SHAPED SHEET METAL PART WITH IMPROVED PROCESSING PROPERTIES

Description

The invention relates to a shaped sheet metal part having improved processing 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”, “sheet metal component” and “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 explicitly 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 6982-1, sample form 2 (Annex B Tab. B1) (version of 2020-06), unless explicitly stated otherwise. Bending angle is determined according to VDA Standard 238-100 for the force maximum. Vickers hardness HV5 was determined to DIN EN ISO 6507 (2018.07). Yield point in the context of this application, in the case of a pronounced yield point, means the yield point Re. In the case of a continuous yield point, by contrast, yield point means the value for the Rp0.2 yield point.

The microstructure was determined on 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 process for 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 shaped sheet metal component comprises an anticorrosion coating containing aluminum.

WO 2006/128821 and WO 2007/122230 A1 disclose processes for producing shaped sheet metal parts having improved processing properties. These involve using forming tools having different temperature zones.

Against the background of the prior art, the problem addressed was that of further developing a shaped sheet metal part such that improved processing properties are achieved in conjunction with an aluminum-based anticorrosion coating. Furthermore, the intention was to specify a process by which such shaped sheet metal parts can be produced practically.

The invention solves this problem by a process for producing a shaped sheet metal part having at least one first zone and one second zone having different material properties, comprising the following steps:

• • a. providing a sheet metal blank made from a flat steel product comprising a steel substrate composed of steel consisting of, aside from iron and unavoidable impurities, (in % by weight)
    • C: 0.27-0.5%,
    • Si: 0.05-0.6%,
    • Mn: 0.4-3.0%,
    • 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: 0.001-1.0%;
  • b. heating the sheet metal blank such that the AC3 temperature of the sheet metal blank is at least partly exceeded and the temperature T_ins of the sheet metal blank on insertion into a forming tool provided for a hot press forming operation (step c)) is at least partly at a temperature above Ms+100° C. where Ms is the martensite start temperature;
  • c. inserting the heated sheet metal blank into a forming tool, where the forming tool has a temperature control device for closed-loop control of the temperature of at least one of its sections that comes into contact with the sheet metal blank during the hot press forming, and where the transfer time t_trans required for the removing from the heating device and the inserting 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 and optionally held at a first target temperature in the first zone and a second target temperature in the second zone;
  • e. removing the cooled shaped sheet metal part from the tool.

Compared to known flat steel products, the steel substrate of the flat steel product used in accordance with 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 0.16% 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 0.16% 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. Furthermore, 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 to higher temperatures 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 flow pathways in the material on resistance point welding that have an adverse effect on suitability for welding and hence 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 a risk that Al oxides will form at the surface of a product manufactured from a steel material alloyed in accordance with the invention, which would worsen the wetting characteristics in the hot dip coating operation. 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 process 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 of 400° C. and 300° C. for a certain period of time. Within the temperature range, there is still a certain diffusion rate of carbon in the steel substrate, while thermodynamic stability 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 seeds. 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 ≤ Al / 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, and that 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 an Al/Nb ratio for which:

Al / Nb ≤ 20. ,

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:

Al / Nb ≤ 30. .

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:

Al / Nb ≤ 20. .

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 0.27-0.5% 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 0.50% by weight, preferably to not more than 0.45% 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 0.30% by weight, preferably 0.32% by weight, especially at least 0.33% by weight, especially at least 0.34% by weight, preferably at least 0.35% by weight, may be provided. With these contents, taking account of 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, especially at least 0.8% by weight, preferably of at least 0.9% 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 any 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 due to the residual 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 perlite 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.

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 0.020% 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 in 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 thereof 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 above 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 shaped sheet metal part and to the shaped sheet metal part itself.

The next stage in the process of the invention is to heat the sheet metal blank thus provided (step a)) in a manner known per se such that the Act temperature of the blank is exceeded by the entire blank and the Ac3 temperature of the blank is preferably at least partly exceeded, and the temperature T_ins of the blank on insertion into a forming tool provided for a hot press forming operation (step c)) is at least partly a temperature above Ms+100° C., especially above Ms+300° C. Partial exceedance of a temperature (here Ac3 or Ms+100° C.) in the context of this application means that at least 30%, especially at least 60%, of the volume of the blank, preferably at least 90% of the volume of the blank, exceeds a corresponding temperature. In all cases in which reference is made to partial exceedance of a temperature, it is preferable that the entire blank exceeds the corresponding temperature. On insertion into the forming 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 yet be complete on insertion into the forming tool. Instead, up to 70% of the volume of the blank on insertion into the forming tool may consist of other 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 remaining 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 is determined by the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Band 1: Grundlagen [Materials Science Steel Volume 1: Principles], Verlag Stahleisen GmbH, Dusseldorf, 1984, p. 229, is

Ac ⁢ 3 = ( 902 - 225 * % ⁢ C + 19 * % ⁢ Si - 11 * % ⁢ Mn - 5 * % ⁢ Cr + 13 * % ⁢ Mo - 20 * % ⁢ Ni + 55 * % ⁢ V ) ⁢ °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. 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 0 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 0 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:

Θ std = T furnace , ref 4 T furnace 4 · Θ

where the furnace temperatures should each be inserted in kelvin.

