Cold-rolled and heat-treated steel sheet and method for producing same

Abstract

A cold-rolled and heat-treated steel sheet comprising: 0.19% < = C < = 0.45%; 1% < = Mn < = 2.0%; 0.1% < = Si < = 0.6%; 0.01% < = Al < = 0.1%; 0.12% < = Cr < = 0.8%; 0.01% < = Ti < = 0.1%; 0.0001% < = B < = 0.010%; 0% < = P < = 0.02%; 0% < = S < = 0.03%; 0% < = N < = 0.09%; 0% < = Nb < = 0.09%; 0% < = Mo < = 0.9%; 0% < = V < = 0.1%; 0% < = Ni < = 2%; 0% < = Cu < = 2%; 0% < = Ca < = 0.005%; 0% < = Ce < = 0.1%; 0% < = Mg < = 0.05%; 0% < = Zr < = 0.05%, the remainder being composed of iron and unavoidable impurities resulting from machining, the microstructure of said steel comprising, in area percentage, 1-6% bainite, 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] Automotive components are subject to two conflicting demands: ease of formability and strength. However, in recent years, global environmental concerns have placed a third requirement on automobiles: improved fuel consumption. Consequently, automotive components must now be made of materials with high formability to meet standards for easy assembly within complex automotive assemblies, while also requiring increased strength for vehicle crashworthiness and durability, while also reducing vehicle weight to improve fuel efficiency. Furthermore, steel components must be weldable without suffering from liquid metal embrittlement.

[0003] Therefore, a lot of research and development efforts have been invested in reducing the amount of material used in automobiles by increasing the strength of the material. In contrast, an increase in the strength of steel sheets reduces formability, and therefore there is a need to develop materials with both high strength and high formability.

[0004] Early research and development in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are listed herein for a clear understanding of the present invention:

[0005] EP3561119 provides a tempered martensitic steel having a low yield ratio and excellent uniform elongation, the tempered martensitic steel comprising, by weight %, 0.2% to 0.6% C, 0.01% to 2.2% Si, 0.5% to 3.0% Mn, 0.015% or less P, 0.005% or less S, 0.01% to 0.1% Al, 0.01% to 0.1% Ti, 0.05% to 0.5% Cr, 0.01% to 0.1% Ti, 0.05% to 0.5% Cr, 0.01% to 0.1% S, 0.01% to 0.1% S, 0.01% to 0.1% S, 0.01% to 0.1% Al, 0.01% to 0.1% Ti, 0.05% to 0.5% Cr, 0.01% to 0.1% S ... The invention relates to a product comprising 0.0005% to 0.005% B, 0.05% to 0.5% Mo, 0.01% or less N, and the balance Fe and unavoidable impurities; a product having a yield ratio of 0.4 to 0.6; a product having a tensile strength and uniform elongation (TS*U-EI) of 10,000 MPa% or more; and a microstructure comprising, by area fraction, 90% or more tempered martensite, 5% or less ferrite, and the balance bainite. However, the YS / TS ratio is not achieved.

[0006] The known prior art related to the production of high strength and high formability steel sheets suffers from one drawback or another: there is therefore a need for cold rolled steel sheets having a strength greater than 1300 MPa and a method for their production. Summary of the Invention

[0007] The object of the present invention is to solve these problems by making available a cold rolled and heat treated steel sheet which simultaneously:

[0008] - an ultimate tensile strength greater than or equal to 1300 MPa and preferably higher than 1350 MPa,

[0009] - A yield strength greater than or higher than 1050 MPa and preferably higher than 1140 MPa.

[0010] - Total elongation of 6% or more and preferably greater than 7%.

[0011] - A hole expansion ratio greater than or higher than 40% and preferably higher than 45%.

[0012] In a preferred embodiment, the cold rolled and heat treated steel sheet exhibits a YS / TS ratio greater than 0.75.

[0013] In a preferred embodiment, the cold rolled and heat treated steel sheet exhibits a bendability of less than 2.5 r / t at 90°C.

[0014] Preferably, such a steel may also have good suitability for forming, in particular for rolling, with good weldability and paintability.

