WO2024110306A1

WO2024110306A1 - Method of hot press forming, with improved process window

Info

Publication number: WO2024110306A1
Application number: PCT/EP2023/082054
Authority: WO
Inventors: Stefan BIENHOLZ; Sebastian STILLE; Maria KÖYER; Janko Banik; David Hoffmann
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2022-11-21
Filing date: 2023-11-16
Publication date: 2024-05-30
Also published as: DE102022130775A1

Abstract

The invention relates to a method for producing a sheet metal part (9) from a sheet metal blank having an aluminum-based anti-corrosion coating. According to the invention, the sheet metal blank is heated in a furnace and formed and cooled in a forming tool. By setting an annealing parameter Ψ advantageous bending properties are achieved.

Description

Hot press forming process with improved process window

The invention relates to a method for producing a sheet metal part by hot forming a steel sheet blank.

“Steel sheet blanks” or “sheet metal blanks” are understood here to mean blanks from flat steel products, such as blanks. When we talk about a “steel flat product” or a “sheet metal product”, we mean rolled products such as steel strips or sheets from which “sheet metal blanks” (also called blanks) are cut for the manufacture of, for example, bodywork components. “Formed sheet metal parts” or “sheet metal components” are made from such sheet metal blanks, whereby the terms “formed sheet metal part” and “sheet metal component” are used synonymously here.

All information on the contents of the steel compositions specified in the present application is based on weight, unless expressly stated otherwise. All unspecified "% data" in connection with a steel alloy are therefore to be understood as data in "wt. %". With the exception of the data on the residual austenite content of the structure of a sheet metal molding according to the invention, which is based on the volume (data in "vol. %), data on the contents of the various structural components (e.g. martensite) relate to the area of a cross-section of a sample of the respective product (data in area percent "area %"), unless expressly stated otherwise. Data on the contents of the components of an atmosphere given in this text relate to the volume (data in "vol. %).

Where formulae or conditions are given in this text in which values are calculated or formed on the basis of contents of certain alloying elements, the relevant contents of alloying elements are inserted in these formulae or conditions in % by weight, unless otherwise stated.

The bending angle is determined according to VDA standard 238-100 for the maximum force. For the purposes of this application, the bending angle is understood to be the bending angle along the rolling direction.

EP 3 611 288 A1 discloses various methods for producing a sheet metal part by hot forming a sheet metal blank. The sheet metal blank has a coating made of aluminium or an aluminium alloy on at least one side and has a thickness of 6 to 26 pm.

Investigations into such processes have shown that, despite the same process conditions, variations in the properties of the sheet metal parts produced can occur. In particular, it was shown that the bending angle can vary both across the sheet metal part and from sheet metal part to sheet metal part. Deviations in the range of +/- 3° are not uncommon. However, the bending angle is a particularly critical value in automotive engineering, as it is a good indicator for describing the ductility behavior of the material. A minimum bending angle is often specified for safety-relevant components. Excessively large ranges in the bending angle are therefore particularly disadvantageous. In order to be able to reliably guarantee the minimum bending angle over a large range, the components must be oversized accordingly, which is disadvantageous.

The object of the present invention is to further develop such processes in such a way that a stable process window is created in which sheet metal parts with optimal properties can be produced. In particular, the bending angle should vary as little as possible.

This object is achieved by a method for producing a sheet metal part, comprising the following work steps: a. Providing a sheet metal blank from a flat steel product with a thickness d of at least 0.7 mm and a maximum of 4.0 mm, comprising a steel substrate consisting of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the sheet metal blank has an aluminum-based anti-corrosion coating on at least one side with a one-sided coating weight of 10-40

wherein the anti-corrosive coating has an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum; b. Heating the sheet metal blank in a furnace with a furnace temperature Tof _en in an annealing time tG such that the AC3 temperature of the blank is at least partially exceeded and the temperature T _E jnig of the blank when placed in a a forming tool provided for hot press forming (work step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t _Tran s required for removing the blank from the heating device and inserting the blank is at most 20 s, preferably at most 15 s; d. Hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled to the target temperature Tziei over a duration twz of more than 1s at a cooling rate rwz which is at least partially more than 30 K/s and is optionally held there; e. Removing the sheet metal part cooled to the target temperature T _Ziei from the tool; characterized in that the sheet metal part has a corrosion protection coating with a thickness dA, which comprises an alloy layer with a thickness d|_, wherein for an annealing parameter ip with

calculated from the standardized glow time t _norm according to

applies: * > 0.10 min ^-1 .

