Method for producing a hot-dip-coated high-strength steel strip, and corresponding hot-dip-coated high-strength steel strip

Abstract

The invention relates to a method for producing a hot-dip-coated high-strength steel strip with an Rm tensile strength of at least 1180 MPa, wherein the method comprises the following steps: (i) providing a rolled steel strip made of a steel having the following composition in % by weight: C: from 0.08 to 0.140; Mn: from 1.70 to 2.75; Cr: from 0.050 to 1.00; Mo: from 0.15 to 0.45; and optionally one or more of the following elements in % by weight: Si: from 0.050 to 1.00; Nb: from 0.010 to 0.100; Ti: from 0.010 to 0.100; V: from 0.001 to 0.100; B:≤ 0.0060, in particular from 0.0001 to 0.0060; N: from 0.0001 to 0.016; Ni: 10 ≤ 0.20; Al: ≤ 0.20; Cu: from 0.01 to 0.30; S: ≤ 0.03; P: ≤ 0.02; the remainder being iron, including customary steel-accompanying elements, where the following applies to the numerical values of the proportions of the elements Mn, Cr, Mo and C in % by weight: [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80, (ii) annealing the rolled steel strip in the course of a continuous hot-dip-coating process by continuous annealing, wherein the annealing is carried out at a maximum temperature between 730°C and 780°C, inclusive, for a time period of 5 s to 1200 s and is carried out in particular in such a way that the proportion of austenite in the microstructure of the steel is at most 70% by volume, (iii) subsequently cooling the steel strip and maintaining the temperature in the temperature range between 200°C and 500°C, inclusive, for 15 to 500 s, (iv) subsequently hot-dip coating the steel strip at a temperature between 380 and 500°C, (v) subsequently finally cooling the hot-dip-coated steel strip at an average cooling rate of 1 K / s to 50 K / s to ambient temperature, wherein, after final cooling, 30 to 60% by volume martensite is present in the microstructure. The invention also relates to a corresponding hot-dip-coated high-strength steel strip.

Description

[0001] Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0002] 1

[0003] Method for producing a hot-dip coated high-strength steel strip and corresponding hot-dip coated high-strength steel strip

[0004] The present invention relates to a method for producing a hot-dip coated high-strength steel strip with a R m -Tensile strength of at least 1180 MPa, the method comprising the following steps: providing a rolled steel strip,

[0005] Annealing of the rolled steel strip as part of a continuous hot-dip coating process by means of continuous annealing, subsequent cooling and hot-dip coating of the steel strip at a temperature between 380 and 500 °C, followed by final cooling of the hot-dip coated steel strip.

[0006] The invention relates not only to this method but also to a corresponding hot-dip coated high-strength steel strip with a tensile strength (R m ) of at least 1180 MPa.

[0007] Sheets made from such hot-dip coated, high-strength steel strips with tensile strengths of 1000 MPa and above are required, for example, for automotive applications. For most of these applications, sufficient cold formability of the sheet is essential. The steel of the actual steel strip or sheet is typically a dual-phase steel (DP steel), meaning it has a dual-phase microstructure (for example, a martensitic-ferritic dual-phase microstructure). According to VDA 239-100, corresponding dual-phase steels are standardized up to a tensile strength of 1350 MPa.

[0008] One challenge in the use of hot-dip coated high-strength steel strips with an R mA tensile strength of 1180 MPa or more often leads to the occurrence of so-called hydrogen-induced cracking. In this process, atomic hydrogen penetrates the material and causes embrittlement / brittle fracture under mechanical stress, resulting in sudden component failure. This phenomenon is particularly critical for high-strength steels > 1000 MPa and restricts the use of high-strength steel grades with increasing strength class. Therefore, it is of fundamental importance to assess the material's resistance to harmful hydrogen embrittlement. Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0009] 2

[0010] (hydrogen-induced cracking).

[0011] It should be briefly mentioned here that when specifying ranges based on limits, such as the limits a and b (i.e., ranges of values), the specification "from a to b" means that the values ​​a and b are included in the range defined by them, while the specification "between a and b" means that they are excluded from the range.

[0012] Document EP 4 317 515 A1 describes a method for producing a hot-dip coated high-strength steel strip with an R m -Tensile strength of at least 980 MPa and a martensitic-ferritic dual-phase structure, the process comprising the following steps:

[0013] Providing a rolled steel strip made of steel with the following composition in wt%: C: 0.05 to 0.10, Si: 0.1 to 0.5, Mn: 1.6 to 2.5, Cr: 0.2 to 0.6, Mo: 0.1 to 0.4, Ti: 0.01 to 0.05, P < 0.015, S < 0.003, Al: 0.0 to 0.05 and optionally one or two of the elements Nb and V, where Cr + Mo + Ti + Nb + V < 0.5, the remainder being iron and unavoidable accompanying elements; annealing the steel strip in a continuous hot-dip coating process by continuous annealing at a maximum temperature in the range of 780 to 820 °C; subsequent cooling of the steel strip to a temperature in the range of 460 °C to 470 °C; subsequent hot-dip coating of the steel strip at this temperature in the range of 460 °C to 470 °C, followed by final cooling of the hot-dip coated steel strip, whereby after final cooling a martensitic-ferritic dual phase structure is present.The heating rate to the maximum temperature during annealing is relatively high ("rapid heat treatment"). The document further describes the resulting hot-dip coated high-strength steel strip, in which the actual steel strip possesses the aforementioned martensitic-ferritic dual-phase structure and which, in some examples, exhibits a R. m -has a tensile strength of 1180 MPa and more.

[0014] The invention is based on the objective of providing a method for producing a hot-dip coated high-strength steel strip and a corresponding hot-dip coated high-strength steel strip, in which the Salzgitter Flachstahl GmbH 27 October 2025 124097WO / 53491

[0015] 3 hot-dip coated steel strips, a total of high R m -Exhibits a tensile strength of at least 1180 MPa with a low tendency to hydrogen embrittlement.

[0016] The problem is solved by the respective subject matter of the independent patent claims. Advantageous further developments of the invention result from the features of the dependent claims.

