Galvanized alloy steel material having excellent corrosion resistance after processing and method for manufacturing the same

Abstract

The present invention relates to a plated steel material that can be used for automobiles, home appliances, building materials, etc., and more particularly, to a zinc alloy plated steel material having excellent corrosion resistance after processing and a method for manufacturing the same.

Description

[0001] This application is a divisional application of Chinese patent application filed on December 13, 2018, with Chinese patent application number 201880084547.0 and the invention title "Zinc-plated alloy steel with excellent corrosion resistance after processing and a method for manufacturing the same". This application claims priority to Korean application with application number 10-2017-0180328. Technical Field

[0002] This invention relates to a plated steel that can be used in automobiles, home appliances, building materials, etc., and more specifically, to a galvanized alloy steel with excellent corrosion resistance after processing and a method for manufacturing the same. Background Technology

[0003] Galvanizing, which inhibits iron corrosion through cathodic corrosion prevention, offers excellent corrosion resistance and is economical, making it widely used in the manufacture of steel with high corrosion resistance. In particular, compared to electro-galvanized steel, hot-dip galvanized steel, which immerses steel in molten zinc to form a coating, has a simpler manufacturing process and lower prices, leading to increasing demand across all industries, including automobiles, home appliances, and building materials.

[0004] When hot-dip galvanized steel is exposed to a corrosive environment, it exhibits the characteristic of sacrificial corrosion protection, where zinc, with its lower oxidation-reduction potential than iron, is corroded first, thus inhibiting the corrosion of the steel. Furthermore, the zinc coating is oxidized and forms dense corrosion products on the steel surface, isolating the steel from the oxidizing atmosphere and thereby improving the steel's corrosion resistance.

[0005] However, with increasing industrialization, air pollution and corrosive environments are worsening, and due to strict regulations on resource and energy conservation, the demand for steels with superior corrosion resistance compared to existing galvanized steels is increasing. As part of this, various studies have been conducted on manufacturing technologies for galvanized alloy steels that improve the corrosion resistance of steel sheets by adding elements such as aluminum (Al) and magnesium (Mg) to the galvanizing bath. As a representative galvanized alloy steel, research is actively underway related to the technology of manufacturing Zn-Al-Mg coated steels by further adding Mg to the Zn-Al coating composition (Patent Document 1).

[0006] Furthermore, when cracks appear in the coating during the processing and forming of galvanized steel, the coating's corrosion-blocking effect weakens, and sacrificial corrosion protection of the coating rapidly occurs at the cracked areas, thus shortening the coating's lifespan and reducing its corrosion resistance. This is especially true for galvanized alloy steel used in automobiles and other applications, where components are manufactured and used through various forming processes; therefore, the demand for solutions that minimize post-processing corrosion resistance degradation is increasing.

[0007] (Patent Document 1) Japanese Patent Publication No. 2002-332555 Summary of the Invention

[0008] Technical problems to be solved One aspect of the present invention is to provide a galvanized alloy steel and a method for manufacturing the same, wherein the galvanized alloy steel ensures excellent corrosion resistance and prevents a decrease in corrosion resistance by reducing the generation of coating cracks during processing, thereby ensuring excellent corrosion resistance after processing.

[0009] The technical problem to be solved by the present invention is not limited to the problems mentioned above. Those skilled in the art can clearly understand other problems not mentioned from the following description.

[0010] Technical solution One embodiment of the present invention relates to a galvanized alloy steel with excellent corrosion resistance after processing, the galvanized alloy steel comprising: a base iron; a zinc alloy coating formed on the base iron; and an inhibition layer formed between the base iron and the zinc alloy coating, wherein the zinc alloy coating comprises, by weight %, Mg: 0.5-3.5%, Al: 0.5-11.0%, Si: 10-350ppm, the balance being Zn and unavoidable impurities, and the inhibition layer comprising a Si enrichment layer.

