Method for producing a hardened sheet steel component

Abstract

The invention relates to a method for producing components made of sheet steel. A sheet steel blank is cut from a flat steel strip coated with a zinc-iron-containing coating, and the sheet steel blank is either heated at least in some regions to a temperature above Ac3 and subsequently shaped and quench-hardened in a hot state in a press-hardening tool or the coated sheet steel blank is cold-formed in order to form a sheet steel pre-component and the sheet steel pre-component is heated at least in some regions to a temperature above Ac3 and subsequently quench-hardened in a form-hardening tool. The heating process is carried out in the temperature range from room temperature to 530 °C at an average heating rate r1 and in the temperature range between 530 °C and 670 °C at an average heating rate r2, wherein (I).

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 0.10 to 0.30 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 up 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 heated to the required austenitizing temperature and, if necessary, held at this temperature. This blank is then transferred to a forming press, where the blank is formed while hot, preferably in a single stroke, and then quenched by the cold press-hardening tool. Since only one forming stroke and only one tool is 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 usually do not have an overly complex shape, since only one forming stroke is 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. A blank is cut from 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 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, there are also so-called ZF coatings, also known as GA coatings or galvannealed coatings7, in which steel sheets are heat-treated in the continuous hot-dip galvanizing plant shortly after the zinc coating, typically in the temperature range 460°C to 570°C, thus creating a zinc-iron alloy layer on the steel strip. The iron contained in the substrate thus reacts with the zinc from the coating material. However, this is only the case with so-called ZF coatings. With these coatings, zinc is first deposited on the steel substrate and then subjected to a heat treatment. This heat treatment is referred to as galvannealing. The layer itself is also called galvannealed.

[0016] During heat treatment, an alloying reaction occurs between the iron from the steel and the zinc, so that after heat treatment the layer is a zinc-iron alloy layer.

[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 stamping and press hardening, cracks typically form in this very thin ALOs layer, resulting in minimal zinc evaporation and the formation and deposition of zinc oxide on the surface. After press hardening or hot stamping, 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.

[0020] It was further discovered that the silver color 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 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.

[0021] Different and fluctuating coloring of the surface of a steel component is not necessarily a quality feature of the surface itself, but can lead to problems.

[0022] Furthermore, it is known that circuit boards or components heat up differently and not entirely homogeneously in the furnace. Furthermore, different heating rates can result in different iron contents in the coating, which in turn leads to different emissivities and, in turn, differences in heating behavior.

[0023] Inhomogeneous heating behavior of blanks during press hardening or components during die hardening is detrimental to energy-optimized, process-reliable and economical production. During production, it must be ensured that the blanks or components are fully austenitized in every case so that they can then be fully hardened. Therefore, the minimum furnace residence time during production must be based on the worst case scenario, i.e. the slowest heating or the lowest emissivities, even if these cases only make up a fraction of the production quantity. This means that in production with inhomogeneous heating behavior, the vast majority of blanks or components are usually in the furnace longer than necessary, which is detrimental to the vast majority of blanks or components.Components lead to a lower output and a reduced process window and thus increasing rejects than would be the case with a more homogeneous heating behavior.

[0024] Furthermore, during hot stamping, varying and fluctuating surface coloration can lead to dimensional stability problems, as the size of the finished hardened component depends on the size of the austenitized pre-component inserted into the hot stamping tool. The size of the austenitized pre-component inserted into the hot stamping tool depends on the cooling behavior of the pre-component between the furnace and the hot stamping tool, which in turn depends on the emissivity of the surface of the pre-component due to radiant heat losses. The formation of the surface zinc oxide layer is also influenced by this. This occurs particularly with thin zinc-iron layers and thin sheets, especially with sheet thicknesses below 1.5 mm and zinc layer thicknesses such as ZF 80 according to EN 10346 - this means that the layer thickness must be approximately 35 g / m 2 per page.

[0025] 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.

[0026] 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.

[0027] From CN 115125439 A it is known to heat a steel 34MnB8 with a zinc-iron coating with holding zones and comparatively low heating rates.

[0028] The opposite is known from EP 2611945 B1, namely a rapid heating of sheets with a zinc-iron coating, whereby heating is to take place from room temperature to 500 °C at a minimum of 15 K / s to 50 K / s.

