High yield ratio 800mpa grade hot-dip galvanizing steel sheet and method for manufacturing the same

Abstract

The application discloses a high yield ratio 800MPa grade hot-base hot-dip galvanized steel plate, which comprises a base plate and a hot-dip galvanizing layer, the base plate contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows: C: 0.04-0.069%, Mn: 1.3-1.8%, 0 The microstructure of the base plate comprises a complex phase structure of granular bainite and ferrite, and second phase particle precipitates, the volume fraction of the second phase particle precipitates is not less than 0.2%, and the proportion of the second phase particle precipitates with a size not greater than 10nm in the volume fraction of all the second phase particle precipitates is greater than 80%.

Description

Technical Field

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

[0002] With the development of automotive lightweighting technology, advanced high-strength steel is playing an increasingly larger role in automotive structural components. Multiphase steel, with its ultra-high strength and excellent formability, has become the preferred choice for chassis components such as control arms and torsion beams.

[0003] For chassis components with complex shapes, the steel plates are required to have good flanging and hole-expanding capabilities and corrosion resistance.

[0004] Chinese patent document with publication number CN104513930A and publication date of 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", mainly discloses the performance design and manufacturing method of hot-rolled pickled plate, but does not disclose the influence of hot-dip galvanizing process on the performance of hot-dip galvanized steel plate.

[0005] Chinese patent document with publication number CN109055867A and publication date of December 21, 2018, entitled "A method for producing hot-dip galvanized sheet with high tensile strength of 540MPa", discloses a method for producing hot-dip galvanized sheet with high tensile strength, but its strength level is only 540MPa.

[0006] Chinese patent document CN113215485A, published on August 6, 2021, entitled "A 780MPa Grade Hot-Dip Coated Duplex Steel and Its Preparation Method", discloses a 780MPa hot-dip coated duplex steel and its manufacturing method. Its chemical composition is as follows: C: 0.04%~0.08%, Si: 0.5%~0.8%, Mn: 1.4%~2.1%, Al: 0.02%~0.08%, Nb: 0.04~0.07%, Ti: 0.05~0.14%, Cr: 0~0.25%, Cu: 0~0.01%, Ni: 0~0.01%, B: 0~0.001%, P: 0~0.015%, S: 0~0.001%, with the remainder being Fe and unavoidable impurities. The patent has a relatively high Si content, which makes it easy for red iron scale to form on the surface during hot rolling, resulting in poor galvanized surface quality and thus affecting corrosion resistance.

[0007] It can be seen that none of the above patent documents have achieved the effect of high yield strength ratio in hot-dip galvanized duplex steel. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high yield strength ratio 800MPa grade hot-dip galvanized steel sheet. This high yield strength ratio 800MPa grade hot-dip galvanized steel sheet has a high yield strength ratio, high hole expansion performance and high corrosion resistance. It can be used in automotive structural parts and chassis components, as well as in other application fields that require high strength, weight reduction and corrosion resistance.

[0009] To achieve the above objectives, the present invention provides a high yield strength ratio 800MPa grade hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer, wherein the substrate contains Fe and unavoidable impurities, and the substrate further contains the following chemical elements in the following mass percentages:

[0010] C: 0.04-0.069%, Mn: 1.3-1.8%, 0<Cr≤0.5%, Ti: 0.05-0.12%, 0<Nb≤0.05%, Mo: 0.1-0.25%, Al: 0.02-0.08%;

[0011] The substrate does not contain Si.

[0012] The microstructure of the substrate includes a multiphase structure of granular bainite and ferrite, and a second phase particle precipitate, wherein the volume fraction of the second phase particle precipitate is not less than 0.2%, and the proportion of the second phase particle precipitate with a size not greater than 10 nm to the total volume fraction of the second phase particle precipitate is greater than 80%.

[0013] In this invention, when the silicon content in the steel is high, a fir olivine (2FeO-SiO2) oxide scale easily forms on the surface of the slab during heating, which adversely affects the surface quality of the zinc layer and thus impacts corrosion resistance. Therefore, to ensure that the steel plate possesses both ultra-high strength and good corrosion resistance, this invention employs a silicon-free design. It should be noted that a silicon-free design means that no Si element needs to be intentionally added during the smelting process. Since iron ore contains a small amount of Si, even if the finished steel plate contains trace amounts of Si, it is an unavoidable impurity in this invention.

