A 220mpa grade low-ripple galvannealed bake hardening steel sheet and a method of manufacturing and use thereof

Abstract

This invention relates to the field of automotive steel technology, specifically to a 220MPa grade low-wavyness hot-dip galvanized bake-hardening steel sheet, its manufacturing method, and its application. The main chemical elements are C, Si, Mn, P, Als, Nb, N, and S. The dissolved C content in the steel is 0.0008%–0.0015%, with the balance being iron and unavoidable impurities. This invention also discloses a manufacturing method for the above-mentioned bake-hardening hot-dip galvanized steel sheet, including steelmaking, continuous casting, slab heating, hot rolling, laminar flow cooling, pickling, hot-dip galvanizing, and leveling processes. The surface waviness Wsa of the produced steel sheet after 5% forming is... (1‑5) ≤0.35μm, increase in surface waviness ΔWsa (1‑5) With a thickness of ≤0.05μm and a yield strength of 220~280MPa, it meets the application requirements of water-based no-primer coating process for automotive exterior body panels.

Description

Technical Field

[0001] This invention relates to the field of automotive steel technology, specifically to a 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet and its manufacturing method. Background Technology

[0002] In the process of automotive lightweighting, high-strength steel is often used as a substitute. However, high-strength steel is more difficult to form. Bake-hardening steel has a lower yield strength before baking, making it easy to stamp and form. After painting and baking, its yield strength increases, and it has good dent resistance. It is often used in the manufacture of automotive outer panels. 180MPa grade bake-hardening steel has been widely used in automotive outer body panels. With the trend of automotive lightweighting, the higher strength 220MPa grade bake-hardening steel is increasingly being used in parts such as engine hood panels and door panels.

[0003] With the advancement of green and energy-saving technologies in the automotive industry, the latest key technology developed by automobile manufacturers is water-based no-primer coating. This process reduces the paint film thickness by 20-30 μm compared to conventional processes, achieving energy conservation and emission reduction in both manufacturing and usage, and significantly lowering production costs. It is increasingly being adopted by automobile manufacturers and is considered one of the most promising coating technologies currently available. However, the elimination of the primer layer, which provides high coverage for surface defects in the steel sheet, reduces the paint film's ability to conceal these defects, resulting in a decrease in surface clarity. There is a significant correlation between surface waviness and coating effect; generally, lower waviness leads to better coating results. According to the PV1045 standard, the waviness Wsa of a no-primer automotive exterior panel after 5% coating is achieved... (1-5) It must not exceed 0.35μm. In actual coating processes, waviness usually increases with forming, except for the waviness Wsa after forming 5%. (1-5) In addition, the increase in waviness before and after forming also has a significant impact on the paint clarity. The smaller the increase in waviness after forming, the less pronounced the "orange peel texture" on the steel plate surface, and the higher the paint clarity quality.

[0004] CN115369314B discloses a bake-hardening hot-dip galvanized steel sheet with low waviness and an anti-aging process and its manufacturing method. However, this technical solution uses Nb-Ti composite addition, and the coarse TiN inclusions may have a potential impact on the surface quality of the steel sheet, which is not conducive to the uniform refinement of the grain structure. It only controls the absolute waviness Wsa after deformation, and fails to limit the waviness increment ΔWsa, which is a more stringent indicator that reflects the uniformity of material deformation, and cannot fundamentally guarantee the appearance quality requirements of the no-primer coating.

[0005] CN111101067A discloses a hot-dip galvanized steel sheet with stable bake-hardening performance and its production method. The technical solution adopts an ultra-low carbon composition design with Ti-Nb composite micro-alloying, and precisely controls the solid solution C content at 0.0005%~0.0015%. The solid solution C content is finely adjusted by adjusting the annealing temperature, so that the product's bake-hardening value is stable at 30~50MPa within 6 months. However, this technical solution only focuses on the stability of the bake-hardening value and does not involve the control of the surface waviness of the steel sheet at all, which cannot meet the stringent requirements of the no-intermediate-coating process for the surface quality after forming.

