UA126725C2 - Cold rolled and coated steel sheet and a method of manufacturing thereof - Google Patents

Abstract

A cold rolled and heat treated steel sheet having a composition comprising of 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 can 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 remainder 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 % Residual Austenite, and 0 % to 5 % Martensite, wherein the cumulated amounts of Residual Austenite and Ferrite is between 70 % and 80 %.

Description

The field of technology to which the invention belongs

The invention relates to cold-rolled and coated steel sheets, suitable for use as a steel sheet for cars.

Automotive parts must satisfy two incompatible requirements, namely, ease of forming and strength, but in recent years, a third requirement has also appeared - the reduction of fuel consumption by cars in connection with global environmental protection. That is, now automotive parts must be made of material that has excellent formability in order to meet the criteria of lightness with a complex car layout and at the same time have increased strength, resistance to shock loads and wear resistance of the car while reducing the weight of the car to improve fuel efficiency.

Therefore, intensive research and development is being carried out to reduce the amount of materials used in cars by increasing the strength of the materials. On the other hand, increasing the strength of steel sheets reduces their formability, and thus necessitates the development of materials that have high strength and excellent formability.

Technical level

Early research and development in the field of high strength and excellent formability steel sheets led to several methods of producing high strength and excellent formability steel sheets, some of which are listed in the description for the final evaluation of the invention.

US Patent 05 20140234657 protects a hot dip coated galvanized steel sheet having a microstructure (in volume fractions) equal to or greater than 20 95 and equal to or less than 99 95 of a sum of one or two martensite and bainite , the residual structure contains one or two components of ferrite, austenite less than 8 95 (volume fraction) and pearlite fraction equal to or less than 10 95 by volume. Additionally, in US Patent 20140234657, a tensile strength of 980 MPa was achieved, but an elongation of 25 95 was not achieved.

U.S. Patent 05,865,969 protects a high-strength galvanized steel sheet that has a tensile strength of 590 MPa or higher and excellent processability. The composition contains the following components, in 9o mass., C: from 0.05 95 to 0.3 95, 5i: from 0.7 95 to 2.7 905, Mp: from 0.5 95 to

Zo 2.8 95, P: 0.1 95 or less, 5: 0.01 95 or less, AI: 0.1 95 or less, and M: 0.008 95 or less, and other iron and unavoidable impurities. The microstructure contains, in fractions of the area ratio, the ferrite phase: from 30 95 to 90 95, the bainite phase: from C 95 to 30 95 and the martensitic phase: from 5 95 to 40 95, where, among the martensitic phases, there are martensitic phases that have a dimensional ratio C or higher, in the amount of 30 95 or higher.

Disclosure of the essence of the invention

The purpose of the present invention is to solve these problems by providing cold-rolled steel and coated sheets that simultaneously possess: ultimate tensile strength greater than or equal to 600 MPa and preferably greater than 620 MPa, total elongation greater than or equal to 31 95 and preferably greater than 33 95 .

In a preferred embodiment, the steel sheets claimed in the invention may also have a yield strength of 320 MPa or higher.

In a preferred embodiment, the steel sheets claimed in the invention may also have a yield strength to tensile strength ratio of 0.6 or higher.

Preferably, in addition, said steel can have good formability, in particular for rolling, with good weldability and coating suitability.

Another goal of the present invention is the development of an accessible method of production of the specified sheets, which are compatible with traditional industrial use and at the same time are resistant to changes in production parameters.

The cold-rolled and heat-treated steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or aluminum, or aluminum alloys to increase corrosion resistance.

Carbon is present in steel in amounts between 0.13 95 and 0.18 95. Carbon is an element required to increase the strength of steel sheet by producing low-temperature transformation phases such as bainite, in addition, carbon plays a crucial role in stabilizing austenite , therefore, is a necessary element for the preservation of residual austenite. Therefore, carbon plays two crucial roles, one is to increase strength and the other is to preserve austenite, which provides ductility. However, with a carbon content of less than 0.13 95, it is impossible to stabilize austenite in the appropriate amount, which is required for the steel of the invention.

