METHOD FOR PRODUCING A HARDENED STEEL PRODUCT

Description

[0001] The invention relates to a method for producing coated hardened steel products, in particular for use as structural components of a motor vehicle.

[0002] It is well known to coat metallic components for corrosion protection and to convert them into molded parts by hot forming. For safety-relevant vehicle body components, for example, aluminum-silicon-coated high-strength and ultra-high-strength heat-treatable steels, especially manganese-boron-containing heat-treatable steels such as 22MnB5 or 34MnB5, are used in practice.

[0003] For example, WO 2009 / 090555 A1 discloses a method for producing a hot-stamped coated steel section, comprising the steps of: pre-coating a steel strip with aluminum or aluminum alloy by hot-dip coating, wherein the thickness of the pre-coating is 20 to 33 micrometers on each side, cutting the pre-coated steel strip into a steel section, heating the steel section in a furnace, transferring the heated steel section into a pressing tool, hot-stamping the steel section in the pressing tool, and cooling the steel section.

[0004] DE 10 2007 019 196 A1 discloses a process for producing flexibly rolled strip material with a cathodic corrosion protection coating. The strip material is coated at elevated strip temperature in a zinc pot (hot-dipped galvanized steel). The zinc coating is rolled off in the same ratio as the actual strip thickness, with the goal of achieving a final coating thickness of greater than or equal to 7.5 micrometers after flexible rolling.

[0005] EP 3 181 248 A1 discloses a method for producing a sheet metal blank, in which a strip material is flexibly rolled with a target thickness profile for sheet metal blanks to be cut from it. The blanks are cut out depending on at least two consecutive regions of a generated measured thickness profile. It is described that the strip material can be coated before or after the flexible rolling, with coating before the flexible rolling resulting in a different coating thickness along the length.

[0006] From US 2018 / 142 337 A1, the production of a hot-formed component is known which, after hot forming, has a diffusion layer with a thickness of 2 to 40 micrometers, preferably 2 to 10 micrometers.

[0007] From the publication Widmann M. et al.: "Formation of intermetallic phases in Alcoated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content" a 22MnB5 heat-treatable steel is known which is provided with an AS31405110 coating with a thickness of 24 to 35 micrometers.

[0008] WO 2008 / 113426 A2 discloses a method for producing a sheet metal component, in which a hot-rolled or cold-rolled strip is hot-dip coated or electrolytically coated and then subjected to a flexible rolling process. In the flexible rolling process, different thicknesses of the flexibly rolled steel strip are produced by varying rolling pressures. Depending on the sheet thickness after flexible rolling, the coating thickness is applied to varying thicknesses during coating. Depending on the rolling pressure, the coating thickness increases with the expected rolling pressure.

[0009] WO 2006 097 237 A1 discloses a method and system for hot-dip coating hot-rolled steel strip. The steel strip passes through a pickling station, a rinsing station, a drying station, a heating furnace, and then a molten bath. The finished thickness and thickness tolerance of the hot-dip coated steel strip are achieved by a controlled thickness reduction in a rolling stand in the process line. The finished thickness is monitored by a thickness gauge at the exit of the rolling stand, and deviations from the target thickness are fed back as a control signal to the rolling stand adjustment.

[0010] WO 2016 / 198186 A1 discloses a method for hot forming a steel component. The steel component is coated with a corrosion-resistant scale protection layer, and prior to hot forming, surface oxidation occurs, forming a corrosion-resistant oxidation layer on the scale protection layer.

[0011] EP 3 489 386 A1 discloses a method for producing a hardened component from a coated steel substrate. A first coating containing at least 85 percent aluminum by weight, with a thickness between 5 and 50 micrometers, preferably between 10 and 35 micrometers, is applied to the steel substrate. A nanocrystalline zinc-copper coating is then applied to the coated steel substrate.

[0012] WO 2017 / 017621 A1, corresponding to EP 3 329 029 B1, discloses a method for producing a phosphate-coatable part from a steel sheet with a metallic aluminum-based coating. A steel sheet is provided with a precoating containing 4.0 to 20.0 wt.% zinc, 1.0 to 3.5 wt.% silicon, optionally other alloying elements, each containing less than 0.3 wt.%, and the remainder being aluminum. The coating thickness is between 5 and 50 micrometers. The coated steel sheet is then hot-formed.

