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 hot-rolled and cold-rolled steel strip coated with a zinc-based 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 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 at a heating rate which varies over the furnace dwell time and which is based on the zinc layer thickness, wherein 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 a maximum heating rate r2, wherein r1 < r2.

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 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. 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 quickly transferred to a hot stamping press, where the 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 consisting essentially of zinc with elements with a higher affinity for oxygen, so-called ZF coatings, also called GA coatings, are also known. In these coatings, a heat treatment follows the zinc coating, thus creating a zinc-iron alloy layer on the steel strip. A coating consisting predominantly of zinc, within the meaning of the invention, comprises no less than 90 wt.% zinc. Advantageously, such a zinc-based coating contains no less than 92 wt.%, preferably no less than 94 wt.%, and particularly preferably no less than 96 wt.% zinc.

[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 ALOs 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, inconsistently and unpredictably, exhibit a silver coloration, either entirely or only in certain areas, in addition to the familiar greenish-brownish 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. It has also been 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 there is a lack of oxygen at higher temperatures. The temperature of approximately 530 °C marks the end of the existence of the so-called zeta-zinc-iron phase.

[0020] 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.

[0021] Furthermore, it is known that circuit boards or components heat up differently and not entirely homogeneously in the furnace. For example, heating from the outside to the inside is not homogeneous. 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.

[0022] 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.

[0023] Furthermore, during hot-press hardening, 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-press hardening tool. The size of the austenitized pre-component inserted into the hot-press hardening tool depends on the cooling behavior of the pre-component between the furnace and the hot-press hardening tool, which, due to radiant heat losses, in turn depends on the emissivity of the surface of the pre-component.

[0024] The formation of the surface zinc oxide layer is also affected. This occurs particularly with comparatively thin zinc coatings and thin sheets, especially with sheet thicknesses below 1.5 mm and zinc layer thicknesses such as Z 80 according to EN 10346 – this means that the layer thickness is approximately 40 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 CN115125439 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 Bl, namely a rapid heating of sheets with a zinc-iron coating, whereby heating from room temperature to 500°C is to take place at a rate of at least 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 developments are characterized in the dependent subclaims. According to the invention, heating is carried out in such a way that, tailored to the sheet and coating, optimized heating with variable heating rates is always ensured over the furnace residence time, resulting in more homogeneous properties. A formula is provided for this purpose, which leads to reliable process control.

[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 Z40, Z60, 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 wt.% to 99 wt.% and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2 wt.%.

[0035] Particularly preferably, the zinc-based metallic corrosion protection layer can be applied using a hot-dip galvanizing process. This can be a simple and robust application method.

[0036] According to the invention, a press-hardened component coated with a zinc-based coating is heated in such a way that it has homogeneous zinc oxide layers, a uniform heating of the circuit board is possible, resulting in excellent mechanical properties and ensuring optimal processing properties.

[0037] 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 small batch sizes, also economically advantageous (rapid heating even at low furnace temperatures). The heat requirement when heating sheet steel blanks or sheet steel prefabricated components in a furnace is composed of the heat quantity required for the desired heating of the annealing material (the blank or part) plus the heat quantity due to undesirable 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. On the one hand, for example, the first furnace zone, i.e. the zone up to which 530 °C is reached at the blank or part, can be set "cooler" than in prior 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.

[0038] An energy-optimized, process-reliable and economically advantageous operation is provided especially for runs of Z100 and smaller.

[0039] Steel grades such as 34MnB5 or 20MnB8 can be processed in an even more energy-optimized, process-reliable and economical manner, as they have lower austenitizing temperatures than other steel grades.

[0040] Reduced energy consumption has a direct impact on CO2 emissions and can therefore be implemented in a sustainable and environmentally friendly manner.

[0041] According to the invention, it was found that the most homogeneous emissivity possible over the entire surface and thus a uniform heating and cooling behavior for heated circuit boards or pre-assembled parts can be ensured if the heating rates are selected in an optimally coordinated manner, but at least the first heating rate rl until 530°C is reached is below a certain average heating rate, for example the maximum heating rate r2.

[0042] Heating can occur through radiation or convection.

[0043] In another embodiment, it can be done by radiation and convection.

[0044] Heating by radiation can be achieved, for example, by an infrared radiator. Furthermore, the invention recognizes that one or more heating rates should be set that vary over the furnace residence time and depend on the zinc layer thickness in order to advantageously ensure high-quality layer and oxide formation of the zinc-based layer.

[0045] 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.

