1180MPa-grade ultrahigh-strength steel plate for automobile and preparation method thereof

Abstract

The invention discloses a 1180MPa-grade ultrahigh-strength steel plate for an automobile and a preparation method of the 1180MPa-grade ultrahigh-strength steel plate, and belongs to the technical field of cold-rolled strip production. The steel plate comprises the following chemical components in percentage by mass: 0.17%-0.20% of C, 1.2%-1.6% of Si, 1.70%-2.10% of Mn, less than or equal to 0.008% of P, less than or equal to 0.005% of S, 0.30%-0.50% of Al, 0.15%-0.30% of Cr, 0.08%-0.12% of V, less than or equal to 0.0045% of N and the balance of Fe and impurities, wherein Ceq = C + Si / 30 + Mn / 20 + 2P + 4S < = 0.41. The preparation method comprises the steps of smelting, continuous casting, hot rolling, pickling cold rolling and continuous annealing. Through alloy component design and process collaborative optimization, the prepared ultrahigh-strength steel plate has high strength, high plasticity and high reaming performance, and the problem that existing bainite complex-phase steel is poor in comprehensive performance can be effectively solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of cold-rolled plate strip production, and relates to a 1180MPa-grade ultra-high-strength steel plate for automobiles and a preparation method thereof, in particular to a method for preparing a 1180MPa-grade high-plasticity high-hole-expanding-performance ultra-high-strength steel plate based on continuous annealing dynamic regulation technology. BACKGROUND

[0002] Under the pressure of environment and energy, the demand for energy saving and emission reduction and the development of new energy vehicles put forward higher requirements for automobile lightweight. New energy vehicles, due to the relatively large weight of the battery, adopt higher strength materials and advanced structure design, realize the high integration of parts through integrated body structure and modularized assembly design, and reduce the number of parts, thereby reducing the weight while ensuring the safety performance of the automobile. This puts forward higher performance requirements for the ultra-high-strength steel (UHSS) for automobiles: ① complex part forming: high elongation; ② high local forming performance: high hole expanding performance; ③ ensure the connection strength of the part under high load: high strength, balanced elongation and hole expanding. Bainite steel with bainite and residual austenite as the matrix structure has attracted much attention. It avoids the problem of hardness difference between soft and hard phases in dual-phase steel, and can improve the ductility and local forming performance of the steel.

[0003] For ultra-high-strength carbon-free bainite steel, the size and proportion of carbon-free bainite determine the strength and hole expanding performance of the carbon-free bainite steel, and the morphology, content and carbon content of residual austenite also have important influence on the plasticity and hole expanding performance. The content of fresh martensite formed in the final cooling process will deteriorate the plasticity and hole expanding performance. For carbon-free bainite steel, there is a contradictory relationship between elongation and hole expanding performance, therefore, the proportion of the structure needs to be finely regulated to obtain balanced strength-plasticity-hole expanding performance. By designing the unique structure of carbon-free bainite steel, excellent strength and plasticity can be ensured while excellent local forming performance (such as flanging and hole expanding) is achieved. Therefore, the carbon-free bainite steel with TRIP effect can meet the demand of the automobile industry for new steel plates with high strength, high plasticity and high local forming performance.

[0004] Through relevant patent query, the patents similar to the 1180MPa-grade super bainite steel plate are as follows: CN110724877A discloses a 1180MPa grade high plasticity bainite complex phase steel plate for automobile and a preparation method thereof. In the method, the content ranges of C, Si and Mn in the chemical composition of the steel plate are very wide. When the content of C is at the lower limit of its limited range, the strength of 1180MPa cannot be obtained. When the content of Si is at the lower limit of its limited range, the effect of inhibiting the precipitation of carbide cannot be achieved, and the precipitation of carbide will significantly reduce the stability of residual austenite, so that good plasticity index cannot be obtained. In the method, the hot rolling process only provides heating temperature, finish rolling temperature and coiling temperature, and the cooling path and intermediate temperature control are not specified, which is not conducive to the stable control of the product organization performance and surface quality. The yield strength of the obtained product is low, and the application scene is limited.

