Ultrahigh-formability 300MPa-grade alloying hot galvanizing phosphorus-containing high-strength steel for automobile outer plate and preparation method of ultrahigh-formability 300MPa-grade alloying hot galvanizing phosphorus-containing high-strength steel

Abstract

The invention discloses ultrahigh-formability 300MPa-grade alloying hot galvanizing phosphorus-containing high-strength steel for an automobile outer plate and a preparation method of the ultrahigh-formability 300MPa-grade alloying hot galvanizing phosphorus-containing high-strength steel, relates to the technical field of automobile steel manufacturing, and skillfully utilizes coupling addition among alloy elements to enable component design of a steel plate to have very high economical efficiency. By combining with an optimized alloying hot-dip galvanizing process, the alloying hot-dip galvanized steel plate of which the yield strength is 300-340 MPa, the tensile strength is greater than or equal to 460 MPa, the percentage elongation after fracture is greater than or equal to 36%, the plastic strain ratio r value is greater than or equal to 1.8, the work hardening index n value is greater than or equal to 0.17, the steel plate forming performance index anisotropy index delta r value is less than or equal to 0.35, and the FLC0 is greater than or equal to 0.32 is prepared; the automobile outer plate has the characteristics of high strength, high plasticity, high forming performance, high surface quality and low anisotropy index.

Description

Technical Field

[0001] This invention relates to the field of automotive steel manufacturing technology, and more specifically, to a high-strength, phosphorus-containing alloyed hot-dip galvanized steel with ultra-high formability (300MPa grade) for automotive outer panels and its preparation method. Background Technology

[0002] In the automotive manufacturing industry, steel occupies an irreplaceable position in the automotive metal materials system. High-strength steel and ultra-high-strength steel, with their superior mechanical properties, are widely used in the manufacture of automotive safety and structural components, significantly improving vehicle safety performance and structural stability. However, the current situation of steel used in automotive exterior panels is noteworthy, as this type of steel accounts for approximately one-quarter of the total steel used in automobiles. Automotive exterior panels encompass body surface parts such as door panels, side panels, roof panels, and hood panels. These parts have spatial curved surface structures with extremely complex shapes, making them difficult to form. This necessitates that automotive exterior panel steel not only possess high surface quality but also excellent formability. Currently, the main steels used in automotive exterior panels are interstitial steel (IF steel) and bake-hardening steel (BH steel) with a pure zinc coating. These two types of steel have good formability and can meet the complex usage requirements of automotive exterior panels. However, due to their relatively low strength, they are significantly insufficient in achieving automotive lightweighting.

[0003] Furthermore, the widely used pure zinc coating currently has many drawbacks. When stamping automotive parts, the low hardness of pure zinc coating makes it prone to wear and scratches, which can damage the integrity of the coating and reduce its protective performance. Moreover, during welding, the pure zinc coating easily reacts chemically with the welding electrode tip, accelerating electrode wear and reducing the lifespan of the welding equipment. Therefore, to adapt to the fierce competition in the automotive steel market, developing a hot-dip galvanized alloyed steel sheet for automotive outer panels that combines high strength, high plasticity, high formability, high surface quality, and low anisotropy index has become a critical issue that urgently needs to be addressed in the automotive manufacturing and metal materials fields.

[0004] 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. The tensile strength of the steel plate can reach over 380 MPa. However, this phosphorus-containing high-strength steel has a low elongation rate and does not contain any alloying elements, resulting in the inability to fix C and N atoms in the steel, leading to a low r-value and poor formability. Furthermore, the anisotropy index of the steel plate is not addressed. Additionally, the steel plate is produced using a hot-dip galvanizing annealing process, without any alloying 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. The steel plate has a yield strength of 180~320MPa, a tensile strength of 340~440MPa, an elongation of 35~46%, and an average plastic strain ratio ≥1.8. The steel plate does not mention its formability or anisotropy index. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a 300MPa grade alloyed hot-dip galvanized high-strength phosphorus steel with ultra-high formability for automotive outer panels and its preparation method. Through reasonable composition design and optimization of smelting, hot rolling, pickling, cold rolling, alloyed hot-dip galvanizing and finishing processes, a 300MPa grade alloyed hot-dip galvanized high-strength phosphorus steel with high strength, high plasticity, high formability, high surface quality and low anisotropy index is achieved.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A high-strength, phosphorus-containing alloyed hot-dip galvanized steel with ultra-high formability (300MPa grade) for automotive exterior panels comprises the following components by mass percentage: C: 0.0061%~0.0090%, Si: 0.08%~0.28%, Mn: 0.71%~1.00%, P: 0.066%~0.095%, Cu: 0.40%~0.95%, Nb: 0.020%~0.075%, Ti: 0.015%~0. 0.035%, Ca: 0.0010%~0.0030%, Mg: 0.0010%~0.0030%, Al: 0.010%~0.030%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; among which, calculated by the corresponding mass percentage of each element, 1.05≤(Nb+2Ti) / (8C+7N+3S)≤1.30, 5≤(Cu / P)≤10.

