Method for producing hardened steel components

Abstract

The invention relates to a method for producing hardened steel components, wherein a sheet metal blank is cut out from a galvanized or alloy-galvanized strip made of a hardenable steel alloy and the sheet metal blank is then cold-formed to produce a component blank and then at least partly heated to a temperature which causes a microstructural change to austenite, wherein the at least partly austenitized component blank is then fed to a press hardening tool in which the component blank is interlockingly held by means of an upper and lower die which have a shape that corresponds substantially to the component blank, wherein, due to the contact of the component blank material with the dies, which are in particular cooled, the heat is extracted from the steel material so quickly that martensitic hardening occurs due to the cooling at a cooling rate above the critical cooling rate. The galvanized or alloy-galvanized strip is provided with a coil lubrication and sheet metal blanks cut from the steel strip are at least partly provided with an additional lubrication prior to cold forming, the additional lubrication being carried out using a drawing lubricant in an amount of 0.1 g / m² to 2 g / m².

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 raising an object made of a hardenable steel alloy, such as a boron-manganese steel, to a temperature at which the microstructure is largely or entirely in the form of austenite or gamma iron. Austenite has a face-centered cubic structure with a maximum solubility for carbon of 2.06%.

[0005] Significantly more carbon can be dissolved in austenite than in the ferrite present in the initial state of the steel material. During heating for the purpose of austenite formation, the carbon of the existing cementite (iron carbide FeaC) dissolves in the austenite. If this carbon-rich austenite is then quenched, the kinetic inhibition prevents it from transforming back into ferrite and cementite. The iron lattice can no longer transform into body-centered cubic o-iron. Instead, it transforms into a body-centered cubic lattice, which is also tetragonally strained by the carbon, as carbon is precipitated because the carbon solubility is reduced in this structure. This structure is called martensite. However, for this to happen, the cooling rate must be high enough for the kinetic inhibition to take effect. This minimum rate is the so-called critical hardening rate.

[0006] 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 cold rolling the resulting slabs 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.

[0007] 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. Thickness reduction does not constitute forming within the meaning of this disclosure.

[0008] It is not uncommon to galvanize such cold-rolled strip in a galvanizing plant. This can be done by electrolytic coating, PVD coating, or hot-dip galvanizing. For this, the strip is unwound from the coil and rewound at the end.

[0009] To produce sheet steel components, the steel strip is unwound from the coil and cut into pieces, called blanks. These blanks are thus flat steel sheets with a limited dimension. To produce hardened sheet steel components, these blanks can be further processed in two ways.

[0010] In the so-called direct process, also known as 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.

[0011] In the so-called indirect process, also known as the hot stamping process, the blank is formed into a component blank in a usually multi-stage forming process, usually mainly through a combination of deep drawing, trimming and / or post-forming when 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 does not usually carry out any further 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.

[0012] In both the direct and indirect processes, the product is a hardened steel component. In the direct process, steel strips with an aluminum-silicon coating, galvanized or alloy-galvanized steel strips, or uncoated steel strips are typically 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, as defined by the invention, is any steel strip or steel sheet coated with zinc or a zinc-based alloy. Zinc-based means that zinc constitutes the largest alloying component of the coating and, in particular, accounts for more than 50% by weight of the coating. However, alloys such as ZnNi, ZnCr, ZnMg, or others are certainly conceivable.

[0015] This layer can advantageously have a thickness of 5 μm to 20 μm per side. This can ensure good corrosion protection. In particular, the coating can be a Z80, Z120, Z140, or Z180 according to DIN EN 10346.

[0016] Zinc-based corrosion protection coatings 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 may also contain other oxygen-affine elements such as magnesium.

[0017] Particularly preferably, the metallic corrosion protection layer can be applied using a hot-dip galvanizing process. This can be a simple and robust application method.

