AÇO DÚCTIL LAMINADO A QUENTE DE ULTRA-ALTA RESISTÊNCIA DE ALTO GRAU DE FLANGEAMENTO, SEU MÉTODO DE PRODUÇÃO E SEU USO

Abstract

HOT-ROLLED ULTRA-HIGH STRENGTH DUCTILE STEEL WITH HIGH FLANGING QUALITY, METHOD OF PRODUCING SAID HOT-ROLLED STEEL AND USE THEREOF. The present invention relates to a hot-rolled (HR) steel strip with high flanging capacity at ultra-high strength levels with high total elongation, bending capacity and toughness values, to a method of producing said hot-rolled steel and use thereof.

Description

1 / 33 ULTRA-HIGH STRENGTH HIGH-GRADE FLANGING HOT-ROLLED DUCTILE STEEL, ITS PRODUCTION METHOD AND ITS USE Field of Invention

[001] The present invention relates to a hot-rolled (HR) steel strip with high flanging capacity at ultra-high strength levels with high values ​​of total elongation, flexibility and toughness, to a method of producing said hot-rolled steel and to its use. Background of the invention

[002] It is known that as the strength of hot-rolled (HR) steel increases, its formability decreases. One of the main areas of application for HR steels in transportation and automotive applications is chassis and suspension (C&S), such as in the lower control arm. Other areas include truck frame rails, bumper beams, or battery boxes for electric vehicles. The typical thickness of HR steels used for these applications is less than 4.5 mm. Thicker gauge HR steels, up to 12 mm, can be used in engineering applications, such as crane booms or in transportation applications for heavy truck frames.

[003] From a weight reduction perspective, it is imperative that higher strength steels be used for the above applications in order to reduce the gauge of the steel strip. Therefore, ultra-high strength (UHSS) steels with a maximum tensile strength (Rm) typically greater than 1000 MPa would be useful for this purpose.

[004] These applications of HR steels require mechanical properties that are difficult to reconcile. In addition to high strength, the steel must also have good formability for the production of the component through cold forming, as this is a route of Petition 870260062179, dated 06 / 25 / 2026, page 5 / 109 2 / 33 energy-efficient production compared to hot forming. Furthermore, good impact resistance or energy absorption capacity is also required for applications such as bumper beams, battery housings, crane booms, or structural rails. For component assembly, good weldability is also necessary, typically characterized by a low carbon equivalent of the steel.

[005] However, as the tensile strength of steels increases, the formability parameters decrease. Formability is a generic term for steel sheets that is seen as a combination of material behavior during various mechanical operations, such as stretching, bending, drawing, and flanging. Depending on the component geometry, any one or a combination of two or more material attributes is important during sheet metal forming. For typical C&S automotive parts, flanging capability in stretching is also important. This type of formability requires high hole expansion capacity (HEC) and good total elongation. For making structural rails, bumper beams, or battery housings, which are typically manufactured by roll forming, bending capability is important. The production of crane booms also requires good HEC, good bending capability, and elongation.

[006] Achieving high formability and high impact resistance values ​​in ultra-high strength steels is a challenge. Objectives of the invention

[007] It is an object of the invention to provide a hot-rolled steel strip with ultra-high strength combined with high flanging capacity, good elongation, and capacity to Petition 870260062179, dated 06 / 25 / 2026, p. 6 / 109 3 / 33 folding and impact resistance.

[008] It is also an objective of the invention to provide a hot-rolled steel strip with excellent weldability.

[009] It is also an objective of the invention to provide a method for producing such steels. Description of the invention

[010] One or more of the objectives is achieved in hot-rolled steel according to claim 1. Preferred embodiments are described in any of the dependent claims.

[011] According to a second aspect, the invention is also configured in the method according to claim 10 for producing the steels according to the invention.

[012] According to a third aspect, the invention is also configured in the use of hot-rolled steel for the production of a part for transport or an engineering application.

[013] The steel according to the invention contains carbon, silicon, aluminum and manganese as essential elements. The ranges of the content of these alloying elements (in % by weight) in the steel are as follows:

[014] C: 0.10 -0.30;

[015] Si: 0.50 -1.50;

[016] Al: 0.010 - 1.00;

[017] Mn: 1.00 -3.00;

[018] where (Si + Al) > 0.80;

[019] and optionally any one or more of the following linking elements:

[020] V: less than 0.10;

[021] Nb: less than 0.10;

[022] Ti: less than 0.10;

[023] Mo: less than 0.50;

[024] Cr: less than 1.50; Petition 870260062179, dated 06 / 25 / 2026, p. 7 / 109 4 / 33

[025] Cu: less than 1.00;

[026] Ni: less than 0.50;

[027] B: less than 0.0030 (30 ppm);

[028] also inevitably including

[029] N: less than 0.0100 (100 ppm),

[030] S: less than 0.005;

[031] P: less than 0.020;

[032] the remainder being Fe and other unavoidable impurities resulting from the iron and steel production process. Note that all compositional percentages are given as % by weight unless otherwise indicated.

[033] Carbon is present in steel in an amount of 0.10-0.30%, preferably 0.10-0.26%, more preferably 0.10-0.23%. Carbon, which causes strong hardening of the solid solution in iron, is added primarily for its strength and hardenability. Carbon ensures that during cooling on the exit table after hot rolling, austenite does not transform into ferrite and / or pearlite above a critical cooling rate (20°C / s). Less than 0.10% C will not give the desired Rm level of 1000 MPa or more, preferably 1200 MPa or more, and if the C content is greater than 0.30%, the weldability of the formed parts may be insufficient. Weldability is also improved by a low equivalent carbon value. A suitable minimum value for carbon is 0.16%.

[034] Silicon is added in an amount less than 1.50% to increase strength by reinforcing the replacement solid solution in the iron mesh. The other important effect of Si in steels is that Si retards the precipitation of carbides (cementite and other carbides). As a result, the martensite phase when subjected to quenching will not form harmful iron carbides in its matrix. Petition 870260062179, dated 06 / 25 / 2026, p. 8 / 109 5 / 33 When the Si content is less than 0.50%, the strengthening and carbide formation suppression effects are insufficient to achieve the intended benefits. On the other hand, when the Si content is above 1.50%, excessive oxide formation may occur during the thermomechanical processing (reheating of the plate, hot rolling, coiling, etc.) of the steel. These oxide scales are detrimental to hot rolling, pickling, coating, and overall surface appearance. Furthermore, rolling forces during hot rolling increase, and the steel becomes brittle when the Si content exceeds 1.50% to such a level that it makes the steel very difficult to hot roll. Thus, the amount of Si according to the invention is typically greater than 0.50% and less than 1.50%, preferably in the range of 0.60% to 1.30%, or more preferably in the range of 0.70% to 1.10%.