In a preferred execution variant, the heating is effected in a furnace having 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.

The dewpoint of the furnace atmosphere in the furnace is preferably at least −25° C., especially 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. In this case, the heating is effected 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. In the execution variants in which the shaped sheet metal part comprises an aluminum-based anticorrosion coating, an additional effect of keeping the part above Ac3 is that the thickness of the alloy layer (also frequently referred to as interdiffusion zone) grows too much. This has an adverse effect on the weldability of the anticorrosion coating.

The blank thus heated is taken 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 Act temperature. In all these variants, the temperature is especially not more than 900° C. These temperature ranges 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 transfer is required in order to avoid excessive cooling prior to forming.

This forming tool has a temperature control device for controlling the temperature at least of one of its sections that comes into contact with the sheet metal blank during the hot press forming operation. In this way, it is possible to adjust the temperature of the forming tool to different temperatures section by section.

In the subsequent step d), the sheet metal blank is subjected to hot press forming to give the shaped sheet metal part, where the blank, in the course of hot press forming, is cooled down to and optionally held at a first target temperature in the first zone and a second target temperature in the second zone. The cooling to different target temperatures achieves establishment of different microstructures in the different zones. In this way, in turn, different material properties are assured in the first and second zones. For example, there may be a difference in tensile strength, hardness and/or ductility in the two zones.

The first zone may, for example, be regions of components having a higher strength that are intended for weld points or flanges. Such second zones typically have an area of at least 100 cm², especially of 100 to 5000 cm².

The second zone may, for example, be regions of the component having relatively high ductility. Such first zones typically have an area of not more than 5000 cm², especially of 1 to 5000 cm².

The tool thus not only shapes the blank to the shaped sheet metal part but simultaneously also quenches the respective target temperature in the different zones. The cooling rate in the first zone r_tool,1 to the first target temperature is especially at least 20 K/s, preferably at least 30 K/s, especially at least 50 K/s, in a particular execution at least 100 K/s. The cooling rate in the second zone r_tool,2 to the second target temperature is especially at least 10 K/s, preferably at least 15 K/s. An average cooling rate r_tool,1 in the first zone is preferably higher than an average cooling rate r_tool,2 in the second zone. The average cooling rate relates in each case to the temperature range from the insertion temperature T_ins to the respective first or second target temperature.

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.

In a preferred execution variant of the process, the sections that come into contact with the sheet metal blank during the hot press forming include at least one first section and one second section. The first section comes into contact with the first zone during the hot press forming, and the second section comes into contact with the second zone during the hot press forming. In addition, the first section is adjusted to a first tool temperature and the second section to a second tool temperature, where the first tool temperature is preferably lower than the second tool temperature. These different sections with different tool temperature ensure that the respective zones of the sheet metal blank that come into contact with the different sections are cooled down to different target temperatures during the hot press forming.

The tool temperature in a tool section that comes into contact with the sheet metal blank means the surface temperature of that section immediately before insertion of the sheet metal blank. The temperature should be averaged over that section. Such a temperature is measured, for example, by means of a thermal imaging camera. Image analysis can then be used to average the temperature across the section.

In a preferred variant, the first tool temperature is not more than 200° C. The first tool temperature is preferably between room temperature (RT) and 200° C., preferably between 20° C. and 180° C., especially between 50° C. and 150° C. These low first tool temperatures permit reliable achievement of a correspondingly low target temperature.

The second tool temperature is preferably at least 200° C., preferably at least 300° C., especially at least 400° C., preferably at least 450° C., especially at least 500° C. In addition, the second tool temperature is preferably not more than 600° C., especially not more than 550° C. The second tool temperatures enable procedurally reliable establishment of the desired second target temperature.

The first target temperature is especially 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 first target temperature 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. This low first target temperature can ensure sufficient formation of hardness microstructure.

The second target temperature is preferably at least 200° C., preferably at least 300° C., especially at least 400° C., preferably at least 450° C., especially at least 500° C. In addition, the second target temperature is preferably not more than 650° C., more preferably not more than 600° C., especially not more than 550° C. This ensures that a correspondingly soft microstructure is formed. In particular, the second target temperature is greater than the first target temperature in order to establish the different microstructure of the two zones.

