Cold rolled and double annealed steel sheets

Abstract

The present invention relates to a cold-rolled and double-annealed steel sheet, which is made of steel having the following composition, wherein the composition comprises, by weight percentage: C: 0.03% to 0.18%, Mn: 6.0% to 11.0%, 0.2%≤Al<3%, Mo: 0.05% to 0.5%, B: 0.0005% to 0.005%, S≤0.010%, P≤0.020%, N≤0.008%, and the composition optionally comprises, by weight percentage, one or more of the following elements: Si≤1.20%, Nb≤0.050%, Ti≤0.050%, Cr≤0.5%, V≤0.2%, the remainder of the composition being iron and unavoidable impurities resulting from smelting, the steel plate having a microstructure comprising, by surface fraction, the following: 0% to 45% ferrite, 20% to 50% retained austenite, 5% to 80% annealed martensite, less than 5% fresh martensite, and the carbon content [C]A and the manganese content [Mn]A in the austenite, expressed in weight %, such that the ratio ([C]A 2 ×[Mn]A) / (C% 2 × Mn%) is 4.5 to 11.0, C% and Mn% are the nominal C weight percentage and Mn weight percentage in the steel, and is less than 4×106 / mm 2 carbide density.

Description

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

[0002] For the production of various parts, such as components of body structural members and body panels of motor vehicles, it is known to use plates made of DP (Dual Phase) steel or TRIP (Transformation Induced Plasticity) steel.

[0003] In view of global environmental protection, one of the main challenges in the automotive industry is to reduce the weight of vehicles to improve their fuel efficiency without neglecting safety requirements. To meet these requirements, the steelmaking industry continues to develop new high-strength steels to provide steels with improved yield strength and tensile strength, as well as good ductility and formability.

[0004] One development aimed at improving mechanical properties is increasing the manganese content in steel. The presence of manganese helps increase the steel's ductility by stabilizing the austenite. However, these steels suffer from brittleness. To overcome this, elements such as boron are added. These boron-added chemicals are very strong during the hot rolling stage, but the strips become too hard to be processed further. The most effective method for softening the strips is batch annealing, but this results in a loss of toughness.

[0005] In addition to these mechanical requirements, such steel sheets must also exhibit good resistance to liquid metal embrittlement (LME). Zinc-coated steel sheets or zinc alloy-coated steel sheets are very effective for corrosion resistance and are therefore widely used in the automotive industry. However, experience has shown that arc welding or resistance welding of certain steels can lead to 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 zinc along the grain boundaries of the underlying steel substrate under applied stress or internal stress caused by restraint, thermal expansion, or phase transformation. Additions of elements such as carbon or silicon are known to be detrimental to LME resistance.

[0006] The automotive industry typically assesses such resistance by placing an upper limit on the so-called LME index, calculated according to the following formula:

[0007] LME index = C% + Si% / 4, where C% and Si% represent the weight percentages of carbon and silicon in steel, respectively.

[0008] Publication WO2020011638 relates to a method for providing a medium-manganese (Mn 3.5% to 12%) cold-rolled steel with a reduced carbon content. Two process routes are described. The first involves intercritical annealing of the cold-rolled steel sheet. The second involves double annealing of the cold-rolled steel sheet, the first annealing being fully austenitic and the second annealing being intercritical. Due to the choice of annealing temperature, a good compromise between tensile strength and elongation is obtained. By lowering the annealing temperature, an enrichment of austenite is obtained, which means good fracture thickness strain values. However, the low amounts of carbon and manganese used in this invention limit the tensile strength of the steel sheet to a value not higher than 980 MPa.

[0009] Therefore, the object of the present invention is to solve the above problems and provide a steel plate having a combination of high mechanical properties of a tensile strength TS higher than or equal to 900, a uniform elongation UE higher than or equal to 11.0%, a yield strength higher than or equal to 700 MPa and satisfying the formula [(YS-200)×UE+(TS-300)×TE] / (C%×Mn%) higher than 29000, TE being the total elongation of the plate expressed in %, the tensile strength TS expressed in MPa, the yield strength YS expressed in MPa, the uniform elongation UE expressed in %, and C% and Mn% being the nominal weight % of C and Mn of the steel.

