DE102022102111A1 - Uncoated cold-rolled steel sheet for hot forming, method of manufacturing a hot-formed sheet steel component and hot-formed sheet steel component - Google Patents

Abstract

The invention relates to an uncoated, cold-rolled steel sheet (1) made from a hardenable steel material for hot forming and having a deterministic surface structure (1.1). 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 an uncoated cold-rolled steel sheet made from a hardenable steel material for hot forming with a deterministic surface structure. Furthermore, the invention 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 using hot forming has already become industrially established, in particular for the production of body parts such as, for example, the production of safety-relevant B-pillars, etc. Sheet steel components can be produced using direct or indirect hot forming processes. Flat blanks (directly) or already preformed or (cold) formed semi-finished products/parts (indirectly) made of a hardenable steel material are heated to a temperature at which, depending on the composition of the steel sheet used, a structural transformation occurs. The structural transformation to austenite begins with Ac1 and when Ac3 is reached or above Ac3, an essentially completely austenitic structure is present. The heating above at least Ac1 is also called "austenitizing" in technical circles, especially if a complete transformation into austenite is to take place (>= Ac3). After heating, the hot (austenitized) sheet steel is placed in a forming tool and hot-formed. In the course of or after the end of hot forming, the still warm steel sheet is cooled in such a way, preferably within the forming tool, which is preferably actively cooled, that the structure in the steel sheet (component) changes to a hard structure of martensite and/or bainite, preferably essentially from martensite, transformed. In professional circles, the cooling or quenching of the sheet steel within the forming tool or by the action of a (hardening) tool that 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 depend on the chemical composition of the hardenable steel material used and can be taken 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 strength. A good balance between strength and weight has been found with classic hot forming or press hardening, particularly of manganese-boron steels for the manufacture of structural components in the automotive sector.

Conventionally, steel sheets made from a hardenable steel material and coated with a metallic coating are hot dip coated, usually with an Al-based coating. A specific layer structure forms within the coating. This structure is determined based on an Al-Fe boundary layer that is established, which is essential for good adhesion, among other things, the thickness of which depends on the immersion temperature and the composition of the melt. The steel sheets coated in this way are further processed into a sheet steel component in a hot forming process. Due to austenitization during hot forming, the coating is transformed by diffusion and/or alloying. In addition, the structure of the coating can essentially be adjusted depending on the requirements in terms of corrosion, weldability and paintability. The substantial breakdown of the metallic coating occurs due to diffusion between the metallic coating and the steel sheet at the intermetallic layer. Corresponding examples are disclosed in the prior art, including in the publications EP 2 086 755 B1 , EP 2 242 863 B1 .

From the international disclosure document WO 2019/111301 A1 discloses a cold-rolled steel sheet of the generic type and a method for producing a hot-formed sheet steel component, the teaching being aimed at a special combination of a defined coiling temperature after hot rolling and a specific roughness of the cold-rolled steel sheet after cold rolling, so that a defined intermetallic layer with a specific standard deviation forms can form in the coating.

In the disclosure document DE 10 2020 201 451 A1 describes the production of a sheet steel textured with a deterministic surface structure, which is coated with an aluminum-based coating, which is used to form a hot-formed sheet steel component.

Furthermore, it is known that during the austenitization of blanks in furnace systems, in particular the furnace rollers in roller hearth furnaces for hot forming (in particular the first part of the furnace length), adhesion of Al-Si coating that has become partially liquid during the austenitization can result. In particular, this correlates directly with the austenitization rate, since due to the constant diffu sion speed with higher austenitization speed, the amount of partially liquid Al-Si coating increases, so that there is a need to adjust the austenitization conditions (temperature setting and flow rate through the individual furnace zones) by means of a special furnace design (requirement of several furnace zones with partitions) and/or special furnace control.

Furthermore, it is also known that a high rate of austenitization prior to hot forming or press hardening correlates with a high pore density in the coating layer, primarily in the interdiffusion zone. Pronounced pores in the coating layer lead to spatters in the resistance spot welding process and thus to surface defects in body construction.

