900 mpa-grade hot-rolled hot-dip galvanized steel plate and manufacturing method therefor

Abstract

Disclosed in the present invention is a 900 MPa-grade hot-rolled hot-dip galvanized steel plate which comprises a substrate and a hot-dip galvanizing layer. The substrate contains Fe and inevitable impurities, and the substrate further comprises the following chemical elements in percentage by mass: 0.06%-0.10% of C, 1.5-2.15% of Mn, 0.02-0.08% of Al, 0<Cr≤0.5%, 0.05-0.30% of Mo, 0.11-0.16% of Ti, and 0＜Nb≤0.05%. The substrate does not contain Si element, and the microstructure of the substrate comprises granular bainite and second-phase particle precipitates, the volume fraction of the second-phase particle precipitates being not less than 0.2%, and the volume fraction ratio of the second-phase particle precipitates having the size not larger than 10 nm to all the second-phase particle precipitates being greater than 80%.

Description

A 900MPa grade hot-dip galvanized steel sheet and its manufacturing method Technical Field

[0001] The present 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 advancement of lightweighting technology, advanced high-strength steels are increasingly used in automotive structural components. Duplex steel, due to its exceptional strength and excellent formability, has become the preferred choice for chassis components such as control arms and torsion beams.

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

[0004] Chinese patent document CN104513930A, published on April 15, 2015, and entitled "Ultra-High-Strength Hot-Rolled Complex Phase Steel Sheet and Strip with Excellent Bending and Hole Expansion Properties and Method for Manufacturing Same," discloses that the composition of ultra-high-strength hot-rolled complex phase steel sheet and strip with excellent bending properties 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 also includes at least one alloying element of Nb ≤ 0.05% and Ti: 0.06-0.15%, with the remainder being Fe and unavoidable impurities. This patent document discloses the performance design and manufacturing methods of hot-rolled pickled steel sheet, but does not address the impact of the hot-dip galvanizing process on the properties of hot-dip galvanized steel sheet.

[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] A Chinese patent document, published on August 6, 2021 and numbered CN113215485A, titled "A 780MPa Grade Hot-Base Coated Dual-Phase Steel and Its Preparation Method," discloses a 780MPa hot-base coated duplex steel and its manufacturing method. The steel's chemical composition is as follows: C: 0.04%-0.08%, Si: 0.5%-0.8%, Mn: 1.4%-2.1%, Al: 0.02%-0.08%, Nb: 0.04-0.07%, Ti: 0.05-0.14%, Cr: 0-0.25%, Cu: 0-0.01%, Ni: 0-0.01%, B: 0-0.001%, P: 0-0.015%, S: 0-0.001%, and the remainder is Fe and unavoidable impurities. The patent claims a relatively high Si content, making it prone to the formation of red iron scale on the surface during hot rolling.

[0007] Summary of the Invention

[0008] One of the objects of the present invention is to provide a 900MPa grade hot-dip galvanized steel sheet having high strength, high formability and high corrosion resistance, which can be used for automotive structural parts and chassis components, as well as other application fields requiring high strength, weight reduction and corrosion resistance.

[0009] To achieve the above object, the present invention provides a 900 MPa grade hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer, wherein the substrate contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages:

[0010] C: 0.060%-0.100%, Mn: 1.50-2.15%, Al: 0.020-0.080%, 0<Cr≤0.50%, Mo: 0.05-0.30%, Ti: 0.11-0.16%, 0<Nb≤0.050%;

[0011] The substrate does not contain Si element;

[0012] 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 volume fraction of the second phase particle precipitates with a size of not more than 10 nm accounts for more than 80% of the total second phase particle precipitates.

[0013] Generally speaking, in order to achieve ultra-high strength, the steel sheet needs to be heated to a higher temperature in the hot-dip galvanizing process, partially or completely austenitized, and then a martensite phase is formed during the cooling process, thereby achieving the purpose of improving strength. However, this structure containing some martensite has poor hole expansion performance and cannot meet the forming requirements of the present invention. Therefore, the present invention optimizes and controls the chemical composition and metallographic structure, adopts Nb and Ti microalloying, forms a granular bainite structure during the hot rolling process, and then forms a second phase precipitation with a sufficient volume fraction during the annealing hot-dip galvanizing process, further strengthening the bainite structure, thereby obtaining 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 of more than 600°C in the hot-dip galvanizing process, the second phase precipitation is prone to coarsening, resulting in a reduction in the precipitation strengthening effect, which reduces the strength of the steel sheet. To this end, the present invention adds Mo element to form alloy carbides with Ti and Nb microalloying elements, reducing the interfacial energy with the matrix, making it difficult for the second phase precipitation to grow, maintaining a smaller size, and achieving ultra-high strength.

