Coated steel sheet and its manufacturing method

Abstract

The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly to a plated steel sheet having excellent heat resistance and sacrificial corrosion resistance and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly to a plated steel sheet having excellent heat resistance and sacrificial corrosion resistance and a method for manufacturing the same. [Background technology]

[0002] Steel used in automotive exhaust systems must be heat-resistant and corrosion-resistant. When using conventional galvanized steel sheets, the low melting point of the coating layer can lead to alloying and discoloration when high-temperature exhaust gases are emitted. Furthermore, aluminum plating lacks sacrificial corrosion protection, meaning that if the coating layer is damaged during vehicle operation, carbon steel may corrode, necessitating the use of expensive stainless steel.

[0003] To solve the above problems, Patent Document 1 proposes a manufacturing method in which a zinc foil is pressure-bonded to the surface of a cold-rolled steel sheet using a cladding method, and at the same time, an aluminum foil is pressure-bonded onto the zinc foil, and then the steel sheet is passed through an oven at a temperature ranging from 300 to 400° C. However, although this method has the advantage of ensuring corrosion resistance and heat resistance, the process is complicated and therefore difficult to apply in practice.

[0004] Patent Document 2 proposes an aluminum-plated steel sheet that has excellent heat resistance and discoloration resistance by forming a plating layer containing Al and Si on carbon steel. This technology has the problem that the aluminum plating layer has excellent surface corrosion resistance but does not have sacrificial corrosion protection and cannot effectively prevent corrosion of the base steel.

[0005] Patent Document 3 proposes a ferritic stainless steel for aluminum-plated steel sheets that has excellent resistance to condensed water corrosion and high-temperature oxidation when high-sulfur fuel is used. However, this also has the problem of increased costs because expensive stainless steel is used as the base steel. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Registration No. 10-0580047 [Patent Document 2] Korean Patent Registration No. 10-1197955 [Patent Document 3] Korean Patent Registration No. 10-1485643 Summary of the Invention [Problem to be solved by the invention]

[0007] According to one embodiment of the present invention, a plated steel sheet and a manufacturing method thereof are provided.

[0008] According to one embodiment of the present invention, there is provided a plated steel sheet having excellent heat resistance and sacrificial corrosion resistance, and a method for manufacturing the same.

[0009] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding the further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0010] According to one embodiment of the present invention, a base steel sheet; and a coating layer formed on the base steel sheet and containing, by weight, 15 to 35% Zn, 5.0 to 15.0% Si, 1.0 to 7.0% Fe, the balance being Al and other impurities; Other impurities in the plating layer are 5.0% or less by weight, It is possible to provide a plated steel sheet in which the ratio of the Si content in the region from the surface to the 1 / 3 point of the plating layer in the thickness direction to the Si content in the entire plating layer is 60 to 85%.

[0011] The above-mentioned base steel sheet can contain, by weight, C: 0.1% or less, Mn: 2.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Si: 1.5% or less, Al: 0.5% or less, P: 0.01% or less, S: 0.01% or less, the balance being Fe and other unavoidable impurities.

[0012] The plated steel sheet may have a difference in whiteness (ΔL) of 5 or less after exposure at 500°C for 24 hours, and a cross-sectional red rust occurrence rate of 5% or less after 200 hours of salt spray.

[0013] According to one embodiment of the present invention, the method includes the steps of: preparing a base steel sheet; heat treating the base steel sheet; dipping the heat-treated base steel sheet in a coating bath containing, by weight, 17 to 38% Zn, 7 to 18% Si, the balance being Al and other impurities, to coat the steel sheet; and cooling the plated steel sheet to a temperature range of 300°C or less at an average cooling rate of 10 to 50°C / s; It is possible to provide a method for producing a plated steel sheet in which the other impurities in the plating bath are 6.0% by weight or less.

[0014] The above-mentioned base steel sheet can contain, by weight, C: 0.1% or less, Mn: 2.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Si: 1.5% or less, Al: 0.5% or less, P: 0.01% or less, S: 0.01% or less, the balance being Fe and other unavoidable impurities.

[0015] During the heat treatment, the base steel sheet can be maintained at a temperature in the range of 750 to 850°C for 50 to 500 seconds.

