Method for producing hardened steel components

Abstract

The invention relates to a method for producing hardened steel components, wherein: galvanised sheet steel blanks made of a quench-hardenable steel alloy are subjected to a hot stamping or press hardening process so as to produce deep-drawn hardened steel components; after the hardening process, the hardened steel components are subjected to a surface conditioning process by means of centrifugal wheel blasting; and, at least in the case of sheet metal components in which the potential difference between the zinc coating and the steel substrate is less than 150 mV, the centrifugal wheel blasting is performed with an Almen intensity N of at least 0.08 mm.

Description

[0001] Process for producing hardened steel components

[0002] The invention relates to a method for producing hardened steel components.

[0003] It is known to produce high-strength components from sheet steel by hardening. In particular, such components are also produced by quench hardening or transformation hardening from the austenitic state.

[0004] Quench hardening involves bringing an object made of a hardenable steel alloy, such as a boron-manganese steel, to a temperature at which the structure is largely or entirely in the form of austenite or gamma iron. Subsequently, by cooling above the so-called critical cooling rate, the austenite is converted to martensite, thereby achieving high tensile strength.

[0005] To produce such a hardenable steel sheet material, a steel strip is produced by melting a hardenable steel alloy, casting it into slabs, usually using a continuous casting process, hot rolling the resulting slabs in a hot strip mill, and then cold rolling them in a cold rolling mill. The steel strip is typically several hundred to over a thousand meters long, a few millimeters or fractions of a millimeter thick, and wound into coils or bundles.

[0006] During hot and cold rolling, the slab's thickness is significantly reduced, resulting in a corresponding elongation. However, the width is largely maintained through appropriate measures.

[0007] The thickness reduction is not a forming process within the meaning of this disclosure.

[0008] It is not uncommon to galvanize such cold-rolled strip in a galvanizing plant. For this, the strip is unwound from the coil and rewound at the end. Galvanizing can be carried out by electrolytic coating, PVD coating, and hot-dip galvanizing. Directly after hot-dip galvanizing (Z according to EN 10346 or Gl according to VDA 239-100), a brief heat treatment can be carried out to create a so-called galvannealed coating (ZF according to EN 10346 or GA according to VDA 239-100). To produce sheet steel components, the steel strip is unwound from the coil and pieces are cut from this steel strip, so-called blanks. These blanks are thus flat steel sheets with limited expansion. To produce hardened sheet steel components, these blanks can be further processed in two ways.

[0009] In the so-called direct process, also known as press hardening or press hardening, the blanks are austenitized by heating them, usually in a furnace, and then formed into a sheet metal component in a relatively colder forming tool in a single step. The forming takes place while the sheet metal is still hot. Once the forming process is complete in the tool and the tool is fully closed, the tool surfaces are in contact with the sheet metal component. This dissipates the heat into the (cooled) tool at a rate that exceeds the critical hardening rate. This achieves the hardening outlined above by transforming it into a martensitic structure.

[0010] In the so-called indirect process, also known as the hot stamping process or hot stamping, the blank is formed into a component blank in a typically multi-stage forming and trimming process, usually mainly through a combination of deep drawing, trimming and / or post-forming while the sheet is cold. This component blank is then usually austenitized in a furnace and placed in a tool (hot stamping tool) while still hot. The hot stamping tool usually no longer carries out any actual forming, although this is possible to a limited extent with a spring-loaded component blank and / or final forming. When the tool is fully closed, the surface of the tool rests against the surface of the component blank and dissipates the heat, so that the component blank is hardened into the component.

[0011] In both cases, i.e. the direct and indirect process, the product is a hardened sheet steel component.

[0012] In the direct process, steel strips with an aluminum-silicon coating or galvanized or alloy-galvanized steel strips or uncoated steel strips are usually used.

[0013] In the indirect process with upstream cold forming, galvanized or alloy-galvanized steel strips or uncoated steel strips are typically used. Since the aluminum-silicon coating is very brittle at room temperature, the coating can flaking off during cold forming, which is why this coating variant is typically only used for the direct process without upstream cold forming.

[0014] Galvanized or alloy-galvanized steel strip or steel sheet within the meaning of the invention is that steel strip or steel sheet that has a coating with zinc or a zinc-based alloy. Zinc-based means that zinc is the largest alloying component of the coating and, in particular, accounts for more than 50% by weight of the coating. Alloys such as ZnNi, ZnCr, ZnMg, or others can also be used.