[035] Aluminum behaves comparably to Si in steel, according to the invention. It acts as a solid solution strengthening element in steel when deliberately added. It also retards the kinetics of carbide precipitation during martensite quenching. When the Al content is less than 0.030%, the strengthening and carbide formation suppression effects are negligible. Aluminum values ​​below 0.030% are considered residual from the deoxidation step during steel production, and therefore a minimum content of 0.030% is preferable. On the other hand, when the Al content is above 1.00%, excessive oxide formation may occur during the thermomechanical processing (plate reheating, hot rolling, coiling, etc.) of the steel.Furthermore, Al increases the transformation temperature of ferrite into austenite, necessitating hot rolling of the steel at higher temperatures to complete the hot rolling process in the austenitic phase, since at higher temperatures... Petition 870260062179, dated 06 / 25 / 2026, p. 9 / 109 6 / 33 lower, intercritical ferrite appears. Higher amounts of oxidation can occur at higher temperatures. These oxide scales are detrimental to hot rolling, pickling, coating, and overall surface appearance. Furthermore, rolling forces during hot rolling increase when Al exceeds 1.00% in combination with the presence of Si at a level that makes the steel very brittle and difficult to hot roll. Additionally, an Al content above 1.00% can also promote ferrite formation during cooling on the exit table, reducing the incubation time for ferrite formation during continuous cooling. Ferrite is a detrimental phase in this invention because it will introduce brittle interfaces with fresh martensite and tempered martensite. These interfaces will act as nucleation points for the initiation of deformation damage, reducing the formability, elongation, and impact strength of the steel.Therefore, the Al in the present invention is present in an amount of 0.010-1.00%, preferably 0.030-1.00%, preferably 0.20-0.80%, more preferably in the range of 0.30-0.80%.

[036] Although Si or Al individually can give the effects of strengthening the solid solution and inhibiting carbide precipitation during martensite quenching, when both elements are present the synergistic effect of these elements is also similar to their individual effects. Therefore, the total content of (Si + Al) in this invention should be at least 0.80%, preferably at least 1.00% to achieve the desired carbide suppression effects and the desired strength levels. When Al and Si are both present, there can be several advantages facilitating the processing of the steel, particularly during hot rolling, pickling and coating. The presence of Petition 870260062179, dated 06 / 25 / 2026, page 10 / 109 7 / 33 Some amount of Al with Si alters the oxide characteristics in the scale during high-temperature processing. This facilitates the removal of the scale after hot rolling.

[037] As will be described later, the initial martensite that forms in hot-rolled steel during the coiling phase reduces the hardness during coil cooling in this invention. Suppressing carbide formation during this self-hardening (coil cooling) of the steel due to the individual or synergistic effects of Si and Al is important to the invention. As a result, martensite reduces its dislocation density only without forming carbides. Carbides are detrimental to the elongation, formability, and impact resistance of steel because they are brittle in nature and act as nucleation sites for the initiation of damage during deformation.

[038] Manganese is present in an amount of 1,003.00%. The main effect of Mn is to increase strength and toughness. At levels below 1.00% by weight, the intended effects are not achieved, while at amounts above 3.00%, casting and segregation problems will be caused. Furthermore, the deformation mechanism in the steel may change to transformation-induced plasticity (TRIP) due to the stabilization of austenite by Mn at room temperature, which is not conducive to obtaining a good combination of all the mechanical properties targeted in the product (i.e., impact strength, formability, strength). Preferably, the Mn content is in the range of 1.20-2.70%. In one embodiment, the Mn is between 1.40-2.60%, preferably between 1.50-2.50%, more preferably between 1.60-2.50%. In one embodiment, the minimum adequate amount of Mn is 1.65% and the maximum adequate amount of Mn would be 1.95%.

[039] In addition to the effects described above of the alloying elements Petition 870260062179, dated 06 / 25 / 2026, page 11 / 109 8 / 33 essential elements in the steel of the invention, namely C, Si, Al, and Mn, another collective effect of these alloying elements is to increase the hardenability of the steel. They help prevent the formation of pearlite or ferrite phases during cooling after austenitization. This characteristic allows the steel to avoid these phases above a certain cooling rate during the cooling of the exit table after hot rolling and before coiling. The presence of these softer (ferrite) and non-homogeneous (pearlite) phases is detrimental to obtaining good mechanical properties and formability in the final product, as they promote brittle and incoherent interfaces in the microstructure.

[040] One or more microalloying elements, selected from group V, Nb, Ti and Mo, are optionally present. These microalloying elements increase strength through precipitation hardening by their carbides, nitrides or carbonitrides. They also improve the weldability of the steel. Chromium, another optional element for this invention, also increases the hardenability of the steel.

[041] Chromium, another optional element for this invention, also increases the hardening capacity of the steel.

[042] Copper, when present, increases the strength of steel both by strengthening the solid solution and by precipitation hardening through copper precipitates. Nickel increases impact resistance and compensates for any lack of heat that may occur during hot working of the steel, due to the presence of copper.

[043] If present as linking elements, the preferred additions of these optional alloying elements (in % by weight) are:

[044] V: 0.010-0.10

[045] Nb: 0.010-0.10 Petition 870260062179, dated 06 / 25 / 2026, page 12 / 109 9 / 33

[046] Ti: 0.010-0.10

[047] Mo: 0.050-0.50

[048] Cr: 0.10-1.50

[049] Cu: 0.030-1.00

[050] Ni: 0.020-0.50

[051] Nitrogen, sulfur, and phosphorus are residual elements present in steel as a result of the steel production and refining process. Their quantities are limited to S < 0.005%, P < 0.020%, and N < 0.0100%. Quantities exceeding these are detrimental to mechanical properties, formability, and weldability. Preferably, S < 0.002% and N is between 0.0005% and 0.0100%. Nitrogen in the specified range gives a similar effect to C and contributes to strength by forming carbonitrides of microalloying elements.

[052] The optional alloying elements and the elements nitrogen, sulfur and phosphorus can be varied independently of each other within the specified ranges. They have been found to have an additive effect, not a synergistic effect, in the steels according to the invention.

[053] According to a second aspect, the invention is also configured in a process for producing a hot-rolled strip that achieves the desired microstructure in the final product. Consequently, the method according to the invention is a method for producing hot-rolled steel with the chemical composition described above.

[054] The steel production method comprises the following steps:

[055] - casting molten steel into slabs;

[056] - reheat the plates, preferably to a temperature of 1100°C or more and preferably for a time of 30 minutes or more; Petition 870260062179, dated 06 / 25 / 2026, page 13 / 109 10 / 33

[057] - perform roughing lamination of the slab at an intermediate gauge, typically in the range of 35-45 mm, to break the structure as cast;

[058] - to roll the hot steel into a strip, preferably with a hot finishing rolling temperature (FRT) above the Ar3 temperature of the steel, where Ar3 is the temperature at which the transformation of austenite into ferrite begins during cooling;

[059] - perform accelerated cooling of the hot-rolled strip on the exit table with a cooling rate greater than 20°C / s;

[060] - coiling hot-rolled and cooled steel strip to a temperature in the range of (Ms-50)°C to (Ms-160)°C, where Ms is the initial martensite temperature (in °C) of the steel;

[061] - cool the steel coil to room temperature;

[062] - to strip hot-rolled steel strip;

[063] - optionally, coat the hot-rolled steel strip with Zn or a Zn-based alloy or an Al-based alloy or any other coating;

[064] To avoid misunderstanding, Ms is expressed in °C. Preferably, the FRT is above Ar3 + 50°C. Figure 1 shows a schematic representation of the hot rolling and cooling process to ambient temperature superimposed on a schematic diagram of continuous cooling transformation (CCT). Ambient temperature is defined as around 20°C. Reheating is preferably carried out for a time of 60 minutes or more, particularly when the hot rolling process according to the invention is carried out in a conventional hot strip mill based on thick plates.

