Cold rolled annealed steel sheets or hot pressed annealed steel parts

Abstract

The present invention relates to a cold rolled, annealed and tempered 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%, Al: <3%, Mo: 0.05% to 0.5%, B: 0.0005% to 0.005%, S≤0.010%, P≤0.020%, N≤0.008%, and optionally comprises, by weight percentage, one or more of the following elements: Si≤1.20%, Ti≤0.050%, Nb≤0.050%, Cr≤0.5%, V≤0.2%, the remainder of the composition being iron and inevitable impurities resulting from smelting, the steel plate having a microstructure comprising, by surface fraction, the following: 0% to 30% ferrite, such ferrite having a grain size of less than 1.0 μm; 3% to 30% retained austenite; 40% to 95% tempered martensite; less than 5% fresh martensite, and the carbon content in austenite expressed as a weight percentage [C] A and manganese content [Mn] A So that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 C% and Mn% are the nominal values ​​of carbon and manganese in wt%.

Description

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

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

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

[0004] One of the developments made to improve the mechanical properties is to increase the manganese content in the steel. The presence of manganese helps to improve the ductility of the steel due to the stabilization of austenite. But these steels have a weakness of brittleness. To overcome this problem, elements such as boron are added. These boron-added chemicals are very strong at the hot rolling stage, but the hot strip is too hard to be processed further. The most effective way to soften the hot strip is batch annealing, but it results in a loss of toughness.

[0005] In addition to these mechanical requirements, such steel sheets must also show 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 may lead to the appearance 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 the grain boundaries of the underlying steel substrate under applied stress or internal stress caused by restraint, thermal expansion or phase transformation. It is known that the addition of elements such as carbon or silicon is detrimental to resistance to LME.

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

[0007] LME index = C% + Si% / 4,

[0008] Where C% and Si% represent the nominal weight percentages of carbon and silicon in the steel, respectively.

[0009] Publication WO2020011638 relates to a method for providing an intermediate medium manganese (3.5% to 12% Mn) cold-rolled steel with a reduced carbon content. Two process routes are described. The first involves intercritical zone 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 small amount of carbon and manganese used in the invention limits the tensile strength of the steel sheet to a value not higher than 980 MPa.

[0010] Therefore, the object of the present invention is to solve the above problems and to provide a steel sheet having a combination of high mechanical properties of a yield strength higher than or equal to 1000 MPa, a tensile strength TS higher than or equal to 1450 MPa, a uniform elongation UE higher than or equal to 6.5% and a total elongation TE higher than or equal to 9%.

[0011] Preferably, the steel sheet according to the present invention satisfies TS×TE>13700 MPa.%.

[0012] Preferably, the LME index of the steel plate according to the present invention is less than 0.36.

[0013] Preferably, the carbon equivalent Ceq of the steel sheet according to the present invention is less than 0.4%, and the carbon equivalent is defined as

[0014] Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo%

[0015] The elements are expressed in weight percentage.

[0016] Preferably, the alpha value of the resistance spot welded weld of two steel parts of a steel sheet according to the invention is at least 30 daN / mm2.

[0017] 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 according to claims 2 to 10, alone or in combination.

[0018] Another object of the invention is a resistance spot welded seam of two steel components according to claim 11 .

[0019] Another object of the invention is a press-hardened and tempered steel component according to claim 12 .

[0020] The present invention will now be described in detail and will be illustrated by way of examples without introducing limitations.

[0021] According to the present invention, the carbon content is 0.03% to 0.18% to ensure satisfactory strength and good weldability characteristics. Above 0.18% carbon, 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 for stabilizing the austenite. If the carbon content is below 0.03%, the strength of the tempered martensite is insufficient to obtain a UTS above 1450 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%.

[0022] The manganese content is 6.0% to 11.0%. An addition above 11.0% may reduce the weldability of the steel sheet and may reduce the productivity of the component assembly. In addition, the risk of center segregation increases, thereby impairing the mechanical properties. Since the soaking temperature also depends on the manganese content, the minimum amount of manganese is defined to stabilize the austenite so as to obtain the target microstructure and strength after soaking. Preferably, the manganese content is 6.0% to 9%.