[0014] Furthermore, the precipitation of second-phase particles is a key strengthening mechanism in this invention. Only when the precipitated second-phase particles have a sufficient volume fraction and are small in size can both the porosity and strength be improved simultaneously. When the volume fraction of second-phase particles is less than 0.2%, the tensile strength of the steel plate will be less than 800 MPa. Second-phase particles precipitate at various stages of cooling. In the hot rolling process, the particles precipitated during the high-temperature rolling stage are larger in size, generally above 20 nm, and the strengthening effect is weak. The second-phase particles precipitated during the cooling after coiling and the heating process in the hot-dip galvanizing process have smaller sizes (less than 10 nm) and have a significant strengthening effect. Therefore, this invention controls the volume fraction of second-phase particles with a size no greater than 10 nm to account for more than 80% of the total precipitation volume fraction, so that the tensile strength of the steel plate reaches more than 800 MPa.

[0015] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage content of each chemical element is as follows:

[0016] C: 0.04-0.069%, Mn: 1.3-1.8%, 0 < Cr ≤ 0.5%, Ti: 0.05-0.12%, 0 < Nb ≤ 0.05%, Mo: 0.1-0.25%, Al: 0.02-0.08%; balance is Fe and other unavoidable impurities.

[0017] The design principles of each chemical element in the hot-dip galvanized steel sheet of this invention are as follows:

[0018] C: In the hot-dip galvanized steel sheet described in this invention, carbon (C) is the most basic element in steel and the most commonly used strengthening element. The carbon content determines the tensile strength level of the steel sheet. Simultaneously, carbon stabilizes austenite and promotes the formation of bainite and other structures; carbon and microalloying elements form sufficient precipitated strengthening phases to ensure the strength of the steel. However, an excessively high carbon percentage can lead to deterioration in formability and weldability. Therefore, in the hot-dip galvanized steel sheet described in this invention, the carbon percentage is controlled between 0.04% and 0.069%.

[0019] Mn: In the hot-dip galvanized steel sheet of the present invention, manganese is a solid solution strengthening element that can improve hardenability, delay the pearlite transformation and lower the bainite transformation temperature, and refine the bainite structure. If the mass percentage of manganese is too low, it will lead to insufficient strength. However, if the mass percentage of manganese is too high, it will reduce the plasticity of the steel sheet and easily lead to segregation. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of Mn is controlled at 1.3-1.8%.

[0020] Cr: In the hot-dip galvanized steel sheet of this invention, chromium improves hardenability and solid solution strengthening, inhibits the formation of pearlite, and is beneficial to the formation of bainite. Furthermore, chromium is less prone to segregation. A higher Cr mass percentage leads to increased costs and also tends to produce more martensite, affecting the steel's strength. Therefore, in the hot-dip galvanized steel sheet of this invention, the Cr mass percentage is controlled to be 0 < Cr ≤ 0.5%.

[0021] Titanium (Ti): In the hot-dip galvanized steel sheet described in this invention, titanium is one of the important precipitation strengthening and grain refinement strengthening elements. Especially during the hot-dip galvanizing annealing process, Ti precipitates in the form of TiC, which can significantly improve the strength of the ferrite matrix, thus benefiting the increase in yield strength ratio and elongation. When the Ti content is less than 0.05%, the tensile strength will not reach 800 MPa. When the Ti content is greater than 0.12%, its strengthening effect tends to saturate. From an economic perspective, the upper limit of the Ti content in this invention is 0.12%. Therefore, in the hot-dip galvanized steel sheet described in this invention, the mass percentage of Ti is controlled between Ti: 0.05-0.12%.

[0022] Niobium (Nb): In the hot-dip galvanized steel sheet described in this invention, niobium 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 the hot-dip galvanized steel sheet described in this invention, the mass percentage of Nb is controlled at 0 < Nb ≤ 0.05%.

