DE102020113397A1 - Coated sheet metal blank for the production of a hot-formed and press-hardened sheet steel component - Google Patents

Abstract

Coated sheet metal plate (2, 2a) for the production of a hot-formed and press-hardened sheet steel part, the sheet metal plate (2, 2a) having a hardenable steel as base material (1) with a coating (5, 7) on at least one side, the coating being titanium and aluminum and / or at least one further alloy element comprising magnesium, silicon, zinc and / or copper and that an austenitizing step (Ac) takes place during a hot forming process of the coated sheet metal blank (2, 2a), the alloying elements of the coating (5, 7) being within segregate the coating after the austenitizing step (Ac) and thereby the coating has at least one surface layer (3) and one connection layer (4), the surface layer (3) essentially comprising aluminum, silicon, magnesium, zinc and / or copper and the connection layer (4) consists essentially of titanium.

Description

The invention relates to a coated sheet metal blank for the production of a hot-formed and press-hardened sheet steel part according to the preamble of claim 1 and a method according to claim 13.

Hot-formed steels, such as 22MnB5, have an almost complete martensitic structure after the hot-forming process with a nominal tensile strength Rm of an average of 1500MPa. The strengths of these hot-formed steels are currently being increased further up to 2000MPa, for example by increasing the carbon content in the steel base material.

A sheet metal blank is used as the starting material for the production of a hot-formed and press-hardened sheet steel component in a hot-forming process. The coated sheet metal blank, which has not yet been heat-treated, has a hardenable steel as the base material, which is coated on one or both sides with a metallic anti-scale coating. In common practice, this is an aluminum-silicon (AISi) coating. Such an AISi coating, however, favors an entry of hydrogen from the steam atmosphere in the heat treatment furnace into the steel base material in the heat treatment step, as a result of which steel sheet components of higher strength react more brittle when exposed to stress. In addition, an AISi coating forms a molten phase in the heat treatment step, which can lead to heavy soiling of the transport rollers, especially in a roller hearth furnace.

In order to avoid the above disadvantages of the AISi coating, the hot forming process can be carried out with an uncoated sheet metal blank. In this case, however, scaling in the form of an oxide layer forms in the heat treatment step, which flakes off during hot forming in the forming tool and thus contaminates the forming tool. In addition, the layer of scale still adhering to the sheet steel component has to be removed from the sheet steel component in a laborious manner after the hot forming process has been completed, for example by blasting, in order to ensure that the component can be welded.

From the US 2018/0274062 A1 a sheet steel coating made of zinc, titanium, silicon, magnesium, lanthanum, cerium, tin and aluminum is known, the microstructure of the coating not having a binary phase of aluminum and zinc.

From the US 7527876 B2 a surface-treated sheet metal with a film of titanium, nickel, aluminum and fluorine is known.

The object of the invention is to provide a coated sheet metal blank for producing a sheet steel component in a hot forming process, with which a sheet steel component can be produced more simply and / or more energy-efficiently compared to the prior art.

This object is achieved by a coated sheet metal blank having the features of claim 1 and by a method according to claim 13. Preferred developments of the invention are disclosed in the subclaims.

The invention relates to a coated sheet metal blank for the production of a hot-formed and press-hardened sheet steel part, the sheet metal blank having a hardenable steel with a coating on at least one side as the base material, the coating being titanium and aluminum and / or at least one further alloy element, comprising magnesium, silicon, zinc and / or copper and that an austenitizing step takes place during a hot forming process of the coated sheet metal blank, the alloying elements of the coating segregating within the coating after the austenitizing step and thereby the coating having at least one surface layer and one bonding layer, the surface layer essentially being aluminum, silicon, Has magnesium, zinc and / or copper and the connection layer has essentially titanium.

