A cold-rolled steel sheet and a method of manufacturing and use thereof

Abstract

The application discloses a cold-rolled steel plate and a preparation method and application thereof, and relates to the technical field of automobile steel manufacturing. The cold-rolled steel plate adopts a C-Si-Mn basic system composition, fully utilizes the phase change law of basic alloy elements, stabilizes residual austenite by strengthening C instead of Mn, and simultaneously adds a small amount of Ti and Nb elements to obtain a fine martensite matrix structure. In the preparation method, the first continuous annealing obtains a fine lath structure, and the second continuous annealing promotes the growth of reverse transformation austenite on the lath structure to obtain thin film-shaped stable residual austenite with high content, uniform distribution and submicron level. The preparation method is suitable for conventional and batch production on the existing production line, and is helpful to the manufacture of vehicle structural parts with complex shapes and high formability, such as A / B columns, longitudinal beams, bumpers and door anti-collision rods.

Description

Technical Field

[0001] This invention relates to the field of high-strength and high-ductility automotive steel manufacturing technology, and more specifically, to a cold-rolled steel sheet, its preparation method, and its application. Background Technology

[0002] As the automotive industry further increases its requirements for safety performance during service and forming performance during manufacturing, automotive steel must achieve high strength-ductility product. Traditional high-strength and high-ductility automotive steels are mainly represented by quenched ductile steel, medium-manganese steel, and nano-bainitic steel. However, medium-manganese steel relies on the addition of high Mn to obtain more retained austenite to achieve high strength and ductility, but the excessively high Mn content leads to brittleness and Mn segregation in the cast billet during continuous casting, making industrial production difficult; nano-bainitic steel has a high carbon content, resulting in poor welding and processing performance; only quenched ductile steel is currently the third generation of steel that has achieved large-scale industrial production.

[0003] Currently, commercially available quenched ductile steels have low alloy content, with retained austenite content mostly below 15%, and the retained austenite is mostly distributed in blocks. Its stability is low, and it is difficult to obtain ultra-high elongation while ensuring gigap strength, so as to meet the requirement of 30 GPa·% strength-ductility product.

[0004] To prepare 30 GPa·% high strength and ductility product steel, existing technologies often employ high Mn, high C, or the addition of a large amount of precious alloying elements. For example, Chinese patent CN110055466A proposes a method for manufacturing hot-rolled high-strength medium-manganese steel with a high strength and ductility product of 30 GPa·% and a chemical composition of C: 0.18–0.22%, Si: 0.10–0.30%, Mn: 4.80–5.20%, Al: 0.02–0.05%, P≤0.020%, S≤0.0030%, with the balance being Fe and unavoidable impurities. Although the product described in this patent has superior performance, its Mn content far exceeds the capacity for stable industrial production.

[0005] Chinese patent CN 106244924A proposes "a cold-rolled quenched ductile steel and its preparation method", with the following chemical composition: C: 0.18-0.23%; Mn: 1.5-2.2%; Si: 1.3-1.8%; P: ≤0.02%; S: ≤0.008%; Nb: 0-0.05%; Ti: 0-0.1%, with the balance being Fe and unavoidable impurities. Although the chemical composition of this patent is relatively simple, the strength-ductility product of the final annealed product is only about 25 GPa·%, and the aging time in the annealing process is relatively long, reaching 400-1000 s, which increases the difficulty of industrial application.

[0006] Therefore, how to prepare cold-rolled steel sheets with higher strength and ductility, so that the cold-rolled steel sheets have excellent mechanical properties and are easy to produce in a routine and mass manner, has become a technical problem that urgently needs to be solved in this field.

[0007] In view of this, the present invention is proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a cold-rolled steel sheet, its preparation method, and its application, so as to solve or improve the above-mentioned technical problems.

[0009] This invention is implemented as follows: In a first aspect, embodiments of the present invention provide a cold-rolled steel sheet, the chemical composition of which, by mass percentage, comprises the following components: C: 0.25wt%-0.33wt%, Mn: 1.5wt%-2.5wt%, Si: 1.5wt%-2.0wt%, Cr: 0wt%-0.3wt%, Ti: 0wt%-0.03wt%, Nb: 0wt%-0.03wt%, N≤0.005wt%, P≤0.01wt%, S≤0.005wt%, balance Fe and unavoidable impurities; wherein, C and Mn satisfy the following relationship: C+Mn≤2.7wt%; The microstructure of cold-rolled steel sheet includes network martensite and retained austenite, with the volume fraction of network martensite >60%, and its basic unit is martensitic strips; The retained austenite is distributed along the interface of the martensite bands, forming a thin film, and the volume fraction of the thin film austenite is ≥80%, with the equivalent circle diameter of the retained austenite ≤1μm.

