WO2024179942A1 - Method for producing a hardened sheet steel component - Google Patents

Abstract

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

Description

Method for producing a hardened steel sheet component

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

It is known in automobile construction to use sheet steel components with increased stability to increase the rigidity of the passenger cell. 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 than sheet steel components made of conventional steel. Such hardened sheet steel components are used because the hardening enables components that can have a reduced wall thickness compared to components made of non-hardenable steel types. This makes it possible to provide steel bodies for motor vehicles that are comparatively light while being extremely stable.

The processes for producing hardened sheet steel components are known.

The hardening mechanism used is known as quench hardening, in which the steel material is first converted into its high-temperature phase, known 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. For example, the critical cooling rate for the boron-manganese steel 22MnB5 is around 23 Kelvin per second.

At these cooling rates, the austenitic phase does not transform back into ferrite, but rather into martensite. The rapid cooling essentially freezes this martensitic phase. Since austenite can dissolve significantly more carbon than martensite, carbon precipitation occurs, which distorts the lattice and leads to the high hardness. The prerequisite for this is that the steel material or steel alloy contains sufficient carbon, which is ensured by using suitable alloys. The steel industry usually provides so-called boron-manganese steels for this purpose, with one of the most common steels being 22MnB5, which is, however, part of a comparatively large family of steel grades. In particular, a steel material with the following composition in mass percent is used:

Carbon up to 0.4, preferably 0.10 to 0.30 Silicon up to 1.9, preferably 0.11 to 1.5 Manganese up to 3.0, preferably 0.8 to 2.5 Chromium up to 1.5, preferably 0.1 to 0.9 Molybdenum up to 0.9, preferably 0.001 to 0.1 Nickel up to 0.9, preferably up to 0.2 Titanium up to 0.2, preferably 0.02 to 0.1 Vanadium up to 0.2 Tungsten up to 0.2, Aluminium up to 0.2, preferably 0.02 to 0.07 Boron up to 0.01, preferably 0.0005 to 0.005 Sulfur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01

Rest iron and impurities.

In the past, two basic methods for processing have emerged.

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

For components with more complex geometries and/or higher quantities, the applicant has developed the so-called form hardening, also known as the indirect process, in which a blank is cut out of a steel sheet and this is first formed and trimmed in conventional cold forming processes to form a steel sheet pre-assembly part. This pre-assembly part is It is formed in such a way that it is slightly smaller in all three spatial directions than the target geometry, as it is then heated to the austenitizing temperature and increases in size due to thermal expansion. After austenitizing, this pre-assembly is quickly transferred to a hot-dip galvanizing press, in which the already fully formed pre-assembly is in contact with the hot-dip galvanizing tool in the closed press and is only held in place to cool it down quickly. Only minor calibrations or adjustments are usually carried out in the press. As the components are usually completely trimmed in the cold forming process, trimming in the hardened state is usually no longer necessary. Hot-dip galvanizing allows for more complex components, as the upstream, usually multi-stage forming and trimming process in the cold state allows for more freedom in shaping.

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

It is known to use galvanized steel sheets for the two aforementioned processes, whereby these galvanized steel sheets usually have a zinc-based coating that contains a certain proportion of one or more elements with an even higher affinity for oxygen. These elements with an even higher affinity for oxygen diffuse to the surface during the high-temperature process required for austenitization and form a glassy layer there, an oxide skin, which protects the underlying zinc layer from evaporating during the high-temperature process.

The applicant uses suitable galvanized steel sheets in both processes.

In addition to steel sheets with an almost pure zinc layer, i.e. steel sheets with a coating essentially made of zinc with elements that have a higher affinity for oxygen, so-called ZF coatings, also called GA coatings, are also known, in which the zinc coating is followed by a heat treatment, thus creating a zinc-iron alloy layer on the steel strip. A coating consisting predominantly of zinc comprises, within the meaning of the invention, not less than 90% by weight of zinc. Advantageously, such a zinc-based coating contains not less than 92% by weight, preferably not less than 94% by weight, particularly preferably not less than 96% by weight of zinc.