[0013] EP 2 393 953 B1 discloses a method for producing a coated steel component by hot forming. A flat steel product made from an alloyed heat-treatable steel is coated with an aluminum coating containing at least 85% aluminum by weight. The flat steel product coated with the aluminum coating is then coated with a zinc coating containing at least 90% zinc. A top layer is further applied, and the coated and top-coated flat steel product is then hot-formed.

[0014] The object of the present invention is to propose a method for producing a coated and hardened component, in particular as a structural component for a motor vehicle, which has good corrosion protection resistance in areas with different thicknesses.

[0015] The invention is defined in the appended claims.

[0016] The object is achieved by means of a method for producing a hardened steel product comprising the steps of: providing a steel substrate with a base material made of a hardenable steel; coating the steel substrate with an aluminum-containing precoating with at least 85 weight percent aluminum to produce a precoated steel substrate, wherein the coating of the precoated steel substrate has a thickness (d1) of at least 36 micrometers (µm);Flexible rolling of the pre-coated steel substrate such that successive sections of the pre-coated steel substrate are rolled to different thicknesses with rolling reductions of at least 1% and / or a maximum of 60%, starting from the initial thickness of the steel substrate (2), in order to produce a variable thickness over the length of the pre-coated steel substrate, wherein the pre-coating after the flexible rolling has a reduced first thickness (d2a) of less than 33 micrometers in thinner first sections and a reduced second thickness (d2b) of more than 33 micrometers in thicker second sections, which is thicker than the reduced first thickness (d2a); machining a blank from the flexibly rolled strip material; heating the blank such that the base material of the blank is at least partially austenitized, wherein diffusion processes take place between the base material and the pre-coating as a result of the heating;Hot forming the heated blank, wherein the heated blank is formed and cooled so rapidly that a hardened steel product with a coating is produced, wherein the coating is formed from the pre-coating by heating and hot forming, with a first final thickness in the thinner first sections which is greater than the reduced first thickness and is between 15 to 50 micrometers, and with a second final thickness in the thicker second sections which is greater than the reduced second thickness and is between 30 to 60 micrometers;

[0017] One advantage is that the substrate still has a sufficiently thick coating after flexible rolling. It has been shown that the coating increases during heating for subsequent hot forming due to diffusion processes, so that the final coating thickness after hot forming is greater than the respective coating thickness after flexible rolling and before heating for hot forming. Because the pre-coating has a thickness of at least 36 micrometers, the coating is sufficiently thick even in the thinner initial sections to achieve good corrosion protection due to the subsequent heating for hot forming, despite the thickness reduction that occurs during flexible rolling. In the thicker sections of the finished component, which generally have to withstand higher loads, the coating is also correspondingly thicker, so that these sections are particularly well protected.Overall, this results in a load-optimized and weight-reduced component with excellent coating protection in all thickness ranges.

[0018] The steel substrate can, for example, be a hardenable or temperable steel material, particularly one containing manganese. This can contain other microalloying elements in addition to manganese. The steel material can, for example, contain the following proportions of alloying elements, each in percent by weight: Carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0.4%; Manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%; Aluminium (Al) with at most 0.1%; Silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%; Chromium (Cr) with at least 0.01% and at most 1.0%; Titanium (Ti) with at most 0.02%, in particular at most 0.01%; Boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%; Phosphorus (P) with at most 0.1%, in particular at most 0.01%; Sulphur (S) with at most 0.05%, in particular at most 0.01%; Optionally, additional alloying elements with a content of up to 1.55% (1550 ppm); the remainder iron (Fe) and unavoidable impurities.

[0019] As optional additional alloying elements, the substrate may contain in particular at least one of the following in weight percent: Copper (Cu) with a maximum of 0.1%; nickel (Ni) with a maximum of 0.1%; niobium (Nb) with a maximum of 0.1%; molybdenum (Mo) with a maximum of 1.0%; vanadium (V) with a maximum of 0.25%; include, without being restricted to this. The mass fraction of the optional alloying elements can also be lower; for example, molybdenum can also be included at a maximum of 0.8%, 0.5% or 0.25%. The mass fraction of the optional alloying elements is a maximum of 1.55% in total, in particular a maximum of 1.0%, in particular a maximum of 0.8%. The alloying element niobium advantageously results in a fine-grained structure in a component hot-formed from the alloy. In particular, in combination with molybdenum, which can inhibit grain growth, a particularly fine-grained structure is produced, which in turn has a positive effect on the strength of the component produced from it.

