High-yield-ratio 800MPa-grade hot-based hot-dip galvanized steel plate and manufacturing method thereof

Abstract

The invention discloses a high-yield-ratio 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: 0.04-0.069% of C, 1.3-1.8% of Mn, 0 < Cr < = 0.5%, 0.05-0.12% of Ti, 0 < Nb < = 0.05%, 0.1-0.25% of Mo and 0.02-0.08% of Al; the substrate does not contain an Si element; 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%.

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 ultra-high strength and good forming performance, and has become the first choice for chassis parts such as control arms and torsion beams.

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

[0004] The Chinese patent document with publication number CN104513930A and publication date April 15, 2015, entitled "Ultra-high strength hot-rolled complex phase steel plate and strip with good bending and hole expansion performance and manufacturing method thereof" focuses on the performance design and manufacturing method of hot-rolled pickled plate, but does not disclose the effect of hot-dip galvanizing process on the performance of hot-dip galvanized steel plate.

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

[0006] The Chinese patent document with publication number CN113215485A and publication date August 6, 2021, and titled "A 780MPa grade hot-base coated dual-phase steel and its preparation method", discloses a 780MPa hot-base coated dual-phase steel and its manufacturing method, whose chemical composition 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 are Fe and unavoidable impurities. The Si content of this patent is relatively high, and red iron scale is easily formed on the surface during hot rolling, and the galvanized surface quality is poor, which in turn affects the corrosion resistance.

[0007] It can be seen that none of the above patent documents achieves the effect of high yield strength ratio of hot-dip coated dual-phase steel. Summary of the invention

[0008] One of the purposes of the present invention is to provide a high yield ratio 800MPa grade hot-dip galvanized steel plate, which has a high yield ratio, high hole expansion performance and high corrosion resistance, 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.

[0009] In order to achieve the above object, the present invention provides a high yield ratio 800MPa grade hot-dip galvanized steel sheet, which includes a substrate and a hot-dip galvanized layer, wherein the substrate contains Fe and inevitable impurities, and the substrate further contains the following chemical elements with the following mass percentages:

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

[0011] The substrate does not contain Si element;

[0012] 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.

[0013] In the present invention, when the silicon content in the steel is high, iron olivine (2FeO-SiO2) scale is easily formed on the surface of the slab 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 good corrosion resistance while having ultra-high strength, the present invention adopts a silicon-free design. It should be noted that the silicon-free design means that there is no need to deliberately add Si elements during the smelting process. Since iron ore contains a small amount of Si elements, even if the finished steel plate contains trace amounts of Si elements, it is also an unavoidable impurity in the present invention.

[0014] 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. When the volume fraction of the second-phase particles is less than 0.2%, the tensile strength of the steel plate will be less than 800MPa. The second-phase particles are precipitated at all stages of cooling. In the hot rolling process, the particles precipitated in the high-temperature rolling stage are relatively large in size, generally more than 20nm, and the strengthening effect is relatively weak. The second-phase particles precipitated during the cooling after coiling and the heating process of the hot-dip galvanizing process have a smaller size (less than 10nm) and have a significant strengthening effect. Therefore, the present invention controls the volume fraction of second-phase particles with a size not greater than 10nm to account for more than 80% of the total volume fraction of precipitation, so that the tensile strength of the steel plate reaches more than 800MPa.

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

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

[0017] In the hot-dip galvanized steel sheet of the present invention, the design principles of each chemical element are specifically described as follows:

[0018] C: In the hot-dip galvanized steel sheet of the present invention, the carbon 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. At the same time, carbon can stabilize austenite and promote the formation of bainite and other structures; carbon and microalloying elements form sufficient precipitation strengthening phases to ensure the strength of the steel. However, too high a mass percentage of carbon will lead to deterioration of forming and welding properties. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of C is controlled to be 0.04-0.069%.

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

[0020] Cr: In the hot-dip galvanized steel sheet of the present invention, chromium has the effect of improving hardenability and solid solution strengthening, can inhibit the formation of pearlite, is conducive to the formation of bainite structure, and is not easy to segregate. When the mass percentage of Cr is high, it will lead to increased costs and it is also easy to produce more martensitic structure, which affects the strength of the steel body. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of Cr is controlled to 0<Cr≤0.5%.