[0046] The average heating rate is advantageously set to a maximum of 14 K / s in the temperature range from room temperature (RT) to 530 °C. This applies particularly to coatings Z40 to Z180 on 20MnB8 to 34MnB8 with a sheet thickness of 0.85 mm to 3 mm. Other boron-manganese grades in the strength range of 400 to 1200 MPa Rm after the hardening process are also conceivable.

[0047] 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.

[0048] The average heating rate rl within the meaning of the invention is the average of the 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, especially during the first 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 in each case over the duration of the heating from room temperature to 530 °C, i.e. if, for example, 51 seconds are needed to reach 530 °C in the furnace at a room temperature of 20 °C, the average heating rate is 10 K / s. In contrast, a maximum heating rate is determined via the highest value at a specific point in the temperature range. 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.

[0049] It was found that the heating rate rl in the range from room temperature to 530 °C must be adjusted in such a way that, for the most homogeneous heating possible across the blank or pre-component, this depends on the sheet thickness d on the one hand, but also on the layer thickness s in g / m 2 .

[0050] Therefore, the following relationship arises: valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g / m 2 per page.

[0051] The inventors surprisingly discovered that a heating rate rl lower than the above-mentioned formula results in homogeneous heating and optimal layer formation, depending on the layer thickness and sheet thickness. Advantageously, a comparatively low heating rate can compensate for existing inhomogeneities on the blank or pre-component, such as locally varying oil residues, surprisingly well.

[0052] To optimize the cycle time, the heating of the steel sheets or steel prefabricated components should not be too slow and should therefore advantageously fulfill the following relationship: valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g / m 2 per side. Furthermore, it was discovered that starting with the decay of the zeta phase, which decays at 530°C, the heating rate should be increased to improve the quality of the layer formation.

[0053] Therefore, heating can advantageously have a maximum heating rate r2 in the temperature range from 530 °C to 670 °C, where r2 > rl. This means that the highest heating rate in the temperature range from 530 °C to 670 °C should be higher than the average heating rate in the range from room temperature to 530 °C. This allows delta phase decomposition to be achieved earlier and oxide formation to be favorably influenced for further processing properties such as welding or phosphating.

[0054] Particularly favorable properties can be achieved if the heating rate r2 is set between 15 K / s and 25 K / s.

[0055] In order to ensure the most homogeneous heating of the surface possible and to make the subsequent cooling behavior more homogeneous, heating can be carried out in the temperature range from 670 °C to 780 °C with an average heating rate r3 that is below the average heating rate rl, i.e., r3 < rl. In addition, the condition r3 < r2 must also be met.

[0056] 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.

[0057] 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.

[0058] Regarding oxygen saturation, it was found that the oxygen available for zinc oxidation is influenced by burning oil, dew point, weather conditions, new furnace elements, and the strength of convection in the furnace, especially during long production intervals. Oxygen consumption by burning oil is influenced by the amount of corrosion protection oil on the blank or pre-component surface and, if necessary, also by the amount of cold forming oil present on the pre-component surface.

[0059] The dew point is determined in particular by the climate zone and the system environment.

[0060] 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.

[0061] It was found that the composition of the furnace atmosphere, especially with regard to the oxygen content, can be locally inhomogeneous.

[0062] 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.

[0063] 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.

[0064] 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 coated with a zinc-based 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 takes place at a heating rate which is variable over the furnace residence time and dependent on the zinc layer thickness,where in the temperature range from room temperature to 530 °C the heating takes place with an average heating rate rl and in the temperature range between 530 °C and 670 °C with a maximum heating rate r2, where rl < r2 .,

[0065] In particular, by setting rl < r2, the delta phase decomposition can advantageously be achieved earlier and the oxide formation can be favorably influenced for further processing properties such as welding or phosphatability.

[0066] An advantageous further development provides that the maximum heating rate r2 is set to be greater than 14 K / s, in particular between 15 K / s and 25 K / s.

[0067] An advantageous further development provides that the average heating rate rl is set to less than 14 K / s.

[0068] An advantageous further development provides that the average heating rate rl is chosen in K / s, which has the following formula with d in mm and s in g / m 2 fulfilled: valid for the sheet thickness range d from 0.85 mm to 3 mm and the layer thickness range s from 20 to 100 g / m 2 per side. This advantageously allows both the sheet thickness and the layer thickness of the zinc-based coating to be taken into account to determine the highest average heating rate rl.

[0069] An advantageous further development provides that an average heating rate rl in K / s is selected, which has the following formula with d in mm and s in g / m 2 fulfilled: valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g / m 2 per side. This advantageously allows both the sheet thickness and the thickness of the zinc-based coating to be taken into account to determine a minimum average heating rate, which can improve cycle time.

