Aluminum-plated steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet suitable for hot-press forming and, more specifically, to an aluminum-plated steel sheet, particularly, an aluminum-plated steel sheet having excellent surface quality and bendability, and a method for manufacturing same.

Description

Aluminum-coated steel sheet and manufacturing method thereof

[0001] The present invention relates to a steel sheet suitable for hot forming, and more particularly, to an aluminum-plated steel sheet, particularly an aluminum-plated steel sheet having excellent surface quality and bendability, and a method for manufacturing the same.

[0002] Recently, automobile manufacturers have been conducting ongoing research into vehicle weight reduction to meet demands such as improved fuel efficiency and CO2 emission regulations. Consequently, various alloying elements are being incorporated into steel to improve strength at a given weight. However, this has led to increased hardenability and poor workability, such as springback.

[0003]

[0004] To address the aforementioned issues, hot press forming has been proposed. This technique involves forming steel of a given strength within the austenitic single-phase region, then rapidly cooling it to a low temperature to form a low-temperature structure, such as martensite, within the steel, dramatically improving the strength of the product. This process minimizes the loss of workability during forming of high-strength components.

[0005]

[0006] Meanwhile, bendability is emerging as a representative material property for evaluating the crashworthiness of components manufactured through hot press forming. However, the bendability of components manufactured through hot press forming is known to be poor due to the high strength of the components themselves and the presence of low-temperature structures, such as martensite, which are inherently vulnerable to bending.

[0007] To address this, Patent Document 1 discloses a technique for improving bendability by controlling the number density of inclusions in steel. Specifically, Patent Document 1 discloses that bendability is improved by reducing the number density of inclusions by controlling the temperature and temperature retention time of the slab during the steel plate manufacturing process, particularly during hot rolling. However, Patent Document 1 does not describe a technical idea for improving the surface quality of the steel plate.

[0008] As another technology, Patent Document 2 discloses that a Ni alloy layer is formed on the surface by performing Ni electroplating before annealing of a cold-rolled steel sheet, and that an austenite structure is uniformly formed on the surface during a subsequent annealing process, thereby exhibiting excellent bendability regardless of the internal structure. However, Ni may cause contact dermatitis, allergy, and edema to workers during the work process, and therefore, in Korea, there are restrictions on the amount of dissolution (for one week, per unit area (cm 2 ) The amount of Ni dissolved from the surface of the product is 0.5㎍ or less.

[0009] (Patent Document 1) Korean Patent Publication No. 10-2021-0116594

[0010] (Patent Document 2) Korean Patent Publication No. 10-2020-0142372

[0011] One aspect of the present invention is to provide a steel sheet suitable for hot forming, which can exhibit excellent bendability even after hot forming, specifically an aluminum-plated steel sheet having excellent surface quality and bendability. The present invention also provides a method for manufacturing the aluminum-plated steel sheet.

[0012] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0013] An aluminum-plated steel sheet according to one aspect of the present invention comprises: a base steel sheet; and an aluminum-based plating layer on at least one surface of the base steel sheet; and when the base steel sheet is defined as a surface layer at a point up to 200 µm in the thickness direction from the surface, the surface layer may include a ferrite phase having an area fraction of 80% or more.

[0014] In one embodiment of the present invention, in the GDS (Glow Discharge Spectrometer) profile of C and Si observed in the thickness direction of the steel sheet, the ratio of f represented by the following [formula] may be 0.2 to 0.9.

[0015] [ceremony]

[0016] f = Area from the interface between the base steel plate and the plating layer to the point where carbon (C) is decarburized by 50% / (Length (depth) from the interface between the base steel plate and the plating layer to the point where carbon (C) is decarburized by 50% × Carbon (C) content / 2)

[0017] (Here, carbon (C) refers to the weight content contained in the steel plate.)

[0018] In one embodiment of the present invention, the steel sheet contains, in wt%, carbon (C): 0.02 to 0.60%, silicon (Si): 0.001 to 2.000%, aluminum (Al): 0.001 to 1.000%, manganese (Mn): 0.1 to 4.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0.1% or less, boron (B): 0.0001 to 0.0100%, copper (Cu): 1.0% or less, molybdenum (Mo): 1.0% or less, chromium (Cr): 1.0% or less, nickel (Ni): 1.0% or less, vanadium (V): 1.0% or less, calcium (Ca): 0.01% or less, niobium (Nb): 0.1% or less, Tin (Sn): 1.0% or less, Tungsten (W): 1.0% or less, Antimony (Sb): 1.0% or less, Magnesium (Mg): 0.1% or less, Cobalt (Co): 1% or less, Arsenic (As): 1.0% or less, Zirconium (Zr): 1.0% or less, Bismuth (Bi): 1.0% or less, Rare Earth Elements (REM): 0.3% or less, and the remainder may contain Fe and unavoidable impurities.

