WO2023020931A1

WO2023020931A1 - Steel having improved processing properties for working at elevated temperatures

Info

Publication number: WO2023020931A1
Application number: PCT/EP2022/072555
Authority: WO
Inventors: Thomas Gerber; Janko Banik; Dr. Stefan KREBS; Dr. Bernd Linke; Dr. Cássia CASTRO MÜLLER; Tayfun DAGDEVIREN; Maria KÖYER
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2021-08-19
Filing date: 2022-08-11
Publication date: 2023-02-23

Abstract

The invention relates to a flat steel product for hot working, a worked steel sheet part, a process for manufacturing the flat steel product and a process for manufacturing the worked steel sheet product, the flat steel product and the steel sheet part having improved properties in particular in combination with an aluminum-based anticorrosive coating.

Description

Steel with improved processing properties for forming at elevated temperatures

The invention relates to a flat steel product for hot forming and a method for producing such a flat steel product. Furthermore, the invention relates to a shaped sheet metal part with improved properties and a method for producing such a shaped sheet metal part from a flat steel product.

Whenever a "flat steel product" or a "sheet metal product" is mentioned below, this means rolled products such as steel strips or sheets, from which "sheet metal blanks" (also called blanks) are separated for the manufacture of body components, for example. “Shaped sheet metal parts” or “sheet metal components” of the type according to the invention are made from sheet metal blanks of this type, the terms “shaped sheet metal part” and “sheet metal component” being used synonymously here.

All information on contents of the steel compositions given in the present application are based on the weight, unless expressly stated otherwise. All "% figures" in connection with a steel alloy that are not specified in more detail are therefore to be understood as figures in "% by weight". With the exception of the information on the volume (in "% by volume") related to the retained austenite content of the structure of a sheet metal part according to the invention, information on the contents of the various structural components relate to the area of a section of a sample of the respective product (indication in Percentage by area "area-%"), unless expressly stated otherwise. Information given in this text on the contents of the components of an atmosphere refers to the volume (specified in "% by volume").

Mechanical properties, such as tensile strength, yield point, elongation, which are reported here, have been determined in the tensile test according to DIN-EN ISO 6982-1, specimen form 2 (Annex B Tab. Bl) (status 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 had been etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffractometry. WO 2019/223854 A1 discloses a shaped sheet metal part and a method for producing such a shaped sheet metal part which has a tensile strength of at least 1000 MPa. The shaped sheet metal part consists of a steel which, in addition to iron and unavoidable impurities, contains (in % by weight) 0.10 - 0.30% C, 0.5 - 2.0% Si, 0.5 - 2.4% Mn, 0.01 - 0.2% Al, 0.005 - 1.5% Cr, 0.01 - 0.1% P and optionally further optional elements, in particular 0.005 - 0.1% Nb. In addition, the sheet metal component includes an anti-corrosion coating that contains aluminum.

A shaped sheet metal part and a method for producing such a shaped sheet metal part are also known from EP 2 553 133 B1.

Against the background of the prior art, the task was to further develop a flat steel product for hot forming in such a way that, in conjunction with an aluminum-based anti-corrosion coating, improved processing properties of the hot-formed sheet metal part can be achieved. In addition, a method should be specified with which such shaped sheet metal parts can be produced in a practice-oriented manner.

The invention achieves this object by means of a flat steel product for hot forming, 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%,

AI: 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 -1.00% passes.

Compared to known flat steel products, the steel substrate of the flat steel product according to the invention has an aluminum content of at least 0.06% 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 0.140% by weight, in particular at least 0.15% 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 0.140% by weight, in particular at least 0.15% by weight, preferably at least 0.16% by weight. The maximum aluminum content in this variant is at most 0.50% by weight, in particular at most 0.35% by weight, preferably at most 0.25% by weight, in particular at most 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. In this variant, the maximum aluminum content is at most 1.0% by weight, in particular at most 0.9% by weight, preferably at most 0.80% by weight. Aluminum ("AI") is known to be added as a deoxidizing agent during the production of steel. At least 0.01% by weight of Al is required to reliably bind the oxygen contained in the molten steel. In addition, Al can also be used to bind undesired but production-related unavoidable N contents. Comparatively high aluminum contents have so far been avoided because the Ac3 temperature also shifts upwards with the aluminum content. 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 connection with an aluminum-based anti-corrosion coating.

When the flat steel product is coated with an aluminum-based anti-corrosion coating and during the subsequent hot forming of sheet metal blanks separated from it into shaped sheet metal parts, iron diffuses from the steel substrate into the liquid anti-corrosion coating. In the interdiffusion zone, iron-aluminide compounds with a higher density are formed via a multi-stage phase transformation (Fe2Al5^Fe2AI^FeAl^Fe3AI). The formation of such dense phases is associated with higher aluminum consumption than in less dense phases. This locally higher aluminum consumption leads to the formation of pores (voids) in the phase obtained. These pores preferably form in the transition area between the steel substrate and the anti-corrosion coating, where the proportion of available aluminum 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 transition area.

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

- Due to the pores, the mechanical integrity in this area is reduced. Layer detachment can occur more quickly under corrosive stress.

In addition, the transmittable 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 have a negative effect on the suitability for welding and thus reduce the welding area.

- The pores themselves facilitate crack initiation and crack propagation during static and dynamic bending. Surprisingly, it has been shown that by increasing the aluminum content ("AI") in the steel substrate to the lower limits described and beyond, a significant reduction in pore formation during coating with an aluminum-based anti-corrosion coating and the subsequent hot forming can be achieved. Especially in the transition area between steel substrate and anti-corrosion coating, the locally higher aluminum consumption in the formation of denser iron-aluminide compounds can be at least partially compensated 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 of Al oxides forming on the surface of a product made from steel material alloyed according to the invention, which would impair 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 selected below the upper limits already mentioned.

In particular, the bending behavior of the sheet metal component is supported by the niobium content (“Nb”) according to the invention of at least 0.001% by weight. The Nb 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 Nb content leads to a distribution of niobium carbonitrides, particularly in the method described below for producing a flat steel product for hot forming with an anti-corrosion coating, which leads to a particularly fine hardened structure during subsequent hot forming. During cooling after hot dip coating, the coated steel flat 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 ("C") in the steel substrate, while the thermodynamic solubility is very low. Thus, carbon diffuses to lattice defects and accumulates there. Lattice defects are caused in particular by dissolved niobium atoms, which expand the atomic lattice due to their significantly higher atomic volume and thus enlarge the tetrahedron and octahedron gaps in the atomic lattice, so that the local solubility of C is increased. Consequently, there are clusters of carbon and niobium in the steel substrate, which are then formed in the subsequent austenitizing to very fine precipitates during the hot forming step and act as additional austenite nuclei. This results in a refined austenite structure with smaller austenite grains and thus also a refined hardened structure.

This applies in particular 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.

The higher Nb content has another advantage. Surprisingly, it has been shown that the higher Nb content in the steel substrate leads to a shift in the electrochemical potential in the final sheet metal part towards a more positive (i.e. nobler) potential. The Nb content in the interdiffusion layer has proven to be a good indicator for the shift in the electrochemical potential. When the Nb content in the interdiffusion layer is at least 0.010%, the potential is about 100-150 mV higher than that of a comparative substrate with a lower Nb content. The shaped sheet metal part made in this way therefore has a higher resistance to corrosion.

However, if the Nb content is too high, the recrystallizability is impaired. Therefore, the Nb content is 0.2% by weight at maximum. Furthermore, the Nb content is preferably at most 0.20% by weight, in particular at most 0.15% by weight, preferably at most 0.10% by weight, in particular at most 0.05% by weight.

Aluminum and niobium both have an impact on grain refinement during austenitization in the hot forming process. It has been found that, in addition to niobium, aluminum in particular refines grain growth at elevated temperatures in austenite (e.g. at over 1200 °C) through relatively early formation of AlN, i.e. at relatively high temperatures. The formation of AlN is thermodynamically favored over the formation of NbN or NbC. The precipitation of AlN 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 from the Al content to the Nb content:

1<Al/Nb, the Al/Nb ratio >2, in particular >3, is preferred. At the same time, if the Al/Nb ratio is too high, AlN formation is no longer as advantageously fine, but rather increasingly coarser AlN particles occur, which reduces the grain refinement effect again. It has been shown that this effect occurs earlier with low manganese contents than with higher ones Manganese content, since the AC3 temperature decreases with increasing manganese content. It is therefore advantageous, optionally with low manganese contents of <1.6% by weight, to have a ratio of Al/Nb

set, for which applies:

Al/Nb < 20.0, which roughly corresponds to an atomic ratio of both elements < 6. For Mn<1.6% by weight, the Al/Nb ratio 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.

With higher manganese contents of Mn > 1.7% by weight, on the other hand, higher ratios are also possible. It is therefore advantageous, optionally with higher manganese contents of 1.7% by weight or more, to set an Al/Nb ratio for which the following applies:

Al/Nb < 30.0,

For Mn>1.7% by weight, the Al/Nb ratio is preferably <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, particularly preferably <12.0, in particular <10.0, preferably <9.0, in particular <8.0, preferably <7.0.