[0006] CN109844159A discloses a steel for painted parts, whose composition system is a Nb\Ti\Mo composite system. The alloy composition is difficult to control, and the control of waviness relies on the roughness of the rolls of the cold rolling mill. It does not involve hot rolling and hot-dip galvanizing processes. The waviness increases significantly before and after forming, and there is no essential improvement in the painting quality.

[0007] CN108531819B discloses a hot-dip galvanized steel sheet that meets the requirements for paint-free automotive outer panels and its manufacturing method. However, the yield strength of the steel is 180MPa, which can only meet the requirements of grade HC180BD+Z and cannot meet the requirements for higher strength automotive outer panel parts such as grade HC220BD+Z.

[0008] CN116219323B discloses a hot-dip galvanized alloyed H220BD steel plate with uniform pores and low surface waviness, and its preparation method. Its surface is an alloyed GA coating, which is not suitable for parts using hot-dip galvanized GI coating.

[0009] Therefore, there is an urgent need to provide a hot-dip galvanized bake-hardening steel sheet that can meet the 220MPa strength level, has low waviness increment, is suitable for intermediate coating process, and its manufacturing method and application. Summary of the Invention

[0010] To address the technical problems of existing bake-hardening steel sheets having large waviness increments, low strength grades, and inability to meet the requirements of intermediate coating-free coating processes, this invention provides a 220MPa-grade low-waviness hot-dip galvanized bake-hardening steel sheet, its manufacturing method, and its application. By employing a pure Nb microalloyed ultra-low carbon steel composition system instead of an Nb / Ti composite system, the invention fully leverages the dual effects of Nb precipitation strengthening and grain refinement strengthening. Furthermore, it synergistically optimizes the entire process, including hot-rolling high-temperature final rolling and coiling, high-reduction cold rolling, and refined parameter control of the finishing rolls. The resulting hot-dip galvanized bake-hardening steel sheet achieves a yield strength exceeding 220MPa, and after forming (5%), the surface waviness Wsa... (1-5) ≤0.35μm, increase in surface waviness ΔWsa after forming (1-5) ≤0.05μm.

[0011] The technical solution of this invention is as follows: In a first aspect, the present invention provides a 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet, the chemical composition by mass percentage of which is: C: 0.0015%~0.003%, Si≤0.06%, Mn: 0.40%~0.60%, P: 0.035%~0.045%, S≤0.01%, Als: 0.02%~0.06%, Nb: 0.005%~0.015%, N≤0.003%, solution C: 0.0008~0.0015%, with the balance being iron and unavoidable impurities.

[0012] Furthermore, the mechanical properties of the steel plate meet the following requirements: yield strength 220~280MPa, tensile strength 320~440MPa, elongation A80≥32%, n90 value≥0.15, r90 value≥1.20, and bake hardening value BH2≥30MPa.

[0013] Furthermore, the surface waviness Wsa after the steel plate has been formed by 5% (1-5) ≤0.35μm, and the increase in surface waviness ΔWsa after forming 5% (1-5) ≤0.05μm.

[0014] The design principles of each chemical element in the 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet described in this invention are as follows: Carbon (C) is a key element for achieving bake-hardening properties. Solid-solution C pins dislocations and increases yield strength during baking. If the C content in the steel is too low, the BH2 value will be insufficient; correspondingly, the C content should not be too high either. If the C content is too high, the solid-solution C exceeds the limit, leading to severe strain aging and poor aging resistance. Therefore, in the hot-dip galvanized bake-hardening steel sheet described in this invention, the mass percentage of C is controlled within the range of 0.0015% to 0.003%.

[0015] Si: Si is a solid solution strengthening element, but excessive Si content will worsen the wettability of hot-dip galvanized surfaces and produce silicon spot defects. At the same time, silicon spots will damage surface smoothness and indirectly increase waviness. Based on this, in the hot-dip galvanized bake-hardening steel sheet of the present invention, the mass percentage of Si element is controlled within the range of no more than 0.06%.

