Shaped sheet metal part having improved forming properties

Abstract

The invention relates to a shaped sheet metal part formed from a flat steel product, the steel substrate of which comprises a structural proportion of at least 5% lower bainite. A shaped sheet metal part of this kind has improved processing properties. The invention also relates to a method for producing such a shaped sheet metal part.

Description

[0001] Sheet metal forming part with improved forming properties

[0002] The invention relates to a sheet metal forming part formed from a flat steel product and to a method for manufacturing such a sheet metal forming part.

[0003] When the terms "flat steel product" or "sheet metal product" are used below, these refer to rolled products such as steel strips or sheets, from which "sheet blanks" (also called blanks) are cut for the production of, for example, body parts. "Sheet metal forming parts" or "sheet metal components" of the type according to the invention are manufactured from such sheet metal blanks, whereby the terms "sheet metal forming part" and "sheet metal component" are used synonymously.

[0004] All information regarding the content of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified percentages relating to a steel alloy are therefore to be understood as wt.%. With the exception of the information relating to the retained austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (expressed in vol.%), information regarding the content of the various microstructural constituents refers to the area of ​​a section of a sample of the respective product (expressed in area percent, "area %"), unless expressly stated otherwise.

[0005] Vickers hardness is determined according to DIN EN ISO 6507 (2018.07).

[0006] The microstructure was determined from longitudinal sections that had been etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffraction.

[0007] WO 2020 / 239905 A1 discloses a sheet metal forming part and a method for manufacturing such a sheet metal forming part. The sheet metal forming part shown there has a microstructure with at least 95% martensite. In practice, it has been found that for many applications in the automotive sector, higher ductility and thus a softer material is required, which is not achievable with a purely martensitic sheet metal forming part.

[0008] Therefore, the object of the present invention is to provide a sheet metal forming part that has improved forming properties.

[0009] This task is solved by a sheet metal part formed from a flat steel product comprising a steel substrate consisting of steel, which, in addition to iron and unavoidable impurities (in wt.%), consists of

[0010] C: 0.05 -0.20%,

[0011] Si: 0.01 -2.0%,

[0012] Mn: 0.1 -2.0%,

[0013] AI: 0.01 -0.5%

[0014] Note: 0.003 -0.070%

[0015] B: 0.0005-0.01%

[0016] P: < 0.03%

[0017] S: <0.02%,

[0018] N: <0.02%,

[0019] Sn: < 0.03%

[0020] As: < 0.01 %,

[0021] Sb: < 0.02%, and optionally one or more of the elements “Cr, Co, Cu, Mo, Ni, V, Ti, Ca, W” in the following concentrations

[0022] Cr: 0.01 - 1.0%, Co: 0.005 - 1.0%, Cu: 0.01 - 0.2%, Mo: 0.002 - 1.0%, Ni: 0.01 - 2.0%, V: 0.02 - 0.25%,

[0023] Ti: 0.005-0.10%, Ca: 0.0005 - 0.01%,

[0024] W: 0.001 - 1.0 %, consisting of the microstructure of the steel substrate comprising at least 5% lower bainite.

[0025] Lower bainite is defined as a microstructure of dislocation-rich (bainitic) ferrite in the form of fine needles or plates with a carbon-richer second phase embedded within these needles or plates. A carbon-richer second phase may also be present between the needles or plates. This carbon-rich second phase is typically the iron carbide Fe3C (cementite) or the electron carbide Fe2,4C.

[0026] However, the austenite phase (γ-Fe) and the structural constituent martensite are also possible. In these cases, it is referred to as carbide-free lower bainite.

[0027] Upper bainite is also a microstructure of dislocation-rich (bainitic) ferrite in the form of fine needles or plates. It forms during cooling in the upper temperature range. Due to the more favorable diffusion conditions, carbon diffuses to the grain boundaries of the ferrite needles or plates. Carbon-rich secondary phases form between the ferrite units. This carbon-rich secondary phase is typically the iron carbide Fe3C. However, the austenite phase (γ-Fe) and the microstructural constituent martensite are also possible. In a two-dimensional view, the carbide or cementite particles in upper bainite are arranged between the plates or bands of bainitic ferrite. However, the particle formation in the form of bands occurs unevenly in three dimensions, especially at higher formation temperatures.Due to the preferential formation of upper bainite at austenite grain boundaries, in contrast to the formation of lower bainite from enriched austenite or in fine form within ferrite plates, and the higher formation temperatures, the formation of upper bainite can generally be classified as coarser.

[0028] Martensite forms at extremely high cooling rates through a sudden, diffusion-free flipping of the austenite lattice when the so-called martensite start temperature Ms is reached. During this process, dissolved carbon is forcibly dissolved, leading to strain in the crystal lattice and thus to the high hardness of the material. The microstructure then consists of needle-, lanceolate, or plate-shaped units. If martensite is heated to temperatures that allow carbon diffusion, the forcibly dissolved atoms redistribute themselves within the lattice and form carbides within the needle-, lanceolate, or plate-shaped units, which, after carbon precipitation, are themselves ferritic. This process is called tempering.In the manganese-boron steels described here with the mentioned chemical composition, the martensite start temperature Ms is so high that this process can take place below this temperature even without additional heating; this effect is called "self-tempering".

[0029] Therefore, in the manganese-boron steels described here, both (tempered) martensite and lower bainite can form as needle- or plate-shaped units of ferrite containing embedded carbides. The orientation of the linear carbides is used in the literature to distinguish between lower bainite and tempered martensite. If these carbides are oriented parallel to each other in only one set within the needle- or plate-shaped ferrite units, the microstructure is lower bainite (compare Figure 2b). In contrast, tempered martensite shows several (usually three) sets of carbides oriented parallel to each other, but not parallel between these sets, within each needle- or plate-shaped ferrite unit. In carbide-free lower bainite, retained austenite or martensite form the sets of linear inclusions.

[0030] The inclusion of lower bainite offers several advantages: Firstly, high strength can be achieved, as lower bainite almost reaches the strength of martensite. The precipitation of the carbon-rich phase (especially the carbides, primarily cementite) within the ferrite units results in a significantly finer microstructure (i.e., a high degree of structural refinement) than in upper bainite. This higher grain size of bainitic ferrite plates leads to an increase in both strength and toughness. This finer microstructure results in high strength, only slightly below that of martensite. Simultaneously, the formation of bainite leads to lower residual stresses in the material compared to martensite. This results in good ductility and reduced crack susceptibility.Likewise, the risk of hydrogen embrittlement is reduced by the generally lower strength of the lower bainite compared to martensite and the reduced extent of internal stresses in the component due to the lower martensite content.

[0031] Another positive effect of the microstructure according to the invention is its tempering resistance. As explained, the manganese-boron steels described here undergo self-tempering of martensite. This effect persists throughout the service life of the sheet metal part, causing the mechanical properties, such as strength, to degrade over time. This degradation occurs more rapidly the higher the temperatures to which the sheet metal part is exposed during its service life. The more martensite is replaced by bainitic phases (upper and lower bainite), the more this effect is reduced. As a result, a sheet metal part can be achieved whose mechanical properties change only slightly over its service life.

[0032] In a preferred embodiment, the steel substrate has a microstructure consisting of the following phases (in %):

[0033] Martensite: 10 to 95%, lower bainite: 5 to 35%, upper bainite: 0 to 80%

[0034] Retained austenite: 0 to 5%

[0035] Remaining 0 to 3%

[0036] These microstructural components, with the exception of the distinction between martensite and lower bainite, can be identified using light microscopy. To determine the phase fraction, a comparison with reference images is performed. These reference images schematically show the image-analytically determined fraction of a phase in a homogeneous matrix at the following levels: 3%, 5%, 10%, 15%, 20%, 35%, and 50%. The distinction between martensite and lower bainite is made using scanning electron microscopy according to the criteria outlined above. The determination of the phase fractions is then identical to the light microscopy method.

[0037] The remainder consists of unavoidable impurities of other structural components. These can include, for example, titanium carbonitrides and coarsely precipitated cementite.

[0038] Preferred ranges for the martensite content are 10 to 90%, particularly 10 to 80%, preferably 10 to 70%, particularly 20 to 65%, and preferably 25 to 65%.

[0039] Preferred ranges for the lower bainite content are 10 to 35%, particularly 15 to 35%, and preferably 20 to 35%. Preferred ranges for the upper bainite content are 5 to 70%, particularly 5 to 60%, and preferably 10 to 55%.

[0040] In a particular preferred embodiment, the steel substrate has a microstructure consisting of the following phases (in %):

[0041] Martensite: 10 to 80%, lower bainite: 10 to 35%, upper bainite: 5 to 70%

[0042] Retained austenite: 0 to 5%

[0043] Remaining 0 to 3%.

[0044] In another special preferred embodiment, the steel substrate has a microstructure consisting of the following phases (in %):

[0045] Martensite: 10 to 70%, lower bainite: 15 to 35%, upper bainite: 5 to 60%

[0046] Retained austenite: 0 to 5%

[0047] Remaining 0 to 3%.

[0048] In another special preferred embodiment, the steel substrate has a microstructure consisting of the following phases (in %):

[0049] Martensite: 25 to 65% lower bainite: 20 to 35% upper bainite: 5 to 55%

[0050] Retained austenite: 0 to 5%

[0051] Remaining 0 to 3%.

[0052] A particularly high proportion of lower bainite leads to good forming properties on the one hand and still to a relatively high strength on the other.

