STEEL HAVING IMPROVED PROCESSING PROPERTIES FOR WORKING AT ELEVATED TEMPERATURES

Abstract

A flat steel product for hot forming to a formed shaped sheet metal part and processes of making same. The flat steel product and the shaped sheet metal part have improved properties, especially in conjunction with an aluminum-based anticorrosion coating.

Description

The invention relates to a flat steel product for hot forming and to a process for producing such a flat steel product. The invention further relates to a shaped sheet metal part having improved properties and to a process for producing such a shaped sheet metal part from a flat steel product.

Where a “flat steel product” or else a “sheet metal product” is discussed hereinafter, this means rolled products such as steel strips or sheets from which “sheet metal blanks” (also called blanks) are divided for the production of bodywork components, for example. “Shaped sheet metal parts” or “sheet metal components” of the type according to the invention have been produced from such sheet metal blanks; the terms “shaped sheet metal part” and “sheet metal component” are used synonymously here.

All FIGURES relating to contents of the steel compositions that are reported in the present application are based on weight, unless expressly stated otherwise. All indeterminate percentage FIGURES associated with a steel alloy should therefore be regarded as FIGURES in “% by weight”. With the exception of the FIGURES for the residual austenite content of the microstructure of a shaped sheet metal part of the invention that are based on volume (reported in “% by volume”), FIGURES relating to the contents of the different microstructure constituents are each based on the area of a section of a sample of the respective product (reported in area percent, “area %”), unless explicitly stated otherwise. FIGURES given in this text for the contents of the constituents of an atmosphere are based on volume (reported in “% by volume”).

Mechanical properties, such as tensile strength, yield point, elongation, that are reported here have been ascertained by the tensile test according to DIN-EN ISO 6892-1, sample form 2 (Annex B Tab. B1) (version of 2020-06), unless explicitly stated otherwise. The bending angle is determined according to VDA Standard 238-100 for the force maximum.

The microstructure was produced from longitudinal sections that had been subjected to etching with 3% Nital (alcoholic nitric acid). The proportion of residual austenite was determined by x-ray diffractometry.

WO 2019/223854 A1 discloses a shaped sheet metal part and a method of producing such a shaped sheet metal part, which has a tensile strength of at least 1000 MPa. This shaped sheet metal part consists of a steel composed of, as well as iron and unavoidable impurities, (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 any further optional elements, especially 0.005-0.1% Nb. Moreover, the sheet metal component comprises an anticorrosion coating containing aluminum.

EP 2 553 133 B1 likewise discloses a shaped sheet metal part and a method of producing such a shaped sheet metal part.

Against the background of the prior art, the object was that of further developing a flat steel product for hot forming such that improved processing properties of the hot-formed shaped sheet metal part are achieved in conjunction with an aluminum-based anticorrosion coating. Furthermore, a method by which such shaped sheet metal parts can be produced in practice was to be specified.

The invention achieves this object by means of a flat steel product for hot forming, comprising a steel substrate composed of steel consisting of, as well as iron and unavoidable impurities, (in % by weight)

• • C: 0.30-0.50%,
  • Si: 0.05-0.6%,
  • Mn: 0.5-3.0%,
  • Al: 0.10-1.0%,
  • 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%,
  • Mo: 0.002-0.3%,
  • Ni: 0.01-0.5%,
  • V: 0.001-0.3%,
  • Ca: 0.0005-0.005%,
  • W: 0.001-1.00%.

Compared to known flat steel products, the steel substrate of the flat steel product of the invention has an aluminum content of at least 0.10% by weight, more preferably at least 0.11% by weight, especially at least 0.12% by weight, preferably at least 0.140% by weight, especially at least 0.15% by weight, preferably at least 0.16% by weight. The maximum aluminum content is 1.0% by weight, especially not more than 0.8% by weight.

In a first developed variant, the aluminum content is at least 0.10% by weight, more preferably at least 0.11% by weight, especially at least 0.12% by weight, preferably at least 0.140% by weight, especially at least 0.15% by weight, preferably at least 0.16% by weight. The maximum aluminum content in this variant is not more than 0.50% by weight, especially not more than 0.35% by weight, preferably not more than 0.25% by weight, especially not more than 0.24% by weight.

In a second developed variant, the aluminum content is at least 0.50% by weight, preferably at least 0.60% by weight, preferably at least 0.70% by weight. The maximum aluminum content in this variant is not more than 1.0% by weight, especially not more than 0.9% by weight, preferably not more than 0.80% by weight.

It is well known that aluminum (“Al”) is added as deoxidant in the production of steel. Reliable binding of the oxygen present in the steel melt requires at least 0.01% by weight of Al. Al may additionally be used for binding of contents of N that are unwanted but unavoidable for production-related reasons. Comparatively high aluminum contents have been avoided to date since the Ac3 temperature also moves upward with the aluminum content. This has an adverse effect on austenitization, which is important for hot forming. However, it has been found that elevated aluminum contents surprisingly lead to positive effects in conjunction with an aluminum-based anticorrosion coating.

In the coating of the flat steel product with an aluminum-based anticorrosion coating and in the subsequent hot forming of sheet metal blanks divided therefrom to give shaped sheet metal parts, there is diffusion of iron from the steel substrate into the liquid anticorrosion coating. This forms, in the interdiffusion zone, iron aluminide compounds having relatively high density via a multistage phase transformation (Fe2Al5→Fe2Al→FeAl→Fe3Al). The formation of such denser phases is associated with higher consumption of aluminum than in the case of lower-density phases. This locally higher aluminum consumption leads to formation of pores (vacancies) in the resultant phase. These pores are preferably formed in the transition region between steel substrate and anticorrosion coating, where the proportion of aluminum available is shaped to a significant degree 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 region.

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

• • The pores reduce mechanical integrity in this region. This can result in faster layer detachment under corrosive stress.
  • Moreover, there is a reduction in the transmissible force at the connection site of two components after bonding or welding.
  • The pores lead to altered current pathways in the material on resistance point welding that have an adverse effect on suitability for welding and thus reduce the welding range.
  • Even the pores themselves facilitate initiation of cracking and propagation of cracking on static and dynamic bending.

It has been found that, surprisingly, increasing the aluminum content (“Al”) in the steel substrate to the lower limits described or higher can achieve a distinct reduction in pore formation on coating with an aluminum-based anticorrosion coating and subsequent hot forming. Especially in the transition region between steel substrate and anticorrosion coating, the locally higher aluminum consumption in the case of formation of denser iron aluminide compounds can be at least partly compensated for by the aluminum content of the steel substrate, such that the formation of pores, especially a band of pores, is suppressed.

In the case of an excessively high Al content, especially in the case of contents of more than 1.0% by weight of Al, there is the risk that Al oxides will form at the surface of a product manufactured from steel material alloyed in accordance with the invention, which would worsen the wetting characteristics in the case of hot dip coating. Moreover, in the case of relatively high Al contents, the formation of nonmetallic Al-based inclusions is favored, which, as coarse inclusions, have an adverse effect on crash characteristics. Therefore, the Al content chosen is preferably below the upper limits already mentioned.

The bending characteristics of the sheet metal component are supported in particular by the inventive niobium content (“Nb”) of at least 0.001% by weight. The niobium content is preferably at least 0.005% by weight, especially at least 0.010% by weight, preferably at least 0.015% by weight, more preferably at least 0.020% by weight, especially at least 0.024% by weight, preferably at least 0.025% by weight.

The niobium content specified leads more particularly, in the method described hereinafter for production of a flat steel product for hot forming with an anticorrosion coating, to a distribution of niobium carbonitrides that leads to a particularly fine hardening microstructure in the subsequent hot forming operation. During the cooling after the hot dip coating, the coated flat steel product is kept within a temperature range between 400° C. and 300° C. for a certain period of time. Within this temperature range, there is still a certain diffusion rate of carbon in the steel substrate, while thermodynamic solubility is very low. Carbon thus diffuses to and accumulates at lattice defects. Lattice defects are caused in particular by dissolved niobium atoms which widen out the atomic lattice by virtue of their much higher atomic volume, and hence increase the size of the tetrahedral and octahedral gaps in the atomic lattice, such that the local solubility of C is increased. Consequently, clusters of C and Nb arise in the steel substrate, which are then transformed to very fine precipitates in the subsequent austenitization step of hot forming and act as additional austenite grains. The result is therefore a refined austenite microstructure with relatively small austenite grains and hence also a refined hardening microstructure.

This also relates in particular to the ferritic interdiffusion layer that forms in the hot forming operation. The refined ferritic microstructure in the interdiffusion layer promotes the reduction of tendencies to initiate cracking under flexural loads.

However, too high an Nb content leads to worsened recrystallizability. Therefore, the Nb content is not more than 0.2% by weight. Further preferably, the niobium content is not more than 0.20% by weight, especially not more than 0.15% by weight, preferably not more than 0.10% by weight, especially not more than 0.05% by weight.

Aluminum and niobium both have an influence on grain refining in austenitization in the hot forming process. It has been found that Al, as well as Nb, especially refines grain growth at elevated temperatures in austenite (for example at more than 1200° C.) via comparatively early formation (i.e. taking place at relatively high temperatures) of AlN. The formation of AlN is thermodynamically favored over the formation of NbN or NbC. The precipitation of AlN has a grain-refining effect here in austenite and hence a toughness-improving effect. Rising Al/Nb ratios improve this effect. It is therefore optionally the case that, for the Al/Nb ratio of Al content to Nb content:

$1\le \mathrm{Al}/\mathrm{Nb};$

the Al/Nb ratio is preferably ≥2, especially ≥3. At the same time, an excessively high ratio of Al/Nb has the effect that AlN formation is no longer as advantageously fine, but rather increasingly coarser AlN particles occur, which again reduces the grain-refining effect. It has been found that this effect occurs earlier in the case of low manganese contents than in the case of higher manganese contents since the Ac3 temperature decreases with rising manganese content. It is therefore advantageous, optionally in the case of low manganese contents of not more than 1.6% by weight, to establish a ratio of Al/Nb for which:

$\mathrm{Al}/\mathrm{Nb}\le 20.0,$

which corresponds roughly to an atomic ratio of the two elements of ≤6. Preferably, when Mn≤1.6% by weight, the Al/Nb ratio is ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

In the case of higher manganese contents of Mn≥1.7% by weight, by contrast, higher ratios are also possible. It is therefore advantageous, optionally in the case of higher manganese contents of 1.7% by weight or more, to establish a ratio of Al/Nb for which:

$\mathrm{Al}/\mathrm{Nb}\le 30.0.$

Preferably, when Mn≥1.7% by weight, the Al/Nb ratio is ≤28.0, especially ≤26.0, preferably ≤24.0, more preferably ≤22.0, preferably ≤20.0, especially ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

Irrespective of the manganese content, it is thus optionally preferable to establish a ratio of Al/Nb for which:

$\mathrm{Al}/\mathrm{Nb}\le 20.0.$

The Al/Nb ratio is preferably ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

Carbon (“C”) is present in the steel substrate of the flat steel product in contents of 0.30-0.50% by weight. C contents set at such a level contribute to the hardenability of the steel in that they delay ferrite and bainite formation and stabilize the residual austenite in the microstructure.

