Method for producing a hardened steel sheet part

Abstract

The invention relates to a method for producing a hardened steel sheet part, in particular to a method for producing a hardened steel sheet part by means of a so-called indirect process, for which the applicant has also coined the term "press-hardening". The object of the invention is to establish a relationship between the sheet thickness, the coating weight and the part blank delivery temperature in order to determine the free cooling time (FAZ) which can be used in each case for parts produced in this way.

Description

Technical Field

[0001] This invention relates to a method for producing hardened steel plate components.

[0002] In particular, the present invention relates to a method for producing hardened steel sheet components by a so-called indirect process, for which the applicant has coined the term "form hardening". Background Technology

[0003] It is known that high-strength components can be produced from steel sheets through hardening, for example, in vehicle manufacturing. In particular, such components are obtained from the austenitic state through quenching hardening or transformation hardening.

[0004] Quenching hardening refers to heating a product made of hardenable steel to a temperature above the so-called Ac3 point of the steel, at which the microstructure exists essentially or entirely in the form of austenite or γ-iron, and then quenching it at a cooling rate above the so-called critical cooling rate of the steel, so as to transform the austenite into martensite and thereby achieve high tensile strength.

[0005] In this case, the steel plate material used is therefore a steel that can be hardened by quenching, such as boron-manganese steel of 22MnB5, 34MnB5 or 20MnB8 alloy.

[0006] To produce this type of hardenable steel sheet material, steel strip is manufactured through the following steps: melting a hardenable steel alloy, casting the alloy into a slab (usually using a continuous casting process), hot-rolling the resulting slab in a hot-rolling strip mill, and optionally subsequently cold-rolling the strip in a cold-rolling mill. The steel strip is typically hundreds to over a thousand meters long, a few millimeters or less than one millimeter thick, and is wound into what are called coils or strip coils.

[0007] During hot and cold rolling, the slab thickness is significantly reduced, while its length increases accordingly. However, the width of the slab remains essentially unchanged through appropriate measures. The reduction in thickness does not constitute forming in the sense of this disclosure.

[0008] Such hot-rolled or cold-rolled strips are typically galvanized on a galvanizing line. For this purpose, the strip is unwound from the steel coil and rewound after galvanizing. Galvanizing can be performed by hot-dip galvanizing, electrolytic galvanizing, or PVD coating. In the context of this invention, galvanized or zinc-alloy steel strip or sheet is a steel strip or sheet blank having a zinc coating or a zinc-based alloy coating.

[0009] "Zinc-based" means that zinc is the largest component in the coating, especially when it accounts for more than 80% of the coating weight.

[0010] In addition to the Z coatings (such as Z40, Z60, Z80, Z100, Z120, Z140, Z160 or Z180) according to DIN EN 10346, zinc-iron coatings may also be specified, for example, with an iron content of 8 to 12% by weight, the balance being zinc. The iron content can advantageously be within this range to optimize heating performance. According to one option, the coating is selected from the ZF60 to ZF180 specifications according to DIN EN 10346. ZF60 is approximately 25 g / m² per side. 2 The zinc-containing iron coating already provides a certain degree of corrosion protection and allows for further processing. Based on corrosion protection requirements, each side has a coating strength of up to 90g / m². 2 A larger coating weight may be advantageous.

[0011] These hot-rolled or cold-rolled strips may also have an aluminum-silicon coating, or they may be processed without a coating.

[0012] To produce sheet steel components, steel strip is uncoiled from a coil and further processed, or sheets called billets are cut or punched from the uncoiled strip. These billets are therefore flat steel sheets with defined profiles. To produce hardened sheet steel components, the steel strip or billets can be further processed in two different ways.

[0013] In the so-called direct process (also known as compression hardening or die hardening), the billet is austenitized by heating (usually in a furnace) and then typically hot-formed into a sheet metal part in a one-step process in a relatively cool die (compression hardening die). Once hot forming is complete and the die is fully closed, the die surface adheres to the sheet metal part. Thus, heat dissipates into the normally cooled die at a rate above the critical cooling rate, thereby achieving the aforementioned hardening by transforming into a martensitic microstructure.

[0014] In compression molding, blanks with aluminum-silicon coatings, galvanized blanks, or blanks without coatings are typically used.

[0015] When die-hardening is performed on galvanized billets with higher coating weights, especially when the coating grade is ≥Z100 or ZF100 according to EN 10346 and the double-sided coating weight of zinc or zinc-iron alloy is 100 g / m² under three-point testing,... 2 The billet must be pre-cooled to a temperature between 450°C and 600°C before forming. This is done to avoid unwanted microcracks in the steel matrix due to so-called liquid metal embrittlement (LME), and also to avoid excessive zinc adhesion in the forming mold (caused by zinc vapor sublimation), during which zinc vapor evaporates from the liquid zinc phase through open surfaces.

[0016] Because only one die can be used for forming and hardening during the compression molding process, final trimming (i.e., final trimming of the outer edges and hole edges) can only be performed after the formed and hardened part has been removed from the die. Final trimming is done in an additional process step, and due to the extremely high strength of the part, it is typically not done through trimming and punching dies for wear reasons, but rather through laser cutting. Final trimming is time-consuming and expensive, especially when the trimming length is long, there are many holes, the cycle time is short, or the production volume is high, with multiple parts produced, and a large number of hardened parts.

