Cold-rolled annealed steel sheet and its manufacturing method

Abstract

The present invention provides a cold-rolled annealed steel sheet, which is made of a steel having a composition comprising, 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 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 being iron and unavoidable impurities resulting from smelting, and the steel sheet has a surface fraction of: - 25% to 54% retained austenite, - 46% to 75% ferrite, - less than 8% fresh martensite, - [C] A *√[Mn] A Carbon in austenite [C] expressed as weight percent is 0.48 to 1.8 A and manganese [Mn] A The present invention deals with cold-rolled and annealed steel sheets having a microstructure including heterogeneous redistribution of manganese, characterized by a manganese content and a manganese distribution with a gradient of -50 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 evaluated 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 medium and intermediate manganese (3.5-12% Mn) cold-rolled steel with reduced carbon content. Two process routes are described. The first involves a single intercritical annealing of the cold-rolled steel sheet. The second involves double annealing of the cold-rolled steel sheet, the first fully austenitic and the second intercritical. Depending on the annealing temperature, a good compromise between tensile strength and elongation can be achieved. By reducing the annealing temperature, austenite enrichment is obtained, which implies good strain-to-thickness values. However, the low amounts of carbon and manganese used in the present invention limit the tensile strength of the steel sheet to values ​​below 980 MPa. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2020 / 011638 Summary of the Invention [Means for solving the problem]

[0009] Therefore, an object of the present invention is 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 980 MPa or more, a uniform elongation UE of 15% or more, and a total elongation TE of 20.0% or more.

[0010] Preferably, the cold rolled annealed steel sheet has a total elongation TE and hole expandability HE that satisfy TExHE>670, where TE and HE are expressed as %.

[0011] Preferably, the cold-rolled and annealed steel sheet according to the present invention has a yield strength YS of 800 MPa or more.

[0012] Preferably, the cold rolled and annealed steel sheet according to the present invention has an LME index of less than 0.36.

[0013] Preferably, the cold rolled and annealed steel sheet has a hole expansion ratio HE of 25 or more.

[0014] 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% and the elements are defined as mass Expressed as a percentage.

[0015] Preferably, the resistance spot welds of two steel parts of cold rolled annealed steel sheet according to the invention have a strength of at least 30 daN / mm 2 has an α value of

[0016] Another object of the present invention is to provide a method for producing a Charpy impact energy of 0.4 J / mm at 20°C. 2The object of the present invention is to obtain a hot-rolled heat-treated steel sheet having a high toughness exceeding 1000 kJ / s.

[0017] The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet may also comprise any of the features of claims 2 to 11, either alone or in combination.

[0018] Another object of the invention is the resistance spot welding of two steel parts according to claim 12. [Brief explanation of the drawings]

[0019] [Figure 1] 1 shows test pieces of hot-rolled heat-treated steel sheets of Test 1 and Test 4. [Figure 2] Curves representing cumulative area fraction for Test 1 and Test 4 are plotted. DETAILED DESCRIPTION OF THE INVENTION

[0020] The invention will now be described in detail and illustrated by examples, without introducing any limitations.

[0021] 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 on the carbon content: the higher the carbon content, the lower the soaking temperature required to stabilize 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 0.05% to 0.10%.

[0022] 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 6.0% to 9%.

[0023] 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 result in reduced weldability and castability of the steel sheet. 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 0.7% to 2.2%.

[0024] 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%.

[0025] According to the present invention, the boron content is 0.0005% to 0.005% 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%.

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

[0027] The maximum silicon content 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 added is 0.5%.

[0028] 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.

[0029] 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%.

[0030] Optionally, chromium and vanadium may be added up to 0.5% and 0.2%, respectively, to improve strength.

[0031] 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.

[0032] 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: - 25% to 54% retained austenite, - 46% to 75% ferrite, - less than 8% fresh martensite, - Product [C] A × √[Mn] A is between 0.48 and 1.8, mass Carbon in austenite [C] expressed as a percentage A and manganese [Mn] A Content, and - Heterogeneous redistribution of manganese, characterized by a manganese distribution with a gradient of more than -50 Contains:

[0033] The microstructure of the steel sheet according to the present invention contains 25% to 54% retained austenite, preferably 30 to 50% austenite. If the austenite is below 25% or above 54%, the uniform and total elongation cannot reach the minimum values ​​of 15% and 20.0%, respectively.