[0070] An advantageous further development provides for heating in the temperature range from 670 °C to 780 °C at an average heating rate r3, where r3 < rl. This ensures homogeneous heating of the entire surface and makes the subsequent cooling behavior more homogeneous.

[0071] An advantageous further development provides that the heating rate rl is reduced with increasing layer thickness s, so that if sl < s2 < s3, the heating rate rl is set such that rl(sl) > rl(s2) > rl(s3).

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

[0073] An advantageous further development provides that the zinc-based coating has a layer thickness in the range of 20 to 100 g / m 2 per page.

[0074] An advantageous further development provides that the steel strip has a thickness of 0.85 mm to 3 mm.

[0075] An advantageous further development provides that the coating is selected from specifications ranging from Z40 to Z200, in particular Z40 to Z180. Specifications Z40 to Z100 are particularly preferred, as these allow coating thicknesses of 20 g / m 2 or 50 g / m 2 can usually already provide sufficiently good corrosion protection and exhibit good processing properties. An advantageous further development provides for the furnace heating power to be reduced in the furnace area corresponding to a blank or pre-component temperature of up to 530 °C in a continuous furnace so that the average heating rate of 14 K / s is not exceeded in the temperature range up to 530 °C.

[0076] 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 advantageously allows the cycle time to be kept as short as possible.

[0077] An advantageous further development provides for the total furnace temperature to be reduced to a range of 860 °C to 890 °C in order to keep energy consumption and CO2 emissions low.

[0078] An advantageous development provides for the furnace heating power to be reduced or kept constant in furnace areas where the blank or pre-assembled part has a temperature of more than 530 °C. This can advantageously keep energy consumption and CO2 emissions low.

[0079] 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.

[0080] 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.

[0081] An advantageous development provides that in heating systems 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. An advantageous development provides that the zinc-based coating has a layer thickness of 3 μm to 14 μm per side.

[0082] An advantageous further development provides for the zinc-based coating to have a zinc content of 85 wt.% to 99 wt.%, preferably 94 wt.% to 98 wt.%, aluminum in the range of 0.2 to 2 wt.%, and unavoidable impurities. A high zinc content can ensure cathodic corrosion protection. Furthermore, good processability can be ensured if the layer consists predominantly of zinc and the remainder contains aluminum. Other elements, such as magnesium, can potentially affect the emissivity and thus influence the heating rate.

[0083] 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.

[0084] 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.

[0085] An advantageous further development provides that a steel strip with the following composition is used (all data in % by weight):

[0086] Carbon up to 0.4, preferably 0.10 to 0.30 and

[0087] Silicon up to 1.9, preferably 0.11 to 1.5 and

[0088] Manganese up to 3.0, preferably 0.8 to 2.5 and

[0089] Chromium up to 1.5, preferably 0.1 to 0.9 and

[0090] Molybdenum up to 0.9, preferably 0.001 to 0.1 and

[0091] Nickel up to 0.9, preferably up to 0.2 and

[0092] Titanium up to 0.2 preferably 0.02 to 0.1 and

[0093] Vanadium up to 0.2 and

[0094] Tungsten up to 0.2 and

[0095] Aluminium up to 0.2, preferably 0.02 to 0.07 and boron up to 0.01, preferably 0.0005 to 0.005 and

[0096] Sulphur max. 0.01, preferably max. 0.008 and

[0097] Phosphorus max. 0.025, preferably max. 0.01 and

[0098] Rest iron and smelting-related impurities.

[0099] The invention is explained by way of example with reference to the figures. They show:

[0100] Figure 1: three maximum average heating rates depending on the layer thickness s and

[0101] Sheet thickness d;

[0102] Figure 2: three minimum average heating rates depending on the layer thickness s and

[0103] Sheet thickness d;

[0104] Figure 3: the preferred average heating rate range for the layer thickness s = 20 g / m 2 (Z40);

[0105] Figure 4: the preferred average heating rate range for the layer thickness s = 50 g / m 2 (Z100);

[0106] Figure 5: the preferred average heating rate range for the layer thickness s = 90 g / m 2 (Z180);

[0107] Figure 6: two coatings in cross-section and different heating rates;

[0108] Figure 7: an exemplary heating curve from room temperature to 900°C for a

[0109] Z140 coated, oiled steel plate

[0110] According to the invention, a press-hardened component coated with a zinc-based coating is heated until it has a homogeneous zinc oxide layer. This enables uniform heating, i.e., a homogeneous heating behavior, of the blank or sheet steel component, thus ensuring excellent mechanical properties and optimal processing characteristics.

