Economical 800MPa class hot-rolled hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

The present invention discloses an economical 800 MPa class hot-rolled hot-dip galvanized steel sheet comprising a substrate containing Fe and unavoidable impurities, and a hot-dip galvanized layer, wherein the substrate further contains the following chemical elements in the following mass percentages: C: 0.05~0.10%, Mn: 1.4~2.0%, Ti: 0.1~0.15%, Al: 0.02~0.08%; the substrate does not contain Si, Cr, Mo, and Ni elements; the microstructure of the substrate comprises a granular bainite + ferrite biphase structure and second-phase particle precipitates, the volume fraction of the second-phase particle precipitates being 0.2% or more, and the volume fraction of second-phase particle precipitates with a size of 10 nm or less in relation to the total second-phase particle precipitates being more than 80%. Accordingly, the present invention further discloses a method for manufacturing the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and particularly to a hot-dip galvanized steel sheet and a method for manufacturing the same.

Background Art

[0002] With the development of lightweight technologies for automobiles, the proportion of advanced high-strength steel in automotive structural parts has been increasing. Dual-phase steel has become the first choice for chassis parts such as control arms and torsion beams due to its ultra-high strength and excellent formability. However, hot-rolled galvanized steel sheet is one of the effective means to improve the corrosion resistance of automotive chassis.

[0003] In a Chinese patent document with publication number CN104513930A, publication date April 15, 2015, and title "Ultra-high strength hot-rolled dual-phase steel sheet and strip excellent in bending performance and hole expansion performance and method for manufacturing the same", an ultra-high strength hot-rolled dual-phase steel sheet and strip excellent in bending performance are disclosed. Their composition is in weight percentages: 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 further contains at least one alloy element among Nb ≤ 0.05%, Ti: 0.06 - 0.15%, and the balance is Fe and inevitable impurities. This patent focuses on the performance design and manufacturing method of hot-rolled pickled sheets.

[0004] A Chinese patent document with publication number CN113215485A, published on August 6, 2021, and titled "780MPa Class Hot-Rolled Plated Duplex Steel and Method for Manufacturing the Same" discloses that the 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%, with the remainder being Fe and unavoidable impurities. The Si content is relatively high in this patent.

[0005] Currently, automobile manufacturers are placing increasingly high demands on automotive steel, not only regarding mechanical performance, formability, and corrosion resistance, but also on reducing the cost of steel sheets. Therefore, this invention aims to provide an economical, high-strength hot-rolled galvanized steel. [Overview of the project] [Problems that the invention aims to solve]

[0006] One objective of the present invention is to provide an economical 800MPa class hot-rolled hot-dip galvanized steel sheet, which has low cost, good formability and strength, and can be used for automotive structural parts and chassis parts, as well as in other application fields where high strength, lightweight, corrosion resistance, etc., are required. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides an economical 800 MPa class hot-rolled hot-dip galvanized steel sheet comprising a substrate containing Fe and unavoidable impurities, and a hot-dip galvanized layer, wherein the substrate further contains the following chemical elements in the following mass percentages: It contains C: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, and Al: 0.020-0.080%; The aforementioned substrate does not contain the elements Si, Cr, Mo, and Ni; The microstructure of the substrate comprises a granular bainite + ferrite multiphase structure and second-phase particle precipitates, the volume fraction of the second-phase particle precipitates being 0.2% or more, and the volume fraction of second-phase particle precipitates with a size of 10 nm or less being 80% or more of the total second-phase particle precipitates.

[0008] Furthermore, in the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention, the mass percentage content of each chemical element in the substrate is: The composition is C: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, Al: 0.020-0.080%; the remainder is Fe and other unavoidable impurities.

[0009] In the economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention, the substrate does not contain noble metal elements such as Cr, Mo, and Ni.