[0015] Another object of the invention is also to provide a method for manufacturing these panels that is compatible with conventional industrial applications and at the same time is robust to manufacturing parameter variations.

[0016] 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.

[0017] Other features and advantages of the present invention will become apparent from the following detailed description of the invention. DETAILED DESCRIPTION

[0018] Carbon is present in steel at 0.19% to 0.45%. Carbon is an element essential for increasing the strength of steel sheets by delaying the formation of bainite during cooling after annealing. A content of less than 0.19% would not allow the steel of the present invention to possess sufficient tensile strength and ductility. On the other hand, a carbon content exceeding 0.45% significantly hardens the weld zone and heat-affected zone, impairing the mechanical properties of the weld zone. The preferred limit for carbon is 0.21% to 0.4%, and the more preferred limit is 0.22% to 0.3%.

[0019] The steel of the present invention has a manganese content of 1% to 2.0%. Manganese is a strength-imparting element, and an amount of at least 1% is essential for providing strength and hardenability to the steel sheet. Therefore, higher percentages of manganese are preferred, such as 1.1% to 1.9%, and more preferably 1.1% to 1.6%. However, when manganese exceeds 2.0%, this can have adverse effects, such as slowing the transformation of austenite to martensite, preventing the martensite from transforming into tempered martensite during tempering, and thus preventing the steel from achieving the target mechanical properties. In addition, manganese contents above 2.0% can cause centerline segregation and reduce the weldability of the steel of the present invention. Furthermore, high manganese contents are disadvantageous in terms of hydrogen-induced delayed fracture, an important criterion for steelmakers and the automotive industry.

[0020] The steel of the present invention contains a silicon content of 0.1% to 0.6%. Silicon is an element that contributes to increased strength through solid solution strengthening. Silicon is a component that slows carbide precipitation during cooling after annealing, thus promoting martensite formation. However, silicon is also a ferrite former and raises the Ac3 transformation point, which pushes the annealing temperature to a higher temperature range. This is why the silicon content is kept to a maximum of 0.6%. Silicon contents above 0.6% may also cause temper embrittlement and impair coatability. The preferred limit for silicon is 0.15% to 0.5%, and more preferably 0.2% to 0.4%.

[0021] The aluminum content of the steel of the present invention is 0.01% to 0.1%. Aluminum can be added during steelmaking to deoxidize the steel to capture oxygen. Above 0.1% will increase the Ac3 point, thereby reducing productivity. In addition, within such a range, aluminum combines with nitrogen in the steel to form aluminum nitride, thereby reducing the grain size, and aluminum also delays the precipitation of cementite. However, in the present invention, when the aluminum content exceeds 0.1%, the amount and size of aluminum nitride are not conducive to hole expansion and bending, and also push Ac3 to a higher temperature range (which is very expensive to achieve industrially), and also cause grain coarsening during annealing soaking. The preferred limit of aluminum is 0.01% to 0.06%, and more preferably 0.01% to 0.05%.

[0022] Chromium is an essential element of the steel of the present invention, present in an amount of 0.12% to 0.8%. Chromium provides strength to the steel through solid solution strengthening and hardening, but degrades the surface finish of the steel when used above 0.8%. The preferred limit for chromium is 0.15% to 0.7%, and more preferably 0.2% to 0.6%.

[0023] Titanium is an element added to the steel of the present invention at 0.01% to 0.1%, preferably 0.01% to 0.09%. It is useful for forming carbides, nitrides, and carbonitrides, which impart strength to the steel of the present invention through precipitation hardening during the annealing soaking temperature range, and thus is finer after full annealing, which results in hardening of the product. However, when the titanium content is above 0.1%, titanium consumes carbon by forming a large number of precipitates, which is disadvantageous for the present invention because a large number of precipitates tends to reduce the ductility of the steel.

[0024] Boron is an essential element that can be added at 0.0001% to 0.010%, preferably 0.001% to 0.004%, to harden the steel. Boron captures nitrides to form boron nitride, which imparts strength to the steel of the present invention. Boron also imparts hardenability to the steel of the present invention. However, when boron is added above 0.010%, the rollability of the steel sheet is significantly reduced. In addition, boron segregation may occur at grain boundaries, which is detrimental to formability.