While the standardized annealing time is typically given in s (seconds), it is more useful to specify the annealing parameter » in min ^-1 (1/minutes). To do this, the standardized annealing time in minutes is inserted into the upper equation. The thickness of the anti-corrosive coating dA and the thickness di. of the alloy layer are determined in the metallographic micrograph of the component.

Surprisingly, it has been shown that the annealing parameter ip correlates with the standard deviation of the bending angle. Compliance with this condition ensures that there is not too great a variation in the bending angle across the sheet metal part produced or between successively produced sheet metal parts. The starting material used in the process according to the invention is a sheet metal blank made from a flat steel product with a thickness d of at least 0.7 mm and a maximum of 4.0 mm, comprising a steel substrate made of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The sheet metal blank has an aluminum-based corrosion protection coating on at least one side with a one-sided coating weight of

wherein the anti-corrosive coating has an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. The thickness d is preferably a maximum of 3.5 mm, in particular a maximum of 3.3 mm, preferably a maximum of 2.5 mm, in particular a maximum of 2.0 mm, preferably a maximum of 1.8 mm. Furthermore preferably, the thickness d is at least 0.9 mm, in particular at least 1.1 mm, preferably at least 1.3 mm. In a special embodiment, the thickness d is 1.5 mm.

In a preferred embodiment, the sheet metal blank has an aluminum-based corrosion protection coating on both sides with a one-sided coating weight of 10-40

on the

9 The weight of the print on both sides in this case is 20-80

Preferably, the one-sided coating weight (both in the one-sided coated variant and in the two-sided coated variant) is at least 15-^ and/or a maximum of 30— , in particular a maximum of 25 -7 The two large surfaces of the sheet metal blank that are opposite one another are referred to as the two sides of the sheet metal blank. The narrow surfaces are referred to as the edges.

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 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

In a preferred variant, the Si content of the melt is 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, in particular 0.2-0.4 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.

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 when it solidifies.

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially made of aluminum and iron. The other 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 a-iron, optional further components, the total contents of which 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 components include in particular the other components 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.

The Al base layer lies on the alloy layer and is directly adjacent to it. The alloy layer is therefore located between the Al base layer and the steel substrate. In individual cases with particularly thin anti-corrosive coatings, the Al base layer is not continuous, so that there are areas in which the alloy layer is not covered by the Al base layer. The composition of the Al base layer preferably corresponds to the composition of the melt of the melt bath. This means that it consists of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 5 wt.% alkali or alkaline earth metals, preferably up to 1.0% wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

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, in particular 0.2-0.4 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. In another 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.

In a preferred variant, the sheet metal blank comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer finish of the anti-corrosive coating.

The oxide layer consists in particular of more than 80% by weight of oxides, the majority of the oxides (i.e. more than 50% by weight 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 taken up by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and/or magnesium in metallic form.

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.

In the method according to the invention, such a sheet metal blank is provided (work step a)), which is then heated in an oven with an oven temperature Tof _en for an annealing time tc such that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einig of the blank when placed in a forming tool intended for hot press forming (work step c)) is at least partially above Ms+100°C, in particular above Ms+300°C. In particular, the temperature T _Einig of the blank when placed at least partially exceeds 600°C. In a particularly preferred variant, the temperature T _Einig of the blank when placed is at least partially, in particular completely, in the range 600°C to 850°C, in order to ensure good formability and sufficient hardenability. 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, preferably the entire blank, exceeds a corresponding temperature. The same applies to the at least partial presence of a temperature in the range 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 completed 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 level than others during heating. To do this, the heat supply can be 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 structure there is significantly softer than in the other parts that have a martensitic structure. In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by ensuring optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

Maximum strength properties of the resulting 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.

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.

AC3[°C] = (902 wt.% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[ ^o C/wt.%] 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.

