Cold-rolled coated steel sheet and its manufacturing method

Abstract

Cold rolled coated steel sheet, the steel having the following composition: 0.15%≦carbon≦0.25%, 1.5%≦manganese≦2.5%, 1%≦silicon≦2%, 0%≦aluminum≦0.09%, 0.1%≦chromium≦0.6%, 0%≦phosphorus≦0.02%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.09%, 0%≦molybdenum≦0.5%, 0.001%≦niobium≦0.09%, 0%≦titanium≦0.06%, 0%≦vanadium≦0.1%, 0%≦nickel≦1%, 0%≦copper≦1%, 0% ≦calcium≦0.005%, 0%≦boron≦0.010%, 0%≦magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦cerium≦0.1%, and the remainder including iron and unavoidable impurities, the steel plate having a microstructure including, by area ratio, a cumulative abundance ratio of bainite and distributed martensite of 35% to 70%, 9% to 15% retained austenite, 12% to 38% ferrite, and 5% to 15% fresh martensite.

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 technology]

[0002] Automotive parts must satisfy two contradictory requirements: ease of formability and strength. Recently, environmental concerns have placed a third demand on automobiles: improved fuel economy. Therefore, automotive parts must now be made from materials with high formability to meet the criteria for ease of incorporation into complex automotive assemblies, while simultaneously improving strength for vehicle crashworthiness and durability while reducing vehicle weight for improved fuel economy. Furthermore, steel parts must be weldable without the drawbacks of liquid metal embrittlement.

[0003] Therefore, vigorous research and development efforts are being made to reduce the amount of material used inside the vehicle by increasing the strength of the material. Conversely, increasing the strength of steel sheet reduces its formability, so there is a need to develop 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 final understanding of the present invention.

[0005] EP3187608 provides a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 1,300 MPa or more and excellent ductility and in-plane uniformity of material properties, and also provides a method for manufacturing the steel sheet. The high-strength hot-dip galvanized steel sheet has a specific composition of C, Si, Mn, etc. In this chemical composition, the Ti content [Ti] and the N content [N] satisfy the relationship [Ti]>4[N]. The high-strength hot-dip galvanized steel sheet has a microstructure containing an area fraction of 60% to 90% martensite, an area fraction of more than 5% to 40% polygonal ferrite, and an area fraction of less than 3% (including 0%) retained austenite. The average Vickers hardness of the martensite is 450 to 600, and the average grain size of the martensite is 10 μm or less. The standard deviation of the grain size of the martensite is 4.0 μm or less. EP3187608 can provide a tensile strength of over 980 MPa, but does not have an elongation of more than 14%.

[0006] EP3473741 discloses a steel plate having a tensile strength of 950 MPa or more and good toughness, and a manufacturing method thereof. The steel plate has a specific composition and metal structure in which the area fraction of ferrite is 30% or less (including 0%), the area fraction of tempered martensite is 70% or more (including 100%), the area fraction of retained austenite is 4.5% or less (including 0%), and the average aspect ratio of iron-based carbides precipitated within the tempered martensite grains and having a maximum particle size of 10% is 3.5 or more. However, the steel of EP3473741 cannot provide an ultimate tensile strength of 950 MPa or more.

[0007] Known prior art related to the production of high strength, highly formable steel plates suffers from one or other deficiency, and therefore, there is a need for cold rolled steel plates having strengths in excess of 1000 MPa and methods for producing the same. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent Application Publication No. 3187608 [Patent Document 2] European Patent Application Publication No. 3473741 Summary of the Invention

[0009] The object of the present invention is to solve these problems by making available a cold rolled coated steel sheet which at the same time has: an ultimate tensile strength of 1000 MPa to 1180 MPa, preferably 1050 MPa to 1180 MPa, - a yield strength of 700 MPa to 850 MPa, preferably 700 MPa to 840 MPa, - a hole expansion rate of at least 20%, preferably at least 25%, - Total elongation of at least 14%, preferably at least 15%.