[0019] In one embodiment of the present invention, the microstructure of the steel plate in the remaining area excluding the surface layer may be a mixed phase of ferrite and pearlite.

[0020] In one embodiment of the present invention, the steel plate may have a maximum three-point bending angle of 55° or more.

[0021]

[0022] A method for manufacturing an aluminum-plated steel sheet according to another aspect of the present invention, comprising the steps of preparing a base steel sheet; preparing the base steel sheet at a dew point temperature of 5 to 30°C and 1 to 70% hydrogen (H 2) -The remaining nitrogen (N 2) The method may include a step of annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 1000 seconds; and a step of loading the annealed steel sheet into a galvanizing facility to form a plating layer on at least one surface of the steel sheet.

[0023] In one embodiment of the present invention, the molten plating equipment may sequentially be provided with a snout section connected to the annealing furnace and a molten plating bath.

[0024] In one embodiment of the present invention, the snout section may have a dew point temperature of -40°C or lower.

[0025] In one embodiment of the present invention, the molten plating bath may be an aluminum-based plating bath.

[0026] In one embodiment of the present invention, the steel plate may have the above-described alloy composition.

[0027] In one embodiment of the present invention, the base steel sheet can be obtained through a step of reheating a steel slab to a temperature range of 1050 to 1300°C; a step of finish-rolling the reheated steel slab to a temperature range of Ar3 to 1000°C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet to a temperature range of 750°C or lower; and a step of cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 30 to 90%.

[0028] According to the present invention, not only can the bendability of an aluminum-plated steel sheet be improved, but also the surface quality and plating adhesion can be improved.

[0029] The aluminum-plated steel sheet of the present invention is suitable for hot forming for manufacturing products.

[0030] Figure 1 shows the GDS measurement results of an aluminum-plated steel sheet (invention example 3) according to one embodiment of the present invention.

[0031] The terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms, unless the context clearly dictates otherwise.

[0032] The meaning of "comprising" as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0033] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0034]

[0035] Hereinafter, the present invention will be described in detail.

[0036]

[0037] An aluminum-plated steel sheet according to one aspect of the present invention may include a base steel sheet and an aluminum-based plating layer on at least one surface of the base steel sheet.

[0038]

[0039] In one embodiment of the present invention, the base steel sheet may be any material that is a high-strength steel suitable for use as an automotive material and is applicable to obtaining a plated steel sheet through hot-dip galvanizing. Accordingly, there are no particular limitations on the alloy composition of the base steel sheet.

[0040] However, as a non-limiting example, the above-mentioned steel plate is carbon steel containing a certain amount of carbon (C), for example, in wt%, carbon (C): 0.02 to 0.60%, silicon (Si): 0.001 to 2.000%, aluminum (Al): 0.001 to 1.000%, manganese (Mn): 0.1 to 4.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0.1% or less, boron (B): 0.0001 to 0.0100%, copper (Cu): 1.0% or less, molybdenum (Mo): 1.0% or less, chromium (Cr): 1.0% or less, nickel (Ni): 1.0% or less, vanadium (V): 1.0% or less, calcium (Ca): 0.01% or less, niobium (Nb): 0.1% or less, tin (Sn): 1.0% or less, tungsten (W): 1.0% or less, antimony (Sb): 1.0% or less, magnesium (Mg): 0.1% or less, cobalt (Co): 1% or less, arsenic (As): 1.0% or less, zirconium (Zr): 1.0% or less, bismuth (Bi): 1.0% or less, rare earth elements (REM): 0.3% or less, and the remainder may contain iron and unavoidable impurities.

[0041] Among the alloy compositions described above, C, Mn, etc. can be added to secure the strength of the steel, Si can greatly contribute to stabilizing the strength, and Al has a deoxidation effect. P, S, N, etc. may be elements that are inevitably introduced during the steel manufacturing process, but it is to be noted that they are not limited thereto. In addition, it will be apparent to those skilled in the art that Ti, B, Cu, Mo, Cr, Ni, V, Ca, Nb, Sn, W, Sb, Mg, Co, As, Zr, Bi, REM, etc. may be additionally included in addition to the aforementioned composition, taking into account the target properties of the final product, etc.