[0029] The object of the invention is to create a method for producing sheet steel components and to ensure homogeneous properties of the components.

[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, heating is carried out in such a way that optimized heating is always ensured, tailored to the sheet and coating, resulting in more homogeneous properties. A formula is provided for this purpose, simplifying the process. Heating can be achieved by radiation or convection.

[0033] In another embodiment, it can be done by radiation and convection.

[0034] Heating by radiation can be achieved, for example, by an infrared radiator.

[0035] In order to achieve optimal heating, a heating rate is set that is variable over the furnace residence time.

[0036] In particular, the temperature differences upon reaching the target temperature across the entire steel sheet blank or the steel sheet pre-component should not exceed 25 K, preferably 18 K, and more preferably 15 K. This can further homogenize the layer formation, in particular the reaction, which occurs particularly pronounced in the temperature range up to 530 °C.

[0037] According to the invention, it was found that heating according to EP 2611945 B1 does not lead to success, but on the contrary increases the problems.

[0038] According to the invention, a steel sheet plate or a steel strip is used which has a zinc-iron-containing coating (ZF).

[0039] This layer can advantageously have a thickness of 4 μm to 14 μm per side. This can ensure good corrosion protection. In particular, the coating can be a ZF60, ZF80, ZF90, ZF100, ZF120, ZF140, or ZF180 according to DIN EN 10346.

[0040] Particularly preferably, the metallic corrosion protection layer can be applied using a hot-dip process. It is then galvannealed. This can be a simple and robust application method. According to the invention, a press-hardened component coated with a zinc-iron layer (ZF) is heated to a level that provides a homogeneous zinc oxide layer, allowing for uniform heating of the blank or sheet steel component. This results in excellent mechanical properties and ensures optimal processing characteristics.

[0041] In addition, the invention enables an energy-optimized (reduced heat input into the annealing material and / or reduced heat losses) and process-reliable (assured austenitization) and, with low loads, also economically advantageous (rapid heating even at low furnace temperatures) operation.

[0042] The heat requirement for heating sheet steel blanks or sheet steel prefabricated components in a furnace consists of the amount of heat required for the desired heating of the annealing material (the blank or the component), plus the amount of heat due to unwanted furnace losses, e.g., at furnace doors, furnace shell, or transport elements. Both can be minimized by the reduced heating rate according to the invention.

[0043] On the one hand, for example, the first furnace zone, i.e. the zone until 530 °C is reached on the blank or part, can be set "cooler" than in state-of-the-art processes. Alternatively or additionally, the entire furnace temperature can also be reduced, for example, to a range of 860 °C to 890 °C compared to the previously usual furnace temperatures of 900 °C to 920 °C.

[0044] An energy-optimized, process-reliable and economically advantageous operation is provided especially for runs ZF100 and smaller.

[0045] Steel grades such as 34MnB5 or 20MnB8 can be processed with even greater energy efficiency, process reliability, and cost-effectiveness, as they have lower austenitizing temperatures than other steel grades. Other boron-manganese grades with tensile strengths of 400 to 1200 MPa after the hardening process are also conceivable.

[0046] Reduced energy consumption has a direct impact on CO2 emissions and can therefore be implemented in a sustainable and environmentally friendly manner. The invention has shown that the most uniform emission levels possible, and thus a uniform heating and cooling behavior of the circuit boards or prefabricated components, can be ensured by selecting a heating rate that is below a certain average heating rate in a specific temperature range. Furthermore, a different heating rate can be deliberately set in the other temperature ranges to ensure optimal heating.

[0047] The basic idea is to reduce the temperature of at least the first furnace zones of a continuous furnace or the furnace chamber temperature of a multi-layer chamber furnace and thus to reduce the average heating rates.

[0048] It is advantageous to set the average heating rate to a maximum of 14 K / s in the temperature range from room temperature (RT) to 530 °C. This applies in particular to coatings ZF60 to ZF180, preferably on steel strip material 22MnB5 or 20MnB8 or 34MnB8.

[0049] In an advantageous embodiment, the average heating rate can be set to a maximum of 13 K / s, 12 K / s, 11 K / s, 10 K / s or 9 K / s.