In the course of hot press forming, the sheet metal blank can cool down at most to the temperature of the adjoining tool. The first target temperature therefore corresponds at least to the first tool temperature. The second target temperature likewise corresponds at least to the second tool temperature. By removing the shaped sheet metal part from the tool at the correct time (i.e. establishment of the residence time in the tool t_tool), it is possible in principle to adjust how far the target temperature is above the assigned tool temperature. For example, in the case of a tool temperature of 50° C., the shaped sheet metal part can also be removed only after it has cooled down to 350° C. For this purpose, the shaped sheet metal part must merely be removed at the correct time. However, since the current cooling rate decreases with the extent to which the temperature of the shaped sheet metal part approaches the temperature of the tool, the target temperature can be established in a more stable manner when there is no great difference between target temperature and tool temperature. In the case of large differences, the current cooling rate is comparatively high, and so the process is very sensitive to small variations in the residence time in the tool. For that reason, the first target temperature is preferably not more than 250 K, preferably not more than 200 K, especially not more than 150 K, preferably not more than 100 K, especially not more than 80 K, above the first tool temperature. It is likewise the case that the second target temperature is preferably not more than 100 K, especially not more than 70 K, preferably not more than 50 K, especially not more than 30 K, above the second tool temperature. More preferably, both criteria are observed simultaneously.

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, especially not more than 15 s, preferably not more than 10 s. In this way, it is possible to achieve efficient production on an industrial scale.

In the context of this application, the specified transformation temperatures are fixed as follows:

The martensite start temperature of a steel within the scope of the provisions of the invention should be calculated in accordance with the formula:

Ms [ °C . ] = ( 490.85 % ⁢ by ⁢ weight - 302.6 % ⁢ C - 30.6 % ⁢ Mn - 
 16.6 % ⁢ Ni - 8.9 % ⁢ Cr + 2.4 % ⁢ Mo - 11.3 % ⁢ Cu + 8.58 % ⁢ Co + 7.4 % ⁢ W - 14.5 % ⁢ Si ) [ °C . / ⁢ % ⁢ by ⁢ weight ]

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

The Act temperature and the Ac3 temperature of a steel within the scope of the provisions of the invention should be calculated according to the formulae:

Ac ⁢ 1 [ °C . ] = ( 739 ⁢ % ⁢ by ⁢ weight - 22 * % ⁢ C - 7 * % ⁢ Mn + 
 2 * % ⁢ Si + 14 * % ⁢ Cr + 13 * % ⁢ Mo - 
 13 * % ⁢ Ni + 20 * % ⁢ V ) [ °C . / ⁢ % ⁢ by ⁢ weight ] Ac ⁢ 3 [ °C . ] = ( 902 ⁢ % ⁢ by ⁢ weight - 225 * % ⁢ C + 19 * % ⁢ Si - 
 11 * % ⁢ Mn - 5 * % ⁢ Cr + 13 * % ⁢ Mo - 
 20 * % ⁢ Ni + 55 * % ⁢ V ) [ °C . / ⁢ % ⁢ by ⁢ weight ]

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

In a preferred variant of the process, the cooling characteristics of the blank are adjusted in step d) at least partly via the contact pressures applied by the forming tool. Especially in regions of low temperatures in the forming tool, i.e. in the first section with the first tool temperature, variation of the contact pressure leads to distinctly different cooling rates, such that the microstructure of the blank, especially in the assigned first zone, is variable via the contact pressure. The forming tool in such a case preferably has means of varying the contact pressure.

In a further preferred variant of the process, a forming rate in the hot press forming operation in step d) is controlled with respect to the duration with which the section of the forming tool which is controlled in respect of its temperature comes into contact with the blank during the hot press forming operation. For example, the forming rate can be controlled such that the first zone comes into contact as quickly as possible with the first section of the forming tool. This achieves the effect that the first zone cools down as quickly as possible since the first section has been set to a low first tool temperature. This in turn leads to high strength in the first zone since a high martensite content is formed in that first zone. Conversely, the forming rate is reduced, for example, when a particular zone of the shaped sheet metal part is to cool down particularly slowly in order to create a softer microstructure there. The forming rate is thus preferably controlled such that the first section comes into contact with the first zone before the first zone has cooled down to a temperature below Ms+300° C. Subsequently, the forming rate is reduced. This preferably achieves the effect that the second section comes into contact with the first zone only after the second zone has cooled down to a temperature below Ms+300° C. This further assists the slower cooling in the second zone, which is assured in any case by the higher second tool temperature.

In a specific variant, the process is developed such that the sheet metal blank has regions of different thickness. The sheet metal blank elucidated hereinafter is likewise developed such that it has regions of different thickness.

