Gigaba-grade cold-rolled automobile steel plate with multiple functions and preparation method thereof

Abstract

The invention discloses a one-steel multi-energy gigaba-grade cold-rolled automobile steel plate and a preparation method thereof. The one-steel multi-energy gigaba-grade cold-rolled automobile steel plate comprises 0.145%-0.165% of C, 0.32%-0.52% of Si, 1.68%-1.78% of Mn, 0.46%-0.56% of Cr, 0.012%-0.026% of Ti, 0.030%-0.045% of Al, smaller than or equal to 0.018% of P, smaller than or equal to 0.004% of S and the balance Fe and other inevitable impurities. According to the method, through reasonable, economical and low-cost component design and effective control of key process parameters of each process section, one component system and hot rolling and cold rolling design are adopted, through different continuous annealing processes, development of 980 MPa-grade cold-rolled dual-phase steel DP980, 980 MPa-grade cold-rolled quenching partition steel QP980 and 1180 MPa-grade cold-rolled martensitic steel MS1180 is achieved, and the design and development requirements of the Gigabar steel are met.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more specifically, to a multi-functional GPa-grade cold-rolled automobile steel plate and a preparation method thereof. Background Art

[0002] With the continuous development of the automobile industry, under the environmental trend of low carbon and low energy consumption, high safety, low cost and lightweight have become the most concerned direction of major automobile manufacturers. Relevant research shows that for every 10% reduction in the weight of the whole vehicle, the fuel efficiency can be improved by 6% to 8%; for every 100kg reduction in the curb weight of the vehicle, the fuel consumption per 100 kilometers can be reduced by 0.3 to 0.6L; for every 100kg reduction in the vehicle weight, the carbon dioxide emissions can be reduced by about 5g / km, and the emissions of pollutants such as nitrides and sulfides will also be reduced accordingly. In addition, a 10% reduction in curb weight can reduce the braking distance by 5% and the steering force by 6%, thereby significantly improving the active safety of the vehicle. Therefore, the use of lightweight materials has become a consensus among major automobile companies. High-strength steel for automobiles can not only improve the strength of body parts, but also has the advantage of being recyclable. Compared with other materials such as carbon fiber, its comprehensive cost is significantly lower.

[0003] As a typical representative of high-strength steel for automobiles, Jipa steel is mainly used in structural parts and safety parts of automobile bodies. In recent years, the output of Jipa steel has gradually increased and has been widely promoted and applied. At present, Jipa steel includes dual-phase steel DP, complex-phase steel CP, quenching and partitioning steel QP, martensitic steel MS, phase transformation induced plasticity steel TRIP and its derivatives. Its tensile strength exceeds 980MPa, and the strength level is still increasing, which provides strong support for the low-carbon development of the automobile industry, and the market demand is also increasing year by year. Most of Jipa steel products contain martensite, among which dual-phase steel has good forming properties and is mainly composed of martensite and ferrite. With the increase of strength, the proportion of martensite gradually increases; quenching and partitioning steel significantly improves the plasticity of steel by introducing residual austenite, and its structure is martensite + residual austenite, or martensite + residual austenite + a small amount of ferrite; while martensitic steel is mainly composed of martensite structure, and relatively simple parts are usually processed by roll forming.

[0004] In recent years, some people have proposed that in the process of steel production, the same chemical composition of the billet is used, and it undergoes a forming process with different deformation temperatures, different deformation degrees, and different deformation paths, and then different heat treatment temperatures, holding times, and cooling methods are used to obtain a variety of different material properties, so that it can be applied to different load conditions and achieve "one steel with multiple functions". The concept of one steel with multiple functions has moved the differentiation point of steel products to the steelmaking process, simplified the smelting operation process, and provided new ideas for steel grade development. Considering that the GIP steel for automobiles mainly obtains the required structure and properties through phase transformation strengthening, especially martensite as the main hard phase, it can significantly improve the strength of steel. Both the dual-phase steel and quenching and partitioning steel in GIP steel contain a large amount of martensite, while martensitic steel has the organizational characteristics of full martensite. If we can reasonably apply the design concept of "one steel with multiple functions" and combine it with the flexible control technology of organizational properties, and use the continuous annealing process to adjust the GIP steel to meet different organizational and performance requirements while ensuring the required content of martensite and without increasing the investment in production line equipment, it will be convenient for automobile users to meet the needs of processing and forming different parts, and can also greatly reduce the production costs of steel companies.

