Cold-rolled and coated steel sheet and method of manufacturing the same

Abstract

To provide cold-rolled and coated steel sheets with high strength and high formability.SOLUTION: A cold-rolled and coated steel sheet has composition containing the following elements, expressed in percentage by weight: 0.140%≤carbon≤0.2%, 1.5%≤manganese≤2.15%, 0.5%≤silicon≤0.8%, 0.4%≤aluminum≤0.8%, 0%≤phosphorus≤0.09%, 0%≤sulfur≤0.09%, 0%≤nitrogen≤0.09%, 0.01%≤niobium≤0.1%, and 0.01%≤titanium≤0.1%; with the remainder of composition being composed of iron and unavoidable impurities caused by processing. The microstructure of the steel sheet contains, in area fraction, 40 to 60% of intercritical ferrite, 25 to 45% of transformed ferrite, 8% to 20% and 5% to 20% of fresh martensite, 0 to 10% of bainite, where the cumulated amount of the intercritical ferrite and transformed ferrite is between 75% and 85%.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to cold rolled coated steel sheet suitable for use as automotive steel sheet. [Background technology]

[0002] Automotive parts are required to satisfy two conflicting requirements, namely, ease of molding and strength, but in recent years, a third requirement of improved fuel efficiency has also been imposed on automobiles due to consideration of the global environment. Therefore, automotive parts must now be manufactured from materials with high moldability to meet the criteria of ease of fitting into complex automobile assemblies, and at the same time, the strength of the vehicle's impact resistance and durability must be improved while reducing the vehicle's weight to improve fuel efficiency.

[0003] Therefore, vigorous research and development is being conducted to reduce the amount of material used in automobiles by increasing the strength of the materials. Conversely, as the strength of steel sheets increases, their formability decreases, so there is a demand for the development of materials that combine high strength and high formability.

[0004] Previous research and development in the field of high strength and highly formable steel plates has resulted in several methods for producing high strength and highly formable steel plates, some of which are listed herein for a clear understanding of the present invention.

[0005] EP2768989 claims to have a high-strength hot-dip galvanized steel strip which, by mass percentage, consists of the following elements: 0.13 - 0.19% C, 1.70 - 2.50% Mn, up to 0.15% Si, 0.40 - 1.00% Al, 0.05 - 0.25% Cr, 0.01 - 0.05% Nb, up to 0.10% P, up to 0.004% Ca, up to 0.05% S, up to 0.007% N, and optionally at least one of the following elements: up to 0.50% Ti, up to 0.40% V, up to 0.50% Mo, up to 0.50% Ni, up to 0.50% Cu, up to 0.005% B, with the balance being Fe and unavoidable impurities, where 0.40% < Al + SI < 1.05% and Mn + Cr > 1.90%. Here, the hot-dip galvanized steel strip has a microstructure containing 8 - 12% retained austenite and 10 - 20% martensite, with the balance being a mixture of ferrite and bainite, and the hot-dip galvanized steel strip contains 10% or less bainite. Here, the hot-dip galvanized steel strip has a minimum ultimate tensile strength Rm of 700 MPa, a minimum 0.2% proof stress Rp of 400 MPa, and a minimum total elongation of 18%. The steel of EP2768989 does not assume a steel having a strength of 780 MPa or more while predicting an elongation of more than 20%.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] The object of the present invention is an ultimate tensile strength of 780 MPa or more, preferably exceeding 800 MPa, a yield strength of 400 MPa or more, preferably exceeding 450 MPa, a total elongation of 18% or more, preferably exceeding 20%, a hole expansion rate of 20% or more, preferably exceeding 22% The objective of the present invention is to solve these problems by making available a cold rolled steel sheet which simultaneously has:

[0008] Preferably, such steels also have good suitability for forming, rolling, good weldability and coatability.

[0009] Another object of the invention is also to make available a manufacturing process for these steel sheets that is robust to changes in manufacturing parameters, while being compatible with conventional industrial applications. [Means for solving the problem]

[0010] The cold rolled and heat treated steel sheet of the present invention is coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve its corrosion resistance.

