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

Abstract

The present invention provides a cold-rolled annealed steel sheet, which is made of a steel having a composition including, in weight percent, C: 0.03-0.18%, Mn: 6.0-11.0%, Al: 0.2-3%, Mo: 0.05-0.5%, B: 0.0005-0.005%, S≦0.010%, P≦0.020%, N≦0.008%, and optionally includes one or more of the following elements in weight percent: Si≦1.20%, Ti≦0.050%, Nb≦0.050%, Cr≦0.5%, V≦0.2%, with the remainder of the composition being iron and inevitable impurities resulting from smelting, and the steel sheet has, in surface fractions, 25%-55% retained austenite, 45%-75% ferrite, less than 5% fresh martensite, and a ratio ([C] A ×[Mn] 2 A Carbon in austenite [C], expressed as weight percent, such that ) / (C% × Mn%) is 19.0 to 41.0 wt.%, where C% and Mn% are the nominal values ​​of carbon and manganese in weight percent. A and manganese [Mn] A Content, 3×10 6 / mm 2 The present invention deals with cold-rolled annealed steel sheets having a microstructure including a heterogeneous redistribution of manganese, characterized by a carbide density below 1000 kJ / cm2 and a manganese distribution with a gradient of -30 or more.

Description

[Technical Field] 【0001】 The present invention relates to a high-strength steel plate having good weldability and a method for obtaining such a steel plate. [Background technology] 【0002】 It is known that thin sheets made of DP (dual-phase) steel or TRIP (transformation-induced plasticity) steel are used to manufacture various items such as automotive body structural components and body panels. 【0003】 One of the major challenges in the automotive industry is reducing vehicle weight to improve fuel efficiency from an environmental perspective, without neglecting safety requirements. To meet these requirements, the steel industry is continuously developing new high-strength steels with improved yield strength and tensile strength, as well as good ductility and formability. 【0004】 One of the developments undertaken to improve mechanical properties is increasing the manganese content in steel. The presence of manganese helps increase the ductility of steel thanks to the stabilization of austenite. However, these steels exhibit a weakness in brittleness. To overcome this problem, elements such as boron are added. These boron-added chemicals make the steel very tough during the hot-rolling stage, but the hot band becomes too hard to process further. The most efficient way to soften the hot band is batch annealing, but this leads to a loss of toughness. 【0005】 In addition to these mechanical requirements, such steel sheets must exhibit good resistance to liquid metal embrittlement (LME). Zinc or zinc alloy coated steel sheets are highly effective in corrosion resistance and are therefore widely used in the automotive industry. However, it has been observed that arc welding or resistance welding of certain steels can cause the development of certain cracks due to a phenomenon called liquid metal embrittlement ("LME") or liquid metal-promoting cracking ("LMAC"). This phenomenon is characterized by the penetration of liquid zinc along the grain boundaries of the underlying steel substrate under applied or internal stress resulting from constraints, thermal expansion, or phase transformation. The addition of elements such as carbon or silicon is known to be detrimental to LME resistance. 【0006】 The automotive industry typically uses the following formula: LME index=%C+%Si / 4 Such resistance is evaluated by limiting the upper limit of the so-called LME index, which is calculated according to the formula, where %C and %Si represent the weight percentages of carbon and silicon in the steel, respectively. 【0007】 Publication WO2020011638 relates to a method for providing medium and intermediate manganese (Mn between 3.5 and 12%) cold-rolled steel with reduced carbon content. Two process pathways are described. The first relates to single transformation-section annealing of cold-rolled steel sheets. The second relates to double annealing of cold-rolled steel sheets, with the first being full austenite and the second being transformation-section. Thanks to the selection of the annealing temperature, a good compromise between tensile strength and elongation can be obtained. Lowering the annealing temperature results in austenite enrichment, which implies a good fracture thickness strain value. However, with the low amounts of carbon and manganese used in this invention, the tensile strength of the steel sheet is limited to a value of 980 MPa or less. [Prior art documents] [Patent Documents] 【0008】 [Patent Document 1] International Publication No. 2020 / 011638 【Summary of the Invention】 【Means for Solving the Problems】 【0009】 Therefore, an object of the present invention is to solve the above problems, and to provide a cold-rolled annealed steel sheet having a tensile strength of 950 MPa or more, a uniform elongation UE of 12.0% or more, a total elongation TE of 15% or more, and YS, UE, TS, and TE satisfying the following formula: (YS × UE + TS × TE) / (C% × Mn%) > 34000, where TE is the total elongation of the steel sheet represented by %, tensile strength TS is represented by MPa, yield strength YS is represented by MPa, uniform elongation UE is represented by %, and C% and Mn% are the nominal weight percentages of C and Mn in the steel, and having a combination of high mechanical properties. 