Cold-rolled and heat-treated steel sheets and their manufacturing methods

Abstract

Cold-rolled and heat-treated steel sheet containing the following elements: 0.3%≦C≦0.45%, 0.4%≦Mn≦1.4%, 0.1%≦Si≦0.9%, 0.01%≦Al≦0.1%, 0.1%≦Cr≦0.8%, 0.01%≦Nb≦0.1%, 0.1%≦Ni≦0.9%, 0.01%≦Mo≦0.9%, 0.01%≦Ti≦0.1%, 0.0001%≦B≦0.010%, 0%≦P≦0.02%, 0%≦S≦0.0 Cold-rolled and heat-treated steel sheet containing 3%, 0%≦N≦0.09%, 0%≦V≦0.1%, 0%≦Cu≦2%, 0%≦Ca≦0.005%, 0%≦Ce≦0.1%, 0%≦Mg≦0.05%, and 0%≦Zr≦0.05%, with the remainder of the composition consisting of iron and unavoidable impurities resulting from processing, wherein the microstructure of the steel contains 1-5% bainite by area percentage, with the remainder being tempered martensite.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet suitable for use as a steel sheet for vehicles.

Background Art

[0002] Automobile parts are required to meet two conflicting requirements, namely ease of forming and strength. In recent years, however, a third requirement of improving fuel efficiency has also been imposed on automobiles from the perspective of global environmental problems. Therefore, currently, automobile parts must be made of materials having high formability in order to meet the standards that easily fit into complex automobile assemblies. At the same time, while reducing the weight of the vehicle and improving fuel efficiency, the strength must be improved for the impact resistance and durability of the vehicle. Furthermore, steel parts must be weldable without suffering from liquid metal embrittlement.

[0003] Therefore, research and development for reducing the amount of materials used in vehicles by increasing the strength of the materials is being actively conducted. On the contrary, increasing the strength of the steel sheet reduces the formability, and thus the development of materials having both high strength and high formability is required.

[0004] Previous research and development in the field of high-strength and high-formability steel sheets have produced several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed herein for a decisive understanding of the present invention.

[0005] EP3486346 presents a steel sheet having a specific chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure containing martensite and bainite. The total area fraction of martensite and bainite with respect to the entire microstructure is 95% or more and 100% or less. The remainder of the microstructure is at least one of ferrite and retained austenite. The microstructure contains a specific inclusion cluster, and the content of the inclusion cluster in the microstructure is 5 clusters / mm 2The following conditions apply: The microstructure contains prior austenite grains with an average size exceeding 5 μm. The steel sheet has a tensile strength of 1320 MPa or higher. However, the EP3486346 steel cannot achieve a bendability of 2.5t or less.

[0006] Known prior art for the manufacture of high-strength and highly formable steel sheets suffers from deficiencies in either of these aspects; therefore, there is a need for cold-rolled steel sheets with strengths exceeding 1500 MPa and methods for manufacturing them. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] European Patent No. 3486346 [Overview of the project]

[0008] The object of the present invention is to solve these problems by making available cold-rolled and heat-treated steel sheets that simultaneously possess the following properties.

[0009] -Ultimate tensile strength of -1500MPa to 1750MPa, preferably 1580MPa to 1720MPa. Yield strength of -1250MPa to 1450MPa, preferably 1280MPa to 1400MPa. Overall growth of over -6%.

[0010] Hole expansion ratio equal to -40% or more, preferably more than 45%.

[0011] In a preferred embodiment, the cold-rolled and heat-treated steel sheet exhibits a YS / TS ratio greater than 0.60.

[0012] Preferably, such steel may also have particularly good weldability and coating ability, and may have good suitability for forming, and especially for rolling.

[0013] In a preferred embodiment, cold-rolled and heat-treated steel sheets exhibit a bendability of less than 3.0 when measured at 90° for r / t. Bending tests were performed according to the ASTM E290-22 standard. The ratio of punch die radius to material thickness was calculated as an expression of the material's bendability.

[0014] Another objective of the present invention is to make available a method for manufacturing these plates that are robust to changes in manufacturing parameters while being suitable for conventional industrial applications.

