Cold rolled and annealed steel sheet and method of manufacturing the same

Abstract

To provide a cold rolled and annealed steel sheet having a combination of high mechanical properties.SOLUTION: A cold rolled and annealed steel sheet is made of a steel having a composition comprising, by weight percent (wt.%): 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%, the remainder of the composition being iron and unavoidable impurities. The steel sheet features a microstructure featuring, in surface fraction, from 25% to 55% of retained austenite, from 45% to 75% of ferrite, less than 5% of fresh martensite, and carbon [C]A and manganese [Mn]A contents in austenite, expressed in weight percent, such that the ratio ([C]A×[Mn]2A) / (C%×Mn%) is from 19.0 to 41.0 wt.%, and C% and Mn% denote the nominal values in carbon and manganese in wt.%, and comprising an inhomogeneous repartition of manganese.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] In order to manufacture various items such as parts of automobile body structure members and vehicle body panels, it is known to use thin plates made of DP (dual-phase) steel or TRIP (transformation-induced plasticity) steel.

[0003] One of the major issues in the automotive industry is to reduce the weight of vehicles in order to improve the fuel efficiency of vehicles from the perspective of global environmental conservation without neglecting safety requirements. To meet these requirements, new high-strength steels having steel sheets with improved yield strength and tensile strength and good ductility and formability are continuously being developed by the steelmaking industry.

[0004] One of the developments carried out to improve mechanical properties is to increase the manganese content in steel. The presence of manganese helps to increase the ductility of steel thanks to the stabilization of austenite. However, these steels exhibit brittle weakness. To overcome this problem, an element as boron is added. These boron-added chemical actions are very tough at 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 exhibit good resistance to liquid metal embrittlement (LME). Steel sheets coated with zinc or zinc alloys are very effective in terms of corrosion resistance and are therefore widely used in the automotive industry. However, it has been experienced that arc welding or resistance welding of certain steels can cause the occurrence of certain cracks due to a phenomenon called 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 an applied or internal stress resulting from restraint, 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 evaluates such resistance by limiting the upper limit value of a so-called LME index calculated according to the following formula: LME index = %C + %Si / 4 wherein %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 medium-manganese (Mn between 3.5 and 12%) cold-rolled steels with reduced carbon content. Two process routes are described. The first one relates to single intercritical annealing of cold-rolled steel sheets. The second one relates to double annealing of cold-rolled steel sheets, the first one being full austenite and the second one being intercritical. Thanks to the selection of the annealing temperature, a good compromise between tensile strength and elongation is obtained. By lowering the annealing temperature, enrichment of austenite is obtained, which implies a good fracture thickness strain value. However, when the amounts of carbon and manganese used in the present invention are low, 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

Summary of the Invention

Means for Solving the Problems

[0009] Therefore, an object of the present invention is to solve the above-mentioned problems, and the steel sheet has 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 satisfy the following formula: (YS×UE + TS×TE) / (C%×Mn%) > 34000, where TE is the total elongation of the steel sheet represented by %, the tensile strength TS is represented by MPa, the yield strength YS is represented by MPa, the uniform elongation UE is represented by %, and C% and Mn% are the nominal weight percentages of C and Mn in the steel. To provide a cold-rolled annealed steel sheet having a high combination of 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 Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133*Mn%*Mo% as defined, and the elements are represented by weight percentages.

[0013] Preferably, the resistance spot welded portion 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 one 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

Figure 2

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%. If 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%. If 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, in order to reduce manganese segregation during casting, the aluminum content is 0.2% to 3%. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. When the addition amount exceeds 3%, the weldability of the steel plate may decrease and the castability may also decrease. Furthermore, it is difficult to achieve a tensile strength exceeding 980 MPa. Moreover, the higher the aluminum content, the higher the soaking temperature for stabilizing austenite. Aluminum is added in an amount of at least 0.2% to improve the robustness of the product by expanding 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% in order to reduce manganese segregation during casting. Furthermore, the addition of at least 0.05% of molybdenum provides resistance to brittleness. When it exceeds 0.5%, the addition of molybdenum is costly and not effective considering the required properties. In a preferred embodiment 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% to improve the toughness of the hot-rolled steel sheet and the spot weldability of the cold-rolled steel sheet. When it exceeds 0.005%, the formation of boron carbide at the prior austenite grain boundaries is promoted and the steel becomes more brittle. In a preferred embodiment of the present invention, the boron content is 0.001% to 0.003%.

