Cold-rolled, annealed and tempered steel sheet and its manufacturing method

Abstract

The present invention relates to a cold rolled, annealed and tempered steel sheet made of a steel, the steel having the following properties expressed in percentages by weight: C: 0.03% to 0.18% Mn: 4.5% to 10.0% B: 0.0005% to 0.005% Ti: 0.010% to 0.050% Si: 0.1 to 1.20% S≦0.010% P≦0.020% N≦0.010% Including, and optionally the following elements, expressed in weight percent: Al≦2.5% Mo≦0.4% Nb≦0.050% Cr≦0.5% V≦0.2% The remainder of the composition is iron and unavoidable impurities resulting from smelting, and in surface fraction: - 0% to 30% ferrite, - 3% to 30% retained austenite F γ and the manganese content in austenite expressed in weight percent [Mn] γ But, F γ ×([Mn] γ -1.3×%Mn) 2 >1.00, such as retained austenite, - Residual tempered martensite The present invention deals with cold-rolled, annealed, and tempered steel sheets made of steel having a microstructure including

Description

[Technical Field]

[0001] The present invention relates to a steel sheet having high strength and high ductility and to a method for obtaining such a steel sheet. [Background technology]

[0002] It is known to use sheets made from DP (dual phase) steels or TRIP (transformation induced plasticity) steels to manufacture various articles such as automotive body structural members and body panel components.

[0003] One of the main challenges in the automotive industry is to reduce the weight of vehicles in order to improve fuel efficiency and protect the global environment, without ignoring safety requirements. To meet these requirements, new high-strength steels are continuously developed by the steel industry to provide plates with improved yield strength and tensile strength, as well as good ductility and formability.

[0004] One of the developments made to improve mechanical properties is to increase the manganese content in steel. The presence of manganese helps to increase the ductility of steel by stabilizing austenite. However, these steels exhibit a weakness of brittleness. To overcome this problem, elements such as boron are added. These boron-added chemicals make the hot band very tough during the hot rolling stage, but the hot band is too hard for further processing. The most efficient way to soften the hot band is batch annealing, but this can lead to a loss of toughness.

[0005] Publication WO2022018568 describes a cold-rolled, annealed, and tempered steel sheet having a combination of good weldability and high mechanical properties, a yield strength of 1000 MPa or more, a tensile strength (TS) of 1450 MPa or more, a uniform elongation (UE) of 6.5% or more, and a total elongation (TE) of 9% or more, in which the hot-rolled steel sheet is annealed before cold rolling to promote the diffusion of manganese and reduce the hardness while maintaining the toughness of the hot-rolled steel sheet. Nevertheless, this manufacturing method does not allow for a steel sheet with a high hole expansion ratio. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2022 / 018568 Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, an object of the present invention is to solve the above problems and to provide a steel sheet having high mechanical properties and a combination of tensile strength TS, yield strength YS, uniform elongation UE, total elongation TE and hole expansion ratio HE that satisfies TS*TE+YS*UE+(TE*HE*100)>49000%MPa. [Means for solving the problem]

[0008] Preferably, the steel sheet according to the invention has a carbon equivalent Ceq of less than 0.4%, the carbon equivalent being defined as follows: Ceq=%C+%Si / 55+%Cr / 20+%Mn / 19-%Al / 18+2.2%P-3.24%B-0.133*%Mn*%Mo Here, the elements are expressed as weight percent.

[0009] The object of the present invention is achieved by providing a steel sheet as set forth in claim 1. Another object is achieved by providing a steel sheet as set forth in claim 2. The steel sheet may also have the features of claim 3.

[0010] Another object is achieved by providing a method as claimed in claim 4. The method may also have the features of claim 5.

[0011] The invention will now be described in detail and illustrated by way of example, without introducing any limitations, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view of a steel sheet according to the present invention. [Figure 2] FIG. 2 is a cross-sectional view of a steel sheet not according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The invention will now be described and illustrated in detail by means of examples, without introducing any limitations.

[0014] Next, the composition of the steel sheet according to the present invention will be described, with the contents expressed as weight percent (wt%).

[0015] According to the present invention, to ensure satisfactory strength and good weldability, the carbon content is 0.03% to 0.18%. If the carbon content exceeds 0.18%, the weldability of the steel sheet may be reduced. If the carbon content is less than 0.03%, the content of tempered martensite is insufficient to obtain a TS exceeding 1450 MPa. In a preferred embodiment of the present invention, the carbon content is 0.05% to 0.18%. In another preferred embodiment of the present invention, the carbon content is 0.10 to 0.18%.