Regions of different thickness of the sheet metal blank (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 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 suitably 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 forming tool may be any kind of tool which, taking account of the respective shaping of the shaped sheet metal part to be created, is capable of exerting the required forming and pressing forces on the respectively formed sheet metal blank. Suitable forming tools for this purpose are especially those that have a die and a ram that can be inserted into the die for forming.

The temperature control device may especially take the form of a cooling device. In such a case, the temperature control device is preferably disposed adjacent to the first section, on or in the forming tool.

In addition, the temperature control device may especially take the form of a heating device. In such a case, the temperature control device is preferably disposed adjacent to the second section, on or in the forming tool. In particular, the heating device may take the form of one or more heating cartridges inserted into a bore in the forming tool.

In a particularly preferred execution variant, the temperature control device comprises channels introduced into the forming tool, through which a medium flows. This variant has the advantage that it is possible both to introduce heat into the tool through the flow of a hot medium through the channels, or else to remove heat through the flow of a cooler medium through the channels. Depending on the desired temperature, the medium is, for example, water, ice-water, oil, a cryogenic salt solution, liquid nitrogen or another fluid. Consequently, both a temperature control device in the form of a cooling device and a temperature control device configured as a heating device may have such a design with channels introduced into the forming tool through which a medium flows. A temperature control device in the form of a heating device may alternatively or additionally have electrical heating elements disposed on or in the forming tool. Electrical heating elements have the advantage of enabling faster temperature changes.

In a particularly preferred specific execution variant, the sections that come into contact with the sheet metal blank during the hot press forming include at least one first section and one second section. The first section comes into contact with the first zone during the hot press forming, and the second section comes into contact with the second zone during the hot press forming. In addition, the first section is adjusted to the first tool temperature by means of the temperature control device configured as a cooling device, and the second section is adjusted to the second tool temperature by means of a temperature control device configured as a heating device. The forming tool thus comprises a cooling device in order to cool the first section to the first tool temperature, and simultaneously a heating device in order to heat the second section to the second tool temperature.

The flat steel product preferably comprises an anticorrosion coating in order to prevent 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%, 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 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 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 5.0% 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 can especially comprise 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 - 160 ⁢ g m 2

in the case of double-sided anticorrosion coatings, or

15 - 180 ⁢ g m 2

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

100 - 200 ⁢ g m 2

in the case of double-sided coatings, or

50 - 100 ⁢ g m 2

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

120 - 180 ⁢ g m 2

in the case of double-sided coatings, or

60 - 90 ⁢ 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 optional 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. The 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.

The invention further relates to a shaped sheet metal part formed from a flat steel product comprising a flat steel product elucidated above in connection with the process. This flat steel product comprises a steel substrate composed of steel consisting of, aside from iron and unavoidable impurities, (in % by weight)

• • C: 0.06-0.5%,
  • Si: 0.05-0.6%,
  • Mn: 0.4-3.0%,
  • 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: 0.001-1.0%.

The preferred configurations of the steel substrate of the flat steel product that have been elucidated above in connection with the process are likewise preferred configurations for the steel substrate of the shaped sheet metal part.

The shaped sheet metal part of the invention comprises at least one first zone and one second zone having different material properties.

In the first zone, the shaped sheet metal part has:

• • a yield point of at least 1200 MPa, especially at least 1300 MPa,
  • and/or a tensile strength of at least 1400 MPa, especially at least 1600 MPa,
  • and/or an elongation at break A80 of at least 3.5%, especially at least 4%, especially at least 4.5%, preferably at least 5%,
  • and/or a bending angle of at least 30°, especially at least 40°, preferably at least 45°,
  • and/or a yield point ratio of at least 60% and at most 85%,
  • and/or a Vickers hardness of at least 500 HV5, especially at least 540 HV5.

In a developed variant, the shaped sheet metal part has a yield point in the first zone of at least 1200 MPa, preferably at least 1300 MPa, more preferably at least 1400 MPa, especially at least 1500 MPa.

In a developed variant, the shaped sheet metal part in the first zone has a tensile strength of 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 in the first zone has an elongation at break A80 of at least 3.5%, especially at least 4%, especially at least 4.5%, preferably at least 5%.

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

bending ⁢ angle corrected = bending ⁢ angle found · sheet ⁢ metal ⁢ thickness

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

In a particularly preferred variant, the shaped sheet metal part in the first zone 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%.

In a preferred execution variant, the shaped sheet metal part has a Vickers hardness in the first zone of at least 500 HV5, preferably at least 550 HV5, especially at least 570 HV5, preferably at least 580 HV5.