[0010] In a preferred embodiment, the cold rolled coated steel sheet exhibits a YS / TS ratio greater than 0.6.

[0011] Preferably, such steels also have good weldability and coatability, as well as good suitability for forming, especially rolling.

[0012] Another object of the invention is also to make available a method for manufacturing these plates that is robust to shifts in manufacturing parameters, while being compatible with conventional industrial applications.

[0013] The cold-rolled coated steel sheet of the present invention is coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve corrosion resistance.

[0014] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0015] Carbon is present in steel at 0.15% to 0.25%. Carbon is an essential element for increasing the strength of steel sheets by delaying the formation of ferrite and promoting the formation of bainite during cooling after annealing. Carbon also plays a central role in the formation of partitioned martensite. If the carbon content is less than 0.15%, adequate amounts of bainite and partitioned martensite cannot be formed, resulting in reduced strength and ductility. On the other hand, if the carbon content exceeds 0.25%, the weld and heat-affected zones become significantly hardened, impairing the mechanical properties of the weld. The preferred carbon limit is 0.17% to 0.23%, and the more preferred limit is 0.18% to 0.21%.

[0016] The manganese content of the steel of the present invention is 1.5% to 2.5%. Manganese is an element that stabilizes austenite, resulting in retained austenite and imparts strength. At least 1.5% manganese is necessary to retard ferrite formation, thereby imparting strength and hardenability to the steel sheet, and to stabilize austenite. Therefore, a higher manganese content, such as 1.7% to 2.3%, and more preferably 2% to 2.3%, is preferred. However, manganese content exceeding 2.5% has adverse effects, such as slowing the austenite-to-bainite transformation during isothermal bainite transformation, resulting in reduced ductility. Furthermore, manganese content exceeding 2.5% results in insufficient bainite formation, and the formation of fresh martensite exceeds the target limit, resulting in reduced elongation. Furthermore, a manganese content exceeding 2.5% causes center segregation and reduces the weldability of the steel.

[0017] The silicon content of the steel of the present invention is 1% to 2%. Silicon as a constituent retards cementite precipitation in martensite. Furthermore, silicon retards carbon precipitation as cementite in bainite during soaking after cooling from high temperatures. Therefore, during the formation of carbide-free bainite, austenite is enriched with carbon, and the presence of 0.9% silicon stabilizes austenite at room temperature. In both cases, cementite in bainite or martensite also contributes to reduced elongation. While the prevention of cementite formation by the presence of silicon is important, adding more than 2% silicon does not improve the above effect and can cause problems such as hot-rolling embrittlement. In the steel of the present invention, silicon exceeding 2% prevents Zn from dissolving in the grains. Therefore, during welding, liquid Zn moves along the grain boundaries instead of entering the grains, causing liquid metal embrittlement. Therefore, the silicon concentration is controlled to an upper limit of 2%. The preferred silicon limit for the steel is 1.1% to 1.9%, more preferably 1.3% to 1.7%.

[0018] 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. Above 0.09%, the Ac3 point increases, resulting in reduced productivity. Furthermore, within this range, aluminum combines with nitrogen in the steel to form aluminum nitride, reducing grain size. Aluminum also retards cementite precipitation. However, in the present invention, if the aluminum content exceeds 0.09%, the amount and size of aluminum nitride adversely affect hole expansion and bending, pushes Ac3 into a higher temperature range that is industrially prohibitively expensive to reach, and causes grain coarsening during annealing and soaking. The preferred limit for aluminum is 0 to 0.06%, more preferably 0 to 0.05%.

[0019] Chromium is an essential element of the steel of the present invention and is present in an amount of 0.1% to 0.6%. Chromium provides strength and hardening to the steel, but if used in amounts exceeding 0.6%, it will impair the surface finish of the steel. The preferred limit for chromium is 0.1% to 0.5%, more preferably 0.1% to 0.4%.

