DE102021128545A1 - Steel flat product with quick-heating coating - Google Patents

Abstract

The invention relates to a flat steel product for producing a steel component by hot forming. The flat steel product comprises a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and an aluminum-based anti-corrosion coating lying on the steel substrate . In this case, an absorption layer, which comprises lithium aluminum hydroxide, is arranged on the anti-corrosion coating. Furthermore, the invention relates to the production of such a flat steel product with an absorption layer.

Description

The invention relates to a flat steel product for producing a steel component by hot forming, comprising a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and a Aluminum-based anti-corrosion coating on the steel substrate.

The term “flat steel product” is understood here to mean all rolled products whose length is much greater than their thickness. This includes steel strips and sheets as well as blanks and blanks made from them.

During hot forming, also known as hot forming, press hardening or hot press form hardening, flat steel products, such as steel blanks, which are separated from cold or hot-rolled steel strip, are heated to a deformation temperature that is generally above the austenitization temperature (AC3) of the respective steel and, in the heated state, are tool of a forming press. In the course of the subsequent forming, the sheet metal blank or the component formed from it experiences rapid cooling through contact with the cool tool. The cooling rates are set in such a way that a hardened structure results in the steel substrate. The microstructure is transformed into an at least partially martensitic microstructure. The result is a hardened steel component.

The flat steel product is typically heated in a preheated roller hearth furnace, which the flat steel product runs through. From the point of view of optimal energy use, including reduced CO ₂ emissions, the minimized process duration and the optimized options for process control, there is a requirement for the most effective possible transfer of the thermal energy, which is usually introduced as thermal radiation, into the flat steel product. A shortened heating time means that the corresponding roller hearth furnaces can be dimensioned shorter, which has a positive effect on costs and space requirements.

From the WO 2012/120081 A2 flat steel products with various anti-corrosion coatings are known. Various cover layers are proposed to improve the heating behavior.

The object of the present invention is to provide a particularly suitable composition for the absorption layer for flat steel products with an aluminum-based anti-corrosion coating.

This object is achieved by a flat steel product for producing a steel component by hot forming, comprising a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and an aluminum-based anti-corrosion coating overlying the steel substrate. In this case, an absorption layer comprising lithium aluminum hydroxide is arranged on the anti-corrosion coating.

Within the meaning of the invention, lithium aluminum hydroxides are mixed hydroxides or double hydroxides in which metal cations share hydroxide ions with their respective neighbors in the crystal structure. The excess positive charge is compensated by anions.

These anions are preferably carbonate anions. The latter are referred to as hydroxycarbonates or hadroxide carbonates. The absorption layer therefore preferably comprises carbonates of lithium aluminum hydroxide, in particular lithium hydrotalcite.

Lithium hydrotalcite is the simplified name for lithium aluminum hydroxide carbonate with the chemical formula [LiAl ₂ (OH) ₆ ] ₂ (CO ₃ ) 4H ₂ O

In particular, the lithium hydrotalcite is in crystalline form.

In a specific alternative of the invention, the lithium aluminum hydroxides form layers in the crystal structure with interlayers of anions which may contain water of crystallization. The H ₂ O molecules in the intermediate layer stabilize the linking of the layers via hydrogen bridges.

In particular, the absorption layer consists of 80% by weight to 100% by weight lithium hydrotalcite or 80% by weight to 100% by weight of a mixture of lithium hydrotalcite and hydrotalcite and optional components in each case, the total content of which does not exceed 20 % by weight.

Lithium aluminum hydroxides, in particular their carbonates, preferably lithium hydrotalcite, have the advantage that even small amounts result in significant absorption in the infrared range and thus a reduction in reflectivity in the infrared range, i.e. the range of the radiation spectrum in which the interior of the furnace emits radiation that essentially leads to heating of the sheet metal blanks. In addition, lithium-based salt layers passivate the surface and thus improve corrosion protection.

In the context of this application, the infrared range is understood to mean the wave number range from 1000 to 10,000 cm ^-1 . This corresponds to the wavelength range of 1 - 10 µm.

In addition, a well-adhering absorption layer with lithium aluminum hydroxide can be produced in a simple manner by immersing the flat steel product in an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) or by spraying it with such a solution or by coil coating it. During the subsequent drying of the steel flat product, lithium hydrotalcite crystals form on the aluminum-containing surface of the anti-corrosion coating. An absorption layer with lithium hydrotalcite crystals is thus formed.

The absorption layer lies on top of the anti-corrosion coating and is directly adjacent to it. In particular, the absorption layer is a cover layer which closes off the layer structure formed on the flat steel product on its outer side.

In a preferred variant, the coating weight of the absorption layer is 20-750 mg/m ² . The print run weight is the print run weight per page. In the case of steel flat products coated on both sides with an absorption layer on both sides, the coating weight on each of the two sides is 20-750 mg/m ² . It has been shown that even such low coating weights lead to a significant reduction in the degree of reflection.

