Cold-rolled, annealed and partitioned steel sheet and method of manufacturing same

Abstract

The invention relates to a cold-rolled, annealed and partitioned steel sheet made of steel having a composition comprising in weight percent: C: 0.05% 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 comprising one or more of the following elements in weight percent: Al < 3%, Si < 1.20%, Ti < 0.050%, Nb < 0.050%, Cr < 0.5%, V < 0.2%, the remainder of said composition being iron and unavoidable impurities resulting from melting, said steel sheet having a microstructure comprising in surface fraction: - 0% to 30% of ferrite, when such ferrite is present, having a grain size smaller than 1.0 pm; - 8% to 40% of retained austenite, the fraction of austenite islands having a size larger than 0.5 pm being smaller than or equal to 5%; - 30% to 92% of partitioned martensite; - less than 3% of fresh martenite; - [C] in austenite expressed in weight percent A content and manganese [Mn] A content such that the ratio ([C] A 2 x [Mn] A ) / (C% 2 x Mn%) is smaller than 18.0, C% and Mn% being the nominal values of carbon and manganese in weight %.

Description

[0001] The present invention relates to a high-strength steel sheet having good weldability properties and to a method for 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] Considering the 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 the safety requirements. To meet these requirements, the steelmaking industry continuously develops new high-strength steels to obtain sheets having improved yield and tensile strengths, as well as good ductility and formability.

[0004] One development made to improve the mechanical properties is to increase the content of manganese in the steel. The presence of manganese contributes to improve the ductility of the steel due to the stabilization of the 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 at the hot rolling stage, but the hot band is too hard to be further processed. The most efficient method 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 against corrosion 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 matrix under an applied stress or internal stress 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 WO 2020011638 relates to a method to provide medium-manganese and medium-manganese (Mn from 3.5% to 12%) cold-rolled steel with reduced carbon content. Two process routes are described. The first one involves intercritical annealing of the cold-rolled steel sheet. The second one involves double annealing of the cold-rolled steel sheet, the first one being fully austenitic and the second one 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 in austenite is obtained, which means a good fracture thickness strain value. However, the low 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 steel sheet having a combination of high mechanical properties with a tensile strength TS greater than or equal to 1270, an uniform elongation UE greater than or equal to 10.0%, a total elongation TE greater than or equal to 14.0%, a hole expansion ratio of at least 15%; and which satisfies the formula (TS x TE) / (C% + Si% / 4) > 50000 MPa.% where C% and Si% refer to the nominal weight % of C and Si in the steel.

[0011] Preferably, the yield strength of the steel sheet is greater than or equal to 1000 MPa.

[0012] Preferably, the LME index of the steel sheet 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 lower than 0.4%, the carbon equivalent being defined as

[0014]

[0015] where the elements are expressed in weight percent.

[0016] Preferably, the alpha value of the resistance spot weld of two steel parts of the steel sheet according to the present invention is at least 30 daN / mm 2 .

[0017] The present invention will now be described in detail and illustrated by examples, without introducing limitations.

[0018] The object of the present invention is achieved by providing a cold-rolled, annealed and partitioned steel sheet. The steel sheet is made of a steel having the following composition, in weight percent:

[0019] C: 0.05% to 0.18%

[0020] Mn: 6.0% to 11.0%

[0021] Mo: 0.05% to 0.5%

[0022] B: 0.0005% to 0.005%

[0023] S < 0.010%

[0024] P < 0.020%

[0025] N < 0.008%

[0026] and optionally comprises one or more of the following elements in weight percent:

[0027]

[0028] the remainder of the composition being iron and unavoidable impurities resulting from smelting,

[0029] the steel sheet has a microstructure comprising, in surface fraction,

[0030] - 0% to 30% of ferrite, when such ferrite is present, having a grain size less than 1.0 pm,

[0031] - 8% to 40% of retained austenite, the fraction of austenite islands having a size greater than 0.5 pm being less than or equal to 5%,

[0032] - 30% to 92% of partitioned martensite,

[0033] - less than 3% of fresh martensite,

[0034] - the carbon [C]A content and the manganese [Mn]A content in the austenite expressed in weight percent are such that the ratio is less than 18.0, C% and Mn% being the nominal values of carbon and manganese in weight %.