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

In a preferred variant, the oven has a constant oven temperature. However, it can also be advantageous to have an oven with different zones with different temperatures. (or different ovens with different temperatures). In such a case, the oven temperature T _O f _e n is determined as follows:

where N is the number of furnace zones, T _{t is} the temperature of the i-th furnace zone and t _t is the time that the sheet metal blank spends in the i-th furnace zone, i.e. the time that the sheet metal blank is exposed to the i-th temperature. In such a case, the furnace temperature is calculated as a time-weighted average. In such a case, the annealing time tc is the sum of the times that the sheet metal blank is exposed to the i-th temperature. This means:

In a preferred embodiment, the average heating rate r _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 _of is to be understood as the average heating rate from 30°C to 700°C.

In a preferred embodiment, the standardized average heating 0_norm is at least 5 Kmm/s, in particular at least 8 Kmm/s, preferably at least 10 Kmm/s. The maximum standardized average heating is 15 Kmm/s, in particular a maximum of 14 Kmm/s, preferably a maximum of 13 Kmm/s.

The average heating rate 0 is the product of the average heating rate in Kelvin per second from 30°C to 700°C and the sheet thickness in millimeters.

In the normalized average heating, this product 0 is normalized by the present furnace temperature T _O fen in relation to a reference furnace temperature T _O f _e n, reference of 900°C = 1173.15 K in the following way:

The oven temperatures must be entered in Kelvin. In a preferred embodiment, the heating takes place in a furnace with a furnace temperature T _O fen of at least Ac3+10°C, preferably at least 850°C, preferably at least 880°C, particularly preferably at least 900°C and a maximum of 1000°C, preferably a maximum of 960°C, particularly preferably a maximum of 940°C, in particular a maximum of 930°C.

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

In a special embodiment, the heating in step b) takes place step by step 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 with 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.

In order to better describe the processes during heating, the total time in the furnace tc, which is added up from the individual times as explained above, is standardized with respect to the thickness of the sheet metal blank and with respect to the furnace temperature T _O fen. Since the diffusion behavior in the corrosion protection coating in the temperature range for the furnace temperature T _O f _e n from 880°C to 960°C depends essentially linearly on the furnace temperature Tof _en , a correction term can be used that depends linearly on the furnace temperature Tof _en . Likewise, the diffusion behavior in the sheet thickness range from 0.7 mm to 3 mm depends essentially linearly on the sheet thickness d. Therefore, a correction term that depends linearly on the sheet thickness can also be used here. Empirical studies have shown that the following approximation is well suited to describing the diffusion processes:

In a preferred embodiment, the standardized annealing time is at least 140 seconds, in particular 2.5 minutes. Furthermore, the standardized annealing time is preferably a maximum of 8 minutes, preferably a maximum of 6 minutes, in particular a maximum of 5 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 for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties. It has also been shown that the maximum bending angle decreases again if the annealing times are too long (see e.g. EP 3 611 288 Al)

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, in particular above Ms+300°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms refers to the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACl temperature. In all of these variants, the temperature is in particular a maximum of 900°C. These temperature ranges ensure good formability of the material overall.

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 a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling before forming.

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 heated 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 Twz is preferably a maximum of 600°C, in particular a maximum of 550°C. It only has to be ensured that the tool temperature Twz is below the desired target temperature T^i. 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 25s, in particular a maximum of 20s, preferably a maximum of 10s. The target temperature Taei 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 Taei 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.

The martensite start temperature of a steel within the scope of the inventive specifications is according to the formula:

Ms [°C] = (490.85 wt.% — 302.6 %C — 30.6 %Mn - 16.6 %Ni — 8.9 %Cr + 2.4 %Mo — 11.3 %Cu + 8.58 %Co + 7.4 %W — 14.5 %Si) [°C/wt.%], 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 is the Co content, %W is the W content and %Si is the Si content of the respective steel in wt.%.

The ACl temperature and the AC3 temperature of a steel within the scope of the inventive specifications are according to the formulas:

AC1[°C] = (739 wt.% — 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni + 20*%V)[ ^o C/wt.%]

AC3[°C] = (902 wt.% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[ ^o C/wt.%], where %C denotes the C content, %Si the Si content, %Mn the Mn content, %Cr the Cr content, %Mo the Mo content, %Ni the Ni content and +%V the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8 10).

In the tool, the blank is not only formed into the sheet metal part, but is also quenched to the target temperature at the same time. The cooling rate in the tool r _wz 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, in a special design at least 100 K/s. The cooling rate is iwz is the cooling rate for martensite formation. The cooling rate r _wz is therefore determined between the insertion temperature and the martensite start temperature. Further cooling from the martensite start temperature to the target temperature can also be carried out at a lower cooling rate.