[0010] Preferably, the total elongation TE of the steel plate is higher than or equal to 15.0%.

[0011] Preferably, the LME index of the steel plate according to the present invention is less than 0.36.

[0012] Preferably, the carbon equivalent Ceq of the steel sheet according to the present invention is less than 0.4%, said carbon equivalent being defined as

[0013] Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo%

[0014] The elements are expressed in percentage by weight.

[0015] Preferably, the α value of the resistance spot weld of two steel parts of a steel sheet according to the invention is at least 30 daN / mm2.

[0016] The object of the present invention is achieved by providing a cold-rolled and double-annealed steel plate made of steel having the following composition, wherein the composition comprises, by weight percentage:

[0017] C: 0.03% to 0.18%

[0018] Mn: 6.0% to 11.0%

[0019] 0.2%≤Al<3%

[0020] Mo: 0.05% to 0.5%

[0021] B: 0.0005% to 0.005%

[0022] S≤0.010%

[0023] P≤0.020%

[0024] N≤0.008%

[0025] And the composition optionally comprises one or more of the following elements in weight percentage:

[0026] Si≤1.20%

[0027] Nb≤0.050%

[0028] Ti≤0.050%

[0029] Cr≤0.5%

[0030] V≤0.2%

[0031] The remainder of the composition is iron and unavoidable impurities resulting from smelting.

[0032] The steel plate has a microstructure comprising, by surface fraction, the following:

[0033] -0% to 45% ferrite,

[0034] -20% to 50% retained austenite,

[0035] -5% to 80% annealed martensite,

[0036] - less than 5% fresh martensite,

[0037] -Carbon content in austenite expressed in wt% [C] A and manganese content [Mn] A , so that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 × Mn%) is 4.5 to 11.0, C% and Mn% are the nominal C weight percentage and Mn weight percentage in the steel, and

[0038] -Less than 4×10 6 / mm 2 The steel plate may further include any of the following features, alone or in combination:

[0039] Optionally, the carbon content is 0.05% to 0.15%;

[0040] Optionally, the manganese content is 6.0% to 9%;

[0041] Optionally, the aluminum content is 0.2% to 2.2%;

[0042] Optionally, the microstructure comprises 5% to 25% ferrite, 25% to 50% retained austenite, and 25% to 70% annealed martensite;

[0043] Optionally, ferrite is present and is equiaxed;

[0044] Optionally, the microstructure contains no ferrite, 25% to 45% retained austenite, and 55% to 75% annealed martensite;

[0045] Optionally, the tensile strength is higher than or equal to 900 MPa, the uniform elongation UE is higher than or equal to 11.0%, the yield strength YS is higher than or equal to 700 MPa, and the total elongation TE, YS, UE, TS is such that [(YS-200)×UE+(TS-300)×TE] / (C%×Mn%) is higher than 29000;

[0046] Optionally, the LME index is below 0.36;

[0047] Optionally, the carbon equivalent Ceq of the steel is less than 0.4%, said carbon equivalent being defined as

[0048] Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133xMn%xMo%

[0049] The elements are expressed in percentage by weight.

[0050] Another object of the present invention is a resistance spot weld of two steel parts made of cold rolled and double annealed steel sheets according to any of the above solutions, wherein the resistance spot weld has an alpha value of at least 30 daN / mm. 2 .

[0051] The present invention will now be described in detail and illustrated by way of examples without introducing any limitations.

[0052] According to the present invention, the carbon content is between 0.03% and 0.18% to ensure satisfactory strength and good weldability. Carbon contents above 0.18% may reduce the steel's weldability and resistance to LME. 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 below 0.03%, the strength of the annealed martensite is insufficient to achieve a UTS above 900 MPa. In a preferred embodiment of the present invention, the carbon content is between 0.05% and 0.15%. In another preferred embodiment, the carbon content is between 0.08% and 0.12%, or even more preferably, between 0.08% and 0.10%.