[0023] Optionally, some elements can be added to the composition of the steel according to the present invention.

[0024] The maximum addition of the silicon content is limited to 1.20% to improve LME resistance. In addition, this low silicon content makes it possible to simplify the process by eliminating the step of pickling the hot-rolled steel sheet before hot band annealing. Preferably, the maximum silicon content added is 0.5%.

[0025] To provide precipitation strengthening, titanium can be added up to 0.050%. Preferably, in order to protect boron from the formation of BN, at least 0.010% of titanium is added in addition to boron.

[0026] Niobium can optionally be added up to 0.050% to refine austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.010%.

[0027] Optionally, chromium and vanadium can be added up to 0.5% and 0.2% respectively to improve the strength.

[0028] The remaining composition of the steel is iron and impurities resulting from steelmaking. In this regard, P, S and N are regarded as residual elements which are at least inevitable impurities. Their contents are 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, in surface fraction: 25% to 55% retained austenite, Ratio ([C] A × [Mn] 2 A ) / (C% × Mn%) is 19.0 to 41.0% by weight, where C% and Mn% are the nominal values of carbon and manganese in % by weight, and the carbon [C] and manganese [Mn] contents in austenite, expressed in % by weight, such that A and manganese [Mn] A content, 45% to 75% ferrite, less than 5% fresh martensite, 3 × 10 6 / mm 2 below carbide density, and a non-uniform redistribution of manganese, characterized by a manganese distribution having a gradient of -30 or more is contained.

[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 A) / (C%×Mn%) is such that it is 19.0 to 41.0% by weight. When the ratio is below 19.0, the retained austenite is not sufficiently stable, resulting in a decrease in both the yield strength and elongation due to the rapid transformation of the retained austenite to martensite. When it exceeds 41.0, the retained austenite is too stable to generate a sufficient TRIP - TWIP effect during deformation. Such a TWIP - TRIP effect is 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, Volume 46A, June 2015 (S. LEE, K. LEE, and B. C. DE 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 annealing in the transformation range of the hot - rolled steel sheet, but is also formed during the first and second annealing in the transformation range of the cold - rolled steel sheet.

[0035] Fresh martensite can be present with 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 low carbon and manganese contents results in a martensite start temperature Ms above 20°C. To obtain the final mechanical properties, the fresh martensite is limited to a maximum of 5%, preferably a maximum of 3%, or even better reduced to 0.

[0036] Finally, the carbide density is 3×10 in order to ensure that the overall property combination index [YSxUE + TSxTE] / (C%xMn%) remains above 34000.6 / mm 2 It must remain below. Such carbides can be formed during the first annealing after cold rolling when the T1 temperature is too low.

[0037] The cold-rolled 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 annealed steel sheet has a yield strength of 780 MPa or more.

[0039] Preferably, the cold-rolled annealed steel sheet has an LME index below 0.36.

[0040] Preferably, the cold-rolled annealed steel sheet has a carbon equivalent Ceq of less than 0.4% for improving 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%, and the elements are expressed in weight percentages.

[0041] The welded assembly can be manufactured by producing 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 weld joining the first thin sheet to the second thin sheet is characterized by high resistance in the cross-tensile test 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, and those skilled in the art can define it. However, it is preferable to use the method according to the present invention including the following steps:

[0044] The semi-finished product that can be further hot-rolled is provided with the above steel composition. The semi-finished product is heated to a temperature of 1150°C to 1300°C so that hot rolling can be facilitated, and the final hot rolling temperature FRT is 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0045] Then, the hot-rolled steel is cooled and wound at a temperature T of 20°C to 650°C, preferably 300°C to 500°C. coil and taken up.

