Heat-treated cold-rolled steel sheet and method for producing same

Abstract

A cold-rolled steel sheet having a composition comprising 0.05% < = carbon < = 0.15%, 1.8% < = manganese < = 2.7%, 0.1% < = silicon < = 1%, 0.01% < = aluminum < = 0.8%, 0.1% < = chromium < = 0.9%, 0% < = phosphorus < = 0.09%, 0.0001% < = titanium < = 0.1%, 0.0005% < = boron < = 0.003%, 0.01% < = niobium < = 0.1%, 0% < = sulfur < = 0.09%, 0% < = nitrogen < = 0.09%, 0% < = vanadium < = 0.2%, 0% < = molybdenum < = 0.2%, 0% < = nickel < = 2%, 0% < = copper < = 2%, 0% < = calcium < = 0.005%, 0% < = cerium < = 0.1%, 0% < = magnesium < = 0.05%, the microstructure of the steel sheet contains, in area fraction, 40% to 60% of martensite, 15% to 40% of critical zone ferrite, 10% to 35% of cumulant transformed ferrite and bainite, and 0% to 5% of retained austenite.

Description

[0001] This patent application is a divisional application of the patent application with application number 202180039907.7, filed on July 1, 2021, and with the title “Heat treated cold rolled steel sheet and method of manufacturing the same”. TECHNICAL FIELD

[0002] The present invention relates to a cold rolled steel sheet having high strength and high formability, said cold rolled steel sheet having a tensile strength of 980 MPa or more and total elongation of more than 14% which is suitable for use as a steel sheet for vehicles. BACKGROUND

[0003] Automobile parts are required to meet two inconsistent demands, i.e., easy formability and high strength, but in recent years, a third requirement of improving fuel consumption is also given to automobiles in view of global environmental problems. Therefore, now automobile parts must be made of a material having high formability in order to meet the standard of easy assembly of complex automobile components, and at the same time, the strength must be increased for vehicle crashworthiness and durability, while reducing the weight of the vehicle to improve fuel efficiency.

[0004] Therefore, a lot of research and development efforts have been made to reduce the amount of material used in automobiles by increasing the strength of the material. Conversely, the increase in the strength of the steel sheet reduces the formability, and thus a material having both high strength and high formability must be developed.

[0005] Early research and development in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are listed herein for a clear understanding of the present invention:

[0006] US 9074 272 describes a steel having the following chemical composition: 0.1% to 0.28% of C, 1.0% to 2.0% of Si, 1.0% to 3.0% of Mn, and the remainder consisting of iron and unavoidable impurities. The microstructure contains 5% to 20% of residual austenite, 40% to 65% of bainitic ferrite, 30% to 50% of polygonal ferrite, and less than 5% of martensite. US 9074 272 relates to a cold rolled steel sheet having excellent elongation, but the invention described in US 9074 272 fails to achieve a strength of 900 MPa as a requirement to reduce the weight while maintaining the robustness of complex automobile parts.

[0007] The known prior art related to the manufacture of high strength and high formability steel sheets has one or the other defect: there is therefore a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same. SUMMARY

[0008] The object of the present invention is to solve these problems by making available a cold-rolled steel sheet having simultaneously:

[0009] - an ultimate tensile strength greater than or equal to 950 MPa, and preferably higher than 980 MPa or even higher than 1000 MPa,

[0010] - an overall elongation greater than or equal to 14%, and preferably greater than or equal to 15%.

[0011] In a preferred embodiment, the steel sheet according to the present invention can have a yield strength value greater than or higher than 540 MPa or even better higher than 550 MPa.

[0012] Preferably, such a steel can also have a good aptitude for forming, in particular for rolling, and good weldability and coatability.