[0020] Examples of usable boron-manganese-containing steel materials are 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5, or 34MnB5. The starting material (strip material) can have a tensile strength of, for example, at least 450 MPa. A molded part produced from the coated steel substrate can have a final tensile strength of, for example, at least 1100 MPa, in particular at least 1500 MPa. It is also possible for certain sub-regions of the molded part, where necessary, to be adjusted to a lower tensile strength of less than 1100 MPa and thus higher ductility. The steel substrate can have an initial thickness of, for example, between 1.0 and 4.0 mm.

[0021] The coating contains at least 85% by weight of aluminum, which includes the possibility of using a pure aluminum coating (100% by weight Al), as well as the use of an alloy which contains aluminum as the main alloying component with at least 85% by weight and optionally other alloying components, for example silicon with, for example, between 5 and 15% by weight and / or iron with up to 5% by weight and / or one or more other alloying elements in smaller proportions. The proportion of the other alloying elements, for example at least one from the group consisting of Mn, Cr, Ti, B, P, S, Cu, Ni, Nb, Mo and V, can together amount to, for example, up to 1.5% by weight. In the context of the present disclosure, the term aluminum coating or aluminum-based coating is generally used due to the main component being aluminum, which also encompasses the aforementioned possibilities of other.Alloy compositions are to be included conceptually. The aluminum coating can be applied to the steel substrate, for example, by hot-dip plating in a molten bath containing at least 85 percent aluminum by weight and optionally other alloying components, or by other conventional coating processes. An exemplary composition of the molten bath or the applied coating can contain up to 3 percent iron by weight, 9 to 12 percent silicon by weight, optionally one or more other alloying elements totaling up to 1.5 percent by weight, and the remainder aluminum. It is understood that unavoidable impurities may also be present.

[0022] The precoating is applied to the steel substrate with a thickness (d1) of at least 36 micrometers, in particular at least 40 micrometers. The precoated steel substrate forms the basis for a hardened component with variable thicknesses to be produced from it. The coating of the steel substrate can be applied, for example, by hot-dip coating, whereby the steel substrate is immersed in a tank of molten coating material. It is understood that other known coating methods can also be used.

[0023] After pre-coating, the pre-coated steel substrate is flexibly rolled. It is understood that further steps such as heating, coiling, uncoiling, straightening, cleaning, or the like may be interposed. Optionally, it can be provided, in particular, that the steel substrate is heated in the coating system after application of the pre-coating in order to achieve pre-diffusion between the pre-coating and the steel substrate. Pre-diffusion heating is carried out at temperatures below the melting temperature of the coating material, for example, in a temperature window between 0.5 and 0.9 times the melting temperature of the coating material. Pre-diffusion causes a thicker interdiffusion zone to form between the base material of the steel substrate and the coating material during the coating process.This makes it possible to heat the parts more quickly during hot forming, which has an overall positive effect on the cycle times during hot forming.

[0024] In flexible rolling, strip material with a substantially uniform sheet thickness is rolled out into strip material with variable sheet thickness along its length by varying the roll gap during the process. The sections of varying thickness created by flexible rolling extend transversely to the longitudinal direction or rolling direction of the strip material. After flexible rolling, the strip material can be easily rewound into a coil and sent elsewhere for further processing, or it can be processed directly, for example, by cutting the strip material to length into individual sheet elements.

[0025] Flexible rolling is carried out with reductions of at least 1% and / or a maximum of 60% based on the initial thickness (d1) of the pre-coated steel substrate, in particular with reductions between 3% and 55%. Flexible rolling reduces the thickness of the pre-coating along with the steel substrate. In particular, the pre-coating can have a reduced first thickness (d2a) of less than 20 micrometers in thinner first sections after flexible rolling.Alternatively or in addition, the flexible rolling is carried out in such a way that the pre-coating after the flexible rolling has a reduced second thickness (d2b) of more than 33 micrometers, in particular of more than 36 micrometers, in thicker second sections. It is understood that, depending on the desired component geometry, any other thickness ranges or transition areas can lie between the thinnest sections and the thickest sections of the strip material.