[0005] CN109778062A discloses a 1200MPa grade cold-rolled complex phase steel and a preparation method thereof. The method uses Nb-Ti micro-alloying to prepare a 1200MPa grade cold-rolled complex phase steel, and the elongation is about 7%. However, the hole flanging performance is not tested, which is not conducive to the forming of complex parts, and the application scene is severely limited.

[0006] CN113528946A discloses a 1200MPa grade reinforced forming complex phase steel and a preparation method thereof. In the method, the contents of C, Si and Mn are relatively high, which are 0.21-0.24%, 1.0-1.4% and 2.1-2.6% respectively, resulting in strong crack sensitivity of the casting blank, and the casting blank cannot be cold charged and heated. At the same time, Cu element is added in the composition, the alloy cost is high, and the steel plate needs to be softened by cover annealing after hot rolling, which significantly increases the process cost. SUMMARY

[0007] The technical problem to be solved by the present application is the poor comprehensive performance of the existing 1180MPa grade bainite complex phase steel.

[0008] The purpose of the present application is to provide a method for preparing a 1180MPa grade steel plate for automobile based on continuous annealing dynamic regulation technology. Through precise control of the organization composition, a bainite complex phase steel with a strength level greater than 1180MPa and high plasticity and hole flanging performance is obtained. The production process is stable, which is conducive to batch production of the product, thereby improving the competitiveness of the product under the same strength level condition and widening the application range of the product in the field of automobile steel.

[0009] The technical solution adopted by the present application to solve the technical problem is as follows: In a first aspect, the present application provides a preparation method of a 1180MPa grade ultra-high strength steel plate for automobile, comprising the following steps: S1. Melting the molten steel according to the chemical composition of the 1180MPa grade super bainite steel for automobile, and continuously casting the slab; The chemical composition is as follows in percentage by mass: C 0.17%-0.20%, Si 1.2%-1.6%, Mn 1.70%-2.10%, P≤0.008%, S≤0.005%, Al 0.30%-0.50%, Cr 0.15%-0.30%, V 0.08%-0.12%, N≤0.0045%, the balance being Fe and inevitable impurities, and satisfying Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41; S2. The slab is heated, and then subjected to phosphorus removal, rough rolling, finish rolling, laminar cooling and coiling to obtain a hot-rolled coil; S3. The hot-rolled coil is subjected to pickling and cold rolling to become a cold-rolled thin strip coil; S4. After the cold-rolled thin strip coil is subjected to continuous annealing, a 1180 MPa grade ultra-high strength steel plate for automobiles is prepared; The continuous annealing is as follows: the cold-rolled thin strip coil is heated to 860-900℃ for soaking and holding for 260-340s, then slowly cooled to 760-780℃ at a rate of 1-5℃ / s, and then rapidly cooled to 330-360℃ at a rate of 16-20℃ / s, and overaging treatment is performed; The overaging treatment is implemented in three stages: the holding time of the first and second stages of overaging is 390-412s, and the overaging temperature is dynamically adjusted based on the soaking temperature, when the soaking temperature is at the lower limit of 860℃, the overaging temperature is controlled to be 330℃, and when the soaking temperature is increased by 10℃, the corresponding overaging temperature is increased by 10℃, and when the soaking temperature is at the upper limit of 900℃, the overaging temperature is controlled to be 370℃; the third stage is subjected to cooling treatment to cool to 240-280℃ at a rate of 3-5℃ / s, and the holding time is 345-370s.

[0010] In the step S1, the temperature of the casting is 1520-1555℃.

[0011] In the step S1, the thickness of the slab is 260-240mm.

[0012] In the step S2, the temperature of the slab heating is 1240-1280℃, and the total soaking time is 280-440min.

[0013] In the step S2, the whole length of the rolling line is fully scaled, and the scaling water pressure is≥22Mpa, and the scaling temperature is≥1173℃.

[0014] In the step S2, soft blowing water is used for pre-cooling the iron sheet during rough rolling.

[0015] In the step S2, the roughing temperature is 1040-1130℃, the finish rolling temperature is 860-920℃, the intermediate temperature is 720-760℃, the finish rolling pass is 7 passes, the last pass reduction is ≤15% and the lubrication rolling is adopted.