[0023] Mo: In the hot-dip galvanized steel sheet described in this invention, Mo is a key additive element. Molybdenum improves hardenability and solid solution strengthening, inhibits pearlite formation, and is beneficial for bainite formation. Additionally, when using Nb, V, and Ti microalloying, fine carbides or carbonitrides can form during cooling, providing precipitation strengthening. However, these carbides or carbonitrides tend to coarsen strongly, and the strengthening effect decreases sharply as the second-phase particle size increases. When Mo is added to the steel, it partially replaces the microalloying elements in the second phase, thereby reducing the interfacial energy of the second phase and making it less prone to growth. When the Mo content is less than 0.1%, sufficient bainite cannot be formed, and the effect of preventing second-phase growth is not significant. When the Mo content is greater than 0.25%, its effect tends to saturate. Therefore, in the hot-dip galvanized steel sheet described in this invention, the mass percentage of Mo is controlled at 0.1-0.25%.

[0024] Al: In the hot-dip galvanized steel sheet described in this invention, Al is a deoxidizing element of steel, reducing oxide inclusions and purifying the steel, which is beneficial to improving the formability of the steel sheet. However, the high mass percentage of aluminum can cause oxidation, further affecting continuous casting production. Therefore, in the hot-dip galvanized steel sheet described in this invention, the mass percentage of Al is controlled at 0.02-0.08%.

[0025] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the substrate also contains at least one of the following chemical elements:

[0026] 0 < V ≤ 0.2%;

[0027] 0 < B ≤ 0.003%.

[0028] Adding V and B can further optimize the performance of the hot-dip galvanized steel sheet described in this invention. Wherein:

[0029] V: In the hot-dip galvanized steel sheet of the present invention, vanadium is one of the important precipitation strengthening and grain refinement strengthening elements. The precipitation of vanadium carbides in ferrite is beneficial to improving the strength of ferrite. However, when the mass percentage of V is higher than 0.2%, the strengthening effect is close to saturation, and the cost is high. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of V is controlled at 0 < V ≤ 0.2%.

[0030] B: In the hot-dip galvanized steel sheet described in this invention, boron 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, excessive boron 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 the hot-dip galvanized steel sheet described in this invention, the mass percentage of boron is controlled to be 0 < B ≤ 0.003%.

[0031] Furthermore, in the hot-dip galvanized steel sheet of the present invention, among the unavoidable impurities in the substrate, P ≤ 0.02%, N ≤ 0.005%, and S ≤ 0.002%.

[0032] In the hot-dip galvanized steel sheet of the present invention, nitrogen (N), phosphorus (P), and sulfur (S) are all impurity elements in the steel. Therefore, when technical conditions permit, the content of impurity elements in the steel should be reduced as much as possible to obtain steel with better performance and higher quality. S, in particular, easily forms MnS inclusions. When S > 0.002%, the porosity will be less than 85%. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of S is preferably controlled to S ≤ 0.002%.

[0033] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the volume proportion of the granular bainite is not less than 95%.

[0034] In the hot-dip galvanized steel sheet of this invention, the microstructure of the substrate comprises a multiphase structure of granular bainite and ferrite. Since ferrite is relatively soft and has low strength, when the ferrite content is high, microcracks easily form at the interface between ferrite and bainite under external force, reducing the porosity. Therefore, in this invention, the microstructure is mainly bainite, and the volumetric phase of bainite is not less than 95%.

[0035] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the grain size of the granular bainite is less than 6 μm.

[0036] Generally, a larger grain size results in a lower yield strength. In this invention, the ferrite content is relatively low, therefore the bainite grain size has a significant impact on the yield strength. To achieve the technical effects of this invention, the grain size of the granular bainite is preferably controlled to be less than 6 μm.

[0037] Furthermore, in the hot-dip galvanized steel sheet of the present invention, in its hot-rolled state: the volume ratio of granular bainite is not less than 95%, and the size of the M / A islands in the granular bainite is less than 1 μm.

