Zinc alloy plated steel material with excellent corrosion resistance after processing and its manufacturing method

Abstract

The present invention relates to a plated steel material that can be used in automobiles, home appliances, building materials, etc., and more particularly to a zinc alloy plated steel material that has excellent corrosion resistance after processing and a method for producing the same. [Solution] A zinc alloy plated steel material with excellent corrosion resistance after processing is provided, which includes base iron (11), a zinc alloy plated layer formed on the base iron, and an inhibition layer (12) formed between the base iron and the zinc alloy plated layer, wherein the zinc alloy plated layer contains, by weight, 0.5 to 3.5% Mg, 0.5 to 11.0% Al, 10 to 350 ppm Si, the balance being Zn and unavoidable impurities, and the inhibition layer includes a Si-enriched layer.

Description

[Technical Field]

[0001] The present invention relates to a plated steel material that can be used in automobiles, home appliances, building materials, etc. The present invention relates to a zinc alloy plated steel material having excellent corrosion resistance after processing and a method for producing the same. [Background technology]

[0002] The zinc plating method, which inhibits iron corrosion through cathodic protection, is excellent in corrosion prevention performance and economy. It is widely used to produce steel with high corrosion resistance. Hot-dip galvanized steel, which is formed by immersing steel in lead, has a higher corrosion resistance than electro-galvanized steel. The manufacturing process is simple and the product is inexpensive, making it suitable for a wide range of applications, including automobiles, home appliances, and building materials. Demand is increasing across industries, including automotive and electronics.

[0003] When hot-dip galvanized steel is exposed to a corrosive environment, zinc, which has a lower oxidation-reduction potential than iron, Sacrificial corrosion protection, which corrodes first and inhibits the corrosion of steel. Furthermore, the zinc in the plating layer is oxidized. This allows the formation of dense corrosion products on the steel surface, isolating the steel from the oxidizing atmosphere. Improves the corrosion resistance of steel.

[0004] However, with the advancement of industry, air pollution and corrosive environments have worsened, and resources and energy are becoming scarce. -Strict regulations on thrifting have led to the development of steel with better corrosion resistance than conventional galvanized steel. As part of this effort, aluminum (Al) is being added to the zinc plating bath. Zinc alloy-based steels that improve the corrosion resistance of steel by adding elements such as magnesium (Mg). There has been extensive research into the manufacturing technology of zinc alloy plated steel. Manufacturing of Zn-Al-Mg plated steel by adding Mg to the Zn-Al plating composition Research into this technology is being actively conducted (Patent Document 1).

[0005] On the other hand, if cracks occur in the coating layer during processing of galvanized steel material, The corrosion prevention effect of the plating layer weakens, and sacrificial corrosion of the plating layer at the cracked area progresses rapidly. This reduces the lifespan of the plating layer and reduces corrosion resistance. In the case of steel materials, parts are manufactured through various forming processes, so corrosion resistance decreases after processing. In reality, there is an increasing demand for a method that can reduce the [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-332555 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is not only to ensure excellent corrosion resistance but also to prevent the plating layer from breaking down during processing. It reduces crack occurrence and prevents deterioration of corrosion resistance, ensuring excellent corrosion resistance after processing. The present invention aims to provide a zinc alloy plated steel material and a method for manufacturing the same.

[0008] The problems to be solved by the present invention are not limited to the problems mentioned above, and may include problems not mentioned above. Still other problems will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0009] One aspect of the present invention is a steel substrate, a zinc alloy plating layer formed on the steel substrate, and a steel sheet comprising: Inhibition layer formed between the steel substrate and the zinc alloy plating layer The zinc alloy plating layer contains, by weight, Mg: 0.5 to 3.5%, Al: 0. 5 to 11.0%, Si: 10 to 350 ppm, the balance being Zn and unavoidable impurities. The present invention relates to a zinc alloy plated steel material having excellent corrosion resistance after processing, which contains a silicon-enriched layer within the coating.

