A cold-rolled hot-dip galvanized steel sheet having high surface gloss and a method for manufacturing the same

Abstract

The application discloses a cold-rolled hot-dip galvanized steel plate with high surface gloss, which comprises a steel base plate, a hot-dip galvanized layer and a transition layer between the steel base plate and the hot-dip galvanized layer; the transition layer comprises zinc-iron intermetallic compounds, and the average thickness of the transition layer is 2-30% of the thickness of the hot-dip galvanized layer; the grains in the hot-dip galvanized layer are in an equiaxed crystal morphology, and the average grain size is less than 30% of the thickness of the hot-dip galvanized layer; the steel base plate contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows: C: 0.001-0.06%, Mn: 0.15-1.5%, P: 0.02-0.07%, Nb: 0-0.03%, and Ti: 0-0.03%. Correspondingly, the application further discloses a manufacturing method of the cold-rolled hot-dip galvanized steel plate, which comprises the following steps: (1) preparing a steel base plate; (2) continuous annealing; and (3) hot-dip galvanizing. The cold-rolled hot-dip galvanized steel plate prepared by the application has excellent performance, and has the advantages of high surface reflectivity, high formability, low cost and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a manufacturing method thereof, in particular to a cold-rolled hot-dip galvanized steel sheet and a manufacturing method thereof. BACKGROUND

[0002] In recent years, with the rapid development of industrial production, the application scenarios of cold-rolled hot-dip galvanized steel sheets have become more and more.

[0003] One of the characteristics of hot-dip galvanized steel sheets is good corrosion resistance. In the currently obtained hot-dip galvanized steel sheets, the zinc plating layer on the surface of the hot-dip galvanized steel sheet can isolate the steel substrate from the external corrosive environment, and can form a primary cell between the zinc plating layer and the steel substrate, and can realize the improvement of the corrosion resistance of the steel substrate through the principle of sacrificial anode protection.

[0004] In the actual preparation of hot-dip galvanized steel sheets, researchers usually adopt a method of continuously annealing and hot-dip galvanizing the cold-rolled steel substrate to prepare the hot-dip galvanized steel sheet. This preparation method has high production efficiency and can effectively reduce production cost. In addition, in this manufacturing process, the person skilled in the art can also adjust the strip temperature, cooling rate and other process parameters in the continuous annealing process, and change the composition of the steel substrate, to realize the adjustment of the mechanical properties of the steel substrate in a wide range, to meet the requirements of users on the strength, elongation and other aspects of the steel substrate.

[0005] However, there are not many technical solutions for the appearance characteristics of hot-dip galvanized steel sheets, especially how to solve the surface gloss. Two patent documents with publication numbers CN106795612A and CN107075653A, published on May 31, 2017 and August 18, 2017, respectively, and named "High-strength hot-dip galvanized steel sheet" and "High-strength hot-dip galvanized steel sheet" disclose two high-strength hot-dip galvanized steel sheets with different surface gloss. SUMMARY

[0006] One of the purposes of the present application is to provide a cold-rolled hot-dip galvanized steel sheet with high surface gloss. The cold-rolled hot-dip galvanized steel sheet can have high surface light reflection performance without subsequent treatment through the optimization design of the steel substrate, the hot-dip galvanized layer and the transition layer between the steel substrate and the hot-dip galvanized layer. The cost is low, and the steel sheet can obtain excellent formability and high corrosion resistance.

[0007] In order to achieve the above-mentioned purpose, the present application provides a cold-rolled hot-dip galvanized steel sheet with high surface gloss, which comprises a steel substrate, a hot-dip galvanized layer and a transition layer between the steel substrate and the hot-dip galvanized layer:

[0008] The transition layer comprises zinc-iron intermetallic compounds, and the average thickness of the transition layer is 2-30% of the thickness of the hot-dip galvanized layer.

[0009] the grains in the hot-dip galvanized layer are equiaxed, and the average grain size is less than 30% of the thickness of the hot-dip galvanized layer;

[0010] The steel substrate contains Fe and inevitable impurities, and further contains the following chemical elements in the mass percentage: C: 0.001-0.06%, Mn: 0.15-1.5%, P: 0.02-0.07%, Nb: 0-0.03%, Ti: 0-0.03%.

