DE102018118015A1 - Process for producing a hardened steel product - Google Patents

Abstract

The invention relates to a method for producing a hardened steel product, comprising the steps of: providing a steel substrate (2) from a hardenable steel; Coating the steel substrate (2) with an aluminum pre-coating (4), the pre-coating (4) having a thickness (d1) of at least 34 μm being applied to the steel substrate (2); Flexible rolling of the precoated steel substrate (2) to produce a variable thickness along the length of the precoated steel substrate (2), the precoating in thinner first sections to a reduced first thickness (d2a) of less than 33 µm and in thicker second sections rolled to a reduced second thickness (d2b) that is thicker than the reduced first thickness (d2a); Working out a circuit board (22) from the flexibly rolled steel substrate (2); Heating the circuit board (22) to the austenitizing temperature, diffusion processes taking place between the base material and the precoating (4); and hot forming (S4) the heated blank (22), the heated blank (22) being deformed and cooled so rapidly that a hardened steel product (42) with a coating is produced.

Description

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

It is known to coat metallic components for corrosion protection and to form them into molded parts by means of hot forming. In practice, aluminum-silicon coated high-strength and ultra-high-strength tempering steels are used in practice for safety-relevant body components of motor vehicles, in particular manganese-boron-containing tempering steels such as 22MnB5 or 34MnB5.

For example, from the WO 2009/090555 A1 discloses a method of making a hot stamped coated steel section, comprising the steps of: precoating a steel strip with aluminum or aluminum alloy by hot dipping, the thickness of the precoating being 20 to 33 microns on each side, cutting the precoated steel strip into a steel section, heating the steel section in an oven, transferring the heated steel section into a press tool, hot stamping the steel section in the press tool, and cooling the steel section.

From the WO 2008/113426 A2 a method for producing a sheet metal component is known, a hot or cold strip being hot-dip coated or electrolytically coated and then subjected to a flexible rolling process.

In the flexible rolling process, different sheet thicknesses of the flexibly rolled steel strip are produced by different rolling pressures. Coated to the sheet thickness after the flexible rolling, the coating is formed to different thicknesses during coating, the coating thickness being made larger depending on the rolling pressure as the rolling pressure to be expected increases.

From the WO 2006 097 237 A1 a method and a system for hot-dip coating of hot-rolled steel strip are known. The steel strip passes through a pickling station, a rinsing station, a drying station, a heating furnace and then a melting bath. The finished thickness and the thickness tolerance of the hot-dip coated steel strip are achieved through a controlled reduction in thickness in a roll stand in the process line by checking the finished thickness in the outlet of the roll stand by a thickness measuring device and deviations from the target thickness as a control signal due to the position of the roll stand.

From the WO 2016/198186 A1 a method for hot forming a steel component is known. The steel component is provided with a corrosion-resistant scale protection layer and before hot forming there is a surface oxidation in which a corrosion-resistant oxidation layer is formed on the scale protection layer.

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 resistance to corrosion protection in areas with different thicknesses.

The object is achieved by means of a method for producing a hardened steel product, comprising the steps: providing a steel substrate with a base material made of a hardenable steel; Coating the steel substrate with a pre-coating containing aluminum to produce a pre-coated steel substrate, the coating of the pre-coated steel substrate having a thickness (d1) of at least 34 micrometers (µm); Flexible rolling of the precoated steel substrate in such a way that successive sections of the precoated steel substrate are rolled out to different degrees to produce a variable thickness over the length of the precoated steel substrate, the precoating after the flexible rolling in thinner first sections having a reduced first thickness (d2a) less than 33 microns and in thicker second sections has a reduced second thickness (d2b) that is thicker than the reduced first thickness (d2a); Working out a board from the flexibly rolled strip material; Heating the board in such a way that the base material of the board is at least partially austenitized, diffusion processes taking place between the base material and the precoating due to the heating; Hot forming of the heated blank, the heated blank being deformed and cooled so quickly that a hardened steel product with a coating is produced.