[0016] The temperature of the plating bath can be 40° C. or more above the melting point of the plating component system. [Effects of the Invention]

[0017] According to one embodiment of the present invention, a plated steel sheet and a manufacturing method thereof can be provided.

[0018] According to one embodiment of the present invention, it is possible to provide a plated steel sheet having excellent heat resistance and sacrificial corrosion resistance, and a method for manufacturing the same.

[0019] According to one embodiment of the present invention, an aluminum-based plated steel sheet that can be used for automobile exhaust systems (mufflers) and a method for manufacturing the same can be provided. [Brief explanation of the drawings]

[0020] [Figure 1] 10 is a photograph showing red rust formation in Example 5 according to one embodiment of the present invention. [Figure 2] 10 is a photograph of the Si distribution of Example 5 according to one embodiment of the present invention, taken by EPMA. [Figure 3] 10 is a photograph showing red rust formation in Comparative Example 7, which deviates from one embodiment of the present invention. [Figure 4] 10 shows a photograph of the Si distribution of Comparative Example 7, which deviates from one embodiment of the present invention, taken by EPMA. DETAILED DESCRIPTION OF THE INVENTION

[0021] Preferred embodiments of the present invention will be described below. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to explain the present invention in more detail to those skilled in the art to which the invention pertains.

[0022] In the present invention, the inventors have confirmed that by applying to carbon steel the sacrificial corrosion resistance and heat resistance that cannot be achieved by conventional aluminum-plated steel sheets made from stainless steel, it is possible to provide a plated steel sheet that is inexpensive yet has excellent heat resistance and sacrificial corrosion resistance, and have thus completed the present invention.

[0023] The present invention will be described in detail below.

[0024] A plated steel sheet according to one embodiment of the present invention may include a base steel sheet; and a plating layer formed on the base steel sheet.

[0025] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0026] Base steel sheet In the present invention, the composition of the base steel sheet is not particularly limited, however, the base steel sheet according to one embodiment of the present invention may have a composition, in weight percent, of C: 0.1% or less, Mn: 2.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Si: 1.5% or less, Al: 0.5% or less, P: 0.01% or less, and S: 0.01% or less.

[0027] In addition to the above-described composition, the steel material of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities may be unintentionally mixed in during the normal manufacturing process and therefore cannot be excluded. Since such impurities are known to any engineer in the field of normal steel manufacturing, the details of all of them will not be specifically mentioned in this specification.

[0028] plating layer The plating layer according to one embodiment of the present invention contains, by weight, 15-35% Zn, 5.0-15.0% Si, 1.0-7.0% Fe, and the remainder being Al and other impurities, and the other impurities may be 5.0% or less.

[0029] If the Zn content of the plating layer exceeds 35%, there is a problem of poor plating adhesion. In one embodiment of the present invention, Zn can be contained at 30% or less. On the other hand, if the Zn content is less than 15%, there is a problem of insufficient sacrificial corrosion protection. In one embodiment of the present invention, Zn can be contained at 20% or more.

[0030] During Al plating, the addition of Si has the advantage of suppressing excessive reaction between Al and Fe, thereby ensuring plating adhesion. If the Si content exceeds 15.0%, the plating adhesion may actually be poor. In one embodiment of the present invention, the Si content may be 120% or less. However, to effectively ensure the above-mentioned advantages, the Si content may be limited to 5.0% or more. In one embodiment of the present invention, the Si content may be 7.0% or more.

[0031] Although Fe is not artificially added during plating, the Fe component of the base steel sheet reacts with the Al in the coating layer, potentially forming an Al-Fe alloy phase from the interface between the coating layer and the base steel sheet toward the coating layer. If the Fe content of the coating layer is less than 1.0%, the Al-Fe alloy phase may be insufficient, resulting in poor coating adhesion. On the other hand, if the Fe content exceeds 7.0%, excessive Al-Fe alloy phase is formed, which can lead to peeling of the coating layer during processing.

[0032] Other impurities may be contained up to 5.0%. Impurities include Mg, Mn, etc. If the Mg content exceeds 5.0%, Mg oxide may be formed on the surface of the plating bath during plating, which may cause surface defects and discoloration at high temperatures.