[0007] The present invention also discloses a method for preparing ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels as described above, including smelting, hot rolling, pickling, cold rolling, alloyed hot-dip galvanizing and finishing. 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 a billet. In the hot rolling process, the billet is heated to 1250℃~1320℃ and held in the furnace for 60min~100min; then it is hot rolled into steel plate with an initial rolling temperature of 1140℃~1190℃, a final rolling temperature of ≥940℃, and a coiling temperature of 740℃~780℃. In the alloyed hot-dip galvanizing process, the cold-rolled steel sheet, obtained by sequential pickling and cold rolling, is heated to the annealing isothermal temperature of 860℃~930℃ for 40s~80s. The steel sheet is then slowly cooled to 760℃~820℃ at a rate of 2℃ / s~7℃ / s. Next, the steel sheet is rapidly cooled to 470℃~490℃ at a rate of ≥30℃ / s before being placed in a zinc pot at a temperature of 450℃~460℃. This ensures that the temperature of the rapidly cooled steel sheet before entering the zinc pot is higher than the zinc pot temperature but not exceeding 30℃. The hot-dip galvanizing time is 3s~6s. The zinc... The liquid composition, by mass percentage, is: Al: 0.075%~0.150%, with the remainder being Zn and unavoidable impurity elements; subsequently, the steel plate enters an alloying furnace for alloying treatment at an alloying temperature of 500℃~550℃ for 15s~35s, obtaining an alloyed hot-dip galvanized steel plate with an alloyed coating on the surface; wherein, the iron mass percentage content of the alloyed coating is controlled at 7%~11%; finally, the alloyed hot-dip galvanized steel plate is cooled to room temperature to obtain a cooled steel plate, which is then subjected to the aforementioned finishing treatment to obtain the ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels.

[0008] Implementing the embodiments of the present invention will have the following beneficial effects: (1) The present invention has good economic efficiency by rationally designing the chemical composition of the steel plate to a C, Si, Mn, P system and adding alloying elements such as Nb, Ti, Ca, Cu, and Mg.

[0009] (2) By adding a reasonable amount of Cu and P coupling, the present invention strengthens the grain boundary, reduces the negative impact of the segregation of P element at the grain boundary, and thus improves the steel plate’s resistance to secondary processing brittleness; at the same time, it also improves the steel plate’s resistance to atmospheric corrosion.

[0010] (3) By adding a reasonable amount of Nb and Ti, Ti can firstly form carbon-sulfur compounds such as TiS and Ti4C2S2 with S. On the one hand, this can control the morphology and distribution of sulfides to a certain extent, reducing the adverse effects of S on the performance of the steel plate. On the other hand, TiC will grow epitaxially on Ti4C2S2 particles, making the size of TiC larger, which is conducive to the formation of dispersed TiC compounds, thereby improving the formability of the steel plate. Secondly, it is necessary to ensure that there is sufficient Ti and Nb in the steel plate to combine with carbon and nitrogen to form stable carbonitrides, effectively removing solid solution atoms in the interstitial positions of the steel plate, greatly reducing the obstruction of interstitial atoms to crystal slip. The {111} texture inside the steel plate is significantly enhanced, greatly increasing the r value of the steel plate and effectively improving the formability of the steel plate. In addition, after fixing the carbonitrides, Ti and Nb still retain a certain amount of residual amount. These residual Ti and Nb elements exist in the steel plate matrix in a solid solution state, producing a solid solution strengthening effect and enhancing the strength of the steel plate. At the same time, appropriate amounts of residual Ti and Nb elements can reduce the anisotropy of the steel plate, making the mechanical properties of the steel plate more uniform in all directions and reducing the anisotropy index Δr value of the steel plate.

