Cold-rolled heat-treated steel sheet and method for manufacturing the same

Abstract

To provide steel sheets that possess both high strength and high formability. [Solution] Cold-rolled heat-treated steel sheet contains 0.05% ≤ carbon ≤ 0.12%, 1.0% ≤ manganese ≤ 2%, 0.01% ≤ silicon ≤ 0.5%, 0.01% ≤ aluminum ≤ 0.1%, 0.01% ≤ niobium ≤ 0.1%, 0% ≤ phosphorus ≤ 0.09%, 0% ≤ sulfur ≤ 0.09%, 0% ≤ nitrogen ≤ 0.09%, 0.1% ≤ chromium ≤ 0.5%, 0% ≤ nickel ≤ 3%, 0% ≤ titanium ≤ 0.1%, 0% ≤ calcium ≤ 0.005%, 0 The steel contains %≦copper≦2%, 0%≦molybdenum≦0.5%, 0%≦vanadium≦0.1%, 0%≦boron≦0.003%, 0%≦cerium≦0.1%, 0%≦magnesium≦0.010%, and 0%≦zirconium≦0.010%, and the microstructure of the steel sheet contains, by area fraction, 50-90% recrystallized ferrite, 10-50% non-recrystallized ferrite, 0-15% cementite, and 0.5-2% niobium carbides.

Description

Technical Field

[0001] The present invention relates to a cold-rolled heat-treated steel sheet suitable for use as an automobile steel sheet.

Background Art

[0002] Structural steels need to meet two conflicting requirements, namely, ease of forming and strength. In recent years, due to concerns about global environmental problems, a third requirement of improving the impact on CO2 consumption has been imposed on these structural steels intended to be used for constructing solar frames, lacquering, silos, roofing materials, cladding materials, and other similar purposes. Therefore, currently, structural steels must be made of materials with high strength in order to meet the standards of durability and lifespan.

[0003] Therefore, intensive research and development efforts have been made to reduce the amount of materials used in automobiles by increasing the strength of the materials. Conversely, increasing the strength of the steel sheet reduces the formability. Therefore, there is a need to develop materials that have both high strength and high formability.

[0004] Prior research and development in the field of high-strength and high-formability steel sheets have led to several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed herein for the final evaluation of the present invention.

[0005] US10920293 is a steel sheet with a composition of mass% C: 0.07~0.19%, Si: 0.09% or less, Mn: 0.50~1.60%, P: 0.05% or less, S: 0.01% or less, Al: 0.01~0.10%, N: 0.010% or less, with the remainder being Fe and unavoidable impurities. It has a microstructure containing ferrite as the main phase, 2~12% by volume of pearlite, 3% or less of martensite, and the remainder being a low-temperature generated phase. The ferrite has an average grain size of 25 μm or less, the pearlite has an average grain size of 5 μm or less, the martensite has an average grain size of 1.5 μm or less, and the mean free path of the pearlite is 5.5 μm or more. However, the steel of US10920293 cannot achieve a tensile strength of 600 MPa or more. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] U.S. Patent No. 10920293 [Overview of the project] [Problems that the invention aims to solve]

[0007] The object of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have the following: A TS / YS ratio of 1.10 or higher. Extreme tensile strength of 600 MPa or more, A total elongation of 14% or more, preferably 15% or more.

[0008] Preferably, such steel has a yield strength of 550 MPa or more, preferably more than 580 MPa.

[0009] Preferably, such steel can also have good suitability for rolling, with good formability, good weldability, bendability, and coating properties.

[0010] Preferably, such steel can also have a hole expansion ratio of more than 40%.

[0011] Another objective of the present invention is to make available a method for manufacturing these plates that is robust to changes in manufacturing parameters while being compatible with conventional industrial applications. [Means for solving the problem]

[0012] The cold-rolled heat-treated steel sheet of the present invention is coated with zinc or a zinc alloy, or with aluminum or an aluminum alloy, in order to improve its corrosion resistance.