On the other hand, with a carbon content exceeding 0.18 95, there is poor weldability of 60 steel by the spot welding method, which limits its use for automotive parts.

The manganese content in the steel of the invention is between 1.195 and 1.895. This element is gammagenic. The main purpose of adding manganese is to obtain a structure that contains austenite and gives strength to the steel. It has been established that manganese in an amount of at least 1.1 95 by mass provides strength and the ability to harden steel sheet, as well as stabilization of austenite. However, when the Mn content exceeds 1.8 95, detrimental effects are observed because it inhibits the transformation of austenite to bainite during aging at an elevated temperature for bainite transformation. In addition, a manganese content above 1.8 956 also reduces ductility and, in addition, deteriorates the weldability of the steel according to the invention, so the specified elongation cannot be achieved. The desired manganese content for the present invention can be maintained between 1.2 95 and 1.8 95, more preferably from 1.3 95 to 1.7 9.

The content of silicon in the steel of the invention is between 0.5 95 and 0.9 95. Silicon is a component that can slow down the precipitation of carbides during aging at elevated temperature, therefore, due to the presence of silicon, the carbon-rich austenite is stabilized at room temperature. In addition, due to the low solubility of silicon in carbide, it effectively suppresses or slows down the formation of carbides, therefore it also contributes to the formation of the bainite structure, which is desirable according to the present invention, since it provides permanent essential characteristics. However, the unmatched silicon content does not give the mentioned effect and leads to such a problem as brittleness when released. Therefore, the concentration is adjusted to the upper limit of 0.9 95. The desired silicon content for the present invention can be maintained between 0.6 95 and 0.8 Vol.

Aluminum is an essential element and is present in steel between 0.6 90 and 1 to. Aluminum is an alphagenic element and increases the overall elongation of the steel of the invention. A minimum amount of 0.6 95 aluminum is required in order to have a minimum of ferrite in the steel and increase the elongation of the steel of the invention.

In addition, aluminum is used to remove oxygen from the steel in the molten state to purify the steel of the invention and also prevents oxygen from entering the gas phase. However, in the case when aluminum is more than 1 95, it forms nitrile AIM, which has a bad effect on the steel of the invention, so the predominant aluminum content is in the range between 0.6 9b5 and 0.8 Ob.

The content of phosphorus in the steel of the invention is between 0.002 95 and 0.02 95. Phosphorus reduces the weldability by the method of spot welding and the ductility of the steel in the hot state, especially due to the tendency to segregate along grain boundaries or to co-segregate with manganese. Of these

For reasons, the phosphorus content is limited to 0.02 95 and preferably lower than 0.014 95.

Sulfur is not an essential element, but can be contained in steel as an impurity, and from the point of view of the present invention, the lowest possible sulfur content is preferred, but it is 0.003 95 or less, from the point of view of production costs. In addition, if more sulfur is present in the steel, it forms sulfides especially with manganese and reduces its beneficial effect on the steel of the invention.

The nitrogen content is limited to 0.007 95, in order to avoid aging of the material and to minimize the deposition of nitrides during solidification, which have a negative effect on the mechanical properties of steel.

Chromium is an optional element for this invention. The content of chromium present in the steel of the invention can be between 0.05 95 and 195. Chromium is an essential element that provides strength and hardening of steel, but when more than 1 95 is used, it deteriorates the surface proofing of the steel. In addition, when the chromium content is up to 1 95, the dispersion configuration of carbides in bainite structures is coarsened, therefore the low density of carbides in bainite remains.

Molybdenum is an optional element, which is from 0.001 95 to 0.595 in the steel of the invention; molybdenum plays a significant role in determining hardenability and hardness, delays the formation of bainite and eliminates the precipitation of carbides in bainite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, thus, for economic reasons, its content is limited to 0.5 95.

