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

Abstract

To provide a cold-rolled heat-treated steel sheet suitable for use as a steel sheet for an automobile.SOLUTION: The cold-rolled heat-treated steel sheet has a composition that includes the following elements expressed in percent by weight: 0.10%≤carbon≤0.5%, 1%≤manganese≤3.4%, 0.5%≤silicon≤2.5%, 0.03%≤aluminum≤1.5%, 0%≤sulfur≤0.003%, 0.002%≤phosphorus≤0.02% and 0%≤nitrogen≤0.01%, and that optionally can include one or more of chromium, molybdenum, niobium, titanium, copper, nickel, calcium, vanadium, boron, cerium, magnesium and zirconium. The residual composition is composed of iron and unavoidable impurities; the fine structure includes, in terms of area fraction, 10 to 30% retained austenite, 10 to 40% bainite, 5 to 50% annealed martensite, 1 to 20% quenched martensite, and smaller than 30% tempered martensite; and a cumulative amount of bainite and retained austenite is 25% or more.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Automotive parts must meet two contradictory requirements: ease of formability and strength, but in recent years, due to considerations of the global environment, automobiles have also been given a third requirement: improved fuel efficiency. Therefore, automotive parts must be made of highly formable materials to meet the standards of ease of fit for complex automobile assemblies, while at the same time reducing vehicle weight and improving fuel efficiency while improving the strength of the vehicle's crashworthiness and durability.

[0003] Therefore, intensive research and development efforts are being made to reduce the amount of material used in automobiles by increasing the strength of the materials. Conversely, since increasing the strength of steel sheets reduces their formability, there is a demand for the development of materials that combine 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 here for the final evaluation of the present invention.

[0005] EP3128023 describes a high-strength cold-rolled steel sheet having excellent elongation, hole expandability, delayed fracture resistance, and a high yield ratio, and a manufacturing method thereof. The high-yield-ratio high-strength cold-rolled steel sheet has a composition, by mass%, containing 0.13% to 0.25% C, 1.2% to 2.2% Si, 2.0% to 3.2% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, 0.008% or less N, 0.055% to 0.130% Ti, with the balance being Fe and unavoidable impurities. The steel plate has a microstructure containing 2% to 15% ferrite by volume having an average crystal grain size of 2 μm or less, 5 to 20% retained austenite by volume having an average crystal grain size of 0.3 to 2.0 μm, 10% or less (including 0%) martensite by volume having an average grain size of 2 μm or less, and the remainder being bainite and tempered martensite, where the average crystal grain size of the bainite and tempered martensite is 5 μm or less.

[0006] EP3009527 provides a high-strength cold-rolled steel sheet having excellent elongation, stretch flangeability, and a high yield ratio, and a manufacturing method thereof. The high-strength cold-rolled steel sheet has a certain composition and a certain microstructure. The composition contains, on a mass basis, 0.15% to 0.27% C, 0.8% to 2.4% Si, 2.3% to 3.5% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, and 0.010% or less N, with the balance being Fe and unavoidable impurities. The microstructure contains ferrite with an average grain size of 5 μm or less and a volume fraction of 3% to 20%, retained austenite with a volume fraction of 5% to 20%, and martensite with a volume fraction of 5% to 20%, with the balance being bainite and / or tempered martensite. The total number of retained austenite with a grain size of 2 μm or less, martensite with a grain size of 2 μm or less, or a mixture of these phases is 150 or more per 2,000 μm2 of the plate thickness cross section parallel to the rolling direction of the steel plate.

[0007] EP3144406 claims a high strength cold rolled steel sheet with excellent ductility, the steel sheet containing, by weight, 0.1% to 0.3% carbon (C), 0.1% to 2.0% silicon (Si), 0.005% to 1.5% aluminum (Al), 1.5% to 3.0% manganese (Mn), 0.04% or less phosphorus (P) (except 0%), 0.015% or less sulfur (S) (except 0%), 0.02% or less nitrogen (N) (except 0%), and 0.05% or less sulphur (S) (except 0%). The total of Si and Al (Si+Al) (wt%) is 1.0% or more, and the microstructure contains, in terms of area fraction, 5% or less of polygonal ferrite having a minor axis to major axis ratio of 0.4 or more, 70% or less (excl. 0%) of acicular ferrite having a minor axis to major axis ratio of 0.4 or less, 25% or less (excl. 0%) of acicular retained austenite, and the balance is martensite. Furthermore, EP3144406 foresees a high strength steel having a tensile strength of 780 MPa or more but unable to reach a yield strength of 600 MPa or more, and therefore lacks formability, particularly for automotive exterior and anti-intrusion parts. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent No. 3128023 [Patent Document 2] European Patent No. 3009527 [Patent Document 3] European Patent No. 3144406 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to - an ultimate tensile strength of at least 900 MPa, preferably greater than 980 MPa; a total elongation of at least 14%, preferably more than 18%, - Yield strength of 550 MPa or more, The object of the present invention is to solve these problems by making available a cold rolled steel sheet having both of the above properties. [Means for solving the problem]