[0013] Carbon is present in steel at a concentration of 0.05% to 0.12%. Carbon is an essential element for increasing the strength of steel sheets, not only through the formation of fine alloy precipitates but also through interstitial strengthening. If the carbon content is less than 0.05 wt%, it becomes difficult to simultaneously achieve the required yield strength of 550 MPa or more and total elongation of more than 14%. Whenever the carbon content exceeds 0.12%, it reduces coating properties and exhibits insufficient adhesion at the steel-coating interface. A carbon content exceeding 0.12% lowers the Ac1 temperature, so that secondary phases such as pearlite, bainite, and martensite can form at a relatively low soaking temperature, which reduces hole expansion and increases work hardening during bending, and is therefore undesirable. Accordingly, the preferred range of carbon in the steel of the present invention is 0.05% to 0.11%, more preferably 0.07% to 0.095%.

[0014] The manganese content of the steel of the present invention is 1.0% to 2%; the purpose of adding manganese is essentially to impart strength to the steel through solid solution strengthening. If the Mn content is less than 1%, it becomes difficult to simultaneously achieve the required yield strength of 550 MPa or more and total elongation of more than 14%. If the Mn content exceeds 2%, the transformation from austenite to pearlite is suppressed, martensite and / or bainite are formed, resulting in poor weldability in the sense that the hardness in the heat-affected zone (HAZ) increases, and surface cracking is more likely to occur during welding. The preferred content of the present invention can be maintained at 1.1% to 1.9%, and more preferably 1.2% to 1.8%, in order to ensure good bendability of the steel of the present invention.

[0015] The silicon content of the steel of the present invention is 0.01% to 0.5%. Silicon adds strength to ferrite through solid solution strengthening, and this effect tends to increase the hole expansion rate and ensure good ductility. However, if the content exceeds 0.5%, silicon concentrates on the surface of the steel sheet in the form of an oxide during annealing, worsening the coating properties and causing embrittlement. Furthermore, an excessive silicon content exceeding 0.5% also impairs toughness at high temperatures and often causes surface cracking during welding. For this reason, the silicon content is limited to 0.5% or less. The silicon content is preferably 0.01% to 0.4%, more preferably 0.01% to 0.3%.

[0016] Aluminum is an essential element and is present in the steel of the present invention at a concentration of 0.01% to 0.1%. Aluminum promotes ferrite formation and increases the Ms temperature, thereby enabling the present invention to have ferrite in the appropriate amount required for the steel to impart ductility and strength to the steel. However, if the presence of aluminum exceeds 0.1%, the Ac3 temperature increases, making annealing and hot rolling finish temperatures in the fully austenitic region economically unreasonable. The aluminum content is preferably limited to 0.01% to 0.09%, more preferably to 0.01% to 0.05%.

[0017] Niobium is an essential element for the steel of the present invention at a concentration of 0.01% to 0.1%, and is suitable for forming carbides and carbonitrides, thereby imparting the strength of the steel of the present invention through precipitation hardening. Niobium is also thought to influence the size of microstructural components by precipitation as carbides and by delaying recrystallization during the heating process. Therefore, a finer microstructure is formed in the final product, and as a result, the steel of the present invention can reach the target strength. However, niobium content exceeding 0.1% is not economically interesting and forms coarser precipitates that are detrimental to properties such as hole expansion rate and elongation of the steel. Furthermore, when the niobium content is 0.1% or higher, niobium is also detrimental to the hot ductility of the steel, causing difficulties during casting and rolling. The preferred limit for niobium content is 0.01% to 0.09%, and the more preferred limit is 0.01% to 0.05%.

[0018] Although phosphorus is not an essential element, it may be present in steel as an impurity. From the viewpoint of the present invention, it is preferable that the phosphorus content be as low as possible, preferably 0.09% or less. Phosphorus tends to segregate particularly at grain boundaries or co-segregate with manganese, thus reducing spot weldability and hot ductility. For these reasons, its content is limited to less than 0.09%, preferably less than 0.03%, and more preferably less than 0.014%.

[0019] Although sulfur is not an essential element, it may be present in steel as an impurity. From the viewpoint of the present invention, it is preferable that the sulfur content be as low as possible, but from the viewpoint of manufacturing costs, it should be 0.09% or less. Furthermore, if a higher amount of sulfur is present in the steel, it will combine with manganese in particular to form sulfides, reducing its beneficial effect on the steel of the present invention.

[0020] To avoid material degradation over time and minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of steel, nitrogen is limited to 0.09%.