[0034] Such austenite forms during intercritical annealing of hot-rolled steel sheets, but also during the first and second intercritical annealing of cold-rolled steel sheets. During intercritical annealing of hot-rolled steel sheets, regions containing manganese above and below the nominal value form, 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.

[0035] Due to the inhomogeneous redistribution of manganese in austenite after hot band annealing and the slow diffusion kinetics of manganese in austenite, the manganese inhomogeneity formed during hot band annealing still exists after the first and second intercritical annealing of cold rolled steel sheets, which can be evidenced by a gradient of manganese distribution in the microstructure that is greater than -50.

[0036] mass Carbon in austenite [C] expressed as a percentage A and manganese [Mn] A The content is the product [C] A × √[Mn] AThe ratio is such that the ratio is between 0.48 and 1.8. If the ratio is less than 0.48, the retained austenite is not stable enough to provide a continuous TRIP-TWIP effect during deformation. If the ratio is greater than 1.8, the retained austenite is too stable to generate sufficient TRIP-TWIP effect during deformation. Such TWIP-TRIP effect is described inter alia 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 B.C. D. Cooman).

[0037] The microstructure of the steel sheet according to the present invention contains 46-75% ferrite, preferably 50-70% ferrite, which is formed during the second intercritical annealing of the cold-rolled steel sheet.

[0038] Fresh martensite can be present in a surface fraction of up to 8%, but is not a desired phase in the microstructure of the steel sheet according to the invention. It can be formed during the final cooling step to room temperature by the transformation of unstable austenite. Indeed, this unstable austenite, with its 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 8%, preferably a maximum of 5%, or better still, a maximum of 3%, or even better reduced to 0.

[0039] The cold-rolled and annealed steel sheet according to the present invention has a tensile strength TS of 980 MPa or more, a uniform elongation UE of 15% or more, and a total elongation of 20.0% or more.

[0040] Preferably, the cold rolled annealed steel sheet is TE ×It has a total elongation TE and hole expandability HE that meet HE>670.

[0041] Preferably, the cold-rolled and annealed steel sheet according to the present invention has a yield strength YS of 800 MPa or more.

[0042] Preferably, the cold rolled and annealed steel sheet according to the present invention has an LME index of less than 0.36.

[0043] Preferably, the cold rolled and annealed steel sheet has a hole expansion ratio HE of 25 or more.

[0044] 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:

[0045] The welded assembly can be manufactured by producing two parts from a sheet of cold rolled annealed steel according to the invention and then performing resistance spot welding of the two steel parts.

[0046] The resistance spot welds joining the first sheet to the second sheet shall have a strength of at least 30 daN / mm 2 It is characterized by a high resistance in a cross tensile test, defined by an α value of

[0047] 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 1,150°C to 1,300°C to facilitate hot rolling, and the final hot rolling temperature FRT is 800°C to 980°C. Preferably, the FRT is 850°C to 950°C.

[0048] The hot-rolled steel is then cooled and coiled at a temperature Tcoil of 20°C to 600°C, preferably 300°C to 500°C.

[0049] The hot rolled steel sheet can then be cooled to room temperature and pickled.

[0050] The hot-rolled steel sheet is then annealed at an annealing temperature T between Ac1 and Ac3. HBA More precisely, it is annealed to T HBA is selected to promote heterogeneous redistribution of manganese. This manganese heterogeneity is measured by the gradient of the manganese distribution in the hot rolled steel sheet, which must be greater than or equal to -30. Preferably, the temperature T HBA is composed of Ac1+5°C to Ac3. Preferably, the temperature T HBA is 580℃ to 680℃.

[0051] 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 heterogeneous manganese distribution. HBA During this time, the temperature T HBA Furthermore, this heat treatment of the hot rolled steel sheet is maintained at 0.4 J / mm 2 This allows the hardness to be reduced while maintaining toughness above 100%.

[0052] The hot rolled heat treated steel sheet can then be cooled to room temperature and pickled to remove oxidation.

[0053] The hot-rolled heat-treated steel sheet is then cold-rolled at a reduction rate of 20% to 80%.