[0014] In addition, the present invention adopts a silicon-free design, so it has good surface quality and corrosion resistance. In the present invention, when the silicon content in the steel is high, fayalite (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.

[0015] In addition, the precipitation of second-phase particles is a key strengthening mechanism in the present invention. Only when the precipitated second-phase particles have a sufficient volume fraction and are small in size can they have a 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 900 MPa. In some embodiments, the volume fraction of the second-phase particles is 0.2-0.35%. Second-phase particles precipitate at all stages of cooling. The particles precipitated during the high-temperature rolling stage of the hot rolling process are larger in size, generally more than 20 nm, and the strengthening effect is weaker. 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 10 nm) and have a significant strengthening effect. Therefore, the present invention controls the volume fraction of second-phase particles with a size of no more than 10 nm to be greater than 80% of the total precipitation volume fraction, so that the tensile strength of the steel plate reaches more than 900 MPa. In some embodiments, the volume fraction of second-phase particles with a size of no more than 10 nm to be 81-95% of the total precipitation volume fraction.

[0016] Furthermore, in the 900 MPa grade hot-dip galvanized steel sheet of the present invention, the mass percentage of each chemical element of the substrate is:

[0017] C: 0.060%-0.100%, Mn: 1.50-2.15%, Al: 0.020-0.080%, 0<Cr≤0.50%, Mo: 0.05-0.30%, Ti: 0.11-0.16%, 0<Nb≤0.05%; the balance is Fe and other inevitable impurities.

[0018] In the hot-dip galvanized steel sheet of the present invention, the design principles of each chemical element are specifically described as follows:

[0019] Carbon (C): In the hot-dip galvanized steel sheet described herein, carbon is the most essential element and the most commonly used strengthening element. The carbon content determines the tensile strength of the steel. Carbon also stabilizes austenite and promotes the formation of microstructures such as bainite. Carbon and microalloying elements form sufficient precipitation-strengthening phases to ensure steel strength. However, excessive carbon content can degrade formability and weldability. Therefore, in the hot-dip galvanized steel sheet described herein, the carbon content is controlled within a range of 0.060% to 0.100%.

[0020] Mn: In the hot-dip galvanized steel sheet described herein, manganese is a solid solution strengthening element that improves hardenability, delays pearlite transformation, lowers the bainite transformation temperature, and refines the bainite structure. However, a low manganese content can result in insufficient strength. Furthermore, a high manganese content can reduce the steel sheet's plasticity and easily lead to segregation. Therefore, in the hot-dip galvanized steel sheet described herein, the manganese content is controlled within a range of 1.50-2.15%.

[0021] Al: In the hot-dip galvanized steel sheet described herein, aluminum acts as a deoxidizing element, reducing oxide inclusions, purifying the steel, and improving the sheet's formability. However, excessive aluminum content can lead to oxidation, further impacting continuous casting production. Therefore, the aluminum content in the hot-dip galvanized steel sheet described herein is controlled within a range of 0.020-0.080%.

[0022] Cr: In the hot-dip galvanized steel sheet described herein, chromium improves hardenability and solid solution strengthening, inhibits the formation of pearlite, and is beneficial to the formation of bainite. Furthermore, chromium is less likely to segregate. A high Cr content increases costs and tends to produce a higher amount of martensite. Therefore, in the hot-dip galvanized steel sheet described herein, the Cr content is controlled within the range of 0 < Cr ≤ 0.50%. In some embodiments, the Cr content is controlled within the range of 0.15-0.50%.

[0023] Mo: In the hot-dip galvanized steel sheet described herein, Mo is a key additive element. Molybdenum improves hardenability and solid solution strengthening, inhibits the formation of pearlite, and promotes the formation of bainite. Furthermore, when microalloyed with Nb, V, and Ti, fine carbides or carbonitrides can form during cooling, contributing to precipitation strengthening. However, these carbides or carbonitrides have a strong tendency to coarsen, and the strengthening effect decreases sharply as the size of the second-phase particles increases. When Mo is added to the steel, it partially replaces the microalloying elements in the second phase, thereby reducing the interfacial energy of the second phase and preventing its growth. When the Mo content is less than 0.05%, sufficient bainite cannot be formed, and the effect of inhibiting the growth of the second phase is not significant. When the Mo content exceeds 0.30%, the effect tends to saturate. Therefore, in the hot-dip galvanized steel sheet described herein, the mass percentage of Mo is controlled to 0.05-0.30%.

