Economical 800 mpa-grade hot-rolled hot-dip galvanized steel sheet and manufacturing method therefor

Abstract

Disclosed in the present invention is an economical 800 MPa-grade hot-rolled hot-dip galvanized steel sheet, comprising a substrate and a hot-dip galvanized layer. The substrate contains Fe and inevitable impurities. In addition, the substrate further contains the following chemical elements in percentage by mass: 0.05-0.10% of C, 1.4-2.0% of Mn, 0.10-0.15% of Ti, and 0.02-0.08% of Al. The substrate does not contain elements Si, Cr, Mo and Ni. Microstructures of the substrate include a complex-phase structure of granular bainite and ferrite, and second-phase particle precipitates; the volume fraction of the second-phase particle precipitates is not less than 0.2%; and the ratio of the volume fraction of the second-phase particle precipitates having a size not greater than 10 nm to the volume fraction of all the second-phase particle precipitates is greater than 80%. Correspondingly, further disclosed in the present invention is a manufacturing method for the economical 800 MPa-grade hot-rolled hot-dip galvanized steel sheet.

Description

An economical 800MPa 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, with its exceptional strength and excellent formability, has become the preferred choice for chassis components such as control arms and torsion beams. Hot-dip galvanized steel is an effective way to improve the corrosion resistance of automotive chassis.

[0003] Chinese patent publication number CN104513930A, published on April 15, 2015, and titled "Ultra-High-Strength Hot-Rolled Complex Phase Steel Plate and Strip with Excellent Bending and Hole Expansion Properties and Method for Manufacturing Same," discloses that the composition, by weight, of ultra-high-strength hot-rolled complex phase steel plate and strip with excellent bending properties is as follows: C 0.07% to 0.14%, Si 0.1% to 0.4%, Mn 1.55% to 2.00%, P ≤ 0.015%, S ≤ 0.004%, Al 0.01% to 0.05%, N ≤ 0.005%, Cr 0.15% to 0.50%, and also includes at least one alloying element of Nb ≤ 0.05% and Ti 0.06% to 0.15%, with the remainder being Fe and unavoidable impurities. This patent focuses on the performance design and manufacturing methods of hot-rolled pickled steel.

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

[0005] Currently, automobile manufacturers have increasingly higher requirements for automobile steel. In addition to requirements for mechanical properties, formability, and corrosion resistance, they also require steel sheets to have lower costs. Based on this, the present invention aims to provide an economical high-strength hot-dip galvanized steel.

[0006] Summary of the Invention

[0007] One of the purposes of the present invention is to provide an economical 800MPa grade hot-rolled base galvanized steel sheet, which has low cost and good formability and strength, and can be used for automotive structural parts and chassis components, as well as other application fields requiring high strength, weight reduction and corrosion resistance.

[0008] To achieve the above object, the present invention provides an economical 800 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:

[0009] C: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, Al: 0.020-0.080%;

[0010] The substrate does not contain Si, Cr, Mo and Ni elements;

[0011] The microstructure of the substrate includes a complex phase 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 not greater than 10 nm accounts for greater than or equal to 80% of the total second-phase particle precipitates.

[0012] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, the mass percentage of each chemical element of the substrate is:

[0013] C: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, Al: 0.020-0.080%; the balance is Fe and other inevitable impurities.

[0014] The economical 800MPa grade hot-dip galvanized steel sheet of the present invention does not contain precious metal elements such as Cr, Mo and Ni in its substrate.

[0015] Existing 800MPa-grade ultra-high-strength steels typically contain some martensite. Therefore, the hot-dip galvanizing process requires the addition of more alloying elements such as Cr and Mo to delay phase transformation, reducing the critical cooling rate and allowing some of the supercooled austenite to transform into martensite after galvanizing. However, the presence of martensite can lead to uneven microstructure hardness and a low hole expansion rate. Therefore, unlike conventional martensitic transformation-strengthened steels, the present invention achieves high strength without the addition of precious alloying elements by designing a microstructure composed of granular bainite, second-phase particle precipitates, and a small amount of ferrite.