[0033] According to one embodiment of the present invention, the plating amount is 20 to 200 g / m on one side. 2 It can be.

[0034] According to one embodiment of the present invention, the ratio of the Si content in the region from the surface to the 1 / 3 point of the plating layer in the thickness direction to the Si content in the entire plating layer may be 60 to 85%.

[0035] The distribution of Si and Si compounds in the coating layer can have a significant effect on corrosion resistance, and in particular, the greater the distribution on the surface of the coated steel sheet, the more advantageous it is for ensuring corrosion resistance. In the present invention, the Si distribution can be confirmed by analyzing the cross section of the steel sheet using an EPMA (Electron Probe Micro Analyzer).

[0036] If the ratio of the Si content in the region from the surface to the one-third point in the thickness direction of the plating layer to the Si content in the entire plating layer is less than 60%, it will be difficult to ensure the desired corrosion resistance.On the other hand, if this ratio exceeds 85%, Si will be excessively distributed on the surface, which may result in poor surface quality of the plating layer.

[0037] The method for producing a steel sheet according to the present invention will be described in detail below.

[0038] The plated steel sheet according to one embodiment of the present invention can be produced by heat treating, plating, and cooling a base steel sheet that satisfies the alloy composition of the present invention.

[0039] Preparation of base steel sheet A base steel sheet satisfying the alloy composition according to one embodiment of the present invention can be prepared. The alloy composition of the base steel sheet according to one embodiment of the present invention is not particularly limited, but it is more preferable that the composition satisfies the composition proposed in the present invention.

[0040] The composition of the base steel sheet can be the same as that explained above.

[0041] Heat Treatment In the present invention, a heat treatment may be performed before plating. The conditions for the heat treatment are not particularly limited, and ordinary conditions usable in the same technical field may be applied. According to one embodiment of the present invention, the base steel sheet may be heat treated at a temperature range of 750 to 850°C for 50 to 500 seconds.

[0042] plating The heat-treated base steel sheet can be immersed in a plating bath containing, by weight, 17 to 38% Zn, 7 to 18% Si, the balance being Al and other impurities, for plating.

[0043] If the Zn content of the plating bath exceeds 38%, a large amount of ash will be generated during plating, resulting in problems such as reduced workability due to dust generation. Furthermore, the Zn content of the plating layer may exceed the target Zn content of the present invention. In one embodiment of the present invention, the Zn content may be 37% or less. On the other hand, if the Zn content is less than 17%, the target Zn content of the plating layer cannot be ensured, resulting in problems such as insufficient sacrificial corrosion protection. In one embodiment of the present invention, the Zn content may be 20% or more.

[0044] During Al plating, the addition of Si has the advantage of suppressing excessive reaction between Al and Fe, thereby ensuring plating adhesion. If the Si content exceeds 18%, the melting point of the plating bath increases during plating, which may result in a problem of reduced durability of the structure in the plating bath. In one embodiment of the present invention, Si may be contained at 16% or less. However, to effectively ensure the above-mentioned advantages, the Si content may be limited to 7% or more. In one embodiment of the present invention, Si may be contained at 9% or more.

[0045] In the case of Fe, it is not artificially added to the coating bath, but the steel sheet reacts with the Al in the coating layer to form an Al-Fe alloy phase from the interface between the base steel sheet and the coating layer toward the coating layer, and this can be contained in the coating layer. Meanwhile, during the coating process, the steel sheet is immersed in the coating bath, and the Fe component of the steel sheet can dissolve in the coating bath. In other words, while Fe is not intentionally added to the coating bath, a certain amount can inevitably be present.

[0046] Other impurities may be contained up to 6.0%. Impurities include Mg, Mn, etc. If the Mg content exceeds 6.0%, Mg oxide may be formed on the surface of the plating bath during plating, which may cause surface defects and discoloration at high temperatures.

[0047] According to one embodiment of the present invention, the plating amount is 20 to 200 g / m on one side. 2 It can be.

[0048] cooling The plated steel sheet can be cooled to a temperature range of 300°C or lower at an average cooling rate of 10 to 50°C / s.