[0011] (4) By adding a certain amount of Ca, this invention reduces the surface tension of molten steel, improves its fluidity, and helps remove inclusions by flotation, thereby further improving the purity of the steel. On the other hand, Ca has a good desulfurization effect, and CaS preferentially precipitates and disperses in the steel matrix, inhibiting the precipitation and growth of harmful sulfides such as MnS.

[0012] (5) By adding a certain amount of Mg, the present invention has the following advantages: First, Mg has a good deoxidation and desulfurization effect, which can reduce the number and size of inclusions in steel. Second, Mg refines the grains of steel plate, which can improve the strength and plasticity of steel, promote the formation of {111} texture, improve the deep drawing performance of steel, and reduce the anisotropy index Δr value of steel plate.

[0013] (6) The 300MPa grade alloyed hot-dip galvanized steel sheet involved in this invention introduces a certain proportion of critical zone ferrite into the ferrite matrix, combined with dispersed NbC, Ti(C,N) main precipitates and Cu-rich phases, so that the yield strength of the steel sheet reaches 300MPa~340MPa, tensile strength ≥460MPa, elongation after fracture ≥36%, plastic strain ratio r value ≥1.8, work hardening index n value ≥0.17, and anisotropy index Δr value ≤0.35 and FLC0 ≥0.32. Furthermore, by cleverly utilizing the fact that the temperature of the steel sheet before entering the zinc pot is greater than the temperature of the zinc pot but does not exceed 30°C, the zinc liquid can better spread and adhere to its surface, reducing the influence of surface tension, thereby improving the wetting effect of the zinc liquid on the steel sheet and benefiting the surface quality of the steel sheet. Meanwhile, an appropriate temperature difference can make the heat transfer process of the steel plate relatively slow and the chemical reaction rate faster when the steel plate enters the zinc pot, reducing the adhesion of impurities in the zinc liquid to the surface of the steel plate and preventing the phenomenon of zinc solidification or dragging; thus realizing a 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with high strength, high plasticity, high formability, high surface quality and low anisotropy index.

[0014] (7) By controlling the content of Al element in zinc liquid to 0.075%~0.150%, the present invention ensures that the coating has good gloss, promotes the uniformity of alloying reaction, and improves the quality stability of alloyed coating; and generates a very thin inhibition layer composed of Fe2Al5 and a small amount of FeAl2 and FeAl5 at the interface between steel plate and plating solution, which hinders the reaction between Fe-Zn. During subsequent alloying treatment, the inhibition layer will be destroyed, the reaction between Fe-Zn will begin, which helps to form a dense δ phase coating. The Fe-Zn grains on the coating surface are fine and uniform in size distribution. The coating is firmly bonded to the steel plate substrate and is not prone to powdering and peeling during use. Attached Figure Description

[0015] Figure 1 This is a SEM image of the 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels according to Embodiment 1 of the present invention. Detailed Implementation

[0016] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0017] This invention discloses an ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels, comprising the following components by mass percentage: C: 0.0061%~0.0090%, Si: 0.08%~0.28%, Mn: 0.71%~1.00%, P: 0.066%~0.095%, Cu: 0.40%~0.95%, Nb: 0.020%~0.075%, Ti: 0.015 %~0.035%, Ca: 0.0010%~0.0030%, Mg: 0.0010%~0.0030%, Al: 0.010%~0.030%, N≤0.003%, S≤0.003%, the balance being Fe and unavoidable impurities; among which, calculated by the corresponding mass percentage of each element, 1.05≤(Nb+2Ti) / (8C+7N+3S)≤1.30, 5≤(Cu / P)≤10.

[0018] 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 increasing the strength of steel plates. However, excessive C content can lead to the formation of too many carbides and dissolved C in the steel, reducing its elongation, r-value, and n-value, thus decreasing the formability of the steel plate. It also increases the aging risk of the steel plate, affecting its usability. Therefore, this invention requires a C content range of 0.0061% to 0.0090%.

