Flat steel product having an al coating, method for the production thereof, steel component, and method for production thereof

Abstract

The invention relates to a steel component consisting of a steel substrate made of a steel comprising 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and of an Al-based protective coating applied to the steel substrate. The protective coating has, in total, an iron-free and manganese-free mass fraction of up to 30% of additional alloy constituents. The iron-free and manganese-free mass fraction of the protective coating of Si as an additional alloy constituent is between 3% and 15% Si. In addition, the iron-free and manganese-free mass fraction of the protective coating of Mg as an additional alloy constituent is up to 1.0% Mg. The iron-free and manganese-free fraction of the protective coating of Zn as an additional alloy constituent is between 0.4% and 25.0% Zn. The protective coating also has a near-surface first Zn-rich layer which has a Zn content higher than an average Zn content of the protective layer, and wherein the Zn content of the first Zn-rich layer is more than 5 wt.%.

Description

[0001] Flat steel product with an Al coating, method for its production, steel component and method for its production

[0002] The invention relates to a flat steel product for hot forming, which consists of a steel substrate consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and a protective coating based on Al applied to the steel substrate, which optionally contains a total of up to 30 wt.% of other alloying elements.

[0003] The invention also relates to a steel component and a method for producing such a steel component.

[0004] The term "flat steel product" refers to all rolled products whose length is significantly greater than their thickness. This includes steel strips and sheets, as well as cut pieces and blanks obtained from them.

[0005] During hot forming, also known as hot forming, press hardening, or hot press hardening, steel blanks cut from cold- or hot-rolled steel strip are heated to a deformation temperature generally above the austenitizing temperature (Ac3) of the respective steel and placed in the heated state into the tool of a forming press. During the subsequent forming process, the sheet metal blank or the component formed from it undergoes rapid cooling due to contact with the cool tool. The cooling rates are adjusted to create a hardened structure in the steel substrate. This structure is converted into a martensitic structure, which is referred to as the hardened structure.

[0006] Typical steels suitable for hot press hardening are the AE steels, whose chemical composition is listed in Table 10.

[0007] For hot-rolled MnB steel sheets provided with an Al coating, which are intended for the production of steel components by hot forming, EP 0971 044 B1 specifies an alloying specification according to which an MnB steel, in addition to iron and unavoidable impurities (in wt. %), should have a carbon content of more than 0.20% but less than 0.5%, a manganese content of more than 0.5% but less than 3%, a silicon content of more than 0.1% but less than 0.5%, a chromium content of more 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 more than 0.001% but less than 0.08%. The Al coating is a so-called AISi coating, which consists of 9-10 wt% Si, 2-3.5 wt% iron and the remainder aluminum.The flat steel products thus prepared and coated are heated to a heating temperature of over 700 °C and held at that temperature for a specific holding time. During this heating and holding process, the protective coating melts and alloys through. Iron diffuses from the steel substrate into the protective coating, forming phases with greater temperature stability. The molten protective coating then solidifies. A solidified, temperature-stable protective coating is a prerequisite for the subsequent forming step. During the forming step, the flat steel product is placed in a press die, where it is warm-formed into the steel component, and cooled so rapidly that a hardened microstructure is created in the steel substrate of the flat steel product.

[0008] The AISi coating described has the disadvantage that the temperatures involved during hot press forming lead to severe oxidation of the aluminum-rich surface. This aluminum oxide layer, in turn, leads to insufficient performance of the subsequent phosphating step. The aluminum oxide layer prevents the formation of firmly adhering metal phosphates during phosphating, as the process parameters are typically insufficient to break down the aluminum oxide layer. The resulting steel component is highly susceptible to cosmetic corrosion. While this does not affect the structural stability of the steel component, it does result in a reduced aesthetic appearance, which is undesirable for the end customer.

[0009] DE 10 2009 007 909 A1 discloses a process for producing high-strength steel components which, on the one hand, ensures optimized deformation behavior of the coating during hot press forming of the respective flat steel product and, on the other hand, in which the resulting steel components have optimized cathodic corrosion protection. The steel component according to the invention has a zinc-alloyed surface with a zinc content of at least 60 wt. %, in particular of at least 80 wt. This results in cathodic corrosion protection which is clearly detectable electrochemically. EP 2 045 360 A1 discloses a process for producing high-strength steel components which have optimized corrosion protection and are particularly suitable for use in automobile bodies. After hot forming, a steel component according to the invention has a zinc-alloyed surface with a zinc content of at least 60 wt.-%, in particular at least 80 wt.%. This results in cathodic corrosion protection that is clearly detectable electrochemically. This results in corrosion resistance that is at least comparable to pure zinc coatings.

[0010] Zn contents above 25 wt% increase the risk of liquid metal embrittlement. Zinc in the protective coating has the disadvantage of its relatively low melting point. During the production of the steel component by hot forming, liquid Zn phases can form. These can, on the one hand, penetrate the steel substrate and contribute to cracking (liquid metal embrittlement), and, on the other hand, adhere to forming tools or furnace rollers, leading to contamination and delamination.

