Aluminum-plated steel sheet and method for producing same

Abstract

The present invention relates to an aluminized steel sheet for automobiles and the like and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to an aluminized steel sheet for automobiles and the like and a method for manufacturing the same. Background Art

[0002] In recent years, fuel efficiency has been improved by reducing the weight of automobiles. For this purpose, the thickness of steel sheets can be reduced, but when the thickness is reduced, problems with the stability of automobiles may occur, so it is necessary to support by increasing the strength of the steel. For this reason, the demand for high-strength steel sheets has been increasing, and various types of steel sheets have been developed. However, since such steel sheets have high strength, there are problems such as poor workability such as springback phenomena.

[0003] To solve such problems, the hot press forming process has been proposed. The hot press forming process is also called hot stamping, hot working, etc. The hot press forming process is a method of processing a steel sheet at a high temperature suitable for processing (800 °C or higher), then performing stamping in a mold, and rapidly cooling to a low temperature, thereby forming a low-temperature structure such as martensite in the steel sheet, thereby increasing the strength of the final product. In the above-described case, when manufacturing a component having high strength, the problem of workability can be minimized.

[0004] When using such hot forming, not only can complex shapes be easily formed, but also the manufactured components (hot formed components) have the advantage of ensuring high strength, so its usability has been increasing in recent years. In particular, by using a plated steel sheet using aluminum, zinc, etc. on the surface of the steel sheet, corrosion resistance is also attempted to be ensured. As an example, Patent Document 1 discloses the content of using an aluminized steel sheet for the hot press forming process.

[0005] When performing hot forming using the aluminized steel sheet, there is a problem of poor corrosion resistance because aluminum lacks sacrificial corrosion resistance. Due to the demand of customer companies for high-corrosion-resistant aluminized steel sheets, new coated steel sheets adding elements such as zinc and magnesium are being developed in the aluminizing process.

[0006] However, existing automobile component manufacturers use hot forming, so they have a hot forming process optimized for existing aluminized steel sheets. Therefore, there are limitations when applying the hot press forming process using the new coated steel sheet adding the elements such as Zn and Mg.

[0007] Therefore, it is necessary to use existing aluminized steel sheets. Since the aluminized steel sheet usually contains a small amount of Si, it is also called an AlSi coated steel sheet. Since the AlSi coated steel sheet is an alloy coated steel sheet of Al and Si, it usually consists of an Al structure and an Si structure. In particular, since Si solidifies in a form of separate precipitation during the cooling process of the coated steel sheet, spangles are formed as the surface feature of the AlSi coated steel sheet.

[0008] If spangles are formed on the surface of the aluminized steel sheet, there is a problem that the surface becomes unaesthetic, and it has a negative impact on corrosion resistance.

[0009] (Patent Document 1) U.S. Patent No. 6,296,805 Summary of the Invention

[0010] (I) Technical Problems to be Solved

[0011] One aspect of the present invention relates to an aluminized steel sheet, and to an aluminized steel sheet having excellent surface quality and corrosion resistance and a method for manufacturing the same.

[0012] The technical problems of the present invention are not limited to the above. Those skilled in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention based on the entire text of the specification of the present invention.

[0013] (II) Technical Solutions

[0014] One aspect of the present invention relates to an aluminized steel sheet, the aluminized steel sheet including a base steel sheet and an Al plating layer formed on the base steel sheet, and after surface etching, the size of spangles on the surface of the Al plating layer is 2.5 mm or less.

[0015] After surface etching of the plated steel sheet, the fraction of Si phase observed in the optical image phase may be 40 area% or less.

[0016] The whiteness of the aluminized steel sheet may be 85% or more.

[0017] The glossiness of the aluminized steel sheet may be 60 GU or more.