[0017] The invention thus relates to a method for producing a hot-dip coated high-strength steel strip with an R m -Tensile strength of at least 1180 MPa, particularly in a range of 1180 MPa to 1350 MPa, wherein the method comprises the following steps:

[0018] (i) Providing a rolled steel strip made of a steel having the following composition in wt%: C: from 0.08 to 0.140, preferably 0.10 to 0.122; Mn: from 1.70 to 2.75, preferably 2.00 to 2.40; Cr: from 0.050 to 1.00, preferably 0.20 to 0.40; Mo: from 0.15 to 0.45, preferably 0.15 to 0.35; and optionally one or more of the following elements in wt%: Si: from 0.050 to 1.00, preferably 0.40 to 0.60; Nb: from 0.010 to 0.100, preferably 0.020 to 0.050; Ti: from 0.010 to 0.100, preferably 0.020 to 0.400; V: from 0.001 to 0.100; B: < 0.0060, in particular from 0.0001 to 0.0060; N: from 0.0001 to 0.016; Ni: < 0.20; Al: < 0.20; Cu: from 0.01 to 0.30; S: < 0.03; P: < 0.02; balance iron, including usual steel-associated elements, wherein the numerical values ​​of the proportions of the elements Mn, Cr, Mo and C in weight % are as follows: [M eqU j] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80,

[0019] (ii) Annealing of the rolled steel strip as part of a continuous hot-dip coating process by continuous annealing, wherein the annealing is carried out at a maximum temperature between 730 °C up to and including 780 °C, for a total duration of 5 s to 1200 s (holding time in the said temperature range), in particular from 10 s to 250 s, and in particular such that the proportion of austenite in the microstructure of the steel is a maximum of 70 volume-%,

[0020] (iii) subsequent cooling of the steel strip and holding the temperature in the temperature range between 200 °C and 500 °C inclusive for 15 to 500 s,

[0021] (iv) subsequently hot-dip coating of the steel strip at a temperature between 380 and 500 °C,

[0022] (v) subsequent final cooling of the hot-dip coated steel strip at an average cooling rate of 1 K / s to 50 K / s to ambient temperature, whereby after final cooling 30 to 60 volume% martensite (fresh and / or self Salzgitter Flachstahl GmbH 27 October 2025 124097WO / 53491

[0023] 4. (set to dry) is present in the structure and

[0024] (vi) optional post-treatment of the hot-dip coated steel strip in the form of a dressing and / or stretch bending and / or a heat treatment at a maximum temperature of < 400 °C.

[0025] Through the specified composition of the steel for the steel strip (steel chemistry) in combination with a corresponding manufacturing process in which the annealing temperature is limited to a maximum of 780 °C within a continuous hot-dip coating process, a hot-dip coated high-strength steel strip with an R can be produced. mA tensile strength > 1180 MPa is provided, which is highly resistant to H2 embrittlement (hydrogen-induced stress corrosion cracking). Annealing is carried out, in particular, at a maximum temperature between 730 °C and 780 °C. In the finished hot-dip coated high-strength steel strip, this is achieved through a specific combination of the aforementioned steel chemistry with precisely controlled phase fractions of the microstructure. In such a hot-dip coated high-strength steel strip, the steel of the actual strip preferably exhibits a microstructure with a first main phase of 30–60 volume% fresh and / or self-tempered martensite and a second main phase of ferrite and / or bainitic ferrite with a total of 30–60 volume%, as well as optionally a residual microstructure consisting of pearlite, retained austenite, and other microstructural components, wherein the total retained austenite content is < 5 volume%.The other structural components include, in particular, one or more of the following: lower bainite, carbide-free bainite, MA phase, non-self-tempered martensite (single or multiple tempering), carbides, sulfides, oxides and nitrides, and their mixed phases.

[0026] The steel thus exhibits a dual- or multi-phase microstructure. Due to the relatively low annealing temperature, microstructures of the supplied steel strip are partially retained and present in the hot-dip coated high-strength steel strip. The second main phase of the steel, in particular, contains a proportion of recovered but solid "old ferrite" (voreutectoid and / or bainitic) from the microstructure of the supplied steel strip, thereby enabling high resistance to hydrogen-induced cracking (hydrogen resistance). Salzgitter Flachstahl GmbH, October 27, 2025

[0027] 124097WO / 53491

[0028] 5. The relatively low annealing temperature prevents the complete microstructure from reforming through conversion to austenite and subsequent re-transformation. During austenitization and microstructure reformation, so-called former austenite grain boundaries are formed, which are particularly susceptible to hydrogen embrittlement. Since a complete re-transformation is avoided in the present steel strip in favor of incorporating a portion of the microstructure from the rolled state, the network of former austenite grain boundaries does not extend continuously through the final microstructure. This improves resistance to hydrogen embrittlement and limits potential crack propagation. A common limitation of utilizing a microstructure component from the old, rolled state is an overall lower strength, as the old components ("old ferrite") are annealed.This is prevented in the steel according to the invention by the specific selection of alloying elements. In particular, molybdenum (Mo) delays the recovery kinetics, so that the microstructural component from the old rolled structure does not completely lose its roll-hard strength. The microstructure thus created also has a high density of so-called subgrain boundaries / low-angle grain boundaries, i.e., a pronounced substructure of the grains, which further increases the strength of the softer phase. Overall, this results in a high-strength steel strip with high resistance to hydrogen-induced stress corrosion cracking.

[0029] The microstructural constituents were determined in longitudinal sections perpendicular to the rolling surface by means of electron backscatter diffraction measurements using Kikuchi band contrast and light-optical imaging. The grain structure was also determined from the electron backscatter diffraction measurements, whereby a grain is defined by having a grain boundary with a disorientation angle of > 15° (so-called high-angle grain boundary - HGPP, see G. Gottstein, Physikalische Grundlagen der Materialskunde [Physical Foundations of Materials Science], Springer-Verlag Berlin Heidelberg, 2007).

[0030] In the aforementioned microstructure of the steel of the actual steel strip, the grains bounded by high-angle grain boundaries with a disorientation angle of > 15° contain a volume fraction of grains in which the longitudinal axis of an ellipse of a respective grain is perpendicular to the transverse direction (area spanned by sheet or strip normal and rolling direction). Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0031] 6. The grains must be oriented within 10° of the rolling direction and comprise at least 50% of the total volume of all grains. The grain orientation results from the fact that a portion of the rolled structure is inherited in the final structure, and in the rolled structure itself, the grains are stretched by the rolling process so that their longitudinal axis aligns parallel to the rolling direction. The longitudinal axis of a grain corresponds to the long axis of an ellipse, which, according to the article "KF Mulchrone, K. Roy Choudhury: 'Fitting an ellipse to an arbitrary shape: implications for strain analysis'; Journal of Structural Geology 26 (2004) 143-153", can be adapted to any grain shape with regard to its properties during passive deformation. Thus, the ellipse describes the two-dimensional (2D) grain shape in the plane of the image, which is defined by the rolling direction and the sheet normal.The orientation of the grain with respect to the rolling direction can be described via the longitudinal axis of this ellipse.