[0035] The steel sheet can also comprise, alone or in combination, any of the following features.

[0036] Optionally, wherein the carbon content is 0.08% to 0.15%.

[0037] Optionally, wherein the manganese content is 6.0% to 9%.

[0038] Optionally, wherein the aluminum content is 0.2% to 2.2%.

[0039] Optionally, wherein the microstructure comprises 5% to 25% of ferrite, 15% to 30% of retained austenite and 45% to 80% of partitioned martensite.

[0040] Optionally, wherein the microstructure does not comprise ferrite, comprises 20% to 30% of retained austenite and 70% to 80% of partitioned martensite.

[0041] Optionally, wherein the tensile strength is greater than or equal to 1270 MPa, the uniform elongation UE is greater than or equal to 10.0%, the total elongation TE is greater than or equal to 14.0%, and wherein TS, TE and the carbon content and the silicon content satisfy the following formula: where C% and Si% refer to the nominal weight% of C and Si of the steel.

[0042] Optionally, wherein the hole expansion ratio is greater than or equal to 15%.

[0043] Optionally, wherein the yield strength YS is greater than or equal to 1000 MPa.

[0044] Optionally, wherein the LME index is less than 0.36.

[0045] Optionally, wherein the carbon equivalent Ceq of the steel is below 0.4%, the carbon equivalent being defined as

[0046]

[0047] where the elements are expressed in weight percent.

[0048] Another object of the application is a resistance spot welded joint of two steel parts according to any of the embodiments described above, the resistance spot welded joint having an alpha value of at least 30 daN / mm 2 .

[0049] Another object of the application is a press hardened and partitioned steel part according to any of the embodiments described above.

[0050] According to the application, the carbon content is comprised between 0.05% and 0.18% to ensure satisfactory strength and good weldability properties. Carbon greater than 0.18% can decrease the weldability of the steel sheet and the resistance to LME. 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 less than 0.05%, the strength of the partitioned martensite is not sufficient to have a UTS greater than 1270 MPa. In a preferred embodiment of the application, the carbon content is comprised between 0.08% and 0.15%. In another preferred embodiment of the application, the carbon content is comprised between 0.10% and 0.15%.

[0051] The manganese content is comprised between 6.0% and 11.0%. Adding more than 11.0% can decrease the weldability of the steel sheet and can decrease the productivity of the parts assembly. Moreover, the risk of centerline segregation increases, impairing the mechanical properties. Since the soaking temperature also depends on the manganese content, the minimum of manganese is defined to stabilize the austenite to obtain the target microstructure and strength after soaking. Preferably, the manganese content is comprised between 6.0% and 9%.

[0052] According to the present invention, the aluminum content is less than 3% to reduce manganese segregation during casting. Aluminum is an element very effective for deoxidizing the steel in liquid phase during refining. With more than 3%, the weldability of the steel sheet can be reduced, to the point that the castability is also reduced. Moreover, it is difficult to achieve a tensile strength greater than 1270 MPa. Furthermore, the higher the aluminum content, the higher the soaking temperature to stabilize the austenite. Preferably, aluminum is added at least up to 0.2% to improve the product robustness by enlarging the intercritical range and to improve the weldability. Moreover, aluminum can be added to avoid the appearance of inclusions and oxidation problems. In a preferred embodiment of the present invention, the aluminum content is between 0.2% and 2.2% and more preferably between 0.7% and 2.2%.

[0053] The molybdenum content is between 0.05% and 0.5% to reduce manganese segregation during casting. Moreover, the addition of at least 0.05% of molybdenum provides resistance to brittleness. More than 0.5%, the addition of molybdenum is expensive and ineffective in view of the required properties. In a preferred embodiment of the present invention, the molybdenum content is between 0.15% and 0.35%.