[0011] Carbon is present in the steel at 0.140% to 0.2%. Carbon is a necessary element for increasing the strength of the steel sheet by producing low-temperature transformation phases such as martensite and bainite, and carbon also plays a vital role in stabilizing austenite and is therefore a necessary element for ensuring retained austenite. Carbon therefore plays two vital roles, one in increasing strength and the other in retaining austenite and imparting ductility. However, a carbon content below 0.140% does not allow austenite to be stabilized in the appropriate amount required for the steel of the present invention. On the other hand, if the carbon content exceeds 0.2%, the steel exhibits poor spot weldability and is limited in its application to automotive parts. If carbon is present in excess of 0.2%, the presence of fresh martensite may be high, which is detrimental to ductility. The preferred range of carbon for the steel of the present invention is 0.140% to 0.19%, more preferably 0.140% to 0.18%.

[0012] The manganese content of the steel of the present invention is between 1.5% and 2.15%. This element is gammagenous. The purpose of adding manganese is to obtain a structure essentially containing austenite and to impart strength to the steel. It has been found that an amount of manganese of at least 1.5% by weight provides strength and hardenability of the steel plate, as well as stabilizing the austenite. Furthermore, a manganese content of more than 2.15% also reduces the ductility and also reduces the weldability of the steel of the present invention, so that the elongation target may not be achieved. The preferred content of the present invention may be maintained between 1.7% and 2.15%, even more preferably between 1.8% and 2.15%.

[0013] The silicon content of the steel of the present invention is 0.5%-0.8%. Silicon is an element that can delay the precipitation of carbides during annealing temperature and cooling after overaging, so the presence of silicon stabilizes the carbon-rich austenite at room temperature, but an unbalanced content of silicon will not produce the aforementioned effect and will bring problems such as temper embrittlement and liquid metal embrittlement. Therefore, the concentration is controlled within the upper limit of 0.8%. The preferred content of the present invention can be maintained at 0.5%-0.7%.

[0014] Aluminum is an essential element and is present in the steel of the present invention at 0.4% to 0.8%. Aluminum promotes ferrite formation and increases the Ms temperature, which allows the present invention to have the appropriate amount of both martensite and ferrite required to impart ductility and strength to the steel of the present invention. However, when aluminum is present at more than 0.8%, it increases the Ac3 temperature, making annealing and hot rolling finishing temperatures in the fully austenitic field economically unreasonable. The aluminum content is preferably limited to 0.4% to 0.7%, more preferably 0.4% to 0.6%.

[0015] In order to simultaneously ensure the amount of retained austenite and ferrite according to the invention and a total elongation of 18%, or even 20%, and a hole expansion ratio of 20%, the cumulative amount of silicon and aluminum is preferably 0.9% to 1.2%, more preferably 1% to 1.2%.

[0016] Phosphorus is not an essential element, but may be contained in the steel as an impurity, and from the standpoint of the present invention, the phosphorus content is as low as possible, preferably less than 0.09%. Phosphorus has a tendency to segregate, particularly at grain boundaries, or to co-segregate with manganese, thereby reducing spot weldability and hot ductility. For these reasons, its content is limited to less than 0.09%, preferably less than 0.03%, more preferably less than 0.014%.

[0017] Although sulfur is not an essential element, it may be contained in the steel as an impurity, and from the viewpoint of the present invention, it is preferable that the sulfur content is as low as possible, but from the viewpoint of production costs, it is 0.09% or less. Furthermore, if more sulfur is present in the steel, it will combine with manganese in particular to form sulfides, reducing its beneficial effect on the steel of the present invention.

[0018] Nitrogen is limited to 0.09% to avoid changes in the material over time and to minimize the precipitation of nitrides during solidification, which are detrimental to the mechanical properties of the steel.

[0019] Chromium is an optional element of the present invention. The chromium content is present in the steel of the present invention from 0% to 0.1%. Chromium provides strength and hardening to the steel, but if used in excess of 0.1%, it will damage the surface finish of the steel.

[0020] Nickel may be added as an optional element in amounts up to 3% to increase the strength of the steel and improve its toughness. A minimum of 0.01% is preferred to produce such an effect. However, at contents above 3%, nickel causes a decrease in ductility.

[0021] Niobium is an essential element of the present invention. The niobium content is present in the steel of the present invention between 0.01% and 0.1% and is added to the steel of the present invention to form carbonitrides to give the steel strength by precipitation hardening. Niobium also influences the size of the components of the microstructure by its precipitation as carbonitrides and by retarding recrystallization during heat treatment. In this way, a fine microstructure is formed at the end of the holding temperature and, as a consequence, after the completion of the annealing which gives the steel of the present invention the hole expansion ratio. However, a niobium content above 0.1% is not economically interesting since a saturation effect of its influence is observed, which means that additional amounts of niobium do not bring any strength improvement to the product.