[0010] In a preferred embodiment, the steel sheet according to the invention may also exhibit a ratio of yield strength to tensile strength of 0.5 or more.

[0011] Preferably, such steels are also weldable and paintable and well suited to forming, especially rolling.

[0012] Another object of the invention is also to make available a manufacturing process for these steel sheets that is robust to variations in manufacturing parameters, while being compatible with conventional industrial applications.

[0013] The cold rolled heat treated steel sheet of the present invention may be optionally coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve its corrosion resistance.

[0014] Carbon is present in the steel at between 0.10% and 0.5%. Carbon is an element necessary for generating low-temperature transformation phases such as martensite to increase the strength of the steel sheet, and furthermore, it is an element necessary for ensuring the retained austenite, since it plays a central role in austenite stabilization. Thus, carbon plays two important roles, one is to increase the strength, and the other is to retain the austenite and give it ductility. However, if the carbon content is less than 0.10%, it will not be possible to stabilize the appropriate amount of austenite required by the steel of the present invention. On the other hand, if the carbon content exceeds 0.5%, the spot weldability of the steel will decrease, limiting its application to automotive parts.

[0015] The manganese content of the steel of the present invention is between 1% and 3.4%. This element is of the gamma type. The purpose of adding manganese is essentially to obtain a structure containing austenite and giving strength to the steel. Manganese in an amount of at least 1% by weight has been found to provide strength and hardenability of the steel sheet, as well as to stabilize the austenite. Therefore, in the presented invention, a higher percentage of manganese is preferred, such as up to 3.4%. However, a manganese content above 3.4% has adverse effects, such as retarding the transformation of austenite to bainite during the isothermal hold of the bainite transformation. Furthermore, a manganese content above 3.4% reduces the ductility and also the weldability of the steel, so that the ductility target may not be achieved. The preferred range of manganese is between 1.2% and 2.3%, and more preferably between 1.2% and 2.2%.

[0016] The silicon content of the steel of the invention is between 0.5% and 2.5%. Silicon is an element that retards the precipitation of carbides during overaging, so that the presence of silicon stabilizes the carbon-rich austenite at room temperature. Furthermore, due to the low solubility of silicon in carbides, it effectively inhibits or retards their formation, thus promoting the formation of low-density carbides in the bainite structure that is sought to give the steel its essential characteristics according to the invention. However, a disproportionate content of silicon would not bring about the mentioned effects and would cause problems such as temper embrittlement. The concentration is therefore limited within the upper limit of 2.5%.

[0017] The aluminum content is between 0.03 and 1.5%. In the present invention, aluminum removes the oxygen present in the molten steel to prevent it from forming a gas phase. Aluminum also fixes the nitrogen in the steel to form aluminum nitride and reduces the size of the particles. If the aluminum content exceeds 1.5%, the Ac3 point becomes high temperature and the productivity decreases. In the present invention, when adding a high manganese content, an aluminum content between 1.0 and 1.5% is used to offset the effect of manganese on the transformation points such as Ac3 and the progress of austenite formation with temperature.

[0018] The chromium content of the steel of the present invention is between 0.05% and 1%. Chromium is an essential element that gives the steel strength and hardness, but if used in excess of 1%, it impairs the surface finish of the steel. Furthermore, at chromium contents below 1%, the carbide distribution pattern of the bainite structure becomes coarse, and therefore the carbide density of the bainite remains low.

[0019] The phosphorus content of the steel of the invention is between 0.002% and 0.02%. Phosphorus has a tendency to segregate or co-segregate with manganese, especially at grain boundaries, which reduces spot weldability and hot ductility. For these reasons, its content is limited to less than 0.02%, preferably less than 0.013%.

[0020] Sulfur is not an essential element, but may be present in the steel as an impurity, and from the standpoint of the present invention, the sulfur content is preferably as low as possible, but not more than 0.003%, from the standpoint of production costs. Furthermore, if more sulfur is present in the steel, it will combine, especially with manganese, to form sulfides, reducing the beneficial effect of manganese on the steel of the present invention.

