Heat-treated cold-rolled steel sheet and its manufacturing method

Abstract

1. Heat-treated cold-rolled steel sheet containing the following elements, expressed in weight percent: 0.1%≦carbon≦0.25%, 2.15%≦manganese≦3.0%, 1%≦silicon≦0.8%, 0.1%≦aluminum≦0.9%, 0.05%≦chromium≦0.5%, 0%≦phosphorus≦0.09%, 0%≦sulphur≦0.09%, 0%≦nitrogen≦0.09%, 2.4%≦C+Mn≦3%, 0%≦niobium≦0.1%, 0%≦titanium≦0.1%, 0%≦vanadium≦0.1%, 0%≦molybdenum≦1%, 0%≦nickel≦1%, 0%≦calcium a heat-treated cold-rolled steel sheet comprising 0.005% or less of zinc, 0% or less of boron, 0.01% or less of cerium, 0% or less of magnesium, 0.05% or less of zirconium, and the remainder consisting of iron and unavoidable impurities; a microstructure of the steel sheet comprising 20% ​​to 70% of martensite, 5 to 60% of intercritical ferrite, 5 to 30% of transformed ferrite, 8% to 20% of retained austenite, and 1 to 20% of bainite, with a cumulative amount of intercritical ferrite and transformed ferrite being 15% to 65%.

Description

[Technical field]

[0001] The present invention relates to a cold-rolled steel sheet having a tensile strength of 950 MPa or more and a total elongation of 14.0% or more, and having high strength and high formability suitable for use as a steel sheet for vehicles. [Background technology]

[0002] Automotive parts are required to satisfy two conflicting needs, namely ease of forming and strength, but in recent years, a third requirement, improved fuel efficiency, has been placed on automobiles due to concerns about the global environment. Thus, automotive parts must now be made of materials with high formability to meet the criteria of ease of fitting into complex automobile assemblies, while at the same time improving strength for the automobile's impact resistance and durability, while reducing the automobile's weight to improve fuel efficiency.

[0003] For this reason, research and development is being actively pursued to reduce the amount of material used in automobiles by improving the strength of materials. Conversely, increasing the strength of steel sheets reduces their formability, so it is necessary to develop materials that have both high strength and high formability.

[0004] Previous research and development in the field of high strength and highly formable steel plates has led to several methods for producing high strength and highly formable steel plates, some of which are listed herein for a final understanding of the present invention.

[0005] EP 2971209 is a patent related to a high-strength hot-dip galvanized duplex-phase steel strip with a duplex-phase microstructure expressed in volume percentage, which has improved formability for use in the automotive industry, has an essential element composition of C: 0.13 - 0.19%, Mn: 1.70 - 2.50%, Si: 0 - 0.15%, Al: 0.40 - 1.00%, Cr: 0.05 - 0.25%, Nb: 0.01 - 0.05%, P: 0 - 0.10%, Ca: 0 - 0.004%, S: 0 - 0.05%, N: 0 - 0.007%, with the balance being Fe and inevitable impurities, 0.40% < Al + Si < 1.05% and Mn + Cr > 1.90%, contains 8 - 12% retained austenite, 20 - 50% bainite, less than 10% martensite, and the balance being ferrite. However, this licensed patent cannot reach a tensile strength exceeding 900 MPa.

[0006] Known prior arts related to the manufacture of high-strength and high-formability steel sheets lack either one or the other. Therefore, there is a need for cold-rolled steel sheets having high strength and high formability and a method for manufacturing the same.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

[0008] An object of the present invention is to solve these problems by making available a cold-rolled steel sheet having the following simultaneously. - An ultimate tensile strength of 950 MPa or more, preferably exceeding 980 MPa, - An elongation at break of 14.0% or more, - A yield strength of 600 MPa or more, preferably 630 MPa or more.

[0009] In a preferred embodiment, the steel sheet according to the present invention can have a YS / TS ratio exceeding 0.55.

[0010] Preferably, such a steel also has good weldability and coatability, as well as good suitability for forming, especially rolling.

