Cold-rolled doubly annealed steel sheet

Abstract

To provide a high strength steel sheet having satisfactory weldability.SOLUTION: 0.03 to 0.18% C, 6.0 to 11.0% Mn, 0.2 ≤ Al <3%, 0.05 to 0.5% Mo, 0.0005 to 0.005% B, S ≤ 0.010%, P ≤ 0.020%, N ≤ 0.008%, and optionally one or more of the elements Si ≤ 1.20%, Nb ≤ 0.050%, Ti ≤ 0.050%, Cr ≤ 0.5%, V ≤ 0.2%, in weight percentages, the remainder being iron and unavoidable impurities; A microstructure of 0-45% ferrite, 20-50% retained austenite, 5-80% annealed martensite, and less than 5% fresh martensite in surface fractions, wherein the C and Mn contents in the austenite and the nominal C and Mn weight percentages in the steel satisfy a predetermined relation, and further comprising a concentration of carbides below 4 * 106 / mm2.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] It is known to use steel sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels to manufacture various items such as automotive body structural components and body panels.

[0003] One of the major challenges in the automotive industry is to reduce vehicle weight to improve fuel efficiency and protect the global environment, without compromising safety. To meet these requirements, the steel industry is continually developing new high-strength steels with improved yield strength, tensile strength, 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 in brittleness. To overcome this problem, elements such as boron are added. These boron-added chemistries are very tough at the hot rolling stage, but the hot band is too hard to be further processed. The most efficient way to soften the hot band is batch annealing, but it leads to a loss of toughness.

[0005] In addition to these mechanical requirements, such steels must also exhibit good resistance to liquid metal embrittlement (LME). Zinc- or zinc-alloy-coated steels are highly effective in corrosion resistance and are therefore widely used in the automotive industry. However, it has been observed that arc or resistance welding of certain steels can cause the development of specific cracks due to a phenomenon known as liquid metal embrittlement ("LME") or liquid metal-assisted cracking ("LMAC"). This phenomenon is characterized by the penetration of liquid Zn along grain boundaries of the underlying steel substrate under applied or internal stresses resulting from restraint, thermal expansion, or phase transformation. The addition of elements such as carbon or silicon is known to be detrimental to LME resistance.

[0006] In the automotive industry, the following formula is usually used: LME index=C%+Si% / 4, Such resistance is assessed by limiting the upper limit of the so-called LME index, calculated according to: where C% and Si% represent the weight percentages of carbon and silicon in the steel, respectively.

[0007] Publication WO2020011638 relates to a method for providing medium and intermediate manganese (3.5-12% Mn) cold-rolled steel with reduced carbon content. Two process routes are described. The first involves a single intercritical annealing of the cold-rolled steel sheet. The second involves double annealing of the cold-rolled steel sheet, the first fully austenitic and the second intercritical. Depending on the annealing temperature, a good compromise between tensile strength and elongation can be achieved. By reducing the annealing temperature, austenite enrichment is obtained, which implies good strain-to-thickness values. However, the low amounts of carbon and manganese used in the present invention limit the tensile strength of the steel sheet to values ​​below 980 MPa. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2020 / 011638 Summary of the Invention

[0009] It is therefore an object of the present invention to solve the above-mentioned problems and to provide a steel sheet having high mechanical properties in combination with a tensile strength TS of 900 or more, a uniform elongation UE of 11.0% or more and a yield strength of 700 MPa or more, and satisfying the formula [(YS-200)xUE+(TS-300)xTE] / (C%xMn%) greater than 29000, where TE is the total elongation of the sheet expressed in %, the tensile strength TS is expressed in MPa, the yield strength YS is expressed in MPa, the uniform elongation UE is expressed in %, and C% and Mn% are the nominal weight % of C and Mn in the steel. [Means for solving the problem]

[0010] Preferably, the steel sheet has a total elongation TE of 15.0% or more.

[0011] Preferably, the steel sheet according to the invention has an LME index of less than 0.36.

[0012] Preferably, the steel sheet according to the invention has a carbon equivalent Ceq of less than 0.4%, the carbon equivalent being: Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo% and elements are expressed as weight percent.