[0005] In the existing patents of high-strength steel for automobiles with multiple uses or multiple grades of steel, only the production and preparation methods of a single type of steel are described and explained, and most of them use the addition of a large amount of alloy elements such as Mn, Cr, Mo, and Nb, which results in high alloy costs. In addition, there are few patents for high-strength steel produced by cold rolling using multiple uses or multiple grades of steel, and they are also limited to a certain type of steel. One component is used to achieve different strengths of the same steel. Although this can achieve the development of multiple grades of one steel, it wastes alloy costs for mature low-strength products, increases the difficulty of process production for high-strength products, and makes it difficult to ensure the control accuracy of batch production, which is not conducive to industrial production applications and market promotion; hot rolling production lines need to have ultra-fast cooling equipment to produce high-strength steel or even ultra-high-strength steel, but the processes of most ordinary hot rolling production lines can only meet the production of different grades of low-strength products, and it is difficult to meet industrial production without equipment modification or increased equipment investment.

[0006] Publication number CN114959478A discloses a multi-purpose 800MPa grade complex phase steel and its control method, but does not achieve the transition of different steel grades or strength levels. Publication number CN106929758A discloses a multi-grade cold-rolled low-alloy high-strength steel strip and its production method. Through Nb micro-alloying, only three low-alloy high-strength steels are obtained with one component, and the yield strength is at a relatively low strength level of 420 to 500MPa. Publication number CN109355573A discloses a multi-grade hot-rolled steel plate based on carbon distribution technology and its manufacturing method. The Si content is high, and the surface quality of the product is difficult to control. After the air cooling section and the ultra-fast cooling section, it is cooled to between 320 and 540°C and then coiled. Conventional hot rolling production lines need to add additional ultra-fast cooling equipment, which is limited to production lines with extremely high stable control capabilities. Publication No. CN116219282AA discloses a one-steel multi-grade cold-rolled ultra-high-strength strip steel and its preparation method. Its composition design has jumped from low-carbon steel to medium-carbon steel, and it is not "one steel". The content range of important strengthening elements Mn, Cr, Nb and Als that affects casting is too wide, which does not conform to the concept of "one steel multi-grade" or "one steel multi-purpose". Publication No. CN114086065AA discloses a one-steel multi-grade multi-specification beam steel and its production method. The Nb and Ti content is high, the alloy cost is high, and it is limited to hot rolling production. Its tensile strength is less than 780MPa, and for low-strength 500L grade products, it wastes and increases the alloy cost, and for high-strength 700L grade products, it increases the difficulty of production. Publication No. CN117701992A5198173AA discloses a one-steel multi-stage cold-rolled continuous annealing quenching partitioning steel and a production method thereof, wherein the C, Mn, Si, Nb, and Ti contents are relatively high, the strip structure is ferrite + martensite + bainite + residual austenite, the yield strength is Rp0.2: 500-800MPa, the tensile strength is ≥980MPa, the elongation after fracture A50 is ≥18%, and the final tensile strength does not change much, all of which belong to the QP980 grade product, and the "one steel multi-grade" concept is not truly realized. The yield strength can be controlled based on process adjustment. The use of such a wide composition is not conducive to batch and stable production, and industrial production is extremely difficult. Publication No. CN118497615AA discloses a 980MPa grade multi-phase steel plate with multiple uses and its production method. The amount of Mn, Mo, and Nb elements added is relatively large, and the alloy cost is high. The multiple uses of one steel are limited to the same type and same level of 980MPa grade multi-phase steel. Publication No. CN118272736AA discloses a multi-stage cold-rolled continuous annealing enhanced formability dual-phase steel and its production method. The content range of important strengthening elements Mn, Si, Cr, and Nb is too wide, and the composition design itself exceeds the steel type and strength level. It does not conform to the concept of "one steel with multiple levels" or "one steel with multiple uses", and it is difficult to achieve stable industrial mass production.Publication number CN108517468AA discloses an economical cold-rolled dual-phase steel with multiple grades and a production method thereof. The composition is limited to cold-rolled dual-phase steel below 780MPa strength level. For low-strength dual-phase steel, the C and Mn alloy content is higher than that of ordinary 490MPa and 590MPa dual-phase steels, which is not conducive to reducing production costs. Summary of the invention