[0018] During heat treatment for the purpose of austenitization, a zinc layer transforms into a layer known as a zinc-iron layer. Under the influence of heat, the base material (iron) diffuses into the zinc layer, forming a heterogeneous zinc-iron layer. Zinc oxides and oxides of alloying elements present in the zinc coating, such as aluminum, may also be present on the surface. The mechanisms for this are known and described in the applicant's patents. Furthermore, it is known that coils can be treated with so-called prelubes both at the steelworks for storage and transport, and that drawing oils or spot lubricants can be applied later to the blanks, usually shortly before forming, as forming or drawing aids.

[0019] However, in contrast to conventional cold forming, in the indirect process, after prior cold forming of the galvanized or alloy-galvanized blanks into component blanks, the oil present on the component blanks is burned or decomposed at the high temperatures prevailing in the austenitizing furnace. Particularly when drawing oils are used in cold forming, an undesirable surface effect, the so-called "elephant skin," can arise, which is still visible even after cleaning with, for example, wheel blasting (SRS) or dry ice blasting (according to a patent owned by the applicant). The cause appears to be that oil accumulates locally during heating, similar to a frying pan, with increased oxide formation on the Zn-Fe surface of the former zinc layer in areas of the thinner oil layer.This effect occurs exclusively in the indirect process during the heat treatment of galvanized or alloy-galvanized component blanks for the purpose of austenitization when using drawing oils as drawing aids in the cold forming of the component blanks.

[0020] For conventional cold-formed parts that do not undergo further heat treatment for the purpose of austenitization with subsequent hardening, the amount and composition of oiling are not critical with regard to the undesirable surface effect of the so-called "elephant skin" of severe severity, since the heat treatment factor is missing here.

[0021] The same applies analogously to uncoated so-called bare cold-formed parts, even if they undergo heat treatment for the purpose of austenitization with subsequent hardening as part of the indirect process, since the factor of zinc or zinc alloy coating is missing here.

[0022] In components manufactured using the direct process, these undesirable surface effects also do not occur, regardless of the coating of the steel strips, since no cold forming takes place, only hot forming, and thus the factor of using drawing oil as a drawing aid in cold forming is missing. In components with an aluminum-silicon coating and uncoated components, the factor of zinc or zinc alloy coating would also be missing, even with the use of drawing oil. This is shown in the overview in Figure 5.

[0023] Galvanised sheets are treated with a steelworks oiling / coil oiling, a so-called prelube (e.g. ANTICORIT PL 3802 39 S), particularly to prevent white rust during transport and storage. The applied quantity is usually between 0.5 g / m 2 and 1 g / m 2 .

[0024] In the cold forming of sheet steel components in general, including for example in the indirect process outlined above, at least local additional lubrication with forming lubricants, so-called drawing oils (e.g. KTL N16), is required for critical forming components / areas. The applied quantity is usually between 1.5 g / m 2 (for lighter) and 3 g / m 2 during heavy forming.

[0025] Forming-critical components / areas within the meaning of the invention exist, for example, if, after forming, there is a risk of a sheet thickness reduction of greater than 15%, in particular greater than 20%, in the subsequent finished part area and / or if, in the finished part and / or waste area, there is a risk of a safety margin against necking of less than 25%, in particular less than 20%, relative to the forming limit curve in the forming limit diagram of the forming analysis. While a maximum permissible sheet thickness reduction for components in vehicle construction is defined, for example, based on crash requirements, necking as a precursor to crack formation must also be avoided for reasons of process reliability during cold forming.

[0026] Forming-critical components / areas within the meaning of the invention also exist, for example, if during the forming process there is a risk of zinc abrasion, i.e. abrasion of more than 0.1 g / m 2or the risk of zinc deposits of more than 0.1 g / m 2 in the forming tool. This is particularly the case for forming processes with small radii of less than 20 mm and / or large wrap angles of greater than 40° and / or steep sides with side angles of less than 20° and / or large drawing depths of greater than 50 mm.