During heat treatment for the purpose of austenitizing press-hardening steels, the formation of a thin oxide skin made of elements with a higher affinity for oxygen only occurs in the case of so-called Z coatings, also known as GI coatings, i.e. coatings that are essentially based on zinc. In typical Z coatings, the layer contains not only zinc but also around 0.2 to 2% by weight of aluminum.

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

During die hardening and press hardening, cracks usually form in this very thin ALOs layer, resulting in a small amount of zinc evaporating and the formation and deposition of zinc oxide on the surface. After press hardening or die hardening, such components typically have a light green to brownish surface, which is caused by the coating of these zinc oxides.

It has been observed that press-hardened components or form-hardened components with zinc coatings sometimes, in a fluctuating and unpredictable manner, either completely or only in certain areas, have a silver coloring in addition to the familiar greenish-brownish color. A silver coloring in the sense of the invention can also be perceived as light gray or light grayish and can also appear matt or shiny in places. Experience has shown that the frequency of silver coloring increases in particular with the thickness of the coating. It was also discovered that the silver coloration develops when the entire zinc layer has not been converted into solid zinc-iron phases up to a temperature of 530 °C, i.e. when there is still liquid zinc left on the surface at 530 °C and/or there is a lack of oxygen at higher temperatures. The temperature of around 530 °C marks the end of the existence of the so-called zeta zinc-iron phase.

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

It is also known that circuit boards or components heat up differently and not really homogeneously in the oven. For example, the heating from the outside to the inside is not homogeneous. In addition, different heating progresses can lead to different iron contents in the coating, which in turn leads to different emissivities and thus to differences in the heating behavior.

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

In addition, during the die hardening process, a different and fluctuating coloration of the surface can lead to dimensional stability problems, since the size of the finished hardened component depends on the size of the austenitized pre-component inserted into the die hardening tool. The size of the austenitized pre-component inserted into the die hardening tool is depends on the cooling behavior of the pre-component between the furnace and the hardening tool, which in turn depends on the emissivity of the surface of the pre-component due to the radiant heat losses.

The formation of the surface zinc oxide layer is also affected by this. This occurs particularly with comparatively thin zinc coatings and thin sheets, especially with sheet thicknesses of less than 1.5 mm and zinc layer thicknesses such as Z 80 according to EN 10346 - this means that the layer thickness is approximately 40 g/m ² per side.

From DE 10 2020 113287 Al a process for producing hardened sheet steel components is known in which the dew point is adjusted in the furnace to avoid silvery spots.

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

From CN115125439 A it is known to heat a steel 34MnB8 with a zinc-iron coating with holding zones and comparatively low heating rates.

The opposite is known from EP 2611945 Bl, namely a rapid heating of sheets with a zinc-iron coating, whereby heating from room temperature to 500°C with at least 15 K/s to 50 K/s is to be carried out.

The object of the invention is to provide a method for producing sheet steel components and to ensure homogeneous properties of the components.

The object is achieved by a method having the features of claim 1.

Advantageous further developments are identified in the dependent subclaims. According to the invention, heating is carried out in such a way that optimized heating with variable heating rates over the furnace residence time is always guaranteed, tailored to the sheet and the coating, which leads to more homogeneous properties. A formula is provided for this purpose, which leads to reliable process control.

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

This layer can advantageously have a thickness of 5 pm to 20 pm 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 according to DIN EN 10346.

Zinc-based corrosion protection coatings can have a comparatively high zinc content of 85 wt.% to 99 wt.% and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2 wt.%.

Particularly preferably, the metallic anti-corrosive layer based on zinc can be applied using a hot-dip galvanizing process. This can be a simple and robust method of application.

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

In addition, the invention enables an energy-optimized (reduced heat input into the annealing material and/or reduced heat losses) and process-safe (secured austenitization) and, with small quantities, also economically advantageous (fast heating even at low furnace temperatures). The heat requirement when heating sheet steel blanks or sheet steel prefabricated parts in a furnace is made up of the amount of heat required for the desired heating of the annealing material (the blank or the part) plus the amount of heat due to undesirable furnace losses, e.g. Furnace doors or furnace casing or transport elements. Both can be minimized by the reduced heating rate according to the invention. On the one hand, for example, the first furnace zone, i.e. the one until 530 °C is reached on the board or part, can be set "cooler" than in prior art processes. Alternatively or additionally, the entire furnace temperature can also be reduced, for example to a range of 860 °C to 890 °C compared to the previously usual furnace temperatures of 900 °C to 920 °C.

