Zinc alloy-plated steel and manufacturing method therefor

Abstract

The present invention provides a zinc alloy-plated steel and a manufacturing method therefor, the zinc alloy-plated steel comprising: a base iron; a zinc alloy plating layer formed on at least one surface of the base iron; and an Fe-Al-based alloy interface layer provided between the base iron and the zinc alloy plating layer, wherein the zinc alloy plating layer comprises, by wt%, 1-3.5% of Mg, 1-4% of Al, and the balance of Zn and other inevitable impurities, and the average particle size of intermetallic compounds in the Fe-Al-based alloy interface layer is 81 nm or less.

Description

Zinc alloy-plated steel and its manufacturing method

[0001] The present invention relates to a zinc alloy plated steel and a method for manufacturing the same, and more particularly, to a highly corrosion-resistant plated steel having excellent corrosion resistance in a processed part and a method for manufacturing the same.

[0002] Hot-dip galvanized steel sheets are widely used in construction materials and home appliances due to their excellent self-sacrificing properties. When hot-dip galvanized steel sheets are exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode where the iron of the base metal is exposed, resulting in zinc loss from the plating layer. This sacrificial anode action of zinc effectively inhibits rust formation on the iron of the base metal in a corrosive environment, but reduces anode efficiency. To address this issue, aluminum (Al) and magnesium (Mg) are being added to zinc (Zn). This creates dense corrosion products in corrosive environments, enhancing anode efficiency and producing highly corrosion-resistant plating products.

[0003] However, zinc-plated alloy steel has the disadvantage of poor workability and corrosion resistance at the machined area. This is because zinc (Zn), aluminum (Al), and magnesium (Mg) form an alloy phase with high hardness, and this alloy phase with high hardness causes cracks during the machining process, which reduces the corrosion resistance at the machined area.

[0004] The present invention aims to address these conventional problems, and provides a zinc alloy-plated steel material with excellent corrosion resistance at the machined portion, and a method for manufacturing the same. However, these tasks are exemplary and should not be construed as limiting the scope of the present invention.

[0005] According to one aspect of the present invention, a zinc alloy plated steel is provided.

[0006] The zinc alloy-plated steel material includes: a base steel; a zinc alloy plating layer formed on at least one surface of the base steel; and an Fe-Al alloy interface layer provided between the base steel and the zinc alloy plating layer; wherein the zinc alloy plating layer includes, in wt%, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn, and other unavoidable impurities, and the average grain size of the Fe-Al alloy interface layer may be 81 nm or less.

[0007] In one embodiment, the average particle size of the Fe-Al alloy interface layer may be 65 nm or less.

[0008] In one embodiment, the Fe-Al alloy interface layer may include one or more kinds selected from the group consisting of Fe2Al5, FeAl3, and FeAl.

[0009] In one embodiment, after bending, the average distance between cracks in the Fe-Al alloy interface layer may be 20 to 30 μm.

[0010] According to another aspect of the present invention, a method for manufacturing a zinc alloy plated steel is provided.

[0011] The method for manufacturing the above zinc alloy plated steel may include the steps of: preparing a zinc alloy plating bath containing, in wt%, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn, and other unavoidable impurities; immersing a base iron in the zinc alloy plating bath and performing plating to form a zinc alloy plating layer on the base iron; and cooling the base iron on which the zinc alloy plating layer is formed at a rate of 8 to 20°C / s.

[0012] In one embodiment, the cooling step may be a step of cooling the base steel on which the zinc alloy plating layer is formed at a rate of 10 to 12°C / s.

[0013] In one embodiment, the zinc alloy plating bath can be maintained at a temperature increased from the melting point of the molten zinc alloy to a range of 20°C to 80°C.

[0014] In one embodiment, the temperature of the immersion iron may be equal to or 20°C higher than the temperature of the zinc alloy plating bath.

[0015] According to the embodiment of the present invention as described above, by controlling the cooling rate during the plating process to change the crystal size of the Fe-Al alloy interface layer located between the base iron and the plating layer, the exposed area of ​​the base iron to each crack after processing is minimized, thereby providing a high-corrosion-resistant zinc alloy plating steel having excellent corrosion resistance in the processed part.

[0016] Of course, the scope of the present invention is not limited by these effects.

[0017] Figure 1 is a process flow diagram showing a method for manufacturing a zinc alloy-plated steel according to an embodiment of the present invention.

[0018] Figure 2 is a schematic diagram showing the crystal size of the alloy interface layer of a zinc alloy-plated steel according to an embodiment of the present invention.

