Flat steel product, method for the production and use thereof

Abstract

The invention relates to flat steel products composed of, in weight percent, in short: wt.%: C: 0.010-0.060; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Nb: 0.040-0.090; V: up to 0.12; and further optional components: up to 0.30; the remainder being Fe and unavoidable impurities. The invention also relates to a process for producing such flat steel products and the use of such flat steel products, particularly in connection with the storage or transport of hydrogen.

Description

[0001] The invention relates to a flat steel product, a process for its production and a use.

[0002] In this text, the term flat steel product is understood to mean steel strips or steel sheets produced in a rolling process, as well as blanks, plates or similar products obtained therefrom, the thickness of which is significantly smaller than their width and length.

[0003] With the increasing importance of hydrogen for industrial processes, there is a need for piping systems suitable for the transport of molecular hydrogen. Currently, austenitic stainless steels or normalized annealed ferritic steels with low strength are often used for hydrogen transport.

[0004] The steel materials for the storage, transmission, and transport of hydrogen, for example in pipelines and tanks made of such steel materials, are specified in several international standards, e.g., ISO 3183, API 5L, EIGA Guideline IGC Doc 121 / 14.

[0005] The materials considered suitable for the transport and use of hydrogen are unalloyed quality steels with high hydrogen resistance, which, however, often have low strength. Commonly used steels are grades X 42, X 46, and X 52 according to the API 5L standard or their European equivalents. These grades, as well as those commonly used for hydrogen pipelines, often have the problem that, due to their comparatively high carbon content of up to approximately 0.26 percent by weight, they require an energy-intensive post-annealing process at temperatures of up to 900 °C after pipe production. A further problem is the frequently formed second phase, pearlite, whose presence leads to reduced local formability. Due to the frequent need for post-annealing, as explained above, the achievable values ​​for mechanical properties such as yield strength and strength are limited.

[0006] Against this background, the task is to develop steels that can be used in the field of hydrogen pipelines and, in particular, offer a higher level of strength than the steels known to date.

[0007] The object is achieved with a flat steel product having the features of claim 1. The object is further achieved with a method having the features of claim 16 and with a use according to one of claims 18 to 20.

[0008] A flat steel product according to the invention is composed of, in weight percent, in short: wt.%: C: 0,010-0,060; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Note: 0.040-0.090, preferably 0.040-0.085; V: up to 0.12; and other optional components: up to 0.30; balance Fe and unavoidable impurities.

[0009] The comparatively low carbon content ensures that when the steel strip is welded to form pipes and tanks, particularly using HF, MAG, and laser welding, only a slight change in material strength is observed in the weld seam area compared to conventional pipe materials. In connection with the use in hydrogen pipelines, this offers the advantage that stress-induced cracking under a hydrogen atmosphere can be largely avoided. Furthermore, it ensures that the formation of second phases is largely avoided. At carbon contents above 0.060 wt. %, the formation of second phases was observed beyond an accepted level, so that the volume fraction of the ferritic matrix fell below 94 volume percent, and sometimes even below 90 volume percent, which were defined as the relevant lower limits for the volume fraction of fine-grained ferritic matrix during development.Carbon contents below 0.010 wt.%, on the other hand, led to a significant deterioration in mechanical properties, particularly low yield and tensile strengths. For this reason, the specified carbon content range was chosen.

[0010] The presence of manganese, Mn, as an alloying component is essential for the strength of the flat steel product. Within appropriately selected limits, the presence of Mn promotes a reduction in the steel's transformation temperature to a level that makes a thermomechanical rolling process feasible, allowing the desired fine-grained microstructure to be achieved. With the aim of limiting alloying costs and increasing the recyclability of the manufactured flat steel products, the Mn content was kept to a minimum, resulting in the above-mentioned Mn content range.Due to the comparatively low Mn content, it is possible to mix scrap potentially arising at a later date from the flat steel products and products manufactured from them with other types of scrap, so that it can be used both as cooling scrap in oxygen steelmaking plants and as feedstock scrap in electric steelmaking plants.

[0011] The addition of the alloying element aluminum, Al, serves primarily to fix nitrogen and prevent embrittlement after the hot rolling process. Furthermore, the addition of Al promotes continuous castability for the production of steel slabs. To achieve sufficient nitrogen fixation in flat steel products, a minimum Al content of 0.020 wt.% was found. To limit the cost increase caused by the Al addition while adequately meeting these nitrogen fixation requirements, but also to avoid impairing the castability of the steel, the upper limit of the Al content was set at 0.050 wt.%.

[0012] The alloying element niobium (Nb) was used as a carbide former. The alloying element vanadium is optionally used as an additional carbide former. The upper limit was chosen at 0.12 weight percent to achieve an economical alloying concept, since above this limit, no further effect on increasing strength was observed. With a weight fraction of the carbide former Nb in the alloy composition within the specified range, a sufficiently high density of precipitated, extremely fine carbides with a size ranging from less than 5 nanometers up to 100 nanometers could be achieved, as well as a homogeneous distribution of these carbides in the microstructure.

