Method for producing a hardened sheet steel component

Abstract

The invention relates to a method for manufacturing components from sheet steel, wherein a sheet steel blank is cut out from a flat steel strip with a zinc-iron-containing coating and either the sheet steel blank is heated in at least some regions 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 into a preformed sheet steel component and the preformed sheet steel component is heated in at least some regions to a temperature above Ac3 and then quench-hardened in a form hardening tool, wherein the heating takes place at an average heating rate r1 in the temperature range from room temperature to 530° C. and takes place at an average heating rate r2 in the temperature range between 530° C. and 670° C., wherer⁢1≤r⁢20.4.

Description

[0001] The invention relates to a method for manufacturing a hardened sheet steel component.

[0002] 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 usually hardened sheet steel components, i.e. sheet steel components that have a significantly higher hardness and tensile strength compared to sheet steel components made of conventional steel. Such hardened sheet steel components are used because the hardening process enables components to be produced that have a reduced wall thickness compared to components made from non-hardenable grades of steel. This makes it possible to provide steel bodies for motor vehicles that are comparatively light while offering extremely high stability.

[0003] The methods for manufacturing hardened sheet steel components are known.

[0004] The hardening mechanism used is known as quench hardening, in which the steel material is first transformed into its high-temperature phase, referred to as austenite or gamma iron, by heating. This fully or partially austenitic steel structure is then cooled at a cooling rate that is higher than the so-called critical cooling rate of the steel alloy. The critical cooling rate for the boron-manganese steel 22MnB5, for example, is around 23 Kelvin per second.

[0005] At these cooling rates, the austenitic phase does not transform back into ferrite, but instead, a transformation into martensite occurs. This martensitic phase is frozen in place, so to speak, by the rapid cooling. Since austenite can dissolve considerably more carbon than martensite, carbon precipitation occurs, which distorts the lattice and results in the high degree of hardness. The prerequisite for this is that the steel material or steel alloy must contain sufficient carbon, which is ensured by the use of suitable alloys.

[0006] The steel industry usually provides so-called boron-manganese steels for this purpose; one of the most common steels is 22MnB5, but it is part of a relatively large family of steel grades. In particular, a steel material with the following composition in mass percent is used:carbonup to 0.4,preferably 0.10 to 0.30siliconup to 1.9,preferably 0.11 to 1.5manganeseup to 3.0,preferably 0.8 to 2.5chromiumup to 1.5,preferably 0.1 to 0.9molybdenumup to 0.9,preferably 0.001 to 0.1nickelup to 0.9,preferably up to 0.2titaniumup to 0.2,preferably 0.02 to 0.1vanadiumup to 0.2tungstenup to 0.2aluminumup to 0.2,preferably 0.02 to 0.07boronup to 0.01,preferably 0.0005 to 0.005sulfurmax. 0.01,preferably max. 0.008phosphorusmax. 0.025,preferably max. 0.01Remainder iron and impurities.

[0007] In the past, two basic processing methods have become established.

[0008] The first and older process is known as press hardening, also called the direct method, in which a blank is cut out from a flat steel sheet and then brought to the required austenitizing temperature and, where appropriate, is kept at this temperature. This blank is then transferred to a forming press, in which the blank is formed in the hot state, preferably in a single stroke, and quenched by the cold press hardening tool. Since only one forming stroke and only one tool are available, the final trimming of outer edges and provision of holes in press-hardened components usually can only be carried out in the hardened state, usually by means of laser cutting. This press hardening yields hardened components that usually have a shape that is not too complex since only one forming stroke is available.

[0009] For components that have more complex geometries and / or are produced in higher quantities, the applicant has developed what is known as form hardening, also called the indirect method, in which a blank is cut out from a steel sheet and is first formed and trimmed using conventional cold forming processes to form a preformed sheet steel component. This preformed component is formed in such a way that it is slightly smaller than the target geometry in all three spatial directions since it is then heated to the austenitizing temperature and expands accordingly due to thermal expansion. After the austenitization, this preformed component is then transferred to a form hardening press, in which this already finished preformed component is in contact with the form hardening tool in the closed press and is only held in place in order to cool it down quickly. Normally, only minor calibrations or adjustments are made in the press. Since the components usually have already been completely trimmed in the cold forming process, a trimming is usually no longer necessary in the hardened state. Form hardening allows for more complex components since the preceding, usually multi-step forming and trimming process in the cold state affords more degrees of freedom in terms of shape.

[0010] Since body components of modern motor vehicles are often not made of uncoated sheet steel, but are instead coated with metallic corrosion protection layers to improve their corrosion protection properties, it makes sense to assume that metal-coated sheet steel should also be used for hardened components.

[0011] It is also known to use galvanized sheet steels for the two methods mentioned above; these galvanized sheet steels usually have a zinc-based coating that has a particular proportion of one or more elements with an even higher affinity for oxygen. These elements with a higher oxygen affinity diffuse to the surface during the high-temperature process that is required for the austenitization and form a glassy layer there, an oxide skin that protects the underlying zinc layer from evaporation during the high-temperature process.