[0019] Figure 3 is a scanning electron microscope photograph showing the microstructure of the alloy interface layer of a zinc alloy-plated steel according to a comparative example of the present invention.

[0020] Fig. 4 is a scanning electron microscope photograph showing the microstructure of a zinc alloy-plated steel according to a comparative example and an embodiment of the present invention.

[0021] Figure 5 is a scanning electron microscope photograph of the surface of a bent portion of a zinc alloy-plated steel material after bending according to a comparative example and an embodiment of the present invention.

[0022] Hereinafter, various embodiments of the present invention will be described in detail with reference to the attached drawings. These embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely explain the present disclosure and to fully convey the spirit of the present invention to those skilled in the art. In addition, the thickness and size of each layer in the drawings are exaggerated for convenience and clarity of explanation.

[0023] Hereinafter, a zinc alloy plated steel material and a manufacturing method thereof according to an embodiment of the present invention will be described.

[0024] A zinc alloy-plated steel material, which is one aspect of the present invention, includes a base iron and a zinc alloy-plated layer formed on at least one surface of the base iron.

[0025] There is no particular limitation on the type of the above-mentioned base steel, and for example, the base steel may be a hot-rolled steel sheet or cold-rolled steel sheet used as the base steel of a typical zinc-based or zinc alloy-based plated steel sheet.

[0026] In the above zinc alloy-plated steel, the zinc alloy plating layer may be formed on only one side of the base steel, or may be formed on both sides of the base steel. In this case, the zinc alloy plating layer refers to a plating layer that is formed of a Zn-Mg-Al alloy and includes Mg and Al, but in which the excess amount is Zn.

[0027] Specifically, it is preferable that the zinc alloy plating layer contains, in wt%, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn, and unavoidable impurities.

[0028] Magnesium (Mg) is an element added for corrosion resistance. In the present invention, the Mg content is controlled to 1.0% or more to ensure the desired level of corrosion resistance. However, excessive addition may cause dross due to oxidation, which may cause problems during operation. Therefore, it is preferable to control the Mg content to 3.5% or less.

[0029] Aluminum (Al) suppresses the formation of magnesium oxide dross within the plating bath and forms an alloy interface with iron, thereby ensuring plating adhesion. Therefore, the Al content within the plating layer should be controlled to at least 1.0%. However, excessive Al content requires an increase in the plating bath temperature, which increases process costs and shortens the life of the plating equipment. Therefore, it is recommended to control the upper limit of the Al content to 4%.

[0030] An Fe-Al alloy interface layer is formed between the base iron and the zinc alloy plating layer. The Fe-Al alloy interface layer is formed by alloying iron contained in the base iron with aluminum contained in the zinc alloy plating bath, and refers to a layer formed at the interface between the base iron and the zinc alloy plating layer.

[0031] In one embodiment, the Fe-Al alloy interface layer is a layer including an intermetallic compound of Fe and Al, and examples of the intermetallic compound of Fe and Al include Fe2Al5, FeAl3, and FeAl. In addition, other effective impurities such as Zn may be included.

[0032] The above Fe-Al alloy interface layer not only plays a role in improving the adhesion between the base iron and the zinc alloy plating layer, but also plays a role as an inhibitory layer that prevents Fe diffusion from the base iron to the zinc alloy plating layer.

[0033] According to the present invention, in order for the Fe-Al alloy interface layer to play a role in securing corrosion resistance, the average particle size of the intermetallic compound must be 81 nm or less (greater than 0), more preferably 65 nm or less.

[0034] At this time, the average particle size of the intermetallic compound in the Fe-Al alloy interface layer can be measured by taking a SEM photograph so that the surface of the alloy interface layer is observed. The particle size of the intermetallic compound referred to here means the average value of the diameters of each crystal grain (in the case of polygonal crystal grains, the diameter of the circle assuming the smallest circle containing the crystal grains) obtained for 10 crystal grains of any intermetallic compound in the observation field of the SEM.

[0035] Figure 1 is a schematic diagram exemplarily showing the crystal size of an alloy interface layer of a zinc alloy-plated steel according to an embodiment of the present invention.

[0036] Referring to Fig. 1, an Fe-Al alloy interface layer (20) is provided between the base steel (10) and the zinc alloy plating layer (30). As one moves from (a) to (c) of Fig. 1, the size of the Fe-Al crystal (22), which is an intermetallic compound, becomes smaller.