The possible alloying elements of the coating include titanium, aluminum, magnesium, silicon, zinc and / or copper. The coating, in particular a metallic coating, can for example also be an aluminum-titanium-zinc layer or an aluminum-copper-titanium layer or an aluminum-magnesium-titanium layer or an aluminum-silicon-titanium layer or a mixture of these alloying elements. Furthermore, the coating can consist of two alloy elements, for example titanium and aluminum, titanium and zinc, titanium and magnesium, titanium and silicon or titanium and copper. The coating can comprise two, three, four, five or more of the alloy elements. Furthermore, impurities or secondary alloy elements can occur as a residual proportion. The coating can be designed as a scale protection coating on one or both sides of the sheet metal plate.
The advantage here is that the formation of scale in the furnace and the associated contamination of the tools can be avoided. Furthermore, the coating can act actively as a sacrificial material against undesired corrosion in permanently wet areas. The coating has a melting temperature above approximately 1000 ° C and melts therefore not in the heat treatment step. Compared to an AISi coating, the oven dwell time can be shortened considerably to 3-10 minutes, since there is no need to wait for any additional holding times due to the diffusion processes, as is necessary, for example, with the AISi coating. This increases the output and reduces the energy requirement. In contrast to a layer of scale, the coating adheres well and is weldable to the hot-formed component. The coating can therefore remain on the finished sheet steel component and does not need to be removed, so that a processing step involving cleaning blasting can be omitted. The coating can also act as a barrier layer, by means of which hydrogen entry into the base material during the heat treatment step can be prevented or at least reduced. The coating means that there is no need for a protective gas atmosphere or dew point control in the furnace or subsequent heat treatment, which simplifies the process and reduces costs. Furthermore, the alloying elements of the coating can segregate within the coating during the hot forming process. An austenitizing step takes place during the hot forming process. In the austenitizing step, the coated sheet metal blank is heated to austenitizing temperature in a furnace, preferably a roller hearth furnace, the alloying elements of the coating segregating within the coating and the coating, after segregation, having at least one surface layer and one bonding layer, the surface layer being essentially aluminum, Silicon, magnesium, zinc and / or copper and the connection layer essentially comprises titanium. In particular, the alloying elements of the base material can diffuse into the coating. The surface layer can bring about a constant coefficient of friction during forming in the tool, which can result in a more uniform flow behavior of the material, which can result in a reduction in tears, for example. Furthermore, as a scale-resistant layer, the surface layer can prevent scale formation. The connection layer can ensure a good connection or adhesion to the sheet metal blank so that subsequent forming does not lead to flaking. In part, this mechanical bonding effect can also be supported by diffusion processes of alloying elements of the bonding layer and the base material. The connection layer thus forms a ductile connection to the sheet metal plate.

It is also preferred that the coating additionally has a transition layer between the surface layer and the connection layer, the transition layer being designed as a graded layer and having alloying elements comprising titanium, aluminum, magnesium, silicon, zinc and / or copper and alloying elements of the base material. The transition layer is a third layer which is located between the surface layer and the connection layer. For the purposes of this invention, graded describes that the concentration of the alloying elements varies over the layer thickness.

The diffusion processes of the various alloy elements within the coating and their segregation and reaction with the surrounding atmosphere also lead to a graded transition layer between the surface layer and the connection layer, in addition to a scale-resistant surface layer and a ductile connection layer to the sheet metal blank.

The graded transition layer functions as a transition zone between the segregating alloying elements, the concentration of the alloying elements of the surface layer and the connecting layer in the transition layer being lower than in the surface layer or the connecting layer itself.

The surface layer usually has a brittle material behavior and poor electrical conductivity. However, the scale protection requirements can also be met with a layer thickness of the surface layer of 0.05 µm. The surface layer is connected to the underlying transition layer by diffusion processes, which ensures sufficient electrical conductivity and the associated weldability. The connection layer also ensures a good connection or adhesion to the base material through diffusion processes with the base material. As a result, flaking can be avoided in a subsequent reshaping. The advantage of this coating structure is that by applying a single coating, which is separated into a bonding layer, a surface layer and a transition layer, both the requirements for the coating with regard to scale protection with simultaneous formability and weldability can be met, as well as adaptability with regard to various Treatment and aging temperatures and times are available. When applying a multi-layer system, each individual layer can be designed for overload protection, which in practice can only be achieved with thicker layers.

It is preferred that the transition layer essentially comprises zinc, titanium and / or aluminum. With the element zinc as an alloy substance component, the weldability and / or the corrosion resistance can be increased. Titanium can increase the melting temperature. Aluminum can increase the resistance to oxidation.

It is preferred that the coating has elements made from the base material. Through diffusion processes, elements, i.e. components of the base material, can get into the coating by diffusion. Through diffusion processes with the base material, the bonding layer can lead to a good connection or adhesion of the coating to the base material.

It is preferred that a second heat treatment step takes place during the hot forming process of the coated sheet metal blank between the austenitizing step and a press hardening step and / or a third heat treatment step takes place after the press hardening step, so that the structure of the base material consists essentially of martensite with retained austenite and preferably bainite. As a result, in addition to the martensite content, the content of retained austenite and possibly bainite in the structure can also be set in order to improve the ductility and the bending angle of hot-formed workpieces. In one embodiment, the coated sheet metal blank can be processed from a base material to produce a sheet steel component, in which a coated sheet metal blank undergoes a first heat treatment process in a first austenitizing step, the coated sheet metal blank is subjected to a quenching process step in a second step and the coated sheet metal blank is subjected to a third bainitizing step goes through a second heat treatment process and in a fourth press hardening step the sheet metal blank is press hardened to form the steel sheet component and in a fifth treatment step the steel sheet component is subjected to a third heat treatment step (that is to say partitioning heat treatment step). In the first heat treatment process, the coated sheet metal blank is preferably heated above a material-specific austenitizing temperature, for example to around 900 ° C. In a subsequent quenching process, the cooling is preferably carried out at a cooling rate of more than 27 ° C./s, so that the steel sheet can be rapidly cooled. A second heat treatment process preferably starts immediately after the quenching process. By starting the second heat treatment process, the formation of bainite in the structure can be achieved. By abruptly delaying the transformation process from austenite to bainite, a high proportion of bainite and stabilized retained austenite can be set in the structure. The holding time of the temperature during the second heat treatment step is designed in such a way that a structural transformation can take place in the base material essentially to bainite, retained austenite and martensite. In a third heat treatment step (partitioning heat treatment step), the structure is stabilized by evenly redistributing the carbon. This results in the high ductility of the sheet steel material.

