DE102020204356A1 - Hardened sheet metal component, produced by hot forming a flat steel product and process for its production - Google Patents

Abstract

A hardened sheet metal component, produced by hot forming, comprising a steel substrate and an anti-corrosion coating that contacts the steel substrate, is proposed, the anti-corrosion coating of which has a first alloy layer that contacts the steel substrate and a second alloy layer that contacts the first alloy layer. Furthermore, a flat steel product is proposed, comprising a steel substrate and an anti-corrosion coating which contacts the steel substrate and has the main components manganese and zinc. In addition, a method for producing a hardened sheet metal component is proposed.

Description

The present invention relates to a hardened sheet metal component comprising a steel substrate and an anti-corrosion coating; the anti-corrosion coating includes manganese, iron and zinc. The present invention also relates to a flat steel product for producing the hardened sheet metal component according to the invention; the flat steel product comprises a steel substrate and a metallic layer contacting the steel substrate.

The present invention also relates to a method for producing a hardened sheet metal component according to the invention.

Finally, the invention also relates to the use of a flat steel product according to the invention for producing a hardened sheet metal component according to the invention.

The flat steel products according to the invention are rolled products, such as steel strips, steel sheets or blanks and blanks obtained therefrom, the thickness of which is significantly less than their width and length.

A hardened sheet metal component according to the invention is preferably a shaped steel part which is produced from a corresponding flat steel product by hot forming.

Alloy elements with the highest weight percentage are referred to as the main component or components. This means that the alloying element is contained in a larger amount than any further alloying constituent that may be contained.

Information on the components of the structure of a steel, a flat steel product or a sheet metal component formed from it, on the other hand, always relate to the volume (information in% by volume). If mentioned, the proportions of austenite were measured via XRD with Fe-filtered Co-Kα radiation. The XRD measurement method is described in the following source: DIN EN 13925 X-ray diffractometry of polycrystalline and amorphous materials Part 1 and 2 from 2003_7, Part 3 from 2005. The other structural components, if mentioned, have each been identified by light microscopy after nital etching.

The mechanical properties mentioned in the present text tensile strength Rm, yield strength Rp0.2 and elongation at break A80 are according to DIN EN ISO 6892-1: 2017-02 has been determined.
In DE 10 2012 024 616 A1 it is described that hardened components with extremely high strength can be manufactured from press-hardening steel sheets by heating the steel sheet above the austenitizing temperature and obtaining an essentially purely martensitic structure by cooling it during pressing. A steel sheet is disclosed, in particular a press-hardening steel sheet, consisting of a steel substrate and an anti-corrosion layer containing zinc and manganese.

EP 3 483 297 A1 discloses a press hardened component with excellent ductility and corrosion resistance. The press-hardened component consists of a steel substrate and a zinc-based corrosion layer. The coated flat steel product is heated above the austenitizing temperature and held for 5-10000 s, cooled to 400-650 ° C and hot-formed in this area.

In EP 2 778 247 A1 discloses a steel sheet and a method for producing a component by hot press forming the steel sheet after heating to a temperature between the Ac1 and Ac3 temperatures of the respective steel alloy. According to this method, the steel sheet is heated to a temperature which is between the Ac1 and Ac3 temperatures of the respective steel alloy, and hot-press formed.

From the EP 2 383 353 B1 Examples of high-strength, Mn-containing steels are known which, as coated or uncoated hot or cold strip, have an elongation at break A80 of at least 4% and a tensile strength of 900-1500 MPa. Furthermore, a method for producing a coated or uncoated hot or cold strip is presented.

In WO 2019/020169 a sheet metal component and a method for its production is disclosed, which leads to an increased residual elongation of the component at high strengths. The structure consists of 5-50% by volume of austenite and the remainder of martensite, tempered martensite or ferrite, whereby the ferrite content can also be "0".

In the hot forming of known hot forming steels, corrosion protection coatings are usually used, for example Al-Si coatings, in order to prevent scaling of the surface. Such Al-Si coatings, however, only passively protect against corrosion due to their barrier effect. Passive corrosion protection means that if the coating is damaged in the affected areas, the corrosion protection effect does not apply. The use of actively protective Zn coatings is only possible to a limited extent due to the comparatively low melting point of pure zinc of approx. 419 ° C. Active corrosion protection means that the corrosion protection coating is less noble than the steel substrate and reacts with the oxygen in the environment in order to protect the steel substrate and, even if the corrosion protection coating is locally damaged, the protective effect is retained over a certain distance. It has been observed that when forming, liquid zinc penetrates into the base material (the steel substrate) and leads to crack formation there (so-called liquid metal embrittlement). In some cases this problem is already circumvented by substituting the Zn coating with a higher melting alloy, so that this is only available in liquid form to a sufficiently small extent during hot forming or by using alternative processes. In that regard, let me return to the document DE 10 2012 024 616 A1 referenced.

Typical manganese-boron steels used for hot forming have a low elongation at break and are therefore only able to absorb energy to a limited extent in the event of a crash.

Corrosion protection coatings for steel sheets (flat steel products) and hot-formed steel sheets (hardened sheet metal components) which contain zinc and manganese are already known from the prior art. The already known anti-corrosion coatings, flat steel products or hardened sheet metal components and the processes for their respective production do not yet appear to be optimal. There is a need for an anti-corrosion coating on a flat steel product that has or takes on a dark color during heating, so that radiation energy can be absorbed particularly efficiently in the furnace and the substrate can thus be heated particularly quickly to a temperature above Ac3.

This corresponds to the existing need for sheet metal components with an anti-corrosion coating that can be hot-formed after only a short period in which the furnace has been in place, without this having an impact on the desired structure of the steel substrate after the hot-forming. This is accompanied by the requirement for a low Ac3 temperature to save energy during the heating and cooling process. At the same time, the aim is to increase the throughput when using existing ovens or to be able to design future ovens smaller for a given throughput when designing or purchasing new ovens.

In addition, there is a need for anti-corrosion layers that enable cathodic and active corrosion protection, with active corrosion protection preferably provided by an electrochemical potential difference between the anti-corrosion coating and substrate of greater than 0 mV, preferably greater than 50 mV, particularly preferably greater than 100 mV and very particularly preferably greater than 150 mV is.

In addition, there is also a need for anti-corrosion coatings, flat steel products or hardened sheet metal components, in which tool and furnace roller adhesions are suppressed or completely avoided. If a coating becomes liquid, as is often the case with corrosion coatings from the prior art, part of the coating always adheres to the furnace rollers or to the tool, which reduces the service life of the equipment. Corrosion protection coatings are therefore advantageous which have as little or no portion of the liquid phase as possible on the surface.

Against the background of the prior art explained above, the task was to create a sheet metal component which, compared to conventionally manufactured sheet metal components, enables energy savings and a reduction in CO ₂ emissions through lower forming temperatures, which absorbs radiation energy particularly efficiently and thus heats it up quickly can be, allows an increased residual elongation at high strengths and with which the highest possible potential for cathodic corrosion protection is preserved. Furthermore, the task consisted of reducing the cooling speed required for martensite formation in the steel substrate, which should increase process reliability during hot forming and also enable energy savings here. In addition, the disadvantages associated with the respective prior art referred to above should preferably be overcome or at least weakened. The focus was also on ensuring that no cracks in the anti-corrosion coating caused by liquid zinc occur during hot forming, even at temperatures above Ac1.

In addition, a method for producing such a sheet metal component should be specified.

According to a primary aspect of the present invention, numerous of the objects specified above are achieved by a hardened sheet metal component comprising a steel substrate and an anti-corrosion coating, the steel substrate consisting of (in% by weight) C: up to 0.5%, Si: 0, 05 - 1%, Mn: 4 - 12%, Cr: 0.1 - 4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, Cu and Ni: in total up to 2%, Ti, Nb, V: in total up to 0.5%, rare earths: up to 0.1% and the remainder of Fe and unavoidable impurities and wherein the anti-corrosion coating comprises a first alloy layer, the two main components of which are iron and zinc, which contact the steel substrate, and a second alloy layer which contacts the first alloy layer, the two main components of which are manganese and zinc.

