Economical 800MPa-grade heat-based hot-dip galvanized steel plate and manufacturing method thereof

Abstract

The invention discloses an economical 800MPa-grade hot-based hot-dip galvanized steel plate which comprises a base plate and a hot-dip galvanized layer, the base plate contains Fe and inevitable impurities, and the base plate further contains the following chemical elements in percentage by mass: 0.05-0.10% of C, 1.4-2.0% of Mn, 0.10-0.15% of Ti, 0.02-0.08% of Al, 0.05-0.10% of Cr, 0.05-0.10% of Mo, 0.05-0.10% of Ni, 0.05-0.10% of Mo, 0.05-0.10% of Mo, 0.05-0.10% of Mo, 0.05-0.10% of Mo and the balance of Fe. The substrate does not contain elements of Si, Cr, Mo and Ni; the microstructure of the base plate comprises a multiphase structure of granular bainite and ferrite and second-phase particle precipitates, the volume fraction of the second-phase particle precipitates is not smaller than 0.2%, and the volume fraction proportion of the second-phase particle precipitates with the size not larger than 10 nm to all the second-phase particle precipitates is larger than 80%. Correspondingly, the invention further discloses a manufacturing method of the economical 800MPa-grade hot-based hot-dip galvanized steel plate.

Description

Technical Field

[0001] The invention relates to a steel plate and a manufacturing method thereof, and in particular to a hot-dip galvanized steel plate and a manufacturing method thereof. Background Art

[0002] With the development of automobile lightweight technology, the proportion of advanced high-strength steel in automobile structural parts is increasing. Duplex steel has super high strength and good formability, and has become the first choice for chassis parts such as control arms and torsion beams. Among them, hot-dip galvanized steel sheet is one of the effective ways to improve the corrosion resistance of automobile chassis.

[0003] The Chinese patent document with publication number CN104513930A and publication date April 15, 2015, and titled "Ultra-high strength hot-rolled complex phase steel plate and strip with good bending and hole expansion performance and manufacturing method thereof" discloses that the composition weight percentage of ultra-high strength hot-rolled complex phase steel plate and strip with good bending performance is: C 0.07% to 0.14%, Si 0.1% to 0.4%, Mn 1.55% to 2.00%, P ≤ 0.015%, S ≤ 0.004%, Al 0.01% to 0.05%, N ≤ 0.005%, Cr 0.15 to 0.50%, and also includes at least one alloy element of Nb ≤ 0.05%, Ti 0.06 to 0.15%, and the rest is Fe and unavoidable impurities. The patent focuses on the performance design and manufacturing method of hot-rolled pickled plates.

[0004] The Chinese patent document "A 780MPa hot-base coated dual-phase steel and its manufacturing method" with publication number CN113215485A and publication date August 6, 2021, discloses the chemical composition as follows: C: 0.04% to 0.08%, Si: 0.5% to 0.8%, Mn: 1.4% to 2.1%, Al: 0.02% to 0.08%, Nb: 0.04 to 0.07%, Ti: 0.05 to 0.14%, Cr: 0 to 0.25%, Cu: 0 to 0.01%, Ni: 0 to 0.01%, B: 0 to 0.001%, P: 0 to 0.015%, S: 0 to 0.001%, and the rest is Fe and unavoidable impurities. The Si content in this patent is relatively high.

[0005] At present, automobile factories have higher and higher requirements for automobile steel. In addition to the requirements of mechanical properties, formability and corrosion resistance, they also require steel plates to have lower costs. Based on this, the present invention is expected to provide an economical high-strength hot-dip galvanized steel. Summary of the invention

[0006] One of the purposes of the present invention is to provide an economical 800MPa grade hot-rolled base galvanized steel sheet, which has low cost and good formability and strength, and can be used for automotive structural parts and chassis parts, and can also be used in other application fields requiring high strength, weight reduction and corrosion resistance.

[0007] In order to achieve the above object, the present invention provides an economical 800MPa grade hot-dip galvanized steel sheet, which comprises a substrate and a hot-dip galvanized layer, wherein the substrate contains Fe and inevitable 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 composite structure of granular bainite + 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 the second phase particle precipitates with a size of no more than 10 nm accounts for more than 80% of the total second phase particle precipitates.