[0021] Niobium is present in the steel between 0.001 and 0.1% and is added to the steel of the present invention in order to form carbonitrides and thereby give the steel its strength by precipitation hardening. Niobium also influences the size of the components of the microstructure through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, a finer microstructure is formed at the end of the temperature hold, which results after the completion of the annealing, which leads to the hardening of the steel of the present invention. However, when the niobium content exceeds 0.1%, a saturation effect of its influence is observed, which means that the addition of niobium does not improve the strength of the product and is not of economic interest.

[0022] Titanium is added to the steel of the present invention between 0.001% and 0.1%. Like niobium, titanium plays a role in hardening the steel of the present invention, since it participates in carbonitride formation. In addition, titanium also forms titanium nitrides that appear during solidification of the casting. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides that are detrimental to formability. If the titanium content is less than 0.001%, it does not have any effect on the steel of the present invention.

[0023] The calcium content of the steel of the invention is between 0.0001% and 0.005%. Calcium is added to the steel of the invention as an optional element, especially during inclusion treatment. Calcium contributes to the refining of the steel by trapping the harmful sulphur content in globular form and retarding the harmful effects of sulphur.

[0024] Copper may be added as an optional element in amounts of 0.01% to 2% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.001% copper is required to achieve this effect. However, if the copper content exceeds 2%, the surface quality may deteriorate.

[0025] Nickel may be added as an optional element in amounts of 0.01 to 3% to increase the strength and toughness of the steel. A minimum of 0.01% is required to obtain this effect. However, if its content exceeds 3%, nickel causes a decrease in ductility.

[0026] Molybdenum is an optional element constituting 0.001% to 0.5% of the steel of the present invention, and it plays an effective role in determining the hardenability and hardness, delaying the appearance of bainite and avoiding the precipitation of carbides in bainite, but since the addition of molybdenum excessively increases the cost of adding alloying elements, its content is limited to 0.5% for economic reasons.

[0027] Nitrogen is limited to 0.01% to avoid aging of the material, which is detrimental to the mechanical properties of the steel, and to minimize the precipitation of aluminum nitride during solidification, which has a detrimental effect on the mechanical properties of the steel.

[0028] Vanadium has the effect of increasing the strength of the steel by forming carbides or carbonitrides and for economic reasons is limited to an upper limit of 0.1%. Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following weight proportions: cerium ≦ 0.1%, boron ≦ 0.003%, magnesium ≦ 0.010%, zirconium ≦ 0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grains during solidification. The balance of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of the steel plate is as follows:

[0030] The annealed martensite present in the steel of the invention has an area fraction between 5% and 50%. The main component of the steel of the invention in terms of the microstructure after the first annealing cycle is quenched or tempered martensite obtained during the continuous cooling from the holding temperature and the final tempering. This quenched or tempered martensite is annealed during the second annealing. Depending on the soaking temperature of the second annealing, the area fraction of the annealed martensite is at least 5% in the case of annealing close to a fully austenitic domain or limited to 50% in the case of intercritical holding.

[0031] Quenched martensite constitutes between 1% and 20% of the microstructure by area fraction. Quenched martensite imparts strength to the steel of the present invention. Quenched martensite is formed during the final cooling of the second anneal. Although no minimum is required, more than 20% quenched martensite will provide excessive strength but degrade other mechanical properties beyond acceptable limits.

[0032] Tempered martensite constitutes between 0 and 30% of the microstructure by area. Martensite occurs when the steel reaches Tc min ~Tc maxTempered martensite can be formed when the steel is cooled between 0.25 and 1.05°C and tempered during the overaging hold. Tempered martensite imparts ductility and strength to the steel of the present invention. More than 30% tempered martensite provides excessive strength but reduces elongation beyond acceptable limits. Furthermore, tempered martensite reduces the hardness gap between soft phases such as retained austenite and hard phases such as hardened martensite.

[0033] Bainite constitutes 10% to 40% of the microstructure of the steel of the present invention in terms of area fraction. In the present invention, bainite is cumulatively composed of lath bainite and granular bainite, and the granular bainite has very low density carbides. The low density carbides referred to here are those having a number of carbides per unit area of ​​100 μm 2 It has a high dislocation density, meaning that there are less than 100 carbides per lath, which gives the steel of the present invention high strength and elongation. Lath bainite is in the form of thin ferrite laths with austenite or carbides formed between the laths. The lath bainite of the steel of the present invention gives the steel adequate formability. To ensure an elongation of 14% and preferably 15% or more, it is necessary to have 10% bainite.