[0050] Advantageously, heating takes place in the temperature range from room temperature to 530 °C with an average heating rate rl and in the temperature range between 530 °C and 670 °C with an average heating rate r2 where the heating rates are set such that: rl < —

[0051] 7*2

[0052] In particular, the following relationship should apply: rl < —

[0053] The average heating rate within the meaning of the invention is the average heating rate of the steel sheet blank or steel sheet pre-assembly during heating from room temperature to 530°C. This means that the heating rate can be comparatively high, particularly during the initial heating phase to 100°C and more, and then decreases continuously as the temperature difference between the steel sheet blank or steel sheet pre-assembly and the furnace temperature decreases. The average heating rate is calculated over the heating period from room temperature to 530°C. This means that if, for example, 51 seconds are required to reach 530°C in the furnace at a room temperature of 20°C, the average heating rate is 10 K / s.

[0054] The sheet thickness range of the steel strip can be selected from 0.85 mm to 3 mm, preferably 0.9 mm to 2 mm.

[0055] It was found that an optimal zinc oxide layer is achieved by selecting an appropriate average heating rate rl in the range of 3 to 14 K / s, particularly 6 to 14 K / s, in the target temperature range from ambient to 530 °C. The optimal heating rate range can be determined by calculating the sheet thickness.

[0056] The formula according to the invention for the average heating rate rl from RT to 530 °C depending on the sheet thickness d (in mm) is:

[0057] According to the invention, the constant a (in K / s) is 6.37 K / s and is valid in the sheet thickness range d from 0.85 mm to 3 mm.

[0058] According to the invention, the temperature homogeneity of the steel sheet blank can be increased across the entire blank when heated to 530 °C. The maximum temperature difference across the blank is less than 25 K.

[0059] In addition, it was found that it is useful, alternatively or additionally, to ensure a certain oxygen saturation on the sheet surface above a certain temperature.

[0060] Particularly in a temperature range above 800 °C, it can be advantageous to adjust the furnace atmosphere so that at least 5 vol.% oxygen is present at least at the surface of the components to be heated. Regarding oxygen saturation, it has been found that the oxygen available for zinc oxidation is influenced, especially during long production intervals, by burning oil, dew point, weather conditions, new furnace elements, and the strength of convection in the furnace.

[0061] 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.

[0062] The dew point is determined in particular by the climate zone and the system environment.

[0063] 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.

[0064] It was found that the composition of the furnace atmosphere, especially with regard to the oxygen content, can be locally inhomogeneous.

[0065] 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.

[0066] 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.

[0067] The invention thus relates to a method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat steel strip with a zinc-iron-containing coating and the sheet steel blank is either heated at least in part to a temperature above Ac3 and then formed in a press-hardening tool in the hot state and quench-hardened, or the coated sheet steel blank is cold-formed to 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 is carried out at a heating rate that is variable over the furnace residence time, wherein the heating is carried out in the temperature range from room temperature to 530 °C with an average heating rate rl and in the temperature range between 530 °C and 670 °C with an average heating rate r2, where rl < r2 0.4

[0068] An advantageous further development provides that the average heating rate r2 is set below 7 K / s.

[0069] An advantageous further development provides that the average heating rate r2 is set between 1.6 K / s and 5.6 K / s.

[0070] An advantageous further development provides that the average heating rate rl is set below 14 K / s.

[0071] An advantageous further development provides that the average heating rate rl is set between 3 K / s and 14 K / s.

[0072] An advantageous further development provides that with increasing sheet thickness d the heating rate rl is reduced accordingly, so that if dl < d2 < d3, the heating rate rl is set such that rl(dl) > rl(d2) > rl(d3).

[0073] An advantageous further development provides that the maximum average heating rate rl is selected in the range from room temperature to 530 °C, which obeys the following formula for the sheet thickness d in mm: rl < O (l + i) , where: a = 6.37 K / s

[0074] Valid for the sheet thickness range d from 0.85 mm to 3 mm

[0075] This can advantageously reduce the maximum temperature difference on the steel sheet heated to 530 °C to below 25 K, thus increasing the homogeneity of the temperature distribution on the sheet or pre-component. This can have a beneficial effect on layer formation, particularly on the reaction. In a preferred variant, a = 5.32 K / s is selected. This can reduce the maximum temperature difference on the steel sheet heated to 530 °C to below 18 K, further enhancing the aforementioned effects.

