Cold-rolled and hot-treated steel sheet and method for producing a cold-rolled and hot-treated steel sheet.

Description

"Cold-rolled and hot-treated steel sheet and method for producing a cold-rolled and hot-treated steel sheet" Field of Invention

[001] The present invention relates to cold-rolled and heat-treated steel sheet that is suitable for use as steel sheets for automobiles. Background of the Invention

[002] Automotive parts are needed to satisfy two inconsistent needs, namely ease of forming and strength, but in recent years a third need for improved fuel consumption has also been added to automobiles in view of global environmental concerns. Thus, automotive parts must now be made of material that has high formability in order to meet the criteria of ease of fitting in complex automobile assembly and at the same time must improve strength for vehicle collision protection and durability while reducing vehicle weight to improve fuel efficiency.

[003] Therefore, intensive development and research efforts are directed toward reducing the amount of material used in cars by increasing material strength. On the other hand, an increase in the strength of steel sheets decreases formability, and thus the development of materials that have both high strength and high formability is necessary.

[004] Initial research and developments in the field of high-strength, high-formability steel sheets have resulted in several methods for producing high-strength, high-formability steel sheets, some of which are numbered in this document for conclusive appreciation of the present invention: Petition 870260036885, dated 04 / 20 / 2026, page 11 / 40 2 / 19 EP3144406 is a patent claiming a high-strength cold-rolled steel sheet having excellent ductility comprising, in % by weight, Carbon (C): 0.1% to 0.3%, Silicon (Si): 0.1% to 2.0%, Aluminum (Al): 0.005% to 1.5%, Manganese (Mn): 1.5% to 3.0%, Phosphorus (P): 0.04% or less (excluding 0%), Sulfur (S): 0.015% or less (excluding 0%), Nitrogen (N): 0.02% or less (excluding 0%), and a remainder of Iron (Fe) and unavoidable impurities, wherein the sum of Silicon and Aluminum (Si+Al) (% by weight) satisfies 1.0% or more, and wherein a microstructure It comprises: by area fraction, 5% or less of polygonal ferrite having a ratio between minor geometric axis and major geometric axis of 0.4 or greater, 70% or less (excluding 0%) of acicular ferrite having a ratio between minor geometric axis and major geometric axis of 0.4 or less, 25% or less (excluding 0%) of acicular retained austenite, and a remainder of martensite.Additionally, patent no. EP3144406 describes a high-strength steel with a tensile strength of 780 MPa or more.

[005] Document No. EP3009527 provides a high-strength cold-rolled steel sheet that has excellent elongation, excellent draw flangability, and a high yield strength, and a method for manufacturing the same. The high-strength cold-rolled steel sheet has the following composition and microstructure. The composition contains C 0.15% to 0.27%, Si 0.8% to 2.4%, Mn 2.3% to 3.5%, P 0.08% or less, S 0.005% or less, Al 0.01% to 0.08%, and N 0.010% or less on a mass basis, the remainder being Fe and unavoidable impurities. The microstructure comprises: ferrite having an average grain size of 5 µm or less and a volume fraction of 3% to 20%, retained austenite having a volume fraction of 5% to 20%, and martensite having a volume fraction of 5% to 20%, the remainder being bainite and / or tempered martensite. The total number of retained austenite with a grain size of 2 µm or less, martensite with Petition 870260036885, dated 20 / 04 / 2026, p. 12 / 40 3 / 19 a grain size of 2 µm or less, or a mixed phase thereof is 150 or more by 2000 µm² of a cross-sectional thickness parallel to the direction of rotation of the steel sheet. The steel sheet of document no. EP3009527 can achieve a tensile strength of 960 MPa or more, but cannot achieve an elongation of 20% or more. Description of the Invention

[006] The purpose of the present invention is to solve these problems by making available cold-rolled, heat-treated steel sheets that simultaneously have: - an ultimate tensile strength greater than or equal to 960 MPa and preferably above 980 MPa, and - a total elongation greater than or equal to 20% and preferably above 21%.

[007] In a preferred embodiment, the steel sheet according to the invention has a yield strength greater than or equal to 475 MPa.

[008] In a preferred embodiment, the steel sheet according to the invention has a yield strength / tensile strength ratio of 0.45 or greater.

[009] Preferably, such action may also have satisfactory suitability for forming, in particular, for Iam ination with satisfactory weldability and coating capacity.

[010] Another objective of the present invention is also to make available a method for manufacturing these sheets that is compatible with conventional industrial applications while being robust with respect to changes in manufacturing parameters.