[0016] The manganese content is 4.5% to 10.0%. Addition of more than 10.0% may reduce the weldability of the steel plate and reduce the productivity of component assembly. Furthermore, the risk of center segregation increases to the point where mechanical properties are impaired. The minimum manganese content is defined to stabilize austenite and obtain the target microstructure and strength after soaking. Preferably, the manganese content is 5.5% to 9.0%, more preferably 6.0% to 9.0%.

[0017] According to the present invention, the boron content is 0.0005% to 0.005% to improve the toughness of the hot-rolled steel sheet. If the boron content exceeds 0.005%, the formation of boron carbides at the prior austenite grain boundaries is promoted, making the steel more brittle. In a preferred embodiment of the present invention, the boron content is 0.001% to 0.003%.

[0018] Titanium can be added up to 0.050% to provide precipitation strengthening. A minimum of 0.010% titanium is added in addition to boron to protect the boron from the formation of BN.

[0019] According to the present invention, the silicon content is 0.1% to 1.20% to simplify the process by eliminating the step of pickling the hot-rolled steel sheet before hot band annealing. The maximum addition of silicon content is limited to 1.20% to improve LME resistance. Preferably, the maximum silicon content added is 1.0%.

[0020] Optionally, some elements can be added to the composition of the steel according to the invention.

[0021] Aluminum can be added up to 2.5% to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. Addition of more than 2.5% may reduce the weldability, e.g., castability, of the steel plate. Furthermore, it is difficult to achieve a tensile strength of more than 1450 MPa. Preferably, the maximum aluminum content added is 0.5%, more preferably 0.3%, and even more preferably 0.1%.

[0022] Molybdenum can be added up to 0.4% to reduce manganese segregation during casting. Above 0.4%, the addition of molybdenum becomes costly and ineffective given the properties required. Preferably, a minimum of 0.1% molybdenum is added to provide resistance to embrittlement.

[0023] Niobium may optionally be added up to 0.05% to refine the austenite grains during hot rolling and provide precipitation strengthening.

[0024] Chromium and vanadium may optionally be added up to 0.5% and 0.2%, respectively, to provide improved strength.

[0025] The remainder of the steel composition consists of iron and impurities resulting from smelting. In this respect, P, S and N are considered to be residual elements, at least unavoidable impurities. Their contents are: P max 0.020%, S max 0.010% and N max 0.010%.

[0026] Next, the microstructure of the cold-rolled, annealed, and tempered steel sheet of the present invention will be described. This steel sheet contains the following in its surface fraction: - 0% to 30% ferrite, - 3% to 20% retained austenite F γ (Manganese content in austenite [Mn] γ and the nominal manganese content %Mn expressed as weight percent, F γ ×([Mn] γ -1.3×%Mn) 2 >1.00) - The remainder is tempered martensite.

[0027] The microstructure of the cold-rolled, annealed, and tempered steel sheet according to the present invention contains 0 to 30% ferrite. Such ferrite can be formed during annealing of the hot-rolled steel sheet and the cold-rolled steel sheet when the annealing is performed at a temperature of Ac1 to Ac3. When the annealing of the cold-rolled steel sheet is performed above Ac3, no ferrite is present.

[0028] The microstructure of the steel sheet according to the present invention is 3% to 20% of retained austenite (F γ Below 3% or above 20% austenite, the uniform elongation UE and total elongation TE cannot reach the target values ​​of 6.5% and 9%.

[0029] Such austenite is formed not only during the intercritical multi-stage annealing of hot-rolled steel sheets, but also during the annealing of cold-rolled steel sheets. During the intercritical annealing of hot-rolled steel sheets, regions with manganese content higher than the nominal value (%Mn) and regions with manganese content lower than the nominal value are formed, resulting in a heterogeneous distribution of manganese. The lower the annealing temperature, the lower the manganese content [Mn] in the austenite. γ Therefore, the temperature T X The first step of the multi-step annealing at a temperature T1 lower than 0.1 makes it possible to create regions with a higher amount of manganese compared to the final step of the multi-step annealing.

[0030] A large amount of manganese in the austenite favors the stability of the austenite, and this manganese heterogeneity helps to achieve the mechanical properties.

