Method for producing a flat steel product provided with a zn-based Anti-corrosion coating, and flat steel product

Abstract

The invention relates to a method for producing a flat steel product provided with a Zn-based anti-corrosion coating and having a reduced susceptibility to liquid metal embrittlement, and to a corresponding flat steel product.

Description

[0001] ThyssenKrupp Steel Europe AG 247016P10WO

[0002] August 7, 2025

[0003] 1 / 17

[0004] Method for producing a steel flat product and steel flat product provided with a zinc-based corrosion protection coating

[0005] The invention relates to a method for producing a steel flat product provided with a zinc-based corrosion protection coating with reduced susceptibility to liquid metal embrittlement, and to a corresponding steel flat product.

[0006] Here, "flat steel products" refers to rolled products whose length and width are significantly greater than their thickness. These include, in particular, steel strips, steel sheets, and cut pieces derived from them.

[0007] Unless explicitly stated otherwise, all information regarding the content of alloying elements in this text is given in wt.%.

[0008] Liquid metal embrittlement (LME) describes a mechanism in which a liquid phase penetrates a substrate and causes embrittlement. LME frequently occurs during the thermal joining of high-strength steels used in automotive body-in-white construction and can manifest as complete separation of the substrate along grain boundaries, which is associated with a significant reduction in performance characteristics. In automotive body-in-white construction, GMAW brazing, GMAW welding, laser beam welding, and resistance spot welding are primarily used, and LME can occur in all of these processes. The necessary conditions for LME formation include not only the presence of mechanical stresses and a liquid phase, but also the susceptibility of the material to this process.

[0009] It is known from the literature that small amounts of boron in steel can reduce the tendency to form LME, cf. Abari et al 2024 “Insight into grain boundaries with reduced liquid metal embrittlement susceptibility in a boron-added 3rd generation advanced high strength steel - ScienceDirect“, https: / / www.sciencedirect.com / science / article / pii / S 0264127523010006.

[0010] The object of the invention is therefore to provide a method and a correspondingly manufactured flat steel product with which the tendency to liquid metal embrittlement can be reduced. ThyssenKrupp Steel Europe AG 247016P10WO

[0011] August 7, 2025 2 / 17

[0012] The problem with regard to the method is solved by the features of claim 1 and with regard to the steel flat product by the features of claim 10.

[0013] According to a first teaching, the invention relates to a method for producing a steel flat product provided with a zinc-based corrosion protection coating and with a reduced tendency to liquid metal embrittlement, comprising the following steps: a) Providing a steel flat product which, in wt.%, consists of

[0014] C: 0.10 - 1.0 %,

[0015] Si: 0.010 - 5.0%, Mn: 0.30 - 10.0%, and optionally one or more of the alloying elements summarized in the group “V, Cr, Mo, B, Ti, Nb, Al, Ni, Cu, W, As, Sn, Sb, REM, Mg, Ca, Zr, Ta”, with the proviso that the contents of the respective optionally present alloying element are to be dimensioned as follows:

[0016] V: up to 0.30%

[0017] Cr: up to 1.50%

[0018] Mon: up to 1.0%

[0019] B: up to 0.010%

[0020] Ti: up to 0.30%

[0021] Note: up to 0.30%

[0022] AI: up to 10.0%

[0023] Ni: up to 1.0%

[0024] Cu: up to 1.0%, W: up to 1.0%, As: up to 0.1%, Sn: up to 0.1%, Sb: up to 0.1%, REM: up to 0.1%, Mg: up to 0.015%, Ca: up to 0.015%, Zr: up to 0.1%, ThyssenKrupp Steel Europe AG 247016P10WO

[0025] August 7, 2025

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[0027] Ta: up to 0.1%, and consisting as a remainder of iron and unavoidable impurities, wherein the unavoidable impurities include contents of less than 0.20% P, less than 0.050% S, and less than 0.020% N, wherein the sum of the impurity content is <= 0.50%; b) Producing a layer on the surface of the steel flat product by applying and drying a carrier fluid to the surface of the steel flat product, which contains at least one compound of a water-soluble or dispersible salt of boric acid or boric acid, wherein the layer thus produced on the surface has a coating weight of 1 to 100 mg / m² and covers 20 to 100% of the surface of the steel flat product; c) Coating the steel flat product with a corrosion-resistant coating consisting of zinc and unavoidable impurities, and optionally, in wt.%, up to 8.0% Al and / or consists of up to 8.0% Mg.