[011] The cold-rolled, hot-treated steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminum or aluminum alloys to improve its corrosion resistance. Petition 870260036885, dated 20 / 04 / 2026, page 13 / 40 4 / 19 Description of Embodiments of the Invention

[012] Carbon is present in steel from 0.1% to 0.5%. Carbon is a necessary element to increase the strength of the steel of the present invention by producing low-temperature transformation phases such as martensite. Additional carbon also has a crucial function in stabilizing austenite, so it is a necessary element to ensure residual austenite. Therefore, carbon has two crucial functions: one is to increase strength and the other is to retain austenite to confer ductility. But a carbon content lower than 0.1% will not be able to stabilize austenite in an adequate amount required by the steel of the present invention. On the other hand, at a carbon content exceeding 0.5%, the steel exhibits unsatisfactory spot weldability, which limits its application to automotive parts. The preferred limit for carbon is 0.15% to 0.45% and the most preferred limit is 0.15% to 0.3%.

[013] The manganese content of the steel of the present invention is from 1% to 3.4%. This element is gamma-containing. The purpose of adding manganese is essentially to obtain a structure containing austenite. Manganese is an element that stabilizes austenite at room temperature to obtain residual austenite. An amount of at least about 1% by weight of manganese is required to provide the strength and hardenability to the steel of the present invention as well as to stabilize austenite. Thus, a higher percentage of manganese is preferred by the invention presented, such as 3%. But when the manganese content is greater than 3.4%, it produces adverse effects, such as delaying the transformation of austenite into bainite during isothermal holding for bainite transformation. In addition, a manganese content above 3.4% also deteriorates the weldability of the present steel, and the ductility targets may not be achieved.The preferred range for Manganese is between 1.2% and 2.8%, and the most preferred range is between 1.3% and 2.4%.

[014] The silicon content of the steel of the present invention is 0.5% to Petition 870260036885, dated 20 / 04 / 2026, page 14 / 40 5 / 19 2.5%. Silicon is a constituent that can delay carbide precipitation during excessive aging; therefore, due to the presence of silicon, carbon-rich austenite is stabilized at room temperature. Additionally, due to the unsatisfactory solubility of silicon in carbide, it effectively inhibits or delays carbide formation, thus also promoting the formation of low-density carbides in a bainitic structure, which is sought in accordance with the present invention to impart the steel of the present invention its essential mechanical properties. However, a disproportionate silicon content does not produce the aforementioned effect and leads to problems such as temper embrittlement. Therefore, the concentration is controlled within an upper limit of 2.5%. The preferred limit for silicon is 0.8% to 2%, and the most preferred limit is 1.3% to 1.9%.

[015] The aluminum content is from 0.01% to 1.5%. In the present invention, aluminum removes oxygen present in molten steel to prevent oxygen from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride in order to reduce grain size. Higher aluminum content, above 1.5%, increases the Ac3 point at high temperatures, thus reducing productivity. The preferred limit for aluminum is from 0.01% to 1% and the most preferred limit is from 0.01% to 0.5%.

[016] The chromium content of the steel of the present invention is from 0.05% to 1%. Chromium is an essential element that provides strength and hardens steel, but when used above 1% it imparts a surface finish to the steel. Additional chromium content below 1% thickens the carbide dispersion pattern in bainitic structures, thus maintaining a low carbide density in bainite. The preferred limit for chromium is from 0.1% to 0.8% and the most preferred limit is from 0.2% to 0.6%.

[017] Niobium is present in the steel of the present invention. Petition 870260036885, dated 20 / 04 / 2026, page 15 / 40 6 / 19 Niobium, with a concentration of 0.001% to 0.1%, is suitable for forming carbonitrides to impart strength to the steel of the present invention through precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbonitrides and by delaying recrystallization during the heating process. Thus, the finer microstructure formed at the end of the temperature holding period and consequently after complete annealing will lead to product hardening. However, a niobium content above 0.1% is not economically interesting since a saturation effect of its influence is observed, meaning that the additional amount of niobium does not result in any improvement in product strength. The preferred limit for niobium is 0.001% to 0.09%, and the most preferred limit is 0.001% to 0.07%.

[018] Sulfur is not an essential element, but it can be contained as an impurity in steel and, from the point of view of the present invention, the sulfur content is preferably as low as possible, but is 0.003% or less from the point of view of manufacturing cost. Additionally, if sulfur is present in steel, it combines to form sulfides, especially with manganese, and reduces its beneficial impact on the present invention.

[019] The phosphorus constituent of the steel of the present invention is between 0.002% and 0.02%. Phosphorus reduces spot weldability and hot ductility, particularly due to its tendency to segregate grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02% and preferably lower than 0.013%.