[0010] Conventional 800MPa class ultra-high-strength steels typically contain a partial martensitic structure. Therefore, the hot-dip galvanizing process requires the addition of phase transformation-delaying alloying elements such as Cr and Mo to reduce the critical cooling rate, thereby delaying the phase transformation of some supercooled austenite to martensite after galvanizing. However, the presence of martensite leads to non-uniformity of microstructural hardness, resulting in a decrease in hole expansion rate. Thus, unlike conventional martensitic transformation-strengthened steels, the present invention designs the microstructure as granular bainite + second-phase particle precipitates + a small amount of ferrite in order to obtain high strength without adding expensive alloying elements.

[0011] Bainite structures lack strength, and strong precipitation strengthening is necessary to improve their strength. However, during the hot rolling, coiling, and hot-dip galvanizing processes, the second phase particles tend to coarse after precipitation, weakening the precipitation effect. Normally, this coarsening tendency can be suppressed by adding a predetermined amount of Mo. However, since Mo is very expensive, the present invention aims to further reduce costs by strictly controlling the hot rolling, annealing, and hot-dip galvanizing processes without adding Mo, thereby increasing the volume fraction of the second phase while reducing its coarsening tendency. As a result, the microstructure characteristics are obtained in which the volume fraction of second phase particle precipitates is 0.2% or more, and the volume fraction of second phase particle precipitates with a size of 10 nm or less accounts for 80% or more of the total second phase particle precipitates, thus realizing the technical effects of the present invention.

[0012] 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 force, and the hole expansion rate decreases. Therefore, in this invention, the microstructure is mainly granular bainite.

[0013] Furthermore, in this invention, the precipitation of second-phase particles is a crucial strengthening mechanism. The precipitated second-phase particles can simultaneously improve hole expansion and strength only if they have a sufficient volume fraction and are fine in size. In this invention, if the volume fraction of second-phase particles falls below 0.2%, the yield strength of the steel sheet will be less than 680 MPa. Second-phase particle precipitates occur at any stage of cooling, but the particles precipitated during the high-temperature rolling stage in the hot rolling process are generally large (20 nm or more in size) and have a weak strengthening effect. Second-phase particles precipitated during the cooling and annealing heating processes after hot rolling have a small size (less than 10 nm) and have a significant strengthening effect. For this invention, the yield strength of the steel sheet can reach 680 MPa or more only if the volume fraction of second-phase particle precipitates with a size of 10 nm or less accounts for 80% or more of the total volume fraction of second-phase particle precipitates.

[0014] From the perspective of compositional design, the design principles for each chemical element in this invention are as follows.

[0015] The carbon (C) content significantly affects the tensile strength level of steel sheets. Carbon can stabilize austenite and promote bainite formation. Carbon ensures the strength of steel by forming a sufficient precipitation strengthening phase with trace alloying elements. However, a high mass percentage of carbon leads to deterioration of formability and weldability. Therefore, in this invention, the mass percentage of C is controlled to 0.050-0.100%.

[0016] Mn: Manganese is a solid solution strengthening element that improves hardenability, delays the phase transformation of pearlite, lowers the bainite transformation temperature, and refines the bainite structure. In this invention, if the mass percentage of manganese is too low, it leads to insufficient strength, but if the mass percentage of manganese is too high, the plasticity of the steel sheet decreases and segregation becomes more likely. Therefore, in this invention, the mass percentage of Mn is controlled to 1.400 to 2.000%.

[0017] Ti: In this invention, titanium is an important precipitation strengthening and fine-grain strengthening element. In particular, during the annealing process of hot-dip galvanizing, Ti precipitates in the form of TiC, improving the strength of the ferrite matrix and contributing to improvements in yield ratio and elongation. Therefore, in this invention, the mass percentage of Ti is controlled to Ti:0.100~0.150%.

[0018] Al: Al is a deoxidizing element in steel, and it can purify the steel by reducing the presence of oxides, which is advantageous in improving the formability of steel sheets. However, if the mass percentage of aluminum is high, oxidation occurs, which further affects continuous casting production. Therefore, in this invention, the mass percentage of Al is controlled to 0.020 to 0.080%.

[0019] Furthermore, in the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention, the substrate further contains at least one of the following chemical elements: 0 <Nb≦0.030%; 0 < B ≤ 0.003%.