[0028] According to a second teaching, the invention relates to a steel flat product provided with a zinc-based corrosion protection coating with reduced susceptibility to liquid metal embrittlement, wherein the steel flat product consists, in wt.%, of

[0029] C: 0.10 - 1.0 %,

[0030] Si: 0.010 - 5.0 %,

[0031] Mn: 0.30 - 10.0 %, and optionally one or more of the alloying elements summarized in the group “V, Cr, Mo, B, Ti, Nb, Al, Ni, Cu, W, As, Sn, Sb, REM, Mg, Ca, Zr, Ta”, with the proviso that the contents of the respective optionally present alloying element are to be dimensioned as follows:

[0032] V: up to 0.30%

[0033] Cr: up to 1.50%

[0034] Mon: up to 1.0%

[0035] B: up to 0.010%

[0036] Ti: up to 0.30%, ThyssenKrupp Steel Europe AG 247016P10WO

[0037] August 7, 2025

[0038] 4 / 17

[0039] Note: up to 0.30%

[0040] AI: up to 10.0%

[0041] Ni: up to 1.0%

[0042] Cu: up to 1.0%

[0043] W: up to 1.0%

[0044] As: up to 0.1%

[0045] Sn: up to 0.1%

[0046] Sb: up to 0.1%

[0047] REM: up to 0.1%

[0048] Mg: up to 0.015%

[0049] Approximately: up to 0.015%

[0050] Zr: up to 0.1%

[0051] Ta: up to 0.1%, and the remainder consists of iron and unavoidable impurities, wherein the unavoidable impurities include contents of less than 0.20% P, less than 0.050% S and less than 0.020% N, wherein the sum of the impurity content is <= 0.50%, wherein the corrosion protection coating consists of zinc and unavoidable impurities and optionally, in wt%, up to 8.0% Al and / or up to 8.0% Mg, wherein a maximum boron concentration, determined by GDOES analysis, is present in a transition zone between the corrosion protection coating and the steel flat product.

[0052] In one embodiment, the steel flat product provided with a zinc-based corrosion protection coating and reduced susceptibility to liquid metal embrittlement is a steel flat product that can be manufactured or produced using the method according to the invention.

[0053] Glow discharge optical emission spectrometry (GDOES) is a spectrometric method for the atomic analysis of layer structures. The information contained here and in the claims regarding the structure of the corrosion coating, the transition zone, and the near-surface boundary layer of the steel flat product refers to a layer profile determined by a GDOES measurement. The GDOES measurement method described, for example, in the VDI Lexicon of Materials Engineering, edited by Hubert Gräfen, VDI-Verlag GmbH, Düsseldorf 1993, is a standard method for the rapid acquisition of a concentration profile of layer structures; see also Bauch, J., Rosenkranz, R. (2017). GDOES - Glow Discharge Optical Emission Spectrometry. ThyssenKrupp Steel Europe AG 247016P10WO

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[0056] Spectroscopy: Physical Materials Diagnostics. Springer Vieweg, Berlin, Heidelberg, https: / / doi.org / 10.1007 / 978-3-662-53952-l 42.

[0057] The steel flat product consists of (all values ​​in wt.%) in addition to iron and unavoidable impurities:

[0058] 0.10 to 1.0, preferably 0.110 to 0.50, particularly preferably 0.130 to 0.40, C, 0.010 to 5.0, preferably 0.030 to 2.50, particularly preferably 0.10 to 2.0, Si, 0.30 to 10.0, preferably 0.50 to 4.0, particularly preferably 0.70 to 3.50, Mn, optionally up to 0.30, preferably 0.0001 to 0.250, particularly preferably 0.0010 to 0.20, V, optionally up to 1.50, preferably 0.0001 to 1.0, particularly preferably 0.0010 to 0.60, Cr, optionally up to 1.0, preferably 0.0001 to 0.80, particularly preferably 0.0010 to 0.50, Mo, optionally up to 0.010, preferably 0.0001 to 0.0045, particularly preferably 0.0005 to 0.0040; B, optionally up to 0.30, preferably 0.0001 to 0.20, particularly preferably 0.0010 to 0.10; Ti, optionally up to 0.30, preferably 0.0001 to 0.20, particularly preferably 0.0010 to 0.10; Nb, optionally up to 10.00, preferably 0.0010 to 2.00, particularly preferred 0.010 to 1.00; AI, optionally up to 1.0, preferably 0.0001 to 0.60, particularly preferred 0.0010 to 0.40; Ni, optionally up to 1.0, preferably 0.0001 to 0.60, particularly preferred 0.0.0010 to 0.40, Cu, optionally up to 1.00, preferably 0.0001 to 0.700, particularly preferably 0.0010 to 0.500, W, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.01, As, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.02, Sn, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.01, Sb, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.02, SEM, optionally up to 0.015, preferably up to 0.01, particularly preferably up to 0.005, Mg, optionally up to 0.015, preferably up to 0.01, particularly preferably up to 0.008, Ca, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.01, Zr, optionally up to 0.1, preferably up to 0.05, particularly preferably up to 0.01, Ta, and as a remainder of iron and unavoidable impurities, wherein the unavoidable impurities are up to 0.020, preferably 0.0002 to 0.0150, particularly preferably 0.0002 to 0.010, N, up to 0.20, preferably 0.0002 to 0.15, particularly preferably 0.0010 to 0.10, P and up to 0.050, preferably 0.0005 to 0.0300,Particularly preferably, the impurities are 0.0005 to 0.0200. The proportions of the impurities are so small that they have no effect on the properties of the steel flat product and the sheet produced from it. Furthermore, the sum of the impurity levels is ≤ 0.50%, preferably ≤ 0.30%, and particularly preferably ≤ 0.20%. ThyssenKrupp Steel Europe AG 247016P10WO

