METHOD FOR PRODUCING A ZINC-MAGNESIUM-GALVANNEALED HOT-DIP COATING AND FLAT STEEL PRODUCT PROVIDED WITH SUCH A COATING

Abstract

A process may be utilized to produce a galvannealed Zn—Mg hot dip coating that provides improved corrosion protection on a steel substrate, and to a flat steel product having such characteristics. To this end, a steel substrate may be subjected to hot dip coating in an Fe-saturated melt bath heated to 350-650° C., wherein the melt bath comprises in percent by weight 0.1-0.16% Al, 0.1-0.6% Mg, and also <2% Si, <1% each of Ni or Cu, <0.3% Co, <0.0001% Mn, <0.5% Sr, <0.1% each of Pb, B, Bi or Cd, <0.2% each of REM, Ti or Cr, or <0.5% Sn, with the balance being zinc and unavoidable impurities. Further, a galvannealing treatment may be conducted in which the steel substrate is kept at 450-800° C. for 10-25 seconds to produce a galvannealed Zn—Mg hot dip coating on the steel substrate. The coating may comprise Zn, unavoidable impurities, and in percent by weight 0.10-0.5% Al, 5.0-15.0% Fe, 0.10-0.8% Mg and <2 % Si, <1% each of Ni, Cu, <0.3% Co, <0.0001% Mn, <0.5% Sr, <0.1% each of B, Bi, Cd, Pb, <0.2% each of Cr, Ti, REM, <0.01% Sn.”

Description

The invention relates to a process for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, wherein the steel substrate is typically a flat steel product. A “flat steel product” refers to rolled steel products such as strips, sheets, and (pre-cut) blanks obtained therefrom.

The invention further relates to a flat steel product coated with a galvannealed zinc-magnesium hot dip coating.

A process and a flat steel product of a corresponding nature are known from WO 2009/059950 A2. In the known process, the flat steel product is annealed at an annealing temperature of 500-900° C., then cooled down to a bath entry temperature in the range of 360−710° C. and then guided through a Zn—Mg melt bath which has been heated to a melt bath temperature of 350-650° C. and may comprise, as well as zinc and unavoidable impurities (in % by weight), 4-8% Mg and 0.5-1.8% Al, and optionally one or more of the following elements with a content below the upper limit specified for each of these elements: 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%. In the flat steel product that exits from the melt bath, the layer thickness of the metallic coating is adjusted by removing excess Zn—Mg melt. The flat steel product thus obtained has a Zn—Mg—Al coating which, as well as zinc and unavoidable impurities, comprises (in % by weight) Mg: 4-8% and Al: 0.5-1.8%, and optionally one or more of the following elements with a content below the upper limit specified for each of these elements: 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%, REM <0.2%, Sn<0.5%. The flat steel product thus coated has excellent protection against corrosion and good suitability for welding.

Practical experience shows that, in the case of a galvannealed hot dip coating system, owing to the elevated Fe content in the coating resulting from the galvannealing treatment, there is premature occurrence of iron corrosion products that impair the anticorrosive action.

Against the background of the prior art, it was an object of the invention to name a process that allows the coating of steel substrates with a galvannealed hot dip coating which offers improved corrosion protection. A flat steel product correspondingly provided with an optimized galvannealed coating was likewise to be named.

In relation to the process, this object has been achieved by the invention in that, in a process of this kind, the steps specified in claim 1 are performed.

A flat steel product that achieves the aforementioned object in accordance with the invention is specified in claim 10.

Advantageous configurations of the invention are specified in the dependent claims and are elucidated individually hereinafter, as is the general concept of the invention.

The invention provides a process for producing steel products, especially flat steel products, that have been provided with a zinc-based hot dip coating and have improved anticorrosion properties. For this purpose, magnesium is introduced into the coating. To maintain the processing properties of the galvannealed hot dip coating, the maximum magnesium content that can be included in the alloy is limited here in order to still assure the alloy-forming characteristics of the galvannealed hot dip coating.