[0034] The retained austenite constitutes 10% to 30% of the steel by area fraction. Retained austenite is known to have a higher solubility of carbon than bainite and therefore acts as an effective carbon trap, retarding the formation of carbides in bainite. The carbon percentage in the retained austenite of the present invention is preferably higher than 0.9% and preferably lower than 1.1%. The retained austenite of the steel according to the present invention confers enhanced ductility.

[0035] In addition to the above microstructures, the microstructure of the cold rolled heat treated steel sheet does not contain microstructural components such as pearlite, ferrite and cementite without compromising the mechanical properties of the steel sheet.

[0036] The steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel having a chemical composition according to the invention. Casting can be carried out in ingots or continuously in the form of thin slabs or thin strip, i.e. with thicknesses ranging from about 220 mm for slabs to up to several tens of millimeters for thin strip.

[0037] For example, slabs having the above chemical composition are produced by continuous casting, and the slabs are optionally soft-reduced directly during the continuous casting process to avoid central segregation and to keep the local carbon to nominal carbon ratio below 1.10. The slabs resulting from the continuous casting process can be used directly at high temperature after continuous casting, or they may first be cooled to room temperature and then reheated for hot rolling.

[0038] The temperature of the slab subjected to hot rolling should preferably be at least 1200°C and less than 1280°C. If the temperature of the slab is less than 1200°C, excessive load is applied to the rolling mill, and furthermore, the temperature of the steel is lowered to the ferrite transformation temperature during the finish rolling, so that the steel is rolled in a state in which the structure contains transformed ferrite. Therefore, the temperature of the slab is preferably high enough so that the hot rolling can be completed in the temperature range of Ac3 to Ac3 + 100°C, and the final rolling temperature exceeds Ac3. Reheating at temperatures above 1280°C is industrially expensive and should be avoided.

[0039] A final rolling temperature range between Ac3 and Ac3+100°C is preferred in order to have a structure favorable for recrystallization and rolling. It is necessary to carry out the final rolling pass at a temperature above Ac3, since below this temperature the rollability of the steel sheet is significantly reduced. The steel sheet thus obtained is then cooled at a cooling rate of more than 30°C / s to the coiling temperature, which must be less than 600°C. Preferably, the cooling rate is less than or equal to 200°C / s.

[0040] The hot rolled steel sheet is then coiled at a coiling temperature below 600° C. to avoid ovalization, and preferably below 570° C. to avoid scale formation. A preferred range for such coiling temperatures is between 350° C. and 570° C. The coiled hot rolled steel sheet may be cooled to room temperature before being subjected to any hot strip annealing, or may be directly transferred to be subjected to any hot strip annealing.

[0041] The hot rolled steel sheet may be subjected to an optional descaling step prior to any hot strip annealing in order to remove any scale formed during hot rolling. The hot rolled steel sheet may then be subjected to an optional hot strip annealing at a temperature between 400°C and 750°C for at least 12 hours and not more than 96 hours, the temperature being kept below 750°C to avoid partial hot rolled microstructural transformation and thus loss of microstructural homogeneity. An optional descaling step of this hot rolled steel sheet may then be carried out, for example via pickling of such steel sheet. This hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet with a thickness reduction of between 35 and 90%. The cold rolled steel sheet obtained from the cold rolling step is then subjected to a two step annealing to impart the microstructural and mechanical properties to the steel of the present invention.

[0042] In the first annealing, the cold-rolled steel sheet is heated to a soaking temperature between Ac3 and Ac3+100°C at a heating rate of more than 3°C / s, and Ac3 of the steel is expressed by the following formula: Ac3=901-262*C-29*Mn+31*Si-12*Cr-155*Nb+86*Al where the elemental contents are expressed as weight percent.

[0043] The steel sheet is held at the soaking temperature for 10 to 500 seconds to fully recrystallize and transform into the strongly work-hardened initial austenite structure. The steel sheet is then cooled at a rate of more than 20°C / s until a temperature of less than 500°C, preferably less than 400°C, is reached. Furthermore, a cooling rate of at least 30°C / s is preferred to ensure robustness of the formation of the single-phase martensite structure after this first anneal.

[0044] The cold rolled steel sheet may then be optionally tempered at between 120°C and 250°C.

[0045] The cold rolled annealed steel sheet is then subjected to a second annealing at a heating rate of more than 3°C / s at T soaking by heating to a soaking temperature between Ac3 and Ac4, T soaking =830-260*C-25*Mn+22*Si+40*Al where the elemental contents are expressed as weight percent.

