Cold rolled and heat treated steel sheet and method of manufacturing thereof

Abstract

To provide a cold rolled and heat treated steel sheet having a maximum tensile strength of 960 MPa or more and a total elongation of 20% or more.SOLUTION: The cold rolled and heat treated steel sheet of the present invention contains, in mass%, C: 0.1 to 0.5%, Mn: 1 to 3.4%, Si: 0.5 to 2.5%, Al: 0.01 to 1.5%, Cr: 0.05 to 1%, Nb: 0.001 to 0.1%, S: ≤0.003%, P: 0.002 to 0.02%, and N: ≤0.01%, further contains one or more of Mo, Ti, Cu, Ni, Ca, V, B, Ce, Mg, and Zr, with the balance being iron and unavoidable impurities. The microstructure of the rolled steel sheet comprises, by area fraction, Bainite: 10 to 60%, Ferrite: 5 to 50%, Residual Austenite: 5 to 25%, Martensite: 2 to 20%, and Tempered Martensite: ≤25%, the balance being Annealed Martensite from 1% to 45%.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 satisfy two conflicting requirements: ease of formability and strength. Recently, however, environmental concerns have placed a third demand on automobiles: improved fuel economy. Therefore, automotive parts must now be manufactured from materials with high formability to meet standards for ease of fit in complex automotive assemblies, while simultaneously improving strength for vehicle crashworthiness and durability while reducing vehicle weight for improved fuel efficiency.

[0003] Therefore, vigorous research and development is being conducted to increase the strength of materials and reduce the amount of material used in automobiles.However, since increasing the strength of steel sheets reduces their formability, it is necessary to develop materials that combine high strength and high formability.

[0004] Early research and development in the field of high strength and highly formable steel plates has resulted in several methods for producing high strength and highly formable steel plates, some of which are listed here for final evaluation of the present invention.

[0005] EP3144406, which claims a high-strength cold-rolled steel sheet with excellent ductility, contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 0.1% to 2.0%, aluminum (Al): 0.005% to 1.5%, manganese (Mn): 1.5% to 3.0%, phosphorus (P): 0.04% or less (excluding 0%), sulfur (S): 0.015% or less (excluding 0%), nitrogen (N): 0.02% or less (excluding 0%). The steel contains residual silicon (Si), iron (Fe) and unavoidable impurities, and the sum of silicon and aluminum (Si+Al) (by weight) is 1.0% or more. The microstructure contains, by area fraction, 5% or more of polygonal ferrite with a minor axis to major axis ratio of 0.4 or more, 70% or less (excluding 0%) of acicular ferrite with a minor axis to major axis ratio of 0.4 or less, 25% or less (excluding 0%) of acicular retained austenite, and residual martensite. Furthermore, EP3144406 envisions high-strength steel with a tensile strength of 780 MPa or more.

[0006] EP3009527 provides a high-strength cold-rolled steel sheet with excellent elongation, excellent stretch-flangeability, and a high yield ratio, and a manufacturing method thereof. This high-strength cold-rolled steel sheet has a certain composition and microstructure. The composition contains, by mass, 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 has an average grain size of 5 μm or less and a volume fraction of 3% to 20% ferrite, a volume fraction of 5% to 20% retained austenite, and a volume fraction of 5% to 20% martensite, with the remainder 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 2000 μm across the thickness section parallel to the rolling direction of the steel sheet. 2 The steel plate of EP3009527 can reach a strength of 960 MPA or more, but cannot achieve an elongation of 20% or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent Application Publication No. 3144406 [Patent Document 2] European Patent Application Publication No. 3009527 Summary of the Invention

[0008] The object of the present invention is to solve these problems by making available a cold-rolled, heat-treated steel sheet which simultaneously: - an ultimate tensile strength of at least 960 MPa, preferably greater than 980 MPa; - Total elongation of at least 20%, preferably more than 21%

[0009] In a preferred embodiment, the steel sheet according to the invention has a yield strength of 475 MPa or more.

[0010] In a preferred embodiment, the steel sheet according to the invention has a yield strength / tensile strength ratio of 0.45 or more.

[0011] Preferably, such steels also have good weldability and coatability, as well as good suitability for forming, especially rolling.

[0012] Another object of the present invention is to make available a method for manufacturing these plates that is stable towards shifts in manufacturing parameters, while still being compatible with conventional industrial applications.