[0016] Mn: Mn is also a solid solution strengthening element in steel and can combine with S to form MnS, preventing hot brittleness and avoiding brittle cracking of steel sheets during rolling and forming. However, it should be noted that excessive Mn content will reduce the deep drawing formability of the steel sheet, lower the r-value, and affect the adhesion of the galvanized layer. Based on this, in the hot-dip galvanized bake-hardening steel sheet described in this invention, the mass percentage content of Mn element is controlled within the range of 0.40% to 0.60%.

[0017] P: P is one of the main solid solution strengthening elements in this invention. P has a significant solid solution strengthening effect, improving the strength of steel while having a relatively small impact on formability. However, it should be noted that when the P content in the steel is too low, it is difficult to achieve an effective strengthening effect; the P content should also not be too high, as excessive P will cause grain boundary segregation, reduce the toughness of the steel plate, and cause spot welding difficulties. Based on this, in the hot-dip galvanized bake-hardening steel plate described in this invention, the mass percentage content of P element is controlled within the range of 0.035% to 0.045%.

[0018] Sulfur (S): Sulfur is a harmful element in steel, easily causing brittle fracture. In particular, S forms MnS inclusions, which disrupt the continuity of the steel matrix, affecting surface quality and formability. This leads to strain concentration around the inclusions during forming, increasing surface waviness and waviness increment, while also reducing the steel's toughness and weldability. Therefore, the lower the S content, the better. Based on this, in the hot-dip galvanized bake-hardening steel sheet described in this invention, the S element mass percentage is controlled below 0.01%.

[0019] Als: Als is used for deoxidation and nitrogen fixation, forming AlN precipitates to prevent nitrogen-induced aging problems and avoid decreased formability and increased surface waviness in steel plates due to aging. However, it should be noted that if the Als content is too low, it is difficult to achieve the desired deoxidation and nitrogen fixation effect; correspondingly, if the Als content is too high, it will increase smelting costs, and excessive AlN will lead to a decrease in the toughness of the steel plate, making it prone to cracking during forming and indirectly affecting surface smoothness. Based on this, in the hot-dip galvanized bake-hardening steel plate described in this invention, the mass percentage of Al element is controlled within the range of 0.02% to 0.06%.

[0020] Nb: Nb is the core microalloying element of this invention and has the following multiple functions: 1) Precipitation strengthening: Nb can form dispersed NbC and Nb(C,N) precipitates with C and N in steel, which improves the strength of the steel plate through precipitation strengthening and helps stabilize the microstructure; 2) Grain refinement strengthening: Nb can effectively inhibit austenite grain growth and delay recrystallization during hot rolling, thereby refining the final ferrite grain structure. The fine and uniform grain structure fundamentally improves the material deformation coordination and effectively reduces the phenomenon of local strain concentration during forming. It is the key metallurgical mechanism to control the waviness increment ΔWsa(1-5) after forming within the stringent index range of ≤0.05μm, which can significantly improve the paint clarity of the outer plate; 3) Solid solution carbon regulation: By controlling the atomic ratio of Nb to C to be less than 1, some C is retained in the matrix in a solid solution state, realizing the precise regulation of solid solution carbon content, thereby obtaining stable and high bake hardening response performance. Furthermore, this invention employs a pure Nb microalloying system without adding Ti. This completely eliminates the potential adverse effects of coarse TiN inclusions on the surface quality and uniformity of the steel plate. It also avoids fluctuations in the dissolved carbon content caused by Ti preferentially combining with C and N, ensuring stable bake-hardening performance. However, it should be noted that if the Nb content is too low, precipitation strengthening and grain refinement strengthening are insufficient, making it difficult to effectively regulate the microstructure. Conversely, if the Nb content is too high, excessive precipitates are easily formed, leading to a decrease in dissolved carbon content, which is detrimental to the stability of bake-hardening performance and increases production costs. Therefore, in the hot-dip galvanized bake-hardening steel plate described in this invention, the mass percentage of Nb is controlled within the range of 0.005% to 0.015%.

