180MPa-grade alloyed hot-galvanized phosphorus-containing high-strength steel with excellent surface quality and preparation method thereof

Abstract

The invention discloses 180MPa-grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality and a preparation method thereof, and the component design of a steel plate has very high economical efficiency by ingeniously utilizing coupling addition among alloy elements; by combining an optimized alloying hot-dip galvanizing process, the alloying hot-dip galvanized steel plate with the yield strength being 180-220 MPa, the tensile strength being larger than or equal to 320 MPa, the percentage elongation after fracture being larger than or equal to 45%, the plastic strain ratio r value being larger than or equal to 2.3, the work hardening index n value being larger than or equal to 0.23 and the roughness of the upper surface and the lower surface of the steel plate being 0.8-1.8 microns respectively is prepared, and the pulverization resistance of the 60-degree V-bending experiment is of the first level. The automobile outer plate has the advantages of being high in plasticity, high in concave resistance, high in forming performance and high in surface quality.

Description

Technical Field

[0001] This invention relates to the field of automotive steel manufacturing technology, and more specifically, to a 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality and its preparation method. Background Technology

[0002] As the automotive industry continues to expand, customers' demands for vehicle body quality and appearance are increasing. As the "face" of a car, the design, manufacturing technology, and processes of automotive outer panels are becoming more complex. Automotive outer panels are often large, curved, and complex-shaped structures, and are exposed to the elements for extended periods. Currently, the most common steels used for automotive outer panels are IF steel and BH steel, primarily with a pure zinc coating. IF steel is made by adding a certain amount of alloying elements to ultra-low carbon steel, completely fixing the carbon and nitrogen atoms into carbonitride compounds. This eliminates interstitial atoms in the steel matrix, resulting in pure ferrite and excellent formability and no aging. However, IF steel has drawbacks such as insufficient dent resistance and poor lightweighting. BH steel retains some interstitial solid solution carbon atoms. During the coating and baking process, the solid solution carbon pins dislocations, hindering their movement and increasing the steel's strength, giving it excellent dent resistance. However, BH steel suffers from significant natural aging, leading to a decline in formability. Furthermore, pure zinc coatings currently have significant drawbacks. Firstly, their low hardness makes them susceptible to wear and scratches during the stamping of automotive parts, affecting their integrity and protective performance. Secondly, they are prone to reacting with the welding electrode during welding, reducing the lifespan of the welding electrode. To overcome these drawbacks, alloyed hot-dip galvanizing has emerged. Alloyed hot-dip galvanizing involves further heating after zinc plating, causing iron in the substrate to diffuse into the coating, transforming the pure zinc layer into an alloyed layer. This results in alloyed hot-dip galvanized steel sheets possessing excellent corrosion resistance, paintability, weldability, and high surface strength. Therefore, there is an urgent need to develop a hot-dip galvanized alloyed steel sheet for automotive outer panels that combines high plasticity, high dent resistance, high formability, and high surface quality to meet the fierce competition in the automotive steel market.

[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. This leads to a low r-value in the steel sheet, poor formability, and a tendency for natural aging. Furthermore, the steel sheet is produced using a hot-dip galvanizing annealing process without alloying. Patent application number 201610728440.8 discloses an iron-zinc coated steel sheet with a yield strength of 210 MPa and its production method. The iron-zinc coated steel sheet with a yield strength of 210 MPa has the following chemical composition (wt%): C: 0.03–0.04%, Si: 0.007–0.015%, Mn: 0.32–0.4%, P≤0.02%, S≤0.01%, Als: 0.015–0.05%, N≤0.005%. The remainder is Fe and unavoidable impurities. However, this steel sheet does not use added alloying elements, resulting in the inability to fix C and N atoms in the steel, leading to a low r-value, poor formability, and susceptibility to natural aging. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality and its preparation method. Through the rational design of chemical composition and production process, combined with alloyed hot-dip galvanizing process, the high-strength steel obtained has high plasticity, high dent resistance, high formability and high surface quality.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A high-strength, 180MPa grade alloyed hot-dip galvanized phosphorus-containing steel with excellent surface quality comprises the following components by mass percentage: C: 0.0005%~0.0020%, Si: 0.003%~0.020%, Mn: 0.10%~0.25%, P: 0.015%~0.030%, V: 0.010%~0.050%, Cu: 0.05%~0.30%, Al: 0.010%~0.050%, Ca: 0.0020%~0.0060%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; wherein, calculated by the corresponding mass percentage of each element, 15≤(V / C)≤25, 4≤(Cu / P)≤8, 10≤(Al / N)≤20.

