Cold-rolled and annealed steel sheet and method for manufacturing the same

Abstract

The present invention relates to a cold rolled and annealed steel sheet made of a steel having a composition comprising in percentage by weight: C: 0.03% to 0.18% Mn: 6.0% to 11.0% Al: 0.2% to 3% Mo: 0.05% to 0.5% B: 0.0005% to 0.005% S < 0.010% P < 0.020% N < 0.008%, and optionally one or more of the following elements in percentage by weight: Si < 1.20% Ti < 0.050% Nb < 0.050% Cr < 0.5% V < 0.2%, the remainder of the composition being iron and unavoidable impurities resulting from melting, the steel sheet having a microstructure comprising in surface fraction: - 25% to 55% of retained austenite, - 5% to 50% of ferrite, - 5% to 70% of partitioned martensite, - less than 5% of fresh martensite, - carbon in austenite expressed in percentage by weight [C] A and manganese [Mn] A content, such that the ratio ([C] A 2 x [Mn] A ) / (C% 2 x Mn%) is 3.0 to 8.0, C% and Mn% being the nominal values in % by weight of carbon and manganese, and - a heterogeneous re-partitioning of manganese characterized by a slope of manganese distribution higher than or equal to -40.

Description

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

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

[0003] One of the main challenges in the automotive industry, in view of global environmental protection, is to reduce the weight of vehicles to improve their fuel efficiency without neglecting safety requirements. To meet these requirements, the steelmaking industry continuously develops new high-strength steels to obtain sheets having improved yield and tensile strengths and good ductility and formability.

[0004] One development made to improve mechanical properties is to increase the content of manganese in the steel. The presence of manganese contributes to improving the ductility of the steel due to the stabilization of austenite. But these steels have the weak point of being brittle. To overcome this problem, elements such as boron are added. These boron-added chemicals are very tough during 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 steel sheets must also exhibit good resistance to Liquid Metal Embrittlement (LME). Steel sheets coated with zinc or zinc alloys are very effective for corrosion resistance and are therefore widely used in the automotive industry. However, experience shows that the electric arc welding or resistance welding of certain steels can 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 stresses or internal stresses resulting from constraints, thermal expansion or phase transformations. It is known that the addition of elements such as carbon or silicon is detrimental to the resistance to LME.

[0006] The automotive industry generally assesses such resistance by limiting the upper value of the so-called LME index calculated according to the following formula:

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

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

[0009] Publication WO2020011638 relates to a method for providing an intermediate manganese (Mn 3.5% to 12%) cold rolled steel with reduced carbon content. Two process routes are described. The first one is about intercritical annealing of the cold rolled steel sheet. The second one is about double annealing of the cold rolled steel sheet, the first annealing being fully austenitic and the second annealing being intercritical. Thanks to the choice of the 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 a good fracture thickness strain value. But the small amount of carbon and manganese used in this invention limits the tensile strength of the steel sheet to a value not higher than 980 MPa.

[0010] It is therefore an object of the present invention to solve the above-mentioned problems and to provide a cold rolled and annealed steel sheet having the following combination of high mechanical properties: wherein the tensile strength TS is higher or equal to 1000 MPa, the uniform elongation UE is higher or equal to 13% and the total elongation TE is higher or equal to 16%.

[0011] Preferably, the yield strength of the cold rolled and annealed steel sheet is higher or equal to 850 MPa.

[0012] Preferably, the cold rolled and annealed steel sheet according to the present invention satisfies YS x UE + TS x TE > 31000 MPa.%.

[0013] Preferably, the LME index of the cold rolled and annealed steel sheet according to the present invention is lower than 0.36.

[0014] Preferably, the carbon equivalent Ceq of the cold rolled and annealed steel sheet according to the present invention is lower than 0.4%, the carbon equivalent being defined as

[0015] Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133*Mn%*Mo%

[0016] where the elements are expressed in weight percent.

