Low-anisotropy hot-dip aluminum-silicon 180 MPa grade phosphorus-containing high-strength steel and preparation method thereof

Abstract

According to the low-anisotropy hot-dip aluminum-silicon 180 MPa grade phosphorus-containing high-strength steel and the preparation method thereof, a hot-dip aluminum-silicon automobile outer plate is prepared through low-cost component design and ingenious combination of heat treatment and an aluminum-silicon plating process, the yield strength of a steel plate of the hot-dip aluminum-silicon automobile outer plate ranges from 180 MPa to 220 MPa, the tensile strength is larger than or equal to 320 MPa, the percentage elongation after fracture is larger than or equal to 43%, the plastic strain ratio r value is larger than or equal to 2.2, and the hot-dip aluminum-silicon automobile outer plate is suitable for being used as a steel plate. According to the high-strength steel and the manufacturing method thereof, the isotropic special-shaped index delta r value is smaller than or equal to 0.2, the work hardening index n value is larger than or equal to 0.23, the structure of the manufactured high-strength steel comprises ferrite, a Ti (N, C) precipitated phase, an NbC precipitated phase and a Cu-rich phase, and the automobile outer plate has excellent forming performance, high surface quality, no timeliness and small anisotropy value.

Description

Technical Field

[0001] This invention relates to the field of automotive steel manufacturing technology, and more specifically, to a low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel and its preparation method. Background Technology

[0002] Under the national dual-carbon policy, lightweighting has become an inevitable trend for new energy vehicles to increase driving range, improve durability and safety. Although high-strength steel and ultra-high-strength steel are widely used in safety and structural components, the steel used for automotive outer panels, which accounts for nearly a quarter of automotive steel, is still mainly low-strength mild steel, indicating huge potential for lightweighting. Automotive outer panels mainly refer to the surface parts of the car body, such as door panels, side panels, roof panels, and hood panels. These surface parts, as the "face" of the car, must meet the requirements of personalized styling and high visibility. Automotive outer panels are mostly large spatial curved surface structures with complex shapes, and are exposed to the elements for a long time. At present, the most common steel used for automotive outer panels is IF steel and BH steel with hot-dip galvanized coating. IF steel is made by adding a certain amount of alloying elements to ultra-low carbon steel, so that the carbon and nitrogen atoms in the steel are completely fixed into carbonitride compounds. In this way, there are no interstitial atoms in the steel matrix, resulting in pure ferrite, which gives the steel excellent formability and no aging. However, IF steel has drawbacks such as insufficient anti-dip resistance and significant anisotropy. BH steel retains some interstitial dissolved carbon atoms. During the coating and baking process, the dissolved carbon diffuses and accumulates near dislocations, pinning them and hindering their movement, thus increasing the steel's strength and giving it excellent anti-dip resistance. However, BH steel suffers from significant natural aging during storage, leading to a decline in formability. Furthermore, the hot-dip galvanizing coating used in the aforementioned automotive outer panel steels to meet corrosion resistance requirements is becoming increasingly uneconomical due to dwindling zinc resources and rising prices. Therefore, there is an urgent need to develop a high-strength outer panel steel that possesses both low anisotropy and excellent deep-drawing performance. Simultaneously, it is necessary to explore a new hot-dip galvanizing method that matches the heat treatment regime of high-strength outer panel steel to meet future market demands for high-strength outer panel steel with good surface quality and superior performance.

