Method for hot press forming by way of electric resistance heating and with improved process window

Abstract

The invention relates to the production of a sheet metal part by hot forming. Heating prior to hot forming is at least partly done by electric resistance heating. The invention provides for a process window which defines reliable heating rates depending on the coating weight of an aluminum-based anti-corrosion coating.

Description

[0001] Process for hot press forming using electrical resistance heating and improved process window

[0002] The invention relates to a method for producing a sheet metal part by hot forming a steel sheet blank.

[0003] "Steel sheet blanks" or "sheet metal blanks" are understood here to mean blanks from flat steel products, such as blanks. When a "steel flat product" or "sheet metal product" is mentioned, this refers to rolled products, such as steel strips or sheets, from which "sheet metal blanks" (also called blanks) are cut for the production of, for example, car body components. "Formed sheet metal parts" or "sheet metal components" are made from such sheet metal blanks, whereby the terms "formed sheet metal part" and "sheet metal component" are used synonymously here.

[0004] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" data relating to a steel alloy are therefore to be understood as data in "wt%." With the exception of the data relating to the residual austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (specified in "vol%"), data on the contents of the various microstructure components (e.g., martensite) refer to the area of ​​a microsection of a sample of the respective product (specified in area percent, "area%"), unless expressly stated otherwise. Information provided in this text regarding the contents of the constituents of an atmosphere refers to the volume (specified in "vol%").

[0005] Where formulas or conditions are mentioned in this text in which values ​​are calculated or formed on the basis of contents of certain alloying elements, the respective contents of alloying elements are entered in these formulas or conditions in wt.%, unless otherwise stated.

[0006] EP 2 086 755 B1 discloses a method for producing a shaped sheet metal part by hot forming a sheet metal blank. The sheet metal blank has a coating of aluminum or an aluminum alloy on at least one side. EP 3 456 427 B1 discloses the heating of such sheet metal blanks by induction in order to shorten the heating time. It is also explained that heating by induction can be problematic because the coating becomes molten above a certain temperature. From this point on, the electromagnetic forces cause the coating to agglomerate in certain areas. The consequence of this is that the final coating exhibits significant local fluctuations in thickness. EP 3 456 427 B1 therefore teaches that from the point at which the coating becomes molten, the heating rate should be reduced to such an extent that agglomeration behavior is avoided.

[0007] This procedure has the disadvantage that the process time is extended because the material heats up much more slowly.

[0008] The object of the present invention is to identify a process window that enables heating by means of electrical resistance heating with a high heating rate.

[0009] Electrical resistance heating is defined as heating using the Joule effect. The material to be heated (here, a sheet metal blank) is subjected to electrical currents. The current flow against the material's resistivity generates heat directly in the material. Two types of electrical resistance heating are distinguished:

[0010] - Conductive heating: Applying an electric current to the material, which flows through the material and heats it

[0011] - Inductive heating: Exposure to the material with alternating electromagnetic fields that induce electrical currents in the material, whereby the induced currents heat the material

[0012] As explained with inductive heating, electromagnetic forces acting on a molten coating can cause agglomerations of the coating to form in certain areas. The same effect can also occur with conductive heating, since in this case, strong fields act on the molten coating.

[0013] This object is achieved by a method for producing a shaped 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 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 on at least one side an aluminum-based corrosion protection coating with a one-sided coating weight A of 10-100 ^7, 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 whose total contents are limited to a maximum of 2.0 wt.%, and the remainder being aluminum; b.Heating the sheet metal blank at least temporarily by means of electrical resistance heating such that a final temperature of the sheet metal blank during heating is at least partially greater than the AC3 temperature of the sheet metal blank and the temperature T. E ini g the blank, when placed in a forming tool intended for hot press forming (step d)), at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c. Optionally, holding the sheet blank at the final temperature for a holding time where the holding time t HaU at least 3 s and a maximum of 60 s; d. Inserting the heated sheet metal blank into a forming tool, whereby the transfer time required for removing the blank from the heating device and inserting it is t Trans is at most 20 s, preferably at most 15 s; e. hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is heated to a target temperature T during the hot press forming for a period twz of more than 1 s. Z iei is cooled and optionally kept there, whereby the cooling depends on the temperature T E ini g at least up to the martensite start temperature with a cooling rate at least partially exceeding 25 K / s; f. removing the sheet metal part cooled to the target temperature Tziei from the tool; characterized in that for the heating rate r Ind of the sheet metal blank during heating by means of electrical resistance heating: 30 < r Ind < 11 ■ 10 9 ■ yl -4 ' 364 + 20 where the coating weight A is in units and the heating rate r Ind deployed in the unit becomes.