[0046] Then, the hot-rolled steel plate can be cooled to room temperature and pickled.

[0047] Then, the hot-rolled steel plate is annealed at an annealing temperature T of Tc1 to 680°C. HBA Tc1 is the temperature at which all carbides dissolve for a hot-rolled plate having a homogeneous nominal carbon and manganese distribution. Since Tc1 is the boundary line between the ferrite / austenite / carbide three-phase region and the ferrite / austenite two-phase region, Tc1 is higher than Ac1, which is higher than the Ac1 temperature because Ac1 is the boundary line between the ferrite / carbide region and the ferrite / austenite / carbide region. Preferably, the temperature T HBA is 580°C to 680°C.

[0048] The steel plate is maintained at the temperature T for a holding time t of 0.1 to 120 hours to promote manganese diffusion. Furthermore, this heat treatment of the hot-rolled steel plate makes it possible to reduce the hardness while maintaining toughness exceeding 0.4 J / mm HBA of the hot-rolled steel plate. HBA and. 2 maintain.

[0049] Then, the hot-rolled and heat-treated steel plate is cooled to room temperature and pickled to remove oxidation.

[0050] Then, the hot-rolled and heat-treated steel plate is cold-rolled with a reduction ratio of 20% to 80%.

[0051] Next, the cold-rolled steel sheet is subjected to a first annealing for a holding time t1 of 1 to 120 hours at a transformation range temperature T1 from Tc2 to T HBA up to. Tc2 is the temperature at which all carbides dissolve for a cold-rolled sheet having a heterogeneous carbon and manganese distribution. Tc2 is usually lower than Tc1 due to the presence of C and Mn enrichment zones. When T1 is lower than Tc2, high-density carbides that cannot be completely dissolved during the second annealing remain. 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 must be lower than T HBA . Therefore, the presence of a high carbide density results in a decrease in the overall property combination index [YSxUE + TSxTE] / (C%xMn%) below 34000.

[0052] Preferably, the transformation range temperature T1 is 500 to 650 °C, more preferably 540 °C to 630 °C, and the time t1 soak is 1 to 30 hours. Such a first annealing can be carried out by batch annealing.

[0053] Next, the cold-rolled steel sheet is subjected to a second annealing at a transformation range temperature T2 of 650 to 750 °C for a holding time t2 of 10 seconds to 1000 seconds.

[0054] The second annealing is carried out at a higher temperature than the first annealing in order to increase the retained austenite fraction, dilute the carbon and manganese in the retained austenite, and ensure an appropriate mechanical stability of the austenite to guarantee a continuous TRIP-TWIP effect during deformation.

[0055] Preferably, the transformation range 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 annealed steel sheet is cooled below 80°C, preferably to room temperature. Upon cooling, a portion of the austenite that is not overly rich in manganese and carbon can transform into fresh martensite.

[0057] Next, the steel sheet can be coated by any suitable method including hot-dip coating with zinc or a zinc-based alloy or aluminum or an aluminum-based alloy, electroplating, or vacuum coating.

[0058] The present invention will be described by the following examples, which are in no way limiting.

Examples

[0059] Three grades with the compositions summarized in Table 1 were cast into semi-finished products and processed into steel sheets.

[0060] Table 1 - Composition The compositions tested are summarized in the following table, with the elemental contents expressed in weight percent.

[0061]

Table 1

[0062] The Ac1 and Ac3 temperatures were determined by the diametral test and metallographic analysis of the cold-rolled thin sheet.

[0063] Table 2 - Process Parameters of the Hot-Rolled Heat-Treated Steel Sheet 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 heat-treated at temperature T HBA and maintained at said temperature for a holding time t HBA . The following specific conditions were applied to obtain the hot-rolled heat-treated steel sheet:

[0064]

Table 2

[0065] The hot-rolled and heat-treated steel plates are analyzed, and the corresponding properties are summarized in Table 3.