[0017] In so-called indirect processes (also known as form hardening processes or form hardening (FH)), steel strip or billets are formed into part blanks in a cold state through a series of forming and trimming processes, typically multi-stage (usually primarily through a combination of drawing, stretching, trimming, and / or shaping, collectively referred to herein as cold forming (CF)), in a manner known per se. Cold forming is typically carried out in a set of cold forming dies (collectively referred to herein as cold forming dies), for example in a stamping production line with five forming presses and so-called station dies, or in a multi-station press with so-called transfer dies. The part blank, which has been fully formed and finally trimmed in the cold state, is then typically austenitized in a furnace and, while still hot, inserted into a die that is typically cooled (form hardening die). The part blank is usually placed on a spring-loaded support device, which is positioned within the form hardening die and moved by the upper part of the press, also known as an ejector. Placing the part blank on the ejector prevents uncontrolled and uneven cooling of the part blank before the form hardening die closes. During the closing process, the forming hardening die typically ceases to perform any actual shaping on the part blank. Once the die is fully closed, the die surface adheres to the surface of the part blank and dissipates heat from the part blank at a rate exceeding the critical cooling rate, thereby hardening the part blank into the part. After cooling, the part is removed, at which point it becomes the completed hardened steel part.

[0018] In form hardening, galvanized billets are typically used, although uncoated billets can theoretically be used. However, billets with aluminum-silicon coatings cannot be form hardened because the aluminum-silicon coating is very brittle at room temperature and may peel off undesirably during cold forming into part blanks.

[0019] In form hardening, trimming (laser) is usually no longer necessary in the hardened state, as the final trimming is done entirely during the cold forming process.

[0020] In both cases, namely in the direct process and the indirect process, the product is a hardened, final-trimmed steel plate component. In the direct process, the final trimming is performed after hot forming and hardening, i.e., on the hardened component, while in the indirect process, it is performed as part of the cold forming of the component blank, and therefore before hardening.

[0021] In both cases, from its pre-heating state until insertion into the corresponding mold, the steel undergoes several volumetric or length changes in all spatial directions. This is particularly important for component blanks in indirect processes. These length changes can, in principle, be described based on dilatometer curves, i.e., based on the temperature-length change curve of the steel sample used, for example, according to... Figure 1 As shown: In the furnace, thermal expansion occurs first as the temperature is gradually increased until austenitization begins (a); shrinkage occurs during the transformation of the microstructure to austenite (b); thermal expansion occurs again during further heating until the furnace is removed (c); and after the furnace is removed, i.e. during the transfer from the furnace to the forming and hardening mold, corresponding shrinkage occurs during cooling to the corresponding mold entry temperature (d).

[0022] In the context of this invention, in an indirect process, the mold entry temperature refers to the average temperature of the component blank, weighted by its surface area, after it has been inserted into the mold at the moment the mold is just closed (i.e., at the start of cooling). The transfer between the furnace and the mold typically takes place in ambient air at ambient temperature. The cooling of the component blank from the furnace exit temperature to the corresponding mold entry temperature depends particularly on radiative heat loss, and also on heat losses caused by forced convection and free convection. This heat loss, in turn, depends particularly on the component blank's furnace exit temperature (i.e., the temperature at which the component blank leaves the heated furnace chamber shortly before the furnace door begins to open), the so-called "free cooling time," and the plate thickness, and may also additionally depend on surface emissivity, the geometry of the component blank (which exhibits a self-radiative effect that delays cooling compared to the blank), the airflow during the transfer process, and the ambient temperature.

[0023] Based on the ratio of length at the entry temperature to length before heating begins, it is possible to determine in the indirect process how much the cold-forming die used to produce the part blank must be scaled down—that is, made smaller in all spatial directions—so that the part blank produced in the cold-forming die, after heating in the furnace and subsequently cooling during transfer from the furnace to the form-hardening die, fits into the form-hardening die when the press closes. It must be taken into account that the form-hardening die must, in turn, be scaled up relative to the desired final geometry of the part at room temperature—that is, made larger in all spatial directions.

[0024] As mentioned above, no final trimming is performed after hardening in indirect processes. Therefore, to ensure dimensional accuracy of parts in indirect processes, the austenitic part blank must be inserted into the form-hardening die at the correct temperature and thus the correct size (i.e., appropriately), or hardened at that temperature when the form-hardening die is closed. If the die temperature is too high, and the part blank is therefore too large, the outer trim profile of the hardened part is typically too large, and the hole pattern is correspondingly distorted. If the die temperature is too low, and the part blank is therefore too small, the outer trim profile of the hardened part is typically too small, and the hole pattern is correspondingly distorted. Inserting part blanks that are too large or too small into the form-hardening die can also cause other problems. For example, if the part blank tilts on the centering pin, or if sensitive areas of the part blank (typically, especially the sidewall areas) are undesirably re-formed, it can lead to so-called part misalignment or a so-called double-plate condition, subsequently resulting in production interruptions, die damage, or even die breakage.

[0025] Furthermore, for galvanized steel sheets, since the sheet undergoes a re-forming operation while still hot during the closing of the form-hardening die, part blanks inserted into the form-hardening die in an excessively large or particularly small state may develop undesirable microcracks in the steel matrix due to so-called liquid metal embrittlement (LME).

