STEEL WITH IMPROVED PROCESSING PROPERTIES FOR FORMING AT ELEVATE TEMPERATURES

Description

[0001] The invention relates to a sheet metal part with improved properties and a method for producing such a sheet metal part from a flat steel product.

[0002] When reference is made to a "flat steel product" or a "sheet metal product" below, this refers to rolled products such as steel strips or sheets, from which "sheet metal blanks" (also called blanks) are cut for the production of, for example, car body components. "Formed sheet metal parts" or "sheet metal components" of the type according to the invention are made from such sheet metal blanks, whereby the terms "formed sheet metal part" and "sheet metal component" are used synonymously here.

[0003] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" data relating to a steel alloy are therefore to be understood as data in "wt%." With the exception of the data relating to the residual austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (specified in "vol%"), data on the contents of the various microstructure constituents refer to the area of ​​a microsection of a sample of the respective product (specified in area percent, "area%"), unless expressly stated otherwise. Information given in this text regarding the contents of the constituents of an atmosphere refers to the volume (specified in "vol%").

[0004] Mechanical properties reported here, such as tensile strength, yield strength, and elongation, were determined in tensile tests according to DIN-EN ISO 6892-1, specimen shape 2 (Appendix B Table B1) (as of June 2020), unless explicitly stated otherwise. The bending angle is determined according to VDA standard 238-100 for the maximum force.

[0005] The microstructure was determined on longitudinal sections etched with 3% Nital (alcoholic nitric acid). The content of retained austenite was determined by X-ray diffraction.

[0006] WO 2019 / 223854 A1 discloses a sheet metal part and a method for producing such a sheet metal part, which has a tensile strength of at least 1000 MPa. The sheet metal part consists of a steel that, in addition to iron and unavoidable impurities, contains (in wt. %) 0.10–0.30% C, 0.5–2.0% Si, 0.5–2.4% Mn, 0.01–0.2% Al, 0.005–1.5% Cr, 0.01–0.1% P, and optionally further optional elements, in particular 0.005–0.1% Nb. Furthermore, the sheet metal component comprises a corrosion protection coating containing aluminum.

[0007] EP 2 555 133 B1 also discloses a sheet metal part and a method for producing such a sheet metal part. EP 3 655 560 A1 discloses a method for producing a steel sheet suitable for a sheet metal part.

[0008] Given the state of the art, the task was to further develop a flat steel product for hot forming in such a way that, in combination with an aluminum-based anti-corrosive coating, improved processing properties of the hot-formed sheet metal part could be achieved. Furthermore, a process was to be specified that would enable such sheet metal parts to be manufactured in a practical manner.

[0009] The sheet metal part of the invention is set out in claim 1. The method for producing the sheet metal part of the invention is set out in claim 8.

[0010] Compared to known flat steel products, the steel substrate of the flat steel product according to the invention has an aluminum content of at least 0.06 wt.%, preferably at least 0.07 wt.%, in particular at least 0.08 wt.%. The aluminum content is preferably at least 0.10 wt.%, particularly preferably at least 0.11 wt.%, in particular at least 0.12 wt.%, preferably at least 0.140 wt.%, in particular at least 0.15 wt.%, preferably at least 0.16 wt.%. The maximum aluminum content is 1.0 wt.%, in particular a maximum of 0.8 wt.%.

[0011] In a first further developed variant, the aluminum content is at least 0.07 wt.%, in particular at least 0.08 wt.%, preferably at least 0.10 wt.%, particularly preferably at least 0.11 wt.%, in particular at least 0.12 wt.%, preferably at least 0.140 wt.%, in particular at least 0.15 wt.%, preferably at least 0.16 wt.%. The maximum aluminum content in this variant is a maximum of 0.50 wt.%, in particular a maximum of 0.35 wt.%, preferably a maximum of 0.25 wt.%, in particular a maximum of 0.24 wt.%.

[0012] In a second further developed variant, the aluminum content is at least 0.50 wt.%, preferably at least 0.60 wt.%, preferably at least 0.70 wt.%. The maximum aluminum content in this variant is a maximum of 1.0 wt.%, in particular a maximum of 0.9 wt.%, preferably a maximum of 0.80 wt.%.

[0013] Aluminum ("Al") is known to be added as a deoxidizer during steel production. At least 0.01 wt.% Al is required to reliably capture the oxygen contained in the molten steel. Al can also be used to capture undesirable, yet unavoidable, nitrogen contents during production. Relatively high aluminum contents have been avoided to date, as increasing aluminum also causes the Ac3 temperature to shift upwards. This has a negative impact on austenitization, which is important for hot forming. However, it has been shown that increased aluminum contents, surprisingly, lead to positive effects when combined with an aluminum-based corrosion protection coating.

[0014] During the coating of flat steel products with an aluminum-based corrosion protection coating and the subsequent hot forming of cut sheet metal blanks into formed sheet metal parts, iron diffuses from the steel substrate into the liquid corrosion protection coating. In the interdiffusion zone, iron-aluminide compounds with higher density are formed via a multi-stage phase transformation (Fe2Al5→Fe2Al→FeAl→Fe3Al). The formation of such denser phases is associated with higher aluminum consumption than with less dense phases. This locally higher aluminum consumption leads to the formation of pores (vacancies) in the resulting phase. These pores preferentially form in the transition zone between the steel substrate and the corrosion protection coating, where the proportion of available aluminum is strongly influenced by the aluminum content of the steel substrate.In particular, an accumulation of pores in the form of a band in the transition area can occur.

[0015] Such pores, and especially a band of pores, cause a variety of problems: The pores reduce the mechanical integrity in this area. This can lead to faster delamination under corrosive stress. Furthermore, the transferable force at the joint between two components after bonding or welding is reduced. The pores lead to altered current paths in the material during resistance spot welding, which negatively impacts weldability and thus reduces the weld area. The pores themselves facilitate crack initiation and propagation during static and dynamic bending.

[0016] Surprisingly, it has been shown that by increasing the aluminum content ("Al") in the steel substrate to the described lower limits and beyond, a significant reduction in pore formation can be achieved during the coating with an aluminum-based corrosion protection coating and subsequent hot forming. Particularly in the transition region between the steel substrate and the corrosion protection coating, the locally higher aluminum consumption during the formation of denser iron-aluminide compounds can be at least partially compensated by the aluminum content of the steel substrate, thus suppressing the formation of pores, especially a band of pores.

[0017] If the Al content is too high, especially at contents exceeding 1.0 wt% Al, there is a risk of Al oxides forming on the surface of a product made from steel alloyed according to the invention, which would impair the wetting behavior during hot-dip coating. Furthermore, higher Al contents promote the formation of non-metallic Al-based inclusions, which, as coarse inclusions, negatively impact crash behavior. Therefore, the Al content is preferably selected below the aforementioned upper limits.

[0018] The bending behavior of the sheet metal component is particularly supported by the inventive niobium ("Nb") content of at least 0.001 wt.%. The Nb content is preferably at least 0.005 wt.%, in particular at least 0.010 wt.%, preferably at least 0.015 wt.%, particularly preferably at least 0.020 wt.%, in particular at least 0.024 wt.%, preferably at least 0.025 wt.%.

[0019] The specified Nb content leads, particularly in the process described below for producing a flat steel product for hot forming with a corrosion protection coating, to a distribution of niobium carbonitrides, which results in a particularly fine hardened microstructure during subsequent hot forming. During cooling after hot-dip coating, the coated flat steel product is kept for a certain time in a temperature range between 400 °C and 300 °C. In this temperature range, a certain diffusion rate of carbon ("C") still exists in the steel substrate, while the thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there. Lattice defects are caused in particular by dissolved niobium atoms, which, due to their significantly higher atomic volume, expand the atomic lattice and thus enlarge the tetrahedral and octahedral gaps in the atomic lattice, thus increasing the local solubility of C.As a result, clusters of carbon and niobium form in the steel substrate, which then transform into very fine precipitates in the subsequent austenitizing step of hot forming and act as additional austenite nuclei. This results in a refined austenite microstructure with smaller austenite grains and thus also a refined hardening microstructure.

[0020] This particularly applies to the ferritic interdiffusion layer that forms during hot forming. The refined ferritic microstructure in the interdiffusion layer helps reduce the tendency for crack initiation under bending loads.

[0021] The higher Nb content has another advantage. Surprisingly, it has been shown that the higher Nb content in the steel substrate leads to a shift in the electrochemical potential of the final sheet metal part toward a more positive (i.e., more noble) potential. The Nb content in the interdiffusion layer has proven to be a good indicator of the shift in the electrochemical potential. If the Nb content in the interdiffusion layer is at least 0.010%, the potential is approximately 100-150 mV higher than in a comparable substrate with a lower Nb content. The sheet metal part thus obtained has greater corrosion resistance.

[0022] However, an excessively high Nb content leads to impaired recrystallizability. Therefore, the Nb content is a maximum of 0.2 wt.%. Furthermore, the Nb content is preferably a maximum of 0.20 wt.%, in particular a maximum of 0.15 wt.%, preferably a maximum of 0.10 wt.%, in particular a maximum of 0.05 wt.%.

[0023] Aluminum and niobium both influence grain refinement during austenitization in the hot forming process. It has been found that aluminum, along with niobium, particularly refines grain growth at elevated temperatures in austenite (e.g., above 1200 °C) through the relatively early, i.e., relatively high-temperature, formation of AlN. AlN formation is thermodynamically favored over NbN or NbC. The precipitation of AlN has a grain-refining effect in austenite and thus improves toughness. Increasing Al / Nb ratios enhance this effect. Therefore, the following optionally applies to the Al / Nb ratio based on Al content to Nb content: 1 ≤ Al / Nb The preferred Al / Nb ratio is >_ 2, especially >_ 3. At the same time, an excessively high Al / Nb ratio results in the AlN formation no longer being as advantageously fine, but rather in the formation of increasingly coarse AlN particles, which in turn reduces the grain refinement effect. It has been shown that this effect occurs earlier at low manganese contents than at higher manganese contents, since the AC3 temperature decreases with increasing manganese content. Therefore, at low manganese contents of ≤ 1.6 wt.%, an Al / Nb ratio is set for which the following applies: Al / Nb ≤ 20.0 , which corresponds approximately to an atomic ratio of both elements ≤ 6. For Mn ≤ 1.6 wt.%, the ratio Al / Nb is preferably ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, particularly preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0.

[0024] However, with higher manganese contents of Mn >_ 1.7 wt.%, higher ratios are also possible. For higher manganese contents of 1.7 wt.% or more, a ratio of Al / Nb is set for which the following applies: Al / Nb ≤ 30.0 ,

[0025] Preferably, for Mn >_ 1.7 wt.%, the ratio Al / Nb is ≤ 28.0, in particular ≤ 26.0, preferably ≤ 24.0, particularly preferably ≤ 22.0, preferably ≤ 20.0, in particular ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, particularly preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0.

