DE102020201451A1 - Sheet steel for hot forming, method for producing a hot-formed sheet steel component and hot-formed sheet steel component - Google Patents

Abstract

The invention relates to a steel sheet for hot forming, comprising a substrate (1) made of a hardenable steel material with an aluminum-based coating (2), an intermetallic layer (3) being formed between the substrate (1) and the aluminum-based coating (2). The invention also relates to a method for producing a hot-formed sheet steel component (10) and a hot-formed sheet steel component (10).

Description

The invention relates to a steel sheet for hot forming, comprising a substrate made of a hardenable steel material with an aluminum-based coating, an intermetallic layer being formed between the substrate and the aluminum-based coating. The invention also relates to a method for producing a hot-formed sheet steel component and a hot-formed sheet steel component.

The production of sheet steel components by means of hot forming has already established itself industrially, in particular for the production of body parts such as for example for the production of safety-relevant B-pillars etc. Sheet steel components can be produced in direct as well as in indirect hot forming processes. Flat blanks (directly) or pre-formed or near-net-shape (cold) formed semi-finished products / parts (indirect) made from a hardenable steel material are heated to a temperature at which, depending on the composition of the substrate used, a structural change occurs within the substrate of the steel sheet. With Ac1, the structural transformation into austenite begins and when Ac3 or above Ac3 is reached, an essentially completely austenitic structure is present. The heating above at least Ac1 is also called "austenitizing" in specialist circles, especially if a complete conversion into austenite is to take place (> = Ac3). After heating, the warm (austenitized) sheet steel is placed in a forming tool and hot formed. In the course of or after the hot forming process, the still warm steel sheet is cooled in such a way, preferably within the forming tool, which is preferably actively cooled, so that the structure in the substrate turns into a hard structure made of martensite and / or bainite, preferably essentially made of martensite, converts. In specialist circles, the cooling or quenching of the sheet steel within the forming tool or by the action of a (hardening) tool, which has the final contour of the sheet steel component to be produced, is also called “press hardening”. Alternatively, cooling / quenching can also take place outside of a forming tool / hardening tool, in particular in a (cold) medium, for example in an oil bath, and is referred to as “hardening”. Heating and cooling curves for setting the required microstructure are dependent on the chemical composition of the hardenable steel material used and can be seen or derived from so-called ZTA or ZTU diagrams. By means of hot forming, it is possible to set an essentially martensitic microstructure with high strengths. A good balance between strength and weight has been found with the classic hot forming or press hardening of manganese-boron steels in particular for the production of structural components in the vehicle sector, for example in motor vehicles, land vehicles and rail vehicles.

Conventionally, steel sheets coated with a metallic coating made of a hardenable steel material are hot-dip coated, usually with an Al-based coating. A certain layer structure is formed within the coating. This structure is determined on the basis of an Al-Fe intermetallic layer that is established, which is essential for good adhesion, among other things, with its thickness depending on the immersion temperature and the composition of the melt. The steel sheets coated in this way are further processed into a steel sheet component in a hot forming process. As a result of the austenitization during hot forming, the coating is converted into an n-layer structure by diffusion and / or alloying through, where n is dependent on the composition of the coating and can be at least 2 or more. In addition, the structure of the coating can be essentially adapted depending on the requirements with regard to corrosion, weldability and paintability. The substantial alloying through of the metallic coating results from diffusion between the metallic coating and the substrate at the intermetallic layer. Corresponding examples are disclosed in the prior art, for example in the publications EP 2 086 755 B1 , EP 2 242 863 B1 .

During the austenitizing process, the metallic coating is essentially alloyed through. Diffusion of the iron from the substrate into the coating and of the aluminum / silicon from the coating into the substrate takes place essentially homogeneously, so that components are formed, in particular close to the surface within the alloyed coating, which have an essentially unsatisfactory influence on the paintability and / or weldability, in particular due to the fact that there is an essentially porous surface with predominantly closed pores. The poor weldability results from an explosion-like expansion of the gas or gases enclosed in the pores and thus an increased tendency for spatter. An open porosity, on the other hand, would be advantageous for paintability, since the paint can better get caught in the surface structure and there is a relative enlargement of the surface.

The object is therefore to provide a sheet steel coated with an aluminum-based coating which enables the production of a hot-formed sheet steel component in a manner with which a hot-formed sheet steel component can be produced with good weldability, corrosion resistance and / or paintability.

The object is achieved with a sheet steel for hot forming with the features of claim 1, with a method for producing a hot-formed sheet steel component with the features of claim 9 and with a hot-formed sheet steel component with the features of claim 12.

According to a first teaching of the invention, the steel sheet for hot forming comprises a substrate made of a hardenable steel material with an aluminum-based coating, an intermetallic layer being formed between the substrate and the aluminum-based coating, the substrate having a surface structure with valley areas, flank areas and mountain areas.

The inventors have surprisingly found that by structuring the surface of the substrate with valley areas, flank areas and mountain areas, which can be designed stochastically, quasi-stochastically or deterministically, an enlargement of the surface compared to a standard surface for hot forming, i.e. to a surface which is not provided with a structure can be provided, whereby a positive influence on the corrosion resistance, weldability and / or paintability can be exerted. Due to the different, in particular non-linear diffusion paths of the iron from the substrate into the coating, an inhomogeneity occurs at least close to the surface within the essentially fully alloyed coating, which in turn can be advantageously noticeable in comparison to a homogeneous distribution of the components from the prior art, in that the closed porosity of the surface can be substantially reduced compared to the prior art. The structure can be introduced into the surface of the substrate in a targeted manner by suitable means, in particular by embossing and / or rolling in, in particular, wherein the surface structure can have a geometrically defined structure. There are essentially no limits to the design freedom of the surface structure. These can be introduced individually and as required. The valley and mountain areas are connected to one another via flank areas, so that the diffusion of the iron from the substrate into the coating is preferably set locally in a targeted manner by setting the flank area angles and / or via the depth or the distance between the valley and mountain areas can.