[0042]

[0043] Meanwhile, the researchers of the present invention conducted in-depth research to address the problems of prior art, particularly the insufficient bendability of galvanized steel sheets for automobiles. As a result, they discovered that softening the surface of the base steel sheet and applying this softened base steel sheet as a plating material can significantly improve the bendability of the final material.

[0044] Accordingly, in the present invention, a method for softening the surface of a steel plate is provided, which is different from the conventionally known method, and this will be described in detail below.

[0045]

[0046] In one embodiment of the present invention, the base steel plate may have a softened surface. As an example, when the surface of the base steel plate is defined as a point up to 200 μm in the thickness direction from the surface, the surface layer may include a ferrite phase with an area fraction of 80% or more. Here, the surface layer may be both sides or one side of the base steel plate.

[0047] In one embodiment of the present invention, if the fraction of the ferrite phase within the surface layer is less than 80%, the level of improvement in the bendability of the member obtained through hot forming may not be significant. The structure other than the ferrite phase is not particularly limited, but as an example, it may be a pearlite phase.

[0048] In one embodiment of the present invention, in order to further improve the bendability of the plated steel sheet, the structure of the surface layer may be a single-phase ferrite.

[0049]

[0050] In one embodiment of the present invention, the microstructure of the remaining region excluding the surface layer of the base steel plate is not particularly limited, but as an example, it may be composed of a mixed phase of ferrite and pearlite. In this case, when composed of a mixed phase, the fraction of each phase will be determined according to the alloy composition contained in the base steel plate, and is not particularly limited.

[0051]

[0052] According to one embodiment of the present invention, an aluminum-based plating layer may be included on at least one surface of the aforementioned steel plate, preferably on a surface portion where a ferrite phase is formed with an area fraction of 80% or more.

[0053] In one embodiment of the present invention, the aluminum-based plating layer may be a plating layer containing aluminum (Al) as a main element, and as one example, the aluminum-based plating layer may include, in weight %, 0 to 30% of zinc (Zn), 7 to 10% of silicon (Si), 5 to 10% of iron (Fe), the remainder being aluminum (Al) and unavoidable impurities.

[0054] In one embodiment of the present invention, the aluminum-based plating layer can be formed by immersing the base steel sheet in an aluminum-based molten plating bath, as will be described in detail later, and it is well known that the composition of the aluminum-based plating layer is determined according to the composition of the aluminum-based molten plating bath.

[0055] In one embodiment of the present invention, zinc (Zn) among the alloy compositions constituting the aluminum-based plating layer is an element that is advantageous in securing the corrosion resistance of the plating layer, and a sacrificial corrosion protection effect can be obtained by Zn. As one example, if the content of Zn exceeds 30%, there is a problem in that the Zn diffuses into the base steel sheet during hot forming of the base steel sheet on which the plating layer is formed, causing cracks to occur due to LME. Therefore, it is to be noted that the Zn may be included up to 30%, and even if it is 0%, there is no problem in securing the physical properties of the aluminum-based plating layer. In another embodiment of the present invention, the Zn in the plating layer may be included at 27% or less.

[0056] In one embodiment of the present invention, silicon (Si) among the alloy compositions forming the aluminum-based plating layer can play a role in suppressing diffusion of aluminum present in the plating bath into the interior of the base steel sheet during plating of the base steel sheet. As an example, in order to obtain the above-described effect, the content of Si may be 7% or more. On the other hand, if the content exceeds 10%, there is a concern that the melting point of the plating bath may rapidly increase, and in this case, there is a problem that the amount of ash generated from the plating bath rapidly increases.

[0057] In one embodiment of the present invention, iron (Fe) among the alloy compositions constituting the aluminum-based plating layer is an impurity in the plating bath. As an example, the Fe may be present at approximately 3% or less. In addition, Fe may be included in the aluminum-based plating layer as Fe present in the base steel sheet is eluted into the plating layer during the plating process. As an example, if the Fe content is less than 5%, there is a concern that Al in the plating bath and Fe of the base steel sheet may form an uneven Al-Fe alloy phase on the surface of the base steel sheet during plating, which may result in a disadvantage of a rough plating surface. On the other hand, if the Fe content exceeds 10%, there is a problem that the Al-Fe alloy phase is excessively formed, which increases the possibility of the plating layer being peeled off.

[0058]

[0059] In one embodiment of the present invention, a steel sheet having a soft phase in a surface portion and an aluminum-based plating layer formed thereon according to one embodiment of the present invention may have a GDS (Glow Discharge Spectrometer) profile of carbon (C) and silicon (Si) observed in the thickness direction and may have the following characteristics. Here, the GDS concentration profile refers to the concentration and concentration profile of a specific element measured using a glow discharge optical emission spectrometer.