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 Vickers hardness is also possible for smaller component sections. In this way, it is possible to specifically examine individual regions of the component where tensile tests are impossible because of the geometry (for example bent workpieces or regions with variation in sheet thickness). Vickers hardness is determined to DIN EN ISO 6507 (2018.07). The number “5” relates to the testing force in kiloponds (kp). In the case of HV5, the testing force is 5 kiloponds (kp). In a standardized test, however, no significant differences are found in the measurement of HV1 to HV30. The values with different testing forces are thus likewise within the ranges specified for HV5.

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

In the second zone, the shaped sheet metal part has:

• • a yield point of not more than 800 MPa, especially of not more than 600 MPa, especially not more than 580 MPa,
  • and/or a tensile strength of not more than 1000 MPa, especially of not more than 800 MPa,
  • and/or an elongation at break A80 of at least 8%, especially of at least 10%, especially at least 12%,
  • and/or a bending angle of at least 80°, especially at least 90°, preferably at least 100,
  • and/or a yield point ratio of at least 60% and at most 85%,
  • and/or a Vickers hardness of not more than 320 HV5, especially of not more than 300 HV5, especially not more than 270 HV5.

In a developed variant, the shaped sheet metal part has a yield point in the second zone of not more than 800 MPa, especially of not more than 600 MPa, preferably not more than 580 MPa, more preferably 560 MPa, most preferably 540 MPa.

In a developed variant, the shaped sheet metal part has a tensile strength in the second zone of not more than 1000 MPa, especially of not more than 800 MPa, preferably 780 MPa, more preferably 760 MPa, most preferably 740 MPa.

In particular, the shaped sheet metal part has an elongation at break A80 in the second zone of at least 8%, especially of at least 10%, especially at least 12%, most preferably at least 14%.

In addition, the shaped sheet metal part, in a preferred variant, may have a bending angle in the second zone of at least 80°, especially at least 90°, more preferably at least 100. The bending angle here means the bending angle corrected with respect to the sheet metal thickness. The corrected bending angle is found from the bending angle at a particular thickness from the following formula:

bending ⁢ angle corrected = bending ⁢ angle found · sheet ⁢ metal ⁢ thickness

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

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

In a preferred execution variant, the shaped sheet metal part in the second zone has a Vickers hardness of not more than 320 HV5, especially of not more than 300 HV5, especially not more than 270 HV5, preferably not more than 260 HV5, especially not more than 250 HV5, preferably not more than 240 HV5, preferably not more than 230 HV5.

A shaped sheet metal part having such a configuration of the first zone and second zone has significant advantages. Firstly, the first zone has a high-strength region which is particularly resistant to deformation. Secondly, the second zone has a relatively soft region of particularly good suitability for absorption of energy via deformation. The result is a shaped sheet metal part having particularly good crash performance since, firstly, energy absorption of impact energy, for example, is assured and, secondly, there is a high-stability region in order to protect particular sensitive parts (example: the passenger cabin) from deformation. The shaped sheet metal part is especially a B pillar having a soft base or a front or rear longitudinal beam with a soft region.

The real mechanical indices of the shaped sheet metal part are ascertained by first subjecting the shaped sheet metal part to cathodic coating with dipcoating paint or to an analogous heat treatment. Cathodic dipcoating operations are generally conducted for corresponding components in the automotive industry. In the case of cathodic dipcoating, 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 affect the mechanical indices, in the context of this application, the mechanical indices (yield point, tensile strength, yield point ratio, elongation at break A80, bending angle, Vickers hardness) should be understood such that they exist in a component that has been cathodically dipcoated or in a component which, after forming, has been subjected to a heat treatment analogous to a cathodic dipcoating operation. In practice, there is a slight variation in the heat treatment of the cathodic dipcoating. Customary temperatures are 165°180°, and customary hold times 12-30 minutes. However, changes in the mechanical indices because of these variations are negligible.

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

In a preferred execution variant, the shaped sheet metal part has a microstructure in the first zone with more than 95% martensite, especially more than 98%.

By virtue of the high martensite content in the first zone, very high tensile strengths and yield points can be achieved.

In a preferred embodiment, the former austenite grains of the martensite in the first zone have an average grain diameter of less than 14 μm, especially less than 12 μm, preferably less than 10 μm, preferably less than 8 μm. As a result of the fine microstructure, it is more homogeneous. The result is an improvement in mechanical properties, especially lower crack sensitivity and hence improved bending properties and higher elongation at break.

Further preferably, the shaped sheet metal part has a microstructure in the second zone with less than 95% annealed martensite and bainite and optionally up to 60% pearlite. The residual austenite content here is especially less than 3%, preferably less than 1%. Since annealed martensite and bainite are difficult to differentiate, the sum total of annealed martensite and bainite is considered here. This sum total is less than 95%, preferably less than 90%, especially less than 80%, preferably less than 70%. Bainite in this case means both lower bainite and upper bainite. The proportion of pearlite is preferably not more than 50%, especially not more than 40%. In particular, the proportion of pearlite is at least 35%, preferably at least 30%. The sum total of annealed martensite and bainite is preferably at least 40%, especially at least 50%, preferably at least 60%.