[0021] Ti: In the hot-dip galvanized steel sheet described in the present invention, titanium is one of the important precipitation strengthening and fine grain strengthening elements. Especially in the hot-dip galvanizing annealing process, Ti will precipitate in the form of TiC, which can significantly improve the strength of the ferrite matrix, which is beneficial to improve the yield strength ratio and elongation. When the Ti content is less than 0.05%, the tensile strength will not reach 800MPa. When the Ti content is greater than 0.12%, its strengthening effect tends to saturation. From an economic point of view, the upper limit of the Ti content of the present invention is 0.12%. Therefore, in the hot-dip galvanized steel sheet described in the present invention, the mass percentage of Ti is controlled at Ti: 0.05-0.12%.

[0022] Nb: In the hot-dip galvanized steel sheet of the present invention, 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 is close to saturation and the cost is high. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of Nb is controlled to be 0<Nb≤0.05%.

[0023] Mo: In the hot-dip galvanized steel sheet described in the present invention, Mo is a key additive element in the present invention. Molybdenum has the effect of improving hardenability and solid solution strengthening, can inhibit the generation of pearlite, and is conducive to the formation of bainite structure. In addition, when Nb, V and Ti are used for microalloying, fine carbides or carbonitrides can be formed during the cooling process, which plays a role in precipitation strengthening, but these carbides or carbonitrides have a very strong tendency to coarsen, and the strengthening effect drops sharply after the size of the second phase particles increases. When the Mo element is added to the steel, Mo will partially replace the microalloying elements in the second phase, thereby reducing the interfacial energy of the second phase, making it difficult for the second phase to grow. When the Mo content is less than 0.1%, sufficient bainite cannot be formed, and the effect of preventing the second phase from growing is not significant. When the Mo content is greater than 0.25%, its effect tends to be saturated. Therefore, in the hot-dip galvanized steel sheet described in the present invention, the mass percentage of Mo is controlled to be 0.1-0.25%.

[0024] Al: In the hot-dip galvanized steel sheet of the present invention, Al is a deoxidizing element of steel, which reduces oxide inclusions in steel, purifies steel, and is beneficial to improving the formability of the steel sheet. However, the mass percentage of aluminum is high, which will cause oxidation and further affect the continuous casting production. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of Al is controlled to 0.02-0.08%.

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

[0026] 0<V≤0.2%;

[0027] 0<B≤0.003%.

[0028] Adding V and B can further optimize the performance of the hot-dip galvanized steel sheet of the present invention.

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

[0030] B: In the hot-dip galvanized steel sheet of the present invention, boron is beneficial to expand the bainite phase region, ensuring that the steel sheet 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 sheet, resulting in a decrease in the hole expansion rate and elongation of the steel. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of B is controlled to 0<B≤0.003%.

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

[0032] In the hot-dip galvanized steel sheet of the present invention, N, P and S are all impurity elements in the steel. Therefore, under the condition that technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the steel should be reduced as much as possible. In particular, S is easy to form MnS inclusions. When S>0.002%, the hole expansion rate will be less than 85%. Therefore, in the hot-dip galvanized steel sheet of the present invention, the mass percentage of S is preferably controlled to S≤0.002%.

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

[0034] In the hot-dip galvanized steel sheet of the present invention, the microstructure of the substrate includes a complex phase structure of granular bainite + ferrite. Since 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 bainite, and the volume phase of bainite is not less than 95%.

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

[0036] 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 preferably controlled to be less than 6 μm.

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

[0038] Granular bainite is defined as a mixture of bainitic ferrite and M / A islands. Generally speaking, the hardness of the M / A islands in the granular bainite is much higher than that of the matrix structure. During the punching process, microcracks will form at the interface between the M / A islands and the matrix. As the size of the M / A islands increases, the number and size of the microcracks formed increase. When the size of the M / A islands is greater than 1 μm, the hole expansion rate of the steel plate will be less than 85%. In order to achieve the technical effect of the present invention, in the hot-dip galvanized steel sheet described in the present invention, the size of the M / A islands in the granular bainite in the hot-rolled state can be controlled to be less than 1 μm.