[0011] The object of the present invention is to provide a steel component that can be produced by hot forming and has improved phosphating properties.

[0012] This problem is solved by a steel component consisting of a steel substrate containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and an Al-based protective coating applied to the steel substrate, which optionally has a total iron- and manganese-free mass fraction of up to 30% of additional alloying components. The iron- and manganese-free mass fraction of the protective coating of Si as an additional alloying component is between 3% and 15% Si, in particular between 6% and 12% Si. In addition, the iron- and manganese-free mass fraction of the protective coating of Mg as an additional alloying component is up to 1.0% and the iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying component is between 0.4% and 25.0% Zn.Furthermore, the protective coating has a near-surface first Zn-rich layer with a Zn content that is higher than the average Zn content of the protective coating. Furthermore, the Zn content of the first Zn-rich layer is more than 5 wt.%. The protective coating is, in particular, an AlSi coating.

[0013] For the purposes of this application, an iron- and manganese-free mass fraction of an alloy component in the protective coating is understood to be the proportion of the total mass of this alloy component to the total mass of all elements in the protective coating except iron and manganese. Using the iron- and manganese-free mass fraction to characterize the protective coating has the advantage that the numerical values ​​are not changed by the diffusion of iron and manganese from the steel substrate. Iron- and manganese-free mass fractions are given in [%]. In contrast, the total mass-related element contents are usually given in [wt. %].

[0014] The protective coating therefore comprises a proportion of up to 30%, preferably up to 20%, based on the total mass without iron and manganese, of additional alloying elements, of which 3 to 15% is Si, up to 1.0% is Mg and between 0.4 and 25% is Zn.

[0015] In particular, the mass fraction of Mn is more than 0.1 wt.% and less than 2.5 wt.%, preferably more than 0.2 wt.% and preferably less than 2.0 wt.%, particularly preferably more than 0.4 wt.% and preferably less than 1.8 wt.%. During hot forming, a certain amount of Mn diffuses from the steel substrate into the protective coating, where it forms an additional alloying component.

[0016] Preferably, the protective coating consists solely of Al and unavoidable impurities in addition to Fe, Si, Mg, Zn, and optionally Mn. For the purposes of this application, an element is an unavoidable impurity of the protective coating if the iron- and manganese-free mass fraction in the protective coating is less than 0.5%. This definition refers only to unavoidable impurities in the protective coating. For the unavoidable impurities in the steel substrate mentioned later, the standard limits apply.

[0017] For the purposes of this application, a near-surface layer is understood to be a layer that extends over a region with a thickness of 500 nm that borders the surface of the protective coating. The first Zn-rich layer therefore extends over a region with a thickness of 500 nm that borders the surface of the protective coating. In other words, the first Zn-rich layer extends into the topmost 500 nm of the protective coating.

[0018] It has been shown that an increased zinc content in this near-surface area enables reliable phosphating. The zinc near the surface is converted to zinc phosphates during phosphating. A zinc phosphate layer also offers better corrosion protection than an iron phosphate layer that would otherwise form. Furthermore, zinc phosphate is an excellent base for paint adhesion.

[0019] The iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying component is preferably at least 0.6%, particularly preferably at least 1.0%, in particular at least 1.5%.

[0020] Further preferably, the Zn content of the first Zn-rich layer is more than 10.0 wt.%, particularly preferably more than 18.0 wt.%.

[0021] The higher the Zn content of the first Zn-rich layer, the better the phosphating behavior, as more zinc phosphates and fewer iron phosphates are formed. With a higher iron- and manganese-free mass fraction of the protective Zn coating, a high Zn content in the first Zn-rich layer can be achieved more easily and reliably. Therefore, a high iron- and manganese-free mass fraction of the protective Zn coating is preferred.

[0022] However, zinc in the protective coating has the disadvantage of its relatively low melting point. During the production of the steel component by hot forming, liquid Zn phases can form. These can, on the one hand, penetrate the steel substrate and contribute to cracking (liquid metal embrittlement), and on the other hand, adhere to forming tools or furnace rollers, leading to contamination and delamination. Therefore, the zinc content cannot be chosen too high. Tests have shown that iron- and manganese-free mass fractions of the protective coating of Zn of no more than 5.0%, in particular of less than 5%, preferably of no more than 2.3%, and particularly preferably of no more than 1.8%, are particularly advantageous for the flat steel product.

