EP4283003A1 - Method for producing a sheet metal part - Google Patents

Abstract

The invention relates to a method for producing a sheet metal part with a diffusible hydrogen H_diff content of up to 0.4 ppm. In this method, a WOB value is set according to a rolling degree to sheet thickness ratio. The invention further relates to a sheet metal part produced in this way.

Description

The present invention relates to a method for producing a sheet metal part comprising a substrate with a corrosion protection coating, a corresponding sheet metal part and its use in the automotive sector.

In order to offer the combination of low weight, maximum strength and protective effect required in modern body construction, components that are hot-formed from high-strength steel are now used in areas of the body that can be exposed to particularly high loads in the event of a crash. During hot forming, also known as hot press hardening, steel blanks (also referred to as sheet metal blanks), which have previously been separated from cold or hot-rolled steel strip, are heated to a deformation temperature that is generally above the austenitization temperature of the respective steel, and in the heated state are placed in the tool of a forming press placed. During the subsequent forming process, the sheet metal blank or the component formed from it experiences rapid cooling through contact with the cool tool. The cooling rates are set so that a hardness structure (i.e. martensitic structure) results in the component.

When we refer to a “flat steel product” or a “sheet metal product” below, this refers to rolled products, such as steel strips or sheets, from which “sheet metal blanks” (also called blanks) are cut off for the production of body components, for example. “Sheet metal parts” or “sheet metal components” of the type according to the invention are made from such sheet metal blanks, the terms “sheet metal part” and “sheet metal component” being used synonymously here.

All information on the contents of the steel compositions specified in the present application are based on weight, unless expressly stated otherwise. All unspecified “% data” relating to a steel alloy are therefore to be understood as “% by weight” data. With the exception of the information on the residual austenite content of the structure of a sheet metal part according to the invention based on the volume (indicated in "% by volume"), information on the content of the various structural components refers to the area of a section of a sample of the respective product (indicated in Area percentage "Area%"), unless expressly stated otherwise.

Mechanical properties such as tensile strength, yield strength, elongation, which are reported here, were determined in tensile tests in accordance with DIN-EN ISO 6982-1, sample form 2 (Appendix B, Tab. B1) (as of 2020-06), unless expressly stated otherwise . The bending angle is determined according to the VDA standard 238-100 for the maximum force.

The structure was determined on longitudinal sections that were etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffractometry.

WO 2015/036151 A1 discloses a method for producing a sheet metal part provided with a metallic coating that protects against corrosion and a corresponding sheet metal part. The method according to this document includes coating a flat steel product with an alloy of aluminum, zinc, magnesium and optionally silicon and iron, cutting a blank from the flat steel product, heating the blank and forming the blank to obtain the desired sheet metal part.

DE 699 07 816 T2 discloses a method for producing a coated hot and cold rolled steel sheet with very high strength after thermal treatment. For this purpose, a flat steel product is provided with a coating and thermally treated. During thermal treatment, the workpiece is heated to a temperature of over 750 °C.

EP 2 993 248 A1 discloses a flat steel product with an aluminum-containing coating, which contains 0.005 to 0.7% by weight of at least one alkali and / or alkaline earth metal, and a process for its production. In this process, the coated flat steel product is heated to a temperature of 700 to 900 °C for 360 s, 600 s or 800 s and then formed.

When the sheet metal blanks, consisting of a steel substrate and an aluminum-based, metallic corrosion protection coating, are heated, hydrogen diffuses through the metallic coating into the steel substrate as a result of the surface reaction of the moisture present in the oven with the aluminum coating. After press hardening, the hydrogen can no longer escape from the steel substrate because the metallic coating represents a barrier for the diffusible hydrogen H _diff at room temperature. The H _diff content reduces the stresses that the steel can bear in the long term, and spontaneous "hydrogen-induced" fractures can occur in the presence of tensile stresses in the sheet, so-called hydrogen embrittlement. Around To avoid cracks due to the stresses that are usually present in body-in-white construction, the diffusible hydrogen content should be below a component-specific value. This value depends, among other things, on the complexity of the hot forming operation, the post-processing by, for example, laser cutting, punching, mechanical cutting or hot trimming and the installation situation and joining concept and thus the state of stress in the body. Depending on the processing mentioned, the amount H _diff remaining after processing should preferably be ≤ 0.4 ppm (parts per million) before critical body-in-white processes.

Furthermore, there are manufacturing processes in which areas of coated steel strips are rolled to a smaller sheet thickness than other areas and corresponding sheet metal blanks with different degrees of rolling are then taken from them. This allows weight-optimized and load-adapted components to be produced. The ratio of the reduction in thickness during rolling to the initial thickness is called the rolling ratio. According to the invention, the degree of rolling only applies to a rolling process in which the coating with the anti-corrosion coating is already present on the substrate. The rolled areas with a smaller sheet thickness compared to the sheet thickness existing before rolling have a significantly higher density of defects in the steel substrate due to the rolling. This allows diffusible hydrogen to accumulate better in the rolled areas than in the unrolled areas, so that after hot forming and press hardening there is a higher diffusible hydrogen content. As a result, hydrogen-induced cracking can occur much more quickly after hot forming and press hardening in material rolled after coating. A known method of reducing the content of diffusible hydrogen in the component is to lower the dew point in the furnace in which the steel sheet is heated before forming, in order to thereby prevent the formation of diffusible hydrogen from the existing moisture during the oxidation of the substrate To reduce the furnace atmosphere and thereby also lower the H _diff recording of the sheet metal part. However, lowering the dew point is more complex the lower the dew point has to be set. It is therefore desirable not to influence the dew point if possible and, if necessary, not to lower it too much.

The present invention is therefore based on the object of providing a method for producing sheet metal parts comprising a substrate with a corrosion protection coating, with which corresponding sheet metal parts can be obtained which have the lowest possible H _diff content in order to reduce the risk of hydrogen-induced Minimize cracking after hot forming and during subsequent use. Furthermore, it is one The object of the present invention is to provide a method with which it is possible not to exceed a certain H _diff content in a hot-formed component by selecting different furnace parameters depending on the degree of rolling and the sheet thickness of the flat steel product used.

eingestellt werden, und
(D) Umformen des aufgeheizten Stahlflachproduktes aus Schritt (C) in einem Formwerkzeug unter gleichzeitigem Abkühlen, um das Blechformteil zu erhalten.This object is achieved by the method according to the invention for producing a sheet metal part with a content of diffusible hydrogen H _diff of up to 0.4 ppm, comprising at least the steps:
(A) Providing a flat steel product comprising
a steel substrate made of steel, which in addition to iron and unavoidable impurities (in% by weight). C: 0.06-0.5%, Si: 0.05-0.6%, Mn: 0.4-3.0%, Al: 0.06-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10% B: 0.0005-0.01% P: ≤0.03%, S: ≤0.02%, N: ≤0.02%, Sn: ≤0.03%, As: ≤0.01% and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ca, W” in the following contents CR: 0.01-1.0%, Cu: 0.01-0.2%, Mon: 0.002-0.3%, Ni: 0.01-0.5%, V: 0.001-0.3%, Approx.: 0.0005-0.005%, W: 0.001-1.0% consists,
wherein the steel substrate has an aluminum-based corrosion protection coating on at least one side
and wherein the flat steel product has a degree of rolling to sheet thickness ratio (WGB) of greater than 0.8 to 200;
(B) Determination of a WOP value depending on the degree of rolling to sheet thickness ratio WGB within an area spanned by straight connecting distances between the points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value on the y-axis and the degree of rolling to sheet thickness ratio are plotted on the x-axis;
(C) treating the flat steel product at a mean furnace temperature T _furnace (in K) for a period t _furnace (in h) in a heating device, where the dew point temperature of the furnace atmosphere of the furnace T dew _point (in K), the mean furnace temperature T _furnace (in K) and the duration t _oven (in h) according to the following equation of the general formula (1) $$\mathit{WOP}=\frac{T_{\mathrm{Oven}}}{\mathrm{K}}\cdot \log \left(\frac{t_{\mathrm{Oven}}}{\mathrm{H}}+1.15\right)+{\left(\frac{T_{\mathrm{dew\; point}}}{\mathrm{K}}-243.15\right)}^{1.6}$$

be set, and
(D) Forming the heated flat steel product from step (C) in a mold with simultaneous cooling in order to obtain the sheet metal part.

Furthermore, these tasks are also solved by a corresponding sheet metal part and by using the sheet metal part according to the invention in the automotive sector, in particular as a bumper support/reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill.

The method according to the invention serves to produce a sheet metal part with a content of diffusible hydrogen H _diff of up to 0.4 ppm, preferably 0.01 to 0.4 ppm, particularly preferably 0.05 to 0.4 ppm, for example 0.1, 0 ,2, 0.3, or 0.4 ppm, each in the material after hot forming. H _diff describes the amount of hydrogen atoms that are present in dissolved form in the steel substrate after hot forming. The diffusible hydrogen content is based on the mass (analogous to the weight percent information for alloy elements). Methods for determining the H _diff content are known to those skilled in the art, for example desorption mass spectrometry with heated samples (thermal desorption mass spectrometry (TDMS)). For the purposes of this application, the content of diffusible hydrogen H _diff is determined within 48 hours after hot forming.

Compared to known flat steel products, the steel substrate has a steel with an aluminum content that is at least 0.06% by weight, preferably at least 0.07% by weight, in particular at least 0.08% by weight. The aluminum content is preferably at least 0.10% by weight, particularly preferably at least 0.11% by weight, in particular at least 0.12% by weight, preferably at least 0.16% by weight. The maximum aluminum content is 1.0% by weight, in particular a maximum of 0.8% by weight.

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

In a second, further developed variant, the aluminum content is at least 0.50% by weight, preferably at least 0.60% by weight, preferably at least 0.70% by weight. The maximum aluminum content in this variant is a maximum of 1.0% by weight, in particular a maximum of 0.90% by weight, preferably a maximum of 0.80% by weight.

Aluminum ("Al") is known to be added as a deoxidizer in the production of steel. At least 0.01% by weight of Al is required to securely bind the oxygen contained in the steel melt. Al can also be used to bind undesirable but unavoidable levels of N due to production reasons. Comparatively high aluminum contents have so far been avoided because the aluminum content also shifts the Ac3 temperature upwards. This has a negative effect on austenitization, which is important for hot forming. However, it has been shown that increased aluminum contents surprisingly lead to positive effects in conjunction with an aluminum-based corrosion protection coating.