[0013] Preferably, the resistance spot weld between two steel parts of a steel sheet according to the invention has an α value of at least 30 daN / mm 2 .

[0014] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also comprise any of the features of claims 2 to 10, either alone or in combination.

[0015] Another object of the invention is the resistance spot welding of two steel parts according to claim 11. DETAILED DESCRIPTION OF THE INVENTION

[0016] The invention will now be described in detail and illustrated by examples, without introducing any limitations.

[0017] According to the present invention, the carbon content is 0.03% to 0.18% to ensure satisfactory strength and good weldability. If the carbon content exceeds 0.18%, the weldability and resistance to LME of the steel plate may be reduced. The soaking temperature depends on the carbon content: the higher the carbon content, the lower the soaking temperature required to stabilize the austenite. If the carbon content is less than 0.03%, the strength of the annealed martensite is not sufficient to obtain a UTS of more than 900 MPa. In a preferred embodiment of the present invention, the carbon content is 0.05% to 0.15%. In another preferred embodiment of the present invention, the carbon content is 0.08 to 0.12%, or even better, 0.08 to 0.10%.

[0018] The manganese content is 6.0% to 11.0%. If the addition amount exceeds 11.0%, the weldability of the steel sheet may deteriorate, which may reduce the productivity of component assembly. Furthermore, the risk of center segregation increases to the extent that it impairs mechanical properties. Since the soaking temperature also depends on the manganese content, a minimum manganese content is specified to stabilize austenite in order to obtain the target microstructure and strength after soaking. Preferably, the manganese content is 6.0% to 9%.

[0019] According to the present invention, the aluminum content is 0.2% to 3% to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. Addition of aluminum in excess of 3% can reduce the weldability and castability of the steel sheet. Furthermore, it is difficult to achieve a tensile strength of more than 900 MPa. Furthermore, the higher the aluminum content, the higher the soaking temperature required to stabilize austenite. Aluminum is added up to at least 0.2% to improve product robustness and weldability by widening the transformation interval. Furthermore, aluminum can be added to avoid inclusion and oxidation problems. In a preferred embodiment of the present invention, the aluminum content is 0.2% to 2.2%, more preferably 0.7 to 2.2%.

[0020] The molybdenum content is 0.05% to 0.5% to reduce manganese segregation during casting. Furthermore, the addition of at least 0.05% molybdenum provides resistance to embrittlement. Above 0.5%, the addition of molybdenum is costly and ineffective considering the required properties. In a preferred embodiment of the present invention, the molybdenum content is 0.15% to 0.35%.

[0021] According to the present invention, the boron content is 0.0005% to 0.005% to improve the toughness of hot-rolled steel sheets and the spot weldability of cold-rolled steel sheets. 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%.

[0022] Optionally, some elements can be added to the composition of the steel according to the invention.

[0023] The maximum silicon content is limited to 1.20% to improve LME resistance. In addition, this low silicon content allows the process to be simplified by eliminating the step of pickling the hot-rolled steel sheet before hot band annealing. Preferably, the maximum silicon content added is 0.8%.

[0024] Titanium can be added up to 0.050% to provide precipitation strengthening. Preferably, a minimum of 0.010% titanium is added in addition to boron to protect the boron from forming BN.

[0025] Niobium may optionally be added up to 0.050% to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.010%.

[0026] Optionally, chromium and vanadium may be added up to 0.5% and 0.2%, respectively, to improve strength.

[0027] The remainder of the steel's 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 limited to 0.010% for S, 0.020% for P, and 0.008% for N.

[0028] Next, the microstructure of the steel sheet according to the present invention will be described, which has a surface fraction of: - 0% to 45% ferrite, - 20% to 50% retained austenite, - 5-80% annealed martensite, - less than 5% fresh martensite, - Ratio([C] A 2 ×[Mn] A ) / (C% 2Carbon in austenite [C], expressed in weight percent, such that C% × Mn%) is 4.5 to 11.0, and C% and Mn% are the nominal C and Mn weight percents in the steel. A and manganese [Mn] A Content, and - 4×10 6 / mm 2 Carbide density below Contains:

[0029] The microstructure of the steel sheet according to the present invention contains 20% to 50% retained austenite. If the austenite content is less than 20%, the uniform elongation (UE) cannot reach the minimum value of 11.0%. If the austenite content is more than 50%, the yield strength will be less than 700 MPa.