[0025] The phosphorus content of the steel of the present invention is limited to 0.02%. Phosphorus is a hardening element in solid solution. Therefore, a small amount of at least 0.002% phosphorus can be beneficial. However, phosphorus also has its 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%.

[0026] Sulfur is not an essential element, but may be included in steel as an impurity. From the perspective of manufacturing costs, the sulfur content is preferably as low as possible, but is 0.03% or less, and preferably no more than 0.005%. Furthermore, if sulfur is present in high concentrations in the steel, it combines with Mn and Ti, in particular, to form sulfides, which are detrimental to the bending, hole expansion, and elongation properties of the steel of the present invention.

[0027] Nitrogen is limited to 0.09% to avoid material aging and to minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of the steel.

[0028] Niobium is an optional element and may be present in an amount of 0% to 0.09%, preferably 0.001% to 0.08%, and more preferably 0.01% to 0.07%. It is useful for forming carbonitrides that impart strength to the steel according to the present invention through precipitation hardening during the annealing soaking temperature range, and is therefore finer after full annealing, which results in hardening of the product. However, when the niobium content is above 0.09%, niobium consumes carbon by forming a large amount of carbonitrides, which is disadvantageous for the present invention because large amounts of carbonitrides tend to reduce the ductility of the steel.

[0029] Molybdenum is an optional element present in the steel of this invention in an amount of 0% to 0.9%. When added in an amount of at least 0.01%, molybdenum effectively improves hardenability and hardness, delaying the formation of ferrite and bainite during cooling after annealing. Mo also contributes to the toughness of hot-rolled products, making them easier to manufacture. However, excessive addition of molybdenum increases the cost of alloying elements, so for economic reasons, its content is limited to 0.9%. The preferred limit for molybdenum is 0% to 0.7%, and more preferably 0% to 0.6%.

[0030] Vanadium is an optional element that can be added to the steel of the present invention at a level of 0% to 0.1%, preferably 0.001% to 0.1%. Like niobium, it participates in the formation of carbonitrides and therefore plays a role in hardening. However, it also contributes to the formation of VN, which occurs during solidification of the cast product. Therefore, the amount of V is limited to 0.1% to avoid coarse VN that would negatively affect hole expansion. Vanadium contents below 0.001% have no effect on the steel of the present invention.

[0031] Nickel can be added as an optional element in an amount of 0% to 2% to increase the strength and improve the toughness of the steel of the present invention. A minimum of 0.01% is preferred to achieve this effect. However, when its content is higher than 2%, nickel causes deterioration of ductility.

[0032] Copper can be added as an optional element in an amount of 0% to 2% 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 these effects. However, when its content exceeds 2%, it may deteriorate the surface appearance.

[0033] Calcium is an optional element that can be added to the steel of the present invention at 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 aids in steel refining by capturing harmful sulfur content during the spheroidizing process.

[0034] 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 maximum levels indicated, these elements allow for refinement of the inclusion grains during solidification.

[0035] The remainder of the steel's composition consists of iron and unavoidable impurities caused by processing.

[0036] The microstructure of the steel sheet according to the present invention comprises, by area percentage, at least 90% tempered martensite and 1% to 6% bainite.

[0037] The area fractions of the phases in the microstructure are determined by cutting a specimen from the steel plate, polishing it and etching it with reagents known per se to expose the microstructure, and then examining the section by scanning electron microscopy, for example using a scanning electron microscope in secondary electron mode at a magnification greater than 500×.

[0038] The ferrite fraction is determined by SEM observation after etching with Nital or Picral / Nital reagents.The tempered martensite and bainite are measured according to the E228-17 ASTM standard.

[0039] Tempered martensite constitutes the matrix phase of the microstructure. Tempered martensite is formed from martensite that forms during cooling after annealing, particularly at temperatures below Ms-50°C. This martensite is then tempered while being held at a tempering temperature, Ttemper, of 180°C to 380°C. The tempered martensite of the present invention imparts ductility and strength to the steel. The martensite content is 94% to 99%, preferably 95% to 99%, and more preferably 96% to 99%.