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

[0012] C: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%; the balance is Fe and other inevitable 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] In the existing 800MPa level ultra-high strength steel, some martensitic structures are usually contained. Therefore, for the hot-dip galvanizing process, it is necessary to add more alloy elements such as Cr and Mo to delay the phase transformation, reduce the critical cooling rate, so that some supercooled austenite can be delayed until after galvanizing before the phase transformation to martensite, but the presence of martensite will make the microstructure hardness uneven, resulting in a lower hole expansion rate. Therefore, unlike conventional martensitic phase transformation strengthening steel, in order to obtain high strength without adding precious alloy elements, the present invention designs the microstructure as granular bainite + second phase particle precipitates + a small amount of ferrite.

[0015] The strength of bainite structure is insufficient, and strong precipitation strengthening is required to improve the strength. However, during hot rolling and hot-dip galvanizing, the second phase particles are easily coarsened after precipitation, resulting in a weakened precipitation effect. Generally speaking, a certain amount of Mo element can be added to reduce the coarsening tendency. However, Mo element is very expensive. In order to further reduce costs, the present invention does not add Mo element. Therefore, the purpose of increasing the volume fraction of the second phase and reducing its coarsening tendency is achieved by strictly controlling the hot rolling, annealing and hot-dip galvanizing processes, thereby obtaining a volume fraction of the second phase particle precipitates of not less than 0.2%, and the second phase particle precipitates with a size of not more than 10nm account for a volume fraction ratio of more than 80% of the total second phase particle precipitates. The organizational characteristics are to achieve the technical effects of the present invention.

[0016] From the perspective of microstructure design, ferrite is relatively soft and has low strength. When the ferrite content is high, microcracks are easily formed on the interface between ferrite and bainite under the action of external force, reducing the hole expansion rate. Therefore, the microstructure in the present invention is mainly granular bainite.

[0017] In addition, the precipitation of second-phase particles in the present invention is a key strengthening mechanism. Only when the precipitated second-phase particles have a sufficient volume fraction and are small in size, can the hole expansion rate and strength be improved at the same time. In the present invention, when the volume fraction of the second-phase particles is less than 0.2%, the yield strength of the steel plate will be less than 680MPa. The second-phase particle precipitates are precipitated at various stages of cooling. In the hot rolling process, the particle size precipitated in the high-temperature rolling stage is relatively large, generally more than 20nm, and the strengthening effect is relatively weak. The second-phase particles precipitated during the cooling and annealing process after hot rolling have a smaller size (less than 10nm) and have a significant strengthening effect. For the present invention, the yield strength of the steel plate can only reach more than 680MPa when the volume fraction of the second-phase particle precipitates with a size not greater than 10nm accounts for more than 80% of the total volume fraction of the second-phase particle precipitates.

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

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

[0020] Mn: Manganese is a solid solution strengthening element that improves hardenability, delays pearlite transformation, reduces bainite transformation temperature, and refines bainite structure. In the present invention, a lower mass percentage of manganese will result in insufficient strength, but a higher mass percentage of manganese will result in reduced plasticity of the steel plate and easily lead to segregation. Based on this, the present invention controls the mass percentage of Mn to 1.4-2.0%.

[0021] Ti: In the present invention, titanium is an important precipitation strengthening and fine grain strengthening element. Especially in the hot dip galvanizing annealing process, Ti will precipitate in the form of TiC, which improves the strength of the ferrite matrix and is conducive to improving the yield strength ratio and elongation. Therefore, the mass percentage of Ti in the present invention is controlled to Ti: 0.10-0.15%.

[0022] Al: Al is a deoxidizing element for steel, which can reduce oxide inclusions in steel, purify steel, and improve the formability of steel plates. However, a high mass percentage of aluminum will cause oxidation, further affecting continuous casting production. Based on this, the present invention controls the mass percentage of Al 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 of the present invention.

