900mpa grade hot-dip galvannealed steel sheet and method for manufacturing the same

Abstract

The application discloses a 900MPa-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: 0.06%-0.10%, Mn: 1.5-2.15%, Al: 0.02-0.08%, 0 The microstructure of the base plate comprises granular bainite 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] The present application relates to a steel sheet and a manufacturing method thereof, and in particular to a hot-dip galvanized steel sheet and a manufacturing method thereof. BACKGROUND

[0002] With the development of automobile lightweight technology, the proportion of advanced high-strength steel in automobile structural parts is also increasing. Duplex steel has become the first choice for control arms, torsion beams and other chassis parts due to its ultra-high strength and good forming performance.

[0003] For chassis parts with complex shapes, the steel sheet is required to have good flanging and hole expanding ability and corrosion resistance.

[0004] The Chinese patent document with publication number CN104513930A, publication date April 15, 2015, and title "Ultra-high strength hot-rolled duplex steel sheet and strip with good bending and hole expanding performance and manufacturing method thereof" discloses that the composition of the ultra-high strength hot-rolled duplex steel sheet and strip with good bending performance is as follows: 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 at least one of Nb≤0.05%, Ti: 0.06-0.15% alloying elements, and the rest is Fe and unavoidable impurities. The patent document discloses the performance design and manufacturing method of hot-rolled pickled sheet, and does not involve the influence of hot-dip galvanizing process on the performance of hot-dip galvanized steel sheet.

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

[0006] A Chinese patent document with publication number CN113215485A, publication date August 6, 2021, and title "780MPa grade hot base plated dual-phase steel and its preparation method" discloses a 780MPa hot base plated dual-phase steel and its manufacturing method. The chemical composition of the steel is: 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%, and the rest is Fe and inevitable impurities. The Si content of the steel is relatively high, and red scale is easily formed on the surface during hot rolling. SUMMARY

[0007] One of the purposes of the present application is to provide a 900MPa grade hot base hot galvanized steel sheet having high strength, high formability and high corrosion resistance, which can be used for automobile structural parts and chassis parts, and can also be used in other application fields requiring high strength, weight reduction and corrosion resistance.

[0008] To achieve the above purpose, the present application provides a 900MPa grade hot base hot galvanized steel sheet, which comprises a substrate and a hot galvanizing layer, the substrate contains Fe and inevitable impurities, and further contains the following chemical elements in mass percentage:

[0009] C: 0.06%-0.10%, Mn: 1.5-2.15%, Al: 0.02-0.08%, 0

[0010] The substrate does not contain Si element;

[0011] The microstructure of the substrate includes granular bainite and second phase particle precipitates, the volume fraction of the second phase particle precipitates is not less than 0.2%, and the proportion of second phase particle precipitates with a size not greater than 10nm in the volume fraction of all second phase particle precipitates is greater than 80%.

[0012] Generally, in order to achieve ultra-high strength, the steel plate needs to be heated to a high temperature in the hot galvanizing process, partially or completely austenitized, and then martensite phase is formed in the cooling process, so as to achieve the purpose of improving the strength. However, the structure containing part of the martensite has poor hole expansion performance and cannot meet the forming requirements of the present application. Therefore, the present application optimizes and controls the chemical composition and microstructure, adopts Nb, Ti micro-alloying, forms granular bainite structure in the hot rolling process, and then forms a sufficient volume fraction of the second phase precipitation in the annealing and hot galvanizing process, further strengthens the bainite structure, so as to obtain ultra-high strength. In this process, the size of the second phase particle precipitate needs to be strictly controlled. When the steel strip is heated to a high temperature above 600℃ in the hot galvanizing process, the second phase precipitates are easy to coarsen, which reduces the precipitation strengthening effect and reduces the strength of the steel plate. Therefore, the present application adds Mo element to form alloy carbide with Ti and Nb micro-alloying elements, reduces the interface energy with the matrix, so that the second phase precipitates are not easy to grow, and the size is kept small, so as to achieve ultra-high strength.

[0013] In addition, the present application adopts a silicon-free design, so it has good surface quality and corrosion resistance. In the present application, when the silicon content in the steel is high, the slab is easy to form fayalite (2FeO-SiO2) iron oxide scale on the surface during the heating process, which has an adverse effect on the surface quality of the zinc layer, thereby affecting the corrosion resistance. Therefore, in order to ensure that the steel plate has ultra-high strength while having good corrosion resistance, the present application adopts a silicon-free design. It should be noted that the silicon-free design means that Si element does not need to be intentionally added during smelting, and since the iron ore contains a small amount of Si element, even if the finished steel plate contains a small amount of Si element, it is an unavoidable impurity of the present application.

