Method for producing hardened steel sheet parts

Abstract

The invention relates to a method for producing a steel sheet component, in which a steel sheet blank is cut from a hot-rolled and cold-rolled flat steel strip coated with a zinc-based coating, and the steel sheet blank is at least partially heated to a temperature above Ac3 and then formed and quench-hardened in the hot state in a press hardening tool, or a steel sheet blank is cold-formed into a steel sheet preform part, the steel sheet preform part is quenched and hardened in a forming and hardening tool, the heating speed is set in a temperature range of 420 DEG C to 530 DEG C, so that the silver coloring coefficient S is greater than 1, and the heating is carried out at a heating speed which varies with the holding time of the furnace, and the heating speed is related to the thickness of the zinc layer.

Description

Technical Field

[0001] The invention relates to a method for producing a hardened sheet steel component. Background Art

[0002] In automotive engineering, it is known to increase the rigidity of a passenger compartment by using sheet steel components with increased stability. These sheet steel components with increased stability are usually hardened sheet steel components, i.e. hardened sheet steel components have a substantially higher hardness and tensile strength than sheet steel components made of conventional steels. Such hardened sheet steel components are used because hardening allows the production of components with reduced wall thickness compared to components made of non-hardened steel grades. Thus, a relatively light automotive steel body can be provided which at the same time provides extremely high stability.

[0003] Methods of manufacturing hardened sheet steel components are well known.

[0004] A hardening mechanism commonly known as quench hardening is used, in which the steel is first transformed by heating to its high temperature phase, called austenite or gamma iron. This fully or partially austenitic steel structure is then cooled at a cooling rate that is higher than the critical cooling rate of the alloy steel. The critical cooling rate of boron manganese steel 22MnB5 is, for example, about 23K / s.

[0005] At this cooling rate, the austenite phase does not transform back into ferrite, but into martensite. Rapid cooling effectively freezes this martensite phase. Since austenite can dissolve more carbon than martensite, this causes carbon precipitation, which distorts the crystal lattice and leads to high hardness. A prerequisite for this is that the steel or steel alloy must contain sufficient carbon, which is ensured by using a suitable alloy.

[0006] The steel industry usually provides boron manganese steel for this purpose; one of the most common steels is 22MnB5, but this is only part of a large family of steel grades. Specifically, steels with the following composition (mass percentage) are used: Carbon up to 0.4, preferably 0.15-0.35 Silicon up to 1.9, preferably 0.11-1.5 Manganese up to 3.0, preferably 0.8-2.5 Chromium up to 1.5, preferably 0.1-0.9 Molybdenum up to 0.9, preferably 0.001-0.1 Nickel up to 0.9, preferably 0.01-0.2 Titanium up to 0.2, preferably 0.02-0.1 Vanadium up to 0.2; Tungsten up to 0.2; Aluminum up to 0.2, preferably 0.02-0.07; Boron up to 0.01, preferably 0.0005-0.005; Sulfur: maximum 0.01, preferably maximum 0.008; Phosphorus at most 0.025, preferably at most 0.01; and Residual iron and impurities.

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

[0008] The first and older process is press hardening, also known as the direct process, in which a blank is cut from a flat sheet and then brought to the desired austenitizing temperature and, if necessary, kept at this temperature. The blank is then transferred to a forming press where it is formed in the hot state, preferably in a single stroke, and then quenched with a cold press hardening tool. Since there is only one forming stroke and therefore only one tool, final trimming of the outer edges and holes of the press hardened parts can usually only be carried out in the hardened state, usually by means of laser cutting. Due to the single forming stroke, this type of press hardening can only be used to produce hardened parts that are usually not too complex in shape.

[0009] For parts with more complex geometries and / or larger production quantities, the applicant has developed a form hardening process, also known as an indirect process, in which a blank is cut from a steel sheet and first formed and trimmed in a conventional cold forming process to produce a preformed part from steel sheet. The preformed part is formed in such a way that it is slightly smaller than the target geometry in all three spatial directions because it is subsequently heated to the austenitizing temperature and expands accordingly due to thermal expansion. After austenitizing, the preformed part is quickly transferred to a form hardening press, where the already formed preformed part comes into contact with a form hardening tool in a closed press, simply to cool it down quickly. Usually only minor calibrations or adjustments are made in the press. Since the parts are usually completely trimmed during the cold forming process, no trimming is usually required in the hardened state. Since the above-mentioned multi-stage forming and trimming process, which is usually carried out in the cold state, allows a higher degree of design freedom, form hardening can produce more complex parts.