Regardless of the manganese content, it is therefore optionally preferable to set a ratio of Al/Nb for which the following applies:

Al/Nb < 20.0,

The Al/Nb ratio 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 steel flat 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 retained austenite in the structure. A C content of at least 0.06% by weight is required in order to achieve adequate hardenability and the associated high strength.

However, the weldability can be adversely affected by high C contents. In order to improve the weldability, the C content can be reduced to 0.5% by weight, preferably to a maximum of 0.5% by weight, in particular to a maximum of 0.45% by weight, preferably to 0.42% by weight. %, particularly preferably 0.40% by weight, preferably at most 0.38% by weight, in particular at most 0.35% by weight. In order to be able to use the positive effects of the presence of carbon in a particularly reliable manner, 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. With these contents, tensile strengths of the sheet metal part of at least 1000 MPa, in particular at least 1100 MPa, can be reliably achieved after hot pressing, taking into account the further provisions of the invention.

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. In this variant, the maximum C content is at most 0.30% by weight, in particular at most 0.25% by weight, preferably at most 0.25% by weight. With this maximum C content, the weldability can be significantly improved and a good ratio of force absorption and maximum bending angle in the bending test according to VDA238-100 in the press-hardened state can be achieved.

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 at most 0.5% by weight, in particular at most 0.50% by weight, preferably at most 0.40% by weight, preferably at most 0.38% by weight, in particular at most 0.35% by weight.

In a third further developed variant, the C content is at least 0.30% by weight, preferably at least 0.32% by weight, in particular at least 0.33% by weight, preferably at least 0.34% by weight at least 0.35% by weight, in particular at least 0.40% by weight, preferably at most 0.44% by weight. In this variant, the maximum C content is at most 0.5% by weight, in particular at most 0.50% by weight, preferably at most 0.48% by weight. With these contents, tensile strengths of the sheet metal part of at least 1700 MPa, in particular at least 1800 MPa, can be reliably achieved after hot pressing, taking into account the further provisions of the invention.

Silicon ("Si") is used to further increase the hardenability of the steel flat 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. From an Si content of 0.05% by weight, this is already the case hardening effect on. From an Si content of at least 0.15% by weight, in particular at least 0.20% by weight, there is a significant increase in strength. Si contents above 0.6% by weight have an adverse effect on the coating behavior, particularly in the case of 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 the formation of ferrite and bainite. With manganese contents of less than 0.4% by weight, significant proportions of ferrite and bainite are formed during press hardening, even with very rapid cooling rates, which should be avoided. Mn contents of at least 0.5% by weight, in particular at least 0.7% by weight, preferably at least 0.8% by weight, in particular at least 0.9% by weight, preferably at least 1.00% by weight %, in particular at least 1.05% by weight, particularly preferably at least 1.10% by weight, are advantageous if a martensitic structure is to be ensured, in particular in areas of greater deformation. Mn 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 at most 3.0% by weight, preferably at most 2.5% by weight. Above all, the weldability is severely restricted, which is why the Mn content is preferably limited to at most 1.6% by weight and in particular to 1.30% by weight, preferably to 1.20% by weight. Mn contents of less than or equal to 1.6% by weight are also preferred for economic reasons.

Titanium ("Ti") is a microalloying element that is alloyed to contribute to grain refinement, with at least 0.001% by weight Ti, particularly at least 0.004% by weight, preferably at least 0.010% by weight Ti, added for sufficient availability should be. Above 0.10% by weight Ti, the cold-rollability and recrystallizability deteriorate significantly, which is why larger Ti contents should be avoided. In order to improve cold-rollability, the Ti content may preferably be limited to 0.08% by weight, more preferably 0.038% by weight, more preferably 0.020% by weight, particularly 0.015% by weight. Titanium also has the effect of binding nitrogen and thus enabling boron to exert 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 adequate binding of nitrogen. Boron (“B”) is added to improve the hardenability of the flat steel product by allowing boron atoms or boron precipitates attached to the austenite grain boundaries to reduce the grain boundary energy, thereby suppressing nucleation of ferrite during press hardening. A significant effect on the hardenability occurs with B contents 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. on. With B contents above 0.01% by weight, on the other hand, 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 B content is limited 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 from 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 notched bar impact work deteriorate with increasing P or S content. From P contents of 0.03% by weight, the martensite also 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 at most 0.02% by weight, preferably at most 0.0010% by weight, in particular at most 0.005% by weight.

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

Other typical impurities are tin ("Sn") and arsenic ("As"). The Sn content is at most 0.03% by weight, preferably at most 0.02% by weight. The As content is at most 0.01% by weight, in particular at most 0.005% by weight.

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

Chromium, copper, molybdenum, nickel, vanadium, calcium and tungsten can optionally be added to the steel of a flat steel product according to the invention, either individually or in combination with one another.

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 increasing 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 reliable process control . Too high a content of Cr, however, impairs the coatability of the steel. Therefore, the Cr content of the steel of the steel substrate is limited to at most 1.0% by weight, preferably at most 0.80% by weight, in particular at most 0.75% by weight, preferably at most 0.50% by weight. , In particular limited to a maximum of 0.30% by weight.

Vanadium ("V") can optionally be alloyed in amounts of 0.001 - 1.0% by weight. The vanadium content is preferably at most 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 the 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 sheet metal 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 at most 0.2% by weight, preferably at most 0.1% by weight, in particular at most 0. 10 wt .-% is limited. Molybdenum ("Mo") can be optionally added to improve process stability as it significantly slows down ferrite formation. From contents of 0.002% by weight, dynamic molybdenum-carbon clusters form up to ultra-fine molybdenum carbides on the grain boundaries, which significantly slow down the mobility of the grain boundary and thus diffusive phase transformations. In addition, molybdenum 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 an alloy of molybdenum, the Mo 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 ferrite and bainite formation. Nickel also has a positive effect on hot-rollability, especially when the steel contains copper. Copper degrades hot-rollability. In order to counteract the negative influence of copper on hot-rollability, 0.01% by weight of nickel can be alloyed with the steel; the Ni content is preferably at least 0.015% by weight, in particular 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 at most 0.10% by weight.

Calcium ("Ca") is used in steel to form non-metallic inclusions, especially manganese sulfides. The rounded indentation significantly reduces the negative effect of the inclusions on hot workability, fatigue strength and toughness. In order to also utilize 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 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 the Ca content is too high, the probability increases that non-metallic inclusions involving calcium will form, which impair the degree of 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 observed.

Tungsten ("W") can optionally be added in amounts of 0.001 - 1.0% by weight to slow down the formation of ferrite. A positive effect on the hardenability already results from 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. Irrespective 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 of both elements are created by ensuring the coating performance and to ensure sufficient welding behavior.

The above explanations of element contents and their preferred limits apply correspondingly to the method described below for producing a flat steel product, for the shaped sheet metal part and for the method for producing a shaped sheet metal part.

The flat steel product preferably comprises an anti-corrosion coating in order to protect the steel substrate from oxidation and corrosion during hot forming and when the steel component produced is used.

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

Such an anti-corrosion coating is preferably produced by hot-dip coating the steel flat product. The flat steel product is passed through a liquid melt consisting of 0.1-15% by weight Si, preferably more than 1.0% by weight Si, optionally 2-4% by weight Fe, optionally up to 5% by weight % alkali or alkaline earth metals, preferably up to 1.0% by weight alkali or alkaline earth metals, and optionally up to 15% by weight Zn, preferably up to 10% by weight Zn and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 5-15% by weight, preferably 7-12% by weight, in particular 8-10% by weight.

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

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

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

The Al base layer lies on top of the alloy layer and is immediately adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt in the molten bath. That is, it consists of 0.1 - 15 wt.% Si, optionally 2 - 4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt. - % alkali metals or alkaline earth metals, optionally up to 15% by weight Zn, preferably up to 10% by weight Zn and optional other components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder Aluminum.

In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0 .5% by weight Mg. Furthermore, the optional content of alkali or alkaline earth metals in the AI The base layer comprises 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 anti-corrosion coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

The anti-corrosion coating preferably has a thickness of 5 to 60 μm, in particular 10 to 40 μm. The coating weight of the anti-corrosion coating is in particular 30 - 360^ m for anti-corrosion coatings on both sides, or 15 - 180^ for the one-sided variant. m

The coating weight of the anti-corrosion coating is preferably 100-200% for m double-sided coatings, or 50-100% for one-sided coatings. The m is particularly preferably

Coating weight of the anti-corrosion coating 120-180^ with coatings on both sides, or m

60-90^ for one-sided covers. m

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 thicknesses of the anti-corrosion coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 μm, even in the case of thin anti-corrosion coatings.

In a preferred variant, the flat steel product comprises an oxide layer arranged on the anti-corrosion coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer end of the anti-corrosion 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, hydroxides and/or magnesium oxide are present alone or as a mixture in the oxide layer in addition to aluminum oxide. The remainder of the oxide layer not occupied by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and/or magnesium in 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 at most 500 nm.