[0013] Another object of the present invention is also to make available a process for manufacturing these sheets, which is compatible with conventional industrial applications while being robust to manufacturing parameter variations. DETAILED DESCRIPTION

[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 the steel between 0.05% and 0.15%. Carbon is an essential element for increasing the strength of the steel sheet by generating a low temperature transformation phase such as martensite. Moreover, carbon also plays a key role in austenite stabilization. A content less than 0.05% would not ensure the formation of martensite, thus reducing the strength. On the other hand, when the carbon content exceeds 0.15%, the weld zone and the heat-affected zone are significantly hardened and thus impair the mechanical properties of the weld zone. Therefore, the preferred limit is between 0.07% and 0.12%, and more preferably between 0.08% and 0.11%.

[0016] The manganese content of the steel of the present invention is between 1.8% and 2.7%. Manganese is an element which gives strength to the steel by solid solution strengthening. Manganese is required in an amount of at least about 1.8 wt% to provide strength and hardenability of the steel sheet and to form ferrite. Therefore, a higher percentage of manganese is preferred, for example between 1.9% and 2.5%, and more preferably between 2.1% and 2.5%. But when manganese is more than 2.7%, it generates an adverse effect such as slowing down the transformation of austenite during cooling after annealing, resulting in a decrease in ductility. Moreover, a manganese content higher than 2.7% also decreases the weldability of the steel of the present invention.

[0017] The silicon content of the steel of the present invention is 0.1% to 1%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. Silicon promotes ferrite transformation. However, adding more than 1% of silicon does not improve the mentioned effects and causes problems such as hot shortness. Therefore, the concentration is controlled within the upper limit of 1%. The preferred limit of the presence of silicon is maintained as 0.2% to 0.9%, and more preferably as 0.3% to 0.7%.

[0018] The aluminum content of the steel of the present invention is 0.01% to 0.8%. Within such a range, aluminum combines with nitrogen in the steel to form aluminum nitride, thereby reducing the size of the grains. However, in the present invention whenever the content of aluminum exceeds 0.8%, it increases the Ac3 point, thereby decreasing the productivity. Therefore, the preferred range of aluminum is maintained as 0.01% to 0.7%, and more preferably as 0.01% to 0.6%.

[0019] In a preferred embodiment, the cumulative amount of silicon and aluminum is at least 0.6% since both elements are ferrite phase producing elements, thereby participating in the formation of ferrite which is beneficial for both elongation and ductility.

[0020] The chromium content of the steel of the present invention is 0.1% to 0.9%. Chromium is an essential element for providing strength and hardening to the steel, but above 0.9% it impairs the surface finish of the steel when used. Therefore, for the best realization of the effects of chromium, the preferred limit is 0.2% to 0.8%, and more preferably 0.2% to 0.7%.

[0021] Titanium is an essential element that can be added to the steel of the present invention in the range of 0.0001% to 0.1%, and preferably 0.01% to 0.08%. Similar to niobium, titanium participates in the carbonitride, thereby playing a role in hardening. However, it also participates in the formation of TiN which occurs during the solidification of the cast product. The amount of Ti is therefore limited to 0.1% to avoid coarse TiN which is harmful to the hole expansion. In case the titanium content is below 0.0001%, it does not have any effect on the steel of the present invention.

[0022] Boron is an essential element of the present invention and is added in very small amounts, as well as in the range of 0.0005% to 0.003%. Boron imparts hardenability and strength to the steel of the present invention. However, when boron is added more than 0.003%, it is observed that the rollability of the steel sheet decreases significantly. Moreover, boron segregation can occur at the grain boundaries, which is harmful to the formability.

[0023] Niobium is an essential element that can be added to the steel in an amount of 0.01 to 0.1 %, preferably 0.01 to 0.06 %. Niobium is suitable for forming carbonitrides by precipitation hardening to impart strength to the steel according to the present application. Since niobium delays recrystallization during heating, the microstructure formed at the end of the holding temperature and thus after full annealing is finer, which leads to hardening of the product. However, when the niobium content is higher than 0.1 %, the amount of carbonitrides is detrimental to the present application, as a large amount of carbonitrides tends to decrease the ductility of the steel.