The task is therefore to first provide an uncoated cold-rolled steel sheet which is coated with an aluminium-based coating which tends to have fewer adhesions during austenitization, so that no complex adjustment of the furnace systems and/or the furnace control is necessary, and ultimately a hot-formed one To specify sheet steel component, which has no pronounced pore formation, so that a process-reliable resistance weldability can be ensured.

The object is achieved with an uncoated cold-rolled sheet steel having the features of claim 1, with a method for producing a hot-formed sheet steel component with the features of claim 5 and with a hot-formed sheet steel component with the features of claim 9.

According to a first teaching, the invention relates to an uncoated cold-rolled steel sheet made of a hardenable steel material for hot forming with a deterministic surface structure, the surface of the steel sheet having a roughness Ra between 1.5 and 6.0 µm and a peak number RPc between 30 and 150 1 /cm.

The inventors have surprisingly found that a defined structuring of the surface of the uncoated cold-rolled steel sheet with a deterministically designed surface structure, the surface of the steel sheet having a roughness Ra between 1.5 and 6.0 μm and a peak number RPc between 30 and 150 1/ cm, a positive effect can be had on the subsequent manufacturing processes. In particular, the roughness Ra can be adjusted to at least 1.8 μm, preferably at least 2.0 μm. In particular, the roughness Ra can be adjusted to a maximum of 5.5 μm, preferably to a maximum of 5.0 μm. In particular, the number of peaks RPc can be set to at least 35 l/cm, preferably to at least 40 l/cm. In particular, the number of peaks RPc can be set to a maximum of 120 l/cm, preferably to a maximum of 90 l/cm.

The roughness Ra in µm and the number of peaks RPc in 1/cm can be determined along a defined measuring section, cf. EN ISO 4287 .

Under sheet steel is u. to understand a steel flat product as strip or sheet metal or plate. The steel sheet has a longitudinal extent (length), a transverse extent (width) and a vertical extent (thickness). The surface of the steel sheet is structured in a defined manner by means of one or more rolls, for example in the last rolling stand in a cold rolling train or separately in a (post) rolling stand. The deterministic surface structure on the surface of the steel sheet (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 peak area on the surface of the roller. The setting of the roughness Ra and the peak number RPc on the surface of the steel sheet depends on the one hand on the roughness Ra and the peak number RPc of the surface of the roll and on the other hand on the transfer rate, which depends on the degree of rolling and/or the rolling force and can therefore be specifically controlled.

The thickness of the uncoated cold-rolled 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.

A deterministic surface structure is to be understood as meaning recurring surface structures which have a defined shape and/or design, cf. EP 2 892 663 B1 . In particular, this also includes surfaces with a (quasi) stochastic appearance, which, however, are applied using a deterministic texturing process and are therefore composed of deterministic form elements. The method for the deterministic texturing of rollers, thus for the production of rollers with a corresponding positive impression, is in EP 2 892 663 B1 described.

According to one embodiment, the deterministic surface structure of the steel sheet has a structure depth Rt between 1 and 25 μm, in particular between 5 and 25 μm, preferably between 9 and 21 μm. The structure depth Rt in µm is the maximum distance between the highest peak in a peak area and the deepest valley in a valley area of the surface structure of the steel sheet along a defined measuring section, cf. EN ISO 4287 . The deterministic surface structure thus preferably has at least one mountain area and at least one valley area, wherein the deterministic surface structure can in particular be designed as an open structure, i.e. there is a continuous valley area and several mountain areas that are not connected to one another. Furthermore, the deterministic surface structure can preferably be designed as a closed structure, ie there is a continuous mountain area and several valley areas that are not connected to one another. Alternatively, a mixed structure of mountain and valley areas is also possible. The flank area connects the mountain and valley areas. Thus, there can also be several edge areas.