[0020] By adding Nb and B, the performance of the hot-dip galvanized steel sheet according to the present invention can be further optimized. However,

[0021] Nb: Niobium is one of the important precipitation strengthening and grain refinement strengthening elements. However, when the mass percentage of Nb exceeds 0.030%, the increase in the strengthening effect by Nb weakens, and the cost increases. Therefore, in the present invention, the mass percentage of Nb is controlled to be Nb ≤ 0.030%.

[0022] B: Boron contributes to the expansion of the bainite phase region, ensuring that the steel sheet obtains a bainite structure during the cooling process after rolling, and bringing a significant effect on the improvement of the strength and hardness of the steel. However, when the B element is excessive, martensite structure occurs excessively in the steel sheet, leading to a decrease in the hole expansion rate and elongation rate of the steel. Therefore, in the present invention, the mass percentage of B is controlled to be B ≤ 0.003%.

[0023] The inevitable impurities in the present invention are mainly S, P, and N. As long as it is technically acceptable, the lower their contents, the better.

[0024] Furthermore, in the economic type 800 MPa grade hot-rolled hot-dip galvanized steel sheet according to the present invention, among the inevitable impurities of the substrate, it may be controlled such that P ≤ 0.02%, N ≤ 0.005%, and S ≤ 0.005%.

[0025] Furthermore, in the economic type hot-rolled hot-dip galvanized steel sheet of 800 MPa grade according to the present invention, the volume ratio of the granular bainite is 95% or more. Excluding the granular bainite and the second-phase particle precipitates, the balance is ferrite.

[0026] Furthermore, in the economic type hot-rolled hot-dip galvanized steel sheet of 800 MPa grade according to the present invention, the volume fraction of the second-phase particle precipitates is 0.2 - 0.35%.

[0027] Furthermore, in the economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention, the volume fraction of second-phase particle precipitates with a size of 10 nm or less to the total second-phase particle precipitates is 80-95%.

[0028] Furthermore, in the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention, the crystal grain size of the granular bainite is less than 5 μm, for example, 3.0 to 4.8 μm.

[0029] Generally, yield strength decreases as the grain size increases. In this invention, because the ferrite content is low, the grain size of bainite has a significant impact on yield strength. However, in order to realize the technical effects of this invention, the grain size of granular bainite is controlled to less than 5 μm.

[0030] Furthermore, the economical 800MPa class hot-rolled galvanized steel sheet according to the present invention has a yield strength of ≥680MPa, a tensile strength of ≥800MPa, an elongation ratio A80 of ≥15%, and a hole expansion ratio of ≥50%.

[0031] In some embodiments, the yield strength of the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention is ≥700 MPa or ≥730 MPa. In some embodiments, the yield strength of the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention is 680 to 800 MPa, for example, 700 to 800 MPa.

[0032] In some embodiments, the tensile strength of the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention is 800 to 850 MPa.

[0033] In some embodiments, the elongation A80 of the economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention is 15.0 to 19.5%.

[0034] In some embodiments, the hole expansion ratio of the economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention is 50.0 to 80.0%.

[0035] Another objective of the present invention is to provide a method for manufacturing an economical 800 MPa class hot-rolled hot-dip galvanized steel sheet. This method, when combined with the composition design of the present invention, can acquire the required microstructure characteristics of the present invention, thereby providing a hot-dip galvanized steel sheet with good formability and strength at a low cost.

[0036] To achieve the above objective, the method for manufacturing the economical 800 MPa class hot-rolled hot-dip galvanized steel sheet provided by the present invention includes the following steps: To manufacture slabs; Heat and maintain the temperature of the slab; After hot rolling, the material is cooled to a winding temperature of 450-500°C at a cooling rate of ≥50°C / s and then wound into a coil. Pickling is performed to obtain a pickled coil; Annealing and Hot-Dip Galvanizing: Pickled coils are annealed in a direct-burn, oxidation-free continuous annealing furnace, however, the direct-fire section temperature is controlled to 550-630°C, the heating section temperature to 630-720°C, the soaking section temperature to 630-720°C, the soaking section holding time to 50-100 s, the slow-cooling section temperature to 550-670°C, and after exiting the slow-cooling section, the coils are cooled to the hot-dip galvanizing temperature.

