Cold-rolled annealed steel sheet and its manufacturing method

Abstract

The present invention relates to 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%, the remainder of the composition being iron and unavoidable impurities resulting from smelting, and the steel sheet has a surface fraction of -30% to 55% retained austenite, -45% to 70% ferrite, and less than -5% fresh martensite; and carbon [C] in austenite, expressed in weight percent. A and manganese [Mn] A The content is [C] A *[Mn] A / ((0,1+C% 2 The present invention deals with cold-rolled and annealed steel sheets having a microstructure including heterogeneous redistribution of manganese, characterized by a manganese distribution satisfying the relationship of (Mn%)*(Mn%)+2)≧1.10 and having 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 to use steel sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels to manufacture various items such as automotive body structural components and body panels.

[0003] One of the major challenges in the automotive industry is to improve vehicle fuel economy in order to protect the global environment, without neglecting safety requirements. Mass To meet these requirements, new high-strength steels with improved yield strength and tensile strength, as well as steel plates with good ductility and formability, are continuously developed by the steelmaking industry.

[0004] One of the developments made to improve mechanical properties is to increase the manganese content in steel. The presence of manganese helps to increase the ductility of steel by stabilizing austenite. However, these steels exhibit a weakness in brittleness. To overcome this problem, elements such as boron are added. These boron-added chemistries 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 steels must also exhibit good resistance to liquid metal embrittlement (LME). Zinc- or zinc-alloy-coated steels are highly effective in corrosion resistance and are therefore widely used in the automotive industry. However, it has been observed that arc or resistance welding of certain steels can cause the development 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 grain boundaries of the underlying steel substrate under applied or internal stresses 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] In the automotive industry, the following formula is usually used: LME index=C%+Si% / 4, Such resistance is assessed by limiting the upper limit of the so-called LME index, which is calculated according to: where %C and %Si are the percentages of carbon and silicon in the steel, respectively. Mass Expressed as a percentage.

[0007] Publication WO2020011638 relates to a method for providing a medium to medium manganese (3.5-12% Mn) cold-rolled steel with a reduced carbon content. Two process routes are described. The first involves a single intercritical annealing of the cold-rolled steel sheet. The second involves a double annealing of the cold-rolled steel sheet, the first being fully austenitic and the second being intercritical. Depending on the selection of the annealing temperature, a good compromise between tensile strength and elongation is obtained. However, the tensile strength of the steel sheet does not exceed 980 MPa. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2020 / 011638 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, it is an object of the present invention to solve the above-mentioned problems and to provide a cold-rolled annealed steel sheet having a combination of high mechanical properties, including a tensile strength TS of 1050 MPa or more, a yield strength YS of 780 MPa or more, a uniform elongation UE of 13% or more, and a total elongation TE of 15% or more, without degrading weldability characteristics. Preferably, the cold-rolled annealed steel sheet according to the present invention has an LME index of less than 0.36. Preferably, the cold-rolled annealed steel sheet has a hole expansion ratio HE of 15% or more. [Means for solving the problem]

[0010] 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% Here, the elements are Mass Expressed as a percentage, is defined as:

[0011] Preferably, the resistance spot weld between two steel parts of cold rolled and annealed steel sheet according to the invention has an α value of at least 30 daN / mm 2 .

[0012] Preferably, the cold rolled and annealed steel sheet according to the present invention is [(TS-800) × (YS-300) × UE × TE] / [(0 . 1+C%) × Mn%]>3.3 × 10 7 In the formula, TS and YS are expressed in MPa, UE and TE are expressed in %, C% and Mn% are Mass The nominal concentration in % Meet the following.

[0013] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also comprise the properties according to any one of claims 2 to 10. Another object of the present invention is the resistance spot welding of two steel parts according to claim 11. [Brief explanation of the drawings]

[0014]

Figure 1

Figure 2

[0015] The invention will now be described in detail and illustrated by examples, without introducing any limitations.

[0016] According to the present invention, the carbon content is 0.03% to 0.18% to ensure sufficient strength and good weldability. If the carbon content exceeds 0.18%, the weldability and resistance to LME of the steel plate may be reduced. The soaking temperature depends particularly on the carbon content: the higher the carbon content, the lower the soaking temperature required to stabilize the austenite. If the carbon content is less than 0.03%, the austenite fraction will not be 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 0.07% to 0.12%.