The steps that are typically implemented in a process for hot dip coating of a flat steel product with a Zn—Mg—Al coating, which are not mentioned individually here, are elucidated in WO 2009/059950 A2 which has already been mentioned above, the contents of which are hereby incorporated by reference into the present application for completion of the disclosure.

According to the invention, in a departure from the prior art known from WO 2009/059950 A2, in accordance with the invention, the composition of the melt bath is chosen such that, in the subsequent galvannealing treatment, a galvannealing coating with optimal use and anticorrosion properties is established. The aim of the procedure of the invention is the production of a hot dip coating with a Fe content typical of a galvannealed coating of 5-15% by weight, especially 7-15% by weight.

Accordingly, in a process of the invention for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, at least the following steps are implemented:

• a) providing a steel substrate,
• b) hot dip coating of the steel substrate in an Fe-saturated melt bath heated to a bath temperature of 350-650° C., consisting of (in % by weight) 0.1-0.16% Al and 0.1-0.6% Mg, and in each case optionally <2% Si, <0.1% Pb, <0.2% Ti, <1% Ni, <1% Cu, <0.3% Co, <0.0001% Mn, <0.2% Cr, <0.5% Sr, <0.1% B, <0.1% Bi, <0.1% Cd, <0.2% REM or <0.5% Sn, the balance being zinc and unavoidable impurities from the production, and
• c) performing a galvannealing treatment in which the hot dip-coated steel substrate is kept at a galvannealing temperature of 450-800° C. over a galvannealing time of 10-25 s in order to produce a galvannealed zinc-magnesium hot dip coating on the steel substrate, comprising zinc and unavoidable impurities (in % by weight) 0.10-0.5% Al, 5.0-15.0% Fe, 0.10-0.8% Mg and optionally one or more of the following elements with the following proviso regarding their contents: Si: <2%, Pb: <0.1%, Ti: <0.2%, Ni: <1%, Cu: <1%, Co: <0.3%, Mn: <0.0001%, Cr: <0.2%, Sr: <0.5%, B: <0.1%, Bi: <0.1%, Cd: <0.1%, REM: <0.2%, Sn <0.01%.

The selection of the parameters in the process of the invention is justified as follows:

