METHOD FOR MANUFACTURING A COATED STEEL SHEET, GALVANIZED STEEL SHEET, GALVANIZED STEEL SHEET FOLLOWED BY ANNEALING, SPOT WELDED JOINT OF AT LEAST TWO METAL SHEETS AND USE OF A GALVANIZED STEEL SHEET FOLLOWED BY ANNEALING

Abstract

METHOD FOR MANUFACTURING A COATED STEEL SHEET, GALVANIZED STEEL STRIP, GALVANIZED STEEL STRIP, SPOT-WELDED JOINT OF AT LEAST TWO METAL SHEETS AND USE OF A COATED STEEL SHEET. The present invention relates to a method for manufacturing a steel strip, a steel strip with controlled decarburization depth, a spot-welded joint and the use of said steel strip or said spot-welded joint. This invention is particularly suitable for the automotive industry due to the improved resistance to liquid metal embrittlement (LME) along with the target mechanical properties.

Description

1 / 26 “METHOD FOR MANUFACTURING A COATED STEEL SHEET, GALVANIZED STEEL SHEET, GALVANIZED STEEL SHEET FOLLOWED BY ANNEALING, SPOT WELDED JOINT OF AT LEAST TWO METAL SHEETS AND USE OF A GALVANIZED STEEL SHEET FOLLOWED BY ANNEALING” Field of Invention

[001] The present invention relates to a method of manufacturing a steel strip, a spot-welded joint, and the use of said steel strip or said spot-welded joint. This invention is particularly suitable for the automotive industry due to the improvement of the liquid metal embrittlement (LME) resistance property of advanced high-strength steels. Background of the Invention

[002] In order to reduce the weight of vehicles, high-strength steels are used in the automotive industry, mainly in structural parts. These steel grades comprise alloying elements to greatly improve their mechanical properties.

[003] During their manufacture, before coating, fully hardened steels undergo an annealing step that increases their balance between strength and ductility. In this step, the steel is heated and held above its recrystallization temperature in a controlled atmosphere and then cooled to a galvanizing temperature for zinc coating on the steel surface by the hot-dip galvanizing method.

[004] For example, a common practice is to heat all hard steel from room temperature to a recrystallization temperature (heating step) and then hold that temperature (immersion step). Both steps being carried out in an atmosphere comprising, for example, 5% by volume of H2 together with 95% of N2, having a dew point of -20 °C. Petition 870260014660, dated 02 / 13 / 2026, page 13 / 44 2 / 26 or higher. Then, the steel is rapidly cooled to the desired temperature.

[005] In the heating and immersion sections, above about 700 °C, the dew point is controlled in such a way that the oxygen present in the high dew point atmosphere in the furnace diffuses into the subsurface of the steel at a higher rate than the diffusion of oxide-forming steel alloying elements such as Manganese (Mn), Aluminum (Al), Silicon (Si) or Chromium (Cr) towards the surface of the steel.

[006] The presence of C together with other oxide-forming alloying elements in steel, such as Mn, Si, Cr and Al, leads to at least two types of reaction.

[007] Firstly, as represented in Figure 1, oxygen reacts with carbon and forms gases (images A and B), such as CO2 and CO, leading to a depletion of carbon atoms in the subsurface of the steel and creating a decarburized layer (1) (images C and D). Carbon depletion is stronger near the surface (2). In addition to what has been said above, carbon atoms from the bulk (3) diffuse into the carbon depletion zone (1) (image E). All these phenomena occur at the same time (image F). If more carbon atoms leave the subsurface layer of steel than carbon atoms diffuse into said layer, the subsurface layer of steel will be decarburized and / or form carbon-depleted areas compared to the carbon level of the bulk steel.

[008] Secondly, as represented in Figure 2, oxygen reacts with the alloying elements of steel, such as manganese (Mn), aluminum (Al), silicon (Si) and chromium (Cr), having a greater affinity for oxygen than iron, leading to the formation of oxides mainly in the subsurface of the steel which are known as internal selective oxides, reported as (4), and a much smaller amount on the surface known as selective oxides Petition 870260014660, dated 02 / 13 / 2026, page 14 / 44 3 / 26 external, reported as (5). These oxides, being, for example, elemental oxides such as MnO, SiO2. In addition, it forms complex mixed oxides such as MnSiO3, MnSiO4. These oxides may be present in the form of discontinuous nodules or a continuous layer at the grain boundaries in the subsurface of the steel. These internal oxides are present mainly along the grain boundaries and also within the grain.

[009] In a subsequent step of the process, these steels are usually coated with another metal or metal alloy, such as a zinc-based coating, to improve their properties, such as corrosion resistance, phosphatizability, etc. Metallic coatings can be deposited by the hot-dip method or the electroplating method. Hot-dip zinc-based coating, also known as hot-dip galvanizing, usually contains about 0.1 to 0.4 percent by weight of aluminum. This aluminum reacts preferentially with iron and forms an inhibition layer between the steel / coating interface. This inhibition layer is mainly composed of Fe and Al and forms Fe2Al5-xZnx (0 < x < 1), an intermetallic compound. This inhibition layer may contain some Zn atoms.

[0010] When used in the automotive industry, zinc-coated steel sheets are generally welded using the resistance spot welding (RSW) method. During this process, liquid zinc or liquid zinc alloy penetrates the subsurface area of ​​the steel and causes Liquid Metal Embrittlement (LME) of the steel. This leads to a decrease in the ductility of the steel and causes premature failure.