A substrate is to be understood as a flat steel product in the form of a strip or sheet metal / plate. It has a longitudinal extension (length), a transverse extension (width) and a vertical extension (thickness). The substrate can be a hot strip (hot-rolled steel strip) or a cold strip (cold-rolled steel strip), or it can be made from a hot strip or from a cold strip. The substrate can consist of a material (monolith) or a composite material (2 or more layers). The surface of the substrate can preferably be structured by means of one or more rollers, for example the surface structure can be introduced in a rolling stand in a rolling train or separately in a (subsequent) rolling stand. The valley, flank and mountain areas of the surface structure can depend in their dimensions (depth, width, etc.), inter alia, on the degree of rolling, which is for example up to 20%, in particular up to 10%, preferably up to 5%, preferably up to 4%, particularly preferably up to 3%, the degree of rolling expressing the ratio of the decrease in thickness (input thickness minus output thickness in the roll stand) of the rolled substrate to the input thickness, in particular taking into account the reduction in thickness. The degree of rolling is, for example, at least 0.2%, in particular at least 0.5%, preferably at least 1%.

The surface structure with valley, flank and mountain areas on the surface of the substrate (negative impression) is essentially found on the roller or rollers as a positive impression, with a valley area on the surface of the substrate corresponding to a mountain area on the surface of the roller. The surface structure can be designed stochastically, in particular quasi-stochastically or preferably deterministically.

Following the introduction of the surface structure, the substrate is coated, on one side or preferably on both sides, with an aluminum-based coating in a hot-dip coating system to form a sheet steel (strip / sheet / plate shape).

The aluminum-based coating has the following chemical composition in% by weight: optionally one or more alloy elements from the group (Si, Fe, Mg, Zn): Si until 15.0, Fe until 5.0,

Remainder AI and unavoidable impurities. In addition to aluminum and unavoidable impurities, additional elements such as silicon with a content of up to 15.0% by weight and / or iron with a content of up to 5.0% by weight and / or magnesium with a content of up to to 5.0% by weight and / or zinc with a content of up to 30.0% by weight in the coating. Si can be present in particular with at least 0.1% by weight, preferably with at least 2.0% by weight, preferably with at least 7.0% by weight, the content in particular being limited to a maximum of 12.0% by weight , can preferably be limited to a maximum of 11.0% by weight. Si in the coating can contribute to improved processability in hot-dip coating. Alternatively or additionally, Fe can be present in particular with at least 0.1% by weight, preferably with at least 0.5% by weight, preferably with at least 1.0% by weight, the content in particular being limited to a maximum of 4.0% by weight .-%, preferably to a maximum of 3.5 wt .-% can be limited. Fe in the coating can increase the melting temperature of the coating, which can be advantageous when austenitizing. Alternatively or in addition, Mg can in particular be present at at least 0.1% by weight, preferably at least 0.2% by weight, the content being in particular to a maximum of 3.0% by weight, preferably to a maximum of 1.5% by weight .-%, preferably to a maximum of 0.8 wt .-% can be limited. Mg in the coating can contribute to a reduction in the uptake of diffusible hydrogen into the substrate. In order to bring about an improvement in the corrosion properties, in particular to reduce the formation of red rust, Zn can be present in the coating with at least 1% by weight, in particular at least 4% by weight, its content to a maximum of 50% by weight, in particular can be limited to a maximum of 22% by weight, preferably to a maximum of 16% by weight.

Due to the process, an intermetallic layer forms between the substrate and the coating. The intermetallic layer has at least iron and aluminum compounds. The intermetallic layer can have a thickness between 0.01 and 10 μm, in particular up to 8 μm, preferably between up to 6 μm.

The intermetallic layer follows this course due to the surface structure with valley, flank and mountain areas, which was previously introduced on at least one side of the surface of the substrate. The aluminum-based coating of the steel sheet shows on the free surface, more or less on the side facing away from the substrate and the intermetallic layer, Mg until 5.0, Zn until 30.0, preferably an essentially flat or planar surface. Thus, the metallic coating varies in its thickness in the longitudinal and transverse extent of the steel sheet depending on the course of the valley and mountain areas, the height extent (thickness) of the steel sheet remains essentially constant if it is a substrate with a uniform thickness, or wherein the vertical extension (thickness) of the steel sheet remains essentially constant, at least in sections, if it is a substrate with a varying thickness in the longitudinal or transverse extension.

The thickness of the steel sheet is, for example, 0.5 to 4.0 mm, in particular 0.6 to 3.0 mm, preferably 0.7 to 2.5 mm.

The thickness of the aluminum-based coating is, for example, 3 to 40 μm before hot forming, in particular 10 to 40 μm, preferably 11 to 35 μm, preferably 12 to 30 μm, more preferably 13 to 27 μm, the thickness corresponding to the mean thickness. Due to the surface structure and the thickness of the coating, which varies in the longitudinal and transverse extent of the steel sheet, the thickness of the coating of the steel sheet according to the invention is not constant throughout, so that an average thickness is given.

Further advantageous configurations and developments emerge from the following description. One or more features from the claims, the description and also the drawing can be combined with one or more other features from them to form further embodiments of the invention. One or more features from the independent claims can also be linked by one or more other features.

According to one embodiment of the steel sheet according to the invention, the hardenable steel material has the following chemical composition in% by weight: C = 0.05 until 0.5, Mn = 0.3 until 3.0, Si = 0.05 until 1.7, P. until 0.1, S. until 0.1, N until 0.1, and optionally one or more alloy elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca): Al until 1.0, Ti until 0.2, V until 0.5, Nb until 0.5, B. until 0.01, Cr until 1.0, Mon until 1.0, Cu until 1.0, Ni until 1.0, Approx until 0.1,

Remainder Fe and unavoidable impurities.

