Thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and preparation method

Abstract

The present invention provides a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method, which belongs to the field of automobile steel and preparation. The preparation method includes adjusting the heat treatment process in the hot rolling and finishing rolling stage to obtain a thin-gauge strip containing a martensite structure with a volume fraction of ≥90%, and then performing critical zone continuous annealing regulation, heating the strip to 680°C to 720°C at 5 to 50°C / s, and then heating to 800 to 850°C at 0.5 to 7°C / s, isothermal for 80 to 150 seconds, then slowly cooling to 680°C to 720°C at 5°C / s, and then rapidly cooling to 240 to 320°C at more than 30°C / s for 10 to 60 seconds, and then heating to 370 to 450°C at 10°C / s for 120 to 180 seconds, and finally cooling to room temperature at 10°C / s to obtain a thin-gauge high-strength and high-plasticity QP steel containing a mixed structure of martensite, ferrite and residual austenite. The high-strength and high-plasticity QP steel prepared by the present invention has improved performance, and the preparation process is simple and efficient, reducing energy consumption and carbon emissions.

Description

Technical Field

[0001] The present invention belongs to the field of automobile steel and its preparation, and specifically relates to a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method thereof. Background Art

[0002] As the country continues to pay attention to environmental, energy and climate issues, the automotive industry is developing in the direction of energy conservation, emission reduction, high safety and low cost. The main material of the car body is steel. The steel material consumed in the production of a car accounts for about 72-88% of the total amount of automotive materials. Therefore, it is necessary to continuously optimize the quality and cost performance of automotive steel. Among the advanced high-strength steels (AHSS), quenching-partitioning steel (QP steel) is the representative of the third generation of advanced high-strength automotive steel and the main trend of future automotive body development. QP steel gives full play to the high strength of martensite and the superplasticity of austenite, mainly through phase transformation strengthening, combined with fine grain strengthening and precipitation strengthening, to form a composite microstructure with coexistence of hard phase and soft phase, so that the material with low alloy content and mainly composed of cheap alloy elements has comprehensive mechanical properties between the first and second generation advanced high-strength steels. The production of traditional QP steel is affected by the minimum thickness limit of hot-rolled plates, and further thinning is required in the cold rolling process. The structure before heat treatment is ferrite + pearlite after cold deformation. Taking QP980 as an example, the plasticity of the finished plate usually does not exceed 25%, which seriously limits its application in parts with high formability requirements. How to further improve the strength and plasticity of QP steel is the focus of QP steel research and development.

[0003] Compared with the traditional hot rolling process, the thin slab continuous casting and rolling process has the characteristics of compact process, reduced equipment, shortened production line, and only local heating after continuous casting, which significantly reduces the energy consumption and comprehensive cost of the manufacturing process. At the same time, the thin slab continuous casting and rolling process has certain advantages in the production of automotive high-strength steel due to its high temperature uniformity, especially in the production of thin-gauge hot-rolled strip. The thin slab continuous casting and rolling process can directly produce thin-gauge QP steel, eliminating the subsequent cold rolling, which not only shortens the process but also saves energy, which is in line with the future development trend of the steel industry. Because there is no need for subsequent cold rolling, there is no need to adjust the hot-rolled structure to a softer ferrite + pearlite structure, so that the thin slab continuous casting and rolling process can give full play to the hot rolling production line organization control ability to further improve the strength and plasticity of QP steel.

[0004] In the prior art, Chinese patent with authorization number CN111321281A discloses a method for achieving reinforcement and plasticization of I&QP steel by microstructure regulation, producing I&QP steel with an initial structure of bainite, and the produced slab has a tensile strength of more than 1000MPa and a total elongation of more than 26%. However, the above method is only applicable to the production of some QP steels, and cannot achieve the corresponding effect for steels with initial structures of other components; Chinese patents with authorization numbers CN107475627, CN107488814 and CN107557692 respectively disclose methods for producing hot-rolled 600MPa, 800MPa and 1000MPa TRIP steels based on thin slab continuous casting and rolling processes, and obtain the best mechanical properties by regulating the chemical composition and process technology, but the degree of improvement in the high strength and high plasticity of the steel is limited. Summary of the invention

[0005] In view of the above-mentioned defects or deficiencies in the prior art, the present invention aims to provide a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method thereof.