[0021] Excessive nitrogen (N) content leads to the formation of coarse AlN, which damages the surface smoothness of the steel sheet, increases surface waviness, and reduces the stamping formability of the steel sheet, making it prone to cracking during forming and affecting surface quality. Furthermore, excessive N content exacerbates aging problems, resulting in unstable steel sheet performance. Therefore, in this invention, the N content should be controlled as low as possible. Based on this, in the hot-dip galvanized bake-hardening steel sheet described in this invention, the mass percentage of N element is controlled below 0.003%.

[0022] Secondly, the present invention provides a method for manufacturing a 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet, comprising the following steps: (1) Steelmaking and continuous casting process: After desulfurization of molten iron, smelting in a converter, and RH vacuum treatment, the molten steel is continuously cast into slabs; (2) Hot rolling process: The slab is heated and then hot rolled. The roughing exit temperature of the hot rolling is 1030-1090℃, and the final rolling temperature of the 7-stand finishing rolling is controlled at 910-950℃. (3) Cooling process: Laminar flow cooling mode is adopted after rolling; (4) Coiling process: The coiling temperature is controlled at 680-730℃ to obtain hot-rolled steel coils; (5) Pickling and rolling process: The hot-rolled steel coil is pickled and cold-rolled, and the cold rolling reduction rate is controlled at 80% to 90%; (6) Hot-dip galvanizing and finishing process: The cold-rolled steel coils are continuously annealed, galvanized and finished using a continuous hot-dip galvanizing production line. The continuous annealing temperature is 780-820℃, the holding time is 60-90s, the zinc liquid temperature is 450-460℃, and the finishing elongation is 1.2%-1.6%.

[0023] In the manufacturing method described above in this invention, the hot rolling process has a significant impact on the mechanical properties of ultra-low carbon bake-hardening steel. In this invention, the final hot rolling temperature of the ultra-low carbon bake-hardening steel plate should be above 910°C, and the higher coiling temperature of 680-730°C is conducive to the precipitation of Nb, forming dispersed NbC and Nb(C,N) precipitates, which fully exerts the precipitation strengthening effect, improves the strength of the steel plate, and at the same time refines the final ferrite grains, improves the formability of the steel plate, and reduces the waviness increment during forming.

[0024] In the pickling and rolling process, the cold rolling reduction rate is controlled at 80% to 90%. The high reduction rate can increase the dislocation density in the ferrite body, provide a strong driving force for annealing recrystallization, and make the grains finer and more uniform after annealing. This is one of the key process measures to reduce ΔWsa and can also improve the strength of the steel plate.

[0025] In the hot-dip galvanizing and finishing processes, the annealing temperature is 780–820℃, and the holding time is 60–90 seconds. At this temperature, only a small amount of NbC precipitated during the hot rolling stage undergoes re-dissolution. The solid solution C content is mainly ensured by the design of the excess C composition. After annealing, cooling at an appropriate rate inhibits the over-aging precipitation of solid solution C, ensuring the BH value after exiting the zinc bath. The galvanizing process can be regarded as a low-temperature over-aging treatment. At the zinc bath temperature of 450–460℃, some solid solution C is captured by carbides, reaching the equilibrium solid solution C content at this temperature. The cooling rate after exiting the zinc bath should not be too low to avoid excessive diffusion of solid solution C atoms to dislocations and grain boundaries, which would affect the stability of the BH value.

[0026] Furthermore, the metallographic structure of the hot-rolled steel sheet is ferrite with a grain size of 6-7. This microstructure ensures excellent formability of the steel sheet. The fine and uniform ferrite grains reduce strain concentration during forming and decrease waviness increment. At the same time, the ferrite structure works synergistically with the solid solution C and Nb precipitates to ensure that the steel sheet strength reaches the 220MPa level, achieving a balance between strength, formability, and surface properties.