The degree of reflection R in the infrared range is determined within the meaning of this application by using a black body radiator as a reference. The black body has a temperature of T=920°C, which corresponds to an average oven temperature. The spectral radiant power i _λ (T) of the blackbody radiator at the temperature T is multiplied by the measured spectral reflectivity ρ _λ and integrated over the wavelength range. This integral is normalized to the spectral radiant power integrated over the same wavelength range. So it applies $$\overline{R}\left(T\right)=\frac{{\displaystyle {\int }_{\mathrm{<figure-callout\; id="1"\; label="\lambda "\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">\lambda </figure-callout>}1}^{\mathrm{<figure-callout\; id="2"\; label="\lambda "\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">\lambda </figure-callout>}2}{\rho }_{\lambda }\cdot i_{\lambda }\left(T\right)\mathrm{i.e}\lambda }}{{\displaystyle {\int }_{\mathrm{<figure-callout\; id="1"\; label="\lambda "\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">\lambda </figure-callout>}1}^{\lambda 2}{i}_{\lambda }\left(T\right)\mathrm{i.e}\lambda }}$$

mit der Lichtgeschwindigkeit c, dem Planck'schen Wirkungsquantum h und der Boltzmann-Konstante k_B. Die Integration erfolgt jeweils über den Bereich der Wellenlänge, der den Wellenzahlen von 1000 - 10000 cm^-1 entspricht, d.h. von λ₁ = 1 µm. bis λ₂ = 10 µm. Der nachfolgend verwendete Reflexionsgrad R ist definiert als R(920°C).i _λ (T) results from Planck's law of radiation $$i_{\lambda }\left(T\right)=2\mathrm{<figure-callout\; id="2"\; label="\pi \; H\; c"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">\pi </figure-callout>}\frac{H{c}^{}}{}\frac{{}^2}{{\mathrm{<figure-callout\; id="5"\; label="\lambda "\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}^5}\frac{1}{e^{\frac{Hc}{\lambda k_{B}T}-1}}$$

with the speed of light c, Planck's constant h and the Boltzmann constant k _B . The integration takes place in each case over the wavelength range that corresponds to the wave numbers from 1000 to 10000 cm ^-1 , ie from λ ₁ = 1 μm. up to λ ₂ = 10 µm. The reflectance used below R is defined as R (920°C).

The aluminum-based anti-corrosion coating can be applied to one or both sides of the flat steel product. The two opposite large surfaces of the flat steel product are referred to as the two sides of the flat steel product. The narrow faces are called edges.

Such an anti-corrosion coating is preferably produced by hot-dip coating the steel flat product. The flat steel product is passed through a liquid melt consisting of 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% by weight of Zn, preferably up to 10% by weight of Zn and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 7-12% by weight, in particular 8-10% by weight.

In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0. 5% by weight Mg. Furthermore, the optional content of alkali metals or alkaline earth metals in the melt can include in particular at least 0.0015% by weight Ca, in particular at least 0.01% by weight Ca.

During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the anti-corrosion coating of the flat steel product has an alloy layer and an Al base layer in particular when it solidifies.

The alloy layer rests on the steel substrate and is directly adjacent to it. The alloy layer is essentially made up of aluminum and iron. The remaining elements from the steel substrate or the melt composition do not enrich significantly in the alloy layer. The alloy layer preferably consists of 35-60% by weight Fe, preferably α-iron, optional further components whose contents are limited to a maximum of 5.0% by weight, preferably 2.0%, and the remainder aluminum. the Al content preferably increasing towards the surface. The optional further components include in particular the remaining components of the melt (ie silicon and optionally alkali metals or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron.

The Al base layer lies on top of the alloy layer and is immediately adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt in the melt bath. That is, it consists of 1.0-15% by weight Si, optionally 2-4% by weight Fe, optionally up to 5% by weight alkali or alkaline earth metals, preferably up to 1.0% by weight Alkaline or alkaline earth metals, optionally up to 15% Zn and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder aluminum.

In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0 .5% by weight Mg. Furthermore, the optional content of alkali metals or alkaline earth metals in the Al base layer can include in particular at least 0.0015% by weight Ca, in particular at least 0.1% by weight Ca.

In a further preferred variant of the anti-corrosion coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

bei beidseitigen Korrosionsschutzüberzügen bzw. $15-180\frac{g}{m^2}$

bei der einseitigen Variante. Bevorzugt beträgt das Auflagengewicht des Korrosionsschutzüberzuges $100-200\frac{g}{m^2}$

bei beidseitigen Überzügen bzw. $50-100\frac{g}{m^2}$

für einseitige Überzüge. Besonders bevorzugt beträgt das Auflagengewicht des Korrosionsschutzüberzuges $120-180\frac{g}{m^2}$

bei beidseitigen Überzügen bzw. $60-90\frac{g}{m^2}$

für einseitige Überzüge.The anti-corrosion coating preferably has a thickness of 5-60 μm, in particular 10-40 μm. The coating weight of the anti-corrosion coating is in particular $30-360\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

with anti-corrosion coatings on both sides or $15-180\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

in the unilateral variant. The coating weight of the anti-corrosion coating is preferably $100-200\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

in the case of double-sided coatings or $50-100\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for one-sided coatings. The coating weight of the anti-corrosion coating is particularly preferred $120-180\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

in the case of double-sided coatings or $60-90\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="DE102021128545A1\_0014.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for one-sided coatings.