[0053] The manganese content is 6.0% to 11.0%. Additions above 11.0% may reduce the weldability of the steel sheet and the productivity of component assembly. Furthermore, the risk of centerline segregation increases, impairing mechanical properties. Since the soaking temperature also depends on the manganese content, a minimum manganese content is defined to stabilize the austenite and achieve the desired microstructure and strength after soaking. Preferably, the manganese content is 6.0% to 9%.

[0054] 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. An addition of more than 3% may reduce the weldability of the steel plate, thereby reducing castability. In addition, it is difficult to achieve a tensile strength higher than 900 MPa. In addition, the higher the aluminum content, the higher the soaking temperature for stabilizing austenite. Aluminum is added at least up to 0.2% to improve product strength by expanding the critical zone range and improve weldability. In addition, 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%, and more preferably 0.7% to 2.2%.

[0055] Molybdenum content ranges from 0.05% to 0.5% to reduce manganese segregation during casting. Furthermore, adding at least 0.05% molybdenum provides resistance to embrittlement. Above 0.5%, the cost of adding molybdenum is high and unsuitable considering the desired properties. In a preferred embodiment of the present invention, the molybdenum content ranges from 0.15% to 0.35%.

[0056] According to the present invention, a boron content of 0.0005% to 0.005% improves the toughness of hot-rolled steel sheets and the spot weldability of cold-rolled steel sheets. Above 0.005%, boron carbides are formed at the grain boundaries of the prior austenite, making the steel more brittle. In a preferred embodiment of the present invention, the boron content is 0.001% to 0.003%.

[0057] Optionally, some elements may be added to the composition of the steel according to the invention.

[0058] The maximum silicon content added is limited to 1.20% to improve LME resistance. Furthermore, this low silicon content allows for a simpler process by eliminating the need for pickling the hot-rolled steel sheet prior to hot strip annealing. Preferably, the maximum silicon content added is 0.8%.

[0059] Titanium can be added up to 0.050% to provide precipitation hardening. Preferably, a minimum of 0.010% titanium is added when boron is added to protect the boron from forming BN.

[0060] Niobium may optionally be added up to 0.050% to refine the austenite grains during hot rolling and provide precipitation hardening. Preferably, the minimum amount of niobium added is 0.010%.

[0061] Chromium and vanadium may optionally be added up to 0.5% and 0.2% respectively to provide improved strength.

[0062] The remainder of the steel composition is iron and impurities resulting from smelting. In this regard, P, S, and N are at least considered to be residual elements that are unavoidable impurities. Their contents are less than or equal to 0.010% for S, less than or equal to 0.020% for P, and less than or equal to 0.008% for N.

[0063] The microstructure of the steel plate according to the present invention will now be described. It comprises, in terms of surface fraction:

[0064] -0% to 45% ferrite.

[0065] -20% to 50% retained austenite.

[0066] -5% to 80% annealed martensite.

[0067] - Less than 5% fresh martensite.

[0068] -Carbon content in austenite expressed in wt% [C] A and manganese content [Mn] A , so that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 × Mn%) is 4.5 to 11.0, C% and Mn% are the nominal C weight percentage and Mn weight percentage in the steel, and

[0069] -Less than 4×10 6 / mm 2 carbide density.

[0070] The microstructure of the steel sheet according to the present invention contains 20% to 50% retained austenite. Below 20% retained austenite, the uniform elongation (UE) cannot reach the minimum value of 11.0%. Above 50%, the yield strength is less than 700 MPa.

[0071] Such austenite may be formed during intercritical annealing of a hot-rolled steel sheet, and may also be formed by transformation of a portion of martensite at a high temperature during the first annealing or the second annealing of a cold-rolled steel sheet.

[0072] Carbon content in austenite expressed as a percentage by weight [C] A and manganese content [Mn] A So that the ratio ([C] A 2×[Mn] A ) / (C% × Mn%) is between 4.5 and 11.0, where C% and Mn% are the nominal carbon and manganese weight percentages in the steel. This formula indicates the level of partitioning of carbon and manganese into retained austenite. When the ratio is below 4.5, the yield strength cannot reach the minimum level of 700 MPa. When the ratio is above 11.0, the retained austenite is too stable to provide a sufficient TRIP-TWIP effect during deformation. Such a TWIP-TRIP effect is clearly demonstrated 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, page 2356, Vol. 46A, June 2015 (S. LEE, K. LEE and BCDE COOMAN).