Niobium is an optional element for this invention. The content of niobium present in the steel of the invention can be between 0.001 and 0.195, and niobium is added to the steel of the invention to form carbonitrides, which give the strength of the steel of the present invention by dispersion hardening. In addition, niobium can affect the size of microstructural components by precipitation in the form of carbonitrides and inhibition of recrystallization during the heating process.

That is, a finer-grained microstructure is formed at the end of temperature exposure and, as a result, after the completion of annealing, which leads to the hardening of the steel of the invention. However, a niobium content above 0.195 is economically impractical, as there is a saturation effect of its influence; this means that the additional amount of niobium does not lead to any improvement in the strength of the product.

Titanium is an optional element and can be added to the steel of the invention in an amount between 0.001 95 and 0.1 95. Like niobium, Ti is involved in the formation of carbonitrides, so it plays a role in the hardening of the steel of the invention. In addition, titanium forms nitrides that appear during the solidification of the casting product. Therefore, the amount of titanium is limited to 0.1 95 to avoid the formation of coarse-grained titanium nitrides, which harm formability. When the titanium content is less than 0.001905, Ti has no effect on the steel of the invention.

Copper can be added as an optional element in amounts from 0.01 95 to 2 95 to increase the strength and improve the corrosion resistance of the steel. To achieve the specified effect, 0.01 95 copper is required. However, when the Si content exceeds 2 95, it may deteriorate the appearance of the surface.

Nickel can be added as an optional element in amounts from 0.01 95 to C 95 to increase the strength and improve the impact toughness of the steel. A minimum of 0.01 95 nickel is required to achieve the specified effect. However, when the nickel content exceeds 395, Mi causes ductility to deteriorate.

The content of calcium in the steel of the invention is between 0.0001 95 and 0.005 95. Calcium is added to the steel of the invention as an optional element, especially during the processing of inclusions. Calcium contributes to the cleaning of steel by binding, which causes damage to sulfur, which is contained in globular form, and in this way, slows down the harmful effect of sulfur.

Vanadium is effective in increasing the strength of steel by forming carbides or carbonitrides, and the upper limit of its content is 0.1 95 for economic reasons. Other elements such as cerium, boron, magnesium or zirconium may be added individually or in combination in the following proportions by weight: cerium 20.1 95, boron 20.003 95, magnesium 20.010 95 and zirconium 20.010 95. Up to the specified maximum level of content, these elements enable cleaning of the grain during hardening. Another part of the steel composition is iron and inevitable impurities that appeared during processing.

The microstructure of the steel sheet includes: ferrite is from 60 95 to 7595 microstructure per area fraction for the steel of the invention.

Ferrite is the primary phase of steel as a matrix. In this invention, the ferrite is collectively composed of polygonal ferrite and acicular ferrite. Ferrite gives the steel of the invention high strength, as well as elongation. In order to provide an elongation of 31 95 and preferably 33 95 or higher, it is necessary to have 60 956 ferrite. In the steel of the invention, ferrite is formed during cooling after annealing. However, when the ferrite content is higher than 75 95, the strength of the steel of the invention is not achieved.

Zo Bainite is from 2095 to 3095 microstructures per area for the steel of the present invention. In this invention, the bainite consists collectively of lamellar bainite and granular bainite. To provide a tensile strength of 620 MPa and preferably 630 MPa or higher, 2095 bainite is required. Bainite is formed during aging at an elevated temperature.

Residual austenite is from 10 95 to 15 95 of the share of the area for steel. Residual austenite is known to have a higher carbon solubility than bainite and therefore acts as an effective carbon trap and therefore inhibits the formation of carbides in bainite. Preferably, the percentage of carbon inside the residual austenite of the invention exceeds 0.9 95 and preferably less than 1.1 95. According to the invention, the residual austenite in the steel gives it increased plasticity.

Martensite is an optional component and can be present in an amount between О 95 and 5 95 of the fraction of the area of the microstructure, and is present in a small amount. Martensite for the present invention includes fresh martensite as well as tempered martensite. In the present invention, martensite is formed by cooling after annealing and becomes tempered upon exposure during aging at an elevated temperature. Fresh martensite also forms during cooling after coating a cold-rolled steel sheet. Martensite gives plasticity and strength to the steel of the invention when its content is less than 5 95. When the content of martensite exceeds 5 95, the steel acquires excessive strength, but outside the acceptable limit, martensite reduces elongation. The preferred limits for martensite are between 0 9b and З 95.