[0006] In order to achieve the above purpose, the embodiment of the present invention adopts the following technical solution:

[0007] The embodiment of the present invention provides a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method. The initial structure in the steel is martensite. By adjusting the heating rate during the heat treatment process, thin-gauge high-strength and high-plasticity QP steel is produced based on the thin slab continuous casting and rolling process to achieve the effect of enhancing plasticization. Related studies have shown that different initial structures have a great influence on the mechanical properties of TRIP steel. The present invention uses martensite as the initial structure, based on the martensitic hot-rolled plate, and obtains a thin-gauge strip with a thickness of 0.8 to 1.8 mm through a thin slab continuous casting and rolling process, and obtains high-strength and high-plasticity QP steel through corresponding heat treatment, and the strength-plasticity product can reach up to 34 GPa·%.

[0008] The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling has the following chemical compositions by weight percentage: C: 0.15-0.3%, Si: 1.5-2.5%, Mn: 1.5-2.75%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.03-0.05%, and the rest is iron and unavoidable impurities; and the slab is regulated by the hot rolling process of thin slab continuous casting and rolling to obtain a hot-rolled plate with an initial structure of martensite, and then regulated by critical zone continuous annealing to obtain a thin-gauge high-strength and high-plasticity QP steel with a three-phase mixed structure of martensite, ferrite and residual austenite, wherein the residual austenite is in a film form.

[0009] In the hot rolling process of the thin slab continuous casting and rolling, the temperature of the slab is controlled at 1100℃~1150℃ before entering the finishing mill, and the final rolling temperature is controlled at 860℃~880℃. After rolling in the finishing mill, the slab is cooled to below 230℃ at a cooling rate of ≥30℃ / s by a laminar cooling process to obtain a martensitic hot-rolled plate. Through the thin slab continuous casting and rolling process, a thin-gauge strip with a thickness of 0.8~1.8mm is directly rolled on the hot rolling mill, and the volume fraction of the lath martensite structure in the strip is ≥90%.

[0010] The critical zone continuous annealing control adopts a two-step method for heat treatment, wherein the quenching temperature is 240-320°C and the partitioning temperature is 370-450°C. The heating stage adopts two-stage heating, in which the austenite phase transformation does not occur in the first stage of heating, and the second stage of heating is in the austenite phase transformation range to obtain austenite, and the heating rate is strictly controlled in this range to make the austenite morphology thin film-like. The specific heat treatment system is as follows: the thin-gauge strip is heated to 680°C-720°C at a rate of 5-50°C / s, then heated to 800-850°C at a rate of 0.5-7°C / s, isothermally heated for 80-150 seconds, then slowly cooled to 680°C-720°C at a rate of 5°C / s, then quickly cooled to 240-320°C at a rate greater than 30°C / s and kept warm for 10-60 seconds, then heated to 370-450°C at a rate of 10°C / s and kept warm for 120-180 seconds, and finally cooled to room temperature at a rate of 10°C / s to obtain a thin-gauge high-strength and high-plasticity QP steel containing a three-phase mixed structure of martensite, ferrite and residual austenite. Among them, the ferrite content is controlled by the two-phase zone holding temperature and time, and the volume fraction is 47-55%; the martensite content is controlled by the quenching temperature, and the volume fraction is 31-38%; the residual austenite is controlled by the quenching temperature, the partitioning temperature and its corresponding time, and the volume fraction is 13-19%.