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

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

Reduced energy requirements have a direct impact on CO2 emissions and can therefore be implemented in a sustainable and environmentally friendly manner.

According to the invention, it was found that the most homogeneous emissivity possible over the entire surface and thus a uniform heating and cooling behavior for heated circuit boards or prefabricated parts can be ensured if the heating rates are selected so as to be optimally coordinated with one another, but at least the first heating rate rl until 530°C is reached is below a certain average heating rate, for example the maximum heating rate r2.

Heating can occur through radiation or convection.

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

Heating by radiation can be achieved, for example, by an infrared heater. Furthermore, it was recognized according to the invention that one or more heating rates should be set that are variable over the furnace residence time and dependent on the zinc layer thickness in order to advantageously ensure a high-quality layer and oxide formation of the zinc-based layer.

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

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 applies in particular to coatings Z40 to Z180 on 20MnB8 to 34MnB8 with a sheet thickness of 0.85 mm to 3 mm. Other boron-manganese grades in the strength range of 400 to 1200 MPa Rm after the hardening process are also conceivable.

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.

The average heating rate rl in the sense of the invention is the average of the heating rate of the sheet steel blank or sheet steel pre-assembly when heating from room temperature to 530 °C. This means that the heating rate can be comparatively high, especially in the first heating phase to 100 °C and more, and then decreases continuously as the temperature difference between the sheet steel blank or sheet steel pre-assembly and the furnace temperature decreases. The average heating rate is calculated over the duration of the heating from room temperature to 530 °C, i.e. if, for example, 51 seconds are needed to reach 530 °C in the furnace at a room temperature of 20 °C, the average heating rate is 10 K/s. In contrast, a maximum heating rate is determined using the highest value at a specific point in the temperature range. The sheet thickness range of the steel strip can be selected from 0.85 mm to 3 mm, preferably 0.9 mm to 2 mm.

It was found that the heating rate rl in the range from room temperature to 530 °C must be adjusted in such a way that, in order to achieve the most homogeneous heating possible across the board or prefabricated part, this depends on the one hand on the sheet thickness d, but also on the layer thickness s in g/m ² .

gültig für den Blechdickenbereich d von 0,85 mm bis 3 mm und Schichtdickenbereich s von 20 bis 100 g/m² je Seite. This results in the following relationship:

valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g/m ² per side.

The inventors have surprisingly discovered that a heating rate rl smaller than the formula given above results in homogeneous heating and optimal layer formation, depending on the layer thickness and sheet thickness. Advantageously, the existing inhomogeneities on the board or prefabricated part, such as locally different oil residues, can be compensated surprisingly well with a comparatively low heating rate.

gültig für den Blechdickenbereich d von 0,85 mm bis 3 mm und Schichtdickenbereich s von 20 bis 100 g/m² je Seite. Des Weiteren wurde herausgefunden, dass ab dem Zerfall der zeta-Phase, welche bei 530°C zerfällt, die Heizrate erhöht werden soll, um die Qualität der Schichtausbildung zu erhöhen. In order to optimize the cycle time, the heating of the steel sheets or steel components should not be too slow and should therefore advantageously fulfill the following relationship:

valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g/m ² per side. Furthermore, it was found that starting from the decay of the zeta phase, which decays at 530°C, the heating rate should be increased in order to improve the quality of the layer formation.

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

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

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

In addition, it was found that it is alternatively or additionally useful to ensure a certain oxygen saturation on the sheet surface from a certain temperature. The solution according to the invention therefore lies in eliminating the factors that lead to silver discoloration.

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

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

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

The weather conditions have an influence through chimney effects or through the displacement of oxygen by gaseous combustion products. It has also been found that new oven elements, such as stainless steel, ceramic or other insulating materials, can consume or bind oxygen or have an influence on the oxygen saturation.

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

Advantageously, at least 5 vol.% oxygen can be made available close to the surface starting at a temperature of 800 °C. Close to the surface here means in an area closer than 10 mm to the surface of the circuit board or the pre-assembled part.

