Plated steel sheet and manufacturing method therefor

Abstract

The present invention relates to a plated steel sheet and a manufacturing method therefor, the plated steel sheet comprising: a base steel sheet comprising, by weight%, 0.03-0.1% of C, 1.5% or less of Si, 0.3-2.7% of Mn, 0.5-1.5% of Cr, 0.2-1.0% of Cu, 0.05-0.1% of Ni, 0.1% or less of Al, 0.1% or less of Nb, 0.1% or less of Ti, 0.03% or less of P, 0.02% or less of S, and 0.015% or less of N, with the remainder being Fe and other inevitable impurities; and a plating layer formed on at least one surface of the base steel sheet, wherein the area of non-plated regions is 0.1% or less by area%, and the difference in tensile strength before and after a corrosion test is 10% or less. More specifically, the present invention relates to a plated steel sheet having excellent corrosion resistance and durability, and a manufacturing method therefor.

Description

Galvanized steel sheet and its manufacturing method

[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 corrosion resistance and durability and a method for manufacturing the same.

[0002] Zinc-based galvanized steel exhibits a sacrificial nature. When exposed to a corrosive environment, zinc, with a lower redox potential than iron, corrodes first, inhibiting corrosion of the steel. Furthermore, as the zinc in the plating layer oxidizes, it forms a dense corrosion product on the steel surface, shielding it from the oxidizing environment and enhancing its corrosion resistance. Thanks to these advantageous properties, zinc-based galvanized steel has recently been expanding its application to include building materials, home appliances, and automotive steel.

[0003] However, due to the increase in air pollution caused by industrialization, the corrosive environment is gradually worsening, and due to strict regulations on resource and energy conservation, there is a growing need to develop steel with better corrosion resistance than conventional galvanized steel.

[0004] To address these issues, research is being conducted on manufacturing technologies for zinc alloy-based galvanized steel sheets, which enhance the corrosion resistance of steel by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. A representative example is the Zn-Mg-Al zinc alloy-based galvanized steel sheet, which is prepared by adding magnesium to the Zn-Al plating composition.

[0005] However, Zn-Mg-Al zinc alloy-plated steel sheets are often processed and used, and when subjected to shearing and bending processing, the base steel sheet is exposed at naturally occurring cross-sections, cracks in the plating layer, and processing damage areas, which reduces corrosion resistance and accelerates the propagation of corrosion, making it difficult to maintain the durability of the steel sheet.

[0006] When a typical zinc alloy-coated steel sheet is exposed to a corrosive environment, zinc hydroxide (Zn(OH2)) is mainly generated on the surface. On the other hand, in the case of a Zn-Al-Mg zinc alloy-coated steel sheet, a very dense corrosion product, Mg-Al LDH (Layered double hydroxide, Mg6Al2(OH)), is generated due to the influence of Mg and Al in the coating surface layer. 16 CO3) is formed. These corrosion products flow down to the exposed parts of the steel plate and cover them, which helps maintain the initial corrosion resistance.

[0007] However, when the uneven formation of corrosion products and the base steel plate are thick, the exposed areas are not sufficiently covered, resulting in partial corrosion potential differences and the development of weak points of corrosion. This uneven corrosion propagation reduces durability. Especially when used as structural steel, maintaining durability is especially crucial, necessitating quality improvement.

[0008] According to one embodiment of the present invention, a plated steel sheet and a method for manufacturing the same are provided.

[0009] According to one embodiment of the present invention, it is an object to provide a plated steel sheet having excellent corrosion resistance and durability and a method for manufacturing the same.

[0010] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0011] According to one embodiment of the present invention, a steel sheet comprising, in wt%, C: 0.03 to 0.1%, Si: 1.5% or less, Mn: 0.3 to 2.7%, Cr: 0.5 to 1.5%, Cu: 0.2 to 1.0%, Ni: 0.05 to 0.1%, Al: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the remainder being Fe and other unavoidable impurities; and

[0012] A plating layer formed on at least one surface of the above steel plate;

[0013] The area where plating occurs is 0.1% or less in terms of area%,

[0014] It may be a plated steel sheet with a difference in tensile strength before and after corrosion test of 10% or less.

[0015] The above steel plate may have an oxide containing at least one of Cr, Cu, Ni, Nb, and Ti in an area % of 10% or less within a 50 nm region from the surface.

[0016] The above plating layer may be a Zn-based plating layer containing, in weight %, Mg: 4 to 7% and Al: 8.2 to 20%.