[0053] In addition to the previously described method for identifying lower bainite, the grain boundaries of former austenite grains can also be used. In the case of a completely martensitic microstructure, the grain boundaries of former austenite grains are not visible during Nital etching. Other etching methods, such as EBSD using orientation relationships, are required for this. In contrast, it should be noted that when lower bainite is present in the microstructure, the grain boundaries of former austenite grains are indeed visible during Nital etching. They can therefore be used to quantify the proportion of the microstructure.

[0054] In a special embodiment, the surface area fraction of the visible former austenite grains is greater than 15%, preferably greater than 20%, and particularly greater than 25%.

[0055] To determine the area fraction of visible former austenite grains, the microstructure is imaged under a light microscope using an objective with a numerical aperture of at least 0.90 at a magnification of 1000:1 (according to DIN 50600:2017-10) and digitally photographed with an image resolution of at least 2700 x 1800 pixels at an image field of 145 µm x 97 µm (see Figure 1a). Subsequently, the identifiable former austenite grain boundaries are manually traced using image processing software or, preferably, image analysis software (see Figure 1b). The line thickness (brush size) is a maximum of three pixels (in Figure 1b, the lines are thicker to ensure visibility in the patent publication). Areas completely surrounded by former austenite grain boundaries are then considered former austenite grains for the purposes of this analysis. The area fraction X AThe number of visible former austenite grains is determined as the number of pixels within these former austenite grains P. A based on the total number of pixels in the image P geS) which in turn is calculated as the product of the number of pixels in the x-direction N P , X and in the y-direction N P , y results in:

[0056] P A P A

[0057] Xa ~ ~ N P x - N P v

[0058] The pixels of the drawn grain boundaries of completely enclosed former austenite grains are referred to as the area P. A Pixels belonging to grain boundaries that do not enclose grains are counted as belonging to the grains, but not those of grain boundaries. Image analysis software can be used to simplify the evaluation.

[0059] In a particular embodiment, the total surface area of ​​carbides in the lower bainite is a maximum of 20%, particularly a maximum of 15%, and preferably a maximum of 10%. Typically, the surface area of ​​carbides in the lower bainite is at least 1%, 2%, 3%, or 4%.

[0060] In a further special embodiment, the areal density of carbides in the lower Bai- 1 li nit is a maximum of 120, in particular a maximum of 100, preferably a maximum of 95 ^.

[0061] 1

[0062] Typically, the areal density of carbides in the lower bainite is at least 10 — , 20

[0063] The mean surface area of ​​a carbide in the lower bainite is preferably less than 1500 nm². 2 preferably less than 1200 nm 2 , especially less than 1000 nm 2 .

[0064] The area fraction X is then c and the area density p cof carbides in the lower bainite as follows:

[0065] The microstructure is imaged using a scanning electron microscope with a lateral resolution of at least 3 nm and a magnification of at least 20,000:1 (according to DIN 50600:2017-10) using an in-lens detector, such that the carbides appear bright compared to the ferrite, and digitally photographed with an image resolution of at least 2000 x 1400 pixels and a field of view of 6 pm x 4 pm (see Figure 2a). In the digital image, the ferrite units of the lower bainite are marked as polygons using an image analysis program. Within each polygon (called a Region of Interest or ROI, plural ROIs), the carbides are then selected based on their brightness by setting threshold values.

[0066] Two parameters are determined using image analysis. The first is the area fraction X. cThe number of carbides within the ferrite unit (ROI) is calculated as the quotient of the sum of the pixel counts of all detected carbides, P. c in the image field and the total number of pixels PROI of all considered ROIs in the image field:

[0067] PC

[0068] PROI On the other hand, the area density p c The number of carbides within the ferrite unit (ROI) is calculated as the quotient of the number of all carbides detected within the ROIs in the image field N. c and the total area A RO I of all ROIs in the image field is determined. This area is in turn derived from the product of the total number of pixels PROI of all ROIs in the image field with the area A. Px of a single pixel in the image, which is determined by the calibration of the image field:

[0069] From these two quantities, the mean area A can also be calculated. Karbid The density of a carbide in the lower bainite is determined as the ratio of surface area to surface density.

[0070] The formation of carbides has detrimental effects on the microstructure. Firstly, carbide particles are responsible for initiating failure during deformation, thus reducing the toughness and ductility of the materials. Furthermore, carbon is lost through carbide formation, hindering the stabilization of retained austenite and impeding ductility through carbide precipitation. Therefore, a particularly high carbide content reduces ductility. (In principle, the volume fraction of carbides is relevant for these effects. However, the volume fraction can be related to the area fraction, which can be more easily determined using micrographs. The underlying theory for the relationship between area and volume fraction can be found, for example, in [1].)Exner, Hans Eckart; Hougardy, Hans Paul; "Introduction to Quantitative Microstructure Analysis"; DGM Informationsgesellschaft Verlag; Oberursel 1986; ISBN 3-88355-108-2; pp. 15ff or 2. J.E. Hilliard "Measurement of Volume in Volume" in DeHoff, Robert T.; Rhines, Frederick N.; "Quantitative Microscopy"; McGraw-Hill Book Company, New York ... 1968; Library of Congress Catalog Card Number 68-22766; pp. 45ff).

[0071] Therefore, it is advantageous to reduce or completely eliminate carbides as much as possible, depending on the concept and / or process. In many cases, such as the present one, the formation of carbides in bainite cannot be completely ruled out due to the analytical approach. In principle, in addition to the total surface area fraction, their distribution parameters (number, size, and spacing of the particles) also play a role.

[0072] A small mean surface area of ​​a carbide within a carbide group indicates that many separate carbides have precipitated. This results in a particularly fine microstructure within the ferrite units. This leads to high strength and good ductility.

[0073] Low areal fractions and low areal density of carbides in lower bainite, especially carbide-free lower bainite, represent an advantageous microstructural component for several reasons. The absence of carbides increases resistance to cleavage fracture and reduces susceptibility to microcracking. The retained austenite content in carbide-free lower bainite also contributes to increased toughness through deformation-induced transformation to martensite under externally applied deformation. For the purposes of this application, lower bainite is referred to as "carbide-free" if, in the previously described measurement method for X, the following applies: cNo carbides could be detected.

[0074] Preferably, the sheet metal part has a Vickers hardness of at most 420 HV5, in particular the Vickers hardness is at most 400 HV5, more preferably at most 380 HV5, and most preferably at most 370 HV5. Furthermore preferably the Vickers hardness is at least 250 HV5, in particular at least 280 HV5, and more preferably at least 300 HV5.

[0075] Vickers hardness is a qualitative measure of resistance to penetration by a test indenter and thus resistance to plastic deformation. Characterization using Vickers hardness has the advantage that it allows for the determination of Vickers hardness even for smaller component sections. This enables targeted investigation of specific areas of the component where tensile tests are not possible due to the geometry (e.g., curved workpieces or areas with varying sheet thickness). Vickers hardness is determined according to DIN EN ISO 6507 (2018.07). The designation "5" refers to the test force.

[0076] Carbon (“C”) is present in the steel substrate of the flat steel product in concentrations of 0.05–0.20 wt.%. Such C concentrations represent a good compromise between good hardenability on the one hand and good formability on the other. Preferably, the C content is at least 0.08 wt.%, particularly at least 0.10 wt.%. More preferably, the C content is a maximum of 0.18 wt.%, particularly a maximum of 0.14 wt.%. Silicon (“Si”) is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silicon manganese as an alloying agent, which has a positive effect on production costs. Silicon is present in the steel substrate of the flat steel product in concentrations of 0.01–2.0 wt.%. A hardening effect occurs even at a Si content of 0.05 wt.%. From a silicon content of at least 0.15 wt.A significant increase in strength occurs with silicon contents of less than 0.20 wt.%, particularly at least 0.20 wt.%. Silicon contents above 0.65 wt.% have a detrimental effect on coating behavior, especially with aluminum-based coatings. Silicon contents of at most 0.55 wt.%, particularly at most 0.30 wt.%, are preferred to improve the surface quality of the coated steel flat product.

[0077] Manganese (“Mn”) is present in the steel substrate in concentrations of 0.1–2.0 wt.%. Mn increases the hardenability of the steel by lowering the Ac3 and / or Ar3 transformation temperature from ferrite to austenite. This allows the furnace temperature required for complete conversion to austenite during the heat treatment of the flat steel product from which a sheet metal part according to the invention is formed to be reduced. Compared to carbon, manganese has the advantage of achieving higher ductility in the hardened state, which manifests itself, for example, in higher impact toughness. Furthermore, the reduction of the critical cooling rate with increasing manganese content is associated with an increase in hardening capacity.Fluctuations in cooling conditions or differing contact conditions during the cooling of sheet metal parts manufactured from the steel alloy according to the invention can be better compensated for, and the variation in properties is limited. However, excessively high Mn contents increase the carbon segregation behavior and can lead to inhomogeneous hardening behavior across the cross-section of the respective product and the formation of hardening cracks. Furthermore, increasing Mn contents increase the risk of external Mn oxides or Mn-based mixed oxides forming on the surface of the product manufactured from the steel alloy according to the invention. As in the case of excessive Si contents, this would pose a risk of deteriorating the wetting behavior of a flat steel product manufactured from the steel alloy according to the invention during hot-dip coating.To avoid these negative effects, the Mn content of a steel flat product intended for forming a sheet metal part according to the invention is limited to a maximum of 2.0 wt.%, whereby adverse effects of the presence of Mn can be particularly reliably avoided by limiting the Mn content to a maximum of 1.50 wt.%. Conversely, the positive effects of Mn can be utilized particularly reliably if the Mn content is at least 0.40 wt.%, and in particular at least 0.60 wt.%. In particular, the Mn content is at least 0.80 wt.%, preferably at least 1.00 wt.%. Furthermore, the Mn content is preferably a maximum of 1.4 wt.%, more preferably a maximum of 1.3 wt.%, and in particular a maximum of 1.30 wt.%, and more preferably a maximum of 1.20 wt.%.

