Cold-rolled heat-treated steel sheet and method for manufacturing the same

Abstract

To obtain a cold-rolled heat-treated steel sheet suitable for use as a steel sheet for an automobile.SOLUTION: The cold-rolled heat-treated steel sheet has a composition that includes the following elements expressed in percent by weight: 0.10%≤carbon≤0.5%, 1%≤manganese≤3.4%, 0.5%≤silicon≤2.5%, 0.03%≤aluminum≤1.5%, sulfur≤0.003%, 0.002%≤phosphorus≤0.02%, and nitrogen≤0.01%, and that can include one or more of chromium, molybdenum, niobium, titanium, copper, nickel, calcium, vanadium, boron, cerium, magnesium and zirconium, as optional elements. The residual composition is composed of iron and unavoidable impurities; the fine structure includes, in terms of area fraction, 10 to 30% retained austenite, 5 to 50% annealed bainite, 10 to 40% bainite, 1 to 20% quenched martensite, and smaller than 30% tempered martensite; and a total amount of bainite and retained austenite is 30% or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a cold-rolled, heat-treated steel sheet suitable for use as an automotive steel sheet. [Background technology]

[0002] Automotive parts must meet two contradictory requirements: ease of formability and strength, but in recent years, due to considerations of the global environment, automobiles have also been given a third requirement: improved fuel efficiency. Therefore, automotive parts must be made of highly formable materials to meet the standards of ease of fit for complex automobile assemblies, while at the same time reducing vehicle weight and improving fuel efficiency while improving the strength of the vehicle's crashworthiness and durability.

[0003] Therefore, intensive research and development efforts are being made to reduce the amount of material used in automobiles by increasing the strength of the materials. Conversely, since increasing the strength of steel sheets reduces their formability, there is a demand for the development of materials that combine high strength and high formability.

[0004] Previous research and development in the field of high strength and highly formable steel plates has led to several methods for producing high strength and highly formable steel plates, some of which are listed here for the final evaluation of the present invention.

[0005] EP3144406 claims a high strength cold rolled steel sheet with excellent ductility, the steel sheet containing, by weight, 0.1% to 0.3% carbon (C), 0.1% to 2.0% silicon (Si), 0.005% to 1.5% aluminum (Al), 1.5% to 3.0% manganese (Mn), 0.04% or less phosphorus (P) (except 0%), 0.015% or less sulfur (S) (except 0%), 0.02% or less nitrogen (N) (except 0%). The balance is iron (Fe) and unavoidable impurities, the sum of silicon and aluminum (Si+Al) (wt%) is 1.0% or more, and the microstructure contains, in area fraction, 5% or less of polygonal ferrite having a minor axis to major axis ratio of 0.4 or more, 70% or less (excluding 0%) of acicular ferrite having a minor axis to major axis ratio of 0.4 or less, 25% or less (excluding 0%) of acicular retained austenite, and the balance is martensite. Furthermore, EP3144406 envisions a high strength steel having a tensile strength of 780 MPa or more.

[0006] EP3128023 describes a high-strength cold-rolled steel sheet having excellent elongation, hole expandability, delayed fracture resistance, and a high yield ratio, and a manufacturing method thereof. The high-strength cold-rolled steel sheet having a high yield ratio has a composition, in mass%, containing 0.13% to 0.25% C, 1.2% to 2.2% Si, 2.0% to 3.2% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, 0.008% or less N, 0.055% to 0.130% Ti, with the balance being Fe and unavoidable impurities. The steel plate has a microstructure containing 2% to 15% ferrite by volume having an average crystal grain size of 2 μm or less, 5 to 20% retained austenite by volume having an average crystal grain size of 0.3 to 2.0 μm, 10% or less (including 0%) martensite by volume having an average grain size of 2 μm or less, and the remainder being bainite and tempered martensite, where the average crystal grain size of the bainite and tempered martensite is 5 μm or less.