[0011] Another embodiment of the present invention relates to a method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing, the method comprising the following steps: preparing hot-rolled steel with a grain size of 1-100 μm; cold-rolling the hot-rolled steel to produce cold-rolled steel with a surface roughness of 0.2-1.0 μm and a slope of 0.2-1.2 μm; immersing iron, the base material of the cold-rolled steel, in a plating bath for plating, the plating bath comprising, by weight, Mg: 0.5-3.5%, Al: 0.5-11.0%, Si: 10-350 ppm, the balance being Zn and unavoidable impurities; wiping and cooling the hot-dip galvanized alloy steel after plating.

[0012] Beneficial effects According to the present invention, a Zn-Al-Mg based zinc-plated alloy steel with excellent corrosion resistance after processing and a method for manufacturing the same can be provided. This offers the advantage of expanding the application areas to those previously limited. Attached Figure Description

[0013] Figure 1 This is a photograph analyzing the composition of the inhibition layer of a galvanized alloy steel manufactured according to a specific embodiment of the present invention.

[0014] Figure 2 This is a photograph of the coating cross-section of Example 3 of the present invention.

[0015] Figure 3 This is a photograph of the coating cross-section of Comparative Example 3 in the embodiments of the present invention. Best practice

[0016] Conventional zinc plating solidifies into a single Zn phase, but Zn-Al-Mg zinc alloys contain Zn, an alloy phase of Mg and Zn, and an Al phase. Depending on the trace elements in the plating bath and the physical and chemical states of the iron substrate surface, the plating structure becomes very complex.

[0017] In the microstructure of Zn-Al-Mg zinc alloy coatings (hereinafter, zinc alloy coatings or coatings), the alloy phases of Zn and Mg can be MgZn2, Mg2Zn... 11 It is composed of various intermetallic compounds, and their hardness reaches Hv 250-300. Furthermore, an inhibition layer composed of intermetallic compounds of Fe and Al can be formed at the interface between the coating and the iron substrate. The intermetallic compounds of Fe and Al include Fe4Al. 13 Fe2Al5, etc. These intermetallic compounds also have high brittleness, so the coating is prone to cracking during physical deformation.

[0018] Therefore, as a solution to ensure excellent corrosion resistance after processing by reducing the generation of coating cracks during the processing of galvanized alloy steel, the inventors have developed a method for forming the inhibitory layer robustly and uniformly, and have completed the present invention. The present invention will now be described in detail.

[0019] The galvanized alloy steel of the present invention comprises: a base iron; a zinc alloy coating formed on the base iron; and an inhibition layer formed between the base iron and the zinc alloy coating.

[0020] The zinc alloy coating preferably comprises, by weight percent: magnesium (Mg): 0.5-3.5%, aluminum (Al): 0.5-11.0%, silicon (Si): 10-350 ppm, with the balance being Zn and unavoidable impurities. The composition is described in detail below.

[0021] Mg plays a crucial role in improving the corrosion resistance of galvanized steel. In corrosive environments, it forms a dense layer of zinc hydroxide corrosion products on the coating surface, effectively preventing corrosion of the galvanized steel. Therefore, the Mg content is preferably 0.5% by weight or more, more preferably 0.8% by weight or more. However, when the Mg content is too high, the Mg oxide scum on the plating bath surface increases dramatically, thus negating the antioxidant effect of adding trace elements. To prevent this problem, Mg is preferably 3.5% by weight or less, more preferably 2.0% by weight or less.

[0022] The Al inhibits the formation of Mg oxide scum in the plating bath and reacts with Zn and Mg in the plating bath to form Zn-Al-Mg intermetallic compounds, thereby improving the corrosion resistance of the plated steel. In this invention, to achieve the above-mentioned effects, Al preferably contains 0.5% by weight or more, more preferably 0.8% by weight or more. However, when the Al content is too high, the weldability and phosphate treatment properties of the plated steel may deteriorate. To prevent this problem, Al is preferably 11.0% by weight or less, more preferably 6.0% by weight or less.

[0023] The Si, dissolved in the Fe-Al compound, imparts ductility when forming the inhibition layer of the zinc alloy coating. When the Si is enriched in the inhibition layer without precipitating as Mg₂Si, it is beneficial to improve the adhesion and fracture toughness of the coating. For these effects, the Si content is preferably 10 ppm by weight or more. However, when the amount of Si increases, if the surface roughness or shape of the steel sheet is uneven, coarse and brittle Mg₂Si precipitates form at the interface between the coating and the base iron, thus initiating cracks due to external stress. Therefore, the Si content is preferably no more than 350 ppm by weight.