• • Al content in the melt bath:
  • The Al content in the melt bath is 0.10-0.16% by weight, in order to assure good adhesion of the coating produced in accordance with the invention on the steel substrate. The addition of aluminum to the melt bath brings about the formation of an Fe2Al5 inhibition layer between the base material and the coating. This Fe2Al5 inhibition layer firstly assures the bonding of the coating to the steel substrate. Secondly, the Fe2Al5 inhibition layer inhibits the diffusion of the iron from the base material into the coating. The Fe2Al5 layer must not be too indistinct because the bonding between substrate and coating is otherwise impaired. However, it must not be too pronounced either because the diffusion of the iron into the coating is otherwise hindered too significantly. This mechanism is applicable both to the limitation of the Al content in the melt bath and to the limitation of the Al content in the coating. Since Al is accumulated in the coating to a greater degree than it is present in the melt bath, different upper limits are applicable to bath and coating. The Al contents of the melt bath that are envisaged in accordance with the invention are such as to result in a layer structure which is optimal for the adhesion of the coating. If the aluminum content in the melt bath exceeds the upper limit of 0.16% by weight, a covering Fe₂Al₅ inhibition layer will form between the base material and the coating, which is so thick that no proper galvannealed coating can be formed. If the aluminum content is less than 0.10% by weight, adhesion of the coating is inadequate. Harmful effects of the Al content on the formation of an optimal galvannealed coating can be reliably avoided in that the Al content of the melt bath is limited to not more than 0.15% by weight, and the positive effect of the presence of Al in the melt bath occurs particularly reliably when the Al content is more than 0.1% by weight, i.e. for the purposes of the person skilled in the art there is distinctly more than 0.1% by weight of Al present in the melt bath.
  • Fe content in the melt bath:
  • The melt bath should be saturated in Fe in order that the driving force for diffusion of iron out of the steel base material into the melt bath is hindered. The provision that the melt bath is to be saturated in Fe corresponds in practice, with the other specifications that are applicable here, to an Fe content in the melt bath of 0.01-0.5% by weight. If the iron content in the melt bath is less than 0.01%, iron can diffuse out of the steel base material into the melt bath. This damages the structure of the base material. The upper limit in the Fe content in the melt bath is determined by the solubility limit of the iron in the melt bath. If this limit is exceeded, there can be increased formation of slag resulting from precipitating iron phases.
  • Mg content in the melt bath:
  • The Mg content in the melt bath is 0.10-0.6% by weight. The upper limit of 0.6% by weight is applicable in order to achieve alloy formation in the coating at technically and economically favorable galvannealing temperatures of up to 800° C. It is found to be particularly practical in this aspect when the Mg content of the melt bath is restricted to up to 0.4% by weight. In the case of contents of less than 0.1% by weight, however, the improvement in corrosion protection which is the aim of the addition of Mg to the melt bath is achieved only inadequately. It can therefore be advantageous to add distinctly more than 0.1% by weight to the melt bath for the purposes of the person skilled in the art.
  • Other elements present in the melt bath:
  • The balance not accounted for by the contents of Al, Fe and Mg in the melt bath provided in accordance with the invention consists of zinc, where it is optionally possible in each case for the melt bath to include (in % by weight), less than 2% Si, less than 0.1% Pb, less than 0.2% Ti, less than 1% Ni, less than 1% Cu, less than 0.3% Co, less than 0.0001% Mn, less than 0.2% Cr, less than 0.5% Sr, less than 0.1% B, less than 0.1% Bi, less than 0.1% Cd, less than 0.2% REM and less than 0.5% Sn. These optionally present elements are present in the form of impurities from the preparation that are unavoidable but inactive with regard to the behavior of the melt bath and the coating to be produced therefrom, or may be deliberately added to establish particular properties of the coating. What is noteworthy here is that the Mn content of the melt bath is always restricted to a maximum value at which Mn is not present in the melt bath for technical purposes and as such displays no effect at all.
  • Melt bath temperature TB:
  • The melt bath temperature TB should be 350-650° C. At temperatures below 350° C., the melt bath begins to solidify and the steel substrate can no longer be coated. At temperatures greater than 650° C., there is increased evaporation of zinc. This is hazardous to health and leads to soiling of the coating plants. Melt bath temperatures of at least 430° C. have been found to be particularly useful in practice. Over and above a melt bath temperature of at least 430° C., it is possible to guarantee an optimally fluid melt bath with which coatings with optimized runoff properties and likewise optimized surface quality and homogeneity of the coating thickness can be achieved on the respective steel substrate. By limiting the melt bath temperature to 490° C., it is additionally possible to reduce the zinc loss resulting from evaporation, the endangerment of the environment and soiling to a minimum. Optimal melt bath temperatures of the Zn—Mg—Al—Fe melt bath envisaged in accordance with the invention are accordingly in the range of 430-490° C.
  • Galvannealing temperature TG:
  • The galvannealing temperature TG in the process of the invention is to be 450-800° C. The galvannealing temperature has to be at least 450° C. in order to activate the diffusion of iron out of the steel substrate into the hot dip coating. In order to be able to utilize this effect in an operationally reliable manner, the galvannealing temperature TG can be adjusted to at least 540° C. At temperatures above 800° C., by contrast, there is the risk that the zinc layer will evaporate off to an enhanced degree. This unwanted effect can be avoided in a particularly reliable manner in that the galvannealing temperature TG is restricted to a maximum of 720° C. Experiments have shown that, with the Mg contents that have been envisaged in accordance with the invention in the melt bath, the temperature range of 540-720° C. is optimal when the galvannealing times are chosen in the manner elucidated hereinafter. It has been found that particularly reliably zeta phase-free coatings can be produced with galvannealing temperatures of 540-720° C.
