800mpa grade high-holes expansion hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

This invention discloses an 800MPa grade high-perforation hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer coated on at least one surface of the substrate, wherein the substrate contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages: C: 0.03-0.08%, 0<Si≤0.45%, Mn: 1.3-1.8%, Al: 0.02-0.1%, Cr: 0.2-0.6%, Ti: 0.05-0.15%, Nb≤0.05%, B≤0.003%; wherein the mass percentages of N, Ti, and Nb also satisfy: 0.01%≤(Ti-3.43N+0.52Nb) / 4≤0.03%. Accordingly, this invention also discloses a method for manufacturing the above-mentioned steel sheet, comprising the steps of: (1) smelting and casting; (2) hot rolling; (3) pickling; (4) annealing; (5) hot-dip galvanizing; and (6) leveling. This 800MPa grade high-expansion hot-dip galvanized steel sheet has excellent mechanical properties, with a longitudinal yield strength ≥660MPa, tensile strength ≥780MPa, elongation A50 ≥15%, punching expansion rate ≥50%, and reaming expansion rate ≥80%.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more particularly to a hot-dip galvanized steel plate and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of the automotive industry, the market and users have increasingly higher requirements for lightweight vehicles. Lightweighting has become a trend in the automotive industry, and the proportion of high-strength steel plates in automotive structural components is also increasing.

[0003] In the actual production of high-strength steel, an increasing number of automotive models are using 80kg-grade steel plates to manufacture chassis components. This not only places certain requirements on the steel's strength and elongation but also on its hole-expanding performance. Furthermore, in addition to the aforementioned requirements for strength, elongation, and hole-expanding performance, to meet the corrosion resistance requirements of automotive components in different operating environments and extend the service life of vehicles, it is necessary to further improve the corrosion resistance of the steel. Currently, ordinary hot-rolled and cold-rolled plates and pickled plates can no longer meet the requirements of the automotive industry; hot-dip galvanized plates are one of the effective ways to improve the corrosion resistance of automotive components.

[0004] Based on this, in order to further consider the processing performance, corrosion resistance and manufacturability of the material, and to meet the current market and user needs, the inventors hope to obtain a new type of 800MPa high-expansion hot-dip galvanized steel sheet and its manufacturing method.

[0005] In the existing technology, although researchers have designed some steel plates with excellent performance, they still cannot meet the performance requirements of specific strength and manufacturing process parameters, and there are still shortcomings in performance.

[0006] For example, Chinese patent document CN109055867A, published on December 21, 2018, entitled "A Method for Producing Hot-Dip Galvanized Steel Sheets with a Tensile Strength of 540 MPa," discloses a method for producing hot-dip galvanized steel sheets with a tensile strength of 540 MPa. Its characteristic is that the pickled sheet is directly galvanized without cold rolling, which is a short-process method. However, the tensile strength of the steel sheet produced by this method is 540 MPa, which is different from the 800 MPa strength level of the steel designed in this invention.

[0007] For example, Chinese patent document CN108396259A, published on August 14, 2018, entitled "A Hot-Rolled Galvanized Steel Sheet with High Hole-Expansion Performance and Its Manufacturing Method," discloses a hot-rolled galvanized steel sheet with high hole-expansion performance and its manufacturing method. The sheet has a yield strength ≥600 MPa and a hole expansion rate ≥40%. Its composition contains 0.5-2.5% Si, which is relatively high, making it prone to forming a red iron scale on the surface, hindering the control of the galvanized surface. In this technical solution, the high Si content of the steel is drastically different from the technical solution designed in this invention.