[0023] According to the present invention, the aluminum content is less than 3% to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. An addition above 3% may reduce the weldability of the steel sheet, as does the castability. In addition, it is difficult to achieve a tensile strength above 1450MPa. In addition, the higher the aluminum content, the higher the soaking temperature for stabilizing austenite. It is preferred to add at least up to 0.2% aluminum to improve product robustness by expanding the critical zone range and to 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%.

[0024] The molybdenum content is 0.05% to 0.5% to reduce manganese segregation during casting. In addition, the addition of at least 0.05% molybdenum provides resistance to brittleness. Above 0.5%, the addition of molybdenum is expensive and inappropriate considering the required properties. In a preferred embodiment of the invention, the molybdenum content is 0.15% to 0.35%.

[0025] According to the present invention, the boron content is 0.0005% to 0.005% to improve the toughness of the hot rolled steel sheet and the spot weldability of the cold rolled steel sheet. Above 0.005%, the formation of boron carbides at the original 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%.

[0026] Optionally, some elements may be added to the composition of the steel according to the invention.

[0027] The maximum addition of silicon content is limited to 1.20% to improve LME resistance. In addition, such low silicon content makes it possible to simplify the process by eliminating the step of pickling the hot rolled steel sheet before tropical annealing. Preferably, the maximum silicon content added is 0.5%.

[0028] Titanium can be added up to 0.050% to provide precipitation hardening. Preferably, when adding boron a minimum of 0.010% titanium is also added to prevent boron from forming BN.

[0029] Niobium may optionally be added up to 0.050% to refine the austenite grains and provide precipitation hardening during hot rolling. Preferably, the minimum amount of niobium added is 0.010%.

[0030] Chromium and vanadium may optionally be added up to 0.5% and 0.2% respectively to provide improved strength.

[0031] The remainder of the steel composition is iron and impurities resulting from smelting. In this respect, P, S and N are at least considered as 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.

[0032] The microstructure of the steel plate according to the present invention will now be described. It comprises, in terms of surface fraction:

[0033] - 0% to 30% ferrite, such ferrite having a grain size below 1.0 μm,

[0034] -3% to 30% retained austenite,

[0035] -40% to 95% tempered martensite

[0036] - less than 5% fresh martensite,

[0037] - the carbon content [C] A and the manganese content [Mn] A in austenite expressed in weight percentages such that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 C% and Mn% are the nominal values ​​of carbon and manganese in wt%.

[0038] The microstructure of the steel sheet according to the present invention contains 3% to 30% of retained austenite. Below 3% or above 30% of austenite, the uniform elongation UE and the total elongation TE cannot reach their respective minimum values ​​of 6.5% and 9%.

[0039] Such austenite is formed during intercritical annealing of hot rolled steel sheets, and also during annealing of cold rolled steel sheets. During intercritical annealing of hot rolled steel sheets, regions containing manganese contents above the nominal value and regions containing manganese contents below the nominal value are formed, resulting in an inhomogeneous distribution of manganese. Carbon co-segregates with manganese accordingly. This manganese inhomogeneity is measured by the slope of the manganese distribution of the hot rolled steel sheet, which must be higher than or equal to -30, such as Figure 2 As shown in and described later.

[0040] The carbon content [C]A and the manganese content [Mn]A in austenite expressed in weight percentages are such that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 ×Mn%) is less than 7.80, C% and Mn% are nominal values ​​of carbon and manganese in weight %. When the ratio is higher than 7.80, the retained austenite is too stable to provide sufficient TRIP-TWIP effect during deformation. Such a TWIP-TRIP effect is particularly 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, page 2356, Vol. 46A, June 2015 (S. LEE, K. LEE and BCDE COOMAN).

[0041] The microstructure of the steel sheet according to the present invention contains 0% to 30% ferrite, such ferrite having a grain size below 1.0 μm. Such ferrite may be formed during annealing of the cold rolled steel sheet when the annealing of the cold rolled steel sheet is performed at a temperature of Ac1 to Ac3 of the cold rolled steel sheet. When the annealing of the cold rolled steel sheet is performed at a temperature higher than Ac3 of the cold rolled steel sheet, no ferrite is present. Preferably, the ferrite content is 0% to 25%.