[0018] By weight%, the base steel sheet may contain: C: 0.02 - 0.6%, Si: 0.001 - 2%, Al: 0.001 - 1%, Mn: 0.1 - 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 - 0.1%, B: 0.0001 - 0.01%, Cu: 0 - 1.00%, Mo: 0 - 1.00%, Cr: 0 - 1.00%, Ni: 0 - 1.00%, V: 0 - 1.00%, Ca: 0 - 0.01%, Nb: 0 - 0.1%, Sn: 0 - 1%, W: 0 - 1%, Sb: 0 - 1%, Mg: 0 - 0.1%, Co: 0 - 1%, As: 0 - 1%, Zr: 0 - 1%, Bi: 0 - 1%, REM: 0 - 0.3%, the balance being Fe and impurities.

[0019] Another aspect of the present invention relates to a method for manufacturing an aluminized steel sheet, the manufacturing method comprising the steps of: immersing a base steel sheet in an aluminum (Al) plating bath containing silicon (Si) and attaching a plating solution to the surface of the base steel sheet; adjusting the amount of plating attachment on the surface of the base steel sheet with an air knife (A / K); and transferring the base steel sheet with the attached plating solution to a cooling device, wherein the K value defined by the following formula 1 satisfies 200 to 400.

[0020] (Formula 1)

[0021] where a: the content of Si in the plating bath (wt%), b: the line speed (meters per minute (mpm)), c: the A / K interval (mm), d: the A / K pressure (kPa), e: the A / K height (mm)

[0022] (III) Beneficial effects

[0023] The aluminized steel sheet as an example of the present invention can provide an aluminized steel sheet with a beautiful surface and excellent corrosion resistance.

[0024] The multiple beneficial advantages and effects of the present invention are not limited to the above content and can be more easily understood during the description of specific embodiments of the present invention. Description of the drawings

[0025] Figure 1 are photographs of the surface of the plated steel sheet of Comparative Example 8 before (a) and after (b) etching.

[0026] Figure 2 are the analysis results of the electron probe X-ray microanalyzer (EPMA) of Comparative Example 8.

[0027] Figure 3 (a) to (c) are photographs of the surface of Invention Example 1, Invention Example 3 and Comparative Example 8 before and after etching.

[0028] Figure 4 (a) to (c) are the optical images and image analysis results of Invention Example 1, Invention Example 3 and Comparative Example 8. Best mode for carrying out the invention

[0029] The terms used in this specification are for explaining the present invention and are not intended to limit the present invention. In addition, unless the relevant definitions show a clearly opposite meaning, the singular forms used in this specification also include the plural forms.

[0030] As used in the specification, the meaning of "comprising" is used to specifically illustrate the composition and does not exclude the existence or addition of other compositions.

[0031] Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art. Terms defined in the dictionary are interpreted to conform to the meaning in the relevant technical literature and the currently disclosed content.

[0032] The spangle formed on the surface of an aluminized steel sheet, i.e., an AlSi-coated steel sheet, is a phenomenon caused by the separate precipitation of Si during the cooling of the coated steel sheet. In other words, during the cooling process of the coated steel sheet, needle-shaped Si is formed on the surface of the coated steel sheet, and due to the needle-shaped Si, the spangle phenomenon occurs (see Figure 1 and Figure 2 ). The spangle not only impairs the appearance of the surface of the coated steel sheet, but also has a negative impact on the corrosion resistance because galvanic corrosion occurs between the spangle and adjacent Al and Si.

[0033] To control the spangle, various methods are utilized. Among them, the scheme of adding other elements to the plating bath during plating is widely used. However, due to the many difficulties in controlling the plating bath, a technique is needed to control the spangle size without any special additional composition change in the plating bath.

[0034] Hereinafter, an aluminized steel sheet as a specific embodiment of the present invention will be described in detail.

[0035] An aluminized steel sheet according to an embodiment of the present invention may include a base steel sheet and an Al plating layer formed on the base steel sheet, and the size of the spangle on the surface of the Al plating layer may be 2.5 mm or less when visually confirmed after surface etching.