[0011] Another object of the invention is to make available a manufacturing process for these plates that is stable towards variations in the manufacturing parameters, whilst being compatible with conventional industrial applications. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0013] Carbon is present in steel between 0.1 and 0.25%. Carbon is a necessary element for increasing the strength of steel sheets by generating low-temperature transformation phases such as martensite. In addition, carbon also plays a crucial role in austenite stabilization. If the content is less than 0.1%, it will not be possible to stabilize austenite and ensure at least 20% martensite, which will result in a decrease in strength and ductility. On the other hand, if the carbon content exceeds 0.25%, the weld zone and heat-affected zone will be significantly hardened, impairing the mechanical properties of the weld zone. The preferred limit of carbon is between 0.12 and 0.22%, more preferably between 0.15 and 0.20%.

[0014] The manganese content of the steel of the present invention is between 2.15 and 3.0%. Manganese is an element that stabilizes austenite to obtain retained austenite and imparts strength. A manganese amount of at least 2.15% by weight has been found to stabilize austenite and provide strength and hardenability to the steel sheet. Therefore, a higher percentage of manganese, such as 2.2-2.9%, is preferred. However, if manganese exceeds 3.0%, adverse effects occur, such as retarding the transformation of austenite to bainite during the isothermal hold of the bainite transformation, resulting in reduced ductility. Also, if the manganese content exceeds 3.0%, the weldability of the steel will be reduced. Therefore, the preferred limits for the steel of the present invention are between 2.2% and 2.9%, more preferably between 2.3% and 2.6%.

[0015] Silicon is an essential element of the steel of the present invention, and is present between 0.1 and 0.8%. Silicon is added to the steel of the present invention to impart strength by solid solution strengthening. Silicon plays a part in the formation of the microstructure by preventing the precipitation of carbides and promoting the formation of martensite. However, if the silicon content exceeds 0.8%, the surface properties and weldability of the steel deteriorate, so that the silicon content is preferably between 0.15% and 0.7%, more preferably between 0.2% and 0.6%.

[0016] The aluminum content of the present invention is between 0.1 and 0.9%. Aluminum is added to deoxidize the steel of the present invention. Aluminum is an alpha-forming element and also promotes the stabilization of austenite by retarding the formation of carbides. This can improve the formability and ductility of the steel. To obtain such an effect, an aluminum content of 0.1% or more is necessary. However, if the aluminum content exceeds 0.9%, the Ac3 point rises beyond the permissible range, making it very difficult to achieve austenite single phase industrially, and therefore hot rolling in the complete austenite region cannot be performed. Therefore, the aluminum content should not exceed 0.9%. The preferred limit of the presence of aluminum is between 0.2 and 0.8%, more preferably between 0.3 and 0.8%.

[0017] The chromium content of the steel of the invention is between 0.05% and 0.5%. Chromium is an essential element that gives the steel its strength and hardness, but if used in excess of 0.5%, it impairs the surface finish of the steel. The preferred limits for chromium are between 0.1% and 0.4%, more preferably between 0.1% and 0.3%.

[0018] The phosphorus content of the steel of the invention is limited to 0.09%. Phosphorus is an element that hardens in solid solution and also prevents the formation of carbides. Small amounts of phosphorus, at least 0.002%, can therefore be advantageous, but phosphorus also has adverse effects such as reduced spot weldability and hot ductility due to its tendency to segregate, especially at grain boundaries, or to co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.05%.

[0019] Although sulfur is not an essential element, it may be present in steel as an impurity up to 0.09%. The sulfur content is preferably as low as possible, but is preferably between 0.001 and 0.03% from the viewpoint of production costs. Furthermore, if a larger amount of sulfur is present in the steel, it will combine with, in particular, Mn and Ti to form sulfides, reducing their beneficial effects on the present invention.

[0020] Nitrogen is limited to 0.09% to avoid ageing of the material; it forms nitrides which impart strength to the steel of the present invention by precipitation strengthening with vanadium and niobium, but whenever nitrogen is present in excess of 0.09%, it can form large amounts of aluminum nitride which are detrimental to the present invention, so the preferred upper limit for nitrogen is 0.01%.