[0111] In addition, the invention enables energy-optimized operation. This reduces energy losses and increases process efficiency.

[0112] According to the invention, it was found that the most uniform emissivity possible and thus a uniform heating behavior for heated blanks or prefabricated components can be ensured if the heating rates are variably adjusted depending on the sheet and layer thickness.

[0113] It is also particularly advantageous if the heating rates are selected to match each other.

[0114] It is advantageous if the first heating rate rl is below a certain average heating rate r until 530°C is reached. m lies.

[0115] In an advantageous embodiment, r m 14 K / s, so that rl < r m , ie rl < 14 K / s.

[0116] Heating can be achieved by radiation or convection. Alternatively, heating can be achieved by both radiation and convection.

[0117] Furthermore, it was recognized that one or more heating rates should be set that are variable over the furnace residence time and dependent on the zinc layer thickness in order to advantageously ensure the most uniform heating possible of the blanks or pre-assembled parts and a high-quality formation of the zinc-based layer.

[0118] The basic idea here is to reduce the temperature of the first furnace zones and thus lower the average heating rates rl from room temperature (RT) to 530 °C to a maximum of 14 K / s. This applies in particular to coatings Z40 to Z180 on 20MnB8 to 34MnB8 with a sheet thickness of 0.8 mm to 3 mm. A sheet thickness range of 0.9 mm to 2.5 mm is particularly preferred. The room temperature in the sense of the invention is the initial temperature that sheets or pre-assembled parts have before they are placed in a furnace. It is the temperature that prevails in the production facility or the temperature at which the blank or pre-assembled parts are preheated by warm product carriers. This initial temperature can be 50 °C or 60 °C, for example. This temperature can also fluctuate seasonally, but its fluctuation range is irrelevant for the subsequent process.

[0119] In case a clear definition is needed, the room temperature can be set to the standard temperature of 20°C.

[0120] 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-%).

[0121] It was found that the invention can always be easily replicated if a formulaic relationship with the sheet thickness and layer thickness is given: valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g / m 2 per page.

[0122] Thus, the heating rate rl, especially in the temperature range up to 530 °C, is variably adjusted depending on the sheet thickness (d) and the layer thickness (s).

[0123] As already explained, blanks or formed prefabricated components heat up with varying degrees of homogeneity during heating for the purpose of austenitization, particularly from the edge inward. This also influences the formation of the zinc oxide layer due to the locally inhomogeneous heating. Figure 1 shows three maximum average heating rates rl as a function of the layer thickness s and sheet thickness d according to the formula given above. It can be seen that the highest average heating rate rl decreases with increasing sheet thickness.

[0124] This means that with increasing sheet thickness d the heating rate is reduced accordingly, so that if dl < d2 < d3, the heating rate is adjusted such that rl(dl) > rl (d2) > rl(d3).

[0125] Furthermore, the connection with the layer thickness becomes clear: the curves also decrease with increasing layer thickness.

[0126] Figure 2 shows three minimum average heating rates rl as a function of the coating thickness s and sheet thickness d. Here, too, the interplay of these factors is clearly visible.

[0127] Thus, the heating rate rl is reduced accordingly with increasing layer thickness s, so that if sl < s2 < s3, the heating rate rl is adjusted such that rl(sl) > rl (s2) > rl(s3).

[0128] Figure 3 shows the preferred average heating rate range rl for the layer thickness s = 20 g / m 2 per side. This represents the typical value for the Z40 coating. It can be seen that, for example, with a sheet thickness of 1.5 mm, a preferred average heating rate range rl for Z40, i.e. s = 20 g / m 2 per side, from room temperature to 530°C of about 3.4 to 7.8 K / s.

[0129] Figure 4 and Figure 5 represent the preferred average heating rate range rl for the layer thickness s = 50 g / m 2i.e. Z100 or s = 90 g / m 2 which corresponds to Z180.

[0130] The effect of the invention can also be seen visually, shown in Figure 6, where on the left the Z80 coating was applied with an average heating rate according to the prior art from room temperature to 530°C at 20 Kelvin per second. Compared to the right-hand illustration, which shows the same coating but with a heating rate rl according to the invention, in this case 7.7 Kelvin per second, it can be seen that the coating is considerably more uniform, including in terms of surface morphology. Figure 7 shows an example heating curve from room temperature to 900°C for an oiled steel blank coated with Z140. The three different heating rates rl to r3 can be clearly seen. It is clear that the average heating rate rl from room temperature to 530°C should be set comparatively low in order to ensure the most homogeneous heating possible across the entire surface of the blanks or pre-component.From the decay of the zeta phase at 530°C, heating should occur more rapidly. Therefore, the maximum heating rate r2 in the range from 530°C to 670°C should be selected higher than the average heating rate rl to ensure good layer formation. From 670°C to 780°C, however, heating should preferably occur comparatively slower to enable robust operation.