[0032] In particular, it is provided that heating to the maximum temperature during annealing takes place in one or more stages, and that the heating rate is generally, or at least in the temperature range from 550°C up to the maximum temperature during annealing, in the range of 0.1 K / s to 28 K / s. Unlike the "rapid heat treatment" known from EP 4 317 515 A1, the present concept can therefore also utilize significantly lower heating rates.

[0033] Regarding the composition of the steel, the optional proportion of element B is preferably in the range of < 0.0005 wt%. In other words, no boron is added, and the stated boron content of < 5 ppm can de facto be considered part of the usual elements found in steel.

[0034] According to a preferred embodiment of the method according to the invention, the steel strip provided is a cold- or hot-rolled steel strip, the production of which the method comprises the following steps:

[0035] (a) Producing a hot-rolled steel strip from steel of the aforementioned composition,

[0036] (b) optional pickling of the hot-rolled steel strip,

[0037] (c) optional cold rolling of the hot-rolled steel strip to the cold-rolled steel strip with a reduction of 10% - 60% and

[0038] (d) Optional hood annealing of the cold- or hot-rolled steel strip at Salzgitter Flachstahl GmbH, October 27, 2025

[0039] 124097WO / 53491

[0040] 7. Maximum holding temperature between 400 and 700 °C for an annealing period of 12 hours to 6 days, the annealing period including the time for heating and cooling to room temperature, and the hood annealing of the hot-rolled steel strip is carried out, in particular, before the cold rolling of the hot-rolled steel strip. Hood annealing of the hot-rolled steel strip can, on the one hand, reduce the strength for the subsequent cold rolling and thus improve rollability, and on the other hand, the strength of the final product (hot-dip coated high-strength steel strip) can be increased by targeted fine precipitates. It is specifically intended that the hot-rolled steel strip is produced by hot rolling from a steel slab heated to a temperature of 1100 °C to 1300 °C.Preferably, the hot-rolled steel strip produced in this way is subsequently coiled at a temperature > 300 °C, preferably between 300 °C and 650 °C, and particularly between 300 °C and 580 °C (coiling temperature). Coiling at a coiling temperature above 300 °C ensures good rollability during the subsequent optional cold rolling. On the other hand, if the coiling temperature is too high, important microalloying elements can precipitate too coarsely, thus negating their beneficial effect on the hot-dip coated high-strength steel strip.

[0041] According to a further preferred embodiment of the process according to the invention, the hot-rolled steel strip has a microstructure consisting of more than 30 volume% bainitic ferrite as the main phase and a residual microstructure of ferrite, martensite, retained austenite, and less than 5 volume% pearlite. The aforementioned "old ferrite" originates from the microstructure of the hot-rolled steel strip. The proportion of recovered but solid "old ferrite" in the microstructure of the hot-dip coated high-strength steel strip enables—as already mentioned—the high hydrogen resistance. The (coil) temperature after hot rolling should, in particular, be < 650 °C to avoid excessive pre-eutectoid ferrite in the hot-rolled steel strip and to reliably achieve the aforementioned microstructure of the hot-rolled steel strip.

[0042] According to yet another preferred embodiment of the inventive method, the hot-dip coating produced on the steel strip is zinc- or aluminum-based. The zinc-based coating is [Salzgitter Flachstahl GmbH, October 27, 2025].

[0043] 124097WO / 53491

[0044] 8

[0045] The hot-dip coating is preferably a zinc or a zinc-magnesium or a zinc-magnesium-aluminium hot-dip coating, and the aluminum-based hot-dip coating is an aluminum-silicon or an aluminum-zinc-silicon hot-dip coating.

[0046] The invention further relates to a hot-dip coated high-strength steel strip with an R m-Tensile strength of at least 1180 MPa, particularly in the range of 1180 MPa to 1350 MPa, preferably produced by the aforementioned method, with the actual rolled steel strip and a hot-dip coating on this steel strip, wherein the steel of the steel strip has the following composition in wt.%: C: from 0.08 to 0.140, preferably 0.10 to 0.122; Mn: from 1.70 to 2.75, preferably 2.00 to 2.40; Cr: from 0.050 to 1.00, preferably 0.20 to 0.40; Mo: from 0.15 to 0.45, preferably < 0.35; and optionally one or more of the following elements in wt.%: Si: from 0.050 to 1.00, preferably 0.40 to 0.60; Nb: from 0.010 to 0.100, preferably 0.020 to 0.050; Ti: from 0.010 to 0.100, preferably 0.020 to 0.400; V: from 0.001 to 0.100; B: < 0.0060, in particular from 0.0001 to 0.0060, preferably < 0.0005 wt%; N: from 0.0001 to 0.016; Ni: < 0.20; Al: < 0.20; Cu: from 0.01 to 0.3; S: < 0.03; P: < 0.02, balance iron.including usual steel-associated elements, wherein the numerical values ​​of the proportions of the elements Mn, Cr, Mo and C in weight % are: [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80, wherein the steel of the steel strip has a microstructure with a first main phase of 30 - 60 volume % fresh and / or self-tempered martensite and a second main phase of ferrite and / or bainitic ferrite with a total of 30 - 60 volume %, as well as a residual microstructure consisting of pearlite, retained austenite and optionally other microstructural components and a total retained austenite content < 5 volume %, with grains bounded by high-angle grain boundaries with a disorientation angle of > 15°, wherein a volume fraction of these grains, in which in a plane spanned by the strip normal and the rolling direction (i.e. the strip longitudinal extent) the longitudinal axis of the respective grain is aligned within 10° to the rolling direction (strip longitudinal extension),constitutes at least 50% by volume of the total volume of all grains.