[0021] In the steel, carbon and manganese are cumulatively present between 2.4 and 3%. Both carbon and manganese stabilize the austenite in the steel of the invention and provide strength to the steel of the invention. To ensure an elongation of 14.0% while reaching a tensile strength of 950 MPa in the steel of the invention, the cumulative amount is a minimum of 2.4% to have 8% retained austenite, but whenever the cumulative amount of carbon and manganese exceeds 3%, the strengthening effect prevails and the balance of elongation and tensile strength is no longer interesting. The preferred limits of the cumulative presence of carbon and manganese are between 2.5 and 2.9%, more preferably between 2.5 and 2.8%.

[0022] Niobium is an optional element that can be added to the steel in amounts up to 0.1%, preferably between 0.0010 and 0.1%. Niobium is suitable for forming carbonitrides that impart strength to the steel of the invention by precipitation hardening. Since niobium retards recrystallization during heating, the microstructure formed at the end of the holding temperature and, as a consequence, after complete annealing, is finer, which leads to a hardening of the product. However, when the niobium content exceeds 0.1%, a large amount of carbonitrides tends to reduce the ductility of the steel, so that amount of carbonitrides is not preferred for the invention.

[0023] Titanium is an optional element that can be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.1%. Niobium plays a role in hardening, since it is included in carbonitrides as niobium. However, it is also involved in forming TiN, which appears during solidification of the cast product. The amount of Ti is limited to 0.1% to avoid coarse TiN, which is detrimental to hole expansion. If the titanium content is less than 0.001%, titanium has no effect on the steel of the present invention.

[0024] Vanadium is an optional element that can be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.01%. Vanadium plays a role in hardening, since as niobium it participates in carbonitrides. But it also participates in the formation of VN, which appears during solidification of the cast product. The amount of V is limited to 0.1% to avoid coarse VN, which is detrimental to hole expansion. If the vanadium content is less than 0.001%, vanadium has no effect on the steel of the present invention.

[0025] Molybdenum is an optional element constituting between 0% and 1% of the steel of the invention, it enhances the hardenability of the steel of the invention and influences the transformation of austenite to ferrite and bainite during cooling after annealing, however, the addition of molybdenum excessively increases the cost of adding alloying elements, so that its content is limited to 1% for economic reasons.

[0026] Nickel can be added as an optional element in amounts between 0% and 1% to increase the strength of steel and improve its toughness. A minimum of 0.01% is needed to produce such an effect. However, when its content exceeds 1%, nickel causes a deterioration of ductility.

[0027] Calcium is an optional element that can be added to the steel of the present invention in amounts up to 0.005%, preferably between 0.001% and 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 refinement of the steel by preventing harmful sulfur inclusions when spheroidizing it.

[0028] Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in proportions of Ce≦0.1%, B≦0.01%, Mg≦0.05% and Zr≦0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grains during solidification.

[0029] The balance of the steel composition consists of unavoidable impurities resulting from the steel and processing.

[0030] The microstructure of the steel sheet according to the present invention comprises 20%-70% martensite, 5%-60% intercritical ferrite, 5%-30% transformed ferrite, 8%-20% retained austenite, 1%-20% bainite and a cumulative amount of intercritical ferrite and transformed ferrite between area fractions of 15%-65%.

[0031] Martensite constitutes between 20% and 70% of the microstructure by area fraction. The martensite of the present invention can include both fresh and tempered martensite, as well as island MA morphology. However, tempered martensite is an optional microconstituent that is preferably limited in the steel in an amount between 0% and 10%, preferably between 0 and 5%. Tempered martensite can form during cooling after annealing. Fresh martensite forms during cooling after overaging. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the martensite content is between 20% and 60%, more preferably between 24% and 56%.

[0032] Interval ferrite constitutes between 5% and 60% of the microstructure by area fraction of the steel of the invention. This interval ferrite confers a total elongation of at least 14.0% to the steel of the invention. Interval ferrite results from annealing at temperatures below Ac3. As will be described below, interval ferrite differs from the ferrite that may be produced after annealing, which we term "transformed ferrite". In particular, contrary to transformed ferrite, interval ferrite is polygonal. Moreover, transformed ferrite is enriched in carbon and manganese, i.e. has a higher carbon and manganese content than that of interval ferrite. Interval ferrite and transformed ferrite can therefore be distinguished by observing micrographs with a FEG-TEM microscope using secondary electrons after etching with metabisulfite. On such micrographs, interval ferrite appears in a medium grey colour, whereas transformed ferrite appears in a dark grey colour due to its higher carbon and manganese content. The preferred limit of the presence of intercritical ferrite in the steel of the invention is between 5% and 50%, more preferably between 10% and 50%.