[0007] The purpose of the present invention is to overcome the above-mentioned defects of the prior art, to provide a multi-functional GPa-grade cold-rolled automobile steel plate and a preparation method thereof, in view of the personalized product demand characteristics of GPa-grade cold-rolled automobile steel and the problems of different alloy designs of different steel grades in the prior art, which make it difficult to make steel and continuously cast, and the high production cost, the present invention combines the common characteristics of three types of steel, studies the chemical composition and production process, and designs a reasonable, economical, and low-cost composition and effectively controls the key process parameters of each process section, so as to realize the use of a component system, hot rolling and cold rolling process design, and realize the development of 980MPa-grade cold-rolled dual-phase steel DP980, 980MPa-grade cold-rolled quenching and partitioning steel QP980 and 1180MPa-grade cold-rolled martensitic steel MS1180 through different continuous annealing process systems, and the tensile strength of all of them exceeds 1000MPa, meeting the design and development requirements of GPa steel. In addition, the present invention improves the utilization rate of billets and coils in the steel production process, solves the problem of destocking three types of GIP steel continuous casting billets, hot-rolled coils and cold-rolled coils, greatly reduces the high cost problem caused by the need to increase single-furnace smelting and independent casting production for small batch order contracts, and is beneficial for enterprises to reduce production costs.

[0008] To achieve the above object, the technical solution of the present invention is as follows:

[0009] A multifunctional GPa-grade cold-rolled automobile steel plate comprises the following components in mass percentage: C: 0.145-0.165%, Si: 0.32-0.52%, Mn: 1.68-1.78%, Cr: 0.46-0.56%, Ti: 0.012-0.026%, Al: 0.030-0.045%, P≤0.018%, S≤0.004%, and the balance is Fe and other inevitable impurities.

[0010] The present invention also discloses a method for preparing a multi-functional GPa-grade cold-rolled automobile steel plate as described above, comprising the following steps: smelting and continuous casting process, converter smelting, LF refining, RH refining, continuous casting, hot rolling process, cold rolling process and annealing process;

[0011] In the smelting and continuous casting process, converter smelting, LF refining and RH refining are sequentially adopted, and then slabs with a thickness of 230-233 mm are prepared by continuous casting;

[0012] In the hot rolling process, the slab is heated, rough rolled, finish rolled, laminar cooled and coiled to obtain a hot rolled steel coil;

[0013] In the cold rolling process, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 1.0 to 2.0 mm;

[0014] In the annealing step, the cold-rolled steel material is continuously annealed to obtain the GPa-grade cold-rolled automobile steel sheet.

[0015] Implementing the embodiments of the present invention will have the following beneficial effects:

[0016] 1. The present invention fully considers the production organization characteristics of small batches of GIP steel by steel enterprises, and through reasonable, economical and low-cost component design and effective control of key process parameters of each process section, realizes the development of 980MPa grade cold-rolled dual-phase steel DP980, 980MPa grade quenching and partitioning steel QP980 and 1180MPa grade martensitic steel MS1180 by adopting a component system, hot rolling and cold rolling process design, and solves the high cost problem caused by the need to increase single-furnace smelting and independent casting production of small batches of GIP steel.

[0017] 2. Compared with similar products in the prior art, the present invention adopts a low alloy cost design and does not add precious alloy elements such as Mo and Nb. By controlling the addition amount and reasonable proportion of important alloy elements such as C, Mn, Cr, the production cost is not high, which meets the requirements of green environmental protection, energy conservation and emission reduction. The tensile strength of the manufactured product exceeds 1000MPa.

[0018] 3. The chemical composition and heat treatment process of the steel plate designed by the present invention are feasible for production in conventional steel production lines, provided that the requirements of organization and performance are met, without the need for additional investment and equipment transformation and upgrading. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a typical microstructure diagram of DP980 in Example 1 of the present invention.

[0020] Figure 2 This is a typical microstructure diagram of QP980 according to Example 1 of the present invention.

[0021] Figure 3 This is a typical microstructure diagram of MS1180 according to Example 1 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0023] The invention discloses a multifunctional GPa-grade cold-rolled automobile steel plate, which comprises the following components in mass percentage: C: 0.145-0.165%, Si: 0.32-0.52%, Mn: 1.68-1.78%, Cr: 0.46-0.56%, Ti: 0.012-0.026%, Al: 0.030-0.045%, P≤0.018%, S≤0.004%, and the balance is Fe and other inevitable impurities.

[0024] In a specific embodiment, the thickness of the GPa-grade cold-rolled automobile steel sheet is 1.0-2.0 mm.

