WO2024179668A1 - Method for producing a hardened sheet steel component - Google Patents

Abstract

The invention relates to a method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat, hot- and cold-rolled steel strip coated with a zinc-based coating, and at least sub-regions of the sheet steel blank are either heated to a temperature above Ac3 and then shaped in a hot state in a press-hardening tool and quench-hardened, or the sheet steel blank is cold-formed into a sheet steel preform and the sheet steel preform is then quench-hardened in a hot-stamping tool, wherein, for heating between 400°C and 550°C, in particular between 420°C and 530°C, the heating rate is set such that the degree of discoloration towards silver S is greater than 1.

Description

Method for producing a hardened sheet steel 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 hardening enables components to be made 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 0.15 to 0.35 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 0.01 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 into a sheet steel pre-assembly using conventional cold forming processes. This pre-assembly is formed in such a way that it is slightly smaller in all three spatial directions than the target geometry, as it is then heated to the austenitizing temperature and expands accordingly due to thermal expansion. After austenitizing, this pre-assembly is quickly transferred to a hot-dip quenching press, in which the already fully formed pre-assembly is in contact with the hot-dip quenching tool in the closed press and is only held in place to cool it quickly. Only minor calibrations or adjustments are usually made 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 quenching 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, ie steel sheets with a coating consisting essentially of zinc with elements that are more affine to oxygen, so-called ZF coatings, also called GA coatings, are also known, in which the zinc coating A heat treatment follows and a zinc-iron alloy layer is created on the steel strip.

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 known that the nature and color of a surface influence the absorption and emission of heat radiation via the surface. This property is described by the emissivity; different and fluctuating colors can therefore lead to different and fluctuating emissivity of the surface. Surfaces with silver coloring usually have significantly lower emissivity than surfaces with the typical greenish to brownish coloring, which, due to the reduced heat flow, leads on the one hand to slower heating in the oven and on the other hand to slower cooling during transfer from the oven to the press.

Such different and fluctuating emission levels can lead to different and fluctuating heating behavior in the furnace, which can be technically and especially economically problematic for several reasons. Since it must be ensured in every case that the steel sheet is fully austenitized for the subsequent hardening for the production of the components, the minimum furnace dwell time of all blanks or pre-assembled parts must be extended so that even slowly heating surfaces with silver coloring are reliably austenitized, even though these may only make up a fraction of all blanks or pre-assembled parts; extending the minimum furnace dwell time of all blanks or pre-assembled parts usually results in a lower output of hardened components per production time and is therefore a disadvantage. Since the maximum furnace dwell time, given the faster heating surfaces of the typical greenish to brownish coloring, cannot be extended to the same extent, the existing furnace process window is also reduced. This can lead to an increased amount of scrap in the event of production disruptions, which is of course also a disadvantage.

The different and fluctuating emission levels can cause a further problem, particularly for hot stamping, due to the resulting different and fluctuating cooling behavior of the pre-assembled parts between the furnace and the press. After austenitizing, the pre-assembled parts are transferred to the corresponding hot stamping press. As already mentioned, differently colored areas or areas with different degrees of emission can show different cooling behavior. Since during form hardening the component is no longer trimmed or influenced in any other way after hardening, the insertion temperature of the austenitized pre-component must be correct, since the insertion temperature ultimately determines the size of the component as the sum of the pre-formed shape and the thermal expansion. Different and particularly fluctuating insertion temperatures therefore lead to fluctuations in the dimensional accuracy of the components during form hardening, which represents a quality problem because it may mean that specified dimensional tolerances cannot be met. Since silver coloring usually occurs irregularly and unpredictably and sometimes only individual components or component areas of a batch of components transported together from the furnace to the press are affected, correction or compensation via the transfer time between the furnace and the press is not possible. It might also be tempting to remove the silver staining by increasing the oven dwell time, but it has been shown that increasing the oven dwell time tends to aggravate the problem rather than solving it, since areas with silver staining remain stable even with a longer oven dwell time, while the remaining areas become increasingly darker with increasing oven dwell time; a longer oven dwell time would also be uneconomical and therefore disadvantageous for the reasons already mentioned.

It is also known that such galvanized high-temperature treated steel sheets or steel sheet components are often further treated after press hardening or form hardening. The process mainly used is so-called wheel blasting. In the case of a pronounced silver coloration, it has been found that paint adhesion problems can arise if the wheel blasting was insufficient.

The silver coloring mentioned is also often undesirable on the customer side, as it does not provide a uniform surface appearance of the components.