[0019] Si: Si is an important additive element in this invention. Si acts as a solid solution strengthening element, increasing the resistance to dislocation movement and thus improving the strength of the steel plate. Simultaneously, an appropriate amount of Si helps refine the grains, making the steel plate's microstructure more uniform; furthermore, it improves the wettability of the zinc bath on the steel plate surface, allowing the zinc bath to spread better. However, excessive Si content will reduce the fluidity of the zinc bath, leading to uneven zinc coating thickness, with some areas potentially being too thick or too thin. Therefore, this invention requires a Si content range of 0.08% to 0.28%.

[0020] 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 reduce the plasticity and weldability of the steel plate. Therefore, this invention requires the Mn content to be in the range of 0.71% to 1.00%.

[0021] P: P is an important additive element in this invention. P plays a role in solid solution strengthening and is also one of the most economical elements for improving the strength of steel plates. P has a certain influence on the fluidity and wettability of zinc liquid. An appropriate amount of P helps improve the wettability of zinc liquid on the steel plate surface, allowing the zinc liquid to spread better on the steel plate surface, thus obtaining a more uniform galvanized layer. Furthermore, when P is used in combination with Cu, it 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; and it can also increase the surface tension of the zinc liquid, reducing its fluidity and causing defects such as uneven thickness and incomplete plating in the galvanized layer. Therefore, this invention requires the P content to be in the range of 0.066%~0.095%.

[0022] 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 a Cu content range of 0.40% to 0.95%, and it must satisfy the condition: 5 ≤ (Cu / P) ≤ 10, 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.

[0023] 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(C,N), which plays a role in grain refinement and precipitation strengthening, ensuring good plasticity and formability of the steel plate 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 0.015%~0.035%.

[0024] Nb: Nb is an important additive element in this invention. Nb forms precipitates with interstitial elements such as C to eliminate interstitial atoms, thus playing a precipitation strengthening role. Nb can also refine grains and reduce the anisotropy index of the steel plate. However, excessive addition of Nb can lead to excessively high strength and decreased plasticity of the steel plate. Therefore, this invention requires the Nb content to be in the range of 0.020%~0.075%. Furthermore, it needs to satisfy: 1.05≤(Nb+2Ti) / (8C+7N+3S)≤1.30, ensuring sufficient Ti and Nb to fix carbonitrides, effectively remove interstitial solid solution atoms in the steel plate, eliminate the influence of harmful S elements, enhance the {111} texture, increase the r-value of the steel plate, improve the formability of the steel plate, and have a certain amount of residual amount to reduce the anisotropy index of the steel plate, while also playing a solid solution strengthening role. However, the residual amount cannot be too high, as excessive amounts will lead to a decrease in the plasticity of the steel plate.

[0025] Ca: Ca is an important additive element in this invention. Ca can be used as a deoxidizer and inoculant, playing a role in microalloying, which can significantly refine the grains and improve the overall properties of the steel plate, such as plasticity and weldability. Simultaneously, Ca has excellent desulfurization effects; Ca can combine with S to form high-melting-point CaS. During the solidification process of molten steel, CaS preferentially precipitates and disperses in the steel matrix, inhibiting the precipitation and growth of harmful sulfides such as MnS. Therefore, this invention requires the Ca content to be in the range of 0.0010% to 0.0030%.

[0026] Mg: Mg is an important additive element in this invention. Mg is a good deoxidizer and desulfurizer. It can reduce the number of inclusions in steel, decrease their size, and improve their uniform distribution and morphology. Trace amounts of Mg can improve the size and distribution of carbides in steel, promoting fine and uniform carbide particles, thereby refining the grain size of the steel plate. Refined grains can improve the strength and plasticity of the steel. Furthermore, Mg can promote the formation of {111} texture, which is beneficial for improving the deep-drawing performance of steel and reducing the anisotropy index Δr of the steel plate. Therefore, this invention requires the Mg content to be in the range of 0.0010% to 0.0030%.

[0027] 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 0.030%.

[0028] S: S is a harmful element in steel, which easily consumes Mn to form MnS inclusions, impairing the performance of the steel plate. The lower its content, the better, but too low a content will lead to production difficulties and increased costs. Therefore, this invention requires the S content to be controlled within the range of ≤0.003%.