[0021] Chromium is an optional element of the present invention. The chromium content that can be present in the steel of the present invention is 0.1% to 0.5%. Chromium provides strength and hardening to the steel, but when used in excess of 0.5%, it impairs the surface finish of the steel. The preferred limit of chromium for the present invention is 0.1% to 0.4%, more preferably 0.2% to 0.4%.

[0022] Nickel may be added as an optional element in an amount up to 3% to enhance the strength of the steel and improve its toughness. To achieve such an effect, a minimum of 0.01% is preferred. However, when its content exceeds 3%, nickel causes a decrease in ductility.

[0023] Titanium is an optional element and can be added up to 0.1% to the steel of the present invention. Niobium plays a role in the hardening of the steel of the present invention because it is involved in the formation of carbonitrides. Furthermore, titanium also forms titanium nitride that appears during the solidification of the casting. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitride that is harmful to formability. If the titanium content is less than 0.001%, it has no effect on the steel of the present invention.

[0024] The calcium content in the steel of the present invention is up to 0.005%. Calcium is added to the steel of the present invention as an optional element in a preferred minimum amount of 0.0001%, particularly during the treatment of inclusions. Calcium contributes to the refining of the steel by preventing the harmful sulfur content in the globular form, thereby delaying the harmful effects of sulfur.

[0025] Copper may be added as an optional element in an amount up to 2% to enhance the strength of the steel and improve its corrosion resistance. To obtain such an effect, a minimum of 0.01% of copper is preferred. However, when its content exceeds 2%, it may deteriorate the appearance of the surface.

[0026] Molybdenum is an optional element that constitutes up to 0.5% of the steel of the present invention. Molybdenum plays an effective role in determining hardenability and hardness, delays the appearance of bainite, and avoids carbide precipitation in bainite. However, since the addition of molybdenum excessively increases the addition cost of alloying elements, its content is limited to 0.5% for economic reasons.

[0027] Vanadium is effective in increasing the strength of steel by forming carbides or carbonitrides, and the upper limit is 0.1% for economic reasons. Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination at the following weight ratios: cerium ≤ 0.1%, boron ≤ 0.003%, magnesium ≤ 0.010% and zirconium ≤ 0.010%. Up to the indicated maximum content levels, these elements enable the refinement of grain size during solidification. The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

[0028] Next, the microstructure of the steel plate will be described.

[0029] Recrystallized ferrite constitutes 50% - 90% of the microstructure in terms of the area fraction of the steel of the present invention, preferably having an average grain size of 3.6 microns or less, and more preferably an average grain size of 2 - 3.6 microns. This recrystallized ferrite imparts at least 14% total elongation to the steel of the present invention. However, when the recrystallized ferrite content exceeds 90% in the matrix of the steel of the present invention, it is impossible to achieve a yield strength of 550 MPa. Recrystallized ferrite grains are defined as dislocation-free equiaxed grains that nucleate and grow during heating and soaking below the Ac1 temperature during annealing after cold rolling. Therefore, the preferred limit of the presence of recrystallized ferrite in the matrix of the present invention is 54% - 85% in terms of area fraction, more preferably 54% - 80%.

[0030] Non-recrystallized ferrite constitutes 10% to 50% of the microstructure in the area fraction of the steel of the present invention. Non-recrystallized ferrite grains are defined as dislocations containing elongated ferrite grains that formed during cold rolling and did not recrystallize during heating and soaking below the Ac1 temperature during annealing after cold rolling. Non-recrystallized ferrite contributes to the high strength of the steel of the present invention, and at least 10% non-recrystallized ferrite is necessary to ensure a yield strength of 550 MPa or higher. However, if the non-recrystallized ferrite content in the matrix of the steel of the present invention exceeds 50%, it is impossible to achieve a total elongation of at least 14%. Therefore, the preferred limit for the presence of non-recrystallized ferrite in the present invention is 15% to 50%, more preferably 20% to 48%, in area fraction.

[0031] The cumulative presence of non-recrystallized ferrite and recrystallized ferrite may be at least 85%, preferably at least 90%, and more preferably at least 98% or 99.5%. Recrystallized ferrite microstructure elements and non-recrystallized ferrite microstructure elements are distinguished by optical microscopy using non-etching and Dino etching solutions (140 ml distilled water, 100 ml H2O2, 4 g oxalic acid, 2 ml H2SO4, and 1.5 ml HF). The area fraction of each component is measured according to ASTM E562.

