BR112021003583A2 - cold rolled steel sheet, production method, use and vehicle - Google Patents

Abstract

COLD LAMINATED STEEL SHEET, METHOD OF PRODUCTION, USE AND VEHICLE. A cold-rolled and heat-treated steel sheet with a composition comprising the following elements 0.13% = carbon = 0.18%, 1.1% = manganese = 1.8%, 0.5% = silicon = 0 .9%, 0.6% = aluminum = 1%, 0.002% = phosphorus = 0.02%, 0% = sulfur = 0.003%, 0% = nitrogen = 0.007% and may contain one or more of the following optional 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%, the remaining composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 60 to 75% ferrite, 20 to 30% bainite, 10 to 15% austenite residual and 0% to 5% martensite, where the accumulated amounts of residual austenite and ferrite are between 70% and 80%.

Description

"COLD LAMINATED STEEL SHEET, METHOD OF PRODUCTION, USE AND VEHICLE"

[001] The present invention relates to cold rolled and coated steel sheets suitable for use as steel sheet for automobiles.

[002] Automotive parts are needed to satisfy two inconsistent needs, namely, ease of conformation and strength, but in recent years a third requirement for improving fuel consumption is also applied to automobiles in view of global environmental concerns. Thus, now automotive parts must be made of material with high formability, in order to meet the criteria of ease of adjustment in complex car assembly and, at the same time, improve the strength for collision protection and the vehicle's durability, reducing vehicle weight to improve fuel efficiency.

[003] Therefore, intense research and development efforts are made to reduce the amount of material used in the car, increasing the strength of the material. On the other hand, an increase in the strength of steel sheets decreases formability and, therefore, it is necessary to develop materials with high strength and high formability.

[004] Previous research and developments in the field of high strength and high formability steel sheets have resulted in various methods for producing high strength and high formability steel sheets, some of which are enumerated here for conclusive consideration of the present invention.

[005] Document US 20140234657 is a patent application claiming a hot-dip galvanized steel sheet having a microstructure, in volume fraction, equal to or greater than 20% and equal to or less than 99% in the total of one or two of martensite and bainite, a residual structure contains one or two of ferrite, residual austenite less than 8% by volume fraction and perlite equal to or less than 10% by volume fraction. Furthermore, US 20140234657 achieves a tensile strength of 980 MPa, but is unable to achieve 25% elongation.

[006] The document US 8657969 claims a high strength galvanized steel sheet with a tensile strength of 590 MPa or more and excellent processability. The composition of the component contains, in % by mass, C: 0.05% to 0.3%, Si: 0.7% to 2.7%, Mn: 0.5% to 2.8%, P: 0 .1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.008% or less, and the rest: Fe or unavoidable impurities. The microstructure contains, in terms of area ratio, ferrite phases: 30% to 90%, bainite phases: 3% to 30% and martensite phases: 5% to 40%, in which, between the martensite phases, martensite phases having an aspect ratio of 3 or more are present in a proportion of 30% or more.

[007] The purpose of the present invention is to solve these problems, providing cold-rolled steel and coated sheets that, simultaneously, have: - a maximum tensile strength greater than or equal to 600 MPa and preferably above 620 MPa; - a total elongation greater than or equal to 31% and preferably above 33%.

[008] In a preferred embodiment, the steel sheets according to the invention can also have a yield point of 320 MPa or more.

[009] In a preferred embodiment, the steel sheets according to the invention may also have a yield strength to tensile strength ratio of 0.6 or more.

[010] Preferably, this steel can also have a good suitability for forming, in particular for rolling with good weldability and coating capacity (coatability).

[011] Another objective of the present invention is also to provide a method for the manufacture of these sheets that is compatible with conventional industrial applications, being robust in the sense of changes in manufacturing parameters.

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

[013] Carbon is present in steel between 0.13% and 0.18%. Carbon is a necessary element to increase the strength of the steel sheet, producing low-temperature transformation phases, such as bainite; in addition, carbon also plays a key role in stabilizing austenite, therefore, a necessary element to guarantee residual austenite. Therefore, carbon plays two key roles, one in increasing strength and the other in retaining austenite to impart ductility. However, a carbon content of less than 0.13% will not be able to stabilize the austenite in an adequate amount required by the steel of the present invention. On the other hand, with a carbon content greater than 0.18%, steel has low spot weldability, which limits its application in automotive parts.