[0036] Since the Al plating layer of the aluminized steel sheet is an alloy of Al and Si, it usually contains an Al structure and an Si structure. During the cooling process of the coated steel sheet, Si may solidify in a separate precipitation form, thus possibly causing the spangle phenomenon, which is a surface feature of the coated steel sheet. At this time, in the spangle area on the surface of the coated steel sheet, Si and Al do not mix but exist in the form of aggregates of the same kind of particles, and galvanic corrosion occurs at the grain boundaries where Si and Al are in contact, which may have a negative impact on the corrosion resistance. In addition, if the formed spangle is large and protrudes from the surface, it may have a negative impact on the surface appearance of the coated steel sheet. Therefore, the size of the spangle is preferably 2.5 mm or less when visually confirmed after surface etching. In addition, the shape of the spangle observed after etching is a rhombus with two attached triangles, and at this time, the length of the major axis of the rhombus can be defined as the size of one spangle.

[0037] There is no particular limitation on the surface etching. As a specific example, it can be etched in 4 g of NaCl + 200 mL (40% H3PO4) at room temperature for 1 to 3 minutes.

[0038] The plated steel sheet can have a whiteness of more than 85% and a glossiness of more than 60 GU. When the plated steel sheet has a whiteness of more than 85% and a glossiness of 60 GU, a beautiful surface can thus be ensured.

[0039] Figure 1 It is a photograph showing the surface of the plated steel sheet of Comparative Example 8 before (a) and after (b) etching. Figure 2 It is the analysis result of the electron probe X-ray microanalyzer (EPMA) of Comparative Example 8. Refer to the following Figure 1 and Figure 2 for a further description of an embodiment of the present invention.

[0040] The spangle is a phenomenon that occurs when Si precipitates alone during the cooling of the plated steel sheet. When observing the surface of the plated steel sheet with an electron probe X-ray microanalyzer (EPMA), it can be needle-shaped Si. Figure 1 and Figure 2 It is a photograph showing the surface of Comparative Example 8 in the following examples.

[0041] In addition, the fraction of the Si phase in the optical image phase of the surface of the Al-coated layer after etching can be 40 area% or less. When there is too much Si phase on the surface, galvanic corrosion occurs between the Al phase and the Si phase on the surface, which may reduce the corrosion resistance.

[0042] There is no particular limitation on the base steel sheet as long as it is a steel sheet that can be used for hot forming. There is no particular limitation on the classification according to the manufacturing process such as hot-rolled steel sheets and cold-rolled steel sheets, nor on the steel grades and alloy composition systems such as dual-phase steel (DP), complex structure steel, TRIP steel, and TWIP steel.

[0043] As a specific example, taking the 22MnB5 steel grade as a representative, in terms of weight% (wt.%), the specific contents can be: carbon (C): 0.1 - 0.3%, manganese (Mn): 1.0 - 2.0%, silicon (Si): 0.02 - 0.30%, boron (B): 5 - 45 ppm, and the rest contains inevitable impurities and iron (Fe).

[0044] In addition, the base steel plate may be a steel grade composed of, by weight %, carbon (C): 0.02 - 0.6%, silicon (Si): 0.001 - 2%, aluminum (Al): 0.001 - 1%, manganese (Mn): 0.1 - 4%, phosphorus (P): below 0.05%, sulfur (S): below 0.02%, nitrogen (N): below 0.02%, titanium (Ti): 0 - 0.1%, boron (B): 0.0001 - 0.01%, copper (Cu): 0 - 1.00%, molybdenum (Mo): 0 - 1.00%, chromium (Cr): 0 - 1.00%, nickel (Ni): 0 - 1.00%, vanadium (V): 0 - 1.00%, calcium (Ca): 0 - 0.01%, niobium (Nb): 0 - 0.1%, tin (Sn): 0 - 1%, tungsten (W): 0 - 1%, antimony (Sb): 0 - 1%, magnesium (Mg): 0 - 0.1%, cobalt (Co): 0 - 1%, arsenic (As): 0 - 1%, zirconium (Zr): 0 - 1%, bismuth (Bi): 0 - 1%, rare earth metals (REM): 0 - 0.3%, with the balance being Fe and impurities.

[0045] Next, a specific embodiment of the method for manufacturing an aluminized steel plate according to an example of the present invention will be described in detail.