[0012] The applicant uses suitable galvanized sheet steels in both processes.

[0013] In addition to sheet steels with an almost pure zinc layer, i.e. sheet steels with a coating composed mainly of zinc with elements that have a higher oxygen affinity, so-called ZF coatings, also known as GA coatings or galvannealed coatings, are also known, in which sheet steels are typically heat-treated in the temperature range from 460° C. to 570° C. in the continuous hot-dip galvanizing line shortly after application of the zinc coating, thus producing a zinc-iron-containing alloy layer on the steel strip.

[0014] This causes the iron contained in the substrate to react with the zinc from the coating material. But this is only the case with so-called ZF coatings. With these coatings, zinc is first deposited on the steel substrate and is then subjected to a heat treatment. This heat treatment is referred to as galvannealing. The coating itself is also referred to as galvannealed.

[0015] During the heat treatment, an alloying reaction occurs between the iron from the steel and the zinc so that after the heat treatment, the layer is a zinc-iron alloy layer.

[0016] The most commonly used element with a higher oxygen affinity is aluminum, which forms a very thin aluminum oxide skin on the surface during the austenitization heat treatment, which, as already mentioned, prevents large-scale evaporation of zinc.

[0017] During the form hardening and press hardening, cracks usually form in this very thin Al2O3 layer, resulting in the evaporation of a small amount of zinc and the formation and deposition of zinc oxide on the surface. After the press hardening or form hardening, such components typically have a light green to brownish surface, which is caused by their being coated with these zinc oxides.

[0018] It has been observed that in a fluctuating and unpredictable way, press-hardened components or form-hardened components with zinc coatings, in addition to the known greenish-brownish color, sometimes also have a silver coloration either all over or only in certain regions. For purposes of the invention, a silver coloration can also be perceived as light gray or light grayish and can also appear matt or shiny in places.

[0019] It has also been discovered that the silver coloration forms when the entire zinc layer is not transformed into solid zinc-iron phases up to a temperature of 530° C., i.e. liquid zinc still remains on the surface at 530° C. and / or there is a lack of oxygen at higher temperatures. The temperature of approx. 530° C. marks the end of the existence of what is known as the zetazinc-iron phase.

[0020] Different and fluctuating colorations of the surface of a steel component are not necessarily a quality feature with regard to the surface itself, but can lead to problems.

[0021] It is also known that blanks or components heat up differently and not really homogeneously in the furnace. In addition, different heating progressions can result in different iron contents in the coating, which in turn leads to different emissivities and thus to differences in the heating behavior.

[0022] A non-homogeneous heating behavior of blanks during press hardening or of components during form hardening is a disadvantage for an energy-optimized, 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. The minimum furnace dwell time for production must therefore be based on the most unfavorable case, i.e. the slowest heating or the lowest emissivities, even if these cases account for only a fraction of the production quantity. This means that in production with non-homogeneous heating behavior, the vast majority of the blanks or components are usually in the furnace for longer than necessary, which leads to a lower output and reduced processing window for the vast majority of the blanks or components and thus to a higher number of rejects than would be the case with a more homogeneous heating behavior.

[0023] In addition, a different and fluctuating coloring of the surface during form hardening can lead to dimensional accuracy problems since the size of the finished hardened component depends on the size of the austenitized preformed component that is inserted into the form hardening tool. The size of the austenitized preformed component that is inserted into the form hardening tool depends on the cooling behavior of the preformed component between the furnace and the form hardening tool, which in turn, due to radiant heat losses, depends on the emissivity of the surface of the preformed component.

[0024] This also influences the formation of the superficial zinc oxide layer. This particularly occurs with thin zinc-iron coatings and thin sheets, especially below a 1.5 mm sheet thickness and a zinc coating thickness such as ZF 80 according to standard EN 10346—this means that the coating layer is about 35 g / m2 per side.

[0025] DE 10 2020 113287 A1 discloses a method for manufacturing hardened sheet steel components in which the dew point in the furnace is set so as to prevent silvery patches.

[0026] DE 10 2020 106996 A1 discloses a device and a method for heating zinc-coated boron-manganese steels in which active or passive gas circulation is proposed to prevent silver coloration.

[0027] CN 115125439 A discloses heating a 34MnB8 steel with a zinc-iron coating through the use of holding zones and comparatively low heating rates.

[0028] EP 2611945 B1 discloses the opposite, namely a rapid heating of sheets with a zinc-iron coating in which the heating from room temperature to 500° C. should be carried out at a rate of at least 15 K / s and up to 50 K / s.

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

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

[0031] Advantageous modifications are disclosed in the dependent claims thereof.

