Cold-rolled coated steel sheet and manufacturing method thereof

Abstract

To provide a high-strength and high-formability steel sheet and a manufacturing method thereof.SOLUTION: A cold-rolled coated steel sheet having, in mass percent, C: 0.30% to 0.45%, Mn: 1% to 2.5%, Si: 0.9% to 2.2%, Al: 0.09% or less, Nb: 0.001% to 0.09%, P: 0.02% or less, S: 0.03% or less, N: 0.09% or less, and optionally one or more of Mo: 0.5% or less, Cr: 0.6% or less, Ti: 0.06% or less, V: 0.1% or less, Ca: 0.005% or less, B: 0.010% or less, Mg: 0.05% or less, Zr: 0.05% or less, and Ce: 0.1% or less, with the balance consisting of iron and unavoidable impurities; the steel sheet having a microstructure, in area fraction, comprising 35% to 65% carbon-enriched martensite, 15% to 40% bainite, 14% to 30% retained austenite, 4% to 15% ferrite, and 0% to 10% fresh martensite, the remainder being carbon-enriched martensite.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a cold-rolled coated steel sheet suitable for use as a steel sheet for vehicles.

Background Art

[0002] Automobile parts are required to meet two conflicting needs, namely, ease of forming and strength. In recent years, however, a third requirement of improving fuel efficiency has been imposed on automobiles in consideration of the global environment. Therefore, automobile parts must now be made of materials having high formability so as to meet the criteria of ease of fitting in complex automobile assemblies, and at the same time, while reducing the weight of the vehicle to further improve fuel efficiency, the strength for impact resistance and durability of the vehicle must be improved, and steel parts must be weldable without suffering from liquid metal embrittlement.

[0003] Therefore, intensive research and development has been carried out to reduce the amount of materials used in automobiles by increasing the strength of the materials. On the contrary, since increasing the strength of the steel sheet reduces the formability, it is necessary to develop a material having both high strength and high formability.

[0004] Initial research and development in the field of high-strength and high-formability steel sheets has led to several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed here for the final evaluation of the present invention.

[0005] EP3412786 is a high-strength steel sheet having a specific composition, wherein the metallographic structure of the steel sheet includes polygonal ferrite, bainite, tempered martensite, and retained austenite. When the metallographic structure is observed with a scanning electron microscope, the metallographic structure satisfies 10 to 50 area% of polygonal ferrite, 10 to 50 area% of bainite, and 10 to 80 area% of tempered martensite with respect to the entire metallographic structure. When the metallographic structure is measured by X-ray diffraction, the metallographic structure satisfies 5.0 volume% or more of retained austenite, 3.5 volume% or more of retained austenite with a carbon concentration of 1.0 mass% or less, and 2.4 volume% or less of retained austenite with a carbon concentration of 0.8 mass% or less with respect to the entire metallographic structure. However, in EP3412786, a tensile strength of 1180 MPa and an overall elongation of 18% cannot be achieved simultaneously.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

[0007] Known prior art related to the production of high-strength high-formability steel sheets is caused by one or the other dropout. Therefore, a cold-rolled steel sheet having a strength exceeding 1150 MPa and a method for producing the same are required.

[0008] The object of the present invention is to solve these problems by making available a cold-rolled and heat-treated steel sheet having the following simultaneously. - An ultimate tensile strength of 1170 MPa or more, preferably exceeding 1180 MPa, or exceeding 1200 MPa , - A yield strength of 730 MPa or more, preferably exceeding 760 MPa%, - An overall elongation of 18% or more, preferably 19% or more.

[0009] In a preferred embodiment, the cold-rolled and heat-treated steel sheet exhibits a YS / TS ratio greater than 0.7.

[0010] Preferably, such steel can also have good compatibility with forming, particularly rolling, along with good weldability and coating ability.

[0011] Another object of the present invention is to make available a method for manufacturing these plates that is stable towards shifts in manufacturing parameters while being suitable for conventional industrial applications.

[0012] 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 in order to improve its corrosion resistance.

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention.