[0027] Furthermore, in the finishing process, the surface roughness Ra of the finishing roller is controlled between 1.8 and 3.0 μm, and the peak density Rpc is controlled above 90. Since the roughness of the finishing roller is replicated on the steel plate surface at a rate of approximately 40% to 60%, these Ra and Rpc values ​​can create a suitable roughness on the steel plate surface. The axial Ra deviation at six points on the finishing roller is controlled to not exceed 5% to ensure the uniformity of roughness. The relationship between Rmax and Ra of the finishing roller is controlled to satisfy Rmax / Ra < 6 because the ratio of Rmax to Ra represents the degree of vibration during the roller grinding process; the lower the ratio, the more stable the processing and the more uniform the roughness. The surface waviness Wsa of the steel plate is also considered. (1-5) The surface roughness Ra is positively correlated with the uniformity of the roughness, and the uniformity of the roughness also has a significant impact on the waviness. By controlling the parameters of the finishing roll, the surface waviness Wsa of the finished steel plate can be reduced. (1-5) The size should be controlled below 0.35μm.

[0028] Furthermore, in the hot rolling process, rolling lubrication is introduced. This effectively ensures the uniformity of strip crown and shape, avoids local stress concentration caused by uneven shape during rolling, reduces surface waviness, and also reduces frictional loss during rolling, minimizes scratches on the steel plate surface, and improves surface quality.

[0029] Furthermore, in the pickling and rolling process, a 5-stand continuous rolling mill is used.

[0030] Furthermore, in the hot-dip galvanizing process, the temperature of the strip steel entering the zinc bath is controlled at 460–470°C. This temperature ensures a moderate temperature difference between the strip steel and the molten zinc, avoiding defects such as uneven adhesion of the galvanized layer and zinc flow marks caused by excessive temperature difference. At the same time, it ensures the stability of the low-temperature over-aging effect during the galvanizing process, ensuring a balanced C content in the solid solution. This improves the quality of the galvanized layer, stabilizes the baking hardening performance, and prevents galvanized layer defects from affecting surface smoothness.

[0031] Thirdly, the present invention also provides an application of the above-mentioned 220MPa grade low-wavy hot-dip galvanized bake-hardened steel sheet, for example, for manufacturing automotive exterior body panels.

[0032] The beneficial effects of this invention are as follows: This invention employs an ultra-low carbon steel composition system with added Nb instead of an Nb / Ti composite system, which results in finer grain size and effectively reduces the increase in waviness ΔWsa after steel plate forming. (1-5)Nitrogen (Nb) is a key element in controlling the solid solution C content to achieve bake-hardening properties. It is used in combination with C and N, and with Mn and S, employing an excess C composition design, meaning the atomic ratio of Nb to C is less than 1. Simultaneously, Nb can refine the grain structure, resulting in better mechanical and surface properties of the steel sheet. This is determined by the temperature control range of the hot-dip galvanizing production line. Because the annealing temperature is low, the carbides formed after hot rolling rarely decompose, and the solid solution C content in the steel sheet will be ensured by the excess C composition design. The solid solution strengthening element is primarily P, using a low Mn and low Si alloy design to ensure that the requirements for mechanical properties, formability, and surface quality of the steel sheet are met.

[0033] Furthermore, the hot rolling process has a significant impact on the mechanical properties of ultra-low carbon bake-hardening steel. The final rolling temperature of ultra-low carbon bake-hardening steel plates should be above Ar3, and a higher coiling temperature is beneficial to the comprehensive mechanical properties of the steel plates. This invention employs a "three-high" process system for hot rolling: a heating temperature above 1250℃, a final rolling temperature above 910℃, and a coiling temperature of 680℃. The microstructure after hot rolling is ferrite with a grain size of 6-7, achieving precise control of the microstructure and properties.

[0034] Compared with existing technologies, the steel plate prepared by this invention can meet the requirements of PV1045 standard, and the surface waviness index Wsa is [not specified] under the condition of 5% forming. (1–5) ≤0.35μm, waviness increment ΔWsa after forming (1–5) With a thickness of ≤0.05μm, it is perfectly suited to the stringent application requirements of water-based, no-primer coating processes for automotive exterior panels; at the same time, the yield strength of the steel plate reaches 220~280MPa, achieving a balance between low waviness performance and high strength grade, and has broad prospects for promotion and application value. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a three-dimensional surface morphology diagram of the steel plate after 5% forming in Embodiment 1 of the present invention.