[0014] Surprisingly, it has been shown that the permissible heating rate r Inddepends on the coating weight of the corrosion protection coating. This results from the processes that take place during heating. During heating, iron diffuses from the steel substrate into the coating. The increasing iron content in the coating leads to an increase in the liquidus temperature of the coating. In particular, the diffusion leads to the formation or growth of a preferably ferritic alloy layer at the transition between the steel substrate and the coating at the expense of the Al base layer. The alloy layer consists in particular of 35-90 wt.% Fe, 0.1-12 wt.% Si, optionally up to 0.5 wt.% Mg and optionally further components, the total contents of which are limited to a maximum of 3.5 wt.%, and the remainder is aluminum. This alloy layer, which is also referred to as the ternary phase, therefore has a melting point that is significantly higher than the melting point of the Al base layer. The agglomeration behavior therefore occurs only to a reduced extent in the alloy layer.However, the melting point of the Al base layer also increases successively with increasing iron content. Thus, two competing and coupled processes are present. Agglomeration essentially begins as soon as the Al base layer becomes molten. At the same time, however, diffusion also increases significantly at this point, which in turn dampens agglomeration due to the increase in melting point. Furthermore, the electromagnetic fields lead to increased movement in the molten phases, which in turn influences the diffusion behavior. Practice has shown that, depending on the coating weight, there is a favorable range for the heating rate r. Ind which is given by the formula described above. The examples described below demonstrate that layer thickness variations of less than 50% can be achieved by adhering to the formula described above.

[0015] In a preferred embodiment, the heating rate r Ind of the sheet metal blank during heating by electrical resistance heating: r Ind < 11 ■ 10 9 ■ A“ 4 ' 488 + 20

[0016] As can be seen from the examples described below, by adhering to this additional limit, layer thickness variations can be achieved to be less than 30%. In a preferred embodiment, the heating rate r Indat least 30 K / s, in particular at least 50 K / s, in particular at least 80 K / s, preferably at least 120 K / s, particularly preferably at least 150 K / s, in particular at least 200 K / s, preferably at least 300 K / s. A uniformly high heating rate enables a particularly economical process, as the sheet metal blanks can be processed quickly. In addition, there is less scrap in the event of disruptions in the process. If, for example, there are disruptions in the hot press forming process, all sheet metal blanks that were in the heating step at the time of the disruption must be disposed of. These sheet metal blanks cannot be heated again, as this would result in other diffusion processes in the coating. The hot forming process must always be carried out as a continuous process and cannot be interrupted in the meantime.With a fast heating rate, fewer sheet metal blanks are in the process at the same time, so less material needs to be disposed of in the event of a malfunction, making the process more efficient.

[0017] The heating rate r Ind is defined as the average heating rate during heating by electrical resistance heating. That is, r Ind is determined as the difference between final temperature T Ind of heating by means of electrical resistance heating and the starting temperature (typically room temperature) divided by the duration t Ind heating by electrical resistance heating.

[0018] 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 corrosion protection 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 whose total contents 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. 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 whose total contents are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

[0019] In a preferred variant, the Si content of the melt is 7-12 wt.%, in particular 8-10 wt.%.

[0020] 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.

[0021] During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer upon solidification.

[0022] The alloy layer lies on the steel substrate and directly borders it. The alloy layer is essentially formed from aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60 wt.% Fe, preferably α-iron, optional further constituents whose total contents are limited to a maximum of 5.0 wt.%, preferably 2.0%, and the remainder aluminum, with the Al content preferably increasing towards the surface. The optional further constituents include in particular the remaining constituents of the melt (i.e., silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

[0023] The Al base layer lies on top of the alloy layer and directly borders it. The alloy layer thus lies between the Al base layer and the steel substrate. In individual cases, with particularly thin corrosion protection coatings, the Al base layer is not continuous, so that there are areas where 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 molten 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 constituents whose total contents are limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

[0024] 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 in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0025] In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

[0026] In a preferred variant, the sheet metal blank comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located particularly on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.

[0027] The oxide layer consists in particular of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, either alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form.

[0028] Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm. In particular, the maximum thickness of the oxide layer is 500 nm.