[0017] The above plating layer may further include one or more of the following groups (a) to (h).

[0018] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0019] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0020] (c) Ti: 0.1% or less

[0021] (d) W: 0.5% or less

[0022] (e) Cu: 2.0% or less

[0023] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0024] (g) B: 0.1% or less, P: 0.1% or less

[0025] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0026] The above plating layer may include at least one of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn22 eutectic structure, an Al single-phase structure in which Zn is dissolved, and a Zn single-phase structure.

[0027] According to one embodiment of the present invention, there is provided a step of preparing a steel sheet, which comprises, in wt%, C: 0.03 to 0.1%, Si: 1.5% or less, Mn: 0.3 to 2.7%, Cr: 0.5 to 1.5%, Cu: 0.2 to 1.0%, Ni: 0.05 to 0.1%, Al: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the remainder being Fe and other unavoidable impurities;

[0028] A step of annealing the above steel plate;

[0029] A step of pretreating the annealed steel plate; and

[0030] A step of plating the above pretreated steel plate;

[0031] The above preprocessing step may be a method for manufacturing a plated steel sheet in which the R value defined in the following relational expression 1 is 4 or more and the number of passes is 3 or more.

[0032] [Relationship 1]

[0033] R = (Bt 2 ) × (Bn / 2)

[0034] (In the formula, Bt is the brush hair thickness (mm) and Bn is the number of brush hairs (pieces).)

[0035] In the above plating step, the plating bath may be a Zn-based plating bath containing, in weight %, Mg: 4 to 7% and Al: 8.2 to 20%.

[0036] The above plating bath may further include one or more of the following groups (a) to (h).

[0037] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0038] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0039] (c) Ti: 0.1% or less

[0040] (d) W: 0.5% or less

[0041] (e) Cu: 2.0% or less

[0042] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0043] (g) B: 0.1% or less, P: 0.1% or less

[0044] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0045] The R value defined in the above relational expression 1 can be 4 to 6.

[0046] The number of passes in the above preprocessing step may be 3 to 5.

[0047] According to one embodiment of the present invention, a plated steel sheet and a method for manufacturing the same can be provided.

[0048] According to one embodiment of the present invention, a plated steel sheet having excellent corrosion resistance and durability and a method for manufacturing the same can be provided.

[0049] According to one embodiment of the present invention, by initially forming a dense corrosion product and an oxide layer containing chromium on an exposed portion of a base steel plate during processing, the propagation of corrosion is inhibited, thereby providing a plated steel plate having excellent durability and a method for manufacturing the same.

[0050] According to one embodiment of the present invention, when exposed to a corrosive environment, a plated steel sheet and a method for manufacturing the same can be provided, which can form a stable oxide layer by concentrating Cr in a corrosion layer formed on an exposed portion of a base steel sheet, thereby inhibiting corrosion propagation, and can delay the rate of loss of the plated layer by forming a barrier layer, along with the initial corrosion delay effect of a Mg-Al based LDH.

[0051] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0052] Figures 1 (a) and 1 (b) are photographs of the surface of a steel plate before and after pretreatment of Invention Example 2 according to one embodiment of the present invention, observed using an energy dispersive spectrometer (EDS).

[0053] Figures 2 (a) and (b) are optical photographs of cross-sections after composite corrosion tests of Comparative Example 15 and Invention Example 2, respectively, according to one embodiment of the present invention.

[0054] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0055] Hereinafter, the present invention will be described in detail.

[0056] A plated steel sheet according to one embodiment of the present invention may include a base steel sheet; and a plated layer formed on at least one surface of the base steel sheet.

[0057] Below, the composition of the steel plate of the present invention is described in detail.

[0058] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0059] According to one embodiment of the present invention, the base steel sheet of the plated steel sheet may include, in weight %, C: 0.03 to 0.1%, Si: 1.5% or less, Mn: 0.3 to 2.7%, Cr: 0.5 to 1.5%, Cu: 0.2 to 1.0%, Ni: 0.05 to 0.1%, Al: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, P: 0.03% or less, S: 0.02% or less, and N: 0.015% or less.

