Method for preparing hardened steel components

Abstract

This invention relates to a method for preparing hardened steel components, comprising cutting sheet blanks from galvanized or alloy-galvanized steel strips of hardenable alloy steel, cold-forming the sheet blanks into component blanks, heating them in at least some areas to a temperature that at least causes the structure to transform to austenite, wherein the component blanks, already austenitized in at least some areas, are then supplied to a forming hardening tool, the component blanks being held in the forming hardening tool in a form-fitting manner by upper and lower tools having a shape substantially consistent with the component blanks, wherein the material of the component blanks abuts against, in particular, the cooled upper and lower tools, and martensitic hardening occurs due to the rapid extraction of heat from the steel material by cooling at a rate higher than the critical cooling rate, wherein, prior to cold forming, the galvanized or alloy-galvanized steel strip is coated with a coil lubricant, and at least some areas of the sheet blanks cut from the steel strip are coated with an additional lubricant, the additional lubricant being 0.1 g / m 2 -2g / m 2 Stretching lubricant.

Description

Technical Field

[0001] This invention relates to a method for preparing hardened steel components. Background Technology

[0002] It is known that high-strength components can be produced from steel plates through hardening. These components are produced, in particular, through quenching hardening and transformation hardening from the austenitic state.

[0003] Quenching hardening refers to the temperature at which products made of hardenable alloy steel (such as boron-manganese steel) reach a structure that is predominantly or entirely austenitic or γ-ferrite. Austenite has a face-centered cubic structure and a maximum carbon solubility of 2.06%.

[0004] Carbon dissolves more readily in austenite than in ferrite, which is present in the initial state of steel. During heating to form austenite, carbon from the existing cementite (iron carbide, Fe3C) dissolves into the austenite. If the carbon-rich austenite is then quenched, ferrite and cementite reversion will not occur due to kinetic inhibition. The iron lattice no longer transforms into a body-centered cubic α-iron lattice. Instead, it folds into a body-centered cubic lattice because this structure reduces the solubility of carbon, causing carbon to deposit and thus also subject to tetragonal strain. This structure is called martensite. However, for kinetic inhibition to take effect, rapid cooling is required. The minimum rate is called the critical hardening rate.

[0005] To prepare such hardenable steel sheet materials, steel bars are prepared by dissolving hardenable alloy steel, which is then cast into ingots, typically through continuous casting. The resulting ingots are hot-rolled in a hot strip mill and cold-rolled in a cold strip mill to form steel strips. These steel strips are typically hundreds to over a thousand meters long, a few millimeters or only a fraction of a millimeter thick, and are wound into what are known as coils.

[0006] During the cold and hot rolling processes, the thickness of the steel ingot is significantly reduced, thus making it longer. However, the width remains essentially unchanged through appropriate measures.

[0007] For the purposes of this disclosure, the reduction in thickness is not considered part of the forming process.

[0008] It is not uncommon to galvanize such cold-rolled steel strips on galvanizing lines; this can be achieved through electrolytic coating, PVD coating, and hot-dip galvanizing. For this purpose, the steel strip is unwound from the coil and rewound at the end.

[0009] To prepare sheet metal components, steel strip is unwound from the coil and cut into segments called sheet blanks. These sheet blanks are therefore flat steel sheets of finite dimensions. These sheet blanks can be further processed in two ways to prepare hardened sheet metal components.

[0010] In the so-called direct method (also known as the molding hardening method), sheet metal blanks are typically austenitized by heating in a furnace and then formed into a sheet metal part in a single step in a relatively cool forming tool. In this case, the forming takes place while the steel sheet is still hot. Once forming occurs in the tool and the tool is completely sealed, the tool surface comes into contact with the sheet metal part. This causes heat to dissipate to the (cooled) tool at a rate higher than the critical hardening rate, thereby achieving the aforementioned hardening by transforming into a martensitic structure.

[0011] In the so-called indirect method (also known as the form hardening method), sheet metal blanks are typically formed into part blanks in a multi-step forming process. While cooled, the sheet metal blanks are usually formed primarily through a combination of deep drawing, shaping, and / or post-forming processes. The part blank is then typically austenitized in a furnace and placed in a tool (form hardening tool) while still hot. The form hardening tool typically does not perform further actual forming, although this is possible to a very small extent in the case of spring-loaded part blanks and / or final forming. Once the tool is completely sealed, heat dissipation occurs through contact between the tool surface and the surface of the part blank, causing the part blank to harden and form the part.