[0029] In the method according to the invention, such a sheet metal blank is provided (working step a)), which is then heated at least temporarily by means of electrical resistance heating (working step b)) in such a way that a final temperature of the sheet metal blank is at least partially greater than the AC3 temperature of the blank and the temperature T E ini g of the blank when placed in a forming tool intended for hot pressing (step c)) is at least partially at a temperature above Ms+100°C, in particular above Ms+300°C. In particular, the temperature T E ini g of the blank during insertion at least partially 600 °C. In a particularly preferred variant, the temperature T E ini gof the blank during insertion at least partially, in particular completely in the range 600 °C to 850 °C, in order to ensure good formability and sufficient hardenability. Partially exceeding a temperature (here AC3 or Ms+100 °C) in the sense of this application is understood to mean that at least 30%, in particular at least 60% of the volume of the blank, preferably the entire blank, exceeds a corresponding temperature. The same applies to the at least partial presence of a temperature in the interval 600 °C to 850 °C in the preferred variant explained above. When inserted into the forming tool, at least 30% of the blank therefore has an austenitic structure, i.e. the transformation from the ferritic to the austenitic structure does not have to be complete when inserted into the forming tool.Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other microstructure components, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be deliberately kept at a lower temperature level than others during heating. To do this, the heat can be specifically directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat supply. In the part of the blank material whose temperature remains lower, no or only significantly less martensite is formed during forming in the tool, so that the microstructure there is significantly softer than in the other parts that have a martensitic microstructure.In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by providing optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0030] Maximum strength properties of the obtained sheet metal part can be achieved by the final temperature, at least partially achieved in the sheet metal blank, being between Ac3 and 1200 °C, in particular between Ac3 and 1100 °C, preferably between Ac3 and 1000 °C, particularly preferably between 850 °C and 950 °C.

[0031] 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.

[0032] AC3[°C] = (902% by weight - 225*%C + 19*%Si - ll*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55* o / oV)[°C / wt.-° / o] 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.

[0033] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0034] Heating the sheet metal blank, at least temporarily, to a final temperature by means of electrical resistance heating means that the sheet metal blank is heated from a temperature below 100 °C (typically room temperature) to a final temperature above AC3. This heating can be achieved entirely by electrical resistance heating. Alternatively, the heating can take place in several phases, for example, in a first heating phase by means of electrical resistance heating and, from a certain temperature, in a second heating phase by means of infrared radiation. Of course, more than two phases are also possible. Heating by means of electrical resistance heating means that at least 90% of the energy absorbed by the sheet metal blank is introduced via the Joule effect.

[0035] In a preferred embodiment, heating by electrical resistance heating is by induction heating. Heating by induction means that at least 90% of the energy absorbed by the sheet metal blank is provided via the induction coils. Heating by induction has the advantage that the energy can be introduced into the material without contact, which facilitates large-scale industrial use.

[0036] In an alternative preferred embodiment, the heating by means of electrical resistance heating is a conductive heating. Conductive heating has the advantage that no large system technology such as induction coils is required. Therefore, the space requirement and complexity are also reduced. Following heating of the sheet metal blank to the final temperature (step b), the sheet metal blank can optionally be held for a holding time t HaUbe kept at the final temperature, with the holding time t HaU at least 3 seconds, preferably at least 5 seconds, in particular at least 8 seconds. Furthermore, the holding time is a maximum of 60 seconds, preferably a maximum of 40 seconds, in particular a maximum of 20 seconds. During the holding time at the final temperature above AC3, the austenite grains in the steel substrate grow. This may be necessary to achieve the desired mechanical properties of the steel substrate.

[0037] Holding can be achieved by storing the material in a furnace at the same furnace temperatures. Alternatively, holding can also be achieved by electrical resistance heating, in which the applied power is reduced so that only the usual heat loss is compensated (e.g., reducing the induction power or reducing the applied external current).

[0038] The blank heated in this way is removed from the respective heating device and transported into the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms+100°C, in particular above Ms+300°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACl temperature. In all of these variants, the maximum temperature is in particular 900°C. These temperature ranges ensure good formability of the material overall.

[0039] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably no more than 20 seconds, especially within a maximum of 10 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0040] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C. When the blank is inserted, the tool can also have a temperature slightly below room temperature if, for example, the cooling water used is slightly colder (e.g. 15 °C). In some embodiments, the tool therefore has a temperature between 10 °C and 200 °C when the blank is inserted. Optionally, in a special embodiment, the tool can be tempered, at least in some areas, to a temperature 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 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 Tziei.The residence time in the tool twz is preferably at least 2 s, in particular at least 3 s, particularly preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, in particular at most 20 s, preferably at most 10 s.

[0041] The tool can be cooled with a cooling fluid, particularly cooling water, through channels running inside the tool. The cooling channels can also be partially open for more efficient cooling, allowing the cooling fluid to come into contact with the sheet metal part and resulting in faster cooling.

[0042] The target temperature Tziei 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 Tziei 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.

[0043] The martensite start temperature of a steel within the scope of the invention is according to the formula:

[0044] 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 denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%.

[0045] The ACl temperature and the AC3 temperature of a steel within the scope of the invention specifications are according to the formulas: AC 1[°C] = (739 wt.% - 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni + 20* 0 / oV)[°C / wt.-° / o]

[0046] AC3[°C] = (902 wt.% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[°C / wt.-° / o], 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).