[0046] The method for manufacturing the aluminized steel plate includes the following steps: immersing the base steel plate in a plating bath and attaching a plating solution to the surface of the base steel plate; adjusting the plating attachment amount on the surface of the base steel plate; and transferring the base steel plate with the attached plating solution to a cooling device. Hereinafter, each step will be described in detail.

[0047] First, the base steel plate is immersed in a plating bath and a plating solution is attached to the surface of the base steel plate. Specifically, the base steel plate is immersed in an aluminum (Al) plating bath containing silicon (Si) to attach the plating solution to the surface of the base steel plate.

[0048] It is effective when the plating bath introduction temperature of the base steel plate is 620 - 680°C. Before immersing the base steel plate in the plating bath, the base steel plate can be heated to a certain temperature (introduction temperature). At this time, it is effective when the introduction temperature is 620 - 680°C. Heating the base steel plate to the introduction temperature can prevent the deviation of the plating attachment amount or the problem of non - plating caused by the reduction in fluidity by making the molten aluminum solidify rapidly on the steel plate surface. However, excessive heating promotes the melting of the steel plate instead and may accelerate the generation of dross.

[0049] The plating bath may contain: silicon (Si): 6 wt% or more and less than 11.5 wt%, iron (Fe): 1 - 4 wt%, the balance being aluminum (Al) and inevitable impurities. The plating bath may be an Al-based plating bath, and the Al-based plating bath is used to form an Al plating layer. As long as it can be applied to the plating of hot-formed plated steel sheets, it can be applied without limitation in the present invention. As a preferred example, the plating bath composition may contain: silicon (Si): 6 wt% or more and less than 11.5 wt%, iron (Fe): 1 - 4 wt%, the balance being aluminum (Al) and inevitable impurities.

[0050] As the Si content in the plating bath increases, the melting point of the plating bath decreases. Therefore, it is considered that the plated steel sheet is easier to cool. As the Si content increases, a plated steel sheet with a beautiful surface can be obtained, but in fact, a plated steel sheet with a poor surface will be obtained instead. This is because when the Si content is 11.5 wt% or more, it is near the eutectic point of the binary phase of Al-Si, and a large amount of Si immediately aggregates and solidifies, resulting in an increase in the size of the spangle, so a plated steel sheet with a poor surface may be obtained.

[0051] The temperature of the plating bath being 630 - 680 °C is effective. When the temperature of the plating bath is too low, the fluidity of the plating solution in the plating bath may decrease. On the other hand, when the temperature of the plating bath is too high, the generation of scum in the plating bath may increase.

[0052] Next, adjust the plating adhesion amount of the base steel sheet. The device or method for adjusting the plating adhesion amount is not particularly limited and can be carried out by a method commonly used in the technical field to which the present invention pertains. As an example, an air knife (A / K) is used.

[0053] The plating adhesion amount is 8 - 80 g / m based on one side 2 , and the coating thickness being 3 - 30 μm is effective. The plating adhesion amount of Al plating can usually be converted to the plating adhesion amount when the coating thickness is ×2.7. When the coating thickness of 3 - 30 μm is converted to the plating adhesion amount, it is about 8 - 80 g / m 2 . A more preferred thickness is 4 - 26 μm, and the plating adhesion amount is 10 - 70 g / m 2 .

[0054] Transfer the base steel sheet with the adjusted plating adhesion amount to a cooling device and cool it. When the base steel sheet with the plating solution attached enters the cooling device, rapid cooling starts, and the reaction of the alloy layer in the Al plating layer may stop. As an example of the cooling device, it includes a cooling tower.

[0055] In the manufacturing method, the K value defined by the following formula (1) is effective when it is 200 to 400.

[0056] The formula (1) can be calculated as follows.

[0057] (Formula (1))

[0058] Where a: the content of Si in the plating bath (wt%), b: the wire speed (m / min), c: the A / K interval (mm), d: the A / K pressure (kPa), e: the A / K height (mm). The wire speed refers to the speed at which it passes through and transfers in the plating bath. The interval of the air knife (A / K) used to adjust the plating adhesion amount refers to the interval between the steel plate and the air knife. The A / K height refers to the distance from the plating bath to the A / K.