[0011] Regarding the decarburized layer, the thicker the decarburized layer, the better the resistance against LME. However, the decarburized layer deteriorates the mechanical properties of the steel. This is mainly due to the formation of a soft ferrite phase in the area of Petition 870260014660, dated 02 / 13 / 2026, page 15 / 44 4 / 26 subsurface of steel. The thickness of the decarburized layer must be controlled in such a way as to provide excellent LME resistance, in addition to satisfying the desired mechanical properties. In general, the annealing atmosphere needs to be controlled in such a way as to produce an ideal depth of decarburized layer, satisfying both excellent LME resistance and specific mechanical properties. The objective of this invention is to provide a solution addressing the aforementioned problems. Description of the Invention

[0012] This objective is achieved by providing a method according to claim 1. The method may also comprise any features of claims 2 to 7. This objective is also achieved by providing a steel plate according to claims 8 to 11, a spot welded joint according to claim 12. This objective is also achieved by providing a preferred use for the claimed steel plate or spot welded joint.

[0013] Other features and advantages of the invention will become apparent from the following detailed description of the invention.

[0014] To illustrate the invention, various embodiments and non-limiting example tests will be described, particularly with reference to the following figures. Brief Description of the Drawings Figure 1 illustrates several reactions that occur in an annealing furnace; Figure 2 illustrates the internal and external oxidation of alloying elements in steel; Figure 3 illustrates one embodiment of an annealing furnace and a hot-dip coating installation; Petition 870260014660, dated 02 / 13 / 2026, page 16 / 44 5 / 26 Figure 4 illustrates a second embodiment of an annealing furnace and a hot-dip coating installation; Figure 5 illustrates one embodiment of an annealing cycle according to the invention; Figure 6 illustrates a second embodiment of an annealing cycle according to the invention; Figure 7 shows a first embodiment of a claimed steel sheet with galvanized coating; Figure 8 presents a second embodiment of a claimed steel sheet with galvanized coating; Figure 9 shows two SEM images demonstrating the influence of the claimed process on the decarburized layer in the first grade of steel (experiments A1 and A2*); Figure 10 displays two SEM images showing the influence of the claimed process on internal oxides, inhibition layer and galvanized coating on first-grade steel [experiment A1 (image on the left) and experiment A2* (image on the right)]; Figure 11 shows two SEM images illustrating the influence of the claimed process on the decarburized layer (left image) and on the internal oxides, inhibition layer and galvanized coating (right image) in a second grade of steel (experiment B1*); Figure 12 shows two SEM images demonstrating the influence of the claimed process on the decarburized layer (left image) and on the internal oxides and galvanized coating followed by annealing (galvannealed) (right image) on the first grade of steel (experiment A3*); Figure 13 shows a SEM image illustrating the influence of the claimed process on the decarburized layer (left image) and on the internal oxides and galvannealed coating (right image) in the second degree. Petition 870260014660, dated 02 / 13 / 2026, page 17 / 44 6 / 26 of steel (experiment B2*); Figure 14 illustrates the resistance spot welding process in the 3-layer stacking condition, showing the likely location of LME crack formation; Figure 15 illustrates one method for performing resistance spot welding tests. Description of Embodiments of the Invention

[0015] The invention relates to a method for manufacturing a coated steel sheet, coated with a zinc-based or aluminum-based coating, comprising: A) The supply of steel sheet with the following chemical composition, in percentage by weight: 0.01 < Al < 1.0% 0.07 < C < 0.50% 0.3 < Mn < 5.0% V < 0.2%, 0.01 < Si < 2.45% 0.35 < Si + Al < 3.5 N < 0.01%, P < 0.02%, S < 0.01% and, optionally, at least one of the following elements, in weight percentage: B < 0.004%, Co < 0.1%, 0.001 < Cr < 1.00% Cu < 0.5%, 0.001 < Mo < 0.5% Petition 870260014660, dated 13 / 02 / 2026, p. 18 / 44 7 / 26 Nb < 0.1%, Ni < 1.0%, Ti < 0.1%, with the remainder of the composition consisting of iron and unavoidable impurities resulting from the manufacturing process; B) The annealing of the aforementioned steel plate, comprising, in this order: i) a preheating stage in which the said steel plate is heated from ambient temperature to a temperature Ti between 550 °C and Ac1+50 °C, ii) a heating stage in which the said steel plate is heated from a temperature Ti to a recrystallization temperature T2 between 720 °C and 1000 °C in an atmosphere A1, comprising between 0.1 and 15% by volume of H2 with the remainder composed of an inert gas, H2O, O2 and unavoidable impurities, with a dew point DP1 between -10 °C and +30 °C, iii) an immersion stage in which the said steel plate is maintained at the said recrystallization temperature T2 in an atmosphere A2, comprising between 0.1 and 15% by volume of H2 with the remainder composed of an inert gas, H2O, O2 and unavoidable impurities, having a dew point DP2 between -30 °C and 0 °C, with the aforementioned dew point DP1 being higher than the aforementioned dew point DP2, and iv) a cooling stage; C) Coating the aforementioned steel sheet with a zinc- or aluminum-based coating.

[0016] In the following paragraphs, the scope of the claimed invention will be discussed and explained.

[0017] The supplied steel has the claimed composition for the following reasons: Petition 870260014660, dated 13 / 02 / 2026, p. 19 / 44 8 / 26 - 0.01 < Al < 1.0% by weight of Al increases the Ms temperature and thus destabilizes the retained austenite. Furthermore, with an increase in Al content above 1.0%, the Ac3 temperature increases, hindering industrial production; - 0.07 < C < 0.50% by weight: if the carbon content is less than 0.07%, there is a risk of insufficient tensile strength. Furthermore, if the steel microstructure contains retained austenite, its stability, necessary to achieve sufficient elongation, cannot be obtained. If the C content is greater than 0.5%, the hardenability of the weld increases; - 0.3 < Mn < 5.0% by weight. Manganese is a solid solution hardening element that contributes to high tensile strength. However, when the Mn content is above 5.0%, it can contribute to the formation of a structure with excessively marked segregated zones that can negatively affect the mechanical properties of welds. Preferably, the manganese content is in the range between 1.5 and 3.0% by weight. This makes it possible to obtain satisfactory mechanical strength without increasing the difficulty of industrial steel manufacturing and without increasing the hardenability of the welds; - V < 0.2% by weight. The vanadium forms precipitate, achieving hardening and strengthening; - 0.01 < Si < 2.45% by weight. Si retards carbide formation and stabilizes austenite. When the Si content is greater than 2.45%, the plasticity and toughness of the steel are significantly reduced.