[0047] In the tool, the blank is not only formed into the sheet metal part, but simultaneously quenched to the target temperature. The cooling rate in the tool r W z is in particular at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, in a special embodiment at least 100 K / s. The cooling rate rwz is defined as the average cooling rate between the temperature T E ini g when inserted into the forming tool and the martensite start temperature.

[0048] In a preferred embodiment, the cooling rate from the insertion temperature T E ini g to the martensite finish temperature of at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s. The martensite finish temperature is determined using dilatometer measurements.

[0049] The further cooling from the martensite start temperature or the martensite finish temperature to the target temperature T Z iei can also be carried out with lower cooling rates, since this cooling no longer has a significant effect on the microstructure formation.

[0050] After removal of the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature TAB of less than 100 °C within a cooling time t A B from 0.5 to 600s. This is usually achieved by air cooling.

[0051] In a preferred embodiment, the sheet metal blank has a corrosion protective coating on an aluminum base with a one-sided coating weight of 10-60. The double-sided coating weight in this case is 20-120. The preferred coating weight is one-sided coating weight (both in the one-sided coated version and in the double-sided coated version) at least 15^ and / or maximum 40^, preferably maximum The two sides of the sheet blank are the two large, opposing surfaces of the sheet metal blank. The narrow surfaces are called edges. From one-sided coating weights of or 15, a A significant improvement in corrosion protection can be observed. Smaller maximum one-sided coating weights such as those mentioned, preferably 40, especially 30, are preferred.

[0052] 25 allow higher heating rates and are therefore preferable.

[0053] In a preferred embodiment, the sheet metal blanks are exposed to an atmosphere with a dew point of a maximum of 5 °C, preferably a maximum of 0 °C, in particular a maximum of -5 °C, and particularly preferably a maximum of -10 °C, during heating by means of electrical resistance heating. This allows the formation of oxides and the introduction of diffusible hydrogen into the steel substrate to be controlled.

[0054] In a preferred embodiment, the heating of the sheet metal blank comprises a plurality of successive heating phases, wherein at least two heating phases from the plurality of heating phases differ by the heating method used.

[0055] A specific example of this embodiment is the design with a first heating phase and a second heating phase, wherein the sheet metal blank is heated by longitudinal field induction during the first heating phase and by transverse field induction or in a furnace with a furnace temperature of <0.05 in the second phase. The first and second heating phases therefore form the majority of the heating phases. Furthermore, the first and second heating phases differ in the heating method used. While longitudinal field induction is used as the heating method in the first heating phase, transverse field induction or heating in a furnace with a furnace temperature of <0.05 in the second heating phase is alternatively used. When heating in a furnace with a furnace temperature, the heat transfer occurs essentially by radiation. This specific example with two heating phases is described in more detail below.Other examples include preheating in a furnace to a first temperature and subsequent further heating by longitudinal or transverse field induction to the final temperature. Likewise, more than two heating phases can be provided. For example, the heating can initially involve a first heating phase to a first temperature, followed by a second heating phase to an intermediate temperature T. Zw , and then a third heating phase to the final temperature, wherein the sheet metal blank is preheated in a furnace during the first heating phase, heated by longitudinal field induction during the second heating phase, and heated in the third phase by transverse field induction or in a furnace at a furnace temperature of 100 °C. More than three heating phases are also possible.

[0056] Between two consecutive heating phases, an intermediate holding phase can optionally be provided in which the sheet metal blank is kept at the temperature reached.

[0057] In a preferred embodiment, the heating of the sheet metal blank comprises a first heating phase to an intermediate temperature T Zwand a second heating phase to the final temperature, wherein the sheet metal blank is heated during the first heating phase by means of longitudinal field induction and in the second phase by means of transverse field induction or in a furnace with a furnace temperature of 10 °C. In longitudinal field induction, the induction coils are oriented such that the resulting magnetic field lines run essentially parallel to the direction of extension of the sheet metal blank. In transverse field induction, the induction coils are oriented such that the resulting magnetic field lines run essentially perpendicular to the direction of extension of the sheet metal blank. The following preferably applies to the intermediate temperature T Zw :

[0058] T c - 50 K < T Zw < T C + 50K where T cThe Curie temperature of the steel substrate is referred to as the Curie temperature. Until the Curie temperature is reached, the steel substrate is ferromagnetic. Therefore, longitudinal field induction is very efficient below the Curie temperature. Above the Curie temperature, however, it is more economical to continue heating using transverse field induction or infrared radiation in a furnace. The Curie temperature is typically in the range of 730 °C to 770 °C. The intermediate temperature is particularly preferably located very close to the Curie temperature, meaning:

[0059] T c - 30K < T Zw < T C + 30K especially

[0060] T c - IOC < T Zw < T C + IOK In this way, the longitudinal field induction is used particularly efficiently.