[0014] In addition, the second phase particle precipitation is the key strengthening mechanism in the present application. Only when the precipitated second phase particles have sufficient volume fraction and small size, can they have strong strengthening effect. When the volume fraction of the second phase particles is less than 0.2%, the tensile strength of the steel plate is less than 900MPa. The second phase particles are precipitated at each stage of cooling, and the particle size of the particles precipitated in the high temperature rolling stage in the hot rolling process is large, generally more than 20nm, and the strengthening effect is weak. The second phase particles precipitated in the cooling after coiling and the heating process of the hot galvanizing process have small size (less than 10nm) and have significant strengthening effect. Therefore, the present application controls the proportion of the volume fraction of the second phase particles with a size not greater than 10nm in the total volume fraction of the precipitates to be greater than 80%, so that the tensile strength of the steel plate is greater than 900MPa.

[0015] Further, in the 900MPa grade hot base hot galvanizing steel plate described in the present application, the mass percentage of each chemical element of the base plate is:

[0016] C: 0.06-0.10%, Mn: 1.5-2.15%, Al: 0.02-0.08%, 0

[0017] The design principles of each chemical element in the hot-based hot-dip galvanized steel sheet described in the present application are specifically described as follows:

[0018] C: In the hot-based hot-dip galvanized steel sheet described in the present application, the C element is the most basic element of the steel grade and the most commonly used strengthening element. The carbon content can determine the tensile strength level of the steel sheet. Carbon element can also stabilize austenite and promote the formation of bainite and other organizations; carbon and micro-alloying elements form sufficient precipitated strengthening phases to ensure the strength of the steel. However, too high a percentage of carbon will lead to deterioration of the forming and welding performance. Therefore, in the hot-based hot-dip galvanized steel sheet described in the present application, the mass percentage of C element is controlled between 0.06% and 0.10%.

[0019] Mn: In the hot-based hot-dip galvanized steel sheet described in the present application, manganese is a solid solution strengthening element that can improve the hardenability, delay pearlite transformation and reduce bainite transformation temperature, and refine bainite structure. When the mass percentage of manganese is too low, it will lead to insufficient strength. And when the mass percentage of manganese is too high, it will lead to a decrease in the plasticity of the steel sheet, and also easily lead to segregation. Therefore, in the hot-based hot-dip galvanized steel sheet described in the present application, the mass percentage of Mn is controlled between 1.5% and 2.15%.

[0020] Al: In the hot-based hot-dip galvanized steel sheet described in the present application, aluminum is a deoxidizing element of steel, which can reduce oxide inclusions in steel, purify steel, and help improve the forming performance of the steel sheet. However, when the mass percentage of aluminum is too high, oxidation will occur, further affecting continuous casting production. Therefore, in the hot-based hot-dip galvanized steel sheet described in the present application, the mass percentage of Al is controlled between 0.02% and 0.08%.

[0021] Cr: In the hot-based hot-dip galvanized steel sheet described in the present application, the chromium element has the effect of improving the hardenability and solid solution strengthening, can inhibit the generation of pearlite, is beneficial to the formation of bainite structure element, and chromium element is not easy to segregate. When the mass percentage of Cr is too high, it will lead to an increase in cost, and also easily produce a lot of martensite structure. Therefore, in the hot-based hot-dip galvanized steel sheet described in the present application, the mass percentage of Cr is controlled between 0

[0022] Mo: In the hot-dip galvanizing steel plate described in the present application, Mo element is a key additive element. Molybdenum has the effect of improving hardenability and solid solution strengthening, and can inhibit the generation of pearlite and is beneficial to the formation of bainite. In addition, when Nb, V and Ti are used for micro-alloying, fine carbides or carbonitrides can be formed during cooling, which plays a role in precipitation strengthening. However, the tendency of these carbides or carbonitrides to coarsen is very strong, and after the size of the second phase particles increases, the strengthening effect decreases sharply. When Mo element is added to the steel, Mo will partially replace the micro-alloying elements in the second phase, thereby reducing the interface energy of the second phase, making the second phase less likely to grow. When the Mo content is less than 0.05%, not enough bainite can be formed, and the effect of preventing the growth of the second phase is not significant. When the Mo content is greater than 0.30%, its effect tends to be saturated. Therefore, in the hot-dip galvanizing steel plate described in the present application, the mass percentage of Mo is controlled to be 0.05-0.30%.