[0026] Regardless of the manganese content, it is therefore preferable to set a ratio of AI / Nb for which the following applies: Al / Nb ≤ 20.0 ,

[0027] Preferably, the ratio Al / Nb is ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, particularly preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0.

[0028] Carbon ("C") is present in the steel substrate of the flat steel product in concentrations of 0.06–0.5 wt.%. Such adjusted C contents contribute to the hardenability of the steel by delaying the formation of ferrite and bainite and stabilizing the residual austenite in the microstructure. A C content of at least 0.06 wt.% is required to achieve sufficient hardenability and the associated high strength.

[0029] However, high C contents can negatively impact weldability. To improve weldability, the C content can be adjusted to 0.5 wt.%, preferably to a maximum of 0.5 wt.%, in particular to a maximum of 0.45 wt.%, preferably to 0.42 wt.%, particularly preferably to 0.40 wt.%, preferably to a maximum of 0.38 wt.%, in particular to a maximum of 0.35 wt.%.

[0030] To ensure particularly reliable utilization of the positive effects of the presence of carbon, C contents of at least 0.10 wt.%, preferably 0.11 wt.%, in particular at least 0.13 wt.%, preferably at least 0.15 wt.% can be provided. At these contents, tensile strengths of the sheet metal part of at least 1000 MPa, in particular at least 1100 MPa, can be reliably achieved after hot press forming, subject to the further provisions of the invention.

[0031] In a first further developed variant, the C content is at least 0.10 wt.%, preferably 0.11 wt.%, in particular at least 0.13 wt.%, preferably at least 0.15 wt.%. The maximum C content in this variant is a maximum of 0.30 wt.%, in particular a maximum of 0.25 wt.%, preferably a maximum of 0.25 wt.%. With these maximum C contents, weldability can be significantly improved, and a good ratio of force absorption to maximum bending angle can be achieved in the bending test according to VDA238-100 in the press-hardened state.

[0032] In a second further developed variant, the C content is at least 0.25 wt.%, preferably at least 0.30 wt.%, in particular at least 0.32 wt.%. The maximum C content in this variant is a maximum of 0.5 wt.%, in particular a maximum of 0.50 wt.%, preferably a maximum of 0.40 wt.%, preferably a maximum of 0.38 wt.%, in particular a maximum of 0.35 wt.%.

[0033] In a third further developed variant, the C content is at least 0.30 wt.%, preferably at least 0.32 wt.%, in particular at least 0.33 wt.%, preferably at least 0.34 wt.%, preferably at least 0.35 wt.%, in particular at least 0.40 wt.%, preferably at most 0.44 wt.%. The maximum C content in this variant is at most 0.5 wt.%, in particular at most 0.50 wt.%, preferably at most 0.48 wt.%. At these contents, tensile strengths of the sheet metal part of at least 1700 MPa, in particular at least 1800 MPa after hot press forming, can be reliably achieved, subject to the further provisions of the invention.

[0034] Silicon ("Si") is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product through solid solution strengthening. Silicon also enables the use of ferro-silicon-manganese as an alloying agent, which has a beneficial effect on production costs. A hardening effect is already evident at a Si content of 0.05 wt.%. A significant increase in strength occurs at a Si content of at least 0.15 wt.%, and in particular at least 0.20 wt.%. Si contents above 0.6 wt.% have a detrimental effect on the coating behavior, especially with Al-based coatings. Si contents of no more than 0.50 wt.%, and in particular no more than 0.30 wt.%, are preferred to improve the surface quality of the coated flat steel product.

[0035] Manganese ("Mn") acts as a hardening element by significantly retarding the formation of ferrite and bainite. At manganese contents below 0.4 wt. %, significant amounts of ferrite and bainite are formed during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents of at least 0.5 wt. %, in particular at least 0.7 wt. %, preferably at least 0.8 wt. %, in particular at least 0.9 wt. %, preferably at least 1.00 wt. %, in particular at least 1.05 wt. %, particularly preferably at least 1.10 wt. %, are advantageous if a martensitic microstructure is to be ensured, particularly in areas of greater deformation. Mn contents of more than 3.0 wt. % have a detrimental effect on the processing properties, which is why the Mn content of flat steel products according to the invention is limited to a maximum of 3.0 wt. %, preferably a maximum of 2.5 wt. %.Weldability, in particular, is severely limited, which is why the Mn content is preferably limited to a maximum of 1.6 wt%, particularly to 1.30 wt%, and preferably to 1.20 wt%. Mn contents of less than or equal to 1.6 wt% are also preferred for economic reasons.

[0036] Titanium ("Ti") is a microalloying element that is added to contribute to grain refinement. At least 0.001 wt.% Ti, in particular at least 0.004 wt.%, and preferably at least 0.010 wt.% Ti, should be added to ensure sufficient availability. Above 0.10 wt.% Ti, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided. To improve cold rollability, the Ti content can preferably be limited to 0.08 wt.%, in particular to 0.038 wt.%, particularly preferably to 0.020 wt.%, and especially 0.015 wt.%. Titanium also has the effect of binding nitrogen, thus enabling boron to exert its strong ferrite-inhibiting effect. Therefore, in a preferred embodiment, the titanium content is more than 3.42 times the nitrogen content to achieve sufficient nitrogen binding.

[0037] Boron ("B") is added to improve the hardenability of the steel flat product by reducing the grain boundary energy through boron atoms or boron precipitates deposited on the austenite grain boundaries, thereby suppressing the nucleation of ferrite during press hardening. A significant effect on hardenability occurs at B contents of at least 0.0005 wt.%, preferably at least 0.0007 wt.%, in particular at least 0.0010 wt.%, in particular at least 0.0020 wt.%. At B contents above 0.01 wt.%, however, increased formation of boron carbides, boron nitrides, or boron nitrocarbides occurs, which in turn represent preferred nucleation sites for the nucleation of ferrite and reduce the hardening effect. For this reason, the B content is limited to at most 0.01 wt%, preferably at most 0.0100 wt%, preferably at most 0.0050 wt%, in particular at most 0.0035 wt%, in particular at most 0.0030 wt%, preferably at most 0.0025 wt%.

[0038] Phosphorus ("P") and sulfur ("S") are elements that are introduced into steel as impurities by iron ore and cannot be completely eliminated in the large-scale steelmaking process. The P and S contents should be kept as low as possible, since mechanical properties such as notch impact energy deteriorate with increasing P and S contents. Furthermore, at P contents of 0.05 wt.% and above, embrittlement of the martensite begins to occur, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.03 wt.%, in particular a maximum of 0.02 wt.%. The S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably a maximum of 0.0010 wt.%, in particular a maximum of 0.005 wt.%.

[0039] Nitrogen ("N") is also present in small amounts as an impurity in steel due to the steelmaking process. The N content should be kept as low as possible and should not exceed 0.02 wt.%. Nitrogen is particularly harmful to alloys containing boron, as it inhibits the transformation-retarding effect of boron through the formation of boron nitrides. Therefore, the N content in this case should preferably not exceed 0.010 wt.%, and in particular not exceed 0.007 wt.%.

[0040] Other typical impurities are tin ("Sn") and arsenic ("As"). The Sn content is a maximum of 0.05 wt.%, preferably a maximum of 0.02 wt.%. The As content is a maximum of 0.01 wt.%, in particular a maximum of 0.005 wt.%.

[0041] In addition to the previously discussed impurities P, S, N, Sn, and As, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "unavoidable impurities." The total content of these "unavoidable impurities" is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, Cu, Mo, Ni, V, Ca, and W described below, 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," whose total content is limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%.

[0042] Chromium, copper, molybdenum, nickel, vanadium, calcium and tungsten can optionally be alloyed to the steel of a flat steel product according to the invention, either individually or in combination with one another.

[0043] Chromium ("Cr") suppresses the formation of ferrite and pearlite during accelerated cooling of a flat steel product according to the invention and enables complete martensite formation even at lower cooling rates, thereby increasing hardenability.

[0044] These effects occur starting at a content of 0.01 wt.%, with a content of at least 0.10 wt.%, preferably at least 0.15 wt.%, having proven effective in practice for safe process control. However, excessive Cr contents impair the coatability of the steel. Therefore, the Cr content of the steel or the steel substrate is limited to a maximum of 1.0 wt.%, preferably a maximum of 0.80 wt.%, in particular a maximum of 0.75 wt.%, preferably a maximum of 0.50 wt.%, in particular a maximum of 0.30 wt.%.

[0045] Vanadium ("V") can optionally be added in amounts of 0.001–1.0 wt.%. The vanadium content is preferably a maximum of 0.3 wt.%. For cost reasons, a maximum of 0.2 wt.% vanadium is added.

[0046] Copper ("Cu") can optionally be added to the alloy to increase hardenability with additions of at least 0.01 wt.%, preferably at least 0.010 wt.%, especially at least 0.015 wt.%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. If the Cu content is too high, hot-rollability deteriorates significantly due to low-melting Cu phases on the surface, which is why the Cu content is limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%, especially a maximum of 0.10 wt.%.

[0047] Molybdenum ("Mo") can optionally be added to improve process stability, as it significantly slows ferrite formation. Starting at concentrations of 0.002 wt.%, dynamic molybdenum-carbon clusters, including ultrafine molybdenum carbides, form at the grain boundaries. These clusters significantly slow grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces grain boundary energy, which reduces the nucleation rate of ferrite. The Mo content is preferably at least 0.004 wt.%, in particular at least 0.01 wt.%. Due to the high costs associated with a molybdenum alloy, the Mo content should be at most 0.3 wt.%, in particular at most 0.10 wt.%, and preferably at most 0.08 wt.%.

[0048] Nickel ("Ni") stabilizes the austenitic phase and can optionally be added to the alloy to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive influence on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative influence of copper on hot rollability, 0.01 wt.% nickel can be added to the steel; preferably, the Ni content is at least 0.015 wt.%, in particular at least 0.020 wt.%. For economic reasons, the nickel content should be limited to a maximum of 0.5 wt.%, in particular a maximum of 0.20 wt.%. The Ni content is preferably a maximum of 0.10 wt.%.

[0049] Calcium ("Ca") is used in steels to form non-metallic inclusions, particularly manganese sulfides. The rounded shape significantly reduces the negative effect of these inclusions on hot formability, fatigue strength, and toughness. To utilize this effect in a flat steel product according to the invention, a flat steel product according to the invention can optionally contain at least 0.0005 wt.% Ca, in particular at least 0.0010 wt.%, preferably at least 0.0020 wt.%. The maximum Ca content is 0.01 wt.%, in particular a maximum of 0.007 wt.%, preferably a maximum of 0.005 wt.%. Excessively high Ca contents increase the likelihood of non-metallic inclusions involving calcium forming, which impair the purity of the steel and also its toughness. For this reason, the maximum Ca content should not exceed 0.005 wt%, preferably not exceed 0.003 wt%, in particular not exceed 0.002 wt%.-%, preferably a maximum of 0.001 wt.%.