[0023] Magnesium has proven to be an advantageous additional alloying component in the protective coating and can be easily alloyed into aluminum protective coatings of the type in question here. The added amount of Mg is adjusted so that the iron- and manganese-free mass fraction of magnesium totals at least 0.10%, preferably at least 0.15%. In particular, iron- and manganese-free mass fractions of magnesium of less than 0.50%, less than 0.4%, or less than 0.35% have proven particularly advantageous in practice. The magnesium added in large quantities to the protective coating is characterized by a higher oxygen affinity than the main component, aluminum, of the protective coating. Even in the presence of such small amounts of magnesium, a thin oxide layer forms on the surface of the protective coating, covering the aluminum lying between it and the steel substrate.This thin layer prevents the aluminum from reacting with the moisture present in the atmosphere of the furnace used to heat the flat steel product during the heating required for hot forming the flat steel product. Oxidation of the aluminum in the coating and the associated release of hydrogen, which could diffuse into the coating and the steel substrate of the flat steel product, are thus effectively prevented. Surprisingly, this is particularly true when the aluminum-based coating becomes locally molten as a result of heating and its surface cracks, allowing molten coating material to come into contact with the furnace atmosphere. Particularly with longer annealing times, flat steel products according to the invention exhibit a lower hydrogen concentration in the component hot-formed from the flat steel product than in conventionally coated aluminum-coated flat steel products.

[0024] Therefore, the addition of magnesium to the alloy has the advantage of reducing hydrogen ingress into the steel substrate. If a very high hydrogen concentration is reached locally, this weakens the bonding at the grain boundaries of the steel substrate structure to such an extent that, during use, the resulting stress causes a crack along the grain boundary.

[0025] The elemental content of the protective coating is determined primarily by glow discharge optical emission spectroscopy (GD-OES). Alternatively, wet chemical stripping followed by ICP-OES (inductively coupled plasma optical emission spectrometry) is used to determine the elemental content.

[0026] The layer thickness of the protective coating is typically in the range of at least 5 pm and a maximum of 35 pm, in particular at least 10 pm and a maximum of 25 pm.

[0027] The steel substrate is made of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. In particular, the structure of the steel can be converted into a martensitic or partially martensitic structure by hot forming. The structure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic structure, as this has a particularly high hardness. The steel substrate is particularly preferably a steel which, in addition to iron and unavoidable impurities (in wt.%), consists of

[0028] C: 0.04 - 0.45 wt%,

[0029] Si: 0.02 - 1.2% by weight, Mn: 0.5 - 2.6% by weight, Al: 0.02 - 1.0% by weight, P: <0.05% by weight,

[0030] S: < 0.02 wt%,

[0031] N: < 0.02 wt%,

[0032] Sn: < 0.03 wt%,

[0033] As: < 0.01 wt%,

[0034] Ca: < 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents:

[0035] Cr: 0.08 - 1.0 wt.%,

[0036] B: 0.001 - 0.005 wt.%,

[0037] Mo: <0.5 wt%,

[0038] Ni: <0.5 wt%,

[0039] Cu: <0.2 wt%,

[0040] Nb: 0.02 - 0.08 wt.%,

[0041] Ti: 0.01 - 0.08 wt.%,

[0042] V: <0.1 wt%.

[0043] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows:

[0044] Cr: < 0.050 wt%,

[0045] B: < 0.0005 wt.%

[0046] Nb: < 0.005 wt%,

[0047] Ti: < 0.005 wt%

[0048] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0049] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and / or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.%, or in the range of 0.12-0.25 wt.%, or in the range of 0.33-0.37 wt.%.

[0050] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or at least 0.06 wt.%.

[0051] In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt.% and / or at least 0.75 wt.%. In particularly preferred embodiments, the Mn content is in the range of 0.75-0.85 wt.% or in the range of 1.0-1.6 wt.%.

[0052] In a preferred variant, the Al content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, preferably a maximum of 0.25 wt.%. Alternatively or additionally, the Al content is preferably at least 0.02%.

[0053] It has also been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the Si and Al contents (commonly referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0054] The elements P, S, and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Irrespective of this, the S content is preferably a maximum of 0.012%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0055] Optionally, the steel also contains chromium at a content of 0.08–1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0056] In the case of an optional alloying of chromium, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0057] Preferably, the steel optionally also contains boron at a content of 0.001-0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0058] Optionally, the steel may contain molybdenum in a maximum content of 0.5 wt%, in particular a maximum of 0.1 wt%.

[0059] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0060] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0061] In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti and V. The optional Nb content is at least 0.02 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.1 wt.%, preferably at most 0.05 wt.% In the case of an optional alloying of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The sum is at most 0.1 wt.%, in particular at most 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0062] The steel substrate is particularly preferably a steel from the group of steels AE, whose chemical analysis is given in Table 10. Table 10 should be understood as meaning that for each steel from the group of steels AE, the elemental content is given in weight percent. A minimum and a maximum weight percentage are given. For example, steel A contains a carbon content of C: 0.05 wt.% - 0.10 wt.%.

[0063] The above explanations regarding preferred steel substrates naturally also apply to the steel substrate of the flat steel product described below, as well as the manufacturing processes described.

[0064] In a preferred embodiment of the steel component, the surface of the protective coating is formed predominantly from Zn enrichments in metallic and / or oxidic form. For the purposes of this application, the surface of the protective coating is formed predominantly from Zn enrichments in metallic and / or oxidic form if more than 50% (area percent), preferably more than 70%, of the surface of the protective coating exhibits localized Zn enrichments. This proportion can be determined, for example, using electron microscopy images followed by image analysis. For each point, both Al and Zn are recorded according to their signal intensity, so that zinc enrichments can be identified.