Alternatively, in a further exemplary embodiment, the quenching process step and the bainitizing step can be omitted and the press hardening step and partitioning heat treatment step can be carried out simultaneously in a temperature-controlled tool. With the forming tool closed, the sheet steel component can be cooled down to a removal temperature which is in a temperature range close to the material-specific martensite finish temperature at which conversion of austenite to martensite is largely complete, in particular in a temperature range of the martensite finish -Temperature of +/- 10% and that, in particular, a forming tool is heated during the press hardening step to a tool temperature below the martensite finish temperature.

In a further embodiment variant, following direct or indirect hot forming, a partitioning heat treatment step can take place in a heating device in such a way that the temperature range is below the material-specific martensite start temperature and above the martensite finish temperature, in particular with a removal temperature from the opened forming tool in a temperature range above the martensite finish temperature + 10% to + 30%. The advantage of this removal temperature is that component distortion is no longer to be expected.

It is preferred that the coating of the sheet metal plate has a layer thickness between 0.1 μm and 10 μm, preferably between 0.5 μm and 5 μm, particularly preferably not greater than 1 μm. The greatly reduced layer thickness leads to a significantly reduced diffusion time in the heat treatment furnace. Furthermore, the diffusion time is reduced due to the smaller diffusion paths.

It is preferred that the surface layer and the connection layer each have a layer thickness of greater than or equal to 0.05 μm to less than or equal to 5 μm. The surface layer meets high scale protection requirements with a low layer thickness. For processability in the joining process, for example, and under economical conditions From a point of view of viewpoints, a very small layer thickness is of great benefit.

• • Titan: bis 90 Gew.-%
• • Aluminium: bis 60 Gew.-%

und/oder mindestens ein weiteres Legierungselement, umfassend

• • Magnesium: bis 50 Gew.-%,
• • Silizium: bis 35 Gew.-%,
• • Zink: bis 90 Gew.-%

und/oder

• • Kupfer: bis 60 Gew.-%,

bevorzugt:

• • Titan: 15-60 Gew.-%,
• • Aluminium: 5-40 Gew.-%,
• • Zink: 15-60 Gew.-%,

und/oder

• • Magnesium: 5-30 Gew.-%,
• • Silizium: bis 35 Gew.-%,

besonders bevorzugt

• • Titan: 30-40 Gew.-%,
• • Aluminium: 25-35 Gew.-%,

und

• • Zink: 35-45 Gew.-%.

It is preferred that the coating has the following alloy elements:

• • Titanium: up to 90% by weight
• • Aluminum: up to 60% by weight

and / or comprising at least one further alloy element

• • Magnesium: up to 50% by weight,
• • silicon: up to 35% by weight,
• • Zinc: up to 90% by weight

and or

• • Copper: up to 60% by weight,

preferred:

• • titanium: 15-60% by weight,
• • aluminum: 5-40% by weight,
• • Zinc: 15-60% by weight,

and or

• • Magnesium: 5-30% by weight,
• • silicon: up to 35% by weight,

particularly preferred

• • titanium: 30-40% by weight,
• • aluminum: 25-35% by weight,

and

• • Zinc: 35-45% by weight.

Furthermore, impurities or secondary alloying elements can occur as a residual proportion, provided that the sum of all of the aforementioned alloying elements of the coating is less than 100% by weight. Experiments have shown that zinc, as an alloy component, can increase the weldability and / or the corrosion resistance. Titanium as an alloy component is necessarily included in any case to raise the melting temperature. In addition, the melting point of the coating is already above 1000 ° C from a titanium content of approx. 5% by weight and thus well above the process temperature in the heat treatment furnace (approx. 900 ° C). Therefore, the coating remains in the solid phase during the heat treatment step without melting, whereby the heat treatment furnace can be prevented from being contaminated. According to the invention, the titanium content of the alloy is between 5% by weight and
90% by weight, according to the invention the aluminum content of the alloy is between 5% by weight and 60% by weight.

It is preferred that the base material has a zinc coating on at least one side for applying the coating. This results in a wide range of base materials that can be used, so that a suitable or available material can be selected for the respective specific application. Furthermore, the base material can also have a coating other than a zinc coating, for example a coating made of zinc-magnesium or zinc-iron.

It is preferred that the coating after the austenitizing step has a silicon content of up to 35% by weight, preferably up to 15% by weight, and / or a zinc content of up to 90% by weight, preferably up to 60% by weight. The coated sheet metal blank can be heated to the austenitizing temperature in the austenitizing step. Tests have shown that the element silicon diffuses from the base material into the coating at the austenitizing temperature. The proportion of silicon in the surface layer can be adjusted or increased by diffusion processes from the base material. As a result, the scale protection mechanism and / or corrosion protection of the layer can be increased. The alloying element zinc can increase the weldability and the corrosion resistance.

