Semi-finished product for hot forming

Abstract

The invention relates to a semi-finished product (1) for hot forming, the product being a tailor-welded blank, a tailor-rolled blank or a patch-work blank, wherein the semi-finished product (1) comprises one first section or multiple first sections having a first thickness (d1) and at least one second or multiple second sections having a second thickness (d2), the first thickness (d1) being less than the second thickness (d2), wherein the semi-finished product (1) comprises a steel sheet (2) which is coated with an aluminum-based overcoat (2.1) at least in the section having the second thickness (d2), and wherein the semi-finished product (1) has a deterministically formed surface structure at least in the section having the second thickness (d2) with the aluminum-based overcoat (2.1).

Description

[0001] Semi-finished products for hot forming

[0002] The invention relates to a semi-finished product for hot forming, which is a tailor-welded blank, a tailor-rolled blank or a patchwork blank, wherein the semi-finished product comprises a first or more first sections with a first thickness and at least one second or more second sections with a second thickness, wherein the first thickness is less than the second thickness, wherein the semi-finished product comprises a steel sheet coated with an aluminum-based coating at least in the section of the second thickness.

[0003] In the hot forming of hardenable steels, for example manganese-boron steels, hot-dip aluminizing (FAL) coatings are generally used nowadays, see also EP 2 086 755 B1 and EP 2 242 863 B1 for example. These offer effective scale protection during the annealing process prior to hot forming and thus ensure the further processability of the press-hardened component without the need for an additional process step to remove scale deposits. During the above-mentioned annealing process, the longer the annealing time, the more metallurgical transformations occur in the FAL layer. Since the typical process temperatures during hot forming, at approximately 900°C, are significantly higher than the melting temperature of the FAL layer, the coating melts. However, this effect is mitigated by the fact that iron diffusing in from the substrate significantly raises the melting point of the coating.If the annealing process is successfully completed, a multi-layer structure is usually formed, which has a good range of properties during subsequent pressing and later processing.

[0004] For adequate further processing of the press-hardened material, it is essential that the transformation process in the coating described above is fully completed. Therefore, a sufficiently long annealing period prior to press hardening is technically essential. This annealing process, in turn, requires furnace capacity and energy resources. For example, for an FAL-coated 22MnB5 sheet with a thickness of 1.5 mm, an annealing time of approximately 4 to 5 minutes at temperatures slightly above 900°C is typically required. This annealing time is comprised of two phases: First, some time passes in the furnace until the previously cold sheet material reaches the desired target temperature. The second phase then consists of holding the blank at the target temperature until the layer has undergone the transformations described above.The first phase (heating) is particularly important because as long as the material has not yet reached the target temperature, the diffusion processes involved in the transformation also proceed at a significantly slower rate.

[0005] In order to combine different functions in one component, so-called tailored products have been developed; these can be designed to suit the load required for each task. A tailor-welded blank usually consists of at least two sheets / (shaped) blanks welded together in a butt joint. The sheets preferably have different thicknesses and can additionally or alternatively be made of different materials. As an alternative to a butt joint, a tailor-welded blank can also consist of at least two sheets / (shaped) blanks welded together in an overlap joint. A patchwork blank usually consists of a base sheet which is locally reinforced on its surface by joining (welding) to at least one or more additional sheets which are smaller than the base sheet.A tailor-rolled blank is typically a single-piece sheet metal with varying thicknesses, produced by what is known in the industry as "flexible rolling." The semi-finished products in question are well-known in the industry. Components can be manufactured from these semi-finished products using either cold or hot forming.

[0006] The annealing process for hardenable steels is time- and energy-intensive. Reducing heating times opens up significant potential for cost reduction. This can be achieved, for example, through shorter cycle times and / or reduced process technology, such as shorter heating furnaces or capacity expansions, e.g., using greater sheet thicknesses with the same parameters and furnace length. As the thickness increases, longer holding times are necessary to allow the phases described above to complete. Thus, when using semi-finished products with different thicknesses, the annealing process and the associated holding time always depend on the greatest thickness of the semi-finished product, which inevitably leads to an undesirably high or increased cycle time. It would therefore be desirable to enable more efficient processing of semi-finished products with different thicknesses on existing systems with predetermined cycle times.