[0073] The microstructure of the steel sheet according to the present invention comprises 0% to 45% ferrite. Such ferrite may form during the first annealing of the cold-rolled steel sheet when the first annealing is performed at a temperature below the Ac3 of the cold-rolled steel sheet. When the first annealing of the cold-rolled steel sheet is performed at a temperature above the Ac3 of the cold-rolled steel sheet, ferrite is absent. In a preferred embodiment, such ferrite recrystallizes and exhibits equiaxed grains with an aspect ratio below 2.

[0074] The microstructure of the steel sheet according to the present invention comprises 5% to 80% annealed martensite. Such martensite can be formed by transformation of a portion of austenite enriched below nominal carbon and manganese during cooling after intercritical annealing of the hot-rolled steel sheet. However, the annealed martensite is primarily formed during cooling after the first annealing of the cold-rolled steel sheet and is then annealed during the second annealing of the cold-rolled steel sheet. Such annealed martensite can be tempered martensite and / or regenerated martensite and / or recrystallized martensite. When the second annealing is performed in a lower temperature range, the martensite can preferably be tempered martensite and regenerated martensite. When the second annealing is performed in a higher temperature range, the martensite can preferably be regenerated martensite and recrystallized martensite.

[0075] Fresh martensite may be present at a surface fraction of less than 5%, but it is not a desirable phase in the microstructure of the steel sheet according to the present invention. It may form during the final cooling step to room temperature through the transformation of unstable austenite, which is low in manganese and carbon. In fact, this unstable austenite with low carbon and manganese contents leads to a martensite start temperature, Ms, above 20°C. To achieve the final mechanical properties, the fresh martensite content must be less than 5%, and preferably less than 3%, or even better, reduced to 0%.

[0076] Finally, the carbide density should be kept below 4×10 6 / mm 2 To ensure that the formula [(YS-200)×UE+(TS-300)×TE] / (C%×Mn%) remains above 29,000.

[0077] In a first embodiment, the microstructure comprises 5% to 25% ferrite, 25% to 50% retained austenite, and 25% to 70% annealed martensite.

[0078] In another embodiment, the microstructure comprises no ferrite, 25% to 45% retained austenite, and 55% to 75% annealed martensite.

[0079] The steel plate according to the present invention has a tensile strength TS higher than or equal to 900 MPa, a uniform elongation UE higher than or equal to 11.0%, a yield strength higher than or equal to 700 MPa and satisfies the formula [(YS-200)×UE+(TS-300)×TE] / (C%×Mn%) higher than 29000, where TE is the total elongation of the plate.

[0080] Preferably, the total elongation TE of the steel plate is higher than or equal to 15.0%.

[0081] Preferably, the LME index of the steel plate according to the present invention is less than 0.36.

[0082] Preferably, the carbon equivalent Ceq of the steel sheet according to the present invention is less than 0.4%, said carbon equivalent being defined as

[0083] Ceq=C%+Si% / 22+Cr% / 20+Mn% / 19-Al%18+2.2P%-3.24B%-0.33*Mn%*Mo% wherein the elements are expressed in weight percentage.

[0084] The welded assembly can be manufactured by producing two components from the steel sheet according to the invention and then performing resistance spot welding of the two steel components.

[0085] The resistance spot weld joining the first plate to the second plate is characterized by a high resistance in a transverse tensile test defined by an α value of at least 30 daN / mm2.

[0086] The steel sheet according to the present invention can be produced by any suitable manufacturing method and the method can be defined by a person skilled in the art. However, it is preferred to use a method according to the present invention comprising the following steps:

[0087] A semi-finished product having the above steel composition that can be further hot rolled is provided. The semi-finished product is heated to a temperature of 1150°C to 1300°C to facilitate hot rolling, wherein the final hot rolling temperature FRT is 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0088] The hot rolled steel is then cooled and heated to a temperature T of 20°C to 650°C, and preferably 300°C to 500°C. 卷取 Roll up.