This can be done, for example, by blowing in fresh air, whereby the fresh air is preferably preheated accordingly. In multi-layer chamber furnaces, i.e. without a transport movement of the blanks or prefabricated parts during heating as occurs in a continuous furnace, convection can also be forced by flushing, flow, circulation or temporarily opening furnace doors.

The invention thus relates to a method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat steel strip coated with a zinc-based coating and the sheet steel blank is either heated at least in part to a temperature above Ac3 and then formed in a press hardening tool in a hot state and quench hardened, or the coated sheet steel blank is cold formed into a sheet steel pre-component and the sheet steel pre-component is heated at least in part to a temperature above Ac3. and then quench hardened in a die hardening tool, wherein the heating is carried out at a heating rate which is variable over the furnace residence time and dependent on the zinc layer thickness, wherein in the temperature range from room temperature to 530 °C the heating is carried out at an average heating rate rl and in the temperature range between 530 °C and 670 °C at a maximum heating rate r2, where rl < r2 .

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

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

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

gültig für den Blechdickenbereich d von 0,85 mm bis 3 mm und den Schichtdickenbereich s von 20 bis 100 g/m² je Seite. Vorteilhafterweise kann dadurch einerseits die Blechdicke als auch die Schichtdicken der Beschichtung auf Basis von Zink zur Ermittlung der höchsten mittleren Aufheizrate rl berücksichtigt werden. An advantageous further development provides that the average heating rate rl in K/s is selected, which satisfies the following formula with d in mm and s in g/m ² :

valid for the sheet thickness range d from 0.85 mm to 3 mm and the layer thickness range s from 20 to 100 g/m ² per side. Advantageously, this allows both the sheet thickness and the layer thickness of the zinc-based coating to be taken into account to determine the highest average heating rate rl.

gültig für den Blechdickenbereich d von 0,85 mm bis 3 mm und Schichtdickenbereich s von 20 bis 100 g/m² je Seite. Vorteilhafterweise kann dadurch einerseits die Blechdicke als auch die Schichtdicken der Beschichtung auf Basis von Zink zur Ermittlung einer minimalen mittleren Aufheizrate berücksichtigt werden, welche die Taktzeit verbessern kann. An advantageous further development provides that an average heating rate rl in K/s is selected, which satisfies the following formula with d in mm and s in g/m ² :

valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g/m ² per side. Advantageously, this allows both the sheet thickness and the layer thickness of the zinc-based coating to be taken into account to determine a minimum average heating rate, which can improve the cycle time.

An advantageous further development provides that the heating takes place in the temperature range from 670 °C to 780 °C with an average heating rate r3, where r3 < rl. This ensures homogeneous heating of the entire surface and also makes the subsequent cooling behavior more homogeneous.

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

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

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

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

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

An advantageous further development provides that the furnace heating power is increased in the areas in which the board or the pre-assembled part has a temperature of more than 530 °C. This advantageously allows the cycle time to be kept as short as possible.

An advantageous further development provides that the entire furnace temperature is reduced to a range of 860 °C to 890 °C in order to keep energy consumption and CO2 emissions low.

An advantageous further development provides that in the furnace areas in which the board or the pre-assembled part has a temperature of more than 530 °C, the furnace heating power is reduced or kept constant. This can advantageously keep the energy requirement and CO2 emissions low.

An advantageous further development provides that in furnace areas in which the blank or the pre-assembled part is at 800 °C or more, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere.

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

An advantageous further development provides that in heating devices in which the blank or the steel pre-component is heated in a stationary manner, such as multi-layer chamber furnaces, the furnace heating power is reduced permanently or over time in such a way that the average heating rate of 14 K/s is not exceeded in the temperature range from room temperature to 530 °C. An advantageous further development provides that the zinc-based coating has a layer thickness of 3 pm to 14 pm per side.

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

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

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

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

Carbon up to 0.4, preferably 0.10 to 0.30 and

Silicon up to 1.9, preferably 0.11 to 1.5 and

Manganese up to 3.0, preferably 0.8 to 2.5 and

Chromium up to 1.5, preferably 0.1 to 0.9 and

Molybdenum up to 0.9, preferably 0.001 to 0.1 and

Nickel up to 0.9, preferably up to 0.2 and

Titanium up to 0.2 preferably 0.02 to 0.1 and

Vanadium up to 0.2 and

Tungsten up to 0.2 and

Aluminium up to 0.2, preferably 0.02 to 0.07 and Boron up to 0.01, preferably 0.0005 to 0.005 and

Sulphur max. 0.01, preferably max. 0.008 and

Phosphorus max. 0.025, preferably max. 0.01 and

Rest iron and smelting-related impurities.

