An economical 800MPa grade hot-dip galvanized steel sheet and its manufacturing method

Abstract

This invention discloses an economical 800MPa grade hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer. The substrate contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%. The substrate does not contain Si, Cr, Mo, or Ni. The microstructure of the substrate comprises a multiphase structure of granular bainite and ferrite, and second-phase particle precipitates, wherein the volume fraction of the second-phase particle precipitates is not less than 0.2%, and the volume fraction of second-phase particle precipitates with a size not greater than 10 nm accounts for more than 80% of the total volume fraction of second-phase particle precipitates. Accordingly, this invention also discloses a method for manufacturing this economical 800MPa grade hot-dip galvanized steel sheet.

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. Hot-dip galvanized steel sheets are one of the effective ways to improve the corrosion resistance of automotive chassis.

[0003] 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-expanding properties and manufacturing method thereof," discloses the following composition by weight percentage for ultra-high strength hot-rolled multiphase steel plate and strip with good bending properties: C 0.07%–0.14%, Si 0.1%–0.4%, Mn 1.55%–2.00%, P ≤0.015%, S ≤0.004%, Al 0.01%–0.05%, N ≤0.005%, Cr 0.15%–0.50%, and also includes at least one alloying element selected from Nb ≤0.05% and Ti 0.06%–0.15%, with the remainder being Fe and unavoidable impurities. This patent focuses on the performance design and manufacturing method of hot-rolled pickled steel plates.

[0004] Chinese patent document CN113215485A, published on August 6, 2021, entitled "A 780MPa hot-dip galvanized duplex steel and its manufacturing method," discloses the following chemical composition: 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 Si content in this patent is relatively high.

[0005] Currently, automakers have increasingly higher requirements for automotive steel, demanding not only superior mechanical properties, formability, and corrosion resistance, but also lower costs. Therefore, this invention aims to provide an economical high-strength hot-dip galvanized steel. Summary of the Invention

[0006] One of the objectives of this invention is to provide an economical 800MPa grade hot-rolled base plate hot-dip galvanized steel sheet, which has low cost, good formability and strength, and can be used in automotive structural parts and chassis components, as well as other application fields that require high strength, weight reduction and corrosion resistance.

[0007] To achieve the above objectives, the present invention provides an economical 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 further contains the following chemical elements in the following mass percentages:

[0008] C: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%;

[0009] The substrate does not contain Si, Cr, Mo and Ni elements;

[0010] 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%.

[0011] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, the mass percentage content of each chemical element in the substrate is as follows:

[0012] C: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%; balance is Fe and other unavoidable impurities.

[0013] The economical 800MPa grade hot-dip galvanized steel sheet of the present invention does not contain precious metal elements such as Cr, Mo, and Ni in its substrate.

[0014] Existing 800MPa-level ultra-high strength steels typically contain some martensite. Therefore, for hot-dip galvanizing, it is necessary to add a significant amount of alloying elements such as Cr and Mo to delay phase transformation and reduce the critical cooling rate, allowing some supercooled austenite to transform into martensite after galvanizing. However, the presence of martensite leads to uneven microstructure hardness and a low porosity. Therefore, unlike conventional martensitic transformation strengthened steels, this invention designs the microstructure as granular bainite + second-phase particle precipitates + a small amount of ferrite to achieve high strength without adding expensive alloying elements.

[0015] Bainitic microstructures lack sufficient strength and require strong precipitation strengthening to improve it. However, during hot rolling and hot-dip galvanizing, the precipitated second-phase particles easily coarsen, leading to a weakened precipitation effect. Typically, a certain amount of Mo can be added to reduce this coarsening tendency. However, Mo is very expensive. To further reduce costs, this invention does not add Mo. Therefore, by strictly controlling the hot rolling, annealing, and hot-dip galvanizing processes, the volume fraction of the second phase is increased, and its coarsening tendency is reduced. This results in a microstructure where the volume fraction of second-phase precipitates is not less than 0.2%, and the proportion of second-phase precipitates with a size not greater than 10 nm to the total volume fraction of all second-phase precipitates is greater than 80%, thus achieving the technical effect of this invention.

[0016] From a microstructure design perspective, ferrite is relatively soft and has low strength. When the ferrite content is high, microcracks easily form at the ferrite-bainite interface under external force, reducing the porosity. Therefore, the microstructure in this invention is mainly granular bainite.