[0026] In a process step following flexible rolling, blanks are produced from the flexibly rolled strip material. This process step is also referred to as singulation. Singulation can be performed by mechanical cutting or laser cutting. For the purposes of this disclosure, the term "blanks" encompasses both rectangular sheet metal panels cut from the strip material and shaped cuts. Shaped cuts are sheet metal elements machined from the strip material, the outer contour of which is already adapted to the shape of the final product.

[0027] After separation, the sheet metal blanks are hot-formed, with additional process steps possibly being inserted between them. For hot-forming, at least a partial area of the blank is heated to the austenitizing temperature; it is then placed in a hot-forming tool, formed in the hot-forming tool, and quickly cooled to produce a hardened part. Heating takes place in a suitable heating device, for example a continuous furnace. Heating to the austenitizing temperature refers to a temperature range in which at least partial austenitization occurs or is present, i.e. a microstructure in the two-phase region of ferrite and austenite. For this purpose, the blank is heated to a temperature above Ac1, i.e. the temperature at which the formation of austenite begins. For example, the blank can be heated to a temperature of over 880°C and / or up to 960°C.According to one possible design, the blank is heated for austenitizing at a temperature of at least 700°C at a heating rate of more than 12 K / s. This rapid heating reduces production time. After heating to the austenitizing temperature and placing it in the hot forming tool, the blank is formed and quickly cooled. The rapid cooling of the formed part in the forming tool creates a hardened, at least partially martensitic microstructure in the component. This process of hot forming and rapid cooling in a forming tool is also known as press hardening.

[0028] Through heating and hot forming, the coating is formed from the precoating and the underlying steel substrate, which increases in thickness compared to the precoating due to diffusion processes. The first final thickness in the thinner first sections of the finished component is more than 15 micrometers, in particular more than 20 micrometers, and less than 50 micrometers, in particular less than 40 micrometers. In the thicker second sections, the coating after hot forming has a second final thickness of less than 60 micrometers, in particular less than 50 micrometers, and more than 30 micrometers, in particular more than 35 micrometers.

[0029] It has surprisingly been shown that the coating increases more in the thinner areas during hot forming than in the thicker areas. In particular, the coating is formed with a final thickness ratio (d3a / d3b) of the first final thickness (d3a) to the second final thickness (d3b) that is greater than an intermediate thickness ratio (d2a / d2b) of the reduced first thickness (d2a) to the reduced second thickness (d2b). In this way, the different coating thicknesses are advantageously equalized, so that good corrosion protection is achieved in all sections of the component.

[0030] Hot forming can be carried out as an indirect process, comprising the sub-steps of cold preforming, subsequent heating of the cold preformed component to austenitizing temperature, and subsequent hot forming to create the final contour of the product. Hot forming can also be carried out as a direct process, characterized by the component being heated directly to austenitizing temperature and then hot-formed to the desired final contour in one step. No prior (cold) preforming takes place in this case.

[0031] According to one possible embodiment, the coating can be produced in the forming tool prior to forming, forming a metal oxide layer on the surface. A metal oxide layer is corrosion-resistant and inert, thus reducing tool wear during forming. If a metal oxide layer is formed on the coating surface, the layer thicknesses specified in the present disclosure for the state after hot forming refer to the total coating thickness, i.e., including the oxide layer.

[0032] Preferred embodiments are explained below with reference to the drawing figures. Figure 1 schematically shows a method according to the invention for producing a coated, hardened molded part; Figure 2A shows a section of the coated steel substrate after pre-coating in an enlarged schematic representation; Figure 2B shows a section of the coated steel substrate after flexible rolling in an enlarged schematic representation; and Figure 2C shows a section of the coated and flexibly rolled steel substrate after hot forming in an enlarged schematic representation.

[0033] The <h2 style=";text-align:left;direction:ltr">Figure 1 and <h2 style=";text-align:left;direction:ltr"> 2A to 2C are described together below.

[0034] <h2 style=";text-align:left;direction:ltr"> Figure 1 shows a method according to the invention for producing a hardened product from a coated steel substrate 2. The steel substrate in strip form is also referred to as steel strip or generally as strip material. In the individual state, the steel substrate is also referred to as a blank.