[0003] Patent application number 201810795677.7 discloses a phosphorus-containing high-strength steel and its preparation method. The phosphorus-containing high-strength steel has the following composition by mass percentage: C: 0.002%~0.008%, Si: 0.06%~0.1%, Mn: 0.9%~1.3%, P: 0.06%~0.10%, S: ≤0.005%, Alt: 0.025%~0.035%, with the remainder being iron and unavoidable impurities. This phosphorus-containing high-strength steel has a low elongation rate and does not employ alloying elements, resulting in the inability to fix C and N atoms in the steel, leading to a low r-value and poor formability of the steel sheet. Furthermore, the steel sheet is produced using a hot-dip galvanizing annealing process, without involving a hot-dip aluminizing-silicon annealing process. Patent application number 202311387544.3 discloses a 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel and its manufacturing method. The high-strength IF steel composition by mass percentage is: C: ≤0.003%, Si: ≤0.03%, Mn: 0.35~0.65%, P: 0.03~0.075%, S≤0.010%, Als: 0.020~0.045%, Nb: 0.008~0.015%, Ti: 0.02~0.03%, B: 0.0004~0.0010%, N≤0.0035%, with the remainder being Fe and unavoidable impurities. This steel plate does not mention anisotropy index or natural aging, and the steel plate uses a hot-dip galvanized iron alloy annealing process, without involving a hot-dip aluminized silicon annealing process. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel and its preparation method. Through the rational design of chemical composition and production process, combined with hot-dip aluminized silicon process, a hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel for automotive outer panels with both high surface quality and low anisotropy is prepared.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel comprises the following components by mass percentage: C: 0.0010%~0.0045%, Si: 0.010%~0.070%, Mn: 0.10%~0.40%, P: 0.020%~0.060%, Cu: 0.05%~0.50%, Ti: 0.010%~0.090%, Nb: 0.010%~0.050%, Al: 0.010%~0.050%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; wherein, calculated by the corresponding mass percentage of each element, 5≤(Ti+Nb) / (C+N+S)≤15, 4≤(Cu / P)≤12.

[0006] This invention also discloses a method for preparing the low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel as described above, comprising smelting, hot rolling, pickling, cold rolling, hot-dip aluminized silicon coating, and finishing; in the hot-dip aluminized silicon coating process, the cold-rolled steel sheet obtained by the smelting, hot rolling, pickling, and cold rolling processes is heated to an annealing isothermal temperature of 820℃~880℃ for 40s~120s; subsequently, the steel sheet is slowly cooled to a slow cooling temperature of 630℃~730℃. The time is 5s~20s; then the slowly cooled steel plate is put into the aluminum-silicon plating bath to obtain the steel plate coating. The temperature of the aluminum-silicon plating bath is 620℃~730℃, the aluminum-silicon plating time is 5s~15s, and the total time of aluminum-silicon plating time and slow cooling time is ≥15s; and the slow cooling temperature is greater than or equal to the temperature of the aluminum-silicon plating bath but does not exceed 10℃; after the aluminum-silicon plating is completed, the steel plate is cooled to room temperature at a cooling rate of ≥20℃ / s to obtain a cooled steel plate, which is then subjected to the above-mentioned finishing treatment to obtain the low anisotropy hot-dip aluminum-silicon 180MPa grade phosphorus-containing high-strength steel.

[0007] Implementing the embodiments of the present invention will have the following beneficial effects: (1) This invention controls the alloy cost of steel plates by rationally designing the chemical composition of the steel plates without adding expensive alloying elements. Furthermore, by rationally adding Ti and Nb, Ti can react with S to generate TiS and Ti4C2S2. This reaction can regulate the morphology and distribution of sulfides, thereby reducing the harmful effects of sulfur on the mechanical properties of steel plates. TiC will grow epitaxially with Ti4C2S2 particles as the core, which will increase the size of TiC particles. This process helps to form a dispersed titanium carbide phase, ultimately improving the plastic forming ability of steel plates. Secondly, sufficient amounts of Ti and Nb are ensured to fix the carbonitrides, effectively removing interstitial solid solution atoms in the steel plate, effectively preventing natural aging, and ensuring that the yield strength of the steel plate remains within the range of 180MPa to 225MPa within 6 months, with an elongation after fracture ≥42.5%, and the increase in yield strength ≤5MPa, and the decrease in elongation after fracture ≤0.5%. Furthermore, by adding appropriate amounts of Cu and P, grain boundaries are strengthened, reducing the adverse effects of P segregation at grain boundaries and lowering the risk of brittleness during secondary processing of the steel plate. Simultaneously, reasonable production processes are employed to control the precipitation of precipitates and obtain a {111} texture beneficial to deep drawing performance, giving the steel plate excellent deep drawing properties and an anisotropy value ≤0.20.

[0008] (2) The present invention places the hot-dip aluminum-silicon process in the slow cooling section. On the one hand, it reasonably controls the temperature and time of the hot-dip aluminum-silicon process and the slow cooling process to ensure the formation of Cu-rich phase in the steel plate and strengthen the grain boundary; on the other hand, it reduces energy consumption in the production process and achieves low carbon and environmental protection.