[0013] The precipitate size of the finest carbides can be determined, for example, with a transmission electron microscope (TEM) using the replica method via carbon pull-out impressions, as described, for example, in H. Schumann and H. Oettel "Metallography," 14th edition, 2005, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Section 2.7.4, pages 317f. In the method used here, the sample sections are selectively etched so that the ferritic matrix of the samples is dissolved and the precipitate particles on the surface are exposed. Subsequently, thin carbon layers with a thickness of between 10 nm and 20 nm are applied to the sample surface by vapor deposition under high vacuum and reinforced with a pioloform carrier film. The carbon deposition is carried out at an inclination angle of 45° to image the surface topography of the sample. After detachment from the sample material, the layer exhibits a geometric negative of the sample surface.The layer is removed by dissolving the original sample in diluted nitric acid (10 vol.% Nital). During removal of the layer, the particles are extracted from the sample. The carbon imprints are analyzed in a TEM using scattering absorption contrast. The resolution limit of this method is approximately 5 nm.

[0014] Because the number of necessary alloying components is comparatively small, a resource-saving manufacturing process for the flat steel products according to the invention is possible.

[0015] Depending on the desired setting of further properties, vanadium can be provided up to a content of 0.12 wt.% as a component of flat steel products according to the invention.

[0016] In addition to the aforementioned elements, further optional components with a total content of up to 0.30 percent by weight may be included.

[0017] The additional optional ingredients mentioned are composed as follows, all information in weight percentage: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100.

[0018] Within the stated upper limits, it has been proven that the optional components have no noticeable influence on the properties tested for the manufactured flat steel products. A flat steel product according to the invention achieves a combination of properties that has a particularly advantageous effect on the usability of the manufactured flat steel products for the production of pipelines for hydrogen transport. The alloy components selected according to the invention in the alloy specification specified according to the invention ensure that hardening in the weld seam area does not occur or only occurs to a minimal extent, and therefore embrittlement due to hydrogen contact is not observed to a significant extent. These positive properties for the stated application go hand in hand with the good formability required for the production of pipelines.

[0019] In an advantageous further development of the flat steel product according to the invention, the alloying specification is as follows: The flat steel product is composed of, in weight percent, in short: wt.%: C: 0,010-0,040; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,040-0,060; Optional components: up to 0.30; balance Fe and unavoidable impurities.

[0020] With the set upper limit for carbon and the correspondingly adjusted restricted range of the Nb content, it is achieved that, on the one hand, carbide formation is enabled to increase strength, while the undesirable formation of second phases is largely suppressed.

[0021] Preferred ranges may be for C: 0.010-0.030 wt%, and / or for Mn: 0.150-0.250 wt%, and / or for Al: 0.025-0.040 wt%.

[0022] The mechanical properties are such that in a preferred embodiment the flat steel product has: a yield strength Rp0.2 between 380 MPa and 520 MPa, preferably ≤ 510 MPa; and / or a tensile strength Rm 450-600 MPa, preferably 450-540 MPa; and / or an elongation at break A80 ≥ 20 percent, preferably ≥ 22 percent; and / or an elongation at break A5 ≥ 24 percent, preferably ≥ 26 percent, each determined longitudinally to the rolling direction and each determined according to DIN EN ISO 6892-1:2019.

[0023] In particular, the yield strength Rp0.2 and the tensile strength Rm must not be too low or too high, so that both sufficient load-bearing capacity of the manufactured pipe or hydrogen storage device and good manufacturability can be assumed.

[0024] In an alternative advantageous development of the flat steel product according to the invention, the alloying specification is as follows: The flat steel product is composed of, in weight percent, in short: wt.%: C: 0,010-0,040; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,054-0,070; Optional components: up to 0.30; balance Fe and unavoidable impurities.

[0025] By increasing the required addition of Nb compared to the previously described further development, a steel flat product is obtained which has increased strength compared to a steel flat product with a lower mandatory addition of Nb; by limiting the upper limit to the still moderate content here, it is also ensured that the formability of the steel flat product, for example in terms of the elongation at break A 5 and / or A 80 , is not significantly impaired for the desired application purposes.

[0026] Preferred ranges may be for C: 0.010-0.030 wt%, and / or for Mn: 0.150-0.250 wt%.

[0027] The optional ingredients mentioned are composed as follows, all information in weight percentage: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100.

[0028] The optional components mentioned are alternatively composed as follows: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100; V: up to 0.020.

[0029] Within the above-mentioned upper limits, it has been shown that the optional components have no noticeable influence on the investigated properties of the manufactured flat steel products.

[0030] In an alternative advantageous development of the flat steel product according to the invention, the alloying specification is as follows: The flat steel product is composed of, in weight percent, in short: wt.%: C: 0,015-0,050; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,065-0,085; V: 0,020-0,060; Optional components: up to 0.30; balance Fe and unavoidable impurities.

[0031] The alternative refinement specifically stipulates that vanadium, abbreviation: V, is present as a required component of the alloy, rather than as an optional one. By slightly increasing the carbon limits, the microstructure of the flat steel product is modified in conjunction with the adjusted other components, particularly the carbide formers Nb and V, resulting in increased strength. The formability of the flat steel product is not significantly impaired within the intended usage scenario. However, the increased strength is achieved at the cost of increased use of carbon and microalloying elements, which is why these are capped at the specified upper limits.

[0032] Preferred ranges may be for C: 0.020-0.040 wt%, and / or for Mn: 0.150-0.250 wt%, and / or for Al: 0.025-0.040 wt%, and / or for Nb: 0.070-0.085 wt%, and / or for V: 0.025-0.050 wt%.

[0033] In an alternative advantageous development of the flat steel product according to the invention, the alloying specification is as follows: the flat steel product is composed of, in weight percent, in short: wt.%: C: 0,020-0,060; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,070-0,090; V: 0,080-0,12; Optional components: up to 0.30; balance Fe and unavoidable impurities.

