800 MPa grade hot-expanded molten zinc-plated steel sheet and its manufacturing method

Abstract

The present invention discloses an 800 MPa grade hot hole expansion galvanized steel sheet and a manufacturing method thereof, which include a substrate and a hot-dip galvanized layer plated on at least one surface of the substrate. The substrate contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.03 to 0.08%, 0 < Si ≤ 0.45%, Mn: 1.3 to 1.8%, Al: 0.02 to 0.1%, Cr: 0.2 to 0.6%, Ti: 0.05 to 0.15%, Nb ≤ 0.05%, B ≤ 0.003%; provided that the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%. The 800 MPa grade hot hole expansion galvanized steel sheet has extremely excellent mechanical properties, with its longitudinal yield strength ≥ 660 MPa, tensile strength ≥ 780 MPa, elongation at break A50 ≥ 15%, punching hole expansion rate ≥ 50%, and reaming hole expansion rate ≥ 80%.

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] In recent years, with the rapid development of the automobile industry, the market and user requirements for automobile lightweighting have become increasingly high. Lightweighting has become a trend in the development of the automobile industry, and the proportion of high-strength steel sheets in automobile structural parts is also increasing.

[0003] In the actual preparation of high-strength steel materials, in more automobile models, there are not only certain requirements for the strength and elongation rate of the steel materials, but also certain requirements for the hole-expanding performance of the steel materials. 80 kg-class steel sheets are used in the production of automobile chassis parts. In addition to the above requirements for strength, elongation rate, and hole-expanding performance, in order to meet the corrosion resistance requirements of automobile parts corresponding to different usage environments and extend the service life of automobiles, it is necessary to further improve the corrosion resistance of the steel materials. However, current ordinary hot-rolled and cold-rolled sheets, pickled and rolled sheets can no longer meet the requirements of the automobile industry, and hot-dip galvanized sheets are one of the effective methods to improve the corrosion resistance of automobile parts.

[0004] Based on this, in order to further balance the processing performance, corrosion resistance and manufacturability of the material and meet the current market needs and user needs, the inventors hope to obtain a novel 800 MPa-class high hole-expanding hot-dip galvanized steel sheet and a method for manufacturing the same.

[0005] In the prior art, researchers and developers have already designed several steel sheets with excellent performance, but still cannot meet performance requirements such as specific strength and manufacturing process parameters, and there is still a lack of performance.

[0006] For example, in a Chinese patent document with publication number CN109055867A, publication date December 21, 2018, and title "Method for Producing a Hot-Dip Galvanized Steel Sheet with High Hole Expansion Property and Tensile Strength of 540 MPa", a method for producing a hot-dip galvanized steel sheet with high hole expansion property and tensile strength of 540 MPa is disclosed. It is characterized by directly galvanizing pickled steel sheets without cold rolling, and it is a method with a short process. However, the tensile strength of the steel sheet produced by such a solution is only 540 MPa.

[0007] Also, for example, in a Chinese patent document with publication number CN108396259A, publication date August 14, 2018, and title "Hot-Rolled Galvanized Steel Sheet with High Hole Expansion Property and Its Manufacturing Method", a hot-rolled galvanized steel sheet with high hole expansion property and its manufacturing method are disclosed. Its yield strength is ≥600 MPa, the hole expansion rate is ≥40%, its composition contains 0.5 - 2.5% Si, the Si content is relatively high, red scale is likely to form on the surface, which is disadvantageous for the control of the galvanized surface. In this technical solution, the Si content of the steel material is high.

[0008] Furthermore, for example, 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 with Good Bendability and Hole Expansion Property and Their Manufacturing Methods", an ultra-high strength hot-rolled dual-phase steel sheet and strip with good bendability and hole expansion property and their manufacturing methods are disclosed. In this technical solution, the performance design and manufacturing method of hot-rolled pickled steel sheets are disclosed, but the role of element B is not considered in its chemical composition, and the influence of the hot-dip galvanizing process on the performance of the hot-dip galvanized steel sheet is not disclosed either.

Summary of the Invention

Problems to be Solved by the Invention

[0009] One object of the present invention is to provide a hot-dip galvanized steel sheet with high hole-expanding property of 800 MPa grade. The hot-dip galvanized steel sheet with high hole-expanding property of 800 MPa grade adopts a reasonable chemical composition design, can obtain good comprehensive mechanical properties, has high strength and high corrosion resistance, and at the same time has the characteristic of high hole-expanding rate. It can be used as automotive body structure parts and automotive chassis parts, and can also be used in other application fields where high strength and weight reduction are required, and has good application prospects.

Means for Solving the Problems

[0010] To achieve the above object, the present invention includes a substrate and a hot-dip galvanized layer plated on at least one surface of the substrate. The substrate contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.03 - 0.08%, 0 < Si ≤ 0.45%, Mn: 1.3 - 1.8%, Al: 0.02 - 0.1%, Cr: 0.2 - 0.6%, Ti: 0.05 - 0.15%, Nb ≤ 0.05%, B ≤ 0.003%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%. A hot-dip galvanized steel sheet with high hole-expanding property of 800 MPa grade is provided.