[0017] The manganese content is 6.0% to 11.0%. If the addition amount exceeds 11.0%, the weldability of the steel sheet may deteriorate, which may reduce the productivity of component assembly. Furthermore, the risk of center segregation increases to the extent that it impairs mechanical properties. Since the soaking temperature also depends on the manganese content, a minimum manganese content is specified to stabilize austenite in order to obtain the target microstructure and strength after soaking. Preferably, the manganese content is 6.5% to 9.0%.

[0018] According to the present invention, the aluminum content is 0.2% to 3% to reduce manganese segregation during casting. Aluminum is a very effective element for deoxidizing steel in the liquid phase during refining. Addition of more than 3% may reduce the weldability of the steel plate and reduce its castability. Furthermore, it is difficult to achieve a tensile strength of more than 980 MPa. Furthermore, the higher the aluminum content, the higher the soaking temperature required to stabilize austenite. Aluminum is added at least 0.2% to improve product robustness and weldability by expanding the transformation range. Furthermore, aluminum is added to avoid inclusion and oxidation problems. In a preferred embodiment of the present invention, the aluminum content is 0.5% to 1.5%.

[0019] The molybdenum content is 0.05% to 0.5% to reduce manganese segregation during casting. Furthermore, the addition of at least 0.05% molybdenum provides resistance to embrittlement. Above 0.5%, the addition of molybdenum is costly and ineffective considering the required properties. In a preferred embodiment of the present invention, the molybdenum content is 0.1% to 0.3%.

[0020] 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. Above 0.005%, the formation of boron carbides at the preceding 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%.

[0021] Optionally, some elements can be added to the composition of the steel according to the invention.

[0022] To improve LME resistance, the maximum silicon content is limited to 1.20%. In addition, this low silicon content allows simplifying 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%.

[0023] Titanium can be added up to 0.050% to provide precipitation strengthening. Preferably, a minimum of 0.010% titanium is added in addition to boron to protect the boron from forming BN.

[0024] Niobium may optionally be added up to 0.050% to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.010%.

[0025] Optionally, chromium and vanadium may be added up to 0.5% and 0.2%, respectively, to improve strength.

[0026] The remainder of the steel composition is iron and impurities resulting from smelting. In this respect, P, S, and N are considered to be residual elements, at least unavoidable impurities. Their contents are not more than 0.010% for S, 0.020% for P, and 0.008% for N.

[0027] Next, the microstructure of the cold rolled and annealed steel sheet according to the present invention will be described, which has a surface fraction of: - 30% to 55% retained austenite, - 45% to 70% ferrite, - Less than 5% fresh martensite - Mass Carbon in austenite [C], expressed as a percentage A and manganese [Mn] A The content is [C] A × [Mn] A / ((0 . 1+C%2 ) × (Mn%+2))≧1.10 C% and Mn% Mass are the nominal values ​​of carbon and manganese in percent, Fulfilling - and heterogeneous redistribution of manganese, characterized by manganese distribution with gradients of -30 or greater. Contains:

[0028] The microstructure of the steel sheet according to the present invention contains 30% to 55% retained austenite, preferably 30 to 50% austenite. If the austenite content is below 30% or above 55%, the uniform and total elongation cannot reach the target values.

[0029] Such austenite is formed during intercritical annealing of hot-rolled steel sheets, but also during intercritical annealing of cold-rolled steel sheets. During intercritical annealing of hot-rolled steel sheets, regions containing manganese above and below the nominal value are formed, resulting in a heterogeneous distribution of manganese. Carbon therefore co-segregates with manganese. This manganese heterogeneity is measured by the slope of the manganese distribution in the hot-rolled steel sheet, which should be greater than or equal to -30, as shown in Figure 2 and discussed below.

[0030] The microstructure of the steel sheet according to the present invention contains 45% to 70% ferrite, preferably 50% to 70% ferrite, which is formed during intercritical annealing of hot-rolled steel sheets, but also during intercritical annealing of cold-rolled steel sheets.

[0031] Fresh martensite can be present in a surface fraction of up to 5%, but is not a desired phase in the microstructure of the steel sheet according to the invention. It can form during the final cooling step to room temperature by transformation of unstable austenite. Indeed, this unstable austenite with a low carbon and manganese content leads to a martensite start 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.