[0024] 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. In particular, during the hot-dip galvanizing annealing process, Ti will precipitate in the form of TiC, which will significantly increase the strength of the ferrite matrix, which is beneficial to improving the yield strength ratio and elongation. When the Ti content is less than 0.11%, the tensile strength does not reach 900MPa, and when the Ti content exceeds 0.16%, its strengthening effect tends to saturation. From an economic perspective, the upper limit of the Ti content in the present invention is 0.16%. Therefore, in the technical solution described in the present invention, the mass percentage of Ti is controlled to Ti: 0.11-0.16%.

[0025] Nb: Niobium is a key element in the hot-dip galvanized steel sheet of the present invention, contributing to precipitation and grain refinement. However, when the mass percentage of Nb exceeds 0.050%, the strengthening effect of Nb approaches saturation, and the cost is high. Therefore, in the high hole expansion hot-dip galvanized steel sheet of the present invention, the mass percentage of Nb is controlled within the range of 0 < Nb ≤ 0.050%. In some embodiments, the mass percentage of Nb is controlled within the range of 0.010-0.05%.

[0026] Furthermore, in the 900 MPa grade hot-dip galvanized steel sheet of the present invention, the substrate further contains 0<V≤0.20%.

[0027] Adding V element can further optimize the performance of the hot-dip galvanized steel sheet of the present invention.

[0028] V: In the hot-dip galvanized steel sheet described herein, vanadium contributes to precipitation strengthening and grain refinement, with the precipitation of vanadium carbides further improving strength. However, when the mass percentage of V exceeds 0.2%, the strengthening effect approaches saturation, and the cost is high. Therefore, in the high-hole-expansion hot-dip galvanized steel sheet described herein, the mass percentage of V is controlled within the range of 0 < V ≤ 0.20%.

[0029] Furthermore, among the inevitable impurities in the substrate of the 900 MPa grade hot-dip galvanized steel sheet of the present invention, P≤0.02%, N≤0.005%, and S≤0.005%.

[0030] In the hot-dip galvanized steel sheet described in the present invention, P, N and S are all impurity elements in the steel. Therefore, if technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible.

[0031] Furthermore, in the 900 MPa grade hot-dip galvanized steel sheet of the present invention, the volume phase ratio of the granular bainite is not less than 90%.

[0032] In the hot-dip galvanized steel sheet described herein, the granular bainite structure exhibits uniform properties, excellent hole expansion, and contains a large amount of supersaturated carbon atoms. During the annealing and hot-dip galvanizing steps described herein, it precipitates in large quantities, thereby improving the strength and formability of the steel sheet. Therefore, the microstructure of the present invention is primarily granular bainite. When the granular bainite content falls below 90%, the precipitation strengthening effect is weakened, and the tensile strength does not reach 900 MPa.

[0033] In addition to granular bainite, the 900 MPa grade hot-dip galvanized steel sheet of the present invention may also contain a small amount of ferrite and / or martensite.

[0034] Ferrite is relatively soft, characterized by low strength and good plasticity. However, when the ferrite content is high, the tensile strength of the steel plate does not reach 900 MPa. Martensite has high strength but low plasticity. When the martensite content is high, cracks are easily formed at the interface between martensite and the weaker bainite, reducing the steel plate's hole expansion performance. Therefore, the content of martensite and ferrite must be strictly controlled to ensure that the granular bainite content is above 90%.

[0035] Furthermore, in the 900 MPa grade hot-dip galvanized steel sheet of the present invention, the grain size of the granular bainite is less than 5 μm, such as 2.5-4.9 μm.

[0036] In the present invention, the grain size of granular bainite significantly affects yield strength. Larger granular bainite grains result in lower yield strength, while grain sizes greater than 5 μm can reduce the yield strength of the steel plate. To achieve the technical benefits of the present invention, the grain size of the granular bainite is preferably controlled to be less than 5 μm.

[0037] Furthermore, in the 900 MPa grade hot-dip galvanized steel sheet of the present invention, its longitudinal yield strength is ≥780 MPa, tensile strength is ≥900 MPa, elongation A80 is ≥16%, and hole expansion ratio is ≥50%.