However, high C contents can adversely affect weldability. In order to improve weldability, the carbon content can be adjusted to 0.45% by weight, preferably to not more than 0.42% by weight, more preferably 0.40% by weight, preferably not more than 0.38% by weight, especially not more than 0.35% by weight.

In order to be able to utilize the positive effects of the presence of C particularly reliably, C contents of at least 0.32% by weight, preferably 0.33% by weight, especially at least 0.34% by weight, preferably at least 0.35% by weight, may be provided. At these contents, with regard to the further provisions of the invention, it is possible to reliably achieve tensile strengths of the shaped sheet metal part of at least 1700 MPa, especially at least 1800 MPa, after hot press forming.

Silicon (“Si”) is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silico-manganese as alloying agent, which has a beneficial effect on production costs. A hardening effect is already established over and above an Si content of 0.05% by weight. A significant rise in strength occurs over and above an Si content of at least 0.15% by weight, especially at least 0.20% by weight. Si contents above 0.6% by weight have a disadvantageous effect on coating characteristics, especially in the case of Al-based coatings. Si contents of not more than 0.50% by weight, especially not more than 0.30% by weight, are preferably established in order to improve the surface quality of the coated flat steel product.

Manganese (“Mn”) acts as a hardening element in that it significantly delays ferrite and bainite formation. In the case of manganese contents of less than 0.4% by weight, during press hardening, significant proportions of ferrite and bainite are formed even in the case of very rapid cooling rates, which should be avoided. Mn contents of at least 0.5% by weight, preferably of at least 0.7% by weight, especially of at least 0.8% by weight, preferably of at least 0.9% by weight, especially of at least 1.00% by weight, preferably of at least 1.05% by weight, more preferably of at least 1.10% by weight, are advantageous when a martensitic microstructure is to be ensured, especially in regions of relatively high forming. Manganese contents of more than 3.0% by weight have an adverse effect on processing properties, and therefore the Mn content of flat steel products of the invention is limited to not more than 3.0% by weight, preferably not more than 2.5% by weight. Weldability in particular is greatly restricted, and therefore the Mn content is limited preferably to not more than 1.6% by weight and especially to 1.30% by weight, especially to not more than 1.20% by weight. Manganese contents of not more than 1.6% by weight are additionally also preferred for economic reasons.

Titanium (“Ti”) is a microalloy element which is included in the alloy in order to contribute to grain refining, and at least 0.001% by weight of Ti, especially at least 0.004% by weight, preferably at least 0.010% by weight of Ti, should be added for sufficient availability. There is a distinct deterioration in cold rollability and recrystallizability over and above 0.10% by weight of Ti, and therefore greater Ti contents should be avoided. In order to improve cold rollability, the Ti content may be restricted preferably to 0.08% by weight, especially to 0.038% by weight, more preferably to 0.020% by weight, especially 0.015% by weight. Titanium additionally has the effect of binding nitrogen and hence making it possible for boron to display its greatly ferrite-inhibiting effect. Therefore, in a preferred development, the titanium content is more than 3.42 times the nitrogen content in order to achieve sufficient binding of nitrogen.

Boron (“B”) is included in the alloy in order to improve the hardenability of the flat steel product in that boron atoms or boron precipitates adjoining austenite grain boundaries reduce the grain boundary energy, which suppresses the nucleation of ferrite during press hardening. A distinct effect on hardenability occurs in the case of contents of at least 0.0005% by weight, preferably at least 0.0007% by weight, especially at least 0.0010% by weight, especially at least 0.0020% by weight. In the case of contents exceeding 0.01% by weight, by contrast, there is increased formation of boron carbides, boron nitrides or boron nitrocarbides, which in turn constitute preferred nucleation sites for the nucleation of ferrite and lower the hardening effect again. For that reason, the boron content is limited to not more than 0.01% by weight, preferably not more than 0.0100% by weight, preferably not more than 0.0050% by weight, especially not more than 0.0035% by weight, especially not more than 0.0030% by weight, preferably not more than 0.0025% by weight.

Phosphorus (“P”) and sulfur (“S”) are elements that are introduced into the steel as impurities by iron ore and cannot be eliminated entirely in the industrial scale steelworks process. The P content and the S content should be kept as low as possible since there is deterioration in mechanical properties, for example notched impact resistance, with increasing P content or S content. Moreover, there is incipient embrittlement of the martensite over and above P contents of 0.03% by weight, and therefore the P content of a flat steel product of the invention is limited to not more than 0.03% by weight, especially not more than 0.02% by weight. The S content of a flat steel product of the invention is limited to not more than 0.02% by weight, preferably not more than 0.0010% by weight, especially not more than 0.005% by weight.

Nitrogen (“N”) is likewise present as an impurity in the steel in small amounts owing to the steel manufacturing process. The N content should be kept as low as possible and should be not more than 0.02% by weight. Especially in the case of alloys containing boron, nitrogen is harmful since the formation of boron nitrides prevents the transformation-retarding effect of boron, and therefore the nitrogen content in this case should preferably be not more than 0.010% by weight, especially not more than 0.007% by weight.

Further typical impurities are tin (“Sn”) and arsenic (“As”). The Sn content is not more than 0.03% by weight, preferably not more than 0.02% by weight. The As content is not more than 0.01% by weight, especially not more than 0.005% by weight.

As well as the above-elucidated impurities P, S, N, Sn and As, it is also possible for further elements to be present in the steel as impurities. These further elements are combined under the “unavoidable impurities”. The content of these “unavoidable impurities” preferably adds up to not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloy elements Cr, Cu, Mo, Ni, V, Ca and W described hereinafter for which a lower limit is specified may also be present in the steel substrate as unavoidable impurities in contents below the respective lower limit. In that case, they are likewise counted among the “unavoidable impurities”, the total content of which is limited to not more than 0.2% by weight, preferably not more than 0.1% by weight.

Chromium, copper, molybdenum, nickel, vanadium, calcium and tungsten may optionally each be included in the alloy individually or in combination with one another as part of the steel of a flat steel product of the invention.

Chromium (“Cr”) suppresses the formation of ferrite and pearlite during accelerated cooling of a flat steel product of the invention and enables complete martensite formation even in the case of relatively low cooling rates, which achieves an increase in hardenability.

These stated effects are established over and above a content of 0.01% by weight, and a content of at least 0.10% by weight, preferably at least 0.15% by weight, has been found to be useful for reliable processing in practice. However, excessively high contents of Cr impair the coatability of the steel. Therefore, the Cr content of the steel of a steel substrate is limited to not more than 1.0% by weight, preferably not more than 0.80% by weight, especially not more than 0.75% by weight, preferably not more than 0.50% by weight, especially not more than 0.30% by weight.

Vanadium (V) may optionally be included in the alloy in contents of 0.001-1.0% by weight. The vanadium content is preferably not more than 0.3% by weight. For reasons of cost, not more than 0.2% by weight of vanadium is included in the alloy.

Copper (Cu) may optionally be included in the alloy in order to increase hardenability in the case of additions of at least 0.01% by weight, preferably at least 0.010% by weight, especially at least 0.015% by weight. In addition, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. In the case of an excessively high Cu content, there is a distinct deterioration in hot rollability owing to low-melting Cu phases at the surface, and therefore the Cu content is limited to not more than 0.2% by weight, preferably not more than 0.1% by weight, especially not more than 0.10% by weight.

Molybdenum (Mo) may optionally be added in order to improve process stability, since it distinctly slows ferrite formation. Over and above contents of 0.002% by weight, there is dynamic formation of molybdenum-carbon clusters up to and including ultrafine molybdenum carbides at the grain boundaries, which distinctly slow the mobility of the grain boundary and hence diffusive phase transformations. Moreover, molybdenum reduces grain boundary energy, which reduces the nucleation rate of ferrite. The Mo content is preferably at least 0.004% by weight, especially at least 0.01% by weight. Because of the high costs associated with alloying of molybdenum, the content should be not more than 0.3% by weight, especially not more than 0.10% by weight, preferably not more than 0.08% by weight.

Nickel (Ni) stabilizes the austenitic phase and may optionally be included in the alloy in order to reduce the Ac3 temperature and to suppress the formation of ferrite and bainite. Nickel additionally has a positive influence on hot rollability, especially when the steel contains copper. Copper worsens hot rollability. In order to counter the adverse effect of copper on hot rollability, it is possible to include 0.01% by weight of nickel in the alloy as part of the steel; the Ni content is preferably at least 0.015% by weight, preferably at least 0.020% by weight. For economic reasons, the nickel content should remain limited to not more than 0.5% by weight, especially not more than 0.20% by weight. The Ni content is preferably not more than 0.10% by weight.

Calcium (Ca) in steels serves for indentation of nonmetallic inclusions, especially of manganese sulfides. Rounded indentation distinctly reduces the adverse effect of the inclusions on hot formability, sustained strength and toughness. In order to utilize this effect in the case of a flat steel product of the invention as well, a flat steel product of the invention may optionally contain at least 0.0005% by weight of Ca, especially at least 0.0010% by weight, preferably at least 0.0020% by weight. The maximum Ca content is 0.01% by weight, especially not more than 0.007% by weight, preferably not more than 0.005% by weight. In the case of excessively high Ca contents, there is a growing probability that nonmetallic inclusions involving Ca will form, which worsen the purity of the steel and also the toughness thereof. For that reason, an upper limit of the Ca content of not more than 0.005% by weight, preferably not more than 0.003% by weight, especially not more than 0.002% by weight, preferably not more than 0.001% by weight, should be observed.

Tungsten (W) may optionally be included in the alloy in contents of 0.001-1.0% by weight in order to slow ferrite formation. A positive effect on hardenability already arises in the case of W contents of at least 0.001% by weight. For reasons of cost, not more than 1.0% by weight of tungsten is included in the alloy.

In preferred developments, the sum total of the Mn content and the Cr content (“Mn+Cr”) is more than 0.7% by weight, especially more than 0.8% by weight, preferably more than 1.1% by weight. Below a minimum sum total of the two elements, the necessary transformation-inhibiting action is lost. Irrespective of that, the sum total of the Mn content and the Cr content is less than 3.5% by weight, preferably less than 2.5% by weight, especially less than 2.0% by weight, more preferably less than 1.5% by weight. The upper limits for the two elements are the result of assurance of coating performance and for assurance of adequate welding characteristics.

The above elucidations relating to element contents and preferred limits thereof are correspondingly applicable to the process described hereinafter for production of a flat steel product, to the shaped sheet metal part and to the process for producing a shaped sheet metal part.