[0010] Another aspect of the present invention is a method for manufacturing a steel sheet by a method comprising the steps of: providing a hot-rolled steel material having a grain size of 1 to 100 μm; The hot-rolled steel material is cold-rolled to have a surface roughness of 0.2 to 1.0 μm and a steepness of 0.2 to 1.2. and a step of manufacturing a cold-rolled steel material having a composition of, by weight, Mg: 0. 5~3.5%, Al: 0.5~11.0%, Si: 10~350ppm, balance Zn and A step of plating by immersing in a plating bath containing unavoidable impurities, and the plated molten zinc alloy The zinc alloy plating process provides excellent corrosion resistance after the processing step, which includes wiping and cooling the plated steel material. This relates to a manufacturing method for steel materials. [Effects of the Invention]

[0011] According to the present invention, a Zn-Al-Mg zinc alloy plated steel material having excellent corrosion resistance after processing and This allows the use of the material to be expanded to areas where its use was previously limited. The advantage is that it can expand the area. [Brief explanation of the drawings]

[0012] [Figure 1] 1(a) to 1(d) are photographs showing the analysis of the components of the inhibition layer of a zinc alloy plated steel material produced in one embodiment of the present invention. [Figure 2] 1 is a photograph showing a cross section of a plating layer of Example 3 among the examples of the present invention. [Figure 3] 1 is a photograph showing a cross section of a plating layer in Comparative Example 3 of the examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] While ordinary zinc plating solidifies as a single phase of zinc, Zn-Al-Mg zinc alloy In gold plating, Zn phase, Mg and Zn alloy phase, Al phase, etc. coexist. The plating structure is the result of the physical and chemical changes on the surface of the base iron caused by trace elements in the plating bath and the manufacturing process. Depending on the conditions, very complex plating structures can be formed.

[0014] Zn-Al-Mg zinc alloy plating layer (hereinafter referred to as zinc alloy plating layer or plating layer) plating The alloy phases of Zn and Mg in the structure are MgZn2, Mg2Zn 11 Various intermetallic materials such as The hardness of these compounds can reach Hv250 to 300. At the interface between the coating layer and the base steel, an inhibitor layer (Inhib) consisting of an intermetallic compound of Fe and Al is formed. The intermetallic layer of Fe and Al can be formed. Fe4Al as a compound 13 , Fe2Al5, etc. These intermetallic compounds also have high Because of its brittleness, cracks are likely to occur in the plating layer when physically deformed.

[0015] Therefore, the inventors have developed a method for reducing the occurrence of cracks in the plating layer during processing of zinc alloy plated steel material. As a method for ensuring excellent corrosion resistance after processing, the above-mentioned inhibitor layer is The present invention has been developed by devising a method for forming the same uniformly. do.

[0016] The zinc alloy plated steel material of the present invention comprises a base steel and a zinc alloy plated layer formed on the base steel. and an inhibition layer formed between the base steel and the zinc alloy plating layer. layer).

[0017] The zinc alloy plating layer contains, by weight %, magnesium (Mg): 0.5 to 3.5%, aluminum (Al): Aluminum (Al): 0.5 to 11.0%, Silicon (Si): 10 to 350 ppm, balance It is preferable that Zn and inevitable impurities are contained. Each component will be described in detail below.

[0018] The above Mg plays a very important role in improving the corrosion resistance of zinc-plated steel. , forming dense zinc hydroxide corrosion products on the surface of the plating layer in a corrosive environment. This effectively prevents corrosion of zinc-plated steel. The content is preferably 0.5% by weight or more, and more preferably 0.8% by weight or more. However, if the content is too high, Mg oxide may form on the surface of the plating bath. The oxidation inhibitory effect of adding trace elements is cancelled out by the rapid increase in the amount of reactive dross on the surface of the plating bath. To prevent this, it is preferable that Mg is contained in an amount of 3.5 wt % or less, and 2.0 wt % or less. % or less is more preferable.