【0010】 Preferably, the cold-rolled annealed steel sheet has a yield strength of 780 MPa or more. 【0011】 Preferably, the cold-rolled annealed steel sheet according to the present invention has an LME index of less than 0.36. 【0012】 Preferably, the cold-rolled annealed steel sheet according to the present invention has a carbon equivalent Ceq of less than 0.4, and the 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 represented by weight percentages. 【0013】 Preferably, the resistance spot weld of two steel parts of the cold-rolled annealed steel sheet according to the present invention has an α value of at least 30 daN / mm2. 【0014】 The object of the present invention is achieved by providing the steel sheet according to claim 1. The steel sheet can also include any of the features of claims 2 to 8, either alone or in combination. 【0015】 Another object of the present invention is the resistance spot welding of two steel parts according to claim 9. 【Brief Description of the Drawings】 【0016】 [Figure 1] It is a figure showing the cross section of the hot rolled and heat treated steel sheets of Test 17 and Test 1. [Figure 2] It is a figure showing the cumulative area fraction of three maps as a function of the manganese content for Test 17 and Test 1. 【Embodiments for Carrying Out the Invention】 【0017】 Next, the present invention will be described in detail without introducing limitations, and will be explained by way of examples. 【0018】 According to the present invention, in order to ensure satisfactory strength and good weldability, the carbon content is 0.03% to 0.18%. When the carbon exceeds 0.18%, the weldability of the steel sheet and the resistance to LME may decrease. The soaking temperature depends on the carbon content: the higher the carbon content, the lower the soaking temperature for stabilizing austenite. When the carbon content is less than 0.03%, the austenite fraction is not sufficiently stabilized to obtain the desired tensile strength and elongation after soaking. 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 0.05% to 0.12%. 【0019】 The manganese content is 6.0% to 11.0%. When the addition amount exceeds 11.0%, the weldability of the steel sheet may decrease, and the productivity of component assembly may decrease. Furthermore, the risk of center segregation increases to the extent that it impairs the mechanical properties. Since the soaking temperature also depends on the manganese content, a minimum value of manganese is defined to stabilize austenite in order to obtain the target microstructure and strength after soaking. Preferably, the manganese content is 6.0% to 9%. 【0020】 According to the present invention, the aluminum content is 0.2% to 3% in order to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. If the amount added exceeds 3%, the weldability and castability of the steel sheet may decrease. Furthermore, it becomes difficult to achieve a tensile strength exceeding 980 MPa. In addition, the higher the aluminum content, the higher the soaking temperature required to stabilize the austenite. Aluminum is added at least 0.2% to improve the robustness of the product by extending the transformation interval range and to improve weldability. Furthermore, aluminum is added to avoid the occurrence of inclusion and oxidation problems. In a preferred embodiment of the present invention, the aluminum content is 0.7% to 2.2%. 【0021】 The molybdenum content is 0.05% to 0.5% to reduce manganese segregation during casting. Furthermore, the addition of at least 0.05% of molybdenum provides resistance to brittleness. Above 0.5%, the addition of molybdenum becomes costly and ineffective considering the required properties. In preferred embodiments of the present invention, the molybdenum content is 0.1% to 0.3%. 【0022】 According to the present invention, the boron content is 0.0005% to 0.005% in order to improve the toughness of hot-rolled steel sheets and the spot weldability of cold-rolled steel sheets. If the content exceeds 0.005%, the formation of boron carbides at the prior austenite grain boundaries is promoted, making the steel more brittle. In a preferred embodiment of the present invention, the boron content is 0.001% to 0.003%. 