[0017] Preferably, the resistance spot weld of two steel parts of the cold rolled and annealed steel sheet according to the present invention has an alpha value of at least 30 daN / mm 2 .

[0018] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet can also comprise any feature of claims 2 to 10 alone or in combination.

[0019] Another object of the present invention is a resistance spot weld of two steel parts according to claim 11.

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

[0021] According to the application, the carbon content is comprised between 0.03% and 0.18% to ensure satisfactory strength and good weldability properties. Above 0.18%, the weldability and the LME resistance of the steel sheet can decrease. The soaking temperature depends on the carbon content: the higher the carbon content, the lower the soaking temperature to stabilize the austenite. If the carbon content is below 0.03%, the fraction of austenite is not stable enough to reach the desired tensile strength and elongation after soaking. In a preferred embodiment of the application, the carbon content is comprised between 0.05% and 0.15%. In another preferred embodiment of the application, the carbon content is comprised between 0.05% and 0.10%.

[0022] The manganese content is comprised between 6.0% and 11.0%. Above 11.0%, the addition of manganese can decrease the weldability of the steel sheet and the productivity of the assembly of parts can decrease. Moreover, the risk of centerline segregation increases and thus impairs the mechanical properties. Since the temperature of soaking also depends on the manganese content, the minimum of manganese is defined to stabilize the austenite to reach the targeted microstructure and strength after soaking. Preferably, the manganese content is comprised between 6.0% and 9%.

[0023] According to the application, the aluminum content is comprised between 0.2% and 3% to reduce the manganese segregation during casting. Aluminum is a very efficient element to deoxidize the steel in liquid phase during the refining. Above 3%, the addition of aluminum can decrease the weldability of the steel sheet and thus the casting ability. Moreover, it is difficult to reach a tensile strength above 980 MPa. Moreover, the higher the aluminum content, the higher the soaking temperature to stabilize the austenite. The addition of at least 0.2% of aluminum improves the product robustness by enlarging the intercritical range and improves the weldability. Moreover, the addition of aluminum avoids the appearance of inclusions and oxidation problems. In a preferred embodiment of the application, the aluminum content is comprised between 0.7% and 2.2%.

[0024] The molybdenum content is comprised between 0.05% and 0.5% to reduce the manganese segregation during casting. Moreover, the addition of at least 0.05% of molybdenum provides a resistance to embrittlement. Above 0.5%, the addition of molybdenum is expensive and not appropriate considering the required properties. In a preferred embodiment of the application, the molybdenum content is comprised between 0.15% and 0.35%.

[0025] According to the application, the boron content is comprised between 0.0005% and 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 prior austenite grain boundaries is promoted, making the steel more brittle. In a preferred embodiment of the application, the boron content is comprised between 0.001% and 0.003%.

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

[0027] The maximum addition of silicon content is limited to 1.20% to improve the resistance to LME. Moreover, this low silicon content makes it possible to simplify the process by omitting the pickling step of the hot-rolled steel sheet prior to the intercritical annealing. Preferably, the maximum silicon content added is 0.5%.

[0028] Titanium can be added up to 0.050% to provide precipitation strengthening. Preferably, at least 0.010% of titanium is added in addition to boron to protect boron from the formation of BN.

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

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

[0031] The remaining part of the composition of the steel is iron and impurities resulting from the smelting. In this respect, P, S and N are at least considered as residual elements, which are unavoidable impurities. Their content is 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 cold-rolled and annealed steel sheet according to the application will now be described. Said microstructure comprises, in terms of surface fraction:

[0033] - 25% to 55% of retained austenite,

[0034] - 5% to 50% of ferrite,

[0035] - 5% to 70% of partitioned martensite

[0036] - less than 5% of fresh martensite,

[0037] - carbon [C] in the austenite expressed in weight percentage, A and manganese [Mn] A content, such that the ratio ([C] A 2 x [Mn] A ) / (C% 2 x Mn%) is between 3.0 and 8.0, C% and Mn% being the nominal values in weight % of carbon and manganese, and

[0038] - a heterogeneous re-partitioning of manganese characterized by a slope of the manganese distribution in the microstructure higher than or equal to -40.