[0046] 10 to 500 seconds to ensure adequate recrystallization and transformation and to obtain a minimum of 50% austenite in the microstructure. The steel is then cooled to Tc at a rate greater than 20°C / s. max ~Tc min These are cooled to temperatures in the range between Tc max and Tc min is the following: Tc max =565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb Tc min =565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb where the content of the elements is expressed in weight percent. The annealed cold rolled steel sheet is then brought to a temperature range of 350-550°C and held within that temperature range for 5-500 seconds to ensure the formation of an adequate amount of bainite and temper the martensite to impart the target mechanical properties to the steel of the present invention. The annealed cold rolled steel sheet is then cooled to room temperature at a cooling rate of at least 1°C / s to obtain the cold rolled heat treated steel sheet.

[0047] The cold rolled heat treated steel sheet may then be optionally coated by any of the known industrial methods such as electrogalvanizing, JVD, PVD, hot dip galvanizing (GI / GA), etc. Electrogalvanizing is merely exemplified for a proper understanding of the present invention. Electrogalvanizing does not change or modify the mechanical properties or microstructure of the claimed cold rolled heat treated steel sheet. Electrogalvanizing can be performed by conventional industrial methods, such as electroplating. EXAMPLES

[0048] The following tests, examples, figurative illustrations and tables presented herein are to be considered as non-limiting in nature and for illustrative purposes only, illustrating advantageous features of the present invention.

[0049] The steel plates made with different steel compositions are summarized in Table 1, each of which is manufactured according to the process parameters specified in Table 2. The following Table 3 summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0050] [Table 1]

[0051] Table 2 summarizes the annealing process parameters carried out on the steels of Table 1. Steel compositions I1 to I5 are useful for the manufacture of steel sheets according to the invention. The table also specifies reference steels designated in the table by R1 to R5. Table 2 also lists the annealing process parameters for the steels of Table 1. min and Tc max A table of these Tc min and Tc max For the steels of the invention and the reference steels, Tc max =565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb Tc min=565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb It is defined as follows:

[0052] Furthermore, before the annealing treatment was performed for the steels of the invention and the reference steels, the steels were heated to a temperature between 1000°C and 1280°C, then subjected to hot rolling with a finishing temperature above 850°C and coiled at a temperature below 600°C. The hot rolled coils were then processed as claimed and then cold rolled to a thickness reduction between 30 and 95%. These cold rolled steel sheets were subjected to a heat treatment, the heating rate of the second anneal was 6°C / s for all steels listed in Table 2, and the cooling rate after the soaking of the second anneal was 70°C / s for all steels listed in Table 2.

[0053] [Table 2]

[0054] Table 3 illustrates the results of tests carried out according to standards using various microscopes, such as a scanning electron microscope, to determine the microstructure of both the steel of the invention and the reference steel.

[0055] The results are presented here.

[0056] [Table 3]

[0057] Table 4 illustrates the mechanical properties of both the steels of the invention and the reference steels. Tensile tests are carried out according to the JIS Z2241 standard to measure the tensile strength, yield strength and total elongation.

[0058] The results of various mechanical tests carried out in accordance with the standard are summarized.

[0059] [Table 4]

Claims

1. 2. A cold rolled heat treated steel sheet comprising the following elements expressed in weight percent: 0.10%≦Carbon≦0.5% 1%≦Manganese≦3.4% 0.5%≦Silicon≦2.5% 0.03%≦Aluminum≦1.5% 0%≦sulfur≦0.003% 0.002%≦Phosphorus≦0.02% 0%≦Nitrogen≦0.01% and any of the following elements: 0.05%≦Chromium≦1% 0.001%≦Molybdenum≦0.5% 0.001%≦Niobium≦0.1% 0.001%≦Titanium≦0.1% 0.01%≦Copper≦2% 0.01%≦Nickel≦3% 0.0001%≦Calcium≦0.005% 0%≦Vanadium≦0.1% 0%≦Boron≦0.003% 0%≦Cerium≦0.1% 0%≦Magnesium≦0.010% 0%≦Zirconium≦0.010% a residual composition consisting of iron and unavoidable impurities produced by processing, and a microstructure of the steel sheet containing, in area fractions, 10 to 30% retained austenite, 10 to 40% bainite, 5 to 50% annealed martensite, 1 to 20% hardened martensite, and less than 30% tempered martensite, and a cumulative amount of bainite and retained austenite is 25% or more.