[0010] Since the body parts of modern cars are usually not made of uncoated steel sheets, but have a metallic anti-corrosion layer to improve their corrosion protection, it makes sense to assume that metallic coated steel sheets should also be used for hardened components.

[0011] For both processes, it is also known to use galvanized steel sheets; galvanized steel sheets usually have a zinc-based coating that contains a certain proportion of one or more elements with high oxygen affinity. These high oxygen affinity elements diffuse to the surface during the high temperatures required for austenitization and form a glass layer on the surface, which is an oxide layer that protects the underlying zinc layer from evaporation during the high temperature process.

[0012] Applicants used suitable galvanized steel sheets in both processes.

[0013] In addition to almost pure zinc-coated steel sheets, ie steel sheets with a coating consisting predominantly of zinc, an element with a high oxygen affinity, there are also known ZF coatings, also called GA coatings, in which the zinc coating process is followed by a heat treatment to produce a zinc-iron alloy layer on the steel strip.

[0014] During the heat treatment of press-hardened steel for austenitizing, the formation of a thin oxide layer consisting of elements with a higher oxygen affinity occurs only in Z coatings, also called GI coatings, i.e., essentially zinc-based coatings. In a typical Z coating, the coating contains, in addition to zinc, about 0.2-2% by weight of aluminum.

[0015] The most commonly used element with a higher oxygen affinity is aluminum. In order to austenitize, during the heat treatment process, aluminum will form a very thin aluminum oxide layer on the surface. As mentioned above, the aluminum oxide layer can prevent the zinc from evaporating in large quantities.

[0016] During the form-hardening and press-hardening processes, cracks often form in this very thin aluminum oxide layer, which leads to a slight evaporation of the zinc and the formation and deposition of zinc oxide on the surface. After press-hardening or form-hardening, such parts often have a light green-brown surface, which is caused by the zinc oxide layer.

[0017] It has been observed that die-hardened or form-hardened parts with a zinc coating, in addition to the known green-brown color, sometimes exhibit a silver coloration in a fluctuating and unpredictable manner, either completely or only in certain areas. A silver coloration for the purposes of the present invention is also to be understood as a light off-white or light grey color and can also have a matte appearance or even be shiny in places. Experience has shown that the frequency of silver coloration increases in particular with increasing coating thickness.

[0018] It was also found that silver coloration forms when the entire zinc layer is not converted to a solid ZnFe phase at a temperature of 530°C, i.e. liquid zinc remains on the surface at 530°C and / or oxygen is depleted at high temperatures. A temperature of about 530°C marks the end of the presence of the ζ ZnFe phase.

[0019] In itself, different and fluctuating colours on the surface of a steel component are not necessarily a quality characteristic, but can lead to problems.

[0020] It is well known that the texture and color of a surface have an influence on the absorption and emission of thermal radiation through the surface. This property is described by the emissivity; therefore, different and fluctuating colors can lead to different and fluctuating surface emissivities. The emissivity of a silver-colored surface is usually much lower than that of a typical green-brown surface, because the heat flow is thereby reduced, leading to slower heating in the furnace on the one hand and slower cooling during transfer from the furnace to the press on the other hand.

[0021] Such different and fluctuating emission levels can therefore lead to different and fluctuating heating behavior in the furnace, which can be technically and especially economically problematic for a number of reasons. Since it must always be ensured during the production of components that the steel sheet is completely austenitized during the subsequent hardening process, the minimum furnace holding time for all blanks or preforms must be extended so that even the slowly heated silver-colored surfaces can be reliably austenitized, even though these surfaces may only constitute a small proportion of all blanks or preforms; extending the minimum furnace holding time for all blanks or preforms generally leads to a lower output of hardened components per unit of production time and is therefore a disadvantage. Since the typical green-brown surfaces heat up more quickly, the maximum furnace holding time cannot be extended to the same extent, so that the available furnace process window is also reduced. In the event of a production interruption, this can lead to an increased generation of scrap, which is of course also a disadvantage.

[0022] Different and fluctuating emissivities can lead to further problems, especially for form hardening, due to different and fluctuating cooling behavior of the preformed components between furnace and press. After austenitizing, the preformed components are transferred to the corresponding form hardening press. Here, as mentioned before, areas with different colors, or more precisely, areas with different emissivities, show different cooling behaviors. But since the components are not trimmed or otherwise affected after hardening, the insertion temperature of the austenitized preformed components must be exactly right, because the insertion temperature ultimately determines the dimensions of the component, i.e. the result of the preform shape and thermal expansion. Different and especially fluctuating insertion temperatures can therefore lead to fluctuations in the dimensional accuracy of the components in form hardening, which is a quality problem, since in some cases the specified dimensional tolerances may not be met. Since silver coloring is often irregular and unpredictable and in some cases only individual components or component areas of a certain number of components that are transported together from the furnace to the press are affected, correction or compensation by means of the transport time between furnace and press is not possible. It may also be reasonable to eliminate silver coloration by means of longer furnace holding times, but it has been shown that longer furnace holding times make the problem worse rather than solve it, since silver-colored areas are stable even at longer furnace holding times, while other areas become increasingly darker with increasing furnace holding times; longer furnace holding times are also uneconomical and therefore disadvantageous for the reasons already mentioned above.