[0039] The microstructure of the steel sheet according to the present invention contains 25% to 55% retained austenite, and preferably 30% to 50% austenite. Below 25% or above 55% austenite, the uniform elongation UE and the total elongation TE may not reach their respective minimum values ​​of 13% and 16%.

[0040] Such austenite is formed during intercritical annealing of hot-rolled steel sheets, but also during the first and second intercritical 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 uneven distribution of manganese. As a result, carbon and manganese co-segregate. This manganese unevenness is measured by the manganese distribution slope of the hot-rolled steel sheet, which must be greater than or equal to -30, as shown in Figure 2. Figure 2 shown and explained later.

[0041] Due to the uneven redistribution of manganese in austenite after hot strip annealing and the slow diffusion kinetics of manganese in austenite, the manganese inhomogeneity formed during hot strip annealing still exists after the first intercritical annealing and the second intercritical annealing of the cold rolled steel sheet. This can be evidenced by the slope of the manganese distribution in the microstructure (which is greater than or equal to -40).

[0042] Carbon in austenite expressed as a weight percentage [C] A and manganese [Mn] A The content makes the ratio ([C] A 2×[Mn] A ) / (C%2×Mn%) is between 3.0 and 8.0. When the ratio is below 3.0, the retained austenite is not stable enough to provide a sustained TRIP-TWIP effect during deformation. When the ratio is above 8.0, the retained austenite is too stable to produce a sufficient TRIP-TWIP effect during deformation. This TWIP-TRIP effect is prominently 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, Vol. 46A, p. 2356, June 2015 (S. Lee, K. Lee, and B.C. De Cooman).

[0043] The microstructure of the steel sheet according to the application comprises 5% to 50% of ferrite, preferably 10% to 45% of ferrite. Such ferrite forms during the intercritical annealing of the hot-rolled steel sheet, but also during the first and second intercritical annealings of the cold-rolled steel sheet.

[0044] The microstructure of the steel sheet according to the application comprises 5% to 70% of partitioned martensite, preferably 8% to 50% of partitioned martensite. Such martensite can form upon cooling after the intercritical annealing of the hot-rolled steel sheet by transformation of the partial austenite which is less rich in carbon and martensite than the nominal value. But it forms mainly upon cooling after the first annealing of the cold-rolled steel sheet, then gets partitioned during the second annealing of the cold-rolled steel sheet.

[0045] Fresh martensite can exist up to 5% in terms of surface fraction, but is not a phase expected in the microstructure of the steel sheet according to the application. It can form during the step of final cooling to room temperature by transformation of unstable austenite. Indeed, such unstable austenite with low carbon and low manganese content leads to a martensite start temperature Ms higher than 20°C. In order to obtain final mechanical properties, fresh martensite is limited to a maximum of 5% and preferably reduced down to 0%.

[0046] Partitioned martensite can be distinguished from fresh martensite on a cross-section polished and etched by agents known per se, such as Nital reagent, observed by scanning electron microscopy (SEM) or on a polished cross-section analyzed by electron backscattered diffraction (EBSD). The average C content of the partitioned martensite is strictly lower than the nominal C content of the steel. Such low C content is due to the partitioning of carbon from the martensite formed upon quenching below the Ms temperature of the steel to the austenite during the intercritical annealing. P The fresh martensite (which results from the transformation of the carbon-rich austenite into martensite after the partitioning step) has a C content higher than the nominal carbon content of the steel and a dislocation density higher than the partitioned martensite.

[0047] In contrast, fresh martensite (which results from the transformation of the carbon-rich austenite into martensite after the partitioning step) has a C content higher than the nominal carbon content of the steel and a dislocation density higher than the partitioned martensite.