According to one embodiment, the steel sheet has a surface with an Sdr value of at least 2.8%, in particular at least 3.2%, preferably at least 5%, preferably at least 10%. The Sdr value is also referred to as the developed interface ratio or can be seen as the increase in surface area in relation to the "projection area". 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 dimensions of the surface structure and on the number or distribution of the structure, with the Sdr value being up to 100%, in particular up to 50%, preferably up to 20% can.

According to one embodiment, the hardenable steel material has the following chemical composition in % by weight: C = 0.05 until 0.5, Mn = 0.3 until 5.0, Si = 0.05 until 1.7, P until 0.1, S until 0.1, N until 0.1, and optionally one or more alloying 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, no 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 a content of 0.1% by weight, preferably wise at least a content of 0.15 wt .-% is present. Furthermore, C contributes to a large extent to a higher CEV value (CEV = carbon equivalent), which negatively affects weldability, so that a content of up to a maximum of 0.5% by weight, to reduce the tendency to cracks, in particular up to a maximum of 0 45% by weight, preferably up to a maximum of 0.4% by weight. Furthermore, negative influences with regard to the toughness properties, the forming properties and the suitability for welding can be avoided by the specified upper limit. 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 to undesirably form pearlite during cooling and lowers the critical cooling rate, thereby increasing hardenability. In addition, Mn can be used to bind S in order to prevent the hot-rollability from being impaired too much by an FeS eutectic and/or to reduce the proportion of pearlite, so that in particular a content of at least 0.5% by weight is present. On the other hand, an excessively high Mn concentration has a negative effect on 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, 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 alloyed in particular.

Silicon (Si) is an alloying element that contributes to deoxidation. A level of at least 0.05% by weight is used to ensure effectiveness. However, Si can also contribute to an increase in strength, so that a content of at least 0.1% by weight, preferably at least 0.2% by weight, is alloyed in. 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, 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 to retard cementite formation in amounts up to 0.1% by weight. To ensure the desired delay and stabilization, levels of in particular at least 0.002% by weight, preferably at least 0.004% by weight, are set. However, P has a strong impact on toughness and thus has an unfavorable effect on formability. Due to its low diffusion rate, P can also lead to severe segregation when the melt solidifies. Negative influences on formability and/or weldability can be ruled out with certainty if the content is limited in particular to a maximum of 0.05% by weight, and preferably to a maximum of 0.05% by weight to additionally reduce the segregation effects.

Sulfur (S) is an alloying element that can be present in amounts up to 0.1% by weight. Since S in steel has a strong tendency to segregate and can adversely affect formability or toughness as a result of the excessive formation of FeS, MnS or (Mn,Fe)S, the content is therefore limited 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 amounts of up to 0.1% by weight to form nitrides 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. N can be used just like C to increase the martensite hardness, but weakens the grain boundaries less than C. In order to achieve this effect, in particular contents of at least 0.0005% by weight, preferably at least 0.001% by weight, preferably at least 0.002% by weight, can be set. However, N, especially in combination with Al and/or Ti, leads to the formation of coarse nitrides, which can have a negative impact 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, the N content should particularly preferably be set to between 0.001%<N<=0.004% in the case of Ti contents>0.01% by weight.

The steel sheet can optionally contain one or more alloying 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 added boron can unfold its strength-enhancing effect. Therefore, in particular alloyed with a content of at least 0.005% by weight, preferably at least 0.01% by weight. To avoid technical casting problems, the content is limited in particular to a maximum of 0.5% by weight, preferably to a maximum of 0.2% by weight.

Titanium (Ti), as an optional alloying element, can increase strength through the formation of carbides, nitrides and/or carbonitrides and act as a microsegregation element. Furthermore, formation of coarse austenite structure can be suppressed. Also, Ti can be used for grain refinement and/or nitrogen fixation and, if boron is present, can increase the effectiveness of boron. Since it can also contribute to increasing the effectiveness of Cr, it can optionally be alloyed 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, and 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 alloyed in. Contents of at least 0.01% by weight, preferably at least 0.015% by weight, can preferably be used to utilize the strength-increasing effect.