[0015] The cold-rolled and heat-treated steel sheet of the present invention may optionally be coated with zinc or a zinc alloy or aluminum or an aluminum alloy to improve its corrosion resistance. (Electrochemical, JVD, CVD, hot dip, and other coating processes as used herein)

[0016] Other features and advantages of the present invention will become apparent from the following detailed description of the invention. [Modes for carrying out the invention]

[0017] Carbon is present in steel at a concentration of 0.3% to 0.45%. Carbon is an essential element for increasing the strength of steel sheets by delaying the formation of ferrite and bainite during cooling after annealing. A carbon content of less than 0.3% prevents the steel of the present invention from having sufficient tensile strength and ductility. On the other hand, if the carbon content exceeds 0.45%, the weld and heat-affected zone harden significantly, and therefore the mechanical properties of the weld are impaired. The preferred limit for carbon is 0.31% to 0.4%, and the more preferred limit is 0.32% to 0.38%.

[0018] The manganese content of the steel of the present invention is 0.4% to 1.4%. Manganese is an element that provides strength, and at least 0.4% of manganese is necessary to provide strength and hardenability to the steel sheet by delaying the formation of ferrite. Therefore, manganese in proportions such as 0.5% to 1.3% is preferred, and more preferably 0.55% to 0.9%. However, if the manganese content exceeds 1.4%, this will cause adverse effects such as a delay in the transformation of austenite to martensite, leading to a decrease in the ductility of the final product. Furthermore, a manganese content exceeding 1.4% will cause central segregation and will also reduce the weldability of the steel. Moreover, a high manganese content is detrimental in terms of hydrogen-delayed fracture, which is an important criterion for steel manufacturers and the automotive industry.

[0019] The silicon content of the steel of the present invention is 0.1% to 0.9%. Silicon is an element that contributes to strength improvement through solid solution strengthening. Silicon is a component that can delay the precipitation of carbides during cooling after annealing, and therefore promotes the formation of martensite. However, silicon is also a ferrite-forming agent and raises the Ac3 transformation point, which pushes the annealing temperature into a higher temperature range, which is why the silicon content is kept at a maximum of 0.9%. A silicon content exceeding 0.9% can also cause tempering embrittlement, and furthermore, silicon also impairs coating properties. The preferred limit for the presence of silicon is 0.2% to 0.8%, more preferably 0.3% to 0.7%.

[0020] The aluminum content of the steel of this invention is 0.01 to 0.1%. During the manufacture of steel, aluminum can be added to deoxidize the steel and capture oxygen. If the aluminum content exceeds 0.1%, the Ac3 point rises, thereby reducing productivity. Furthermore, within this range, aluminum combines with nitrogen in the steel to form aluminum nitride, reducing the size of the crystal grains, and aluminum also delays cementite precipitation. However, in this invention, if the aluminum content exceeds 0.1%, the amount and size of the aluminum nitride are detrimental to hole widening and bending, and it also pushes the Ac3 point to a much higher temperature range that is very expensive to achieve industrially, and also causes grain coarsening during annealing soaking. The preferred limit for aluminum is 0.01% to 0.08%, and more preferably 0.01% to 0.06%.

[0021] Chromium is an essential element of the steel of this invention and is present in an amount of 0.1% to 0.8%. Chromium provides strength and hardness to the steel, but if used in amounts exceeding 0.8%, the surface finish of the steel is impaired. The preferred limit for the presence of chromium is 0.2% to 0.7%, more preferably 0.2% to 0.6%.

[0022] Niobium is an essential element and may be present in amounts of 0.01% to 0.1%, preferably 0.01% to 0.09%, and more preferably 0.01% to 0.07%. Niobium is suitable for providing strength to the steel according to the present invention by forming carbonitrides through precipitation hardening within the annealing sonic temperature range, which leads to the hardening of the product. However, if the niobium content exceeds 0.1%, niobium consumes carbon by forming a large amount of carbonitrides, and a large amount of carbonitrides tends to reduce the ductility of the steel, which is undesirable for the present invention.

[0023] Nickel is an essential element and is present in an amount of 0.1% to 0.9% to increase the strength and improve the toughness of the steel of the present invention. To obtain such effects, a minimum of 0.01% is preferred. The preferred limit for the presence of nickel is 0.2% to 0.7%, more preferably 0.3% to 0.6%.