[0008] For example, Chinese patent document CN104513930A, published on April 15, 2015, entitled "Ultra-high strength hot-rolled multiphase steel plate and strip with good bending and hole expansion properties and manufacturing method thereof," discloses an ultra-high strength hot-rolled multiphase steel plate and strip with good bending and hole expansion properties and manufacturing method thereof. The technical solution discloses the performance design and manufacturing method of hot-rolled pickled plate, but does not consider the role of element B in chemical composition, and does not disclose the influence of hot-dip galvanizing process on the performance of hot-dip galvanized steel plate. Summary of the Invention

[0009] One of the objectives of this invention is to provide an 800MPa grade high-expansion hot-dip galvanized steel sheet. This 800MPa grade high-expansion hot-dip galvanized steel sheet adopts a reasonable chemical composition design, which can obtain good comprehensive mechanical properties. While having high strength and high corrosion resistance, it also has the characteristics of high hole expansion rate. It can be used as automotive body structural parts and automotive chassis parts, as well as other application fields that require high strength and weight reduction, and has good application prospects.

[0010] To achieve the above objectives, the present invention provides an 800MPa grade high-perforation hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer coated on at least one surface of the substrate, wherein the substrate contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages:

[0011] C: 0.03-0.08%, 0<Si≤0.45%, Mn: 1.3-1.8%, Al: 0.02-0.1%, Cr: 0.2-0.6%, Ti: 0.05-0.15%, Nb≤0.05%, B≤0.003%;

[0012] The mass percentage content of N, Ti, and Nb also satisfies the following condition: 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%.

[0013] Furthermore, in the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, the mass percentage content of each chemical element is as follows:

[0014] C: 0.03-0.08%, 0 < Si ≤ 0.45%, Mn: 1.3-1.8%, Al: 0.02-0.1%, Cr: 0.2-0.6%, Ti: 0.05-0.15%, Nb ≤ 0.05%, B ≤ 0.003%; balance is Fe and unavoidable impurity elements;

[0015] The mass percentage content of N, Ti, and Nb also satisfies the following condition: 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%.

[0016] The design principles of each chemical element in the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet of this invention are as follows:

[0017] C: In the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention, considering that the carbon content largely determines the tensile strength level of the steel sheet, carbon is used for solid solution strengthening and the formation of sufficient precipitated strengthening phases to ensure the strength of the steel; however, a high mass percentage of carbon will result in coarse carbide particles, which is detrimental to the expansion performance. Therefore, in order to ensure both high expansion performance and good forming and welding properties at the same strength level, the mass percentage of carbon element in the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention is controlled between 0.03% and 0.08%.

[0018] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be preferably controlled between 0.04% and 0.07%.

[0019] Si: In the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, Si can play a solid solution strengthening role and improve the strength of the steel sheet. Simultaneously, the addition of Si can increase the work hardening rate and the uniform elongation and total elongation at a given strength, which helps to improve the elongation of the steel sheet. Furthermore, Si can prevent the precipitation of carbides and reduce the appearance of pearlite phase. However, it should be noted that silicon in steel easily leads to the formation of iron oxide scale (2FeO-SiO2) on the surface of the steel sheet, which has an adverse effect on surface quality. Therefore, in the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, the mass percentage of Si is controlled to 0 < Si ≤ 0.45%.

[0020] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Si element can be preferably controlled to 0 < Si ≤ 0.2%.

[0021] Mn: In the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, Mn is a solid solution strengthening element. When the mass percentage of Mn in the steel is low, it will lead to insufficient strength of the steel. However, when the mass percentage of Mn is too high, it will lead to a decrease in the plasticity of the steel sheet. In addition, Mn also delays the pearlite transformation, improves the hardenability of the steel, and lowers the bainite transformation temperature, thus refining the substructure of the steel and ensuring the acquisition of a lath substructure. This ensures good formability while maintaining the tensile strength of the product. Therefore, considering the influence of Mn content on the performance of steel, the mass percentage of Mn in the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention is controlled between 1.3% and 1.8%.

[0022] Al: In the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention, Al is a deoxidizing element in steel, which can reduce oxide inclusions and purify the steel, thus improving the formability of the steel sheet. However, it should be noted that the content of Al in the steel should not be too high. When the mass percentage of Al in the steel is too high, oxidation will occur, which will further affect continuous casting production. Therefore, considering the influence of Al on the performance of the steel sheet in this technical solution, the mass percentage of Al in the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention is controlled between 0.02% and 0.1%.

