High-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss and method for manufacturing the same

Abstract

The application discloses a high-strength 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, wherein 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; 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 the mass percentage as follows: C: 0.09-0.25%, Mn: 2.3-3.0%, Si: 0.3-2.0%, Cr: 0-0.7%, Nb: 0-0.03%, and Ti: 0-0.03%. Correspondingly, the application further discloses a manufacturing method of the high-strength 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] This invention relates to a steel plate and its manufacturing method, and more particularly to a cold-rolled hot-dip galvanized steel plate and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of industrial production, cold-rolled hot-dip galvanized steel sheets have begun to be widely used in building materials, automobiles, home appliances and other fields, and the application scenarios of cold-rolled hot-dip galvanized steel sheets are becoming more and more diverse.

[0003] One of the characteristics of hot-dip galvanized steel sheets is their good corrosion resistance. In currently available hot-dip galvanized steel sheets, the zinc coating on the surface isolates the steel substrate from the external corrosive environment, and a galvanic cell can be formed between the zinc coating and the steel substrate. This improves the corrosion resistance of the steel substrate through the principle of sacrificial anode protection.

[0004] In actual production of hot-dip galvanized steel sheets, they are typically obtained by continuously annealing and hot-dip galvanizing cold-rolled steel substrates. This method offers high production efficiency and effectively reduces production costs. Furthermore, in this manufacturing process, those skilled in the art can adjust a wide range of mechanical properties of the steel substrate by modifying process parameters such as strip temperature and cooling rate during continuous annealing, as well as changing the composition of the steel substrate, to meet user requirements for strength, elongation, and other aspects.

[0005] However, there are few existing technologies that address the appearance characteristics of hot-dip galvanized steel sheets, especially how to improve surface gloss. Two patent documents, CN106795612A (published May 31, 2017, entitled "High-strength Hot-dip Galvanized Steel Sheet") and CN107075653A (published August 18, 2017, also entitled "High-strength Hot-dip Galvanized Steel Sheet"), disclose two types of high-strength hot-dip galvanized steel sheets with different surface gloss levels. Summary of the Invention

[0006] One of the objectives of this invention is to provide a high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss. This high-strength cold-rolled hot-dip galvanized steel sheet, through optimized design of its own steel substrate, hot-dip galvanized layer and transition layer located between the steel substrate and the hot-dip galvanized layer, can obtain the advantage of high surface reflectivity without subsequent processing. It has a beautiful appearance, fewer surface defects and lower cost. The steel sheet can simultaneously obtain excellent formability, high strength and high corrosion resistance.

[0007] To achieve the above objectives, the present invention provides a high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss, comprising a steel substrate, a hot-dip galvanized layer, and a transition layer located between the steel substrate and the hot-dip galvanized layer:

[0008] The transition layer comprises a zinc-iron intermetallic compound, 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 have an equiaxed crystal morphology, 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 unavoidable impurities, as well as the following chemical elements in the following mass percentages: C: 0.09-0.25%, Mn: 2.3-3.0%, Si: 0.3-2.0%, Cr: 0-0.7%, Nb: 0-0.03%, Ti: 0-0.03%.

[0011] In existing technologies, the surface gloss of the hot-dip galvanized layer of conventional hot-dip galvanized steel sheets is relatively low. This is because there are microscopic undulations on the surface of the hot-dip galvanized layer, which affect the light reflectivity. In addition, the grain size of the hot-dip galvanized layer of conventional hot-dip galvanized steel sheets is relatively large, and the grain size in a plane parallel to the surface of the steel substrate is usually greater than the coating thickness.

[0012] To address this problem in the existing technology, the inventors discovered through extensive research and practice that when the microstructure of the hot-dip galvanized layer is altered, i.e., the grain size of the hot-dip galvanized layer is reduced, the grains in the hot-dip galvanized layer will exhibit an equiaxed crystal morphology, and the surface micro-undulations of the hot-dip galvanized layer will be significantly reduced. This design can effectively increase the gloss of the hot-dip galvanized layer surface.

[0013] The inventors discovered that the key to changing the microstructure and surface micro-undulation of the hot-dip galvanized layer lies in adjusting the hot-dip galvanizing process so that a transition layer containing zinc-iron intermetallic compounds is formed at the zinc-steel substrate interface before the zinc liquid (plating solution) solidifies on the steel substrate surface. This effectively increases the nucleation points during the solidification of the zinc liquid and ultimately reduces the grain size of the hot-dip galvanized layer.