[0042] The microstructure of the steel sheet according to the invention contains 40% to 95% tempered martensite. Such martensite can be formed by transformation of a portion of austenite enriched below nominal carbon and manganese when cooling after intercritical annealing of the hot rolled steel sheet. However, it is mainly formed when cooling after annealing of the cold rolled steel sheet and then tempered during tempering of the cold rolled steel sheet.

[0043] Fresh martensite may be present up to 5% in surface fraction, but is not a desired phase in the microstructure of the steel sheet according to the invention. It may be formed by transformation of unstable austenite during the final cooling step to room temperature. In practice, this unstable austenite with low carbon and manganese content leads to a martensite start temperature Ms above 20° C. In order to obtain the final mechanical properties, the fresh martensite is limited to a maximum of 5%, and preferably below 2%, or even better reduced to 0%.

[0044] Tempered martensite can be distinguished from fresh martensite by observation on a section polished and etched with a reagent known per se, such as Nital, or by electron backscatter diffraction (EBSD) analysis on a polished section. Tempered martensite has a lower dislocation density than fresh martensite.

[0045] In contrast, fresh martensite produced by the transformation of carbon-rich austenite to martensite after the tempering step has a C content higher than the nominal carbon content of the steel and a higher dislocation density than the tempered martensite.

[0046] In a first embodiment, the microstructure comprises 5% to 25% ferrite, 10% to 25% retained austenite, and 50% to 85% tempered martensite.

[0047] In another embodiment, the microstructure contains no ferrite, 5% to 15% retained austenite, and 85% to 95% tempered martensite.

[0048] The steel sheet according to the present invention has a yield strength YS higher than or equal to 1000 MPa, a tensile strength TS higher than or equal to 1450 MPa, a uniform elongation UE higher than or equal to 6.5% and a total elongation TE higher than or equal to 9%.

[0049] Preferably, the LME index of the cold rolled and annealed steel sheet is lower than 0.36.

[0050] Preferably, the steel sheet has a carbon equivalent Ceq below 0.4% to improve weldability. Carbon equivalent is defined as Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133*Mn%*Mo%, where the elements are expressed in weight percentage.

[0051] 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.

[0052] 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.

[0053] 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:

[0054] The above steel composition provides 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 so that it can be easily hot rolled, wherein the final hot rolling temperature FRT is 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0055] 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 the bottom.

[0056] The hot rolled steel sheet is then cooled to room temperature and may be pickled.

[0057] The hot rolled steel sheet is then annealed to an annealing temperature T of Ac1 to Ac3. HBA More precisely, choose T HBA To minimize the area fraction of precipitated carbides to less than 0.8% and promote inhomogeneous redistribution of manganese. This manganese inhomogeneity is measured by the slope of the manganese distribution of the hot rolled steel sheet, which must be higher than or equal to -30. Preferably, the temperature T HBA More preferably, the temperature T HBA 580℃ to 680℃.

[0058] The steel plate is placed at a temperature T HBA Keep at 0.1 hours to 120 hours of holding time t HBA , to promote manganese diffusion and the formation of an uneven manganese distribution. 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.

[0059] The hot rolled and heat treated steel plate is then cooled to room temperature and may be pickled to remove oxidation.

[0060] The hot rolled and heat treated steel sheets are then cold rolled at a reduction ratio of 20% to 80%.

[0061] Then, the cold-rolled steel sheet is subjected to a temperature T1 to (Ac3+50×C% / 0.1). 均热 The annealing was continued for 10 seconds to 3600 seconds. 均热 , T1 is the temperature at which 30% of ferrite is formed in terms of surface fraction at the end of soaking, Ac3 is determined by dilatometric method for cold rolled steel sheets, and C% refers to the nominal concentration of carbon. 均热Above this limit, sufficient austenite cannot be made stable at room temperature. 均热 is 720°C to 860°C, and more preferably 720°C to 820°C, and the time t 均热 The annealing time is 100 to 1000 seconds. Such annealing can be performed by continuous annealing.