[0007] Different sheet thicknesses and / or coating weights of the (individual) semi-finished products can also lead to different process windows in the annealing process.

[0008] From the international publication WO 2020 / 130401 Al, a steel sheet coated with an aluminum-based coating is known, which is tempered with a deterministic texture in order to provide a component with good paintability and a low Wsa value.

[0009] The task is therefore to provide a semi-finished product with different thicknesses for hot forming, which does not require a disadvantageous increase in cycle time during the annealing process.

[0010] The problem is solved with a semi-finished product having the features of claim 1.

[0011] The invention relates to a semi-finished product for hot forming, which is a tailor-welded blank, a tailor-rolled blank or a patchwork blank, wherein the semi-finished product comprises a first or more first sections with a first thickness and at least one second or more second sections with a second thickness, wherein the first thickness is less than the second thickness, wherein the semi-finished product comprises a steel sheet coated with an aluminum-based coating at least in the section of the second thickness, wherein the semi-finished product has a deterministically formed surface structure on the aluminum-based coating at least in the section of the second thickness.

[0012] The inventors have surprisingly discovered that a defined surface structuring with a deterministic surface structure on the aluminum-based coating can positively influence the heating behavior by promoting heating during the annealing process, allowing the semi-finished product to be heated more quickly in the second thickness section. Thus, the cycle time is no longer necessarily dependent on the largest thickness for a semi-finished product with different thicknesses, so that a semi-finished product with different thicknesses can be provided for hot forming, which enables more efficient processing of semi-finished products with different thicknesses on existing systems with predetermined cycle times.

[0013] At least the portion of the second thickness of the semi-finished product, which has the aluminum-based coating with the deterministically formed surface structure, is directly exposed to or faces the furnace atmosphere during the annealing process. In other words, this means that at least one side of the semi-finished product has a portion with a second thickness with an aluminum-based coating and with a deterministically formed surface structure on the aluminum-based coating. Thus, the second thickness is designed with a deterministically formed surface structure on the aluminum-based coating either on one side or on both sides.

[0014] During the annealing process, iron diffuses from the steel (sheet) into the aluminum-based coating, preventing it from becoming molten and maintaining the positive effect of the deterministically formed surface structure on the aluminum-based coating even at high temperatures (up to just over 900°C). Therefore, the different heating rates can be maintained throughout the entire heating interval.

[0015] In their investigations, the inventors discovered the following relationship. Assuming that, during furnace heating with homogeneous heat distribution, the heat flow is homogeneously distributed across the entire semi-finished product surface after entering the furnace, and that heat transfer is essentially by thermal radiation, neglecting edge effects, and considering only the influence of texture on the heating rate, the effective heating rates, where temperatures are measured, for example, using thermocouples within a recorded period, in [K / s] or [°C / s], can be divided into three zones, each of which is accompanied by a sharp drop in the heating rate:

[0016] • Zone 1: heating rate approx. 0.02 and higher up to approx. 570°C;

[0017] • Zone 2: reduced heating rate with different gradients between 570°C and

[0018] 740°C

[0019] • Zone 3: rapidly increasing heating rate > 740°C

[0020] In the range between approximately 570°C and 600°C, the heating rate drops significantly, only to exhibit a plateau-like curve again up to a temperature of approximately 740°C. The phase diagram for Al and Si, which is usually present between 3 and 15 wt.% in the aluminum-based coating, however, shows a transition from zone 1 to zone 2 between the phases Al and Si and Liquid+a. The reduction in the heating rate can therefore be explained by the fusion enthalpy of the Al and Si system. Melting point of AS is approximately 580°C. Between 730°C and 750°C, the heating rates drop to a value of around 0.01, only to increase significantly to values ​​above 0.05 with further temperature increases. Here, A ciof the steel material used. At the transition from zone 2 to zone 3, the coating contains the ternary system of Al, Si, and Fe, the Fe content of which decreases towards the surface. This is due to the diffusion of Fe into the Al and Si layer. The enthalpy of fusion of the ternary system only contributes to the observed drop in heating rate during the transition from zone 2 to zone 3 and further along in zone 3 to a limited extent. From the transition from zone 2 to zone 3, the main influence on the heating rate lies in the phase transformation from ferritic to austenitic iron. This results in a significant drop in the heating rate. With increasing temperature and advancing time, the proportion of austenite increases, meaning that less and less heat is required for the phase transition. The heating rate therefore increases accordingly. Here, too, the surface structure has an influence on the heating rate.