It is preferred that the base material is assigned to a material class for the production of high-strength sheet steel components, the strength of which is Rm greater than 1180MPa, preferably Rm greater than 1650MPa, particularly preferably Rm greater than 1800MPa.

The tensile strength determines the maximum mechanical tensile stress that the base material can withstand. In the case of a sheet metal body component, for example, the base material thus has at least the same ductility and higher tensile strength than the cold forming grades usually used, such as DP780 or DP980. In addition, the higher strength of the components results in a weight saving of up to 20% compared to base materials such as 22MnB5 with an AISi coating.

It is preferred that the coating of the base material can be applied by means of gas phase deposition, preferably physical vapor deposition (PVD) or chemical vapor deposition (CVD), or electrochemically, in particular galvanically or chemically, or the application from the powdery state, preferably by means of powder coating or plasma spraying is feasible.

The PVD process is a process for applying thin layers. It serves as a prototype coating for applying, for example, AITi or AITi (X) layers (with X = further alloy elements). In contrast to CVD technologies, PVD technologies are characterized by the fact that layers are removed from a solid material (so-called target) and produced. In CVD techniques, a layer is deposited from the gas phase. Typical PVD processes are, for example, evaporation or sputtering (atomization). Magnetron sputtering is another variant.

• 0,20 Gew.-% bis 0,42 Gew.-% Kohlenstoff
• bis 2,2 Gew.-% Silizium
• 0,3 Gew.-% bis 2,5 Gew.-% Mangan
• bis 0,06 Gew.-% Niob
• 0,001 Gew.-% bis 0,01 Gew.-% Bor
• bis 0,5 Gew.-% Chrom
• bis 0,5 Gew.-% Nickel
• bis 0,5 Gew.-% Molybdän
• bis 0,1 Gew.-% Vanadium
• bis 0,06 Gew.-% Aluminium
• bis 0,05 Gew.-% Titan
• bis 0,01 Gew.-% Stickstoff
• bis 0,01 Gew.-% Schwefel
• bis 0,02 Gew.-% Phosphor

Furthermore, the invention relates to a method for producing a sheet steel component comprising the following steps: heating and holding a coated sheet metal sheet in an austenitizing step for an austenitizing time above a material-specific austenitizing temperature, inserting the sheet metal sheet into a forming tool at an inserting temperature, cooling the one inserted in the forming tool Sheet metal blank in a press hardening step in a cooling time with formation of the sheet steel component, removing the sheet steel component in a removal step with a removal temperature from the forming tool, the base material having at least the following components in percent by weight:

• 0.20 wt% to 0.42 wt% carbon
• up to 2.2% by weight silicon
• 0.3 wt% to 2.5 wt% manganese
• up to 0.06% by weight niobium
• 0.001 wt% to 0.01 wt% boron
• up to 0.5% by weight of chromium
• up to 0.5% by weight nickel
• up to 0.5% by weight of molybdenum
• up to 0.1% by weight vanadium
• up to 0.06 wt% aluminum
• up to 0.05 wt% titanium
• up to 0.01% by weight nitrogen
• up to 0.01% by weight sulfur
• up to 0.02% by weight of phosphorus

The steel alloy can have iron and impurities, for example, as further constituents. Due to the relatively high carbon content of up to 0.42% by weight, the residual austenite of the base material can be stabilized. In order to also be able to achieve good weldability of the base material, the carbon content should not exceed 0.42% by weight. In particular, the silicon content of 0.8% by weight to 2.2% by weight can stabilize the intermediate structure and retained austenite in the base material, whereby the ductility of the base material can be improved. The alloying element silicon helps to improve the tempering behavior of high-strength steels, so that increased ductility can be achieved. Since silicon is insoluble in iron carbides, the formation of carbides is suppressed and retained austenite is stabilized. In the case of high-strength and at the same time ductile steels, this occurs in small quantities as a thin film between and within the martensite lancets. In order to further improve the ductility of the base material, the manganese content is also limited so that the steel alloy contains a maximum of 2.5% by weight of manganese, preferably only up to 1.0%, in order to achieve a homogeneous structure and disadvantageous carbide formation to prevent the base material. By adding up to 0.06% by weight of niobium, grain refinement can be achieved, which can also contribute to improving ductility. Due to the strong reduction or elimination of titanium, the martensitic transformation in the press hardening step is delayed and the bainite transformation is supported in the bainitization step. Boron is used to bind nitrogen, which also improves ductility. With the alloy components according to the invention, a base material can be made available which is characterized by high strength and, at the same time, high ductility. The steel alloy preferably has contents of chromium, nickel and / or molybdenum, the sum total of chromium, nickel and / or molybdenum preferably being less than 0.5% by weight, preferably less than 0.35% by weight. The steel alloy thus preferably has only a very small proportion of chromium, nickel and molybdenum, as a result of which the amount of more expensive alloying elements in the steel alloy can be reduced.