[0012] In both methods, namely the direct and indirect methods, the product obtained is a hardened steel component.

[0013] In the direct method, aluminum-silicon coated steel strip, galvanized or alloy-galvanized steel strip, or uncoated steel strip are typically used.

[0014] In indirect methods of pre-cold forming, galvanized or alloy-galvanized steel strips or uncoated steel strips are typically used. Because the aluminum-silicon coating is very brittle at room temperature and may peel off during cold forming, this type of coating is usually only used in direct methods without pre-cold forming.

[0015] For the purposes of this invention, galvanized or alloy-galvanized steel strip refers to steel strip or sheet with a zinc-containing or zinc-based alloy coating. "Zinc-based" means that zinc is the main alloying component in the coating, particularly accounting for more than 50% of the coating weight. However, it is readily conceivable to use ZnNi, ZnCr, ZnMg, or other alloys.

[0016] Advantageously, the coating thickness on each side can be 5μm-20μm. This ensures good corrosion resistance. In particular, the coating can be Z80, Z120, Z140, or Z180 according to DIN EN 10346.

[0017] Zinc-based anti-corrosion coatings can have a relatively high zinc content of 85wt%-99wt%, and contain 0.2-2wt% aluminum in addition to unavoidable impurities. They may also contain other elements with an affinity for oxygen, such as magnesium.

[0018] Particularly preferably, the metal corrosion-resistant layer can be applied by a hot-dip galvanizing method, such as hot-dip zinc plating. This constitutes a simple and practical application method.

[0019] During the heat treatment process to achieve austenitization, the 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 and forms a non-uniform zinc-iron layer, thus allowing zinc oxides and oxides of alloying elements such as aluminum, which are present in the zinc coating, to also exist on the surface. This mechanism is known and described in the applicant's patent.

[0020] It is also known that, for storage and transportation, coils are lubricated at steel mills with so-called prelubes, and later, usually before forming, forming or stretching aids in the form of stretching oil or spot lubricant are applied to the sheet blanks.

[0021] However, compared to traditional cold forming, in indirect methods, after the galvanized or alloy-galvanized sheet blanks are pre-cooled and formed into part blanks, the lubricating oil present in the part blanks is burned or decomposed at the generally high temperatures in the furnace for austenitization. This is especially true when drawing oil is used in cold forming, potentially resulting in an undesirable surface effect known as "elephant skin," which remains visible even after cleaning, for example, by vacuum blasting (ABC) or dry ice blasting (according to one of the applicant's patents). This may be caused by localized accumulation of lubricating oil during heating, such as in a frying pan, where increased oxide formation occurs on the Zn-Fe surface of the previously zinc-coated layer in areas with thinner oil layers. This effect only occurs in indirect methods of heat treatment of galvanized or alloy-galvanized part blanks for austenitization when drawing oil is used as a drawing aid in the cold-formed part blank.

[0022] In the case of conventional cold-formed parts that do not undergo any further heat treatment for austenitization and subsequent hardening, the amount and composition of lubricant are not important for the undesirable surface effect of "elephant skin" because there is no heat treatment factor in this case.

[0023] This also applies to uncoated, so-called glossy cold-formed parts, even if these parts undergo heat treatment as part of an indirect method to achieve austenitization and subsequently harden, since there is no factor of zinc or zinc alloy coating in this case.

[0024] For parts produced using direct methods, regardless of the steel strip coating, such undesirable surface effects do not occur because only hot forming is performed, not cold forming. Therefore, the factor of using drawing oil as a drawing aid in cold forming is eliminated. For parts with aluminum-silicon coatings and no coating, even if drawing oil is used, the factor of zinc coating or alloy zinc coating is eliminated. This demonstrates... Figure 5In the overview.

[0025] To prevent white rust during transportation and storage, galvanized steel sheets are coated with steel mill lubricating oil / coil lubricating oil, known as pre-lubricating oil (such as ANTICORIT PL 3802 39S); the amount used in this case is typically 0.5 g / m³. 2 -1g / m 2 .

[0026] In typical cold forming of sheet metal parts, such as the indirect methods described above, additional lubrication, at least locally, is required for forming critical components / areas using forming lubricant, known as drawing oil (e.g., KTL N16); in this case, the amount applied is typically between 1.5 g / m² for light forming. 2 and recrystallization 3g / m 2 between.