[0034] By slightly increasing the carbon limits even further, the microstructure of the flat steel product is modified in conjunction with the further adjusted components, particularly the carbide formers Nb and V, resulting in further strength increases. The formability of the flat steel product is not significantly impaired within the scope of the usage scenario. However, the increased strength comes at the cost of increased use of carbon and microalloying elements, which is why these are capped at the specified limits.

[0035] Preferred ranges may be for C: 0.035-0.055 wt%, and / or for Mn: 0.150-0.250 wt%, and / or for Al: 0.025-0.040 wt%, and / or for Nb: 0.075-0.090 wt%.

[0036] The optional components can be composed of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100.

[0037] It has been shown that within these limits the optional components do not noticeably change the investigated properties of the flat steel product.

[0038] For the purposes of this application, the following definitions apply: Ultrafine carbides refer to carbides that cannot be detected by light microscopy using the procedure described below for light microscopic microstructural determination, and that occur particularly in a diameter range from less than 5 nanometers to 100 nanometers, for example, particularly within ferrite grains. Fine carbides refer to carbides that can be detected by light microscopy using the procedure described below and that are measured in diameters down to one micrometer.

[0039] Within the scope of the present development, the presence of ultrafine carbides and their homogeneous distribution in all inventive samples and their further developments were qualitatively demonstrated using the TEM examination described above. However, the quantitative contribution to the microstructure could not be determined due to their insufficient detectability by light microscopy. Phenomenologically and qualitatively, it was found that ultrafine carbides, i.e., carbides not detectable by light microscopy in the diameter range from less than 5 nanometers to 100 nanometers, are present in the inventive flat steel products and their further developments. It is assumed that the ultrafine carbides contribute to the strength of the produced inventive samples and their further developments. The presumed mechanism is the obstruction of dislocation movements.Furthermore, it is assumed that the finest carbides contribute to the observed yield strength and ductility of the materials via the grain refinement mechanism by hindering recrystallization during hot rolling.

[0040] As stated above, the suitability of the flat steel product for its intended use benefits from a microstructure such as that found in the developed flat steel products, namely a microstructure consisting of a ferritic matrix with extremely fine carbides in the size range of up to 100 nanometers that are not detectable by light microscopy and with fine carbides detectable by light microscopy in the size range of up to one micrometer. The proportion of the ferritic matrix in the microscopically detectable microstructure is at least 90 percent by volume, preferably at least 94 percent by volume, and the proportion of fine carbides in the microscopically detectable microstructure is a maximum of 2 percent by volume.

[0041] In addition to the ferritic matrix and the fine carbides, the microstructure, which can be determined by light microscopy, shows either no further phases, or further phases which may be, for example, grain boundary cementite, fine lamellar pearlite and / or bainite, and which, in the accepted phase proportion according to the above provision, do not noticeably limit the technological properties of the flat steel products.

[0042] Both the finest carbides and the fine carbides are homogeneously distributed in the ferritic matrix.

[0043] The structure, which can be determined by light microscopy, therefore consists of at least 90 volume percent, preferably at least 94 volume percent, of a ferritic matrix, as well as a maximum of 2 volume percent of homogeneously distributed fine carbides in the size range of up to one micrometer, and, if present, the remainder of other phases such as grain boundary cementite, fine lamellar pearlite and / or bainite.

[0044] In the context of this application, with the sole exception of the finest carbides detected by TEM, any reference to a structure or a phase present in a structure is to be understood as an indication of a structure or phase that is identifiable by light microscopy, whereby the method of light microscopy examination described below was used: Metallographic examinations are carried out on samples taken across the strip width at half the distance between the longitudinal edge and the center line of the product in accordance with DIN EN 10149-3:2013. One longitudinal section is taken per sample, with the metallographic examination being carried out by light microscopy at 200x and 1000x magnification. The 200x magnification provides an overview of the type of structural components present, while the 1000x magnification is used for a detailed examination of the structural components.The ground surface is etched with Nital, i.e., with alcoholic nitric acid containing 3 vol.% nitric acid, as described in H. Schumann and H. Oettel "Metallography," 14th edition, 2005, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Section 2.3.4.1, page 232. The microstructure components are determined by summing the area fractions of the individual microstructure components across the entire sheet thickness. For this purpose, 14 image sections per mm of sheet thickness are examined for their microstructure components at 1000x magnification, using the determination method described in H. Schumann and H. Oettel "Metallography," 14th edition, 2005, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Section 2.4.5, page 271: "In a measuring field . W the area of ​​the structural component is determined (measured). A measured value A A ^ of the area share AAis obtained as the ratio of the measured area to the total area A(W) of the measuring field. Typically, an average must be calculated over several measured values.

[0045] It is assumed that the area proportions of the phases correspond to the respective volume proportions, so that in this application phase proportions are given in volume percent.

[0046] In particular, the grain size of the ferritic matrix was found to be very small, namely finer than ASTM 6 as required by DIN EN 10149-1:2013-12. The grain sizes are determined according to EN ISO 643:2020-06 by comparison with standardized image series tables.