[0037] In some embodiments, the slab is manufactured by smelting and casting. The thickness of the slab is preferably 220 to 250 mm.

[0038] Furthermore, in this invention, after exiting the slow-cooling section, the material may be cooled to the hot-dip galvanizing temperature at a cooling rate of ≥5°C / s. Since this invention does not form austenite during the soaking process and does not undergo phase transformation during the cooling process, the cooling rate after exiting the slow-cooling section does not affect performance, and therefore, in this invention, this cooling rate is not particularly limited. The reason for setting the rate to ≥5°C / s is to ensure the cooling rate when the machine group is operating normally.

[0039] In the manufacturing method according to the present invention, by controlling the cooling process after hot rolling and the heating process in the annealing step, the precipitated second-phase particle precipitate has a small size (less than 10 nm) and exhibits a significant strengthening effect.

[0040] In this invention, the cooling rate after rolling is controlled to ≥50°C / s in order to quickly bring the hot-rolled steel sheet into the bainite transformation region and obtain a fine bainite structure. If the cooling rate is below 50°C / s, a large amount of ferrite and pearlite will be generated during the cooling process. In some embodiments, the cooling rate after rolling is set to 50-80°C / s. Furthermore, the coiling temperature after cooling is controlled to 450-500°C in order to obtain a granular bainite structure with fine M / A islands. If the coiling temperature is high, pearlite is more likely to form, the grain size will increase, and at the same time, a large amount of second-phase particles will precipitate and coarseen during the cooling process after coiling, reducing the strength of the hot-dip galvanized steel after annealing. If the coiling temperature is low, martensite will be formed, leading to a decrease in the hole expansion rate.

[0041] Furthermore, in this invention, the reason for controlling the direct-fire section temperature to 550-630°C is that if the direct-fire section temperature exceeds 630°C, the heating rate decreases, the second-phase particle precipitates tend to coarseen, the strengthening effect weakens, and the yield strength becomes insufficient. However, if the direct-fire section temperature falls below 550°C, the heating load on the subsequent radiating tube increases, making it difficult to stably control the uniform heating temperature, which is disadvantageous for obtaining a steel coil with uniform performance at the tip, middle, and end.

[0042] In this invention, both the heating section temperature and the soaking section temperature during annealing are 630 to 720°C. If the heating section temperature and soaking section temperature fall below 630°C, the volume fraction of the second phase precipitate decreases during the soaking process, leading to a decrease in strength. If the heating section temperature and soaking section temperature exceed 720°C, martensite is formed during the cooling process, leading to a decrease in hole expansion and a decrease in yield strength.

[0043] Furthermore, the reason for setting the annealing soaking section warming time to 50-100 seconds is that if the annealing soaking section warming time is less than 50 seconds, the internal stresses of the steel sheet cannot be completely eliminated during the hot rolling process, resulting in a poor surface condition and making it difficult to obtain a good hot-dip galvanized surface. However, if the soaking warming time exceeds 100 seconds, the second phase particles become coarser and melt in large quantities, reducing the strength and making it impossible to ensure the yield strength of the steel sheet.

[0044] In this invention, the reason for controlling the slow-cooling section temperature to 550-670°C is that as the slow-cooling section temperature increases, the second phase particles precipitated during the heating process gradually coarseen, thus reducing the yield strength.

[0045] Furthermore, in the manufacturing method according to the present invention, the slab is heated to 1230-1280°C and kept warm for 1-3 hours.

[0046] In this embodiment, the purpose of setting the heating temperature to ≥1230°C is to dissolve as much (Ti,Nb)(C,N) as possible that is generated during the continuous casting process. Trace alloys such as Ti dissolved in austenite precipitate as nanoscale second-phase particles during hot rolling and winding, particularly during annealing and hot-dip galvanizing, effectively improving the strength of the steel sheet. On the other hand, if the temperature exceeds 1280°C, the austenite grains become coarser, which is unfavorable for the toughness of the steel sheet, and the oxide scale becomes thicker, which is unfavorable for dephosphorization from the oxide scale, ultimately affecting the surface quality of the hot-dip galvanized steel. Therefore, it is preferable to set the heating temperature to 1230~1280°C.