[0078] Compared to known flat steel products, the steel substrate of the flat steel product according to the invention has an aluminum content of at least 0.06 wt.%, preferably at least 0.07 wt.%, and particularly at least 0.08 wt.%. Preferably, the aluminum content is at least 0.10 wt.%, particularly preferably at least 0.11 wt.%, particularly at least 0.12 wt.%, and preferably at least 0.16 wt.%. The maximum aluminum content is 1.0 wt.%, and particularly at most 0.8 wt.%.

[0079] Aluminum (“Al”) is known to be added as a deoxidizing agent in steel production. The aluminum content of the steel substrate is at least 0.01 wt.%, in particular at least 0.04 wt.%, and preferably at least 0.06 wt.%. The maximum aluminum content is 0.50 wt.%, preferably at most 0.20 wt.%, in particular at most 0.15 wt.%, preferably at most 0.12 wt.%, and in particular at most 0.10 wt.%. At least 0.01 wt.% Al is required to reliably bind the oxygen contained in the molten steel. Al can also be used to bind undesirable, but unavoidable, nitrogen content due to the manufacturing process.In this way, the formation of AIN or NbN is promoted in competition with the nitrogen binding by TiN that is classically known in heat-treatable steels, and, insofar as Ti is present in the steel of a flat steel product intended for forming a sheet metal part according to the invention, the formation of comparatively coarse TiN is avoided. The aim is to prevent the formation of boron nitrides so that B, as explained below, can exert its beneficial influence on delaying the transformation into the form dissolved in the crystal lattice. Furthermore, the presence of Al within the content limits specified by the invention results in grain refinement. Comparatively high aluminum contents have been avoided so far because the Ac3 temperature also shifts upwards with the aluminum content. This has a negative effect on the austenitization, which is important for hot forming.If the aluminum content is too high, particularly above 0.50 wt% Al, there is a risk that aluminum oxides will form on the surface of a product made from steel alloyed according to the invention. These oxides would impair the wetting behavior during hot-dip coating. Furthermore, higher aluminum contents promote the formation of non-metallic aluminum-based inclusions, which, as coarse inclusions, negatively affect crash performance. Therefore, the aluminum content is preferably selected below the aforementioned upper limits.

[0080] Niobium (“Nb”) is a microalloying element added to contribute to grain refinement, with at least 0.003 wt.%, and in particular at least 0.005 wt.%, being added. Preferably, the Nb content is at least 0.010 wt.%, in particular at least 0.015 wt.%, and most preferably at least 0.020 wt.%. The maximum niobium content is 0.070 wt.%, in particular at most 0.050 wt.%, and preferably 0.030 wt.%.

[0081] The specified niobium content, particularly in the following described process for manufacturing a steel flat product for hot forming with a corrosion-resistant coating, leads to a distribution of niobium carbides and niobium carbonitrides, resulting in a particularly fine, hardened microstructure during subsequent hot forming. During cooling after hot-dip coating, the coated steel flat product is held for a certain period of time within a temperature range of 400 °C to 300 °C. Within this temperature range, a certain diffusion rate of carbon still exists in the steel substrate, while its thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there.Lattice defects are primarily caused by dissolved niobium atoms, whose significantly larger atomic volume expands the atomic lattice, thereby enlarging the tetrahedral and octahedral voids and increasing the local solubility of carbon. Consequently, clusters of carbon and niobium form in the steel substrate. During the subsequent austenitization step of hot forming, these clusters transform into very fine precipitates of niobium carbides and niobium carbonitrides, acting as additional austenite nuclei. This results in a refined austenite microstructure with smaller austenite grains and thus a refined hardened microstructure. Furthermore, these precipitates act as traps for free hydrogen, thereby promoting resistance to hydrogen embrittlement.

[0082] This applies particularly to the ferritic interdiffusion layer that forms during hot forming. The refined ferritic microstructure in the interdiffusion layer helps reduce crack initiation tendencies under bending loads, and the precipitates in the ferritic interdiffusion layer trap free hydrogen before it can concentrate within the substrate. However, excessively high Nb contents pose a risk that the Nb may not be completely dissolved when alloy steel slabs are heated at minimum furnace temperatures of 1100 °C. Furthermore, excessively high Nb contents can negatively affect coating behavior in the hot-dip process.

[0083] Boron (“B”) is added to improve the hardenability of the steel flat product by reducing the grain boundary energy through boron atoms or boron precipitates deposited at the austenite grain boundaries, thereby suppressing ferrite nucleation during press hardening. A significant effect on hardenability occurs at contents of at least 0.0005 wt.%, preferably at least 0.0007 wt.%, particularly at least 0.0010 wt.%, and especially at least 0.0020 wt.%. Furthermore, B improves grain boundary strength by preferentially adhering to grain boundaries and displacing detrimental elements, such as phosphorus (P), from these areas. This improves toughness and fracture reduction. However, at contents above 0.01 wt.%, boron carbides, boron nitrides, or boron nitrocarbides are formed in greater quantities, which in turn provide preferred nucleation sites for ferrite and reduce the hardening effect.For this reason, the boron content is limited to a maximum of 0.01 wt.%, preferably a maximum of 0.0100 wt.%, preferably a maximum of 0.0050 wt.%, in particular a maximum of 0.0035 wt.%, in particular a maximum of 0.0030 wt.%, preferably a maximum of 0.0025 wt.%.

[0084] Phosphorus (“P”) and sulfur (“S”) are elements that are introduced into steel as impurities from iron ore and cannot be completely removed in large-scale steelmaking processes. The P and S content should be kept as low as possible, since mechanical properties such as impact strength deteriorate with increasing P and S content, respectively. Furthermore, embrittlement of the martensite begins to occur at P contents of 0.03 wt.%, which is why the P content of a flat steel product according to the invention is preferably limited to a maximum of 0.02 wt.%, and in particular to a maximum of 0.015 wt.%. The S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably to a maximum of 0.0020 wt.%, and in particular to a maximum of 0.0010 wt.%.

[0085] Nitrogen (“N”) is also present in steel in small quantities as an impurity due to the steelmaking process. The N content should be kept as low as possible and should not exceed 0.03 wt.%. Nitrogen is particularly detrimental in alloys containing boron, as it inhibits the conversion-retarding effect of boron by forming boron nitrides. Therefore, in this case, the nitrogen content should preferably be no more than 0.010 wt.%, and more specifically, no more than 0.007 wt.%.

[0086] Other typical impurities are tin (“Sn”), arsenic (“As”), and antimony (“Sb”). The Sn content is a maximum of 0.03 wt.%, preferably a maximum of 0.02 wt.%. The As content is a maximum of 0.01 wt.%, particularly a maximum of 0.005 wt.%. The Sb content is a maximum of 0.02 wt.%, particularly a maximum of 0.01 wt.%, and particularly a maximum of 0.005 wt.%.

[0087] In addition to the previously described impurities P, S, N, Sn, As, and Sb, other elements may also be present as impurities in the steel. These other elements are collectively referred to as "unavoidable impurities." Preferably, the total content of these "unavoidable impurities" is a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, Cu, Mo, Ni, V, Ti, Ca, and W described below, for which a lower limit is specified, may also occur as unavoidable impurities in the steel substrate at levels below the respective lower limit. In this case, they are also counted among the "unavoidable impurities," the total content of which is limited to a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%.

[0088] Chromium, copper, molybdenum, nickel, vanadium, titanium, calcium and tungsten can be optionally alloyed individually or in combination with each other to the steel of a flat steel product according to the invention.

[0089] Chromium (“Cr”) suppresses the formation of ferrite and pearlite during accelerated cooling of a steel flat product according to the invention and enables complete conversion to martensite or bainite even at lower cooling rates, thereby increasing hardenability.

[0090] These effects occur from a chromium content of 0.01 wt.%, with a content of at least 0.10 wt.%, preferably at least 0.15 wt.%, proving effective in practice for reliable process control. Cr also increases tensile strength without significantly impairing elongation. This is also explained by the formation of chromium carbides, which increase strength and tempering resistance. However, excessively high chromium contents impair the coating properties of the steel. Therefore, the chromium content of the steel or the steel substrate is limited to a maximum of 1.0 wt.%, in particular a maximum of 0.75 wt.%, preferably a maximum of 0.50 wt.%.

[0091] Copper (Cu) can optionally be added to increase hardenability at additions of at least 0.01 wt.%, preferably at least 0.010 wt.%, and particularly at least 0.015 wt.%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. If the Cu content is too high, hot rolling deteriorates significantly due to low-melting-point Cu phases on the surface; therefore, the Cu content is limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%, and particularly a maximum of 0.10 wt.%.