Vanadium (V) and/or niobium (Nb) can be added as optional alloying elements individually or in combination for grain refinement and/or to delay hydrogen-induced cracking. Like Ti, these optional alloying elements can be used as micro-alloying elements to form strength-enhancing carbides, nitrides and/or carbonitrides. To ensure their effectiveness, V and/or Nb can be used in particular in amounts of (in each case) at least 0.005% by weight, preferably at least 0.01% by weight, preferably at least 0.015% by weight. The minimum content for Nb and V, 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 a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight, since higher contents have a disadvantageous effect on the material properties, in particular can have a negative effect on the toughness properties of the steel.

Boron (B), as an optional alloying element, can segregate on the phase boundaries and prevent their movement. This can lead to a fine-grain structure, which can have a beneficial effect on the mechanical properties. In order to ensure the effectiveness of these effects and to increase hardenability, a content of up to 0.01% by weight, in particular for cost reasons up to a maximum of 0.005% by weight, and to reliably avoid embrittlement at grain boundaries, preferably up to a maximum of 0.004% by weight %, and in particular to ensure reliable effectiveness even in the presence of N, for example in the form of technically unavoidable impurities in the steel melt with N, in particular of at least 0.0005% by weight, to increase the fine grain size, preferably of 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 for binding N.

Chromium (Cr) can be added as an optional alloying element to adjust the hardness and strength, in particular with a content of at least 0.01% by weight, since like C it can support the transformation into austenite. For reasons of cost, the upper limit is defined as 1.0% by weight. If the content is too high, the suitability for welding 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 carbon diffusion and thus promote a conversion far from equilibrium, in particular contents of at least 0.01% by weight, preferably at least 0.1% by weight, preferably at least 0.15% by weight, can be alloyed in.

As an optional alloying element, molybdenum (Mo) 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 added with a content of up to 1.0% by weight, in particular between 0.01 and 0.8% by weight the greatest possible hardness and to reduce carbon diffusion, preferably between 0.1 and 0.5% by weight.

If, for example, the optional alloying elements Cr and Mo are alloyed together, their 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 in particular of at least 0.01% by weight, preferably of at least 0.05% by weight, can be alloyed in. In particular, the content is limited to a maximum of 0.5% by weight, preferably to a maximum of 0.2% by weight, in order to avoid negative To avoid influences on the suitability for welding and the toughness properties in the heat-affected zone of a weld made on the steel sheet 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 alloyed in. Contents of at least 0.02% by weight can preferably be alloyed in to increase toughness. To improve the 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 in the melt as a desulfurizing agent and for the targeted influencing of sulfides 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, which can lead to a change in the 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 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 the properties of the steel material, preferably not worsening them. P, S and/or N can also be tolerated as impurities if they are not specifically alloyed.

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 which has been coated with an aluminium-based coating; austenitizing the steel sheet at a temperature of at least Ac1; Hot forming and cooling of the austenitized sheet steel to a temperature between 20°C and Ms.

After the introduction/embossing of the deterministic surface structure by means of rollers, the steel sheet is coated on one side or preferably on both sides with an aluminum-based coating in a hot-dip coating system.

The steel sheet coated with an aluminium-based coating is heated or austenitized to a temperature of at least Ac1 or above, in particular at least Ac3 or above, to form austenite in the steel sheet, preferably in a period of time which is sufficient, in particular depending on the Thickness and/or composition of the steel material used to fully 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 period, so that complete austenitization > Ac3 is preferred.

As a result of the deterministic surface structure, there are different, in particular non-linear, diffusion paths of the iron from the steel sheet into the coating during austenitizing, so that it was surprisingly found that the tendency of the coating to adhere to the furnace rollers could be reduced.

In the context of the invention, in addition to complete heating of the steel sheet, only partial heating is to be understood if a sheet steel component is to be produced with partially different properties.