[0014] Carbon is present in the steel in an amount of 0.30% to 0.45%. Carbon is an element necessary for increasing the strength of the steel sheet by delaying the formation of ferrite and bainite during cooling after annealing. Further, carbon plays an extremely important role in austenite stabilization. At a content rate of less than 0.25%, austenite cannot be stabilized, and the strength and ductility decrease. On the other hand, when the carbon content rate exceeds 0.45%, the weld zone and the heat-affected zone are significantly hardened, and the mechanical properties of the weld zone are impaired. The preferable limit of carbon is 0.32% to 0.45%, and the more preferable limit is 0.35% to 0.42%.

[0015] The manganese content of the steel of the present invention is 1% to 2.5%. Manganese is an element that stabilizes austenite to obtain retained austenite and imparts strength. A manganese amount of at least 1% is necessary to provide the strength and hardenability of the steel sheet by delaying the formation of ferrite as well as stabilizing austenite. Therefore, a higher percentage of manganese, such as 1.2 to 2.5%, is preferred, more preferably 1.2% to 2.1%. However, when manganese exceeds 2.5%, this causes an adverse effect of delaying the transformation of austenite to bainite during isothermal holding for bainite transformation, leading to a decrease in ductility. Furthermore, when manganese exceeds 2.5%, sufficient bainite is not formed, the formation of martensite exceeds the target limit, and thus the elongation decreases. Also, a manganese content exceeding 2.5% causes center segregation and will also reduce the weldability of the present steel.

[0016] The silicon content of the steel of the present invention is 0.9% to 2.2%. Silicon as a constituent delays the precipitation of cementite in martensite. Furthermore, silicon delays the precipitation of carbon as cementite in bainite during soaking after cooling from high temperature. Thus, during the formation of carbide-free bainite, carbon is enriched in austenite, and therefore, due to the presence of 0.9% silicon, austenite is stabilized at room temperature. In any case, cementite in bainite or cementite in martensite is the cause of the decrease in elongation. Preventing cementite formation due to the presence of silicon is important, but adding more than 2.2% silicon does not improve the above effects, leading to problems such as hot rolling embrittlement. Similarly, silicon exceeding 2.2% in the steel of the present invention prevents Zn from dissolving into the crystal grains. Therefore, during welding, instead of entering the crystal grains, the liquid Zn progresses along the grain boundaries, causing liquid metal embrittlement. Therefore, its concentration is controlled within the upper limit of 2.2%. The preferred limit of silicon for the present steel is 1% to 2.1%, more preferably 1.2% to 2.1%.

[0017] The aluminum content of the steel of the present invention is 0 to 0.09%. Aluminum is added during steelmaking to deoxidize the steel and capture oxygen. When it exceeds 0.09%, the Ac3 point rises, thereby reducing productivity. Further, within such a range, aluminum combines with nitrogen in the steel to form aluminum nitride, reducing the size of the crystal grains, and aluminum also delays the precipitation of cementite. However, when the aluminum content in the present invention exceeds 0.09%, the amount and size of aluminum nitride have an adverse effect on reaming and bending, and also push it into a higher temperature range that is very costly industrially to reach Ac3, and also cause coarsening of the crystal grains during annealing soaking. The preferred limit of aluminum is 0% to 0.06%, more preferably 0% to 0.05%.

[0018] Niobium is present between 0.001% and 0.09%, preferably between 0.001% and 0.08%, more preferably between 0.01% and 0.07%. Niobium is suitable for forming carbonitrides and imparts strength to the steel according to the present invention by precipitation during the annealing soaking temperature range. As a result, after complete annealing, the microstructure is finer, resulting in hardening of the product. However, when the niobium content exceeds 0.09%, niobium consumes carbon by forming a large amount of carbonitrides, and a large amount of carbonitrides tend to reduce the ductility of the steel and also tend to consume carbon during the formation of carbonitrides, reducing the availability of carbon for austenite stabilization, which is not preferred in the present invention.

[0019] The phosphorus content of the steel of the present invention is limited to 0.02%. Phosphorus is an element that hardens in solid solution. Therefore, a small amount of phosphorus of at least 0.002% can be advantageous, but due to the tendency of segregation at grain boundaries or co-segregation with manganese in particular, phosphorus has adverse effects such as a decrease in spot weldability and hot ductility. For these reasons, its content is preferably limited to a maximum of 0.015%.