[0024] Vanadium is an optional element that can be added to the steel of the present application in an amount of up to 0.2 %, preferably 0.001 to 0.01 %. Like niobium, vanadium is involved in carbonitrides, thus playing a role in hardening. But vanadium is also involved in the formation of VN that appears during solidification of the cast product. The amount of V is thus limited to 0.2 % to avoid coarse VN that are detrimental to hole expansion. In case of vanadium content lower than 0.001 %, vanadium does not have any effect on the steel of the present application.

[0025] The phosphorus content of the steel of the present application is limited to 0.09 %. Phosphorus is an element that hardens in solid solution and also interferes with the formation of carbides. Thus, a small amount of at least 0.002 % of phosphorus can be advantageous, but phosphorus also has its detrimental effects, such as a decrease in spot weldability and hot ductility, in particular because of its tendency to segregate at the grain boundaries or co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.02 %.

[0026] Sulfur is not an essential element but can be included in the steel as an impurity. The sulfur content is preferably as low as possible, but from the point of view of manufacturing costs, it is 0.09 % or less, and preferably less than 0.03 %. Furthermore, if higher sulfur is present in the steel, it forms sulfides in particular in combination with Mn and Ti, and reduces the beneficial effects of Mn and Ti on the present application.

[0027] Nitrogen is limited to 0.09 % to avoid aging of the material and to minimize the precipitation of aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel.

[0028] Molybdenum is an optional element that constitutes 0 to 0.2 % of the steel of the present application; when added in an amount of at least 0.01 %, molybdenum improves the hardenability and the hardness, delaying the appearance of bainite, thus promoting the formation of martensite. Molybdenum also promotes the formation of ferrite. However, the addition of molybdenum in excess increases the cost of the added alloying elements, so for economic reasons, its content is limited to 0.2 %. The preferred limit for molybdenum is 0.01 to 0.2 %.

[0029] Nickel can be added as an optional element in an amount of 0% to 2% to increase the strength of the steel of the present invention and to improve its toughness. A minimum of 0.01% is preferred to obtain such effects. However, when its content is higher than 2%, nickel causes deterioration of ductility.

[0030] Copper can be added as an optional element in an amount of 0% to 2% to increase the strength of the steel of the present invention and to improve its corrosion resistance. A minimum of 0.01% is preferred to obtain such effects. However, when its content is higher than 2%, it can deteriorate the surface appearance.

[0031] Calcium is an optional element that can be added to the steel of the present invention up to 0.005%, preferably 0.0001% to 0.005%. Calcium is added to the steel of the present invention as an optional element especially during inclusion treatment. Calcium helps the refining of the steel by capturing the harmful sulphur content during spheroidizing of the steel.

[0032] Other elements such as cerium, magnesium or zirconium can be added alone 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 grains during solidification.

[0033] The remainder of the composition of the steel consists of iron and unavoidable impurities resulting from the processing.

[0034] The microstructure of the steel sheet according to the present invention comprises, in area fraction: 40% to 60% of martensite, 5% to 40% of intercritical ferrite, 10% to 35% cumulative amount of transformed ferrite and bainite, and 0% to 5% of retained austenite.

[0035] The martensite constitutes 40% to 60% of the microstructure in area fraction. The martensite is formed significantly during cooling after annealing and in particular after exceeding the Ms temperature and in particular between Ms - 10°C and 20°C, or during cooling after overaging. The martensite gives the present invention its strength. The preferred limit of the martensite is 42% to 58%, and more preferably 43% to 56%.