[0007] EP3009527 provides a high-strength cold-rolled steel sheet having excellent elongation, stretch flangeability, and a high yield ratio, and a manufacturing method thereof. The high-strength cold-rolled steel sheet has a certain composition and a certain microstructure. The composition contains, on a mass basis, 0.15% to 0.27% C, 0.8% to 2.4% Si, 2.3% to 3.5% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, and 0.010% or less N, with the balance being Fe and unavoidable impurities. The microstructure contains ferrite with an average grain size of 5 μm or less and a volume fraction of 3% to 20%, retained austenite with a volume fraction of 5% to 20%, and martensite with a volume fraction of 5% to 20%, with the balance being bainite and / or tempered martensite. The total number of retained austenite with a grain size of 2 μm or less, martensite with a grain size of 2 μm or less, or a mixture of these phases is 150 or more per 2,000 μm2 of the plate thickness cross section parallel to the rolling direction of the steel plate. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent No. 3144406 [Patent Document 2] European Patent No. 3128023 [Patent Document 3] European Patent No. 3009527 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to - an ultimate tensile strength of at least 950 MPa, preferably greater than 980 MPa; - a total elongation of at least 20%, preferably more than 21%; The object of the present invention is to solve these problems by making available a cold-rolled, heat-treated steel sheet having both of the above properties. [Means for solving the problem]

[0010] In a preferred embodiment, the steel sheet according to the invention has a yield strength / tensile strength ratio of 0.60 or more.

[0011] In a preferred embodiment, the steel sheet according to the invention may also exhibit a yield strength of 600 MPa or more.

[0012] Preferably, such steels are also weldable and paintable and well suited to forming, especially rolling.

[0013] Another object of the invention is also to make available a manufacturing process for these steel sheets that is robust to variations in manufacturing parameters, while being compatible with conventional industrial applications.

[0014] The cold rolled, heat treated steel sheet of the present invention may be optionally coated with zinc or a zinc alloy, or aluminum or an aluminum alloy, to improve its corrosion resistance.

[0015] Carbon is present in the steel at between 0.10% and 0.5%. Carbon is an element necessary to generate low-temperature transformation phases such as martensite to increase the strength of the steel of the present invention, and is also an element necessary to ensure retained austenite, since it plays a central role in austenite stabilization. Thus, carbon plays two important roles: one is to increase strength, and the other is to retain austenite and provide ductility. However, if the carbon content is less than 0.10%, it will not be possible to stabilize the appropriate amount of austenite required by the steel of the present invention. On the other hand, if the carbon content exceeds 0.5%, the spot weldability of the steel will decrease, limiting its application to automotive parts.

[0016] The manganese content of the steel of the present invention is between 1% and 3.4%. This element is of the gamma type. The purpose of adding manganese is to essentially obtain a structure containing austenite. Manganese is an element that stabilizes austenite at room temperature to obtain retained austenite. A quantity of at least 1% by weight of manganese is essential to provide the strength and hardenability of the steel sheet of the present invention as well as to stabilize the austenite. Therefore, in the presented invention, a higher percentage of manganese is preferred, such as 3.4%. However, a manganese content exceeding 3.4% has adverse effects such as retarding the transformation of austenite to bainite during the isothermal hold of the bainite transformation. Furthermore, if the manganese content exceeds 3.4%, the weldability of the steel is reduced and the ductility target may not be achieved. The preferred range of manganese is between 1.2% and 2.3%, and more preferably between 1.2% and 2.2%.

[0017] The silicon content of the steel of the invention is between 0.5% and 2.5%. Silicon is an element that retards the precipitation of carbides during overaging, and therefore, due to its presence, the carbon-rich austenite is stable at room temperature. Furthermore, due to the low solubility of silicon in carbides, it effectively inhibits or retards the formation of carbides, thus promoting the formation of low-density carbides in the bainite structure that is required according to the invention to give the steel of the invention its essential mechanical properties. However, a disproportionate content of silicon would not bring about the mentioned effects and would cause problems such as temper embrittlement. The concentration is therefore limited within the upper limit of 2.5%.