[0032] According to the invention, heating is carried out in such a way that an optimized heating is always ensured in a way that is matched to the sheet and the coating, which results in more homogeneous properties. A formula is provided for this purpose, which facilitates implementation of the method.

[0033] The heating can be carried out by means of radiation or convection.

[0034] In another embodiment, it can be carried out by means of radiation and convection.

[0035] The heating by means of radiation can be carried out using an infrared heater, for example.

[0036] In order to achieve optimum heating, a variable heating rate over the furnace dwell time is set.

[0037] In particular, when the target temperature is reached, the temperature differences across the entire sheet steel blank or preformed sheet steel component should not exceed 25 K, preferably 18 K, and more preferably 15 K. This can further homogenize the layer formation, in particular the complete reaction, which takes place to a particularly pronounced degree in the temperature range up to 530° C.

[0038] According to the invention, it has been discovered that a heating according to EP 2611945 B1 does not lead to success, but on the contrary makes the problems even worse.

[0039] According to the invention, a sheet steel blank or steel strip that has a zinc-iron-containing coating (ZF) is used.

[0040] Advantageously, this layer can 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 or ZF80 or ZF90 or ZF100 or ZF120 or ZF140 or ZF180 in accordance with DIN EN 10346.

[0041] Particularly preferably, the metallic corrosion protection layer can be applied using hot-dip methods. It is then galvannealed. This can be a simple and robust application method.

[0042] According to the invention, a press-hardened component coated with a zinc-iron layer (ZF) is heated in such a way that it has a homogeneous zinc oxide layer, thus enabling a homogenized heating of the blank or preformed sheet steel component. This results in outstanding mechanical properties and ensures optimum processing characteristics.

[0043] In addition, the invention enables a mode of operation that is energy-optimized (reduced heat input into the annealed material and / or reduced heat losses), reliable (ensured austenitization), and, in the case of small runs, also economically advantageous (heating that is still rapid, even at low furnace temperatures).

[0044] When heating sheet steel blanks or preformed sheet steel components in a furnace, the required heat is made up of the amount of heat required for the desired heating of the annealed material (the blank or component) plus the amount of heat lost due to unwanted furnace losses, e.g. involving furnace doors, the furnace casing, or transport elements. Both can be minimized by means of the reduced the heating rate according to the invention.

[0045] On the one hand, for example the first furnace zone, i.e. the zone extending up to when the blank or component reaches 530° C., can be set to be “cooler” than in methods according to the prior art. Alternatively or additionally, however, the overall furnace temperature can also be reduced, for example to a range from 860° C. to 890° C. compared to the furnace temperatures of 900° C. to 920° C. that have been conventional up to now.

[0046] This yields an energy-optimized, reliable, and economically advantageous mode of operation, particularly for coatings of ZF100 and below.

[0047] In this connection, steel grades such as 34MnB5 or 20MnB8 can be processed in an even more energy-optimized, reliable, and economical way since they have lower austenitizing temperatures than other steel grades. It is also conceivable to use other boron-manganese grades with tensile strengths of from 400 to 1200 MPa after the hardening process.

[0048] A reduced energy requirement has a direct impact on CO2 emissions, thus permitting sustainable and environmentally friendly implementation.

[0049] According to the invention, it has been discovered that the most uniform possible emissivities and thus a uniform heating and cooling behavior of the blanks or preformed components can be ensured if in a particular temperature range, a heating rate is selected that is below a particular average heating rate. A different heating rate can also be deliberately set in the other temperature ranges in order to ensure optimum heating.

[0050] The basic idea here is to reduce the temperature of at least the first furnace zones of a continuous furnace or to reduce the furnace chamber temperature of a multi-layer chamber furnace and thus reduce the average heating rates.

[0051] It is advantageous if the average heating rate is set to a maximum of 14 K / s in the temperature range from room temperature (RT) to 530° C. This is particularly true for coatings ZF60 to ZF180, preferably on 22MnB5, 20MnB8, or 34MnB8 steel strip material.

[0052] 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.

[0053] Advantageously, the heating takes place at an average heating rate r1 in the temperature range from room temperature to 530° C. and takes place at an average heating rate r2 in the temperature range between 530° C. and 670° C., the heating rates being set in such a way that the following applies:r⁢1≤r⁢20.4.

[0054] In particular, the following relationship should apply:r⁢1≤r⁢20.5.

[0055] For purposes of the invention, the average heating rate is the average of the heating rate of the sheet steel blank or preformed sheet steel component when heating from room temperature to 530° C. This means that the heating rate can be comparatively high, especially during the first heating phase up to 100° C. and more, and then continuously decreases since the temperature difference between the sheet steel blank or preformed sheet steel component and the furnace temperature decreases.

[0056] The average heating rate is calculated over the duration of the heating from room temperature to 530° C., i.e. if it takes for example 51 seconds to reach 530° C. in the furnace with a room temperature of 20° C., then the average heating rate is 10 K / s.