[0023] It is known that such galvanized high temperature treated steel sheets or steel sheet parts are usually subjected to other post-treatments after die hardening or form hardening. The main process used is shot blasting. In cases where the silver coloration is very obvious, it has been found that paint adhesion problems may occur in the case of insufficient shot blasting.

[0024] The aforementioned silver coloration is also often undesirable to customers because it gives the part an uneven surface appearance.

[0025] Figure 1 and Figure 2 A silver-grey surface and a green surface are compared; the silver-grey surface is mainly aluminium oxide, while the green surface is mainly zinc oxide.

[0026] EP 2 611 945 B1 discloses a method for producing hardened sheet steel components, in which a predetermined heating regime is maintained with the aim of maintaining a heating rate of 16-50 K / s at temperatures up to 500° C.; this is to ensure faster heating.

[0027] DE 10 20 2011 3 287 A1 describes a method for producing hardened sheet steel components, in which the dew point in the furnace is set to prevent the formation of silver spots.

[0028] DE 10 20 20 10 6 996 A1 describes a device and a method for heating zinc-coated boron-manganese steel, proposing active or passive gas circulation to prevent silver staining. Summary of the invention

[0029] The problem underlying the present invention is to invent a method for producing sheet steel parts which are free of silver coloration and have a uniform emissivity.

[0030] This problem is solved by a method having the features of claim 1 .

[0031] Advantageous modifications are described in the dependent claims.

[0032] According to the invention, a steel sheet blank or steel strip is used which has a zinc-based coating.

[0033] Advantageously, the zinc-based coating may have a thickness of 5 μm-20 μm per side. This ensures good corrosion protection. Specifically, the coating may be a Z80 or Z120 or Z140 or Z180 coating according to DIN EN 10346.

[0034] The zinc-based anti-corrosion layer may have a relatively high zinc content of 85% to 99% by weight and contain, besides unavoidable impurities, 0.2% to 2% by weight of aluminum. Other oxygen-philic elements, such as magnesium, may also be present.

[0035] Particularly preferably, the zinc-based metal anti-corrosion layer can be applied using a hot-dip method. This is a simple and robust coating method.

[0036] According to the invention, it has been found that the most uniform possible emissivity and thus uniform heating and cooling behavior of the blank or preform can be ensured if a heating rate is selected within a certain temperature range which is below a critical heating rate.

[0037] It has also been found to be useful, alternatively or additionally, to ensure that the sheet surface has a certain oxygen saturation above a certain temperature.The solution of the invention is therefore to eliminate the factors causing silver coloration.

[0038] In particular, it is advantageous to avoid heating at excessively high heating rates in the temperature range of 420°C-530°C. For this reason, the heating rate can be increased below or above this temperature range in order to avoid extending the furnace holding time and the cycle time. Thus, the heating rate can have different values ​​within the holding time.

[0039] In particular, in the temperature range above 800° C., it is advantageous to adjust the atmosphere in the furnace so that at least 5% by volume of oxygen exists on the surface of the component to be heated.

[0040] Therefore, according to the invention, between 420° C. and 530° C., a heating rate below the limit value should be established; the most important influencing variables for the heating rate of the steel sheet blank or steel sheet preformed part in the furnace are the furnace temperature, the sheet thickness, the zinc coating thickness, the oiling condition and the steel alloy. In particular, the zinc coating thickness is a relevant influencing factor for the possible heating rate.

[0041] With regard to oxygen saturation, it has been determined that oxygen available for zinc oxidation is affected by the combustion oil, dew point, weather conditions, new furnace elements and the intensity of convection in the furnace, especially during production intervals.

[0042] The oxygen consumption due to burning oil is affected by the amount of corrosion protection oil on the surface of the blank or preformed component and, where applicable, by the amount of cold forming oil present on the surface of the preformed component.

[0043] The dew point is determined by the climate zone and the factory environment.

[0044] Weather conditions have an impact due to the stack effect and displacement of oxygen by gaseous combustion products. It is also determined that new furnace elements such as stainless steel, ceramic or other insulating materials can bind or consume oxygen or have an impact on oxygen saturation.