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

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

[0041] The hardness uniformity index (HI) refers to the standard deviation of the hardness distribution in the thickness direction of the steel plate / the average value of the hardness distribution in the thickness direction of the steel plate. The size of the HI value reflects the uniformity of the structure in the thickness direction.

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

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

[0044] In addition, another object of the present invention is to disclose a method for manufacturing a high yield ratio 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 hot-dip galvanized steel sheet with a high yield ratio.

[0045] In order to achieve the above object, the present invention proposes a method for manufacturing a high yield ratio 800MPa grade hot-dip galvanized steel sheet, which comprises the following steps:

[0046] obtaining a slab;

[0047] heating and keeping the slab warm;

[0048] Hot rolling, followed by cooling at a cooling rate of ≥50°C / s to a coiling temperature of 450-500°C;

[0049] Pickling to obtain pickled coils;

[0050] Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion non-oxidizing continuous annealing furnace, where the soaking section temperature is controlled at 630-720°C, the soaking section holding time is 30-120s, the slow cooling section temperature is 550-670°C, and after leaving the slow cooling section, they are cooled to the hot-dip galvanizing temperature.

[0051] In the present invention, the cooling rate after rolling is ≥50°C / s, so as to make the hot-rolled steel plate quickly enter the bainite transformation zone, obtain fine bainite structure and reduce the size of M / A island. If the cooling rate is less than 50°C / s, more ferrite or pearlite will be generated during the cooling process, and the size of M / A island in granular bainite will be larger (>1μm).

[0052] The coiling temperature is controlled at 450-500℃ in order to obtain a granular bainite structure with refined M / A islands. When the coiling temperature is high, pearlite is easily formed, and the grain size is also large. At the same time, a large number of second phase particles will precipitate and coarsen during the cooling process after coiling, resulting in a yield strength ratio of less than 0.9 after hot-dip galvanizing annealing. When the coiling temperature is too low, martensite will be formed, resulting in a lower hole expansion rate.

[0053] In the present invention, the annealing soaking temperature is controlled to be 630-720°C because: when the soaking temperature is less than 630°C, the volume fraction of the second phase precipitated during the soaking process is low, resulting in lower strength. When the soaking temperature exceeds 720°C, martensite will be generated during the cooling process, resulting in a larger hardness uniformity index (HI), resulting in a hole expansion rate of less than 85%.

[0054] In addition, the present invention controls the annealing soaking section holding time to 30-120s because: when the annealing soaking section holding time is less than 30s, the internal stress generated in the previous process of the steel plate cannot be completely eliminated, and the recovery and precipitation process of the steel plate is uneven, resulting in a larger hardness uniformity index (HI) and a lower hole expansion rate. However, when the soaking temperature exceeds 120s, 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.

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

[0056] 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. Therefore, 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 conventional units.

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

[0058] In some embodiments of the present invention, controlling the heating temperature of the slab can further improve the performance and surface quality. The heating temperature is set to be higher than 1230°C, the purpose of which is to dissolve the (Ti, Nb) (C, N) produced in the continuous casting process as much as possible. The micro-alloys such as solid-solubilized Ti 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 improve the strength of the steel plate. When the temperature exceeds 1280°C, the austenite grains coarsen, which is not conducive to the toughness of the steel plate; at the same time, the iron oxide scale is thicker, which is not conducive to the dephosphorization of the iron oxide scale, and ultimately affects the surface quality of hot-dip galvanizing. Therefore, it is preferred to set the heating temperature to 1230-1280°C.

[0059] Furthermore, in the method for manufacturing hot-dip galvanized steel sheet described in 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 840-880°C.

[0060] In some embodiments of the present invention, the rough rolling temperature control of the hot rolling process has a greater impact on micro-alloys 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 the improvement of the final 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.