It is preferred that the hot forming process is an indirect hot forming in which a cutting operation and / or shaping operation in a cold forming step is carried out before the heat treatment step and the press hardening step. Indirect hot forming makes it possible to achieve more complex geometries than, for example, with direct hot forming.

It is also preferred that the sheet steel component is a vehicle body component and / or structural components and / or vehicle chassis component. If the base material is used or used, for example, as a sheet steel component, For example, for a vehicle body component or structural component or a vehicle chassis component, this can lead to a weight reduction. After hot forming, the base material has a yield strength Rp greater than 800MPa, particularly preferably Rp greater than 1350MPa. In a material, the yield point characterizes the stress up to which the material exhibits almost no permanent plastic deformation with a moment-free and uniaxial tensile load. This means that the material deforms, but returns to its original shape when the load is removed. The deformation remains reversible or elastic. The bending angle of the base material after hot forming is preferably equal to or greater than 65 °, particularly preferably greater than or equal to 80 °. The elongation at break of a base material formed, for example, using the quenching and partitioning method is preferably greater than 8%, particularly preferably equal to or greater than 12%. Due to this high bending angle and / or the high elongation at break, the base material can have a particularly high ductility.

• 1 Prinzipskizze eines zweilagigen Schichtaufbaus nach der Warmumformung, nach einem ersten Ausführungsbeispiel.
• 2 Prinzipskizze eines dreilagigen Schichtaufbaus nach der Warmumformung, nach einem zweiten Ausführungsbeispiel.
• 3 REM/EDX Untersuchung einer Ausführungsform der Beschichtung mit 1µm Stärke nach der Presshärtung.
• 4 REM/EDX Untersuchung einer weiteren Ausführungsform der Beschichtung mit 1µm Stärke nach der Presshärtung.
• 5 Prozessabfolge für eine direkte Warmumformung zur Herstellung eines warmumgeformten und pressgehärteten Stahlblechteils als Blockschaubild veranschaulicht, nach einem ersten Warmumformverfahren.
• 6 Prozessabfolge für eine indirekte Warmumformung zur Herstellung eines warmumgeformten und pressgehärteten Stahlblechteils als Blockschaubild veranschaulicht, nach einem zweiten Warmumformverfahren.
• 7 Prozessabfolge einer Quenching und Partinioning Warmumformung zur Herstellung eines warmumgeformten und pressgehärteten Stahlblechteils, nach einem dritten Warmumformverfahren.
• 8 Temperatur-Zeit-Verlauf einer Q&P-Prozessroute.

The various embodiments are explained in more detail below with reference to the following figures. Show it:

• 1 Basic sketch of a two-layer layer structure after hot forming, according to a first embodiment.
• 2 Schematic sketch of a three-layer layer structure after hot forming, according to a second exemplary embodiment.
• 3 SEM / EDX investigation of an embodiment of the coating with a thickness of 1 µm after press hardening.
• 4th SEM / EDX investigation of a further embodiment of the coating with a thickness of 1 µm after press hardening.
• 5 Process sequence for a direct hot forming for the production of a hot formed and press-hardened sheet steel part illustrated as a block diagram, after a first hot forming process.
• 6th Process sequence for an indirect hot forming for the production of a hot formed and press-hardened sheet steel part illustrated as a block diagram, after a second hot forming process.
• 7th Process sequence of quenching and partinioning hot forming for the production of a hot-formed and press-hardened sheet steel part, according to a third hot-forming process.
• 8th Temperature-time history of a Q&P process route.

In 1 is the structure according to a first embodiment of a coated sheet metal plate 2 shown. As a base material 1 carbon, boron and / or manganese-containing quenched and tempered steel can be used, for example. On this base material 1 becomes a coating on both sides 5 with the layer thickness S _G2 upset. The coating has two layers. The layer thickness S _G2 is greater than or equal to 0.5 µm to less than or equal to 10 µm. The coating 5 can be in a tie layer 4th and a surface layer 3 be subdivided. The surface layer 3 can be a layer thickness S ₁ from greater than or equal to 0.05 µm to less than or equal to 5 µm. The connection layer 4th can be a layer thickness S ₂ from greater than or equal to 0.05 µm to less than or equal to 5 µm. The surface layer 3 essentially comprises aluminum or silicon, zinc, copper or magnesium. The connection layer 4th essentially comprises titanium. The coating 5 can act as a barrier layer, by means of which an excessive hydrogen entry from the furnace atmosphere into the base material during the heat treatment 1 is avoided. The coating 5 does not melt in the furnace because the melting point of the alloy at 1000 ° C is well above the process temperatures. The coating 5 can also be spot welded. The coating can therefore 5 remain on the finished sheet steel component and need not be removed.