[0054] Next, the cold-rolled steel sheet is soaked at a temperature T1 of Ac3 to 950 ° C. soakHold time t1 from 10 seconds to 1000 seconds soak The steel is subjected to a first annealing for a period of time during which Ac3 is measured by dilatometric testing of the cold-rolled steel sheet. Such a first annealing allows the manganese inhomogeneity formed during the hot band annealing to be partially preserved. This is evidenced by the steel sheet showing a gradient of manganese distribution in the microstructure of at least -60. In a preferred embodiment, this temperature is selected to obtain an austenite grain size below 25 μm. Preferably, the annealing temperature T1 soak is 780 to 900°C, more preferably 780 to 870°C, and time t1 soak The first annealing time is 100 to 500 seconds. Such a first annealing can be carried out by continuous annealing.

[0055] The cold-rolled and annealed steel sheet is then cooled to a temperature below 80°C, preferably to room temperature.

[0056] Upon cooling, most of the austenite, which is less rich in manganese and carbon, transforms into fresh martensite, which contains manganese and carbon rich regions and manganese and carbon depleted regions.

[0057] Next, the cold-rolled steel sheet is subjected to a transformation temperature T2 soak and holding time t2 from 10 seconds to 1800 seconds soak The second annealing is performed for a period of time Tc, which corresponds to the temperature at which carbides completely dissolve, and can be measured by FEG-SEM observation after the heat treatment. Preferably, the second annealing is performed for a period of time Tc. soak is 650℃ to 700℃, and t2 soak The second annealing time is 100 to 500 seconds. Such a second annealing can be carried out by continuous annealing.

[0058] The temperature for the second anneal is selected based on the grade's composition so that the austenite formed is sufficiently stable and the formation of fresh martensite upon cooling is minimized. The higher the aluminum content, the higher the temperature. The higher the manganese content, the lower the temperature.

[0059] The cold rolled duplex annealed steel sheet is then cooled to below 80° C., preferably to room temperature. Upon cooling, some of the austenite, which is less rich in manganese and carbon, may transform into a limited amount of fresh martensite.

[0060] The steel sheet may then be coated by any suitable method, including hot dip coating, electrodeposition or vacuum coating of zinc or zinc-based alloys or aluminum or aluminum-based alloys.

[0061] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0062] Five grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates.

[0063] Table 1 - Composition The tested compositions are summarized in the table below, with elemental contents mass Expressed as a percentage.

[0064] [Table 1]

[0065] The Ac1 and Ac3 temperatures are determined by dilatometric testing and metallographic analysis.

[0066] Table 2 - Process parameters for hot-rolled heat-treated steel sheets The cast steel semi-finished products were reheated at 1200° C., hot rolled and then coiled at 450° C. The following specific conditions for obtaining hot rolled heat treated steel sheets were applied:

[0067] [Table 2]

[0068] The hot-rolled heat-treated steel sheets were analyzed, and the corresponding properties are summarized in Table 3.

[0069] Table 3 - Microstructure and properties of hot-rolled heat-treated steel sheets The gradient of manganese distribution and Charpy impact energy at 20°C, which indicate the toughness of the sheet, were measured.

[0070] Charpy impact energy is measured according to ISO Standard 148-1:2006(F) and ISO Standard 148-1:2017(F).

[0071] 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 profile.

[0072] Figure 1 shows the hot-rolled heat-treated steel specimens of Test 1 and Test 4. The black areas correspond to areas with low manganese content, and the gray areas correspond to areas with high manganese content.

[0073] 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.

[0074] 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. 2It 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.

[0075] This curve is plotted in Figure 2 for Test 1 and Test 4: 100% of the sheet specimens contain more than 1% manganese. In Test 1, 20% of the sheet specimens contain more than 10% manganese.

[0076] The slope of the resulting curve is then calculated between the point representing 80% of the cumulative area fraction and the point representing 20% ​​of the cumulative area fraction.

[0077] In Test 1, the slope was higher than -30, indicating that the redistribution of manganese was heterogeneous, with regions of low and high manganese content.

[0078] In contrast, in Test 4, the absence of heat treatment after hot rolling implies that the redistribution of manganese is not inhomogeneous, as evidenced by the slope of the manganese distribution being less than -30.

[0079] [Table 3]

[0080] Table 4 - Process parameters for cold rolled annealed steel sheets The resulting hot-rolled heat-treated steel sheet is then cold-rolled. The cold-rolled steel sheet is then cooled to a temperature T1 soak The first annealing is performed at t1 soak The steel plate is then heated to a temperature T2 before being cooled to room temperature. soak The second annealing is performed at t2 soak The following specific conditions were applied to obtain cold-rolled annealed steel sheets:

[0081] [Table 4]

[0082] Tests 2, 9, 11, 16 and 20 were subjected to a second anneal at too high a temperature.