Carbon (C) performs several important functions. First and foremost, C is a martensite former and is therefore essential for setting a desired hardness in the hot-formed sheet steel component, so that at least a content of 0.05% by weight, in particular at least 0.1% by weight, preferably at least one Content of 0.15% by weight is present. Furthermore, C makes a major contribution to a higher CEV value (CEV = carbon equivalent), which has a negative impact on the weldability, so that a content of up to a maximum of 0.5% by weight, to reduce the tendency to crack, in particular up to a maximum of 0 , 45% by weight, preferably up to a maximum of 0.4% by weight. Furthermore, the specified upper limit can avoid negative influences on the toughness properties, the forming properties and the suitability for welding. Depending on the required formability and toughness, the C content can be set individually within the specified ranges.

Manganese (Mn) is an alloying element that contributes to hardenability. At the same time, Mn reduces the tendency for undesired formation of pearlite during cooling and lowers the critical cooling rate, whereby the hardenability is increased. In addition, Mn can be used to set S, in order to prevent the hot-rollability from being excessively impaired by an FeS eutectic, and / or to reduce the pearlite content, so that in particular a content of at least 0.5% by weight is present. An excessively high Mn concentration, on the other hand, has a negative effect on the weldability, so that Mn is limited to a maximum of 3.0% by weight. To ensure the desired formability, the content is limited in particular to a maximum of 2.7% by weight, and to improve the toughness properties, preferably to a maximum of 2.5% by weight. In order to set the desired strength properties, a content of at least 0.8% by weight is added in particular.

Silicon (Si) is an alloying element that contributes to deoxidation. To ensure effectiveness, a content of at least 0.05% by weight is used. However, Si can also contribute to increasing the strength, so that a content of at least 0.1% by weight, preferably of at least 0.2% by weight, is preferably added. If too much Si is added to the steel, this can have a negative impact on the toughness properties, formability and weldability. The content is therefore limited to a maximum of 1.7% by weight, in order to improve the surface quality, in particular to a maximum of 0.9% by weight, preferably to a maximum of 0.7% by weight.

Phosphorus (P) is an alloying element that can be adjusted in contents of up to 0.1% by weight to delay the formation of cementite. To ensure the desired delay and stabilization, contents of in particular at least 0.002% by weight, preferably at least 0.004% by weight, are set. However, P has a strong toughness-reducing effect and therefore has an unfavorable effect on formability. In addition, due to its low diffusion rate, P can lead to severe segregation when the melt solidifies. Negative influences on the formability and / or weldability can be safely ruled out if the content is limited to a maximum of 0.05% by weight in particular, and preferably to a maximum of 0.03% by weight to further reduce the segregation effects.

Sulfur (S) is an alloying element which can be present in contents up to 0.1% by weight. Since S in steel has a strong tendency to segregate and can negatively affect the formability or toughness as a result of the excessive formation of FeS, MnS or (Mn, Fe) S, the content is therefore in particular to a maximum of 0.05% by weight. , preferably limited to a maximum of 0.05% by weight, preferably to a maximum of 0.01% by weight.

Nitrogen (N) can be used as an alloying element in contents of up to 0.1% by weight to form nitride and / or improve hardenability. In principle, N cannot be completely avoided in steel production due to the N-containing earth's atmosphere, but it can be very advantageous, depending on other alloying elements. Like C, N can be used to increase the martensite hardness, but it does not weaken the grain boundaries as much as C does. In order to achieve this effect, contents of at least 0.0005% by weight, preferably of at least 0.001% by weight, preferably of at least 0.002% by weight, can be set. However, especially in combination with Al and / or Ti, N leads to the formation of coarse nitrides, which can have a negative effect on formability. The content is therefore limited in particular to a maximum of 0.015% by weight, preferably to a maximum of 0.01% by weight, preferably to a maximum of 0.007% by weight. If Ti is present, in the case of Ti contents> 0.01% by weight, the N content should particularly preferably be set between 0.001% <N <= 0.004%.

The steel sheet can optionally contain one or more alloy elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca).

Aluminum (Al) can be added as an optional alloying element up to a maximum of 1.0% by weight. In particular, Al can be used to bind any nitrogen that may be present, so that optionally alloyed boron can develop its strength-increasing effect. In particular, a content of at least 0.005% by weight, preferably at least 0.01% by weight, is therefore added. To avoid problems with casting technology, the content is limited in particular to a maximum of 0.5% by weight, preferably to a maximum of 0.2% by weight.

As an optional alloying element, titanium (Ti) can increase strength through the formation of carbides, nitrides and / or carbonitrides and act as a micro-segregation element. Furthermore, the formation of a coarse austenite structure can be suppressed. Ti can also be used for grain refinement and / or nitrogen binding and, if boron is present, to increase the effectiveness of boron. Since it can also contribute to enhancing the effectiveness of Cr, it can optionally be added with a content of up to 0.2% by weight. For reasons of cost, the content is limited in particular to a maximum of 0.15% by weight, to reliably avoid the formation of excessively large titanium nitrides, preferably to a maximum of 0.1% by weight, preferably to a maximum of 0.05% by weight. To ensure effectiveness, a content of at least 0.005% by weight can be added. To utilize the strength-increasing effect, contents of at least 0.01% by weight, preferably of at least 0.015% by weight, can preferably be used.

Vanadium (V) and / or niobium (Nb) can be added as optional alloying elements individually or in combination to refine the grain and / or to delay the hydrogen-induced crack formation. These optional alloying elements, like Ti, can be used as micro-alloying elements in order to form strength-increasing carbides, nitrides and / or carbonitrides. To ensure their effectiveness, V and / or Nb can be used in particular with contents of (each) at least 0.005% by weight, preferably of at least 0.01% by weight, preferably of at least 0.015% by weight. For Nb and V, the minimum content, individually or in total, is particularly preferably at least 0.02% by weight. The optional alloying elements are (each) limited to a maximum of 0.5% by weight, in particular to a maximum of 0.2% by weight, preferably to a maximum of 0.1% by weight, since higher contents have a detrimental effect on the material properties, in particular can have a negative effect on the toughness properties of the steel.