[0018] The aluminum content is between 0.03% and 1.5%. In the present invention, aluminum removes the oxygen present in the molten steel to prevent it from forming a gas phase during the solidification process. Aluminum also fixes the nitrogen in the steel to form aluminum nitride and reduces the size of the particles. If the aluminum content exceeds 1.5%, the Ac3 point becomes high and the productivity decreases. When adding a high manganese content, an aluminum content between 1.0% and 1.5% can be used to offset the effect of manganese on the transformation point and the progress of austenite formation with temperature.

[0019] The chromium content of the steel of the present invention is between 0.05% and 1%. Chromium is an essential element that gives the steel strength and hardness, but if used in excess of 1%, it impairs the surface finish of the steel. Furthermore, at chromium contents below 1%, the carbide distribution pattern of the bainite structure becomes coarse, and therefore the carbide density of the bainite remains low.

[0020] Niobium is present in the steel of the invention between 0.001 and 0.1% and is suitable for forming carbonitrides and imparting the strength of the steel of the invention by precipitation hardening. Niobium also influences the size of the components of the microstructure through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, a finer microstructure is formed at the end of the temperature hold, as a result after the completion of the annealing which leads to hardening of the product. However, when the niobium content exceeds 0.1%, a saturation effect of its influence is observed, which means that the addition of niobium does not improve the strength of the product and is not of economic interest.

[0021] Similar to niobium, titanium is added to the steel of the present invention between 0.001% and 0.1%, which plays a role in hardening the steel of the present invention, since titanium participates in carbonitrides. However, titanium also forms titanium nitrides that appear during solidification of the casting. The amount of titanium is limited to 0.1% to avoid the formation of coarse titanium nitrides that are detrimental to formability. If the titanium content is less than 0.001%, it does not have any effect on the steel of the present invention.

[0022] The phosphorus content of the steel according to the invention is between 0.002% and 0.02%, and since phosphorus has a tendency to segregate or co-segregate with manganese, especially at grain boundaries, reducing spot weldability and hot ductility, for these reasons its content is limited to 0.02%, preferably less than 0.013%.

[0023] Sulfur is not an essential element, but may be contained in the steel as an impurity, and from the viewpoint of the present invention, the sulfur content is preferably as small as possible, but is not more than 0.003% from the viewpoint of production costs. Furthermore, if more sulfur is present in the steel, the sulfur will combine with manganese to form sulfides, particularly with manganese, reducing the beneficial effect of manganese on the present invention.

[0024] The calcium content of the steel of the invention is between 0.001% and 0.005%. Calcium is added to the steel of the invention as an optional element, especially during inclusion treatment. Calcium contributes to the refining of the steel by trapping the harmful sulphur content in globular form and retarding the harmful effects of sulphur.

[0025] Nitrogen is limited to 0.01% to avoid aging of the material, which is detrimental to the mechanical properties of the steel, and to minimize the precipitation of aluminum nitride during solidification.

[0026] Molybdenum is an optional element constituting 0% to 0.5% of the steel of the present invention, and it plays an effective role in improving the hardenability and hardness, delaying the appearance of bainite and avoiding the precipitation of carbides in bainite, but since the addition of molybdenum excessively increases the cost of adding alloying elements, its content is limited to 0.5% for economic reasons.

[0027] Copper may be added as an optional element in amounts between 0.01% and 2% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% is required to achieve this effect. However, if the copper content exceeds 2%, the surface properties may deteriorate.

[0028] Nickel may be added as an optional element in amounts of 0.01% to 3% to increase the strength and toughness of the steel. A minimum of 0.01% is required to obtain this effect. However, if its content exceeds 3.0%, nickel causes a decrease in ductility.