When the flat steel product is coated with an aluminum-based anti-corrosion coating and the subsequent hot forming of sheet metal blanks divided into sheet metal parts, iron diffuses from the steel substrate into the liquid anti-corrosion coating. In the interdiffusion zone, iron-aluminide compounds with higher density are formed via a multistage phase transformation (Fe ₂ Al ₅ →Fe ₂ Al→FeAl→Fe ₃ Al). The formation of such denser phases is associated with higher aluminum consumption than with less dense phases. This locally higher aluminum consumption leads to the formation of pores (vacancies) in the resulting phase. These pores preferably form in the transition area between the steel substrate and the corrosion protection coating, where the proportion of aluminum available is strongly influenced by the aluminum content of the steel substrate. In particular, there may be an accumulation of pores in the form of a band in the transition area.

• Durch die Poren ist die mechanische Integrität in diesem Bereich reduziert. Es kann schneller zu Schichtablösung bei korrosiver Beanspruchung kommen.

Such pores, and in particular a band of pores, cause a variety of problems:

• The pores reduce the mechanical integrity in this area. Layer detachment can occur more quickly under corrosive stress.

• Die Poren führen zu veränderten Strombahnen im Material beim Widerstandpunktschweißen, die die Schweißeignung negativ beeinflussen und so den Schweißbereich reduzieren.
• Bereits die Poren selbst erleichtern die Risseinleitung und Rissausbreitung beim statischen und dynamischen Biegen.

In addition, the transferable force at the connection point between two components is reduced after gluing or welding.

• The pores lead to changed current paths in the material during resistance spot welding, which negatively influence the weldability and thus reduce the welding area.
• The pores themselves facilitate crack initiation and propagation during static and dynamic bending.

Surprisingly, it has been shown that by increasing the aluminum content ("Al") in the steel substrate to the lower limits described and beyond, a significant reduction in pore formation can be achieved when coating with an aluminum-based corrosion protection coating and subsequent hot forming. Particularly in the transition area between the steel substrate and the anti-corrosion coating, the locally higher aluminum consumption can at least partially result in the formation of denser iron-aluminide compounds be compensated for by the aluminum content of the steel substrate, so that the formation of pores, in particular a band of pores, is suppressed.

If the Al content is too high, in particular if the Al content is more than 1.0% by weight, there is a risk that Al oxides will form on the surface of a product made from steel material alloyed according to the invention, which would worsen the wetting behavior during hot-dip coating . In addition, higher Al contents promote the formation of non-metallic Al-based inclusions, which, as coarse inclusions, have a negative impact on crash behavior. The Al content is therefore preferably chosen to be below the upper limits already mentioned.

The bending behavior of the sheet metal component is particularly supported by the niobium content ("Nb") according to the invention of at least 0.001% by weight. The niobium content is preferably at least 0.005% by weight, in particular at least 0.010% by weight, preferably at least 0.015% by weight, particularly preferably at least 0.020% by weight, in particular at least 0.024% by weight, preferably at least 0.025% by weight .-%.

The specified niobium content leads to a distribution of niobium carbonitrides, which leads to a particularly fine hardened structure during subsequent hot forming. During cooling after hot-dip coating, the coated flat steel product is kept in a temperature range of 400 °C and 300 °C for a certain time. In this temperature range there is still a certain rate of diffusion of carbon in the steel substrate, while the thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there. Lattice disturbances are caused in particular by dissolved niobium atoms, which expand the atomic lattice due to their significantly higher atomic volume and thus increase the tetrahedral and octahedral gaps in the atomic lattice, so that the local solubility of C is increased. As a result, clusters of C and Nb arise in the steel substrate, which then transform into very fine precipitates in the subsequent austenitization step of hot forming and act as additional austenite nuclei. This results in a refined austenite structure with smaller austenite grains and thus also a refined hardening structure.

This particularly applies to the ferritic interdiffusion layer that forms during hot forming. The refined ferritic structure in the interdiffusion layer supports the reduction of crack initiation tendencies under bending loads.

However, too high an Nb content leads to delayed recrystallizability. Therefore, the Nb content is a maximum of 0.2% by weight. Furthermore, the niobium content is preferably a maximum of 0.20% by weight, in particular a maximum of 0.15% by weight, preferably a maximum of 0.10% by weight, in particular a maximum of 0.05% by weight.

bevorzugt ist das Verhältnis Al/Nb ≥ 2, insbesondere ≥ 3. Gleichzeitig führt ein zu großes Verhältnis von Al/Nb, dass die AlN-Bildung nicht mehr so vorteilhaft fein erfolgt, sondern zunehmend gröbere AlN-Partikel auftreten, was den Kornfeinungseffekt wieder schmälert. Es hat sich gezeigt, dass dieser Effekt bei niedrigen Mangan-Gehalten früher auftritt als bei höheren Mangan-Gehalten, da mit steigendem Mangan-Gehalt die AC3-Temperatur abnimmt. Daher ist es vorteilhaft, optional bei niedrigen Mangan-Gehalten von kleiner gleich 1,6 Gew.-% ein Verhältnis von Al/Nb einzustellen für das gilt: $$\mathrm{Al/Nb}\le \mathrm{20}\mathrm{.0},$$

was etwa einem atomaren Verhältnis beider Elemente ≤ 6 entspricht. Bevorzugt ist für Mn≤1,6 Gew.-% das Verhältnis Al/Nb ≤ 18.0, insbesondere ≤ 16.0, bevorzugt ≤ 14.0, besonders bevorzugt ≤ 12.0, insbesondere ≤10.0, bevorzugt ≤ 9.0, insbesondere ≤ 8.0, bevorzugt ≤ 7.0.Aluminum and niobium both have an influence on grain refinement during austenitization in the hot forming process. It has been found that Al, in addition to Nb, particularly refines the grain growth at elevated temperatures in the austenite (for example at over 1200 ° C) via a relatively early formation of AlN, that is, taking place at relatively high temperatures. The formation of AIN is thermodynamically favored compared to the formation of NbN or NbC. The precipitation of AIN has a grain-refining effect in the austenite and thus improves toughness. Increasing Al/Nb ratios improve this effect. Therefore, the following optionally applies to the Al/Nb ratio of Al content to Nb content: $$1\le \mathrm{Al/Nb}$$

The preferred ratio is Al/Nb ≥ 2, in particular ≥ 3. At the same time, if the ratio of Al/Nb is too high, the AlN formation is no longer as advantageously fine, but increasingly coarser AlN particles appear, which again reduces the grain refining effect . It has been shown that this effect occurs earlier at low manganese levels than at higher manganese levels, as the AC3 temperature decreases as the manganese level increases. It is therefore advantageous to optionally set a ratio of Al/Nb for low manganese contents of less than or equal to 1.6% by weight, for which the following applies: $$\mathrm{Al/Nb}\le \mathrm{20}\mathrm{.0},$$

which roughly corresponds to an atomic ratio of both elements ≤ 6. For Mn ≤ 1.6% by weight, the ratio Al/Nb ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, particularly preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0 is preferred.

However, with higher manganese contents of Mn ≥ 1.7% by weight, higher ratios are also possible. It is therefore advantageous to optionally set a ratio of Al/Nb for higher manganese contents of 1.7% by weight or more, for which the following applies: $$\mathrm{Al/Nb}\le 3\mathrm{0}\mathrm{.0},$$

For Mn ≥ 1.7% by weight, the ratio Al/Nb ≤ 28.0, in particular ≤ 26.0, preferably ≤ 24.0, particularly preferably ≤ 22.0, preferably ≤ 20.0, in particular ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, especially is preferred preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0.

Regardless of the manganese content, it is optionally preferable to set a ratio of Al/Nb for which the following applies: $$\mathrm{Al/Nb}\le \mathrm{20}\mathrm{.0},$$

The ratio Al/Nb is preferably ≤ 18.0, in particular ≤ 16.0, preferably ≤ 14.0, particularly preferably ≤ 12.0, in particular ≤ 10.0, preferably ≤ 9.0, in particular ≤ 8.0, preferably ≤ 7.0.

Carbon ("C") is contained in the steel substrate of the flat steel product in amounts of 0.06 - 0.5% by weight. C contents set in this way contribute to the hardenability of the steel by delaying the formation of ferrite and bainite and stabilizing the residual austenite in the structure. A carbon content of at least 0.06% by weight is required to achieve sufficient hardenability and the associated high strength.

However, high C contents can have a negative impact on weldability. In order to improve weldability, the carbon content can be set to 0.5% by weight, preferably at most 0.50% by weight, particularly preferably at most 0.40% by weight, preferably at most 0.38% by weight, in particular at most 0.35% by weight can be set.

In order to be able to use the positive effects of the presence of C particularly safely, C contents of at least 0.10% by weight, preferably 0.11% by weight, in particular at least 0.13% by weight, preferably at least 0 .15% by weight can be provided. At these levels, taking into account the further requirements of the invention, tensile strengths of the sheet metal part of at least 1000 MPa, in particular at least 1100 MPa, can be safely achieved after hot pressing.

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

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

In a third, further developed variant, the C content is at least 0.30% by weight, preferably at least 0.35% by weight, in particular at least 0.40% by weight, preferably at most 0.44% by weight. The maximum C content in this variant is a maximum of 0.5% by weight, in particular a maximum of 0.50% by weight, preferably a maximum of 0.48% by weight.

Silicon ("Si") is used to further increase the hardenability of the flat steel product as well as the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silizio-manganese as an alloying agent, which has a beneficial effect on production costs. A hardening effect occurs from an Si content of 0.05% by weight. From an Si content of at least 0.15% by weight, in particular at least 0.20% by weight, a significant increase in strength occurs. Si contents above 0.6% by weight have a detrimental effect on the coating behavior, especially with Al-based coatings. Si contents of at most 0.50% by weight, in particular at most 0.30% by weight, are preferably set in order to improve the surface quality of the coated flat steel product.

Manganese ("Mn") acts as a hardening element by greatly retarding ferrite and bainite formation. With manganese contents of less than 0.4% by weight, significant proportions of ferrite and bainite are formed during press hardening, even at very fast cooling rates, which should be avoided. Mn contents of at least 0.7% by weight, preferably at least 0.8% by weight, in particular at least 0.9% by weight, particularly preferably at least 1.10% by weight, are advantageous if A martensitic structure should be ensured, especially in areas of larger forming. Manganese contents of more than 3.0% by weight have a disadvantageous effect on the processing properties, which is why the Mn content of flat steel products according to the invention is limited to a maximum of 3.0% by weight, preferably a maximum of 2.5% by weight. In particular, weldability is severely limited, which is why the Mn content is preferably limited to a maximum of 1.6% by weight and in particular to 1.3% by weight. Manganese contents of less than or equal to 1.6% by weight are also preferred for economic reasons.

