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, hot and cold rolled steel strip coated with a zinc-based 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 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 and depends on the zinc layer thickness, 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 a maximal heating rate r2 in the temperature range between 530° C. and 670° C., where r1<r2.

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 quickly 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 called GA coatings, are also known, in which the application of the zinc coating is followed by a heat treatment, thus producing a zinc-iron alloy layer on the steel strip.

[0014] For purposes of the invention, a coating that is said to “primarily consist” of zinc consists of 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.

[0015] In the heat treatment for purposes of austenitizing press-hardened steels, the formation of the thin oxide skin composed of high oxygen affinity elements occurs only with so-called Z coatings, also called GI coatings, i.e. coatings that are essentially zinc-based. In addition to zinc, typical Z coatings contain for example 0.2 to 2 wt % aluminum.

[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. Experience shows that the frequency of silver coloration increases with the thickness of the coating.

[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 zeta-zinc-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. For example, the heating from the outside to the inside is not homogeneous. 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 comparatively thin zinc coatings and thin sheets, especially below a 1.5 mm sheet thickness and a zinc coating thickness such as Z 80 according to standard EN 10346—this means that the coating layer is about 40 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] CN115125439 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 disclosing 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 with variable heating rates over the furnace dwell time 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 results in a reliable process control.

[0033] According to the invention, a sheet steel blank or steel strip that has a zinc-based coating is used.

[0034] Advantageously, this layer can 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 or Z60 or Z80 or Z120 or Z140 or Z180 in accordance with DIN EN 10346.

[0035] Zinc-based corrosion protection layers can have a comparatively high zinc content of 85 wt % to 99 wt % and in addition to inevitable impurities, also contain aluminum in the range from 0.2 to 2 wt %.

[0036] Particularly preferably, the metallic zinc-based corrosion protection layer can be applied using hot-dip methods, i.e. hot-dip galvanization. This can be a simple and robust application method.

[0037] 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, thus enabling a homogenized heating of the blank, which results in outstanding mechanical properties and ensures optimum processing characteristics.

[0038] 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). 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. 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.

[0039] This yields an energy-optimized, reliable, and economically advantageous mode of operation, particularly for coatings of Z100 and below.

[0040] 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.

[0041] A reduced energy requirement has a direct impact on CO2 emissions, thus permitting sustainable and environmentally friendly implementation.

[0042] According to the invention, it has been discovered that the most homogeneous possible emissivities across the entire surface and thus a uniform heating and cooling behavior of heated blanks or preformed components can be ensured if the heating rates are selected so that they are optimally matched to one another, but at least so that the first heating rate r1 up to when 530° C. is reached is below a particular average heating rate, for example the maximum heating rate r2.

[0043] The heating can be carried out by means of radiation or convection.

[0044] In another embodiment, the heating can be carried out by radiation and convection.

[0045] The heating by means of radiation can be carried out using an infrared heater, for example.

[0046] It has also been discovered according to the invention that one or more heating rates, which are variable over the furnace dwell time and depend on the zinc layer thickness, should be set in order to advantageously be able to ensure a high-quality layer formation and oxide formation of the zinc-based layer.

[0047] 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.

[0048] Advantageously, 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 Z40 to Z180 on 20MnB8 to 34MnB8 with a sheet thickness of 0.85 mm to 3 mm. It is also conceivable to use other boron-manganese grades with strengths in the range from 400 to 1200 MPa Rm after the hardening process.

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

[0050] For purposes of the invention, the average heating rate r1 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. The average heating rate is respectively 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. By contrast, a maximum heating rate is determined by the highest value at a certain point in the temperature range.

[0051] 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.

[0052] It has been discovered that the heating rate r1 in the range from room temperature to 530° C. must be set in such a way that the most homogeneous possible heating across the blank or preformed component depends on both the sheet thickness d and the coating layer s in g / m2.