One advantage is that the substrate has a sufficiently thick coating even after flexible rolling. It has been shown that the coating grows when heated for the subsequent hot forming due to the diffusion processes, so that the final thickness of the coating after the hot forming is greater than the respective coating thickness after the flexible rolling and before the heating for the hot forming. Due to the fact that the precoating has a thickness of at least 34 micrometers, the coating is also in spite of the reduction in thickness that results in the context of flexible rolling, due to the subsequent heating for hot forming the thinner first sections are sufficiently thick to achieve good corrosion protection. 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 or weight-reduced component with excellent coating protection in all thickness ranges.

• Kohlenstoff (C) mit mindestens 0,15 % und höchstens 0,5 %, insbesondere höchstens 0,4 %;
• Mangan (Mn) mit mindestens 0,5 % und höchstens 5,0 %, insbesondere mindestens 0,8 % und höchstens 2,5 %;
• Aluminium (AI) mit höchstens 0,1 %;
• Silizium (Si) mit mindestens 0,1 % und höchstens 0,9 %, insbesondere höchstens 0,5 %;
• Chrom (Cr) mit mindestens 0,01 % und höchstens 1,0 %;
• Tital (Ti) mit höchstens 0,02 %, insbesondere höchstens 0,01 %;
• Bor (B) mit mindestens 0,0005 und höchstens 0,080 %, insbesondere mindestens 0,002 % und höchstens 0,006 %;
• Phosphor (P) mit höchstens 0,1 %, insbesondere höchstens 0,01 %;
• Schwefel (S) mit höchstens 0,05 %, insbesondere höchstens 0,01;
• optional weitere Legierungselemente mit einem Anteil von bis zu 1,55 % (1550 ppm); den Rest Eisen (Fe) und unvermeidbare Verunreinigungen.

The steel substrate can be, for example, a hardenable or temperable, in particular manganese-containing, steel material. In addition to manganese, this can contain further microalloying elements. The steel material can contain, for example, the following proportions of alloy elements 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%;
• Aluminum (AI) with a maximum of 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%;
• Tital (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%;
• Sulfur (S) with at most 0.05%, in particular at most 0.01;
• optionally further alloying elements with a share of up to 1.55% (1550 ppm); the rest iron (Fe) and unavoidable impurities.

• Kupfer mit höchstens 0,1 %;
• Nickel mit höchstens 0,1 %;
• Niob mit höchstens 0,1 %;
• Molybdän mit höchstens 1,0 %;
• Vanadium mit höchstens 0,25 %;

beinhalten, ohne hierauf eingeschränkt zu sein. Der Masseanteil der optionalen Legierungselemente kann auch geringer sein, beispielsweise kann Molybdän auch mit höchstens 0,8 %, 0,5 % oder 0,25 % enthalten sein. Der Masseanteil der optionalen Legierungselemente beträgt in Summe maximal 1,55 %, insbesondere maximal 1,0 %, insbesondere maximal 0,8 %. Das Legierungselement Niob bewirkt in vorteilhafter Weise eine feinkörnige Struktur eines aus der Legierung warmumgeformten Bauteils. Insbesondere im Zusammenwirken mit Molybdän, das ein Kornwachstum hemmen kann, ergibt sich eine besonders feinkörnige Struktur, was sich wiederum günstig auf die Festigkeit des hieraus hergestellten Bauteils auswirkt.As optional additional alloying elements, the substrate can in each case in percent by weight in particular at least one of:

• Copper with at most 0.1%;
• Nickel with a maximum of 0.1%;
• Niobium with at most 0.1%;
• Molybdenum with a maximum of 1.0%;
• Vanadium with a maximum of 0.25%;

include without being limited to this. The mass fraction of the optional alloy elements can also be lower, for example molybdenum can also be present at a maximum of 0.8%, 0.5% or 0.25%. The mass fraction of the optional alloy elements is a maximum of 1.55% in total, in particular a maximum of 1.0%, in particular a maximum of 0.8%. The alloy element niobium advantageously brings about a fine-grained structure of a component that is hot-formed from the alloy. In particular, in cooperation with molybdenum, which can inhibit grain growth, a particularly fine-grained structure results, which in turn has a favorable effect on the strength of the component produced therefrom.