[0018] Steels may optionally contain elements such as Nb, B, Ni, Ti, Cu, Mo and / or Co for the following reasons.

[0019] Boron may optionally be contained in steel in an amount less than or equal to 0.004% by weight. By segregating at the grain boundary, B decreases the grain boundary energy and is therefore beneficial for increasing the embrittlement resistance of the liquid metal. Petition 870260014660, dated 02 / 13 / 2026, p. 20 / 44 9 / 26

[0020] Chromium may be present in a content of less than or equal to 1.00% by weight. Chromium allows for the delay of pro-eutectoid ferrite formation during the cooling stage after being held at maximum temperature during the annealing cycle, making it possible to achieve a higher level of strength. Its content is limited to 1.00% by weight for cost reasons and to avoid excessive hardening.

[0021] Copper may be present in a content of less than or equal to 0.5% by weight for hardening steel by precipitation of metallic copper.

[0022] Molybdenum in amounts less than or equal to 0.5% by weight is effective in increasing hardenability and stabilizing retained austenite, as this element retards austenite decomposition.

[0023] Nickel may optionally be contained in the steel in an amount less than or equal to 1.0% by weight to improve toughness.

[0024] Titanium and niobium are also elements that can optionally be used to obtain hardening and strengthening by precipitate formation. However, when the amount of Nb is greater than 0.1% and / or the Ti content is greater than 0.1% by weight, there is a risk of excessive precipitation causing a reduction in toughness, which should be avoided.

[0025] P and S are considered as residual elements resulting from steelmaking. P may be present in an amount less than or equal to 0.04% by weight. S may be present in an amount less than or equal to 0.01% by weight.

[0026] Preferably, the chemical composition of the steel does not include Bismuth (Bi). In fact, without wanting to be limited to any theory, it is believed that if the steel sheet contains Bi, the wettability decreases and therefore the adhesion of the coating.

[0027] For a proper understanding of the invention described, a few terms will be defined. The dew point is the temperature at which the air Petition 870260014660, dated 02 / 13 / 2026, p. 21 / 44 10 / 26 must be cooled to become saturated with water vapor. In steelmaking, Ac1 corresponds to the temperature at which austenite begins to form during heating. Ms corresponds to the temperature at which, after rapid cooling, austenite begins to form martensite.

[0028] The various stages of the process can take place in furnaces as represented in Figure 3 or Figure 4. Both furnaces comprise a preheating section (6), a heating section (7), an immersion section (8) and a cooling section (9). The furnace as illustrated in Figure 4 also comprises a partitioning section (10).

[0029] The preheating stage generally occurs after the steel has been cold-rolled, also known as the Full Hard condition. During this preheating, the steel sheet is heated from ambient temperature to a temperature Ti between 550 °C and Ac1 +50 °C in a non-oxidizing atmosphere. It can be done in any heating medium capable of heating the steel to a temperature Ti without producing iron oxide or in limited quantities. For example, this stage can be done in an RTF (Radiant Tubular Furnace) with an atmosphere composed of N2, H2 and unavoidable impurities, in induction heating, or in a DFF (Direct-Fired Furnace) with an atmosphere with an air / fuel gas ratio < 1. However, it is possible in a DFF composed of several zones, for example, 5 zones, to have an air / fuel gas ratio > 1 in the last or last two zones.

[0030] During the heating stage, the steel plate is heated from a temperature Ti to a recrystallization temperature T2 between 720 °C and 1000 °C in an atmosphere A1, comprising between 0.1 and 15% by volume of H2 with the remainder consisting of an inert gas, H2O, O2 and unavoidable impurities with a dew point DP1 between -10 °C and +30 °C. Nitrogen may be used as the inert gas. Petition 870260014660, dated 13 / 02 / 2026, p. 22 / 44 11 / 26

[0031] During the immersion stage, the steel plate is heated to the aforementioned recrystallization temperature T2 in an atmosphere A2, comprising between 0.1 and 15% by volume of H2 with the remainder consisting of an inert gas, H2O, O2 and unavoidable impurities with a dew point DP2 between -30 °C and 0 °C, the aforementioned dew points DP1 being higher than the aforementioned dew point DP2. Nitrogen may be used as the inert gas.

[0032] Atmospheres A1 and A2 can be obtained by using preheated steam and incorporating N2-H2 gases in a furnace equipped with a pyrometer, H2 and dew point detectors in different sections monitoring the H2, the atmospheric dew point and the temperature.

[0033] Cooling can be achieved in an atmosphere comprising 20 to 50% H2 together with N2. This gas mixture was blown onto the steel surface using a high-speed fan. Cooling can also be achieved by any other cooling means, such as cooling rollers.

[0034] In the following section, without being limited to any theory, the physical phenomenon in the heating and immersion stages will be explained in order to understand the core of the invention.

[0035] In the heating stage, the gradual increase in temperature along with the comparatively high dew point allows for a high pO2 (partial pressure of oxygen), leading to oxygen diffusion in the steel. This increased oxygen diffusion has two main consequences. Firstly, it allows for deep decarburization of the steel subsurface by reaction with the interstitial element carbon. Secondly, oxygen reacts with substitutional oxide-forming elements, such as Mn, Si, Al, and Cr, and forms internal oxide in the subsurface area of ​​the steel, which reduces the amount of alloying element available to form surface oxides. These internal oxides preferentially form in the grain boundary area due to a Petition 870260014660, dated 02 / 13 / 2026, page 23 / 44 12 / 26 faster diffusion of these alloying elements.