[0059] August 7, 2025

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[0061] The transition zone between corrosion protection coating and steel flat product is, in the sense of the invention, the zone which, determined by means of a GDOES analysis, can be defined below the maximum value of zinc of the corrosion protection coating and below the maximum value of iron of the steel flat product.

[0062] The flat steel product, which can be hot-rolled or preferably cold-rolled, undergoes heat treatment to achieve desired material properties, which depend in particular on the composition. The heat treatment methods used are those known from the production of thin steel sheets, especially continuous annealing in a continuous furnace or discontinuous annealing (hood annealing), but also any other heat treatment suitable for achieving the desired microstructure and / or ensuring the functionality of a surface finish, for example, annealing equipment that can be operated inductively and / or conductively, preferably in conjunction with rapid cooling.

[0063] Alternatively, heat treatment, using hot-rolled strip as an example, can also be carried out by wetting the hot-rolled strip with the aforementioned carrier fluid after it leaves the last rolling stand of a hot rolling mill. Drying can then occur gradually through the warm strip, so that, for example, the desired coating has already formed before coiling.

[0064] Depending on its composition and heat treatment, the steel flat product can be produced with a tensile strength of at least 600 MPa, in particular at least 700 MPa, preferably at least 800 MPa up to 1800 MPa, in particular up to 1700 MPa, preferably up to 1650 MPa. The tensile strength can be determined according to DIN EN ISO 6892:2020-06 (specimen type 2, longitudinal specimens). The produced steel flat product, preferably a cold-rolled strip, is intended for the manufacture of components, in particular automotive components, by means of cold forming. Depending on the composition of the steel flat product and a desired property such as tensile strength, yield strength, elongation at break, hole expansion ratio, etc., those skilled in the art know the annealing conditions after preferably cold-rolling the strip in order to selectively adjust these properties, depending on the composition (chemistry) of the steel flat product, by means of heat treatment prior to hot-dip coating.These range from soft steels (deep-drawing steels) to high-strength, high-strength and ultra-high-strength steels, see ThyssenKrupp Steel Europe AG 247016P10WO.

[0065] August 7, 2025

[0066] 7 / 17, for example, also material data sheet VDA 239-100, edition May 2016. The latter exhibit a high tendency towards LME and can be improved by applying the invention.

[0067] The invention is particularly suitable for so-called and known high-strength, higher-strength and ultra-high-strength DP, CP, Q&P steels, bainitic and martensitic steels with a tensile strength Rm between 780 and 1650 MPa.

[0068] Q&P steels are third-generation steels of the so-called "advanced high strength steels," abbreviated as ahss (see also https: / / ahssinsights.org / ?s=gp+steel). The production of this type of steel is well-known to experts. An example of a generic process for producing a Q&P steel is disclosed in EP 2 524 970 Al.

[0069] The steel flat product is provided with a zinc-based corrosion protection coating, which, in addition to zinc and unavoidable impurities, may optionally contain additional elements such as aluminum with a content of up to 8.0 wt.%, in particular up to 5.0 wt.%, and / or magnesium with a content of up to 8.0 wt.%, in particular up to 5.0 wt.%. Steel flat products with a zinc-based corrosion protection coating exhibit very good cathodic corrosion protection and have been used in automotive manufacturing for decades. Hot-dip galvanizing and electrolytic galvanizing are particularly suitable for producing this corrosion protection. If improved corrosion protection is required, the corrosion protection coating additionally contains magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt.%.Aluminium can be present as an alternative or in addition to magnesium with a content of at least 0.01 wt.%, in particular at least 0.1 wt.%, for example to improve the bonding of the corrosion protection coating to the steel flat product.

[0070] The coating thickness in the solid state can range from 1.0 pm to 60.0 pm per side. The coating thickness can be adjusted independently for each side, particularly between at least 4.0 pm, preferably at least 5.0 pm, and a maximum of 58.0 pm, preferably between 5.0 pm and a maximum of 55.0 pm. In a particular embodiment, the coating thickness is at least 1.0 pm, preferably at least 2.0 pm, particularly preferably at least 3.0 pm, and especially at least 5.0 pm. ThyssenKrupp Steel Europe AG 247016P10WO

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[0073] 25.0 pm, preferably a maximum of 20.0 pm and particularly preferably a maximum of 15.0 pm, especially a maximum of 10.0 pm, independently of each other, each per side.