[0027] For the purposes of this invention, for example, if, after forming, there is a risk of a reduction in sheet thickness greater than 15%, particularly greater than 20%, in the subsequent finished part area, and / or, relative to the forming limit curve of the finished part area and / or the forming limit diagram of the scrap area in the forming analysis, there is a safety margin of less than 25%, particularly less than 20%, of necking risk, then a critical forming part / area is considered to exist. Although the maximum permissible reduction in sheet thickness of components in a vehicle structure is defined, for example, according to crash requirements, necking must also be avoided as an initial stage of crack formation for the reliability of cold forming.

[0028] For the purposes of this invention, for example, if there is a risk of zinc abrasion during the molding process, i.e., exceeding 0.1 g / m 2 Wear, or the presence of more than 0.1 g / m in the forming tool. 2 The risk of zinc adhesion is considered to exist in critical molding components / areas. In particular, in molding processes with small radii less than 20 mm and / or large wrap angles greater than 40° and / or steep sidewalls with sidewall angles less than 20° and / or large draw depths greater than 50 mm.

[0029] WO2018060082A1 discloses a method for manufacturing components from steel products coated with an Al-Si protective coating, comprising the following steps:

[0030] -Provide substrates composed of steel products coated with an Al-Si protective coating.

[0031] - The substrate is heated to temperature T1, so that the Al-Si protective coating is pre-alloyed only with the iron portion from the steel product.

[0032] - Cool the pre-alloyed substrate to room temperature.

[0033] - An anti-corrosion oil is applied to the surface of the pre-alloyed substrate, wherein the anti-corrosion oil is composed of a component containing fatty acid esters.

[0034] - Convey the pre-alloyed substrate coated with anti-corrosion oil.

[0035] -Heating the pre-alloyed substrate coated with anti-corrosion oil to temperature T2 allows the Al-Si protective coating to fully alloy with the iron from the steel product, removing the anti-corrosion oil without any residue.

[0036] - The reheated substrate is molded into a component.

[0037] EP3278895B1 discloses a steel sheet for hot forming, its production method, and hot-formed parts produced by this method. The steel sheet is a hardenable steel sheet comprising the following composition (mass percentage): carbon: 0.100%-0.600%, silicon: 0.50%-3.00%, manganese: 1.20%-4.00%, titanium: 0.005%-0.100%, boron: 0.0005%-0.0100%, phosphorus: 0.100% or less, sulfur: 0.0001%-0.0100%, aluminum: 0.005%-1.000%, and nitrogen: 0.0100% or less, as well as residual iron and impurities. The roughness of the steel strip satisfies Rz > 2.5 μm, and the oil coating on the surface has a concentration of 50 mg / m². 2 -1500mg / m 2 .

[0038] DE10000138A1 discloses a method and apparatus for preparing a molded part. In the method of preparing a molded part, a molded part material is deformed to contact a mold, wherein: after preforming the molded part material, until it substantially contacts the mold, a sliding medium is introduced between the molded part material and the mold at at least one point of contact between the molded part material and the mold, and then the molded part material is shaped into its final shape by molding pressure. In the apparatus for implementing this method, for this purpose, during the deformation process prior to achieving the final shape, a sliding medium is introduced between the molded material (3,12) and the mold (2,10,11) at at least one point where the molded material (3,12) abuts against the mold (2,10,11).

[0039] Vehicle manufacturers have put forward their own specifications and standards regarding the application and differentiation of the corresponding lubricants; it should be emphasized here that these specifications and standards concern conventional cold-formed parts and their further processing as cold-formed parts in the vehicle construction, and do not concern cold-formed parts that have undergone heat treatment as part of an indirect hardening method to achieve austenitization before further processing in the vehicle construction.

[0040] The German Association of the Automotive Industry (VDA) has developed recommendations for the use of lubricants in cold forming, publishing them as VDA 230-213. This standard specifies the quality requirements for the outermost coatings of uncoated and coated steel and aluminum materials used in body construction; according to VDA 230-213, these can be prelube, prelube 2, or hot melt adhesive. For re-limitations or extensions related to materials or applications, this standard generally also applies to sheet metal cleaning oils, stretching oils, and anti-corrosion oils.