[0045] It has been found that the furnace atmosphere may have local inhomogeneities in its composition, especially in its oxygen content.

[0046] Advantageously, starting from a temperature of 800 °C, at least 5 % by volume of oxygen can be provided in the vicinity of the surface. In the context of this invention, "in the vicinity of the surface" means in the range less than 10 mm from the surface of the blank or preform part.

[0047] For example, this can be achieved by blowing in fresh air, which is preferably preheated accordingly. In a multi-chamber furnace, i.e. where the blank or preform part is not transported during the heating process as in a continuous furnace, forced convection can also be achieved by purging, flowing, circulating or temporarily opening the furnace door.

[0048] For the heating rate, especially in the temperature range between 420 °C and 530 °C, the following factors need to be considered.

[0049] During heating, the zinc layer melts at about 420 °C. In order for the entire layer thickness d to be converted into the solid Zn-Fe phase, there must be at least a time t of d 2 = D * t for diffusion, where D is the temperature-dependent diffusion coefficient, which has the following form:

[0050] where the activation energy Q = 271 kJ / mol and the ideal gas constant R = 8.31 J / mol / K.

[0051] To take into account the influence of the Fe-Al inhibition layer, the layer thickness d is replaced by d + d0.

[0052] Therefore, when heating from 420 °C to 530 °C, the diffusion coefficient increases. When Q = 271 kJ / mol, it increases by about 600 times. Therefore, the temperature-time curve is crucial for the possible diffusion path and this must be taken into account, for example in the form of a temperature-dependent heating rate r(T). Then, the following dimensionless silver coloring coefficient S can be defined: In the formula, T1 = 693 K, T2 = 803 K, d is the layer thickness in meters (m), and b and d0 are constants, where b = 4.0 * 10 7 m 2 / s, d0 = 4.4 * 10 -6 m.

[0053] This formula only applies to the case where r(T) > 0 in the temperature range T1 < T < T2, i.e. the heating rate must be positive because, as described above, the steel is heated and the temperature range is 420 °C - 530 °C.

[0054] Therefore, for a given temperature-time curve, if the silver coloring coefficient S defined above is less than 1, a silver-colored surface is produced.

[0055] This formula gives the upper limit of silver coloring for the zinc layer Z140 (9.8 μm thick) between 420 °C and 530 °C as a constant heating rate of 8.6 K / s, and the maximum possible heating rate for Z180 (12.6 μm thick) is 6 K / s. This has been experimentally verified: below this heating rate, the silver coloring coefficient is greater than 1, which does not result in any silver coloring, while in the above temperature range, above this heating rate, unwanted silver coloring occurs.

[0056] Therefore, the present invention relates to a method for producing steel sheet components, in which steel sheet blanks are cut from hot-rolled and cold-rolled flat steel strips coated with a zinc-based coating, and then the steel sheet blanks are at least partially heated to a temperature above Ac3, then formed in a hot state in a die-hardening tool and quenched and hardened, or the steel sheet blanks are cold-formed into steel preformed components, the steel preformed components are at least partially heated to a temperature above Ac3, and then quenched and hardened in a forming and hardening tool, wherein in the temperature range of 420 °C - 530 °C, heating is carried out in such a way that the silver coloring coefficient S is greater than 1, and the silver coloring coefficient S is applicable to the following formula: where r(T) is the heating rate in Kelvin per second (K / s); T is the temperature in Kelvin (K); in the temperature range of T1 < T < T2, r(T) > 0, and T1 = 693 K, T2 = 803 K; d is the zinc layer thickness in meters (m); constant b = 4.0 * 10 7 m 2 / s; constant d0 = 4.4 * 10 -6 m; Q is the activation energy Q = 271 kJ / mol, and R is the ideal gas constant R = 8.31 J / mol / K, and, especially in the temperature range of 420 °C - 530 °C, heating is carried out at a heating rate that varies with the soaking time in the furnace and is related to the zinc layer thickness.

[0057] Heating can be carried out by radiation or convection.

[0058] In another embodiment, heating can be carried out by radiation and convection.

[0059] For example, an infrared heater can be used for radiation heating.

[0060] According to an advantageous refinement, the heating rate r decreases as the thickness d of the zinc coating increases, such that when d1 < d2 < d3, the heating rate is set to r(d1) > r(d2) > r(d3).

[0061] According to an advantageous refinement, between 420 °C and 530 °C, the average heating rate of the zinc coating Z80 is less than 15 K / s, the average heating rate of the zinc coating Z140 is less than 8.6 K / s, and the average heating rate of the zinc coating Z180 is less than 6 K / s.