[0020] Sulfur is not an essential element but may be contained as an impurity in steel. The sulfur content is preferably as low as possible, but from the viewpoint of manufacturing cost, it is preferably 0.03% or less, and at most 0.005%. When more sulfur is present in the steel, sulfur forms sulfides with Mn and Ti, which particularly have an adverse effect on the bending, hole expansion and elongation of the steel of the present invention.

[0021] Nitrogen is limited to 0.09% in order to avoid the aging change of the material and to minimize the precipitation of nitrides during solidification, which has an adverse effect on the mechanical properties of the steel.

[0022] Molybdenum is an optional element present in the steel of the present invention in an amount of 0% to 0.5%. When molybdenum is added in an amount of at least 0.01%, it plays an effective role in improving hardenability and hardness, and delays the formation of ferrite and bainite during cooling after annealing. Also, Mo is beneficial to the toughness of hot-rolled products and facilitates manufacturing. However, the addition of molybdenum excessively increases the addition cost of alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit of molybdenum is 0% to 0.4%, more preferably 0% to 0.3%.

[0023] Chromium is an optional element of the steel of the present invention and is 0% to 0.6%. Chromium imparts strength and hardening to the steel, but when used in excess of 0.5%, it impairs the surface finish of the steel. The preferred limit of chromium is 0.01% to 0.5%, more preferably 0.01% to 0.2%.

[0024] Titanium is an optional element and can be added to the steel of the present invention in an amount of 0% to 0.06%, preferably 0.001% to 0.03%. Similar to niobium, titanium plays a role in hardening because it is involved in carbonitrides. However, titanium is also involved in forming TiN that appears during the solidification of cast products. The amount of Ti is limited to 0.06% to avoid coarse TiN that has an adverse effect on hole expansion. When the titanium content is less than 0.001%, titanium has no effect on the steel of the present invention.

[0025] Vanadium is an optional element and can be added to the steel of the present invention in an amount of 0% to 0.1%, preferably 0.001% to 0.1%. Similar to niobium, vanadium plays a role in hardening because it is involved in carbonitrides. However, vanadium is also involved in forming VN that appears during the solidification of cast products. The amount of V is limited to 0.1% to avoid coarse VN that has an adverse effect on hole expansion. When the vanadium content is less than 0.001%, vanadium has no effect on the steel of the present invention.

[0026] Calcium is an optional element and can be added to the steel of the present invention in an amount of 0% to 0.005%, preferably 0.001% to 0.005%. Calcium is added to the steel of the present invention as an optional element, particularly during inclusion treatment. Calcium contributes to steelmaking by capturing sulfur-containing substances that have an adverse effect during spheroidization.

[0027] Boron is an optional element and can be added in an amount of 0 to 0.010%, preferably 0.001 to 0.004% to harden the steel.

[0028] Other elements such as cerium, magnesium, or zirconium can be added individually or in combination at the following ratios, i.e., Ce ≤ 0.1%, Mg ≤ 0.05%, and Zr ≤ 0.05%. Up to the indicated maximum content level, these elements enable the refinement of inclusion particles during solidification.

[0029] The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

[0030] The microstructure of the steel sheet according to the present invention includes, in terms of area fraction, 35% to 65% carbon-enriched martensite, 5% to 35% bainite, 14% to 30% retained austenite, 4% to 15% ferrite, and 0% to 10% fresh martensite.

[0031] The surface fraction of the phases in the microstructure is determined by the following method. A test piece is cut from a steel plate, polished, etched with a reagent of known composition to expose the microstructure. This cross-section is then examined by a scanning electron microscope, for example, a scanning electron microscope with a field emission electron gun (a "FEG-SEM") at a magnification greater than 5000 times in the secondary electron mode.

[0032] The measurement of the ferrite fraction is carried out by SEM observation after etching with Nital or Picral / Nital reagent.

[0033] The measurement of the retained austenite is carried out by XRD, and for the carbon-enriched martensite, the dilatometry test was carried out according to S.M.C. Van Bohemen and J. Sietsma in the publication of Metallurgical and materials transactions, Volume 40A, May 2009 - 1059.