[0050] Tungsten ("W") can optionally be added to the alloy in amounts of 0.001–1.0 wt.% to slow ferrite formation. A positive effect on hardenability is already achieved at W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0051] In preferred embodiments, the sum of the Mn content and the Cr content ("Mn+Cr") is more than 0.7 wt.%, in particular more than 0.8 wt.%, preferably more than 1.1 wt.%. Below a minimum sum of both elements, their necessary transformation-inhibiting effect is lost. Irrespective of this, the sum of the Mn content and the Cr content is less than 3.5 wt.%, preferably less than 2.5 wt.%, in particular less than 2.0 wt.%, particularly preferably less than 1.5 wt.%. The upper limit values ​​for both elements arise to ensure coating performance and to guarantee adequate welding behavior.

[0052] The above explanations regarding element contents and their preferred limits apply accordingly to the process described below for producing a flat steel product, for the sheet metal part and for the process for producing a sheet metal part.

[0053] The flat steel product preferably comprises a corrosion protection coating to protect the steel substrate from oxidation and corrosion during hot forming and during use of the produced steel component.

[0054] In a specific embodiment, the flat steel product preferably comprises an aluminum-based anti-corrosive coating. The anti-corrosive coating can be applied to one or both sides of the flat steel product. The two large, opposing surfaces of the flat steel product are referred to as the two sides of the flat steel product. The narrow surfaces are referred to as the edges.

[0055] Such a corrosion protection coating is preferably produced by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt consisting of 0.1-15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.

[0056] In a preferred variant, the Si content of the melt is 1.0 - 3.5 wt.% or 5-15 wt.%, preferably 7 - 12 wt.%, in particular 8-10 wt.%.

[0057] In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0058] During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer upon solidification.

[0059] The alloy layer lies on the steel substrate and directly borders it. The alloy layer is essentially formed from aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60 wt.% Fe, preferably α-iron, optional further constituents whose total contents are limited to a maximum of 5.0 wt.%, preferably 2.0%, and the remainder aluminum, with the Al content preferably increasing towards the surface. The optional further constituents include in particular the remaining constituents of the melt (i.e., silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

[0060] The Al base layer lies on top of the alloy layer and directly adjoins it. The composition of the Al base layer preferably corresponds to the composition of the melt of the molten bath. This means that it consists of 0.1-15 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.

[0061] In a preferred variant of the Al base layer, the optional content of alkali 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. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0062] In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

[0063] The corrosion protection coating preferably has a thickness of 5 to 60 µm, in particular 10 to 40 µm. The coating weight of the corrosion protection coating is in particular 30 − 360 g m 2 with corrosion protection coatings on both sides, or 15 − 180 g m 2 in the one-sided variant. The coating weight of the corrosion protection coating is preferably 100 − 200 g m 2 for double-sided coatings, or 50 − 100 g m 2 for one-sided coatings. The coating weight of the corrosion protection coating is particularly preferred 120 − 180 g m 2 for double-sided coatings, or 60 − 90 g m 2 for one-sided covers.

[0064] The thickness of the alloy layer is preferably less than 20 µm, more preferably less than 16 µm, especially less than 12 µm, particularly preferably less than 10 µm, preferably less than 8 µm, especially less than 5 µm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 µm, even with thin anti-corrosive coatings.

[0065] In a preferred variant, the flat steel product comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located in particular on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.

[0066] The oxide layer consists in particular of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form. For the optional embodiment with zinc as a component of the Al base layer, zinc oxide components are also present in the oxide layer.

[0067] Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm.

[0068] In an alternative design, the flat steel product includes a zinc-based anti-corrosive coating. The anti-corrosive coating can be applied to one or both sides of the flat steel product. The two large, opposing surfaces of the flat steel product are referred to as the two sides of the flat steel product. The narrow surfaces are referred to as the edges.

[0069] Such a zinc-based corrosion protection coating preferably comprises 0.2-6.0 wt.% Al, 0.1-10.0 wt.% Mg, optionally 0.1-40 wt.% manganese or copper, optionally 0.1-10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and the remainder zinc. In particular, the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%. The Mg content is in particular a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%. The corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition, or by electrolytic processes.

[0070] A further developed flat steel product preferably has a high uniform elongation Ag of at least 10.0%, in particular at least 11.0%, preferably at least 11.5%, in particular at least 12.0%.

[0071] Furthermore, the yield strength of a specially designed flat steel product exhibits a continuous curve or only a slight degree of variation. For the purposes of the application, "continuous curve" means that there is no pronounced yield strength. A yield strength with a continuous curve can also be referred to as a proof strength Rp0.2. A yield strength with a slight degree of variation is understood here as a pronounced yield strength at which the difference ΔRe between the upper yield strength value ReH and the lower yield strength value ReL is at most 45 MPa. The following applies: Δ Re = ReH − ReL ≤ 45 MPa mit ReH = obere Streckgrenze in MPa und ReL = untere Streckgrenze in MPa .

[0072] Particularly good ageing resistance can be achieved with flat steel products for which the difference ΔRe is not more than 25 MPa.

[0073] A specially developed flat steel product has an elongation at break A80 of at least 15%, in particular at least 18%, preferably at least 19%, particularly preferably at least 20%.

[0074] The process for producing a steel flat product for hot forming with a corrosion protection coating comprising the following steps: a) Providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in wt%), consists of C: 0,06 - 0,5 %, Si: 0,05 - 0,6 %, Mn: 0,4-3,0%, Al: 0,06 - 1,0 %, Note: 0,001 - 0,2 %, Ti: 0,001 - 0,10 % B: 0,0005 - 0,01% P: ≤ 0,03 %, S: ≤ 0,02 %, N: ≤ 0,02 %, Sn: ≤ 0,03 % Ace: ≤ 0,01 % and optionally one or more of the elements "Cr, Cu, Mo, Ni, V, Ca, W" in the following contents Cr: 0,01 - 1,0 %, Cu: 0,01 - 0,2 %, Mon: 0,002 - 0,5 %, Ni: 0,01 - 0,5 % V: 0,001 - 0,3% Approx: 0,0005 - 0,005 % W: 0,001 -1,00 % consists; b) heating the slab or thin slab at a temperature (T1) of 1100 - 1400 °C; c) Optional pre-rolling of the soaked slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) hot rolling to a hot-rolled flat steel product, the final rolling temperature (T3) being 750 - 1000 °C; (e) optional coiling of the hot-rolled flat steel product, with a coiling temperature (T4) of not more than 700 °C; f) Optional descaling of the hot-rolled flat steel product; g) Optional cold rolling of the flat steel product, with a cold rolling degree of at least 30%; h) annealing the flat steel product at an annealing temperature (T5) of 650 - 900 °C; i) cooling the flat steel product to an immersion temperature (T6) which is 650 - 800 °C, preferably 670 - 800 °C; j) coating the flat steel product cooled to the immersion temperature with a corrosion protection coating by hot-dip coating in a molten bath with a melt temperature (T7) of 660 - 800 °C, preferably 680 - 740 °C; k) cooling the coated flat steel product to room temperature, the first cooling time being t mT in the temperature range between 600 °C and 450 °C is more than 10s, in particular more than 14s and the second cooling time t nT in the temperature range between 400 °C and 300 °C is more than 8s, in particular more than 12s; l) Optional skin passing of the coated flat steel product.

[0075] In step a), a semi-finished product composed according to the alloy specified for the steel flat product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin slab casting.

[0076] In step b), the semi-finished product is thoroughly heated to a temperature (T1) of 1100–1400 °C. If the semi-finished product has cooled after casting, it is first reheated to 1100–1400 °C for thorough heating. The thorough heating temperature should be at least 1100 °C to ensure good formability for the subsequent rolling process. The thorough heating temperature should not exceed 1400 °C to avoid the presence of molten phases in the semi-finished product.

[0077] In the optional step c), the semi-finished product is pre-rolled into an intermediate product. Thin slabs are not usually subjected to pre-rolling. Thick slabs to be rolled into hot strip can be pre-rolled if required. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C to ensure that the intermediate product retains sufficient heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product. To keep grain growth low during the rolling process, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200 °C.

[0078] In step d), the slab or thin slab, or, if step c) has been performed, the intermediate product, is rolled into a hot-rolled flat steel product. If step c) has been performed, the intermediate product is typically finish-rolled immediately after rough rolling. Typically, finish rolling begins no later than 90 seconds after the end of rough rolling. The slab, the thin slab, or, if step c) has been performed, the intermediate product, are rolled to a final rolling temperature (T3). The final rolling temperature, i.e., the temperature of the finished hot-rolled flat steel product at the end of the hot rolling process, is 750–1000 °C. At finish rolling temperatures below 750 °C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those performed before hot-dip coating. The final rolling temperature is limited to a maximum of 1000 °C to prevent coarsening of the austenite grains. Furthermore, final rolling temperatures of a maximum of 1000 °C are relevant for the process to achieve coiling temperatures (T4) below 700 °C.

[0079] Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or reversing rolling. In the case of continuous hot strip rolling, step e) provides for the optional coiling of the hot-rolled steel flat product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. The cooling medium used for this purpose can be water, air, or a combination of both. The coiling temperature (T4) should not exceed 700 °C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500 °C have proven favorable for cold rolling. The coiled hot strip is then cooled to room temperature in air using the conventional method.

[0080] In step f), the hot-rolled flat steel product is optionally descaled in a conventional manner by pickling or by another suitable treatment.

[0081] The scale-cleaned hot-rolled flat steel product can optionally be subjected to cold rolling before annealing in step g), for example, to meet more stringent thickness tolerance requirements. The cold rolling degree (CTC) should be at least 30% to inject sufficient deformation energy into the flat steel product for rapid recrystallization. The CTC is defined as the quotient of the thickness reduction during cold rolling ΔdCTC divided by the hot strip thickness d: KWG = Δ dKW / d where ΔdKW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, where the thickness reduction ΔdKW results from the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is usually a hot strip with a hot strip thickness of d. The flat steel product after cold rolling is also commonly referred to as cold strip. The cold rolling degree can, in principle, assume very high values ​​of over 90%. However, cold rolling degrees of up to 80% have proven to be beneficial for preventing strip breakage.

[0082] In step h), the flat steel product undergoes an annealing treatment at annealing temperatures (T5) of 650–900 °C. For this purpose, the flat steel product is first heated to the annealing temperature within 10 to 120 seconds and then held at that temperature for 30 to 600 seconds. The annealing temperature is at least 650 °C, preferably at least 720 °C. Annealing temperatures above 900 °C are undesirable for economic reasons.