[0027] From W02018060082A1 a method is known for producing a component from a steel product coated with an Al-Si protective coating, comprising the following steps:

[0028] - Providing a substrate consisting of a steel product coated with an Al-Si protective coating, - Heating the substrate to a temperature T1 such that the Al-Si protective coating is only partially pre-alloyed with Fe of the steel product, - Cooling the pre-alloyed substrate to room temperature, - Applying an anti-corrosive oil to the surface of the pre-alloyed substrate, wherein the anti-corrosive oil consists of a composition containing fatty acid esters, - Transporting the pre-alloyed substrate provided with the anti-corrosive oil, - Heating the pre-alloyed substrate provided with the anti-corrosive oil to a temperature T2 such that the Al-Si protective coating is completely alloyed with Fe of the steel product and the anti-corrosive oil is removed without residue, and - Forming the reheated substrate into the component.

[0029] EP3278895B1 discloses a steel sheet for hot forming, a method for its production, and a hot-formed component produced in this way. The steel sheet is a hardenable steel sheet with the composition in mass % C: 0.100% to 0.600%, Si: 0.50% to 3.00%, Mn: 1.20% to 4.00%, Ti: 0.005% to 0.100%, B: 0.0005% to 0.0100%, P: 0.100% or less, S: 0.0001% to 0.0100%, Al: 0.005% to 1.000%, and N: 0.0100% or less, the remainder being iron and impurities, wherein the roughness of the steel strip satisfies Rz>2.5 pm and 50 mg / m 2 up to 1500 mg / m 2 an oil coating is applied to the surface.

[0030] A method and a device for producing molded parts are known from DE10000138A1. In the method for producing molded parts, in which a molded part material is deformed while coming into contact with a molding tool, it is provided that after a pre-deformation of the molded part material until it largely comes into contact with the molding tool, a lubricant is introduced between the molded part material and the molding tool at at least one point where the molded part material rests against the molding tool, and the molding material is then brought into its final shape by the forming pressure. In a device for carrying out the method, a lubricant can be introduced between the molded part material (3, 12) and the molding tool (2, 10, 11) at at least one point where the molded part material (3, 12) rests against the molding tool (2, 10, 11, 16, 17) during the deformation before the final shape is reached.

[0031] Vehicle manufacturers have issued their own regulations and standards regarding the application and differentiation of the corresponding lubricants. It should be emphasized here that these regulations and standards all refer to conventional cold-formed parts and their further processing as cold-formed components in vehicle construction, and not to cold-formed parts that undergo heat treatment for the purpose of austenitization as part of an indirect hardening process before further processing in vehicle construction. The VDA (German Association of the Automotive Industry) has developed a recommendation for the use of oils in cold forming and published it as VDA 230-213. This standard specifies the quality requirements for final coatings for uncoated and coated steel and aluminum materials used in body construction, so-called prelubes, prelubes 2, and hotmelts, as defined in VDA 230-213.With material- or application-related restrictions or extensions, this standard also applies to blank washing oils, drawing oils and corrosion protection oils of a general nature.

[0032] Examples of prelubes include Multidraw PL 61 from Zeller+Gmelin, Eislingen, and Anticorit PL 3802-39 S from Fuchs, Mannheim. Examples of spot lubricants for partial relubrication include AP 167 / 22 from Hermann Bantieon GmbH, Ulm, and KTL N16 from Zeller+Gmelin, Eislingen. The general requirement is that the total amount of all lubricants applied does not exceed 3 g / m². 2 , in exceptional cases after consultation 4 g / m 2, must not be exceeded. This applies to every point on the component and is not to be determined as an average value. Further restrictions on the (total) oiling quantity are noted in sheet metal identification cards, technical delivery specifications or component drawings and must be given priority accordingly. The application method and layer weight or degree of oiling of the products must be selected so that, on the one hand, there is even, closed surface wetting of the parts to be protected, and, on the other hand, no hindrance or annoyance occurs during processing due to excessive film thickness (e.g. adhesion, oil vapors during welding, etc.). It is pointed out that in each case, all effects with regard to the resulting properties must be observed and evaluated (e.g.A lower layer weight can impair corrosion protection and friction behavior, and a high layer weight can lead to undesirable oil leakage from the stored coils as well as to impaired compatibility with adhesives and removability).