[0057] The sheet thickness range of the steel strip can be selected to be from 0.85 mm to 3 mm, preferably from 0.9 mm to 2 mm.

[0058] It has been discovered that with the appropriate selection of an average heating rate r1 in the range from 3 to 14 K / s, in particular 6 to 14 K / s, in the target temperature range from RT to 530° C., an optimum zinc oxide layer is achieved. The optimum heating rate range can be determined calculating it based on the sheet thickness.

[0059] The formula according to the invention for the average heating rate r1 from RT to 530° C. depending on the sheet thickness d (in mm) is:r⁢1≤a⁢ (1+1d).

[0060] According to the invention, the constant a (in K / s) is 6.37 K / s and applies within the sheet thickness range d from 0.85 mm to 3 mm.

[0061] According to the invention, the homogeneity of the temperature of the sheet steel blank can thus be increased across the entire blank in a heating up to 530° C. In this case, the maximum temperature difference across the blank is less than 25 K.

[0062] It has also been discovered that alternatively or additionally, it makes sense to ensure a particular oxygen saturation at the sheet surface above a particular temperature.

[0063] Particularly in a temperature range above 800° C., it can be advantageous to adjust the furnace atmosphere so that at least 5% oxygen by volume is present at least at the surface of the components to be heated.

[0064] With regard to the oxygen saturation, it has been determined that the oxygen that is available for the zinc oxidation, particularly during long production intervals, is influenced by burning oil, the dew point, weather conditions, new furnace elements, and the intensity of convection in the furnace.

[0065] The oxygen consumption due to burning oil is influenced by the amount of corrosion protection oil on the surface of the blank or preformed component and, where applicable, also by the amount of cold forming oil that is present on the surface of the preformed component.

[0066] The dew point is in particular affected by the climate zone and the environment of the steel plant.

[0067] Weather conditions have an influence due to chimney effects and the displacement of oxygen by gaseous combustion products. It has also been determined that new furnace elements such as stainless steel, ceramic, or other insulating materials can consume or bind to oxygen and can therefore influence the oxygen saturation.

[0068] It has been discovered that the composition of the furnace atmosphere, particularly with regard to the oxygen content, can be locally non-homogeneous.

[0069] Advantageously, at least 5% oxygen by volume can be provided close to the surface starting from a temperature of 800° C. In this context, “close to the surface” means in a region closer than 10 mm to the surface of the blank or prefabricated part.

[0070] This can be achieved, for example, by blowing in fresh air, with the fresh air preferably being correspondingly preheated. In multi-layer chamber furnaces, i.e. without a transport movement of the blanks or preformed components during heating as is the case in a continuous furnace, convection can also be forced by means of purging, blowing, or circulating, or by means of a temporary opening of the furnace doors.

[0071] The invention thus relates to a method for manufacturing components from sheet steel, wherein a sheet steel blank is cut out from a flat steel strip with a zinc-iron-containing coating and either the sheet steel blank is heated in at least some regions to a temperature above Ac3 and then formed in a hot state in a press hardening tool and quench hardened or the coated sheet steel blank is cold-formed into a preformed sheet steel component and the preformed sheet steel component is heated in at least some regions to a temperature above Ac3 and then quench-hardened in a form hardening tool, wherein the heating takes place at a heating rate that is variable over the furnace dwell time, wherein the heating takes place at an average heating rate r1 in the temperature range from room temperature to 530° C. and takes place at an average heating rate r2 in the temperature range between 530° C. and 670° C., wherer⁢1≤r⁢20.4.

[0072] In an advantageous modification, the average heating rate r2 is set to below 7 K / s.

[0073] In an advantageous modification, the average heating rate r2 is set to between 1.6 K / s and 5.6 K / s.

[0074] In an advantageous modification, the average heating rate r1 is set to below 14 K / s.

[0075] In an advantageous modification, the average heating rate r1 is set to between 3 K / s and 14 K / s.

[0076] In an advantageous modification, the heating rate r1 is correspondingly reduced as the sheet thickness d increases so that if d1<d2<d3, the heating rate r1 is set such that r1(d1)>r1(d2)>r1(d3).

[0077] In an advantageous modification, in the range from room temperature to 530° C., the maximum average heating rate r1 is selected, which obeys the following formula for the sheet thickness d in mm:r⁢1≤a⁡(1+1d),where:a=6.37 K / sThis applies to the sheet thickness range d from 0.85 mm to 3 mm.By means of this, the maximum temperature difference in the sheet steel blank that has been heated to 530° C. can be advantageously reduced to less than 25 K, thus making it possible to increase the homogeneity of the temperature distribution throughout the blank or preformed component. This can have an advantageous effect on the layer formation, in particular on the complete reaction. In a preferred variant, a=5.32 K / s is selected. By means of this, the maximum temperature difference in the sheet steel blank that has been heated to 530° C. can be reduced to below 18 K, which further increases the effects mentioned above.