The thickness of the alloy layer is preferably less than 20 μm, particularly preferably less than 16 μm, particularly preferably less than 12 μm, in particular less than 10 μm. The thickness of the Al base layer results from the difference in the thicknesses of the anti-corrosion coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 μm, even in the case of thin anti-corrosion coatings.

In _{the preferred variant with} 0.1-1.0 _wt Hydrotalcite forms when the flat steel product is immersed in, sprayed with or coil-coated in an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) and In addition, the Mg content described is present in the Al base layer. In such a case, the absorbent layer comprises a mixture of hydrotalcite and lithium hydrotalcite. In particular, the absorption layer consists of 80% by weight to 100% by weight of a mixture of lithium hydrotalcite and hydrotalcite and optional components whose total contents are not more than 20% by weight. The proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is preferably at least 5% by weight, preferably at least 10% by weight. In particular, the proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is at most 90% by weight, preferably at most 75% by weight.

In a preferred variant of the flat steel product, the degree of reflection is R in the infrared range less than 0.55, in particular less than 0.50, preferably less than 0.45, in particular less than 0.40, preferably less than 0.35, particularly preferably less than 0.30, in particular less than 0.25 less than 0.20, in particular less than 0.15. The lower the degree of reflection in the infrared range, the greater the heating during the subsequent production of a steel component.

The invention also relates to the use of lithium aluminum hydroxide, in particular its carbonates, preferably lithium hydrotalcite, in an absorption layer on an aluminum-based anti-corrosion coating to reduce the reflectivity in the infrared range. The use has the same advantages that were explained above in relation to the flat steel product. The invention also relates to the use of the specially developed absorption layers mentioned above and in particular to the use of a mixture of lithium hydrotalcite and hydrotalcite in an absorption layer on an anti-corrosion coating with 0.1-1.0% by weight Mg in the Al base layer.

The steel substrate is a steel containing 0.1-3 wt% Mn and optionally up to 0.01 wt% B. In particular, the microstructure of the steel can be converted into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic microstructure, since this has a particularly high degree of hardness.

• C: 0,04 - 0,45 Gew.-%,
• Si: 0,02 - 1,2 Gew.-%,
• Mn: 0,5 - 2,6 Gew.-%,
• Al: 0,02 - 1,0 Gew.-%,
• P: ≤ 0,05 Gew.-%,
• S: ≤ 0,02 Gew.-%,
• N: ≤ 0,02 Gew.-%,
• Sn: ≤ 0,03 Gew.-%
• As: ≤ 0,01 Gew.-%
• Ca: ≤ 0,005 Gew.-%

sowie optional einem oder mehreren der Elemente „Cr, B, Mo, Ni, Cu, Nb, Ti, V“ in folgenden Gehalten

• Cr: 0,08 - 1,0 Gew.-%,
• B: 0,001 - 0,005 Gew.-%
• Mo: ≤ 0,5 Gew.-%
• Ni: ≤ 0,5 Gew.-%
• Cu: ≤ 0,2 Gew.-%
• Nb: 0,02 - 0,08 Gew.-%,
• Ti: 0,01 - 0,08 Gew.-%
• V: ≤ 0,1 Gew.-%

besteht.The steel substrate is particularly preferably a steel which, in addition to iron and unavoidable impurities (in % by weight), consists of

• C: 0.04 - 0.45% by weight,
• Si: 0.02 - 1.2% by weight,
• Mn: 0.5 - 2.6% by weight,
• Al: 0.02 - 1.0% by weight,
• P: ≤ 0.05% by weight,
• S: ≤ 0.02% by weight,
• N: ≤ 0.02% by weight,
• Sn: ≤ 0.03% by weight
• As: ≤ 0.01% by weight
• Ca: ≤ 0.005% by weight

and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents

• Cr: 0.08 - 1.0% by weight,
• B: 0.001 - 0.005% by weight
• Mo: ≤ 0.5% by weight
• Ni: ≤ 0.5% by weight
• Cu: ≤ 0.2% by weight
• Nb: 0.02 - 0.08% by weight,
• Ti: 0.01 - 0.08 wt%
• V: ≤ 0.1% by weight

consists.

• Cr: ≤ 0,050 Gew.-%,
• B: ≤ 0,0005 Gew.-%
• Nb: ≤ 0,005 Gew.-%,
• Ti: ≤ 0,005 Gew.-%

The elements P, S, N, Sn, As, Ca are impurities that cannot be completely avoided in steel production. In addition to these elements, other elements can also be present as impurities in the steel. These other elements are summarized under the "unavoidable impurities". The total content of unavoidable impurities is preferably not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloying elements Cr, B, Nb, Ti, for which a lower limit is given, can also occur in contents below the respective lower limit as unavoidable impurities in the steel substrate. In that case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The individual upper limits for the respective contamination of these elements are preferably as follows:

• Cr: ≤ 0.050% by weight,
• B: ≤ 0.0005% by weight
• Nb: ≤ 0.005% by weight,
• Ti: ≤ 0.005% by weight

These preferred upper limits are to be considered as alternatives or together. Preferred variants of the steel therefore meet one or more of these four conditions.