[0027] Nb: Niobium is one of the important precipitation strengthening and fine grain strengthening elements, but when the mass percentage of Nb is higher than 0.03%, the added value of the strengthening effect of Nb is weakened and the cost is high. Therefore, the mass percentage of Nb is controlled to Nb≤0.03% in the present invention.

[0028] B: Boron is beneficial to expand the bainite phase region, ensuring that the steel plate can obtain bainite structure during cooling after rolling, and significantly improving the strength and hardness of the steel. However, too much B element will cause too much martensite structure to appear in the steel plate, resulting in a decrease in the hole expansion rate and elongation of the steel. Therefore, the mass percentage of B in the present invention is controlled to B≤0.003%.

[0029] In the present invention, the inevitable impurities are mainly S, P and N, and it is desired that the content thereof is as low as possible if technical conditions permit.

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

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

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

[0033] Generally speaking, when the grain size is larger, the yield strength is lower. In the present invention, the ferrite content is relatively low, so the bainite grain size has a greater impact on the yield strength. In order to achieve the technical effect of the present 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 described in the present invention, its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and hole expansion ratio is ≥50%.

[0035] In addition, another object of the present invention is to provide an economical method for manufacturing 800MPa grade hot-dip galvanized steel sheet. This method, combined with the component design of the present invention, can obtain the organizational characteristics to be obtained by the present invention, thereby obtaining a low-cost hot-dip galvanized steel sheet with good formability and strength.

[0036] In order to achieve the above object, the present invention proposes a method for manufacturing the above economical 800MPa grade hot-dip galvanized steel sheet, which comprises the steps of:

[0037] obtaining a slab;

[0038] heating and keeping the slab warm;

[0039] Hot rolling, followed by cooling at a cooling rate of ≥50°C / s to a coiling temperature of 450-500°C;

[0040] Pickling to obtain pickled coils;

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

[0042] In addition, the present invention can be cooled to the hot-dip galvanizing temperature at a cooling rate of ≥5°C / s after exiting the slow cooling section. The present invention does not specifically limit the cooling rate because the present invention does not form austenite during the soaking process and does not undergo phase change during the cooling process, so the cooling rate after exiting the slow cooling section has no effect on the performance. The rate ≥5°C / s is the cooling rate to ensure the normal operation of a conventional unit.

[0043] The manufacturing method of the present invention controls the cooling and annealing heating processes after hot rolling so that the precipitated second phase particles have a smaller size (less than 10 nm) and have a significant strengthening effect.

[0044] In the present invention, the cooling rate after rolling is controlled to be ≥50℃ / s in order to make the hot-rolled steel plate quickly enter the bainite transformation zone and obtain a fine bainite structure. When the cooling rate is less than 50℃ / s, more ferrite or pearlite will be generated during the cooling process. In addition, the coiling temperature after cooling is controlled to be 450-500℃ in order to obtain a granular bainite structure with refined M / A islands. Pearlite is easily formed when the coiling temperature is high, and the grain size is also larger. At the same time, a large number of second phase particles will precipitate and coarsen during the cooling process after coiling, resulting in a decrease in strength after hot-dip galvanizing annealing. Martensite will be formed when the coiling temperature is low, resulting in a lower hole expansion rate.

[0045] In addition, the present invention controls the direct-fire section temperature to 550-630°C because: when the direct-fire section temperature is higher than 630°C, the heating rate is reduced, the second phase particles are easily coarsened, the strengthening effect is weakened, and the yield strength is insufficient. If the direct-fire section temperature is lower than 550°C, the subsequent radiant tube heating load increases, and the soaking temperature is difficult to stably control, which is not conducive to obtaining a steel coil with uniform performance at the head, middle and tail.

[0046] In the present invention, the heating section temperature and the soaking section temperature of annealing are both 630-720°C. When the heating section temperature and the soaking section temperature are less than 630°C, the volume fraction of the second phase precipitated during the soaking process is low, resulting in lower strength. When the heating section temperature and the soaking section temperature exceed 720°C, martensite will be generated during the cooling process, resulting in lower hole expansion rate and lower yield strength.