The invention is explained by way of example using figures. They show:

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

Sheet thickness d;

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

Sheet thickness d;

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

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

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

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

Figure 7: an example heating curve from room temperature to 900°C for a

Z140 coated, oiled steel plate

According to the invention, a press-hardened component coated with a zinc-based coating is heated so that it has a homogeneous zinc oxide layer. This enables uniform heating, ie a homogeneous heating behavior, of the circuit board or the component. of the sheet steel component and thus ensures excellent mechanical properties and optimal processing characteristics.

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

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

It is also particularly advantageous if the heating rates are chosen to match each other.

It is advantageous if the first heating rate rl is below a certain average heating rate r _m until 530°C is reached.

In an advantageous embodiment, r _m is 14 K/s, so that rl < r _m , i.e. rl < 14 K/s.

Heating can be achieved by radiation or convection. Alternatively, heating can be achieved by radiation and convection.

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

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

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

The temperatures given below are in °C. Temperature differences can be given in K (Kelvin). Unless otherwise stated, contents in alloys are given in mass percent (M-%).

gültig für den Blechdickenbereich d von 0,85 mm bis 3 mm und Schichtdickenbereich s von 20 bis 100 g/m² je Seite. It was found that the invention can always be easily reproduced if a formulaic relationship with the sheet thickness and layer thickness is given:

valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g/m ² per side.

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

As already mentioned, blanks or formed prefabricated parts heat up to varying degrees of homogeneity during heating for the purpose of austenitization, particularly from the edge inwards. This also influences the formation of the zinc oxide layer due to the locally inhomogeneous heating. Figure 1 shows three maximum average heating rates rl depending on the layer thickness s and sheet thickness d according to the formula given above. It can be seen that the highest average heating rate rl decreases over the sheet thickness.

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

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

Figure 2 shows three minimum average heating rates rl depending on the layer thickness s and sheet thickness d. Here, too, the interaction of the factors can be clearly seen.

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

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

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

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

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

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

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

A suitable unit for heating, in particular by means of radiation or convection, is, for example, a continuous furnace, in which jacketed radiant tubes, which are heated by gas, for example, 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 rl is not exceeded, while in the range above this, in particular from 530 °C to 670 °C, significantly higher heating rates are also possible. It is particularly advantageous if r2 is larger than rl.

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

A multi-layer chamber furnace is also suitable, whereby the furnace temperature is preferably kept constant and a reduction in the furnace temperature may result in the most critical temperature range up to 530°C being passed through with a correspondingly low heating rate rl. However, a furnace temperature that varies over the furnace residence time, i.e. a furnace temperature that is low at the beginning and then increased from 530°C, is also conceivable. The advantage of the invention is that with the inventive setting of the heating rate rl in the critical temperature range from RT to 530°C, repeatable results are achieved with regard to a uniform, homogeneous coating.

The variable setting of the heating rate ensures homogeneous heating behavior in the furnace depending on the layer thickness. The ability to variably set the heating rates means that the heating rates can be reduced below 530 °C and increased above 530 °C, especially between 530 °C and 670 °C. This shortens the furnace residence time, reduces energy consumption and CO2 emissions. Overall, this significantly increases the process efficiency.

Claims

Claims 1. A method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat steel strip coated with a zinc-based coating and the sheet steel blank is either heated at least in part to a temperature above Ac3 and then formed in a press hardening tool while hot and quench hardened, or the coated sheet steel blank is cold formed into a sheet steel pre-component and the sheet steel pre-component is heated at least in part to a temperature above Ac3 and then quench hardened in a die hardening tool, characterized in that the heating takes place at a heating rate which is variable over the furnace residence time and dependent on the zinc layer thickness, wherein in the temperature range from room temperature to 530 °C the heating takes place at an average heating rate rl and in the temperature range between 530 °C and 670 °C at a maximum heating rate r2, where rl < r2 . 2. The method according to claim 1, wherein the maximum heating rate r2 is set greater than 14 K/s, in particular between 15 K/s and 25 K/s. 3. The method according to claim 1, wherein the average heating rate rl is set to less than 14 K/s. 4. Method according to claim 1 or 2, characterized in that the average heating rate rl in K/s is selected which satisfies the following formula with d in mm and s in g/m ² :