[0060] In one embodiment of the present invention, in the GDS (Glow Discharge Spectrometer) profile of C and Si observed in the thickness direction of the steel plate, the ratio of f represented by [Formula 1] below may be 0.2 to 0.9.

[0061] [Formula 1]

[0062] f = Area from the interface between the base steel plate and the plating layer to the point where carbon (C) is decarburized by 50% / (Length (depth) from the interface between the base steel plate and the plating layer to the point where carbon (C) is decarburized by 50% × (Carbon (C) / 2))

[0063] (Here, carbon (C) refers to the weight content contained in the steel plate.)

[0064]

[0065] In one embodiment of the present invention, the ratio of f in [Formula 1] can be described in detail with reference to FIG. 1.

[0066] Figure 1 shows a GDS profile observing the components of C and Si in the depth direction from the surface of an invention steel (coated steel sheet) according to one embodiment of the present invention. Here, the x-axis represents the depth (㎛) from the surface of the plating layer in the thickness direction of the base steel sheet, and the y-axis represents the concentration (weight %) of the components.

[0067] In Fig. 1, a represents the thickness of the plating layer, and the point where the descending curve indicated by the tangent line in the Si profile ends can be regarded as the plating layer thickness. At this time, the Al-Fe alloy layer formed by the temperature of the plating bath after plating and the latent heat after plating is completed is also included in the plating layer. Generally, the intersection of the Al and Fe profiles is defined as the thickness of the plating layer, but since this does not take into account the aforementioned alloy layer, in one embodiment of the present invention, the plating layer thickness is measured using the tangent line of the Si profile.

[0068] Meanwhile, b represents a point (depth in the thickness direction) at which the C content on the surface of the plated steel sheet (surface of the plating layer) becomes 50% of the C content contained in the base steel sheet, and c represents a depth (ba) from the interface between the plating layer and the base steel sheet to a point at which the C content in the base steel sheet becomes 50%. d represents a 50% concentration of the C content in the base steel sheet on the y-axis, and e represents a portion of the area (point area) in the c section, which corresponds to the area where C is decarburized on the surface of the base steel sheet.

[0069] In a GDS profile according to one embodiment of the present invention, the area (e) from the interface between the base steel sheet and the plating layer to the point where carbon (C) is decarburized by 50% may be defined as f, which is the value (e / (c×d)) obtained by dividing the area by the product of the depth (c) from the interface between the base steel sheet and the plating layer to the point where carbon (C) is decarburized by 50% and the 50% concentration (d) of the carbon (C) content in the base steel sheet.

[0070] If the value of f above is less than 0.2, the depth of decarburization on the surface of the steel sheet is insufficient, and thus the bendability is not improved. On the other hand, if the value of f above exceeds 0.9, the strength of the member obtained after hot forming is likely to be inferior due to excessive decarburization on the surface of the steel sheet, and thus it is impossible to secure bendability while maintaining strength.

[0071]

[0072] An aluminum-plated steel sheet according to one embodiment of the present invention is obtained by using a base steel sheet having a soft structure on the surface as a plating material, and thus not only has excellent surface quality and bendability, but also has excellent plating adhesion.

[0073] In one embodiment of the present invention, the steel plate may have a characteristic in which the maximum three-point bending angle is 55° or more.

[0074] Additionally, the above steel plate may have high strength, and as a non-limiting example, may have a tensile strength of 900 MPa or more.

[0075]

[0076] Hereinafter, a method for manufacturing an aluminum-plated steel sheet according to another aspect of the present invention will be described in detail.

[0077]

[0078] In one embodiment of the present invention, an aluminum-plated steel sheet can be manufactured by preparing a cold-rolled steel sheet as a base steel sheet through a series of processes, then annealing the base steel sheet under certain conditions, and then hot-dip galvanizing it. Each process step according to one embodiment of the present invention is described in detail below, and it is to be noted that the following manufacturing process is an example for manufacturing the steel sheet of the present invention.

[0079]

[0080] In one embodiment of the present invention, the steel plate is a high-strength steel suitable as an automobile material, and its alloy composition is not particularly limited and is replaced with the above-mentioned content.

[0081] In one embodiment of the present invention, the base steel sheet is a cold-rolled steel sheet manufactured through a series of processes, and as an example, the cold-rolled steel sheet can be obtained through a step of reheating a steel slab to a temperature range of 1050 to 1300°C; a step of finish-rolling the reheated steel slab to a temperature range of Ar3 to 1000°C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 750°C or lower; and a step of cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 30 to 90%. However, it should be noted that the present invention is not limited to a cold-rolled steel sheet obtained through the above-described conditions and processes.