[0060] Carbon (C): 0.03~0.1%

[0061] Carbon (C) is an element that can be added to improve strength, and is the most effective and economical way to secure strength. If the carbon (C) content is less than 0.03%, it is difficult to secure sufficient strength of the steel, and other alloying elements must be added to secure the strength of the steel. According to one embodiment of the present invention, the carbon (C) content may be 0.04% or more. On the other hand, if the content exceeds 0.1%, weldability may deteriorate, and local corrosion resistance may be adversely affected due to the promotion of precipitation of carbides, etc. According to one embodiment of the present invention, the carbon (C) content may be 0.09% or less.

[0062] Silicon (Si): 1.5% or less

[0063] Silicon (Si) generally acts as a deoxidizer and can increase the strength of steel through solid solution strengthening. Furthermore, silicon (Si) forms a protective film (H4SiO4) on the surface, enhancing overall corrosion resistance. However, if the silicon (Si) content exceeds 1.5%, it can coarsen precipitates, hindering the yield strength achieved through precipitation strengthening.

[0064] Manganese (Mn): 0.3~2.7%

[0065] Manganese (Mn) plays a role in increasing strength through solid solution strengthening. However, if the content is excessive, it can cause embrittlement of the steel and reduce corrosion resistance due to the formation of MnS. If the manganese (Mn) content is less than 0.3%, it may be difficult to maintain the strength of the steel. According to one embodiment of the present invention, it may be 0.4% or more. On the other hand, if the content exceeds 2.7%, when the slab is cast, a segregation zone develops in the center of the thickness, which reduces weldability, and when the ferrite phase transformation in which C, Mn, Cr, etc. are redistributed is performed, the enrichment phenomenon of the above elements becomes severe, which may reduce corrosion resistance. According to one embodiment of the present invention, it may be 2.5% or less.

[0066] Chromium (Cr): 0.5~1.5%

[0067] Chromium (Cr) is a fundamental alloying element for ensuring corrosion resistance of base steel sheets. Chromium (Cr) can stabilize the corrosion layer by concentrating at the interface of corrosion products. In addition, when the base steel sheet is exposed to a corrosive environment such as a salt water environment, chromium (Cr) is precipitated and concentrated together with iron oxide, which forms a dense layer and can act as a barrier layer (stable rust) that reduces the spread of rust. In the case of a plated steel sheet, when exposed to a corrosive environment, the plating layer is lost as a sacrificial corrosion prevention layer to prevent rust at the exposed part of the base metal. However, if a barrier layer in which chromium (Cr) is concentrated at the exposed part of the base metal is provided, the loss rate of the plating layer due to the sacrificial function of the plating can be delayed. To secure the above-described effect, chromium (Cr) may be added in an amount of 0.5% or more. According to one embodiment of the present invention, 0.6% or more may be included. The higher the additive content, the higher the corrosion resistance effect of the corrosion product interface. However, if the content exceeds 1.5%, the structure may become coarse, which may reduce the formation of a stable oxide layer. According to one embodiment of the present invention, the content may be 1.4% or less.

[0068] Copper (Cu): 0.2~1.0%

[0069] Copper (Cu) can be added to improve the corrosion resistance of steel sheets. However, if the copper content exceeds 1.0%, it can form a liquid phase at the base metal interface at high temperatures, causing high-temperature embrittlement. Therefore, when adding more than 1.0%, nickel (Ni) must be added to prevent this problem. On the other hand, if the copper (Cu) content is less than 0.2%, it may be difficult to ensure corrosion resistance.

[0070] Nickel (Ni): 0.05~0.1%

[0071] Nickel (Ni) can generally be added to prevent red-hot embrittlement in copper-containing steel. Adding nickel (Ni) is effective in resisting both general and localized corrosion. To achieve the above-described effects, nickel (Ni) may be added in an amount of 0.05% or more, according to one embodiment of the present invention. However, since nickel (Ni) is an expensive element, the upper limit of its content may be limited to 0.1%, taking into account the relative input effect. In one embodiment of the present invention, the nickel content may be 0.09% or less.

[0072] Aluminum (Al): 0.1% or less

[0073] Aluminum (Al) is an element added to molten steel for deoxidation. It reacts with nitrogen in the steel, effectively refining austenite grains and improving toughness. It also enhances corrosion resistance. However, if its content exceeds 0.1%, aluminum oxide can form coarse inclusions in the steel, degrading its brittleness.

[0074] Niobium (Nb): 0.1% or less

[0075] Niobium (Nb) precipitates NbC, thereby imparting a precipitation strengthening effect. When NbC is precipitated, it inhibits the formation of carbides of Cr, thereby increasing the solid solution Cr. In order to secure the above-described effect, according to one embodiment of the present invention, niobium (Nb) may be added in an amount of 0.05% or more. On the other hand, if the content exceeds 0.1%, there is a risk of forming coarse carbonitrides.