[0006] This invention also discloses a method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality as described above, including smelting, hot rolling, pickling, cold rolling, alloyed hot-dip galvanizing, and finishing; in the alloyed hot-dip galvanizing process, the cold-rolled steel sheet obtained by sequentially undergoing pickling and cold rolling is heated to the annealing isothermal temperature, which is 810℃~870℃, and the annealing isothermal time is 40s~120s; then the steel sheet is slowly cooled to 680℃~750℃ at a slow cooling rate of 3℃ / s~10℃ / s; then the steel sheet is rapidly cooled to 460℃~480℃ at a rapid cooling rate of ≥30℃ / s before being placed in a zinc pot at a temperature of 450℃~460℃ to ensure the rapid cooling of the steel sheet. The temperature of the steel sheet before entering the zinc pot is higher than the zinc pot temperature but not more than 20°C, and the hot-dip galvanizing time is 3s~5s. The composition of the zinc liquid, by mass percentage, is: Al: 0.05%~0.15%, with the remainder being Zn and unavoidable impurity elements. Subsequently, the steel sheet enters the alloying furnace for alloying treatment at an alloying temperature of 480°C~530°C for 10s~30s, obtaining an alloyed hot-dip galvanized steel sheet with an alloyed coating on the surface. The iron mass percentage content of the alloyed coating is controlled at 7%~11%. Finally, the alloyed hot-dip galvanized steel sheet is cooled to room temperature to obtain a cooled steel sheet, which is then subjected to the aforementioned finishing treatment to obtain the 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality.

[0007] Implementing the embodiments of the present invention will have the following beneficial effects: (1) This invention achieves good economic efficiency by rationally designing the chemical composition of the steel plate to a C, Si, Mn, P system and adding trace amounts of alloying elements such as V, Ca, and Cu. (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.

[0008] (3) This invention effectively removes interstitial solid solution atoms in steel plates by adding a reasonable amount of V and C coupling, which not only eliminates the natural aging of the steel plates but also improves their formability. Secondly, the formation of carbides (VC) serves the purpose of precipitation strengthening and grain refinement, and can also further enhance the {111} texture, which is beneficial to both the strength and plasticity of the steel plates. Finally, a certain amount of V residue is ensured to balance the strength and plasticity of the steel plates.

[0009] (4) By adding a reasonable amount of Al and N in a coupled manner, the present invention first ensures that sufficient Al is added as a deoxidizer to remove oxygen dissolved in the molten steel during oxygen blowing smelting, and secondly ensures that after being used as a deoxidizer, there is a sufficient amount of Al to fix interstitial N atoms, thereby improving the plasticity and forming performance of the steel plate.

[0010] (5) By adding a reasonable 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, with CaS preferentially precipitating and dispersing in the steel matrix, inhibiting the precipitation and growth of harmful sulfides such as MnS.

[0011] (6) The 180MPa 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 VC precipitates, AlN precipitates, and Cu-rich phases. The yield strength of the steel sheet is 180MPa~220MPa, tensile strength ≥320MPa, elongation after fracture ≥45%, plastic strain ratio r ≥2.3, and work hardening index n ≥0.23. Furthermore, by cleverly utilizing the fact that the temperature of the steel sheet before entering the zinc pot is higher than the temperature of the zinc pot but not more than 20°C, the zinc liquid can be better spread and adhered 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. At the same time, an appropriate temperature difference can make the heat transfer process of the steel sheet relatively slow and the chemical reaction rate faster when the steel sheet enters the zinc pot, reducing the adhesion of impurities in the zinc liquid to the surface of the steel sheet and preventing the phenomenon of zinc solidification or zinc dragging. This process ensures that the roughness of the upper and lower surfaces of the steel plate is between 0.8μm and 1.8μm, respectively, and that the anti-powdering performance in the 60°V bending test is Grade 1. This results in the preparation of 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel, which possesses high plasticity, high anti-denting performance, high formability, and high surface quality.