[0066] Table 3 - Microstructure of Hot-Rolled and Heat-Treated Steel Plates The gradient of the manganese distribution was determined.

[0067] The heat treatment of the hot-rolled steel plate enables manganese to diffuse into austenite: the redistribution of manganese is inhomogeneous in the regions with low manganese content and high manganese content. This manganese inhomogeneity helps to achieve mechanical properties and can be measured thanks to the manganese profile.

[0068] Figure 1 shows the cross-sections of the hot-rolled and heat-treated steel plates of Test 17 and Test 1. The black regions correspond to the regions with low manganese content, and the gray regions correspond to the regions with high manganese content.

[0069] This figure is obtained by the following method: specimens are cut from the hot-rolled and heat-treated steel plates at a thickness of 1 / 4 and polished.

[0070] Then, the cross-section is characterized by an electron probe microanalyzer equipped with a field emission electron gun (``FEG'') at a magnification exceeding 10,000 times to determine the manganese content. Three maps of 10 μm × 10 μm of different parts of the cross-section are obtained. These maps are composed of pixels of 0.01 μm 2 and the manganese content in weight percent 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 manganese content.

[0071] This curve is plotted in Figure 2 for Test 17 and Test 1, and 100% of the thin plate cross-section contains more than 1% manganese. In Test 1, 20% of the thin plate cross-section contains more than 10% manganese.

[0072] Then, the gradient of the obtained 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 is higher than -30, indicating that the redistribution of manganese is inhomogeneous, having regions with low manganese content and regions with high manganese content.

[0074] In contrast, in Test 17, the absence of heat treatment after hot rolling implies that the redistribution of manganese is not inhomogeneous, which can be seen from the value of the gradient of the manganese distribution less than -30.

[0075] [Table 3]

[0076] Table 4 - Process Parameters of Cold-Rolled Annealed Steel Sheets Next, the obtained hot-rolled and heat-treated steel sheet is cold-rolled. Then, before being cooled to less than 80°C, the cold-rolled steel sheet is first annealed at temperature T1 and maintained at said temperature for a holding time t1. Then, before being cooled to room temperature, the steel sheet is annealed a second time at temperature T2 and maintained at said temperature for a holding time t2. The following specific conditions for obtaining the cold-rolled annealed steel sheet were applied:

[0077] [Table 4]

[0078] Next, the cold-rolled annealed steel sheet was 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 Annealed Steel Sheets The phase percentages of the microstructure of the obtained cold-rolled annealed steel sheet after the second annealing were determined.

[0080] [C] A and [Mn] Acorresponds 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 the phases in the microstructure is determined by the following method: The specimen is cut from the cold-rolled annealed steel sheet, polished, and etched with a reagent known per se to reveal the microstructure. Thereafter, the cross-section is examined in the secondary electron mode at a magnification exceeding 5000 times using a scanning electron microscope, for example, a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM").

[0082] The determination of the surface fraction of ferrite is carried out thanks to SEM observation after etching with Nital or Picral / Nital reagent.

[0083] The measurement of the volume fraction of retained austenite is carried out thanks to X-ray diffraction.

[0084] The density of carbides is determined thanks to the cross-section of the thin sheet examined by a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") and image analysis at a magnification exceeding 15000 times.

[0085]

Table 5

[0086] In addition to retained austenite and ferrite, Tests 17 to 19 contain distributed martensite in amounts of 46, 49 and 62%, respectively.

[0087] The inhomogeneity of the manganese distribution obtained after annealing of the hot-rolled steel sheet is maintained as much as possible after both annealing steps of the cold-rolled steel sheet. It can be seen by comparing the gradient of the manganese distribution obtained after annealing of the hot-rolled steel sheet (Table 3) with the gradients of the manganese distributions obtained after the first and second annealing steps of the cold-rolled steel sheet (Table 5).

