Method for producing a hardened sheet steel component

Abstract

The invention relates to a method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat, hot- and cold-rolled steel strip coated with a zinc-based coating, and at least sub-regions of the sheet steel blank are either heated to a temperature above Ac3 and then shaped in a hot state in a press-hardening tool and quench-hardened, or the sheet steel blank is cold-formed into a sheet steel preform and the sheet steel preform is then quench-hardened in a hot-stamping tool, wherein, for heating from 420°C to 530°C, the heating rate is set such that the degree of discoloration towards silver S is greater than 1, and wherein the heating takes place with a heat-up rate that is variable over the furnace dwell time and dependent on the zinc layer thickness.

Description

[0001] Method for producing a hardened steel sheet component

[0002] The invention relates to a method for producing a hardened sheet steel component.

[0003] It is known in automotive engineering to use sheet steel components with increased stability to increase the rigidity of the passenger compartment. These sheet steel components with increased stability are typically hardened sheet steel components, i.e., steel components that exhibit significantly higher hardness and tensile strength than steel components made of conventional steel. Such hardened sheet steel components are used because the hardening process enables components to be produced that can have a reduced wall thickness compared to components made of non-hardenable steel grades. This makes it possible to provide steel bodies for motor vehicles that are comparatively lightweight yet extremely stable.

[0004] The processes for producing hardened sheet steel components are known.

[0005] The hardening mechanism used is quench hardening, in which the steel material is first heated to its high-temperature phase, known as austenite or gamma iron. This fully or partially austenitic steel structure is then cooled at a cooling rate that exceeds the so-called critical cooling rate of the steel alloy. For example, the critical cooling rate for the boron-manganese steel 22MnB5 is approximately 23 Kelvin per second.

[0006] At these cooling rates, the austenitic phase does not reconvert to ferrite, but rather to martensite. Rapid cooling essentially freezes this martensitic phase. Since austenite can dissolve significantly more carbon than martensite, carbon precipitation occurs, distorting the lattice and resulting in high hardness. This requires that the steel material or steel alloy contains sufficient carbon, which is ensured by the use of suitable alloys. The steel industry typically provides so-called boron-manganese steels for this purpose. One of the most common is 22MnB5, which, however, is part of a comparatively large family of steel grades. Specifically, a steel material with the following composition in mass percent is used:

[0007] Carbon up to 0.4, preferably 0.15 to 0.35 Silicon up to 1.9, preferably 0.11 to 1.5 Manganese up to 3.0, preferably 0.8 to 2.5 Chromium up to 1.5, preferably 0.1 to 0.9 Molybdenum up to 0.9, preferably 0.001 to 0.1 Nickel up to 0.9, preferably 0.01 to 0.2 Titanium up to 0.2, preferably 0.02 to 0.1 Vanadium up to 0.2 Tungsten up to 0.2, Aluminum up to 0.2, preferably 0.02 to 0.07 Boron up to 0.01, preferably 0.0005 to 0.005 Sulphur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01

[0008] Rest iron and impurities.

[0009] In the past, two basic processing methods have emerged.

[0010] The first and older process is press hardening, also known as the direct process. In this process, a blank is cut out of a flat steel sheet and then brought to the required austenitizing temperature and, if necessary, held at this temperature. This blank is then transferred to a forming press, in which the blank is formed while hot, preferably with a single stroke, and then quenched by the cold press-hardening tool. As there is only one forming stroke and only one tool available, the final trimming of outer edges and holes in press-hardened components can usually only be carried out in the hardened state. This is usually done by laser cutting. This press hardening process results in hardened components that do not usually have a too complex shape, as there is only one forming stroke available.

[0011] For components with more complex geometries and / or higher production runs, the applicant has developed what is known as hot stamping, also known as the indirect process. In this process, a blank is cut out of a steel sheet and then formed and trimmed into a sheet steel pre-component using conventional cold forming processes. This pre-component is formed in such a way that it is slightly smaller in all three spatial directions than the target geometry, as it is then heated to the austenitizing temperature and expands accordingly due to thermal expansion. After austenitizing, this pre-component is quickly transferred to a hot stamping press, where the already fully formed pre-component is in contact with the hot stamping tool in the closed press and is merely held in place to cool it quickly. Typically, only minor calibrations or adjustments are carried out in the press.Since the components are usually already fully trimmed in the cold forming process, trimming in the hardened state is usually no longer necessary. Hot stamping allows for more complex components, as the upstream, usually multi-stage, forming and trimming process in the cold state allows for greater design freedom.

