Shaped Sheet-Metal Part with Improved Welding Properties

Abstract

The invention relates to a shaped sheet metal part having a steel substrate and an aluminum-based anticorrosion coating disposed on at least one side of the steel substrate. In this context, the anticorrosion coating has an Al base layer and an alloy layer. In addition, the shaped sheet metal part has an area-based impedance of not more than 1.0 Ωcm2. The invention additionally relates to a process for producing such a shaped sheet metal part.

Description

[0001] The invention 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.

[0002] 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.

[0003] 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”. Figures given in this text for the contents of the constituents of an atmosphere are based on volume (reported in “% by volume”).

[0004] WO 2022 / 048990 A1 and EP 2 993 248 B1 disclose shaped sheet metal parts having similar aluminum-based coatings and processes for production thereof.

[0005] Even though such an aluminum-based coating significantly improves the further processibility of the hot-formed shaped sheet metal parts, further technical challenges remain in downstream process steps. It is true that aluminum is a self-passivating material that automatically limits the growth of an oxide layer.

[0006] Nevertheless, especially at high temperatures, even in the case of an aluminum surface, a native oxide layer is formed, which cannot be ignored from a technical point of view. As a result, joining in particular—for example by resistance point welding methods—is problematic, being impaired by elevated volume resistances in the presence of such an oxide layer. This makes it necessary to carefully select the welding current required according to the specific material composite, which leads overall to a limited welding range and hence to a lower level of process reliability.

[0007] It is therefore an object of the present invention to match the composition of the coating and the production parameters to one another so as to result in improved welding properties.

[0008] This object is achieved by a shaped sheet metal part comprising

[0009] a steel substrate consisting of a steel including 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B

[0010] and an aluminum-based anticorrosion coating disposed on at least one side of the steel substrate.

[0011] This anticorrosion coating has an Al base layer and an alloy layer, where the alloy layer lies atop the steel substrate and the Al base layer lies atop the alloy layer. The alloy layer consists of 35-90% by weight of Fe, 2-12% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance. The Al base layer consists of 35-55% by weight of Fe, 4-10% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance. In addition, the shaped sheet metal part has an area-based magnitude of impedance ZA of not more than 1.0 Ωcm2.

[0012] An optional content of up to 3% by weight of alkali metals or alkaline earth metals means that alkali metals and / or alkaline earth metals may be present, where the total content of all alkali metals and alkaline earth metals is up to 3% by weight.

[0013] Preferably, the optional content of alkali metals or alkaline earth metals in the alloy layer consists of up to 0.5% by weight of Mg and / or the optional content of alkali metals or alkaline earth metals in the Al base layer consists of up to 0.5% by weight of Mg. This means that the alloy layer preferably consists of 35-90% by weight of Fe, 2-12% by weight of Si, optionally up to 0.5% by weight of Mg, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance. The Al base layer preferably consists of 35-55% by weight of Fe, 4-10% by weight of Si, optionally up to 0.5% by weight of Mg, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance.

[0014] Impedance is determined by pressing two shaped sheet metal parts that have been coated on both sides two-dimensionally against one another with the aid of two gold-plated copper electrodes, where the contact area of the two copper electrodes has an area A and the contact area of the two shaped sheet metal parts is at least twice the contact area of the copper electrodes (i.e. the contact area of the two shaped sheet metal parts has an area of at least 2*A). The two copper electrodes are circular and have a diameter of 45 mm. Area-based impedance is determined essentially by the flow of current in the region directly between the two copper electrodes and to a smaller degree by currents in the immediate environment of the copper electrodes. Because the contact area of the two shaped sheet metal parts is at least twice the size of the contact area of the copper electrodes, it is ensured that the currents in the immediate environment of the copper electrodes are unaffected by field lines at the sample edges. In addition, measurements have shown that, at the typical thicknesses of the shaped sheet metal parts (0.5 mm to 4 mm), measurements are not dependent on sheet thickness since the resistance is dominated by the contact area.

[0015] For the measurement, the shaped sheet metal parts must have a flush contact over the area. In the simplest case of flat samples of the shaped sheet metal parts, these must thus be plane-parallel. The samples of the shaped sheet metal parts are pressed against one another in the measurement so as to give rise to a conductive contact over the full area. In practice, 3 kN has been found to be suitable here. An AC voltage having an amplitude U of, for example, 10 mV about an average (polarization value) of 0 V is applied to the two copper electrodes. The frequency in the example described here is 10 mHz. Subsequently, the amplitude and phase shift of the resulting current I are measured. Area-based impedance is the ratio of AC voltage to current multiplied by the area of the contact area:ZA=UI·A

[0016] An area-based impedance of not more than 1.0 Ωcm2, especially not more than 0.9 Ωcm2, preferably not more than 0.8 Ωcm2, has the advantage that reliable or optimal joining is possible by resistance point welding and, in particular, there is a sufficient process window for the choice of welding current in processing. The magnitude of the phase shift between voltage and current is not more than 3°, preferably not more than 1°, especially not more than 0.1°. This has the advantage that it is essentially the ohmic fraction of complex resistance which is effective and is measured.