[0053] This yields the following relationship:r⁢1<3⁢2d*s3this being 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 / m2 per side.In this connection, the inventors have surprisingly discovered that with a heating rate r1 lower than that in the above formula, a homogeneous heating and an optimal layer formation are achieved as a function of the coating layer and the sheet thickness. Advantageously, with a comparatively low heating rate, the existing non-homogeneities in the blank or preformed component, for example locally different oiling residues, can be evened out surprisingly well.

[0055] In order to optimize the cycle time, however, the heating of the sheet steels or preformed sheet steel components also should not take place too slowly and should therefore advantageously fulfill the following relationship:r⁢1>1⁢4d*s3this being 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 / m2 per side.It has also been discovered that starting from the decay of the zeta phase, which decays at 530° C., the heating rate should be increased in order to increase the quality of the layer formation.

[0057] For this reason, the heating in the temperature range from 530° C. to 670° C. can advantageously have a maximum heating rate r2, where r2>r1. 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. The delta phase decay can therefore be reached earlier and the oxide formation can be favorably influenced for other processing properties such as welding and phosphatability.

[0058] Particularly favorable properties can be achieved if the heating rate r2 is set between 15 K / s and 25 K / s.

[0059] In order to ensure the most homogeneous possible heating of the surface and in order to be able to make the subsequent cooling behavior more homogeneous, the heating in the temperature range from 670° C. to 780° C. can take place at an average heating rate r3, which is lower than the average heating rate r1, i.e. the following applies: r3<r1. In addition, the condition r3<r2 should also be fulfilled.

[0060] 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. The solution according to the invention therefore lies in eliminating the factors that lead to the silver coloration.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] The dew point is in particular affected by the climate zone and the environment of the steel plant.

[0065] 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.

[0066] It has been discovered that the composition of the furnace atmosphere, particularly with regard to the oxygen content, can be locally non-homogeneous.

[0067] 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.

[0068] 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.

[0069] 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 coated with a zinc-based 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 and depends on the zinc layer thickness, 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 a maximal heating rate r2 in the temperature range between 530° C. and 670° C., where r1<r2.

[0070] In particular setting r1<r2 advantageously permits the delta phase decay to be reached earlier and permits the oxide formation to be favorably influenced for other processing properties such as welding and phosphatability.

[0071] In an advantageous modification, the maximum heating rate r2 is set to above 14 K / s, in particular between 15 K / s and 25 K / s.

[0072] In an advantageous modification, the average heating rate r1 is set to below 14 K / s.

[0073] In an advantageous modification, the average heating rate r1 in K / s is selected that fulfills the following formula where d is expressed in mm and s is expressed in g / m2:r⁢1<3⁢2d*s3this being 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 / m2 per side. This advantageously makes it possible to take into account on the one hand, the sheet thickness and on the other, the layer thicknesses of the zinc-based coating in order to determine the maximum average heating rate r1.In an advantageous modification, an average heating rate r1 in K / s is selected that fulfills the following formula where d is expressed in mm and s is expressed in g / m2:r⁢1>1⁢4d*s3this being 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 / m2 per side. This advantageously makes it possible to take into account on the one hand, the sheet thickness and on the other, the layer thicknesses of the zinc-based coating in order to determine a minimum average heating rate, which can improve the cycle time.In an advantageous modification, the heating takes place at an average heating rate r3 in the temperature range from 670° C. to 780° C., where the following applies: r3<r1. This can ensure a homogeneous heating of the entire surface as well as making the subsequent cooling behavior more homogeneous.In an advantageous modification, the heating rate r1 is reduced as the coating layer s increases so that if s1<s2<s3, then the heating rate r1 is set in such a way that r1(s1)>r1 (s2)>r1(s3).

[0077] In an advantageous modification, the heating rate r1 is reduced as the sheet thickness d increases so that if d1<d2<d3, then the heating rate r1 is set in such a way that r1(d1)>r1(d2)>r1(d3).

[0078] In an advantageous modification, the zinc-based coating has a coating layer in the range from 20 to 100 g / m2 per side.