[0016] In the step S2, the laminar cooling is adopted by the front concentrated cooling mode, and the upper and lower headers are 45% and 60% respectively.

[0017] In the step S2, the U-shaped coiling mode is adopted in the coiling, the strip head and tail are adjusted by the hot coiling box before entering the finish rolling mill, and the coiling temperature of the head 30m and the tail 70m is controlled to be 680-710℃, and the coiling temperature of the middle section is 640-680℃.

[0018] In the step S2, the thickness of the hot-rolled coil is 2.3-4.0mm.

[0019] In the step S3, the hot-rolled strip is subjected to pickling-5-rack cold rolling, and the cold rolling reduction is 40-70%.

[0020] Further, the cold rolling reduction is 45-65%.

[0021] In the step S3, the thickness of the cold-rolled thin strip is 1.0-2.0mm.

[0022] In the step S4, the continuous annealing is first heated to 300℃ at 5-10℃ / s, and then heated to 860-900℃ at 0.2-2.0℃ / s.

[0023] In the step S4, after the continuous annealing is completed, the strip shape is adjusted by the skin pass mill, the unit speed is controlled to be 50-60m / min, and the skin pass elongation is 0.3-0.5%; when the thickness of the cold-rolled thin strip is at the lower limit of 1.0mm, the skin pass elongation is controlled to be 0.5%, and the skin pass elongation is reduced by 0.05% for each increase of 0.1mm of the thickness of the cold-rolled thin strip.

[0024] In the second aspect, the application provides a 1180MPa-grade ultra-high strength steel plate for automobiles prepared by the preparation method, and the chemical components are as follows in terms of mass percentage: C 0.17%-0.20%, Si 1.2%-1.6%, Mn 1.70%-2.10%, P≤0.008%, S≤0.005%, Al 0.30%-0.50%, Cr 0.15%-0.30%, V 0.08%-0.12%, N≤0.0045%, the balance is Fe and inevitable impurities, and Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 is met.

[0025] Further, the microstructure of the 1180 MPa grade ultra-high strength steel sheet for automobile is composed of 70% to 80% lath bainite, 8% to 15% residual austenite, 5% to 10% tempered martensite and a small amount of martensite.

[0026] Further, the yield strength of the 1180 MPa grade ultra-high strength steel sheet for automobile is 920 to 1020 MPa, the tensile strength is 1180 to 1240 MPa, the elongation A 50 is 14% to 18%, and the hole expansion ratio is 30% to 50%.

[0027] The 1180 MPa grade ultra-high strength steel sheet for automobile provided by the application has the beneficial effects that: the chemical composition of the 1180 MPa grade ultra-high strength steel sheet for automobile mainly contains C, Si and Mn as main elements, the content of Si is controlled to be less than 1.6%, and the content of Mn is controlled to be less than 2.10%, thereby further reducing the surface defects and element segregation problems of the ultra-high strength steel sheet in the production process, and simultaneously playing the effects of inhibiting cementite precipitation and stabilizing austenite, compared with other automobile ultra-high strength steel products with a strength grade of ≥1180 MPa, the design of alloy composition and the synergistic optimization of process form a cold-rolled steel sheet with a higher strength grade and taking into account plasticity and formability.

[0028] The application further provides a preparation method of the 1180 MPa grade ultra-high strength steel sheet for automobile, and the continuous annealing dynamic regulation technology is used to ensure the accurate control of the 1180 MPa ultra-high strength steel structure. Compared with the prior art, the overaging stage in the continuous annealing does not need to use the quenching and partitioning technical means, and a wide range of traditional acid rolling-continuous annealing production lines are met, and the application has the advantages of low production cost, stable process and the like.