[0017] The area-based impedance of the component is influenced by various effects. As well as the composition and thickness of the anticorrosion coating, the characteristics of the surface also affect impedance. For example, rough and uneven surfaces have the effect that the size of the effective contact areas is reduced. When two uneven surfaces are pressed against one another, this will inevitably result in effective contact areas that truly adjoin one another, and regions in which a certain separation remains. The proportion of effective contact areas in the contact area has an important influence on impedance. It has been found that the production process described hereinafter leads to a layer composition and a surface topology that has such a low impedance.

[0018] The measurement method described (impedance spectroscopy) is particularly suitable for determining the effects of the surface structure and chemistry. In the literature, suitability for welding is frequently ascertained via cold transfer resistance. However, this is done using a comparatively high DC current of 10 A, and the resistance is ascertained only after current flow for 15 seconds. As a result of these high currents, however, any factors influencing impedance as a result of oxides on the surface, for example, are no longer fully measurable by at least partly breaking them up. However, it has been found in practice that specifically these oxides can be troublesome in the welding operation. It is likewise possible for the comparatively high measurement current to result in melting of the surface and hence a change in the surface structure. By the test method described here, it is possible to quantify the influence of surface properties without any significant influence on the properties to be measured by the test method. In addition, in the customary measurements of cold transfer resistance, ball electrodes having a diameter of 16 mm are used. Because of the ball shape, the diameter of the contact areas of electrode and metal sheet is thus about 2 to 5 mm. Instead, in the test method described here, planar electrodes of comparatively large area with a diameter rounded to 45 mm are used. This increased area likewise allows improved detectability of surface properties (in particular, for example, of the troublesome oxides).

[0019] Such a shaped sheet metal part is produced with the aid of a specific coating and forming process. Such a production process of the invention comprises the following steps:

[0020] a) providing a slab or a thin slab consisting of steel including 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B;

[0021] b) through-heating the slab or thin slab at a temperature (T1) of 1100-1400° C.;

[0022] c) optionally pre-rolling the through-heated slab or thin slab to give an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;

[0023] d) hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.;

[0024] e) optionally coiling the hot-rolled flat steel product, where the coiling temperature (T4) is not more than 700° C.;

[0025] f) descaling the hot-rolled flat steel product;

[0026] g) optionally cold-rolling the flat steel product, where the degree of cold rolling is at least 30%;

[0027] h) annealing the flat steel product at an annealing temperature (T5) of 650-900° C. under an atmosphere consisting of 2-15% by volume of hydrogen, with a balance of nitrogen and unavoidable impurities, and at a dewpoint between −50° C. and 25° C.;

[0028] i) cooling the flat steel product to an immersion temperature (T6) of 650-800° C., preferably 670-800° C.;

[0029] j) coating the flat steel product that has been cooled to the immersion temperature with an anticorrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C., preferably 680-740° C., where the melt in the melt bath consists of 5-15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance;

[0030] k) cooling the coated flat steel product to room temperature, where the first cooling period tMT in the temperature range between 600° C. and 450° C. is more than 10 s, especially more than 14 s, and the second cooling period tLT in the temperature range between 400° C. and 300° C. is more than 8 s, especially more than 12 s, in an atmosphere containing 16-25% by volume of oxygen and at a dewpoint between −5° C. and 25° C.;

[0031] l) optionally skin pass rolling the coated flat steel product;

[0032] m) optionally coiling the coated flat steel product to a coil with a tensile force of 500-5000 daN;

[0033] n) dividing a sheet metal blank from the coated flat steel product;

[0034] o) heating the sheet metal blank such that the AC3 temperature of the blank is exceeded at least to some degree and the temperature Tins of the blank on insertion into a forming tool provided for a hot press forming operation is at least partly at a temperature above Ms+100° C. where Ms denotes the martensite start temperature;

[0035] p) inserting the heated sheet metal blank into a forming tool, where the transfer time ttrans 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;

[0036] q) 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 to the target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least partly more than 30 K / s and optionally held at that temperature;

[0037] r) removing the shaped sheet metal part that has been cooled to the target temperature Ttarget from the tool.

[0038] In step a), a semifinished product of a composition in accordance with 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] In step f), the hot-rolled flat steel product is descaled in a conventional manner by pickling or by another suitable treatment.

[0044] 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:DCR=Δ⁢dCR / dwith 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.

[0046] The annealing in step h) is effected under an atmosphere consisting to an extent of 2-15% by volume of hydrogen, preferably 2-10% by volume of hydrogen, especially 2-4% by volume of hydrogen, with a balance of nitrogen and unavoidable impurities. The dewpoint in the annealing is between −50° C. and 25° C. Preferably, the dewpoint is greater than 0° C., preferably greater than 4° C. This low-oxygen atmosphere in combination with the dewpoint established ensures that the formation of an oxide layer is sufficiently limited to achieve the desired surface topology that leads to the specific impedance of the invention.

[0047] 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 lower 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.

[0048] 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.

[0049] 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. “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. 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 5-15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 3% by weight of alkali metals or alkaline earth metals, 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 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. Further preferably, the optional content of alkali metals or alkaline earth metals in the melt consists of 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, and optionally at least 0.0015% by weight of Ca, especially at least 0.01% by weight of Ca. In a preferred variant, the optional content of alkali metals or alkaline earth metals consists of up to 0.5% by weight of Mg.