The flat steel product preferably comprises an anticorrosion coating in order to protect the steel substrate from oxidation and corrosion on hot forming and in the use of the steel component produced.

In a specific embodiment, the flat steel product preferably comprises an aluminum-based anticorrosion coating. This anticorrosion coating may have been applied to one or both sides of the flat steel product. Both sides of the flat steel product refer to the two opposite large faces of the flat steel product. The narrow faces are referred to as edges.

Such an anticorrosion coating is preferably produced by hot dip coating of the flat steel product. This involves passing the flat steel product through a liquid melt consisting of up to 15% by weight of Si, preferably more than 1.0% by weight of Si, optional 2-4% by weight of Fe, optionally up to 5% by weight of alkali metals 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, preferably up to 10% by weight of Zn and optionally further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance.

In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 5-15% by weight, especially 7-12% by weight, especially 8-10% by weight.

In a preferred variant, the optional content of alkali metals or alkaline earth metals in the melt comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals in the melt may especially comprise at least 0.0015% by weight of Ca, especially at least 0.01% by weight of Ca.

In the course of hot dip coating, iron diffuses out of the steel substrate into the liquid coating, such that the anticorrosion coating of the flat steel product on solidification especially has an alloy layer and an Al base layer.

The alloy layer lies atop and directly adjoins the steel substrate. The alloy layer is formed essentially from aluminum and iron. The other elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists 35-60% by weight of Fe, preferably α-iron, optional further constituents, the total contents of which are limited to not more than 5.0% by weight, preferably 2.0%, and aluminum as the balance, where the Al content preferably rises in the surface direction. The optional further constituents especially include the other constituents of the melt (i.e. silicon, with or without alkali metals or alkaline earth metals, especially Mg and/or Ca) and the other constituents of the steel substrate in addition to iron.

The Al base layer lies atop and directly adjoins the alloy layer. The composition of the Al base layer preferably corresponds to the composition of the melt in the melt bath. This means that it consists of 0.1-15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5% by weight of alkali metals or alkaline earth metals, preferably up to 1.0% by weight of alkali metals or alkaline earth metals, optionally up to 15% by weight of Zn, preferably up to 10% by weight of Zn, and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance.

In a preferred variant of the Al base layer, the optional content of alkali metals or alkaline earth metals comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals in the Al base layer especially comprises at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca.

In a further-preferred variant of the anticorrosion coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

The anticorrosion coating preferably has a thickness of 5-60 μm, especially of 10-40 μm. The coatweight of the anticorrosion coating is especially

$30-360\frac{g}{m^2}$

in the case of double-sided anticorrosion coatings, or

$15-180\frac{g}{m^2}$

in the case of the single-sided variant. The coatweight of the anticorrosion coating is preferably

$100-200\frac{g}{m^2}$

in the case of double-sided coatings, or

$50-100\frac{g}{m^2}$

for single-sided coatings. The coatweight of the anticorrosion coating is more preferably

$120-180\frac{g}{m^2}$

in the case of double-sided coatings, or

$60-90\frac{g}{m^2}$

for single-sided coatings.

The thickness of the alloy layer is preferably less than 20 μm, more preferably less than 16 μm, especially less than 12 μm, more preferably less than 10 μm, preferably less than 8 μm, especially less than 5 μm. The thickness of the Al base layer is found from the difference in the thicknesses of anticorrosion coating and alloy layer. The thickness of the Al base layer even in the case of thin anticorrosion coatings is preferably at least 1 μm.

In a preferred variant, the flat steel product comprises an oxide layer atop the anticorrosion coating. The oxide layer especially lies atop the Al base layer and preferably forms the outer conclusion of the anticorrosion coating.

The oxide layer especially consists to an extent of more than 80% by weight of oxides, where the main proportion of the oxides (i.e. more than 50% by weight of the oxides) is aluminum oxide. The oxide layer optionally includes, in addition to aluminum oxide, hydroxides and/or magnesium oxide alone or as a mixture. The remainder of the oxide layer which is not composed of 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 constituent of the Al base layer, zinc oxide constituents are also present in the oxide layer.

The oxide layer of the flat steel product preferably has a thickness of greater than 50 nm. In particular, the thickness of the oxide layer is not more than 500 nm.

In an alternative configuration, the flat steel product comprises a zinc-based anticorrosion coating. This anticorrosion coating may have been applied to one or both sides of the flat steel product. Both sides of the flat steel product refer to the two opposite large faces of the flat steel product. The narrow faces are referred to as edges.

Such a zinc-based anticorrosion coating preferably comprises 0.2-6.0% by weight of Al, 0.1-10.0% by weight of Mg, optionally 0.1-40% by weight of manganese or copper, optionally 0.1-10.0% by weight of cerium, optionally not more than 0.2% by weight of further elements, unavoidable impurities, and zinc as the balance. In particular, the Al content is not more than 2.0% by weight, preferably not more than 1.5% by weight. The Mg content is especially not more than 3.0% by weight, preferably not more than 1.0% by weight. The anticorrosion coating may be applied by hot dip coating or by physical gas phase deposition or by electrolytic methods.

A further-developed flat steel product preferably has a high uniform expansion Ag of at least 10.0%, especially at least 11.0%, preferably at least 11.5%, especially at least 12.0%.

In addition, the yield point of a particularly refined flat steel product has a continuous progression or is only slightly pronounced. What is meant by a continuous progression in the context of the application is that there is no pronounced yield point. A yield point with continuous progression can also be referred to as yield point Rp0.2. A slightly pronounced yield point in the present context is understood to mean a pronounced yield point where the difference ΔRe between the upper yield point limit ReH and lower yield point limit ReL is not more than 45 MPa. Accordingly:

$\Delta Re=\left(\mathrm{ReH}-R\mathrm{eL}\right)\le 45\mathrm{MPa}\mathrm{with}\mathrm{ReH}=\mathrm{upper}\mathrm{yield}\mathrm{point}\mathrm{in}\mathrm{MPa}\mathrm{and}\mathrm{ReL}=\mathrm{lower}\mathrm{yield}\mathrm{point}\mathrm{in}\mathrm{MPa}.$

Particularly good aging resistance can be achieved in the case of flat steel products for which the difference ΔRe is not more than 25 MPa.

A specifically further-developed flat steel product has an elongation at break A80 of at least 15%, especially at least 18%, preferably at least 19%, more preferably at least 20%.

In a preferred execution variant, the flat steel product has fine precipitates in the microstructure, especially in the form of niobium carbonitrides and/or titanium carbonitrides.

Fine precipitates in the context of this application refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates.

In a preferred configuration, the fine precipitates in the microstructure are round precipitates having a diameter of up to 20 nm. In particular, the diameter is at least 2 nm. Further preferably, the diameter is not more than 15 nm, especially not more than 12 nm.

In a further preferred configuration, the flat steel product has largely fine precipitates in the microstructure. In the context of this application, largely fine precipitates is understood to mean that more than 80%, preferably more than 90%, of all precipitates are fine precipitates. This means that more than 80%, preferably more than 90%, of all precipitates have a diameter of less than 30 nm.

In a preferred execution variant, the density of the fine precipitates is at least 0.018 per 100 nm², preferably at least 0.020 per 100 nm².

The fine precipitates result in a particularly fine microstructure having small grain diameters. The fine microstructure makes it more homogeneous. The result is an improvement in the mechanical properties, especially lower crack sensitivity and hence improved bending properties and higher elongation at break. This also results in better toughness with more marked necking characteristics on fracture.

The precipitates in the flat steel product and in the shaped sheet metal part (see below) are determined with the aid of electron scattering and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas are produced on longitudinal sections (20×30 mm). The resolution of the measurement is between 10 000-fold and 200 000-fold. Using these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates. By simple counting, the proportion of fine precipitates in the total number of precipitates in the measurement field and the total number of fine precipitates in the measurement field are ascertained. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis.

The flat steel product is especially developed such that it has regions of different thickness. The process for producing a shaped sheet metal part which is described below is preferably likewise developed such that such a flat steel product with regions of different thickness is used. In addition, the shaped sheet metal part elucidated hereinafter is developed such that it has regions of different thickness.

Regions of different thickness of the flat steel product (called “tailored blanks”) can be created in various ways:

• • Special cold rolling passes in which individual regions are more intensely or frequently rolled result in a lower material thickness in these regions and hence a lower thickness (called “tailor rolled blanks”).
  • By welding (typically by laser welding), sheet metal blanks of different thickness or/and different material are bonded to one another in order to achieve a coherent sheet metal blank having regions of different thickness (called “tailor welded blanks”).
  • By resistance spot welding or laser welding, patches are applied to an existing sheet metal blank in order to thicken regions thereof. Alternatively, the patches may also be applied by means of structural adhesives.

Regions of different thickness have the advantage that individual areas of the final shaped sheet metal part (see below) can be specifically strengthened. In this way, it is possible to give those parts that are subject to exceptional stresses (for example during a crash) a more stable configuration, whereas other parts have a thinner configuration in order to reduce the weight of the component. The result is thus a weight-optimized component that has specific strengthening in the regions of high stresses.

The process of the invention for production of a flat steel product for hot forming having an anticorrosion coating comprises the following steps:

• • a) providing a slab or thin slab consisting of steel consisting of, as well as iron and unavoidable impurities, (in % by weight)
    • C: 0.30-0.50%,
    • Si: 0.05-0.6%,
    • Mn: 0.5-3.0%,
    • Al: 0.10-1.0%,
    • 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%,
    • Mo: 0.002-0.3%,
    • Ni: 0.01-0.5%
    • V: 0.001-0.3%
    • Ca: 0.0005-0.005%
    • W: 0.001-1.00%;
  • b) through-heating the slab or thin slab at a temperature (T1) of 1100-1400° C.;
  • c) optionally pre-rolling the through-heated slab or thin slab to an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;
  • d) hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.;
  • e) optionally coiling the hot-rolled flat steel product, where the coiling temperature (T4) is at most 700° C.;
  • f) optionally descaling the hot-rolled flat steel product;
  • g) optionally cold rolling the flat steel product, where the degree of cold rolling is at least 30%;
  • h) annealing the flat steel product at an annealing temperature (T5) of 650-900° C.;
  • i) cooling the flat steel product to a dipping temperature (T6) of 650-800° C., preferably 670-800° C.;
  • j) coating the flat steel product cooled to the dipping temperature with an anticorrosion coating by hot dip coating in a melt bath with a melt temperature (T7) 660-800° C., preferably 680-740° C.;
  • k) cooling the coated flat steel product to room temperature, where the first cooling time t_MT in the temperature range between 600° C. and 450° C. is more than 10 s, especially more than 14 s, and the second cooling time tur in the temperature range between 400° C. and 300° C. is more than 8 s, especially more than 12 s;
  • l) optionally skin pass rolling the coated flat steel product.

In step a), a semifinished product of a corresponding composition to the alloy defined in accordance with the invention for the flat steel product is provided. This may be a slab produced by conventional continuous slab casting or by continuous thin slab casting.