[0033] The transformed ferrite constitutes 5% to 30% of the microstructure by area fraction for the steel of the present invention. The transformed ferrite of the present invention is composed of ferrite formed after annealing and bainitic ferrite formed during soaking treatment to coat the steel. The transformed ferrite gives the steel of the present invention high strength and elongation. To ensure an elongation of 14.0%, preferably 15% or more, it is necessary to have 5% transformed ferrite. The transformed ferrite of the present invention is formed during cooling after annealing and during soaking treatment to coat the steel. The transformed ferrite of the present invention is richer in carbon and manganese compared to the intercritical ferrite. However, whenever the transformed ferrite content is present in the steel of the present invention above 30%, it is not possible to have both tensile strength and total elongation at the same time. The preferred limit of the presence of ferrite for the present invention is between 6% and 25%, more preferably between 7% and 20%.

[0034] The retained austenite constitutes 8% to 20% by area fraction of the steel. The retained austenite of the steel according to the invention increases the ductility due to the TRIP effect. The retained austenite of the invention may also be present in the form of islands of MA. The preferred limits of the presence of austenite are between 8% and 18%, more preferably between 8% and 15%. In a preferred embodiment, the retained austenite comprises carbon in an amount higher than 0.8% by weight and lower than 1.1% by weight, more preferably between 0.9% and 1.1% by weight, even more preferably between 0.95% and 1.05% by weight.

[0035] Bainite constitutes 1% to 20% of the microstructure in terms of area fraction for the steel of the present invention. In the present invention, bainite is cumulatively composed of lath bainite and granular bainite. In order to ensure a tensile strength of 950 MPa or more, it is necessary to have at least 1% bainite. Bainite is formed during overaging.

[0036] The cumulative amount of transformed ferrite and intercritical ferrite must be between 15% and 65%, which ensures that the steel of the present invention always has a total elongation of at least 14.0% as well as a tensile strength of 950 MPa.

[0037] The steel sheet of the invention can be obtained by any suitable method, but it is preferable to use a method according to a preferred embodiment of the invention, comprising the following successive steps:

[0038] Such a method includes providing a steel semi-finished product having a chemical composition according to the invention, which can be cast into ingots or continuously cast in the form of thin slabs or thin strip (i.e., thicknesses ranging, for example, from about 220 mm for slabs to a few tens of millimeters for thin strip).

[0039] For the purpose of simplifying the present invention, the slab is considered as a semi-finished product. The slab having the above chemical composition is produced by continuous casting, and the slab is preferably subjected to direct soft reduction during casting to ensure the elimination of center segregation and the reduction of porosity. The slab provided by the continuous casting process can be used directly at high temperature after continuous casting, or it can be first cooled to room temperature and then reheated for hot rolling.

[0040] The temperature of the slabs subjected to hot rolling must be at least 1000°C, preferably at least 1050°C, preferably above 1100°C and below 1250°C. If the temperature of the slab is lower than 1000°C, excessive loads are applied to the rolling mill and, furthermore, the temperature of the steel may drop to the ferrite transformation temperature during the finish rolling, so that the steel is rolled with transformed ferrite in the structure. Furthermore, the temperature must not exceed 1250°C, since there is a risk of forming coarse ferrite grains, which leads to coarse ferrite grains that reduce the ability of these grains to recrystallize during hot rolling. The larger the initial ferrite grain size, the less it recrystallizes, which means that reheating temperatures above 1250°C must be avoided, since this is industrially expensive and unfavorable in terms of ferrite recrystallization.

[0041] The temperature of the slab is preferably high enough to complete the hot rolling in the fully austenitic range and to allow hot rolling to occur between Ac3 and Ac3+200°C, with the finish hot rolling temperature remaining above Ac3, preferably above Ac3+50°C. Final rolling must be performed above Ac3, since below this temperature the steel sheet shows a significant loss of rollability. The final rolling temperature should preferably be above Ac3+50°C in order to have a structure suitable for recrystallization and rolling.