[0017] 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 porosity and strength be improved simultaneously. In this invention, when the volume fraction of second-phase particles is less than 0.2%, the yield strength of the steel plate will be less than 680 MPa. Second-phase particle precipitates are produced at various stages of cooling. During the high-temperature rolling stage of the hot rolling process, the precipitated particles are larger, generally above 20 nm, and the strengthening effect is weak. The second-phase particles precipitated during the cooling and annealing processes after hot rolling have smaller sizes (less than 10 nm) and have a significant strengthening effect. For this invention, the yield strength of the steel plate can only reach above 680 MPa when the volume fraction of second-phase particle precipitates with a size not greater than 10 nm accounts for more than 80% of the total volume fraction of second-phase particle precipitates.

[0018] From the perspective of component design, the design principles of each chemical element in this invention are as follows:

[0019] C: The carbon content largely determines the tensile strength grade of the steel plate. Carbon can stabilize austenite and promote the formation of bainite. Carbon and microalloying elements form sufficient precipitated strengthening phases to ensure the strength of the steel. However, an increase in the mass percentage of carbon will lead to a deterioration in formability and weldability. Based on this, the present invention controls the mass percentage of C to be 0.05-0.10%.

[0020] Mn: Manganese is a solid solution strengthening element that improves hardenability, delays the pearlite transformation, lowers the bainite transformation temperature, and refines the bainite microstructure. In this invention, a low mass percentage of manganese would lead to insufficient strength, while a high mass percentage would reduce the plasticity of the steel plate and easily cause segregation. Therefore, this invention controls the mass percentage of Mn to 1.4-2.0%.

[0021] Ti: In this invention, titanium is an important precipitation strengthening and grain refinement strengthening element, especially during hot-dip galvanizing annealing, where Ti precipitates as TiC, increasing the strength of the ferrite matrix and thus improving the yield strength ratio and elongation. Therefore, the mass percentage of Ti in this invention is controlled at 0.10-0.15%.

[0022] Al: Al is a deoxidizing element in steel, which can reduce oxide inclusions and purify the steel, thus improving the formability of steel plates. However, a high percentage of aluminum by mass can lead to oxidation, further affecting continuous casting production. Therefore, this invention controls the percentage of Al by mass to 0.02-0.08%.

[0023] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, the substrate further contains at least one of the following chemical elements:

[0024] 0 < Nb ≤ 0.03%;

[0025] 0 < B ≤ 0.003%.

[0026] Adding Nb and B can further optimize the performance of the hot-dip galvanized steel sheet described in this invention. Wherein:

[0027] Niobium (Nb) is an important precipitation strengthening and grain refinement element, but when the mass percentage of Nb exceeds 0.03%, the strengthening effect of Nb decreases, and the cost is also high. Therefore, this invention controls the mass percentage of Nb to ≤0.03%.

[0028] B: Boron helps expand the bainite phase region, ensuring that the steel plate can obtain bainite structure during post-rolling cooling, which significantly improves the strength and hardness of the steel. However, excessive B will lead to excessive martensite structure in the steel plate, resulting in a decrease in the expansion rate and elongation of the steel. Therefore, this invention controls the mass percentage of B to B≤0.003%.

[0029] In this invention, the unavoidable impurities are mainly S, P and N, and their content is expected to be as low as possible when technical conditions permit.

[0030] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, the unavoidable impurities in the substrate can be controlled to P≤0.02%, N≤0.005%, and S≤0.005%.

[0031] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, the volume proportion of granular bainite is not less than 95%. Apart from granular bainite and second-phase particle precipitates, the remainder is ferrite.

[0032] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, the grain size of the granular bainite is less than 5μm.

[0033] 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 controlled to be less than 5 μm.

[0034] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet of the present invention, its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and hole expansion rate is ≥50%.

[0035] Furthermore, another objective of this invention is to provide an economical method for manufacturing 800MPa grade hot-dip galvanized steel sheet. This method, in conjunction with the composition design of this invention, can obtain the microstructure characteristics desired by this invention, thereby obtaining a low-cost hot-dip galvanized steel sheet with good formability and strength.

[0036] To achieve the above objectives, the present invention proposes a method for manufacturing the aforementioned economical 800MPa grade hot-dip galvanized steel sheet, comprising the following steps:

[0037] A slab is obtained;

[0038] Heat and hold the slab at that temperature;

[0039] Hot-rolled, then cooled at a cooling rate of ≥50℃ / s to a coiling temperature of 450-500℃;

[0040] Pickling is performed to obtain pickled rolls;

[0041] Annealing and hot-dip galvanizing: Pickled coils are annealed directly in a combustion-free, non-oxidizing continuous annealing furnace. The temperature of the direct-fire section is controlled at 550-630℃, the temperature of the heating section is 630-720℃, the temperature of the soaking section is 630-720℃, the holding time in the soaking section is 50-100s, the temperature of the slow cooling section is 550-670℃, and after exiting the slow cooling section, the coils are cooled to the hot-dip galvanizing temperature.