[0047] Furthermore, in the manufacturing method according to the present invention, in the hot rolling process, the rough rolling start temperature is controlled to 1130-1190°C, the finish rolling start temperature is controlled to 980-1080°C, and the finish rolling exit temperature is controlled to 840-940°C.

[0048] In this invention, controlling the rough rolling temperature during the hot rolling process has a significant impact on trace alloys such as Ti. Ti carbides and carbonitrides precipitate at low rough rolling temperatures and during the finish rolling process. However, the size of these precipitates increases during this process, which is detrimental to further improving strength. Therefore, the starting temperature for rough rolling is controlled to 1130-1190°C, and the starting temperature for finish rolling is controlled to 980-1080°C.

[0049] Furthermore, the finish rolling exit temperature during the hot rolling process also affects the microstructure. When the finish rolling exit temperature is low, the austenite grains become cake-like, and a large amount of deformation energy is accumulated, which contributes to grain refinement during the subsequent cooling process and also contributes to an improvement in hole expansion rate. However, if the finish rolling exit temperature is too low, ferrite is generated, leading to a decrease in hole expansion rate. If the finish rolling exit temperature is too high, it is unfavorable for surface quality, so preferably, the finish rolling exit temperature during hot rolling is controlled to 840-940°C.

[0050] In some embodiments, the cumulative deformation of rough rolling and finish rolling during the hot rolling process is controlled to be ≥90%.

[0051] In some embodiments, after the winding process is completed, the hot-rolled coil is placed in a heat-insulating pit to cool slowly, and the average cooling rate is set to ≤15°C / h.

[0052] In some embodiments, the pickling tension-leveling elongation may be 0.2-2% to further control the plate shape and obtain better surface quality; to obtain even better surface quality, a pickling rate of 60-150 m / min may be employed, the temperature of the final pickling bath in the pickling process may be controlled to 80-90°C, and the iron ion concentration may be controlled to 30-40 g / L.

[0053] Furthermore, in the manufacturing method according to the present invention, the hot-dip galvanizing temperature in the hot-dip galvanizing step is 440 to 480°C.

[0054] The reason for setting the hot-dip galvanizing temperature to 440-480°C is that the bonding between the steel plate surface and the zinc solution is good within this temperature range. If the temperature is below or above this range, the suitability of the substrate surface for plating decreases, which may result in surface defects after plating.

[0055] Hot-dip galvanizing may be performed using a plating solution well known in this field. In some embodiments, the weight of one side of the hot-dip galvanized layer is 20 to 380 g / m². 2 That's fine.

[0056] Furthermore, in the manufacturing method according to the present invention, a leveling step is further included after the hot-dip galvanizing step, and the leveling rate is controlled to 0.05 to 1.3%.

[0057] In this embodiment, in order to obtain even better surface quality in terms of the effects of implementing the present invention, the leveling rate after hot-dip galvanizing is controlled to 0.05 to 1.3%. If the leveling rate is below 0.05%, the surface quality of the zinc layer will be poor, but if the leveling rate is above 1.3%, significant work hardening will occur, which tends to reduce the elongation rate of the steel sheet.

[0058] In some embodiments, the thickness of the hot-dip galvanized sheet according to the present invention is 1.8 to 3.5 mm. [Effects of the Invention]

[0059] The economical 800 MPa class hot-rolled hot-dip galvanized steel sheet and its manufacturing method according to the present invention have the following advantages and beneficial effects:

[0060] The economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention does not contain precious metal elements such as Cr, Mo, and Ni, thus having a low cost while simultaneously ensuring a strength level of 800MPa.

[0061] In some embodiments, the economical 800 MPa class hot-rolled galvanized steel sheet according to the present invention has a yield strength of ≥680 MPa, a tensile strength of ≥800 MPa, an elongation A80 of ≥15%, and a hole expansion ratio of ≥50%. Therefore, it can be used as an automobile chassis and structural component, meeting the technical requirements for flanging, press working, and automobile weight reduction of complex automobile parts.