[0019] The Al suppresses the formation of Mg oxide dross in the plating bath and reduces the amount of Zn and M in the plating bath. It reacts with magnesium to form a Zn-Al-Mg intermetallic compound, which improves the corrosion resistance of plated steel. In the present invention, in order to obtain the above effect, it is preferable that the content be 0.5% by weight or more. It is preferable that the content is 0.8% by weight or more. However, if the content is excessive, This may result in a decrease in the weldability and phosphate treatability of the plated steel. In this respect, the Al content is preferably 11.0% by weight or less, and more preferably 6.0% by weight or less. It is more preferable that:

[0020] The above-mentioned Si is used to form an inhibition layer on the zinc alloy plating layer. At times, Si is dissolved in Fe-Al compounds to provide ductility. If it is not precipitated as a metal but is concentrated in the inhibitor layer, it is advantageous for improving the adhesion and fracture toughness of the plating layer. For the above effects, it is preferable that the content be 10 ppm by weight or more. If the amount is too large, the interface between the coating layer and the base steel may become uneven if the roughness or shape of the steel sheet surface is uneven. Coarse, brittle Mg2Si precipitates are formed, which in turn induces cracks in response to external stress. Therefore, it is preferable that the Si content does not exceed 350 ppm by weight. .

[0021] The remainder includes zinc (Zn) and unavoidable impurities.

[0022] On the other hand, some Fe in the plating bath may be included in the plating layer, but the Fe mentioned above is mainly contained in the plating bath. It is present in the inhibition layer at the interface between the plating layer and the base steel.

[0023] The zinc alloy plating layer is made of MgZn2 phase, Mg2Zn 11 Mg and Zn alloys containing phases The first phase to solidify during solidification is the Zn phase, and the internal After the solidification of the primary Zn, Zn and MgZn2 are solidified in the form of lamellae. The ternary phases of Zn, MgZn2, and Al are finally solidified. In the slower cases, in addition to MgZn2, Mg2Zn 11 can be formed in the Zn phase. In comparison, MgZn2 and Mg2Zn 11 Because of its high hardness, it becomes the starting point for cracks when subjected to external stress. However, the inclusion of Mg can improve corrosion resistance.

[0024] The suppression layer includes a layer in which Si is enriched (Si-enriched layer). In particular, the Si-enriched layer includes: The Si-enriched layer is preferably located below the suppression layer. This means that Si is dissolved in the suppression layer and is contained in the lower part of the suppression layer. The suppression layer contains a Si-enriched layer in which Si is dissolved, and thus the suppression layer can suppress the stress like an external stress. Even if the control layer is given mechanical toughness, the suppression layer reduces mechanical damage, and the plating layer The suppression layer is uniformly formed and the S In order to distribute i uniformly within the inhibition layer, the plating bath components are controlled, and the surface of the base steel is Metallographic control, surface structure and plate shape are required. If the amount of Si is sufficient, the Si content will be large and the zinc alloy coating layer will be thicker than the steel substrate. When external stress is applied, a stress concentration phenomenon occurs in the Mg2Si alloy phase, and the cracks Therefore, the occurrence and growth of cracks may be accelerated. The interface formed between the The number of Mg2Si alloy phases is preferably 5 or less. More preferably, the number of Mg2Si alloy phases is more than 500 nm. As shown in Figure 3, the number of Mg2Si alloy phases is suppressed. can be observed just above the layer.

[0025] When the suppression layer is formed uniformly, Si is also dissolved uniformly along the suppression layer. However, the uneven shape of the steel surface and the presence of the uneven shape on the steel surface make it possible. If the oxides are not decomposed smoothly, the thickness of the inhibition layer may become uneven or may be partially In this case, coarse Mg2Si may be formed. Therefore, it is preferable that the suppression layer is uniformly formed between the base steel and the zinc alloy plating layer. .