In step b), the semifinished product is through-heated at a temperature (T1) of 1100-1400° C. If the semifinished product is to be cooled after the casting, the semifinished product is first reheated to 1100-1400° C. for through-heating. The through-heating temperature should be at least 1100° C. in order to ensure good formability for the subsequent rolling process. The through-heating temperature should not be more than 1400° C. in order to avoid fractions of molten phases in the semifinished product.

In the optional step c), the semifinished product is pre-rolled to an intermediate product. Thin slabs are typically not subjected to any pre-rolling. Thick slabs that are to be rolled out to hot strips can be subjected to pre-rolling if required. In that case, the temperature of the intermediate product (T2) at the end of the pre-rolling should be at least 1000° C. in order that the intermediate product contains sufficient heat for the subsequent step of finish rolling. However, high rolling temperatures can also promote grain growth during the rolling operation, which has an adverse effect on the mechanical properties of the flat steel product. In order to minimize grain growth during the rolling operation, the temperature of the intermediate product at the end of the pre-rolling should not be more than 1200° C.

In step d), the slab or thin slab or, if step c) has been performed, the intermediate product is rolled to give a hot-rolled flat steel product. If step c) has been performed, the intermediate product is typically finish-rolled immediately after the pre-rolling. The finish-rolling typically commences no later than 90 s after the end of the pre-rolling. The slab, the thin slab or, if step c) has been performed, the intermediate product are rolled to completion at a final rolling temperature (T3). The final rolling temperature, i.e. the temperature of the completely hot-rolled flat steel product at the end of the hot rolling operation, is 750-1000° C. In the case of final rolling temperatures of less than 750° C., the amount of free vanadium decreases, since relatively large amounts of vanadium carbides are precipitated. The vanadium carbides that precipitate in the course of 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, as conducted prior to hot dip coating, for example. The final rolling temperature is limited to values of not more than 1000° C. in order to prevent coarsening of the austenite grains. Moreover, final rolling temperatures of not more than 1000° C. are of relevance for process technology purposes in order to establish coiling temperatures (T4) of less than 700° C.

The hot rolling of the flat steel product can be effected in the form of a continuous hot strip rolling operation or of a reversing rolling operation. Step e) in the case of continuous hot strip rolling provides for optional coiling of the hot-rolled flat steel product. For this purpose, the hot strip, after the hot rolling, is cooled down to a coiling temperature (T4) within less than 50 s. The cooling medium used for this purpose may, for example, be water, air or a combination of the two. The coiling temperature (T4) should be not more than 700° C. in order to avoid the formation of large vanadium carbides. There is in principle no lower limit to the coiling temperature. However, coiling temperatures of at least 500° C. have been found to be favorable for cold rollability. Subsequently, the coiled hot strip is cooled down to room temperature in a conventional manner under air.

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

The descaled hot-rolled flat steel product, prior to the annealing treatment in step g), may optionally be subjected to cold rolling in order, for example, to meet higher demands on the thickness tolerances of the flat steel product. The degree of cold rolling (DCR) should be at least 30% in order to introduce sufficient deformation energy into the flat steel product for rapid recrystallization. The degree of cold rolling DCR is understood to mean the quotient of the decrease in thickness on cold rolling ΔdCR divided by the hot strip thickness d:

$\mathrm{DCR}=\Delta \mathrm{dCR}/d$

with ΔdCR=decrease in thickness on cold rolling in mm and d=hot strip thickness in mm, where the decrease in thickness ΔdCR is calculated from the difference in thickness of the flat steel product before cold rolling relative to the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is typically a hot strip of hot strip thickness d. The flat steel product after cold rolling is typically also referred to as cold strip. The degree of cold rolling may in principle assume very high values of more than 90%. However, degrees of cold rolling of not more than 80% have been found to be favorable for avoidance of 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 kept 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, after the annealing, is cooled down to a dipping temperature (T6) in order to prepare it for the subsequent coating treatment. The dipping temperature is less than the annealing temperature and is matched to the temperature of the melt bath. The dipping temperature is 600-800° C., preferably at least 650° C., more preferably at least 670° C., more preferably at most 700° C.

For particularly homogeneous interfacial layer formation, it is important that there is adequate thermal energy in the interfacial layer between steel substrate and aluminum melt. This is not the case at temperatures lower than 600° C., such that unwanted compounds can form, the later reconversion of which can lead to pores. Over and above the preferred dipping temperatures, there is another significant increase in the diffusion rate of iron into aluminum, such that more iron can diffuse into the still-liquid interfacial layer even at the start of the coating process. The duration of the cooling of the annealed flat steel product from the annealing temperature T5 to the dipping temperature T6 is preferably 10-180 s. In particular, the dipping temperature T6 differs from the temperature of the melt bath T7 by not more than 30 K, especially not more than 20 K, preferably not more than 10 K.

The flat steel product is subjected to a coating treatment in step j). The coating treatment is preferably effected by continuous hot dip coating. The coating can be applied only on one side, on both sides or on all sides of the flat steel product. The coating treatment is preferably effected as a hot dip coating process, especially as a continuous process. The flat steel product typically comes into contact with the melt bath on all sides, such that it is coated on all sides. The melt bath containing the alloy to be applied to the flat steel product in liquid form is typically at a temperature (T7) of 660-800° C., preferably 680-740° C. Aluminum-based alloys have been found to be particularly suitable for coating of aging-resistant flat steel products with an anticorrosion coating. In such a case, the melt bath contains up to 15% by weight of Si, preferably more than 1.0%, optionally 2-4% by weight, of Fe, optionally up to 5% by weight of alkali metals 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, preferably up to 10% by weight of Zn and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance. In a preferred variant, the Si content of the melt is 1.0-3.5% by weight or 7-12% by weight, especially 8-10% by weight. In a preferred variant, the optional content of alkali metals or alkaline earth metals in the melt comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals in the melt may especially comprise at least 0.0015% by weight of Ca, especially at least 0.01% by weight of Ca.

After the coating treatment, the coated flat steel product is cooled down to room temperature in step k). A first cooling time t_MT in the temperature range between 600° C. and 450° C. (moderate temperature range MT) is more than 5 s, preferably more than 10 s, especially more than 14 s, and a second cooling time tur in the temperature range between 400° C. and 300° C. (low temperature range LT) is more than 4 s, preferably more than 8 s, especially more than 12 s.

The first cooling time t_MT in the temperature range between 600° C. and 450° C. (moderate temperature range MT) may be achieved by gradual, continuous cooling or else by holding at a temperature for a certain time within this temperature range. Intermediate heating is even possible. All that is important is that the flat steel product remains within the temperature range between 600° C. and 450° C. at least for a period of time of cooling time t_MT. Within this temperature range, there is on the one hand a significant diffusion rate of iron into aluminum, and on the other hand the diffusion of aluminum into steel is inhibited since the temperature is below half the melting temperature of steel. This enables diffusion of iron into the anticorrosion coating without significant diffusion of aluminum into the steel substrate.

The diffusion of iron into the anticorrosion coating has several advantages: firstly, the melting of the anticorrosion coating is delayed on austenitization prior to press hardening. Secondly, there is homogenization of the coefficients of thermal expansion of anticorrosion coating and substrate. This means that the transition region between the coefficients of thermal expansion of substrate and surface becomes broader, which reduces thermal stresses on reheating.

At the same time, the diffusion of aluminum into the steel substrate would have considerable disadvantages: by virtue of the very high affinity of aluminum for nitrogen, a high aluminum content can have the effect that nitrogen is removed from fine precipitates, such as niobium carbonitrides or titanium carbonitrides, and there is instead preferential formation of coarse precipitates, such as aluminum nitrides, at the grain boundaries. This would worsen crash performance, and also reduce the bending angle. Moreover, this destabilizes the fine precipitates (for example the niobium-containing precipitates) in the uppermost substrate region, which are important for many preferred properties. In addition, the inhomogeneous diffusion rate of aluminum in the steel substrate into ferrite compared to pearlite/bainite/martensite would lead to an inhomogeneous distribution of Al in the edge layer of the steel substrate. This should likewise be avoided in order to improve crash performance and bending performance. These disadvantages of the diffusion of aluminum into the steel substrate are therefore reduced or avoided by inhibition.

By virtue of the preferred first cooling time t_MT (14 s), there is an increase in the iron concentration in the interfacial transition layer to such an extent that this further reduces the activity of aluminum in the coating directly at the substrate boundary. This then leads to an even further decrease in aluminum uptake into the substrate on austenitization prior to the press hardening with the associated advantages described above.

The second cooling time tur in the temperature range between 400° C. and 300° C. (low temperature range LT) can likewise be implemented by gradual, continuous cooling or else by holding at a temperature for a certain time within this temperature range. Intermediate heating is even possible. All that is important is that the flat steel product remains within the temperature range between 400° C. and 300° C. at least for a period of time of cooling time t_LT.

Within this temperature range, there is still a certain diffusion rate of carbon within the steel substrate, while thermodynamic solubility is very low. Carbon thus diffuses to and collects at lattice defects, for example dissolved Nb atoms. These widen the atomic lattice by virtue of their much higher atomic volume and hence increase the size of the tetrahedra and octahedral gaps in the atomic lattice, such that the local solubility of C is increased. This results in clusters of C and Nb that are then transformed to very fine precipitates in the austenitization step of the hot forming and lead to a refined austenite microstructure and hence also hardening microstructure, and to a reduction in the free hydrogen content.

In the case of the preferred hold time of more than 12 s, very fine iron carbides (called transition carbides) are additionally formed, which in turn dissolve very quickly again on austenitization and lead to additional austenite seeds and hence to an even finer austenite microstructure and hence also hardening microstructure.

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

The invention further relates to a shaped sheet metal part formed from a flat steel product comprising an above-elucidated steel substrate and an anticorrosion coating. The anticorrosion coating has the advantage that it prevents scale formation during austenitization in the course of hot forming. In addition, such an anticorrosion coating protects the shaped sheet metal part from corrosion.

In a specific embodiment, the shaped sheet metal part preferably comprises an aluminum-based anticorrosion coating. The anticorrosion coating of the shaped sheet metal part preferably comprises an alloy layer and an Al base layer. In the shaped sheet metal part, the alloy layer is also frequently referred to as interdiffusion layer.

The thickness of the anticorrosion coating is preferably at least 10 μm, more preferably at least 20 μm, especially at least 30 μm.

The thickness of the alloy layer is preferably less than 30 μm, more preferably less than 20 μm, especially less than 16 μm, more preferably less than 12 μm. The thickness of the Al base layer is found from the difference in the thicknesses of anticorrosion coating and alloy layer.

The alloy layer lies atop and directly adjoins the steel substrate. The alloy layer of the shaped sheet metal part preferably consists of 35-90% by weight of Fe, 0.1-10% by weight of Si, optionally up to 0.5% by weight of Mg and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance. As a result of the further diffusion of iron into the alloy layer, the proportions of Si and Mg are correspondingly lower than the respective proportion thereof in the melt of the melt bath.