[0047] The mechanical properties are such that in a preferred embodiment, especially in the samples containing vanadium as a non-optional, i.e. mandatory, element, the flat steel product has: a yield strength Rp0.2 between 460 MPa and 670 MPa, particularly preferably ≤ 660 MPa; and / or a tensile strength Rm 520-750 MPa, preferably 520-730 MPa; and / or an elongation at break A80 ≥ 12 percent, preferably ≥ 14 percent, particularly preferably ≥ 20 percent; and / or elongation at break A5 ≥ 22 percent; each of the above-mentioned sizes is determined along the rolling direction and is determined in accordance with DIN EN ISO 6892-1:2019.

[0048] Due to the comparatively high strength of the described flat steel products, it is possible to produce them in comparatively thin thicknesses without compromising the mechanical stability required for the specific application. Therefore, a flat steel product with a thickness between 1.5 and 6 mm is preferred, and a thickness between 1.7 and 4 mm is particularly preferred.

[0049] Furthermore, the flat steel products obtained according to the observations are preferably

[0050] Hole expansion capacity ≥ 80 percent, preferably > 90 percent, according to ISO 16630:2017-09, and / or a folding capacity of 180 degrees when bent according to DIN EN ISO 7438:2016-07.

[0051] The observed parameters "hole expansion capacity," also referred to analogously as "hole expansion ratio," and "folding capacity" are evidence of the high degree of formability of the flat steel products. The described flat steel products are therefore particularly well suited for the production of pipes and tanks, as they can be subjected to the forming processes typically required for the production of pipes and tanks, such as folding, bending, expanding, and shear forming.

[0052] Preference is given to flat steel products which have a reduction in area at fracture Z according to DIN EN ISO 6892-1:2020-06 of greater than or equal to 50.0%, preferably 60.0%.

[0053] It is particularly preferred if, in further training, the product of [Cf] and [Rm] applies: 15 ≥ Cf × Rm , with the following parameters: [Rm]: the numerical value of the tensile strength of the steel flat product in MPa, [Cf]: the numerical value of a proportion Cf of free carbon in the total content of carbon C in the steel flat product in percent by weight, where the proportion Cf is the proportion of carbon C in the steel flat product, i.e. the weight percent from the melt analysis without a unit, less the proportion Ageb of carbon bound to Ti, Nb, Mo and V in the total content of carbon C in the steel flat product. The proportion Ageb is in turn determined as A geb = 1 C × 12,01 47,86 × Ti − 47,86 14,01 × N + 12,01 92,91 × Nb + 12,01 2 × 95,95 × Mo + 12,01 50,94 × V

[0054] The factor 12,01 47,86 × Ti − 47,86 14,01 × N describes the C content bound as TiC by subtracting the portion of Ti that is previously bound as TiN, but this C content cannot be less than 0. The factor 12,01 92,91 × Nb describes the C content bound as NbC. The factor 12,01 2 × 95,95 × Mo describes the C content bound as Mo2C. The factor 12,01 50,94 × V describes the bound C content as VC. The numerical values ​​used in the formula come from the atomic masses of the respective elements, namely 12.01 u for C; 47.86 u for Ti; 14.01 u for N; 92.91 u for Nb; 95.95 u for Mo; and 50.94 u for V. [C] describes the numerical value of the C content in wt.%, [Ti] describes the numerical value of the Ti content in wt.%, [N] describes the numerical value of the N content in wt.%, [Nb] describes the numerical value of the Nb content in wt.%, [Mo] describes the numerical value of the Mo content in wt.%, [V] describes the numerical value of the V content in wt.%.

[0055] With Ageb, [Cf] can be calculated, namely with: Cf = C − C × Ageb .

[0056] For the manufactured flat steel products, a proportion Cf of free carbon in the total carbon content C in the flat steel product is calculated as a weight percent based on the melt analysis of the respective sample and using the formulas mentioned above. The numerical value of this proportion is multiplied by the numerical value of the experimentally determined tensile strength in MPa. For particularly preferred samples according to the training, this product is rounded to a maximum of 15. Conversely, the selection of samples for which this product is rounded to a maximum of 15 can qualify the particularly preferred samples from the total sample pool.

[0057] It has been shown that a value of this product greater than 15 is associated with unsatisfactory formability. One reason for this may be that an excessively high free carbon content relative to the tensile strength has a negative impact on formability. Unsatisfactory formability manifests itself, in particular, in an insufficiently high reduction in area at fracture Z, as described above.

[0058] In order to produce flat steel products that meet the desired requirements, a process according to the following sequence can be used: a) Providing a precursor product, for example a solidified melt in the form of a slab, wherein the steel is composed of, in weight percent, in short: wt.%: C: 0,010-0,060; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Note: 0,040-0,070; V: up to 0.12; and other optional ingredients: up to 0.30; Rest Fe and unavoidable impurities, where the optional components consist of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100; preferably with a composition according to the alloying specifications of the type mentioned at the outset or one of the above-mentioned developments, i.e. each of the alloying specifications mentioned in this text within the limits of the element ranges mentioned here, in particular those mentioned in one of the patent claims 2 to 4; b) heating, preferably in a walking beam furnace, at a furnace temperature between 1200 degrees Celsius and 1350 degrees Celsius, preferably between 1250 degrees Celsius and 1350 degrees Celsius; c) hot rolling the heated precursor product with a final hot rolling temperature of at least 910 degrees Celsius, preferably between 910 degrees Celsius and 970 degrees Celsius; d) Coiling at a coiling temperature between 530 degrees Celsius and 670 degrees Celsius, preferably between 530 degrees Celsius and 650 degrees Celsius.