[065] The invention is not limited to the ingot casting method. The steel can be ingot-cast as a conventional thick plate with Petition 870260062179, dated 06 / 25 / 2026, p. 14 / 109 11 / 33 a casting thickness between 150 and 350 mm, and typically from 225 to 250 mm, as well as a thin slab with a casting thickness between 50 and 150 mm in a direct strip mill. Schematic examples of a process involving a conventional hot strip mill and a direct slab / direct casting mill are shown in Figures 2a and 2b, respectively.For conventional slab casting, slab reheating is necessary to reheat the slab from ambient temperature (usually thin cast slabs have cooled from the casting temperature to ambient temperature in a slab yard) and homogenize the slab composition. Therefore, the reheating temperature must be above 1100°C. This is also necessary to dissolve any precipitations when microalloying elements are present and to bring the slab to a temperature such that the final hot rolling in the finishing mill can still be performed at FRT>Ar3. This often requires a reheating temperature (slab) between 1150 and approximately 1250°C.For thin slab casting, the cast slab is subjected to a homogenization treatment in a homogenization furnace immediately after the thin slab casting where the homogenization temperature must be above 1100°C, and is normally around 1125 to 1150°C. This would also prevent the formation of any precipitates when microalloyed elements are present and would also bring the thin slab to a temperature such that the final hot rolling in the finishing mill can still be carried out at FRT>Ar3. According to the invention, the reheating or homogenization time for the thin slab casting path is preferably 30 minutes or more.

[066] The hot rolling of the steel should be carried out in the phase. Petition 870260062179, dated 06 / 25 / 2026, page 15 / 109 12 / 33 austenitic to ensure that no ferrite is present in the final microstructure. Another objective of hot rolling in the austenitic phase is to reduce the hot rolling force and therefore the final rolling temperature (FRT) is preferably maintained at a temperature that is at least 50°C higher than the Ar3 of the steel.

[067] After hot rolling, the steel strip is cooled on an exit table. Here the requirement is that the steel be cooled at a rate greater than the critical cooling rate to avoid any undesirable phase transformation from austenite. In particular, ferrite and pearlite must not form because they are detrimental to the mechanical properties and formability of the final product. Therefore, the ROT-CR must exceed the critical cooling rate to avoid the formation of ferrite and pearlite. There is no maximum critical ROT-CR because to ensure the transformation of austenite as long as the aforementioned critical cooling rate is exceeded up to the strip thickness.An unnecessarily high ROT-CR can affect the flatness of the strip after cooling and cause control problems in stopping at the correct cooling stop temperature; therefore, a suitable maximum ROT-CR is around 300°C / s, preferably around 200°C / s and more preferably around 150°C / s. A practical ROT-CR range is 20 to 100°C / s, as this is achievable through air cooling, laminar cooling, or water jet cooling, depending on the strip thickness. For practical reasons, the output table cooling rate (ROT-CR) is defined as the average cooling rate of the strip surface.

[068] Next, the hot-rolled steel strip is coiled at a temperature below the Ms of the steel, in the temperature range of (Ms-50)°C to (Ms-160)°C. Coiling below Ms is to ensure that subsequent cooling of the coil begins with Petition 870260062179, dated 06 / 25 / 2026, p. 16 / 109 13 / 33 a mixture of martensite and austenite phases, with the initial martensite content in the range of 40-85% by volume. If the initial martensite content is greater than this amount, or in other words, if the coiling temperature (CT) is below Ms-160°C, then the necessary quenching effect of the initial martensite is not obtained and it does not achieve high ductility, formability and impact resistance in the steel as a result of less time available during coil cooling and a temperature too low for effective quenching to occur. If the initial martensite content is less than 40% by volume, then excessive quenching of the martensite may occur, with the product not being an ultra-high strength steel in the context of this invention.

[069] During coil cooling, the tempering of the initial martensite occurs continuously. Simultaneously, as the steel cools in the coil, new fresh martensite forms. Due to the presence of Si and Al in the steel, no carbides are formed in the tempered martensite. Furthermore, due to some splitting of the carbon from the martensite into austenite, very small amounts of austenite may remain untransformed at room temperature (also known as retained austenite), but its quantity is preferably limited to a maximum of 1% by volume, including 0% by volume.

[070] After the steel has cooled to room temperature, the oxides (scale) in hot-rolled steels are removed by pickling in an acidic solution (e.g., HCl) at hot temperatures (80-120°C) or by a combination of pickling and mechanical brushing of the surface. This step is necessary to make the steel surface suitable for direct use as uncoated HR steel or to make it amenable to the coating process, when optionally required for corrosion resistance. Petition 870260062179, dated 06 / 25 / 2026, p. 17 / 109 14 / 33

[071] Optionally, the HR steel strip can be coated, for example by hot-dip or electro-coating, with Zn or a Zn-based alloy, or an Al-based alloy or any other coating technique to give good corrosion resistance in service.

[072] The above process results in the desired microstructure to obtain the desired mechanical properties. The invention is also configured to a steel article manufactured according to the above process and the chemical composition of the steel containing the following microstructure (in % by volume):

[073] - Tempered martensite (initial martensite during winding): 40-85%, preferably at least 50%, more preferably at least 60%;

[074] - Fresh martensite (martensite formed during coil cooling after winding): 15-60%, preferably 50% maximum, more preferably 40% maximum;

[075] - Retained austenite: maximum 1% by volume, including 0% by volume.

[076] - Cementite or any other metallic carbides: 0% by volume.

[077] The chemical composition of the steel, the processing and the microstructure result, according to the invention, in the following mechanical and formability properties.

[078] - Yield strength (Rp): at least 1100 MPa

[079] - Maximum tensile strength (Rm): at least 1200 MPa

[080] - Rate of return (Rp / Rm): at least 0.85

[081] - Total lengthening: at least 6.0% JIS5

[082] - Hole expansion capacity: at least 30%.

[083] - Bending angle at 1 mm thickness: at least 70°

[084] Preferably, the Charpy impact resistance is at Petition 870260062179, dated 06 / 25 / 2026, page 18 / 109 15 / 33 minus 40 Joules at -40°C and at least 100 Joules at room temperature.

[085] The chemical composition of the steel, processing and microstructure result, according to the invention, preferably in the following mechanical and formability properties.

[086] - Yield strength (Rp): at least 1100 MPa

[087] - Maximum tensile strength (Rm): at least 1200 MPa

[088] - Rate of return (Rp / Rm): at least 0.85

[089] - Total lengthening: at least 8.5% JIS5

[090] - Hole expansion capacity: at least 50%.

[091] - Bending angle at 1 mm thickness: at least 80°

[092] - Charpy impact resistance: at least 40 Joules at 40°C and at least 100 Joules at room temperature.

[093] The strength values ​​of steel result primarily from the presence of its hard constituents in the microstructure. Martensite is a strong phase in steel, and due to the low quenching temperature below Ms during coil cooling, martensite does not lose much of its strength. Therefore, both fresh martensite and tempered martensite in this invention are responsible for achieving ultra-high strength values ​​in this invention. Furthermore, the absence of carbides due to the presence of Si and Al in the steel reduces the initiation of damage to the steel during deformation, giving a high total elongation value. Retained austenite is minimized to less than 1% by volume, as it is detrimental due to its low stability for impact resistance.