[0032] Niobium carbide is present in the steel of the present invention. According to the present invention, it is advantageous that the size of the niobium carbide precipitates is 2 nm to 200 nm, more preferably 2 nm to 20 nm. The niobium carbide of the present invention includes both intragranular niobium carbide (i.e., called intragranular niobium carbide because it precipitates inside ferrite particles) and intergranular niobium carbide (i.e., called intergranular niobium carbide because it precipitates at ferrite grain boundaries). Homogeneous and tightly packed precipitation of niobium carbide enhances the strength of the steel. The limit of niobium carbide presence is 0.5% to 2% in area fraction, more preferably 0.5% to 1.5% in area fraction.

[0033] The amount of cementite that can optionally be present in the steel of this invention is between 0% and 15%. While cementite imparts strength to this invention, if the presence of cementite exceeds 15%, the full elongation will not be achieved.

[0034] In addition to the microstructure described above, the microstructure of cold-rolled heat-treated steel sheets does not contain microstructure components such as pearlite, bainite, and martensite, without impairing the mechanical properties of the steel sheets.

[0035] The steel sheet according to the present invention can be manufactured by any suitable method. A preferred method consists of providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be carried out in the form of an ingot or continuously in the form of thin slabs or thin strips, i.e., in thicknesses ranging from about 220 mm in the case of slabs to several tens of millimeters in the case of thin strips.

[0036] For example, slabs having the above-mentioned chemical composition are manufactured by continuous casting, and the slabs are optionally subjected to direct light compression during the continuous casting process to avoid central segregation, ensuring that the local carbon to nominal carbon ratio is maintained below 1.10. Slabs provided by the continuous casting process can be used directly at high temperatures after continuous casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0037] The temperature of the slab subjected to hot rolling must be at least 1000°C and below 1280°C. If the slab temperature is below 1000°C, niobium dissolution will not occur completely, and as a result, the niobium will not form appropriate carbides during annealing. Furthermore, temperatures below 1000°C may place an excessive load 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 ferrites incorporated into the structure. Therefore, the slab temperature is preferably high enough to allow hot rolling to be completed in the temperature range of Ac3 to Ac3+100°C, and to allow the final rolling temperature to remain above Ac3. Reheating at temperatures above 1280°C should be avoided as it is industrially expensive.

[0038] The final rolling temperature must be in the range of Ac3 to Ac3 + 100°C to have a microstructure favorable for recrystallization and rolling. The final rolling pass is preferably carried out at a temperature above 850°C, below which the steel sheet shows a significant decrease in rollability. The hot-rolled steel thus obtained is then cooled at a cooling rate of more than 20°C / second to a coiling temperature that must be between 450°C and 650°C. The purpose of maintaining the coiling temperature between 450°C and 650°C is to keep microalloy elements such as niobium in the solid solution of the hot band in order to maximize precipitation during annealing after cold rolling. Preferably, the cooling rate is 200°C / second or less.

[0039] Next, the hot-rolled steel is wound at a winding temperature of 450°C to 650°C, preferably 450°C to 625°C, to avoid ellipsis. A more preferred range for such winding temperatures is 460°C to 625°C. The wound hot-rolled steel is cooled to room temperature and then subjected to an optional hot strip annealing.

[0040] The hot-rolled steel may be subjected to an optional descaling step to remove scale formed during hot rolling, prior to optional hot strip annealing. The hot-rolled sheet may then be subjected to an optional hot strip annealing at a temperature of, for example, 400°C to 750°C for at least 12 to 96 hours, the temperature being kept below 750°C to avoid partial transformation of the hot-rolled microstructure, and thus loss of microstructure uniformity. Subsequently, the optional descaling step of this hot-rolled steel may be carried out, for example, by pickling such a sheet. This hot-rolled steel is then subjected to cold rolling to obtain a cold-rolled steel sheet with a thickness reduced by 35 to 90%. The cold-rolled steel sheet obtained from the cold-rolling process is then annealed to impart the microstructure and mechanical properties of the steel of the present invention.