[0059] The said (Formula (1)), for manufacturing a plated steel sheet with a beautiful surface, takes into account the respective characteristics of various variables that determine the characteristics of the coating layer. The technical significance lies in considering the influencing factors and their correlations when manufacturing the plated steel sheet for the purpose of a beautiful surface.

[0060] To manufacture a plated steel sheet with a beautiful surface, as a result of considering various process factors such as the content of Si in the plating bath, the plating bath temperature, the introduction temperature, the wire speed (Line Speed), the A / K interval, the A / K pressure, the A / K height, and the cooling speed of the cooling tower, it is necessary to optimize the content of Si in the plating bath, the wire speed, the A / K interval, the A / K pressure, and the A / K height, thereby obtaining the said (Formula (1)). When the K value of the said (Formula (1)) is 200 to 400, a plated steel sheet with a beautiful surface can thus be obtained.

[0061] Specifically, when the K value is less than 200, the A / K pressure is insufficient to reduce the spangle size, or the wire speed, the A / K interval, and the A / K height are low, so it is difficult to reduce the spangle size. When the K value exceeds 400, the A / K pressure is too high or the wire speed, the A / K interval, and the A / K height are higher than the appropriate values, so it is difficult to reduce the spangle size by cooling the plated steel sheet.

[0062] Before immersing the base steel sheet in the plating bath, it can be provided in various ways. As an example, after heating the billet, it can be manufactured through processes including hot rolling, coiling, cold rolling, annealing, etc. The specific process conditions such as the heating, hot rolling, coiling, cold rolling, annealing, etc. of the required billet can vary according to the characteristics required for the base steel sheet, so there is no particular limitation. Detailed implementation mode

[0063] Hereinafter, embodiments of the present invention will be described. Of course, those skilled in the art to which the present invention pertains can make various changes to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of rights of the present invention is not limited to the following embodiments. The scope of rights of the present invention should be determined by the claims and their equivalents.

[0064] (Example)

[0065] In an embodiment of the present invention, a conventional 22MnB5 base steel plate is prepared and immersed in a plating bath containing about 7.5 - 11.5 wt% of Si and the rest containing Al and inevitable impurities to produce an aluminized steel plate. At this time, the temperature of the plating bath is 640 - 670 °C, and the introduction temperature of the base steel plate is 650 - 680 °C.

[0066] To manufacture the plated steel plate, at this time, the content of Si in the plating bath, the steel plate transfer speed (linear speed) are adjusted, and the single-sided plating adhesion amount is adjusted to 10 - 80 g / m 2 . The plating adhesion amount is adjusted with an air knife (A / K) and transferred to a cooling tower as a cooling device for cooling. At this time, the process conditions are shown in Table 1.

[0067] [Table 1]

[0068]

[0069] The formula 1 can be calculated as follows.

[0070] (Formula 1)

[0071] Among them, a: the content of Si in the plating bath (wt%), b: the linear speed (m / min), c: the A / K interval (mm), d: the A / K pressure (kPa), e: the A / K height (mm).

[0072] For the test pieces manufactured as described above, after surface etching, the spangle size was confirmed, and the whiteness, glossiness, and surface appearance of each test piece were confirmed. The results are shown in Table 2 below.

[0073] The whiteness and glossiness are the average values of the values measured 3 times on the front / back side after each test piece is cut into 150 × 150 mm. The whiteness is the L value of the measured value, and the glossiness is the comparison value of the value at 60 degrees. The whiteness is expressed as a fraction (%), and complete white is regarded as 100. The glossiness uses the gloss unit (GU).

[0074] The surface etching is carried out with a solution of 4 g of NaCl + 200 mL (40% H3PO4) and etched for 1 - 3 minutes.

[0075] The spangle size is measured by taking a photograph of a 50×50 mm coated steel sheet and analyzing the photograph through image analysis. The shape of the spangle observed after etching is a rhombus with two triangles attached. At this time, the length of the major axis of the rhombus is defined as the size of one spangle, and the average value of the lengths of the major axes of 25 such rhombuses is defined as the spangle size during image analysis. In particular, for ease of measurement and clear standards, the average value of the major axes of 25 large rhombuses is measured and expressed as the spangle size, rather than small rhombuses.