[0038] In some embodiments, the longitudinal yield strength of the 900 MPa grade hot-dip galvanized steel sheet of the present invention is ≥810 MPa. In some embodiments, the longitudinal yield strength of the 900 MPa grade hot-dip galvanized steel sheet of the present invention is 780-880 MPa.

[0039] In some embodiments, the tensile strength of the 900 MPa grade hot-dip galvanized steel sheet of the present invention is 900-950 MPa.

[0040] In some embodiments, the elongation A80 of the 900 MPa grade hot-dip galvanized steel sheet of the present invention is 16-20%.

[0041] In some embodiments, the 900 MPa grade hot-dip galvanized steel sheet of the present invention has a hole expansion ratio of 50-90%.

[0042] In addition, another object of the present invention is to provide a method for manufacturing a 900MPa grade hot-dip galvanized steel sheet. This method, combined with the composition 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 high strength, high formability and high corrosion resistance.

[0043] In order to achieve the above object, the present invention proposes a method for manufacturing a 900MPa grade hot-dip galvanized steel sheet, which comprises the following steps:

[0044] obtaining a slab;

[0045] heating the slab and keeping it warm;

[0046] Hot rolling, followed by cooling at a cooling rate of ≥40°C / s to a coiling temperature of 440-520°C;

[0047] pickling to obtain pickled coils;

[0048] Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion-free continuous annealing furnace, with the soaking section temperature controlled at 630-720°C, the soaking section holding time at 30-120s, the slow cooling section temperature at 550-670°C, and then cooled to the hot-dip galvanizing temperature after exiting the slow cooling section.

[0049] In the present invention, the hot-rolled coil is water-cooled to 440-520°C at a cooling rate of ≥40°C / s and coiled. The cooling rate after rolling is ≥40°C / s so that the hot-rolled steel plate can quickly enter the bainite transformation zone. When the cooling rate is less than 40°C / s, more ferrite or pearlite will be generated during the cooling process. The coiling temperature is controlled to 440-520°C after cooling in order to obtain a bainite structure. When the coiling temperature is high, 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 will be formed, resulting in a hole expansion rate of less than 50%. In some embodiments, the cooling rate is 40-80°C / s, such as 45-75°C / s. In some embodiments, the coiling temperature is 450-500°C.

[0050] In some embodiments, after the coiling process is completed, the hot coil can be placed in a holding pit for slow cooling, and the average cooling rate can be ≤15°C / h.

[0051] In the present invention, the annealing soaking zone temperature is controlled to 630-720°C because: when the soaking zone 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 hole expansion ratio of less than 50%.

[0052] Furthermore, the present invention controls the annealing soaking period to 30-120 seconds because, when the soaking period is less than 30 seconds, the internal stress of the steel sheet cannot be completely eliminated during hot rolling, resulting in poor surface condition and difficulty in achieving a good hot-dip galvanized surface. However, when the soaking period exceeds 120 seconds, the second-phase particles coarsen and dissolve extensively, resulting in reduced strength and an inability to maintain the yield strength of the steel sheet. In some embodiments, the soaking period is 40-100 seconds.

[0053] In this invention, the slow cooling section temperature is controlled at 550-670°C because: if 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. Due to the limited cooling capacity of the unit in the slow cooling section, when the soaking section is 630-720°C, it is generally difficult to cool the slow cooling section below 550°C.

[0054] Furthermore, after exiting the slow cooling stage, the present invention can cool to the hot-dip galvanizing temperature at a cooling rate of ≥5°C / s. This cooling rate is not specifically limited because austenite is not formed during the soaking process and phase transformation does not occur during the cooling process. Therefore, the cooling rate after exiting the slow cooling stage has no effect on performance. A cooling rate of ≥5°C / s is required to ensure normal operation of conventional units.

[0055] Furthermore, in the manufacturing method of the present invention, the slab is produced by smelting and casting. In some embodiments, the slab has a thickness of 220 mm to 250 mm.

[0056] Furthermore, in the manufacturing method of the present invention, the slab is heated to 1230-1280° C. and kept warm for 1-3 hours.