[0049] In the present invention, by concentrating Si in the surface layer of the coating layer, the cooling rate can be limited to form dense corrosion products during corrosion and improve corrosion resistance. The slower the cooling rate, the more Si and Si compounds can migrate to the surface layer, but the present invention can limit their distribution.

[0050] If the average cooling rate is less than 10°C / s, silicon oxides may form inside the coating layer, promoting galvanic corrosion within the coating layer and potentially reducing corrosion resistance. Furthermore, the coating layer may not be cooled sufficiently, resulting in partial shedding. On the other hand, if the cooling rate exceeds 50°C / s, the coating layer cools too quickly, inhibiting the migration of silicon and silicon oxides to the surface, resulting in a deterioration in corrosion resistance.

[0051] In the present invention, the cooling end temperature may refer to the temperature reached by the top roll. If the temperature of the steel sheet before reaching the top roll during cooling exceeds 300°C, there is a problem that coating peeling occurs at the top roll.

[0052] The plated steel sheet of the present invention can ensure excellent properties such as heat resistance, sacrificial corrosion resistance, and plating adhesion. [Example]

[0053] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0054] (Example) A base steel sheet having a composition (by weight): C: 0.01%, Mn: 0.5%, Ti: 0.01%, Nb: 0.01%, Si: 0.3%, Al: 0.045%, P: 0.01%, and S: 0.01% was heat treated and then coated under the conditions shown in Table 1 below to produce a coated steel sheet. Here, the cooling temperature may specifically refer to the temperature reached by the top roll. Meanwhile, as shown in the composition of the coating bath in Table 1, Fe was not intentionally added to the coating bath, but during the manufacturing process, the Fe in the steel sheet was dissolved at the specified weight % level when the steel sheet was immersed in the coating bath. Other components may include Zn, Si, and inevitable impurities other than Fe, with the balance being Al.

[0055] In the present invention, the heat treatment conditions were a temperature of 800°C for 50 seconds in a furnace maintained in a reducing atmosphere, followed by immersion in a plating bath. The immersion temperature was maintained at the same temperature as the plating bath, and the plating bath temperature was maintained at a temperature 40°C higher than the melting point of the plating component system. The plating amount was 60 g / m on one side using air wiping. 2 was applied uniformly.

[0056] [Table 1]

[0057] The coating layer composition and Si ratio of the manufactured coated steel sheets are shown. The coating layer composition was measured using ICP (Inductively Coupled Plasma), and the Si ratio was measured by analyzing the cross section of the steel sheet using EPMA (Electron Probe Micro Analyzer) to show the ratio of Si and Si compounds. Here, the surface region refers to the region from the surface of the coating layer to the 1 / 3 point of the coating layer in the thickness direction, and the interior region refers to the remaining region of the coating layer excluding the surface region.

[0058] Furthermore, the physical properties of heat resistance, sacrificial corrosion resistance, plating adhesion and roll pickup were observed and shown. The measurement standards and methods for each property are as follows:

[0059] Heat resistance: The plated test piece was exposed to an oven at 500°C for 24 hours, and the difference in whiteness (ΔL) before and after exposure was measured and evaluated according to the following criteria. Whiteness was measured according to ISO 2469. O: Δ is 5 or less X:Δ exceeds 5

[0060] Sacrificial corrosion resistance: After mounting the plated test piece so that the cross section is exposed, it is subjected to a 200-hour salt spray test, and the occurrence of red rust on the surface is observed. Here, the salt spray test is carried out using a 5% NaCl solution at 35°C, and the occurrence of red rust can be observed visually. O: Cross-sectional red rust occurrence rate 5% or less X: Cross-section red rust occurrence rate exceeds 5%

[0061] Plating Adhesion: After the 0-t bending experiment, a taping experiment was conducted to observe whether the plating layer peeled off. In the taping experiment, the sample was unfolded after bending, tape was attached to the bent area, and the tape was then peeled off again to observe whether the plating layer was still attached to the tape. O: No peeling of the plating layer X: Peeling of plating layer occurs

[0062] Roll pickup: When the steel sheet coming out of the coating bath passed through the top roll, the presence or absence of adhesion of the coating layer to the surface of the top roll was observed. O: No adsorption occurs X: Adsorption occurs

[0063] [Table 2]

[0064] As shown in Table 1, it was confirmed that in the examples of the present invention that satisfied the conditions of the present invention, all of the physical properties aimed at by the present invention could be secured.