[0076] An advantageous further development provides for the furnace heating power to be reduced, at least in the first furnace zones, so that the average heating rate is reduced or set to a maximum of 14 K / s from room temperature to 530°C. This can advantageously minimize energy losses in the furnace inlet area, since the steel sheet blank or prefabricated components are fed into the furnace here, and losses can be minimized by lowering the temperature in the first furnace zone.

[0077] An advantageous further development provides for heating in the temperature range from 530°C to 670°C at an average heating rate r2, where r2 < 0.9 * rl . This advantageously allows for the decomposition of the delta-ZnFe phase to be evened out and for oxide formation to be favorably influenced for further processing properties such as welding or phosphating.

[0078] An advantageous further development provides that the heating in the temperature range above r2

[0079] 670°C to 780°C with an average heating rate r3, where: — < r3 < r2. This advantageously evens out the decomposition of the gamma-ZnFe phase, ensuring more homogeneous heating of the entire surface and making the subsequent cooling behavior more homogeneous. An advantageous further development provides for the zinc-iron-containing coating to have an iron content of 8 to 14%, in particular 8 to 12%, with the remainder being zinc. The iron content can advantageously be within this range to ensure optimal heating.

[0080] A further development stipulates that the coating was selected from the specifications ZF60 to ZF180 according to DIN EN 10346. ZF60, i.e. a zinc-iron-containing coating with about 30 g / m 2Each side can already provide a certain degree of corrosion protection and is also easy to process. Depending on the corrosion protection requirements, a thicker layer of up to 90 g / m 2 be advantageous for each side.

[0081] An advantageous further development provides that the heating takes place in the temperature range from room temperature to 530 °C at an average heating rate rl of at least 3 K / s, preferably at least 6 K / s. Advantageously, the average heating rate rl can be above 6 K / s in order to shorten the heating time and keep the cycle time low.

[0082] 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.

[0083] An advantageous further development provides that in a continuous furnace 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 heating rate of 14 K / s is not exceeded in the temperature range up to 530 °C.

[0084] An advantageous further development provides for the furnace heating power to be increased in areas where the blank or pre-assembled part has a temperature of more than 530 °C. This can keep the cycle time as short as possible.

[0085] An advantageous refinement provides for the overall furnace temperature to be reduced to a range of 860 °C to 890 °C. This advantageously minimizes energy consumption and CO2 emissions. An advantageous refinement provides for the furnace heating power to be reduced or kept constant in the furnace areas where the blank or pre-assembled part has a temperature of more than 530 °C. This advantageously minimizes energy consumption and CO2 emissions.

[0086] 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.

[0087] 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.

[0088] 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 average heating rate of 14 K / s is not exceeded in the temperature range from room temperature to 530 °C.

[0089] An advantageous further development provides that the zinc-iron-containing coating has a layer thickness of 4 pm to 14 pm per side.

[0090] An advantageous further development provides for the zinc-iron-containing coating to be applied using a hot-dip galvanizing process, particularly hot-dip galvanizing, and then galvannealed. This can be a simple and robust process with outstanding reproducibility.

[0091] An advantageous further development provides that the steel strip has a sheet thickness of 0.85 mm to 3 mm.

[0092] An advantageous development provides that the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel, and particularly preferably a 22MnB5 or 20MnB8 or 34MnB5. An advantageous development provides that the steel strip is made of a strip with the following composition (all data in wt.%):

[0093] Carbon up to 0.4, preferably 0.10 to 0.30 and

[0094] Silicon up to 1.9, preferably 0.11 to 1.5 and

[0095] Manganese up to 3.0, preferably 0.8 to 2.5 and

[0096] Chromium up to 1.5, preferably 0.1 to 0.9 and

[0097] Molybdenum up to 0.9, preferably 0.001 to 0.1 and

[0098] Nickel up to 0.9, preferably up to 0.2 and

[0099] Titanium up to 0.2 preferably 0.02 to 0.1 and

[0100] Vanadium up to 0.2 and

[0101] Tungsten up to 0.2 and

[0102] Aluminium up to 0.2, preferably 0.02 to 0.07 and

[0103] Boron up to 0.01, preferably 0.0005 to 0.005 and

[0104] Sulphur max. 0.01, preferably max. 0.008 and

[0105] Phosphorus max. 0.025, preferably max. 0.01 and

[0106] Rest iron and smelting-related impurities.