【0023】 Several elements can be optionally added to the composition of the steel according to the present invention. 【0024】 The maximum silicon content to be added is limited to 1.20% to improve LME resistance. In addition, this low silicon content allows for process simplification by eliminating the step of pickling the hot-rolled steel sheet before hot band annealing. Preferably, the maximum silicon content to be added is 0.5%. 【0025】 To provide precipitation strengthening, titanium can be added up to 0.050%. Preferably, at least 0.010% titanium is added in addition to boron to protect boron from BN formation. 【0026】 Niobium may be optionally added to up to 0.050% during hot rolling to purify austenite grains and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.010%. 【0027】 The strength can be improved by optionally adding chromium and vanadium to concentrations of up to 0.5% and 0.2%, respectively. 【0028】 The remaining composition of steel consists of iron and impurities resulting from smelting. In this regard, P, S, and N are considered residual elements that are at least unavoidable impurities. Their content is 0.010% or less for S, 0.020% or less for P, and 0.008% or less for N. 【0029】 Next, the microstructure of the cold-rolled annealed steel sheet according to the present invention will be described. It is as follows, in terms of surface fraction: 25% to 55% retained austenite, Ratio ([C] A ×[Mn] 2 A The carbon content in austenite, expressed as a weight percentage, is such that ) / (C%×Mn%) is between 19.0 and 41.0 wt%, and C% and Mn% are the nominal values ​​of carbon and manganese in weight percent. A and manganese [Mn] A content, 45% to 75% ferrite, Fresh martensite of less than 5%, 3 x 10 6 / mm 2 Carbide density below, and A heterogeneous redistribution of manganese characterized by a manganese distribution with a gradient of -30 or greater. It contains. 【0030】 The fine structure of the steel sheet according to the present invention contains 25% to 55% retained austenite, preferably 30 to 50% austenite. When the austenite is less than 25% or more than 55%, the uniform elongation UE and the total elongation TE cannot reach their respective minimum values of 12% and 15%. 【0031】 Such austenite is formed during the intercritical annealing of the hot-rolled steel sheet, but also during the first and second intercritical annealing of the cold-rolled steel sheet. During the intercritical annealing of the hot-rolled steel sheet, regions containing a manganese content higher than the nominal value and regions containing a manganese content lower than the nominal value are formed, resulting in a non-uniform distribution of manganese. Therefore, carbon co-segregates with manganese. This manganese inhomogeneity is measured due to the gradient of the manganese distribution in the hot-rolled steel sheet, which must be -30 or more, as shown in and described later in FIG. 2. 【0032】 Due to the non-uniform redistribution of manganese in austenite after hot band annealing and the low diffusion rate theory of manganese in austenite, the manganese inhomogeneity formed during hot band annealing still exists after the first and second intercritical annealing of the cold-rolled steel sheet. This can be proven by the gradient of the manganese distribution in the fine structure, which is -30 or more. 【0033】 Carbon [C] in austenite A and manganese [Mn] A contents are expressed in weight percent, and the ratio ([C] A × [Mn] 2 AThe ratio of ) / (C%×Mn%) is between 19.0% and 41.0% by weight. If the ratio is below 19.0%, the retained austenite is not sufficiently stable, leading to a decrease in both yield strength and elongation due to the rapid transformation of retained austenite to martensite. If it is above 41.0%, the retained austenite is too stable and cannot generate a sufficient TRIP-TWIP effect during deformation. Such TWIP-TRIP effects are specifically described in "Observation-of-the-TWIP-TRIP-Plasticity-Enhancement-Mechanism-in-Al-Added-6-Wt-Pct-Medium-Mn-Steel" DOI:10.1007 / s11661-015-2854-z, The Minerals, Metals & Materials Society and ASM International 2015, p.2356, Vol. 46A, June 2015 (S. LEE, K. LEE, and BCDE COOMAN). 【0034】 The microstructure of the steel sheet according to the present invention contains 45% to 75% ferrite, preferably 45% to 70% ferrite. Such ferrite is formed during the transformation section annealing of the hot-rolled steel sheet, but also during the first and second transformation section annealing of the cold-rolled steel sheet. 