Plated steel sheet and manufacturing method therefor

Abstract

According to exemplary embodiments of the present invention, a plated steel sheet having improved platability and LME resistance, and a manufacturing method therefor can be provided.

Description

galvanized steel sheet and method of manufacturing the same

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same.

[0002] Steel sheets can be exposed to various environments from the storage stage after manufacturing, through their application and formation into products, until the products are used. Due to external environmental factors, oxidation and corrosion may occur, leading to a deterioration in the quality of these steel sheets. To prevent this, a plating layer can be formed on the surface of the steel sheet. This plating layer can provide a physical barrier against external oxygen, moisture, and salt, and the materials within the plating layer can act as an electrochemical sacrificial anode to prevent corrosion or oxidation of the steel sheet.

[0003] Various studies are being conducted to improve the quality of such galvanized steel sheets. In particular, during processing where heat and stress are simultaneously applied to the material, such as welding, the zinc in the plating layer can penetrate the substrate steel sheet and cause Liquid Metal Embrittlement (LME). Furthermore, since plating performance can deteriorate depending on the composition of the substrate steel sheet, there is a need to develop technologies to address these issues.

[0004] (Patent Document 1) Japanese Published Patent Application No. 2001-323355

[0005] The problem that the technical concept of the present invention aims to solve is to provide a high-strength plated steel sheet with improved plating properties and LME resistance, and a method for manufacturing the same.

[0006] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall details of the specification.

[0007] According to exemplary embodiments for solving the problem of the present invention, a plated steel sheet is provided. The plated steel sheet comprises, in weight percent, C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, the remainder being Fe and unavoidable impurities; and a plating layer disposed on the surface of the plated steel sheet, wherein the tensile strength is 590 MPa or more, and the plated steel sheet may satisfy the following equation 1.

[0008] [Relationship 1]

[0009] A1 / A2<4

[0010] In the above relationship 1, A1 is the maximum value of element B analyzed by GDS in the region from the surface of the base steel plate to the center of thickness up to 100 nm, and A2 is the average value of element B analyzed by GDS in the region from the surface of the base steel plate to the center of thickness up to 100 nm.

[0011] The above-mentioned steel plate may further satisfy the following relationship 2.

[0012] [Relationship 2]

[0013] 0 ≤ [Si] / (100*[B]) ≤ 10

[0014] In the above equation 2, [B] and [Si] represent the content (weight%) of B and Si, respectively.

[0015] The above-mentioned steel sheet may further contain titanium (Ti): 0.005~0.04% by weight.

[0016] The above-mentioned steel plate may further satisfy the following relationship 3.

[0017] [Relationship 3]

[0018] [Ti] / 3.4[N] > 1

[0019] In the above equation 2, [Ti] and [N] represent the content (weight%) of Ti and N, respectively.

[0020] The plating layer may contain, in weight percent, magnesium (Mg): 0.1~2.5%, aluminum (Al): 0.1~3.0%, the remainder being Zn and unavoidable impurities.

[0021] The above plating layer may further include one or more of the following groups (a) to (h).

[0022] (a) At least one of Si: 0.5% or less, Ni: 0.5% or less

[0023] (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

[0024] (c) Ti: 0.1% or less

[0025] (d) W: 0.5% or less

[0026] (e) Cu: 2.0% or less

[0027] (f) At least one of Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less

[0028] (g) At least one of B: 0.1% or less, P: 0.1% or less

[0029] (h) At least one of Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less

[0030] The above-mentioned plated steel sheet may have an LME maximum crack length of 30 μm or less.

[0031] According to other exemplary embodiments of the present invention, a method for manufacturing a plated steel sheet is provided. The method for manufacturing the plated steel sheet comprises the steps of: preparing a hot-rolled steel sheet by hot-rolling a steel slab comprising, in weight percent, C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, the remainder being Fe and unavoidable impurities; pickling the hot-rolled steel sheet at an average through-speed of 80~300 mpm; and providing a cold-rolled steel sheet by cold-rolling the pickled hot-rolled steel sheet at a reduction rate of 20~80%. The method may include the step of continuously annealing the above cold-rolled steel sheet at a temperature of 700 to 900°C to a dew point temperature of -20 to +30°C; and the step of immersing the continuously annealed cold-rolled steel sheet in a plating bath and then cooling it at a cooling rate of 3 to 30°C / s to obtain a plated steel sheet.