[0041] Annealing of cold-rolled steel sheets is carried out in two steps. The first step begins by heating the steel sheet from room temperature to a temperature T1 of 580°C to 650°C at a heating rate HR1 of at least 20°C / second. It is advantageous to keep the T1 temperature lower than the recrystallization onset temperature calculated by differential scanning calorimetry experiments as described on pages 765-773 of the paper "Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery, recrystallization and ferrite to austenite phase transformation" by Taylor and Francis, published on December 6, 2013. Subsequently, the second step begins by further heating the steel sheet from T1 to a soaking temperature T2 of 700°C to 760°C at a heating rate HR2 of at least 2°C / second, where HR2 is lower than HR1, and then annealing is performed at T2 for 10 to 500 seconds. In a preferred embodiment, the heating rate in the second step is less than 10°C / second, more preferably less than 8°C / second. A preferred temperature T2 for soaking is 700°C to Ac1-50°C.

[0042] Next, the cold-rolled steel is cooled from T2 to a temperature range T3 of 400°C to 500°C, preferably 420°C to 490°C, at an average cooling rate of at least 10°C / second, preferably at least 15°C / second, and the cooling step may include an optional slow-cooling substep within the T3 temperature range at a cooling rate of 2°C / second or less, preferably 1°C / second or less. The cold-rolled steel sheet is held within the temperature range T3 for 10 to 500 seconds.

[0043] Next, in order to facilitate the hot-dip galvanizing of cold-rolled steel sheets, the temperature of the coating bath can be set to 420°C to 480°C depending on the properties of the coating.

[0044] Cold-rolled steel sheets can also be coated by any known industrial process such as electro-galvanizing, JVD, or PVD, and it is not necessary to bring them to the aforementioned temperature range before coating.

[0045] Next, optional post-batch annealing may be performed at a temperature of 150°C to 300°C for 30 minutes to 120 hours.

[0046] Subsequently, skin pass rolling can be performed on the cold-rolled steel sheet with a minimum skin pass reduction of 1.3% or more, preferably 1.4% or more. [Examples]

[0047] The following tests, examples, diagrams, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and illustrate advantageous features of the present invention.

[0048] Table 1 summarizes steel sheets made from steel of different compositions, and each of these steel sheets is manufactured according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the steel sheets obtained during testing, and Table 4 summarizes the evaluation results of the obtained properties.

[0049] [Table 1] Underlined values: Not according to the present invention.

[0050] Table 2 Table 2 summarizes the parameters of the annealing process performed on the steels in Table 1. Steel compositions I1 to I3 and R1 to R5 are used in the manufacture of the plates according to the present invention. Table 2 also shows the summaries of Ac1 and Ac3. These Ac1 and Ac3 are defined for the steels of the present invention and the reference steel by expansion measurement studies conducted in accordance with the ASTM A1033-04 standard.

[0051] The following processing parameters are the same for all steels in Table 1. All steels in Table 1 were heated to a temperature of 1200°C before hot rolling and then to a temperature of 460°C before final hot-dip galvanizing.

[0052] Table 2 is as follows:

[0053] [Table 2]

[0054] Table 3 Table 3 illustrates the results of tests performed according to standards using different microscopes, such as scanning electron microscopes, to determine the microstructure of both the steel of the present invention and the reference steel.

[0055] The results are specified herein:

[0056] [Table 3] I = According to the present invention; R = Reference; Underlined value: Not according to the present invention.

[0057] Table 4 Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. Tensile tests were performed on A80 specimens according to NBN EN ISO 6892-1, Method B, to determine the tensile strength, yield strength, and total elongation.

[0058] This document summarizes the results of various mechanical tests conducted in accordance with the standards.

[0059] [Table 4] I = According to the present invention; R = Reference; Underlined value: Not according to the present invention.