[0010] Secondly, embodiments of the present invention provide a method for preparing cold-rolled steel sheet as described in any of the foregoing embodiments, comprising the following steps: The cold-rolled hardened coil is subjected to a first continuous annealing and a second continuous annealing in sequence.

[0011] Thirdly, embodiments of the present invention provide the application of cold-rolled steel sheet as described in any of the foregoing embodiments or cold-rolled steel sheet prepared by the preparation method as described in any of the foregoing embodiments in vehicle structural components, wherein the vehicle structural components are selected from at least one of A / B pillars, longitudinal beams, bumpers, and door anti-collision bars.

[0012] The present invention has the following beneficial effects: The cold-rolled steel sheet provided in this invention, including its chemical composition and preparation method, is suitable for routine, mass production on existing production lines. It facilitates the manufacture of complex-shaped, highly formable vehicle structural components, such as A / B pillars, longitudinal beams, bumpers, and door anti-collision bars. A detailed analysis follows: Cold-rolled steel sheets adopt a simple C-Si-Mn basic system composition, making full use of the phase transformation law of these three basic alloying elements. The residual austenite is stabilized mainly by an appropriate amount of strengthening element C rather than Mn, while a small amount of Ti, Nb and other elements are added to obtain a fine martensitic matrix structure.

[0013] The method for preparing cold-rolled steel sheets provided in this invention involves two consecutive annealing processes, which differs from conventional single-phase continuous annealing. Specifically, the first continuous annealing yields a fine lath structure, which is beneficial for providing high matrix strength. The second continuous annealing promotes the growth of reverse-transformed austenite on the lath structure, resulting in thin-film, stable, high-content, uniformly distributed, and submicron-sized retained austenite. This is more effective in alleviating stress concentration in the network martensite matrix during deformation because the transformation-induced plasticity (TRIP) effect is more pronounced during deformation. Furthermore, the low ferrite content, as a soft phase, further improves the strength and plasticity of the product, promoting lightweighting of automotive steel. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 These are microscopic tissue photographs of Example 3 of the present invention; Figure 2 This is an EBSD diagram of Embodiment 3 of the present invention; Figure 3 This is a diagram showing the distribution of the equivalent circle diameter of the retained austenite in Embodiment 3 of the present invention; Figure 4 This is a diagram showing the distribution of the equivalent circle diameter of the martensite stripes in Embodiment 3 of the present invention. Figure 5 This is a microscopic tissue photograph of Comparative Example 14 of the present invention; Figure 6 The following is a process flow diagram of the first and second continuous annealing in the cold-rolled steel sheet preparation method of the present invention: the left diagram shows the first continuous annealing; the right diagram shows the second continuous annealing. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0017] The cold-rolled steel sheet provided in this invention achieves a reasonable distribution of microstructure through rational design of its chemical composition. The preparation method is suitable for routine, mass production on existing production lines, facilitating the manufacture of complex-shaped, highly formable vehicle structural components, such as A / B pillars, longitudinal beams, bumpers, and door anti-collision bars. The specific implementation process is analyzed below: In a first aspect, embodiments of the present invention provide a cold-rolled steel sheet, the chemical composition of which, by mass percentage, comprises the following components: C: 0.25wt%-0.35wt%, Mn: 1.5wt%-2.5wt%, Si: 1.5wt%-2.0wt%, Cr: 0wt%-0.3wt%, Ti: 0wt%-0.03wt%, Nb: 0wt%-0.03wt%, N≤0.005wt%, P≤0.01wt%, S≤0.005wt%, balance Fe and unavoidable impurities; wherein, C and Mn satisfy the following relationship: C+Mn≤2.7wt%; The microstructure of cold-rolled steel sheet includes network martensite and retained austenite, with the volume fraction of network martensite >60%, and its basic unit is martensitic strips; The retained austenite is distributed along the interface of the martensite bands, forming a thin film, and the volume fraction of the thin film austenite is ≥80%, with the equivalent circle diameter of the retained austenite ≤1μm.

[0018] Among them, network refers to the continuous distribution of the microstructure along the grains or phase boundaries, which are interconnected to form a network or vein-like microstructure; martensite bands refer to the martensite matrix distributed in bands, with the smallest unit being a martensite band.

[0019] The cold-rolled steel sheet provided in this embodiment of the invention adopts a simple C-Si-Mn basic system composition, making full use of the phase transformation law of these three basic alloying elements. It mainly stabilizes the residual austenite by using an appropriate amount of strengthening element C rather than Mn, avoiding the reduction of manufacturability due to high Mn content in the alloy. At the same time, a small amount of Ti, Nb and other elements are added to obtain a fine martensitic matrix structure.