[0025] In a specific embodiment, the yield strength of 980MPa grade cold-rolled dual-phase steel DP980 is 550-720MPa, the tensile strength is ≥980MPa, and the post-fracture productivity is ≥9%; the yield strength of 980MPa grade cold-rolled quenched and partitioned steel QP980 is 600-850MPa, the tensile strength is ≥980MPa, and the post-fracture productivity is ≥15%; the yield strength of 1180MPa grade cold-rolled martensitic steel MS1180 is 950-1200MPa, the tensile strength is ≥1180MPa, and the post-fracture productivity is ≥4%.

[0026] The present invention also discloses a method for preparing a multi-functional GPa-grade cold-rolled automobile steel plate as in any embodiment of the present invention, comprising the following steps: smelting and continuous casting process, converter smelting, LF refining, RH refining, continuous casting, hot rolling process, cold rolling process and annealing process, which are specifically as follows:

[0027] In the smelting and continuous casting process, converter smelting, LF refining and RH refining are used in sequence, and then continuous casting is used to prepare a slab with a thickness of 230-233 mm. Specifically, the addition amount of hardenability elements C, Mn and Cr is strictly controlled during the production process to ensure that the final product obtains a sufficient amount of martensitic structure.

[0028] S2. In the hot rolling process, the slab is heated, rough rolled, finish rolled, laminar cooled and coiled to obtain a hot rolled steel coil.

[0029] In a specific embodiment, in the hot rolling process, the heating temperature of the slab is 1190-1220°C, and the holding time is 180-220min to ensure the uniformity of the structure during the heating process of the steel slab and avoid grain coarsening. The cumulative rolling reduction rate in the austenite zone is ≥97%, and the final rolling temperature is 875-905°C to avoid mixed crystal phenomenon in the two-phase zone rolling. The hot rolling thickness is 2.3-4.5mm, and the laminar cooling is to a coiling temperature of 585-615°C. In the hot rolling process, the coiling adopts a U-type coiling method to improve the stability of the hot rolling coil through coiling performance, wherein the coiling temperature at both ends of the slab, 30m from the slab head and 40m from the slab tail, is controlled to be 50-80°C higher than the coiling temperature in the middle of the slab. After coiling, it is placed in a slow cooling pit for slow cooling to room temperature.

[0030] S3. In the cold rolling process, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 1.0 to 2.0 mm.

[0031] In a specific embodiment, in the cold rolling process, the hot rolled steel coil is pickled with hydrochloric acid and then cold rolled using a CVC five-stand six-roll mill, with a cold rolling reduction rate of 55-58% and a cold rolling thickness of 1.0-2.0 mm.

[0032] In S4, in the annealing process, the cold-rolled steel material is continuously annealed to obtain a GPa-grade cold-rolled automobile steel sheet.

[0033] In a specific embodiment, in the annealing process, when preparing 980MPa grade cold-rolled dual-phase steel DP980, the process parameters specifically include: the strip annealing temperature is 795-805°C, the holding time is 100-160s, the slow cooling outlet temperature is 670-690°C, the slow cooling time is 50-80s, the rapid cooling outlet temperature is 290-300°C, and the rapid cooling rate is 40-60°C / s; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 235-245°C; the final cooling temperature is controlled at 115-125°C, and then the strip enters the water quenching tank to cool to room temperature, the strip adopts 0.3% flat elongation control, and finally is coiled by the coiler.

[0034] In a specific embodiment, in the annealing process, when preparing 980MPa grade cold-rolled quenched and partitioned steel QP980, the process parameters specifically include: strip annealing temperature of 840-850°C, holding time of 110-170s, slow cooling outlet temperature of 710-730°C, slow cooling time of 60-90s, rapid cooling cooling rate of 40-60°C / s, and rapid cooling outlet temperature of 275-285°C; in order to achieve martensite tempering and austenite carbon enrichment, over-aging is controlled by segmentation, and the first over-aging The first stage is heated by induction heater, and the temperature is controlled at 335-345℃ for 170-280s. The second stage of over-aging starts to be cooled by fan, and the temperature is controlled at 300-310℃ for 170-280s. The third stage of over-aging continues to be cooled, and the temperature is controlled at 260-270℃ for 170-280s. The final cooling temperature is controlled at 115-125℃, and then the strip enters the water quenching tank to cool to room temperature. The strip is controlled by 0.2% flat elongation, and finally coiled by the coiler.

[0035] In a specific embodiment, in the annealing process, when preparing 1180MPa grade cold-rolled martensitic steel MS1180, the process parameters specifically include: the strip annealing temperature is 870-880°C, the holding time is 110-170s, the slow cooling outlet temperature is 750-770°C, the slow cooling time is 60-90s, the rapid cooling rate is 45-65°C / s, and the rapid cooling outlet temperature is 260-270°C; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 220-230°C; the final cooling temperature is controlled at 115-125°C, and then the strip enters the water quenching tank to cool to room temperature, the strip adopts 0.2% flat elongation control, and finally is coiled by the coiler.