[0062] According to an advantageous refinement, for heating above 800 °C, within a range less than 10 mm from the surface of the blank or steel preform, the oxygen content of the furnace atmosphere is ensured to be at least 5% by volume.

[0063] According to an advantageous refinement, in the temperature range of 420 °C - 530 °C, the heating rate is set between 3 K / s and 15 K / s and is below the corresponding critical heating rate, where the critical heating rate is related to the thickness of the zinc base layer and decreases as the thickness increases.

[0064] According to an advantageous refinement, in the case of a continuous furnace, in the furnace zone corresponding to a blank or preform temperature of 420 - 530 °C or in the furnace zone corresponding to a blank or preform temperature up to 530 °C, the heating capacity of the furnace is reduced in such a way that the critical heating rate is not exceeded in the temperature range of 420 °C - 530 °C.

[0065] According to an advantageous refinement, the heating capacity of the furnace is increased in the zone where the temperature of the blank or preform is below 420 °C and / or especially in the zone where the temperature of the blank or preform exceeds 530 °C, so that the cycle time is as short as possible.

[0066] According to an advantageous refinement, in the furnace zone where the temperature of the blank or preform is 800 °C or above, fresh air, especially preheated fresh air or oxygen, is supplied to the furnace atmosphere.

[0067] According to an advantageous refinement, in the furnace zone where the temperature of the blank or preform is 800 °C or above, the existing furnace atmosphere is circulated.

[0068] According to an advantageous refinement, in a heating device for heating a blank or steel preform in a fixed manner, such as a multi - layer box furnace, the heating capacity of the furnace is continuously or over time reduced in such a way that the critical heating rate is not exceeded in the temperature range of 420 °C - 530 °C.

[0069] According to an advantageous refinement, the thickness of the zinc - based coating layer is 5 μm - 16 μm per side.

[0070] According to an advantageous development, the zinc-based coating is applied by a hot-dip coating process, in particular by hot-dip galvanizing.

[0071] According to an advantageous development, the steel strip is produced from a hardenable steel alloy, in particular a boron-manganese steel, particularly preferably 22MnB5 or 20MnB8 or 34MnB5.

[0072] According to an advantageous development, a strip having the following composition is used as the steel strip (all figures are in wt %): Carbon up to 0.4, preferably 0.15-0.35; Silicon up to 1.9, preferably 0.11-1.5; Manganese up to 3.0, preferably 0.8-2.5; Chromium up to 1.5, preferably 0.1-0.9; Molybdenum up to 0.9, preferably 0.001-0.1; Nickel up to 0.9, preferably 0.01-0.2; Titanium up to 0.2, preferably 0.02-0.1; Vanadium up to 0.2; Tungsten up to 0.2; Aluminum up to 0.2, preferably 0.02-0.07; Boron up to 0.01, preferably 0.0005-0.005; Sulfur: maximum 0.01, preferably maximum 0.008; Phosphorus at most 0.025, preferably at most 0.01; and Residual iron and smelting related impurities. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The present invention will be explained by way of examples and drawings. In the drawings:

[0074] Figure 1 A 15×20 cm plate is shown which is silver after austenitization;

[0075] Figure 2 Show Figure 1 SEM image of the plate shown;

[0076] Figure 3 A 15×20 cm plate is shown which is green after austenitization;

[0077] Figure 4 Show Figure 3 SEM image of the plate shown;

[0078] Figure 5 The critical heating rate is shown as a function of zinc coating thickness;

[0079] Figure 6 shows the temperature-furnace position curve for a continuous furnace at a constant throughput rate;

[0080] Figure 7 To show the graphs of the silver point for given heating rates at 420°C and 530°C, the curve in the middle is assumed to be a linear curve and the zinc coating thickness is given;

[0081] Figure 8 is a graph showing two heating rate curves with and without silver coloration;

[0082] Fig. 9 Schematic showing the effect of the temperature range of 420°C-530°C on the heating rate in a continuous furnace (DLO);

[0083] Fig.10 Schematic diagram showing the influence of the temperature range of 420°C-530°C on the heating rate in the chamber of a multi-layer box furnace (MLK);

[0084] Fig.11 Schematic representation of the effect of starting from a temperature of 800°C on the oxygen content near the surface in a continuous furnace (DLO);

[0085] Fig.12 Schematic representation of the effect of a temperature of 800° C. on the oxygen content near the surface in the chamber of a multi-layer chamber furnace (MLK);

[0086] Fig.13 shows the conventional heating rate according to the prior art, compared with the heating possible according to the invention;

[0087] Fig.14 Two possible heating paths according to the invention with different heating rates after reaching 530° C. are shown. DETAILED DESCRIPTION

[0088] To visually illustrate the differences in surface coloring above, Figure 1 shows a steel sheet coated with zinc and having a silver appearance after austenitization, Figure 3 The corresponding steel plate having a green appearance after austenitization is shown.

