn 0.2 0.9 - - 455 450

39 W6 50 30 55 15 X3 60 30 4 1 .29 180 Zn 0.2 1 .1 - - 455 500

40 W6 50 30 55 15 X4 70 30 4.3 1 .41 180 Zn 0.2 1 .1 - - 455 200

45 W7 50 30 55 15 X9 70 30 4.3 1 .32 180 Zn 0.8 0.9 - - 465 450

49 W5 50 30 50 15 X10 60 30 4.4 1 .32 180 Al - - 9.5 3.5 670 100

50 W5 50 30 50 15 X10 60 30 4.4 1 .32 180 Al - - 9.5 3.5 670 100

51 W7 50 30 50 15 X1 1 60 30 4.3 1 .28 220 Al - - 9.5 3.5 670 100

52 W6 50 30 50 15 X1 1 60 30 4.3 1 .28 220 Al - - 10.5 3.2 670 150

53 W7 50 30 50 15 X1 1 60 30 4.3 1 .28 220 Al - - 10.5 3.2 670 150

Table 3a (Tests according to the invention)

Test Steel TB tB TS ts Flux TF tF pH- r F TE Melt bath [% by weight] TBad AG

ra [s] ra [s] ra [s] F [g/cm³] ra Base Al Mg Si Fe ra [g/m²] metal

1 W1 50 30 50 15 X1 60 30 4.6 1 .3 180 Zn 0.8 0.9 - - 465 400

2 W1 50 30 50 15 X2 60 30 4.3 1 .29 180 Zn 0.8 0.9 - - 465 400

5 W1 50 30 50 15 X5 55 30 4.2 1 .24 180 Zn 0.8 0.9 - - 465 450

6 W1 20 30 20 15 X6 20 30 4.2 1 .28 180 Zn 0.8 0.9 - - 465 450

7 W1 55 30 50 15 X7 60 30 4.2 1 .29 180 Zn 0.8 0.9 - - 465 450

8 W1 50 30 50 15 X8 60 30 4.4 1 .28 180 Zn 0.8 0.9 - - 465 500

9 W2 50 30 50 15 X9 60 30 4.3 1 .32 180 Zn 0.8 0.9 - - 465 500

12 W2 60 30 50 15 X12 60 30 4.4 1 .37 180 Zn 0.8 0.9 - - 465 100

13 W2 65 30 50 15 X1 60 30 4.6 1 .3 180 Zn 0.8 0.9 - - 465 400

14 W2 65 30 50 15 X2 60 30 4.3 1 .29 80 Zn 0.8 0.9 - - 465 400

17 W3 50 30 50 15 X5 60 30 4.2 1.24 180 Zn 0.8 0.9 - - 465 550

19 W3 50 30 50 15 X7 60 30 4.2 1.29 180 Zn 0.8 0.9 - - 465 550

20 W3 50 30 50 15 X8 60 30 4.4 1.28 180 Zn 0.8 0.9 - - 465 400

21 W3 50 30 50 15 X9 60 30 4.3 1.32 180 Zn 0.8 0.9 - - 465 400

24 W4 50 30 50 15 X12 60 30 4.4 1.37 180 Zn 0.8 0.9 - - 465 100

25 W4 50 30 50 15 X1 60 30 4.6 1.3 180 Zn 0.6 0.9 - - 465 400

26 W4 50 30 50 15 X2 60 30 4.3 1.29 80 Zn 0.6 0.9 - - 465 400

29 W4 50 30 50 15 X5 60 30 4.2 1.24 180 Zn 0.2 1.1 - - 455 100

31 W5 50 30 50 15 X7 60 30 4.2 1.29 180 Zn 1.5 4.8 - - 455 450

32 W5 50 30 50 15 X8 60 30 4.4 1.28 180 Zn 1.5 4.8 - - 455 450

36 W5 50 30 50 15 X12 60 30 4.4 1.37 180 Zn 0.2 0.9 - - 455 100

37 W6 50 30 55 15 X1 60 30 4.6 1.3 180 Zn 0.2 1.1 - - 455 500

38 W6 50 30 55 15 X2 60 30 4.3 1.29 180 Zn 0.2 1.1 - - 455 500

Table 3b (Tests not according to the invention)

Test Steel TB tB TS ts Flux TF tF pH- r F TE Melt bath [% by weight] TBad AG

[°C] [s] ra [s] ra [s] F [g/cm³] ra Base Al Mg Si Fe ra [g/m²] metal

41 W6 50 30 55 15 X5 70 30 4.2 1 .24 180 Zn 1 .5 9.5 - - 455 400

42 W6 50 30 55 15 X6 70 30 4.2 1 .28 180 Zn 1 .5 9.5 - - 455 450

43 W6 50 30 55 15 X7 70 30 4.2 1 .29 180 Zn 0.8 0.9 - - 465 450

44 W7 50 30 55 15 X8 70 30 4.4 1 .28 180 Zn 0.8 0.9 - - 465 450

46 W7 50 30 50 15 X10 60 30 4.4 1 .32 250 Zn 0.8 0.9 - - 465 400

47 W7 50 30 50 15 X1 1 60 30 4.3 1 .28 250 Zn 0.8 0.9 - - 500 400

48 W7 50 30 50 15 X12 60 30 4.4 1 .37 180 Zn 0.8 0.9 - - 465 100

54 W6 50 30 50 15 X2 60 30 4.3 1 .29 180 Al - - 10.5 3.2 670 100

55 W7 50 30 50 15 X2 60 30 4.3 1 .29 180 Al - - 9.5 3.5 670 150

Table 3c (Tests not according to the invention)