[0061] In the case of heating during the second heating phase by means of a furnace, the heating takes place in a furnace with a furnace temperature Toten of at least Ac3+10°C, in particular at least 840 °C, preferably at least 850 °C, preferably at least 880 °C, particularly preferably at least 900 °C and at most 1000 °C, preferably at most 960 °C, particularly preferably at most 940 °C, in particular at most 930 °C.

[0062] The second heating phase (or, if there are more than two heating phases, the last heating phase) can preferably be followed by the optional holding phase, in which the sheet metal blank is held for a holding time t HaUThe final temperature is maintained above AC3. This can be achieved by storing the material in an oven at the same temperatures as explained in the previous paragraph. Alternatively, the holding can be achieved by electrical resistance heating, in which the applied power is reduced so that only the usual heat loss is compensated.

[0063] The sheet metal part produced using the method described above has a corrosion protection coating with a thickness d A This corrosion protection coating also comprises an alloy layer with a thickness d .

[0064] 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, the thickness of the corrosion protection coating d is preferably Amaximum 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.

[0065] The thickness of the alloy layer d i is preferably at most 15 pm, in particular at most 10 pm, particularly preferably at most 8 pm, in particular at most 6 pm. Furthermore, the thickness of the alloy layer d is preferably at least 2 pm, preferably at least 4 pm.

[0066] The corrosion protection coating of the sheet metal part preferably comprises an alloy layer and an optional Al base layer. In the sheet metal part, the alloy layer is also frequently referred to as an interdiffusion layer or ternary phase. The thickness of the optional Al base layer results from the difference between the thicknesses of the corrosion protection coating and the alloy layer. Depending on the strength of the diffusion during the preceding heat treatment, the original Al base layer may also have been completely converted into an alloy layer. In this case, the corrosion protection coating of the sheet metal part comprises an alloy layer but no Al base layer. Alternatively, a portion of the original Al base layer remains. Only the composition of the Al base layer changes to the composition described below.

[0067] In a preferred embodiment of the process, the ratio of the thickness d.i. 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 dA is a maximum of 70%.

[0068] In a preferred variant of the process, the sheet metal part produced is designed in such a way that the alloy layer lies on the steel substrate and is directly adjacent to it.

[0069] 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.

[0070] 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.%, with the remainder being aluminum. The optional additional components are preferably the elements present in the steel of the steel substrate alongside 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.

[0071] The alloy layer preferably has a ferritic structure.

[0072] The optionally present Al base layer of the sheet metal part preferably lies on top of the alloy layer of the steel component and is directly adjacent to it. The Al base layer of the steel component preferably consists of 35-55 wt.% Fe, 0.4-10 wt.% Si, optionally up to 0.5 wt.% Mg, and optionally other components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum.

[0073] The optionally present Al base layer can have a homogeneous element distribution, with local element contents varying by no more than 10%. Preferred variants of the Al base layer, however, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are regions whose average Si content is at least 20% less than the average Si content of the Al base layer. Silicon-rich phases are regions whose average Si content is at least 20% higher than the average Si content of the Al base layer.

[0074] 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 bordered by silicon-poor regions. A continuous layer of silicon-rich phases is understood to mean that, in the vertical micrograph, a line can be drawn 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 is understood to mean that, in the vertical micrograph, a line can be drawn parallel to the surface of the steel substrate such that it runs at least X% within the silicon-rich phases.In this case, the silicon-rich phases are arranged in such a coherent manner that, in the vertical micrograph, a line can be drawn parallel to the surface of the steel substrate, such that at least 40% of it runs within the silicon-rich phases. In an alternative design variant, the silicon-rich phases are arranged in islands within the silicon-poor phase.

[0075] 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.

[0076] In a preferred variant, the steel component comprises an oxide layer arranged on the corrosion protection coating. The oxide layer lies in particular on the optionally present Al base layer and preferably forms the outer edge of the corrosion protection coating. If no Al base layer is present, the oxide layer preferably lies on the alloy layer and preferably forms the outer edge of the corrosion protection coating. The oxide layer of the steel component consists in particular of more than 80 wt. % oxides, with the main proportion of the oxides (i.e. more than 50 wt. % of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer alone or as a mixture. The remainder of the oxide layer not taken up by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and / or magnesium in metallic form.

[0077] 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.

[0078] In a specific development, the steel substrate of the sheet metal part has a microstructure 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 having" is to be understood as meaning that there are regions of the sheet metal part that have the mentioned microstructure. In addition, there may also be regions of the sheet metal part that have a different microstructure. The sheet metal part therefore has the mentioned microstructure in sections or regions.

[0079] Due to the high content of martensite or lower bainite, very high tensile strengths and yield points can be achieved.