[0062] The economical 800MPa class hot-rolled galvanized steel sheet according to the present invention does not contain Si and Cr elements and has superior surface quality and corrosion resistance compared to similar galvanized steel sheets. It is necessary to explain that the absence of Si and Cr elements in the present invention means that there is no need to intentionally add them during the smelting process. Since iron ore and scrap steel contain trace amounts of Si and Cr, even if trace amounts (e.g., less than 0.1%) of Si and Cr are present in the finished steel sheet, these are residual elements and not intentionally added by the present invention.

[0063] The economical 800MPa class hot-rolled hot-dip galvanized steel sheet according to the present invention can be applied to conventional hot continuous rolling production lines and hot-dip galvanizing production lines. Compared to conventional methods for producing hot-dip galvanized steel sheets, the present invention eliminates the cold rolling process, shortens the process, improves production efficiency, and saves energy. [Modes for carrying out the invention]

[0064] The following interpretation and explanation will be based on specific examples to further describe the economical 800MPa class hot-rolled hot-dip galvanized steel sheet and its manufacturing method according to the present invention, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0065] Each of the economical 800 MPa class hot-rolled hot-dip galvanized steel sheets according to the embodiments of the present invention was manufactured by the following process:

[0066] (1) Slabs were manufactured by smelting and casting, with a slab thickness of 220 mm to 250 mm. The mass percentage content of each chemical element in the slabs for each example and comparative example of the present invention is shown in Table 1.

[0067] (2) The slab was heated to 1230-1280°C and kept warm for 1-3 hours.

[0068] (3) Hot rolling: The rough rolling start temperature was controlled to 1130-1190°C, the finish rolling start temperature to 980-1080°C, and the finish rolling exit temperature to 840-940°C, and the cumulative deformation amount of both rough rolling and finish rolling during the hot rolling process was set to ≥90%.

[0069] (4) After hot rolling, the material was cooled to a coiling temperature of 450-500°C at a cooling rate of ≥50°C / s.

[0070] In some embodiments, after the winding process is completed, the hot-rolled coil may be selectively placed in a heat-insulating pit for slow cooling, and the average cooling rate may be ≤15°C / h.

[0071] (5) Pickling was performed to obtain pickled coils.

[0072] In some embodiments, the pickling tension-leveling elongation may be 0.2-2% to further control the plate shape and obtain better surface quality; to obtain even better surface quality, a pickling rate of 60-150 m / min may be employed, the temperature of the final pickling bath in the pickling process may be controlled to 80-90°C, and the iron ion concentration may be controlled to 30-40 g / L.

[0073] (6) Annealing and Hot-Dip Galvanizing: Pickled coils are annealed directly in a combustion, oxidation-free continuous annealing furnace, provided that the direct-fire section temperature is controlled to 550-630°C, the heating section temperature to 630-720°C, the soaking section temperature to 630-720°C, the soaking section holding time to 50-100 s, the slow-cooling section temperature to 550-670°C, and after exiting the slow-cooling section, the coils are cooled to a hot-dip galvanizing temperature of 440-480°C at a cooling rate of ≥5°C / s to perform hot-dip galvanizing, with a single-sided weight of 20-380 g / m². 2 That would have been fine too.

[0074] (7) Leveling: The leveling rate was controlled to 0.05 to 1.3%, and hot-dip galvanized sheets with a thickness of 1.8 to 3.5 mm were obtained.

[0075] The mass percentage composition of each chemical element in the substrates for each example and comparative example is shown in Table 1.

[0076] [Table 1]

[0077] Table 2 shows the specific process parameters for each example and comparative example of the present invention in the process steps described above.

[0078] [Table 2]

[0079] Samples were taken from the hot-dip galvanized steel sheets obtained in Examples 1-16 and Comparative Examples 1-18. The microstructure was observed using a combination of optical and scanning electron microscopy, and the grain size was measured using the equivalent area method. The size distribution and volume fraction of the second phase precipitate were measured using the carbon extraction replica method, and the measurement results are shown in Table 3.