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

The sheet metal part produced in this way has a corrosion protection coating with a thickness d _A. This corrosion protection coating also comprises an alloy layer with a thickness du

The thickness of the corrosion protection coating d _A is preferably at least 6 pm, particularly preferably at least 8 pm, in particular at least 10 pm. Furthermore preferably, the thickness of the corrosion protection coating d _A is maximum 28 pm, preferably maximum 25 pm, in particular maximum 22 pm, in particular maximum 20 pm, preferably maximum 18 pm, in particular maximum 16 pm.

The thickness of the alloy layer d|_ is preferably a maximum of 15 pm, in particular a maximum of 10 pm, particularly preferably a maximum of 8 pm, in particular a maximum of 6 pm. Furthermore preferably, the thickness of the alloy layer d _L is at least 2 pm, preferably at least 4 pm.

The anti-corrosive coating of the sheet metal part preferably comprises an alloy layer and an optional Al base layer. In the case of the sheet metal part, the alloy layer is also often referred to as an interdiffusion layer. The thickness of the optional Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. Depending on the strength of the diffusion during the previous heat treatment, the original Al base layer may also have been completely converted into an alloy layer. In this case, the anti-corrosive coating of the sheet metal part comprises an alloy layer but no Al base layer. Alternatively, part of the original Al base layer remains. Only the composition of the Al base layer changes to the composition described below. Extensive tests in the production of sheet metal parts have shown that a particularly advantageous process window exists when an annealing parameter with

applies: * > 0.10 min ^-1

In such a case, the process described above reliably produces sheet metal parts whose bending angles exhibit a small scatter.

In a preferred embodiment of the method, the ratio of the thickness d|_ of the alloy layer of the sheet metal part to the thickness dA of the corrosion protection coating of the sheet _metal part is at least 30%. In a further preferred variant, the ratio is a maximum of 70%.

In a preferred variant of the process, the sheet metal part produced is designed in such a way that the alloy layer rests on the steel substrate and is directly adjacent to it.

Furthermore, the corrosion protection coating of the sheet metal part preferably comprises an Al base layer, wherein the alloy layer of the corrosion protection coating of the sheet metal part is arranged between the Al base layer and the steel substrate.

The alloy layer of the sheet metal part preferably consists of 35-90 wt.% Fe, 0.1-12 wt.% Si, optionally up to 0.5 wt.% Mg and optional additional components, the total contents of which are limited to a maximum of 3.5 wt.%, and the remainder aluminum. The optional additional components are preferably the elements present in the steel of the steel substrate in addition to iron. Due to the further diffusion of iron into the alloy layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten bath.

The alloy layer preferably has a ferritic structure. The optionally present Al base layer of the sheet metal part preferably lies on 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 further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

The optionally present Al base layer can have a homogeneous element distribution in which the local element contents do not vary by more than 10%. Preferred variants of the Al base layer, on the other hand, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are areas whose average Si content is at least 20% less than the average Si content of the Al base layer. Silicon-rich phases are areas whose average Si content is at least 20% more than the average Si content of the Al base layer.

In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form a layer that is at least 40% continuous and is bordered by silicon-poor regions. A continuous layer of silicon-rich phases means that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs completely through the silicon-rich phases. In contrast, a layer that is at least X% continuous means that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs at least X% within the silicon-rich phases. In the present case, the silicon-rich phases are therefore arranged in such a connected manner that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs at least 40% within the silicon-rich phases. In an alternative embodiment, the silicon-rich phases are arranged in islands in the silicon-poor phase.

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.

In a preferred variant, the steel component comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer lies in particular on the optionally present Al base layer and preferably forms the outer finish of the anti-corrosive coating. If no Al base layer is present, the oxide layer preferably lies on the alloy layer and preferably forms the outer finish of the corrosion protection coating.

The oxide layer of the steel component consists in particular of more than 80% by weight of oxides, with the majority of the oxides (i.e. more than 50% by weight 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 taken up by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and/or magnesium in metallic form.

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 pm, in particular a maximum of 2 pm.