In Figures 1 and 2, a silvery-gray surface and a greenish surface are contrasted, with the silvery-gray surface being dominated by aluminum oxide, while the greenish surface is dominated by zinc oxide. EP 2 611 945 B1 discloses a method for producing hardened sheet steel components, in which a predetermined heating regime is to be maintained, whereby a heating rate of 16 to 50 K/s is to be maintained up to a temperature of 500 °C; this is intended to ensure faster heating.

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

The object of the invention is to provide a method for producing sheet steel components without silver coloring with a uniform emissivity.

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, 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 Z80 or Z120 or Z140 or Z180 according to DIN EN 10346.

Zinc-based corrosion protection layers can have a comparatively high zinc content of 85% to 99% by weight and, in addition to unavoidable impurities, also contain aluminum in the range of 0.2 to 2% by weight. They can also contain other oxygen-affine elements such as magnesium. Particularly preferably, the metallic anti-corrosive layer based on zinc can be applied using a hot-dip process. This can be a simple and robust method of application.

According to the invention, it was found that the most uniform possible emission levels and thus a uniform heating and cooling behavior of the circuit boards or prefabricated components can be ensured if a heating rate is selected in a certain temperature range that is below a critical heating rate.

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.

In particular, it can be advantageous not to heat the material at too high a rate in a temperature range between 400 °C and 550 °C and especially in a temperature range between 420 °C and 530 °C. In temperature ranges below and above this, the heating rate can be increased in order not to extend the time spent in the furnace and thus the cycle time.

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.

According to the invention, a heating rate between 420 °C and 530 °C is thus defined, which should be below a limit value, whereby the furnace temperature, the sheet thickness, the zinc layer thickness, the oiling condition and the steel alloy are the most important influencing factors for the heating rate of a steel sheet blank or a steel sheet prefabricated part in a furnace. 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 from 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.

Regarding the heating rate, particularly in the temperature range between 400 °C and 550 °C and especially between 420 °C and 530 °C, some considerations are necessary below. When heated, the zinc layer melts at about 420 °C. In order for the entire layer thickness d to be converted into solid Zn-Fe phases, at least the time t with d ² =D* t must be available for diffusion, where D is the temperature-dependent diffusion coefficient for which the shape

QD = D ₀ * e RT with the activation energy Q=271 kJ/mol and the universal gas constant R=8.31 J/mol/K .

To take into account the influence of the Fe-Al inhibition layer, the layer thickness d is replaced by d+do.

wobei Ti = 693 K, T₂ = 803 K , d die Schichtdicke in Meter, b und do Konstanten mit b = 4,0 * 10⁷ m²/s und d₀ = 4,4 * 10“⁶ m Sind. When heating from 420 °C to 530 °C, the diffusion coefficient increases. For Q=271 kJ/mol, it increases by about 600 times. The temperature-time curve is therefore crucial for the possible diffusion path and must be taken into account, for example in the form of the temperature-dependent heating rate r(T). A dimensionless silveriness number S can then be defined as follows:

where Ti = 693 K, T ₂ = 803 K , d is the layer thickness in meters, b and do are constants with b = 4.0 * 10 ⁷ m ² /s and d ₀ = 4.4 * 10“ ⁶ m.

This formula is only valid for r(T) > 0 in the temperature range T _± < T < T ₂ , i.e. the heating rate must be positive since the sheet is heated and the temperature range from 420 °C to 530 °C is described as explained above. A silvery surface is obtained if the silveriness number S defined above is less than 1 for a given temperature-time curve.

The formula gives a constant heating rate of 8.6 K/s as the limit for silver coloration for the zinc layer Z140 (9.8 pm thick) between 420 °C and 530 °C and 6 K/s as the maximum possible for Z180 (12.6 pm thick). This could be verified experimentally, so that at a heating rate below this, the silveriness number is greater than 1 and this does not lead to silver coloration, and at a heating rate above this in the temperature range mentioned, an undesirable silver coloration occurred.