The anti-corrosive coating consists of a single or multi-layer metallic layer that makes material contact with the steel substrate. According to the invention, the cathodic corrosion protection coating of the hardened sheet metal component offers active corrosion protection, with active corrosion protection preferably being provided by an electrochemical potential difference between the corrosion protection coating and the substrate of greater than 0 mV, preferably greater than 50 mV, particularly preferably greater than 100 mV and very particularly preferably greater than 150 mV.

The expert decides on the basis of microscopic examinations on the cross-section whether a layer to be viewed has one or more layers.

The hardened sheet metal component is characterized in that, in the flat steel product, the proportion of the total mass of manganese in the anti-corrosion coating of the total mass of manganese and zinc in the anti-corrosion coating is equal to or greater than 3%, preferably equal to or greater than 5%, preferably equal to or greater than 10%, preferably equal to or greater than 20% and particularly preferably equal to or greater than 27% and / or the proportion of iron in the second alloy layer of the anti-corrosion coating is up to 20% and / or it contains a remainder of unavoidable impurities.

It has been shown that with such a high total proportion of manganese in the total mass of manganese and zinc in the anti-corrosion coating, comprising the first alloy layer, the second alloy layer and optionally an outer layer comprising manganese oxide, it is surprisingly particularly clear that only evaporate extremely small amounts of zinc during heating, radiation energy is absorbed particularly efficiently, the oven dwell time can be advantageously reduced, particularly efficient corrosion protection can be achieved and the disadvantageous effects associated with liquid metal embrittlement are not observed or are only observed to a reduced extent.

The present invention makes it possible to select the manganese content in such a way that liquid zinc does not cause cracks or defects in the anti-corrosion coating even at temperatures above Ac1. This gives the flexibility to adapt the properties of the anti-corrosion coating to the required hot forming temperature and to vary the hot forming temperature.

A hardened sheet metal component is particularly preferred, the proportion of the total mass of manganese in the anti-corrosion coating in the total mass of manganese and zinc in the anti-corrosion coating being equal to or less than 40%, preferably equal to or less than 35%. It has been shown that when a manganese content of 40%, based on the total mass of manganese and zinc in the corrosion protection coating as a whole, is exceeded, the individual advantages discussed above are in some cases no longer reproducible.

In addition, the present invention relates to a flat steel product, in particular for producing a hardened sheet metal component according to the invention, as defined in the claims and discussed above. Such a flat steel product according to the invention comprises a steel substrate and a single-layer or multi-layer anti-corrosion coating, the two main components of which are zinc and manganese, which contact the steel substrate.

The steel substrate of the flat steel product consists of (in% by weight) C: up to 0.5%, Si: 0.05-1%, Mn: 4-12%, Cr: 0.1-4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, Cu and Ni: in total up to 2%, Ti, Nb, V: in Sum up to 0.5%, rare earths: up to 0.1% and the remainder of Fe and unavoidable impurities.

In the single or multi-layer anti-corrosion coating of the hardened flat steel product, which makes contact with the steel substrate, the proportion of the total mass of manganese in the total mass of manganese is and zinc in the anti-corrosion coating is equal to or greater than 3%, preferably equal to or greater than 5%, preferably equal to or greater than 10%, preferably equal to or greater than 20% and particularly preferably equal to or greater than 27% and preferably equal to or less than 40%, and particularly preferably equal to or less than 35%.

The anti-corrosive coating consists of a single or multi-layer metallic layer that makes material contact with the steel substrate. According to the invention, the cathodic corrosion protection coating of the flat steel product offers active corrosion protection, with active corrosion protection preferably being provided by an electrochemical potential difference between the corrosion protection coating and the substrate of at least greater than 0 mV, preferably greater than 50 mV, particularly preferably greater than 100 mV and very particularly preferably greater than 150 mV.

It has been shown that with such a high total proportion of manganese in the total mass of manganese and zinc in the corrosion protection coating, comprising the first alloy layer, the second alloy layer and optionally further layers, preferably comprising manganese oxide, it is surprisingly particularly clearly ensured that only extremely small amounts of zinc evaporate during heating, radiation energy is absorbed particularly efficiently, the furnace dwell time can be advantageously reduced, particularly efficient corrosion protection can be achieved and the disadvantageous effects associated with liquid metal embrittlement are not observed or are only observed to a reduced extent. If a manganese content of 40%, based on the total mass of manganese and zinc in the corrosion protection coating as a whole, is exceeded, the individual advantages discussed above can in some cases no longer be achieved in a reproducible manner.

A flat steel product according to the invention is particularly preferred, the total proportion of manganese and zinc in the single or multi-layer contacting corrosion protection coating contacting the steel substrate being greater than 90% by weight, preferably greater than 95% by weight, particularly preferably greater than 99 % By weight and very particularly preferably greater than 99.9% by weight, and / or the proportion of iron is less than 10% by weight, preferably less than 2% by weight, and / or no aluminum or Aluminum is contained in a maximum amount of 0.05% by weight and / or a remainder of unavoidable impurities is contained.

For the purposes of this application, an element is an unavoidable contamination of the anti-corrosion coating if the mass fraction in the anti-corrosion coating is less than 1.0% by weight, preferably less than 0.8% by weight and particularly preferably less than 0.5% by weight. This definition only refers to unavoidable contamination of the anti-corrosion coating. For the unavoidable impurities in the steel substrate mentioned later, the customary limit values apply.

It goes without saying that although the transformation of the flat steel product according to the invention into a hardened sheet metal component according to the invention may result in a slight change in the mass fraction of zinc, for example because zinc evaporates or diffuses into the steel substrate, the person skilled in the art will, however, recognize such effects when designing the component according to the invention, taking into account the process parameters when converting the flat steel product into the hardened sheet metal component.

It goes without saying that the hardened sheet metal components according to the invention discussed above can be produced by treating a flat steel product according to the invention.

The person skilled in the art can, for example, by hot forming, convert a preferred flat steel product according to the invention into a preferred hardened sheet metal component according to the invention; for selected details, reference is made to the following statements on the method according to the invention.

The first alloy layer and the second alloy layer are, because the production of the hardened sheet metal component according to the invention regularly starts from a flat steel product according to the invention and a production method according to the invention is carried out, usually the result of a segregation of a starting alloy which comprises manganese and zinc as main components; Due to diffusion processes, the first alloy layer usually contains a high proportion of iron, which comes from the steel substrate. On components according to the invention, methods and uses in the further course below.

The first alloy layer contacting the steel substrate, the two main components of which are iron and zinc, cathodically protects the substrate from corrosion. The second alloy layer, the two main components of which are manganese and zinc, has a melting point well above the temperatures normally used in hot forming. As a result, there is advantageously less tool and oven roller buildup. It should be noted that in the second alloy layer, the two main components of which are manganese and zinc, the proportion of the total mass of manganese is significantly higher than in the first alloy layer. This is because the main components in the first alloy layer are iron and zinc, that is to say that both the proportion of iron and the proportion of zinc in this first alloy layer are each higher than the proportion of any other metal, including manganese. In the second alloy layer, on the other hand, manganese and zinc are the two main components, that is, the proportion of manganese and the proportion of zinc are each higher than the proportion of other alloy components, including iron.

In the second alloy layer, the proportion of iron in the total mass of the second alloy layer can be up to 20% by weight, preferably up to 15% by weight. In some cases, the proportion of the total iron mass may even be higher than the proportion of the total zinc mass. Such local peculiarities are usually unproblematic; however, for the purposes of the present invention, it is necessary that in the second alloy layer, manganese and zinc as a whole are the two main components.