[0050] After the coating treatment, the coated flat steel product is cooled down to room temperature in step k). A first cooling period tMT in the temperature range between 600° C. and 450° C. (moderate temperature range MT) is more than 10 s, especially more than 14 s, and a second cooling period tLT in the temperature range between 400° C. and 300° C. (low temperature range LT) is more than 8 s, especially more than 12 s.

[0051] The cooling is effected in an atmosphere containing 16-25% by volume of oxygen, preferably 18-22% by volume of oxygen, especially in ambient air, and at a dewpoint between −5° C. and 25° C.

[0052] This relates both to the cooling in the moderate temperature range and to the cooling in the low temperature range.

[0053] The first cooling period tMT 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 period tMT. 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.

[0054] The diffusion of iron into the anticorrosion coating has several advantages.

[0055] Firstly, the melting of the anticorrosion coating is delayed on austenitization prior to press hardening (see below). 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.

[0056] At the same time, the diffusion of aluminum into the steel substrate would have considerable disadvantages.

[0057] 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.

[0058] By virtue of the preferred first cooling time tMT (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.

[0059] The second cooling period tLT 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 period tLT.

[0060] 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.

[0061] 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 on austenitization and lead to additional austenite seeds and hence to an even finer austenite microstructure and hence also hardening microstructure.

[0062] 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.

[0063] In the subsequent step m), the coated flat steel product is coiled to a coil in order to enable storage or transport. The coiling is effected with a tensile force of at least 500 daN (decanewtons). The tensile force is preferably not more than 5000 daN, especially not more than 1500 daN.

[0064] Subsequently, in step n), a sheet metal blank is divided from the coated flat steel product and subjected to further processing.

[0065] This sheet metal blank is then heated in the manner known per se (step o) such that the Ac3 temperature of the blank is exceeded at least to some degree. In particular, the temperature is at least partly between Ac3 and 1000° C., preferably between 850° C. and 950° C. In addition, the temperature Tins of the blank on insertion into a forming tool provided for a hot press forming operation (step p)) is at least partly at a temperature above Ms+100° C., especially above Ms+300° C. In particular, the temperature Tins of the blank on insertion is at least partly 600° C. In a particularly preferred variant, the temperature Tins of the blank on insertion is at least partly, especially completely, within a range of 600° C. to 850° C., in order to assure good formability and sufficient hardenability. What is meant in the context of this application by “partial exceedance of a temperature” (here, AC3 or Ms+100° C. or 600° C.) is 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 existence of a temperature in the interval of 600° C. to 850° C. in the preferred variant elucidated above. 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 austenitic microstructure need not 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 specifically kept at a lower temperature level during the heating than others. For this purpose, the supply of heat may be directed specifically only to particular sections of the blank, or the parts that are to be heated to a lesser degree may be shielded against the supply of heat. In the part of the blank material wherein the temperature remains lower, only a distinctly smaller amount of martensite, if any, is formed in the course of forming in the tool, such that the microstructure there is much softer than in the respective other parts in which there is a martensitic microstructure. In this way, in the respectively formed shaped sheet metal part, it is possible to specifically establish a softer region in which, for example, toughness is optimal for the respective end use, while the other regions of the shaped sheet metal part have maximized strength.

[0066] Maximum strength properties of the resultant shaped sheet metal part may be enabled in that the temperature attained at least to some degree in the sheet metal blank is between Ac3 and 1000° C., preferably between 850° C. and 950° C.

[0067] The minimum temperature Ac3 to be exceeded is determined by the following formula specified by HOUGARDY, HP. in Werkstoffkunde Stahl, Band 1: Grundlagen [Materials; Steel; Volume 1: Principles], Verlag Stahleisen GmbH, Dusseldorf, 1984, p. 229:Ac⁢3=(902⁢%⁢ by⁢ wt. -225*%⁢ C+19*%⁢ Si-11*%⁢ Mn-5*%⁢ Cr+13*%⁢ Mo-20* %⁢ Ni+55*%⁢ V)[°⁢ C. / %⁢ by⁢ wt.]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 optimally uniform distribution of properties can be achieved in that the blank is fully through-heated in step o).

[0069] In a preferred execution variant, the average heating rate rfurnace of the sheet metal blank on heating in step o) 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 rfurnace here is the average heating rate from 30° C. to 700° C.

[0070] In a preferred execution variant, the normalized average heating Θnorm is at least 5 Kmm / s, especially at least 8 Kmm / s, preferably at least 10 Kmm / s. At maximum, the normalized average heating is 15 Kmm / s, especially at most 14 Kmm / s, preferably at most 13 Kmm / s.

[0071] Average heating Θ means the product of average heating rate in kelvin per second from 30° C. to 700° C. and sheet thickness in millimeters.

[0072] For normalized average heating, this product Θ is normalized by the current furnace temperature Tfurnace relative to a reference furnace temperature Tfurnace, reference of 900° C.=1173.15 K in the following manner:Θ norm=T furnace,reference4T furnace4·Θwhere the furnace temperatures should in each case be inserted in kelvin.In a preferred execution variant, the heating is effected in a furnace having a furnace temperature Tfurnace 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.

[0074] Preferably, the dewpoint of the furnace atmosphere in the furnace here is at least −20° C., preferably at least −15° C., more preferably at least −10° 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., preferably not more than +10° C., especially not more than +5° C.