[0026] Furthermore, when a blank part with an unsuitable mold entry temperature is inserted, further dimensional deviations, particularly sidewall angle deviations and twisting, may occur due to the resulting change in local cooling conditions within the mold (especially in the positive and negative radius regions).

[0027] EP 1 651 789 B1 discloses a method for producing hardened parts from steel sheets, wherein the parts are cold-formed in a cold-forming die and subsequently transferred to a forming hardening die. In this case, the part's entry temperature is 740°C to 910°C, preferably 780°C to 840°C. The preferred entry temperature can also be achieved when the annealing temperature of the cold-formed part is between 800°C and 850°C, because the part is galvanized, and the zinc plating prevents the part from cooling too quickly. The part is inserted into the forming die at high speed. During the transfer from the furnace to the forming hardening die, the part temperature decreases by about 10°C to 80°C, particularly by 40°C.

[0028] EP 2 177 641 B1 discloses a steel sheet component with a cathodic anti-corrosion coating, which is cold-formed and undergoes final trimming and the desired punching cuts or hole patterns before, during, or after cold forming. The component is cold-formed and trimmed, and the punching cuts and hole patterns are arranged so that the component to be hardened is 0.5% to 2% smaller than the finally hardened component.

[0029] EP 1 786 936 B1 discloses a method for producing form-hardened steel sheet components, wherein the components are heated to an annealing temperature above 780°C, particularly 800°C to 950°C, after cold forming. The components expand by 1%, resulting in a positive dimensional deviation of 0.2% after annealing and just before insertion into the form-hardening die. For this purpose, prior to annealing, the components are formed to be approximately 0.95% to 0.4%, particularly 0.8%, smaller than the desired final geometry in all three spatial axes. During the transfer into the form-hardening die, the component temperature decreases by 10°C to 80°C, particularly 40°C.

[0030] EP 2 655 674 B1 discloses a method for forming and hardening zinc-plated or zinc alloy-plated steel sheets, wherein, to avoid zinc adhesion to the forming die, the steel alloy used is configured for delayed transformation, such that forming is performed at a forming temperature of 500°C to 800°C and below the peritectic temperature of the iron-zinc system. The target forming temperature is between 450°C and 800°C, such that quenching hardening achieved by delayed transformation from austenite to martensite is performed at a temperature below 800°C. For forming, for example, a steel sheet with a thickness of 1.5 mm is used, wherein, prior to quenching hardening, the steel sheet is supplied to an intermediate cooling station and transferred to a forming hardening die as soon as possible after cooling. Intercooling is performed such that the forming temperature is between 450°C and 800°C, preferably between 450°C and 700°C, and more preferably between 450°C and 600°C. Summary of the Invention

[0031] The object of this invention is to create a method for producing hardened steel components that ensures the components have the desired dimensional accuracy and quality, and that the method has higher process reliability.

[0032] This objective is achieved by a method having the features described in claim 1.

[0033] Advantageous improvements are described in detail in the dependent claims of claim 1.

[0034] The inventors recognized that, contrary to the conventional assumption that the "free cooling time" should be kept as short as possible in indirect processes, it is necessary to have a minimum free cooling time, i.e., the lowest temperature at which the mold is inserted into the hardening die. The inventors further recognized that a maximum free cooling time is also necessary, and that these maximum values ​​may vary depending on the mechanical properties. For example, the bending angle in a bending test according to VDA 238-100 is more sensitive to large FAZ values ​​than Rp0.2, Rm, or elongation at break in a tensile test according to DIN EN ISO 6892-1.

[0035] The term "Free Cooling Time (FAZ)" refers to the time during which the blank of the part to be hardened is "freely" cooled in the surrounding environment (i.e., cooled only by thermal radiation and free and forced convection caused by the transfer operation) during the transfer from the furnace to the forming hardening die. FAZ is defined as the time period from the moment the furnace door begins to open to the moment the die closes. Full-surface die contact is typically present at the moment of die closure, which usually occurs at the bottom dead center of the press.

[0036] Unless otherwise stated, FAZ means FAZ. PROD This refers to the FAZ (French Alternating Zone) set during the production process. PROD Typically, it roughly corresponds to FAZ (i.e., within ±1 second). FRÄS This refers to the FAZ upon which the manufacture of cold forming dies is based. When the dimensions of the hardened part deviate from the target value, the FAZ... PROD It can be set to be compatible with FAZ FRÄS Slightly different.

[0037] Therefore, the free cooling time (FAZ) may already be affecting the manufacturing process of cold forming dies. Once the cold forming die for indirect processes has been scaled and milled and adjusted according to the entry temperature, the cost of correction is extremely high and time-consuming because all dies in the cold forming die group are affected. Therefore, it is important to correctly select the FAZ for parts with a given sheet thickness and plating weight. FRÄS Cold forming scaling (KU-SKAL) and cold forming die manufacturing are based on this free cooling time (FAZ). FRÄS For example, if FAZ FRÄS Too close to one of the FAZ limits, i.e., FAZ MIN or FAZ MAX In adverse circumstances, when adjustments are needed during production due to dimensional deviations (parts being too large or too small), it may be impossible to adjust the production FAZ (i.e., FAZ) within the required range within the FAZ limit. PROD This is used to eliminate dimensional deviations.

[0038] In particular, FAZ PROD It can be advantageously adapted to parameters such as ambient temperature, airflow, and surface emissivity of component blanks.