[0041] Examples of pre-lubricating oils include Multidraw PL 61 from Zeller+Gmelin in Eislingen, Germany, and Anticorit PL 3802-39S from Fuchs in Mannheim, Germany. Examples of drawing oils (i.e., spot lubricants for local relubrication) include AP 167 / 22 from Hermann Bantleon GmbH in Ulm, Germany, and KTL N16 from Zeller+Gmelin in Eislingen, Germany. Generally, the total amount of all lubricating oils should not exceed 3 g / m³. 2 In special circumstances, the amount may be negotiated to be 4g / m 2 This applies to every location on the component and is not determined by an average value. Further limits on the (total) lubricant quantity specified in the sheet metal identification card, technical delivery specifications, or component drawings must be prioritized accordingly. The application method and layer weight or lubrication level of the product must be selected in such a way that, on the one hand, uniform wetting protects the closed surfaces of the component, but on the other hand, excessive layer thickness does not cause obstruction or failure during processing (e.g., oil vapor during bonding, welding, etc.). It should be noted that in each case, all effects must be considered and evaluated for the resulting performance (e.g., lower layer weight may negatively impact corrosion resistance and friction behavior, while higher layer weight may lead to undesirable oil leakage in stored rolls and may negatively impact compatibility and removability with adhesives).

[0042] For example, Zeller+Gmelin produces a drawing oil called MULTIDRAW KTL N 16, which is immiscible with water and contains synthetic lubricants as well as phosphorus- and sulfur-containing additives. Its technical specifications are: according to DIN 51 757, density is 900 kg / m³ at 15°C. 3 According to DIN 51 562, the viscosity at 40°C is 160 mm. 2 / s, flash point >200℃ according to DIN ISO 2592. For light molding, use no more than 1.5g / m². 2This type of oil should be used at a rate not exceeding 3g / m during recrystallization. 2 This kind of oil.

[0043] Such stretching oils also have a corrosion-inhibiting effect, but are suitable for lubrication of the forming area, especially with additives.

[0044] Before hardening, the part blanks that are cold-formed indirectly (and usually not cleaned for cost reasons) are heated to above Ac3 (e.g., to 900°C) during the austenitization process in the furnace.

[0045] During austenitization in a furnace, the oil is heated and burns / decomposes, which can result in a very noticeable undesirable surface effect known as "elephant skin," which remains visible even after cleaning. This is likely caused by localized oil buildup during heating, such as in a frying pan, where increased oxide formation occurs on the previously zinc-coated Zn-Fe surface in areas with a thinner oil layer. Summary of the Invention

[0046] The purpose of this invention is to create a method for preparing hardened steel parts with superior surface quality from galvanized or alloy-galvanized steel strips.

[0047] This objective is achieved by a method including the features of claim 1. Other advantageous embodiments correspond to the features of the dependent claims.

[0048] The inventors have discovered that not all lubricants work in the same way. For example, drawing oils, obviously due to their additives, are more effective at lower concentrations (especially those greater than 2 g / m³). 2 It already produces very obvious undesirable elephant skin from the beginning, therefore, its content must be treated more strictly compared to pre-lubricating oils or steel mill lubricants used for corrosion protection.

[0049] However, sufficient lubrication is required in cold forming for the fabrication of critical components / areas.

[0050] According to the present invention, the amount of drawing oil is therefore limited to 0.1 g / m 2 -2g / m 2 .

[0051] Typically, the indicated amounts of pre-lubricating oil, stretching oil, and their combinations apply to each location of the component and are not considered as average values.

[0052] If satisfactory forming cannot be achieved using this amount of drawing oil and existing pre-lubricating oil, and drawing oil is not used in critical forming areas or the entire area, then the amount of pre-lubricating oil should be set to 1 g / m. 2 -3g / m 2Compared to existing technologies, this pre-lubrication amount is exceptionally high, but combined with reduced stretching oil, it ensures both good formability and a good surface quality with a low rate of elephant skin formation. Furthermore, during the heat treatment process to achieve austenitization, any amount of oil can be burned off or decomposed at the high temperatures prevalent in the furnace, thus advantageously ensuring that no residual oil can be introduced into subsequent processes.