[0036] The intercritical ferrite constitutes 15% to 40% of the microstructure of the steel of the invention in area fraction. This intercritical ferrite imparts to the steel of the invention a total elongation of at least 14%. The intercritical ferrite results from annealing at a temperature below Ac3. The intercritical ferrite is different from the ferrite which can be produced after annealing, hereinafter referred to as "transformation ferrite", which will be described below. In contrast to the transformation ferrite, the intercritical ferrite is polygonal. Furthermore, the transformation ferrite is rich in carbon and manganese, i.e. has a carbon content and a manganese content higher than the carbon content and the manganese content of the intercritical ferrite. The intercritical ferrite and the transformation ferrite can thus be distinguished by observation of micrographs with a SEM microscope using secondary electrons after etching with a 2% nitric acid in ethanol etching solution. On such micrographs, the intercritical ferrite appears in medium grey, while the transformation ferrite appears in dark grey due to its higher carbon content and manganese content. It is preferred to have 20% to 40%, and more preferably 25% to 38% of intercritical ferrite.

[0037] The total amount of transformation ferrite and bainite constitutes 10% to 35% of the microstructure of the steel of the invention in area fraction. The transformation ferrite of the present invention constitutes the ferrite formed during cooling after annealing and the steel according to the invention always contains transformation ferrite, i.e. the presence of transformation ferrite is always more than 0%. The transformation ferrite imparts to the steel of the invention a high strength as well as an elongation. The transformation ferrite of the steel of the invention is rich in carbon and manganese compared to the intercritical ferrite and it is mandatory to have transformation ferrite in the steel. The bainite is formed during the overaging holding, especially at 400°C to 480°C. To ensure an elongation of 14%, it is necessary to have 10% of transformation ferrite and bainite. But whenever the total amount is higher than 35% in the steel of the invention, it is not possible to have both the tensile strength and the total elongation. The preferred limit of the transformation ferrite and bainite of the present invention is 15% to 30%.

[0038] The retained austenite is an optional microstructure and can be present in the steel from 0% to 5%.

[0039] In addition to the above microstructure, the microstructure of the cold-rolled and heat-treated steel sheet does not contain microstructural components such as pearlite, tempered martensite and cementite without prejudice to the mechanical properties of the steel sheet.

[0040] The steel sheet according to the present invention can be produced by any suitable method. The preferred method comprises providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be made as a ingot or continuously made in the form of a thin slab or thin strip, i.e. with a thickness ranging from about 220 mm (for a slab) up to a few tens of millimeters (for a thin strip).

[0041] For example, the slab having the above chemical composition is manufactured by continuous casting, wherein the slab is optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and to ensure that the local carbon to nominal carbon ratio remains below 1.10. The slab provided by the continuous casting process can be used directly at high temperature after continuous casting or can be first cooled to room temperature and then reheated for hot rolling.

[0042] The temperature of the slab subjected to hot rolling is at least 1000°C and must be lower than 1280°C. In case the temperature of the slab is lower than 1000°C, excessive load is applied to the rolling mill, moreover, the temperature of the steel can decrease to the ferrite transformation temperature during the finishing rolling, whereby the steel will be rolled in a state containing transformed ferrite in the microstructure. Reheating at a temperature higher than 1280°C must be avoided because it is industrially expensive. Therefore, the finishing rolling temperature of the slab is higher than Ac3, and preferably high enough so that the hot rolling can be completed in a temperature range of Ac3+150°C to Ac3+250°C.

[0043] In order to have a microstructure that is favorable to recrystallization and rolling, a finishing rolling temperature range of Ac3 to Ac3+200°C is necessary. It is preferred that the finishing pass is carried out at a temperature higher than 850°C, and better at least 950°C.

[0044] The hot rolled steel obtained in this way is then cooled to coiling temperature at a cooling rate of at least 30°C / s. Preferably, the cooling rate will be less than or equal to 200°C / s.

[0045] The hot rolled steel is then coiled at a temperature of 475°C to 650°C to avoid ovalization, and preferably at a temperature of 475°C to 625°C to avoid scale formation. A more preferred range of such coiling temperature is 500°C to 625°C. The coiled hot rolled steel is then cooled to room temperature, and then subjected to an optional interanneal.