[0016] The strength of bainite structure is insufficient, and strong precipitation strengthening is required to improve the strength. However, during the hot rolling and hot-dip galvanizing process, the second phase particles are easily coarsened after precipitation, resulting in a weakening of the precipitation effect. Generally speaking, a certain amount of Mo element can be added to reduce the coarsening tendency. However, Mo element is very expensive. In order to further reduce costs, the present invention does not add Mo element. Therefore, by strictly controlling the hot rolling, annealing and hot-dip galvanizing processes, the purpose of increasing the volume fraction of the second phase and reducing its coarsening tendency is achieved, thereby obtaining the organizational characteristics of the volume fraction of the second phase particle precipitates being not less than 0.2%, and the volume fraction of the second phase particle precipitates with a size not greater than 10nm accounting for greater than or equal to 80% of the total second phase particle precipitates, so as to achieve the technical effect of the present invention.

[0017] From the perspective of microstructure design, ferrite is relatively soft and has low strength. When the ferrite content is high, microcracks are easily formed at the interface between ferrite and bainite under external forces, reducing the hole expansion rate. Therefore, the microstructure of the present invention is mainly granular bainite.

[0018] 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 the hole expansion rate and strength be improved at the same time. In the present invention, when the volume fraction of the second-phase particles is less than 0.2%, the yield strength of the steel plate will be less than 680MPa. Second-phase particle precipitates are precipitated in all stages of cooling. The particle size precipitated in the high-temperature rolling stage of the hot rolling process is relatively large, generally more than 20nm, and the strengthening effect is relatively weak. The second-phase particles precipitated during the cooling and annealing process after hot rolling have a smaller size (less than 10nm) and have a significant strengthening effect. For the present invention, the yield strength of the steel plate can reach more than 680MPa only when the volume fraction of the second-phase particle precipitates with a size not greater than 10nm accounts for more than or equal to 80% of the total volume fraction of the second-phase particle precipitates.

[0019] From the perspective of composition design, the design principles of the chemical elements in the present invention are as follows:

[0020] C: The carbon content largely determines the tensile strength of the steel plate. Carbon stabilizes austenite and promotes the formation of bainite. Carbon and microalloying elements form sufficient precipitation-strengthening phases to ensure the steel's strength. However, increasing the mass percentage of carbon degrades formability and weldability. Therefore, the present invention limits the mass percentage of carbon to 0.050-0.100%.

[0021] Mn: Manganese is a solid solution strengthening element that improves hardenability, delays pearlite transformation, lowers the bainite transformation temperature, and refines the bainite structure. In the present invention, a lower manganese mass percentage results in insufficient strength, while a higher manganese mass percentage reduces the steel plate's plasticity and can easily lead to segregation. Therefore, the present invention controls the Mn mass percentage to 1.400-2.000%.

[0022] Ti: In the present invention, titanium is an important precipitation-strengthening and grain-refining strengthening element. Especially during hot-dip galvanizing annealing, Ti precipitates as TiC, increasing the strength of the ferrite matrix and contributing to improved yield strength and elongation. Therefore, the mass percentage of Ti in the present invention is controlled to 0.100-0.150%.

[0023] Al: Al is a deoxidizing element in steel, reducing oxide inclusions and purifying the steel, which helps improve the formability of the steel plate. However, a high percentage of aluminum can cause oxidation, further impacting continuous casting production. Therefore, the present invention controls the Al percentage to 0.020-0.080%.

[0024] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, the substrate further contains at least one of the following chemical elements:

[0025] 0<Nb≤0.030%;

[0026] 0 <B≤0.003%。

[0027] Adding Nb and B can further optimize the performance of the hot-dip galvanized steel sheet of the present invention.

[0028] Nb: Niobium is one of the most important elements for precipitation strengthening and grain refinement. However, when the mass percentage of Nb exceeds 0.030%, the added value of Nb weakens and the cost is high. Therefore, the present invention controls the mass percentage of Nb to Nb ≤ 0.030%.

[0029] Boron (B): Boron helps expand the bainite phase, ensuring that the steel plate forms a bainite structure during post-rolling cooling, significantly improving the steel's strength and hardness. However, excessive B can lead to excessive martensite in the steel plate, reducing the steel's hole expansion and elongation. Therefore, the present invention controls the B content to ≤ 0.003% by weight.

[0030] In the present invention, the inevitable impurities are mainly S, P and N, and it is desired that their contents are as low as possible when technical conditions permit.

[0031] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, among the inevitable impurities of the substrate, P≤0.02%, N≤0.005%, and S≤0.005% can be controlled.

[0032] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, the volume ratio of the granular bainite is not less than 95%.Except for the granular bainite and the second phase particle precipitates, the rest is ferrite.