[0062] The cold rolled and annealed steel sheet is then quenched at an average cooling rate of at least 0.1° C. / sec and preferably at least 1° C. / sec to below 80° C. and preferably below 50° C. A portion of the austenite present at the end of soaking will transform into fresh martensite.

[0063] After quenching, the steel plate is then heated to a temperature T 回火 The tempering step is continued for a period of time t 回火 , the temperature T 回火 and the holding time t 回火 So that (T 回火 +273)×(13+log t 回火 ) is 6000 to 8700, and preferably 7000 to 8200. Preferably, T 回火 Below 300°C, and t 回火 From 100 seconds to 1800 seconds.

[0064] At the end of this tempering step, the fresh martensite is transformed into tempered martensite.

[0065] The cold rolled, annealed and tempered steel sheet is then cooled to room temperature. It may then be coated by any suitable method, including hot-dip coating, electrodeposition or vacuum coating of zinc or a zinc-based alloy or aluminum or an aluminum-based alloy.

[0066] In another embodiment, the process can be stopped after annealing of the hot rolled sheet or after cold rolling or after coating, and the corresponding steel sheet can be cut into blanks which will then be used to manufacture components by press hardening. If the coating is carried out by hot dip galvanizing, it is generally preferred to carry out annealing to prepare the surface of the sheet just before immersing it in a hot melt bath.

[0067] Such a press hardening operation includes an austenitizing step, in which the steel blank is heated in a furnace to a temperature of T1 to (Ac3 + 50 × C% / 0.1) similar to the above-mentioned annealing of the cold-rolled steel sheet. Preferably, the austenitizing temperature is 720°C to 860°C, and more preferably 720°C to 820°C, and the austenitizing time is 30 seconds to 1000 seconds. Then, the heated blank is transferred to a hot stamping die in which hot stamping is performed.

[0068] The part is then held in the die while being hardened by a quenching operation. The quenching is performed to achieve a cooling rate of at least 0.1C / sec until the Ms temperature is reached. During this quenching, the part will acquire a microstructure identical to the target microstructure of the cold rolled and annealed steel sheet.

[0069] The steel component is then subjected to a tempering operation which requires that the steel component be tempered at a temperature T 回火 The component is heated and maintained for time t 回火 , the temperature T 回火 and the holding time t 回火 So that (T 回火 +273)×(13+log t 回火 ) is 6000 to 8700, and preferably 7000 to 8200. Preferably, T 回火 Below 300°C, and t 回火 The time is 10 to 1800 seconds. The part will then obtain the same microstructure as the target microstructure of the cold rolled, annealed and tempered steel plate.

[0070] Such tempering can advantageously be carried out during the baking process for curing the paint when the steel component is painted.

[0071] The invention will now be illustrated by the following examples, which are in no way limiting. Example

[0072] Seven grades whose compositions are summarized in Table 1 were cast into semi-finished products and processed into steel plates.

[0073] Table 1 - Composition

[0074] The compositions tested are summarized in the table below, where the element contents are expressed in weight percent.

[0075]

[0076] Underlined values: outside the scope of this invention

[0077] The Ac1, Ac3 and Ms temperatures of the cold rolled sheets were determined by dilatometry testing and metallographic analysis.

[0078] Table 2 - Process parameters of hot rolled and heat treated steel plates

[0079] The cast steel semi-finished product is reheated at 1200°C, hot rolled, and then coiled. HBA The hot rolled and coiled steel sheet is heat treated at the temperature and kept at the temperature for a holding time t HBA The following specific conditions apply to obtain hot rolled and heat treated steel plates:

[0080]

[0081] Underlined value: Access to target attribute parameters is not allowed

[0082] The hot rolled and heat treated steel plates were analyzed and the corresponding properties are summarized in Table 3.

[0083] Table 3 - Microstructure and properties of hot rolled and heat treated steel plates

[0084] Determine the slope of the manganese distribution and the fraction of precipitated carbides.

[0085] The fraction of precipitated carbides was determined from cross sections of the plates examined at a magnification greater than 15000x by scanning electron microscopy with a field emission gun ("FEG-SEM") and image analysis.