In a special development, the steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite or lower bainite, preferably at least partially more than 90% martensite or lower bainite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%. In this context, “partially have” is to be understood as meaning that there are areas of the sheet metal part that have the mentioned structure. In addition, there can also be areas of the sheet metal part that have a different structure. The sheet metal part therefore has the mentioned structure in sections or in regions.

Due to the high content of martensite or lower bainite, very high tensile strengths and yield strengths can be achieved.

The sheet metal part according to the invention is preferably a component for a land vehicle, sea vehicle or aircraft. It is particularly preferably an automobile part, in particular a body part. The component is preferably a B-pillar, longitudinal member, A-pillar, sill or cross member or a component of the vehicle side structure.

The steel substrate of the flat steel product and thus also of the sheet metal part produced is made of a steel that contains 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. In particular, the structure of the steel can be converted into a martensitic or partially martensitic structure by hot forming. The structure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic structure, since this has a particularly high hardness.

Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt. %), consists of

C: 0.04 - 0.45 wt.%,

Si: 0.02 - 1.2 wt.%,

Mn: 0.5 - 2.6 wt.%,

Al: 0.02 - 1.0 wt.%,

P: < 0.05 wt.%,

S: < 0.02 wt.%,

N: < 0.02 wt.%,

Sn: < 0.03 wt.%

As: < 0.01 wt.%

Ca: < 0.005 wt.% 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 wt.%,

B: 0.001 - 0.005 wt.%

Mo: <0.5 wt.%

Ni: <0.5 wt%

Cu: <0.2 wt.%

Nb: 0.01 - 0.08 wt.%,

Ti: 0.01 - 0.08 wt.%

V: <0.2 wt.%.

The elements P, S, N, Sn, As, Ca are impurities that cannot be completely avoided in steel production. Ca is also sometimes deliberately added to Sulfur is added to the alloy to bind it. In such a case, the Ca content is at least 0.001% by weight. The maximum Ca content in this case is also 0.005% by weight.

In addition to these elements, other elements may 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 a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The optional alloying elements Cr, B, Nb, Ti, for which a lower limit is specified, can also be present in the steel substrate as unavoidable impurities in contents below the respective lower limit. In this case, they are also counted as 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 wt.%,

B: < 0.0005 wt.%

Nb: < 0.005 wt.%,

Ti: < 0.005 wt.%

These preferred upper limits should be considered alternatively or together. Preferred variants of the steel therefore meet one or more of these four conditions.

In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and/or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.% or in the range of 0.11-0.25 wt.% or in the range of 0.32-0.37 wt.%.

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

In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt.% and/or at least 0.75 wt.%. In particularly preferred embodiments, the Mn content is in the range of 0.75-0.85 wt.% or in the range of 1.0-1.6 wt.%. In a preferred variant, the Al content of the steel is a maximum of 0.75% by weight, in particular a maximum of 0.5% by weight, preferably a maximum of 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 contents of silicon and aluminum is limited. In a preferred variant, the sum of the contents of Si and Al (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. In addition or alternatively, the sum of the contents of Si and Al is at least 0.06 wt.%, preferably at least 0.08 wt.%.

The elements P, S and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03% by weight. Irrespective of this, the S content is preferably a maximum of 0.012%. In addition or in addition, the N content is preferably a maximum of 0.009% by weight.

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

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

Preferably, the steel optionally also contains boron in a content of 0.001-0.005 wt.%. In particular, the B content is a maximum of 0.004 wt.%.

Optionally, the steel may contain molybdenum in a content of not more than 0.5% by weight, in particular not more than 0.1% by weight.

Furthermore, the steel can optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

Optionally, the steel may also contain copper with a maximum content of 0.2 wt.%, preferably a maximum of 0.15 wt.%. In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti and V. The optional Nb content is at least 0.01 wt.%, in particular at least 0.02 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional V content is a maximum of 0.2 wt.%, in particular a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

In the case of an optional alloying of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The sum is a maximum of 0.1 wt.%, in particular a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