[0014] In this invention, the high-strength cold-rolled hot-dip galvanized steel sheet designed by the inventor consists of three layers: a hot-dip galvanized layer, a transition layer, and a steel substrate. The transition layer includes a zinc-iron intermetallic compound. When the thickness of the transition layer is less than 2% of the thickness of the hot-dip galvanized layer, its influence on the microstructure of the hot-dip galvanized layer is insignificant. However, when the thickness of the transition layer is greater than 30% of the thickness of the hot-dip galvanized layer, the transition layer affects the flow of liquid zinc on the surface of the steel substrate during air knife cleaning, causing uneven local thickness of the hot-dip galvanized layer, thereby reducing the surface quality and gloss of the hot-dip galvanized layer. Therefore, in this invention, the average thickness of the transition layer is specifically controlled to be 2-30% of the thickness of the hot-dip galvanized layer.

[0015] Accordingly, based on this design of the present invention, the grains in the hot-dip galvanized layer of the steel plate can exhibit an equiaxed crystal morphology, and it is necessary to specifically control the average grain size in the hot-dip galvanized layer 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, it will result in poor micro-undulation of the surface of the hot-dip galvanized layer.

[0016] Furthermore, in the above-mentioned technical solution of the present invention, the inventors have further designed the chemical element composition of the steel substrate. The purpose of the above composition design is to obtain a steel substrate with a tensile strength higher than 980 MPa, while having good formability and low cost.

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

[0018] C: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate described in this invention, the content of element C directly affects the strength and plasticity of the substrate. When the C content in the steel is too low, it is difficult to obtain a steel substrate with a tensile strength higher than 980 MPa. Appropriately increasing the C content can stabilize austenite and form retained austenite at room temperature, thereby improving the work hardening rate and elongation of the steel substrate. Conversely, when the C content in the steel is too high, it will lead to a decrease in the toughness and plasticity of the steel. Therefore, considering the influence of element C on the properties of steel, in this invention, the mass percentage of element C is controlled between 0.09% and 0.25%.

[0019] Mn: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate described in this invention, Mn is one of the main solid solution strengthening elements. It can not only increase the strength of the steel substrate, but also improve its hardenability. When the Mn content in the steel is too low, the strengthening effect of Mn is weakened; while when the Mn content in the steel is too high, it will adversely affect the plating properties of the steel substrate. Based on this, considering the influence of Mn content on the performance of steel, the Mn content must be strictly controlled. In this invention, the mass percentage of Mn is controlled between 2.3% and 3.0%.

[0020] Si: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate described in this invention, Si is also one of the solid solution strengthening elements in the substrate, which can effectively improve the strength of the substrate. Simultaneously, Si can inhibit carbide precipitation, promote the enrichment of C atoms in austenite, improve the stability of austenite, and thus increase the work hardening rate of the substrate. When the Si content in the steel is too low, its effect on improving the ductility of the substrate is poor; while when the Si content in the steel is too high, it easily leads to the formation of red iron scale defects on the hot-rolled sheet, affecting the surface quality of the cold-rolled sheet and the finished product, and increasing the difficulty of strip steel manufacturing. Therefore, in order to maximize the beneficial effects of Si, in this invention, the mass percentage of Si is controlled between 0.3% and 2.0%.

[0021] Cr: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate described in this invention, adding an appropriate amount of Cr can effectively improve the hardenability of the steel substrate, reduce the critical cooling rate for quenching, and increase the strength of the steel substrate. However, it should be noted that the Cr content in the steel should not be too high. When the Cr content in the steel is too high, it will reduce the ductility of the steel substrate, and since Cr is expensive, it will also significantly increase production costs. Therefore, in this invention, the mass percentage of Cr is controlled between 0-0.7%.

[0022] Nb: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate 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%.

[0023] Ti: In the high-strength cold-rolled hot-dip galvanized steel sheet substrate 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, excessive Ti will increase the size of the 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 is controlled between 0-0.03% in this invention.

[0024] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the zinc-iron intermetallic compound occupies more than 70% of the volume in the transition layer.

[0025] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the transition layer also includes an iron-aluminum intermetallic compound.

[0026] In the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss 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 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 solution, and their formation cannot be completely avoided.

[0027] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the thickness of the hot-dip galvanized layer is 5-25 μm.

[0028] 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.

