A method and product for improving the comprehensive performance of multiphase steel through organizational regulation.

Abstract

This invention relates to the field of advanced high-strength cold-rolled steel materials for automobiles, specifically to a method and product for improving the comprehensive performance of multiphase steel through microstructure control. The method includes: Step 1: initial microstructure control during hot rolling; Step 2: control through a two-stage cold rolling process followed by a bell-type annealing; Step 3: microstructure control during annealing. The multiphase steel prepared through these steps has a yield strength of 795±20 MPa, a tensile strength of 1255±30 MPa, an elongation after fracture >19.0%, a porosity λ≥45%, and a strength-ductility product >23.0 GPa%. The method used in this invention significantly improves the elongation after fracture and porosity of multiphase steel while simultaneously increasing its yield strength and tensile strength.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength cold-rolled steel materials for automobiles, specifically to a method and product for improving the comprehensive performance of multiphase steel through microstructure regulation. Background Technology

[0002] In recent years, environmental pollution, oil shortages, and climate change have become significant factors restricting the development of gasoline-powered vehicles, which can no longer meet people's transportation needs. New energy vehicles refer to automobiles that use unconventional fuels as their power source or, while using conventional fuels, employ new onboard power systems and advanced technologies in vehicle power control and drive, resulting in vehicles with advanced technological principles, new technologies, and new structures. New energy vehicles provide a completely new "track" for promoting the high-quality, sustainable development, and transformation and upgrading of the automotive industry, and are a trendsetter leading global automotive industry change. Given the unique safety and lightweight requirements of new energy vehicles, reducing vehicle weight and improving collision safety have become key focuses for automakers.

[0003] Ultra-high strength steel refers to steel with a tensile strength greater than or equal to 1000 MPa. This type of steel possesses excellent toughness and plasticity. Applying ultra-high strength steel to the manufacture of new energy vehicles can not only increase the load-bearing capacity of the vehicle structure while reducing its weight, but also absorb and disperse energy, improving the collision safety of the vehicle structure, thus possessing enormous development potential. Among them, ultra-high strength multiphase steel is an ultra-high strength steel whose microstructure mainly consists of ferrite / bainite, with small amounts of martensite, retained austenite, and pearlite. Ultra-high strength multiphase steel has complex microstructures and small strength differences between its phases. It not only has excellent cold forming properties but also superior hole expansion and flanging performance, making it suitable for manufacturing various complex parts. At the same time, ultra-high strength multiphase steel has a high yield strength ratio, which gives it higher deformation resistance compared to high-strength materials of the same strength level. It can absorb more energy during a collision, which helps to further improve the safety of automobiles. It is particularly suitable for the manufacture of safety components and is therefore widely used in chassis suspension parts, B-pillars, bumpers, and other parts of new energy vehicles, attracting high attention from technical researchers in the industry.

[0004] Existing technologies typically employ the method of adding microalloying elements such as Cr, Mo, Nb, and Ti to steel structures to prepare ultra-high strength multiphase steel. While this method can improve the yield strength and tensile strength of ultra-high strength multiphase steel, it is difficult to comprehensively improve the elongation after fracture and the hole expansion rate. Summary of the Invention

[0005] To address the technical problem in existing technologies where the yield strength and tensile strength of ultra-high strength multiphase steel are improved, but the elongation after fracture and the expansion rate are not significantly improved, this invention provides a method and product for improving the comprehensive performance of multiphase steel through microstructure control. By sequentially performing initial microstructure control during hot rolling, two cold rolling + annealing process controls, and annealing microstructure control on the steel billet, the yield strength and tensile strength of the multiphase steel are improved, while its elongation after fracture and expansion rate are significantly increased.

[0006] In a first aspect, the present invention provides a method for improving the comprehensive performance of multiphase steel through microstructure regulation, comprising the following steps:

[0007] Step 1: Initial microstructure control during hot rolling. The initial microstructure control during hot rolling involves sequentially heating the billet, rough rolling, finish rolling, and coiling to obtain a hot-rolled coil. In the heating process, the soaking temperature is 1230±10℃, the soaking time is 35 min, and the total furnace time is 315±5 min. In the rough rolling process, the roughing exit temperature is 1075±15℃, and the final rolling temperature is 880±15℃. In the coiling process, a front-end laminar flow cooling process is adopted, and U-shaped cooling is performed on the head, middle, and tail sections of the rolled plate obtained from the finish rolling process to ensure that the coiling temperature in the middle of the rolled plate is 575±15℃.

