Method for producing a steel component from at least one first steel sheet molded part and a second steel sheet molded part, and steel component

Abstract

The invention relates to a method for manufacturing a steel component from at least one first steel sheet metal part and a second steel sheet metal part. The method comprises the following steps: providing the first steel sheet metal part and the second steel sheet metal part; placing the steel sheet metal parts next to each other, whereby the amount of aluminum-based corrosion protection coating in an overlap area must not be too high; welding the steel sheet metal parts together, for example by laser beam welding. A further aspect of the invention relates to the steel component.

Description

[0001] The invention relates to a method for manufacturing a steel component from at least one first sheet metal part and a second sheet metal part. The invention further relates to a steel component.

[0002] When the terms "flat steel product" or "sheet metal product" are used below, these refer to rolled products such as steel strips or sheets, from which "sheet blanks" (also called blanks) are cut for the production of, for example, bodywork components. "Sheet metal forming parts" of the type according to the invention are produced from such sheet metal blanks by forming. "Steel components" are, in turn, composed of several "sheet metal forming parts" that are joined together at one or more joints.

[0003] All information regarding the concentrations of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified percentages relating to a steel alloy are therefore to be understood as values ​​in wt.%. Information in this text regarding the concentrations of the constituents of an atmosphere refers to volume (expressed in vol.%).

[0004] Steel flat products for hot forming into sheet metal parts are known from the prior art, featuring an aluminum-based or zinc-based coating. For example, WO 2022 / 048990 A1 and EP 2 993 248 B1 disclose such steel flat products and sheet metal parts with corresponding coatings. These coatings offer good corrosion protection. Steel flat products coated in this way play a significant role in hot forming processes known as press hardening.

[0005] However, the processing of such sheet metal components has revealed difficulties in joining them to steel components. For example, resistance spot welding (WPS), a process regularly used in the automotive industry, has been shown to reduce the strength of the components in the weld area. This is reflected in destructive material testing, which reveals a detrimentally low force transmission across the weld.

[0006] As a measure of the quality of maximum force transmission, the shear tensile strength is measured using a tensile test specimen according to the specifications of SEP 1220-3:2011-08, where shear tensile strength is the shear force. The shear force is the maximum force applied at failure in the shear tensile strength test. SEP 1220-3:2011-08 refers to ISO 14273 for the execution of the quasi-static shear tensile test. The title of ISO 14273 is: "Resistance welding - Destructive testing of welds - Specimen dimensions and methods for tensile shear testing of resistance spot and projection welds." There is no corresponding standard for laser welds. The test is carried out in accordance with ISO 14273:2000.The measurement involves taking a rectangular strip from the steel component such that the lap weld runs parallel to the short edge of the rectangular strip along its entire length, in the direction parallel to the short edge, and is located in the center of the strip. The short edge of the strip has a length of 45 mm. The exact length of the long extension direction is not relevant; a length of 200 mm has proven practical. This strip is then clamped at both ends and subjected to a force in the direction of the long extension direction. The force is gradually increased until the weld fails. The value of the maximum force before failure is referred to as the maximum force transmission. This is the maximum force applied at failure in the shear tensile strength test.In the context described, this is also referred to as shear strength, even though strictly speaking it is a force and not an area-related force.

[0007] Regular measurements conducted in practice have shown that the maximum force transmission varies considerably depending on the materials being joined. While welded specimens without a corrosion protection coating exhibit a maximum force transmission, i.e., shear tensile strength, of more than 45 kN, specimens with known corrosion protection coatings, for example, those based on aluminum, show a significantly reduced maximum force transmission, often below 30 kN. This is considered insufficient for use in relevant sectors such as automotive engineering.

[0008] Since the use of coated components is required in many applications, for example in automotive engineering in wet areas of vehicles, the task is to provide joined steel components with high-quality corrosion coating and good strength.

[0009] The problem is solved by a method having the features of claim 1. The problem is further solved by a steel component having the features of claim 14.

[0010] A method for manufacturing a steel component from at least one first steel sheet metal part and one second steel sheet metal part is proposed.

[0011] The method according to the invention comprises several steps: First, the first and second sheet metal forming parts are provided. The first and second sheet metal forming parts can, for example, be blanks cut from steel strip and subsequently formed. The provided first and / or second sheet metal forming parts are each obtained by hot forming, preferably hot forming as press hardening, of a precursor product, wherein the precursor product is a flat steel product provided with a corrosion protection coating by hot-dip coating. The provided first and / or second sheet metal forming parts are each, preferably by hot-dip coating, provided with a corrosion protection coating and subsequently formed by hot forming.Preferably, the hot forming process includes press hardening or is carried out as a press hardening process. This means that the sheet steel parts are obtained by first applying a corrosion-resistant coating to the flat steel product to create a semi-finished product, and then hot forming the semi-finished product into the desired shape to obtain the sheet steel part. Preferably, the hot forming process is carried out as a press hardening process, which is now an important method, particularly in the automotive industry, for producing high-strength formed parts.

[0012] A first semi-finished product, from which the first sheet metal part is produced, and a second semi-finished product, from which the second sheet metal part is produced, can be identical. However, the first semi-finished product and the second semi-finished product can also be different, in particular having different thicknesses of the corrosion protection coating.

[0013] A first section of the first steel sheet component and a second section of the second steel sheet component are placed adjacent to each other to form an overlap area. The surfaces of the first and second sections facing each other in this overlap area combine a total amount of aluminum-based corrosion protection coating that does not exceed a maximum total coating weight. This maximum total coating weight is 100 g / m² or less. The maximum total coating weight is a defined upper limit for the sum of the coating weights of the corrosion protection coatings placed between the steel sheet components. The surfaces on the outer side of the stack of sheets and the corrosion protection coatings on them are not considered when calculating the maximum total coating weight.In particular, the maximum total bearing weight is the defined upper limit for the sum of a first bearing weight of a first corrosion protection coating on the surface of the first section facing the second section, and a second bearing weight of a second corrosion protection coating on the surface of the second section facing the first section. If both sheet metal components have a corrosion protection coating, they form a stack of two corrosion protection coating layers, and the maximum total bearing weight of the stack of layers forms the upper limit that, according to the invention, is not exceeded by the sum of the bearing weights, which are composed of the bearing weights of the amount of aluminum corrosion protection coating applied between the sheets in the stack of layers.

[0014] The first steel sheet metal part and the second steel sheet metal part are welded together in the overlap area to form a weld seam.

[0015] For joining, the first and second sheet metal parts are placed next to each other. They are placed together in such a way that an overlap area is created, in which a section of the first sheet metal part and a section of the second sheet metal part overlap. The first and second sheet metal parts overlap in a section, preferably an overlap strip, with an overlap extent of at least 10 mm, preferably at least 15 mm, and particularly preferably at least 16 mm. The overlap strip is preferably a maximum of 100 mm wide, and particularly preferably a maximum of 50 mm wide. Thus, the first and second sheet metal parts overlap, for example, in a strip with a width corresponding to one of the aforementioned overlap extents.

[0016] The corrosion protection coating of the first steel sheet component and / or the corrosion protection coating of the second steel sheet component face each other in the overlap area and lie together between the two substrates. In the overlap area, a section of the corrosion protection coating of the first steel sheet component and a section of the corrosion protection coating of the second steel sheet component, oriented towards each other, form a stack of corrosion protection layers, providing corrosion protection layers with a total thickness that is the sum of the individual thicknesses.According to the invention, the stack of corrosion protection layers of the respective adjacent sections of the mutually facing corrosion protection coatings together comprises a quantity of Al-based corrosion protection coating for which the following applies: The total weight of the two mutually facing corrosion protection layers, i.e., the two layers stacked on top of each other by overlap, has a total weight that is less than or equal to the maximum total weight, where the maximum total weight is less than or equal to 100 g / m².If the maximum total coating weight is set at 100 g / m², this can be achieved, for example, by having each of the two sheet steel parts have a corrosion protection coating with a coating weight of 100 g / m² on both sides, so that the sum of the layers facing each other adds up to twice half of 100 g / m², i.e. a total of 100 g / m², and thus achieves the maximum total coating weight of 100 g / m².

[0017] In all considerations in this text, the following applies to the bearing weight of the steel sheet forming part produced by hot forming: the values ​​given for the bearing weight refer to the condition of the corrosion protection coating on the flat steel product used for the manufacture of the steel sheet forming part immediately after coating, but before heat treatment or hot forming. The bearing weight is therefore the value before hot forming.