[0082] In one embodiment of the present invention, the step of reheating the steel slab is a process for minimizing the load during subsequent hot rolling and securing a sufficient reduction ratio, and can be performed within the temperature range described above. By hot-rolling the reheated steel slab in this manner, a hot-rolled steel sheet can be obtained. At this time, in order to suppress the formation of mixed grain structures during the hot rolling process and minimize malfunctions due to fluctuations in the hot rolling load, the finishing rolling can be performed within the temperature range described above. Meanwhile, in coiling the hot-rolled steel sheet obtained as described above, the coiling process can be performed at the temperature described above in order to minimize material deviation in the width direction during the coiling process and suppress scale defects on the coil surface. Thereafter, by uncoiling the coiled hot-rolled steel sheet and performing cold rolling, a cold-rolled steel sheet can be obtained. At this time, the cold rolling reduction rate can be set according to the thickness of the cold rolled steel sheet to be obtained, and cold rolling can be performed at a reduction rate within the aforementioned range by taking into consideration the rolling load, etc.

[0083]

[0084] In one embodiment of the present invention, a prepared steel plate is loaded into an annealing furnace and annealing heat treatment is performed, and at this time, the atmosphere, temperature, annealing time, etc. within the annealing furnace can be controlled.

[0085] In one embodiment of the present invention, the steel sheet is heated at a dew point temperature of 5 to 30°C and 1 to 70% hydrogen (H 2) -The remaining nitrogen (N 2) In a gas atmosphere annealing furnace, annealing can be performed by maintaining the temperature in the range of 600 to 950°C for 5 to 1000 seconds. Here, the gas atmosphere concentration is volume % (vol%).

[0086] By performing annealing heat treatment according to one embodiment of the present invention, decarburization can be caused on the surface of the steel sheet, thereby allowing the surface portion of the steel sheet to be configured as a soft structure.

[0087] As an example, during the annealing heat treatment process, water vapor contained in the moist nitrogen dissociates into O atoms on the surface of the steel sheet, and this O atom reacts with carbon (C) in the steel sheet to decarburize into carbon monoxide (CO). As a result, a ferrite layer may form on the surface as the surface structure decomposes into cementite and carbide.

[0088] In one embodiment of the present invention, if the dew point temperature in the annealing furnace is less than 5°C during annealing heat treatment, there is a risk that oxidizing elements (e.g., Mn, Si, etc.) contained in the steel may form oxides on the surface, whereas if the temperature exceeds 30°C, the critical oxygen content for oxidizing Fe is reached, and there is a risk that Fe surface oxidation may occur.

[0089] In one embodiment of the present invention, if the hydrogen content in the atmosphere within the annealing furnace is less than 1%, the oxide layer of the base steel sheet may not be sufficiently reduced, resulting in poor plating adhesion, and the surface oxide may inhibit the reaction between oxygen and carbon in the steel, potentially preventing sufficient decarburization of the surface layer. On the other hand, even if the hydrogen content exceeds 70%, there is no problem in achieving the purpose of the present invention; however, as the hydrogen content increases, the cost and risk of explosion increase, and therefore, taking this into consideration, the content may be limited to 70% or less.

[0090] In one embodiment of the present invention, when annealing a steel sheet loaded into an annealing furnace, if the temperature is less than 600°C, there is a concern that recrystallization of the steel sheet may not occur sufficiently, and on the other hand, if the temperature exceeds 950°C, there is a problem that the life of the annealing furnace is reduced due to equipment load and process costs increase.

[0091] In one embodiment of the present invention, it is advantageous to perform the annealing heat treatment for 5 seconds or longer to ensure sufficient recrystallization; however, if it exceeds 1000 seconds, productivity may decrease.

[0092]

[0093] In one embodiment of the present invention, an annealed heat-treated steel sheet may be loaded into a hot-dip galvanizing facility to form a plating layer on at least one surface of the steel sheet. The hot-dip galvanizing facility may sequentially be equipped with a snout section connected to the annealing furnace and a hot-dip galvanizing bath.

[0094] In one embodiment of the present invention, when transporting the annealed heat-treated steel sheet within a hot-dip galvanizing facility, the snout section can be controlled to a dew point temperature of -40°C or lower in order to minimize defects such as non-plating during subsequent hot-dip galvanizing.

[0095] In one embodiment of the present invention, if the dew point temperature of the snout section exceeds -40°C, a thin oxide film is formed on the surface of the molten plating bath due to moisture present in the section, resulting in poor plating adhesion during plating of the base steel sheet, and thus there is a risk of excessive occurrence of under-plating on the surface of the base steel sheet. The lower the dew point temperature of the snout section, the more advantageous it is for minimizing the occurrence of under-plating, but the lower limit may be set to -60°C in consideration of equipment specifications.