[0011] In the prior art, the surface glossiness of the hot-dip galvanized layer of the conventional hot-dip galvanized steel sheet is low, because the surface of the hot-dip galvanized layer has micro-undulations, thereby affecting the reflectivity of light. In addition, the grain size of the hot-dip galvanized layer of the conventional hot-dip galvanized steel sheet is large, and the grain size in the plane parallel to the surface of the steel sheet is usually greater than the thickness of the plated layer. However, the inventors have found through a large number of researches and practices that when the microstructure of the hot-dip galvanized layer is changed, i.e. the grain size of the hot-dip galvanized layer is reduced, the grains in the hot-dip galvanized layer are equiaxed, and the surface micro-undulations of the hot-dip galvanized layer are significantly reduced, thereby increasing the surface glossiness.

[0012] The key to changing the microstructure of the hot-dip galvanized layer and the degree of surface micro-undulations is to form a transition layer with an appropriate thickness at the zinc liquid-steel substrate interface before the zinc liquid (i.e. the plating liquid) solidifies on the surface of the steel substrate by adjusting the hot-dip galvanizing process, and the transition layer contains zinc-iron intermetallic compounds to increase the nucleation points when the zinc liquid solidifies, thereby ultimately reducing the grain size of the hot-dip galvanized layer.

[0013] In the cold-rolled hot-dip galvanized steel sheet designed in the present application, which is composed of a hot-dip galvanized layer, a transition layer and a steel substrate, and the transition layer contains zinc-iron intermetallic compounds, when the thickness of the transition layer is less than 2% of the thickness of the hot-dip galvanized layer, the influence of the transition layer on the microstructure of the hot-dip galvanized layer is not significant, and when the thickness of the transition layer is greater than 30% of the thickness of the hot-dip galvanized layer, the transition layer will affect the flow of liquid zinc on the surface of the steel substrate when the steel substrate is blown by the air knife, causing the local thickness of the hot-dip galvanized layer to be uneven, thereby reducing the surface quality and glossiness of the plated layer. Therefore, in the present application, the average thickness of the transition layer is specifically controlled to be 2-30% of the thickness of the hot-dip galvanized layer.

[0014] Correspondingly, based on the design of the present application, the grains in the hot-dip galvanized layer can be equiaxed, and the average grain size in the hot-dip galvanized layer needs to be controlled to be less than 30% of the thickness of the hot-dip galvanized layer. This is because when the average grain size in the hot-dip galvanized layer is greater than 30% of the thickness of the hot-dip galvanized layer, the surface micro-undulations of the hot-dip galvanized layer will not be effective.

[0015] Further, the high surface gloss cold-rolled hot-dip galvanized steel sheet of the present application has the following mass percentage of each chemical element in the steel substrate: C: 0.001-0.06%, Mn: 0.15-1.5%, P: 0.02-0.07%, Nb: 0-0.03%, Ti: 0-0.03%, and the balance of Fe and inevitable impurities

[0016] In addition, in the above technical solution of the present application, the inventors further design the chemical element composition of the steel substrate, and the purpose of the above composition design is to obtain a steel substrate with a wide strength level range, good formability and low cost.

[0017] Specifically, in the steel substrate of the cold-rolled hot-dip galvanized steel sheet of the present application, the design principles of each chemical element are as follows:

[0018] C: In the steel substrate of the cold-rolled hot-dip galvanized steel sheet of the present application, the content of C element directly affects the strength and plasticity of the steel substrate. When the content of C element in the steel is too low, it is not easy to accurately control its content; and when the content of C element in the steel is too high, it will lead to the decrease of toughness and plasticity of the steel. Therefore, considering the influence of C element on the performance of the steel, the mass percentage of C element is controlled between 0.001-0.06% in the present application.

[0019] Mn: In the steel substrate of the cold-rolled hot-dip galvanized steel sheet of the present application, Mn is one of the main solid solution strengthening elements in the steel substrate, which can not only increase the strength of the steel substrate, but also improve the hardenability of the steel substrate. When the content of Mn element in the steel is too low, the strengthening effect of Mn is weakened; and when the content of Mn element in the steel is too high, the strength of the prepared steel substrate is too high, the elongation is reduced, and too high Mn content is not conducive to the galvanizability of the steel substrate. Based on this, considering the influence of Mn element content on the performance of the steel, the content of Mn element must be strictly controlled, and in the present application, the mass percentage of Mn element is controlled between 0.15-1.5%.

[0020] P: In the steel substrate of the cold-rolled hot-dip galvanized steel sheet of the present application, P is one of the solid solution strengthening elements, which can effectively improve the strength of the steel substrate. However, it should be noted that the content of P element in the steel should not be too high, and too high content of P element in the steel will reduce the galvanizability of the steel substrate and the toughness of the steel substrate. In addition, too high P content will inhibit the formation of zinc-iron alloy phase, which is not conducive to the control of the gloss of the coating. Therefore, considering the influence of P element on the performance of the steel substrate in the present technical solution, the mass percentage of P in the steel substrate of the cold-rolled hot-dip galvanized steel sheet of the present application is controlled between 0.02-0.07%.