[0037] Depending on the particle size of the Fe-Al crystal (22), the interval at which cracks (21) occur in the alloy interface layer (20) during processing differs. As shown in Fig. 1 (a) to (c), the Fe-Al crystal (22) becomes finer and the interval at which cracks (21) occur in the alloy interface layer during bending processing narrows. That is, the Fe-Al crystal (22) is a mixture of phases including Fe2Al5, FeAl3, and FeAl, and when these structures are refined, there is an effect of preventing stress concentration in the crack portion of the base steel (10) during bending processing.

[0038] In the present invention, when the average grain size of the Fe-Al crystal (22) exceeds 81 nm, fine cracks do not form in the intermetallic compound, but cracks form locally, easily concentrating stress in that part.

[0039] The average grain size of the above Fe-Al crystals (22) is controlled in the cooling step after the zinc alloy plating layer is formed, as will be described later. When the cooling rate is 8 to 20°C / s, preferably 10 to 12°C / s, an appropriate alloy interface layer can grow between the zinc alloy plating layer and the base iron. In this case, the crystal size is formed to be the minimum, so that when processing, the number of cracks (21) is large, but the crack (21) area is small, thereby exposing less of the base iron (10). Although a smaller crystal size is preferable, the adhesion of the plating layer may be reduced, and therefore, the average grain size of the intermetallic compound preferred in the present invention is 81 nm or less.

[0040] In the case of Fig. 1(a), crystal growth occurs sufficiently due to the low cooling rate, and accordingly, the crystal size of the Fe-Al crystal (22) is relatively large, the number of cracks (21) is small, but the spacing between the cracks (21) is wide.

[0041] Meanwhile, in the case of Fig. 1(c), the size of the Fe-Al crystals (22) in the alloy interface layer (20) is relatively small and the area of ​​the cracks (21) is narrow. This has the effect of preventing stress concentration in the base iron (10) and improving corrosion resistance. In the present invention, after bending, the average distance between cracks (21) in the alloy interface layer (20) may be 20 to 30 ㎛.

[0042] According to another aspect of the present invention, a method for manufacturing a zinc alloy plated steel is provided.

[0043] Figure 2 is a process flow chart showing a method for manufacturing a zinc alloy-plated steel according to an embodiment of the present invention.

[0044] Referring to FIG. 2, the method for manufacturing the zinc alloy-plated steel includes a step (S110) of preparing a zinc alloy plating bath containing, in wt%, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn, and other unavoidable impurities; a step (S120) of immersing a base iron in the zinc alloy plating bath and performing plating to form a zinc alloy plating layer on the base iron; and a step (S130) of cooling the base iron on which the zinc alloy plating layer has been formed at a rate of 8 to 20°C / s.

[0045] The above step (S110) is a step for preparing a zinc alloy plating bath.

[0046] Magnesium (Mg) is an element added for corrosion resistance. In the present invention, the Mg content is controlled to 1.0% or more to ensure the desired level of corrosion resistance. However, excessive addition may cause dross due to oxidation, which may cause problems during operation. Therefore, it is preferable to control the Mg content to 3.5% or less.

[0047] Aluminum (Al) suppresses the formation of magnesium oxide dross within the plating bath and forms an alloy interface with iron, thereby ensuring plating adhesion. Therefore, the Al content within the plating layer should be controlled to at least 1.0%. However, excessive Al content requires an increase in the plating bath temperature, which increases process costs and shortens the life of the plating equipment. Therefore, it is recommended to control the upper limit of the Al content to 4%.

[0048] Zinc alloy plating is performed under normal plating conditions, and according to one embodiment of the present invention, the plating bath temperature may be 450 to 460°C.

[0049] More specifically, the zinc alloy plating bath may be maintained at a temperature increased by 20°C to 80°C above the melting point of the molten zinc alloy. Preferably, the operation should be conducted at a temperature 30°C higher than the melting temperature to prevent solidification, and if the temperature exceeds 80°C, zinc ash may be generated, which may cause problems with surface quality. Therefore, it is preferable to maintain the temperature at a temperature increased by 20°C to 80°C, preferably 30°C to 80°C, above the melting temperature.

[0050] The above step (S120) is a step of immersing the base iron in a zinc alloy plating bath and performing plating to form a zinc alloy plating layer on the base iron.

[0051] In one embodiment, the temperature of the immersion iron may be equal to or 30°C, preferably 20°C, higher than the temperature of the zinc alloy plating bath.

[0052] Afterwards, the zinc alloy-plated steel is subjected to a gas wiping treatment to adjust the plating adhesion amount. The gas wiping treatment is intended to adjust the plating adhesion amount, and there are no particular limitations on the method. Air or nitrogen may be used as the gas, with nitrogen being more preferred. This is because, if air is used, Mg oxidation occurs preferentially on the surface of the plating layer, which may cause surface defects in the plating layer.