[0029] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the mass percentage of each chemical element in the steel substrate is as follows: C: 0.09-0.25%, Mn: 2.3-3.0%, Si: 0.3-2.0%, Cr: 0-0.7%, Nb: 0-0.03%, Ti: 0-0.03%, with the balance being Fe and unavoidable impurities.

[0030] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the composition mass percentage of the hot-dip galvanized layer is: Al: 0.1-0.5%, with the remainder being Zn and unavoidable impurities.

[0031] In the above-described technical solution of this invention, the inventors, through designing the plating solution, can further ensure that the formed hot-dip galvanized layer contains 0.1-0.5% Al element by mass, with the remainder being Zn and unavoidable impurities. In this design, because a small amount of Al element is added to the plating solution, the hot-dip galvanized layer will inevitably contain Al element.

[0032] It should be noted that when the Al content in the hot-dip galvanized layer is too high, the Al will accumulate on the surface of the hot-dip galvanized layer and form an aluminum oxide film, thereby reducing the surface gloss. Therefore, it is preferable to control the mass percentage of Al in the hot-dip galvanized layer of the present invention between 0.1% and 0.5%.

[0033] Furthermore, in the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention, the average gloss value of its surface is greater than 400 gloss units, and its tensile strength is greater than 980 MPa.

[0034] Accordingly, another objective of the present invention is to provide a method for manufacturing the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss as described above. The inventors have optimized the manufacturing process. The high-strength cold-rolled hot-dip galvanized steel sheet obtained by this manufacturing method has the advantages of high surface reflectivity and fewer surface defects, as well as low cost, high strength, good formability, and high corrosion resistance.

[0035] To achieve the above objectives, the present invention provides a method for manufacturing high-strength cold-rolled hot-dip galvanized steel sheets with high surface gloss, comprising the following steps:

[0036] (1) A steel substrate is prepared;

[0037] (2) Continuous annealing: Heat the strip to a uniform temperature of 750-900℃ and hold for 30-180s;

[0038] (3) Hot-dip galvanizing: The mass percentage of Al element in the galvanizing solution is 0.10-0.25%; the total immersion time of the steel substrate in the galvanizing solution is 1-5s; after the steel substrate is removed from the zinc pot, it is cooled to ≤250℃ at a cooling rate of greater than 20℃ / s.

[0039] In this technical solution designed by the present invention, the inventors have not imposed any special limitations on the manufacturing process of the steel substrate. Those skilled in the art can use conventional technical means to design and produce the corresponding plate according to the chemical composition of the designed steel substrate. In some specific embodiments, those skilled in the art can carry out smelting according to the chemical composition designed by the present invention, specifically by heating the billet at 1150-1250℃, controlling the holding time to be 0.5-3h, hot rolling final rolling temperature to 850-950℃, then coiling at 500-600℃, followed by pickling and cold rolling of the hot-rolled coil, and controlling the final cold rolling reduction to be between 30-90% to prepare the required steel substrate.

[0040] In the continuous annealing process of step (2) of the present invention, the selection of the above-mentioned homogenization temperature and holding time, controlling the homogenization temperature to 750-900℃ and controlling the holding time to 30-180s is mainly to obtain suitable mechanical properties of the steel substrate and excellent surface plating properties. When the homogenization temperature is lower than 750℃ and the holding time is lower than 30s, the austenite content formed in the steel substrate during the homogenization process is low, resulting in a lower proportion of strengthening phase formed after cooling, making it difficult to achieve a tensile strength higher than 980MPa; while when the homogenization temperature is higher than 900℃ and the holding time is greater than 180s, grain coarsening will occur in the steel substrate, reducing the strength and toughness of the steel substrate, and high-temperature long-term annealing will promote the diffusion of alloying elements in the steel substrate to the surface, increasing the oxide thickness on the surface of the steel substrate, which is detrimental to the surface plating properties of the steel substrate.

[0041] Furthermore, in the hot-dip galvanizing process of step (3) of this invention, the effective Al content in the plating solution can be specifically controlled to be 0.10-0.25% by mass. This is because when the effective Al content in the plating solution is too low, below 0.10%, the chemical reaction rate between the steel substrate and the zinc liquid (plating solution) is too fast, and the thickness of the zinc-iron intermetallic compound formed at the interface between the steel substrate and the zinc liquid is difficult to control, thereby reducing the uniformity of the coating thickness and the gloss of the coating surface. In addition, when the Al content in the plating solution is too low, the Fe element dissolved in the plating solution from the steel substrate is prone to form bottom slag in the plating solution, ultimately increasing the incidence of surface defects in the coating. When the Al content in the plating solution is too high, above 0.25%, the steel substrate will form a thicker and denser iron-aluminum intermetallic compound layer with the Al element in the plating solution after entering the plating solution, thereby inhibiting the formation of zinc-iron intermetallic compounds, and thus failing to achieve the effect of improving the reflectivity of the coating surface.