In a preferred embodiment, the carbon content of the steel is at most 0.37% by weight and/or at least 0.06% by weight. In particularly preferred variants, the C content is in the range from 0.06 to 0.09% by weight or in the range from 0.12 to 0.25% by weight or in the range from 0.33 to 0.37% by weight %.

In a preferred embodiment, the Si content of the steel is at most 1.00% by weight and/or at least 0.06% by weight.

In a preferred variant, the Mn content of the steel is at most 2.4% by weight and/or at least 0.75% by weight. In particularly preferred variants, the Mn content is in the range of 0.75-0.85% by weight or in the range of 1.0-1.6% by weight.

In a preferred variant, the Al content of the steel is at most 0.75% by weight, in particular at most 0.5% by weight, preferably at most 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02%.

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

The elements P, S, N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the maximum P content is 0.03% by weight. Regardless, the S content is preferably at most 0.012%. In addition or as a supplement, the N content is preferably at most 0.009% by weight.

Optionally, the steel also contains chromium with a content of 0.08 - 1.0% by weight. The Cr content is preferably at most 0.75% by weight, in particular at most 0.5% by weight.

In the case of an optional addition of chromium, the sum of the chromium and manganese contents is preferably limited. The total is at most 3.3% by weight, in particular at most 3.15% by weight. Furthermore, the sum is at least 0.5% by weight, preferably at least 0.75% by weight.

The steel preferably also optionally contains boron with a content of 0.001-0.005% by weight. In particular, the B content is at most 0.004% by weight.

The steel can optionally contain molybdenum with a content of at most 0.5% by weight, in particular at most 0.1% by weight.

Furthermore, the steel can optionally contain nickel with a content of at most 0.5% by weight, preferably at most 0.15% by weight.

Optionally, the steel can also contain copper with a content of at most 0.2% by weight, preferably at most 0.15% by weight.

In addition, the steel can optionally contain one or more of the micro-alloying elements Nb, Ti and V. The optional Nb content is at least 0.02% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional Ti content is at least 0.01% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional V content is at most 0.1% by weight, preferably at most 0.05% by weight.

In the case of an optional addition of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The total is at most 0.1% by weight, in particular at most 0.068% by weight. Furthermore, the sum is preferably at least 0.015% by weight.

Of course, the above explanations for preferred steel substrates also apply to the steel substrates in the production processes described below.

1. a) Bereitstellen eines Stahlflachproduktes umfassend ein Stahlsubstrat, das aus einem Stahl, der 0,1 - 3 Gew.-% Mn und optional bis zu 0,01 Gew.-% B aufweist, besteht, und einen auf dem Stahlsubstrat aufliegenden Korrosionsschutzüberzug auf Aluminium-Basis,
2. b) Applizieren einer Lithiumcarbonat (Li₂CO₃) umfassenden wässrigen Lösung auf das Stahlflachprodukt, insbesondere Eintauchen des Stahlflachproduktes in eine Lithiumcarbonat (Li₂CO₃) umfassende wässrige Lösung oder Besprühen des Stahlflachproduktes mit einer Lithiumcarbonat (Li₂CO₃) umfassenden wässrigen Lösung oder Belegen des Stahlflachproduktes mit einer Lithiumcarbonat (Li₂CO₃) umfassenden wässrigen Lösung im Coil-Coating-Verfahren.

The invention also relates to a method for producing a flat steel product as described above, comprising at least the following work steps:

1. a) Providing a flat steel product comprising a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and an anti-corrosion coating on aluminum lying on the steel substrate -Base,
2. b) Applying an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) to the flat steel product, in particular immersing the flat steel product in an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) or spraying the flat steel product with an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ). or coating the flat steel product with an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) in the coil coating process.

As a result of this treatment, the aqueous solution with lithium carbonate is distributed evenly over the entire surface, so that a homogeneous, surface-covering absorption layer is formed, which comprises lithium aluminum hydroxide, in particular its carbonates, preferably lithium hydrotalcite.

In a preferred further development, the pH of the aqueous solution is at least 11 and at most 15, in particular at most 13, preferably at most 12. This ensures that sufficient Li ₂ CO ₃ is dissolved so that sufficient lithium aluminum hydroxide forms can. In addition, the pH value in this range leads to good wettability and thus to particularly even distribution.

In particular, the aqueous solution contains 0 to 20 g/L Li ₂ CO ₃ . The content of Li ₂ CO ₃ is preferably at least 5 g/L. Furthermore, the content of Li ₂ CO ₃ is preferably not more than 15 g/L. Optionally, the solution can contain LiOH. The LiOH content is in particular 0-10 g/L. The LiOH content is preferably at least 1 g/L. Furthermore, the content of LiOH is preferably not more than 5 g/L.

In a preferred development, the aqueous solution also contains an oxidizing agent to improve the homogeneity and to accelerate the reaction. The oxidizing agent is preferably nitrate with a concentration of 0.1-15 g/L, preferably 2-10 g/L or chlorate with a concentration of 0.05-2 g/L, preferably 0.2-1.5 g/L. The salts of the alkali metals, preferably LiNO3 and KClO3, are suitable as nitrates and chlorates.