[0020] Specifically, the chemical composition design of the cold-rolled steel sheet provided by this invention has the following functional characteristics: Carbon (C) promotes lath structure formation during the first annealing process; higher C content results in finer laths and helps refine the network martensite matrix structure after the second annealing. Sufficient C facilitates rapid partitioning of the network martensite matrix with the surrounding thin-film austenite, thereby increasing the austenite content and stability, and better leveraging the TRIP effect of retained austenite to improve plasticity. However, excessively high carbon content can easily induce high levels of twinned martensite. Twinned martensite has high strength but significantly reduces the plasticity of the steel, and also greatly reduces weldability. Insufficiently low carbon content leads to low martensite content and high ferrite content, resulting in low tensile strength and strength-ductility product. Therefore, in this invention, the C content is controlled between 0.25wt% and 0.35wt%.

[0021] Mn: Mn promotes the distribution of the network martensite matrix and the surrounding thin film austenite during the second annealing, thereby stabilizing the high content of thin film austenite. However, excessive Mn content can easily lead to rapid growth of austenite, reducing the mechanical stability of austenite and also reducing plasticity. Therefore, in this invention, the Mn content is controlled at 1.5wt%-2.5wt%.

[0022] C and Mn satisfy the relationship C+Mn≤2.7wt%. On the one hand, controlling the C and Mn content to a sufficient amount ensures strength while maintaining a moderately high level of stability of residual austenite to exert the TRIP effect and achieve a high strength-ductility product. On the other hand, excessive C and Mn content will lead to an imbalance in the austenite phase transformation of the film, and the austenite morphology will easily change from film to block, with the size increasing from submicron to micron, thereby reducing plasticity.

[0023] Si: Si improves the stability of residual austenite by inhibiting the decomposition of C-rich thin-film austenite during the second annealing process. In addition, Si also improves the strength of the network martensite matrix. However, excessive Si content can also induce matrix brittleness. Therefore, the Si content in this invention is controlled at 1.5wt%-2.0wt%.

[0024] Cr: Cr can promote the formation of fine lath structure and strengthen the network martensite matrix, but if the Cr content is too high, it is easy for C to react and form carbides, which consumes C in the retained austenite and reduces the stability of the retained austenite. Therefore, the Cr content in this invention is controlled at 0wt%-0.3wt%.

[0025] Ti and Nb: Ti can remove free N in steel and react with C to form nano-sized TiC particles, resulting in precipitation strengthening. At the same time, Nb can also refine the microstructure through precipitation strengthening, thereby controlling the size of martensite bands in the microstructure to within 2 μm. Microstructure refinement helps to improve strength and plasticity, but excessive Ti and Nb will lead to a significant increase in rolling resistance. Therefore, in this invention, the content of Ti and Nb is controlled at 0 wt%-0.03 wt%.

[0026] Elements S, P, and N are impurity elements in cold-rolled steel sheets. They can impair the plasticity of steel by segregating at grain boundaries and forming harmful inclusions. The lower their content, the better.

[0027] In an optional embodiment, the microstructure of the cold-rolled steel sheet, by volume fraction, comprises: 65%-85% network martensite, 10%-20% retained austenite, and 0%-15% ferrite.

[0028] It should be noted that the fine lath structure obtained in the first continuous annealing provides nucleation sites for the thin-film austenite in the second continuous annealing. The retained austenite is further promoted in the second continuous annealing. Specifically, the reverse-transformed austenite grows on the lath structure, making it easier to obtain thin-film, stable, high-content, uniformly distributed, and submicron-sized retained austenite. This is more effective in alleviating stress concentration in the network martensite matrix during deformation, as its TRIP effect is more pronounced during deformation. Furthermore, the low ferrite content, as a soft phase, further improves the strength and plasticity of the product, promoting lightweighting of automotive steel.

[0029] Residual austenite distributed within ferrite or at ferrite grain boundaries undergoes martensitic transformation earlier during plastic deformation; while residual austenite distributed between martensitic strips is more stable and can gradually undergo martensitic transformation with increasing strain. This is the basis of the microstructure design of this invention.

[0030] In this invention, the retained austenite mainly grows on a lath structure, resulting in a film-like, stable, high-content austenite that is more effective in the TRIP effect during deformation, while the network martensitic matrix provides strength. Furthermore, a certain amount of ferrite between the martensitic matrix also ensures excellent plasticity; however, excessive ferrite content leads to a strength below 1000 MPa, therefore the ferrite content is controlled between 0% and 15%.

[0031] In an optional embodiment, the retained austenite is distributed along the martensite strip interface in the form of a thin film, and the volume fraction of the thin film austenite is ≥80%, and the equivalent circle diameter of the retained austenite is ≤1μm.

[0032] It should be noted that the mechanical stability of thin-film austenite is significantly higher than that of blocky austenite. It can continuously exhibit the TRIP effect during deformation, alleviating stress concentration, delaying crack initiation and propagation, and thus contributing to high elongation. Specifically, the key guarantee for austenite stability and deformability lies in the distribution of austenite along the martensite strip interface, ensuring a volume fraction of thin-film austenite ≥80% and an equivalent circle diameter of retained austenite ≤1μm. This is because only austenite at the martensite strip interface can adequately provide "lubrication" for the deformation of the martensite matrix, while the fine, thin-film austenite can maintain high mechanical stability during deformation and exert a continuous TRIP effect to achieve high elongation.