[0054] According to the present invention, the boron content is 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. More than 0.005%, the formation of borocarbides at the prior austenite grain boundaries is promoted, making the steel more brittle. In a preferred embodiment of the present invention, the boron content is between 0.001% and 0.003%.

[0055] Some elements can be optionally added to the composition of the steel according to the present invention.

[0056] 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 eliminating the step of pickling the hot rolled steel sheet before the heat band annealing. Preferably, the maximum silicon content added is 1.0%.

[0057] Titanium can be added up to 0.050% to provide precipitation strengthening. Preferably, a minimum of 0.010% of titanium is added in addition to boron to protect the boron from forming BN.

[0058] 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%.

[0059] Chromium and vanadium can be optionally added up to 0.5% and 0.2% respectively to provide improved strength.

[0060] The remainder 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 as 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.

[0061] The microstructure of the steel sheet according to the application will now be described. It comprises, in terms of surface fraction:

[0062] - 0% to 30% of ferrite, when such ferrite is present, having a grain size less than 1.0 pm.

[0063] - 8% to 40% of retained austenite, the fraction of austenite islands having a size greater than 0.5 pm being less than or equal to 5%,

[0064] - 30% to 92% of partitioned martensite

[0065] - less than 3% of fresh martensite,

[0066] - the carbon [C]A content and the manganese [Mn]A content in the austenite, expressed in weight percent, are such that the ratio is less than 18.0, C% and Mn% being the nominal values of carbon and manganese in weight %.

[0067] The microstructure of the steel sheet according to the application comprises 8% to 40% of retained austenite. Less than 8% or more than 40% of austenite, the uniform elongation UE and the total elongation TE cannot reach the minimum values of 10.0% and 14.0% respectively.

[0068] Such austenite forms during the intercritical annealing of the hot-rolled steel sheet but also during the annealing of the cold-rolled steel sheet. During the intercritical annealing of the hot-rolled steel sheet, regions comprising a higher than nominal manganese content and regions comprising a lower than nominal manganese content are formed, resulting in a non-uniform distribution of manganese. Carbon is therefore co-segregated with manganese. This manganese inhomogeneity is measured by the slope of the manganese distribution of the hot-rolled steel sheet, which must be greater than or equal to -50, as shown later and explained. Figure 3

[0069] The carbon [C]A content and the manganese [Mn]A content in the austenite, expressed in weight percent, are such that the ratio ​less than 18.0. When this ratio is greater than 18.0, the retained austenite is too stable to provide a sufficient TRIP-TWIP effect during deformation. Such TWIP-TRIP effect is particularly explained 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, Volume 46A, June 2015 (S. LEE, K. LEE and B. C. DE COOMAN).

[0070] Furthermore, the fraction of austenite islands having a size greater than 0.5 pm must remain less than or equal to 5% to ensure that the hole expansion ratio will remain at least equal to 15%. Indeed, such large austenite islands are not stable enough.

[0071] The microstructure of the steel sheet according to the application comprises 0% to 30% of ferrite, when such ferrite is present, having a grain size less than 1.0 pm. Such ferrite can be formed during annealing of the cold-rolled steel sheet when the annealing of the cold-rolled steel sheet is performed at a temperature between Ac1 and Ac3 of the cold-rolled steel sheet. When the annealing of the cold-rolled steel sheet is performed above Ac3 of the cold-rolled steel sheet, there is no ferrite. Preferably, the ferrite content is 0% to 25%.

[0072] The microstructure of the steel sheet according to the application comprises 30% to 92% of partitioned martensite. Such martensite is mostly formed upon cooling after annealing of the cold-rolled steel sheet, then partitioned during the partitioning of the cold-rolled steel sheet.