Examples of usable steel materials containing boron manganese 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 that certain sub-areas of the molded part, where necessary, are set to a lower tensile strength of less than 1100 MPa and, therefore, higher ductility. The steel substrate can have an initial thickness of, for example, between 1.0 and 4.0 mm.

The coating preferably contains at least 85% by weight of aluminum, which also includes the possibility of using a pure aluminum coating (100% by weight of Al), as well as the use of an alloy which contains aluminum as a main alloy component with at least 85% by weight and optionally further alloy components. for example silicon with, for example, between 5 and 15 percent by weight and / or iron with up to 5 percent by weight and / or other alloying elements in smaller proportions. In the context of the present disclosure, the term aluminum coating or aluminum-based coating is also generally used due to the main constituent aluminum, which conceptually encompasses the possibilities mentioned for other alloy compositions. The aluminum coating can, for example, be applied to the steel substrate in a hot-dip process in a melt bath with at least 85 percent by weight of aluminum and, if appropriate, further alloy components or other conventional coating processes. An exemplary composition of the molten bath or the applied coating can contain up to 3% by weight of iron, 9 to 12% by weight of silicon and the rest of aluminum.

According to a preferred embodiment, the precoating is applied to the steel substrate with a thickness (d1) of at least 36 micrometers, in particular at least 40 micrometers. The steel substrate pre-coated in this way forms the basis for a hardened component with variable thicknesses to be produced therefrom. The coating of the steel substrate can be applied, for example, by means of hot-dip coating, the steel substrate being immersed in a basin with molten coating material. It is understood that other known coating methods can also be used.

The precoated steel substrate is rolled flexibly after the precoating, it being understood that further steps, such as heating, winding up to and from the coil, straightening, cleaning or the like can be interposed. Optionally, it can be provided in particular that the steel substrate is heated in the coating system after the application of the precoating in order to achieve a pre-diffusion between the precoating and the steel substrate. The 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 times and 0.9 times the melting temperature of the coating material. The pre-diffusion already forms a thicker interdiffusion zone between the base material of the steel substrate and the coating material during the coating process. This makes it possible to carry out the heating more quickly in the course of hot forming, which has an overall favorable effect on the cycle times during hot forming.

In flexible rolling, strip material with a substantially uniform sheet thickness is rolled out by changing the roll gap during the process to form strip material with a variable sheet thickness over the length. The sections of different thicknesses produced by flexible rolling extend transversely to the longitudinal direction or rolling direction of the strip material. After the flexible rolling, the strip material can be easily rewound into the coil and fed to further processing at another point, or it can be further processed directly, for example by cutting the strip material into individual sheet metal elements.

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 precoated steel substrate, in particular with rolling degrees between 3% and 55%. Flexible rolling also reduces the thickness of the pre-coating with the steel substrate. The pre-coating after the flexible rolling in thinner first sections can in particular have a reduced first thickness (d2a) of less than 20 micrometers. Alternatively or in addition, the flexible rolling is carried out such that the precoating after the flexible rolling in thicker second sections has a reduced second thickness (d2b) of more than 33 micrometers, in particular of more than 36 micrometers. It goes without saying that between the thinnest sections and the thickest sections of the strip material, depending on the desired component geometry, there may be any other thickness ranges or transition ranges in between.

In a process step downstream of the flexible rolling, blanks are produced from the flexibly rolled strip material. This process step is also called separating. The separation can be carried out by mechanical cutting or by laser cutting. In the context of the present disclosure, the term circuit boards is intended to include both rectangular metal sheets that have been cut out of the strip material and shape cuts. Form cuts are sheet metal elements made from the strip material, the outer contour of which is already adapted to the shape of the end product.

The sheet metal blanks are hot-formed after the separation, it being possible for further process steps to be interposed. For hot forming, the blank is heated to austenitizing temperature at least in a partial area; then placed in a thermoforming tool and reshaped in the thermoforming tool and rapidly cooled, so that a hardened molded part is produced. The heating is carried out in a suitable heating device, for example in a continuous furnace. Heating to austenitizing temperature means a temperature range in which at least partial austenitization takes place or is present, that is to say a microstructure in the two-phase region of ferrite and austenite. For this purpose, the board is heated to a temperature above Ac1, that is, the temperature at which austenite begins to form. For example, the board can be heated to a temperature of over 880 ° C and / or up to 960 ° C. According to a possible embodiment, the circuit board is heated for austenitizing at least up to a temperature of 700 ° C. with a heating rate of more than 12 K / sec. The manufacturing time is reduced by rapid heating. After heating to the austenitizing temperature and placing it in the thermoforming tool, the board is formed and quickly cooled. The rapid cooling of the molded part in the forming tool produces 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.