[0074] Below the minimum limit, sufficient cathodic corrosion protection cannot be guaranteed, and above the maximum limit, joining problems may occur when connecting a component made from it to another component; in particular, if the thickness of the corrosion protection coating exceeds the specified maximum limit, a stable thermal joining process cannot be ensured.

[0075] If the corrosion protection coating contains magnesium and aluminum in addition to zinc and unavoidable impurities, it is known in the industry as zinc-magnesium, ZM or Zn-Al-Mg.

[0076] If improved corrosion protection is required, the coating contains magnesium with a content of at least 0.8 wt.%, in particular at least 1.0 wt.%, preferably at least 1.1 wt.%, and aluminum with a content of at least 0.8 wt.%, in particular at least 1.0 wt.%. The coating contains magnesium with a content of up to 8.0 wt.%, preferably up to 7.0 wt.%, particularly preferably 5.0 wt.%, in particular up to 4.0 wt.%, and aluminum with a content of up to 8.0 wt.%, preferably up to 7.0 wt.%, in particular up to 5.0 wt.%, in particular up to 4.0 wt.%.

[0077] The corrosion protection coating can also consist solely of zinc with small amounts of aluminum (> 0 to 0.50 wt%) in addition to unavoidable impurities; this is also known in the industry as "Z" or "Gl" (see, for example, DIN EN ISO 10346:2015-10, Chapter 7.4, Section 7.4.2). If subsequent heat treatment, also known in the industry as "galvannealing", is carried out, it is referred to as "ZF" or "GA" (see, for example, DIN EN ISO 10346:2015-10, Chapter 7.4, Section 7.4.3).

[0078] Impurities in the corrosion protection coating may include elements from the group consisting of Si, Sb, Bi, Zr, Ni, Cr, Pb, Ti, Ca, Mn, Sn, La, Ce, Fe, and Cr in concentrations individually or cumulatively up to 0.50 wt.%, in particular up to 0.40 wt.%, preferably up to 0.30 wt.%, whereby, alternatively, the concentration of Fe may be higher due to the diffusion described above. The remainder is zinc, or, as disclosed above, magnesium and / or aluminum. ThyssenKrupp Steel Europe AG 247016P10WO

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[0081] Therefore, in step c) the corrosion protection coating is applied by hot-dip coating.

[0082] The layer structure of the corrosion protection coating can be designed such that the optional elements Mg and Al can be present in enriched oxide form in its surface boundary layer immediately adjacent to the surface, the thickness of which is limited to a maximum of 10% of the total thickness of the corrosion protection coating. If Al and especially Mg are components of the corrosion protection coating, the Mg oxides can essentially dominate at the surface. In addition, Zn oxide is present at and / or below the surface, so that the oxide layer forming on the corrosion protection coating passivates the surface. On the opposite side of the corrosion protection coating, towards the flat steel product, there is a transition zone in which iron and zinc predominate. This transition zone ends at the surface boundary layer of the flat steel product.

[0083] Before applying the corrosion protection coating, a layer is created on the surface of the flat steel product by applying and drying a carrier fluid to the surface of the flat steel product, which contains at least one compound of a water-soluble or dispersible salt of boric acid or boric acid, wherein the layer thus created on the surface has a coating weight of 1 to 100 mg / m² 2 exhibits and covers 20 to 100% of the surface of the steel flat product.

[0084] The drying process results in a very thin boron-containing layer on the surface of the flat steel product. The resulting layer has a coating weight of at least 2 mg / m². 2 , preferably at least 3 mg / m³ 2 , preferably at least 5 mg / m³ 2 up to a maximum of 80 mg / m² 2 , preferably a maximum of 70 mg / m³ 2 , preferably a maximum of 60 mg / m³ 2The coverage of the surface of the steel flat product of the layer produced can be, in particular, at least 25%, 30%, 35%, 40%, preferably at least 45%, 50%, 55%, 60%, preferably at least 65%, 70%, 75%, 80%, and especially preferably at least 85%, 90%, 93%, or 96%.

[0085] The inventive method allows existing alloy concepts to be individually optimized by enriching the near-surface region with boron, thereby advantageously influencing the tendency for liquid metal embrittlement. Boron thus does not need to be added during the manufacturing process of the steel substrate, which could potentially lead to problems. Conventionally alloyed boron has a work-hardening effect in steel and reduces the tendency for liquid metal embrittlement. ThyssenKrupp Steel Europe AG 247016P10WO

[0086] August 7, 2025

[0087] 10 / 17 often reduces the ductility of the steel. The method according to the invention offers the possibility of effectively reducing LME without reducing the ductility of the steel.