[0018] Furthermore, in all considerations in this text, the term "bilateral bearing weight" refers to the total weight of the coatings on both surfaces of the flat steel product or sheet metal part. It is assumed that, inherent in the process of hot-dip coating for corrosion protection coatings, and well-known in the relevant field, the bilateral bearing weight is distributed equally on both sides of the substrate within the tolerances of industrial manufacturing.Particularly preferably, an equal distribution on both sides means that the deviation of the distribution of the contact weight in g / m² is less than 20%, preferably less than 10%, and most preferably less than 5%, where the deviation is defined as the difference between the contact weight A_1 on the first substrate side and the contact weight A_2 on the second substrate side, and this difference divided by the smaller of the two values, i.e., deviation = Abs(A_1-A_2) / A_1 if A_1 < A_2 and deviation = Abs(A_1-A_2) / A_2 if A_2 < A_1. "Abs" denotes the absolute value function.

[0019] In addition to the entire preceding and subsequent description, it should be noted that reference is made to the respective original coating weight, that is, the respective coating weight in the respective pre-product. The original coating weight refers to the coating weight immediately after application of the coating, but before hot forming. This nomenclature is used because the specific hot forming process, preferably press hardening, which according to the invention produces the properties of the steel sheet metal parts to be provided, results in a change to the corrosion protection coating.The corrosion protection coating, in direct contact with the substrate surface, possibly with slight interdiffusion, has an alloy layer that forms, among other things, through the diffusion of iron, in particular, into the corrosion protection coating applied during hot-dip coating. This diffusion already occurs during the hot-dip coating process in the molten bath and continues, especially during subsequent annealing steps, particularly during hot forming. With increasing distance from the substrate surface, the iron content in the corrosion protection coating decreases, and there is a point beyond which the iron content drops rapidly with increasing distance.Microscopic examination of a transverse fracture reveals this phenomenon as the apparent presence of a top layer that has formed on top of the alloy layer. The composition of this top layer is highly dependent on the composition of the melt bath and the subsequent heat treatment. In the case of hot-dip coating of a flat steel product in a bath containing 80% or more aluminum by weight, this top layer appears as an aluminum-rich layer covered with a very thin oxide passivation layer. A possible embodiment of such a layer is described in WO 2022 / 048990 A1. The formation of the various layers is highly dependent on several parameters, and the exact nature of this dependence is not yet fully understood. Therefore, the layers cannot be defined by a single, definitive specification of their properties.Therefore, the reference to the coating weights chosen in this text was to the time after coating and before hot forming, as this representation is unambiguous and corresponds to the representation usually chosen in the field.

[0020] Although this is the standard method, the coating weight can, of course, also be determined after hot forming of the steel sheet metal part or the steel component manufactured from it. For this purpose, the entire corrosion protection coating is removed using wet chemical methods and analyzed. Since the iron content of the applied coating is generally negligible (e.g., max. 4% for an aluminum-based corrosion protection coating), the iron content that diffuses in during hot forming can be subsequently subtracted. Therefore, for an aluminum-based corrosion protection coating, the coating weight is calculated by considering the sum of the aluminum and silicon contents in the wet-chemically removed coating.

[0021] Finally, according to the invention, the first sheet metal part and the second sheet metal part are welded together in the overlap area by forming a weld seam.

[0022] A method for manufacturing a steel component from at least one first steel sheet forming part and one second steel sheet forming part is proposed, the method comprising the following steps: providing the first steel sheet forming part, which is designed as a hot-formed steel flat product, and providing the second steel sheet forming part, which is also designed as a hot-formed steel flat product. At least one of the first steel sheet forming part and one of the second steel sheet forming part have a corrosion protection coating.

[0023] A first section of the first steel sheet metal part and a second section of the second steel sheet metal part are joined together to form an overlapping area. The surfaces of the first and second sections, facing each other in this overlapping area, accumulate a total amount of aluminum-based corrosion protection coating. The total coating weight of this coating is a maximum of 100 g / m² or less. Each coating weight refers to the coating weight before heat treatment, such as press hardening.

[0024] Next: Welding the first and second sheet metal components together in the overlap area to form a weld seam. This welding is carried out, for example, using laser beam welding.

[0025] Surprisingly for the developers, it has been shown that welding sheet metal components as described above results in steel components whose weld strength, assessed here by the shear tensile strength as defined in SEP 1220-3:2011-08 (i.e., the maximum tensile force at failure in the shear tensile strength test), is highly sensitive to the total amount of coating material present in the overlap area. Surprisingly and counterintuitively, it has been found that the strength can be significantly increased by precisely controlling the amount of coating material present in the overlap area.The developers found that it is crucial that the total amount of coating material contributed by the first steel sheet component and the second steel sheet component, forming a stack of corrosion protection layers due to the overlapping arrangement of the coatings facing each other, does not exceed a limit value for aluminum-based corrosion protection coatings. This limit value is referred to above as the maximum total coating weight and, according to the invention, must not exceed 100 g / m², preferably 90 g / m².

[0026] SEP 1220-3:2011-08 refers to ISO 14273 for the execution of the quasi-static tensile shear test. The title of ISO 14273 is: "Resistance welding - Destructive testing of welds - Specimen dimensions and methods for tensile shear testing of resistance spot and projection welds." There is no corresponding standard for laser welds. The test is performed in accordance with ISO 14273:2000.

[0027] The developers attribute the decisive influence of limiting the amount of aluminum-based corrosion protection material used to join the steel sheet metal components in the overlap area to the processes occurring during the joining process. When the two sheet metal components are welded, a liquid alloy forms in the weld seam area. Observations have shown that the proportion of aluminum in this liquid alloy originating from the coatings has a significant impact on the mechanical properties of the cooled weld. Based on their observations described here, the developers suspect that an excessively high aluminum content in the weld seam alloy leads to a degradation of the weld's mechanical stability. One possible explanation, according to the developers' hypothesis, could lie in aluminum's tendency to develop brittle phases.Since no closed scientific theory has yet been found for the empirically observed phenomenon, reference is initially made to the experiments carried out by the developers and the observations made on them.

[0028] In a preferred embodiment, the method for manufacturing a steel component from at least one first steel sheet forming part and a second steel sheet forming part is provided such that it comprises the following steps: providing the first steel sheet forming part with a first aluminum-based corrosion protection coating, which is designed as a hot-formed steel flat product, and the second steel sheet forming part with a second aluminum-based corrosion protection coating, which is designed as a hot-formed steel flat product. Preferably, the first steel sheet forming part is designed as a press-hardened steel flat product and / or the second steel sheet forming part is designed as a press-hardened steel flat product.

[0029] A first section of the first sheet metal part and a second section of the second sheet metal part are placed next to each other to form an overlap area formed by the first and second sheet metal parts, wherein the surfaces of the first and second sections facing each other in the overlap area together form a stack consisting of a section of the first corrosion protection coating and a section of the second corrosion protection coating. The stack as a whole has a total coating weight of at most 100 g / mm², preferably at most 50 g / mm², more preferably at most 40, 39, 38, 37, 36, or, most preferably, 35 g / mm², in that order of increasing preference; welding the first and second sheet metal parts together in the overlap area to form a weld seam.Welding is carried out, for example, using laser beam welding.

[0030] The bearing weight refers to the bearing weight before hot forming. If both sheet steel parts have a double-sided aluminum-based corrosion protection coating with, within the usual range of deviation, an equal distribution of the coating on both sides of the substrate, then the invention can also be defined as follows: A method for manufacturing a steel component from at least one first sheet steel part and a second sheet steel part is provided, comprising the following steps: Providing the first sheet steel component with a first Al-based corrosion protection coating with a first double-sided support weight, which is designed as a hot-formed, in particular press-hardened, steel flat product, and the second sheet steel component with a second Al-based corrosion protection coating with a second double-sided support weight, which is designed as a hot-formed, in particular press-hardened, steel flat product;A first section of the first steel sheet metal part and a second section of the second steel sheet metal part are placed next to each other to form an overlap area consisting of the first steel sheet metal part and the second steel sheet metal part, wherein the surfaces of the first section and the second section facing each other in the overlap area together form a stack consisting of a section of the first corrosion protection coating and a section of the second corrosion protection coating.