Titanium ("Ti") is a microalloy element that is alloyed to contribute to grain refinement, with at least 0.001% by weight of Ti, in particular at least 0.004% by weight, preferably at least 0.010% by weight of Ti, should be added for sufficient availability. From 0.10% by weight of Ti, cold rolling and recrystallizability deteriorate significantly, which is why larger Ti contents should be avoided. In order to improve the cold-rollability, the Ti content can preferably be limited to 0.08% by weight, in particular to 0.038% by weight, particularly preferably to 0.020% by weight, in particular 0.015% by weight. Titanium also has the effect of binding nitrogen and thus allowing boron to develop its strong ferrite-inhibiting effect. Therefore, in a preferred development, the titanium content is more than 3.42 times the nitrogen content in order to achieve sufficient binding of nitrogen.

Boron ("B") is alloyed to improve the hardenability of the flat steel product by having boron atoms or boron precipitates attached to the austenite grain boundaries reduce the grain boundary energy, thereby suppressing the nucleation of ferrite during press hardening. A clear effect on hardenability occurs at levels of at least 0.0005% by weight, preferably at least 0.0007% by weight, in particular at least 0.0010% by weight, in particular at least 0.0020% by weight. At levels above 0.01% by weight, however, boron carbides, boron nitrides or boron nitrocarbides are increasingly formed, which in turn represent preferred nucleation sites for the nucleation of ferrite and reduce the hardening effect again. For this reason, the boron content is set to at most 0.01% by weight, preferably at most 0.0100% by weight, preferably at most 0.0050% by weight, in particular at most 0.0035% by weight, in particular at most 0. 0030% by weight, preferably at most 0.0025% by weight.

Phosphorus ("P") and sulfur ("S") are elements that are introduced into steel as impurities by iron ore and cannot be completely eliminated in the large-scale steelworks process. The P content and the S content should be kept as low as possible, since the mechanical properties such as the impact energy deteriorate with increasing P content or S content. From P contents of 0.03% by weight, the martensite begins to become brittle, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.03% by weight, in particular a maximum of 0.02% by weight is. The S content of a flat steel product according to the invention is limited to a maximum of 0.02% by weight, preferably a maximum of 0.0010% by weight, in particular a maximum of 0.005% by weight.

Nitrogen ("N") is also present in small amounts in steel as an impurity due to the steel manufacturing process. The N content should be kept as low as possible and should not exceed 0.02% by weight. Nitrogen is particularly harmful for alloys that contain boron, since it prevents the conversion-retarding effect of boron through the formation of boron nitrides, which is why the nitrogen content in this case should preferably be at most 0.010% by weight, in particular at most 0.007% by weight.

Other typical impurities are tin (“Sn”) and arsenic (“As”). The Sn content is a maximum of 0.03% by weight, preferably a maximum of 0.02% by weight. The As content is a maximum of 0.01% by weight, in particular a maximum of 0.005% by weight.

In addition to the previously explained impurities P, S, N, Sn and As, other elements can also be present as impurities in the steel. These other elements are grouped under the “unavoidable impurities”. The total content of these “unavoidable impurities” is preferably a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The optional alloying elements Cr, Cu, Mo, Ni, V, Ca and W described below, for which a lower limit is specified, can also occur in levels below the respective lower limit as unavoidable impurities in the steel substrate. In this case, they are also counted among the "unavoidable impurities", the total content of which is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight.

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

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

These effects mentioned occur from a content of 0.01% by weight, with a content of at least 0.10% by weight, preferably at least 0.15% by weight, having proven itself in practice for safe process management . However, excessive Cr contents impair the ability of the steel to be coated. Therefore, the Cr content of the steel of one of the steel substrate is limited to a maximum of 1.0% by weight, in particular a maximum of 0.75% by weight, preferably a maximum of 0.50% by weight.

Vanadium (V) can optionally be alloyed in amounts of 0.001 - 1.0% by weight. The vanadium content is preferably a maximum of 0.3% by weight. For cost reasons, a maximum of 0.2% by weight of vanadium is added.

Copper (Cu) can optionally be alloyed in order to increase hardenability with additions of at least 0.01% by weight, preferably at least 0.010% by weight, in particular at least 0.015% by weight. In addition, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. If the Cu content is too high, the hot-rollability deteriorates significantly due to low-melting Cu phases on the surface, which is why the Cu content is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight, in particular a maximum of 0. 10% by weight is limited.

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

Nickel (Ni) stabilizes the austenitic phase and can optionally be alloyed to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive influence on hot-rollability, especially if the steel contains copper. Copper impairs hot-rollability. In order to counteract the negative influence of copper on hot-rollability, 0.01% by weight of nickel can be alloyed into the steel; the Ni content is preferably at least 0.020% by weight. For economic reasons, the nickel content should remain limited to a maximum of 0.5% by weight, in particular a maximum of 0.20% by weight. The Ni content is preferably a maximum of 0.10% by weight.

Calcium (Ca) is used in steels to form non-metallic inclusions, especially manganese sulfides. The rounded shape significantly reduces the negative effect of the inclusions on hot formability, fatigue strength and toughness. In order to also use this effect in a flat steel product according to the invention, a flat steel product according to the invention can optionally contain at least 0.0005% by weight of Ca, in particular at least 0.0010% by weight, preferably at least 0.0020% by weight. The maximum Ca content is 0.01% by weight, in particular a maximum of 0.007% by weight, preferably a maximum of 0.005% by weight. If too high Ca content increases the likelihood that non-metallic inclusions involving Ca will form, which will worsen the purity of the steel and also its toughness. For this reason, an upper limit of the Ca content of at most 0.005% by weight, preferably at most 0.003% by weight, in particular at most 0.002% by weight, preferably at most 0.001% by weight, should be maintained.

Tungsten (W) can optionally be alloyed in amounts of 0.001 - 1.0% by weight to slow ferrite formation. A positive effect on hardenability occurs with W contents of at least 0.001% by weight. For cost reasons, a maximum of 1.0% by weight of tungsten is added.

In preferred developments, the sum of the Mn content and the Cr content ("Mn+Cr") is more than 0.7% by weight, in particular more than 0.8% by weight, preferably more than 1.1% by weight .-%. Below a minimum sum of both elements, their necessary conversion-inhibiting effect is lost. Regardless of this, the sum of the Mn content and the Cr content is less than 3.5% by weight, preferably less than 2.5% by weight, in particular less than 2.0% by weight, particularly preferably less than 1.5% by weight. The upper limit values for both elements are created to ensure coating performance and to ensure sufficient welding behavior.

The above explanations regarding element contents and their preferred limits apply accordingly to the sheet metal part described below.

The flat steel product includes an anti-corrosion coating to protect the steel substrate from oxidation and corrosion during hot forming and use of the formed sheet metal part.

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

Such a corrosion protection coating is preferably produced by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt, which consists of up to 15% by weight of Si, preferably more than 1.0%, optionally 2-4% by weight of Fe, optionally up to 5% by weight of alkali or alkaline earth metals, preferably up to 1.0% by weight. % alkali or alkaline earth metals, and optionally up to 15% by weight of Zn, preferably up to 10% by weight of Zn and optional further components, the total contents of which are limited to a maximum of 2.0% by weight, and as The rest is aluminum.

In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 7-12% by weight, in particular 8-10% by weight.

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

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

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially formed from aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60% by weight of Fe, preferably α-iron, optional further components, the total content of which is limited to a maximum of 5.0% by weight, preferably 2.0%, and the balance is aluminum, whereby the Al content increases preferably towards the surface. The optional further components include in particular the remaining components of the melt (i.e. silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron.

The Al base layer lies on the alloy layer and is directly adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt of the melt pool. This means that it consists of 0.1 - 15% by weight of Si, optionally 2-4% by weight of Fe, optionally 5% by weight of alkali or alkaline earth metals, preferably up to 1.0% by weight. Alkaline or alkaline earth metals, optionally up to 15% Zn and optional other components, the total content of which is limited to a maximum of 2.0% by weight, and the balance aluminum.

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

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.

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

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

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

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

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

für einseitige Überzüge.The anti-corrosion coating preferably has a thickness of 5 - 60 µm, in particular 10 - 40 µm. The application weight of the corrosion protection coating is in particular $30-360\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

with corrosion protection coatings on both sides or $15-180\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

in the one-sided variant. The coating weight of the corrosion protection coating is preferred $100-200\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for double-sided covers or $50-100\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for one-sided covers. The coating weight of the corrosion protection coating is particularly preferred $120-180\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for double-sided covers or $60-90\frac{\mathrm{<figure-callout\; id="2"\; label="G\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">G</figure-callout>}}{m^{}}$

for one-sided covers.

The thickness of the alloy layer is preferably less than 20 µm, particularly preferably less than 16 µm, in particular less than 12 µm, particularly preferably less than 10 µm, preferably less than 8 µm, in particular less than 5 µm. The thickness of the Al base layer results from the difference in the thickness of the corrosion protection coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 μm, even with thin corrosion protection coatings.

In a preferred variant, the flat steel product comprises an oxide layer arranged on the corrosion protection coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer end of the corrosion protection coating.

The oxide layer consists in particular of more than 80% by weight of oxides, with the majority of the oxides (ie more than 50% by weight of the oxides) being 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 metallic form. For the optional embodiment with zinc as a component of the Al base layer, zinc oxide components are also present in the oxide layer.

The oxide layer of the flat steel product preferably has a thickness that is greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm.

In a preferred variant, the flat steel product has a diffusible hydrogen H _diff content of up to 0.4 ppm, preferably 0.01 to 0.4 ppm, particularly preferably 0.05 to 0.4 ppm, for example 0.1, 0.2 , 0.3, or 0.4 ppm. H _diff describes the amount of hydrogen atoms that are present in dissolved form in the steel substrate after the corrosion protection coating has been applied. The diffusible hydrogen content is based on the mass (analogous to the weight percent information for alloy elements). Methods for determining the H _diff content are known to those skilled in the art, for example desorption mass spectrometry with heated samples (thermal desorption mass spectrometry (TDMS)). For the purposes of this application, the content of diffusible hydrogen H _diff is determined within 48 hours after application of the corrosion protection coating.

