800MPa-grade complex phase steel and preparation method thereof

Abstract

The invention relates to 800 MPa-grade complex phase steel and a preparation method, and belongs to the technical field of steel manufacturing. The complex phase steel comprises the following chemical components in percentage by mass: 0.05%-0.09% of C, 0.2%-0.5% of Si, 1.4%-1.8% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 0.02%-0.05% of Nb, 0.05%-0.10% of Ti, 0.10%-0.25% of Mo and a matrix element Fe, the microstructure of the complex-phase steel comprises bainite, ferrite and retained austenite. Through a specific alloy system design, Si and Mo are used for inhibiting pearlite formation and refining grains, and meanwhile, a dispersion strengthening phase is separated out by means of Ti and Nb; according to the technology, multi-stage temperature control rolling is implemented based on high-temperature heating, dislocation strengthening is introduced in combination with non-recrystallization rolling, and the bainite phase change process is precisely regulated and controlled through an intermittent cooling path; and finally, the retained austenite is stabilized through a low-elongation pickling withdrawal and straightening process, collaborative optimization of the bainite and retained austenite proportion is achieved, and collaborative improvement of high strength and high elongation is achieved.

Description

Technical Field

[0001] This application relates to the field of steel manufacturing technology, and in particular to an 800MPa grade multiphase steel and its preparation method. Background Technology

[0002] With the automotive industry's increasing demands for energy conservation, environmental protection, and safety, lightweighting and high strength have become the core development trends for automotive steel. Advanced high-strength steels, represented by multiphase steels, are increasingly widely used in complex structural components such as chassis control arms and connecting arms. These components require rigorous deformation processes such as hole expansion, flanging, and drawing during processing, which places higher demands on the material's strength, plasticity (elongation), and hole expansion performance. At the same time, the requirement for consistency of the material's transverse and longitudinal properties (low anisotropy) is becoming increasingly stringent.

[0003] However, existing multiphase steel technology has significant drawbacks: the volume fraction ratio of bainite, ferrite, and retained austenite in microstructure control is unclear, leading to an imbalance between strength and plasticity; performance control overemphasizes longitudinal indicators while neglecting plasticity and the synergistic effect of transverse and longitudinal differences, restricting stamping stability; excessively high Cr content in composition design increases costs and hardenability, resulting in a narrowed cooling process window and reduced yield; the cooling process uses continuous cooling rate in the front stage and lacks air cooling segmentation, hindering precise control of multiphase ratios. Therefore, there is an urgent need to develop a new type of multiphase steel that combines precise microstructure control capabilities with low anisotropy, achieving simultaneous improvement in strength, plasticity, hole expansion performance, and transverse and longitudinal consistency through composition-process synergistic optimization, meeting the stringent requirements of the automotive industry for high-performance steel. Summary of the Invention

[0004] This application provides an 800MPa grade multiphase steel and its preparation method to solve the following technical problem: how to achieve synergistic optimization of improving the strength and plasticity of multiphase steel and reducing its anisotropy. In a first aspect, embodiments of this application provide an 800MPa grade multiphase steel, wherein the chemical composition of the multiphase steel, by mass fraction, includes: C: 0.05%~0.09%, Si: 0.2%~0.5%, Mn: 1.4%~1.8%, P≤0.015%, S≤0.003%, Nb: 0.02%~0.05%, Ti: 0.05%~0.10%, Mo: 0.10%~0.25%, and the matrix element Fe; The microstructure of the multiphase steel includes bainite, ferrite, and retained austenite.

[0005] Optionally, the volume fraction of bainite is ≥60%, the volume fraction of retained austenite is 5%~10%, and the remainder is ferrite.

[0006] Optionally, the multiphase steel meets at least one of the following properties: tensile strength ≥800MPa, yield strength ≥680MPa, elongation ≥18%, hole expansion rate ≥80%, and difference in tensile strength between transverse and longitudinal directions ≤40MPa.