[0012] (7) By controlling the mass percentage of Al element in the zinc bath to 0.05%~0.15%, the present invention ensures that the coating has good gloss, promotes the uniformity of the alloying reaction, and improves the quality stability of the alloyed coating. It also generates a very thin inhibition layer composed of Fe2Al5 and a small amount of FeAl2 and FeAl5 at the interface between the steel plate and the plating solution, which hinders the reaction between Fe and Zn. During the subsequent alloying treatment, the inhibition layer will be destroyed, and the reaction between Fe and Zn will begin, which helps to form a dense δ phase coating. The Fe-Zn grains on the coating surface are small and uniform in size. The coating is firmly bonded to the steel plate substrate and is not prone to powdering and peeling during use. Detailed Implementation

[0013] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0014] This invention discloses a 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality, comprising the following components by mass percentage: C: 0.0005%~0.0020%, Si: 0.003%~0.020%, Mn: 0.10%~0.25%, P: 0.015%~0.030%, V: 0.010%~0.050%, Cu: 0.05%~0.30%, Al: 0.010%~0.050%, Ca: 0.0020%~0.0060%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; wherein, calculated by the corresponding mass percentage of each element, 15≤(V / C)≤25, 4≤(Cu / P)≤8, 10≤(Al / N)≤20.

[0015] 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 the C content to be in the range of 0.0005% to 0.0020%.

[0016] 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 can have adverse effects on hot-dip galvanizing. It can cause a dramatic increase in the Fe-Zn reaction in the galvanized layer, resulting in a thicker coating and reduced adhesion. Therefore, this invention requires the Si content to be in the range of 0.003% to 0.020%.

[0017] 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 0.10% to 0.25%.

[0018] 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. 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 0.015% to 0.030%.

[0019] 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. Furthermore, when Cu is used in combination with P, it can improve the atmospheric corrosion resistance of the steel plate, enhancing its corrosion resistance in harsh environments to a certain extent. 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%~0.30%, and it must satisfy: 4≤(Cu / P)≤8, ensuring sufficient Cu segregation at grain boundaries, reducing the adverse effects of P segregation at grain boundaries, and reducing the risk of brittleness during secondary processing of the steel plate.

[0020] V: V is an important additive element in this invention. V is a strong carbide-forming element, forming VC carbides with interstitial carbon atoms, which plays a precipitation strengthening role, thus increasing the strength of the steel plate. Simultaneously, the VC precipitates exhibit a fine and dispersed distribution, effectively hindering grain boundary migration and inhibiting grain growth, ultimately achieving a grain refinement effect and contributing to improved plasticity of the steel plate. However, excessive V can lead to increased strength but decreased plasticity. Therefore, this invention requires a V content range of 0.010% to 0.050%, and it must satisfy: 15 ≤ (V / C) ≤ 25, ensuring sufficient V to fix C, effectively removing interstitial solid solution atoms in the steel plate, ensuring the formation of a {111} texture, increasing the r-value of the steel plate, and having a certain amount of residual V to balance the strength and plasticity of the steel plate.

[0021] Al: Al is an important additive element in this invention. Al has two main functions. First, it is added as a deoxidizer during steelmaking, primarily to remove oxygen dissolved in the molten steel during oxygen blowing. Second, Al has a strong affinity for nitrogen (N), fixing interstitial N atoms to form AlN, thus improving the plasticity and formability of the steel plate. However, excessively high Al content can lead to excessive inclusions. Therefore, this invention requires an Al content range of 0.010% to 0.050%, and it must satisfy the condition: 10 ≤ (Al / N) ≤ 20, ensuring that after removing oxygen dissolved in the molten steel during oxygen blowing, sufficient Al is available to fix interstitial N atoms, thereby improving the plasticity and formability of the steel plate.