The proportion of iron in the anti-corrosion coating is limited in order to avoid the formation of red rust in the event of corrosion. The origin of the red rust is difficult to distinguish and is therefore to be regarded as critical. Red rust, which occurs when the anti-corrosion coating corrodes, is not critical, but it is difficult to distinguish from red rust on the steel substrate, which should be prevented in order to ensure the function of the component. Therefore, red rust in the coating is unacceptable for the customer, even if in the case of the cathodic dissolution of the coating, this has no significance for the function of the component under the coating. The iron content can also be set by pre-alloying the flat steel product, which can be done by furnace annealing before hot forming. Further possibilities for the master alloy would be to go through a continuous annealing or a hood annealing in order to produce a master alloy with iron. Pre-alloying means separate heating as a sub-process before the actual heating to the heating temperature for hot forming.

Preferably, in the second alloy layer, manganese and zinc are the two main components and the three main components are manganese, zinc and iron. As stated above, local fluctuations in concentration are irrelevant for the determination of the main constituents in the second alloy layer, since it depends on the alloy layer as a whole and on the total masses contained in it.

The sheet metal component according to the invention offers cathodic and active corrosion protection through the anti-corrosion coating, active corrosion protection preferably being provided by an electrochemical potential difference between the anti-corrosion coating and the substrate of greater than 0 mV, preferably greater than 50 mV, particularly preferably greater than 100 mV and very particularly preferably greater than 150 mV .

In a preferred hardened sheet metal component according to the invention, the thickness of the first alloy layer is preferably greater than or equal to the thickness of the second alloy layer. This applies preferably in at least 80% of the total area of the anti-corrosion coating, preferably in at least 85%, particularly preferably in at least 90% of the total area of the anti-corrosion coating. In this way, particularly effective protection against corrosion is ensured. Starting from components according to the invention and using production methods according to the invention (see in detail below), the person skilled in the art can adjust the thickness of the alloy layers in the desired manner, in particular by varying the residence time at selected high temperatures.

A hardened sheet metal component according to the invention is preferred, in the anti-corrosion coating further comprising an outer layer contacting the second alloy layer comprising manganese oxide, the thickness of the first alloy layer preferably being greater than or equal to the total thickness of the second alloy layer and the outer layer.

Such a hardened sheet metal component according to the invention thus also comprises in the anti-corrosion coating, in addition to the first alloy layer and the second alloy layer, at least the outer layer comprising manganese oxide. During the production of the hardened component, this is usually caused by the reaction of manganese (as is also present in the second alloy layer in particular) with oxygen, e.g. B. the oxygen in the ambient air. Such an outer layer comprising manganese oxide is distinguished by the fact that, due to its dark color, it only receives little radiation in an oven reflected. As a result, the surface absorbs more radiant energy and can be heated up quickly. Corresponding advantages also relate in particular to the production method according to the invention.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, like the general inventive concept.
A sheet metal component according to the invention is accordingly produced by hot forming a flat steel product, the steel substrate of which consists of (in% by weight) C: up to 0.5%, Si: 0.05 - 1%, Mn: 4 - 12%, Cr: 0, 1 - 4%, AI: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, in total up to 2% Cu or Ni , in total up to 0.5% of Ti, Nb or V, rare earths: up to 0.1% and the remainder consists of Fe and unavoidable impurities and the contacting corrosion protection coating has the two main components iron and zinc, the proportion of Total mass of manganese in the anti-corrosion coating of the total mass of manganese and zinc in the anti-corrosion coating is equal to or greater than 3%, preferably equal to or greater than 5%, preferably equal to or greater than 10%, preferably equal to or greater than 20% and particularly preferred is equal to or greater than 27% and preferably equal to or less than 40% and particularly preferably equal to or less than 35%.

The steel material used for the steel substrate can be used both in its pure form and in combination in the form of layers (preferably 3 to 5 layers, sheets on top of one another, which are previously connected to a steel strip by a rolling process) or joined steel sheets (e.g. a combination of two steel strips attached to each other, which are e.g. joined to a steel strip by a laser weld seam or are used in the form of tailored blanks

The thickness of the steel substrate is preferably in the range from 0.6 mm to 7 mm, preferably in the range from 0.8 to 4 mm and particularly preferably in the range from 0.8 to 3 mm.

The carbon content is decisive for the strength of the material and must therefore be set and limited in a targeted manner. The maximum content of% C is determined by formula (1)% C <0.55% by weight -% Cr / 10, whereby the% C content in% by weight and% Cr in% by weight cannot be negative. C and Cr both increase the strength of the steel. At the same time, however, the proportion of these two elements must not be too high, otherwise there is a risk of excessive formation of Cr carbides, which increase the brittleness of the material. Therefore, the contents of Cr and C are adjusted according to formula (1).

At the same time, the flat steel product according to the invention has a bending angle of more than 60 °, determined according to VDA 238-100: 2010-12, after hot forming into the sheet metal component.

The flat steel product formed according to the invention into the sheet metal component consists of a steel substrate which is assigned to the class of so-called "medium manganese steels", which usually have Mn contents of 4 to 12% by weight, in particular an Mn content of greater than 4% by weight, preferably greater than 5% by weight and an Mn content of less than 9% by weight, preferably less than 7% by weight. Manganese "Mn" lowers the austenitizing temperature and delays the transformation of ferrite, pearlite and bainite. This also allows the holding temperature in the furnace to be reduced before hot forming. The advantages obtained are further enhanced by holding and hot forming in the two-phase area. During the subsequent cooling process, a high proportion of austenite is retained. This leads to a very high residual elongation at break as well as a high possible bending angle up to the first cracks in the plate bending test according to VDA 238-100 and thus a higher energy absorption in the event of a crash. The Mn contents of a flat steel product processed according to the invention are 4 to 12% by weight, in particular an Mn content greater than 4% by weight, preferably greater than 5% by weight and an Mn content less than 9% by weight, preferably less than 7% by weight so that the required minimum strengths of a steel according to the invention are reliably achieved and at the same time a sufficiently high residual austenite content is retained, which ensures optimal elongation properties.

In the steel substrate of a flat steel product formed according to the invention to form the component, carbon “C” determines, on the one hand, the strength of martensite and, on the other hand, the amount and stability of the retained austenite. If the carbon content is too high, the weldability and toughness of the steel, e.g. B. negatively influenced by the formation of Cr carbides. The carbon content of Mn steels of the type selected according to the invention is therefore at most 0.5% by weight, with lower C contents of less than 0.5% by weight, preferably of up to 0.3% by weight, in particular of up to 0.2 wt .-% prove to be particularly favorable. If the carbon content is too low, however, the amount and stability of the remaining retained austenite is impaired. The C content of a steel according to the invention is therefore at least 0.02% by weight, preferably at least 0.04% by weight.

Aluminum "AI" and silicon "Si" are strong ferrite formers. Both elements counteract the influence of the austenite formers C and Mn. The essential task of the elements Si and Al in the steel of a flat steel product hot-formed according to the invention into the sheet metal component is to suppress the carbide precipitation and thus to promote the stability of the retained austenite. At the same time, Si and Al lead to solid solution hardening and reduce the specific weight of the steel. However, if the Si and Al content is too low, the carbide precipitation may not be effectively suppressed. If the Si and Al contents are too high, on the other hand, processing is made more difficult both in the case of production via a continuous casting process and in the case of production via a strip casting process. The invention therefore provides for the Si content to be limited to a maximum of 1% by weight, preferably to a maximum of 0.8% by weight, whereby the positive effects of the presence of Si can then already be used effectively, when the Si content of the steel of the flat steel product from which the component according to the invention is hot-formed is at least 0.05% by weight. In particular, Al contents greater than 3.5% by weight of the flat steel product used according to the invention for the hot forming of the sheet metal component significantly reduce the density of the steel, but lead to increased proportions of ferrite in the structure and an associated decrease in strength. If the Al content is too high, the suitability for welding is also reduced, as stable welding slag is formed during the welding process and the electrical welding resistance is increased. At the same time, the Ac3 temperature is increased by high Al contents to such an extent that a low hot forming temperature, as aimed at by the invention, can no longer be achieved.