[0081] In an advantageous modification, the furnace heat output is reduced at least in the first furnace zones so that the average heating rate is reduced or is set to a maximum of 14 K / s from room temperature to 530° C. This can advantageously minimize the energy losses in the region of the furnace entrance since the sheet steel blanks or preformed components are correspondingly fed into the furnace here and the losses can be minimized by lowering the temperature in the first furnace zone.

[0082] In an advantageous modification, the heating takes place at an average heating rate r2 in the temperature range from 530° C. to 670° C., where the following applies: r2<0.9*r1. In this way, the delta-ZnFe phase decay can be advantageously evened out and the oxide formation can be favorably influenced for other processing properties such as welding and phosphatability.

[0083] In an advantageous modification, the heating takes place at an average heating rate r3 in the temperature range above 670° C. up to 780° C., where the following applies:r⁢24<r⁢3<r 2.This can advantageously even out the decomposition of the gamma-ZnFe phase and can thus ensure a more homogeneous heating of the entire surface as well as making the subsequent cooling behavior more homogeneous.In an advantageous modification, the zinc-iron-containing coating has an iron content of 8 to 14%, in particular 8 to 12%, and zinc as the remainder. The iron content can advantageously be in this range in order to ensure optimum heating.

[0085] In a modification, the coating has been selected from the specifications ZF60 to ZF180 in accordance with DIN EN 10346. ZF60, i.e. a zinc-iron-containing coating with approximately 30 g / m2 per side, can already provide a certain corrosion protection and is also easy to process. Depending on the corrosion protection requirements, a thicker coating layer of up to 90 g / m2 per side can be advantageous.

[0086] In an advantageous modification, the heating takes place at an average heating rate r1 of at least 3 K / s, preferably at least 6 K / s, in the temperature range from room temperature to 530° C. Advantageously, the average heating rate r1 can be above 6 K / s in order to shorten the heating time and minimize the cycle time.

[0087] In an advantageous modification, for the heating above 800° C., an oxygen content of at least 5% by volume in the furnace atmosphere is ensured in a region closer than 10 mm to the surface of the blank or preformed steel component.

[0088] In an advantageous modification, in the case of a continuous furnace, in the region of the furnace corresponding to a blank or preformed component temperature of up to 530° C., the furnace heat output is reduced in such a way that the heating rate of 14 K / s is not exceeded in the temperature range up to 530° C.

[0089] In an advantageous modification, the furnace heat output is increased in the regions in which the blank or preformed component has a temperature of more than 530° C. This can minimize the cycle time to the greatest possible extent.

[0090] In an advantageous modification, the overall furnace temperature is reduced to a range from 860° C. to 890° C. This has the advantage of minimizing energy needs and CO2 emissions.

[0091] In an advantageous modification, the furnace heat output is reduced or kept constant in the furnace regions in which the blank or preformed component has a temperature of more than 530° C. This has the advantage of minimizing energy needs and CO2 emissions.

[0092] In an advantageous modification, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere in furnace regions in which the blank or preformed component has a temperature of 800° C. or more.

[0093] In an advantageous modification, the existing furnace atmosphere is circulated in furnace regions in furnace regions in which the blank or preformed component has a temperature of 800° C. or more.

[0094] In an advantageous modification, in heating units in which the blank or preformed steel component is heated in a stationary manner such as in multi-layer chamber furnaces, the furnace heat output 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.

[0095] In an advantageous modification, the zinc-iron-containing coating has a layer thickness of 4 μm to 14 μm per side.

[0096] In an advantageous modification, the zinc-iron-containing coating has been applied by means of a hot-dip process, in particular hot-dip galvanizing, and has then been galvannealed. This can be a simple and robust process with outstanding reproducibility.

[0097] In an advantageous modification, the steel strip has a sheet thickness of 0.85 mm to 3 mm.

[0098] In an advantageous modification, the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably, a 22MnB5, 20MnB8, or 34MnB5 boron-manganese steel.

[0099] In an advantageous modification, a strip with the following composition is used as the steel strip (all figures in wt %):

[0100] carbon up to 0.4, preferably 0.10 to 0.30, and

[0101] silicon up to 1.9, preferably 0.11 to 1.5, and

[0102] manganese up to 3.0, preferably 0.8 to 2.5, and

[0103] chromium up to 1.5, preferably 0.1 to 0.9, and

[0104] molybdenum up to 0.9, preferably 0.001 to 0.1, and

[0105] nickel up to 0.9, preferably up to 0.2, and

[0106] titanium up to 0.2, preferably 0.02 to 0.1, and

[0107] vanadium up to 0.2 and

[0108] tungsten up to 0.2 and

[0109] aluminum up to 0.2, preferably 0.02 to 0.07, and

[0110] boron up to 0.01, preferably 0.0005 to 0.005, and

[0111] sulfur max. 0.01, preferably max. 0.008, and

[0112] phosphorus max. 0.025, preferably max. 0.01, and

[0113] Remainder iron and smelting-related impurities.

