Method for producing a flat steel product having an aluminum-based anti-corrosion coating and flat steel product having an aluminum-based anti-corrosion coating

Abstract

The invention relates to a method for producing a flat steel product (9) having a prealloyed anti-corrosion coating (13), comprising at least the following steps: providing a coated flat steel product (9), comprising a steel substrate (11) having an aluminum-based anti-corrosion coating (13) present on at least one side of the steel substrate (11); subjecting the coated flat steel product (9) to a heat treatment, comprising the following sub-steps: i. heating the coated flat steel product (9) in a furnace at a furnace temperature T between 950° C. and 1150° C. for a residence time t in the furnace. v between 40s and 150s, wherein the furnace temperature is selected such that the coated flat steel product (9) is heated at a rate exceeding 10K / s in a temperature range of 500°C to 700°C; ii. maintaining the coated flat steel product (9) at a temperature above Ac3 for a period of between 20s and 60s.

Description

Technical Field

[0001] The invention relates to a method for producing a flat steel product having a pre-alloyed corrosion protection coating.

[0002] The invention further relates to a flat steel product having a pre-alloyed corrosion protection coating.

[0003] Furthermore, the invention relates to a method for producing a steel component having a fully alloyed corrosion-protection coating.

[0004] Finally, the invention relates to a steel component having a fully alloyed corrosion-resistant coating. Background Art

[0005] The term "flat steel products" is used here to include all rolled products whose length is significantly greater than their thickness. These products include strips, plates, and blanks and slabs derived therefrom.

[0006] Unless expressly stated otherwise, any numerical values ​​given herein for alloy contents are to be understood as values ​​in "% by weight".

[0007] Unless otherwise stated, the percentage values ​​of the structural components and the percentage values ​​of the phases in the layers are based on volume (vol %). However, these values ​​are determined as area percentages in the metallographic sections.

[0008] A pre-alloyed corrosion resistant coating is understood to be a coating in which a certain proportion of iron is diffused, but complete and sufficient alloying has not yet been achieved. Therefore, a pre-alloyed corrosion resistant coating is characterized by an iron content of between 30 and 45% by weight, preferably between 30 and 40% by weight, and particularly preferably between 35 and 40% by weight. In contrast, the iron content of a fully alloyed corrosion resistant coating is much higher, at least 45% by weight, in particular at least 50% by weight, and preferably between 50 and 70% by weight. As will be explained below, the corrosion resistant coating may comprise different layers and phases. In this case, the iron content of the corrosion resistant coating is to be understood as an average value across the individual layers and phases.

[0009] The steel base material of the flat steel product according to the invention consists, in particular, of so-called "MnB steels." This type of steel is standardized in EN 10083-3. These steels have good hardenability and allow for reliable processing during hot pressing. This allows for economical martensitic hardening during hot forming, even in the tool, without the need for additional cooling.

[0010] Typical steel grades suitable for hot press hardening are steels AE whose chemical compositions are listed in Table 2.

[0011] The steel substrate can also be a composite material, for example with three steel layers, at least one of which is temperable, in particular hardenable, cf. EP 2 886 332 B1.

[0012] For hot-rolled MnB steel sheets provided with an aluminum-based corrosion protection coating and intended for the production of steel components by hot press hardening, EP 0 971 044 B1 specifies an alloy formulation, according to which the MnB steel, apart from iron and unavoidable impurities, should have (in wt. %) a carbon content of greater than 0.20% but less than 0.5%, a manganese content of greater than 0.5% but less than 3%, a silicon content of greater than 0.1% but less than 0.5%, a chromium content of greater than 0.01% but less than 1%, a titanium content of less than 0.2%, an aluminum content of less than 0.1%, a phosphorus content of less than 0.1%, a sulfur content of less than 0.05%, and a boron content of greater than 0.0005% but less than 0.08%.

[0013] Aluminum-based anti-corrosion coatings are so-called AlSi coatings, which contain 3-15% by weight of Si and up to 3.5% by weight of iron. The anti-corrosion coating preferably contains 9-10% by weight of silicon and 2-3.5% by weight of iron. These percentages are based on the anti-corrosion coating before heat treatment. In a special variant, in addition to the alloying elements Si and Fe, the anti-corrosion coating contains only aluminum and unavoidable impurities. Therefore, the balance is aluminum and unavoidable impurities. The flat steel product thus constructed and coated is heated to a heating temperature above the Ac1 temperature, then placed in a press-forming tool, where it is hot-formed into a steel component, and then cooled so rapidly while keeping the press-forming tool closed that a hardened microstructure is formed in the steel matrix of the flat steel product. During this process, especially during the heating process, Fe diffuses from the steel substrate into the aluminum coating. As a result, iron is introduced into the alloy, resulting in complete hardening of the coating.

[0014] However, the typical heating process for hot press hardening of aluminum-coated MnB steel is significantly longer than for uncoated material—for example, 4-6 minutes instead of 3-4 minutes. This is due, firstly, to the poor thermal coupling of the temporary reflective coating and, secondly, to the time required for complete alloying of the coating. In principle, this time can be shortened by selecting a higher furnace temperature for hot press hardening. However, furnace temperatures above 940°C, combined with rapid heating rates, result in a reduced surface roughness of the press-hardened material, resulting in insufficient adhesion of the paint layer during subsequent further processing.

[0015] Therefore, DE 10 2008 006 771 B3 proposes a two-stage process, in which the coated steel substrate undergoes a first heating step and a second heating step. In the first heating step, the aluminum-based coating is pre-alloyed. In this pre-alloyed state, the flat steel product is shipped from the steel producer to the processor. At the processor, the second heating step, which involves hot press hardening, takes place. By splitting the process in two, the second heating step at the processor can be significantly shortened. According to DE 10 2008 006 771 B3, the first heating step takes an hour or more and is carried out at temperatures of 550-723°C.

[0016] DE 10 2014 112 448 B4 also proposes a method, however, the purpose of which is to completely and fully alloy a hardenable steel sheet with an aluminum-silicon coating. The alloying process is carried out until the coating is saturated with the iron in the steel, which leaves only a small process window for subsequent heating, as this results in a thick layer that has a negative impact on the weldability of the hardened steel sheet component. Summary of the Invention

[0017] The object of the present invention is to further develop the above-mentioned method in such a way that the overall process time can be shortened while at the same time producing a flat steel product having improved properties.

[0018] This object is achieved by a method for producing a flat steel product having a pre-alloyed corrosion-protection coating, the method comprising at least the following steps:

[0019] - providing a coated flat steel product comprising a steel substrate having an aluminum-based anti-corrosion coating present on at least one side of the steel substrate;

[0020] - The coated flat steel product is subjected to a heat treatment comprising the following steps:

[0021] i. Heating the coated flat steel product in a furnace at a temperature T between 950°C and 1150°C, preferably between 960°C and 1080°C, in particular between 980°C and 1060°C, with a furnace residence time t v between 40 s and 150 s, wherein the furnace temperature is selected such that the coated flat steel product is heated at a rate exceeding 10 K / s in the temperature range of 500° C. to 700° C.;

[0022] ii. Maintaining the coated flat steel product at a temperature above Ac3 for a period of time between 20s and 60s.

[0023] This process window offers numerous advantages. Relatively short furnace dwell times combined with high furnace temperatures allow for reliable pre-alloying of the corrosion protection coating, while the mechanical and technical properties of the steel product remain intact and the basic microstructure of the steel substrate is preserved in terms of its composition.