[0009] (3) Based on a reasonable low-cost alloy composition design and using hot-dip aluminum-silicon galvanizing process, this invention achieves a 180MPa-level automotive outer panel with excellent formability, high surface quality, no aging, and small anisotropy value. The high-strength steel produced has a yield strength of 180MPa~220MPa, tensile strength ≥320MPa, elongation after fracture ≥43%, plastic strain ratio r ≥2.2, anisotropy index Δr ≤0.2, work hardening index n ≥0.23, yield strength maintained in the range of 180MPa~225MPa, elongation after fracture ≥42.5%, and the increase in yield strength ≤5MPa, the decrease in elongation after fracture ≤0.5%. The microstructure includes ferrite and Ti(N,C) precipitates, NbC precipitates and Cu-rich phase. Detailed Implementation

[0010] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0011] This invention discloses a low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel, comprising the following components by mass percentage: C: 0.0010%~0.0045%, Si: 0.010%~0.070%, Mn: 0.10%~0.40%, P: 0.020%~0.060%, Cu: 0.05%~0.50%, Ti: 0.010%~0.090%, Nb: 0.010%~0.050%, Al: 0.010%~0.050%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; wherein, calculated by the corresponding mass percentage of each element, 5≤(Ti+Nb) / (C+N+S)≤15, 4≤(Cu / P)≤12.

[0012] Specifically, the rationale for the alloy design of this invention is as follows: C: Carbon (C) is an important additive element in this invention. C is one of the most economical elements for improving the strength of steel plates. However, excessive C content leads to the formation of too many carbides and dissolved C in the steel, reducing its elongation, r-value, and n-value, resulting in decreased formability. It also increases the aging risk of the steel plate, affecting its usability. Conversely, excessively low C content significantly increases steelmaking costs in industrial production. Therefore, balancing performance and cost, this invention requires a C content range of 0.0010% to 0.0045%.

[0013] Si: Si is an important additive element in this invention. Si plays a role in solid solution strengthening, improving the strength of the steel plate. However, excessive Si content will reduce the plasticity and toughness of the steel and seriously affect the weldability of the steel plate. Therefore, this invention requires the Si content to be in the range of 0.010% to 0.070%.

[0014] Mn: Mn is an important additive element in this invention. Mn plays a role in solid solution strengthening, improving the strength of the steel plate. However, excessive Mn content will alter the internal microstructure of the steel plate, leading to increased grain size inhomogeneity and poorer deformation coordination during plastic deformation. Simultaneously, excessive Mn content will promote the formation of more hard and brittle phases in the weld and heat-affected zone, altering the chemical composition and microstructure of the weld metal. Therefore, this invention requires the Mn content to be in the range of 0.10% to 0.40%.

[0015] P: P is an important additive element in this invention. P plays a role in solid solution strengthening and is one of the most economical elements for increasing the strength of steel plates. Furthermore, when used in combination with Cu, P can improve the atmospheric corrosion resistance of the steel plate, enhancing its corrosion resistance in harsh environments to a certain extent. However, excessive P content can easily cause grain boundary embrittlement, leading to brittleness during secondary processing. Therefore, this invention requires the P content to be in the range of 0.020%~0.060%.

[0016] Cu: Cu is an important additive element in this invention. Cu exists in the steel matrix as a Cu-rich phase, precipitating as nano-elemental particles during the aging stage, thus playing a precipitation strengthening role and improving grain boundary strength. However, excessive Cu content will result in coarse Cu-rich phases, while insufficient content will affect the precipitation amount of Cu-rich phases, both of which are detrimental to the strength of the steel plate. Therefore, this invention requires the Cu content to be in the range of 0.05%~0.50%, and it must satisfy: 4≤(Cu / P)≤12, ensuring sufficient Cu segregation at grain boundaries, reducing the adverse effects of P segregation at grain boundaries, and lowering the risk of brittleness during secondary processing of the steel plate.

[0017] Ti: Ti is an important additive element in this invention. Ti is a strong carbide and nitride forming element, capable of combining with interstitial carbon and nitrogen atoms to form stable and fine carbide and nitride Ti(N,C), which plays a role in grain refinement and precipitation strengthening, eliminating the natural aging of the steel plate. This ensures the steel plate has good plasticity and formability while significantly improving its strength. However, excessive Ti content will dissolve in the ferrite matrix, increasing strength but decreasing plasticity. Therefore, this invention requires the Ti content to be in the range of 0.010%~0.090%.