[0042] The sheet obtained in this way is then cooled to a coiling temperature below 600° C. at a cooling rate of at least 30° C. / s. Preferably, the cooling rate is less than or equal to 65° C. / s and greater than 35° C. / s. The coiling temperature is preferably greater than 350° C. in order to avoid the transformation of austenite into ferrite and pearlite and to favor the formation of a homogeneous bainite and martensite microstructure.

[0043] The coiled hot rolled steel sheet may be cooled to room temperature before being subjected to the optional hot band annealing, or may be sent directly to the optional hot band annealing.

[0044] If necessary, the hot rolled steel sheet may be subjected to an optional pickling in order to remove scale formed during hot rolling. The hot rolled sheet is then subjected to an optional hot band annealing at a temperature between 400 and 750°C for a period of preferably 1 to 96 hours.

[0045] Thereafter, the hot-rolled steel sheet may be pickled as necessary to remove scale.

[0046] The hot rolled steel sheet is then cold rolled at a reduction ratio of between 35 and 90%. The cold rolled steel sheet is then annealed to impart the desired microstructure and mechanical properties to the steel of the present invention.

[0047] The cold rolled steel sheet is then subjected to a two-step annealing, where the first step begins with heating the steel sheet from room temperature to a temperature T1 between 600°C and 750°C, with a heating rate HR1 of at least 2°C / s, with a preferred range of HR1 being 2°C / s to 40°C / s, more preferably 3°C / s to 25°C / s, and then the second step begins with further heating the steel sheet from T1 to a soaking temperature T2 between Ac1 and Ac3, with a heating rate HR2 of 15°C / s or less, with HR2 being lower than HR1, and then annealing at T2 for 10 to 500 seconds. In a preferred embodiment, the heating rate of the second step is less than 5°C / s, more preferably less than 3°C / s. The preferred temperature T2 for soaking is between Ac1+30°C and Ac3, more preferably between Ac1+30°C and Ac3-20°C.

[0048] The second heating step ensures that the steel of the present invention has sufficient time at high temperature to completely dissolve all precipitates such as cementite formed in the previous processing steps. This results in the austenite of the present invention with a homogeneous carbon content between 0.8% and 1.1% by weight, and an intercritical ferrite area fraction of 5-60%.

[0049] Next, the cold-rolled steel sheet is annealed at a soaking temperature T2 between Ac1 and Ac3.

[0050] In a preferred embodiment, the soaking temperature is selected to ensure that the microstructure of the steel sheet at the end of the soaking comprises at least 50% austenite, more preferably at least 60% austenite.

[0051] The cold rolled steel is then cooled from T2 to an overaging hold temperature Tover between Ms-50°C and 500°C, preferably between Ms-40°C and 490°C, at an average cooling rate of at least 5°C / s, preferably at least 10°C / s, more preferably 15°C / s, the cooling step may comprise an optional slow cooling sub-step from T2 to a temperature Tsc between 600°C and 750°C, at a cooling rate of at most 2°C / s, preferably at most 1°C / s.

[0052] The cold rolled steel sheet is then held in the Tover for 5 to 500 seconds.

[0053] In a first embodiment, the cold rolled steel sheet is then cooled to room temperature to obtain the heat treated cold rolled steel sheet according to the invention. In another embodiment, the cold rolled steel sheet can be post batch annealed at a temperature between 150 and 300°C for a period between 30 minutes and 120 hours. In another embodiment, the cold rolled steel sheet can be brought to the temperature of the coating bath as required to facilitate hot dip coating of the cold rolled steel sheet and to perform any coating, depending on the nature of the coating. In the case of zinc coatings, the temperature of such steel can be kept between 420 and 460°C.

[0054] The cold rolled steel sheet may also be coated by any of the known industrial methods such as electrogalvanizing, JVD, PVD, etc., which may not require the cold rolled steel sheet to be brought to the above temperature ranges prior to coating. EXAMPLES

[0055] The following tests and examples presented herein are non-limiting in nature and must be considered for illustrative purposes only, showing the advantageous features of the invention, explaining the significance of the parameters selected by the inventors after extensive experimentation, and further establishing the properties that can be achieved with the steel according to the invention.