[0007] Secondly, embodiments of this application provide a method for preparing the multiphase steel described in the first aspect, the method comprising: A slab with the following chemical composition was obtained: C: 0.05%~0.09%, Si: 0.2%~0.5%, Mn: 1.4%~1.8%, P≤0.015%, S≤0.003%, Nb: 0.02%~0.05%, Ti: 0.05%~0.10%, Mo: 0.10%~0.25%, and the matrix element Fe; The slab is sequentially heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate; The hot-rolled plate is subjected to laminar flow cooling, coiling, pickling, and straightening in sequence to obtain multiphase steel.

[0008] Optionally, the total time the slab spends in the furnace is 180 min to 220 min, and the heating temperature is 1180℃ to 1280℃.

[0009] Optionally, the initial rolling temperature of the roughing mill is 1150℃~1220℃, and the final rolling temperature of the roughing mill is 1050℃~1100℃.

[0010] Optionally, the initial rolling temperature of the finishing mill is 1020℃~1080℃, and the final rolling temperature of the finishing mill is 860℃~920℃.

[0011] Optionally, the laminar flow cooling adopts segmented cooling control, which includes front-stage air cooling, middle-stage water cooling, and rear-stage air cooling.

[0012] Optionally, the initial air cooling time is 2s to 5s, and the initial air cooling temperature is 790℃ to 830℃.

[0013] Optionally, the temperature of the mid-section water cooling is 530℃~580℃.

[0014] Optionally, the winding temperature is 500℃~530℃.

[0015] Optionally, the elongation rate of the acid-washed straightening is 0.5% to 1.5%.

[0016] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides an 800MPa grade multiphase steel. The chemical composition of the multiphase steel, by mass fraction, includes: C: 0.05%~0.09%, Si: 0.2%~0.5%, Mn: 1.4%~1.8%, P≤0.015%, S≤0.003%, Nb: 0.02%~0.05%, Ti: 0.05%~0.10%, Mo: 0.10%~0.25%, and the matrix element Fe. The microstructure of the multiphase steel includes: bainite, ferrite, and retained austenite. By adding alloying elements such as Si and Mo, and combining this with non-recrystallization rolling in the finishing stage, high-density dislocations are introduced into the deformed austenite. Post-rolling air cooling refines the ferrite grains and increases the grain boundary area. Subsequently, rapid mid-stage water cooling to the bainite phase region suppresses unfavorable microstructures and increases the bainite volume fraction. The subsequent air cooling stage promotes microstructure homogenization and the diffusion of C elements into untransformed austenite, enhancing the stability of residual austenite and reducing internal stress. Finally, a low-elongation pickling and straightening process is used to maintain the stability of residual austenite and avoid microstructure inhomogeneity and differences in transverse and longitudinal properties caused by local stress concentration. Attached Figure Description

[0017] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0019] Figure 1 A flowchart illustrating an 800MPa grade multiphase steel and its preparation method is provided for embodiments of this application. Figure 2 The image shows the microstructure of multiphase steel provided in the embodiments of this application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0021] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges between 1 and 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including" and "contains" used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship. "And / or" indicates that multiple situations can exist individually or simultaneously. Expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0022] In a first aspect, embodiments of this application provide an 800MPa grade multiphase steel, wherein the chemical composition of the multiphase steel, by mass fraction, includes: C: 0.05%~0.09%, Si: 0.2%~0.5%, Mn: 1.4%~1.8%, P≤0.015%, S≤0.003%, Nb: 0.02%~0.05%, Ti: 0.05%~0.10%, Mo: 0.10%~0.25%, and the matrix element Fe; The positive effects of limiting the mass fraction of carbon (C) to 0.05%~0.09% include: optimizing toughness and residual austenite stability while ensuring material strength, thus achieving a synergistic improvement in strength and plasticity. For example, the mass fraction of C can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, etc.