2 shows, according to a second embodiment, the layer structure of the coated sheet metal blank 2a . The three-layer coating 7th with a layer thickness S _G3 between 0.5 µm and 10 µm has next to the surface layer 3 and the connection layer 4th a transition layer 6th with the layer thickness S ₃ on. During exposure in a furnace, the various components within the layer can diffuse and separate and react with the surrounding atmosphere, creating a scale-resistant surface layer 3 , a graded transition layer 6th and a ductile bonding layer 4th to the base material 1 forms.

In 3 is an SEM / EDX investigation of a TiAI coating with a layer thickness S _G3 of essentially 1 µm in thickness shown after press hardening. The proportions of the elements iron (Fe), aluminum (Al), titanium (Ti), silicon (Si) and oxygen (O) are shown in arbitrary units [au] over the thickness of the layer in [µm]. A thin layer smaller than 1 µm was applied to the board using the PVD process, which protects the sheet from oxidation and hydrogen embrittlement after heating and from forming. In the example shown, a TiAl layer was applied to a Sheet metal plate applied. The sheet metal blank was made after the press hardening step P. examined. There is a segregation of iron, aluminum, titanium and silicon. Aluminum is enriched with oxygen on the surface, thereby forming a protective AlOx layer with the oxygen in the environment. The oxygen peak does not penetrate the entire layer thickness S _G3 , therefore thin layers smaller than 1 µm are sufficient for the application. Oxidation does not take place. The layer can fulfill its function.

After a transition shift 6th with a layer thickness S ₃ a titanium-rich layer can be seen under the AlOx layer. The titanium signal is superimposed here with the silicon, which has diffused from the sheet material into the layer. The superimposition of silicon and oxygen has another positive effect, which speaks in favor of a SiOx layer within the newly formed layer: The SiOx layer under the AlOx has a good corrosion protection barrier. Iron diffuses from the base material into the middle layer.

In 4th is an SEM / EDX investigation of a TiAlZn coating with a layer thickness S _G3 of essentially 1 µm in thickness shown after press hardening. The proportions of the elements iron (Fe), titanium (Ti), zinc (Zn), aluminum (Al) and oxygen (O) are shown in arbitrary units [au] over the thickness of the layer in [µm]. A thin layer of approx. 1 µm was applied to the board using the PVD process, which protects the sheet from oxidation and hydrogen embrittlement after heating and from forming. In the example shown, a TiAlZn layer was applied to a sheet metal plate. The sheet metal blank was made after the press hardening step P. examined. There is a segregation of aluminum, titanium and zinc. Aluminum is enriched with oxygen from the surrounding atmosphere on the surface, creating an AlOx layer that protects against scale. The oxygen peak does not penetrate the entire layer thickness S _G3 , therefore thin layers of approx. 1 µm are sufficient for the application. Oxidation does not take place. The layer can fulfill its function. In a transitional shift 6th with the layer thickness S ₃ zinc has accumulated and forms a good anti-corrosion barrier. During the welding process, the zinc layer partially liquefies and makes the coating easy to weld. A titanium-rich layer can be seen after the transition layer. This bonding layer ensures good adhesion and deformability of the coating in the hot forming process.

In 5 After a first hot forming process, the process sequence of direct hot forming to produce a hot formed and press-hardened sheet steel part is shown schematically. In a first insertion step E _i1 becomes the initially undeformed coated sheet metal blank 2 , 2a transferred from a base material to an oven. In one austenitizing step A _c becomes the undeformed coated sheet metal blank 2 , 2a to a process temperature above the tool-specific austenitizing temperature A _c3 the coated sheet metal blank 2 , 2a heated, which can be around 900 ° C, for example. In a second insertion step E _i2 becomes the as yet undeformed coated sheet metal blank 2 , 2a Conveyed directly from the furnace in the hot state into a forming tool, whereby no cold forming has been carried out beforehand. A press hardening step takes place in the forming tool P. hot forming with simultaneous press hardening with the formation of the sheet steel component. In one removal step E _o the sheet steel component formed in this way is removed from the forming tool after hot forming / press hardening. In one filing step A _b the sheet steel component is stored for further processing. In one processing step B. the sheet steel component is further processed in a post-processing station, for example in the form of cutting operations.

In 6th After a second hot forming process, the process sequence of indirect hot forming for the production of a hot formed and press-hardened sheet steel part is shown schematically. In a cold forming step K _u before a heat treatment, the coated sheet metal blank, which has not yet been deformed, becomes 2 , 2a cold formed. Then in a first insertion step E _i1 the cold-formed coated sheet metal blank 2 , 2a transported to an oven. In one austenitizing step A _c is the cold-formed, coated sheet metal blank 2 , 2a to a process temperature above the tool-specific austenitizing temperature A _c3 the coated sheet metal blank 2 , 2a warmed up.