[0022] Titanium is an essential element and can be added to the steel of the present invention in an amount of 0.01% to 0.1%. Like niobium, titanium plays a role in the hardening of the steel of the present invention, since it participates in the formation of carbonitrides. In addition, titanium also forms titanium nitrides that appear during solidification of the cast product. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides that are detrimental to formability. If the titanium content is less than 0.01%, it has no effect on the steel of the present invention.

[0023] The calcium content in the steel of the present invention is a maximum of 0.005%. Calcium is added to the steel of the present invention as an optional element, especially during inclusion treatment, with a preferred minimum amount of 0.0001%. Calcium contributes to the refining of the steel by suppressing the harmful sulfur components in spheroidal form, thereby retarding the harmful effects of sulfur.

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

[0025] Molybdenum is an optional element constituting a maximum of 0.5% of the steel of the present invention. It plays an effective role in determining hardenability and hardness, delaying the appearance of bainite and avoiding the precipitation of carbides in bainite. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so that its content is limited to 0.5% for economic reasons.

[0026] Vanadium is effective in increasing the strength of the steel by forming carbides or carbonitrides and is limited to an upper limit of 0.1% for economic reasons. Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following weight proportions: cerium ≦0.1%, boron ≦0.003%, magnesium ≦0.010% and zirconium ≦0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grains during solidification. The balance of the steel composition consists of iron and inevitable impurities resulting from processing.

[0027] Next, the microstructure of the steel sheet will be described.

[0028] Inter-critical ferrite constitutes 40% to 60% of the microstructure by area fraction of the steel of the invention. This inter-critical ferrite confers a hole expansion ratio of at least 20% to the steel of the invention. The inter-critical ferrite results from annealing at a temperature below Ac3. The inter-critical ferrite is different from the ferrite that may be produced after annealing, hereinafter referred to as "transformed ferrite", as described below. In particular, in contrast to transformed ferrite, the inter-critical ferrite is polygonal. Furthermore, the transformed ferrite is enriched in carbon and manganese, i.e. has a higher carbon and manganese content than that of the inter-critical ferrite. Thus, the inter-critical ferrite and the transformed ferrite can be distinguished by observing the micrographs with a FEG-TEM microscope using secondary electrons after etching with metabisulfite. In such micrographs, the inter-critical ferrite appears medium grey, whereas the transformed ferrite appears dark grey due to its higher carbon and manganese content.

[0029] The transformed ferrite constitutes 25% to 45% of the microstructure by area fraction for the steel of the present invention. The transformed ferrite of the present invention is composed of ferrite after annealing and bainitic ferrite formed during the soaking treatment for coating the steel. The transformed ferrite gives the steel of the present invention high strength and elongation. To ensure an elongation of 18%, preferably 20% or more, it is necessary to have 25% transformed ferrite. The transformed ferrite of the present invention is formed during the cooling performed after annealing and during the soaking treatment for coating the steel, the transformed ferrite of the steel of the present invention is rich in carbon and manganese compared to the intercritical ferrite. However, whenever the transformed ferrite content is present in the steel of the present invention more than 45%, it is not possible to have both tensile strength and total elongation at the same time. The preferred limit of the presence of ferrite of the present invention is 25% to 40%, more preferably 25% to 35%.

[0030] The retained austenite constitutes 8% to 20% by area fraction of the steel. The retained austenite of the steel according to the invention imparts improved ductility due to the TRIP effect. The preferred limit of the presence of austenite is 8% to 18%, more preferably 8% to 15%. The retained austenite of the invention preferably contains carbon in an amount of 0.8% to 1.1%, more preferably 0.9% to 1.1%, even more preferably 0.95% to 1.05%.

[0031] The cumulative amount of transformed ferrite and intercritical ferrite must be between 75% and 85%, which ensures that the steel of the present invention always has a total elongation greater than 18% and a hole expansion ratio of 20% at the same time.

[0032] Fresh martensite constitutes 5%-20% of the microstructure by area fraction. Fresh martensite is formed during cooling after cladding of cold rolled steel sheets. Martensite imparts strength to the steel of the present invention. However, if the presence of martensite exceeds 20%, it imparts excess strength but reduces elongation and hole expansion ratio beyond the allowable limits of the steel of the present invention. The preferred limit of martensite in the steel of the present invention is 5%-15%, more preferably 10%-15%.