[0021] Nb: In the steel substrate of the cold-rolled hot-dip galvanized steel sheet described in this invention, Nb combines with C and N to form Nb(C,N), which can effectively suppress grain coarsening during hot working, refine ferrite grains, and improve the strength and toughness of the steel substrate. However, it should be noted that the Nb content in the steel should not be too high, as excessive Nb will increase the recrystallization temperature and increase the production cost of the steel substrate. Therefore, in this invention, the mass percentage of Nb is controlled between 0-0.03%.

[0022] Ti: In the steel substrate of the cold-rolled hot-dip galvanized steel sheet described in this invention, Ti, through its combination with C and N to form Ti(C,N), can effectively refine the microstructure of the steel substrate. However, it should be noted that the Ti content in the steel should not be too high. Excessive Ti will increase the size of the aforementioned precipitates, thereby reducing the ductility of the steel substrate and increasing production costs. Therefore, in order to maximize the beneficial effects of Ti, the mass percentage of Ti in the steel substrate of the cold-rolled hot-dip galvanized steel sheet described in this invention is controlled between 0-0.03%.

[0023] Furthermore, in the high surface gloss cold-rolled hot-dip galvanized steel sheet of the present invention, the zinc-iron intermetallic compound occupies more than 70% of the volume in the transition layer.

[0024] Furthermore, in the high surface gloss cold-rolled hot-dip galvanized steel sheet of the present invention, the transition layer further includes an iron-aluminum intermetallic compound.

[0025] In the high-gloss cold-rolled hot-dip galvanized steel sheet described in this invention, the transition layer can specifically be composed of zinc-iron intermetallic compounds and iron-aluminum intermetallic compounds. Among them, zinc-iron intermetallic compounds affect the nucleation and solidification of zinc liquid and the microstructure of zinc coating layer. Therefore, the volume proportion of zinc-iron intermetallic compounds in the transition layer must be greater than 70%. In addition, iron-aluminum intermetallic compounds are formed due to the reaction between the steel substrate and Al element in the plating bath, and their formation cannot be completely avoided.

[0026] Furthermore, in the high surface gloss cold-rolled hot-dip galvanized steel sheet of the present invention, the thickness of the hot-dip galvanized layer is 5-25 μm.

[0027] In the cold-rolled hot-dip galvanized steel sheet designed in this invention, when the thickness of the hot-dip galvanized layer is less than 5 μm, the steel sheet cannot achieve excellent corrosion resistance; while when the thickness of the hot-dip galvanized layer is greater than 25 μm, the uniformity of the coating thickness at the edges and center of the steel substrate in the width direction is difficult to control, and the cost is too high. Therefore, in this invention, the thickness of the hot-dip galvanized layer can preferably be controlled between 5-25 μm.

[0028] Further, in the cold-rolled hot-dip galvanized steel sheet with high surface glossiness, the hot-dip galvanized layer has a composition with Al 0.1-0.3% and the rest being Zn and inevitable impurities.

[0029] In the technical scheme above, the inventors can further ensure that the hot-dip galvanized layer contains Al 0.1-0.3% and the rest being Zn and inevitable impurities. In this design, a small amount of Al is added to the plating solution, so the hot-dip galvanized layer inevitably contains Al.

[0030] It should be noted that if the content of Al in the hot-dip galvanized layer is too high, the Al will be enriched on the surface of the hot-dip galvanized layer and form an aluminum oxide film, thereby reducing the surface glossiness. Therefore, the content of Al in the hot-dip galvanized layer is preferably controlled to be between 0.1-0.3%.

[0031] Further, in the cold-rolled hot-dip galvanized steel sheet with high surface glossiness, the average glossiness value of the surface is greater than 400 gloss units.

[0032] Correspondingly, another object of the present application is to provide a manufacturing method of the cold-rolled hot-dip galvanized steel sheet with high surface glossiness above. The cold-rolled hot-dip galvanized steel sheet obtained by using the manufacturing method has the advantages of high surface reflectivity, low production cost, excellent formability and high corrosion resistance.