[0089] Figure 1 and Figure 4 The corresponding SEM images of the surfaces are shown.

[0090] As mentioned previously, hardened parts with zinc-based coatings will appear as Figure 1 This silver coloration is shown. The likelihood of silver coloration increases with the thickness of the zinc coating.

[0091] Figure 1 and Figure 2The silver-colored parts shown have a zinc oxide layer that is reduced or does not completely cover an existing aluminum oxide layer, or have a large area or surface area that is free of zinc oxide.

[0092] This silver coloring on the parts can be like Figure 1 It can occur globally as shown, or only locally; it can be caused by variations in layer thickness or by differences in the substrate. As already mentioned, silver coloration is not a purely aesthetic issue in nature. While customers do want to receive parts with a uniform appearance, fluctuating silver coloration and the resulting fluctuating emissivity can lead to fluctuating heating behavior in the furnace and fluctuating cooling behavior between the furnace and the press, with adverse effects on economic efficiency and / or part quality.

[0093] It was determined that very noticeable silver coloration in combination with a possibly inadequate blasting process could lead to subsequent problems during the painting process.

[0094] As mentioned above, as the inventors have discovered, silver coloration occurs when the heating rate in the temperature range of 420°C-530°C exceeds a certain critical heating rate. Above this average critical heating rate, a silver surface will definitely appear.

[0095] As mentioned before, this is also related to the thickness of the zinc-based coating.

[0096] exist Figure 5 In the graph, the average critical heating rate (in K / s) from 420°C to 530°C is plotted as a function of the thickness of the zinc-based coating (in μm). The average heating rate from 420°C to 530°C can be determined from the heating curve by dividing the temperature difference Δθ=530°C-420°C=110 K by the required time Δt=t(530°C)-t(420°C). The curve here is the limit line, above which a silver appearance cannot be excluded or must occur, and below which a green or green-brown surface appearance is achieved.

[0097] exist Figure 6 In the diagram, not only is the temperature profile of the sheet steel preform or sheet steel blank depicted during the furnace holding time as usual, but also the temperature profile over the length of a continuous furnace with a constant throughput rate is schematically depicted. This is permissible for a continuous furnace with a constant throughput rate, since the furnace holding time is proportional to the furnace position. Figure 6 The top area of ​​the furnace is schematically divided into four furnace zones: zone A with a temperature range from ambient temperature to 420°C, zone B with a critical temperature range from 420°C to 530°C, zone C from 530°C to 800°C, and zone D above 800°C up to the required final temperature.

[0098] Below this, the temperature curves for the furnace holding time and the furnace position are plotted. In such a continuous furnace, as previously mentioned, it is important that the heating rate of the billet or preformed part in zone B does not exceed a certain heating rate related to the zinc layer thickness. However, due to the cycle time in the furnace, especially in a continuous furnace, it is not possible to fix in one zone without changing the whole process in time, so the heating rate in the non-critical zone A, especially in zone C, increases accordingly.

[0099] In zone D, according to the present invention, a further solution can be provided starting from 800 °C, that is, at a distance of at least less than 10 mm from the surface of the billet or steel preformed part, the atmosphere is adjusted so that the oxygen content is higher than 5% by volume.

[0100] This can be achieved in several ways. On the one hand, fresh air, especially preheated fresh air, can be blown into this zone; on the other hand or alternatively, the existing furnace atmosphere can be circulated sufficiently.

[0101] Therefore, it would be advantageous to set the heating rates of zones A, B, C, and D in a variable and flexible manner. Specifically, it is advantageous to set the heating rate variably and flexibly according to the zinc layer thickness or the zinc coating thickness.

[0102] Therefore, the heating rate defined as r is r1 in zone A, r2 in zone B, r3 in zone C, and r4 in zone D.

[0103] In an advantageous embodiment, r1 > r2.

[0104] In another advantageous embodiment, r2 > r3.

[0105] In a particularly advantageous embodiment, r1 > r2 > r3.

[0106] As the zinc coating thickness d increases, the heating rate r decreases variably. If d1 < d2 < d3, the heating rate is set to r(d1) > r(d2) > r(d3).