[0029] Vanadium has the effect of increasing the strength of the steel by forming carbides or carbonitrides, and from an economical point of view, its upper limit is 0.1%. Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions: cerium ≦0.1%, boron ≦0.003%, magnesium ≦0.010%, zirconium ≦0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grains during solidification. The balance of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0030] The microstructure of the steel sheet claimed in the present invention is composed of bainite, which constitutes between 10% and 40% of the microstructure of the steel in terms of area fraction. In the present invention, the bainite of the present invention is cumulatively composed of lath bainite and granular bainite. 10% bainite is essential to achieve a total elongation of 20%.

[0031] The retained austenite constitutes 10-30% of the steel by area fraction. It is known that the retained austenite has a higher solubility of carbon than bainite and therefore acts as an effective carbon trap, retarding the formation of carbides in bainite. The carbon percentage in the retained austenite of the present invention is preferably higher than 0.9% and preferably lower than 1.1%. The retained austenite of the steel according to the present invention confers enhanced ductility.

[0032] The annealed bainite constitutes 5% to 50% of the microstructure of the steel of the present invention in terms of area fraction. The annealed bainite imparts strength and formability to the steel of the present invention. The annealed bainite is T soakingIt is formed during the second anneal at temperatures between 0.1 and 0.5% annealed bainite. The steel of the present invention needs 5% annealed bainite to reach the target elongation, but if the amount of annealed bainite exceeds 50%, the steel of the present invention cannot reach its strength.

[0033] Quenched martensite constitutes 1% to 20% of the microstructure by area fraction. Quenched martensite provides strength to the invention. Quenched martensite forms during cooling of the second anneal. Although no minimum is required, more than 20% quenched martensite provides excessive strength but degrades other mechanical properties beyond acceptable limits.

[0034] Tempered martensite constitutes 0% to 30% of the microstructure by area. Martensite occurs when the steel is at Tc min ~Tc max Tempered martensite can be formed when the steel is cooled between 0.25 and 1.05° C. and tempered during the overaging hold. Tempered martensite provides ductility and strength to the present invention. More than 30% tempered martensite provides excessive strength but reduces elongation beyond acceptable limits.

[0035] In addition to the above microstructures, the microstructure of the cold rolled heat treated steel sheet does not contain microstructural components such as pearlite, ferrite and cementite without compromising the mechanical properties of the steel sheet.

[0036] The 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 a chemical composition according to the invention. 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 up to several tens of millimeters for thin strip.

[0037] For example, slabs having the above chemical composition are produced by continuous casting, and the slabs are optionally soft-reduced directly during the continuous casting process to avoid central segregation and to keep the local carbon to nominal carbon ratio below 1.10. The slabs resulting from the continuous casting process can be used directly at high temperature after continuous casting, or they may first be cooled to room temperature and then reheated for hot rolling.

[0038] The temperature of the slab subjected to hot rolling should preferably be at least 1200°C and less than 1280°C. If the temperature of the slab is less than 1200°C, excessive load is applied to the rolling mill, and furthermore, the temperature of the steel is lowered to the ferrite transformation temperature during the finish rolling, so that the steel is rolled in a state in which the structure contains transformed ferrite. Therefore, the temperature of the slab is preferably high enough so that the hot rolling can be completed in the temperature range of Ac3 to Ac3 + 100°C, and the final rolling temperature exceeds Ac3. Reheating at temperatures above 1280°C is industrially expensive and should be avoided.

[0039] A final rolling temperature range between Ac3 and Ac3+100°C is preferred in order to have a structure favourable for recrystallization and rolling. It is necessary to carry out the final rolling pass at a temperature above Ac3, since below this temperature the rollability of the steel sheet is significantly reduced. The steel sheet thus obtained is then cooled at a cooling rate of more than 30°C / s to the coiling temperature, which must be less than 600°C. Preferably, the cooling rate is less than or equal to 200°C / s.

[0040] The hot rolled steel sheet is coiled at a coiling temperature below 600° C. to avoid ovalization of the hot rolled steel sheet, and preferably below 570° C. to avoid scale formation. The preferred range of coiling temperatures is between 350 and 570° C. The coiled hot rolled steel sheet is cooled to room temperature before being subjected to an optional hot strip anneal.