As a result of the heating and hot forming, the coating is formed from the pre-coating and the underlying steel substrate, which increases due to diffusion processes compared to the thickness of the pre-coating. The first final thickness (d3a) is preferably formed in the thinner first sections of the finished component with more than 15 micrometers, in particular more than 20 micrometers, and less than 50 micrometers, in particular less than 40 micrometers. Alternatively or in addition, the coating after the hot forming in the thicker second sections can have a second final thickness (d3b) of less than 60 micrometers, in particular less than 50 micrometers and / or more than 30 micrometers, in particular more than 35 micrometers.

It has surprisingly been found that the coating grows more strongly in the thinner areas in the course of hot forming than in 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), which 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 in general adapt to one another in an advantageous manner, so that overall good corrosion protection is achieved in all sections of the component.

According to a first possibility, the hot forming can be carried out as an indirect process, which comprises the sub-steps cold preforming, subsequent heating of the cold preformed component to the austenitizing temperature, and subsequent hot forming to produce the final contour of the product. According to a second possibility, hot forming can also be carried out as a direct process, which is characterized in that the component is heated directly to the austenitizing temperature and then hot formed to the desired final contour in one step. A previous (cold) preforming does not take place here.

According to a possible embodiment, the coating can be produced in the forming tool before the forming, in such a way that a metal oxide layer is formed on the surface. A metal oxide layer is corrosion-resistant and inert, so tool wear during forming is reduced. If a metal oxide layer is formed on the coating surface, the layer thicknesses given in the present disclosure for the state after hot forming relate to the total coating thickness, that is to say including the oxide layer.

• 1 schematisch ein erfindungsgemäßes Verfahren zur Herstellung eines beschichteten, gehärteten Formteils;
• 2A einen Ausschnitt des beschichteten Stahlsubstrat nach dem Vorbeschichten in vergrößerter schematischer Darstellung;
• 2B einen Ausschnitt des beschichteten Stahlsubstrat nach dem Flexiblen Walzen in vergrößerter schematischer Darstellung; und
• 2C einen Ausschnitt des beschichteten und flexibel gewalzten Stahlsubstrats nach dem Warmumformen in vergrößerter schematischer Darstellung.

Preferred exemplary embodiments are explained below with reference to the drawing figures. Here shows

• 1 schematically an inventive method for producing a coated, hardened molded part;
• 2A a section of the coated steel substrate after precoating in an enlarged schematic representation;
• 2 B a section of the coated steel substrate after the flexible rolling in an enlarged schematic representation; and
• 2C a section of the coated and flexibly rolled steel substrate after hot forming in an enlarged schematic representation.

The 1 and 2A to 2C are described below together.

1 shows an inventive method 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. When isolated, the steel substrate is also referred to as a circuit board.

• Kohlenstoff (C) mit mindestens 0,15 % und höchstens 0,5 %, insbesondere höchstens 0,4 %;
• Mangan (Mn) mit mindestens 0,5 % und höchstens 5,0 %, insbesondere mindestens 0,8 % und höchstens 2,5 %;
• Aluminium (AI) mit höchstens 0,1 %;
• Silizium (Si) mit mindestens 0,1 % und höchstens 0,9 %, insbesondere höchstens 0,5 %;
• Chrom (Cr) mit mindestens 0,01 % und höchstens 1,0 %;
• Tital (Ti) mit höchstens 0,02 %, insbesondere höchstens 0,01 %;
• Bor (B) mit mindestens 0,0005 und höchstens 0,080 %, insbesondere mindestens 0,002 % und höchstens 0,006 %;
• Phosphor (P) mit höchstens 0,1 %, insbesondere höchstens 0,01 %;
• Schwefel (S) mit höchstens 0,05 %, insbesondere höchstens 0,01;
• optional weitere Legierungselemente mit einem Anteil von bis zu 1,5 % (1550 ppm); den Rest Eisen (Fe) und unvermeidbare Verunreinigungen.