[020] Nitrogen is limited to 0.01% in order to avoid material aging and minimize the precipitation of aluminum nitrides during solidification, which are detrimental to the mechanical properties of the steel. Molybdenum is an optional element that constitutes 0% to 0.5% of the steel of the present invention; molybdenum has an effective function in improving hardenability and hardness, delaying the appearance of bainite and preventing the precipitation of carbides in Petition 870260036885, dated 20 / 04 / 2026, page 16 / 40 7 / 19 Bainite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%.

[021] Titanium is an optional element that can be added to the steel of the present invention from 0.001% to 0.1% equal to Niobium. It is involved in carbonitrides, so it has a hardening function. But it also forms titanium nitrides that appear during the solidification of the molten product. The amount of titanium is therefore limited to 0.1% to avoid the formation of coarse titanium nitrides for formability. If the titanium content is below 0.001%, it does not confer any effect on the steel of the present invention. The preferred limit for titanium is 0.001% to 0.09% and the most preferred limit is 0.001% to 0.07%.

[022] Copper can be added as an optional element in an amount of 0.01% to 2% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to obtain such effects. However, when its content is above 2%, it can degrade the surface aspects.

[023] Nickel can be added as an optional element in an amount of 0.01% to 3% to increase the strength of steel and to improve its toughness. A minimum of 0.01% is required to obtain such effects. However, when its content is above 3%, nickel causes deterioration of ductility.

[024] Calcium content is an optional element that can be added to the steel of the present invention from 0.0001% to 0.005%. Calcium is added to the steel of the present invention as an optional element especially during the inclusion treatment. Calcium contributes to the refining of the steel by retaining the harmful sulfur content in globular form, thus delaying the harmful effect of sulfur. Petition 870260036885, dated 20 / 04 / 2026, page 17 / 40 8 / 19

[025] Vanadium is an optional element that can be added as it is effective in enhancing the strength of steel by forming carbides or carbonitrides and the upper limit is 0.1% from an economic point of view.

[026] Other elements, such as Cerium, Boron, Magnesium or Zirconium, may be added individually or in combination in the following proportions: Cerium 0.1%, Boron 0.003%, Magnesium 0.010% and Zirconium 0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[027] The microstructure of the steel sheet according to the invention comprises 10% to 50% Bainite, 5% to 50% Ferrite, 5% to 25% Residual Austenite, 2% to 20% Martensite, 0% to 25% Tempered Martensite and 1% to 45% Annealed Martensite by area fraction.

[028] The surface fractions of phases in the microstructure are determined by the following method: a specimen is cut from the steel sheet, polished and etched with a known reagent to reveal the microstructure. The section is then examined by scanning electron microscopy, for example, with a Field Emission Scanning Electron Microscope (“FEG-SEM”) at a magnification greater than 5000x, in secondary electron mode.

[029] The determination of the ferrite fraction is carried out using SEM observations after recording with Nital or Picral / Nital reagent.

[030] Residual Austenite determination is done by XRD and, for tempered Martensite, dilatometry studies were conducted according to the publication by SMC Van Bohemen and J. Sietsma in Metallurgical and materials transactions, volume 40A, May 2009-1059.

[031] Bainite constitutes between 10% and 60% of the microstructure Petition 870260036885, dated 20 / 04 / 2026, page 18 / 40 9 / 19 area fraction for the steel of the present invention. To guarantee a total elongation of 20%, it is mandatory to have 10% bainite. Preferably, the presence of bainite is between 12% and 55%, and more preferably between 13% and 52%.

[032] Ferrite constitutes 5% to 50% of the microstructure by area fraction for the steel of the present invention. Ferrite imparts elongation to the steel of the present invention. The ferrite of the present steel may comprise polygonal ferrite, lap ferrite, acicular ferrite, plate ferrite, or epitaxial ferrite. To ensure an elongation of 20% or more, it is necessary to have 5% ferrite. The ferrite of the present invention is formed during annealing and cooling after annealing. However, when the ferrite content is present above 50% in the steel of the present invention, it is not possible to have both the yield strength and the total elongation simultaneously because ferrite decreases both tensile strength and yield strength, and also increases the hardness gap with hard phases such as martensite and bainite, and reduces local formability. The preferred limit for the presence of ferrite for the present invention is 6% to 49%.

[033] Residual Austenite constitutes 5% to 25% by area fraction of the steel. Residual Austenite is known to have a higher carbon solubility than Bainite and therefore acts as an effective carbon trap, thus retarding the formation of carbides in Bainite. The percentage of carbon within the Residual Austenite of the present invention is preferably higher than 0.9%, and preferably lower than 1.2%. The Residual Austenite of the steel according to the invention confers enhanced ductility. The preferred range for residual austenite is between 8% and 24%, and more preferably between 12% and 20%.