[0031] The sum of the first and second double-sided printing weights is a maximum of 200 g / mm², preferably a maximum of 100 g / mm², more preferably a maximum of 80 g / mm², or a maximum of 78 g / mm², or a maximum of 76 g / mm², or a maximum of 74 g / mm², or a maximum of 72 g / mm², or, most preferably, a maximum of 70 g / mm², in that order of increasing preference. It is particularly preferred if, as an additional condition, the sum of the first and second double-sided printing weights is at least 35 g / mm², or at least 40 g / mm², or at least 50 g / mm², or at least 55 g / mm², or at least 60 g / mm², in that order of increasing preference.

[0032] The sum of the first and second double-sided printing weights is particularly preferably between 35 g / mm² and 200 g / mm², more preferably between 40 g / mm² and 100 g / mm², even more preferably between 50 g / mm² and 80 g / mm², or, in order of preference: between 55 g / mm² and 78 g / mm², or from 60 g / mm² to 76 g / mm², or from 60 g / mm² to 74 g / mm², or from 60 g / mm² to 72 g / m². It is particularly preferred if each of the first and second double-sided printing weights has at least 20 g / mm², and even more preferably at least 25 g / mm².

[0033] As a final process step, the first steel sheet metal part and the second steel sheet metal part are welded together in the overlap area to form a weld seam.

[0034] According to an advantageous embodiment, the flat steel product used as a steel substrate consists of a steel containing 0.50 to 3.0 wt.% Mn, 0.02 to 1.2 wt.% Si, preferably 0.10 to 0.50 wt.% Si, and 0.01 to 1.00 wt.% Cr. In particular, the microstructure of the steel can be transformed into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the sheet metal part is therefore preferably a martensitic or at least partially martensitic microstructure, since this exhibits particularly high hardness.

[0035] The steel substrate is particularly preferably made of a steel which, in addition to iron and unavoidable impurities (in wt.%), meets the following alloy specification: C: 0.04 - 0.45 wt.%, Yes: 0.02 - 1.2 wt.%, preferably 0.10 - 0.50 wt.%, Mn: 0.50 - 3.0 wt.%, preferably 0.5 - 2.6 wt.%, Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.% S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.010 wt.% Ca: ≤ 0.005 wt.% and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V, W" in the following concentrations: Cr: 0.01 - 1.0 wt.%, preferably 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.% Mon: ≤ 0.5 wt.%, Ni: ≤ 0.5 wt.%, Cu: ≤ 0.2 wt.%, Note: 0.01 - 0.2 wt.% Ti: 0.008 - 0.10 wt.% V: ≤ 0.3 wt.%, W: 0.001 - 1.00 wt.%.

[0036] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided in steel production. Occasionally, Ca is also deliberately added to bind sulfur. In such cases, the Ca content is at least 0.001 wt.%. The maximum Ca content, even in this case, is 0.005 wt.%.

[0037] In addition to these elements, other elements may also be present as impurities in the steel. These other elements are collectively referred to as "unavoidable impurities." Preferably, the total content of unavoidable impurities is a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also occur as unavoidable impurities in the steel substrate at levels below the respective lower limit. In this case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. Preferably, the individual upper limits for the respective impurities of these elements are as follows: Cr: ≤ 0.050 wt.%, B: ≤ 0.0005 wt.% Note: ≤ 0.005 wt.% Ti: ≤ 0.005 wt.%.

[0038] These preferred upper limits should be considered as alternatives or in combination. Preferred steel variants therefore fulfill one or more of these four conditions.

[0039] In a preferred embodiment, the carbon content of the steel is a maximum of 0.37 wt.% and / or a minimum of 0.06 wt.%. In particularly preferred embodiments, the carbon content is in the range of 0.06–0.09 wt.%, or in the range of 0.11–0.25 wt.%, or in the range of 0.32–0.37 wt.%.

[0040] In the steel flat products according to the invention, carbon has a retarding effect on the formation of ferrite and bainite. Simultaneously, retained austenite is stabilized and the Ac3 temperature is reduced. A carbon content of at least 0.06 wt.% is advantageous to ensure the hardenability of the steel flat product and the tensile strength of the press-hardened product of at least 1000 MPa. If a higher strength level is desired, carbon contents > 0.12 wt.% are preferred. If the carbon content is further increased to values ​​of at least 0.19 wt.%, the hardenability can also be improved, so that the steel flat product exhibits a very good combination of hardenability and strength. However, carbon contents greater than 0.45 wt.% have a detrimental effect on the mechanical properties of the steel flat product, since carbon contents greater than 0.45 wt.% promote the formation of brittle martensite during press hardening.Furthermore, high carbon contents can negatively affect weldability. To improve weldability, the carbon content can preferably be adjusted to values ​​below 0.40 wt.%, particularly 0.3 wt.%. Especially with carbon contents below 0.25 wt.%, weldability can be significantly improved again, and a good ratio of force absorption to maximum bending angle can also be achieved in the bending test according to VDA 238-100 in the press-hardened condition.

[0041] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or a minimum of 0.06 wt.%.

[0042] Silicon is used to further increase the hardenability of the steel flat product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silicon manganese as an alloying agent, which has a positive effect on production costs. A hardening effect occurs even at a silicon content of 0.06 wt.%. A significant increase in strength occurs at a silicon content of at least 0.15 wt.%. Silicon contents above 0.5 wt.% have a detrimental effect on coating behavior, especially with aluminum-based coatings. Silicon contents of up to 0.4 wt.% are preferred to improve the surface quality of the coated steel flat product.

[0043] In a preferred embodiment, the manganese content of the steel is a maximum of 2.6 wt.%, more preferably a maximum of 2.4 wt.%, and / or a minimum of 0.75 wt.%. In particularly preferred embodiments, the manganese content is in the range of 0.75–0.85 wt.% or in the range of 1.0–1.6 wt.%.

[0044] Manganese acts as a hardening element by significantly delaying ferrite and bainite formation. At manganese contents below 0.5 wt.%, ferrite and bainite form during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents above 0.75 wt.%, particularly 0.9 wt.%, are preferred when a martensitic microstructure is desired, especially in areas subject to significant deformation. Manganese contents above 2.6 wt.% adversely affect processing properties. In particular, weldability is severely restricted, which is why the Mn content of preferred flat steel products is limited to a maximum of 2.6 wt.%, and particularly to a maximum of 1.6 wt.%. Manganese contents below 1.6 wt.% are also preferred for economic reasons.

[0045] In a preferred variant, the aluminum content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, and preferably a maximum of 0.25 wt.%. Alternatively or additionally, the aluminum content is preferably at least 0.02 wt.%.

[0046] Aluminum is used as a deoxidizing agent to bind oxygen. It also inhibits cementite formation. At least 0.02 wt% Al in the steel is required for reliable oxygen binding. However, since the Ac3 temperature also increases significantly with rising Al alloy content, the Al content is preferably limited to 0.25 wt%. Above 0.25 wt%, Al significantly hinders the conversion to austenite before press hardening, making austenitization inefficient in terms of time and energy. For typical furnace temperatures between 850 and 950°C in hot forming, an Al content of no more than 0.1 wt% is preferably maintained to ensure complete austenitization of the steel.

[0047] Furthermore, it has been shown that limiting the sum of the silicon and aluminum contents can be beneficial. In a preferred embodiment, the sum of the Si and Al contents (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0048] The elements P, S, and N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Independently of this, the S content is preferably a maximum of 0.012 wt.%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0049] Phosphorus (P) and sulfur (S) are elements that are introduced into steel as impurities from iron ore and cannot be completely removed in large-scale steelmaking processes. The P and S content should be kept as low as possible, since mechanical properties such as impact strength deteriorate with increasing P and S content, respectively. Furthermore, embrittlement of the martensite increases with P contents above 0.1 wt.%, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.05 wt.%, preferably a maximum of 0.03 wt.%. The S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably a maximum of 0.012 wt.%.

[0050] Nitrogen (N) is present in small quantities in steel due to the steelmaking process. The N content should be kept as low as possible and should be less than 0.02 wt.%. Nitrogen is particularly detrimental in alloys containing boron, as it inhibits the conversion-retarding effect of boron by forming boron nitrides. Therefore, in this case, the nitrogen content should preferably be a maximum of 0.010 wt.%, and more preferably a maximum of 0.009 wt.%.

[0051] The tin content is a maximum of 0.03 wt.%, preferably a maximum of 0.02 wt.%. The arsenic content is a maximum of 0.010 wt.%, particularly a maximum of 0.005 wt.%.

[0052] Optionally, the steel also contains chromium in a content of 0.01–1.0 wt.%, preferably 0.08–1.0 wt.%. Preferably, the chromium content is a maximum of 0.75 wt.%, particularly a maximum of 0.5 wt.%.