[0024] Here, the heating rate is measured close to the surface, for example using near-surface thermocouples, because for the basic process the temperature of the layer or the temperature of the contact area between the layer and the substrate is relevant, rather than the core temperature of the steel substrate.

[0025] The steel substrate is typically a steel with a ferrite-pearlite structure, preferably a manganese-boron steel with a ferrite-pearlite structure, particularly preferably a manganese-boron steel with a ferrite-pearlite structure that can be converted to a martensitic structure by heat treatment in the form of a hot hardening treatment.

[0026] The steel material consists of a hardenable steel material. The steel material preferably has the following chemical composition in wt %:

[0027] C=0.05 to 0.5, preferably 0.1 to 0.4% by weight,

[0028] Mn=0.3 to 3.0 wt%,

[0029] Si=0.05 to 1.7 wt %,

[0030] P is at most 0.1 wt%,

[0031] S is at most 0.1 wt%,

[0032] N is at most 0.1 wt%,

[0033] and optionally one or more alloying elements from (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca):

[0034] Al is up to 1.0 wt%.

[0035] Ti is at most 0.2 wt%,

[0036] V is at most 0.5 wt%,

[0037] Nb is at most 0.5 wt%,

[0038] B is at most 0.01 wt%,

[0039] Cr is up to 1.0 wt%,

[0040] Mo is up to 1.0 wt%.

[0041] Cu is up to 1.0 wt%.

[0042] Ni is at most 1.0 wt%.

[0043] Ca is at most 0.1 wt%,

[0044] The rest is iron and inevitable impurities.

[0045] The steel substrate is particularly preferably a steel from steel groups AE, the chemical analysis of which is given in Table 2. Table 2 should be understood to mean that for each steel from steel groups AE, the element proportions are given in weight percent. Minimum and maximum weight proportions are given. For example, the carbon content of steel A is C: 0.05% to 0.10% by weight. If the lower limit is 0, the element is considered optional. If there is no entry in the table, there is no restriction for this element. For steels CE, an upper limit is set for the elements chromium and molybdenum, only for the sum of the elemental contents of chromium and molybdenum. In addition to the elements listed in the table, steels AE may also contain other optional elements, such as Cu, N, Ni, V, Sn, and Ca. The remainder is iron.

[0046] The aluminum-based corrosion protection coating is preferably a so-called AlSi coating, which contains 0.5-15% by weight of Si, optionally up to 5% by weight of iron, optionally up to 5% by weight of alkali metals or alkaline earth metals, preferably up to 1.0% by weight of alkali metals or alkaline earth metals, and optionally up to 15% by weight of Zn, preferably up to 10% by weight of Zn, and optionally further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum.

[0047] In a preferred variant, the selective content of alkali metals or alkaline earth metals comprises 0.1-1.0 wt.-% Mg, in particular 0.1-0.7 wt.-% Mg, preferably 0.1-0.5 wt.-% Mg. The selective content of alkali metals or alkaline earth metals in the melt may further comprise, in particular, at least 0.0015 wt.-% Ca, in particular at least 0.01 wt.-% Ca.

[0048] The aluminum-based anti-corrosion coating is preferably a so-called AlSi coating, which contains 0.5-15% by weight of Si, optionally up to 5% by weight of iron, optionally up to 5% by weight of magnesium, the remainder being aluminum. The anti-corrosion coating preferably contains 3-15% by weight of Si, in particular 5-11% by weight of Si, especially 7-10% by weight of Si, and 2-3.5% by weight of iron. The magnesium content is preferably 0.05-1% by weight, in particular 0.1-0.5% by weight.

[0049] The first heat treatment is divided into a first sub-step and a second sub-step. In the first sub-step, the flat steel product is heated. In the second sub-step, the flat steel product is maintained at a temperature above Ac3. This has the advantage of preventing accidental overheating of the corrosion protection coating. Overheating can reduce the spot weldability and paint adhesion of the hot press-hardened steel component. Therefore, in the second sub-step, the coated flat steel product is preferably maintained at a temperature between Ac3 and 950°C.

[0050] The advantage of using a two-stage method with the described first and second heat treatments for actual hot press hardening is that, in the second step, carried out at the flat steel product processor, the flat steel product already has a pre-alloyed corrosion protection coating. Heat treatment at the flat steel product processor typically takes place in a roller-hearth furnace. Untreated aluminum-based coatings have the disadvantage that they become partially fluid during the heat treatment and can lead to deposits on the furnace rollers. This, in turn, leads to an unstable heating process, increased scrap due to incorrect positioning of the heated flat steel product, and increased repair costs due to damage to the furnace rollers. All of this can be avoided by further processing the flat steel product with the pre-alloyed corrosion protection coating produced according to the present invention. Due to the iron content introduced into the aluminum-based corrosion protection coating by diffusion, the melting point of the corrosion protection coating is increased, so that no further fluidization occurs during the subsequent heating process for hot press hardening. Deposits on the furnace rollers are thus avoided.

[0051] The advantage of heating rates above 10 K / s in the temperature range of 500°C to 700°C is the formation of both low-silicon and silicon-rich phases within the corrosion-resistant coating. The silicon-rich phase is distributed as islands within the low-silicon phase. Heating rates exceeding 10 K / s ensure that the islands of silicon-rich phase are evenly distributed within the low-silicon phase. This ensures that hot-pressed hardened steel components have uniformly distributed silicon-rich regions within the low-silicon phase, even near the surface of the corrosion-resistant coating. Consequently, the steel component exhibits improved spot weldability.

[0052] Furthermore, the average roughness value R of the pre-alloyed flat steel product is reliably determined. a The roughness is between 0.3 μm and 2.0 μm, preferably between 0.5 μm and 1.6 μm. This ensures that the hot press hardened steel component also has a sufficiently large roughness for good paint adhesion.

[0053] In the sense of this application, "islanded" refers to an arrangement in which discrete, discontinuous areas are surrounded by another material, that is, there are "islands" of one particular material within another material.

[0054] The low-silicon phase refers to a phase whose silicon content is 1 to 10% by weight, preferably 1 to 6% by weight.

[0055] The silicon-rich phase refers to a phase whose silicon content is greater than 10% by weight, preferably 10-15%.

[0056] In a preferred variant, the heating rate is less than 20 K / s in the temperature range of 500° C. to 700° C. This results in a stable process window, so that the desired flat steel product can be reliably obtained even with slight variations in the dwell time.

[0057] In a particular variant of the method, the anti-corrosion coating is arranged on both sides of the steel substrate and the applied weight r on both sides is 50 g / m 2 and 200g / m 2 In this case, the residence time t v Preferably it conforms to the following formula:

[0058]

[0059] in

[0060] T = furnace temperature in °C,

[0061] d = thickness of the flat steel product in mm,

[0062] t v = residence time in the furnace in seconds,

[0063] r=g / m 2 Apply weight to both sides of the unit.

[0064] This relationship can provide a reliable process window for corrosion resistant cladding of varying thickness and applied weight.

[0065] The double-sided application weight is the sum of the application weights on both coated sides. Therefore, in the case of double-sided coated flat steel products, this value is not the application weight per coated side, but the sum of the application weights per side.

[0066] The thickness of the flat steel product is in particular 0.5-3.5 mm, preferably 0.8-2.8 mm.