In an alternative embodiment, the flat steel product includes a zinc-based anti-corrosion coating. The anti-corrosion coating can be applied to one side or both sides of the flat steel product. The two opposite large surfaces of the flat steel product are referred to as the two sides of the flat steel product. The narrow faces are called edges.

Such a zinc-based anti-corrosive coating preferably comprises 0.2-6.0 wt% Al, 0.1-10.0 wt% Mg, optionally 0.1-40 wt% manganese or copper, optionally 0.1 - 10.0% by weight cerium, optionally at most 0.2% by weight other elements, unavoidable impurity and the balance zinc. In particular, the Al content is at most 2.0% by weight, preferably at most 1.5% by weight. The Mg content is in particular at most 3.0% by weight, preferably at most 1.0% by weight. The anti-corrosion coating can be applied by hot dip coating or by physical vapor deposition or by electrolytic processes.

A further developed flat steel product preferably 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 shows a continuous course or only a small extent. For the purposes of the application, continuous progression means that there is no pronounced yield point. A continuous yield point can also be referred to as a yield point Rp0.2. In the present case, a low yield point is understood to mean a pronounced yield point in which the difference ARe between the upper yield point value ReH and the lower yield point value ReL is at most 45 MPa. The following applies:

ARe = (ReH - ReL) < 45 MPa with ReH = upper yield point in MPa and ReL = lower yield point in MPa.

A particularly good resistance to aging can be achieved with steel flat products, for which the difference ARe is at most 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 method according to the invention for the production of a flat steel product for hot forming with an anti-corrosion coating comprising the following work steps: a) Providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of 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 -1.00% passes; b) through heating of the slab or thin slab at a temperature (TI) of 1100 - 1400 °C; c) optional pre-rolling of the through-heated slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) hot-rolling into a hot-rolled steel flat product, the finish rolling temperature (T3) being 750 - 1000 °C; e) optional coiling of the hot-rolled steel flat product, the coiling temperature (T4) being at most 700 °C; f) optional descaling of the hot-rolled flat steel product; g) optional cold rolling of the steel flat product, the degree of cold rolling being at least 30%; h) annealing of the steel flat product at an annealing temperature (T5) of 650 - 900 °C; i) cooling the flat steel product to an immersion temperature (T6) which is 650-800° C., preferably 670-800° C.; j) coating the flat steel product, which has been cooled to the immersion temperature, with an anti-corrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C., preferably 680-740° C.; k) Cooling of the coated flat steel product to room temperature, the first cooling time t _mT in the temperature range between 600 °C and 450 °C being more than 10 s, in particular more than 14 s, and the second cooling time t _n in the temperature range between 400 °C and 300 °C is more than 8s, in particular more than 12s; l) Optional skin-passing of the coated steel flat product. In step a), a semi-finished product composed according to the alloy specified according to the invention for the flat steel product is made available. This can be a slab produced in conventional continuous slab casting or in thin slab continuous casting.

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

In the optional work step c), the semi-finished product is pre-rolled into an intermediate product. Thin slabs are usually not subjected to pre-rolling. Thick slabs that are to be rolled into hot strip can be pre-rolled if necessary. In this case, the temperature of the intermediate product (T2) at the end of rough rolling should be at least 1000°C so that the intermediate product contains enough heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the flat steel product. In order to keep grain growth low during the rolling process, the temperature of the intermediate product should not exceed 1200 °C at the end of rough rolling.

In step d), the slab or thin slab or, if step c) has been carried out, the intermediate product is rolled to form a hot-rolled flat steel product. If step c) has been carried out, the intermediate product is typically finish-rolled immediately after rough-rolling. Typically, finish rolling begins no later than 90 s after the end of rough rolling. The slab, the thin slab or, if step c) has been carried out, the intermediate product are rolled at a finish rolling temperature (T3). The final rolling temperature, i.e. the temperature of the finished hot-rolled steel flat product at the end of the hot-rolling process, is 750 - 1000 °C. At final rolling temperatures below 750 °C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large. They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those carried out before hot-dip coating. The final rolling temperature is limited to a maximum of 1000 °C prevent coarsening of the austenite grains. In addition, final rolling temperatures of no more than 1000 °C are process-technically relevant for setting coiling temperatures (T4) below 700 °C.

The hot rolling of the steel flat product can take place as continuous hot strip rolling or as reversing rolling. In the case of continuous hot strip rolling, step e) provides for an optional coiling of the hot-rolled flat steel product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 s after hot rolling. For example, water, air or a combination of both can be used as a cooling medium for this purpose. The coiling temperature (T4) should not exceed 700 °C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500 °C have proven to be favorable for cold-rollability. The coiled hot strip is then cooled in air to room temperature in a conventional manner.

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

The hot-rolled flat steel product that has been cleaned of scale can optionally be subjected to cold rolling before the annealing treatment in step g), in order, for example, to meet higher requirements for the thickness tolerances of the flat steel product. The degree of cold rolling (KWG) should be at least 30% in order to introduce sufficient deformation energy into the steel flat product for rapid recrystallization. The degree of cold rolling KWG is the quotient of the reduction in thickness during cold rolling AdKW divided by the hot strip thickness d:

KWG = AdKW/d with AdKW = reduction in thickness during cold rolling in mm and d = hot strip thickness in mm, where the reduction in thickness AdKW results from the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is usually a hot strip with a hot strip thickness d. The flat steel product after cold rolling is usually also referred to as cold strip. In principle, the degree of cold rolling can assume very high values of over 90%. However, degrees of cold rolling of at most 80% have proven to be beneficial for avoiding strip cracks. In step h), the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650 - 900 °C. For this purpose, the flat steel product is first heated to the annealing temperature within 10 to 120 s and then held at the annealing temperature for 30 to 600 s. The annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are not desirable for economic reasons.

In step i), the flat steel product is cooled to an immersion temperature (T6) after annealing in order to prepare it for the subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the melt pool. The immersion temperature is 600-800°C, preferably at least 650°C, particularly preferably at least 670°C, particularly preferably at most 700°C. For a particularly homogeneous formation of the boundary layer, it is important that there is sufficient thermal energy in the boundary layer between the steel substrate and the molten aluminum. This is not the case at temperatures below 600 °C, so that undesired compounds can form whose later reconversion can lead to pores. Above the preferred immersion temperatures, the rate of diffusion of iron in aluminum increases significantly again, so that more iron can diffuse into the still liquid boundary layer right from the start of the coating process. The duration of the cooling of the annealed steel flat product from the annealing temperature T5 to the immersion temperature T6 is preferably 10-180 s. In particular, the immersion temperature T6 differs from the temperature of the melt bath T7 by no more than 30K, in particular no more than 20K, preferably no more than IOK away.

In step j), the flat steel product is subjected to a coating treatment. The coating treatment is preferably carried out by continuous hot dip coating. The coating can be applied to only one side, to both sides or to all sides of the steel flat product. The coating treatment preferably takes place as a hot-dip coating process, in particular as a continuous process. The flat steel product usually comes into contact with the molten bath on all sides, so that it is coated on all sides. The molten bath, which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660-800°C, preferably 680-740°C. Aluminum-based alloys have proven to be particularly suitable for coating age-resistant flat steel products with an anti-corrosion coating. In such a case, the molten bath contains up to 15% by weight Si, preferably more than 1.0%, optionally 2-4% by weight Fe, optionally up to 5% by weight alkali or alkaline earth metals, preferably up to 1.0% by weight of alkali metals or alkaline earth metals, and optionally up to 15% by weight of Zn, in particular up to 10% by weight of Zn and optional other components, the total content of which does not exceed 2.0% by weight. -% are limited, and the remainder aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 7-12% by weight, in particular 8-10% by weight. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0. 5% by weight Mg. Furthermore, the optional content of alkali metals or alkaline earth metals in the melt can include in particular at least 0.0015% by weight Ca, in particular at least 0.01% by weight Ca.

After the coating treatment, the coated flat steel product is cooled to room temperature in step k). A first cooling time t _mT in the temperature range between 600 °C and 450 °C (average temperature range mT) is more than 5 s, preferably more than 10 s, in particular more than 14 s and a second cooling time t _nT in the temperature range between 400 °C and 300 °C (low temperature range nT) more than 4s, preferably more than 8s, in particular more than 12s.

The first cooling time t _mT can be implemented in the temperature range between 600° C. and 450° C. (average temperature range mT) by slow, continuous cooling or by holding at a temperature in this temperature range for a certain time. Even intermediate heating is possible. It is only important that the flat steel product remains in the temperature range between 600 °C and 450 °C for at least a cooling period t _mT . In this temperature range, on the one hand, there is a significant rate of diffusion of iron in aluminum and, on the other hand, the diffusion of aluminum in steel is inhibited because the temperature is below half the melting point of steel. This allows diffusion of iron into the anti-corrosion coating without extensive diffusion of aluminum into the steel substrate.