【0035】 Fresh martensite can be present in a surface fraction of up to 5%, but it is not a desirable phase in the microstructure of the steel sheet according to the present invention. It can be formed during the final cooling step to room temperature by the transformation of unstable austenite. In fact, this unstable austenite, with its low carbon and manganese content, results in a martensite onset temperature Ms above 20°C. To obtain the final mechanical properties, fresh martensite is limited to a maximum of 5%, preferably a maximum of 3%, or even better, reduced to 0. 【0036】 Finally, to ensure that the carbide density remains above 34000 in the overall property combination index [YSxUE+TSxTE] / (C%xMn%), 3 × 10⁻¹⁰6 / mm 2 It must remain below this level. Such carbides can form during the first annealing after cold rolling if the T1 temperature is too low. 【0037】 The cold-rolled and annealed steel sheet according to the present invention has a tensile strength TS of 950 MPa or more, a uniform elongation UE of 12.0% or more, and a total elongation TE of 15% or more. 【0038】 Preferably, the cold-rolled and annealed steel sheet has a yield strength of 780 MPa or more. 【0039】 Preferably, the cold-rolled and annealed steel sheet has an LME index of less than 0.36. 【0040】 Preferably, the cold-rolled and annealed steel sheet has a carbon equivalent (Ceq) of less than 0.4% to improve weldability. The 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 percent. 【0041】 A welded assembly can be manufactured by generating two parts from a thin sheet of cold-rolled annealed steel according to the present invention, and then performing resistance spot welding of the two steel parts. 【0042】 The resistance spot welds joining the first thin plate to the second thin plate are characterized by high resistance in cross-tensile tests defined by an α value of at least 30 daN / mm2. 【0043】 The steel sheet according to the present invention can be produced by any suitable manufacturing method, which a person skilled in the art can specify. However, it is preferable to use the method according to the present invention, which includes the following steps: 【0044】 The semi-finished product, which can be further hot-rolled, is provided with the steel composition described above. The semi-finished product is heated to a temperature of 1150°C to 1300°C to facilitate hot rolling, and the final hot-rolling temperature (FRT) is 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C. 【0045】 Next, the hot-rolled steel is cooled to a temperature of 20°C to 650°C, preferably 300°C to 500°C T coil It gets wound up. 【0046】 Next, the hot-rolled steel sheet can be cooled to room temperature and then pickled. 【0047】 Next, the hot-rolled steel sheet is annealed at an annealing temperature of Tc1 to 680℃. HBA Annealing is performed to Tc1. Tc1 is the temperature at which all carbides melt in a hot-rolled sheet having a homogeneous nominal carbon and manganese distribution. Since Tc1 is the boundary between the ferrite / austenite / carbide three-phase region and the ferrite / austenite two-phase region, Tc1 is higher than Ac1, because Ac1 is the boundary between the ferrite / carbide region and the ferrite / austenite / carbide region, and therefore it is higher than the Ac1 temperature. Preferably, temperature T HBA The temperature range is 580°C to 680°C. 【0048】 The steel plate is held for 0.1 to 120 hours to promote manganese diffusion. HBA During the period, the temperature T HBA This is maintained. Furthermore, this heat treatment of hot-rolled steel sheets results in a temperature of 0.4 J / mm² for hot-rolled steel sheets. 2 This makes it possible to reduce hardness while maintaining toughness that exceeds the limit. 【0049】 Next, the hot-rolled steel sheet can be cooled to room temperature and then pickled to remove oxidation. 【0050】 Next, the hot-rolled heat-treated steel sheet is cold-rolled with a reduction ratio of 20% to 80%. 【0051】 Next, the cold-rolled steel sheet is processed from Tc2 to T HBA The sheet is subjected to a first annealing at a transformation interval temperature T1 for a holding time t1 of 1 to 120 hours. Tc2 is the temperature at which all carbides dissolve in a cold-rolled sheet with a heterogeneous carbon and manganese distribution. Tc2 is usually lower than Tc1 due to the presence of C and Mn enriched zones. If T1 is lower than Tc2, high-density carbides remain that cannot be completely dissolved during the second annealing. The carbon and manganese trapped in the carbides cannot contribute to the formation and stabilization of retained austenite. Furthermore, in order to concentrate more carbon and manganese in the austenite, the first annealing is performed at T HBA It must be lower. Therefore, the presence of a high carbide density results in a decrease in the overall property combination index [YSxUE+TSxTE] / (C%xMn%) to below 34000. 