[0067] The flat steel product provided with the pre-alloyed corrosion protection coating according to the present invention can then be cooled to room temperature and stored until it can be further processed to produce the corresponding steel component. This is typically achieved by cooling in ambient air. The cooling rate between the furnace temperature and 200°C is preferably less than 5 K / s, particularly less than 3.5 K / s. Because the corrosion protection coating is only partially alloyed during the first heating stage (i.e., the Fe content is less than 5% in the area adjacent to the surface and with a thickness of 1.0 μm, i.e., up to 1.0 μm below the surface of the corrosion protection coating), the corrosion protection coating exhibits low corrosion sensitivity even after the first heating stage. Therefore, its storage, transportation, and further processing steps before the second heating stage can be carried out smoothly without requiring additional measures. At the same time, the pre-alloyed corrosion protection coating retains properties that allow the resulting flat steel product to be cut or trimmed using simple cutting operations even after the first heating stage, without causing any permanent damage to the coating.

[0068] The present invention further relates to a flat steel product with a prealloyed corrosion protection coating, comprising a steel substrate having a prealloyed aluminum-based corrosion protection coating present on at least one side of the steel substrate. The steel substrate preferably has a martensite content of less than 10% by volume, preferably less than 5% by volume, and the corrosion protection coating has an average iron content of 30-45% by weight. Furthermore, the corrosion protection coating comprises:

[0069] a low-silicon phase which, in addition to unavoidable impurities, contains 1-10% by weight of Si, 10-50% by weight of Fe, a maximum of 1% by weight of Mn and 40-80% by weight of Al, the sum of these components being 100% by weight, and

[0070] - a silicon-rich phase, which contains, in addition to unavoidable impurities, 10-15 wt. % Si, 25-50 wt. % Fe, a maximum of 1 wt. % Mn and 40-80 wt. % aluminum, the sum of the components being 100 wt. %, wherein the silicon-rich phase is distributed in the form of islands in the low-silicon phase.

[0071] Such flat steel products with a pre-alloyed corrosion protection coating can be produced, for example, using the above-described method and offer the aforementioned advantages. In particular, the average roughness Ra of the pre-alloyed flat steel product is in the range of 0.3 μm to 2.0 μm, preferably 0.5 μm to 1.6 μm. This ensures that the hot-pressed hardened steel component also has a sufficiently high roughness for good paint adhesion.

[0072] The thickness of the flat steel product is in particular 0.5-3.5 mm, preferably 0.8-2.8 mm.

[0073] In flat steel products with pre-alloyed corrosion protection coatings, the silicon-rich phase is distributed in the low-silicon phase in an island-like manner, i.e., the silicon-rich phase has discrete, unconnected areas surrounded by the low-silicon phase. In this case, in the grinding disc, the area is less than 100 μm 2 The discrete, unconnected regions account for more than 80% of the total silicon-rich phase. In particular, the area is less than 50μm 2 In particular, the discrete, unconnected regions account for more than 50% of the total silicon-rich phase. In other words, more than 80% of the silicon-rich phase is smaller than 100 μm in area. 2 It exists in the form of small "islands", preferably more than 50% of the silicon-rich phase exists in an area less than 50μm 2 They exist in the form of small "islands".

[0074] In a preferred variant, the silicon-rich phase is distributed in the low-silicon phase in an island-like manner, that is, the region with the silicon-rich phase is distributed over an area whose thickness perpendicular to the surface is at least greater than 50% of the thickness of the anti-corrosion coating. Therefore, instead of forming a single narrow strip with a silicon-rich phase region, the silicon-rich region is distributed in an island-like manner over a strip that occupies at least half of the anti-corrosion coating. In addition, it is ensured that in the hot press hardened steel component, the anti-corrosion coating, especially the near-surface region of the anti-corrosion coating, has a silicon-rich phase of 10% to 25% by volume. This in turn enables the hot press hardened steel component to have uniformly distributed silicon-rich regions in the low-silicon phase in the near-surface region of the anti-corrosion coating. This achieves improved spot weldability of the steel component.

[0075] In a preferred variant, the proportion of the silicon-rich phase in the prealloyed corrosion protection coating is greater than 5% by volume, preferably greater than 10% by volume.

[0076] In a preferred variant of the flat steel product with a prealloyed corrosion protection coating, the Fe content of the corrosion protection coating at each point exceeds 10% by weight of Fe. This has the advantage that the melting point at each point is sufficiently high to prevent fluidization of parts of the corrosion protection coating during the subsequent hot press hardening process.

[0077] In a preferred variant of a flat steel product with a pre-alloyed corrosion protection coating, the flat steel product includes a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness between 1 μm and 6 μm, which is arranged adjacent to the steel substrate. The diffusion layer thus directly adjoins and contacts the steel substrate. The diffusion layer is often referred to as a ferrite seam. Due to the soft transition between the hard coating and the (relatively) softer substrate, this diffusion layer improves the cutting behavior when cutting the flat steel product with the pre-alloyed corrosion protection coating.

[0078] Furthermore, in a preferred embodiment, the flat steel product includes a layer of a silicon-rich phase having a thickness between 1 μm and 3 μm, which is arranged adjacent to the diffusion layer. Thus, the silicon-rich phase layer directly adjoins and contacts the diffusion layer. The chemical composition of the silicon-rich phase layer is identical to that of the regions having isolated islands of silicon-rich phase. During the pre-alloying process, the silicon-rich phase layer grows similarly to the diffusion layer.

[0079] A further development of flat steel products with a prealloyed corrosion protection coating includes an oxide layer on the surface of the corrosion protection coating. This oxide layer forms spontaneously by reaction with atmospheric oxygen and consists primarily of aluminum oxide (Al2O3) or, in embodiments where the corrosion protection coating contains magnesium, MgO and Al2O3. Typically, the proportion of MgO in the oxide layer is 55% to 65%, and the proportion of aluminum oxide is approximately 35% to 45%.

[0080] In both cases, the thickness of the oxide layer is generally 20 nm to 300 nm, preferably 50 nm to 200 nm, and furthermore protects the flat steel product against corrosion.

[0081] The invention also relates to a method for producing a steel component having a fully alloyed corrosion-resistant coating, comprising at least the following steps:

[0082] - providing the above flat steel product with a pre-alloyed corrosion resistant coating;

[0083] - heating the flat steel product with the pre-alloyed corrosion protection coating to a forming temperature T between Ac3 and 950°C U , heating time t E Between 2s and 600s, especially between 5s and 600s, preferably between 20s and 400s, more preferably between 60s and 240s.

[0084] During the heating time t E The process of internal heating of flat steel products involves raising the temperature to the forming temperature T within the temperature range U and keep the flat steel product at the forming temperature T U .

[0085] Heating can be carried out by means of a short roller hearth furnace, a chamber furnace or a rapid heating device (conduction, induction or contact heating device).

[0086] Since flat steel products with a pre-alloyed corrosion protection coating are used, subsequent heating within the short heating time mentioned is sufficient. No additional time is required for the corrosion protection coating to fully alloy. Therefore, in some cases, it is even possible to synchronize the process steps downstream of the heating, such as transfer to the press, forming, press hardening, cutting if necessary, and transfer to the receiving station, with the heating time, so that there is no or only minimal time delay.

[0087] Another advantage of using flat steel products with a pre-alloyed corrosion protection coating is that they can be heated in a roller-hearth furnace without having to worry about deposits on the rollers. Since the iron content is introduced into the aluminum-based corrosion protection coating by diffusion, the melting point of the corrosion protection coating is increased. Therefore, there is no fluidization during the subsequent heating process for hot press hardening. This also prevents deposits on the rollers.

