ES2946086T3 - Cold-rolled and coated steel sheet and a manufacturing method thereof - Google Patents

Abstract

A cold-rolled, heat-treated steel sheet having a composition consisting 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 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% <= Vain dio <= 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 inevitable 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% of residual austenite and 0% to 5% of martensite, where the cumulative amounts of residual austenite and ferrite are between 70% and 80%. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Cold-rolled and coated steel sheet and a manufacturing method thereof

[0001] The present invention relates to cold rolled and coated steel sheets suitable for use as a steel sheet for automobiles.

[0002] Automotive parts are required to satisfy two inconsistent needs, namely ease of formability and strength, but a third requirement of improvement in fuel consumption in recent years is also given to automobiles in view of global environmental concerns . Therefore, now automotive parts must be made of a material having high formability in order for them to fit the criteria of ease of fit in complex automobile assembly and at the same time have to improve their strength in terms of to the crash energy absorption capacity and durability of the vehicle while reducing the weight of the vehicle to improve efficiency in terms of fuel consumption.

[0003] Therefore, intensive research and development efforts have been made to reduce the amount of material used in automobiles by increasing the strength of the material. On the contrary, an increase in the strength of steel sheets decreases the formability and therefore the development of materials having both high strength and high formability is required.

[0004] Previous research and development in the field of high-strength, high-formability steel sheets have resulted in various methods of producing high-strength, high-formability steel sheets, some of which are listed in this application. for a conclusive appreciation of the present invention:

Document US20140234657 is a patent application that claims 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 total of one or two martensite and bainite, a residual structure contains one or two of ferrite, residual austenite of less than 8% by volume fraction and pearlite equal to or less than 10% by volume fraction. Furthermore, US20140234657 reaches a tensile strength of 980 MPa but is unable to reach 25% elongation.

US8657969 claims that the high-strength galvanized steel sheet has a tensile strength of 590 MPa or more and excellent processability. The component composition contains, in 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: from 30% to 90%, bainite phases: from 3% to 30%, and martensite phases: from 5% to 40%, in which, between the martensite phases, the martensite phases having an aspect ratio of 3 or more are present in a proportion of 30% or more.

WO 2017125809 describes a coated cold rolled steel sheet comprising 1.9-2.2 manganese.

[0005] The purpose of the present invention is to solve these problems by making available cold-rolled and clad steel sheets having simultaneously:

- A maximum tensile strength greater than or equal to 600 MPa and preferably greater than 620 MPa,

- A total elongation greater than or equal to 31% and preferably greater than 33%.

[0006] In a preferred embodiment, the steel sheets according to the invention can also have a yield strength of 320 MPa or more.

[0007] 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. Preferably, such a steel may also have good formability, in particular for rolling with good weldability and coating ability.

[0008] Another object of the present invention is also to provide a process for manufacturing these sheets that is compatible with conventional industrial applications while being robust with respect to changes in manufacturing parameters.

[0009] The heat-treated, cold-rolled steel sheet of the present invention can optionally be coated with zinc or zinc alloys, or aluminum or aluminum alloys to improve its corrosion resistance.

[0010] Carbon is present in steel between 0.13% and 0.18%. Carbon is a necessary element to increase the resistance of the steel sheet by producing phases in the transformation at low temperature. such as bainite. In addition, carbon also plays a fundamental role in stabilizing austenite, therefore it is a necessary element to ensure residual austenite. Thus, carbon plays two critical roles, one in increasing strength and one in retaining austenite to impart ductility. But a carbon content 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%, the steel has poor spot weldability that limits its application in automotive parts.

[0011] The manganese content of the steel of the present invention is between 1.1% and 1.8%. This element is gammagenic. The purpose of adding manganese is essentially to obtain an austenite-containing structure and to impart strength to the steel. It has been found that an amount of at least 1.1% by weight of manganese provides the strength and hardenability of the steel sheet, as well as stabilizing the austenite. However, when the manganese content is greater than 1.8%, adverse effects occur, such as delaying the transformation of austenite to bainite during overaging retention for bainite transformation. Additionally, manganese content above 1.8% also reduces ductility and also deteriorates weldability of the present steel, so elongation targets may not be achieved. A preferable content for the present invention can be kept between 1.2% and 1.8%, more preferably between 1.3% and 1.7%.