[0033] In an optional embodiment, the microstructure of the cold-rolled steel sheet has at least one of the following characteristics: Feature 1: The average width of the martensitic strips is ≤2μm; Feature 2: The equivalent circle diameter of the martensitic strips is ≤2μm.

[0034] It should be noted that the average width of the martensite strips is set to be ≤2μm and the equivalent circle diameter of the martensite strips is set to be ≤2μm. This is because only when the martensite strips are sufficiently fine can the coordinated deformation capacity of the martensite matrix be guaranteed. If the average width of the martensite strips is >2μm and / or the equivalent circle diameter is >2μm, the plasticity of the martensite matrix itself is greatly reduced. In this case, even if the thin film austenite can accommodate deformation through the TRIP effect, the martensite matrix will induce stress concentration and cracking within a certain strain range, resulting in poor elongation.

[0035] In an optional embodiment, the chemical composition of the cold-rolled steel sheet, by mass percentage, further includes at least one of the following components: B: 0wt%-0.005wt%, Cu: 0wt%-0.5wt%, Ni: 0wt%-0.5wt%, Mo: 0wt%-0.2wt%, V: 0.0001wt%-0.20wt%, Zr: 0.0001wt%-0.20wt%, W: 0.000 1wt%-0.20wt%, Ca: 0wt%-0.050wt%, Ce: 0.0001wt%-0.0050wt%, La: 0.0001wt%-0.0050wt%, Sn: 0wt%-0.1wt% and Mg: 0wt%-0.0030wt%.

[0036] B: B can increase the bonding force of grain boundaries. The network martensite matrix contains a large number of grain boundaries. The addition of B element is crucial to improving the elongation of the network martensite matrix. However, if the B content is too high, it is easy to form brittle phases such as BN. Therefore, the B content in this invention is controlled at 0wt%-0.005wt%.

[0037] Cu and Ni: Cu and Ni can promote the formation of austenite to a certain extent and have a certain effect on increasing the content of thin film austenite after two annealings. However, Cu and Ni elements significantly increase the cost of alloys. Therefore, the content of Cu and Ni in this invention is controlled at 0wt%-0.5wt%.

[0038] Mo: Mo can refine the network martensite matrix structure through segregation and increase the bonding force of grain boundaries, both of which can improve the elongation of the network martensite matrix. However, excessive Mo will lead to an increase in rolling deformation resistance. Therefore, the Mo content in this invention is controlled at 0wt%-0.2wt%.

[0039] V, Zr and W: These elements can refine the network martensite matrix structure through nanoscale precipitation and improve the coordinated deformation ability of the network martensite matrix. However, these elements will increase the cost of the alloy. Therefore, the contents of V, Zr and W in this invention are controlled at 0.0001wt%-0.20wt% respectively.

[0040] Ca, Ce, La, and Sn: These elements increase grain boundary bonding by preventing impurity elements such as P from segregating at grain boundaries, thereby enhancing the plasticity of the network martensite matrix. In addition, they can reduce the size of inclusions in steel to further optimize the elongation of the material and obtain a high strength-ductility product. However, adding too many elements can lead to problems such as smelting difficulties and reduce manufacturability. Therefore, in this invention, the contents of the above elements are controlled as follows: Ca: 0wt%-0.050wt%, Ce: 0.0001wt%-0.0050wt%, La: 0.0001wt%-0.0050wt%, and Sn: 0wt%-0.1wt%.

[0041] Mg: Mg can reduce the size of inclusions in steel to optimize the elongation and strength-ductility product of steel, but a higher Mg content will significantly increase the difficulty of steelmaking. Therefore, the Mg content in this invention is controlled at 0wt%-0.0030wt%.

[0042] In an optional embodiment, the yield strength of the cold-rolled steel sheet is 600MPa-850MPa, and the tensile strength is 980MPa-1100MPa. 50 The total elongation is 30%-36%, and the strength-plasticity product is ≥30GPa·.

[0043] Secondly, embodiments of the present invention provide a method for preparing cold-rolled steel sheet as described in any of the foregoing embodiments, comprising the following steps: The cold-rolled hardened coil is subjected to a first continuous annealing and a second continuous annealing. The process flow diagrams for the first and second continuous annealing are shown below. Figure 6 The left image shows the first continuous annealing; the right image shows the second continuous annealing.

[0044] In an optional implementation, the first continuous annealing is selected from any of the following processes: Process 1 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, and then rapidly cooling it to room temperature at a cooling rate of 30℃ / s-60℃ / s to obtain the annealed coil. Process 2 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 190℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain the annealed coil. The second continuous annealing process includes the following steps: heating the annealed coil to Ac1 to Ac3-50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 380℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain a cold-rolled steel sheet.