[0021] If the semi-finished product is designed as a tailor-welded blank, the semi-finished product comprises at least two steel sheets of different thicknesses welded together in a butt joint or lap joint, wherein one of the steel sheets forms a section of the semi-finished product with a second thickness and is coated with an aluminum-based coating. The other steel sheet forms a section of the semi-finished product with a first thickness and can preferably also be coated with an aluminum-based coating.

[0022] If the semi-finished product is designed as a tailor-rolled blank, the semi-finished product comprises a substantially flexibly rolled steel sheet with an aluminum-based coating, wherein the semi-finished product has a first or more first sections with a first thickness and at least one second or more second sections with a second thickness.

[0023] If the semi-finished product is designed as a patchwork blank, the semi-finished product comprises a base sheet made of steel which is locally reinforced on its surface by connecting (welding) to at least one or more additional steel sheets which are smaller in size than the base sheet. The additional steel sheet(s) for the reinforcement are provided with an aluminum-based coating and, when connected to the base sheet, form the section with the second thickness of the semi-finished product. The base sheet forms the section with the first thickness of the semi-finished product (with the exception of the section(s) in which the additional steel sheets are connected to the base sheet), whereby the base sheet can preferably also consist of a steel sheet coated with an aluminum-based coating.

[0024] Steel sheet refers to a flat steel product in the form of a strip, sheet, or plate. The steel sheet has a longitudinal dimension (length), a transverse dimension (width), and a vertical dimension (thickness). The thickness of the steel sheet can be, for example, between 0.50 and 6.0 mm, in particular 0.60 and 5.0 mm, preferably 0.70 and 4.0 mm. The steel sheet can be hot- or cold-rolled.

[0025] The aluminum-based coating is conventionally applied via a hot-dip coating route.

[0026] A deterministic surface structure is introduced during skin passing on the surface of the steel sheet coated with an aluminum-based coating as a negative imprint by applying force with a positive imprint on the skin passing roll with a defined texture, whereby a valley area on the surface of the coating corresponds to a mountain area on the surface of the roll.

[0027] Deterministic surface structures are defined as recurring surface structures that have a defined shape and / or configuration, cf. EP 2 892 663 B1 and WO 2020 / 130401 A1. In particular, this also includes surfaces with a (guasi-)stochastic appearance, which, however, are applied using a deterministic texturing process and are thus composed of deterministic form elements.

[0028] The deterministically formed surface structure on the aluminum-based coating, at least in the second thickness section of the semi-finished product, is essentially directly exposed to or facing the furnace atmosphere during the annealing process. In particular, the semi-finished product formed as a patchwork blank can have a stochastically formed surface structure in the second thickness section on the shielded side of the steel sheet intended for reinforcement or a deterministically formed surface structure that is different from the side directly facing the furnace atmosphere.

[0029] Further advantageous embodiments emerge from the subclaims.

[0030] According to one embodiment, the deterministic surface structure has an average roughness Ra between 1.0 and 6.0 pm. In particular, the average roughness Ra can be at least 1.30 pm, preferably at least 1.50 pm, more preferably at least 1.70 pm. In particular, the roughness Ra can be a maximum of 5.0 pm, preferably a maximum of 4.0 pm, more preferably a maximum of 3.0 pm. According to one embodiment, the deterministic surface structure has a peak number RPc between 100 and 250 1 / cm. In particular, the peak number RPc can be at least 110 1 / cm, preferably at least 130 1 / cm. In particular, the peak number RPc can be a maximum of 220 1 / cm, preferably a maximum of 200 1 / cm, more preferably a maximum of 180 1 / cm.