[0088] Table 6 - Mechanical Properties of Cold Rolled Annealed Steel Sheets The mechanical properties of the obtained cold rolled annealed steel were determined and summarized in the following table.

[0089] The yield strength YS, tensile strength TS, total elongation TE, and uniform elongation UE are measured in accordance with the ISO standard ISO6892-1 issued in October 2009.

[0090] [Table 6]

[0091] Tests 2 and 4 were not subjected to a second annealing to dilute manganese and carbon in austenite. Therefore, the obtained austenite was too stable after the first annealing, resulting in a decrease in elongation.

[0092] Test 9 was subjected to a second annealing temperature T2 that was too high, resulting in the formation of too high a proportion of austenite, a part of which transformed to fresh martensite during cooling, resulting in 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 that was not high enough, resulting in the formation of high-density carbides, which could not be easily dissolved during the second annealing. The carbon and manganese trapped in the carbides could not contribute to the formation and stabilization of retained austenite. Therefore, the presence of a high carbide density resulted in a decrease in the overall property combination index (YSxUE + TSxTE) / / (C% x Mn%) that was too low.

[0094] Test 16 was not subjected to the first annealing and did not show 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 resulted in a value of the overall property combination index that was too low.

[0095] Table 7 - Weldability characteristics of cold-rolled annealed steel sheets Spot welding under the conditions of ISO standard 18278-2 was carried out on cold-rolled annealed steel sheets.

[0096] In the test used, the sample consists of two steel sheets and is in the form of equivalent cross-welding. 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. Thereby, it becomes 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 expressed in daN / mm 2 and is represented by.

[0097] The weldability of the obtained cold-rolled annealed steel was determined and summarized in the following table.

[0098]

Table 7

Claims

1. 1. A cold rolled annealed steel sheet comprising, in weight percent: C: 0.03-0.18% Mn: 6.0-11.0% Al: 0.2 to 3% Mo: 0.05-0.5% B: 0.0005-0.005% S≦0.010% P≦0.020% N≦0.008% and containing, by weight percentage, one or more of the following elements: Si≦1.20% Ti≦0.050% Nb≦0.050% Cr≦0.5% V≦0.2% The remainder of the composition is iron and inevitable impurities resulting from smelting, The steel plate has a surface fraction of 25% to 55% retained austenite; 45% to 75% ferrite, less than 5% fresh martensite; Ratio ([C] Ａ ×[Mn] ２ Ａ Carbon in austenite [C] expressed as weight percent such that [C% x Mn%] / (C% x Mn%) is 19.0-41.0 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 A carbide density below Heterogeneous redistribution of manganese, characterized by a manganese distribution with a gradient of -30 or greater 2. A cold rolled annealed steel sheet having a microstructure comprising:

2. The cold rolled annealed steel sheet according to claim 1, having a carbon content of 0.05% to 0.15%.

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

4. The cold rolled annealed steel sheet according to any one of claims 1 to 3, wherein the aluminum content is 0.7% to 2.2%.

5. The cold rolled annealed steel sheet according to any one of claims 1 to 4, wherein the tensile strength is 950 MPa or more, the uniform elongation UE is 12.0% or more, the total elongation TE is 15% or more, and YS, UE, TS and TE satisfy the following formula (YSxUE+TSxTE) / (C%xMn%)>34000, and C% and Mn% are nominal values ​​for carbon and manganese in weight%.

6. The cold-rolled annealed steel sheet according to any one of claims 1 to 5, having a yield strength of 780 MPa or more.

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.

8. The steel has a carbon equivalent Ceq of less than 0.4, said carbon equivalent being Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133xMn%xMo% The cold rolled annealed steel sheet according to any one of claims 1 to 7, wherein the elements are expressed in weight percent.

9. A resistance spot weld of two steel parts made of cold rolled annealed steel sheet according to any one of claims 1 to 8, wherein the resistance spot weld has a strength of at least 30 daN / mm ２ A resistance spot weld having an α value of

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