[0088] In particular, the method for producing a steel component having a fully alloyed corrosion-resistant coating is further developed such that for a heating time t E The following formula holds:

[0089]

[0090] and / or

[0091]

[0092] in

[0093] T U = Molding temperature,

[0094] d = thickness of the flat steel product in mm,

[0095] t E = Heating time in seconds.

[0096] The thickness of the flat steel product is in particular 0.5-3.5 mm, preferably 0.8-2.8 mm.

[0097] The lower limit of the heating time ensures that the thickness of the existing or newly formed diffusion layer adjacent to the steel substrate is at least 1 μm. A diffusion layer thickness of more than 1 μm is a good indicator that a sufficient degree of complete alloying has been achieved.

[0098] The upper limit of the heating time ensures that the thickness of the existing or newly formed diffusion layer adjacent to the steel substrate does not exceed 10 μm. A greater diffusion layer thickness is detrimental to paint adhesion and the spot weldability of the steel component.

[0099] The method for producing a steel component having a fully alloyed corrosion-resistant coating comprises in particular the following steps:

[0100] - Forming of flat steel products into steel components in forming tools.

[0101] In the forming tool, the flat steel product is not only formed into a steel component but also quenched to the target temperature. The cooling rate to the target temperature in the forming tool is, in particular, at least 20 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and particularly preferably at least 100 K / s. The target temperature is below 400°C, preferably below 300°C. Furthermore, the target temperature is preferably at least 50°C. This results in a martensite content in the base material exceeding 50% by volume, preferably exceeding 80% by volume, in particular exceeding 90% by volume, and in particular exceeding 95% by volume.

[0102] After the steel component has been removed from the forming tool, it is cooled to a cooling temperature T of less than 50° C. within a cooling time of 0.5 to 600 s. This is usually done by air cooling.

[0103] The present invention also relates to a steel component with a fully alloyed anti-corrosion coating, comprising a steel substrate with a fully alloyed aluminum-based anti-corrosion coating present on at least one side of the steel substrate. Such a steel component can be produced in particular according to the above-described method. The fully alloyed anti-corrosion coating comprises:

[0104] a low-silicon phase which, in addition to unavoidable impurities, contains 1-10% by weight of Si, preferably 1-6% by weight of Si, 40-60% by weight of Fe, a maximum of 1% by weight of Mn and 30-60% by weight of aluminum,

[0105] A silicon-rich phase (R) which, in addition to unavoidable impurities, contains 10-15% by weight of Si, 40-70% by weight of Fe, a maximum of 1% by weight of Mn and 40-80% by weight of aluminum, wherein the silicon-rich phase is distributed in the form of islands in the low-silicon phase.

[0106] The Fe content of the fully alloyed corrosion protection coating is at least 45% by weight, in particular at least 50% by weight, preferably 50% to 70% by weight.

[0107] Here, the silicon-rich phase is distributed in the low-silicon phase in an island-like manner, so that the corrosion protection coating, especially the near-surface region of the corrosion protection coating, contains 10% to 25% by volume of the silicon-rich phase. The silicon-rich islands ensure good spot weldability. In addition, they also increase the roughness of the steel component, resulting in an average roughness R of more than 1 μm. a The average roughness is preferably greater than 1.2 μm, particularly preferably greater than 1.5 μm. This ensures good paint adhesion.

[0108] The martensite content of the steel substrate is more than 50% by volume, preferably more than 80% by volume, in particular more than 90% by volume, and especially more than 95% by volume.

[0109] The corrosion protection coating optionally comprises up to 5% by weight of magnesium, preferably 0.05 to 1% by weight, in particular 0.1 to 0.5% by weight of magnesium.

[0110] The magnesium content of the corrosion-resistant coating is preferably 0.1 to 0.5% by weight. At the same time, the regions of the low-silicon phase and the silicon-rich phase at a distance of more than 1.0 μm from the surface of the corrosion-resistant coating each have a magnesium content of up to 0.5% by weight. Due to the possible enrichment of magnesium in the surface region during the forming process, the magnesium content in the two phases close to the surface may be higher than 0.5% by weight. However, the magnesium content in the deeper phases does not exceed 0.5% by weight.

[0111] A further developed variant of a steel component with a fully alloyed corrosion resistant coating includes an oxide layer at the surface of the corrosion resistant coating. This oxide layer forms spontaneously by reaction with atmospheric oxygen and consists essentially of aluminum oxide (Al2O3) or, in variants in which the corrosion resistant coating contains magnesium, MgO and Al2O3. Typically, the proportion of MgO in the oxide layer is 55% to 65%, and the proportion of aluminum oxide is approximately 35% to 45%.

[0112] In both cases, the oxide layer typically has a thickness of 20 nm to 300 nm, preferably 50 nm to 200 nm, and additionally protects the steel component from corrosion.

[0113] In the sense of the present invention, the near-surface region of the (pre-alloyed or fully alloyed) corrosion resistant coating refers to the region below the surface of the corrosion resistant coating to a depth of 200 nm. The surface of the corrosion resistant coating here refers to the region in contact with the surrounding atmosphere or oxide layer.

[0114] In a preferred variant, the proportion of silicon-rich phase in the fully alloyed corrosion protection coating is less than 20% by volume, preferably less than 15% by volume.

[0115] In steel components with a fully alloyed corrosion protection coating, the silicon-rich phase is particularly distributed in the low-silicon phase in an island-like manner, i.e., the silicon-rich phase has discrete, unconnected areas surrounded by the low-silicon phase. In this case, the area is less than 100 μm 2 The discrete, unconnected regions account for more than 80% of the total silicon-rich phase. In particular, the area is less than 50 μm 2 The discrete, unconnected regions account for more than 50% of the total silicon-rich phase. In other words, more than 80% of the silicon-rich phase is smaller than 100 μm in area. 2It exists in the form of small "islands", preferably more than 50% of the silicon-rich phase has an area of ​​less than 50μm 2 They exist in the form of small "islands".

[0116] In a preferred embodiment, the steel component includes a diffusion layer having a thickness between 1 μm and 20 μm, specifically comprising Fe3Al and Fe2Al5, which is arranged adjacent to the steel substrate. The diffusion layer thus directly adjoins and contacts the steel substrate. In a preferred embodiment, the diffusion layer has a thickness of at least 3 μm. In particular, the thickness does not exceed 16 μm, and particularly preferably does not exceed 12 μm. The thickness of the diffusion layer is a good indicator of the degree of complete alloying. It has been shown that the desired complete alloying can be achieved within the indicated thickness range.

[0117] In a preferred variant, the porosity in the anti-corrosion coating is less than 5% by volume. In the case of a diffusion layer, the porosity is less than 5% by volume based on the anti-corrosion coating and the diffusion layer. Like the proportions of the individual phases, the porosity is determined by measuring the area in the polishing image.