[0008] Step Two: The process involves two cold rolling cycles followed by a bell-type annealing. This process involves sequentially pickling, initial cold rolling, bell-type annealing, and a second cold rolling of the hot-rolled coil to obtain a cold-rolled coil with a thickness of 0.8-2.5 mm. After pickling, the surface reflectivity of the hot-rolled coil is ≥70%. In the initial cold rolling process, the reduction rate is 40%-45%. In the bell-type annealing process, the annealing temperature is 690±10℃, and the holding time is 10-12 hours. In the second cold rolling process, the reduction rate is 35%-40%.

[0009] Step 3: Annealing microstructure control. Annealing microstructure control involves continuously annealing the cold-rolled coil obtained in Step 2 to obtain strip steel. The continuous annealing process includes an austenitizing homogenization zone, a slow cooling zone, a rapid cooling zone, an over-aging zone, and a final cooling zone. The homogenization temperature of the austenitizing homogenization zone is 910±10℃; the cooling rate of the slow cooling zone is 5-8℃ / s, and the slow cooling exit temperature is 690±5℃; the cooling rate of the rapid cooling zone is ≥35℃ / s, and the rapid cooling exit temperature is 400±10℃; the over-aging temperature of the over-aging zone is 390±10℃; and the final cooling exit temperature of the final cooling zone is ≤150℃.

[0010] Furthermore, in step one, the chemical composition of the steel billet by mass percentage is: C: 0.10%-0.14%, Si: 0.6%-0.8%, Mn: 2.2%-2.5%, V: 0.19%-0.23%, N: 0.020%-0.032%, Alt: 0.60%-0.80%, B: 0.0025%-0.0030%, P, S, and O are all not greater than 0.005%, and the balance is Fe and other unavoidable impurities.

[0011] Furthermore, in step one, U-shaped cooling involves cooling the head, middle, and tail sections of the rolled plate obtained from the finishing rolling process at different temperatures. The head and tail sections of the rolled plate are 30-35m long, and the cooling temperature of the head and tail sections is 600±15℃.

[0012] Furthermore, in step one, the hot-rolled coil obtained after winding is subjected to slow cooling for 72 hours.

[0013] Furthermore, the hot-rolled coil obtained in step one comprises a ferrite microstructure with a volume fraction of 75%-85% and a pearlite microstructure with a volume fraction of 15%-25%, wherein acicular ferrite accounts for more than 45% of the ferrite microstructure by volume, and polygonal ferrite accounts for 20%-35% of the ferrite microstructure by volume. The ferrite microstructure mainly exists in two forms: acicular ferrite and polygonal ferrite.

[0014] Furthermore, in step three, the running speed of the cold-rolled coil is controlled according to its thickness specifications. Specifically, the running speed of cold-rolled coils with a thickness of 0.8-1.2mm is ≥95m / min; the running speed of cold-rolled coils with a thickness greater than 1.2mm and less than 2.0mm is 90±5m / min; and the running speed of cold-rolled coils with a thickness ≥2.0mm is 85±5m / min.

[0015] Furthermore, the strip steel obtained in step three is leveled. The leveling process adopts a rolling force control mode, with the rolling force controlled at 7800±200KN. After the leveling process, the strip steel is obtained as multiphase steel.

[0016] Furthermore, the strip steel obtained in step three includes a lath bundle bainitic ferrite matrix with a volume fraction of 60%-70%, a proeutectoid ferrite matrix with a volume fraction of 15%-20%, and a lamellar retained austenite matrix with a volume fraction of 15%-20%. The lath bundle bainitic ferrite matrix contains high-density dislocations and V(C,N) phases with a particle diameter ≤9nm.