[0033] For example, Zeller and Gmelin produces a drawing oil called MULTIDRAW KTL N 16, which is immiscible with water and contains synthetic fatliquoring agents as well as phosphorus and sulfur-containing additives. Its technical properties are: density / 15°C kg / m 3 900 DIN 51 757, viscosity / 40°C mm 2 / s 160 DIN 51 562, flash point °C > 200 DIN ISO 2592. This oil should be used for light forming up to 1.5 g / m 2 and for heavy forming up to 3 g / m 2 be applied.

[0034] Such drawing oils also have a corrosion-inhibiting effect, but are specifically tailored to lubrication in forming areas through additives. During austenitization in a furnace prior to hardening, the component blanks cold-formed using the indirect process (and usually not washed for cost reasons) are heated to temperatures above Ac3 (e.g., 900°C).

[0035] During austenitization in the furnace, the oil heats up and burns / decomposes, resulting in an undesirable surface effect, the so-called "elephant skin," which is still visible even after cleaning. The cause appears to be that oil accumulates locally during heating, similar to a frying pan, with increased oxide formation on the Zn-Fe surface of the former zinc layer in areas with a thinner oil layer.

[0036] The object of the invention is to provide a method by which such hardened steel components are produced from galvanized or alloy-galvanized steel strips with a better surface quality.

[0037] The problem is solved by a method having the features of claim 1. Advantageous further developments are characterized in subclaims.

[0038] The inventors discovered that not all oils react the same way. Drawing oils, apparently due to their additives, produce a significant effect even at lower concentrations, particularly above 2 g / m 2 an undesirable elephant skin of a strong nature and are therefore to be viewed more critically in terms of quantity than the prelubes or steel mill oils which serve to protect against corrosion.

[0039] However, to produce components / areas that are critical for forming, sufficient lubrication is required during cold forming.

[0040] According to the invention, the amount of drawing oil is therefore limited to 0.1 g / m 2 up to 2 g / m 2 limited.

[0041] In general, the quantities of Prelube and drawing oil and combinations thereof apply to each part of the component and should not be considered as an average value.

[0042] In the event that satisfactory forming is not possible with such a quantity of drawing oil and the existing prelube, the quantity of prelube between 1 g / m 2 and 3 g / m 2While this amount of prelube is unusually high compared to the state of the art, it, in combination with the reduced amounts of drawing oil, ensures good formability and a good surface quality with minimal sagging. Furthermore, during heat treatment for the purpose of austenitization, any amount of oil can be burned or decomposed at the prevailing high temperatures in the furnace, advantageously preventing any residual oil from being introduced into subsequent processes.

[0043] For the purposes of the invention, a coil lubrication or prelube is the lubrication defined in the VDA 230-213 standard under chapter "3.2.1 Prelube". A drawing lubricant or drawing oil or forming oil or additional lubrication within the meaning of the invention is the lubrication defined in the VDA 230-213 standard under chapter "3.3 Spot lubricant". The invention thus relates in particular to a method for producing hardened steel components, wherein a blank is cut out of a strip of a hardenable, galvanized or alloy-galvanized steel alloy and the blank is then cold formed into a component blank and then heated at least in part to a temperature that causes a structural change towards austenite, wherein the at least partially austenitized component blank is then fed to a form hardening tool,in which the component blank is held in a form-fitting manner by means of an upper and lower tool which has a shape which essentially corresponds to the component blank, whereby the heat is extracted from the steel material so quickly by the material of the component blank being in contact with the particularly cooled tools that martensitic hardening occurs as a result of cooling at a cooling rate above the critical cooling rate, whereby the galvanised strip is provided with a coil oiling and blanks cut out of the steel strip are provided with an additional oiling at least in part before cold forming, whereby the additional oiling is carried out with a drawing lubricant in an amount of 0.lg / m, 2 up to 2g / m2.