[0042] In addition, in step (3), the immersion time of the steel substrate in the plating solution needs to be specifically controlled to be 1-5 seconds. If the immersion time of the steel substrate in the plating solution is too short, the thickness of the zinc-iron intermetallic compound transition layer will be too thin or the proportion of zinc-iron intermetallic compounds in the transition layer will be too low, thus failing to improve the reflectivity. If the immersion time of the steel substrate in the plating solution is too long, the amount of iron-aluminum intermetallic compounds formed between the steel substrate and the plating solution will increase, and the iron-aluminum intermetallic compounds will inhibit the growth of zinc-iron intermetallic compounds, resulting in uneven thickness of the iron-zinc intermetallic compounds and the coating thickness, ultimately reducing the surface gloss of the coating.

[0043] Accordingly, in the hot-dip galvanizing process designed in this invention, it is also necessary to control the cooling rate of the steel substrate to ≤250℃ after it exits the zinc bath at a rate greater than 20℃ / s. This is because if the cooling rate is too slow, martensitic structure cannot be formed, resulting in the tensile strength of the steel plate not reaching 980MPa or higher.

[0044] Furthermore, in the manufacturing method described in this invention, in step (2), the atmosphere in the heating section and the heat preservation section is a mixture of N2, H2 and H2O, wherein the volume content of H2 is 1-20% and the dew point is -30-20℃.

[0045] In the manufacturing method designed in this invention, a mixture of N2, H2, and H2O can be used as the atmosphere in the heating and heat preservation sections. The control of the H2 volume content and the selection of the dew point are to ensure good plating suitability of the steel substrate. Specifically, when the designed H2 volume content is less than 1%, the residual iron oxide film on the surface of the cold-rolled steel substrate cannot be effectively reduced, which is detrimental to the wetting of the plating solution on the steel substrate surface; while when the H2 volume content is greater than 20%, it will increase safety hazards and costs.

[0046] Furthermore, since the presence of gaseous impurities (H2O) cannot be completely avoided in the annealing furnace, and the H2 in the annealing atmosphere reacts with the unavoidable gaseous impurities (O2) in the furnace to form H2O, it is difficult to guarantee that the dew point in the annealing furnace is below -50°C. When the dew point is between -50°C and -30°C, the steel substrate tends to undergo external oxidation, which is detrimental to the formation of chemical bonds between the steel substrate and the plating solution, resulting in plating defects such as incomplete plating and uneven plating thickness, affecting the product's appearance and corrosion resistance. When the dew point is above 20°C, the Fe 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 plating adhesion. Therefore, in the technical solution designed in this invention, the dew point in the annealing atmosphere can be limited to between -30°C and 20°C.

[0047] Furthermore, in the manufacturing method described in this invention, in step (2), when the Si mass percentage content in the steel substrate is greater than 0.7%, the dew point of the atmosphere in the heating section and the heat preservation section is controlled to be greater than -10°C.

[0048] In the above technical solution of the present invention, when the Si mass percentage content in the steel substrate is greater than 0.7%, the reason for further controlling the dew point of the atmosphere in the heating section and the heat preservation section to be greater than -10°C is as follows: When the Si content in the steel substrate is high and the dew point of the annealing atmosphere is low, the Si element in the steel substrate is prone to segregation on the surface of the steel substrate and react with H2O in the atmosphere to form a Si-containing oxide film on the surface of the steel substrate, reducing the wettability and adhesion of the plating solution on the surface of the steel substrate; while when the dew point of the annealing atmosphere is high, the diffusion flux of O element into the interior of the steel substrate increases, and the Si element combines with the O element in the steel substrate to form an oxide precipitate phase, reducing the amount of Si element segregation on the surface of the steel substrate, thereby reducing the thickness of the Si-containing oxide formed on the surface of the steel substrate during the annealing process.