In a condition heated to above austenitizing temperature, in a second insertion step E _i2 is then the coated sheet metal blank 2 , 2a fed to a forming tool. In one press hardening step P. hot forming and press hardening takes place with the formation of the sheet steel component. Then in a removal step E _o the sheet metal blank is removed from the forming tool and in a storage step A _b for further processing, for example in the form of a cutting operation, in one processing step B. filed. What is not shown is that, as an alternative, the processing step B. the austenitizing step A _c can be upstream.

In 7th is the schematic process sequence of a third hot forming process Quenching and partinioning hot forming for the production of a hot-formed and press-hardened sheet steel part shown. In one austenitizing step A _c becomes an undeformed coated sheet metal blank 2 , 2a above the tool-specific austenitizing temperature A _c3 warmed up. This is followed by a quenching process step A _s . This is followed by a bainitization step B _s . Then in one insertion step E _i the undeformed coated sheet metal blank 2 , 2a fed to a forming tool. In one press hardening step P. hot forming and press hardening takes place with the formation of the sheet steel component. In one removal step E _i the sheet steel component is removed and placed in a storage step A _b filed. This is followed by a partitioning heat treatment step P _w at.

8th exemplifies the course of the temperature T in ° C over time t for quenching and partitioning hot forming. At a first
Heat treatment process step A _c becomes the coated sheet metal blank 2 , 2a preferably to a process temperature T _A within a time Δt _A , via the material-specific austenitizing temperature A _c3 , for example heated to about 900 ° C. The deterrent process A _c is preferably at a cooling rate of greater than 27 ° C / s over a period of time Δt _s carried out so that a rapid cooling of the coated
Sheet metal blank 2 , 2a can be done before the second heat treatment process B _s starts. The second heat treatment process B _s preferably starts when the workpiece has a temperature Tein less than the bainite start temperature B _start and greater the bainite finish temperature B _fin having. The holding phase Δt _B during the second heat treatment step is designed so that in the base material 1 a structural transformation can essentially take place to bainite, retained austenite and martensite. It can be a ferrite area Fer , a bainite area Ba and a pearlite area By form. The holding phase Δt _B can be between 10 seconds and 30 minutes, preferably between 30 seconds and 6 minutes. During the press hardening step P. cools the workpiece within the time Δt _H further on T _off away. During the filing step A _b air cooling continues for a while Δt _LK .
In a third heat treatment step P _w , a partitioning heat treatment step, is used within the time span Δt _p the frozen structure consisting essentially of bainite, stabilized retained austenite and possibly martensite is partitioned, ie stabilized by uniform redistribution of the carbon. The temperatures can be in the temperature range greater than 100-225 ° C. for 30 seconds to 60 minutes, preferably greater than 150-200 ° C. for 5-30 minutes.

The removal temperature T _off can be used in a temperature range of -10% to + 30% around the material-specific martensite finish temperature M _f and below the martensite start temperature M _s in which a transformation from austenite to martensite has largely been completed. The forming tool can be heated in the press hardening step, in particular to a tool temperature below the martensite finish temperature M _f .

List of reference symbols

1

Base material

2

coated sheet metal blank with a two-layer coating

2a

coated sheet metal plate with a three-layer coating

3

Surface layer

4th

Connection layer

5

two-layer coating

6th

Transition layer

7th

three-layer coating

away

Filing step

Ac

Austenitizing step

Ac3

material-specific austenitizing temperature

As

Quenching process step

B.

Machining step

Ba

Bainite area

Bfin

Bainite finish temperature

Bs

Bainitization step

Bstart

Bainite start temperature

egg

Insertion step

Egg1

first insertion step

Egg2

second insertion step

Eo

Removal step

Fer

Ferrite area

Ku

Cold forming step

Mf

Martensite finish temperature

Ms

Martensite start temperature

P.

Press hardening step

Pw

Partitioning heat treatment step

By

Pearlite area

S1

Layer thickness of the surface layer

S2

Layer thickness of the connection layer

S3

Layer thickness of the transition layer

SG2

Layer thickness of the two-layer coating

SG3

Layer thickness of the three-layer coating

T

temperature

t

Time

TA

Process temperature during the austenitizing step A _c

Thou

Workpiece temperature at the end of the press hardening step P

Tein

Workpiece temperature during the bainitization step B _s

ΔT

Temperature difference between the martensite start temperature M _s and the workpiece temperature during the bainitizing step Bs

ΔtA,

Duration of the austenitizing step A _c

ΔtB

Holding phase during the bainitization step

ΔtH

Duration of the press hardening step P

ΔtLK

Duration of the air cooling during the depositing step A _b

Δtp

Duration of the partitioning heat treatment step P _w

Δts

Duration of the quenching process A _c

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely 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

• US 2018/0274062 A1 [0005]
• US 7527876 B2 [0006]

Claims (15)