[034] Martensite constitutes 2% to 20% by area fraction of steel. Martensite imparts tensile strength to the steel of the present invention. Martensite is formed during cooling by overaging. The Petition 870260036885, dated 20 / 04 / 2026, page 19 / 40 10 / 19 The preferred limit for martensite is 3% to 18%, and more preferably 4% to 15%.

[035] Tempered martensite constitutes 0% to 25% of the microstructure by area fraction. Martensite can be formed when steel is cooled between Tcmin and Tcmmax and is tempered during the retention of excessive aging. Tempered martensite imparts ductility and strength to the present invention. When tempered martensite is in excess of 25%, it imparts excessive strength but decreases elongation beyond the acceptable limit. The preferred limit of tempered martensite is 0% to 20% and more preferably 0% to 18%.

[036] Annealed martensite constitutes 1% to 45% of the microstructure of the steel of the present invention by area fraction. Annealed martensite confers strength and formability to the steel of the present invention. Annealed martensite is formed during the second annealing at a temperature between TS and Ac3. It is necessary to have at least 1% of these microstructural constituents to achieve the elongation targeted by the steel of the present invention, but when the amount exceeds 45% of the steel, the present invention is not capable of achieving strength and elongation simultaneously. The preferred limit for its presence is 2% to 40%, and more preferably 2% to 35%.

[037] In addition to the microstructure mentioned above, the microstructure of cold-rolled and heat-treated steel sheet is free of microstructural components such as pearlite without imparting the mechanical properties of steel sheets.

[038] A steel sheet according to the invention can be produced by any suitable method. A preferred method consists of providing a semi-finished steel casting with a chemical composition according to the invention. The casting can be made in ingots or continuously in the form of thin sheets or thin strips, i.e., with a thickness in the range of Petition 870260036885, dated 04 / 20 / 2026, page 20 / 40 11 / 19 approximately 220 mm for blades up to several tens of millimeters for thin strips.

[039] For example, a blade having the chemical composition described above is manufactured by continuous casting where the blade is optionally subjected to direct soft reduction during the continuous casting process to avoid central segregation and ensure a ratio between local carbon and nominal carbon maintained below 1.10. The blade supplied by continuous casting process can be used directly at a high temperature after continuous casting or can first be cooled to room temperature and then reheated for hot smelting. The reheating temperature is between 1100 °C and 1280 °C.

[040] The temperature of the sheet undergoing hot rolling is preferably at least 1200 °C and should be below 1280 °C. If the sheet temperature is below 1200 °C, excessive load is imposed on a rolling mill, and additionally, the steel temperature may decrease to a ferrite transformation temperature during the final rolling, where the steel will be rolled in a state where the transformed ferrite is contained within the structure. Therefore, the sheet temperature is also preferably high enough so that hot rolling can be completed in the temperature range of Ac3 to Ac3+200 °C and the final rolling temperature remains above Ac3. Reheating at temperatures above 1280 °C should be avoided due to the fact that it is industrially expensive.

[041] A final rolling temperature range between Ac3 and Ac3+200 °C is preferred to have a structure that is favorable to recrystallization and rolling. It is necessary that the final rolling pass be carried out at a temperature higher than Ac3, due to the fact that below this temperature the steel sheet exhibits a significant drop in Petition 870260036885, dated 20 / 04 / 2026, page 21 / 40 12 / 19 laminator capacity. The sheet obtained in this way is then cooled at an average cooling rate above 30 °C / s to the winding temperature which must be below 600 °C. Preferably, the cooling rate will be less than or equal to 200 °C / s and the winding temperature is preferably below 570 °C.

[042] Hot-rolled steel sheet is coiled at a coiling temperature below 600 °C to avoid ovalization of the hot-rolled steel sheet and preferably below 570 °C to avoid scale formation. The preferred coiling temperature range is between 350 °C and 570 °C. The coiled hot-rolled steel sheet is cooled to ambient temperature before undergoing optional hot band annealing.

[043] Hot-rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during hot rolling. The hot-rolled sheet may then be subjected to optional Hot Band Annealing at temperatures between 400 °C and 750 °C for at least 12 hours and at most 96 hours, but the temperature must be kept below 750 °C to avoid partially transforming the hot-rolled microstructure and thus losing microstructure homogeneity. After this, an optional scale removal step may be performed to remove the scale, for example, by pickling such steel sheet. This hot-rolled steel sheet is cold-rolled with a thickness reduction between 35 and 90%. The cold-rolled steel sheet obtained from the cold rolling process is then subjected to two annealing cycles to impart to the steel of the present invention the microstructure and mechanical properties.