[0114] The invention is explained by way of example below with the aid of drawings. In the drawings:

[0115] FIG. 1: shows the preferred average heating rate range from RT to 530° C. over the sheet thickness for a first constant a;

[0116] FIG. 2: shows the preferred average heating rate range from RT to 530° C. over the sheet thickness for a second constant a;

[0117] FIG. 3: shows a possible furnace temperature in the zone up to 530° C. for a first constant a;

[0118] FIG. 4: shows the temperature distribution across the blank with a heating rate r1 according to the prior art;

[0119] FIG. 5: shows a top view of two sheet metal samples approx. 220 mm×300 mm with different heating rates;

[0120] FIG. 6: shows transverse-section images of two sheet metal samples with different heating rates;

[0121] FIG. 7: shows a sample heating curve for ZF110.

[0122] According to the invention, a press-hardened component coated with a zinc-iron layer (ZF) is heated such that it has a homogeneous zinc oxide layer and such that uniform heating of the blank is possible. This ensures excellent mechanical properties and optimum processing characteristics.

[0123] In addition, the invention enables a mode of operation that is energy-optimized (reduced heat input into the annealed material and / or reduced heat losses) since the furnace zone extending until the sheet metal blank reaches 530° C. can be operated at a reduced temperature, thus making it possible to minimize the energy losses in the first furnace zone. This reduces energy costs and increases process efficiency.

[0124] According to the invention, it has been discovered that the most uniform possible emissivities and thus a uniform heating and cooling behavior of the blanks or preformed components can be ensured if a heating rate r1 that is below a critical heating rate is selected in a particular temperature range.

[0125] The basic idea here is to reduce the temperature of the first furnace zones and thus reduce the average heating rates r1 from room temperature (RT) to 530° C. Consequently, the average heating rate r1 should be set in such a way that it fulfills the following relationship, where r2 represents the average heating rate r2 in the range from 530° C. to 670° C., where the following applies:r⁢1≤r⁢20.4in particular the following should apply:r⁢1≤r⁢20.5.It is particularly advantageous if the heating rate r1 is set to a maximum of 14 K / s. This applies in particular to coatings ZF60 to ZF180 with a sheet thickness of 0.85 mm to 3 mm.For purposes of the invention, room temperature is the initial temperature that sheets or preformed components have before they are placed into a furnace. It is the temperature that prevails in the production plant or the temperature after a slight warming by a warm product carrier, i.e. an initial temperature of 50° C. or 60° C. is also conceivable. This temperature naturally can fluctuate seasonally, but its fluctuation range in itself is irrelevant to the subsequent process.

[0128] In the event that a clear definition is required, room temperature is established at the standard temperature of 20° C.

[0129] The temperatures indicated below are to be understood to be in ° C. Temperature differences may be indicated in K (Kelvin). Unless otherwise stated, contents in alloys are indicated in mass percent (M-%).

[0130] It has been discovered that with the appropriate selection of an average heating rate r1 in the range from 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.

[0131] The formula according to the invention for the average heating rate r1 as a function of the sheet thickness d (in mm) is:r⁢1≤a⁡(1+1d)where:a=6.37 K / sThis applies to the sheet thickness range d from 0.85 mm to 3 mm.The average heating rate r1 is therefore set as a function of the sheet thickness. As the sheet thickness increases, the heating rate r1 is reduced accordingly so that if d1<d2<d3, then the heating rate r1 is set so that r1(d1)>r1(d2)>r1(d3).

[0135] This can advantageously reduce the maximum temperature difference to below 25 K and thus increase homogeneity in the sheet steel blank that has been heated to 530° C. In a preferred variant, a=5.32 K / s is selected. This can reduce the maximum temperature difference to below 18 K in the sheet steel blank that has been heated to 530° C.

[0136] As explained above, during the heating by means of radiation or convection for purposes of the austenitization, blanks or shaped preformed components heat up in a non-homogeneous way, in particular from the outside to the inside. This effect occurs to a greater extent with a zinc-iron alloy (ZF) due to locally differing iron contents. This also influences the formation of the zinc oxide layer due to the locally non-homogeneous heating.

[0137] As explained above, it is surprising that with an average heating rate r1 in the range from 4 to 14 Kelvin per second, an optimum zinc oxide layer is formed in the target temperature range from room temperature to 530° C.

[0138] This effect also occurs if the heating rate r2 is set to between 1.6 and 5.6 Kelvin per second in the temperature range from 530° C. to 670° C.

[0139] In a particularly advantageous embodiment, the heating rate r1 is set in the range from 4 to 14 Kelvin per second in the target temperature range from room temperature to 530° C. and the heating rate r2 is set to between 1.6 and 5.6 Kelvin per second in the temperature range from 530° C. to 670° C. in order to achieve an optimum zinc oxide layer.