[0011] Furthermore, in the hot-dip galvanized steel sheet with high hole-expanding property of 800 MPa grade according to the present invention, the mass percentage contents of its respective chemical elements are: C: 0.03 - 0.08%, 0 < Si ≤ 0.45%, Mn: 1.3 - 1.8%, Al: 0.02 - 0.1%, Cr: 0.2 - 0.6%, Ti: 0.05 - 0.15%, Nb ≤ 0.05%, B ≤ 0.003%; the balance is Fe and inevitable impurity elements; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%.

[0012] In the substrate of the 800 MPa grade hot hole expanding galvanized steel sheet according to the present invention, the design principle of each chemical element is as follows.

[0013] C: In the substrate of the 800 MPa grade hot hole expanding galvanized steel sheet according to the present invention, the content of C greatly determines the tensile strength level of the steel sheet. C contributes to solid solution strengthening and forms sufficient precipitation strengthening phases to ensure the strength of the steel. However, when the mass percentage content of C is high, carbide particles coarsen, which is disadvantageous to the hole expanding performance. Therefore, on the premise of ensuring the strength of the steel grade, in order to achieve both high hole expansion rate and good formability and weldability, in the substrate of the 800 MPa grade hot hole expanding galvanized steel sheet according to the present invention, the mass percentage content of C element is controlled to be 0.03 - 0.08%.

[0014] Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of C element may be controlled to be 0.04 - 0.07%.

[0015] Si: In the substrate of the 800 MPa grade hot hole expanding galvanized steel sheet according to the present invention, Si element plays a role in solid solution strengthening to improve the strength of the steel sheet. By adding Si, the work hardening rate and the uniform elongation rate and total elongation rate at a predetermined strength can be improved, contributing to the improvement of the elongation rate of the steel sheet. Furthermore, Si can also prevent the precipitation of carbides and reduce the appearance of pearlite phase. However, it should be noted that when the steel contains silicon, surface defects of fayalite (2FeO - SiO2) oxide scale are likely to form on the surface of the steel sheet, which has an adverse effect on the surface quality. Therefore, in the substrate of the 800 MPa grade hot hole expanding galvanized steel sheet according to the present invention, the mass percentage content of Si element is controlled to be 0 < Si ≤ 0.45%.

[0016] Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of the Si element may be controlled to be 0.05 < Si ≦ 0.45%. Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of the Si element may be controlled to be 0 < Si ≦ 0.2%. Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of the Si element may be controlled to be 0.05 < Si ≦ 0.2%.

[0017] Mn: In the substrate of the 800 MPa grade high hole expansion property hot-dip galvanized steel sheet according to the present invention, the Mn element is a solid solution strengthening element. When the mass percentage content of the Mn element in the steel is low, it will lead to insufficient strength of the steel material. However, when the mass percentage content of the Mn element is too high, it will lead to a decrease in the plasticity of the steel sheet. In addition, Mn delays the pearlite transformation, improves the hardenability of the steel, lowers the bainite transformation temperature, refines the structure of the lower structure of the steel, and ensures the acquisition of the lath substructure, and brings good formability on the premise of ensuring the tensile strength of the product. Therefore, considering the influence of the Mn element content on the steel material performance, in the substrate of the 800 MPa grade high hole expansion property hot-dip galvanized steel sheet according to the present invention, the mass percentage content of the Mn element is controlled to be 1.3 - 1.8%.

[0018] Al: In the substrate of the 800 MPa grade high hole expansion property hot-dip galvanized steel sheet according to the present invention, Al is a deoxidizing element in the steel, which can reduce the oxide-based inclusions in the steel, purify the steel quality, and contribute to the improvement of the formability of the steel sheet. However, it should be noted that if the mass percentage content of the Al element in the steel is too high, oxidation will occur, which will further affect the continuous casting production. Therefore, in this technical solution, considering the influence of the Al element on the steel sheet performance, in the substrate of the 800 MPa grade high hole expansion property hot-dip galvanized steel sheet according to the present invention, the mass percentage content of the Al element is controlled to be 0.02 - 0.1%.

[0019] Cr: In the substrate of the 800 MPa grade hot hole-expanding galvanized steel sheet according to the present invention, the Cr element suppresses the formation of pearlite and contributes to the formation of bainite structure, and can ultimately contribute to strength and hole expansion rate. According to the research of the present inventors, when the mass percentage content of the Cr element in the steel is less than 0.15%, the influence on the CCT curve becomes insignificant. However, when the mass percentage content of Cr in the steel is too high, it not only causes an increase in the cost of the alloy, but also makes it easier to generate a large amount of martensite structure. Therefore, in the substrate of the 800 MPa grade hot hole-expanding galvanized steel sheet according to the present invention, the mass percentage content of the Cr element is controlled to be 0.2 - 0.6%.

[0020] Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of Cr may be controlled to be 0.2 - 0.35%.

[0021] Ti: In the substrate of the 800 MPa grade hot hole-expanding galvanized steel sheet according to the present invention, Ti is one of the important precipitation strengthening and grain refinement strengthening elements. Especially in the hot dip galvanized annealing process, further precipitation strengthening by Ti and immobilization of C can contribute to the improvement of the strength and elongation of the steel sheet. Therefore, in order to exert the beneficial effects of the Ti element, in the present invention, the mass percentage content of the Ti element is controlled to be 0.05 - 0.15%.