[0092] Molybdenum (“Mo”) can be optionally added to improve process stability, as it significantly slows down ferrite formation. From contents of 0.002 wt.%, molybdenum-carbon clusters, up to and including ultrafine molybdenum carbides, dynamically form at the grain boundaries, significantly slowing down grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces the grain boundary energy, which decreases the nucleation rate of ferrite. Preferably, the Mo content is at least 0.004 wt.%, particularly at least 0.01 wt.%. Due to the high costs associated with a molybdenum alloy, the Mo content should be at most 0.3 wt.%, preferably at most 0.20 wt.%, particularly at most 0.15 wt.%, particularly at most 0.10 wt.%, and preferably at most 0.08 wt.%.

[0093] Nickel (“Ni”) stabilizes the austenitic phase and can be optionally added to lower the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative effect of copper on hot rollability, 0.01 wt% nickel can be added to the steel; preferably, the Ni content is at least 0.015 wt%. For economic reasons, the nickel content should be limited to a maximum of 0.50 wt%, and preferably to a maximum of 0.20 wt%. Preferably, the Ni content is a maximum of 0.10 wt%.

[0094] Vanadium (V) can optionally be added in amounts of 0.02–0.25 wt.%. Preferably, the vanadium content is at least 0.04 wt.%, and particularly at least 0.10 wt.%. For cost reasons, a maximum of 0.25 wt.% vanadium is added. Vanadium has the same effect on grain refinement and precipitation as already explained with regard to Nb.

[0095] From a vanadium content of 0.010 wt.% upwards, increased amounts of vanadium carbonitrides are formed. These act as traps for the hydrogen present in the steel, which would otherwise embrittle the steel. Preferably, the vanadium content is at least 0.020 wt.%, particularly at least 0.030 wt.%, and most preferably at least 0.050 wt.%.

[0096] Titanium (“Ti”) is a microalloying element that is optionally added to contribute to grain refinement, with at least 0.005 wt% Ti, preferably at least 0.008 wt%, particularly at least 0.010 wt%, and preferably at least 0.015 wt% Ti being added to ensure sufficient availability. Similar to Nb, Ti forms precipitates in the form of carbides and carbonitrides, which contribute to grain refinement and act as traps for free hydrogen.

[0097] From 0.10 wt% Ti onwards, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided. To improve cold rollability, the Ti content can preferably be limited to 0.08 wt%, more preferably to 0.050 wt%, particularly preferably to 0.040 wt%, and especially 0.020 wt%. Titanium also has the effect of binding nitrogen, thus enabling boron to exert its strong ferrilytic effect. Therefore, in a preferred embodiment, the titanium content is more than 3.42 times the nitrogen content to achieve sufficient nitrogen binding.

[0098] Calcium (“Ca”) is used in steels to form non-metallic inclusions, particularly manganese sulfides. The rounded formation of these inclusions significantly reduces their negative impact on hot formability, fatigue strength, and toughness. To utilize this effect in a flat steel product according to the invention, such a product can optionally contain at least 0.0005 wt.% Ca, in particular at least 0.0010 wt.%, preferably at least 0.0020 wt.%. The maximum Ca content is 0.01 wt.%, in particular a maximum of 0.007 wt.%, preferably a maximum of 0.005 wt.%. Excessively high Ca contents increase the likelihood of the formation of non-metallic inclusions involving calcium, which impair the purity and toughness of the steel. For this reason, the Ca content should be limited to a maximum of 0.005 wt%, preferably a maximum of 0.0030 wt%, and in particular a maximum of 0.0020 wt%.The -% must be adhered to. Tungsten (W) can optionally be added in amounts of 0.001–1.0 wt.% to slow down ferrite formation. A positive effect on hardenability is already achieved with W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.%, and in particular a maximum of 0.30 wt.%, of tungsten is added.

[0099] In preferred embodiments, the sum of the Mn and Cr content ("Mn+Cr") is greater than 0.7 wt.%, particularly greater than 0.8 wt.%, and preferably greater than 1.1 wt.%. Below a minimum sum of both elements, their necessary conversion-inhibiting effect is lost. Regardless of this, the sum of the Mn and Cr content is less than 3.5 wt.%, preferably less than 2.5 wt.%, particularly less than 2.0 wt.%, and most preferably less than 1.5 wt.%. The upper limits for both elements are determined by ensuring coating performance and adequate weldability.

[0100] The steel flat product preferably includes a corrosion protection coating on at least one side to protect the steel substrate from oxidation and corrosion during hot forming and use of the produced steel component.

[0101] The foregoing explanations regarding element contents and their preferred limits for the steel substrate of the sheet metal part apply accordingly to the process for manufacturing a sheet metal part described below, as well as to the steel flat product used in this process.

[0102] In a particular embodiment, the sheet metal part preferably comprises a corrosion protection coating, especially one based on aluminum. The corrosion protection coating can be applied to one or both sides of the steel flat product. The two sides of the steel flat product are defined as the two opposing large surfaces of the steel flat product. The narrow surfaces are referred to as edges.

[0103] Such a sheet metal part with a corrosion-resistant coating is produced by hot forming a flat steel product. Therefore, the following section first explains the flat steel product and its preferred embodiments, which serve as the starting material in the manufacturing process of the sheet metal part. Further preferred properties of the sheet metal part with a corrosion-resistant coating are then explained. Such a flat steel product comprises a steel substrate consisting of steel, which, in addition to iron and unavoidable impurities (in wt.%), consists of

[0104] C: 0.05 -0.20%,

[0105] Si: 0.01 -2.0%,

[0106] Mn: 0.1 -2.0%,

[0107] Al: 0.01 -0.50%,

[0108] Note: 0.003 -0.070%

[0109] B: 0.0005-0.01%

[0110] P: < 0.03%

[0111] S: <0.02%,

[0112] N: <0.03%,

[0113] Sn: < 0.03%

[0114] As: < 0.01 %,

[0115] Sb: < 0.02%, and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ti, Ca, W” in the following concentrations

[0116] Cr: 0.01 -1.0%,

[0117] Cu: 0.01 - 0.2%

[0118] Mo: 0.002 - 0.3%

[0119] Ni: 0.01 -0.50%,

[0120] V: 0.02 - 0.25%

[0121] Ti: 0.005-0.10%

[0122] Ca: 0.0005 - 0.01%, W: 0.001 - 1.0%.

[0123] The steel flat product preferably includes a corrosion protection coating to protect the steel substrate from oxidation and corrosion during hot forming and use of the produced steel component. In a particular embodiment, the steel flat product preferably includes an aluminum-based corrosion protection coating. The corrosion protection coating can be applied to one or both sides of the steel flat product. _,The two large, opposing surfaces of the flat steel product are referred to as the product's faces. The narrow surfaces are called edges.

[0124] Such a corrosion protection coating is preferably produced by hot-dip coating the steel flat product. The steel flat product is passed through a liquid melt consisting of 0.1–15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2–4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. In a preferred variant, the Si content of the melt is 1.0–3.5 wt.% or 7–12 wt.%, particularly 8–10 wt.%.

[0125] In a preferred embodiment, the optional content of alkali or alkaline earth metals in the melt comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0126] The content of optional additional components is preferably limited to 1.5 wt.%, particularly to 1.0 wt.%, preferably to 0.5 wt.%, particularly to 0.3 wt.%, preferably to 0.10 wt.%, and particularly to 0.05 wt.%. In particular, the optional additional components correspond to unavoidable impurities and are present only in technically unavoidable amounts. This applies to all previously described melt compositions.

[0127] In 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.

[0128] The alloy layer lies on top of the steel substrate and is directly adjacent to it. The alloy layer is essentially composed of aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. Preferably, the alloy layer consists of 35–60 wt.% Fe, more preferably α-iron, optional additional components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0%, and aluminum as the remainder, with the Al content preferably increasing towards the surface. The optional additional components include, in particular, the remaining components of the melt (i.e., silicon and optionally alkali or alkaline earth metals, especially Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

[0129] The aluminum base layer lies on top of the alloy layer and is directly adjacent to it. Preferably, the composition of the aluminum base layer corresponds to the composition of the melt of the molten pool. That is, it consists of 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn, and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Preferred compositions of the aluminum base layer correspond to the preferred melt compositions. Similarly, the content of optional further components in the aluminum base layer is preferably limited to 1.5 wt.%, in particular to 1.0 wt.%, preferably to 0.5 wt.%, in particular to 0.3 wt.%, preferably to 0.10 wt.%, and in particular to 0.05 wt.%.In particular, the optional additional components correspond to unavoidable impurities and are only present in technically unavoidable levels.

[0130] In a preferred embodiment of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can, in particular, comprise at least 0.0015 wt.% Ca, more preferably at least 0.1 wt.% Ca. More preferably, the optional content of alkali or alkaline earth metals consists of 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, more preferably 0.1–0.5 wt.% Mg, and optionally at least 0.0015 wt.% Ca, more preferably at least 0.1 wt.% Ca.

[0131] 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.

[0132] The corrosion protection coating preferably has a thickness of 5–60 µm, particularly 10–40 µm. The coating weight is particularly 30–360 g / m² for double-sided coatings and 15–180 g / m² for single-sided coatings. Preferably, the coating weight is 100–200 g / m² for double-sided coatings and 50–100 g / m² for single-sided coatings. Most preferably, the coating weight is 120–180 g / m² for double-sided coatings and 60–90 g / m² for single-sided coatings.