[0012] The silicon content of the steel of the present invention is between 0.5% and 0.9%. Silicon is a constituent that can retard carbide precipitation during overaging, therefore, due to the presence of silicon, carbon-rich austenite stabilizes at room temperature. In addition, 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 that is sought according to the present invention to impart its essential characteristics to the steel. However, the disproportionate content of silicon does not produce the above-mentioned effect and causes 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%.

[0013] Aluminum is an essential element and is present in steel between 0.6% and 1%. Aluminum is an alphagen element and imparts full elongation to the steel of the present invention. A minimum of 0.6% aluminum is required to have a minimum of ferrite thereby imparting the 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 gas phase. But as long as the aluminum is more than 1%, it forms AIN, which is detrimental to the steel of the present invention, therefore, the preferable range for the presence of the aluminum is between 0.6% and 0.8%.

[0014] The phosphorous constituent of the steel of the present invention is between 0.002% and 0.02%. Phosphorus reduces spot weldability and hot ductility, particularly because of its tendency to segregate at grain boundaries or cosegregate with manganese. For these reasons, its content is limited to 0.02% and is preferably less than 0.014%.

[0015] Sulfur is not an essential element, but it may be contained as an impurity in the steel, and from the point of view of the present invention, the sulfur content is preferably as low as possible, but it is 0.003% or less from the point of view of manufacturing cost. Also, if more sulfur is present in the steel, it combines to form sulfides, especially with manganese, and reduces its beneficial impact on the steel of the present invention.

[0016] Nitrogen is limited to 0.007% in order to avoid aging of the material and to minimize the precipitation of nitrides during solidification that are detrimental to the mechanical properties of the steel.

[0017] 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 in excess of 1%, it impairs the surface finish of the steel. Other chromium contents below 1% coarsen the dispersion pattern of carbide in bainite structures, thus keeping the density of carbides in bainite at a low level.

[0018] Molybdenum is an optional element constituting 0.001% to 0.5% of the steel of the present invention; Molybdenum plays an effective role in determining hardenability and hardness, retards the appearance of bainite, and prevents 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%.

[0019] Niobium is an optional element for the present invention. Niobium content may be present in the steel of the present invention between 0.001 and 0.1% and is added in 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, a finer microstructure is formed at the end of the holding temperature and, as a consequence, after completion of the annealing, which will lead to the hardening of the steel of the present invention. However, the niobium content above 0.1% is not economically interesting since a saturation effect of its influence is observed, this means that an additional amount of niobium does not produce any improvement in the resistance of the product.

[0020] 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, therefore it plays a role in the hardening of the steel of the present invention. Additionally, titanium also forms titanium nitrides that appear during the solidification of the cast product. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides detrimental to formability. In case the titanium content is below 0.001%, it has no effect on the steel of the present invention.

[0021] Copper can be added as an optional element in an amount of 0.01% to 2% to increase the strength of the steel and to improve its resistance to corrosion. A minimum of 0.01% copper is required to obtain this effect. However, when its content is more than 2%, it can degrade the surface aspects.

[0022] Nickel can be added as an optional element in an amount of 0.01 to 3% to increase the strength of the steel and to improve its toughness. A minimum of 0.01% is required to produce such effects. However, when its content is higher than 3%, nickel causes deterioration of ductility.

[0023] 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 the refining of steel by stopping the harmful sulfur content in globular form, thus retarding the harmful effects of sulfur.

[0024] Vanadium is effective in improving the strength of steel by 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 maximum content levels indicated, these elements allow the grain to be refined during solidification. The rest of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0025] The microstructure of the steel sheet comprises:

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, the ferrite cumulatively comprises polygonal ferrite and acicular ferrite. The 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. But as long as the ferrite content is present above 75% in the steel of the present invention, the strength is not achieved.