[0017] The matrix structure of the strip steel is bainitic ferrite, which belongs to the BCC phase structure. As a hard phase, the BCC phase structure has a higher resistance to crack propagation and grain boundary movement during plastic deformation. The grain boundary resistance of the BCC phase is twice that of the FCC phase, making a significant contribution to the improvement of strength during tensile testing. In this invention, the volume fraction of lath bundle bainitic ferrite matrix structure is as high as 60%-70%. The lath bundle bainitic ferrite is distributed in different orientations. These irregularly oriented structures have more grain boundaries. During deformation, when the steel generates a crack initiation and begins to migrate under applied stress, on the one hand, the resistance to the crack tip passing through the grain boundary is large; on the other hand, after passing through the grain boundary, the change in crack propagation direction also requires a large amount of energy. Therefore, by controlling the volume fraction of lath bundle bainitic ferrite in the matrix structure to 60%-70%, it is beneficial to improve the strength and plasticity of the steel.

[0018] Compared to dispersed precipitation, interphase precipitation in strip steel has a superior strengthening effect. The interaction between interphase precipitation and the high-density dislocations in the lath bundle bainitic ferrite matrix leads to a rapid increase in dislocation density during plastic deformation or pore-expanding deformation of the multiphase steel under external force, accompanied by a "dislocation multiplication mechanism." When dislocations in the BCC bainitic phase have the conditions to slip, they migrate towards the interface. Due to the large number of lath bundle bainitic grain boundaries and the complex structure at the phase interface, with a high proportion of large-angle grain boundaries, only a small number of mobile dislocations can cross the phase interface and enter the retained austenite or a small amount of ferrite in the adjacent FCC structure. Most dislocations accumulate at the phase interface. During this process, on the one hand, the bainitic structure, due to its high dislocation density... The softening caused by the decrease in hardness helps to coordinate the strength-ductility difference between the "hard phase" and the "soft phase," which, from a macroscopic performance perspective, can simultaneously improve the strength and ductility of the steel. On the other hand, the entanglement and accumulation of dislocations at the phase interface not only hinders or even stops the movement of dislocations, but also weakens the dislocation multiplication effect, leading to a sudden increase in stress at the phase interface. In other words, this also increases the risk factor of local stress concentration. However, due to the presence of 15%-20% lamellar retained austenite in the microstructure, martensitic transformation can be induced under stress, i.e., the "TRIP effect" occurs. This can effectively release the stress in the local area, and at the same time, it can delay or even inhibit the generation and propagation of microcracks, thereby hindering the generation of necking during plastic deformation and greatly improving the ductility of the steel. As external stress continues to increase, with the formation and propagation of microcracks, when they move to the phase interface, the energy required for the cracks to continue propagating is greatly increased due to the presence of dislocation pile-up and the pinning effect of precipitated particles. This forces the cracks to find a path and continue to move slowly, thereby delaying the occurrence of steel fracture behavior and effectively improving its strength and plasticity.

[0019] Secondly, the present invention provides a multiphase steel prepared by the method described above, wherein the multiphase steel has a yield strength of 795±20MPa, a tensile strength of 1255±30MPa, an elongation after fracture of >19.0%, a porosity λ≥45%, and a strength-ductility product of >23.0GPa.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention first involves hot rolling initial microstructure control of the slab. After hot rolling initial microstructure control, the grains are fully broken during cold rolling, and the distortion stored energy in the matrix increases accordingly, thereby greatly improving the recrystallization driving force. During the subsequent continuous annealing heating process, the recrystallization nucleation and growth of γ grains proceed in the following three stages to ensure the orderliness of the recrystallization process, which is conducive to achieving excellent matching comprehensive performance and at the same time achieving its performance stability.

[0022] First stage: As the heating temperature increases, 15%-25% of the pearlite P structure in the matrix gradually dissolves. The grain boundary energy storage is relatively high at the cementite and ferrite phase interface. In addition, the C content of the cementite phase structure is also high. Therefore, the energy fluctuation, structural fluctuation and concentration fluctuation required for γ recrystallization nucleation are easily satisfied at the two-phase interface. γ first nucleates and grows here.

[0023] Second stage: During the heating process, >45% of the AF structure grows rapidly along the direction perpendicular to the needle tip, causing the "needle-like" shape to gradually disappear and evolve into an irregular polygonal shape, resulting in a change in the structure morphology. At the same time, solid solution C atoms diffuse to the grain boundaries, thus satisfying the conditions required for nucleation at the grain boundaries. γ grains nucleate here and grow along the AF phase interface.