[0047] It is specifically intended that the large-angle grain boundaries will be defined by Salzgitter Flachstahl GmbH on October 27, 2025, 124097WO / 53491

[0048] Nine limited grains of the body-centered cubic and / or tetragonally distorted cubic phase(s) exhibit a substructure defined by small-angle grain boundaries with a disorientation angle between 4° and 15°, and for these grain boundaries, the ratio AS / AL of the area fraction of the small-angle grain boundaries AS to the area fraction of the large-angle grain boundaries AL is greater than 0.40, i.e., AS / AL > 0.40. For the determination of AL, all large-angle grain boundaries with a disorientation angle > 15° in the microstructure of the body-centered cubic and / or tetragonally distorted cubic phase(s) are considered. The disorientation angle is defined as the smallest rotation angle of all crystallographically equivalent misorientations.

[0049] According to a preferred embodiment of the hot-dip coated high-strength steel strip according to the invention, the steel strip has an R p 0.2 yield strength greater than 700 MPa and / or an elongation at break > 6.

[0050] According to a further preferred embodiment of the hot-dip coated high-strength steel strip according to the invention, the hot-dip coating is a zinc- or aluminum-based hot-dip coating. As already mentioned, the zinc-based hot-dip coating is preferably a zinc- or a zinc-magnesium- or a zinc-magnesium-aluminum hot-dip coating, and the aluminum-based hot-dip coating is an aluminum-silicon or an aluminum-zinc-silicon hot-dip coating.

[0051] Preferably, the hot-dip coated high-strength steel strip exhibits such high resistance to hydrogen-induced cracking that, in at least five test specimens made from the steel strip with a laser-cut central hole, each of these test specimens passes a load test of at least 85% in a step test according to / analogous to the standard VDA 238-201.

[0052] Finally, in the case of hot-dip coated high-strength steel strip, the adhesion quality of the hot-dip coating to the steel strip is particularly high, corresponding to grade 2 or better in the ball impact test according to SEP 1931.

[0053] The effect of the elements in the high-strength steel according to the invention is described below. Salzgitter Flachstahl GmbH, October 27, 2025

[0054] 124097WO / 53491

[0055] 10

[0056] Steel band described in more detail. Accompanying elements are unavoidable and their effects are considered in the analysis concept where necessary.

[0057] Trace elements are elements that are already present in iron ore or enter the steel during the manufacturing process. Due to the undesirable effects of these elements, attempts are made to remove them to a tolerable level or to convert them into less harmful forms.

[0058] Nitrogen (N) is an accompanying element in steel production. Steels containing free nitrogen are prone to severe aging. Even at low temperatures, nitrogen diffuses across dislocations and blocks them. This results in an increase in strength coupled with a rapid loss of toughness. The nitrogen can be bound in the form of nitrides by adding aluminum or titanium. For the aforementioned reasons, the optional nitrogen content is limited to < 0.016 wt% or to amounts unavoidable in steel production.

[0059] Phosphorus (P) is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness and improves hardenability through solid solution strengthening. However, attempts are generally made to reduce the phosphorus content as much as possible, since, among other things, its low diffusion rate makes it highly prone to segregation and significantly reduces toughness. The deposition of phosphorus at the grain boundaries leads to grain boundary fractures. Furthermore, phosphorus raises the transition temperature from tough to brittle behavior to up to 300 °C. During hot rolling, near-surface phosphorus oxides at the grain boundaries can cause fracture cracking. The negative effects of phosphorus can be partially compensated for by adding small amounts of boron. It is believed that boron increases grain boundary cohesion and reduces phosphorus segregation at the grain boundaries.However, in some steels, phosphorus is used in small quantities (< 0.1%) as a microalloying element due to its low cost and significant strength increase, for example in higher-strength IF (interstitial-free) steels. For the aforementioned reasons, the phosphorus content is limited to < 0.02% or to amounts unavoidable during steel production.

[0060] Sulfur (S), like phosphorus, is bound as a trace element in iron ore. It is found in Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0061] 11

[0062] Steel containing sulfur is undesirable (except for free-cutting steels) because it is prone to strong segregation and has a highly embrittlement effect. Therefore, every effort is made to minimize the amount of sulfur in the melt (e.g., through deep vacuum treatment). Furthermore, any sulfur present is converted into the relatively harmless compound manganese sulfide (MnS) by adding manganese. During the rolling process, the manganese sulfides are often rolled out in rows and act as nucleation sites for the transformation. This leads, especially with diffusion-controlled transformation, to a row-like microstructure and, if the row structure is very pronounced, can result in impaired mechanical properties (e.g., prominent martensite fragments instead of distributed martensite islands, anisotropic material behavior, reduced elongation at break). For the aforementioned reasons, the sulfur content is limited to < 0.03% by weight.

[0063] Alloying elements are typically added to steel to selectively influence specific properties. However, a single alloying element can affect different properties in different types of steel. The relationships are multifaceted and complex. The following section will examine the effects of alloying elements in more detail.

[0064] Carbon (C) is considered the most important alloying element in steel. Its targeted addition (up to 2.06%) is what transforms iron into steel. Often, the carbon content is drastically reduced during steel production. Due to its relatively small atomic radius, carbon is dissolved interstitially within the iron lattice. Its solubility is a maximum of 0.02% in α-iron and a maximum of 2.06% in γ-iron. In dissolved form, carbon significantly increases the hardenability of steel. This differing solubility necessitates pronounced diffusion processes during phase transitions, which can lead to very different kinetic conditions. Furthermore, carbon increases the thermodynamic stability of austenite, which is reflected in the phase diagram as an extension of the austenite region to lower temperatures and allows for the stabilization of higher retained austenite concentrations at room temperature within the microstructure.With increasing dissolved carbon content in martensite, the lattice distortions increase, and consequently, so does the strength of the diffusion-free phase. To ensure sufficient strength and retained austenite content, the minimum carbon content is therefore set at 0.08 wt%. Since excessively high carbon contents lead to [a situation where...] Salzgitter Flachstahl GmbH, October 27, 2025.

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[0066] 12

[0067] Since the retained austenite phase is stabilized, which impairs hydrogen resistance, the maximum carbon content in the steel according to the invention is limited to 0.140 wt%. Excessively high carbon contents also typically prove detrimental to weldability and liquid metal embrittlement. Therefore, the carbon content is limited to values ​​of 0.08 to 0.140 wt%, preferably 0.10 to 0.122 wt%.