Claims

1. Cold-rolled and heat-treated steel sheet, containing the following elements expressed in weight percentage: 0.05% ≤ Carbon ≤ 0.12% 1.0% ≤ Manganese ≤ 2% 0.01% ≤ Silicon ≤ 0.5% 0.01% ≤ Aluminum ≤ 0.1% 0.01% ≤ Niobium ≤ 0.1% 0% ≤ Phosphorus ≤ 0.09% 0% ≤ Sulfur ≤ 0.09% 0% ≤ Nitrogen ≤ 0.09% It includes, as well as any of the following optional elements 0.1% ≤ Chromium ≤ 0.5% 0% ≤ Nickel ≤ 3% 0% ≤ Titanium ≤ 0.1% 0% ≤ Calcium ≤ 0.005% 0%≦Copper≦2% 0% ≤ Molybdenum ≤ 0.5% 0% ≤ Vanadium ≤ 0.1% 0% ≤ Boron ≤ 0.003% 0% ≤ Cerium ≤ 0.1% 0% ≤ Magnesium ≤ 0.010% 0% ≤ Zirconium ≤ 0.010% Having a composition that can contain one or more of the following, The remaining composition consists of iron and unavoidable impurities resulting from processing, and the microstructure of the steel sheet contains, by area fraction, 50-90% recrystallized ferrite, 10-50% non-recrystallized ferrite, 0-15% cementite, and 0.5-2% niobium carbide, wherein the cumulative amount of recrystallized ferrite and non-recrystallized ferrite is at least 85%, in a cold-rolled and heat-treated steel sheet.

2. The cold-rolled heat-treated steel sheet according to claim 1, wherein the composition contains 0.01% to 0.4% silicon.

3. The cold-rolled heat-treated steel sheet according to claim 1 or 2, wherein the composition contains 0.05% to 0.11% carbon.

4. A cold-rolled heat-treated steel sheet according to any one of claims 1 to 3, wherein the composition comprises 0.01% to 0.09% aluminum.

5. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 4, wherein the cumulative amount of recrystallized ferrite and non-recrystallized ferrite is at least 90%.

6. A cold-rolled heat-treated steel sheet according to any one of claims 1 to 5, wherein the amount of recrystallized ferrite is 54% to 85%.

7. A cold-rolled heat-treated steel sheet according to any one of claims 1 to 6, wherein the non-recrystallized ferrite content is 20% to 48%.

8. A cold-rolled heat-treated steel sheet according to any one of claims 1 to 8, having an ultimate tensile strength of 600 MPa or more, a total elongation of 14% or more, and a yield strength-to-tensile strength ratio of 1.10 or more.

9. A cold-rolled heat-treated steel sheet according to any one of claims 1 to 9, having a yield strength of 550 MPa or more.

10. A method for manufacturing cold-rolled heat-treated steel sheets, comprising the following series of steps: - A step of providing a steel composition according to any one of claims 1 to 4; - A step of reheating the semi-finished product to a temperature of 1000°C to 1280°C; - A step of rolling a semi-finished product in a temperature range of Ac3 to Ac3 + 100°C, wherein the hot-rolling finish temperature exceeds Ac3, thereby obtaining hot-rolled steel; - A step of cooling the hot-rolled steel to a winding temperature of 450°C to 650°C at a cooling rate of more than 20°C / second, and a step of winding the hot-rolled steel; - A step of cooling the hot-rolled steel to room temperature; - A step of optionally performing a scale removal process on the hot-rolled steel sheet; - Optionally, perform annealing on hot-rolled steel sheets at 400°C to 750°C; - A step of optionally performing a scale removal process on the hot-rolled steel sheet; - The hot-rolled steel sheet is cold-rolled at a reduction ratio of 35 to 90% to obtain a cold-rolled steel sheet; - Heating the cold-rolled steel sheet in two steps: The first step begins with heating the steel plate from room temperature to a temperature T1 of 580°C to 650°C at a heating rate HR1 of at least 20°C / second. The second step begins by further heating the steel plate from T1 to a soaking temperature T2 of 700°C to 760°C at a heating rate HR2 of 2°C / second or more, where HR2 is lower than HR1, and then annealing is performed at T2 for 10 to 500 seconds. The annealing step; - Next, the cold-rolled steel sheet is cooled from T2 to a holding temperature T3 of 400°C to 500°C at an average cooling rate of at least 10°C / second; Next, the cold-rolled steel sheet is held at T3 for 10 to 500 seconds to bring it to a temperature range of 420°C to 480°C; - Next, the cold-rolled sheet is covered to obtain a cold-rolled heat-treated steel sheet. Methods that include...

11. The method according to claim 11, wherein the winding temperature is 450°C to 625°C.

12. The method according to claim 11 or 12, wherein the finishing rolling temperature exceeds 850°C.

13. Use of a steel sheet according to any one of claims 1 to 10 or a steel sheet manufactured according to the method of claims 11 to 13 for the manufacture of structural steel.

14. A steel structure comprising a component obtained according to claim 14.

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