[0026] 1(a) to 1(d) show the suppression layer (I) of the zinc alloy plated steel material manufactured in one embodiment of the present invention. This is a photograph of the analysis of the inhibition layer. The plating layer is analyzed by a wet analysis method in which it is dissolved in a hydrochloric acid solution, and by GDOES, GDMS, etc. Analysis using a plasma source or direct analysis of the components via TEM The above-mentioned Figures 1(a) to (d) show the components observed through TEM. As shown in Figure 1(a) to (d), the suppression of the zinc alloy plated steel material It can be seen that the Si layer is enriched at the bottom of the suppression layer. It can be seen that an inhibiting layer 12 is formed on the base iron 11. Surface reference numeral 13 indicates a protective film for TEM observation. 1 shows the results of analyzing the Al, Fe and Si components in the suppression layer. In particular, it can be seen from FIG. 1(d) that Si is enriched in the suppression layer.

[0027] An embodiment of the method for producing a zinc alloy plated steel material according to the present invention will be described in detail below. The zinc alloy plated steel material of the present invention is prepared by providing a base iron and immersing the base iron in a plating bath. After plating, the process involves wiping, adjusting the thickness of the plating layer, and cooling.

[0028] When preparing the base steel, it is preferable to first make the metal structure of the hot-rolled steel uniform. The grains of the above hot-rolled steel material are preferably in the surface layer (within 1 / 8 of the total thickness based on the surface). When the structure of hot-rolled steel, especially the surface structure, becomes uneven, the surface shape during cold rolling is The unevenness of the layer and the uneven diffusion of Fe from the base steel required for the formation of the layer cause the layer to form unevenly. The Si concentration is not uniform, and the coarse Mg2Si alloy phase is locally formed. For this reason, the average grain size of the above hot-rolled steel material is 1 to 100 μm. More preferably, the crystal grain size is 1 to 50 μm or 5 to 3 μm. It is more preferable that the thickness is 0 μm.

[0029] When the grain size of the hot-rolled steel is less than 1 μm, it is advantageous for securing strength, but In some cases, the surface roughness due to the crystal grains may increase. Although this method is advantageous in terms of homogenizing the shape, there is concern that excessive increases in the hot rolling temperature may cause scale defects. In order to obtain the above-mentioned grain size of the hot-rolled steel material, the manufacturing cost of the product may increase. As an example of a method for this purpose, the hot rolling temperature is set to at least 800°C or more, or after hot rolling, For example, the coiling temperature is set to 550°C or higher.

[0030] When the above hot-rolled steel material is cold-rolled to produce cold-rolled steel material, the surface roughness (Ra ) is 0.2 to 1.0 μm, and the steepness is preferably 0.2 to 1.2. I wish.

[0031] The surface roughness is determined by the pressure and surface shape of the roll when the roll rolls the material. If the surface roughness exceeds 1.0 μm, the roughness will increase and the plating layer will not form. Therefore, it is difficult to form a uniform Si-enriched layer. On the other hand, if the thickness is less than 0.2 μm, the surface friction coefficient decreases and the steel material There is a risk of slipping on the slope.

[0032] The steepness measurement is carried out by placing a steel material 1m or more in width and 2m or more in length on a flat surface plate. The steel surface is placed firmly in contact with the surface, and the degree of bending of the steel is measured. The steepness is expressed as the value obtained by dividing the bending height (H) by the wavelength (P) and then multiplying it by 100. In other words, it is expressed by the formula height (H) / wavelength (P) x 100. The smaller the steepness, the Furthermore, if the steepness exceeds 1.2, the bending of the steel material will be large. When the steel material passes through the plating bath, the surface flow is deviated, and the inhibiting layer The lower the steepness, the better. Controlling it below 0.2 is not desirable because it would require excessive process costs.

[0033] The method for controlling the roughness and steepness within the appropriate range is not limited to any one method. It is preferable that the reduction ratio in the final stage of cold rolling is in the range of 2 to 5%. It is necessary to apply an appropriate tension to the steel plate inside. For example, the steel surface can be plasma treated. Since the final shape is determined by the rolling rolls, the rolling ratio is preferably 5% or less. However, in the case of thin plates with a thickness of 0.5 mm, in order to reduce the overload of the first stage rolling, It is preferable to set it to % or more.