[0033] Bainite constitutes 0%-10% of the microstructure by area fraction for the steel of the present invention, which may be any microstructure. Bainite may be a form of bainitic ferrite and / or carbide-free bainite. Bainite may be formed during overaging hold prior to coating. Bainite imparts strength to the steel of the present invention. To achieve tensile strengths of 780 MPa or more, it is preferred to have 2% bainite, but with more than 10% bainite the steel will not achieve the hole expansion ratio. The preferred limit for bainite is 2%-10%, and the more preferred limit is 5%-10%.

[0034] In addition to the above microstructures, the microstructure of the cold rolled and heat treated steel plate does not contain microstructural components such as pearlite, tempered martensite and cementite without compromising the mechanical properties of the steel plate.

[0035] The steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a casting of a semi-finished product of steel having a chemical composition according to the invention. The casting can be carried out in ingots or continuously in the form of thin slabs or thin strip, i.e. with thicknesses ranging from about 220 mm for slabs to several tens of millimeters for thin strip.

[0036] For example, slabs having the above-mentioned chemical composition are produced by continuous casting, and the slabs are optionally subjected to direct soft reduction during the continuous casting process to avoid central segregation and to maintain a local carbon to nominal carbon ratio of less than 1.10. The slabs provided by the continuous casting process can be used directly at high temperature after continuous casting, but can also be first cooled to room temperature and then reheated for hot rolling.

[0037] The temperature of the slab subjected to hot rolling must be at least 1000°C and less than 1280°C. If the temperature of the slab is lower than 1000°C, the rolling mill will be overloaded and the temperature of the steel may drop to the ferrite transformation temperature during the finish rolling, so that the steel is rolled with transformed ferrite contained in the structure. Therefore, the temperature of the slab is preferably high enough so that the hot rolling can be completed in the temperature range of Ac3+100°C to Ac3+200°C and the final rolling temperature remains above Ac3. Reheating at temperatures above 1280°C must be avoided because it is industrially expensive.

[0038] A final rolling temperature range of Ac3 to Ac3+200°C is necessary to have a favorable structure for recrystallization and rolling. The final rolling pass is preferably carried out at a temperature above 950°C, since below this temperature the steel sheet shows a significant decrease in rollability. The hot-rolled steel thus obtained is then cooled at a cooling rate of more than 30°C / s to the coiling temperature, which must be between 475°C and 650°C. Preferably, the cooling rate is equal to or lower than 200°C / s.

[0039] The hot rolled steel is then coiled at a coiling temperature of 475°C to 650°C to avoid ovalization, preferably 475°C to 625°C to avoid scale formation. A more preferred range for such coiling temperature is 500°C to 625°C. The coiled hot rolled steel is cooled to room temperature before being subjected to an optional hot zone anneal.

[0040] The hot rolled steel may be subjected to an optional descaling step to remove the scale formed during hot rolling prior to the optional hot zone anneal. The hot rolled sheet may then be subjected to an optional hot zone anneal for at least 12 hours to not more than 96 hours at a temperature of, for example, 400°C to 750°C, the temperature remaining below 750°C to avoid partial transformation of the hot rolled microstructure and thus loss of microstructural homogeneity. An optional descaling step of this hot rolled steel may then be carried out, for example by pickling such sheet.

[0041] This hot rolled steel is cold rolled to obtain a cold rolled steel sheet with a rolling reduction of 35 to 90%. Next, the cold rolled steel sheet obtained from the cold rolling process is annealed to impart a fine structure and mechanical properties to the steel of the present invention.

[0042] To anneal the cold rolled steel sheet, it is heated to a soaking temperature of Ac1-Ac3 with a heating rate of at least 3°C / s, and then annealing is carried out at that temperature for 5-500 seconds, preferably 50-250 seconds. In a preferred embodiment, the heating is at least 10°C / s, more preferably at least 15°C / s.

[0043] The annealing temperature is preferably Ac1+30°C to Ac3, and more preferably Ac1+30°C to Ac3-30°C.

[0044] In a preferred embodiment, the soaking time and temperature are selected to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 40% austenite, more preferably at least 50% austenite.