[0015] Zinc coatings on steel sheets are so-called cathodic corrosion protection layers, since a potential difference between the electrochemically less noble layer, formed from pure zinc or based on zinc, and the electrochemically more noble substrate, i.e. the iron-based steel sheet carrier material, protects the substrate in the event of damage to the layer and contact with a corrosive substance such as water by firstly the less noble layer serving as a sacrificial anode and being dissolved, thus preventing corrosion of the more noble substrate.

[0016] The potential difference between the layer and the substrate can be determined via the galvanostatic dissolution in a galvanostatic holding test of a classic three-electrode arrangement (with reference electrode: Ag / AgCl, counter electrode: graphite, electrolyte: 200 g / l NaCl + 100 g / l ZnSO4 x 7 H2O, current density: 10 mA / cm 2, Gassing: using an "aquarium pump" or similar, temperature: 23 °C ± 2 K, measured value: voltage in mV as a function of the measurement time in s). A constant current is applied between the component section or the sample with the layer and the counter electrode, and simultaneously the potential between the reference electrode and the component section or the sample with the layer is measured. The layer to be characterized is slowly dissolved down to the substrate. From the temporal progression of the potential, characteristic quantities of the layer and, among other things, the potential difference between the layer and the substrate can be easily determined.

[0017] While in cold-formed components made of galvanized steel sheets, the layer of the finished component essentially corresponds to the layer of the galvanized cold-rolled strip and is usually present as a zinc layer with a small aluminum content or as a pure zinc layer, in form- or press-hardened components made of galvanized steel sheets, the layer on the finished component is present as a zinc-iron diffusion layer with a significant iron content and a modified surface, particularly one coated with oxides. To improve paint adhesion and weldability, the surface can generally be further conditioned by cleaning or leveling steps following the hardening process, i.e., on the hardened steel component, for example, by wheel blasting. This is known to the person skilled in the art and is set out in several of the applicant's patent applications.

[0018] It is known that the intensity of wheel blasting can be determined using so-called Almen test strips. The Almen test strips, made of spring steel, are available in three different thicknesses: "N", "A", and "C" strips. The "N" strips are 0.79 mm, "A" strips 1.29 mm, and "C" strips 2.39 mm thick. The Almen test strips are clamped in a holder, which is attached to the test sheet or test component at the position to be tested, for example, by welding, and are blasted on one side together with the test sheet or test component using the settings to be tested. The Almen test strips bulge towards the blasted side. The resulting arc height of the strip is measured with a dial indicator and specified as the beam intensity as a value in mm. The Almen measuring strip used must always be specified, e.g., intensity = 0.25 mm A.

[0019] Accordingly, the Almen type "N" is used here in Class 1 with a strip thickness of 0.79 mm. Class 1 defines the pre-bending as + / - 0.025 mm maximum. The length and width of the strip are 76.1 x 19.0 mm, and the hardness of type "N" is 72.5-76 HRA, with the measurement being carried out according to SAE AMS 2430.

[0020] The reason for both the increased iron content of the coating and the altered, oxide-coated surface is primarily the high-temperature treatment for the purpose of austenitization—that is, the heating of the galvanized steel sheet to temperatures above the Ac3 point and the associated alloying reaction of the coating with the substrate. In particular, at temperatures above 780 °C, the coating becomes increasingly enriched with iron from the substrate.

[0021] For shorter oven residence times (OVZ) and / or higher layer thicknesses, for example layer thickness Z140 according to EN 10346 with 140 g / m on both sides2 Zinc alloy in the three-surface test or the layer thickness GI60 / 60 according to VDA 239-100 with 60-90 g / m 2 Zinc alloy on each side, the ZnFe diffusion layer of the hardened component usually consists of two phases: the zinc-rich r-ZnFe phase with a zinc content of approximately 75 wt.% and the iron-rich zinc ferrite with a zinc content of approximately 40 wt.%. If such a layer is dissolved in a galvanostatic holding test, the dissolution curve initially shows the very low potential of the zinc-rich T-ZnFe phase, which dissolves first. Once the T-ZnFe phase is used up, the dissolution curve shows the slightly higher potential of the zinc ferrite, which is then dissolved until it too is used up, and the dissolution curve finally shows the potential of the steel substrate.