[0075] In a specific execution variant, the heating in step o) 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 the furnace entry temperature) of at least 650° C., preferably at least 680° C., especially at least 720° C. The temperature in the first heating zone is preferably not more than 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.

[0076] The total time in the furnace tfurnace, 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. Prolonged total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is assured. On the other hand, holding above Ac3 for an excessively long period leads to grain coarsening, which has an adverse effect on mechanical properties.

[0077] The blank which has thus been heated is removed from the respective heating device, which may, for example, be a conventional heating furnace, an induction heating device which is likewise known per se, or a conventional device for keeping shaped sheet metal parts 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 temperature ranges assure good formability of the material overall.

[0078] In step p), the transfer of the austenitized blank from the respectively used heating device to the forming tool is performed within preferably not more than 20 s, especially not more than 15 s. Such rapid transport is required in order to avoid excessive cooling prior to shaping.

[0079] 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 have a temperature slightly below room temperature if, for example, the cooling water used is slightly colder (e.g. 15° C.). This means that the tool in individual execution variants has a temperature between 10° C. and 200° C. on insertion of the blank. The dwell time in the tool ttool is preferably at least 2 s, especially at least 3 s, more preferably at least 5 s. The dwell time in the tool is preferably not more than 25 s, especially not more than 20 s, preferably not more than 10 s.

[0080] The target temperature Ttarget of the shaped sheet metal part is at least to some degree 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 Ttarget 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.

[0081] The martensite start temperature of a steel within the provisions of the invention should be calculated by the formulaMs [°⁢ C.]=(490.85%⁢ by⁢ wt. -302.6⁢%⁢ C-30.6%⁢ Mn-16.6%⁢ Ni-8.9%⁢ Cr+2.4%⁢ Mo--11.3 %⁢ Cu+8.58%⁢ Co+7.4%⁢ W-14.5%⁢ Si)[°⁢ C. / %⁢ by⁢ wt.]where C % here 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 formulaeAC⁢1 [°⁢ C.]=(739⁢%⁢ by⁢ wt. -22*%⁢ C-7*%⁢ Mn+2*%⁢ Si+14*%⁢ Cr+13*%⁢ Mo-13* %⁢ Ni+20*%⁢ V)[°⁢ C. / %⁢ by⁢ wt.]andAC⁢3 [°⁢ C.]=(902⁢%⁢ by⁢ wt. -225*%⁢ C+19*%⁢ Si-11*%⁢ Mn-5*%⁢ Cr+13*%⁢ Mo-20* %⁢ Ni+55*%⁢ V)[°⁢ C. / %⁢ by⁢ wt.]where, here too, % 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).In the tool, the blank is thus not just shaped to give the shaped sheet metal part, but simultaneously also quenched to the target temperature. The cooling rate in the tool rtool to the 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 removal of the shaped sheet metal part in step r) is followed by cooling of the shaped sheet metal part to a cooling temperature TAB of less than 100° C. within a cooling period tAB of 0.5 to 600 s. This is generally accomplished by air cooling.

[0085] The steel used in the process includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B. The same applies to the steel of the shaped sheet metal part. The preferred steel compositions described hereinafter should be considered to be preferred both for the shaped sheet metal part and for the process for production thereof.

[0086] In particular, the microstructure of the steel is convertible by hot forming to a martensitic or partly martensitic microstructure. The microstructure of the steel substrate of the shaped sheet metal part is thus preferably a martensitic or at least partly martensitic microstructure, since this has particularly high hardness.

[0087] The steel substrate is more preferably a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)

[0088] C: 0.04-0.45% by weight,

[0089] Si: 0.02-1.2% by weight,

[0090] Mn: 0.5-2.6% by weight,

[0091] Al: 0.02-1.0% by weight,

[0092] P: ≤0.05% by weight,

[0093] S: ≤0.02% by weight,

[0094] N: ≤0.02% by weight,

[0095] Sn: ≤0.03% by weight

[0096] As: ≤0.01% by weight

[0097] Ca: ≤0.005% by weightand optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents:

[0098] Cr: 0.08-1.0% by weight,

[0099] B: 0.001-0.005% by weight

[0100] Mo: ≤0.5% by weight

[0101] Ni: ≤0.5% by weight

[0102] Cu: ≤0.2% by weight

[0103] Nb: 0.02-0.08% by weight,

[0104] Ti: 0.01-0.08% by weight

[0105] V: ≤0.1% by weight.

[0106] The elements P, S, N, Sn, As, Ca are impurities that cannot be entirely avoided in steelmaking. As well as these elements, further elements may also be present in the steel as impurities. These further elements are consolidated as the “unavoidable impurities”. The content of unavoidable impurities preferably adds up to not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloy constituents Cr, B, Nb, Ti for which a lower limit is reported may also occur in contents below the respective lower limit as unavoidable impurities in the steel substrate. 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. The individual upper limits for the respective contamination by these elements are as follows:

[0107] Cr: ≤0.050% by weight,

[0108] B: ≤0.0005% by weight

[0109] Nb: ≤0.005% by weight,

[0110] Ti: ≤0.005% by weight

[0111] These preferred upper limits should be considered alternatively or collectively. Preferred variants of the steel thus fulfill one or more of these four conditions.