[0039] The cooling of austenitic part blanks during the transfer from the furnace to the form-hardening die depends not only on the part blank exit temperature, the part blank plate thickness, and especially the FAZ (freezing zone), but also on the surface emissivity of the part blank, the ambient temperature, and the flow conditions of the surrounding air (e.g., airflow). This means that cooling varies, particularly with the FAZ value, and the part blank may therefore have different temperatures at the moment the form-hardening die closes.

[0040] The mold entry temperature also affects the final microstructure and / or mechanical properties of the part, because cooling begins once the part is removed from the furnace. Furthermore, the mold entry temperature is crucial for the final dimensions of the finished part, and therefore also critical for dimensional accuracy.

[0041] The mold entry temperature is largely affected by the transfer time from the furnace to the form-hardening mold. Specific information regarding the transfer time and the resulting free cooling time is not typically provided. The transfer time is kept as short as possible, or the part is inserted into the form-hardening mold at high speed. This is based on the common assumption in the art that the transfer should be performed as quickly as possible.

[0042] The transfer time and the resulting free cooling time are also usually kept short because it is generally assumed that the unwanted zinc adhesion of galvanized parts in the cooling mold only occurs in the direct process involving hot forming, i.e., only in compression hardening, and not in the indirect process (form hardening) that does not involve hot forming.

[0043] Zinc adhesion affects component quality and can potentially damage cooling molds. If galvanized parts are affected by FAZ... PROD Inserting the die into the form-hardening mold with an excessively short length and excessively high temperature will result in severe mold contamination. In form hardening, this is not caused by wear of the hot-dip galvanized coating, but by the sublimation of zinc vapor evaporating from the zinc phase. The consequences of zinc deposits accumulating in the form-hardening mold include surface damage in the form of scratches, abrasions, and adhesion transfer on the hardened part, as well as downtime due to parts getting stuck in the mold, or the risk of mold breakage due to double-piece pressing caused by parts not being detected in time. Because of the need for downtime, the necessary periodic removal of zinc deposits reduces the productivity of the forming equipment.

[0044] On the other hand, excessively long FAZ PROD This can lead to abnormal microstructure and / or undesirable mechanical properties. This is because, if free cooling continues for too long, the critical cooling rate required to reliably achieve the desired microstructure and / or desired mechanical properties can no longer be met.

[0045] Furthermore, if FAZ PROD Too short or too long, or if the chosen FAZ FRÄS The FAZ is so close to the upper or lower limit that it can no longer be adjusted upwards or downwards when length deviations occur in mass production. PROD If this is not the case, the dimensional accuracy of the finished parts cannot be guaranteed.

[0046] The mold entry temperature is usually described in a general way, specifying a wide range of values. Important parameters such as plate thickness, component blank exit temperature, and plating weight are not taken into account.

[0047] Free cool-down time is not usually specified, and the lower limit of free cool-down time is only limited by the equipment, because transfer cannot be performed at arbitrarily high speeds.

[0048] The free cooling time is neither defined nor set according to the various influencing parameters, which may lead to various problems, such as undesirable microstructure and / or undesirable mechanical properties in the hardened part, zinc adhesion in the form-hardening die, damage to the form-hardening die and part, dimensional accuracy problems of the hardened part, and process uncertainties.

[0049] Adjusting the mold temperature by regulating the furnace temperature to adjust the part's dimensional accuracy is not easy to implement because the furnace reacts slowly and this method is relatively inaccurate.

[0050] Therefore, the inventors recognized that the dimensional accuracy of the components could be advantageously adjusted through free cooling time. They realized that this could be achieved faster, more efficiently, and more accurately than changing the furnace temperature.

[0051] According to the present invention, it has also been found that different plate thicknesses, different coating weights, and different component blank exit temperatures require different free cooling times.

[0052] The inventors further recognized that, contrary to the assumption that zinc adhesion only occurs in the direct process, i.e., during molding hardening, significant zinc adhesion can also occur during form hardening. This problem cannot be eliminated by mold coating and is based on the fact that strong zinc adhesion can still form on the mold even when no forming operation is performed on the part or only a small amount of forming operation is performed on the part, and therefore no openings are created on the hot zinc or zinc-iron surface due to the forming operation.

[0053] Therefore, the present invention relates to a method for producing hardened steel sheet components, wherein a steel sheet blank is cut from a steel strip made of a hardenable steel alloy and having a zinc-based coating, the steel sheet blank is then cold-formed into a component blank, the component blank is then heated in a furnace to a hardening temperature above the austenitizing temperature and subsequently pressed in a forming hardening die and thereby quenched and hardened. The component blank is inserted into the forming hardening die after heating at a defined entry temperature, wherein the entry temperature is controlled by a free cooling time FAZ. PROD Based on the plate thickness s (mm), the furnace exit temperature θ (°C) of the component blank, and the double-sided coating weight ZX (g / m²) of the zinc-based coating... 2 This is used to set the following relationships:

[0054] in

[0055]

[0056] in

[0057]

[0058] .

[0059] In one implementation, FAZ MAX Determined based on the following relationship: .

[0060] This can advantageously improve the mechanical properties of hardened steel plate components, especially the bending angle.

[0061] In one implementation, an initial free cool time (FAZ) is specified. FRÄS It serves as the base for producing cold forming dies. This facilitates the scaling of cold forming dies.