[0022] With longer OZ and / or thinner coating thicknesses, such as Z80 coating thickness according to EN 10346 or GI35 / 35 coating thickness according to VDA 239-100, the ZnFe diffusion layer of the hardened component typically consists solely of zinc ferrite. As OZ increases, the zinc ferrite becomes increasingly enriched with iron from the substrate, making the coating electrochemically more similar to the substrate.

[0023] Accordingly, the dissolution curves of samples of the same initial layer thickness also show increasingly higher potentials of the zinc ferrite with increasing OZ, and thus increasingly smaller potential differences between the zinc ferrite and the substrate. The cathodic corrosion protection of the layer thus becomes increasingly weaker with increasing OZ.

[0024] In cases where very high demands are placed on the cathodic corrosion protection of hardened components made of galvanized steel sheets, the presence of the zinc-rich r-ZnFe phase is required.

[0025] Until now, it was assumed that for hot-form-hardened components made of galvanized steel sheets, sufficient cathodic corrosion protection, as well as good paint adhesion and weldability, was only achieved with a potential difference between the coating and the substrate of at least 150 mV during galvanostatic dissolution. A minimum potential difference of 150 mV between the coating and the substrate as a reasonable limit for corrosion protection can be derived, for example, from Table 1 on page 1762 of the textbook "Korrosion und Korrosionsschutz" Volume 3 by Egon Kunze, published by WILEY-VCH.

[0026] In some cases, against the background of the above-mentioned assumption, a potential difference of greater than 150 mV between the layer and the substrate is also required and accordingly the sheets for components meeting this requirement are provided with correspondingly thick zinc layers and yet limitations and problems in handling still exist.

[0027] In both of the processes described above, namely hot stamping and press hardening, a minimum furnace residence time (MIN / DVZ) is typically set on the production line for each component during production, so that the parts may only leave the furnace when the actual furnace residence time (ACTUAL / DVZ) of the parts in the furnace exceeds the specified MIN / DVZ. For galvanized parts, the criterion for this is usually the assured austenitization of the substrate. The term "parts" within the meaning of the inventions includes, on the one hand, preformed sheet steel components for the hot stamping process, but also, on the other hand, sheet steel blanks still to be formed for the press hardening process.

[0028] In addition, in both hot stamping and press hardening processes, a maximum furnace dwell time (MAX_DVZ) is typically set on the production line during production, so that parts are rejected as nonconforming if the actual furnace dwell time of the parts exceeds the specified MAX_OVZ. For galvanized parts, the criterion for this is usually the condition of the coating.

[0029] The theoretical furnace residence time process window (VRT-PF) is defined using MIN_VRT and MAX_VRT as the lower and upper limits. The current state of the art presents several problems with its implementation.

[0030] If system malfunctions or other faults occur in the production plant when heating the galvanized steel sheet to temperatures above the Ac3 point in the furnace or in plant components downstream of the furnace on the process side, for example when transferring parts from the furnace to the press or in the press in the tool, etc., the ISTJDVZ of parts in the furnace can quickly become excessive. One example is a so-called layup in a hot stamping or press hardening tool. This occurs when a hot part is placed into the tool from the transfer device, e.g. a robot, and becomes jammed on a centering pin, tilts, or is not positioned correctly for some other reason. To avoid tool breakage, the press cycle must be stopped immediately. In order for the tool to be usable again, the hot parts that have not been pressed out must usually be removed from the tool manually using pliers.During the time required to remove the misplaced parts from the tool, including checking in and out in the press safety area, the OVZ continues to run for the parts in the furnace.

[0031] In general, slippages occur more frequently in hot forming or press hardening than in cold forming, as cold parts center and align themselves better on the mold or against stops than hot parts. Furthermore, hot parts are more likely than cold parts to get caught on the grippers of the automation device and thus slip or tip over after being deposited. If the ISTJDVZ then exceeds the MAX_OVZ or the ISTJDVZ exceeds the specified OVZ-PF, the entire furnace load is considered scrap and disposed of. This, of course, incurs unwanted costs.

[0032] In the case of galvanized steel sheet parts which have areas of different sheet thicknesses and thus different heating rates, for example formed by sheets welded together, so-called tailored welded blanks (TWB) and / or sheets welded together in areas, so-called patched blanks and / or sheets with rolled areas of different thicknesses, so-called tailored rolled blanks (TRB), the resulting OVZ-PF of the entire part is significantly smaller than the respective OVZ-PF of the individual sheet thickness areas, since it only includes the overlap area of ​​the OVZ-PF of the slowest and fastest heating sheet areas.