[0083] In step i), the flat steel product is cooled to an immersion temperature (T6) after annealing to prepare it for subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the molten bath. The immersion temperature is 600–800°C, preferably at least 650°C, more preferably at least 670°C, and most preferably at most 700°C. For particularly homogeneous boundary layer formation, it is important that sufficient thermal energy is present in the boundary layer between the steel substrate and the aluminum melt. This is not the case at temperatures lower than 600°C, so that undesirable compounds can form, the subsequent reconversion of which can lead to pores.Above the preferred immersion temperatures, the diffusion rate of iron into aluminum increases significantly again, allowing increased iron diffusion into the still-liquid boundary layer right at the beginning of the coating process. The cooling time of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10–180 s. In particular, the immersion temperature T6 should not deviate from the temperature of the molten bath T7 by more than 30 K, especially not more than 20 K, and preferably not more than 10 K.

[0084] The flat steel product is subjected to a coating treatment in work step j). The coating treatment is preferably carried out by means of continuous hot-dip coating. The coating can be applied to just one side, both sides, or all sides of the flat steel product. The coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process. The flat steel product usually comes into contact with the molten bath on all sides, so that it is coated on all sides. The molten bath, which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660 - 800 °C, preferably 680 - 740 °C. Aluminum-based alloys have proven particularly suitable for coating ageing-resistant flat steel products with an anti-corrosive coating. In such a case, the molten bath contains up to 15 wt.-% Si, preferably more than 1.0%, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, in particular up to 10 wt.% Zn and optionally further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0085] After the coating treatment, the coated flat steel product is cooled to room temperature in step k). A first cooling time t mT in the temperature range between 600 °C and 450 °C (medium temperature range mT) is more than 10 s, in particular more than 14 s, and a second cooling time t nT in the temperature range between 400 °C and 300 °C (low temperature range nT) is more than 8 s, in particular more than 12 s.

[0086] The first cooling time t mT can be achieved in the temperature range between 600 °C and 450 °C (medium temperature range mT) by slow, continuous cooling or by holding at a temperature for a certain time within this temperature range. Intermediate heating is even possible. The only important thing is that the flat steel product remains in the temperature range between 600 °C and 450 °C for at least the cooling time t mT. In this temperature range, on the one hand, there is a significant diffusion rate of iron into aluminum and, on the other hand, the diffusion of aluminum into steel is inhibited because the temperature is below half the melting temperature of steel. This allows diffusion of iron into the corrosion protection coating without strong diffusion of aluminum into the steel substrate.

[0087] The diffusion of iron into the corrosion protection coating has several advantages: Firstly, it delays the melting of the corrosion protection coating during austenitizing before press hardening. Secondly, it homogenizes the thermal expansion coefficients of the corrosion protection coating and the substrate. This means that the transition area between the substrate's thermal expansion coefficient and the surface becomes wider, which reduces thermal stresses during reheating.

[0088] At the same time, the diffusion of aluminum into the steel substrate would have significant disadvantages: Due to the very high affinity of aluminum for nitrogen, a high aluminum content can lead to nitrogen dissolving from fine precipitates, such as niobium carbonitrides or titanium carbonitrides, and instead, coarse precipitates, such as aluminum nitrides, forming preferentially at the grain boundaries. These would impair crash performance and reduce the bending angle. Furthermore, this destabilizes the fine precipitates (e.g., the Nb-containing precipitates) in the uppermost substrate region, which are important for many preferred properties. Furthermore, the inhomogeneous diffusion rate of aluminum in the steel substrate in ferrite versus pearlite / bainite / martensite would lead to an uneven distribution of Al in the surface layer of the steel substrate. This should also be prevented to improve crash and bending performance.These disadvantages of aluminum diffusion into the steel substrate are therefore reduced or avoided by inhibition.

[0089] Due to the preferred initial cooling time t mT (14 s), the iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum uptake into the substrate during austenitization prior to press hardening, with the associated advantages described above.

[0090] The second cooling time t nT in the temperature range between 400 °C and 300 °C (low temperature range nT) can also be achieved by slow, continuous cooling or by holding the product at a temperature within this temperature range for a certain period of time. Intermediate heating is even possible. The only important thing is that the flat steel product remains in the temperature range between 400 °C and 300 °C for at least the cooling time t nT.

[0091] In this temperature range, carbon still diffuses to a certain extent in the steel substrate, while its thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there, for example, in dissolved Nb atoms. Due to their significantly higher atomic volume, these atoms expand the atomic lattice and thus enlarge the tetrahedral and octahedral interstices in the atomic lattice, increasing the local solubility of C. This results in clusters of C and Nb, which then transform into very fine precipitates during the austenitization step of hot forming, leading to a refined austenite microstructure and thus also a hardening microstructure, as well as a reduction in the free hydrogen content.

[0092] With the preferred holding time of more than 12s, very fine iron carbides (so-called transition carbides) are also formed, which in turn dissolve very quickly during austenitizing and lead to additional austenite nuclei and thus an even finer austenite structure and thus also a hardening structure.

[0093] The coated flat steel product can optionally be subjected to skin passing with a skin passing degree of up to 2% to improve the surface roughness of the flat steel product.

[0094] The invention further relates to a sheet metal part formed from a flat steel product comprising a previously described steel substrate and a corrosion protection coating. The corrosion protection coating has the advantage of preventing scale formation during austenitization during hot forming. Furthermore, such a corrosion protection coating protects the formed sheet metal part against corrosion.

[0095] In a specific embodiment, the sheet metal part preferably comprises an aluminum-based corrosion protection coating. The corrosion protection coating of the sheet metal part preferably comprises an alloy layer and an Al base layer. In the sheet metal part, the alloy layer is often also referred to as an interdiffusion layer.

[0096] The thickness of the corrosion protection coating is preferably at least 10 µm, particularly preferably at least 20 µm, in particular at least 30 µm.

[0097] The thickness of the alloy layer is preferably less than 30 µm, particularly preferably less than 20 µm, especially less than 16 µm, and particularly preferably less than 12 µm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer.

[0098] The alloy layer lies directly on the steel substrate. The alloy layer of the sheet metal part preferably consists of 35–90 wt.% Fe, 0.1–10 wt.% Si, optionally up to 0.5 wt.% Mg, and optional additional components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum. Due to the further diffusion of iron into the alloy layer, the Si and Mg contents are correspondingly lower than their respective contents in the melt of the molten bath.

[0099] The alloy layer preferably has a ferritic structure.

[0100] The Al base layer of the sheet metal part lies on top of the alloy layer of the steel component and is directly adjacent to it. The Al base layer of the steel component preferably consists of 35–55 wt.% Fe, 0.4–10 wt.% Si, optionally up to 0.5 wt.% Mg, and optionally other components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.

[0101] The Al base layer can have a homogeneous element distribution, with local element contents varying by no more than 10%. Preferred variants of the Al base layer, however, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are regions whose average Si content is at least 20% lower than the average Si content of the Al base layer. Silicon-rich phases are regions whose average Si content is at least 20% higher than the average Si content of the Al base layer.

[0102] In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form at least a 40% continuous layer bordered by silicon-poor regions. In an alternative embodiment, the silicon-rich phases are arranged in islands within the silicon-poor phase.

[0103] For the purposes of this application, "island-shaped" means an arrangement in which discrete, unconnected areas are enclosed by another material - i.e., "islands" of a particular material are located within another material.

[0104] In a preferred variant, the steel component comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located in particular on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.

[0105] The oxide layer of the steel component consists, in particular, of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, either alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form.

[0106] The oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is a maximum of 4 µm, in particular a maximum of 2 µm.

[0107] In a special design, the sheet metal part includes a zinc-based anti-corrosion coating.

[0108] Such a zinc-based corrosion protection coating preferably comprises up to 80 wt.% Fe, 0.2 - 6.0 wt.% Al, 0.1 - 10.0 wt.% Mg, optionally 0.1 - 40 wt.% manganese or copper, optionally 0.1 - 10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and the remainder zinc. In particular, the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%. The Fe content, which results from diffusion, is preferably more than 20 wt.%, in particular more than 30 wt. Furthermore, the Fe content is in particular a maximum of 70 wt.%, in particular a maximum of 60 wt.% The Mg content is in particular a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%. The corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition or by electrolytic processes.

[0109] In a specific development, the steel substrate of the sheet metal part has a microstructure with at least partially more than 80% martensite and / or lower bainite, preferably at least partially more than 90% martensite and / or lower bainite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%. In a preferred development, the steel substrate of the sheet metal part has a microstructure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%. In this context, "partially having" is to be understood as meaning that there are regions of the sheet metal part that have the mentioned microstructure. In addition, there may also be regions of the sheet metal part that have a different microstructure. The sheet metal part therefore has the mentioned microstructure in sections or regions.

[0110] The high martensite content allows very high tensile strengths and yield points to be achieved.

[0111] In an alternative development, the steel substrate of the sheet metal part has a microstructure with a ferrite content of more than 5%, preferably more than 10%, in particular more than 20%. Furthermore, the ferrite content is preferably less than 85%, in particular less than 70%. The martensite content is less than 80%, in particular less than 50%. In addition, the microstructure can optionally contain bainite and / or pearlite. The exact ratio of the microstructure components depends on the level of the C content and the Mn content as well as on the cooling conditions during forming. The microstructure designed in this way has greater ductility and therefore leads to improved forming behavior. A corresponding sheet metal part preferably has an elongation at break A80 in a range of 8% to 25%, preferably between 10% and 22%, in particular between 12% and 20%.

[0112] In a preferred embodiment, the former austenite grains of the martensite have an average grain diameter of less than 14 µm, in particular less than 12 µm, preferably less than 10 µm. The fine microstructure makes it more homogeneous. This results in an improvement in the mechanical properties, in particular, a lower crack susceptibility and thus improved bending properties and higher elongation at break.

[0113] In a further developed variant, the sheet metal part has at least partially a yield strength of at least 950 MPa, in particular at least 1100 MPa, in particular at least 1200 MPa, preferably at least 1300 MPa, preferably at least 1400 MPa, in particular at least 1500 MPa.

[0114] In a further developed variant, the sheet metal part has at least partially a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, preferably at least 1400 MPa, in particular at least 1600 MPa, preferably at least 1700 MPa, in particular at least 1800 MPa.

[0115] In particular, the sheet metal part has at least partially an elongation at break A80 of at least 3.5%, in particular at least 4%, in particular at least 4.5%, preferably at least 5%, particularly preferably at least 6%.

[0116] In addition, in a preferred variant, the sheet metal part can at least partially have a bending angle of at least 30°, in particular at least 40°, in particular at least 45°, preferably at least 50°. The bending angle here is understood to be the bending angle corrected for the sheet thickness. The corrected bending angle results from the determined bending angle at the maximum force (measured according to VDA standard 238-100) (also referred to as the maximum bending angle) from the formula Biegewinkel korrigiert = Biegewinkel ermittel ⋅ Blechdicke where the sheet thickness in mm is to be inserted into the formula. This applies to sheet thicknesses greater than 1.0 mm. For sheet thicknesses less than 1.0 mm, the corrected bending angle corresponds to the determined bending angle. In this context, "partially exhibit" means that there are areas of the sheet metal part that exhibit the specified mechanical property. In addition, there may also be areas of the sheet metal part whose mechanical properties are below the limit value. The sheet metal part therefore exhibits the specified mechanical property in sections or regions. This is because different areas of the sheet metal part can undergo different heat treatments. For example, individual areas can be cooled more quickly than others, resulting in more martensite, for example, forming in the faster-cooled areas. Therefore, different mechanical properties also arise in the different areas.