[0083] Test 4 was subjected to neither hot band annealing nor cold rolling, but only a second anneal.

[0084] Test 5 was not subjected to hot band annealing or cold rolling.

[0085] Test 12 was subjected to a second anneal at a temperature below Tc.

[0086] 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.

[0087] Table 5 - Microstructure of cold rolled annealed steel sheets The phase percentages in the microstructure of the resulting cold-rolled and annealed steel sheets, as well as the gradient of manganese distribution after the first annealing and after the second annealing, were measured.

[0088] [C] A and [Mn] A teeth, 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%).

[0089] 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.

[0090] The measurement of the surface fraction of ferrite is carried out by SEM observation after Nital or Picral / Nital reagent etching.

[0091] The measurement of the volume fraction of retained austenite is carried out by X-ray diffraction.

[0092] 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.

[0093] [Table 5]

[0094] The heterogeneity of the manganese distribution obtained after annealing of the hot-rolled steel sheet is preserved after both annealings of the steel sheet, as 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 both annealings of the cold-rolled steel sheet (Table 5).

[0095] 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.

[0096] The yield strength YS, tensile strength TS, and total and uniform elongations TE and UE are measured according to the ISO standard ISO6892-1 published in October 2009. The hole expansion ratio test is carried out in accordance with the ISO16630 standard.

[0097] [Table 6]

[0098] Tests 2, 9, and 11 fall below the minimum target [C] because the carbon concentration in the austenite is too low due to the high temperature of the second anneal. A × √[Mn] A In addition, tests 9 and 11 show too high an amount of austenite.

[0099] Furthermore, Tests 2, 16 and 20 contain a large amount of fresh martensite due to the second annealing temperature being too high.

[0100] Test 12 exceeded the maximum target [C] because the second anneal was too low, resulting in high carbon in the austenite. A × √[Mn] A Shows.

[0101] Test 4 falls below the minimum target due to the lack of hot band annealing [C] A × √[Mn] A and homogeneous manganese redistribution.

[0102] Test 5 shows the manganese redistribution, which is homogeneous and contains less than the target amount of austenite when not properly stabilized due to the absence of a hot band anneal.

[0103] Table 7 - Weldability characteristics of cold rolled annealed steel sheets Spot welding according to ISO standard 18278-2 was carried out on cold-rolled annealed steel sheets.

[0104] 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:

[0105] The weldability of the resulting cold rolled and annealed steels was measured and is summarized in the table below.

[0106] [Table 7]

[0107] LME index = C% + Si% / 4 ( mass %).

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 - 25% to 54% retained austenite, - 46% to 75% ferrite, - less than 8% fresh martensite, However, the total of retained austenite, ferrite and fresh martensite is 100%. - [C] Ａ ×√[Mn] Ａ Carbon in austenite [C] expressed in mass percent is 0.48 to 1.36 Ａ and manganese [Mn] Ａ content, - and heterogeneous redistribution of manganese, characterized by a manganese distribution with a gradient of 27 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 0.7% to 2.2%.

5. The microstructure is 0.8 × 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, having a tensile strength of 980 MPa or more, a uniform elongation UE of 15% or more, and a total elongation TE of 20.0% or more.

7. The cold-rolled annealed steel sheet according to any one of claims 1 to 6, having a yield strength of 800 MPa or more.

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

36.

9. The cold-rolled annealed steel sheet according to any one of claims 1 to 8, wherein the hole expansion ratio HE is 25% or more.

10. The total elongation TE in % and the hole expansion ratio HE in % are calculated using the following formula: TE x HE>670 The cold-rolled annealed steel sheet according to any one of claims 1 to 9, which satisfies the above.

11. 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% The cold rolled annealed steel sheet according to any one of claims 1 to 10, wherein the elements are defined as follows:

12. A resistance spot weld of two steel parts of cold rolled and annealed steel sheet according to any one of claims 1 to 11, wherein the resistance spot weld has a strength of at least 30 daN / mm ２ (where the α value is the ratio of the cross tensile strength value to the product of the diameter of the weld and the thickness of the base material).

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