[0045] It should be noted that the first continuous annealing, which produces a fine lath structure, serves to provide nucleation sites for the thin-film austenite in the second annealing. Specifically, the first continuous annealing temperature is controlled between Ac3-30℃ and Ac3+50℃, primarily to allow the alloy to austenitize slightly below or above Ac3, followed by rapid cooling to room temperature or 190℃-250℃ to promote the formation of the fine lath structure. Maintaining a rapid cooling temperature below 250℃ further promotes the formation of the fine lath structure. Simultaneously, the heating temperature should not exceed Ac3+50℃ to avoid abnormal growth of austenite at high temperatures. Here, Ac3 is the austenite transformation termination temperature.

[0046] The rapid cooling temperature of the second annealing should still be maintained within the range of 190℃-250℃, close to the formation of the fine lath structure, to promote the transformation of unstable austenite, which is detrimental to mechanical properties. Simultaneously, the reheating temperature should be maintained within the range of 380℃-420℃ to promote the distribution of C and Mn elements from martensite to austenite, thereby improving the stability of austenite. Specifically, the second continuous annealing temperature is controlled between Ac1 and Ac3-50℃. The purpose is to control the austenite to obtain a large number of nucleation sites along the interface of the fine lath structure obtained in the first annealing at a lower annealing temperature, resulting in a predominantly thin-film morphology. At this point, the austenite size is small, and C and Mn rapidly distribute within the smaller austenite to obtain a large amount of stable retained austenite. This austenite exhibits high thermodynamic and mechanical stability and displays high homogeneity in its microstructure. At the same time, the second continuous annealing should avoid the formation of large-sized blocky austenite at high annealing temperatures, which would reduce the product's plasticity and strength-ductility product. Ac1 is the austenite transformation initiation temperature.

[0047] In an optional embodiment, the preparation method further includes the following steps: Smelting and continuous casting, hot rolling, pickling and cold rolling; Hot rolling includes the following process: heating temperature is 1150℃-1250℃, furnace time is 30min-90min, slab is hot rolled after exiting the heating furnace, and the final rolling temperature is controlled at 870℃-920℃. After hot rolling, laminar flow cooling and coiling are carried out, with the cooling rate controlled at 10℃ / s-40℃ / s and the coiling temperature at 450℃-600℃.

[0048] Furthermore, the heating temperature is 1180℃-1230℃, and the winding temperature is 550℃-600℃.

[0049] It should be noted that, in the embodiments of the present invention, smelting and continuous casting are carried out according to the chemical composition of cold-rolled steel plates, and the steel plates are cast into slabs with a thickness of 30mm-50mm at the export slab; the deformation of cold rolling is 40%-60%. The thickness of the export slab and the deformation of cold rolling are reasonably adjusted according to actual needs.

[0050] The hot-rolled coils are then pickled and cold-rolled. Pickling is used to remove the iron oxide scale and rust layer that are generated during the transportation or storage of the hot-rolled coils, making their surface smooth. If impurities such as oxide scale or rust layer are not removed, they may be pressed into the surface of the steel plate during the cold rolling process, resulting in defects such as strip breakage, steel sticking, and black spots, which in turn affect the continuity of production and the yield.

[0051] In summary, the method for preparing cold-rolled steel sheet provided in this embodiment of the invention includes the following steps: (1) Smelting and continuous casting According to the chemical composition of cold-rolled steel sheet, it is smelted and cast into slabs. The thickness of the slabs obtained after continuous casting is 30mm-50mm. (2) Hot rolling Hot rolling includes the following process: heating temperature is 1180℃-1230℃, furnace time is 30min-90min, slab is hot rolled after exiting the heating furnace, and the final rolling temperature is controlled at 870℃-920℃.

[0052] After hot rolling, laminar flow cooling and coiling are carried out, with the cooling rate controlled at 10℃ / s-40℃ / s and the coiling temperature at 550℃-600℃, to obtain hot-rolled coils.

[0053] (3) Pickling and cold rolling The hot-rolled coil obtained in step (2) is pickled and cold-rolled to obtain a hard-rolled coil. The deformation during cold rolling is 40%-60%.

[0054] (4) First continuous annealing The first continuous annealing is selected from any of the following processes: Process 1 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, and then rapidly cooling it to room temperature at a cooling rate of 30℃ / s-60℃ / s to obtain the annealed coil. Process 2 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 190℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain the annealed coil.

[0055] (5) Second continuous annealing The second continuous annealing process includes the following steps: heating the annealed coil to Ac1 to Ac3-50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 380℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain a cold-rolled steel sheet.