[0045] The cold rolled steel sheet is then cooled from the soaking temperature to an overaging hold temperature of 425°C to 500°C, preferably 440°C to 480°C, at an average cooling rate of at least 5°C / s, preferably at least 10°C / s.

[0046] Next, the cold-rolled steel sheet is held at the overaging temperature for 5 to 500 seconds.

[0047] To facilitate the hot-dip coating of the cold-rolled steel sheet, the cold-rolled steel sheet may then be brought to a coating bath temperature of 420°C to 680°C, depending on the nature of the coating.

[0048] The coated cold rolled steel sheet is then cooled from the coating bath temperature to room temperature at a cooling rate of at least 5°C / s, preferably at least 9°C / s, to ensure the formation of fresh martensite in the steel of the present invention.

[0049] After cooling to room temperature, a cold rolled and coated steel sheet is obtained. The cold rolled steel sheet can also be coated by any of the known industrial processes such as electrogalvanizing, JVD, PVD, etc., which may not require bringing the steel sheet to the above temperature range before coating. EXAMPLES

[0050] The following studies, examples, schematic illustrations and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0051] Steel sheets manufactured from steels with different compositions are summarized in Table 1, the steel sheets are manufactured according to the processing parameters specified in Table 2, respectively. Then, in Table 3, the microstructures of the steel sheets obtained during the tests are summarized, and in Table 4, the evaluation results of the obtained properties are summarized.

[0052] [Table 1]

[0053] Table 2 Table 2 summarizes the annealing treatment parameters carried out on the steels of table 1. Steel compositions A to D are suitable for the manufacture of steel sheets according to the invention. Table 2 also lists Ac1 and Ac3, which are defined for the steels of the invention and for which the reference steels are empirically determined by dilatometry studies.

[0054] The following processing parameters are the same for all steels in Table 1. All steels in Table 1 are heated to a temperature of 1120°C prior to hot rolling and the hot rolling finish temperature for all steels is 900°C. All steels in Table 1 are then coiled at 620°C.

[0055] Table 2 is as follows.

[0056] [Table 2]

[0057] Table 3 Table 3 illustrates the results of tests carried out according to standards with various microscopes, such as a scanning electron microscope, for determining the microstructure of both the steel of the invention and the reference steel.

[0058] The results are set forth herein.

[0059] [Table 3]

[0060] Table 4 Table 4 illustrates the mechanical properties of both the steels of the invention and the reference steels. Tensile tests are carried out according to ASTM standards to determine the tensile strength, yield strength and total elongation.

[0061] The results of various mechanical tests carried out in accordance with the standard are summarized.

[0062] [Table 4]

Claims

1. Cold rolled and coated steel sheet containing the following elements, expressed in weight percent: 0.140%≦Carbon≦0.2% 1.5%≦Manganese≦2.15% 0.5%≦Silicon≦0.8% 0.4%≦Aluminum≦0.8% 0%≦phosphorus≦0.09% 0%≦sulfur≦0.09% 0%≦Nitrogen≦0.09% 0.01%≦Niobium≦0.1% 0.01%≦Titanium≦0.1% and any one or more of the following elements: 0%≦Chromium≦0.1% 0%≦Nickel≦3% 0%≦Calcium≦0.005% 0%≦Copper≦2% 0%≦Molybdenum≦0.5% 0%≦Vanadium≦0.1% 0%≦Boron≦0.003% 0%≦Cerium≦0.1% 0%≦Magnesium≦0.010% 0%≦Zirconium≦0.010% and the balance being composed of iron and unavoidable impurities resulting from processing, the microstructure of the steel sheet comprising, by area fraction, 40-60% intercritical ferrite, 25-45% transformed ferrite, 8-20% retained austenite, 5-20% fresh martensite, and 0-10% bainite, and the cumulative amount of intercritical ferrite and transformed ferrite is 75-85%.

2. 2. The cold rolled and coated steel sheet of claim 1, wherein the composition comprises 0.5% to 0.7% silicon.

3. Cold rolled and coated steel sheet according to claim 1 or 2, the composition of which comprises 0.14% to 0.19% carbon.

4. Cold rolled and coated steel sheet according to any one of claims 1 to 3, the composition of which comprises 0.4% to 0.7% aluminium.

5. Cold rolled and coated steel sheet according to any one of claims 1 to 4, the composition of which comprises 1.7% to 2.15% manganese.