[0018] Nb: Nb is an important additive element in this invention. Nb forms precipitates with interstitial elements such as C, eliminating interstitial atoms and playing a precipitation strengthening role. This eliminates the natural aging of the steel plate, improves its cold forming performance, and refines the grain size, reducing the anisotropy of the steel plate. Nb also expands the rolling process window, improving the uniformity of the coil's continuous rolling performance. However, excessive addition of Nb can lead to excessively high strength and decreased plasticity in the steel plate. Therefore, this invention requires the Nb content to be in the range of 0.010%~0.050%. Furthermore, it must satisfy the following condition: 5≤(Ti+Nb) / (C+N+S)≤15, ensuring sufficient Ti and Nb to fix carbonitrides, effectively remove interstitial solid solution atoms in the steel plate, effectively prevent natural aging, and ensure that the yield strength increase is ≤5MPa and the elongation after fracture decreases by ≤0.5% within 6 months. It is beneficial to further enhance the {111} texture, increase the r value of the steel plate, and have a certain amount of residual amount, which reduces the anisotropy value of the steel plate and also plays a role in precipitation strengthening. However, the residual amount should not be too high, otherwise it will lead to a decrease in the plasticity of the steel plate.

[0019] Al: Al is an important additive element in this invention. Al is added as a deoxidizer during the steelmaking process, mainly to remove oxygen dissolved in the molten steel during oxygen blowing smelting. However, when the Al content is too high, it will form too many inclusions. Therefore, this invention requires the Al content to be in the range of 0.010% to 0.050%.

[0020] S: S is a harmful element in steel, easily consuming Mn to form MnS inclusions, which impairs the performance of the steel plate. Its content should be as low as possible. Considering the production cost of steel, this invention requires the S content to be controlled within the range of ≤0.003%.

[0021] Nitrogen (N): Nitrogen is a harmful element in steel. It forms nitrides with Nb and Ti, reducing the combined strengthening effect of Nb and Ti. Simultaneously, nitrogen deteriorates the elongation and weldability of steel plates. Lower nitrogen content is better, but excessively low levels can lead to production difficulties and increased costs. This invention requires the nitrogen content to be controlled within the range of ≤0.003%.

[0022] In one specific embodiment, the high-strength steel has a yield strength of 180MPa~220MPa, a tensile strength ≥320MPa, an elongation after fracture ≥43%, a plastic strain ratio r ≥2.2, an anisotropy index Δr ≤0.2, and a work hardening index n ≥0.23. Within 6 months after the steel plate is manufactured, the yield strength remains within the range of 180MPa~225MPa, the elongation after fracture is ≥42.5%, and the increase in yield strength is ≤5MPa, and the decrease in elongation after fracture is ≤0.5%.

[0023] In one specific embodiment, the microstructure of the high-strength steel includes ferrite as well as Ti(N,C) precipitates, NbC precipitates and Cu-rich phases.

[0024] The present invention also discloses a method for preparing the low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel as described above, including smelting, hot rolling, pickling, cold rolling, hot-dip aluminized silicon and finishing. S1. In the smelting process, the raw materials are smelted in a converter and refined with RH to obtain molten steel, and then the molten steel is continuously cast into billets.

[0025] S2. In hot rolling, the heating temperature is 1200℃~1280℃, and the furnace time is 150min~210min; then hot rolling is carried out into steel plates, with an initial rolling temperature of 1050℃~1130℃, a final rolling temperature of ≥900℃, and a coiling temperature of 690℃~770℃.

[0026] Specifically, the principle of setting key parameters for the hot rolling process in this invention is as follows: (1) The present invention controls the heating temperature between 1200℃ and 1280℃. This temperature range helps to ensure that the added Ti element fixes the N element, so that TiN precipitation is complete; at the same time, it ensures that the billet is fully austenitized, which is convenient for subsequent rolling. The furnace time is 150min to 210min. The appropriate heating temperature and holding time ensure that the alloying elements in the billet are completely dissolved and evenly distributed, ensuring the uniformity of the billet composition, and playing a role in controlling the original austenite grain size. When the heating temperature is higher than 1280℃, the austenite grains are prone to grow excessively, which affects the mechanical properties of the final product; when the temperature is lower than 1200℃, austenitization will be incomplete, which will affect the rolling performance and the uniformity of the final product. A furnace time of more than 210min is prone to overheating.