[0062] In one implementation, FAZ is specified. FRÄS With relative to FAZ MIN and FAZ MAX A reserve distance R, where R ≥ 1 second, and specifically, R ≥ 2 seconds. This reserve distance R advantageously facilitates possible adjustments within the range of production parameters, as the mold will not approach its limits from the outset. This allows for a wider production window.

[0063] In one implementation, the same FAZ is specified for different plate thicknesses s. FRÄS This simplifies the operation for equipment operators, allowing them to always set the same FAZ even for components with different plate thicknesses. PROD This does not require checking whether the setting is feasible on the device side.

[0064] In one implementation, FAZ is specified for different plate thicknesses s. FRÄS Select distance from FAZ MAX The same distance R is maintained. This allows for the part to be inserted as coldly as possible, thus requiring less heat to dissipate in the form-hardening mold. This minimizes wear on the form-hardening mold and enables the use of cheaper mold materials and simpler, cheaper mold cooling configurations.

[0065] In one implementation, the following relationship is specified to apply to plate thickness s: .

[0066] In one implementation, the following relationship applies to the furnace exit temperature θ of the component blank: .

[0067] In one implementation, the following relationship applies to the total double-sided coating weight ZX (g / m²) of the zinc-based coating. 2 ): .

[0068] In one implementation, it is specified that at the start of production, i.e., for each new production batch of the blank for the part to be hardened produced using the corresponding cold forming die set, FAZ PROD Roughly corresponds to FAZ FRÄS (i.e., within ±1 second), and, depending on the size of the hardened component, FAZ PROD Able to limit FAZ MIN and FAZ MAX Adjust within the specified range.

[0069] In one implementation, the dimensions of the hardened component are specified to be measured online, and in the event of deviation, the FAZ is checked. MIN and FAZ MAX Change FAZ online within the scope PROD This is to compensate for the dimensional deviation. This allows the equipment to operate as efficiently as possible and to minimize waste.

[0070] Furthermore, it is particularly advantageous if the closed-loop control system is automated, especially if it is operated using artificial intelligence.

[0071] In one implementation, the steel sheet used is specified as having a metallic anti-corrosion coating, particularly a zinc-based or zinc-made metallic anti-corrosion coating. Zinc-based coatings can be advantageously used to ensure cathodic protection of the steel components.

[0072] In one embodiment, the zinc-based coating is specified to contain 85% to 99% by weight of zinc, particularly 94% to 98% by weight of zinc, 0.2% to 2% by weight of aluminum, and unavoidable impurities.

[0073] In one implementation, the zinc-based coating is specified as a coating containing zinc and iron, wherein the iron content is 8% to 12% by weight and the balance is zinc.

[0074] In one embodiment, the steel strip is specified to be made of a hardenable steel alloy, particularly boron-manganese steel, especially preferably 22MnB5, 20MnB8 or 34MnB5.

[0075] Advantageous embodiments of the invention specify the use of transformation-delayed steel grades, wherein hot-rolled or cold-rolled steel strips are used, and the contents of the following alloying elements in the hot-rolled or cold-rolled steel strips are within the following limits (in weight %): Carbon: not exceeding 0.4%, preferably 0.15 to 0.3%, Silicon: not exceeding 1.9%, preferably 0.11 to 1.5%, Manganese: not exceeding 3.0, preferably 0.8 to 2.5. Chromium: not exceeding 1.5%, preferably 0.1 to 0.9%, Molybdenum: not exceeding 0.9, preferably 0.1 to 0.5. Nickel: not exceeding 0.9, Titanium: not exceeding 0.2, preferably 0.02 to 0.1. Vanadium: not exceeding 0.2%, Tungsten: not exceeding 0.2, Aluminum: not exceeding 0.2, preferably 0.02 to 0.07. Boron: not exceeding 0.01, preferably 0.0005 to 0.005. Sulfur: up to 0.01, preferably up to 0.008. Phosphorus: up to 0.025, preferably up to 0.01. The balance consists of iron and impurities. Attached Figure Description

[0076] The invention is explained by way of example with reference to the accompanying drawings. In the drawings: Figure 1 Exemplary steel hardening curves can be obtained by quenching.

[0077] Figure 2 The highly schematic depiction of the form hardening process includes cold forming (KU), continuous furnace (FURNACE), linear transfer from furnace to press (TRANSFER), and form hardening (FH), which is carried out in a quadruple production manner, with four pieces per batch, wherein the four pieces are arranged side by side with each other in the passing direction.

[0078] Figure 3 Zinc adhesion in the hardened die (lower die) after 4 strokes.

[0079] Figure 4 Zinc adhesion in the hardened die (lower and upper dies) after 600 strokes.

[0080] Figure 5 :according to Figure 4 An illustration of a local area of ​​a hardened part produced in a forming hardening mold.

[0081] Figure 6 Two microstructure images are used for comparison. One image shows the correct martensite content according to the invention, and the other image shows the microstructure after a free cooling time of FAZ. PROD Excessive length results in an undesirable ferrite content.

[0082] Figure 7 General FAZ PROD Process window.

[0083] Figure 8 Example FAZ PROD The process window is subject to additional constraints, on the one hand, by the equipment's shortest potential FAZ, and on the other hand, by the production FAZ, from which the cycle time is adversely affected.