[0096]

[0097] In one embodiment of the present invention, the annealed heat-treated steel sheet can be plated by being immersed in a molten plating bath through a snout section.

[0098] In one embodiment of the present invention, the molten plating bath may be an aluminum-based plating bath, which may be a plating bath containing aluminum (Al) as a main component. As an example, the aluminum-based plating bath may contain 7 to 10% of silicon (Si) in addition to Al, and by adding the Si, the effects of suppressing the fluidity of the molten metal and the diffusion of Al into the base steel sheet during plating may be obtained. In addition, zinc (Zn): 0 to 30%, iron (Fe): may be further contained as impurities within 3%, and it is to be noted that these contents are replaced with the contents described above.

[0099] In one embodiment of the present invention, the plating can be performed in an aluminum-based plating bath at 600 to 680°C.

[0100]

[0101] According to one embodiment of the present invention, prior to manufacturing a base steel sheet into a plated steel sheet, an annealing heat treatment is performed under specific conditions to form a soft structure on the surface of the base steel sheet, thereby improving the bendability of the final material. In addition, when the base steel sheet passes through a hot-dip galvanizing facility for the plating process of the base steel sheet after the annealing heat treatment, the dew point temperature of the snout section can be controlled to prevent defects such as under-plating. Accordingly, a plated steel sheet, particularly an aluminum-plated steel sheet, with excellent surface quality and bendability can be provided.

[0102]

[0103] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0104] (Example)

[0105] Cold rolled steel sheets (non-annealed steel sheets) having the alloy composition shown in Table 1 below were prepared as base steel sheets, and then annealed and plated under the conditions shown in Table 1 below to manufacture each aluminum-plated steel sheet. At this time, the plating treatment was performed according to the plating conditions below.

[0106] [Plating Conditions]

[0107] Plating bath A: 8.8%Si-1.0%Fe, balance Al and unavoidable impurities, 660℃

[0108] Plating bath B: 7.6%Si-26.6%Zn-1.0%Fe, balance Al and unavoidable impurities, 630℃

[0109]

[0110] Afterwards, the microstructure and mechanical properties of each aluminum-plated steel sheet were measured, and the results are shown in Table 2 below.

[0111] At this time, the microstructure was observed on the cross-section of the base steel plate (cold-rolled steel plate) that had undergone annealing heat treatment and plating. Specifically, the specimen was cut in a direction perpendicular to the rolling direction to obtain a specimen, which was then mounted and etched using a Nital solution. The cross-section (cross-section in the thickness direction) was then observed using a scanning electron microscope (SEM).

[0112] In addition, tensile strength was measured through a tensile test at room temperature (approximately 25℃) according to ISO6892 after manufacturing specimens according to JIS-5 standard. Bending property (C direction and L direction) was measured through 3-point bending evaluation (VDA238-100) 5 times each, and the average value of the bending outer angle converted from the maximum bending strength was recorded.

[0113] And, the plating adhesion was evaluated using structural adhesive SA-1607E from Bogwangsa. First, 30×80mm 2 Using a Teflon jig on a specimen of size 50×10×10mm 3 After applying the adhesive in the shape of a rectangular solid, baking was performed at a firing temperature of 170℃ for 20 minutes. After baking, the adhesive and the specimen were stored at room temperature for one day, then the specimen was bent at a 90° angle to forcibly separate from the adhesive. At this time, if peeling occurred inside the adhesive, it was judged as ‘normal’, and if separation occurred between the plating layer and the adhesive, it was judged as ‘peeling’. In addition, if the peeled area had a diameter of 3 mm or more, it was judged as ‘peeling’, and if peeling occurred but in a very small area with a diameter of 3 mm or less, it was re-evaluated through a re-experiment. A total of 5 tests were performed, and if peeling occurred 3 or more times, it was marked as poor plating adhesion (×), and if it occurred less than 3 times, it was marked as good plating adhesion (○).

[0114]

[0115] Meanwhile, for elemental analysis such as carbon and silicon, the GDS (Glow Discharge Spectrometer) method, which enables quantitative analysis of various components in the depth (thickness) direction, was used to conduct concentration analysis of major elements such as carbon at a sufficient depth from the surface of the plating layer.

[0116] Afterwards, the f value was calculated based on the result graph as in Fig. 1. Each measurement value is shown in Table 2 below.