[0046] The hot rolled steel can be subjected to an optional scale removal step to remove the scale formed during hot rolling prior to the optional interanneal. The hot rolled sheet can then be subjected to an optional interanneal at a temperature of, for example, 400°C to 750°C for preferably at least 12 hours and not more than 96 hours, the temperature being kept below 750°C to avoid partially transforming the hot rolled microstructure and thus losing microstructure homogeneity. Thereafter, the optional scale removal step of the hot rolled steel can be carried out by, for example, pickling of such steel sheet.

[0047] The hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet at a thickness reduction of 35% to 90%. The cold rolled steel sheet obtained from the cold rolling process is then subjected to annealing to impart the steel of the present invention with microstructure and mechanical properties.

[0048] To anneal the cold rolled steel sheet, it is heated to an soaking temperature of Ac1+60°C to Ac3, preferably at a heating rate of at least 3°C / s, and then annealed at this temperature for a time of 5 seconds to 500 seconds, preferably for a time of 50 seconds to 250 seconds. In a preferred embodiment, the heating is performed at least at 10°C / s, and more preferably at least at 15°C / s. During this annealing, critical zone ferrite is formed.

[0049] The preferred annealing soaking temperature is Ac1+70°C to Ac3, and more preferably Ac1+80°C to Ac3-30°C.

[0050] In a preferred embodiment, the time and temperature of the soaking are chosen to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 50% of austenite, and more preferably at least 60% of austenite.

[0051] The cold rolled steel sheet is then cooled in a two step cooling process, wherein the first step starts from the soaking temperature to a temperature T1 of 550°C to 650°C at a cooling rate CR1 of at least 3°C / s, preferably at least 5°C / s, and more preferably at least 10°C / s. During this step, transformed ferrite is formed. The cold rolled steel is then held at T1 for a time of 1 second to 20 seconds, and preferably for a time of 2 seconds to 15 seconds, and more preferably for a time of 5 seconds to 12 seconds.

[0052] Thereafter, the second step starts from further cooling the cold rolled steel sheet from T1 to a overaging temperature T2 of 400°C to 480°C at a cooling rate CR2 of at least 3°C / s, preferably at least 5°C / s, and more preferably at least 7°C / s.

[0053] The overaging is then performed at T2 for a time of 5 seconds to 100 seconds. During the overaging, some bainite is formed. The preferred overaging temperature T2 is 420°C to 475°C. The preferred time of the overaging temperature is during 15 seconds to 75 seconds, and more preferably 20 seconds to 75°C.

[0054] The cold rolled steel sheet can then be cooled to room temperature, or can be brought to a hot dip bath temperature of 420°C to 680°C depending on the nature of the coating to facilitate the hot dip coating of the cold rolled steel sheet.

[0055] In either case, the final cooling to room temperature is done at a cooling rate of at least 5°C / s, and preferably at least 9°C / s to ensure the formation of fresh martensite in the steel of the application.

[0056] The cold rolled steel sheet can also be coated by any known industrial process such as electrogalvanizing, JVD, PVD, etc. which can not require bringing the steel sheet to the above mentioned temperature range before coating.

[0057] Example

[0058] The following tests and examples provided herein are non-limiting in nature and must be considered for illustrative purposes only, and will show the advantageous features of the present application and illustrate the importance of the parameters chosen by the inventors after a large number of experiments, and further determine the properties that can be achieved by the steel according to the present application.

[0059] Steel plate samples according to the present application and steel plate samples according to some comparative grades were prepared with the compositions summarized in Table 1 and the process parameters summarized in Table 2. The corresponding microstructures of these steel plates are summarized in Table 3 and the properties in Table 4.

[0060] Table 1: Compositions of the tests

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

[0062]

[0063] Table 1 also shows the Ac1 and Ac3 temperature points calculated by dilatometry.