"Hot forming and cooling" means that the steel sheet, as a result of targeted austenitization, includes hot forming as indirect or direct hot forming combined with cooling in a tool (press hardening) or in a medium (hardening). If the austenitized sheet steel is cooled to a temperature below Ms, it can be ensured that the formation of a hard structure from austenite to martensite is forced, in particular by 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 °C, more preferably up to a temperature of 200 °C.

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 hot forming takes place in a hot forming tool and cooling in a cooling tool or hot forming and cooling in one tool, the cooling tool and tool correspond to a press-hardening tool, which has the advantage that a particularly dimensionally accurate hot-formed sheet steel component is produced, since the austenitized sheet steel is in contact with a shaping contour of the press-hardening tool. In the case of indirect hot forming, the press-hardening tool causes only minor shaping (= hot forming) as part of a calibration and/or correction to the target size or final geometry of the hot-formed 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 indirect hot forming, a preformed part can be provided as the steel sheet with the aluminum-based coating, which part essentially has a geometry close to the final dimensions.

Alternatively and preferably, in the case of direct hot-forming, the steel sheet with the aluminium-based coating is 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, hot forming can also be performed in two or more hot forming tools. The one hot forming tool or the hot forming tool in the last stage of several tools in the process is also designed for cooling at the same time, so that the one hot forming tool or the last hot forming tool arranged in the process chain is designed as a press-hardening tool, which is actively cooled and has corresponding (critical) cooling speeds provides in order to be able to set a hard structure in the hot-formed sheet steel component. If there are 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.

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

According to one embodiment of the method, the austenitizing is carried out in a furnace at a temperature of at least Ac3 for a period of between 50 and 900 s, in particular between 80 and 600 s, preferably between 100 and 400 s, preferably between 180 and 360 s. The temperature for austenitization is a maximum of 980°C, in particular a maximum of 940°C. Depending on the temperature in the furnace and the thickness of the steel sheet used, in addition to the transformation of the steel workpiece completely into austenite, essentially complete alloying of the coating can be ensured within the specified time periods. “Through alloying” means that > 50% by weight Fe is present in the coating, or < 70% by weight Al is present in all phases.

The aluminium-based coating can be alloyed with at least 30 wt. A heat treatment prior to hot forming can be used to carry out targeted "pre-alloying" depending on the temperatures (< Ac1) and duration (up to approx. 600s), in order to enable, for example, a reduction in the time until the coating alloys through during austenitization during hot forming , cf. for example DE 10 2008 006 771 B3 .

The aluminium-based coating has the following chemical composition in % by weight: optionally one or more alloying elements from the group (Si, Fe, Mg, Zn): si until 15.0, feet until 5.0, mg until 5.0, Zn until 30.0,

remainder Al and unavoidable impurities. In the aluminium-based coating, in addition to aluminum and unavoidable impurities, additional elements such as silicon with a content of up to 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 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 4.0% by weight, with the content being limited in particular to a maximum of 12.0% by weight. , preferably can 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 at least 0.1% by weight, preferably at least 0.5% by weight, preferably at least 1.0% by weight, with the content being limited in particular to a maximum of 4.0% by weight .-%, preferably to a maximum of 5.5 wt .-% can be limited. Fe in the plating can increase the melting temperature of the plating, which can be beneficial in austenitizing. Alternatively or additionally, Mg can be present in particular with at least 0.1% by weight, preferably with at least 0.2% by weight, with the content being limited 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. Alternatively or additionally, Zn can be present in an amount of at least 0.1% by weight, preferably at least 0.2% by weight, with the content in particular being limited to a maximum of 20.0% by weight, preferably a maximum of 10.0% by weight .-%, preferably to a maximum of 5.0 wt .-% can be limited. Zn can contribute to the improvement of corrosion resistance in the coating.

The thickness of the aluminum-based coating is, for example, 5 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 average thickness. Due to the deterministic surface structure with the thickness of the coating varying in the longitudinal and transverse extent of the steel sheet, the thickness of the coating on the steel sheet is not constant throughout, so that an average thickness is given. The aluminium-based coating on the steel sheet, on the other hand, is essentially level or planar on its (free) surface.