[0033] The MINJDVZ with the criterion of complete austenitization of the part is determined by the slowest heating and naturally thickest area.

[0034] The MAX_OVZ with the criterion of a layer that meets the customer's criteria on the entire part is determined by the fastest heating, naturally thinnest area, which has already lost part of its OVZ-PF at the time of the MINJDVZ of the thickest area.

[0035] If the MAX_OVZ is generally limited by the requirement of a minimum potential difference of 150 mV measured in the dissolution test, this limit value can be reached disadvantageously quickly due to the alloying processes between substrate and layer during high-temperature treatment, in particular the enrichment of iron in the layer, or the OVZ-PF can be disadvantageously small.

[0036] For galvanized sheet steel parts, both with uniform sheet thickness and with sheet thicknesses that vary in certain areas, attempts have already been made to counteract this circumstance by varying the zinc coating.

[0037] In particular, attempts have been made to increase the MAX_OVZ or to ensure a larger OVZ-PF with a longer stability of the minimum value of 150 mV potential difference by increasing the zinc layer. The higher zinc layer acts in two different ways. Firstly, it ensures that the enrichment of the layer with iron from the steel substrate, which occurs at higher temperatures, takes longer due to the larger amount of zinc. Secondly, the higher zinc layer ensures that the sheet heats up more slowly due to the slower Zn-Fe reaction and the associated slower increase in emissivity in the furnace. Thus, the enrichment of the layer with iron from the steel substrate, which occurs at higher temperatures, begins later. In the case of composite sheets, this is particularly relevant for the MAX_OVZ of the thinner partner.However, these attempts were unsuccessful. Increasing the coating layer often leads to undesirable zinc buildup in the tool.

[0038] In addition, attempts have been made to reduce the MINJDVZ of the thicker partner of a composite sheet by reducing the zinc coating. However, the lower zinc content also leads to a faster enrichment of the layer with iron from the substrate at higher temperatures, resulting in a potential difference of less than 150 mV. These attempts were also unsuccessful. Reducing the coating often results in the required potential difference of 150 mV being undercut in the galvanostatic dissolution. In the case of composite sheets, this negates the advantage of faster heating of the thicker partner, or the OVZ-PF of the composite sheet is often even lower.

[0039] The object of the invention is to enlarge the OVZ-PF between the MINJDVZ and the MAX_OVZ and to enable greater flexibility in a process for producing hardened steel components, even in the event of disruptions and a low level of rejects.

[0040] The object is achieved by a method for producing hardened steel components having the features of claim 1.

[0041] Advantageous further training is indicated in the dependent subclaims.

[0042] The inventors have surprisingly found that a subsequent process can be used to influence the surface in such a way that, for components whose zinc-coated surface has a potential difference to the substrate of less than 150 mV, a surface can still be created that can be adjusted with regard to further processing properties, such as paint adhesion, scratch penetration, weldability, bondability, expanded foam adhesion and corrosion protection.

[0043] A potential difference of less than 150 mV between the steel substrate and the layer can occur with long furnace residence times in general, relatively longer furnace residence times of the thinner sheet thickness areas of parts with different sheet thickness areas, for example the areas of the thin TWB partner, with larger sheet thickness jumps,

[0044] Disturbances, lower zinc deposits, a combination of these.

[0045] According to the invention, finished hardened sheet metal components with sufficient or good properties for further processing can be achieved if the wheel blasting, with which the finished sheet metal components are surface cleaned and conditioned, is set in a specific range of the Almen intensity N of greater than 0.08 mm.

[0046] According to the invention, turbine speeds of more than 1400 revolutions per minute, in particular more than 1800 revolutions per minute and in particular more than 2200 revolutions per minute are aimed for at throughput speeds between 3 m per minute and 15 m per minute.

[0047] The invention makes it possible to significantly increase or expand the furnace residence time process window, allowing sheets with a uniform thickness to remain in the furnace longer, thus making the overall process more tolerant to disruption. This also applies to composite sheets, where the combined process window is also significantly enlarged.