[0036] At the end of the heating stage, the subsurface area of ​​steel comprises: - a partially decarburized layer with a thickness between 10 and 30 µm and a carbon percentage by weight between 5 and 20 percent of the carbon percentage by volume of the steel; - a decarburized layer, external to the partially decarburized layer, with a thickness between 30 and 70 µm and a carbon percentage by weight of less than 5 percent of the carbon percentage by volume of the steel.

[0037] These values ​​are provided only to obtain an order of magnitude. Parameters such as heating time, temperature at the end of heating, carbon content of the steel, as well as the dew point which determines the pO2, influence the thickness of the aforementioned complete and partially decarburized layers.

[0038] In the immersion stage, compared to the heating stage, the temperature is higher, but the dew point is lower. It has several effects on the subsurface area of ​​the steel.

[0039] Due to the comparatively lower dew point in the immersion section, the amount of oxygen is also lower and therefore can only diffuse to a limited (smaller) depth in the subsurface area of ​​the steel, causing a decarburization reaction at a limited depth in the subsurface area of ​​the steel. Meanwhile, carbon atoms diffuse from the bulk to the carbon-depleted area of ​​the subsurface area of ​​the steel (partially decarburized layer followed by a decarburized layer). In effect, the carbon atoms present in the partially decarburized area diffuse to the decarburized area, and the partially decarburized area is again filled with carbon atoms from the bulk. Petition 870260014660, dated 02 / 13 / 2026, page 24 / 44 13 / 26 Thus, it produces a decarburized layer very close to the surface of the steel. This decarburization reaction depends on several factors such as the immersion temperature, the dew point (pO2), the duration of decarburization, and the amount of carbon present in the raw steel.

[0040] Consequently, at the end of the immersion stage, the subsurface area of ​​steel comprises: - a partially decarburized layer with a thickness of about 30 µm and a carbon percentage by weight between 5 and 20 percent of the carbon percentage by volume of the steel; - a decarburized layer, external to the partially decarburized layer, with a thickness of about 20 µm and a carbon percentage by weight of less than 5 percent of the carbon percentage by volume of the steel.

[0041] These values ​​are provided only to obtain an order of magnitude.

[0042] Due to a higher partial pressure of oxygen (pO2) in the heating section, a greater amount of O2 can easily diffuse into the subsurface area of ​​the steel and form internal oxide, thus trapping Si, Mn, Cr, Al much deeper in the subsurface area. This phenomenon occurs in the initial recrystallization stage in the heating section. In the immersion section, grain growth and the formation of large ferrite grains mainly occur in the subsurface area of ​​the steel.

[0043] Due to the formation of deeper internal oxides in the subsurface area of ​​the steel followed by grain growth, a layer of ferrite free of internal oxides was formed on the surface of the steel. This layer can readily react with aluminum in the plating bath during galvanizing and forms a satisfactory inhibiting layer.

[0044] Unlike the state of the art, in this process of Petition 870260014660, dated 13 / 02 / 2026, page 25 / 44 14 / 26 annealing, the dew point of the heating stage is higher than that of the immersion stage, allowing for improved steel properties in terms of resistance to liquid metal embrittlement (LME) as explained previously. Apparently, the invention also has the advantage of producing a controlled depth of complete decarburized layer, having a carbon weight percentage of less than 5 percent of the carbon weight percentage of the steel by volume.

[0045] Preferably, the dew point DP2 is between -25 °C and +10 °C. Preferably, the dew point DP2 is between -20 °C and 0 °C. Preferably, the dew point DP2 is between -25 °C and -5 °C. Even more preferably, the dew point is between -25 °C and -5 °C.

[0046] Preferably, in the said cooling step, said steel plate is cooled to a temperature T3 between Ms and Ms+150 °C and held at T3 for at least 40 seconds in an atmosphere A3 comprising between 1 and 30% by volume of H2 and an inert gas, with a dew point DP3 less than or equal to -40 °C. Even more preferably, said temperature T3 is between Ms+10 °C and Ms+150 °C. This allows for a partitioned microstructure.

[0047] Preferably, after the aforementioned cooling step iv), the said steel plate is further cooled to a temperature Tqt between (Ms-5 °C) and (Ms-170 °C) and then undergoes a reheating step v) in which the said steel plate is reheated to a temperature T4 between 300 and 550 °C for 30 s to 300 s. This step is also known as the partitioning step. Even more preferably, the said steel plate is optionally held at Tqt for a duration between 2 and 8 s. Even more preferably, the said steel plate is reheated to a temperature T4 between 330 and 490 °C.

[0048] Preferably, after the aforementioned cooling stage iv) and the aforementioned reheating stage v), in an equalization stage vi) the aforementioned Petition 870260014660, dated 02 / 13 / 2026, page 26 / 44 A 15 / 26 strip of steel is heated to a temperature between 300 °C and 500 °C in an A4 atmosphere comprising between 1 and 30% by volume of H2 and at least one inert gas, with a dew point DP4 less than or equal to -40 °C.

[0049] Preferably, the steel plate in step A) has at least the following percentage by weight: 0.001 < Cr+Mo < 1.000%.

[0050] Preferably, the aforementioned heating and immersion steps last between 100 and 500 seconds. Preferably, in the aforementioned heating and immersion steps, the A1 and A2 atmosphere comprises between 3 and 8% by volume of H2.

[0051] Preferably, the aforementioned DP1 is between 5 °C and 40 °C higher than DP2. Even more preferably, the aforementioned DP1 is between 10 °C and 30 °C higher than DP2.

[0052] Preferably, in the aforementioned step C), the aforementioned coating is made by electroplating or hot-dip coating.

[0053] Preferably, in said step C), said coating is made by the hot-dip coating method and said steel strip is adjusted to a temperature between 5 °C and 10 °C above a galvanizing bath, having an aluminum content between 0.15 and 0.40 percent by weight, being maintained at a temperature between 450 °C and 470 °C.