[014] The manganese content of the steel of the present invention is between 1.1% and 1.8%. This element is gammageneous. The purpose of adding manganese is essentially to obtain a structure that contains austenite and gives strength to the steel. An amount of at least 1.1% by weight of manganese has been found to provide the strength and hardenability of the steel sheet, as well as to stabilize the austenite. However, when the manganese content is greater than 1.8%, it has adverse effects, such as delaying the transformation of austenite to bainite during the retention of overaging (overaging) for the transformation of bainite. Furthermore, manganese content above 1.8% also reduces ductility and also deteriorates the weldability of the steel present, so elongation targets may not be achieved. A preferred content for the present invention can be maintained between 1.2% and 1.8%, even more preferably 1.3% and 1.7%.

[015] The silicon content of the steel of the present invention is between 0.5% and 0.9%. Silicon is a constituent that can retard the precipitation of carbides during superaging, therefore, due to the presence of silicon, the carbon-rich austenite is stabilized at room temperature.

Furthermore, due to the poor solubility of silicon in carbide, it effectively inhibits or retards the formation of carbides, therefore, it also promotes the formation of the bainitic structure which, according to the present invention, is sought to give the steel its essential characteristics. However, the disproportionate silicon content does not produce the mentioned effect and leads to a problem such as temper embrittlement. Therefore, the concentration is controlled within an upper limit of 0.9%. A preferable content for the present invention can be kept between 0.6% and 0.8%

[016] Aluminum is an essential element and is present in steel between 0.6% and 1%. Aluminum is an alpha-gen element and gives total elongation to the steel of the present invention. A minimum of 0.6% aluminum is required to have a minimum of ferrite, thus imparting elongation to the steel of the present invention. Aluminum is also used to remove oxygen from the molten state of steel to clean the steel of the present invention and also prevents oxygen from forming a gaseous phase. However, whenever aluminum is greater than 1%, it forms AlN, which is detrimental to the steel of the present invention, therefore, the preferable range for the presence of aluminum is between 0.6% and 0.8%.

[017] The phosphorus constituent of the steel of the present invention is between 0.002% and 0.02%. Phosphorus reduces spot weldability and hot ductility, mainly due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02% and preferably less than 0.014%.

[018] Sulfur is not an essential element, but may be contained as an impurity in steel and, from the point of view of the present invention, the sulfur content is preferably as low as possible, but is 0.003% or less of the point. view of the cost of manufacturing. Furthermore, if superior sulfur is present in steel, it combines to form sulfides, especially with manganese, and reduces its beneficial impact on the steel of the present invention.

[019] Nitrogen is limited to 0.007% to prevent material aging and minimize the precipitation of nitrides during solidification, which are detrimental to the mechanical properties of steel.

[020] Chromium is an optional element for the present invention.

The chromium content that can be present in the steel of the present invention is between 0.05% and 1%. Chromium is an essential element that provides strength and hardening to steel, but when used above 1% it damages the surface finish of the steel. Additional chromium contents below 1% make the carbide dispersion pattern in bainitic structures coarser, therefore; keep the density of carbides low in bainite.

[021] Molybdenum is an optional element that constitutes 0.001% to 0.5% of the steel of the present invention; molybdenum plays an effective role in determining hardness and hardness, delays the appearance of bainite and prevents the precipitation of carbides into bainite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so that for economic reasons its content is limited to 0.5%.

[022] Niobium is an optional element for the present invention.

The niobium content may be present in the steel of the present invention between 0.001 and 0.1% and is added to the steel of the present invention to form carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium will also affect the size of microstructural components through its precipitation as carbonitrides and by delaying recrystallization during the heating process. Thus, the finer microstructure formed at the end of the holding temperature and, as a consequence, after completion of annealing which will lead to hardening of the steel of the present invention. However, the niobium content above 0.1% is not economically interesting, as a saturation effect of its influence is observed, which means that the additional amount of niobium does not result in any improvement in the strength of the product.

[023] Titanium is an optional element and can be added to the steel of the present invention between 0.001% and 0.1%. Like niobium, it is involved in the formation of carbonitrides and therefore plays a role in hardening the steel of the present invention. Additionally, titanium also forms titanium nitrides that appear during solidification of the molten product. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides, which are detrimental to formability. In case the titanium content is less than 0.001%, it does not give any effect to the steel of the present invention.

[024] Copper can be added as an optional element in an amount of 0.01% to 2% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% copper is required to achieve this effect. However, when its content is above 2%, it can degrade the surface aspects.