The diffusion of iron into the anti-corrosion coating has several advantages:

On the one hand, the melting of the anti-corrosion coating is delayed during austenitizing before press hardening. On the other hand, the thermal expansion coefficients of the anti-corrosion coating and the substrate are homogenized. This means that the transition area between the coefficient of thermal expansion of the substrate and the surface becomes wider, which reduces the thermal stresses during reheating. At the same time, diffusing aluminum into the steel substrate would have considerable disadvantages: Due to the very high affinity of aluminum for nitrogen, a high aluminum content can lead to nitrogen separating from fine precipitations, such as niobium carbonitrides or titanium carbonitrides, and coarse precipitations forming instead. such as aluminum nitrides, preferentially form on the grain boundaries. These would worsen the crash performance as well as reduce the bending angle. In addition, this destabilizes the fine precipitates (eg the Nb-containing precipitates) in the uppermost substrate region, which are important for many preferred properties. Furthermore, the inhomogeneous diffusion rate of aluminum in the steel substrate in ferrite compared to pearlite/bainite/martensite would lead to an uneven distribution of Al in the surface layer of the steel substrate. This should also be prevented to improve crash and bending performance. These disadvantages of aluminum diffusing into the steel substrate are therefore reduced or avoided by inhibition.

Due to the preferred first cooling time t _mT (14s), the iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum absorption in the substrate during austenitization before press hardening, with the associated advantages described above.

The second cooling time t _n in the temperature range between 400° C. and 300° C. (lower temperature range nT) can also be realized by slow, continuous cooling or by holding at a temperature in this temperature range for a certain time. Even intermediate heating is possible. It is only important that the flat steel product remains in the temperature range between 400 °C and 300 °C for at least a cooling period t _nT .

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 collects there, eg to dissolved Nb atoms. Due to their significantly higher atomic volume, these expand the atomic lattice and thus enlarge the tetrahedron and octahedron gaps in the atomic lattice, so that the local solubility of C is increased. This results in clusters of C and Nb, which are then converted into very fine precipitations in the austenitization step of hot forming and lead to a refined austenite structure and thus also a hardened structure, as well as a reduction in the free hydrogen content. With the preferred holding time of more than 12s, very fine iron carbides (so-called transition carbides) are also formed, which in turn dissolve very quickly during austenitizing and lead to additional austenite nuclei and thus an even finer austenite structure and thus also hardened structure.

The coated steel flat product can optionally be skin-passed with a skin-pass degree of up to 2% in order to improve the surface roughness of the steel flat product.

The invention further relates to a shaped sheet metal part formed from a flat steel product, comprising a steel substrate as explained above and an anti-corrosion coating. The anti-corrosion coating has the advantage that it prevents scale formation during austenitization during hot forming. Furthermore, such an anti-corrosion coating protects the shaped sheet metal part against corrosion.

In a special embodiment, the shaped sheet metal part preferably comprises an aluminum-based anti-corrosion coating. The anti-corrosion coating of the sheet metal part preferably comprises an alloy layer and an Al base layer. In the case of the formed sheet metal part, the alloy layer is also often referred to as the 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 thicknesses of the anti-corrosion coating and the alloy layer.

The alloy layer rests on the steel substrate and is directly adjacent to it. The alloy layer of the shaped sheet metal part preferably consists of 35 - 90% by weight Fe, 0.1 - 10% by weight Si, optionally up to 0.5% by weight Mg and optional other components, the total content of which is at most 2 .0% by weight, and the remainder aluminum. As iron continues to diffuse into the alloy layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten bath.

The alloy layer preferably has a ferritic structure. The aluminum base layer of the shaped sheet metal part lies on the alloy layer of the steel component and is directly adjacent to it. The Al base layer of the steel component preferably consists of 35-55% by weight Fe, 0.4-10% by weight Si, optionally up to 0.5% by weight Mg and optional other components, the total content of which is at most 2.0% by weight, and the remainder aluminum.

The Al base layer can have a homogeneous element distribution in which the local element contents vary by no more than 10%. In contrast, preferred variants of the Al base layer have low-silicon phases and high-silicon 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 islands in the silicon-poor phase.

In the context of this application, “island-shaped” is understood to mean an arrangement in which discrete unconnected areas are surrounded by another material—that is, “islands” of a specific material are located in another material.

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

The oxide layer of the steel component consists in particular of more than 80% by weight of oxides, with the main proportion of the oxides (i.e. more than 50% by weight of the oxides) being aluminum oxide. Optionally, hydroxides and/or magnesium oxide are present alone or as a mixture in the oxide layer in addition to aluminum oxide. The remainder of the oxide layer not occupied by the oxides and optionally present hydroxides preferably 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 embodiment, the shaped sheet metal part includes a zinc-based anti-corrosion coating.

Such a zinc-based anti-corrosion coating preferably comprises up to 80% by weight Fe, 0.2 - 6.0% by weight Al, 0.1 - 10.0% by weight Mg, optionally 0.1 - 40 % by weight of manganese or copper, optionally 0.1-10.0% by weight of cerium, optionally at most 0.2% by weight of other elements, unavoidable impurities and the remainder zinc. In particular, the Al content is at most 2.0% by weight, preferably at most 1.5% by weight. The Fe content, which comes about as a result of indiffusion, is preferably more than 20% by weight, in particular more than 30% by weight. In addition, the Fe content is in particular a maximum of 70% by weight, in particular a maximum of 60% by weight. The Mg content is in particular at most 3.0% by weight, preferably at most 1.0% by weight. The anti-corrosion coating can be applied by hot dip coating or by physical vapor deposition or by electrolytic processes.

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

Due to the high martensite content, very high tensile strength and yield points can be achieved.

In an alternative development, the steel substrate of the shaped 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 microstructure 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 a higher ductility and therefore leads to improved forming behavior. A corresponding shaped sheet metal part preferably has an elongation at break A80 in a range from 8% to 25%, preferably between 10% and 22%, in particular between 12% and 20%.

In a preferred embodiment, the former austenite grains of the martensite have an average grain diameter of less than 14 μm, in particular less than 12 μm, preferably less than 10 μm. Due to the fine structure, this is more homogeneous. There is an improvement in the mechanical properties, in particular a lower susceptibility to cracking and thus improved bending properties and a higher elongation at break.

In a further developed variant, the shaped sheet metal part at least partially has a yield point of at least 950 MPa, in particular at least 1100 MPa, in particular at least 1200 MPa, preferably at least 1300 MPa, preferably at least 1400 MPa, in particular at least 1500 MPa.

In a further developed variant, the shaped 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, preferably at least 1400 MPa, in particular at least 1600 MPa, preferably at least 1700 MPa, in particular at least 1800 MPa.

In particular, the shaped sheet metal part at least partially has an elongation at break A80 of at least 3.5%, in particular at least 4%, in particular at least 4.5%, preferably at least 5%, particularly preferably at least 6%.

In addition, in a preferred variant, the shaped sheet metal part can at least partially have a bending angle of at least 30°, in particular at least 40°, in particular at least 45°, preferably at least 50°. The bending angle is to be understood here as the bending angle corrected by the sheet thickness. The corrected bending angle results from the determined bending angle in the maximum force (measured according to VDA standard 238-100) (also referred to as maximum bending angle) from the formula

Bending angle _corrected = bending angle _guessed ■ Sheet thickness where the sheet thickness in mm is to be entered in 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 have” is to be understood as meaning that there are areas of the sheet metal part that have the mechanical property mentioned. In addition, there can also be areas of the sheet metal part whose mechanical properties are below the limit value. The shaped sheet metal part therefore has the mechanical properties mentioned in sections or in regions. This is because different areas of the sheet metal part can receive different heat treatments. For example, individual areas can be cooled more quickly than others, as a result of which more martensite forms, for example, in the areas that have cooled more quickly. This is why different mechanical properties also appear 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 shaped sheet metal part has fine precipitations in the structure, in particular in the form of niobium carbonitrides and/or titanium carbonitrides.

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

In a preferred embodiment, the mean diameter of the fine precipitates is at most 11 nm, preferably at most 10 nm, in particular at most 8 nm, preferably at most 6 nm.

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

The fine precipitations require a particularly fine structure with small grain diameters. Due to the fine structure, this is more homogeneous. There is an improvement in the mechanical Properties, in particular a lower susceptibility to cracking and thus improved bending properties and a higher elongation at break. This also results in better toughness with more pronounced fracture necking behavior.

The real mechanical characteristics of the sheet metal part are determined by first cathodically coating the sheet metal part with dip paint or by subjecting it to an analogous heat treatment. Cathodic dip coatings are usually carried out for corresponding components in the automotive industry. In cathodic dip painting, the components are first coated in an aqueous solution. This coating is then burned in during a heat treatment. The shaped sheet metal parts are heated to 170° C. and kept at this temperature for 20 minutes. The components are then cooled to room temperature in ambient air. Since this heat treatment can affect the mechanical parameters, the mechanical parameters (yield point, tensile strength, yield point ratio, elongation at break A80, bending angle, Vickers hardness) are to be understood in the sense of this application that they are present on a component with a cathodic dip coating or on a Component that, after forming, has undergone a heat treatment analogous to cathodic dip painting. In practice, the heat treatment of the cathodic dip coating varies slightly. Temperatures of 165°C-180°C and holding times of 12 - 30 minutes are usual. However, the change in the mechanical parameters due to these variations (165°C-180°C; 12 - 30 minutes) are negligible.