[0029] The microstructure of the 1180 MPa grade ultra-high strength steel sheet for automobile prepared according to the chemical composition and the preparation method is composed of 70% to 80% lath bainite, 8% to 15% residual austenite, 5% to 10% tempered martensite and a small amount of martensite. The mixed structure of a high proportion of bainite and tempered martensite can improve the strength grade of the steel sheet to 1180 MPa and take into account high yield, high elongation and high forming characteristics, the residual austenite improves the ductility of the steel sheet, so that the prepared 1180 MPa grade ultra-high bainite steel sheet meets the requirements of high strength and ductility and hole expansion and flanging performance, and can be used for automobile structural parts, such as auxiliary frames, B columns, front longitudinal beams and the like. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The metallographic photo of the 1180 MPa grade ultra-high strength steel sheet for automobile prepared in Example 1 is shown in the figure; Figure 2 The scanning electron microscope photo of the 1180 MPa grade ultra-high strength steel sheet for automobile prepared in Example 1 is shown in the figure. DETAILED DESCRIPTION

[0031] In order to make the technical problems, technical solutions and beneficial effects to be solved in the present application more clearly, the present application is further described in detail below in combination with embodiments. Unless otherwise defined, all technical terms used herein have the same meanings as understood by those skilled in the art.

[0032] The application discloses a 1180MPa-grade ultra-high-strength steel plate for automobiles, and a preparation method thereof. The preparation method comprises a smelting process, a continuous casting process, a hot rolling process, an acid pickling and cold rolling process and a continuous annealing process. The chemical components of the steel plate are as follows in percentage by mass: C 0.17%-0.20%, Si 1.2%-1.6%, Mn 1.70%-2.10%, P≤0.008%, S≤0.005%, Al 0.30%-0.50%, Cr 0.15%-0.30%, V 0.08%-0.12%, N≤0.0045%, and the balance of Fe and inevitable impurities, and Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41 is satisfied. The microstructure of the steel plate is composed of 70%-80% lath bainite, 8%-15% residual austenite, 5%-10% tempered martensite and a small amount of martensite. The yield strength of the steel plate is 920-1020MPa, the tensile strength is 1180-1240MPa, the elongation A 50 is 14%-18%, and the hole expansion ratio is 30%-50%.

[0033] In an embodiment of the application, in the hot rolling process, the slab is heated by hot charging or cold charging.

[0034] In an embodiment of the application, in the hot rolling process, the rough rolling start temperature is 1040-1130℃, the finish rolling temperature is 860-920℃, the deformation amount in the unrecrystallization zone is controlled to refine the austenite grains; the number of finish rolling passes is 7, the reduction rate of the last pass is≥12% and lubrication rolling is adopted to refine the surface layer austenite grains and improve the surface quality and the plate shape. As a non-limiting example, the rough rolling start temperature can be 1040℃, 1070℃, 1100℃, 1130℃ or within a range formed by any two of the above values; and the finish rolling temperature can be 860℃, 880℃, 900℃, 920℃ or within a range formed by any two of the above values.

[0035] In an embodiment of the present application, in the hot rolling process, the coiling adopts a U-shaped control mode, the coiling temperature of the front 30 m of the strip head and the rear 70 m of the strip tail is controlled to be 680-710 ℃, and the coiling temperature of the middle section is controlled to be 640-680 ℃, so as to ensure the stability of the longitudinal microstructure and performance of the strip steel, and to obtain ferrite + pearlite microstructure in the hot rolled state, thereby improving the cold rolling workability. As a non-limiting example, the coiling temperature of the front 30 m of the strip head and the rear 70 m of the strip tail can be 680 ℃, 690 ℃, 700 ℃, 710 ℃, or within a range consisting of any two of the above values; the coiling temperature of the middle section can be 640 ℃, 650 ℃, 660 ℃, 670 ℃, 680 ℃, or within a range consisting of any two of the above values.

[0036] In an embodiment of the present application, in the pickling and cold rolling process, the cold rolling reduction is controlled to be 40-70%, so as to provide energy storage for recrystallization and phase change. As a non-limiting example, the cold rolling reduction can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, or within a range consisting of any two of the above values. Preferably, the cold rolling reduction is 45-65%.

[0037] In an embodiment of the present application, in the continuous annealing process, heating to 300 ℃ and 860-900 ℃ at speeds of 5-10 ℃ / s and 0.2-2.0 ℃ / s respectively, and soaking for 260-340 s, so that the steel plate is heated at a complete austenitizing temperature, to ensure complete austenitization and eliminate undissolved carbides, and to obtain uniform austenite microstructure. Too low a temperature will result in incomplete austenitization, residual ferrite or carbides, and significantly reduced strength and bainite content; too high a temperature will cause austenite grain coarsening, and reduce toughness and formability. As a non-limiting example, the soaking temperature can be 860 ℃, 870 ℃, 880 ℃, 890 ℃, 900 ℃, or within a range consisting of any two of the above values.