[0033] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, the volume fraction of the second phase particle precipitates is 0.2 to 0.35%.

[0034] Furthermore, in the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention, the volume fraction of second phase particle precipitates with a size of no more than 10 nm accounts for 80 to 95% of all second phase particle precipitates.

[0035] Furthermore, in the economical 800 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 3.0 to 4.8 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 controlled to be less than 5μm.

[0037] Furthermore, in the economical 800MPa grade hot-dip galvanized steel sheet described in the present invention, its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15%, and hole expansion ratio is ≥50%.

[0038] In some embodiments, the yield strength of the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention is ≥700 MPa or ≥730 MPa. In some embodiments, the yield strength of the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention is 680-800 MPa, such as 700-800 MPa.

[0039] In some embodiments, the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention has a tensile strength of 800-850 MPa.

[0040] In some embodiments, the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention has an elongation A80 of 15.0 to 19.5%.

[0041] In some embodiments, the economical 800 MPa grade hot-dip galvanized steel sheet of the present invention has a hole expansion ratio of 50.0 to 80.0%.

[0042] In addition, another object of the present invention is to provide an economical method for manufacturing 800MPa grade hot-dip galvanized steel sheets. 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 low-cost hot-dip galvanized steel sheets with good formability and strength.

[0043] In order to achieve the above object, the present invention proposes a method for manufacturing the above economical 800MPa grade hot-dip galvanized steel sheet, which comprises the 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 ≥50°C / s to a coiling temperature of 450-500°C and coiling;

[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 direct annealing section temperature controlled at 550-630°C, the heating section temperature at 630-720°C, the soaking section temperature at 630-720°C, the soaking section holding time at 50-100s, 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 some embodiments, the slab is produced by smelting and casting, and the thickness of the slab is preferably 220 to 250 mm.

[0050] 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 no phase transformation occurs during cooling. Therefore, the cooling rate after the slow cooling stage has no impact on performance. A cooling rate of ≥5°C / s is required to ensure normal operation of conventional units.

[0051] The manufacturing method of the present invention controls the cooling and annealing heating processes after hot rolling, so that the precipitated second phase particles have a smaller size (less than 10 nm), and have a significant strengthening effect.

[0052] In the present invention, the cooling rate after rolling is controlled to be ≥50°C / s in order to allow the hot-rolled steel plate to quickly enter the bainite transformation zone and obtain a fine bainite structure. When the cooling rate is less than 50°C / s, more ferrite or pearlite will be generated during the cooling process. In some embodiments, the cooling rate after rolling is 50-80°C / s. In addition, the coiling temperature after cooling is controlled to be 450-500°C 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 larger. At the same time, a large number of second phase particles will precipitate and coarsen during the cooling process after coiling, resulting in a decrease in strength after hot-dip galvanizing annealing. When the coiling temperature is low, martensite will be formed, resulting in a lower hole expansion ratio.

[0053] Furthermore, the present invention controls the direct-fire section temperature to 550-630°C because, when the direct-fire section temperature exceeds 630°C, the heating rate decreases, the second-phase particles tend to coarsen, and the strengthening effect is weakened, resulting in insufficient yield strength. If the direct-fire section temperature is lower than 550°C, the subsequent radiant tube heating load increases, making it difficult to stably control the soaking temperature, which is not conducive to obtaining a steel coil with uniform performance from the beginning, middle, and end.

[0054] In the present invention, the annealing temperature in the heating and soaking sections is 630-720°C. When the heating and soaking section temperatures are less than 630°C, the volume fraction of the second phase precipitated during the soaking process is low, resulting in lower strength. When the heating and soaking section temperatures exceed 720°C, martensite is generated during the cooling process, resulting in lower hole expansion and lower yield strength.

[0055] Furthermore, the annealing soaking period is set at 50-100 seconds because, if the soaking period is less than 50 seconds, the internal stresses in the steel plate cannot be completely eliminated during hot rolling, resulting in poor surface condition and difficulty in achieving a good hot-dip galvanized surface. However, if the soaking period exceeds 100 seconds, the second-phase particles coarsen and dissolve significantly, resulting in a decrease in strength and an inability to maintain the yield strength of the steel plate.

[0056] In the present invention, the temperature of the slow cooling section is controlled to be 550-670° C. because: when the temperature of the slow cooling section is high, the second phase particles precipitated during the heating process will gradually coarsen, resulting in a decrease in yield strength.