[0012] Since body components of modern motor vehicles are often not made of uncoated sheet steel, but are provided with metallic corrosion protection layers to improve corrosion protection properties, it is logical to assume that metallic coated sheet steel is also used for hardened components.

[0013] It is known to use galvanized steel sheets for the two aforementioned processes. These galvanized steel sheets typically have a zinc-based coating containing a certain proportion of one or more elements with even greater oxygen affinity. These elements with even greater oxygen affinity diffuse to the surface during the high-temperature process required for austenitizing, forming a glassy layer, an oxide skin, which protects the underlying zinc layer from evaporation during the high-temperature process.

[0014] The applicant uses suitable galvanized steel sheets in both processes.

[0015] In addition to steel sheets with an almost pure zinc layer, i.e. steel sheets with a coating essentially consisting of zinc with elements that have a higher affinity for oxygen, so-called ZF coatings, also called GA coatings, are also known, in which the zinc coating is followed by a heat treatment, thus creating a zinc-iron alloy layer on the steel strip.

[0016] During heat treatment for the purpose of austenitizing press-hardening steels, the formation of a thin oxide layer composed of elements with a higher affinity for oxygen only occurs in so-called Z-coatings, also known as GI coatings, i.e., coatings primarily based on zinc. In typical Z-coatings, the layer contains approximately 0.2 to 2% aluminum by weight in addition to zinc.

[0017] The most commonly used element with a higher affinity for oxygen is aluminum, which, during heat treatment for the purpose of austenitizing, forms a very thin aluminum oxide skin on the surface, which, as already explained, prevents large-scale zinc evaporation.

[0018] During hot-press hardening and press-hardening, cracks typically form in this very thin Al2O3 layer, resulting in minimal zinc evaporation and the formation and deposition of zinc oxide on the surface. After hot-press hardening or hot-press hardening, such components typically exhibit a light green to brownish surface, which is caused by the coating of these zinc oxides.

[0019] It has been observed that press-hardened or form-hardened components with zinc coatings sometimes, intermittently and unpredictably, exhibit a silver coloration, either entirely or only in certain areas, in addition to the familiar greenish-brown color. A silver coloration within the meaning of the invention can also be perceived as light gray or light grayish and can also appear matte or shiny in places. Experience has shown that the frequency of silver coloration increases, particularly with the coating thickness.

[0020] It was further discovered that the silver coloration develops when the entire zinc layer has not been converted into solid zinc-iron phases up to a temperature of 530 °C, i.e., when liquid zinc still remains on the surface at 530 °C and / or when oxygen is lacking at higher temperatures. The temperature of approximately 530 °C marks the end of the existence of the so-called zeta-zinc-iron phase. Different and fluctuating colorations on the surface of a steel component are not necessarily a quality characteristic of the surface itself, but can lead to problems.

[0021] It is known that the nature and color of a surface influence the absorption and emission of thermal radiation across the surface. This property is described by the emissivity; different and fluctuating colors can thus lead to different and fluctuating emissivities of the surface. Surfaces with silver colorings typically have significantly lower emissivities than surfaces with the typical greenish to brownish coloring. Due to the resulting reduced heat flow, this leads to slower heating in the furnace and slower cooling during transfer from the furnace to the press.

[0022] Such varying and fluctuating emissivity can lead to varying and fluctuating heating behavior in the furnace, which can be technically and, in particular, economically problematic for several reasons. Since it must be ensured in every case that the steel sheet is fully austenitized for subsequent hardening before manufacturing the components, the minimum furnace residence time of all blanks or pre-assembled parts must be extended such that even slow-heating surfaces with silver coloration are reliably austenitized, even though these may only represent a fraction of all blanks or pre-assembled parts. Extending the minimum furnace residence time of all blanks or pre-assembled parts usually results in a lower output of hardened components per production time and is therefore disadvantageous.Since the maximum furnace residence time cannot be extended to the same extent due to the faster heating of the surfaces with their typical greenish to brownish color, the available furnace process window also shrinks. This can lead to increased scrap generation in the event of production disruptions, which is obviously also a disadvantage.