[0038] Granular bainite is defined as a mixture of bainitic ferrite and M / A islands. Generally, the M / A islands within granular bainite have a much higher hardness than the matrix structure. During punching, microcracks form at the interface between the M / A islands and the matrix; as the size of the M / A islands increases, the number and size of the microcracks also increase. When the M / A island size is greater than 1 μm, the hole expansion rate of the steel sheet will be less than 85%. To achieve the technical effects of this invention, in the hot-dip galvanized steel sheet described in this invention, the size of the M / A islands within the granular bainite in the hot-rolled state can be controlled to below 1 μm.

[0039] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the hardness uniformity index HI ≤ 3%.

[0040] Furthermore, in the hot-dip galvanized steel sheet of the present invention, the hardness uniformity index HI ≤ 2%.

[0041] The Hardness Uniformity Index (HI) is calculated by dividing the standard deviation of the hardness distribution along the thickness direction of a steel plate by the average value of the hardness distribution along the thickness direction. The HI value reflects the uniformity of the microstructure along the thickness direction.

[0042] In the hot-dip galvanized steel sheet of this invention, when the HI value is high, microcracks are easily generated in the tear zone at the cut during punching, leading to deterioration of the hole expansion and flanging performance. When the HI index > 3%, the hole expansion rate will be less than 85%. Therefore, in order to achieve the technical effect of this invention, in the hot-dip galvanized steel sheet of this invention, the hardness uniformity index is controlled at (HI) ≤ 3%. In some preferred embodiments, the hardness uniformity index can be controlled at (HI) ≤ 2%.

[0043] Furthermore, in the hot-dip galvanized steel sheet of the present invention, its yield strength is ≥720MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, hole expansion rate is ≥85%, and yield strength ratio is ≥0.9.

[0044] Furthermore, another objective of this invention is to disclose a method for manufacturing a hot-dip galvanized steel sheet with a high yield strength ratio of 800MPa. This method, in conjunction with the composition design of this invention, can obtain the microstructure characteristics desired by this invention, thereby obtaining a hot-dip galvanized steel sheet with a high yield strength ratio.

[0045] To achieve the above objectives, this invention proposes a method for manufacturing a high yield strength ratio (800 MPa) hot-dip galvanized steel sheet, comprising the following steps:

[0046] A slab is obtained;

[0047] Heat and hold the slab at that temperature;

[0048] Hot-rolled, then cooled at a cooling rate of ≥50℃ / s to a coiling temperature of 450-500℃;

[0049] Pickling is performed to obtain pickled rolls;

[0050] Annealing and hot-dip galvanizing: Pickled coils are annealed directly in a combustion-free, non-oxidizing continuous annealing furnace. The temperature of the soaking zone is controlled at 630-720℃, the holding time in the soaking zone is 30-120s, the temperature of the slow cooling zone is 550-670℃, and after exiting the slow cooling zone, the coils are cooled to the hot-dip galvanizing temperature.

[0051] In this invention, the post-rolling cooling rate is ≥50℃ / s to enable the hot-rolled steel sheet to quickly enter the bainite transformation region, obtain a fine bainite structure, and reduce the M / A island size. If the cooling rate is less than 50℃ / s, more ferrite or pearlite will be generated during the cooling process, and the M / A island size in the granular bainite will be larger (>1μm).

[0052] Controlling the coiling temperature to 450-500℃ is to obtain a granular bainite microstructure with refined M / A islands. Higher coiling temperatures tend to form pearlite with larger grain sizes. Furthermore, during the cooling process after coiling, a large number of second-phase particles precipitate and coarsen, resulting in a yield strength ratio of less than 0.9 after hot-dip galvanizing annealing. Conversely, excessively low coiling temperatures lead to martensite formation, resulting in a lower porosity.

[0053] In this invention, the annealing homogenization zone temperature is controlled at 630-720℃ because: when the homogenization zone temperature is below 630℃, the volume fraction of the second phase precipitated during homogenization is low, resulting in lower strength. When the homogenization temperature exceeds 720℃, martensite will be generated during cooling, leading to a higher hardness uniformity index (HI) and a porosity below 85%.