[0107] Figure 5 The variation of the average critical heating rate from 420 °C to 530 °C with the zinc coating thickness is shown, Figure 7 A graph showing the silver point curve of the variation of the critical heating rate at 530 °C with the heating rate at 420 °C for different zinc coating thicknesses is shown. Silver coloring will definitely occur above the silver point curve, and each curve between 420 °C and 530 °C is assumed to be a linear curve. Here it can be clearly seen that the silver point is related to the zinc coating thickness and also to the heating rate curve. If it is assumed that between the temperatures T1 = 693 K and T2 = 803 K, the curve of the heating rate is linear, expressed as Then for different zinc layer thicknesses, the formula for S is: Figure 7 Silver point curve chart.

[0108] Figure 8 Two heating rates are shown as a function of temperature, the first starting at about 10 K / s and representing the classical heating rate ramping down to the final temperature.

[0109] The second heating rate starts at about 5 k / s; this lower heating rate is maintained until the critical temperature of 530° C. is exceeded. After the zinc-iron crystals reach the surface, the heating rate increases sharply, and then decreases again as the temperature difference between the sample and the furnace decreases.

[0110] Fig. 9 A longitudinal cross-section of a continuous furnace is shown, in which heating is carried out by radiant tubes (e.g. gas heating). In the critical temperature range between 420° C. and 530° C., heating is carried out, if necessary also in the upstream furnace region, in such a way that the critical heating rate is not exceeded, while significantly higher heating rates are possible in other regions.

[0111] Fig.10 The same heating process is shown in a multi-layer box furnace, wherein the furnace temperature is preferably kept constant and, if necessary, is reduced to allow passage through the critical temperature range with a correspondingly low heating rate. However, it is also conceivable that the furnace temperature varies with the furnace holding time, i.e. the furnace temperature is very low at the beginning and increases from 530°C, as shown on the right side of the figure.

[0112] Fig.11 It shows where in the continuous furnace the atmosphere contains more than 5% oxygen by volume at temperatures above 800°C; this is also plotted in the graph showing the temperature profile of the blank or preformed part within the furnace.

[0113] Fig.12 A comparison curve for a multi-layer chamber furnace is shown.

[0114] Fig.13 The heating rate curve according to the prior art is shown in comparison with a possible heating process according to the present invention, taking a blank coated with Z180 as an example, the dot-dash line indicating a possible heating process according to the prior art.

[0115] The silvery surface appearance results from the selection of an excessively high heating rate in the critical temperature range of 420° C. to 530° C. In this case, the silver coloration factor S=0.8.

[0116] To the right of it is a possible variant of the heating curve according to the invention, with a silver coloration factor of S=1.2. It is clear that in this example, both heating curves reach the target temperature of 905°C at the same time, i.e. after about 210 seconds. However, the heating process according to the invention achieves a uniform non-silver surface appearance, because the change in slope starting from 530°C indicates that zinc-iron crystals have reacted to the surface. This prevents silver coloration.

[0117] This clearly shows that correctly setting the heating rate is crucial for the formation of a silver surface in the critical temperature range of 420°C–530°C.

[0118] Fig.14 Two possible variants of the heating rate setting according to the invention for two blanks coated with Z180 are shown. In both variants, the critical temperature range of 420° C.-530° C. is passed at the same relatively low heating rate. This ensures that silver coloration is avoided (the silver coloration factor in each case is about 1.2 here).

[0119] The line on the left shows an increase in heating rate in the temperature range above 530°C. By increasing the heating capacity in this range, the furnace holding time is reduced by about 50 seconds. It is easy to see that at constant heating capacity, as shown by the dotted line (right), the furnace holding time is actually longer, but this reduces energy consumption and CO2 emissions compared to the other variants.

[0120] An advantage of the invention is that within the critical temperature range of 420° C. to 530° C., the heating rate setting according to the invention has reproducible results in terms of a uniform non-silver surface appearance.

[0121] The variable setting of the heating rate ensures uniform heating behavior in the furnace, related to the zinc coating thickness.

[0122] The ability to variably adjust the heating rate means that the heating rate can be increased below 420°C and above 530°C without increasing the risk of silver coloration. This shortens the furnace holding time, thereby reducing energy consumption and CO2 emissions. Overall, this greatly improves process efficiency.