[0023] Cr: In the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention, Cr is an element that inhibits the formation of pearlite and promotes the formation of bainite, ultimately contributing to improved strength and porosity. The inventors found that when the mass percentage of Cr in the steel is less than 0.15%, its effect on the CCT curve is not significant. However, when the mass percentage of Cr in the steel is high, it not only leads to increased alloy costs but also easily generates more martensite. Based on this, in the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention, the mass percentage of Cr is controlled between 0.2% and 0.6%.

[0024] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Cr can be preferably controlled between 0.2-0.35%.

[0025] Ti: In the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, Ti is one of the important precipitation strengthening and grain refinement strengthening elements. Especially during the hot-dip galvanizing annealing process, the further precipitation strengthening of Ti and the fixation of C are beneficial to improving the strength and elongation of the steel sheet. Therefore, in order to give full play to the beneficial effects of Ti, in this invention, the mass percentage content of Ti is controlled between 0.05% and 0.15%.

[0026] Nb: In the substrate of the 800MPa grade high-perforation hot-dip galvanized steel sheet described in this invention, Nb is one of the important precipitation strengthening and grain refinement strengthening elements. However, when the mass percentage of Nb is higher than 0.05%, the strengthening effect of Nb approaches saturation, and the cost is also high. Therefore, in order to maximize the beneficial effects of Nb while controlling production costs, in this invention, the mass percentage of Nb is controlled to be Nb≤0.05%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage of Nb can be preferably controlled to Nb≤0.02%.

[0028] B: In the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet described in this invention, B is beneficial for expanding the bainite phase region, ensuring that the steel sheet can obtain bainite structure during post-rolling cooling, which significantly improves the strength and hardness of the steel. However, it should be noted that the content of element B in the steel should not be too high. Excessive B will lead to excessive martensite structure in the steel sheet, resulting in a decrease in the expansion rate and elongation of the steel. Therefore, in this invention, the mass percentage content of element B is controlled to B≤0.003%.

[0029] Furthermore, it should be noted that in this technical solution designed in this invention, while controlling the mass percentage content of a single chemical element in the substrate, the inventors further controlled the mass percentage content of N, Ti, and Nb in the substrate to satisfy: 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%. Here, N is an impurity element in the substrate.

[0030] In this design approach, the main purpose of adding high levels of Ti and Nb to the steel is to ensure that the strip can precipitate dispersed, fine nano-sized carbides during the annealing and hot-dip galvanizing process, thereby achieving a strong precipitation strengthening effect. In this invention, the C content needs to be designed in conjunction with the Ti and Nb contents to ensure sufficient precipitation of Ti and Nb. Simultaneously, the addition of an appropriate amount of Cr is to obtain a bainitic structure without pearlite that affects porosity during hot rolling and annealing and hot-dip galvanizing. Cr also has good tempering resistance, which helps maintain the strength of bainite during annealing. Therefore, only when the mass percentages of N, Ti, and Nb satisfy the above relationship of "0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%", and are combined with the required manufacturing process, can a hot-dip galvanized steel sheet with high strength and high porosity be obtained.

[0031] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, among the unavoidable impurity elements, P≤0.02%, S≤0.005%, and N≤0.005%.

[0032] In the above technical solution, P, S and N are all impurity elements in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention. Under the condition that the technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the steel sheet should be reduced as much as possible.

[0033] Therefore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the content of P element is controlled to be P≤0.02%, the content of S element is controlled to be S≤0.005%, and the content of N element is controlled to be N≤0.005%.

[0034] Furthermore, in the 800MPa grade high-perforation hot-dip galvanized steel sheet of the present invention, the mass percentage content of each chemical element in the substrate further satisfies at least one of the following:

[0035] C: 0.04-0.07%;

[0036] 0 < Si ≤ 0.2%;

[0037] Cr: 0.2-0.35%;

[0038] Nb≤0.02%.