In the case of immersion, the immersion preferably takes place for an immersion time of 60 s to 300 s, preferably 120 s to 240 s. A longer immersion time has the advantage that the reaction to form lithium hydrotalcite can take place reliably. However, a shorter immersion time is advantageous for industrial production in order to make the manufacturing process efficient. The times mentioned have proved to be a good compromise in this respect.

In a preferred development of the method, the flat steel product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, when the aqueous solution is applied, in particular when it is immersed or sprayed or covered in the coil coating method . A higher temperature accelerates the chemical reaction in film formation, but if the temperature is too high, the aqueous solution evaporates too quickly, so that film formation is not reliably completed.

In a preferred variant, before step b), the flat steel product is subjected to an activation treatment analogous to a phosphating process. In addition, the surface can be activated before the treatment, analogous to a phosphating process. For this purpose, an activation solution based on phosphates, in particular based on sodium titanyl phosphate, or based on copper is applied to the flat steel product. Suitable activating agents are, for example, under the trade names SurTec® 145, SurTec® 610 V, SurTec® 615 V, SurTec® 616 V, Fixodine®X, Fixodine®50, Fixodine®50CF (now Bonderite® M-AC 50CF), Fixodine®950 (now Bonderite® M-AC 950), Fixodine®G 3039, Fixodine®C 5020 A, Fixodine®G 5020 B, Fixodine®C 9114, Fixodine®9112, Gardolene® Z26, Gardolene®V 6599, Gardolene® V 6560 A , Gardolene® V 6559, Gardolene® V 6526, Gardolene® V 6522, Gardolene® V 6520, Gardolene® V 6518, Gardolene® V 6513, Prepalene® X ® 163 available.

1. a) Bereitstellen eines Blechzuschnitts aus einem zuvor erläuterten Stahlflachprodukt;
2. b) Erwärmen des Blechzuschnitts derart, dass zumindest teilweise die AC3 Temperatur des Zuschnitts überschritten ist und die Temperatur des Zuschnitts beim Einlegen in ein für ein Wampressformen vorgesehenes Umformwerkzeug (Arbeitsschritt c)) zumindest teilweise eine Temperatur oberhalb von Ms+100°C aufweist, wobei Ms die der Martensitstarttemperatur bezeichnet, wobei die mittlere Aufheizung größer ist als 15 Kmm/s;
3. c) Einlegen des erwärmten Blechzuschnitts in ein Umformwerkzeug, wobei die für das Entnehmen aus der Erwärmungseinrichtung und das Einlegen des Zuschnitts benötigte Transferdauer t_Trans höchstens 20s, bevorzugt höchstens 15s, beträgt;
4. d) Warmpressformen des Blechzuschnitts zu dem Blechformteil, wobei der Zuschnitt im Zuge des Warmpressformens über eine Dauer twz von mehr als 1s mit einer zumindest teilweise mehr als 30 K/s betragenden Abkühlgeschwindigkeit rwz auf die Zieltemperatur T_Ziel abgekühlt und optional dort gehalten wird;
5. e) Entnehmen des auf die Zieltemperatur abgekühlten Blechformteils aus dem Werkzeug.

The invention also relates to a method for producing a shaped sheet metal part, comprising the following work steps:

1. a) providing a sheet metal blank from a previously explained flat steel product;
2. b) heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature of the blank when it is placed in a forming tool provided for hot-press forming (step c)) at least partially has a temperature above Ms+100°C, wherein Ms denotes the martensite start temperature, the mean heating being greater than 15 Kmm/s;
3. c) inserting the heated sheet metal blank into a forming tool, the transfer time t _Trans required for removing it from the heating device and inserting the blank being at most 20 s, preferably at most 15 s;
4. d) Hot pressing of the sheet metal blank to form the shaped sheet metal part, the blank being cooled to the target temperature T target over a period twz of more than 1 s at a cooling rate rwz, at least in some cases exceeding 30 K/s, during the _course of the hot pressing and optionally being held there;
5. e) removing the shaped sheet metal part, which has cooled to the target temperature, from the tool.

In the method according to the invention, a blank is thus provided (step a)), which consists of steel suitably composed in accordance with the above explanations and has an absorption layer, which is then heated with an average heating rate of more than 15 km/s in such a way that at least the AC3 temperature of the blank is partially exceeded and the temperature T _eing of the blank when it is placed in a forming tool provided for hot-press forming (work step c)) is at least partially a temperature above Ms+100°C.

The mean heating is to be understood as the product of the mean heating rate from 30° C. to 700° C. and the sheet metal thickness. The mean heating is more than 15 Kmm/s, in particular more than 20 Kmm/s, preferably more than 25 Kmm/s, in particular more than 30 Kmm/s, particularly preferably more than 35 Kmm/s. The heating in step a) preferably takes place in an oven, in particular a roller hearth oven. Therefore, thermal radiation dominates over thermal conduction when heating the sheet metal blanks. The absorption layer according to the invention increases the proportion of the absorbed thermal radiation, so that such high average heating results.