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

[0058] In this embodiment, the heating temperature is set to ≥1230°C to dissolve the (Ti, Nb)(C, N) generated during the continuous casting process as much as possible. Microalloys such as solid-solubilized Ti in the austenite precipitate as nanoscale second-phase particles during hot rolling and coiling, particularly during annealing and hot-dip galvanizing, effectively further improving the strength of the steel sheet. However, temperatures exceeding 1280°C cause coarsening of the austenite grains, which is detrimental to the toughness of the steel sheet. Furthermore, the resulting oxide scale becomes thicker, hindering its dephosphorization and ultimately affecting the surface quality of the hot-dip galvanized steel. Therefore, the heating temperature is preferably set between 1230°C and 1280°C.

[0059] 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.

[0060] In the present invention, the rough rolling temperature control in the hot rolling process has a greater impact on microalloys such as Ti. At a lower rough rolling temperature and in the finishing rolling process, Ti carbides and carbonitrides will precipitate. The precipitation size of this process is large, which is not conducive to further improvement of strength. Therefore, the rough rolling start temperature is controlled at 1130-1190°C, and the finishing rolling start temperature is controlled at 980-1080°C.

[0061] The hot rolling exit temperature also affects the microstructure. When the exit temperature is low, the austenite grains form a pancake shape, accumulating a large amount of deformation energy. This helps reduce grain size during the subsequent cooling process and also improves the hole expansion ratio. However, when the exit temperature is too low, ferrite is produced, resulting in a decrease in hole expansion ratio. Excessively high exit temperatures can negatively impact surface quality, so the optimal exit temperature for hot rolling is 840-940°C.

[0062] In some embodiments, the cumulative deformation of rough rolling and finish rolling during the hot rolling process is controlled to be ≥90%.

[0063] In some embodiments, after the coiling process is completed, the hot coil is placed in a holding pit for slow cooling, with an average cooling rate of ≤15°C / h.

[0064] In some embodiments, the elongation of the pickling straightening can be 0.2-2% to further control the plate shape and thus obtain better surface quality; a pickling speed of 60-150 m / min can also be used, the temperature of the last pickling tank in the pickling process is controlled at 80-90°C, and the iron ion concentration is controlled at 30-40 g / L to further obtain better surface quality.

[0065] Furthermore, in the manufacturing method of the present invention, in the hot-dip galvanizing step, the hot-dip galvanizing temperature is 440-480°C.

[0066] The hot-dip galvanizing temperature is 440-480℃ because the bonding between the steel plate surface and the zinc liquid is better within this temperature range. If the temperature is lower or higher than this range, the galvanizing performance of the substrate surface will deteriorate, which may lead to poor surface quality after galvanizing.

[0067] Hot dip galvanizing can be performed using a bath well known in the art. In some embodiments, the weight of the hot dip galvanized layer on one side can be 20-380 g / m 2 .

[0068] 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%.

[0069] In this embodiment, in order to further achieve better surface quality based on the effects of the present invention, the flatness after hot-dip galvanizing is controlled to 0.05-1.3%. If the flatness is less than 0.05%, the surface quality of the zinc layer is poor. If the flatness is greater than 1.3%, significant work hardening will occur, which tends to reduce the elongation of the steel sheet.

[0070] In some embodiments, the hot-dip galvanized sheet of the present invention has a thickness of 1.8-3.5 mm.

[0071] The economical 800MPa grade hot-dip galvanized steel sheet and its manufacturing method of the present invention have the following advantages and beneficial effects:

[0072] The economical 800 MPa grade hot-dip galvanized steel sheet of the present invention does not contain precious metal elements such as Cr, Mo and Ni, and thus has a low cost while still ensuring a strength level of 800 MPa.

[0073] In some embodiments, the economical 800 MPa-grade hot-dip galvanized steel sheet of the present invention has a yield strength of 680 MPa or greater, a tensile strength of 800 MPa or greater, an elongation (A80) of 15%, and a hole expansion ratio of 50% or greater. Therefore, it can be used in automotive chassis and structural components, meeting the technical requirements for flanging and stamping complex automotive parts and lightweighting.

[0074] The economical 800MPa-grade hot-dip galvanized steel sheet described in this invention is free of Si and Cr elements, and exhibits superior surface quality and corrosion resistance compared to similar hot-dip galvanized steel sheets. It should be noted that the absence of Si and Cr in this invention means that no Si and Cr are intentionally added during the smelting process. Because iron ore and scrap steel contain small amounts of Si and Cr, even if trace amounts, for example, less than 0.1%, are present in the finished steel sheet, these are residual elements and not intentionally added by this invention.