[0039] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the substrate microstructure is bainite + ferrite, and the substrate has precipitates, including nano-sized precipitates.

[0040] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the bainite volume fraction is ≥95%.

[0041] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the nano-scale precipitates include TiC and (Ti,Nb)C, and the diameter of the nano-scale precipitates is between 3-20nm.

[0042] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the precipitates also include TiN precipitates with larger particles and a diameter of <10µm.

[0043] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, the average weight of the hot-dip galvanized layer on one side is 20-600 g / m². 2 .

[0044] Furthermore, in the 800MPa grade high-expansion hot-dip galvanized steel sheet of the present invention, its performance meets the following requirements: longitudinal yield strength ≥660MPa, tensile strength ≥780MPa, elongation A50 ≥15%, punching expansion rate ≥50%, and reaming expansion rate ≥80%.

[0045] Accordingly, another objective of the present invention is to provide a method for manufacturing the 800MPa grade high-expansion hot-dip galvanized steel sheet described above. The 800MPa grade high-expansion hot-dip galvanized steel sheet obtained by this manufacturing method has high strength and excellent corrosion resistance, as well as a high hole expansion rate, and has good application prospects.

[0046] To achieve the above objectives, the present invention proposes a method for manufacturing the aforementioned 800MPa grade high-perforation hot-dip galvanized steel sheet, comprising the following steps:

[0047] (1) Smelting and casting;

[0048] (2) Hot rolling: The slab is heated to 1200-1280℃ and held at that temperature; then it is rolled, wherein the roughing exit temperature is controlled to be ≥1000℃, the finishing exit temperature is 840-950℃, and the finishing rolling speed is ≥7.5m / s; after finishing rolling, the steel plate is water-cooled to the coiling temperature of 430-540℃ at a cooling rate of 40-150℃ / s and then coiled.

[0049] (3) Pickling;

[0050] (4) Annealing: After pickling, the steel coil is annealed and heated in a combustion non-oxidizing continuous annealing furnace. The heating rate is ≥5℃ / s, the annealing and heating temperature is 480-740℃, the holding time of the annealing and heating section is 30-300s, and the cooling rate after heating is ≥3℃ / s.

[0051] (5) Hot-dip galvanizing;

[0052] (6) Flat.

[0053] In the above technical solution of the present invention, in step (2), for Ti-containing hot-dip galvanized steel, the heating temperature of the slab is particularly important for performance and surface finish. During continuous casting, Ti produces a large number of large-sized (Ti, Nb)(C, N) precipitates. Setting the heating temperature to ≥1200℃ is primarily to ensure that alloying elements such as Ti can dissolve as much as possible during the heating process, ensuring that subsequent microalloying elements such as Ti can precipitate at the nanoscale during hot rolling, especially during the annealing and hot-dip galvanizing process. However, it should be noted that the heating temperature should not be too high. When the heating temperature exceeds 1280℃, there is a tendency for grain coarsening, which is detrimental to the toughness of the steel plate; simultaneously, the iron oxide scale is thicker, which is not conducive to descaling of the iron oxide scale, ultimately affecting the surface quality of the hot-dip galvanized steel. Therefore, in the hot rolling process of the present invention, the heating temperature is preferably controlled between 1200-1280℃.

[0054] Furthermore, the roughing temperature control and rolling speed during hot rolling have a significant impact on microalloying elements such as Ti. At lower roughing temperatures and during finishing rolling, Ti carbides and carbonitrides precipitate, and these precipitates are relatively large, which is detrimental to improving the final strength. Therefore, in the hot rolling process of this invention, the roughing exit temperature is controlled to be ≥1000℃, and the finishing rolling speed is controlled to be ≥7.5m / s.

[0055] Furthermore, the finishing mill exit temperature, coiling temperature, and water cooling rate during the hot rolling process have a significant impact on the microstructure. Lower finishing mill exit temperatures and lower water cooling rates tend to result in blocky ferrite. Higher coiling temperatures also lead to higher ferrite and pearlite content, while lower coiling temperatures may result in martensite. Therefore, in the hot rolling process of this invention, the finishing mill exit temperature is controlled at 840-950°C; after finishing milling, the steel plate is water-cooled to 430-540°C at a cooling rate of 40-150°C / s before coiling to control a more sufficient bainitic transformation phase.