[0040] Bainite is included in an amount of 1% to 6%. In the context of the present invention, bainite may include carbide-free bainite and / or lath bainite and granular bainite. When present, lath bainite is in the form of laths with a thickness of 1 μm to 5 μm. When present, carbide-free bainite is bainite with a very low carbide density of 1 μm per 100 μm. 2 The area unit contains fewer than 100 carbides and may contain islands of austenite. When present, granular bainite takes the form of grains with carbides located within the grain interior. Bainite provides improved elongation. The preferred bainite content is 1% to 5%, and more preferably 1% to 4%.

[0041] In addition to the above-mentioned microstructures, the microstructure of the cold-rolled and heat-treated steel sheet does not contain microstructural components such as pearlite, cementite, ferrite, fresh martensite, and retained austenite without impairing the mechanical properties of the steel sheet.

[0042] Cold-rolled steel and heat-treated plate according to the present invention can be produced by any suitable method. A preferred method comprises providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be made into an ingot or continuously produced in the form of a thin slab or thin strip, i.e., having a thickness ranging from about 220 mm (for slabs) to tens of millimeters (for thin strip).

[0043] For example, slabs are considered semi-finished products. Slabs with the above-mentioned chemical composition are produced by continuous casting, preferably undergoing direct soft reduction during casting to eliminate center segregation and reduce porosity. The slabs provided by the continuous casting process can be used directly at high temperatures after casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0044] The temperature of the slab undergoing hot rolling is preferably at least 1000°C, preferably higher than 1150°C, and must be lower than 1300°C. If the slab temperature is lower than 1150°C, excessive load is placed on the rolling mill. Furthermore, the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, causing the steel to be rolled while containing transformed ferrite in its structure. Furthermore, the temperature must not exceed 1300°C because it is industrially expensive.

[0045] The temperature of the slab is high enough so that hot rolling can be completed entirely in the austenitic range, with the finishing hot rolling temperature being kept above 850° C. The final rolling must be carried out above 850° C., since below this temperature the steel plate exhibits a significant drop in rollability.

[0046] The plate obtained in this manner is then cooled to a temperature of less than or equal to 680°C at a cooling rate of at least 5°C / second. Preferably, the cooling rate will be less than or equal to 100°C / second and greater than 10°C / second. Thereafter, the hot-rolled steel plate is coiled at a coiling temperature of less than 680°C, preferably between 500°C and 680°C, and more preferably between 520°C and 670°C. Thereafter, the coiled hot-rolled steel plate is cooled, preferably to room temperature. The hot-rolled steel plate may then be subjected to an optional descaling process, such as pickling, to remove the scale formed during hot rolling and ensure that no scale remains on its surface before the hot-rolled steel plate is subjected to an optional hot strip annealing.

[0047] The hot rolled sheet may be subjected to an optional hot strip annealing at a temperature of 350°C to 750°C for a period of 1 to 96 hours. The temperature and duration of such hot strip annealing are selected to ensure softening of the hot rolled sheet, thereby facilitating cold rolling of the hot rolled sheet. The hot rolled sheet may then be subjected to an optional descaling process, such as pickling, to remove scale formed during the hot strip annealing.

[0048] The hot-rolled steel sheet is then cooled to room temperature, after which the hot-rolled steel sheet is then cold-rolled at a thickness reduction ratio of 35% to 90% to obtain a cold-rolled steel sheet.

[0049] The cold rolled steel sheet is then subjected to annealing to impart the target microstructure and mechanical properties to the steel of the present invention.

[0050] During annealing, the cold-rolled steel sheet is heated from room temperature to a soaking temperature TA of Ac3+10°C to Ac3+150°C at a heating rate HR1 of 1°C / s to 30°C / s. Preferably, the HR1 rate is 1°C / s to 20°C / s, and more preferably 1°C / s to 10°C / s. A preferred TA temperature is 800°C to 900°C.