[0026] Bainite constitutes 20% to 30% of the microstructure per area fraction for the steel of the present invention. In the present invention, the bainite cumulatively consists 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 forms during overaging retention.

[0027] Residual austenite constitutes 10 to 15% by area fraction of the steel. Residual austenite is known to have a higher carbon solubility than bainite and therefore acts as an effective carbon trap, thereby retarding 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 improved ductility.

[0028] Martensite is an optional constituent and can be present between 0% and 5% of the microstructure by area fraction and be found in trace amounts. The martensite for the present invention includes both fresh martensite and tempered martensite. The present invention forms martensite due to cooling after annealing and tempers during overage retention. Fresh martensite is also formed during cooling after cold rolled steel sheet coating. The martensite imparts ductility and strength to the steel of the present invention when it is below 5%. When the martensite exceeds 5%, it provides excess strength but decreases the elongation beyond the acceptable limit. The preferable limit for martensite is between 0% and 3%.

[0029] 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 the 31 % as long as it has a tensile strength of 600 MPa. Ferrite and residual austenite are soft phases compared to martensite and bainite, therefore they impart elongation and ductility, but as long as the cumulative presence is greater than 80%, the strength will drop beyond acceptable limits.

[0030] In addition to the microstructure mentioned above, the microstructure of the cold-rolled and heat-treated steel sheet is free of microstructural components, such as perlite and cementite without impairing the mechanical properties of the steel sheets.

[0031] A steel sheet according to the invention can be produced by any suitable process. A preferred process is to provide a semi-finished casting of steel with a chemical composition according to the invention. Casting can be done in ingots or continuously in the form of thin slabs or thin strips, ie with a thickness ranging from about 220 mm for slabs to several tens of millimeters for thin strip.

[0032] For example, a slab having the chemical composition described above is manufactured by continuous casting where the slab was optionally subjected to direct mild reduction during the continuous casting process to avoid central segregation and to ensure a local carbon ratio. a nominal carbon maintained below 1.10. The slab provided by the continuous casting process can be used directly at high temperature after continuous casting or it can be first cooled to room temperature and then reheated for hot rolling.

[0033] The temperature of the slab, which is subjected to hot rolling, is at least 1150 °C and must be below 1280 °C. In case the slab temperature is less than 1150 °C, excessive load is imposed on a rolling mill. Therefore, the slab temperature is preferably high enough that hot rolling can be completed in the temperature range from Ac1 50 °C to Ac1+250 °C and preferably between Ac1+50 °C and Ac1+200 °C while always keeping the final lamination temperature above Ac1+50 °C. Overheating to temperatures above 1280 °C should be avoided as it is industrially expensive.

[0034] It is preferred that a final rolling temperature range between Ac1 50 °C and Ac1+250 °C has a structure that is favorable for recrystallization and rolling. It is necessary that the last rolling pass be carried out at a temperature higher than Ac1 50 °C, since below this temperature the steel sheet shows a significant drop in rolling capacity. The sheet thus obtained is then cooled at a cooling rate above 30 °C/s to the winding temperature which should be below 625 °C. Preferably, the cooling rate will be less than or equal to 200 °C/s.

[0035] Next, the hot-rolled steel sheet is coiled at a coiling temperature of less than 625°C to avoid out-of-roundness and preferably less than 600°C to avoid scaling. The preferred range for such a winding temperature is between 350°C and 600°C. Coiled hot rolled steel sheet can be cooled to room temperature before subjecting it to optional hot strip annealing.

[0036] Hot rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling prior to optional "hot strip annealing". The hot rolled sheet can then undergo optional hot band annealing at temperatures between 400°C and 750°C for at least 12 hours and not more than 96 hours, with the temperature remaining below 750°C to avoid partially transform the hot rolled microstructure and therefore lose the homogeneity of the microstructure. Subsequently, an optional step for removing scale from this hot-rolled steel sheet can be carried out, for example, by pickling said sheet. This hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 and 90%. Next, the cold rolled steel sheet obtained from the cold rolling process is subjected to annealing to impart the microstructure and mechanical properties to the steel of the present invention.