[0065] In a preferred embodiment of the steel component, the protective coating comprises a silicon-poor phase and a silicon-rich phase, with the silicon-rich phase being distributed in islands within the silicon-poor phase. The Zn content of the silicon-rich phase is less than 90%, preferably less than 80%, of an average Zn content of the protective coating. At the same time, the Zn content of the silicon-poor phase is more than 105%, preferably more than 110%, of the average Zn content of the protective coating. In other words, the ratio of the mass fraction of Zn (specified in wt. %) in the silicon-rich phase to the mass fraction of Zn (specified in wt. %) in the entire protective coating is less than 90%, preferably less than 80%. Likewise, the ratio of the mass fraction of Zn (expressed in wt.%) in the low-silicon phase to the mass fraction of Zn (expressed in wt.%) in the entire protective coating is greater than 105%, preferably greater than 110%.

[0066] There is therefore an increase in the concentration of zinc in the silicon-poor phases, while the zinc content in the silicon-rich phases drops below the average value.

[0067] In special embodiments, the Si content of the low-silicon phases is less than 10 wt.%, preferably less than 7 wt.%, particularly preferably less than 5 wt.%.

[0068] Further special embodiments have a Si content of the silicon-rich phase that is greater than 10 wt.%, preferably greater than 12 wt.%.

[0069] For the purposes of this application, "island-shaped" is understood to mean an arrangement in which discrete, unconnected regions are enclosed by another material – i.e., "islands" of a particular material are located within another material. In vertical metallographic section, the "islands" can have length-to-width aspect ratios in the range of 1:1. Here, the length is measured parallel to the surface and the width perpendicular to the surface. The "islands" therefore have a similar length to width. However, significantly larger aspect ratios can also occur, in which the length is significantly greater than the width. The islands are therefore strip-shaped.

[0070] The object of the invention is also achieved by a flat steel product for hot forming, which consists of a steel substrate made of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and an Al-based protective coating applied to the steel substrate, which optionally has a total iron- and manganese-free mass fraction of up to 30% of additional alloying constituents. Furthermore, the iron- and manganese-free mass fraction of Si in the protective coating as an additional alloying constituent is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of Mg in the protective coating as an additional alloying constituent is between 0.10% and 0.50% Mg, preferably between 0.1 and 0.35% Mg. In addition, the iron- and manganese-free mass fraction of Zn in the protective coating as an additional alloying component is between 0.4 and 5.0% Zn, preferably between 0.4 and 2.3% Zn.

[0071] The protective coating therefore comprises a proportion of up to 30%, preferably up to 20%, based on the total mass without iron, of additional alloying elements, of which 3 to 15% is Si, between 0.10% and 0.50% is Mg and between 0.4 and 2.3% is Zn.

[0072] In particular, the mass fraction of Mn is more than 0.1 wt.% and less than 2.5 wt.%, preferably between 0.2 wt.% and 2.0 wt.%, and particularly preferably between 0.4 wt.% and 1.8 wt.%. Even during the application of the protective coating, a certain amount of Mn can diffuse from the steel substrate into the protective coating, where it forms an additional alloying component. This is particularly the case with hot-dip coating due to the high temperatures in the molten bath.

[0073] Preferably, the protective coating of the flat steel product consists solely of Al and unavoidable impurities, in addition to Fe, Si, Mg, Zn, and optionally Mn. For the purposes of this application, an element is an unavoidable impurity of the protective coating if the iron- and manganese-free mass fraction in the protective coating is less than 0.5%. This definition refers only to unavoidable impurities in the protective coating. For the unavoidable impurities in the steel substrate mentioned later, the standard limits apply.

[0074] It has been shown that starting with an iron- and manganese-free mass fraction of 0.4% of zinc after hot forming, a sufficiently high zinc content is present near the surface to enable reliable phosphating. During phosphating, zinc near the surface bonds to form zinc phosphates, which offer better corrosion protection than an iron phosphate layer that would otherwise form. Furthermore, zinc phosphate is an excellent base for paint adhesion.

[0075] The iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying component is preferably at least 0.6%, particularly preferably at least 1.0%, in particular at least 1.5%. The higher the iron- and manganese-free mass fraction of zinc, the higher the Zn content in the near-surface area after hot forming and the better the phosphating behavior, since more zinc phosphates and fewer iron phosphates are formed.

[0076] With a higher iron- and manganese-free mass fraction of the protective Zn coating, a high Zn content of the first Zn-rich layer can be reliably achieved. Therefore, a high iron- and manganese-free mass fraction of the protective Zn coating is preferred.

[0077] However, zinc in the protective coating has the disadvantage of its relatively low melting point. During the production of the steel component by hot forming, liquid Zn phases can form. These phases can, on the one hand, penetrate the steel substrate and contribute to cracking (liquid metal embrittlement), and on the other hand, adhere to forming tools or furnace rollers, leading to contamination and delamination. Therefore, the zinc content cannot be chosen too high. Tests have shown that iron- and manganese-free Zn mass fractions of the protective coating of no more than 2.3%, preferably no more than 1.8%, are particularly favorable.