[0031] The mean roughness Ra in pm and the peak count RPc in 1 / cm can be determined along a defined measuring section, see DIN EN ISO 4287.

[0032] According to one embodiment, the deterministic surface structure has a structure depth Rz between 4.0 and 25.0 pm, in particular between 5.0 and 22.0 pm, preferably between 6.0 and 18.0 pm, preferably a maximum of 15 pm. The structure depth Rz in pm is the maximum distance between the highest peak and the deepest point of the deterministic surface structure along a defined measuring section, cf. DIN EN ISO 4287.

[0033] The setting of the roughness Ra and / or the peak count RPc on the surface of the steel sheet depends on the roughness Ra and the peak count RPc of the surface of the roll and on the transfer rate, which depends on the rolling degree and / or the rolling force, and can therefore be controlled specifically.

[0034] According to one embodiment, the surface of the aluminum-based coating has an Sdr value of > 4.0%. The Sdr value is also referred to as the developed interface ratio or can be seen as the magnification of the surface in relation to the “projection surface”. This means that one or more defined areas are observed, for example images taken using a confocal white light microscope, and the determined or measured actual surface is set in relation to the projection surface (flat or planar surface) in the defined area(s). The magnification of the surface depends on the shape, design and / or dimensions of the surface structure as well as on the number or distribution of the structure, whereby the Sdr value can be up to 35%, for example. The Sdr value can in particular be at least 5.0%, preferably at least 7.0%, more preferably at least 9.0%. Method for determining orDetermining the SDR value is familiar to those skilled in the art, particularly based on DIN EN ISO 25178. For example, the SDR value can be determined using atomic force microscopy (AFM). AFM, for example, enables a resolution of up to 90 x 90 pm. 2 or even higher if necessary. One available technology for determining / capturing surface parameters is known as "psurf." Details are available at: www.nano-focus.de / technoloqie / messprinzipien / usurf-technoloqie / .

[0035] According to one embodiment, the deterministic surface structure has a skewness Rsk between +1.0 and -2.0. In particular, the skewness can be between +1.0 and >0, with positive values ​​indicating profiles with a high peak content. Rsk evaluates the asymmetry of the amplitude density, with positive values ​​indicating profiles with a high peak content, cf. DIN EN ISO 4287.

[0036] Suitable steel sheets coated with an aluminum-based coating include all hardenable steel alloys known to those skilled in the art and used in practice. Examples include manganese-boron steels or, in particular, other steels for hot forming, such as micro-alloyed steels, with tensile strengths in the hardened state of at least 500 MPa, in particular at least 600 MPa, preferably at least 1200 MPa, and more preferably at least 1500 MPa and higher. Depending on the alloy or the carbon content of the hardenable steel, a maximum tensile strength of up to 2500 MPa or higher can be achieved, in particular a maximum of 2300 MPa, preferably a maximum of 2200 MPa.

[0037] According to one embodiment, the steel sheet can have the following chemical composition in wt.%, at least in the section with the second thickness of the semi-finished product:

[0038] C = 0.05 to 0.5, in particular at least 0.10, preferably at least

[0039] 0.20, preferably at least 0.250, particularly preferably at least 0.280, Mn = 0.3 to 3.0,

[0040] Si = 0.05 to 1.7,

[0041] P to 0.1,

[0042] S to 0.1,

[0043] N up to 0.1, and optionally one or more alloying elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca):

[0044] AI up to 1.0,

[0045] Ti up to 0.2,

[0046] V up to 0.5, Nb up to 0.5,

[0047] B to 0.01,

[0048] Cr up to 1.0,

[0049] Mo to 1.0,

[0050] Cu up to 1.0,

[0051] Ni up to 1.0,

[0052] Ca to 0.1,

[0053] Rest Fe and unavoidable impurities.