  • Galvannealing Time:
  • The galvannealing time, i.e. the time over which the flat steel product provided with the hot dip coating is kept at the galvannealing temperature, is 10-25 s. The galvannealing time has to be at least 10 s in order to achieve the minimum content of Fe envisaged in accordance with the invention in the coating and the associated formation of the characteristic galvannealing coating phases. In order to assure this in an operationally reliable manner, a galvannealing time of at least 12 s can be provided. At the same time, in accordance with the invention, the galvannealing time is not to exceed 25 s in order to avoid overalloying of the coating, i.e. a rise in the Fe contents of the coating to values of more than 15% by weight. The galvannealing time is variable in principle within the limits defined in accordance with the invention and can be influenced by the rate with which the steel substrate coated in each case is passed through the galvannealing treatment.
  • An overalloyed coating would not result in a sufficient anticorrosive effect. In order to particularly reliably avoid overalloying, the galvannealing time can be limited to not more than 24 s. Coatings having optimal properties can be produced unerringly in an economically viable manner when the galvannealing time is 12-24 s and the galvannealing temperature is 540-720° C.
  • Layer thickness of the coating obtained:
  • The layer thickness of the metallic coating produced in accordance with the invention is typically 3-20 μm. Layer thicknesses within this range, with optimal use properties and forming characteristics of the steel substrate coated in accordance with the invention, achieve corrosion protection which is likewise optimal.
  • Degree of alloy formation in the coating obtained:
  • It is basically the case that the degree to which an alloy is formed is defined via the Fe content in the coating. The coating is considered to be alloyed when the iron content in the coating is 9% by weight or more, whereas it is considered to be “overalloyed” when the Fe content in the coating is greater than 15% by weight.
  • Al content in the coating:
  • The Al content in the coating is 0.10-0.5% by weight. Al contents of this kind are necessary to ensure bonding of the coating, and are established as a result of the Al content of the melt bath envisaged in accordance with the invention. The Fe₂Al₅ inhibition layer formed in the coating ensures good bonding of the coating and improves the forming characteristics. The reason for the higher content in the coating compared to the melt bath likewise lies in the Fe₂Al₅ inhibition layer, since Al from the bath is increasingly accumulated in reaction with the Fe from the steel here. If the aluminum content in the coating is less than 0.10% by weight, what is formed is an inadequate Fe₂Al₅ inhibition layer that is required for sufficient adhesion. If the aluminum content in the coating is greater than 0.5% by weight, what is formed is an Fe₂Al₅ inhibition layer which is so thick that the diffusion of iron into the coating is so significantly inhibited that no proper galvannealed coating can be formed.
  • Fe content in the coating obtained:
  • The Fe content in the coating is 5.0-15.0% by weight. In order to count as a galvannealed coating, the iron content in the coating in customary production has to be at least 5.0% by weight. As elucidated above, alloy formation can be assumed when the iron content of the coating is at least 9% by weight. Advantageously, the Fe content in a flat steel product coated with a hot dip coating in accordance with the invention is 9.0-13.0% by weight.
  • Mg content in the coating obtained:
  • The Mg content in the coating is 0.10-0.8% by weight. At this Mg content, improved corrosion protection compared to a standard galvannealed coating without Mg is achieved. If the Mg content in the coating is less than 0.10% by weight, no improvement in corrosion protection is detectable. If the Mg content in the coating is more than 0.8%, the galvannealing temperatures necessary have to be increased to such an extent that performance in conventionally available production plants can no longer be implemented viably in technical and economic terms. As a result of the formation of Zn—Mg phases, Mg is incorporated into the coating with higher proportions than it is dissolved in the melt bath. This effect corresponds to the processes already elucidated above in connection with the Al content of a coating created in accordance with the invention.
  • Other elements present in the coating obtained:
  • The main constituent of the coating produced in accordance with the invention is zinc. In addition, as well as the main alloy elements Al, Mg and Fe that have already been mentioned, the coating may include the unavoidable impurities from the preparation and optionally one or more of the following elements each with a content below the upper limit specified for each of these elements (in % by weight): Si: <2%, Pb: <0.1%, Ti: <0.2%, Ni: <1%, Cu: <1%, Co: <0.3%, Mn: <0.0001%, Cr: <0.2%, Sr: <0.5%, B: <0.1%, Bi: <0.1%, Cd: <0.1%, REM <0.2%, Sn<0.01%. In accordance with the comments that have already been made above with regard to the other elements present in the melt bath, the elements in question may be present in the manner of impurities in contents that are ineffective for alloying purposes or may be deliberately added at elevated contents for establishment of particular properties of the coating. The contents of Mn should in each case be kept so low that they display no effect. Ca, Be and Li are also permitted merely as impurities in a coating of the invention, where the contents of Ca, Be and Li should be limited to less than 0.0001% by weight in each case.