[044] In the first annealing of cold-rolled steel sheet, the cold-rolled steel sheet is heated at a heating rate UR1 that is greater than 3 °C / s and preferably greater than 5 °C / s, at a temperature Petition 870260036885, dated 20 / 04 / 2026, p. 22 / 40 13 / 19 of the TS1 imbibition between TS and Ac3, where Ac3 and TS for the present steel are calculated using the following formula: Equation 1 TS = 830 -260*C -25*Mn + 22*Si + 40*AI Equation 2 Ac3 = 901 - 262*C - 29*Mn + 31*Si - 12*Cr - 155*Nb + 86*Al where the element contents are expressed as a percentage by weight.

[045] The steel sheet is held at TS1 for 10 to 500 seconds to ensure proper recrystallization and at least 50% austenite transformation of the heavily worked hardened initial structure. The sheet is then cooled at a cooling rate CR1 that is greater than 25 °C / s and preferably greater than 50 °C / s at room temperature. During this cooling, the cold-rolled steel sheet may optionally be held at a temperature range between 350 °C and 480 °C and preferably at a range between 380 °C and 450 °C and the holding time is 10 to 500 seconds, then cool the cold-rolled steel sheet to room temperature to obtain the annealed cold-rolled steel sheet.

[046] Then, for a second annealing, the cold-rolled and annealed steel sheet is heated at a heating rate UR2 that is greater than 3 °C / s, to a second annealing soak temperature TS2 between TS and Ac3 where: TS = 830 -260*C -25*Mn + 22*Si + 40*AI (1) Ac3 = 901 - 262*C - 29*Mn + 31*Si - 12*Cr - 155*Nb + 86*AI (2) where element contents are expressed as a percentage by weight.

[047] For 10 seconds to 500 seconds to ensure proper recrystallization and transformation to obtain a minimum of Petition 870260036885, dated 20 / 04 / 2026, p. 23 / 40 14 / 19 Austenite microstructure 50%. The temperature TS2 is always less than or equal to the temperature TS1. The sheet is then cooled at a cooling rate CR2 that is greater than 20 °C / s, preferably greater than 30 °C / s, and more preferably greater than 50 °C / s, at a temperature in the Tbreak range that is between Tcmáx and Tcmáx. These Tcmáx and Tcmáx are defined as follows: Equation 3 Tcmáx = 565 - 601 * (1 - Exp(-0.868*C)) - 34*Mn - 13*Si - 10*Cr + 13*Al - 361 *Nb Equation 4 Tcmin = 565 - 601 * (1 - Exp(-1,736*C)) - 34*Mn -13*Si -10*Cr + 13*Al - 361 *Nb wherein the element content is expressed as a percentage by weight.

[048] After this, the cold-rolled and annealed steel sheet is brought to a TOA temperature range of 380 °C to 580 °C and held for 10 seconds to 500 seconds to ensure the formation of an adequate amount of Bainite, as well as to temper the Martensite to impart the desired mechanical properties to the steel of the present invention. After this, the cold-rolled and annealed steel sheet is cooled to room temperature at a cooling rate of at least 1 °C / s to form Martensite to obtain cold-rolled and heat-treated steel sheet. The preferred temperature range for TOA is from 380 °C to 500 °C, and more preferably from 380 °C to 480 °C.

[049] The hot-treated cold-rolled steel sheet can then optionally be coated by any of the known industrial processes, such as electrogalvanizing, JVD, PVD, hot-dip galvanizing (GI / GA), etc. Electrogalvanizing does not alter or modify any of the mechanical properties or microstructure of the hot-treated cold-rolled steel sheet as claimed. Electrogalvanizing can be done by any conventional industrial process, for example, by electroplating.

[050] The following tests, examples, figurative examples and tables presented in this document are not restrictive in Petition 870260036885, dated 04 / 20 / 2026, page 24 / 40 15 / 19 nature and should be considered for illustrative purposes only, and will exhibit the advantageous features of the present invention.

[051] Steel sheets made of steels with different compositions are numbered and grouped in Table 1, where the steel sheets are produced according to the process parameters as stipulated in Table 2, respectively. After that, Table 3 gathers the microstructure of the steel sheets obtained during the tests and Table 4 gathers the result of property evaluations obtained.