[0054] Furthermore, the present invention controls the holding time of the annealing homogenization zone to be 30-120s because: when the holding time of the annealing homogenization zone is less than 30s, the internal stress generated in the steel plate in the previous process cannot be completely eliminated, and the recovery and precipitation process of the steel plate is uneven, resulting in a large hardness uniformity index (HI) and a low porosity. However, when the homogenization temperature exceeds 120s, the second phase particles coarsen and dissolve in large quantities, leading to a decrease in strength, and the yield strength of the steel plate cannot be guaranteed.

[0055] In this invention, the temperature of the slow cooling section is controlled at 550-670℃ because: when the temperature of the slow cooling section is too high, the second-phase particles precipitated during the heating process will gradually coarsen, leading to a decrease in yield strength. Since the unit has limited cooling capacity in the slow cooling section, when the temperature of the soaking section is 630-720℃, it is usually difficult to cool the slow cooling section to below 550℃.

[0056] Furthermore, this invention can cool to the hot-dip galvanizing temperature at a cooling rate of ≥5℃ / s after exiting the slow cooling section. This invention does not impose a specific limitation on this cooling rate because it does not form austenite during the homogenization process, nor does it undergo phase transformation during cooling. Therefore, the cooling rate after exiting the slow cooling section has no impact on performance. A rate of ≥5℃ / s is the cooling rate required to ensure normal operation of a conventional unit.

[0057] Furthermore, in the manufacturing method of the hot-dip galvanized steel sheet of the present invention, the slab is heated to 1230-1280°C and held at that temperature for 1-3 hours.

[0058] In some embodiments of the present invention, controlling the heating temperature of the slab can further improve performance and surface quality. Setting the heating temperature above 1230°C aims to dissolve as much (Ti, Nb)(C, N) as possible during the continuous casting process. The microalloying of Ti and other solidified particles in the austenite precipitates as nanoscale second-phase particles during hot rolling, especially during annealing and hot-dip galvanizing, effectively improving the strength of the steel plate. However, when the temperature exceeds 1280°C, the austenite grains coarsen, which is detrimental to the toughness of the steel plate; simultaneously, the iron oxide scale becomes thicker, hindering descaling and ultimately affecting the surface quality of the hot-dip galvanized steel. Therefore, it is preferable to set the heating temperature to 1230-1280°C.

[0059] Furthermore, in the manufacturing method of hot-dip galvanized steel sheet of the present invention, in the hot rolling step, the rough rolling start temperature is controlled at 1130-1190℃, the finish rolling start temperature is controlled at 980-1080℃, and the finish rolling exit temperature is controlled at 840-880℃.

[0060] In some embodiments of the present invention, the roughing temperature control of the hot rolling process has a significant impact on microalloys such as Ti. When Ti is at a lower roughing temperature and during the finishing rolling process, Ti carbides and carbonitrides will precipitate. The precipitates in this process are large in size, which is not conducive to the improvement of the final strength. Therefore, the roughing temperature is controlled at 1130-1190℃ and the finishing temperature is controlled at 980-1080℃.

[0061] Furthermore, the finishing mill exit temperature in the hot rolling process is a key process for controlling grain size and M / A island size. At lower finishing mill exit temperatures, the austenite grains are disc-shaped, accumulating a large amount of deformation energy, which is beneficial for reducing grain size and M / A island size during subsequent cooling, and also helps to increase the porosity. However, if the finishing mill exit temperature is too low, mixed grains will occur, resulting in a higher hardness uniformity index (HI) and a lower porosity. Therefore, the finishing mill exit temperature in hot rolling is controlled at 840-880℃.

[0062] Furthermore, in the manufacturing method of the hot-dip galvanized steel sheet of the present invention, the hot-dip galvanizing temperature is 440-480°C during the hot-dip galvanizing step.

[0063] In this embodiment, the temperature of the hot-dip galvanizing zinc pot is controlled at 440-480℃ because the bonding between the steel plate surface and the zinc liquid is better within this temperature range. If the temperature is lower or higher than this range, the plating susceptibility of the substrate surface deteriorates, resulting in poor surface finish after plating.

[0064] Furthermore, in the manufacturing method of hot-dip galvanized steel sheet of the present invention, a leveling process is included after the hot-dip galvanizing step, and the leveling rate is controlled to be 0.05-1.3%.