[0033] To achieve the above objects, the present application provides a manufacturing method of the cold-rolled hot-dip galvanized steel sheet with high surface glossiness above, which comprises the following steps:

[0034] (1) preparing a steel substrate;

[0035] (2) continuous annealing: heating the steel strip to a soaking temperature of 650-850℃ and maintaining for 30-180s;

[0036] (3) hot-dip galvanizing: the plating solution contains Al 0.10-0.14% by mass; the total time of the steel substrate immersed in the plating solution is 1-5s; and the steel substrate is cooled to ≤250℃ at a cooling rate of >6℃ / s after leaving the zinc pot.

[0037] In this technical scheme designed by the inventors, the manufacturing process of the steel substrate is not particularly limited, and those skilled in the art can use conventional technical means to design the steel substrate according to the chemical composition of the designed steel substrate.

[0038] In some specific embodiments, the skilled in the art can smelt the chemical composition designed according to the present application, and specifically heat the casting blank at 1150-1250 ℃, control the holding time to be 0.5-3 h, the final rolling temperature of hot rolling to be 850-950 ℃, then perform coiling at 500-700 ℃, and then perform pickling and cold rolling on the hot-rolled coil, and control the final cold rolling reduction to be 30-90%, so as to prepare the required steel substrate.

[0039] In the above technical scheme of the present application, in the continuous annealing process of step (2), the selection of soaking temperature and holding time is mainly to obtain appropriate mechanical properties and excellent surface platability. When the soaking temperature is lower than 650 ℃ and the holding time is lower than 30 s, the steel substrate after cold rolling cannot be fully recrystallized, which reduces the formability of the steel substrate. When the soaking temperature is higher than 850 ℃ and the holding time is greater than 180 s, grain coarsening occurs in the steel substrate, which reduces the strength and toughness of the steel substrate, and the amount of alloying elements diffusing to the surface of the steel substrate is large, which is not conducive to the surface platability of the steel substrate.

[0040] In addition, in the hot dip galvanizing process of step (3) of the present application, the mass percentage of effective Al in the plating solution can be specifically controlled to be 0.10-0.14%. This is because, when the effective Al content in the plating solution is too low, lower than 0.10%, the chemical reaction speed between the steel substrate and the zinc liquid (i.e. the plating solution) is too fast, the thickness of the zinc-iron intermetallic compound formed at the interface between the steel substrate and the zinc liquid is not easy to control, thereby affecting the flowability of the zinc liquid on the surface of the steel substrate, and finally reducing the thickness uniformity of the coating and the gloss of the coating surface. In addition, when the Al content in the plating solution is too low, the Fe element dissolved from the steel substrate into the plating solution is easy to form dross in the plating solution, increasing the defect rate of the coating surface. When the Al content in the plating solution is too high, higher than 0.14%, the steel strip entering the plating solution will form a thick and dense iron-aluminum intermetallic compound layer with the Al element in the plating solution, thereby inhibiting the formation of zinc-iron intermetallic compound, and thus failing to achieve the effect of improving the gloss of the coating surface.

[0041] In addition, in step (3), the time for the steel substrate to be immersed in the plating solution needs to be specifically controlled to be 1-5 s. If the steel substrate is immersed in the plating solution for too short a time, the thickness of the zinc-iron intermetallic compound transition layer is too thin or the volume fraction of the zinc-iron intermetallic compound in the transition layer is too low, so that the effect of improving the reflectivity cannot be achieved. If the steel substrate is immersed in the plating solution for too long a time, the amount of iron-aluminum compound formed between the steel substrate and the plating solution will increase, and the production of zinc-iron intermetallic compound will be inhibited, thereby causing the thickness of the zinc-iron intermetallic compound on the surface of the steel strip to be unevenly distributed, and reducing the gloss of the coating surface.

[0042] Correspondingly, in the hot-dip galvanizing process designed in the present application, the steel substrate is also required to be cooled at a cooling rate greater than 6 ℃ / s to 250 ℃ or below after being taken out of the zinc pot. This is because: when the cooling rate is too slow, the thickness of the formed zinc-iron intermetallic compound is prone to be high, and the grain size in the zinc metal layer is prone to be large, thereby being unfavorable to the improvement of the glossiness.

[0043] Further, in the manufacturing method described in the present application, in step (2), the atmosphere in the heating section and the holding section is N2, H2 and H2O mixed gas, wherein the H2 volume content is 1-20%, and the dew point is -50-20 ℃.