[0049] Therefore, in order to suppress the segregation of Si on the surface of the steel substrate and its adverse effect on the plating properties of the steel substrate, when the mass percentage content of Si in the steel substrate is greater than 0.7%, the dew point of the atmosphere in the heating section and the heat preservation section can preferably be controlled above -10°C.

[0050] Furthermore, in the manufacturing method described in this invention, in step (3), the temperature of the plating solution is controlled to be 450-470°C, and the temperature difference between the steel substrate and the plating solution is less than 10°C when the substrate is placed in the zinc pot.

[0051] In the above-described technical solution of this invention, the plating bath temperature can be controlled at 450-470℃. This is because: when the plating bath temperature is too low, the fluidity of the plating bath is poor, making it difficult to control the uniformity of the coating thickness; when the plating bath temperature is too high, the evaporation rate of the zinc liquid is accelerated, easily generating zinc ash in the furnace nose, thus increasing coating defects such as exposed iron; in addition, high plating bath temperature leads to increased energy consumption for heating the zinc pot, which is not conducive to reducing production costs. Therefore, preferably, the plating bath temperature can be controlled between 450-470℃.

[0052] Furthermore, a large temperature difference between the steel substrate and the plating solution when the substrate enters the zinc bath increases the difficulty of managing the zinc bath's thermal balance and promotes the formation of zinc dross, leading to coating defects and affecting the surface quality of the finished product. Therefore, it is preferable to control the temperature difference between the steel substrate and the plating solution when the substrate enters the zinc bath to be within 10°C.

[0053] Furthermore, in the manufacturing method described in this invention, in step (3), when the mass percentage content of Si in the steel substrate is ≥0.7%, the mass percentage content of Al element in the plating solution is controlled to be 0.10-0.14%.

[0054] In the above-described technical solution of this invention, when the Si mass percentage content in the steel substrate is ≥0.7%, the Al mass percentage content in the plating solution can be further controlled to be 0.10-0.14%. This is because: when the Al mass percentage content in the plating solution is greater than 0.14%, the reaction rate between the steel substrate surface and the Al in the plating solution is high, which easily forms a dense iron-aluminum intermetallic compound layer, hindering the formation of zinc-iron intermetallic compounds, thereby reducing the surface gloss of the plating layer.

[0055] Furthermore, in the manufacturing method described in this invention, in step (3), when the mass percentage content of Si in the steel substrate is <0.7%, the mass percentage content of Al element in the plating solution is controlled to be 0.16-0.25%.

[0056] In the above-described technical solution of this invention, when the mass percentage of Si in the steel substrate is less than 0.7%, the proportion of manganese oxide in the oxide on the surface of the steel substrate after annealing in an atmosphere with a dew point greater than -30°C is relatively large. Manganese oxide has high chemical activity and reacts quickly with Al in the plating solution. This easily leads to the rapid consumption of Al elements in the plating solution near the surface of the steel substrate, thereby accelerating the reaction between the steel substrate and Zn in the plating solution to form zinc-iron intermetallic compounds. Therefore, when the effective Al content in the plating solution is low, the rate of zinc-iron intermetallic compound formation is faster, easily leading to a thicker transition layer and difficulty in controlling the uniformity of the transition layer thickness, thus affecting the uniformity of the coating thickness and ultimately reducing the reflectivity of the finished coating surface. Therefore, preferably, when the mass percentage of Si in the steel substrate is less than 0.7%, the mass percentage of Al elements in the plating solution can be controlled to be 0.16-0.25%.

[0057] Compared with existing technologies, the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0058] The high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss described in this invention achieves high surface reflectivity without subsequent processing through optimized design of its own steel substrate, hot-dip galvanized layer, and transition layer between the steel substrate and the hot-dip galvanized layer, as well as reasonable control of the manufacturing process. Its hot-dip galvanized layer has fewer defects and lower cost. This high-strength cold-rolled hot-dip galvanized steel sheet has high strength, excellent formability, and high corrosion resistance.

[0059] Unlike conventional hot-dip galvanized steel sheets designed in the prior art, the high-strength cold-rolled hot-dip galvanized steel sheet designed in this invention has a tensile strength >980MPa and a surface gloss value greater than 400 gloss units (the gloss value of polished black glass with a refractive index of 1.567 is set to 100 gloss units at a geometric angle of 60 degrees). It has a very broad prospect for promotion and application and has great value for use. Attached Figure Description

[0060] Figure 1 The diagram schematically illustrates the structure of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss according to the present invention.

[0061] Figure 2 This is a metallographic scanning electron microscope backscattered electron image of the actual cross-section of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss in Example 1.