[0054] According to one embodiment, the aluminum-based coating may have the following chemical composition in wt.%:

[0055] Si up to 15.0,

[0056] Fe up to 5.0,

[0057] Mg up to 5.0,

[0058] Zn up to 30.0,

[0059] Rest AI and unavoidable impurities.

[0060] In addition to aluminum and unavoidable impurities, the aluminum-based coating may contain additional elements such as silicon with a content of up to 15.0 wt.% and / or iron with a content of up to 5.0 wt.% and / or magnesium with a content of up to 5.0 wt.% and / or zinc with a content of up to 30.0 wt.%. Si may be present in particular at a content of at least 0.1 wt.%, preferably at least 2.0 wt.%, more preferably at least 4.0 wt.%, whereby the content may in particular be limited to a maximum of 12.0 wt.%, preferably to a maximum of 11.0 wt.%. Si in the coating can contribute to improved processability during hot-dip coating. Alternatively or additionally, Fe may be present in particular at a content of at least 0.1 wt.%, preferably at least 0.5 wt.%, more preferably at least 1.0 wt.%, whereby the content may in particular be limited to a maximum of 4.0 wt.%, preferably to a maximum of 3.5 wt.%.Fe in the coating can increase the melting point of the coating, which can be advantageous during austenitizing (annealing process). Alternatively or additionally, Mg can be present in particular at a level of at least 0.1 wt.%, preferably at least 0.2 wt.%, whereby the content can be limited in particular to a maximum of 3.0 wt.%, preferably to a maximum of 1.5 wt.%, more preferably to a maximum of 0.8 wt.%. Mg in the coating can contribute to a reduction in the absorption of diffusible hydrogen into the substrate. Alternatively or additionally, Zn can be present in particular at a level of at least 0.1 wt.%, preferably at least 0.2 wt.%, whereby the content can be limited in particular to a maximum of 20.0 wt.%, preferably to a maximum of 10.0 wt.%, more preferably to a maximum of 5.0 wt.%. Zn in the coating can contribute to improving corrosion resistance.

[0061] The thickness of the aluminum-based coating is, for example, 3.0 to 40.0 pm before hot forming, in particular 10.0 to 40.0 pm, preferably 11.0 to 35.0 pm, more preferably 12.0 to 30.0 pm, more preferably 13.0 to 27.0 pm.

[0062] A 1.50 mm thick cold-rolled steel sheet of grade 22MnB5 coated with an FAL coating (Si: 7%, Fe: 2%, balance Al and unavoidable impurities, thickness 25 μm) was skin-passed on both sides using various textured skin-pass rolls. A stochastic surface structure was embossed into the surface of the FAL coating of a first coated steel sheet (VI). The skin-pass rolls were textured in a known manner using the EDT process (cf. EP 2 006 037 B1). Another coated steel sheet (2) was skin-passed with a deterministic surface structure with a double-I structure (cf. EP 2 892 663 B1).

[0063] Another cold-rolled steel sheet of grade 22MnB5 with a thickness of 1.40 mm, coated with an FAL coating (Si: 7%, Fe: 2%, balance Al and unavoidable impurities, thickness 25 μm), was skin-passed on both sides using different textured skin-pass rolls. A stochastic surface structure was imprinted into the surface of the FAL coating in a second to fourth coated steel sheet (V2) to (V4). Further coated steel sheets (4) to (10) and (13) were each skin-passed with a deterministic surface structure, with double-I structures of different sizes being selected.

[0064] Ten samples were taken from a total of thirteen steel sheets of different thicknesses and the surface structure parameters were determined according to DIN EN ISO 4287 and the mean value was calculated in each case, see Table 1.

[0065] One of the samples VI to 13 was butt welded to another steel sheet (22MnB5, 1.0 mm) with an aluminum-based coating (Si: 7%, Fe: 2%, balance Al and unavoidable impurities, thickness 25 μm) with a stochastically skin-treated surface structure, so that a semi-finished product could be prepared as a tailor-welded blank for hot forming. The additional steel sheet thus forms the section with the first thickness, and the sample thus forms the section with the second thickness of the semi-finished product.