[0041] The hot rolled steel sheet may be subjected to an optional descaling step to remove the scale formed during hot rolling. The hot rolled sheet may then be subjected to an optional hot strip annealing at a temperature between 400°C and 750°C for at least 12 hours and not more than 96 hours, but the temperature is kept below 750°C to avoid partial hot rolled microstructural transformation and thus loss of microstructural homogeneity. An optional descaling step may then be carried out to remove the scale, for example via pickling of such sheet. This hot rolled steel sheet is cold rolled with a thickness reduction of 35-90%. The cold rolled steel sheet obtained from the cold rolling step is then subjected to a two step annealing to impart the microstructural and mechanical properties to the steel of the present invention.

[0042] In the first annealing of the cold-rolled steel sheet, the cold-rolled steel sheet is heated to a soaking temperature between Ac3 and Ac3+100°C at a heating rate of more than 3°C / s, and Ac3 of the steel is expressed by the following formula: Ac3=901-262*C-29*Mn+31*Si-12*Cr-155*Nb+86*Al where the elemental contents are expressed as weight percent.

[0043] The steel sheet is held at the soaking temperature for 10 to 500 seconds to completely recrystallize and completely transform into austenite of the initial structure that is strongly work-hardened. The steel sheet is then cooled at a cooling rate of more than 25°C / s to a temperature range of 380°C to 480°C, preferably 380 to 450°C, and the cold-rolled steel sheet is held for 10 to 500 seconds, and then cooled to room temperature to obtain an annealed cold-rolled steel sheet.

[0044] In the second annealing of cold-rolled annealed steel sheets, at heating rates exceeding 3°C / s, T soaking Heat to a soaking temperature between Ac3 and T soaking =830-260*C-25*Mn+22*Si+40*Al where the elemental contents are expressed as weight percent.

[0045] To ensure proper recrystallization and transformation and to obtain a minimum of 50% austenite in the microstructure, the steel plate is then cooled to Tc at a cooling rate of more than 20°C / s for 10 to 500 seconds. max ~Tc min These are cooled to temperatures in the range between Tc max and Tc min is the following: Tc max =565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb Tc min =565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb where the elemental contents are expressed as weight percent.

[0046] The cold rolled annealed steel sheet is then brought to a temperature range of 350-550°C and held for 5-500 seconds to ensure the formation of an adequate amount of bainite and temper the martensite to impart the steel of the present invention with the targeted mechanical properties. The cold rolled annealed steel sheet is then cooled to room temperature at a cooling rate of at least 1°C / sec to form quenched martensite to obtain the cold rolled heat treated steel sheet.

[0047] The cold rolled heat treated steel sheet may then be optionally coated by any of the known industrial methods such as electrogalvanizing, JVD, PVD, hot dip galvanizing (GI / GA), etc. Electrogalvanizing does not change or modify the mechanical properties or the microstructure of the claimed cold rolled heat treated steel sheet. Electrogalvanizing can be done by conventional industrial methods, e.g. electroplating. EXAMPLES

[0048] The following tests, examples, figurative illustrations and tables presented herein are to be considered as non-limiting in nature and for illustrative purposes only, illustrating advantageous features of the present invention.

[0049] The steel plates made with different steel compositions are summarized in Table 1, each of which is manufactured according to the process parameters specified in Table 2. The following Table 3 summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0050] Table 1 shows the compositions, expressed in weight percent, of steels. Steel compositions I1 to I5 for the manufacture of steel sheets according to the invention, and the table also specifies reference steel compositions, designated in the table by R1 to R4. Table 1 also serves as a comparison table between the steels of the invention and the reference steels. Table 1 is as follows:

[0051] [Table 1]