[0080] The sheet metal part according to the invention is preferably a component for a land vehicle, marine vehicle, or aircraft. It is particularly preferably an automotive part, in particular a body part. The component is preferably a B-pillar, longitudinal member, A-pillar, sill, or cross member, or a component of the vehicle's side structure.

[0081] The steel substrate of the flat steel product and thus also of the produced sheet metal part is made of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. In particular, the structure of the steel can be converted into a martensitic or partially martensitic structure by hot forming. The structure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic structure, as this has a particularly high hardness. The steel substrate is particularly preferably a steel which, in addition to iron and unavoidable impurities (in wt.%), consists of

[0082] C: 0.04-0.45 wt%,

[0083] Si: 0.02-1.2 wt%,

[0084] Mn: 0.5-2.6 wt%,

[0085] AI: 0.02-1.0 wt%,

[0086] P: < 0.05 wt%,

[0087] S: < 0.02 wt%,

[0088] N: < 0.02 wt%,

[0089] Sn: < 0.03 wt%,

[0090] As: < 0.01 wt%,

[0091] Ca: < 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents

[0092] Cr: 0.08-1.0 wt.%,

[0093] B: 0.001-0.005 wt%,

[0094] Mo: <0.5 wt%,

[0095] Ni: <0.5 wt%,

[0096] Cu: <0.2 wt%,

[0097] Nb: 0.01-0.08 wt%,

[0098] Ti: 0.01-0.08 wt%,

[0099] V: <0.2 wt.%.

[0100] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. Occasionally, Ca is also deliberately added to the alloy to bind sulfur. In such a case, the Ca content is at least 0.001 wt.%. The maximum Ca content in this case is also 0.005 wt.%.

[0101] In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows:

[0102] Cr: < 0.050 wt%,

[0103] B: < 0.0005 wt%,

[0104] Nb: < 0.005 wt%,

[0105] Ti: < 0.005 wt%.

[0106] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0107] 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.%.

[0108] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt% and / or at least 0.06 wt%.

[0109] 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.%.

[0110] In a preferred variant, the Al content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, preferably a maximum of 0.25 wt.%. Alternatively or additionally, the Al content is preferably at least 0.02%.

[0111] It has also been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the Si and Al contents (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.% The elements P, S, and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Irrespective of this, the S content is preferably a maximum of 0.012%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0112] Optionally, the steel also contains chromium at a content of 0.08–1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0113] In the case of an optional alloying of chromium, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0114] Preferably, the steel optionally also contains boron at a content of 0.001-0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0115] Optionally, the steel may contain molybdenum in a content of not more than 0.5 wt%, in particular not more than 0.1 wt%.

[0116] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0117] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0118] 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 at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.2 wt.%, in particular at most 0.1 wt.%, preferably at most 0.05 wt.% In the case of an optional alloying of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The sum is at most 0.1 wt.%, in particular at most 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0119] To demonstrate the effectiveness of the invention, several tests were conducted. Slabs with the compositions specified in Table 1, a thickness of 240 mm and a width of 1200 mm, were produced and heated in a pusher-type 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 thoroughly heated. The slabs were discharged from the pusher-type furnace at their respective through-heating temperatures 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. At the end of the pre-rolling phase, the intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 of 1100°C.The pre-rolled strips were fed to the finish rolling immediately after rough rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The pre-rolled strips were rolled into hot strips with a final thickness of 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 the conventional manner by pickling before being subjected to cold rolling until the thickness specified in Table 3 was achieved. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 of 870 °C and held at annealing temperature for 100 s each before being cooled at a cooling rate of 1 K / s to the immersion temperature T6 of 690 °C.The cold-rolled strips were passed through a molten coating bath at temperature T7 of 676 °C at their respective immersion temperature T6. The strip speed was 76 m / min in all cases. The composition of the coating bath is given in Table 2. After coating, the coated strips were blown off to adjust the coating weights. An air stream was used for this purpose. The temperature of the air stream was 70 °C in all cases. In this process, all strips were coated on both sides, with the coating weight on both sides 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 TmT of 18 s and Tn? of lös.Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5-15 K / s each.

[0120] From the resulting steel strips, blanks were cut and used for further tests. In these tests, sheet metal samples in the form of blanks for a generic B-pillar were hot-pressed from the blanks. Table 3 shows the heating parameters. The first three columns show the test number (No.), the sheet thickness in mm, and the one-sided coating weight. Column 4 indicates the type of electrical resistance heating. "Longitudinal / transverse" means that the heating is carried out as induction heating, initially in a first heating phase using longitudinal field induction and in a second phase using transverse field induction. At the end of the first phase, the boards had the intermediate temperature T Zwassumed, which is specified in column 5. The entry "transverse" for samples 3 and 21 means that the entire heating was carried out as induction heating using transverse field induction. Therefore, no intermediate temperatures are specified for these samples. The entry "conductive" for samples 53-55 means that the samples were heated conductively. Specifically, the samples had a contact area on opposite sides measuring 3 cm. In these contact areas, the samples were contacted and subjected to a direct current, which led to heating of the samples.