[0020] The phosphorus content of the steel of the present invention is limited to 0.02%. Phosphorus is a hardening element in solid solution. Therefore, small amounts of at least 0.002% phosphorus can be advantageous, but phosphorus also has its negative effects, such as reducing spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.015%.

[0021] Sulfur is not an essential element, but may be present in the steel as an impurity. The lower the sulfur content, the better. From the viewpoint of production costs, the sulfur content is 0.03% or less, preferably 0.005% at most. Furthermore, if more sulfur is present in the steel, it will combine with Mn and Ti to form sulfides, which have a negative effect on the bending, hole expansion, and elongation of the steel of the present invention.

[0022] Nitrogen is limited to 0.09% to avoid ageing of the material and to minimize the precipitation of nitrides during solidification, which is detrimental to the mechanical properties of the steel.

[0023] Niobium is an optional element that can be added to the steel of the present invention in an amount of 0.001% to 0.09%, preferably 0.001% to 0.08%, and more preferably 0.01% to 0.07%. Niobium is suitable for forming carbonitrides, imparting strength to the steel of the present invention by precipitation hardening during the annealing soak temperature range, resulting in a finer product after full annealing. However, niobium contents greater than 0.09% are not preferred for the present invention because niobium consumes carbon by forming large amounts of carbonitrides, which tends to reduce the ductility of the steel and consumes carbon during carbonitride formation, reducing the availability of carbon for austenite stabilization.

[0024] Titanium is an optional element that may be added to the steel of the present invention in a range of 0% to 0.06%, preferably 0.001% to 0.03%. Like niobium, titanium plays a role in hardening by participating in carbonitrides. However, titanium also participates in the formation of TiN, which appears during solidification of cast products. Therefore, the amount of Ti is limited to 0.06% to avoid coarse TiN, which is detrimental to hole expansion. If the titanium content is less than 0.001%, titanium does not have any effect on the steel of the present invention.

[0025] Vanadium is an optional element that may be added to the steel of the present invention in an amount of 0% to 0.1%, preferably 0.001% to 0.1%. Like niobium, vanadium participates in carbonitrides and therefore plays a role in hardening. However, it also participates in the formation of VN, which appears during solidification of cast products. Therefore, the amount of V is limited to 0.1% to avoid coarse VN, which is detrimental to hole expansion. If the vanadium content is less than 0.001%, vanadium does not provide any benefit to the steel of the present invention.

[0026] Molybdenum is an optional element present in the steel of the present invention in an amount of 0% to 0.5%. Molybdenum plays an effective role in improving hardenability and hardness, and an addition of at least 0.01% retards the formation of ferrite and bainite during cooling after annealing. Mo is also beneficial to the toughness of hot-rolled products, making them easier to manufacture. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit for molybdenum is 0% to 0.4%, more preferably 0% to 0.3%.

[0027] Nickel can be added as an optional element in amounts between 0% and 1% to increase the strength and toughness of steel. A minimum of 0.01% is needed to produce this effect. However, above 1%, nickel reduces ductility.

[0028] Copper can be added as an optional element in amounts of 0% to 1% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to produce such an effect. However, above 1%, copper causes a decrease in hot ductility during hot rolling.

[0029] Calcium is an optional element that may 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 as an optional element to the steel of the present invention, particularly during inclusion treatment. Calcium contributes to the refining of the steel by inhibiting sulfur content, which has a negative effect on the spheroidization of the steel.

[0030] Boron is an optional element that can be added in the range of 0 to 0.010%, preferably 0.001% to 0.004%, to harden the steel.

[0031] Other elements such as cerium, magnesium or zirconium can be added individually or in combination in the following proportions: Ce≦0.1%, Mg≦0.05% and Zr≦0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the inclusion particles during solidification.

[0032] The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0033] The microstructure of the steel plate according to the present invention contains, by area ratio, 35% to 70% cumulatively present distributed martensite and bainite, 9% to 15% retained austenite, 12% to 38% ferrite, and 5% to 15% fresh martensite.