[0023] The positive effects of limiting the mass fraction of Si to 0.2%~0.5% include: improving strength through solid solution strengthening and suppressing carbide precipitation, while maintaining ferrite purity to ensure plasticity and the stability of retained austenite. For example, the mass fraction of Si can be 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0024] The positive effects of limiting the mass fraction of Mn to 1.4%~1.8% include: significantly improving hardenability to suppress pearlite formation, refining grains, and promoting bainitic phase transformation, thus providing a microstructure basis for high strength and high elongation. For example, the mass fraction of Mn can be 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, etc.

[0025] The positive effects of limiting the mass fraction of phosphorus (P) to ≤0.015% include: effectively avoiding intergranular segregation and cold brittleness, ensuring the low-temperature toughness and weldability of the material, and suppressing hot cracking defects during the hot rolling process. For example, the mass fraction of P can be 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, etc.

[0026] The positive effects of limiting the mass fraction of sulfur (S) to ≤0.003% include: maximally suppressing the formation of MnS inclusions, avoiding hot brittleness during hot rolling, and significantly improving the material's ductility, impact toughness, and surface quality. For example, the mass fraction of S can be 0.001%, 0.002%, 0.003%, etc.

[0027] The positive effects of limiting the Nb mass fraction to 0.02%~0.05% include: synergistic improvement of material strength through grain refinement and precipitation strengthening, while suppressing austenite grain coarsening to ensure plasticity and low-temperature toughness. For example, the Nb mass fraction can be 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0028] The positive effects of limiting the mass fraction of Ti to 0.05%~0.10% include: achieving grain refinement through Ti(C,N) precipitates pinning austenite grain boundaries and synergistically enhancing precipitation strengthening; simultaneously, fixing free nitrogen to improve toughness and optimize the strength-ductility balance of steel. For example, the mass fraction of Ti can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, etc.

[0029] The positive effects of limiting the Mo mass fraction to 0.10%~0.25% include: significantly improving hardenability and inhibiting pearlite transformation, precisely controlling the bainite phase transformation kinetics to optimize the volume fraction, and enhancing the precipitation strengthening effect, thus synergistically ensuring a high strength and toughness balance. For example, the Mo mass fraction can be 0.10%, 0.15%, 0.20%, 0.25%, etc.

[0030] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is: The sum of the percentages of all components in a composition should equal 100%, and the content ranges of several components should meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100. Furthermore, the specific content of Fe is made up to 100% by the actual detected values ​​of the other chemical components mentioned above, together with any unlisted active elements and / or impurity elements, and Fe must constitute the absolute proportion as a matrix element.

[0031] The microstructure of the multiphase steel includes bainite, ferrite, and retained austenite.

[0032] Bainite: A non-lamellar structure formed by diffusion-shear coupled phase transformation in the intermediate temperature range after austenitization. It is a multiphase structure composed of carbon-supersaturated ferrite and cementite (or retained austenite). Ferrite: A body-centered cubic solid solution of pure iron or low carbon, which usually precipitates from austenite through diffusion-type phase transformation in the high to intermediate temperature range. Retained austenite: Face-centered cubic austenite that remains after cooling to room temperature without undergoing martensitic or bainitic transformation.

[0033] In some embodiments, the volume fraction of bainite is ≥60%, the volume fraction of retained austenite is 5%~10%, and the remainder is ferrite.

[0034] A bainite volume fraction ≥60% serves as the dominant high-strength matrix, contributing to the material's load-bearing capacity. It also synergistically constrains ferrite grain refinement and regulates the stability of retained austenite. Examples of bainite volume fractions include 60%, 65%, 70%, and 75%. A retained austenite volume fraction between 5% and 10% significantly enhances the material's elongation and work hardening capacity through the TRIP effect (transformation-induced plasticity). Simultaneously, it synergistically optimizes the strength-plasticity balance within the bainite matrix, ensuring a high strength-toughness equilibrium. Examples of retained austenite volume fractions include 5%, 6%, 7%, 8%, 9%, and 10%.