As a steel substrate 2 Within the scope of the present disclosure, a hardenable flat steel product is also included, which can contain, for example, the following proportions of alloy elements in each case 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%;
• Aluminum (AI) with a maximum of 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%;
• Tital (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%;
• Sulfur (S) with at most 0.05%, in particular at most 0.01;
• optional additional alloying elements with a share of up to 1.5% (1550 ppm); the rest iron (Fe) and unavoidable impurities.

• Kupfer mit höchstens 0,1 %;
• Nickel mit höchstens 0,1 %;
• Niob mit höchstens 0,1 %;
• Molybdän mit höchstens 1,0 %;
• Vanadium mit höchstens 0,25 %;

ohne hierauf eingeschränkt zu sein, wobei sich die genannten Prozentangaben jeweils auf Masseprozent des Stahlsubstrats beziehen. Es können ein oder mehrere der genannten optionalen Legierungselemente verwendet werden. Der Masseanteil der optionalen Legierungselemente beträgt in Summe maximal 1,55 %, insbesondere maximal 1,0 %, vorzugsweise maximal 0,8 %.This alloy composition includes, for example, boron-manganese-containing steel materials such as 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5 and 34MnB5. In the initial state, 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. The optional further alloying elements can be selected from the group:

• Copper with at most 0.1%;
• Nickel with a maximum of 0.1%;
• Niobium with at most 0.1%;
• Molybdenum with a maximum of 1.0%;
• Vanadium with a maximum of 0.25%;

without being limited to this, the percentages given refer in each case to the percentage by mass of the steel substrate. One or more of the optional alloy elements mentioned can be used. The mass fraction of the optional alloy elements is a maximum of 1.55% in total, in particular a maximum of 1.0%, preferably a maximum of 0.8%.

In the procedural step S1 becomes the steel substrate 2 which is in the initial state on a coil 3 can be wound with a precoat 4 Mistake. When applied to the steel substrate, the precoat contains 4 Aluminum with at least 85 percent by weight and silicon with up to 15 percent by weight. It goes without saying that other alloying elements can 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 pre-coating 4 can be applied to the steel substrate using generally known methods 2 be applied. One possibility is the hot dip application. The steel substrate passes through 2 in a coating system 6 a melt pool 5 made of liquid coating material 4 which is on the surface of the substrate 2 adheres so that a pre-coated steel substrate is produced. The melt of the coating material can contain, for example, 8 to 15 percent by weight of silicon, 2 to 4 percent by weight of iron and the remainder aluminum as well as unavoidable impurities.

The pre-coating 4 is applied to the steel substrate with a thickness d1 of at least 36 micrometers, in particular at least 40 micrometers 2 applied. The coating thickness d1 can have a maximum thickness of 60 micrometers, in particular up to 50 micrometers. 2A shows schematically a section of the steel substrate 2 with pre-coating 4 , the combination of steel substrate with precoating with reference numerals 2 ' is provided.

After applying the first coating 4 becomes the coated steel substrate 2 ' rolled flexibly. For this the coated steel band 2 ' , a largely constant sheet thickness before flexible rolling D1 over the length, by means of rollers 7 . 8th rolled such that it receives a variable sheet thickness D2a, D2b, D2c along the rolling direction. The coated and flexibly rolled steel substrate is marked with the reference number 12 Mistake.

The process is monitored and controlled during rolling, using a sheet thickness measurement 9 determined data as an input signal for controlling the rollers 7 . 8th be used. Flexible rolling becomes one from the strip material in accordance with the desired target thickness profile 12 to be cut board or a component to be produced therefrom. Flexible rolling can be carried out with a rolling degree of at least 1% and / or a maximum of 60% based on the initial thickness D1 of the precoated steel substrate 2 ' be carried out, in particular with rolling edges between 3% and 55%. In 2 B is a section of the pre-coated steel substrate 12 shown after flexible rolling. It can be seen that the flexibly rolled strip material 12 first rolled out areas after rolling a with a first thickness D2a and less rolled out second areas b with a second thickness D2b as well as intermediate transition areas c with variable thickness D2c having. As part of flexible rolling, the thickness is reduced both in the substrate 2 as well as in the pre-coating 4 , With increasing roller pressure, the thickness of the substrate increases 2 as well as the thickness of the precoat applied to it 4 from. The pre-coating 4 has after the flexible rolling in the thinner first sections a a reduced first thickness d2a less than 20 microns in particular, and in thicker second sections b a reduced second thickness d2b in particular more than 33 micrometers, preferably more than 36 micrometers.