[0057] In the present invention, the heating temperature of the slab can further improve the performance and surface quality. The heating temperature is set to ≥1230℃, the purpose of which is to dissolve the Ti (C, N) produced during the continuous casting process as much as possible. The micro-alloys such as solid-solution Ti in the austenite are precipitated as nano-scale second-phase particles during hot rolling and coiling, especially during the annealing and hot-dip galvanizing process, which can effectively improve the strength of the steel plate. When the temperature exceeds 1280℃, the austenite grains coarsen, 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 ultimately affects the surface quality of the hot-dip galvanizing. Therefore, it is preferred to set the heating temperature to 1230-1280℃.

[0058] 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.

[0059] In the hot rolling process described in the present invention, the control of the roughing temperature has a greater impact on microalloys such as Ti. During the roughing and finishing rolling processes, carbides and carbonitrides will precipitate, and the precipitation size of this process is relatively large. When the roughing and finishing temperatures are high, a large amount of precipitation will occur, which is not conducive to the improvement of the final strength. When the roughing and finishing temperatures are low, the microalloying elements dissolved in the austenite are reduced, which is not conducive to precipitation during the cooling process. Therefore, the roughing start temperature is controlled to be 1130-1190°C, and the finishing start temperature is controlled to be 980-1080°C.

[0060] In the hot rolling process described herein, the finish rolling exit temperature significantly influences the microstructure. A lower finish rolling exit temperature produces pancake-shaped austenite grains, accumulating a large amount of deformation energy and facilitating the achievement of a fine grain size during the subsequent cooling process. However, excessively low finish rolling exit temperatures can induce ferrite transformation, adversely affecting formability. Therefore, the hot rolling finish rolling exit temperature is controlled between 840°C and 940°C.

[0061] In some embodiments, the pickling and straightening elongation can be 0.2-2%; the pickling speed can be 60-150 m / min, the temperature of the last pickling tank in the pickling process can be 80-90°C, and the iron ion concentration can be 30-40 g / L. 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 a negative impact on the plate shape. In addition, a lower pickling speed and a higher pickling tank temperature will result in over-pickling, while a higher pickling speed and a lower pickling tank temperature will result in under-pickling, both of which may affect the surface of the pickled plate and, in turn, the surface quality after hot-dip galvanizing.

[0062] In some embodiments, the pickled coils are annealed directly in a combustion-free continuous annealing furnace, where both the direct annealing section and the heating section employ conventional processes. In some embodiments, the direct annealing section may have a temperature of 550-660°C. In some embodiments, the heating section may have a temperature of 630-720°C.

[0063] Furthermore, in the manufacturing method of the present invention, in the hot-dip galvanizing step, the hot-dip galvanizing temperature is 440-480°C.

[0064] In the hot-dip galvanizing step of the present invention, the temperature of the hot-dip galvanizing pot can be cooled to 440-480°C. Within this temperature range, the steel sheet surface has good adhesion to the zinc solution. Temperatures below or above this range can result in poor galvanizability of the substrate surface and a poor post-galvanizing surface. The plating solution can be any conventional plating solution used in the art for preparing hot-dip galvanized layers.

[0065] Furthermore, in the manufacturing method of the present invention, the hot-dip galvanizing step further includes leveling, and the leveling rate is controlled to be 0.05-1.3%.

[0066] In some embodiments of the present invention, a flattening rate less than 0.05% results in poor surface quality of the zinc layer. A flattening rate greater than 1.3% results in significant work hardening, reducing the elongation of the steel sheet. Therefore, in the present invention, the flattening rate is controlled to be between 0.05% and 1.3%. In some embodiments, the flattening rate is controlled to be between 0.05% and 1.1%.

[0067] In some embodiments, the single-side weight of the hot-dip galvanized layer of the present invention is 20-380 g / m 2 .

[0068] In some embodiments, the hot-dip galvanized sheet of the present invention has a thickness of 1.8-3.5 mm.

[0069] Compared with the prior art, the 900MPa grade hot-dip galvanized steel sheet and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0070] The hot-base hot-dip galvanized complex phase steel of the present invention has better corrosion resistance than conventional hot-rolled pickling plates and is suitable for conventional hot rolling production lines and hot-dip galvanizing production lines.

[0071] 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.

[0072] The hot-dip galvanized steel sheet described in the present invention has the characteristics of high strength, high formability, and high corrosion resistance, wherein the high formability includes high elongation and high hole expansion rate. As mentioned above, the microstructure of the present invention is mainly granular bainite, the dislocation density is reduced during the heating process in the hot-dip galvanizing process, and the size of the second phase particles is small, which is conducive to the improvement of the hole expansion rate and elongation. Therefore, excellent hole expansion performance and elongation can be obtained. In some embodiments, its longitudinal yield strength is ≥780MPa, the tensile strength is ≥900MPa, the elongation A80 is ≥16%, and the hole expansion rate is ≥50%.