[0035] Within the scope of the present disclosure, the steel substrate 2 includes a hardenable flat steel product which may, for example, contain the following proportions of alloying elements in weight percent: Carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0.4%; Manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%; Aluminium (Al) with at most 0.1%; Silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%; Chromium (Cr) with at least 0.01% and at most 1.0%; Titanium (Ti) with at most 0.02%, in particular at most 0.01%; Boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%; Phosphorus (P) with at most 0.1%, in particular at most 0.01%; Sulphur (S) with at most 0.05%, in particular at most 0.01%; Optionally, additional alloying elements with a content of up to 1.55% (1550 ppm); the remainder iron (Fe) and unavoidable impurities.

[0036] This alloy composition includes, for example, boron-manganese-containing steel materials such as 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5, and 34MnB5. The steel material can have a yield strength of, for example, 150 to 1100 MPa and / or a tensile strength of at least 450 MPa in its initial state. The optional additional alloying elements can be selected from the group: Copper (Cu) with a maximum of 0.1%; nickel (Ni) with a maximum of 0.1%; niobium (Nb) with a maximum of 0.1%; molybdenum (Mo) with a maximum of 1.0%; vanadium (V) with a maximum of 0.25%; without being limited thereto, the stated percentages each refer to a mass percentage of the steel substrate. One or more of the optional alloying elements mentioned may be used. The total mass fraction of the optional alloying elements is a maximum of 1.55%, in particular a maximum of 1.0%, preferably a maximum of 0.8%.

[0037] In process step S1, the steel substrate 2, which in its initial state may be wound on a coil 3, is provided with a precoating 4. When applied to the steel substrate, the precoating 4 contains aluminum at a weight ratio of at least 85 percent and silicon at a weight ratio of up to 15 percent. It is understood that other alloying elements may be included at the expense of the silicon content, for example iron and / or other alloying elements with a total of up to 5 percent by weight. The precoating 4 can be applied to the steel substrate 2 using generally known methods. One possibility is application by hot-dip plating. In this process, the steel substrate 2 passes through a molten bath 5 of liquid coating material 4 in a coating system 6, which adheres to the surface of the substrate 2, thus producing a precoated steel substrate.The melt of the coating material can contain, for example, 8 to 15 weight percent silicon, 2 to 4 weight percent iron, optionally one or more further alloying elements, such as at least one from the group of Mn, Cr, Ti, B, P, S, Cu, Ni, Nb, Mo, V, together up to 1.5 weight percent, and as the remainder aluminum as well as unavoidable impurities.

[0038] The precoating 4 is applied to the steel substrate 2 with a thickness d1 of at least 36 micrometers, in particular at least 40 micrometers. The coating thickness d1 can have a maximum thickness of 60 micrometers, in particular up to 50 micrometers. <h2 style=";text-align:left;direction:ltr"> Figure 2A shows schematically a section of the steel substrate 2 with pre-coating 4, wherein the combination of steel substrate with pre-coating is provided with reference numeral 2'.

[0039] After applying the first coating 4, the coated steel substrate 2' is flexibly rolled (S2). For this purpose, the coated steel strip 2', which has a largely constant sheet thickness D1 along its length prior to flexible rolling, is rolled by means of rollers 7, 8 such that it acquires a variable sheet thickness D2a, D2b, D2c along the rolling direction. The coated and flexibly rolled steel substrate is designated by reference numeral 12.

[0040] During rolling, the process is monitored and controlled, with the data determined by a sheet thickness measurement 9 being used as an input signal to control the rolls 7, 8. Flexible rolling is carried out according to the desired target thickness profile of a blank to be cut from the strip material 12 or a component to be manufactured therefrom. Flexible rolling can be carried out with rolling degrees of at least 1% and / or a maximum of 60% based on the initial thickness D1 of the pre-coated steel substrate 2', in particular with rolling degrees between 3% and 55%. <h2 style=";text-align:left;direction:ltr"> Figure 2Ba section of the pre-coated steel substrate 12 after flexible rolling is shown. It can be seen that the flexibly rolled strip material 12 after rolling has more heavily rolled first regions a with a first thickness D2a and less heavily rolled second regions b with a second thickness D2b as well as transition regions c in between with a variable thickness D2c. During the flexible rolling process, a thickness reduction takes place both in the substrate 2 and accordingly in the pre-coating 4. As the roller pressure increases, both the thickness of the substrate 2 and the thickness of the pre-coating 4 applied thereto decrease. After flexible rolling, the pre-coating 4 has a reduced first thickness d2a of in particular less than 20 micrometers in the thinner first sections a, and a reduced second thickness d2b of in particular more than 33 micrometers, preferably more than 36 micrometers, in the thicker second sections b.