[0022] Nb: In the substrate of the 800 MPa grade hot hole-expanding galvanized steel sheet according to the present invention, Nb is one of the important precipitation strengthening and grain refinement strengthening elements. However, when the mass percentage of Nb exceeds 0.05%, the strengthening effect of Nb reaches saturation and the cost becomes high. Therefore, in order to exert the beneficial effects of the Nb element and suppress the production cost, in the present invention, the mass percentage content of the Nb element is controlled such that Nb ≤ 0.05%.

[0023] Of course, in some preferred embodiments, in order to obtain better implementation effects, preferably, the mass percentage content of Nb may be controlled such that Nb ≤ 0.02%.

[0024] B: In the substrate of the 800 MPa grade hot expandable hot-dip galvanized steel sheet according to the present invention, B contributes to the expansion of the bainite phase region and can ensure the acquisition of the bainite structure in the steel sheet during cooling after rolling, which is effective in improving the strength and hardness of the steel material. However, excessive B element leads to the appearance of excessive martensite structure in the steel sheet and also leads to the decrease of the hole expansion rate and elongation rate of the steel material. Therefore, it is necessary to pay attention that the content of B element in the steel should not be too high. Thus, in the present invention, the mass percentage content of B element is controlled to B≤0.003%.

[0025] Furthermore, in this technical solution designed according to the present invention, the inventors need to pay attention that while controlling the mass percentage content of a single chemical element in the substrate, the mass percentage contents of N, Ti, and Nb in the substrate are further controlled to satisfy 0.01%≤(Ti - 3.43N + 0.52Nb) / 4≤0.03%. However, N is an impurity element in the substrate.

[0026] In such a design idea, the purpose of adding high Ti and high Nb in the steel is mainly to ensure the precipitation of finely dispersed nano-scale carbides in the annealing hot-dip galvanizing process of the strip steel, so that a strong precipitation strengthening effect can be achieved. In the present invention, in order to ensure sufficient precipitation of Ti and Nb, the C content needs to be designed in matching with the Ti and Nb contents. Therefore, only when the mass percentage contents of N, Ti, and Nb elements satisfy the above relationship of "0.01%≤(Ti - 3.43N + 0.52Nb) / 4≤0.03%", and an appropriate amount of Cr element is added (moreover, Cr has good tempering resistance and contributes to maintaining the strength of bainite during the annealing process) to obtain a bainite structure without containing pearlite that affects the hole expansion performance in the hot rolling coiling and annealing hot-dip galvanizing processes, it becomes possible for the first time to obtain a hot-dip galvanized steel sheet with high strength and high hole expansion rate in combination with the desired manufacturing process.

[0027] Furthermore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, among the inevitable impurity elements, P ≤ 0.02%, S ≤ 0.005%, and N ≤ 0.005%.

[0028] In the above technical solution, P, S, and N are all impurity elements in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention. In order to obtain a steel material with better performance and superior quality as much as technically permitted, the content of impurity elements in the steel sheet should be reduced as much as possible.

[0029] Therefore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, the content of P element is controlled to P ≤ 0.02%, the content of S element is controlled to S ≤ 0.005%, and the content of N element is controlled to N ≤ 0.005%.

[0030] Furthermore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, the mass percentage content of each chemical element in the substrate further satisfies at least one of the following: C: 0.04 - 0.07%; 0 < Si ≤ 0.2%, preferably 0.05 ≤ Si ≤ 0.2%; Cr: 0.2 - 0.35%; Nb ≤ 0.02%.

[0031] Furthermore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, the matrix of the microstructure of the substrate is bainite + ferrite, and the matrix has precipitates including nanoscale precipitates.

[0032] Furthermore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, its bainite volume fraction is ≥ 95%.

[0033] Furthermore, in the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to the present invention, its ferrite volume fraction is ≤ 5%.

[0034] Furthermore, in the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention, the matrix of the microstructure of the substrate is bainite with a volume fraction of 95 to 99% and ferrite with a volume fraction of 1 to 5%.

[0035] Furthermore, in the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention, the nano-scale precipitates include TiC and (Ti,Nb)C, and the diameter of the nano-scale precipitates is 3 to 20 nm.

[0036] Furthermore, in the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention, the precipitates further include TiN precipitates with a diameter of <10 μm.

[0037] Furthermore, in the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention, the weight average value of the single side of the galvanized layer is 20 to 600 g / m 2 is.

[0038] Furthermore, in the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention, its performance is longitudinal yield strength ≥ 660 MPa, tensile strength ≥ 780 MPa, elongation at break A50 ≥ 15%, punching hole expansion rate ≥ 50%, and reaming hole expansion rate ≥ 80%.

[0039] In some embodiments, the longitudinal yield strength of the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention is ≥ 675 MPa, preferably 675 to 810 MPa.

[0040] In some embodiments, the tensile strength of the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention is ≥ 800 MPa, preferably 800 to 870 MPa.

[0041] In some embodiments, the elongation at break A50 of the 800 MPa grade hot hole expandable galvanized steel sheet according to the present invention is ≥ 18%, preferably 18 to 20%.

[0042] In some embodiments, the punching hole expansion rate of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention is ≧55%, preferably ≧60%. In some embodiments, the punching hole expansion rate of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention is 50 - 80%.