[0036] The following are specific embodiments

[0037] The present invention is now further described in detail by using the same composition steel billet, hot rolling and cold rolling processes to prepare three types of GPa steels (980MPa grade cold rolled dual-phase steel DP980, 980MPa grade cold rolled quenching and partitioning steel QP980 and 1180MPa grade cold rolled martensitic steel MS1180).

[0038] Example 1

[0039] 1.1 In the smelting and continuous casting process: A multi-functional GPa-grade cold-rolled automobile steel plate, the chemical composition of which includes by weight percentage: C: 0.156%, Si: 0.46%, Mn: 1.73%, Cr: 0.52%, Ti: 0.023%, Al: 0.036%, P: 0.011%, S: 0.001%, and the balance is Fe and other inevitable impurities. After converter smelting, LF refining, and RH refining, the molten steel with chemical composition meeting the standard requirements is continuously cast into a slab with a thickness of 230 mm, which is used as a multi-functional GPa-grade automobile cold-rolled high-strength steel raw material.

[0040] 1.2 Hot rolling process: The slab is heated, rough rolled, finished rolled, laminar cooled and coiled to obtain hot rolled steel coils; the slab heating temperature is 1205°C, the heat preservation is 192min, the cumulative rolling reduction rate in the austenite zone is ≥97%, the final rolling temperature is 889°C, the hot rolling thickness is 2.3mm, and the laminar cooling to the coiling temperature is 596°C. Furthermore, the U-type coiling method is adopted, and the coiling temperatures of the 30-meter head and 40-meter tail of the strip are increased by 50°C and 80°C respectively. After coiling, the strip is placed in a slow cooling pit for slow cooling to room temperature.

[0041] 1.3 Cold rolling process: After the strip is pickled with hydrochloric acid, it is cold rolled using a CVC five-stand six-roll mill. The cold rolling reduction rate is 56.5% and the cold rolling thickness is 1.0 mm.

[0042] 1.4 Continuous annealing process:

[0043] 1) 980MPa grade cold-rolled dual-phase steel DP980: the strip annealing temperature is 799°C, the holding time is 103s, the slow cooling outlet temperature is 681°C, the slow cooling time is 52s, the rapid cooling outlet temperature is 292°C, and the rapid cooling rate is 60°C / s; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of overaging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of overaging is 236°C; the final cooling temperature is controlled at 120°C, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.3% flat elongation control and is finally coiled by the coiler.

[0044] 2) 980MPa grade cold-rolled quenched and partitioned steel QP980: the strip annealing temperature is 843℃, the holding time is 119s, the slow cooling outlet temperature is 715℃, the slow cooling time is 60s, the rapid cooling rate is 60℃ / s, and the rapid cooling outlet temperature is 277℃; in order to achieve martensite tempering and austenite carbon enrichment, overaging adopts segmented control. The first stage of overaging is heated by an induction heater, and the temperature is controlled at 339℃ for 171s; the second stage of overaging starts to be cooled by a fan, and the temperature is controlled at 302℃ for 171s; the third stage of overaging continues to cool, and the temperature is controlled at 261℃ for 171s; the final cooling temperature is controlled at 120℃, and then the strip enters the water quenching tank to cool to room temperature. The strip is controlled by 0.2% flat elongation, and finally coiled by a coiler.

[0045] 3) 1180MPa grade cold-rolled martensitic steel MS1180: the strip annealing temperature is 875℃, the holding time is 121s, the slow cooling outlet temperature is 754℃, the slow cooling time is 61s, the rapid cooling rate is 65℃ / s, and the rapid cooling outlet temperature is 263℃; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 223℃; the final cooling temperature is controlled at 119℃, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.2% flat elongation control and is finally coiled by the coiler.

[0046] 1.5 Mechanical properties test

[0047] The finished mechanical properties test results of 980MPa grade cold-rolled dual-phase steel DP980, 980MPa grade cold-rolled quenching and partitioning steel QP980 and 1180MPa grade cold-rolled martensitic steel MS1180 are shown in Table 1. The tensile strength of all of them exceeds 1000MPa, meeting the performance requirements of GPa steel.