[0024] The third stage: In the PF microstructure with 20%-35% of the average C content, the thermal motion of C atoms can be intensified as the heating temperature increases, which may lead to the existence of high C microregions at grain boundaries or in grains, that is, there are concentration fluctuations in PF grains; at the same time, the irregular arrangement of atoms at grain boundaries and the presence of more crystal defects satisfy the structural fluctuations required for nucleation; in addition, after hot rolling deformation and cold rolling with large reduction, a large amount of distortion energy is accumulated in the grains, which can easily meet the energy fluctuation requirements for nucleation. Therefore, in the high C microregions in the PF microstructure, γ begins to nucleate and grow.

[0025] This invention provides a method and product for improving the comprehensive performance of multiphase steel through microstructure regulation. The method involves regulating the initial microstructure of a hot-rolled slab to a volume fraction of 75%-85% ferrite and 15%-25% pearlite. By subjecting the hot-rolled coil with the regulated initial microstructure to two cold-rolling and annealing processes, a multiphase steel is obtained. This multiphase steel has a matrix of 60%-70% lath bundle bainitic ferrite, and also contains 15%-20% proeutectoid ferrite and 15%-20% lamellar retained austenite. This significantly improves the yield strength and tensile strength of the multiphase steel, while also enhancing its elongation after fracture, porosity, and strength-ductility. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a microstructure diagram of the steel billet after step one in Example 1.

[0028] Figure 2 This is a diagram of the acicular ferrite structure in the ferrite structure of the steel billet after step one in Example 1.

[0029] Figure 3 This is a diagram of the pearlite structure in the ferrite structure of the steel billet after step one in Example 1.

[0030] Figure 4 This is an enlarged view of the pearlite structure in the ferrite structure of the steel billet after step one treatment in Example 1.

[0031] Figure 5 This is a microscopic scanning image of the multiphase steel in Example 1.

[0032] Figure 6 This is a bright-field morphology image of the lamellar residual austenite structure in the multiphase steel of Example 1.

[0033] Figure 7 This is a dark field morphology image of the lamellar residual austenite structure in the multiphase steel in Example 1.

[0034] Figure 8 yes Figure 6 The diffraction pattern of the lamellar residual austenite structure marked by the middle circle.

[0035] Figure 9This is a morphology diagram of the V(C,N) phase precipitates in the lath bundle bainitic ferrite matrix of the multiphase steel in Example 1.

[0036] Figure 10 This is a diagram of the high-density dislocation morphology in the lath bundle bainitic ferrite matrix of the multiphase steel in Example 1.

[0037] In the figure, PF represents polygonal ferrite; AF represents acicular ferrite; P represents pearlite; F represents ferrite; and B represents lath bundle bainite. Detailed Implementation

[0038] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0039] The present invention employs the following method to prepare multiphase steel:

[0040] Step 1: Initial Microstructure Control in Hot Rolling. The initial microstructure control of hot rolling involves sequentially heating the billet, roughing, finishing, and coiling to obtain a hot-rolled coil. In the heating process, the soaking temperature is 1230±10℃, the soaking time is 35 min, and the total furnace time is 315±5 min. In the roughing process, the exit temperature is 1075±15℃, and the final rolling temperature is 880±15℃. The pass allocation and reduction rate for each pass in the roughing and finishing processes are calculated by the secondary system roughing setting model. In the coiling process, a front-end laminar flow cooling process is adopted. U-shaped cooling is performed on the head, middle, and tail sections of the rolled plate obtained from the finishing process. The cooling temperature of the head and tail sections is controlled at 600±15℃, and the coiling temperature in the middle section is controlled at 575±15℃. The head and tail sections of the rolled plate are 30-35 m long. The hot-rolled coil obtained after coiling is then subjected to slow cooling for 72 hours.

[0041] Step Two: The process involves two cold rolling cycles followed by a bell-type annealing. This process involves sequentially pickling, initial cold rolling, bell-type annealing, and a second cold rolling of the hot-rolled coil to obtain a cold-rolled coil with a thickness of 0.8-2.5 mm. After pickling, the surface reflectivity of the hot-rolled coil is ≥70%. In the initial cold rolling process, the reduction rate is 40%-45%, resulting in an pickled coil. In the bell-type annealing process, the annealing temperature is 690±10℃, and the holding time is 10-12 hours. In the second cold rolling process, the reduction rate is 35%-40%.