[0043] In some embodiments, the reaming hole expansion rate of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention is ≧90%, preferably ≧95%. In some embodiments, the reaming hole expansion rate of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention is 80 - 120%, preferably 90 - 120%.

[0044] In some embodiments, the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention has a longitudinal yield strength of 675 - 810 MP, a tensile strength of 800 - 870 MPa, an elongation rate of 18 - 20%, a punching hole expansion rate of 50 - 80%, and a reaming hole expansion rate of 80 - 120%.

[0045] In some embodiments, the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention has a yield strength of 678 - 801 MP, a tensile strength of 803 - 862 MPa, an elongation rate A50 of 18 - 20%, a punching hole expansion rate of 52 - 82%, and a reaming hole expansion rate of 85 - 117%.

[0046] Correspondingly, another object of the present invention is to provide a manufacturing method of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to the present invention. The 800 MPa grade hot-dip galvanized steel sheet obtained by adopting this manufacturing method has high strength and excellent corrosion resistance, and at the same time has the characteristic of high hole expansion rate, and has good application prospects.

[0047] To achieve the above object, the manufacturing method of the above 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property provided by the present invention includes the following steps: (1) Smelting and casting; (2) Hot rolling: Heat the slab to 1200 - 1280 °C and hold for heat preservation; then perform rolling, but control the rough rolling outlet temperature to be ≥ 1000 °C, control the finish rolling outlet temperature to be 840 - 950 °C, and control the rolling speed of finish rolling to be ≥ 7.5 m / s; after finish rolling, water-cool the steel plate to a coiling temperature of 430 - 540 °C at a cooling rate of 40 - 150 °C / s and coil it. (3) Pickling; (4) Annealing: After pickling, anneal and soak the steel coil in a non-oxidizing continuous annealing furnace, where the heating rate is ≥ 5 °C / s, the annealing and soaking temperature is 480 - 740 °C, the holding time at the annealing and soaking stage is 30 - 300 s, and the cooling rate after soaking is ≥ 3 °C / s. (5) Hot-dip galvanizing; (6) Levelling.

[0048] In the above technical solution of the present invention, in step (2), in the case of Ti-containing hot-dip galvanized steel, the heating temperature of the slab is particularly important for performance and surface. In the continuous casting process, Ti precipitates in large amounts as (Ti, Nb)(C, N) precipitates with large sizes. However, the main purpose of setting the heating temperature to ≥ 1200 °C is to ensure that alloying elements such as Ti are dissolved as much as possible to ensure the precipitation of nano-scale microalloys such as Ti in the subsequent hot rolling coiling process, especially in the annealing and hot-dip galvanizing process, during the overheating process of the slab. However, when the heating temperature exceeds 1280 °C, the grain size tends to coarsen, which is disadvantageous to the toughness of the steel plate. At the same time, the oxide scale thickens, which is disadvantageous to the removal of phosphorus from the oxide scale and ultimately affects the surface quality of the hot-dip galvanizing. Therefore, it is necessary to pay attention that the heating temperature should not be too high. Therefore, in the hot rolling process of the present invention, preferably, the heating temperature is controlled to be 1200 - 1280 °C.

[0049] In addition, the rough rolling temperature control and rolling speed in the hot rolling process have a great influence on microalloying elements such as Ti. In the case of low rough rolling temperature and finish rolling process, carbides and carbonitrides of Ti precipitate, but the precipitation size in this process is large, which is disadvantageous for the improvement of the final strength. Therefore, in the hot rolling process of the present invention, the rough rolling outlet temperature is controlled to be ≥ 1000 °C, and the rolling speed of the finish rolling is controlled to be ≥ 7.5 m / s. In some embodiments, the rough rolling outlet temperature is controlled to be 1000 - 1080 °C. In some embodiments, the rolling speed of the finish rolling is controlled to be 7.5 - 11 m / s.

[0050] In addition, the finish rolling outlet temperature, coiling temperature, and water cooling speed in the hot rolling process also have a great influence on the microstructure. When the finish rolling outlet temperature is low and the water cooling speed is slow, massive ferrite is likely to be generated. When the coiling temperature is high, the contents of ferrite and pearlite also increase, but when the coiling temperature is low, there is a risk of generating a martensite structure. Therefore, in the hot rolling process of the present invention, in order to control the bainite phase transformation to be sufficient, the finish rolling outlet temperature is controlled to be 840 - 950 °C. After finish rolling, the steel plate is water-cooled to 430 - 540 °C at a cooling rate of 40 - 150 °C / s and then coiled. In some embodiments, the finish rolling outlet temperature is controlled to be 870 - 930 °C. In some embodiments, the cooling rate after finish rolling is 50 - 110 °C / s.

[0051] Correspondingly, in the annealing process of step (4) of the present invention, the annealing soaking temperature is limited to 480 - 740 °C, and the precipitation of (Ti,Nb)(C,N) is the strongest in this temperature range. When annealing below the austenite transformation point Ac1 during heating, the steel plate maintains the bainite single-phase structure hot-rolled. When annealing above the austenite transformation point Ac1 during heating and the cooling rate after soaking is ≥ 3 °C / s, excessive formation of the ferrite structure is avoided, contributing to the improvement of the hole expansion rate.