[0053] For the purposes of this invention, coil lubricant or pre-lubricant is the lubricant defined in section 3.2.1, Pre-lubrication, of standard VDA 230-213. For the purposes of this invention, stretching lubricant, stretching oil, forming oil, or auxiliary lubricant is the lubricant defined in chapter 3.3, Pre-lubricants, of standard VDA 230-213. Therefore, the present invention particularly relates to a method for preparing hardened steel components, wherein a sheet blank is cut from a strip of hardenable galvanized or alloy-galvanized steel, the sheet blank is then cold-formed into a component blank, and then heated in at least some areas to a temperature that causes the structure to change to austenite, wherein the component blank, which has already been austenitized in at least some areas, is supplied to a forming hardening tool, the component blank being held in the forming hardening tool in a form-fitting manner by an upper tool and a lower tool having a shape substantially consistent with the component blank, wherein heat is rapidly extracted from the steel material by contact with the material of the component blank against, in particular, the cooled upper and lower tools, and martensitic hardening occurs due to cooling at a rate higher than the critical cooling rate, wherein, prior to the cold forming, a coil lubricant is applied to the galvanized steel strip, and an additional lubricant is applied to at least some areas of the sheet blank cut from the steel strip, the additional lubricant being a stretching lubricant at a dosage of 0.1 g / m 2 -2g / m 2 .

[0054] This invention relates in particular to a method for preparing hardened steel components, wherein a sheet blank is cut from a galvanized or alloy-galvanized steel strip of hardenable alloy steel, the sheet blank is then cold-formed into a component blank, and heated in at least some areas to a temperature that at least causes the structure to transform to austenite. The component blank, already austenitized in at least some areas, is then supplied to a forming hardening tool, the component blank being held in the forming hardening tool by form-fitting an upper tool and a lower tool having a shape substantially consistent with the component blank. Heat is rapidly extracted from the steel material due to the contact between the component blank material and, in particular, the cooled upper and lower tools, and martensitic hardening occurs due to cooling at a rate higher than the critical cooling rate. Prior to the cold forming, a coil lubricant is applied to the galvanized or alloy-galvanized steel strip, and an additional lubricant, using a stretching lubricant at a dosage of 0.1 g / m², is applied to at least some areas of the sheet blank cut from the steel strip. 2 -2g / m 2 .

[0055] According to another embodiment, the amount of additional lubricating oil is 0.1 g / m. 2 -1.5g / m 2 .

[0056] According to another embodiment, the amount of coil lubricant exceeds 0.5 g / m. 2 And preferably exceeding 1.0 g / m 2 Especially preferably, exceeding 1.5 g / m 2 .

[0057] According to another embodiment, the ratio of coil lubricant to additional lubricant is 3:1-1:2, preferably 2:1-1:1, depending on the molding setting. This ratio initially shifts towards favoring the additional lubricant as the critical molding area increases, until the amount of additional lubricant reaches 2 g / m². 2 Then it switches to a lubricant that is beneficial to the coil. Advantageously, this further aids in molding without creating an "elephant skin".

[0058] According to another embodiment, the total amount of lubricating oil from the coil lubricating oil and the auxiliary lubricating oil does not exceed 4 g / m. 2 This allows for more targeted prevention of negative impacts on the surface.

[0059] According to another embodiment, additional lubricant is applied only to key molding areas, while the remaining areas are lubricated with roll lubricant. The additional lubricant in some areas is set at 2 g / m². 2 Or less, preferably 1.5g / m 2Or less, while the coil lubricant is set at 2g / m 2 Or less.

[0060] According to another embodiment, when the entire surface is covered with additional lubricating oil, the amount of additional lubricating oil on the entire surface is set to 2 g / m. 2 Or less, especially 1.5g / m 2 Or less, especially 1g / m 2 Or less, while the coil lubricant is set at 1.5 g / m. 2 or more, especially 2g / m 2 Or less.

[0061] Another implementation provides that the additional lubricant is applied directly to the coil lubricant. This eliminates the need for cost-intensive and labor-intensive cleaning processes, namely any cleaning of the coil lubricant after the coil is transported from the steel manufacturer to the processor.

[0062] In another implementation, the auxiliary lubricant and the coil lubricant have different chemical compositions. This allows for an optimal combination of corrosion protection, typically the main component of the coil lubricant, and improved molding performance, provided by the auxiliary lubricant.

[0063] In another embodiment, the zinc-plated or alloy-galvanized coating has a layer thickness of 5 μm-20 μm on each side. This improves corrosion protection.

[0064] According to another embodiment, the galvanized or alloy galvanized coating is applied by hot-dip galvanizing, particularly hot-dip galvanizing. This method ensures a simple and cost-effective coating process.