[0117]

[0118] Steel alloy composition (weight%) Manufacturing conditions CSiMnAl Annealing temperature (℃) Annealing furnace dew point temperature (℃) Snout dew point temperature (℃) Plating bath Comparative example 10.220.261.150.04779-37.1-38.9 A Comparative example 20.220.251.140.04779-40.1-33.7 A Comparative example 30.220.251.140.04779-45.5-38.2 A Comparative example 40.080.301.240.0381915.0-32.8 A Invention example 10.080.301.240.0381915.1-44.7 A Invention example 20.080.301.240.0381914.9-43.9A Comparative Example 50.220.261.150.0381915.1-36.4A Invention Example 30.220.261.150.0381914.6-53.2A Invention Example 40.220.261.150.0381915.3-53.5A Comparative Example 60.220.261.150.0378818.0-31.4A Invention Example 50.210.261.160.0478817.8-46.3A Invention Example 60.210.261.160.0478818.1-51.7A Invention Example 70.080.301.240.0382015.0-49.4A invention example 80.080.301.240.0382014.8-41.1A invention example 90.210.261.160.0478917.7-45.8A invention example 100.210.261.160.0478918.2-55.1A comparative example 70.220.261.150.04776-59.2-39.4B comparative example 80.220.261.150.04774-58.3-39.1B invention example 110.210.261.160.047775.1-50.3B invention example 120.210.261.160.047905.2-49.8B

[0119]

[0120] Steel grade Surface layer Microstructure GDS Mechanical properties Plating Adhesion FP a b c def Tensile strength (MPa) Bending C (°) Bending L (°) Comparative example 1762437.834.2-3.60.1100150849.355.9×Comparative example 2732720.519.87-0.60.1100151150.156.1×Comparative example 3772339.434.6-4.80.1100150348.154.3×Comparative example 499.20.828.971.842.90.040.970.571022140.3140.2×Invention example 199.10.923.17248.90.040.890.461011140.1140.3○Invention Example 299.20.831.57240.50.040.910.561001140.2140.3○Comparative Example 599.40.62848.920.90.111.370.60148857.062.6×Invention Example 399.30.718.241.623.40.111.530.59151957.263.2○Invention Example 499.10.937.460.923.50.111.490.58150357.861.3○Comparative Example 699.40.633.369.636.30.112.530.63151055.662.6×Invention Example 599.30.721.956.434.50.1052.390.66151158.065.1○Invention Example 699136.57134.50.1052.550.70151355.769.1○Invention Example 799.10.931.571.940.40.040.950.591005140.0140.2○Invention Example 899.40.630.37241.70.040.930.561006140.3140.3○Invention Example 999.10.930.958.727.80.1051.70.58150956.364.5○Invention Example 1099.30.731.458.126.70.1052.20.78151058.369.9○Comparative Example 775251716.9-0.10.1100157859.062.4×Comparative Example 8782220.419.8-0.60.1100157558.261.1×Invention Example 1199.20.818.449.4431.00.1051.60.49151377.786.6○Invention Example 1299.30.72160.939.90.1051.770.42151081.286.5○

[0121]

[0122] As shown in Tables 1 and 2 above, it can be confirmed that Invention Examples 1 to 12, which satisfy both the alloy composition and manufacturing conditions proposed in the present invention, have high tensile strength of 1000 MPa or more and excellent bending performance. In addition, plating adhesion was also excellent.

[0123] On the other hand, Comparative Examples 1 to 3, in which the dew point of the annealing furnace and the dew point of the snout section were outside the scope of the present invention, had inferior bendability and plating adhesion.

[0124] In addition, Comparative Examples 4 to 6 showed improved bendability due to surface decarburization caused by a high dew point temperature during annealing, but poor plating adhesion due to the dew point in the snout section being outside the range of the present invention.

[0125] In Comparative Examples 7 and 8, the plating adhesion was poor because the dew point of the snout section was outside the scope of the present invention. On the other hand, the improved bendability in Comparative Examples 7 and 8 compared to Comparative Examples 1 to 3 is due to the plating being performed using a plating bath containing a large amount of Zn.