The third teaching of the invention relates to a hot-formed sheet steel component comprising a steel sheet with an aluminum-based coating alloyed with Fe, the steel sheet having a deterministic surface structure, the surface of the coating having a roughness Ra between 2.0 and 10.0 μm and a peak number RPc between 40 and 170 1/cm.

Through the targeted adjustment of the roughness Ra and peak number RPc on the uncoated cold-rolled sheet steel with a defined roughness Ra between 1.5 and 6.0 µm and a defined peak number RPc between 30 and 150 1/cm, and in particular a structure depth Rt between 1 and 25 µm, the process results in a roughness Ra between 2.0 and 10.0 µm and a peak number RPc between 40 and 170 1/cm on the surface of the coating (20) on the hot-formed sheet steel component (10), so that a surface finish is provided , which can be excellently and reliably resistance spot welded.

In particular, the roughness Ra can be at least 2.1 μm, preferably at least 2.2 μm. In particular, the roughness Ra can be a maximum of 9.0 μm, preferably a maximum of 8.0 μm.

In particular, the structure depth Rt can be at least 6 μm, preferably at least 7 μm. In particular, the structural depth Rt can be a maximum of 21 μm, preferably a maximum of 20 μm.

In particular, the number of peaks RPc can be at least 45 l/cm, preferably at least 50 l/cm. In particular, the number of peaks RPc can be a maximum of 160 l/cm, preferably a maximum of 155 l/cm.

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

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

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

The aluminium-based coating of the hot-formed sheet steel component is essentially through-alloyed with iron (> 50% by weight Fe based on the entire coating) and has a mixed form, which includes depends on the chemical composition of the coating.

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

Specific configurations of the invention are explained in more detail below with reference to the drawing. The drawing and accompanying description of the resulting features are not to be read as limiting to the respective configurations, but serve to illustrate exemplary configurations. Furthermore, the respective features can be used with each other as well as with features of the above description for possible further developments and improvements of the invention, especially in additional configurations that are not shown.

• 1a, b) eine Draufsicht auf einen Teilbereich eines unbeschichteten kaltgewalzten Stahlblechs mit einer deterministischen Oberflächenstruktur a) und ein Schliffbild entlang der Linie I-I b),
• 2) ein Schliffbild eines Teilabschnitts an einem mit einem aluminiumbasierten Überzug beschichteten Stahlblech und
• 3) ein Schliffbild eines Teilabschnitts nach dem Warmumformen und Abkühlen eines warmumgeformten Stahlblechbauteils.

The drawing shows in

• 1a, b ) a top view of a portion of an uncoated cold-rolled steel sheet with a deterministic surface structure a) and a micrograph along line II b),
• 2 ) a micrograph of a section of a steel sheet coated with an aluminium-based coating and
• 3 ) a micrograph of a section after hot forming and cooling of a hot-formed sheet steel component.

1 shows in 1a) a plan view of a portion of an uncoated, cold-rolled steel sheet (1) with a deterministic surface structure (1.1), the steel sheet (1) consisting 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 alloying 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; Mon to 1.0; Cu to 1.0; Ni to 1.0; Approx to 0.1; remainder Fe and unavoidable impurities. In the micrograph in 1b) the deterministic surface structure (1.1) with at least one valley area (1.12) and at least one mountain area (1.11), which are connected to one another by at least one flank area (1.13), can be seen. The deterministic surface structure (1.1) can be designed as an open, closed or mixed structure. Depending on the design and/or shape of the surface structure (1.1), the surface of the steel sheet (1) has an Sdr value of at least 2.8% and thus an enlarged surface. Other three-dimensional configurations of the structure, which can be represented repeatedly in the longitudinal and/or transverse direction of the steel sheet, are also conceivable. The arrow in 1a) symbolizes the rolling direction. A deterministic surface structure (1.1) is embossed into the uncoated, cold-rolled steel sheet (1) by means of rolling, with the surface of the steel sheet (1) having a roughness Ra between 1.5 and 6.0 µm and a peak number RPc between 30 and 150 according to the invention 1/cm. Furthermore, the deterministic surface structure (1.1) has a structure depth Rt between 1 and 25 μm.