[0064] Table 2: Process parameters

[0065] Table 2 summarizes the annealing process parameters implemented on the steel samples of Table 1, all of them re-heated at 1230°C, hot-rolled with a finishing temperature of 875°C, coiled at 550°C and cold-rolled with a reduction of 50%, then subjected to annealing and a two-step cooling schedule including overaging:

[0066]

[0067] Table 3 summarizes the results of the tests performed according to standards on different microscopes such as scanning electron microscope for determining the microstructure composition of both the steel tests of the present application and the reference tests.

[0068] Table 3: Microstructure of the tests

[0069]

[0070] Table 4 summarizes the mechanical properties of both the steel of the present application and the reference steels. The tensile strength, yield strength and total elongation tests were performed according to the JIS Z2241 standard.

[0071] Table 4: Mechanical properties of the tests

[0072]

[0073] The examples show that the steel plate according to the present application is the only one showing all the targeted properties due to its specific composition and microstructure.

[0074] The present application also discloses the following remarks:

[0075] Remark 1 : A cold-rolled steel sheet, the composition of which, expressed in weight percent, comprises the following elements:

[0076] 0.05% < Carbon < 0.15%

[0077] 1.8% < Manganese < 2.7%

[0078] 0.1% < Silicon < 1%

[0079] 0.01% < Aluminum < 0.8%

[0080] 0.1% < Chromium < 0.9%

[0081] 0% < Phosphorus < 0.09%

[0082] 0.0001% < Titanium < 0.1%

[0083] 0.0005% < Boron < 0.003%

[0084] 0.01% < Niobium < 0.1%

[0085] 0% < Sulfur < 0.09%

[0086] 0% < Nitrogen < 0.09%

[0087] and can comprise one or more of the following optional elements:

[0088] 0% < Vanadium < 0.2%

[0089] 0% < Molybdenum < 0.2%

[0090] 0% < Nickel < 2%

[0091] 0% < Copper < 2%

[0092] 0% < Calcium < 0.005%

[0093] 0% < Cerium < 0.1%

[0094] 0% < Magnesium < 0.05%

[0095] 0% < Zirconium < 0.05%

[0096] the remainder consisting of iron and unavoidable impurities resulting from the processing, the microstructure of said steel sheet comprising, in area fraction: 40% to 60% of martensite, 15% to 40% of intercritical ferrite, 10% to 35% cumulative amount of transformed ferrite and bainite, and 0% to 5% of retained austenite.

[0097] Note 2: The cold-rolled coated steel sheet, wherein the composition contains 0.2% to 0.9% silicon.

[0098] Note 3: The cold-rolled coated steel sheet, wherein the composition contains 0.07% to 0.12% carbon.

[0099] Note 4: The cold-rolled coated steel sheet, wherein the composition contains 0.01% to 0.7% aluminum.

[0100] Note 5: The cold-rolled coated steel sheet, wherein the composition contains 1.9% to 2.5% manganese.

[0101] Note 6: The cold-rolled coated steel sheet, wherein the composition contains 0.2% to 0.8% chromium.

[0102] Note 7: The cold-rolled coated steel sheet contains more than 0.6% silicon and aluminum cumulatively.

[0103] Note 8: The cold-rolled coated steel sheet wherein the cumulative amount of transformed ferrite and bainite is 15% to 30%.

[0104] Note 9: The cold-rolled coated steel sheet, wherein the amount of martensite is 42% to 58%.

[0105] Note 10: The cold-rolled coated steel sheet, wherein the steel sheet has an ultimate tensile strength of 950 MPa or greater and a total elongation of 14% or greater.

[0106] Note 11: The cold-rolled coated steel sheet, wherein the yield strength of the steel sheet is 540 MPa or greater.