[0117] The mechanical characteristics mentioned have proven to be particularly advantageous in ensuring use in a vehicle with good crash performance.

[0118] In a special further development, the sheet metal part has fine precipitations in the structure, particularly in the form of niobium carbonitrides and / or titanium carbonitrides.

[0119] For the purposes of this application, fine precipitates are defined as all precipitates with a diameter of less than 30 nm. The remaining precipitates are referred to as coarse precipitates.

[0120] In a preferred embodiment, the average diameter of the fine precipitates is a maximum of 11 nm, preferably a maximum of 10 nm, in particular a maximum of 8 nm, preferably a maximum of 6 nm. For the purposes of this invention, largely fine precipitates means that more than 80%, preferably more than 90%, of all precipitates are fine precipitates. This means that more than 80%, preferably more than 90%, of all precipitates have a diameter of less than 30 nm.

[0121] The fine precipitates result in a particularly fine microstructure with small grain diameters. This fine microstructure makes the material more homogeneous. This results in improved mechanical properties, particularly reduced crack susceptibility, resulting in improved flexural properties and higher elongation at fracture. This also results in improved toughness with more pronounced fracture necking behavior.

[0122] The actual mechanical properties of the sheet metal part are determined by first coating it cathodically with a dip coating or subjecting it to a similar heat treatment. Cathodic dip coatings are typically used for corresponding components in the automotive industry. In cathodic dip coating, the components are first coated in an aqueous solution. This coating is then baked in a heat treatment. The sheet metal parts are heated to 170°C and held at this temperature for 20 minutes. The components are then cooled to room temperature in ambient air.Since this heat treatment can influence the mechanical parameters, for the purposes of this application, the mechanical parameters (yield strength, tensile strength, yield strength ratio, elongation at break A80, bending angle, Vickers hardness) are to be understood as existing on a component with a cathodic dip coating or on a component that, after forming, was subjected to a heat treatment analogous to a cathodic dip coating. In practice, the heat treatment for cathodic dip coating varies slightly. Temperatures of 165°C–180°C and holding times of 12–30 minutes are typical. However, the changes in the mechanical parameters due to these variations (165°C–180°C; 12–30 minutes) are negligible.

[0123] In a preferred variant, the sheet metal part comprises a cathodic dip coating.

[0124] In a particularly preferred embodiment of the sheet metal part, the electrochemical potential of the surface of the sheet metal part in a corrosive medium is at least -0.50 V. The electrochemical potential is therefore -0.50 V or greater, i.e. more positive.

[0125] The electrochemical potential is determined according to DIN standard "DIN 50918 (2018.09) ("Rest potential measurement on homogeneous mixed electrodes"). Where absolute values ​​are given for the electrochemical potential instead of differential values, this refers to the reference to the standard hydrogen electrode. The corrosive medium used in the measurement is an aqueous 5% NaCl solution with a pH of 7, which represents typical corrosion conditions in the automotive sector. In other words, the electrochemical potential of the surface of the sheet metal part in an aqueous 5% NaCl solution with a pH of 7 is at least -0.50 V.

[0126] The electrochemical potential is preferably at least -0.45 V, particularly preferably at least -0.40 V, in particular at least -0.39 V, particularly preferably at least -0.38 V, in particular at least -0.36 V, preferably at least -0.34 V. Furthermore, the electrochemical potential is preferably at most -0.1 V, preferably at most -0.20 V, in particular -0.25 V, preferably at most -0.30 V.

[0127] A higher, i.e., more positive, electrochemical potential has the advantage that the sheet metal part exhibits a lower tendency to corrosion. Surprisingly, it has been shown that the higher Nb content in the steel substrate leads to a shift in the electrochemical potential toward a more positive (i.e., more noble) potential. This potential is typically about 100-150 mV higher than for a comparable substrate with a lower Nb content.

[0128] A further developed variant of the sheet metal part is characterized in that the anti-corrosive coating is an aluminum-based anti-corrosive coating, and the sheet metal part comprises an alloy layer and an aluminum base layer. Furthermore, in the cross-section of the alloy layer over a measuring length of 500 µm, the porous area in the alloy layer is less than 250 µm 2<, preferably less than 200 µm 2<, in particular less than 180 µm 2<, particularly preferably less than 100 µm 2<, in particular less than 75 µm 2<.

[0129] Pores are cavities that can arise within the alloy layer for various reasons. One mechanism is the formation of iron aluminide compounds with a higher density via a multi-stage phase transformation (Fe2Al5→Fe2Al→FeAl→Fe3Al). The formation of such denser phases is associated with higher aluminum consumption than with less dense phases. This locally higher aluminum consumption leads to the formation of pores (vacancies) in the resulting phase. These pores preferentially form in the alloy layer in the transition zone between the steel substrate and the corrosion protection coating, where the proportion of available aluminum is strongly influenced by the aluminum content of the steel substrate. In particular, an accumulation of pores in the form of a band can occur in the alloy layer in the transition zone, i.e. in the third of the alloy layer closest to the substrate.

[0130] By reducing the pore area, a variety of problems can be reduced or prevented: The pores reduce the mechanical integrity in this area. Layer detachment can occur more quickly under corrosive stress. In addition, the transferable force at the connection point between two components after bonding or welding is reduced. With the reduction in the number of pores achieved according to the invention, the area over which the forces of the adhesive bond are transferred is increased by over 60% during bonding. Consequently, the risk of delamination failure is correspondingly reduced. The pores lead to altered current paths in the material during resistance spot welding, which negatively affect the weldability and thus reduce the weld area. By reducing the pores, an increased weld area and thus stable further processing of the sheet metal component can be enabled. The pores themselves facilitate crack initiation and crack propagation during static and dynamic bending.By reducing the pore area, a higher bending angle can be achieved.

[0131] In a special embodiment, the proportion of the surface area occupied by pores in the alloy layer with a diameter greater than or equal to 0.1 µm is less than 10%, in particular less than 5%, preferably less than 3%. Smaller pores have a significantly lesser impact on the aforementioned reduction in mechanical integrity. Therefore, a particularly fine-pored alloy layer is preferred.

[0132] In a particularly preferred embodiment, the welding range is at least 0.9 kA, preferably at least 1.0 kA, particularly preferably at least 1.1 kA, in particular at least 1.2 kA. The welding range is determined according to SEP 1220-2. In particular, the welding range is a maximum of 1.6 kA, in particular a maximum of 1.4 kA. These ranges enable particularly stable further processing of the sheet metal parts.

[0133] In a special embodiment, the Nb content in the alloy layer is greater than 0.010 wt.%, preferably greater than 0.015 wt.%, in particular greater than 0.018 wt.%.

[0134] The sheet metal part according to the invention is preferably a component for a land vehicle, marine vehicle, or aircraft. It is particularly preferably an automotive part, in particular a body part. The component is preferably a B-pillar, side member, A-pillar, sill, or cross member.

[0135] The inventive method for producing a sheet metal part is set out in independent claim 8.

[0136] In the method according to the invention, a blank is thus provided which consists of a steel which has been suitably composed in accordance with the above explanations (working step a)), which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg of the blank when placed in a forming tool intended for hot press forming (working step c)) is at least partially above Ms+100°C, in particular above Ms+300°C. In particular, the temperature T Einlg of the blank during insertion at least partially exceeds 600°C. In a particularly preferred variant, the temperature T Einlg of the blank during insertion is at least partially, in particular completely in the range 600°C to 850°C, in order to ensure good formability and sufficient hardenability. Partially exceeding a temperature (here AC3 orMs+100°C) is understood in the context of this application to mean that at least 30%, in particular at least 60% of the volume of the blank, preferably the entire blank, exceeds a corresponding temperature. The same applies to the at least partial presence of a temperature in the interval 600°C to 850°C in the preferred variant explained above. When placed in the forming tool, at least 30% of the blank therefore has an austenitic structure, i.e. the transformation from the ferritic to the austenitic structure does not have to be complete when placed in the forming tool. Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite.For this purpose, certain areas of the blank can be kept at a lower temperature than others during heating. To do this, the heat can be specifically directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat supply. In the part of the blank material whose temperature remains lower, no or only significantly less martensite is formed during forming in the tool, so that the microstructure there is significantly softer than in the other parts that have a martensitic microstructure. In this way, a softer area can be specifically set in the formed sheet metal part, which, for example, has optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0137] Maximum strength properties of the obtained sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

[0138] The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Volume 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229., with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

[0139] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0140] In a preferred embodiment, the average heating rate r furnace of the sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, in particular at least 6 K / s, preferably at least 8 K / s, in particular at least 10 K / s, preferably at least 15 K / s. The average heating rate r furnace is to be understood as the average heating rate from 30 °C to 700 °C.

[0141] In a preferred embodiment, the standardized mean heating Θ standard at least 5 Kmm / s, in particular at least 8 Kmm / s, preferably at least 10 Kmm / s. The maximum standardized average heating rate is 15 Kmm / s, in particular a maximum of 14 Kmm / s, preferably a maximum of 13 Kmm / s.

[0142] The average heating Θ is the product of the average heating rate in Kelvin per second from 30 °C to 700 °C and the sheet thickness in millimeters.

[0143] In the normalized average heating, this product Θ is normalized by the present furnace temperature T furnace in relation to a reference furnace temperature T furnace, reference of 900°C = 1173.15 K in the following way: Θ norm = T Ofen , Referenz 4 T Ofen 4 Θwhere the oven temperatures are to be entered in Kelvin.

[0144] In a preferred embodiment, the heating takes place in a furnace with a furnace temperature T furnace of at least Ac3+10°C, preferably at least 850 °C, preferably at least 880 °C, particularly preferably at least 900 °C, in particular at least 920 °C, and at most 1000 °C, preferably at most 950 °C, particularly preferably at most 930 °C.

[0145] The dew point of the furnace atmosphere in the furnace is preferably at least -20 °C, preferably at least -15 °C, in particular at least -5 °C, particularly preferably at least 0 °C and at most +25 °C, preferably at most +20 °C, in particular at most +15 °C.

[0146] In a specific embodiment, the heating in step b) takes place stepwise in areas with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. Here, the heating takes place in a first heating zone at a temperature (so-called furnace inlet temperature) of at least 650 °C, preferably at least 680 °C, in particular at least 720 °C. The maximum temperature in the first heating zone is preferably 900 °C, in particular a maximum of 850 °C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably a maximum of 1200 °C, in particular a maximum of 1000 °C, preferably a maximum of 950 °C, particularly preferably a maximum of 930 °C.