[0131] Thus, the heating rates rl, r2 and r3 are adjusted such that the following relationship applies: rl < r2; r2 > r3.

[0132] In a particularly advantageous embodiment, the heating rates can be set such that r2 > rl > r3.

[0133] For example, r2 can be set greater than 14 K / s, in particular between 15 K / s and 25 K / s. rl can preferably be in the range from 3 K / s to 14 K / s, and r3 can accordingly preferably be below the value of rl.

[0134] A suitable heating unit, particularly by radiation or convection, is a continuous furnace, for example, where 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 heating rate rl is not exceeded. In the higher temperature range, particularly from 530 °C to 670 °C, significantly higher heating rates are possible. It is particularly advantageous if r2 is greater than rl.

[0135] In another particularly advantageous embodiment, r3 is always lower than rl and r2 in order to minimize the risk of furnace overheating and the risk of burnout.

[0136] A multi-layer chamber furnace is also suitable, whereby the furnace temperature is preferably kept constant, and a reduction in the furnace temperature may result in the most critical temperature range up to 530°C 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 and then increased from 530°C, is also conceivable. The invention has the advantage that, with the inventive setting of the heating rate rl in the critical temperature range from RT to 530°C, repeatable results with regard to a uniform, homogeneous coating are achieved.

[0137] The variable heating rate setting ensures homogeneous heating behavior in the furnace depending on the layer thickness. The ability to variably adjust the heating rates allows heating rates to be reduced below 530 °C and increased above 530 °C, particularly between 530 °C and 670 °C. This shortens the furnace residence time, reduces energy consumption, and reduces CO2 emissions. Overall, this significantly increases 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 coated with a zinc-based 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, characterized in that the heating takes place at a heating rate which is variable over the furnace residence time and dependent on the zinc layer thickness,where in the temperature range from room temperature to 530 °C the heating takes place with an average heating rate rl and in the temperature range between 530 °C and 670 °C with a maximum heating rate r2, where rl < r2 ., 2. The method according to claim 1, wherein the maximum heating rate r2 is set greater than 14 K / s, in particular between 15 K / s and 25 K / s.

3. The method according to claim 1, wherein the average heating rate rl is set to be less than 14 K / s.

4. A method according to claim 1 or 2, characterized in that the average heating rate rl in K / s is chosen which has the following formula with d in mm and s in g / m 2 fulfilled: valid for the sheet thickness range d from 0.85 mm to 3 mm and the layer thickness range s from 20 to 100 g / m 2 per page.

5. Method according to one of the preceding claims, characterized in that an average heating rate rl in K / s is selected which has the following formula with d in mm and s in g / m 2 fulfilled: valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g / m 2 per page.

6. Method according to one of the preceding claims, characterized in that the heating takes place in the temperature range from 670 °C to 780 °C with an average heating rate r3, where r3 < rl.

7. Method according to one of the preceding claims, wherein the heating rate rl is reduced with increasing layer thickness s, so that if sl < s2 < s3, the heating rate rl is set such that rl(sl) > rl(s2) > rl(s3).

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

9. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness in the range of 20 to 100 g / m 2 per page.

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

11. Method according to one of the preceding claims, characterized in that the coating was selected from specifications from Z40 to Z200, in particular Z40 to Z180.

12. 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 so that the average heating rate of 14 K / s is not exceeded in the temperature range up to 530 °C.

13. 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.

14. 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 in order to keep energy consumption and CO2 emissions low.

15. 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.

16. 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.

17. 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.

18. Method according to one of the preceding claims, characterized in that in heating devices in which the blank or the steel pre-component is stationary is heated up, 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.

19. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness of 3 pm to 14 pm per side.

20. Method according to one of the preceding claims, characterized in that the zinc-based coating has a zinc content of 85 wt.% to 99 wt.%, in particular 94 wt.% to 98 wt.%, aluminum in the range of 0.2 to 2 wt.% and unavoidable impurities.

21. 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.

22. 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.

23. 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 aluminum up to 0.2, preferably 0.02 to 0.07 and Boron up to 0.01, preferably 0.0005 to 0.005 and sulfur max. 0.01, preferably max. 0.008 and phosphorus max. 0.025, preferably max. 0.01 and the remainder iron and smelting-related impurities.

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