[0084] Figure 9 According to the present invention, the method for selecting FAZ FRÄS The first variant, in which FAZ is used in different plate thicknesses FRÄS same.

[0085] Figure 10 According to the present invention, the method for selecting FAZ FRÄS The second variant, in which FAZ is used for different plate thicknesses FRÄS The settings have changed and are consistent with FAZ. MAX Maintain the same distance.

[0086] Figure 11 FAZ during production PROD Compared to the FAZ of milled cold forming dies FRÄS An exemplary adjustment, the initial manufacturing of the cold forming die is based on the FAZ. FRÄS of.

[0087] Figure 12 The coating weight is 140g / m 2 At that time, minimum free cooling time FAZ MIN and maximum free cooldown time FAZ MAX Different curves showing the variation of sheet thickness and billet tapping temperature with different plate thicknesses and components, and an exemplary selection of acceptable FAZ for a sheet thickness of 1.8 mm. PROD The illustration.

[0088] Figure 13 The table in the example contains the corresponding minimum free cool time (FAZ). MIN Maximum free cooldown time FAZ MAX and the free cooldown time (FAZ) selected in the corresponding test. PROD , and the results.

[0089] Figure 14 When potential dimensional deviations occur, adjust the FAZ during the production process. PROD One possible solution. Detailed Implementation

[0090] Figure 1 The dilatometer curve is shown, which is the temperature-length change curve of a steel sample hardened by quenching during heating and subsequent cooling. To determine this dilatometer curve, the sample was induction heated at a heating rate as close as possible to that of mass production, and cooled by blowing gas into the induction coil if necessary. The length change caused by the heating and cooling of the sample was detected by measuring pins and displacement sensors. The temperature of the dilatometer was controlled by thermocouples placed on the sample surface. The temperature-length change graph recorded throughout the measurement process, i.e., the dilatometer curve, is the measurement result. Based on the labeled dilatometer curve, it is possible to understand what length or volume changes the cold-formed part billet underwent in the furnace and subsequently during its transfer from the furnace to the forming hardening die until the forming hardening die was closed. In the furnace, thermal expansion occurs first until austenitization begins (a); contraction occurs during the transformation of the microstructure to austenite (b); thermal expansion occurs again during further heating until it is removed from the furnace (c); and after removal from the furnace, i.e., during the transfer from the furnace to the molding hardening or forming hardening die, corresponding contraction occurs during cooling to the corresponding entry temperature (d). Based on the ratio between the entry length (i.e., the length at the entry temperature) and the initial length (i.e., the length before heating), it is possible to determine how much the cold forming die used to produce part blanks must be reduced in size so that the part blanks can be fitted to the form-hardened die after being heated in the furnace and subsequently cooled during transfer.

[0091] Figure 2 The form hardening process is illustrated schematically in the upper image area, where a flat steel strip is unwound from a steel coil, a blank is then cut from the flat steel strip, and the flat blank is formed in cold forming (KU) (typically in a multi-stage forming and trimming process), with final trimming at the outer contour and hole pattern. The part blank produced in this manner is then fed into a furnace, removed from the furnace while hot, and inserted into a form hardening die, where the part blank is rapidly cooled by bringing the upper and lower dies into contact with the part to be formed.

[0092] For example Figure 2 As shown in the lower image area, the individual parts do not pass through the furnace sequentially. Instead, to increase output and make the process as economical as possible, multiple part blanks (in...) are... Figure 2 In the example, four pieces are fed into the furnace, then simultaneously removed from the furnace, and inserted into the forming hardening mold in the same arrangement according to the through direction (DLR) and hardened.

[0093] For galvanized or zinc alloy coated parts, an excessively short FAZ PROD Zinc adhesion may occur on the forming and hardening mold during the forming and hardening process. Figure 3 and Figure 4Zinc adhesion on the forming hardening die can lead to damage to the hardened part, as well as undesirable surface (NiO) defects in the form of scratches, abrasions, and adhesion transfer. Figure 5 ).like Figure 3 As shown, this can happen in just a few strokes and can become increasingly severe. Figure 4 ).

[0094] Furthermore, zinc adhesion in the forming hardened die can cause the hardened part to get stuck and remain in the die, potentially leading to double-piece pressing or even die breakage. Zinc adhesion is particularly prevalent in FAZ. PROD This occurs when the free cooling time is too short, for example, when the part is inserted into the forming and hardening mold as quickly as possible after annealing. If the free cooling time is too short, the mold entry temperature will be too high for a given coating weight, resulting in significant zinc adhesion.

[0095] On the other hand, if FAZ PROD If the time is too long, the critical cooling rate cannot be achieved. This results in undesirable mechanical properties and / or undesirable abnormal microstructure. Figure 6 ).exist Figure 6 On the right, you can see the percentage of undesirable ferrite, which is due to FAZ. PROD It is formed due to excessive length.

[0096] Since the free cooling time is particularly dependent on the plate thickness, it is advantageous to set the free cooling time according to the plate thickness used. Figure 7 ). Figure 7 A general process window is shown, with the plate thickness range from the lower limit of plate thickness s. u (e.g., 0.7mm or 1.0mm) to the upper limit of the plate thickness s o (e.g., 2.0mm or 2.8mm). If the cooling time is too short, zinc adhesion will occur, potentially leading to surface defects and production interruptions; while if the cooling time is too long, it may result in abnormal microstructure or cause parts to fail in mechanical properties. Furthermore, the process window for free cooling time depends on the coating weight (g / m²) of the zinc-based coating. 2 ) and the furnace exit temperature θ (°C) of the component blank.