The uncoated cold-rolled steel sheet (1) with the deterministic surface structure (1.1) according to the invention is coated with an aluminum-based coating (2) in a hot-dip coating system, the aluminum-based coating (2) optionally containing one or more alloying elements from the group (Si , Fe, Mg, Zn): with Si up to 15.0% by weight; Fe up to 5.0% by weight; Mg to 5.0% by weight; Zn may contain up to 30.0% by weight. A correspondingly coated sheet steel (1) is shown in a micrograph in a partial section 2 to see. The thickness of the steel sheet (1) 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 coating (2) on both sides).

A steel sheet (1) coated with an aluminum-based coating (2) is austenitized at a temperature of at least Ac1 and then hot-formed and cooled to a temperature between 20 °C and Ms. 5 shows a micrograph of a section after hot forming and cooling of a hot-formed steel sheet component (10), which comprises a steel sheet (1) with an aluminum-based coating (20) alloyed with Fe, the steel sheet (1) having a deterministic surface structure (1.1). According to the invention, the surface of the coating (20) has a roughness Ra between 2.0 and 10.0 μm and a peak number RPc between 40 and 170 1/cm.

In a series of tests, 5 hot-formed sheet steel components (reference: 1* to 5*) were produced conventionally and 5 hot-formed sheet steel components (invention: 6 to 10) were produced according to the invention. A hardenable steel of the quality 22MnB5 was used as the steel material, which was cold-rolled to a thickness of 1.5 mm for all investigations, each with an aluminum-based coating with 10.7% by weight Si, 2.6% by weight Fe, remainder Al and unavoidable impurities was coated on both sides in a hot-dip coating system with an (average) thickness of 25 µm. In the last rolling stand of the cold rolling train, pairs of rolls were used which had a surface texture textured using EDT, so that in reference 1* to 5* the surface of the uncoated steel sheet was structured with a stochastic surface structure. In embodiments 6 and 7 according to the invention, the surface of the uncoated sheet steel is textured by means of a laser with a pair of rollers with a deterministic surface structure in the form of a closed rectangular structure, cf. 1a) , been structured. The structuring of the surface in versions 8 to 10 according to the invention was carried out analogously to versions 5 and 6, but with the difference that a coarser, deterministic surface texture was generated in the pair of rollers, resulting in a rectangular structure in the surface of the uncoated steel sheet, which was twice as was made large. The rolling forces [t/m] applied in the last roll stand for versions 1* to 10 and the characteristic values Ra, Rt, RPc and Sdr value of the surface structure produced in the surface of the uncoated steel sheet have been determined and listed in Table 1. Furthermore, the characteristic values Ra and RPc after hot-dip coating were determined for versions 1* to 10, which are also listed in Table 1. The coated steel sheets provided for versions 1* to 10 were austenitized under the same conditions in a furnace, namely at 920° C. for approx. 300 s. The warm steel sheets were subjected to a hot forming tool, which was actively cooled, within a transfer time of approx. 7 s and was therefore also designed as a press-hardening tool at the same time, supplied and inserted, in which hot forming and cooling to form hot-formed sheet steel components 1* to 10 was carried out. The average cooling rate was at least 40 K/s and was thus above the critical cooling rate. A martensite microstructure with >98 area % was established in all sheet steel components. Table 1 uncoated sheet steel coated sheet steel hot-formed sheet steel component Ra to µm Rt in µm rolling force in t/m RPc in 1/cm Sdr value in % Ra to µm RPc in 1/cm Ra to µm RPc in 1/cm WPS 1* 0.4 3.1 1090 11.7 0.8 0.9 37.2 2.1 181.1 0 2* 0.6 3.7 1020 15.6 1.4 2.5 52.7 1.8 204.5 0 3* 0.9 4.4 926 33.5 2 1.1 43.5 2.2 187.4 0 4* 1.1 5.7 970 37.1 2.5 1.8 40.9 1.6 201.8 0 5* 1.5 6.9 1076 45.9 2.5 1.6 45.4 1.9 175.9 0 6 2.5 10.2 1017 59.8 3.6 0.9 26.8 2.9 154.3 ** 7 2.8 10.6 1250 60.7 5.6 1.4 32.5 5.0 140.6 ** 8th 3.7 14.2 951 60.8 11.2 1.8 17.1 4.1 133.1 ** 9 4.4 17.1 1017 61.3 12.9 2.4 27.2 4.9 92.5 ** 10 4.6 18.8 1250 62.2 13.5 2.6 25.4 7.0 60.8 *