[0107] The invention is explained by way of example with reference to the drawings, which show:

[0108] Figure 1: the preferred average heating rate range from RT to 530 °C across the sheet thickness for a first constant a;

[0109] Figure 2: the preferred average heating rate range from RT to 530 °C across the sheet thickness for a second constant a; Figure 3: a possible furnace temperature in the zone up to 530 °C for a first constant a;

[0110] Figure 4: the temperature distribution over the board with a heating rate rl after

[0111] State of the art;

[0112] Figure 5: two sheet metal samples approx. 220 mm x 300 mm with different heating rates in

[0113] top view;

[0114] Figure 6: two sheet metal samples with different heating rates in cross-section;

[0115] Figure 7: an example heating curve for ZF110.

[0116] According to the invention, a press-hardened component coated with a zinc-iron layer (ZF) is heated to form a homogeneous zinc oxide layer, allowing for uniform heating of the circuit board. This ensures excellent mechanical properties and optimal processing characteristics.

[0117] Furthermore, the invention enables energy-optimized operation (reduced heat input into the annealing material and / or reduced heat losses) because the furnace zone can be operated at a reduced temperature until the sheet metal blank reaches 530 °C, thus minimizing energy losses in the first furnace zone. This reduces energy costs and increases process efficiency.

[0118] 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 rl is selected in a certain temperature range which is below a critical heating rate.

[0119] The basic idea here is to reduce the temperature of the first furnace zones and thus reduce the average heating rates rl from room temperature (RT) to 530 °C. Thus, the average heating rate rl should be set such that it satisfies the following relationship, where r2 is the average heating rate r2 in the range from 530°C to 670°C, where: ' 3 rl < — 0.4

[0120] 7*2 in particular the following shall apply: rl < —

[0121] It is particularly advantageous if the heating rate rl is set to a maximum of 14 K / s. This applies especially to coatings ZF60 to ZF180 with a sheet thickness of 0.85 mm to 3 mm.

[0122] For the purposes of the invention, room temperature is the initial temperature at which sheets or prefabricated components are placed before they are placed in a furnace. It is the temperature prevailing in the production facility or slightly warmed by a warm workpiece carrier; i.e., an initial temperature of 50°C or 60°C is also conceivable. This temperature can, of course, fluctuate seasonally, but its fluctuation range is irrelevant for the subsequent process.

[0123] In case a clear definition is needed, the room temperature is set to the standard temperature of 20 °C.

[0124] The temperatures listed below are in °C. Temperature differences may be indicated in K (Kelvin). Unless otherwise stated, alloy contents are given in mass percent (M-%).

[0125] It was found that with the appropriate selection of an average heating rate rl in the range of 4 to 14 K / s, in particular 6 to 14 K / s, a uniform zinc oxide layer is formed in the target temperature range from RT to 530 °C.

[0126] The formula according to the invention for the average heating rate rl depending on the sheet thickness d (in mm) is: where: a = 6.37 K / s valid for the sheet thickness range d from 0.85 mm to 3 mm.

[0127] Thus, the average heating rate rl is adjusted depending on the sheet thickness. As the sheet thickness increases, the heating rate rl is reduced accordingly, so that when dl < d2 < d3, the heating rate rl is adjusted such that rl(dl) > rl(d2) > rl(d3).

[0128] This can advantageously reduce the maximum temperature difference on the steel sheet heated to 530 °C to below 25 K, thus increasing homogeneity. In a preferred variant, a = 5.32 K / s is selected. This can reduce the maximum temperature difference on the steel sheet heated to 530 °C to below 18 K.

[0129] As already mentioned, blanks or formed prefabricated components heat up inhomogeneously during heating by radiation or convection for the purpose of austenitization, particularly from the outside in. This effect is amplified in a zinc-iron alloy (ZF) due to locally varying iron contents. This also influences the formation of the zinc oxide layer due to the locally inhomogeneous heating.

[0130] As explained, it is surprising that an optimal zinc oxide layer is formed at an average heating rate rl in the range of 4 to 14 Kelvin per second in the target temperature range from room temperature to 530 °C.

[0131] This effect also occurs when the heating rate r2 is set between 1.6 and 5.6 Kelvin per second in the temperature range from 530°C to 670°C.