[0133] The thickness of the alloy layer is preferably less than 20 pm, particularly preferably less than 16 pm, particularly less than 12 pm, particularly preferably less than 10 pm, preferably less than 8 pm, and particularly less than 5 pm. The thickness of the Al base layer is the difference between the thicknesses of the corrosion protection coating and the alloy layer. Preferably, the thickness of the Al base layer is at least 1 pm, even with thin corrosion protection coatings.

[0134] In a preferred embodiment, the steel flat product comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located, in particular, on the aluminum base layer and preferably forms the outer layer of the corrosion protection coating.

[0135] The oxide layer consists in particular of more than 80 wt.% oxides, wherein the main proportion of the oxides (i.e., more than 50 wt.% of the oxides) is aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide, alone or as a mixture, are present in the oxide layer. 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. In the optional embodiment with zinc as a component of the aluminum base layer, zinc oxide components are also present in the oxide layer.

[0136] Preferably, the oxide layer of the steel flat product has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm.

[0137] In an alternative design, the steel flat product includes a zinc-based corrosion protection coating. This coating can be applied to one or both sides of the steel flat product. The two sides of the steel flat product are defined as the two opposing large surfaces. The narrow surfaces are referred to as edges.

[0138] Such a zinc-based corrosion protection coating preferably comprises 0.2–6.0 wt.% Al, 0.1–10.0 wt.% Mg, optionally 0.1–40 wt.% manganese or copper, optionally 0.1–10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and zinc as the remainder. In particular, the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%. The Mg content is particularly a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%. The corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition, or by electrolytic processes.

[0139] Such a flat steel product can be produced, for example, by the following work steps: a) Providing a slab or a thin slab made of steel, which, in addition to iron and unavoidable impurities (in wt.%), consists of

[0140] C: 0.05 - 0.20%

[0141] Si: 0.01 - 2.0%

[0142] Mn: 0.1 - 2.0%

[0143] AI: 0.01 - 0.50%

[0144] Note: 0.003 - 0.070%

[0145] B: 0.0005 - 0.01%

[0146] P: < 0.03%

[0147] S: < 0.02%

[0148] N: < 0.03 %,

[0149] Sn: < 0.03%

[0150] As: < 0.01 %,

[0151] Sb: < 0.02%, and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ti, Ca, W” in the following concentrations

[0152] Cr: 0.01 - 1.0 %,

[0153] Cu: 0.01 - 0.2%

[0154] Mo: 0.002 - 0.3%

[0155] Ni: 0.01 - 0.50%

[0156] V: 0.02 - 0.25%

[0157] Ti: 0.005 - 0.10%

[0158] Ca: 0.0005 - 0.01%, W: 0.001 - 1.0%; b) Through-heating of the slab or thin slab at a temperature (TI) of 1100 - 1400 °C; c) Optional pre-rolling of the through-heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) Hot rolling to a hot-rolled steel flat product, wherein the final rolling temperature (T3) is 750 - 1000 °C; e) Optional coiling of the hot-rolled steel flat product, wherein the coiling temperature (T4) is at most 700 °C; f) Optional descaling of the hot-rolled steel flat product; g) optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%; h) annealing of the steel flat product at an annealing temperature (T5) of 650–900 °C; i) cooling of the steel flat product to an immersion temperature (T6) of 650–800 °C, preferably 670–800 °C;j) Coating the steel flat product, cooled to the immersion temperature, with a corrosion protection coating by hot-dip coating in a melt bath with a melt temperature (T7) of 660–800 °C, preferably 680–740 °C; k) Cooling the coated steel flat product to room temperature, wherein the first cooling time t; mT in the temperature range between 600 °C and 450 °C more than 10s, in particular more than 14s, and the second cooling duration t n in the temperature range between 400 °C and 300 °C more than 8s, in particular more than 12s; I) optional dressing of the coated steel flat product.

[0159] In step a), a semi-finished product composed according to the alloy specified for the steel flat product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin continuous slab casting.

[0160] In step b), the semi-finished product is heated through at a temperature (TI) of 1100–1400 °C. If the semi-finished product has cooled after casting, it is first reheated to 1100–1400 °C for thorough heating. The heating temperature should be at least 1100 °C to ensure good formability for the subsequent rolling process. The heating temperature should not exceed 1400 °C to avoid the presence of molten phases in the semi-finished product.

[0161] In optional step c), the semi-finished product is pre-rolled to an intermediate product. Thin slabs are not usually pre-rolled. Thick slabs intended for hot-rolled strip can be pre-rolled if necessary. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C to ensure it contains sufficient heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which negatively impacts the mechanical properties of the finished steel product. To minimize grain growth during rolling, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200 °C.

[0162] In step d), the slab or thin slab, or, if step c) was performed, the intermediate product, is rolled into a hot-rolled steel flat product. If step c) was performed, the intermediate product is typically finish-rolled immediately after pre-rolling. Finish rolling typically begins no later than 90 seconds after the end of pre-rolling. The slab, thin slab, or, if step c) was performed, the intermediate product, is rolled to a final rolling temperature (T3). The final rolling temperature, that is, the temperature of the finished hot-rolled steel flat product at the end of the hot rolling process, is 750–1000 °C. At final rolling temperatures below 750 °C, the amount of free vanadium decreases because larger quantities of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have a mean grain size of 30 nm or more and are not dissolved in subsequent annealing processes, such as those performed before hot-dip coating. The final rolling temperature is limited to a maximum of 1000 °C to restrict coarsening of the austenite grains. Furthermore, final rolling temperatures of no more than 1000 °C are relevant for process engineering purposes in setting coil temperatures (T4) below 700 °C.

[0163] Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or as reversing rolling. Step e) provides for optional coiling of the hot-rolled steel flat product in the case of continuous hot strip rolling. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. Water, air, or a combination of both can be used as the cooling medium. The coiling temperature (T4) should not exceed 700 °C to prevent the formation of large vanadium carbides. In principle, there is no lower limit to the coiling temperature. However, coiling temperatures of at least 500 °C have proven advantageous for cold rolling. Subsequently, the coiled hot strip is cooled to room temperature in the conventional manner using air.

[0164] In step f), the hot-rolled steel flat product is optionally descaled in a conventional manner by pickling or by another suitable treatment.

[0165] The hot-rolled steel flat product, cleaned of scale, can optionally undergo cold rolling before the annealing treatment in step g) to meet, for example, higher requirements for the thickness tolerances of the steel flat product. The cold rolling degree (CW) should be at least 30% to introduce sufficient deformation energy into the steel flat product for rapid recrystallization. The cold rolling degree (CW) is defined as the quotient of the thickness reduction during cold rolling (AdW) divided by the hot strip thickness (d).

[0166] KWG = AdKW / d, where AdKW = thickness reduction during cold rolling in mm and d = hot-rolled strip thickness in mm. The thickness reduction AdKW is the difference between the thickness of the steel flat product before cold rolling and the thickness of the steel flat product after cold rolling. The steel flat product before cold rolling is typically hot-rolled strip with a thickness d. The steel flat product after cold rolling is also commonly referred to as cold-rolled strip. In principle, the degree of cold rolling can reach very high values ​​of over 90%. However, degrees of cold rolling of no more than 80% have proven advantageous for preventing strip cracking.

[0167] In step h), the steel flat product undergoes an annealing treatment at annealing temperatures (T5) of 650–900 °C. For this purpose, the steel flat product is first heated to the annealing temperature within 10 to 120 s and then held at the annealing temperature for 30 to 600 s. The annealing temperature is at least 650 °C, preferably at least 720 °C. Annealing temperatures above 900 °C are not desirable for economic reasons.

[0168] In step i), the steel flat product is cooled to an immersion temperature (T6) after annealing to prepare it for subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the melt pool. The immersion temperature is 600–800 °C, preferably at least 650 °C, particularly preferably at least 670 °C, and particularly preferably at most 700 °C.

[0169] For a particularly homogeneous boundary layer formation, it is important that sufficient thermal energy is present in the boundary layer between the steel substrate and the aluminum melt. This is not the case at temperatures below 600 °C, so that undesirable compounds can form, the subsequent reconversion of which can lead to pores. Above the preferred immersion temperatures, the diffusion rate of iron in aluminum increases significantly again, so that more iron can diffuse into the still liquid boundary layer right from the start of the coating process. The cooling time of the annealed steel flat product from the annealing temperature T5 to the immersion temperature T6 is preferably 10–180 s. In particular, the immersion temperature T6 deviates from the temperature of the melt bath T7 by no more than 30 K, more specifically no more than 20 K, and preferably no more than 10 K.

[0170] In step j), the steel flat product undergoes a coating treatment. This coating treatment is preferably carried out by continuous hot-dip coating. The coating can be applied to one side, both sides, or all sides of the steel flat product. The coating treatment is preferably carried out as a hot-dip coating process, particularly as a continuous process. In this process, the steel flat product typically comes into contact with the molten metal bath on all sides, so that it is coated on all sides. The molten metal bath, which contains the alloy to be applied to the steel flat product in liquid form, typically has a temperature (T7) of 660–800 °C, preferably 680–740 °C. Aluminum-based alloys have proven particularly suitable for coating age-resistant steel flat products with a corrosion-resistant coating. In such a case, the molten metal bath contains up to 15 wt.-% Si preferably more than 1.0%, optionally 2–4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally other constituents, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. In a preferred variant, the Si content of the melt is 1.0–3.5 wt.% or 7–12 wt.%, in particular 8–10 wt.%. In a preferred embodiment, the optional content of alkali or alkaline earth metals in the melt comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0171] After the coating treatment, the coated steel flat product is cooled to room temperature in step k). The initial cooling time is t. mT in the temperature range between 600 °C and 450 °C (mean temperature range mT) more than 10s, in particular more than 14s, and a second cooling period t nT in the temperature range between 400 °C and 300 °C (low temperature range nT) more than 8s, especially more than 12s.