[0147] The total time in the furnace t furnace , which consists of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, preferably at least 4 minutes for both variants (constant furnace temperature, step-by-step heating). Furthermore, the total time in the furnace for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties.

[0148] The blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an equally known induction heating device or a conventional device for keeping steel components hot, and transported into the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms+100°C, in particular above Ms+300°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the AC1 temperature. In all of these variants, the maximum temperature is in particular 900°C. These temperature ranges ensure good formability of the material overall.

[0149] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably a maximum of 20 seconds, in particular within a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0150] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C. When the blank is inserted, the tool can also have a temperature slightly below room temperature if, for example, the cooling water used is slightly colder (e.g. 15 °C). In some embodiments, the tool therefore has a temperature between 10 °C and 200 °C when the blank is inserted. Optionally, in a special embodiment, the tool can be tempered, at least in some areas, to a temperature T WZ of at least 200 °C, in particular at least 300 °C, in order to only partially harden the component. Furthermore, the tool temperature T WZ is preferably a maximum of 600 °C, in particular a maximum of 550 °C. It only has to be ensured that the tool temperature T WZ is below the desired target temperature T Ziel.The residence time in the tool t WZ is preferably at least 2 s, in particular at least 3 s, particularly preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, in particular at most 20 s, preferably at most 10 s.

[0151] The target temperature T target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C. Alternatively, the target temperature T target of the sheet metal part is particularly preferably below Ms-50°C, where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.

[0152] The martensite start temperature of a steel which is within the scope of the specifications according to the invention is to be calculated according to the formula: where %C denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%.

[0153] The AC1 temperature and the AC3 temperature of a steel which lies within the scope of the specifications according to the invention are to be calculated according to the formulas:, where %C denotes the C content, %Si denotes the Si content, %Mn denotes the Mn content, %Cr denotes the Cr content, %Mo denotes the Mo content, %Ni denotes the Ni content and +%V denotes the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10).

[0154] In the tool, the blank is not only formed into the sheet metal part, but is also simultaneously quenched to the target temperature. The cooling rate in the tool to the target temperature is in particular at least 20 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s.

[0155] After removal of the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature T AB of less than 100 °C within a cooling time t AB of 0.5 to 600 s. This is usually done by air cooling.

[0156] In the following, the invention is explained in more detail using exemplary embodiments.

[0157] The figures show: Figure 1a shows a schematic representation of a sample according to the invention with a low number of pores, obtained from a micrograph. Figure 1b shows a schematic representation of a reference sample with an increased number of pores, obtained from a micrograph. Figure 2a shows a schematic representation of a sample according to the invention with a low number of pores, obtained from a micrograph after a corrosion test. Figure 2b shows a schematic representation of a reference sample with an increased number of pores, obtained from a micrograph after a corrosion test. Figure 3 shows a grain representation of the reconstructed austenite.

[0158] To demonstrate the effectiveness of the invention, several tests were conducted. Slabs with the compositions listed in Table 1, thicknesses of 200–280 mm and widths of 1000–1200 mm, were produced. They were heated in a pusher-type furnace to a respective temperature T1 and held at T1 for between 30 and 450 minutes until the temperature T1 in the core of the slabs was reached and the slabs were thus thoroughly heated. The production parameters are listed in Table 2. The slabs were discharged from the pusher-type furnace at their respective thorough heating temperatures T1 and subjected to hot rolling. The tests were carried out as continuous hot strip rolling. For this purpose, the slabs were first pre-rolled to an intermediate product with a thickness of 40 mm. The intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 at the end of the pre-rolling phase.The pre-rolled strips were fed to the finish rolling immediately after rough rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The pre-rolled strips were rolled into hot strips with a final thickness of 3-7 mm and the respective final rolling temperatures T3 specified in Table 2, cooled to the respective coiling temperature, wound into coils at the respective coiling temperatures T4, and then cooled in still air. The hot strips were descaled in a conventional manner by pickling before being subjected to cold rolling with the cold rolling grades specified in Table 2. The cold-rolled flat steel products were heated in a continuous annealing furnace to a respective annealing temperature T5 and held at annealing temperature for 100 s each before being cooled to their respective immersion temperature T6 at a cooling rate of 1 K / s.The cold-rolled strips were passed through a molten coating bath at temperature T7 at their respective immersion temperature T6. The composition of the coating bath is given in Table 3. After coating, the coated strips were blown off in a conventional manner, producing deposits with varying layer thicknesses (see Table 3). The strips were first cooled to 600 °C at an average cooling rate of 10 - 15 K / s. During the further cooling process between 600 °C and 450 °C and between 400 °C and 300 °C, the strips were cooled for the cooling times T mT and T nT given in Table 2. Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5 - 15 K / s in each case.

[0159] Table 4 shows which steel variant (see Table 1) was combined with which process variant (see Table 2) and which coating (see Table 3).

[0160] Steel compositions D, E, and F are reference examples that are not in accordance with the invention. Accordingly, tests 3, 10, 11, 12, 13, 17, and 18 are not in accordance with the invention.

[0161] The thickness of the steel strips produced was between 1.4 mm and 1.7 mm in all tests.

[0162] After cooling to room temperature, samples were taken from the cooled steel strips transverse to the rolling direction in accordance with DIN EN ISO 6892-1 sample shape 2 (Appendix B Table B1). The samples were subjected to a tensile test in accordance with DIN EN ISO 6892-1 sample shape 2 (Appendix B Table B1). The results of the tensile test are given in Table 4. The following material properties were determined during the tensile test: the type of yield point, which is designated Re for a pronounced yield point and Rp for a continuous yield point, and for a continuous yield point the value for the proof stress Rp0.2, for a pronounced yield point the values ​​for the lower yield point ReL, the upper yield point ReH and the difference between the upper and lower yield points ΔRe, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80.All specimens exhibit a continuous yield strength Rp or a slightly pronounced yield strength with a difference ΔRe between the upper and lower yield strengths of no more than 45 MPa and a uniform elongation Ag of at least 11.5%. Specimens 3 and 17 exhibit a pronounced yield strength Re, while all other specimens exhibit a continuous yield strength Rp. For specimens 3 and 17, the lower yield strength ReL and the upper yield strength ReH are given in Table 4. For all other specimens, the yield strength Rp0.2 is given.

[0163] From the 20 steel strips produced in this way, blanks were cut off and used for further tests. In these tests, sheet metal parts 1 - 24 in the form of 200 x 300 mm 2< plates were hot-pressed from the respective blanks. Table 7 shows which of the 20 coating tests corresponds to which forming test. For this purpose, the blanks were heated in a heating device, for example a conventional heating furnace, from room temperature at an average heating rate r furnace (between 30 °C and 700 °C) in a furnace with a furnace temperature T furnace. The total time in the furnace, which includes heating and holding, is designated t furnace. The dew point of the furnace atmosphere was -5 °C in all cases. The blanks were then removed from the heating device and placed in a forming tool which has the temperature T wz.At the time of removal from the furnace, the blanks had reached the furnace temperature. The transfer time t Trans , comprising the time required for removal from the heating device, transport to the tool, and insertion into the tool, was between 5 and 14 s. The temperature T Einlg of the blanks upon insertion into the forming tool was above the respective martensite start temperature +100°C in all cases. The blanks were formed into the respective sheet metal parts in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate r WZ . The residence time in the tool is denoted by t WZ . Finally, the samples were cooled in air to room temperature. Table 5 lists the parameters mentioned for various variants, where "RT" abbreviates room temperature.

[0164] Table 5 shows very different variants for the forming process. While, for example, variant II results in almost complete formation of a martensitic microstructure (see Table 8, Test 1), the comparatively slow cooling of variant X with the high tool temperature T WZ leads to a modified microstructure formation with high ferrite contents, which results in a higher elongation at fracture A80.

[0165] Table 6 lists the key parameters for a further developed process variant. In these tests, the sheet metal blanks were not heated in a furnace with a constant furnace temperature as in the previously described tests, but rather the sheet metal blanks were heated step by step in areas with different temperatures. The tests were carried out in a roller hearth furnace with different heating zones. In principle, however, the process can also be implemented in several separate furnaces. The blanks were first brought into an inlet area of ​​the furnace with an inlet temperature T inlet. From there, the blanks were moved through a central area to an outlet area of ​​the furnace with an outlet temperature T outlet. Table 6 shows the inlet temperature T inlet, the outlet temperature T outlet and the maximum furnace temperature T max through which the blanks pass.In most cases, the maximum furnace temperature was assumed to be in the outlet zone. However, in variant AX, the maximum furnace temperature was assumed to be in the central zone. The further procedure was identical to the previously described process. The corresponding parameters are listed in Table 6.

[0166] Table 7 summarizes the overall results for the sheet metal parts obtained. The first columns indicate the sample number, the steel grade according to Table 1, the process variant according to Table 2, the coating according to Table 2, and the hot forming variant according to Table 5 and Table 6, respectively. The following columns show the yield strength, the tensile strength, and the elongation at break A80. These values ​​were determined according to DIN EN ISO 6892-1 specimen shape 2 (Appendix B Table B1) on specimens transverse to the rolling direction. The determined bending angle was determined according to VDA standard 238-100 with a bending axis transverse to the rolling direction. The determined bending angle is calculated from the punch travel using the formula specified in the standard (the determined bending angle (also referred to as the maximum bending angle) is the bending angle at which the force in the bending test is at its maximum).In order to eliminate the influence of the sheet thickness on the bending angle, the corrected bending angle was calculated from the determined bending angle using the formula . Biegewinkel korrigiert = Biegewinkel ermittelt ⋅ Blechdicke where the sheet thickness in mm must be inserted into the formula. This applies to sheet thicknesses greater than 1.0 mm. For sheet thicknesses less than 1.0 mm, the corrected bending angle corresponds to the determined bending angle. The measured maximum bending angle is given in Table 7. To determine the corrected bending angle, these numerical values ​​must be multiplied by the square root of the sheet thickness, which is given in Table 4.

[0167] The mechanical characteristics in Table 7 were determined after a cathodic dip coating was applied to the formed sheet metal part. During this coating process, the sheet metal parts were heated to 170 °C and held at this temperature for 20 minutes. The components were then cooled to room temperature in ambient air.

[0168] The microstructural properties of the sheet metal part are listed in Table 8. The microstructural fractions are given in area %. All examples according to the invention have a martensite content of more than 90%.

[0169] Table 8 also lists the properties of the fine precipitates in the microstructure. These precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refinement. The precipitates are determined using electron-optical and X-ray images (TEM and EDX) based on carbon extraction replicas (known in the technical literature as "carbon extraction replicas"). The carbon extraction replicas were created on longitudinal sections (20 x 30 mm). The measurement resolution la is between 10,000 and 200,000 times. Based on these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates are all precipitates with a diameter of less than 30 nm. The remaining precipitates are referred to as coarse precipitates. The proportion of fine precipitates to the total number of precipitates in the measurement field is determined by simple counting.The average diameter of the fine precipitates is also calculated using computer-assisted image analysis. In the samples according to the invention, the proportion of fine precipitates is more than 90%. The average diameter of the fine precipitates is less than 11 nm.