[0013] The cold rolled 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. DETAILED DESCRIPTION OF THE INVENTION

[0014] Carbon is present in the steel at 0.1% to 0.5%. Carbon is an essential element for increasing the strength of the steel of the present invention by forming low-temperature transformation phases such as martensite. Furthermore, carbon also plays a crucial role in austenite stabilization, thus ensuring retained austenite. Thus, carbon plays two important roles: increasing strength and retaining austenite to provide ductility. However, a carbon content of less than 0.1% does not stabilize the sufficient amount of austenite required for the steel of the present invention. On the other hand, a carbon content greater than 0.5% results in poor spot weldability, limiting its application to automotive parts. The preferred carbon limit is 0.15% to 0.45%, with a more preferred limit being 0.15% to 0.3%.

[0015] The manganese content of the steel of the present invention is 1% to 3.4%. This element is gamma-generating. The purpose of adding manganese is to obtain a structure that is essentially austenite-containing. Manganese stabilizes austenite at room temperature, resulting in retained austenite. A manganese content of at least about 1% by weight is essential for providing strength and hardenability to the steel of the present invention, as well as for stabilizing austenite. Therefore, according to the presented invention, a higher manganese content, such as 3%, is preferred. However, manganese contents higher than 3.4% have adverse effects, such as delaying the austenite-to-bainite transformation during the isothermal hold for bainite transformation. Furthermore, manganese contents greater than 3.4% not only fail to achieve the ductility target, but also deteriorate the weldability of the steel. The preferred range of manganese is 1.2% and 2.8%, with a more preferred range being between 1.3% and 2.4%.

[0016] The silicon content of the steel of the present invention is 0.5% to 2.5%. Silicon is an element that can delay the precipitation of carbides during overaging, and therefore, the presence of silicon stabilizes carbon-rich austenite at room temperature. Furthermore, due to the low solubility of silicon in carbides, it effectively inhibits or delays the formation of carbides, thus promoting the formation of low-density carbides in the bainite structure, which is required by the present invention to impart the steel with its basic mechanical properties. However, an unbalanced silicon content does not produce the above effect and leads to problems such as temper embrittlement. Therefore, its concentration is limited to an upper limit of 2.5%. The preferred silicon limit is 0.8% to 2%, and the more preferred limit is 1.3% to 1.9%.

[0017] The aluminum content is 0.01% to 1.5%. In the present invention, aluminum removes oxygen present in molten steel, preventing it from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing the grain size. An aluminum content higher than 1.5% raises the Ac3 point to a high temperature, reducing productivity. The preferred limit of aluminum is 0.01% to 1%, and the more preferred limit is 0.01% to 0.5%.

[0018] The chromium content of the steel of the present invention is 0.05% to 1%. Chromium is an essential element that provides strength and hardness to the steel, but if used in excess of 1%, it will impair the surface finish of the steel. Chromium contents below 1% coarsen the carbide distribution pattern in the bainite structure, thereby keeping the carbide density in the bainite low. The preferred limit for chromium is 0.1% to 0.8%, with a more preferred limit being 0.2% to 0.6%.

[0019] Niobium is present in the steel of the present invention at 0.001% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention through precipitation hardening. Niobium also influences the size of microstructural components through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, the resulting finer microstructure formed at the end of the holding temperature and after full annealing leads to hardening of the product. However, a saturation effect of niobium content above 0.1% is observed, which means that additional amounts of niobium do not result in any strength improvement in the product, making it economically unattractive. The preferred limit for niobium is 0.001% to 0.09%, and the more preferred limit is 0.001% to 0.07%.

[0020] Although sulfur is not an essential element, it may be contained in steel as an impurity, and from the viewpoint of the present invention, the sulfur content is preferably as low as possible, but from the viewpoint of production costs, it is 0.003% or less. Furthermore, if higher sulfur is present in the steel, it will reduce its beneficial effect on the present invention, especially since it combines with manganese to form sulfides.

[0021] The phosphorus constituent of the steel of the present invention is between 0.002% and 0.02%, and phosphorus has a tendency to segregate, particularly at grain boundaries and to co-segregate with manganese, thereby reducing spot weldability and hot ductility. For these reasons, its content is limited to 0.02%, preferably less than 0.013%.

[0022] Nitrogen is limited to 0.01% to avoid material aging and minimize the precipitation of aluminum nitride during solidification, which adversely affects the mechanical properties of the steel. Molybdenum is an optional element that constitutes 0% to 0.5% of the steel of the present invention. Molybdenum plays an effective role in improving hardenability and hardness, delaying the appearance of bainite, and preventing the precipitation of carbides in bainite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%.