[0032] The holding time of the above continuous annealing step may be 100 to 300 s.

[0033] The above plating bath may contain, in weight percent, magnesium (Mg): 0.1~2.5%, aluminum (Al): 0.1~3.0%, the remainder being Zn and unavoidable impurities.

[0034] According to exemplary embodiments of the present invention, a high-strength plated steel sheet with improved plating properties and LME resistance and a method for manufacturing the same can be provided.

[0035] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0036] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0037] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0038] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0039] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0040] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0041] The present invention will be described in detail below through each embodiment. It should be noted that each embodiment described in this specification is not limited to a single embodiment but may also be combined with other embodiments. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0042] The present invention will be described in detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0043] [Plated Steel Sheet]

[0044] According to exemplary embodiments, the plated steel sheet can have a tensile strength of 590 MPa or more. In this way, by securing sufficient tensile strength, it can be used for automotive structural materials and the like where high strength characteristics are required.

[0045] According to exemplary embodiments, the plated steel sheet may have an LME maximum crack length of 30 μm or less. In this case, the LME maximum crack length may refer to the maximum crack length of type B that occurs when spot welding is performed using a sample prepared according to SEP 1220-2.

[0046] A plated steel sheet comprises a base steel sheet and a plating layer disposed on the surface of the base steel sheet.

[0047] The base steel sheet may contain, in weight%, C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, and the remainder being Fe and unavoidable impurities.

[0048] The composition of the aforementioned base steel sheet is explained in detail below. Unless otherwise defined, the content of each alloy component below is based on weight percent.

[0049] Carbon (C): 0.04~0.20%

[0050] Carbon (C) is an element that plays a role in improving the strength of the steel sheet. If the content of C is less than 0.04%, it may be difficult to secure the level of strength targeted by the present invention. If the content of C exceeds 0.20%, the martensite strength of the base steel sheet increases, but the difference in strength between phases with ferrite, which has a low carbon concentration, may increase. In this regard, C may be 0.04 to 0.20%. More specifically, C may be 0.04 to 0.15%.

[0051] Manganese (Mn): 0.8~3.0%

[0052] Manganese (Mn) is an element that can contribute to securing the final martensite fraction by suppressing the formation of ferrite and promoting the formation of austenite. If the Mn content is less than 0.8%, the martensite fraction decreases, making it difficult to secure strength. If the Mn content exceeds 3.0%, Mn may segregate in the thickness direction of the steel sheet, easily forming manganese bands within the slab. Consequently, the probability of defects occurring during rolling, along with continuous casting cracks, may increase. In this regard, Mn may be between 0.8% and 3.0%.

[0053] Silicon (Si): 0.3% or less (including 0%)

[0054] If the silicon (Si) content in the steel is excessive, the LME resistance of the plated steel sheet may decrease, and the plating performance may deteriorate due to reduced plating wettability. Therefore, the upper limit of the Si content may be limited to 0.3%. Since a lower Si content is preferable, the lower limit may be 0%. That is, the plated steel sheet may substantially not contain Si. More specifically, the Si content may be 0 to 0.3%. Even more specifically, the Si content may be 0 to 0.2%. Even more specifically, the Si content may be 0 to 0.1%.

[0055] Chrome (Cr): 0.8% or less (excluding 0%)

[0056] Chromium (Cr) can contribute to improving the hardenability of steel and securing high strength. Additionally, by contributing to the formation of martensite, it can contribute to minimizing the decrease in elongation relative to the increase in strength. If the Cr content exceeds 0.8%, not only are the above effects saturated, but cold rolling may also become difficult due to an excessive increase in hot-rolled strength. In this regard, the Cr content may be 0.8% or less (excluding 0%). More specifically, the Cr content may be 0.10 to 0.8%.

[0057] Aluminum (Al): 0.01~0.8%

[0058] Aluminum (Al) can be added to remove oxygen from molten steel and can contribute to improving austenite refinement and hardenability by combining with nitrogen in the steel to form AlN. Additionally, it can be utilized as a substitute element for Si as it can suppress some carbides. If the Al content is less than 0.01%, it may be difficult to sufficiently secure the aforementioned effects. If the Al content exceeds 0.8%, the likelihood of surface defects in plated steel sheets increases due to the excessive formation of inclusions during steelmaking / continuous casting, and manufacturing costs may rise. In this regard, the Al content may be between 0.01% and 0.8%.