[0056] Thirdly, embodiments of the present invention provide the application of cold-rolled steel sheet as described in any of the foregoing embodiments or cold-rolled steel sheet prepared by the preparation method as described in any of the foregoing embodiments in vehicle structural components, wherein the vehicle structural components are selected from at least one of A / B pillars, longitudinal beams, bumpers, and door anti-collision bars.

[0057] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0058] Examples 1-20 and Comparative Examples 1-11 The preparation method of cold-rolled steel sheet with a strength-ductility product of 30 GPa·% provided in Examples 1-20 and Comparative Examples 1-11 of the present invention includes the following steps: (1) Smelting and continuous casting The cold-rolled steel sheet is smelted according to its chemical composition and cast into slabs. The thickness of the slabs obtained after continuous casting is 30mm-50mm.

[0059] (2) Hot rolling Hot rolling includes the following process: heating temperature is 1180℃-1230℃, furnace time is 30min-90min, slab is hot rolled after exiting the heating furnace, and the final rolling temperature is controlled at 870℃-920℃.

[0060] After hot rolling, laminar flow cooling and coiling are carried out, with the cooling rate controlled at 10℃ / s-40℃ / s and the coiling temperature at 550℃-600℃, to obtain hot-rolled coils.

[0061] (3) Pickling and cold rolling The hot-rolled coil obtained in step (2) is pickled and cold-rolled to obtain a hard-rolled coil. The deformation during cold rolling is 40%-60%.

[0062] (4) First continuous annealing The first continuous annealing is selected from any of the following processes: Process 1 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, and then rapidly cooling it to room temperature at a cooling rate of 30℃ / s-60℃ / s to obtain the annealed coil. Process 2 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 190℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain the annealed coil.

[0063] (5) Second continuous annealing The second continuous annealing process includes the following steps: heating the annealed coil to Ac1 to Ac3-50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 380℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain a cold-rolled steel sheet.

[0064] The chemical compositions of each embodiment and comparative example are summarized in Tables 1-1 and 1-2 (chemical compositions). The chemical compositions of Examples 1-20 are selected from steel composition A, B, C, D and E, and the chemical compositions of Comparative Examples 1-14 are selected from steel composition a, b, c, A, D and E.

[0065] The process parameters involved in the hot rolling, pickling, and cold rolling processes in the preparation method are summarized in Table 2 (Preparation Process Parameter 1); the process parameters for the first continuous annealing in the preparation method are summarized in Table 3 (Preparation Process Parameter 2) and the process parameters for the second continuous annealing are summarized in Table 4 (Preparation Process Parameter 3); the process flow diagrams for the first and second continuous annealing are shown below. Figure 6 The left figure shows the first continuous annealing; the right figure shows the second continuous annealing. It should be noted that the process parameters listed in the preparation method of this invention can be achieved through Tables 2-4 below, or used in combination, and are not limited to these methods.

[0066] Table 1-1 Chemical Composition (wt%)

[0067] Note: " / " in the table indicates that the corresponding element has not been added.

[0068] Table 1-2 Chemical composition (wt%)

[0069] Note: " / " in the table indicates that the corresponding element has not been added.

[0070] Table 2 Preparation process parameters 1

[0071] Table 3 Preparation process parameters II

[0072] Note: " / " in the table indicates that the corresponding preparation process was not implemented. The absence of process parameters for Comparative Examples 5-6 and 14 indicates that Comparative Examples 5-6 and 14 did not undergo the first continuous annealing.

[0073] Table 4 Preparation process parameters (III)

[0074] Note: The absence of process parameters for Comparative Example 3-4 indicates that Comparative Example 3-4 did not undergo a second continuous annealing.

[0075] Test Example 1 This test example provides the following test analysis for the obtained cold-rolled steel sheet: (1) The microstructure of cold-rolled steel sheets was tested using Example 3 and Comparative Example 14 as examples. The microstructure photograph of Example 3 is shown in [image missing]. Figure 1 See the micrograph of Comparative Example 14. Figure 5 .

[0076] from Figure 1 The microstructure photographs of Example 3 show that the small number of "depressed" blocky regions inside the network martensite matrix are ferrite, and their volume fraction was determined by the area method. The volume fraction of retained austenite was mainly determined by X-ray diffraction based on GB / T 29706-2012. The volume fraction of martensite matrix = 100% - volume fraction of ferrite - volume fraction of retained austenite. The relevant results are shown in Table 5.

[0077] from Figure 5 The microstructure photographs of Comparative Example 14 show that, as Comparative Example 14 is a cold-rolled steel sheet produced by a conventional composition and a single continuous annealing process, its microstructure is mainly blocky martensite, with low martensite and retained austenite content, and contains a relatively large amount of ferrite. Figure 1 The network martensitic matrix structure of Example 3 forms a stark contrast.