[0078] Magnesium has proven to be an advantageous additional alloying component in the protective coating and can be easily alloyed into aluminum protective coatings of the type in question here. The amount of Mg added is adjusted so that the iron- and manganese-free mass fraction of magnesium is at least 0.10%, preferably at least 0.15%. In particular, iron- and manganese-free mass fractions of magnesium of less than 0.50%, less than 0.4%, or less than 0.35% have proven particularly advantageous in practice. The magnesium added in small quantities to the protective coating is characterized by a higher oxygen affinity than the main component, aluminum, of the protective coating. Even in the presence of such small amounts of magnesium, a thin oxide layer forms on the surface of the protective coating, covering the aluminum lying between it and the steel substrate.This thin layer prevents the aluminum from reacting with the moisture present in the atmosphere of the furnace used to heat the flat steel product during the heating required for hot forming the flat steel product. Oxidation of the aluminum in the coating and the associated release of hydrogen, which could diffuse into the coating and the steel substrate of the flat steel product, are thus effectively prevented. Surprisingly, this is particularly true when the aluminum-based coating becomes locally molten as a result of heating and its surface cracks, allowing molten coating material to come into contact with the furnace atmosphere. Particularly with longer annealing times, flat steel products according to the invention exhibit a lower hydrogen concentration in the component hot-formed from the flat steel product than in conventionally coated aluminum-coated flat steel products.

[0079] Therefore, adding magnesium to the alloy has the advantage of reducing hydrogen ingress into the steel substrate. If a very high hydrogen concentration is reached locally, this weakens the bonding at the grain boundaries of the steel substrate structure to such an extent that, during use, the resulting stress causes a crack along the grain boundary.

[0080] In a particularly preferred variant of the flat steel product, the protective coating is applied by hot-dip coating. Hot-dip coating, also known in technical terms as "hot-dip aluminizing," is a particularly economical method for applying a protective coating.

[0081] The layer thickness of the protective coating is typically in the range of at least 5 pm and a maximum of 35 pm, in particular at least 10 and a maximum of 25 pm.

[0082] The object of the invention is also achieved by a method for producing a phosphated steel component comprising the following steps:

[0083] - Heating the above-described flat steel product to a heating temperature for hot forming and holding at the heating temperature, so that diffusion leads to a concentration of Zn, in particular with the formation of metallic and oxidic Zn enrichments, in the near-surface area of ​​the protective coating

[0084] - Hot forming the coated steel substrate into the steel component

[0085] - Phosphating of the hot-formed steel component

[0086] Heating the previously described flat steel product to a heating temperature for hot forming and holding it at that temperature leads to a concentration of Zn in the near-surface region. In particular, a first Zn-rich layer forms, as already explained in connection with the steel component, and to the formation of oxidic and / or metallic Zn enrichments on the surface of the protective coating, which were also explained in connection with the steel component. This concentration of Zn in the near-surface region leads to significantly improved phosphating behavior and, in particular, to the formation of zinc phosphates instead of iron phosphates.

[0087] The heating temperature is preferably between 830°C and 980°C, more preferably between 830°C and 910°C. Furthermore, the holding time for the flat steel product at the heating temperature is at least 1 and at most 18 minutes. The heating and holding take place in a furnace.

[0088] Hot forming includes the following two steps in particular:

[0089] - Removing the flat steel product from the furnace and placing it in a forming tool

[0090] - Forming the flat steel product into a steel component in the forming tool

[0091] To avoid major heat losses, the transfer time between the furnace and the forming tool is typically no more than 10 seconds.

[0092] Optionally, the steel flat product can be cooled in the forming tool during forming at cooling rates of 20-1000 K / s, preferably 25-500 K / s, to harden the steel substrate.

[0093] Alternatively, the flat steel product can first be formed into a steel component in the forming tool and then the steel component can be cooled at cooling rates of 20-1000 K / s, preferably 25-500 K / s, to harden the steel substrate.

[0094] Furthermore, the object of the invention is also achieved by a method for producing a phosphated steel component comprising the following work steps:

[0095] - Providing a steel substrate made of a steel containing 0.1-3 wt% Mn and optionally up to 0.01 wt% B

[0096] - Hot-dip coating of the steel substrate by means of a melt with a protective coating based on Al, which optionally has a total iron- and manganese-free mass fraction of up to 30% of additional alloying components, wherein the iron- and manganese-free mass fraction of the protective coating of Si as an additional alloying component is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of the protective coating of Mg as an additional alloying component is up to 1% and wherein the iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying component is between 0.4% and 25.0% Zn

[0097] - Heating the coated steel substrate to a heating temperature for hot forming and holding at the heating temperature so that diffusion leads to a concentration of Zn, in particular with the formation of metallic and oxidic Zn enrichments, on the surface of the protective coating

[0098] - Hot forming the coated steel substrate into the steel component

[0099] - Phosphating the hot-formed steel component.