[0112] In a preferred embodiment, the C content of the steel is not more than 0.37% by weight and / or at least 0.06% by weight. In particularly preferred execution variants, the C content is in the range of 0.06-0.09% by weight or in the range of 0.12-0.25% by weight or in the range of 0.33-0.37% by weight.

[0113] In a preferred embodiment, the Si content of the steel is not more than 1.00% by weight and / or at least 0.06% by weight.

[0114] The Mn content of the steel in a preferred variant is not more than 2.4% by weight and / or at least 0.75% by weight. In particularly preferred execution variants, the Mn content is in the range of 0.75-0.85% by weight or in the range of 1.0-1.6% by weight.

[0115] The Al content of the steel in a preferred variant is not more than 0.75% by weight, especially not more than 0.5% by weight, preferably not more than 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02% by weight.

[0116] In addition, it has been found that it can be helpful when the sum total of the contents of silicon and aluminum is limited. In a preferred variant, the sum total of the contents of Si and Al (typically referred to as Si+Al) is therefore not more than 1.5% by weight, preferably not more than 1.2% by weight. Supplementarily or alternatively, the sum total of the contents of Si and Al is at least 0.06% by weight, preferably at least 0.08% by weight.

[0117] The elements P, S, N are typical impurities that cannot be entirely avoided in steelmaking. In preferred variants, the P content is not more than 0.03% by weight. Independently thereof, the S content is preferably not more than 0.012%. Additionally or supplementarily, the N content is preferably not more than 0.009% by weight.

[0118] Optionally, the steel additionally contains chromium with a content of 0.08-1.0% by weight. The Cr content is preferably not more than 0.75% by weight, especially not more than 0.5% by weight.

[0119] In the case of optional inclusion of chromium in the alloy, the sum total of the contents of chromium and manganese is preferably limited. The sum total is not more than 3.3% by weight, especially not more than 3.15% by weight. In addition, the sum total is at least 0.5% by weight, preferably at least 0.75% by weight.

[0120] The steel preferably optionally additionally contains boron with a content of 0.001-0.005% by weight. In particular, the B content is not more than 0.004% by weight.

[0121] Optionally, the steel may contain molybdenum with a content of not more than 0.5% by weight, especially not more than 0.1% by weight.

[0122] In addition, the steel may optionally contain nickel with a content of not more than 0.5% by weight, preferably not more than 0.15% by weight.

[0123] Optionally, the steel may additionally contain copper with a content of not more than 0.2% by weight, preferably not more than 0.15% by weight.

[0124] In addition, the steel may optionally contain one or more of the microalloy elements Nb, Ti and V. The optional Nb content here is at least 0.02% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional Ti content is at least 0.01% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional V content is preferably not more than 0.1% by weight, preferably not more than 0.05% by weight.

[0125] In the case of optional inclusion of two or more of the elements Nb, Ti and V in the alloy, the sum total of the contents of Nb, Ti and V is preferably limited. The sum total is not more than 0.1% by weight, especially not more than 0.068% by weight. In addition, the sum total is preferably at least 0.015% by weight.

[0126] In a preferred variant, the anticorrosion coating of the cooled flat steel product obtained in step k) has an Al base layer, the chemical composition of which corresponds to the composition of the melt used in step j). This means that it consists in particular of 5-15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance. This results from diffusion of iron out of the steel substrate into the liquid coating (out of the melt) in steps j) and k), such that the anticorrosion coating of the cooled flat steel product on solidification especially includes an alloy layer and an Al base layer. 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 may especially comprise at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca. Further preferably, the optional content of alkali metals or alkaline earth metals consists of 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, and optionally at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca. Further preferably, the optional content of alkali metals or alkaline earth metals consists of up to 0.5% by weight of Mg.

[0127] 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 of 35-60% by weight of Fe, preferably α-iron, optional further constituents, the contents of which are limited to a total of not more than 5.0% by weight, preferably 2.0% by weight, and aluminum as the balance, where the Al content preferably rises in surface direction. The optional further constituents especially include the other constituents of the melt (i.e. silicon and optionally alkali metals or alkaline earth metals, especially Mg and Ca) and the other components of the steel substrate in addition to iron.

[0128] 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 in particular of 5-15% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance.

[0129] Preferred compositions of the Al base layer correspond to the preferred melt compositions. 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 may especially comprise at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca. Further preferably, the optional content of alkali metals or alkaline earth metals consists of 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, and optionally at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca. Further preferably, the optional content of alkali metals or alkaline earth metals consists of up to 0.5% by weight of Mg.

[0130] 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.