[0034] The carbon-enriched martensite is the parent phase of the steel and is contained in an amount of 35% to 65% to achieve a strength level of 1170 MPa or more. If the amount of carbon-enriched martensite exceeds 65%, it will have an adverse effect on ductility. The carbon-enriched martensite of this steel can be in the form of laths with a lath thickness thicker than 0.1 micron. The martensite formed during cooling after annealing transforms into carbon-enriched martensite during heating to the overaging temperature. The preferred presence of carbon-enriched martensite in the steel of the present invention is 35% to 63%, more preferably 35% to 60%.

[0035] Bainite is contained in an amount of from 15% to 40%. In the framework of the present invention, the bainite may include carbide-free bainite. The carbide-free bainite is 100μm 2It is bainite that has a very low density of carbides, less than 100 carbides per unit area, and may contain island-like austenite in some cases. When carbide-free bainite exists in the form of lath bainite, the thickness of the laths is 1 to 5 microns. Bainite provides improved elongation. The preferred presence of bainite is 15% to 35%, more preferably 18% to 35%.

[0036] Retained austenite is contained in an amount of 14% to 30%, imparting ductility to the steel. In the framework of the present invention, the retained austenite of the present invention preferably contains more than 0.8% carbon, and more preferably the carbon content exceeds 0.9%. The austenite range can impart mechanical properties such as formability and elongation. Furthermore, austenite also imparts ductility to the steel. The preferred range of retained austenite is 14% to 28%, more preferably 14% to 26%.

[0037] Ferrite constitutes 4% to 15% of the microstructure by area fraction with respect to the steel of the present invention. Ferrite imparts strength to the steel of the present invention as well as elongation. The ferrite of this steel can include polygonal ferrite, lath ferrite, acicular ferrite, plate-like ferrite or epitaxial ferrite. In order to ensure an elongation of 18% or more, preferably 20% or more, it is necessary to have 4% ferrite. The ferrite of the present invention is formed during annealing and during cooling after annealing. However, whenever the ferrite content exceeds 15% in the steel of the present invention, due to the fact that ferrite increases the hardness gap with hard phases such as carbon-enriched martensite and bainite, reduces local ductility, and results in a decrease in total elongation and yield strength, it is not possible to have both total elongation and yield strength simultaneously. The preferred limit for the presence of ferrite in the present invention is 5% to 15%, more preferably 6% to 14%.

[0038] Fresh martensite constitutes 0% to 10% of the microstructure in terms of area fraction. The present invention can form fresh martensite by cooling after overaging retention and also during the cooling after coating of the cold-rolled steel sheet. Fresh martensite imparts strength to the steel of the present invention. However, when the presence of fresh martensite exceeds 10%, it imparts excessive strength, but since fresh martensite has the same carbon content as retained austenite, fresh martensite is brittle and hard, thus reducing elongation beyond the allowable limit for the steel of the present invention. The preferable limit of martensite for the steel of the present invention is 0% to 8%, more preferably 0% to 5%.

[0039] The cold-rolled coated steel sheet according to the present invention can be manufactured by any suitable method. The preferable method consists of providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be performed on an ingot or continuously in the form of a thin slab or a thin strip, that is, the thickness ranges from about 220 mm in the case of a slab to several tens of mm in the case of a thin strip.

[0040] For example, the slab is regarded as a semi-finished product. The slab having the above chemical composition is manufactured by continuous casting, where the slab is preferably directly subjected to soft reduction during casting to ensure removal of central segregation and reduction of porosity. The slab provided by the continuous casting process can be directly used at a high temperature after continuous casting or can first be cooled to room temperature and then reheated for hot rolling.

[0041] The temperature of the slab to be hot-rolled should preferably be at least 1000 °C, preferably exceeding 1150 °C and less than 1300 °C. If the temperature of the slab is lower than 1150 °C, an excessive load will be applied to the rolling mill, and during finish rolling, the temperature of the steel may drop to the ferrite transformation temperature, whereby the steel will be rolled with transformed ferrite in the structure. It is preferable to keep the temperature of the slab above 1150 °C to retain all the fine alloying elements, especially niobium, in solid solution. Further, since it is industrially costly, the temperature should not exceed 1300 °C.