[0059] The provision of step a) can preferably be carried out with one of the above-mentioned alternative and / or preferred alloy compositions.

[0060] Steps a), b), c), d) are carried out in their alphabetical order, whereby intermediate steps are not required but are not excluded either.

[0061] The precursor product is, for example, a slab with a thickness between 180 mm and 280 mm, preferably between 220 mm and 280 mm, particularly preferably between 260 mm and 280 mm. The furnace time is preferably set between 60 minutes and 500 minutes, particularly preferably between 140 minutes and 300 minutes. Empirically determined, the precursors are thoroughly heated at these furnace times.

[0062] In order to produce a flat steel product which meets the desired requirements, a process according to the following sequence can alternatively be used: a) Providing a precursor product, for example a solidified melt in the form of a slab, wherein the steel is composed of, in weight percent, in short: wt.%: C: 0,015-0,060; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,065-0,090; V: 0,020- 0,12; optional ingredients: up to 0.30; Rest Fe and unavoidable impurities, where the optional components are composed of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100; preferably with a composition according to the alloying specifications mentioned in one of claims 5 to 7; b) heating, preferably in a walking beam furnace, at a furnace temperature between 1200 degrees Celsius and 1350 degrees Celsius, preferably between 1230 degrees Celsius and 1350 degrees Celsius; c) hot rolling the heated precursor product with a final hot rolling temperature of at least 920 degrees Celsius, preferably between 920 degrees Celsius and 970 degrees Celsius; d) Coiling at a coiling temperature between 500 degrees Celsius and 730 degrees Celsius, preferably between 560 and 710.

[0063] The provision of step a) can preferably be carried out with one of the above-mentioned alternative and / or preferred alloy compositions.

[0064] Steps a), b), c), d) are carried out in their alphabetical order, whereby intermediate steps are not required but are not excluded either.

[0065] A preferred type of use of the flat steel products according to the invention or their further developments is the production of pipes for hydrogen pipelines or hydrogen tanks.

[0066] A pipe made from a flat steel product according to the invention is preferably used for conducting and storing hydrogen. Furthermore, a tank made from a flat steel product according to the invention can be used for storing or temporarily storing or transporting hydrogen.

[0067] The following tests were conducted, which confirm the findings described above: Steel flat products were manufactured from alloys whose melt analyses are summarized in Tables 1 and 3. First, a slab of the corresponding composition was produced. b) The slab was heated in a furnace section at a furnace temperature listed in Tables 2 and 4, respectively, in the "T_Furnace" column. The furnace time, i.e., the duration of the slab's stay in the furnace, is given in the "Furnace Time" column. c) This was followed by hot rolling, with a final hot rolling temperature listed in Tables 2 and 4, in the "WET" column; d) This was followed by coiling at a coiling temperature specified in the "HT" column.

[0068] The microstructure was determined using the following light microscopic procedure: A metallographic examination of the material samples was performed. The samples were taken across the strip width at half the distance between the longitudinal edge and the centerline of the product in accordance with DIN EN 10149-3:2013. Longitudinal sections of each sample were taken, with the metallographic examination being performed at 200x and 1000x magnification using light microscopy. The 200x magnification provided an overview of the structural components present, while the 1000x magnification allowed for a detailed examination of the structural components. The ground surfaces were etched with Nital, i.e., with alcoholic nitric acid containing 3 vol.% nitric acid, as described in H. Schumann and H. Oettel "Metallography," 14th edition, 2005, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Section 2.3.4.1, page 232.The microstructure components were determined by summing the area fractions of the individual microstructure components across the entire sheet thickness. For this purpose, 14 image sections per mm of sheet thickness were examined for their microstructure components at 1000x magnification using the determination method described in H. Schumann and H. Oettel "Metallography," 14th edition, 2005, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Section 2.4.5, page 271. "In a measuring field... W the area of ​​the structural component is determined (measured). A measured value A A ^ of the area share AA is obtained as the ratio of the measured area to the total area A(W) of the measuring field. Typically, an average must be calculated over several measured values.

[0069] Determination of the microstructure using light microscopy revealed that samples according to the invention have a microstructure consisting of homogeneously distributed fine carbides in a size range of up to one micrometer in diameter and a proportion of up to 2 percent by volume of the microstructure in a ferritic matrix, with the proportion of the ferritic matrix in the microstructure being at least 90 percent by volume, preferably at least 94 percent by volume. The remaining portion of the microstructure was found to be secondary phases under light microscopy, for example, grain boundary cementite, finely lamellar pearlite, and / or bainite.

[0070] In particular, the grain sizes of the ferritic matrix were found to be very small, namely finer than ASTM 6 as required by DIN EN 10149-1:2013-12. The grain sizes were determined according to EN ISO 643:2020-06 by comparison with standardized image series tables.

[0071] Using the method described above for determining the finest carbide precipitates, TEM investigations showed that all samples contained homogeneously distributed finest carbides with diameters in the range of less than 5 nanometers up to 100 nanometers.

[0072] In the tensile test according to DIN EN ISO 6892-1:2019, very good strength and elongation at break values ​​of Rp 0.2 , R m , A 80 and A 5 were demonstrated, which are given in Tables 2 and 4.

[0073] Furthermore, the hole expansion ratio was determined according to ISO 16630:2017-09 and the reduction in area at fracture Z according to DIN EN ISO 6892-1:2020-06, respectively, given in Tables 2 and 4.

[0074] Furthermore, the proportion Ageb and the value [Cf] x [Rm] are given, which result from the melt analyses in the manner described in detail above.