wobei r(T) die Aufheizrate in Kelvin pro Sekunde, T die Temperatur in Kelvin, r(T) > 0 im Temperaturbereich T_± < T < T₂ und T_± = 693 K, T₂ = 803 K , d die Zinkschichtdicke in m, Konstante b = 4,0 * 10⁷ m² /s, Konstante d₀ = 4,4 * 10^-6 m, Q die Aktivierungsenergie Q = 271 kJ/Mol und R die universelle Gaskonstante R = 8,31 J/Mol/K ist. Eine Ausführungsform sieht vor, dass die mittlere Heizrate zwischen 420 °C und 530 °C für die Zinkauflage Z80 kleiner 15 K/s, für die Zinkauflage Z140 kleiner 8,6 K/s und für die Zinkauflage Z180 kleiner 6 K/s ist. The invention thus relates in particular to a method for producing sheet steel components, wherein a sheet steel blank is cut out of a flat hot- and cold-rolled steel strip coated with a zinc-based coating and the sheet steel blank is either heated in at least some areas 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 to form a sheet steel pre-component and the sheet steel pre-component is heated to a temperature above Ac3 and then quench-hardened in a form hardening tool, wherein for heating from 400 °C to 550 °C, in particular from 420 °C to 530 °C, the critical heating rate is set so that the silveriness number S is greater than 1, where:

where r(T) is the heating rate in Kelvin per second, T is the temperature in Kelvin, r(T) > 0 in the temperature range T _± < T < T ₂ and T _± = 693 K, T ₂ = 803 K , d is the zinc layer thickness in m, constant b = 4.0 * 10 ⁷ m ² /s, constant d ₀ = 4.4 * 10 ^-6 m, Q is the activation energy Q = 271 kJ/mol and R is the universal gas constant R = 8.31 J/mol/K. One embodiment provides that the average heating rate between 420 °C and 530 °C is less than 15 K/s for the zinc coating Z80, less than 8.6 K/s for the zinc coating Z140 and less than 6 K/s for the zinc coating Z180.

One embodiment provides that for heating above 800 °C, an oxygen content in the furnace atmosphere in a region closer than 10 mm to the surface of the blank or the steel pre-component of at least 5 vol.% is ensured.

One embodiment provides that the heating rate in the temperature range from 400 °C to 550 °C, in particular in the temperature range from 420 °C to 530 °C, is set between 3 K/s and 15 K/s and below the respective critical heating rate, wherein the critical heating rate depends on the layer thickness of the zinc-based layer and becomes smaller with increasing thickness.

One embodiment provides that in a continuous furnace in the furnace area corresponding to a blank or pre-component temperature of 420 to 530 °C or in the furnace area corresponding to a blank or pre-component temperature of up to 530 °C, the furnace heating power is reduced such that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded.

One embodiment provides that in the furnace dwell time ranges in which the blank or the pre-component has a temperature of less than 420 °C and/or in particular in the furnace dwell time ranges in which the blank or the pre-component has a temperature of more than 530 °C, the furnace heating power is increased in order to keep the cycle time as short as possible.

One embodiment provides that in furnace areas in which the blank or the pre-assembled part has a temperature of 800 °C or more, fresh air, in particular preheated fresh air or oxygen, is supplied to the furnace atmosphere.

One embodiment 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. One embodiment 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 critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded.

One embodiment provides that the zinc-based coating has a layer thickness of 5 pm to 16 pm per side.

One embodiment provides that the zinc-based coating was applied by means of a hot-dip galvanizing process.

One embodiment provides that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel, particularly preferably a 22MnB5 or 20MnB8 or 34MnB5.

One embodiment provides that a steel strip with the following composition is used (all data in % by weight):

Carbon up to 0.4, preferably 0.15 to 0.35 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 0.01 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 a drawing. In the drawing:

Figure 1: a sheet measuring 15 x 20 cm with a silvery colour after austenitization; Figure 2: the sheet according to Figure 1 in an SEM image;

Figure 3: a sheet measuring 15 x 20 cm with a greenish colour according to

austenitization;

Figure 4: the sheet according to Figure 3 in a SEM image;

Figure 5: the critical heating rate as a function of the zinc layer thickness; Figure 6: a temperature-furnace location curve of a continuous furnace with constant throughput speed; Figure 7: a diagram showing the silveriness limits at given heating rates at

420°C and 530°C with the progression assumed to be linear in between and given zinc layer thicknesses;

Figure 8: a diagram showing two heating rate curves with and without silver staining;

Figure 9: schematically the influence on the heating rate in the temperature range of

420°C to 530°C in a continuous flow furnace (DLO);

Figure 10: schematically the influence on the heating rate in the temperature range of

420°C to 530°C in a chamber of a multi-layer chamber furnace (MLK);

Figure 11: schematically showing the influence on the near-surface oxygen content from a temperature of 800 °C in a continuous flow furnace (DLO);

Figure 12: schematically showing the influence on the near-surface oxygen content from a temperature of 800 °C in a chamber of a multi-layer chamber furnace (MLK);

To visually illustrate the differences in surface coloration already described, Figure 1 shows a steel sheet which has been coated with zinc and has a silvery appearance after austenitization, and Figure 3 shows a corresponding steel sheet which has a greenish appearance after austenitization.