[0042] The temperature of the slab should preferably be high enough to complete hot rolling completely within the austenite range, and the finish hot rolling temperature should remain above 850 °C. Below this temperature, the steel sheet shows a significant decrease in rollability, so finish rolling needs to be carried out above 850 °C.

[0043] Next, the sheet obtained by this method is cooled to a temperature of 650 °C or lower at a cooling rate exceeding 3 °C / second. Preferably, the cooling rate is not more than 65 °C / second and exceeds 10 °C / second. Then, the hot-rolled steel sheet is coiled at a coiling temperature below 650 °C, preferably below 600 °C, more preferably below 575 °C. Then, the coiled hot-rolled steel sheet is cooled, preferably to room temperature. Next, the hot-rolled sheet is subjected to an optional scale removal process such as pickling to remove the scale and intergranular oxidation formed during hot rolling, ensuring that there is no scale on the surface of the hot-rolled steel sheet before being subjected to any hot band annealing.

[0044] The hot-rolled sheet can be optionally subjected to hot band annealing at a temperature of 350 °C to 750 °C for 1 to 96 hours. The temperature and time of such hot band annealing are selected to ensure softening of the hot-rolled steel sheet to facilitate cold rolling of the hot-rolled steel sheet. Next, the hot-rolled sheet can be subjected to an optional scale removal process such as pickling to remove the scale and intergranular oxidation formed during hot band annealing.

[0045] Next, the hot-rolled steel sheet is cooled to room temperature, and then the hot-rolled sheet is cold-rolled at a thickness reduction rate of 35 to 70% to obtain a cold-rolled steel sheet.

[0046] Next, the cold-rolled steel sheet is subjected to annealing to impart the target microstructure and mechanical properties to the steel of the present invention.

[0047] In annealing, the cold-rolled steel sheet is heated, and the cold-rolled steel sheet is heated at a heating rate HR1 of 2 °C / sec to 70 °C / sec until it reaches a soaking temperature TA of Ac3 - 10 °C to Ac3 - 50 °C from room temperature. The HR1 rate is 5 °C / sec to 60 °C / sec, more preferably 10 °C / sec to 50 °C / sec. The preferred TA temperature is 760 °C to 840 °C.

[0048] The Ac3 temperature is calculated from the measurement of the expansion rate.

[0049] Next, the cold-rolled steel sheet is held at the annealing soaking temperature TA for 10 to 1000 seconds to ensure sufficient transformation to form at least 90% austenite at the end of soaking. Then, the cold-rolled steel sheet is cooled at an average cooling rate CR1 between 1 °C / sec and 1000 °C / sec, preferably between 8 °C / sec and 900 °C / sec, more preferably between 8 °C / sec and 100 °C / sec, to a cooling stop temperature range CS1 of Ms - 40 °C to Ms - 130 °C, preferably 190 °C to 250 °C, more preferably 185 °C to 240 °C. The steel is held at CS1 for 1 second to 200 seconds. During this cooling step, the martensite of the present invention is formed. If the CS1 temperature exceeds Ms - 40 °C, the steel of the present invention has too much austenite, which has an adverse effect on its stability and thus the total elongation. If CS1 is less than Ms - 130 °C, the amount of retained austenite is too small to achieve the target of total elongation.

[0050] In subsequent steps, the cold-rolled steel sheet is heated at a heating rate HR3 of 1°C / second to 100°C / second from the CS1 temperature to the overaging temperature range TOA of 350°C to 450°C. A preferred TOA temperature is 360°C to 440°C. During heating to the TOA temperature and during holding at the TOA temperature, the martensite formed during annealing after cooling is transformed into carbon-enriched martensite by rejecting the carbon consumed by austenite for its stabilization as retained austenite at room temperature. A still somewhat amount of carbon from the martensite remains in the carbon-enriched martensite, and this carbon is present in the carbon-enriched martensite in the form of precipitates. At the same time, the unstable austenite is also transformed into bainite without cementite, and the bainite rejects carbon due to the presence of silicon, thereby also contributing to the stabilization of the retained austenite. Thereafter, the cold-rolled steel sheet is held at the TOA temperature for 5 to 500 seconds for overaging.