[0048] In addition, selecting lower zinc coatings can reduce the minimum furnace residence time and thus the cycle time. This can increase plant productivity.

[0049] The invention thus relates in particular to a method for producing hardened and surface-conditioned steel components, wherein galvanized steel sheet blanks made of a quench-hardenable steel alloy are subjected to form hardening or press hardening so that hardened steel components are produced, wherein the hardened steel components are subjected to surface conditioning by means of wheel blasting after hardening, wherein at least in the case of sheet metal components in which the potential difference between the zinc coating and the steel substrate is less than 150 mV, the wheel blasting is carried out with an Almen intensity N of at least 0.08 mm.

[0050] A further development provides that the Almen intensity N is more than 0.09 mm, in particular more than 0.1 mm and at most 0.25 mm, in particular at most 0.2 mm.

[0051] Method according to one of the preceding claims, characterized in that the throughput speed of the components through the wheel blasting process is 3 m / min to 15 m / min.

[0052] A further development provides that the galvanized or alloy-galvanized steel sheets or steel sheet components have a coating made of zinc or based on zinc with proportions of iron and / or copper and / or nickel and / or aluminum and / or magnesium.

[0053] A further development provides that the zinc coating has a thickness of 3 pm to 14 pm before heat treatment.

[0054] A further development provides that the maximum furnace residence time in seconds per mm sheet thickness during heating for the purpose of at least partial austenitization is between 2.85 and 5.87 times the numerical value of the zinc layer in g per m 2as the sum of the top and bottom surfaces before heat treatment. This means that for a 1 mm steel sheet with a zinc coating Z140 of 140 g / m 2 the maximum furnace residence time is between 399 and 822 seconds.

[0055] A further development calls for turbine speeds of more than 1400, especially more than 1800, and especially more than 2200 revolutions per minute for wheel blasting. The higher the turbine speed, the more effective the cleaning can be. However, the blasting material is subject to increased wear, so the turbine speed should be selected as low as possible and as high as necessary.

[0056] A further development proposes that the throughput speed of the components through the wheel blasting process be between 3 m / min and 15 m / min. A slower throughput can achieve a greater cleaning effect, but this increases the cycle time of the cleaning process and reduces efficiency.

[0057] A further development provides that the sheet steel component or the sheet steel blank has the following composition (all data in wt.%): Carbon up to 0.4, preferably 0.15 to 0.3

[0058] Silicon up to 1.9, preferably 0.11 to 1.5

[0059] Manganese up to 3.0, preferably 0.8 to 2.5

[0060] Chromium up to 1.5, preferably 0.1 to 0.9

[0061] Molybdenum up to 0.9, preferably 0.1 to 0.5

[0062] Nickel up to 0.9,

[0063] Titanium up to 0.2 preferably 0.02 to 0.1

[0064] Vanadium up to 0.2

[0065] Tungsten up to 0.2,

[0066] Aluminum up to 0.2, preferably 0.02 to 0.07

[0067] Boron up to 0.01, preferably 0.0005 to 0.005

[0068] Sulphur max. 0.01, preferably max. 0.008

[0069] Phosphorus max. 0.025, preferably max. 0.01

[0070] Rest iron and impurities.

[0071] The invention is explained by way of example with reference to some drawings.

[0072] They show:

[0073] Figure 1: different pasture intensities N;

[0074] Figure 2: Minimum and maximum furnace residence time and furnace residence time process window according to the state of the art;

[0075] Figure 3: minimum and maximum furnace residence time and furnace residence time process window according to the invention; Figure 4: minimum and maximum furnace residence times and furnace residence time

[0076] Process window of a sheet metal component with different sheet thickness ranges using the example of a state-of-the-art TWB;

[0077] Figure 5: minimum and maximum furnace residence times and furnace residence time

[0078] Process window of a sheet metal component with different sheet thickness ranges using the example of a TWB according to the invention;

[0079] Figure 6: Schematic representation of the theoretical furnace residence time process window and possible real furnace residence time process windows

[0080] Figure 7: Potential curves of the galvanostatic dissolution of samples of a Z140-

[0081] Zinc coating at different furnace residence times;

[0082] Figure 8: Potential curves of the galvanostatic dissolution of samples of the same relative furnace residence time with different zinc deposits;

[0083] Figure 9: Paint adhesion and infiltration results after 6 cycles VDA 233-102 of cured samples of a 22MnB5+Z140 with different oven residence times and potential differences of less than 150 mV (corresponding to Figure 7) and surface conditioning according to the invention;

[0084] Figure 10: Representation of the scratched samples from Figure 9 after the end of the test;

[0085] Figure 11: Surface resistance values ​​according to furnace residence times and radiation intensities.