[0034] The surface fraction of the phases in the microstructure is determined by the following method: a sample is cut from the steel sheet, polished and etched with reagents known per se to reveal the microstructure, and the section is then examined in a scanning electron microscope, for example a field emission electron microscope ("FEG-SEM"), in secondary electron mode at a magnification of more than 5000 times.

[0035] The ferrite content is determined by SEM observation after etching with Nital or picral / Nital reagents.

[0036] The determination of retained austenite is made by sigmametry, and the determination of bainite and distributed martensite is made by image analysis.

[0037] The determination of the proportions of bainite, fresh martensite and partitioned martensite is carried out by analysis of images from the FEG-SEM.

[0038] Bainite and distributed martensite form the matrix of the steel and are present in amounts of 35% to 70% to achieve strength levels of 1000 MPa or higher. A cumulative amount of bainite and distributed martensite exceeding 70% will adversely affect ductility. The distributed martensite in the present steel may be in the form of laths with lath thicknesses greater than 0.1 microns. Martensite formed during cooling after annealing transforms into distributed martensite during heating to the overaging temperature. Within the scope of the present invention, bainite may include carbide-free bainite and / or lath bainite. When present, lath bainite is in the form of laths with a thickness of 1 micron to 5 microns. When present, carbide-free bainite has a very low density of carbides and is present in laths with a thickness of 100 μm. 2 The bainite has less than 100 carbides per area unit and optionally contains austenite islands. The bainite provides improved elongation. The preferred presence of bainite and distributed martensite for the steel of the present invention is 40% to 68%, more preferably 42% to 65%.

[0039] The retained austenite is present in an amount of 9-15% and confers ductility to the steel. In the framework of the present invention, the retained austenite preferably contains a carbon content of more than 0.8%, more preferably a carbon content of more than 0.9%. The austenite range makes it possible to impart mechanical properties such as formability and elongation. Furthermore, austenite also imparts ductility to the steel. The preferred range of retained austenite is 10-14%, more preferably 11-14%.

[0040] Ferrite constitutes 12% to 38% of the microstructure of the steel of the present invention by area fraction. Ferrite imparts strength and elongation to the steel of the present invention. The ferrite in the steel may include polygonal ferrite, lath ferrite, acicular ferrite, tabular ferrite, or epitaxial ferrite. To ensure elongation of 14%, preferably 15%, or greater, 12% ferrite is necessary. The ferrite of the present invention is formed during annealing and cooling after annealing. However, whenever the ferrite content exceeds 38% in the steel of the present invention, it is impossible to simultaneously achieve both yield strength and total elongation due to the fact that ferrite increases the hardness gap with hard phases such as distributed martensite and bainite, reducing local ductility and resulting in a decrease in total elongation and yield strength. The preferred limit of ferrite in the present invention is 14% to 36%, more preferably 15% to 35%.

[0041] Fresh martensite constitutes 5% to 15% of the microstructure by area. In the present invention, fresh martensite is formed by cooling after overaging, and may also be formed during cooling after coating of cold-rolled steel sheets. Fresh martensite imparts ductility and strength to the steel of the present invention. However, if the presence of fresh martensite exceeds 15%, the fresh martensite has the same carbon content as retained austenite and is therefore brittle and hard. While the fresh martensite imparts excess strength, it also reduces elongation beyond the allowable limit of the steel of the present invention. The preferred martensite limit for the steel of the present invention is 5% to 14%, more preferably 8% to 13%.

[0042] In addition to the above microstructure, the microstructure of the cold rolled steel sheet does not contain microstructural components such as pearlite and cementite.

[0043] The cold rolled coated steel sheet according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel having the 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.

[0044] For example, slabs are considered semi-finished products. Slabs having the above chemical composition are produced by continuous casting, and the slabs are preferably subjected to soft reduction directly during casting to ensure the elimination of center segregation and reduced porosity. Slabs provided by the continuous casting process can be used directly at high temperature after continuous casting, or they can be first cooled to room temperature and then reheated for hot rolling.