[0030] Such austenite can be formed not only during intercritical annealing of hot-rolled steel sheets, but also during the first annealing of cold-rolled steel sheets or during the second annealing by partial transformation of martensite at high temperatures.

[0031] Carbon in austenite [C] A and manganese [Mn] A The content is expressed as a weight percent and the ratio ([C] A 2 ×[Mn] A ) / (C% 2The formula is such that C% × Mn%) is 4.5 to 11.0, where C% and Mn% are the nominal C and Mn weight percents in the steel. This formula indicates the level of carbon and manganese partitioning into retained austenite. If the ratio is below 4.5, the yield strength cannot reach the minimum level of 700 MPa. If the ratio is above 11.0, the retained austenite is too stable to provide sufficient TRIP-TWIP effect during deformation. Such TWIP-TRIP effects are notably described in "Observation of the TWIP-TRIP Plasticity Enhancement Mechanism in Al-Added 6 Wt Pct Medium Mn Steel," DOI: 10.1007 / s11661-015-2854-z, The Minerals, Metals & Materials Society and ASM International 2015, p. 2356, Vol. 46A, June 2015 (S. Lee, K. Lee, and B. C. De Cooman).

[0032] The microstructure of the steel sheet according to the present invention contains 0-45% ferrite. Such ferrite can be formed during the first annealing of the cold-rolled steel sheet if the annealing is performed at a temperature below Ac3 of the cold-rolled steel sheet. If the first annealing of the cold-rolled steel sheet is performed above Ac3 of the cold-rolled steel sheet, no ferrite is present. In a preferred embodiment, such ferrite is recrystallized and exhibits equiaxed grains with a shape ratio below 2.

[0033] The microstructure of the steel sheet according to the present invention contains 5 to 80% annealed martensite. Such martensite can be formed during cooling after intercritical annealing of the hot-rolled steel sheet by the transformation of a portion of austenite with less carbon and manganese than the nominal value. However, it is primarily formed during cooling after the first annealing of the cold-rolled steel sheet and then annealed during the second annealing of the cold-rolled steel sheet. Such annealed martensite can be tempered martensite and / or recovered and / or recrystallized martensite. If the second annealing is performed at a lower temperature range, the martensite can preferably be tempered martensite and recovered martensite. If the second annealing is performed at a higher temperature range, the martensite can preferably be recovered and recrystallized martensite.

[0034] Fresh martensite can be present at a surface fraction below 5%, but it is not a desired phase in the microstructure of the steel sheet according to the invention. It can be formed during the final cooling step to room temperature by the transformation of unstable austenite, which is poor in manganese and carbon. In fact, this unstable austenite, which has a low carbon and manganese content, leads to a martensite start temperature Ms above 20°C. To obtain the final mechanical properties, the fresh martensite must be reduced to below 5%, preferably below 3%, or even better to 0%.

[0035] Finally, to ensure that the formula [(YS-200)xUE+(TS-300)xTE] / (C%xMn%) remains above 29000, the carbide density is 4x10 6 / mm 2 must remain below

[0036] In a first embodiment, the microstructure comprises 5% to 25% ferrite, 25% to 50% retained austenite, and 25% to 70% annealed martensite.

[0037] In another embodiment, the microstructure is ferrite-free and contains 25% to 45% retained austenite and 55% to 75% annealed martensite.

[0038] The steel sheet according to the present invention has a tensile strength TS of 900 MPa or more, a uniform elongation UE of 11.0% or more, a yield strength of 700 MPa or more, and satisfies the formula [(YS-200)xUE+(TS-300)xTE] / (C%xMn%) greater than 29000, where TE is the total elongation of the steel sheet.

[0039] Preferably, the steel sheet has a total elongation TE of 15.0% or more.

[0040] Preferably, the steel sheet according to the invention has an LME index of less than 0.36.

[0041] Preferably, the steel sheet according to the invention has a carbon equivalent Ceq of less than 0.4%, the carbon equivalent being: Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo% and elements are expressed as weight percent.