The alloy layer preferably has a ferritic microstructure.

The Al base layer of the shaped sheet metal part lies atop and directly adjoins the alloy layer of the steel component. The Al base layer of the steel component preferably consists of 35-55% by weight of Fe, 0.4-10% by weight of Si, optionally up to 0.5% by weight of Mg and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance.

The Al base layer may have a homogeneous element distribution where the local element contents vary by not more than 10%. Preferred variants of the Al base layer, by contrast, have low-silicon phases and silicon-rich phases. Low-silicon phases here are regions wherein the average Si content is at least 20% less than the average Si content of the Al base layer. Silicon-rich phases here are regions wherein the 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 disposed within the low-silicon phase. In particular, the silicon-rich phases form at least a 40% continuous layer bounded by the low-silicon regions. In an alternative execution variant, the silicon-rich phases are arranged in the form of islands in the low-silicon phase.

What is meant by “in the form of islands” in the context of this application is an arrangement in which discrete noncoherent regions are surrounded by another material—i.e. there are “islands” of a particular material in another material.

In a preferred variant, the steel component comprises an oxide layer atop the anticorrosion coating. The oxide layer especially lies atop the Al base layer and preferably forms the outer conclusion of the anticorrosion coating.

The oxide layer of the steel component especially consists to an extent of more than 80% by weight of oxides, where the main proportion of the oxides (i.e. more than 50% by weight of the oxides) is aluminum oxide. The oxide layer optionally includes, in addition to aluminum oxide, hydroxides and/or magnesium oxide alone or as a mixture. The remainder of the oxide layer which is not composed of 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, especially of at least 100 nm. In addition, the thickness is not more than 4 μm, especially not more than 2 μm.

In a specific configuration, the shaped sheet metal part comprises a zinc-based anticorrosion coating.

Such a zinc-based anticorrosion coating preferably comprises up to 80% by weight of Fe, 0.2-6.0% by weight of Al, 0.1-10.0% by weight of Mg, optionally 0.1-40% by weight of manganese or copper, optionally 0.1-10.0% by weight of cerium, optionally not more than 0.2% by weight of further elements, unavoidable impurities, and zinc as the balance. In particular, the Al content is not more than 2.0% by weight, preferably not more than 1.5% by weight. The Fe content that arises from inward diffusion is preferably more than 20% by weight, especially more than 30% by weight. In addition, the Fe content is especially not more than 70% by weight, especially not more than 60% by weight. The Mg content is especially not more than 3.0% by weight, preferably not more than 1.0% by weight. The anticorrosion coating may be applied by hot dip coating or by physical gas phase deposition or by electrolytic methods.

In a specific development, the steel substrate of the shaped sheet metal part has a microstructure having at least in part more than 80% martensite and/or lower bainite, preferably at least in part more than 90% martensite and/or lower bainite, especially at least in part more than 95%, more preferably at least in part more than 98%. In a preferred development, the steel substrate of the shaped sheet metal part has a microstructure having at least in part more than 80% martensite, preferably at least in part more than 90% martensite, especially at least in part more than 95%, more preferably at least in part more than 98%. What is meant in this context by “having in part” is that there are regions of the shaped sheet metal part having the microstructure specified. In addition, there may also be regions of the shaped sheet metal part that have a different microstructure. The shaped sheet metal part thus has the specified microstructure in sections or regions.

By virtue of the high martensite content, it is possible to achieve very high tensile strengths and yield points.

In a preferred embodiment, the former austenite grains of the martensite have an average grain diameter of less than 14 μm, especially less than 12 μm, preferably less than 10 μm. As a result of the fine microstructure, it is more homogeneous. The result is an improvement in mechanical properties, especially a lower crack sensitivity and hence improved bending properties and higher elongation at break.

The shaped sheet metal part, in a further-developed variant, at least partly has a yield point of at least 950 MPa, especially at least 1100 MPa, especially at least 1200 MPa, preferably at least 1300 MPa, more preferably at least 1400 MPa, especially at least 1500 MPa.

In a further-developed variant, the shaped sheet metal part at least partly has a tensile strength of at least 1000 MPa, especially at least 1100 MPa, preferably at least 1300 MPa, preferably at least 1400 MPa, especially at least 1600 MPa, preferably of 1700 MPa, more preferably 1800 MPa.

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

In addition, the shaped sheet metal part, in a preferred variant, may at least partly have a bending angle of at least 30°, especially at least 40°, more preferably at least 45°, more preferably at least 50°. The bending angle is understood here to mean the bending angle corrected with respect to the sheet thickness. The corrected bending angle is found from the bending angle ascertained at the force maximum (measured to VDA standard 238-100) (also referred to as maximum bending angle) by the formula

$\mathrm{Bending}{\mathrm{angle}}_{\mathrm{corrected}}=\mathrm{Bending}{\mathrm{angle}}_{\mathrm{asce}rtained}·\sqrt{\mathrm{Sheet}\mathrm{thickness}}$

where the sheet thickness should be inserted into the formula in mm. This applies to sheet thicknesses greater than 1.0 mm. In the case of sheet thicknesses less than 1.0 mm, the corrected bending angle corresponds to the bending angle ascertained.

In this connection, “partly having” is understood to mean that there are regions of the shaped sheet metal part that have the specified mechanical property. In addition, there may also be regions of the shaped sheet metal part having the mechanical property below the limit. The shaped sheet metal part thus has the specified mechanical property in sections or regions. The reason for this is that different regions of the shaped sheet metal part can undergo different heat treatments. For example, individual regions can be cooled down more quickly than others, which results, for example, in formation of more martensite in the more rapidly cooled regions. Therefore, different mechanical properties are also established in the different regions. The same applies to the Vickers hardness elucidated hereinafter.

In a particularly preferred variant, the shaped sheet metal part at least in part has a yield point ratio (ratio of yield point to tensile strength) of at least 60% and at most 85%. The yield point ratio is preferably at least 65%, especially at least 70%.

The mechanical indices specified have been found to be particularly advantageous in order to assure use in an automobile with good crash performance.

In a specific development, the shaped sheet metal part has fine precipitates in the microstructure, especially in the form of niobium carbonitrides and/or titanium carbonitrides.

Fine precipitates in the context of this application refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates.

In a preferred configuration, the average diameter of the fine precipitates is not more than 11 nm, preferably not more than 10 nm, especially not more than 8 nm, preferably not more than 6 nm.

In a further preferred configuration, the shaped sheet metal part has largely fine precipitates in the microstructure. In the context of this application, largely fine precipitates is understood to mean that more than 80%, preferably more than 90%, of all precipitates are fine precipitates.

This means that more than 80%, preferably more than 90%, of all precipitates have a diameter of less than 30 nm.

The fine precipitates result in a particularly fine microstructure with small grain diameters. The fine microstructure makes it more homogeneous. The result is an improvement in mechanical properties, especially lower crack sensitivity and hence improved bending properties and higher elongation at break. This also establishes better toughness with more marked necking characteristics on fracture.

In a preferred execution variant, the shaped sheet metal part at least partly has a Vickers hardness of at least 500 HV1, preferably at least 540 HV1.

In qualitative terms, Vickers hardness is the resistance to the penetration of a test specimen and hence the resistance to plastic deformation. Characterization by Vickers hardness has the advantage that determination of the Vickers hardness is also possible for relatively small component sections. In this way, it is possible to specifically examine individual regions of the component where tensile tests are not possible because of the geometry (for example curved workpieces or regions with variation in layer thickness). Vickers hardness is determined to DIN EN ISO 6507 (2018.07). The identifier “1” relates to the testing force in kiloponds (kp), i.e. 1 kp here. In a test in accordance with the standard, however, no significant differences are found in the measurement from HV1 to HV30. The values with different testing forces are thus likewise within the ranges specified for HV1.

The real mechanical indices of this shaped sheet metal part are ascertained by first subjecting the shaped sheet metal part to cathodic coating with dip-coating lacquer or an analogous heat treatment. Cathodic dip-coating operations are generally conducted for corresponding components in the automotive industry. In cathodic dip coating, the components are first coated in an aqueous solution. This coating is then baked in a heat treatment. This involves heating the shaped sheet metal parts to 170° C. and keeping them at that temperature for 20 minutes. Subsequently, the components are cooled down to room temperature under ambient air. Since this heat treatment can influence mechanical indices, the mechanical indices (yield point, tensile strength, yield point ratio, elongation at break A80, bending angle, Vickers hardness) in the context of this application should be regarded as existing in a component with a cathodic dip coating or in a component which, after forming, has been subjected to a heat treatment which is analogous to a cathodic dip coating. In practice, the heat treatment of the cathodic dip coating varies slightly. Customary temperatures are 165° C.-180° C. and hold times 12-30 minutes. However, the change in the mechanical indices because of 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.

It is a feature of a further-developed variant of the shaped sheet metal part that the anticorrosion coating is an aluminum-based anticorrosion coating and the shaped sheet metal part comprises an alloy layer and an Al base layer.

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

The shaped sheet metal part of the invention is preferably a component for a land vehicle, marine vehicle or aircraft. It is more preferably an automobile part, especially a bodywork part. The component is preferably a B pillar, longitudinal beam, A pillar, doorsill or transverse member.

The process of the invention for producing a shaped sheet metal part of the invention as elucidated above comprises at least the following steps:

• • a) providing a sheet metal blank made of a flat steel product comprising a steel substrate composed of steel consisting of, as well as iron and unavoidable impurities, (in % by weight)
    • C: 0.30-0.50%,
    • Si: 0.05-0.6%,
    • Mn: 0.5-3.0%,
    • Al: 0.10-1.0%,
    • 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%,
    • Mo: 0.002-0.3%,
    • Ni: 0.01-0.5%
    • V: 0.001-0.3%
    • Ca: 0.0005-0.005%
    • W: 0.001-1.00%;
  • b) heating the sheet metal blank in such a way that at least in part the AC3 temperature of the blank is exceeded and the temperature T_ins of the blank on insertion into a forming tool provided for hot press forming (step c)) at least in part has a temperature above Ms+100° C., especially above MS+300° C., where Ms denotes the martensite start temperature;
  • c) inserting the heated sheet metal blank into a forming tool, where the transfer time t_trans required for the removal from the heating device and the insertion of the blank is not more than 20 s, preferably not more than 15 s;
  • d) hot press forming the sheet metal blank to the shaped sheet metal part, where the blank, in the course of hot press forming, is cooled down to the target temperature T_target over a period t_tool of more than 1 s at a cooling rate r_tool of at least in part more than 30 K/s, and optionally kept at that temperature;
  • e) removing the shaped sheet metal part cooled to the target temperature from the tool.