For the purposes of this application, partially exceeding a temperature (here AC3 or Ms+100°C) means that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature. When it is placed in the forming tool, at least 30% of the blank has an austenitic structure, ie the transformation from the ferritic to the austenitic structure does not have to be complete when it is placed in the forming tool. Rather, up to 70% of the volume of the blank when it is placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or non-recrystallized or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature level than others during heating. For this, the The heat supply can only be directed to certain sections of the blank or the parts that are to be heated less are shielded from the heat supply. In the part of the cut material whose temperature remains lower, no martensite or only significantly less martensite is formed in the course of forming in the tool, so that the structure there is significantly softer than in the other parts in which a martensitic structure is present. In this way, a softer area can be set in a targeted manner in the formed sheet metal part, for example by having optimum toughness for the respective application, while the other areas of the sheet metal part have maximum strength.

Maximum strength properties of the formed sheet metal part obtained can be made possible by the temperature at least partially reached in the sheet metal blank being between AC3 and 1000°C, preferably between 850°C and 950°C.

mit %C = jeweiliger C-Gehalt, %Si = jeweiliger Si-Gehalt, %Mn = jeweiliger Mn-Gehalt, %Cr = jeweiliger Cr-Gehalt, %Mo = jeweiliger Mo-Gehalt, %Ni =jeweiliger Ni-Gehalt und %V = jeweiliger V-Gehalt des Stahls, aus dem der Zuschnitt besteht, bestimmt.The minimum temperature to be exceeded is AC3 according to that of HOUGARDY, HP. in Materials Science Steel Volume 1: Basics, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229., given formula $$\text{AC}$$$$3=\left(902-225*\%\text{C}+19*\%\text{si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mon}-20*\%\text{no}+55*\%\text{V}\right)°\text{C}$$

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 = specific V content of the steel from which the blank is made determined.

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

In a preferred embodiment, the heating takes place in an oven with an oven temperature T _oven of 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 a maximum of 930°C.

The dew point in the oven is preferably at least -20°C, preferably at least -15°C, in particular at least -5°C, preferably at least 0°C, particularly preferably at least +5°C and at most +25°C, preferably at most +20 °C, in particular a maximum of +15°C.

In a special embodiment, the heating in step b) takes place in stages 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 at most 1200°C, in particular at most 1000°C, preferably at most 950°C, particularly preferably at most 930°C.

The total time in the oven t _oven , which is made up of a heating time and a holding time, is preferably at least 1 minute, in particular at least 2 minutes, preferably at least 3 minutes, in both variants (constant oven temperature, gradual heating). Furthermore, the total time in the oven in both variants is preferably a maximum of 12 minutes, in particular a maximum of 10 minutes, preferably a maximum of 8 minutes, in particular a maximum of 6 minutes. Longer total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is ensured. On the other hand, holding above AC3 for too long leads to grain coarsening, which has a negative effect on the mechanical properties.

The blank heated in this way is removed from the respective heating device and transported into the forming tool so quickly that its temperature when it arrives in the tool is at least partially above Ms+100°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 temperature is in particular a maximum of 900°C. Overall, these temperature ranges ensure good formability of the material.

In work step c), the transfer of the austenitized blank from the heating device used in each case to the forming tool is completed within preferably no more than 20 seconds, in particular no more than 15 seconds. Such rapid transport is necessary to avoid excessive cooling before deformation.

When inserting the blank, 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. In a special embodiment, the tool can optionally be tempered at least in regions to a temperature Twz of at least 200° C., in particular at least 300° C., in order to only partially harden the component. Furthermore, the tool temperature Twz is preferably at most 600°C, in particular at most 550°C. It is only necessary to ensure that the tool temperature Twz is below the desired target temperature T _target . The residence time in the tool twz is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s.

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 shaped sheet metal part is particularly preferably below Ms−50° C., with Ms denoting 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.

zu berechnen, wobei auch hier mit C% der C-Gehalt, mit %Mn der Mn-Gehalt, mit %Mo der Mo-Gehalt, mit %Cr der Cr-Gehalt, mit %Ni der Ni-Gehalt, mit %Cu der Cu-Gehalt, mit %Co der Co-Gehalt, mit %W der W-Gehalt und mit %Si der Si-Gehalt des jeweiligen Stahls in Gew.-% bezeichnet sind.The martensite start temperature of a steel within the specifications of the invention is according to the formula: $$\begin{array}{l}\text{Ms}\left[°\text{C}\right]=(490.85-302.6\%\text{C}-30.6\%\text{Mn}-16.6\%\text{no}-8.9\%\text{Cr}+2.4\%\text{Mon}-11.3\\ \%\text{Cu}+8.58\%\text{co}+7.4\%\text{W}-14.5\%\text{si})\left[°\text{C}/\text{weight}.-\%\right]\end{array}$$

to be calculated, where C% is the C content, %Mn is the Mn content, %Mo is the Mo content, %Cr is the Cr content, %Ni is the Ni content, %Cu is the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in % by weight.