[0011] By mixing the three phases and proportioning the volume fraction, the obtained thin-gauge high-strength and high-plasticity QP steel has a tensile strength greater than 986MPa, a total elongation of 30.2% to 34.6%, and a strength-plasticity product of 31.2 to 34.1GPa·%. Compared with the conventional hot-rolled + cold-rolled QP steel with an initial hot-rolled structure of ferrite + pearlite, the strength-plasticity product is increased by more than 40%.

[0012] It can be seen from the above technical scheme that the thin-gauge high-strength and high-plasticity QP steel and preparation method based on thin slab continuous casting and rolling provided in the embodiment of the present invention include adjusting the heat treatment process in the hot rolling and finishing rolling stage to obtain a martensitic structure with a volume fraction of ≥90%, and then performing two-step critical zone continuous annealing regulation. The heating stage adopts two-stage heating, and the thin-gauge strip is heated to 680°C~720°C at a rate of 5~50°C / s, and then heated to 800~850°C at a rate of 0.5~7°C / s, isothermally heated for 80~150 seconds, and then slowly cooled to 680°C~720°C at a rate of 5°C / s, and then rapidly cooled to 240~320°C at a rate greater than 30°C / s and kept warm for 10~ The steel is heated to 370-450 ℃ for 60 seconds, then heated to 370-450 ℃ at a rate of 10 ℃ / s and kept warm for 120-180 seconds, and finally cooled to room temperature at a rate of 10 ℃ / s to obtain a thin-gauge high-strength and high-plasticity QP steel containing a three-phase mixed structure of martensite, ferrite and residual austenite; the prepared thin-gauge high-strength and high-plasticity QP steel has a tensile strength greater than 986MPa, a total elongation of 30.2%-34.6%, and a strength-plasticity product of 31.2-34.1GPa·%. Compared with the QP steel with an initial hot-rolled structure of ferrite + pearlite produced by conventional hot rolling + cold rolling, the strength-plasticity product is increased by 40%; the preparation process is simpler and more efficient, and energy consumption and carbon emissions are greatly reduced, which has good application prospects under the background of carbon neutrality.

[0013] Of course, it is not necessary to achieve all of the advantages described above at the same time to implement any product or method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0015] Figure 1 This is a SEM image of the annealed microstructure of the thin-gauge high-strength and high-plasticity QP steel described in Example 1 of the present invention;

[0016] Figure 2 This is a SEM image of the annealed microstructure of the thin-gauge high-strength and high-plasticity QP steel described in Example 2 of the present invention;

[0017] Figure 3 This is a SEM image of the annealed microstructure of the thin-gauge high-strength and high-plasticity QP steel described in Example 3 of the present invention;

[0018] Figure 4 This is the SEM image of the annealed structure of the comparative example QP steel of the present invention. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations. It should be noted that the embodiments of the present invention and the features in the embodiments can also be combined with each other without conflict.

[0020] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. In the description of the present invention, the terms "first", "second", "third", "fourth", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0021] Example 1

[0022] This embodiment provides a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method.

[0023] The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling has the following chemical composition by weight percentage:

[0024] C: 0.15%, Si: 1.5%, Mn: 1.5%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.03%; the rest are iron and inevitable impurities; and the final annealing structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is in a film form.

[0025] In the three-phase mixed structure, the volume fraction of ferrite is 55%, the volume fraction of martensite is 31.8%, the volume fraction of retained austenite is 13.2%, and the thickness of the thin-gauge QP steel is 1.6 mm. This embodiment also provides a method for preparing thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling as described above, the preparation method comprising:

[0026] Based on thin slab continuous casting and rolling, the slab is hot rolled to control the initial structure, the slab temperature is set to 1100°C before entering the finishing mill, the final rolling temperature is controlled to 870°C, and the slab is cooled to 200°C at a cooling rate of 45°C / s through the front-end laminar cooling process after rolling for coiling, and a hot-rolled plate with a thickness of 1.6 mm is produced, and the initial structure of the hot-rolled plate is a lath martensite structure with a volume fraction of 95%; and,