【0052】 Preferably, the transformation interval temperature T1 is 500 to 650°C, more preferably 540 to 630°C, and time t1 soak This takes 1 to 30 hours. Such a first annealing can be performed by batch annealing. 【0053】 Next, the cold-rolled steel sheet is subjected to a second annealing process at a transformation zone temperature T2 of 650-750°C for a holding time t2 of 10 to 1000 seconds. 【0054】 The second annealing is performed at a higher temperature than the first annealing to increase the residual austenite fraction and dilute the carbon and manganese in the residual austenite, thereby ensuring adequate mechanical stability of the austenite and guaranteeing a sustained TRIP-TWIP effect during deformation. 【0055】 Preferably, the transformation interval temperature T2 is 670°C to 720°C, and t2 is 80 to 500 seconds. Such a second annealing can be carried out by continuous annealing. 【0056】 Next, the cold-rolled and annealed steel sheet is cooled to below 80°C, preferably to room temperature. Upon cooling, some of the austenite that is not very rich in manganese and carbon may transform into fresh martensite. 【0057】 Next, the steel sheet can be coated by any suitable method, including hot-dip galvanizing, electrodeposition, or vacuum coating of zinc or a zinc-based alloy, or aluminum or an aluminum-based alloy. 【0058】 The present invention will be explained by the following examples, but these are by no means limiting. [Examples] 【0059】 The three grades whose compositions are summarized in Table 1 were cast into semi-finished products and then processed into steel plates. 【0060】 Table 1 - Composition The tested compositions are summarized in the table below, with elemental content expressed as weight percentage. 【0061】 [Table 1] 【0062】 The Ac1 and Ac3 temperatures were determined by diametral testing and metallographic analysis of cold-rolled sheets. 【0063】 Table 2 - Process parameters for hot-rolled and heat-treated steel sheets The as-cast steel semi-finished product was reheated at 1200°C, hot-rolled, and then coiled at 450°C. Next, the hot-rolled heat-treated steel sheet was subjected to a temperature T HBA Heat treatment is performed, and the holding time is t HBA The temperature is maintained during this time. The following specific conditions were applied to obtain hot-rolled heat-treated steel sheets: 【0064】 [Table 2] 【0065】 The hot-rolled and heat-treated steel sheets were analyzed, and their corresponding properties are summarized in Table 3. 【0066】 Table 3 - Microstructure of hot-rolled and heat-treated steel sheet The gradient of the manganese distribution was determined. 【0067】 The heat treatment of hot-rolled steel sheets allows manganese to diffuse into the austenite; the redistribution of manganese is heterogeneous in areas with low and high manganese content. This manganese heterogeneity helps achieve the desired mechanical properties and can be measured thanks to the manganese profile. 【0068】 Figure 1 shows cross-sections of the hot-rolled steel sheets from Test 17 and Test 1. The black areas correspond to regions with low manganese content, and the gray areas correspond to regions with high manganese content. 【0069】 This figure is obtained by the following method: A test specimen is cut from a hot-rolled, heat-treated steel sheet to a thickness of 1 / 4 and then polished. 【0070】 Subsequently, the cross-section was characterized using an electron probe microanalyzer equipped with a field emission electron gun ("FEG") at a magnification exceeding 10,000x to determine the manganese content. Three 10 μm × 10 μm maps were obtained from different parts of the cross-section. These maps were 0.01 μm 2 It consists of pixels. The amount of manganese in weight percentage is calculated for each pixel and then plotted on a curve representing the cumulative area fraction of the three maps as a function of the amount of manganese. 【0071】 This curve is plotted in Figure 2 for Test 17 and Test 1, showing that 100% of the plate cross-sections contain more than 1% manganese. In Test 1, 20% of the plate cross-sections contain more than 10% manganese. 【0072】 Next, the gradient of the resulting curve is calculated between the point representing 80% of the cumulative area fraction and the point representing 20% ​​of the cumulative area fraction. 【0073】 In Test 1, this gradient was higher than -30, indicating that the manganese redistribution was heterogeneous, with regions having both low and high manganese content. 