Steel plate having hot-dip aluminized coating for hot forming

Abstract

The invention relates to a steel plate made of a hardenable steel material and coated with a hot-dip aluminized coating for hot forming, wherein the surface of the hot-dip aluminized coating has an SDR value of at least 3.0%, wherein the hot-dip aluminized coating has a deterministic surface structure.

Description

[0001] FAL-coated steel sheet for hot forming

[0002] The invention relates to a steel sheet coated with an FAL coating made of a hardenable steel material for hot forming, wherein the surface of the FAL coating has an Sdr value of at least 3.0%.

[0003] In the hot forming of manganese-boron steels, for example, hot-dip aluminizing (FAL) coatings are generally used today. This provides effective scale protection during the annealing process prior to hot forming, thus ensuring the further processability of the press-hardened component without the need for an additional process step to remove scale deposits. During the aforementioned annealing process, the FAL layer undergoes metallurgical transformations as the annealing time increases. 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. This effect is mitigated, however, 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 above-described conversion process in the coating 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 a 1.5 mm thick FAL-coated 22MnB5 sheet, an annealing time of approximately 4 to 5 minutes at temperatures slightly above 900 °C is typically required.

[0005] This annealing time is made up of two phases: First, some time passes in the furnace until the previously cold sheet material has reached 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. Steels or steel sheets with FAL coatings for hot forming are disclosed, for example, in EP 1 013 785 A1. Furthermore, a stochastic skin pass process for FAL-coated steel sheets for hot forming is described in EP 3 239 337 B1.Furthermore, WO 2020 / 130401 A1 discloses a deterministically textured skin pass roll for FAL-coated steel sheets in order to obtain a visually good surface with an excellent paint appearance.

[0006] From DE 10 2020 124 488 A1 it is further known to prepare a steel sheet in the cold rolling process in such a way that an enlarged surface is produced on the steel sheet, which is then coated with an FAL coating, the FAL-coated steel sheet is press-hardened to form a component and the component has an Sdr value of between 3% and 30% at least in one bonded section.

[0007] It is advantageous and desirable to improve the heating behavior of the material so that the furnace heat can be introduced into the workpiece as efficiently as possible. This could save furnace capacity (e.g., by operating shorter furnaces), enable more efficient use of the forming press, especially with thick sheet thicknesses or patchwork blank joints, and achieve more effective use of energy resources during furnace operation, preferably to ensure reproducibility.

[0008] The teaching of the invention relates to a steel sheet coated with an FAL coating made of a hardenable steel material for hot forming, wherein the surface of the FAL coating has an Sdr value of at least 3.0%, wherein the FAL coating has a deterministic surface structure.

[0009] The SDR value refers to a developed threshold ratio or is also a measure of the surface enlargement, which indicates the percentage of the additional area of ​​a definition area that is attributable to a structure compared to the absolutely flat definition area. Methods for determining the SDR value are familiar to the expert, in particular based on DIN EN ISO 25178. For example, the SDR value can be determined by or 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.nanofocus.de / technologie / messprinzi- pien / usurf-technoloqie / .

[0010] The deterministic surface structure of the FAL coating has, in particular, an Sdr value of at least 3.5%, 4.0%, 4.3%, 4.6%, 5.0%, preferably of at least 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, preferably of at least 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, particularly preferably of at least 14.0%, 15.0%. The Sdr value can be limited to a maximum of 35.0%, in particular a maximum of 32.0%.

[0011] A flat surface has or would have an Sdr value of 0%.

[0012] Surprisingly, it has been found that a deterministic surface structure in combination with an Sdr value of the FAL coating of at least 3.0% results in a significant increase in the efficiency of the coupled thermal radiation in a furnace into the material system for heating to hot forming temperature.

[0013] Steel sheet refers, among other things, 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 steel sheet can be hot-rolled or, preferably, cold-rolled. Hot rolling and, optionally preferred, cold rolling are known to those skilled in the art.

[0014] The thickness of the coated steel sheet can be, for example, 0.50 to 6.0 mm, in particular 0.60 to 4.0 mm, preferably 0.70 to 3.50 mm.

[0015] Deterministic surface structure refers to recurring structures, such as embossings, which have a defined shape and / or design, cf. EP 2 892 663 B1. In particular, this also includes surfaces with a (quasi-) stochastic appearance, which, however, are applied using a deterministic texturing process and are thus composed of deterministic form elements.

[0016] An FAL coating is an aluminum-based coating. The FAL coating is applied to the steel sheet conventionally in known devices or by known methods. The introduction of a deterministic surface structure takes place after the application and solidification of the FAL coating using deterministically textured skin-pass rolls. In contrast to DE 10 2020 124 488 Al, the surface enlargement does not take place in the cold rolling process on an uncoated steel sheet, but according to the invention only after the application of the FAL coating. This ensures reproducibility and thus a targeted introduction of a deterministic surface structure into the FAL coating.

[0017] 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.

[0018] According to one embodiment, the deterministic surface structure has a peak count RPc between 100 and 250 1 / cm. In particular, the peak count RPc can be at least 110 1 / cm, preferably at least 130 1 / cm. In particular, the peak count RPc can be a maximum of 220 1 / cm, preferably a maximum of 200 1 / cm, and more preferably a maximum of 180 1 / cm.

[0019] 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.

[0020] 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 measurement distance, cf. DIN EN ISO 4287.

[0021] 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.