[094] Retained austenite results from the splitting of carbon from martensite into austenite during coil cooling. Carbon increases the stability of austenite by lowering the temperature. Petition 870260062179, dated 06 / 25 / 2026, p. 19 / 109 16 / 33 However, in the present invention, retained austenite is deliberately avoided because it is difficult to control the mechanical stability of retained austenite during different deformation and forming processes. Retained austenite should have very high mechanical stability for its beneficial effect of increasing elongation (i.e., elasticity) and impact resistance. It needs very high carbon saturation along with a thin-film type morphology to enhance these properties. High carbon supersaturation during a continuous cooling process at low temperature, such as in coil cooling, is very difficult to achieve. When the mechanical stability of austenite is low, it rapidly transforms into martensite and creates brittle interfaces with the matrix phase, which affects the total elongation.Retained austenite with low mechanical stability transforms even more rapidly in a dynamic loading process, such as impact, and reduces impact resistance. Therefore, in this invention, a more homogeneous microstructure was created by tempered martensite and fresh martensite, without the presence of a large amount of retained austenite. In other words, the presence of retained austenite is intentionally avoided, and its maximum quantity was limited to 1% by volume.

[095] Another motivation for avoiding or minimizing the retained austenite phase in this invention is to reduce the propensity for liquid metal embrittlement (LME) during welding of Zn-coated steel or Zn alloy steel. It is known that Zn-coated steels or Zn alloy steels with retained austenite phase in their microstructures are more prone to LME during welding.

[096] This was achieved by using low temperature winding in the range of (Ms-50)°C to (Ms-160)°C, which is a temperature range where substantial carbon splitting is not expected. Petition 870260062179, dated 06 / 25 / 2026, page 20 / 109 17 / 33 stabilize large amounts of austenite.

[097] Rp, Rm and total elongation were determined from quasi-static tensile tests (strain rate 3 x 10-4 s-1) at room temperature with the JIS No. 5 specimen geometry with tensile tests parallel to the rolling direction according to EN 10002-1 / 150 6892-1. The geometry of the tensile specimens consisted of 50 mm length in the rolling direction, 25 mm width and a thickness of 3.2 mm. The strength of the steel at 0.2% strain is measured as tensile strength (Rp or YS). The ratio between the yield strength and the ultimate tensile strength (Rp / Rm) is expressed as the yield ratio.

[098] The bending capacity was determined by three-point bending tests following the VDA 238-100 standard on 3.2 mm thick, 40 mm x 30 mm samples in both longitudinal and transverse directions. The bending axis was along the 30 mm dimension and the bending radius was 0.4 mm. The bending angles obtained from the strips with different thicknesses (2.8, 3.2 and 3.5 mm thick respectively) were converted to the angles corresponding to 1.0 mm thickness using the following formula: bending angle at 1.0 mm thickness = measured angle x square root of actual thickness in mm. From these converted bending angles, for a specific heat treatment condition, the lowest value from the longitudinal and transverse specimens was taken to claim the strips of this invention.

[099] The flanging capacity of the steel or the hole expansion capacity (HEC) was determined by hole expansion tests. Specimens with dimensions of 90 mm χ 90 mm x 3.2 mm were cut from coiled coated steel. A 10 mm diameter hole was drilled in the middle of the specimens, and hole expansion tests were performed according to the standard. Petition 870260062179, dated 06 / 25 / 2026, page 21 / 109 18 / 33 ISO / TS 16630:2003(E). The HEC value was determined by the formula: HEC = (initial hole diameter expansion / initial diameter) χ 100%.

[0100] Charpy impact resistance was measured using full-size (55 mm x 10 mm x 10 mm) Charpy V-notch (CVN) specimens according to ASTM A370. Tests were performed in both directions of the sheet, machining the V-groove parallel and perpendicular to the rolling direction.

[0101] For all the mechanical tests above, at least three samples were tested for each condition and the average values ​​are reported.

[0102] The microstructure was analyzed using a combination of techniques - optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and dilatometry. Dilatometry tests on (I x w x t) specimens of 10 mm x 5 mm x 3.2 mm were performed by heating the specimens at a rate of 10°C / s up to 950°C, holding for 2 minutes, and cooling to room temperature at a rate of 100°C / s (slow cooling for Ms) or 0.3°C / s (slow cooling for Ar3). From the dilatometry data, the Ms and Ar3 temperatures were determined. The amount of initial martensite (that is, also the tempered martensite that is quenched after cooling the coil) after coiling the steel was determined using the Koistinen-Marburger formula given in the literature below: “A general equation prescribing the extent of the austenite-martensite transformation in pure iron-carbon alloys and plain carbon steels” by D.P. Koistinen, R.E. Marburger, Acta Metallurgica, vol. 7, 1959, pp.59-60. f = 100 {1 - exp(-(1.10 x 10-2· (¾ - CT))}

[0103] where Ms is the initial temperature of the martensite (in °C) and CT is the temperature of the coil (in °C), thus (Ms-CT) reflects the subcooling below Ms at the beginning of the coil cooling and is, Petition 870260062179, dated 06 / 25 / 2026, page 22 / 109 19 / 33 therefore, a measure for the initial amount of martensite.

[0104] The amount of retained austenite was determined by XRD at a location ¼ of the sample thickness. XRD patterns were recorded in the range of 45° to 165° (2Q) on an Xpert PRO Panalytical standard powder diffractometer (CoKa radiation). Quantitative determination of phase proportions was performed by Rietveld analysis using the Bruker Topas software package for Rietveld refinement. The amounts of carbides, ferrite, pearlite, and bainites in the microstructure were determined through high-resolution SEM image analysis. By subtracting the initial martensite and other phase fractions as applicable (retained austenite, carbide, and other determined phases) from the total amount, the fresh martensite fractions were obtained.

[0105] The composition of the zinc or zinc alloy coating is not limited. Although the coating can be applied in various ways, hot-dip galvanizing is preferred using a standard Gl coating bath. The Zn-based coating may comprise a Zn alloy containing Al as a bonding element. The preferred composition of the zinc bath contains 0.100.35% by weight of Al, the remainder being zinc and unavoidable impurities.

[0106] Other Zn coatings can also be applied. An example comprises a zinc alloy coating according to WO 2008 / 102009, in particular a zinc alloy coating layer consisting of 0.3–4.0% by weight of Mg and 0.05–6.0% by weight of Al, preferably 0.1–5.0% of Al, and optionally a maximum of 0.2% by weight of one or more additional elements together with unavoidable impurities, the remainder being zinc. A preferred Zn bath composed of Mg and Al as the main alloying elements has the composition: 0.5–3.8% by weight of Al, 0.5–3.0% by weight of Mg, optionally a maximum of 0.2% of a Petition 870260062179, dated 06 / 25 / 2026, p. 23 / 109 20 / 33 or more additional elements; the remainder being zinc and unavoidable impurities. An additional element normally added in a small amount of less than 0.2% by weight could be selected from the group consisting of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi. Pb, Sn, Bi and Sb are generally added to form flowers (crystals). Preferably, the total amount of additional elements in the zinc alloy is at most 0.2% by weight. These small amounts of an additional element do not alter the properties of the coating or the bath to any significant extent for usual applications. Preferably, when one or more additional elements are present in the coating, each is present in an amount < 0.02% by weight, preferably each is present in an amount < 0.01% by weight.Additional elements are typically added only to prevent the formation of impurities in the molten zinc alloy bath for hot-dip galvanizing, or to form crystals in the coating layer.