[0038] In an embodiment of the present application, in the continuous annealing process, cooling to a slow cooling end temperature of 760-780 ℃ at a cooling speed of 1-5 ℃ / s, to avoid proeutectoid ferrite formation and ensure that the microstructure is dominated by bainite; too high a temperature or too slow a cooling speed will promote ferrite precipitation, reducing the strength and bainite proportion; too fast a cooling speed may cause local martensite to form prematurely. As a non-limiting example, the slow cooling speed can be 1 ℃ / s, 2 ℃ / s, 3 ℃ / s, 4 ℃ / s, 5 ℃ / s, or within a range consisting of any two of the above values; the slow cooling end temperature can be 760 ℃, 770 ℃, 780 ℃, or within a range consisting of any two of the above values.

[0039] In an embodiment of the present application, in the continuous annealing process, the cooling rate is 16-20 ℃ / s to the fast cooling end point temperature of 330-360 ℃, the bainite transformation range is precisely controlled, and a lath bainite dominant structure is achieved. This cooling rate can inhibit the formation of pearlite and promote the medium-temperature bainite phase transformation. If the temperature is too high or the cooling rate is insufficient, coarse upper bainite or pearlite will be generated, reducing the strength and toughness. If the temperature is too low or the cooling rate is too fast, excessive martensite will be easily formed, reducing the target bainite content and the plasticity. As a non-limiting example, the fast cooling rate can be 16 ℃ / s, 17 ℃ / s, 18 ℃ / s, 19 ℃ / s, 20 ℃ / s, or within a range formed by any two of the above values. The fast cooling end point temperature can be 330 ℃, 340 ℃, 350 ℃, 360 ℃, or within a range formed by any two of the above values.

[0040] In an embodiment of the present application, in the continuous annealing process, the overaging temperature is dynamically adjusted according to the soaking temperature. When the soaking temperature is at the lower limit (860 ℃), the overaging temperature is 330 ℃. For every 10 ℃ increase in the soaking temperature, the corresponding overaging temperature increases by 10 ℃. When the soaking temperature is at the upper limit of control (900 ℃), the overaging temperature is controlled at 370 ℃. The overaging temperature is controlled in three stages. In the first two stages, the overaging temperature is controlled according to the corresponding overaging temperature of the soaking temperature. Each stage is held for 390-412 s. In the third stage, the temperature is lowered at a cooling rate of 3-5 ℃ / s and controlled at 240-280 ℃. The holding time is 345-370 s. Dynamic overaging can control the phase ratio of carbon-free bainite + tempered martensite and residual austenite in the final structure, promote the diffusion of carbon from the supersaturated bainite / martensite to the residual austenite, stably form 8-15% of austenite and 5-10% of tempered martensite, and avoid the precipitation of η-carbide, to ensure that the strength and plasticity and forming performance are balanced. If dynamic control or segmented control is not performed, insufficient residual austenite or excessive carbide precipitation will occur, reducing the elongation, hole expansion rate, and strength and plasticity.

[0041] The following specific examples will be used to explain the scheme of the present application. Those skilled in the art will understand that the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. If the specific technology or conditions are not specified in the examples, the technology or conditions described in the literature in the art or according to the product instructions are used. If the reagents or instruments are not specified by the manufacturer, they are all conventional products that can be obtained by purchase.

[0042] Examples: The present application provides three groups of 1180 MPa grade ultra-high strength steel plates for automobiles prepared by the preparation method of the present application, the chemical composition of which is shown in Table 1, and the balance is Fe and unavoidable impurities.

[0043] Table 1 Chemical composition (wt. %) The preparation method of the 1180MPa grade ultra-high strength steel plate for automobile is as follows.

[0044] (a) Smelting process: the molten steel is smelted according to the chemical composition of the ultra-high strength steel plate shown in Table 1.