[0031] Manganese content in retained austenite expressed in weight percent [Mn] γ is F γ ×([Mn] γ -1.3×%Mn) 2 >1.00. Below 1.00, the austenite is not stabilized sufficiently to achieve the desired compromise of high strength, high ductility, and high hole expansion ratio. Preferably, the manganese content [Mn] in the retained austenite is γ is 9.6% by weight or more, more preferably 9.7% by weight or more, and even more preferably 9.8% by weight or more.

[0032] The remainder of the microstructure is tempered martensite. The martensite formed during cooling of the cold rolled steel sheet after annealing is tempered during tempering of the cold rolled steel sheet.

[0033] The steel sheet according to the present invention has high mechanical properties and a combination of tensile strength TS, yield strength YS, uniform elongation UE, total elongation TE and hole expansion ratio HE that meet TS*TE+YS*UE+(TE*HE*100)>49000%MPa.

[0034] Preferably, the steel sheet has a TS greater than 1450 MPa, a YS greater than 1000 MPa, a uniform elongation greater than 6.5%, and a total elongation equal to or greater than 9.0%. Preferably, the steel sheet has a hole expansion ratio HE greater than 15%, more preferably greater than 18%.

[0035] Preferably, the steel sheet according to the invention has a carbon equivalent Ceq of less than 0.4%, the carbon equivalent being defined as follows: Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo% Here, the elements are expressed as weight percent.

[0036] The steel sheet of the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use a method according to the invention, which comprises the following steps:

[0037] The above steel composition is provided in a semi-finished product which can be further hot rolled. The semi-finished product is heated to a temperature of 1100°C to 1300°C to facilitate hot rolling, with a final hot rolling temperature FRT of 800°C to 1000°C. Preferably, the FRT is 800°C to 900°C.

[0038] Next, the hot-rolled steel sheet is cooled to a temperature T below 650 ° C, preferably 300 to 550 ° C. coil The steel sheet is then coiled at a temperature of 1000°C, and hot rolled to obtain a coiled steel sheet.

[0039] The hot rolled and coiled steel sheet can then be cooled to room temperature and pickled.

[0040] The hot-rolled steel sheet is then annealed in at least two steps (hereinafter referred to as multi-step annealing) to promote heterogeneous manganese distribution.

[0041] In the first step, the hot-rolled steel sheet is heated to an annealing temperature T1 between Ac1 and Tc, where Tc is the carbide dissolution temperature at equilibrium conditions, which can be determined by thermodynamic calculations performed using software such as Thermo-Calc®.

[0042] In order to form at least 10% austenite at equilibrium according to Thermo-Calc® calculations, with the remainder being ferrite and carbides, the steel sheet is maintained at the T1 temperature for a holding time t1 of 0.1 to 120 hours.

[0043] In the final step, the hot-rolled steel sheet is heated from T1 to a temperature T higher than Tc and lower than Ac3, preferably higher than Tc and lower than 680 ° C. X and hold for 0.1 to 40 hours. X and cooling at room temperature, a hot-rolled annealed steel sheet is obtained having a microstructure comprising, in surface fraction: - 15% to 50% of retained austenite (more than 6.0% of the retained austenite is [Mn] γ >1.35×(%Mn) and [Mn] γ and %Mn are the manganese content and nominal manganese content in austenite, respectively, expressed in weight percent; - less than 1% carbides, - The remainder being ferrite.

[0044] Preferably, more than 6.5%, more preferably more than 7.0%, and even more preferably more than 8.5% of the retained austenite is [Mn] γ Meets >1.35×(%Mn).

[0045] This multi-step annealing promotes manganese diffusion and the formation of a non-uniform manganese distribution, and also reduces the hardness of the hot-rolled steel sheet while maintaining its toughness.

[0046] In a preferred embodiment of the present invention, one additional step of annealing can be added between the first and final step of annealing. In this additional step, the steel sheet is annealed from T1 to T1-T X is heated to a temperature T2, and X The temperature is maintained at T2 for a holding time t2 of 0.1 to 120 hours before being heated to T2.

[0047] In another preferred embodiment of the present invention, up to three additional annealing steps can be added between the first and final step of annealing.

[0048] The hot rolled heat treated steel sheet can then be cooled to room temperature and pickled to remove oxidation.

[0049] Next, the hot-rolled heat-treated steel sheet is cold-rolled at a reduction ratio of 20% to 80%.