[052] Table 1 represents the steels with compositions expressed as weight percentages. The steel compositions 11 to 15 for the manufacture of sheets according to the invention, this table also specifies the reference steel compositions which are designated in the table by R1 to R4. Table 1 also serves as a comparison tabulation between the steel of the invention and reference steel. Table 1 also shows the tabulation of Ac3 defined for steel samples by the following equation: Ac3 = 901 - 262*0 - 29*Mn + 31*Si - 12*Cr - 155*Nb + 86*AI (2) Table 1 is in this document. Table 1 Samples of Steel C Mn Si Al Cr Nb SP Ca N Mo Cu Ni VB Ti TS Ac 3 A 0.1 8 1.4 5 1.8 5 0.03 0 0.30 0 0.06 0.01 0 0.00 1 0.000 4 0.003 0 78 8 85 8 B 0.2 2 1.4 5 1.8 5 0.02 7 0.30 0 0.06 2 0.001 0 0.01 1 0.000 0.00 6 70 0.00 7 0.01 0 0.00 1 0.000 4 0.003 0 78 0 84 9 C 0.2 1 2.1 0 1.4 7 0.02 7 0.34 6 0.00 1 0.001 7 0.01 0.020 70.06 0.00 4 0.00 8 0.02 3 0.00 1 0.000 5 0.005 0 75 6 82 9 D 0.2 1 2.2 2 1.4 4 0.04 0 0.21 2 0.00 2 0.00 0.01 0.01 0.80 0.006 0 0.00 2 0.00 9 0.02 5 0.00 4 0.000 8 0.002 7 75 4 82 8 E 0.2 0 1.8 2 1.6 3 0.02 7 0.30 2 0.01 0.01 1 0.01 0.000 8 0.003 4 0.00 1 0.00 5 0.01 3 0.00 1 0.000 7 0.004 76 9 84 5 F 0.2 0 1.8 2 1.6 1 0.02 5 0.00 0.02 0.02 0.026 0.01 0 0.000 8 0.003 6 0.00 1 0.00 5 0.01 3 0.00 1 0.000 7 0.004 76 9 84 1 G 0.2 0 1.8 2 1.6 3 0.00.2 30.3 43 0.001 9 0.01 0 0.000 8 0.003 6 0.00 1 0.00 5 0.01 3 0.00 1 0.000 7 0.004 77 0 83 7 Petition 870260036885, dated 20 / 04 / 2026, p. 25 / 40 16 / 19 Table 2

[053] Table 2 lists the annealing process parameters implemented in the steels of Table 1. The steel compositions 11 to 17, which are used for the manufacture of sheets according to the invention, are also specified in this table as the reference steel, designated in the table by R1 to R5. Table 2 also shows the tabulation of Tcmin and Tcmmax. These Tcmmax and Tcmin are defined for the steels of the invention and reference steels as follows: Tcmax = 565 - 601 * (1 - Exp(-0.868*C)) - 34*Mn -13*Si -10*Cr + 13*AI - 361 *Nb (3) Tcmmin = 565 - 601 * (1 - Exp(-1.736*C)) - 34*Mn -13*Si -10*Cr + 13*AI - 361 *Nb (4)

[054] Additionally, before performing the annealing treatment on the steels of the invention, as well as on the reference steels, all steels were cooled after hot rolling at an average cooling rate of 40 °C / s. The hot-rolled coils were then processed as claimed and, after that, cold-rolled with a thickness reduction between 30 and 95%. The final cooling rate is above 1 °C / s.

[055] These cold-rolled steel sheets, both of the invention steel and the reference steel, were subjected to heat treatments as numbered in Table 2 in this document: Table 2 First Annealing Sample of Steel Tests Tde Reheating (°C) T Final HR (°C) Tde Coiling HR (°C) Reduction of CR (%) HR1 (°C / s) TS1 (°C) Soaking time (s) CR1 (°C / s) Holding temperature T (°C) tde holding (s) 11 A 1250 915 554 49 6 830 120 70 400 200 I2 B 1245 930 546 51 6 830 120 70 400 200 I3 C 1240 920 446 49 8 820 330 70 400 370 I4 D 1220 937 546 55 6 770 80 70 XXX XXX I5 E 1239 910 545 53 6 830 120 70 XXX XXX I6 F 1239 910 545 53 6 830 120 70 XXX XXX I7 G 1239 910 545 53 6 810 120 70 XXX XXX R1 A 1250 915 554 49 - - - - - - R2 B 1245 930 546 51 - - - - - - R3 C 1240 920 446 49 - - - - - - Petition 870260036885, dated 20 / 04 / 2026, p. 26 / 40 17 / 19 First Annealing Sample of Steel Tests Tde Reheating (°C) T Final HR (°C) Tde Coiling HR (°C) Reduction of CR (%) HR1 (°C / s) TS1 (°C) Soaking time (s) CR1 (°C / s) Holding temperature T (°C) tde holding (s) R4 C 1240 920 446 49 - - - - - - R5 D 1220 937 546 55 - - - - - - Second Annealing Sample of Steel Tests HR 2 (°C / s) TS2 (°C) Soaking time (s) CR2 (°C / s) Interruption time (°C) Heating rate to TOA (°C / s) TOA (°C) Holding time (s) Ac3 temperature (°C) TS (°C) TCmax T (°C) TCmin T (°C) 11 A 6 800 100 70 270 20 400 200 858 788 379 263 I2 B 6 820 100 70 300 20 400 200 849 722 364 239 I3 C 6 790 100 70 310 10 400 200 829 756 371 247 I4 D 6 770 80 70 290 20 400 200 828 754 370 247 E I6 F 6 790 100 70 290 20 400 200 841 769 375 255 400 200 858 788 379 263 R2 B 6,800 100 70 270 20,400 200 849 722,364,239 R3 330 70 400 10 400 370 829 756 371 247 R5 D 6 770 80 70 290 20 400 200 828 754 370 247 I = according to the invention; R = reference; underlined values: not in accordance with the invention. Table 3