[0044] In the manufacturing method designed in the present application, N2, H2 and H2O mixed gas can be used as the atmosphere in the heating section and the holding section, wherein the H2 volume content and the dew point are selected to make the steel substrate have better galvanizability. When the designed H2 volume content is less than 1%, the residual thin film of iron oxide on the surface of the cold-rolled steel substrate cannot be effectively reduced, which is unfavorable to the immersion of the plating solution on the surface of the steel substrate; and when the H2 volume content is greater than 20%, the safety hazard and cost will be increased.

[0045] In addition, since the gas impurity H2O cannot be completely avoided in the annealing furnace, and the H2 in the annealing atmosphere will react with the unavoidable gas impurity O2 in the furnace to form H2O, it is difficult to guarantee that the dew point in the annealing furnace is below -50 ℃; when the dew point is higher than 20 ℃, the iron element in the steel substrate will react with H2O to form iron oxide, thereby reducing the wettability between the steel substrate and the plating solution and the adhesion of the coating. Therefore, in the technical solution designed in the present application, the dew point in the annealing atmosphere is limited to -50-20 ℃.

[0046] Further, in the manufacturing method described in the present application, in step (2), if the mass percentage content of Mn in the steel substrate is greater than 1.0%, the dew point of the atmosphere in the heating section and the holding section is controlled to be greater than -10 ℃.

[0047] In the above technical solution of the present application, when the mass percentage content of Mn in the steel substrate is greater than 1.0%, the dew point of the atmosphere in the heating section and the holding section is further controlled to be greater than -10 ℃. The reason is that: when the dew point of the annealing atmosphere is low, the Mn element in the steel substrate is prone to be segregated on the surface of the steel substrate and react with H2O in the atmosphere to form a thin film of Mn-containing oxide on the surface of the steel substrate, thereby reducing the wettability of the plating solution on the surface of the steel substrate, which will lead to defects such as plating leakage and reduce the surface quality of the coating; and when the dew point of the annealing atmosphere is high, the diffusion flux of O element into the steel substrate increases, and the Mn element will combine with the O element in the steel substrate to form an oxide precipitated phase, which will to some extent prevent the Mn element from being segregated on the surface of the steel substrate.

[0048] Therefore, in order to reduce the amount of Mn-containing oxides on the surface of the steel substrate as much as possible, when the mass percentage of Mn in the steel substrate is greater than 1.0%, the dew point of the atmosphere in the heating section and the holding section is controlled to be above -10°C.

[0049] Further, in the manufacturing method described in the present application, in step (3), the temperature of the plating solution is controlled to be 450-470°C, and the temperature difference between the steel substrate when entering the zinc pot and the temperature of the plating solution is less than 10°C.

[0050] In the above technical solution of the present application, the temperature of the plating solution can be controlled to be 450-470°C, because: when the temperature of the plating solution is too low, the flowability of the plating solution is poor, and it is not easy to control the uniformity of the plating layer thickness; and when the temperature of the plating solution is too high, the evaporation rate of the zinc solution increases, and zinc ash is easily produced in the furnace nose, thereby increasing defects such as scale and increasing the production cost. Therefore, preferably, the temperature of the plating solution is controlled to be between 450-470°C.

[0051] In addition, when the temperature difference between the strip temperature when entering the zinc pot and the temperature of the plating solution is large, it will increase the difficulty of heat balance management of the zinc pot, and also promote the formation of zinc slag, thereby affecting the surface quality of the finished product. Therefore, preferably, the temperature difference between the steel substrate temperature when entering the zinc pot and the temperature of the plating solution can be controlled to be within 10°C.

[0052] Compared with the prior art, the cold-rolled hot-dip galvanized steel sheet with high surface glossiness and the manufacturing method thereof described in the present application have the following advantages and beneficial effects:

[0053] The cold-rolled hot-dip galvanized steel sheet with high surface glossiness described in the present application has the advantage of high surface light reflection performance without subsequent treatment, low cost, and excellent formability and high corrosion resistance of the steel sheet.

[0054] Unlike the conventional hot-dip galvanized steel sheet designed in the prior art, the cold-rolled hot-dip galvanized steel sheet designed in the present application has lower cost and higher surface glossiness, the surface glossiness value is greater than 400 gloss units (the mirror glossiness value of polished black glass with a refractive index of 1.567 is set to be 100 gloss units at a geometric angle of 60 degrees), and has high corrosion resistance and excellent formability, has very broad application prospects and use value. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 The structure of the cold-rolled hot-dip galvanized steel sheet with high surface glossiness described in the present application is schematically shown.

[0056] Figure 2A practical cross-section metallographic scanning electron microscope backscattered electron image of the high surface gloss cold rolled hot dip galvanized steel sheet of Example 1.