In the process of the invention, a blank consisting of a steel of suitable composition in accordance with the above elucidations is thus provided (step a)), which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partly exceeded and the temperature T_ins of the blank on insertion into a shaping tool provided for hot press forming (step c)) is at least partly a temperature above Ms+100° C., especially above Ms+300° C. In particular, the temperature T_ins of the blank on insertion at least in part exceeds 600° C. In a particularly preferred variant, the temperature T_ins of the blank on insertion is at least partly, especially completely, in the range of 600° C. to 850° C. in order to assure good formability and sufficient hardenability. Partial exceedance of a temperature (AC3 or Ms+100° C. or 600° C. here) in the context of this application is understood to mean that at least 30%, especially at least 60%, of the volume of the blank, preferably the whole blank, exceeds a corresponding temperature. The same applies to the at least partial presence of a temperature in the 600° C. to 850° C. interval in the above-elucidated preferred variant. On insertion into the shaping tool, at least 30% of the blank thus has an austenitic microstructure, meaning that the transformation from the ferritic to the austenitic microstructure need not be complete on insertion into the shaping tool. Instead, up to 70% of the volume of the blank on insertion into the shaping tool may consist of different microstructure constituents, such as annealed bainite, annealed martensite and/or non-recrystallized or partly recrystallized ferrite. For this purpose, particular regions of the blank may be kept at a lower temperature level than others in a controlled manner during the heating. For this purpose, the supply of heat may be directed only to particular sections of the blank in a controlled manner, or the parts that are to be heated to a lesser degree may be shielded from the supply of heat. In the part of the blank material at a lower temperature, in the course of forming in the tool, only distinctly less martensite, if any, is formed, such that the microstructure at that point is distinctly softer than in the respective other regions in which there is a martensitic microstructure. In this way, in the respectively formed sheet metal part, it is possible to establish a softer region in a controlled manner, in which, for example, toughness is optimal for the respective end use, whereas the other regions of the shaped sheet metal part have maximized strength.

Maximum strength properties of the resultant shaped sheet metal part may be enabled in that the temperature attained at least partly in the sheet metal blank is between Ac3 and 1000° C., preferably between 850° C. and 950° C.

The minimum temperature Ac3 to be exceeded, according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Band 1: Grundlagen [Materials Science Steel Volume 1: Principles], Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229, is

$\mathrm{Ac}3=\left(902-{225}^{*}\%C+{19}^{*}\%\mathrm{Si}-{11}^{*}\%\mathrm{Mn}-5^{*}\%\mathrm{Cr}+{13}^{*}\%\mathrm{Mo}-{20}^{*}\%\mathrm{Ni}+{55}^{*}\%V\right)°C.$

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

An optimal uniform distribution of properties can be achieved in that the blank is completely through-heated in step b).

In a preferred execution variant, the average heating rate r_furnace of the sheet metal blank on heating in step b) is at least 3 K/s, preferably at least 5 K/s, especially at least 6 K/s, preferably at least 8 K/s, especially at least 10 K/s, preferably at least 15 K/s. The average heating rate r_furnace should be regarded here as the average heating rate from 30° C. to 700° C.

In a preferred execution variant, the standardized average heating Θ_std is at least 5 Kmm/s, especially at least 8 Kmm/s, preferably at least 10 Kmm/s. The maximum standardized average heating is 15 Kmm/s, especially not more than 14 Kmm/s, preferably not more than 13 Kmm/s.

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

For the standardized average heating, this product Θ is corrected by the present furnace temperature T_furnace in relation to a reference furnace temperature T_furnace, ref of 900° C.=1173.15 K in the following manner:

${\Theta }_{std}=\frac{T_{furna\mathrm{ce},\mathrm{ref}}^4}{T_{furnace}^4}·\Theta$

where the furnace temperatures should each be inserted in kelvin.

In a preferred execution variant, the heating is effected in a furnace with a furnace temperature T_furnace of at least Ac3+10 K, preferably at least 850° C., preferably at least 880° C., more preferably at least 900° C., especially at least 920° C., and not more than 1000° C., preferably not more than 950° C., more preferably not more than 930° C.

Preferably, the dew point of the furnace atmosphere in the furnace here is at least −20° C., preferably at least −15° C., especially at least −5° C., more preferably at least 0° C. and not more than +25° C., preferably not more than +20° C., especially not more than +15° C.

In a specific execution variant, the heating in step b) is effected stepwise in regions with different temperature. In particular, the heating is effected in a roller hearth furnace with different heating zones. The heating is effected here in a first heating zone with a temperature (called furnace intake temperature) of at least 650° C., preferably at least 680° C., especially at least 720° C. The maximum temperature in the first heating zone is preferably 900° C., especially not more than 850° C. Further preferably, the maximum temperature of all heating zones in the furnace is not more than 1200° C., especially not more than 1000° C., preferably not more than 950° C., more preferably not more than 930° C.

The total time in the furnace t_furnace, composed of a heating time and a hold time, in both variants (constant furnace temperature, stepwise heating) is preferably at least 2 minutes, especially at least 3 minutes, preferably at least 4 minutes. In addition, the total time in the furnace in both variants is preferably not more than 20 minutes, especially not more than 15 minutes, preferably not more than 12 minutes, especially not more than 8 minutes. Longer total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is assured. On the other hand, holding for an excessively long period above Ac3 leads to grain coarsening, which has an adverse effect on the mechanical properties.

The blank thus heated is removed from the respective heating device, which is, for example, a conventional heating furnace, an induction heating device which is likewise known per se or a conventional device for keeping steel components hot, and transported into the forming tool with sufficient speed that its temperature on arrival in the tool is at least partly above Ms+100° C., especially above Ms+300° C., preferably above 600° C., especially above 650° C., more preferably above 700° C. Ms here denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partly above the AC1 temperature. In all these variants, the temperature is especially not more than 900° C. These differences in temperature overall assure good formability of material.

In step c), the transfer of the austenitized blank from the heating device used in the particular case to the forming tool is carried out within preferably not more than 20 s, especially not more than 15 s. Such a rapid transport is required in order to avoid excessive cooling prior to forming.

The tool on insertion of the blank is typically at a temperature between room temperature (RT) and 200° C., preferably between 20° C. and 180° C., especially between 50° C. and 150° C. The tool on insertion of the blank may also be at a temperature slightly below room temperature, for example if the cooling water used is slightly colder (e.g. 15° C.). This means that the tool in individual execution variants on insertion of the blank is at a temperature between 10° C. and 200° C. It is optionally possible that the tool, in a particular embodiment, has been heated at least in regions to a temperature T_tool of at least 200° C., especially at least 300° C., in order to merely partially harden the component. In addition, the tool temperature T_tool is preferably not more than 600° C., especially not more than 550° C. It should merely be ensured that the tool temperature T_tool is below the desired target temperature T_target. The residence time in the tool t_tool is preferably at least 2 s, especially at least 3 s, more preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, especially not more than 20 s, preferably not more than 10 s.

The target temperature T_target of the shaped sheet metal part is at least partly below 400° C., preferably below 300° C., especially below 250° C., preferably below 200° C., more preferably below 180° C., especially below 150° C. Alternatively, the target temperature T_target of the shaped sheet metal part is more preferably below Ms−50° C. where Ms denotes the martensite start temperature. In addition, the target temperature of the shaped sheet metal part is preferably at least 20° C., more preferably at least 50° C.

The martensite start temperature of a steel within the provisions of the invention should be calculated by the formula:

$\mathrm{Ms}\left[\mathrm{°C}.\right]=\left(490.85\%\mathrm{by}\mathrm{weight}-302.6\%C-30.6\%\mathrm{Mn}-16.6\%\mathrm{Ni}-8.9\%\mathrm{Cr}+2.4\%\mathrm{Mo}-‐11.3\%\mathrm{Cu}+8.58\%\mathrm{Co}+7.4\%W-14.5\%\mathrm{Si}\right)\left[\mathrm{°C}./\%\mathrm{by}\mathrm{weight}\right]$

where here % C denotes the C content, % Mn the Mn content, % Mo the Mo content, % Cr the Cr content, % Ni the Ni content, % Cu the Cu content, % Co the Co content, % W the W content and % Si the Si content of the respective steel in % by weight.

The AC1 temperature and the AC3 temperature of a steel within the provisions of the invention should be calculated by the formulae:

$AC1\left[\mathrm{°C}.\right]=\left(739\%\mathrm{by}\mathrm{weight}-{22}^{*}\%C-7^{*}\%\mathrm{Mn}+2^{*}\%\mathrm{Si}+{14}^{*}\%\mathrm{Cr}+{13}^{*}\%\mathrm{Mo}-{13}^{*}\%\mathrm{Ni}+{20}^{*}\%V\right)\left[\mathrm{°C}./\%\mathrm{by}\mathrm{weight}\right]$ $AC3\left[\mathrm{°C}.\right]=\left(902\%\mathrm{by}\mathrm{weight}-{225}^{*}\%C+{19}^{*}\%\mathrm{Si}-{11}^{*}\%\mathrm{Mn}-5^{*}\%\mathrm{Cr}+{13}^{*}\%\mathrm{Mo}-{20}^{*}\%\mathrm{Ni}+{55}^{*}\%V\right)\left[\mathrm{°C}./\%\mathrm{by}\mathrm{weight}\right]$

where it is also the case here that % C denotes the C content, % Si the Si content, % Mn the Mn content, % Cr the Cr content, % Mo the Mo content, % Ni the Ni content and +% V the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10).

The tool thus not only shapes the blank to the shaped sheet metal part but simultaneously also quenches to the target temperature. The cooling rate in the tool r_tool to the target temperature is especially at least 20 K/s, preferably at least 30 K/s, especially at least 50 K/s, and in a particular execution at least 100 K/s.

After the shaped sheet metal part has been removed in step e), the shaped sheet metal part is cooled to a cooling temperature T_cool of less than 100° C. within a cooling time t_cool of 0.5 to 600 s. This is generally accomplished by air cooling.

The invention is elucidated in detail hereinafter by working examples.

FIG. 1 shows a grain diagram of reconstructed austenite.