[0023] The different and fluctuating emissivity can pose a further problem, particularly for hot stamping, due to the resulting different and fluctuating cooling behavior of the pre-components between the furnace and the press. After austenitizing, the pre-components are transferred to the corresponding hot stamping press. As already mentioned, differently colored areas or areas with different emissivity can exhibit different cooling behavior. Since the component is no longer trimmed or otherwise influenced after hardening during hot stamping, the insertion temperature of the austenitized pre-component must be correct, as the insertion temperature ultimately determines the size of the component as the sum of the pre-formed shape and the thermal expansion.Different and especially fluctuating insertion temperatures lead to fluctuations in the dimensional accuracy of the components during hot-form hardening, which poses a quality problem because specified dimensional tolerances may not be met. Since silvering typically occurs irregularly and unpredictably, and sometimes only affects individual components or component areas within a batch of components transported from the furnace to the press, correction or compensation via the transfer time between the furnace and the press is not possible.It might also be tempting to eliminate the silver staining by increasing the kiln residence time, but it has been shown that increasing the kiln residence time tends to exacerbate the problem rather than solving it, since areas with silver staining remain stable even with a longer kiln residence time, while the remaining areas become increasingly darker with increasing kiln residence time; moreover, increasing the kiln residence time would be uneconomical and therefore disadvantageous for the reasons already mentioned.

[0024] It is also known that such galvanized, high-temperature-treated steel sheets or steel sheet components are often subjected to post-treatment after press hardening or hot stamping. The most commonly used process is wheel blasting. In cases of severe silver discoloration, it has been observed that paint adhesion problems can arise if the wheel blasting was inadequate.

[0025] The silver coloring mentioned is also often undesirable for customers, as it does not provide a uniform surface appearance of the components.

[0026] Figures 1 and 2 contrast a silvery-gray surface with a greenish surface, with the silvery-gray surface being dominated by aluminum oxide, while the greenish surface is dominated by zinc oxide. EP 2 611 945 B1 discloses a method for producing hardened sheet steel components, in which a predetermined heating regime is to be maintained, with a heating rate of 16 to 50 K / s being maintained up to a temperature of 500 °C; this is intended to ensure faster heating.

[0027] From DE 10 2020 113287 A1 a method for producing hardened sheet steel components is known in which the dew point in the furnace is adjusted to avoid silvery spots.

[0028] From DE 10 2020 106996 Al a device and a method for heating zinc-coated boron-manganese steels is known, in which an active or passive gas circulation is proposed to avoid silver staining.

[0029] The object of the invention is to provide a method for producing sheet steel components without silver coloring with a uniform emissivity.

[0030] The problem is solved by a method having the features of claim 1.

[0031] Advantageous further training is indicated in the dependent subclaims.

[0032] According to the invention, a steel sheet plate or a steel strip is used which has a zinc-based coating.

[0033] This layer can advantageously have a thickness of 5 μm to 20 μm per side. This can ensure good corrosion protection. In particular, the coating can be a Z80, Z120, Z140, or Z180 according to DIN EN 10346.

[0034] Zinc-based corrosion protection coatings can have a comparatively high zinc content of 85% to 99% by weight and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2% by weight. They can also contain other oxygen-affine elements such as magnesium. Particularly preferably, the zinc-based metallic corrosion protection coating can be applied using a hot-dip process. This can be a simple and robust application method.

[0035] According to the invention, it was found that the most uniform emissivity possible and thus a uniform heating and cooling behavior of the circuit boards or pre-assembled parts can be ensured if a heating rate is selected in a certain temperature range that is below a critical heating rate.

[0036] Furthermore, it was discovered that, alternatively or additionally, it is useful to ensure a certain oxygen saturation on the sheet surface above a certain temperature. The solution according to the invention thus lies in eliminating the factors that lead to silver coloration.

[0037] In particular, it may be advantageous not to heat the material at an excessively high rate in a temperature range between 420 °C and 530 °C. However, the heating rate can be increased in temperature ranges below and above this to avoid extending the residence time in the furnace and thus the cycle time. The heating rates can therefore be varied over the furnace residence time.

[0038] Particularly in a temperature range above 800 °C, it may be advantageous to adjust the furnace atmosphere so that at least 5 vol.% oxygen is present at least on the surface of the components to be heated.

[0039] According to the invention, a heating rate between 420 °C and 530 °C is thus specified, which should be below a limit value. The furnace temperature, sheet thickness, zinc layer thickness, oiling condition, and steel alloy are the most important influencing factors for the heating rate of a steel sheet blank or steel sheet pre-component in a furnace. In particular, the zinc layer thickness is a relevant factor influencing the possible heating rate. Regarding oxygen saturation, it has been determined that the oxygen available for zinc oxidation is influenced, particularly during long production intervals, by burning oil, dew point, weather conditions, new furnace elements, and the strength of convection in the furnace.