[0073] 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 and weight reduction, and is particularly suitable for molding designs with complex shapes. DETAILED DESCRIPTION

[0074] The 900 MPa grade hot-dip galvanized steel sheet and its manufacturing method described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0075] Examples 1-13 and Comparative Examples 1-9

[0076] The 900 MPa grade hot-dip galvanized steel sheets in each embodiment of the present invention are prepared by the following steps:

[0077] (1) Smelting and casting to produce slabs with a thickness of 220 mm to 250 mm. Table 1 lists the mass percentages of the chemical elements in the slabs of the embodiments of the present invention and the comparative examples.

[0078] (2) Heat the slab to 1230-1280℃ and keep it warm for 1-3h.

[0079] (3) Hot rolling: The starting temperature of rough rolling is controlled at 1130-1190℃, the starting temperature of finishing rolling is controlled at 980-1080℃, and the outlet temperature of finishing rolling is controlled at 840-940℃. During the hot rolling process, the cumulative deformation of rough rolling and finishing rolling can be controlled to be ≥90%.

[0080] (4) After hot rolling, the steel is cooled at a cooling rate of ≥40°C / s to a coiling temperature of 440-520°C.

[0081] In some embodiments, optionally, after the coiling process is completed, the hot coil can be placed in a heat preservation pit for slow cooling, and the average cooling rate can be ≤15°C / h.

[0082] (5) Pickling to obtain pickled coils.

[0083] In some embodiments, the pickling tensile elongation can be 0.2-2%; the pickling speed can be 60-150 m / min, the temperature of the last pickling tank in the pickling process can be 80-90° C., and the iron ion concentration can be 30-40 g / L.

[0084] 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 a negative impact on the plate shape. In addition, a lower pickling speed and a higher acid tank temperature will lead to over-pickling, while a higher pickling speed and a lower acid tank temperature will lead to under-pickling, which may affect the surface of the pickled plate and, in turn, the surface quality after hot-dip galvanizing.

[0085] (6) Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion-free continuous annealing furnace, in which the direct annealing section and the heating section adopt conventional processes, the soaking section temperature is controlled to be 630-720℃, and the soaking section holding time is 30-120s; the slow cooling section temperature is 550-670℃, and after leaving 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 single-side weight of the hot-dip galvanized layer can be 20-380g / m 2 .

[0086] (7) Flatness: The flatness 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.

[0087] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S and N)

[0088] Note: In Table 1, steel grade AG is the steel grade used in the embodiment of the present invention, while HK is the steel grade used in the comparative example.

[0089] Table 2 lists the specific process parameters in the above process steps of various embodiments of the present invention and comparative examples.

[0090] Table 2.

[0091] Samples of the hot-dip galvanized steel sheets obtained in Examples 1-13 and Comparative Examples 1-9 were taken and their microstructures were observed using optical microscopy and scanning electron microscopy. Grain size was measured using the equivalent area method. The size distribution and volume fraction of the second phase precipitation were measured using the extraction carbon replica method. The results are listed in Table 3.

[0092] Table 3.

[0093] In addition, JIS 13B tensile test specimens were taken longitudinally for each example and comparative example, with a gauge length of 80 mm, to measure yield strength and tensile strength. Mechanical property testing was performed in accordance with GB / T 228.1-2010. Hole expansion testing was performed in accordance with GB / T 24524-2021. The results of these tests are listed in Table 4.

[0094] As shown in Tables 3 and 4, Examples 1-17 of the present invention achieve ideal microstructure characteristics and grain refinement through reasonable chemical element composition design, combined with optimized hot rolling process and annealing and hot-dip galvanizing process. The annealing and hot-dip galvanizing heating process promotes the precipitation of a large number of second-phase particles to improve strength without reducing plasticity. During the annealing process, the temperature of each section is strictly controlled to avoid coarsening of the second-phase particles, thereby achieving good performance. The yield strength finally obtained by each embodiment of the present invention is ≥793MPa, the tensile strength is ≥902MPa, the elongation A80 is ≥16.5%, and the hole expansion rate is ≥51.2%.

[0095] Unlike the present invention, the comparative examples 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:

[0096] In Comparative Example 1, since the annealing 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 the strength is reduced.