In a preferred variant, the shaped sheet metal part comprises a cathodic dip coating.

In a particularly preferred embodiment of the sheet metal part, the electrochemical potential of the surface of the sheet metal part is at least −0.50 V in a corrosive medium. The electrochemical potential is therefore −0.50 V or greater, ie more positive.

The electrochemical potential is determined according to the DIN standard "DIN 50918 (2018.09) ("resting potential measurement on homogeneous mixed electrodes"). Insofar as absolute instead of difference values for the electrochemical potential are given, this refers to the reference to the standard hydrogen electrode. An aqueous 5% NaCl solution with a pH value of 7, which represents typical corrosion conditions in the automotive sector, is used as the corrosive medium in the measurement. In other words, the electrochemical potential is Surface of the sheet metal part in an aqueous 5% NaCl solution with a pH value of 7 at least -0.50 V.

The electrochemical potential is preferably at least −0.45 V, particularly preferably at least −0.40 V, in particular at least −0.39 V, particularly preferably at least −0.38 V, in particular at least −0.36 V, preferably at least −0 34 V. Furthermore, the electrochemical potential is preferably at most -0.1 V, preferably at most -0.20 V, in particular -0.25 V, preferably at most -0.30 V.

A larger, ie more positive, electrochemical potential has the advantage that the sheet metal part has a lower tendency to corrode. Surprisingly, it has been shown that the higher Nb content in the steel substrate leads to a shift in the electrochemical potential to a more positive (i.e. nobler) potential. The potential is typically about 100-150 mV higher than a comparison substrate with lower Nb content.

A further developed variant of the shaped sheet metal part is characterized in that the anti-corrosion coating is an aluminum-based anti-corrosion coating and the shaped sheet metal part comprises an alloy layer and an Al base layer. In addition, in the cross section of the alloy layer over a measuring length of 500 μm, the area occupied by pores in the alloy layer is less than 250 μm ² , preferably less than 200 μm ² , in particular less than 180 μm ² , particularly preferably less than 100 μm ² , in particular less than 75 μm ² .

Pores are cavities that could arise within the alloy layer for a variety of reasons. One mechanism is the formation of higher density iron aluminide compounds via a multi-step phase transformation (Fe2Al5^Fe2AI^FeAl^Fe3AI). The formation of such dense phases is associated with higher aluminum consumption than in less dense phases. This locally higher aluminum consumption leads to the formation of pores (voids) in the phase obtained. These pores preferably form in the alloy layer in the transition area between the steel substrate and the anti-corrosion coating, where the proportion of available aluminum 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 area, ie 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:

- Due to the pores, the mechanical integrity in this area is reduced. Layer detachment can occur more quickly under corrosive stress. In addition, the transmittable 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, on the other hand, the area over which the forces of the adhesive connection are transmitted is increased by more than 60% when gluing. 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 have a negative effect on the suitability for welding 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 crack propagation during static and dynamic bending. A higher bending angle can consequently be achieved by reducing the pore area.

In a special configuration, the proportion of the area occupied by pores in the alloy layer with a diameter greater than or equal to 0.1 μm is less than 10%, in particular less than 5%, preferably less than 3%. Smaller pores have a much smaller impact on the reduction in mechanical integrity discussed. A particularly fine-pored alloy layer is therefore preferred.

In a particularly preferred embodiment, the welding range is at least 0.9 kA, preferably at least 1.0 kA, particularly preferably at least 1.1 kA, in particular at least 1.2 kA. The welding area is determined according to SEP 1220-2. In particular, the welding range is a maximum of 1.6 kA, in particular a maximum of 1.4 kA. The areas mentioned enable particularly stable further processing of the shaped sheet metal parts.

In a special configuration, the Nb content in the alloy layer is greater than 0.010% by weight, preferably greater than 0.015% by weight, in particular greater than 0.018% by weight.

The shaped sheet metal part according to the invention is preferably a component for a land vehicle, sea vehicle or aircraft. It is particularly preferably a Automobile part, in particular a body part. The component is preferably a B-pillar, side member, A-pillar, rocker panel or cross member.

In the method according to the invention for producing a shaped sheet metal part according to the invention in the manner explained above, at least the following work steps are carried out: a) Preparing a sheet metal blank from a steel flat product comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of

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%

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 -1.00% passes; a) heating of the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T _unit of the blank when it is placed in a forming tool provided for hot-press forming (step c)) at least partially a temperature above Ms+100°C, in particular above Ms+300°C, where Ms designates the martensite start temperature. b) inserting the heated sheet metal blank into a forming tool, the transfer time t _{Tr an} required for removing it from the heating device and inserting the blank being at most 20 s, preferably at most 15 s; c) Hot-press forming of the sheet metal blank to form the shaped sheet metal part, the blank being cooled to the target temperature T target temperature T _target and optionally held there over a period t _wz of more than 1 s with a cooling rate r _wz at least in part of more than 30 K/s in the course of the hot-press forming becomes; d) removing the shaped sheet metal part, which has been cooled to the target temperature, from the tool;

In the method according to the invention, a blank is thus provided (step a)), which consists of steel suitably composed in accordance with the explanations above, which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einig of the blank when it is placed in a forming tool provided for hot-press forming (work step c)) is at least partially a temperature above Ms+100°C, in particular above Ms+300°C. In particular, the temperature T _eing of the blank during insertion at least partially exceeds 600°C. In a particularly preferred variant, the temperature T _Ein ig of the blank during insertion is at least partially, in particular completely, in the range from 600° C. to 850° C. in order to ensure good formability and sufficient hardenability. In the context of this application, partially exceeding a temperature (here AC3 or Ms+100°C) means that at least 30%, in particular at least 60% of the volume of the blank, preferably the entire blank, exceeds a corresponding temperature. The same applies to the at least partial presence of a temperature in the interval 600° C. to 850° C. in the preferred variant explained above. When placed in the forming tool, at least 30% of the blank has an austenitic structure, ie the transformation from the ferritic to the austenitic structure does not have to be completed when it is placed in the forming tool. Rather, up to 70% of the volume of the blank when it is placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or non-recrystallized or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature level than others during heating. For this purpose, the supply of heat can be directed only to specific sections of the blank, or the parts that are to be heated less can be shielded from the supply of heat. In the part of the cut material whose temperature remains lower, no martensite or only significantly less martensite is formed in the course of forming in the tool, so that the structure there is significantly softer than in the other parts in which a martensitic structure is present. In this way, a softer area can be specifically set in the formed sheet metal part in which, for example, there is optimal toughness for the respective application, while the other areas of the sheet metal part have maximized strength.

Maximum strength properties of the formed sheet metal part obtained can be made possible if the temperature at least partially reached in the sheet metal blank is between Ac3 and 1000° C., preferably between 850° C. and 950° C.

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

AC3[°C] = (902 wt% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[° C/wt%] where %C = each C content, %Si = each Si content, %Mn = each Mn content, %Cr = each Cr content, %Mo = each Mo content, %Ni respective Ni content and %V = respective V content of the steel from which the blank is made.

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

In a preferred embodiment variant, the mean heating rate r _O f _en of the sheet metal blank during heating in step b) is at least 3 K/s, preferably at least 5 K/s, in particular at least 6 K/s, preferably at least 8 K/s, in particular at least 10 K/s, preferably at least 15 K/s. The average heating rate of _a _furnace is to be understood as the average heating rate from 30 °C to 700 °C.

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

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

With normalized mean heating, this product 0 is normalized around the present furnace temperature T _furnace in relation to a reference _furnace temperature T _furnace , reference of 900°C=1173.15 K in the following way:

T oven, R ⁴ reference

®norm ⁼ 4 ©the oven temperatures are to be entered in Kelvin.

^Oven

In a preferred embodiment, the heating takes place in an oven with an oven temperature T _O f _en of at least Ac3+10° C., preferably at least 850° C., preferably at least 880° C., particularly preferably at least 900° C., in particular at least 920° C. and at most 1000°C, preferably at most 950°C, particularly preferably at most 930°C.

The dew point of the furnace atmosphere in the furnace is preferably at least −20° C., preferably at least −15° C., in particular at least −5° C., particularly preferably at least 0° C. and at most +25° C., preferably at most +20° C., in particular at most +15°C.

In a special embodiment, the heating in step b) takes place in stages in areas with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. In this case, heating takes place in a first heating zone at a temperature (so-called furnace inlet temperature) of at least 650.degree. C., preferably at least 680.degree. C., in particular at least 720.degree. The maximum temperature in the first heating zone preferably 900 °C, in particular a maximum of 850 °C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably not more than 1200° C., in particular not more than 1000° C., preferably not more than 950° C., particularly preferably not more than 930° C.