[0042] A welded assembly can be manufactured by producing two parts from a steel plate according to the invention and then performing resistance spot welding of the two steel parts.

[0043] The resistance spot weld joining the first sheet to the second sheet is characterized by a high resistance in a cross tensile test defined by an α value of at least 30 daN / mm 2 .

[0044] The steel sheet according to the invention can be produced 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 comprising the following steps: The above-mentioned steel composition is provided in a semi-finished product that can be further hot rolled. The semi-finished product is heated to a temperature of 1150°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 850°C to 950°C.

[0045] The hot-rolled steel is then cooled and coiled at a temperature Tcoil of 20°C to 650°C, preferably 300°C to 500°C.

[0046] The hot rolled steel sheet can then be cooled to room temperature and pickled.

[0047] Next, the hot-rolled steel sheet is annealed at an annealing temperature Tc of 680°C. HBA The steel is annealed to 680°C. Tc corresponds to the temperature at which carbides completely dissolve, and can be measured by FEG-SEM observation after heat treatment. In this range, annealing minimizes the area fraction of precipitated carbides and promotes manganese distribution into austenite. Furthermore, below 680°C, the microstructure does not coarsen. Tc is higher than Ac1 because Tc is the boundary between the ferrite / austenite / carbide three-phase region and the ferrite / austenite two-phase region, which is higher than the Ac1 temperature because Ac1 is the boundary between the ferrite / carbide region and the ferrite / austenite / carbide region. Preferably, the temperature T HBA is 600℃ to 680℃.

[0048] The steel sheet is subjected to a holding time t of 0.1 to 120 hours to promote manganese diffusion. HBA During this time, the temperature T HBA Furthermore, this heat treatment of the hot-rolled steel sheet makes it possible to reduce the hardness while maintaining the toughness of the hot-rolled steel sheet.

[0049] The hot rolled heat treated steel sheet can then be cooled to room temperature and pickled to remove oxidation.

[0050] The hot-rolled heat-treated steel sheet is then cold-rolled at a reduction rate of 20% to 80%.

[0051] The cold-rolled steel sheet is then subjected to a first annealing at a temperature T1 between (Ac1 + Ac3) / 2 and (Ac3 + 80) for a holding time t1 between 10 and 1800 seconds. If T1 is higher than this limit, sufficient austenite cannot be stabilized at room temperature. Preferably, T1 is between 720 and 900°C, more preferably between 720 and 870°C, and the time t1 is between 100 and 1000 seconds. Such annealing can be carried out by continuous annealing.

[0052] The cold rolled annealed steel sheet is then cooled to below 80°C, preferably at an average cooling rate of at least 0.1°C / s, preferably at least 1°C / s. The microstructure of the steel sheet then consists of austenite and martensite, and may also contain ferrite if the annealing temperature is below Ac3. Such ferrite is not present if the annealing is carried out above Ac3.

[0053] After cooling, the steel sheet is then subjected to a second annealing step at a temperature T2 of 350-650°C for a time t2 of 1-100 hours. Preferably, T2 is 400-650°C and t2 is 1-50 hours. This step can be carried out by batch annealing.

[0054] The primary purpose of the second anneal is to temper the martensite at the beginning of the annealing, while the temperature is still low. Then, as the temperature increases, the partitioning of carbon and manganese from the adjacent martensite to the austenite continues. Finally, as the temperature reaches T2, some of the martensite transforms to austenite.

[0055] The second annealing temperature T2 depends on the chemical composition, intermediate batch anneals, and first annealing. It must be low enough to limit the formation of unstable austenite, which then transforms to fresh martensite with small deformations, resulting in a loss of both yield strength and elongation. It must be low enough to avoid the formation of unstable austenite, which transforms to fresh martensite during final cooling, resulting in a loss of elongation. It must also be high enough to avoid the formation of excess carbides, which consume carbon and manganese and result in a loss of strength. This carbide formation is particularly likely when the second annealing temperature T2 is below the Tc value of the steel plate.

[0056] The second annealing temperature T2 must also be high enough to avoid the formation of too stable austenite, which would result in a loss of elongation due to the lack of the TRIP-TWIP effect.

[0057] The cold rolled duplex annealed steel sheet is then cooled to room temperature, during which time a small amount of fresh martensite may be formed by transformation of some of the austenite which is poorer in manganese and carbon.