[0032] Carbon in austenite [C] A and manganese [Mn] A The content is Mass Expressed as a percentage, [C] A × [Mn] A / ((0 . 1+C% 2 ) × (Mn%+2))≧1.10, C% and Mn% are Mass These are the nominal values ​​of carbon and manganese in percent. When the value of the formula is below 1.10, it is not possible to ensure sufficient elongation for the steel plate.

[0033] Preferably, the concentration of carbides in the cold rolled annealed steel sheet is 1×10 6 / mm 2 The following is the result.

[0034] The cold-rolled and annealed steel sheet according to the present invention has a tensile strength of 1050 MPa or more, a uniform elongation UE of 13% or more, and a total elongation TE of 15% or more.

[0035] Preferably, the cold-rolled and annealed steel sheet has a yield strength of 780 MPa or more.

[0036] Preferably, the cold rolled and annealed steel sheet has an LME index of less than 0.36.

[0037] Preferably, the cold-rolled and annealed steel sheet has a hole expansion ratio HE of 15% or more.

[0038] According to the present invention, the cold rolled and annealed steel sheet preferably has a carbon equivalent Ceq of less than 0.4% to improve weldability. Carbon equivalent Ceq = C% + Si% / 55 + Cr% / 20 + Mn% / 19 - Al% / 18 + 2.2P% - 3.24B% - 0.133 × Mn% × Mo% Here, the elements are Mass Expressed as a percentage, is defined as:

[0039] In a preferred embodiment, the tensile strength TS expressed in MPa, the yield strength YS expressed in MPa, the uniform elongation UE expressed in % and the total elongation TE expressed in % of the cold rolled annealed steel sheet are calculated by the following formula: [(TS-800) × (YS-300) × UExTE] / [(0.1+C%) × Mn%]>3.3 × 10 7 In the formula, C% and Mn% are Mass Corresponding to the nominal carbon and manganese content in percent, Meet the following.

[0040] The welded assembly can be manufactured by producing two thin sheets of cold rolled annealed steel and resistance spot welding the two steel parts together.

[0041] The resistance spot weld joining the first sheet to the second sheet is characterized by a high resistance in a cross tensile test defined by an α value of at least 30 daN / mm 2 .

[0042] The steel sheet according to the invention can be produced by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use a method according to the invention comprising the following steps: The above-mentioned steel composition is provided in a semi-finished product that can be further hot rolled. The semi-finished product is heated to a temperature of 1150°C to 1300°C to facilitate hot rolling, with a final hot rolling temperature FRT of 800°C to 1000°C. Preferably, the FRT is 850°C to 950°C.

[0043] The hot-rolled steel is then cooled and coiled at a temperature Tcoil of 20° C. to 600° C. The hot-rolled steel sheet can then be cooled to room temperature and pickled.

[0044] The hot-rolled steel sheet is then annealed at an annealing temperature T between Ac1 and Ac3. HBA Preferably, the temperature is T HBA is composed of Ac1+5°C to Ac3. Preferably, the temperature T HBA The temperature is 580℃ to 680℃. The steel sheet is subjected to a holding time t of 0.1 to 120 hours to promote manganese diffusion and the formation of a non-uniform manganese distribution. HBA During this time, the temperature T HBA is maintained.

[0045] T HBA The temperature and time for such intercritical annealing are selected to obtain 10-60% austenite and 40-90% ferrite after cooling, with the proportion of precipitated carbides being kept below 0.8%. In particular, the selection of an appropriate time and temperature for such intercritical annealing must take into account the maximum carbide fraction that can be tolerated according to the present invention. In particular, T HBA is T HBA It is noted that an increase in Cr will limit carbide precipitation and is selected by those skilled in the art to limit carbide precipitation.

[0046] With respect to chemical composition, the higher the amount of carbon and aluminum in the steel, the higher the concentration of carbides for a given temperature. This means that for carbon and aluminum contents at the top of the claimed range, T is adjusted accordingly to limit carbide precipitation. HBA This means that we must increase

[0047] Furthermore, the lower the amount of manganese in the steel, the higher the carbide concentration at a given temperature. This means that for manganese contents at the lower end of the claimed range, T must be increased accordingly to limit carbide precipitation. HBA This means that we must increase

[0048] The hot rolled heat treated steel sheet can then be cooled to room temperature and pickled to remove oxidation.

[0049] The hot-rolled heat-treated steel sheet is then cold-rolled at a reduction rate of 20% to 80%.