[0041] After flexible rolling, the strip material 12 can be rewound into coil 3 so that it can be transported to a subsequent processing station. After the rolling process, the steel strip 12 can be smoothed in a subsequent process step, which takes place in a strip straightening device. The smoothing process step is optional and can also be omitted.

[0042] After flexible rolling (S2) or smoothing (if provided), the coated and flexibly rolled steel strip 12 is separated in process step S3. Individual sheet metal blanks 22 are machined from the steel strip 12, for example, using a punching and / or cutting device 10. Depending on the shape of the sheet metal blanks 22 to be produced, these can be punched out of the strip material 12 as a shaped cut, with any unused edge being discarded as scrap, or the strip material 12 can simply be cut into sections.

[0043] The blanks 22 are hot-formed in a subsequent step S4, which can also be referred to as press hardening. During hot-forming or press hardening, the blank 22 is heated to a temperature that is generally above the AC1 or AC3 temperature of the material, for example, between 750°C and 1000°C. The heating can be carried out by suitable methods, such as inductive heating, conductive heating, heating in a roller hearth furnace, contact heating using hot plates, infrared, or other known methods. After heating to austenitizing temperature, the blank 22 is then placed in a hot-forming tool 11, formed therein, and cooled or quenched so rapidly that a martensitic hardened structure is at least partially created in the thus-produced molded part.

[0044] Hot forming (S4) can be performed as a direct process according to a first option. The blank 22 is heated directly to the austenitizing temperature and then hot formed in a single step to the desired final contour. No prior (cold) preforming takes place. A second option is also possible, which involves hot forming as an indirect process, comprising the sub-steps of cold preforming, subsequent heating of the cold preformed component to the austenitizing temperature, and subsequent hot forming to produce the final contour of the formed part.

[0045] Due to the heating of the blank 22 during hot forming, diffusion processes occur between the base material of the steel substrate 2 and the coating material 4. Iron diffuses from the steel substrate 2 into the coating material 4, so that the overall thickness d3 of the coating 4 increases compared to the thickness d2 present after flexible rolling, i.e., the coating thicknesses d3a, d3b of the hot-formed component 32 are each thicker than the corresponding coating thicknesses d2a, d2b before hot forming. The holding time for austenitizing the coated blank 22 depends on the selected temperature and can be between 4 and 10 minutes. Preferably, the coating 4 of the hot-formed product 32 has a final coating thickness d3a of more than 15 micrometers, in particular more than 20 micrometers, in the thinner first sections a.In the thicker second sections b, the coating 4 can have a second final thickness d3b after heating or hot forming, in particular of more than 30 micrometers, preferably more than 35 micrometers. For good weldability of the manufactured component, it is advantageous if the final thickness d3a of the coating 4 is less than 50 micrometers, in particular less than 40 micrometers, in the thinner regions a, and less than 60 micrometers, in particular less than 50 micrometers, in the thicker regions b.

[0046] Optionally, surface oxidation of the coated and flexibly rolled substrate 2 can be performed prior to hot forming (S4). This creates an oxidation layer on the coating 4. This leads to greater heat absorption, allowing heating times to be shortened. In a favorable process, the blank can be heated during hot forming at least until a temperature of 700°C is reached, with a heating rate of more than 12 K / s. List of reference symbols

[0047] 2Steel substrate 3Coil 4Coating 5Melting bath 6Coating device 7Rollers 8Rollers 9Thickness control 10Cutting device 11Hot forming tool 12flexibly rolled substrate 22Blank 32Hot forming part afirst section or further section ctransition section Dthickness (2+4) dthickness (4) S1-S4process steps