[025] Nickel can be added as an optional element in an amount of 0.01 to 3% to increase the strength of the steel and improve its toughness. A minimum of 0.01% is required to produce such effects. However, when its content is above 3%, nickel causes deterioration of ductility.

[026] The calcium content in the steel of the present invention is between 0.0001% 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 steel refining by retaining the harmful sulfur content in globular form, thus delaying the harmful effects of sulfur.

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

[028] The microstructure of the steel sheet comprises:

[029] Ferrite constitutes 60% to 75% of the microstructure per area fraction for the steel of the present invention. Ferrite constitutes the primary phase of steel as a matrix. In the present invention, ferrite comprises, cumulatively, polygonal ferrite and acicular ferrite. Ferrite imparts high strength, as well as elongation, to the steel of the present invention. To ensure an elongation of 31%, and preferably 33% or more, it is necessary to have 60% Ferrite. Ferrite is formed during cooling after annealing in the steel of the present invention. However, whenever the ferrite content is present above 75% in the steel of the present invention, strength is not achieved.

[030] Bainite constitutes 20% to 30% of the microstructure per area fraction for the steel of the present invention. In the present invention, bainite consists cumulatively of lath bainite and granular bainite. To ensure a tensile strength of 620 MPa and preferably 630 MPa or more, it is necessary to have 20% bainite. Bainite is formed during superaging retention.

[031] The residual austenite constitutes 10% to 15% per area fraction of the steel. Residual austenite is known to have greater carbon solubility than bainite and therefore acts as an effective carbon trap, delaying the formation of carbides in bainite. The percentage of carbon within the residual austenite of the present invention is preferably greater than 0.9% and preferably less than 1.1%. The residual austenite of the steel, according to the invention, imparts an increased ductility.

[032] Martensite is an optional constituent and may be present between 0% and 5% of the microstructure per area fraction and found in traces. Martensite, for the present invention, includes both fresh martensite and tempered martensite. The present invention forms martensite due to cooling after annealing and is tempered during overaging retention. Fresh martensite also forms during the cooling after coating of cold rolled steel sheet. Martensite imparts ductility and strength to the steel of the present invention when it is below 5%. When martensite is greater than 5%, it provides excess strength but decreases elongation beyond the acceptable limit. The preferable range for martensite is between 0% and 3%.

[033] A total amount of residual ferrite and austenite must always be between 70% and 80% to have a total elongation of 31% and a minimum of 70% is required to ensure a total elongation above 31%, having a resistance to 600MPa traction. Residual ferrite and austenite are soft phase compared to martensite and bainite, therefore, they impart elongation and ductility, but whenever the cumulative presence is greater than 80%, the strength falls beyond acceptable limits.

[034] In addition to the aforementioned microstructure, the microstructure of the cold-rolled and heat-treated steel sheet is free from microstructural components, such as perlite and cementite, without harming the mechanical properties of the steel sheets.

[035] A steel sheet, according to the invention, can be produced by any suitable method. A preferred method is to provide a semi-finished steel casting having a chemical composition in accordance with the invention. The casting can be made in ingots or continuously in the form of thin slabs or thin strips, that is, with a thickness varying from approximately 220 mm for slabs to several tens of millimeters for thin strips.

[036] For example, a board with the chemical composition described above is manufactured by continuous casting, in which the board optionally underwent gentle direct reduction during the continuous casting process to avoid central segregation and to ensure a local carbon to nominal carbon kept below 1.10. The board supplied 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.

[037] The temperature of the slab, which is subjected to hot rolling, is at least 1150 °C and must be less than 1280 °C. In case the slab temperature is below 1150 °C, excessive load is imposed on a laminator. Therefore, the temperature of the slab is preferably high enough so that the hot rolling can be completed in the temperature range of Ac1+50 °C to Ac1+250 °C and preferably between Ac1+50 °C and Ac1+200 °C, while always having the final lamination temperature remaining above Ac1+50 °C. Reheating at temperatures above 1280 °C should be avoided as they are industrially expensive.

[038] A final lamination temperature range between Ac1+50 °C to Ac1+250 °C is preferred to have a structure that is favorable for recrystallization and lamination. It is necessary that the final rolling cycle be performed at a temperature higher than Ac1+50 °C, as below this temperature the steel plate presents a significant drop in rolling capacity. The sheet thus obtained is then cooled at a cooling rate above 30 °C/s until the winding temperature which must be below 625 °C. Preferably, the cooling rate will be less than or equal to 200 °C/s.