[0059] Phosphorus (P): 0.10% or less

[0060] Phosphorus (P) is a solid solution strengthening element, but if its content exceeds 0.10%, weldability may be reduced and the risk of steel brittleness may increase. Therefore, the P content can be controlled to 0.10%. More specifically, the P content may be greater than 0% and less than or equal to 0.10%.

[0061] Sulfur (S): 0.020% or less

[0062] Sulfur (S) is an impurity element that can impair the ductility and weldability of steel sheets. Therefore, as the S content increases, the ductility and weldability of steel sheets may become inferior, so the upper limit may be limited to 0.020% in consideration of this. More specifically, the S content may be greater than 0% and less than or equal to 0.020%.

[0063] Boron (B): 0.0003~0.010%

[0064] Boron (B) is an element that improves hardenability and can contribute to strength improvement by promoting the formation of martensite. In particular, according to exemplary embodiments, by securing a relatively excessive amount of B, the strength of the plated steel sheet can be secured even while minimizing the Si content. In this regard, the B content may be 0.0003% or more. According to exemplary embodiments, a higher B content is preferable. However, if the B content increases excessively, it may become concentrated on the surface during continuous annealing, which may reduce plating performance. In this regard, the B content may be 0.010% or less. In summary, the B content may be 0.0003% to 0.010%. More specifically, the B content may be 0.0005% to 0.010%.

[0065] Niobium (Nb): 0.001~0.04%

[0066] Niobium (Nb) can contribute to an increase in strength by precipitating in the form of carbides at the austenite grain boundaries and inhibiting the growth of the austenite grain size during annealing. If the Nb content is less than 0.0010%, it may be difficult to obtain the aforementioned effect sufficiently. If the Nb content exceeds 0.04%, it may increase manufacturing costs. In this regard, the Nb content may be 0.0010% to 0.04%. More specifically, the Nb content may be 0.001% to 0.030%.

[0067] Nitrogen (N): 0.01%

[0068] Nitrogen (N) is an impurity that is inevitably introduced during the steel manufacturing process. As the N content increases, uncontrolled, non-uniform nitrides may precipitate in the base steel sheet, forming BN and potentially lowering the effective B content in the steel. Therefore, the upper limit of N can be controlled to 0.01%. While a lower N content is preferable, considering that it is an element that may inevitably be added during the manufacturing process, the lower limit may exceed 0%. In summary, the N content may be 0.01% or less. More specifically, the N content may be greater than 0% and less than or equal to 0.01%.

[0069] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the conventional steel manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the conventional manufacturing process, all details thereof are not specifically mentioned in this specification.

[0070] Optionally, according to exemplary embodiments, the base steel sheet may further include titanium (Ti).

[0071] Titanium (Ti): 0.005~0.04%

[0072] Titanium (Ti) can form nitrides to reduce the concentration of N in steel and contribute to more easily securing an effective B content. If the Ti content is less than 0.0050%, it may be difficult to sufficiently obtain the aforementioned effects. If the Ti content exceeds 0.04%, manufacturing costs increase, and strength may actually decrease due to a reduction in the carbon concentration of martensite.

[0073] According to exemplary embodiments, the substrate steel plate can satisfy the following relationship 1.

[0074] [Relationship 1]

[0075] A1 / A2<4

[0076] In the above Equation 1, A1 is the maximum value of element B analyzed by GDS in the region from the surface of the substrate steel plate to the center of thickness up to 100 nm, and A2 is the average value of element B analyzed by GDS in the region from the surface of the substrate steel plate to the center of thickness up to 100 nm. By satisfying the above-described Equation 1, the segregation of B in the extreme surface region up to a depth range of 100 nm from the surface of the substrate steel plate can be minimized. As a result, plating wettability can be improved, and furthermore, the effect of improving LME resistance due to B can be enhanced. The lower limit of the value of A1 / A2 according to the above Equation 1 is not specifically limited and may be greater than 0. More specifically, the value of A1 / A2 according to Equation 1 may be greater than 0 and less than 4. Even more specifically, the value of A1 / A2 according to Equation 1 may be greater than 0 and less than 3.