[0076] In addition, in Table 2 below, if the spangle size is very small and difficult to confirm with the naked eye, it is indicated as not detected.

[0077] In addition, in terms of judging whether the surface is aesthetically pleasing, the surface shape of the aluminized steel sheet is expressed as ordinary. When the spangle is too large and its shape is prominent, it is expressed as poor. When the surface is more aesthetically pleasing compared to the surface of a conventional aluminized steel sheet, it is expressed as excellent.

[0078] [Table 2]

[0079]

[0080] From Tables 1 to 2 and Figures 1 to 4 it can be seen from the results that for the aluminized steel sheets corresponding to the inventive examples that meet the scope proposed by the present invention, no spangles are observed on the surface after etching, or they have a size of 2.5 mm or less. At this time, they have a whiteness of 85% or more and a glossiness of 60 GU or more, so excellent surface characteristics can be ensured.

[0081] Figure 1 (a) and (b) of are photographs of observing the surface of Comparative Example 8 before and after etching, Figure 2 is the result of the EPMA analysis of the said Comparative Example 8.

[0082] Figure 1 (a) of is a photographic image of Comparative Example 8, and it can be confirmed that a pattern is formed on the surface of the coated steel sheet. To further confirm this, Figure 1 (b) of shows a photographic image of the AlSi-coated steel sheet with the surface etched.

[0083] The spangle pattern of the AlSi-coated steel sheet is a phenomenon that occurs when Si precipitates separately during the cooling of the coated steel sheet. Figure 2 shows the result of observing the surface of the coated steel sheet of the said Comparative Example 8 with an electron probe X-ray microanalyzer (EPMA), and it can be confirmed that the acicular structure of Si shows the spangle pattern.

[0084] Figure 3 (a) to (c) of are respectively photographs of observing the surface of Inventive Example 1, Inventive Example 3, and Comparative Example 8 before and after etching. Figure 4(a) to (c) of this are the optical images and image analysis results of Invention Example 1, Invention Example 3, and Comparative Example 8, respectively. From Figure 3 and Figure 4 results, it can be seen that in the invention examples, the fraction of the surface Si phase is reduced to 40 area% or less. As described above, it can be expected that as the size of the Si phase decreases, this will have an additional effect on the improvement of future corrosion resistance.

Claims

1. An aluminized steel sheet, which comprises a base steel sheet and an Al plating layer formed on the base steel sheet. After surface etching, the size of the spangles on the surface of the Al plating layer is 2.5 mm or less.

2. The aluminized steel sheet according to claim 1, wherein, After surface etching of the plated steel sheet, the fraction of the Si phase observed in the optical image phase is 40 area% or less.

3. The aluminized steel sheet according to claim 1, wherein, The whiteness of the aluminized steel sheet is 85% or more.

4. The aluminized steel sheet according to claim 1, wherein, The glossiness of the aluminized steel sheet is 60 GU or more.

5. The aluminized steel sheet according to claim 1, wherein, By weight%, the base steel sheet consists of C: 0.02 - 0.6%, Si: 0.001 - 2%, Al: 0.001 - 1%, Mn: 0.1 - 4%, P: 0.05% or less, S: 0.02% or less, N: 0.02% or less, Ti: 0 - 0.1%, B: 0.0001 - 0.01%, Cu: 0 - 1.00%, Mo: 0 - 1.00%, Cr: 0 - 1.00%, Ni: 0 - 1.00%, V: 0 - 1.00%, Ca: 0 - 0.01%, Nb: 0 - 0.1%, Sn: 0 - 1%, W: 0 - 1%, Sb: 0 - 1%, Mg: 0 - 0.1%, Co: 0 - 1%, As: 0 - 1%, Zr: 0 - 1%, Bi: 0 - 1%, REM: 0 - 0.3%, and the balance of Fe and impurities.

Images