[0052] It is necessary to explain that in this technical solution designed according to the present invention, in some embodiments, before annealing, first, the strip steel is supplied to a straight-through furnace for preliminary oxidation to obtain a steel plate with a preliminary oxide film formed on the surface, and the thickness of the preliminary oxide film is controlled to be 60-120 nanometers. After preliminary oxidation, it is further put into an annealing furnace for annealing to obtain a steel plate whose surface is reduced by hydrogen gas.

[0053] Furthermore, in the manufacturing method according to the present invention, in step (2), the heating temperature is controlled to be 1250-1280°C.

[0054] Furthermore, in the manufacturing method according to the present invention, in step (2), the heat preservation time is 1-3 hours.

[0055] Furthermore, in the manufacturing method according to the present invention, in step (2), the finishing rolling exit temperature is controlled to be 840-920°C, and / or the coiling temperature is controlled to be 430-500°C.

[0056] Furthermore, in the manufacturing method according to the present invention, in step (3), the elongation rate of pickling tension leveling is controlled to be 0.2-2%, the pickling speed is controlled to be 60-150 m / min, the temperature of the final pickling tank in the pickling process is controlled to be 80-90°C, and the iron ion concentration is controlled to be 30-40 g / L.

[0057] Furthermore, in the manufacturing method according to the present invention, in step (4), the annealing soaking temperature is 650-730°C, and / or the soaking time in the annealing soaking stage is 30-120 s.

[0058] Furthermore, in the manufacturing method according to the present invention, in step (4), the heating rate is 5-20°C / s, preferably 5-15°C / s.

[0059] Furthermore, in the manufacturing method according to the present invention, in step (4), the cooling rate after soaking is 3-25°C / s; in some embodiments, the cooling rate is 12-25°C / s.

[0060] Furthermore, in the manufacturing method according to the present invention, in step (5), the temperature of the hot dip galvanizing pot is 440 to 480 °C.

[0061] Furthermore, in the manufacturing method according to the present invention, in step (6), the leveling rate is 0.05 to 1.3%, preferably, the leveling rate is 0.2% to 0.4%.

[0062] Furthermore, in the manufacturing method according to the present invention, in step (6), the thickness of the finally obtained steel sheet is ≤ 5 mm.

[0063] The 800 MPa grade hot dip galvanized steel sheet with high hole expansion property and its manufacturing method according to the present invention have the following advantages and beneficial effects as compared with the prior art: In the present invention, an economical and reasonable chemical composition design is adopted, and in combination with the existing hot continuous rolling production line and hot dip galvanizing production line, it is possible to produce an 800 MPa grade hot dip galvanized steel sheet with ultra-high strength and high hole expansion rate. Compared with the conventional manufacturing method of hot dip galvanized sheets, this manufacturing process optimized according to the present invention can omit the cold rolling process, shorten the process steps, improve the production efficiency, save energy, and effectively reduce the production cost.

[0064] The 800 MPa grade hot dip galvanized steel sheet manufactured by the present invention has characteristics such as high hole expansion rate, high strength, and high corrosion resistance. It has a longitudinal yield strength ≥ 660 MPa, a tensile strength ≥ 780 MPa, an elongation rate A50 ≥ 15%, a punching hole expansion rate ≥ 50%, and a reaming hole expansion rate ≥ 80%. Therefore, it can be used as automotive body structural parts and automotive chassis parts, and can also be used in other application fields where high strength and weight reduction are required, having good application prospects.

Brief Description of the Drawings

[0065]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0066] Hereinafter, based on specific examples and drawings, the 800 MPa grade high hole expansion molten zinc plated steel sheet according to the present invention and its manufacturing method will be further interpreted and explained. However, such interpretation and explanation do not unduly limit the technical solution of the present invention.

Examples

[0067] Examples 1 to 11 and Comparative Examples 1 to 10 The mass percentage ratios of each chemical element of the substrates of the 800 MPa grade high hole expansion molten zinc plated steel sheets according to Examples 1 to 11 and the comparative steel sheets according to Comparative Examples 1 to 10 are shown in Table 1.

[0068]

Table 1

[0069] Remarks: In Table 1 above, the formula "(Ti - 3.43N + 0.52Nb) / 4" represents the TiC precipitation equivalent, and in the formula, the corresponding mass percentage contents of each chemical element are substituted for N, Ti, and Nb.

[0070] The 800 MPa grade high hole expansion molten zinc plated steel sheets according to Examples 1 to 11 of the present invention and the comparative steel sheets according to Comparative Examples 1 to 10 were all prepared by the following steps: (1) Smelting and casting were carried out according to the chemical compositions shown in Table 1.

[0071] (2) Hot rolling: The slab obtained through smelting and continuous casting is heated to 1200 - 1280 °C and kept warm for 1 - 3 hours; then rolling is carried out. However, the rough rolling exit temperature is controlled to be ≥1000 °C, the finish rolling exit temperature is controlled to be 840 - 950 °C, preferably 840 - 920 °C, and the rolling speed of finish rolling is controlled to be ≥7.5 m / s; after finish rolling, the steel plate is water-cooled to a coiling temperature of 430 - 540 °C at a cooling rate of 40 - 150 °C / s and coiled. Preferably, the coiling temperature could also be controlled to be 430 - 500 °C.