The total time in the oven t _oven , which is made up of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, preferably at least 4 minutes, in both variants (constant oven temperature, gradual heating). Furthermore, the total time in the oven in both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is ensured. On the other hand, holding above Ac3 for too long leads to grain coarsening, which has a negative effect on the mechanical properties.

The blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an induction heating device that is also known per se, or a conventional device for keeping steel components warm, and transported into the forming tool so quickly that its temperature during Arrival in the tool is at least partially above Ms+100°C, in particular above Ms+300°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACI temperature. In all of these variants, the temperature is in particular a maximum of 900°C. Overall, these temperature ranges ensure good formability of the material.

In work step c), the austenitized blank is transferred from the heating device used in each case to the forming tool within 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 the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200.degree. C., preferably between 20.degree. C. and 180.degree. C., in particular between 50.degree. C. and 150.degree. When inserting the blank, the tool can also have a temperature slightly below room temperature, for example if the is used cooling water is slightly colder (e.g. 15°C). The tool thus has a temperature of between 10°C and 200°C in individual design variants when inserting the blank. In a special embodiment, the tool can optionally be tempered at least in regions to a temperature T _w of at least 200° C., in particular at least 300° C., in order to only partially harden the component. Furthermore, the tool temperature T _wz is preferably at most 600°C, in particular at most 550°C. It is only necessary to ensure that the tool temperature T _wz is below the desired target temperature T _target . The residence time in the tool t _wz is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The residence time in the tool is preferably a maximum of 25 s, in particular a maximum of 20 s, preferably a maximum of 10 s.

The target temperature T _target of the sheet metal part is at least partially below 400° C., preferably below 300° C., in particular below 250° C., preferably below 200° C., particularly preferably below 180° C., in particular below 150° C. Alternatively, the target temperature T _target of the shaped sheet metal part is particularly preferably below Ms−50° C., with Ms denoting the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.

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

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

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

AC1[°C] = (739 wt% - 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni +20*%V )[° C/wt%] AC3[°C] = (902 wt% - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[° C/% by weight], with %C being the C content, %Si being the Si content, %Mn being the Mn content, %Cr being the Cr content, and %Mo being the Mo content , the Ni content is denoted by %Ni and the vanadium content of the respective steel is denoted by +%V (Brandis H 1975 TEW-Techn. Ber. 1 8-10).

In the tool, the blank is not only formed into the shaped sheet metal part, but is also quenched to the target temperature at the same time. The cooling rate in the tool r _W z to the target temperature is in particular at least 20 K/s, preferably at least 30 K/s, in particular at least 50 K/s, in a particular embodiment at least 100 K/s.

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

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

The figures show:

FIG. 1a shows a schematic representation of a sample according to the invention with a low number of pores obtained from a micrograph.

FIG. 1b shows a schematic representation of a reference sample with an increased number of pores obtained from a micrograph.

FIG. 2a shows a schematic representation, obtained from a micrograph, of a sample according to the invention with a low number of pores after a corrosion test.

FIG. 2b shows a schematic representation, obtained from a micrograph, of a reference sample with an increased number of pores after a corrosion test.

FIG. 3 shows a grain representation of the reconstructed austenite.

Several tests were carried out to demonstrate the effect of the invention. For this purpose, slabs with the compositions given in Table 1 with a thickness of 200-280 mm and a width of 1000-1200 mm were produced, heated to a respective temperature TI in a pusher furnace and kept at TI for between 30 and 450 min until the temperature TI was reached in the core of the slabs and the slabs were thus heated through. The manufacturing parameters are given in Table 2. The slabs were discharged from the pusher furnace at their respective through heating temperature TI and subjected to hot rolling. The tests were carried out as continuous hot strip rolling. For this purpose, the slabs were first pre-rolled into an intermediate product with a thickness of 40 mm, with the intermediate products, which can also be referred to as pre-strips in hot strip rolling, each having an intermediate product temperature T2 at the end of the pre-rolling phase. The pre-strips were fed to finish-rolling immediately after rough-rolling, so that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish-rolling phase. The pre-strips were rolled out to hot strips with a final thickness of 3-7 mm and the respective final rolling temperatures T3 given in Table 2, cooled to the respective coiling temperature and wound up into coils at the respective coiling temperatures T4 and then cooled in still air. The hot strips were descaled in a conventional manner by means of pickling before they were subjected to cold rolling with the cold rolling grades given in Table 2. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 and held at the annealing temperature for 100 s before they were cooled to their respective immersion temperature T6 at a cooling rate of 1 K/s. The cold strips were passed through a molten coating bath at temperature T7 at their respective immersion temperature T6. The composition of the coating bath is given in Table 3. After coating, the coated tapes were blown off in a conventional manner, producing overlays with different layer thicknesses (see Table 3). The strips were first cooled to 600 °C at an average cooling rate of 10 - 15 K/s. In the further course of cooling between 600 °C and 450 °C and between 400 °C and 300 °C, the strips were cooled over the cooling times T _mT and T _nT given in Table 2. Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5 - 15 K/s.

Table 4 summarizes which steel variant (see Table 1) was combined with which process variant (see Table 2) and which coating (see Table 3).

Steel compositions D, E and F are reference examples not according to the invention. Accordingly, runs 3, 10, 11, 12, 13, 17 and 18 are not in accordance with the invention.

The thickness of the steel strips produced was between 1.4 mm and 1.7 mm in all tests. After cooling to room temperature, samples were taken transversely to the rolling direction from the cooled steel strips in accordance with DIN EN ISO 6892-1 sample form 2 (Annex B Tab. B1). The samples were subjected to a tensile test in accordance with DIN EN ISO 6892-1 sample form 2 (Annex B Tab. Bl). Table 4 gives the results of the tensile test. The following material parameters were determined as part of the tensile test: the type of yield point, which is denoted by Re for a pronounced yield point and Rp for a continuous yield point, as well as the value for the yield point Rp0.2 in the case of a continuous yield point, and the values for in the case of a pronounced yield point the lower yield point ReL, the upper yield point ReH and the difference between the upper and lower yield point ARe, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80. All specimens have a continuous yield point Rp or an only slightly pronounced yield point with a difference ARe between the upper and lower yield point of at most 45 MPa and a uniform elongation Ag of at least 11.5%. There is a pronounced yield point Re for samples 3 and 17 and a continuous yield point Rp for all other samples. Table 4 shows the lower yield point ReL and the upper yield point ReH for samples 3 and 17. The yield point Rp0.2 is specified for all other samples.

From each of the 20 steel strips produced in this way, blanks were cut and used for further tests. In these tests, sheet metal part samples 1-24 in the form of 200×300 mm ² large plates were hot-press formed from the respective blanks. Table 7 shows which of the 20 coating tests corresponds to which forming test. For this purpose, the blanks are heated in a heating device, for example in a conventional heating oven, from room temperature at 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 furnace, which includes heating and holding, is _denoted by t _furnace . The dew point of the furnace atmosphere was -5°C in all cases. The blanks are then removed from the heating device and placed in a forming tool, which has the temperature T _w z . By the time they were removed from the oven, the blanks had reached oven temperature. The transfer time t _Tr ans made up of the removal from the heating device, transport to the tool and insertion into the tool was between 5 and 14 s. The temperature T _Einig of the blanks when they were inserted into the forming tool was above the respective temperature in all cases Martensite start temperature+100°C. The blanks have been formed into the respective shaped sheet metal part in the forming tool, with the shaped sheet metal parts being cooled in the tool at a cooling rate r _wz . The dwell time in the tool is given by t _wz designated. Finally, the samples were cooled to room temperature in air. Table 5 shows the parameters mentioned for the various variants, with "RT" abbreviating the room temperature.

Table 5 shows very different variants for the forming process. For example, while Variant II almost completely forms a martensitic structure (see Table 8, Experiment 1), the comparatively slow cooling of Variant X with the high mold temperature T _w leads to a modified structure with high ferrite content, which is expressed in Form a higher elongation at break A80.

Table 6 lists the essential parameters for a further developed process variant. In these tests, the sheet metal blank was not heated in an oven with a constant oven temperature as in the experiments described above, but the sheet metal blanks were heated in stages in areas with different temperatures. The tests were carried out in a roller hearth furnace with different heating zones. In principle, however, the process can also be implemented in several separate furnaces. The blanks were first brought into an inlet area of the oven with an inlet temperature T _inlet . From there the blanks were moved through a central area into an outlet area of the oven with an outlet temperature T _{Aus iauf} . Table 6 shows the inlet temperature T _iniauf , the outlet temperature T _out iauf and the maximum oven temperature T _max through which the blanks pass. In most cases, the maximum furnace temperature was assumed in the outlet area. With variant AX, however, the maximum oven temperature was assumed to be in the central area. The rest of the process was identical to the process described above. The corresponding parameters are given in Table 6.