[0100] The iron- and manganese-free mass fractions have preferred minimum and maximum values, which are explained above with reference to the steel component together with their advantages.

[0101] In particular, the hot-dip coating of the steel substrate is carried out by means of a melt with a protective coating based on Al, which in total has an iron- and manganese-free mass fraction of up to 30% of additional alloying components, wherein the additional alloying components comprise Si, Mg and Zn, wherein the iron- and manganese-free mass fraction of the protective coating of Si as an additional alloying component is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of Mg of the protective coating as an additional alloying component is between 0.10-0.50% Mg, preferably between 0.1-0.35% Mg, and that the iron and loose mass fraction of Zn of the protective coating as an additional alloying component is between 0.4-5% Zn, preferably between 0.4-2.3% Zn.

[0102] Heating the previously described flat steel product to a heating temperature for hot forming and holding it at that temperature leads to a concentration of Zn in the near-surface region. In particular, a first Zn-rich layer forms, as already explained in connection with the steel component, and to the formation of oxidic and metallic Zn enrichments on the surface of the protective coating, which were also explained in connection with the steel component. This concentration of Zn in the near-surface region leads to significantly improved phosphating behavior and, in particular, to the formation of zinc phosphates instead of iron phosphates.

[0103] The heating temperature is preferably between 830°C and 980°C, more preferably between 830°C and 910°C. Furthermore, the holding time for the flat steel product at the heating temperature is at least 1 and at most 18 minutes. The heating and holding take place in a furnace.

[0104] Hot forming includes the following two steps in particular:

[0105] - Removing the flat steel product from the furnace and placing it in a forming tool

[0106] - Forming the flat steel product into a steel component in the forming tool

[0107] To avoid major heat losses, the transfer time between the furnace and the forming tool is typically no more than 10 seconds.

[0108] Optionally, the steel flat product can be cooled in the forming tool during forming at cooling rates of 20-1000 K / s, preferably 25-500 K / s, to harden the steel substrate.

[0109] Alternatively, the flat steel product can first be formed into a steel component in the forming tool and then the steel component can be cooled at cooling rates of 20-1000 K / s, preferably 25-500 K / s, to harden the steel substrate.

[0110] Phosphating refers to the full-surface phosphating of a steel component, in which a phosphate layer is deposited on the steel component, for example, with a thickness of 0.5 to 3.0 pm. The phosphate layer increases paint adhesion and prevents corrosion of the steel substrate in the event of paint damage.

[0111] In a special embodiment of the aforementioned process for producing a phosphated steel component, a silicon-poor phase and a silicon-rich phase form by diffusion during heating and holding, with the silicon-rich phase being distributed in islands within the silicon-poor phase. The Zn content of the silicon-poor phase is between 25% and 60%, preferably between 30% and 50% of the Zn content in the melt.

[0112] In another specific embodiment of the aforementioned method for producing a phosphated steel component, the heating temperature is higher than the Ac3 temperature, so that the microstructure of the steel substrate is austenitic. Furthermore, the coated steel substrate is quenched after hot forming or during hot forming, so that a hardened microstructure forms in the microstructure of the steel substrate of the flat steel product.

[0113] The invention is explained in more detail using the following exemplary embodiments in conjunction with the figures. In the figures:

[0114] Fig. 1 is a scanning electron microscope image of the surface of a hot-formed steel component before phosphating;

[0115] Fig. 2 is a scanning electron microscope image of the surface of a hot-formed steel component after phosphating;

[0116] Fig. 3 is a schematic representation of a micrograph.

[0117] The steel substrate used was a 1.5 mm thick steel sheet made of a steel with a composition as listed in Table 1, with the remainder being iron and unavoidable impurities. The steel is therefore a steel from Group D of Table 10.

[0118] Table 1: Steel composition of the steel substrate (in wt%)

[0119] Based on this steel substrate, four different variants were examined in more detail. First, the steel substrate was coated with an Al-based protective coating by hot-dip coating. The thickness of the protective coating was 20 μm in each case. Table 2 below shows the target composition of the respective melt, with the remainder being iron and unavoidable impurities: Table 2: Melt compositions

[0120] For the steel substrates coated in this way, the elemental contents of the protective coating were then determined using GD-OES (Glow Discharge Optical Emission Spectroscopy). This allowed the iron- and manganese-free mass fractions of the respective elements to be determined. The results are presented in Table 3.

[0121] Table 3: Iron- and manganese-free mass fractions of the protective coating of the coated steel substrates (flat steel products)

[0122] Variant A is therefore not in accordance with the invention, since the iron- and manganese-free mass fraction of zinc, at 0.2%, is outside the inventive range. This variant serves as a reference sample.