[0131] The anticorrosion coating preferably has a thickness of 5-60 m, especially of 10-40 μm. The coat weight of the anticorrosion coating is especially3⁢0-3⁢6⁢0⁢9m2in the case of double-sided anticorrosion coatings or1⁢5-1⁢8⁢0⁢gm2in the case of the single-sided variant. The coat weight of the anticorrosion coating is preferably100-200⁢gm2in the case of double-sided coatings or50-100⁢gm2for single-sided coatings. The coat weight of the anticorrosion coating is more preferably 1201⁢8⁢0⁢gm2in the case of double-sided coatings or60-90⁢gm2for single-sided coatings.The thickness of the alloy layer is preferably less than 20 am, more preferably less than 16 am, more preferably less than 12 μm, especially less than 10 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 disposed atop the anticorrosion coating. The oxide layer lies in particular atop the Al base layer and preferably forms the outer conclusion of the anticorrosion coating.The oxide layer consists especially to an extent of more than 80% by weight of oxides, where the majority 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 oxides of the alkali metals and alkaline earth metals (especially magnesium oxide) on their own or as a mixture. The remainder of the oxide layer not accounted for by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and / or magnesium in metallic form.The oxide layer of the flat steel product preferably has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is not more than 500 nm.When the flat steel product is reheated in step o), there is further diffusion of iron into the anticorrosion coating. Thus, even within a short heating time, an anticorrosion coating fully alloyed with iron is established, having an Fe content of at least 35% by weight.The shaped sheet metal part of the invention which is obtained in step r) and also corresponds to the shaped sheet metal part as claimed in claim 1 preferably has the features described below.The anticorrosion coating of the shaped sheet metal part produced comprises an alloy layer and an Al base layer.The alloy layer here 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, 2-12% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the contents of which are limited to a total of 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 the alkali metals or alkaline earth metals (especially Mg in the preferred variant) are correspondingly lower than the respective proportion thereof in the melt in the melt bath. The optional content of alkali metals or alkaline earth metals in the alloy layer preferably consists of up to 0.5% by weight of Mg. The alloy layer preferably comprises at least 0.1% by weight of Mg and at most 0.3% by weight of Mg.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 shaped sheet metal part. The Al base layer of the shaped sheet metal part preferably consists of 35-55% by weight of Fe, 4-10% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance. The optional content of alkali metals or alkaline earth metals in the Al base layer preferably consists of up to 0.5% by weight of Mg. The Al base layer preferably comprises at least 0.1% by weight of Mg and at most 0.3% by weight of Mg.The Al base layer may have a homogeneous element distribution in which 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.

[0143] 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.

[0144] 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.

[0145] In a preferred variant, the shaped sheet metal part comprises an oxide layer disposed atop the anticorrosion coating. The oxide layer lies in particular atop the Al base layer and preferably forms the outer conclusion of the anticorrosion coating.

[0146] The oxide layer of the shaped sheet metal part consists especially to an extent of more than 80% by weight of oxides, where the majority 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 oxides of the alkali metals and alkaline earth metals (especially magnesium oxide) on their own or as a mixture. The remainder of the oxide layer not accounted for by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and / or magnesium in metallic form.

[0147] The oxide layer preferably has a thickness of at least 50 nm, especially of at least 100 nm. In addition, the thickness is preferably not more than 4 m, especially not more than 2 m.

[0148] The steel substrate of the shaped sheet metal part preferably has a microstructure having more than 80% martensite at least in part, preferably more than 90% martensite at least in part, especially more than 95% at least in part, more preferably more than 98% at least in part. What is meant in this context by “partly having” or “having . . . in part” is that there are regions of the shaped sheet metal part that have the microstructure mentioned. 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 microstructure mentioned in sections or regions.

[0149] As a result of the high martensite content, it is possible to achieve very high tensile strengths and yield points.

[0150] The invention is elucidated in detail in association with the tables that follow.

[0151] The effect of the invention was shown by conducting multiple experiments. For this purpose, slabs having the compositions specified in table 1 were produced with a thickness of 200-280 mm and width of 1000-1200 mm, 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 had been attained in the core of the slabs and hence the slabs were through-heated. The production parameters are reported 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 performed 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 hot strip operation, each had an intermediate product temperature T2 at the end of the preliminary rolling phase. Immediately after preliminary rolling, the preliminary strips were sent to finish rolling, such that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The preliminary strips were rolled out 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 under stationary air. The hot strips were descaled by pickling in a conventional manner, before being subjected to cold rolling with the degrees of cold rolling specified in table 2. The cold-rolled flat steel products were heated in a tunnel annealing furnace to a respective annealing temperature T5 at a dewpoint TP1 of −40° C. 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 with their respective dipping temperature T6 were conducted through a molten coating bath at temperature T7. The composition of the coating bath is reported in table 3. After the coating operation, the coated strips were blown in a conventional manner, producing coats with 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. In the course of further cooling between 600° C. and 450° C. and between 400° C. and 300° C., the strips were cooled down over the cooling periods tMT and tLT specified in table 2. Oxygen content and dewpoint TP2 here were as specified in table 2. Between 450° C. and 400° C. and below 220° C., the strips were cooled down at a cooling rate of 5-15 K / s in each case. Subsequently, the strips were coiled to coils, using a tensile force specified in table 2.

[0152] 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).

[0153] The thickness of the steel strips produced was between 1.4 mm and 1.7 mm in all the experiments.