[0024] The balance includes zinc (Zn) and unavoidable impurities.

[0025] In addition, a portion of the Fe in the plating bath may be included in the coating, but the Fe is mainly present in the inhibition layer at the interface between the coating and the iron substrate.

[0026] The zinc alloy coating comprises an alloy phase of Mg and Zn, a Zn phase, an Al phase, etc., and the alloy phase of Mg and Zn includes MgZn2 phase and Mg2Zn. 11 Phase. During solidification, the Zn phase solidifies first, containing a small amount of dissolved Al. After the primary Zn solidifies, Zn and MgZn2 solidify in layers, eventually forming a ternary phase of Zn, MgZn2, and Al. When the cooling rate is relatively slow, Mg2Zn may form in addition to MgZn2. 11Compared to Zn, MgZn2 or Mg2Zn 11 Its high hardness can make it a potential initiation point for cracks when external stress is applied, but its corrosion resistance can be improved by including Mg.

[0027] The suppression layer comprises a Si-enriched layer (Si-enriched layer). In particular, the Si-enriched layer is preferably located at the bottom of the suppression layer. The Si-enriched layer refers to Si dissolved in the suppression layer composed of Fe-Al compounds and contained at the bottom of the suppression layer. The suppression layer includes a Si-enriched layer, which is a Si-solution morphology; therefore, even when mechanical toughness such as external stress is imparted to the suppression layer, mechanical damage can be reduced from the suppression layer, thereby preventing crack propagation to the coating. To uniformly form the suppression layer and ensure uniform Si distribution within it, control of the plating bath composition, the microstructure of the iron substrate, the surface structure, and the plate shape is required. When this control is insufficient, Si forms as a coarse Mg2Si alloy phase between the iron substrate and the zinc alloy coating, and stress concentration occurs in the Mg2Si alloy phase when external stress is applied, which may promote crack initiation and growth. Therefore, between the iron substrate and the zinc alloy coating, preferably, five or fewer Mg2Si alloy phases with a diameter exceeding 1000 nm are formed per 100 μm of interface length. More preferably, five or fewer Mg2Si alloy phases with a diameter exceeding 500 nm are formed. Figure 3 As shown, the Mg2Si alloy phase can be observed directly above the suppression layer.

[0028] When the inhibition layer is formed uniformly, Si can also be uniformly dissolved along the inhibition layer. However, when the shape of the steel surface is non-uniform and the oxides present on the steel surface cannot decompose smoothly due to the non-uniformity, the thickness of the inhibition layer may become non-uniform, or some parts may not form an inhibition layer. In this case, coarse Mg2Si may be formed. Therefore, preferably, the inhibition layer is formed uniformly between the iron substrate and the zinc alloy coating.

[0029] Figure 1 This is a photograph analyzing the inhibition layer of a galvanized alloy steel manufactured according to a specific embodiment of the present invention. The composition of the inhibition layer can be analyzed using wet analysis methods that dissolve the coating in hydrochloric acid solution, methods using plasma sources such as GDOES and GDMS, or methods using TEM for direct compositional analysis, etc. Figure 1 It utilizes TEM (Transient Electron Microscopy) to analyze the components. For example... Figure 1 As shown, it can be seen that a layer of Si enriched at the bottom of the inhibition layer is formed in the inhibition layer of the galvanized alloy steel. Figure 1(a) is a photograph showing the inhibition layer, which is formed on the iron substrate 11. Additionally, reference numeral 13 indicates a protective film used for TEM observation. Figure 1 Images (b) to (d) show photographs illustrating the analysis of the composition of Al, Fe, and Si in the suppression layer. In particular, through... Figure 1 From (d), we can see that Si is enriched in the suppression layer.

[0030] The following is a detailed description of a specific embodiment of the method for manufacturing galvanized alloy steel according to the present invention. The method for manufacturing galvanized alloy steel according to the present invention includes the following processes: preparing a base iron, immersing the prepared base iron in a plating bath for plating, adjusting the plating thickness by wiping, and cooling.