[0037] Figure 2 This is a three-dimensional surface morphology diagram of the steel plate after 5% forming in Comparative Example 1 of this invention.

[0038] Figure 3 This is an optical micrograph of the surface of the steel plate in Embodiment 1 of the present invention.

[0039] Figure 4This is an optical micrograph of the surface of the steel plate in Comparative Example 1 of this invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0041] It should be noted that the calculation method for C in the solid solution in this invention is C=C-Nb / 7.75.

[0042] Example 1 A 220MPa grade low-wavyness hot-dip galvanized bake-hardening steel sheet, the chemical composition and mass percentage of which are shown in Table 1. The manufacturing method of the low-wavyness galvanized bake-hardening steel sheet includes the following steps: (1) Steelmaking and continuous casting process: After desulfurization of molten iron, smelting in a converter and RH vacuum treatment, the molten steel is continuously cast into slabs. After the slabs are inspected and cleaned, they enter the next process. (2) Hot rolling process: The slab is heated and then hot rolled. The exit temperature of the roughing mill is 1060℃ and the exit temperature of the finishing mill is 935℃. (3) Cooling process: Laminar flow cooling mode is adopted after rolling; (4) Coiling process: The coiling temperature is controlled at 705℃ to obtain hot-rolled steel coils; (5) Pickling and rolling process: The hot-rolled steel strip is pickled using a pickling and rolling combined unit, and then cold-rolled in 5 passes to obtain the strip steel. The cold rolling reduction rate is controlled at 80% to 90%. (6) Hot-dip galvanizing and finishing process: After continuous annealing, galvanizing and finishing of cold-rolled steel coils using a continuous hot-dip galvanizing production line, continuous annealing and baking hardening steel is produced; during the continuous hot-dip galvanizing process, the continuous annealing temperature is 812℃, the continuous annealing time is 60s, the metallographic structure is ferrite, the zinc bath temperature is 455℃, and the strip temperature entering the zinc pot is 465℃; during the finishing process, the finishing elongation is 1.4%, the finishing roll roughness Ra is 1.8μm, and Rpc100.

[0043] Example 2-3 A 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet, the chemical composition and mass percentage of which are shown in Table 1. The production method is the same as in Example 1, and the specific parameters in the production method are different as shown in Table 2.

[0044] Comparative Examples 1-3 A hot-dip galvanized bake-hardening steel sheet has the chemical composition and mass percentage as shown in Table 1, and the specific parameters in the production method are shown in Table 2.

[0045] The chemical composition design of Examples 1-3 and Comparative Examples 1-3 of the present invention is shown in Table 1. The main process parameters of Examples 1-3 and Comparative Examples 1-3 of the present invention are shown in Table 2. The mechanical properties and waviness test results of the steel plates produced by Examples 1-3 and Comparative Examples 1-3 of the present invention are shown in Table 3.

[0046] Table 1. Chemical composition design of Examples 1-3 and Comparative Examples 1-3 of the present invention (mass percentage, %)

[0047] Table 2. Main process parameters of Examples 1-3 and Comparative Examples 1-3 of the present invention

[0048] Table 3. Mechanical properties and waviness test results of Examples 1-3 and Comparative Examples 1-3 of the present invention

[0049] As shown in Table 3, the steel plates prepared in Examples 1-3 of this invention all have a yield strength of over 220 MPa, a BH2 value of over 30 MPa, and a surface waviness Wsa after forming 5% of the steel. (1-5) All are less than 0.35 μm, and the waviness increment ΔWsa (1-5) All are less than 0.05μm, fully meeting the requirements of the no-intermediate-coat process.

[0050] Comparative Example 1, due to the addition of Ti element and the high roughness and low Rpc of the finishing roller, resulted in a BH2 value of only 19MPa, and the waviness and its increment seriously exceeded the standard, resulting in poor surface quality.

[0051] Comparative Example 2, due to its low Nb content, resulted in insufficient fine-grain strengthening and precipitation strengthening, leading to low yield strength and BH2 value, which could not meet the requirements of 220MPa level and high BH2 value, and the increase in waviness was also large.