The presence of chromium “Cr” in contents of 0.1 to 4% by weight in a steel according to the invention specifically reduces the risk of stress corrosion cracking. Cr and Al prevent hydrogen-induced cracking. In addition, Cr contributes to the increase in strength. Furthermore, Cr also lowers the Ms temperature (martensite start temperature) and thus supports the stabilization of retained austenite. These positive effects can be observed from a Cr content of 0.1% by weight, but in particular from a Cr content of at least 2.2% by weight. From a Cr content of 2.2% by weight, the scaling resistance is also improved in the uncoated state. In the case of flat steel products that are provided with a metallic corrosion protection coating, a positive effect on the coating can be used, such as the effect as a diffusion barrier for the diffusion of iron into the protective coating. The Cr content of the steel of a flat steel product hot-formed into the component according to the invention is limited to a maximum of 4% by weight, because higher contents could result in Cr carbides which would negatively affect the ductility of the steel.

Also with a view to avoiding the formation of higher amounts of Cr carbide, the invention prescribes that the “% C” content of carbon “C” in% by weight and the “% Cr” content of chromium “Cr” in% by weight. -% of the steel of a flat steel product formed according to the invention to form the sheet metal component must comply with the condition% C <0.55% by weight -% Cr / 10.

By adding copper “Cu” or nickel “Ni” to the steel of the hot-formed flat steel product according to the invention, the resistance to various corrosion mechanisms can be improved. The positive effect of Cu and Ni can be used particularly safely by adding these elements in amounts in which they are technically effective. This is to be expected if the sum of the Cu and Ni contents in the steel of the component according to the invention is greater than 0.04% by weight. In contrast, negative effects such as higher costs and hot crack brittleness with high Cu contents of the individual or combined presence of Cu or Ni in steels according to the invention are reliably avoided by limiting the sum of the Cu and Ni contents to a maximum of 2% by weight.

The micro-alloy elements Ti, Nb and V can be present in the steel of the flat steel product from which the component according to the invention is formed, in contents of up to 0.5% by weight in total. These micro-alloy elements help to refine the grain and increase strength. In total, contents of Ti, Nb and V exceeding 0.5% by weight do not lead to an increase in this effect, whereas the positive effects of Ti, Nb and V in the steel of the component according to the invention can safely be used if their content in total is at least 0.05% by weight.

The austenitic structure can be additionally stabilized by adding nitrogen "N" in contents of up to 0.05% by weight. If the N content is too high, the processability during continuous casting is impaired and an embrittling amount of nitrides is created.

The phosphorus “P” content of the steel of a component according to the invention is limited to a maximum of 0.05% by weight in order to reliably exclude negative influences of this element due to embrittlement.

For the same reason, the sulfur “S” content of a steel according to the invention is limited to a maximum of 0.01% by weight in order to prevent embrittlement.

Rare earths “REM” can contribute to grain refinement in the steel of the component according to the invention by forming oxides and improve the isotropy of the mechanical-technological properties via the texture. The two rare earths cerium and lanthanum are chemically almost identical and therefore always occur in a community in nature. Due to their chemical similarity, they are very difficult and therefore difficult to separate. They have the same effect. The rare earths can be freely substituted for use in steel. At contents above 0.1% by weight, however, there is a risk of so-called “clogging”, i.e. clogging of the casting mold by locally solidifying melt, among other things when casting the steel on an industrial scale. The advantages of the presence of the SEM can nevertheless be used safely if the steel content of a component according to the invention is at least 0.0005% by weight.

The bending angle determined in accordance with VDA 238-100: 2010-12 is a measure of the folding behavior of the material in the event of a crash and thus an indicator of the ductility that a hot-formed component has. Components according to the invention are characterized by a high bending angle of at least 60 °, in particular at least 80 ° or more than 80 °, such as at least 85 °, after hot forming. The even, very fine structure plays a supporting role. A high austenite content, as is the case when the hot forming takes place at temperatures which are in the two-phase mixing area of the steel (or lower), from which the flat steel product from which the component is formed, has advantageous effects.

Sheet metal components hardened according to the invention are characterized by the fact that they have a structure which consists of at least 1% by volume of retained austenite, the retained austenite content of the structure being at most 50.0% by volume, preferably at most 40.0% by volume .-% and particularly preferably can be at most 35.0% by volume. The rest of the structure of the sheet metal component consists of strength-increasing components of martensite and tempered martensite. It can also contain ferrite, whereby the ferrite content can also be “0”. The amount of other technically unavoidable structural components present is so small that they are ineffective with regard to the properties of the component according to the invention. The mean grain diameter of the grains of the structure is less than 5 µm.

Sheet metal components hardened according to the invention have, after hot forming, a tensile strength Rm of the steel substrate with anti-corrosion coating of at least 800 MPa, preferably at least 900 MPa, particularly preferably at least 1000 MPa.

Sheet metal components hardened according to the invention have, after hot forming, an elongation at break A80 of more than 6%, preferably more than 8%, particularly preferably more than 11%.

Sheet metal components hardened according to the invention have, after hot forming, a product Rm * A80 formed from its tensile strength Rm and its elongation at break A80 of more than 10,000 MPa%, preferably more than 12,000 MPa%, particularly preferably more than 13,000 MPa%. The increased elongation at break serves to absorb increased energy in the event of a crash.

A hardened sheet metal component according to the invention is preferred, the steel substrate being a steel with the structure described. Starting from components according to the invention and using the method according to the invention (see below in each case), the described microstructure can be achieved, which guarantees the strengths aimed at by the person skilled in the art. Due to the anti-corrosion coating, there is excellent protection against corrosion.

A hardened sheet metal component according to the invention is preferably a hot-formed component, preferably a component of a motor vehicle, preferably selected from the group consisting of bumper cross members, side impact beams, pillars and body reinforcements.

1. (a) Bereitstellen eines Stahlsubstrats aus einem Stahl, der (in Gew.-%) aus
   • C: bis zu 0,5 %,
   • Si: 0,05 - 1 %,
   • Mn: 4-12%,
   • Cr: 0,1 - 4 %,
   • Al: bis zu 3,5 %,
   • N: bis zu 0,05 %,
   • P: bis zu 0,05 %,
   • S: bis zu 0,01 %,
   • Cu, Ni: in Summe bis zu 2 %,
   • Ti, Nb, V: in Summe bis zu 0,5 %
   • Seltene Erden: bis zu 0,1 %
   und als Rest aus Fe und unvermeidbaren Verunreinigungen besteht,
2. (b) Applizieren einer ein- oder mehrlagigen metallischen kathodischen Korrosionsschutzbeschichtung, deren zwei Hauptbestandteile Zink und Mangan sind, auf das Stahlsubstrat, wobei in der ein- oder mehrlagigen metallischen Schicht der Anteil der Gesamtmasse an Mangan an der Gesamtmasse an Mangan und Zink gleich oder größer ist als 3%, bevorzugt gleich oder größer 5%, bevorzugt gleich oder größer 10%, bevorzugt gleich oder größer 20%, besonders bevorzugt gleich oder größer 27% ist und dabei bevorzugt gleich oder kleiner 40 %, und besonders bevorzugt gleich oder kleiner 35 % ist.
3. (c) Durcherwärmen des Stahlsubstrats mit kathodischer Korrosionsschutzbeschichtung auf eine Erwärmungstemperatur, wobei die Erwärmungstemperatur in einer Alternative mindestens 400°C und höchstens gleich der Ac3 -Temperatur - 30°C des Stahlsubstrats beträgt, oder die Erwärmungstemperatur in einer anderen Alternative mindestens gleich der Ac3 -Temperatur - 30°C und höchstens gleich der Ac3 -Temperatur + 100°C des Stahls beträgt, aus dem das Stahlsubstrat jeweils besteht;
4. (d) Umformen des auf die Erwärmungstemperatur erwärmten Stahlsubstrats mit Korrosionsschutzbeschichtung zu dem Blechbauteil und optional
5. (e) Abkühlen des Blechbauteils mit einem durchschnittlichen Temperaturgradienten von maximal 1.000 K/s bis auf Raumtemperatur.