[0062] Figure 3 This is a secondary scanning electron microscope (SEM) image of the transition layer surface of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss in Example 1 after the hot-dip galvanized layer has been removed. Detailed Implementation

[0063] The high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments and accompanying drawings. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0064] Examples 1-6 and Comparative Examples 1-10

[0065] In this invention, the steel substrates corresponding to the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10 are all designed with the chemical element composition shown in Table 1 below.

[0066] Table 1. (wt%, balance Fe and other unavoidable impurity elements besides impurity P)

[0067]

[0068]

[0069] The high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10 of this invention were all prepared using the following steps:

[0070] (1) Steel substrate is prepared by existing technology: smelting is carried out according to the chemical composition shown in Table 1, and the billet is heated at 1150-1250℃, the holding time is controlled at 0.5-3h, the hot rolling final rolling temperature is 850-950℃, and then the coil is rolled at 500-600℃. The hot rolled coil is then pickled and cold rolled, and the final cold rolling reduction is controlled between 30-90%.

[0071] (2) Continuous annealing of the steel substrate: Heating to 750-900℃ at an average heating rate of 10℃ / s, and then holding at that temperature for 30-180s. The atmosphere in the heating and holding sections is a mixture of N2, H2, and H2O, with the volume content of H2 fixed at 1-20%, and the dew point of the atmosphere being -30-20℃. Specifically, when the Si mass percentage content in the steel substrate is greater than 0.7%, the dew point of the atmosphere in the heating and holding sections is controlled to be greater than -10℃.

[0072] (3) Hot-dip galvanizing: After annealing, the strip steel is cooled at 40℃ / s in a mixed gas of N2, 10% H2 and H2O to the temperature at which the strip steel enters the zinc pot. After holding at the zinc pot temperature for 15s, the strip steel is immersed in the zinc pot plating solution for hot-dip galvanizing. The plating solution temperature is maintained at 450-470℃, the difference between the zinc pot temperature and the plating solution temperature does not exceed 10℃, and the mass percentage of Al in the plating solution is 0.10-0.25%, with the balance being Zn and unavoidable impurities. The immersion time of the steel substrate in the plating solution is controlled at 1-5s. After the steel substrate leaves the plating solution, it is quickly purged with an air knife to control the thickness of the hot-dip galvanized layer to be between 5-25 micrometers. Subsequently, the strip steel is cooled to 250℃ or below at an average cooling rate of higher than 20℃ / s.

[0073] In this hot-dip galvanizing process, when the Si mass percentage content in the steel substrate is ≥0.7%, the Al mass percentage content in the plating solution is specifically controlled to be 0.10-0.14%; when the Si mass percentage content in the steel substrate is <0.7%, the Al mass percentage content in the plating solution is specifically controlled to be 0.16-0.25%.

[0074] In this invention, the hot-dip galvanizing process in step (3) 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 will also be formed between the hot-dip galvanized layer and the steel substrate.

[0075] It should be noted that the chemical composition design and related processes of the high-strength cold-rolled hot-dip galvanized steel sheets in Examples 1-6 all meet the specifications designed in this invention. Although the comparative hot-dip galvanized steel sheets in Comparative Examples 1-10 are also prepared using the above steps (1)-(3), there are parameters in the specific processes used in the comparative hot-dip galvanized steel sheets in Comparative Examples 1-10 that do not meet the design requirements of this invention.

[0076] Tables 2-1 and 2-2 list the specific process parameters for the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10.

[0077] Table 2-1.

[0078]

[0079] Table 2-2.

[0080]

[0081]

[0082] Samples were taken from the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10 obtained through the above process steps. The galvanized steel sheets of each example and comparative example were observed and analyzed in order to accurately analyze the characteristics of the prepared galvanized steel sheets.

[0083] In this invention, the inventors visually assessed the appearance of the hot-dip galvanized steel sheets obtained in Examples 1-6 and Comparative Examples 1-10. A uniform coating with few missed spots and minimal slag defects was classified as "good," or "OK." Uneven coating, large missed areas, and numerous slag defects were classified as "bad," or "NG." The relevant observation results are listed in Table 3 below.

[0084] Furthermore, based on the ASTM D 523 standard, the inventors used a BYK4561 miniature gloss meter to measure the specular gloss of the hot-dip galvanized steel sheet surface at a 60-degree angle to obtain the average gloss value of the steel sheet surfaces in each embodiment and comparative example. Specifically, the specular gloss value of polished black glass with a refractive index of 1.567 at a geometric angle of 60 degrees was set to 100 gloss units.