The flat steel product provided in step (A) of the method according to the invention has a rolling degree to sheet thickness ratio of 0.8 to 200, preferably greater than 0.8 to 180, particularly preferably greater than 0.8 to 150.

mit Δh gleich der Dickenabnahme durch das Walzen, d. h. Startdicke - Enddicke (Δh = h ₀ - h ₁) und h₀ gleich der Startdicke des Stahlflachprodukts, jeweils in mm. In einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens wird in Schritt (A) ein Stahlflachprodukt eingesetzt, welches Bereiche aufweist, die auf eine geringere Blechdicke gewalzt werden als andere Bereiche. In diesem erfindungsgemäß bevorzugten Fall wird für das jeweilige Bauteil der größte vorliegende Abwalzgrad zugrunde gelegt.The flat steel product provided according to the invention preferably has a degree of rolling of 0.5% to 75%, particularly preferably 2.5% to 60%. According to the invention, the degree of rolling is given in%. In the context of the present invention, the degree of rolling means the ratio of the reduction in thickness through rolling to the initial thickness of the flat steel product; in particular, the degree of rolling is determined according to the following formula (2): $$\mathit{degree\; of\; rolling}=\frac{\mathit{\Delta h}}{H_O}$$

with Δh equal to the reduction in thickness through rolling, ie starting thickness - final thickness ( Δh = h ₀ - h ₁ ) and h ₀ equal to the starting thickness of the flat steel product, each in mm. In a preferred embodiment of the method according to the invention, a flat steel product is used in step (A), which has areas that are rolled to a smaller sheet thickness than other areas. In this case, which is preferred according to the invention, the greatest degree of rolling available is used as a basis for the respective component.

wobei die Blechdicke in mm angesetzt wird und identisch mit h ₁, der Enddicke des Stahlflachproduktes nach dem Walzen ist.The dimensionless degree of rolling to sheet thickness ratio (WGB) is determined according to the invention using the following formula (3): $$\mathit{WGB}=1.5\cdot \frac{1+\mathit{degree\; of\; rolling}\cdot 100}{\frac{1}{2}\cdot \left(1+\sqrt{\mathit{sheet\; thickness}/\mathrm{mm}}\right)}$$

where the sheet thickness is set in mm and is identical to h ₁ , the final thickness of the flat steel product after rolling.

According to the invention, the flat steel products used in step (A) of the method according to the invention are preferably present in a sheet thickness (final thickness h ₁ ) of 0.5 to 6 mm, in particular 0.5 to 4 mm, particularly preferably 0.8 to 3 mm. (particularly in the areas with the greatest degree of rolling).

According to the invention, the coated flat steel product from step (A), after which process step (B) has been carried out, is preferably transferred directly into process step (C) according to the invention. However, it is also possible for further steps to be carried out between steps (A) and (B) or (C), for example cutting off areas, in particular sheets or circuit boards, of the flat steel product, for example by shear cutting or laser cutting, introducing holes by laser processing or punching, and/or previous heat treatments to change the properties of the coating or substrate.

Step (B) of the method according to the invention includes the determination of a WOP value depending on the degree of rolling to sheet thickness ratio WGB within an area spanned by straight connecting sections between points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system, in which the WOP value is plotted on the y-axis and the degree of rolling ratio to sheet thickness ratio on the x-axis, as preferred in Figure 1 shown. According to the invention, a suitable WOP value range is determined from which a WOP value can then be selected. According to the invention, however, all WOP values lying in the specific WOP value range meet the condition that a sheet metal part with a diffusible hydrogen content of a maximum of 0.4 ppm is obtained.

Step (B) of the method according to the invention serves to determine a WOP value depending on the degree of rolling to sheet thickness ratio of the flat steel product used, where WOP means "hydrogen-related furnace parameter" and is unitless. The WOP value then provides information about the process parameters with which the heat treatment in step (C) should be carried out so that sheet metal parts with diffusible hydrogen contents of a maximum of 0.4 ppm are obtained.

When determining the WOP value according to the present invention, a range for suitable WOP values is determined via the degree of rolling to sheet thickness ratio. A WOP value can then preferably be selected from this range, which is then used to determine the corresponding value for T _oven , t _oven and T t _point using the equation of the general formula (1). In general, however, all values present in the correspondingly determined range of WOP values are suitable for being used in the equation of the general formula (1) in order to determine the corresponding values for T _oven , t _oven and T _{dew point} .

Step (B) of the method according to the invention is preferably carried out in that the WOP value is spanned within an area by straight connecting distances between the points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 ( WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP Value on the y-axis and the degree of rolling sheet thickness ratio are plotted on the x-axis, is determined graphically to a predetermined degree of rolling sheet thickness ratio (area A). The corresponding diagram is in Figure 1 shown, area A results from a union of the shown partial areas "3", "4" and "5" in Figure 1 .

In a preferred embodiment of the method according to the invention, the WOP value is determined according to step (B) of the method according to the invention within an area spanned by straight connecting sections between the points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200), P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) as well as P33 (WGB 50, WOP 300) and P12 (WGB 0.8, WOP 300) in a coordinate system in which the WOP value on the y-axis and the rolling degree Sheet thickness ratio (WGB) is plotted on the x-axis (area B). The corresponding diagram is in Figure 1 shown, area B is the shown partial area "5" without the partial areas "3" and "4" in Figure 1 .

With the WOP value determined in step (B) of the method according to the invention, it can then be determined according to the invention at which dew point temperature of the oven atmosphere T _{dew point} , at which average oven temperature T _oven and for what duration t _oven step (C) of the method according to the invention is carried out.

so eingestellt werden, dass sich der WOP-Wert in dem mithilfe von Figur 1 festgelegten Intervall zwischen dem minimalen und dem maximalen WOP-Wert befindet.Step (C) of the method according to the invention comprises treating the flat steel product at an average oven temperature T _oven (in K) for a duration t _oven (in h), whereby the dew point temperature of the oven atmosphere T _{dew point} (in K), the average oven temperature T _oven ( in K) and the duration t _oven (in h) according to the following equation of the general formula (1) $$\mathit{WOP}=\frac{T_{\mathrm{Oven}}}{\mathrm{K}}\cdot \log \left(\frac{t_{\mathrm{Oven}}}{\mathrm{H}}+1.15\right)+{\left(\frac{T_{\mathrm{dew\; point}}}{\mathrm{K}}-243.15\right)}^{1.6}$$

can be set so that the WOP value is in the using Figure 1 defined interval between the minimum and maximum WOP value.

The oven temperature T _oven (in K) is the temperature that prevails on average in the oven in which step (C) of the method according to the invention takes place. According to the invention, T _oven can take on any value that a person skilled in the art considers appropriate. In the process according to the invention, T _oven AC1 is preferably up to 1373 K, preferably 1113 to 1253 K, particularly preferably 1133 to 1223 K, very particularly preferably 1153 to 1193 K. AC1 means the first austenitization temperature, which depends on the alloy composition.

The duration t _oven (in h) is the time over which the stated oven temperature T _oven prevails in step (C). According to the invention, t _oven can assume any value that a person skilled in the art considers suitable. In the method according to the invention, t _furnace describes in particular the period in which the flat steel product is moved through a continuous furnace or remains in a stationary furnace. In the process according to the invention, t _oven is preferably 0.04 to 0.5 h, in particular at least 0.060 h, preferably at least 0.067 h. The maximum time in the _oven is preferably 0.4 h, in particular a maximum of 0.25 h.

In one embodiment, oven temperature T _oven , duration t _oven , and WOP value are used to calculate and then set the dew point temperature of the oven atmosphere of the oven T dew _point using equation (1). The dew point temperature of the oven T _{dew point} (in K) is, for example, 238.15 to 308.15 K, preferably 253.15 to 293.15 K, particularly preferably 263.15 to 283.15 K.

In a further preferred embodiment, the dew point temperature of the oven atmosphere of the oven T _{dew point} , duration t _oven and WOP value are used to calculate and then set the oven temperature T _oven using equation (1).

In a further preferred embodiment, the dew point temperature of the oven atmosphere of the oven T _{dew point} , oven temperature T _oven and WOP value are used to calculate and then set the duration t _oven using equation (1).

Step (C) of the process according to the invention can generally be carried out in any furnace known to those skilled in the art, for example roller hearth furnaces, chamber furnaces, multi-layer chamber furnaces, walking beam furnaces.

Step (D) of the method according to the invention includes forming the heated flat steel product from step (C) in a molding tool with simultaneous cooling in order to obtain the sheet metal part.

In general, all methods for hot forming known to those skilled in the art can be used in step (D) of the method according to the invention, for example described in hot forming in Automobile construction - processes, materials, surfaces, Landsberg/Lech: Verlag Moderne Industrie, 2012 , The Library of Technology.

In step (D) of the method according to the invention, the desired sheet metal part is obtained from the flat steel product from step (C) by forming. So that the desired hardness structure, for example at least 80% martensite, remainder bainite, ferrite and retained austenite, is formed in the sheet metal part, the forming takes place with simultaneous cooling. The cooling in step (C) of the method according to the invention takes place, in particular over a period of twz more than 1 s, preferably with a cooling rate r _WZ of 10 to 1000 K/s, preferably at least 27 K/s, in particular at least 30 K/s. The maximum cooling rate is preferably up to 500 K/s. The present invention therefore preferably relates to the method according to the invention, wherein the cooling in step (D) takes place at a cooling rate of 27 to 500 K/s.

The flat steel product used is preferably a strip, in particular a hot strip or a cold strip, a sheet, i.e. H. a piece of hot strip or cold strip, or a circuit board made of hot strip or a circuit board made of cold strip. The present invention preferably relates to the method according to the invention, wherein the flat steel product is a blank made from a hot strip or a blank made from a cold strip.

Methods for producing a hot strip or a cold strip are known to those skilled in the art and are described, for example, in ( Hoffmann, Hartmut; Neugebauer, Reimund; Spur, Günter (2012): Manual Forming. Munich: Carl Hanser Verlag GmbH & Co. KG. Pages 109 to 165 and pages 196 to 207 ).

Methods for producing a corresponding coated flat steel product are known to those skilled in the art; for example, the coating with the anti-corrosion coating can be carried out by fire coating, electrolytic coating or by means of a piece coating process. The present invention therefore preferably relates to the method according to the invention, wherein the coating is carried out by hot-dip coating, electrolytic coating or by means of a piece coating process.

The aluminum-silicon-iron alloy is preferably applied using a continuous hot-dip coating process. During the coating, the temperature of the aluminum molten bath is preferably between 660 °C and 720 °C.

Silicon in the coating acts as a diffusion blocker and serves to calm the melt pool when the coating formed from the aluminum alloy is applied using hot-dip coating.