[0035] In some embodiments, the multiphase steel satisfies at least one of the following properties: tensile strength ≥800MPa, yield strength ≥680MPa, elongation ≥18%, hole expansion rate ≥80%, and difference in tensile strength between transverse and longitudinal directions ≤40MPa.

[0036] Tensile strength: The maximum engineering stress a material can withstand in a tensile test. Yield strength: The stress value corresponding to 0.2% plastic deformation of the material. Elongation: The percentage increase in gauge length after the tensile specimen breaks. Hole expansion rate: The percentage increase in diameter of the material when the hole expands to the point of cracking in a hole expansion test. Difference in tensile strength between transverse and longitudinal directions: The absolute value of the difference between the tensile strength of the material in the transverse direction (perpendicular to the rolling direction) and the longitudinal direction (parallel to the rolling direction). For example, the tensile strength can be 800MPa, 820MPa, 840MPa, 860MPa, 880MPa, 900MPa, etc.; the yield strength can be 680MPa, 700MPa, 720MPa, 740MPa, 760MPa, 780MPa, etc.; the elongation can be 18%, 20%, 22%, 24%, 26%, etc.; the hole expansion rate can be 80%, 82%, 84%, 86%, 88%, 90%, etc.; and the difference between transverse and longitudinal tensile strength can be 20MPa, 25MPa, 30MPa, 35MPa, 40MPa, etc.

[0037] Figure 1A flowchart illustrating an 800MPa grade multiphase steel and its preparation method, provided as an embodiment of this application.

[0038] Please see Figure 1 Secondly, this application provides a method for preparing the multiphase steel described in the first aspect, the method comprising: S1. Obtain a slab with the following chemical composition: C: 0.05%~0.09%, Si: 0.2%~0.5%, Mn: 1.4%~1.8%, P≤0.015%, S≤0.003%, Nb: 0.02%~0.05%, Ti: 0.05%~0.10%, Mo: 0.10%~0.25%, and the matrix element Fe; S2. The slab is sequentially heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate; S3. The hot-rolled plate is sequentially subjected to laminar flow cooling, coiling, pickling, and straightening to obtain multiphase steel.

[0039] After pretreatment, the molten iron is smelted in a converter, refined in an LF furnace, refined in an RH furnace, and continuously cast to obtain a slab with the aforementioned chemical composition, ensuring compositional uniformity.

[0040] In some embodiments, the total time the slab spends in the furnace is 180 min to 220 min, and the heating temperature is 1180℃ to 1280℃.

[0041] The total time the slab spends in the furnace includes preheating, heating, and homogenization. The total furnace time is between 180 and 220 minutes to ensure uniform solidification of alloying elements. Examples of possible furnace times include 180, 190, 200, 210, and 220 minutes. Heating temperatures are between 1180°C and 1280°C, allowing for sufficient element diffusion to effectively eliminate compositional segregation remaining from melting, ensuring uniform slab microstructure. Simultaneously, this improves material plasticity and reduces deformation resistance, ensuring stable implementation of subsequent rolling processes. Examples of possible heating temperatures include 1180°C, 1200°C, 1220°C, 1240°C, 1260°C, and 1280°C.

[0042] In some embodiments, the initial rolling temperature of the roughing mill is 1150℃~1220℃, and the final rolling temperature of the roughing mill is 1050℃~1100℃.

[0043] The initial rolling temperature of rough rolling is between 1150℃ and 1220℃ to reduce the rolling load and lay a good foundation for the subsequent finishing rolling process. For example, the initial rolling temperature of rough rolling can be 1150℃, 1170℃, 1190℃, 1210℃, etc. The final rolling temperature of rough rolling is between 1050℃ and 1100℃, which can moderately refine the austenite grains, providing a uniform and fine initial microstructure for finishing rolling. For example, the final rolling temperature of rough rolling can be 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc.

[0044] In some embodiments, the initial rolling temperature of the finishing mill is 1020℃~1080℃, and the final rolling temperature of the finishing mill is 860℃~920℃.