[0023] Ti: In the hot-dip galvanizing steel plate described in the present application, titanium is one of the important precipitation strengthening and fine-grain strengthening elements, especially during the hot-dip galvanizing annealing process, Ti will precipitate in the form of TiC, which can significantly improve the strength of the ferrite matrix, and is beneficial to the improvement of the yield ratio and the elongation. When the Ti content is less than 0.11%, the tensile strength cannot reach 900 MPa, and when the Ti content is greater than 0.16%, the strengthening effect tends to be saturated, and from the economic point of view, the upper limit of Ti content in the present application is 0.16%. Therefore, in the technical solution described in the present application, the mass percentage of Ti is controlled to be Ti: 0.11-0.16%.

[0024] Nb: In the hot-dip galvanizing steel plate described in the present application, niobium is one of the important precipitation strengthening and fine-grain strengthening elements, but when the mass percentage of Nb is higher than 0.05%, the strengthening effect of Nb approaches saturation, and the cost is relatively high. Therefore, in the high-hole-expanding hot-dip galvanizing steel plate described in the present application, the mass percentage of Nb is controlled to be 0

[0025] Further, in the 900 MPa grade hot-dip galvanizing steel plate described in the present application, the substrate further contains 0

[0026] The addition of V element can further optimize the performance of the hot-dip galvanizing steel plate described in the present application. Among them:

[0027] V: In the hot-dip galvanizing steel plate described in the present application, vanadium plays a role in precipitation strengthening and fine-grain strengthening, and the precipitation of vanadium carbide is beneficial to further improving the strength. However, when the mass percentage of V is higher than 0.2%, the strengthening effect approaches saturation, and the cost is relatively high. Therefore, in the high-hole-expanding hot-dip galvanizing steel plate described in the present application, the mass percentage of V is controlled to be 0

[0028] Further, in the base plate of the 900MPa grade hot-dip galvannealed steel sheet according to the present application, P is ≤0.02%, N is ≤0.005%, and S is ≤0.005%.

[0029] In the hot-dip galvannealed steel sheet according to the present application, P, N, and S are impurity elements in the steel, and thus, the contents of the impurity elements in the steel are desirably reduced as much as possible, if the technical conditions permit, in order to obtain a steel material having better performance and quality.

[0030] Further, in the 900MPa grade hot-dip galvannealed steel sheet according to the present application, the volume fraction of the granular bainite is not less than 90%.

[0031] In the hot-dip galvannealed steel sheet according to the present application, the granular bainite has uniform performance and excellent hole expansion performance, and contains a large amount of supersaturated carbon atoms. The carbon atoms are precipitated in a large amount during the heating process of the annealing and hot-dip galvanizing process according to the present application, thereby improving the strength and formability of the steel sheet. Thus, the microstructure in the present application is mainly the granular bainite, and when the content of the granular bainite is less than 90%, the precipitation strengthening effect is weakened, and the tensile strength does not reach 900MPa.

[0032] In addition to the granular bainite, the 900MPa grade hot-dip galvannealed steel sheet according to the present application can contain a small amount of ferrite and / or martensite.

[0033] The ferrite is relatively soft, and has the characteristics of low strength and good plasticity. However, when the content of the ferrite is high, the tensile strength of the steel sheet does not reach 900MPa. The martensite has high strength, but low plasticity, and when the content of the martensite is high, cracks are easily formed at the interface between the martensite and the bainite having low strength, thereby reducing the hole expansion performance of the steel sheet. Thus, the contents of the martensite and the ferrite need to be strictly controlled, so that the content of the granular bainite is higher than 90%.

[0034] Further, in the 900MPa grade hot-dip galvannealed steel sheet according to the present application, the grain size of the granular bainite is less than 5μm.

[0035] In the present application, the grain size of the granular bainite has a large influence on the yield strength. When the grain size of the granular bainite is large, the yield strength is low, and when the grain size of the bainite is greater than 5μm, the yield strength of the steel sheet is reduced. In order to achieve the technical effects of the present application, the grain size of the granular bainite is desirably controlled to be less than 5μm.