[0047] In addition, the annealing soaking section holding time is 50-100s because: when the annealing soaking section holding time is less than 50s, the internal stress of the steel plate cannot be completely eliminated during the hot rolling process, and the poor surface condition of the steel plate makes it difficult to obtain a good hot-dip galvanized surface. However, when the soaking time exceeds 100s, the second phase particles coarsen and dissolve in large quantities, resulting in reduced strength, and the yield strength of the steel plate cannot be guaranteed.

[0048] In the present invention, the temperature of the slow cooling section is controlled to be 550-670° C. 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 of the present invention, the slab is heated to 1230-1280° C. and kept warm for 1-3 hours.

[0050] In this embodiment, the heating temperature is set to ≥1230°C, the purpose of which is to dissolve (Ti, Nb) (C, N) produced in the continuous casting process as much as possible, and the micro-alloys such as Ti dissolved in austenite are precipitated as nano-scale second phase particles during hot rolling and coiling, especially during annealing and hot-dip galvanizing, which can effectively further improve the strength of the steel sheet. When the temperature exceeds 1280°C, the austenite grains coarsen, which is not conducive to the toughness of the steel sheet; at the same time, the iron oxide scale is thick, which is not conducive to the dephosphorization of the iron oxide scale, and ultimately affects the surface quality of hot-dip galvanizing, so it is preferred to set the heating temperature to 1230-1280°C.

[0051] Furthermore, in the manufacturing method of the present invention, in the hot rolling step, the rough rolling start temperature is controlled to be 1130-1190°C, the finishing rolling start temperature is controlled to be 980-1080°C, and the finishing rolling outlet temperature is controlled to be 840-940°C.

[0052] In the present invention, the rough rolling temperature control in the hot rolling process has a greater impact on microalloys such as Ti. During the relatively low rough rolling temperature and the finishing rolling process, Ti carbides and carbonitrides will precipitate. The precipitation size of this process is relatively large, which is not conducive to further improvement of strength. Therefore, the rough rolling start temperature is controlled at 1130-1190°C, and the finishing rolling start temperature is controlled at 980-1080°C.

[0053] In addition, the finishing exit temperature of the hot rolling process also affects the microstructure. When the finishing exit temperature is low, the austenite grains are in a pancake shape, accumulating a large amount of deformation energy, which is conducive to reducing the grain size and improving the hole expansion rate in the subsequent cooling process. However, when the finishing exit temperature is too low, ferrite will be produced, resulting in a decrease in the hole expansion rate. Too high a finishing exit temperature is not good for the surface quality, so it is preferred to control the hot rolling finishing exit temperature to 840-940℃.

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

[0055] The hot-dip galvanizing temperature is 440-480℃ because the bonding between the steel plate surface and the zinc liquid is better in this temperature range. If the temperature is lower or higher than this range, the galvanizability of the substrate surface will deteriorate, which may lead to poor surface quality after plating.

[0056] Furthermore, in the manufacturing method of the present invention, after the hot-dip galvanizing step, it also includes flattening, and the flattening rate is controlled to be 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 after hot-dip galvanizing is controlled to be 0.05-1.3%. When the flatness is less than 0.05%, the surface quality of the zinc layer is poor, and when the flatness is greater than 1.3%, greater work hardening will occur, which is easy to reduce the elongation of the steel plate.

[0058] The economical 800MPa grade hot-dip galvanized steel sheet and the manufacturing method thereof of the present 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, and thus has a 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 of ≥680MPa, a tensile strength of ≥800MPa, an elongation A80 of ≥15%, and a hole expansion rate of ≥50%. Therefore, it can be used as an automobile chassis and structural parts to meet the technical requirements of flanging, stamping and lightweighting of complex automobile parts.

[0061] The economical 800MPa grade hot-dip galvanized steel sheet of the present invention does not contain Si and Cr elements, and has better surface quality and corrosion resistance than the same type of hot-dip galvanized steel sheet. It should be noted that the present invention does not contain Si and Cr means that no special addition is required during the smelting process. Since iron ore and scrap steel contain a small amount of Si and Cr, even if there is a trace amount of Si and Cr in the finished steel sheet, for example, less than 0.1%, they are residual elements, not specially added by the present invention.