In a preferred embodiment, the former austenite grains of the annealed martensite in the second zone have an average grain diameter of less than 14 μm, especially less than 12 μm, preferably less than 10 μm, preferably less than 8 μm. As a result of the fine microstructure, it is more homogeneous. The result is an improvement in mechanical properties, especially lower crack sensitivity and hence improved bending properties and higher elongation at break.

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. This relates both to the first zone and to the second zone.

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.

In a further preferred configuration, the shaped sheet metal part has largely fine precipitates in the microstructure. This relates both to the first zone and to the second zone. What is meant by “largely fine precipitates” in the context of this application is 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. As a result of the fine microstructure, it is 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 results in establishment of better toughness with more marked necking characteristics on fracture.

In a developed variant of the shaped sheet metal part, the shaped sheet metal part comprises 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 low-silicon regions. In an alternative execution variant, the silicon-rich phases are arranged in insular form in the low-silicon phase.

What is meant by “in insular form” 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 contamination, 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.

The invention is elucidated in detail hereinafter by working examples.

The figures show:

FIG. 1 a schematic diagram of a sheet metal blank in a forming tool at the time of insertion into the forming tool,

FIG. 2 a schematic diagram of a sheet metal blank in a forming tool after forming,

FIG. 3 a grain illustration of the reconstructed austenite.

The working examples that follow serve to further elucidate the invention.

Multiple experiments were conducted to demonstrate the effect of the invention. For this purpose, in a conventional manner, steel strips (i.e. flat steel products) having the compositions specified in table 1 were produced. Empty cells in table 1 mean that the respective element was not deliberately included in the alloy. However, the elements may nevertheless be present as an unavoidable impurities. Steel compositions F are a reference example which is not in accordance with the invention.

The steel strips thus generated were hot dip coated in a conventional manner, using the melts specified in table 2. Table 2 in each case gives the layer thickness of the anticorrosion coating on one side, with the top side and bottom side in coated form.

The thickness of the steel strips created in all experiments was between 1.4 mm and 1.6 mm. After cooling to room temperature, for one steel strip per steel type, samples were taken transverse to rolling direction in accordance with DIN EN ISO 6892-1 sample shape 2 (annex B tab. B1). According to DIN EN ISO 6892-1 sample shape 2 (annex B tab. B1), the samples were subjected to a tensile test. Table 3 gives the results of the tensile test on the flat steel product. The following material indices were ascertained in the course of the tensile test: yield point type, and, in the case of a continuous yield point, the yield strength value Rp0.2 (referred to herein as yield point), tensile strength Rm, uniform elongation Ag and elongation at break A80. All samples have a continuous yield point and a uniform elongation Ag of at least 10%.

Blanks have been divided from each of the steel strips created, and these have been used for the further experiments. Corresponding steel components were produced by hot press forming from the blanks (pattern plates). The samples for further mechanical tests were taken at flat points on these components. In the course of further processing, 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 duration in the furnace, which comprises heating and holding, is defined as t_furnace. The dewpoint of the furnace atmosphere is defined as T_dewpoint. Subsequently, the blanks have been removed from the heating device and placed into a forming tool within a transfer time. At the time of removal from the furnace, the blanks had assumed the furnace temperature. The transfer time t_trans composed of that for the removing from the heating device, the transfer to the tool and the insertion into the tool was between 5 and 15 s. On insertion into the forming tool, the blanks had assumed a temperature T_ins. In the forming tool, the blanks have been formed to the respective shaped sheet metal part. The residence time in the closed tool after the forming is defined as t_tool. Finally, the samples have been cooled to room temperature under air. Table 4a gives these general parameters for the hot forming operation.

FIG. 1 shows a schematic diagram of a sheet metal blank 1 on insertion into the forming tool 3. The forming tool comprises a die 5 having a recess 7 and a ram 9. The ram 9 has a footprint of trapezoidal cross section with an end face, and lateral faces 16 that taper obliquely to the end face. The ram 9 is borne by a carrier 10 connected thereto in one-piece form, the lateral edge regions 12, 14 of which protrude laterally beyond the lateral faces 16 of the ram 9 at the upper edge thereof in the manner of a collar. The lower edge faces 18 of the edge regions 12, 14 are connected in horizontal alignment to the lateral face 16 of the ram 9. There is a first section 11 of the forming tool 3 at the end face of the ram 9, and a second section 13 at the transition of the lateral face 16 of the ram 9 to the lower edge face 18 of the edge region 14. The first section 11 is adjusted to a first tool temperature by means of a temperature control device 15 configured as a cooling device. The temperature control device 15 is shown in FIG. 1 in the form of cooling channels. The second section 13 is adjusted to a second tool temperature by means of a temperature control device 17 configured as a heating device. The temperature control device 17 is shown in FIG. 1 in the form of heating coils. The die 5 has a further section 19 adjusted to a third tool temperature by means of a temperature control device 21 configured as a cooling device.