[0053] Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.01–1.0 wt.%, preferably 0.08–1.0 wt.%. Chromium influences the hardenability of the flat steel product by slowing down the diffusive transformation during press hardening. In flat steel products according to the invention, chromium has a beneficial effect on hardenability from a content of 0.08 wt.%, whereby a Cr content > 0.1 wt.% is preferred for reliable process control, especially to prevent bainite formation. If the steel contains more than 1.0 wt.% chromium, the coating properties deteriorate. To obtain good surface quality, the Cr content can preferably be limited to a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0054] In the case of optional chromium alloying, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0055] Preferably, the steel also optionally contains boron in a content of 0.001–0.010 wt.%. In particular, the boron content is a maximum of 0.005 wt.%, preferably a maximum of 0.004 wt.%.

[0056] Boron can be optionally added to improve the hardenability of the steel flat product. Boron atoms deposited on the austenite grain boundaries, or boron precipitates, reduce the grain boundary energy, thereby suppressing ferrite nucleation during press hardening. A significant effect on hardenability occurs at boron contents of at least 0.001 wt.%. However, at boron contents above 0.010 wt.%, boron carbides, boron nitrides, or boron nitrocarbides are formed more frequently. These, in turn, represent preferred nucleation sites for ferrite and reduce the hardening effect. For this reason, the boron content is limited to a maximum of 0.010 wt.%.

[0057] When boron is added, titanium is preferably also added to bind nitrogen. In this case, the titanium content should preferably be at least 3.42 times the nitrogen content in wt.%.

[0058] Optionally, the steel may contain molybdenum with a maximum content of 0.5 wt.%, in particular a maximum of 0.1 wt.%.

[0059] Molybdenum (Mo) can be added optionally to improve process stability, as it significantly slows down ferrite formation. From contents as low as 0.002 wt%, molybdenum-carbon clusters, up to and including ultrafine molybdenum carbides, form dynamically at the grain boundaries, significantly slowing down grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces the grain boundary energy, which decreases the nucleation rate of ferrite. Due to the high costs associated with a molybdenum alloy, the Mo content should be no more than 0.5 wt%, preferably no more than 0.3 wt%.

[0060] Optionally, the steel may also contain copper with a content of a maximum of 0.2 wt.%, preferably a maximum of 0.15 wt.%.

[0061] Copper (Cu) can be added as an optional alloy to increase hardenability at additions of at least 0.01 wt%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges.

[0062] Furthermore, the steel may optionally contain nickel with a content of a maximum of 0.5 wt.%, preferably a maximum of 0.15 wt.%.

[0063] Nickel (Ni) stabilizes the austenitic phase and can be optionally added to lower the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative effect of copper on hot rollability, at least 0.01 wt% nickel can be added to the steel. For economic reasons, the nickel content should be limited to a maximum of 0.5 wt%, preferably a maximum of 0.4 wt%.

[0064] Furthermore, the steel may optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.01 wt.%, in particular at least 0.02 wt.% and at most 0.2 wt.%, preferably at most 0.08 wt.%, and more preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.10 wt.%, in particular at most 0.08 wt.%, and more preferably at most 0.04 wt.%. The optional V content is at most 0.3 wt.%, preferably at most 0.2 wt.%, in particular at most 0.1 wt.%, and more preferably at most 0.05 wt.%.

[0065] Niobium (Nb) can be added as an optional alloying element to contribute to grain refinement from a content of 0.01 wt.%, and particularly from 0.010 wt.%. However, niobium impairs the recrystallizability of the steel. At an Nb content above 0.1 wt.%, the steel can no longer be recrystallized in conventional continuous furnaces before hot-dip coating.

[0066] Titanium (Ti) is a microalloying element that can be optionally added to contribute to grain refinement. Titanium also forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept relatively low. Titanium binds nitrogen, thus enabling boron to exert its strong ferrilytic effect. Sufficient nitrogen binding requires at least 3.42 times the nitrogen content, with at least 0.008 wt% Ti being added to ensure adequate availability. Preferably, the titanium content is at least 0.010 wt%, more preferably at least 0.015 wt%. From 0.1 wt% Ti onwards, cold rolling and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided.

[0067] Vanadium (V) is a highly carbon-affine element. When vanadium is free, that is, in an unbound or dissolved state, it can bind supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its diffusion rate. Crucially, the presence of V in a dissolved state is essential. Surprisingly, very low V contents have proven particularly beneficial for aging resistance. At higher V contents, larger vanadium carbides can precipitate even at elevated temperatures, and these carbides then do not dissolve at temperatures of 800–900°C, which are typical for continuous annealing in hot-dip coating systems. Even minute amounts of vanadium, as low as 0.001 wt%, can hinder the attachment of free carbon to dislocations. Above a V content of 0.2 wt%, vanadium no longer improves aging resistance.The anti-aging effect of vanadium is particularly pronounced at vanadium contents up to 0.009 wt.%, with a maximum effect occurring at a preferred vanadium content of 0.002 wt.%. At vanadium contents greater than 0.009 wt.%, vanadium carbides are increasingly formed. Vanadium carbides cannot be dissolved at temperatures of 860°C, which are typical, for example, for annealing temperatures in a hot-dip coating system, when the vanadium content in the steel exceeds 0.009 wt.%. The vanadium content of the steel in a flat steel product according to the invention is limited to a maximum of 0.1 wt.% for cost reasons. Furthermore, higher vanadium contents do not result in a significant improvement in the mechanical properties.

[0068] In the case of optional alloying with several of the elements Nb, Ti, and V, the sum of the Nb, Ti, and V contents is preferably limited. The sum is a maximum of 0.1 wt.%, particularly a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0069] Tungsten (W) can optionally be added in amounts of 0.001–1.0 wt.% to slow down ferrite formation. A positive effect on hardenability is already observed with W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0070] The above explanations regarding preferred steel substrates naturally also apply to the steel substrate or steel of the flat steel product described below, as well as to the steel substrates or steels in the described manufacturing processes.

[0071] According to an advantageous further development, the provision of the first sheet metal part or the second sheet metal part, preferably both sheet metal parts, comprises: A) Providing the uncoated steel flat product, B) Hot-dip coating the steel flat product to apply a corrosion protection coating, C) Hot forming, preferably additionally press hardening, of the steel flat product to obtain the first steel sheet forming part or the second steel sheet forming part.

[0072] The process steps A) to C) are carried out in alphabetical order. After step B) and before step C), the coated steel flat product is available as a semi-finished product.

[0073] The provision of the steel flat product may in particular include the provision of a steel strip that has been manufactured using the following steps: a) Providing a slab or thin slab made of steel having, for example, one of the alloy compositions mentioned above; b) Heating the slab or thin slab through at a temperature (T1) of 1000-1400 degrees Celsius; c) Optionally pre-rolling the heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000-1200 degrees Celsius; d) Hot-rolling to a hot-rolled steel flat product, wherein the final rolling temperature (T3) is 750-1000 degrees Celsius; e) Optionally coiling the hot-rolled steel flat product, wherein the coiling temperature (T4) is not more than 700 degrees Celsius; f) Descaling the hot-rolled steel flat product; g) optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%;

[0074] Hot-dip coating is preferably carried out in a melt bath.

[0075] The steel flat product is passed through a liquid melt consisting of 0.1–15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2–4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optional other components, the total content of which is limited to a maximum of 2.0 wt.%, and, besides unavoidable impurities, aluminum. Preferably, the optional content of alkali or alkaline earth metals is at least 0.1 wt.%.

[0076] In a preferred variant, the Si content of the melt is 0.5 - 3.5 wt.% or 7 - 12 wt.%, in particular 8-10 wt.%.

[0077] In a preferred embodiment, the optional content of alkali or alkaline earth metals in the melt comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, more preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can, in particular, comprise at least 0.0015 wt.% Ca, more preferably at least 0.01 wt.% Ca. More preferably, the optional content of alkali or alkaline earth metals in the melt consists of 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, more preferably 0.1–0.5 wt.% Mg, and optionally at least 0.0015 wt.% Ca, more preferably at least 0.01 wt.% Ca.

[0078] The content of optional additional components is preferably limited to 1.5 wt.%, in particular to 1.0 wt.%, preferably to 0.5 wt.%, in particular to 0.3 wt.%, preferably to 0.10 wt.%, and in particular to 0.05 wt.%. In particular, the optional additional components correspond to unavoidable impurities and are present only in technically unavoidable amounts. This applies to all melt compositions described above.