[0040] The oxygen consumption by burning oil is influenced by the amount of corrosion protection oil on the blank or pre-component surface and, if necessary, additionally by the amount of cold forming oil present on the pre-component surface.

[0041] The dew point is determined in particular by the climate zone and the system environment.

[0042] Weather conditions influence the combustion process through chimney effects and the displacement of oxygen by gaseous combustion products. Furthermore, it has been observed that new furnace elements, such as stainless steel, ceramic, or other insulating materials, can consume or bind oxygen, or influence oxygen saturation.

[0043] It was found that the composition of the furnace atmosphere, especially with regard to the oxygen content, can be locally inhomogeneous.

[0044] Advantageously, at least 5 vol.% oxygen can be provided near the surface starting at a temperature of 800 °C. Near the surface means in an area closer than 10 mm to the surface of the circuit board or pre-assembled part.

[0045] This can be achieved, for example, by blowing in fresh air, with the fresh air preferably being preheated accordingly. In multi-layer chamber furnaces, i.e., without transport movement of the blanks or pre-assembled parts during heating, as occurs in a continuous furnace, convection can also be forced by purging, flow, circulation, or temporarily opening the furnace doors.

[0046] Regarding the heating rate, especially in the temperature range between 420 °C and 530 °C, some considerations are necessary below. During heating, the zinc layer melts at approximately 420 °C. In order for the entire layer thickness d to be converted into solid Zn-Fe phases, the diffusion time t with d 2 =D* t are available, where D is the temperature-dependent diffusion coefficient for which the shape

[0047] _ o_ D = D0* e RT with the activation energy Q=271 kJ / Mol and the universal gas constant R=8.31 ​​J / Mol / K .

[0048] To account for the influence of the Fe-Al inhibition layer, the layer thickness d is replaced by d+do.

[0049] When heated from 420 °C to 530 °C, the diffusion coefficient increases. For Q=271 kJ / mol, it increases approximately 600-fold. The temperature-time curve is therefore crucial for the possible diffusion path, and this must be taken into account, for example, in the form of the temperature-dependent heating rate r(T). A dimensionless silveriness number S can then be defined as follows: where Ti = 693 K, T2= 803 K , d is the layer thickness in meters, b and do are constants with b = 4.0 * 10 7 m 2 / s and d0= 4.4 * 10“ 6 m Are.

[0050] This formula is only valid for r(T > 0 in the temperature range T ± < T < T2, i.e., the heating rate must be positive, since the sheet is heated and the temperature range from 420 °C to 530 °C is described as above. Thus, a silvery surface is obtained if the silveriness number S defined above is less than 1 for a given temperature-time curve.

[0051] The formula yields a constant heating rate of 8.6 K / s as the limit for silver coloration for the zinc layer Z140 (9.8 pm thick) between 420 °C and 530 °C, and 6 K / s as the maximum possible for Z180 (12.6 pm thick). This could be verified experimentally: at a lower heating rate, the silveriness number is greater than 1 and this did not lead to silver coloration, while at a higher heating rate in the specified temperature range, undesirable silver coloration occurred.

[0052] The invention thus relates to a method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat hot- and cold-rolled steel strip coated with a zinc-based coating, and the sheet steel blank is heated, at least in part, either to a temperature above Ac3 and then formed in a press-hardening tool in the hot state and quench-hardened, or the sheet steel blank is cold-formed to form a sheet steel pre-component, and the sheet steel pre-component is heated, at least in part, to a temperature above Ac3 and then quench-hardened in a press-hardening tool, wherein the heating takes place in the temperature range from 420 °C to 530 °C in such a way that the silveriness number S is greater than 1, where: where r(T) is the heating rate in Kelvin per second,

[0053] T is the temperature in Kelvin, r(T) > 0 in the temperature range 7^ < T < T2 and 7^ = 693 K, T2 = 803 K , d is the zinc layer thickness in m, constant b = 4.0 * 10 7 m 2 / s, constant d0= 4.4 * 10“ 6 m,

[0054] Q is the activation energy Q = 271 kJ / mol and

[0055] R is the universal gas constant R = 8.31 J / mol / K, and the heating occurs particularly in the temperature range from 420 °C to 530 °C at a heating rate that varies over the furnace residence time and depends on the zinc layer thickness. Heating can occur by radiation or convection.

[0056] In another embodiment, it can be done by radiation and convection.

[0057] Heating by radiation can be achieved, for example, by an infrared radiator.

[0058] An advantageous further development provides that with increasing zinc coating thickness t / the heating rate r is reduced, so that if dl < d2 < d3, the heating rate is set such that r(dl) > r(d2) > r(d3).