[0052] In Comparative Example 3, the low winding temperature was not conducive to the full precipitation and grain refinement of Nb, resulting in a waviness increment ΔWsa of 0.11 μm after forming, which exceeded the control target of 0.05 μm.

[0053] Figure 1 and Figure 2 The surface three-dimensional morphology comparison shows that the surface micromorphology of the steel plate in Example 1 is more uniform and delicate, while the surface of the steel plate in Comparative Example 1 has obvious unevenness. Figure 3 and Figure 4The optical micrographs show that the surface of the steel plate in Example 1 has more uniform and denser burr marks, and the individual burr marks are shallower and larger in area, while the surface of the steel plate in Comparative Example 1 has obvious unevenness and granular texture.

[0054] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.0015%~0.003%, Si≤0.06%, Mn: 0.40%~0.60%, P: 0.035%~0.045%, S≤0.01%, Als: 0.02%~0.06%, Nb: 0.005%~0.015%, N≤0.003%, C in solid solution: 0.0008~0.0015%, with the balance being iron and unavoidable impurities.

2. The 220MPa grade low-wavy hot-dip galvanized bake-hardening steel sheet as described in claim 1, characterized in that, The mechanical properties of the steel plate meet the following requirements: yield strength 220~280MPa, tensile strength 320~440MPa, elongation A80≥32%, n90 value≥0.15, r90 value≥1.20, and bake hardening value BH2≥30MPa.

3. A 220MPa grade low-wavyness hot-dip galvanized bake-hardening steel sheet as described in claim 1 or 2, characterized in that, Surface waviness Wsa after steel plate forming 5% (1-5) ≤0.35μm, and the increase in surface waviness ΔWsa after forming 5% (1-5) ≤0.05μm.

4. A method for manufacturing a 220MPa grade low-wavyness hot-dip galvanized bake-hardening steel sheet as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Steelmaking and continuous casting process: After desulfurization of molten iron, smelting in a converter, and RH vacuum treatment, the molten steel is continuously cast into slabs; (2) Hot rolling process: The slab is heated and then hot rolled. The roughing exit temperature of the hot rolling is 1030-1090℃, and the final rolling temperature of the 7-stand finishing rolling is controlled at 910-950℃. (3) Cooling process: Laminar flow cooling mode is adopted after rolling; (4) Coiling process: The coiling temperature is controlled at 680-730℃ to obtain hot-rolled steel coils; (5) Pickling and rolling process: hot-rolled steel coils are pickled and cold-rolled, with the cold rolling reduction rate controlled at 80% to 90%; (6) Hot-dip galvanizing and finishing process: The cold-rolled steel coils are continuously annealed, galvanized and finished using a continuous hot-dip galvanizing production line. The continuous annealing temperature is 780-820℃, the holding time is 60-90s, the zinc liquid temperature is 450-460℃, and the finishing elongation is 1.2%-1.6%.

5. The manufacturing method as described in claim 4, characterized in that, The metallographic structure of the hot-rolled steel plate is ferrite with a grain size of 6-7.

6. The manufacturing method as described in claim 4, characterized in that, In the finishing process, the surface roughness Ra of the finishing roller is controlled between 1.8 and 3.0 μm, the peak density Rpc is controlled above 90, and the axial Ra deviation of the finishing roller at 6 points does not exceed 5%. The relationship between Rmax and Ra of the finishing roller is controlled to satisfy Rmax / Ra<6.

7. The manufacturing method as described in claim 4, characterized in that, In the hot rolling process, rolling lubrication is applied during the rolling process.

8. The manufacturing method as described in claim 4, characterized in that, In the pickling and rolling process, a 5-stand continuous rolling mill is used.

9. The manufacturing method as described in claim 4, characterized in that, During the hot-dip galvanizing process, the temperature of the strip steel entering the zinc pot is controlled at 460-470°C.

10. An application of the 220MPa grade low-wavyness hot-dip galvanized bake-hardening steel sheet as described in any one of claims 1-3, characterized in that, The steel plate is used to manufacture automotive exterior body panels.

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