After flexible rolling, the strip material can 12 back to the coil 3 be wound up so that it can be transported to a subsequent processing station. After the rolling process, the steel strip can 12 are smoothed in a subsequent process step, which takes place in a strip straightening device. The Smoothing step is optional and can also be omitted.

After flexible rolling ( S1 ) or smoothing (if provided) is the coated and flexibly rolled steel strip 12 in the procedural step S3 sporadically. In doing so, the steel strip 12 individual sheet metal boards 22 worked out, for example by means of a punching and / or cutting device 10 , Depending on the shape of the sheet metal to be manufactured 22 can this from the tape material 12 are punched out as a shape cut, whereby an edge that is no longer used is removed as scrap, or the strip material 12 can easily be cut to length.

The boards 22 are in a subsequent step S4 hot formed, which can also be called press hardening. The board is used for hot forming or press hardening 22 heated to a temperature which 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 for example by means of inductive heating, conductive heating, heating in the roller hearth furnace, contact heating by hot plates, infrared, or other known methods. After warming to austenitizing temperature, the board 22 then in a thermoforming tool 11 inserted and reshaped therein and cooled or quenched so quickly that at least partially a martensitic hardness structure occurs in the molded part produced in this way.

Hot forming ( S5 ) can be carried out as a direct process. The board 2 heated directly to the austenitizing temperature and then hot formed to the desired final contour in one step. A previous (cold) preforming does not take place here. According to a second possibility, hot forming can also be carried out as an indirect process which comprises 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 molded part.

Due to the warming of the blank as part of the hot forming 22 find diffusion processes between the base material of the steel substrate 2 and the coating material 4 instead of. Iron diffuses out of the steel substrate 2 in the coating material 4 so the thickness d3 the coating 4 compared to the thickness existing after flexible rolling d2 overall increases, that is, the coating thicknesses d3a . d3b of the hot-formed component 32 are each thicker than the corresponding coating thickness d2a . d2b before hot forming. The holding time for austenitizing the coated board 22 depends on the selected temperature and can be between 4 and 10 minutes. Preferably the coating 4 of the hot-formed product 32 in the thinner first sections a a final coating thickness d3a of more than 15 micrometers, in particular more than 20 micrometers. In the thicker second sections b can the coating 4 after heating or hot forming, a second final thickness d3b in particular of more than 30 micrometers, preferably more than 35 micrometers. For a good weldability of the manufactured component, it is favorable if the final thickness d3a the coating 4 in the thin areas a is less than 50 micrometers, in particular less than 40 micrometers, and in the thicker areas b is less than 60 micrometers, in particular less than 50 micrometers.

Optionally, before hot forming ( S4 ) surface oxidation of the coated and flexibly rolled substrate 2 be made. This creates an oxidation layer on the coating 4 educated. This leads to a higher heat absorption, so that the heating-up times can be shortened. In a favorable process, the blank can be heated at least up to a temperature of 700 ° C. with a heating rate of more than 12 K / sec in the course of hot forming.