In a departure from the figure, in general, sections of the ram 9 and of the die 5 that are opposite one another and hence come into contact with the same zone of the sheet metal blank 1 are both provided with temperature control devices having the same effect. For example, in general, a temperature control device configured as a heating device would also be provided in the die 5 opposite the temperature control device 17 configured as a heating device. For better visibility, no such illustration has been included in the figure.

In the hot press forming of the sheet metal blank 1 that then follows, the die 9 is brought to bear on the sheet metal blank 1 at high speed, such that the greatly cooled first section 11 rapidly comes into intense contact with the first zone 31 assigned thereto (see FIG. 2) of the sheet metal blank. In this way, the sheet metal blank 1 is quenched in its first zone 31 so quickly that different material properties are established there than in a second zone 33 of the sheet metal blank 1. Subsequently, the forward displacement of the ram 9 is reduced, in order not to bring about excessively rapid cooling in the second zone 33 in particular, which could lead to formation of hardness microstructure. In the second section 13 in particular, which has been heated to the second, higher tool temperature, there is only reduced removal of heat via the ram 9, such that a softer, tougher microstructure is conserved in the second zone 33 of the sheet metal blank 1 which comes into contact with that second section 13.

FIG. 2 shows the sheet metal blank 1 in the forming tool 3 on conclusion of the forming. The sheet metal blank 1 has thus become the shaped sheet metal part. It is clearly apparent that the sheet metal blank 1 (or the shaped sheet metal part) has a first zone 31 that comes into contact with the first section 11 of the forming tool 3 during the hot press forming. In addition, the sheet metal blank 1 (or the shaped sheet metal part) has a second zone 33 that comes into contact with the second section 13 of the forming tool 3 during the hot press forming. The shaped sheet metal part that has thus been created by forming the sheet metal blank 1 thus has a first zone 31 and a second zone 33. The material properties are different in the first zone 31 than in the second zone 33.

Table 4b gives the hot forming parameters, which differ in the different sections or zones. These are the first tool temperature T_tool,1 to which the first section has been adjusted and the second tool temperature T_tool,2 to which the second section has been adjusted. Because of the different tool temperatures, there are inevitably different cooling rates r_tool,1 and r_tool,2 in the first and second zones of the sheet metal blank. On removal of the shaped sheet metal part from the forming tool, there are still different target temperatures in the first and second zones, defined as T_target,1 and T_target,2.

Tables 5a and 5b indicate the mechanical properties of the shaped sheet metal part in the first and second zones, as result from the preceding process. It is clearly apparent that a higher strength has been established in the first zone, while ductility is higher in the second zone.

In addition to the determination of the mechanical properties, the microstructure in the first and second zones was additionally determined. The results in this regard are reported in tables 6a and 6b. While the microstructure in the first zone consists of martensite to an extent of more than 99% and has only small proportions of residual austenite, a microstructure has been established in the second zone with less than 95% annealed martensite and bainite and up to 60% pearlite. Residual austenite was undetectable in this case and is therefore below 1%.

In addition, tables 6a and 6b give 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 electrooptical and x-ray images (TEM and EDX) using carbon extraction replicas. The carbon extraction replicas were produced on longitudinal sections (20×30 mm). The resolution of the measurement was between 10 000-fold and 200 000-fold. These images can be used to classify the precipitates 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. The proportion of fine precipitates in the total of precipitates in the measurement field is ascertained by counting. For the fine precipitates, in addition, the average diameter is calculated by computer-assisted image analysis. In the samples of the invention, the proportion of fine precipitates both in the first zone and in the second zone is more than 90%. The average diameter of the fine precipitates is additionally below 11 nm. The precipitates have not been determined in one experiment. The entry in the table is therefore “n.d.” (not determined).

In addition, tables 6a and 6b report 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 orientation of 5° and a minimum grain diameter of 5 pixels was assumed in accordance with DIN EN ISO 643.