[0048] Table 1 Mechanical properties test results of three kinds of GIP steel finished products in Example 1

[0049] Steel Type Thickness mm Yield strength MPa Tensile strength MPa Elongation after break % DP980 1.0 631 1038 13 QP980 1.0 673 1055 18.5 MS1180 1.0 1046 1249 8.5

[0050] Example 2

[0051] 2.1 Steelmaking process: A multi-functional GPa grade cold-rolled automobile steel plate, the chemical composition of which includes by weight percentage: C: 0.147%, Si: 0.51%, Mn: 1.75%, Cr: 0.47%, Ti: 0.018%, Al: 0.039%, P: 0.015%, S: 0.001%, and the balance is Fe and other inevitable impurities. After converter smelting, LF refining, and RH refining, the molten steel with chemical composition meeting the standard requirements is continuously cast into a steel billet with a thickness of 232 mm, which is used as a multi-functional automobile cold-rolled GPa steel raw material.

[0052] 2.2 Hot rolling process: The slab is heated, rough rolled, finished rolled, laminar cooled and coiled to obtain a hot rolled steel coil; the slab heating temperature is 1213°C, the heat preservation is 203min, the cumulative rolling reduction rate in the austenite zone is ≥97%, the final rolling temperature is 898°C, the hot rolling thickness is 3.5mm, and the laminar cooling is to a coiling temperature of 602°C. Furthermore, the U-type coiling method is adopted, and the coiling temperatures of the 30m head and 40m tail of the strip are increased by 50°C and 80°C respectively, and the coiling is completed and placed in a slow cooling pit for slow cooling to room temperature.

[0053] 2.3 Cold rolling process: After the strip is pickled with hydrochloric acid, it is cold rolled using a CVC five-stand six-roll mill. The cold rolling reduction rate is 57.1% and the cold rolling thickness is 1.5 mm.

[0054] 2.4 Continuous annealing process:

[0055] 1) 980MPa grade cold-rolled dual-phase steel DP980: the strip annealing temperature is 801°C, the holding time is 130s, the slow cooling outlet temperature is 678°C, the slow cooling time is 65s, the rapid cooling outlet temperature is 295°C, and the rapid cooling rate is 48°C / s; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of overaging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of overaging is 239°C; the final cooling temperature is controlled at 122°C, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.3% flat elongation control and is finally coiled by the coiler.

[0056] 2) 980MPa grade cold-rolled quenched and partitioned steel QP980: the strip annealing temperature is 847℃, the holding time is 141s, the slow cooling outlet temperature is 719℃, the slow cooling time is 71s, the rapid cooling rate is 49℃ / s, and the rapid cooling outlet temperature is 276℃; in order to achieve martensite tempering and austenite carbon enrichment, overaging adopts segmented control. The first stage of overaging is heated by an induction heater, and the temperature is controlled at 340℃ for 203s; the second stage of overaging starts to be cooled by a fan, and the temperature is controlled at 303℃ for 203s; the third stage of overaging continues to cool, and the temperature is controlled at 264℃ for 203s; the final cooling temperature is controlled at 121℃, and then the strip enters the water quenching tank to cool to room temperature. The strip is controlled by 0.2% flat elongation, and finally coiled by a coiler.

[0057] 3) 1180MPa grade cold-rolled martensitic steel MS1180: the strip annealing temperature is 878℃, the holding time is 141s, the slow cooling outlet temperature is 761℃, the slow cooling time is 71s, the rapid cooling rate is 55℃ / s, and the rapid cooling outlet temperature is 266℃; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 225℃; the final cooling temperature is controlled at 121℃, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.2% flat elongation control and is finally coiled by the coiler.

[0058] 2.5 Mechanical properties test

[0059] The test results of finished mechanical properties of 980MPa grade cold-rolled dual-phase steel DP980, 980MPa grade cold-rolled quenching and partitioning steel QP980 and 1180MPa grade cold-rolled martensitic steel MS1180 are shown in Table 2. The tensile strength of all of them exceeds 1000MPa, meeting the performance requirements of GPa steel.

[0060] Table 2 Test results of mechanical properties of three kinds of GIP steel finished products in Example 2

[0061] Steel Type Thickness mm Yield strength MPa Tensile strength MPa Elongation after break % DP980 1.5 619 1020 14.5 QP980 1.5 664 1037 19 MS1180 1.5 1028 1219 9

[0062] Embodiment 3:

[0063] 3.1 Steelmaking process: A multi-functional GPa grade cold-rolled automobile steel plate, the chemical composition of which includes by weight percentage: C: 0.163%, Si: 0.33%, Mn: 1.69%, Cr: 0.54%, Ti: 0.025%, Al: 0.032%, P: 0.012%, S: 0.002%, and the balance is Fe and other inevitable impurities. After converter smelting, LF refining, and RH refining, the molten steel with chemical composition meeting the standard requirements is continuously cast into a steel billet with a thickness of 233 mm, which is used as a raw material for multi-functional automobile cold-rolled GPa steel.