[0034] On the other hand, the above cold-rolled material can be annealed at a temperature of 600 to 850°C as required. The atmosphere in the furnace during the annealing is nitrogen (N2) with 1 to 10% by volume of hydrogen (H2). When the hydrogen concentration is less than 1% by volume, the surface of the steel is It is difficult to reduce the oxide on the surface, and if it exceeds 10% by volume, the manufacturing cost increases. Therefore, it is preferable that the hydrogen content be 1 to 10% by volume.

[0035] The dew point temperature in the atmosphere during the annealing process varies, so the oxide film formed on the surface of the base steel is Not only are the ratios of constituent components different, but the rate of internal oxidation is also different, so the dew point temperature is It is preferable to control the temperature at -60 to -10°C. When the dew point temperature is less than -60°C, This is undesirable because it may require excessive costs to control the purity of the raw material gas. If the dew point temperature exceeds -10°C, the reduction of contaminants on the surface of the raw steel will not proceed smoothly. There is a possibility that an oxide film is formed by trace elements and impurities such as B and Mn contained in the steel. This may result in impairing plating wettability.

[0036] The base steel prepared as described above is immersed in a plating bath to produce a zinc alloy plated steel material. The plating bath contains, by weight, 0.5 to 3.5% Mg, 0.5 to 11.0% Al, and Si. : 10 to 350 ppm, the balance being Zn and unavoidable impurities. This is the same as the above description of the zinc alloy plating layer.

[0037] On the other hand, the plating bath may further contain 10 to 80 ppm by weight of iron (Fe). The above Fe is mainly dissolved from the base iron and included in the plating bath. Any Fe that exceeds the limit of its capacity will combine with Al to form FeAl compounds. In addition, some of the Si in the plating bath is absorbed into the FeAl formed in the plating bath. In this way, the FeAl compounds generated in the plating bath form a solid phase called dross. These exist in the form of metals, and can be mixed into the plating layer during production, causing defects. In addition, the dross absorbs Si in the plating bath, reducing the concentration of soluble Si in the plating bath. In this case, Si is not uniformly concentrated in the inhibition layer. The total Fe content in the bath preferably does not exceed 80 ppm by weight. As an example of a method for controlling the Fe content to 80 ppm or less, an inert gas is placed at the bottom of the plating bath. The inert gas is injected to make the compounds consisting of Fe and Al in the plating bath rise to the top. The Fe concentration in the plating bath can be reduced. The Fe content is not particularly limited, but Controlling it too low would incur excessive process costs, so it is best to keep it at 10 ppm or higher. stomach.

[0038] On the other hand, the amounts of Al, Mg, and Zn contained in the plating bath are determined depending on the composition of the plating layer. The temperature of the plating bath during the plating process is set at 10°C or more above the melting point of the plating bath composition. It is preferable that the temperature of the plating bath is 90°C or less. If this is not the case, the fluidity of the plating bath will decrease, which may hinder uniform plating coverage. On the other hand, if the temperature of the plating bath exceeds 90°C above the melting point of the plating bath, Increase of oxide on plating bath surface due to oxidation of Mg and refractory of plating bath with Al and Mg Erosion of the soil can be a problem.

[0039] If the temperature of the base iron immersed in the plating bath is equal to or higher than the temperature of the plating bath, the surface oxide This is advantageous in terms of decomposition and Al concentration. The temperature of the raw iron being drawn in is preferably 5°C or higher than the temperature of the plating bath. However, if the temperature of the base iron drawn into the plating bath is excessively high, If the temperature is too high, it may be difficult to control the temperature of the plating pot, and the composition of the base steel may change. There is a possibility that excessive elution may occur in the plating bath, so do not exceed 30°C in relation to the temperature of the plating bath. It is preferable to avoid this, and the temperature should not exceed 20°C above the plating bath temperature. It is more preferable to do so.

[0040] The zinc alloy plated steel material was gas wiped with the above plating bath to obtain a coating weight of Adjust and cool. [Example]

[0041] The present invention will be described in more detail below with reference to examples. The following examples are provided to aid in understanding the present invention. The present invention is not intended to limit the scope of the present invention.