[0022] 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 0.0020%~0.0060%.

[0023] 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%.

[0024] 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%.

[0025] In one specific embodiment, the high-strength steel has a yield strength of 180MPa~220MPa, a tensile strength ≥320MPa, an elongation after fracture ≥45%, a plastic strain ratio r value ≥2.3, a work hardening index n value ≥0.23, and the roughness of the upper and lower surfaces of the steel plate is 0.8μm~1.8μm, respectively. The steel plate also has a grade 1 anti-powdering performance in a 60° V-bending test.

[0026] In one specific embodiment, the microstructure of the high-strength steel includes ferrite and dispersed VC precipitates, AlN precipitates and Cu-rich phases; wherein the volume fraction of critical ferrite in the ferrite is 3% to 10%.

[0027] This invention also discloses a method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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.

[0028] S2. In hot rolling, the heating temperature is 1150℃~1210℃, and the furnace time is 60min~150min; then hot rolling is carried out into steel plates, with an initial rolling temperature of 1010℃~1080℃, a final rolling temperature of ≥880℃, and a coiling temperature of 660℃~720℃.

[0029] Specifically, the principles of the key parameters of the hot rolling process of this invention are as follows: (1) The present invention controls the heating temperature between 1150℃ and 1210℃, which ensures that the billet is fully austenitized. The furnace time is 60min to 150min. 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 controlling the original austenite grain size. When the heating temperature is higher than 1210℃, the austenite grains are prone to grow excessively, affecting the mechanical properties of the final product; when the temperature is lower than 1150℃, austenitization will be incomplete, affecting the rolling performance and the uniformity of the final product. A furnace time of less than 60min is prone to uneven composition, and a furnace time of more than 150min is prone to coarse austenite grains.

[0030] (2) In this invention, the initial rolling temperature is controlled between 1010℃ and 1080℃, and the final rolling temperature is ≥880℃. 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 ≥880℃, while a final rolling temperature ≥880℃ 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.

[0031] (3) The present invention controls the coiling temperature between 660℃ and 720℃, which is conducive to the formation of relatively coarse carbonitrides VC and AlN. During the annealing process, these carbonitrides can act as nucleation sites for grains and promote recrystallization. This is conducive to the formation of uniform and coarse equiaxed ferrite grains, thereby improving the deep drawing performance of the steel plate.

[0032] 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.

[0033] 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.

[0034] S4. In cold rolling, the cold rolling reduction rate is controlled at 75%~90% to obtain cold-rolled steel sheet.

[0035] Specifically, this invention controls the cold rolling reduction rate to 75%~90%. Cold rolling allows ferrite to acquire sufficient deformation energy, creating favorable conditions for recrystallization and thus ensuring the recrystallization effect. During recrystallization, the {111} texture component grows rapidly with the help of the stored energy. Since there is a positive correlation between the {111} texture and the plastic strain ratio r of the steel sheet, its rapid growth is beneficial to improving the deep-drawing performance of the steel sheet.

[0036] S5. In alloy hot-dip galvanizing, the cold-rolled steel sheet, which has undergone pickling and cold rolling processes, is heated to the annealing isothermal temperature of 810℃~870℃ for 40s~120s. The steel sheet is then slowly cooled to 680℃~750℃ at a rate of 3℃ / s~10℃ / s. Next, the steel sheet is rapidly cooled to 460℃~480℃ 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 but not higher than the zinc pot temperature. The temperature exceeds 20℃, and the hot-dip galvanizing time is 3s~5s. The composition of the zinc liquid, by mass percentage, is: Al: 0.05%~0.15%, 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 480℃~530℃ for 10s~30s, obtaining an alloyed hot-dip galvanized steel plate with an alloyed coating on the surface. 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.

[0037] Specifically, the key parameters are explained below: (1) The present invention controls the annealing isothermal temperature to be 810℃~870℃ and the annealing isothermal time to be 40s~120s. The reasonable annealing isothermal temperature and annealing isothermal time ensure that the steel plate is heated to above the critical zone, so that a small 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 sufficiently, 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.