[0107] In another embodiment, the metallic coating comprises a (commercially pure) layer of aluminum or a layer of aluminum alloy. A typical metal bath for hot-dip coating such as an aluminum layer comprises an aluminum alloy with silicon, for example, an aluminum alloy with 8 to 11 ppm of silicon and a maximum of 4% iron, optionally a maximum of 0.2% of one or more additional elements such as calcium, unavoidable impurities, the remainder being aluminum. Silicon is present to prevent the formation of a thick ferrometallic intermetallic layer that reduces adhesion and formability. Iron is preferably present in amounts between 1 and 4%, more preferably at least 2%. Examples Petition 870260062179, dated 06 / 25 / 2026, page 24 / 109 21 / 33

[0108] Steel ingots of seven chemical compositions of the invention Steels AB and DH, and a comparative steel C with dimensions 200 mm x 100 mm x 100 mm, were cast by melting the charges in a vacuum induction furnace. The chemical compositions of these steels are presented in Table 1. Steels AB and DH contain C, Si, and Al within the limits defined in the invention, while the comparative steel has Al and Si outside the limits defined in the invention. All ingots were reheated for 1 hour at 1200°C and rolled on roughing rolling to a thickness of 25 mm. Then, the strips were reheated again at 1200°C for 30 minutes and hot-rolled to their final thicknesses of 2.8, 3.2 mm, 3.5, and 12 mm with FRTs above 900°C, which are in the austenitic phase range for all steels. The Ar3 and Ms values ​​for the steels, measured by dilatometry, are also presented in Table 1.

[0109] After hot rolling, the steels were immediately subjected to exit table cooling at various cooling rates, and then coil cooling simulations were performed in a muffle furnace by cooling to ambient temperature from different starting CTs. Subsequently, the strips were pickled to remove oxides in the conventional manner.

[0110] The various processing conditions for the steels are summarized in Table 2. Steels A, B, and C have similar Ms and Ar3. The FRT temperature for steel A was 953°C, for steel B 939°C, and for steel C 945°C, all at least 50°C above Ar3. For steels A and B, a slow table cooling rate (ROT-CR) of 3°C / s was used, which is outside the defined lower limit of the invention. In addition, two additional coiling temperatures (200°C and 480°C), outside the defined limits of this invention, were used for steels A and B with FRT and CT within the limits required in the invention. The TC of 200°C is much lower than (Ms-160)°C and Petition 870260062179, dated 06 / 25 / 2026, p. 25 / 109 22 / 33 480°C is above the Ms of these steels. These conditions were used for comparison purposes. For steel C, which has a chemical composition outside the scope of this invention, all processing conditions (FRT, ROT-CR, and CT) were chosen within the limits defined by this invention. For DF steels, one set of processing parameters was within the ranges claimed for this invention (FRT, ROT-CR, and CT), but for the other set, FRT and ROT-CR were kept the same, but only CT was kept higher for comparison purposes. In this case, a CT of 375°C was used, which was higher than (Ms-50°C) for DF steels. In fact, this CT was slightly higher than their Ms temperatures. For GH steels, all processing parameters were kept within the limits required by the present invention.

[0111] Samples were extracted from the final steel strips for various mechanical and microstructural characterizations, as described. Hot-rolled steels 12 mm thick were used for the preparation of Charpy impact specimens, while steel strips 2.8, 3.2, and 3.5 mm thick were used for all other characterizations.

[0112] For various processing conditions, the phase content in the microstructures is presented in Table 3, tensile properties are given in Table 4, bending and HEC test results in Table 5, and Charpy impact strength in Table 6. The following are the abbreviations and symbols used in the tables to present the tensile and bending test results: Rp = yield strength, Rm = maximum tensile strength, AJIS5 = total elongation using JIS5 specimens, BA = bending angle, L = longitudinal specimen where the bending axis is parallel to the rolling direction, T = transverse specimen where the bending axis is perpendicular to the rolling direction. Petition 870260062179, dated 06 / 25 / 2026, page 26 / 109 23 / 33 lamination.

[0113] Table 3 shows that steel A and steel B achieved microstructures consisting of less than 85% and at least 15% by volume of tempered martensite and fresh martensite, respectively, using FRT and ROT-CR within the defined limits for a CT ranging from 275 to 375°C, which is within the range required for this invention. Furthermore, the steels do not contain carbide in their microstructures, and the retained austenite content was less than 1% by volume for these processing conditions. Their microstructures did not contain any other phases, such as ferrite, bainite, or pearlite.

[0114] AB steels, when subjected to a ROT-CR>20°C / s with FRT at a temperature above 50°C above Ar3, produced microstructures with substantial amounts of bainite and retained austenite with some amounts of fresh martensite forming below Ms when wound at 480°C, which is above the Ms of these steels. There were no carbides present in these steels for this winding condition because of their Al and Si content. The high content of retained austenite was caused by carbon enrichment in the austenite during the bainitic transformation above Ms and in the fresh martensite formed during coil cooling below Ms. These bainitic microstructures with high amounts of retained austenite are different from the microstructures intended in this invention.

[0115] Similarly, when steels A and B are wound to At 200°C, which is much lower than the Ms-160°C of these steels, the microstructure of the steels also becomes different from that required for this invention. This CT condition has more than 85% by volume of tempered martensite and less than 15% by volume of fresh martensite. On the other hand, steels A and B, when they had FRT and Petition 870260062179, dated 06 / 25 / 2026, p. 27 / 109 24 / 33 CT within the range required for this invention, but with a slower ROT-CR of 3°C / s (less than 20°C / s), showed considerable amounts of ferrite and pearlite in their microstructures, in addition to a bainite matrix and substantial amounts of retained austenite. Ferrite, pearlite, and bainite formed due to the slow ROT-CR before being rolled.

[0116] C steel formed a considerable amount of carbides (2.3% by volume) during processing with all parameters within the range required for this invention due to the low amounts of Si and Al in C steel.

[0117] As a result of the microstructures described above, the properties were obtained as indicated in Tables 4 to 6. Steels A and B achieved Rp above 1100 MPa and Rm above 1200 MPa with a yield ratio greater than 0.85 along with a total elongation (AJIS5) greater than 8.5%. When the CT is very high (480°C), the minimum Rp and Rm levels targeted in this invention are not achieved in steels A and B due to the presence of softer phases – bainite and retained austenite – although the total elongation is high. The low Rp values ​​for CT of 480°C also caused the yield ratio to be less than 0.85. On the other hand, when the CT is very low (CT = 200°C), the Rp and Rm are above the target values ​​with a high yield ratio, but the total elongation is very low (<8.5%).The low total elongation is caused by a very high amount of initial martensite (= tempered martensite > 85% by volume) present in the microstructure and a lack of quenching effect during coil cooling due to less time available and a temperature too low for effective quenching to occur.

[0118] With a slow ROT-CR of 3°C / s, the Rp and Rm values ​​in steels A and B are less than 1100 MPa and 1200 MPa, respectively, Petition 870260062179, dated 06 / 25 / 2026, p. 28 / 109 25 / 33 due to the formation of the softer phases of bainite, ferrite, pearlite, and retained austenite. Furthermore, the yield ratio is less than 0.85, although the total elongation is high.

[0119] Due to the presence of carbides in its microstructure and the absence of carbide-suppressing elements Si and Al, C steel achieved low values ​​of Rp, Rm, yield ratio, and total elongation. Carbides are detrimental to mechanical properties and promote damage during deformation. Therefore, low levels of tensile properties were obtained in C steel.