[0042] Step 3: Annealing microstructure control. Annealing microstructure control involves continuously annealing the cold-rolled coil obtained in Step 2 to obtain strip steel. The continuous annealing process includes an austenitizing homogenization zone, a slow cooling zone, a rapid cooling zone, an over-aging zone, and a final cooling zone. The homogenization temperature of the austenitizing homogenization zone is 910±10℃; the cooling rate of the slow cooling zone is 5-8℃ / s, and the slow cooling exit temperature is 690±5℃; the cooling rate of the rapid cooling zone is ≥35℃ / s, and the rapid cooling exit temperature is 400±10℃; the over-aging temperature of the over-aging zone is 390±10℃; and the final cooling exit temperature of the final cooling zone is ≤150℃. In the continuous annealing process, the running speed of the cold-rolled coil is controlled according to its thickness specifications. Specifically, the running speed of cold-rolled coils with a thickness of 0.8-1.2mm is ≥95m / min; the running speed of cold-rolled coils with a thickness greater than 1.2mm and less than 2.0mm is 90±5m / min; and the running speed of cold-rolled coils with a thickness ≥2.0mm is 85±5m / min.

[0043] Step 4: The strip steel obtained in Step 3 is leveled. The leveling process adopts the rolling force control mode, and the rolling force is controlled at 7800±200KN. After the strip steel is processed by the leveling process, the multiphase steel product is obtained.

[0044] Step 5: Samples of the hot-rolled coils obtained in Step 1 and the multiphase steel products obtained in Step 4 are taken for microstructure analysis and mechanical property testing.

[0045] The chemical composition and corresponding mass percentage of the steel billets used in Step 1 of Examples 1-8 are shown in Table 1. The specific process parameters for preparing the multiphase steel in Examples 1-8 are shown in Table 2. The microstructure analysis and mechanical property test results of the hot-rolled coils obtained in Step 1 and the multiphase steel obtained in Step 3 of Examples 1-8 are shown in Table 3. In Table 3, PF is the volume fraction of polygonal ferrite, AF is the volume fraction of acicular ferrite, P is the volume fraction of pearlite, F is the volume fraction of ferrite, B is the volume fraction of lath bundle bainite, and γ is the volume fraction of lamellar retained austenite. The microstructure diagrams, acicular ferrite diagrams, pearlite diagrams, and magnified pearlite diagrams of the steel billets after Step 1 in Example 1 are shown in Table 3. Figure 1 , Figure 2 , Figure 3 , Figure 4 As shown, the microstructure of the multiphase steel product obtained in step four of Example 1 includes: a bright-field morphology of the lamellar retained austenite structure; a dark-field morphology of the lamellar retained austenite structure; a diffraction pattern of the lamellar retained austenite structure; a morphology of V(C,N) phase intercalation in the lath bundle bainitic ferrite matrix; and a morphology of high-density dislocations in the lath bundle bainitic ferrite matrix. Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 As shown.

[0046] Table 1. Chemical composition and corresponding mass percentage of steel billets in Examples 1-8

[0047]

[0048] Table 2. Specific process parameters for preparing multiphase steel in Examples 1-8

[0049]

[0050] Table 3. Microstructure analysis and mechanical property test results of multiphase steel

[0051]