[0050] Next, the cold rolled steel sheet is i Temperature T of ~(Ac3+100*%C / 0.1) A Hold time t from 1 second to 3600 seconds A Annealed over T i is the temperature above which less than 30% surface fraction of ferrite remains at the end of this anneal, the remainder being austenite, and T i is determined by dilatometry tests and metallographic analysis, Ac3 is determined by dilatometry of cold rolled steel sheet, and %C refers to the nominal concentration of carbon. Above (Ac3 + 100 * %C / 0.1), the austenite formed at the end of the soaking will result in too little retained austenite being stabilized during cooling to room temperature. Preferably, T A is 750°C to 850°C. Preferably, t A The annealing time is 100 to 1000 seconds. Such annealing can be carried out by continuous annealing.

[0051] The cold-rolled annealed steel sheet is then quenched at an average cooling rate of at least 5°C / s to below Ms-100°C. Part of the austenite present at the end of the soaking is converted to fresh martensite during this cooling. Preferably, the average cooling rate is higher than 10°C / s to promote martensite formation.

[0052] After quenching, the steel plate is heated to a temperature T h The tempering process is carried out at 1 second to 2 hours. h Between the T h Preferably, the temperature is maintained at T h is 150°C to 300°C, more preferably 150°C to 280°C, and even more preferably 150°C to 250°C. h is 100 seconds to 1800 seconds, more preferably 100 seconds to 500 seconds.

[0053] The fresh martensite is transformed into tempered martensite at the end of this tempering process.

[0054] The cold rolled, annealed and tempered steel sheet is then cooled to room temperature and can then be coated by any suitable process, including hot dip coating, electrodeposition or vacuum coating of zinc or a zinc-based alloy or an aluminum or aluminum-based alloy.

[0055] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0056] Four grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates.

[0057] The compositions tested are summarized in the table below, with their elemental contents expressed as weight percent.

[0058] [Table 1]

[0059] Ac1, Ac3, Ms and T i The temperature was determined by dilatometry testing and metallographic analysis. c was determined by thermodynamic calculations.

[0060] <Table 2 - Processing parameters for cold-rolled, annealed, and tempered steel sheets> The cast steel semi-finished products were heated to 1200°C, hot rolled at a finish rolling temperature of 850°C, and then coiled at a temperature of 450°C.

[0061] In trials 1 and 2, the hot-rolled steel sheet was then annealed in three steps: a first step at temperature T1 and held at that temperature for a holding time t1; a second step of heating from T1 to temperature T2 for a holding time t2; and a third step of heating from T1 to temperature T2 for a holding time t3. X Between T2 and temperature T X The final step is annealing to

[0062] In trials 3 to 10, the hot-rolled steel sheet is then annealed in one step at a temperature T1 and maintained at that temperature for a holding time t1.

[0063] Except for trial 3, the steel plates were held for a holding time of t A Between the temperatures T A The cold-rolled annealed steel sheet is then cooled to a temperature of 30°C and then annealed at a temperature T h and hold for a holding time t before cooling to room temperature. h The temperature is maintained for .

[0064] The following specific conditions are applied to obtain cold rolled annealed and tempered steel sheets:

[0065] [Table 2]

[0066] The hot-rolled annealed steel sheets were analyzed before the cold rolling step, and the corresponding microstructures and properties are summarized in Tables 3 and 4, respectively.

[0067] The percentages of the phases in the microstructure of the resulting hot rolled and annealed steel sheets were determined.

[0068] The surface fraction of the phases in the microstructure is determined by the following method: Samples are cut from hot-rolled annealed steel sheets, polished and etched with reagents known per se to reveal the microstructure, and the sections are then examined in a scanning electron microscope, for example a field emission scanning electron microscope ("FEG-SEM"), in secondary electron mode at a magnification of more than 5000 times.

[0069] The surface fraction of ferrite is determined by SEM observation after Nital or Picral / Nital reagent etching.

[0070] The surface fraction of retained austenite is determined by X-ray diffraction.

[0071] Figures 1 and 2 show the cross sections of the hot-rolled and annealed steel sheets from Trial 1 and Trial 3, respectively. The black areas correspond to areas with low manganese content, and the gray areas correspond to areas with high manganese content.

[0072] This diagram was obtained by cutting a sample to a thickness of 1 / 4 of the hot-rolled annealed steel sheet and polishing it.