The method according to the invention for producing a sheet metal component provided in accordance with the preceding claims comprises the following work steps:

1. (a) Providing a steel substrate made from a steel which (in% by weight) consists of
   • C: up to 0.5%,
   • Si: 0.05 - 1%,
   • Mn: 4-12%,
   • Cr: 0.1-4%,
   • Al: up to 3.5%,
   • N: up to 0.05%,
   • P: up to 0.05%,
   • S: up to 0.01%,
   • Cu, Ni: in total up to 2%,
   • Ti, Nb, V: in total up to 0.5%
   • Rare earths: up to 0.1%
   and the remainder consists of Fe and unavoidable impurities,
2. (b) Applying a single or multi-layer metallic cathodic corrosion protection coating, the two main components of which are zinc and manganese, to the steel substrate, with the proportion of the total mass of manganese in the total mass of manganese and zinc being equal to or greater in the single or multi-layer metallic layer is than 3%, preferably equal to or greater than 5%, preferably equal to or greater than 10%, preferably equal to or greater than 20%, particularly preferably equal to or greater than 27% and preferably equal to or less than 40%, and particularly preferably equal to or less than 35 % is.
3. (c) Heating the steel substrate with the cathodic anti-corrosion coating to a heating temperature, the heating temperature in one alternative being at least 400 ° C and at most equal to the Ac3 temperature - 30 ° C of the steel substrate, or the heating temperature in another alternative at least equal to the Ac3 - Temperature - 30 ° C and at most equal to the Ac3 temperature + 100 ° C of the steel of which the steel substrate is made in each case;
4. (d) Forming the steel substrate heated to the heating temperature with an anti-corrosion coating to form the sheet metal component and optionally
5. (e) Cooling the sheet metal component with an average temperature gradient of a maximum of 1,000 K / s down to room temperature.

The basic possibilities of producing flat steel products which are suitable for the purposes according to the invention and which are provided in step a) of the method according to the invention are shown in FIG EP 2 383 353 B1 described, the content of which is incorporated into the present application by reference. In the diagram reproduced there and the associated sections [0031] to [0037] of FIG EP 2 383 353 B1 the various ways available in practice for the production of flat steel products are shown, which are suitable for the production of components according to the invention.

It is also possible to feed the rolled strip directly into the hot forming process, i.e. without prior heating from step (c).

The single or multi-layer metallic cathodic corrosion protection coating, the two main components of which are zinc and manganese, can be carried out using a method selected from the group consisting of: electrolytic deposition, galvanic deposition, physical vapor deposition, chemical vapor deposition, immersion method, slurry method , thermal spraying and combinations thereof, preferably electrolytically or by means of physical vapor deposition, and particularly preferably by means of physical vapor deposition.

Due to the single or multi-layer metallic layer provided in the flat steel product according to the invention and contacting the steel substrate, the two main components of which are zinc and manganese, scaling of the surface of the steel substrate during hot forming is largely or completely suppressed, even in the presence of oxygen. Liquid metal embrittlement takes place only to a small extent or is avoided. In an advantageous embodiment, a is formed on the surface Manganese oxide layer, so that a particularly high level of radiation energy can be absorbed, so that the component as a whole, and in particular its steel substrate, adjusts to the furnace temperature more quickly. This in turn can reduce the dwell time in the oven. Reference is made to the above statements.

In comparison to the hot forming process with conventional hot forming steels at forming temperatures between 880 and 950 ° C., the flat steel product according to the invention exhibits active corrosion protection at relatively low forming temperatures. Compared to pure zinc coatings with a melting point of approx. 419 ° C, the flat steel product exhibits active corrosion protection at comparatively high forming temperatures. The use of pure zinc would lead to severe cracking due to liquid metal-induced embrittlement of the base material, even at higher forming temperatures.

The increased Mn content of the steel substrate not only lowers the necessary hot forming temperature but also the cooling rate required for martensite formation, which increases process reliability during hot forming and enables energy savings, since the forming tool has to be cooled less. Due to the reduced hot forming temperatures, the Zn content of the anti-corrosion coating can also be increased, which improves the active anti-corrosion protection. Due to the complex oxidation states of manganese, manganese is not as suitable as an active protection against corrosion like zinc.

At one of the heating-through temperatures from step (c), the second alloy layer is preferably at least 70% by volume, preferably at least 80% by volume, in the solid state of aggregation. This can be adjusted via the proportion of the total mass of manganese in the total mass of manganese and zinc. Preferably from 20% the coating is suitable for the high temperature range of hot forming, smaller proportions are sufficiently temperature stable for the low temperature range. A sufficiently high proportion of solid aggregate is important in order to avoid cracking during hot forming.

The soaking temperature from step (c) preferably does not exceed the temperature which is derived from the manganese content via formula (2) T annealing [° C.] 458 ° C. + 14.31 ° C. / wt% *% Mn [wt%] % Mn of the coating calculated. Adhering to this formula (2) ensures that the coating is in the solid state during forming.

The person skilled in the art can set the heating through temperature from step (c) of a preferred hardened sheet metal component according to the invention by varying the proportions of manganese and zinc and by varying the parameters of the manufacturing process in the usual way. The setting of the preferred soaking temperature from step (c) has the result that only a small part of the second alloy layer is molten even at the soaking temperature, so that there is no significant liquid metal embrittlement.

Based on this, the invention shows a way in which a sheet metal component can be produced by means of resource-saving hot forming that, after its hot forming, has optimal mechanical properties, in particular a high elongation at break, and, due to these properties and its other usage properties, also meets high requirements when the component is subjected to crashes .

The heating through can advantageously take place in a conventional oven in the presence of atmospheric oxygen without this being detrimental to the desired success.

On the contrary: The method according to the invention is preferably carried out in such a way that, during the thermal hardening treatment, an outer layer comprising manganese oxide is formed which contacts the second alloy layer of the finished hardened component through reaction of manganese present in the metallic layer of the component with atmospheric oxygen. This outer layer of manganese oxide has a dark color and allows the efficient absorption of radiation energy in the manner already described above.

The high manganese content of flat steel products processed according to the invention enables lower hot forming temperatures than with conventional hot forming steels. The invention thus makes it possible to save energy and costs.

Since the steel substrate comprises a steel with a structure which consists of at least 1% by volume of retained austenite, the retained austenite content of the structure being at most 50.0% by volume, preferably at most 40.0% by volume can and particularly preferably can be at most 35.0% by volume. The rest of the structure of the sheet metal component consists of strength-increasing components of martensite and tempered martensite. It can also contain ferrite, whereby the ferrite content can also be “0”. The amount of other technically unavoidable structural components present is so small that they are ineffective with regard to the properties of the component according to the invention. The mean grain diameter of the grains of the structure is less than 5 µm.