[0037] In annealing, cold rolled steel sheet subjected to two heating stages to reach the homogenization temperature between Ac1+30 °C and Ac3, where Ac1 and Ac3 for the present steel is calculated using the following formula:

Ac1 =723- 10.7[Mn] - 16[Ní] 29.1 [Sí] 16.9[Cr] 6.38[W] 290[As] Ac3 = 910 - 203[C]A(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 [AI] 120[As] 400[Ti]

Where the content of the elements is expressed in percentage by weight.

[0038] In the step, a 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. Subsequently, in the second subsequent heating stage, the cold rolled steel sheet is heated at a heating rate between 1 °C/s and 5 °C/s to the annealing homogenization temperature.

[0039] The cold-rolled steel sheet is then preferably held at the homogenization temperature for 10 to 500 seconds to ensure at least 30% transformation to austenite microstructure of the highly work-hardened initial structure. The cold-rolled steel sheet is then cooled in two cooling stages to an overaging holding temperature. In the first cooling stage, the cold rolled steel sheet is cooled at a cooling rate of less than 5 °C/s and preferably less than 3 °C/s to a temperature range between 600 °C and 720 °C and preferably between 625 °C and 720 °C. During this first stage of cooling, a ferrite matrix of the present invention is formed. Subsequently, in a second post-cooling stage, the cold rolled steel sheet is cooled to an overaging temperature range between 250 °C and 470 °C at a cooling rate between 10 °C/s and 100 °C/s. . The cold rolled steel sheet is then held in the overaging temperature range for 5 to 500 seconds. Next, the cold rolled steel sheet is brought to the temperature of a coating bath temperature range of 400 °C and 480 °C to facilitate coating of the cold rolled steel sheet. The cold rolled steel sheet is then coated by any of the well-known industrial processes such as electro-galvanizing, JVD, PVD, hot dip (GI), etc.

EXAMPLES

[0040] The following tests, examples, figurative exemplification and tables presented in this application are not restrictive in nature and should be considered for illustrative purposes only, and will show the advantageous features of the present invention.

[0041] The steel sheets made of steels with different compositions are collected in Table 1, where the steel sheets are produced according to the process parameters as stipulated in Table 2, respectively. Subsequently, Table 3 lists the microstructures of the steel sheets obtained during the tests and Table 4 lists the results of the evaluations of the properties obtained.

Tabla 1

Table 1

Table 2

[0042] Table 2 lists the parameters of the annealing process implemented in the steels of Table 1. The steel compositions A and B are used for the manufacture of sheets 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]A(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 [AI] 120[As] 400[Ti]

Where the content of the elements is expressed in percentage by weight.

[0043] All sheets were cooled at a cooling rate of 34 °C/s after rolling in hot and finally brought to a temperature of 460 °C before coating. All plates have a 65% cold rolling reduction.

[0044] Table 2 is as follows:

Table 3

[0045] Table 3 exemplifies the results of the tests carried out in accordance with the standards in different microscopes such as a scanning electron microscope to determine the microstructures of both the steels of the invention and those of reference.

[0046] The results are stipulated in this request:

Table 4

[0047] Table 4 exemplifies the mechanical properties of both the steels of the invention and those of reference. In order to determine the tensile strength, yield strength and total elongation, tensile tests are carried out in accordance with JIS Z2241 standards.