[0075] Tests were conducted on specimens with a hole expansion ratio of 80% or more to demonstrate their suitability for use in hydrogen storage and conduction: C-shaped test specimens with a diameter of 28 mm and a material thickness of 3 mm were laser-cut from the material to ensure that no surface hardening occurred. These specimens were used for two investigations to measure the effect of atomic hydrogen on material-induced softening by deforming the specimens in the plasticity range, particularly to investigate the influence of the so-called HELP (hydrogen-enhanced localized plasticity) mechanism.

[0076] Some of the samples were galvanized without cyanide, and another sample was stored in hydrogen at 350 bar for 10 days. The expansion of the C-ring was then determined as a function of the force applied using a constant strain rate test. Force-displacement curves were measured to determine the deformation energy.

[0077] In parallel, the sensitivity of the material to possible hydrogen embrittlement was measured on identically prepared samples in step tests with a combination of constant loading in the elastic range and step-by-step mechanical loading in the plastic material range.

[0078] As a reference, untreated samples, i.e. samples not exposed to hydrogen, were subjected to the same procedure.

[0079] By comparing the test results, it was shown that there is no statistically significant correlation between the hydrogen exposure and the test results. Thus, based on the test results on these samples, it was demonstrated that the produced steels are insensitive to hydrogen exposure. Table 1 Nr. C Mn Al Nb V Si P S Cr Ti B Sn Ni N Mon Cu % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight 1 0,022 0,19 0,038 0,058 0,001 0,041 0,01 0,0026 0,06 0,002 0,0001 0,006 0,05 0,005 0,02 0,03 2 0,022 0,19 0,038 0,058 0,001 0,041 0,01 0,0026 0,06 0,002 0,0001 0,006 0,05 0,005 0,02 0,03 3 0,026 0,19 0,034 0,042 0,001 0,026 0,005 0,0014 0,041 0,002 0,0001 0,008 0,045 0,0049 0,01 0,036 V4 0,015 0,17 0,03 0,05 0,001 0,043 0,011 0,0037 0,081 0,003 0,0001 0,002 0,053 0,0055 0,016 0,055 V5 0,017 0,19 0,03 0,028 0,001 0,039 0,012 0,0025 0,072 0,003 0,0002 0,004 0,038 0,0045 0,009 0,033 V6 0,03 0,18 0,033 0,025 0,001 0,039 0,01 0,0015 0,058 0,002 0,0001 0,003 0,035 0,0036 0,014 0,043 V7 0,03 0,18 0,033 0,025 0,001 0,039 0,01 0,0015 0,058 0,002 0,0001 0,003 0,035 0,0036 0,014 0,043 8 0,026 0,19 0,03 0,06 0,001 0,04 0,007 0,0033 0,046 0,002 0,0001 0,006 0,039 0,0046 0,006 0,036 9 0,02 0,16 0,038 0,056 0,001 0,03 0,008 0,0009 0,036 0,002 0,0001 0,003 0,038 0,0049 0,008 0,047 10 0,027 0,18 0,031 0,06 0,001 0,051 0,009 0,0026 0,068 0,002 0,0001 0,005 0,06 0,0036 0,011 0,048 V11 0,063 0,71 0,041 0,04 0,001 0,034 0,01 0,001 0,044 0,002 0,0002 0,002 0,034 0,0085 0,007 0,024 V12 0,064 0,69 0,039 0,038 0,001 0,046 0,013 0,0009 0,053 0,002 0,0001 0,003 0,032 0,006 0,006 0,053 V: Comparison sample Table 2 Nr. Hole expansion ratio Rm Rp0.2 A80 A5 Z Age [Cf] x [Rm] Ferrite content T_Ofe n Furnace life WET HT MPa MPa % % % 1 1 % °C min °C °C 1 125% 502 443 29,0 40,0 74,8 0,41 6,53 96 1289 183 918 574 2 119% 502 408 27,1 36,0 82,2 0,41 6,53 96 1284 177 922 535 3 108% 489 407 29,0 43,8 73,9 0,24 9,64 95 1298 188 921 612 V4 118% 452 378 29,0 40,0 74,5 0,51 3,30 95 1256 176 951 708 V5 119% 444 403 29,9 38,2 72,8 0,26 5,59 99 1298 445 890 662 V6 124% 407 352 33,9 45,5 72,0 0,14 10,45 98 1274 171 904 668 V7 120% 438 361 29,9 38,9 72,8 0,14 11,24 97 1271 172 893 672 8 101% 512 461 25,5 32,4 69,6 0,32 9,03 95 1251 167 958 636 9 101% 497 479 25,8 32,2 72,1 0,40 5,98 95 1302 195 929 613 10 108% 531 459 24,7 36,1 75,3 0,32 9,73 96 1304 188 923 614 V11 71% 536 465 19,9 25,2 59,9 0,09 30,64 92 1282 196 890 630 V12 70 % 543 490 24,9 34,1 61,8 0,09 31,75 92 1277 197 891 616 V: Reference sample; bold and underlined parameters are not in accordance with the requirements Table 3 Nr. C Mn Al Nb V Si P S Cr Ti B Sn Ni N Mon Cu % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight % by weight 13 0,029 0,19 0,034 0,076 0,036 0,04 0,007 0,001 0,038 0,002 0 0,002 0,041 0,005 0,012 0,031 14 0,029 0,19 0,034 0,076 0,036 0,04 0,007 0,001 0,038 0,002 0 0,002 0,041 0,005 0,012 0,031 V15 0,091 1,31 0,041 0,041 0,035 0,165 0,011 0,0036 0,058 0,002 0,0003 0,004 0,062 0,006 0,013 0,047 V16 0,094 1,34 0,037 0,042 0,035 0,199 0,011 0,0024 0,041 0,003 0,0002 0,002 0,036 0,0066 0,008 0,036 17 0,03 0,17 0,033 0,073 0,037 0,038 0,01 0,001 0,04 0,002 0 0,002 0,039 0,004 0,008 0,044 18 0,029 0,19 0,034 0,076 0,036 0,04 0,007 0,001 0,038 0,002 0 0,002 0,041 0,005 0,012 0,031 19 0,029 0,19 0,034 0,076 0,036 0,04 0,007 0,001 0,038 0,002 0 0,002 0,041 0,005 0,012 0,031 V20 0,07 1,71 0,026 0,086 0,045 0,025 0,01 0,0048 0,046 0,016 0,0001 0,004 0,029 0,0072 0,008 0,021 V21 0,095 1,33 0,037 0,044 0,037 0,197 0,013 0,0021 0,062 0,004 0,0003 0,004 0,067 0,0062 0,014 0,056 V22 0,097 1,33 0,034 0,044 0,037 0,199 0,01 0,0015 0,042 0,003 0,0002 0,005 0,028 0,0079 0,005 0,031 23 0,049 0,18 0,04 0,084 0,101 0,037 0,011 0,0022 0,039 0,002 0,0002 0,003 0,037 0,0035 0,007 0,043 24 0,049 0,18 0,04 0,084 0,101 0,037 0,011 0,0022 0,039 0,002 0,0002 0,003 0,037 0,0035 0,007 0,043 V25 0,069 1,67 0,037 0,089 0,042 0,081 0,01 0,0013 0,045 0,028 0,0002 0,003 0,055 0,005 0,011 0,033 V: Comparison sample Table 4 Nr. Hole expansion ratio Rm Rp0.2 A80 A5 Z Age [Cf] x [Rm] Ferrite content T_Oven Furnace life WET HT % MPa MPa % % % 1 1 % °C min °C °C 13 130 550 501 24,5 36,0 68,6 0,66 5,47 94 1260 183 957 576 14 103 578 496 23,4 34,4 68,7 0,66 5,74 94 1270 194 944 584 V15 57 624 548 16,5 21,4 49,2 0,16 47,82 89 1251 173 904 655 V16 55 638 565 18,9 24,7 51,8 0,15 50,92 88 1269 188 913 657 17 103 623 581 19,4 25,2 66,0 0,62 7,06 96 1296 176 957 674 18 95 580 517 22,2 26,7 66,3 0,66 5,76 96 1237 145 969 673 19 103 617 555 20,2 26,8 66,3 0,66 6,13 97 1295 188 941 680 V20 53 670 638 21,9 28,7 51,6 0,32 32,01 85 1284 210 913 648 V21 47 646 572 23,3 33 58,7 0,16 51,49 89 1278 265 889 681 V22 62 652 592 19,1 27,3 61,9 0,15 53,64 89 1297 187 89 1 667 23 94 705 608 18,4 24,1 60,9 0,72 9,79 96 1304 222 950 683 24 94 670 590 17,6 22,3 60,3 0,72 9,31 96 1307 223 954 694 V25 59 685 639 15,2 20,0 51,9 0,36 30,25 85 1269 164 917 610 V: Reference sample; bold and underlined parameters are not in accordance with the requirements