To demonstrate the effect of the invention, several tests were carried out. For this purpose, slabs with the compositions given in Table 1 with a thickness of 240 mm and a width of 1200 mm were produced and heated in a pusher furnace to a temperature TI of 1200°C. The slabs were then held at TI for between 30 and 450 minutes until the temperature TI in the core of the slabs was reached and the slabs were thus heated through. The slabs were discharged from the pusher furnace at their respective through-heating temperature TI 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, with the intermediate products, which can also be referred to as pre-strips in hot strip rolling, each having an intermediate product temperature T2 of 1100°C at the end of the pre-rolling phase. The pre-strips were fed to the finish rolling immediately after the pre-rolling so that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish rolling phase. The pre-strips were rolled out to hot strips with a final thickness of 4 mm and a final rolling temperature T3 of 890°C, cooled to the respective coiling temperature and wound into coils at a coiling temperature T4 of 580°C and then cooled in still air. The hot strips were descaled in a conventional manner by pickling before they were subjected to cold rolling until they reached the thickness specified in Table 3. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 of 870°C and kept at annealing temperature for 100s each before they were cooled at a cooling rate of 1 K/s to the immersion temperature T6 of 690°C. The cold strips were immersed at their respective immersion temperature T6 through a molten coating bath with a temperature T7 of 676°C. The strip speed was 76 m/min in all cases. The compositions of the coating bath are given in Table 2. After coating, the coated strips were blown off to adjust the coating weights. An air stream was used for this. The temperature of the air stream was 70°C in all cases. During the process, all of the strips were coated on both sides, with the one-sided coating weight being given in Table 3. The coating weight is identical on both sides in this process. 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 over cooling times T _mT of 18s and T _nT of 15s. Between 450°C and 400°C and below 220°C, the strips were cooled at a cooling rate of 5-15 K/s each.

Blanks were cut from the steel strips produced in this way and used for further tests. In these tests, sheet metal part samples in the form of 200 x 300 mm ² plates were hot-pressed from the respective blanks. The parameters explained below are given in Tables 3 and 4 for each of the 26 tests. For hot-press forming, the blanks were heated in a heating device, for example in a conventional heating furnace, from room temperature with an average heating rate rof _en (between 30°C and 700°C) in an oven with an oven temperature Tofen. The annealing time in the oven, which includes heating and holding, is designated te. The dew point of the oven 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 Twz. At the time of removal from the oven, the blanks had reached the oven temperature. The transfer time t _Tr ans, which consists of the removal from the heating device, the transport to the tool and the insertion into the tool, was 5s. The temperature T _Einig _of the blanks when inserted into the forming tool was in all cases above the respective martensite start temperature +100°C. The blanks were formed into the respective sheet metal part in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate rwz to a target temperature T^i. The residence time in the tool is designated as twz. Finally, the samples were cooled in air to room temperature.

The results are summarized in Table 5 in the form of the properties of the sheet metal parts obtained. The thickness dA of the anti-corrosive coating and the thickness d|_ of the alloy layer were determined for this purpose in the metallographic micrograph. Figure 1 shows a schematic representation of such a micrograph of a sheet metal part 9. A steel substrate 1 is shown. A corrosion protection coating 3 is arranged on the steel substrate 1. The corrosion protection coating 3 has a thickness dA. The corrosion protection coating 3 comprises an alloy layer 5 and an optional Al base layer 7. The alloy layer 5 has a thickness d|_. The alloy layer 5 lies on the steel substrate 1 and is directly adjacent to it. The Al base layer 7 lies on the alloy layer 5 and is directly adjacent to it.

Also given in Table 5 is the annealing parameter with

In addition, the martensite content, the mean value of the bending angle and the standard deviation of the bending angle are given in Table 5. The martensite content was determined on a cross-section. The bending angle was determined on 6 samples along the rolling direction in accordance with VDA standard 238-100 for the maximum force. The mean value and the standard deviation of the bending angle were determined from these 6 bending tests of the same sheet metal part. It can be clearly seen that the standard deviation of the bending angle is lower with the annealing parameters according to the invention. By complying with the condition for it is ensured that there is not too great a variation in the bending angle across the sheet metal part produced or between sheet metal parts produced one after the other.