[0086] Typical sheet thicknesses, which are used in both the direct process (press hardening) and the indirect process (form hardening), are between 0.5 and 4.0 mm. The usual zinc coating thicknesses are typically between 3 and 14 μm, which corresponds to coating weights of approximately 40 to 200 g per m². 2 in sum corresponds to the top and bottom.

[0087] As already explained, for example, with long furnace residence times, particularly when using sheets with regions of different sheet thicknesses, the potential difference between the layer and the substrate can drop to below 150 mV, particularly due to the enrichment of the ZnFe diffusion layer with iron from the substrate. This usually leads to such parts being considered scrap. Figure 1 shows various Almen intensities N in mm of deflection: on the one hand, Almen intensities according to the prior art and, on the other hand, an Almen intensity according to the invention. In each case, the mean value across the conveyor belt width was used as the reference value for the bar; the error bar indicates the standard deviation for the respective example.

[0088] According to the invention, it was found that at AI intensities N of greater than 0.08 mm, in particular more than 0.09 mm and in particular above 0.1 mm, the surface of the zinc layer is so good even at a potential difference of less than 150 mV that sufficient corrosion protection is ensured, in particular with regard to paint adhesion, paint infiltration, crater formation in the KT paint layer and surface corrosion, while at the same time providing excellent weldability and bondability.

[0089] Figure 2 shows the temperature-time curve of a uniformly thick sheet in a furnace for austenitizing. After the minimum furnace residence time (MIN) has elapsed, the sheet has reached the temperature required for successful hardening. After the maximum furnace residence time (MAX), sufficient quality is no longer guaranteed with the current technology. Therefore, the process window (OVZ-PF) within which production can take place is determined, from the time the MIN_DVZ expires until the time the MAX_OVZ expires.

[0090] In general, the smaller the MINJDVZ, the shorter the furnace cycle, which is advantageous in itself. For the OVZ-PF, the smaller it is, the greater the expected scrap, especially in the event of malfunctions. Scrap should therefore be avoided or minimized.

[0091] With surface conditioning according to the invention by wheel blasting with the inventive Almen intensities N, a significantly larger MAX_OVZ and thus a significantly increased OVZ-PF is achieved as shown in Figure 3. The expanded, additional area is referred to as the process window extension (OVZ-PF extension).

[0092] Figure 4 shows that the heating times for a composite sheet made up of a thicker and a thinner partner sheet, for example, a TWB and / or a thicker and a thinner sheet section with state-of-the-art surface conditioning, differ. The process windows also differ accordingly. The minimum furnace residence time of the thinner sheet is reached first, which means that the process window of the thinner sheet begins first. However, due to the faster heating, the maximum furnace residence time is also reached more quickly, which means that the process window of the thinner sheet is also passed through or expired more quickly.

[0093] For the thicker sheet, the minimum furnace residence time is reached later due to the slower heating, so that the process window of the thicker partner is shifted towards longer furnace residence times.

[0094] The process window of the composite sheet, which is heated as a unit in the furnace, is formed from the intersection of the two individual process windows, since only in this window are the properties of both sheets simultaneously satisfactory for a certain period of time. The MIN_DVZ of the composite sheet is determined by the thicker sheet, and the MAX_OVZ of the composite sheet is determined by the thinner sheet. The resulting process window of the composite sheet, labeled as "OVZ-PF TWB" in Figure 4, is therefore significantly smaller overall for tailored welded blanks (TWB), tailored rolled blanks (TRB), or patched blanks than each of the process windows of the individual partners or sheet thickness ranges.

[0095] Figure 5 shows that, with surface conditioning according to the invention, the process window for both the thinner partner and the thicker partner is individually larger within the inventive surface intensity ranges. In the example shown, the minimum furnace residence times are also closer together due to faster achievement of the MINJDVZ of the thicker sheet, for example, by reducing the zinc coating on the thicker sheet. This results in a significantly larger process window for the composite sheet and also advantageously shortens the cycle time of the composite sheet. Therefore, the possible process window "OVZ-PF TWB" in Figure 5 is significantly wider than that of the prior art in Figure 4, namely by the two ranges of the "OVZ-PF extensions."