[0056] Accordingly, in the annealing process of step (4) of the present invention, the annealing homogenization temperature is limited to 480-740℃, within which (Ti, Nb) and (C, N) precipitation is most intense. When annealing below the austenite transformation point Ac1 during heating, the steel plate retains the single-phase structure of hot-rolled bainite. When annealing above the austenite transformation point Ac1 during heating, and the cooling rate after homogenization is ≥3℃ / s, the generation of excessive ferrite structure is avoided, which is beneficial to improving the porosity.

[0057] It should be noted that, in some embodiments of the technical solution designed in this invention, before annealing, the strip steel can be pre-oxidized in an open flame furnace to obtain a steel plate with a pre-oxidized film on the surface, and the thickness of the pre-oxidized film is controlled to be 60-120 nanometers; after pre-oxidation, it is then annealed in an annealing furnace to obtain a steel plate with a surface reduced by hydrogen.

[0058] Furthermore, in the manufacturing method described in this invention, in step (2), the finishing mill exit temperature is controlled to be 840-920°C; and the coiling temperature is either 430-500°C.

[0059] Furthermore, in the manufacturing method described in this invention, in step (4), the annealing temperature is 650-730°C, and / or the holding time of the annealing temperature range is 30-120 seconds.

[0060] Furthermore, in the manufacturing method described in this invention, in step (5), the temperature of the hot-dip galvanizing pot is 440-480°C.

[0061] Furthermore, in the manufacturing method described in this invention, in step (6), the flatness ratio is 0.05-1.3%.

[0062] Compared with the prior art, the 800MPa grade high-expansion hot-dip galvanized steel sheet and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0063] This invention employs an economical and rational chemical composition design, which, when combined with existing hot rolling and hot-dip galvanizing production lines, can produce 800MPa-grade high-expansion hot-dip galvanized steel sheets with ultra-high strength and high porosity. Compared with traditional hot-dip galvanized sheet production methods, the optimized manufacturing process of this invention eliminates the cold rolling step, shortens the process flow, improves production efficiency, saves energy, and effectively reduces production costs.

[0064] The 800MPa grade high-expansion hot-dip galvanized steel sheet prepared by this invention has the characteristics of high expansion rate, high strength, and high corrosion resistance. Its longitudinal yield strength is ≥660MPa, tensile strength is ≥780MPa, elongation A50 is ≥15%, punching expansion rate is ≥50%, and reaming expansion rate is ≥80%. It can be used as automotive body structural parts and automotive chassis parts, as well as other application fields that require high strength and weight reduction, and has good application prospects. Attached Figure Description

[0065] Figure 1 A typical metallographic photograph of the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet of Example 1.

[0066] Figure 2This is a photograph of the large TiN particles contained in the comparative steel plate of Comparative Example 8. Detailed Implementation

[0067] The following will further explain and illustrate the 800MPa grade high-expansion hot-dip galvanized steel sheet and its manufacturing method according to the present invention with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0068] Examples 1-11 and Comparative Examples 1-10

[0069] Table 1 lists the mass percentage of each chemical element in the substrates of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 and the comparative steel sheets of Comparative Examples 1-10.

[0070] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S and N)

[0071]

[0072]

[0073] Note: In Table 1 above, the formula “(Ti-3.43N+0.52Nb) / 4” represents the equivalent of TiC precipitation. In the formula, N, Ti and Nb are all substituted with the mass percentage of each chemical element.

[0074] The 800MPa grade high-perforation hot-dip galvanized steel sheets of Examples 1-11 and the comparative steel sheets of Comparative Examples 1-10 of the present invention were all prepared using the following steps:

[0075] (1) Smelting and casting according to the chemical composition shown in Table 1.