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

valid for the sheet thickness range d from 0.85 mm to 3 mm and layer thickness range s from 20 to 100 g/m ² per side. 6. Method according to one of the preceding claims, characterized in that the heating takes place in the temperature range from 670 °C to 780 °C with an average heating rate r3, where r3 < rl. 7. Method according to one of the preceding claims, wherein the heating rate rl is reduced with increasing layer thickness s, so that if sl < s2 < s3, the heating rate rl is adjusted such that rl(sl) > rl(s2) > rl(s3). 8. Method according to one of the preceding claims, wherein the heating rate rl is reduced with increasing sheet thickness d, so that when dl < d2 < d3, the heating rate rl is adjusted such that rl(dl) > rl (d2) > rl(d3). 9. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness in the range of 20 to 100 g/m ² per side. 10. Method according to one of the preceding claims, characterized in that the steel strip has a thickness of 0.85 mm to 3 mm. 11. Method according to one of the preceding claims, characterized in that the coating was selected from specifications from Z40 to Z200, in particular Z40 to Z180. 12. Method according to one of the preceding claims, characterized in that in a continuous furnace in the furnace area corresponding to a blank or pre-component temperature of up to 530 °C, the furnace heating power is reduced so that the average heating rate of 14 K/s is not exceeded in the temperature range up to 530 °C. 13. Method according to one of the preceding claims, characterized in that the furnace heating power is increased in the regions in which the board or the pre-component has a temperature of more than 530 °C. 14. Method according to one of the preceding claims, characterized in that the total furnace temperature is reduced to a range of 860 °C to 890 °C in order to keep the energy requirement and the CO2 emissions low. 15. Method according to one of the preceding claims, characterized in that in the furnace regions in which the blank or the pre-component has a temperature of more than 530 °C, the furnace heating power is reduced or kept constant. 16. Method according to one of the preceding claims, characterized in that in furnace areas in which the blank or the pre-component is at 800 °C or more, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere. 17. Method according to one of the preceding claims, characterized in that in furnace areas in which the blank or the pre-component has 800 °C or more, the existing furnace atmosphere is circulated. 18. Method according to one of the preceding claims, characterized in that in heating devices in which the blank or the steel pre-component is stationary is heated up, such as multi-layer chamber furnaces, the furnace heating power is reduced permanently or over time in such a way that the average heating rate of 14 K/s is not exceeded in the temperature range from room temperature to 530 °C. 19. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness of 3 pm to 14 pm per side. 20. Method according to one of the preceding claims, characterized in that the zinc-based coating has a zinc content of 85 wt.% to 99 wt.%, in particular 94 wt.% to 98 wt.%, aluminum in the range of 0.2 to 2 wt.% and unavoidable impurities. 21. Method according to one of the preceding claims, characterized in that the zinc-based coating was applied by means of a hot-dip galvanizing process. 22. Method according to one of the preceding claims, characterized in that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably a 22MnB5 or 20MnB8 or 34MnB5. 23. Method according to one of the preceding claims, characterized in that the steel strip used is a strip with the following composition (all data in % by weight): Carbon up to 0.4, preferably 0.10 to 0.30 and Silicon up to 1.9, preferably 0.11 to 1.5 and Manganese up to 3.0, preferably 0.8 to 2.5 and Chromium up to 1.5, preferably 0.1 to 0.9 and Molybdenum up to 0.9, preferably 0.001 to 0.1 and Nickel up to 0.9, preferably up to 0.2 and Titanium up to 0.2, preferably 0.02 to 0.1 and vanadium up to 0.2 and tungsten up to 0.2 and aluminium up to 0.2, preferably 0.02 to 0.07 and Boron up to 0.01, preferably 0.0005 to 0.005 and sulfur max. 0.01, preferably max. 0.008 and phosphorus max. 0.025, preferably max. 0.01 and the rest iron and smelting-related impurities.