The effect of the invention was shown by conducting multiple experiments. For this purpose, slabs having the compositions specified in table 1 and having a thickness of 200-280 mm and width of 1000-1200 mm were created, heated up to a respective temperature T1 in a pusher furnace and kept at T1 for between 30 and 450 min until the temperature T1 in the core of the slabs had been attained and the slabs were thus through-heated. The production parameters are specified in table 2. The slabs with their respective through-heating temperature T1 were discharged from the pusher furnace and subjected to hot rolling. The experiments were executed in the form of a continuous hot strip rolling operation. For this purpose, the slabs were first pre-rolled to an intermediate product of thickness 40 mm, and the intermediate products, which can also be referred to as preliminary strips in the case of hot strip rolling, each had an intermediate product temperature T2 at the end of the pre-rolling phase. Immediately after the pre-rolling, the preliminary strips were sent to finish rolling, such that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish-rolling phase. The preliminary strips were rolled to hot strips having a final thickness of 3-7 mm and the respective final rolling temperatures T3 specified in table 2, cooled down to the respective coiling temperature and wound up to coils at the respective coiling temperatures T4 and then cooled down in stationary air. The hot strips were descaled in a conventional manner by pickling before being subjected to a cold rolling operation with the degrees of cold rolling specified in table 2. The cold-rolled flat steel products were heated to a respective annealing temperature T5 in a tunnel annealing furnace and kept at annealing temperature for 100 s in each case, before being cooled down to their respective dipping temperature T6 at a cooling rate of 1 K/s. The cold strips were guided at their respective dipping temperature T6 through a molten coating bath at temperature T7. The composition of the coating bath is specified in table 3. After the coating, the coated strips were blown dry in a conventional manner, which produced coating layers having different layer thicknesses (see table 3). The strips were first cooled down to 600° C. at an average cooling rate of 10-15 K/s. Later on in the cooling between 600° C. and 450° C. and between 400° C. and 300° C., the strips were cooled down over the cooling periods T_MT and T_LT specified 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 in each case.

Table 4 is a collation of which steel variant (see table 1) was combined with which process variant (see table 2) and which coating (see table 3).

Steel compositions F is a reference example that is not in accordance with the invention. Correspondingly, experiments 10, 11 and 18 are not in accordance with the invention.

The thickness of the steel strips produced in all experiments was between 1.4 mm and 1.7 mm.

After cooling to room temperature, samples were taken transverse to rolling direction from the cooled steel strips according to DIN EN ISO 6892-1 sample form 2 (annex B table B1). The samples were subjected to a tensile test according to DIN EN ISO 6892-1 sample form 2 (annex B table B1). Table 4 gives the results of the tensile test. In the course of the tensile test, the following material indices were ascertained: yield point type, which is referred to as Re for a pronounced yield point and as Rp for a continuous yield point, and the yield point value Rp0.2 in the case of a continuous yield point, the values for the lower yield point ReL, the upper yield point ReH and the difference of upper and lower yield point ΔRe in the case of a pronounced yield point, tensile strength Rm, uniform elongation Ag and elongation at break A80. All samples have a continuous yield point Rp and a uniform elongation Ag of at least 11.5%. Therefore, the Rp0.2 yield point is reported for all samples.

In addition, table 4 reports the properties of the fine precipitates in the microstructure of the flat steel product. The precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refinement. The precipitates are determined with the aid of electron scattering and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas were produced on longitudinal sections (20×30 mm). The magnification in the measurement is between 10 000-fold and 200 000-fold. Using these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates. By simple counting, the proportion of fine precipitates in the total number of precipitates in the measurement field is ascertained. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis. In the inventive samples, the proportion of fine precipitates is more than 90%. The average diameter of the fine precipitates is additionally below 12 nm.

Blanks have been divided from each of the steel strips thus produced, and these have been used for the further experiments. In these experiments, shaped sheet metal part samples 1-8 have been shaped by hot press forming from the respective blanks in the form of sheets of size 200×300 mm². For this purpose, the blanks have been heated in a heating device, for example in a conventional heating furnace, from room temperature at an average heating rate r_furnace (between 30° C. and 700° C.) in a furnace with a furnace temperature T_furnace. The total time in the furnace, comprising heating and holding, is referred to as t_furnace. The dew point of the furnace atmosphere in all cases was −5° C. Subsequently, the blanks have been removed from the heating device and inserted into a forming tool at temperature T_tool. At the juncture of removal from the furnace, the blanks had assumed the furnace temperature. The transfer time t_trans, composed 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_ins of the blanks on insertion into the forming tool was in all cases above the respective martensite start temperature +100° C. In the forming tool, the blanks have been formed to the respective shaped sheet metal part, and the shaped sheet metal parts were cooled in the tool at a cooling rate r_tool. The residence time in the tool is referred to as t_tool. Finally, the samples have been cooled down to room temperature. Table 5 gives the parameters mentioned for various variants, where “RT” is an abbreviation of room temperature.

Table 5 shows very different variants for the forming process. While there is virtually complete formation of martensitic microstructure, for example, in the case of variant II, the comparatively slow cooling of the variants X with the high tool temperature T_tool leads to altered microstructure formation with high ferrite contents, the effect of which takes the form of a higher elongation at break A80.

Table 6 is a collation of the overall results for the shaped sheet metal parts obtained. The first columns give the sample number, the steel type 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. The further columns give the yield point Rp0.2, tensile strength Rm, the ratio of yield point to tensile strength (yield point ratio), and elongation at break A80. These values were ascertained according to DIN EN ISO 6892-1 sample form 2 (annex B table B1) on samples transverse to rolling direction. The bending angle ascertained has been ascertained according to VDA standard 238-100 with a bending axis transverse to rolling direction. The bending angle ascertained is calculated in each case by the formula specified in the standard from the path of the ram (the bending angle ascertained (also referred to as maximum bending angle) is the bending angle at which the force has its maximum in the bending experiment). In order to eliminate the effect of the sheet thickness on the bending angle, the corrected bending angle was calculated from the ascertained bending angle by the formula

$\mathrm{Bending}{\mathrm{angle}}_{\mathrm{corrected}}=\mathrm{Bending}{\mathrm{angle}}_{\mathrm{asce}rtained}·\sqrt{\mathrm{Sheet}\mathrm{thickness}}$

where the sheet thickness should be inserted into the formula in mm. Table 7 gives the bending angle ascertained. In order to determine the corrected bending angle, these numerical values should accordingly be multiplied by the root of the sheet thickness specified in table 4. In addition, table 7 gives the Vickers hardness HV1. This was ascertained in accordance with DIN EN ISO 6507 (2018.07).

The mechanical indices in table 6 were ascertained after a cathodic dip coating had been applied to the formed shaped sheet metal part. During this coating process, the shaped sheet metal parts were heated to 170° C. and kept at that temperature for 20 minutes. Subsequently, the components are cooled down to room temperature under ambient air.

Table 7 reports the microstructure properties of the shaped sheet metal part. The contents in the microstructure are reported in area %. All inventive examples have a martensite content of more than 90%.

In addition, table 7 reports the properties of the fine precipitates in the microstructure. The precipitates are niobium carbonitrides and titanium carbonitrides, both of which contribute to grain refining. The precipitates are determined with the aid of electron scattering and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas were produced on longitudinal sections (20×30 mm). The magnification in the measurement is between 10 000-fold and 200 000-fold. Using these images, the precipitates can be divided into coarse and fine precipitates. Fine precipitates refer to all precipitates having a diameter of less than 30 nm. The other precipitates are referred to as coarse precipitates. By simple counting, the proportion of fine precipitates in the total number of precipitates in the measurement field is ascertained. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis. In the inventive samples, the proportion of fine precipitates is more than 90%. The average diameter of the fine precipitates is additionally below 11 nm.

In addition, table 7 reports the grain diameter of the former austenite grains. For this purpose, the austenite grains were reconstructed by means of the ARPGE software from EBSD measurements. The software parameters were:

• • Nishiyama-Wassermann orientation relationship
  • Tolerance for grain identification 7°
  • Tolerance for parent growth nucleation 7°
  • Tolerance for parent grain growth 15°
  • Minimum accepted grain size 10 pixels

For grain identification, a maximum variance in the orientation of 5° and a minimum grain diameter of 5 pixels according to DIN EN ISO 643 were assumed.

By way of example, FIG. 1 shows a corresponding reconstruction of austenite. In this case, the average diameter of the former austenite grains is 7.5 μm. In all inventive examples, the average grain diameter of the former austenite grains is below 14 μm.

TABLE 1
(steel types)
Steel	C	Si	Mn	Al	Cr	Nb	Ti	B	P	S	N	Sn	As	Cu	Mo	Ca	Ni	Al/Nb
A	0.35	0.16	1.1	0.21	0.118	0.026	0.0096	0.0025	0.005	<0.0005	0.0035	0.005	0.003	0.019	0.005	0.001	0.032	8.1
B*	0.37	0.3	1.4	0.05	0.18	0.003	0.040	0.0035	0.015	0.003	0.007	0.03	0.01	0.05	0.035	0.003	0.03	16.7
C	0.46	0.20	0.80	0.20	0.12	0.03	0.010	0.0025	0.005	0.0005	0.0035	0.005	0.003	0.019	0.005	0.001	0.019	6.7
D	0.36	0.15	0.8	0.19	0.18	0.025	0.009	0.0024	0.011	<0.0005	0.0036	0.005	0.002	0.02	0.004	0.001	0.031	7.6
Balance: iron and unavoidable impurities. Figures each in % by weight;
*noninventive reference examples

TABLE 2
(production conditions for flat steel product)
Process	T1	T2	T3	T4	DCR	T5	T6	T7	tMT	tLT
variant	[° C.]	[° C.]	[° C.]	[° C.]	[%]	[° C.]	[° C.]	[° C.]	[s]	[s]
a	1250	1075	850	630	55	773	685	678	15	13
b	1280	1110	860	620	50	787	708	686	15	13
c	1205	1060	820	550	55	768	684	683	18	15
d	1210	1120	910	580	55	770	685	685	18	15
e	1205	1065	830	555	45	655	650	680	15	13
f	1210	1110	900	575	70	760	720	700	20	18
g	1320	1080	840	620	50	805	715	710	23	20
h	1315	1090	910	655	55	870	800	720	25	23
i	1300	1120	830	585	35	895	800	725	30	28
j	1310	1095	915	630	65	830	740	710	25	25
k	1240	1080	870	620	50	823	728	710	35	30
l	1280	1100	870	630	55	808	713	708	18	15
m	1280	1085	870	580	50	793	715	708	22	20
n	1285	1105	885	640	65	755	685	675	13	11
Some figures are rounded

TABLE 3
(coating variants)
						Layer
						thickness
Coating	Melt analysis	(single-sided)
variant	Si	Fe	Mg	Others	Al	[μm]
α	9.5	3	0.3	<1%	balance	10
β	8	3.5	0.5	<1%	balance	40
γ	10	3	<0.01	<1%	balance	25
δ	8.2	3.8	0.25	<1%	balance	27
ε	10.5	3.1	0.33	<1%	balance	30
ϕ	8.1	3.9	<0.01	<1%	balance	25

TABLE 4
(flat steel products)
		Thickness								Fine (Nb, Ti)(C, N)
		of the						Elongation	Uniform	precipitates
Coating		steel			Yield	Rp0.2 or		at break	elongation	Propor-	Average	Number
experi-		strip	Process	Coating	point	ReL	Rm	A80	Ag	tion	diameter	[per
ment no.	Steel	[mm]	variant	variant	type	[MPa]	[MPa]	[%]	[%]	[%]	[nm]	100 nm2]
1	A	1.5	a	γ	continuous	493	717	20	12	95	6.3	0.0293
2	A	1.5	j	α	continuous	436	682	21	13	94	10	0.0217
3	A	1.5	b	β	continuous	451	693	20	12	93	7.2	0.0221
4*	B	1.6	a	γ	continuous	403	591	24	13	Coarse	0.0172
										precipitates only
5*	B	1.6	f	ε	continuous	411	603	20	13	Coarse	0.0161
										precipitates only
6	C	1.4	e	γ	continuous	511	723	16	10	94	9	0.0275
7*	B	1.5	i	γ	continuous	371	553	26	14	Coarse	0.0124
										precipitates only
8	D	1.5	e	δ	continuous	443	651	22	12	92	7	0.0223
*noninventive reference examples