[0089] The hot rolled steel sheet is then cooled to room temperature and can be pickled.

[0090] The hot rolled steel sheet is then annealed to an annealing temperature Tc of 680°C. HBA . Tc corresponds to the temperature at which carbides are completely dissolved and can be determined by FEG-SEM observation after heat treatment. Within this range, annealing will allow minimizing the area fraction of precipitated carbides and promoting the distribution of manganese into austenite. In addition, below 680°C, the microstructure is not coarsened. Tc is higher than Ac1 because Tc is the boundary line 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 line between the ferrite / carbide region and the ferrite / austenite / carbide region. Preferably, the temperature T HBA 600℃ to 680℃.

[0091] The steel plate is placed at the temperature T HBA Keep the holding time t at 0.1 hours to 120 hours HBATo promote the diffusion of manganese. In addition, such heat treatment of the hot rolled steel sheet allows the hardness to be reduced while maintaining the toughness of the hot rolled steel sheet.

[0092] The hot rolled and heat treated steel plate is then cooled to room temperature and may be pickled to remove oxidation.

[0093] The hot rolled and heat treated steel sheets are then cold rolled at a reduction ratio of 20% to 80%.

[0094] The cold-rolled steel sheet is then subjected to a first annealing at a temperature T1 of (Ac1+Ac3) / 2 to (Ac3+80) for a holding time t1 of 10 to 1800 seconds. When T1 is higher than this limit, sufficient austenite cannot be stabilized at room temperature. Preferably, T1 is 720°C to 900°C, and more preferably 720°C to 870°C, and the time t1 is 100 to 1000 seconds. Such annealing can be performed by continuous annealing.

[0095] The cold rolled and annealed steel sheet is then cooled to below 80°C, preferably at an average cooling rate of at least 0.1°C / second, and more preferably at least 1°C / second. The sheet's microstructure then consists of austenite and martensite and may also contain ferrite if the annealing temperature is below Ac3. If annealing is performed above Ac3, such ferrite will not be present.

[0096] After cooling, the steel sheet is then subjected to a second annealing step at a temperature T2 of 350° C. to 650° C. for a time t2 of 1 to 100 hours. Preferably, T2 is 400° C. to 650° C. and t2 is 1 to 50 hours. This step can be performed by batch annealing.

[0097] The primary purpose of the second annealing step is to temper the martensite at the start of the annealing process while the temperature is still low. Then, as the temperature rises, the distribution of carbon and manganese from the adjacent martensite to the austenite continues. Finally, when the temperature reaches T2, part of the martensite transforms into austenite.

[0098] The second annealing temperature T2 depends on the chemical composition, the intermediate batch annealing, and the first annealing. The second annealing temperature T2 should be low enough to limit the formation of unstable austenite, which will subsequently transform into fresh martensite under small deformation conditions, resulting in a reduction in both yield strength and elongation. The second annealing temperature T2 should be low enough to avoid the formation of unstable austenite, which will transform into fresh martensite during final cooling, resulting in a reduction in elongation. The second annealing temperature T2 should also be high enough to avoid the formation of excessive carbides, which consume carbon and manganese and lead to a reduction in strength. This carbide formation may occur in particular when the second annealing temperature T2 is below the Tc value of the steel sheet.

[0099] The second annealing temperature T2 should also be high enough to avoid the formation of overstable austenite, which would lead to a reduction in elongation due to the lack of the TRIP-TWIP effect.

[0100] The cold rolled and double annealed steel sheet is then cooled to room temperature, and during such cooling a small proportion of fresh martensite can be formed by transformation of a portion of the austenite which is low in manganese and carbon.

[0101] The panels may 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.

[0102] The invention will now be illustrated by the following examples which are in no way limiting. Example

[0103] Three grades, the compositions of which are summarized in Table 1, were cast into semi-finished products and processed into steel plates.