[0059] An advantageous further development provides that the average heating rate between 420 °C and 530 °C is less than 15 K / s for the zinc coating Z80, less than 8.6 K / s for the zinc coating Z140 and less than 6 K / s for the zinc coating Z180.

[0060] An advantageous further development provides that for heating above 800 °C an oxygen content in the furnace atmosphere of at least 5 vol.% is ensured in an area closer than 10 mm to the surface of the blank or the steel pre-component.

[0061] An advantageous further development provides that the heating rate is set in the temperature range from 420 °C to 530 °C, between 3 K / s and 15 K / s and below the respective critical heating rate, whereby the critical heating rate depends on the thickness of the zinc-based layer and becomes smaller with increasing thickness.

[0062] An advantageous development provides that in a continuous furnace in the furnace area corresponding to a blank or pre-component temperature of 420 to 530 °C or in the furnace area corresponding to a blank or pre-component temperature of up to 530 °C, the furnace heating power is reduced such that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded. An advantageous development provides that in the areas in which the blank or pre-component has a temperature of less than 420 °C and / or in particular in the areas in which the blank or pre-component has a temperature of more than 530 °C, the furnace heating power is increased in order to keep the cycle time as short as possible.

[0063] An advantageous further development provides that in furnace areas in which the blank or the pre-component has a temperature of 800 °C or more, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere.

[0064] An advantageous further development provides that in furnace areas in which the blank or the pre-assembled part is at 800 °C or more, the existing furnace atmosphere is circulated.

[0065] An advantageous further development provides that in heating devices in which the blank or the steel pre-component is heated stationary, such as multi-layer chamber furnaces, the furnace heating power is reduced permanently or over time in such a way that the critical heating rate is not exceeded in the temperature range from 420 °C to 530 °C.

[0066] An advantageous further development provides that the zinc-based coating has a layer thickness of 5 pm to 16 pm per side.

[0067] An advantageous further development provides that the zinc-based coating was applied by means of a hot-dip galvanizing process, in particular by hot-dip galvanizing.

[0068] An advantageous further development provides that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably a 22MnB5 or 20MnB8 or 34MnB5.

[0069] An advantageous further development provides that a steel strip with the following composition is used (all data in % by weight):

[0070] Carbon up to 0.4, preferably 0.15 to 0.35 and silicon up to 1.9, preferably 0.11 to 1.5 and

[0071] Manganese up to 3.0, preferably 0.8 to 2.5 and

[0072] Chromium up to 1.5, preferably 0.1 to 0.9 and

[0073] Molybdenum up to 0.9, preferably 0.001 to 0.1 and nickel up to 0.9, preferably 0.01 to 0.2 and

[0074] Titanium up to 0.2 preferably 0.02 to 0.1 and

[0075] Vanadium up to 0.2 and

[0076] Tungsten up to 0.2 and

[0077] Aluminium up to 0.2, preferably 0.02 to 0.07 and boron up to 0.01, preferably 0.0005 to 0.005 and

[0078] Sulphur max. 0.01, preferably max. 0.008 and

[0079] Phosphorus max. 0.025, preferably max. 0.01 and

[0080] The remainder is iron and impurities resulting from the melting process. The invention is explained using a drawing as an example. It shows:

[0081] Figure 1: a sheet measuring 15 x 20 cm with a silvery color after austenitization; Figure 2: the sheet according to Figure 1 in an SEM image;

[0082] Figure 3: a sheet measuring 15 x 20 cm with a greenish coloration after

[0083] austenitization;

[0084] Figure 4: the sheet according to Figure 3 in an SEM image;

[0085] Figure 5: the critical heating rate as a function of the zinc layer thickness;

[0086] Figure 6: a temperature-furnace location curve of a continuous furnace with constant throughput speed;

[0087] Figure 7: a diagram showing the silveriness limits at given heating rates at

[0088] 420°C and 530°C with the course assumed to be linear in between and given zinc layer thicknesses;

[0089] Figure 8: a diagram showing two heating rate curves with and without silver staining;

[0090] Figure 9: schematically the influence on the heating rate in the temperature range of

[0091] 420°C to 530°C in a continuous furnace (DLO);

[0092] Figure 10: schematically showing the influence on the heating rate in the temperature range from 420°C to 530°C in a chamber of a multi-layer chamber furnace (MLK);

[0093] Figure 11: schematically showing the influence on the near-surface oxygen content from a temperature of 800 °C in a continuous furnace (DLO);

[0094] Figure 12: Schematic representation of the influence on the near-surface oxygen content starting at a temperature of 800 °C in a chamber of a multi-layer chamber furnace (MLK); Figure 13: A comparison of the conventional heating rate curve according to the prior art with a possible heating process according to the invention;

[0095] Figure 14: two possible heating routes according to the invention with different heating rates after reaching 530°C.