[0068] Aluminum (Al) is typically alloyed with steel to bind the oxygen and nitrogen dissolved in the iron. This process converts the oxygen and nitrogen into aluminum oxides and aluminum nitrides. These precipitates can refine the grain structure by increasing the number of nucleation sites, thereby improving toughness and strength. In its dissolved state, aluminum, like silicon, delays ferrite formation, thus enabling the formation of sufficient amounts of ferrite. It also suppresses carbide formation, resulting in a delayed transformation of austenite. For this reason, Al is also used as an alloying element in retained austenite steels to replace some of the silicon with aluminum. This approach is based on the fact that Al is less critical to the zinc plating reaction than silicon. However, Al can negatively impact hot ductility and castability in continuous casting.Al also causes an undesirable increase in the Ac3 transformation temperature. The optional Al content is therefore limited to 0.20 wt%, particularly to avoid a retained austenite phase, which negatively affects hydrogen resistance.

[0069] Silicon (Si) increases the strength and yield strength of ferrite through solid solution strengthening, with only a slight decrease in elongation at break. Another important effect is that silicon delays ferrite formation, thus enabling its formation before quenching. Ferrite formation enriches and stabilizes the austenite with carbon. At higher concentrations, silicon significantly stabilizes the austenite in the lower temperature range, particularly in the bainite formation range, by preventing carbide formation. During hot rolling, highly adhering scale can form at high silicon concentrations, which can impair further processing. In continuous zinc plating, silicon can diffuse to the surface during annealing and form film-like oxides, either alone or together with manganese. Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0070] 13

[0071] Oxides impair coating properties by interfering with the hot-dip reaction (iron dissolution and inhibitory layer formation) when the steel strip is immersed in the (zinc) melt. This manifests as poor adhesion and uncoated areas. However, good coating properties of the steel strip and good coating adhesion can be ensured through appropriate furnace operation with an adjusted moisture content in the annealing gas and / or a low Si / Mn ratio and / or the use of moderate amounts of silicon. Si is an optional alloying element. For the aforementioned reasons, the minimum Si content is set at 0.050 wt% and the maximum Si content at 1.00 wt%. An optimum is achieved with a Si content limited to values ​​of 0.40 to 0.60 wt% to avoid reducing hydrogen resistance due to retained austenite.

[0072] Manganese (Mn) is added to almost all steels for desulfurization, converting the harmful sulfur into manganese sulfides. Furthermore, manganese increases the strength of the ferrite through solid solution strengthening and delays the transformation to lower temperatures. A primary reason for alloying with manganese is the significant improvement in hardenability. Due to the inhibition of diffusion, the pearlite and bainite transformations are delayed for longer periods, and the martensite initiation temperature is lowered. Like silicon, manganese tends to form oxides on the steel surface during annealing. Depending on the annealing parameters and the contents of other alloying elements (especially Si and Al), manganese oxides (e.g., MnO) and / or mixed Mn oxides (e.g., sulfur-133) can form.

[0073] Mn₂SiO₄). However, manganese is considered less critical at low Si / Mn or Al / Mn ratios, as globular oxides rather than oxide films tend to form. Nevertheless, high manganese contents can negatively affect the appearance of the zinc layer and zinc adhesion. Therefore, the Mn content is specified as 1.70 wt% to 2.75 wt%, preferably 2.00 wt% to 2.40 wt%.

[0074] Chromium (Cr): The addition of chromium primarily improves hardenability. In its dissolved state, chromium delays the pearlite and bainite transformations, thereby lowering the martensite start temperature. Another important effect is that chromium significantly increases tempering resistance, resulting in almost no loss of strength in the zinc bath. Chromium is also a Salzgitter Flachstahl GmbH 27 October 2025 124097WO / 53491

[0075] 14

[0076] Carbide former. If chromium is present in carbide form, the austenitizing temperature before hardening must be high enough to dissolve the chromium carbides. Otherwise, the increased nucleation rate can impair hardenability. Chromium also tends to form oxides on the steel surface during annealing, which can reduce the quality of the zinc plating. Therefore, the Cr content is specified at values ​​of 0.050 to 1.00 wt%, preferably 0.20 to 0.40 wt%.

[0077] Molybdenum (Mo): The addition of molybdenum is similar to that of chromium to improve hardenability. The pearlite and bainite transformations are delayed, and the martensite start temperature is lowered. Molybdenum also significantly increases tempering resistance, so no strength loss is expected in the zinc bath, and increases the strength of the ferrite through solid solution strengthening. The Mo content is added depending on the dimensions, plant configuration, and microstructure. By slowing down carbon diffusion, Mo can also counteract the enrichment of carbon in retained austenite. High Mo contents also lead to high strength of the hot-rolled strip, which negatively affects cold rolling. For these reasons, the Mo content is set at values ​​of 0.15 to 0.45 wt%, preferably 0.15 to 0.35 wt%, to avoid excessive recovery and softening of the old ferrite.

[0078] Copper (Cu): The addition of copper can increase tensile strength and hardenability. In combination with nickel, chromium, and phosphorus, copper can form a protective oxide layer on the surface, which can significantly reduce the corrosion rate. In combination with oxygen, copper can form harmful oxides at the grain boundaries, which can have particularly negative effects on hot forming processes. The optional copper content is therefore limited to 0.01 to 0.3 wt%.

[0079] Nickel (Ni): Nickel can increase tensile strength and hardenability. However, in combination with oxygen, nickel can form harmful oxides at the grain boundaries, which can have particularly negative effects on hot forming processes. The optional nickel content is therefore limited to 0.20% by weight. Salzgitter Flachstahl GmbH, October 27, 2025

[0080] 124097WO / 53491

[0081] 15

[0082] Microalloying elements are typically added in very small quantities (< 0.1%). Unlike alloying elements, they primarily act through precipitation, but can also influence properties in solution. Despite their small additions, microalloying elements significantly affect the manufacturing conditions as well as the processing and final properties. Carbide and nitride formers soluble in the iron lattice are generally used as microalloying elements. The formation of carbonitrides is also possible due to the complete intersolubility of nitrides and carbides. The tendency to form oxides and sulfides is usually most pronounced in microalloying elements, but this is typically prevented by other alloying elements. This property can be used to advantage by binding the generally detrimental elements sulfur and oxygen.However, setting can also have negative consequences if it results in insufficient microalloying elements being available for carbide formation. Typical microalloying elements are vanadium, titanium, niobium, and boron. These elements can dissolve in the iron lattice and form carbides or nitrides with carbon and nitrogen.