[0024] Molybdenum is an essential element and is present in the steel of the present invention in an amount of 0.01% to 0.9%. Molybdenum plays an effective role in improving hardenability and hardness. When added in an amount of at least 0.01%, it delays the formation of ferrite and bainite during cooling after annealing. Mo is also beneficial to the toughness of hot-rolled products and makes manufacturing easier. However, the addition of molybdenum excessively increases the addition cost of alloying elements. Therefore, its content is limited to 0.9% for economic reasons. The preferred limit of molybdenum is 0.01% to 0.7%, more preferably 0.01% to 0.6%.

[0025] Titanium is an essential element added to the steel of the present invention in an amount of 0.01% to 0.1%, preferably 0.01% to 0.09%. Titanium is suitable for forming carbides, nitrides and carbonitrides by precipitation hardening in the annealing soaking temperature range to impart strength to the steel according to the present invention. As a result, the product is hardened. However, when the titanium content exceeds 0.1%, titanium consumes carbon by forming a large amount of precipitates, and the large amount of precipitates tend to reduce the ductility of the steel, so it is not preferable for the present invention. The preferred limit of the presence of titanium is 0.01% to 0.08%, more preferably 0.01% to 0.06%.

[0026] Boron is an essential element and is added in an amount of 0.0001% to 0.010%, preferably 0.001% to 0.004% to harden the steel. Boron captures nitrides to form boron nitride that imparts strength to the steel of the present invention. Boron also imparts hardenability to the steel of the present invention. However, it has been found that when more than 0.010% of boron is added, the rolling property of the steel sheet significantly decreases. Furthermore, boron segregation may occur at the grain boundaries, which is harmful to formability.

[0027] The phosphorus content of the steel of the present invention is limited to 0.02%. Phosphorus is an element that hardens in solid solutions. Therefore, a small amount of phosphorus, at least 0.002%, may be advantageous, but phosphorus also has adverse effects such as reduced spot weldability and hot ductility, particularly due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.015%.

[0028] Sulfur is not an essential element, but it may be present in the steel as an impurity. The sulfur content should be as low as possible, preferably 0.03% or less, and more preferably 0.005% or less, from the standpoint of manufacturing costs. Furthermore, if there is more sulfur present in the steel, it will combine with Mn and Ti in particular to form sulfides, which is detrimental to the bending, hole expansion, and elongation of the steel of the present invention.

[0029] Nitrogen is limited to 0.09% to avoid material degradation over time and to minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of steel.

[0030] Vanadium is an optional element that may be added to the steel of the present invention in an amount of 0% to 0.1%, preferably 0.001% to 0.1%. Similar to niobium, vanadium plays a role in hardening because it is involved in carbonitrides. However, vanadium is also involved in the formation of VN that appear during the solidification of the cast product. The amount of V is limited to 0.1% to avoid coarse VN that is detrimental to hole expansion. If the vanadium content is less than 0.001%, it has no effect on the steel of the present invention.

[0031] Copper may be added in an amount of 0% to 2% as an optional element to increase the strength of the steel of the present invention and improve its corrosion resistance. A minimum of 0.01% is preferred to achieve such effects. However, if the content exceeds 2%, it may degrade the surface appearance.

[0032] Calcium is an optional element that can be added to the steel of the present invention in an amount of 0% to 0.005%, preferably 0.001% to 0.005%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium contributes to the purification of the steel by capturing harmful sulfur content during spheroidization.

[0033] Other elements such as cerium, magnesium, or zirconium may be added individually or in combination in the following proportions: Ce ≤ 0.1%, Mg ≤ 0.05%, and Zr ≤ 0.05%. Up to the indicated maximum content levels, these elements can refine the inclusion grains during solidification.

[0034] The remainder of steel's composition consists of iron and unavoidable impurities resulting from processing.

[0035] The microstructure of the steel sheet according to the present invention contains 1% to 5% bainite by area fraction, with the remainder being tempered martensite.

[0036] The area fraction of a phase in the microstructure is determined by the following method: The sample is cut from a steel plate, polished, and etched using a reagent of known properties to reveal the microstructure. The cross-section is then examined using a scanning electron microscope, for example, in secondary electron mode at a magnification of over 500x.