[0052] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for improving the comprehensive performance of multiphase steel through microstructure regulation, characterized in that, Includes the following steps: Step 1: Initial microstructure control during hot rolling. The initial microstructure control during hot rolling involves sequentially heating the billet, rough rolling, finish rolling, and coiling to obtain a hot-rolled coil. In the heating process, the soaking temperature is 1230±10℃, the soaking time is 35 min, and the total furnace time is 315±5 min. In the rough rolling process, the roughing exit temperature is 1075±15℃, and the final rolling temperature is 880±15℃. In the coiling process, a front-end laminar flow cooling process is adopted, and U-shaped cooling is performed on the head, middle, and tail sections of the rolled plate obtained from the finish rolling process to ensure that the coiling temperature in the middle of the rolled plate is 575±15℃. Step Two: The process involves two cold rolling cycles followed by a bell-type annealing. This process involves sequentially pickling, initial cold rolling, bell-type annealing, and a second cold rolling of the hot-rolled coil to obtain a cold-rolled coil with a thickness of 0.8-2.5 mm. After pickling, the surface reflectivity of the hot-rolled coil is ≥70%. In the initial cold rolling process, the reduction rate is 40%-45%. In the bell-type annealing process, the annealing temperature is 690±10℃, and the holding time is 10-12 hours. In the second cold rolling process, the reduction rate is 35%-40%. Step 3: Annealing microstructure control. Annealing microstructure control involves continuously annealing the cold-rolled coil obtained in Step 2 to obtain strip steel. The continuous annealing process includes, in sequence, an austenitizing homogenization zone, a slow cooling zone, a rapid cooling zone, an over-aging zone, and a final cooling zone. The homogenization temperature of the austenitizing zone is 910±10℃; the cooling rate of the slow cooling zone is 5-8℃ / s, and the slow cooling exit temperature is 690±5℃; the cooling rate of the rapid cooling zone is ≥35℃ / s, and the rapid cooling exit temperature is 400±10℃; the over-aging temperature of the over-aging zone is 390±10℃; and the final cooling exit temperature of the final cooling zone is ≤150℃. The strip steel obtained in step three includes a lath bundle bainitic ferrite matrix with a volume fraction of 60%-70%, a proeutectoid ferrite with a volume fraction of 15%-20%, and a lamellar retained austenite with a volume fraction of 15%-20%. The lath bundle bainitic ferrite matrix contains high-density dislocations and V(C,N) phases with a particle diameter ≤9nm. In step one, the chemical composition of the steel billet by mass percentage is: C: 0.10%-0.14%, Si: 0.6%-0.8%, Mn: 2.2%-2.5%, V: 0.19%-0.23%, N: 0.020%-0.032%, Alt: 0.60%-0.80%, B: 0.0025%-0.0030%, P, S, and O are all not greater than 0.005%, and the balance is Fe and other unavoidable impurities.

2. The method for improving the comprehensive performance of multiphase steel through microstructure regulation as described in claim 1, characterized in that, In step one, U-shaped cooling involves cooling the head, middle and tail sections of the rolled plate obtained from the finishing rolling process at different temperatures. The head and tail sections of the rolled plate are 30-35m long, and the cooling temperature of the head and tail sections is 600±15℃.

3. The method for improving the comprehensive performance of multiphase steel through microstructure regulation as described in claim 1, characterized in that, In step one, the hot-rolled coil obtained after winding is slowly cooled for 72 hours.

4. The method for improving the comprehensive performance of multiphase steel through microstructure regulation as described in claim 1, characterized in that, The hot-rolled coil obtained in step one includes a ferrite structure with a volume fraction of 75%-85% and a pearlite structure with a volume fraction of 15%-25%, wherein the acicular ferrite structure accounts for more than 45% of the volume fraction of the ferrite structure and the polygonal ferrite structure accounts for 20%-35% of the volume fraction of the ferrite structure.

5. The method for improving the comprehensive performance of multiphase steel through microstructure regulation as described in claim 1, characterized in that, In step three, the running speed of the cold-rolled coil is controlled according to its thickness specifications. Specifically, the running speed of cold-rolled coils with a thickness of 0.8-1.2mm is ≥95m / min; the running speed of cold-rolled coils with a thickness greater than 1.2mm and less than 2.0mm is 90±5m / min; and the running speed of cold-rolled coils with a thickness ≥2.0mm is 85±5m / min.

6. The method for improving the comprehensive performance of multiphase steel through microstructure regulation as described in claim 1, characterized in that, The strip steel obtained in step three is leveled. The leveling process adopts a rolling force control mode, with the rolling force controlled at 7800±200KN. After the leveling process, the strip steel is obtained as multiphase steel.

7. A multiphase steel prepared by the method according to any one of claims 1-6, characterized in that, The yield strength of the multiphase steel is 795±20MPa, the tensile strength is 1255±30MPa, the elongation after fracture is >19.0%, the porosity λ≥45%, and the strength-ductility product is >23.0GPa.

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