2. 2. The cold rolled heat treated steel of claim 1, wherein said composition comprises 1% to 2% silicon.

3. 3. Cold rolled heat treated steel according to claim 1 or 2, wherein the composition comprises 0.03% to 1.0% aluminium.

4. 4. The cold rolled heat treated steel of claim 3, wherein the composition comprises 0.03% to 0.6% aluminum.

5. 5. Cold rolled heat treated steel according to any one of claims 1 to 4, wherein the composition comprises from 1.2% to 2.3% manganese.

6. A cold rolled heat treated steel according to any one of the preceding claims, wherein the composition comprises between 0.03% and 0.5% chromium.

7. The cold rolled heat treated steel according to any one of claims 1 to 6, wherein the cumulative amount of tempered martensite, quenched martensite and annealed martensite is 20% or more, and the proportion of annealed martensite is more than 10%.

8. 8. Cold rolled heat treated steel according to any one of the preceding claims, wherein the carbon content of the retained austenite is between 0.9 and 1.1%.

9. 9. The cold rolled heat treated steel according to any one of claims 1 to 8, wherein the steel sheet has an ultimate tensile strength of at least 950 MPa and a total elongation of at least 15%.

10. 10. The cold rolled heat treated steel of claim 9, wherein the steel sheet has an ultimate tensile strength of 1000 MPa or more and a ratio of yield strength to ultimate tensile strength of 0.5 or more.

11. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 10, which is free of ferrite.

12. A method for producing a cold rolled heat treated 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 1200°C and 1280°C, - rolling said semi-finished product in the austenitic range with a hot rolling finishing temperature above Ac3 to obtain a hot rolled steel sheet; - cooling the steel sheet at a cooling rate of more than 30°C / s to a coiling temperature of less than 600°C and coiling the hot-rolled sheet; - cooling the hot rolled sheet to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling process, - optionally carrying out annealing on the hot-rolled steel sheet at a temperature between 400°C and 750°C, - optionally subjecting the hot rolled steel sheet to a descaling process, - cold rolling the hot rolled steel sheet with a reduction ratio of between 35% and 90% to obtain a cold rolled steel sheet; - then performing a first annealing by heating the cold rolled steel sheet at a rate of more than 3°C / s to a soaking temperature between Ac3 and Ac3+100°C and holding it for 10 to 500 seconds; - then cooling the steel plate at a rate of more than 20°C / s to a temperature of less than 500°C; - optionally carrying out a tempering step of said annealed steel sheet between 120°C and 250°C, - Next, the annealed cold rolled steel sheet is heated at a rate of more than 3 ° C. / sec. ｓｏａｋｉｎｇ performing a second anneal by heating to a soaking temperature between Ac1 and Ac2 and holding it for 10 to 500 seconds; - The steel sheet is then heated to Tc at a rate of more than 20 ° C / sec. ｍａｘ ~Tc ｍｉｎ and cooling to a temperature range between ・Tc ｍａｘ =565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb ・Tc ｍｉｎ =565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb where C, Mn, Si, Cr, Al and Nb are the weight percentages of the elements in the steel; - then bringing said annealed cold rolled steel sheet to a temperature range between 350°C and 550°C for 5-500 seconds and cooling said annealed cold rolled steel sheet to room temperature at a cooling rate of at least 1°C / s to obtain a cold rolled heat treated steel sheet. A manufacturing method comprising:

13. 13. The method of claim 12, wherein the coiling temperature is less than 570°C.

14. The method according to claim 12 or 13, wherein the finish rolling temperature is between Ac3 and Ac3+100°C.

15. A method according to any one of claims 12 to 14, wherein the cooling rate after the first anneal is greater than 30°C / s to a temperature below 500°C.

16. The annealed cold rolled steel sheet is T ｓｏａｋｉｎｇ The cold rolled heat treated steel sheet according to any one of claims 12 to 15, wherein the steel sheet is continuously annealed for 10 seconds to 500 seconds at an annealing temperature between Ac3m ti,e and Ac3m ti,e such that the ratio of austenite to annealed martensite is between 50:50 and 90:

10. The method for producing the cold rolled heat treated steel sheet according to any one of claims 12 to 15.

17. Use of a steel sheet according to any one of claims 1 to 11 or produced according to the method according to claims 12 to 16 for the manufacture of structural or safety parts of a vehicle.

18. Part obtained according to claim 18 by flexible rolling of said steel sheet.

19. A vehicle comprising a part obtained according to any one of claims 12 to 19.

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