[0096] To visually illustrate the differences in surface coloration already described, Figure 1 shows a steel sheet which has been coated with zinc and has a silvery appearance after austenitization, and Figure 3 shows a corresponding steel sheet which has a greenish appearance after austenitization.

[0097] Figures 2 and 4 show the corresponding SEM images of the surfaces.

[0098] As already explained, hardened components with a zinc-based coating can exhibit silver staining, as shown in Figure 1. The probability of silver staining increases with the thickness of the zinc coating.

[0099] Components with a silver coloration according to Figures 1 and 2 have a reduced or incompletely covering zinc oxide layer over the existing aluminum oxide layer or have larger areas or surface portions without zinc oxide.

[0100] Such silver coloring on the component can occur globally, as in Figure 1, or only locally, whereby this can be caused by variations in layer thickness or by differences in the substrate. As already explained, the silver coloring is not a problem of a purely cosmetic nature. Although it is customer desire to obtain components with a uniform appearance, the fluctuating silver coloring can, due to the resulting fluctuating emissivity, lead to fluctuating heating behavior in the furnace and fluctuating cooling behavior between the furnace and the press, both of which have adverse consequences for economic efficiency and / or component quality. It has also been found that pronounced silver coloring in combination with possibly inadequate wheel blasting can lead to problems during subsequent painting.

[0101] As already explained, the inventors discovered that silver coloration occurs when the heating rate exceeds a certain critical heating rate in a temperature range of 420 °C to 530 °C. Above this average critical heating rate, a silvery surface is to be expected.

[0102] As already mentioned, this also depends on the thickness of the zinc-based coating.

[0103] Figure 5 plots the average critical heating rate from 420 °C to 530 °C in K / s against the thickness of the zinc-based layer in micrometers. The average heating rate between 420 °C and 530 °C can be determined from a heating curve by dividing the temperature difference ΔΔΩ = 530 °C - 420 °C = 110 K by the time required ΔΩ = t(530 °C) - 1(420 °C). The curve represents the boundary line above which a silvery appearance cannot be ruled out or must be expected, while below this value a greenish or greenish-brownish surface appearance is achieved.

[0104] Figure 6 shows the temperature curve of a steel sheet prefabricated component or a steel sheet blank not only plotted against the furnace residence time as usual, but also schematically plotted against the length of a continuous furnace with a constant throughput speed. This is permissible for continuous furnaces with a constant throughput speed, as the furnace residence time is directly proportional to the furnace position. In the upper section of Figure 6, the furnace length is schematically divided into four furnace sections: section A with a temperature range from ambient temperature to 420 °C, section B with the critical temperature range from 420 °C to 530 °C, section C with 530 °C to 800 °C, and section D above 800 °C up to the desired final temperature.

[0105] Below, the temperature curve is plotted as a function of the furnace residence time and furnace position. In such a continuous furnace, it is important that the heating rate of the blank or pre-component in area B does not exceed a certain heating rate, as already explained, depending on the thickness of the zinc layer. However, since in a furnace, especially a continuous furnace, the cycle times cannot be tied to a specific area without changing the entire process in terms of time, the heating rates in areas A and especially C, where they are not critical, are increased accordingly.

[0106] In the area D, starting at 800 °C, further solutions can be provided according to the invention, namely that the atmosphere is adjusted at least closer than 10 mm to the surface of the blank or the steel pre-component so that the oxygen content is above 5 vol.%.

[0107] This can be achieved through several measures. Firstly, fresh air, especially preheated fresh air, can be blown into this area; secondly, or alternatively, the existing furnace atmosphere can be sufficiently circulated.

[0108] It is therefore advantageous if the heating rate can be adjusted variably and flexibly in areas A, B, C, and D. In particular, it is advantageous if the heating rate can be adjusted variably and flexibly depending on the zinc layer thickness or zinc coating thickness.

[0109] The heating rate, which is defined as r, can thus have values ​​ri in area A, r2in area B, r3in area C and r4in area D.

[0110] In an advantageous embodiment, ri > r2.

[0111] In a further advantageous embodiment, r2> r3.

[0112] In a particularly advantageous embodiment, ri > r2> r3.