Claims

1. Flat steel product, composed of, in weight percent, in short: wt.%: C: 0,010-0,060; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Note: 0,040-0,090; V: up to 0.12; and other optional components: up to 0.30; balance Fe and unavoidable impurities.

2. Flat steel product according to claim 1, composed of, in weight percent, in short: wt.%: C: 0.010-0.040; preferably 0.010-0.030; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Note: 0,040-0,060; Optional components: up to 0.30; balance Fe and unavoidable impurities.

3. Flat steel product according to claim 1, composed of, in weight percent, in short: wt.%: C: 0.010-0.040; preferably 0.010-0.030; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0,020-0,050; Note: 0,054-0,070; Optional components: up to 0.30; balance Fe and unavoidable impurities.

4. Flat steel product according to one of claims 1 to 3, wherein the optional components are composed of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.

100.

5. Flat steel product according to claim 1, composed of, in weight percent, in short: wt.%: C: 0.015-0.050, preferably 0.020-0.040; Mn: 0.150-0.300, preferably 0.150-0.250; Al: 0.020-0.050, preferably 0.025-0.040; Note: 0.065-0.085; preferably 0.070-0.085; V: 0.020-0.060; preferably 0.025-0.050; Optional components: up to 0.30; balance Fe and unavoidable impurities.

6. Flat steel product according to claim 1, composed of, in weight percent, in short: wt.%: C: 0.020-0.060, preferably 0.035-0.055; Mn: 0.150-0.300, preferably 0.150-0.250; Al: 0.020-0.050, preferably 0.025-0.040; Note: 0.070-0.090, preferably 0.075-0.090; V: 0,080-0,12; Optional components: up to 0.30; balance Fe and unavoidable impurities.

7. Steel flat product according to claim 5 or claim 6, wherein the optional components are composed of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.

100.