Figures 2 and 4 show the corresponding SEM images of the surfaces.

As already stated, hardened components that have a zinc-based coating can have silver colorings such as those in Figure 1, with the probability increasing with the thickness of the zinc coating. Components with a silver coloring according to Figures 1 and 2 have a reduced or incompletely covering zinc oxide layer. above the existing aluminum oxide layer or have larger areas or surface areas without zinc oxide.

Such silver coloring on the component can occur globally, as in Figure 1, or only locally, and can be caused by variations in layer thickness or by differences in the substrate. As already explained, silver coloring is not a problem of a purely cosmetic nature. Although customers want components to appear uniform, the fluctuating silver coloring can lead to fluctuating heating behavior in the furnace and fluctuating cooling behavior between the furnace and the press due to the resulting fluctuating emissivity, with adverse consequences in terms of cost-effectiveness and/or component quality.

It was also found that a strong silver coloration in combination with possibly insufficient wheel blasting can lead to problems during subsequent painting.

As already stated, the silver coloration, as discovered by the inventors, occurs when the heating rate exceeds a certain critical heating rate in a temperature range of 400 °C to 550 °C, in particular in a temperature range of 420 °C to 530 °C. Above this average critical heating rate, a silvery surface must be expected.

As already mentioned, this also depends on the thickness of the zinc-based coating.

In Figure 5, the average critical heating rate from 420 °C to 530 °C in K/s is plotted against the thickness of the zinc-based layer in micrometers. The average heating rate between 420 °C and 530 °C can be determined from a heating curve by dividing the temperature difference A3 = 530 °C - 420 °C = 110 K by the time required for this At = t(530 °C) - 1(420 °C). The curve is the boundary line above which a silvery appearance cannot be excluded or must be expected, while below which a greenish or greenish-brownish surface appearance is achieved. In Figure 6, the temperature curve of a steel sheet prefabricated part or a steel sheet blank is not only shown as usual over the furnace residence time, but also schematically over the length of a continuous furnace with a constant throughput speed. This is permissible for continuous furnaces with a constant throughput speed, since the furnace residence time is directly proportional to the furnace position. In the upper part of Figure 6, the furnace length is schematically divided into four furnace areas: an area A with a temperature range from ambient temperature to 420 °C, an area B with the critical temperature range from 420 °C to 530 °C, an area C with 530 °C to 800 °C and an area D above 800 °C up to the desired final temperature.

Below, the temperature curve is plotted as a function of the furnace dwell time and the furnace position. In such a continuous furnace, it is important that the heating rate of the blank or the pre-component in area B does not exceed a certain heating rate, as already explained, depending on the thickness of the zinc layer. However, since in a furnace, particularly a continuous furnace, the cycle times cannot be tied to one area without changing the entire course in terms of time, the heating rates in area A and especially C, where they are not critical, are increased accordingly.

In region D, starting at 800 °C, further solutions can be provided according to the invention, namely that the atmosphere is adjusted at least closer than 10 mm to the surface of the blank or the steel prefabricated part so that the oxygen content is above 5 vol.%.

This can be achieved by several measures. On the one hand, fresh air, and in particular preheated fresh air, can be blown into this area, on the other hand or alternatively, the existing oven atmosphere can be sufficiently circulated.

= 693 K und T₂ = 803 K den Verlauf der Heizrate linear als r(T) = + (r₂ - ri)-^— an, so ergibt sich für ver- TZ ~T-L schiedene Zinkschichtdicken aus der Formel für S die Graphik nach Figur 7 für die Silbrig- keitsgrenzen. While Figure 5 shows the average critical heating rate from 420 °C to 530 °C over the thickness of the zinc layer, Figure 7 shows a diagram in which the silveriness limit curves, above which silver coloration must be expected, are plotted as a critical heating rate at 530 °C as a function of the heating rate at 420 °C with an assumed linear progression between 420 °C and 530 °C for various zinc layer thicknesses. It is clear that the silveriness limit depends on the zinc layer thickness on the one hand and on the heating rate progression on the other. If one takes between the temperatures

= 693 K and T ₂ = If the heating rate is linearly represented as r(T) = + (r ₂ - ri)-^— at 803 K, the graph shown in Figure 7 for the silveriness limits results from the formula for S for different zinc layer thicknesses.