Claims

1. Method for producing a hardened steel product comprising: providing a steel substrate (2) having a base material of a hardenable steel; coating (S1) the steel substrate (2) with an aluminum-containing precoating (4) containing at least 85% by weight of aluminum, wherein the precoating (4) is applied to the steel substrate (2) with a thickness (d1) of at least 34 micrometers (µm); flexible rolling (S2) of the precoated steel substrate (2), wherein successive portions (a, b, c) of the precoated steel substrate (2) are rolled out to different degrees, with rolling degrees of at least 1% and / or at most 60% starting from the initial thickness of the steel substrate (2), wherein through the flexible rolling the precoating obtains in thinner first portions (a) a reduced first thickness (d2a) of less than 33 micrometers, and in thicker second portions (b) a reduced second thickness (d2b) which is thicker than the reduced first thickness (d2a); working (S3) a blank (22) from the flexibly rolled steel substrate (2); heating the blank (22) such that the base material of the blank (22) is at least partially austenitized, wherein diffusion processes take place between the base material and the precoating (4) through the heating; and hot forming (S4) the heated blank (22), wherein the heated blank (22) is formed and rapidly cooled such that a hardened steel product (32) with coating is produced, wherein the coating is formed from the precoating (4) through the heating and hot forming (S4), with a first end thickness (d3a) in the thinner first portions (a) which is greater than the reduced first thickness (d2a) and is between 15 to 50 micrometers, and with a second end thickness (d3b) in the thicker second portions (b) which is greater than the reduced second thickness (d2b) and is between 30 to 60 micrometers.

2. Method according to claim 1, characterised in that the precoating (4) is applied to the steel substrate (2) with a thickness (d1) of at least 40 micrometers.

3. Method according to claim 1 or 2, characterised in that the precoating (4) obtains in the thinner first portions (a) a reduced first thickness (d2a) of less than 20 micrometers by the flexible rolling (S2).

4. Method according to any one of claims 1 to 3, characterised in that the precoating (4) obtains in the thicker second portions (b) a reduced second thickness (d2b) of more than 36 micrometers by the flexible rolling (S2).

5. Method according to any one of claims 1 to 4, characterised in that the coating is formed with a final thickness ratio (d3a / d3b) of the first final thickness (d3a) to the second final thickness (d3b) being greater than an intermediate thickness ratio (d2a / d2b) of the reduced first thickness (d2a) to the reduced second thickness (d2b).

6. Method according to any one of claims 1 to 5, characterised in that steel substrate (2) with an initial thickness of 1.0 to 4.0 mm is used.

7. Method according to any one of claims 1 to 6, characterised in that that the coating (4) is applied to the steel substrate (2) by means of hot-dip coating.

8. Method according to claim 7, characterised in that that the steel substrate (2) is heated in the coating device (6) after application of the precoating (4) in order to achieve prediffusion between the precoating (4) and the steel substrate (2).

9. Method according to any one of claims 1 to 8, characterised in that the steel substrate (2) is a hardenable steel which contains the following proportions of alloying elements, respectively in percent by weight: carbon (C) with more than 0.15% and less than 0.5%; manganese (Mn) with more than 0.5% and less than 5.0%; aluminum (Al) with less than 0.1%; silicon (Si) with more than 0.1% and less than 0.9%; chromium (Cr) with more than 0.01% and less than 1.0%; tital (Ti) with less than 0.2%; boron (B) with more than 0.0005 and less than 0.080%; phosphorus (P) with less than 0.1%; sulfur (S) with less than 0.05%; optionally other alloying elements with a content of up to 1.55%; the rest iron (Fe) and unavoidable impurities.

10. Method according to claim 9, characterised in that as optionally further alloying element at least one of: copper (Cu) with at most 0.1%; nickel (Ni) with at most 0.1%; niobium (Nb) with at most 0.1%; molybdenum (Mo) with at most 1.0%; vanadium (V) with at most 0.25%; respectively in percent by weight, is used.

11. Method according to any one of claims 1 to 10, characterised in that that the precoating (4) contains at least 85% by weight of aluminum, and optionally may contain 5 to 15% by weight of silicon.

12. Method according to any one of claims 1 to 11, characterised in that that a metal oxide layer is formed on the surface of the coated steel substrate (2) before hot forming.

13. Method according to any one of claims 1 to 12, characterised in that that the blank (22) is heated for austenitizing at a heating rate of more than 12 K / sec at least until a temperature of 700°C is reached.