[0073] Fresh martensite can be present less than 3% in terms of surface fraction, but is not a desired phase in the microstructure of the steel sheet according to the application. It can be formed during the step of final cooling to room temperature by transformation of unstable austenite. Indeed, such unstable austenite having a low carbon and manganese content leads to a martensite start temperature Ms higher than 20°C. In order to obtain final mechanical properties, fresh martensite must be less than 3% and preferably less than 2% or even better reduced to 0%.

[0074] The partitioned martensite can be distinguished from the fresh martensite on a cross section polished and etched with reagents 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. This low C content results from the partitioning of carbon from the martensite formed upon quenching below the Ms temperature of the steel to austenite during the holding at the partitioning temperature TP.

[0075] In contrast, the fresh martensite, which results from the transformation of carbon-enriched austenite to 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. In a first embodiment, the microstructure comprises 5% to 25% of ferrite, 15% to 30% of retained austenite and 45% to 80% of partitioned martensite.

[0076] In another embodiment, the microstructure does not comprise ferrite, comprises 20% to 30% of retained austenite and 70% to 80% of partitioned martensite.

[0077] The steel sheet according to the application has a tensile strength TS greater than or equal to 1270, an uniform elongation UE greater than or equal to 10.0%, a total elongation TE greater than or equal to 14.0%, a hole expansion ratio of at least 15% and satisfies the formula .

[0078] Preferably, the steel sheet has a yield strength greater than or equal to 1000 MPa.

[0079] Preferably, the cold rolled and annealed steel sheet has an LME index less than 0.36.

[0080] Preferably, the steel sheet has a carbon equivalent Ceq less than 0.4% to improve weldability. The carbon equivalent is defined as

[0081] Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133 x Mn% x Mo%, with the elements expressed in weight percent.

[0082] A welded assembly can be manufactured by producing two parts with a steel sheet according to the application and then resistance spot welding the two steel parts.

[0083] The resistance spot weld joint that joins the first sheet to the second sheet is characterized by a high resistance in transverse tensile test defined by an alpha value of at least 30 daN / mm 2 .

[0084] The steel sheet according to the application can be produced by any appropriate manufacturing method and one can be defined by the person skilled in the art. However, it is preferred to use a method according to the application comprising the following steps:

[0085] A semi-finished product able to be further hot-rolled is provided having the above steel composition. The semi-finished product is heated to a temperature of 1150°C to 1300°C so that it can be easily hot-rolled, with a final hot-rolling temperature FRT of 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0086] The hot-rolled steel is then cooled and coiled at a temperature Tcoiling of 20°C to 600°C, and preferably 300°C to 500°C.

[0087] The hot-rolled steel sheet is then cooled to room temperature and can be pickled.

[0088] The hot-rolled steel sheet is then annealed to an annealing temperature THBA of Ac1 to Ac3. More precisely, THBA is chosen so as to minimize the area fraction of precipitated carbides to less than 0.8% and to promote a heterogeneous redistribution of manganese. This heterogeneity of manganese is measured by the slope of the manganese profile of the hot-rolled steel sheet, which must be greater than or equal to -50. Preferably, the temperature THBA is Ac1 + 5°C to Ac3. Preferably the temperature THBA is 580°C to 680°C.

[0089] The steel sheet is held at said temperature THBA for a holding time tHBA of 0.1 hour to 120 hours to promote the diffusion of manganese and the formation of a heterogeneous manganese profile. Moreover, this heat treatment of the hot-rolled steel sheet allows to decrease the hardness while maintaining the toughness of the hot-rolled steel sheet.

[0090] The hot-rolled and heat-treated steel sheet is then cooled to room temperature and can be pickled to remove the oxides.

[0091] The hot-rolled and heat-treated steel sheet is then cold-rolled with a reduction of area of 20% to 80%.

[0092] The cold-rolled steel sheet is then subjected to annealing at a temperature Tanneal of T1 to 930°C for a holding time tanneal of 3 seconds to 1000 seconds, T1 being the temperature at which 30% of ferrite is formed at surface fraction at the end of the annealing. When Tanneal is higher than 930°C, not enough austenite can be stabilized at room temperature. Preferably, Tanneal is 720°C to 900°C, and more preferably 720°C to 870°C, and the time tanneal is 100 seconds to 1000 seconds. Such annealing can be performed by continuous annealing.