[0065] Figure 1 shows a photograph of red rust formation in Example 5 according to an embodiment of the present invention. As shown in Figure 1, during an experiment to determine whether or not red rust occurred, it was confirmed that no red rust occurred. Figure 2 shows a photograph of the Si distribution in Example 5 according to an embodiment of the present invention, taken with an EPMA. As shown in Figure 2, it can be confirmed that a large amount of Si is distributed on the surface of the plating layer.

[0066] In contrast, Comparative Examples 1 to 11 are examples in which the plating bath composition does not satisfy the ranges proposed by the present invention. During plating, the plating bath composition does not satisfy all of the ranges proposed by the present invention, and the composition of the plating layer during the production of the plated steel sheet deviates from the range of the present invention. As a result, it was not possible to ensure all of the physical properties targeted by the present invention.

[0067] 3 shows a photograph of red rust formation in Comparative Example 7, which deviates from an embodiment of the present invention. As shown in FIG. 3, it can be seen that red rust has formed.

[0068] 4 shows an EPMA photograph of the Si distribution in Comparative Example 7, which deviates from an embodiment of the present invention. As shown in FIG. 4, it can be seen that the Si distribution is more abundant inside the plating layer than on the surface.

[0069] Comparative Examples 12 and 13 are examples in which the cooling rate during cooling after coating deviated from the range proposed by the present invention. In Comparative Example 12, the cooling rate was not sufficient, resulting in excessive Si diffusion to the steel sheet surface, and as a result, a problem occurred in which the coating layer was adsorbed when passing through the top roll. In Comparative Example 13, the cooling rate was excessive, resulting in insufficient Si diffusion to the surface. As a result, the sacrificial corrosion protection was poor.

[0070] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. Base steel sheet; and a plating layer formed on the base steel sheet and containing, by weight, 15 to 35% Zn, 5.0 to 15.0% Si, 1.0 to 7.0% Fe, the balance being Al and other impurities; Other impurities in the plating layer are 5.0% or less by weight, The plated steel sheet, wherein the ratio of the Si content in the region from the surface to a point one-third of the plating layer in the thickness direction to the Si content in the entire plating layer is 60 to 85%.

2. 2. The plated steel sheet according to claim 1, wherein the base steel sheet contains, by weight%, C: 0.1% or less, Mn: 2.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Si: 1.5% or less, Al: 0.5% or less, P: 0.01% or less, S: 0.01% or less, the balance being Fe and other unavoidable impurities.

3. 2. The plated steel sheet according to claim 1, wherein, after exposure at 500°C for 24 hours, the difference in whiteness (ΔL) between before and after exposure is 5 or less, and the cross-sectional red rust occurrence rate after 200 hours of salt water spraying is 5% or less.

4. preparing a base steel sheet; heat treating the base steel sheet; dipping the heat-treated base steel sheet in a coating bath containing, by weight, 17 to 38% Zn, 7 to 18% Si, the balance being Al and other impurities, to coat the steel sheet; and cooling the plated steel sheet to a temperature range of 300°C or less at an average cooling rate of 10 to 50°C / s; The method for producing a plated steel sheet, wherein the plating bath contains 6.0% or less other impurities by weight.

5. 5. The method for producing a plated steel sheet according to claim 4, wherein the base steel sheet contains, by weight%, C: 0.1% or less, Mn: 2.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Si: 1.5% or less, Al: 0.5% or less, P: 0.01% or less, S: 0.01% or less, the balance being Fe and other unavoidable impurities.

6. The method for manufacturing a steel sheet according to claim 4, wherein the base steel sheet is held at a temperature in the range of 750 to 850°C for 50 to 500 seconds during the heat treatment.

7. The method for producing a plated steel sheet according to claim 4, wherein the temperature of the plating bath is 40°C or more higher than the melting point of the plating component system.

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