[0132] In a particularly advantageous embodiment, the heating rate rl is set in the range of 4 to 14 Kelvin per second in the target temperature range from room temperature to 530 °C, and the heating rate r2 is set between 1.6 and 5.6 Kelvin per second in the temperature range from 530 °C to 670 °C in order to achieve an optimal zinc oxide layer. Figure 1 shows the preferred average heating rate range rl across the sheet thickness, whereby it can be seen that the maximum permissible average heating rate rl decreases across the sheet thickness. Figure 1 shows the representation for the constant a = 6.37 K / s - Figure 2 shows the average heating rate rl across the sheet thickness for a = 5.32 K / s. This can further increase the homogeneity of the heating by reducing the maximum permissible average heating rate rl. In addition, the particularly preferred lower limit of 6 K / s is shown as a dashed line.

[0133] The effect according to the invention is also visually visible in Figure 5. Here, two 1.5 mm thick steel sheet samples measuring approximately 220 mm x 300 mm are heated at different heating rates. In the left image, the layer is absolutely uniform due to the low heating rate rl of approximately 6.5 K / s up to 530 °C used according to the invention. While in the right image, at a heating rate rl of approximately 20 K / s up to 530 °C, an edge effect is evident due to uneven layer growth.

[0134] This can also be seen in Figure 6, where the ZF80 coating was applied on the right with a heating rate rl according to the state of the art from room temperature to 530 °C at 20 Kelvin per second. Compared to the left-hand illustration, which shows the same coating but with a heating rate rl according to the invention, in this case at 7.7 Kelvin per second, it can be seen that the coating and surface morphology are considerably more uniform. Here, too, the sheet thickness was 1.5 mm.

[0135] Figure 4 shows how the temperatures behave in different areas of the sheet at a heating rate rl of approximately 18 K / s, and significant differences are evident, particularly in the upper temperature ranges. However, even at temperatures of around 530 °C, temperature differences of up to 40 K are possible on a blank. This can also significantly influence the coating reaction and thus lead to undesirable effects in the coating.

[0136] A suitable heating unit is, for example, a continuous furnace, in which jacketed radiant tubes, which are, for example, gas-fired, perform the heating. In the critical temperature range between room temperature and 530 °C, heating is carried out in such a way that the critical heating rate rl is not exceeded, while in the ranges above this, a significantly higher heating rate r2 and / or r3 is also possible.

[0137] Thus, the heating rate rl is set to satisfy the following relationship: especially 0.5

[0138] A multi-layer chamber furnace is also suitable; in any case, heating should be carried out by radiation and convection. In this case, the furnace temperature is preferably kept constant and, if necessary, a reduction in the furnace temperature leads to the critical temperature range being passed through at a correspondingly low heating rate rl. However, a furnace temperature that varies over the furnace residence time, i.e., a low furnace temperature at the beginning (i.e., rl) and then increased from 530 °C to adjust r2 and / or r3, is also conceivable. This is shown, for example, in Figure 3; the furnace temperature is reduced in the first zone and adjusted accordingly depending on the sheet thickness.

[0139] Figure 7 shows an example heating curve for an oiled steel plate coated with ZF110. It can be seen that the heating rate rl from room temperature to 530°C is set relatively low in order to achieve the most homogeneous heating possible. rl and r2 were chosen such that the relationship rl < — applies. For example, rl can be set at 10 K / sec and r2 at 5 K / sec and r3 at 3 K / sec.

[0140] From 530°C to 670°C the heating rate r2 is set, which is set lower than rl * 0.9, so that the relationship r2 < 0.9 * rl applies In this example

[0141] From 670°C to 780°C, the heating rate r3 was further reduced to achieve the most homogeneous heating of the entire surface of the blank. At the same time, the heating rate should not be too low to avoid excessively extending the furnace residence time. Therefore, r3 2 r2 was chosen between — and r2, so that the relationship — < r3 < r2 holds.

[0142] Accordingly, it is advantageous if the heating rates rl, r2, and r3 are variably adjusted over the furnace residence time. It is particularly advantageous if the heating rate rl is adjusted depending on the sheet thickness in the temperature range from room temperature to 530 °C.