[0088] The wetting of the surface(s) of the flat steel product with a carrier liquid containing at least one compound of a water-soluble or dispersible salt of boric acid or boric acid is carried out by a process selected from the group consisting of or comprising spraying, coating (coil-coating process), or dipping. Typical suitable wetting times range from > 0 s to 120 s, in particular at least 1 s, preferably at least 2 s to a maximum of 100 s, and preferably a maximum of 90 s.

[0089] Prior to wetting with a carrier fluid, the surface or surfaces of the steel flat product can be subjected to conventional cleaning in one or more process steps.

[0090] Preferably, the carrier liquid contains at least one compound of a water-soluble or dispersible salt of boric acid or boric acid in a concentration of 10 g / l to 200 g / l, the remainder being water and unavoidable impurities.

[0091] In one embodiment, the carrier liquid contains one or more substances selected from the group consisting of wetting agents (surfactants, emulsifiers), network formers, and / or rheological additives, and combinations thereof, preferably wetting agents and / or network formers. This ensures the formation of a densely covering layer evenly distributed on the surface of the flat steel product. For example, bismuth nitrate, particularly in a concentration of 0.1 g / l to 30 g / l, with the remainder being water and unavoidable impurities, can be added as a network former. Alternatively or additionally, a surfactant, for example, can be added as a wetting agent. A preferably added wetting agent can be present in a concentration of 0.01 g / l to 1 g / l, with the remainder being water and unavoidable impurities.

[0092] In another version, the carrier fluid contains no organic solvents, in particular no VOCs (Volatile Organic Compounds).

[0093] Suitable water-soluble or dispersible salts of boric acid are therefore borates that are dispersible or soluble in water, i.e., molecularly dispersed. In accordance with the invention, borates have the borate ion (BO3)3- or a condensed form thereof as an anion in their ionic lattice. ThyssenKrupp Steel Europe AG 247016P10WO

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[0096] (e.g. (B4O5(OH)4) 2JTetraborate). The boron-containing metal salt is selected from the group containing or consisting of magnesium chloroborate (as the mineral boracite), sodium tetraborate (as the mineral borax or kernite), calcium borate (as the mineral pandermite or priceite), calcium borate (as the mineral colemanite), and calcium sodium borate (as the mineral ulexite). The aforementioned compounds may contain water of crystallization.

[0097] If sodium borate, particularly sodium tetraborate, is used, a maximum sodium concentration, determined by GDOES analysis, may also be present in a transition zone between the corrosion protection coating and the steel flat product. Furthermore, a maximum oxygen concentration, also determined by GDOES analysis, may be present in a transition zone between the corrosion protection coating and the steel flat product.

[0098] Particularly preferably, the maximum boron concentration can correspond to a boron content between 0.30 and 4.0% (by weight). Depending on the requirements, this content can vary, so that the maximum boron concentration can have a boron content of, in particular, at least 0.50%, 0.60%, 0.70%, preferably at least 0.80%, 0.90%, 1.0%, preferably at least 1.10%, 1.20%, 1.30%, up to, in particular, 3.80%, 3.70%, 3.60%, preferably 3.50%, 3.40%, 3.30%.

[0099] The maximum sodium concentration can have a sodium content between 0.050 and 1.0% (by weight). Depending on the specific requirements, this content can vary, so that the maximum sodium concentration can have a sodium content of, in particular, at least 0.10%, 0.12%, 0.14%, preferably at least 0.16%, 0.18%, 0.20%, preferably at least 0.22%, 0.24%, 0.26%, up to, in particular, 0.80%, preferably 0.60%.

[0100] The maximum oxygen concentration can have an oxygen content between 2.0 and 10.0% (by weight). Depending on the specific requirements, this content can vary, so that the maximum oxygen concentration can have an oxygen content of, in particular, at least 2.20%, 2.50%, 2.80%, preferably at least 3.10%, 3.40%, 3.70%, up to, in particular, 9.50%, 9.20%, 8.80%, preferably 8.50%, 8.10%, or 7.50%.

[0101] The carrier fluid can be, for example, an aqueous solution or dispersion, which can be applied conventionally at the end of a rolling mill during hot rolling or, preferably, during cold rolling, and thus inline before coiling. ThyssenKrupp Steel Europe AG 247016P10WO

[0102] August 7, 2025

[0103] 12 / 17 nem Coil. Alternatively, the carrier fluid can also be applied in a separate step after rolling and before coating, for example in a coil-to-coil step. In another alternative, the carrier fluid can also be applied before coating and thus inline in the galvanizing step.

[0104] After applying a carrier fluid to the surface(s) of the flat steel product, it is dried to form the coating. In step b), the flat steel product can pass through a drying device to dry and form the coating. Drying can take place at temperatures from 30 to 250 °C, particularly from 50 to 180 °C. Typical suitable drying times range from > 0 s to 120 s, particularly at least 1 s, preferably at least 2 s to a maximum of 90 s, and preferably a maximum of 60 s.