[0036] Further, in the 900MPa grade hot-dip galvannealed steel sheet according to the present application, the longitudinal yield strength is ≥780MPa, the tensile strength is ≥900MPa, the elongation A80 is ≥16%, and the hole expansion ratio is ≥50%.

[0037] Furthermore, another object of the present application is to provide a manufacturing method of a 900MPa grade hot-dip galvannealed steel sheet, which is matched with the composition design of the present application, can obtain the microstructure characteristics desired by the present application, and thus obtain a hot-dip galvannealed steel sheet having high strength, high formability, and high corrosion resistance.

[0038] To achieve the above object, the present application proposes a manufacturing method of a 900MPa grade hot-dip galvannealed steel sheet, which comprises the steps of:

[0039] manufacturing a slab;

[0040] heating and holding the slab;

[0041] hot-rolling, and then cooling at a cooling rate of ≥40℃ / s to a coiling temperature of 440-520℃;

[0042] pickling to obtain a pickled coil;

[0043] annealing and hot-dip galvanizing: directly annealing the pickled coil in a combustion non-oxidizing continuous annealing furnace, wherein the soaking section temperature is controlled to be 630-720℃, the soaking section holding time is 30-120s, the slow cooling section temperature is 550-670℃, and the temperature is cooled to the hot-dip galvanizing temperature after leaving the slow cooling section.

[0044] In the present application, the hot-rolled coil is water-cooled at a cooling rate of ≥40℃ / s to a coiling temperature of 440-520℃. The cooling rate after rolling is ≥40℃ / s, in order to make the steel sheet after hot-rolling quickly enter the bainite transformation zone. When the cooling rate is less than 40℃ / s, more ferrite or pearlite will be generated during the cooling process. The coiling temperature after cooling is controlled to be 440-520℃, in order to obtain bainite structure. When the coiling temperature is higher, pearlite is easily formed, and the grain size is also larger, resulting in a strength of less than 900MPa after hot-dip galvanizing annealing. When the coiling temperature is too low, martensite is formed, resulting in an expansion rate of less than 50%.

[0045] In the present application, the annealing soaking section temperature is controlled to be 630-720℃, because: when the soaking section temperature is less than 630℃, the volume fraction of the second phase precipitated during the soaking process is low, which will result in low strength. When the soaking temperature exceeds 720℃, martensite will be generated during the cooling process, resulting in an expansion rate of less than 50%.

[0046] In addition, the present application controls the annealing soaking section holding time to be 30-120s, because: when the annealing soaking section holding time is less than 30s, the internal stress of the steel sheet during hot-rolling cannot be completely eliminated, and the surface state of the steel sheet is poor, which makes it difficult to obtain a good hot-dip galvanizing surface. However, when the soaking time exceeds 120s, the second phase particles are coarsened and dissolved in large quantities, resulting in a decrease in strength, and the yield strength of the steel sheet cannot be guaranteed.

[0047] In the present application, the temperature of the slow cooling section is controlled to be 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, resulting in a decrease in the yield strength. Since the cooling capacity of the unit in the slow cooling section is limited, when the soaking section is 630-720℃, it is usually difficult to cool the slow cooling section to below 550℃.

[0048] In addition, the present application can be cooled to the hot galvanizing temperature at a cooling rate of ≥5℃ / s after leaving the slow cooling section, and the present application does not particularly limit the cooling speed, because the present application does not form austenite during the soaking process, and no phase change occurs during the cooling process, so the cooling rate after leaving the slow cooling section has no effect on the performance. The rate of ≥5℃ / s is to ensure the cooling speed when the conventional unit is normally operated.

[0049] Further, in the manufacturing method described in the present application, the slab is heated to 1230-1280℃ and held for 1-3h.

[0050] In the present application, the heating temperature of the slab can further improve the performance and surface quality. The heating temperature is set to be ≥1230℃, which aims to dissolve Ti(C, N) generated during the continuous casting process as much as possible, and the microalloying elements such as Ti in solid solution in austenite are precipitated as nanoscale second phase particles during hot rolling and coiling, especially during annealing and hot galvanizing, which can effectively improve the strength of the steel plate. However, when the temperature exceeds 1280℃, the austenite grains are coarsened, which is not conducive to the toughness of the steel plate; at the same time, the iron oxide scale is thick, which is not conducive to the dephosphorization of the iron oxide scale, and finally affects the surface quality of the hot galvanizing, therefore, the heating temperature is preferably set to 1230-1280℃.