[0107] Appendix 12: A method for producing cold-rolled coated steel sheets, comprising the following sequential steps:

[0108] - Provides the steel composition according to any one of claims 1 to 7;

[0109] - The semi-finished product is then reheated to a temperature of 1000°C to 1280°C;

[0110] - The semi-finished product is rolled, wherein the hot rolling finishing temperature should be higher than Ac3 to obtain hot-rolled steel;

[0111] - Cooling the hot-rolled steel to a coiling temperature of 475°C to 650°C at a cooling rate of at least 30°C / second; and coiling the hot-rolled steel;

[0112] - Cool the hot-rolled steel to room temperature;

[0113] - Optionally, the hot-rolled steel sheet is subjected to an oxide scale removal process;

[0114] - optionally annealing the hot-rolled steel sheet;

[0115] - optionally performing a scale removal process on the hot-rolled steel sheet;

[0116] - cold rolling the hot-rolled steel sheet at a reduction rate of 35% to 90% to obtain a cold-rolled steel sheet;

[0117] - heating the cold-rolled steel sheet from room temperature to a soaking temperature of Ac1+60°C to Ac3,

[0118] - then performing annealing at the soaking temperature for a time of 5 seconds to 500 seconds,

[0119] - then cooling the cold-rolled steel in a two-step cooling process, wherein:

[0120] o a first step of cooling from the soaking temperature to a temperature T1 of 550°C to 650°C at a cooling rate CR1 of at least 3°C / s,

[0121] o then holding the cold-rolled steel at T1 for a time of 1 second to 20 seconds,

[0122] o then a second step of further cooling the cold-rolled steel sheet from T1 to a peak-aging temperature T2 of 400°C to 480°C at a cooling rate CR2 of at least 3°C / s,

[0123] - then performing peak-aging at T2 for a time of 5 seconds to 100 seconds,

[0124] - then optionally bringing the cold-rolled steel sheet to a temperature range of 420°C to 680°C to facilitate coating and optionally coating the cold-rolled steel sheet,

[0125] - thereafter, cooling the cold-rolled steel sheet to room temperature at a cooling rate of at least 5°C / s to obtain a cold-rolled coated steel sheet.

[0126] Note 13: The method, wherein the coiling temperature is 475°C to 625°C.

[0127] Note 14: The method, wherein the soaking temperature is chosen to ensure that at least 50% austenite is present at the end of soaking.

[0128] Note 15: The method, wherein the peak-aging temperature is 420°C to 475°C.

[0129] Note 16: The method, wherein the cooling rate after coating is at least 9°C / s.

[0130] Note 17: Use of the steel sheet or steel sheet produced according to the method for manufacturing a structural or safety part of a vehicle.

[0131] Comment 18: A vehicle comprising a component obtained according to the method described.

Claims

1. A cold-rolled steel sheet, expressed as a weight percentage, wherein the composition of the cold-rolled steel sheet comprises the following elements: 0.05% ≤ Carbon ≤ 0.15% 1.8% ≤ Manganese ≤ 2.7% 0.1% ≤ Silicon ≤ 1% 0.01% ≤ Aluminum ≤ 0.8% 0.1% ≤ Chromium ≤ 0.9% 0% ≤ Phosphorus ≤ 0.09% 0.0001%≤Titanium≤0.1% 0.0005% ≤ Boron ≤ 0.003% 0.01%≤Niobium≤0.1% 0% ≤ Sulfur ≤ 0.09% 0% ≤ Nitrogen ≤ 0.09% And can include one or more of the following optional elements: 0%≤vanadium≤0.2% 0% ≤ Molybdenum ≤ 0.2% 0%≤Nickel≤2% 0%≤Copper≤2% 0% ≤ Calcium ≤ 0.005% 0%≤Cerium≤0.1% 0% ≤ Magnesium ≤ 0.05% 0%≤Zirconium≤0.05% The remaining portion consists of iron and unavoidable impurities resulting from processing. The microstructure of the steel plate comprises, by area fraction: 40% to 60% martensite, 15% to 40% critical zone ferrite, 10% to 35% cumulative transformed ferrite and bainite, and 0% to 5% retained austenite.