[0044] The invention particularly relates to a method for producing hardened steel components, wherein a blank is cut out of a galvanized or alloy-galvanized strip made of a hardenable steel alloy and the blank is then cold formed into a component blank and then heated at least in part to at least a temperature that causes a structural change towards austenite, wherein the at least partially austenitized component blank is then fed to a form-hardening tool in which the component blank is held in a form-fitting manner by means of an upper and lower tool which have a shape substantially corresponding to the component blank, wherein the contact of the material of the component blank against the particularly cooled tools causes the heat to be extracted from the steel material so quickly that cooling at a cooling rate above the critical cooling rate results in martensitic hardening,wherein the galvanized or alloy-galvanized strip is provided with a coil oiling and blanks cut out of the steel strip are provided with an additional oiling at least in part before cold forming, wherein the additional oiling is carried out with a drawing lubricant in an amount of 0.1 g / m, 2 up to 2 g / m 2 occurs.

[0045] A further development stipulates that the additional oiling should be between 0.1 g / m 2 and 1.5 g / m 2 amounts.

[0046] A further development stipulates that the amount of coil oiling must be more than 0.5 g / m 2 and preferably more than 1.0 g / m 2 and particularly preferably more than 1.5 g / m 2 amounts.

[0047] A further development provides that a ratio of coil oiling to additional oiling of 3: 1 to 1: 2, preferably 2: 1 to 1: 1, is set, depending on the forming process, whereby with an increasing proportion of critical forming areas, the ratio is initially adjusted in favor of the additional oiling until it reaches 2 g / m 2 and is subsequently shifted to favor coil lubrication. This can advantageously further facilitate the forming process without causing the "elephant skin" to form.

[0048] A further development stipulates that the total amount of oiling from coil oiling and additional oiling is 4 g / m 2 This allows for even more targeted prevention of negative surface damage.

[0049] A further development provides that the additional oiling is only applied in areas that are particularly critical for forming, while the remaining areas are lubricated by the coil oiling, whereby the partial additional oiling is limited to 2 g / m 2 or less, particularly preferably 1.5 g / m 2 or less, while the coil oiling should be set to 2 g / m 2 or less.

[0050] A further development provides that in the case of full-surface additional oiling, the full-surface additional oiling is limited to 2 g / m 2 or less, in particular 1.5 g / m 2 or less and in particular 1 g / m 2 or less, while the coil oiling should be set to 1.5 g / m 2 or more, in particular 2 g / m 2 or less, in particular 2 g / m 2 or less.

[0051] A further development proposes applying the additional oil directly to the coil oil. This can eliminate the need for a costly and labor-intensive cleaning process, i.e., any cleaning of the coil oil after transport of the coil from the steel manufacturer to the processor.

[0052] A further development provides for the additional lubrication and the coil lubrication to have different chemical compositions. This advantageously allows for an optimal combination of corrosion protection, which is usually the main component of the coil lubrication, with improved formability, which is usually provided by the additional lubrication.

[0053] A further development stipulates that the galvanized or alloy-galvanized coating has a layer thickness of 5 μm to 20 μm per side. This can lead to improved corrosion protection properties.

[0054] A further development provides for the galvanized or alloy-galvanized coating to be applied using a hot-dip galvanizing process. This process can ensure a simple and cost-effective coating method.

[0055] A further development 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.

[0056] A further development provides that the sheet steel component or sheet steel plate with the following composition is used (all data in wt.%):

[0057] 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 Tungsten up to 0.2,

[0065] Aluminum up to 0.2, preferably 0.02 to 0.07

[0066] Boron up to 0.01, preferably 0.0005 to 0.005

[0067] Sulphur max. 0.01, preferably max. 0.008

[0068] Phosphorus max. 0.025, preferably max. 0.01

[0069] Rest iron and impurities.