[0170] Furthermore, the grain diameter of the former austenite grains is given in Table 8. For this purpose, the austenite grains were reconstructed from EBSD measurements using the ARPGE software. The software parameters were: Orientation relationship Nishiyama-Wassermann Tolerance for grain identification 7° Tolerance for parent growth nucleation 7° Tolerance for parent grain growth 15° Minimum accepted grain size 10 Pixel

[0171] For grain identification, a maximum orientation deviation of 5° and a minimum grain diameter of 5 pixels were assumed according to DIN EN ISO 643.

[0172] An example is Figure 3a corresponding reconstruction of the austenite from test no. 1. In this case, the mean diameter of the former austenite grains is 7.5 µm. In all examples according to the invention, the mean grain diameter of the former austenite grains is below 14 µm. In two tests, the grain diameter of the former austenite grains was not determined. The entry in Table 8 is therefore "nb" (not determined).

[0173] Table 9 shows the application-specific properties of the sheet metal part. Firstly, the porous area in the alloy layer is given over a measuring length of 500 µm. In all examples according to the invention, this area is less than 250 µm 2< . It is clearly evident that more pores form in the coating variants γ and φ, which do not contain Mg. This applies to tests 1, 3, 4, 5, 7, 10, 12, 16, and 18. In contrast, the other Mg-containing layers show fewer pores. The Nb content in the alloy layer given in Table 9 is an average value of the Nb content in this layer. The Nb content in the alloy layer decreases slightly towards the surface and is approximately characterized by a linear decrease in the layer.

[0174] Furthermore, Table 9 shows the proportion of the area occupied by pores with a diameter greater than or equal to 0.1 µm. In all examples according to the invention, this proportion is less than 10%. The total area of ​​the pores and the proportion of pores larger than 0.1 µm were determined from micrographs using computer-assisted image analysis. As an example, Figure 1a a micrograph of test 1 with a fine pore structure and Figure 1b For comparison, a micrograph of test 12 with a coarser pore structure in the alloy layer. Figure 1b The larger pores are visible as black spots in the alloy layer. Figures 2a and 2b The effects of the larger pores after a corrosion test are shown. Figures 2a and 2bshow micrographs of the same tests after a corrosion test. For this purpose, the samples were placed in a corrosive medium and subjected to a current to simulate prolonged electrochemical corrosion. The corrosive medium used was an aqueous 5% NaCl solution with a pH of 7. The current was 1 mA / cm 2 for a period of 6 hours. It is clearly evident that Figure 2b the layer was almost completely removed, while in the Figure 2a the layer is still well bonded to the substrate. The inventive examples with finer pores thus resist corrosion significantly better than the reference examples with the coarser pore structure.

[0175] Table 8 also shows the welding range according to SEP 1220-2. For all variants according to the invention, the welding range is at least 0.9 and at most 1.6 kA.

[0176] The electrochemical potential is also given in Table 8. The electrochemical potential is determined according to DIN standard "DIN 50918 (2018.09) ("Rest potential measurement on homogeneous mixed electrodes"). The given absolute value is to be understood as a reference to the standard hydrogen electrode. The corrosive medium used in the measurement is an aqueous 5% NaCl solution with a pH of 7, which represents typical corrosion conditions in the automotive sector. It is clearly evident that all samples exhibit an electrochemical potential greater than -0.50 V. Table 1 (steel grades) Steel C Si Mn Al Cr Nb Ti B P S N Sn Ace Cu Mon Ca Ni Al / Nb A 0,22 0,145 1,1 0,18 0,2 0,032 0,017 0,0024 0,004 0,0007 0,0034 0,03 5,6 B 0,35 0,16 1,1 0,21 0,118 0,026 0,0096 0,0025 0,005 <0,0005 0,0035 0,005 0,003 0,019 0,005 0,001 0,032 8,1 C 0,131 0,20 1,17 0,13 0,21 0,021 0,004 0,0021 0,011 0,005 0,0059 0,04 0,025 0,0012 0,06 6,2 D* 0,087 0,12 1,52 0,05 0,1 0,04 0,008 0,0010 0,015 0,003 0,009 0,02 0,1 0,05 0,034 1,3 E* 0,235 0,3 1,3 0,05 0,28 0,003 0,040 0,0035 0,02 0,003 0,007 0,03 0,01 0,03 0,03 0,005 0,025 16,7 F* 0,37 0,3 1,4 0,05 0,18 0,003 0,040 0,0035 0,015 0,003 0,007 0,03 0,01 0,05 0,035 0,003 0,03 16,7 G 0,15 0,3 1,15 0,1 0,3 0,028 0,015 0,0030 0,015 0,005 0,0060 0,03 0,01 0,12 0,05 0,003 0,027 3,6 H 0,165 0,45 2,4 0,75 0,75 0,03 0,035 0,002 0,02 0,003 0,005 0,03 0,01 0,1 0,048 25,0 I 0,46 0,2 0,8 0,2 0,12 0,03 0,010 0,0025 0,005 0,0005 0,0035 0,005 0,003 0,019 0,005 0,001 0,019 6,7 J 0,23 0,17 0,6 0,07 0,25 0,005 0,021 0,0007 0,006 0,0007 0,0034 0,015 0,006 0,041 0,01 0,001 0,03 14,0 Remainder iron and unavoidable impurities. All values ​​are in wt.%; * Reference examples not according to the invention Table 2 (Manufacturing conditions for flat steel products) Process variant T1 [°C] T2 [°C] T5 [°C] T4 [°C] KWG [%] T5 [°C] T6 [°C] T7 [°C] t mT [s] t nT [S] a 1250 1075 850 630 55 773 685 678 15 13 b 1280 1110 860 620 50 787 708 686 15 13 c 1205 1060 820 550 55 768 684 683 18 15 d 1210 1120 910 580 55 770 685 685 18 15 e 1205 1065 830 555 45 655 650 680 15 13 f 1210 1110 900 575 70 760 720 700 20 18 g 1320 1080 840 620 50 805 715 710 23 20 h 1315 1090 910 655 55 870 800 720 25 23 i 1300 1120 830 585 55 895 800 725 30 28 j 1310 1095 915 630 65 830 740 710 25 25 k 1240 1080 870 620 50 823 728 710 55 30 l 1280 1100 870 630 55 808 713 708 18 15 m 1280 1085 870 580 50 793 715 708 22 20 n 1285 1105 885 640 65 755 685 675 13 11 Figures partially rounded Table 3 (coating variant) Coating variant Melt analysis Layer thickness (one-sided) [µm] Si Fe Mg Other Al α 9,5 3 0,5 <1% rest 10 β 8 3,5 0,5 <1% rest 40 γ 10 3 <0,01 <1% rest 25 δ 8,2 3,8 0,25 <1% rest 27 ε 10,5 3,1 0,33 <1% rest 30 φ 8,1 3,9 <0,01 <1% rest 25 Table 4 (flat steel product) Coating test no. Steel Thickness of the steel strip [mm] Process variant Coating variant Yield strength type Rp0.2 or ReL [MPa] ReH [MPa] Rm [MPa] Elongation at break A80 [%] Uniform elongation Ag [%] 1 A 1,5 m γ continuously 536 634 20 13 2 A 1,5 l δ continuously 488 618 21 14 3* D 1,4 a φ pronounced 436 447 600 19 13 4 C 1,5 b γ continuously 443 636 22 14 5 C 1,5 j φ continuously 464 671 21 12 6 C 1,6 k δ continuously 427 588 23 13 7 B 1,5 a γ continuously 493 717 20 12 8 B 1,5 j α continuously 436 682 21 13 9 B 1,5 b β continuously 451 693 20 12 10* F 1,6 a γ continuously 403 591 24 13 11* F 1,6 f ε continuously 411 603 20 13 12* E 1,6 c γ continuously 452 657 18 11 13* E 1,7 c δ continuously 395 602 19 12 14 G 1,6 h δ continuously 401 555 23 12 15 H 1,6 d ε continuously 483 622 20 11 16 I 1,4 e γ continuously 511 723 16 10 17* D 1,6 g β pronounced 408 418 527 21 13 18* F 1,5 i γ continuously 371 553 26 14 19 J 1,5 n α continuously 444 632 19 13 20 A 1,5 m δ nB nB nB nB nB nB * non-inventive reference examples Table 5 (Hot forming parameters) Hot forming variant Average heating rate r furnace [30 - 700 °C] [K / s] T oven [°C] t oven [min.] Transfer time [s] Dew point oven [°C] T Einlg [°C] T WZ [°C] t WZ [s] Cooling rate rwz [K / s] T Target [°C] I 8 925 6 8 -5 800 RT 15 50 50 II 5 920 6 6 -5 815 RT 6 300 40 III 15 920 5 5 -5 850 RT 15 50 50 IV 10 880 6 7 -5 740 100 10 50 120 V 8 950 3 12 -5 770 100 10 50 120 VI 10 925 4 7 -5 810 RT 10 450 40 VII 5 920 10 7 -5 807 RT 15 50 50 VIII 5 920 12 8 -5 796 RT 15 100 50 IX 5 920 12 14 -5 728 100 10 200 110 X 5 920 6 10 -5 792 550 15 30 560 Figures partially rounded Table 6 (Hot forming parameters) Warmum form variant Average heating rate r furnace [30 - 700 °C] [K / s] T inlet [°C] Tmax [°C] T Outlet [°C] t oven [min.] Transfer time [s] Dew point [°C] T Einlg [°C] T WZ [°C] t WZ [s] Cooling rate r WZ [K / s] T Target [°C] AI 4 700 930 930 6 8 -5 795 RT 15 100 50 A.II 5 750 910 910 6 8 -5 775 RT 15 100 50 A.III 6 800 930 930 5 6 -5 820 RT 15 100 50 A.IV 7,5 850 930 930 5 10 -5 770 RT 15 100 50 AV 8 850 950 950 4,5 8 -15 810 RT 15 100 50 A.VI 6 750 955 910 5 6 +5 792 RT 15 100 50 A.VII 7 850 950 950 4,5 12 +5 776 RT 15 100 50 A.VIII 4,5 750 880 880 6 8 +15 761 RT 15 100 50 A.IX 5 800 880 880 6 10 +15 745 RT 15 100 50 AX 5,5 800 920 880 6 8 +20 760 RT 15 100 50 A.XI 6 800 930 930 12 7 -20 804 RT 15 100 50 Figures partially rounded Table 7 (Sheet metal part) Forming test no. Coating test no. Steel Process variant Coating variant Hot forming variant Yield strength [MPa] Tensile strength [MPa] A80 [%] Bending angle [°] 1 1 A m γ II 1203 1451 5,4 64 2 2 A l δ I 1152 1390 5,8 67 3* 3 D a φ I 444 683 16,6 130 4 4 C b γ I 1002 1224 6,1 87 5 5 C j φ V 971 1182 5,4 84 6 6 C k δ VII 993 1212 5,3 82 7 7 B a γ II 1422 1856 5,3 45 8 8 B j α III 1411 1846 5,5 46 9 9 B b β IV 1391 1823 5,0 43 10* 10 F a γ II 1400 1854 5 43 11* 11 F f ε IX 1380 1830 5,2 44 12* 12 E c γ II 1134 1486 5,6 60 13* 13 E c δ I 1136 1484 5,8 54 14 14 G h δ III 982 1197 5,5 82 15 15 H d ε X 853 1107 7,2 87 16 16 I e γ VIII 1622 1893 4,5 36 17* 17 D g β V 420 660 17,2 135 18* 18 F i γ IX 1361 1816 5,4 45 19 20 A m δ X 449 634 18,2 134 20 20 A m δ VI 1218 1509 5,1 62 21 19 J n α I 1189 1478 5,5 58 22 19 J n α IV 1158 1402 5,8 63 23 1 A m γ A.III 1162 1471 5,4 61 24 1 A m γ A.VI 1195 1482 5,1 59 Table 8 (microstructure) Forming test no. structure Martensite Bainite ferrite retained austenite Fine (Nb,Ti)(C,N) precipitates proportion [%] / average diameter Grain diameter of the former austenite grains 1 99,9 - - 0,1 94% / 7nm 7,5 µm 2 99,5 - - 0,5 nb 6,8 µm 3* 38.5 Martensite + Bainite 60% 1,5 25% / 12 nm 21 µm 4 100 - - - 97% / 5 nm 9,5 µm 5 99 - - 1 96% / 5 nm 11 µm 6 100 - - - 98% / 5 nm 10 µm 7 99,9 - - 0,1 96% / 6 nm 7,1 µm 8 99,9 - - 0,1 94% / 8 nm 6,4 µm 9 99,9 - - 0,1 94% / 6 nm 6,1 µm 10* 99,9 - - 0,1 Only coarse excretions 9 µm 11* 100 - - - Only coarse excretions 11 µm 12* 100 - - - Only coarse excretions 14,8 µm 13* 99,9 - - 0,1 Only coarse excretions 13,7 µm 14 99,5 - - 0,5 97% / 7 nm 9,5 µm 15 95.5% bainite+martensite - 4,5 98% / 7 nm nb 16 100 - - 0 nb 10.8 mm 17* 42% Martensite + Bainite 57 1,0 23% / 13µm 19 µm 18* 100 - - - Only coarse excretions 12 µm 19 32% Martensite + Bainite 67% 1,0 nB nb 20 99,5 - 0,5 - nB 5,8 µm 21 99,9 - 0,1 - nb nb 22 97,5 - 2,5 . nb nb 23 100 - - nb nb nb 24 100 - - nb nb nb * non-inventive reference examples Table 9 (Properties of the sheet metal part) Forming test no. Pore-occupied area over a measuring length of 500 µm in the alloy layer in µm 2< Percentage of the area occupied by pores with a diameter greater than or equal to 0.1µm in % Average Nb content in the alloy layer in wt.% Electrochemical potential in V Welding range in kA 1 200 7 0,027 - 0,325 1,2 2 85 4 0,029 - 0,351 1,4 3* 175 35 0,030 - 0,301 0,8 4 180 7 0,018 -0,338 1,2 5 200 8 0,019 - 0,378 1,1 6 75 4 0,020 - 0,370 1,5 7 190 5 0,021 - 0,325 1,5 8 100 2,5 0,022 -0,338 1,4 9 60 2 0,019 - 0,384 1,4 10* 185 23 0,001 - 0,447 0,8 11* 135 42 0,002 - 0,418 0,6 12* 185 30 0,001 - 0,417 0,8 13* 100 25 0,001 - 0,435 0,8 14 110 4 0,024 -0, 338 1,6 15 125 7 0,026 - 0,383 1,4 16 190 7 0,025 - 0,412 0,9 17* 110 27 0,031 - 0,305 0,8 18* 200 45 0,001 - 0,407 0,5 19 135 6 0,025 - 0,418 1,5 20 74 4 0,029 - 0,364 1,5 * non-inventive reference examples