[0045] (b) Continuous casting process: the casting temperature is 1520-1550℃, and the slab thickness obtained by continuous casting is 230-240mm.

[0046] (c) Hot rolling process: the slab obtained by continuous casting is sequentially subjected to heating, dephosphorization, rough rolling, coiling, finish rolling and laminar cooling to obtain a hot rolling coil; the total heating time of the slab in the furnace is 280-440min; the rough rolling is put into soft blowing water to pre-cool the iron sheet; the whole length and the number of the descaling are full, the descaling water pressure is ≥22MPa, and the descaling temperature is ≥1173℃; the finish rolling is 7 passes, the last pass reduction is ≥12% and lubrication rolling is adopted; the coiling adopts U-shaped coiling mode, and the strip head and tail are adjusted by using a hot coiling box before entering the finish rolling unit; the laminar cooling adopts a front concentrated cooling mode, and the upper and lower headers are 45% and 60% respectively; the main hot rolling process parameters are shown in Table 2.

[0047] Table 2 Hot rolling process parameters (d) Pickling and cold rolling process: after the hot rolling coil is subjected to pickling to remove the surface iron oxide scale, it is cold-rolled into a cold-rolled thin strip steel by 5 racks, wherein the thicknesses of the strip steels of examples 1-3 are 1.6mm, 1.2mm and 1.0mm respectively, and the cold rolling reductions thereof are 46.7%, 53.8% and 58.3% respectively.

[0048] (e) Continuous annealing process: the cold-rolled thin strip steel is treated by a continuous annealing process to produce the required product; first, the strip steel is heated to 300℃ and the soaking temperature (complete austenitization temperature) at a speed of 5-10℃ / s and 0.2-2.0℃ / s respectively, and the soaking time is kept for a period of time to completely austenitize; then, it is cooled to the slow cooling endpoint temperature at a speed of 1-5℃ / s, and then to the fast cooling endpoint temperature at a speed of 16-20℃ / s; the overaging temperature is controlled in three stages, the first two stages are controlled according to the overaging temperature corresponding to the soaking temperature, each stage is kept for 390-412s, the third stage is cooled at a speed of 3-5℃ / s to the endpoint temperature, and kept for 345-370s, the austenite to lath bainite transformation occurs in the overaging stage; finally, it is cooled to room temperature, and the strip shape is adjusted by a skin pass mill; the specific continuous annealing process parameters are shown in Table 3.

[0049] Table 3 Continuous annealing process parameters The microstructure of the 1180MPa grade ultra-high strength steel plate prepared by the above process is as shown in Figure 1 The scanning electron microscope photos are as shown in Figure 2 The performance of the above 1180MPa grade ultra-high strength steel plate for automobiles is tested according to GB / T228-2010 "Metallic Materials Tensile Test Method at Room Temperature" and GB / T24524-2021 "Metallic Materials Thin Plate and Thin Strip Hole Expansion Test Method", and the mechanical properties are as shown in Table 4.

[0050] Table 4 Mechanical properties

Claims

1. A method for preparing 1180MPa grade ultra-high strength steel plates for automobiles, characterized in that: The steps include: S1. According to the chemical composition of 1180MPa grade automotive super bainite steel, the molten steel is smelted and continuously cast into slabs; The chemical composition is as follows by mass percentage: C 0.17%-0.20%, Si 1.2%-1.6%, Mn 1.70%-2.10%, P≤0.008%, S≤0.005%, Al 0.30%-0.50%, Cr 0.15%-0.30%, V 0.08%-0.12%, N≤0.0045%, the balance being Fe and unavoidable impurities, and satisfies Ceq=C+Si / 30+Mn / 20+2P+4S≤0.41; S2. The slab is heated, then dephosphorized, rough rolled, finished rolled, laminar cooled and coiled to obtain a hot rolled coil; S3. The hot-rolled coil is pickled and cold-rolled to form a cold-rolled thin strip steel coil; S4. After continuous annealing, the cold-rolled thin strip steel coil is produced into 1180 MPa grade ultra-high-strength steel plate for automobiles; The continuous annealing is as follows: the cold-rolled thin strip steel coil is heated to 860-900°C and soaked for 260-340 seconds, then slowly cooled to 760-780°C at a rate of 1-5°C / s, and then rapidly cooled to 330-360°C at a rate of 16-20°C / s for over-aging treatment; The overaging treatment is carried out in three stages: the holding time of the first and second overaging stages is 390-412 seconds, and the overaging temperature is dynamically adjusted based on the ambulating temperature. When the ambulating temperature is at the lower limit of 860°C, the overaging temperature is controlled to 330°C. For every 10°C increase in the ambulating temperature, the corresponding overaging temperature is increased by 10°C. When the ambulating temperature is at the upper limit of 900°C, the overaging temperature is controlled to 370°C. In each stage; the third stage is a cooling treatment, cooling to 240-280°C at a rate of 3-5°C / s, and the holding time is 345-370 seconds.

2. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S1, the casting temperature is 1520-1555°C.

3. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S2, at least one of the following conditions is met: Slab heating: heating temperature is 1240~1280℃, total furnace time is 280~440min; Descaling: Descaling of the entire rolling mill, descaling water pressure ≥ 22Mpa, descaling temperature ≥ 1173℃; Rough rolling: soft water is blown into the rough rolling to pre-cool the iron sheet; Finishing rolling: the starting rolling temperature is 1040~1130℃, the final rolling temperature is 860~920℃, the intermediate temperature is 720~760℃, the finishing rolling passes are 7 passes, the final pass reduction rate is ≤15% and lubrication rolling is adopted; Laminar cooling: adopts front-end centralized cooling method, with upper and lower headers at 45% and 60% respectively; Coiling: U-type coiling mode is adopted. The head and tail of the strip are adjusted by using a hot coil box before the strip enters the finishing mill. The coiling temperature of the strip 30m before the head and 70m after the tail is controlled at 680~710℃, and the coiling temperature of the middle section is 640~680℃.

4. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S3, the hot-rolled strip is cold-rolled in a pickling-5 stand with a cold-rolling reduction ratio of 40-70%.

5. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 4, characterized in that: The cold rolling reduction rate is 45-65%.

6. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S1, the slab thickness is 260-240 mm; In step S2, the thickness of the hot-rolled coil is 2.3-4.0 mm; In step S3, the thickness of the cold-rolled thin strip steel is 1.0-2.0 mm.

7. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S4, during continuous annealing, the material is first heated to 300° C. at a rate of 5-10° C. / s, and then heated to 860-900° C. at a rate of 0.2-2.0° C. / s for soaking.

8. The method for preparing an 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 1, wherein: In step S4, after continuous annealing is completed, the strip is cooled to room temperature, the strip shape is adjusted through the skin pass mill, the unit speed is controlled to 50-60 m / min, and the flattening elongation is 0.3-0.5%; when the thickness of the cold-rolled thin strip is at the lower limit of 1.0 mm, the flattening elongation is controlled to be 0.5%, and for every 0.1 mm increase in the thickness of the cold-rolled thin strip, the corresponding flattening elongation is reduced by 0.05% until it is reduced to the lower limit of the flattening elongation.

9. The 1180 MPa grade ultra-high strength steel plate for automobiles produced by the method according to any one of claims 1 to 8, characterized in that: The chemical composition of the steel plate is as follows, by mass percentage: C 0.17%-0.20%, Si 1.2%-1.6%, Mn 1.70%-2.10%, P≤0.008%, S≤0.005%, Al 0.30%-0.50%, Cr 0.15%-0.30%, V 0.08%-0.12%, N≤0.0045%, the balance being Fe and unavoidable impurities, and satisfies Ceq=C+Si / 30+Mn / 20+2P+4S≤0.

41.

10. The 1180 MPa grade ultra-high strength steel plate for automobiles according to claim 9, characterized in that: The microstructure of the steel plate is composed of 70% to 80% lath bainite, 8% to 15% retained austenite, 5% to 10% tempered martensite and trace martensite; The yield strength of the steel plate is 920~1020MPa, the tensile strength is 1180~1240MPa, and the elongation A 50 The value is 14%~18%, and the hole expansion rate is 30%~50%.

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