[0093] The cold-rolled and annealed steel sheet is then quenched to Tq, Tq being set in the range of (Ms70%-75) to (Ms70%-20). Ms70% is the temperature at which the steel sheet reaches a 70% martensite content by the quenching operation. This value is determined by dilatometry testing of a sample cooled to room temperature and re-heated to 120°C by plotting the martensite transformation kinetics curve during cooling to room temperature. As shown in Figure 1, the value corresponding to 70% of the martensite percentage (normalized to 0.7 compared to 1 at room temperature) is defined as Ms70%. Figure 1

[0094] Such quenching is performed at an average cooling rate of at least 0.1 °C / s and preferably at least 1 °C / s. A fraction of the austenite present at the end of soaking will transform into fresh martensite, the exact proportion depending on the value of Tq.

[0095] After quenching, the steel sheet is then subjected to a partitioning step at a temperature Tp of 300°C to 550°C during a time tp of 5 seconds to 1000 seconds. Preferably, Tp is 350°C to 500°C and tp is 100 seconds to 300 seconds.

[0096] Fresh martensite transforms into partitioned martensite at the end of this partitioning step. Austenite is further enriched in carbon.

[0097] The cold-rolled, annealed and partitioned steel sheet is then cooled to room temperature and a small fraction of fresh martensite can form during such cooling. The sheet can then be coated by any suitable process, including hot dip coating of zinc or zinc-based alloys or aluminum or aluminum-based alloys, electrodeposition or vacuum coating.

[0098] In another embodiment, the above process can be stopped after annealing of the hot-rolled sheet, cold-rolling or after coating and the corresponding steel sheet can be cut into blanks which will then be used for manufacturing parts by press hardening. If coating is performed by hot dip coating, it is generally preferred to anneal the sheet just before dipping the sheet into the hot bath to prepare the surface of the sheet.

[0099] Such press hardening operation includes an austenitizing step in which the steel blank is heated in an oven to a temperature of T1 to 930°C, similar to the above-mentioned annealing of the cold-rolled steel sheet. Preferably, this austenitizing temperature is 720°C to 900°C and more preferably 720°C to 870°C and the austenitizing time is 30 seconds to 1000 seconds. The heated blank is then transferred into a hot stamping die in which hot stamping is performed.

[0100] ​The part is then kept in the mould while hardening is performed by quenching operation in a manner known to the person skilled in the art. Quenching is performed to reach a cooling rate of at least 0.1 °C / s until reaching a temperature Tq ranging from (Ms70% - 75) to (Ms70% - 20). During this quenching, the part will acquire the same microstructure as for a cold-rolled and annealed steel sheet.

[0101] The steel part is then transferred, generally within 2 to 100 seconds, into an oven to undergo a partitioning operation requiring reheating of the part at a temperature Tp ranging from 300 to 550°C for a holding time tp ranging from 2 to 1000 seconds. Preferably, Tp ranges from 350 to 500°C and tp ranges from 100 to 300 seconds. The part will then acquire the same microstructure as for a cold-rolled, annealed and partitioned steel sheet.

[0102] The application will now be illustrated by the following examples, which are in no way limitative.

[0103] Example 1 - Steel sheets for cold forming

[0104] Six grades, the composition of which is summarized in Table 1, were cast as semi-products and processed into steel sheets.

[0105] Table 1 - Composition

[0106] The compositions tested are summarized in the following table, in which the element contents are expressed in weight percent.

[0107]

[0108] Underlined values: outside the application

[0109] The Ac1, Ac3 and Ms temperatures of the cold-rolled sheets were determined by dilatometry tests and metallographic analysis.