TABLE 5
(hot forming parameters)
	Average heating				Furnace
	rate rfurnace				dew				Cooling
Hot forming	[30-700° C.]	Tfurnace	tfurnace	Transfer	point	Tins	Ttool	ttool	rate rtool	Ttarget
variant	[K/s]	[° C.]	[min.]	time [s]	[° C.]	[° C.]	[° C.]	[s]	[K/s]	[° C.]
I	8	925	6	8	−5	800	RT	15	50	50
II	5	920	6	6	−5	815	RT	6	300	40
III	15	920	5	5	−5	830	RT	15	50	50
IV	10	880	6	7	−5	740	100	10	50	120
V	8	950	3	12	−5	770	100	10	50	120
VI	10	925	4	7	−5	810	RT	10	450	40
VII	5	900	5	7	−5	806	RT	15	100	50
VIII	5	920	12	8	−5	796	RT	15	100	50
IX	5	920	12	14	−5	728	100	10	200	110
X	5	920	6	10	−5	792	550	15	30	560
Some figures are rounded

TABLE 6
(shaped sheet metal part)
Experi-				Hot	Yield	Tensile	Yield			Vickers
ment		Process	Coating	forming	point	strength	point	A80	Bending	hardness
no.	Steel	variant	variant	variant	[MPa]	[MPa]	ratio	[%]	angle [°]	[HV1]
1	A	a	γ	II	1422	1856	77%	5.3	45	595
2	A	j	α	III	1411	1846	76%	5.5	46	592
3	A	b	β	IV	1391	1823	76%	5.0	43	589
4*	B	a	γ	II	1400	1854	76%	5	43	592
5*	B	f	ε	IX	1380	1830	75%	5.2	44	598
6	C	e	γ	VIII	1622	1893	86%	4.5	36	607
7*	B	i	γ	IX	1361	1816	75%	5.4	45	586
8	D	e	δ	VII	1413	1862	76%	5.6	42	604

TABLE 7
(microstructure)
	Microstructure
					Fine (Nb, Ti)	Grain
					(C, N)	diameter
					precipitates	of the
Forming					Proportion	former
experiment	Marten-			Residual	[%]/Average	austenite
no.	site	Bainite	Ferrite	austenite	diameter	grains
1	99.9	—	—	0.1	96%/6 nm	7.1 μm
2	99.9	—	—	0.1	94%/8 nm	6.4 μm
3	99.9	—	—	0.1	94%/6 nm	6.1 μm
4*	99.9	—	—	0.1	Coarse	9 μm
					precipitates
					only
5*	100	—	—	—	Coarse	11 μm
					precipitates
					only
6	100	—	—	0	95%	10.8 mm
7*	100	—	—	—	Coarse	12 μm
					precipitates
					only
8	99.8			0.2	93%	7.4
* noninventive reference examples

Claims

1.-18. (canceled)

19. A flat steel product for hot forming, comprising:

a steel substrate composed of steel comprising iron and by weight comprising:

C: 0.30-0.50%,

Si: 0.05-0.6%,

Mn: 0.5-3.0%,

Al: 0.10-1.0%,

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

unavoidable impurities ≤0.2%,

wherein an Al/Nb ratio of Al content to Nb content is ≤20.0.

20. The flat steel product of claim 19, wherein the steel by weight further comprises one or more elements comprising:

Cr: 0.01-1.0%,

Cu: 0.01-0.2%,

Mo: 0.002-0.3%,

Ni: 0.01-0.5%,

V: 0.001-0.3%,

Ca: 0.0005-0.005%, and

W: 0.001-1.0%.

21. The flat steel product of claim 20, wherein at least one of following conditions is applicable to the steel:

Ti<3.42N; and

0.7% by weight<Mn+Cr<3.5% by weight.

22. The flat steel product of claim 19, further comprising an anticorrosion coating on at least one side of the steel substrate.

23. The flat steel product of claim 22, wherein the anticorrosion coating is an aluminum-based anticorrosion coating and comprises an alloy layer and an Al base layer.

24. The flat steel product of claim 23, wherein

the alloy layer comprises 35%-60% by weight of Fe, constituents limited to a total of not more than 5.0% by weight, and balanced aluminum, and

the Al base layer comprises 1.0%-15% by weight of Si, 2%-4% by weight of Fe, up to 5.0% by weight of alkali metals or alkaline earth metals, up to 15% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.

25. The flat steel product of claim 19, comprising one or more of following properties:

a yield point with a continuous progression (Rp0.2) or a yield point with a difference (ΔRe) between an upper yield point limit (ReH) and a lower yield point limit (ReL) of not more than 45 MPa;

a uniform elongation Ag of at least 10%; and

an elongation at break A80 of at least 15% or at least 20%.

26. The flat steel product of claim 19, further comprising fine precipitates in a microstructure of the steel substrate in a form of niobium carbonitrides and/or titanium carbonitrides.

27. The flat steel product of claim 26, wherein the fine precipitates in the microstructure are round precipitates having a diameter of up to 20 nm.

28. A shaped sheet metal part formed from a flat steel product, comprising:

a steel substrate composed of steel comprising iron and by weight comprising: C: 0.30-0.50%, Si: 0.05-0.6%, Mn: 0.5-3.0%, Al: 0.10-1.0%, 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 unavoidable impurities ≤0.2%; and

an anticorrosion coating, wherein an Al/Nb ratio of Al content to Nb content is ≤20.0.

29. The shaped sheet metal part of claim 28, wherein the steel by weight further comprises one or more elements comprising:

Cr: 0.01-1.0%,

Cu: 0.01-0.2%,

Mo: 0.002-0.3%,

Ni 0.01-0.5%,

V: 0.001-0.3%,

Ca: 0.0005-0.005%, and

W 0.001-1.0%.

30. The shaped sheet metal part of claim 28, wherein the steel substrate of the shaped sheet metal part has a microstructure having:

at least in part more than 80% martensite and/or lower bainite, or

at least in part more than 90% martensite and/or lower bainite, and wherein a former austenite grains of the martensite have an average grain diameter of less than 14 μm, less than 12 μm, or less than 10 μm.

31. The shaped sheet metal part of claim 28, further comprising one or more characterizations of:

at least in part having a yield point of at least 1200 MPa or at least 1300;

at least in part having a tensile strength of at least 1400 MPa or at least 1600 MPa;

at least in part having an elongation at break A80 of at least 3.5%, at least 4%, at least 4.5%, or at least 5%;

at least in part having a bending angle of at least 30°, at least 40°, or at least 45°; and

at least in part having a yield point ratio of at least 60% and at most 85%.

32. The shaped sheet metal part of claim 28, further comprising fine precipitates in a microstructure in a form of niobium carbonitrides and/or titanium carbonitrides.

33. The shaped sheet metal part of claim 28, wherein the shaped sheet metal part at least partly has a Vickers hardness of at least 500 HV1 or at least 540 HV1.

34. A process for producing a shaped sheet metal part, comprising steps of:

a) providing a sheet metal blank made of a flat steel product, comprising: a steel substrate composed of steel comprising iron and by weight comprising: C: 0.30-0.50%, Si: 0.05-0.6%, Mn: 0.5-3.0%, Al: 0.10-1.0%, 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 unavoidable impurities ≤0.2%, wherein an Al/Nb ratio of Al content to Nb content is ≤20.0;

b) heating the sheet metal blank such that at least in part an AC3 temperature of the sheet metal blank is exceeded and a temperature Tins of the sheet metal blank on insertion into a forming tool provided for hot press forming at least in part has a temperature above Ms+100° C., wherein Ms is a martensite start temperature;

c) inserting the heated sheet metal blank into the forming tool, wherein a transfer time ttrans required for removal from a heating device and insertion of the sheet metal blank is not more than 20 s or not more than 15 s;

d) hot press forming the sheet metal blank to the shaped sheet metal part, wherein the sheet metal blank during the hot press forming, is cooled down to a target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least in part more than 30 K/s and kept at the target temperature Ttarget; and

e) removing the shaped sheet metal part cooled to the target temperature Ttarget from the forming tool.

35. The process of claim 34, wherein the steel steel by weight further comprises one or more elements comprising:

Cr: 0.01-1.0%,

Cu: 0.01-0.2%,

Mo: 0.002-0.3%,

Ni: 0.01-0.5%,

V: 0.001-0.3%,

Ca: 0.0005-0.005%, and

W: 0.001-1.0%.

36. The process of claim 34, wherein a temperature at least partly obtained in the sheet metal blank in the step b) is between the AC3 temperature and 1000° C., or between 850° C. and 950° C.

37. The process of claim 34, wherein the target temperature Ttarget of the shaped sheet metal part is at least partly below 400° C. or below 300° C.

38. The process of claim 34, wherein the step a) of providing the sheet metal blank made of the flat steel product, further comprising:

providing a slab or thin slab comprising the steel;

through-heating the slab or thin slab at a temperature (T1) of 1100-1400° C.;

hot rolling the slab or thin slab to produce a hot-rolled flat steel product, wherein a final rolling temperature (T3) is 750-1000° C.;

annealing the flat steel product at an annealing temperature (T5) of 650-900° C.;

cooling the flat steel product to a dipping temperature (T6) of 650-800° C. or 670-800° C.; and

coating the flat steel product cooled to the dipping temperature with an anticorrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C. or 680-740° C.; and

cooling the coated flat steel product to room temperature, wherein a first cooling time tMT in a temperature range between 600° C. and 450° C. is more than 10 s or more than 14 s, and a second cooling time tLT in a temperature range between 400° C. and 300° C. is more than 8 s or more than 12 s.

39. The process of claim 38, further comprising:

pre-rolling the through-heated slab or thin slab to an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;

coiling the hot-rolled flat steel product, wherein a coiling temperature (T4) is at most 700° C.;

descaling the hot-rolled flat steel product;

cold rolling the flat steel product, wherein a degree of cold rolling is at least 30%; and

skin pass rolling the coated flat steel product.

40. The process of claim 38, wherein the anticorrosion coating on the flat steel product is applied to the flat steel product in liquid form and comprises one or more of:

up to 15% by weight of Si,

2-4% by weight of Fe,

up to 5% by weight of alkali metals or alkaline earth metals,

up to 15% Zn,

constituents limited to a total of not more than 2.0% by weight, and

balanced aluminum.