[0053] The above step (S130) is a step of cooling the base steel on which the zinc alloy plating layer has been formed at a rate of 8 to 20°C / s.

[0054] In the above cooling step, the cooling rate should be set to a range in which the crystal size within the alloy interface layer is formed to the minimum, so that during processing, the number of cracks is large but the crack area is small, thereby exposing less of the base iron. In the present invention, the cooling rate is set to 8 to 20°C / s, preferably 10 to 12°C / s, so that an appropriate alloy interface layer grows between the zinc alloy plating layer and the base iron.

[0055] When the cooling rate is less than 8℃ / s, the slow cooling rate ensures time for grain growth, and the grain size of the alloy interface layer is formed large. The large grain size creates a small number of cracks during processing, but a large crack area, exposing a large area of ​​the base steel. The large crack area generated after processing makes the corrosion resistance vulnerable. In addition, the growth of the brittle MgZn2 phase causes cracks in the plating layer during processing.

[0056] When the cooling rate exceeds 20℃ / s, the rapid cooling rate rapidly terminates grain growth and forms an alloy interface layer with a small grain size. However, excessively fast cooling rates can reduce plating adhesion due to insufficient growth of the Fe-Al alloy interface layer between the base iron and the zinc alloy plating layer. Furthermore, cracks at grain boundaries can be induced due to differences in shrinkage between phases in the zinc alloy plating layer composed of various phases. Therefore, the cooling rate should be limited to 8 to 20℃ / s.

[0057] Below, preferred experimental examples are presented to aid understanding of the present invention. However, these examples are provided solely to aid understanding of the present invention and are not intended to limit the present invention. Any details not described herein are technically feasible for those skilled in the art, and therefore, their description will be omitted.

[0058] <Experimental Example>

[0059] A 0.7 mm cold rolled material was prepared using a base steel. The base steel contains 0.1 wt% carbon (C), 0.01 wt% silicon (Si), 0.3 wt% manganese (Mn), 0.05 wt% phosphorus (P), 0.03 wt% sulfur (S), and the remainder iron (Fe) and impurities inevitably contained during the steelmaking process. However, the composition and content of the base steel are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0060] The above-mentioned steel is annealed at 680 to 880°C in a nitrogen atmosphere, and then the annealed specimen is cooled to a temperature 0 to 20°C higher than the plating bath temperature and immersed in the plating bath for 1 to 5 seconds. At this time, the temperature of the plating bath is maintained at a temperature 20 to 80°C higher than the melting temperature (Tm). After immersion in the plating bath, the plating thickness is adjusted to approximately 20 μm by nitrogen wiping, and then cooled at the cooling rate shown in Table 1 below to produce a high-corrosion-resistant alloy-plated steel sheet. The cooling rate is set in the range from the plating bath immersion temperature of 450 to 460°C to 300°C, which is the solidification point of the plating layer, and the average cooling rate is measured.

[0061] It is difficult to observe the alloy interface layer unless only the zinc alloy plating layer is removed. Therefore, the zinc alloy plating layer can be removed without damaging the alloy interface layer. The zinc alloy plating layer was dissolved by immersing the sample in a solution of 80 ml of distilled water and 20 ml of 35% hydrochloric acid for 5 minutes, and then the grain size of each crystal in the alloy interface layer was measured by photographing with a field emission scanning electron microscope (FE-SEM). The crystal size was then determined through the average value.

[0062] Classification Composition (wt%) Tm (℃) Cooling rate (℃ / s) Fe-Al alloy interface layer average crystal size (nm) ZnAlMg Comparative Example 196.821.24604156.07 Comparative Example 26110.15 Example 1880.59 Example 21063.54 Example 31254.05 Comparative Example 322-

[0063] In Table 1 above, the average crystal size of the Fe-Al alloy interface layer was measured by the following method. First, a sample was prepared by dissolving and removing the zinc alloy plating layer by immersing the zinc alloy-plated steel in an etching solution containing 20 ml of 36.46% hydrochloric acid (HCl) and 80 ml of distilled water for 5 minutes. During dissolution, bubbles are generated due to the corrosion reaction of the plating layer, but the bubbles stop generating as the dissolution is completed. Therefore, the immersion was stopped when the bubbles disappeared. The average particle size of the remaining intermetallic compound was measured by observing the surface of the sample from which the plating layer was dissolved and removed using an SEM.

[0064] Figure 3 is a scanning electron microscope photograph showing the microstructure of the alloy interface layer of a zinc alloy-plated steel according to Comparative Example 3 of the present invention.