8. A flat steel product according to any one of claims 2 to 4, having a yield strength Rp0.2 between 380 MPa and 520 MPa, preferably ≤ 510 MPa; and / or a tensile strength Rm of 450-600 MPa, preferably 450-540 MPa; and / or an elongation at break A80 of ≥ 20 percent, preferably ≥ 22 percent; and / or an elongation at break A5 of ≥ 24 percent, preferably ≥ 26 percent; each of the aforementioned values ​​is determined along the rolling direction.

9. A flat steel product according to any one of claims 5 to 7, having a yield strength Rp0.2 between 460 MPa and 670 MPa, particularly preferably ≤ 660 MPa; and / or a tensile strength Rm 520-750 MPa, preferably 520-730 MPa; and / or an elongation at break A80 ≥ 12 percent, preferably ≥ 14 percent, particularly preferably ≥ 20 percent; and / or elongation at break A5 ≥ 22 percent, each of the aforementioned values ​​being determined in each case along the rolling direction.

10. Steel flat product according to one of the preceding claims, comprising fine carbides in a ferritic matrix, wherein the proportion of the ferritic matrix in the structure is at least 90 volume percent, preferably at least 94 volume percent, and the proportion of fine carbides in the structure is at most 2 volume percent.

11. Steel flat product according to one of the preceding claims, having a grain size finer than ASTM 6 according to DIN EN 10149-1:2013-12.

12. Steel flat product according to one of the preceding claims, having a hole expansion capacity ≥ 80 percent, preferably ≥ 90 percent, according to ISO 16630:2017-09, and / or a folding capacity of 180 degrees when bent according to EN ISO 7438:2016-07.

13. Flat steel product according to one of the preceding claims, having a Z-value measuring the reduction in area at fracture according to DIN EN ISO 6892-1:2020-06 of greater than or equal to 50.0%, preferably greater than or equal to 60.0%.

14. Flat steel product according to one of the preceding claims, characterized in that for the product of [Cf] and [Rm]: 15 ≥ Cf × Rm , with the following parameters: [Rm]: the numerical value of the tensile strength of the flat steel product in MPa, [Cf]: the numerical value of a proportion Cf of free carbon in the total content of carbon C in the flat steel product in percent by weight, where the proportion Cf is the proportion of carbon C in the flat steel product less the proportion of carbon bound to Ti, Nb, Mo and V in the total content of carbon C in the flat steel product.

15. Flat steel product according to one of the preceding claims, having a thickness between 1.5 millimeters and 6 millimeters, preferably between 1.7 millimeters and 4 millimeters.

16. A method for producing a flat steel product according to one of claims 1 to 15, comprising the following steps: a) Providing a precursor product consisting of a steel composed of, in weight percent, in short: wt.%: C: 0,010-0,060; Mn: 0.150-0.300; preferably 0.150-0.250; Al: 0.020-0.050; preferably 0.025-0.040; Note: 0,040-0,070; V: up to 0.12; and further optional components: up to 0.30; remainder Fe and unavoidable impurities, whereby the optional components are composed of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100; preferably with a composition according to the alloying specifications mentioned in one of claims 2 to 4; b) heating, preferably in a walking beam furnace, preferably for a furnace time of at least 60 minutes, particularly preferably at least 140 minutes, at a furnace temperature between 1200 degrees Celsius and 1350 degrees Celsius, preferably between 1250 degrees Celsius and 1350 degrees Celsius; c) hot rolling with a final hot rolling temperature of at least 910 degrees Celsius, preferably between 910 degrees Celsius and 970 degrees Celsius; d) coiling at a coiling temperature between 530 degrees Celsius and 670 degrees Celsius, preferably between 530 degrees Celsius and 650 degrees Celsius.

17. A method for producing a flat steel product according to one of claims 1 to 15, comprising the following steps: a) Providing a precursor product consisting of a steel composed of, in weight percent, in short: wt.%: C: 0,015-0,060; Mn: 0,150-0,300; Al: 0,020-0,050; Note: 0,065-0,090; V: 0,020- 0,12; optional components: up to 0.30; balance Fe and unavoidable impurities, whereby the optional components consist of, in weight percent, in short: wt.%: Si: up to 0.060; P: up to 0.0120; S: up to 0.0050; Cr: up to 0.100; Ti: up to 0.010; B: up to 0.0010; Sn: up to 0.0200; No: up to 0.100; N: up to 0.0080; Mon: up to 0.030; Cu: up to 0.100; preferably with a composition according to the alloying specifications mentioned in one of claims 5 to 7; b) heating, preferably in a walking beam furnace, preferably for a furnace time of at least 60 minutes, particularly preferably at least 140 minutes, at a furnace temperature between 1200 degrees Celsius and 1350 degrees Celsius, preferably between 1230 degrees Celsius and 1350 degrees Celsius; c) hot rolling with a final hot rolling temperature of at least 920 degrees Celsius, preferably between 920 degrees Celsius and 970 degrees Celsius; d) coiling at a coiling temperature between 500 degrees Celsius and 730 degrees Celsius, preferably between 560 and 710.

18. Use of a flat steel product according to one of claims 1 to 17 for the production of pipes for hydrogen pipelines and / or hydrogen tanks.

19. Use of a pipe made from a flat steel product according to any one of claims 1 to 17 for conducting and storing hydrogen.

20. Use of a tank made from a flat steel product according to any one of claims 1 to 17 for storing and transporting hydrogen.

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