[0054] Preferably in said step C), said coating is made by the hot-dip coating method and said steel strip is adjusted to a temperature between 5 °C and 10 °C above a galvanizing bath, having an aluminum content between 0.09 and 0.15 percent by weight, being maintained at a temperature between 450 °C and 470 °C and is then heated to a temperature between 470 °C and 550 °C after exiting said galvanizing bath. Such process steps allow the production of a galvannealed steel strip.

[0055] Figures 5 and 6 illustrate two typical thermal cycles Petition 870260014660, dated 02 / 13 / 2026, p. 27 / 44 16 / 26 described above. In Figure 5, the preheating of the complete hard steel sheet begins from room temperature and lasts 146 seconds until the steel reaches 575 °C. Then, during the heating stage, the steel is heated from 575 °C to 715 °C in 131 seconds and then from 715 °C to the immersion temperature (800 °C) in 174 seconds. Next, a strip goes through the immersion stage where its temperature is maintained at 800 °C for 146 seconds. Finally, the strip is rapidly cooled, by quenching, to a temperature of 190 °C. After that, the sheet goes through a reheating stage also known as the heat treatment partition stage at 365 °C for 105 seconds and then cools to 465 °C. The steel is finally galvanized in a Zn-0.2% by weight Al bath maintained at 460 °C.

[0056] As shown in Figure 6, the preheating of the complete hard steel sheet begins from room temperature and lasts 146 seconds until the steel reaches 675 °C. Then, during the heating stage, the steel is heated from 675 °C to 815 °C in 131 seconds and then from 815 °C to the immersion temperature (880 °C) in 174 seconds. Next, the strip undergoes an immersion stage where its temperature is maintained at 880 °C so that immersion is carried out for 146 seconds. Finally, the strip is rapidly cooled, by quenching, to a temperature of 280 °C. After that, the sheet goes through a reheating stage also known as the heat treatment partition stage at 450 °C for 105 seconds and then cools to 460 °C. The steel is finally galvanized in a Zn-0.2% by weight Al bath maintained at 460 °C.

[0057] As illustrated in Figure 7, the invention also relates to a galvanized steel strip, manufactured as described above, comprising: - a volume of steel (18) with a composition as described above; Petition 870260014660, dated 02 / 13 / 2026, page 28 / 44 17 / 26 - a partially decarburized layer (17), on top of said volume of steel (18), having a thickness between 20 and 40 μm and a carbon percentage by weight between 5 and 20 percent of the carbon percentage by weight of the steel by volume and having a microstructure comprising at least 50 percent ferrite and at least one of the following constituents: bainite, martensite and / or retained austenite; - a decarburized layer (16) on top of said partially decarburized layer (17), having a thickness between 5 and 40 μm and a carbon percentage by weight of less than 5 percent of the carbon percentage by volume of the steel and having a microstructure comprising at least 90 percent ferrite, the upper part of said decarburized layer (16) comprising an inner oxide layer (15), having a thickness between 2 and 12 μm, and containing elemental oxides based on Mn, Si, Al and Cr and mixed oxides of Mn, Si, Al and Cr; - an inhibition layer (14) on top of said inner oxide layer (15), having a thickness between 100 nm and 500 nm; - a zinc-based coating layer (13) on top of said inhibition layer (14) with a thickness between 3 and 30 μm.

[0058] The aforementioned inner oxide layer is in the outer portion of the decarburized layer, closer to the inhibition layer, as illustrated in Figure 7. The inner oxide layer comprises the aforementioned oxides and has a carbon weight percentage of less than 5 percent of the carbon weight percentage of the steel by volume and has at least 90 percent ferrite.

[0059] As illustrated in Figure 8, the invention also relates to a galvannealed steel strip, manufactured as described above, comprising: - a volume of steel (18) with a composition as Petition 870260014660, dated 13 / 02 / 2026, page 29 / 44 18 / 26 described previously; - a partially decarburized layer (17) on top of said steel mass (18) with a thickness between 20 and 40 μm and a carbon percentage by weight between 5 and 20 percent of the carbon percentage by volume of the steel and having a microstructure comprising at least 50 percent ferrite and at least one of the following constituents: bainite, martensite and / or retained austenite; - an outer decarburized layer (16) to the partially decarburized layer (17), having a thickness between 5 and 40 μm and a carbon percentage by weight of less than 5 percent of the carbon percentage by volume of the steel and having a microstructure comprising at least 90 percent ferrite, the upper part of said decarburized layer (16) comprising an inner oxide layer (15), having a thickness between 2 and 12 μm, and containing elemental oxides based on Mn, Si, Al and Cr and mixed oxides of Mn, Si, Al and Cr; - a zinc-iron-based coating layer (12) on top of said inner oxide layer (15) with a thickness between 3 and 30 μm and containing between 10 and 20 percent by weight of iron.

[0060] The inner oxide layer cannot be thicker than the decarburized layer. Consequently, if the decarburized layer has a thickness of “x” μm, where x is between 5 and 12 μm, the inner oxide layer has a thickness between 2 and “x”. The aforementioned inner oxide layer is in the outer portion of the decarburized layer, closer to the inhibition layer, as illustrated in Figure 8. The inner oxide layer comprises the aforementioned oxides and has a carbon weight percentage of less than 5 percent of the carbon weight percentage of the steel by volume and has at least 90 percent ferrite.

[0061] Preferably, the said steel strip has a thickness Petition 870260014660, dated 13 / 02 / 2026, pp. 30 / 44 19 / 26 between 0.5 mm and 3.0 mm.

[0062] Preferably, the said steel strip has an ultimate tensile strength (UTS) greater than 900 MPa.

[0063] The invention also relates to a spot-welded joint of at least two metal sheets comprising at least one steel sheet as described above, said joint containing zero cracks having a size greater than 100 µm.

[0064] Preferably, the said spot-welded joint comprises two or three metal plates. Preferably, the said spot-welded joint also comprises an aluminum plate or a steel plate.