[0064] 3.2 Hot rolling process: The slab is heated, rough rolled, finished rolled, laminar cooled and coiled to obtain a hot rolled steel coil; the slab heating temperature is 1195°C, the heat preservation is 211min, the cumulative rolling reduction rate in the austenite zone is ≥97%, the final rolling temperature is 902°C, the hot rolling thickness is 4.5mm, and the laminar cooling is to a coiling temperature of 611°C. Furthermore, the U-type coiling method is adopted, and the coiling temperatures of the 30-meter head and 40-meter tail of the strip are increased by 50°C and 80°C respectively. After coiling, the strip is placed in a slow cooling pit for slow cooling to room temperature.

[0065] 3.3 Cold rolling process: After the strip is pickled with hydrochloric acid, it is cold rolled using a CVC five-stand six-roll mill. The cold rolling reduction rate is 55.6% and the cold rolling thickness is 2.0 mm.

[0066] 3.4 Continuous annealing process:

[0067] 1) 980MPa grade cold-rolled dual-phase steel DP980: the strip annealing temperature is 796℃, the holding time is 157s, the slow cooling outlet temperature is 686℃, the slow cooling time is 79s, the rapid cooling outlet temperature is 296℃, and the rapid cooling rate is 40℃ / s; in order to prevent the strength reduction caused by martensite tempering, the temperature of each stage of overaging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of overaging is 244℃; the final cooling temperature is controlled at 121℃, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.3% flat elongation control and is finally coiled by the coiler.

[0068] 2) 980MPa grade cold-rolled quenched and partitioned steel QP980: the strip annealing temperature is 842℃, the holding time is 170s, the slow cooling outlet temperature is 726℃, the slow cooling time is 87s, the rapid cooling rate is 49℃ / s, and the rapid cooling outlet temperature is 281℃; in order to achieve martensite tempering and austenite carbon enrichment, overaging adopts segmented control. The first stage of overaging is heated by an induction heater, and the temperature is controlled at 343℃ for 251s; the second stage of overaging starts to be cooled by a fan, and the temperature is controlled at 309℃ for 251s; the third stage of overaging continues to cool, and the temperature is controlled at 269℃ for 251s; the final cooling temperature is controlled at 124℃, and then the strip enters the water quenching tank to cool to room temperature. The strip is controlled by 0.2% flat elongation, and finally coiled by a coiler.

[0069] 3) 1180MPa grade cold-rolled martensitic steel MS1180: the strip annealing temperature is 873℃, the holding time is 170s, the slow cooling outlet temperature is 766℃, the slow cooling time is 89s, the rapid cooling rate is 45℃ / s, and the rapid cooling outlet temperature is 269℃; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 228℃; the final cooling temperature is controlled at 125℃, and then the strip enters the water quenching tank to cool to room temperature. The strip adopts 0.2% flat elongation control and is finally coiled by the coiler.

[0070] 3.5 Mechanical properties test

[0071] The test results of finished mechanical properties of 980MPa grade cold-rolled dual-phase steel DP980, 980MPa grade cold-rolled quenching and partitioning steel QP980 and 1180MPa grade cold-rolled martensitic steel MS1180 are shown in Table 3. The tensile strength of all of them exceeds 1000MPa, meeting the performance requirements of GPa steel.

[0072] Table 3 Mechanical properties test results of three kinds of GIP steel finished products in Example 3

[0073] Steel Type Thickness mm Yield strength MPa Tensile strength MPa Elongation after break % DP980 2.0 664 1049 13 QP980 2.0 686 1048 19 MS1180 2.0 1077 1266 8

[0074] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A multi-functional GPa grade cold-rolled automobile steel plate, characterized in that: The following components are included in mass percentage: C: 0.145~0.165%, Si: 0.32~0.52%, Mn: 1.68~1.78%, Cr: 0.46~0.56%, Ti: 0.012~0.026%, Al: 0.030~0.045%, P≤0.018%, S≤0.004%, the balance is Fe and other inevitable impurities.

2. The multi-functional GPa grade cold-rolled automobile steel sheet according to claim 1, characterized in that: The thickness of the GPa-grade cold-rolled automobile steel plate is 1.0-2.0 mm.