In principle, the steel substrate processed in accordance with the invention may be any steel component, for example a steel profile or the like. However, the invention is of particularly good suitability for the processing of flat steel products as steel substrate since flat steel products of this kind can be processed in the manner of the invention with high economic viability in plants established in practice. More particularly, plants suitable for this purpose are those which are passed by the respective flat steel product in a manner known in principle in a continuous run.

The application of the invention is not limited to steel products that are produced from a particular steel type, but is suitable for coating of all steel strips and sheets on which particular demands are made with regard to corrosion protection. For the treatment by the process of the invention, therefore, suitable steel products are all of those which consist of steels which can be coated at all with a galvannealed Zn—Mg—Al coating of the type to be produced in accordance with the invention. This especially include IF steels, especially soft or higher-strength IF steels, bake-hardening steels, microalloyed steels and multiphase steels. A selection of alloy specifications which is illustrative of these steels is summarized in table 6.

In accordance with the above elucidations, a flat steel product of the invention has a galvannealed zinc-magnesium hot dip coating consisting of (in % by weight)

• • Al: 0.10-0.5%
  • Fe: 5.0-15.0%
  • Mg: 0.10-0.8%
  • optionally of one or more of the following elements with the following proviso regarding their contents:
    • Si: <2%,
    • Pb: <0.1%,
    • Ti: <0.2%,
    • Ni: <1%,
    • Cu: <1%,
    • Co: <0.3%,
    • Mn: <0.0001%,
    • Cr: <0.2%,
    • Sr: <0.5%,
    • B: <0.1%,
    • Bi: <0.1%,
    • Cd: <0.1%,
    • REM: <0.2%,
    • Sn: <0.01%,
  • the balance being zinc and unavoidable other impurities.

A particular product feature that should be emphasized is that it is possible to produce coatings created in the manner of the invention in the above-defined mode of the invention that are free of zeta phases and therefore show relatively low abrasion in forming operations.

The invention is elucidated in detail hereinafter with reference to working examples. The figures show:

FIG. 1 a schematic diagram of an annealing cycle performed on flat steel product samples of the invention;

FIG. 2 a diagram showing, by way of example, the shift in the current density potential curve of flat steel product samples consisting of an IF steel of variant 2 specified in table 1, these having been provided in the manner of the invention with galvannealed coatings with a rising Mg content.

Cold-rolled flat steel product samples consisting of IF steels of variants 1 and 2 specified in table 1 have been provided.

The flat steel product samples of corresponding composition have been pretreated in a manner which is known per se and has been described in detail in WO 2009/059950 A2 for the hot dip coating.

As shown schematically in FIG. 1, the samples, after degreasing, in a continuous run, have first undergone a heat treatment in which they have been subjected to recrystallization annealing, in order then to run through a melt bath at a particular intake temperature TE. Different melt baths have been used here in different experiments, the compositions of which are specified in table 2. The melt bath temperature in each case was 460° C.