[039] The hot rolled steel sheet is then wound at a winding temperature below 625°C to avoid ovalization and preferably below 600°C to avoid scale formation. The preferred range for such a winding temperature is between 350 °C and 600 °C. The rolled hot rolled steel sheet can be cooled to room temperature before being subjected to an optional hot band annealing.

[040] Hot rolled steel sheet can be subjected to an optional scale removal step to remove scale formed during hot rolling prior to optional hot band annealing. The hot rolled sheet can then be subjected to an optional hot band annealing at temperatures between 400 °C and 750 °C for at least 12 hours and no more than 96 hours, the temperature remaining below 750 °C to prevent transformation partial of the hot-rolled microstructure and, therefore, the loss of homogeneity of the microstructure. Thereafter, an optional scale removal step from this hot-rolled steel sheet can be carried out by, for example, stripping such sheet. This hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%. The cold rolled steel sheet obtained from the cold rolling process is then subjected to annealing to impart microstructure and mechanical properties to the steel of the present invention.

[041] In annealing, the cold rolled steel sheet is subjected to two heating steps to reach the immersion temperature between Ac1+30 °C and Ac3, where Ac1 and Ac3 for the present steel are calculated using the following formula : Ac1 = 723 - 10.7[Mn] - 16[Ni] + 29.1[Si] + 16.9[Cr] + 6.38[W] + 290[As] Ac3 = 910 - 203[C] ^(1/2) - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W] - 30[Mn] - 11[Cr] - 20[Cu] + 700[P] + 400[Al] + 120[As] + 400[Ti] where the contents of the elements are expressed in percentage by weight.

[042] In step one, the cold rolled steel sheet is heated at a heating rate between 10 °C/s and 40 °C/s at a temperature range between 550 °C and 650 °C. Thereafter, in the second subsequent heating step, the cold rolled steel sheet is heated at a heating rate between 1 °C/s and 5 °C/s to the annealing immersion temperature.

[043] Thereafter, the cold rolled steel sheet is preferably kept at the immersion temperature for 10 to 500 seconds to ensure at least 30% transformation into austenite microstructure of the initial heavily work-hardened structure. Then, the cold rolled steel sheet is then cooled in two cooling steps to a super-aging holding temperature. In cooling step one, the cold rolled steel sheet is cooled to a cooling rate of less than 5 °C/s and preferably less than 3 °C/s at a temperature range between 600 °C and 720 °C and preferably between 625°C and 720°C. During this cooling step, a ferrite matrix of the present invention is formed. Subsequently, in a second subsequent cooling step, the cold rolled steel sheet is cooled to a super aging temperature range between 250 °C and 470 °C at a cooling rate between 10 °C/s and 100 °C /s. Then the cold rolled steel sheet is held in the super aging temperature range for 5 to 500 seconds. The cold rolled steel sheet is then heated to a coating bath temperature range of 400 °C and 480 °C to facilitate the coating of the cold rolled steel sheet. Then the cold rolled steel sheet is coated by any of the known industrial processes such as electroplating, JVD, PVD, hot dip (GI) etc.

EXAMPLES

[044] The following tests, examples, figurative examples and tables that are presented in this document are of a non-restrictive nature and should be considered for illustrative purposes only and will exhibit the advantageous characteristics of the present invention.

[045] The steel sheets made of steels of different compositions are gathered in Table 1, where the steel sheets are produced according to the process parameters stipulated in Table 2, respectively. Next, Table 3 gathers the microstructures of the steel sheets obtained during the tests and Table 4 gathers the results of the evaluations of the properties obtained.

TABLE 1 Steel sample C Mn Si Al PSN Other elements present A 0.155 1.54 0.696 0.728 0.014 0.002 0.003 - B 0.157 1.54 0.690 0.721 0.014 0.002 0.003 - C 0.148 1.54 0.698 0 0.013 0.0027 0.0044 - D 0.114 1.62 0.293 0.031 0.027 0.0028 0.005 -Ni:0.025, Cr: 0.345 Underlined values: not according to the invention.

TABLE 2

[046] Table 2 gathers the parameters of the annealing process implemented in the steels in Table 1. The steel compositions A and B serve for the manufacture of plates according to the invention. This table also specifies the reference steels that are designated in the table as C and D. Table 2 also shows the tabulation of Ac1 and Ac3. These Ac1 and Ac3 are defined for the steels of the invention and the reference steels as follows: Ac1 = 723 - 10.7[Mn] - 16[Ni] + 29.1[Si] + 16.9[Cr] + 6.38[W] + 290[As] Ac3 = = 910 - 203[C]^(1/2) - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[ Mo] + 13.1[W] - 30[Mn] - 11[Cr] - 20[Cu] + 700[P] + 400[Al] + 120[As] + 400[Ti] in which the contents of the elements are expressed as a percentage by weight.