[0079] A preferred melt composition consists of: 9 to 11 Si; 2 to 3.5 Fe; optionally 0.1-1.0 alkali and / or alkaline earth metals; remainder Al and unavoidable impurities, all values ​​in wt. percent, abbreviated: wt.%.

[0080] In hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer when it solidifies.

[0081] The result of step B) is then a steel flat product that has the Al-based corrosion protection coating and can be used as a semi-finished product for the production of the first steel sheet forming part and / or the second steel sheet forming part.

[0082] The hot-dip coating measure summarized above under step B) includes, according to an advantageous further development, in particular the following sub-steps: B1) Preheating the steel flat product to a preheating temperature (VWT) of 650°C to 900°C; B2) Cooling the steel flat product to a bath immersion temperature (BET) of 650°C to 800°C, preferably 680°C to 720°C; B3) Immersion of the steel flat product, having a BET, into the molten bath having a melt temperature of 660°C to 800°C, preferably 670°C to 710°C; B4) Blowing off the steel flat product after exiting the molten bath to establish a double-sided bearing weight between 30 and 200 g / m², preferably between 30 and 160 g / m², particularly preferably between 35 and 100 g / m²; B5) Cooling the steel flat product;Preferably, a first cooling period in the temperature range between 600 degrees Celsius and 450 degrees Celsius is more than 10 s, in particular more than 14 s, and a second cooling period in the temperature range between 400 degrees Celsius and 300 degrees Celsius is more than 8 s, in particular more than 12 s; B6) optional: dressing. ;

[0083] Upon exiting the molten pool, the flat steel product is preferably blown off using a gas stream. This controls the thickness of the aluminum-based corrosion protection coating, ensuring in particular that both sides have the same layer thickness and that the combined coating weight of both sides is preferably between 30 and 100 g / m², and particularly preferably between 30 and 90 g / m².

[0084] The double-sided printing weight of the intermediate product is particularly preferred to be between 30 and 65 g / m². Even more preferred, in order of increasing preference, is a weight between 30 and 45 g / m², or between 30 and 39 g / m², or between 30 and 38 g / m², or between 30 and 35 g / m².

[0085] The flat steel product, preferably formed as sections of steel strip or as steel sheets and provided with a corrosion protection coating, is subjected to hot forming as a semi-finished product, preferably also to press hardening. The hot forming or the hot forming including press hardening preferably comprises the following steps: C1) Heating the steel flat product at a temperature between 840°C and 1000°C, preferably between 880°C and 950°C. The heating time is preferably between 240 and 900 seconds. A roller hearth furnace or a chamber furnace can be used for heating, for example. C2) Transferring the heated steel flat product to a hot forming tool. The transfer time is preferably between 4 and 15 seconds. C3) Hot forming, preferably also press hardening, of the coated steel flat product in the hot forming tool. The tool is closed, for example, at a closing speed of 20 to 500 mm / s. To achieve press hardening, which is preferred, the tool is water-cooled, preferably at a temperature between 10 and 80°C.This results in a cooling rate of the steel flat product between 30 and 300 K / s, causing the hot forming process to transition into a microstructural transformation in the formed product. This leads to martensite formation and consequently an increase in the strength of the substrate material, resulting in press hardening. C4) Removal of the steel flat product, which preferably has a temperature between 50 and 250 degrees Celsius.

[0086] Hot forming and press hardening also alter the corrosion protection coating applied to the surface of the steel flat product, for example, an aluminum-based corrosion protection coating; it now exists as an alloy layer resting on the substrate with an aluminum-rich top layer, optionally capped with a thin oxide passivation layer. This layer initially exhibits a comparatively high iron content near the substrate, for example, between 55 wt.% and 90 wt.%, which decreases rapidly with increasing distance from the substrate surface. As a result, the alloy layer is covered with an aluminum-rich top layer, which, at high aluminum content levels, is also covered with a very thin oxide passivation layer.

[0087] According to preferred design variants, the maximum total surface weight is 70 g / m².

[0088] According to a preferred training method, the maximum total surface weight is 50 g / m².

[0089] According to a preferred training method, the maximum total surface weight is 40 g / m².

[0090] According to a preferred training method, the maximum total surface weight is 39 g / m².

[0091] According to a preferred training method, the maximum total surface weight is 38 g / m².

[0092] According to a preferred training method, the maximum total surface weight is 37 g / m².

[0093] According to a preferred training method, the maximum total surface weight is 36 g / m².

[0094] According to a preferred training method, the maximum total surface weight is 35 g / m².

[0095] In a preferred embodiment, the surfaces of the first and second sections, facing each other in the overlap area, are arranged to accumulate a total amount of aluminum-based corrosion protection coating that does not fall below a lower limit. This lower limit is referred to as the minimum total coating weight.

[0096] This means that the sections of corrosion protection coatings facing each other that are laid next to each other in the overlap area, in total, contain a quantity of Al-based corrosion protection coating applied to and further treated on the pre-product, which is greater than the minimum total coating weight.

[0097] For example, it may be provided that the minimum total printing weight of the layer stack is 30.0 g / m², 27.5 g / m², 25 g / m², 20 g / m², or 17.5 g / m². This is particularly preferred in a case where the maximum total printing weight on one side is 100 g / m², 90 g / m², or 70 g / m².

[0098] Alternatively, for example, the minimum total single-sided printing weight of the layer stack can be specified as 20 g / m² or 17.5 g / m². This is particularly preferred in a case where the maximum total printing weight is 50 g / m².

[0099] The total amount of aluminum-based corrosion protection coating present in the stack of corrosion protection layers is also limited at the lower end, since observations have shown that the observed surprising effect is no longer present at excessively low amounts of aluminum-based corrosion protection coating. This is hypothetically attributed to the fact that below the minimum total coating weight, the steel sheet exhibits similar welding properties to an uncoated steel sheet, or at least approximates them. Specifying these lower limits also takes into account the fact that, when producing the coatings using hot-dip coating, certain minimum thicknesses of the resulting layers cannot be undercut for purely technical reasons.

[0100] The stated values ​​for the maximum total bearing weight of the layer stack limit the total thickness of the corrosion protection coatings of the first steel sheet component and the second steel sheet component, which form a corrosion protection layer stack in the overlap area, to the specified upper limits of the bearing weight considered as a one-sided bearing weight of 100 g / m², preferably 90 g / m², particularly preferably 70 g / m², particularly preferably 50 g / m², and even more preferably, in order of increasing preference, 40 g / m², 39 g / m², 38 g / m², 37 g / m², 36 g / m², or 35 g / m².

[0101] Preferably, both the corrosion protection coating of the first steel sheet forming part and the corrosion protection coating of the second steel sheet forming part are Al-based corrosion protection coatings, particularly preferably applied by means of a hot-dip process in the same melt composition to the still uncoated steel flat product before hot forming, in order to obtain a pre-product for the subsequent hot forming, preferably also press hardening.

[0102] Particularly preferably, the first and second sheet metal components are identical. This means, for example, that both the first and second sheet metal components consist of hot-formed, preferably additionally press-hardened, flat steel products, which, prior to hot forming, have an aluminum-based corrosion protection coating with the same coating weight A_1 on both sides, with A_1 between 30 and 100 g / m², preferably between 30 and 90 g / m². Each side of the first and second sheet metal components thus has a coating that corresponds to a coating weight of half A_1, i.e., (A_1) / 2, on one side of the precursor product.The corrosion protection layer stack thus has two sections of aluminum-based corrosion protection coating which, in the area of ​​the overlap, combine a quantity of aluminum-based corrosion protection coating facing each other, corresponding to a one-sided original coating weight of aluminum-based corrosion protection coating of twice (A_1) / 2, i.e. a total of A_1.

[0103] As mentioned, according to the invention it is provided that the first layer section and the second layer section together bring together a quantity of Al-based corrosion protection coating facing each other, the sum of which does not exceed a maximum total coating weight of the layer stack, wherein the maximum total coating weight is 100 g / m² or less, preferably 90 g / m² or less.