[0027] (2) In this invention, the initial rolling temperature is controlled between 1050℃ and 1130℃, and the final rolling temperature is ≥900℃. An excessively high initial rolling temperature will result in coarse austenite grains, which may form coarse ferrite grains after rolling, reducing the plasticity and toughness of the steel plate. An excessively low initial rolling temperature cannot guarantee a final rolling temperature ≥900℃, while a final rolling temperature ≥900℃ is necessary to ensure that the entire hot rolling process is carried out within the single-phase austenite region of the steel plate, avoiding rolling in the austenite-ferrite two-phase region and preventing mixed grain formation.

[0028] (3) The present invention controls the coiling temperature between 690℃ and 770℃, which is conducive to the precipitation of carbides NbC and TiC and refines the grains. Too low a coiling temperature will cause interstitial atoms to be not sufficiently fixed, which will lead to aging brittleness and reduce the formability of the steel plate. Too high a coiling temperature will produce more iron oxide scale, which will reduce the efficiency of subsequent pickling and will cause uneven cooling rate of hot-rolled plate, resulting in poor head and tail performance.

[0029] S3. In pickling, the steel plate is pickled to remove the iron oxide scale formed on the surface of the steel plate due to oxidation, so that the surface of the steel plate is smooth and provides good surface conditions for the subsequent cold rolling process.

[0030] S4. In cold rolling, the cold rolling reduction rate is controlled at 75%~90% to obtain cold-rolled steel sheet.

[0031] Specifically, this invention uses cold rolling to fully crush and elongate the grains, increasing energy storage. The energy stored in cold rolling deformation is the driving force for recrystallization during subsequent heat treatment. A sufficient reduction rate ensures the effectiveness of ferrite recrystallization, which is beneficial for the rapid growth of the {111} texture component. The {111} texture positively affects the plastic strain ratio r-value of the steel sheet. Therefore, controlling the cold rolling reduction rate at 75%~90% is beneficial for obtaining a higher r-value and excellent deep-drawing performance. However, an excessively high cold rolling reduction rate will increase the load on the cold rolling mill.

[0032] S5. In hot-dip aluminized silicon plating, the cold-rolled steel sheet, which has undergone smelting, hot rolling, pickling, and cold rolling processes in sequence, is heated to the annealing isothermal temperature of 820℃~880℃ for 40s~120s. The steel sheet is then slowly cooled to the slow cooling temperature of 630℃~730℃ for 5s~20s. The slowly cooled steel sheet is then immersed in an aluminized silicon plating bath to obtain a coating. The bath temperature is 620℃~730℃, and the plating time is 5s~15s, with the total plating time and slow cooling time ≥15s. The slow cooling temperature is greater than or equal to the bath temperature but not exceeding 10℃. After aluminized silicon plating, the steel sheet is cooled to room temperature at a cooling rate ≥20℃ / s to obtain a cooled steel sheet.

[0033] Specifically, the key parameters and principles of the hot-dip aluminum-silicon plating process of this invention are as follows: (1) The annealing isothermal temperature is 820℃~880℃ and the annealing time is 40s~120s to ensure that the recrystallization behavior of the cold-rolled deformation structure occurs fully, which is conducive to grain growth, that is, the process of {111} oriented grains swallowing grains of other orientations. Therefore, as the grains grow, the {111} texture becomes stronger and the r value becomes higher.

[0034] (2) The slow cooling temperature is 630℃~730℃, and the slow cooling time is 5s~20s. Limiting the slow cooling temperature and slow cooling time has the following two effects: First, it ensures the formation of Cu-rich phases in the steel plate, strengthens the grain boundaries, and promotes the strength of the steel plate. Second, it can ensure the precipitation of Nb and Ti carbides, effectively remove interstitial solid solution atoms in the steel plate, and at the same time help to enhance the {111} texture.