Table 7 summarizes the overall results for the sheet metal parts obtained. The first columns indicate the sample number, the steel grade according to Table 1, the process variant according to Table 2, the coating according to Table 2 and the hot-forming variant according to Table 5 or Table 6. The yield point, tensile strength and elongation at break A80 are given in the other columns. These values were determined according to DIN EN ISO 6892-1 specimen form 2 (Annex B Tab. Bl) on specimens perpendicular to the rolling direction. The determined bending angle has been determined according to the VDA standard 238-100 with a bending axis transverse to the rolling direction. The determined bending angle is calculated according to the formula specified in the standard from the stamp path (the determined bending angle (also referred to as maximum bending angle) is the bending angle at which the force in the bending test has its maximum). To the influence of To eliminate sheet thickness on the bending angle, the corrected bending angle was calculated from the determined bending angle according to the formula

Bending angle _corrected = bending angle _determined ■ Sheet thickness where the sheet thickness in mm is to be entered in 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. Table 7 gives the measured maximum bending angle. To determine the corrected bending angle, these numerical values must therefore be multiplied by the square root of the sheet thickness, which is given in Table 4.

The mechanical characteristics in Table 7 were determined after a cathodic dip coating was applied to the formed sheet metal part. During this coating process, the shaped sheet metal parts were heated to 170° C. and held at this temperature for 20 minutes. The components are then cooled to room temperature in ambient air.

Table 8 gives the structural properties of the sheet metal part. The structural proportions are given in area %. All examples according to the invention have a martensite content of more than 90%.

Table 8 also shows the properties of the fine precipitations in the structure. The precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refinement. The excretions are determined with the help of electron-optical and X-ray images (TEM and EDX) using carbon extraction replicas (known in the technical literature as “carbon extraction replicas”). The carbon pull-out impressions were made on longitudinal sections (20x30mm). The resolution of the measurement la is between 10,000 and 200,000 times. Based on these recordings, the excretions can be divided into coarse and fine excretions. All precipitations with a diameter of less than 30 nm are referred to as fine precipitations. The remaining exudates are referred to as coarse excretions. The proportion of fine waste in the total number of waste in the measuring field is determined by simply counting. The average diameter of the fine waste is also calculated using computer-aided image analysis. In the samples according to the invention, the proportion of fine precipitates is more than 90%. The mean diameter of the fine precipitates is also less than 11 nm. Table 8 also shows the grain diameter of the former austenite grains. For this purpose, the austenite grains would be reconstructed from EBSD measurements using the ARPGE software. The software parameters were:

• Orientation relationship Nishiyama-Aquarius

• Tolerance for grain identification 7°

• Tolerance for parent growth nucleation 7°

• Tolerance for parent grain growth 15°

• Minimum accepted grain size 10 pixels

A maximum orientation deviation of 5° and a minimum grain diameter of 5 pixels according to DIN EN ISO 643 were assumed for grain identification.

By way of example, FIG. 3 shows a corresponding reconstruction of the austenite from test no. 1. In this case, the mean diameter of the former austenite grains is 7.5 μm. In all examples according to the invention, the average grain diameter of the former austenite grains is less than 14 μm. The grain diameter of the former austenite grains was not determined in two tests. The entry in Table 8 is therefore "n.b." (not determined).

Table 9 gives the application-related properties of the sheet metal part. On the one hand, the area occupied by pores in the alloy layer is specified over a measuring length of 500 μm. In all examples according to the invention, this area is less than 250 μm ² .

It can be clearly seen that the coating variants and y and (|), which contain no Mg, form more pores. This applies to tests 1, 3, 4, 5, 7, 10, 12, 16 and 18. In contrast, the other layers containing Mg show fewer pores. The Nb content in the alloy layer given in Table 9 is an average of the Nb content in this layer. The Nb content in the alloy layer decreases slightly towards the surface and is approximately characterized by a linear decrease in the layer.

Table 9 also shows the proportion of the area occupied by pores with a diameter greater than or equal to 0.1 μm. In all examples according to the invention, this proportion is less than 10%. The total area of the pores and the proportion of pores larger than 0.1 μm was determined using micrographs using computer-assisted image analysis. By way of example, FIG. 1a shows a micrograph of test 1 with a fine pore structure, and FIG. are clear in FIG. 1b, the coarser pores can be seen as black spots in the alloy layer. The effects of the coarser pores after a corrosion test are shown in FIGS. 2a and 2b. FIGS. 2a and 2b show micrographs of the same tests, each after a corrosion test. For this purpose, the samples were placed in a corrosive medium and subjected to a current to simulate prolonged electrochemical corrosion. An aqueous 5% NaCl solution with a pH of 7 was used as the corrosive medium. The current was ImA/cm ² for a period of 6 hours. It can be clearly seen that in FIG. 2b the layer was almost completely detached, while in FIG. 2a the layer is still well connected to the substrate. The examples according to the invention with finer pores thus withstand corrosion much better than the reference examples with the coarser pore structure.

Table 8 also shows the welding area according to SEP 1220-2. In all variants according to the invention, the welding range is at least 0.9 and at most 1.6 kA.

Table 8 also shows the electrochemical potential. The electrochemical potential is determined according to the DIN standard "DIN 50918 (2018.09) ("resting potential measurement on homogeneous mixed electrodes"). The specified absolute value is to be understood as a reference to the standard hydrogen electrode. An aqueous 5% NaCl solution with a pH value of 7, which represents typical corrosion conditions in the automotive sector, is used as the corrosive medium in the measurement. It can be clearly seen that all samples have an electrochemical potential that is greater than -0.50V.

Preferred embodiments of the invention are described in the following sentences. The sentences are not claims.

1. Flat steel product for hot forming, comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of

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 "Gr, 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% passes.

2. Flat steel product according to sentence 1, characterized in that the following applies to the Al/Nb ratio of Al content to Nb content:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%

3. Flat steel product according to one of sentences 1-2, where at least one of the following conditions applies to the element contents:

Ti > 3.42*N

0.7 wt% <Mn+Cr< 3.5 wt%

4. Flat steel product according to one of the preceding sentences, characterized in that it has an anti-corrosion coating on at least one side.

5. Flat steel product according to sentence 4, characterized in that the anti-corrosion coating is an aluminum-based anti-corrosion coating and has an alloy layer and an Al base layer.

6. Flat steel product according to sentence 5, characterized in that the alloy layer consists of 35-60% by weight Fe, optional further components, the total content of which is limited to a maximum of 5.0% by weight, and the remainder aluminum and/ or the Al base layer of 1.0-15 wt% Si, optionally 2-4 wt% Fe, optionally up to 5.0 wt% alkali or alkaline earth metals, optionally up to 10% Zn and optional other components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum.

7. Flat steel product according to one of sentences 1-6, characterized in that the flat steel product has a continuous yield point (Rp0.2) or a yield point with a difference (ARe) between the upper yield point value (ReH) and the lower yield point value (ReL) of at most 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%.

8. A method for producing a flat steel product for hot forming with an anti-corrosion coating, comprising the following work steps: a) providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of 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%

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% passes b) through heating of the slab or thin slab at a temperature (TI) of 1100 - 1400 °C; c) optional pre-rolling of the through-heated slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) hot-rolling into a hot-rolled steel flat product, the finish rolling temperature (T3) being 750 - 1000 °C; e) optional coiling of the hot-rolled steel flat product, the coiling temperature (T4) being at most 700 °C; f) descaling the hot-rolled flat steel product; g) optional cold rolling of the flat steel product, the degree of cold rolling being at least 30%; h) annealing of the steel flat product at an annealing temperature (T5) of 650 - 900 °C; i) cooling the flat steel product to an immersion temperature (T6) which is 650-800° C., preferably 670-800° C.; j) coating the flat steel product, which has been cooled to the immersion temperature, with an anti-corrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C., preferably 680-740° C.; k) Cooling of the coated flat steel product to room temperature, the first cooling time t _mT in the temperature range between 600 °C and 450 °C being more than 10 s, in particular more than 14 s, and the second cooling time t _n in the temperature range between 400 °C and 300 °C is more than 8s, in particular more than 12s; l) optional skin-passing of the coated steel flat product.

9. procedure according to sentence 8, where the following applies to the Al/Nb ratio from the Al content to the Nb content of the steel of the slab or thin slab:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%. Method according to one of sentences 8-9, characterized in that during hot-dip coating a molten bath is used which contains the corrosion protection to be applied to the steel flat product in liquid form, which consists of up to 15% by weight Si, optionally 2-4% by weight Fe , optionally up to 5 wt. Sheet metal part formed from a flat steel product comprising a steel substrate made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of 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%

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% and an anti-corrosion coating. Sheet metal part according to Clause 11, where the following applies to the Al/Nb ratio from the Al content to the Nb content:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%. Sheet metal part according to one of sentences 11-12, characterized in that the steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite and/or lower bainite, preferably more at least partially more than 90% martensite and/or lower bainite, and wherein the former austenite grains of the martensite preferably have an average grain diameter of less than 14 μm, in particular less than 12 μm, preferably less than 10 μm. Sheet metal part according to one of clauses 11-13, characterized in that the sheet metal part at least partially has a yield point of at least 950 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1500 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 shaped sheet metal part at least partially has a bending angle of at least 30°, in particular at least 40°, preferably at least 50°.