[0105] Before galvanizing / hot-dip galvanizing, the steel flat product undergoes a heat treatment process in which it is annealed at a temperature between 500 and 950 °C to achieve the desired material properties. A conventional hot-dip coating system is particularly well-suited for this purpose, allowing for the economical annealing and subsequent application of a corrosion protection coating. Shortly before hot-dip coating, the steel flat product is cooled to a temperature between -20 K and +100 K relative to the melting bath temperature. Typical melting bath temperatures for "Z" corrosion protection coatings are approximately 465 °C.

[0106] The heat treatment unit can include an integrated reaction chamber in which the steel flat product is annealed in an oxidizing atmosphere. The steel flat product is first annealed in the heat treatment unit, preferably a continuous furnace, in a reducing atmosphere at a temperature, for example, between 500 and 750 °C, at which the alloying elements diffuse to the surface of the steel flat product in only small quantities. The surface, which at this stage consists predominantly of pure iron, is converted into an iron oxide layer by a very short heat treatment at a temperature of up to 850 °C in a reaction chamber integrated into the continuous furnace with an oxidizing atmosphere. This iron oxide layer prevents the alloying elements from diffusing to the surface of the steel flat product during a subsequent annealing treatment at a higher temperature in a reducing atmosphere.In the reducing annealing atmosphere, the iron oxide layer is converted into a pure iron layer, onto which zinc can be bonded in a hot-dip annealing bath. This is ThyssenKrupp Steel Europe AG 247016P10WO.

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[0108] The type of pre- or intermediate oxidation is described by way of example in publications WO 2006 / 061151 Al and WO 2007 / 124781 Al. The O₂ content in an oxidizing annealing atmosphere can be between 0.01% and 5.0%, in particular up to 4.0%, the remainder being N₂. The dew point of an oxidizing annealing atmosphere can be in the range between

[0109] The temperature range for a reducing annealing atmosphere can be between -20 °C and +60 °C. The reducing atmosphere, on the other hand, can contain or consist of an H₂ content of 1% to 15%, particularly 2% to 10%, with the remainder being N₂. The dew point of a reducing atmosphere can lie between -60 °C and +20 °C.

[0110] As a result of the heat treatment, the boron in the produced layer can diffuse at least partially into the near-surface layer of the steel flat product.

[0111] Steel flat product temperature refers to the temperature at the surface of the steel flat product, which can be measured, for example, by means of a pyrometer or other preferably non-contact temperature measuring systems.

[0112] Q&P steels utilize retained austenite as a component of their microstructure to improve work hardening and tensile strength, while increased elongation is achieved through the well-known transformation-induced plasticity effect (TRIP). The retained austenite is embedded in a matrix of tempered martensite (primary martensite). Small amounts of bainite or bainitic ferrite, polygonal ferrite, and fresh martensite (secondary martensite) may also be present in the microstructure of Q&P steels. The steel flat product preferably consists, in addition to iron and unavoidable impurities, of (all values ​​in wt.%):

[0113] C: 0.20 to 0.40%

[0114] Mn: 1.50 to 3.00%.

[0115] Si: 0.90 to 1.50%

[0116] AI: 0.005 to 1.00%, optional Cr: 0.01 to 1.00%, optional Mo: 0.005 to 0.20%, optional B: 0.0001 to 0.002%, optional V, Nb and Ti, wherein the total content of V, Nb and Ti is 0.005 to 0.5%,

[0117] P: up to 0.020%

[0118] S: up to 0.005%

[0119] N: up to 0.008%, ThyssenKrupp Steel Europe AG 247016P10WO

[0120] August 7, 2025

[0121] 14 / 17 and as the remainder Fe and unavoidable impurities, wherein the sum of the impurity fractions is <= 0.8%, particularly preferably having a microstructure or consisting of, in % of the area, 65 to 92% primary (treated) martensite and at least 8% residual austenite, the remainder being filled by up to 27% secondary (untreated) martensite, up to 10% bainite or bainitic ferrite, and / or up to <= 5% polygonal ferrite, wherein the sum of the fractions of secondary (untreated) martensite, bainite or bainitic ferrite and polygonal ferrite is <= 27%.

[0122] The flat product made of Q&P steel can therefore preferably have a tensile strength of at least 1150 MPa to 1650 MPa, determined in the tensile test according to DIN EN ISO 6892 (specimen shape 2, longitudinal specimens).

[0123] For a series of tests, several samples were taken from a cold-rolled steel with a composition that, after suitable heat treatment, can yield a Q&P steel with a minimum tensile strength of 1180 MPa. The samples were tested on a laboratory scale in a Surtec Research Europe GmbH HDPS 2 simulator, in which annealing and coating processes are tested, thus allowing the transfer of findings to continuous annealing and / or hot-dip coating plants. Table 1 lists the parameters of the processes the samples underwent.