[0172] The first cooling period t can be mT Cooling in the temperature range between 600 °C and 450 °C (mean temperature range mT) can be achieved through slow, continuous cooling or by holding the product at a temperature within this range for a certain period of time. Intermediate heating is also possible. The only important factor is that the flat steel product remains cool for at least a cooling period t. mTThe temperature range is between 600 °C and 450 °C. Within this range, iron diffuses significantly into aluminum, while aluminum diffusion into steel is inhibited because the temperature is below half the melting point of steel. This allows iron to diffuse into the corrosion protection coating without significant aluminum diffusion into the steel substrate.

[0173] The diffusion of iron into the corrosion protection coating has several advantages:

[0174] Firstly, the melting of the corrosion protection coating during austenitizing prior to press hardening is delayed. Secondly, the coefficients of thermal expansion of the corrosion protection coating and the substrate become more homogeneous. This means that the transition zone between the coefficient of thermal expansion of the substrate and the surface becomes wider, which reduces thermal stresses during reheating.

[0175] At the same time, the diffusion of aluminum into the steel substrate would have significant disadvantages: Due to the very high affinity of aluminum for nitrogen, a high aluminum content can lead to nitrogen dissolving from fine precipitates, such as niobium carbonitrides or titanium carbonitrides, and instead coarse precipitates, such as aluminum nitrides, preferentially forming at the grain boundaries.

[0176] Due to the preferred first cooling duration t mT (14s) The iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum uptake into the substrate during austenitization prior to press hardening, with the associated advantages described above.

[0177] The second cooling period t nTemperature T in the range between 400 °C and 300 °C (low temperature range nT) can also be achieved through slow, continuous cooling or by holding the product at a temperature within this range for a certain period of time. Intermediate heating is also possible. The only important factor is that the flat steel product remains at this temperature for at least a certain cooling period t. nT remains within the temperature range between 400 °C and 300 °C.

[0178] In this temperature range, carbon still diffuses somewhat into the steel substrate, while its thermodynamic solubility is very low. Carbon diffuses to lattice defects and accumulates there, for example, as dissolved Nb atoms. Due to their significantly larger atomic volume, these Nb atoms expand the lattice, thereby enlarging the tetrahedral and octahedral voids, thus increasing the local solubility of carbon. This results in clusters of carbon and Nb, which then transform into very fine precipitates during the austenitizing step of hot forming, leading to a refined austenitic microstructure and consequently a hardened microstructure, as well as a reduction in the free hydrogen content.

[0179] With a preferred holding time of more than 12 seconds, very fine iron carbides (so-called transition carbides) are formed. These dissolve very quickly during austenitizing, leading to additional austenite nuclei and thus an even finer austenite microstructure and consequently a hardened microstructure. The coated steel flat product can optionally undergo a temper treatment with a tempering degree of up to 2% to improve its surface roughness.

[0180] Alternatively, the steel flat product can also be uncoated, i.e., it can have no corrosion protection coating. In such a case, steps h), i), j) and k) are omitted.

[0181] In the inventive method for producing a sheet metal part of the invention as described above, at least the following steps are carried out: a) providing a sheet metal blank from a previously described flat steel product; b) heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature T E ini g of the blank when placed in a forming tool intended for hot pressing (step c)) has at least a partial temperature above Ms+100 °C, where Ms denotes the martensite start temperature; c) Place the heated sheet blank into a forming tool, wherein the transfer time t required for removing the blank from the heating device and placing it in the tool Tr to the QÜt:

[0182] ^trans.niin — ^trans — ^trans,max[X]\ . CT denotes a dimensionless temperature factor whose maximum value c T>max = 0.92 and its minimum value c T-min = 0.70 and where s denotes the thickness of the sheet metal blank in mm; d) Hot forming of the sheet metal blank to form the sheet metal part, wherein the blank is heated to the target temperature T during hot forming Ziei is cooled down and optionally kept there, and the time t 8 / 5 for cooling from 800 °C to 500 °C, the time is 2–10 s; e) extraction of the product cooled to the target temperature T Z iei cooled sheet metal part from the tool.

[0183] In the inventive method, a blank consisting of a steel composed in a suitable manner according to the preceding explanations is thus provided (step a)), which is then heated in a manner known per se such that at least partially the AC3 temperature of the blank is exceeded and the temperature T Einigthe temperature of the blank when placed in a forming tool intended for hot pressing (step c)) is at least partially above Ms+100 °C, preferably the temperature T EinigWhen inserted into a forming tool intended for hot pressing, the blank must be at least partially at least 800 °C. For the purposes of this application, "partially exceeding or reaching a temperature" (here AC3 or Ms+100 °C or 800 °C) means that at least 30%, and in particular at least 60%, of the blank's volume exceeds the corresponding temperature. Therefore, when inserted into the forming tool, at least 30% of the blank must have an austenitic microstructure; that is, the transformation from a ferritic to an austenitic microstructure need not yet be complete. Rather, up to 70% of the blank's volume may consist of other microstructural constituents, 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 than others during heating. This can be achieved by selectively directing the heat input to specific sections of the blank or by shielding the parts that are to be heated less from the heat input. In the part of the blank whose temperature remains lower, no or significantly less martensite forms during the forming process in the die, resulting in a considerably softer microstructure there compared to the other parts, which exhibit a martensitic structure. In this way, a softer area can be selectively created within the formed sheet metal part, for example, by providing optimal toughness for the intended application, while the other areas of the sheet metal part possess maximized strength.

[0184] 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 lies 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 Houghardy, HP, in *Materialkunde Stahl*, Volume 1: Fundamentals, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229.

[0185] Ac3 = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni + 55*%V) °C where %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.

[0186] An optimally uniform distribution of properties can be achieved by thoroughly heating the blank in step b), preferably ensuring that 100% of the volume is above the specified temperatures (Ac3 or Ms+100°C) or within the specified temperature intervals (e.g., at least 800°C), in particular the temperature T. E ini g The temperature of the blank must be at least 800 °C when placed in a forming tool intended for hot pressing.

[0187] In a preferred embodiment, the average heating rate r is O f en The temperature of the sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, particularly at least 10 K / s, preferably at least 15 K / s. The average heating rate r O fen is to be understood as the average heating rate from 30 °C to 700 °C.

[0188] In a preferred embodiment, the heating takes place in an oven with an oven temperature T. O Temperatures of at least 850 °C, preferably at least 880 °C, particularly preferably at least 900 °C, particularly at least 920 °C, and at most 1000 °C, preferably at most 950 °C, particularly preferably at most 930 °C.

[0189] 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, especially preferably at least 0 °C and at most +25 °C, preferably at most +20 °C, in particular at most +15 °C.

[0190] In a specific embodiment, the heating in step b) takes place in stages in areas with different temperatures. In particular, the heating is carried out in a roller hearth furnace with different heating zones. Here, the heating in a first heating zone occurs at a temperature (so-called furnace inlet temperature) of at least 650 °C, preferably at least 680 °C, and particularly at least 720 °C. The maximum temperature in the first heating zone is preferably 900 °C, and particularly preferably 850 °C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably at most 1200 °C, particularly preferably at most 1000 °C, more preferably at most 950 °C, and most preferably at most 930 °C.

[0191] The total time in the oven t O f en)The heating time, which consists of a heating time and a holding time, is preferably at least 2 minutes, particularly at least 3 minutes, and preferably at least 4 minutes for both variants (constant oven temperature, stepwise heating). Furthermore, the total oven time for both variants is preferably a maximum of 20 minutes, particularly a maximum of 15 minutes, preferably a maximum of 12 minutes, and particularly a maximum of 8 minutes. Longer total oven times have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding the blank above Ac3 for too long leads to grain coarsening, which negatively affects the mechanical properties.

[0192] The pre-heated blank is removed from the respective heating device, which may be, for example, a conventional heating furnace, a known induction heating device, or a conventional device for keeping steel components warm, and transported to the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms + 100 °C, preferably above 600 °C, particularly above 650 °C, and most 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 these variants, the temperature is, in particular, a maximum of 900 °C. These temperature ranges ensure good formability of the material.

[0193] In step c), the austenitized blank is transferred from the heating device used to the forming tool within a transfer time t that depends on the sheet thickness. Tra ns , where t Tra n S according to the following formula

[0194] — ln c7' t trans = -0.01 - Ins + 0.024 M is determined and c T c is a dimensionless temperature factor, and s is the thickness of the sheet metal blank in mm. The notation "[s]" means that the result of the formula is the time in seconds. The dimensionless temperature factor c T Therefore, with its maximum value c T-max the minimum value for the transfer time t tranSimin before and with its minimum value c T-min the maximum value for the transfer time t tranSimax The dimensionless temperature factor c T The value here is 0.70 < c T < 0.92, preferably 0.73 < c T< 0.90. Such rapid transport is necessary to prevent excessive cooling before forming. When dealing with blanks of varying thicknesses, such as tailor-rolled blanks, tailor-welded blanks, or patchwork blanks, the thinnest thickness of the blank must be used to determine the transfer time.