Figure 8 shows two heating rates versus temperature, with a first heating rate starting at about 10 K/s and representing a classical heating rate which decreases to the final temperature.

The second heating rate starts at about 5 K/s, and this lower heating rate is maintained until the critical temperature of 530 °C is exceeded. After the zinc iron crystals have reached the surface, the heating rate increases sharply, only to later decrease again in line with the decreasing temperature difference between the sample and the furnace.

Figure 9 shows a continuous furnace in longitudinal and cross-section, with jacketed radiant tubes, which are heated by gas, for example, providing the heating. In the critical temperature range between 420 °C and 530 °C and, if necessary, in upstream furnace areas, the heating is carried out in such a way that the critical heating rate is not exceeded, while in the other areas a significantly higher heating rate is possible.

Figure 10 shows the same heating process for a multi-layer chamber furnace, whereby the furnace temperature is preferably kept constant and, if necessary, a reduction in the furnace temperature leads to the critical temperature range being passed through at a correspondingly low heating rate. 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, as shown in the figure on the right.

Figure 11 shows where an atmosphere with more than 5 vol.% oxygen is achieved in a continuous furnace above a temperature of 800 °C, this is also entered in the diagram showing the temperature profile of the blanks or prefabricated components in the furnace.

Figure 12 shows a comparable curve for a multi-layer chamber furnace. The invention has the advantage that with the inventive setting of the heating rate in the critical temperature range from 420 °C to 530 °C, repeatable results with regard to a uniform non-silvery surface appearance are achieved.

Claims

Claims 1. A method for producing components from sheet steel, wherein a sheet steel blank is cut out of a flat hot- and cold-rolled steel strip coated with a zinc-based coating and the sheet steel blank is either heated in at least some areas 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 to form a sheet steel pre-component and the sheet steel pre-component is heated in at least some areas to a temperature above Ac3 and then quench-hardened in a form hardening tool, characterized in that the heating takes place in the temperature range from 400 °C to 550 °C, in particular in the temperature range from 420 °C to 530 °C, such that the silveriness number S is greater than 1, where:

where r(T) is the heating rate in Kelvin per second, T is the temperature in Kelvin, r(T) > 0 in the temperature range T _± < T < T ₂ and T _± = 693 K, T ₂ = 803 K , d is the zinc layer thickness in m, constant b = 4.0 * 10 ⁷ m ² /s, constant d ₀ = 4.4 * 10 ^-6 m, Q is the activation energy Q = 271 kJ/mol and R is the universal gas constant R = 8.31 J/mol/K. 2. Process according to claim 1, characterized in that the average heating rate between 420 °C and 530 °C is less than 15 K/s for the zinc coating Z80, less than 8.6 K/s for the zinc coating Z140 and less than 6 K/s for the zinc coating Z180. 3. Method according to claim 1 or 2, characterized in that for heating above 800 °C an oxygen content in the furnace atmosphere in a region closer than 10 mm to the surface of the blank or the steel pre-component of at least 5 vol.% is ensured. 4. Method according to one of the preceding claims, characterized in that the heating rate in the temperature range from 400 °C to 550 °C, in particular in the temperature range from 420 °C to 530 °C, is set between 3 K/s and 15 K/s and below the respective critical heating rate, wherein the critical heating rate depends on the layer thickness of the zinc-based layer and becomes smaller with increasing thickness. 5. Method according to one of the preceding claims, characterized in that in a continuous furnace in the furnace region corresponding to a blank or pre-component temperature of 420 to 530 °C or in the furnace region corresponding to a blank or pre-component temperature of up to 530 °C, the furnace heating power is reduced such that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded. 6. Method according to one of the preceding claims, characterized in that in the areas in which the board or the pre-component has a temperature of less than 420 °C and/or in particular in the areas in which the board or the pre-component has a temperature of more than 530 °C, the furnace heating power is increased in order to keep the cycle time as short as possible. 7. 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. 8. 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. 9. Method according to one of the preceding claims, characterized in that in heating devices in which the blank or the steel pre-component is heated in a stationary manner, such as multi-layer chamber furnaces, the furnace heating power is reduced permanently or over time in such a way that the critical heating rate in the temperature range from 420 °C to 530 °C is not exceeded. 10. Method according to one of the preceding claims, characterized in that the zinc-based coating has a layer thickness of 5 pm to 16 pm per side. 11. 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. 12. 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, particularly preferably a 22MnB5 or 20MnB8 or 34MnB5. 13. Process according to claim 12, 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 0.15 to 0.35 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 0.01 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 impurities from melting.