[0083] Niobium (Nb) typically causes significant grain refinement, as it is the most effective of all microalloying elements in delaying recrystallization and also inhibiting austenite grain growth. Another effect of niobium is the retardation of the α / γ transformation and the lowering of the martensite start temperature in the dissolved state. In principle, the addition of niobium is limited to reaching its solubility limit. While this limit restricts the amount of precipitates, exceeding it primarily results in early precipitation with relatively coarse particles. Precipitation hardening can therefore be particularly effective in steels with low carbon content (greater supersaturation is possible) and in hot forming processes (deformation-induced precipitation). The optional Nb content is therefore limited to values ​​of 0.010 to 0.100 wt% and preferably has values ​​of 0.020 to 0.050 wt% to inhibit the recovery and recrystallization of the old ferrite.

[0084] Titanium (Ti) forms very stable nitrides (TiN) and sulfides even at high temperatures. Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0085] 16

[0086] (TiS₂). Depending on the nitrogen content, some of these only dissolve in the melt. If the resulting precipitates are not removed with the slag, they form coarse particles in the material due to the high formation temperature, which are generally detrimental to the mechanical properties. A positive effect on toughness results from the binding of free nitrogen and oxygen. Titanium thus protects other dissolved microalloying elements such as niobium from being bound by nitrogen. These can then exert their optimal effect. Unbound titanium forms titanium carbides at temperatures above 1150 °C and can thus cause grain refinement (inhibition of austenite grain growth, grain refinement through delayed recrystallization and / or an increase in the nucleation number during α / γ conversion) as well as precipitation hardening.The optional Ti content is therefore limited to values ​​of 0.010 to 0.100 wt% and preferably has values ​​of 0.020 to 0.400 wt%.

[0087] Vanadium (V): The formation of carbides and nitrides of vanadium only begins at temperatures around 1000 °C or even after the α / γ transformation, thus considerably later than with titanium and niobium. Due to the small number of precipitates present in austenite, vanadium has virtually no grain-refining effect. Austenite grain growth is also not inhibited by the late precipitation of vanadium carbides. Therefore, the strength-enhancing effect is based almost entirely on precipitation hardening. In solution, vanadium also acts as a transformation retarder. An advantage of vanadium is its high solubility in austenite and the large volume fraction of fine precipitates resulting from the low precipitation temperature. The optional vanadium content is therefore limited to values ​​of 0.001 to 0.100 wt%.

[0088] Boron (B) forms nitrides and carbides with nitrogen as well as with carbon; however, this is generally not desirable. Firstly, due to its low solubility, only a small amount of precipitates forms, and secondly, these are mostly deposited at the grain boundaries. An increase in surface hardness is not achieved (except in boronizing, which forms FeB and Fe₂B in the surface layer of a workpiece). To prevent nitride formation, attempts are usually made to bind the nitrogen with elements of higher affinity. Titanium, in particular, can ensure the binding of all nitrogen. In dissolved form, boron, even in very small quantities, leads to a significant improvement in hardenability. Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0089] 17

[0090] The mechanism of action of boron can be described as follows: under suitable temperature control, boron atoms attach themselves to the grain boundaries and, by lowering the grain boundary energy, significantly hinder the formation of viable ferrite nuclei. During temperature control, it is important to ensure that boron is predominantly atomically distributed within the grain boundary and not present as precipitates due to excessively high temperatures. The effectiveness of boron decreases with increasing grain size and increasing carbon content (> 0.8%). A concentration above 60 ppm also leads to decreased hardenability, as boron carbides act as nuclei at the grain boundaries. Due to its small atomic diameter, boron diffuses exceptionally well and has a very high affinity for oxygen, which can lead to a reduction in boron content in areas near the surface (down to 0.5 mm). In this context, annealing above 1000 °C is not recommended.This is also recommended because boron can lead to significant grain coarsening at annealing temperatures above 1000 °C. Boron is an extremely critical element in the continuous hot-dip zinc plating process, as even minute quantities, alone or in combination with manganese, can form film-like oxides on the steel surface during annealing. These oxides passivate the strip surface and prevent the zinc plating reaction (iron dissolution and inhibitory layer formation). Whether film-like oxides form depends on both the amount of free boron and manganese and the annealing parameters used (e.g., moisture content in the annealing gas, annealing temperature, annealing time). Higher manganese contents and longer annealing times tend to result in globular and less critical oxides. Furthermore, increasing the moisture content in the annealing gas can reduce the amount of boron-containing oxides on the steel surface.For the aforementioned reasons, the optional B content is kept as low as possible and limited to values ​​< 0.0060 wt% or from 0.0001 to 0.0060 wt%, but preferably to values ​​up to < 0.0005 wt%.

[0091] The numerical values ​​of the proportions of the elements Mn, Cr, Mo and C in weight % are: [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80,

[0092] A value [Mequi] > 6.80 is required on the one hand to achieve a strength > 1180 MPa in the final product and on the other hand to ensure a sufficient bainite content in the steel of the originally hot-rolled steel strip.

[0093] The following describes the features and advantages of the present invention with reference to Salzgitter Flachstahl GmbH, October 27, 2025.

[0094] 124097WO / 53491

[0095] The following 18 examples, with reference to the accompanying drawings and tables, illustrate this. They show:

[0096] Figure 1 shows the microstructure of the actual steel strip in a hot-dip coated high-strength steel strip according to a preferred embodiment of the invention in an electron micrograph.

[0097] Figure 2 is a graphic representation illustrating how, in the case of grains of the structure - spanned in the plane by sheet normal and rolling direction (RD) - the longitudinal axis of a respective grain is determined and

[0098] Figure 3 Large angle grain boundaries (HGB in black) and small angle grain boundaries (LGB in grey) in the microstructure of hot-dip coated high-strength steel strips with two counterexamples a) and b) and an embodiment according to the invention c).