[0048] The results of the various mechanical tests carried out in accordance with the Table 4 standards are collected

Claims (20)

1. A cold-rolled steel sheet having a composition comprising the following elements, expressed in percent by weight: 5 0.13% < carbon < 0.18% 1.1% < manganese < 1.8% 0.5% < silicon < 0.9% 0.6% < aluminum < 1% 10 0.002% < phosphorus < 0.02% 0% < sulfur < 0.003%. 0% < nitrogen < 0.007% And may contain one or more of the following optional elements fifteen 0.05% < chromium < 1% 0.001% < molybdenum < 0.5% 0.001% < niobium < 0.1% 0.001% < titanium < 0.1% 20 0.01% < copper < 2% 0.01% < nickel < 3% 0.0001% < calcium < 0.005% 0% < vanadium < 0.1% 0% < boron < 0.003% 25 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 % residual austenite and 0% to 5% martensite, where the cumulative amounts of residual austenite and ferrite are between 70% and 80%. Cold rolled steel sheet according to claim 1, wherein the composition includes 0.6% to 0.8% silicon. Cold rolled steel sheet according to claim 1 or 2, wherein the composition includes 0.14% to 0.18% carbon. Cold rolled steel sheet according to claim 3, wherein the composition includes 0.6% to 0.8% aluminum. Cold rolled steel sheet according to any one of claims 1 to 4, wherein the composition includes 1.2% to 1.8% manganese. Four. Five Cold rolled steel sheet according to claim 5, wherein the composition includes 1.3% to 1.7% manganese. 7. Cold-rolled steel sheet according to any one of claims 1 to 6, wherein 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, wherein the amount of martensite is between 0% and 3%. 55 Cold-rolled steel sheet according to any one of claims 1 to 8, wherein the residual austenite carbon content is between 0.9 and 1.1%. Cold rolled steel sheet according to any one of claims 1 to 9, wherein said steel sheet 60 has a maximum tensile strength of 600 MPa or more and a total elongation of 31% or more. Cold rolled sheet according to claim 10, wherein said steel sheet has a yield strength 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, wherein the sheet steel is lined. 13. A production process for a cold-rolled steel sheet comprising the following successive stages: - providing a steel composition according to any one of claims 1 to 6; - reheating said semi-finished product at a temperature between 1150 °C and 1280 °C; - rolling said semi-finished product in the austenitic range where the hot rolling finishing temperature will be between Ac1+50 °C and Ac1 250 °C to obtain a hot rolled steel sheet; - cooling the sheet at a cooling rate above 30 °C/s to a winding temperature that is below 625 °C; and coiling said hot rolled sheet; - cooling said hot rolled sheet to room temperature; - optionally performing a scale removal process on said hot rolled steel sheet; - optionally make a hot-rolled steel sheet at a temperature between 400 °C and 750 °C; - optionally performing a 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 annealing at a homogenization temperature between Ac1+30 °C and Ac3 for a duration between 10 and 500 seconds, heating said cold-rolled steel sheet by heating in two stages where: ◦ in the first heating stage, 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; ◦ Next in the second stage, 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 of annealing homogenization to which it is maintained, - then cool the cold-rolled steel sheet in a two-stage cooling where: ◦ In the first stage of cooling, the cold rolled steel sheet is cooled at a cooling rate of less than 5 °C/s at a temperature range between 600 °C and 720 °C ◦ Next, in the second stage, 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 an overaging temperature. - then said cold rolled steel sheet is overaged at a temperature range between 250 °C and 470 °C for 5 to 500 seconds and - then it is cooled to room temperature to obtain a cold-rolled steel sheet. A process according to claim 13, wherein the cooling temperature is below 600°C. 15. A process according to claim 13 or 14, wherein the finishing rolling temperature is between Ac1+50°C and Ac1+200°C. A process according to any one of claims 13 to 15, wherein the cooling rate after annealing is less than 3 °C/s in the temperature range between 625 °C and 720 °C. 17. A process for producing a cold-rolled steel sheet according to any one of claims 13 to 16, wherein the cold-rolled steel sheet is annealed between Ac1+30 °C and Ac3 and the annealing temperature is selected to ensure the presence of at least 30% austenite at the end of the homogenization. A process for producing a cold rolled steel sheet as claimed in any one of claims 13 to 17, wherein the cold rolled steel sheet can be coated in a temperature range between 400 °C and 480 °C. c. Use of a steel sheet according to any one of claims 1 to 12, or of a steel sheet produced according to the process of claims 13 to 18, for the manufacture of structural or safety parts of a vehicle. 20. Vehicle comprising a part obtained according to claim 19.