[0079] In an advantageous modification, the steel strip has a thickness of 0.85 mm to 3 mm.

[0080] In an advantageous modification, the coating has been selected from the specifications Z40 to Z200, in particular Z40 to Z180. Particularly preferably, it has been selected from the specifications Z40 to Z100 since these coating layers of 20 g / m2 to 50 g / m2 usually can already offer extremely good corrosion protection and can have good processing properties.

[0081] 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 so that the average heating rate of 14 K / s is not exceeded in the temperature range up to 530° C.

[0082] 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. It is thus advantageously possible to minimize the cycle time to the greatest possible extent.

[0083] In an advantageous modification, the overall furnace temperature is reduced to a range from 860° C. to 890° C. in order to minimize energy needs and CO2 emissions.

[0084] 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 can advantageously minimize energy needs and CO2 emissions.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] In an advantageous modification, the zinc-based coating has a layer thickness of 3 μm to 14 μm per side.

[0089] In an advantageous modification, the zinc-based coating has a comparatively high zinc content of 85 wt % to 99 wt %, preferably 94 wt % to 98 wt %, and contains aluminum in the range from 0.2 to 2 wt % as well as inevitable impurities. A high zinc content can ensure cathodic corrosion protection. In addition, a good processability can be ensured if the layer predominantly consists of zinc and the remainder is aluminum. Other elements such as magnesium can possibly have an effect on the emissivity and can thus influence the heating rate.

[0090] In an advantageous modification, the zinc-based coating has been applied by means of a hot-dip process, in particular hot-dip galvanizing.

[0091] 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.

[0092] In an advantageous modification, a strip with the following composition is used as the steel strip (all figures in wt %):

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

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

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

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

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

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

[0099] titanium up to 0.2, preferably 0.02 to 0.1, and

[0100] vanadium up to 0.2 and

[0101] tungsten up to 0.2 and

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

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

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

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

[0106] Remainder iron and smelting-related impurities.

[0107] The invention is explained by way of example below with the aid of figures. In the figures:

[0108] FIG. 1: shows three maximum average heating rates as a function of the coating layer s and sheet thickness d;

[0109] FIG. 2: shows three minimum average heating rates as a function of the coating layer s and sheet thickness d;

[0110] FIG. 3: shows the preferred average heating rate range for the coating layer s=20 gm / m2 (Z40);

[0111] FIG. 4: shows the preferred average heating rate range for the coating layer s=50 gm / m2 (Z100);

[0112] FIG. 5: shows the preferred average heating rate range for the coating layer s=90 gm / m2 (Z180);

[0113] FIG. 6: shows transverse-section images of two coatings and different heating rates;

[0114] FIG. 7: shows a sample heating curve from room temperature to 900° C. for an oiled steel blank coated with Z140.

[0115] According to the invention, a press-hardened component coated with a zinc-based coating is heated such that it has a homogeneous zinc oxide layer. This enables a uniform heating of the blank, i.e. a homogeneous heating behavior, of the blank or sheet steel component and thus ensures excellent mechanical properties and optimum processing characteristics.

[0116] In addition, the invention enables a mode of operation that is energy-optimized. This reduces the energy losses and increases process efficiency.

[0117] According to the invention, it has been discovered that the most uniform possible emissivities and thus a uniform heating behavior of heated blanks or preformed components can be ensured if heating rates are variably set as a function of the sheet and layer thickness.

[0118] It is also advantageous if the heating rates are selected so that they are matched to one another.

[0119] In this connection, it is advantageous if the first heating rate r1 until 530° C. is reached is below a certain average heating rate rm.

[0120] In an advantageous embodiment, rm equals 14 K / s so that r1<rm, i.e. r1<14 K / s.

[0121] The heating can be carried out by means of radiation or convection. Alternatively, the heating can be carried out by means of radiation and convection.