[0076] Titanium (Ti): 0.1% or less

[0077] Titanium (Ti) precipitates TiC, imparting a precipitation strengthening effect. Similar to the effect of niobium, when TiC is precipitated, it inhibits the formation of chromium carbides, thereby increasing the solid solution chromium. To secure the above-described effect, according to one embodiment of the present invention, 0.02% or more may be added. On the other hand, if the content exceeds 0.1%, coarse carbonitrides are formed, and the strength improvement effect may be minimal compared to the increase in content.

[0078] Phosphorus (P): 0.03% or less

[0079] Phosphorus (P) is an element that exists as an impurity in steel. If its content is excessive, weldability and toughness deteriorate, so the upper limit of its content can be limited to 0.03%.

[0080] Sulfur (S): 0.02% or less

[0081] Sulfur (S) is an element present as an impurity in steel. Excessive levels can degrade ductility, impact toughness, and weldability, so its upper limit is limited to 0.02%. Furthermore, sulfur (S) acts as a corrosion initiation site when it combines with manganese (Mn) in steel to form MnS inclusions, so its content should be kept to a minimum.

[0082] Nitrogen (N): 0.015% or less

[0083] Nitrogen (N) is an element that exists as an impurity level in steel, and according to one embodiment of the present invention, its content can be limited to 0.015%.

[0084] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0085] Below, the microstructure of the steel plate of the present invention is described in detail.

[0086] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0087] The microstructure of the steel sheet according to one embodiment of the present invention may include at least one of ferrite and bainite as a main phase, and may include a residual structure in an area % of 20% or less.

[0088] According to one embodiment of the present invention, to secure a strength of 600 MPa or more, ferrite and bainite may be included in an amount of 80% or more. If the fraction of ferrite and bainite is less than 80%, pearlite and MA phases may be excessively formed as the remaining structure, which may cause problems such as reduced low-temperature toughness and corrosion resistance.

[0089] According to one embodiment of the present invention, a steel sheet may include an oxide including at least one of Cr, Cu, Ni, Nb, and Ti in an area % of 10% or less within a 50 nm region from the surface.

[0090] The above oxide can be measured by observing at 100x magnification using EPMA within a 50 nm region from the surface, which is the superficial region.

[0091] The steel sheet according to one embodiment of the present invention contains various oxidation-friendly elements such as Cr for forming a stable oxide layer in a corrosion-prone area, Cu for securing additional corrosion resistance, and Si for forming a ferrite or bainite structure and for corrosion resistance, so that the oxides concentrated on the surface can be controlled to secure high corrosion resistance and plating properties.

[0092] When the oxide containing at least one of Cr, Cu, Ni, Nb, and Ti is less than 10%, the plating properties desired in the present invention can be secured. When the oxide content exceeds 10%, the plating wettability may be deteriorated due to the oxides concentrated on the surface, resulting in poor plating properties.

[0093] The plating layer of the plated steel sheet according to one embodiment of the present invention may be a Zn-based plating layer containing, in weight %, Mg: 4 to 7% and Al: 8.2 to 20%.

[0094] Mg is an element that improves the corrosion resistance of the plated steel sheet, and the Mg content in the plated layer can be controlled to 4% or more to secure the desired excellent corrosion resistance. If the Mg content is less than 4%, it may be difficult to secure sufficient corrosion resistance. When the Al-Mg-Zn plated steel sheet is in a neutral, weakly acidic, or weakly alkaline corrosive environment such as salt water or rainwater, Mg in the plated layer is dissolved, and this uniformly forms LDH (Layered Double Hydroxide; (Zn,Mg)6Al2(OH)16(CO3)·4H2O)), thereby improving corrosion resistance, so a higher Mg content is preferable. However, in an acidic corrosive environment, Mg dissolution is too fast, so a larger amount of Mg added may worsen corrosion resistance in an acidic environment. In addition, since there is a possibility that dross in the form of MgO may be generated in the plating bath when excessive Mg is added, the upper limit of the Mg content may be limited to 7% according to one embodiment of the present invention. In addition, the upper limit of the Mg content may be preferably limited to 6%.