[0057] Figure 3 A transition layer surface scanning electron microscope secondary electron image of the high surface gloss cold rolled hot dip galvanized steel sheet of Example 1 after removing the hot dip galvanized layer. DETAILED DESCRIPTION

[0058] The high surface gloss cold rolled hot dip galvanized steel sheet and the manufacturing method thereof according to the present application will be further explained and described in conjunction with specific examples and the accompanying drawings of the specification, however, the explanation and description does not constitute undue limitation on the technical solution of the present application.

[0059] Examples 1-8 and Comparative Examples 1-7

[0060] In the present application, the steel substrate corresponding to the cold rolled hot dip galvanized steel sheet of Examples 1-8 and the comparative galvanized steel sheet of Comparative Examples 1-7 is designed with the chemical element composition shown in Table 1 below.

[0061] Table 1. (wt%, the balance is Fe and other inevitable impurity elements)

[0062]

[0063]

[0064] The cold rolled hot dip galvanized steel sheet of Examples 1-8 and the comparative galvanized steel sheet of Comparative Examples 1-7 according to the present application are both prepared by the following steps:

[0065] (1) Steel substrate preparation: smelting according to the chemical composition shown in Table 1, and controlling the heating of the cast blank at 1150-1250℃, controlling the holding time for 0.5-3h, the final hot rolling temperature for 850-950℃, then coiling at 500-700℃, followed by pickling and cold rolling of the hot rolled coil, and controlling the final cold rolling reduction between 30-90%.

[0066] (2) Continuous annealing of the steel substrate: heating to 650-850℃ at an average heating rate of 10℃ / s, then holding for 30-180s, wherein the atmosphere of the heating section and the holding section is a mixture of N2, H2 and H2O, and the volume content of H2 is 1-20%, and the atmosphere dew point is -50-20℃.

[0067] (3) hot-dip galvanizing: after annealing, the strip steel is cooled to the strip steel entry temperature of the zinc pot at 40 °C / s in a mixed gas of N2, 10% by volume H2 and H2O, and after holding for 15 s at the entry temperature of the zinc pot, the strip steel is put into the plating solution in the zinc pot for hot-dip plating. Among them, the plating solution temperature is kept at 450-470 °C, the difference between the entry temperature of the zinc pot and the plating solution temperature is not more than 10 °C, the mass percentage of Al element in the plating solution is 0.10-0.14%, and the rest is Zn and inevitable impurities. The immersion time of the steel substrate in the plating solution is controlled to be 1-5 s, and after the steel substrate leaves the plating solution, it is quickly passed through the air knife to control the thickness of the hot-dip galvanized layer to be between 5-25 microns, and then the strip steel is cooled to 250 °C or below at an average cooling rate of more than 6 °C / s.

[0068] In the present application, the hot-dip galvanizing process of step (3) described above can effectively form a hot-dip galvanized layer on the surface of the steel substrate, and a transition layer containing zinc-iron intermetallic compounds and iron-aluminum intermetallic compounds is also formed between the hot-dip galvanized layer and the steel substrate.

[0069] It should be noted that the chemical composition design and related processes of the cold-rolled hot-dip galvanized steel sheets of Examples 1-8 meet the specification requirements designed in the present application. Although the steps used for the comparative hot-dip galvanized steel sheets of Comparative Examples 1-7 are also steps (1)-(3) described above, in the specific processes used for the comparative hot-dip galvanized steel sheets of Comparative Examples 1-7, there are parameters that do not meet the design requirements of the present application.

[0070] Table 2-1 and Table 2-2 list the specific process parameters of the cold-rolled hot-dip galvanized steel sheets of Examples 1-8 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-7.

[0071] Table 2-1.

[0072]

[0073]

[0074] Table 2-2.

[0075]

[0076] The finished cold-rolled hot-dip galvanized steel sheets of Examples 1-8 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-7 obtained by the above process steps are sampled respectively, and the zinc-plated steel sheets of each example and comparative sample are observed and analyzed to accurately analyze the properties of the prepared zinc-plated steel sheets.

[0077] In the present application, the inventors determined the plating appearance of the hot-dip galvanized steel sheets of each of Examples 1-8 and Comparative Examples 1-7 by visual observation, and determined that the cases of uniform plating, fewer missed plating points, and fewer point dross defects were "good", i.e., the appearance was "OK". The cases of poor appearance, such as uneven plating, a large area of missed plating, and a large number of point dross defects were determined to be "poor", i.e., the appearance was "NG". The results of the relevant observation are shown in Table 3 below.