[0022] 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. Rsk evaluates the asymmetry of the amplitude density, with positive values ​​indicating profiles with a high peak content, cf. DIN EN ISO 4287. Preferably, the deterministic surface structure has a positive skewness Rsk in the undirected state. In particular, the skewness Rsk can alternatively be between -0.8 and -2.0, with the deterministic surface structure having a negative skewness Rsk, for example, in the directed state.

[0023] Depending on the condition of the coated steel sheet, whether straightened or preferably unstraightened, different parameters can also be set, whereby “straightened” is understood to mean the use of a straightening machine, which is known to the person skilled in the art, and this has an effect on the surface of the FAL coating, in particular through contact between the sheet / strip and the bending rollers, and thus leads to a change in the parameters compared to “unstraightened”.

[0024] Hardenable steel materials are state-of-the-art. 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.

[0025] According to one embodiment, the hardenable steel material can have the following chemical composition in wt.%:

[0026] C = 0.05 to 0.5,

[0027] Mn = 0.3 to 3.0,

[0028] Si = 0.05 to 1.7,

[0029] P to 0.1,

[0030] S to 0.1,

[0031] 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):

[0032] AI up to 1.0,

[0033] Ti up to 0.2,

[0034] V up to 0.5, Nb up to 0.5,

[0035] B to 0.01,

[0036] Cr up to 1.0,

[0037] Mo to 1.0,

[0038] Cu up to 1.0,

[0039] Ni up to 1.0,

[0040] Ca to 0.1,

[0041] Rest Fe and unavoidable impurities.

[0042] The FAL coating has the following chemical composition in wt.%: optionally one or more alloying elements from the group (Si, Fe, Mg, Zn):

[0043] Si up to 15.0,

[0044] Fe up to 5.0,

[0045] Mg up to 5.0,

[0046] Zn up to 30.0,

[0047] Rest AI and unavoidable impurities.

[0048] In addition to aluminum and unavoidable impurities, the FAL 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 in particular be present in a content of at least 0.1 wt.%, preferably at least 2.0 wt.%, more preferably at least 4.0 wt.%, whereby the content can 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 in particular be present in a content of at least 0.1 wt.%, preferably at least 0.5 wt.%, more preferably at least 1.0 wt.%, whereby the content can 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. 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.

[0049] In a preferred embodiment, the Si content in the FAL coating is either 0.2 to 4.5 wt% or 7 to 13 wt%, in particular 8 to 11 wt%.

[0050] In a preferred embodiment, the optional content of Fe in the FAL coating may comprise 0.2 to 4.5 wt.%, in particular 1 to 4 wt.%, preferably 1.5 to 3.5 wt.%.

[0051] In a preferred embodiment, the optional content of Mg in the FAL coating comprises 0.01 to 1.0 wt% Mg, in particular 0.1 to 0.7 wt% Mg, preferably 0.1 to 0.5 wt% Mg.

[0052] In an alternative embodiment, the FAL coating may comprise 2.0 to 24.0 wt% Zn, 1.0 to 7.0 wt% Si, optionally 1.0 to 8.0 wt% Mg if the Si content is between 1.0 and 4.0 wt%, optionally up to 0.3 wt% in total of Pb, Ni, Zr or Hf and in particular impurities whose total contents are limited to a maximum of 2.0 wt%, and the remainder being aluminum.

[0053] The thickness of the FAL coating is 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.

[0054] 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).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.

[0055] Ten samples were taken from a total of thirteen steel sheets of different thicknesses and different finishes. The surface structure parameters were determined according to DIN EN ISO 4287 and the mean value was calculated in each case, see Table 1.

[0056] Samples VI to 13 were fitted with a thermocouple and then heated to a furnace temperature of 920°C. The time required to heat the FAL-coated steel sheet to 910°C is also listed in Table 1. Samples VI to V4 are reference samples and were provided with a stochastic surface structure using EDT-textured temper rolling. Samples 2, 4 to 10, and 13 are inventive samples and were provided with a deterministic surface structure; all samples were provided in the undirected state.

[0057]

[0058] Table 1

[0059] As can be clearly seen, both deterministic surface structures in the FAL coating according to the invention exhibit significantly improved heating behavior compared to the stochastic reference. Depending on the variant, the time savings are approximately 6–53 seconds. 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, for example, a shorter roller hearth furnace or fewer furnace chambers, etc.

[0060] The further steps for producing a component from a heated steel sheet coated with an FAL coating by hardening or press hardening are state of the art and were not investigated further.

Claims

Patent claims 1. Steel sheet made of a hardenable steel material coated with an FAL coating for hot forming, the surface of the FAL coating having an Sdr value of at least 3.0%, characterized in that the FAL coating has a deterministic surface structure.

2. Steel sheet according to claim 1, wherein the deterministic surface structure has an average roughness Ra between 1.0 and 6.0 pm.

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

4. Steel sheet 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. Steel sheet according to one of the preceding claims, wherein the deterministic surface structure has a skewness Rsk between + 1.0 and - 2.

0.

6. Steel sheet according to one of the preceding claims, wherein the hardenable steel material has 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. Steel sheet according to one of the preceding claims, wherein the FAL coating has the following chemical composition in wt.%: optionally one or more alloying elements from the group (Si, Fe, Mg, Zn): Si up to 15.0, Fe up to 5.0, Mg up to 8.0, Zn up to 30.0, Rest AI and unavoidable impurities.

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