[0118] In another preferred embodiment, the steel component includes a silicon-rich phase layer having a thickness between 1 μm and 3 μm, which is arranged adjacent to the diffusion layer. Thus, the silicon-rich phase layer directly adjoins and contacts the diffusion layer on the side facing away from the substrate. On the other side, the diffusion layer contacts the corrosion-resistant coating. The chemical composition of the silicon-rich phase layer is the same as that of the region having the isolated silicon-rich phase islands. BRIEF DESCRIPTION OF THE DRAWINGS

[0119] The present invention will be explained in more detail below with reference to the accompanying drawings, wherein:

[0120] Figure 1a A cross-sectional mill section image of a flat steel product having a prealloyed corrosion protection coating in a first embodiment variant is shown;

[0121] Figure 1b shows a cross-sectional mill section image of a steel component having a fully alloyed corrosion-resistant coating in a first variant embodiment;

[0122] Figure 2a shows a cross-sectional mill section image of a flat steel product having a prealloyed corrosion protection coating in a second embodiment variant;

[0123] Figure 2b shows a cross-sectional mill section image of a steel component having a fully alloyed corrosion-resistant coating in a second variant embodiment;

[0124] Figure 3a shows a cross-sectional mill section image of a flat steel product having a prealloyed corrosion protection coating in a third embodiment variant;

[0125] Figure 3b shows a cross-sectional mill section image of a steel component having a fully alloyed corrosion-resistant coating in a third variant embodiment;

[0126] Figure 4a shows a cross-sectional mill section image of a flat steel product having a prealloyed corrosion protection coating in a fourth embodiment variant;

[0127] Figure 4b A cross-sectional image of a steel component having a fully alloyed corrosion-resistant coating in a fourth variant embodiment is shown. DETAILED DESCRIPTION

[0128] Example 1

[0129] Formed blanks were cut from a 1.8 mm thick steel strip of steel grade D according to Table 2, with a 25 μm thick aluminum-based corrosion protection coating on both sides. The cutting methods used included a punching tool and a laser. The initial composition of the protective coating was 8 wt. % Si, 3 wt. % Fe, 0.3 wt. % Mg, and the remainder Al. The applied weight on each side was 70 g / m². 2 . These formed slabs are first heated in a two-zone continuous furnace in the first zone at a furnace temperature of 1100°C in 105 seconds to above 900°C. The heating rate between 500°C and 700°C is 12K / s. The slab is then kept at a temperature of 920°C for 35 seconds, which is above the Ac3 temperature, which for the selected steel grade is approximately 860°C. During this time, the corrosion protection coating is pre-alloyed. Subsequently, the iron content in the corrosion protection coating is between 35% by weight and 40% by weight. In addition, the iron content at each point of the corrosion protection coating exceeds 10% by weight of iron. The average roughness R in this variant embodiment a 0.9μm.

[0130] Figure 1a The figure shows a cross-sectional mill section image of the resulting flat steel product 9. An anti-corrosion coating 13 is applied to a steel substrate 11. The anti-corrosion coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 within the low-silicon phase 17 is clearly visible. The regions containing the silicon-rich phase 15 are distributed over an area whose thickness perpendicular to the surface is approximately 80% of the thickness of the anti-corrosion coating 13. Consequently, a single narrow strip containing a silicon-rich phase 15 is not formed. Instead, the silicon-rich regions are distributed like islands over a strip that occupies approximately 80% of the anti-corrosion coating. This ensures that in the hot-pressed steel component, the proportion of the silicon-rich phase 15 in the anti-corrosion coating is between 10% and 25% by volume.

[0131] In this variant of a flat steel product with a prealloyed corrosion protection coating, the flat steel product includes a diffusion layer 19 comprising Fe3Al and Fe2Al5 with a thickness of 3 μm, which is arranged adjacent to the steel substrate 11. The diffusion layer 19 thus directly adjoins and is in contact with the steel substrate 11. The corrosion protection coating 13 further includes a layer 21 of a silicon-rich phase 15 with a thickness of 1 μm, which is arranged adjacent to the diffusion layer 19. The layer 21 of the silicon-rich phase 15 thus directly adjoins and is in contact with the diffusion layer 19.

[0132] Furthermore, in this embodiment, the flat steel product comprises an oxide layer at the surface of the corrosion protection coating, the thickness of which is 60 nm, due to Figure 1a At medium resolution, this oxide layer is not visible.

[0133] The formed blanks treated in this way are further processed at another point in the hot forming line. This processing takes place in a short hot forming furnace at a forming temperature of 920°C. A heating period of 150 seconds is sufficient for complete austenitization and alloying of the cladding layer. During this time, the treated formed blanks are brought to and held at forming temperature. Even after processing several 500 blanks, the rollers in the hot forming furnace show no aluminum contamination.

[0134] After hot press hardening, the corrosion resistant coating contains an average of 51% by weight of Fe and has silicon-rich regions with Si contents ranging from 10% to 14%. The corrosion resistant coating does not exhibit distinct stratification between silicon-rich and low-silicon regions. Instead, the silicon-rich phase is distributed in isolated islands within the low-silicon phase. Consequently, both low-silicon and silicon-rich phases coexist near the surface. Consequently, both bonding properties and paint adhesion are superior to typical commercially available aluminum-based corrosion resistant coatings, which form stratification with a continuous low-silicon layer near the surface.

[0135] Figure 1b The cross-sectional grinding image of the formed steel component 23 is shown in FIG. An anti-corrosion coating 13 is arranged on a steel substrate 11. The anti-corrosion coating 13 includes a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 in the low-silicon phase 17 can be clearly seen. There are about 18 volume % of silicon-rich phases 15 in the near-surface area. In addition, the anti-corrosion coating 13 and the diffusion layer 19 have a total porosity of 1.1%. The average surface roughness is Ra = 1.5 μm. The proportion of silicon-rich phases in the anti-corrosion coating is 14 volume%. About 85% of the silicon-rich phases exist in the form of small "islands" with an area of ​​less than 100 μm. 2 .

[0136] The steel component 23 further includes a diffusion layer 19 comprising Fe3Al and Fe2Al5, which is arranged adjacent to the steel substrate 11 and below the corrosion-resistant coating 13. Thus, the diffusion layer 19 directly adjoins and contacts the steel substrate 11. The diffusion layer has a thickness of 7 μm. The steel component 23 further includes a layer 21 of silicon-rich phase 15 having a thickness of 2 μm, which is arranged adjacent to the diffusion layer 19 and below the corrosion-resistant coating 13. Thus, the layer 21 of silicon-rich phase 15 directly adjoins and contacts the diffusion layer 19.

[0137] In this embodiment, the steel component further includes an oxide layer with a thickness of 100 nm on the surface of the anti-corrosion coating. Figure 1b At medium resolution, this oxide layer is not visible.

[0138] Example 2

[0139] Formed blanks were cut from a 1.5 mm thick steel strip of steel grade E according to Table 2, with a 20 μm thick aluminum-based corrosion protection coating on both sides. The cutting method used was laser. The initial composition of the protective coating was 10 wt. % Si, 3 wt. % Fe, the remainder Al. The applied weight on each side was 60 g / m². 2 .

[0140] These formed slabs were first heated in a two-zone continuous furnace in the first zone at a furnace temperature of 1050°C to above 900°C within 90 seconds. The heating rate between 500°C and 700°C was 11 K / s. This furnace section had an O2-containing atmosphere with a dew point TP of -10°C. The slabs were then held at 920°C for 60 seconds, which is above the Ac3 temperature, which for the selected steel grade is approximately 845°C. No dew point control was performed in this furnace section.

[0141] During this time, the AS coating was pre-alloyed. The iron content in the subsequent corrosion protection coating was between 35% and 40% by weight. Furthermore, the iron content of the corrosion protection coating exceeded 10% by weight of iron at every point.

[0142] The resulting cross-sectional grinding image of the flat steel product 9 is shown in Figure 2a. An anti-corrosion coating 13 is arranged on a steel substrate 11. The anti-corrosion coating 13 includes a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 in the low-silicon phase 17 can be clearly seen. The area with the silicon-rich phase 15 is distributed over an area whose thickness perpendicular to the surface is approximately 90% of the thickness of the anti-corrosion coating 13. Therefore, what is formed is not a single narrow strip with a silicon-rich phase 15 area, but rather, the silicon-rich areas are distributed in an island-like manner on a strip that occupies approximately 90% of the anti-corrosion coating. This ensures that in the hot-pressed steel component, the proportion of the silicon-rich phase in the near-surface area of ​​the anti-corrosion coating is between 10% and 25% by volume.