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Table 1 (steel grades)

Remainder iron and unavoidable impurities. Values in % by weight;

Table 2 (Coating variants)

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Table 3 (Process parameters)

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Table 4 (Procedure parameters continued)

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Table 5 (component properties)

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Claims

Patent claims

1. Method for producing a sheet metal part (9) comprising the following steps: a. Providing a sheet metal blank from a flat steel product with a thickness d of at least 0.7 mm and a maximum of 4.0 mm comprising a steel substrate (1) which consists of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the sheet metal blank has an aluminum-based anti-corrosion coating with a one-sided coating weight of 10-40

wherein the corrosion protection coating has an Al base layer which consists of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum; b. heating the sheet metal blank in a furnace with a furnace temperature Tof _en > for an annealing time tc such that the AC3 temperature of the blank is at least partially exceeded and the temperature T^ig of the blank when placed in a forming tool intended for hot press forming (work step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c. Inserting the heated sheet metal blank into a forming tool, the transfer time t _Tran s required for removing it from the heating device and inserting the blank being at most 20s, preferably at most 15s; d. Hot press forming the sheet metal blank to form the sheet metal part, the blank being cooled to the target temperature Tziei during the hot press forming over a period twz of more than ls with a cooling rate rwz that is at least partially more than 30 K/s and optionally being held there; e. Removing the sheet metal part (9) cooled to the target temperature T _Zjei from the tool; characterized in that the sheet metal part (9) has an anti-corrosive coating (3) with a thickness dA, which comprises an alloy layer (5) with a thickness d|_, wherein for an annealing parameter i with

calculated from the standardized glow time t _norm according to

applies: * > 0.10 min ^-1 . Method according to claim 1, characterized in that the furnace temperature is at least 880°C, in particular at least 900°C and/or a maximum of 960°C, in particular a maximum of 940°C. Method according to one of claims 1 to 2, characterized in that the standardized annealing time is at least 140 seconds, in particular 2.5 minutes and/or a maximum of 8 minutes, in particular a maximum of 6 minutes, preferably a maximum of 5 minutes. Method according to one of claims 1 to 3, characterized in that the ratio of the thickness d|_ of the alloy layer (5) of the sheet metal part (9) to the thickness dA of the

Anti-corrosive coating (3) of the sheet metal part

is at least 30% and/or a maximum of 70%. Method according to one of claims 1 to 4, wherein the temperature at least partially reached in the sheet metal blank in step b) is between Ac3 and 1000°C, preferably between 850°C and 950°C. Method according to one of claims 1 to 5, wherein the target temperature T _Zjei of the sheet metal part is at least partially below 400°C, preferably below 300°C. Method according to one of claims 1 to 6, characterized in that the corrosion protection coating of the sheet metal blank provided in step a) has an alloy layer between the Al base layer and the steel substrate. Method according to claim 7, characterized in that the alloy layer of the sheet metal blank consists of 35-60 wt.% Fe, optional further components, the contents of which are limited in total to a maximum of 5.0 wt.%, and the remainder aluminum. Method according to one of claims 1 to 8, characterized in that the anti-corrosive coating (3) of the sheet metal part (9) comprises an Al base layer (7), wherein the alloy layer (5) of the anti-corrosive coating (3) of the sheet metal part (9) is arranged between the Al base layer (7) and the steel substrate (1). Method according to claim 9, characterized in that the Al base layer of the sheet metal part consists of 35-55 wt.% Fe, 0.4-10 wt.% Si, optionally up to 0.5 wt.% Mg and optional further components, the contents of which are limited in total to a maximum of 2.0 wt.%, and the remainder aluminum. Method according to one of claims 1 to 10, characterized in that the alloy layer (5) of the sheet metal part (9) consists of 35-90 wt.% Fe, 0.1-10 wt.% Si, optionally up to 0.5 wt.% Mg and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. Method according to one of claims 1 to 11, characterized in that the steel, in addition to iron and unavoidable impurities (in wt.%), consists of

C: 0.04 - 0.45 wt.%,

Si: 0.02 - 1.2 wt.%,

Mn: 0.5 - 2.6 wt.%,

Al: 0.02 - 1.0 wt.%,

P: < 0.05 wt.%,

S: < 0.02 wt.%,

N: < 0.02 wt.%,

Sn: < 0.03 wt.%

As: < 0.01 wt.%

Ca: < 0.005 wt.% 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 wt.%,

B: 0.001 - 0.005 wt.%

Mo: <0.5 wt.%

Ni: <0.5 wt%

Cu: <0.2 wt.%

Nb: 0.01 - 0.08 wt.%,

Ti: 0.01 - 0.08 wt.%

V: <0.2 wt.%.