As an optional alloying element, boron (B) can segregate on the phase boundaries and prevent their movement. This can lead to a fine-grain structure, which can have beneficial effects on the mechanical properties. In order to ensure the effectiveness of these effects and to increase the hardenability, a content of up to 0.01 wt .-%, and in particular to ensure reliable effectiveness even in the presence of N, for example in the form of technically unavoidable contamination of the steel melt with N, in particular at least 0.0005% by weight, to increase the fine grain size, preferably at least 0, 0010% by weight, preferably at least 0.0015% by weight, are added. With the optional alloying of B, sufficient Ti should also be alloyed to bind N.

Chromium (Cr) can be added as an optional alloying element for setting the hardness and strength, in particular with a content of at least 0.01% by weight, since, like C, it can support the conversion into austenite. For reasons of cost, the upper limit is defined as 1.0% by weight. If the content is too high, the weldability and / or the toughness can be adversely affected, so that the content is limited in particular to a maximum of 0.75% by weight, preferably to a maximum of 0.45% by weight. In order to reduce the carbon diffusion and thus promote a non-equilibrium conversion, contents of at least 0.01% by weight, preferably of at least 0.1% by weight, preferably of at least 0.15% by weight, can be added.

Molybdenum (Mo) as an optional alloying element can increase strength and hardness. Since it can contribute to increasing the effectiveness of Cr or can replace the use of this alloying element, it can optionally be used with a content of up to 1.0% by weight, in particular between 0.01 and 0.8% by weight, to achieve this the greatest possible hardness and preferably between 0.1 and 0.5% by weight to reduce carbon diffusion.

If, for example, the optional alloying elements Cr and Mo are alloyed together, their total contents are limited to a maximum of 1.0% by weight, in particular to a maximum of 0.8% by weight, preferably to a maximum of 0.6% by weight.

Copper (Cu) can be added as an optional alloying element to improve hardenability with a content of up to 1.0% by weight. In order to ensure this effect, contents of at least 0.01% by weight, preferably of at least 0.05% by weight, can be added. The content is limited in particular to a maximum of 0.5% by weight, preferably to a maximum of 0.2% by weight, in order to avoid negative influences on the suitability for welding and the toughness properties in the heat-affected zone of a possible weld made on the sheet steel component.

Nickel (Ni) as an optional alloying element can improve hardenability. To ensure effectiveness, a content of at least 0.01% by weight can be added. To increase the toughness, contents of at least 0.02% by weight can preferably be added. To improve weldability, the content is limited to a maximum of 1.0% by weight, for cost reasons in particular to a maximum of 0.5% by weight, preferably to a maximum of 0.2% by weight.

Calcium (Ca) can be used as an optional alloying element of the melt as a desulfurizing agent and for targeted sulfide influence in contents of up to 0.1% by weight, in particular up to a maximum of 0.05% by weight, preferably up to a maximum of 0.01% by weight, preferably up to a maximum of 0.005% by weight are added, which can lead to a changed plasticity of the sulfides during hot rolling. The effects described can be effective from a content of in particular at least 0.0005% by weight, preferably of at least 0.001% by weight.

The alloying elements specified as optional can alternatively also be tolerated as impurities in contents below the specified minimum limits without influencing, preferably not worsening, the properties of the steel material. P, S and / or N can also be tolerated as impurities if they are not added in a targeted manner.

According to a preferred embodiment of the steel sheet according to the invention, the aluminum-based coating is alloyed with at least 30% by weight of Fe. After the hot dip coating, the coated substrate can be fed to a (further) heat treatment in which the aluminum-based coating can be enriched with iron from the substrate by diffusion processes. A targeted “pre-alloying” can be achieved through heat treatment prior to hot forming, depending on Temperatures (for example <Ac1 or => Ac1) and duration (for example up to approx. 600 s or> 600 s) can be carried out in order, for example, to (further) shorten the time until the coating is alloyed through during the austenitization during hot forming enable, see for example DE 10 2008 006 771 B3 .

According to a preferred embodiment of the steel sheet according to the invention, the surface structure with valley areas, flank areas and mountain areas is designed deterministically. A deterministic surface structure is to be understood in particular as regularly recurring surface structures which have a defined shape and / or configuration or dimensioning. In particular, this also includes surface structures with a (quasi-) stochastic appearance, which are composed of stochastic form elements in a recurring structure. In this way, the valley areas, flank areas and mountain areas can be set in a targeted manner.

According to a further preferred embodiment of the steel sheet according to the invention, the distance or maximum roughness depth Rt between the valley areas and mountain areas in the vertical extent of the substrate is between 1 and 50 μm, in particular between 3 and 40 μm, preferably between 5 and 28 μm, preferably between 8 and 20 µm.

According to a particularly preferred embodiment of the steel sheet according to the invention, the substrate has a surface that is enlarged by at least 3%, in particular by at least 5%, preferably by at least 10%, preferably by at least 13%. The enlargement of the surface of the substrate can be seen in relation to the “projection area” of the surface of the substrate. This means that one or more defined areas are viewed, for example recordings using a confocal white light microscope, and the determined or measured actual surface is set in relation to the projection surface (plane or flat surface) in the defined area or areas. The enlargement of the surface depends on the shape, configuration and / or dimensioning of the valley, flank and mountain areas of the surface structure and also depends on the number or distribution of the structure, with an enlarged surface, for example, up to 500%, in particular up to 200 %, preferably up to 50% can be set. The preferred enlarged surface relates in particular to the as yet uncoated substrate.

The steel sheet according to the invention can optionally be dressed. After the substrate has been coated with an aluminum-based coating, the steel sheet can, for example, be skin-passed by means of skin-pass rollers, in particular with a degree of rolling between 0.2 and 4%, in order to set a targeted roughness and / or structure on the free surface of the coating of the steel sheet be able.