[0122] It has also been discovered that one or more heating rates, which are variable over the furnace dwell time and depend on the zinc layer thickness, should be set in order to be able to advantageously ensure the most uniform possible heating of the blanks or preformed components and ensure a high-quality embodiment of the zinc-based layer.

[0123] The basic idea here is to reduce the temperature of the first furnace zones and thus reduce the average heating rates r1 to a maximum of 14 K / s from room temperature (RT) to 530° C. This particularly applies to coatings Z40 to Z180 on 20MnB8 to 34MnB8 with a sheet thickness of 0.8 mm to 3 mm. A sheet thickness range from 0.9 mm to 2.5 mm is particularly preferable.

[0124] 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 warm product carriers have preheated the blanks or preformed components. This initial temperature can, for example, be 50° C. or 60° C. Furthermore, this temperature can also fluctuate seasonally, but its fluctuation range in itself is irrelevant to the subsequent process.

[0125] In the event that a clear definition is required, room temperature can be established at the standard temperature of 20° C.

[0126] 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-%).

[0127] It has been discovered that the invention can always be reworked in a simple way if a relationship with the sheet thickness and coating layer complies with the formula:r⁢1<3⁢2d*s3this being 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 / m2 per side.The heating rate r1 is therefore set to be variable as a function of the sheet thickness (d) and the layer thickness(s), particularly in the temperature range up to 530° C.

[0129] As explained above, during the heating for purposes of the austenitization, blanks or shaped preformed components heat up in differently homogeneous ways, in particular from the edge to the inside. This also influences the formation of the zinc oxide layer due to the locally non-homogeneous heating.

[0130] FIG. 1 shows three maximum average heating rates r1 as a function of the coating layer s and sheet thickness d in accordance with the above formula. It is evident here that the highest average heating rate r1 decreases as a function of the sheet thickness.

[0131] In other words, as the sheet thickness d increases, the heating rate is correspondingly reduced so that if d1<d2<d3, then the heating rate is set such that r1(d1)>r1(d2)>r1(d3).

[0132] The relationship with the coating layer is also evident such that the curves are likewise lower as the coating layer increases.

[0133] FIG. 2 shows three minimum average heating rates r1 as a function of the coating layer s and sheet thickness d. The interplay among the factors is very evident here as well.

[0134] Consequently the heating rate r1 is correspondingly reduced as the coating layer s increases so that if s1<s2<s3, then the heating rate r1 is set such that r1(s1)>r1(s2)>r1(s3).

[0135] FIG. 3 shows the preferred average heating rate range r1 for the coating layer s=20 g / m2 per side. This shows the typical value for the coating Z40. It is clear that for example with a sheet thickness of 1.5 mm, there is a preferred average heating rate range r1 for Z40, i.e. s=20 g / m2 per side, of approximately 3.4 to 7.8 7.8 K / s from room temperature to 530° C.

[0136] FIG. 4 and FIG. 5 each show the preferred average heating rate range r1 for the coating layer s=50 g / m2, i.e. Z100, and s=90 g / m2, which corresponds to Z180.

[0137] The effect according to the invention is also visually apparent in FIG. 6, where on the left, an average heating rate 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 right, 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 is considerably more uniform, even from the standpoint of the surface morphology.

[0138] FIG. 7 shows a sample heating curve from room temperature to 900° C. for an oiled steel blank coated with Z140. In this case, the three different heating rates r1 to r3 are quite readily apparent. It is clear that the average heating rate r1 from RT to 530° C. should be set comparatively low in order to be able to ensure the most homogeneous possible heating across the entire surface of the blanks or preformed component. Starting from the decay of the zeta phase at 530° C., the heating should take place more quickly; for this reason, the maximum heating rate r2 in the range from 530° C. to 670° C. is selected to be higher than the average heating rate r1 in order to ensure a good layer formation. From 670° C. to 780° C., however, the heating should preferably take place comparatively slowly in order to enable a robust mode of operation.

[0139] Consequently, the heating rates r1, r2, and r3 are set in such a way that the following relationship applies:r⁢1<r⁢2;r⁢2>r 3.