Due to the lower forming temperature of the flat steel product compared to conventional hot forming steels, the hot forming can be carried out in an energy-saving manner with a lower annealing temperature. In an alternative, the heating temperature is at least 400 ° C and at most equal to the Ac3 temperature -30 ° C of the steel (low annealing range), from which the steel substrate is made, to tensile strengths of the hardened sheet metal component greater than 1000 MPa, preferably greater than 1100 MPa and especially preferably greater than 1210 MPa. In another alternative, the heating temperature is at least equal to the Ac3 temperature - 30 ° C and at most equal to the Ac3 temperature + 100 ° C of the steel (high annealing range) from which the steel substrate is made, with tensile strengths of the hardened sheet metal component greater than 1300 MPa , preferably greater than 1400 MPa and particularly preferably greater than 1510 MPa.

The heating temperatures can be particularly low if the forming is to take place in the two-phase region or at temperatures below this (lower annealing range). In this case, the residual austenite content in the component obtained is more than 10% by volume and the elongation at break A80 is more than 14%, preferably more than 15%, particularly preferably more than 16%. The hot forming according to the invention takes place here at heating temperatures that are typically above the Ac1 temperature and 30 ° C below the Ac3 temperature of the respective steel substrate of the flat steel product, whereby in the case of deformation in the two-phase region, heating temperatures of at least 10 ° C are higher than the Ac1 temperature and are at least 50 ° C lower than the Ac3 temperature of the respective steel substrate of the flat steel product.

The procedure according to the invention results in a structure which is characterized by optimized residual austenite components and, as a result, has very good mechanical properties, in particular high residual elongation and high energy absorption in the event of a crash. The comparatively low heating temperatures in this range, at which the hot forming of the sheet metal component according to the invention takes place, also prove to be particularly advantageous if the flat steel product processed according to the invention is to have cathodic corrosion protection. The method according to the invention enables the anti-corrosion layer to provide active protection against corrosion, with active protection preferably being provided by an electrochemical potential difference between the anti-corrosion coating and the substrate of greater than 0 mV, preferably greater than 50 mV, particularly preferably greater than 100 mV and very particularly preferably greater than 150 mV.

The annealing times that are typically required for thorough heating in work step (c) are usually up to 60 minutes, with annealing times of up to 20 minutes, in particular up to 10 minutes, having proven to be particularly economical in practice. Thorough heating can be carried out in conventional chamber furnaces or roller furnaces, in which the flat steel products to be hot-formed are brought to the heating temperature either in batches or in batches. Since in the inventive compositions of the flat steel product deformed to form the component, the properties are formed almost independently of the heating and cooling rate, it can, however, also prove to be beneficial if the heating is carried out by conductive or inductive heating, or also, for example, by means of solid body contact or in a fluidized bed . The alternative to conventional furnace heating means that shorter annealing times can be achieved in comparison to pure radiant heating in a conventional furnace. At the same time, the alternative methods allow more precisely controlled heating cycles, since with them the course of the heating can follow precise specifications. The further advantage of using the alternative heating process is that it is possible to react quickly to changes in production, which are typical for small batch production with different sheet thicknesses. Adjustments of the heating parameters to the changed requirements can be made accordingly quickly.

The hot forming from work step (d) of the flat steel product heated to the respective heating temperature to form the sheet metal component according to the invention can be carried out in conventional hot forming tools available for this purpose in the prior art. The hot forming takes place as immediately as possible after the heating from work step (c), so that the temperature at which the flat steel product enters the hot forming corresponds to the heating temperature, apart from a technically insignificant difference. However, stronger cooling is also permissible as long as the forming forces and springback are advantageous compared to cold forming.

The cooling of the component after the hot forming can also take place in the hot forming tool in a manner known per se. Alternatively, however, after the hot forming, the component can also be removed from the hot forming tool at a suitably short time interval and cooled outside the tool. The cooling rate of the sheet metal component with cathodic corrosion protection coating from the hot forming temperature, preferably down to a temperature of less than 100 ° C., particularly preferably down to room temperature, takes place with an average temperature gradient of a maximum of 1000 K / s up to room temperature.

It goes without saying that the properties of the hardened component produced depend on the properties of the flat steel product used, that is to say provided or produced, and on the design of the thermal hardening treatment. In this respect, the above statements apply accordingly. The person skilled in the art knows how to use a provided or manufactured sheet metal component according to the invention to perform a thermal hardening treatment and, if necessary, a mechanical treatment that preferably takes place simultaneously in order to achieve the desired mechanical properties of the hardened component to be manufactured.

This shows that the comparatively low required heating temperatures at which the hot forming of the flat steel product according to the invention can be carried out, alloying of the protective coating by diffusion of alloy components from the steel substrate takes place at best less so that the protective coating is cathodic even after the hot forming of the component Maintains protective effect. The protective layers present on the flat steel product processed according to the invention and thermoformed into the sheet metal component according to the invention typically have a boundary layer near the surface, bordering the steel substrate of the flat steel product, which is made of metallic and / or oxidic iron, and possibly metallic and / or oxidic iron Manganese and other alloy components of the base material consists. After the hot forming to the sheet metal component, due to the low heating temperatures used according to the invention, at which the hot forming according to the invention takes place, there is a reduced proportion of brittle phases in the boundary layer area compared to the conventional, higher forming temperatures, since there is only one due to the lower heating temperature according to the invention minimized alloying of the protective coating with elements originating from the steel substrate. The potential of cathodic corrosion protection through Zn-rich phases is thus retained.

The parameters of the procedure according to the invention make it possible to maintain the cathodic protective effect of a Zn-containing layer present on the flat steel product and to avoid critical cracks of more than 10 μm during hot forming.

At the comparatively low heating or forming temperatures provided in the method according to the invention, the harmful consequences that would occur if the Zn layer were to be melted are avoided. Due to the diffusion of Fe from the substrate into the layer, its melting point is raised to a sufficient extent. However, in order to maintain cathodic corrosion protection, it is necessary to limit the Fe content in the coating so that sufficient Zn-rich phases are retained after hot forming.

With regard to the proportion of the total mass of manganese selected according to the invention in the total mass of manganese and zinc in the metallic layer of the flat steel product according to the invention, the above statements apply to the proportion of the total mass of manganese in the corrosion protection coating in relation to the total mass of manganese and zinc in the corrosion protection coating of the sheet metal component hardened according to the invention corresponding.

In the case of the reference coating Z100 according to DIN EN 10346, which is conventionally used in the hot forming of medium-manganese steel, a Γ / Γ ₁ phase is formed during annealing, which is comparatively temperature-stable, which limits the proportion of liquid Zn produced during hot forming and thus the risk an occurring liquid metal embrittlement. The high proportion of Fe contained in the Γ / Γ ₁ phase, however, severely restricts the cathodic corrosion protection of the layer.

In the case of the samples according to the invention with a ZnMn coating, the significantly lower furnace temperature for setting the mechanical target properties compared to conventional hot forming also has the significantly Zn-rich δ phase, which leads to an improved corrosion protection potential. The layer system is due to the layer structure caused by the alloy Sufficiently temperature-stable so that at hot forming temperatures according to the invention there is no critical crack formation over 10 μm depth on typical automotive components due to liquid Zn, in which crack propagation would be expected when the component is stressed. Components typical of automobiles are understood to mean so-called omega or hat profiles.

In addition, it is preferably formed on the free surface of the steel substrate in a manner known per se (see Sect. EP 2 290 133 B1 , Paragraphs [0039] to [0043] and [0048]) a manganese-containing layer in metallic and / or oxidic form on the free surface of the flat steel product or sheet metal component, which further increases the effectiveness of the anti-corrosion coating.

Sheet metal components produced according to the invention have, as a result of their deformation at temperatures from 400 ° C to Ac3 temperature - 30 ° C, an optimized combination of high strength values, for which tensile strengths Rm typically stand at at least 900 MPa, preferably at least 1000 MPa, and optimized elongation properties that are express in elongations at break A80 of regularly more than 10%, preferably more than 12%.