[056] Table 3 exemplifies the test results conducted according to standards using different microscopes, such as Scanning Electron Microscopy, to determine the microstructural composition of both the steel of the invention and the reference steel. Residual Austenite is measured by magnetic saturation measurement as per the publication entitled Structure and Properties of Thermally-Mechanically Treated Stainless Steel in Metallurgical Transactions, June 1970, Volume 1. Ferrite, Bainite, Tempered Martensite, and Martensite are observed through image analysis conducted with Aphelion software and with the dilatometry test. Petition 870260036885, dated 20 / 04 / 2026, page 27 / 40 18 / 19 interrupted.

[057] The results are stipulated in this document: Sample of Steel Bainite Ferrite Austenite Residual Martensite Tempered Martensite Annealed Martensite 11 45 25 15 12 0 3 I2 42 25 16 11 0 6 I3 51 7 19 8 4 11 I4 14 46 17 8 7 8 I5 33 16 14 9 17 11 I6 40 19 17 5 9 10 I7 30 29 13 8 13 7 R1 15 60 14 8 3 0 R2 18 58 13 11 0 0 R3 20 28 18 24 10 0 R4 71 7 11 11 0 0 R5 7 48 12 21 12 0 I = according to the invention; R = reference; underlined values: not in accordance with the invention. Table 4

[058] Table 4 exemplifies the mechanical properties of both the steel of the invention and the reference steel. In order to determine tensile strength, yield strength and total elongation, tensile tests are conducted in accordance with the JIS Z2241 standards published in the 11th Edition on October 20, 2020 entitled METALLIC MATERIALS - TENSILE TESTING METHOD OF TEST AT ROOM TEMPERATURE.

[059] From here the result of the various mechanical tests conducted according to the standards is tabulated: Sample Steels Tensile Strength (in MPa) Yield Strength (in MPa) Total Elongation (in %) 11 1025 611 24.4 I2 1029 599 23.7 I3 997 479 24.1 I4 1054 565 22.5 I5 1015 599 23.1 I6 1053 582 21.1 I7 1047 524 22.1 R1 948 492 21.6 R2 947 582 23.3 Petition 870260036885, dated 20 / 04 / 2026, page 28 / 40 19 / 19 Sample Steels Tensile Strength (in MPa) Yield Strength (in MPa) Total Elongation (in %) R3 996 470 18.3 R4 1032 549 15.8 R5 1114 524 15.2 I = according to the invention; R = reference; underlined values: not in accordance with the invention.

Claims

1. COLD-ROLLED AND HOT-TREATED STEEL SHEET characterized by having a composition comprising the following elements, expressed as a percentage by weight: 0.1% < Carbon < 0.5% 1% < Manganese < 3.4% 0.5% < Silicon < 2.5% 0.01% < Aluminum < 1.5% 0.05% < Chromium < 1% 0.001% < Niobium < 0.1% 0% < Sulfur < 0.003% 0.002% < Phosphorus < 0.02% 0% < Nitrogen < 0.01% and may contain one or more of the following optional elements 0% < Molybdenum < 0.5% 0.001% < Titanium < 0.1% 0.01% < Copper < 2% 0.01% < Nickel < 3% 0.0001% < Calcium < 0.005% 0% < Vanadium < 0.1% 0% < Boron < 0.003% 0% < Cerium < 0.1% 0% < Magnesium < 0.010% 0% < Zirconium < 0.010% wherein the remaining composition consists of iron and unavoidable impurities, and a microstructure of the rolled steel sheet comprises by area fraction, Bainite 10% to 60%, Ferrite 5% to 50%, Residual Austenite 5% to 25%, Martensite 2% to 20%, Tempered Martensite 0% to 25%, wherein the Petition 870260036885, dated 20 / 04 / 2026, page.30 / 40 2 / 5 equilibrium is Annealed Martensite, whose content must be from 1% to 45%, in which the steel sheet has a tensile strength greater than 960 MPa and a total elongation of 20% or more and has a yield strength above 475 MPa, which are measured according to JIS Z2241 standards.