[0038] (2) The present invention controls the slow cooling temperature to be 680℃~750℃ and the slow cooling rate to be 3℃ / s~10℃ / s. The reasonable slow cooling temperature and slow cooling rate have the following three effects: First, it ensures that the small amount of austenite generated during the isothermal annealing stage is transformed into critical region ferrite. Critical region ferrite is generally composed of equiaxed fine grains with relatively low dislocation density and a relatively uniform structure. Due to its fine grains, critical region 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, it ensures the precipitation of Cu in the steel plate, forming Cu-rich phase strengthening grain boundaries, and promoting the strength of the steel plate. Third, it ensures the precipitation of carbide VC, effectively removing interstitial solid solution atoms in the steel plate, and at the same time, it is conducive to enhancing the {111} texture and improving the formability of the steel plate.

[0039] (3) In this invention, the steel plate is rapidly cooled to 460℃~480℃ at a rate of ≥30℃ / s before entering the zinc pot. The zinc pot temperature is 450℃~460℃, ensuring that the temperature of the steel plate before entering the zinc pot is greater than the zinc pot temperature but not more than 20℃, and the hot-dip galvanizing time is 3s~5s. Reasonable rapid cooling ensures that the critical zone ferrite with a volume fraction of 3%~10% is retained in the final microstructure. Ensuring that the temperature of the steel plate before entering the zinc pot is greater than the zinc pot temperature but not more than 20℃ has the following two effects: First, it allows the zinc liquid to spread and adhere better on its surface, reducing the influence of surface tension, thereby improving the wetting effect of the zinc liquid on the steel plate and benefiting the surface quality of the steel plate. Second, an appropriate temperature difference can make the heat transfer process of the steel plate relatively slow and the chemical reaction rate faster when it 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 zinc dragging.

[0040] (4) The alloying temperature is 480℃~530℃, and the alloying time is 10s~30s. The alloying process is a process in which the iron content in the coating continuously increases under constant temperature environment. 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% by mass to achieve a dense δ-phase coating with optimal anti-powdering properties. To achieve this, the alloying temperature is controlled at 480℃~530℃, and the alloying time is controlled at 10s~30s. Excessive alloying temperature or time can easily lead to the formation of the Γ-phase (Fe3Zn). 10 ) and Γ1 phase (Fe5Zn 21 This can lead to poor anti-powdering properties of the coating and a deterioration in the surface quality of the galvanized sheet. Additionally, excessively low alloying temperatures and short alloying times can result in low iron content in the alloyed coating, preventing complete alloying and thus deteriorating the surface quality of the galvanized sheet.

[0041] (5) The composition of the zinc bath, by mass percentage, is: Al: 0.05%~0.15%, 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 O than Zn, and an Al2O3 protective film is preferentially formed 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.

[0042] 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%~0.5%, which 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, and finally obtaining 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality.

[0043] The following are specific embodiments. Examples 1-15 The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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.

[0044] 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.

[0045] 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.

[0046] S4. In cold rolling, the cold rolling reduction rate is controlled to obtain cold-rolled steel sheet.

[0047] In S5, during alloyed hot-dip galvanizing, the cold-rolled steel sheet, which has undergone pickling and cold rolling in sequence, is heated to the annealing isothermal temperature, and the annealing isothermal time is controlled. The steel sheet is then slowly cooled to the slow cooling temperature. After rapid cooling, the steel sheet is placed in a zinc pot, ensuring that the temperature of the rapidly cooled steel sheet before entering the zinc pot is higher than the zinc pot temperature but does not exceed 20°C, and the hot-dip galvanizing time is controlled. The steel sheet then enters an alloying furnace for alloying treatment to obtain an alloyed hot-dip galvanized steel sheet with an alloyed coating on the surface. The iron content of the alloyed coating is controlled to be 7%~11%. Finally, the alloyed hot-dip galvanized steel sheet is cooled to room temperature to obtain a cooled steel sheet.

[0048] S6. In the finishing process, the cooled steel plate is fed into the finishing machine for plate shape adjustment and finishing elongation control, ultimately obtaining 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality.