[0037] The fraction of ferrite is determined by SEM observation after etching with Nital or Picral / Nital reagent.

[0038] Tempered martensite constitutes at least 95% of the microstructure by area fraction. Tempered martensite is formed from martensite formed after annealing, particularly during cooling after the temperature drops below Ms, and more specifically below Ms-10°C. Such martensite is then tempered while being held at the tempering temperature. The tempered martensite of the present invention imparts ductility and strength to such steel. Preferably, the tempered martensite content is 96% to 99%.

[0039] Bainite is present in an amount of 1% to 5%, and within the framework of this invention, bainite may include carbide-free bainite and / or lath bainite and granular bainite. If present, lath bainite is in the form of lath with a thickness of 1 to 5 microns. If present, carbide-free bainite has a very low carbide density, 100 μm 2 This is bainite having fewer than 100 carbides per unit area and potentially containing austenite islands. When present, granular bainite is a form of crystal grain in which carbides are present within the crystal grains. Bainite provides improved elongation. A preferred presence of bainite is 1% to 4%.

[0040] In addition to the microstructure described above, the microstructure of cold-rolled and heat-treated steel sheets does not contain microstructure components such as ferrite, fresh martensite, retained austenite pearlite, and cementite, without impairing the mechanical properties of the steel sheets.

[0041] The cold-rolled steel sheet according to the present invention can be manufactured by any preferred method. A preferred method is to provide a semi-finished casting of steel having the chemical composition according to the present invention. Casting may be carried out in the form of an ingot or continuously in the form of a thin slab or thin strip, i.e., having a thickness ranging from about 220 mm in the case of a slab to a maximum of several tens of millimeters in the case of a thin strip.

[0042] For example, a slab would be considered a semi-finished product. A slab having the above-described chemical composition is manufactured by continuous casting, and the slab is preferably subjected to direct light reduction during casting to ensure the elimination of central segregation and a reduction in porosity. The slab provided by the continuous casting process may be used directly at high temperatures after continuous casting, or it may be first cooled to room temperature and then reheated for hot rolling.

[0043] The temperature of the slab subjected to hot rolling is preferably at least 1000°C, preferably above 1150°C, and must not exceed 1300°C. If the slab temperature is below 1150°C, excessive load is placed on the rolling mill, and the steel temperature may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled with transformed ferrite incorporated into its structure. Furthermore, for industrially costly reasons, the temperature must not exceed 1300°C.

[0044] The slab temperature is preferably high enough to allow the hot rolling to be completely completed within the austenite region, and the finish hot rolling temperature remains above 850°C. Below this temperature, the steel sheet exhibits a significant decrease in rollability, so the final rolling must be performed above 850°C.

[0045] Next, the sheet obtained in this manner is cooled to a temperature of 580°C or less at a cooling rate of at least 5°C / second. Preferably, the cooling rate is 100°C / second or less and greater than 10°C / second. After that, the hot-rolled steel sheet is wound up at a winding temperature of 580°C to 500°C, preferably 500°C to 570°C, more preferably 510°C to 550°C. After that, the coiled hot-rolled steel sheet can preferably be cooled to room temperature. Next, the hot-rolled sheet may be subjected to an optional scale removal process, such as pickling, to remove scale formed during hot rolling and to ensure that no scale is present on the surface of the hot-rolled steel sheet before the hot-band annealing of the hot-rolled steel sheet.

[0046] The hot-rolled sheet may be subjected to an optional hot-band annealing at a temperature of 350°C to 750°C for 1 to 96 hours. The temperature and time of such hot-band annealing are selected to ensure that the hot-rolled sheet is softened to facilitate cold rolling of the hot-rolled steel sheet. The hot-rolled sheet may then be subjected to an optional descaling process, such as pickling, to remove the scale formed during the hot-band annealing.

[0047] Next, the hot-rolled steel sheet is cooled to room temperature, and then the hot-rolled sheet is cold-rolled to reduce its thickness by 35-90% to obtain a cold-rolled steel sheet.

[0048] Next, the cold-rolled steel sheet is annealed to impart the target microstructure and mechanical properties to the steel of the present invention.