[0029] Nitrogen (N): Nitrogen is a harmful element in steel, causing a deterioration in the elongation and weldability of steel plates. Lower N content is better, but excessively low levels can lead to production difficulties and increased costs. Therefore, this invention requires the N content to be controlled within the range of ≤0.003%.

[0030] In one specific embodiment, the high-strength steel has a yield strength of 300MPa~340MPa, a tensile strength ≥460MPa, an elongation after fracture ≥36%, a plastic strain ratio r ≥1.8, a work hardening index n ≥0.17, and an anisotropy index Δr ≤0.35 and FLC0 ≥0.32 for its forming performance indicators.

[0031] In one specific embodiment, the microstructure of the high-strength steel includes ferrite and dispersed NbC precipitates, Ti(C,N) precipitates and Cu-rich phases; wherein the volume fraction of critical region ferrite in the ferrite is 20% to 30%.

[0032] This invention also discloses a method for preparing the ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels as described above, including smelting, hot rolling, pickling, cold rolling, alloyed hot-dip galvanizing, and finishing; specifically including the following steps: 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.

[0033] S2. In hot rolling, the billet is heated to 1250℃~1320℃ and held in the furnace for 60min~100min; then it is hot rolled into steel plate with an initial rolling temperature of 1140℃~1190℃, a final rolling temperature of ≥940℃, and a coiling temperature of 740℃~780℃.

[0034] Specifically, the parameter settings in step S2 are explained as follows: (1) The present invention controls the heating temperature between 1250℃ and 1320℃ to ensure that the billet is fully austenitized. At the same time, it ensures that the added Ti element fixes the N element, so that TiN precipitation is complete. The furnace time is 60min to 100min. The appropriate heating temperature and holding time ensure that the alloying elements in the billet are completely dissolved and evenly distributed, ensuring the uniform composition of the billet and playing a role in controlling the original austenite grain size.

[0035] (2) In this invention, the initial rolling temperature is controlled between 1140℃ and 1190℃, and the final rolling temperature is ≥940℃. An excessively high initial rolling temperature will lead to the formation of coarse ferrite grains after rolling, reducing the plasticity of the steel plate. An excessively low initial rolling temperature cannot guarantee a final rolling temperature ≥940℃. A final rolling temperature ≥940℃ is necessary to ensure that the entire hot rolling process is carried out within the austenitic single-phase region of the steel plate, avoiding rolling in the austenitic-ferrite two-phase region and preventing mixed grain formation.

[0036] (3) The present invention controls the coiling temperature between 740℃ and 780℃, which is conducive to the precipitation of carbides NbC and TiC. The dispersed carbides can serve as effective nucleation particles, providing nuclei for the formation of recrystallized grains. These particles not only reduce the critical free energy of nucleation, but also greatly increase the probability of nucleation, accelerate the recrystallization process, and thus significantly improve the strength and plasticity of the material. In addition, the precise control of the coiling temperature plays a key role in the nucleation and growth of ferrite. On the one hand, it is conducive to the formation of ferrite nuclei and increases the number of nuclei; on the other hand, it provides sufficient atomic diffusion channels and energy for the growth of nuclei, so that ferrite can grow uniformly and fully.

[0037] 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, and pickled steel plate is obtained.

[0038] Specifically, after the hot rolling process, a layer of iron oxide scale forms on the surface of the steel plate due to oxidation. Pickling uses chemical etching to remove this oxide scale from the surface of the steel plate. Through pickling, the surface smoothness of the steel plate can be effectively restored, and defects such as oxide film generated during the hot rolling process can be eliminated. This creates good surface quality conditions for the subsequent cold rolling process, ensuring that the steel plate can achieve better deformation uniformity and surface quality during the cold rolling process.

[0039] S4. In cold rolling, the cold rolling reduction rate is controlled at 70%~85% to obtain cold-rolled steel sheet.

[0040] Specifically, this invention controls the cold rolling reduction rate, enabling ferrite to acquire sufficient deformation energy through cold rolling, creating favorable conditions for recrystallization and thus ensuring the recrystallization effect. During recrystallization, the {111} texture component grows rapidly with the help of the energy provided by the stored energy, which is beneficial to improving the plasticity and formability of the steel plate.