[0154] Blanks have been divided from each of the steel steps thus produced, which were used for the further experiments. In these experiments, shaped sheet metal part samples in the form of sheets of size 200×300 mm2 have been hot press formed from the respective blanks. 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 rfurnace (between 30° C. and 700° C.) in a furnace with a furnace temperature Tfurnace. The total duration in the furnace, comprising heating and holding, is designated tfurnace. The dewpoint 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 Ttool. At the time of removal from the furnace, the blanks had assumed the furnace temperature. The transfer duration ttrans composed of the duration for the removal from the heating device, transport to the tool and insertion into the tool was between 5 and 14 s. The temperature Tins of the blanks on insertion into the forming tool in all cases was above the respective martensite start temperature+100° C. The blanks have been formed in the forming tool to the respective shaped sheet metal part, with cooling of the shaped sheet metal parts in the tool at a cooling rate rtool. The dwell time in the tool is designated ttool. Finally, the samples have been cooled to room temperature under air. Table 5 gives the parameters mentioned for various variants, where “RT” is an abbreviation of room temperature.

[0155] Table 6 is a compilation of the overall results for the shaped sheet metal parts obtained. The first columns indicate the sample number, the steel type according to table 1, the process variant according to table 2, the coating according to table 3 and the hot forming variant according to table 5. The further columns show the composition of the alloy layer and Al base layer, the thickness of the oxide layer and the area-based impedance. In addition, the phase shift of current and voltage is reported. The area-based impedance and phase shift were each determined by the method elucidated at the outset. The phase shift was in each case less than 0.05°.TABLE 1(steel types)SteelCSiMnAlCrNbTiBPSNSnAsCuMoCaOthersA0.2350.31.30.050.280.0030.040.00350.020.0030.0070.030.010.030.030.005TABLE 2(production conditions for flat steel product)ProcessT1T2T3T4DCRT5TP1T6T7tMTtLTTP2Oxygen contentTensile force forvariant[° C.][° C.][° C.][° C.][%][° C.][° C.][° C.][° C.][s][s][° C.][% by vol.]coiling [daN]a1205106082055055768−40684683181510211000Some figures roundedTABLE 3(coating variant)Melt analysisCoating variantSiMgFeOthersAlα9.60.253.1<1%Balanceβ2.00.32.9<1%BalanceTABLE 4(flat steel product)Thickness ofthe steelExperimentstripProcessCoatingno.Steel[mm]variantvariant1 A1.6aα2*A1.6aβ*noninventive reference examplesTABLE 5(hot forming parameters)Average heating rateFurnaceHot formingrfurnace [30-700° C.]TfurnacetfurnaceTransferdewpointTinsTtoolttoolCooling rateTtargetvariant[K / s][° C.][min.]time [s][° C.][° C.][° C.][s]rtool [K / s][° C.]l892568−5800RT155050Some figures roundedTABLE 6(shaped sheet metal part)Alloy layer*Al base layer*FeSiMgFeSiMgOxideHotArea-basedPhase[%[%[%[%[%[%layerExperimentProcessCoatingformingimpedanceshiftThicknessbybybyThicknessbybybythicknessNo.Steelvariantvariantvariant[Ω· cm2][°][μm]wt.]wt.]wt.][μm]wt.]wt.]wt.][μm]1Aaαl0.8<0.058854.30.0323464.90.130.62*Aaβl1.2<0.058841.40.0423490.80.150.5*noninventive reference examples

Claims

1-14. (canceled)15. A shaped sheet metal part comprising:a steel substrate comprising a steel comprising 0.1-3% by weight of Mn; andan aluminum-based anticorrosion coating disposed on at least one side of the steel substrate,wherein the anticorrosion coating has an Al base layer and an alloy layer, wherein the alloy layer lies atop the steel substrate and the Al base layer lies atop the alloy layer,wherein the alloy layer comprises 35-90% by weight of Fe, 2-12% by weight of Si, and aluminum as the balance,wherein the Al base layer comprises 35-55% by weight of Fe, 4-10% by weight of Si, and aluminum as the balance, andwherein the shaped sheet metal part has an area-based impedance of not more than 1.0 Ωcm2.

16. The shaped sheet metal part of claim 15, further comprises an oxide layer disposed on the anticorrosion coating.

17. The shaped sheet metal part of claim 15, wherein the steel comprises up to 0.01% by weight of B.

18. The shaped sheet metal part of claim 15, wherein the alloy layer further comprises up to 3% by weight of alkali metals or alkaline earth metals and further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and wherein the Al base layer further comprises up to 3% by weight of alkali metals or alkaline earth metals and further constituents, the contents of which are limited to a total of not more than 2.0% by weight.

19. The shaped sheet metal part of claim 18, wherein the alkali metals or alkaline earth metals in the alloy layer comprise up to 0.5% by weight of Mg and the alkali metals or alkaline earth metals in the Al base layer comprise up to 0.5% by weight of Mg.

20. The shaped sheet metal part of claim 19, further comprising at least one of:the alloy layer comprising at least 0.1% by weight of Mg and at most 0.3% by weight of Mg; andthe Al base layer comprising at least 0.1% by weight of Mg and at most 0.3% by weight of Mg.

21. The shaped sheet metal part of claim 15, wherein the steel substrate has a microstructure having more than 80% or more than 90% martensite.