[0058] The steel sheet can then be coated by any suitable method, including hot dip coating, electrodeposition or vacuum coating of zinc or zinc-based alloys or aluminum or aluminum-based alloys.

[0059] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0060] Three grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates.

[0061] Table 1 - Composition The compositions tested are summarized in the table below, with elemental contents expressed as weight percent.

[0062] [Table 1] The Ac1 and Ac3 temperatures of the cold rolled sheets are measured by dilatometry testing and metallographic analysis.

[0063] Table 2 - Process parameters for hot-rolled heat-treated steel sheets The cast steel semi-finished products were reheated at 1200°C, hot rolled and then coiled. The following specific conditions for obtaining hot rolled heat treated steel sheets were applied:

[0064] [Table 2] Underlined: Parameters that cannot achieve the target characteristics

[0065] Table 3 - Process parameters for cold rolled double annealed steel sheets The resulting hot-rolled heat-treated steel sheet is then cold-rolled. The cold-rolled steel sheet is then first annealed at a temperature T1 and maintained at said temperature for a holding time t1 before being cooled at a cooling rate of 2°C / s. The steel sheet is then heated a second time at a temperature T2 and maintained at said temperature for a holding time t2 before being cooled to room temperature. The following specific conditions for obtaining the cold-rolled annealed steel sheet were applied:

[0066] [Table 3] Underlined: Parameters that cannot achieve the target characteristics

[0067] The cold-rolled and annealed sheets were then analyzed, and the corresponding microstructural elements, mechanical properties and weldability properties were summarized in Tables 4, 5 and 6, respectively.

[0068] Table 4 - Microstructure of cold rolled double annealed steel sheet The phase percentages of the microstructures of the obtained cold rolled duplex annealed steel sheets were measured.

[0069] [C] A and [Mn] Acorresponds to the amount of carbon and manganese in the austenite in weight percent, which are measured using X-ray diffraction for carbon C% and an electron probe microanalyzer equipped with a field emission electron gun for manganese Mn%.

[0070] The surface fraction of the phases in the microstructure is measured by the following method: a specimen is cut from cold-rolled, double-annealed steel sheet, polished, and etched with reagents known per se to reveal the microstructure, and then examined using a scanning electron microscope, for example a scanning electron microscope equipped with a field emission gun ("FEG-SEM"), in secondary electron mode at a magnification of more than 5000x.

[0071] Annealed martensite can be distinguished from fresh martensite by their morphology: annealed martensite has a smooth surface with occasional carbides in the interior, compared to fresh martensite, which has surface roughness and no carbides.

[0072] The measurement of the surface fraction of ferrite is carried out by SEM observation after Nital or Picral / Nital reagent etching.

[0073] The measurement of the volume fraction of retained austenite is carried out by X-ray diffraction.

[0074] The density of precipitated carbides is measured by thin plate specimens examined with a scanning electron microscope equipped with a field emission gun ("FEG-SEM") and image analysis at magnifications in excess of 15,000 times.

[0075] [Table 4] Underlined: Not applicable to the present invention

[0076] Table 5 - Mechanical properties of cold-rolled double-annealed steel sheets The mechanical properties of the obtained cold-rolled double-annealed steel sheets were measured and are summarized in the table below.

[0077] Yield strength YS, tensile strength TS, and uniform and total elongation UE, TE are measured according to ISO standard ISO6892-1 published in October 2009.

[0078] [Table 5] Underlined: Does not match target value

[0079] Tests 1, 2, 3, 4, 8, 19, 26, 27 and 28 were subjected to a temperature T2 that was too low. The austenite formed was ([C] A 2 ×[Mn] A ) / (%C 2 × %Mn) values ​​are too high, resulting in poor uniform elongation.

[0080] In contrast, Tests 5, 9, 18, and 24 were subjected to T2 temperatures high enough to ensure that austenite stability was on target, resulting in very good uniform and total elongation.

[0081] Furthermore, Tests 19, 25, 26, 27, and 28 were below Tc and 4 × 10 6 / mm 2 The specimen was subjected to a temperature T2 containing too high an amount of char, exceeding the maximum allowable value.