[0099] Figures 1 and 3 show the microstructure of the actual steel strips in selected examples of hot-dip coated high-strength steel strips, and Figure 2 shows examples of fitting ellipses E to the grain contour of a microstructure determined by grain boundaries GB to determine the longitudinal axis LA of the grains in order to ascertain their orientation relative to a rolling direction RD in a plane spanned by the strip normal and the rolling direction RD. Tables 1 to 3 document the chemical composition, manufacturing parameters, and properties such as tensile strength R. m , yield strength R p 0,2; Elongation at break Aso as well as microstructure properties of embodiments according to the invention but also of counterexamples.

[0100] Figure 1 shows the microstructure of the actual steel strip in a microscopic image. The contrast between a hard martensitic phase (Phase B) and a softer phase with substructure (Phase A) is clearly visible. Figure 3 shows high-angle grain boundaries (HGB - black) and low-angle grain boundaries (LGB - gray) in the microstructure for selected examples from Tables 2 and 3, namely for cycle steel 7-E (Figure 3a) and 8-F (Figure 3b) as Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0101] 19

[0102] Reference and 5-C according to the invention with a high proportion of low-angle grain boundaries (Figure 3c).

[0103] Table 1 shows the chemical composition of 10 rolled steel samples A to J, of which three steel samples, namely samples C, G and I, exhibit a composition according to the invention. For samples A, B, D, E, F and H, the numerical value [M eqUj] too low. Steel sample F also has a C content that is too high and a Mo content that is too low.

[0104] The cold-rolled steel samples were then treated as follows according to Table 2:

[0105] 1. Heating to a glowing / soaking temperature (Tsoak) at a certain heating rate,

[0106] 2. Annealing of the respective steel sample as part of a continuous process

[0107] Hot-dip coating process via continuous annealing (annealing / soaking temperature Tsoak, soaking time),

[0108] 3. Subsequent cooling of the steel strip and maintenance of the temperature within a suitable temperature range (cooling rate, cooling stop temperature T). q ),

[0109] 4. Subsequently, hot-dip coating of the steel strip at a temperature between 380 and 500 °C (zinc bath temperature),

[0110] 5. Subsequently, final cooling of the hot-dip coated steel strip. The aim of this treatment is to produce a hot-dip coated high-strength steel strip with a specific R-value. m -To produce a tensile strength of at least 1180 MPa.

[0111] For the combinations of steel samples C, G, and I with a maximum annealing temperature in the range of 730 °C to 780 °C (cycles 5, 9, 10, 11, and 13), a corresponding tensile strength was achieved, as shown in Table 2. For almost all other combinations (cycles 1 to 4, 6, 7, and 12), a lower tensile strength was obtained.

[0112] The percentage values ​​given for the microstructural constituents and the proportions of grains with a specific long-axis orientation were determined in longitudinal sections perpendicular to the rolled surface and refer to area fractions (area defined by the sheet or strip normal and the rolling direction), which are also commonly expressed as volume fractions. Furthermore, these values ​​refer to the percentage position over thickness. The retained austenite content can be measured using a magnetic induction method with a magnetizing yoke. Alternatively, Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0113] 20. The proportion of retained austenite can also be determined by X-ray diffraction or electron backscatter diffraction (EBSD) on electropolished samples. Using EBSD measurements in longitudinal sections perpendicular to the rolled surface (area defined by sheet normal and rolling direction), the lengths or length fractions of small-angle and large-angle grain boundaries were determined, which are equivalent to the area fractions.

[0114] To determine the technological characteristics, tensile tests were carried out in accordance with DIN EN ISO 6892-1 :2020-06 on specimens with a gauge length of 80 mm, which were taken longitudinally to the rolling direction.

[0115] Table 3 documents the microstructure properties, the respective volume fraction of the grains with a longitudinal axis in the angle range of a maximum of 10° to the rolling direction, the ratio AS / AL of the area fraction AS of the small-angle grain boundaries to the area fraction AL of the large-angle grain boundaries (compare Figure 3). The steels C and I treated by cycles 5 and 13 are embodiments according to the invention, and the steels D to F and H in combination with the corresponding cycles 6 to 8 and 12 are counterexamples.

[0116] To assess resistance to hydrogen-induced cracking (H2 resistance), step tests were conducted according to the procedure described in VDA 238-201, with the central hole in the test specimens being produced using a laser. The load level refers to the minimum load level passed by all test specimens in a test series of at least five samples. While steel C from cycle 5 exhibits such high resistance to hydrogen-induced cracking that each of the five test specimens produced from the corresponding steel strip passed a load level of 90% in a step test according to VDA 238-201, for each of the steels D to F from cycles 6 to 8, each of the five test specimens produced from the corresponding steel strip passed a load level of only 75% to 80% in a step test according to VDA 238-201.

[0117]

[0118] [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C]

[0119] Table 1

[0120] Salzgitter Flachstahl GmbH, October 27, 2025

[0121] 124097 WO / 53491

[0122]

[0123] Table 2

[0124] Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491

[0125] 24

[0126] 'except for soft tempered martensite, for grain boundaries of the body-centered cubic and / or tetragonally distorted cubic phase(s)'

[0127] Table 3

Claims

Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491 25 Patent claims 1. Method for producing a hot-dip coated high-strength steel strip with a R m -Tensile strength of at least 1180 MPa, particularly in a range of 1180 MPa to 1350 MPa, wherein the method comprises the following steps: Providing a rolled steel strip made from a steel with the following composition in weight %: C: from 0.08 to 0.140, preferably 0.10 to 0.122 Mn: from 1.70 to 2.75, preferably 2.00 to 2.40 Cr: from 0.050 to 1.00, preferably 0.20 to 0.40 Mo: from 0.15 to 0.45, preferably 0.15 to 0.35 and optionally one or more of the following elements in weight %: Si: from 0.050 to 1.00, preferably 0.40 to 0.60 Note: from 0.010 to 0.100, preferably 0.020 to 0.050 Ti: from 0.010 to 0.100, preferably 0.020 to 0.400 V: from 0.001 to 0.100 B: < 0.0060, especially from 0.0001 to 0.0060 N: from 0.0001 to 0.016 Ni: < 0.20 AI: < 0.20 Cu: from 0.01 to 0.30 S: < 0.03 P: < 0.02 Residual iron, including usual steel-associated elements, where the numerical values ​​of the proportions of the elements Mn, Cr, Mo and C in weight % are as follows: [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80, Annealing of the rolled steel strip as part of a continuous hot-dip coating process by means of continuous annealing, wherein the annealing takes place at a maximum temperature between 730 °C and 780 °C inclusive for a duration of 5 s to 1200 s, preferably from 10 s to 250 s, and in particular such that the austenite content in the microstructure of the steel is a maximum of 70 volume%, subsequent cooling of the steel strip and holding of the temperature in a temperature range between 200 °C and 500 °C inclusive for 15 to 500 s, followed by hot-dip coating of the steel strip at a temperature between 380 and 500 °C. Salzgitter Flachstahl GmbH, October 27, 2025 124097WO / 53491 26 Subsequently, final cooling of the hot-dip coated steel strip with an average cooling rate of 1 K / s to 50 K / s to ambient temperature, whereby after final cooling 30 to 60 volume% martensite (fresh and / or self-tempered) is present in the microstructure and optional post-treatment of the hot-dip coated steel strip in the form of a dressing and / or stretch bending and / or a heat treatment at a maximum temperature of less than 400 °C.