[0120] As tensile properties, bending capacity and HEC are also high in steels A and B when processed within the defined processing variables (FRT, ROT-CR and CT) of this invention (Table 5). A minimum bending angle of 80° at 1.0 mm thickness was achieved and a minimum HEC value of 50% was also obtained. However, when the CT is high and above the Ms of the steels (i.e. 480°C), the minimum bending angle and HEC are low and below the target values ​​of 80° at 1.0 mm thickness and 50% respectively. This is because, due to the multiphase nature of the microstructures of these steels containing fresh martensite, bainite and retained austenite (Table 3), numerous damage initiator sites were present at the interfaces of these phases when deformation was performed. Martensite, present as fresh martensite and formed due to the TRIP effect of retained austenite, are both stronger phases than bainite and any untransformed retained austenite.On the other hand, under ideal processing conditions with optimal amounts of tempered martensite and fresh martensite in steels A and B, there was low hardness or a difference in strength between these phases, causing homogeneous deformation during bending and hole expansion. Petition 870260062179, dated 06 / 25 / 2026, page 29 / 109 26 / 33 This resulted in high HEC values ​​and folding under ideal processing conditions.

[0121] Furthermore, with a very low CT of steels A and B below Ms (200°C), the bending and HEC values ​​are also low due to the very high amount of tempered martensite (>85% by volume) present in their microstructures (Table 3). The lack of effective tempering of this initial martensite, present only at the beginning of coiling, caused low ductility, which was also reflected in their total elongation values ​​in Table 4, resulting in the low formability of these steels, measured by bending capacity and HEC.

[0122] When a slow ROT-CR (3°C / s) is employed, the presence of the softer ferrite and pearlite phases also deteriorated the bending capacity and HEC values ​​of steels A and B, as seen in Table 5. This is due to the brittle interfaces of these softer phases and the harder bainite and martensite phases that are obtained after the transformation of the retained austenite during loading.

[0123] Steel C achieved very low bending capacity and HEC values, much lower than the minimum values ​​of 80° at 1.0 mm thickness and 50% respectively. These insufficient formability parameters in steel C were caused by the very low Al and Si content in the steel, which promoted carbide formation (Table 3), although the processing variable was within the range specified for this invention.

[0124] The Charpy impact resistance of steel A, processed according to the invention, in cross-sectional samples (which showed values ​​lower than those of longitudinal samples) is greater than 100 J and 40 J when tested at room temperature and -40°C, respectively. The same values ​​with the elevated CT of 480°C and the Petition 870260062179, dated 06 / 25 / 2026, p. 30 / 109 27 / 33 The lower CT values ​​of 200°C are significantly lower than the minimum values ​​obtained with the ideal processing route described in this invention. As explained previously, these low toughness values ​​are caused by brittle fracture resulting from the spontaneous transformation of retained austenite into martensite, the presence of harder and softer heterogeneous phases, and low tempering effects. Furthermore, the presence of carbides in the C steel also resulted in insufficient Charpy impact resistance values, both at room temperature and cryogenic temperature.

[0125] Therefore, as discussed above, these examples illustrate that when steel is designed according to the composition of the invention and processed according to the invention, the steel achieves high tensile, formability, and toughness properties, as intended, due to its microstructural effects. The same good combination of all properties is not achieved when working outside the limits defined in the invention. Table 1 - Chemical compositions of steels in % by weight (I: invention, C: comparison) Steel C Si Al Mn PSN Nb Ms (oC) Ar3 (oC) A 0.2 1 0.036 1.82 0.01 0.001 0.002 - 428 827 IB 0.2 0.81 0.31 1.82 0.01 0.0012 0.0012 - 435 835 IC 0.21 0.1 0.02 1.85 0.01 0.0012 0.0006 - 430 829 CD 0.2 1 0.03 2.31 0.001 0.0001 0.0012 0.001 366 834 IE 0.21 0.81 0.31 2.3 0.001 0.0001 0.002 0.001 374 854 IF 0.21 0.8 0.03 2.3 0.001 0.0001 0.0008 0.001 374 831 IG 0.18 0.81 0.033 2.39 0.001 0.0001 0.003 0.001 390 840 IH 0.18 0.8 0.032 2.4 0.001 0.0001 0.004 0.019 377 835 I

[0126] The Nb content in AG steels is at a residual level. No niobium was added as an alloying element in these steels. Niobium was added as an alloying element in H steel.

[0127] AB steels have a Mn content of about 1.8% and Petition 870260062179, dated 06 / 25 / 2026, p. 31 / 109 28 / 33 DH steels have a Mn content of approximately 2.35% with varying amounts of Si and Nb. The effect of Mn is a reduction in Ms and causes a change in the ratio of tempered martensite to fresh martensite. Although DH steels have lower values ​​for A JIS5, HEC, and bending angle than A and B steels, DH steels are nevertheless suitable for the purpose.

[0128] As shown in Table 3, the DH steels achieved microstructures consisting of tempered martensite and fresh martensite of less than 85% by volume and at least 15% by volume, respectively, using FRT and ROT-CR within the limits defined for a CT of 275°C (DF steels) and 300°C (GH steels), which fall within the range required for this invention. Furthermore, the steels do not contain carbides in their microstructures, and the retained austenite content was less than 1% by volume for these processing conditions. Their microstructures did not contain any other phases, such as ferrite, bainite, or pearlite.

[0129] DH steels also achieved Rp above 1100 MPa and Rm above 1200 MPa with a yield ratio of 0.85 or higher, along with total elongation (AJIS5) greater than 6% (Table 4). Bending capacity and HEC are also high in DH steels when processed within the defined processing variables (FRT, ROT-CR and CT) of this invention (Table 5). A minimum bending angle of 70° at 1.0 mm thickness and a minimum HEC value of 30% were achieved in these steels.

[0130] However, when the CT of DF steels is high, i.e., At 375°C, which is slightly above the Ms of steels, the microstructures of the steels contain some amounts of bainite and more than 1% by volume of retained austenite (Table 3), which is not intended to be achieved in the microstructures of this invention, and the contents of fresh martensite and tempered martensite are also found outside the Petition 870260062179, dated 06 / 25 / 2026, p. 32 / 109 29 / 33 range defined in this invention. These unintended microstructures do not lead to the desired ultra-high strengths in these steels for 375°C CT. Rp values ​​are below 1100 MPa and Rm values ​​are below 1200 MPa with a yield ratio of less than 0.85 due to the presence of softer bainite and retained austenite phases (Table 4), although good bending and HEC values ​​are achieved (Table 5). Thus, steels that may have a chemical composition within the limits defined in this invention may not achieve all the desired mechanical properties if processing is not carried out within the windows defined in this invention. Table 2 - Processing variables applied to steels Steel Final Thickness (mm) FRT (oC) ROT-CR (oC / s) Coiling Temperature (oC) A 3.2 / 12* 953 31 275, 300, 325, 350, 375 I 3.2 / 12* 953 31 480, 200 C 3.2 / 12* 953 3 325 CB 3.2 939 34 275, 300, 325, 350, 375 I 3.2 939 34 480, 200 C 3.2 939 3 325 CC 3.2 / 12 945 34 350 CD 2.8 962 45 275 I 2.8 45 375 CE 2.8 965 48 275 I 2.8 48 375 CF 2.8 968 53 275 I 2.8 53 375 CG 3.5 973 47 300 IH 3.5 976 46 300 I