The combined content of ferrite and austenite should always be between 70 95 and 80 95 to have a total elongation of 3195, and a minimum of 7095 of ferrite and austenite is required to provide a total elongation above 31 95 and to have a tensile strength of 600 MPa. Ferrite and residual austenite are soft phases compared to martensite and bainite, so they provide elongation and ductility, but when their total content exceeds 80 95, the strength decreases below the permissible limit.

In addition to the above-mentioned microstructure, the cold-rolled and heat-treated steel sheet does not contain microstructural components such as pearlite and cementite without deteriorating the mechanical properties of steel sheets.

The steel sheet according to the invention can be obtained in any suitable way.

The preferred method consists in obtaining a semi-proven steel casting, which has a chemical composition of 60 according to the invention. Casting can be done either in ingots or continuously in the form of thin slabs or thin strips, that is, thicknesses ranging from approximately 220 mm for slabs to tens of millimeters for thin strips.

For example, a slab having the above chemical composition is produced by continuous casting, in which the slab is not necessarily subjected to direct gentle thickness reduction during the continuous casting process to avoid central segregation and to maintain a local carbon to nominal carbon ratio below 1, 10.

The slab obtained using the continuous casting process can be used directly at high temperature after continuous casting or can be first cooled to room temperature and then heated for hot rolling.

The temperature of the hot-rolled slab is at least 1150 °C, and should be below 1280 °C. In the case when the temperature of the slab is lower than 1150 "C, the rolling mill is subjected to an excessive load. Therefore, the temperature of the slab is preferably high enough so that hot rolling can be carried out in the temperature range from As1-50 "C to As12250 "C and preferably between As1-50 "C and As14200 "C, although the final rolling temperature always remains above As1i-50 "C. The temperature of reheating should not exceed 1280 "C, since such a technological mode is expensive.

The temperature range of the final rolling between As1-50 "C and As11250 "C is preferable in order to have a structure that promotes recrystallization and rolling. It is necessary that the final rolling run be carried out at a temperature higher than Ac1--50 "С, because below the indicated temperature, the ability of the steel sheet to deform during rolling is significantly reduced.

Then the sheet obtained in this way is cooled at a cooling rate above 30 °C/s to the temperature of winding into a roll, which should be below 625 °C. Preferably, the cooling rate will be less than or equal to 200 "C/s.

The hot-rolled steel sheet is then wound at a coiling temperature below 625°C to avoid ovalization, and preferably below 600°C to prevent scaling. The preferred coiling temperature range is between 350°C and 600°C. Coiled hot-rolled steel sheet can be cooled to room temperature before being subjected to optional hot-state annealing.

Hot-rolled steel sheet may be subjected to optional descaling at the stage of removing scale formed during hot rolling, to optional hot annealing. The hot-rolled sheet may then be subjected to optional hot annealing at temperatures between 400°C and 750°C for at least 12 hours and no more than 96 hours, with the temperature maintained below 750°C to avoid partial transformation of the hot-rolled microstructure, and, therefore, lose the homogeneity of the microstructure. In the future, an optional step of removing scale from the specified hot-rolled steel sheet can be carried out, for example, by etching this sheet. Said hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet with a thickness reduction between 35 and 90 95. Then, the cold-rolled steel sheet obtained in the cold rolling process is subjected to annealing in order to give the steel of the invention microstructure and mechanical characteristics.

During annealing, the cold-rolled steel sheet is processed at two levels of heating, reaching a quenching temperature between Ac1--30 "C and AcZ3, where the values of Ac! and AcZ for the steel of the invention are calculated using the following formula:

As1-723-10.7|(Mp|-16(M29.1(511-16.9(ISY-6.38 UM |-290IAv)

As3-910-203|С11/2-15.2(М1х-447151--1 04ЎМ1-31.5(Мо1|--13.1 MMM1-30(Мы1-11(Сп- 29IScsh1-7ООГРИ-4ООГАП-- T2O(Az|-400T in which the content of elements is expressed in percent by mass.