[0073] The section is then characterized by an electron probe microanalyzer using a field emission electron gun ("FEG") at a magnification of over 10,000 times to determine the manganese content. Three 10 μm x 10 μm maps of different parts of the section were obtained. These maps were measured at 0.01 μm resolution. 2 The manganese content in weight percent is calculated for each pixel. Mn content [Mn] γPixels with [Mn] higher than 1.35 × (%Mn) correspond to white areas. Black areas correspond to pixels with [Mn] lower than 1.35 × (%Mn). γ It is an area having:

[0074] [Table 3]

[0075] This multi-step annealing promotes the diffusion of manganese in the austenite and the formation of a heterogeneous manganese distribution. The manganese distribution is heterogeneous, with regions of low manganese content and regions of high manganese content. Also, the lower the annealing temperature, the more the manganese content [Mn] in the austenite increases. γ Therefore, the first step of the multi-step annealing at a lower temperature than the final step makes it possible to create regions with a higher amount of manganese compared to the final step of the multi-step annealing.

[0076] A large amount of manganese in the austenite favors the stability of the austenite, and this manganese heterogeneity helps to achieve the mechanical properties.

[0077] In trial 3, the steel sheet was subjected to one-step hot band annealing with temperature and time parameters equivalent to the final step of the multi-step annealing in trials 1 and 2. This resulted in the reduction of [Mn] in the hot-rolled annealed steel sheet. γ >1.35×(%Mn) The Mn-enriched austenite region meeting the condition (%) is much smaller than in Trials 1 and 2.

[0078] Furthermore, as shown in Table 4, this heat treatment can improve the toughness of the hot-rolled annealed steel sheet.

[0079] [Table 4]

[0080] The toughness of the steel plates of Trials 4 to 10 is lower than that of the steel plates of Trials 1 and 2, which were subjected to multi-stage annealing.

[0081] The Charpy impact energy was measured at 20°C according to standards ISO 148-1:2006(F) and ISO 148-1:2017(F).

[0082] The cold rolled annealed and tempered steel sheets were then analyzed, and the corresponding microstructures and properties are summarized in Tables 5 and 6, respectively.

[0083] <Table 5 - Microstructure of cold-rolled, annealed and tempered steel sheets> The percent phases in the microstructure of the resulting cold rolled, annealed and tempered steel sheets were determined.

[0084] The surface fraction of the phases in the microstructure is determined by the following method: a sample is cut from cold-rolled, annealed and tempered 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.

[0085] The surface fractions of tempered martensite and ferrite are determined by SEM observation after etching with Nital or Picral / Nital reagents.

[0086] Retained austenite F γ The volume fraction of is measured by X-ray diffraction.

[0087] [Table 5]

[0088] <Table 6 - Mechanical properties of cold-rolled, annealed and tempered steel sheets> The mechanical properties of the obtained cold rolled, annealed and tempered steel sheets were determined and are summarized in the table below.

[0089] The yield strength YS, tensile strength TS and uniform and total elongations UE and TE are measured according to ISO standard ISO 6892-1 published in October 2009. The hole expansion ratio HE is measured according to ISO standard 16630:2009.

[0090] [Table 6]

[0091] Trials 1 and 2 were subjected to a multi-step annealing that allowed obtaining a large amount of Mn-enriched austenite in the hot-rolled annealed steel sheets, as shown in Table 3. Thanks to this enrichment, after being subjected to cold rolling, annealing and tempering, the microstructure was found to be Mn-enriched according to the formula F γ ×([[ γ -1.3×%Mn) 2 It still contains Mn-enriched regions that make it possible to obtain a higher proportion of stabilized austenite, highlighted by >1.00, which leads to a good compromise between ductility and strength.

[0092] In comparison, in trial 3, the steel sheet was subjected to a one-step hot band annealing with temperature and time parameters equivalent to the final step of the multi-step annealing in trials 1 and 2. This leads to much less Mn-enriched austenite region in the hot rolled annealed steel sheet than in trials 1 and 2, as shown in Table 3, resulting in a lower ductility and strength compromise.

[0093] In trials 4 to 10, the steel sheets were subjected to one-stage hot band annealing. In these cases, the Mn-enriched austenite region in the hot-rolled steel sheets was much smaller than in trials 1 and 2, as shown in Table 3. This is because, according to the formula F γ ×([Mn] γ -1.3×%Mn) 2 >1.00, resulting in less stabilized austenite, where TS*TE+YS*UE+(TE*HE*100)<49000%MPa, resulting in a poor compromise of strength / ductility / hole expansion ratio.