[0143] In this variant of a flat steel product with a prealloyed corrosion protection coating, the flat steel product includes a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 2 μm, which is arranged adjacent to the steel substrate 11. Consequently, the diffusion layer 19 directly adjoins and is in contact with the steel substrate 11. The corrosion protection coating 13 further includes a layer 21 of a silicon-rich phase 15 with a thickness of 1 μm, which is arranged adjacent to the diffusion layer 19. Consequently, the layer 21 of the silicon-rich phase 15 directly adjoins and is in contact with the diffusion layer 19.

[0144] Furthermore, in this embodiment, the flat steel product comprises an oxide layer at the surface of the corrosion protection coating, the thickness of which is 60 nm, due to Figure 1a At medium resolution, this oxide layer is not visible.

[0145] The formed slabs treated in this way are further processed in a separate process step on a rapid induction heating line. The heating time to 900°C is 15 seconds. The slabs are then held at this temperature for a further 30 seconds using infrared irradiation. This completes the austenitization of the base material and the alloying of the coating. The heating time is therefore 45 seconds.

[0146] After hot pressing and hardening, the corrosion-resistant coating contains an average of 56% by weight of iron and has a silicon-rich area with a Si content between 10% and 13.5% by weight. The corrosion-resistant coating does not have a clear stratification of silicon-rich areas and low-silicon areas above the diffusion layer. On the contrary, the silicon-rich phase is distributed in the low-silicon phase in an island-like manner. Therefore, low-silicon phases and silicon-rich phases exist simultaneously in the near-surface area. In this embodiment, the near-surface area of ​​the corrosion-resistant coating has a silicon-rich phase of approximately 18% by volume. In addition, the corrosion-resistant coating 13 and the diffusion layer 19 have a total porosity of 2.3%. Therefore, not only the bonding performance, but also the paint adhesion performance is better than that of the aluminum-based corrosion-resistant coating commonly seen on the market, in which a stratification with a continuous low-silicon layer is formed in the near-surface area.

[0147] Figure 2bThe cross-sectional grinding image of the produced steel component 23 is shown in FIG. An anti-corrosion coating 13 is disposed on a steel substrate 11. The anti-corrosion coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 within the low-silicon phase 17 can be clearly seen. The near-surface region contains approximately 23% by volume of the silicon-rich phase 15. The average surface roughness is R a =1.5μm. The proportion of silicon-rich phase in the anti-corrosion coating is 11% by volume. More than 90% of the silicon-rich phase exists in the form of small "islands" with an area of ​​less than 100μm. 2 .

[0148] The steel component 23 further includes a diffusion layer 19 comprising Fe3Al and Fe2Al5, which is arranged adjacent to the steel substrate 11 and below the corrosion-resistant coating 13. Thus, the diffusion layer 19 directly adjoins and contacts the steel substrate 11. The diffusion layer has a thickness of 6 μm. The steel component 23 further includes a layer 21 of silicon-rich phase 15 having a thickness of 1 μm, which is arranged adjacent to the diffusion layer 19 and below the corrosion-resistant coating 13. Thus, the layer 21 of silicon-rich phase 15 directly adjoins and contacts the diffusion layer 19.

[0149] In this embodiment, the steel component further includes an oxide layer with a thickness of 110 nm on the surface of the anti-corrosion coating. Figure 2b At medium resolution, this oxide layer is not visible.

[0150] Example 3

[0151] Formed slabs were cut from a 1.2 mm thick steel strip of steel grade D according to Table 2, with a 20 μm thick aluminum-based corrosion protection coating on both sides. The cutting method used was laser. The initial composition of the protective coating was 6 wt. % Si, 3 wt. % Fe, 0.3 wt. % Mg, the remainder being Al. The applied weight on each side was 60 g / m². 2 .

[0152] These formed slabs are first heated in the first zone in a two-zone continuous furnace at a furnace temperature of 1000°C to above 900°C in 90 seconds. The heating rate between 500°C and 700°C is 13K / s. This furnace section has an O2-containing atmosphere with a dew point TP of -15°C. The slab is then held at a temperature of 920°C for 30 seconds, which is above the Ac3 temperature, which for the selected steel grade is approximately 860°C. During this time, the corrosion protection coating is pre-alloyed. The Fe content of the corrosion protection coating is then between 35% and 40% by weight. In addition, the Fe content at each point of the corrosion protection coating exceeds 10% by weight of Fe. The average roughness R in this embodiment variant a 1.2μm.

[0153] The resulting cross-sectional grinding image of the flat steel product 9 is shown in Figure 3a . An anti-corrosion coating 13 is arranged on a steel substrate 11. The anti-corrosion coating 13 includes a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 in the low-silicon phase 17 can be clearly seen. The area with the silicon-rich phase 15 is distributed over an area whose thickness perpendicular to the surface is approximately 80% of the thickness of the anti-corrosion coating 13. Therefore, what is formed is not a single narrow strip with an area of ​​silicon-rich phase 15, but rather, the silicon-rich areas are distributed in an island-like manner on a strip that occupies approximately 80% of the anti-corrosion coating. This ensures that in the hot-pressed steel component, the proportion of the silicon-rich phase in the anti-corrosion coating, especially in the near-surface area, is between 10% and 25% by volume.

[0154] In this variant of a flat steel product with a prealloyed corrosion protection coating, the flat steel product includes a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 2.5 μm, which is arranged adjacent to the steel substrate 11. Consequently, the diffusion layer 19 directly adjoins and is in contact with the steel substrate 11. The corrosion protection coating 13 further includes a layer 21 of a silicon-rich phase 15 with a thickness of 0.5 μm, which is arranged adjacent to the diffusion layer 19. Consequently, the layer 21 of the silicon-rich phase 15 directly adjoins and is in contact with the diffusion layer 19.

[0155] Furthermore, in this embodiment, the flat steel product comprises an oxide layer at the surface of the corrosion protection coating, the thickness of which is 70 nm, due to Figure 3a At medium resolution, this oxide layer is not visible.

[0156] The formed slabs treated in this way are further processed in a contact heating line at another location. The heating time to the forming temperature of 900°C is 20 seconds. The slabs are then held at this temperature for a further 75 seconds by infrared irradiation, with the slabs approximately 5 mm away from the slab surface. This fully austenitizes the steel substrate and alloys the corrosion protection coating. The heating time is therefore 95 seconds. Due to the pre-alloyed corrosion protection coating, adhesion to the contact plate is completely eliminated. The thickness of the corrosion protection coating on the plate surface also remains constant.

[0157] After hot press hardening, the corrosion-resistant coating contains an average of 60% Fe by weight and has silicon-rich regions with Si contents between 11% and 15% by weight. The corrosion-resistant coating does not have distinct stratification between silicon-rich and low-silicon regions above the diffusion layer. Instead, the silicon-rich phase is distributed in isolated islands within the low-silicon phase. Therefore, low-silicon and silicon-rich phases coexist in the near-surface region. Consequently, not only the bonding properties but also the paint adhesion properties are better than those of common aluminum-based corrosion-resistant coatings on the market, in which stratification with a continuous low-silicon layer is formed in the near-surface region.