[0065] Referring to Fig. 3, when the cooling rate exceeds 20°C / s, there are parts where the growth of the alloy interface layer is insufficient. In zinc plating, the Fe-Al alloy interface layer formed by the reaction of Al and Fe provides adhesion between the plating and the base iron. In this case, the alloy interface layer is not completely formed, making it difficult to ensure plating adhesion. In addition, it is difficult to measure the average crystal size of the Fe-Al alloy interface layer. Therefore, the cooling rate according to the embodiment of the present invention is limited to 20°C / s or less.

[0066] Fig. 4 is a scanning electron microscope photograph showing the microstructure of a zinc alloy-plated steel according to a comparative example and an embodiment of the present invention.

[0067] Referring to Fig. 4, Fe-Al alloy interface layers formed according to (a) Comparative Example 1, (b) Example 1, and (c) Example 3 are shown. The respective cooling rates are (a) 4°C / s, (b) 8°C / s, and (c) 12°C / s, and it can be seen that the average crystal size of the Fe-Al alloy interface layer decreases to (a) 156.07 nm, (b) 80.59 nm, and (c) 3.54 nm.

[0068] Figure 5 is a scanning electron microscope photograph of the surface of a bent portion of a zinc alloy-plated steel material after bending according to a comparative example and an embodiment of the present invention.

[0069] Figure 5 shows the results of observing the surface of the bent portion using FE-SEM at a magnification of 100x after conducting a 1T bending evaluation on zinc alloy-plated steel manufactured at different cooling rates according to (a) Comparative Example 1, (b) Example 1, and (c) Example 3.

[0070] The spacing and area of ​​cracks after processing were smaller in Examples 1 and 3, which had a high cooling rate, compared to Comparative Example 1, which had a low cooling rate. Referring to Figs. 5 (b) and (c), it can be seen that the average distance between cracks is 20 to 30 μm. This shows that the spacing and area of ​​cracks can be adjusted depending on the cooling rate, and it is expected that the narrower the spacing and area of ​​cracks, the greater the contribution to the corrosion resistance of the processed part.

[0071] According to the embodiment of the present invention as described above, by controlling the cooling rate during the plating process to change the crystal size of the Fe-Al alloy interface layer located between the base iron and the plating layer, the exposed area of ​​the base iron to each crack after processing is minimized, thereby providing a high-corrosion-resistant zinc alloy plating steel having excellent corrosion resistance in the processed part.

[0072] While the present invention has been described with reference to the embodiments illustrated in the drawings, these are merely exemplary, and those skilled in the art will appreciate that various modifications and equivalent alternative embodiments are possible. Therefore, the true scope of technical protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

1. So Ji-cheol; A zinc alloy plating layer formed on at least one surface of the above-mentioned steel; and Including an Fe-Al alloy interface layer provided between the above-mentioned steel and the above-mentioned zinc alloy plating layer; The above zinc alloy plating layer contains, in weight %, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn and other unavoidable impurities. The average particle size of the intermetallic compound in the Fe-Al alloy interface layer is 81 nm or less. Zinc alloy plated steel.

2. In paragraph 1, The average particle size of the above intermetallic compound is 65 nm or less. Zinc alloy plated steel.

3. In paragraph 1, The above intermetallic compound comprises one or more selected from the group consisting of Fe2Al5, FeAl3 and FeAl. Zinc alloy plated steel.

4. In paragraph 1, After bending, The average distance between cracks in the Fe-Al alloy interface layer is 20 to 30 ㎛. Zinc alloy plated steel.

5. A step for preparing a zinc alloy plating bath containing, by weight%, Mg: 1 to 3.5%, Al: 1 to 4%, the remainder Zn and other unavoidable impurities; A step of immersing the base iron in the zinc alloy plating bath and performing plating to form a zinc alloy plating layer on the base iron; and A step of cooling the base steel on which the zinc alloy plating layer is formed at a rate of 8 to 20°C / s; including; Method for manufacturing zinc alloy plated steel.

6. In paragraph 5, The above cooling step is, A step of cooling the base steel on which the zinc alloy plating layer is formed at a speed of 10 to 12°C / s. Method for manufacturing zinc alloy plated steel.

7. In paragraph 5, The above zinc alloy plating bath is, Maintained at a temperature increased from 20℃ to 80℃ above the melting point of the molten zinc alloy. Method for manufacturing zinc alloy plated steel.

8. In paragraph 5, The temperature of the steel during the above immersion is Characterized in that the temperature of the zinc alloy plating bath is equal to or 20℃ higher. Method for manufacturing zinc alloy plated steel.

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