On exit from the melt bath, the thickness of the hot dip coating now present on the particular sample has been adjusted to 7 μm in each case in a manner which is likewise known per se (WO 2009/059950 A2).

Subsequently, the samples have undergone a galvannealing treatment in which they have been kept at a galvannealing temperature TG over a galvannealing time tG.

After the cooling, the Al, Mg and Fe contents of the galvannealing coatings present on the samples after the galvannealing treatment have been determined. The results of this analysis and the galvannealing parameters “galvannealing time tG” and “galvannealing temperature TG” are reported in table 3.

It is found that, in the context of the invention, in the case of selection of a suitable galvannealing temperature TG, depending on the Mg content, it is possible to produce galvannealed hot dip coatings having inventive Fe contents of 8-15% by weight combined with Mg content of 0.1-0.5% by weight.

The analysis for examining the metallic coatings was conducted by means of ICP-OES according to DIN EN ISO 11885.

In further experiments, the effect of the magnesium content of the melt bath used in each case on the alloy formation and anticorrosion properties was examined in the samples consisting of IF steel variant 1 and IF steel variant 2. The contents Al_bath and Mg_bath bath of Al and Mg in the melt bath used in each case, the galvannealing temperature TG and the presence of zeta phases (ζ) are reported for the samples consisting of IF steel variant 1 in table 4 and for the samples consisting of IF steel variant 2 in table 5. In addition, tables 4 and 5 also state in each case whether an alloy has formed in the coating and whether there has been an improvement in the anticorrosion characteristics.

The presence of zeta phases (ζ) was examined by means of XRD (x-ray diffractometry) measurements.

TABLE 1
Steel	C	Si	Mn	P	S	Al	N	Ti
IF steel variant 1	0.0016	0.019	0.14	0.008	0.007	0.029	0.0025	0.075
IF steel variant 2	0.0024	0.076	0.13	0.010	0.008	0.022	0.0025	0.068
Content figures in % by weight, the balance being iron and unavoidable impurities

	TABLE 2
	Al	Fe	Mg	Inventive
	ZF	0.129	0.031	—	NO
	ZF-Mg0.057%	0.144	0.028	0.057	NO
	ZF-Mg0.100%	0.125	0.029	0.100	YES
	ZF-Mg0.189%	0.140	0.029	0.189	YES
	ZF-Mg0.449%	0.136	0.019	0.449	YES
	Content figures in % by weight, the balance being zinc and unavoidable impurities

TABLE 3
Sample		Coating
no.	Steel	tG [s]	TG [° C.]	Melt bath	Al	Fe	Mg	Inventive?
1	IF steel variant 1	15	540	ZF (reference)	0.22	9.09	<0.01	NO
2	IF steel variant 2	15	540	ZF (reference)	0.26	11.58	<0.01	NO
3	IF steel variant 1	10	540	ZF-Mg0.057%	0.18	7.88	0.04	NO
4	IF steel variant 2	10	540	ZF-Mg0.057%	0.28	12.57	0.04	NO
5	IF steel variant 1	10	500	ZF-Mg0.100%	0.31	8.18	0.10	YES
6	IF steel variant 2	10	500	ZF-Mg0.100%	0.29	11.17	0.11	YES
7	IF steel variant 1	15	560	ZF-Mg0.100%	0.30	11.13	0.12	YES
8	IF steel variant 2	15	560	ZF-Mg0.100%	0.29	13.35	0.10	YES
9	IF steel variant 1	20	600	ZF-Mg0.100%	0.30	14.23	0.13	YES
10	IF steel variant 2	20	600	ZF-Mg0.100%	0.26	14.08	0.12	YES
11	IF steel variant 1	15	580	ZF-Mg0.189%	0.47	9.65	0.19	YES
12	IF steel variant 2	15	580	ZF-Mg0.189%	0.42	13.07	0.17	YES
13	IF steel variant 1	15	620	ZF-Mg0.189%	0.44	11.98	0.18	YES
14	IF steel variant 2	30	620	ZF-Mg0.189%	0.48	16.73	0.17	NO
15	IF steel variant 1	15	650	ZF-Mg0.189%	0.47	13.78	0.17	YES
16	IF steel variant 2	30	650	ZF-Mg0.189%	0.43	16.93	0.17	NO
17	IF steel variant 1	15	660	ZF-Mg0.449%	0.30	14.61	0.40	YES
18	IF steel variant 2	15	660	ZF-Mg0.449%	0.21	10.95	0.39	YES
19	IF steel variant 1	20	680	ZF-Mg0.449%	0.23	13.20	0.38	YES
20	IF steel variant 2	20	680	ZF-Mg0.449%	0.22	12.25	0.39	YES
21	IF steel variant 1	30	720	ZF-Mg0.449%	0.32	21.33	0.48	NO
22	IF steel variant 2	15	720	ZF-Mg0.449%	0.24	15.00	0.38	YES