[0065] According to another embodiment, the steel strip is made of hardenable alloy steel, particularly boron-manganese steel, and is especially preferably made of 22MnB5, 20MnB8 or 34MnB5.

[0066] According to another embodiment, a steel plate component or steel plate billet having the following composition (all values ​​are in wt%) is used:

[0067] The carbon content is at most 0.4%, preferably 0.15-0.3%.

[0068] Silicon content is at most 1.9%, preferably 0.11-1.5%.

[0069] Manganese content: maximum 3.0, preferably 0.8-2.5%.

[0070] Chromium content is at most 1.5%, preferably 0.1-0.9%.

[0071] The molybdenum content is at most 0.9%, preferably 0.1-0.5%.

[0072] Nickel at most 0.9,

[0073] The maximum concentration of titanium is 0.2, with a preferred concentration of 0.02-0.1.

[0074] Vanadium at most 0.2

[0075] Tungsten at most 0.2,

[0076] The aluminum content should be at most 0.2, preferably 0.02-0.07.

[0077] Boron content is at most 0.01, preferably 0.0005-0.005.

[0078] Sulfur content maximum 0.01, preferably maximum 0.008.

[0079] Phosphorus content is at most 0.025, preferably at most 0.01.

[0080] Residual iron and impurities. Attached Figure Description

[0081] This invention will be explained through embodiments and accompanying drawings; in the drawings:

[0082] Figure 1 This is a schematic diagram of the standard process for indirect molding and hardening;

[0083] Figure 2 This is a schematic diagram of full-surface coil lubricant and partial surface lubricant applied with subsequent stretching lubricant;

[0084] Figure 3 This is a schematic diagram of the amount of lubricating oil according to the present invention and the prior art;

[0085] Figure 4 The surface areas of two hardened components are shown, with and without undesirable surface effects (obvious "elephant skin");

[0086] Figure 5 This illustrates a comparison of different manufacturing methods for components and the potential problems they may present with surface effects. Detailed Implementation

[0087] This invention utilizes the effect that, if the amounts used are matched, the combination of coil lubricant and stretching oil allows for sufficient forming lubrication without creating an overly noticeable elephant skin effect. Surprisingly, even when the total amount of coil lubricant and stretching oil is greater than in the prior art ( Figure 3 This is also possible because the coil lubricant and stretching oil produce a synergistic effect (not described here), which promotes the surface effect.

[0088] This also eliminates the need for cleaning the coil or sheet blank after transport and before cold forming, a process that is both cost- and labor-intensive.

[0089] Figure 1 The basic sequence is shown. Clearly, cold-rolled steel strip is formed into coils at the steel mill. These coils are coated with a coil lubricant. For the purposes of this invention, the coil lubricant is a lubricant applied to the entire steel strip primarily for corrosion protection. The coil is then cut into slab blanks or slab blanks are cut from the coil. As needed, i.e., in cases where critical components or areas are being formed, these slab blanks are coated with an additional lubricant for forming. According to the invention, the additional lubricant is a lubricant applied and designed for cold forming. This additional lubricant can coat the entire surface or only a portion of the surface.

[0090] Then cold forming is performed, especially in combination with deep drawing, shaping and / or post-forming.

[0091] The component blanks prepared in this way are no longer flat but three-dimensional compared to sheet blanks, and are then heated and molded to harden.

[0092] Figure 2 The diagram shows a sheet blank still adhering to coil lubricant, which is also coated with additional lubricant. However, the additional lubricant is applied only to areas that are subjected to high-intensity forming, while other areas are adequately lubricated with coil lubricant. Such sheet blanks are then formed, particularly deep-drawn.

[0093] The blanks produced in this way are no longer flat but three-dimensional compared to sheet metal blanks, and are then heated and hardened in the same way.

[0094] This is particularly advantageous for molding processes where the deformation degree is high to very high in certain areas and the deformation degree distribution is relatively uneven. In such cases, the additional lubricant in certain areas can be set to 2 g / m. 2 Or less, preferably 1.5g / m 2 Or less, while the coil lubricant is set at 2g / m 2 Or less.

[0095] The purpose of this setting is to ensure that the total fuel volume does not exceed 4g / m³. 2 Preferably, the ratio of coil lubricant to additional lubricant is set to at least 3:1 to 1:2, depending on the degree of deformation. As the degree of deformation increases, this ratio shifts to favor the additional lubricant.