[0023] Titanium, like niobium, is an optional element that can be added to the steel of the present invention at 0.001% to 0.1%. It plays a role in hardening by participating in carbonitrides. However, titanium also forms titanium nitrides that appear during solidification of the cast product. Therefore, the amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides, which have a negative effect on formability. Titanium contents below 0.001% have no effect on the steel of the present invention. The preferred limit for titanium is 0.001% to 0.09%, and the more preferred limit is 0.001% to 0.07%.

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

[0025] Nickel, an optional element, can be added in amounts of 0.01 to 3% to increase the strength of steel and improve its toughness. A minimum of 0.01% is required to achieve this effect. However, above 3%, nickel causes a deterioration in ductility.

[0026] Calcium is an optional element that can be added to the steel of the present invention in a range of 0.0001% to 0.005%. Calcium is added as an optional element to the steel of the present invention, especially during the encapsulation process. Calcium contributes to the refinement of the steel by trapping harmful sulfur particles in a spherical form and reducing the harmful effects of sulfur.

[0027] Vanadium is an optional element that can be added because it is effective in increasing the strength of steel by forming carbides or carbonitrides, and from an economical standpoint, the upper limit is 0.1%.

[0028] Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions: cerium≦0.1%, boron≦0.003%, magnesium≦0.010% and zirconium≦0.010%. These elements make it possible to refine the grains during solidification up to the maximum content levels indicated. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of the steel sheet according to the present invention is composed of the presence of, in area fractions, 10% to 50% bainite, 5% to 50% ferrite, 5% to 25% retained austenite, 2% to 20% martensite, 0% to 25% tempered martensite, and 1% to 45% annealed martensite.

[0030] The surface fraction of the phases in the microstructure is determined in the following way: a specimen is cut from the steel sheet, polished and etched with reagents known per se to reveal the microstructure, the cross section of which is then examined by a scanning electron microscope, for example a scanning electron microscope with a field emission gun ("FEG-SEM") at a magnification of more than 5000 times in secondary electron mode.

[0031] The ferrite fraction is determined by SEM observation after etching with Nital or Picral / Nital reagents.

[0032] The determination of the retained austenite was carried out by XRD and for the tempered martensite, dilatometry studies were carried out according to the publication of S.M.C. Van Bohemen and J. Sietsma, Metallurgical and materials transactions, Vol. 40A, May 2009 - 1059.

[0033] Bainite constitutes between 10% and 60% of the microstructure by area fraction for the steel of the present invention. To ensure a total elongation of 20%, 10% bainite is essential. The presence of bainite is preferably between 12% and 55%, more preferably between 13% and 52%.

[0034] Ferrite constitutes 5% to 50% of the microstructure by area fraction for the steel of the present invention. Ferrite imparts elongation to the steel of the present invention. The ferrite in the steel can include polygonal ferrite, lath ferrite, acicular ferrite, plate ferrite, or epitaxial ferrite. To ensure elongation of 20% or more, 5% ferrite is required. The ferrite of the present invention is formed during annealing and cooling after annealing. However, whenever the ferrite content in the steel of the present invention exceeds 50%, it is impossible to simultaneously achieve both yield strength and total elongation due to the fact that ferrite reduces both tensile strength and yield strength, and also increases the hardness gap with hard phases such as martensite and bainite, reducing local formability. The preferred limit for the presence of ferrite for the present invention is 6% to 49%.

[0035] The retained austenite constitutes 5% to 25% by area fraction of the steel. Retained austenite is known to have a higher solubility of carbon than bainite, and therefore acts as an effective carbon trap, delaying the formation of carbides in bainite. The carbon fraction within the retained austenite of the present invention is preferably higher than 0.9% and lower than 1.2%. The retained austenite of the steel according to the present invention confers increased ductility. The preferred limit of retained austenite is between 8% and 24%, more preferably between 12% and 20%.

[0036] Martensite constitutes 2% to 20% of the steel by area fraction. Martensite imparts tensile strength to the steel of the present invention. Martensite is formed during cooling after overaging. The preferred limit for martensite is 3% to 18%, more preferably 4% to 15%.

[0037] Tempered martensite constitutes 0% to 25% of the microstructure by area fraction. Martensite occurs when the steel reaches Tc min ~Tc maxThis can be formed when the steel is cooled between 0.1 and 1.5°C and then tempered during overaging. Tempered martensite imparts ductility and strength to the present invention. Tempered martensite in excess of 25% imparts excessive strength but reduces elongation beyond the allowable limit. The preferred limit for tempered martensite is 0% to 20%, more preferably 0% to 18%.