Sheet metal components produced according to the invention have an optimized combination of high strength values for which tensile strengths Rm of typically at least 1200 MPa, preferably at least 1300 MPa and optimized elongation properties, when deformed between Ac3 temperature -30 ° C and Ac3 temperature + 100 ° C are expressed in elongations at break A80 of regularly more than 6%, preferably more than 8%.

In the case of components according to the invention, the product Rm x A80 is accordingly also regularly in the range greater than 10,000 MPa%, preferably greater than 11,000 MPa%, particularly preferably greater than 12,000 MPa%.

In contrast, the tensile strengths Rm of sheet metal components made from conventional steels for hot forming are typically at least 1200 MPa at temperatures at which a fully austenitic structure is present, since they are fully martensitic after quenching. However, these components only achieve significantly lower elongation at break A80, so that with these components the product Rm x A80 is regularly only 6,000 - 11,000 MPa%.

Further details, features and advantages of the invention emerge from the figures and from the following description of preferred embodiments. The figures merely illustrate exemplary embodiments of the invention, which do not restrict the essential concept of the invention.

• 1 zeigt in einer lichtmikroskopischen Aufnahme einen oberflächennahen Querschliff des gehärteten Blechbauteils mit einem Stahlsubstrat gemäß Tabelle 1 und einer Korrosionsschutzbeschichtung, die als Hauptbestandteile Mangan und Zink aufweist mit einem Anteil von Mangan an der Gesamtmasse an Mangan und Zink von 30%, die Erwärmungstemperatur betrug 700°C.
• 2 zeigt in einer lichtmikroskopischen Aufnahme einen oberflächennahen Querschliff des gehärteten Blechbauteils mit einem Stahlsubstrat gemäß Tabelle 1 und einer Korrosionsschutzbeschichtung die als Hauptbestandteile Mangan und Zink aufweist mit einem Anteil von Mangan an der Gesamtmasse an Mangan und Zink von 30%, die Erwärmungstemperatur betrug 750°C.

Brief description of the drawings

• 1 shows in a light microscope photo a near-surface cross-section of the hardened sheet metal component with a steel substrate according to Table 1 and an anti-corrosion coating, the main components of which are manganese and zinc with a proportion of manganese in the total mass of manganese and zinc of 30%, the heating temperature was 700 ° C .
• 2 shows in a light microscope photo a near-surface cross-section of the hardened sheet metal component with a steel substrate according to Table 1 and an anti-corrosion coating which has manganese and zinc as main components with a proportion of manganese in the total mass of manganese and zinc of 30%, the heating temperature was 750 ° C.

Embodiments of the invention

A corrosion protection coating with the two main components manganese and zinc is applied to a steel substrate of composition S1, the proportion of the total mass of manganese to the total mass of manganese and zinc being 30%. The average thickness of the anti-corrosion coating is 10 µm. The steel substrate has a structure which consists of at least 1% by volume of retained austenite, the retained austenite content of the structure being at most 50.0% by volume, preferably at most 40.0% by volume, and particularly can preferably be at most 35.0 vol .-% and the remaining structure consists of strength-increasing portions of martensite and tempered martensite. It can also contain ferrite, whereby the ferrite content can also be “0”. The manganese concentration in the coating is chosen so that high-melting phases can develop in the coating. The total proportion of manganese and zinc in the anti-corrosion coating is greater than 99% by weight, preferably greater than 99.9% by weight %. The proportion of iron is less than 0.1% by weight. The anti-corrosion coating can contain aluminum in a maximum amount of 0.05% by weight as well as unavoidable impurities.

The maximum proportion of the total mass of manganese in the total mass of manganese and zinc in the coating (the single-layer metallic layer) is calculated according to formula (2) TGlüh [° C] ≤ 458 ° C + 14.31 ° C / wt% *% Mn [% By weight] adjusted so that the maximum possible heating temperature for hot forming is not exceeded. After the coating has been applied, the resulting component is subjected to a thermal hardening treatment, namely a hot forming process at temperatures of 450 ° C, 500 ° C, 600 ° C, 650 ° C, 700 ° C and 750 ° C. In the examples according to the invention, the formation of liquid phases in the coating is reduced and liquid metal embrittlement during mechanical deformation is reduced to such an extent that the formation of substrate cracks is substantially contained. According to the invention, the previously applied anti-corrosion coating of zinc and manganese separates during the thermal hardening treatment. On the one hand, a first alloy layer is created which makes contact with the steel substrate, the main components of which are iron and zinc. This first alloy layer is suitable for actively protecting the steel substrate from corrosion. On the other hand, a second alloy layer with the main components manganese and zinc is formed which is in contact with the first alloy layer and is near the surface (and thus further removed from the steel substrate). The alloy layer with the main components manganese and zinc is particularly important for the success according to the invention, since it has a melting point which is significantly higher than the temperature reached during hot forming. This at least largely suppresses the buildup of tools and furnace rollers. In addition, the second alloy layer with the main components manganese and zinc suppresses the evaporation of zinc, which is of great importance for the service life of the furnaces used. Furthermore, the high Mn concentration on the surface ensures that hardly any iron can diffuse from the steel substrate to the surface during the heat treatment. This reduces the formation of red rust in the event of corrosion attack.

The structure of the anti-corrosion coating at a heating temperature of 700 ° C is shown in 1 shown, the structure of the anti-corrosion coating at a heating temperature of 750 ° C is shown in 2 shown.

A melt S1 corresponding to the stipulations of the invention has been melted, the composition of which is given in% by weight in Table 1. In addition, the Ac1 and Ac3 temperatures in ° C determined for S1 according to SEP 1680: 1990-12 are given in Table 1. The steel substrate was manufactured in a conventional manner typical for medium manganese steel according to the available routes for the production of flat steel products, as in EP 2 383 353 B1 described in sections [0031] to [0037], hot-rolled and cold-rolled to a pre-product to a thickness “d”. If necessary, hot strip annealing is carried out before cold rolling in order to give the material sufficient ductility for the cold rolling step. According to the invention, the steel substrate was coated with a ZnMn layer. As a reference, the same material was coated with a Z100 anti-corrosion coating.

The coated material was divided into sheet metal samples and these were annealed at different temperatures for 10 minutes in a conventional furnace at temperatures of 450 ° C, 500 ° C, 600 ° C, 650 ° C, 700 ° C and 750 ° C and then in A conventional, water-cooled forming tool is pressed into typical automotive components and flat patterns (omega profiles) and cooled to room temperature. Then tensile tests were carried out on the material and phase proportions were determined on metallographic cross-sections.

The Omega profiles are examples of typical steel components. Finally, the maximum crack depth was determined for the generated Omega profiles. The maximum crack depth was determined using a light microscope, as described above, on a vertical metallographic cross-section, the material for this being taken from the crack-critical areas of the omega profiles. In each case, the crack depth within the steel substrate was evaluated, i.e. without taking into account the crack proportions within the coating.

The tensile strength Rm determined on the flat steel product, the yield strength Rp0.2, the elongation at break A80, the uniform elongation Ag and the product Rm x A80 are given in Table 2. The testing of the values took place according to the specifications of the DIN EN ISO 6892-1: 2017-02 instead of.

The proportion of manganese in the total mass of manganese and zinc in the anti-corrosion coating as well as the tensile strength Rm determined on the hardened sheet metal component, the elongation at break A80, the residual austenite proportion of the respective component obtained and the maximum crack depth at the most critical point of the omega profile are given in Table 3 . The retained austenite proportions and crack depths were cross-sectioned using XRD or determined by light microscope. The corrosion protection coating indicates the proportion of the total mass of manganese in the total mass of manganese and zinc.