[0048] The cold-rolled and annealed steel sheet according to the application has a tensile strength TS higher than or equal to 1000 MPa, a uniform elongation UE higher than or equal to 13% and a total elongation TE higher than or equal to 16%.

[0049] Preferably, the cold-rolled and annealed steel sheet has a yield strength higher than or equal to 850 MPa.

[0050] Preferably, the cold-rolled and annealed steel sheet has an LME index lower than 0.36.

[0051] Preferably, in order to improve the weldability, the cold rolled and annealed steel sheet has a carbon equivalent Ceq lower than 0.4%. The carbon equivalent is defined as Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133*Mn%*Mo%, wherein the elements are expressed in weight percent.

[0052] The welded assembly can be manufactured by producing two parts from the cold rolled and annealed steel sheet according to the application, then resistance spot welding the two steel parts.

[0053] The resistance spot weld joining the first sheet to the second sheet is characterized by a high resistance defined by an alpha value of at least 30 daN / mm 2 in cross tensile test.

[0054] The steel sheet according to the application can be produced by any appropriate manufacturing method and the skilled person can define the method. However, it is preferred to use a method according to the application, said method comprising the following steps:

[0055] A semi-finished product having the above steel composition is provided, which can be further hot rolled. The semi-finished product is heated to a temperature of 1150°C to 1300°C, thus making it possible to hot roll easily, with a final hot rolling temperature FRT of 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0056] The hot rolled steel is then cooled and coiled at a temperature T 卷取 of 20°C to 650°C, and preferably 300°C to 500°C.

[0057] The hot rolled steel sheet is then cooled to room temperature and can be pickled.

[0058] The hot rolled steel sheet is then annealed to an annealing temperature T HBA of Ac1 to Ac3. More precisely, T HBA is chosen to minimize the fraction of precipitated carbides to less than 0.8% and to promote the heterogeneous re-partitioning of manganese. This manganese heterogeneity is measured according to 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 is Ac1 + 5°C to Ac3. Preferably, the temperature T HBA is 580°C to 680°C.

[0059] The steel sheet is maintained at said temperature T HBA for a holding time T HBA of 0.1 hour to 120 hours, to promote the diffusion of manganese and the formation of a heterogeneous manganese distribution. Furthermore, this heat treatment of the hot rolled steel sheet allows to keep the toughness of the hot rolled steel sheet higher than 0.4 J / mm2 while reducing the hardness.

[0060] The hot-rolled and heat-treated steel sheet is then cooled to room temperature and can be pickled to remove the oxidation.

[0061] The hot-rolled and heat-treated steel sheet is then cold-rolled with a reduction of 20% to 80%.

[0062] The cold-rolled steel sheet is then subjected to a first annealing at a critical range temperature T1 均热 of Ac1 to Ac3 of the cold-rolled steel sheet for a holding time t1 均热 of 10 seconds to 1800 seconds. Ac1 and Ac3 are determined by dilatometric test. T1 均热 and t1 均热 are chosen to obtain 50% to 95% of austenite by surface fraction at the end of soaking, which allows to keep as much as possible the manganese inhomogeneities formed during intercritical annealing. This is evidenced by the slope of the manganese distribution shown by the steel sheet in the microstructure which is at least -40. Preferably, the critical range temperature T1 均热 is 650°C to 850°C and more preferably 710°C to 780°C and the time t1 均热 is 100 seconds to 1000 seconds. Such first annealing can be performed by continuous annealing.

[0063] Upon cooling, the part of the austenite which is less rich in manganese and carbon will transform into fresh martensite. This fresh martensite will contain regions rich in manganese and carbon and regions depleted in manganese and carbon.

[0064] Moreover, after cooling following the first annealing, the microstructure will contain 5% to 50% of ferrite.