[0049] 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; Table 4 lists the mechanical properties and metallographic structure of the steels in the examples; and Table 5 lists the properties of the alloying coating of the steels in the examples.

[0050] Table 1 Chemical composition of the steel in the examples, wt%

[0051] Table 2. Process parameters for hot rolling and cold rolling of the steel in the examples.

[0052] Table 3. Alloying hot-dip galvanizing and finishing process parameters for the steels used in the examples.

[0053] Table 4 Properties and metallographic structure of the steels in the examples

[0054] Table 5. Coating properties of the steels in the examples

[0055] 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, alloying hot-dip galvanized steel plates with yield strength of 180MPa~220MPa, tensile strength ≥320MPa, elongation after fracture ≥45%, plastic strain ratio r value ≥2.3, work hardening index n value ≥0.23, roughness of the upper and lower surfaces of the steel plate of 0.8μm~1.8μm respectively, and anti-powdering performance of Grade 1 in 60°V bending test are produced. The microstructure includes ferrite and dispersed VC precipitate phase, AlN precipitate phase and Cu-rich phase. Among them, the volume fraction of critical ferrite in the ferrite is 3%~10%, which realizes the characteristics of high plasticity, high dent resistance, high formability and high surface quality of automobile outer panels.

[0056] 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 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality, characterized in that, Includes the following components by mass percentage: C: 0.0005%~0.0020%, Si: 0.003%~0.020%, Mn: 0.10%~0.25%, P: 0.015%~0.030%, V: 0.010%~0.050%, Cu: 0.05%~0.30%, Al: 0.010%~0.050%, Ca: 0.0020%~0.0060%, N≤0.003%, S≤0.003%, with the balance being Fe and unavoidable impurities; calculated by the mass percentage of each element, 15≤(V / C)≤25, 4≤(Cu / P)≤8, 10≤(Al / N)≤20.

2. The 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality according to claim 1, characterized in that, The high-strength steel has a yield strength of 180MPa~220MPa, tensile strength ≥320MPa, elongation after fracture ≥45%, plastic strain ratio r ≥2.3, work hardening index n ≥0.23, and the roughness of the upper and lower surfaces of the steel plate is 0.8μm~1.8μm, respectively. The steel plate has a grade 1 anti-powdering performance in a 60° V-bending test.

3. The 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality according to claim 1, characterized in that, The microstructure of the high-strength steel includes ferrite and dispersed VC precipitates, AlN precipitates and Cu-rich phases; wherein the volume fraction of critical ferrite in the ferrite is 3% to 10%.

4. A method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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 alloyed hot-dip galvanizing process, the cold-rolled steel sheet, which has undergone pickling and cold rolling treatment in sequence, is heated to the annealing isothermal temperature, which is 810℃~870℃, and the annealing isothermal time is 40s~120s. Then, the steel sheet is slowly cooled to 680℃~750℃ at a slow cooling rate of 3℃ / s~10℃ / s. Then, the steel sheet is rapidly cooled to 460℃~480℃ at a rapid cooling rate of ≥30℃ / s before entering the zinc pot, where the zinc pot temperature is 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 more than 20℃ higher. The hot-dip galvanizing time is 3s~5s. The zinc bath composition, by mass percentage, is: Al: 0.05%~0.15%, with the remainder being Zn and unavoidable impurity elements. The steel plate is then placed in an alloying furnace for alloying treatment at a temperature of 480℃~530℃ for 10s~30s, 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, which is then subjected to the aforementioned finishing treatment to obtain a 180MPa grade alloyed hot-dip galvanized high-strength phosphorus-containing steel with excellent surface quality.

5. The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality according to claim 4, characterized in that, In the hot rolling process, the heating temperature is 1150℃~1210℃, and the furnace time is 60min~150min; then, the steel plate is hot rolled into a steel plate with an initial rolling temperature of 1010℃~1080℃, a final rolling temperature of ≥880℃, and a coiling temperature of 660℃~720℃.

6. The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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.

7. The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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.

8. The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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.

9. The method for preparing 180MPa grade alloyed hot-dip galvanized phosphorus-containing high-strength steel with excellent surface quality 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 0.2%~0.5%.

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