[0051] The cold-rolled steel sheet is then held at the annealing soaking temperature TA for 100 to 1000 seconds to ensure sufficient transformation to form 100% austenite at the end of the soaking. The cold-rolled steel sheet is then cooled to a cooling stop temperature range CS1 of Ms-50°C to Ms-300°C, preferably 100°C to 300°C, and more preferably 120°C to 280°C, at an average cooling rate CR1 of 5°C / s to 200°C / s, preferably 8°C / s to 100°C / s, and more preferably 10°C / s to 70°C / s. During this cooling step, the martensite of the present invention is formed. If the CS1 temperature is higher than Ms-50°C, the steel of the present invention will have too much bainite, which is detrimental to strength.

[0052] Thereafter, the steel is brought to a tempering temperature Ttemper of 180° C. to 380° C., and preferably 200° C. to 350° C., and more preferably 220° C. to 280° C., and then held at the Ttempering temperature for a time of 1 second to 500 seconds for tempering, and during the holding at the tempering temperature, the tempered martensite of the steel of the present invention is formed from the martensite formed during cooling.

[0053] The cold-rolled steel sheet is then cooled to room temperature at a cooling rate of at least 1° C. / second to obtain a cold-rolled and heat-treated steel sheet.

[0054] The cold rolled and heat treated steel sheet obtained may then optionally be coated by any known method.The coating may be made of zinc or an alloy based on zinc or of aluminium or an alloy based on aluminium.

[0055] After coating the product, an optional post-batch annealing may be carried out, preferably at 170°C to 210°C for 12 to 30 hours, to ensure degassing of the coated product. It is then cooled to room temperature to obtain the cold rolled and coated steel sheet.

[0056] Example

[0057] The following tests and examples provided herein are non-limiting in nature and must be considered for illustrative purposes only and will demonstrate the advantageous features of the present invention and illustrate the importance of the parameters selected by the inventors after extensive experimentation and further determining the properties that can be achieved with steel according to the present invention.

[0058] Steel plate samples according to the present invention and some comparative grade steel plate samples were prepared with the compositions summarized in Table 1 and the process parameters summarized in Table 2. The corresponding microstructures of these steel plates are summarized in Table 3, and the properties are summarized in Table 4.

[0059] Table 1 describes the steels having compositions expressed in weight percent and also shows Ac3 and Ms measured according to the A1033-18 ASTM standard.

[0060] Table 1: Composition of the trial

[0061]

[0062] Underlined values: not according to the present invention.

[0063] Table 2 summarizes the annealing process parameters implemented for the steels in Table 1.

[0064] In addition, before annealing the inventive steel and the reference steel, the samples were heated to a temperature of 1150° C. to 1300° C. and hot rolled. All tests were cold rolled at a cold rolling reduction of 55%.

[0065] Table 2: Process parameters of the experiment

[0066]

[0067] I = according to the invention; R = reference; underlined values: not according to the invention.

[0068] Table 3 summarizes the results of tests carried out according to standards on different microscopes, such as a scanning electron microscope, for determining the microstructural composition (in area fractions) of both the steel according to the invention and the reference test.

[0069] Table 3:

[0070] test Steel samples Tempered martensite (%) Bainite (%) Fresh martensite (% Ferrite (%) I1 1 98 2 0 0 I2 2 98 2 0 0 I3 3 98 2 0 0 R1 <![CDATA[ 4 ]]> <![CDATA[ 83 ]]> 2 <![CDATA[ 10 ]]> <![CDATA[ 5 ]]> R2 <![CDATA[ 4 ]]> <![CDATA[ 83 ]]> 4 <![CDATA[ 8 ]]> <![CDATA[ 5 ]]>

[0071] I = according to the invention; R = reference; underlined values: not according to the invention.

[0072] As can be seen from the table above, the microstructural targets were all met for the tests according to the present invention, whereas this was not the case for the reference examples.

[0073] Table 4 summarizes the mechanical and surface properties of both the inventive steel and the reference steel.

[0074] Table 4: Tested mechanical properties

[0075] Yield strength YS, tensile strength TS and total elongation TE are measured according to ISO standard ISO 6892-1 published in October 2009. To evaluate the hole expansion rate, a test called hole expansion is applied, in which the sample is subjected to a 10 mm punch and deformed, after which the hole diameter is measured and HER% is calculated as 100*(Df-Di) / Di.s.