[0062] The economical 800MPa grade hot-dip galvanized steel sheet of the present invention is suitable for conventional hot rolling production lines and hot-dip galvanizing production lines. Compared with the traditional hot-dip galvanized sheet production method, the present invention omits the cold rolling process, shortens the process flow, improves production efficiency, and saves energy. DETAILED DESCRIPTION

[0063] The economical 800 MPa grade hot-dip galvanized steel sheet and its manufacturing method described in the present invention will be further explained and illustrated in conjunction with specific embodiments below. However, such 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 sheets in each embodiment of the present invention are prepared by the following steps:

[0065] (1) Smelting and casting to obtain slabs, the thickness of which is 220 mm to 250 mm. Table 1 lists the mass percentage of each chemical element in the slabs of various embodiments of the present invention and comparative examples.

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

[0067] (3) Hot rolling: the starting temperature of rough rolling is controlled at 1130-1190°C, the starting temperature of finishing rolling is controlled at 980-1080°C, and the outlet temperature of finishing rolling is controlled at 840-940°C. During the hot rolling process, the cumulative deformation of rough rolling and finishing rolling can be ≥90%.

[0068] (4) After hot rolling, the steel is cooled at a cooling rate of ≥50°C / s to a coiling temperature of 450-500°C.

[0069] In some embodiments, optionally, after the coiling process is completed, the hot coil can be placed in a heat preservation pit for slow cooling, with an average cooling rate of ≤15°C / h.

[0070] (5) Pickling to obtain a pickled coil.

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

[0072] (6) Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion non-oxidizing continuous annealing furnace, wherein the direct annealing section temperature is controlled at 550-630°C, the heating section temperature is 630-720°C, the soaking section temperature is 630-720°C, the soaking section holding time is 50-100s, the slow cooling section temperature is 550-670°C, and after exiting the slow cooling section, it can be cooled to the hot-dip galvanizing temperature of 440-480°C at a rate of ≥5°C / s for hot-dip galvanizing. The average single-side weight of the hot-dip galvanized layer can be 20-380g / m 2 .

[0073] (7) Flattening: The flattening rate is controlled to be 0.05-1.3% to obtain a hot-dip galvanized sheet with a thickness of 1.8-3.5 mm.

[0074] Table 1 lists the mass percentages of the chemical elements of the substrates of the embodiments and comparative examples.

[0075] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, N)

[0076]

[0077]

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

[0079] Table 2 lists the specific process parameters of the embodiments and comparative examples of the present invention in the above process steps.

[0080] Table 2.

[0081]

[0082]

[0083] The hot-dip galvanized steel sheets of Examples 1-16 and Comparative Examples 1-18 were sampled, and the microstructures were observed by optical microscopy and scanning electron microscopy, and the grain size was measured by the equivalent area method. The size distribution and volume fraction of the second phase precipitation were measured by the extraction carbon replica method, and the test results are listed in Table 3.

[0084] Table 3.

[0085]

[0086]

[0087] In addition, JIS13B tensile specimens were taken longitudinally for each embodiment and comparative example to measure yield and tensile strength, with a gauge length of 80 mm, and the mechanical property test was performed in accordance with GB / T228.1-2010. The hole expansion rate test was performed in accordance with GB / T24524-2021, 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 the present invention have obtained ideal organizational characteristics and refined grains through reasonable chemical element composition design, combined with optimized hot rolling process and annealing hot-dip galvanizing process. The second phase particles are precipitated in large quantities through the annealing hot-dip galvanizing heating process to improve strength without reducing plasticity. Strictly controlling the temperature of each section during the annealing process to avoid coarsening of the second phase particles is the key to obtaining good performance. The longitudinal yield strength finally obtained in each embodiment of the present invention is above 680MPa, the tensile strength is above 800MPa, the elongation A80 is above 15%, and the hole expansion rate is ≥50%.

[0092] The difference with the present invention is:

[0093] In Comparative Example 1, since the carbon content and the manganese content are higher than the upper limits designed by the present invention, more martensite is formed during the hot rolling process, resulting in less bainite content and reduced hole expansion rate.