[0045] The initial rolling temperature of the finishing mill is between 1020℃ and 1080℃ to precisely match the final rolling temperature. For example, the initial rolling temperature of the finishing mill can be 1020℃, 1040℃, 1060℃, or 1080℃. The final rolling temperature of the finishing mill is between 860℃ and 920℃ to achieve coordinated control of grain refinement and phase transformation processes, ensuring uniform microstructure. For example, the final rolling temperature of the finishing mill can be 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, or 920℃.

[0046] In some implementations, the laminar flow cooling employs segmented cooling control, which includes front-stage air cooling, mid-stage water cooling, and rear-stage air cooling.

[0047] Laminar flow cooling employs segmented cooling control, precisely regulating the bainitic phase transformation and the stability of retained austenite through intermittent temperature control paths of front-stage air cooling, middle-stage water cooling, and rear-stage air cooling, thereby achieving optimized microstructure ratio and synergistic improvement in strength and plasticity.

[0048] In some embodiments, the initial air cooling time is 2s to 5s, and the initial air cooling temperature is 790℃ to 830℃.

[0049] The initial air cooling time is between 2 and 5 seconds, and the initial air cooling temperature is between 790°C and 830°C. This promotes the deformation nucleation of the ferrite phase and improves its equiaxability, while also guiding the precipitation of Ti and Mo carbides. For example, the initial air cooling time can be 2 seconds, 3 seconds, 4 seconds, or 5 seconds; and the initial air cooling temperature can be 790°C, 800°C, 810°C, 820°C, or 830°C.

[0050] In some embodiments, the temperature of the mid-section water cooling is 530°C to 580°C.

[0051] The temperature of the mid-section water cooling is between 530℃ and 580℃, which inhibits the transformation of undesirable structures such as pearlite and increases the volume fraction of bainite. For example, the temperature of the mid-section water cooling can be 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, etc.

[0052] In some embodiments, the winding temperature is 500°C to 530°C.

[0053] The winding temperature is between 500℃ and 530℃, which promotes the diffusion of carbon into the retained austenite to improve its stability, while reducing the internal stress of the microstructure. For example, the winding temperature can be 500℃, 510℃, 520℃, 530℃, etc.

[0054] In some embodiments, the elongation rate of the acid pickling and straightening is 0.5% to 1.5%.

[0055] The elongation rate of pickling and straightening is between 0.5% and 1.5% to ensure the stability of residual austenite and avoid microstructure inhomogeneity and performance differences caused by local stress concentration. For example, the elongation rate of pickling and straightening can be 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, etc.

[0056] The product prepared by the method of preparing multiphase steel is the multiphase steel described above. Since the method of preparing multiphase steel adopts some or all of the technical solutions of the multiphase steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0057] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0058] The chemical composition (mass percentage / %) of the steels in the examples and comparative examples is shown in Table 1.

[0059] Table 1

[0060] The balance consists of impurities other than N, P, and S, and Fe.

[0061] Based on the chemical composition of the molten steel in the examples and comparative examples, this embodiment also provides a method for preparing 800MPa grade multiphase steel, including the following steps: A slab with the chemical composition described in Table 1 was obtained; The slab is sequentially heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate; The hot-rolled sheet was sequentially subjected to laminar flow cooling, coiling, and pickling and straightening to obtain multiphase steel. The process parameters for preparation are shown in Table 2.

[0062] The production methods and process parameters for the examples and comparative examples are shown in Table 2.

[0063] Table 2

[0064] An air cooling time of 0 indicates that the cooling process is carried out directly after rolling.

[0065] The mechanical properties of the embodiments and comparative examples are shown in Table 3.

[0066] Table 3

[0067] The data tables above provide a clear comparison of the differences between various embodiments and comparative examples. The following conclusions can be drawn: As can be seen from the data in Table 3, the multiphase steel provided in this application has a yield strength of 699MPa~769MPa, a tensile strength of 809MPa~847MPa, an elongation of 18.5%~22.4%, a difference in tensile strength between transverse and longitudinal directions of 14~32MPa, and a hole expansion rate of 82.0%~89.8%.