[0070] The invention is explained by way of example using a drawing, which shows:

[0071] Figure 1: highly schematic representation of a standard process for indirect forming and

[0072] hardening;

[0073] Figure 2: highly schematic representation of a full-surface coil lubrication and a subsequent partial additional lubrication with a drawing lubricant;

[0074] Figure 3: schematically shows the oiling quantities according to the invention and according to the

[0075] state of the art;

[0076] Figure 4: Surface areas of two hardened components, one with and one without an undesirable surface effect (pronounced "elephant skin");

[0077] Figure 5: Comparison of different manufacturing processes for components and their possible

[0078] Problems regarding surface effects.

[0079] The invention exploits the fact that the combination of coil lubrication and drawing oil allows for sufficient forming lubrication without the formation of excessively pronounced elephant skin, provided the amounts are coordinated. Surprisingly, this is even possible in cases where the total amount of coil lubrication and drawing oil is greater than that shown in the prior art (Figure 3), as yet unexplained synergistic effects of coil lubrication and drawing oil occur here, which promote the surface effects. In addition, this eliminates a cost- and labor-intensive cleaning step of the coil or blank after delivery and before cold forming.

[0080] Figure 1 shows the basic process. It can be seen that a cold-rolled strip is produced in the steelworks as a coil. This coil is oiled. In the sense of the invention, coil oiling means oiling the entire strip primarily for the purpose of corrosion protection. The coil is then cut into blanks or blanks are cut out of the coil. These blanks receive additional oiling for forming as required, i.e. in the case of components that are critical to forming or in component areas that are critical to forming. According to the invention, additional oiling is oiling with a lubricant that is applied and designed for the purposes of cold forming. This additional oiling can be provided over the entire surface or just part of the surface.

[0081] This is followed by cold forming, particularly in the form of a combination of deep drawing, trimming and / or post-forming.

[0082] The component blanks produced in this way, which, unlike the circuit board, are no longer flat but three-dimensional, are then heated and form-hardened.

[0083] Figure 2 shows a blank with coil oil still adhering to the blank, which receives additional oiling as a blank. However, this oiling is only applied to areas subject to particularly severe forming, while the remaining areas are sufficiently lubricated by the coil oiling. This blank is then formed and, in particular, deep-drawn.

[0084] The component blanks produced in this way, which, in contrast to the circuit board, are no longer flat but three-dimensional, are then also heated and form-hardened.

[0085] This is particularly useful for forming processes where the forming degrees are higher to very high in some areas and the distribution of the forming degrees is relatively uneven. In this case, the additional lubrication in some areas can be increased to 2 g / m 2 or less, particularly preferably 1.5 g / m 2 or less or less, while the coil oiling is set to 2 g / m 2 or less.

[0086] The aim is for the total oiling to be 4 g / m 2and preferably at least a ratio of coil lubrication to additional lubrication of 3:1 to 1:2 is set, depending on the degree of forming. As the degree of forming increases, the ratio is shifted in favor of additional lubrication. However, it is also possible according to the invention to apply an additional, full-surface additional lubrication to the blank and then form it. This is particularly useful for forming in which zinc abrasion and / or zinc adhesions occur in the blank holder and / or drawbead area.

[0087] Figure 3 shows the difference to the state of the art. In the state of the art, the coil oiling is limited to 1 g / m 2 limited, while the additional oiling is usually 2.5 g / m 2 According to the invention, the additional oiling is set to 0.1 g / m 2 up to 2 g / m 2 , which is less than what is actually required for the task of drawing oil. Surprisingly, the coil oiling can also be increased to more than 1 g / m2 in particular more than 1.5 g / m 2 be increased, i.e. more than would be necessary for the corrosion protection task, while the additional oiling remains at 2 g / m 2 or below, i.e. less than would actually be necessary for the task as drawing oil and thus the combination of coil lubrication and drawing oil even exceeds the total amount from the state of the art without the negative surface effects occurring.