[0123] In addition to the element contents in the entire protective coating, the element contents in the near-surface region (i.e., in the uppermost 500 nm) were also determined using GD-OES. The results are shown in Table 4 below: Table 4: Element contents of the protective coating of the coated steel substrates in the near-surface region

[0124] * Levels below the detection limit

[0125] The prepared steel flat products, i.e., the coated steel substrates, were then heated to a heating temperature of 920°C, which is greater than Ac3. They were held at this heating temperature for a holding time of between 5 and 10 minutes and then hot-formed into steel components. The heating and holding were carried out in a roller-hearth furnace with dew point control. The dew point was set to 15°C. The entire process took place in an ambient atmosphere. The steel flat products were then formed into steel components in a tool and quenched to room temperature. The cooling rate was more than 50°C per second. The process parameters for the heat treatment are summarized again in Table 5 below:

[0126] Table 5: Process parameters of heat treatment

[0127] The steel components thus produced were analyzed again in the next step. First, the elemental contents of the protective coating were determined using GD-OES, and from this, the iron- and manganese-free mass fractions were determined. Table 6: Iron- and manganese-free mass fractions of the protective coating of the steel components

[0128] * Levels below the detection limit

[0129] Here, too, it is evident that variant A is not in accordance with the invention, since the iron- and manganese-free mass fraction of zinc, at 0.1%, is outside the range according to the invention.

[0130] In addition to the elemental concentrations in the entire protective coating, the elemental concentrations in the near-surface region (i.e., in the uppermost 500 nm) were also determined using GD-OES. The results are presented in Table 7 below:

[0131] Table 7: Element contents of the protective coating of the steel components in the near-surface area

[0132] It can be clearly seen that variants B, C and D have a first Zn-rich layer near the surface with a Zn content that is higher than the average Zn content of the protective coating. The average Zn content of the protective coating is in any case lower than the Zn content of the melt and is therefore less than 2.26 wt.% for variants B and C, for example. The diffusion process can only cause the average Zn content to decrease further, since no Zn is added. Furthermore, the Zn content of the first Zn-rich layer is 5% or more in all three variants B, C and D. A drastic increase in zinc concentration has occurred in the area near the surface. Figure 1 shows an electron micrograph of the surface of the protective coating for variant B as an example.It is clearly visible that the surface is largely composed of Zn enrichments in metallic and / or oxidic form, present in small crystallites distributed across the surface. The area fraction of Zn enrichments was determined by image analysis to be 56% for variant B and 58% and 16% for variants C and D, respectively.

[0133] Figure 2 shows an electron micrograph of the surface of the protective coating after phosphating, illustrating variant B. A uniform, continuous phosphate layer is clearly visible.

[0134] Figure 3 shows a schematic representation of a micrograph of variant B, which can be captured using scanning electron microscopy. The steel component 11 shown consists of a steel substrate 13 and a protective coating 15. The protective coating 15 comprises a silicon-poor phase A and a silicon-rich phase R, with the silicon-rich phase R distributed in island-like patterns within the silicon-poor phase A. The Zn content in the two phases was determined using energy-dispersive X-ray spectroscopy (EDX). It was shown that, here too, a concentration of zinc occurs in the silicon-poor phases. The results are shown in Table 8 below.

[0135] Table 8: Zn and Si contents of the silicon-rich and silicon-poor phases

[0136] It is clearly evident that the Zn content of the low-silicon phase is between 25% and 60% of the Zn content of the melt. For variant B, the ratio of the Zn content of the low-silicon phase to the melt is 30.1%, for variant B it is 26.5%, and for variant C it is 46.1%. The hot-formed steel components were then phosphated. For this purpose, the samples were subjected to a standard industrial phosphating treatment. This included the following sub-steps:

[0137] - Degreasing in a degreasing bath for 150 seconds at a temperature of 60°C. Chemetall Gardoclean S 5176 was used as the degreasing agent.

[0138] - Activate the surface for 30s with Chemetall Gardolene ZL 6. The activator concentration was 6g / l.

[0139] - Phosphating in a phosphating bath for 150 seconds at a temperature of 60°C. Chemetall Gardobond 24 was used as the phosphating agent.

[0140] After phosphating was completed, the elemental contents in the near-surface region (i.e., the top 500 nm) were determined again. The results are shown in Table 9 below:

[0141] Table 9: Element contents of the protective coating of the phosphated steel components in the near-surface area

[0142] It is clearly visible again that variants B, C, and D have a near-surface first Zn-rich layer with a Zn content that is higher than the average Zn content of the protective coating listed in Table 6. Furthermore, the Zn content of the first Zn-rich layer is 5% or more in all three variants B, C, and D. The values ​​are even higher than before phosphating. However, this is due to the fact that material was applied during phosphating, so that the measuring range of 500 nm thickness has shifted even further into the very zinc-rich region. Furthermore, it is clearly evident that variants B, C, and D exhibit improved phosphating behavior. The near-surface phosphorus contents are more than a factor of 5 higher than those of comparison sample A.