[0140] FIG. 1 shows the preferred average heating rate range r1 over the sheet thickness; it is clear here that the maximum permissible average heating rate r1 decreases over the sheet thickness. FIG. 1 shows the representation for the constant a=6.37 K / s—FIG. 2 shows the average heating rate r1 over 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 r1. In addition, the particularly preferred lower limit of 6 K / s is shown as a dashed line.

[0141] The effect according to the invention is also visually apparent in FIG. 5. Here, two 1.5 mm thick sheet steel samples measuring approximately 220 mm×300 mm are heated at different heating rates. In the image on the left, the layer is absolutely uniform due to the low heating rate r1 of about 6.5 K / s up to 530° C. according to the invention, while in the image on the right, an edge effect is visible due to uneven layer growth at a heating rate r1 of about 20 K / s up to 530° C.

[0142] This is also apparent in FIG. 6, where on the right, a heating rate r1 of 20 Kelvin per second according to the prior art was used on the coating ZF80 from room temperature to 530° C. Compared to the depiction on the left, which shows the same coating, but with a heating rate r1 according to the invention, 7.7 Kelvin per second in this case, it is clear that the coating and surface morphology are considerably more uniform. Here, too, the sheet thickness was 1.5 mm.

[0143] FIG. 4 shows how the temperatures in different regions of the sheet behave at a heating rate r1 of around 18 K / s, and considerable differences are apparent, particularly in the upper temperature ranges. But even when the temperature reaches around 530° C., temperature differences of up to 40 K in a blank are possible. This can also exert a powerful influence on the layer reaction and can therefore lead to undesirable effects in the layer.

[0144] For example, a continuous furnace is a suitable unit for heating, wherein radiant tubes, which are for example gas-heated, carry out the heating. In the critical temperature range between RT and 530° C., the heating is carried out in such a way that the critical heating rate r1 is not exceeded, whereas a significantly higher heating rate r2 and / or r3 is also possible in the ranges above this.

[0145] The heating rate r1 is therefore set so that it fulfills the following relationship:r⁢1≤r⁢20.4in particularr⁢1≤r⁢20.5A multi-layer chamber furnace is also suitable; in any case, the heating should be carried out by means of radiation and convection, where in this case, the furnace temperature is preferably kept constant and, where appropriate, a reduction in the furnace temperature leads to the critical temperature range being passed through at a correspondingly low heating rate r1. It is also conceivable, however, for a furnace temperature to vary over the furnace dwell time, i.e. a furnace temperature that is low at the beginning (i.e. r1) and then above 530° C., is increased in order to adjust it to r2 and / or r3. This is shown in FIG. 3, for example, where the furnace temperature in the first zone is reduced and correspondingly adjusted as a function of the sheet thickness.

[0148] FIG. 7 shows an example of a heating curve for an oiled steel blank coated with ZF110. It is apparent that the heating rate r1 is set comparatively low from RT to 530° C. in order to achieve the most homogeneous heating possible. Here, r1 and r2 were selected so that the relationshipr⁢1≤r⁢20.4applies. For example, r1 can be set to 10 K / sec, r2 can be set to 5 K / sec, and r3 can be set to 3 K / sec.From 530° C. to 670° C., the heating rate r2 is set, which is set to less than r1*0.9 so that the relationship r2<0.9*r1 applies. In this example,

[0150] From 670° C. to 780° C., the heating rate r3 has been further reduced to achieve the most homogeneous heating of the entire surface of the blank. On the other hand, the heating rate should not be too low, in order not to extend the furnace dwell time too much. For this reason, r3 has been selected to be betweenr⁢24and r2 so that the relationshipr⁢24<r⁢3<r⁢2applies.As a result, it is advantageous if the heating rates r1, r2, and r3 are set variably over the furnace dwell time.It is particularly advantageous if the heating rate r1 in the temperature range from RT to 530° C. is set as a function of the sheet thickness.An advantage of the invention is that the setting of the heating rate r1 in the critical temperature range from RT to 530° C. according to the invention achieves repeatable results with regard to a coherent homogeneous coating.

[0154] A variable heating rate over the furnace dwell time advantageously enables a targeted combination of homogeneous heating and optimum furnace dwell time.

[0155] The heating rates r1, r2, and r3 can be set variably without causing undesirable edge effects due to uneven layer growth.

[0156] The ability to increase the heating rates, particularly in the temperature range above 530° C., leads to a shorter furnace dwell time. This reduces costs and CO2 emissions and therefore increases process efficiency.

Claims

1. A method for manufacturing components from sheet steel, wherein a sheet steel blank is cut out from a flat steel strip with a zinc-iron-containing coating and either the sheet steel blank is heated in at least some regions to a temperature above Ac3 and then formed in a press hardening tool in the hot state and quench hardened, orthe coated sheet steel blank is cold-formed into a preformed sheet steel component and the preformed sheet steel component is heated in at least some regions to a temperature above Ac3 and then quench-hardened in a form hardening tool,characterized in that the heating takes place at a heating rate that is variable over the furnace dwell time, wherein the heating takes place at an average heating rate r1 in the temperature range from room temperature to 530° C. and takes place at an average heating rate r2 in the temperature range between 530° C. and 670° C., where r1≤r2 / 0.4.