[0051] Further, in the manufacturing method described in the present application, in the hot rolling step, the rough rolling opening temperature is controlled to be 1130-1190℃, the finish rolling opening temperature is 980-1080℃, and the finish rolling outlet temperature is 840-940℃.

[0052] In the hot rolling process described in the present application, the control of the rough rolling temperature has a greater effect on the microalloying elements such as Ti. During the rough rolling and finish rolling processes, carbides and carbonitrides will be precipitated, and the size of the precipitates during this process is relatively large. When the rough rolling and finish rolling temperatures are high, a large amount of precipitation will occur, which is not conducive to the improvement of the final strength. When the rough rolling and finish rolling temperatures are low, the microalloying elements in solid solution in austenite decrease, which is not conducive to the precipitation during the cooling process. Therefore, the rough rolling opening temperature is controlled to be 1130-1190℃, and the finish rolling opening temperature is 980-1080℃.

[0053] In the hot rolling process described in the present application, the exit temperature of the finish rolling has a great influence on the microstructure. Lower finish rolling exit temperature can obtain cake-shaped austenite grains, accumulate a large amount of deformation energy, and is beneficial to obtaining fine grain size in the subsequent cooling process. However, when the finish rolling exit temperature is too low, ferrite phase transformation will occur, which is not conducive to the forming performance. Therefore, the hot rolling finish rolling exit temperature is controlled to be 840-940℃

[0054] Further, in the manufacturing method described in the present application, in the hot galvanizing step, the hot galvanizing temperature is 440-480℃.

[0055] In the hot galvanizing step described in the present application, the hot galvanizing zinc pot temperature can be cooled to 440-480℃, and the surface of the steel plate has good combination with the zinc liquid in this temperature range. Below or above this temperature range will cause the surface of the substrate to be poor in plating, and the surface after plating is poor.

[0056] Further, in the manufacturing method described in the present application, after the hot galvanizing step, it further includes flattening, and the flattening rate is controlled to be 0.05-1.3%.

[0057] In some embodiments of the present application, when the flattening rate is less than 0.05%, the surface quality of the zinc layer is poor. When the flattening rate is greater than 1.3%, a large work hardening will be produced, reducing the elongation of the steel plate. Therefore, in the present application, the flattening rate is controlled to be 0.05-1.3%.

[0058] Compared with the prior art, the 900MPa grade hot base hot galvanizing steel plate and the manufacturing method thereof described in the present application have the following advantages and beneficial effects:

[0059] The hot base hot galvanizing multiphase steel described in the present application has better corrosion resistance compared with conventional hot rolled and pickled plates, and is suitable for conventional hot continuous rolling production lines and hot galvanizing production lines.

[0060] Compared with the traditional hot galvanizing plate production method, the present application eliminates the cold rolling process, shortens the process flow, improves the production efficiency, and saves energy.

[0061] The hot galvanizing steel plate described in the present application has the characteristics of high strength, high forming performance, and high corrosion resistance. The high forming performance includes high elongation and high hole expansion rate. As described above, the microstructure of the present application is mainly granular bainite, the dislocation density is reduced during the heating process in the hot galvanizing process, and the second phase particle size is small, which is beneficial to the improvement of the hole expansion rate and the elongation. Therefore, excellent hole expansion performance and elongation can be obtained. In some embodiments, the longitudinal yield strength is ≥780MPa, the tensile strength is ≥900MPa, the elongation A80 is ≥16%, and the hole expansion rate is ≥50%.

[0062] Therefore, the application can be used as a structural part of a car body and a chassis part of a car, and can also be used in other application fields requiring high strength and weight reduction, and is particularly suitable for complex shape forming design. DETAILED DESCRIPTION

[0063] The 900MPa grade hot-dip galvanizing steel plate and the manufacturing method thereof will be further explained and described below in combination with specific examples, but the explanation and description does not constitute undue limitation on the technical scheme of the application.

[0064] Examples 1-13 and Comparative Examples 1-9

[0065] The 900MPa grade hot-dip galvanizing steel plate of each embodiment of the application is prepared by the following steps:

[0066] (1) Smelting and casting to prepare a slab, and the thickness of the slab is 220mm-250mm. Table 1 lists the mass percentage of each chemical element of the slab of each embodiment of the application and the comparative examples.

[0067] (2) Heating the slab to 1230-1280℃ and maintaining for 1-3h.