[0027] The hot-rolled plate is subjected to intercritical continuous annealing to adjust the final structure. The specific process is as follows: a two-step heat treatment is adopted, with a quenching temperature of 240°C and a partitioning temperature of 450°C. In the heating stage, a two-stage heating method is adopted, that is, the plate is first heated to 700°C at a rate of 10°C / s, then heated to 850°C at a rate of 0.5°C / s, isothermal for 50 seconds, then slowly cooled to 700°C at a rate of 5°C / s, then quickly cooled to 240°C at 50°C / s and kept warm for 15 seconds, then heated to 450°C at a rate of 10°C / s and kept warm for 150 seconds, and finally cooled to room temperature at a rate of 10°C / s.

[0028] like Figure 1 As shown, the final annealed structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is mostly in a film-like state.

[0029] Example 2

[0030] This embodiment provides a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method.

[0031] The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling has the following chemical composition by weight percentage:

[0032] C: 0.3%, Si: 12.5%, Mn: 2.75%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.05%, and the rest are iron and inevitable impurities; and the final annealing structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is in a film form.

[0033] In the three-phase mixed structure, the volume fraction of ferrite is 47%, the volume fraction of martensite is 34.02%, the volume fraction of retained austenite is 14.13%, and the thickness of thin-gauge QP steel is 0.8 mm.

[0034] This embodiment also provides a method for preparing thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling as described above, the preparation method comprising:

[0035] Based on the thin slab continuous casting and rolling, the slab is hot rolled to control the initial structure, the temperature of the slab is set at 1150°C before entering the finishing rolling unit, the final rolling temperature is controlled at 880°C, and the slab is coiled by cooling to 220°C at a cooling rate of 40°C / s through the front section laminar cooling process after rolling, and the hot rolled plate with a thickness of 0.8 mm is produced through the thin slab continuous casting and rolling process, and the initial structure of the hot rolled plate is a lath martensite structure with a volume fraction of 92%; and,

[0036] The hot-rolled plate is subjected to intercritical continuous annealing to adjust the final structure. The specific process is as follows: a two-step heat treatment is adopted, the quenching temperature is 250°C, and the partitioning temperature is 400°C. Among them, the heating stage adopts a two-stage heating method, the plate is first heated to 720°C at a rate of 30°C / s, then heated to 840°C at a rate of 1°C / s, isothermal for 80 seconds, then slowly cooled to 710°C at a rate of 5°C / s, then quickly cooled to 250°C at 50°C / s for 10 seconds, then heated to 400°C at a rate of 10°C / s for 180 seconds, and finally cooled to room temperature at a rate of 10°C / s.

[0037] like Figure 2 As shown, the final annealed structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is mostly in a film-like state.

[0038] Example 3

[0039] This embodiment provides a thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling and a preparation method.

[0040] The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling has the following chemical composition by weight percentage:

[0041] C: 0.2%, Si: 2.0%, Mn: 2.15%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.04%, and the rest are iron and inevitable impurities; and the final annealing structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is in a film form.

[0042] In the three-phase mixed structure, the volume fraction of ferrite is 47.87%, the volume fraction of martensite is 38%, the volume fraction of retained austenite is 14.13%, and the thickness of thin-gauge QP steel is 1.8 mm.