2. The cold-rolled steel sheet according to claim 1, wherein the composition comprises 0.2% to 0.9% silicon.

3. The cold-rolled steel sheet according to claim 1 or 2, wherein the composition contains 0.07% to 0.12% carbon.

4. The cold-rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.01% to 0.7% aluminum.

5. The cold-rolled steel sheet according to claim 1 or 2, wherein the composition comprises 1.9% to 2.5% manganese.

6. The cold-rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.2% to 0.8% chromium.

7. The cold-rolled steel sheet according to claim 1 or 2, wherein the cumulative amount of silicon and aluminum is more than 0.6%.

8. The cold-rolled steel sheet according to claim 1 or 2, wherein the cumulative amount of transformed ferrite and bainite is 15% to 30%.

9. The cold-rolled steel sheet according to claim 1 or 2, wherein the amount of martensite is 42% to 58%.

10. The cold-rolled steel sheet according to claim 1 or 2, wherein the steel sheet has an ultimate tensile strength of 950 MPa or greater and a total elongation of 14% or greater.

11. The cold-rolled steel sheet according to claim 10, wherein the yield strength of the steel sheet is 540 MPa or greater.

12. A method for producing cold-rolled coated steel sheet, comprising the following sequential steps: - Provides the steel composition according to any one of claims 1 to 7; - The semi-finished product having the steel composition is reheated to a temperature of 1000°C to 1280°C; - The semi-finished product is rolled, wherein the hot rolling finishing temperature should be higher than Ac3 to obtain hot-rolled steel; - Cooling the hot-rolled steel to a coiling temperature of 475°C to 650°C at a cooling rate of at least 30°C / second; and coiling the hot-rolled steel; - Cool the hot-rolled steel to room temperature; - Optionally, the hot-rolled steel sheet is subjected to an oxide scale removal process; - Optional annealing of hot-rolled steel sheets; - Optionally, the hot-rolled steel sheet is subjected to an oxide scale removal process; - The hot-rolled steel sheet is cold-rolled at a reduction rate of 35% to 90% to obtain a cold-rolled steel sheet; - The cold-rolled steel sheet is heated from room temperature to a homogenization temperature ranging from Ac1+60°C to Ac3. - Then anneal at a homogenized temperature for 5 to 500 seconds. - The cold-rolled steel is then cooled in a two-step cooling process, wherein: ○ The first step involves cooling from the homogenization temperature to a temperature T1 of 550°C to 650°C at a cooling rate of at least 3°C / second CR1. ○ Then the cold-rolled steel is held at T1 for 1 to 20 seconds. Then the second step begins by further cooling the cold-rolled steel sheet from T1 to an over-aging temperature T2 of 400°C to 480°C at a cooling rate CR2 of at least 3°C / s. - Then, under T2, the timeout period is 5 to 100 seconds. - The cold-rolled steel sheet is then optionally heated to a temperature range of 420°C to 680°C to facilitate coating and, optionally, to coat the cold-rolled steel sheet. - Subsequently, the cold-rolled steel sheet is cooled to room temperature at a cooling rate of at least 5°C / second to obtain a cold-rolled coated steel sheet.

13. The method of claim 12, wherein the winding temperature is from 475°C to 625°C.

14. The method according to any one of claims 12 or 13, wherein the homogenization temperature is selected to ensure that at least 50% austenite is present at the end of homogenization.

15. The method according to claim 12 or 13, wherein the over-aging temperature is 420°C to 475°C.

16. The method according to claim 12 or 13, wherein the cooling rate after coating is at least 9°C / second.

17. Use of the steel sheet according to any one of claims 1 to 11 or the steel sheet produced by the method according to any one of claims 12 to 16 for manufacturing structural or safety components of a vehicle.

18. A vehicle comprising the component obtained according to claim 17.

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