2. Cold-rolled and hot-treated steel sheet, according to claim 1, characterized by its composition including 0.8% Silicon 2%.

3. Cold-rolled and hot-treated steel sheet, according to any one of claims 1 to 2, characterized by its composition including 1.2% Manganese and 2.8% Manganese.

4. Cold-rolled and hot-treated steel sheet, according to any one of claims 1 to 3, characterized by its composition including 0.01% Aluminum 1%.

5. COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to any one of claims 1 to 4, characterized by its composition including 0.001% Niobium 0.09%.

6. COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to any one of claims 1 to 5, characterized by its composition including 0.1% Chromium 0.8%.

7. COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to any one of claims 1 to 6, characterized by the annealed martensite being from 2% to 40%.

8. COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to any one of claims 1 to 7, characterized by a microstructure containing 12% to 55% Bainite.

9. COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to any one of claims 1 to 8, characterized by the microstructure containing 8% to 24% Residual Austenite. Petition 870260036885, dated 20 / 04 / 2026, p. 31 / 40 3 / 5 10. METHOD FOR PRODUCING A COLD-ROLLED AND HOT-TREATED STEEL SHEET, as defined in claims 1 to 9, characterized by comprising the following steps: - providing a steel composition, as defined in any one of claims 1 to 6; - reheating a semi-finished product to a temperature between 1100 °C and 1280 °C; - rolling the semi-finished product in the austenitic range where the hot-rolling finishing temperature must be above Ac3 to obtain a hot-rolled steel sheet; - cooling the sheet at an average cooling rate above 30 °C / s to a coiling temperature that is below 600 °C; and coiling the hot-rolled sheet; - cooling the hot-rolled sheet to ambient temperature; - optionally performing the step of removing scale from the hot-rolled steel sheet; - Optional annealing is performed on hot-rolled steel sheet at temperatures between 400 °C and 750 °C;- Optionally perform the scale removal step on the hot-rolled steel sheet; - Cold roll the hot-rolled steel sheet with a reduction ratio between 35 and 90% to obtain a cold-rolled steel sheet; - Then perform a first annealing by heating the cold-rolled steel sheet at a rate HR1 greater than 3 °C / s and an imbibition temperature TS1 between TS and Ac3, which is maintained for 10 seconds to 500 seconds;where TS and Ac3 are defined as follows: TS = 830 -260*C -25*Mn + 22*Si + 40*Al (equation 1) Ac3 = 901 - 262*C - 29*Mn + 31*Si - 12*Cr - 155*Nb + 86*Al (equation 2) Petition 870260036885, dated 20 / 04 / 2026, p. 32 / 40 4 / 5 - then cool the sheet at a rate greater than 25 °C / s to ambient temperature where during cooling the cold-rolled steel sheet is optionally held at a temperature in the range between 350 °C and 480 °C for a time between 10 and 500 seconds to obtain the cold-rolled and annealed steel sheet; - then perform a second annealing by heating the cold-rolled and annealed steel sheet at a rate HR2 greater than 3 °C / s and a soaking temperature TS2 between TS and Ac3, which is held for 10 seconds to 500 seconds; - then cool the sheet at a rate CR2 greater than 20 °C / s and a break temperature range Tcmáx between Tcmáx and Tcmáx;where Tcmáx and Tcmín are defined as follows: Tcmáx = 565 - 601 * (1 - Exp(-0.868*C)) - 34*Mn - 13*Si - 10*Cr + 13*Al - 361*Nb (equation 3) Tcmín = 565 - 601 * (1 - Exp(-1.736*C)) - 34*Mn - 13*Si - 10*Cr + 13*Al - 361*Nb (equation 4) where C, Mn, Si, Cr, Al and Nb are in % by weight of the elements in the steel; Then, the cold-rolled and annealed steel sheet is brought to the TOA temperature range, which is between 380 °C and 580 °C, and held at TOA for 10 seconds and 500 seconds, and the cold-rolled annealed steel sheet is cooled to room temperature at a cooling rate higher than 1 °C / s to obtain the cold-rolled and heat-treated steel sheet.

11. METHOD FOR PRODUCING A COLD-ROLLED AND HOT-TREATED STEEL SHEET, according to claim 10, characterized by having a coiling temperature of the hot-rolled steel sheet below 570 °C.

12. METHOD FOR PRODUCING COLD ROLLED AND HOT TREATED STEEL SHEET Petition 870260036885, dated 04 / 20 / 2026, page 33 / 40 5 / 5, according to any one of claims 10 to 11, characterized in that the temperature TS2 is less than or equal to TS1.