[0061] In addition, the finishing exit temperature of the hot rolling process is a key process for controlling the grain size and M / A island size. When the finishing exit temperature is low, the austenite grains are in the shape of cakes, accumulating a large amount of deformation energy, which is beneficial to reducing the grain size and M / A island size in the subsequent cooling process, and is also beneficial to improving the hole expansion rate. However, when the finishing exit temperature is too low, mixed crystals will be generated, resulting in a higher hardness uniformity index (HI) and a lower hole expansion rate. Therefore, the hot rolling finishing exit temperature is controlled at 840-880℃.

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

[0063] In this embodiment, the temperature of the hot-dip galvanizing pot is controlled to be 440-480°C because: within this temperature range, the bonding between the steel plate surface and the zinc liquid is better; below or above this temperature range, the plating property of the substrate surface deteriorates, resulting in a poor surface after plating.

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

[0065] In some embodiments of the present invention, 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, reducing the elongation of the steel sheet. Therefore, in the present invention, the flatness is controlled to be 0.05-1.3%.

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

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

[0068] 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.

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

[0070] Therefore, the present invention can be used as automobile body structural parts and automobile chassis parts, and can also be used in other application fields requiring high strength, weight reduction, etc., and is particularly suitable for the design of parts with complex shapes. DETAILED DESCRIPTION

[0071] The high yield ratio 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.

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

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

[0074] (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.

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

[0076] (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 controlled to be ≥90%.

[0077] (4) After hot rolling, the steel is cooled at a cooling rate of ≥50°C / s to a coiling temperature of 450-500°C.

[0078] In some embodiments, 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.

[0079] (5) Pickling to obtain a pickled coil.

[0080] In some embodiments, the pickling stretching elongation can be 0.2-2%; the pickling speed is 60-150 m / min, 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 30-40 g / L.

[0081] Controlling the pickling and straightening elongation to 0.2-2% can further improve the surface quality. When the straightening elongation is lower than 0.2% or higher than 2%, it will have an adverse effect on the plate shape, resulting in poor surface quality after hot-dip galvanizing. In addition, lower pickling speed and higher acid tank temperature will lead to over-pickling, while higher pickling speed and lower acid tank temperature will lead to under-pickling, both of which will cause surface problems of pickled plates and affect the surface quality after hot-dip galvanizing.

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

[0083] (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.

[0084] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S and N)

[0085]

[0086]

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

[0088] Table 2-1.

[0089]

[0090]

[0091] Table 2-2.

[0092]

[0093]

[0094]

[0095] The hot-dip galvanized steel sheets of Examples 1-17 and Comparative Examples 1-13 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.

[0096] Table 3.

[0097]

[0098]

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

[0100] Take JIS13B tensile specimens along the longitudinal direction to measure the yield and tensile strength, the gauge length is 80mm, and the mechanical properties test is carried out in accordance with GB / T228.1-2010 standard.

[0101] The hole expansion rate test is carried out in accordance with GB / T24524-2021 standard.

[0102] The HI index measurement method is as follows: After grinding and polishing the thickness section of the steel plate, measure the microhardness at equal intervals along the thickness direction of 45°. The test data from the upper surface to the lower surface is a group, and each group is not less than 15 data. At least 3 groups of data are a data set. The "average value of the hardness distribution in the thickness direction of the steel plate" is the average value of the data set, and the "standard deviation of the hardness distribution in the thickness direction of the steel plate" is the standard deviation of the data set. The hardness uniformity index (HI) is calculated by the above formula.

[0103] Table 4.

[0104]

[0105]

[0106] As shown in Tables 3 and 4, Examples 1-17 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 yield strength finally obtained in each embodiment of the present invention is all>720MPa, the tensile strength is all>800MPa, the elongation A80 is all>15%, the hole expansion rate is all>85%, the yield strength ratio is all ≥0.9, and the hardness uniformity index HI is all<3%.

[0107] Unlike the present invention, Comparative Examples 1-13 cannot achieve the technical effects that can be achieved by the present invention because the ingredients or process parameters do not meet the design requirements of the present invention, wherein:

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

[0109] In Comparative Example 2, since the temperature of the annealing soaking section is higher than the upper limit of the present invention, austenite is formed during the heating process and transforms into martensite during the cooling process, resulting in a decrease in the proportion of bainite, an increase in the HI index, and a decrease in the yield strength ratio and hole expansion rate of the steel plate.