[0131] *12 mm thickness is required for Charpy tests, Petition 870260062179, dated 06 / 25 / 2026, p. 33 / 109 30 / 33 other thicknesses were used to determine the microstructure, tensile properties, bending angle and HEC. Table 3 - Microstructure of steels (B = bainite, P = pearlite, F = ferrite) ROTCR Steels (oC / s) Temp. Coiling temperature (°C) Tempered martensite (% by volume) Fresh martensite (% by volume) Retained austenite (% by volume) Carbides (% by volume) Others (% by volume) A 1 31 275 81.4 18.2 0.4 0 ---- I 2 31 300 75.5 23.8 0.7 0 ---- I 3 31 325 67.8 31.9 0.3 0 ---- I 4 31 350 57.6 42.3 0.1 0 ---- I 5 31 375 44.2 55.7 0.1 0 ---- I 6 31 480 — 13.8 11.2 0 B: 75.0 C 7 31 200 91.6 8.3 0.1 0 — C 8 3 325 — — 5.3 0 B=63.5 F+P=31.2 CB 1 34 275 82.8 16.7 0.5 0 ---- I 2 34 300 77.3 22.4 0.3 0 ---- I 3 34 325 70.2 29.8 0 0 ---- I 4 34 350 60.7 39.3 0 0 ---- I 5 34 375 48.3 51.7 0 0 ---- I 6 34 480 — 12.1 10.5 0 B: 77.4 C 7 34 200 92.5 7.5 0 0 — C 8 3 325 — — 6.1 0 B=65.3 F+P=28.6 CC 34 350 39.2 58.5 0 2.3 — CD 45 275 63.2 36.2 0.6 0 --- I 45 375 10.2 63 5.3 0 B=21.5 CE 48 275 66.3 33 0.7 0 --- I 48 375 11.4 74 4.3 0 B=10.3 CF 53 275 66.3 33.2 0.5 0 --- I 53 375 10.4 75 4.5 0 B=10.1 CG 47 300 62.8 36.8 0.4 0 --- I Petition 870260062179, dated 06 / 25 / 2026, p. 34 / 109 31 / 33 ROTCR Steels (°C / s) Coiling Temp. (°C) Tempered Martensite (% by volume) Fresh Martensite (% by volume) Retained Austenite (% by volume) Carbides (% by volume) Others (% by volume) H 46 300 57.1 42.6 0.3 0 --- I Table 4 - Tensile properties of steels ROTCR Steel (°C / s) Coiling Temperature (°C) Rp (MPa) Rm (MPa) A JIS5 (%) Yield Ratio (-) A 1 31 275 1188 1379 10.1 0.86 I 2 31 300 1176 1351 8.8 0.87 I 3 31 325 1178 1333 9.3 0.88 I 4 31 350 1143 1292 9.3 0.89 I 5 31 375 1110 1217 9.2 0.91 I 6 31 480 693 983 16.8 0.7 C 7 31 200 1215 1435 6.3 0.85 C 8 3 325 670 932 11.7 0.72 CB 1 34 275 1148 1327 9 0.87 I 2 34 300 1150 1321 9.1 0.87 I 3 34 325 1148 1300 10 0.88 I 4 34 350 1137 1263 10.4 0.9 I 5 34 375 1127 1228 9.6 0.92 I 6 34 480 705 979 15.3 0.72 C 7 34 200 1220 1428 6.4 0.85 I 8 3 325 663 927 12.1 0.72 CC 34 350 781 1079 6.1 0.73 CD 1 45 275 1140 1331 6.9 0.86 I 2 45 375 755 944 13.6 0.8 EC 1 48 275 1132 1334 7 IH 46 300 1212 1374 8.7 0.88 I Petition 870260062179, dated 06 / 25 / 2026, p. 35 / 109 32 / 33 Table 5 - Bending capacity and HEC of steels ROTCR steel (oC / s) Coiling temperature (oC) BA-L measured (o) BA-T measured at about (o) BA-L at about 1.0 mm (o) BA-T at about 1.0 mm (o) HEC (%) A 1 31 275 45 56.7 80.5 101.4 67 I 2 31 300 56.1 61.4 100.4 109.8 72 I 3 31 325 68.5 67.3 122.5 120.4 79 I 4 31 350 77.8 72.9 139.2 130.4 94 I 5 31 375 62.2 79 111.3 141.3 97 I 6 31 480 35.5 37.2 63.5 66.5 27 C 7 31 200 37.2 38.3 66.5 68.5 19 C 8 3 325 39.2 49.1 70.1 71.7 21 CB 1 34 275 61.4 59.5 109.8 106.4 52 I 2 34 300 59.3 61.5 106.1 110 83 I 3 34 325 48.5 69 86.8 123.4 77 I 4 34 350 69.1 69.3 123.6 124 76 I 5 34 375 55.7 75 99.6 134.2 98 I 6 34 480 33.5 35.1 59.9 62.8 21 C 7 34 200 34.8 36.1 62.3 64.6 18 C 8 3 325 37.2 39.7 66.5 71 23 CC 34 350 35.8 37.2 64 66.5 15 CD* 1 45 275 93.6 76.2 156.6 127.5 41 I 2 45 375 75.8 107.6 126.8 180 57 CE* 1 48 275 53.3 43.4 89.2 72.6 42 I 2 48 375 77 93.6 128.8 156.6 38 CF* 1 53 275 57.7 68 96.6 113.8 32 I 2 53 375 90.6 105.8 151.6 177 35 CG** 47 300 43.9 61.4 82.2 114,8 76 IH** 46 300 45.3 44.1 84.8 82.4 42 I,

[0132] Bending angle AC measured at 3.2 mm; *DF bending angle measured at 2.8 mm thickness; **G, H bending angle measured at 3.5 mm thickness. Petition 870260062179, dated 06 / 25 / 2026, p. 36 / 109 33 / 33 Table 6 - Charpy impact resistance of steels (L = longitudinal sample, T = transverse sample) ROT-CR Steel (oC / s) Temp. Winding temperature (°C) Charpy impact resistance (J) Ambient temperature - 40°C LTLTA 1 31 275 136.9 109.6 90.8 42.1 2 31 300 135.8 111.2 93.2 45.7 3 31 325 140.1 115.8 95.1 46.3 4 31 350 143.7 120 99.8 49.1 5 31 375 144 120.2 100.3 49.5 6 31 480 90.1 75.1 55.1 30.2 7 31 200 76.1 69.2 30.5 26.1 8 3 325 57.8 53.2 23.5 19.1 C 34 350 70.2 75.8 28.3 15.6 Brief description of the drawings

[0133] The invention will now be explained by means of the following drawings, which are not limiting.

[0134] A schematic of the thermomechanical processing of this invention is shown in Figure 1.

[0135] A schematic drawing of a hot strip mill for processing thick slabs is shown in figure 2a, and a thin slab casting plant with a direct mill is shown in figure 2b.