$$\begin{array}{l}\text{AC}3\left[°\text{C}\right]=(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mo}-20*\%\text{Ni }\\ +55*\%\text{V})\left[°\text{C}/\text{Gew}.-\%\right]\end{array}$$

zu berechnen, wobei auch hiermit mit %C der C-Gehalt, mit %Si der Si-Gehalt mit %Mn der Mn-Gehalt mit %Cr der Cr-Gehalt, mit %Mo der Mo-Gehalt, mit %Ni der Ni-Gehalt und mit +%V der Vanadium-Gehalt des jeweiligen Stahls bezeichnet sind (Brandis H 1975 TEW-Techn. Ber. 1 8-10)The AC1 temperature and the AC3 temperature of a steel within the scope of the specifications according to the invention is according to the formulas: $$\begin{array}{l}\text{AC}1\left[°\text{C}\right]=(739-22*\%\text{C}-7*\%\text{Mn}+2*\%\text{si}+14*\%\text{Cr}+13*\%\text{Mon}-13*\%\text{no}+20*\%\text{V}\\ )\left[°\text{C}/\text{weight}.-\%\right]\end{array}$$

$$\begin{array}{l}\text{AC}3\left[°\text{C}\right]=(902-225*\%\text{C}+19*\%\text{si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mon}-20*\%\text{no}\\ +55*\%\text{V})\left[°\text{C}/\text{weight}.-\%\right]\end{array}$$

with %C the C content, with %Si the Si content with %Mn the Mn content with %Cr the Cr content, with %Mo the Mo content, with %Ni the Ni Content and +%V the vanadium content of the respective steel are indicated (Brandis H 1975 TEW-Techn. Ber. 1 8-10)

In the tool, the blank is not only formed into the shaped sheet metal part, but is also quenched to the target temperature at the same time. The cooling rate in the tool rwz 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, particularly preferably at least 100 K/s.

After the sheet metal part has been removed in step e), the sheet metal part is cooled to a cooling temperature T _AB of less than 50° C. within a cooling time t _AB of 0.5 to 600 s. This is usually done by air cooling.

The invention is explained in more detail below using exemplary embodiments.

• 1a eine Rasterelektronenmikroskop-Aufnahme einer Aufsicht auf eine Absorptionschicht in einer ersten Auflösung;
• 1b eine Rasterelektronenmikroskop-Aufnahme einer Aufsicht auf eine Absorptionsschicht in einer zweiten Auflösung;
• 2 den Reflexionsgrad R(k) als Funktion der Wellenzahl k für die Versuche 1-6.

The figures show:

• 1a a scanning electron micrograph of a top view of an absorption layer in a first resolution;
• 1b a scanning electron micrograph of a top view of an absorption layer in a second resolution;
• 2 the reflectance R(k) as a function of the wave number k for experiments 1-6.

Several tests were carried out to demonstrate the effect of the invention. For this purpose, steel blanks with a steel composition according to Table 1 were coated with an Al-based anti-corrosion coating by means of hot-dip coating. Ladle analysis is given in Table 2. The resulting anti-corrosion coating in each case had an Al base layer whose composition corresponds to the melt analysis. The one-sided thickness of the anti-corrosion coating is also listed in Table 2. The steel blanks thus provided were treated with an aqueous solution comprising lithium carbonate to produce an absorption layer on the anti-corrosion coating. The solution contained 13 g/L Li ₂ CO ₃ (saturated solution) and 2 g/L LiOH. The pH of the solution was 12. The steel blanks were immersed in the aqueous solution for an immersion time. Table 3 shows the details of each treatment method. These are the application method, the pH of the aqueous solution, the immersion time and the temperature of the steel blanks during treatment. The resulting properties are listed in Table 2. These are the coating weight of the absorption layer and the average reflectance in the infrared range.

1a and 1b show an example of a top view of the absorption layer from experiment 4. The crystalline form of the lithium hydrotalcite, which contributes to the increased absorption, can be clearly seen.

2 shows a measurement of the spectral reflectivity ρ _k as a function of the wave number k for the experiments 1-6 according to Tables 2 and 3. The experiment number is given as a reference in each case. It can be clearly seen that the spectral reflectivity ρ _k in the infrared range falls significantly compared to reference example 1 as a result of the application of the absorption layer.

The steel blanks produced in this way were then processed into a shaped sheet metal part by means of hot forming. For this purpose, the blanks were heated in a roller hearth furnace from room temperature with an average heating rate r _furnace (between 30°C and 700°C) with a furnace temperature of 920°C. The mean heating rate and the mean heating are given in Table 2. It can be clearly seen that the average heating was more than 15 km/s in all cases according to the invention.