[0088] Thus, the invention cleverly exploits the synergistic lubricating effect.

[0089] Figure 4 shows examples of the negative surface effects that occur, with the left image clearly showing the "elephant skin" surface effect, while the right image shows correspondingly OK components.

[0090] Figure 5 clearly shows the different processes, whereby it becomes clear that the combination of the indirect process, ie cold forming followed by heat treatment for the purpose of austenitization, with galvanized or alloy-galvanized components can lead to the described problem with the surface, which is reduced or prevented according to the invention.

Claims

Claims: A method for producing hardened steel components, wherein a blank is cut from a galvanized or alloy-galvanized strip of a hardenable steel alloy, and the blank is then cold-formed into a component blank and subsequently heated, at least partially, to at least a temperature that causes a microstructure change towards austenite, wherein the at least partially austenitized component blank is then fed into a die hardening tool in which the component blank is positively held by means of an upper and lower tool which have a shape substantially corresponding to the component blank, wherein the contact of the component blank material against the tools, which are in particular cooled, causes the heat to be extracted from the steel material so quickly that, through cooling at a rate above the critical cooling rate, martensitic hardening occurs.wherein the galvanized or alloy-galvanized strip is provided with coil oiling and blanks cut from the steel strip are at least partially provided with additional oiling before cold forming, wherein the additional oiling consists of a drawing lubricant with an amount of 0.1 g / m², 2 up to 2 g / m² 2 The method according to claim 1, characterized in that the additional oiling is less than 1.5 g / m². 2 and especially between 0.1 g / m³ 2 and 1.5 g / m² 2 A method according to one of the preceding claims, characterized in that the amount of coil oiling is more than 0.5 g / m². 2 and preferably more than 1.0 g / m² 2 and in particular preferably more than 1.5 g / m³ 2A method according to one of the preceding claims, characterized in that a ratio of coil oiling to additional oiling of 3:1 to 1:2, preferably 2:1 to 1:1, is set, depending on the forming process, wherein with an increasing proportion of critical forming areas, the ratio is initially increased in favor of the additional oiling until it reaches 2 g / m². 2 a certain amount and is subsequently shifted in favor of coil lubrication. A method according to one of the preceding claims, characterized in that the total amount of lubrication from coil lubrication and additional lubrication is 4 g / m². 2 does not exceed. Method according to one of the preceding claims, characterized in that the additional lubrication is applied only in areas that are particularly critical for forming, while the remaining areas are lubricated by the coil lubrication, wherein the partial additional lubrication is applied to 2 g / m² 2 or less, in particular preferably 1.5 g / m³ 2or set lower, while the coil oiling is set to 2 g / m² 2 or less is set. Method according to one of claims 1 to 5, characterized in that, in the case of full-surface additional oiling, the full-surface additional oiling is set to 2 g / m². 2 or less, in particular 1.5 g / m² 2 or less and in particular 1 g / m³ 2 or set lower, while the coil oiling is set to 1.5 g / m² 2 or more, in particular 2 g / m³ 2 or less, in particular 2 g / m² 2or is adjusted to a lesser degree. A method according to any of the preceding claims, characterized in that the additional oiling is applied directly to the coil oiling. A method according to any of the preceding claims, characterized in that the additional oiling and the coil oiling have different chemical compositions. A method according to any of the preceding claims, characterized in that the galvanized or alloy-galvanized coating has a layer thickness of 5 pm to 20 pm per side. A method according to any of the preceding claims, characterized in that the galvanized or alloy-galvanized coating was applied by means of a hot-dip process, in particular hot-dip galvanizing. A method according to any of the preceding claims, characterized in that the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel, most preferably 22MnB5, 20MnB8, or 34MnB5.Method according to one of the preceding claims, characterized in that the sheet steel component or sheet steel blank is used with the following composition (all values ​​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 Sulfur max. 0.01, preferably max. 0.008 Phosphorus max. 0.025, preferably max. 0.01 Residual iron and impurities.

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