[0042] 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 a 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.

[0043] The manufacturing method described in this invention, through the control of the cooling and annealing processes after hot rolling, results in precipitated second-phase particles with a small size (less than 10 nm), exhibiting a significant strengthening effect.

[0044] In this invention, controlling the cooling rate after rolling to ≥50℃ / s is to ensure that the hot-rolled steel sheet quickly enters the bainite transformation zone, resulting in a fine bainite structure. When the cooling rate is less than 50℃ / s, more ferrite or pearlite will be generated during the cooling process. Furthermore, controlling the coiling temperature after cooling to 450-500℃ is to obtain a granular bainite structure with refined M / A islands. Higher coiling temperatures tend to form pearlite with larger grain sizes, and during the cooling process after coiling, a large number of second-phase particles precipitate and coarsen, leading to reduced strength after hot-dip galvanizing and annealing. Lower coiling temperatures result in the formation of martensite, leading to a lower porosity.

[0045] Furthermore, the present invention controls the direct-fired section temperature to be 550-630℃ because: when the direct-fired section temperature is higher than 630℃, the heating rate decreases, the second-phase particles are more prone to coarsening during precipitation, the strengthening effect is weakened, resulting in insufficient yield strength. If the direct-fired section temperature is lower than 550℃, the subsequent radiant tube heating load increases, the homogenization temperature is difficult to control stably, which is not conducive to obtaining steel coils with uniform properties at the beginning, middle, and end.

[0046] In this invention, the heating zone temperature and the soaking zone temperature during annealing are both 630-720°C. When the heating zone temperature and the soaking zone temperature are below 630°C, the volume fraction of the second phase precipitated during the soaking process is low, resulting in lower strength. When the heating zone temperature and the soaking zone temperature exceed 720°C, martensite will be generated during the cooling process, resulting in lower porosity and lower yield strength.

[0047] Furthermore, the soaking time in the annealing zone is 50-100 seconds because: when the soaking time is less than 50 seconds, the internal stress of the steel plate during hot rolling cannot be completely eliminated, resulting in a poor surface condition and making it difficult to obtain a good hot-dip galvanized surface. However, when the soaking time exceeds 100 seconds, the second-phase particles coarsen and dissolve in large quantities, leading to a decrease in strength and making it impossible to guarantee the yield strength of the steel plate.

[0048] 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 high, the second phase particles precipitated during the heating process will gradually coarsen, resulting in a decrease in yield strength.

[0049] Furthermore, in the manufacturing method described in this invention, the slab is heated to 1230-1280°C and held at that temperature for 1-3 hours.

[0050] In this embodiment, the heating temperature is set to ≥1230℃ to dissolve as much of the (Ti, Nb)(C, N) produced during continuous casting as possible. The microalloying of Ti and other components dissolved in austenite precipitates as nanoscale second-phase particles during hot rolling, especially during annealing and hot-dip galvanizing, effectively further improving the strength of the steel plate. However, when the temperature exceeds 1280℃, 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℃.

[0051] Furthermore, in the manufacturing method described in this invention, in the hot rolling step, 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℃.

[0052] In this invention, the roughing temperature control during 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 further improvement of strength. Therefore, the roughing temperature is controlled at 1130-1190℃ and the finishing temperature is controlled at 980-1080℃.

[0053] Furthermore, the finishing mill exit temperature during hot rolling also affects the microstructure. 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 increasing porosity during subsequent cooling. However, excessively low finishing mill exit temperatures result in the formation of ferrite, leading to a decrease in porosity. Excessively high finishing mill exit temperatures are detrimental to surface quality; therefore, it is preferable to control the hot rolling finishing mill exit temperature to 840-940℃.

[0054] Furthermore, in the manufacturing method described in this invention, the hot-dip galvanizing temperature is 440-480°C during the hot-dip galvanizing step.

[0055] The hot-dip galvanizing temperature is 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 will decrease, which may lead to poor surface finish after plating.

[0056] Furthermore, in the manufacturing method described in this invention, a leveling process is included after the hot-dip galvanizing step, with the leveling rate controlled at 0.05-1.3%.