[0110] In Comparative Example 3, since the annealing soaking time is lower than the lower limit of the present invention, the second phase precipitation is insufficient and the strength is reduced.

[0111] In Comparative Example 4, since the annealing soaking time is higher than the upper limit, the second phase is coarsened and the yield strength ratio is reduced.

[0112] In Comparative Example 5, the temperature in the slow cooling stage is relatively high, which causes the second phase to coarsen and the yield strength ratio to decrease.

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

[0114] In Comparative Example 7, since the coiling temperature is higher than the upper limit of the present invention, the ferrite content is high, the bainite content is reduced, and the grains are coarse, resulting in reduced strength.

[0115] In Comparative Example 8, due to the low cooling rate after rolling, more ferrite is formed during the laminar cooling process, the bainite content is reduced, the size of the M / A island is increased, and the hole expansion rate is reduced.

[0116] In Comparative Example 9, since the carbon content and the manganese content are higher than the upper limit of the present invention, more martensite is formed during the hot rolling process, the bainite content is less, and the hole expansion rate is reduced.

[0117] In Comparative Example 10, since the carbon content and the manganese content are lower than the lower limit of the present invention, although the microstructure meets the requirements of the present invention, the carbon and manganese dissolved in the bainite are insufficient, resulting in insufficient strength.

[0118] In Comparative Example 11, since the S content is higher than the upper limit, although the microstructure meets the requirements of the present invention, it contains more inclusions and segregation, resulting in an increase in the HI index and a decrease in the hole expansion rate of the steel plate.

[0119] In Comparative Example 12, due to the addition of too much B, martensite is generated during hot rolling, resulting in a decrease in the hole expansion rate. In addition, its Mo content is lower than the lower limit of the present invention, resulting in coarsening of the second phase particles during the hot-dip galvanizing heating process, weakening the precipitation strengthening effect, and reducing the strength of the steel plate.

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

[0121] 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.

[0122] 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 the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A high yield ratio 800MPa grade hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer, wherein the substrate contains Fe and unavoidable impurities, characterized in that: The substrate also contains the following chemical elements in the following mass percentages: C: 0.04-0.069%, Mn: 1.3-1.8%, 0<Cr≤0.5%, Ti: 0.05-0.12%, 0<Nb≤0.05%, Mo: 0.1-0.25%, Al: 0.02-0.08%; The substrate does not contain Si element; 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 high yield ratio 800 MPa 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: 0.04-0.069%, Mn: 1.3-1.8%, 0<Cr≤0.5%, Ti: 0.05-0.12%, 0<Nb≤0.05%, Mo: 0.1-0.25%, Al: 0.02-0.08%; the balance is Fe and other inevitable impurities.

3. The high yield ratio 800 MPa 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＜V≤0.2％； 0＜B≤0.003％。 4. The high yield ratio 800 MPa 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≤0.002%.

5. The high yield ratio 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 high yield ratio 800 MPa 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 6 μm.

7. The high yield ratio 800 MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: In the hot-rolled state, the volume phase ratio of the granular bainite is not less than 95%, and the size of the M / A island in the granular bainite is less than 1 μm.

8. The high yield ratio 800 MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its hardness uniformity index HI≤3%.

9. The high yield ratio 800 MPa grade hot-dip galvanized steel sheet according to claim 8, characterized in that: Its hardness uniformity index HI≤2%.

10. The high yield ratio 800 MPa grade hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its yield strength is ≥720MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, hole expansion rate is ≥85%, and yield strength ratio is ≥0.

9.

11. The method for manufacturing a high yield ratio 800 MPa grade hot-dip galvanized steel sheet according to any one of claims 1 to 10, 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 soaking section temperature is controlled at 630-720°C, the soaking section holding time is 30-120s, the slow cooling section temperature is 550-670°C, and after leaving the slow cooling section, they are cooled to the hot-dip galvanizing temperature.

12. The manufacturing method according to claim 11, characterized in that: Heat the slab to 1230-1280°C and keep it at that temperature for 1-3h.

13. The manufacturing method according to claim 11, 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 840-880°C.

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

15. The manufacturing method according to claim 11, 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%.

Images