【0074】 In contrast, in Test 17, the absence of heat treatment after hot rolling implies that the manganese redistribution is not heterogeneous, which can be seen from the manganese distribution gradient value of less than -30. 【0075】 [Table 3] 【0076】 Table 4 - Process parameters for cold-rolled and annealed steel sheets Next, the obtained hot-rolled heat-treated steel sheet is cold-rolled. Then, the cold-rolled steel sheet is subjected to an initial annealing at temperature T1 before cooling to below 80°C, and is held at this temperature for a holding time t1. Then, the steel sheet is subjected to a second annealing at temperature T2 before cooling to room temperature, and is held at this temperature for a holding time t2. The following specific conditions were applied to obtain the cold-rolled annealed steel sheet: 【0077】 [Table 4] 【0078】 Next, cold-rolled and annealed steel sheets were analyzed, and the corresponding microstructure elements, mechanical properties, and weldability properties were summarized in Tables 5, 6, and 7, respectively. 【0079】 Table 5 - Microstructure of cold-rolled and annealed steel sheet The phase percentages of the microstructure of the cold-rolled annealed steel sheet obtained after the second annealing process were determined. 【0080】 [C] A and [Mn] AThese correspond to the amounts of carbon and manganese in austenite in weight percent. They are measured using X-ray diffraction for carbon and an electron probe microanalyzer equipped with a field emission gun for manganese. 【0081】 The surface fraction of phases in the microstructure is determined by the following method: a specimen is cut from a cold-rolled, annealed steel sheet, polished, and etched with a known reagent to reveal the microstructure. The cross-section is then examined using a scanning electron microscope, such as a scanning electron microscope with a field emission electron gun ("FEG-SEM"), at a magnification of over 5000x in secondary electron mode. 【0082】 The determination of the ferrite surface fraction is performed thanks to SEM observation after etching with Nital or Picral / Nital reagents. 【0083】 The volume fraction of retained austenite can be measured thanks to X-ray diffraction. 【0084】 The density of the carbides is determined thanks to the cross-section of the thin plates, which is examined using a scanning electron microscope ("FEG-SEM") equipped with a field emission electron gun and image analysis at magnifications exceeding 15,000x. 【0085】 [Table 5] 【0086】 In addition to retained austenite and ferrite, tests 17-19 contain partitioned martensite in amounts of 46%, 49%, and 62%, respectively. 【0087】 The heterogeneity of the manganese distribution obtained after annealing of hot-rolled steel sheets is maintained as much as possible after both annealing steps of cold-rolled steel sheets. This can be seen by comparing the gradient of the manganese distribution obtained after annealing of hot-rolled steel sheets (Table 3) with the gradient of the manganese distribution obtained after the first and second annealing steps of cold-rolled steel sheets (Table 5). 【0088】 Table 6 - Mechanical properties of cold-rolled and annealed steel sheets The mechanical properties of the obtained cold-rolled and annealed material were determined and summarized in the table below. 【0089】 Yield strength YS, tensile strength TS, total elongation TE, and uniform elongation UE are measured in accordance with ISO standard ISO 6892-1, published in October 2009. 【0090】 [Table 6] 【0091】 Tests 2 and 4 were not subjected to a second annealing to dilute the manganese and carbon in the austenite. Therefore, the resulting austenite became too stable after the first annealing, resulting in reduced elongation. 【0092】 Test 9 was subjected to a second annealing temperature T2 that was too high, resulting in the formation of an excessively high proportion of austenite. Some of this austenite transformed into fresh martensite upon cooling, leading to a decrease in yield strength. Furthermore, the retained austenite was not sufficiently stable, resulting in a decrease in yield strength and elongation. 【0093】 Tests 13, 14, and 15 were subjected to a first annealing at a temperature not sufficiently high, resulting in the formation of high-density carbides, which do not readily dissolve during the second annealing. The carbon and manganese trapped in the carbides cannot contribute to the formation and stabilization of retained austenite. Therefore, the presence of high carbide density leads to a decrease in the overall property combination index (YSxUE+TSxTE) / / (C%xMn%) which is too low. 【0094】 Test 16 was not subjected to the first annealing and did not exhibit sufficient ferrite in its microstructure. Furthermore, it contained 5% fresh martensite, and the retained austenite was not sufficiently stable. These deviations from the objectives of the present invention result in a value of the overall property combination index that is too low. 