[0158] Figure 3b shows a cross-sectional milling image of the resulting steel component 23. An anti-corrosion coating 13 is disposed on a steel substrate 11. The anti-corrosion coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 within the low-silicon phase 17 is clearly visible. The near-surface region contains approximately 15% by volume of the silicon-rich phase 15. Furthermore, the anti-corrosion coating 13 and the diffusion layer 19 have a total porosity of 1.3%. The average surface roughness is R a =1.5μm. The proportion of silicon-rich phase in the corrosion-resistant coating is 13% by volume. More than 90% of the silicon-rich phase exists in the form of small "islands" with an area of ​​less than 100μm. 2 .

[0159] The steel component 23 further includes a diffusion layer 19 comprising Fe3Al and Fe2Al5, which is arranged adjacent to the steel substrate 11 and below the corrosion-resistant coating 13. Thus, the diffusion layer 19 directly adjoins and contacts the steel substrate 11. The diffusion layer has a thickness of 9 μm. The steel component 23 further includes a layer 21 of silicon-rich phase 15 having a thickness of 2 μm, which is arranged adjacent to the diffusion layer 19 and below the corrosion-resistant coating 13. Thus, the layer 21 of silicon-rich phase 15 directly adjoins and contacts the diffusion layer 19.

[0160] In this embodiment, the steel component further includes an oxide layer with a thickness of 220 nm on the surface of the anti-corrosion coating. Figure 3b At medium resolution, this oxide layer is not visible.

[0161] Example 4

[0162] Formed slabs were cut from a 1.6 mm thick steel strip of the D-type steel shown in Table 2, with a 25 μm thick aluminum-based corrosion protection coating on both sides. The cutting method used was laser. The initial composition of the protective layer was 8 wt% Si, 3 wt% Fe, 0.3 wt% Mg, and the remainder Al. The applied weight on each side was 75 g / m². 2 The chemical composition is exactly the same as that of Example 3.

[0163] These formed slabs are first heated to over 900°C in the first zone of a two-zone continuous furnace with the aid of an infrared radiation array (Infrarotstrahlungsfeld) within 60 seconds. The furnace temperature is 1050°C. The heating rate between 500°C and 700°C is 20K / s. This furnace section has an O2-containing atmosphere with a dew point TP of -15°C. The slabs are then kept in a roller hearth furnace at a temperature of 920°C for 45 seconds, which is above the Ac3 temperature, which for the selected steel grade is approximately 860°C. This furnace section has an O2-containing atmosphere with a dew point TP of +20°C. During this time, the corrosion protection coating is pre-alloyed. The Fe content in the corrosion protection coating is then between 40% and 45% by weight. In addition, the Fe content of the corrosion protection coating exceeds 10% by weight of Fe at every point. The average roughness R in this embodiment variant a It is 1.3μm.

[0164] The resulting cross-sectional grinding image of the flat steel product 9 is shown in Figure 4a . An anti-corrosion coating 13 is arranged on a steel substrate 11. The anti-corrosion coating 13 includes a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 in the low-silicon phase 17 can be clearly seen. The area with the silicon-rich phase 15 is distributed over an area whose thickness perpendicular to the surface is approximately 80% of the thickness of the anti-corrosion coating 13. Therefore, what is formed is not a single narrow strip with a silicon-rich phase 15 area, but rather, the silicon-rich areas are distributed in an island-like manner on a strip that occupies approximately 80% of the anti-corrosion coating. This ensures that in the hot-pressed steel component, the proportion of the silicon-rich phase in the near-surface area of ​​the anti-corrosion coating is between 10% and 25% by volume.

[0165] In this variant of a flat steel product with a prealloyed corrosion protection coating, the flat steel product includes a diffusion layer comprising Fe3Al and Fe2Al5 with a thickness of 3 μm, which is arranged adjacent to the steel substrate 11. Consequently, the diffusion layer 19 directly adjoins and is in contact with the steel substrate 11. The corrosion protection coating 13 further includes a layer 21 of a silicon-rich phase 15 with a thickness of 0.5 μm, which is arranged adjacent to the diffusion layer 19. Consequently, the layer 21 of the silicon-rich phase 15 directly adjoins and is in contact with the diffusion layer 19.

[0166] Furthermore, in this embodiment, the flat steel product comprises an oxide layer at the surface of the corrosion protection coating, the thickness of which is 110 nm, due to Figure 4a At medium resolution, this oxide layer is not visible.

[0167] The formed slabs treated in this manner are further processed in a separate process step on a contact heating line using heat conduction. The heating time to the forming temperature of 900°C is 12 seconds. The slabs are then held at this temperature for a further 110 seconds in a roller-hearth furnace. This completes the austenitization of the steel substrate and the alloying of the corrosion-resistant coating. The heating time is therefore 122 seconds. Due to the pre-alloyed corrosion-resistant coating, adhesion to the contact plates or furnace rollers is completely eliminated. The thickness of the corrosion-resistant coating on the slab surface also remains constant.

[0168] After hot press hardening, the corrosion-resistant coating contains an average of 58% Fe by weight and has silicon-rich regions with Si contents between 11% and 16% by weight. The corrosion-resistant coating does not have distinct stratification between silicon-rich and low-silicon regions above the diffusion layer. Instead, the silicon-rich phase is distributed in isolated islands within the low-silicon phase. Therefore, low-silicon and silicon-rich phases coexist in the near-surface region. Consequently, not only the bonding properties but also the paint adhesion properties are better than those of common aluminum-based corrosion-resistant coatings on the market, in which stratification with a continuous low-silicon layer is formed in the near-surface region.

[0169] Figure 4b shows a cross-sectional milling image of the resulting steel component 23. An anti-corrosion coating 13 is disposed on a steel substrate 11. The anti-corrosion coating 13 comprises a silicon-rich phase 15 and a low-silicon phase 17. The island-like distribution of the silicon-rich phase 15 within the low-silicon phase 17 is clearly visible. The near-surface region contains approximately 17% by volume of the silicon-rich phase 15. Furthermore, the anti-corrosion coating 13 and the diffusion layer 19 have a total porosity of 1.2%. The average surface roughness is R a =1.5μm. The proportion of silicon-rich phase in the anti-corrosion coating is 10.5% by volume. More than 90% of the silicon-rich phase exists in the form of small "islands" with an area of ​​less than 100μm. 2 .

[0170] The steel component 23 further includes a diffusion layer 19 comprising Fe3Al and Fe2Al5, which is arranged adjacent to the steel substrate 11 and below the corrosion-resistant coating 13. Thus, the diffusion layer 19 directly adjoins and contacts the steel substrate 11. The diffusion layer has a thickness of 5 μm. The steel component 23 further includes a layer 21 of silicon-rich phase 15 having a thickness of 1 μm, which is arranged adjacent to the diffusion layer 19 and below the corrosion-resistant coating 13. Thus, the layer 21 of silicon-rich phase 15 directly adjoins and contacts the diffusion layer 19.

[0171] In this embodiment, the steel component further includes an oxide layer with a thickness of 170 nm on the surface of the anti-corrosion coating. Figure 4b At medium resolution, this oxide layer is not visible.

[0172] Other embodiments

[0173] The table below lists the process parameters for various thicknesses of formed slabs. The steel grade of the steel substrate is also listed. All formed slabs included a 25 μm thick aluminum-based corrosion protection coating on both sides. The initial composition of the corrosion protection coating was 9 wt. % Si, 3 wt. % Fe, the remainder Al. The applied weight on each side was 70 g / m². 2 .

[0174] These formed slabs were first heated in a two-zone continuous furnace in the first zone at a furnace temperature of 1050°C to above 900°C. The heating rate between 500°C and 700°C exceeded 10K / s in all cases. The slabs were then held in the second zone at 950°C for the times shown in Table 1.