[0110] Table 2 - Process parameters of the steel sheets hot-rolled and heat-treated

[0111] The cast steel semi-products were reheated at 1200°C, hot-rolled and then coiled. The hot-rolled and coiled steel sheets were then heat-treated at a temperature THBA and for a holding time tHBA. The following specific conditions were applied to obtain the hot-rolled and heat-treated steel sheets:

[0112]

[0113] Underlined values: parameters not allowed to obtain the targeted properties

[0114] The hot-rolled and heat-treated steel sheets were analyzed and the corresponding properties are summarized in Table 3.

[0115] Table 3 - Microstructure and properties of the hot-rolled and heat-treated steel sheets

[0116] The slope of the manganese distribution and the fraction of precipitated carbides were determined.

[0117] The fraction of precipitated carbides was determined by examining the cross-section of the sheet with a scanning electron microscope with a field emission gun ("FEG-SEM") and by image analysis at a magnification greater than 15000x.

[0118] The heat treatment of the hot-rolled steel sheet allows the diffusion of manganese in austenite: the redistribution 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 by the manganese profile.

[0119] Figure 2 The figure represents the cross-section of the hot-rolled and heat-treated steel sheet of test 13 and tests 1 to 8. The black areas correspond to regions with a lower amount of manganese and the grey areas correspond to a higher amount of manganese.

[0120] This figure was obtained by cutting a specimen from the hot-rolled and heat-treated steel sheet at 1 / 4 thickness and polishing it.

[0121] Thereafter, this cross-section was characterized by an electron probe micro-analyzer with a field emission gun ("FEG") at a magnification greater than 10000x to determine the amount of manganese. Three 10 pm x 10 pm maps of the different parts of the cross-section were obtained. These maps are composed of pixels of 0.01 pm2. 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 maps as a function of the amount of manganese.

[0122] For tests 13 and 1 to 8, this curve was plotted in Figure 3 : 100% of the cross-section of the sheet contained more than 1% of manganese. For tests 1 to 8, 10% of the cross-section of the sheet contained more than 10% of manganese.

[0123] The slope of the curve obtained was then calculated between the point representing 80% of the cumulative area fraction and the point representing 20% of the cumulative area fraction.

[0124] For tests 1 to 8, the slope was greater than -50, indicating that the redistribution of manganese was not homogeneous, with regions having a low manganese content and regions having a high manganese content.

[0125] Conversely, for test 13, not performing a heat treatment after hot-rolling means that the redistribution of manganese is not heterogeneous, which can be seen by the value of the slope of the manganese distribution being less than -50.

[0126]

[0127] Underlined values: do not match the target value.

[0128] Table 4 - Process parameters of the steel sheets after cold rolling, annealing and partitioning

[0129] For tests 1 to 15, the obtained hot-rolled and heat-treated steel sheets were then cold-rolled. The cold-rolled steel sheets were then annealed at a temperature Tsoak, first, before quenching at Tqwith a cooling rate of 2°C / s, and held at said temperature for a holding time tsoak. Then, the steel sheets were secondarily heated at a temperature Tpand held at said temperature for a holding time tpbefore cooling to room temperature.

[0130] The following specific conditions were applied to obtain the cold-rolled and annealed steel sheets.

[0131]

[0132] Underlined values: parameters for which the target properties cannot be obtained

[0133] Nd: not determined

[0134] The cold-rolled and annealed steel sheets were then analyzed and the corresponding microstructure elements, mechanical properties and weldability properties are respectively summarized in Tables 5, 6 and 7.

[0135] Table 5 - Microstructure of the steel sheets after cold rolling, annealing and partitioning

[0136] The phase percentages of the microstructure of the obtained cold-rolled and partitioned steel sheets were determined.

[0137] [C]Aand [Mn]Acorrespond to the amount of carbon and manganese in the austenite in weight percentage. They were measured both with X-ray diffraction (C%) and with an electron probe micro-analyzer with field emission gun (Mn%).

[0138] 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.

[0139] Determination of the surface fraction of ferrite by SEM observation after etching with Nital or Picral / Nital reagent.