[0121] In column 6 the temperature T Indof the blanks at the end of the electrical resistance heating, i.e., at the end of the heating phases. This also corresponds to the final temperature of the sheet metal blank during heating in all cases except 47. Case 47 is explained in detail in the following paragraph. Column 7 indicates the time for heating by means of electrical resistance heating, i.e., the time in the inductor or the time for conductive heating. For heating using two phases, this is the total time for both phases. Column 8 shows the heating rate r calculated from this. Ind of the sheet metal blank during heating by means of electrical resistance heating. Column 9 gives the inventive upper limit for the heating rate according to the formula

[0122] 11 ■ io 9 ■ A -4 ' 364 + 20. After heating, the boards were kept at the final temperature for a holding time t HaThis holding time is given in column 10. A special case is sample 47. This was first heated to the intermediate temperature T Zw = 740 °C. Since there is no second phase with induction heating in this case, column 6 remains empty. Column 7 specifies the heating time in the inductor, in this case only during the first phase. Likewise, the heating rate only applies to the heating during the first phase. Following the first phase, the blank 47 was transferred to a roller hearth furnace with a furnace temperature of 920 °C within a transfer time of 10 s. In this furnace, the blank was further heated for 90 s to a final temperature of 920 °C. There was no further holding at the final temperature.

[0123] During heating of the boards by electrical resistance heating, the boards were in all cases in an atmosphere with a dew point of -10 °C.

[0124] After the heating described above, the blanks were removed from the heating device (e.g., inductor or furnace) and hot-formed in a forming tool. The process parameters are listed in Table 4. The temperature of the tool is designated Twz. The transfer time t, which consists of the removal from the heating device, transport to the tool, and insertion into the tool, is Tr ans was between 5 and 14s. During this time, the boards cooled down at a rate of r Tr cooled. The temperature T E ini gThe temperature of the blanks when placed in the forming tool was in all cases above the respective martensite start temperature +100°C. In the forming tool, the blanks were formed into the respective sheet metal parts, with the sheet metal parts being cooled in the tool at a cooling rate rwz to the target temperature Tziei. The residence time in the tool is designated twz. The cooling rate to the martensite start temperature is denoted by r Ms Finally, the samples were cooled in air to a temperature below 100 °C (typically room temperature). This time ranged from 0.5 to 600 s.

[0125] Table 5 shows the properties of the resulting sheet metal part. The first column again shows the serial number of the test. Columns two, three, and four show the thickness of the corrosion protection coating d A, the thickness of the alloy layer d, and their ratio. These are average thickness values ​​determined in the dA metallographic cross-section, averaging across three different samples. Column 5 shows the thickness of the oxide layer. The oxide layer thickness was measured using XPS.

[0126] To determine the strength of the agglomerations, it is generally necessary to measure the thickness of the corrosion protection coating over a larger area. Such a measurement is difficult to perform on metallographic cross-sections, as the typical section is not large enough to determine the full extent of the agglomerations. Instead, the measurement is performed using a Fischerscope X-Ray on a measuring field of 20 mm x 20 mm with 66 x 66 measuring points and a measuring spot size of 0.3 mm in diameter according to DIN ISO 3497:2001-12. The Fischerscope X-Ray is operated with its basic calibration. Due to the low mass of aluminum, the aluminum of the coating itself cannot be detected, but the attenuation of the iron signal can be used to determine a distance to the pure steel surface. This results in a measured value that is proportional to the thickness of the corrosion protection coating. This value is shown in Table 5 with d Since the measurement method is not intended to determine the absolute thickness, but only the strength of the layer thickness variation, this measurement quantity is sufficient.

[0127] The measured values ​​obtained for can be displayed in a two-dimensional color representation, where points with the same layer thickness are displayed in the same color. Alternatively, the points with the same values ​​for connected with a line (analogous to isohypses in topographic maps). Figure 1 shows an example of such a representation of test 13. Clearly visible in Figure 1 are vertically running wave-like areas of large and small layer thickness. For example, the vertical structure 1 running in the right third of the image is a "valley" and the diagonal structure in the middle of the image is a "mountain" 3 with a "valley" 5 directly next to it. In the exemplary measuring field, the mean value for The mean value is 4.4 pm, the maximum value is 5.6 pm, and the minimum layer thickness is 3.2 pm. The layer thickness variation relative to the mean value is therefore 28%.