[0031] When preparing the base iron, it is preferable to first homogenize the microstructure of the hot-rolled steel. The grains of the hot-rolled steel are preferably in the surface layer (from the surface to within 1 / 8 of the total thickness). When inhomogeneity occurs in the microstructure of the hot-rolled steel, especially surface inhomogeneity, a uniform suppression layer cannot be formed due to the uneven surface shape during cold rolling and the uneven diffusion of Fe from the base iron necessary for forming the suppression layer, and Si cannot be uniformly enriched. Furthermore, coarse Mg2Si alloy phases tend to form locally. Therefore, the average grain size of the hot-rolled steel is preferably 1-100 μm. The grain size is more preferably 1-50 μm, or even more preferably 5-30 μm.

[0032] When the grain size of the hot-rolled steel is less than 1 μm, it is advantageous to ensure strength, but during cold rolling, the surface roughness may increase due to the grain size. Furthermore, when the grain size of the hot-rolled steel exceeds 100 μm, it is advantageous in terms of shape homogenization, but excessive increases in hot rolling temperature may lead to oxide scale defects and also increase the manufacturing cost of the product. One example of a method for obtaining the grain size of the hot-rolled steel is setting the hot rolling temperature to a minimum of 800°C or higher, or setting the coiling temperature after hot rolling to a minimum of 550°C.

[0033] When hot-rolled steel is cold-rolled to produce cold-rolled steel, it is preferable that the surface roughness (Ra) of the cold-rolled steel is 0.2-1.0 μm and the steepness is 0.2-1.2.

[0034] The surface roughness depends on the pressure of the rolls and the surface shape of the rolls when rolling the material. When the surface roughness exceeds 1.0 μm, the roughness increases, which may lead to the formation of an uneven inhibition layer during coating formation, and there is a problem of difficulty in uniformly forming a Si-enriched layer. On the other hand, when the surface roughness is less than 0.2 μm, the surface friction coefficient decreases, so the steel may slip on the rolls.

[0035] The steepness is measured by placing a steel bar with a width of 1m or more and a length of 2m or more on a flat plate to ensure good adhesion, and then measuring the degree of curvature. The steepness is expressed as the height of curvature (H) divided by the wavelength (P) and multiplied by 100. That is, the steepness is expressed as height (H) / wavelength (P) × 100. A smaller steepness indicates better flatness of the steel. When the steepness exceeds 1.2, the steel has a large degree of curvature, causing deviations in surface flow during plating, negatively impacting the formation of the inhibition layer and the homogenization of the plating. A lower steepness is more advantageous, but controlling the steepness to less than 0.2 requires excessive processing costs and is therefore not preferred.

[0036] The method for controlling the roughness and steepness within an appropriate range is not limited to any one method. In the final rolling step of cold rolling, the reduction rate is preferably set to the range of 2-5%. Appropriate tension needs to be applied to the steel sheet during the rolling process. Furthermore, as an example of imparting surface roughness, plasma treatment can be performed on the steel surface. That is, during the cold rolling, the final shape is determined by the final roll, so the reduction rate is preferably 5% or less. However, in the case of thin sheets with a thickness of 0.5 mm, in order to reduce the overload of shear rolling, the reduction rate is preferably 2% or more.

[0037] Additionally, the cold-rolled material described above can be annealed at a temperature of 600-850°C as needed. The furnace atmosphere during annealing preferably uses a gas containing nitrogen (N2) and 1-10% by volume hydrogen (H2). When the hydrogen concentration is less than 1% by volume, it is difficult to reduce the oxides on the steel surface; when the hydrogen concentration exceeds 10% by volume, manufacturing costs increase. Therefore, a hydrogen concentration of 1-10% by volume is preferred.

[0038] The composition of the oxide film formed on the iron substrate surface varies depending on the dew point temperature of the atmosphere during annealing, and the internal oxidation ratio also differs. Therefore, the dew point temperature is preferably controlled between -60°C and -10°C. When the dew point temperature is below -60°C, controlling the purity of the raw material gas may incur excessive costs, and this is not preferred. On the other hand, when the dew point temperature exceeds -10°C, contaminants on the iron substrate surface may not be reduced effectively, and oxide films containing trace elements or impurities such as B and Mn contained in the steel may form, thus posing a risk of hindering plating wettability.