[0050] Next, the cold-rolled steel sheet is subjected to a transformation temperature T soak The Ac1 and Ac3 are measured by dilatometry. Those skilled in the art will appreciate that the optimum temperature T is sufficiently low to limit the formation of unstable austenite and fresh martensite during the final cooling step. soak This optimum temperature depends, inter alia, on the carbon, manganese and aluminum content. The higher the aluminum content, the higher the soaking temperature required to stabilize the austenite. The higher the carbon or manganese content, the lower the soaking temperature required to stabilize the austenite.

[0051] Preferably, the transformation interval temperature T soak The temperature is 600 to 760°C. The steel sheet is subjected to a holding time t of 10 to 180,000 seconds to obtain a fully recrystallized microstructure. soak During this time, the temperature T soak is maintained.

[0052] The cold-rolled annealed steel sheet is then cooled to room temperature.

[0053] The steel sheet can then be coated by any suitable method, including hot dip coating, electrodeposition or vacuum coating of zinc or zinc-based alloys or aluminum or aluminum-based alloys.

[0054] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0055] Seven grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel sheets.

[0056] [Table 1-Composition] The tested compositions are summarized in the table below, with elemental contents Mass Expressed as a percentage.

[0057] [Table 1]

[0058] The Ac1 and Ac3 temperatures were determined by dilatometric testing and metallographic analysis of cold rolled steel sheets.

[0059] [Table 2 - Process parameters for hot-rolled heat-treated steel sheets] The cast steel semi-finished product was reheated at 1200 °C, hot rolled, and then coiled at 450 °C. The hot rolled and coiled steel sheet was then heated to a temperature T HBA and holding time t HBA The following specific conditions were applied to obtain hot-rolled heat-treated steel sheets:

[0060] [Table 2]

[0061] The hot-rolled heat-treated steel sheets were analyzed, and the corresponding properties are summarized in Table 3.

[0062] [Table 3 - Microstructure and properties of hot-rolled heat-treated steel sheets] The gradient of manganese distribution and the Charpy impact energy at 20°C were measured.

[0063] Charpy impact energy is measured according to ISO Standard 148-1:2006(F) and ISO Standard 148-1:2017(F).

[0064] The heat treatment of hot-rolled steel allows manganese to diffuse into the austenite: the redistribution of manganese is heterogeneous, with regions of low and high manganese content. This manganese heterogeneity helps to achieve mechanical properties and can be measured by the manganese distribution.

[0065] Figure 1 shows the hot-rolled heat-treated steel specimens of Test 1 and Test 10. The black areas correspond to areas with low manganese content, and the grey areas correspond to areas with high manganese content.

[0066] This diagram is obtained by the following method: test specimens are cut at 1 / 4 thickness from hot-rolled heat-treated steel sheets and polished.

[0067] The specimen was then characterized by an electron probe microanalyzer equipped with a field emission electron gun ("FEG") at a magnification of over 10,000x to measure manganese content. Three 10 μm x 10 μm maps of different areas of the specimen were obtained. These maps were measured at 0.01 μm resolution. 2 It is composed of pixels. Mass The manganese content in 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 manganese content.

[0068] This curve is plotted in Figure 2 for Test 1 and Test 10: 100% of the sheet specimens contain more than 1% manganese. In Test 1, 20% of the sheet specimens contain more than 10% manganese.

[0069] The slope of the resulting curve is then calculated between the point representing 80% cumulative area fraction and the point representing 20% ​​cumulative area fraction. In Test 1, this slope is greater than -30, indicating that the redistribution of manganese is heterogeneous, with regions of low and high manganese content.

[0070] In contrast, in Test 10, the absence of heat treatment after hot rolling implies that the redistribution of manganese is not homogeneous, as evidenced by the slope of the manganese distribution being less than -30. This distribution of manganese would not be able to achieve the desired mechanical properties. This is also true for Test 11.

[0071] [Table 3]

[0072] [Table 4 - Process parameters for cold rolled annealed steel sheets] Next, the obtained hot-rolled heat-treated steel sheet is cold-rolled at a reduction rate of 50%. Then, the cold-rolled steel sheet is heated to temperatures T soak and held for a holding time t before being cooled to room temperature. soak The following specific conditions were applied to obtain cold-rolled annealed steel sheets:

[0073] [Table 4]

[0074] The cold-rolled annealed steel sheets were then analyzed, and the corresponding microstructural elements, mechanical properties and weldability properties were summarized in Tables 5, 6 and 7, respectively.