6. 5. The cold rolled and coated steel sheet of claim 4, wherein the composition comprises 0.4% to 0.6% aluminum.

7. 6. The cold rolled and coated steel sheet of claim 5, wherein the composition comprises 1.8% to 2.15% manganese.

8. 4. The cold rolled and coated steel sheet of claim 3, wherein the composition comprises 0.14% to 0.18% carbon.

9. The cold rolled and coated steel sheet according to any one of claims 1 to 8, wherein the cumulative amount of silicon and aluminum is between 0.9% and 1.2%.

10. The cold rolled and coated steel sheet according to any one of claims 1 to 9, wherein the cumulative amount of silicon and aluminum is between 1% and 1.2%.

11. Cold rolled and coated steel sheet according to any one of claims 1 to 10, wherein the carbon content of the retained austenite is between 0.8% and 1.1%.

12. Cold rolled and coated steel sheet according to claims 1 to 12, wherein the intercritical ferrite is between 45% and 55%.

13. Cold rolled and coated steel sheet according to any one of claims 1 to 12, wherein transformed ferrite is between 25% and 40%.

14. Cold rolled and coated steel sheet according to any one of claims 1 to 13, wherein the fresh martensite is between 5% and 15%.

15. The cold rolled and coated steel sheet according to any one of claims 1 to 14, wherein said steel sheet has an ultimate tensile strength of at least 780 MPa and a total elongation of at least 18%.

16. 16. The cold rolled and coated steel sheet of claim 15, wherein the steel sheet has a yield strength of 400 MPa or more and a hole expansion ratio of 20% or more.

17. A method for producing cold rolled and coated steel sheet, comprising the steps of: Providing a steel composition according to any one of claims 1 to 10; reheating the semi-finished product to a temperature of 1000°C to 1280°C; rolling the semi-finished product in a temperature range of Ac3+100°C to Ac3+200°C so that the hot rolling finishing temperature exceeds Ac3 to obtain hot rolled steel; Cooling the hot rolled steel at a cooling rate of at least 30°C / s to a coiling temperature of 475°C to 650°C and coiling the hot rolled steel; Cooling the hot rolled steel to room temperature; Optionally, subjecting the hot rolled steel sheet to a descaling treatment; Optionally, subjecting the hot rolled steel sheet to annealing at 400°C to 750°C; Optionally, subjecting the hot rolled steel sheet to a descaling treatment; cold rolling the hot-rolled steel sheet at a rolling reduction of 35 to 90% to obtain a cold-rolled steel sheet; A step of heating the cold-rolled steel sheet from room temperature to a soaking temperature of Ac1 to Ac3; Next, annealing at a soaking temperature for 5 to 500 seconds; Next, cooling the cold rolled steel sheet from the soaking temperature to an overaging temperature of 425°C to 500°C at an average cooling rate of at least 5°C / s; Next, the cold rolled steel sheet is overaged at an overaging temperature for 5 to 500 seconds to make the temperature range of 420°C to 680°C to facilitate coating; Next, coating the cold rolled sheet; Thereafter, cooling the cold rolled steel sheet to room temperature at a cooling rate of at least 55°C / s to obtain a cold rolled coated steel sheet; A method comprising:

18. The method of claim 17, wherein the coiling temperature is from 475°C to 625°C.

19. 19. The method according to claim 17 or 18, wherein the finish rolling temperature is greater than 950°C.

20. The method according to any one of claims 17 to 19, wherein the average cooling rate between the soaking temperature and the overaging temperature is greater than 10°C / sec.

21. A method according to any one of claims 17 to 20, wherein the soaking temperature is between Ac1 + 30°C and Ac3 - 30°C, such soaking temperature being selected to ensure the presence of at least 40% austenite at the end of the soaking treatment.

22. 22. The method of claim 21, wherein the annealing soak temperature is from Ac1+30°C to Ac3-30°C, the annealing soak temperature being selected to ensure the presence of at least 50% austenite at the end of the annealing.

23. The method according to any one of claims 17 to 22, wherein the temperature for overaging is between 440°C and 480°C.

24. A method according to any one of claims 17 to 23, wherein the cooling rate after coating is at least 9°C / s.

25. Use of a steel sheet according to any one of claims 1 to 16 or produced according to the method of claims 17 to 24 for the manufacture of structural or safety parts of a vehicle.

26. A vehicle comprising a part obtained according to claim 25.

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