[0035] (3) Aluminized silicon plating: After slow cooling, the steel plate enters the aluminum-silicon plating bath to obtain the steel plate coating. The temperature of the aluminum-silicon plating bath is 620℃~730℃, and the aluminum-silicon plating time is 5s~15s. The reasonable aluminum-silicon plating bath temperature and aluminum-silicon plating time further promote the formation of Cu-rich phase in the steel, further strengthen the grain boundaries, and further promote the strength of the steel plate. It can further ensure the precipitation of Nb and Ti carbides, effectively remove interstitial solid solution atoms in the steel plate, and is conducive to further enhancing the {111} texture. It is also necessary to meet the following requirements: slow cooling temperature of steel plate - aluminum-silicon plating bath temperature ≤ 10℃, aluminum-silicon plating time + slow cooling time of steel plate ≥ 15s, to ensure that the ferrite grain growth is more uniform and to avoid the situation of uneven grain size. The uniform grain structure is conducive to improving the uniformity of the mechanical properties of the steel plate, making the performance of the steel plate more consistent in various parts, reducing the anisotropy value of the steel plate; and is conducive to the continuous hot-dip aluminum-silicon plating process, avoiding the formation of a large number of Fe-Al alloy particles adhering to the surface of the steel plate due to temperature fluctuations, which affects the surface quality of the steel plate.

[0036] (4) Rapid cooling: After the aluminum-silicon coating is completed, the steel plate is cooled to room temperature at a rate of ≥20℃ / s. The high cooling rate can effectively control the thickness of the aluminum-silicon coating and make the steel strip structure uniform, while avoiding the formation of crystalline defects on the surface of the steel plate.

[0037] In one specific embodiment, the composition of the aluminum-silicon plating bath, by mass percentage, is: Si: 7%~12%, Al: 88%~93%, with the remainder being Fe and unavoidable impurity elements. Specifically, this limited content of Si can improve the fluidity of the plating bath, which is beneficial for the continuous aluminum-silicon plating process, and makes the aluminum-silicon coating on the steel plate surface uniform without coating defects. At the same time, Si can inhibit the Fe-Al reaction to form alloy phases, effectively reducing the thickness of the alloy layer, thus benefiting the formability of the aluminum-silicon coated steel plate.

[0038] S6. In the finishing process, the cooled steel plate is fed into the finishing machine for plate shape adjustment. The finishing elongation is controlled at 0.2%~0.8%, which can effectively eliminate the yield plateau of the steel plate and improve the surface quality of the steel plate, so that the finished plate is straight and without waviness, and obtains low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel.

[0039] The following are specific embodiments. Examples 1-15 The preparation method of the low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel in this embodiment includes smelting, hot rolling, pickling, cold rolling, hot-dip aluminized silicon and finishing. S1. In the smelting process, the raw materials are smelted in a converter and refined with RH to obtain molten steel, and then the molten steel is continuously cast into billets.

[0040] S2. In hot rolling, the billet is heated to the heating temperature and the time in the furnace is controlled; then it is hot rolled into a steel plate.

[0041] S3. In pickling, the steel plate is pickled to remove the iron oxide scale formed on the surface of the steel plate due to oxidation, so that the surface of the steel plate is smooth and provides good surface conditions for the subsequent cold rolling process.

[0042] S4. In cold rolling, the cold rolling reduction rate is controlled to obtain cold-rolled steel sheet.

[0043] S5. In hot-dip aluminized silicon plating, the cold-rolled steel sheet, which has been successively processed by smelting, hot rolling, pickling and cold rolling, is heated to the annealing isothermal temperature and the annealing isothermal time is controlled; then the steel sheet is slowly cooled to the slow cooling temperature and the slow cooling time is controlled; then the slowly cooled steel sheet is put into the aluminized silicon plating solution to obtain the steel sheet coating, and the total time of aluminized silicon plating time and slow cooling time is ≥15s; and the slow cooling temperature is greater than or equal to the temperature of the aluminized silicon plating solution but does not exceed 10℃; after the aluminized silicon plating is completed, the steel sheet is cooled to room temperature to obtain the cooled steel sheet.

[0044] S6. In the finishing process, the cooled steel plate is fed into the finishing machine for plate shape adjustment and finishing elongation is controlled to obtain low anisotropic hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel.

[0045] Table 1 lists the chemical composition of the steels in the examples, Table 2 lists the hot rolling and cold rolling process parameters of the steels in the examples, Table 3 lists the hot-dip aluminizing and finishing process parameters of the steels in the examples, and Table 4 lists the mechanical properties of the steels in the examples.

[0046] Table 1 Chemical composition of the steel in the examples, wt%

[0047] Table 2. Process parameters for hot rolling and cold rolling of the steel in the examples.