3. A method for preparing a multifunctional GPa grade cold-rolled automobile steel sheet as claimed in any one of claims 1 to 2, characterized in that: The following steps are involved: Smelting and continuous casting process, converter smelting, LF refining, RH refining, continuous casting, hot rolling process, cold rolling process and annealing process; In the smelting and continuous casting process, converter smelting, LF refining and RH refining are sequentially adopted, and then slabs with a thickness of 230-233 mm are prepared by continuous casting; In the hot rolling process, the slab is heated, rough rolled, finish rolled, laminar cooled and coiled to obtain a hot rolled steel coil; In the cold rolling process, the hot rolled steel coil is pickled and then cold rolled to a target thickness of 1.0 to 2.0 mm; In the annealing step, the cold-rolled steel material is continuously annealed to obtain the GPa-grade cold-rolled automobile steel sheet.

4. The preparation method according to claim 3, characterized in that: In the hot rolling process, the heating temperature of the slab is 1190-1220°C, the holding time is 180-220min, the cumulative rolling reduction rate in the austenite zone is ≥97%, the final rolling temperature is 875-905°C, the hot rolling thickness is 2.3-4.5mm, and the laminar cooling to the coiling temperature is 585-615°C.

5. The preparation method according to claim 3, characterized in that: In the hot rolling process, the coiling adopts a U-type coiling method, in which the coiling temperature at both ends of the slab, 30m from the slab head and 40m from the slab tail, is controlled to be 50-80°C higher than the coiling temperature in the middle of the slab. After coiling, the slab is placed in a slow cooling pit for slow cooling to room temperature.

6. The preparation method according to claim 3, characterized in that: In the cold rolling process, the hot rolled steel coil is pickled with hydrochloric acid and then cold rolled using a CVC five-stand six-roller rolling mill, with a cold rolling reduction rate of 55-58% and a cold rolling thickness of 1.0-2.0 mm.

7. The preparation method according to claim 3, characterized in that: In the annealing process, when preparing 980MPa grade cold-rolled dual-phase steel DP980, the process parameters specifically include: the strip annealing temperature is 795-805°C, the holding time is 100-160s, the slow cooling outlet temperature is 670-690°C, the slow cooling time is 50-80s, the rapid cooling outlet temperature is 290-300°C, and the rapid cooling rate is 40-60°C / s; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of overaging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of overaging is 235-245°C; the final cooling temperature is controlled at 115-125°C, and then the strip enters the water quenching tank to cool to room temperature, the strip adopts 0.3% flat elongation control, and finally is coiled by a coiler.

8. The preparation method according to claim 3, characterized in that: In the annealing process, when preparing 980MPa grade cold-rolled quenched and partitioned steel QP980, the process parameters specifically include: strip annealing temperature of 840-850°C, holding time of 110-170s, slow cooling outlet temperature of 710-730°C, slow cooling time of 60-90s, rapid cooling rate of 40-60°C / s, and rapid cooling outlet temperature of 275-285°C; in order to achieve martensite tempering and austenite carbon enrichment, over-aging is controlled by segmentation, and the first segment of over-aging is put into induction heating. The strip is heated by a heater and the temperature is controlled at 335-345°C for 170-280s. The second stage of over-aging begins to be cooled by a fan and the temperature is controlled at 300-310°C for 170-280s. The third stage of over-aging continues to be cooled and the temperature is controlled at 260-270°C for 170-280s. The final cooling temperature is controlled at 115-125°C, and then the strip enters a water quenching tank and is cooled to room temperature. The strip is controlled at a flat elongation of 0.2% and is finally coiled by a coiler.

9. The preparation method according to claim 3, characterized in that: In the annealing process, when preparing 1180MPa grade cold-rolled martensitic steel MS1180, the process parameters specifically include: the strip annealing temperature is 870-880°C, the holding time is 110-170s, the slow cooling outlet temperature is 750-770°C, the slow cooling time is 60-90s, the rapid cooling rate is 45-65°C / s, and the rapid cooling outlet temperature is 260-270°C; in order to prevent the martensite tempering from causing strength reduction, the temperature of each stage of over-aging does not exceed the rapid cooling outlet temperature, and the outlet temperature of the last stage of over-aging is 220-230°C; the final cooling temperature is controlled at 115-125°C, and then the strip enters the water quenching tank to cool to room temperature, the strip adopts 0.2% flat elongation control, and finally is coiled by a coiler.

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