[047] All sheets were cooled to a cooling rate of 34 °C/sec after hot rolling and were finally brought to a temperature of 460 °C before coating. All plates have 65% cold rolling reduction.

[048] Table 2 is as follows:

TABLE 2

Step Step One Two

Warming Temperat Rate Tempera ura Rate Warming Temper Temp Amo Stop Temperatu rt Heating Tempera ture Finishing ture Testing Temperature Finishing Temperature Winding Slow immersion Reheat immersion heat immersion nto HR nto HR fast before the nt steel (°C) then (°C) (°C) (°C) before annealing (s) fast annealing (°C/s) (°C), nt (°C/s) I1 A 1200 850 500 12 600 1.6 770 179

I2 B 1200 870 520 19 600 3.1 800 110

I3 A 1200 850 500 12 600 1.9 800 179

R1 A 1200 850 500 12 600 1.9 800 293

R2 C 1200 850 500 12 600 1.9 800 179

R3 D 1200 920 585 9 600 1,2 770 238

Step one Step two

Temperatur Rate Temperatur Stop Cooling Stop Rate Temperature Time Slow Ac Cool Cooling test for Ac3 1 after cooling fast cooling aging retention (°C) (°C slow annealing (°C/ s) o fast o (°C) o (s) ) o (°C/s) (°C) (°C/s)

111 I1 0.6 700 30 410 410 129 727 7 111 I2 1.5 700 39 460 460 79 727 2 111 I3 0.9 700 31 400 400 129 727 2 111 R1 ----- ----- 34 460 460 129 727 7 R2 0.9 700 31 400 400 129 827 727

R3 0.5 700 27 350 350 172 835 720

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

TABLE 3

[049] Table 3 exemplifies the results of tests performed according to the standards in different microscopes, such as Scanning Electron Microscope to determine the microstructures of both the invention and reference steels.

[050] The results are stipulated here: Ferrite + Austenite Ferrite Bainite Austenite Residual martensite Steel sample (%) (%) residual (%) (%) (%) I1 67 22 11 0 78 I2 65 24 10 1 75 I3 63 27 10 0 73 R1 53 37 10 0 63 R2 62 32 5 1 67 R3 62 33 5 0 67 I = according to the invention; R = reference; underlined values: not according to the invention.

TABLE 4

[051] Table 4 exemplifies the mechanical properties of both the steel of the invention and the reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are performed in accordance with JIS Z2241 standards.

[052] The results of the various mechanical tests performed in accordance with the standards are collated.

TABLE 4 Tensile Strength Sample YS Total Elongation YS/TS Steel (MPa) (MPa) (%) I1 634 386 0.61 34.7 I2 672 401 0.60 33.2 I3 633 411 0.65 36.1 R1 677 389 0.57 27.7 R2 602 365 0.61 28.6 R3 622 343 0.55 22.5 I = according to the invention; R = reference; underlined values: not according to the invention.

Claims (20)