[0095] Al is a component that improves corrosion resistance together with Mg. Since Al is strong against acid, corrosion resistance in acidic environments can increase significantly as the Al content increases. If the Al content is less than 8.2%, there may be problems in securing the target corrosion resistance. In addition, Al has the effect of suppressing the oxidation of Mg in the plating bath, so that the formation of MgO-based dross can be suppressed as the amount of Al added increases. On the other hand, as the amount of Al added increases, corrosion may increase in an alkaline environment. In addition, if the amount of Al added is too large, the melting point of the plating bath increases, so that the plating bath temperature must be maintained high, which aggravates the erosion of structures in the plating bath. Therefore, according to one embodiment of the present invention, the Al content may be limited to 20% or less. In addition, the upper limit of the Al content may be preferably limited to 14.2%.

[0096] According to one embodiment of the present invention, the plating layer may further include at least one of the following groups (a) to (h).

[0097] However, since the elements in each group below are not essential for achieving the tasks of the present invention, their lower limits of content are not limited. Therefore, even if not specifically mentioned below, the lower limit of the content of each element may be 0%.

[0098] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0099] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0100] (c) Ti: 0.1% or less

[0101] (d) W: 0.5% or less

[0102] (e) Cu: 2.0% or less

[0103] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0104] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0105] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0106] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0107] Si has the effect of preventing Fe-Zn alloying due to the formation of interface Mg2Si, and can prevent excessive formation of Fe-Al alloy phase. However, if its content exceeds 0.5%, there is a concern that the melting point of the plating bath will increase, and brittleness will increase due to the excessive formation of Mg2Si. Ni has the effect of preventing Fe diffusion by forming an Al-Ni alloy phase, but if its content exceeds 0.5%, there may be a problem that the cost of auxiliary materials increases excessively.

[0108] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0109] Ca, La, Ce, Y and Sr have the effect of preventing oxidation of Mg in the plating bath by forming an oxide film, but if the content exceeds 1.0%, 0.1%, 0.1%, 0.1% and 1.0%, respectively, Ca may have the problem of increased dross due to increased oxides, and La, Ce, Y and Sr may have the problem of reduced plating properties due to increased viscosity of the plating bath.

[0110] (c) Ti: 0.1% or less

[0111] Ti acts as a nucleation site for Ti-Al intermetallic compounds and has the effect of refining crystal grains (spangles), but if its content exceeds 0.1%, the melting point of the plating bath may increase and there may be a problem of increased dross.

[0112] (d) W: 0.5% or less

[0113] W forms W oxide on the surface, which improves corrosion resistance, but if its content exceeds 0.5%, there may be a problem of the melting point of the plating bath increasing.

[0114] (e) Cu: 2.0% or less

[0115] Cu has the effect of forming an Al-Cu process structure and lowering the hardness of the plating layer, but if its content exceeds 2.0%, there may be a problem of the spangles becoming coarser.

[0116] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0117] Fe, Cr, Mn, and V have the effect of preventing electrode deterioration by suppressing alloying between zinc and the welding electrode due to rapid liquid loss, but if their contents exceed 1.0%, 0.5%, 0.5%, and 0.5%, respectively, there may be a problem in that the melting point of the plating bath increases excessively.

[0118] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0119] B and P have the effect of suppressing LME cracks in welds, but if their contents exceed 0.1% each, there may be a problem of increased dross generation.

[0120] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0121] Sn, Sb, and Bi have the effect of uniformizing spangles and improving pot durability by lowering the plating bath temperature, but if their contents exceed 1.0% each, there may be a problem of coarsening of spangles.

[0122] According to one embodiment of the present invention, the plating layer may contain Zn in addition to the aforementioned composition, and may further include other unavoidable impurities in addition to the aforementioned elements. These unavoidable impurities may be unintentionally incorporated during the typical manufacturing process and therefore cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed herein.

[0123] Below, the method for manufacturing a plated steel sheet of the present invention is described in detail.

[0124] A plated steel sheet according to one embodiment of the present invention can be manufactured by preparing, annealing, pre-treating, and plating a base steel sheet satisfying the above-described alloy composition.

[0125] Preparing the steel plate

[0126] A steel sheet satisfying the alloy composition according to one embodiment of the present invention can be prepared.

[0127] According to one embodiment of the present invention, the steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet, and is not particularly limited.

[0128] Sodun

[0129] The above steel plate can be annealed at a temperature range of 800°C or lower.