According to a second aspect, the invention relates to a method for producing a hot-formed sheet steel component, the method according to the invention comprising the following steps: providing a steel sheet according to the invention; Austenitizing the steel sheet at a temperature of at least Ac1; Hot forming and cooling of the austenitized steel sheet to a temperature between 20 ° C and Ms.

The steel sheet according to the invention is heated or austenitized to a temperature of at least Ac1 or above, in particular to at least Ac3 or above, in order to form austenite in the substrate, preferably in a period of time which is sufficient to, in particular depending on the thickness and / or Composition of the steel material used to completely heat the steel sheet. In the case of partial austenitization between Ac1 and Ac3, the austenite content and the carbon content in the austenite depend on the austenitization time, so that complete austenitization> Ac3 is preferred.

For the purposes of the invention, in addition to complete heating of the sheet steel, only partial heating is to be understood if a sheet steel component with partially different properties is to be produced. In this way, austenitizing the steel sheet at least partially at least at a temperature of at least Ac1 can also take place. In particular, a heating with different, partial-area temperature zones within the steel sheet is to be understood, of which at least one is above Ac3. For example, complete heating of the steel sheet can also take place with temporary, partial-area intermediate cooling, preferably with a fluid, for partial-area-related strength change, for example, the steel sheet being moved out of the furnace for a short time and then moved back into an oven after partial-area intermediate cooling.

“Hot forming and cooling” is to be understood as meaning that the steel sheet includes hot forming as indirect or direct hot forming combined with cooling in a tool (press hardening) or in a medium (hardening) as a result of the targeted austenitization. If the austenitized steel sheet to a temperature below Ms, it can be ensured that the formation of a hard structure of austenite in martensite is forced, in particular by means of suitable cooling rates. The average or critical cooling rate is in particular at least 20K / s, preferably at least 30K / s, preferably at least 40K / s. The conversion to martensite is complete when the Mf temperature is reached or fallen below, cooling to a temperature of 20 ° C., in particular to a temperature of 50 ° C., preferably to a temperature of 80 ° C., being preferred up to a temperature of 100.degree. C., more preferably up to a temperature of 200.degree.

Parameters such as Ac1, Ac3, Ms, Mf, (critical) cooling rates etc. depend on the composition of the steel material used and can be derived from so-called ZTU or ZTA diagrams.

If the hot forming takes place in a hot forming tool and the cooling in a cooling tool or hot forming and cooling in a tool, the cooling tool and tool correspond to a press hardening tool, which has the advantage that a particularly dimensionally stable sheet steel component is produced, since the austenitized steel sheet is in contact with a forming The contour of the press hardening tool arrives. In the case of indirect hot forming, the press hardening tool effects only a slight shaping (= hot forming) within the scope of a calibration and / or correction to the nominal size or final geometry of the sheet steel component to be produced. The press hardening tool is preferably actively cooled and provides corresponding (critical) cooling speeds in order to be able to set a hard structure in the hot-formed sheet steel component. According to one embodiment of the method in the case of indirect hot forming, a preformed part is provided as the steel sheet which essentially has a geometry close to the final dimension.

Alternatively, in the case of direct hot forming, the steel sheet can be provided as a flat blank which, after austenitization, is hot formed in at least one hot forming tool. Depending on the complexity of the sheet steel component to be produced and / or depending on the cycle time, the hot forming can also be hot formed in two or more hot forming tools. The one or the hot forming tool in the last stage of several tools in the process is also designed for cooling, so that the one or the last hot forming tool in the process chain is designed as a press hardening tool, which is actively cooled and corresponding (critical) cooling speeds provides in order to be able to set a hard structure in the hot-formed sheet steel component. In the case of several hot forming tools, the first hot forming tool in the process chain can also be designed for partial cooling, in particular by means of contact cooling.

For the purposes of the invention, in addition to complete cooling, only partial cooling of the sheet steel / sheet steel component is to be understood if a sheet steel component with partially different properties is to be produced.

According to a preferred embodiment of the method, austenitizing is carried out in a furnace at a temperature of at least Ac3 for a duration, in particular dwell time in the furnace, between 50 and 1200 s, in particular between 80 and 720 s, preferably between 100 and 600 s, preferably between 120 and carried out for 360 s. The temperature for austenitizing is a maximum of 980 ° C, in particular a maximum of 940 ° C. Depending on the temperature in the furnace and on the thickness of the steel sheet according to the invention used, in addition to the conversion of the steel workpiece completely into austenite, an essentially complete alloying of the coating can also be ensured within the specified time periods. “Through alloying” is to be understood as meaning that> 50% by weight Fe is present in the coating or Al <70% by weight is present in all phases.

The third teaching of the invention relates to a hot-formed sheet steel component, the sheet steel component having a substrate with an aluminum-based coating and an interdiffusion layer formed between the substrate and the aluminum-based coating, the substrate having a surface structure with valley areas, flank areas and mountain areas.

According to a preferred embodiment of the hot-formed sheet steel component, the interdiffusion layer that is formed follows the course of the surface structure. The surface structure with valley areas, flank areas and mountain areas of the hot-formed sheet steel component essentially corresponds to the basic structure of the surface structure with valley areas, flank areas and mountain areas of the steel sheet, whereby due to the diffusion during austenitization and the mechanical stress in the course During the hot forming, the surface structure of the substrate of the hot-formed sheet steel component can undergo a change in comparison to the surface structure of the substrate of the steel sheet, in particular a relative smoothing in at least some areas.

According to a further preferred embodiment of the hot-formed sheet steel component, the distance or maximum roughness depth Rt between the valley areas and mountain areas in the vertical extent of the substrate is between 1 and 50 μm, in particular between 3 and 40 μm, preferably between 5 and 28 μm, preferably between 8 and 20 µm.

In order to avoid repetition, reference is made to the explanations of the steel sheet and method according to the invention.