[0096] Figure 6 shows three different cases of the furnace residence time process window. The top bar a) shows the entire theoretical process window, which, as previously explained, is defined as the range between the minimum furnace residence time and the maximum furnace residence time. Bars b) and c) also show possible actual furnace residence times and possible actual furnace process windows in a production run. This makes it clear that in large-scale series production, the actual actual furnace residence time is advantageously set at the lower end, i.e. just above the minimum furnace residence time. This is because, on the one hand, if the furnace is the cycle-setter, this shortens the cycle time for the majority of parts and, on the other hand, the time buffer in the event of a disruption until the point at which scrap parts are produced is not wasted, but rather utilized to the greatest extent possible.

[0097] Figure 7 shows the galvanostatic dissolution of hardened samples of the same coating on the 22MnB5 steel substrate with a sheet thickness of 1 mm at different furnace dwell times. The times are given in the form of relative furnace dwell times, i.e., the time from reaching a reference temperature, in this case 870 °C, is given. The furnace dwell time is therefore the sum of the time until 870 °C is reached plus the specified time value. The coating in this case is Z140, i.e., a zinc alloy coating with 140 g / m 2Coating according to DIN EN 10346. This standard is applicable analogously to all coating thicknesses, i.e., from Z80 to Z180. The dissolution curves over the course of the test time show that the potential difference to the steel substrate decreases progressively with increasing furnace residence time. The curve for the very short relative furnace residence time of 45 seconds, starting at 870 °C, initially shows a portion of zinc-rich T-ZnFe phase and, after this phase has dissolved, iron-rich zinc ferrite. The curves for longer furnace residence times show no zinc-rich r-ZnFe phase, but only iron-rich zinc ferrite until dissolution toward the substrate.The curve with a relative furnace residence time of 350 seconds from reaching 870 °C still shows a potential difference of greater than 150 mV to the steel substrate, while the curve with a relative furnace residence time of 400 seconds from reaching 870 °C already shows a potential difference of less than 150 mV to the steel substrate.

[0098] The electrochemical potential difference can vary not only due to the furnace residence time (FRT), but also due to the use of different coating thicknesses. Figure 8 shows a comparison between Z80 and Z100 at the same relative FRT. It can be seen that the potential difference increases with the coating thickness at the same relative FRT. This is demonstrated by the fact that at Z100, the potential difference to the steel substrate is significantly above 150 mV, while at Z80, this is only just the case.

[0099] A specific steel sheet of grade 22MnB5 with a sheet thickness of 1 mm and a Z140 zinc coating was subjected to corrosion testing according to VDA 233-102 ("VDA new"). The same steel sheet was subjected to austenitization and different furnace dwell times, and after subsequent hardening and surface conditioning according to the invention, the paint creepage was measured on samples scratched with the "Clemen" scratching tool after 6 cycles, i.e., 6 weeks according to VDA 233-102. The result for manufactured samples with furnace dwell times according to the invention was a paint creepage of on average well below 1 mm. In addition, very good adhesion of the cathodic dip paint (KT paint) was determined both before and after the 6 VDA 233-102 cycles using a cross-cut test with Gt 0.

[0100] These values ​​were surprisingly positive for the expert and defined as acceptable by customers. These values ​​are shown in Figure 9.

[0101] Figure 10 shows the different sheets according to the invention which were tested according to Figure 9. It can be seen that the samples exhibit surprisingly good properties, since with longer furnace residence times it was assumed that both a stronger paint undercut at the cracks and a stronger crater formation in the KT paint, a stronger surface rust and a stronger paint undercut at the edges would be observed.

[0102] Figure 11 shows the corresponding surface resistance values ​​in mOhm for the samples mentioned. In the table, samples No. 1 to No. 5 were surface-conditioned using a state-of-the-art blast treatment, i.e., with an Almen intensity of 0.060 mm N. The samples below, No. 11 to No. 15, however, were surface-conditioned according to the invention with a higher Almen intensity. In addition, the relative furnace residence time was varied from 45 seconds to 600 seconds from reaching 870 °C.