[0065] The invention also relates to the use of any previously described coated steel sheet or any previously described spot welded joint for the manufacture of automotive vehicles. Experimental Results

[0066] The following section deals with experimental results exhibiting the improved surface and subsurface properties. The experiments were performed on two different types of steel (Steel A and Steel B) with a strip thickness between 1.4 and 1.6 mm.

[0067] The different experimental parameters are reported in Table 1.

[0068] A first set of experiments (A1 and A2*) was conducted to show the influence of the difference in dew points in the heating and immersion sections on the decarburization behavior of steel, in a first grade of steel (Steel A). The steel was annealed followed by galvanizing in a Zn-0.20% wt Al coating bath according to the thermal cycles reported in Figure 5, so that the thermal cycles for both experiments are similar. In Experiment A1, nearly similar dew points were maintained in the heating sections (-5 °C) Petition 870260014660, dated 02 / 13 / 2026, pp. 31 / 44 20 / 26 and immersion (-3 °C). In Experiment A2*, a higher dew point was applied in the heating section (-1 °C) compared to the immersion section (-9 °C). For both experiments, a hydrogen concentration between 4 and 5% was maintained in both sections.

[0069] A second experiment (A3*) was conducted on Steel A. The steel was annealed followed by galvanizing in a Zn 0.129% by weight Al coating bath according to the thermal cycles reported in Figure 5. Immediately after galvanizing, post-coating heat treatment, also known as galvannealing, was performed at 480 °C. In this experiment, a higher dew point was also applied to the heating section (0 °C) compared to the immersion section (-10 °C), and approximately 5% hydrogen was maintained in both sections.

[0070] A third experiment (B1*) was performed on a different steel grade (Steel B). The steel was annealed followed by a galvanized coating bath in Zn-0.20 wt% Al according to the thermal cycles reported in Figure 6. The peak annealing temperature is higher in Steel B compared to Steel A. In this experiment, a higher dew point was also applied to the heating section (-5 °C) compared to the immersion section (-20 °C) and approximately 5% hydrogen was maintained in both sections.

[0071] A fourth experiment (B2*) was also performed on Steel B. The steel was annealed followed by a coating bath of Zn-0.129% by weight Al according to the thermal cycles reported in Figure 6. Immediately after galvanizing, post-coating heat treatment also known as galvannealing was performed at 510 °C. In this experiment, a higher dew point was also applied to the heating section (+4 °C) compared to the immersion section (-5 °C), and approximately 5% hydrogen was maintained in both sections. Petition 870260014660, dated 02 / 13 / 2026, pages 32 / 44 21 / 26

[0072] Experiments A2*, A3*, B1*, and B2* are in accordance with the present invention in that the dew point of the heating section is higher than that of the immersion section. Table 1. Different Experimental Parameters Experiment A1 A2* A3* B1* B2* Steel composition (in weight percent) Steel A: C: 0.24, Mn: 2.02, Si: 1.02, Al: 0.44, Cr: 0.18, Nb: 0.031, V: 0.001, N: 0.003, P: 0.007, S: 0.002 Steel B: C: 0.23, Mn: 2.10, Si: 1.08, Al: 0.47, Mo: 0.30, Nb: 0.02, V: 0.001, N: 0.003, P: 0.007, S: 0.002 Strip thickness (mm) 1.4 1.5 1.4 1.6 1.6 Ti (°C) 575 575 575 675 675 DPi (°C) -5 -1 0 -5 +4 H2 concentration during heating (%) 4 4 4 5 5 T2 (°C) 800 800 800 880 880 DP2 (°C) -3 -9 -10 -20 -5 H2 concentration during immersion (%) 5 4 5 5 5 Cooling temperature (quenching) (°C) 190 190 190 280 280 Reheating temperature after quenching (partitioning) (°C) 365 365 365 450 450 Partitioning duration (s) 105 105 105 105 105 Coating bath composition Zn - 0.20% by weight of Al Zn - 0.20% by weight of Al Zn - 0.129% by weight of Al weight of Al Zn-0.20% by weight of Al Zn-0,129% by weight of Al. Coating Type: Galvanized. Galvannealed. Galvannealing Temperature (°C): 480 - 510. *According to the present invention. Petition 870260014660, dated 02 / 13 / 2026, pages 33 / 44 22 / 26 Decarbonized Layer

[0073] Figure 9 compares SEM micrographs of the decarburized layer formed in the subsurface area of ​​steel produced according to experiment A1 (image on the left) and A2* (image on the right) using Steel A.

[0074] The A2* micrograph of the subsurface area of ​​steel according to the present invention shows: - steel volume (18); - a partially decarburized layer (17) of about pm with a carbon percentage by weight between 5 and 20 percent of the carbon percentage by weight of the steel by volume; - a decarburized layer (16) of about 20 pm, with a carbon percentage by weight of less than 5 percent of the carbon percentage by weight of the steel by volume.

[0075] Conversely, the A1 micrograph of the steel subsurface, according to the prior art, shows only a volume of steel (18) and a partially decarburized layer (17) of about 45 µm. This comparison presents the benefits of the claimed method in forming a decarburized layer in the subsurface area of ​​the steel which is favorable for obtaining the target mechanical strength properties as well as resistance to Liquid Metal Embrittlement and.

[0076] Figure 10 shows SEM micrographs of Steel A samples produced through experiment A1 (image on the left) and A2* (image on the right) showing the presence of internal oxides (15), inhibition layer (14) and galvanized coating (13).

[0077] Figure 11 shows two SEM micrographs of a sample of Steel B produced through experiment B1*. The subsurface micrograph Petition 870260014660, dated 02 / 13 / 2026, pages 34 / 44 23 / 26 steel features: - a volume of steel (18); - a partially decarburized layer (17) of about pm with a carbon percentage by weight between 5 and 20 percent of the carbon percentage by weight of the steel by volume; - a decarburized layer (16) of about 15 pm, with a carbon percentage by weight of less than 5 percent of the carbon percentage by volume of the steel; - the inhibition layer (14), the inner oxide layer (15) and the galvanized coating layer (13).