[0052] Table 2 summarizes the annealing process parameters carried out on the steels of Table 1. Steel compositions I1 to I5 serve for the manufacture of steel sheets according to the invention. The table also specifies reference steels designated in the table by R1 to R4. Table 2 also lists the annealing process parameters for the steels of Table 1. min and Tc max A table of these Tc max and Tc min is as follows for the steel of the present invention and the reference steel: max =565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb Tc min =565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb It is defined as follows:

[0053] Furthermore, before the annealing treatment was performed on the inventive and reference steels, the steels were heated to temperatures between 1000°C and 1280°C, then hot rolled at a finishing temperature above 850°C and coiled at a temperature below 600°C. The hot rolled coils were then processed as claimed and then cold rolled to a thickness reduction between 30 and 95%. These cold rolled steel sheets, both the inventive and reference steels, were subjected to the heat treatments listed in Table 2 herein.

[0054] [Table 2]

[0055] Table 3 illustrates the results of tests carried out according to various microscopic standards, such as scanning electron microscopy, to determine the microstructural composition of both the steel of the invention and the reference steel.

[0056] The results are presented here.

[0057] [Table 3]

[0058] Table 4 illustrates the mechanical properties of both the steels of the invention and the reference steels. Tensile tests are carried out according to the JIS Z2241 standard to measure the tensile strength, yield strength and total elongation.

[0059] The results of the various mechanical tests carried out in accordance with the standard are tabulated below.

[0060] [Table 4]

Claims

1. 2. A cold rolled heat treated steel sheet comprising the following elements expressed in weight percent: 0.10%≦Carbon≦0.5% 1%≦Manganese≦3.4% 0.5%≦Silicon≦2.5% 0.03%≦Aluminum≦1.5% Sulfur≦0.003% 0.002%≦Phosphorus≦0.02% Nitrogen≦0.01% and any of the following elements: 0.05%≦Chromium≦1% 0.001%≦Molybdenum≦0.5% 0.001%≦Niobium≦0.1% 0.001%≦Titanium≦0.1% 0.01%≦Copper≦2% 0.01%≦Nickel≦3% 0.0001%≦Calcium≦0.005% Vanadium≦0.1% Boron≦0.003% Cerium≦0.1% Magnesium≦0.010% Zirconium≦0.010% a residual composition consisting of iron and unavoidable impurities produced by processing, and a microstructure of the rolled steel sheet containing, in area fractions, 10 to 30% retained austenite, 5 to 50% annealed bainite, 10 to 40% bainite, 1% to 20% hardened martensite, and less than 30% tempered martensite, and a total amount of bainite and retained austenite is 30% or more.

2. 2. The cold rolled heat treated steel sheet according to claim 1, wherein the composition comprises 0.7%≦silicon≦2.2%.

3. 3. The cold rolled heat treated steel sheet according to claim 1 or 2, wherein the composition comprises 1%≦silicon≦2.2%.

4. The cold rolled heat treated steel sheet according to any one of claims 1 to 3, wherein the composition includes 0.03%≦aluminum≦1.0%.

5. The cold rolled heat treated steel sheet according to any one of claims 1 to 4, wherein the composition comprises 1.2%≦manganese≦2.3%.

6. The cold rolled heat treated steel sheet according to any one of claims 1 to 5, wherein the composition comprises 0.05%≦chromium≦0.5%.

7. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 6, wherein the sum of the retained austenite phase and the bainite phase exceeds 35%.

8. The cold rolled heat treated steel sheet according to any one of claims 1 to 7, wherein the sum of the annealed bainite phase and the bainite phase exceeds 45%.

9. The cold rolled heat treated steel sheet according to any one of claims 1 to 8, wherein the retained austenite is between 15 and 30%.

10. The cold rolled heat treated steel sheet according to any one of claims 1 to 9, wherein bainite is between 15% and 40%.

11. The cold rolled heat treated steel sheet according to any one of claims 1 to 10, having a tensile strength of more than 950 MPa and a total elongation of 20% or more.