[0195] 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, and particularly between 50 °C and 150 °C. Optionally, in a particular embodiment, the tool can be pre-set to a temperature T, at least in certain areas. W The tool temperature Tz must be at least 200 °C, and in particular at least 300 °C, to ensure partial hardening of the component. Furthermore, the tool temperature Tz is preferably a maximum of 600 °C, and in particular a maximum of 550 °C. It is only necessary to ensure that the tool temperature Tz is not exceeded. wbelow the desired extraction temperature T Entn The frame lies. The residence time in the tool t wz The dwell time in the tool is preferably at least 2s, in particular at least 3s, and most preferably at least 5s. The maximum dwell time in the tool is preferably 25s, in particular at most 20s.

[0196] The extraction temperature T En The removal temperature of the sheet metal part is at least partially below 400 °C, preferably below 300 °C, particularly below 250 °C, preferably below 200 °C, most preferably below 180 °C, and particularly below 150 °C. Alternatively, the removal temperature T is En Removal of the sheet metal part is particularly preferably carried out at 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.

[0197] The martensite start temperature of a steel that meets the requirements of the invention is given by the formula:

[0198] Ms [°C] = (490.85 - 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.%] to be calculated, 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.%.

[0199] The ACl temperature and the AC3 temperature of a steel within the parameters of the invention are given by the following formulas:

[0200] AC1[°C] = (739 - 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni + 20*%V )[°C / wt.- %]

[0201] AC3[°C] = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni + 55*%V)[°C / wt.- %] to be calculated, 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).

[0202] In the tool, the blank is not only formed into the sheet metal part, but also simultaneously quenched to the removal temperature. The cooling process in the tool is controlled so that the time t 8 / 5 The cooling time from 800 °C to 500 °C is 2–10 seconds. The cooling time from 800 °C to 500 °C over time t 8 / 5This process therefore takes place within the tool. This is achieved by appropriately adjusting the transfer time, the tool temperature, and the contact pressure. Both the tool temperature or cooling, as well as the contact pressure during hot pressing, have a decisive influence on heat dissipation from the sheet metal part. These parameters can therefore be used to determine the time t. 8 / 5 The temperature must be adjusted accordingly. Naturally, this adjustment must be made depending on the thickness of the sheet metal part, as more heat needs to be dissipated from thicker parts. The cooling time ts / 5 from 800 °C to 500 °C in the range of 2–10 s ensures that a completely martensitic microstructure does not form, but rather that at least 5% of the microstructure of the sheet metal part is lower bainite. Preferably, the time t is... 8 / 5at least 4s, in particular at least 5s, in particular at least 6s. Furthermore preferably, the time ts / s is a maximum of 9s, in particular a maximum of 8s. This allows the preferred microstructures described above to be achieved.

[0203] After removing 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 period t AB from 0.5 to 600 s. This usually happens through air cooling.

[0204] In a preferred embodiment, the extraction temperature Textraction is at least 50 °C, particularly at least 80 °C, and preferably at least 100 °C. A higher extraction temperature increases the efficiency of the process. Since the structural stability of the bainitic microstructure does not necessitate a particularly low extraction temperature, it is advantageous to select higher extraction temperatures. However, to ensure sufficient dimensional accuracy and / or to reduce distortion during cooling outside the tool geometry, the extraction temperature should be below 250 °C, preferably below 200 °C, particularly preferably below 180 °C, and especially below 150 °C.

[0205] In a particular embodiment, the sheet metal part preferably comprises an aluminum-based corrosion protection coating. Preferably, the corrosion protection coating of the sheet metal part comprises an alloy layer and an aluminum base layer. In the sheet metal part, the alloy layer is also frequently referred to as an interdiffusion layer.

[0206] The thickness of the corrosion protection coating is preferably at least 10 pm, particularly preferably at least 20 pm, and especially at least 30 pm.

[0207] The thickness of the alloy layer is preferably less than 30 pm, particularly preferably less than 20 pm, particularly less than 16 pm, and particularly preferably less than 12 pm. The thickness of the Al base layer is the difference between the thicknesses of the corrosion protection coating and the alloy layer.

[0208] The alloy layer lies on top of the steel substrate and is directly adjacent to it. Preferably, the alloy layer of the sheet metal part consists of 35–90 wt.% Fe, 0.1–12 wt.% Si, and optional additional components, the total content of which is limited to a maximum of 3.5 wt.%, preferably 2.0 wt.%, with aluminum as the remainder. The optional additional components are preferably the elements present in the steel substrate besides iron, and the remaining elements from the melt, such as Zn and alkali or alkaline earth metals. These elements from the melt accumulate in the alloy layer only to a very small extent.

[0209] The alloy layer preferably has a ferritic structure in the area close to the substrate.

[0210] The aluminum base layer of the sheet metal part lies on top of the alloy layer and is directly adjacent to it. Preferably, the aluminum base layer of the sheet metal part consists of 35–55 wt.% Fe, 0.4–10 wt.% Si, optionally up to 3 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Preferably, the optional content of alkali or alkaline earth metals is at least 0.1 wt.%. The same applies to the Al base layer of the sheet metal part: the content of optional additional components is preferably limited to 1.5 wt.%, in particular to 1.0 wt.%, preferably to 0.5 wt.%, in particular to 0.3 wt.%, preferably to 0.10 wt.%, in particular to 0.05 wt.%.In particular, the optional additional components correspond to unavoidable impurities and are only present in technically unavoidable levels.

[0211] In a preferred embodiment of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can, in particular, comprise at least 0.0015 wt.% Ca, more preferably at least 0.1 wt.% Ca. More preferably, the optional content of alkali or alkaline earth metals consists of 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, more preferably 0.1–0.5 wt.% Mg, and optionally at least 0.0015 wt.% Ca, more preferably at least 0.1 wt.% Ca.

[0212] The aluminum base layer can have a homogeneous elemental distribution in which the local elemental contents vary by no more than 10%. Preferred variants of the aluminum base layer, however, exhibit silicon-poor and silicon-rich phases. Silicon-poor phases are regions whose average silicon content is at least 20% lower than the average silicon content of the aluminum base layer. Silicon-rich phases are regions whose average silicon content is at least 20% higher than the average silicon content of the aluminum 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 bounded by silicon-poor regions.A continuous layer of silicon-rich phases is defined as a line drawn parallel to the surface of the steel substrate in a vertical micrograph that passes completely through the silicon-rich phases. Conversely, a layer with at least X% continuousness is defined as a line drawn parallel to the surface of the steel substrate in a vertical micrograph that lies within the silicon-rich phases for at least X% of its length. In the present case, the silicon-rich phases are thus so contiguous that a line drawn parallel to the surface of the steel substrate in a vertical micrograph can lie within the silicon-rich phases for at least 40% of its length. In an alternative configuration, the silicon-rich phases are arranged in island-like formations within the silicon-poor phase.

[0213] For the purposes of this application, "island-shaped" means an arrangement in which discrete, unconnected areas are enclosed by another material – i.e., there are "islands" of a certain material within another material.

[0214] In a preferred embodiment, the sheet metal component comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located, in particular, on the aluminum base layer and preferably forms the outer layer of the corrosion protection coating.

[0215] The oxide layer of the sheet metal part consists in particular of more than 80 wt.% oxides, wherein the main proportion of the oxides (i.e., more than 50 wt.% of the oxides) is 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.

[0216] The oxide layer preferably has a thickness of at least 50 nm, particularly at least 100 nm. Furthermore, the thickness is preferably a maximum of 4 pm, particularly a maximum of 2 pm.

[0217] In a specific embodiment, the sheet metal component includes a zinc-based corrosion protection coating. Such a zinc-based corrosion protection coating preferably comprises up to 80 wt.% Fe, 0.2–6.0 wt.% Al, 0.1–10.0 wt.% Mg, optionally 0.1–40 wt.% manganese or copper, optionally 0.1–10.0 wt.% cerium, optionally at most 0.2 wt.% other elements, unavoidable impurities, and zinc as the remainder. In particular, the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%. The Fe content, which results from diffusion, is preferably more than 20 wt.%, particularly more than 30 wt.%. Furthermore, the Fe content is particularly a maximum of 70 wt.%, particularly a maximum of 60 wt.%. The Mg content is particularly a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%. The corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition, or by electrolytic processes.

[0218] The sheet metal component according to the invention is preferably a component for a land vehicle, sea vehicle, or aircraft. It is particularly preferably an automotive component, especially a body panel. The component is preferably a B-pillar, a longitudinal member, an A-pillar, a sill, or a cross member.

[0219] The invention will now be explained in more detail using exemplary embodiments.

[0220] - Figure aa shows an exemplary light-optical representation of the structure for determining the visible former austenite grain boundaries;

[0221] - Figure lb shows the representation from Figure 1a with the determined visible former austenite grain boundaries;

[0222] - Figure 2a shows a SEM image for determining the carbides and their distribution;

[0223] - Figure 2b shows the representation from Figure 2a with a highlighted ferrite unit of the lower bainite.