[0140] Determination of the volume fraction of retained austenite by X-ray diffraction.

[0141]

[0142] Underlined values: not corresponding to this invention; nd: undetermined.

[0143] Table 6 - Mechanical properties of cold-rolled, annealed and fractionated steel sheets

[0144] The mechanical properties of the obtained cold-rolled, annealed, and fractionated steel sheets are determined and summarized in the table below.

[0145] Yield strength (YS), tensile strength (TS), uniform elongation (UE), and total elongation (TE) were measured according to ISO standard ISO 6892-1, published in October 2009. Pore expansion was tested according to ISO standard 16630.

[0146]

[0147] Underlined values: do not match the target value

[0148] Experiments 4, 6, 9 and 10 were subjected to excessively high quenching temperatures Tq, resulting in the formation of large, unstable austenite islands with a high fraction, thus reducing the porosity.

[0149] Experiment 5 was subjected to a quenching temperature Tq that was too low, resulting in the formation of overly stable austenite during deformation, such as... The values ​​shown are as follows. This results in excessively low total elongation and uniform elongation values.

[0150] Experiment 8 was subjected to a homogenization temperature higher than T1, but Tq was too high, resulting in the significant formation of large, unstable austenite islands with a high fraction. Together with the relatively high fraction of ferrite, this led to a sharp decrease in porosity.

[0151] Experiment 13 was prepared with a composition lacking sufficient manganese and subjected to tropical annealing at too low a temperature. The resulting microstructure consisted of ferrite and carbides, with relatively uniform manganese distribution within the ferrite. Furthermore, the relatively low homogenization temperature led to inadequate dissolution of the carbides. The large ferrite grain size following annealing of the cold-rolled steel sheet was inherited from the very large ferrite size formed during tropical batch annealing. The carbides could not prevent the aberrant grain growth of ferrite during tropical batch annealing. Therefore, the ferrite grain size was too large, and the retained austenite fraction and mechanical stability decreased, leading to a reduction in uniform elongation and total elongation.

[0152] Samples 14 and 15, whose compositions did not contain sufficient manganese, underwent hot annealing at too low a temperature. The resulting microstructure consisted of ferrite and carbides, with relatively uniform manganese distribution within the ferrite. The plates, after quenching and partitioning, did not exhibit a good trade-off between mechanical properties and resistance to LME, as demonstrated by... This is confirmed by the low value.

[0153] Table 7 - Weldability properties of the steel sheets after cold rolling, annealing and partitioning

[0154] Spot welding under standard ISO 18278-2 conditions was performed on the steel sheets after cold rolling, annealing and partitioning.

[0155] In the test used, the sample was composed of two steel sheets in the form of a transverse weld equivalent. A force was applied to break the weld. This force, called transverse tensile strength (CTS), is expressed in daN. It depends on the diameter of the weld and the thickness of the metal, in other words on the thickness of the steel and the metal coating. This makes it possible to calculate a coefficient a, a being the ratio of the value of CTS to the product of the diameter of the weld by the thickness of the base. This coefficient is expressed in daN / mm2.

[0156] The weldability properties after cold rolling, annealing and partitioning were determined and are summarised in the following table:

[0157]

[0158] LME index = C% + Si% / 4 in weight %.

[0159] Nd: not determined

[0160] Example 2 - press-hardened parts

[0161] For trials 16 and 17, the steel sheets obtained after hot rolling and heat treatment were then cold rolled. The cold-rolled steel sheets were then annealed at 860°C for 100 seconds in order to prepare the surface of the sheets for further coating in an aluminium-based hot bath.

[0162] After the coating has solidified and cooled to room temperature, the steel sheets are cut into blanks. Such blanks are then placed in a furnace in which they are annealed at a temperature Tpand held at this temperature for a holding time tpbefore being transferred to a press-hardening die in which they are stamped into a part and quenched at Tqwith a cooling speed of 2°C / s.