[0124] ThyssenKrupp Steel Europe AG 247016P10WO

[0125] August 7, 2025 15 / 17

[0126] Table 1

[0127] Samples 3 and 4 were immersed for 10 s in an aqueous solution containing sodium tetraborate at a concentration of 100 g / l and a surfactant at 0.1 g / l, the remainder being water and unavoidable impurities (M in Table 1) and subsequently dried for 10 s at a temperature of 130 °C (CT in Table 1), so that a layer with a coating weight of 50 mg / m² was deposited on the surface of each sample. 2The coverage was nearly 100%. All samples were annealed at a sample temperature of 840 °C in a reducing atmosphere (G in Table 1) containing 5% H₂, the remainder N₂, and a dew point of -15 °C. Samples 2 and 4 were annealed at a sample temperature of 650 °C for 4 s in an oxidizing atmosphere (V in Table 1) containing 2% O₂, the remainder N₂, and a dew point of -15 °C, and then subsequently returned to the reducing atmosphere. Upon reaching a sample temperature of 840 °C, the holding time in the reducing annealing atmosphere was 30 s. Subsequently, the samples were cooled to a temperature of 490 °C and immersed in a 465 °C hot melt bath containing 0.21 wt% Al, balance Zn and unavoidable impurities and coated (see Table 1), with a corrosion protection coating thickness of 7 pm in each case.

[0128] To investigate the tendency towards liquid metal embrittlement, MIG brazing was performed on the samples. Samples of the same type were always joined together. Analogous to the investigations of the SEP-1220 family for the joining-technical characterization of steel, many OEMs require MIG brazing suitability tests in their approval processes. This process, as a one-sided joining method, is an important procedure in automotive body-in-white construction, where a low tendency towards liquid metal embrittlement (LME) cracking is of particular importance. The characterization of the cracking tendency is generally carried out by brazing a lap joint followed by crack testing.

[0129] Soldering, a standard process, was carried out with 25mm quilting seams in accordance with SEP 1220-4 (dated 2016-04), initially with a wire feed rate of 7.0. ThyssenKrupp Steel Europe AG 247016P10WO

[0130] August 7, 2025, 16 / 17 m / min, taking into account an adjustment of the wire feed to allow differentiation between the various variants. Finally, a visual and material inspection was carried out with a comparative quantification of the crack frequency, see Table 2.

[0131] Table 2

[0132] Samples 3 and 4 treated with metal salts show a reduced number of cracks and a reduction in crack lengths in tests on liquid metal embrittlement tendency using the MIG brazing process.

[0133] Figure 1 is a graphical representation of the distribution of Fe, Zn, O, B, and Na concentrations across the thickness of a corrosion protection coating applied to a sample, as determined by a GDOES measurement on sample 4. All concentrations are given in wt. percent, with the following factors used for clarity: O x 10; B x 50; Na x 100. The depth unit does not correspond to a pm scale, as the GDOES method is not calibrated to spatially resolved depth information. The GDOES profiles on sample 4 show a significantly elevated boron, oxygen, and sodium concentration in the transition zone between the corrosion protection coating and the steel flat product (sample). This zone extends below the maximum zinc concentration of the corrosion protection coating on the left and below the maximum iron concentration of the steel flat product (sample) on the right.The starting and ending points of the transition range can, for example, be defined as the range between the maximum zinc concentration (1.0 wt%) and the maximum iron concentration (1.0 wt%). The maximum boron concentration has a boron content of 1.56 wt%, the maximum oxygen concentration has an oxygen content of 5.7 wt%, and the maximum sodium concentration has a sodium content of 5.7 wt%. ThyssenKrupp Steel Europe AG 247016P10WO.

[0134] August 7, 2025

[0135] The boron content of 0.2 wt.% is present in 17 / 17. The extent to which a diffusion-controlled process influences the near-surface chemical composition of the substrate cannot be definitively determined based on the GDOES profiles. However, it is assumed that at least some of the boron provided in the layer diffuses into the near-surface layer of the steel flat product.