[0077] Meanwhile, when performing GDS analysis on the above-mentioned base steel sheet, the GDS analysis results of the surface of the base steel sheet can be obtained by performing GDS analysis on the surface of a cold-rolled material that has undergone the same heat treatment as the plating material, or by dissolving the plating layer of the plating material and then performing GDS analysis on the surface thereof. When dissolving the plating layer of the plating material as described above, various development solutions can be used, so the type thereof is not specifically limited. However, if the plating layer is excessively dissolved using a strong acid such as hydrochloric acid, some of the base steel and the surface layer concentrations of the base steel may be dissolved. Accordingly, when dissolving the plating layer using the above-mentioned development solution, it is preferable to perform GDS measurement after dissolving in such a way that a reaction inhibition layer (e.g., Fe-Al alloy phase) remains on the surface of the base steel. As one example, when using a development solution based on a chromic acid solution, a sample of base steel with the reaction inhibition layer remaining can be obtained by dissolving for an appropriate time according to the plating amount.

[0078] Optionally, according to exemplary embodiments, the substrate steel sheet may further satisfy the following relationship 2.

[0079] [Relationship 2]

[0080] 0 ≤ [Si] / (100*[B]) ≤ 10

[0081] In the above equation 2, [B] and [Si] represent the content (weight%) of B and Si, respectively.

[0082] In this way, by satisfying the above-described relationship Equation 2 with the content of Si and B in the substrate steel sheet, LME resistance can be improved while simultaneously ensuring good plating wettability. More specifically, the value calculated according to [Si] / (100*[B]) may be 5 or less. More specifically, the value calculated according to [Si] / (100*[B]) may be 1 or less. More specifically, the value calculated according to [Si] / (100*[B]) may be 0.5 or less.

[0083] Optionally, according to exemplary embodiments in which the base steel plate comprises titanium, the base steel plate may further satisfy the following relationship 3.

[0084] [Relationship 3]

[0085] [Ti] / 3.4[N] > 1

[0086] In the above equation 2, [Ti] and [N] represent the content (weight%) of Ti and N, respectively. By satisfying the above equation 3, the effective B content can be secured more easily, and the LME characteristics and plating wettability of the plated steel sheet can be further improved.

[0087] According to exemplary embodiments, the substrate steel sheet may include an internal oxide layer having a thickness in the thickness direction from the interface with the Zn-Mg-Al plating layer to a depth range of 0.01 to 3.0 μm. According to exemplary embodiments, the internal oxide layer may include one or more oxides of Mn, Si, Al, Fe, B, and their alloying elements. This prevents the aforementioned alloying elements from forming oxides on the surface and degrading plating performance. In particular, by minimizing the segregation of B in the extreme surface layer of the substrate steel sheet, the LME resistance and plating wettability of the plated steel sheet can be improved. The average thickness of the internal oxide layer can be determined by calculating the depth using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) on a cross-section of the substrate steel sheet. If the thickness of the internal oxide layer is less than 0.01 μm, it may be difficult to sufficiently secure the aforementioned effects.

[0088] As one example, the plating layer may be disposed on one side of the base steel sheet. As another example, the plating layer may be disposed on both sides of the base steel sheet.

[0089] The plating layer may be one or more of the following, but is not particularly limited: a Zn-based plating layer, a Zn-Mg-based plating layer, a Zn-Mg-Al-based plating layer, a Zn-Mg-Al-Si-based plating layer, an Al-based plating layer, and a combination thereof.

[0090] According to exemplary embodiments, the Zn-Mg-Al plating layer may comprise, in weight percent, magnesium (Mg): 0.1 to 2.5%, aluminum (Al): 0.1 to 3.0%, the remainder being Zn and unavoidable impurities.

[0091] Magnesium (Mg): 0.1~2.5%

[0092] Magnesium (Mg) can contribute to securing a high level of corrosion resistance through the formation of an alloy phase within the plating layer. If the Mg content is less than 0.10%, it may be difficult to sufficiently obtain the aforementioned effect. If the Mg content exceeds 2.5%, excessive dross may be generated due to Mg oxidation in the plating bath. In this regard, the Mg content may be 0.1 to 2.5%. More specifically, the Mg content may be 0.5 to 2.0%.