[0086] Figure 1 Shown are cross sections of hot rolled and heat treated steel sheets of Sample 4 and Sample 28. Black areas correspond to areas with lower amounts of manganese and grey areas correspond to higher amounts of manganese.

[0087] This graph was obtained by cutting a specimen from 1 / 4 thickness of a hot rolled and heat treated steel plate and polishing it.

[0088] The cross section was then characterized by an electron probe microanalyzer with a field emission gun (FEG) at a magnification greater than 10000x to determine the amount of manganese. Three 10 μm*10 μm images of different parts of the cross section were obtained. These images were obtained from 0.01 μm 2 The amount of manganese in each pixel was calculated as a weight percentage and then plotted as a curve representing the cumulative area fraction of the three images as a function of the amount of manganese.

[0089] Plot the curve on Figure 2 For Sample 4 and Sample 28: 100% of the plate cross-sections contained more than 1% manganese. For Sample 4, 20% of the plate cross-sections contained more than 9% manganese.

[0090] The slope of the obtained curve between the point representing a cumulative area fraction of 80% and the point representing a cumulative area fraction of 20% is then calculated.

[0091] For sample 28, the absence of heat treatment after hot rolling means that the redistribution of manganese is not sufficiently heterogeneous, as can be seen by the slope value of the manganese distribution being below -30.

[0092] In contrast, for Sample 4, the redistribution of manganese is clearly non-uniform, as evidenced by the slope value of the manganese distribution being above -30.

[0093]

[0094] Underlined value: does not match the target value

[0095] Heat treatment of the hot rolled steel sheet allows the diffusion of manganese in the austenite: the redistribution of manganese is inhomogeneous with respect to areas with low manganese content and areas with high manganese content. This manganese inhomogeneity contributes to the mechanical properties and can be measured by the manganese distribution.

[0096] Table 4 - Process parameters of cold rolled, annealed and tempered steel sheets

[0097] The obtained hot-rolled and heat-treated steel sheet is then cold-rolled. 均热 The first annealing is performed at the temperature and maintained for a holding time t 均热 , and then quenched at a cooling rate of 2°C / sec to below 80°C, preferably below 50°C. Then, the steel plate is heated to a temperature T 回火 The second heating is carried out and the temperature is maintained for a holding time t 回火 , and then cooled to room temperature. The following specific conditions are applied to obtain the cold rolled and annealed steel sheets:

[0098]

[0099] Underlined value: Access to target attribute parameters is not allowed

[0100] NA: Not applicable

[0101] The cold rolled and annealed plates were then analyzed and the corresponding microstructural elements, mechanical

[0102] The properties and solderability characteristics are summarized in Table 5, Table 6 and Table 7 respectively.

[0103] Table 5 - Microstructure of cold rolled, annealed and tempered steel sheets

[0104] The phase percentages of the microstructure of the obtained cold rolled and tempered steel sheet were determined.

[0105] [C]A and [Mn]A correspond to the amount of carbon and manganese in austenite in weight percent. [C]A and [Mn]A were measured using both X-ray diffraction (C%) and electron probe microanalyzer with a field emission gun (Mn%).

[0106] The surface fraction of the phase in the microstructure is determined by cutting a specimen from a cold rolled and annealed steel sheet, polishing it and etching it with reagents known per se to reveal the microstructure. The section is then examined by scanning electron microscopy, for example with a scanning electron microscope with a field emission gun ("FEG-SEM") in secondary electron mode at a magnification greater than 5000x.

[0107] The determination of the surface fraction of ferrite was carried out by SEM observation after etching with nital or picric acid / nital.

[0108] The volume fraction of retained austenite was determined by X-ray diffraction.

[0109]

[0110] Underlined values: not corresponding to the present invention, nd: not determined

[0111] Table 6 - Mechanical properties of cold rolled, annealed and tempered steel sheets

[0112] The mechanical properties of the obtained cold rolled, annealed and tempered steel sheets were determined and summarized in the following table.

[0113] The yield strength YS, the tensile strength TS as well as the uniform elongation UE and the total elongation TE are measured according to ISO standard ISO 6892-1 published in October 2009.