[0130] According to one embodiment of the present invention, annealing may be performed to secure the desired properties, but since Si undergoes more active surface thickening in a temperature range exceeding 800°C, the annealing temperature may be limited to 800°C or lower.

[0131] Brush pretreatment

[0132] The above annealed steel plate can be pretreated with a brush.

[0133] Pretreatment prior to plating may include shot blasting, pickling, and brushing. However, there are limitations to removing oxides such as Cr, Cu, and Si through pickling, and physically removing the oxides may be effective in removing these surface oxides.

[0134] According to one embodiment of the present invention, pretreatment can be performed using a brush.

[0135] During brush pretreatment, the degree of oxide removal can vary depending on the brush bristles' material, thickness, abrasive type, as well as the amount of pressure, speed, and number of strokes. While stronger brush bristles and higher pressure, speed, and number of strokes result in more effective oxide removal, appropriate control is necessary to ensure surface flatness and appearance after plating. Brush pretreatment removes concentrated oxides from the steel sheet surface before immersion in the plating bath, ensuring plating properties.

[0136] According to one embodiment of the present invention, when pre-treating a brush, the R value defined in the following relational expression 1 may be 4 or more.

[0137] [Relationship 1]

[0138] R = (Bt 2 ) × (Bn / 2)

[0139] (In the formula, Bt is the brush hair thickness (mm) and Bn is the number of brush hairs (pieces).)

[0140] During brush pretreatment, the thickness and number of bristles of the brush may be closely related. According to one embodiment of the present invention, the plating properties desired in the present invention can be secured by appropriately controlling the relationship between the thickness and number of bristles through relational expression 1.

[0141] If the R value defined in the above relational expression 1 is less than 4, there is a concern that the desired plating properties may not be sufficiently secured because the oxides concentrated on the surface may not be sufficiently removed. The upper limit of the R value defined in the relational expression 1 is not particularly limited, but if the pretreatment is excessive, there may be a problem that deep flaws are generated on the surface, resulting in stripes remaining on the surface after plating. Therefore, according to one embodiment of the present invention, the upper limit of the R value may be limited to 6. According to one embodiment of the present invention, even if weak (acceptable level) stripes remain on the surface, if the unplated metal satisfies the conditions of the present invention, the plating properties may not be significantly affected.

[0142] According to one embodiment of the present invention, when pre-treating a brush, the thickness of the hair may be 1 to 1.7 mm. In addition, according to one embodiment of the present invention, the number of brush hairs may be 1 to 7.

[0143] According to one embodiment of the present invention, the number of passes during brush pretreatment may be 3 to 5.

[0144] If the number of passes is less than three, the pretreatment may be ineffective, resulting in poor plating properties. Conversely, if the number of passes exceeds five, streaks may remain on the surface.

[0145] plating

[0146] The above brush-pretreated steel sheet can be plated.

[0147] According to one embodiment of the present invention, during plating, a Zn-based plating bath containing, by weight %, Mg: 4 to 7% and Al: 8.2 to 20% can be used.

[0148] The composition of the plating bath may be limited in scope for the same reasons as described in the composition of the above plating layer, and the same component system may be applied.

[0149] If the magnesium content is less than 4%, it may be difficult to ensure sufficient corrosion resistance. On the other hand, if the content exceeds 7%, there may be a problem with the generation of dross in the form of MgO in the plating bath. Furthermore, to prevent the generation of dross in the form of MgO in the plating bath, the upper limit of the magnesium content may preferably be limited to 6%.

[0150] If the Al content is less than 8.2%, it may be difficult to secure sufficient corrosion resistance. On the other hand, if the content exceeds 20%, the melting point of the plating bath increases, so the plating bath temperature must be maintained high, which may cause the problem of aggravated erosion of structures within the plating bath. In addition, in order to prevent the problem of aggravated erosion of structures within the plating bath due to the increased melting point of the plating bath and the need to maintain a high plating bath temperature, the upper limit of the Al content may preferably be limited to 14.2%.

[0151] According to one embodiment of the present invention, the plating bath may contain other unavoidable impurities. While there are no specific limitations on the impurities, they may be at a level applicable to the same technical field.

[0152] According to one embodiment of the present invention, during plating, the plating bath temperature may be 450 to 480°C, and the temperature difference between the base steel sheet and the plating bath may be 10 to 40°C. In order to increase the interfacial reactivity between the plating solution and the base steel sheet, the plating bath temperature may be controlled within a temperature range of 450 to 480°C.