TABLE 4
					Alloy	Improved
	Albath	Mgbath	Tgalvannealing		formation in	anticorrosion
Sample	[% by wt.]	[% by wt.]	[° C.]	ζ	coating	characteristics
1	0.129	—	540	+	Yes	Reference
3	0.144	0.057	540	+	Incomplete	No
5	0.125	0.100	500	+	Incomplete	Yes
7	0.125	0.100	560	−	Yes	Yes
9	0.125	0.100	600	−	Yes	Yes
11	0.140	0.189	580	−	Yes	Yes
13	0.140	0.189	620	−	Yes	Yes
15	0.140	0.189	650	−	Yes	Yes
17	0.136	0.449	660	−	Yes	Yes
19	0.136	0.449	680	−	Yes	Yes
21	0.136	0.449	720	−	Overalloyed	No
Sample material: IF steel variant 1

TABLE 5
					Alloy	Improved
	Albath	Mgbath	Tgalvannealing		formation in	anticorrosion
Sample	[% by wt.]	[% by wt.]	[° C.]	ζ	coating	characteristics
2	0.129	—	540	+	Yes	Reference
4	0.144	0.057	540	+	Yes	No
6	0.125	0.100	500	+	Yes	Yes
8	0.125	0.100	560	−	Yes	Yes
10	0.125	0.100	600	−	Yes	Yes
12	0.140	0.189	580	−	Yes	Yes
14	0.140	0.189	620	−	Overalloyed	No
16	0.140	0.189	650	−	Overalloyed	No
18	0.136	0.449	660	−	Yes	Yes
20	0.136	0.449	680	−	Yes	Yes
22	0.136	0.449	720	−	Yes	Yes
Sample material: IF steel variant 2

	TABLE 6
	C	Si	Mn	P	S	Al	N	Cu	Cr	Ni	No	Ti	Nb	B	Sn
Soft IF	min	0.001	0.01	0.05	0	0	0.005	0	0	0	0	0	0.03	0	0	0
steels	max	0.006	0.12	0.15	0.019	0.018	0.06	0.006	0.11	0.1	0.1	0.02	0.11	0.02	0.0008	0.03
Higher-	min	0	0.04	0.15	0.025	0	0.02	0	0	0	0	0	0.03	0	0	0
strength	max	0.004	0.14	0.8	0.075	0.012	0.045	0.004	0.08	0.06	0.02	0.02	0.065	0.035	0.0013	0.02
IF steels
Bake-	min	0	0	0.15	0.012	0	0.015	0	0	0	0	0	0	0	0	0
hardening	max	0.08	0.17	0.45	0.051	0.012	0.075	0.004	0.09	0.06	0.06	0.02	0.017	0.012	0.0035	0.015
steels
Micro-	min	0.055	0.03	0.1	0	0	0.02	0	—	—	—	—	0	0.0005	0	0
alloyed	max	0.18	0.48	1.6	0.025	0.012	0.07	0.0085	—	—	—	—	0.033	0.055	0.0005	0.03
steels
Multi-	min	0.02	0	1.2	0	0	0.02	0	0	0	0	0	0	0	0	0
phase	max	0.165	1.6	2.65	0.02	0.003	1.55	0.008	0.12	0.75	0.1	0.28	0.13	0.043	0.002	0.03
steels