[0136] Figures 3 and 4 show the geometry and definitions of the folding samples and Charpy samples. Petition 870260062179, dated 06 / 25 / 2026, page 37 / 109

Claims

1 / 6 CLAIMS 1. Ultra-high strength hot-rolled steel strip, characterized in that it consists of (in % by weight) • C: 0.10 - 0.30; • Si: 0.50 - 1.50; • Al: 0.010 - 1.00; • Mn: 1.00 - 3.00; • (Si + Al): 0.80 - 2.50; and optionally, any one or more of the following alloying elements: • V: less than 0.10; • Nb: less than 0.10; • Ti: less than 0.10; • Mo: less than 0.50; • Cr: less than 1.50; • Cu: less than 1.00; • Ni: less than 0.50; • B: less than 0.0030 (30 ppm); inevitably also consisting of: • N: less than 0.0100 (100 ppm), • S: less than 0.005; • P: less than 0.020;the remainder being Fe and other unavoidable impurities resulting from the iron and steel production process, with a yield strength of at least 1100 MPa, a maximum tensile strength of at least 1200 MPa, a yield ratio of at least 0.85, a total elongation of at least 6.0%, a hole expansion rate of at least 30% and a bending angle at 1 mm thickness of at least 70°; and with a microstructure consisting of 40 to 85% by volume of tempered martensite, 60 to 15% by volume of fresh martensite, less than 1% by volume of retained austenite and without cementite or other carbides.

2. Hot-rolled steel strip according to claim 1, characterized in that it consists of (in % by weight): • - Al: 0.030 - 1.

00.

3. Hot-rolled steel strip according to claim 1 or 2, characterized in that it has a yield strength of at least 1100 MPa, an ultimate tensile strength of at least 1200 MPa, a yield ratio of at least 0.85, a total elongation of at least 8.5%, a hole expansion ratio of at least 50%, a bending angle at 1 mm thickness of at least 80°, and a Charpy impact strength of at least 40 J at -40°C and at least 100 J at room temperature; with a microstructure consisting of 40 to 85% by volume of tempered martensite, 60 to 15% by volume of fresh martensite, less than 1% by volume of retained austenite, and without cementite or other carbides.

4. Hot-rolled steel strip, according to any one of claims 1 to 3, characterized in that it consists of one or more of the following elements in the following amounts (in % by weight): • V: 0.010 - 0.10; • Nb: 0.010 - 0.10; • Ti: 0.010 - 0.10; • Mo: 0.050 - 0.50; • Cr: 0.10 - 1.50; • Cu: 0.030 - 1.00; Petition 870260062179, dated 06 / 25 / 2026, p. 39 / 109 3 / 6 • Ni: 0.020 - 0.50; • N: 0.0005 - 0.0100; • S: at most 0.

002.

5. Hot-rolled steel strip according to any one of claims 1 to 4, characterized in that the microstructure consists of at least 55% by volume of tempered martensite and at most 45% by volume of fresh martensite.

6. Hot-rolled steel strip according to any one of claims 1 to 5, characterized in that the steel consists of at least 1.65% by weight of Mn and at most 2.50% by weight of Mn.

7. Hot-rolled steel strip according to any one of claims 1 to 6, characterized in that the sum of the Al and Si contents is at least 1.00% by weight.

8. Hot-rolled steel strip, according to any one of claims 1 to 7, characterized in that it is provided with a metallic coating layer, such as a Zn layer or a Zn-based alloy layer or an Al-based alloy layer, obtained by hot-dip coating.

9. Hot-rolled steel strip, according to claim 8, characterized in that the zinc alloy coating layer consists of 0.3 - 4.0% by weight of Mg and 0.05 - 6.0% by weight of Al, or 0.1 to 5.0% of Al, and optionally a maximum of 0.2% by weight of one or more additional elements together with unavoidable impurities, the remainder being zinc.

10. Method for producing a hot-rolled steel strip exhibiting ultra-high strength, characterized by the fact that it consists of the following steps: - casting molten steel into thick or thin plates with Petition 870260062179, dated 06 / 25 / 2026, page 40 / 109 4 / 6 a composition (in % by weight) - • C: 0.10 - 0.30; • Si: 0.50 - 1.50; • Al: 0.030 - 1.00; • Mn: 1.00 - 3.00; • (Si + Al): 0.80 - 2.50; and, optionally, any one or more of the following alloying elements: • V: less than 0.10; • Nb: less than 0.10; • Ti: less than 0.10; • Mo: less than 0.50; • Cr: less than 1.50; • Cu: less than 1.00; • Ni: less than 0.50; • B: less than 0.0030 (30 ppm); inevitably also consisting of • N: less than 0.0100 (100 ppm); • S: less than 0.005; • P: less than 0.020; the balance being Fe and other unavoidable impurities resulting from the iron and steel production process; - heating or reheating the plates to a temperature of 1100 °C for a time of 30 minutes or more;- Hot rolling of the slab into a hot-rolled strip by: - ​​roughing the thick slab into an intermediate gauge, typically in the 35-45 mm range, to break the as-cast structure, followed by finished hot rolling into a hot-rolled strip, or Petition 870260062179, dated 06 / 25 / 2026, page 41 / 109 5 / 6 - hot rolling of the thin slab into a hot-rolled sheet by direct rolling: wherein the finished hot rolling temperature (FRT) is above the Ar3 temperature of the steel, where Ar3 is the temperature at which austenite transformation into ferrite begins during cooling; - performing accelerated cooling of the hot-rolled strip on the exit table with a cooling rate greater than 20°C / s; - next, coil the hot-rolled and cooled steel strip to a temperature in the range of (Ms-50)°C to (Ms-160)°C, where Ms is the initial martensite temperature of the steel;- Allow the hot-rolled coiled strip to cool further to ambient temperature; and - Pickle the hot-rolled steel strip.

11. Method according to claim 10, characterized in that the plate consists of one or more of the following elements in the following amounts (in % by weight): • V: 0.010 - 0.10; • Nb: 0.010 - 0.10; • Ti: 0.010 - 0.10; • Mo: 0.050 - 0.50; • Cr: 0.10 - 1.50; • Cu: 0.030 - 1.00; • Ni: 0.020 - 0.50; • N: 0.0005 - 0.0100.

12. Method, according to claim 10 or 11, characterized in that the microstructure consists of at least 55% by volume of tempered martensite and at most 45% by volume of fresh martensite.

13. Method according to any one of claims 10 to 12, characterized in that: - the steel consists of at least 1.65% by weight of Mn and at most 2.50% by weight of Mn, and / or - the sum of Al and Si is at least 1.00% by weight.

14. A method according to any one of claims 10 to 13, characterized in that it is provided with a metallic coating layer, such as a Zn layer or a Zn-based alloy layer or an Al-based alloy layer, obtained by hot-dip coating.

15. Method according to claim 14, characterized in that the zinc alloy coating layer consists of 0.3 - 4.0% by weight of Mg and 0.05% - 6.0% by weight of Al, preferably 0.1 to 5.0% of Al, and optionally a maximum of 0.2% by weight of one or more additional elements together with unavoidable impurities, the remainder being zinc.

16. Use of hot-rolled steel, as defined in any one of claims 1 to 9, characterized in that it is for the preparation of part(s) used for transport or engineering application(s).

17. Use according to claim 16, characterized in that it is for the preparation of chassis(es) or suspension part(s) of a vehicle.

18. Use according to claim 16 or 17, characterized in that it is for preparing a lower control arm, frame rail, bumper beam or battery box or a structure of a heavy truck, or a crane boom. Petition 870260062179, dated 06 / 25 / 2026, pp. 43 / 109

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