[0048] Table 3 Process parameters for hot-dip aluminizing and siliconizing of steel in the examples

[0049] Table 4 Mechanical properties of the steel in the examples

[0050] As can be seen from the above embodiments, by using low-cost composition design and cleverly combining heat treatment and aluminized silicon plating processes, hot-dip aluminized silicon automotive outer panels were prepared. The steel sheet has a yield strength of 180MPa~220MPa, tensile strength ≥320MPa, elongation after fracture ≥43%, plastic strain ratio r ≥2.2, anisotropy index Δr ≤0.2, and work hardening index n ≥0.23. It ensures that the yield strength remains within the range of 180MPa~225MPa within 6 months, the elongation after fracture ≥42.5%, and the increase in yield strength ≤5MPa, and the decrease in elongation after fracture ≤0.5%. The microstructure of the obtained high-strength steel includes ferrite, Ti(N,C) precipitates, NbC precipitates, and Cu-rich phases, achieving excellent formability, high surface quality, and no aging and low anisotropy value for the automotive outer panel.

[0051] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel, characterized in that, Includes the following components by mass percentage: C: 0.0010%~0.0045%, Si: 0.010%~0.070%, Mn: 0.10%~0.40%, P: 0.020%~0.060%, Cu: 0.05%~0.50%, Ti: 0.010%~0.090%, Nb: 0.010%~0.050%, Al: 0.010%~0.050%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; calculated by the mass percentage of each element, 5≤(Ti+Nb) / (C+N+S)≤15, 4≤(Cu / P)≤12.

2. The low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 1, characterized in that, The high-strength steel has a yield strength of 180MPa~220MPa, a tensile strength of ≥320MPa, an elongation after fracture of ≥43%, a plastic strain ratio r of ≥2.2, an anisotropy index Δr of ≤0.2, and a work hardening index n of ≥0.

23. Within 6 months after the steel plate is manufactured, the yield strength is maintained in the range of 180MPa to 225MPa, the elongation after fracture is ≥42.5%, and the increase in yield strength is ≤5MPa, and the decrease in elongation after fracture is ≤0.5%.

3. The low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 1, characterized in that, The microstructure of the high-strength steel includes ferrite, Ti(N,C) precipitates, NbC precipitates, and Cu-rich phases.

4. A method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel as described in any one of claims 1-3, characterized in that, This includes smelting, hot rolling, pickling, cold rolling, hot-dip aluminizing and silicon coating, and finishing. In the hot-dip aluminized silicon process, the cold-rolled steel sheet, obtained by sequential smelting, hot rolling, pickling, and cold rolling, is heated to an annealing isothermal temperature of 820℃~880℃ for 40s~120s. The steel sheet is then slowly cooled to a slow cooling temperature of 630℃~730℃ for 5s~20s. The slowly cooled steel sheet is then immersed in an aluminized silicon plating bath to obtain a coating. The bath temperature is 620℃~730℃, and the plating time is 5s~15s, with the total plating time and slow cooling time ≥15s. The slow cooling temperature is greater than or equal to the bath temperature but not exceeding 10℃. After plating, the steel sheet is cooled to room temperature at a cooling rate ≥20℃ / s to obtain a cooled steel sheet, which is then subjected to a finishing process to obtain the low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel.

5. The method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, The composition of the aluminum-silicon plating solution, by mass percentage, is: Si: 7%~12%, Al: 88%~93%, with the remainder being Fe and unavoidable impurity elements.

6. The method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, In the hot rolling process, the heating temperature is 1200℃~1280℃, and the furnace time is 150min~210min; then, the steel plate is hot rolled into a steel plate with an initial rolling temperature of 1050℃~1130℃, a final rolling temperature of ≥900℃, and a coiling temperature of 690℃~770℃.

7. The method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, In the smelting process, the raw materials are successively smelted in a converter and refined with RH to obtain molten steel, and then the molten steel is continuously cast into billets.

8. The method for preparing low anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, In the pickling process, the steel plate is pickled to remove the iron oxide scale formed on the surface of the steel plate due to oxidation, thereby obtaining a pickled steel plate.

9. The method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, In the cold rolling process, the cold rolling reduction rate is controlled at 75%~90% to obtain the cold-rolled steel sheet.

10. The method for preparing low-anisotropy hot-dip aluminized silicon 180MPa grade phosphorus-containing high-strength steel according to claim 4, characterized in that, In the finishing process, the cooled steel plate is fed into a finishing machine for plate shape adjustment, and the finishing elongation is controlled to be 0.2%~0.8%.

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