[0068] (3) Hot rolling: the rough rolling opening temperature is controlled to be 1130-1190℃, the finish rolling opening temperature is controlled to be 980-1080℃, the finish rolling outlet temperature is controlled to be 840-940℃, and the cumulative deformation of rough rolling and finish rolling in the hot rolling process can be controlled to be ≥90%.

[0069] (4) After hot rolling, cooling to the coiling temperature of 440-520℃ at a cooling rate of ≥40℃ / s.

[0070] In some embodiments, after the coiling process, the hot coil can be placed in a holding pit for slow cooling, and the average cooling rate can be ≤15℃ / h.

[0071] (5) Pickling to obtain a pickled coil.

[0072] In some embodiments, the pickling straightening elongation can be 0.2-2%, the pickling speed can be 60-150m / min, the temperature of the last pickling acid tank in the pickling process can be 80-90℃, and the iron ion concentration can be 30-40g / L.

[0073] Controlling the pickling straightening elongation to be 0.2-2% can further improve the surface quality, and when the straightening elongation is lower than 0.2% or higher than 2%, it will have an adverse effect on the plate shape. In addition, lower pickling speed and higher acid tank temperature will cause over-pickling, and higher pickling speed and lower acid tank temperature will cause under-pickling, which can affect the surface of the pickled plate and further affect the surface quality after hot-dip galvanizing.

[0074] (6) Annealing and hot-dip galvanizing: the pickling coil is directly annealed in a combustion non-oxidizing continuous annealing furnace, wherein the direct fire section and the heating section are both adopted with conventional process, the soaking section temperature is controlled to be 630-720℃, the soaking section holding time is 30-120s; the slow cooling section temperature is 550-670℃, after the slow cooling section, it can be cooled to the hot-dip galvanizing temperature 440-480℃ at a rate of ≥5℃ / s for hot-dip galvanizing, and the average single-side weight of the hot-dip galvanizing layer can be 20-380g / m 2 .

[0075] (7) Skin pass: the skin pass rate is controlled to be 0.05-1.3%, and the hot-dip galvanizing plate with a thickness of 1.8-3.5mm is obtained.

[0076] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S and N)

[0077]

[0078]

[0079] Note: the steel grades A-G in Table 1 are the steel grades used in the embodiments of the present application, and H-K are the steel grades used in the comparative examples.

[0080] Table 2 lists the specific process parameters of the embodiments and the comparative examples in the above process steps.

[0081] Table 2.

[0082]

[0083]

[0084] The hot-dip galvanizing steel plates obtained in Examples 1-13 and Comparative Examples 1-9 are sampled respectively, the microstructure is observed by combining optical microscope and scanning electron microscope, and the grain size is measured by equivalent area method. The size distribution and volume fraction of the second phase precipitation are measured by extraction carbon replica method, and the test results are listed in Table 3.

[0085] Table 3.

[0086]

[0087]

[0088] In addition, the JIS13B tensile samples along the longitudinal direction are taken from each of the example and comparative example samples to measure the yield and tensile strength, the gauge length is 80mm, and the mechanical property test is performed according to the GB / T228.1-2010 standard. The hole expansion rate test is tested according to the GB / T24524-2021 standard, and the test results of the above tests are listed in Table 4.

[0089]

[0090]

[0091] As shown in Table 3 and Table 4, the embodiments 1-17 of the present application obtain ideal microstructure characteristics by reasonable chemical element composition design, combined with optimized hot rolling process and annealing and hot galvanizing process, refine the grain, and promote the second phase particles to precipitate a large amount to improve the strength through annealing and hot galvanizing heating process, while not reducing the plasticity. The temperature of each section is strictly controlled in the annealing process to avoid the coarsening of the second phase particles, thereby obtaining good performance. The yield strength of each embodiment of the present application is ≥793 MPa, the tensile strength is ≥902 MPa, the elongation A80 is ≥16.5%, and the expansion ratio is ≥51.2%.

[0092] Different from the present application, each comparative example cannot achieve the technical effects that the present application can achieve due to the composition or process parameters not meeting the design requirements of the present application, wherein:

[0093] Comparative example 1 has a lower annealing soaking temperature than the lower limit of the present application, resulting in less second phase precipitation during hot galvanizing annealing process, and reduced strength.

[0094] Comparative example 2 has an annealing soaking temperature higher than the upper limit of the present application, resulting in the formation of austenite during heating process, the transformation into martensite during cooling process, the reduction of bainite proportion, and the reduction of yield strength of the steel plate.