[0103] This results, as already shown in Figure 7, in a potential difference of greater than 150 mV occurring with short furnace residence times and less than 150 mV with longer furnace residence times. It can be clearly seen that the surface resistance value increases with longer furnace residence times and rises to a value of approximately 1.6 mOhm with prior art blasting treatment. Customers generally tolerate a maximum value of 1.5 mOhm. Surprisingly, it was found that the values ​​for the parts surface-conditioned according to the invention are lower than those for parts not surface-conditioned according to the invention. This means that surface conditioning according to the invention, i.e. cleaning with high alumina intensity, made it possible to significantly improve the product properties with regard to weldability.This means that longer furnace residence times and thus lower potential differences in the process still do not lead to rejects.

[0104] The invention therefore has the advantage that the heating process for producing hardened steel components, both in the indirect and direct process, becomes considerably more robust against disturbances, while at the same time providing excellent surface and corrosion protection properties.

[0105] A further advantage of the invention can be a reduced cycle time due to a reduction in the layer thickness and the associated reduction in the minimum furnace residence time, as this increases productivity. This is especially true when using tailored property parts, i.e., blanks composed of TWB and / or TRB and / or patched blanks.

Claims

International patent application Voestalpine Metal Forming GmbH P347186WO Claims 1. A method for conditioning the surfaces of heat-treated, galvanized or alloy-galvanized steel sheets or steel sheet components, wherein the steel sheet is heated at least in part for the purpose of austenitization and then formed into a steel sheet component or is first formed into a steel sheet component and then heated at least in part for the purpose of austenitization and after the at least partial austenitization, the steel sheet or the steel sheet component is cooled and hardened at least in part at a rate above the critical cooling rate and the surface of the steel sheet component is then subjected to centrifugal wheel blasting, characterized in that at least in the case of steel sheet components or steel sheet component areas,where the potential difference between the zinc coating and the steel substrate is less than 150 mV, the wheel blasting is carried out with an Almen intensity N of at least 0.08 mm., 2. Method according to claim 1, characterized in that the Almen intensity N is more than 0.09 mm, in particular more than 0.1 mm and at most 0.25 mm, in particular at most 0.2 mm.

3. Method according to claim 1 or 2, characterized in that the galvanized or alloy-galvanized steel sheets or steel sheet components have a coating of zinc or based on zinc with proportions of iron and / or copper and / or nickel and / or aluminum and / or magnesium.

4. Method according to one of the preceding claims, characterized in that the zinc coating has a thickness of 3 pm to 14 pm before the heat treatment.

5. Method according to one of the preceding claims, characterized in that the maximum furnace residence time in seconds per mm of sheet thickness during heating for the purpose of at least partially austenitizing is between 2.85 and 5.87 times the numerical value of the zinc layer in g per m 2 as the sum of the top and bottom sides before heat treatment.

6. Method according to one of the preceding claims, characterized in that turbine speeds of more than 1400, in particular more than 1800 and in particular more than 2200 revolutions per minute are used in wheel blasting.

7. Method according to one of the preceding claims, characterized in that the throughput speed of the components through the wheel blasting process is 3 m / min to 15 m / min.

8. Method according to one of the preceding claims, characterized in that the sheet steel component or the sheet steel blank has the following composition (all data in wt.%): Carbon up to 0.4, preferably 0.15 to 0.3 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 0.1 to 0.5 Nickel up to 0.9, Titanium up to 0.2 preferably 0.02 to 0.1 Vanadium up to 0.2 Tungsten up to 0.2, Aluminum up to 0.2, preferably 0.02 to 0.07 Boron up to 0.01, preferably 0.0005 to 0.005 Sulphur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01 Rest iron and impurities.

9. A hardened steel component with a zinc-based coating, wherein the steel component has a surface conditioned by wheel blasting according to any one of the preceding claims.

10. Use of a welded blank comprising at least one carbon Manganese-boron steel for producing a sheet steel component according to one of claims 1 to 8.

11. Use of a flexibly rolled blank made of a carbon-manganese-boron steel for producing a sheet steel component according to one of claims 1 to 8.

12. Use of a patched blank comprising at least one carbon- Manganese-boron steel for producing a sheet steel component according to one of claims 1 to 8.

13. Patched circuit board according to claim 12, characterized in that the sheet thickness of the circuit board is constant.

14. Use of a hardened steel component according to claim 9 as a vehicle part, in particular as a B-pillar, longitudinal member, cross member, or side wall reinforcement.

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