[0153] 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 explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0154] (Example)

[0155] A galvanized steel sheet was manufactured using a steel grade having the composition shown in Table 1 below under the conditions shown in Table 2 below. The contents of P, S, and N in the steel were included as impurities and were included at a level satisfying the range proposed in the present invention. During plating, a plating bath having the composition of Mg: 5.0%, Al: 11.9%, and the balance Zn was used.

[0156] Steel alloy composition (weight %)CMnSiAlNbTiCuNiCrA0.050.50.30.030.030.030.30.051B0.070.70.250.030.030.030.50.050.5C0.050.50.30.030.050.030.30.051.3D0.180.50.020.03---0.01-E0.070.70.250.030.030.03 0.50.050.4F0.050.50.30.030.050.030.30.051.7G0.070.70.250.030.030.030.30.011.5H0.050.50. 30.030.050.030.30.31.5I0.050.50.30.030.050.030.10.051.5J0.050.50.30.030.050.031.20.051.5

[0157] Specimen number, steel grade, annealing brush, pretreatment temperature (℃), hair thickness (mm), number of hairs (pieces), relational formula, 1 pass, number of times (times), 1A7501.454.94, 2A7501.454.93, 3A7501.454.92, 4A7501.476.86, 35A7501.375.92, 26A7501.332.54, 37A7501.315.28, 8501.432.94, 39A750-50, 310A7501.4-0-11B7501.454.94, 12B7501.454.93, 13B7501.354.23, 214B7501.73.52, 15B7501.432.94, 216B7501.4 32.94317C7501.454.9418C7501.354.23319C750152.5220C750-30221C7501.432.94322D7501.4-0-23E7500.851.6324F7501 .253.6325A7501.253.6426A7501.410.98327B7501.454.9728G7501.454.9329H7501.454.9330I7501.454.9331J7501.454.93

[0158] [Relationship 1]

[0159] R = (Bt 2 ) × (Bn / 2)

[0160] (In the formula, Bt is the brush hair thickness (mm) and Bn is the number of brush hairs (pieces).)

[0161] After the manufactured steel plate was plated under the conditions of Table 3 below, the plating properties and durability of the plated steel plate were evaluated, and the results are shown in Table 3 below.

[0162] The plating properties and durability were evaluated and presented using the following methods and standards.

[0163] (1) Plating performance evaluation

[0164] After plating, the presence of streaks and unplated areas was observed, and the observation results were indicated as follows. The unplated area fraction was observed at a magnification of 100x using the EPMA detection method on the surface of the plated steel sheet, and the presence of streaks was observed with the naked eye. More specific evaluation criteria are as follows. In the present invention, ◎ was the target.

[0165] ◎: No stripes and less than 0.1% of unplated area

[0166] ○: There are weak (acceptable) stripes, and the area where plating occurs is less than 0.1%.

[0167] △: Area where plating occurs: 0.1% or more, but 5% or less

[0168] ×: Unplated area exceeds 5%

[0169] (2) Durability evaluation

[0170] A plated steel sheet of ASTM tensile specimen size was loaded into a salt spray accelerated tester with repeated cycles, where 2 hours of 5% salt water condition, 4 hours of drying, and 2 hours of wetting were considered as 1 cycle, and after 200 cycles, a tensile test was performed after removing corrosion products. The evaluation results were expressed according to the difference in tensile strength before and after the corrosion test. In the present invention, a condition in which the difference in tensile strength before and after the corrosion test was 10% or less was judged as good.

[0171] ○: 10% or less

[0172] △: If it is more than 10% but less than 20%

[0173] ×: If it exceeds 20%

[0174] Specimen number Steel type Property classification Plating durability 1A◎○Invention example 12A◎○Invention example 23A△△Comparative example 14A○○Invention example 35A△△Comparative example 26A××Comparative example 37A××Comparative example 48A××Comparative example 59A△△Comparative example 610A××Comparative example 711B◎○Invention example 412B◎○Invention example 513B△△Comparative example 814B○△Comparative example 915B××Comparative example 1016B××Comparative Example 1117C◎○Invention Example 618C◎○Invention Example 719C△△Comparative Example 1220C××Comparative Example 1321C××Comparative Example 1422D◎×Comparative Example 1523E○×Comparative Example 1624F△△Comparative Example 1725A××Comparative Example 1826A××Comparative Example 1927B○○Invention Example 828G○△Comparative Example 2129H△△Comparative Example 2230I○×Comparative Example 2331J△△Comparative Example 24

[0175] As shown in Table 3 above, in the case of an invention example satisfying the conditions of the present invention, the properties desired in the present invention could be secured.