[0068] As can be seen from Examples 1-6 and Comparative Examples 1-3, the examples exhibit good strength, plasticity, formability, and performance uniformity. However, Comparative Example 1 fails to meet the strength requirements, Comparative Example 2 has poor plasticity and pore-expanding properties, and Comparative Example 3 exhibits relatively large anisotropy, all showing obvious performance defects.

[0069] Appendix Figure 2 Detailed explanation: Figure 2 A micrograph of the multiphase steel microstructure provided in an embodiment of this application. According to... Figure 2 It can be seen that the microstructure of this multiphase steel is dominated by fine-grained bainite, accompanied by fine equiaxed ferrite and retained austenite, which confirms the effectiveness of the segmented cooling process in synergistic effect of strength and plasticity on 800MPa grade multiphase steel.

[0070] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: Performance improvement: Achieves a good match between high strength and high plasticity, with excellent synergistic effect between strength and plasticity; obvious low anisotropy characteristics, small difference between transverse and longitudinal tensile strength, significantly improves stamping stability and increases raw material utilization; Process optimization: By precisely controlling the microstructure ratio through multi-stage cooling, the cost problem caused by high Cr is avoided, and the process window is wide and the yield is high; Wide applicability: It can meet the processing and performance requirements of complex automotive chassis components (such as swing arms and connecting arms), effectively promoting the application of multiphase steel in the automotive industry.

[0071] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A 800 MPa grade multi-phase steel, characterized in that, The chemical composition of the multiphase steel includes, in mass fraction, C: 0.05% to 0.09%, Si: 0.2% to 0.5%, Mn: 1.4% to 1.8%, P: ≤0.015%, S: ≤0.003%, Nb: 0.02% to 0.05%, Ti: 0.05% to 0.10%, Mo: 0.10% to 0.25%, and base element Fe; The microstructure of the multiphase steel includes: bainite, ferrite and residual austenite.

2. The complex phase steel according to claim 1, characterized by The volume fraction of the bainite is ≥60%, the volume fraction of the residual austenite is 5% to 10%, and the rest is ferrite.

3. The complex phase steel as claimed in claim 1, wherein, The multiphase steel satisfies at least one of the following properties: tensile strength ≥800MPa, yield strength ≥680MPa, elongation ≥18%, hole expansion ratio ≥80%, and transverse-longitudinal tensile strength difference ≤40MPa.

4. A method of producing the multiphase steel according to any one of claims 1 to 3, characterized by, The method comprises: obtaining a slab with the chemical composition according to any one of claims 1 to 3; sequentially heating, rough rolling and finish rolling the slab to obtain a hot-rolled plate; sequentially laminar cooling, coiling and pickling straightening the hot-rolled plate to obtain the multiphase steel.

5. The method of claim 4, wherein, The total time of the slab in the furnace is 180min to 220min, and the temperature of the heating is 1180℃ to 1280℃.

6. The method of claim 4, wherein, The rough rolling temperature is 1150℃ to 1220℃, and the finish rolling temperature is 1050℃ to 1100℃.

7. The method of claim 4, wherein, The finish rolling temperature is 1020℃ to 1080℃, and the finish rolling temperature is 860℃ to 920℃.

8. The method of claim 4, wherein, The laminar cooling adopts segmented cooling control, and the segmented cooling includes front air cooling, middle water cooling and rear air cooling; wherein, The time of the front air cooling is 2s to 5s, and the temperature of the front air cooling is 790℃ to 830℃; and / or, The temperature of the middle water cooling is 530℃ to 580℃.

9. The method of claim 4, wherein, The coiling temperature is 500℃ to 530℃.

10. The method of claim 4, wherein, The pickling straightening tension rate is 0.5% to 1.5%.

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