[0038] Annealed martensite constitutes 1% to 45% of the microstructure of the steel of the present invention by area fraction. Annealed martensite imparts strength and formability to the steel of the present invention. Annealed martensite is formed during the second annealing at temperatures between TS and Ac3. To achieve the target elongation with the steel of the present invention, it is necessary to have at least 1% of these microstructural components, but if the amount exceeds 45%, the steel of the present invention will not be able to achieve both strength and elongation simultaneously. The preferred limit for their presence is 2% to 40%, more preferably 2% to 35%.

[0039] In addition to the above microstructure, the microstructure of the cold rolled and heat treated steel sheet does not contain microstructural constituents such as pearlite without impairing the mechanical properties of the steel sheet.

[0040] 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., thicknesses ranging from about 220 mm for slabs to a maximum of several tens of mm for thin strip).

[0041] For example, slabs having the above-mentioned chemical compositions are produced by continuous casting, where the slabs are optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and maintain a local carbon to nominal carbon ratio of less than 1.10. The slabs provided by the continuous casting process may be used directly at high temperature after continuous casting, or may be first cooled to room temperature and then reheated for hot rolling. The reheating temperature is between 1100 and 1280°C.

[0042] The temperature of the slab used for hot rolling is preferably at least 1200°C and should be less than 1280°C. If the slab temperature is lower than 1200°C, excessive loads will be placed on the rolling mill, and the temperature of the steel may drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled with transformed ferrite in the structure. Therefore, the slab temperature should also be high enough so that hot rolling can be completed in the temperature range of Ac3 to Ac3 + 200°C and the final rolling temperature remains above Ac3. Reheating at temperatures above 1280°C is industrially expensive and should be avoided.

[0043] A final rolling temperature range between Ac3 and Ac3+200°C is preferred in order to obtain a structure favorable for recrystallization and rolling. It is necessary to carry out the final rolling pass at a temperature higher than Ac3, since below this temperature the steel sheet shows a significant decrease in rollability. The sheet obtained in this way is cooled at an average cooling rate of more than 30°C / s to a coiling temperature which must be below 600°C. Preferably, the cooling rate is not more than 200°C / s and the coiling temperature is preferably below 570°C.

[0044] The hot rolled steel sheet is coiled at a coiling temperature of less than 600°C to avoid ovalization of the hot rolled steel sheet, and preferably less than 570°C to avoid scale formation. The preferred range of the coiling temperature is between 350°C and 570°C. The coiled hot rolled steel sheet is cooled to room temperature before being subjected to the optional hot band annealing.

[0045] The hot-rolled steel sheet may be subjected to an optional descaling process to remove scale formed during hot rolling. The hot-rolled sheet may then be subjected to an optional hot band annealing process at a temperature between 400 and 750°C for at least 12 hours and up to 96 hours, but the temperature is maintained below 750°C to avoid partially transforming the hot-rolled microstructure and thus losing microstructural homogeneity. An optional descaling step may then be performed to remove the scale, for example, by pickling the steel sheet. This hot-rolled steel sheet is then cold-rolled to a thickness reduction of between 35 and 90%. The cold-rolled steel sheet resulting from the cold-rolling process then undergoes two annealing cycles to impart the microstructure and mechanical properties to the steel of the present invention.

[0046] In the first annealing of the cold-rolled sheet, the cold-rolled sheet is heated to a soaking temperature between TS and Ac3 at a heating rate HR1 greater than 3°C / s, preferably greater than 5°C / s, where Ac3 and TS for the present steel are calculated using the following formula: TS=830-260 * C-25 * Mn+22 * Si+40 * Al Ac3=901-262 * C-29 * Mn+31 * Si-12 * Cr-155 * Nb+86 * Al The elemental contents are expressed as weight percentages.

[0047] The steel sheet is held at TS1 for 10 to 500 seconds to ensure sufficient recrystallization and transformation of at least 50% of the initial structure of severe work hardening to austenite. The sheet is then cooled to room temperature at a cooling rate CR1 greater than 25°C / s, preferably greater than 50°C / s. During this cooling, the cold-rolled steel sheet can optionally be held at a temperature range of 350 to 480°C, preferably 380 to 450°C, for a holding time of 10 to 500 seconds, after which the cold-rolled steel sheet is cooled to room temperature to obtain an annealed cold-rolled steel sheet.