12. The cold rolled heat treated steel sheet according to any one of claims 1 to 11, having a yield strength of more than 600 MPa and a ratio of yield strength to tensile strength of 0.6 or more.

13. 13. The cold rolled heat treated steel sheet according to any one of claims 1 to 12, having a tensile strength between 1000 MPa and 1100 MPa and a total elongation of 23% or more.

14. The cold-rolled heat-treated steel sheet according to any one of claims 1 to 13, characterized in that it does not contain ferrite.

15. A method for producing a cold rolled heat treated steel sheet according to claims 1 to 14, comprising the following successive steps: - Providing a steel composition according to any one of claims 1 to 6, - reheating the semi-finished product to a temperature between 1200°C and 1280°C, - rolling said semi-finished product in the austenitic range with a hot rolling finishing temperature above Ac3 to obtain a hot rolled steel sheet; - cooling the steel sheet at a cooling rate of more than 30°C / s to a coiling temperature of less than 600°C and coiling the hot-rolled sheet; - cooling the hot rolled sheet to room temperature; - optionally subjecting the hot rolled steel sheet to a descaling process, - optionally carrying out annealing on the hot-rolled steel sheet at a temperature between 400°C and 750°C, - optionally subjecting the hot rolled steel sheet to a descaling process, - cold rolling the hot rolled steel sheet with a reduction ratio of between 35% and 90% to obtain a cold rolled steel sheet; - then performing a first annealing by heating the cold rolled steel sheet at a rate of more than 3°C / s to a soaking temperature between Ac3 and Ac3+100°C and holding it for 10 to 500 seconds; - then cooling the sheet at a rate of more than 25°C / s to a temperature between 380°C and 480°C and holding the cold rolled sheet for a time between 10 and 500 seconds; - cooling the cold rolled steel sheet to room temperature to obtain a cold rolled annealed steel sheet; Next, the cold rolled annealed steel sheet is heated at a rate of more than 3 ° C. / sec. ｓｏａｋｉｎｇ performing a second anneal by heating to a soaking temperature between Ac1 and Ac2 and holding it for 10 to 500 seconds; - The steel sheet is then heated to Tc at a rate of more than 20 ° C / sec. ｍａｘ ~Tc ｍｉｎ wherein Tcmax and Tcmin are as follows: ・Tcmax=565-601*(1-Exp(-0.868*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb ・Tcmin=565-601*(1-Exp(-1.736*C))-34*Mn-13*Si-10*Cr+13*Al-361*Nb where C, Mn, Si, Cr, Al and Nb are the weight percentages of the elements in the steel. - then bringing the cold rolled annealed steel sheet to a temperature range between 350 ° C. and 550 ° C. for 5 to 500 seconds and cooling the annealed cold rolled steel sheet to room temperature at a cooling rate of more than 1 ° C. / second to obtain a cold rolled heat treated steel sheet. A manufacturing method comprising:

16. The method for producing a cold rolled heat treated steel sheet according to claim 15, wherein the coiling temperature of the hot rolled steel sheet is less than 570°C.

17. The method for producing a cold-rolled heat treated steel sheet according to claim 15 or 16, wherein the cold-rolled steel sheet is subjected to a first annealing at a soaking temperature between Ac3 and Ac3+75°C for 10 seconds to 500 seconds.

18. The cold-rolled steel sheet that has been subjected to the first annealing is called T ｓｏａｋｉｎｇ The method for producing a cold-rolled heat-treated steel sheet according to any one of claims 15 to 17, wherein the steel is continuously annealed for 10 seconds to 500 seconds between Ac1 and Ac2 so that the ratio of austenite to annealed bainite is between 50:50 and 90:

10.

19. Use of a steel sheet according to any one of claims 1 to 14 or produced according to the method according to claims 15 to 18 for the manufacture of structural or safety parts of a vehicle.

20. A part obtainable according to claim 19 by flexible rolling of said steel sheet.

21. A vehicle comprising a part obtained according to any one of claims 14 to 20.

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