In the first stage, the cold-rolled steel sheet is heated at a rate between 10 "C/s and 40 "C/s to a temperature in the range between 550 "C and 650 "C. After that, in the subsequent second stage of heating, the cold-rolled steel sheet is heated at a rate between 1 "C/s and 5 "C/s to the quenching temperature during annealing.

The cold-rolled steel sheet is then preferably held at an annealing temperature for 10 to 500 s to ensure at least 30 95 conversion of the heavily riveted original structure to an austenitic microstructure. Then, the cold-rolled steel sheet is cooled in two stages of cooling to the aging temperature during long-term exposure. In the first stage of cooling, the cold-rolled steel sheet is cooled at a rate of less than 5 °C/s and preferably less than 3 °C/s to a temperature in the range between 600 °C and 720 °C and preferably between 625 °C and 7200. the specified first degree of cooling forms the ferrite matrix of the invention. After that, in the next second 60 degrees of cooling, the cold-rolled steel sheet is cooled to a long-term aging temperature in the 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 kept at a temperature of long-term aging in the range of up to 500 s. The cold-rolled steel sheet is then brought to a coating bath temperature in the range of 400°C to 480°C to facilitate coating of the cold-rolled steel sheet. The cold-rolled steel sheet is then coated using any known industrial process such as electro-galvanizing, jet vacuum spraying (MO), spraying by condensation of steam (PMO), application by immersion in a heated impregnation composition (galvanized iron), etc.

Examples

The following tests, examples, illustrative examples and tables presented in the invention are essentially non-limiting and should be considered for illustration purposes only, and will demonstrate advantageous features of the invention.

Steel sheets made from steels with different compositions are compiled in Table 1, where steel sheets are obtained according to the technological parameters listed in Table 2, respectively. The following table C shows the microstructures of the steel sheets obtained during the tests, and table 4 summarizes the results of the evaluation of the properties of the obtained sheets.

Table 1

Underlined values: do not correspond to the invention. snnkenn sr di elements

A 10155 0 1.54|0696 0728 |0014) 0002 | 0002 0003 Y.S.S(ЙЙ - 76 0148 154|0698 0 |0013 00027|000441 :Й- 0 Щ|о0л141162|0293)| 0.031 | 0.027

Underlined values: do not correspond to the invention.

Table 2 summarizes the technological parameters of annealing carried out with steels from Table 1.

Steels of composition A and B are used for the production of sheets according to the invention. This table also lists the reference steels, which are designated here as C and Y. Table 2 also lists tabular data

Ac1 and Ac3. These values of Ac1 and Ac3 are determined for steels of the invention and reference steels as follows:

As1-723-10.7|(Mp|-16(M29.1(511-16.9(ISY-6.38 UM |-290IAv)

As3-910-203|С11/2-15.2(М1х-447151--1 04ЎМ1-31.5(Мо1|--13.1 MMM1-30(Мы1-11(Сп- 29IScsh1-7ООГРИ-4ООГАП-- Т2О(Аз|-400T) where the content of elements is expressed in percent by mass.

All sheets were cooled at a cooling rate of 34 "C/s after hot rolling and finally brought to a temperature of 460 "C, before cooling. All sheets were cold rolled at 65 95.