The substrate of the hot-formed sheet steel component has a structure made of martensite with at least 50 area%, in particular at least 60 area%, preferably at least 70 area%, preferably at least 80 area%, particularly preferably at least 90 area% remaining structural components in the form of bainite, austenite, retained austenite, cementite, pearlite and / or ferrite may be present. In particular, the remaining non-martensitic structural component consists for the most part of bainite, it being possible for pearlite and / or ferrite to be present with up to 10% by area, preferably with up to 5% by area. The structure preferably consists of 100% martensite by area, whereby the highest possible hardness, in particular in connection with the corresponding alloying elements used, can be provided. The structure can optionally have production-related, unavoidable structural components such as cementite or other precipitates such as carbides, nitrides and / or oxides and their mixed forms up to a maximum of 2% by area.

The above-mentioned structural components can be present entirely in the sheet steel component or partially, if a sheet steel component with partially different properties is to be produced.

The aluminum-based coating of the hot-formed sheet steel component is essentially through-alloyed and has a mixed form, which, among other things, has the following properties: depends on the chemical composition. The intermetallic layer has, at least locally, a greater thickness compared to the thickness of the intermetallic layer before austenitizing and cooling.

All information on the contents of the alloying elements specified in the present application are based on weight, unless expressly stated otherwise. All contents are therefore to be understood as figures in% by weight. The specified structural components are determined by a suitable evaluation method, e.g. B Evaluation of light or electron microscopic examinations, in particular determined on one or more micrographs and are therefore to be understood as area proportions in area%, unless expressly stated otherwise. An exception to this is the structural component austenite or retained austenite, which is specified as a volume percentage in% by volume, unless expressly stated otherwise.

In the following, specific embodiments of the invention are explained in more detail with reference to the drawing. The drawing and accompanying description of the resulting features are not to be read in a restrictive manner to the respective configurations, but serve to illustrate exemplary configurations. Furthermore, the respective features can be used with one another as well as with features of the above description for possible further developments and improvements of the invention, especially in the case of additional configurations which are not shown.

• 1) einen schematischen Schnitt durch ein Substrat,
• 2a, b) einen schematischen Schnitt durch ein Stahlblech gemäß dem Stand der Technik (a) und durch ein Stahlblech gemäß einer erfindungsgemäßen Ausführungsform (b),
• 3a, b) einen schematischen Schnitt durch ein Stahlblech gemäß einer erfindungsgemäßen Ausführungsform vor dem Austenitisieren (a) und nach dem Warmumformen und Abkühlen (b) und
• 4a, b) jeweils einen Ausschnitt eines Schliffbildes eines warmumgeformten Stahlblechbauteils gemäß dem Stand der Technik (a) und eines warmumgeformten Stahlblechbauteils gemäß einer erfindungsgemäßen Ausführungsform (b).

The drawings show in

• 1 ) a schematic section through a substrate,
• 2a, b ) a schematic section through a steel sheet according to the prior art (a) and through a steel sheet according to an embodiment of the invention (b),
• 3a, b ) a schematic section through a steel sheet according to an embodiment of the invention before austenitizing (a) and after hot forming and cooling (b) and
• 4a, b ) each a section of a micrograph of a hot-formed sheet steel component according to the prior art (a) and a hot-formed sheet steel component according to an embodiment of the invention (b).

1 shows a schematic representation in section of a substrate ( 1 ), where the substrate ( 1 ) consists of a hardenable steel material which has the following chemical composition in% by weight: C = 0.05 to 0.5, Mn = 0.5 to 3.0, Si = 0.05 to 1.7, P to 0.1, S to 0.1, N to 0.1, and optionally one or more alloy elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca): A to 1 , 0, Ti to 0.2, V to 0.5, Nb to 0.5, B to 0.01, Cr to 1.0, Mo to 1.0, Cu to 1.0, Ni to 1.0 , Ca to 0.1, the remainder Fe and unavoidable impurities. The substrate ( 1 ) has a surface structure ( 1.1 ) with valley areas ( 1.12 ), Flank areas ( 1.13 ) and mountain areas ( 1.11 ) on. The valley areas ( 1.12 ) are with the mountain areas ( 1.11 ) over flank areas ( 1.13 ) connected. With (a) the distance in the section between the mountain area ( 1.11 ) and valley area ( 1.12 ), can in particular also be referred to as structure depth or maximum roughness depth Rt, with (b) the width or length in the section of a mountain area ( 1.11 ) and with (d) the width or length in the section of a valley area ( 1.12 ) marked. the 1 shows any shapes and / or configurations or dimensions of the surface structure, which individually or in combination, preferably as a deterministic surface structure ( 1.1 ) are introduced. Depending on the design and / or shape of the surface structure ( 1.1 ) shows the substrate ( 1 ) has a surface that is at least 3% larger. Other three-dimensional configurations that can be represented in the longitudinal and / or transverse direction in a deterministic, quasi-stochastic or stochastic structure, preferably recurring, are also conceivable.

2 shows a schematic representation in section a steel sheet according to the prior art, see FIG. 2a) , and according to an embodiment of the invention, s. 2 B) . The steel sheet in 2a) includes an aluminum-based coating in addition to the substrate ( 2 ) and one between cover ( 2 ) and substrate formed intermetallic layer ( 3 ). The aluminum-based coating ( 2 ) has a constant thickness (dB). The steel sheet according to the invention looks different in section, which is due to the surface structure ( 1.1 ) with valley areas ( 1.12 ), Flank areas ( 1.13 ) and mountain areas ( 1.11 ) also a formed intermetallic layer running along this structure ( 3 ), the aluminum-based coating ( 2 ) on its free surface on which the substrate ( 1 ) and the intermetallic layer ( 3 ) facing away from the side is essentially planar or running parallel to the substrate plane, its thickness varying in the longitudinal extension (L) and transverse extension (Q) of the steel sheet, the thickness corresponding to the averaged thickness (dB, gem), the averaged Thickness (dB, gem) is determined from the sum of the minimum thickness (dB, min) and half the distance (a). In this example, the transverse extension (B) runs perpendicular to the image plane or the longitudinal extension (L), symbolized by a circled X. The aluminum-based coating ( 2 ) In addition to aluminum and unavoidable impurities, one or more alloy elements from the group (Si, Fe, Mg, Zn): Si up to 15.0, Fe up to 5.0, Mg up to 5.0, Zn up to 30.0, contain. The thickness of the steel sheet is, for example, 0.5 to 4.0 mm, in particular 0.6 to 3.0 mm, preferably 0.7 to 2.5 mm (including double-sided coating ( 2 )). The intermetallic layer ( 3 ) includes at least iron and aluminum compounds.