[0072] (3) Pickling: The pickling tension leveling elongation rate is controlled to be 0.2 - 2%, the pickling speed is controlled to be 60 - 150 m / min, the temperature of the final pickling tank in the pickling process is controlled to be 80 - 90 °C, and the iron ion concentration is controlled to be 30 - 40 g / L.

[0073] (4) Annealing: After pickling, the steel coil is annealed and soaked in a combustion-free oxidation continuous annealing furnace. However, the heating rate is ≥5 °C / s, the annealing soaking temperature is controlled to be 480 - 740 °C, the soaking time in the annealing soaking stage is controlled to be 30 - 300 s, and the cooling rate after soaking is controlled to be ≥3 °C / s. Of course, preferably, the annealing soaking temperature could be controlled to be 650 - 730 °C, and the soaking time in the annealing soaking stage could be controlled to be 30 - 120 s.

[0074] (5) Hot-dip galvanizing: The steel plate is put into a zinc pot for hot-rolled plating, and the temperature of the hot-dip galvanizing pot is controlled to be 440 - 480 °C.

[0075] (6) Leveling: Leveling is carried out after plating, the leveling rate is controlled to be 0.05 - 1.3%, and finally a steel plate with a thickness ≤5 mm is obtained.

[0076] In the present invention, the chemical composition design of the 800 MPa grade high hole-expanding hot-dip galvanized steel sheets according to Examples 1 to 11 and the related processes all satisfied the requirements of the standards designed by the present invention. On the other hand, for the comparative steel sheets according to Comparative Examples 1 to 10, although the adopted processes were carried out according to the above processes (1) to (6), there were parameters in the chemical composition design and related processes of the comparative steel sheets according to Comparative Examples 1 to 10 that did not meet the design requirements of the present invention.

[0077] The specific process parameters in the above manufacturing processes of the 800 MPa grade high hole-expanding hot-dip galvanized steel sheets according to Examples 1 to 11 and the comparative steel sheets according to Comparative Examples 1 to 10 are shown in Tables 2-1 and 2-2.

[0078] [Table 2-1]

[0079] [Table 2-2]

[0080] It should be noted that before hot-dip galvanizing, the inventors respectively sampled the substrates of the 800 MPa grade high hole-expanding hot-dip galvanized steel sheets according to Examples 1 to 11 of the finished products obtained through the above processes and steps and the substrates according to Comparative Examples 1 to 10, observed and analyzed the microstructures of the substrates of the steel sheets according to each example and comparative example, and the results obtained from the correlation observation and analysis are shown in Table 3 below.

[0081] The observation and analysis results of the microstructures of the substrates according to Examples 1 to 11 and Comparative Examples 1 to 10 are shown in Table 3.

[0082] [Table 3]

[0083] As can be seen from the observation, in the present invention, the substrates of the 800 MPa grade high hole expansion molten zinc plated steel sheets according to Examples 1 to 11 prepared have a microstructure matrix of bainite + ferrite, and the volume fraction of bainite is in the range of 95 to 99%.

[0084] It should be noted that when actually prepared, the microstructure matrix of the substrates according to Examples 1 to 11 also has nano-scale precipitates containing TiC and (Ti,Nb)C, and the diameters of these nano-scale precipitates are in the range of 3 to 20 nm. At the same time, such precipitates formed on the substrates according to Examples 1 to 11 also include TiN precipitates as large particles, and their specific diameters are all <10 μm.

[0085] In order to obtain the performance of the finally prepared finished zinc plated steel sheet, after the observation of the microstructure of the above substrates was completed, the inventors sampled from the 800 MPa grade high hole expansion molten zinc plated steel sheets according to Examples 1 to 11 of the finished products obtained through the above process steps (1) to (6) and the comparative steel sheets according to Comparative Examples 1 to 10 respectively, measured the mechanical properties, and measured the weight average value of the single side of the molten zinc plating layer of the steel sheets according to each example and comparative example. The obtained measurement results are shown in Table 4 below.

[0086] The measurement means for the related performance was as follows: (1) Measurement of tensile performance: JIS 5# tensile test specimens were taken along the longitudinal direction, and a tensile test was carried out using the standard of GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" to obtain the yield strength, tensile strength and elongation of the steel sheets according to each example and comparative example.

[0087] (2) Hole expansion test: The hole expansion rate was measured by a hole expansion test in which a sample with a center hole was pushed into a die using a punch die until the hole edge of the plate was constricted or a through crack occurred while expanding the center hole of the sample. Since the manufacturing method of the original hole in the center of the sample has a great influence on the measurement results of the hole expansion rate, the original holes in the centers of the samples were manufactured by punching and reaming respectively, and the subsequent test and measurement methods were carried out in accordance with the hole expansion rate measurement method specified in the ISO / DIS 16630 standard. The measurement results are shown in Table 4.

[0088] The measurement results of the mechanical properties of the 800 MPa grade high hole-expandable hot-dip galvanized steel sheets according to Examples 1 to 11 and the comparative steel sheets according to Comparative Examples 1 to 10, and the weight average value of one side of the hot-dip galvanized layer are shown in Table 4.