Claims

1. 1. A hot rolled annealed steel sheet made of a steel having the following properties, expressed in weight percent: C: 0.03% to 0.18% Mn: 4.5% to 10.0% B: 0.0005% to 0.005% Ti: 0.010% to 0.050% Si: 0.1-1.20% P≦0.020% S≦0.010% N≦0.010% Including, and optionally the following elements, expressed in weight percent: Al≦2.5% Mo≦0.4% Nb≦0.05% Cr≦0.5% V≦0.2% The remainder of the composition is iron and unavoidable impurities resulting from smelting, and in surface fraction: 15% to 50% retained austenite, of which more than 6.0% is [Mn] γ > 1.35 × (% Mn) and [Mn] γ and %Mn are the manganese content in austenite and the nominal manganese content in retained austenite, respectively, expressed in weight percent; - less than 1% carbides, - the remainder being ferrite A hot-rolled annealed steel sheet having a microstructure comprising:

2. A cold rolled, annealed and tempered steel sheet made of a steel having the composition according to claim 1, wherein the steel sheet has a surface fraction of: - 0% to 30% ferrite, - 3% to 20% retained austenite F γ The manganese content in austenite [Mn] γ and a nominal manganese content %Mn, both expressed as weight percent, F γ × ([Mn] γ -1.3x% Mn) ２ >1.00, such as retained austenite, - the remainder being tempered martensite A cold-rolled, annealed, and tempered steel sheet having a microstructure comprising:

3. The tensile strength TS, yield strength YS, uniform elongation UE, total elongation TE and hole expansion ratio HE are expressed by the following formula: (TS*TE)+(YS*UE)+(TE*HE*100)>49000MPa% The cold-rolled, annealed, and tempered steel sheet according to claim 2, which satisfies the above.

4. The following consecutive steps - casting steel to obtain slabs, said steel having a composition according to claim 1; - The slab is heated to a temperature T of 1100°C to 1300°C. ｒａｈｅａｔ heating with - hot rolling the heated slab at a finish hot rolling temperature of 800°C to 1000°C; - a coiling temperature T lower than 650 ° C. ｃｏｉｌ and winding the hot-rolled steel sheet. - optionally pickling the hot-rolled steel sheet; - hot-rolled steel sheets in at least two steps, namely: - Temperature T of Ac1 to Tc １ where Tc is the carbide dissolution temperature at equilibrium conditions, and the holding time t is between 0.1 hours and 120 hours. １ During the T １ a first step of maintaining the temperature, - T １ From temperature T Ｘ A final step of heating to T Ｘ is higher than Tc and lower than Ac3, and has a retention time t of 0.1 to 40 hours Ｘ During this time, the temperature T Ｘ Steps to maintain to obtain a hot-rolled annealed steel sheet; - cooling the hot-rolled annealed steel sheet to room temperature; - cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; - optionally pickling the cold rolled steel sheet; - Cold rolled steel sheet, T ｉ Annealing temperature T of (Ac3+100*%C / 0.1) Ａ Heat to T ｉ is the temperature above which at the end of this annealing, less than 30% of the surface fraction of ferrite is formed, the remainder being austenite, and the holding time t Ａ During the T Ａ maintaining the steel sheet at a temperature where %C is the nominal carbon content expressed in weight percent, to obtain a cold rolled annealed steel sheet; - cooling the cold rolled annealed steel sheet to a temperature below Ms-100°C at a cooling rate higher than 5°C / s; - The steel plate is heated to a temperature T of 150°C to 450°C. ｈ and maintaining at that temperature for a holding time of 1 second to 2 hours; - cooling the steel sheet to room temperature to obtain a cold-rolled, annealed and tempered steel sheet; A method for producing a cold-rolled, annealed, and tempered steel sheet, comprising:

5. The annealing of the hot-rolled steel sheet includes an additional step of annealing between the first step and the final step of annealing the hot-rolled steel sheet, and the hot-rolled steel sheet is １ From T １ Higher T Ｘ Lower temperature T ２ and in the final step, T ２ From T Ｘ A holding time t of 0.1 to 120 hours is applied before the temperature is heated to ２ 5. The method for producing a cold rolled, annealed and tempered steel sheet according to claim 4, wherein the temperature is maintained for a period of time.

6. Use of a steel sheet according to any one of claims 1 to 3 or produced according to any one of claims 4 or 5 for the manufacture of structural parts for vehicles.

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