[0085] Accordingly, when testing the sample steel plates of each embodiment and comparative example, tensile tests were specifically performed. The tensile strength of the galvanized steel plates of each embodiment and comparative example was measured at room temperature in accordance with GB / T 228.1 standard.

[0086] In this invention, the average thickness of the transition layer was also measured by taking a scanning electron microscope (SEM) backscattered electron image of the metallographic cross-section of the corresponding hot-dip galvanized steel sheet and measuring it. The measurement of the transition layer thickness, especially the zinc-iron compound thickness, required first etching the cross-sectional metallographic sample with a 0.5% nitric acid-alcohol solution for 5 seconds, then taking a scanning electron microscope backscattered electron image, and finally analyzing the obtained image using image analysis software to obtain the average thickness of the transition layer. The relevant test results are listed in Table 3 below.

[0087] Accordingly, the quantification of the thickness of the iron-aluminum compound in the transition layer of the galvanized steel sheet in each embodiment and comparative example can be specifically achieved by measuring the depth distribution map of Al element using a glow discharge spectrometer, integrating the Al enrichment peaks observed at the interface of the hot-dip galvanized layer and the steel substrate, and converting the integrated area into thickness using the atomic ratio in the Fe2Al5 chemical formula.

[0088] In addition, the observation of grain morphology and the determination of average grain size in the hot-dip galvanized layer can be specifically obtained by measuring the inverse pole figure obtained by EBSD test. It was observed that the grains in the hot-dip galvanized layer of each embodiment have an equiaxed crystal morphology, and their average grain size is less than 30% of the thickness of the hot-dip galvanized layer.

[0089] Regarding the Al content in the hot-dip galvanized layer, the hot-dip galvanized layer can be peeled off using dilute hydrochloric acid containing inhibitors, and then quantitatively determined using ICP emission spectroscopy.

[0090] Table 3 lists the observation and analysis results for the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 and the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10.

[0091] Table 3.

[0092]

[0093]

[0094] 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.

[0095] As can be seen from Table 3 above, compared with Comparative Examples 1-10, the high-strength cold-rolled hot-dip galvanized steel sheets designed in Examples 1-6 have superior comprehensive performance. The surface appearance of the cold-rolled hot-dip galvanized steel sheets in Examples 1-6 is all "OK", that is, their coating is uniform, with fewer uncoated spots and fewer slag defects. In contrast, Comparative Examples 1-10 have poor surface appearance quality.

[0096] Furthermore, through observation and analysis, it was found that in this invention, the transition layer of the high-strength cold-rolled hot-dip galvanized steel sheets in Examples 1-6 all include zinc-iron intermetallic compounds, and the volume proportion of zinc-iron intermetallic compounds in the transition layer is greater than 70%, specifically between 95% and 99%. Additionally, in the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 designed in this invention, the thickness of the hot-dip galvanized layer is specifically between 9.1 and 22.3 μm, the average thickness of the transition layer is 8-18% of the thickness of the hot-dip galvanized layer, corresponding to the equiaxed crystal morphology of the resulting hot-dip galvanized layer, and the average grain size of the hot-dip galvanized layer is less than 30% of the thickness of the hot-dip galvanized layer.

[0097] Accordingly, through research and analysis, it is not difficult to find that the high-strength cold-rolled hot-dip galvanized steel sheets of Examples 1-6 in this invention all have high strength, with tensile strength between 983-1085 MPa, and the Al content in the hot-dip galvanized layer is between 0.17-0.45%. This hot-dip galvanized layer has excellent surface gloss, with an average gloss value greater than 400 gloss units, specifically between 473-502 GU.

[0098] However, Comparative Examples 1-10 had parameters that did not meet the design requirements of this invention during the design process. Therefore, the comparative hot-dip galvanized steel sheets of Comparative Examples 1-10 all had issues such as an average surface gloss value of less than 400 gloss units, NG surface quality, and insufficient tensile strength.

[0099] Figure 1 The diagram schematically illustrates the structure of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss according to the present invention.

[0100] from Figure 1 As can be seen from the present invention, the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss designed in this invention specifically includes: a steel substrate 1 and a hot-dip galvanized layer 3, wherein the hot-dip galvanized layer 3 is located on the surface of the steel sheet, and a transition layer 2 is provided 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.