[0048] Next, the cold-rolled annealed steel sheet is heated for second annealing at a heating rate HR2 greater than 3°C / sec to a second annealing soaking temperature TS2 between TS and Ac3. TS=830-260 * C-25 * Mn+22 * Si+40 * Al Ac3=901-262 * C-29 * Mn+31 * Si-12 * Cr-155 * Nb+86 * Al The elemental contents are expressed as weight percentages.

[0049] Between 10 and 500 seconds, sufficient recrystallization and transformation is ensured to obtain a minimum 50% austenite microstructure. The TS2 temperature is always below the TS1 temperature. The sheet is then cooled to Tc at a cooling rate CR2 greater than 20°C / s, preferably greater than 30°C / s, and more preferably greater than 50°C / s. max ~Tc min The temperature is cooled to a range Tstop between these Tc max and Tc min is defined as follows: 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 The elemental contents are expressed as weight percentages.

[0050] The cold-rolled annealed steel sheet is then brought to a TOA temperature range of 380°C to 580°C and held for 10 to 500 seconds to ensure the formation of an appropriate amount of bainite and temper the martensite to impart the steel of the present invention with the desired mechanical properties. The cold-rolled annealed steel sheet is then cooled to room temperature at a cooling rate of at least 1°C / second to form martensite, resulting in a cold-rolled heat-treated steel sheet. The TOA temperature range is preferably 380°C to 500°C, more preferably 380°C to 480°C.

[0051] The cold rolled heat treated steel sheet can then be optionally coated by any of the known industrial processes such as electrogalvanizing, JVD, PVD, hot dip galvanizing (GI / GA), etc. Electrogalvanizing does not change or modify either the mechanical properties or the microstructure of the claimed cold rolled heat treated steel sheet. Electrogalvanizing can be performed by any conventional industrial process, e.g., electroplating. [Example]

[0052] The following tests, examples, illustrative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0053] Steel plates made from steels of different compositions are listed and summarized in Table 1. Here, the steel plates are manufactured according to the process parameters specified in Table 2. The microstructures of the steel plates obtained during the trail are then summarized in Table 3, and the evaluation results of the obtained properties are summarized in Table 4.

[0054] Table 1 lists steels with compositions expressed in weight percentages. In addition to steel compositions I1 to I5 for the production of plates according to the invention, the table also specifies reference steel compositions designated in the table by R1 to R4. Table 1 also serves as a comparison table between the inventive steels and the reference steels. Table 1 also lists Ac3, which is defined for the steel samples by the following formula: Ac3=901-262 *C-29 * Mn+31 * Si-12 * Cr-155 * Nb+86 * Al

[0055] Table 1 is herein.

[0056] [Table 1]

[0057] Table 2 summarizes the annealing process parameters performed on the steels of Table 1. Steel compositions I1 to I7 are useful for the manufacture of plates according to the invention, and this table also specifies reference steels designated in the table by R1 to R5. Table 2 also lists the Tc min and Tc max These Tcmax and Tcmin are defined as follows for the invention steel and the reference steel: 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

[0058] Furthermore, before the inventive and reference steels were subjected to annealing treatment, all steels were cooled after hot rolling at an average cooling rate of 40°C / s. The hot rolled coils were then processed as claimed and subsequently cold rolled with a reduction of 30-95%. The final cooling rate was above 1°C / s.

[0059] These cold rolled steel sheets, both inventive and reference steels, were subjected to heat treatments as listed in Table 2 herein.

[0060] [Table 2] TIFF2025157446000003.tif255165

[0061] Table 3 illustrates the results of tests carried out according to different microscopic standards, such as scanning electron microscopy, to determine the microstructural composition of both the invention steel and the reference steel. The retained austenite is measured by magnetic saturation measurements according to the publication entitled "Structure and Properties of Thermal-Mechanically Treated 304 Stainless Steel in Metallurgical Transactions," Vol. 1, June 1970. Ferrite, bainite, tempered martensite, and martensite are observed by image analysis carried out using Aphelion software and interrupted expansion rate tests.

[0062] The results are defined herein.

[0063] [Table 3]

[0064] Table 4 illustrates the mechanical properties of both the inventive and reference steels. To determine the tensile strength, yield strength and total elongation, tensile tests are performed according to the JIS Z2241 standard, published in the 11th edition dated October 20, 2020, entitled METALLIC MATERIALS - TENSILE TESTING - METHOD OF TEST AT ROOM TEMPERATURE.