In a preferred embodiment of the method, the treatment in step (C) takes place in such a way that at the end of step (C) the AC3 temperature of the flat steel product is at least partially exceeded in the flat steel product. If a temperature is partially exceeded (here AC3) For the purposes of this application, it is understood that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature. When inserted into the forming tool, at least 30% of the blank has an austenitic structure, ie the conversion from ferritic to austenitic structure does not have to be complete when inserted into the forming tool. Rather, up to 70% of the volume of the blank when inserted into the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or non-or partially recrystallized ferrite. For this purpose, certain areas of the blank can be specifically kept at a lower temperature level than others during heating. For this purpose, the heat supply can be directed only to certain sections of the blank or the parts that should be heated less can be shielded from the heat supply. In the part of the blank material whose temperature remains lower, no or only significantly less martensite is formed during the forming process in the tool, so that the structure there is significantly softer than in the other parts in which a martensitic structure is present. In this way, a softer area can be specifically set in the shaped sheet metal part, for example by having optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

Maximum strength properties of the resulting sheet metal part can be made possible by the fact that the temperature at least partially achieved in the flat steel product in step (C) is preferably between Ac3 and 1000 ° C, particularly preferably between 880 ° C and 950 ° C.

mit %C = jeweiliger C-Gehalt, %Si = jeweiliger Si-Gehalt, %Mn = jeweiliger Mn-Gehalt, %Cr = jeweiliger Cr-Gehalt, %Mo = jeweiliger Mo-Gehalt, %Ni =jeweiliger Ni-Gehalt und %V = jeweiliger V-Gehalt des Stahls, aus dem der Zuschnitt besteht, bestimmt.The minimum temperature to be exceeded is Ac3 according to that of HOUGARDY, HP. in Materials Science Steel Volume 1: Basics, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229 ., given formula $$\mathrm{Ac3}=\left(902-225*\%\mathrm{C}+19*\%\mathrm{Si}-11*\%\mathrm{Mn}-5*\%\mathrm{Cr}+13*\%\mathrm{Mo}-20*\%\mathrm{Ni}+55*\%\mathrm{v}\right)°\mathrm{C}$$

with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and % V = respective V content of the steel from which the blank is made.

An optimally uniform distribution of properties can be achieved by completely heating the flat steel product to the temperatures mentioned in step (C). In a preferred embodiment variant, the standardized average heating Θ _norm is at least 5 Kmm/s, in particular at least 8 Kmm/s, preferably at least 10 Kmm/s. The maximum standardized average heating is 15 Kmm/s, in particular a maximum of 14 Kmm/s, preferably a maximum of 13 Kmm/s.

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

dabei sind die Ofentemperaturen jeweils in Kelvin einzusetzen.In the case of the standardized mean heating, this product Θ is standardized by the existing oven temperature T _oven in relation to a reference oven temperature T _{oven, reference} of 900 ° C = 1173.15 K in the following way: $${\mathrm{\Theta }}_{\mathit{standard}}=\frac{T_{\mathit{Oven},\mathit{reference}}^4}{{\mathrm{<figure-callout\; id="4"\; label="T\; Oven"\; filenames="imgf0001.png"\; state="\{\{state\}\}">T</figure-callout>}}_{\mathit{Oven}}^{\mathit{}}}\cdot \mathrm{\Theta }$$

The oven temperatures must be used in Kelvin.

Between step (C) and step (D), the austenitized flat steel product is transferred from the heating device used in each case to the molding tool within a transfer time (t _Trans ) of preferably a maximum of 20 s, in particular a maximum of 15 s. Such rapid transport is necessary to avoid excessive cooling before deformation.

When inserting the blank, the mold typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C. Optionally, in a special embodiment, the tool can be tempered at least in some areas to a temperature T _WZ of at least 200 ° C, in particular at least 300 ° C, in order to only partially harden the component. Furthermore, the tool temperature Twz is preferably a maximum of 600 ° C, in particular a maximum of 550 ° C. It is only necessary to ensure that the tool temperature Twz is below the desired target temperature T _target . The residence time in the tool twz is preferably at least 2 s, in particular at least 3 s, particularly preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s, preferably a maximum of 15 s, in particular a maximum of 12 s.

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

zu berechnen, wobei hier mit C% der C-Gehalt, mit %Mn der Mn-Gehalt, mit %Mo der Mo-Gehalt, mit %Cr der Cr-Gehalt, mit %Ni der Ni-Gehalt, mit %Cu der Cu-Gehalt, mit %Co der Co-Gehalt, mit %W der W-Gehalt und mit %Si der Si-Gehalt des jeweiligen Stahls in Gew.-% bezeichnet sind.The martensite starting temperature of a steel within the scope of the specifications according to the invention is according to the formula: $$\mathrm{Ms}\left[°\mathrm{C}\right]=490,85°\mathrm{C}+\left(\begin{array}{l}-302,6\%\mathrm{C}-30,6\%\mathrm{Mn}-16,6\%\mathrm{Ni}-8.9\%\mathrm{Cr}+2.4\%\mathrm{Mo}-11,3\%\mathrm{Cu}+\mathrm{8th},58\\ \%\mathrm{Co}+7.4\%\mathrm{W}-14,5\%\mathrm{Si}\end{array}\right)\left[°\mathrm{C}/\mathrm{wt}.-\%\right]$$

to calculate, whereby with C% the C content, with %Mn the Mn content, with %Mo the Mo content, with %Cr the Cr content, with %Ni the Ni content, with %Cu the Cu -Content, with %Co the Co content, with %W the W content and with %Si the Si content of the respective steel in% by weight.

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

zu berechnen, wobei auch hiermit mit %C der C-Gehalt, mit %Si der Si-Gehalt mit %Mn der Mn-Gehalt mit %Cr der Cr-Gehalt, mit %Mo der Mo-Gehalt, mit %Ni der Ni-Gehalt und mit +%V der Vanadium-Gehalt des jeweiligen Stahls bezeichnet sind (Brandis H 1975 TEW-Techn. Ber. 1 8-10).The AC1 temperature and the AC3 temperature of a steel within the scope of the specifications according to the invention is according to the formulas: $$\mathrm{AC}1\left[°\mathrm{C}\right]=739°\mathrm{C}+\left(-22*\%\mathrm{C}-7*\%\mathrm{Mn}+2*\%\mathrm{Si}+14*\%\mathrm{Cr}+13*\%\mathrm{Mo}-13*\%\mathrm{Ni}+20*\%\mathrm{v}\right)\left[°\mathrm{C}/\mathrm{wt}.-\mathrm{<figure-callout\; id="3"\; label="\%\; AC"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\%\; </figure-callout>}\right]$$

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

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

After removing the sheet metal part following step (D), the sheet metal part is cooled to a cooling temperature TAB of less than 100 ° C within a cooling time t _AB of 0.5 to 600 s. This is usually done by air cooling.

In a preferred development of the method, the flat steel product has a high uniform elongation Ag of at least 10.0%, in particular at least 11.0%, preferably at least 11.5%, in particular at least 12.0%.

Furthermore, the yield point of a specially designed flat steel product has a continuous course or only a slight extent. In the sense of the application, continuous progression means that there is no pronounced yield point. A yield point with a continuous course can also be referred to as a yield point Rp0.2. In the present case, a low yield strength is understood to mean a pronounced yield strength in which the difference ΔRe between the upper yield strength value ReH and the lower yield strength value ReL is at most 45 MPa. The following applies: $$\begin{array}{l}\Delta \mathrm{re}=\left(\mathrm{Deer}-\mathrm{ReL}\right)\le 45\mathrm{MPa}\mathrm{with}\mathrm{Deer}=\mathrm{upper}\mathrm{Stretch\; limit}\mathrm{in}\mathrm{MPa}\mathrm{and}\mathrm{Rel}=\mathrm{lower}\\ \mathrm{Stretch\; limit}\mathrm{in}\mathrm{MPa}.\end{array}$$

Particularly good aging resistance can be achieved with flat steel products for which the difference ΔRe is a maximum of 25 MPa.

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

The present invention also relates to a sheet metal part produced by one of the aforementioned methods comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in wt.%) C: 0.06-0.5%, Si: 0.05-0.6%, Mn: 0.4-3.0%, Al: 0.06-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10% B: 0.0005-0.01% P: ≤0.03%, S: ≤0.02%, N: ≤0.02%, Sn: ≤0.03% As: ≤0.01% and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ca, W” in the following contents CR: 0.01-1.0%, Cu: 0.01 -0.2%, Mon: 0.002-0.3%, Ni: 0.01-0.5% V: 0.001-0.3% Approx.: 0.0005-0.005% W: 0.001-1.0% consists,
wherein the steel substrate has an aluminum-based anti-corrosion coating on at least one side and the content of diffusible hydrogen H _diff is up to 0.4 ppm.

The details and preferred embodiments mentioned with regard to the method according to the invention apply accordingly to the sheet metal part according to the invention.

The anti-corrosion coating of the sheet metal part preferably comprises an alloy layer and an Al base layer. In sheet metal parts, the alloy layer is often referred to as an interdiffusion layer.

The thickness of the anti-corrosion coating is preferably at least 10 μm, particularly preferably at least 20 μm, in particular at least 30 μm.

The thickness of the alloy layer is preferably less than 30 μm, particularly preferably less than 20 μm, in particular less than 16 μm, particularly preferably less than 12 μm. The thickness of the Al base layer results from the difference in the thickness of the corrosion protection coating and the alloy layer.

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer of the sheet metal part preferably consists of 35 - 90% by weight of Fe, 0.1 - 10% by weight of Si, optionally up to 0.5% by weight of Mg and optional other components, the total content of which is limited to a maximum of 2.0% by weight, and the balance aluminum. Due to the further diffusion of iron into the alloy layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the melt pool.

The alloy layer preferably has a ferritic structure.

The Al base layer of the sheet metal part lies on the alloy layer of the sheet metal part and is directly adjacent to it. The Al base layer of the sheet metal part preferably consists of 35 - 55% by weight of Fe, 0.4 - 10% by weight of Si, optionally up to 0.5% by weight of Mg and optional other components, their contents in total a maximum of 2.0% by weight is limited, and the remainder is aluminum.

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

In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form at least a 40% continuous layer bounded by silicon-poor regions. In an alternative embodiment variant, the silicon-rich phases are arranged in an island shape in the silicon-poor phase.

For the purposes of this application, “island-shaped” is understood to mean an arrangement in which discrete, unconnected areas are enclosed by another material - i.e. there are “islands” of a certain material in another material.

In a preferred variant, the sheet metal part comprises an oxide layer arranged on the corrosion protection coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer end of the corrosion protection coating.

The oxide layer of the sheet metal part consists in particular of more than 80% by weight of oxides, with the majority of the oxides (ie more than 50% by weight of the oxides) being aluminum oxide. Optional 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.