[0128] The layer thickness variation is determined by first calculating the arithmetic mean of all 66x66 measuring points during the measurement is determined. Furthermore, the maximum and minimum measured values ​​are determined in the same measuring field. The maximum percentage deviation of these two measured values ​​from the mean is the layer thickness variation, which is specified in column 7 of Table 5. The layer thickness variation in the tests processed according to the invention is a maximum of 50%. Preferably, the layer thickness variation is a maximum of 30%, in particular a maximum of 20%.

[0129] Column 8 indicates whether the samples were processed according to the invention. In the tests with a heating rate r Ind for which r applies / nd < 11 ■ 10 9 ■ A -4,364 + 20, the layer thickness variation is generally less than 50%. The resulting agglomerations are therefore acceptable.

[0130] In some of the embodiments, the heating rate is also r Ind < 11 ■ 10 9 ■ A -4 ' 488 + 20.

[0131] These examples show a still small variation in layer thickness of a maximum of 30%.

[0132] Table 1 (steel grades)

[0133] The remainder is iron and unavoidable impurities. All values ​​are in wt.%.

[0134] Table 2 (coating variants)

[0135] Table 3 (Heating process parameters) Table 4 (Forming process parameters)

[0136] Table 5 (Results)

Claims

Patent claims 1. A method for producing a sheet metal part, 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 consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the sheet metal blank has on at least one side an aluminum-based corrosion protection coating with a one-sided coating weight A of 10-100 wherein the corrosion protection 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 at least temporarily by means of electrical resistance heating such that a final temperature of the sheet metal blank during heating is at least partially greater than the AC3 temperature of the sheet metal blank and the temperature T E ini g the blank, when placed in a forming tool intended for hot press forming (step d)), at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c. Optionally, holding the sheet blank at the final temperature for a holding time t HaU , where the holding time t HaUat least 3s and a maximum of 60s; d. Inserting the heated sheet metal blank into a forming tool, whereby the transfer time required for removing the blank from the heating device and inserting it is t T rans is at most 20s, preferably at most lös; e. hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is heated to a target temperature T during the hot press forming for a duration twz of more than 1s Z iei is cooled and optionally kept there, whereby the cooling depends on the temperature T E ini g at least up to the martensite start temperature with a cooling rate rwz that is at least partially more than 25 K / s; f. Removing the target temperature T Z iei cooled sheet metal part from the tool; characterized in that for the heating rate r Ind of the sheet metal blank during heating by means of electrical resistance heating: 30 < r Ind < 11 ■ 10 9 ■ A“ 4 ' 364 + 20 where the coating weight A is in the unit and the heating rate r Ind is used in the unit.

2. Method according to claim 1, characterized in that for the heating rate r Ind of the sheet metal blank during heating by means of electrical resistance heating: r Ind < 11 - 10 9 ■ A -4,488 + 20 3. Method according to one of claims 1 to 2, characterized in that the final temperature of the sheet metal blank during heating is between Ac3 and 1200 °C, preferably between 850 °C and 950 °C.

4. Method according to one of claims 1 to 3, characterized in that the one-sided coating weight is a maximum of 60 in particular a maximum of 40 preferably maximum.

5. Method according to one of claims 1 to 4, characterized in that the sheet metal blanks are exposed to an atmosphere with a dew point of maximum 5 °C, in particular maximum -5 °C, during heating by means of electrical resistance heating.

6. Method according to one of claims 1 to 5, characterized in that the heating of the sheet metal blank comprises a plurality of successive heating phases, wherein at least two heating phases from the plurality of heating phases differ by the heating method used.

7. Method according to one of claims 1 to 6, characterized in that the heating of the sheet metal blank comprises a first heating phase to an intermediate temperature T Zwand a second heating phase to the final temperature, wherein the sheet metal blank is heated during the first heating phase by means of longitudinal field induction and in the second phase by means of transverse field induction or in a furnace with a furnace temperature T 8. Method according to claim 7, characterized in that the intermediate temperature is: T c - 50K < T Zw < T C + 50K where T c the Curie temperature of the steel substrate.

9. Method according to one of claims 1 to 8, characterized in that the electrical resistance heating is heating by induction or conductive heating.

10. The method according to any one of claims 1 to 9, characterized in that the corrosion protection coating of the sheet metal blank provided in step a) comprises an alloy layer between the Al base layer and the steel substrate.

11. The method according to claim 10, characterized in that the alloy layer of the sheet blank consists of 35-60 wt.% Fe, optional further components, the total contents of which are limited to a maximum of 5.0 wt.%, and the remainder being aluminum.

12. Method according to one of claims 1 to 11, characterized in that the corrosion protection coating of the sheet metal part comprises an alloy layer and 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.

13. The method according to claim 12, 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 total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

14. Method according to one of claims 12 to 13, characterized in that the alloy layer of the sheet metal part 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 being aluminium.

15. A method according to any one of claims 1 to 14, 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%, AI: 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.%.

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