[0065] In some embodiments of the present invention, when the flatness ratio is less than 0.05%, the surface quality of the zinc layer is poor; when the flatness ratio is greater than 1.3%, significant work hardening will occur, reducing the elongation of the steel plate. Therefore, the flatness ratio is controlled at 0.05-1.3% in the present invention.

[0066] Compared with the prior art, the high yield strength ratio 800MPa grade hot-dip galvanized steel sheet and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0067] The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet of the present invention has better corrosion resistance than conventional hot-rolled pickled steel sheet and is suitable for conventional hot continuous rolling production lines and hot-dip galvanizing production lines.

[0068] Compared with the traditional hot-dip galvanized sheet production method, the present invention eliminates the cold rolling process, shortens the process flow, improves production efficiency, and saves energy.

[0069] The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet of the present invention has the characteristics of high strength, high hole expansion rate and high corrosion resistance. Its longitudinal yield strength is ≥720MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, hole expansion rate is ≥85%, and yield strength ratio is ≥0.9.

[0070] Therefore, this invention can be used as automotive body structural components and automotive chassis components, as well as in other application fields requiring high strength and weight reduction, and is particularly suitable for the design of parts with complex shapes. Detailed Implementation

[0071] The following will further explain and illustrate the high yield strength ratio 800MPa grade hot-dip galvanized steel sheet and its manufacturing method according to the present invention with reference to specific embodiments. However, this explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0072] Examples 1-17 and Comparative Examples 1-13

[0073] The high yield strength ratio 800MPa grade hot-dip galvanized steel sheets in all embodiments of the present invention are prepared using the following steps:

[0074] (1) Smelting and casting to obtain slabs with a thickness of 220mm-250mm. Table 1 lists the mass percentage of each chemical element in the slabs of the various embodiments and comparative examples of the present invention.

[0075] (2) Heat the slab to 1230-1280℃ and keep it warm for 1-3 hours.

[0076] (3) Hot rolling: The roughing rolling start temperature is controlled at 1130-1190℃, the finishing rolling start temperature is controlled at 980-1080℃, and the finishing rolling exit temperature is controlled at 840-940℃. The cumulative deformation of roughing and finishing rolling during hot rolling can be controlled to be ≥90%.

[0077] (4) After hot rolling, cool to a coiling temperature of 450-500℃ at a cooling rate of ≥50℃ / s.

[0078] In some implementations, after the winding process is completed, the hot roll can be placed in an insulation pit for slow cooling, with an average cooling rate of ≤15℃ / h.

[0079] (5) Pickling to obtain pickled rolls.

[0080] In some implementations, the pickling elongation can be 0.2-2%; the pickling speed is 60-150 m / min; and the temperature of the last pickling tank is controlled at 80-90℃ and the iron ion concentration is controlled at 30-40 g / L.

[0081] Controlling the pickling and straightening elongation rate to 0.2-2% can further improve surface quality. When the elongation rate is below 0.2% or above 2%, it will adversely affect the plate shape, resulting in poor surface quality after hot-dip galvanizing. Furthermore, a lower pickling speed and a higher acid bath temperature will lead to over-pickling, while a higher pickling speed and a lower acid bath temperature will lead to under-pickling, both of which will cause surface problems on the pickled plate and affect the surface quality after hot-dip galvanizing.

[0082] (6) Annealing and Hot-Dip Galvanizing: Pickled coils are annealed directly in a combustion-free, non-oxidizing continuous annealing furnace, with conventional processes used in both the direct-fire and heating sections. The temperature in the soaking section is controlled at 630-720℃, and the holding time in the soaking section is 30-120s; the temperature in the slow cooling section is 550-670℃, and after exiting the slow cooling section, the coils can be cooled at a rate of ≥5℃ / s to the hot-dip galvanizing temperature of 440-480℃ for hot-dip galvanizing. The average single-sided weight of the hot-dip galvanized layer can be 20-380g / m². 2 .

[0083] (7) Flattening: control the flatness rate to 0.05-1.3% to obtain hot-dip galvanized sheet with a thickness of 1.8-3.5mm.

[0084] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S and N)

[0085]

[0086]

[0087] Tables 2-1 and 2-2 list the specific process parameters for each embodiment and comparative example of the present invention in the above process steps.