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

In a special development, the steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%. In this context, the term “partially exhibits” means that there are areas of the sheet metal part that have the structure mentioned. In addition, there may also be areas of the sheet metal part that have a different structure. The sheet metal part therefore has the structure mentioned in sections or areas.

The high martensite content allows very high tensile strengths and yield strengths to be achieved.

In a preferred embodiment variant, the sheet metal part at least partially has a Vickers hardness of at least 340 HV, preferably at least 400 HV, in particular at least 500 HV.

In an alternative development, the steel substrate of the sheet metal part has a structure with a ferrite content of more than 5%, preferably more than 10%, in particular more than 20%. Furthermore, the ferrite content is preferably less than 85%, in particular less than 70%. The martensite content is less than 80%, in particular less than 50%. In addition, the structure can optionally contain bainite and/or pearlite. The exact ratio of the structural components depends on the level of the C content and the Mn content as well as on the cooling conditions during forming. The structure designed in this way has higher ductility and therefore leads to improved forming behavior. A corresponding sheet metal part preferably has an elongation at break A30 in a range of 8% to 25%, preferably between 10% and 22%, in particular between 12% and 20%.

In a further developed variant, the sheet metal part at least partially has a yield strength of at least 750 MPa, in particular at least 950 MPa, preferably at least 1100 MPa, in particular at least 1250 MPa.

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

In particular, the sheet metal part at least partially has an elongation at break A80 of at least 4%, preferably at least 5%, particularly preferably at least 6%.

wobei die Blechdicke in mm in die Formel einzusetzen ist. Dies gilt für Blechdicken größer 1,0 mm. Bei Blechdicken kleiner 1,0 mm entspricht der korrigierte Biegewinkel dem ermittelten Biegewinkel.In addition, in a preferred variant, the sheet metal part can at least partially have a bending angle of at least 30°, in particular at least 40°, preferably at least 50°. The bending angle here means the bending angle corrected with regard to the sheet thickness. The corrected bending angle results from the determined bending angle at the maximum force (measured according to VDA standard 238-100) (also referred to as the maximum bending angle) from the formula $${\mathit{Bend\; angle}}_{\mathit{corrected}}={\mathit{Bend\; angle}}_{\mathit{determined}}\cdot \sqrt{\mathit{sheet\; thickness}}$$

where the sheet thickness in mm is to be inserted into the formula. This applies to sheet thicknesses greater than 1.0 mm. For sheet thicknesses less than 1.0 mm, the corrected bending angle corresponds to the determined bending angle.

In this context, partially exhibiting means that there are areas of the sheet metal part that have the mechanical property mentioned. In addition, there may also be areas of the sheet metal part whose mechanical properties are below the limit value. The sheet metal part therefore has the mechanical property mentioned in sections or areas. This is because different areas of the sheet metal part can undergo different heat treatments. For example, individual areas can be cooled more quickly than others, which means, for example, more martensite is formed in the areas that cooled more quickly. Therefore, different mechanical properties arise in the different areas.

The mechanical key figures mentioned have proven to be particularly advantageous in order to ensure use in an automobile with good crash performance.

In a special development, the sheet metal part has fine precipitates in the structure, in particular in the form of niobium carbonitrides and/or titanium carbonitrides.

For the purposes of this application, fine precipitates are all precipitates with a diameter smaller than 30 nm. The remaining excretions are referred to as coarse excretions.

In a preferred embodiment, the average diameter of the fine precipitates is a maximum of 11 nm, preferably a maximum of 10 nm.

In a further preferred embodiment, the sheet metal part largely has fine precipitates in the structure. For the purposes of this application, largely fine precipitates is to be understood as meaning that more than 80%, preferably more than 90%, of all precipitates are fine precipitates. This means that more than 80%, preferably more than 90%, of all precipitates have a diameter that is smaller than 30 nm.

The fine precipitates create a particularly fine structure with small grain diameters. The fine structure makes it more homogeneous. This results in an improvement in the mechanical properties, in particular a lower sensitivity to cracks and thus improved bending properties and a higher elongation at break. This also results in better toughness with more pronounced fracture penetration behavior.

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

By reducing the pore area, a variety of problems can be reduced or prevented:
The pores reduce the mechanical integrity in this area. Layers can peel off more quickly under corrosive stress. In addition, the transferable force at the connection point between two components is reduced after gluing or welding. With the reduction in the number of pores achieved according to the invention, the area over which the forces of the adhesive connection are transmitted is increased by over 60% during bonding. Consequently, the risk of a delamination fracture is correspondingly reduced.

The pores lead to changed current paths in the material during resistance spot welding, which negatively influence the weldability and thus reduce the welding area. By reducing the pores, an increased welding area and thus stable further processing of the sheet metal component can be made possible.

The pores themselves facilitate crack initiation and propagation during static and dynamic bending. By reducing the pore area, a higher bending angle can be achieved.

The sheet metal part according to the invention is preferably a component for the automotive sector, in particular a body part, in particular a bumper support/reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or a sill. The invention therefore also relates to the use of a previously described sheet metal part in the automotive sector, in particular as a bumper support/reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill.

characters

1

WOP-Wert (wasserstoffbezogener Ofenparameter-Wert)

2

WGB (Abwalzgrad-Blechdickenverhältnis)

3

Teilfläche "3"

4

Teilfläche "4"

5

Teilfläche "5"

Figure 1 shows a diagram in which the WOP value is plotted against the degree of rolling to sheet thickness ratio. In it mean

1

WOP value (hydrogen-related furnace parameter value)

2

WGB (rolling degree to sheet thickness ratio)

3

Partial area "3"

4

Partial area "4"

5

Partial area "5"

Examples

The following exemplary embodiments serve to explain the invention in more detail.

Several tests were carried out to demonstrate the effect of the invention. For this purpose, steel strips (ie flat steel products) with the compositions given in Table 1 were produced in a conventional manner. Blank spaces in Table 1 mean that the respective element was not deliberately added. However, the element may still be present as an unavoidable impurity. The steel compositions D, E and F are reference examples that are not according to the invention.

The steel strips produced in this way were hot-dip coated in a conventional manner, using the melts shown in Table 2 . Table 2 shows the layer thickness of the corrosion protection coating on one side, with the top and bottom being coated.

The thickness of the steel strips produced was between 1.4 mm and 3.3 mm in all tests.

After cooling to room temperature, samples were taken perpendicular to the rolling direction for each steel strip in accordance with DIN EN ISO 6892-1 sample form 2 (Appendix B, Tab. B1). The samples were subjected to a tensile test in accordance with DIN EN ISO 6892-1 sample form 2 (Appendix B Tab. B1). Table 3 shows the results of the tensile test on the flat steel product. As part of the tensile test, the following material parameters were determined: the type of yield point, and in the case of a continuous yield point, the value for the yield strength Rp0.2, in the case of a pronounced yield point, the values for the lower yield point ReL and the upper yield point ReH, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80. All samples have a continuous yield strength Rp or an only slightly pronounced yield strength with a difference ΔRe between the upper and lower yield strength of a maximum of 45 MPa and a uniform elongation Ag of at least 10%. There is a pronounced yield point Re for steel type D and a continuous yield point Rp for all other samples. For steel type D, the lower yield point ReL and the upper yield point ReH are given in Table 3 . For all other types of steel, the yield strength Rp0.2 is given. In addition to the mechanical parameters, the content of diffusible hydrogen H _diff was determined within 48 hours after application of the corrosion protection coating. The results are also shown in Table 3 .

A total of 20 steel strips were made from AJ steel grades and provided with a corrosion protection coating. The steel strips were then rolled in areas with the degree of rolling specified in Table 5 to a sheet thickness after rolling that is specified in Table 6 .

From a total of 20 steel strips produced, blanks were cut out and used for further tests. Corresponding steel components 1 to 20 were hot-press formed from the blanks (formed blanks). The samples for the reeds and mechanical tests were taken from flat areas of these components. Tables 4 and 5 show which type of steel was further processed and examined, in which thickness and with which coating. In Table 5, the degree of rolling is given as a unitless number, as it is also used in formula (3). The percentage is given in the usual way (ie 0.49 corresponds to 49%). During further processing, the blanks were heated in a heating device, for example in a conventional heating oven, from room temperature with an average heating rate r _oven (between 30 ° C and 700 ° C) in an oven with an oven temperature T _oven . The total time in the oven, which includes heating and holding, is denoted by t _oven . The dew point of the furnace atmosphere is designated T _{dew point} . The blanks are then removed from the heating device and inserted into a forming tool which has the temperature Twz within a transfer time. At the time of removal from the oven, the blanks had reached the oven temperature. The transfer time t _Trans , consisting of the removal from the heating device, transport to the tool and insertion into the tool, was between 5 and 13 s. In the forming tool, the blanks were formed into the respective sheet metal part, with the sheet metal parts in the tool cooling down at a rate r _WZ were cooled down. The dwell time in the tool is denoted by twz. Finally, the samples were cooled in air to room temperature. The parameters mentioned are given for different variants in Tables 4 and 5 . Table 5 shows in particular the degree of rolling to sheet thickness ratio (WGB), as it results from the degree of rolling and the sheet thickness after rolling, and WOP value ("hydrogen-related furnace parameter"), which is derived from the dew point, the furnace temperature and the total time in the furnace was determined using the formula. In all cases, the points (WGB, WOP) lie within the area spanned by straight connecting distances between points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling to sheet thickness ratio is plotted on the x-axis. Experiment 11 is located within partial area 3 and therefore in area A. All further experiments are within partial area 5 and therefore both in area A and in the preferred area B. As a result, the content of diffusible hydrogen H _diff is also a maximum of 0 in all cases .4 ppm.