3 each shows, in section, a schematic representation through a steel sheet according to an embodiment of the invention before austenitizing (a) and after hot forming and cooling (b). The arrows symbolize diffusion processes during austenitizing, in particular they show the diffusion paths of the element Fe from the substrate ( 1 ), which due to the heat treatment on the free surface of the aluminum-based coating ( 2 ) diffuses. After hot forming and cooling (b) to a hot formed sheet steel component ( 10 ) the surface structure remains ( 1.1 ) with the valley areas ( 1.12 ), Flank areas ( 1.13 ) and mountain areas ( 1.11 ) also on the hot-formed sheet steel component ( 10 ) essentially obtained. The aluminum-based coating ( 20th ) has a mixed form which results from the heat treatment and depending on the coating system provided (Al and optionally Si and / or Fe and / or Mg and / or Zn). Furthermore, the cover ( 2 ) of the hot-formed sheet steel component have an essentially n-layer, in particular 4- to 5-layer structure.

Steel sheets were provided in an investigation. Two cold-rolled substrates A, B, each 1.5 mm thick, were used as steel materials, the substrates having the chemical alloying elements listed in Table 1 in% by weight, the remainder being Fe and unavoidable impurities. Table 1 Substrate C. Mn Si P. S. N AI Ti Nb B. Cr Mon Approx A. 0.23 1.21 0.21 0.014 0.002 0.004 0.033 0.027 0.001 0.0021 0.11 0.0077 0.001 B. 0.37 1.13 0.42 0.009 0.0009 0.003 0.12 0.01 0.01 0.003 0.12 0.1 0.001

• Nr. 1 - Stahlblech A' mit dB=22 µm;
• Nr. 2 - Stahlblech B' mit dB=22 µm;
• Nr. 3 - Stahlblech A' mit dB=35 µm;
• Nr. 4 - Stahlblech A mit dB, gem=25 µm;
• Nr. 5 - Stahlblech B mit dB, gem= 25 µm;
• Nr. 6 - Stahlblech A mit dB, gem=39 µm.

The substrates A, B were coated with a metallic coating in a hot-dip coating system. The melt was an aluminum-based alloy which was applied to the substrates as an aluminum-based coating with 10.7% by weight Si, 2.6% by weight Fe, the remainder Al and unavoidable impurities. On the one hand, the substrates were processed conventionally, these being identified below as steel sheets with a (') as a reference. Further substrates were produced, with a surface structure with valley areas, flank areas and mountain areas being applied to their surfaces by means of a pair of rollers, which acted on the surfaces of the substrates on both sides, the degree of rolling being up to 2%. The surface structure on the surfaces of the substrates had a deterministic structure, a recurring pattern in the form of an I structure, where a = 12 μm, b = 20 μm and d = 15 μm, cf. 1 . As a result, an enlarged surface was set and provided in a targeted manner, which was at least 5%. In total, the following samples No. 1 to 6 were taken from the following steel sheets for further investigation:

• No. 1 - steel sheet A 'with dB = 22 µm;
• No. 2 - sheet steel B 'with dB = 22 µm;
• No. 3 - sheet steel A 'with dB = 35 µm;
• No. 4 - sheet steel A with dB, gem = 25 µm;
• No. 5 - sheet steel B with dB, gem = 25 µm;
• No. 6 - sheet steel A with dB, according to = 39 µm.

Samples Nos. 1 to 6 were subjected to direct hot working. The samples were austenitized in an oven, with the target temperatures for No. 1 and 4 at 920 ° C, for No. 2 and 5 at 960 ° C and for No. 3 and 6 at 880 ° C. After the austenitization, the warm or austenitized specimens No. 1 to 6 were removed from the furnace after the target temperature had been reached and, within a transfer time of approx. 7 s, a hot forming tool, which was actively cooled and thus also designed as a press hardening tool, fed and inserted, in which hot working and cooling into hot-worked sheet steel components No. 1 to 6 were carried out. The average cooling rate was at least 40 K / s and was thus above the critical cooling rate. A structure of martensite with> 95 surface% was established in all sheet steel components. From the flat samples Nos. 1 to 6, hot-forged steel sheet members Nos. 1 to 6 having a hat-shaped cross section were produced. The samples were completely heated. The cooling also took place completely on the hot-formed sheet steel components.

4th each shows a section of a micrograph, the micrographs being polished for a light microscopic examination and etched with 3% HNO3 acid, pictures being taken with 500x magnification, of a hot-formed sheet steel component according to the prior art 4a) and a hot-formed sheet steel component according to an embodiment of the invention 4b) . 4a) Fig. 11 shows the photograph of the hot-formed steel sheet No. 1 and 4b) the photo of the hot-formed sheet steel no.4. Both photos show through-alloyed coatings ( 20th ) as well as thickened interdiffusion layers ( 30th ). The difference is on the surface of the substrate ( 1 ) to recognize that due to the surface structure ( 1.1 ) with valley areas ( 1.12 ), Flank areas ( 1.13 ) and mountain areas ( 1.11 ) a surface on the alloyed-through coating ( 20th ) can be implemented with a lower porosity compared to a standard surface of a substrate, so that the hot forming, regardless of whether direct or indirect, compared to today's standard hot-formed sheet steel components ( 10 ) can be produced with improved weldability and / or paintability. The formed interdiffusion layer ( 30th ) consequently also follows the course of the surface structure ( 1.11 , 1.12 , 1.13 ) of the substrate of the hot-formed sheet steel component ( 10 ), s. 4b) .