[0088] Table 2-1.

[0089]

[0090]

[0091] Table 2-2.

[0092]

[0093]

[0094]

[0095] Samples of the hot-dip galvanized steel sheets obtained in Examples 1-17 and Comparative Examples 1-13 were taken and their microstructure was observed using optical microscopy and scanning electron microscopy. The grain size was measured using the equivalent area method. The size distribution and volume fraction of the second phase precipitation were measured using the carbon extraction replication method, and the test results are listed in Table 3.

[0096] Table 3.

[0097]

[0098]

[0099] In addition, samples were taken from each embodiment and comparative example, and the following tests were performed. The test results are listed in Table 4:

[0100] The yield and tensile strength were determined by taking JIS13B tensile specimens along the longitudinal direction with a gauge length of 80 mm. The mechanical property tests were performed in accordance with GB / T228.1-2010 standard.

[0101] The porosity test was conducted in accordance with the GB / T24524-2021 standard.

[0102] The HI index measurement method is as follows: After polishing the cross-section of the steel plate, the microhardness is measured at equal intervals along the 45° thickness direction. Test data from the upper surface to the lower surface constitute one set, with each set containing no fewer than 15 data points. At least three sets of data constitute one dataset. The "mean value of the hardness distribution along the thickness direction of the steel plate" is the mean value of the dataset, and the "standard deviation of the hardness distribution along the thickness direction of the steel plate" is the standard deviation of the dataset. The hardness uniformity index (HI) is calculated using the aforementioned formula.

[0103] Table 4.

[0104]

[0105]

[0106] As shown in Tables 3 and 4, Examples 1-17 of this invention, through reasonable chemical element composition design and optimized hot rolling and annealing hot-dip galvanizing processes, achieved ideal microstructure characteristics and refined grains. The annealing hot-dip galvanizing heating process promoted the precipitation of a large number of second-phase particles, increasing strength without reducing plasticity. Strict temperature control at each stage of the annealing process to avoid coarsening of the second-phase particles is key to obtaining good performance. The final yield strength obtained in each embodiment of this invention is all >720MPa, tensile strength is all >800MPa, elongation (A80) is all >15%, porosity is all >85%, yield strength ratio is all ≥0.9, and hardness uniformity index (HI) is all <3%.

[0107] Unlike this invention, Comparative Examples 1-13 cannot achieve the technical effects achievable by this invention because their composition or process parameters do not meet the design requirements of this invention.

[0108] In Comparative Example 1, because the temperature of the annealing soaking zone was lower than the lower limit of the present invention, less second phase was precipitated during the hot-dip galvanizing annealing process, resulting in reduced strength.

[0109] In Comparative Example 2, because the temperature of the annealing soaking zone was higher than the upper limit of the present invention, austenite was formed during the heating process and transformed into martensite during the cooling process, resulting in a decrease in the proportion of bainite, an increase in the HI index, and a decrease in the yield strength ratio and porosity of the steel plate.

[0110] In Comparative Example 3, the annealing homogenization time was lower than the lower limit of the present invention, resulting in insufficient precipitation of the second phase and reduced strength.

[0111] In Comparative Example 4, the second phase coarsened and the yield strength ratio decreased because the annealing homogenization time was higher than the upper limit.

[0112] In Comparative Example 5, the high temperature in the slow cooling section led to coarsening of the second phase and a decrease in the yield strength ratio.

[0113] In Comparative Example 6, the coiling temperature was lower than the lower limit of the present invention, resulting in a higher martensite content and a lower bainite content in the hot coil, which reduced the hole expansion rate of the steel plate.

[0114] In Comparative Example 7, the winding temperature was higher than the upper limit of the present invention, resulting in a higher ferrite content, a lower bainite content, and coarser grains, leading to a decrease in strength.

[0115] Comparative Example 8, due to its lower cooling rate after rolling, resulted in the formation of more ferrite during laminar cooling, a decrease in bainite content, an increase in M / A island size, and a reduction in hole expansion rate.

[0116] Comparative Example 9 shows that because the carbon and manganese contents are higher than the upper limit of this invention, more martensite is formed during hot rolling, the bainite content is lower, and the porosity is reduced.