[0041] S5. In alloyed hot-dip galvanizing, the cold-rolled steel sheet, which has undergone pickling and cold rolling processes sequentially, is heated to the annealing isothermal temperature of 860℃~930℃ for 40s~80s. The steel sheet is then slowly cooled to 760℃~820℃ at a rate of 2℃ / s~7℃ / s. Next, the steel sheet is rapidly cooled to 470℃~490℃ at a rate of ≥30℃ / s before being placed in the zinc pot at a temperature of 450℃~460℃. This ensures that the temperature of the rapidly cooled steel sheet before entering the zinc pot is higher than the zinc pot temperature but not higher. The hot-dip galvanizing process is carried out at 30℃ for 3-6 seconds. The composition of the zinc bath, by mass percentage, is: Al: 0.075%-0.150%, with the remainder being Zn and unavoidable impurity elements. Subsequently, the steel plate enters an alloying furnace for alloying treatment at an alloying temperature of 500℃-550℃ for 15-35 seconds, resulting in an alloyed hot-dip galvanized steel plate with an alloyed coating. The iron content of the alloyed coating is controlled at 7%-11% by mass. Finally, the alloyed hot-dip galvanized steel plate is cooled to room temperature to obtain a cooled steel plate.

[0042] Specifically, this invention employs an alloyed hot-dip galvanizing process. After galvanizing, the steel plate undergoes an alloying treatment, which promotes the diffusion of iron elements from the steel plate into the coating, transforming the pure zinc coating into an alloyed coating. Through this treatment, the alloyed coated steel plate possesses excellent corrosion resistance, paintability, weldability, and high surface strength. The parameter settings in step S5 are explained below: (1) The annealing isothermal temperature is 860℃~930℃, and the annealing isothermal time is 40s~80s. Reasonable annealing isothermal temperature and annealing time ensure that the steel plate is heated above the critical zone, so that an appropriate amount of austenite is formed inside the steel plate. At the same time, it ensures 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 engulfing grains of other orientations. Therefore, as the grains grow, the {111} texture becomes stronger, the r value becomes higher, and the deep drawing performance of the steel plate is better.

[0043] (2) The slow cooling temperature is 760℃~820℃, and the slow cooling rate is 2℃ / s~7℃ / s. The reasonable slow cooling temperature and slow cooling rate have the following three effects: First, to ensure that the appropriate amount of austenite generated during the isothermal annealing stage is transformed into critical zone ferrite. The volume fraction of critical zone ferrite is 20%~30%. Critical zone ferrite is generally composed of equiaxed fine grains with relatively low dislocation density and relatively uniform structure. Due to its fine grains, critical zone ferrite can hinder the movement of dislocations under stress, thereby improving the strength of the steel; at the same time, the fine grains are also conducive to improving the plasticity of the steel. Second, to ensure the precipitation of Cu in the steel plate, forming Cu-rich phase strengthening grain boundaries, and promoting the strength of the steel plate. Third, to ensure the precipitation of carbides NbC and TiC, effectively removing interstitial solid solution atoms in the steel plate, and at the same time, to enhance the {111} texture and improve the formability of the steel plate.

[0044] (3) The steel plate is rapidly cooled to 470℃~490℃ at a rate of ≥30℃ / s and then placed in the zinc pot. The zinc pot temperature is 450℃~460℃. This ensures that the temperature of the steel plate before entering the zinc pot is higher than the zinc pot temperature but not more than 30℃ higher. The hot-dip galvanizing time is 3s~6s. The reasonable rapid cooling rate ensures that the ferrite phase transitions stably to the finished microstructure, effectively promoting the retention of critical ferrite in the final microstructure. The steel plate enters the zinc pot at a temperature higher than the zinc pot temperature but not more than 30℃ higher. Due to the temperature difference, the activity of zinc liquid molecules is significantly enhanced, the surface tension is reduced, and the fluidity is improved. This allows the zinc liquid to spread and adhere more evenly and quickly on the surface of the steel plate, greatly improving the wetting effect of the zinc liquid on the steel plate and facilitating the formation of a high-quality galvanized layer. Furthermore, it inhibits the adhesion of impurities in the zinc liquid to the surface of the steel plate, ensuring the smooth progress of the galvanizing process and the stability of the galvanized layer quality.