[0078] In addition, based on the ASTM D 523 standard, the inventors measured the 60-degree specular gloss of the surface of the hot-dip galvanized steel sheet using a BYK 4561 micro-gloss meter to obtain the average gloss value of the surface of the steel sheets of Examples 1-8 and Comparative Examples 1-7. The specular gloss value of a polished black glass with a refractive index of 1.567 at a geometric angle of 60 degrees was set to 100 gloss units.

[0079] In addition, in the present application, the average thickness of the transition layer was also measured by taking a scanning electron microscope backscattered electron image of the cross-section metallograph of the corresponding hot-dip galvanized steel sheet and measuring it. The measurement of the thickness of the transition layer, particularly the thickness of the zinc-iron compound, required that the cross-section metallograph sample be etched with 0.5% nitric acid alcohol solution for 5 seconds, then a scanning electron microscope backscattered electron image was taken, and finally the obtained image was analyzed by image analysis software to obtain the average thickness of the transition layer. The relevant test results are shown in Table 3 below.

[0080] Correspondingly, the quantification of the thickness of the iron-aluminum compound in the transition layer can be specifically achieved by measuring the depth profile of the Al element using a glow discharge spectrometer, integrating the Al element enrichment peak observed at the hot-dip galvanized layer-steel substrate interface position, and converting the integral area into thickness according to the atomic ratio in the chemical formula Fe2Al5.

[0081] In addition, regarding the observation of the grain morphology and the measurement of the average grain size in the hot-dip galvanized layer, the inverse pole figure measured by EBSD testing can be used, wherein it was observed that the grains in the hot-dip galvanized layer of each of the examples were equiaxed in morphology, and the average grain size was less than 30% of the thickness of the hot-dip galvanized layer. As for the Al content in the hot-dip galvanized layer of the steel sheets of Examples 1-8 and Comparative Examples 1-7, the hot-dip galvanized layer was peeled off using dilute hydrochloric acid containing an inhibitor, and then quantified using ICP emission spectroscopy.

[0082] Table 3 shows the observation and analysis results of the cold-rolled hot-dip galvanized steel sheets of Examples 1-8 and the comparative galvanized steel sheets of Comparative Examples 1-7.

[0083] Table 3.

[0084]

[0085]

[0086] It should be noted that the Al content in the hot-dip galvanized layer listed in Table 3 is higher than the effective Al content in the plating solution in Table 2-2 because when measuring the Al content in the hot-dip galvanized layer, Al from the transition layer will inevitably be introduced.

[0087] As can be seen from Table 3 above, compared with Comparative Examples 1-7, the cold-rolled hot-dip galvanized steel sheets designed in Examples 1-8 have superior comprehensive performance. The surface appearance of the cold-rolled hot-dip galvanized steel sheets in Examples 1-8 is all "OK", that is, their coating is uniform, with fewer uncoated spots and fewer slag defects. In contrast, Comparative Examples 1-7 have poor surface appearance quality.

[0088] Furthermore, through observation and analysis, it was found that in this invention, the transition layer of the cold-rolled hot-dip galvanized steel sheets in Examples 1-8 all include zinc-iron intermetallic compounds, and the volume proportion of zinc-iron intermetallic compounds in the transition layer is greater than 70%. Additionally, in the cold-rolled hot-dip galvanized steel sheets of Examples 1-8 designed in this invention, the thickness of the hot-dip galvanized layer is 5-25 μm, the average thickness of the transition layer is 2-30% of the thickness of the hot-dip galvanized layer, and the grains in the resulting hot-dip galvanized layer exhibit an equiaxed crystal morphology, with an average grain size less than 30% of the thickness of the hot-dip galvanized layer.

[0089] Accordingly, through research and analysis, it is not difficult to find that in the present invention, the content of Al element in the hot-dip galvanized layer of the cold-rolled hot-dip galvanized steel sheet of Examples 1-8 is between 0.1% and 0.3%, which has excellent surface gloss and the average gloss value of the surface is greater than 400 gloss units.

[0090] In contrast, Comparative Examples 1-7 show cases where the average thickness of the transition layer is not 2-30% of the thickness of the hot-dip galvanized layer, and cases where the average surface gloss value is less than 400 gloss units or the appearance is NG.

[0091] Figure 1 A schematic diagram of the structure of the high surface gloss cold-rolled hot-dip galvanized steel sheet described in this invention is shown.