[0045] The temperature of the slab used for hot rolling should preferably be at least 1000°C, preferably above 1150°C, but below 1300°C. If the slab temperature is below 1150°C, excessive loads will be applied to the rolling mill, and furthermore, the temperature of the steel will drop to the ferrite transformation temperature during finish rolling, which may result in the steel being rolled with transformed ferrite in the structure. The slab temperature is preferably kept above 1150°C in order to keep all minor alloying elements, especially niobium, in solid solution. Furthermore, the temperature should not exceed 1300°C due to its industrially expensive nature.

[0046] The temperature of the slab is preferably high enough to allow hot rolling to be completed entirely in the austenitic field, with the finish hot rolling temperature remaining above 850° C. It is necessary that the final hot rolling temperature be at least 850° C., because below this temperature the steel sheet shows a significant decrease in rollability.

[0047] The sheet thus obtained is then cooled to a temperature of 650°C or less at a cooling rate of more than 3°C / s. Preferably, the cooling rate is 65°C / s or less and more than 10°C / s. The hot-rolled steel sheet is then coiled at a coiling temperature of less than 650°C, preferably less than 600°C, more preferably less than 575°C. The coiled hot-rolled steel sheet is then preferably cooled to room temperature. The hot-rolled sheet may then be subjected to an optional descaling process, such as pickling, to remove scale formed during hot rolling and ensure that the surface of the hot-rolled steel sheet is free of scale before the hot-rolled steel sheet is subjected to an optional hot band annealing.

[0048] The hot-rolled sheet is 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 and to facilitate cold rolling of the hot-rolled steel sheet. The hot-rolled sheet may then be subjected to an optional descaling process, such as pickling, to remove scale formed during the hot band annealing.

[0049] Next, this hot-rolled steel sheet is cooled to room temperature and then cold-rolled with a thickness reduction of 35 to 70% to obtain a cold-rolled steel sheet.

[0050] The cold rolled steel sheet is then annealed to impart the desired microstructure and mechanical properties to the steel of the present invention.

[0051] Annealing involves heating the cold-rolled steel sheet in a two-stage heating process, with the first stage of heating starting from room temperature and heating the cold-rolled steel sheet at a heating rate HR1 of at least 10°C / s to a temperature HT1 in the range of 575°C to 770°C. In a preferred embodiment, the heating rate HR1 of such first stage of heating is at least 12°C / s, more preferably at least 13°C / s. A preferred HT1 temperature for such first stage is 600°C to 760°C, more preferably 600°C to 700°C.

[0052] In the second heating step, the cold-rolled steel sheet is heated from HT1 to an annealing temperature Tsoak of 780°C to 880°C, preferably 800°C to 860°C, at a heating rate HR2 of 0.5°C / s to 50°C / s. In a preferred embodiment, the heating rate HR2 in the second heating step is 0.7°C to 25°C / s, more preferably 0.8°C to 20°C / s.

[0053] The cold rolled steel sheet is then held at the annealing soak temperature Tsoak for 100 to 1000 seconds to ensure sufficient transformation to form at least 90% austenite at the end of the soak.

[0054] The cold-rolled steel sheet is then cooled in a two-stage cooling process, the first stage of which is optional, starting from Tsoak, where the cold-rolled steel sheet is cooled at a cooling rate CR1 of 0.1°C / s to 15°C / s to a temperature T1 in the range of 680°C to 820°C. In a preferred embodiment, the cooling rate CR1 of such a first cooling stage is 0.2°C / s to 5°C / s. A preferred T1 temperature for such a first stage is 700°C to 800°C.

[0055] In the second step of cooling, the cold-rolled steel sheet is cooled from T1 or Tsoaking to a temperature T2 of Ms-10°C to 20°C at a cooling rate CR2 of at least 15°C / s. In a preferred embodiment, the cooling rate CR2 of the second step of cooling is at least 20°C / s, more preferably at least 25°C / s. The preferred T2 temperature for such a second step is 300°C to 200°C. Whenever step 1 of cooling is not performed, T1 is equal to Tsoaking.