[0075] [Table 5 - Microstructure of cold rolled and annealed steel sheets] The phase percentages and manganese distribution gradients in the microstructures of the resulting cold-rolled and annealed steel sheets were measured.

[0076] [C] A and [Mn] Ateeth, Mass It corresponds to the amount of carbon and manganese in the austenite in percent, which are measured both by X-ray diffraction (C%) and by electron probe microanalyzer equipped with a field emission gun (Mn%).

[0077] The surface fraction of the phases in the microstructure is measured by the following method: a specimen is cut from cold-rolled annealed steel sheet, polished, and etched with reagents known per se to reveal the microstructure, and then examined using a scanning electron microscope, for example a scanning electron microscope equipped with a field emission gun ("FEG-SEM"), in secondary electron mode at a magnification of more than 5000x.

[0078] The measurement of the surface fraction of ferrite is carried out by SEM observation after Nital or Picral / Nital reagent etching.

[0079] The measurement of the volume fraction of retained austenite is carried out by X-ray diffraction.

[0080] The concentration of precipitated carbides is determined by thin plate specimens examined with a scanning electron microscope equipped with a field emission gun ("FEG-SEM") and image analysis at magnifications in excess of 15,000 times.

[0081] [Table 5]

[0082] The heterogeneity of the manganese distribution obtained after annealing of the hot-rolled steel sheet is preserved after cold-rolling and annealing of the steel sheet. This can be seen by comparing the slope of the manganese distribution obtained after annealing of the hot-rolled steel sheet (Table 3) with the slope of the manganese distribution obtained after annealing of the cold-rolled steel sheet (Table 5). These values ​​are significantly the same.

[0083] [Table 6 - Mechanical properties of cold-rolled annealed steel sheets] The mechanical properties of the resulting cold rolled and annealed steels were measured and are summarized in the table below.

[0084] Yield strength YS, tensile strength TS and uniform elongation TE are measured according to ISO standard ISO6892-1 published in October 2009. Hole expansion ratio HE is measured according to ISO standard 16630:2009.

[0085] [Table 6]

[0086] The examples show that the steel sheets according to the invention, i.e. Examples 1-4, 6-7, 9 and 13-14, are the only ones that exhibit all the target properties due to their specific composition and microstructure.

[0087] Tests 1 to 5 were carried out with steel composition A. In order to find the optimum temperature for limiting the formation of fresh martensite and unstable austenite during the final cooling step, T soak For tests 1 to 4, the selected annealing temperature T soak The stability of austenite is achieved by the amount of carbon and manganese in the austenite, which is greater than 1.10 [C] A × [Mn] A / ((0 . 1+C% 2 ) × In Test 5, the cold-rolled steel sheet was heated to a higher temperature of 720°C. soak Annealed at a temperature less than 1.10°C A × [Mn] A / ((0 . 1+C% 2 ) × This results in a large amount of austenite with less carbon, as can be seen by the (Mn%+2)). This unstable austenite results in a decrease in UE and TE compared to Tests 1-4.

[0088] Tests 6 to 8 are carried out with steel composition B. For tests 6 and 7, T is used to limit the formation of fresh martensite during the final cooling step. soak In Test 8, the cold-rolled steel sheet exhibited a higher T than in Tests 6 and 7. soak The steel is annealed at a high temperature, thus forming more austenite. During the final cooling step, due to the large amount of austenite formed during annealing, 30% fresh martensite is formed. This large amount of fresh martensite does not allow the steel to achieve the desired mechanical properties.

[0089] In Tests 10 and 11, the absence of heat treatment after hot rolling implies that the redistribution of manganese is not inhomogeneous, as can be seen by the slope of the manganese distribution, which is less than -30, even after annealing of the cold-rolled steel sheet. This distribution of manganese makes it impossible to achieve the mechanical properties.

[0090] In Test 12, the hot-rolled steel sheet had a too low T, which resulted in the formation of more than 0.5% precipitated carbides, as seen in Table 3. HBA These precipitated carbides do not dissolve after annealing of cold-rolled steel sheets, and 6 / mm 2 A carbide concentration of 25% is observed. The presence of carbides in cold-rolled steel sheets leads to the formation of 25% fresh martensite during the final cooling step. This large amount of fresh martensite does not allow the desired mechanical properties to be achieved.