Claims

1. A steel plate; and an aluminum-based plating layer on at least one surface of the steel plate, When the surface of the above steel plate is defined as a surface layer from the surface to a maximum of 200 ㎛ in the thickness direction, the surface layer contains a ferrite phase having an area fraction of 80% or more. An aluminum-plated steel sheet having a ratio of f, represented by the following [Equation 1], of 0.2 to 0.9 in the GDS (Glow Discharge Spectrometer) profile of C and Si observed in the thickness direction of the above-mentioned steel sheet. [Formula 1] f = Area from the interface of the base steel plate and the plating layer to the point where 50% of carbon (C) is decarburized / (Length (depth) from the interface of the base steel plate and the plating layer to the point where 50% of carbon (C) is decarburized × (Carbon (C) / 2)) (Here, carbon (C) refers to the weight content contained in the steel plate.) 2. In paragraph 1, The above steel plate contains, in weight %, carbon (C): 0.02 to 0.60%, silicon (Si): 0.001 to 2.000%, aluminum (Al): 0.001 to 1.000%, manganese (Mn): 0.1 to 4.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0.1% or less, boron (B): 0.0001 to 0.0100%, copper (Cu): 1.0% or less, molybdenum (Mo): 1.0% or less, chromium (Cr): 1.0% or less, nickel (Ni): 1.0% or less, vanadium (V): 1.0% or less, calcium (Ca): 0.01% or less, niobium (Nb): 0.1% or less, tin (Sn): Aluminum-plated steel sheet containing: 1.0% or less of W, 1.0% or less of Sb, 0.1% or less of Mg, 1.0% or less of Co, 1% or less of Arsenic (As), 1.0% or less of Zirconium (Zr), 1.0% or less of Bismuth (Bi), 0.3% or less of Rare Earth Elements (REM), and the remainder Fe and unavoidable impurities.

3. In paragraph 1, An aluminum-plated steel sheet comprising, in weight %, zinc (Zn): 0 to 30%, silicon (Si): 7 to 10%, iron (Fe): 5 to 10%, the remainder aluminum (Al) and unavoidable impurities.

4. In paragraph 1, The above steel plate is an aluminum-plated steel plate in which the microstructure of the remaining area, excluding the surface layer, is a mixture of ferrite and pearlite.

5. In paragraph 1, The above steel plate is an aluminum-coated steel plate with a maximum three-point bending angle of 55° or more.

6. Step for preparing the steel plate; The above steel plate is heated at a dew point temperature of 5 to 30°C and 1 to 70% hydrogen (H 2) -The remaining nitrogen (N 2) A step of annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 1000 seconds; and It includes a step of loading the annealed steel plate into a galvanizing facility to form a plating layer on at least one surface of the steel plate. A method for manufacturing an aluminum-plated steel sheet, wherein the above-mentioned molten plating equipment is sequentially equipped with a snout section connected to the annealing furnace and a molten plating bath, the snout section is controlled to a dew point temperature of -40°C or lower, and the molten plating bath is an aluminum-based plating bath.

7. In paragraph 6, The above steel plate is cold rolled steel plate. In weight %, carbon (C): 0.02 to 0.60%, silicon (Si): 0.001 to 2.000%, aluminum (Al): 0.001 to 1.000%, manganese (Mn): 0.1 to 4.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, titanium (Ti): 0.1% or less, boron (B): 0.0001 to 0.0100%, copper (Cu): 1.0% or less, molybdenum (Mo): 1.0% or less, chromium (Cr): 1.0% or less, nickel (Ni): 1.0% or less, vanadium (V): 1.0% or less, calcium (Ca): 0.01% or less, niobium (Nb): 0.1% or less, tin (Sn): 1.0% A method for manufacturing an aluminum-plated steel sheet, comprising: tungsten (W): 1.0% or less, antimony (Sb): 1.0% or less, magnesium (Mg): 0.1% or less, cobalt (Co): 1% or less, arsenic (As): 1.0% or less, zirconium (Zr): 1.0% or less, bismuth (Bi): 1.0% or less, rare earth elements (REM): 0.3% or less, the remainder being iron and unavoidable impurities.

8. In paragraph 6, The above steel plate Step of reheating the steel slab to a temperature range of 1050 to 1300℃; A step of obtaining a hot-rolled steel sheet by final rolling the above-mentioned reheated steel slab in a temperature range of Ar3 to 1000℃; A step of coiling the hot-rolled steel plate at a temperature range of 750℃ or less; and A method for manufacturing an aluminum-plated steel sheet, the method comprising: cold rolling the above-mentioned hot-rolled steel sheet at a reduction ratio of 30 to 90%.

9. In paragraph 6, A method for manufacturing an aluminum-plated steel sheet, wherein the aluminum-based plating bath contains, in weight %, 0 to 30% of zinc (Zn), 7 to 10% of silicon (Si), the remainder being aluminum (Al) and unavoidable impurities.

10. In paragraph 9, A method for manufacturing an aluminum-plated steel sheet, wherein the aluminum-based plating bath further contains 3% or less of iron (Fe) as an impurity.

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