22. The shaped sheet metal part of claim 15, wherein the steel comprises iron and unavoidable impurities, comprising:C: 0.04-0.45% by weight,Si: 0.02-1.2% by weight,Mn: 0.5-2.6% by weight,Al: 0.02-1.0% by weight,P: ≤0.05% by weight,S: ≤0.02% by weight,N: ≤0.02% by weight,Sn: ≤0.03% by weight,As: ≤0.01% by weight, andCa: ≤0.005% by weight, andone or more of elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in following contents:Cr: 0.08-1.0% by weight,B: 0.001-0.005% by weight,Mo: ≤0.5% by weight,Ni: ≤0.5% by weight,Cu: ≤0.2% by weight,Nb: 0.02-0.08% by weight,Ti: 0.01-0.08% by weight, andV: ≤0.1% by weight.

23. A process for producing a shaped sheet metal part, comprising following steps:a) providing a slab or a thin slab comprising steel comprising 0.1-3% by weight of Mn;b) through-heating the slab or thin slab at a temperature (T1) of 1100-1400° C.;d) hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.;f) descaling the hot-rolled flat steel product;h) annealing the flat steel product at an annealing temperature (T5) of 650-900° C. under an atmosphere comprising 2-15% by volume of hydrogen, with a balance of nitrogen and unavoidable impurities, and at a dewpoint between −50° C. and 25° C.;i) cooling the flat steel product to an immersion temperature (T6) of 600-800° C. or 670-800° C.;j) coating the flat steel product that has been cooled to the immersion 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., where the melt in the melt bath comprises 5-15% by weight of Si and aluminum as the balance;k) cooling the coated flat steel product to room temperature, where the first cooling period tMT in the temperature range between 600° C. and 450° C. is more than 10 s or more than 14 s, and the second cooling period tLT in the temperature range between 400° C. and 300° C. is more than 8 s or more than 12 s, in an atmosphere containing 16-25% by volume of oxygen and at a dewpoint between −5° C. and 25° C.;n) dividing a sheet metal blank from the coated flat steel product;o) heating the sheet metal blank such that the AC3 temperature of the blank is exceeded at least to some degree and the temperature Tins of the blank on insertion into a forming tool provided for a hot press forming operation is at least partly at a temperature above Ms+100° C. where Ms denotes the martensite start temperature;p) inserting the heated sheet metal blank into a forming tool, where the transfer time ttrans required for the removal from the heating device and the insertion of the blank is not more than 20 s or not more than 15 s;q) 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 to the target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least partly more than 20 K / s; andr) removing the shaped sheet metal part that has been cooled to the target temperature Ttarget from the tool.

24. The process of claim 23, the steps further comprise one or more of:c) pre-rolling the through-heated slab or thin slab to give an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;e) coiling the hot-rolled flat steel product, where the coiling temperature (T4) is not more than 700° C.;g) cold-rolling the flat steel product, where the degree of cold rolling is at least 30%;l) skin pass rolling the coated flat steel product; andm) coiling the coated flat steel product to a coil with a tensile force of 500-5000 daN.

25. The process of claim 23, wherein in the step a) the steel comprises up to 0.01% by weight of B, and in the step j) the melt in the melt bath comprises 2-4% by weight of Fe, up to 3% by weight of alkali metals or alkaline earth metals, and further constituents, the contents of which are limited to a total of not more than 2.0% by weight.

26. The process of claim 25, wherein in the step j), the content of alkali metals or alkaline earth metals is up to 0.5% by weight of Mg.

27. The process of claim 23, wherein the anticorrosion coating of the cooled flat steel product that has been obtained in the step k) has an Al base layer comprising 5-15% by weight of Si and aluminum as the balance.

28. The process of claim 27, wherein the Al base layer comprises one or more of:2-4% by weight of Fe,up to 3% by weight of alkali metals or alkaline earth metals, andfurther constituents, the contents of which are limited to a total of not more than 2.0% by weight.

29. The process of claim 28, wherein the alkali metals or alkaline earth metals in the Al base layer of the cooled flat steel product obtained in the step k) comprise up to 0.5% by weight of Mg.

30. The process of claim 27, wherein the anticorrosion coating of the cooled flat steel product that has been obtained in the step k) has an alloy layer that lies atop the steel substrate and atop which the Al base layer is disposed, wherein the alloy layer comprises 35-60% by weight of Fe and aluminum as the balance.

31. The process of claim 30, wherein the alloy layer comprises further constituents, the contents of which are limited to a total of not more than 5.0% by weight.

32. The process of claim 23, wherein the temperature attained at least to some degree in the sheet metal blank in step o) is between Ac3 and 1000° C. or between 850° C. and 950° C.

33. The process of claim 23, wherein the target temperature Ttarget of the shaped sheet metal part is at least partly below 400° C. or below 300° C.

34. The process of claim 23, wherein the steel comprises iron and unavoidable impurities, comprising:C: 0.04-0.45% by weight,Si: 0.02-1.2% by weight,Mn: 0.5-2.6% by weight,Al: 0.02-1.0% by weight,P: ≤0.05% by weight,S: ≤0.02% by weight,N: ≤0.02% by weight,Sn: ≤0.03% by weight,As: ≤0.01% by weight, andCa: ≤0.005% by weight, andone or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in following contents:Cr: 0.08-1.0% by weight,B: 0.001-0.005% by weight,Mo: ≤0.5% by weight,Ni: ≤0.5% by weight,Cu: ≤0.2% by weight,Nb: 0.02-0.08% by weight,Ti: 0.01-0.08% by weight, andV: ≤0.1% by weight.

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