Claims

1. Shaped sheet metal part formed from a flat steel product comprising a steel substrate composed of steel consisting of, as well as iron and unavoidable impurities, in % by weight, C:0.06-0.5%,Si:0.05-0.6%,Mn:0.4-3.0%,Al:0.06-1.0%,Nb:0.001-0.2%,Ti:0.001-0.10%,B:0.0005-0.01%,P:≤ 0.03%,S:≤ 0.02%,N:≤ 0.02%,Sn:≤ 0.03%,As:≤ 0.01%, and optionally one or more of the elements Cr, Cu, Mo, Ni, V, Ca, W in the following contents: Cr:0.01-1.0%,Cu:0.01-0.2%,Mo:0.002-0.3%,Ni:0.01-0.5%,V:0.001-0.3%,Ca:0.0005-0.005%,W:0.001-1.0%, where the content of unavoidable impurities adds up to a total of not more than 0.2% by weight, where the following conditions are applicable to the Al / Nb ratio of Al content to Nb content: Al / Nb ≤ 20.0 when Mn ≤ 1.6% by weight and Al / Nb ≤ 30.0 when Mn ≥ 1.7% by weight, and an anticorrosion coating, characterized in that the shaped sheet metal part has precipitates in the microstructure in the form of niobium carbonitrides and / or titanium carbonitrides, where more than 80% of all precipitates have a diameter of less than 30 nm, measured as set out in the description.

2. Shaped sheet metal part according to Claim 1, characterized in that the steel substrate of the shaped sheet metal part has a microstructure having at least in part more than 80% martensite and / or lower bainite, preferably more, at least in part more than 90% martensite and / or lower bainite, and where the former austenite grains of the martensite preferably have an average grain diameter of less than 14 µm, especially less than 12 µm, preferably less than 10 µm.

3. Shaped sheet metal part according to either of Claims 1 and 2, characterized in that the shaped sheet metal part at least in part has a yield point of at least 950 MPa, especially at least 1100 MPa, preferably at least 1500 MPa, especially at least 1500 MPa, and / or the shaped sheet metal part at least in part has a tensile strength of at least 1000 MPa, especially at least 1100 MPa, preferably at least 1300 MPa, especially at least 1800 MPa, and / or the shaped sheet metal part at least in part has an elongation at break A80 of at least 4%, preferably at least 5%, more preferably at least 6%, and / or the shaped sheet metal part at least in part has a bending angle of at least 30°, especially at least 40°, preferably at least 50°.

4. Shaped sheet metal part according to any of Claims 1 to 3, characterized in that the electrochemical potential of the surface of the shaped sheet metal part in a corrosive medium is at least -0.50 V.

5. Shaped sheet metal part according to any of Claims 1 to 4, characterized in that the anticorrosion coating is an aluminium-based anticorrosion coating and comprises an alloy layer and an Al base layer, and wherein, in the cross section of the alloy layer, over a measurement length of 500 µm, the area occupied by pores in the alloy layer is less than 250 µm2, and wherein, in particular, the proportion of the area occupied by pores having a diameter of not less than 0.1 µm is less than 10%.

6. Shaped sheet metal part according to any of Claims 1 to 5, characterized in that the welding range is at least 0.9 kA.

7. Shaped sheet metal part according to any of Claims 1 to 6, characterized in that the Nb content in the alloy layer is greater than 0.010% by weight, preferably greater than 0.015% by weight, especially greater than 0.018% by weight.

8. Process for producing a shaped sheet metal part according to Claims 1-7, comprising the following steps: a) providing a slab or thin slab composed of steel consisting of, as well as iron and unavoidable impurities, in % by weight, C:0.06-0.5%,Si:0.05-0.6%,Mn:0.4-3.0%,Al:0.06-1.0%,Nb:0.001-0.2%,Ti:0.001-0.10%,B:0.0005-0.01%,P:≤ 0.03%,S:≤ 0.02%,N:≤ 0.02%,Sn:≤ 0.03%,As:≤ 0.01%, and optionally one or more of the elements "Cr, Cu, Mo, Ni, V, Ca, W" in the following contents: Cr:0.01-1.0%,Cu:0.01-0.2%,Mo:0.002-0.3%,Ni:0.01-0.5%,V:0.001-0.3%,Ca:0.0005-0.005%,W:0.001-1.0%, where the following conditions are applicable to the Al / Nb ratio of Al content to Nb content: Al / Nb ≤ 20.0 when Mn ≤ 1.6% by weight and Al / Nb ≤ 30.0 when Mn ≥ 1.7% by weight; b) through-heating the slab or thin slab at a temperature (T1) of 1100-1400°C; c) optionally pre-rolling the through-heated slab or thin slab to give an intermediate product having an intermediate product temperature (T2) of 1000-1200°C; d) hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000°C; e) optionally coiling the hot-rolled flat steel product, where the coiling temperature (T4) is not more than 700°C; f) descaling the hot-rolled flat steel product; g) optionally cold-rolling the flat steel product, where the degree of cold rolling is at least 30%; h) annealing the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to a dipping temperature (T6) of 650-800°C, preferably 670-800°C; j) coating the flat steel product cooled to dipping temperature with an anticorrosion coating by hot dip coating in a melt bath at a melt temperature (T7) of 660-800°C, preferably 680-740°C; k) cooling the coated flat steel product to room temperature, where the first cooling time tmT in the temperature range between 600°C and 450°C is more than 10 s, especially more than 14 s, and the second cooling time tnT in the temperature range between 400°C and 300°C is more than 8 s, especially more than 12 s; l) optionally skin pass-rolling the coated flat steel product; m) providing a sheet metal blank made from the flat steel product; n) heating the sheet metal blank in such a way that at least in part the AC3 temperature of the blank is exceeded and the temperature Tins of the blank on insertion into a forming tool provided for hot press forming (step c)) at least in part has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; o) inserting the heated sheet metal blank into a forming tool, where the transfer time ttrans required for the removal from the heating device and the insertion of the blank is not more than 20 s, preferably not more than 15 s; p) hot press forming the sheet metal blank to the shaped sheet metal part, wherein the blank, in the course of hot press forming, is cooled down to the target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least in part more than 30 K / s, and optionally kept at that temperature; q) removing the shaped sheet metal part cooled to the target temperature Ttarget from the tool.

9. Process according to Claim 8, characterized in that a melt bath used in the hot dip coating operation contains the anticorrosion material to be applied to the flat steel product in liquid form, consisting of up to 15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5% by weight of alkali metals or alkaline earth metals and optionally up to 10% Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminium as the balance.