[0039] The prepared base iron is immersed in a plating bath to produce zinc-plated alloy steel. The plating bath, by weight percent, contains: Mg: 0.5-3.5%, Al: 0.5-11.0%, Si: 10-350 ppm, balance Zn, and unavoidable impurities. The composition is the same as described above for the zinc alloy coating.

[0040] Furthermore, the plating bath may further contain 10-80 ppm by weight of iron (Fe). This Fe is primarily dissolved from the base iron and contained within the plating bath. Fe exceeding the limit of solid solubility in the plating bath combines with Al to form FeAl compounds. Additionally, some Si in the plating bath is absorbed by the FeAl formed in the plating bath. These FeAl compounds formed in the plating bath exist in a solid phase form called scum, which can be mixed into the coating during manufacturing and potentially cause defects. Furthermore, the scum absorbs Si in the plating bath, thereby reducing the concentration of soluble Si in the plating bath. In this case, Si cannot be uniformly enriched in the inhibition layer. Therefore, the total Fe content in the plating bath is preferably no more than 80 ppm by weight. As an example of controlling the Fe content in the plating bath to below 80 ppm, an inert gas is injected into the bottom of the plating bath, causing the inert gas to float the compounds formed by Fe and Al in the plating bath to the upper part, thereby reducing the Fe concentration in the plating bath. A low Fe content will not cause problems, but controlling the Fe content to be too low will require excessive processing costs, so 10 ppm or more is sufficient.

[0041] Furthermore, the amounts of Al, Mg, and Zn in the plating bath are determined based on the composition of the coating. The temperature of the plating bath during plating is preferably above +10°C and below +90°C of the melting point of the plating bath composition. When the temperature of the plating bath does not reach above +10°C of the melting point, the fluidity of the plating bath decreases, hindering uniform coating adhesion. Additionally, when the temperature of the plating bath exceeds +90°C of the melting point, the oxides on the surface of the plating bath may increase due to the oxidation of Mg within the bath, and precipitation of refractory materials caused by Al and Mg may occur.

[0042] When the temperature of the substrate iron immersed in the plating bath is above the plating bath temperature, it is advantageous in terms of the decomposition of surface oxides and Al enrichment. To maximize this effect, the temperature of the substrate iron introduced into the plating bath is preferably 5°C or more higher than the plating bath temperature, more preferably 10°C or more higher. However, if the temperature of the substrate iron introduced into the plating bath is too high, it may be difficult to control the temperature of the plating bath, and the components of the substrate iron may dissolve excessively into the plating bath. Therefore, the temperature of the substrate iron is preferably below the plating bath temperature + 30°C, more preferably below the plating bath temperature + 20°C.

[0043] The zinc-plated alloy steel, after being plated in the plating bath, is subjected to gas wiping to adjust the plating adhesion and then cooled. Detailed Implementation

[0044] The embodiments of the present invention will be described in detail below. These embodiments are for understanding the present invention only and do not limit the scope of the invention.

[0045] (Example) A hot-rolled steel sheet with the average grain size shown in Table 1 is prepared, and then cold-rolled to prepare the base steel sheet in Table 1. During the cold rolling, the tension and reduction rate of the steel sheet are adjusted to produce a base steel sheet with the surface roughness (Ra) and steepness shown in Table 1. The composition of the base steel sheet, by weight%, comprises: C: 0.03%, Si: 0.02%, Mn: 0.15%, P: 0.01%, S: 0.01%, balance Fe and unavoidable impurities.

[0046] The prepared base steel plate is immersed in a galvanized alloy bath, and then adjusted to 50 g / m² on one side. 2 After the amount of plating is applied, the material is cooled to produce a Zn-Al-Mg galvanized alloy steel sheet. The Al, Mg, and Si compositions of the produced galvanized alloy steel sheet are then measured and shown in Table 1. The Fe content in the plating bath is also measured and shown in Table 1. The Fe content is analyzed by taking samples of the plating bath from the bottom to the surface of the bath at a distance of 1 / 2.