LIST OF REFERENCE NUMBERS

2

steel substrate

3

coil

4

coating

5

melting bath

6

coater

7

roll

8th

roll

9

thickness control

10

cutter

11

Thermoforming tool

12

flexibly rolled substrate

22

circuit board

32

Hot molding

a

first section

b

second part

c

Transition section

D

Thickness (2 + 4)

d

Thickness ( 4 )

S1-S4

steps

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• WO 2009/090555 A1 [0003]
• WO 2008/113426 A2 [0004]
• WO 2006097237 A1 [0006]
• WO 2016/198186 A1 [0007]

Claims (15)

A method of making a hardened steel product comprising: Providing a steel substrate (2) with a base material made of a hardenable steel; Coating the steel substrate (2) with an aluminum-containing precoat (4), the precoat (4) having a thickness (d1) of at least 34 micrometers (µm) being applied to the steel substrate (2); Flexible rolling of the precoated steel substrate (2), successive sections (a, b, c) of the precoated steel substrate (2) being rolled out to different extents, the precoating being reduced by the flexible rolling in thinner first sections (a) to a reduced first thickness ( d2a) of less than 33 micrometers and in thicker second sections (b) to a reduced second thickness (d2b) which is thicker than the reduced first thickness (d2a); Working out a circuit board (22) from the flexibly rolled steel substrate (2); Heating the circuit board (22) in such a way that the base material of the circuit board (22) is at least partially austenitized, the heating causing diffusion processes between the base material and the precoating (4); and Hot forming (S4) the heated blank (22), the heated blank (22) being shaped and cooled so quickly that a hardened steel product (32) with a coating is produced. Procedure according to Claim 1 , characterized in that the precoating (4) with a thickness (d1) of at least 36 micrometers, in particular at least 40 micrometers, is applied to the steel substrate (2). Procedure according to Claim 1 or 2 , characterized in that the precoating (4) is given a reduced first thickness (d2a) of less than 20 micrometers by the flexible rolling (S2) in the thinner first sections (a). Procedure according to one of the Claims 1 to 3 , characterized in that the precoating (4) is given a reduced second thickness (d2b) of more than 33 micrometers, in particular of more than 36 micrometers, by the flexible rolling (S2) in the thicker second sections (b). Procedure according to one of the Claims 1 to 4 , characterized in that the coating is formed from the precoating (4) by the heating and hot forming (S4), with a first final thickness (d3a) in the thinner first sections (a) which is greater than the reduced first thickness (d2a ) and in particular between 15 to 50 micrometers, and with a second final thickness (d3b) in the thicker second sections (b), which is greater than the reduced second thickness (d2b) and in particular between 30 to 60 micrometers. Procedure according to one of the Claims 1 to 5 , characterized 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), which is greater than an intermediate thickness ratio (d2a / d2b) of the reduced first thickness (d2a) reduced second thickness (d2b). Procedure according to one of the Claims 1 to 6 , characterized in that steel substrate (2) with an initial thickness of 1.0 to 4.0 mm is used. Procedure according to one of the Claims 1 to 7 , characterized in that the flexible rolling (S2) with rolling degrees of at least 1% and / or a maximum of 60% is carried out on the basis of the initial thickness of the steel substrate (2). Procedure according to one of the Claims 1 to 8th , characterized in that the coating (4) is applied to the steel substrate (2) by means of hot-dip coating. Procedure according to Claim 9 , characterized in that the steel substrate (2) is heated after the application of the precoating (4) in the coating system (6) in order to achieve a pre-diffusion between the precoating (4) and the steel substrate (2). Procedure according to one of the Claims 1 to 10 , characterized in that a hardenable steel is used as the steel substrate (2), each containing the following proportions of alloying elements 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 (AI) 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) less than 0.2%; Boron (B) with more than 0.0005 and less than 0.080%; Phosphorus (P) less than 0.1%; Sulfur (S) less than 0.05%; optional additional alloying elements with a share of up to 1.55%; the rest iron (Fe) and unavoidable impurities. Procedure according to Claim 11 , characterized in that at least one of: copper with at most 0.1%; Nickel with a maximum of 0.1%; Niobium with at most 0.1%; Molybdenum with a maximum of 1.0%; Vanadium with a maximum of 0.25%; is used. Procedure according to one of the Claims 1 to 12 , characterized in that the precoating (4), each in weight percent, contains at least 85% aluminum and 5% to 15% silicon. Procedure according to one of the Claims 1 to 13 , characterized in that a metal oxide layer is formed on the surface of the coated steel substrate (2) before hot forming. Procedure according to one of the Claims 1 to 14 , characterized in that the plate (22) for austenitizing is heated at least up to a temperature of 700 ° C with a heating rate of more than 12 K / sec.

Images