[0051] Next, the cold-rolled steel sheet is brought to the temperature of a molten plating bath, which can be 420°C to 680°C, depending on the properties of the coating. This coating can be made of zinc or a zinc-based alloy, or aluminum or an aluminum-based alloy.

[0052] To ensure degassing of the coated product, an optional post-batch annealing can be carried out, preferably at 170 to 210°C for 12 to 30 hours, after annealing of the coated product. Then it is cooled to room temperature to obtain a cold-rolled coated steel sheet.

Example

[0053] The following tests and examples presented in this specification are not essentially limiting and must only be considered for illustrative purposes, showing the advantageous features of the present invention, clarifying the significance of the parameters selected by the inventor after extensive experimentation, and further establishing the properties achievable by the steel according to the present invention.

[0054] Using the compositions summarized in Table 1 and the processing parameters summarized in Table 2, samples of steel plates according to the present invention and samples of steel plates of several comparative grades were produced. The corresponding microstructures of these steel plates were summarized in Table 3, and the properties were summarized in Table 4.

[0055] Table 1 shows steels with compositions expressed in weight percentages, and for each steel, Ac3 is shown, and the Ac3 temperature is calculated from the dilatometry measurement.

[0056]

Table 1

[0057] Table 2 summarizes the annealing process parameters applied to the steels in Table 1.

[0058] Also, Table 2 shows the bainite transformation Bs temperature and the martensite transformation Ms temperature of the inventive steel and the reference steel. The calculation of Bs was performed using the Van Bohemen equation published in Materials Science and Technology (2012), Volume 28, Issue 4, pages 487 - 495, which is as follows. Bs = 839 - (86*[Mn] + 23*[Si] + 67*[Cr] + 33*[Ni] + 75*[Mo]) - 270*(1 - EXP(-1.33*[C]))

[0059] Ms was determined by a dilatometry test carried out according to S.M.C Van Bohemen and J.Siestma in the publication of Metallurguical and Materials Transaction, Volume 40A, May 2009 (pages 1059 - 1068).

[0060] Furthermore, before annealing the steel of the present invention in the same manner as the reference, the sample was heated to a temperature of 1150°C to 1300°C and then subjected to hot rolling at a finishing temperature exceeding 850°C. The cooling rate after hot rolling exceeded 30°C / second until it cooled below 650°C. The steel for all trials (trail) was pickled before cold rolling, and the cold rolling reduction rate for all trials was a 50% reduction. The average heating rate of HR1 was 15°C / second for all trials. All cold-rolled steel sheets were coated in a zinc bath at a temperature of 460°C after overaging retention.

[0061]

Table 2

[0062] Table 3 summarizes the results of tests conducted according to the criteria of different microscopes such as a scanning electron microscope to determine the microstructure composition of both the steel of the invention and the reference trials.

[0063]

Table 3

[0064] From the above table, it can be seen that all the trials according to the present invention meet the microstructure targets.

[0065] Table 4 summarizes the mechanical properties and surface properties of both the steel of the invention and the reference steel.

[0066] <Table 4: Mechanical Properties of Tests> According to the ISO standard ISO6892-1 issued in October 2009, the yield strength YS, tensile strength TS, and total elongation TE are measured.

[0067]

Table 4

Claims

1. 1. A cold rolled coated steel sheet, the steel comprising, by weight percentage, the following: 0.30%≦carbon≦0.45%; 1%≦Manganese≦2.5%, 0.9%≦silicon≦2.2%; 0%≦aluminum≦0.09%, 0.001%≦niobium≦0.09%, 0%≦phosphorus≦0.02%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.09%, and optionally the following elements: 0%≦molybdenum≦0.5%, 0%≦chromium≦0.6%, 0%≦titanium≦0.06%, 0%≦vanadium≦0.1%, 0%≦Calcium≦0.005%, 0%≦Boron≦0.010%; 0%≦Magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦Cerium≦0.1% and the remainder consisting of iron and unavoidable impurities, wherein the steel sheet has a microstructure consisting of, by area fraction, 35% to 65% carbon-enriched martensite, 15% to 40% bainite, 14% to 30% retained austenite, 4% to 15% ferrite, and 0% to 10% fresh martensite.