[0049] In annealing, the cold-rolled steel sheet is heated, specifically at a heating rate HR1 of 1°C / sec to 30°C / sec, until it reaches a soaking temperature TA of Ac3+10°C to Ac3+150°C from room temperature. A preferred HR1 rate is 1°C / sec to 20°C / sec, more preferably 1°C / sec to 10°C / sec. A preferred TA temperature is 800°C to 900°C.

[0050] Next, the cold-rolled steel sheet is held at the annealing soaking temperature TA for 100 to 1000 seconds to ensure sufficient transformation to form at least 90%, preferably 100%, of austenite at the end of soaking. The cold-rolled steel sheet is then cooled to a cooling stop temperature range CS1 of 220°C to 320°C, preferably 230°C to 310°C, more preferably 250°C to 300°C, at an average cooling rate CR1 of 5°C / sec to 200°C / sec, preferably 8°C / sec to 100°C / sec, more preferably 10°C / sec to 70°C / sec. During this cooling step, martensite is formed. If the CS1 temperature exceeds 320°C, the steel of the present invention will have too much austenite, which is detrimental to the total elongation.

[0051] Next, the cold-rolled steel sheet is tempered to a temperature TT of 210°C to 390°C and held at the TT temperature for 1 to 500 seconds. A preferred tempering temperature TT is 220°C to 370°C, more preferably 250°C to 350°C. During this tempering step, the martensite formed during the cooling step is tempered to form tempered martensite. The tempering period is selected so that at the end of tempering, no or a maximum of 2% of retained austenite remains in the cold-rolled steel sheet.

[0052] Next, the cold-rolled steel sheet is cooled to room temperature at a cooling rate of at least 1°C / second to obtain a cold-rolled and heat-treated steel sheet.

[0053] The resulting cold-rolled and heat-treated steel sheet may then be optionally coated by any known method. The coating may be carried out with zinc or a zinc alloy, or with aluminum or an aluminum alloy.

[0054] To ensure degassing of the coated product, an optional post-batch annealing can be performed after coating the product, preferably at 170-210°C for 12-30 hours. The product is then cooled to room temperature to obtain a cold-rolled coated steel sheet. [Examples]

[0055] The following tests and examples presented herein are not limiting in nature and should be considered for illustrative purposes only, to demonstrate the advantageous features of the present invention, to illustrate the importance of parameters selected by the inventors after extensive experimentation, and to further establish the properties that can be achieved by the steel according to the present invention.

[0056] Samples of steel sheets according to the present invention and several comparative grades were prepared with the compositions summarized in Table 1 and the processing parameters summarized in Table 2. The corresponding microstructures of these steel sheets are summarized in Table 3, and their properties are summarized in Table 4.

[0057] Table 1 shows the steels with compositions expressed in weight percentage, along with the Ac3 and Ms values ​​for each steel, with the Ac3 and Ms temperatures measured according to the A1033-18 ASTM standard.

[0058] [Table 1]

[0059] Table 2 summarizes the annealing process parameters applied to the steels shown in Table 1.

[0060] Furthermore, before performing annealing on the steel of the present invention and the reference steel, the samples were heated to a temperature of 1150°C to 1300°C and hot-rolled. All trials were cold-rolled with a cold-rolling ratio of 56%.

[0061] [Table 2]

[0062] Table 3 summarizes the results of tests conducted according to standards using different microscopes, such as scanning electron microscopes, to determine the microstructural composition of the trial.

[0063] [Table 3]

[0064] From the table above, it can be seen that all trials according to the present invention satisfy the microstructure objectives.

[0065] Table 4 summarizes the mechanical and surface properties of steel.

[0066] Table 4: Mechanical properties of the trial Yield strength (YS), tensile strength (TS), and total elongation (TE) were measured according to the ASTM E8 standard issued in July 2022. Hole expansion ratio was measured according to the ISO-16630 standard. Bending tests were performed according to the ASTM E290-22 standard.