[0113] As the zinc coating thickness d increases, the heating rate r is variably reduced. If dl < d2 < d3, the heating rate is adjusted so that r(dl) > r(d2) > r(d3).

[0114] While Figure 5 shows the average critical heating rate from 420 °C to 530 °C over the thickness of the zinc layer, Figure 7 shows a diagram in which the silveriness limit curves, above which silver coloration must be expected, are plotted as a critical heating rate at 530 °C as a function of the heating rate at 420 °C with an assumed linear progression between 420 °C and 530 °C for various zinc layer thicknesses. This clearly shows that the silveriness limit depends on the zinc layer thickness and also on the heating rate progression. If one takes a temperature difference between the temperatures T ± = 693 K and T2= 803 K the heating rate curve is linear as r(T) = i + (r2- ), the following results for different zinc layer thicknesses from the formula for S the graph according to Figure 7 for the silveriness limits.

[0115] Figure 8 shows two heating rates versus temperature, with a first heating rate starting at about 10 K / s and representing a classic heating rate that decreases to the final temperature.

[0116] The second heating rate starts at approximately 5 K / s, and this lower heating rate is maintained until the critical temperature of 530 °C is exceeded. After the zinc iron crystals reach the surface, the heating rate increases sharply, only to later decrease again in accordance with the decreasing temperature difference between the sample and the furnace.

[0117] Figure 9 shows a continuous furnace in longitudinal and cross-sectional views, with jacketed radiant tubes, which are, for example, gas-fired, providing the heating. In the critical temperature range between 420 °C and 530 °C, and if necessary also in upstream furnace areas, heating is carried out in such a way that the critical heating rate is not exceeded, while in the other areas, a significantly higher heating rate is possible.

[0118] Figure 10 shows the same heating curve for a multi-layer chamber furnace, wherein the furnace temperature is preferably kept constant and, if necessary, a reduction in the furnace temperature results in the critical temperature range being passed through at a correspondingly low heating rate. A furnace temperature that varies over the furnace residence time, ie a furnace temperature that is low at the beginning and then increased from 530 °C, is also conceivable, as shown in the figure on the right. Figure 11 shows where, in a continuous furnace, an atmosphere with over 5 vol.% oxygen is achieved above a temperature of 800 °C; this is also shown in the diagram showing the temperature curve of the blanks or preliminary components in the furnace.

[0119] Figure 12 shows a comparable curve for a multi-layer chamber furnace.

[0120] Figure 13 shows a comparison of the heating rate curves according to the prior art and a possible heating process according to the invention, using the example of a board coated with Z180. The dashed line shows a possible heating process using a prior art method.

[0121] Because the heating rate was too high in the critical temperature range of 420 °C to 530 °C, a silvery surface appearance was achieved. The silveriness index S = 0.8.

[0122] To the right, you can see a possible variant of a heating curve according to the invention and a silveriness index of S = 1.2. It can be seen that in this embodiment, both heating curves reach the target temperature of 905°C at the same time, i.e., after approximately 210 seconds. However, the heating according to the invention achieves a uniform, non-silvery surface appearance, since the changing gradient from 530°C onwards indicates the reaction of the zinc-iron crystals all the way to the surface. This prevents silver staining.

[0123] This makes it clear that the correct setting of the heating rate in the critical temperature range from 420 °C to 530 °C is crucial for the formation or non-formation of a silvery surface.

[0124] Figure 14 shows two possible variants of an inventive setting of heating rates on two boards coated with Z180. In both variants, the critical temperature range from 420 °C to 530 °C is passed through at the same, comparatively low heating rate. This reliably prevents silver discoloration (the silveriness number is approximately 1.2 in each case). The left line shows an increased heating rate in the temperature range above 530 °C. By increasing the heating power in this range, the furnace residence time was shortened by approximately 50 seconds. It can be clearly seen that with a constant heating power, which is represented by the dashed variant (right), although the furnace residence time is longer, energy consumption and thus CO2 emissions can be reduced compared to the other variant.

[0125] The invention is advantageous in that the heating rate adjustment according to the invention achieves repeatable results with a uniform, non-silvery surface appearance in the critical temperature range of 420 °C to 530 °C. The variable heating rate adjustment ensures homogeneous heating behavior in the furnace depending on the zinc coating thickness.

[0126] The ability to variably adjust heating rates allows heating rates below 420 °C and above 530 °C to be increased without increasing the risk of silver staining. This shortens furnace residence time, reduces energy consumption, and reduces CO2 emissions. Overall, process efficiency is significantly increased.