2. The method according to claim 1, wherein the average heating rate r2 is set to below 7 K / s, preferably between 1.6 K / s and 5.6 K / s.

3. (canceled)4. The method according to claim 1, wherein the average heating rate r1 is set to below 14 K / s, preferably between 3 K / s and 14 K / s.

5. (canceled)6. The method according to claim 1, wherein the heating rate r1 is correspondingly reduced with increasing sheet thickness d so that when d1<d2<d3, the heating rate r1 is set such that r1(d1)>r1(d2)>r1(d3).

7. The method according to claim 1, characterized in that in the range from room temperature to 530° C., the maximum average heating rate r1 is selected, which obeys the following formula for the sheet thickness d in mm:r⁢1⁢≤a⁡(1+1 / d),where:a=6.37 K / s,wherein the sheet thickness range d is from 0.85 mm to 3 mm.

8. The method according to claim 1, characterized in that the furnace heat output 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. The method according to claim 1, characterized in that the heating is carried out at an average heating rate r2 in the temperature range from 530° C. to 670° C., where the following applies: r2<0.9*r1.

10. The method according to claim 1, characterized in that the heating is carried out at an average heating rate r3 in the temperature range above 670° C. up to 780° C., where the following applies: r2 / 4<r3<r2.

11. The method according to claim 1, characterized in that the zinc-iron-containing coating, preferably selected from the specifications ZF60 to ZF180, has an iron content of 8 to 14%, in particular 8 to 12%, and zinc as the remainder and / or has a layer thickness of 4 μm to 14 μm per side, whereby the zinc-iron-containing coating has been applied by means of a hot-dip process, in particular hot-dip galvanizing, and has then been galvannealed.

12. (canceled)13. The method according to claim 1, characterized in that the heating takes place at an average heating rate r1 of at least 3 K / s, preferably at least 6 K / s, in the temperature range from room temperature to 530° C.

14. The method according to claim 1, characterized in that for the heating above 800° C., an oxygen content of at least 5% by volume in the furnace atmosphere is ensured in a region closer than 10 mm to the surface of the blank or preformed steel component.

15. The method according to claim 1, characterized in that, in the case of a continuous furnace, in the region of the furnace corresponding to a blank or preformed component temperature of up to 530° C., the furnace heat output is reduced in such a way that the heating rate of 14 K / s is not exceeded in the temperature range up to 530° C.

16. The method according to claim 1, characterized in that the furnace heat output is increased in the regions in which the blank or preformed component has a temperature of more than 530° C.

17. The method according to claim 1, characterized in that the overall furnace temperature is reduced to a range of from 860° C. to 890° C.

18. The method according to claim 1, characterized in that the furnace heat output is reduced or kept constant in the furnace regions in which the blank or preformed component has a temperature of more than 530° C.

19. The method according to claim 1, characterized in that fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere in furnace regions in which the blank or preformed component has a temperature of 800° C. or more.

20. The method according to claim 1, characterized in that the existing furnace atmosphere is circulated in furnace regions in furnace regions in which the blank or preformed component has a temperature of 800° C. or more.

21. The method according to claim 1, characterized in that in heating units in which the blank or preformed steel component is heated in a stationary manner such as multi-layer chamber furnaces, the furnace heat output 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. (canceled)23. (canceled)24. The method according to claim 1, characterized in that the steel strip has a sheet thickness of 0.85 mm to 3 mm and / or is made of a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably, a 22MnB5, 20MnB8, or 34MnB5 boron-manganese steel.

25. (canceled)26. The method according to claim 1, characterized in that a strip with the following composition is used as the steel strip:carbon up to 0.4 wt, preferably 0.10 to 0.30 wt %,silicon up to 1.9 wt %, preferably 0.11 to 1.5 wt %,manganese up to 3.0 wt %, preferably 0.8 to 2.5 wt,chromium up to 1.5 wt %, preferably 0.1 to 0.9 wt %,molybdenum up to 0.9 wt %, preferably 0.001 to 0.1 wt %,nickel up to 0.9 wt %, preferably up to 0.2 wt,titanium up to 0.2 wt %, preferably 0.02 to 0.1 wt %,vanadium up to 0.2 wt %,tungsten up to 0.2 wt %,aluminum up to 0.2, wt % preferably 0.02 to 0.07 wt %,boron to 0.01 wt %, preferably 0.0005 to 0.005 wt %,sulfur max. 0.01 wt %, preferably max. 0.008 wt %,phosphorus max. 0.025 wt %, preferably max. 0.01 wt %, andremainder iron and smelting-related impurities.

Images