[0094] In Comparative Example 2, since the carbon content and the manganese content are lower than the lower limits designed by the present invention, the strength is insufficient.

[0095] In Comparative Example 3, when the B element is added, the B content is higher than the upper limit designed by the present invention, so martensite is generated during the hot rolling process, resulting in a decrease in the hole expansion rate.

[0096] In Comparative Example 4, since the Ti content is lower than the lower limit designed by the present invention, the amount of second phase particles precipitated is small and the strength is low.

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

[0098] In Comparative Example 6, since the temperature of the soaking section is higher than the upper limit of the present invention, austenite is formed during the heating process, and phase transformation to martensite occurs during the cooling process. The proportion of bainite decreases, and the yield strength and hole expansion rate of the steel plate decrease.

[0099] In Comparative Example 7, since the temperature of the heating section is lower than the lower limit of the present invention, less second phase is precipitated during the hot-dip galvanizing annealing process, and the yield strength is reduced.

[0100] In Comparative Example 8, since the temperature of the direct firing section and the temperature of the heating section are higher than the upper limit of the present invention, austenite is formed during the heating process, and phase transformation to martensite occurs during the cooling process. The proportion of bainite decreases, and the yield strength and hole expansion rate of the steel plate are reduced.

[0101] In Comparative Example 9, since the soaking time is higher than the upper limit of the present invention, the second phase is coarsened and the yield strength is reduced.

[0102] In Comparative Example 10, since the temperature of the slow cooling stage is higher than the upper limit of the present invention, the second phase is coarsened and the yield strength is reduced.

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

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

[0105] In Comparative Example 13, since the cooling rate after hot rolling is lower than the lower limit of the present invention, more ferrite is 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 various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0107] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and all should belong to 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, wherein the substrate contains Fe and unavoidable impurities, characterized in that: The substrate also 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 and Ni elements; The microstructure of the substrate includes a composite structure of granular bainite + 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 the second phase particle precipitates with a size of no more than 10 nm accounts for more than 80% of the total second phase particle precipitates.

2. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1, characterized in that: The mass percentage of each chemical element of the substrate is: C: 0.05-0.10%, Mn: 1.4-2.0%, Ti: 0.10-0.15%, Al: 0.02-0.08%; The balance is Fe and other inevitable impurities.

3. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The substrate further contains at least one of the following chemical elements: 0<Nb≤0.03%; 0＜B≤0.003％。 4. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Among the inevitable impurities of the substrate, P≤0.02%, N≤0.005%, and S≤0.005%.

5. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The volume phase ratio of the granular bainite is not less than 95%.

6. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The grain size of the granular bainite is less than 5 μm.

7. The economical 800MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and hole expansion rate is ≥50%.

8. The method for manufacturing an economical 800MPa grade hot-dip galvanized steel sheet according to any one of claims 1 to 7, characterized in that: It includes the steps of: obtaining a slab; heating and keeping the slab warm; Hot rolling, followed by cooling at a cooling rate of ≥50°C / s to a coiling temperature of 450-500°C; Pickling to obtain pickled coils; Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion non-oxidizing continuous annealing furnace, where the direct annealing section temperature is controlled at 550-630℃, the heating section temperature is 630-720℃, the soaking section temperature is 630-720℃, the soaking section holding time is 50-100s, the slow cooling section temperature is 550-670℃, and after leaving the slow cooling section, it is cooled to the hot-dip galvanizing temperature.

9. The manufacturing method according to claim 8, characterized in that: Heat the slab to 1230-1280°C and keep it at that temperature for 1-3h.

10. The manufacturing method according to claim 8, characterized in that: In the hot rolling step, the starting temperature of the rough rolling is controlled to be 1130-1190°C, the starting temperature of the finishing rolling is controlled to be 980-1080°C, and the finishing rolling outlet temperature is controlled to be 840-940°C.

11. The manufacturing method according to claim 8, characterized in that: In the hot-dip galvanizing step, the hot-dip galvanizing temperature is 440-480°C.

12. The manufacturing method according to claim 8, characterized in that: After the hot-dip galvanizing step, flattening is also included, and the flattening rate is controlled to be 0.05-1.3%.

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