[0065] The results of various mechanical tests carried out in accordance with the standards are shown in the table below.

[0066]

Table 4

Claims

1. 1. A cold rolled heat treated steel sheet containing the following elements expressed in weight percentage: 0.1%≦Carbon≦0.5% 1%≦Manganese≦3.4% 0.5%≦Silicon≦2.5% 0.01%≦Aluminum≦1.5% 0.05%≦chromium≦1% 0.001%≦niobium≦0.1% 0%≦sulfur≦0.003% 0.002%≦phosphorus≦0.02% 0%≦Nitrogen≦0.01% and any of the following elements: 0%≦molybdenum≦0.5% ０． 0.01%≦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% and a remainder of the composition consisting of iron and unavoidable impurities, and the microstructure of the rolled steel sheet contains, in area fractions, 10% to 60% bainite, 5% to 50% ferrite, 5% to 25% retained austenite, 2% to 20% martensite, and 0% to 25% tempered martensite, with the remainder being annealed martensite, the content of which is 1% to 45%.

2. 2. The cold rolled heat treated steel sheet according to claim 1, wherein the composition comprises 0.8%≦silicon≦2%.

3. The cold-rolled heat-treated steel sheet according to claim 1 or 2, wherein the composition contains 1.2%≦manganese≦2.8%.

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

5. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 4, wherein the composition includes 0.001%≦niobium≦0.09%.

6. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 5, wherein the composition includes 0.1%≦chromium≦0.8%.

7. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 6, wherein the annealed martensite is 2% to 40%.

8. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 7, wherein the microstructure contains 12 to 55% of bainite.

9. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 8, wherein the microstructure contains 8 to 24% retained austenite.

10. The cold rolled heat treated steel sheet according to any one of claims 1 to 10, having a tensile strength of greater than 960 MPa and a total elongation of 20% or greater.

11. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 11, having a yield strength of more than 475 MPa.

12. A method for producing a cold-rolled heat-treated steel sheet according to any one of claims 1 to 11, comprising the following steps: - providing a steel composition according to any one of claims 1 to 6, - reheating the semi-finished product to a temperature between 1100°C and 1280°C, - rolling the semi-finished product in the austenitic range, with a hot rolling finishing temperature above Ac3, to obtain a hot rolled steel sheet; - cooling the sheet at an average 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 step, - optionally annealing the hot-rolled steel sheet at a temperature between 400°C and 750°C; - optionally subjecting the hot rolled steel sheet to a descaling step, - cold rolling the hot-rolled steel sheet at a reduction ratio of 35 to 90% to obtain a cold-rolled steel sheet; - then performing a first annealing by heating the cold-rolled steel sheet at a rate HR1 greater than 3°C / s to a soaking temperature TS1 between TS and Ac3 and holding for 10 seconds to 500 seconds, where TS is defined as follows: TS=830-260 ＊ C-25 ＊ Mn+22 ＊ Si+40 ＊ Al - then cooling the sheet to room temperature at a rate greater than 25°C / s, optionally holding the cold rolled steel sheet in the temperature range between 350 and 480°C during cooling for a time between 10 and 500 seconds to obtain a cold rolled annealed steel sheet; - then, the cold-rolled annealed steel sheet is heated at a rate HR2 greater than 3°C / s to a soaking temperature TS2 between TS and Ac3, and held there for 10 seconds to 500 seconds to perform a second annealing; - The plate is then heated to Tc at a rate CR2 greater than 20°C / s ｍａｘ ~Tc ｍｉｎ and cooling to a temperature range Tstop between Tc ｍａｘ and Tc ｍｉｎ is defined as follows: 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 percent of the elements in the steel. - then bringing the cold rolled annealed steel sheet to a temperature range TOA between 380 and 580°C, holding at TOA for 5 seconds to 500 seconds, and cooling the annealed cold rolled steel sheet to room temperature at a cooling rate higher than 1°C / s to obtain a cold rolled heat treated steel sheet. A manufacturing method comprising:

13. The method for producing a cold rolled heat treated steel sheet according to claim 12, wherein the coiling temperature of the hot rolled steel sheet is less than 570°C.

14. The method for producing a cold-rolled heat-treated steel sheet according to claim 12 or 13, wherein the TS2 temperature is TS1 or lower.

15. Use of a steel sheet according to any one of claims 1 to 11 or produced by the method according to any one of claims 12 to 14 for the production of structural or safety parts of a motor vehicle.

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