[0117] Comparative Example 10, due to its carbon and manganese content being lower than the lower limit of the present invention, although the microstructure meets the requirements of the present invention, the carbon and manganese dissolved in the Bainstein body are insufficient, resulting in insufficient strength.

[0118] Comparative Example 11: Due to the S content being higher than the upper limit, although the microstructure met the requirements of the present invention, it contained more inclusions and segregation, resulting in an increase in the HI index and a decrease in the porosity of the steel plate.

[0119] In Comparative Example 12, the addition of excessive boron resulted in the formation of martensite during hot rolling, leading to a decrease in the porosity. Furthermore, its molybdenum content was below the lower limit of this invention, causing the second-phase particles to coarsen during hot-dip galvanizing, weakening the precipitation strengthening effect and reducing the strength of the steel plate.

[0120] Comparative Example 13: Since the Ti content is lower than the lower limit of the present invention, the amount of second phase particles precipitated is less, resulting in lower strength.

[0121] 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.

[0122] 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. A high yield strength ratio (800 MPa) hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer, characterized in that, The mass percentage of each chemical element in the substrate is as follows: C: 0.04-0.069%, Mn: 1.3-1.8%, 0 < Cr ≤ 0.5%, Ti: 0.05-0.12%, 0 < Nb ≤ 0.05%, Mo: 0.1-0.25%, Al: 0.02-0.08%; the balance is Fe and other unavoidable impurities. The microstructure of the substrate includes a multiphase structure of granular bainite and ferrite, and a second phase particle precipitate, wherein the volume fraction of the second phase particle precipitate is not less than 0.2%, and the proportion of the second phase particle precipitate with a size not greater than 10 nm to the total volume fraction of the second phase particle precipitate is greater than 80%. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet has a hardness uniformity index HI≤3%, a yield strength ≥720MPa, a tensile strength ≥800MPa, an elongation A80 ≥15%, a hole expansion rate ≥85%, and a yield strength ratio ≥0.

9.

2. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, The substrate also contains at least one of the following chemical elements: 0＜V≤0.2%； 0＜B≤0.003%。 3. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, Of the unavoidable impurities in the substrate, P ≤ 0.02%, N ≤ 0.005%, and S ≤ 0.002%.

4. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, The volume proportion of the granular bainite is not less than 95%.

5. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, The grain size of the granular bainite is less than 6 μm.

6. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, In its hot-rolled state, the volume ratio of granular bainite is not less than 95%, and the size of the M / A islands in the granular bainite is less than 1 μm.

7. The high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in claim 1, characterized in that, Its hardness uniformity index HI≤2%.

8. The method for manufacturing high yield strength ratio 800MPa grade hot-dip galvanized steel sheet as described in any one of claims 1-7, characterized in that, It includes the following steps: A slab is obtained; Heat and hold the slab at that temperature; Hot-rolled, then cooled at a cooling rate of ≥50℃ / s to a coiling temperature of 450-500℃; Pickling is performed to obtain pickled rolls; Annealing and hot-dip galvanizing: Pickled coils are annealed directly in a combustion-free, non-oxidizing continuous annealing furnace. The temperature of the soaking zone is controlled at 630-720℃, the holding time in the soaking zone is 30-120s, the temperature of the slow cooling zone is 550-670℃, and after exiting the slow cooling zone, the coils are cooled to the hot-dip galvanizing temperature.

9. The manufacturing method as described in claim 8, characterized in that, Heat the slab to 1230-1280℃ and hold for 1-3 hours.

10. The manufacturing method as described in claim 8, characterized in that, In the hot rolling process, the roughing rolling start temperature is controlled at 1130-1190℃, the finishing rolling start temperature is controlled at 980-1080℃, and the finishing rolling exit temperature is controlled at 840-880℃.

11. The manufacturing method as described in claim 8, characterized in that, In the hot-dip galvanizing process, the galvanizing temperature is 440-480℃.

12. The manufacturing method as described in claim 8, characterized in that, The process after hot-dip galvanizing also includes leveling, with the leveling rate controlled at 0.05-1.3%.

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