[0043] This embodiment also provides a method for preparing thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling as described above, the preparation method comprising:

[0044] Based on the thin slab continuous casting and rolling, the slab is hot rolled to control the initial structure, the temperature of the slab is set at 1130°C before entering the finishing mill, the final rolling temperature is controlled at 880°C, and the slab is coiled by cooling to 220°C at a cooling rate of 40°C / s through the front section laminar cooling process after rolling. The slab with a thickness of 1.8 mm is produced through the thin slab continuous casting and rolling process, and the initial structure of the hot rolled plate is a lath martensite structure with a volume fraction of 95%; and,

[0045] The hot-rolled plate is subjected to intercritical continuous annealing to control the final structure. The specific process is as follows: a two-step heat treatment is adopted, the quenching temperature is 320°C, and the partitioning temperature is 370°C. Among them, the key point is the two-stage heating method used in the heating stage, that is, the plate is first heated to 710°C at a rate of 50°C / s, then heated to 850°C at a rate of 7°C / s, isothermal for 80 seconds, then slowly cooled to 710°C at a rate of 5°C / s, then quickly cooled to 320°C at 50°C / s and kept warm for 10 seconds, then heated to 370°C at a rate of 10°C / s and kept warm for 170 seconds, and finally cooled to room temperature at a rate of 10°C / s.

[0046] like Figure 3 As shown, the final annealed structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is mostly in a film-like state.

[0047] Comparative Example

[0048] The chemical composition corresponding to the thin plate that obtains the highest elongation and the highest strength-ductility product is as follows by weight percentage: C: 0.15%, Si: 1.5%, Mn: 1.5%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.03%; the rest are iron and unavoidable impurities; and the final annealing structure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is blocky.

[0049] In the three-phase mixed structure, the volume fraction of ferrite is 60%, the volume fraction of martensite is 30.51%, and the volume fraction of retained austenite is 9.49%. The thickness of the hot-rolled plate is 2.5 mm, and the thickness after cold rolling is 1.0 mm.

[0050] The preparation process is as follows:

[0051] The temperature is controlled at 1100°C before entering the finishing mill, and the final rolling temperature is controlled at 880°C. By adopting the laminar cooling process at the front section after rolling, the steel is cooled to 600°C at a cooling rate of 30°C / s for coiling. The hot rolling thickness is 2.5mm, and then cold rolled to 1.0mm to obtain a deformed ferrite pearlite structure; and,

[0052] The critical zone continuous annealing control process adopts a two-step heat treatment. The specific process flow is heating to 840℃ at 10℃ / s, isothermal for 80 seconds, then slowly cooling to 710℃ at a rate of 5℃ / s, then fast cooling to 240℃ at 50℃ / s and keeping warm for 30 seconds, then heating to 450℃ at a rate of 10℃ / s and keeping warm for 80 seconds, and finally cooling to room temperature at a rate of 10℃ / s.

[0053] like Figure 4 As shown in the figure, the final annealed microstructure is a three-phase mixed structure of martensite, ferrite and retained austenite, and the retained austenite is mostly blocky.

[0054] The thin-gauge plates obtained in Examples 1-3 and the comparative example were subjected to mechanical property tests, and the test data are listed in Table 1.

[0055] Table 1 Mechanical properties of Examples 1-3 and Comparative Examples

[0056]

[0057]

[0058] As shown in Table 1, the thin-gauge high-strength and high-plasticity QP steel provided by the embodiment of the present invention has a yield strength greater than 600 MPa, a tensile strength greater than 986 MPa, a total elongation of 30.2% to 34.6%, and a strength-plasticity product of 31.2 to 34.1 GPa·%. Compared with the QP steel produced by conventional hot rolling + cold rolling with an initial hot-rolled structure of ferrite + pearlite, the strength-plasticity product is increased by more than 40%.

[0059] It can be seen from the above technical scheme that the thin-gauge high-strength and high-plasticity QP steel and preparation method based on thin slab continuous casting and rolling provided in the embodiment of the present invention include adjusting the heat treatment process in the hot rolling and finishing rolling stage to obtain a martensitic structure with a volume fraction ≥90%, and then performing two-step critical zone continuous annealing regulation to obtain a thin-gauge high-strength and high-plasticity QP steel containing a three-phase mixed structure of martensite, ferrite and residual austenite; the prepared high-strength and high-plasticity QP steel has a tensile strength greater than 986MPa, a total elongation of 30.2% to 34.6%, and a strength-plasticity product of 31.2 to 34.1GPa·%, which is 40% higher than the QP steel with a hot-rolled initial structure of ferrite + pearlite produced by conventional hot rolling + cold rolling.