[0095] Comparative example 3 has a coiling temperature higher than the upper limit of the present application, resulting in higher ferrite content and lower bainite content. In addition, a higher coiling temperature can result in more second phase precipitation, and the second phase particles are prone to coarsening during hot galvanizing heating process, resulting in reduced strength.

[0096] Comparative example 4 has a coiling temperature lower than the lower limit of the present application, resulting in higher martensite content and lower bainite content in hot coil, and reduced expansion ratio of the steel plate.

[0097] Comparative example 5 has a lower cooling speed after rolling than the lower limit of the present application, resulting in reduced bainite content, reduced strength, and insufficient expansion ratio.

[0098] Comparative example 6 has carbon content and manganese content higher than the upper limit of the present application, resulting in more martensite formation during hot rolling process, less bainite content, and reduced expansion ratio.

[0099] Comparative example 7 has C content lower than the lower limit of the present application, resulting in insufficient strength.

[0100] Comparative Example 8 has a Mo content lower than the lower limit of the present application, which results in the second phase particles coarsening during the hot-dip galvanizing heating process, the precipitation strengthening effect is weakened, the strength of the steel sheet is reduced, and the hole expansion rate is lower than 50%.

[0101] Comparative Example 9 has a Ti content lower than the lower limit of the present application, which results in a small amount of second phase particles precipitating, and thus a low strength.

[0102] It should be noted that the combination of the technical features in the present case is not limited to the combination manner described in the claims of the present case or the combination manner described in the specific embodiments. All the technical features described in the present case can be freely combined or integrated in any manner, unless contradictory to each other.

[0103] It should also be noted that the above-mentioned examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above examples, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily conceived by those skilled in the art, and should all fall within the scope of protection of the present application.

Claims

1. A 900 MPa grade hot-dip galvannealed steel sheet comprising a base sheet and a hot-dip galvannealed layer, the base sheet containing Fe and unavoidable impurities, characterized in that, The mass percentage of each chemical element of the substrate is: C: 0.06%-0.10%, Mn: 1.5-2.15%, Al: 0.02-0.08%, 0 The microstructure of the substrate includes granular bainite and second phase particle precipitates, the volume fraction of the second phase particle precipitates is not less than 0.2%, and the proportion of second phase particle precipitates with a size not greater than 10 nm in the volume fraction of all second phase particle precipitates is greater than 80%; the volume fraction of the granular bainite is not less than 90%, and the grain size of the granular bainite is less than 5 μm; The longitudinal yield strength of the 900 MPa grade hot-dip galvanized steel plate is ≥780 MPa, the tensile strength is ≥900 MPa, the elongation A80 is ≥16%, and the hole expansion ratio is ≥50%.

2. The 900 MPa grade hot dip galvannealed steel sheet according to claim 1, characterized by, The substrate also contains 0 3. The 900 MPa grade hot-dipped galvanized steel sheet according to claim 1, characterized by, In the inevitable impurities of the substrate, P≤0.02%, N≤0.005%, and S≤0.005%.

4. The method of producing a 900 MPa grade hot-dipped galvanized steel sheet according to any one of claims 1 to 3, characterized in that, It includes the steps of: A slab is prepared; The slab is heated and held; Hot rolling, followed by cooling at a cooling rate of ≥40 ℃ / s to a coiling temperature of 440-520 ℃; Pickling to obtain a pickled coil; Annealing and hot-dip galvanizing: the pickled coil is directly annealed in a combustion non-oxidizing continuous annealing furnace, wherein the soaking section temperature is controlled to be 630-720 ℃, the soaking section holding time is 30-120 s, the slow cooling section temperature is 550-670 ℃, and the temperature is cooled to the hot-dip galvanizing temperature after leaving the slow cooling section.

5. The production method according to claim 4, wherein The slab is heated to 1230-1280 ℃ and held for 1-3 h.

6. The production method according to claim 4, wherein In the hot rolling step, the rough rolling opening temperature is controlled to be 1130-1190 ℃, the finish rolling opening temperature is 980-1080 ℃, and the finish rolling exit temperature is 840-940 ℃.

7. The production method according to claim 4, wherein In the hot-dip galvanizing step, the hot-dip galvanizing temperature is 440-480 ℃.

8. The production method according to claim 4, wherein After the hot-dip galvanizing step, it also includes skin passing, and the skin passing rate is controlled to be 0.05-1.3%.

Images