[0104] Table 1 - Composition

[0105] The compositions tested are summarized in the table below, where the element contents are expressed in weight percent.

[0106]

[0107] The Ac1 and Ac3 temperatures of the cold-rolled sheets were determined by dilatometry testing and metallographic analysis.

[0108] Table 2 - Process parameters of hot rolled and heat treated steel plates

[0109] The cast steel semi-finished product is reheated at 1200℃, hot rolled, and then coiled. The hot rolled and coiled steel plate is then heated to a temperature of T HBA heat treatment and maintain at the temperature for a holding time t HBA The following specific conditions for obtaining hot rolled and heat treated steel plates apply:

[0110]

[0111] Underlined value: Parameters of the target property are not allowed to be obtained Table 3 - Processing of cold rolled and double annealed steel sheets parameter

[0112] The obtained hot-rolled and heat-treated steel sheet is then cold-rolled. The cold-rolled steel sheet is then first annealed at a temperature T1 and held at that temperature for a holding time t1, and then cooled at a cooling rate of 2°C / second. The steel sheet is then second-heated at a temperature T2 and held at that temperature for a holding time t2, and then cooled to room temperature. The following specific conditions are applied to obtain the cold-rolled and annealed steel sheet:

[0113]

[0114]

[0115] Underlined value: Parameters of the target property are not allowed to be obtained

[0116] The cold rolled and annealed plates were then analyzed, and the corresponding microstructural elements, mechanical properties, and weldability properties are summarized in Tables 4, 5, and 6, respectively.

[0117] Table 4 - Microstructure of cold rolled and double annealed steel plates

[0118] The phase percentages of the microstructure of the obtained cold rolled and double annealed steel sheets were determined.

[0119] [C] A and [Mn] A Corresponds to the amount of carbon and manganese in the austenite in weight percentages. The carbon C% is measured using X-ray diffraction and the manganese Mn% is measured using an electron probe microanalyser with a field emission gun.

[0120] The surface fractions of the phases in the microstructure are determined by cutting a specimen from a cold-rolled and double-annealed steel sheet, polishing the specimen and etching it with reagents known per se to reveal the microstructure. The cross section is then examined by scanning electron microscopy, for example with a field emission gun ("FEG-SEM") in secondary electron mode at a magnification greater than 5000×.

[0121] Annealed martensite and fresh martensite can be distinguished by their morphology: annealed martensite has a smooth surface and sometimes carbides inside, compared to fresh martensite which has a rough surface and no carbides.

[0122] The determination of the surface fraction of ferrite was performed by SEM observation after etching with Nital or picric acid / Nital reagents.

[0123] The volume fraction of retained austenite was determined by X-ray diffraction.

[0124] The density of the precipitated carbides was determined by examining cross sections of the plates at a magnification greater than 15,000× via scanning electron microscopy with a field emission gun (“FEG-SEM”) and image analysis.

[0125]

[0126] Underlined values: not corresponding to the present invention

[0127] Table 5 - Mechanical properties of cold rolled and double annealed steel sheets

[0128] The mechanical properties of the obtained cold rolled and double annealed steel sheets were determined and summarized in the following table.

[0129] The yield strength YS, the tensile strength TS as well as the uniform elongation UE and the total elongation TE are measured in accordance with ISO standard ISO 6892-1, published in October 2009.

[0130]

[0131] Underlined value: does not match the target value

[0132] Test specimens 1, 2, 3, 4, 8, 19, 26, 27 and 28 were subjected to a too low temperature T2. A 2 ×[Mn] A ) / (%C 2 The value of (×% Mn) indicates that the formed austenite is too stable, resulting in a decrease in uniform elongation.

[0133] In contrast, samples 5, 9, 18, 24 were subjected to a sufficiently high T2 temperature to ensure that the stability of the austenite was on target, resulting in very good uniform and total elongation.

[0134] Furthermore, samples 19, 25, 26, 27, and 28 were subjected to a temperature T2 lower than Tc and contained an excessively high amount of carbides, exceeding 4×10 6 / mm 2 The maximum acceptable value.