[0092] from Figure 1 As can be seen from the above, the high-gloss cold-rolled hot-dip galvanized steel sheet designed in this invention specifically includes: a steel substrate 1 and a hot-dip galvanized layer 3 on the surface, and a transition layer 2 between the steel substrate 1 and the hot-dip galvanized layer 3. The transition layer 2 is composed of zinc-iron intermetallic compounds and iron-aluminum intermetallic compounds.

[0093] Figure 2 The image shows a metallographic scanning electron microscope backscattered electron image of the actual cross-section of a cold-rolled hot-dip galvanized steel sheet with high surface gloss, as described in Example 1.

[0094] likeFigure 2 As shown in the scanning electron microscope, the cold-rolled hot-dip galvanized steel sheet of Example 1 still has a three-layer structure, i.e. Figure 2 The steel substrate C, the hot-dip galvanized layer A and the transition layer B between the steel substrate C and the hot-dip galvanized layer A.

[0095] Figure 3 The scanning electron microscope secondary electron image of the surface of the transition layer after removing the hot-dip galvanized layer of the cold-rolled hot-dip galvanized steel sheet with high surface gloss of Example 1.

[0096] As Figure 3 As shown in the embodiment, the cold-rolled hot-dip galvanized steel sheet of Example 1 can remove the surface hot-dip galvanized layer in the dilute hydrochloric acid added with the corrosion inhibitor, and after removing the surface hot-dip galvanized layer, the transition layer mainly composed of zinc-iron intermetallic compound can be observed.

[0097] It should be noted that the combination of the technical features in the case is not limited to the combination of the claims in the case or the combination of the embodiments, and all the technical features disclosed in the case can be freely combined or combined in any way, unless contradictory to each other.

[0098] It should also be noted that the above-mentioned examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above examples, and similar changes or modifications made directly from the disclosure of the present application or easily thought of by those skilled in the art should all belong to the protection scope of the present application.

Claims

1. A cold-rolled hot-dip galvanized steel sheet with high surface glossiness, comprising a steel substrate, a hot-dip galvanized layer, and a transition layer between the steel substrate and the hot-dip galvanized layer, characterized in that: the transition layer comprises zinc-iron intermetallic compounds, the average thickness of the transition layer is 2-30% of the thickness of the hot-dip galvanized layer, and the volume fraction of the zinc-iron intermetallic compounds in the transition layer is greater than 70%; the grains in the hot-dip galvanized layer have an equiaxed crystal morphology, and the average grain size is less than 30% of the thickness of the hot-dip galvanized layer; the hot-dip galvanized layer has a composition of 0.1-0.3% Al and the balance of Zn and inevitable impurities; the steel substrate has a composition of 0.001-0.06% C, 0.15-1.5% Mn, 0.02-0.07% P, 0-0.03% Nb, 0-0.03% Ti, and the balance of Fe and inevitable impurities; the transition layer further comprises iron-aluminum intermetallic compounds; the thickness of the hot-dip galvanized layer is 5-25 μm; the average glossiness value of the surface is greater than 400 gloss units; and the method comprises the steps of: (1) preparing a steel substrate; (2) continuous annealing: heating the strip to a soaking temperature of 650-850°C and maintaining for 30-180 s; (3) hot-dip galvanizing: the mass percentage of Al in the plating solution is 0.10-0.14%, the total time of the steel substrate in the plating solution is 1-5 s, and the cooling rate of the steel substrate after exiting the zinc pot is greater than 6°C / s to ≤250°C; in step (2), the atmosphere in the heating and soaking stages is a mixture of N2, H2, and H2O, wherein the volume content of H2 is 1-20% and the dew point is -50-20°C; in step (2), if the mass percentage of Mn in the steel substrate is greater than 1.0%, the dew point of the atmosphere in the heating and soaking stages is controlled to be greater than -10°C; and in step (3), the plating solution temperature is controlled to be 450-470°C, and the temperature difference between the steel substrate and the plating solution when entering the zinc pot is less than 10°C. ​ ​ ​ 2. The cold rolled and hot dip galvanized steel sheet with high surface gloss according to claim 1, characterized by, ​ 3. The cold rolled and hot dip galvanized steel sheet with high surface gloss according to claim 1, characterized by, ​ 4. A cold rolled and galvannealed steel sheet with high surface gloss according to anyone of claims 1 to 3, characterized in that, ​ 5. The method of producing a cold rolled galvannealed steel sheet having a high surface gloss according to any one of claims 1 to 4, characterized in that, ​ ​ ​ ​ 6. The production method according to claim 5, wherein ​ 7. The production method according to claim 6, wherein ​ 8. The production method according to claim 5, wherein ​

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