[0076] (2) Hot rolling: The slab obtained by smelting and continuous casting is heated to 1200-1280℃ and held for 1-3 hours; then it is rolled, wherein the roughing exit temperature is controlled to be ≥1000℃, the finishing exit temperature is 840-950℃, preferably controlled between 840-920℃, and the finishing rolling speed is ≥7.5m / s; after finishing rolling, the steel plate is water-cooled to a coiling temperature of 430-540℃ at a cooling rate of 40-150℃ / s and then coiled, wherein the coiling temperature is preferably controlled between 430-500℃.

[0077] (3) Pickling: Control the pickling elongation rate to 0.2-2%, control the pickling speed to 60-150m / min, and control the temperature of the last pickling tank to 80-90℃ and the iron ion concentration to 30-40g / L.

[0078] (4) Annealing: After pickling, the steel coil is subjected to homogenization in a combustion-free non-oxidizing continuous annealing furnace, where the heating rate is ≥5℃ / s, the homogenization temperature is controlled at 480-740℃, the holding time in the homogenization section is 30-300s, and the cooling rate after homogenization is ≥3℃ / s. Alternatively, the homogenization temperature can be preferably controlled at 650-730℃, and the holding time in the homogenization section is 30-120s.

[0079] (5) Hot-dip galvanizing: The steel plate is fed into the zinc pot for hot-dip galvanizing, and the temperature of the hot-dip galvanizing zinc pot is controlled at 440-480℃.

[0080] (6) Flattening: After plating, flattening is carried out and the flatness rate is controlled at 0.05-1.3% to finally obtain a steel plate with a thickness of ≤5mm.

[0081] In this invention, the chemical composition design and related processes of the 800MPa grade high-expansion hot-dip galvanized steel sheets in Examples 1-11 all meet the specifications designed in this invention. Although the comparative steel sheets in Comparative Examples 1-10 are also prepared using the above steps (1)-(6), there are parameters in the chemical composition design and related processes of the comparative steel sheets in Comparative Examples 1-10 that do not meet the design requirements of this invention.

[0082] Tables 2-1 and 2-2 list the specific process parameters of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 and the comparative steel sheets of Comparative Examples 1-10 in the above manufacturing process.

[0083] Table 2-1.

[0084]

[0085] Table 2-2.

[0086]

[0087]

[0088] It should be noted that before hot-dip galvanizing, the inventors took samples of the substrates of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 and Comparative Examples 1-10 obtained through the above process steps, and observed and analyzed the microstructure of the substrates of the steel sheets of each example and comparative example. The results of the relevant observation and analysis are listed in Table 3 below.

[0089] Table 3 lists the microstructure observation and analysis results of the substrates of Examples 1-11 and Comparative Examples 1-10.

[0090] Table 3.

[0091]

[0092]

[0093] Observation shows that, in this invention, the microstructure of the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet prepared in Examples 1-11 is bainite + ferrite, and the volume fraction of bainite is between 95% and 99%.

[0094] It should be noted that, in actual preparation, the substrates of Examples 1-11 also have nanoscale precipitates on their microstructure matrix, including TiC and (Ti,Nb)C, with diameters between 3-20 nm. Additionally, the substrates of Examples 1-11 also contain larger TiN precipitates with diameters all <10 μm.

[0095] In order to obtain the performance of the final galvanized steel sheet, after completing the above observation of the microstructure of the substrate, the inventors took samples of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 and the comparative steel sheets of Comparative Examples 1-10 obtained by the above process steps (1)-(6) and conducted mechanical property tests. At the same time, the average weight of the hot-dip galvanized layer on one side of each example and comparative steel sheet was measured. The test results are listed in Table 4.

[0096] The relevant performance testing methods are as follows:

[0097] (1) Tensile property test: Take JIS 5# tensile specimens along the longitudinal direction and conduct tensile tests according to GB / T 228.1-2010 "Metallic materials, tensile test - Part 1: Test method at room temperature" to test and obtain the yield strength, tensile strength and elongation of the steel plates of each embodiment and comparative example.