With steel sheets according to the invention, hot-formed sheet steel components ( 10 ) can be provided with good weldability and / or paintability. By enlarging the surface of the substrate by at least 3%, in particular by at least 5%, preferably by at least 10%, preferably by at least 13%, the shape and / or dimensions of the surface structure ( 1.1 ) with valley areas ( 1.12 ), Flank areas ( 1.13 ) and mountain areas ( 1.11 ) positive influence in particular on the diffusion of iron within the aluminum-based coating ( 2 ) can be taken.

In the designs in the prior art, the rapid diffusion leads to a segregation of atoms in the layers of the coating close to the surface due to the different thermal and chemical properties of the structural components, which ultimately leads to increased pore formation. The uncontrolled formation of pores, especially closed pores, has a negative impact on weldability. A structured and regular nucleation as a result of the structuring of the surface of the substrate leads to a directional diffusion, which leaves less space for the formation of closed pores. The lower thermal and chemical (element distribution) gradient in the layers of the coating close to the surface is essentially responsible for the above-mentioned relationships. The discontinuous course of the thermal and chemical gradients can thus lead to less closed pores and preferably to open pores at the boundary layer with the atmosphere.

Due to the low porosity compared to the prior art with closed pores on the surface, a more even current flow can occur with resistance spot welding, for example, and a lower tendency to spatter, since the lower porosity results in a lower resistance, which is necessary for resistance spot welding and promotes this so that a welded joint with sufficient strength and better reproducibility can be produced. In contrast, a uniform but open porosity on the surface is advantageous for better paintability, since the chemical bond between the surface and an organic coating (paint) applied to the coating has a larger contact area and ensures better interlocking.

To evaluate the weldability and paintability, the sheet steel components hot-formed from the samples / steel sheets No. 1 to 6 were resistance welded and painted, see Table 2. The weldability was assessed on the basis of the width of the welding window. The paintability was assessed on the basis of an optical impression. Table 2 Sheet steel component Weldability Paintability 1 medium medium 2 medium medium 3 bad medium 4th Good Good 5 Good Good 6th Good Good

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

• EP 2086755 B1 [0003]
• EP 2242863 B1 [0003]
• DE 102008006771 B3 [0039]

Claims (14)

Sheet steel for hot forming comprising a substrate (1) made of a hardenable steel material with an aluminum-based coating (2), an intermetallic layer (3) being formed between the substrate (1) and the aluminum-based coating (2), characterized in that the substrate ( 1) has a surface structure (1.1) with valley areas (1.12), flank areas (1.13) and mountain areas (1.11). Sheet steel after Claim 1 , the hardenable steel material having the following chemical composition in% by weight: C. = 0.05 until 0.5, Mn = 0.5 until 3.0, Si = 0.05 until 1.7, P. until 0.1, S. until 0.1, N until 0.1, and optionally one or more alloy elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca): Al until 1.0, Ti until 0.2, V until 0.5, Nb until 0.5, B. until 0.01, Cr until 1.0, Mon until 1.0, Cu until 1.0, Ni until 1.0, Approx until 0.1, Remainder Fe and unavoidable impurities. Steel sheet according to one of the preceding claims, wherein the aluminum-based coating (2) has the following chemical composition in% by weight: optionally one or more alloy elements from the group (Si, Fe, Mg, Zn): Si until 15.0; Fe until 5.0 Mg until 5.0 Zn until 30.0; Remainder Al and unavoidable impurities, and wherein the intermetallic layer has at least iron and aluminum compounds. Steel sheet according to one of the preceding claims, wherein the aluminum-based coating (2) has a thickness between 3 and 40 µm, the thickness corresponding to an average thickness (dB, gem). Steel sheet according to one of the preceding claims, wherein the intermetallic layer (3) has a thickness between 0.01 and 10 µm. Steel sheet according to one of the preceding claims, wherein the surface structure (1.1) is designed deterministically with valley areas (1.12), flank areas (1.13) and mountain areas (1.11). Steel sheet according to one of the preceding claims, wherein in the vertical extent of the substrate (1) the distance (a) between the valley regions (1.12) and mountain regions (1.11) is between 1 and 50 µm. Steel sheet according to one of the preceding claims, wherein the substrate (1) has a surface area enlarged by at least 3%. A method for producing a hot-formed sheet steel component (10), the method comprising the following steps: - Provision of a steel sheet according to one of the preceding claims, - Austenitizing the steel sheet at a temperature of at least Ac1, Hot forming and cooling of the austenitized steel sheet to a temperature between 20 ° C and Ms. Procedure according to Claim 9 , wherein the steel sheet is provided as a flat blank or as a preformed part. Procedure according to Claim 9 or 10 , the austenitizing being carried out in a furnace at a temperature of at least Ac3 for a period of between 50 and 1200 s. Hot-formed sheet steel component (10), in particular manufactured according to one of the Claims 9 until 11 wherein the sheet steel component (10) has a substrate (1) with an aluminum-based coating (20) and an interdiffusion layer (30) formed between the substrate (1) and the aluminum-based coating (20), characterized in that the substrate (1) a surface structure (1.1) with valley areas (1.12), flank areas (1.13) and mountain areas (1.11). Hot-formed sheet steel component according to Claim 12 , the formed interdiffusion layer (30) following the surface structure (1.1). Hot-formed sheet steel component according to Claim 12 or 13th , wherein in the vertical extent of the substrate (1) the distance (a) between the valley areas (1.12) and mountain areas (1.11) is between 1 and 50 μm.

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