[0056] The steel may optionally be held at T2 for a time between 1 and 200 seconds. During this cooling step, the martensite of the present invention is formed. If the T2 temperature is higher than Ms-40°C, the steel of the present invention will have too much austenite which will be detrimental to the total elongation, and if T2 is lower than Ms-130°C, the amount of retained austenite will be too low to achieve the total elongation target.

[0057] In a subsequent step, the cold-rolled steel sheet is heated from the T2 temperature to the overaging temperature range TOA, between 350°C and 550°C, at a heating rate HR3 of 1°C / s to 100°C / s. The preferred TOA temperature is 380°C to 520°C. During heating to the TOA temperature and holding at the TOA temperature, the martensite formed during cooling after annealing transforms into partitioned martensite by eliminating the carbon consumed by austenite for its stabilization as retained austenite at room temperature. Some carbon from martensite still remains in the partitioned martensite, and this carbon is present in the form of precipitates. Simultaneously, the unstable austenite also transforms into cementite-free bainite, which also eliminates carbon due to the presence of silicon, thereby helping to stabilize the retained austenite. The cold-rolled steel sheet is then held at the TOA temperature for 5 to 500 seconds for overaging.

[0058] The cold-rolled steel sheet is then brought to the temperature of the molten coating bath, which may be between 420°C and 680°C depending on the nature of the coating. The coating may be made of zinc or a zinc-based alloy, or aluminum or an aluminum-based alloy. The cold-rolled steel sheet is then cooled to room temperature, yielding a coated cold-rolled steel sheet. [Example]

[0059] The following tests and examples presented herein are non-limiting in nature and must be considered for illustrative purposes only, to illustrate the advantageous features of the present invention, to explain the importance of the parameters selected by the inventors after extensive experimentation, and to establish the properties that can be achieved with steels according to the present invention.

[0060] Steel sheets according to the invention and some comparative grade samples were prepared using the compositions summarized in Table 1 and the processing parameters summarized in Table 2. The corresponding microstructures of these steel sheets are summarized in Table 3 and the properties are summarized in Table 4.

[0061] Table 1 shows the steels with compositions expressed in weight percent.

[0062] [Table 1]

[0063] Table 2 summarizes the annealing process parameters that were applied to the steels in Table 1.

[0064] Table 2 also shows the martensitic transformation Ms temperatures of the steels of the invention and the reference steels.

[0065] Ms was determined by dilatometry tests carried out according to the publication by S. M. C. Van Bohemen and J. Siestma, Metallurgical and Materials Transactions, Vol. 40A, May 2009, pp. 1059-1068.

[0066] Furthermore, before the annealing treatment of the steels of the invention and the reference examples, the samples were heated to a temperature of 1150°C and then hot rolled at a finishing temperature above 850°C. The cooling rate after hot rolling was 30°C / s until the hot rolled steel was cooled to below 650°C for coiling. All the steels in the trials were pickled before cold rolling, and the cold rolling reduction for all trials was 50%. After the overaging hold, all the cold rolled steel sheets were coated in a zinc bath at a temperature of 460°C.

[0067] [Table 2]

[0068] Table 3 summarizes the results of tests carried out according to standards with different microscopes, such as scanning electron microscopes, to determine the microstructural composition of both the steel of the invention and the reference trials.

[0069] [Table 3]

[0070] From the above table it can be seen that all trials according to the present invention met the microstructure goals.

[0071] Table 4 summarizes the mechanical and surface properties of both the inventive and reference steels.

[0072] Table 4: Mechanical properties of the samples Yield strength YS, tensile strength TS and total elongation TE are measured according to ISO standard ISO 6892-1 published in October 2009.