[0104] For example, the first and second sheet metal parts can be identical, namely, in the pre-product stage, i.e., before hot forming, they are each provided with an aluminum-based corrosion protection coating with a coating weight of A_1 = 150 g / m² on both sides. Forming a stack of corrosion protection layers from a section of the first sheet metal part and a section of the second sheet metal part results in the adjacent joining of aluminum-based corrosion protection coatings with a total amount of coating material of 2 x (A_1) / 2 = 2 x 150 / 2 g / m² = 150 g / m², where this value, as explained above, refers to the coating weight of the corrosion protection layer on the pre-product.This value of 150 g / m² is greater than a maximum total coating weight of the layer stack of 100 g / m², or preferably 90 g / m², so that the joining of two hot-formed, preferably additionally press-hardened, sheet steel parts, which each had an Al-based corrosion protection coating with a coating weight on both sides of A_1 = 150 g / m² before hot forming, is not a process according to the invention.

[0105] For example, the first and second sheet metal parts can be identical, namely, both coated with an aluminum-based corrosion protection coating prior to hot forming, with a coating weight of A_1 = 80 g / m² on both sides. Forming a corrosion protection layer stack from a section of the first sheet metal part and a section of the second sheet metal part results in the aluminum-based corrosion protection coating being applied to each other with a total amount of coating material equal to 2 x (A_1) / 2 = 2 x 80 / 2 g / m² = 80 g / m², where, as explained above, this value refers to the coating weight of the corrosion protection layer on the pre-product.This value of 80 g / m² is less than a maximum total coating weight of the layer stack of 100 g / m², or preferably 90 g / m², so that the joining of two hot-formed, preferably additionally press-hardened, sheet steel parts, which each had an Al-based corrosion protection coating with a coating weight of A_1 = 80 g / m² on both sides before hot forming, is a process according to the invention.

[0106] For example, the first and second sheet metal parts can be identical, namely, both coated with an aluminum-based corrosion protection coating prior to hot forming, with a coating weight of A_1 = 40 g / m² on both sides. Forming a corrosion protection layer stack from a section of the first sheet metal part and a section of the second sheet metal part results in the aluminum-based corrosion protection coating being applied to each other with a total amount of coating material equal to 2 x (A_1) / 2 = 2 x 40 / 2 g / m² = 40 g / m², where, as explained above, this value refers to the coating weight of the corrosion protection layer on the pre-product.This value of 40 g / m² is less than a one-sided maximum total bearing weight of the layer stack of 100 g / m², or preferably 90 g / m², such that the joining of two hot-formed, preferably additionally press-hardened, sheet steel parts, each of which had an aluminum-based corrosion protection coating with a two-sided bearing weight of A₁ = 40 g / m² prior to hot forming, is a process according to the invention. This process also fulfills the additional condition of the further developed process according to which the one-sided maximum total bearing weight of the layer stack is 70 g / m². This process also fulfills the additional condition of the further developed process according to which the one-sided maximum total bearing weight of the layer stack is 50 g / m².The additional optional condition that the minimum total surface weight of the corrosion protection coating in the layer stack is at least 17.5 g / m²< , alternatively 20 g / m²< , is also met.

[0107] For example, the first and second steel sheet metal parts can be identical, namely, both coated with an aluminum-based corrosion protection coating prior to hot forming, with a coating weight of A_1 = 35 g / m² on both sides. Forming a corrosion protection layer stack from a section of the first steel sheet metal part and a section of the second steel sheet metal part results in the aluminum-based corrosion protection coating being applied to each other with a total amount of coating material equal to 2 x (A_1) / 2 = 2 x 35 / 2 g / m² = 35 g / m², where, as explained above, this value refers to the coating weight of the corrosion protection layer on the pre-product.This value of 35 g / m² is less than a one-sided maximum total coating weight of the layer stack of 100 g / m², or preferably 90 g / m², or even lower values ​​such as 39 g / m², so that the joining of two hot-formed, preferably additionally press-hardened, sheet steel parts, each of which had an aluminum-based corrosion protection coating with a coating weight of A₁ = 35 g / m² on both sides prior to hot forming, is a process according to the invention. The additional optional condition that the minimum total coating weight of the corrosion protection coating in the layer stack is at least 17.5 g / m², or alternatively 20 g / m², is also fulfilled.

[0108] Alternatively, it is also possible, for example, that the first and second sheet metal parts are not identical. For instance, the first sheet metal part could be coated with an aluminum-based corrosion protection coating with a coating weight of A_1 = 30 g / m² on both sides before hot forming, and the second sheet metal part could be coated with an aluminum-based corrosion protection coating with a coating weight of A_2 = 35 g / m² on both sides before hot forming.Forming a stack of corrosion protection layers from a section of the first steel sheet component and a section of the second steel sheet component results in the merging of aluminum-based corrosion protection coatings with a total amount of aluminum-based corrosion protection coating of (A_1 / 2 + A_2 / 2) = 30 / 2 g / m² + 35 / 2 g / m² = 32.5 g / m², where, as explained above, this value refers to the coating weight of the corrosion protection coating on the respective precursor. This value of 32.5 g / m² is less than a maximum total coating weight of 100 g / m², or preferably 90 g / m², or even more preferably 39 g / m².This means that the joining of two hot-formed, preferably additionally press-hardened, sheet steel parts, one of which had an aluminum-based corrosion protection coating with a coating weight of A_1 = 30 g / m² on both sides prior to hot forming, and the other of which had an aluminum-based corrosion protection coating with a coating weight of A_1 = 35 g / m² on both sides, is a process according to the invention and also fulfills the conditions of further developments of the invention. The additional optional condition that the minimum total coating weight of the corrosion protection coating in the stack of layers is at least 17.5 g / m², or alternatively 20 g / m², is also fulfilled.

[0109] Alternatively, it is also possible that the first steel sheet component has the described Al corrosion protection coating and the second steel sheet component has a Zn-based corrosion protection coating.

[0110] The first sheet metal part and / or the second sheet metal part preferably have a sheet thickness of 0.8 mm to 3.5 mm, preferably of 1.0 mm to 2.8 mm, and particularly preferably of 1.2 to 2.4 mm.

[0111] Welding is preferably carried out by laser beam welding. Using laser beam welding, a weld seam is formed in the overlap area, a so-called overlap seam. The overlap area is preferably between 15 and 20 mm wide, with a width of 16 mm being preferred. In particular, laser beam welding as defined in DIN EN ISO 4063:2023-07 can be used.

[0112] Preferably, the weld seam produced by laser beam welding is a continuous weld seam and has a length between 5 mm and 5 m, preferably from 10 mm to 1 m, and particularly preferably from 20 mm to 50 cm.

[0113] A significant advantage of laser beam welding when welding overlapping sheets is the ability to create deeper and narrower weld seams. This minimizes heat input, reducing the risk of deformation and material damage. Furthermore, the precise energy input allows for accurate weld seam positioning, which is crucial for applications with high quality requirements. Surprisingly, with the steel sheet components in question, which feature corrosion protection coatings, it has been shown that, by appropriately limiting the thickness of the aluminum-based corrosion protection coating, the often-observed adverse reductions in weld strength can be largely avoided during laser beam welding. Observation indicates that, when the coating thickness is limited, the type of coating used in laser beam welding promotes the production of weld seams with comparatively high strength.These advantages are accompanied by process-related advantages of laser beam welding, namely in particular a high process speed, which promotes good economic efficiency through its use, for example in the automotive industry.

[0114] In laser beam welding, one or more of the following laser parameters are particularly preferred: Focus point diameter 0.2 mm-1.0 mm; laser power 2-10 kW, preferably 3-8 kW, particularly preferably 4-6 kW; wavelength range: 800 to 1100 nm; feed rate 1-10 m / min, preferably 2-5 m / min.

[0115] A fiber-guided solid-state laser is preferably used for laser beam welding.

[0116] It has been found that laser beam welding of sheet steel components with aluminum-based corrosion protection coatings, using overlap welds, can produce steel components with surprisingly good strength. Compared to resistance spot welding, which is still widely used in industrial manufacturing, laser beam welding has proven to be very well suited for the application of laser beam welding of sheet steel components with aluminum-based corrosion protection coatings. Manufacturing steel components using laser beam welding thus results in a combination of surprisingly good strength, advantageous process properties, and the resulting beneficial effects on the steel components.Laser beam welding is a force-free process, whereas WPS welding requires a force of, for example, 1 N to 10 N to be applied to the components being welded. Steel components produced by laser beam welding therefore typically exhibit lower induced mechanical stresses compared to WPS welds.

[0117] The weld seam produced by laser beam welding, which is designed as a line connection, improves the mechanical strength of the weld seam produced compared to the point connections produced by WPS.

[0118] Furthermore, laser beam welding is independent of the ohmic resistance of the sheet steel components being welded, resulting in greater flexibility and a higher degree of reproducibility. This is particularly true when welding sheet steel components with corrosion protection coatings of varying thicknesses.