1. COLD LAMINATED STEEL SHEET, characterized by having a composition that comprises the following elements, expressed in percentage by weight: 0.13% ≤ carbon ≤ 0.18% 1.1% ≤ manganese ≤ 1.8% 0, 5% ≤ silicon ≤ 0.9% 0.6% ≤ aluminum ≤ 1% 0.002% ≤ phosphorus ≤ 0.02% 0% ≤ sulfur ≤ 0.003% 0% ≤ nitrogen ≤ 0.007% and may contain one or more of the following elements optional 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% the remaining composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 60 to 75% ferrite, 20 to 30% bainite, 10 to 15% austenite residual and 0% to 5% martensite, where the accumulated amounts of residual austenite and ferrite are between 70% and 80%. 2. COLD LAMINATED STEEL SHEET, according to claim 1, characterized in that the composition includes 0.6% to 0.8% of silicon. 3. COLD LAMINATED STEEL SHEET, according to any one of claims 1 to 2, characterized in that the composition includes 0.14% to 0.18% of carbon. 4. COLD LAMINATED STEEL SHEET, according to claim 3, characterized in that the composition includes 0.6% to 0.8% of aluminum. 5. COLD LAMINATED STEEL SHEET, according to any one of claims 1 to 4, characterized in that the composition includes 1.2% to 1.8% of manganese. 6. COLD LAMINATED STEEL SHEET, according to claim 5, characterized in that the composition includes 1.3% to 1.7% of manganese. 7. COLD ROLLED STEEL SHEET, according to any one of claims 1 to 6, characterized in that the accumulated amounts of ferrite and residual austenite are between 73% and 80% and the percentage of residual austenite is less than 13%. 8. COLD ROLLED STEEL SHEET, according to any one of claims 1 to 7, characterized in that the amount of martensite is between 0% and 3%. 9. COLD ROLLED STEEL SHEET, according to any one of claims 1 to 8, characterized in that the carbon content of the residual austenite is between 0.9 to 1.1%. 10. COLD ROLLED STEEL SHEET, according to any one of claims 1 to 9, characterized in that said steel sheet has a maximum tensile strength of 600 MPa or more, and a total elongation of 31% or more. 11. COLD ROLLED STEEL SHEET, according to claim 10, characterized in that said steel sheet has a yield point of 320 MPa or more and a total elongation of 33% or more. 12. COLD ROLLED STEEL SHEET, according to any one of claims 1 to 11, characterized in that said steel sheet is coated. 13. METHOD OF PRODUCTION of a cold rolled steel sheet, characterized in that it comprises the following successive steps: - providing a steel composition, as defined in any one of claims 1 to 6; - reheating said semi-finished product to a temperature between 1150 °C and 1280 °C; - rolling the said semi-finished product in the austenitic range where the hot rolling finishing temperature must be between Ac1+50 °C and Ac1+250 °C to obtain a hot rolled steel sheet; - cool the sheet at a cooling rate above 30 °C/s to a winding temperature that is less than 625 °C; and winding said hot rolled sheet; - cooling said hot-rolled sheet to room temperature; - optionally carrying out the scale removal process on said hot rolled steel sheet; - optionally, the annealing is carried out in hot rolled steel sheet at temperatures between 400 °C and 750 °C; - optionally carrying out the scale removal process on said hot rolled steel sheet; - cold rolling said hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet; - then carry out an annealing at an immersion temperature between Ac1+30 °C and Ac3 for a period between 10 and 500 seconds, heating the said cold-rolled steel sheet by a two-step heating in which: • in step one of heating, the cold-rolled steel sheet is heated at a heating rate between 10 °C/s and 40 °C/s for a temperature range between 550 °C and 650 °C; • then, in step two, the cold rolled steel sheet is heated at a heating rate between 1 °C/s and 5 °C/s from a temperature range between 550 °C and 650 °C to the temperature immersion annealing stage in which it is held, - then cooling the cold rolled steel sheet in a two-stage cooling in which: • in cooling step one, the cold rolled steel sheet is cooled at a rate of cooling less than 5 °C/s for a temperature range between 600 °C and 720 °C; • then, in step two, the sheet is cooled at a cooling rate between 10 °C/s and 100 °C/s from a temperature range between 600 °C and 720 °C to a super aging temperature ( overaging); - then, said cold-rolled steel sheet is super-aged in a temperature range between 250 °C and 470 °C for 5 to 500 seconds; and - then cooled to room temperature to obtain a cold rolled steel sheet. 14. METHOD, according to claim 13, characterized in that the winding temperature is less than 600 °C. 15. METHOD, according to any one of claims 13 to 14, characterized in that the finishing lamination temperature is between Ac1+50 °C and Ac1+200 °C. 16. METHOD, according to any one of claims 13 to 15, characterized in that the cooling rate after annealing is less than 3 °C/s in the temperature range between 625 °C and 720 °C. 17. METHOD OF PRODUCTION of a cold rolled steel sheet, according to any one of claims 13 to 16, characterized in that the cold rolled steel sheet is annealed between Ac1+30 °C and Ac3 and the annealing temperature is selected in order to guarantee the presence of at least 30% of austenite at the end of the immersion. 18. METHOD OF PRODUCTION of a cold-rolled steel sheet, according to any one of claims 13 to 17, characterized in that the cold-rolled steel sheet can be coated in a temperature range between 400 °C and 480 °C. 19. USE of a steel sheet as defined in any one of claims 1 to 12 or a steel sheet produced according to the method as defined in any one of claims 13 to 18, characterized in that it is for the manufacture structural or safety parts of a vehicle. 20. VEHICLE, characterized in that it comprises a part obtained as defined in claim 19.