By way of example, FIG. 3 shows a corresponding reconstruction of austenite. In that 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
Balance: iron and unavoidable impurities. Figures each in % by weight;
*noninventive reference examples

TABLE 2
(coating variants)

	Layer
	thickness

Melt analysis

(single-sided)

Coating 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 3
(tensile test indices of flat steel products)

		Thick-
		ness of
		the						Uniform
Coating		steel					Elongation at	elongation
experiment		strip	Coating	Yield point	Yield point	Rm	break A80	Ag
No.	Steel	[mm]	variant	type	[MPa]	[MPa]	[%]	[%]

1	A	1.5	γ	continuous	493	717	20	12
2	A	1.5	α	continuous	436	682	21	13
3	A	1.5	β	continuous	451	693	20	12
4*	B	1.6	γ	continuous	403	591	24	13
5*	B	1.6	ε	continuous	411	603	20	13
6	C	1.4	γ	continuous	511	723	16	10
7*	B	1.5	γ	continuous	371	553	26	14
*noninventive reference examples

TABLE 4a
(hot forming parameters in general)

	Average heating rate				Furnace
Hot forming	rfurnace [30-700° C.]		tfurnace	Transfer	dewpoint		ttool
variant	[K/s]	Tfurnace	[min.]	time [s]	[° C.]	Tins [° C.]	[s]

II	5	920	6	6	−5	815	6
III	15	920	5	5	−5	830	15
IV	10	880	6	7	−5	740	10
VIII	5	920	12	8	−5	796	15
IX	5	920	12	14	−5	728	10
Some figures rounded

TABLE 4b
(hot forming parameters of first and second zones)

Hot		Cooling			Cooling
forming	Ttool, 1	rate rtool, 1	Ttarget, 1	Ttool, 2	rate rtool, 2	Ttarget, 2
variant	[° C.]	[K/s]	[° C.]	[° C.]	[K/s]	[° C.]

II	RT	300	40	480	15	530
III	RT	50	50	450	23	500
IV	100	50	120	470	18	520
VIII	RT	100	50	505	20	550
IX	100	200	110	470	19	510
Some figures rounded

TABLE 5a
(indices of shaped sheet metal part in first zone)

Material properties in first zone

Experi-					Tensile			Vickers
ment			Hot forming	Yield point	strength	A80	Bending angle	hardness
No	Steel	Coating variant	variant	[MPa]	[MPa]	[%]	[°]	[HV5]

1	A	γ	II	1422	1856	5.3	45	595
2	A	α	III	1411	1846	5.5	46	592
3	A	β	IV	1391	1823	5.0	43	589
4*	B	γ	II	1400	1854	5	43	592
5*	B	ε	IX	1380	1830	5.2	44	598
6	C	γ	VIII	1622	1893	4.5	36	607
7*	B	γ	IX	1361	1816	5.4	45	586

TABLE 5b
(indices of shaped sheet metal part in second zone)

Material properties in second zone

Experi-					Tensile			Vickers
ment			Hot forming	Yield point	strength	A80	Bending angle	hardness
No.	Steel	Coating variant	variant	[MPa]	[MPa]	[%]	[°]	[HV5]

1	A	γ	II	582	779	11.4	87	209
2	A	α	III	535	767	13.0	98	240
3	A	β	IV	556	739	14.5	101	231
4*	B	γ	II	630	855	9.0	75	283
5*	B	ε	IX	643	856	8.5	71	292
6	C	γ	VIII	587	764	11.0	83	248
7*	B	γ	IX	620	806	9.2	69	275

TABLE 6a
(microstructure of first zone)

				Proportion of fine (Nb, Ti)(C, N)	Grain diameter
Experiment			Residual	precipitates [%]/average	of former
No.	Martensite	Ferrite	austenite	diameter	austenite grains

1	99.9	—	0.1	95%/5 nm	6.5	μm
2	99.9	—	0.1	92%/6.5 nm	6.2	μm
3	99.9	—	0.1	91%/4 nm	5.9	μm
4*	99.9	—	0.1	Coarse precipitates only	10	μm
5*	100	—	—	Coarse precipitates only	11	μm
6	100	—	0	n.d.	10.4	mm
7*	100	—	—	Coarse precipitates only	13	μm
*noninventive reference examples

TABLE 6b
(microstructure of second zone)

Annealed

	martensite			Proportion of fine (Nb, Ti)(C, N)	Grain diameter
Experiment	and		Residual	precipitates [%]/average	of former
No.	bainite	Pearlite	austenite	diameter	austenite grains

1	67	33	<1.0	96%/6 nm	7.1	μm
2	63	37	<1.0	94%/8 nm	6.4	μm
3	62	38	<1.0	94%/6 nm	6.1	μm
4*	85	15	<1.0	Coarse precipitates only	9	μm
5*	79	21	<1.0	Coarse precipitates only	11	μm
6	63	37	<1.0	n.d.	10.8	mm
7*	89	11	<1.0	Coarse precipitates only	14	μm
*noninventive reference examples