[0119] Further advantages of using laser beam welding compared to using WPS are the smaller heat-affected zone, the higher power density, 10^6-10^8 W / cm 2< compared to 10^3-10^5 W / cm 2< for WPS, and the absence of flag formation due to material splashing out of the heat-affected zone and subsequently solidifying, unlike with WPS in laser beam welding.

[0120] Another aspect of the invention relates to a steel component that can be obtained using a method according to the invention and one of its further developments. The steel component is exclusively defined by the interaction of the specific production of a corrosion protection coating in the sheet steel parts from which the steel component is assembled, and the selection of a specific method for joining these sheet steel parts.

[0121] When carrying out the method according to the invention or one of its further developments, it is found that in the area of ​​the weld seam the complex structural processes brought about, for example local melting and diffusion processes, lead to the formation of a structurally complex component, which, however, on the basis of empirical observation, can be regarded as an unexpected improvement in the field of steel components due to its surprisingly good strength.

[0122] In particular, with a suitable choice of the coating weight of the aluminum-based corrosion protection coating, a maximum tensile force at failure in the shear tensile strength test according to SEP 1220-3:2011-08 is achieved, which is at least 25 kN, preferably at least 30 kN, particularly preferably at least 40 kN, and even more preferably at least 42.00 kN. SEP 1220-3:2011-08 refers to ISO 14273 for the execution of the quasi-static shear tensile test. The title of ISO 14273 is: "Resistance welding - Destructive testing of welds - Specimen dimensions and methods for tensile shear testing of resistance spot and projection welds". There is no corresponding standard for laser welds. The test was carried out in accordance with ISO 14273:2000.

[0123] Experiments were conducted that yielded the described findings.

[0124] Steel sheets were produced. The sheets were obtained from a steel strip 1.5 mm thick made of an alloy with the composition listed in Table 1 (all values ​​in wt.%): Table 1 element Salary in wt.% C 0,212 Si 0,26 Mn 1,15 P 0,016 S 0,001 Al 0,029 Cr 0,18 Note 0,001 N 0,003 Ti 0,02 B 0,0028 Fe and unavoidable impurities Remainder to 100 A hot-dip coating process was carried out using a melt composition of 10.9 wt% Si, 3.0 wt% Fe, balance Al, and unavoidable impurities. The melt temperature was 670 °C. The steel strip was first preheated to a temperature of 800 °C within 55 s and held at this temperature for 10 s, then cooled to an immersion temperature of 680 °C and subsequently passed through the bath. After exiting the bath, the coating weight on both sides was adjusted. Six strips were coated using these parameters, with a nominal coating weight of 30 g / m² on each side. 2 , of 35 g / m² 2 , of 40 g / m² 2 , of 60 g / m² 2 , of 80 g / m² 2 and of 150 g / m² 2 The following actual circulation figures were achieved: Table 2 Designation Nominal double-sided printing weight in g / m² Single-sided print run weight, first page in g / m² < Single-sided printing weight, second side in g / m² < Attempt 1 150 73,0 72,2 Attempt 2 80 43,3 44,2 Attempt 3 60 32,5 33,0 Attempt 4 40 21,6 22,2 Attempt 5 35 19,4 18,6 Attempt 6 30 16,4 16,8 Sheet metal sections were cut from the steel strip, which was available as a semi-finished product after coating, and subjected to press hardening. The following parameters were used for this: C1) Heating the steel flat product, which serves as the semi-finished product, at a temperature of 920 degrees Celsius for 300 seconds in a roller hearth furnace. C2) Transferring the heated steel flat product to a hot forming tool. The transfer time was 10 seconds. C3) Hot forming of the steel flat product in the hot forming tool, resulting in press hardening. The tool was closed at a closing speed of 80 mm / s for a closing time of 15 seconds. To achieve press hardening, the tool was water-cooled to an internal wall temperature of 30 degrees Celsius. This resulted in a cooling rate of 46 K / s for the steel flat product, so that the hot forming process was transformed into a press-hardened, formed steel flat product due to a microstructure transformation in the formed product. C4) Removal of the formed steel flat product at a temperature of 75 degrees Celsius.

[0125] The extracted formed steel flat product, after completion of step C4), is a sheet steel component for the production of a steel component according to the invention.

[0126] To investigate the suitability of the manufactured sheet metal parts for welding, weld seams were created. For this purpose, two identical sheet metal blanks, formed according to steps C1) to C4) as described above, were placed overlapping against each other as a first sheet metal part and a second sheet metal part, respectively, according to tests 1, 2, 3, 4, 5, and 6. The first side of each sheet was placed against the second side, with the reference to the respective first and second sides corresponding to the specifications in Table 2. Within the 16 mm overlap area, the first layer of the corrosion protection coating of the first sheet metal part and the second layer of the corrosion protection coating of the second sheet metal part were stacked adjacent to each other and facing each other.The coating present in the raw material has, as a result of hot forming, developed into an aluminum-based corrosion protection coating, which exists as a complex alloy layer with an overlying aluminum-rich top layer. The two layers are brought together so that the surfaces of the first and second sections, facing each other in the overlap area, combine a total amount of aluminum-based corrosion protection coating, which is: for test 1: 145.2 g / m²; for test 2: 87.5 g / m²; for test 3: 65.5 g / m²; for test 4: 43.8 g / m²; for test 5: 38.0 g / m²; for test 6: 33.2 g / m².Here, reference is made to the original coating weight to indicate that the combined layer stack is formed from layers that do not correspond to the original corrosion protection coating, but rather from layers that were created from the corrosion protection coating present in the pre-product by the processes that took place during hot forming and therefore need to be defined more precisely by reference to the pre-product.

[0127] The overlapping sections of the first steel sheet metal part and the second steel sheet metal part were welded using a fiber-guided solid-state laser by means of laser beam welding to form a weld seam.

[0128] The following parameters were set: Focus point diameter 0.6 mm; laser power 4 kW; laser wavelength: 1070 nm; feed rate 3.4 m / min. The welded steel sheet components, forming a steel structural element, were tested for their shear tensile strength analogously to the requirements of SEP 1220-3:2011-08 for WPS-welded specimens. Shear tensile strength is defined as the shear force, which is the maximum force applied at failure in the shear tensile strength test according to SEP 1220-3:2011-08. SEP 1220-3:2011-08 refers to ISO 14273 for the execution of the quasi-static shear tensile test. The title of ISO 14273 is: "Resistance welding - Destructive testing of welds - Specimen dimensions and methods for tensile shear testing of resistance spot and projection welds." There is no corresponding standard for laser welds. The test is performed in accordance with ISO 14273:2000.

[0129] The maximum force transmission is measured using a tensile test. A rectangular strip is taken from the steel component such that the lap weld runs parallel to the short edge of the rectangular strip along its entire length and is located in the center of the strip. The short edge of the strip has a length of 45 mm. The exact length of the long extension direction is not relevant; a length of 200 mm has proven practical. This strip is then clamped at both ends in a testing fixture and subjected to a force in the direction of the long extension direction. The force is successively increased until the weld fractures. The value of the maximum force before fracture is referred to as the maximum force transmission. Three tensile tests are shown to illustrate the procedure. Fig. 1 (Attempt 3), Fig. 2 (Attempt 2) and Fig. 3(Experiment 1) is shown. Five curves of "force / N-extension / mm" are shown. Four of the five curves are shown in the representation of the Figs. 1, 2 and 3 The curves are shown shifted along the x-axis, which represents the extension in mm. This shift is solely for the sake of clarity and can be corrected by shifting each curve to a starting point at the coordinate intersection. The tensile tests for tests 4 to 6 were performed and evaluated analogously.

[0130] For each applied weight, five samples were taken and examined to determine the maximum tensile force at failure in the shear tensile strength test as the arithmetic mean of the maximum force at sample breakage. The results were: Table 3 Designation Shear strength: maximum tensile force at failure in the shear strength test Attempt 1 (28.29 + / - 0.86) kN Attempt 2 (33.52 + / - 0.75) kN Attempt 3 (41.60 + / - 0.52) kN Attempt 4 (42.72 + / - 0.45) kN Attempt 5 (43.04 + / - 0.51) kN Attempt 6 (42.97 + / - 0.41) kN

[0131] It can be seen that for the layer stack in test 1 with the one-sided initial coating weight of Al-based corrosion protection coating of approximately 150 g / m² (composed of two halves of approximately 150 g / m²), a tensile force at failure in the shear tensile strength test of less than 30 kN was determined. This corresponds to previously known expectations that laser welding of sheets with Al-rich corrosion protection coatings is associated with a deterioration in shear tensile strength that is unacceptable for many applications.