Coated sheet metal plate (2, 2a) for the production of a hot-formed and press-hardened sheet steel part, the sheet metal plate (2, 2a) having a hardenable steel as base material (1) with a coating (5, 7) on at least one side, characterized in that the coating is titanium and aluminum and / or at least one further alloy element comprising magnesium, silicon, zinc and / or copper and that an austenitizing step (A _c ) takes place during a hot forming process of the coated sheet metal blank (2, 2a), the alloying elements of the coating (5 , 7) segregate within the coating after the austenitizing step (A _c ) and thereby the coating has at least one surface layer (3) and one bonding layer (4), the surface layer (3) essentially aluminum, silicon, magnesium, zinc and / or Has copper and the connection layer (4) has essentially titanium. Coated sheet metal plate (2, 2a) according to Claim 1 , characterized in that the coating (7) additionally has a transition layer (6) between the surface layer (3) and the connecting layer (4), the transition layer (6) being designed as a graded layer and alloying elements comprising titanium, aluminum, magnesium, Has silicon, zinc and / or copper and alloying elements of the base material (1). Coated sheet metal plate (2, 2a) according to Claim 2 , characterized in that the transition layer (6) essentially comprises zinc, titanium and / or aluminum. Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the coating (5, 7) has elements made from the base material (1). Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized _{in that a second heat treatment step (B s} ) takes place during the hot forming process of the coated sheet metal blank (2, 2a) between the austenitizing _{step (A c} ) and a press hardening step (P) and / or a third heat treatment step (P _w ) takes place after the press hardening step (P), so that the structure of the base material (1) consists essentially of martensite with retained austenite and preferably bainite. Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the coating (5, 7) of the sheet metal blank has a layer thickness (S _G2 , S _G3 ) between 0.1 µm and 10 µm, preferably between 0.5 and 5 µm, particularly preferably not larger than 1 μm. Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the surface layer (3) and the connection layer (4) each have a layer thickness (S ₁ , S ₂ ) of greater than or equal to 0.05 µm to less than or equal to 5 µm. Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the coating (5, 7) has the following alloying elements: • titanium: up to 90% by weight • aluminum: up to 60% by weight and / or at least one further alloy element, comprising • magnesium: up to 50% by weight, • silicon: up to 35% by weight, • zinc: up to 90% by weight and / or • copper: up to 60% by weight, where the Coating preferred: • titanium: 15-60% by weight, • aluminum: 5-40% by weight, • zinc: 15-60% by weight, and / or • magnesium: 5-30% by weight, • Silicon: up to 35% by weight, the coating particularly preferably having • titanium: 30-40% by weight, • aluminum: 25-35% by weight and • zinc: 35-45% by weight. Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the base material (1) has a zinc coating on at least one side for applying the coating (5, 7). Coated sheet metal blank (2, 2a) according to one of the preceding claims, characterized in that the coating (5, 7) after the austenitizing step (A _c ) has a silicon content of up to 35% by weight, preferably up to 15% by weight and / or has a zinc content of up to 90% by weight, preferably up to 60% by weight. Coated sheet metal plate (2, 2a) according to one of the preceding claims, characterized in that the base material (1) is assigned to a material class for the production of high-strength sheet steel components, the strength of which is Rm greater than 1180MPa, preferably Rm greater than 1650MPa, particularly preferably Rm greater than 1800MPa is. Coated sheet metal plate (2, 2a) according to one of the preceding claims, characterized in that the coating (5, 7) of the base material (1) by means of gas phase deposition, preferably PVD or CVD, or electrochemically, in particular can be applied galvanically or chemically or the application can be carried out from the powdery state, preferably by means of powder coating or plasma spraying. A method for producing a sheet steel component, comprising the following steps: heating and holding a coated sheet metal blank (2, 2a) in an austenitizing step (A _c ) for an austenitizing time (Δt _A ,) above a material-specific austenitizing temperature (A _c3 ). - Inserting the sheet metal _{blank (2, 2a) in an inserting step (E i} ) with an insertion temperature (T _A ) in a forming tool - Cooling the sheet metal blank (2, 2a) inserted in the forming tool in a press hardening step (P) in a cooling time (Δt _H ) with formation of the sheet steel component - removing the sheet steel _{component in a removal step (E o} ) with a removal temperature (T _out ) from the forming tool, the base material (1) having at least the following components in percent by weight: 0.20% by weight to 0 , 42% by weight carbon to 2.2% by weight silicon, 0.3% by weight to 2.5% by weight manganese to 0.06% by weight niobium, 0.001% by weight to 0.01 % By weight boron to 0.5% by weight chromium to 0.5% by weight nickel to 0.5% by weight molybdenum to 0.1% by weight vanadium to 0.06% by weight aluminum up to 0.05% by weight titanium to 0.01% by weight nitrogen to 0.01% by weight sulfur to 0.02% by weight phosphorus Method for producing a sheet steel component according to Claim 13 , characterized in that the method is an indirect hot forming, in which a cutting operation and / or shaping operation in a cold forming step (K _u ) is carried out before the austenitizing _{step (A c} ) and the press hardening step (P _w ). Sheet steel component that is produced by a method according to Claims 13 or 14th is produced, characterized in that the sheet steel component is a vehicle body component and / or structural components and / or vehicle chassis component.

Images