[0045] (4) The alloying temperature is 500℃~550℃, and the alloying time is 15s~35s. The alloying process is a process in which the iron content in the coating continuously increases under constant temperature conditions. The most ideal alloyed coating is a dense δ phase (FeZn). 10 Therefore, this invention requires the iron content of the alloyed coating to be controlled at 7%~11% to achieve the formation of a dense δ-phase coating. To achieve this, the alloying temperature is controlled at 500℃~550℃, and the alloying time is controlled at 15s~35s. If the alloying temperature is too high or the time is too long, the coating is prone to forming the Γ-phase (Fe3Zn). 10 ) and Γ1 phase (Fe5Zn 21 If the alloying temperature is too low or the time is too short, the iron content of the alloyed coating will be too low, and the alloying will not be complete, resulting in a deterioration in the surface quality of the galvanized sheet.

[0046] (5) The composition of the zinc bath, by mass percentage, is: Al: 0.075%~0.150%, with the remainder being Zn and unavoidable impurity elements. This content of Al has three functions. First, it ensures that the coating has good gloss because Al has a greater affinity for oxygen than Zn, and an Al2O3 protective film preferentially forms on the surface of the zinc bath, preventing further oxidation. Second, it can reduce the surface tension of the zinc bath, improve its fluidity, help the zinc bath spread evenly on the steel plate surface, promote the uniform alloying reaction, and improve the quality stability of the alloyed coating. Thirdly, during the hot-dip galvanizing process, Fe and Al have a strong affinity, resulting in the formation of a thin inhibitory layer at the interface between the steel plate and the galvanizing bath. This layer consists of Fe2Al5 and small amounts of FeAl2 and FeAl5, hindering the reaction between Fe and Zn. During subsequent alloying, this inhibitory layer is destroyed, allowing the Fe-Zn reaction to begin, which helps form a dense δ-phase coating. The Fe-Zn grains on the coating surface are fine and uniformly distributed, resulting in a strong bond between the coating and the steel substrate. Furthermore, the coating is less prone to powdering and peeling during use. If the Al content is too low, a continuous Fe-Al alloy inhibitory layer cannot be formed, easily leading to defects such as incomplete plating. If the Al content is too high, the Fe-Al alloy inhibitory layer becomes too thick, resulting in incomplete alloying.

[0047] 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 0.6%~1.3% to obtain ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels. This can effectively eliminate the yield plateau of the steel plate and improve the surface quality of the steel plate, making the finished plate straight and without waviness.

[0048] The following are specific embodiments. Examples 1-15 The preparation method of the ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels in this embodiment includes smelting, hot rolling, pickling, cold rolling, alloyed hot-dip galvanizing, and finishing; specifically, it includes the following steps: 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.

[0049] 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.

[0050] 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, and pickled steel plate is obtained.

[0051] S4. In cold rolling, the cold rolling reduction rate is controlled to obtain cold-rolled steel sheet.

[0052] S5. In alloyed hot-dip galvanizing, the cold-rolled steel sheet 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. Then, the steel sheet is rapidly cooled to the temperature before galvanizing and then enters the zinc pot. The temperature of the zinc pot is controlled to ensure that the temperature of the rapidly cooled steel sheet before entering the zinc pot is higher than the temperature of the zinc pot but does not exceed 30°C. After hot-dip galvanizing, the steel sheet enters the alloying furnace for alloying treatment to obtain an alloyed hot-dip galvanized steel sheet with an alloyed coating on the surface. After cooling to room temperature, a cooled steel sheet is obtained.

[0053] S6. In the finishing process, the cooled steel plate is fed into the finishing machine for plate shape adjustment and finishing elongation control to obtain ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels.

[0054] 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 alloying hot-dip galvanizing and finishing process parameters of the steels in the examples, and Table 4 lists the properties and metallographic structure of the steels in the examples.

[0055] Table 1 Chemical composition (wt, %) of the steel in the examples

[0056] Table 2 Hot rolling and cold rolling process parameters for the steel in the examples

[0057] Table 3. Alloying, hot-dip galvanizing, and finishing process parameters for the steels used in the examples.