[0163] The steel pieces are then transferred again to a furnace in which they are re-heated at a temperature Tpand held at this temperature for a holding time tpbefore being cooled to room temperature. The following specific conditions were applied to obtain the steel parts:

[0164]

[0165] The phase percentages of the microstructure of the steel parts obtained were determined:

[0166]

[0167] The mechanical properties of the components were determined and are summarized in the table below.

[0168] The yield strength YS, tensile strength TS, uniform elongation UE and total elongation TE were measured according to ISO standard ISO 6892-1 published in October 2009. The test for hole expansion ratio was performed according to ISO 16630 standard.

[0169]

Claims

1. A cold rolled, annealed and partitioned steel sheet made of a steel having a composition comprising in weight percent: 0.05%≤ C ≤ 0.18 % 6.0% < Mn < 11.0 % 0.05% < Mo < 0.5% 0.0005%≤ B ≤ 0.005 % S≤0.010% P≤0.020% N≤0.008% and optionally comprising in weight percent one or more of the following elements: Al: < 3% Si < 1.20 % Ti < 0.050 % Nb < 0.050 % Cr ≤ 0.5 % V ≤ 0.2 % the remainder of said composition being iron and unavoidable impurities resulting from melting, said steel sheet having a microstructure comprising in surface fraction: - 0% to 30% of ferrite, when such ferrite is present, said ferrite having a grain size lower than 1.0 pm, - 8% to 40% of retained austenite, the fraction of austenite islands having a size greater than 0.5 pm being lower than or equal to 5%, - 30% to 92% of partitioned martensite, - less than 3% of fresh martensite, - Carbon [C] in the retained austenite in weight percent A content and manganese [Mn] content A content such that the ratio ([C] A ² x [Mn] A ) / (C%² x Mn%) is less than 18.0, C% and Mn% being the nominal values of carbon and manganese in weight %.

2. The steel sheet according to claim 1, wherein 0.08% < C < 0.15 %.

3. The steel sheet according to any one of claims 1 or 2, wherein 6.0% < Mn < 9 %.

4. The steel sheet according to any one of claims 1 or 2, wherein 0.2% < Al < 2.2 %.

5. The steel sheet according to any one of claims 1 or 2, wherein said microstructure comprises 5% to 25% of ferrite, 15% to 30% of retained austenite and 45% to 80% of partitioned martensite.

6. The steel sheet according to any one of claims 1 or 2, wherein said microstructure does not comprise ferrite, comprises 20% to 30% of retained austenite and 70% to 80% of partitioned martensite.

7. The steel sheet according to any one of claims 1 or 2, wherein the tensile strength is greater than or equal to 1270 MPa, the uniform elongation UE is greater than or equal to 10.0%, the total elongation TE is greater than or equal to 14.0%, and wherein TS, TE and the carbon content and the silicon content satisfy the following formula: (TS x TE) / (C% + Si% / 4) > 50 000 MPa.% with C% and Si% referring to the nominal weight % of C and Si of the steel.

8. The steel sheet according to any one of claims 1 or 2, wherein the hole expansion ratio is greater than or equal to 15%.

9. The steel sheet according to any one of claims 1 or 2, wherein the yield strength YS is greater than or equal to 1000 MPa.

10. Steel sheet according to either one of claims 1 or 2, wherein the LME index is less than 0.36, wherein, LME index = C% + Si% / 4.

11. The steel sheet according to any one of claims 1 or 2, wherein the carbon equivalent Ceq of the steel is lower than 0.4%, said carbon equivalent being defined as Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133 x Mn% x Mo% with the elements expressed in weight percent.

13. A press hardened and partitioned steel part, the composition and the microstructure of said press hardened and partitioned steel part being according to any one of claims 1 to 11. ​ 12. A resistance spot weld of two steel parts made from a cold rolled, annealed and partitioned steel sheet according to any one of claims 1 to 11, the resistance spot weld having an alpha value of at least 30 daN / mm 2 , a being the ratio of the value of the CTS to the product of the diameter of the weld nugget times the thickness of the base. ​

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