[0060] The above description is only a preferred embodiment of the present invention and an explanation of the technical principles used. It is not intended to limit the scope of the invention claimed for protection, but only represents the preferred embodiment of the present invention. Those skilled in the art should understand that the scope of the invention involved in the present invention is not limited to the technical solution formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

Claims

1. A thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling, characterized in that: The chemical composition by weight percentage is: C: 0.15~0.3%, Si: 1.5~2.5%, Mn: 1.5-2.75%, P≤0.008%, S≤0.008, N≤0.006%, O≤0.005%, Al: 0.03~0.05%, and the rest is iron and unavoidable impurities; and, After the slab is regulated by the hot rolling process of thin slab continuous casting and rolling, the initial structure is martensite; after the slab is regulated by the intercritical zone continuous annealing, the annealed structure is a three-phase mixed structure of martensite, ferrite and retained austenite, in which the retained austenite is in the form of a film; In the three-phase mixed structure, the volume fraction of ferrite is 47-55%, the volume fraction of martensite is 31-38%, and the volume fraction of retained austenite is 13-19%.

2. The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling according to claim 1, characterized in that: The thickness of thin-gauge high-strength and high-plastic QP steel is 0.8~1.8mm.

3. The thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling according to claim 1, characterized in that: The thin-gauge high-strength and high-plasticity QP steel has a tensile strength greater than 986 MPa, a total elongation of 30.2% to 34.6%, and a strength-plasticity product of 31.2 to 34.1 GPa·%.

4. A method for preparing thin-gauge high-strength and high-plasticity QP steel based on thin slab continuous casting and rolling as claimed in any one of claims 1 to 3, characterized in that: include: Based on thin slab continuous casting and rolling, the slab is hot rolled to obtain thin-gauge strips. The temperature is controlled at 1100°C~1150°C before entering the finishing mill, and the final rolling temperature is controlled at 860°C~880°C. After rolling in the finishing mill, the slab is cooled to below 230°C at a cooling rate of ≥30°C / s through a laminar cooling process, and then coiled to obtain a martensitic hot-rolled plate. The hot-rolled plate is subjected to critical zone continuous annealing and heat treatment is performed in a two-step method, wherein the quenching temperature is 240-320°C and the partitioning temperature is 370-450°C; a two-stage heating is adopted in the heating stage, in which no austenite phase transformation occurs in the first stage of heating, and the second stage of heating is in the austenite phase transformation zone to obtain austenite, and the heating rate is controlled in this zone to make the austenite morphology thin-film-like.

5. The preparation method according to claim 4, characterized in that: The hot-rolled plate is subjected to intercritical zone continuous annealing, and the specific heat treatment system is as follows: the thin-gauge strip is heated to 680°C~720°C at a rate of 5~50°C / s, then heated to 800~850°C at a rate of 0.5~7°C / s and isothermal for 80~150 seconds, then slowly cooled to 680°C~720°C at a rate of 5°C / s, then quickly cooled to 240~320°C at a rate greater than 30°C / s and kept warm for 10~60 seconds, then heated to 370~450°C at a rate of 10°C / s and kept warm for 120~180 seconds, and finally cooled to room temperature at a rate of 10°C / s to obtain a thin-gauge high-strength and high-plasticity QP steel containing a three-phase mixed structure of martensite, ferrite and retained austenite.

6. The preparation method according to claim 4, characterized in that: Through the hot rolling process of thin slab continuous casting and rolling, thin strips with a thickness of 0.8~1.8mm are directly rolled on the hot rolling unit.

7. The preparation method according to claim 4, characterized in that: In the martensitic hot-rolled plate, the volume fraction of the lath martensitic structure is ≥90%.

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