[0078] Figure 12 shows two SEM micrographs of a sample of Steel A produced through experiment A3*. The micrograph on the left of the steel subsurface shows: - a volume of steel (18); - a partially decarburized layer (17) of about pm with a carbon percentage by weight between 5 and 20 percent of the carbon percentage by weight of the steel by volume; - a decarburized layer (16) of about 20 pm, with a carbon percentage by weight of less than 5 percent of the carbon percentage by weight of the steel by volume.

[0079] This experiment exhibits a claimed preferable method in which DP1 is between 5 °C and 30 °C higher than DP2.

[0080] Figure 13 shows two SEM micrographs of a sample of Steel B produced through experiment B2*. The micrograph on the left of the steel subsurface shows: - a volume of steel (18); - a partially decarbonized layer (17) of about Petition 870260014660, dated 02 / 13 / 2026, pp. 35 / 44 24 / 26 pm with a carbon percentage by weight between 5 and 20 percent of the carbon percentage by volume of the steel; - a decarburized layer (16) of about 15 pm, with a carbon percentage by weight of less than 5 percent of the carbon percentage by weight of the steel by volume. Galvanized and galvannealed coating

[0081] As shown in Figures 9 and 10 for experiment A2* and in Figure 11 for experiment B1*, the claimed method produces a suitable surface for reactive wetting during galvanizing. As reported in Table 1, during the galvanizing of Steel A and Steel B, the bath composition of Zn-0.20 wt% Al was maintained. During galvanizing, a continuous inhibition layer formed at the steel / coating interface indicates good reactive wetting behavior.

[0082] In experiments A3* and B2*, galvannealed Steel A and Steel B, respectively, were produced after bath galvanizing with Zn-0.129% by weight Al followed by post-coating heat treatment (also known as galvannealing treatment) at 480 °C for Steel A and 510 °C for Steel B. Figure 12 and Figure 13 show cross-sectional SEM micrographs of galvannealed coated Steel A and Steel B, respectively. These micrographs show that the claimed method is suitable for the production of galvannealed coated steel. Evaluation of the Embrittlement Resistance of Liquid Metal

[0083] The susceptibility to Liquid Metal Embrittlement (LME) of the galvanized and galvannealed coated steel produced above according to the thermal cycles reported in Table 1 was evaluated by the method of Petition 870260014660, dated 02 / 13 / 2026, pp. 36 / 44 25 / 26 resistance spot welding on a steel produced under the conditions of experiments A2*, A3*, B1* and B2*. The electrode type was ISO Type B with a face diameter of 6 mm; the electrode force was 5 kN and the water flow rate was 1.5 g.min-1. The welding cycle is reported in Table 2: Table 2. Welding schedule, to determine the property of LME RESISTANCE Welding time (milliseconds) Current level (kilo-Amps) Holding time (milliseconds) 380 Imax 300 380 Imax + 10% d© Imax 300

[0084] The crack resistance behavior of LME was evaluated using the 3-layer stacking condition. In this condition, three coated steel plates were welded together by resistance spot welding, as shown in Figure 14, exhibiting an indentation area (19), a deformed area due to indentation (20), a heat-affected zone (HAZ) area (21), an interfacial HAZ / weld nugget area (22) and contact surfaces in the HAZ area (23). All resistance spot welding tests were performed including severe noise factors, such as Span (24) between two steel plates, Displacement (25) between welding electrode and said steel plate and Electrode angle (26) between welding electrode and said steel plate which is schematically represented in Figure 15. The number of cracks above 100 µm was then evaluated using an optical microscope as reported in Table 3 at all 5 locations, as illustrated in Figure 14.Excellent resistance to LME was observed in steel plate across a wide thickness range, with and without welding noise factors, due to the presence of a specific thickness of the decarburized layer. Petition 870260014660, dated 02 / 13 / 2026, pp. 37 / 44 26 / 26 Table 3. Details of LME cracks after spot welding by RESISTANCE (3-LAYER STACKING CONDITIONS) Experiment Noise Factors Steel Plate Thickness (mm) Number of LME cracks with crack length greater than 100 µm at Welding Current (Imax) Number of LME cracks with crack length greater than 100 µm at Welding Current (Imax + 10% of Imax) A2* None 0.9, 1.6 and 2 mm 0 0 2 mm span, 2 mm displacement and 3 °C electrode angle 0.9, 1.6 and 2 mm 0 0 A3* None 1.4 0 0 2 mm span, 2 mm displacement and 3 °C electrode angle 1.4 0 0 B1* None 1.6 0 0 2 mm span, 2 mm displacement and 3 °C electrode angle 1.6 0 0 B2* None 1.6 0 0 2 mm gap, 2 mm displacement and 3 °C electrode angle 1.6 0 0 Petition 870260014660, dated 02 / 13 / 2026, pp. 38 / 44