2. 2. The cold rolled coated steel sheet of claim 1, wherein the composition comprises 1.2% to 2.5% manganese.

3. 3. The cold rolled coated steel sheet according to claim 1 or 2, wherein the composition comprises 0.32% to 0.45% carbon.

4. 4. Cold rolled coated steel sheet according to any one of claims 1 to 3, wherein the composition comprises 1% to 2.1% silicon.

5. 5. Cold rolled coated steel sheet according to any one of claims 1 to 4, wherein the composition comprises 0.001% to 0.08% niobium.

6. The cold rolled coated steel sheet according to any one of claims 1 to 5, wherein the microstructure comprises 35 to 63% carbon enriched martensite.

7. The cold rolled coated steel sheet according to any one of claims 1 to 6, wherein the microstructure comprises 14% to 28% retained austenite.

8. The cold rolled coated steel sheet according to any one of claims 1 to 7, wherein the microstructure comprises 15 to 35% bainite.

9. The cold rolled coated steel sheet according to any one of claims 1 to 8, having a tensile strength of 1170 MPa or more and a total elongation of 18% or more.

10. The cold-rolled coated steel sheet according to any one of claims 1 to 9, having a yield strength of 730 MPa or more.

11. 1. A method for producing a cold rolled coated steel sheet comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 5 and obtaining a semi-finished product, - reheating the semi-finished product to a temperature above 1000°C, - rolling said semi-finished product completely in the austenitic range, with a hot rolling finishing temperature of 850°C or higher, to obtain a hot rolled steel sheet; - cooling the sheet at a cooling rate of more than 3°C / s to a temperature of not more than 650°C and coiling the hot-rolled sheet at a coiling temperature of less than 650°C, - cooling the hot-rolled sheet; - subjecting said hot rolled steel plate to an optional descaling process, - subjecting said hot-rolled steel sheet to an optional annealing at a temperature between 350 and 750°C for a period of between 1 hour and 96 hours; - subjecting said hot rolled annealed steel sheet to an optional descaling process; - cold rolling the hot-rolled steel sheet at a rolling reduction of 35% to 70% to obtain a cold-rolled steel sheet; - annealing the cold-rolled steel sheet by heating it from room temperature to a soaking temperature TA of Ac3-10°C to Ac3-50°C at a heating rate of 2°C / s to 70°C / s; then carrying out an annealing at TA for 10 to 1000 seconds, the time being chosen so as to obtain a minimum proportion of austenite of 90% at the end of the soak; - then cooling the cold-rolled steel sheet from TA to a cooling stop temperature CS1 of Ms-40°C to Ms-130°C at a cooling rate CR1 of 1°C / s to 1000°C / s and holding the cold-rolled steel sheet at CS1 for 1 to 200 seconds, - then heating said cold-rolled steel sheet from the CS1 temperature to an over-aging temperature TOA of 350°C to 450°C at an average heating rate HR3 of 1°C / s to 100°C / s, - then overaging the cold rolled steel sheet at TOA for 5 to 500 seconds; - then bringing said cold rolled steel sheet to a coating temperature between 420 and 680°C and coating said cold rolled steel sheet; - thereafter cooling the coated cold rolled steel sheet to room temperature to obtain a cold rolled coated steel sheet; The steel plate has a microstructure consisting of, by area fraction, 35% to 65% carbon-enriched martensite, 15% to 40% bainite, 14% to 30% retained austenite, 4% to 15% ferrite, and 0% to 10% fresh martensite.

12. The method of claim 11, wherein the TA temperature is between 760°C and 840°C.

13. 13. The method of claim 11 or 12, wherein the CS1 temperature is between 190°C and 250°C.

14. The method according to any one of claims 11 to 13, wherein the TOA temperature is between 360°C and 440°C.

15. Use of a steel sheet according to any one of claims 1 to 10 or produced by the method according to any one of claims 11 to 14 for the production of structural or safety parts of a vehicle.