[0075] The economical 800MPa hot-dip galvanized steel sheet described in this invention is suitable for conventional hot rolling and hot-dip galvanizing lines. Compared with traditional hot-dip galvanized sheet production methods, this method eliminates the cold rolling process, shortens the process flow, improves production efficiency, and saves energy. DETAILED DESCRIPTION

[0076] The economical 800 MPa grade hot-dip galvanized steel sheet and its manufacturing method described in the present invention will be further explained and illustrated 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.

[0077] The economical 800 MPa grade hot-dip galvanized steel sheets in each embodiment of the present invention are prepared by the following steps:

[0078] (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.

[0079] (2) Heat the slab to 1230-1280℃ and keep it warm for 1-3h.

[0080] (3) Hot rolling: control the starting temperature of rough rolling to 1130-1190℃, the starting temperature of finishing rolling to 980-1080℃, and the outlet temperature of finishing rolling to 840-940℃. The cumulative deformation of rough rolling and finishing rolling during hot rolling can be ≥90%.

[0081] (4) After hot rolling, cool to the coiling temperature of 450-500℃ at a cooling rate of ≥50℃ / s.

[0082] In some embodiments, optionally, after the coiling process is completed, the hot coil can be placed in a heat preservation pit for slow cooling, with an average cooling rate of ≤15°C / h.

[0083] (5) Pickling to obtain pickled coils.

[0084] In some embodiments, the elongation of the pickling straightening can be 0.2-2% to further control the plate shape and thus obtain better surface quality; a pickling speed of 60-150 m / min can also be used, the temperature of the last pickling tank in the pickling process is controlled at 80-90°C, and the iron ion concentration is controlled at 30-40 g / L to further obtain better surface quality.

[0085] (6) Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion-free continuous annealing furnace, wherein the direct annealing section temperature is controlled at 550-630°C, the heating section temperature is 630-720°C, the soaking section temperature is 630-720°C, the soaking section holding time is 50-100s, the slow cooling section temperature is 550-670°C, and after exiting the slow cooling section, it can be cooled at a rate of ≥5°C / s to the hot-dip galvanizing temperature of 440-480°C 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 lists the mass percentages of the chemical elements in the substrates of various embodiments and comparative examples.

[0088] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, and N)

[0089] Note: In Table 1, steel grades AI are steel grades used in the embodiments of the present invention, while JM are steel grades used in the comparative examples.

[0090] Table 2 lists the specific process parameters in the above process steps of various embodiments of the present invention and comparative examples.

[0091] Table 2.

[0092] Samples of the hot-dip galvanized steel sheets obtained in Examples 1-16 and Comparative Examples 1-18 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.

[0093] Table 3.

[0094] In addition, JIS 13B tensile specimens were taken longitudinally for each example and comparative example to measure yield strength and tensile strength. The gauge length was 80 mm. 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 test results are listed in Table 4.

[0095] Table 4.

[0096] As shown in Tables 3 and 4, Examples 1-16 of the present invention achieve ideal structural characteristics and refine the grains 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. Strictly controlling the temperature of each section during the annealing process to avoid coarsening of the second-phase particles is the key to achieving good performance. The longitudinal yield strength ultimately obtained in each embodiment of the present invention is all above 680MPa, the tensile strength is all above 800MPa, the elongation A80 is all above 15%, and the hole expansion rate is all ≥50%.

[0097] Different from the present invention is:

[0098] In Comparative Example 1, since the carbon content and the manganese content are higher than the upper limit designed by the present invention, more martensite is formed during the hot rolling process, resulting in less bainite content and a lower hole expansion rate.

[0099] In Comparative Example 2, since the carbon content and the manganese content are lower than the lower limits designed by the present invention, the strength is insufficient.

[0100] In Comparative Example 3, when B element was added, the B content was higher than the upper limit designed in the present invention, so martensite was generated during the hot rolling process, resulting in a decrease in the hole expansion ratio.

[0101] In Comparative Example 4, since the Ti content is lower than the lower limit designed by the present invention, the amount of second phase particles precipitated is small and the strength is low.