[0065] The cold-rolled steel sheet is then subjected to a second annealing at a critical range temperature T2 均热 of Ac1 to Ac3 of the annealed steel sheet for a holding time t2 均热 of 30 seconds to 3600 seconds. Ac1 and Ac3 are determined by dilatometric test. Preferably, the critical range temperature T2 均热 is 550°C to 650°C and t2 均热 is 100 seconds to 1500 seconds.

[0066] The purpose of this second annealing is to continue the partitioning of carbon and manganese in austenite and martensite. Since a part of the fresh martensite has a higher carbon and manganese than nominal, this part of the martensite can transform into austenite at a temperature lower than T1 均热 with partitioning of manganese and carbon in such austenite. Another part of the martensite organization with less carbon and manganese will not transform into austenite but will induce partitioning of both carbon and manganese in austenite. Thus, T2 均热Lower than T1 均热 t2 均热 Preferably longer than t1 均热 This should allow enough time for the carbon to diffuse in the austenite, but should be kept short enough to avoid that the final content of austenite is higher than 55%, so that the austenite will then contain insufficient amounts of carbon to ensure a TRIP-TWIP effect.

[0067] Preferably, the critical zone temperature T2 均热 500℃ to 650℃ and time t2 均热 The second annealing can be performed by continuous annealing.

[0068] The cold rolled and annealed steel sheet is then cooled to below 80° C., and preferably to room temperature. During cooling, a portion of the austenite that is less rich in manganese and carbon may transform into fresh martensite.

[0069] 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.

[0070] The invention will now be illustrated by the following examples, which are in no way limiting. Example

[0071] Four grades (whose compositions are summarized in Table 1) were cast into semi-finished products and processed into steel plates.

[0072] Table 1 - Compositions

[0073] The compositions tested are summarized in the table below, where the element contents are expressed in weight percent.

[0074]

[0075] The Ac1 and Ac3 temperatures of the cold rolled steel sheets were determined by dilatometry testing and metallographic analysis.

[0076] Table 2 - Process parameters for the hot-rolled and heat-treated steel sheets

[0077] The cast steel semi-finished product is reheated at 1200°C, hot rolled, and then coiled at 450°C. The hot rolled and coiled steel sheet is then heated to T HBA Heat treatment is performed at the temperature and maintained at the temperature for a holding time t HBA The following specific conditions for obtaining hot rolled and heat treated steel plates apply:

[0078]

[0079] Underlined value: does not allow access to target attribute parameters

[0080] The steel sheets after hot rolling and heat treatment were analyzed and the corresponding properties are gathered in Table 3.

[0081] Table 3 - Microstructure and properties of the hot-rolled and heat-treated steel sheets

[0082] The slope of the manganese distribution and the fraction of precipitated carbides were determined.

[0083] The fraction of precipitated carbides was determined from cross sections of the sheets examined by scanning electron microscopy with field emission gun ("FEG-SEM") and image analysis at a magnification greater than 15000x.

[0084] The heat treatment of the hot-rolled steel sheets allows the diffusion of manganese in austenite: the re-partition of manganese is not homogeneous, with regions having a low manganese content and regions having a high manganese content. This manganese heterogeneity contributes to achieving mechanical properties and can be measured as a function of the manganese distribution.

[0085] Figure 1 The cross sections of the hot-rolled and heat-treated steel sheets representative of test 4 and test 15 are represented. The black regions correspond to regions with a lower amount of manganese and the grey regions correspond to regions with a higher amount of manganese.

[0086] This figure was obtained by cutting a specimen at 1 / 4 thickness from the hot-rolled and heat-treated steel sheet and polishing it.