[0175] The formed blanks thus treated are further processed at a different point in the hot forming line. This processing takes place in a short hot forming furnace at a forming temperature of 900°C or 920°C. The thickness of the diffusion layer, weldability, paint adhesion, and tool wear are then tested.

[0176] According to the provisions of SEP1220-2, the weldability is determined to be suitable for resistance spot welding.

[0177] To determine paint adhesion, a stone chip impact test is performed according to EN ISO 20567. Painted specimens are bombarded with a defined amount of corundum in a defined manner and subjected to a VDA corrosion test. Paint adhesion is also determined according to EN ISO 2409 (cross-cut test). In this case, painted specimens are scratched in a defined manner in a cross-cut pattern up to the metal protective layer and are likewise subjected to a VDA corrosion test.

[0178] Tool wear is determined by removing powdery and adhering residues from the tool after a certain number of parts have been produced and measuring them gravimetrically. Table 1 shows the wear mass after 100 molded parts, based on one square meter of coating material. Wear exceeding 100 grams per 1000 parts (i.e. 10 grams per 100 parts) may require unplanned cleaning activities. Therefore, values ​​below 10 grams / 100 parts are considered acceptable.

[0179]

[0180] Table 2

[0181]

Claims

1. A method for producing a flat steel product (9) having a prealloyed corrosion protection coating (13) with an Fe content of between 30 and 45% by weight, the method comprising at least the following steps: - providing a coated flat steel product (9) comprising a steel substrate (11) having an aluminum-based anti-corrosion coating (13) present on both sides of the steel substrate, the aluminum-based anti-corrosion coating having an applied weight r of 50 g / m2 on both sides 2 and 200g / m 2 and containing 0.5-15 wt.-% Si, optionally up to 5 wt.-% Fe, optionally up to 5 wt.-% alkali metal or alkaline earth metal, and optionally up to 15 wt.-% Zn, and optionally further components, the sum of their contents being limited to a maximum of 2.0 wt.-%, and the remainder being aluminum; - subjecting the coated flat steel product (9) to a heat treatment, wherein the heat treatment is divided into a first sub-step, in which the flat steel product is heated, and a second sub-step, in which the flat steel product is kept at a temperature above Ac3, the two sub-steps being designed as follows: i. Heating the coated flat steel product (9) in a furnace at a temperature T between 950°C and 1150°C for a residence time t in the furnace. v between 40 s and 150 s, wherein the furnace temperature is selected such that the coated flat steel product (9) is heated at a rate exceeding 10 K / s in the temperature range of 500° C. to 700° C.; ii. Maintaining the coated flat steel product (9) at a temperature above Ac3 for a period of time between 20s and 60s.

2. The method according to claim 1, characterized in that Residence time t v Conforms to the following formula: in T = furnace temperature in °C, d = thickness of the flat steel product (9) in mm, t v = residence time in the furnace in seconds, r=g / m 2 Apply weight to both sides of the unit.

3. The method according to any one of the preceding claims, characterized in that The aluminum-based anti-corrosion coating (13) contains 0.5-15% by weight of silicon, optionally up to 5% by weight of iron, optionally up to 5% by weight of magnesium, and the remainder is aluminum.

4. Flat steel product (9) with a prealloyed corrosion protection coating (13), comprising a steel substrate (11) having a prealloyed aluminum-based corrosion protection coating (13) present on at least one side of the steel substrate (11), produced according to any one of claims 1 to 2, characterized in that The average iron content of the anti-corrosion coating (13) is 30-45% by weight, wherein the anti-corrosion coating (13) comprises: a low-silicon phase (17) which contains, in addition to unavoidable impurities, 1-10% by weight of Si, 10-50% by weight of Fe, a maximum of 1% by weight of Mn and 40-80% by weight of Al, and - a silicon-rich phase (15) which, in addition to unavoidable impurities, contains 10-15 wt. % Si, 25-50 wt. % Fe, a maximum of 1 wt. % Mn and 40-80 wt. % Al, wherein the silicon-rich phase is distributed in the low-silicon phase in the form of islands.

5. The flat steel product (9) having a pre-alloyed corrosion protection coating (13) according to claim 4, comprising: A diffusion layer (19) comprising Fe3Al and Fe2Al5 with a thickness between 1 μm and 6 μm, arranged adjacent to the steel substrate (11).

6. The flat steel product (9) having a prealloyed corrosion protection coating (13) according to claim 5, comprising: A layer (21) of a silicon-rich phase with a thickness between 1 μm and 3 μm, which is arranged adjacent to the diffusion layer (19).

7. A method for producing a steel component having a fully alloyed corrosion-resistant coating (13), comprising at least the following steps: - providing a flat steel product (9) according to any one of claims 4 to 6, produced according to any one of claims 1 to 3, having a prealloyed corrosion protection coating (13), - heating the flat steel product (9) with the prealloyed corrosion protection coating (13) to a forming temperature T between Ac3 and 950° C. U , heating time t E Between 2s and 600s.

8. Method for producing a steel component having a fully alloyed corrosion protection coating (13) according to claim 7, characterized in that Heating time t E Between the 20s and 400s.

9. Method for producing a steel component having a fully alloyed corrosion protection coating (13) according to claim 7, characterized in that Heating time t E Between the 60s and 240s.

10. Method for producing a steel component having a fully alloyed corrosion protection coating (13) according to claim 7, characterized in that The heating is performed by a conduction-type rapid heating device, an induction-type rapid heating device or a contact-type rapid heating device.

11. Method for producing a steel component having a fully alloyed corrosion protection coating (13) according to claim 7, characterized in that For the heating time t E The following formula holds: in T U = Molding temperature d = thickness of the flat steel product (9) in mm t E = Heating time in seconds.

12. Method for producing a steel component (23) having a fully alloyed corrosion protection coating (13) according to any one of claims 7 to 11, comprising: - forming the flat steel product (9) into a steel component in a forming tool.

13. Steel component (23) with a fully alloyed corrosion protection coating (13), comprising a steel substrate (11) having a fully alloyed aluminum-based corrosion protection coating (13) present at least on one side of the steel substrate (11), produced according to any one of claims 7 to 12, characterized in that The fully alloyed anti-corrosion coating (13) comprises: a diffusion layer (19) comprising Fe3Al and Fe2Al5 with a thickness between 1 μm and 20 μm, arranged adjacent to the steel substrate (11), a low-silicon phase (17) which, in addition to unavoidable impurities, contains 1-10% by weight of Si, 40-60% by weight of Fe, a maximum of 1% by weight of Mn and 30-60% by weight of aluminum, a silicon-rich phase (15) which, in addition to unavoidable impurities, contains 10-15% by weight of Si, 40-70% by weight of Fe, a maximum of 1% by weight of Mn and 40-80% by weight of aluminum, wherein the silicon-rich phase is distributed in the low-silicon phase in the form of islands, The silicon-rich phase (15) is distributed in the low-silicon phase (17) in an island-like manner, that is, the corrosion-resistant coating (13) has 10% to 25% by volume of the silicon-rich phase.

14. The steel component according to claim 13, characterized in that The proportion of the silicon-rich phase in the fully alloyed corrosion-resistant coating (13) is less than 20% by volume.

15. The steel component according to claim 14, characterized in that The steel component (23) comprises: - a silicon-rich phase layer (21) with a thickness between 1 μm and 3 μm, arranged adjacent to the diffusion layer (19).

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