[0067] [Table 4]

Claims

1. Cold-rolled and heat-treated steel sheets, containing the following elements in weight percentage: 0.3% ≤ C ≤ 0.45%, 0.4% ≤ Mn ≤ 1.4%, 0.1% ≤ Si ≤ 0.9% 0.01% ≤ Al ≤ 0.1%, 0.1% ≤ Cr ≤ 0.8%, 0.01% ≤ Nb ≤ 0.1%, 0.1% ≤ Ni ≤ 0.9%, 0.01% ≤ Mo ≤ 0.9%, 0.01% ≤ Ti ≤ 0.1%, 0.0001% ≤ B ≤ 0.010%, 0% ≤ P ≤ 0.02%, 0% ≤ S ≤ 0.03%, 0% ≤ N ≤ 0.09% Includes and any of the following elements 0% ≤ V ≤ 0.1%, 0% ≤ Cu ≤ 2%, 0% ≤ Ca ≤ 0.005%, 0% ≤ Ce ≤ 0.1%, 0% ≤ Mg ≤ 0.05%, 0% ≤ Zr ≤ 0.05% A composition that may contain one or more of the following: The composition has the remainder consisting of iron and unavoidable impurities produced by processing, and the microstructure of the steel contains 1-5% bainite by area percentage, with the remainder being tempered martensite. Cold-rolled and heat-treated steel sheet.

2. A cold-rolled and heat-treated steel sheet according to claim 1, wherein the composition contains 0.31% to 0.4% carbon.

3. A cold-rolled and heat-treated steel sheet according to claim 1 or 2, wherein the composition contains 0.5% to 1.3% manganese.

4. A cold-rolled and heat-treated steel sheet according to any one of claims 1 to 3, wherein the composition contains 0.01% to 0.08% aluminum.

5. A cold-rolled and heat-treated steel sheet according to any one of claims 1 to 4, wherein the composition contains 0.2% to 0.8% silicon.

6. A cold-rolled and heat-treated steel sheet according to any one of claims 1 to 5, wherein the bainite content is 1% to 4%.

7. A cold-rolled and heat-treated steel sheet according to any one of claims 1 to 7, wherein the tempered martensite content is 96% to 99%.

8. The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 8, wherein the sheet has an ultimate tensile strength of 1500 MPa to 1750 MPa and a yield strength of 1250 MPa to 1450 MPa.

9. A method for producing cold-rolled and heat-treated steel sheets, comprising the following series - A step of providing a steel composition according to any one of claims 1 to 5, - A step of reheating the semi-finished product to a temperature of 1000°C to 1300°C. - A step of obtaining a hot-rolled steel sheet by rolling the semi-finished product in the austenite region where the hot-rolling finishing temperature exceeds 850°C. - A step of cooling the plate to a winding temperature of 580°C or less at a cooling rate of at least 5°C / second, and a step of winding the hot-rolled plate in a temperature range of 500°C to 580°C. - A step of cooling the hot-rolled plate to room temperature, -Optionally, a step of performing a scale removal process on the hot-rolled steel sheet, -Optionally, annealing may be performed on a hot-rolled steel sheet. -Optionally, a step of performing a scale removal process on the hot-rolled steel sheet, - The step of obtaining a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction rate of 35% to 90%, - Next, the cold-rolled steel sheet is heated at a heating rate HR1 of 1°C / sec to 30°C / sec, starting from room temperature and reaching a temperature TA of Ac3 + 10°C to Ac3 + 150°C, wherein the cold-rolled steel sheet is held for 100 to 1000 seconds during the heating step. - Next, the cold-rolled steel sheet is cooled at a cooling rate CR1 of 5°C / sec to 200°C / sec, starting from TA and decreasing to a temperature CS1 of 220°C to 320°C. Next, the cold-rolled steel sheet is heated to a T-temper temperature of 210°C to 390°C and held at the T-temper temperature for 1 to 500 seconds. - Next, the process is to cool the steel sheet to room temperature at a cooling rate of at least 1°C / second to obtain a cold-rolled and heat-treated steel sheet. Methods that include...

10. The method according to claim 9, wherein the winding temperature is 500°C to 570°C.

11. The method according to any one of claims 9 to 10, wherein CS1 is 230°C to 310°C.

12. The method according to any one of claims 9 to 11, wherein HR1 is 1°C / sec to 20°C / sec.

13. The method according to any one of claims 9 to 12, wherein TA is 800°C to 900°C.

14. Use of a steel sheet obtained according to any one of claims 1 to 8 or a steel sheet manufactured according to the method described in any one of claims 9 to 13 for manufacturing a structural component of a vehicle.

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