[0087] Then, the cross section was characterized by electron probe micro-analyzer with field emission gun ("FEG") at a magnification greater than 10000x to determine the amount of manganese. Ten 10pm x 10pm images of different parts of the cross section were obtained. These images are composed of 0.01pm pixels. The amount of manganese in weight percent was calculated in each pixel and then plotted as a curve representing the cumulative area fraction of the three images as a function of the amount of manganese. 2

[0088] This curve for test 4 and test 15 is plotted in Figure 2 . 100% of the cross section of the sheet contains more than 1% of manganese. For test 15, 20% of the cross section of the sheet contains more than 10% of manganese.

[0089] The slope of the obtained curve is then calculated between the point representing 80% of the cumulative area fraction and the point representing 20% of the cumulative area fraction.

[0090] For test 4, the absence of heat treatment after hot rolling means that the re-partition of manganese is not sufficiently heterogeneous, which can be seen by the value of the slope of the manganese distribution lower than -30. This is also the case for tests 5 and 6.

[0091] ​On the contrary, for trial 15, the re-partitioning of manganese is clearly heterogeneous, as evidenced by the value of the slope of the manganese distribution being higher than -30. This is also the case for all other trials except 4 to 6.

[0092]

[0093] Underlined values: not allowed to obtain the targeted properties

[0094] Table 4 - Process parameters for the cold-rolled and annealed steel sheets

[0095] The hot-rolled and heat-treated steel sheets obtained were then cold-rolled. The cold-rolled steel sheets were then subjected to a first annealing at a temperature T1 均热 for a holding time t1 均热 , followed by cooling to below 80°C. The steel sheets were then subjected to a second annealing at a temperature T2 均热 for a holding time t2 均热 , followed by cooling to room temperature. The following specific conditions were applied to obtain cold-rolled and annealed steel sheets:

[0096]

[0097] Underlined values: not allowed to obtain the targeted properties

[0098] The cold-rolled and annealed sheets were then analyzed and the corresponding microstructure elements, mechanical properties and weldability properties are respectively summarized in Tables 5, 6 and 7.

[0099] Table 5 - Microstructure of the cold-rolled and annealed steel sheets

[0100] The phase percentages of the microstructure of the cold-rolled and annealed steel sheets obtained were determined as well as the slope of the manganese distribution after the first annealing and after the second annealing.

[0101] The surface fraction of phases in the microstructure was determined by cutting a specimen from the cold-rolled and annealed steel sheet, polishing and etching with reagents known per se to reveal the microstructure. Thereafter, the cross-section was examined by scanning electron microscopy, for example with a scanning electron microscope with field emission gun ("FEG-SEM") at a magnification greater than 5000x in secondary electron mode.

[0102] The determination of the surface fraction of ferrite was made after etching with Nital or Picral / Nital reagent according to SEM observations.

[0103] The determination of the volume fraction of retained austenite was made according to X-ray diffraction.

[0104] [C]A and [Mn]A correspond to the amount of carbon and manganese in austenite in weight percent. They are measured both with X-ray diffraction (C%) and with an electron probe micro-analyzer with field emission gun (Mn%).

[0105]

[0106] Underlined values: do not correspond to the application

[0107] The inhomogeneity of the manganese profile obtained after annealing of the hot-rolled steel sheet is as much as possible preserved after the two annealing steps of the cold-rolled steel sheet. This can be seen by comparing the slope of the manganese profile obtained after annealing of the hot-rolled steel sheet (in Table 3) and the slope of the manganese profile obtained after the first annealing step and after the second annealing step of the cold-rolled steel sheet (Table 5).

[0108] Table 6 - Mechanical properties of the cold-rolled and annealed steel sheets

[0109] The mechanical properties obtained of the cold-rolled and annealed are determined and summarized in the following table.

[0110] The yield strength YS, the tensile strength TS and the total elongation TE and the uniform elongation UE are measured according to ISO standard ISO 6892-1 published in October 2009.

[0111]

[0112] Underlined values: do not match the target values

[0113] Test 2 is performed with a second annealing whose duration is too short to form enough austenite. On the contrary, test 3 is performed with a t2 均热 long enough.