[0078] (2) Based on Figure 2 The EBSD diagram of Example 3 uses the line intercept method to determine the width of the martensite strips, and converts the area of ​​a single martensite strip and the area of ​​retained austenite into the diameter of a circle with the same projected area, i.e., the equivalent circle diameter of retained austenite and martensite strips; related results are shown in [link to relevant results]. Figures 3-4 ,in, Figure 3 This is a diagram showing the distribution of the equivalent circle diameter of the residual austenite in Embodiment 3 of the present invention; Figure 4 This is a diagram showing the distribution of the equivalent circle diameter of the martensite stripes in Embodiment 3 of the present invention. Residual austenite with an equivalent circle diameter < 1 μm is defined as thin-film retained austenite. Figure 4 The proportion of thin-film retained austenite in the medium is 98%.

[0079] Combination Figures 1-4 It can be seen that the microstructure of the cold-rolled steel sheet obtained in Example 3 includes 65%-85% martensite, 10%-20% retained austenite, and 0%-15% ferrite by volume. The martensite has a network structure and is the matrix structure. Its basic unit is martensite bands with an average width of ≤2μm. The content of retained austenite is relatively high, reaching 17%.

[0080] The fine lath structure formed by the first annealing provides a carrier for the growth of retained austenite. The final microstructure is based on a network of micron-sized strip martensite, with high-density, fine, and dispersed submicron retained austenite particles distributed at the microstructure interface. During deformation, these particles act as a "lubricant" to alleviate stress concentration, which helps to improve the overall uniform elongation and total elongation of the material.

[0081] Test Example 2 This test example analyzes the properties of the obtained cold-rolled steel sheet, and the relevant test results are summarized in Tables 5-6. Among them, the test analysis items in Table 5 Test Results (I) include yield strength, tensile strength, uniform elongation, elongation at break, and strength-ductility product. The test methods for yield strength, tensile strength, uniform elongation, and elongation at break in the mechanical properties are based on GB / T 228.1-2010, and the gauge length of the tensile specimen is 50 mm; the strength-ductility product is the product of tensile strength and elongation at break.

[0082] Table 6 shows the test results (II), which include the average width of the martensite strips, the equivalent circle diameter of the martensite strips, the equivalent circle diameter of the retained austenite, and the volume fraction of thin-film austenite. The test methods are the same as those used in Example 3. Comparative Examples 1-14 do not have the microstructure characteristics of the examples, and therefore, the relevant properties in Table 6 were not analyzed and statistically analyzed.

[0083] Table 5 Test Results (I)

[0084] Table 6 Test Results (II)

[0085] As can be seen from the data in Tables 5-6, the cold-rolled steel sheet provided in this embodiment of the invention has a yield strength of 600MPa-800MPa, a tensile strength of 980MPa-1100MPa, a total elongation of A50 of 30%-36%, and a strength-ductility product ≥30GPa·%. Furthermore, this invention has found that to ensure a strength-ductility product of 30GPa·%, the retained austenite content must be maintained above 10%.

[0086] Comparative Examples 1-2 have lower C content, resulting in lower final strength, elongation, and strength-ductility product. This is mainly due to the higher ferrite content and the lower martensite and austenite content.

[0087] The first continuous annealing of 3-4 samples resulted in a very high martensite content, an extremely low austenite content, extremely high tensile strength, but very low elongation.

[0088] The second continuous annealing of 5-6 samples resulted in very low martensite content, low austenite content, and low elongation.

[0089] Comparative Examples 7-9 all underwent two consecutive annealing processes. In Comparative Example 7, the rapid cooling temperature during the first consecutive annealing was too high, resulting in lower final yield strength and elongation at break. In Comparative Example 8, the rapid cooling temperature during the second consecutive annealing was too high, leading to lower final yield strength, tensile strength, and elongation at break. In Comparative Example 9, the reheating temperature during the second consecutive annealing was too low, resulting in lower retained austenite content and elongation at break.

[0090] In Comparative Examples 10-11, the steel composition b has C+Mn=2.8. After two consecutive annealing processes, the final tensile strength is higher than that of steel with lower elongation at break.

[0091] The steel grade in Comparative Examples 12-13 has a higher carbon content in its composition c. The use of two consecutive annealing processes resulted in a higher martensite content, which in turn led to a higher tensile strength and a lower elongation at break.

[0092] Comparative Example 14 had a low C content and was only annealed once, resulting in a low martensite content, a low austenite fraction, and ultimately a low fracture elongation.

[0093] In summary, the cold-rolled steel sheet provided in this invention adopts a simple C-Si-Mn basic system composition, fully utilizing the phase transformation law of the basic alloying elements. It stabilizes the retained austenite by strengthening C rather than Mn, while adding small amounts of Ti and Nb elements to obtain a fine martensitic matrix. The first continuous annealing in the preparation method yields a fine lath structure, and the second continuous annealing promotes the growth of reverse-transformed austenite on the lath structure, resulting in a film-like, stable, high-content, uniformly distributed, and submicron-sized retained austenite. This preparation method is suitable for routine and mass production on existing production lines, and is helpful for manufacturing complex-shaped, highly formable vehicle structural parts, such as A / B pillars, longitudinal beams, bumpers, and door anti-collision bars.