[0058] Table 4 Properties and metallographic structure of the steels used in the examples

[0059] As can be seen from the above embodiments, the clever use of coupling addition between alloying elements makes the composition design of the steel plate highly economical. Combined with the optimized alloying hot-dip galvanizing process, alloyed hot-dip galvanized steel plates with yield strength of 300MPa~340MPa, tensile strength ≥460MPa, elongation after fracture ≥36%, plastic strain ratio r value ≥1.8, work hardening index n value ≥0.17, anisotropy index Δr value ≤0.35, and FLC0 ≥0.32 were prepared. This achieves the characteristics of high strength, high plasticity, high formability, high surface quality, and low anisotropy index for automotive outer panels.

[0060] 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 high-strength, phosphorus-containing alloyed hot-dip galvanized steel with ultra-high formability (300MPa grade) for automotive outer panels, characterized in that... The components include the following percentages by mass: C: 0.0061%~0.0090%, Si: 0.08%~0.28%, Mn: 0.71%~1.00%, P: 0.066%~0.095%, Cu: 0.40%~0.95%, Nb: 0.020%~0.075%, Ti: 0.015%~0.035%, Ca: 0.0010%~0.0030%, Mg: 0.0010%~0.0030%, Al: 0.010%~0.030%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; among which, calculated by the corresponding mass percentage of each element, 1.05≤(Nb+2Ti) / (8C+7N+3S)≤1.30, 5≤(Cu / P)≤10.

2. The ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels according to claim 1, characterized in that, The high-strength steel has a yield strength of 300MPa~340MPa, tensile strength ≥460MPa, elongation after fracture ≥36%, plastic strain ratio r ≥1.8, work hardening index n ≥0.17, anisotropy index Δr ≤0.35, and FLC0 ≥0.

32.

3. The ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels according to claim 1, characterized in that, The microstructure of the high-strength steel includes ferrite and dispersed NbC precipitates, Ti(C,N) precipitates and Cu-rich phases; wherein the volume fraction of critical ferrite in the ferrite is 20% to 30%.

4. A method for preparing ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels as described in any one of claims 1-3, characterized in that, This includes smelting, hot rolling, pickling, cold rolling, alloying hot-dip galvanizing, and finishing; 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 a billet. In the hot rolling process, the billet is heated to 1250℃~1320℃ and held in the furnace for 60min~100min; then it is hot rolled into steel plate with an initial rolling temperature of 1140℃~1190℃, a final rolling temperature of ≥940℃, and a coiling temperature of 740℃~780℃. In the alloyed hot-dip galvanizing process, the cold-rolled steel sheet, obtained by sequential pickling and cold rolling, is heated to the annealing isothermal temperature of 860℃~930℃ for 40s~80s. The steel sheet is then slowly cooled to 760℃~820℃ at a rate of 2℃ / s~7℃ / s. Next, the steel sheet is rapidly cooled to 470℃~490℃ at a rate of ≥30℃ / s before being placed in a zinc pot at a temperature of 450℃~460℃. This ensures that the temperature of the rapidly cooled steel sheet before entering the zinc pot is higher than the zinc pot temperature but not exceeding 30℃. The hot-dip galvanizing time is 3s~6s. The zinc... The liquid composition, by mass percentage, is: Al: 0.075%~0.150%, with the remainder being Zn and unavoidable impurity elements; subsequently, the steel plate enters an alloying furnace for alloying treatment at an alloying temperature of 500℃~550℃ for 15s~35s, obtaining an alloyed hot-dip galvanized steel plate with an alloyed coating on the surface; wherein, the iron mass percentage content of the alloyed coating is controlled at 7%~11%; finally, the alloyed hot-dip galvanized steel plate is cooled to room temperature to obtain a cooled steel plate, which is then subjected to the aforementioned finishing treatment to obtain the ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels.

5. The method for preparing ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels according to claim 4, characterized in that, In the finishing process, the cooled steel plate is fed into the finishing machine for plate shape adjustment, and the finishing elongation is controlled to be 0.6%~1.3%.

6. The method for preparing ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels 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.

7. The method for preparing ultra-high formability 300MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel for automotive outer panels according to claim 4, characterized in that, In the cold rolling process, the cold rolling reduction rate is controlled at 70%~85% to obtain the cold-rolled steel sheet.

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