[0082] Tests 10, 11, 12, 20 and 21 were subjected to a temperature T2 that was too high. The austenite formed was ([C] A 2 ×[Mn] A ) / (%C 2 The austenite was too unstable, as indicated by too low a value of (x%Mn), resulting in a reduction in yield strength. Furthermore, all of these tests showed some fresh martensite formation, with tests 10, 11, and 20 exceeding the maximum allowable value of 5%. In contrast, tests 13 and 22 were subjected to a T2 temperature low enough to ensure that austenite stability was on target without the formation of fresh martensite, resulting in very good properties.

[0083] Table 6 - Weldability characteristics of cold-rolled double-annealed steel sheets Spot welding according to ISO standard 18278-2 was carried out on cold-rolled double-annealed steel sheets.

[0084] In the test used, the sample consists of two steel plates in the form of an identical cross weld. A force is applied to break the weld. This force is known as the cross tensile strength (CTS) and is expressed in daN. It depends on the diameter of the weld and the thickness of the metal, i.e. the thickness of the steel and the metal coating. This makes it possible to calculate a coefficient α, which is the ratio of the value of CTS to the product of the diameter of the weld and the thickness of the base material. This coefficient is expressed in daN / mm 2 It is expressed as:

[0085] The weldability properties of the cold rolled duplex annealed steel sheets were measured and are summarized in the table below.

[0086] [Table 6] LME index=C%+Si% / 4(weight%).

Claims

1. 1. A cold rolled duplex annealed steel sheet comprising, in weight percent: C: 0.03-0.18% Mn: 6.0-11.0% 0.2≦Al<3% Mo: 0.05-0.5% B: 0.0005-0.005% S≦0.010% P≦0.020% N≦0.008% and optionally containing one or more of the following elements in weight percentages: Si≦1.20% Nb≦0.050% Ti≦0.050% Cr≦0.5% V≦0.2% The remainder of the composition is iron and inevitable impurities resulting from smelting, The steel plate has a surface fraction of -0% to 45% ferrite, -20% to 50% retained austenite, -5 to 80% annealed martensite, - less than 5% fresh martensite, -Ratio ([C] Ａ ２ × [Mn] Ａ ) / (C% ２ Carbon in austenite [C] expressed in weight %, such that C% and Mn% are the nominal C and Mn weight percents in the steel. Ａ and manganese [Mn] Ａ Content, and -4 x 10 ６ / mm ２ Carbide density below 1. A cold-rolled, duplex-annealed steel sheet having a microstructure comprising:

2. 2. The steel sheet according to claim 1, wherein the carbon content is 0.05% to 0.15%.

3. 3. The steel sheet according to claim 1, wherein the manganese content is 6.0% to 9%.

4. 4. The steel sheet according to claim 1, wherein the aluminum content is between 0.2% and 2.2%.

5. A steel sheet according to any one of claims 1 to 4, wherein the microstructure comprises 5% to 25% ferrite, 25% to 50% retained austenite and 25% to 70% annealed martensite.

6. The steel sheet according to any one of claims 1 to 5, wherein ferrite is present and is equiaxed.

7. A steel sheet according to any one of claims 1 to 5, wherein the microstructure is ferrite-free and comprises 25% to 45% retained austenite and 55% to 75% annealed martensite.

8. The steel sheet according to any one of claims 1 to 7, wherein the steel sheet has a tensile strength of 900 MPa or more, a uniform elongation UE of 11.0% or more, a yield strength YS of 700 MPa or more, and a total elongation TE, YS, UE, TS such that [(YS - 200) x UE + (TS - 300) x TE] / (C% x Mn%) is greater than 29,000.

9. The steel sheet according to any one of claims 1 to 8, having an LME index below 0.

36.

10. The steel has a carbon equivalent Ceq of less than 0.4%, said carbon equivalent being Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133xMn%xMo% 10. The steel sheet according to any one of claims 1 to 9, wherein the elements are expressed in weight percent.

11. A resistance spot weld of two steel parts made of cold rolled duplex annealed steel sheet according to any one of claims 1 to 10, wherein the resistance spot weld has a strength of at least 30 daN / mm ２ Resistance spot welds having an α value of

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