[0091] Tests 13 to 15 are carried out with steel composition F. For tests 13 and 14, T is used to limit the formation of fresh martensite during the final cooling step. soak In Test 15, the cold-rolled steel sheet exhibited a higher T than in Tests 13 and 14. soakThe steel is annealed at a high temperature, thus forming more austenite. During the final cooling step, 5% fresh martensite is formed due to the large amount of austenite formed during annealing. This amount of fresh martensite does not allow the desired mechanical properties to be achieved.

[0092] [Table 7 - Weldability characteristics of cold-rolled annealed steel sheets] Spot welding under ISO standard 18278-2 conditions is performed on cold-rolled annealed steel sheet.

[0093] In the test used, the sample consists of two steel plates in the form of an identical cross weld. A force is applied to break the weld. This force is known as the cross tensile strength (CTS) and is expressed in daN. It depends on the diameter of the weld 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 and the thickness of the base material. This coefficient is expressed in daN / mm 2 It is expressed as:

[0094] The weldability of the resulting cold rolled and annealed steels was measured and is summarized in the table below.

[0095] [Table 7]

[0096] LME index=C%+Si% / 4( Mass %).

[0097] In Test 16, the weldability characteristics of the present invention cannot be obtained with a chemical composition in which the amount of carbon or silicon in the steel plate is high.

Claims

1. Cold rolled annealed steel sheet comprising, in mass 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 optionally, 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% and the remainder of the composition is steel, which is iron and unavoidable impurities resulting from smelting, The steel plate has a surface fraction of - 30% to 55% retained austenite, - 45% to 70% ferrite, - less than 5% fresh martensite However, the total of retained austenite, ferrite and fresh martensite is 100%. carbon in austenite [C], expressed in mass percent Ａ and manganese [Mn] Ａ The content is [C] Ａ ×[Mn] Ａ / ((0.1+C% ２ )×(Mn%+2))≧1.10 C% and Mn% are the nominal values ​​of carbon and manganese in mass percent; Fulfilling - and heterogeneous redistribution of manganese, characterized by a manganese distribution with a gradient of -30 or more. A cold-rolled annealed steel sheet having a microstructure comprising:

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

3. The cold-rolled annealed steel sheet according to claim 1 or 2, wherein the manganese content is 6.5% to 9.0%.

4. The cold-rolled annealed steel sheet according to any one of claims 1 to 3, wherein the aluminum content is 0.5% to 1.5%.

5. The microstructure is 1×10 ６ / mm ２ The cold-rolled annealed steel sheet according to any one of claims 1 to 4, comprising the following concentrations (number) of carbides:

6. The cold-rolled annealed steel sheet according to any one of claims 1 to 5, wherein the tensile strength is 1050 MPa or more, the yield strength is 780 MPa or more, the uniform elongation UE is 13% or more, and the total elongation TE is 15% or more.

7. The cold-rolled annealed steel sheet according to any one of claims 1 to 6, wherein the LME index (wherein the LME index = C% + Si% / 4, where C% and Si% represent the mass percentages of carbon and silicon in the steel, respectively) is less than 0.

36.

8. The cold-rolled annealed steel sheet according to any one of claims 1 to 7, wherein the hole expansion ratio HE is 15% or more.

9. The steel has a carbon equivalent Ceq of less than 0.4%, Ceq=C%+Si% / 55+Cr% / 20+Mn% / 19-Al% / 18+2.2P%-3.24B%-0.133×Mn%×Mo% where the elements are expressed in mass percent: The cold-rolled annealed steel sheet according to any one of claims 1 to 8, defined as:

10. The tensile strength TS expressed in MPa, the yield strength YS expressed in MPa, the uniform elongation UE expressed in % and the total elongation TE expressed in % are calculated according to the following formula: [(TS-800)×(YS-300)×UExTE] / [(0.1+C%)×Mn%]>3.3x10 ７ where C% and Mn% correspond to the carbon and manganese contents in the bulk in mass percent. The cold-rolled annealed steel sheet according to any one of claims 1 to 9, which satisfies the above.

11. A resistance spot weld of two steel parts of cold rolled and annealed steel sheet according to any one of claims 1 to 10, wherein the resistance spot weld has a strength of at least 30 daN / mm ２ (where α is [CTS] / [weld diameter x base material thickness]).

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