[0143] The invention is advantageous in that the inventive adjustment of the heating rate rl in the critical temperature range from room temperature to 530 °C allows for repeatable results with regard to a uniform, homogeneous coating. A heating rate that can be varied over the furnace residence time advantageously enables a targeted combination of homogeneous heating and optimal furnace residence time.

[0144] The heating rates rl, r2 and r3 can be adjusted variably without causing undesirable edge effects due to uneven layer growth.

[0145] The ability to increase heating rates, especially in the temperature range above 530 °C, leads to a shortened furnace residence time. This reduces costs and CO2 emissions, thus increasing process efficiency.

Claims

Claims 1. A method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat steel strip with a zinc-iron-containing coating and the sheet steel blank is either heated at least in part to a temperature above Ac3 and then formed in a press-hardening tool in the hot state and quench-hardened, or the coated 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 at a heating rate which is variable over the furnace residence time, the heating taking place in the temperature range from room temperature to 530 °C with an average heating rate rl and in the temperature range between 530 °C and 670 °C with an average heating rate r2, where rl < - .

2. The method according to claim 1, wherein the average heating rate r2 is set below 7 K / s.

3. The method according to claim 1 or 2, wherein the average heating rate r2 is set between 1.6 K / s and 5.6 K / s.

4. Method according to one of the preceding claims, wherein the average heating rate rl is set below 14 K / s.

5. Method according to one of the preceding claims, wherein the average heating rate rl is set between 3 K / s and 14 K / s.

6. Method according to one of the preceding claims, wherein with increasing sheet thickness d the heating rate rl is reduced accordingly, so that if dl < d2 < d3, the heating rate rl is set such that rl(dl) > rl(d2) > rl(d3).

7. Method according to one of the preceding claims, characterized in that the maximum average heating rate rl is selected in the range from room temperature to 530 °C, which obeys the following formula for the sheet thickness d in mm: r l £ “ (1 + j) ' where: a = 6.37 K / s Valid for the sheet thickness range d from 0.85 mm to 3 mm 8. Method according to one of the preceding claims, characterized in that the furnace heating power is reduced at least in the first furnace zones, so that the average heating rate is reduced or set to a maximum of 14 K / s from room temperature to 530°C.

9. Method according to one of the preceding claims, characterized in that the heating takes place in the temperature range from 530°C to 670°C with an average heating rate r2, where: r2 < 0.9 * rl 10. Method according to one of the preceding claims, characterized in that the heating takes place in the temperature range above 670°C to 780°C with an average heating rate r3, where:

11. Method according to one of the preceding claims, characterized in that the zinc-iron-containing coating has an iron content of 8 to 14%, in particular 8 to 12%, and the remainder is zinc.

12. Method according to one of the preceding claims, characterized in that the coating was selected from the specification ZF60 to ZF180.

13. Method according to one of the preceding claims, characterized in that the heating takes place in the temperature range from room temperature to 530 °C with an average heating rate rl of at least 3 K / s, preferably at least 6 K / s.

14. 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.

15. 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 up to 530 °C, the furnace heating power is reduced such that the heating rate of 14 K / s is not exceeded in the temperature range up to 530 °C.

16. Method according to one of the preceding claims, characterized in that the furnace heating power is increased in the areas in which the blank or the pre-component has a temperature of more than 530 °C.

17. A method according to any one of the preceding claims, characterized in that the total furnace temperature is reduced to a range of 860 °C to 890 °C.

18. Method according to one of the preceding claims, characterized in that in the furnace areas in which the blank or the pre-component has a temperature of more than 530 °C, the furnace heating power is reduced or kept constant.

19. 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.

20. 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.

21. 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 average heating rate of 14 K / s is not exceeded in the temperature range from room temperature to 530 °C.

22. Method according to one of the preceding claims, characterized in that the zinc-iron-containing coating has a layer thickness of 4 pm to 14 pm per side.

23. Method according to one of the preceding claims, characterized in that the zinc-iron-containing coating was applied by means of a hot-dip galvanizing process, in particular by hot-dip galvanizing, and then galvannealed.

24. Method according to one of the preceding claims, characterized in that the steel strip has a sheet thickness of 0.85 mm to 3 mm.

25. 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.

26. Method according to one of the preceding claims, characterized in that the steel strip used is a strip with the following composition (all data in % by weight): Carbon up to 0.4, preferably 0.10 to 0.30 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 up 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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