2. The method according to claim 1, wherein heating to the maximum temperature during annealing takes place in one or more stages and the heating rate is generally or at least in the temperature range from 550°C to the maximum temperature during annealing in a range of 0.1 K / s to 28 K / s.

3. Method according to claim 1 or 2, wherein the optional proportion of element B in the composition of the steel is in the range of < 0.0005 wt%.

4. A method according to any one of claims 1 to 3, wherein the rolled steel strip is a cold- or hot-rolled steel strip, the production of which the method comprises the following steps: Producing a hot-rolled steel strip from steel with the composition specified in claim 1, optional pickling of the hot-rolled steel strip, optional cold rolling of the hot-rolled steel strip to the cold-rolled steel strip with a thickness reduction of 10-60%, and optional hood annealing of the cold- or hot-rolled steel strip at a maximum holding temperature between 400 and 700 °C for an annealing period of 12 h to 6 days, wherein the annealing period includes the time for heating and the time for cooling to room temperature, and the hood annealing of the hot-rolled steel strip is carried out in particular before the cold rolling of the hot-rolled steel strip.

5. Method according to claim 4, wherein the hot-rolled steel strip is produced by hot rolling from a steel slab heated to a temperature of 1100 °C to 1300 °C and subsequently at a temperature > 300 °C.

6. Method according to claim 5, wherein the hot-rolled steel strip is coiled at a temperature between 300 °C and 650 °C, in particular between 300 °C and 580 °C. Salzgitter Flachstahl GmbH, October 27, 2025, 124097WO / 53491 27 7. Method according to any one of claims 4 to 6, wherein the hot-rolled steel strip has a microstructure consisting of more than 30 volume% bainitic ferrite as a microstructural constituent and a residual microstructure consisting of ferrite, martensite, retained austenite and less than 5 volume% pearlite.

8. Method according to any one of claims 1 to 7, wherein the hot-dip coating produced by hot-dip coating is zinc- or aluminium-based.

9. Hot-dip coated high-strength steel strip with an R m -Tensile strength of at least 1180 MPa, in particular in a range of 1180 MPa to 1350 MPa, preferably produced by a method according to one of claims 1 to 6, with the actual rolled steel strip and a hot-dip coating on the steel strip, wherein the steel of the steel strip has the following composition in weight %: C: from 0.08 to 0.140, preferably 0.10 to 0.122 Mn: from 1.70 to 2.75, preferably 2.00 to 2.40 Cr: from 0.050 to 1.00, preferably 0.20 to 0.40 Mo: from 0.15 to 0.45, preferably 0.15 to 0.35 and optionally one or more of the following elements in weight %: Si: from 0.050 to 1.00, preferably 0.40 to 0.60 Note: from 0.010 to 0.100, preferably 0.020 to 0.050 Ti: from 0.010 to 0.100, preferably 0.020 to 0.400 V: from 0.001 to 0.100 B: < 0.0060, in particular from 0.0001 to 0.0060, preferably < 0.0005 wt% N: from 0.0001 to 0.016 Ni: < 0.20 AI: < 0.20 Cu: from 0.01 to 0.30 S: < 0.03 P: < 0.02 Residual iron, including usual steel-associated elements, where the numerical values ​​of the proportions of the elements Mn, Cr, Mo and C in weight % are as follows: [Mequi] = 2 x [Mn] + [Cr] + 5 x [Mo] + 10 x [C] > 6.80, wherein the steel of the steel strip has a microstructure with a first main phase of 30-60 volume% fresh and / or self-tempered martensite and a second main phase of ferrite and / or bainitic ferrite with a total of 30-60 volume%, as well as Salzgitter Flachstahl GmbH, October 27, 2025 124097WO / 53491 28 has a residual structure consisting of pearlite, retained austenite and optionally other structural components and a total retained austenite content of < 5 volume %, with grains bounded by high-angle grain boundaries with a disorientation angle of > 15°, wherein a volume fraction of these grains, in which the longitudinal axis LA of the respective grain is aligned within 10° to the rolling direction RD in a plane spanned by the strip normal and the rolling direction RD, constitutes at least 50 volume % of the total volume of all grains.

10. Hot-dip coated high-strength steel strip according to claim 9, wherein the grains of the body-centered cubic and / or tetragonally distorted cubic phase(s) bounded by the large-angle grain boundaries have a substructure defined by small-angle grain boundaries with a disorientation angle between 4° and 15° and for these grain boundaries a ratio AS / AL of an area fraction of the small-angle grain boundaries AS to an area fraction of the large-angle grain boundaries AL with a disorientation angle > 15° greater than 0.40, i.e. AS / AL > 0.

40.

11. Hot-dip coated high-strength steel strip according to claim 9 or 10, wherein the steel strip has an R p exhibits a yield strength greater than 700 MPa and / or an elongation at break > 6.

12. Hot-dip coated high-strength steel strip according to any one of claims 9 to 11, wherein the hot-dip coating is a zinc- or aluminium-based hot-dip coating.

13. Hot-dip coated high-strength steel strip according to one of claims 9 to 12, characterized by such high resistance to hydrogen-induced cracking that, in at least five test specimens made from the steel strip with a laser-cut central hole, each of these test specimens passes a load stage of at least 85% in a step test analogous to VDA 238-201.

14. Hot-dip coated high-strength steel strip according to one of claims 9 to 13, characterized by a quality of adhesion of the hot-dip coating to the steel strip that corresponds to grade 2 or better in the ball impact test according to SEP 1931.

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