[0066] Samples VI to 13, or the second thickness sections of the semi-finished products, were equipped with a thermocouple, and the semi-finished products were subsequently heated to a furnace temperature of 920°C. The time required to heat the semi-finished products / samples to 910°C is also listed in Table 1.

[0067]

[0068] Table 1

[0069] As can be clearly seen, the deterministic surface structures according to the invention in the aluminum-based coating exhibit significantly improved heating behavior compared to the stochastic references VI to V4. Depending on the variant, the time savings are approximately 6–53 s. This leads to significant savings in energy consumption in large-scale applications, as well as the option of conducting the annealing process with reduced furnace capacities, e.g., a shorter roller hearth furnace or fewer furnace chambers, etc.

[0070] Comparable effects and reduced cycle times were also observed for semi-finished products such as tailor-rolled blank and patch-work blank, so that on a semi-finished product with different thicknesses, the second section or the thicker section with an aluminum-based coating and a deterministic surface structure can be heated faster compared to undressed sections or sections dressed with a stochastic surface structure.

[0071] Further steps for producing a component from the heated semi-finished products by hardening or press hardening are state of the art and have not been investigated in detail.

[0072] The drawing shows in Figure 1 three different semi-finished products (1) for hot forming, shown schematically in section, designed as tailor-welded blank (see Figure 1a), as tailor-rolled blank (see Figure 1b) and as patch-work blank (see Figure 1c).

[0073] The semi-finished product (1) comprises a first or more first sections with a first thickness (dl) and at least one second or more second sections with a second thickness (d2), wherein the first thickness (dl) is less than the second thickness (d2), wherein the semi-finished product (1) comprises, at least in the section of the second thickness (d2), a steel sheet (2) coated with an aluminum-based coating (2.1), wherein the semi-finished product (1) has, at least in the section of the second thickness (d2), a deterministically formed surface structure with the aluminum-based coating (2.1).

Claims

Patent claims 1. Semi-finished product (1) for hot forming, which is a tailor-welded blank, a tailor-rolled blank or a patchwork blank, wherein the semi-finished product (1) comprises a first or more first sections with a first thickness (dl) and at least one second or more second sections with a second thickness (d2), wherein the first thickness (dl) is less than the second thickness (d2), wherein the semi-finished product (1) comprises, at least in the section of the second thickness (d2), a steel sheet (2) coated with an aluminum-based coating (2.1), characterized in that the semi-finished product (1) has a deterministically formed surface structure with the aluminum-based coating (2.1) at least in the section of the second thickness (d2).

2. Semi-finished product according to claim 1, wherein the deterministic surface structure has an average roughness Ra between 1.0 and 6.0 pm.

3. Semi-finished product according to one of the preceding claims, wherein the deterministic surface structure has a peak number RPc between 100 and 250 1 / cm.

4. Semi-finished product according to one of the preceding claims, wherein the deterministic surface structure has a structure depth Rz between 4.0 and 25.0 pm.

5. Semi-finished product according to one of the preceding claims, wherein the surface of the aluminum-based coating (2.1) has an Sdr value > 4.0%.

6. Semi-finished product according to one of the preceding claims, wherein the steel sheet (2) has at least in the section with the second thickness (d2) of the semi-finished product (1) the following chemical composition in wt.%: C = 0.05 to 0.5, Mn = 0.3 to 3.0, Si = 0.05 to 1.7, P to 0.1, S to 0.1, N up to 0.1, and optionally one or more alloying elements from the group (Al, Ti, V, Nb, B, Cr, Mo, Cu, Ni, Ca): AI up to 1.0, Ti up to 0.2, V to 0.5, Nb up to 0.5, B to 0.01, Cr up to 1.0, Mo to 1.0, Cu up to 1.0, Ni up to 1.0, Ca to 0.1, The remainder is Fe and unavoidable impurities. Semi-finished product according to one of the preceding claims, wherein the aluminum-based coating (2.1) has the following chemical composition in wt.%: Si up to 15.0, Fe up to 5.0, Mg up to 5.0, Zn up to 30.0, Rest aluminum and unavoidable impurities.

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