[0101] Figure 2 This is a metallographic scanning electron microscope backscattered electron image of the actual cross-section of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss in Example 1.

[0102] like Figure 2 As shown, under a scanning electron microscope, the high-strength cold-rolled hot-dip galvanized steel sheet of Example 1 still has a three-layer structure, namely... Figure 2 The steel substrate C, the hot-dip galvanized layer A, and the transition layer B located between the steel substrate C and the hot-dip galvanized layer A are shown.

[0103] Figure 3 This is a secondary scanning electron microscope (SEM) image of the transition layer surface of the high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss in Example 1 after the hot-dip galvanized layer has been removed.

[0104] like Figure 3 As shown in this embodiment, the high-strength cold-rolled hot-dip galvanized steel sheet of Example 1 can have its surface hot-dip galvanized layer removed in dilute hydrochloric acid with added corrosion inhibitor. After removing the surface hot-dip galvanized layer, a transition layer mainly composed of zinc-iron intermetallic compounds can be observed.

[0105] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

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

Claims

1. A high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss, comprising a steel substrate, a hot-dip galvanized layer, and a transition layer located between the steel substrate and the hot-dip galvanized layer, characterized in that: The transition layer comprises a zinc-iron intermetallic compound, and the average thickness of the transition layer is 2-30% of the thickness of the hot-dip galvanized layer; the zinc-iron intermetallic compound occupies more than 70% of the volume of the transition layer. The grains in the hot-dip galvanized layer exhibit an equiaxed crystal morphology, and the average grain size is less than 30% of the thickness of the hot-dip galvanized layer. The mass percentage of each chemical element in the steel substrate is as follows: C: 0.09-0.25%, Mn: 2.3-3.0%, Si: 0.3-2.0%, Cr: 0-0.7%, Nb: 0-0.03%, Ti: 0-0.03%, with the balance being Fe and unavoidable impurities.

2. The high-strength cold rolled galvannealed steel sheet with high surface gloss according to claim 1, characterized in that, The transition layer also includes an intermetallic compound of iron and aluminum.

3. The high strength cold rolled galvannealed steel sheet with high surface gloss according to claim 1, characterized in that, The thickness of the hot-dip galvanized layer is 5-25 μm.

4. The high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss as described in claim 1, characterized in that, The hot-dip galvanized layer has the following composition by mass percentage: Al: 0.1-0.5%, with the remainder being Zn and unavoidable impurities.

5. The high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss as described in any one of claims 1-4, characterized in that, Its surface has an average gloss value greater than 400 gloss units and a tensile strength greater than 980 MPa.

6. The method for manufacturing high-strength cold-rolled hot-dip galvanized steel sheet with high surface gloss as described in any one of claims 1-5, characterized in that, It includes the following steps: (1) A steel substrate is prepared; (2) Continuous annealing: The strip steel is heated to a uniform temperature of 750-900°C and held for 30-180 seconds; (3) Hot-dip galvanizing: The mass percentage of Al element in the galvanizing solution is 0.10-0.25%; the total immersion time of the steel substrate in the galvanizing solution is 1-5s; after the steel substrate is removed from the zinc pot, it is cooled to ≤250°C at a cooling rate of greater than 20°C / s.

7. The manufacturing method as described in claim 6, characterized in that, In step (2), the atmosphere in the heating section and the heat preservation section is a mixture of N2, H2 and H2O, wherein the volume content of H2 is 1-20% and the dew point is -30-20°C.

8. The manufacturing method as described in claim 7, characterized in that, In step (2), when the Si mass percentage content in the steel substrate is greater than 0.7%, the dew point of the atmosphere in the heating section and the heat preservation section is controlled to be greater than -10°C.

9. The manufacturing method as described in claim 6, characterized in that, In step (3), the temperature of the plating solution is controlled at 450-470°C, and the temperature difference between the steel substrate and the plating solution is less than 10°C when the substrate is placed in the zinc pot.

10. The manufacturing method as described in claim 6, characterized in that, In step (3), when the mass percentage content of Si in the steel substrate is ≥0.7%, the mass percentage content of Al in the plating solution is controlled to be 0.10-0.14%.

11. The manufacturing method as described in claim 6, characterized in that, In step (3), when the mass percentage content of Si in the steel substrate is <0.7%, the mass percentage content of Al in the plating solution is controlled to be 0.16-0.25%.

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