[0089]

Table 4

[0090] As shown in Table 4, compared with the comparative steel sheets according to Comparative Examples 1 to 10, the 800 MPa grade high hole-expandable hot-dip galvanized steel sheets according to Examples 1 to 11 of the present application had better comprehensive mechanical properties.

[0091] In the present invention, in the 800 MPa grade high hole-expandable hot-dip galvanized steel sheets according to Examples 1 to 11, the design idea of low-carbon bainite of the present invention was adopted, and at the same time, the addition of Cr improved the resistance to tempering softening during bainite transformation and annealing processes, and the addition of Ti and Nb enhanced the precipitation strengthening effect during the annealing process. The 800 MPa grade high hole-expandable hot-dip galvanized steel sheets according to Examples 1 to 11 finally prepared by the present invention had a yield strength of 678 to 801 MPa, a tensile strength of 803 to 862 MPa, an elongation rate A50 of 18 to 20%, a punching hole expansion rate of 52 to 82%, and a reaming hole expansion rate of 85 - 117%.

[0092] Compared with Examples 1 to 4, Comparative Examples 1 and 2 used the same steel grade A and adopted the same hot rolling process. However, due to the annealing temperature being too high or the cooling rate after soaking being too slow, the proportion of ferrite in the microstructure of the prepared substrate exceeded 5%, which also led to a decrease in the yield strength of the steel and a decrease in the hole expansion rate.

[0093] Compared with Examples 5 to 7, in Comparative Example 3, due to the adoption of a low heating temperature, the solutionization of contained Nb and Ti became insufficient; in Comparative Example 4, the rough rolling exit temperature was low, and in Comparative Example 5, the finish rolling exit temperature was low. As a result, Nb and Ti precipitated coarsely as (Ti,Nb)(C,N) during the hot rolling process, the contribution to strength became small, and the strength of the steel plates all decreased.

[0094] Compared with Examples 5 to 7, in Comparative Example 6, the coiling temperature adopted was high, and due to the high ferrite content in the microstructure of its hot coil, the ferrite content in the final microstructure became high, and the hole expansion rate decreased.

[0095] Correspondingly, in all of Comparative Examples 7 to 10, the design of the chemical composition did not meet the requirements of the present invention. However, in Comparative Example 7, due to the high C content, the contribution to strength increased, which conversely led to a decrease in the hole expansion rate; in Comparative Example 8, due to the high N content, a large amount of Ti element was consumed, and a large amount of massive TiN precipitated. However, the contribution of 5 - 20um TiN to strength was small, which decreased the strength of the steel plate. At the same time, during punching, there were fine cracks caused by large - size TiN at the punching edge, which had a great impact on the punching hole expansion rate; in Comparative Example 9, due to the low Mn element content in the steel during design, the final strength of the steel plate became low; in Comparative Example 10, due to the low Cr element content in the steel, the hardenability became weak. In addition, its low hot rolling cooling rate and high ferrite content in the microstructure of the hot coil also led to a high ferrite content in the final microstructure, and the hole expansion rate of the steel plate decreased.

[0096] Figure 1 is a typical metallographic photograph of the substrate of an 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to Example 1.

[0097] As shown in Figure 1, in this embodiment, the 800 MPa grade hot hole expandable hot-dip galvanized steel sheet according to Example 1 has a microstructure of bainite with a volume fraction of 96% + ferrite with a volume fraction of 4%. The grain sizes of bainite and ferrite are 5.5 μm, the nanoscale precipitation diameter is 3 - 15 nm, and the TiN diameter is <10 μm.

[0098] Figure 2 is a photograph of large TiN particles contained in the comparative steel sheet according to Comparative Example 8. As shown in Figure 2, in this embodiment, Comparative Example 8 has a microstructure of bainite with a volume fraction of 99% + ferrite with a volume fraction of 1%. The nanoscale precipitation diameter is 3 - 20 nm, and the TiN diameter is as large as 10 - 20 μm.

[0099] Also, the combination of each technical feature in this application is not limited to the combination described in the claims of this application or the combination described in the specific embodiments. As long as they do not conflict with each other, all the technical features described in this application can be freely combined or joined in any form.

[0100] Furthermore, it should also be noted that the above-mentioned examples are only specific examples of the present invention. The present invention is not limited to the above examples. It is obvious that any similar changes or modifications that those skilled in the art can directly derive from the disclosure of the present invention or easily think of are included in the protection scope of the present invention.

Claims

1. A steel sheet having a substrate and a hot-dip zinc plating layer plated on at least one surface of the substrate, wherein the substrate contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.03 to 0.08%, 0 < Si ≤ 0.45%, Mn: 1.3 to 1.8%, Al: 0.02 to 0.1%, Cr: 0.2 to 0.6%, Ti: 0.05 to 0.15%, Nb ≤ 0.05%, B ≤ 0.003%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%, and it is characterized as an 800 MPa-class hot-dip zinc plating steel sheet with high hole expansion properties.

2. The mass percentage contents of its respective chemical elements are: C: 0.03 to 0.08%, 0 < Si ≤ 0.45%, Mn: 1.3 to 1.8%, Al: 0.02 to 0.1%, Cr: 0.2 to 0.6%, Ti: 0.05 to 0.15%, Nb ≤ 0.05%, B ≤ 0.003%; the balance is Fe and inevitable impurity elements; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.03%, and it is characterized as the 800 MPa-class hot-dip zinc plating steel sheet according to Claim 1.