[0056] Steel plate samples according to the invention and of some comparative grades were prepared using the compositions summarized in Table 1 and the processing parameters summarized in Table 2. The corresponding microstructures and properties of the steel plates are summarized in Table 3 and Table 4, respectively.

[0057] Table 1 shows said steels having said compositions expressed as percentages by weight.

[0058] [Table 1]

[0059] Table 2 summarizes the annealing treatment parameters performed on the steels in Table 1.

[0060] The bainite transformation Bs temperature and the martensite transformation Ms temperature of the steel of the present invention and the reference steel are shown in Table 1. The calculation of Bs was performed using the Van Bohemen formula published in Materials Science and Technology (2012) Vol. 28, No. 4, pp. 487-495, as follows. Bs=839-(86*[Mn]+23*[Si]+67*[Cr]+33*[Ni]+75*[Mo])-270*(1-EXP(-1.33*[C]))

[0061] The calculation of Ms is performed using the Barbier formula. Ms=545-601.2*(1-Exp(1-0.868*C%))-34.4*Mn%-13.7Si%-9.2Cr%-17.3Ni%-15.4Mo%+10.8V%+4.7Co%-1.4Al%-16.3Cu%-361Nb%-2.44Ti%-3448B%

[0062] It also shows the Ac1 and Ac3 values, which are calculated using the following formulas: Ac1=723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr]+6.38[W]+290[As] Ac3=955-350[C]-25[Mn]+51[Si]+106[Nb]+100[Ti]+68[Al]-11[Cr]-33[Ni]-16[Cu]+67[Mo] The elemental contents are given in weight percent.

[0063] <Table 2: Processing parameters for test examples> All examples and counter examples are reheated to a temperature of 1200°C and then hot rolled with a hot rolling finishing temperature of 920°C, after which the hot rolled steel strip is coiled at 550°C, and the cold rolling reduction for all examples and counter examples is 60%.

[0064] [Table 2]

[0065] Table 3 summarizes the results of tests carried out according to different microscopic standards, such as scanning electron microscopes, for determining the microstructural composition of both the steel of the invention and the reference test examples.

[0066] [Table 3]

[0067] Table 4 summarizes the mechanical properties of both the steels of the invention and the reference steels. Tensile strength, yield strength and total elongation tests are performed according to the ISO 6892-1 standard.

[0068] [Table 4]

[0069] The examples show that the steel sheets according to the invention are unique in that they exhibit all the target properties as a result of their specific composition and microstructure.

Claims

1. Heat treated cold rolled steel sheet containing the following elements expressed in weight percent: 0.1%≦Carbon≦0.25% 2.15%≦Manganese≦3.0% 0.1%≦Silicon≦0.8% 0.1%≦Aluminum≦0.9% 0.05%≦Chromium≦0.5% 0%≦phosphorus≦0.09% 0%≦sulfur≦0.09% 0%≦Nitrogen≦0.09% 2.4%≦C+Mn≦3% Including, Any of the following elements 0%≦Niobium≦0.1% 0%≦Titanium≦0.1% 0%≦Vanadium≦0.1% 0%≦Molybdenum≦1% 0%≦Nickel≦1% 0%≦Calcium≦0.005% 0%≦Boron≦0.01% 0%≦Cerium≦0.1% 0%≦Magnesium≦0.05% 0%≦Zirconium≦0.05% and the remainder being iron and unavoidable impurities resulting from processing, and the microstructure of the steel sheet includes, in area fractions, 20% to 70% martensite, 5% to 60% intercritical ferrite, 5% to 30% transformed ferrite, 8% to 20% retained austenite, the retained austenite having a carbon content of between 0.8% by weight and 1.1% by weight, and 1 to 20% bainite, and the total amount of intercritical ferrite and transformed ferrite is 15% to 65%.

2. A heat-treated cold rolled steel sheet as described in claim 1, wherein the composition includes, by weight percent, 0.15% to 0.7% silicon.

3. The heat treated cold rolled steel sheet according to claim 1, wherein the composition contains, by weight percent, 0.12% to 0.22% carbon.