[0140] In a particularly advantageous embodiment, the heating rates can be set in such a way that r2>r1>r3.

[0141] For example, r2 can be set to above 14 K / s, in particular between 15 K / s and 25 K / s.

[0142] Preferably, r1 can lie in the range from 3 K / s to 14 K / s and correspondingly, r3 can preferably be less than the value of r1.

[0143] For example, a continuous furnace is a suitable unit for heating, particularly by means of radiation or convection, 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 heating rate r1 is not exceeded, whereas significantly higher heating rates are also possible in the range above this, in particular from 530° C. to 670° C.

[0144] It is particularly advantageous if r2 is greater than r1.

[0145] In another particularly advantageous embodiment, r3 is always lower than r1 and r2 in order to minimize the risk of furnace overheating and the risk of fire.

[0146] A multi-layer chamber furnace is also suitable, where in this case, the furnace temperature is preferably kept constant and a reduction in the furnace temperature, where appropriate, leads to the most critical temperature range up to 530° C. 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 and then above 530° C., is increased.

[0147] The invention has the advantage 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.

[0148] The variable setting of the heating rate ensures a homogeneous heating behavior in the furnace as a function of the layer thickness.

[0149] The ability to variably set the heating rates makes it possible to reduce the heating rates below 530° C. and to increase them between 530° C. and 670° C. This reduces the furnace dwell time and reduces energy consumption and CO2 emissions. On the whole, this significantly 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 coated with a zinc-based 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 and depends on the zinc layer thickness, 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 a maximal heating rate r2 in the temperature range between 530° C. and 670° C., where r1<r2.

2. The method according to claim 1, wherein the maximum heating rate r2 is set to above 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 r1 is set to below 14 K / s.

4. The method according to claim 1, characterized in that the average heating rate r1 in K / s is selected that fulfills the following formula where d is expressed in mm and s is expressed in g / m2:r⁢1<3⁢2d*s3this being 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 / m2 per side.

5. The method according to claim 1, characterized in that an average heating rate r1 in K / s is selected that fulfills the following formula where d is expressed in mm and s is expressed in g / m2:r⁢1>1⁢4d*s3this being 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 / m2 per side.

6. The method according to claim 1, characterized in that the heating takes place at an average heating rate r3 in the temperature range from 670° C. to 780° C., where the following applies: r3<r1.

7. The method according to claim 1, wherein the heating rate r1 is reduced as the coating layer s increases so that if s1<s2<s3, then the heating rate r1 is set in such a way that r1(s1)>r1(s2)>r1(s3).

8. The method according to claim 1, wherein the heating rate r1 is reduced as the sheet thickness d increases so that if d1<d2<d3, then the heating rate r1 is set in such a way that r1(d1)>r1(d2)>r1(d3).

9. The method according to claim 1, characterized in that the zinc-based coating has a coating layer in the range from 20 to 100 g / m2 per side.

10. The method according to claim 1, characterized in that the steel strip has a thickness of 0.85 mm to 3 mm.

11. The method according to claim 1, characterized in that the coating has been selected from the specifications Z40 to Z200, in particular Z40 to Z180.

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

13. 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.

14. 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. in order to minimize energy needs and CO2 emissions.

15. 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.

16. 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.

17. 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.

18. 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.

19. The method according to claim 1, characterized in that the zinc-based coating has a layer thickness of 3 μm to 14 μm per side.

20. The method according to claim 1, characterized in that the zinc-based coating has a zinc content of 85 wt % to 99 wt %, in particular 94 wt % to 98 wt %, and contains aluminum in the range from 0.2 to 2 wt % as well as inevitable impurities.

21. The method according to claim 1, characterized in that the zinc-based coating has been applied by means of a hot-dip process, in particular hot-dip galvanizing.

22. The method according to claim 1, characterized in that 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.

23. 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 %,remainder iron and smelting-related impurities.

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