Claims

1. A method for producing a steel plate component, wherein a steel plate blank is cut from a hot-rolled and cold-rolled flat steel strip coated with a zinc-based coating, and the steel plate blank is at least partially heated to a temperature above Ac3, and then formed and quenched in a hot state in a die-hardening tool, or the steel plate blank is cold-formed into a steel preformed component, the steel preformed component is at least partially heated to a temperature above Ac3, and then quenched in a forming and hardening tool, characterized in that heating is carried out in the temperature range of 420 °C - 530 °C in such a way that the silver coloring coefficient S is greater than 1, and the silver coloring coefficient S is applicable to the following formula: wherein, r(T) is the heating rate, in Kelvin per second; T is the temperature, in Kelvin; in the temperature range of T1 < T < T2, r(T) > 0, and T1 = 693K, T2 = 803K; d is the zinc layer thickness, in meters; Constant b = 4.0*10 7 m 2 / s; Constant d0 = 4.4*10 -6 m; Q is the activation energy Q = 271 kJ / mol, and R is the ideal gas constant R = 8.31 J / mol / K, and, in particular, in the temperature range of 420 °C - 530 °C, heating is carried out at a heating rate that varies with the holding time in the furnace and the zinc layer thickness.

2. The method according to claim 1, characterized in that The heating rate r decreases with an increase in the zinc coating thickness d, so that when d1 < d2 < d3, the heating rate is set to r(d1) > r(d2) > r(d3).

3. The method according to any one of the preceding claims, characterized in that Between 420 °C and 530 °C, the average heating rate of the zinc coating Z80 is less than 15 K / s, the average heating rate of the zinc coating Z140 is less than 8.6 K / s, and the average heating rate of the zinc coating Z180 is less than 6 K / s.

4. The method according to any one of the preceding claims, characterized in that For heating above 800 °C, in the range less than 10 mm from the surface of the blank or the steel preformed component, it is ensured that the oxygen content in the furnace atmosphere is at least 5% by volume.

5. The method according to any one of the preceding claims, characterized in that In the temperature range of 420 °C - 530 °C, the heating rate is set to 3 K / s - 15 K / s and is lower than the corresponding critical heating rate, where the critical heating rate is related to the thickness of the zinc base layer and decreases with an increase in thickness.

6. A method according to any one of the preceding claims, characterised in that In the case of a continuous furnace, in the furnace zone corresponding to the temperature of the blank or the preformed component being 420 - 530 °C or in the furnace zone corresponding to the temperature of the blank or the preformed component being up to 530 °C, the heating capacity of the furnace is reduced in such a way that the critical heating rate is not exceeded in the temperature range of 420 °C - 530 °C.

7. A method according to any one of the preceding claims, characterised in that The heating capacity of the furnace is increased in the zone where the temperature of the blank or the preformed component is below 420 °C and / or especially in the zone where the temperature of the blank or the preformed component exceeds 530 °C, so that the cycle time is as short as possible.

8. The method according to any one of the preceding claims, characterized in that In the furnace zone where the temperature of the blank or the preformed component is 800 °C or above, fresh air, especially preheated fresh air or oxygen, is supplied to the furnace atmosphere.

9. The method according to any one of the preceding claims, characterized in that In the furnace zone where the temperature of the blank or the preformed component is 800 °C or above, the existing furnace atmosphere is circulated.

10. The method according to any one of the preceding claims, characterized in that In a heating device for heating the blank or the steel preformed component in a fixed manner, such as a multi-layer box furnace, the heating capacity of the furnace is continuously or over time reduced in such a way that the critical heating rate is not exceeded in the temperature range of 420 °C - 530 °C.

11. The method according to any one of the preceding claims, characterized in that The zinc-based coating has a layer thickness of 5 μm to 16 μm on each side.

12. The method according to any one of the preceding claims, characterized in that The zinc-based coating is applied by a hot-dip coating process, in particular by hot-dip galvanizing.

13. A method according to any one of the preceding claims, characterised in that The steel strip is made of a hardenable alloy steel, in particular a boron-manganese steel, particularly preferably 22MnB5 or 20MnB8 or 34MnB5.

14. A method according to any one of the preceding claims, characterised in that As the steel strip, a strip having the following composition was used (all figures are in wt %): Carbon up to 0.4, preferably 0.15-0.35; Silicon up to 1.9, preferably 0.11-1.5; Manganese up to 3.0, preferably 0.8-2.5; Chromium up to 1.5, preferably 0.1-0.9; Molybdenum up to 0.9, preferably 0.001-0.1; Nickel up to 0.9, preferably 0.01-0.2; Titanium up to 0.2, preferably 0.02-0.1; Vanadium up to 0.2; Tungsten up to 0.2; Aluminum up to 0.2, preferably 0.02-0.07; Boron up to 0.01, preferably 0.0005-0.005; Sulfur: maximum 0.01, preferably maximum 0.008; Phosphorus at most 0.025, preferably at most 0.01; and Residual iron and smelting related impurities.

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