[0042] (Example) A hot-rolled steel sheet having the average grain size shown in Table 1 below was prepared and then cold-rolled to the following The base steel sheets shown in Table 1 were prepared. The tension and reduction of the steel sheets during the cold rolling were adjusted to obtain the steel sheets shown in the table below. The base steel sheets were manufactured having the surface roughness (Ra) and steepness shown in Table 1. The composition is, in weight percent, C: 0.03%, Si: 0.02%, Mn: 0.15%, P: 0 0.01%, S: 0.01%, the remainder includes Fe and unavoidable impurities.

[0043] The above-described base steel sheet was immersed in a zinc alloy plating bath, and one side was coated with 50 g / m 2 Adjust the amount of adhesion After that, the steel sheet was cooled to produce a Zn-Al-Mg zinc alloy plated steel sheet. The Al, Mg and Si contents of the zinc alloy plated steel sheets were measured and shown in Table 1 below. The Fe content was measured and shown in Table 1 below. The sample was taken from halfway between the surface and the boiling water and analyzed.

[0044] The cross section of the coating layer of the zinc alloy coated steel sheet was examined, and the area between the zinc alloy coating layer and the base steel was examined. Formation of a Si-enriched layer in the formed inhibition layer, M The size and number of g2Si alloy phases were measured, and the results are shown in Table 1 below. The formation of the i-enriched layer, the size and number of Mg2Si alloy phases were measured using a cross section of the coating layer using an SEM. After observation, measurements were taken.

[0045] In addition, in order to evaluate the corrosion resistance of the zinc alloy plated steel sheet after processing, After cutting into a circle with a diameter of 100 mm, it was processed into a cup shape using a punch with a diameter of 50 mm. In this case, the edge curvature of the punch was 5 mm, and the drawing ratio ) is 2.0.

[0046] The cup-shaped test piece processed above was subjected to saltwater complex test specified in ISO TC 156. A cyclic corrosion test was conducted. Place the test piece in the corrosion tester with the bottom facing up, and measure the corrosion size while performing the test. After each cycle, it was confirmed whether or not red rust had occurred on the test piece, and the results were shown in Table 1 below. is also shown.

[0047] [Table 1]

[0048] On the other hand, FIG. 2 shows the cross section of the plating layer of the above-mentioned Example 3 of the present invention, and FIG. 3 shows the cross section of the plating layer of the above-mentioned Comparative Example. 2 shows the cross section of the coating layer 22 and the base iron 21. The suppression layer formed between the coating layer 32 and the base iron 31 was uniformly formed. It can be seen that a large amount of Mg2Si 33 was formed in the suppression layer.

[0049] As can be seen from the results of Table 1 and Figures 2 and 3, in the examples of the invention that satisfy the conditions of the present invention, It was confirmed that the steel sheet had excellent corrosion resistance even after processing. In the comparative example in which no corrosion was observed, it was confirmed that the corrosion resistance of the processed portion was reduced.

Claims

1. With bare iron, a zinc alloy plating layer formed on the base steel; an inhibition layer formed between the base steel and the zinc alloy plating layer, The zinc alloy plating layer contains, by weight %, 0.5 to 3.5% Mg, 0.5 to 11.0% Al, 10 to 350 ppm Si, and the balance being Zn and inevitable impurities, the suppression layer includes a Si-enriched layer in which Si is solid-dissolved in an Fe—Al-based compound, A zinc alloy plated steel material having excellent corrosion resistance after processing, wherein the number of Mg 2 Si alloy phases having a diameter exceeding 1000 nm per 100 μm between the zinc alloy plated layer and the base steel is 5 or less.

2. Between the zinc alloy plating layer and the base iron, Mg having a diameter of more than 500 nm per 100 μm ２ 2. The zinc alloy plated steel material having excellent corrosion resistance after working according to claim 1, wherein the number of Si alloy phases is 5 or less.

Images