[0057] In this embodiment, in order to further obtain better surface quality based on the implementation effect of the present invention, the flatness rate after hot-dip galvanizing is controlled to be 0.05-1.3%. When the flatness rate is less than 0.05%, the surface quality of the zinc layer is poor, and when the flatness rate is greater than 1.3%, greater work hardening will occur, which will easily reduce the elongation of the steel plate.

[0058] The economical 800MPa grade hot-dip galvanized steel sheet and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0059] The economical 800MPa grade hot-dip galvanized steel sheet of the present invention does not contain precious metal elements such as Cr, Mo and Ni, so it has low cost while still ensuring a strength level of 800MPa.

[0060] In some embodiments, the economical 800MPa grade hot-dip galvanized steel sheet of the present invention has a yield strength ≥680MPa, tensile strength ≥800MPa, elongation A80 ≥15%, and hole expansion rate ≥50%. Therefore, it can be used as an automobile chassis and structural component, meeting the technical requirements for flanging, stamping, and lightweighting of complex automobile parts.

[0061] The economical 800MPa grade hot-dip galvanized steel sheet described in this invention does not contain Si and Cr elements, and has better surface quality and corrosion resistance compared with similar hot-dip galvanized steel sheets. It should be noted that the absence of Si and Cr in this invention means that they do not need to be intentionally added during the smelting process. Since iron ore and scrap steel contain small amounts of Si and Cr, even if the finished steel sheet contains trace amounts, such as less than 0.1%, of Si and Cr, these are residual elements and not intentionally added by this invention.

[0062] The economical 800MPa grade hot-dip galvanized steel sheet described in this invention is suitable for conventional hot rolling and hot-dip galvanizing production lines. Compared with traditional hot-dip galvanized sheet production methods, this invention eliminates the cold rolling process, shortens the process flow, improves production efficiency, and saves energy. Detailed Implementation

[0063] The following will further explain and illustrate the economical 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.

[0064] The economical 800MPa grade hot-dip galvanized steel sheet in each embodiment of the present invention is prepared by the following steps:

[0065] (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.

[0066] (2) Heat the slab to 1230-1280℃ and keep it warm for 1-3 hours.

[0067] (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 ≥90%.

[0068] (4) After hot rolling, cool to a coiling temperature of 450-500℃ at a cooling rate of ≥50℃ / s.

[0069] In some implementations, optionally, 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.

[0070] (5) Pickling to obtain pickled rolls.

[0071] In some implementations, the elongation rate of pickling and straightening can be 0.2-2% to further control the plate shape and thus obtain better surface quality; a pickling speed of 60-150 m / min can also be used, the temperature of the last pickling tank in the pickling process is controlled at 80-90℃, and the iron ion concentration is controlled at 30-40 g / L to further obtain better surface quality.

[0072] (6) Annealing and hot-dip galvanizing: Pickled coils are annealed directly in a combustion-free, non-oxidizing continuous annealing furnace. The temperature of the direct-fired section is controlled at 550-630℃, the heating section at 630-720℃, the soaking section at 630-720℃, the soaking section holding time at 50-100s, and the slow cooling section at 550-670℃. 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 .

[0073] (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.

[0074] Table 1 lists the mass percentage of each chemical element in the substrates of each embodiment and comparative example.

[0075] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, and N)

[0076]

[0077]

[0078] Note: In Table 1, steel grade AI is the steel grade used in the embodiments of the present invention, while JM is the steel grade used in the comparative examples.

[0079] Table 2 lists the specific process parameters for each embodiment and comparative example of the present invention in the above process steps.

[0080] Table 2.

[0081]

[0082]

[0083] Samples of the hot-dip galvanized steel sheets obtained in Examples 1-16 and Comparative Examples 1-18 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.

[0084] Table 3.

[0085]

[0086]

[0087] In addition, for each embodiment and comparative sample, longitudinal JIS13B tensile specimens were taken to determine the yield and tensile strength, with a gauge length of 80 mm. The mechanical property tests were performed according to GB / T228.1-2010 standard. The porosity test was performed according to GB / T24524-2021 standard, and the test results are listed in Table 4.

[0088] Table 4.

[0089]

[0090]

[0091] As shown in Tables 3 and 4, Examples 1-16 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 longitudinal yield strength obtained in each embodiment of this invention is above 680 MPa, the tensile strength is above 800 MPa, the elongation (A80) is above 15%, and the porosity is ≥50%.

[0092] Unlike this invention:

[0093] In Comparative Example 1, since the carbon and manganese contents are higher than the upper limit designed in this invention, more martensite is formed during hot rolling, resulting in less bainite content and a reduced porosity.