[0135] Test specimens 10, 11, 12, 20 and 21 were subjected to an excessively high temperature T2. A 2 ×[Mn] A ) / (%C 2 The values ​​of (×% Mn) indicate that the austenite that forms is too unstable, resulting in a reduction in yield strength. In addition, all of these samples show some fresh martensite formation, with samples 10, 11, and 20 exceeding the maximum acceptable value of 5%. In contrast, samples 13 and 22 were subjected to a sufficiently low T2 temperature to ensure that the stability of the austenite met the target, without fresh martensite formation, resulting in very good properties.

[0136] Table 6 - Weldability properties of cold rolled and double annealed steel plates

[0137] Cold rolled and double annealed steel plates were spot welded under standard ISO 18278-2 conditions.

[0138] In the test used, the sample consists of two steel plates in the form of a transverse weld equivalent. A force is applied to break the weld. This force, known as the cross tensile strength (CTS), is expressed in daN. It depends on the diameter of the weld and the thickness of the metal, that is, the thickness of the steel and the metal coating. This makes it possible to calculate the coefficient α, which is the ratio of the CTS value to the product of the diameter of the weld multiplied by the thickness of the base material. This coefficient is expressed in daN / mm 2 express.

[0139] The weldability characteristics of the cold rolled and double annealed steel plates were determined and summarized in the following table:

[0140]

[0141] LME index = C% + Si% / 4, in weight %.

Claims

1. A cold rolled and double annealed steel sheet made of steel having the following composition, wherein the composition comprises, by weight percentage: C: 0.03% to 0.18% Mn: 6.0% to 11.0% 0.2％≤Al<3％ Mo: 0.05% to 0.5% B: 0.0005% to 0.005% S≤0.010％ P≤0.020％ N≤0.008％ 0.010%≤Ti≤0.050% And the composition optionally comprises one or more of the following elements in weight percentage: Si≤1.20% Nb≤0.050% Cr≤0.5％ V≤0.2％ The remainder of the composition is iron and unavoidable impurities resulting from smelting. The steel plate has a microstructure comprising, by surface fraction, the following: -0% to 45% ferrite, -20% to 50% retained austenite, -5% to 80% annealed martensite, - less than 5% fresh martensite, -Carbon content in austenite expressed in wt% [C] A and manganese content [Mn] A , so that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 × Mn%) is 4.5 to 11.0, C% and Mn% are the nominal C weight percentage and Mn weight percentage in the steel, and -Less than 4×10 6 Carbide density / mm2 The tensile strength is higher than or equal to 900 MPa, the uniform elongation UE is higher than or equal to 11.0%, the yield strength YS is higher than or equal to 700 MPa, and the total elongation TE, YS, UE, TS makes [(YS-200)×UE+(TS-300)×TE] / (C%×Mn%) higher than 29000.

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

3. The steel sheet according to any one of claims 1 or 2, wherein the manganese content is 6.0% to 9%.

4. The steel sheet according to any one of claims 1 or 2, wherein the aluminum content is 0.2% to 2.2%. 5 . The steel sheet according to claim 1 , wherein the microstructure comprises 5% to 25% ferrite, 25% to 50% retained austenite, and 25% to 70% annealed martensite.

6. The steel plate according to any one of claims 1 or 2, wherein the ferrite is present and is equiaxed. 7 . The steel sheet according to claim 1 , wherein the microstructure does not contain ferrite, and contains 25% to 45% of retained austenite and 55% to 75% of annealed martensite.

8. The steel plate according to any one of claims 1 or 2, wherein the LME index is lower than 0.36, LME index = C% + Si% / 4 in weight%.

9. The steel plate according to any one of claims 1 or 2, wherein the carbon equivalent Ceq of the steel is less than 0.4%, the carbon equivalent being defined as Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133xMn%xMo% The elements are expressed in percentage by weight.

10. A resistance spot weld of two steel parts made from a cold rolled and double annealed steel sheet according to any one of claims 1 to 9, the resistance spot weld having an alpha value of at least 30 daN / mm 2 , the α is the ratio of the value of the transverse tensile strength to the product of the diameter of the welding point and the thickness of the base material.

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