In the case of samples according to the invention, it can be seen that the depth of the crack remains within a limited range of at most 10 μm. Table 1 stole C. Si Mn P. S. Al total Cr Cu N Ni Nb Ti V SEM Ac1 [° C] Ac3 [° C] S1 0.08 0.1 6.9 0.005 0.001 0.05 0.4 0.05 0.01 0.1 0.005 0.004 0.004 0.05 570 ° C 724 ° C Details of the content of the melt S1 in% by weight, the remainder Fe and unavoidable impurities, Table 2 stole Orientation to the rolling direction d Rp0.2 Rm Ag A80 Rm x A80 [mm] [MPa] [MPa] [MPa] [%] [MPa%] S1 across 1.4 1005 1089 14.0 16.9 18,401 S1 along 1.4 956 1102 20.9 22.2 24,464 Mechanical-technological parameters in the initial state

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

• DE 102012024616 A1 [0008, 0013]
• EP 3483297 A1 [0009]
• EP 2778247 A1 [0010]
• EP 2383353 B1 [0011, 0071, 0108]
• WO 2019/020169 [0012]
• EP 2290133 B1 [0098]

Non-patent literature cited

• DIN EN 13925 [0007]
• DIN EN ISO 6892-1: 2017-02 [0008, 0111]

Claims (16)

Flat steel product, comprising a steel substrate and an anti-corrosion coating contacting the steel substrate, the steel substrate consisting of (in% by weight) C: up to 0.5%, Si: 0.05-1%, Mn: 4-12%, Cr: 0.1 - 4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, Cu, Ni: in total up to 2%, Ti, Nb, V: in total up to 0.5%, rare earths: up to 0.1% and the remainder consists of Fe and unavoidable impurities, characterized in that the anti-corrosion coating contacting the steel substrate has the following features The two main components are manganese and zinc, the proportion of the total mass of manganese in the total mass of manganese and zinc is greater than 3%, preferably greater than 10%, particularly preferably greater than 20% and preferably less than 40%, and particularly preferred less than 35%. Flat steel product from Claim 1 , characterized in that the anti-corrosion coating is single-layered. Flat steel product from the Claims 1 until 2 , characterized in that in the corrosion protection coating contacting the steel substrate in one or more layers, the total proportion of manganese and zinc is greater than 90% by weight, preferably greater than 95% by weight, particularly preferably greater than 99% by weight and very particularly preferably greater than 99.9% by weight, and / or the proportion of iron is less than 10% by weight, preferably less than 2% by weight and / or no aluminum or aluminum at a maximum Amount of 0.05 wt .-% is included and / or a remainder of unavoidable impurities is included. Hardened sheet metal component, produced by hot forming, comprising a steel substrate and an anti-corrosion coating contacting the steel substrate, the steel substrate consisting of (in% by weight) C: up to 0.5%, Si: 0.05-1%, Mn: 4- 12%, Cr: 0.1 - 4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, Cu, Ni: in total up to 2%, Ti, Nb, V: in total up to 0.5% Rare earths: up to 0.1% and the remainder consists of Fe and unavoidable impurities, and characterized in that the anti-corrosion coating is as follows Features: a first alloy layer contacting the steel substrate, the two main components of which are iron and zinc, and a second alloy layer contacting the first alloy layer, the two main components of which are manganese and zinc. Hardened sheet metal component Claim 4 , characterized in that the carbon content% C of the steel substrate fulfills the following condition: $$\%\text{C.}<0.55\text{Weight}.-\%-\%\text{Cr /}10.$$

Hardened sheet metal component from the Claims 4 and 5 , characterized in that after the hot forming the tensile strength Rm of the steel substrate with anti-corrosion coating is at least 1000 MPa, its elongation at break A80 is more than 6% and the product Rm * A80 formed from its tensile strength Rm and its elongation at break A80 is more than 10,000 MPa%, preferably more than 12,000 MPa%. Hardened sheet metal component from one of the preceding claims, characterized in that the structure of the hardened steel substrate consists of 1 to 50% by volume of residual austenite and the remainder of martensite, tempered martensite or ferrite, the ferrite content also being “0” can, and wherein the mean grain diameter of the grains of the structure is less than 5 microns. Hardened sheet metal component from one of the preceding claims, characterized in that the thickness of the first alloy layer of the anti-corrosion coating is greater than or equal to the thickness of the second alloy layer of the anti-corrosion coating. Hardened sheet metal component from one of the preceding claims, characterized in that the anti-corrosion coating comprises a further outer layer of manganese oxide contacting the second alloy layer, the thickness of the first alloy layer preferably being greater than or equal to the total thickness of the second alloy layer and the outer layer. Hardened sheet metal component from one of the preceding claims, characterized in that the proportion of the total mass of manganese in the total mass of manganese and zinc in the corrosion protection coating is greater than 3%, preferably greater than 10%, preferably greater than 20% and preferably less than 40%, particularly preferably less than 35%, and / or the second alloy layer of the anti-corrosion coating contains iron in an amount of up to 20% by weight, based on the second alloy layer. Hardened sheet metal component from one of the preceding claims, characterized in that the total proportion of manganese and zinc in the corrosion protection coating contacting the steel substrate is greater than 90% by weight, preferably greater than 95% by weight, particularly preferably greater than 99% by weight .-% and very particularly preferably greater than 99.9% by weight, and / or the proportion of iron is less than 10% by weight, preferably less than 2% by weight and / or no aluminum or aluminum is contained in a maximum amount of 0.05% by weight and / or a remainder of unavoidable impurities is contained. Hardened sheet metal component from one of the preceding claims, characterized in that the anti-corrosion coating offers cathodic corrosion protection. Method for producing a hardened sheet metal component, comprising a steel substrate and an anti-corrosion coating contacting the steel substrate, preferably a hardened sheet metal component with a corrosion coating Claim 4 , comprising the following working steps: (a) providing a steel substrate made of a steel which (in% by weight) consists of C: up to 0.5%, Si: 0.05-1%, Mn: 4-12%, Cr : 0.1 - 4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, Cu, Ni: in total up to 2%, Ti, Nb, V: in total up to 0.5% rare earths: up to 0.1% and the remainder consists of Fe and unavoidable impurities, (b) application of a single or multi-layer metallic cathodic corrosion protection coating , the two main components of which are zinc and manganese, onto the steel substrate, the proportion of the total mass of manganese in the total mass of manganese and zinc in the single or multi-layer metallic layer being greater than 3%, preferably greater than 10%, preferably greater than 20 % and is preferably less than 40%, and particularly preferably less than 35%, (c) heating through the steel substrate with the cathodic corrosion protection coating to a heating temperature, the heating temperature m is at least 400 ° C and at most equal to the Ac3 temperature - 30 ° C of the steel substrate, or the heating temperature is at least equal to the Ac3 temperature - 30 ° C and at most equal to the Ac3 temperature + 100 ° C of the steel from which the Steel substrate each consists of; (d) Forming the steel substrate heated to the heating temperature with a cathodic anti-corrosion coating to form the sheet metal component, and optionally (e) cooling the sheet metal component during and / or after the forming with an average temperature gradient of a maximum of 1,000 K / s to room temperature. Process for the production of a hardened sheet metal component according to Claim 13 , characterized in that the second alloy layer is at least 70% by volume, preferably at least 80% by volume, in the solid state of aggregation at the heating-through temperature from step (c). Procedure Claim 13 , characterized in that the maximum soaking temperature from step (c) fulfills the following condition: $$\text{TGl}\ddot{\text{u}}\text{H}\left[°\text{C.}\right]\le 458°\text{C.}+14.31°\text{C / wt}\%*\%\text{Mn}\left[\text{Weight}\%\right],$$

where T annealing is the heating through temperature in ° C and% Mn is the concentration of Mn in the coating in% by weight. Hardened sheet metal component from one of the preceding claims, characterized in that the hardened sheet metal component is a hot-formed component, preferably a component of a motor vehicle, preferably selected from the group consisting of bumper cross members, side impact beams, pillars and body reinforcements.

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