[0098] (2) Hole Enlargement Test: The hole enlargement rate was determined by the hole enlargement test. A punch was used to press the specimen with a central hole into a die to enlarge the central hole of the specimen until necking or through cracks appeared at the edge of the hole. Since the preparation method of the original central hole of the specimen has a significant impact on the hole enlargement rate test results, punching and reaming were used to prepare the original central hole of the specimen. Subsequent tests and test methods were performed according to the hole enlargement rate test method specified in ISO / DIS 16630 standard. The test results are shown in Table 4.

[0099] Table 4 lists the mechanical property test results of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 and the comparative steel sheets of Comparative Examples 1-10, as well as the average weight of a single side of the hot-dip galvanized layer.

[0100] Table 4.

[0101]

[0102] As shown in Table 4, compared with the comparative steel plates of Comparative Examples 1-10, the 800MPa grade high-expansion hot-dip galvanized steel plates of Examples 1-11 of this case have superior comprehensive mechanical properties.

[0103] In this invention, the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 adopt the low-carbon bainite design concept of this invention, and utilize Cr to improve the bainite transformation and the resistance to tempering softening during the annealing process. Simultaneously, Ti and Nb are added to enhance the precipitation strengthening effect during the annealing process. The yield strength of the 800MPa grade high-expansion hot-dip galvanized steel sheets of Examples 1-11 finally prepared by this invention is between 678-801MPa, its tensile strength is between 803-862MPa, its elongation A50 is between 18-20%, its punching expansion rate is between 52-82%, and its reaming expansion rate is between 85-117%.

[0104] Compared with Examples 1-4, Comparative Examples 1-2 used the same steel grade A and the same hot rolling process, but either the annealing temperature was too high or the cooling rate after homogenization was too slow, which caused the proportion of ferrite in the microstructure of the prepared substrate to exceed 5%, resulting in a decrease in the yield strength of the steel and a reduction in the hole expansion rate.

[0105] Compared with Examples 5-7, Comparative Example 3 used a lower heating temperature, resulting in insufficient solid solution of Nb and Ti. Comparative Example 4 had a lower roughing mill exit temperature, and Comparative Example 5 had a lower finishing mill exit temperature, resulting in coarse precipitation of (Ti, Nb)(C, N) during the hot rolling process, which reduced the contribution to strength and thus reduced the strength of the steel plate.

[0106] Compared with Examples 5-7, Comparative Example 6 used a higher winding temperature, resulting in a higher ferrite content in its hot-rolled microstructure, which in turn led to a higher ferrite content and a lower porosity in the final microstructure.

[0107] Correspondingly, in Comparative Examples 7-10, the chemical composition design did not meet the requirements of this invention. Specifically, in Comparative Example 7, the high C content, while contributing significantly to strength, actually reduced the hole expansion rate. In Comparative Example 8, the high N content consumed a large amount of Ti, leading to the precipitation of a large amount of blocky TiN. Since 5-20 μm TiN contributed little to strength, this reduced the steel plate's strength. Furthermore, the presence of large-sized TiN microcracks on the punched edges during punching significantly impacted the hole expansion rate. In Comparative Example 9, the low Mn content in the steel during design resulted in lower final strength. In Comparative Example 10, the low Cr content in the steel led to weak hardenability, and the low hot-rolling cooling rate resulted in a high ferrite content in the hot-rolled microstructure, leading to a high ferrite content in the final microstructure and a low hole expansion rate.

[0108] Figure 1 A typical metallographic photograph of the substrate of the 800MPa grade high-expansion hot-dip galvanized steel sheet of Example 1.

[0109] like Figure 1 As shown in this embodiment, the microstructure of the 800MPa grade high-expansion hot-dip galvanized multiphase steel plate of Example 1 is 96% bainite + 4% ferrite by volume, the grain size of bainite and ferrite is 5.5um, the diameter of nano-sized precipitates is between 3-15nm, and the diameter of TiN is <10um.

[0110] Figure 2 This is a photograph of the large TiN particles contained in the comparative steel plate of Comparative Example 8.