【0095】 Table 7 - Weldability characteristics of cold-rolled and annealed steel sheets Spot welding was performed on cold-rolled, annealed steel sheets according to ISO standard 18278-2 conditions. 【0096】 In the test used, the sample consists of two steel plates in the form of equivalent cross-welds. A force is applied to break the weld point. This force is known as the cross-tensile strength (CTS) and is expressed in daN. It depends on the diameter of the weld point and the thickness of the metal, i.e., the thickness of the steel and the metal coating. This makes it possible to calculate a coefficient α, which is the ratio of the value of CTS to the product of the diameter of the weld point and the thickness of the base material. This coefficient is daN / mm 2 It is represented as follows. 【0097】 The weldability of the obtained cold-rolled and annealed materials was determined and summarized in the table below. 【0098】 [Table 7]

Claims

[Claim 1] Cold-rolled and annealed steel sheet, The aforementioned steel plate, in mass%, is: C: 0.03-0.18% Mn: 6.0-11.0% Al: 0.2-3% Mo: 0.05-0.5% B: 0.0005-0.005% S ≤ 0.010% P ≤ 0.020% N ≤ 0.008% It includes, optionally, one or more of the following elements: Si ≤ 1.20% Ti ≤ 0.050% Nb ≤ 0.050% Cr ≤ 0.5% V ≤ 0.2% The remainder consists of iron and unavoidable impurities resulting from smelting. The aforementioned steel plate, in terms of surface fraction, 25% to 55% residual austenite, 45% to 75% ferrite, Fresh martensite of less than 5%, It has a microstructure consisting of, The steel plate has a carbide density of less than 3 × 10⁶ / mm² in terms of number density. In the aforementioned steel plate, the ratio ([C] Ａ × [Mn] Ａ ２ ) / (C% × Mn%) is 19.0 to 33.9 mass%, where C% and Mn% are the carbon content and manganese content in the steel in mass%, respectively, and [C] Ａ and [Mn] Ａ These represent the carbon content and manganese content in austenite, respectively, in mass percent. The aforementioned steel plate exhibits a heterogeneous manganese redistribution, characterized by a manganese distribution where the slope calculated between the point representing 80% of the cumulative area fraction and the point representing 20% ​​of the cumulative area fraction of the manganese fraction, obtained from a pixel map of manganese content for different parts of the steel plate at a thickness of 1 / 4 from the surface of the steel plate as a function of manganese content expressed in mass percent, has a slope of -30 or greater. Having, Cold-rolled and annealed steel sheet. [Claim 2] The cold-rolled and annealed steel sheet according to claim 1, wherein the carbon content is 0.05% to 0.15%. [Claim 3] A cold-rolled, annealed steel sheet according to claim 1 or 2, wherein the manganese content is 6.0% to 9%. [Claim 4] A cold-rolled, annealed steel sheet according to any one of claims 1 to 3, wherein the aluminum content is 0.7% to 2.2%. [Claim 5] A cold-rolled, annealed steel sheet according to any one of claims 1 to 4, wherein the tensile strength TS is 950 MPa or more, the uniform elongation UE is 12.0% or more, the total elongation TE is 15% or more, and the yield strengths YS, UE, TS, and TE satisfy the following formula (YS × UE + TS × TE) / (C% × Mn%) > 34000, where C% and Mn% are the mass percent carbon content and manganese content in the steel, respectively. [Claim 6] A cold-rolled annealed steel sheet according to any one of claims 1 to 5, wherein the yield strength YS is 780 MPa or more. [Claim 7] The cold-rolled annealed steel sheet according to any one of claims 1 to 6, wherein the LME index is less than 0.36, where the LME index is calculated according to the formula: LME index = C% + Si% / 4, where C% and Si% are the mass percent carbon content and silicon content in the steel, respectively. [Claim 8] The steel has a carbon equivalent Ceq of less than 0.4, and the carbon equivalent is Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133×Mn%×Mo% A cold-rolled annealed steel sheet according to any one of claims 1 to 7, defined as such, and with elements expressed in mass percent. [Claim 9] A resistance spot weld joint of two steel parts made from cold-rolled annealed steel sheets according to any one of claims 1 to 8, wherein the resistance spot weld joint has a strength of at least 30 daN / mm ２ A resistance spot weld having an α value, where α is the ratio of the value of the cross-tensile strength (CTS) to the product of the diameter of the weld point and the thickness of the steel plate.

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