Claims

ThyssenKrupp Steel Europe AG 247016P10WO August 7, 2025 1 / 4 Patent claims 1. Method for producing a steel flat product with a zinc-based corrosion protection coating and reduced susceptibility to liquid metal embrittlement, comprising the following steps: a) Providing a steel flat product with a steel substrate comprising, in wt.%, of C: 0.10-1.0%, Si: 0.010-5.0%, Mn: 0.30-10.0%, and optionally one or more of the alloying elements summarized in the group "V, Cr, Mo, B, Ti, Nb, Al, Ni, Cu, W, As, Sn, Sb, REM, Mg, Ca, Zr, Ta", with the proviso that the contents of the respective optionally present alloying element are to be dimensioned as follows: V: up to 0.30%, Cr: up to 1.50%, Mon: up to 1.0%, B: up to 0.010%, Ti: up to 0.30%, Note: up to 0.30% AI: up to 10.0%, Ni: up to 1.0%, Cu: up to 1.0%, W: up to 1.0%, As: up to 0.1%, Sn: up to 0.1%, Sb: up to 0.1%, REM: up to 0.1%, Mg: up to 0.015%, Approximately: up to 0.015%, Zr: up to 0.1%, Ta: up to 0.1%, ThyssenKrupp Steel Europe AG 247016P10WO August 7, 2025 2 / 4 and as a remainder consists of iron and unavoidable impurities, wherein the unavoidable impurities include contents of less than 0.20% P, less than 0.050% S and less than 0.020% N, wherein the sum of the impurity content is <= 0.50%, b) Producing a layer on the surface of the steel flat product by applying and drying a carrier liquid to the surface of the steel flat product, which contains at least one compound of a water-soluble or dispersible salt of boric acid or boric acid, wherein the layer thus produced on the surface has a coating weight of 1 to 100 mg / m² 2 exhibits and covers 20 to 100% of the surface of the steel flat product, c) Coating the steel flat product with a corrosion protection coating consisting of zinc and unavoidable impurities as well as optional alloying elements, in wt.%, up to 8.0% Al and / or up to 8.0% Mg.

2. The method according to claim 1, wherein the carrier liquid contains at least one compound of water-soluble or dispersible salt of boric acid or boric acid in a concentration of 10 g / l to 200 g / l, the remainder being water and unavoidable impurities.

3. Method according to any of the preceding claims, wherein the carrier fluid is an aqueous solution or dispersion.

4. Method according to any of the preceding claims, wherein the carrier fluid contains a network former and / or a wetting agent.

5. Method according to one of the preceding claims, wherein the steel flat product passes through a drying device in step b) for drying and producing the layer.

6. Method according to one of the preceding claims, wherein in step c) the corrosion protection coating is applied by hot-dip coating. ThyssenKrupp Steel Europe AG 247016P10WO August 7, 2025 3 / 4 7. Method according to claim 6, wherein the steel flat product undergoes a heat treatment facility prior to hot-dip annealing, in which the steel flat product is annealed at a steel flat product temperature between 500 and 950 °C.

8. Method according to claim 7, wherein a reaction chamber is integrated in the heat treatment device in which the steel flat product is annealed in an oxidizing atmosphere.

9. Method according to any one of claims 7 to 9, wherein the steel flat product is cooled to a temperature between -15 K and +100 K relative to the melt bath temperature shortly before molten immersion.

10. A steel flat product with a zinc-based corrosion protection coating and reduced susceptibility to liquid metal embrittlement, wherein the steel flat product comprises, in wt.%, C: 0.10 - 1.0%, Si: 0.010 - 5.0%, Mn: 0.30 - 10.0%, and optionally one or more of the alloying elements summarized in the group "V, Cr, Mo, B, Ti, Nb, Al, Ni, Cu, W, As, Sn, Sb, REM, Mg, Ca, Zr, Ta", with the proviso that the contents of each optionally present alloying element are to be determined as follows: V: up to 0.30% Cr: up to 1.50% Mon: up to 1.0% B: up to 0.010% Ti: up to 0.30% Note: up to 0.30% AI: up to 10.0% Ni: up to 1.0% Cu: up to 1.0% W: up to 1.0% As: up to 0.1% Sn: up to 0.1% Sb: up to 0.1% REM: up to 0.1% Mg: up to 0.015% Approximately: up to 0.015% ThyssenKrupp Steel Europe AG 247016P10WO August 7, 2025 4 / 4 Zr: up to 0.1% Ta: up to 0.1%, and consisting as a remainder of iron and unavoidable impurities, wherein the unavoidable impurities include contents of less than 0.20% P, less than 0.050% S and less than 0.020% N, wherein the sum of the impurity content is <= 0.50%, wherein the corrosion protection coating consists of zinc and unavoidable impurities and optionally, in wt.%, up to 8.0% Al and / or up to 8.0% Mg, characterized in that a maximum boron concentration, determined by GDOES analysis, is present in a transition zone between the corrosion protection coating and the steel flat product.

11. Steel flat product according to claim 10, wherein a maximum of a sodium concentration, determined by GDOES analysis, is present in a transition area between corrosion protection coating and steel flat product.

12. Steel flat product according to claim 10 or 11, wherein a maximum of an oxygen concentration, determined by GDOES analysis, is present in a transition area between corrosion protection coating and steel flat product.

13. Steel flat product according to claim 10, wherein the maximum boron concentration has a boron content between 0.30 and 4.0%.

14. Steel flat product according to claim 11, wherein the maximum sodium concentration has a sodium content between 0.050 and 1.0%.

15. Steel flat product according to claim 12, wherein the maximum oxygen concentration has an oxygen content between 2.0 and 10.0%.

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