[0224] To demonstrate the effectiveness of the invention, several tests were conducted. For this purpose, a slab with the composition specified in Table 1, measuring 240 mm thick and 1200 mm wide, was produced, heated to a temperature TI in a pusher furnace, and held at TI for 400 minutes until the core temperature TI was reached and the slab was thoroughly heated. The production parameters are given in Table 2. The slab was then removed from the pusher furnace at its heated core temperature TI and subjected to hot rolling. The test was carried out as continuous hot strip rolling. For this, the slab was first pre-rolled to an intermediate product with a thickness of 40 mm, which can also be referred to as a roughing strip in hot strip rolling.The roughing strip was fed to the finish rolling mill immediately after pre-rolling, with the roughing strip temperature T2 corresponding to the temperature of the strip beginning upon entering the rolling mill. The roughing strip was hot-rolled to a final thickness of 4 mm and the final rolling temperatures T3 specified in Table 2. At the coiling temperature T4, it was wound into a coil and then cooled in still air. The hot-rolled strip was descaled conventionally by pickling before being subjected to cold rolling to the degree of cold rolling specified in Table 2. The thickness of the resulting steel strip was 1.5 mm.

[0225] The cold-rolled steel flat product was heated to an annealing temperature T6 in a continuous annealing furnace and held at annealing temperature T5 for 100 s before being cooled to the immersion temperature T6 at a cooling rate of 3 K / s. The cold-rolled strip was then passed through a molten coating bath at temperature T7 at the immersion temperature T6. The composition of the coating bath is given in Table 3. After coating, the coated strip was blown off using conventional methods, producing a coating with the layer thickness specified in Table 3. The strip was initially cooled to 600 °C at an average cooling rate of 10–15 K / s. During the subsequent cooling phases between 600 °C and 450 °C and between 400 °C and 300 °C, the strip was cooled for the cooling times t specified in Table 3. mT and t n The strip was cooled to T. Between 450 °C and 400 °C and below 220 °C, the strip was cooled at a cooling rate in the range of 5 - 15 K / s.

[0226] From the steel strip produced in this way, blanks were cut off and used for further tests. The data for these tests are given in Table 4. In these tests, sheet metal samples in the form of blanks for a generic B-pillar were hot-pressed from the respective blanks. For this purpose, the blanks were heated in a heating device, namely a conventional heating furnace, from room temperature at a mean heating rate r. O fen (between 30 °C and 700 °C) in an oven with an oven temperature T O f e The total time in the furnace, including heating and holding, is denoted by "furnace". The dew point of the furnace atmosphere is given in Table 4. Subsequently, the blanks are removed from the heating unit and placed in a forming tool, which maintains a temperature of T. wzThe blanks were placed in the oven. At the time of removal, the blanks had reached oven temperature. The transfer time t, which consists of the time required for removal from the heating device, transport to the tool, and insertion into the tool, is calculated as follows: Tr The time was 8s. With the given thickness of the sheet metal blank, this results in 0.70 < c. T < 0.92, that for t Tra n S The following should apply: 4.18s < c T < 17.88s. This requirement is met in the present case. The temperature T E ini g The blanks, when placed in the forming tool, were 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 forming in the tool for different times t. 8 / 5 The samples were cooled from 800 °C to 500 °C. Subsequently, the samples were taken at a sampling temperature T. EntnThe workpiece was removed from the mold and cooled to room temperature in air. Table 4 lists the parameters for various variants, where "RT" abbreviates room temperature. The cooling rate between 500 °C and the removal temperature is determined based on the residence time in the mold and the time ts / s.

[0227] From the samples thus prepared, specimens were subsequently taken for tensile tests. Longitudinal sections were also prepared and subjected to etching with 3% Nital. The microstructure of these longitudinal sections was determined according to the explanations in the description. However, for test 1 (see Table 5), the area fraction of the visible former austenite grains could not be determined because the proportion of lower bainite was too low. Therefore, "n / a" is entered in Table 5.

[0228] Figure 1a shows an example of the microstructure used to determine the former austenite grain boundaries. In Figure 1b, the identified former austenite grain boundaries have been manually traced. (The images are 145 µm x 97 µm in size.) It is clearly visible that there are areas completely surrounded by former austenite grain boundaries. These are counted as former austenite grains according to the analysis. The ratio of enclosed pixels to the total number of pixels then yields the area fraction of the visible former austenite grains, as explained previously.

[0229] Figure 2a shows an example of the microstructure for determining the carbides. Using this figure, the ferrite units of the lower bainite are first identified. Lower bainite and upper bainite are distinguished based on the carbides contained in the lower bainite. Furthermore, lower bainite differs from tempered martensite in that the carbides in lower bainite are oriented parallel to each other in only one set, whereas in tempered martensite, the carbides are oriented in several sets (usually three) of parallel carbides within the respective unit, with the carbides not being parallel between these sets. As an example, a ferrite unit of lower bainite is marked and highlighted by a polygonal line in Figure 2b. The properties of the carbides for this ferrite unit can now be determined according to the description.

[0230] Table 5 summarizes the various results regarding the microstructure and carbide distribution of the respective samples.

[0231] Table 1 (steel grade) Iron and unavoidable impurities. Values ​​in wt.%; non-inventive reference examples table 2 (manufacturing conditions for flat steel products) Table 3 (Coating)

[0232] Table 4 (Parameters for hot forming) Table 5 (Properties of sheet metal part)

Claims

Patent claims 1. Sheet metal forming part formed from a flat steel product comprising a steel substrate made of steel, which, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0.05 -0.20%, Si: 0.01 -2.0%, Mn: 0.1 -2.0%, AI: 0.01 -0.50% Note: 0.003 -0.070% B: 0.0005-0.01% P: < 0.03% S: <0.02%, N: <0.03%, Sn: < 0.03% As: < 0.01 %, Sb: < 0.02%, and optionally one or more of the elements “Gr, Cu, Mo, Ni, V, Ti, Ca, W” in the following amounts Cr: 0.01 -1.0%, Cu: 0.01 - 0.2% Mo: 0.002 - 0.3% Ni: 0.01 -0.50%, V: 0.02 - 0.25% Ti: 0.005-0.10% Approximately 0.0005 - 0.01% W: 0.001 -1.0%, consists, characterized in that the microstructure of the steel substrate comprises at least 5% lower bainite.

2. Sheet metal forming part according to claim 1, characterized in that the steel substrate has a structure consisting of the following phases (in %): Martensite: 10 to 95%, lower bainite: 5 to 35%, upper bainite: 0 to 80% Retained austenite: 0 to 5% Remaining 0 to 3%.

3. Sheet metal forming part according to one of claims 1 to 2, characterized in that the area fraction of the visible former austenite grains is greater than 15%.

4. Sheet metal part according to claims 1 to 3, characterized in that the total surface area of ​​carbides in the lower bainite is a maximum of 20%.

5. Sheet metal forming part according to one of claims 1 to 4, characterized in that the surface- 1. Maximum density of carbides in the lower bainite is 120^.

6. Sheet metal forming part according to one of claims 1 to 5, characterized in that the mean area of ​​a carbide in the lower bainite is less than 1500 nm². 2 amounts.

7. Sheet metal part according to one of claims 1 to 6, characterized in that the sheet metal part has a Vickers hardness of a maximum of 420 HV5.

8. Sheet metal forming part according to one of claims 1 to 7, comprising an aluminum-based corrosion protection coating comprising an alloy layer and an Al base layer.

9. Sheet metal forming part according to one of claims 1 to 8, characterized in that the sheet metal forming part is a component for a land vehicle, sea vehicle or aircraft, in particular an automotive part.

10. Method for manufacturing a sheet metal part comprising the following steps: a) Providing a sheet blank from a steel flat product comprising a steel substrate consisting of steel, in addition to iron and unavoidable impurities (in wt.%) consisting of C: 0.05 -0.20%, Si: 0.01 -2.0%, Mn: 0.1 -2.0%, AI: 0.01 -0.50% Note: 0.003 -0.070% B: 0.0005-0.01% P: < 0.03% S: <0.02%, N: <0.03%, Sn: < 0.03% As: < 0.01 %, Sb: < 0.02%, and optionally one or more of the elements “Gr, Cu, Mo, Ni, V, Ti, Ca, W” in the following amounts Cr: 0.01 -1.0%, Cu: 0.01 - 0.2% Mo: 0.002 - 0.3% Ni: 0.01 -0.50%, V: 0.02 - 0.25% Ti: 0.005-0.10% Approximately 0.0005 - 0.01% W: 0.001 -1.0%, exists. b) Heating the sheet metal blank in such a way that at least partially the AC3 temperature of the blank is exceeded and the temperature T Einig of the blank when placed in a forming tool intended for hot pressing (work step c)) at least partially exhibits 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 required for removing the blank from the heating device and inserting it is Tr The following applies: ^trans.niin — ^trans — ^trans,max with fri Cy 1 TI CLX ^trans.min = _ O , O 1 ■ In + 0.024 _ In ^T,min |- ttrans.max - _ O , O 1 ■ In S + 0.024 where c T denotes a dimensionless temperature factor whose maximum value c T>max = 0.92 and its minimum value c T-min = 0.70 and where s denotes the thickness of the sheet metal blank in mm; d) Hot forming of the sheet metal blank to form the sheet metal part, wherein the blank is heated to a removal temperature T during hot forming Ent removal is cooled and optionally held there, with the time t8 / 5 for cooling from 800 °C to 500 °C being 2 - 10 s; e) Removal of the sheet metal part cooled to the removal temperature T removal from the tool; 11. Method according to claim 10, characterized in that the extraction temperature T Ent _ would take at least 50 °C, in particular at least 100 °C.

12. Method according to one of claims 10 to 11, characterized in that the steel flat product has an aluminum-based corrosion protection coating.

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