Claims

1 / 5 Claims 1. METHOD FOR MANUFACTURING A COATED STEEL SHEET, coated with a zinc-based coating (13) characterized by comprising: A) supplying a steel sheet with the following chemical composition, in weight percent: 0.01% < Al < 1.0%, 0.07% < C < 0.50%, 0.3% < Mn < 5.0%, 0% < V < 0.2%, 0.01% < Si < 2.45%, 0.35% < Si + Al < 3.5%, N < 0.01%, P < 0.04%, S < 0.01% and optionally at least one of the following elements, in weight percent: B < 0.004%, Co < 0.1%, Cu < 0.5%, 0.001% < Cr < 1.00%, 0.001% < Mo < 0.5%, Nb < 0.1%, Ni < 1.0%, Ti < 0.1%, the remainder of the composition being made up of iron and unavoidable impurities resulting from the manufacturing process, B) the annealing of the steel sheet comprising, in this order: i) a preheating step in which the steel sheet is heated from ambient temperature to a temperature T1 between 550 °C and Ac1+50 °C,ii) a heating stage in which the steel plate is heated from a temperature T1 to a recrystallization temperature T2 between 720 °C and 1000 °C in an atmosphere A1, comprising between 0.1 and 15% by volume of H2 with the remainder composed of an inert gas, H2O, O2 and unavoidable impurities, with a dew point DP1 between -10 °C and +30 °C, iii) an immersion stage in which the steel plate is maintained at the recrystallization temperature T2 in an atmosphere A2, comprising between 0.1 and 15% by volume of H2 with the remainder composed of an inert gas, H2O, O2 and unavoidable impurities, having a dew point DP2 between -30 °C and 0 °C, with the dew point DP1 being higher than the dew point DP2, iv) a cooling stage in which the steel plate is still cooled to a temperature Tqt between Ms-5 °C and Ms-170 °C, and Petition 870260014660, dated 13 / 02 / 2026, p. 39 / 44 2 / 5 v) a reheating step,where the steel sheet is reheated to a temperature T4 between 300 and 550 °C for 30 seconds to 300 seconds, C) coating the steel sheet with the zinc-based coating (13)., 2. METHOD, according to claim 1, characterized in that the steel plate is optionally held at Tqt for 2 to 8 seconds.

3. METHOD, according to claim 1, characterized by comprising, after the cooling step iv) and the reheating step v), an equalization step vi) in which the steel plate is heated to a temperature between 300 °C and 500 °C in an atmosphere A4 comprising between 1 and 30% by volume of H2 and at least one inert gas, having a dew point DP4 less than or equal to -40 °C.

4. METHOD, according to claim 1, characterized in that DP1 is between 5 °C and 40 °C higher than DP2.

5. METHOD, according to claim 4, characterized in that DP1 is between 10 °C and 30 °C higher than DP2.

6. METHOD, according to claim 1, characterized in that in step C), the coating is made by the hot-dip coating method and the steel sheet is adjusted to a temperature between 5 °C and 10 °C above a galvanizing bath, having an aluminum content between 0.15 and 0.40 percent by weight, being maintained at a temperature between 450 °C and 470 °C.

7. METHOD, according to any one of claims 1 to 5, characterized in that in step C), the coating is made by the hot-dip coating method and the steel sheet is adjusted to a temperature between 5 °C and 10 °C above a galvanizing bath, having an aluminum content between 0.09 and 0.15 percent by weight, being maintained at a temperature between 450 °C and 470 °C and then heated to a temperature between 470 °C and 550 °C after exiting the galvanizing bath.

8. GALVANIZED STEEL SHEET, manufactured by the method as defined in any one of claims 1 to 6, characterized by comprising: - a steel bulk (18) having a composition as defined in claim 1, - a partially decarburized layer (17) on top of the steel bulk (18), having a thickness between 20 and 40 µm and a weight percentage of carbon between 5 and 20 percent of the weight percentage of carbon of the steel bulk (18) and having a microstructure comprising at least 50 percent ferrite and at least one of the following constituents: bainite, martensite and / or retained austenite, - a decarburized layer (16) on top of the partially decarburized layer (17), having a thickness between 5 and 40 µm and a weight percentage of carbon less than 5 percent of the weight percentage of carbon of the steel bulk (18) and having a microstructure comprising at least 90 percent of ferrite,the upper part of the decarburized layer (16) comprising an inner oxide layer (15), having a thickness between 2 and 12 pm, and containing elemental oxides based on Mn, Si, Al and Cr and mixed oxides of Mn, Si, Al and Cr, - an inhibition layer (14) on top of the inner oxide layer (15), having a thickness between 100 nm and 500 nm, - a zinc-based coating layer (13) on top of the inhibition layer (14) with a thickness between 3 and 30 pm., 9. Galvanized steel sheet followed by annealing, manufactured by the method as defined in claim 7, characterized by comprising: Petition 870260014660, dated 02 / 13 / 2026, page. 41 / 44 4 / 5 - a steel bulk (18) having a composition as defined in claim 1, - a partially decarburized layer (17) on top of the steel bulk (18) having a thickness between 20 and 40 pm and a weight percentage of carbon between 5 and 20 percent of the weight percentage of carbon of the steel bulk (18) and having a microstructure comprising at least 50 percent ferrite and at least one of the following constituents: bainite, martensite and / or retained austenite, - a decarburized layer (16) external to the partially decarburized layer (17), having a thickness between 5 and 40 pm and a weight percentage of carbon less than 5 percent of the weight percentage of carbon of the steel bulk (18) and having a microstructure comprising at least 90 percent ferrite,the upper part of the decarburized layer (16) comprising an inner oxide layer (15), having a thickness between 2 and 12 pm, and containing elemental oxides based on Mn, Si, Al and Cr and mixed oxides of Mn, Si, Al and Cr, - an iron-zinc based coating layer (12) on top of the inner oxide layer (15) having a thickness between 3 and 30 pm and containing between 10 and 20 percent by weight of iron.

10. STEEL SHEET, according to any one of claims 8 to 9, characterized in that the steel sheet has a thickness between 0.5 mm and 3.0 mm.

11. STEEL PLATE, according to any one of claims 8 to 10, characterized in that the steel plate has a maximum tensile strength greater than 900 MPa.

12. SPOT WELDED JOINT OF AT LEAST TWO METAL SHEETS, characterized by comprising at least one steel sheet, as defined in any of claims 9 to 11, the joint Petition 870260014660, dated 13 / 02 / 2026, page 42 / 44 5 / 5 containing zero cracks having a size greater than 100 µm.

13. USE OF A GALVANIZED STEEL SHEET FOLLOWED BY ANNEALING, as defined in any of claims 9 to 11, or of a spot-welded joint, as defined in claim 12, characterized by being for the manufacture of automotive vehicles. Petition 870260014660, dated 02 / 13 / 2026, pp. 43 / 44

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