[0176] Figures 1 (a) and (b) are photographs of the surface of a steel plate before and after pretreatment of Invention Example 2 according to one embodiment of the present invention, observed using an energy dispersive spectrometer (EDS). As shown in Figure 1 (b), it can be confirmed that surface oxides have been removed after pretreatment.

[0177] Figures 2(a) and 2(b) are optical photographs of cross-sections after composite corrosion tests of Comparative Example 15 and Inventive Example 2, respectively, according to one embodiment of the present invention. Figure 2(a) shows that no stable oxide layer was formed on the exposed portion of the base steel plate, and thus, a sacrificial reaction of non-uniform oxidation and continuous plating could be observed. Figure 2(b) shows that a passive layer, which is a stable oxide layer, was formed on the exposed portion of the base steel plate.

[0178] On the other hand, in the case of a comparative example that did not satisfy the alloy composition and pretreatment conditions of the steel plate proposed in the present invention, it was confirmed that the desired plating properties and durability were not secured at the same time.

[0179] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. A steel sheet containing, by weight%, C: 0.03 to 0.1%, Si: 1.5% or less, Mn: 0.3 to 2.7%, Cr: 0.5 to 1.5%, Cu: 0.2 to 1.0%, Ni: 0.05 to 0.1%, Al: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the remainder being Fe and other unavoidable impurities; and Including a plating layer formed on at least one surface of the above steel plate; The area where plating occurs is 0.1% or less in area %, Galvanized steel sheet with a difference in tensile strength before and after corrosion test of 10% or less.

2. In claim 1, The above-mentioned steel sheet is a plated steel sheet having an area % of oxide containing at least one of Cr, Cu, Ni, Nb, and Ti within a region of 50 nm from the surface, which is 10% or less.

3. In claim 1, The above plating layer is a zinc-based plating layer containing, in weight %, 4 to 7% Mg and 8.2 to 20% Al, a plated steel sheet.

4. In claim 3, A plated steel sheet, wherein the plating layer further comprises at least one of the following groups (a) to (h). (a) Si: 0.5% or less, Ni: 0.5% or less, or at least one of these (b) Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less, or at least one of these (c) Ti: 0.1% or less (d) W: 0.5% or less (e) Cu: 2.0% or less (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these (g) B: 0.1% or less, P: 0.1% or less, at least one of these (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these 5. In claim 1, A plated steel sheet, wherein the above-mentioned plating layer includes at least one of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn22 eutectic structure, an Al single-phase structure in which Zn is dissolved, and a Zn single-phase structure.

6. A step of preparing a base steel sheet containing, by weight%, C: 0.03 to 0.1%, Si: 1.5% or less, Mn: 0.3 to 2.7%, Cr: 0.5 to 1.5%, Cu: 0.2 to 1.0%, Ni: 0.05 to 0.1%, Al: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the remainder being Fe and other unavoidable impurities; A step of annealing the above steel plate; A step of preprocessing the annealed steel plate; and A step of plating the above pretreated steel plate; A method for manufacturing a plated steel sheet in which the above preprocessing step has an R value of 4 or more as defined in the following relational expression 1 and the number of passes is 3 or more. [Relationship 1] R = (Bt 2 ) × (Bn / 2) (In the formula, Bt is the brush hair thickness (mm) and Bn is the number of brush hairs (pieces).) 7. In claim 6, A method for manufacturing a plated steel sheet, wherein in the above plating step, the plating bath is a Zn-based plating bath containing, in weight %, Mg: 4 to 7% and Al: 8.2 to 20%.

8. In claim 7, A method for manufacturing a plated steel sheet, wherein the plating bath further comprises at least one of the following groups (a) to (h). (a) Si: 0.5% or less, Ni: 0.5% or less, or at least one of these (b) Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less, or at least one of these (c) Ti: 0.1% or less (d) W: 0.5% or less (e) Cu: 2.0% or less (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these (g) B: 0.1% or less, P: 0.1% or less, at least one of these (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these 9. In claim 6, A method for manufacturing a plated steel sheet having an R value of 4 to 6 as defined in the above relational expression 1.

10. In claim 6, A method for manufacturing a galvanized steel sheet having 3 to 5 passes in the above pretreatment step.

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