[0047] The cross-section of the galvanized alloy steel sheet was examined, and the formation of the Si-enriched layer, the size and quantity of the Mg2Si alloy phase in the inhibition layer formed between the zinc alloy coating and the base iron were measured. The results are shown in Table 1. After observing the cross-section of the coating by SEM, the formation of the Si-enriched layer and the size and quantity of the Mg2Si alloy phase were measured.

[0048] Furthermore, to evaluate the corrosion resistance of the galvanized alloy steel sheet after processing, the galvanized alloy steel sheet was cut into a circle with a diameter of 100 mm, and then processed into a cup shape using a punch with a diameter of 50 mm. At this time, the edge curvature of the punch was 5 mm, and the drawing ratio was 2.0.

[0049] The processed cup-shaped specimens were subjected to a saltwater composite corrosion test (cyclic corrosion test) as specified in ISO TC 156. The cup-shaped specimens were placed in a corrosion testing machine with the bottom facing upwards and the test was conducted. The specimens were checked for red rust at each corrosion cycle, and the results are shown in Table 1.

[0050] [Table 1]

[0051] in addition, Figure 2 This is a photograph of the cross-section of the coating in Example 3 of the invention. Figure 3 This is a photograph observing the cross-section of the coating in Comparative Example 3. It can be confirmed that... Figure 2 A uniform inhibition layer is formed between the plating layer 22 and the substrate iron 21, but Figure 3 A large amount of Mg2Si33 is formed in the inhibition layer between the intermediate plating layer 32 and the substrate iron 31.

[0052] From Table 1, Figure 2 and Figure 3 The results show that in the inventive examples that meet the conditions of the present invention, excellent corrosion resistance is confirmed even after processing. In contrast, in the comparative examples that do not meet the conditions of the present invention, poor corrosion resistance after processing is confirmed.

Claims

1. A galvanized alloy steel with excellent corrosion resistance after processing, comprising: Base material iron; A zinc alloy coating is formed on the iron substrate. as well as An inhibitory layer is formed between the iron substrate and the zinc alloy plating. The zinc alloy coating, by weight percent, comprises: Mg: 0.5-3.5%, Al: 0.5-11.0%, Si: 10-350 ppm, with the balance being Zn and unavoidable impurities. The suppression layer includes a Si enrichment layer.

2. The galvanized alloy steel with excellent corrosion resistance after processing according to claim 1, wherein, Between the zinc alloy coating and the iron substrate, the number of Mg2Si alloy phases with a diameter exceeding 1000 nm per 100 μm is less than 5.

3. The galvanized alloy steel with excellent corrosion resistance after processing according to claim 1, wherein, Between the zinc alloy coating and the iron substrate, the number of Mg2Si alloy phases with a diameter exceeding 500 nm per 100 μm is less than 5.

4. A method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing, comprising the following steps: Prepare hot-rolled steel with a grain size of 1-100μm; The hot-rolled steel is cold-rolled to produce cold-rolled steel with a surface roughness of 0.2-1.0 μm and a steepness of 0.2-1.2 μm; The base iron used as the cold-rolled steel is immersed in a plating bath for plating. The plating bath contains, by weight %: Mg: 0.5-3.5%, Al: 0.5-11.0%, Si: 10-350ppm, balance Zn and unavoidable impurities. The hot-dip galvanized alloy steel that has been coated is wiped and cooled.

5. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing according to claim 4, wherein, The plating bath further contains 10-80 ppm of Fe.

6. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing according to claim 4, wherein, The temperature of the plating bath is from the melting point of the plating bath +10℃ to the melting point of the plating bath +90℃.

7. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing according to claim 4, wherein, The temperature of the substrate iron during plating is between the plating bath temperature +5°C and the plating bath temperature +30°C.

8. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing according to claim 4, wherein, The method further includes the step of annealing the cold-rolled steel at a temperature of 600-850°C.

9. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing according to claim 8, wherein, The furnace atmosphere for the annealing heat treatment uses 1-10% by volume hydrogen (H2) with the balance being nitrogen (N2).

10. The method for manufacturing galvanized alloy steel with excellent corrosion resistance after processing, as described in claim 8, wherein, The dew point temperature of the annealing heat treatment is -60°C to -10°C.

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