[0097] The target range for free cooldown time can be determined by the following relationship: FAZ MIN ≤ FAZ PROD ≤ FAZ MAX in

[0098] 。

[0099] Therefore, in order to produce hardened steel sheet components with zinc-based coatings, a minimum FAZ (FAZ) greater than or equal to the specified value should be selected according to the formula above. MIN And less than or equal to the maximum FAZ (FAZ) MAX Free cooldown time FAZ PROD This free cooling time is effective within the following ranges: plate thickness s, coating weight ZX, and component blank exit temperature θ: 0.7mm ≤ s ≤ 2.8mm 870℃ ≤ θ ≤ 930℃, and 60g / m 2 ≤ ZX ≤ 200g / m 2 .

[0100] Figure 12 These calculated values ​​are illustrated as examples. In this case, for a 1.8mm plate (s=1.8mm) and 140g / m², 2 The coating weight (Z140) and the blank exit temperature θ of the component varied between 890℃ and 910℃. It can be seen that FAZ... MIN Line and FAZ MAX The curve shifts with θ. As the part blank temperature increases, the curve shifts to the right, i.e., towards a higher FAZ value. PROD Originally corresponding to FAZ FRÄS In this example, for a 1.8mm board thickness, FAZ FRÄS The time was chosen to be 14 seconds. This shows that a reserve distance R of more than 2 seconds was selected in all directions to allow for corrections to any changes in size.

[0101] Free cooling time can be constrained at its lower limit on the equipment side because transfer cannot be performed arbitrarily quickly. This must be taken into account, especially in slow-speed equipment. However, even in the case of fast-speed equipment, safe process windows can be determined for different plate thicknesses. Furthermore, arbitrarily long cooling times should not be chosen, as this can adversely affect cycle times. Figure 8 These further possible constraints are illustrated by example.

[0102] For different plate thicknesses, the FAZ selected for mold scaling FRÄS They can be set to the same value. It is advantageous to select this FAZ. FRÄS It should reliably ensure a minimum distance between itself and the upper and lower limits of the FAZ. This reserve distance R can be, for example, 2 seconds or 1 second.

[0103] Especially for different plate thicknesses, the same FAZ FRÄS It can simplify the process for production equipment operators, eliminating the need for them to check in advance which FAZ is set for which plate thickness and whether this can be achieved on the equipment side. Figure 9 An example of this is shown.

[0104] Alternatively, FAZ can be selected for different plate thicknesses. FRÄS To make the reserve distance R follow the FAZ MAX curve( Figure 10 In other words, the corresponding FAZ for each plate thickness. FRÄS Corresponding to the calculated FAZ MAX Subtract R. This advantageously ensures that the part is inserted as cold as possible, thus requiring less heat to dissipate in the forming harden mold. This minimizes wear and allows for the use of cheaper mold materials and simpler mold cooling configurations.

[0105] The length correction of the component to be hardened can be achieved by changing the free cooling time. Figure 11 Length deviations may occur during the transfer from the furnace to the forming and hardening mold due to fluctuations in ambient temperature. These fluctuations may be caused, for example, by seasonal variations (summer and winter) or airflow. The free cooling time can be flexibly adjusted by changing the robot speed (at which the part is inserted into the forming and hardening mold) or by incorporating pauses with waiting times into the robot's motion sequence. Accordingly, the free cooling time FAZ in production... PROD Within a safe FAZ process window (i.e., within the defined FAZ) MIN and FAZ MAX The free cooling time (FAZ) is the basis for the manufacture of cold forming dies. FRÄS Variations based on a benchmark. Specifically, when selecting the FAZ upon which the manufacture of the cold forming die is based. FRÄS At this time, a minimum change value can be specified, namely the so-called reserve R, for example by specifying FAZ. FRÄS It is possible to vary in both directions for more than 1 second or more than 2 seconds (R≥1 second or R≥2 seconds) without leaving the safe FAZ process window.

[0106] For example, if the part to be hardened is too short, shorten the free cooling time (FAZ). PROD If the component is too long, extend the free cooling time (FAZ). PROD Therefore, the production free cooling time FAZ PROD With the required initial FAZ FRÄS Variations are made based on a baseline and remain within the defined safety process window. However, for this purpose, FAZ...FRÄS It must be far enough away from the limit.

[0107] Figure 14 A possible process for correcting part length deviations is shown. The conventional process for producing hardened parts in a form hardening process is illustrated here. Initially, the cold forming die (KU) is milled to scale based on a specific entry size into the form hardening die and thus based on a specific FAZ, taking into account future thermal expansion in the furnace, shrinkage during the transfer process, and shrinkage after form hardening (FH), i.e., KU-SKAL based on FAZ. FRÄS In the first step, during production, the production of FAZ is... PROD Basically set to scaled FAZ, i.e., FAZ FRÄS FAZ PROD = FAZ FRÄS ± 1 second. If the part dimensions are incorrect (e.g., as determined as part of Quality Assurance (QS) using inspection gauges or optical measurements), then FAZ... PROD It can vary within the defined FAZ, that is, within the FAZ. MIN and FAZ MAX The variations between these parameters are also possible. As part of online 100% inspection, FAZ can also be performed online after the part dimensions are determined. PROD Any necessary adjustments.