[0114]

[0115] Underlined value: does not match the target value

[0116] Sample 1 was not subjected to any tempering treatment. Its microstructure contained more than 5% of fresh martensite which remained untempered, resulting in poor total elongation values.

[0117] Sample 13 was subjected to annealing at a soaking temperature exceeding (Ac3 + 50 x C% / 0.1). This resulted in too high a carbon value in the retained austenite, leading to uniform and total elongations outside the target.

[0118] Samples 16 and 24 were subjected to the 回火 +273)×(13+log t 回火 ) exceeds the maximum value. This results in too high a value of carbon in the retained austenite, leading to uniform and total elongation outside the target.

[0119] Sample 25 was subjected to hot zone annealing outside the critical range and had too low manganese in its composition compared to the present invention. The corresponding annealed hot zone contained too many carbides and the manganese was not distributed in a non-uniform manner. This resulted in a lower content of retained austenite than the target, which reduced the uniform elongation and total elongation. The size of the ferrite grains was also outside the range, which resulted in a tensile strength far below the target.

[0120] As demonstrated by Table 1, samples 26 to 28 were made using grades that are outside the scope of the invention in terms of composition. In particular, their manganese content is below 6.0% by weight, and their carbon content is above 0.18%. As far as the hot annealing parameters are concerned, as demonstrated by Tables 2 and 3, they are also outside the scope of the invention, indicating that manganese is not distributed in an uneven manner as required by the invention, and that carbide precipitation is too high. This results in a content of retained austenite that is far below the target, and in a uniform elongation and a total elongation that are below the target.

[0121] Table 7 - Weldability characteristics of cold rolled, annealed and tempered steel plates

[0122] Spot welding was performed on cold rolled, annealed and tempered steel plates under standard ISO 18278-2 conditions.

[0123] 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, called 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 to say the thickness of the steel and the metal coating. It makes it possible to calculate a 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 substrate. This coefficient is expressed in daN / mm 2 express.

[0124] The weldability characteristics of cold rolled, annealed and tempered steel plates were determined and summarized in the following table:

[0125]

[0126] LME index = C% + Si% / 4, in weight %.

[0127] Nd: Not determined.

Claims

1. A cold rolled, annealed and tempered steel sheet, the steel sheet being made of steel having a yield strength YS greater than or equal to 1000 MPa and the following composition, the composition comprising by weight percentage: C: 0.03% to 0.18% Mn: 6.0% to 11.0% Mo: 0.05% to 0.5% B: 0.0005% to 0.005% S≤0.010％ P≤0.020％ N≤0.008％ And optionally contain one or more of the following elements by weight percentage: Al<3％ Si≤1.20% Ti≤0.050% 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 30% ferrite, such ferrite having a grain size below 1.0 μm, -3% to 30% retained austenite, -40% to 95% tempered martensite, - less than 5% fresh martensite, -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% 2 C% and Mn% are the nominal values ​​of carbon and manganese in wt%.

2. The steel sheet 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% of ferrite, 10% to 25% of retained austenite, and 50% to 85% of tempered martensite. 6 . The steel sheet according to claim 1 , wherein the microstructure does not contain ferrite, contains 5% to 15% of retained austenite, and 85% to 95% of tempered martensite.

7. The steel plate according to any one of claims 1 or 2, wherein the tensile strength is higher than or equal to 1450 MPa, the uniform elongation UE is higher than or equal to 6.5%, and the total elongation TE is higher than or equal to 9%.

8. The steel plate according to any one of claims 1 or 2, wherein the tensile strength TS and the total elongation TE satisfy the following formula: TS×TE>13700 MPa.%.

9. 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, Where C% and Si% represent the nominal weight percentages of carbon and silicon in the steel, respectively.

10. 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 weight percentage.

11. A resistance spot weld of two steel parts made from a cold rolled, annealed and tempered steel sheet according to any one of claims 1 to 10, the resistance spot weld having an alpha value of at least 30 daN / mm 2 , where α is the ratio of the transverse tensile strength value to the product of the diameter of the weld point multiplied by the thickness of the substrate, expressed in daN / mm 2 express.

12. A press hardened and tempered steel component having a composition and microstructure according to any one of claims 1 to 10.

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