[0094] Comparative Example 2 shows insufficient strength due to carbon and manganese content being below the lower limit designed in this invention.

[0095] Comparative Example 3, in which element B was added, had a B content higher than the upper limit designed in this invention, thus martensite was generated during hot rolling, resulting in a decrease in the porosity.

[0096] In Comparative Example 4, because the Ti content was lower than the lower limit designed in this invention, the amount of second phase particles precipitated was less and the strength was lower.

[0097] In Comparative Example 5, because the temperature of the soaking zone was lower than the lower limit of the present invention, less second phase was precipitated during the hot-dip galvanizing annealing process, thus reducing the yield strength.

[0098] In Comparative Example 6, because the temperature of the soaking zone was higher than the upper limit of the present invention, austenite was formed during the heating process, and the phase transformed into martensite during the cooling process, the proportion of bainite decreased, and the yield strength and porosity of the steel plate decreased.

[0099] In Comparative Example 7, because the temperature of the heating section was lower than the lower limit of the present invention, less second phase was precipitated during the hot-dip galvanizing annealing process, resulting in a decrease in yield strength.

[0100] In Comparative Example 8, because the temperatures of the direct-fired section and the heating section were higher than the upper limit of the present invention, austenite was formed during the heating process, and the phase transformed into martensite during the cooling process, resulting in a decrease in the proportion of bainite and a reduction in the yield strength and porosity of the steel plate.

[0101] In Comparative Example 9, the second phase coarsened and the yield strength decreased because the homogenization time was longer than the upper limit of the present invention.

[0102] In Comparative Example 10, the temperature of the slow cooling section was higher than the upper limit of the present invention, which led to coarsening of the second phase and a decrease in yield strength.

[0103] In Comparative Example 11, because the coiling temperature was lower than the lower limit of the present invention, martensite was generated after hot rolling, resulting in a decrease in bainite content and a decrease in the expansion rate.

[0104] In Comparative Example 12, because the coiling temperature was higher than the upper limit of the present invention, more ferrite was formed during the hot rolling process, accompanied by a large amount of precipitation, which led to a decrease in yield strength.

[0105] In Comparative Example 13, due to the cooling rate after hot rolling being lower than the lower limit of this invention, more ferrite was formed, accompanied by a large amount of precipitation, resulting in a decrease in yield strength.

[0106] 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.

[0107] 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 economical 800MPa grade 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: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%; balance is Fe and other unavoidable impurities; The microstructure of the substrate includes a multiphase 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 volume fraction of second-phase particle precipitates with a size not greater than 10 nm is greater than 80% of the total volume fraction of second-phase particle precipitates. The volume fraction of the granular bainite is not less than 95%, and the grain size of the granular bainite is less than 5 μm. Its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and porosity is ≥50%.

2. The economical 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 ≤ 0.005%.

3. An economical 800MPa grade 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: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%; and at least one of 0 < Nb ≤ 0.03% and 0 < B ≤ 0.003%; the balance being Fe and other unavoidable impurities; The microstructure of the substrate includes a multiphase 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 volume fraction of second-phase particle precipitates with a size not greater than 10 nm is greater than 80% of the total volume fraction of second-phase particle precipitates. The volume fraction of the granular bainite is not less than 95%, and the grain size of the granular bainite is less than 5 μm. Its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and porosity is ≥50%.

4. The economical 800MPa grade hot-dip galvanized steel sheet as described in claim 3, characterized in that, Of the unavoidable impurities in the substrate, P ≤ 0.02%, N ≤ 0.005%, and S ≤ 0.005%.

5. The method for manufacturing the economical 800MPa grade hot-dip galvanized steel sheet as described in any one of claims 1-4, characterized in that, It includes the following steps: A slab is obtained; Heat and hold the slab at that temperature; Hot-rolled, then cooled to a coiling temperature of 450-500℃ at a cooling rate of ≥50℃ / s; 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 direct-fire section is controlled at 550-630℃, the temperature of the heating section is 630-720℃, the temperature of the soaking section is 630-720℃, the holding time in the soaking section is 50-100s, the temperature of the slow cooling section is 550-670℃, and after exiting the slow cooling section, the coils are cooled to the hot-dip galvanizing temperature.

6. The manufacturing method as described in claim 5, characterized in that, Heat the slab to 1230-1280℃ and hold for 1-3 hours.

7. The manufacturing method as described in claim 5, 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 840-940℃.

8. The manufacturing method as described in claim 5, characterized in that, In the hot-dip galvanizing process, the galvanizing temperature is 440-480℃.

9. The manufacturing method as described in claim 5, 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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