15. Sheet metal part according to one of sentences 11-14, characterized in that the sheet metal part has fine precipitations in the structure, in particular in the form of niobium carbonitrides and/or titanium carbonitrides.

16. Sheet metal part according to one of sentences 11-15, characterized in that the electrochemical potential of the surface of the sheet metal part is at least -0.50 V in a corrosive medium.

17. Sheet metal part according to one of sentences 11-16, characterized in that the anti-corrosion coating is an aluminum-based anti-corrosion coating and comprises an alloy layer and an Al base layer, and in the cross-section of the alloy layer over a measuring length of 500 μm the surface occupied by pores in the Alloy layer is less than 250 pm ² and in particular the proportion of the surface occupied by pores with a diameter greater than or equal to 0.1 pm is less than 10%.

18. Sheet metal part according to one of sentences 11-17, characterized in that the welding area is at least 0.9 kA.

19. Sheet metal part according to one of sentences 11-18, characterized in that the Nb content in the alloy layer is greater than 0.010% by weight, preferably greater than 0.015% by weight, in particular greater than 0.018% by weight.

20. A method for producing a shaped sheet metal part comprising the following

Working steps: a) Providing a sheet metal blank from a steel flat product according to one of sentences 1-

7 b) Heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einig of the blank when it is placed in a forming tool provided for hot-press forming (step c)) at least partially has a temperature above Ms+100°C where Ms denotes the martensite start temperature. c) inserting the heated sheet metal blank into a forming tool, the transfer time t _{Tr an} required for removing it from the heating device and inserting the blank being at most 20 s, preferably at most 15 s; d) Hot-press forming of the sheet metal blank to form the shaped sheet metal part, with the blank being cooled to the target temperature T target temperature T _target and optionally held there over a period t _wz of more than 1 s at a cooling rate r _wz , at least in part more than 30 K/s, during the course of the hot-press forming becomes; e) removing the shaped sheet metal part, which has been cooled to the target temperature T _{target ,} from the tool;

21. The method according to sentence 20, wherein the temperature at least partially reached in the sheet metal blank in step b) is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

22. The method according to any one of sentences 20-22, wherein the target temperature T _target of the sheet metal part is at least partially below 400 °C, preferably below 300 °C. hyssenKrupp Steel Europe AG 207086P10WC)

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est iron and unavoidable impurities. All data in % by weight; Reference examples not according to the invention table 1 (steel grades)

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Figures partially rounded table 2 (manufacturing conditions steel flat product)

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table 3 (coating variant)

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Reference examples not according to the invention Table 4 (flat steel product)

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Figures partially rounded table 5 (hot forming parameters)

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Figures partially rounded table 6 (hot forming parameters)

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table 7 (sheet metal part)

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table 8 (structure)

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Reference examples not according to the invention Table 9 (Properties of the shaped sheet metal part)

Claims

patent claims

C: 0.06-0.5%,

Si: 0.05-0.6%,

Mn: 0.4-3.0%,

AI: 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%, where the Al/Nb ratio of Al content to Nb content is:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%

2. Flat steel product according to claim 1, wherein at least one of the following conditions applies to the element contents: 68

Ti < 3.42*N

0.7% by weight <Mn+Cr< 3.5% by weight Flat steel product according to one of the preceding claims, characterized in that it has an anti-corrosion coating on at least one side. Flat steel product according to Claim 3, characterized in that the anti-corrosion coating is an aluminum-based anti-corrosion coating and has an alloy layer and an Al base layer. Flat steel product according to claim 4, characterized in that the alloy layer consists of 35-60 wt Al base layer of 1.0-15% by weight Si, optionally 2-4% by weight Fe, optionally up to 5.0% by weight alkali or alkaline earth metals, optionally up to 10% Zn and optional further components , the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum. Flat steel product according to one of Claims 1 to 5, characterized in that the flat steel product has a continuous yield point (Rp0.2) or a yield point with a difference (ARe) between the upper yield point value (ReH) and the lower yield point value (ReL) of at most 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%. Process for producing a flat steel product for hot forming with an anti-corrosion coating, comprising the following steps: a) providing a slab or a thin slab made of steel which, in addition to iron and unavoidable impurities (in % by weight), consists of

C: 0.06-0.5%,

Si: 0.05-0.6%,

Mn: 0.4-3.0%, 69

AI: 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%, where the Al/Nb ratio of Al content to Nb content is:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%; b) soaking the slab or thin slab at a temperature (TI) of 1100-1400°C; c) optionally pre-rolling the soaked slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000-1200°C; d) hot rolling into a hot-rolled flat steel product, wherein the finish rolling temperature (T3) is 750-1000°C; e) optional coiling of the hot-rolled steel flat product, wherein the coiling temperature (T4) is at most 700°C; f) descaling the hot-rolled flat steel product; 70 g) optional cold rolling of the flat steel product, the degree of cold rolling being at least 30%; h) annealing of the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6) which is 650-800°C, preferably 670-800°C; j) coating the flat steel product, which has been cooled to the immersion temperature, with an anti-corrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C., preferably 680-740° C.; k) Cooling of the coated flat steel product to room temperature, the first cooling time t _mT in the temperature range between 600° C. and 450° C. being more than 10 s, in particular more than 14 s, and the second cooling time t _nT in the temperature range between 400° C. and 300° C is more than 8s, in particular more than 12s; l) optional skin-passing of the coated steel flat product. Method according to claim 7, characterized in that during hot-dip coating a molten bath is used which contains the corrosion protection to be applied to the steel flat product in liquid form, which consists of up to 15% by weight Si, optionally 2-4% by weight Fe, optionally up to 5% by weight alkali metals or alkaline earth metals and optionally up to 10% Zn and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum. Sheet metal part formed from a flat steel product comprising a steel substrate

Steel, which, in addition to iron and unavoidable impurities (in % by weight), consists of

C: 0.06-0.5%,

Si: 0.05-0.6%,

Mn: 0.4-3.0%,

AI: 0.06-1.0%,

Nb: 0.001-0.2%,

Ti: 0.001-0.10%,

B: 0.0005-0.01%,

P: <0.03%, 71

S: <0.02%,

N: <0.02%,

Sn: <0.03%,

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%, where the Al/Nb ratio of Al content to Nb content is:

Al/Nb < 20.0 when Mn < 1.6 wt% and

Al/Nb < 30.0 when Mn > 1.7 wt%, and an anti-corrosion coating. Sheet metal part according to claim 9, characterized in that the steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite and/or lower bainite, preferably more, at least partially more than 90% martensite and/or lower bainite, and wherein preferably the former Austenite grains of martensite have an average grain diameter of less than 14 μm, in particular less than 12 μm, preferably less than 10 μm. Sheet metal part according to one of Claims 9 to 10, characterized in that the sheet metal part at least partially has a yield strength of at least 950 MPa, in particular at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1500 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 shaped sheet metal part at least partially has an elongation at break A80 of at least 4%, preferably 72 has at least 5%, particularly preferably at least 6% and/or the shaped sheet metal part at least partially has a bending angle of at least 30°, in particular at least 40°, preferably at least 50°. Sheet metal part according to one of Claims 9 to 11, characterized in that the sheet metal part has fine precipitations in the structure, in particular in the form of niobium carbonitrides and/or titanium carbonitrides. Sheet metal part according to one of Claims 9 to 12, characterized in that the electrochemical potential of the surface of the sheet metal part is at least -0.50 V in a corrosive medium. Sheet metal part according to one of Claims 9 to 13, characterized in that the anti-corrosion coating is an aluminum-based anti-corrosion coating and comprises an alloy layer and an Al base layer, and in the cross section of the alloy layer over a measuring length of 500 μm the area occupied by pores in the alloy layer is smaller 250 μm ² and in particular the proportion of the area occupied by pores with a diameter greater than or equal to 0.1 μm is less than 10%. Sheet metal part according to one of Claims 9 to 14, characterized in that the welding area is at least 0.9 kA. Sheet metal part according to one of Claims 9 to 15, characterized in that the Nb content in the alloy layer is greater than 0.010% by weight, preferably greater than 0.015% by weight, in particular greater than 0.018% by weight. Method for producing a shaped sheet metal part, comprising the following steps: a) providing a sheet metal blank from a flat steel product according to one of Claims 1-6; b) heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Eini _g of the blank when it is placed in a forming tool provided for hot-press forming (step c)) at least partially has a temperature above Ms+100°C, where Ms denotes that of the martensite start temperature; c) inserting the heated sheet metal blank into a forming tool, the transfer time t _{Tr an} required for removing it from the heating device and inserting the blank being at most 20 s, preferably at most 15 s; d) Hot-press forming of the sheet metal blank to form the shaped sheet metal part, with the blank being cooled to the target temperature T target temperature T _target and optionally kept there over a period t _w z of more than 1s at a cooling rate r _wz that is at least partially greater than 30 K/s during the course of the hot-press forming becomes; e) removing the shaped sheet metal part, which has been cooled to the target temperature T _{target ,} from the tool.