Table 6 shows further mechanical properties of the sheet metal part obtained in this way. The measurements were carried out without a KTL coating. Yield strength, tensile strength and elongation at break A80 were determined in accordance with DIN EN ISO 6892-1 sample form 2 (Appendix B, Tab. B1). The initial measuring length was 80mm. The samples were taken transversely to the rolling direction. In addition, the bending angle at the maximum force was determined in accordance with VDA standard 238-100 and then corrected to the sheet thickness in the manner described. The samples for measuring the bending angle were also taken transversely to the rolling direction. In addition, the Vickers hardness was determined in accordance with DIN EN ISO 6507 (2018.07). Table 1 (steel types) steel C Si Mn Al Cr Nb Ti b P S N Sn As Cu Mo Approx Ni Al/Nb A 0.22 0.145 1.1 0.18 0.2 0.032 0.017 0.0024 0.004 0.0007 0.0034 0.03 5.6 b 0.35 0.16 1.1 0.21 0.118 0.026 0.0096 0.0025 0.005 <0.0005 0.0035 0.005 0.003 0.019 0.005 0.001 0.032 8.1 C 0.131 0.20 1.17 0.13 0.21 0.021 0.004 0.0021 0.011 0.005 0.0059 0.04 0.025 0.0012 0.06 6.2 D* 0.087 0.12 1.52 0.05 0.1 0.04 0.008 0.0010 0.015 0.003 0.009 0.02 0.1 0.05 0.034 1.3 E* 0.235 0.3 1.3 0.05 0.28 0.003 0.040 0.0035 0.02 0.003 0.007 0.03 0.01 0.03 0.03 0.005 0.025 16.7 F* 0.37 0.3 1.4 0.05 0.18 0.003 0.040 0.0035 0.015 0.003 0.007 0.03 0.01 0.05 0.035 0.003 0.03 16.7 G 0.15 0.3 1.15 0.1 0.3 0.028 0.015 0.0030 0.015 0.005 0.0060 0.03 0.01 0.12 0.05 0.003 0.027 3.6 H 0.165 0.45 2.4 0.75 0.75 0.03 0.035 0.002 0.02 0.003 0.005 0.03 0.01 0.1 0.048 25.0 I 0.46 0.20 0.80 0.20 0.12 0.03 0.010 0.0025 0.005 0.0005 0.0035 0.005 0.003 0.019 0.005 0.001 0.019 6.7 J 0.23 0.17 0.6 0.07 0.25 0.005 0.021 0.0007 0.006 0.0007 0.0034 0.015 0.006 0.041 0.01 0.001 0.03 14.0 Residual iron and unavoidable impurities. Information in % by weight;
* reference examples not according to the invention Coating variant Melt analysis Layer thickness (one-sided) [µm] Si Fe Mg Other Al α 9.5 3 0.3 <1% rest 10 β 8th 3.5 0.5 <1% rest 40 γ 10 3 <0.01 <1% rest 25 δ 8.2 3.8 0.25 <1% rest 27 ε 10.5 3.1 0.33 <1% rest 30 φ 8.1 3.9 <0.01 <1% rest 25 Experiment no. Steel Thickness of the steel strip s [mm] Coating s-variant Rp0, 2 or ReL [MPa ] ReH [MPa ] Rm [MPa] Elongation at break A80 [%] Uniform elongation Ag [%] H _diff in ppm after coating 1 A 1.5 δ 469 645 19 12 0.15 2 C 2.9 ε 426 444 585 18 12 0.25 3 b 1.4 φ 492 689 18 12 0.05 4 A 1.5 α 460 639 20 13 0.05 5* D 2.6 γ 429 449 551 21 14 0.3 6 C 2.1 β 449 617 17 12 0.05 7 b 1.5 γ 483 676 19 12 0.06 8th C 3.0 δ 421 578 18 13 0.2 9 b 2.6 δ 489 501 679 19 12 0.08 10 G 1.6 ε 435 598 17 12 0.08 11 I 1.6 φ 514 720 17 11 0.08 12 J 1.6 α 456 639 19 12 0.2 13* E 1.7 α 471 644 19 12 0.21 14 H 3.3 β 471 642 20 12 0.25 15 G 2.0 γ 426 433 572 18 13 0.11 16 C 2.3 δ 412 572 18 12 0.12 17 A 1.6 ε 474 639 19 12 0.15 18 b 1.5 φ 501 707 18 12 0.08 19* D 2.6 φ 439 557 21 14 0.2 20* F 2.0 β 494 700 18 11 0.4 * reference examples not according to the invention Attempt no. steel Thickness of the steel strip [mm] Coating variant Average heating speed r _oven [K/s] standardized mean heating Θ _norm [Kmm/s] Transfer time t _Trans [s] Tool temperature Twz [°C] Tool closing time twz [S] Cooling rate rwz [K/s] T _target [°C] 1 A 1.5 δ 11.2 9.2 7 30 3 197 150 2 C 2.9 ε 7.5 11.9 10 60 6 87 220 3 b 1.4 φ 16.7 10.1 5 40 5 141 100 4 A 1.5 α 10.3 11.5 8th 50 6 108 150 5* D 2.6 γ 9.5 12.1 7 40 6 102 180 6 C 2.1 β 10.5 12.9 10 20 5 104 220 7 b 1.5 γ 8.7 11.4 7 50 8th 65 260 8th C 3.0 δ 6.1 11.7 9 30 8th 76 110 9 b 2.6 δ 10.2 12.2 8th 50 7 90 220 10 G 1.6 ε 10.2 12.6 7 50 10 61 150 11 I 1.6 φ 9.7 12.6 7 40 15 44 150 12 J 1.6 α 10.0 11.6 7 50 10 61 150 13* E 1.7 α 11.2 11.0 5 20 6 107 150 14 H 3.3 β 5.5 10.7 5 80 8th 73 250 15 G 2.0 γ 5.2 10.8 6 20 7 93 130 16 C 2.3 δ 9.5 12.9 13 30 10 66 70 17 A 1.6 ε 14.1 11.7 8th 20 5 146 70 18 b 1.5 φ 11.8 11.6 7 40 4 174 70 19* D 2.6 φ 8.2 12.5 8th 80 15 35 240 20* F 2.0 β 7.8 12.2 10 20 11 44 240 * Reference examples not according to the invention, information partially rounded Attempt no. steel Coating variant Sheet thickness h ₀ [mm] degree of rolling Sheet thickness (after rolling) h ₁ [mm] WGB T _oven [K] t _oven [h] T _{dew point} [K] H _diff [ppm] WOP value Area 1 A δ 1.5 0.49 0.77 80.0 1153.15 0.050 268.15 0.22 264 b 2 C ε 2.9 0.45 1.60 61.0 1173.15 0.100 268.15 0.28 286 b 3 b φ 1.4 0.49 0.71 81.3 1223.15 0.039 273.15 0.15 323 b 4 A α 1.5 0.2 1.20 30.1 1193.15 0.067 273.15 0.1 333 b 5* D γ 2.6 0.48 1.35 68.0 1193.15 0.083 268.15 0.15 281 b 6 C β 2.1 0.35 1.37 49.8 1203.15 0.083 270.15 0.32 305 b 7 b γ 1.5 0.15 1.28 22.5 1163.15 0.083 270.15 0.23 301 b 8th C δ 3 0.38 1.86 49.5 1163.15 0.100 288.15 0.2 554 b 9 b δ 2.6 0.49 1.33 69.7 1203.15 0.083 288.15 0.15 551 b 10 G ε 1.6 0.17 1.33 25.1 1193.15 0.083 273.15 0.15 340 b 11 I φ 1.6 0.13 1.39 19.3 1193.15 0.083 268.15 0.15 281 A 12 J α 1.6 0.23 1.23 34.1 1193.15 0.083 273.15 0.25 340 b 13* E α 1.7 0.38 1.05 57.7 1193.15 0.083 273.15 0.29 340 b 14 H β 3.3 0.49 1.68 65.3 1133.15 0.100 281.15 0.29 447 b 15 G γ 2 0.1 1.80 14.1 1133.15 0.100 280.15 0.15 433 b 16 C δ 2.3 0.3 1.61 41.0 1223.15 0.083 283.15 0.18 477 b 17 A ε 1.6 0.37 1.01 56.9 1233.15 0.067 282.15 0.18 456 b 18 b φ 1.5 0.3 1.05 45.9 1193.15 0.067 280.15 0.25 425 b 19* D φ 2.6 0.37 1.64 50.0 1193.15 0.067 278.15 0.08 397 b 20* F β 2 0.25 1.50 35.1 1163.15 0.100 278.15 0.42 408 b * reference examples not according to the invention Attempt no. Sheet thickness after rolling h ₁ [mm] Yield strength [MPa] Tensile strength[M Pa] A80 [%] Bending angle force maximum[°] Corrected bending angle [°] Hardness Vickers [HV] 1 0.77 1005 1520 4.5 55.9 55.9 478 2 1.60 816 1140 6.0 68.5 86.5 356 3 0.71 1239 1904 4.1 45.5 45.5 617 4 1.20 1008 1511 4.7 54.1 59.3 478 5* 1.35 657 936 8.1 83.9 97.6 285 6 1.37 833 1169 5.7 67.7 79.0 366 7 1.28 1247 1904 4.2 43.7 49.3 616 8th 1.86 802 1124 6.3 68.2 93.0 346 9 1.33 1236 1890 4.1 43.5 50.1 617 10 1.33 853 1261 5.2 64.8 74.7 392 11 1.39 1362 2119 4.0 39.3 46.4 700 12 1.23 1013 1522 4.7 54.0 60.0 485 13* 1.05 1047 1576 4.5 52.6 54.0 497 14 1.68 903 1339 5.2 58.2 75.6 422 15 1.80 852 1249 5.4 61.7 82.7 392 16 1.61 821 1176 5.9 66.5 84.3 363 17 1.01 1010 1515 4.6 54.9 55.1 479 18 1.05 1249 1894 4.0 44.4 45.5 617 19* 1.64 398 504 20.1 136.8 175.0 253 20* 1.50 1269 1929 4.1 42.0 51.4 635 * reference examples not according to the invention

Example

Example of determination of permissible values for T _oven , t _oven and T _{dew point} to maintain an H _diff value of 0.4 ppm in manufactured components made from flat steel products.

$$\mathit{Abwalzgrad}=\frac{\mathit{\Delta h}}{h_0}=\frac{0,735\mathrm{mm}}{\mathrm{1,5}\mathrm{mm}}=0,49=49\%$$

$$\mathit{WGB}=\mathrm{1,5}\cdot \frac{1+\mathit{Abwalzgrad}\cdot 100}{\frac{1}{2}\cdot \left(1+\sqrt{\frac{\mathit{Blechdicke}}{\mathrm{mm}}}\right)}=\mathrm{1,5}\cdot \frac{1+0,49\cdot 100}{\frac{1}{2}\cdot \left(1+\sqrt{\frac{0,765\mathrm{mm}}{\mathrm{mm}}}\right)}=80,0$$