Claims

Claims 1. A method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat hot- and cold-rolled steel strip coated with a zinc-based coating, and the sheet steel blank is heated, at least in part, either to a temperature above Ac3 and then formed in a press-hardening tool in the hot state and quench-hardened, or the sheet steel blank is cold-formed to form a sheet steel pre-component, and the sheet steel pre-component is heated, at least in part, to a temperature above Ac3 and then quench-hardened in a press-hardening tool, characterized in that the heating takes place in the temperature range from 420 °C to 530 °C in such a way that the silveriness number S is greater than 1, where: where r(T) is the heating rate in Kelvin per second, T is the temperature in Kelvin, r(T) > 0 in the temperature range 7^ < T < T2 and 7^ = 693 K, T2 = 803 K , d is the zinc layer thickness in m, constant b = 4.0 * 10 7 m 2 / s, constant d0= 4.4 * 10“ 6 m, Q is the activation energy Q = 271 kJ / mol and R is the universal gas constant R = 8.31 J / mol / K and the heating takes place particularly in the temperature range from 420 °C to 530 °C with a heating rate that is variable over the furnace residence time and dependent on the zinc layer thickness.

2. The method according to claim 1, wherein with increasing zinc coating thickness t / the heating rate r is reduced, so that when dl < d2 < d3, the heating rate is adjusted such that r(dl) > r(d2) > r(d3).

3. Method according to one of the preceding claims, characterized in that the average heating rate between 420 °C and 530 °C is less than 15 K / s for the zinc coating Z80, less than 8.6 K / s for the zinc coating Z140 and less than 6 K / s for the zinc coating Z180.

4. Method according to one of the preceding claims, characterized in that for heating above 800 °C an oxygen content in the furnace atmosphere in a region closer than 10 mm to the surface of the blank or the steel pre-component of at least 5 vol.% is ensured.

5. Method according to one of the preceding claims, characterized in that the heating rate is set in the temperature range from 420 °C to 530 °C, between 3 K / s and 15 K / s and below the respective critical heating rate, wherein the critical heating rate depends on the thickness of the zinc-based layer and becomes smaller with increasing thickness.

6. Method according to one of the preceding claims, characterized in that in a continuous furnace in the furnace region corresponding to a blank or pre-component temperature of 420 to 530 °C or in the furnace region corresponding to a blank or pre-component temperature up to 530 °C, the furnace heating power is reduced such that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded.

7. Method according to one of the preceding claims, characterized in that in the areas in which the blank or the pre-component has a temperature of less than 420 °C and / or in particular in the areas in which the blank or the pre-component has a temperature of more than 530 °C, the furnace heating power is increased in order to keep the cycle time as short as possible.

8. Method according to one of the preceding claims, characterized in that in furnace areas in which the blank or the pre-component has a temperature of 800 °C or more, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere.

9. Method according to one of the preceding claims, characterized in that in furnace areas in which the blank or the pre-component has a temperature of 800 °C or more, the existing furnace atmosphere is circulated.

10. Method according to one of the preceding claims, characterized in that in heating devices in which the blank or the steel pre-component is heated in a stationary manner, such as multi-layer chamber furnaces, the furnace heating power is reduced permanently or over time in such a way that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded.

11. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness of 5 pm to 16 pm per side.

12. Method according to one of the preceding claims, characterized in that the zinc-based coating was applied by means of a hot-dip galvanizing process, in particular by hot-dip galvanizing.

13. Method according to one of the preceding claims, characterized in that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably a 22MnB5 or 20MnB8 or 34MnB5.

14. A method according to claim 13, characterized in that the steel strip used is a strip with the following composition (all data in wt.%): Carbon up to 0.4, preferably 0.15 to 0.35 and Silicon up to 1.9, preferably 0.11 to 1.5 and Manganese up to 3.0, preferably 0.8 to 2.5 and Chromium up to 1.5, preferably 0.1 to 0.9 and Molybdenum up to 0.9, preferably 0.001 to 0.1 and nickel up to 0.9, preferably 0.01 to 0.2 and Titanium up to 0.2 preferably 0.02 to 0.1 and Vanadium up to 0.2 and Tungsten up to 0.2 and Aluminium up to 0.2, preferably 0.02 to 0.07 and boron up to 0.01, preferably 0.0005 to 0.005 and Sulphur max. 0.01, preferably max. 0.008 and Phosphorus max. 0.025, preferably max. 0.01 and Rest iron and smelting-related impurities.

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