[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A cold-rolled steel sheet, characterized in that, The chemical composition of the cold-rolled steel sheet, by mass percentage, includes the following components: C: 0.25wt%-0.33wt%, Mn: 1.5wt%-2.5wt%, Si: 1.5wt%-2.0wt%, Cr: 0wt%-0.3wt%, Ti: 0wt%-0.03wt%, Nb: 0wt%-0.03wt%, N≤0.005wt%, P≤0.01wt%, S≤0.005wt%, balance Fe and unavoidable impurities; wherein, C and Mn satisfy the following relationship: C+Mn≤2.7wt%; The microstructure of the cold-rolled steel sheet includes network martensite and retained austenite, and the volume fraction of network martensite is >60%, with its basic unit being martensitic strips. The retained austenite is distributed along the martensite strip interface, in the form of a thin film, and the volume fraction of the thin film austenite is ≥80%, with the equivalent circle diameter of the retained austenite ≤1μm.

2. The cold-rolled steel sheet according to claim 1, characterized in that, The microstructure of the cold-rolled steel sheet, by volume fraction, comprises: 65%-85% network martensite, 10%-20% retained austenite, and 0%-15% ferrite.

3. The cold-rolled steel sheet according to claim 1, characterized in that, The microstructure of the cold-rolled steel sheet has at least one of the following characteristics: Feature 1: The average width of the martensitic strips is ≤2μm; Feature 2: The equivalent circle diameter of the martensitic strips is ≤2μm.

4. The cold-rolled steel sheet according to claim 1, characterized in that, The chemical composition of the cold-rolled steel sheet, by mass percentage, also includes at least one of the following components: B: 0wt%-0.005wt%, Cu: 0wt%-0.5wt%, Ni: 0wt%-0.5wt%, Mo: 0wt%-0.2wt%, V: 0.0001wt%-0.20wt%, Zr: 0.0001wt%-0.20wt%, W: 0.000 1wt%-0.20wt%, Ca: 0wt%-0.050wt%, Ce: 0.0001wt%-0.0050wt%, La: 0.0001wt%-0.0050wt%, Sn: 0wt%-0.1wt% and Mg: 0wt%-0.0030wt%.

5. The cold-rolled steel sheet according to claim 1, characterized in that, The cold-rolled steel sheet has a yield strength of 600MPa-850MPa and a tensile strength of 980MPa-1100MPa. 50 The total elongation is 30%-36%, and the strength-plasticity product is ≥30GPa·.

6. A method for preparing cold-rolled steel sheet as described in any one of claims 1-5, characterized in that, Includes the following steps: The cold-rolled hardened coil is subjected to a first continuous annealing and a second continuous annealing in sequence.

7. The preparation method according to claim 6, characterized in that, The first continuous annealing is selected from any of the following processes: Process 1: The hard-rolled coil is heated to an annealing temperature of Ac3-30℃ to Ac3+50℃, held at that temperature for 60s-150s, and then rapidly cooled to room temperature at a cooling rate of 30℃ / s-60℃ / s to obtain an annealed coil. Process 2 involves heating the hard-rolled coil to an annealing temperature of Ac3-30℃ to Ac3+50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then reheating it to 190℃-420℃, holding it at that temperature for 200s-250s, and finally cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain the annealed coil. The second continuous annealing includes the following process: heating the annealed coil to Ac1 to Ac3-50℃, holding it at that temperature for 60s-150s, then rapidly cooling it to 190℃-250℃ at a cooling rate of 30℃ / s-60℃ / s, holding it at that temperature for 15s-25s, then heating it to 380℃-420℃, holding it at that temperature for 200s-250s, and then cooling it to room temperature at a cooling rate of 3℃ / s-8℃ / s to obtain a cold-rolled steel sheet.

8. The preparation method according to claim 6, characterized in that, The preparation method further includes the following steps: Smelting and continuous casting, hot rolling, pickling and cold rolling; The hot rolling process includes the following steps: heating temperature is 1150℃-1250℃, furnace time is 30min-90min, hot rolling is performed after the slab exits the heating furnace, and the final rolling temperature is controlled at 870℃-920℃. After hot rolling, laminar flow cooling and coiling are carried out, with the cooling rate controlled at 10℃ / s-40℃ / s and the coiling temperature at 450℃-600℃.

9. The application of a cold-rolled steel sheet as described in any one of claims 1-5 or a cold-rolled steel sheet prepared by the preparation method as described in any one of claims 6-8 in vehicle structural components, characterized in that, The vehicle structural components are selected from at least one of the following: A / B pillars, longitudinal beams, bumpers, and door anti-collision bars.

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