The blanks were then further processed in a conventional manner. For this purpose, the blanks were removed from the roller hearth furnace and placed in a forming tool. When removed from the oven, the blanks had taken on the oven temperature. The transfer time made up of the removal from the heating device, transport to the mold and insertion into the mold was around 10 seconds. The temperature of the blanks when they were placed in the forming tool was in all cases above the respective AC1 temperature and thus also above Ms+100°C. The blanks were formed into the respective shaped sheet metal part in the forming tool, which was heated to room temperature, with the shaped sheet metal parts being cooled in the tool at a cooling rate of approx. 50 K/s for approx. 15 seconds. Finally, the samples were removed from the tool and cooled to room temperature. The cooling took place in still air at a cooling rate of 7 K/s. Table 1 (steel grades) steel C si Mn Al Cr Nb Ti B P s N sn ace Cu Mon Approx no 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 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 C 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 remainder iron and unavoidable impurities. All data in % by weight; Table 2 (properties of steel flat product) Attempt steel grade ladle analysis Layer thickness [µm] Pad weight [mg/m ² ] degree of reflection Heating rate [K/s] Thickness of blanks [mm] Heating [Kmm/s] si feet mg Other Al 1* A 8.1 3.9 <0.01 <1% rest 27 - 0.58 7.1 1.8 12.8 2 A 8.1 3.9 <0.01 <1% rest 27 83 0.43 10.2 1.8 18.4 3 A 8.1 3.9 <0.01 <1% rest 27 106 0.35 14.0 1.8 25.2 4 A 8.1 3.9 <0.01 <1% rest 27 148 0.20 16.1 1.8 28.9 5 A 8.2 3.8 0.25 <1% rest 28 295 0.11 20.6 1.8 37.0 6 A 8.1 3.9 <0.01 <1% rest 27 534 0.09 22.8 1.8 41.0 7 B 8.1 3.9 <0.01 <1% rest 29 158 0.18 16.6 1.8 29.9 8th C 8.1 3.9 <0.01 <1% rest 26 145 0.21 15.9 1.8 28.6 * reference examples not according to the invention Table 3 (manufacturing conditions steel flat product) Attempt application method PH value dive time [s] Temperature [°C] 1* - - - - 2 immersion 12 60 60 3 immersion 12 120 60 4 immersion 12 180 60 5 immersion 12 300 60 6 immersion 12 600 60 7 immersion 12 180 60 8th immersion 12 180 60 * reference examples not according to the invention

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2012/120081 A2 [0005]

Claims (13)

Steel flat product for the production of a steel component by hot forming, comprising a steel substrate consisting of a steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and one overlying the steel substrate Aluminum-based anti-corrosion coating, an absorption layer being arranged on the anti-corrosion coating, characterized in that the absorption layer comprises lithium aluminum hydroxide. flat steel product claim 1 , characterized in that the absorption layer comprises carbonates of lithium aluminum hydroxide, in particular lithium hydrotalcite ([LiAl ₂ (OH) ₆ ] ₂ (CO ₃ ) · 4H ₂ O). Steel flat product according to one of Claims 1 until 2 , characterized in that the coating weight of the absorption layer is 20 - 750 mg/m ² . Steel flat product according to one of Claims 1 until 3 , characterized in that the aluminum-based anti-corrosion coating has an Al base layer consisting of 1.0 - 15% by weight Si, optionally 2-4% by weight Fe, optionally up to 5% by weight alkali or alkaline earth metals, optionally up to 15% by weight Zn and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum. flat steel product claim 4 , characterized in that the Al base layer contains up to 1.0% by weight alkali or alkaline earth metals, which comprises 0.1 - 1.0% by weight Mg and wherein the absorption layer comprises a mixture of hydrotalcite and lithium hydrotalcite. flat steel product claim 5 characterized in that the proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is at least 5% by weight and at most 90% by weight. Steel flat product according to one of Claims 1 until 6 , characterized in that the reflectance R in the infrared range is less than 0.55, preferably less than 0.50, in particular less than 0.40, preferably less than 0.30. Use of lithium aluminum hydroxide in an absorption layer on an aluminum-based anti-corrosion coating to reduce reflectivity in the infrared range. Process for the production of a flat steel product claim 1 comprising the following steps: a) providing a steel flat product comprising a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and one on the steel substrate aluminum-based anti-corrosion coating applied, b) application of an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) to the flat steel product, in particular immersing the flat steel product in an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) or spraying the flat steel product with a lithium carbonate (Li ₂ CO ₃ ) comprising aqueous solution or coating the steel flat product with a lithium carbonate (Li ₂ CO ₃ ) comprising aqueous solution in the coil coating process procedure after claim 9 , characterized in that the pH of the aqueous solution is at least 11 and at most 15, in particular at most 13. Procedure according to one of claims 9 until 10 , characterized in that an aqueous solution comprising lithium carbonate (Li ₂ CO ₃ ) is applied to the flat steel product by immersing the flat steel product in the aqueous solution for an immersion time of 60 s to 300 s, preferably 120 s to 240 s. Procedure according to one of claims 9 until 11 , characterized in that the flat steel product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, when the aqueous solution is applied. A method for producing a shaped sheet metal part, comprising the following steps: a) providing a sheet metal blank from a flat steel product according to one of Claims 1 until 7 ; b) heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature T _Einlg of the blank when it is placed in a forming tool provided for hot-press forming (step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature, with the mean heating being greater than 15km/s; c) inserting the heated sheet metal blank into a forming tool, the transfer time t _Trans required for removing it from the heating device and inserting the blank being at most 20 s, preferably at most 15 s; d) hot-press forming of the sheet metal blank to form the shaped sheet metal part, the blank being cooled down to the target temperature T _target over a period twz of more than 1 s at a cooling rate rwz, at least in part more than 30 K/s, during the course of the hot-press forming and optionally being held there; e) removing the shaped sheet metal part, which has been cooled to the target temperature T _{target ,} from the tool.

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