It is easy to see that targeted structuring of the surface of an uncoated steel sheet with a deterministic surface structure leads not only to an increase in roughness Ra and structure depth Rt, but also to an increase in the surface Sdr value and an increase in the number of peaks RPc. The surface of the aluminium-based coating is essentially identical in all cases before austenitizing and the nature of the coated surface of the steel sheet does not appear to be relevant at first. The influence of the surface quality in the form of the deterministic surface structure only becomes noticeable after hot forming and cooling, so that a surface suitable for resistance spot welding (WPS) is formed. Versions 1* to 5* each show a mediocre welding result with 0, with a lower number of peaks RPc and higher roughness Ra compared to the standard, resulting in a more reliable welding result, with * good and ** very good.

For the micrographs, samples were taken from the respective steel sheets and prepared in the cross-section after etching with 3% HNO3.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited 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]
• WO 2019/111301 A1 [0004]
• DE 102020201451 A1 [0005]
• EP 2892663 B1 [0015]
• DE 102008006771 B3 [0049]

Non-patent Literature Cited

• DIN EN ISO 4287 [0012, 0016]

Claims (9)

Uncoated, cold-rolled steel sheet (1) made from a hardenable steel material for hot forming with a deterministic surface structure (1.1), characterized in that the surface of the steel sheet (1) has a roughness Ra between 1.5 and 6.0 µm and a peak number RPc between 30 and 150 1/cm. sheet steel claim 1 , wherein the deterministic surface structure (1.1) has a structure depth Rt between 1 and 25 µm. Steel sheet according to any one of the preceding claims, wherein the steel sheet (1) has an Sdr value of at least 2.8%. Steel sheet according to one of the preceding claims, wherein the hardenable steel material has 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 alloying 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, no until 1.0, Approx until 0.1, remainder Fe and unavoidable impurities. Method for producing a hot-formed sheet steel component (10), the method comprising the following steps: - Providing a steel sheet (1) according to any one of the preceding claims, which has been coated with an aluminium-based coating (2), - austenitizing the coated steel sheet at a temperature of at least Ac1, - Hot forming and cooling of the austenitized sheet steel to a temperature between 20 °C and Ms. procedure after claim 5 , wherein the steel sheet (1) with the aluminum-based coating (2) is provided as a flat blank or as a preformed part. procedure after claim 5 or 6 , the austenitizing being carried out in a furnace at a temperature of at least Ac3 for a period of between 50 and 600 s. Procedure according to one of Claims 5 until 7 , wherein the aluminium-based coating (2) has the following chemical composition in % by weight: optionally one or more alloying elements from the group (Si, Fe, Mg, Zn): si until 15.0, feet until 5.0, mg until 5.0, Zn until 30.0, remainder Al and unavoidable impurities. Hot-formed sheet steel component (10) comprising a steel sheet (1) with an aluminum-based coating (20) alloyed with Fe, the steel sheet (1) having a deterministic surface structure (1.1), characterized in that the surface of the coating (20) has a roughness Ra between 2.0 and 10.0 µm and a peak count RPc between 40 and 170 1/cm.