4. A heat-treated cold rolled steel sheet as described in any one of claims 1 to 3, wherein the composition includes, by weight percentage, 0.2% to 0.8% aluminum.

5. A heat-treated cold rolled steel sheet as claimed in any one of claims 1 to 4, wherein the composition comprises, by weight percentage, 2.2% to 2.9% manganese.

6. A heat-treated cold rolled steel sheet as claimed in any one of claims 1 to 5, wherein in the composition the total amount of carbon and manganese is between 2.5 and 2.9% by weight.

7. A heat-treated cold rolled steel sheet as claimed in any one of claims 1 to 6, wherein the carbon content of the retained austenite is between 0.9% and 1.1% by weight.

8. A heat-treated cold-rolled steel sheet described in any one of claims 1 to 7, wherein the transformation interval ferrite has an area fraction between 5% and 50%.

9. A heat-treated cold-rolled steel sheet described in any one of claims 1 to 8, wherein the transformed ferrite has an area fraction of between 6% and 25%.

10. A heat-treated cold rolled steel sheet as described in any one of claims 1 to 9, wherein the martensite has an area fraction of between 20% and 60%.

11. The heat treated cold rolled steel sheet according to any one of claims 1 to 10, wherein the steel sheet has an ultimate tensile strength of 950 MPa or more and a total elongation of 14.0% or more.

12. 12. The heat treated cold rolled steel sheet according to claim 11, wherein the steel sheet has a yield strength of 600 MPa or more.

13. A method for producing a cold rolled steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 6, - reheating the semi-finished product to a temperature between 1000°C and 1250°C, where the semi-finished product is a slab; - rolling the semi-finished product at a hot rolling finishing temperature above Ac3 in the temperature range between Ac3 and Ac3+200°C to obtain a hot rolled steel sheet; - cooling the hot-rolled steel sheet at a cooling rate of at least 30 ° C / s to a coiling temperature of less than 600 ° C and coiling the hot-rolled steel sheet, - cooling the hot rolled steel sheet to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling treatment, - optionally annealing the hot rolled steel sheet at between 400°C and 750°C, - optionally subjecting the hot rolled steel sheet to a descaling treatment, - cold rolling the hot rolled steel sheet with a rolling reduction of between 35 and 90% to obtain a cold rolled steel sheet; - a process of annealing the cold-rolled steel sheet by two-stage heating, a first step consists of heating said steel sheet from room temperature to a temperature T1 between 600° C. and 750° C., with a heating rate HR1 of at least 2° C. / s; a second step starting with further heating the steel sheet from T1 to a soaking temperature T2 between Ac1 and Ac3 at a heating rate HR2 of not more than 15°C / s, HR2 being lower than HR1; - followed by annealing at T2 for 10 to 500 seconds; - then cooling said cold-rolled steel sheet from T2 to an overaging temperature Tover between Ms-50°C and 500°C at an average cooling rate of at least 5°C / s, said cooling may include an optional slow cooling sub-step at a slow cooling rate of not more than 2°C / s from T2 to a temperature Tsc between 600°C and 750°C, - the cold rolled steel sheet is then overaged in Tover for 5-500 seconds, to a temperature range between 420°C and 460°C, facilitating any coating; - followed by a post-annealing step, optionally in batches, at a temperature ranging from 150 to 300°C for 30 minutes to 120 hours; - thereafter cooling the cold rolled steel sheet to room temperature to obtain a heat treated cold rolled steel sheet. Including, The resulting heat-treated cold-rolled steel sheet has an ultimate tensile strength of 950 MPa or more, a total elongation of 14.0% or more, and a yield strength of 600 MPa or more. method.

14. The method of claim 13, wherein the coiling temperature is between 350°C and 600°C.

15. 15. The method according to claim 13 or 14, wherein the finish hot rolling temperature is greater than Ac3+50°C.

16. A method according to any one of claims 13 to 15, wherein the soaking temperature T2 is selected to ensure the presence of at least 50% by area of ​​austenite at the end of the soaking.

17. Use of a heat-treated cold rolled steel sheet according to any of claims 1 to 12 or produced by the method according to any of claims 13 to 16 for the manufacture of structural or safety parts of a vehicle.

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