Experiment 1 according to Tables 4-6 is used as an example: $$\begin{array}{l}{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">H</figure-callout>}}_0=1.5\mathrm{mm};H_1=\mathit{sheet\; thickness}=0,765\mathrm{mm};\mathit{\Delta h}\\ =0,73\mathrm{mm};\mathrm{Coating}\mathrm{\delta }\mathrm{with}\mathrm{<figure-callout\; id="0"\; label="Mg"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Mg</figure-callout>}0,25\mathrm{wt}.-\%\end{array}$$

$$\mathit{degree\; of\; rolling}=\frac{\mathit{<figure-callout\; id="0"\; label="\Delta h\; H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Delta h</figure-callout>}}{H_{}}=\frac{0,735\mathrm{mm}}{1.5\mathrm{mm}}=0,49=49\%$$

$$\mathit{WGB}=1.5\cdot \frac{1+\mathit{degree\; of\; rolling}\cdot 100}{\frac{1}{2}\cdot \left(1+\sqrt{\frac{\mathit{sheet\; thickness}}{\mathrm{mm}}}\right)}=1.5\cdot \frac{1+0,49\cdot 100}{\frac{1}{2}\cdot \left(1+\sqrt{\frac{0,765\mathrm{mm}}{\mathrm{mm}}}\right)}=80,0$$

$$\iff 200\le \frac{T_{\mathrm{Ofen}}}{\mathrm{K}}\cdot \log \left(\frac{t_{\mathrm{Ofen}}}{\mathrm{h}}+1,15\right)+{\left(\frac{T_{\mathrm{Taupunkt}}}{\mathrm{K}}-243,15\right)}^{\mathrm{1,6}}\le 584$$

$$\iff 200\le \frac{1153,15\mathrm{K}}{\mathrm{K}}\cdot \log \left(\frac{0,050\mathrm{h}}{\mathrm{h}}+1,15\right)+{\left(\frac{268,15\mathrm{K}}{\mathrm{K}}-243,15\right)}^{\mathrm{1,6}}\le 584$$

• ⇔ 200 ≤ 264 ≤ 584
• ⇔ wahre Aussage

For the WGB value of 80.0, a preferred WOP value of 200 to 584 can be selected Figure 1 read or calculate using the specified points. Now the three parameters T _oven , t _oven and T _{dew point} can be set so that a WOP value results from: 200 ≤ WOP ≤ 584, for example: T _oven = 880 °C = 1153.15 K; t _oven = 180 s = 0.050 h; and T _{dew point} = -5 °C = 268.15 K $$200\le \mathit{WOP}\le 584$$

$$\iff 200\le \frac{T_{\mathrm{Oven}}}{\mathrm{K}}\cdot \log \left(\frac{t_{\mathrm{Oven}}}{\mathrm{H}}+1,15\right)+{\left(\frac{T_{\mathrm{dew\; point}}}{\mathrm{K}}-243,15\right)}^{1.6}\le 584$$

$$\iff 200\le \frac{1153,15\mathrm{K}}{\mathrm{K}}\cdot \mathrm{<figure-callout\; id="0"\; label="log"\; filenames="imgf0001.png"\; state="\{\{state\}\}">log</figure-callout>}\left(\frac{0,050\mathrm{H}}{\mathrm{H}}+1,15\right)+{\left(\frac{268,15\mathrm{K}}{\mathrm{K}}-243,15\right)}^{1.6}\le 584$$

• ⇔ 200 ≤ 264 ≤ 584
• ⇔ true statement

Since the calculated WOP value of 264 is between 200 and 584, a maximum H _diff value of 0.4 ppm can be maintained in the component using the selected parameters.

Commercial applicability

The sheet metal part produced according to the invention has a low tendency to hydrogen-induced fractures under load voltages and can therefore be used advantageously in the automotive sector, aircraft construction or rail vehicle construction.

Claims (17)

Process for producing a sheet metal part with a diffusible hydrogen H _diff content of up to 0.4 ppm, comprising at least the steps: (A) Providing a flat steel product comprising a steel substrate made of steel, which in addition to iron and unavoidable impurities (in% by weight). C: 0.06 -0.5%, Si: 0.05 -0.6%, Mn: 0.4-3.0%, Al: 0.06-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10% B: 0.0005-0.01% P: ≤0.03%, S: ≤0.02%, N: ≤0.02%, Sn: ≤0.03%, As: ≤0.01% and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ca, W” in the following contents CR: 0.01-1.0%, Cu: 0.01-0.2%, Mon: 0.002-0.3%, Ni: 0.01-0.5%, V: 0.001-0.3%, Approx.: 0.0005-0.005%, W: 0.001-1.0% consists, wherein the steel substrate has an aluminum-based corrosion protection coating on at least one side and wherein the flat steel product has a rolling degree to sheet thickness ratio (WGB) of greater than 0.8 to 200; (B) Determination of a WOP value depending on the degree of rolling to sheet thickness ratio WGB within an area spanned by straight connecting distances between the points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value on the y-axis and the degree of rolling to sheet thickness ratio are plotted on the x-axis; (C) treating the flat steel product at a mean furnace temperature T _furnace (in K) for a period t _furnace (in h) in a heating device, where the dew point temperature of the furnace atmosphere of the furnace T _{dew point} (in K), the average furnace temperature T _furnace (in K) and the duration t _oven (in h) according to the following equation of the general formula (1) $$\mathit{WOP}=\frac{T_{\mathrm{Oven}}}{\mathrm{K}}\cdot \log \left(\frac{t_{\mathrm{Oven}}}{\mathrm{H}}+1.15\right)+{\left(\frac{T_{\mathrm{dew\; point}}}{\mathrm{K}}-243.15\right)}^{1.6}$$

be set, and (D) Forming the heated flat steel product from step (C) in a mold with simultaneous cooling in order to obtain the sheet metal part. Method according to claim 1,
characterized in that
the determination of the WOP value according to step (B) within an area spanned by straight connecting distances between the points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200), P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) as well as P33 (WGB 50, WOP 300) and P12 (WGB 0.8 , WOP 300) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling to sheet thickness ratio (WGB) is plotted on the x-axis. Method according to one of claims 1 to 2,
characterized in that
t _{oven is} 0.04 to 0.5 h, preferably 0.060 to 0.4 h, particularly preferably 0.067 to 0.25 h. Method according to one of claims 1 to 3,
characterized in that
The following applies to the Al/Nb ratio of Al content to Nb content: $$\mathrm{Al/Nb}\le \mathrm{20}\mathrm{.0}\mathrm{if}\mathrm{Mn}\le \mathrm{1}\mathrm{,6}\mathrm{wt}\mathrm{.}-\%$$

and $$\mathrm{Al/Nb}\le 3\mathrm{0}\mathrm{.0}\mathrm{if}\mathrm{Mn}\ge \mathrm{1}\mathrm{,7}\mathrm{wt}\mathrm{.}-\%$$

Method according to one of claims 1 to 4, wherein at least one of the following conditions applies to the element contents: Ti < 3.42*N 0.7% by weight <Mn+Cr<3.5% by weight. Method according to one of claims 1 to 5,
characterized in that
the corrosion protection coating has an alloy layer and an Al base layer. Method according to claim 6,
characterized in that the alloy layer consists of 35 - 60% by weight of Fe, optional further components, the total content of which is limited to a maximum of 5.0% by weight, and the balance consists of aluminum
and or the Al base layer made of 1.0 - 15% by weight of Si, optionally 2 - 4% by weight of Fe, optionally up to 5.0% by weight of alkali or alkaline earth metals, optionally up to 15% by weight of Zn and optional others Components whose total content is limited to a maximum of 2.0% by weight, and the remainder consists of aluminum. Method according to one of claims 1 to 7,
wherein the transfer time t _Trans required for removing it from the heating device used in step (C) and inserting the blank into the molding tool used in step D is at most 20 s, preferably at most 15 s. Method according to one of claims 1 to 8,
wherein the cooling in step (D), in particular over a period t _WZ of more than 1 s, with a cooling rate r _WZ of 10 to 1000 K/s, preferably at least 27 K/s, in particular at least 30 K/s, to the Target temperature T _Target occurs. Method according to one of claims 1 to 9, wherein the temperature at least partially achieved in the flat steel product in step (C) is between Ac3 and 1000 °C, preferably between 880 °C and 950 °C. Method according to one of claims 1 to 10, wherein the target temperature T _target of the sheet metal part is at least partially below 400°C, preferably below 300°C. Method according to one of claims 1 to 11
characterized in that the flat steel product has a yield strength with a continuous course (Rp0.2) or a yield strength with a difference (ΔRe) between the upper yield strength value (ReH) and lower yield strength value (ReL) of a maximum of 45 MPa
and or the flat steel product has a uniform elongation Ag of at least 10%
and or the flat steel product has an elongation at break A80 of at least 15%, preferably at least 20%. Sheet metal part produced by the method according to one of claims 1 to 12 comprising a steel substrate made of steel, which in addition to iron and unavoidable impurities (in% by weight). C: 0.06-0.5%, Si: 0.05-0.6%, Mn: 0.4-3.0%, Al: 0.06-1.0%, Nb: 0.001-0.2%, Ti: 0.001-0.10% B: 0.0005-0.01% P: ≤0.03%, S: ≤0.02%, N: ≤0.02%, Sn: ≤0.03% As: ≤0.01%, and optionally one or more of the elements “Cr, Cu, Mo, Ni, V, Ca, W” in the following contents CR: 0.01-1.0%, Cu: 0.01-0.2%, Mon: 0.002-0.3%, Ni: 0.01-0.5% V: 0.001-0.3% Approx.: 0.0005-0.005% W: 0.001-1.0% consists, wherein the steel substrate has an aluminum-based corrosion protection coating on at least one side and wherein the diffusible hydrogen content H _diff is up to 0.4 ppm. Sheet metal part according to claim 13,
where the following applies to the Al/Nb ratio of Al content to Nb content: $$\mathrm{Al/Nb}\le \mathrm{20}\mathrm{.0}\mathrm{if}\mathrm{Mn}\le \mathrm{1}\mathrm{,6}\mathrm{wt}\mathrm{.}-\%$$

and $$\mathrm{Al/Nb}\le 3\mathrm{0}\mathrm{.0}\mathrm{if}\mathrm{Mn}\ge \mathrm{1}\mathrm{,7}\mathrm{wt}\mathrm{.}-\%.$$

Sheet metal part according to one of claims 13 to 14
characterized in that
the sheet metal part at least partially has a Vickers hardness of at least 340 HV, preferably 400 HV, in particular 500 HV. Sheet metal part according to one of claims 13 to 15,
characterized in that the sheet metal part at least partially has a yield strength of at least 750 MPa, in particular at least 950 MPa, preferably at least 1100 MPa, in particular at least 1250 MPa
and or the sheet metal part at least partially has a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1800 MPa
and or the sheet metal part at least partially has an elongation at break A80 of at least 4%, preferably at least 5%, particularly preferably at least 6% and/or the sheet metal part at least partially has a bending angle of at least 30°, in particular at least 40°, preferably at least 50°. Use of a sheet metal part according to one of claims 13 to 16 in the automotive sector, in particular as a bumper support/reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill.

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