[0080] [Table 3]

[0081] Furthermore, JIS 13B tensile test specimens were taken from the samples of each example and comparative example along the longitudinal direction, and the yield strength and tensile strength were measured. The scale distance was set to 80 mm, and the mechanical performance was measured in accordance with the GB / T228.1-2010 standard. Hole expansion ratio was measured in accordance with the GB / T24524-2021 standard, and the measurement results are shown in Table 4.

[0082] [Table 4]

[0083] As shown in Tables 3 and 4, Examples 1 to 16 of the present invention achieved ideal microstructure characteristics by combining a rational chemical element composition design with an optimized hot rolling process and annealing / hot-dip galvanizing process. This refined the crystal grains and promoted the large-scale precipitation of second-phase particles through the heating process of annealing / hot-dip galvanizing, thereby improving strength without reducing plasticity. Strict control of the temperature of each section in the annealing process to avoid coarsening of the second-phase particles was key to achieving good performance. In each example of the present invention, the longitudinal yield strength was 680 MPa or higher, the tensile strength was 800 MPa or higher, the elongation A80 was 15% or higher, and the hole expansion ratio was ≥50%.

[0084] Unlike the present invention, In Comparative Example 1, the carbon and manganese content exceeded the upper limits designed in the present invention, resulting in the formation of a large amount of martensite during the hot rolling process. This led to a decrease in bainite content and a reduction in hole expansion rate.

[0085] Comparative Example 2 suffered from insufficient strength because its carbon and manganese content fell below the lower limits designed in the present invention.

[0086] In Comparative Example 3, although element B was added, the B content exceeded the upper limit designed in the present invention, resulting in the formation of martensite during the hot rolling process, which reduced the hole expansion rate.

[0087] In Comparative Example 4, the Ti content fell below the lower limit designed in the present invention, resulting in a reduced amount of second-phase particle precipitation and decreased strength.

[0088] In Comparative Example 5, the yield strength decreased because the soaking section temperature fell below the lower limit of the present invention, resulting in less second-phase precipitate during the annealing process of hot-dip galvanizing.

[0089] In Comparative Example 6, because the soaking section temperature exceeded the upper limit of the present invention, austenite was formed during the heating process, and then underwent a phase transformation to martensite during the cooling process, resulting in a decrease in the proportion of bainite and a reduction in the yield strength and hole expansion ratio of the steel sheet.

[0090] In Comparative Example 7, the heating section temperature fell below the lower limit of the present invention, resulting in less second-phase precipitate and a decrease in yield strength during the annealing process of hot-dip galvanizing.

[0091] In Comparative Example 8, because the direct-fire section temperature and the heated section temperature exceeded the upper limits of the present invention, austenite was formed during the heating process, and during the cooling process, it underwent a phase transformation to martensite, resulting in a decrease in the proportion of bainite and a reduction in the yield strength and hole expansion ratio of the steel sheet.

[0092] In Comparative Example 9, the soaking section time exceeded the upper limit of the present invention, resulting in coarsening of the second phase and a decrease in yield strength.

[0093] In Comparative Example 10, the second phase became coarser and the yield strength decreased because the slow cooling section temperature exceeded the upper limit of the present invention.

[0094] In Comparative Example 11, because the winding temperature fell below the lower limit of the present invention, martensite was formed after hot rolling, which reduced the bainite content and decreased the hole expansion ratio.

[0095] In Comparative Example 12, the yield strength decreased because the winding temperature exceeded the upper limit of the present invention, resulting in the formation of a large amount of ferrite during the hot rolling process, accompanied by a large amount of precipitation.

[0096] In Comparative Example 13, the yield strength decreased because the cooling rate after hot rolling fell below the lower limit of the present invention, resulting in the formation of a large amount of ferrite and a large amount of precipitation.