Table 2 is below

Table 2 01 st WTO slow | termination | fast | termination | long-term storage (C) (c) cooling slow cooling rapid aging (c) after annealing cooling CCS) cooling CO)

Sess) Sss) es 17777711 A | 7200 | 850 | 500.14 | 6 yuTsl2 | 600 | 1.617701 179 712 | in ' | 1200 | 870 | 520 | 19 | 600 | PLN 800! on

And 1377 7711 A | lgoo | 850 | 500 | 712 | 600 | 79 |8001. 179 11111171 First degree | 0 Дрогыйстулинн.//7/ | 7777/7771 slow termination fast termination | of long |endurance| (SS) | SS) cooling slow cooling rapid aging (s) after annealing | cooling SSS) cooling so

Sss) essay) es 7771717 177717106 | 70 | z0 | "0 | 40 | 7129 | 1117 | 727

1377 09 | 70 2 2 Yushshch | with | 400 | 400 | 129. |t1112| 727

In 7777-7117 111134 | 460 | 460 | 129 7 |1117| 727 782 | 09 2 2 | 70 | with | 400 | 400 | 7129 | 827 | 727

VI GA | 1200 | 850 | 500 | 2 yushf 712 | 600 | 19 | wow | r293 ve | with | Govo 1 850 | 500 | 2 yush 712 | 60 | 19 | in | 179)

And yaz | 0 | l2h00| 920 | 585 | every 9 | 60 | 12 | 770 | 238

And x according to the invention; K - standard; underlined values: do not correspond to the invention.

Table C shows examples of the results of tests carried out in accordance with the standards using various microscopes, such as a scanning electron microscope, to determine the microstructure of the steels of the invention as well as the reference steels.

The results are below:

Table C

Steel samples Ferrite Bainite Residual Martensite periite en r (so) (oo) austenite (96) (oo) U o, o 167 | 22 | 77111110 31116 | 27 | sh-HBm 1... Her 7017 1771717171717171711173 in | 53 | 37 | -.BB lo... HER 1/0 63 4 yaz 1 62 | 33 | Б ЮщХщХІ 0 HER yusch 6 g /

And x according to the invention; K - standard; underlined values: do not correspond to the invention

Table 4 shows examples of mechanical characteristics of steels of the invention, as well as reference steels. In order to determine the tensile strength, yield strength (U5) and total elongation, tensile tests were carried out in accordance with standard 915 22241.

The results of mechanical tests carried out in accordance with the standards are collected in Table 4

Table 4

Tensile strength (MPa) UZ(MPa) | U5/15 | Total elongation (95) 217 1777711111111111111167211111111111111 401 | b60 | .7771717171332 va 77777777171717171717171711602777777777777777711 | 365 | 061 | 2 sh(:M 286. shSHD shRK ( in HER 777777171717171717171717171162277777777771711111 | vase | 055 | 77725

And x according to the invention; K - standard; underlined values: do not correspond to the invention.

Claims (23)