[0096] However, according to the present invention, the sheet blank can also be coated with an additional lubricant on its entire surface before being formed. This is particularly advantageous in forming processes where zinc abrasion and / or zinc adhesion occur in the areas of the steel plate supports and / or tension ribs.

[0097] Figure 3 This demonstrates the difference from existing technologies. In existing technologies, coil lubricants are limited to 1 g / m². 2The amount of additional lubricating oil used is typically 2.5g / m. 2 According to the present invention, the additional lubricating oil is set at 0.1 g / m. 2 -2g / m 2 This is less than the actual amount required for stretching lubrication. Surprisingly, coil lubricant can also be increased to over 1 g / m. 2 Especially those exceeding 1.5g / m 2 That is, the amount exceeds the amount required for corrosion protection, and additional lubricating oil is added as previously adjusted to 2g / m. 2 Or less, that is, less than the amount actually required for stretching lubrication purposes, so the combination of coil lubricant and auxiliary lubricant even exceeds the total amount in the prior art without negative surface effects.

[0098] This invention thus cleverly utilizes the synergistic lubrication effect.

[0099] Figure 4 Examples of negative surface effects are shown; the image on the left clearly shows the "elephant skin" surface effect, while the image on the right shows a normal part.

[0100] Figure 5 Various methods are clearly illustrated, among which it is evident that indirect methods (i.e., cold forming followed by a heat treatment austenitizing step) combined with galvanized or alloy-galvanized parts may result in the surface problems described above, which can be reduced or prevented according to the present invention.

Claims

1. A method for preparing hardened steel components, wherein, Thin sheet blanks are cut from galvanized or alloy-galvanized steel strips of hardenable alloy steel, and then cold-formed into component blanks. These blanks are then heated in at least some areas to a temperature that at least causes an austenitic transformation of the structure. The component blanks, already austenitized in at least some areas, are then supplied to a form-hardening tool. The component blanks are held in the form-fitting tool by upper and lower tools having shapes substantially consistent with the component blanks. The material of the component blanks abuts against the cooled upper and lower tools, and martensitic hardening occurs due to rapid heat extraction from the steel material by cooling at a rate higher than the critical cooling rate. The method is characterized in that, prior to cold forming, the galvanized or alloy-galvanized steel strip is coated with a coil lubricant, and at least some areas of the thin sheet blanks cut from the steel strip are coated with an additional lubricant, the additional lubricant being 0.1 g / m²-2 g / m². 2 Stretching lubricant.

2. The method according to claim 1, characterized in that, The amount of the additional lubricating oil is 0.1 g / m. 2 -1.5g / m 2 .

3. The method according to claim 1, characterized in that, The amount of the lubricating oil in the coil exceeds 0.5 g / m².

4. The method according to claim 1, characterized in that, The total amount of lubricating oil, consisting of coil lubricating oil and auxiliary lubricating oil, shall not exceed 4g / m².

5. The method according to any one of claims 1-4, characterized in that, When all surfaces are coated with additional lubricant, the additional lubricant for all surfaces is set at 2 g / m² or less, while the lubricant for coils is set at 1.5 g / m² or more.

6. The method according to any one of claims 1-4, characterized in that, The additional lubricant is applied directly to the coil lubricant.

7. The method according to any one of claims 1-4, characterized in that, The additional lubricating oil and the coil lubricating oil have different chemical compositions.

8. The method according to any one of claims 1-4, characterized in that, The zinc-plated or alloy zinc-plated coating has a layer thickness of 5μm-20μm on each side.

9. The method according to any one of claims 1-4, characterized in that, Zinc plating or alloy zinc plating is applied by hot-dip coating.

10. The method according to any one of claims 1-4, characterized in that, The steel strip is made of hardenable alloy steel.

11. The method according to any one of claims 1-4, characterized in that, Use steel plate components or steel plate blanks with the following composition (all values ​​are in wt%): Carbon content is at most 0.4%. Silicon up to 1.9 Manganese at most 3.0 Chromium up to 1.5 Molybdenum at most 0.9 Nickel at most 0.9, Titanium is at most 0.2 Vanadium at most 0.2 Tungsten at most 0.2, Aluminum at most 0.2 Boron at most 0.01 Sulfur content up to 0.01 Phosphorus up to 0.025 Residual iron and impurities.

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