[0097] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0098] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. A hot-dip galvanized steel sheet having a tensile strength of ≥ 800 MPa, comprising a substrate containing Fe and unavoidable impurities, and a hot-dip galvanized layer, wherein the substrate further contains the following chemical elements in the following mass percentages: It contains C: 0.050-0.100%, Mn: 1.400-2.000%, Ti: 0.100-0.150%, and Al: 0.020-0.080%; The substrate does not contain Si, Cr, Mo, and Ni elements; The microstructure of the substrate comprises a granular bainite + ferrite biphase structure and second-phase particle precipitates, the volume fraction of the second-phase particle precipitates being 0.2% or more, and the volume fraction of second-phase particle precipitates with a size of 10 nm or less being 80% or more of the total second-phase particle precipitates. A hot-dip galvanized steel sheet having a tensile strength of ≥ 800 MPa, characterized by the above.

2. The mass percentage content of each chemical element in the aforementioned substrate is: The composition is C: 0.050–0.100%, Mn: 1.400–2.000%, Ti: 0.100–0.150%, Al: 0.020–0.080%; the remainder is Fe and other unavoidable impurities. The hot-dip galvanized steel sheet according to claim 1, characterized in that...

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 unavoidable 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 95% or more, and / or the crystal grain size of the granular bainite is less than 5 μm.

6. The weight of one side of the aforementioned hot-dip galvanized layer is 20 to 380 g / m². ２ The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that it is the same as the one described above.

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 to 0.35%, and / or the volume fraction of second phase particle precipitates with a size of 10 nm or less accounts for 80 to 95% of the total volume fraction of the second phase particle precipitates.

8. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that the yield strength is ≥ 680 MPa, the tensile strength is ≥ 800 MPa, the elongation A80 is ≥ 15.0%, and the hole expansion ratio 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 longitudinal yield strength of 680 to 800 MPa, a tensile strength of 800 to 850 MPa, an elongation ratio A80 of 15.0 to 19.5%, and a hole expansion ratio of 50.0 to 80.0%.

10. A method for manufacturing a hot-dip galvanized steel sheet according to any one of claims 1 to 9, characterized by including the following steps. (1) To manufacture slabs; (2) Heat and maintain the temperature of the slab; (3) After hot rolling, the material is cooled to a winding temperature of 450-500°C at a cooling rate of ≥ 50°C / s and then wound; (4) Pickle the coil to obtain a pickled coil; (5) Annealing and hot-dip galvanizing: Pickled coils are annealed directly in a combustion, oxidation-free continuous annealing furnace, however, the direct-fire section temperature is controlled to 550-630°C, the heating section temperature is controlled to 630-720°C, the soaking section temperature is controlled to 630-720°C, the soaking section holding time is controlled to 50-100 s, the slow-cooling section temperature is controlled to 550-670°C, and after exiting the slow-cooling section, the coils are cooled to the hot-dip galvanizing temperature.

11. The manufacturing method according to claim 10, characterized by heating the slab to 1230 to 1280°C and keeping it warm for 1 to 3 hours.

12. The manufacturing method according to claim 10, characterized in that, in the hot rolling process, the rough rolling start temperature is controlled to 1130 to 1190°C, the finish rolling start temperature is controlled to 980 to 1080°C, and the finish rolling exit temperature is controlled to 840 to 940°C, preferably so that the cumulative deformation amount of rough rolling and finish rolling is ≥ 90%.

13. The manufacturing method according to claim 10, characterized in that the hot-dip galvanizing temperature in the hot-dip galvanizing process is 440 to 480°C.

14. The manufacturing method according to claim 10, further comprising a leveling step after the hot-dip galvanizing step, characterized in that the leveling rate is controlled to 0.05 to 1.3%.

15. The manufacturing method according to claim 10, characterized in that, after the winding of step (3) is completed, the hot-rolled coil is placed in a heat-insulating pit and cooled slowly to an average cooling rate of ≤15°C / h; and / or the pickling tension leveling elongation is 0.2 to 2%; and / or a pickling rate of 60 to 150 m / min is adopted, the temperature of the final pickling tank in the pickling process is controlled to 80 to 90°C, and the iron ion concentration is controlled to 30 to 40 g / L.

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