[0132] In contrast, for the layer stacks with a lower amount of Al-based corrosion protection coating, contrary to the developers' expectations derived from practical observations, a significantly better shear tensile strength could be obtained at the weld seam according to test 2 with the one-sided original coating weight of Al-based corrosion protection coating of approximately 80 g / m² (composed of twice half of approximately 80 g / m²) and according to test 3 of approximately 60 g / m² (composed of twice half of approximately 60 g / m²), namely with a tensile force at failure in the shear tensile strength test of over 33 kN in the case of two layers according to test 2 and a value of even over 40 kN in the case according to test 3.As can be seen in Table 3, even higher shear strength values ​​are obtained for layer stacks formed between the sheet substrates with an even lower total coating weight of 43.8 g / m² in test 4, 38.0 g / m² in test 5, or 33.2 g / m² in test 6. This leads to the counterintuitive finding that laser beam welding of sheet steel components with a comparatively low coating weight on aluminum-based corrosion protection coatings results in higher shear strengths than is the case with higher coating weights. Furthermore, the shear strength is significantly higher than typically observed with WPS (Web Processing System). The described findings thus make it possible to produce steel components with lap welds exhibiting higher shear strengths at the lap weld while keeping the coating weight of the aluminum-based corrosion protection coating comparatively low.The latter also brings with it a cost advantage.

[0133] Contrary to previous expectations of experts, it has been shown that by appropriately limiting the aluminum-rich coating material, a welded joint with very good strength can be obtained.

[0134] For reference, a steel sheet component according to test 2 was welded to a steel sheet component with a zinc coating, as specified in standard SEP 1220-3:2011-08, with a distance of 0.1 mm between the aluminum-based layer and the zinc layer, and with different zinc layer thicknesses. The use of a coated steel sheet component according to test 2 corresponded to a one-sided initial coating weight of approximately 40 g / m² of aluminum-based corrosion protection coating. Shear tensile tests yielded results of 43 kN and 45.4 kN.

Claims

1. A method for manufacturing a steel component from at least one first steel sheet component and a second steel sheet component, comprising the following steps: providing the first steel sheet component, which is designed as a hot-formed steel flat product, and the second steel sheet component, which is designed as a hot-formed steel flat product, wherein at least one of the first steel sheet component and the second steel sheet component has a corrosion protection coating; placing a first section of the first steel sheet component and a second section of the second steel sheet component next to each other to form an overlap area formed by the first steel sheet component and the second steel sheet component, wherein the surfaces of the first section and the second section facing each other in the overlap area collectively bring together an amount of Al-based corrosion protection coating;which does not exceed a maximum total printing weight, where the maximum total printing weight is 100 g / m², 2 or less; welding the first sheet steel part and the second sheet steel part in the overlap area to form a weld seam.

2. A method according to any of the preceding claims, wherein the steel flat product follows the alloy composition below: C: 0.04 - 0.45 wt.%, Yes: 0.02 - 1.2 wt.%, Mn: 0.50 - 3.0 wt.%, preferably 0.5 - 2.6 wt.%, Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.% S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.010 wt.% Ca: ≤ 0.005 wt.% and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V, W" in the following concentrations Cr: 0.01 - 1.0 wt.%, preferably 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.% Mon: ≤ 0.5 wt.%, Ni: ≤ 0.5 wt.%, Cu: ≤ 0.2 wt.%, Note: 0.01 - 0.2 wt.% Ti: 0.008 - 0.10 wt.% V: ≤ 0.3 wt.%, W: 0.001 - 1.00 wt.% Residual iron and unavoidable impurities, all values ​​given in wt. percent, abbreviated: wt.%.

3. A method according to any of the preceding claims, wherein the provision of the first steel sheet forming part or the second steel sheet forming part comprises: A) providing the uncoated steel flat product, B) hot-dip coating the steel flat product, C) hot forming, preferably additionally press hardening, of the steel flat product to obtain the first steel sheet forming part or the second steel sheet forming part.

4. The method of claim 3, wherein the hot-dip coating is carried out in a melt bath with the following melt composition: 0.1 to 15 Si, optionally 2 to 4 Fe, optionally up to 5 alkali and / or alkaline earth metals, optionally up to 15 Zn, balance Al and unavoidable impurities, all values ​​being in wt. percent, abbreviated: wt.%.

5. A method according to claim 3 or claim 4, wherein one or more of the following limitations of the melt composition apply: 1.0 to 12, preferably 8 to 10 Si; 2 to 3.5 Fe; optionally 0.1 to 1.0 alkali and / or alkaline earth metals; optionally up to 10 Zn.

6. The method of claim 3 or claim 4, carried out with the following melt composition: 8 to 10 Si, optionally 2 to 4 Fe, optionally up to 1.0 alkali and / or alkaline earth metals, balance Al and unavoidable impurities, all values ​​being in wt. percent, abbreviated as wt.%.

7. A method according to any one of claims 4 to 6, wherein the hot-dip coating comprises the following steps: B1) Preheating the steel flat product to a preheating temperature (VWT) of 650°C to 900°C; B2) Cooling the steel flat product to a bath immersion temperature (BET) of 650°C to 800°C, preferably 680°C to 720°C; B3) Immersing the steel flat product having a BET into the molten bath having a melt temperature of 660°C to 800°C, preferably 670°C to 710°C; B4) Blowing off the steel flat product after exiting the molten bath to establish a coating weight on both sides, which is between 30 and 200 g / m². 2 preferably between 30 and 160 g / m² 2 lies; B5) Cooling of the steel flat product; B6) optional: Dressing.

8. Method according to claim 7, wherein the double-sided support weight is between 30 and 90 g / m² 2lies, and / or wherein the double-sided contact weight is between 30 and 65 g / m² 2 lies, and / or where the double-sided contact weight is between 30 and 45 g / m² 2 lies, and / or where the double-sided contact weight is between 30 and 39 g / m² 2 lies, and / or where the double-sided contact weight is between 30 and 38 g / m² 2 lies, and / or where the double-sided contact weight is between 30 and 35 g / m² 2 lies.

9. A method according to any one of claims 4 to 8, wherein the hot forming, preferably carried out as press hardening or comprising press hardening, comprises the following steps: C1) Heating the steel flat product at a temperature between 840 degrees Celsius and 1000 degrees Celsius, preferably between 880 degrees Celsius and 950 degrees Celsius; C2) Transferring the heated steel flat product into a hot forming tool; C3) Hot forming, preferably press hardening, of the steel flat product in the hot forming tool; C4) Removing the formed steel flat product.

10. Method according to one of the preceding claims, wherein the maximum total printing weight is 70 g / m² 2 is, or where the maximum total surface weight is 50 g / m² 2 amounts.

11. Method according to any one of claims 1 to 9, wherein the maximum total printing weight is 40 g / m² 2 is, or where the maximum total surface weight is 39 g / m² 2 is, or where the maximum total surface weight is 38 g / m²2 is, or where the maximum total surface weight is 37 g / m² 2 is, or where the maximum total surface weight is 36 g / m² 2 is, or where the maximum total surface weight is 35 g / m² 2 amounts.

12. Method according to one of the preceding claims, wherein the first sheet metal part and the second sheet metal part are of the same type, and / or wherein the first sheet metal part and / or the second sheet metal part each have a sheet thickness of 0.8 mm to 3.5 mm, preferably 1.0 mm to 2.8 mm, and particularly preferably 1.2 mm to 2.4 mm.

13. Method according to one of the preceding claims, wherein the welding is carried out as laser beam welding to produce a weld seam designed as an overlap seam in the overlap area, wherein preferably one or more of the following laser parameters are set for the laser beam welding: focus point diameter 0.2 mm-1.0 mm; laser power 2-10 kW, preferably 3-8 kW, particularly preferably 4-6 kW; feed rate 1-10 m / min, preferably 2-5 m / min, and / or wherein the laser beam welding is carried out with a fiber-guided solid-state laser.

14. Steel component obtainable by a method according to any one of claims 1 to 13.

15. Steel component according to claim 14, characterized by the fact that it has a shear strength according to SEP 1220-3:2011-08 of at least 25 kN, preferably at least 30 kN, particularly preferably at least 40 kN, and even more preferably between 41.00 kN and 50.00 kN.

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