[0114] Tests 9 and 10 are performed with a second annealing whose duration is too long and therefore forms too much austenite which is not carbon rich enough, meaning that such austenite will not be stable enough. On the contrary, test 8 is performed with a t2 均热 short enough.

[0115] Tests 11 and 12 are performed with a second annealing whose temperature is too high and whose duration is also too long, therefore forming too much austenite which is not carbon rich enough.

[0116] Tests 13 and 14 are performed with a second annealing whose duration is too long, therefore the carbon content of the austenite is too low.

[0117] Test 18 is performed with a second annealing whose temperature is too low to form enough austenite. On the contrary, test 19 is performed with a T2 均热 high enough.

[0118] Table 7 - Weldability properties of the cold-rolled and annealed steel sheets

[0119] Spot welding under standard ISO 18278-2 conditions was performed on the cold-rolled and annealed steel sheets.

[0120] In the test used, the sample is composed of two steel sheets in the form of a cross-weld equivalent. A force is applied to break the weld. This force, called cross-tensile strength (CTS), is expressed in daN. It depends on the diameter of the weld and the thickness of the metal, i.e. on the thickness of the steel and the metal coating. It makes it possible to calculate a coefficient a, a being the ratio of the value of CTS relative 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 .

[0121] The cold-rolled and annealed weldability properties obtained were determined and are summarized in the table below:

[0122]

[0123] LME index = C% + Si% / 4 in wt%.

Claims

1. A cold-rolled and 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% Al: 0.2% to 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 one or more of the following elements by weight percentage: 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, -25% to 55% retained austenite, -5% to 50% ferrite, -5% to 70% partitioned martensite - less than 5% fresh martensite, -Carbon in austenite expressed as a weight percentage [C] A and manganese [Mn] A content, so that the ratio ([C] A 2 ×[Mn] A ) / (C% 2 × Mn%) is 3.0 to 8.0, C% and Mn% are nominal values ​​of carbon and manganese in % by weight, and - Inhomogeneous redistribution of manganese characterized by a slope of the manganese distribution greater than or equal to -40.

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

3. The cold rolled and annealed steel sheet according to claim 1 or 2, wherein the manganese content is 6.0% to 9%.

4. The cold rolled and annealed steel sheet according to claim 1 or 2, wherein the aluminum content is 0.7% to 2.2%. 5 . The cold rolled and annealed steel sheet according to claim 1 , wherein the microstructure comprises 30% to 50% of retained austenite, 5% to 40% of ferrite, and 8% to 50% of partition martensite.

6. The cold rolled and annealed steel sheet according to claim 1 or 2, wherein the tensile strength is higher than or equal to 1000 MPa, the uniform elongation UE is higher than or equal to 13% and the total elongation TE is higher than or equal to 16%.

7. The cold rolled and annealed steel sheet according to claim 1 or 2, wherein the yield strength is higher than or equal to 850 MPa.

8. The cold-rolled and annealed steel sheet according to claim 1 or 2, wherein YS, UE, TS and TE satisfy the following formula: YS x UE+TS x TE>31000MPa.%, Where YS is the yield strength and TS is the tensile strength.

9. The cold rolled and annealed steel sheet according to claim 1 or 2, wherein the liquid metal embrittlement (LME) index = C% + Si% / 4, wherein the elements are expressed in weight percentage.

10. The cold rolled and annealed steel sheet according to claim 9, wherein the LME index is lower than 0.

36.

11. The cold rolled and annealed steel sheet according to claim 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%-0.133*Mn%*Mo%, The elements are expressed in weight percentage.

12. A resistance spot weld of two steel parts made from the cold rolled and annealed steel sheet according to claim 1 or 2, the resistance spot weld having a strength of at least 30 daN / mm 2 The α value is a ratio of the cross tensile strength (CTS) value to the product of the diameter of the welding point and the thickness of the base material.

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