Claims

1 . A method for coating a flat steel product with a metal, Zn- or Al-based protective covering, comprising the following operation steps that are performed in continuous feed mode:

a) providing the flat steel product;

b) pickling the flat steel product to remove any scale adhering to the flat steel product and to activate the surface thereof; c) removing any mordants still present on the pickled surface of the flat steel product after the pickling process by rinsing the flat steel product with an aqueous medium;

d) conveying the pickled and rinsed flat steel product through a flux bath that consists of an aqueous solution, containing, besides unavoidable impurities created during the production and the process, chloride ions and ions from at least one of the elements of the group "zinc, ammonium and potassium" and optionally added ions of the elements "Na, Ca and Mg", and also optionally trace quantities of ions of the elements Al, Fe, Mn, Mo, Ni, P, Sr, Si and Li, with the qualification that

the total concentration of optionally present sodium ions c(Na⁺) is at least 0.5 g/l and at most 1 .5 g/l,

the total concentration of optionally present calcium ions c(Ca²⁺) is at least 0.5 g/l and at most 1 .5 g/l, the total concentration of optionally present magnesium ions c(Mg⁺) is at most 1 g/l,

the ions of the elements Al, Fe, Mn, Mo, Ni, P, Si, Sr and Li, present in trace quantities at most 10 mg/l, and

the density of the flux bath is at least 1 .25 g/cm³ and at most 1 .45 g/cm³;

e) drying the flat steel product after it emerges from the flux bath and heating the flat steel product to a bath entry temperature of 100 - 230 °C;

f) hot dip coating the flat steel product with a metallic protective covering based on Zn or Al, in a melt bath having the bath entry temperature when the flat steel product enters it;

g) optional performed thermal, chemical or mechanical post- treatment of the flat steel product that has been hot dip coated with the protective covering.

2. The method according to claim 1 , characterised in that the temperature of the flux bath is 40 - 100 °C.

3. The method according to any of the preceding claims, characterised in that the pH value of the flux bath is 4 - 4.5.

4. The method according to any of the preceding claims, characterised in that the immersion period within which the flat steel product is passed through the flux bath is 10 - 120 seconds.

5. The method according to any of the preceding claims, characterised in that the drying period, for which the flat steel product is dried in operation step e), is 10 - 30 seconds.

6. The method according to any of the preceding claims, characterised in that the exposure time of the flat steel product in the melt bath is 1 - 120 seconds.

7. The method according to any of the preceding claims, characterised in that temperature of the melt bath is 430 - 780 °C.

8. The method according to any of the preceding claims, characterised in that the flat steel product is at most 600 mm wide.

9. The method according to any of the preceding claims, characterised in that the melt bath is a Z, ZA, ZM, ZF or AS melt bath.

10. A flat steel product consisting of a structural steel that contains, besides iron and impurities that are unavoidable byproducts of production (in % by weight)

C: 0.001 - 0.7 %,

Mn: 0.10 - 2.0 %,

Al: 0.01 - 2.0 %,

and optionally one or more elements from the group of Si, P, S, Cr, Cu, Mo, N, Ni, Nb, Ti, V, Zr, B, with the qualification that the following values apply for the content of the optionally added elements:

Si: 0.001 - 2.0 %,

P: up to 0.055 %,

S: up to 0.055 %,

Cr: 0.01 - 2.0 %,

Cu: up to 0.6 %,

Mo: up to 0.2 %,

N: up to 0.030 %,

Ni: up to 2.1 %

Nb: up to 0.2 %,

Ti: up to 0.2 %, V: up to 0.2 %,

Zr: up to 0.2 %,

B: up to 0.0060 %, wherein the flat steel product is coated with a metallic protective covering based on Zn or Al by application of the method configured according to one of claims 1 to 9. The flat steel product consisting of a stainless steel that, besides iron and unavoidable production-related impurities also contains (in % by weight): C: 0.001 - 0.5 %,

Mn: 0.10 - 6.0 %,

Al: 0.01 - 2.0 %,

Cr: 5.0 - 30.0 %,

Ni: 2.00 - 30.0 % and, optionally in each case, one or more of the elements from the group Si, Cu, Mo, N, Nb, Ti, V with the condition that the following values apply for the contents of optionally added elements:

Si: 0.001 - 2.0 %,

Cu: up to 2.0 %,

Mo: up to 5.0 %,

N: up to 0.2 %,

Nb: up to 1 .0 %,

Ti: up to 1 .0 %,

V: up to 0.5 %,

wherein the flat steel product is coated with a metallic protective covering based on Zn or Al by application of the method configured according to one of claims 1 to 9.

12. The flat steel product according to claims 10 or 1 1 , characterised in that the metallic covering is a Z covering.

13. The flat steel product according to claims 10 or 1 1 , characterised in that the metallic covering is a ZA covering.

14. The flat steel product according to claims 10 or 1 1 , characterised in that the metallic covering is a ZM covering.

15. The flat steel product according to claims 10 or 1 1 , characterised in that the metallic covering is a ZF covering.

16. The flat steel product according to claims 10 or 1 1 , characterised in that the metallic covering is an AS covering.