[0143]

Claims

Patent claims 1. Steel component (11) comprising a steel substrate (13) of a steel having 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and a protective coating (15) based on aluminum applied to the steel substrate (13), which in total has an iron- and manganese-free mass fraction of up to 30% of additional alloying elements, wherein the additional alloying elements are silicon, magnesium, and zinc, wherein the iron- and manganese-free mass fraction of the protective coating of silicon as an additional alloying element is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of the protective coating of magnesium as an additional alloying element is up to 1.0%, and wherein the iron- and manganese-free mass fraction of the protective coating of zinc as an additional alloying element is between 0.4% and 25.0% Zn. the protective coating (15) has a near-surface first Zn-rich layer with a Zn content,which is higher than the average Zn content of the protective coating, and wherein the Zn content of the first Zn-rich layer is more than 5 wt.%, and wherein the layer thickness of the protective coating is at least 5 pm, and the near-surface layer extends over a region with a thickness of 500 nm adjacent to the surface of the protective coating.

2. Steel component according to claim 1, characterized in that the iron- and manganese-free mass fraction of the protective coating of Mg as an additional alloying element totals at least 0.10%, preferably at least 0.15%, and / or the iron- and manganese-free mass fraction of the protective coating of Mg as an additional alloying element totals at most 0.50%, preferably at most 0.35%, and / or the iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying element totals at least 0.6%, preferably at least 1.0%, and / or the iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying element totals at most 5.0%, preferably at most 2.3%, and / or the Zn content of the first Zn-rich layer is more than 10.0 wt.%, preferably more than 18 wt.%. A steel component according to any one of claims 1 to 2, characterized in that the surface of the protective coating is largely formed from zinc enrichments in metallic and / or oxide form. A steel component according to any one of claims 1 to 3, characterized in that the protective coating (15) comprises a silicon-poor phase (A) and a silicon-rich phase (R), wherein the silicon-rich phase is distributed in an island-like manner within the silicon-poor phase, wherein the zinc content of the silicon-rich phase is less than 90% of the average zinc content of the protective coating and the zinc content of the silicon-poor phase is more than 105% of the average zinc content of the protective coating. A steel component according to claim 4, characterized in that the silicon content of the silicon-poor phase is less than 10 wt.%, preferably less than 7 wt.%, and particularly preferably less than 5 wt.%.Steel component according to one of claims 4 to 5, characterized in that the Si content of the silicon-rich phase is greater than 10 wt.%, preferably greater than 12 wt.%. Steel flat product for hot forming, consisting of a steel substrate (13) of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% Mn.-% B, and a protective coating (15) based on AI applied to the steel substrate (13), which in total has an iron- and manganese-free mass fraction of up to 30% of additional alloying elements, wherein the additional alloying elements are Si, Mg and Zn, wherein the iron- and manganese-free mass fraction of the protective coating of Si as an additional alloying element is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of Mg of the protective coating as an additional alloying element is between 0.10-0.50% Mg, preferably between 0.1-0.35% Mg, characterized in that the iron- and manganese-free mass fraction of Zn of the protective coating as an additional alloying element is between 0.4-5% Zn, preferably between 0.4-2.3% Zn. Steel flat product according to claim 7, characterized in that the protective coating (15) is applied by hot-dip coating. A process for producing a phosphated steel component comprising the following work steps: - Heating the steel flat product according to claim 7 to a heating temperature of at least 700°C for hot forming and holding at the heating temperature, so that diffusion leads to a concentration of Zn with the formation of metallic and oxide Zn enrichments in the near-surface area of ​​the protective coating; - Hot forming of the coated steel substrate into the steel component; - Phosphating of the hot-formed steel component. A process for producing a phosphated steel component comprises the following steps: - Providing a steel substrate made of a steel containing 0.1-3 wt. % Mn and optionally up to 0.01 wt. % B; - Hot-dip coating of the steel substrate using a melt with a protective coating (15) based on Al, which has a total iron- and manganese-free mass fraction of up to 30% of additional alloying elements, wherein the additional alloying elements comprise Si, Mg and Zn, wherein the iron- and manganese-free mass fraction of the protective coating of Si as an additional alloying element is between 3% and 15% Si, in particular between 6% and 12% Si, wherein the iron- and manganese-free mass fraction of the protective coating of Mg as an additional alloying element is up to 1%, wherein the iron- and manganese-free mass fraction of the protective coating of Zn as an additional alloying element is between 0.4% and 25.0% Zn, in particular between 0.4% and 5% Zn, - Heating the coated steel substrate to a heating temperature of at least 700°C for hot forming and holding at the heating temperature, so that diffusion leads to a concentration of Zn with the formation of metallic and oxide Zn enrichments on the surface of the protective coating; - Hot forming of the coated steel substrate into the steel component; - Phosphating of the hot-formed steel component. A method according to claim 9 or 10, characterized in that during heating and holding, a silicon-poor phase (A) and a silicon-rich phase (R) are formed by diffusion, wherein the silicon-rich phase is distributed in an island-like manner within the silicon-poor phase, and wherein the Zn content of the silicon-poor phase is between 25% and 60% of the Zn content of the melt. A method according to any one of claims 9 to 11, characterized in that the heating temperature is higher than the Ac3 temperature, such that the microstructure of the steel substrate is austenitic, and wherein the coated steel substrate (13) is quenched after or during hot forming, such that a hardened microstructure forms in the microstructure of the steel substrate of the flat steel product.

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