3. Among the inevitable impurity elements, P ≤ 0.02%, S ≤ 0.005%, N ≤ 0.005%, and it is characterized as the 800 MPa-class hot-dip zinc plating steel sheet according to Claim 1 or 2.

4. The mass percentage contents of the respective chemical elements in the substrate further satisfy at least one of the following, and it is characterized as the 800 MPa-class hot-dip zinc plating steel sheet according to Claim 1 or 2: C: 0.04 to 0.07%; 0 < Si ≤ 0.2%; Cr: 0.2 to 0.35%; Nb ≤ 0.02%.

5. The matrix of the microstructure of the substrate is bainite + ferrite, and the matrix has precipitates including nano-scale precipitates; preferably, the volume fraction of bainite is ≥ 95%; preferably, the nano-scale precipitates include TiC, (Ti, Nb)C, and the diameter of the nano-scale precipitates is 3 to 20 nm; preferably, the precipitates further include TiN precipitates with a diameter < 10 μm, and it is characterized as the 800 MPa-class hot-dip zinc plating steel sheet according to Claim 1 or 2.

6. The weight average value of one side of the above-mentioned hot-dip galvanized layer is 20 to 600 g / m ２ The 800 MPa-class hot-dip galvanized steel sheet with high hole expansion property according to claim 1 or 2, characterized in that it is as described above.

7. The performance of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to claim 1 or 2 is characterized in that the longitudinal yield strength ≥ 660 MPa, the tensile strength ≥ 780 MPa, the elongation rate A50 ≥ 15%, the punching hole expansion rate ≥ 50%, and the reaming hole expansion rate ≥ 80%.

8. The performance of the 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to claim 1 or 2 is characterized in that the longitudinal yield strength ≥ 675 MPa, preferably 675 - 810 MPa; the tensile strength ≥ 800 MPa, preferably 800 - 870 MPa; the elongation rate A50 ≥ 18%, preferably 18 - 20%; the punching hole expansion rate ≥ 55%, preferably 50 - 80%; and the reaming hole expansion rate ≥ 90%, preferably 80 - 120%.

9. The 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to claim 1 or 2 is characterized in that the yield strength is 678 - 801 MPa, the tensile strength is 803 - 862 MPa, the elongation rate A50 is 18 - 20%, the punching hole expansion rate is 52 - 82%, and the reaming hole expansion rate is 85 - 117%.

10. A method for manufacturing an 800 MPa grade hot-dip galvanized steel sheet with high hole expansion property according to any one of claims 1 - 9, characterized by including the following steps (1) Smelting and casting; (2) Hot rolling: The slab is heated to 1200 - 1280 °C and kept warm; then rolling is carried out, provided that the rough rolling exit temperature is controlled to ≥ 1000 °C, the finish rolling exit temperature is controlled to 840 - 950 °C, and the rolling speed of the finish rolling is controlled to ≥ 7.5 m / s; after finish rolling, the steel sheet is water-cooled to a coiling temperature of 430 - 540 °C at a cooling rate of 40 - 150 °C / s and coiled; (3) Pickling; (4) Annealing: After pickling, the steel coil is annealed and soaked in a combustion-free oxidation continuous annealing furnace, provided that the heating rate is ≥ 5 °C / s, the annealing soaking temperature is 480 - 740 °C, the holding time of the annealing soaking stage is 30 - 300 s, and the cooling rate after soaking is ≥ 3 °C / s; (5) Hot-dip galvanizing; (6) Leveling.

11. In step (2), the heat preservation time is 1 to 3 hours; and / or, the rough rolling outlet temperature is controlled to be 1000 to 1080 °C; and / or, the finish rolling outlet temperature is controlled to be 840 to 920 °C; and / or, the rolling speed of finish rolling is controlled to be 7.5 to 11 m / s; and / or, the cooling rate after finish rolling is controlled to be 50 to 110 °C / s; and / or, the coiling temperature is controlled to be 430 to 500 °C. The manufacturing method according to claim 10, characterized in that.

12. In step (3), the elongation rate of pickling tension leveling is controlled to be 0.2 to 2%, the pickling speed is controlled to be 60 to 150 m / min, the temperature of the final pickling tank in the pickling process is controlled to be 80 to 90 °C, and the iron ion concentration is controlled to be 30 to 40 g / L. The manufacturing method according to claim 10, characterized in that.

13. In step (4), the heating rate is 3 to 25 °C / s; and / or, the annealing soaking temperature is 650 to 730 °C; and / or, the holding time of the annealing soaking stage is 30 to 120 s; and / or, the cooling rate is 12 to 25 °C / s; optionally, before annealing, the steel plate is pre-oxidized to form a pre-oxidized film with a thickness of 60 to 120 nm. The manufacturing method according to claim 10, characterized in that.

14. In step (5), the molten zinc plating pot temperature is 440 to 480 °C. The manufacturing method according to claim 10, characterized in that.

15. In step (6), the leveling rate is 0.05 to 1.3%, preferably, the leveling rate is 0.2% to 0.4%; the thickness of the finally obtained steel plate is ≤ 5 mm. The manufacturing method according to claim 10, characterized in that.

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