[0097] 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 phase changes to martensite during the cooling process, the proportion of bainite decreases, and the yield strength of the steel plate decreases.

[0098] In Comparative Example 3, the coiling temperature exceeded the upper limit of the present invention, resulting in a high ferrite content and a low bainite content. Furthermore, high coiling temperatures lead to the precipitation of more second phases, which tend to coarsen during the hot-dip galvanizing process, leading to reduced strength.

[0099] In Comparative Example 4, since the coiling temperature is lower than the lower limit of the present invention, the martensite content in the hot coil is high and the bainite content is low, resulting in a decrease in the hole expansion rate of the steel plate.

[0100] In Comparative Example 5, the cooling rate after rolling is lower than the lower limit of the present invention, resulting in a decrease in bainite content, reduced strength, and insufficient hole expansion rate.

[0101] In Comparative Example 6, since the carbon content and manganese content are higher than the upper limit of the present invention, more martensite is formed during the hot rolling process, resulting in less bainite content and a lower hole expansion rate.

[0102] Since the carbon content in Comparative Example 7 is lower than the lower limit of the present invention, the strength is insufficient.

[0103] In Comparative Example 8, since the Mo content is lower than the lower limit of the present invention, the second phase particles are coarsened during the hot-dip galvanizing heating process, the precipitation strengthening effect is weakened, the strength of the steel plate is reduced, and the hole expansion rate is lower than 50%.

[0104] In Comparative Example 9, 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.

[0105] 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.

[0106] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A hot-dip galvanized steel sheet with a tensile strength of ≥900 MPa, 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: 0.060%-0.100%, Mn: 1.50-2.15%, Al: 0.020-0.080%, 0<Cr≤0.50%, Mo: 0.05-0.30%, Ti: 0.11-0.16%, 0<Nb≤0.050%; The substrate does not contain Si element; 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 volume fraction ratio of the second phase particle precipitates with a size of not more than 10 nmm to all the second phase particle precipitates is greater than 80%.

2. The hot-dip galvanized steel sheet according to claim 1, characterized in that: The mass percentage of each chemical element of the substrate is: C: 0.060%-0.100%, Mn: 1.50-2.15%, Al: 0.020-0.080%, 0<Cr≤0.50%, Mo: 0.05-0.30%, Ti: 0.11-0.16%, 0<Nb≤0.050%; the balance is Fe and other inevitable impurities.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The substrate further contains 0<V≤0.20%.

4. The 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 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 90%, and / or the grain size of the granular bainite is less than 5 μm.

6. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The single-side weight of the hot-dip galvanized layer is 20-380g / m 2 .

7. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The volume fraction of the second phase particles is 0.2-0.35%, and / or the volume fraction of the second phase particles with a size not greater than 10 nm accounts for 81-95% of the total volume fraction of the precipitation.

8. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its longitudinal yield strength is ≥780MPa, tensile strength is ≥900MPa, elongation A80 is ≥16%, and hole expansion rate is ≥50%.

9. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its longitudinal yield strength is 780-880MPa, tensile strength is 900-950MPa, elongation A80 is 16-20%, and hole expansion rate is 50-90%.

10. The method for manufacturing a hot-dip galvanized steel sheet according to any one of claims 1 to 9, 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 ≥40°C / s to a coiling temperature of 440-520°C and coiling; 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.

11. The manufacturing method according to claim 10, characterized in that: Heat the slab to 1230-1280°C and keep it at that temperature for 1-3h.

12. The manufacturing method according to claim 10, 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.

13. The manufacturing method according to claim 10, characterized in that: In the annealing step, the temperature of the direct annealing section is 550-660°C, and the temperature of the heating section is 630-720°C; and / or, in the hot-dip galvanizing step, the hot-dip galvanizing temperature is 440-480°C.

14. The manufacturing method according to claim 10, 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%.

15. The manufacturing method according to claim 10, characterized in that: In the hot rolling step, the coiling temperature is cooled at a cooling rate of 40 to 80°C / s; and / or, after the coiling process is completed, the hot coil is placed in a heat preservation pit for slow cooling, and the average cooling rate is controlled to be ≤15°C / h; and / or, the pickling and straightening elongation is 0.2-2%, preferably, the pickling speed is 60-150m / min, the temperature of the last pickling acid tank in the pickling process is 80-90°C, and the iron ion concentration is 30-40g / L.

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