[0076]

[0077] I = according to the invention; R = reference; underlined values: not according to the invention.

[0078] As can be seen from the above table, the tests according to the present invention all met the target characteristics, while this was not the case with the reference example.

Claims

1. A cold-rolled and heat-treated steel sheet containing the following elements, expressed in weight percentage: 0.19％≦C≦0.45％； 1%≦Mn≦2.0%; 0.1%≦Si≦0.6%; 0.01％≦Al≦0.1％； 0.12％≦Cr≦0.8％； 0.01%≦Ti≦0.1%; 0.0001％≦B≦0.010％； 0％≦P≦0.02％； 0％≦S≦0.03％； 0％≦N≦0.09％； and can include one or more of the following optional elements: 0%≦Nb≦0.09%; 0%≦Mo≦0.9%; 0％≦V≦0.1％； 0%≦Ni≦2%; 0% ≦ Cu ≦ 2%; 0%≦Ca≦0.005%; 0%≦Ce≦0.1%; 0%≦Mg≦0.05%; 0％≦Zr≦0.05％； The remainder of the composition is made up of iron and unavoidable impurities caused by processing, and the microstructure of the steel comprises, by area percentage, 1 to 6% bainite, the remainder being tempered martensite. 2 . The cold-rolled and heat-treated steel sheet according to claim 1 , wherein the composition comprises 0.21% to 0.4% carbon.

3. The cold rolled and heat treated steel sheet according to claim 1 or 2, wherein the composition contains 1.1% to 1.9% manganese.

4. The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 3, wherein the composition contains 0.01% to 0.06% of aluminum.

5. The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 4, wherein the composition contains 0.15% to 0.5% silicon.

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

7. The cold rolled and heat treated steel sheet according to any one of claims 1 to 7, wherein the tempered martensite is 95% to 99%. 8 . The cold-rolled and heat-treated steel sheet according to claim 1 , wherein the steel sheet has an ultimate tensile strength of 1,300 MPa or more and a yield strength of 1,050 MPa or more.

9. A method for producing cold-rolled and heat-treated steel sheet, comprising the following steps in sequence: - providing a steel composition according to any one of claims 1 to 5; - reheating the semi-finished product to a temperature of 1000° C. to 1300° C.; - rolling the semi-finished product in the austenitic range to obtain hot rolled steel sheets, wherein the hot rolling finishing temperature is higher than 850° C.; - cooling the plate to a coiling temperature less than or equal to 680°C at a cooling rate of at least 5°C / second; and coiling the hot rolled plate; - cooling the hot rolled sheet to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling process; - optionally capable of annealing the hot rolled steel sheet; - optionally subjecting the hot rolled steel sheet to a descaling process; - cold rolling the hot-rolled steel sheet at a reduction ratio of 35% to 90% to obtain a cold-rolled steel sheet; - then heating the cold-rolled steel sheet from room temperature to a temperature TA of Ac3+10°C to Ac3+150°C at a heating rate HR1 of 1°C / s to 30°C / s, and maintaining the cold-rolled steel sheet at the temperature TA for 100 seconds to 1000 seconds; - then cooling the cold rolled steel sheet from TA to a temperature CS1 of Ms-50°C to Ms-300°C at a cooling rate CR1 of 5°C / s to 200°C / s; - then bringing the cold rolled steel sheet to a T tempering temperature of 180° C. to 380° C. and maintaining the temperature at the T tempering temperature for 1 second to 500 seconds; - then cooled to room temperature at a cooling rate of at least 1°C / second to obtain a cold-rolled and heat-treated steel sheet.

10. The method of claim 9, wherein the coiling temperature is 680°C to 500°C.

11. The method according to any one of claims 9 to 10, wherein CS1 is from 100°C to 300°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 to 900°C.

14. Use of a steel sheet obtainable according to any one of claims 1 to 8 or a steel sheet manufactured according to any one of claims 9 to 13 for manufacturing structural parts of a vehicle.

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