[0111] like Figure 2 As shown in this embodiment, the microstructure of Comparative Example 8 is 99% bainite and 1% ferrite by volume, with nanoscale precipitates having a diameter between 3 and 20 nm, and TiN having a larger diameter of 10-20 μm.

[0112] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0113] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. An 800MPa grade high-perforation hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer coated on at least one surface of the substrate, wherein the mass percentage of each chemical element in the substrate is: C: 0.03-0.08%, 0 < Si ≤ 0.45%, Mn: 1.3-1.8%, Al: 0.02-0.1%, Cr: 0.2-0.6%, Ti: 0.05-0.15%, Nb ≤ 0.05%, B ≤ 0.003%; balance is Fe and unavoidable impurity elements; The mass percentage content of N, Ti, and Nb also satisfies the following condition: 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%; The substrate microstructure consists of bainite and ferrite matrix, with precipitates on the matrix. The precipitates include nanoscale precipitates, wherein the volume fraction of bainite is ≥95%, and the nanoscale precipitates include TiC and (Ti,Nb)C, with the diameter of the nanoscale precipitates between 3-20 nm.

2. The 800MPa grade high-perforation hot-dip galvanized steel sheet as described in claim 1, characterized in that, Among the unavoidable impurity elements, P ≤ 0.02%, S ≤ 0.005%, and N ≤ 0.005%.

3. The 800MPa grade high-perforation hot-dip galvanized steel sheet as described in claim 1, characterized in that, The mass percentage content of each chemical element in the substrate further satisfies at least one of the following: C：0.04-0.07％； 0 < Si ≤ 0.2%; Cr：0.2-0.35％； Nb≤0.02%.

4. The 800MPa grade high-perforation hot-dip galvanized steel sheet as described in claim 1, characterized in that, The precipitates also include larger TiN precipitates with a diameter of <10 μm.

5. The 800MPa grade high-expansion hot-dip galvanized steel sheet as described in claim 1, characterized in that, The average weight of the hot-dip galvanized layer on one side is 20–600 g / m². 2 .

6. The 800MPa grade high-perforation hot-dip galvanized steel sheet as described in claim 1, characterized in that, Its performance meets the following requirements: longitudinal yield strength ≥660MPa, tensile strength ≥780MPa, elongation A50 ≥15%, punching expansion rate ≥50%, and reaming expansion rate ≥80%.

7. The method for manufacturing 800MPa grade high-perforation hot-dip galvanized steel sheet as described in any one of claims 1-6, characterized in that, It includes the following steps: (1) Smelting and casting; (2) Hot rolling: The slab is heated to 1200-1280℃ and held at that temperature; then it is rolled, wherein the roughing exit temperature is controlled to be ≥1000℃, the finishing exit temperature is 840-950℃, and the finishing rolling speed is ≥7.5m / s; after finishing rolling, the steel plate is water-cooled to the coiling temperature of 430-540℃ at a cooling rate of 40-150℃ / s and then coiled. (3) Pickling; (4) Annealing: After pickling, the steel coil is annealed and heated in a combustion non-oxidizing continuous annealing furnace. The heating rate is ≥5℃ / s, the annealing and heating temperature is 480-740℃, the holding time of the annealing and heating section is 30-300s, and the cooling rate after heating is ≥3℃ / s. (5) Hot-dip galvanizing; (6) Flat.

8. The manufacturing method as described in claim 7, characterized in that, In step (2), the finishing mill exit temperature is controlled at 840-920℃; and the coiling temperature is controlled at 430-500℃.

9. The manufacturing method as described in claim 7, characterized in that, In step (4), the annealing temperature is 650-730℃, and / or the holding time of the annealing temperature range is 30-120s.

10. The manufacturing method as described in claim 7, characterized in that, In step (5), the temperature of the hot-dip galvanizing pot is 440-480℃.

11. The manufacturing method as described in claim 7, characterized in that, In step (6), the leveling rate is 0.05-1.3%.

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