[0073] [Table 4]

Claims

1. 1. A cold rolled coated steel sheet, the steel comprising, in weight percent: 0.15%≦carbon≦0.25%; 1.5%≦Manganese≦2.5%, 1%≦silicon≦2%; 0%≦aluminum≦0.09%, 0.1%≦chromium≦0.6%, 0%≦phosphorus≦0.02%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.09%, and optionally one or more of the following elements: 0%≦molybdenum≦0.5%, 0.001%≦niobium≦0.09%, 0%≦titanium≦0.06%, 0%≦vanadium≦0.1%, 0%≦Nickel≦1%, 0%≦copper≦1%, 0%≦Calcium≦0.005%, 0%≦Boron≦0.010%; 0%≦Magnesium≦0.05%, 0%≦zirconium≦0.05%, 0%≦cerium≦0.1%, and the remainder being iron and unavoidable impurities, the steel plate having a microstructure including, by area ratio, a cumulative abundance ratio of bainite and distributed martensite of 35% to 70%, 9% to 15% retained austenite, 12% to 38% ferrite, and 5% to 15% fresh martensite.

2. 2. The cold rolled coated steel sheet of claim 1, wherein the composition comprises 1.7% to 2.3% manganese.

3. 3. The cold rolled coated steel sheet according to claim 1 or 2, wherein the composition comprises 0.17% to 0.23% carbon.

4. 4. Cold rolled coated steel sheet according to any one of claims 1 to 3, wherein the composition comprises 1.1% to 1.9% 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. 6. The cold rolled coated steel sheet according to any one of claims 1 to 5, wherein the microstructure comprises a cumulative abundance of bainite and distributed martensite of 40% to 68%.

7. 7. The cold rolled coated steel sheet according to any one of claims 1 to 6, wherein the microstructure comprises 10% to 14% retained austenite.

8. The cold rolled coated steel sheet according to any one of claims 1 to 7, wherein the microstructure comprises 14% to 36% ferrite.

9. The cold-rolled coated steel sheet according to any one of claims 1 to 8, having a total elongation of 14% or more.

10. The cold-rolled coated steel sheet according to any one of claims 1 to 9, having a hole expansion rate of 20% 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 to obtain a semi-finished product, - reheating said semi-finished product to a temperature above 1000°C, - rolling said semi-finished product completely in the austenite range at 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 the hot rolled steel sheet to an optional descaling process; - annealing the hot-rolled steel sheet at a temperature of 350°C to 750°C for 1 hour to 96 hours; - subjecting the hot-rolled annealed steel sheet to an optional descaling step; - cold rolling the hot-rolled steel sheet at a reduction ratio of 35 to 70% to obtain a cold-rolled steel sheet; - then heating the cold-rolled steel sheet in two stages: o) a first step of heating the cold-rolled steel sheet starting from room temperature to a temperature HT1 between 575°C and 770°C at a heating rate HR1 of at least 10°C / s; o) a second heating step starting from HT1 with a heating rate HR2 between 0.5°C / s and 50°C / s to a temperature Tsoak between 780 and 880°C and holding for 100 to 1000 seconds; - then cooling the cold-rolled steel sheet as follows: o) an optional first step of cooling the cold-rolled steel sheet starting from Tsoak to a temperature T1 between 680°C and 820°C, at a cooling rate CR1 between 0.1°C / s and 15°C / s; o) a cooling step starting from T1 or Tsoak to a temperature T2 of Ms-10°C to 20°C at a cooling rate CR2 of at least 15°C / s, wherein the cold-rolled steel sheet can be held at T2 for an optional period of 1 second to 200 seconds; - then heating the cold-rolled steel sheet from the T2 temperature to an over-aging temperature TOA of 350°C to 550°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 the cold-rolled steel sheet to a coating temperature of between 420°C and 680°C in order to coat it; - thereafter cooling the coated cold rolled steel sheet to room temperature to obtain a cold rolled coated steel sheet.

12. The method of claim 11, wherein the Tsoak temperature is between 800°C and 860°C.

13. 13. The method according to claim 11 or 12, wherein the HT1 temperature is between 600°C and 760°C.

14. The method according to any one of claims 11 to 13, wherein the TOA temperature is between 380°C and 520°C.

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

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