Claims

1.-12. (canceled)

13. A process for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, the process comprising:

providing a steel substrate;

hot dip coating the steel substrate in an Fe-saturated melt bath heated to a bath temperature of 350° C.-650° C., wherein the Fe-saturated melt bath comprises 0.1%-0.16% by weight Al, 0.1%-0.6% by weight Mg, zinc, and unavoidable impurities; and

performing a galvannealing treatment in which the steel substrate that has been hot dip-coated is kept at a galvannealing temperature of 450° C.-800° C. over a galvannealing time of 10-25 seconds to produce a galvannealed zinc-magnesium hot dip coating on the steel substrate, wherein the galvannealed zinc-magnesium hot dip coating comprises zinc, unavoidable impurities, and 0.10%-0.5% by weight Al, 5.0%-15.0% by weight Fe, 0.10%-0.8% by weight Mg.

14. The process of claim 13 wherein

the Fe-saturated melt bath further comprises at least one of <2% by weight Si, <0.1% by weight Pb, <0.2% by weight Ti, <1% by weight Ni, <1% by weight Cu, <0.3% by weight Co, <0.0001% by weight Mn, <0.2% by weight Cr, <0.5% by weight Sr, <0.1% by weight B, <0.1% by weight Bi, <0.1% by weight Cd, <0.2% by weight REM, or <0.5% by weight Sn, and

the galvannealed zinc-magnesium hot dip coating further comprises at least one of <2% by weight Si, <0.1% by weight Pb, <0.2% by weight Ti, <1% by weight Ni, <1% by weight Cu, <0.3% by weight Co, <0.0001% by weight Mn, <0.2% by weight Cr, <0.5% by weight Sr, <0.1% by weight B, <0.1% by weight Bi, <0.1% by weight Cd, <0.2% by weight REM, or <0.01% by weight Sn.

15. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.1%-0.15% by weight Al.

16. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.01%-0.5% by weight Fe.

17. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.1%-0.4% by weight Mg.

18. The process of claim 13 wherein the bath temperature of the Fe-saturated melt bath is 430° C.-490° C.

19. The process of claim 13 wherein the galvannealing temperature at which the steel substrate is kept during the galvannealing treatment is 540° C.-720° C.

20. The process of claim 13 wherein the galvannealing time is 12-24 seconds.

21. The process of claim 13 wherein the steel substrate is a flat steel product.

22. The process of claim 13 wherein the steel substrate comprises an IF steel.

23. A flat steel product having a galvannealed zinc-magnesium hot dip coating that comprises:

zinc;

unavoidable impurities;

0.10%-0.5% by weight Al;

5.0-15.0% by weight Fe; and

0.10-0.8% by weight Mg.

24. The flat steel product of claim 23 further comprising at least one of:

<2% by weight Si;

<0.1% by weight Pb;

<0.2% by weight Ti;

<1% by weight Ni;

<1% by weight Cu;

<0.3% by weight Co;

<0.0001% by weight Mn;

<0.2% by weight Cr;

<0.5% by weight Sr;

<0.1% by weight B;

<0.1% by weight Bi;

<0.1% by weight Cd;

<0.2% by weight REM; or

<0.01% by weight Sn.

25. The flat steel product of claim 23 comprising an IF steel.

26. The flat steel product of claim 23 wherein the galvannealed zinc-magnesium hot dip coating is free of zeta phase.