[0102] In Comparative Example 5, since the soaking zone temperature is lower than the lower limit of the present invention, less second phase is precipitated during the hot-dip galvanizing annealing process, and thus the yield strength is reduced.

[0103] In Comparative Example 6, since the temperature of the soaking section is higher than the upper limit of the present invention, austenite is formed during the heating process, and phase changes to martensite during the cooling process, the proportion of bainite decreases, and the yield strength and hole expansion rate of the steel plate are reduced.

[0104] In Comparative Example 7, since the temperature of the heating section is lower than the lower limit of the present invention, less second phase is precipitated during the hot-dip galvanizing annealing process, and the yield strength is reduced.

[0105] In Comparative Example 8, since the temperature of the direct fire section and the heating section is higher than the upper limit of the present invention, austenite is formed during the heating process, and phase transformation is performed into martensite during the cooling process. The proportion of bainite is reduced, and the yield strength and hole expansion rate of the steel plate are reduced.

[0106] In Comparative Example 9, since the soaking period time was higher than the upper limit of the present invention, the second phase was coarsened and the yield strength was reduced.

[0107] In Comparative Example 10, since the temperature of the slow cooling section is higher than the upper limit of the present invention, the second phase is coarsened and the yield strength is reduced.

[0108] In Comparative Example 11, since the coiling temperature is lower than the lower limit of the present invention, martensite is generated after hot rolling, resulting in a decrease in the bainite content and a lower hole expansion ratio.

[0109] In Comparative Example 12, since the coiling temperature is higher than the upper limit of the present invention, more ferrite is formed during the hot rolling process, accompanied by a large amount of precipitation, resulting in a decrease in yield strength.

[0110] In Comparative Example 13, since the cooling rate after hot rolling is lower than the lower limit of the present invention, more ferrite is formed, accompanied by a large amount of precipitation, resulting in a decrease in yield strength.

[0111] 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.

[0112] 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 ≥800 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: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, Al: 0.020-0.080%; The substrate does not contain Si, Cr, Mo and Ni elements; The microstructure of the substrate includes a composite structure of granular bainite + ferrite and second phase particle precipitates, the volume fraction of the second phase particle precipitates is not less than 0.2%, and the volume fraction of the second phase particle precipitates with a size of not more than 10 nm accounts for more than or equal to 80% of the total second phase particle precipitates.

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: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, Al: 0.020-0.080%; 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 at least one of the following chemical elements: 0<Nb≤0.030%; 0<B≤0.003%.

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 95%, 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 20-380 g / 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 particle precipitates is 0.2-0.35%, and / or the volume fraction ratio of the second phase particle precipitates with a size not greater than 10 nm to the total second phase particle precipitates is 80-95%.

8. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: Its yield strength is ≥680MPa, tensile strength is ≥800MPa, elongation A80 is ≥15.0%, and hole expansion rate is ≥50.0%.

9. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that: The hot-dip galvanized steel sheet has a yield strength of 680-800 MPa, a tensile strength of 800-850 MPa, an elongation A80 of 15.0-19.5%, and a hole expansion rate of 50.0-80.0%.

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: (1) obtaining a slab; (2) heating and keeping the slab warm; (3) hot rolling, followed by cooling to a coiling temperature of 450-500°C at a cooling rate of ≥50°C / s and coiling; (4) pickling to obtain a pickled coil; (5) Annealing and hot-dip galvanizing: The pickled coils are directly annealed in a combustion non-oxidizing continuous annealing furnace, where the direct annealing section temperature is controlled at 550-630°C, the heating section temperature is 630-720°C, the soaking section temperature is 630-720°C, the soaking section holding time is 50-100s, the slow cooling section temperature is 550-670°C, and after leaving the slow cooling section, it is 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 rough rolling is controlled to be 1130-1190°C, the starting temperature of finishing rolling is controlled to be 980-1080°C, and the finishing outlet temperature is controlled to be 840-940°C; preferably, the cumulative deformation of rough rolling and finishing rolling is ≥90%.

13. The manufacturing method according to claim 10, characterized in that: 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: After the coiling of step (3) is completed, the hot coil is placed in an insulation pit for slow cooling, and the average cooling rate is controlled to be ≤15°C / h; and / or the elongation of pickling and straightening is 0.2-2%; and / or a pickling speed of 60-150m / min is adopted, and the temperature of the last pickling acid tank in the pickling process is controlled to 80-90°C and the iron ion concentration is controlled to 30-40g / L.

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