[0108] Figure 13 Seven different embodiments are shown. Embodiments 1, 3, 5, 6, and 7 are embodiments of the present invention (“inv.”). Embodiments 2 and 4 are comparative examples.

[0109] The table shows the plate thickness s varying from 1.0 mm to 2.5 mm, the blank exit temperature θ varying from 890℃ to 910℃, and the coating weight ZX varying from 100 g / m². 2 Up to 140g / m 2 Change. FAZ MIN and FAZ MAX Calculate using the following formula:

[0110] 。

[0111] Depends on FAZ PROD Either a qualified part is produced according to the present invention, or a defective part is produced in a comparative example not of the present invention, wherein the reasons for the defect are explained. Therefore, the advantage of the present invention is that the flexible adjustment of the free cooling time ensures part quality, dimensional accuracy, and process reliability during the form hardening process.

[0112] By determining and setting a safe FAZ process window based at least on the plate thickness, zinc adhesion on the mold is avoided. Furthermore, this prevents the occurrence of undesirable microstructures or undesirable mechanical properties.

[0113] Compared to conventional methods (such as changing furnace temperature), the length correction and scaling of components can be set more easily by using free cooling time.

[0114] Furthermore, it is advantageous that energy savings are achieved by controlling the free cooling time, thus enabling a sustainable process.

Claims

1. A method for producing hardened steel sheet components, wherein a steel sheet blank is cut from a steel strip made of a hardenable steel alloy and having a zinc-based coating, the steel sheet blank is then cold-formed into a component blank, the component blank is subsequently heated in a furnace to a hardening temperature above the austenitizing temperature and then pressed in a forming hardening die and thereby quenched and hardened. Its features The component blank is heated and then inserted into the forming and hardening mold at a defined entry temperature, wherein the entry temperature is determined by a free cooling time (FAZ). PROD The values ​​are determined based on the plate thickness s (mm), the furnace exit temperature θ (°C) of the component blank, and the double-sided coating weight ZX (g / m²) of the zinc-based coating, wherein the following relationship applies: in in 。 2. The method according to claim 1, wherein FAZ MAX Determined based on the following relationship: 。 3. The method according to claim 1 or 2, wherein the initial free cooling time FAZ FRÄS Used as a base for producing cold forming molds.

4. The method according to any one of the preceding claims, wherein FAZ FRÄS With respect to FAZ MIN and FAZ MAX The reserve margin R, where R ≥ 1 second, and in particular, R ≥ 2 seconds.

5. The method according to claim 3 or 4, wherein the same FAZ is selected for different plate thicknesses s. FRÄS .

6. The method according to claim 4 or 5, wherein the FAZ is for different plate thicknesses s. FRÄS Select distance from FAZ MAX The same margin R.

7. The method according to any one of the preceding claims, characterized in that, The following relationship applies to the plate thickness s: 。 8. The method according to any one of the preceding claims, characterized in that, The following relationship applies to the furnace exit temperature θ of the component blank: 。 9. The method according to any one of the preceding claims, characterized in that, The following relationship applies to the total double-sided coating weight ZX (g / m²) of the zinc-based coating: 。 10. The method according to any one of claims 3 to 9, characterized in that, At the start of production, FAZ PROD Corresponding to FAZ FRÄS ± 1 second, depending on the size of the hardened part, FAZ PROD Able to limit FAZ MIN and FAZ MAX Adjust within the specified range.

11. The method according to any one of claims 3 to 10, characterized in that, The dimensions of the hardened component are measured online, and in the event of a deviation, the FAZ is checked. MIN and FAZ MAX Change FAZ online PROD This is to compensate for the dimensional deviation.

12. The method according to any one of the preceding claims, characterized in that, The zinc-based coating comprises 85% to 99% by weight of zinc, particularly 94% to 98% by weight of zinc, 0.2% to 2% by weight of aluminum, and unavoidable impurities.

13. The method according to any one of the preceding claims, characterized in that, The zinc-based coating is a coating containing zinc and iron, with an iron content of 8% to 12% by weight and the balance being zinc.

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

15. The method according to any one of the preceding claims, characterized in that, The steel strip is used with the following composition, where all values ​​are in weight percent: Carbon: not exceeding 0.4%, preferably 0.10 to 0.

30. Silicon: not exceeding 1.9%, preferably 0.11 to 1.5%, Manganese: not exceeding 3.0, preferably 0.8 to 2.

5. Chromium: not exceeding 1.5%, preferably 0.1 to 0.9%, Molybdenum: not exceeding 0.9, preferably 0.001 to 0.

1. Nickel: not exceeding 0.9, preferably not exceeding 0.

2. Titanium: not exceeding 0.2, preferably 0.02 to 0.

1. Vanadium: not exceeding 0.2%, Tungsten: not exceeding 0.2, Aluminum: not exceeding 0.2, preferably 0.02 to 0.

07. Boron: not exceeding 0.01, preferably 0.0005 to 0.

005. Sulfur: up to 0.01, preferably up to 0.008, and Phosphorus: up to 0.025, preferably up to 0.

01. The balance consists of iron and impurities generated during the smelting process.

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