FORMULA OF THE INVENTION 1. Cold-rolled steel sheet, which has the following chemical composition, containing the following elements, expressed as a percentage by mass: 0.13 x carbon x 0.18, 11 x manganese x 1.8, O.5 x silicon x 0.9, o b aluminum x 1, o, 002 x phosphorus 0.02 , sulfur0.003, nitrogen:0.007, the rest - iron and inevitable impurities, and the microstructure of the specified steel sheet contains in parts of the area from 60 to 75 95 ferrite, from 20 to 30 95 bainite, from 10 to 15 95 residual austenite and from 0 to 5 95 of martensite, and the total content of residual austenite and ferrite is between 70 and 80 95. 2. Steel sheet according to claim 1, where the chemical composition of the specified sheet additionally contains one or more of the following elements, expressed in percent by mass: . 3. Steel sheet according to claim 1 or 2, the composition of which contains from 0.6 to 0.8 silicon. 4. Steel sheet according to any of claims 1-3, the composition of which contains from 0.14 to 0.18 carbon. 5. Steel sheet according to claim 4, the composition of which contains from 0.6 to 0.8 aluminum. 6. Steel sheet according to any of claims 1-5, the composition of which contains from 1.2 to 1.8 manganese. 7. Steel sheet according to claim 6, the composition of which contains from 1.3 to 1.7 manganese. 8. A steel sheet according to any one of claims 1-7, in which the combined content of retained austenite and ferrite is between 73 and 80 95, and the percentage content of retained austenite is less than 13 95. 9. Steel sheet according to any of claims 1-8, in which the amount of martensite is between 0 and 95. 10. Steel sheet according to any one of claims 1-9, in which the carbon content in the residual austenite is between 0.9 and 1.1 Fo. 11. A steel sheet according to any of claims 1-10, wherein said steel sheet has a tensile strength of 600 MPa or more and a total elongation of 31 95 or more. 12. Steel sheet according to claim 11, where said steel sheet has a yield strength of 320 MPa or higher and a total elongation of 33 95 or more. 13. A steel sheet according to any one of claims 1-12, wherein said steel sheet has a coating formed from zinc or a zinc alloy, or aluminum, or an aluminum alloy. 14. The method of obtaining a cold-rolled steel sheet, which includes the following successive stages: providing a semi-finished product from steel, which has a composition according to any of claims 1-7; heating the specified semi-finished product to a temperature between 1150 and 1280 "C; rolling the specified semi-finished product in the austenitic range, in which the final hot rolling temperature is between As1-50 and As1-250"C, to obtain a hot-rolled steel sheet; cooling the sheet with a cooling rate higher than 30 "C/s to a coiling temperature that is lower than 625 "C; and winding the hot-rolled sheet into a roll; cooling the indicated hot-rolled sheet to room temperature; cold rolling of the specified hot-rolled steel sheet with a degree of compression between 35 and 90 95 to obtain a cold-rolled steel sheet; then the cold-rolled steel sheet is annealed at a holding temperature between As1-30 "C and As3 during a holding period between 10 and 500 s by heating the specified cold-rolled steel sheet in two heating stages, in which: in the first heating stage, the cold-rolled steel sheet is heated at a rate between 10 and 40 "C/s to a temperature in the range between 550 and 650 "C; then, in the second stage, the cold-rolled steel sheet is heated at a rate between 1 and 5 "C/s from a temperature in the range between 550 and 650 "C to the annealing temperature at at the holding temperature at which the sheet is kept, then the cooling of the cold-rolled steel sheet is carried out in two stages of cooling, in which: in the first stage of cooling, the cold-rolled steel sheet is cooled with a cooling rate of less than 5 "C/s to a temperature in the range between 600 and 720 "C , then in the second stage the sheet is cooled at a cooling rate between 10 and 100 "C/s from a temperature in the range between 600 and 720 "C to a temperature of long-term aging, then the specified cold-rolled steel sheet is subjected to long-term aging at a temperature in the range between 250 and 470 " C for 5 to 500 si and then cooled to room temperature to obtain a cold-rolled steel sheet. 60 15. The method according to claim 14, in which, after cooling the specified hot-rolled steel sheet to room temperature, the scale is removed from the specified hot-rolled steel sheet. 16. The method according to claim 14 or 15, in which, after cooling the specified hot-rolled steel sheet to room temperature, the hot-rolled steel sheet is annealed at a temperature between 400 and 750 °C. 17. The method according to claim 16, in which, after the specified annealing of the hot-rolled steel sheet at a temperature between 400 and 750 7C, the scale is removed from the specified hot-rolled steel sheet. 18. The method according to any of claims 14-17, in which the temperature of winding into a roll is below 600 "С. 19. The method according to any of claims 14-18, in which the final rolling temperature is between As1-50 and As14200 "С. 20. The method according to any one of claims 14-19, in which the cooling rate after annealing of the cold-rolled steel sheet is less than C "C/s in the temperature range between 625 and 72076. 21. The method according to any one of claims 14-20, in which the cold-rolled steel sheet is annealed between Ac1-30 C and Ac3, and the annealing temperature is chosen to ensure the presence of at least 30 95 austenite at the end of aging. 22. The method according to any one of claims 14-21, in which the cold-rolled steel sheet can be coated with zinc or a zinc alloy, or aluminum or an aluminum alloy at a temperature in the range between 400 and 480 "С. 23. Use of a steel sheet according to any of claims 1-13 or a steel sheet obtained by a method according to any of claims 14-22 for the production of vehicle structures or parts that are responsible for vehicle safety.