[0093] Aluminum (Al): 0.8~3.0%

[0094] Aluminum (Al) can contribute to suppressing the formation of Mg dross in the plating bath. If the Al content is less than 0.8%, it may be difficult to obtain the aforementioned effect sufficiently. If the Al content exceeds 3.0%, an increase in the temperature of the plating bath is required, which may lead to increased manufacturing costs and severe equipment erosion caused by Al. In this regard, the Al content may be 0.8% to 3.0%. More specifically, the Al content may be 1% to 2.5%.

[0095] The remaining component is zinc (Zn). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0096] Optionally, according to exemplary embodiments, the Zn-Mg-Al plating layer may further comprise one or more of the following groups (a) to (h).

[0097] However, since the elements of each of the following groups are not essential for achieving the objectives of the present invention, the lower limit of their content is not restricted. Therefore, the lower limit of the content of each element may be 0%, even if not specifically mentioned below.

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

[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) At least one of Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less

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

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

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

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

[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 Mg oxidation in the plating bath by forming an oxide film, but if their contents exceed 1.0%, 0.1%, 0.1%, 0.1%, and 1.0%, respectively, Ca may cause problems with increased dross due to increased oxides, and La, Ce, Y, and Sr may cause problems with reduced plating performance due to increased viscosity of the plating bath.

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

[0111] Although Ti acts as a nucleation site for Ti-Al intermetallic compounds and has a grain (spangle) refinement effect, if its content exceeds 0.1%, the melting point of the plating bath increases and there may be a problem with an increase in Dross.

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

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

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

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

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

[0117] Fe, Cr, Mn, and V have the effect of preventing electrode degradation by suppressing alloying between zinc and the welding electrode due to rapid liquid phase loss, but if their content exceeds 1.0%, 0.5%, 0.5%, and 0.5%, respectively, there may be a problem where the melting point of the plating bath rises excessively.

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

[0119] B and P have the effect of suppressing LME cracks in the weldment, but if their content exceeds 0.1% each, there may be a problem of increased Dross generation.

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

[0121] Sn, Sb, and Bi have the effect of homogenizing spangles and improving the composition within the pot by lowering the plating bath temperature, but if their content exceeds 1.0% each, there may be a problem with the coarsening of spangles.

[0122] [Method for manufacturing galvanized steel sheets]

[0123] A method for manufacturing a galvanized steel sheet may include the steps of preparing a hot-rolled steel sheet, pickling and cold-rolling the hot-rolled steel sheet, continuously annealing, and obtaining a galvanized steel sheet.

[0124] The step of preparing hot-rolled steel sheets can be carried out by hot-rolling steel slabs.

[0125] The composition of the steel slab may refer to the description of the base steel sheet of the galvanized steel sheet described above. For example, the steel slab may contain C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb: 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, and the remainder being Fe and unavoidable impurities. Regarding the composition of the steel slab, the description of the base steel sheet of the galvanized steel sheet described above may be referred to. That is, the alloy composition of the steel slab may further satisfy the relationship Equation 2 described above. The alloy composition of the steel slab may further satisfy the relationship Equation 3 described above. The alloy composition of the steel slab can further satisfy the aforementioned Equations 2 and 3.

[0126] Furthermore, the hot rolling process is not particularly limited, and conditions commonly used in the relevant technical field may be applied without restriction.

[0127] The pickling step can be performed on hot-rolled steel sheets at an average passing speed of 80 to 300 mpm. As a result, scale present on the surface of the hot-rolled steel sheets can be removed.

[0128] If the average through-plate speed is 80 mpm or less, excessive pickling may occur, resulting in loss of the base steel sheet and a decrease in productivity due to reduced production speed. On the other hand, if the average through-plate speed is 300 mpm or more, scale may remain, which may adversely affect plating quality and place an excessive load on the pickling equipment (rolls, motors, etc.).

[0129] The type of acid solution used for pickling is not specifically limited, but as a non-limiting example, an acid solution of 3 to 20 weight percent at 30 to 80 ℃ can be used.

[0130] The cold rolling step may be a step of providing a cold-rolled steel sheet by cold-rolling a pickled hot-rolled steel sheet with a reduction rate of 20 to 80%. When the reduction rate is 20% or less, there may be insufficient driving force for recrystallization during subsequent continuous annealing, and when it is 80% or more, the load on the rolling equipment is heavy and excessively high equipment costs may be incurred, so a level of 20 to 80% is desirable.

[0131] According to exemplary embodiments, cold rolling can be performed immediately after pickling. There is a risk that additional oxides may form on the surface of the steel sheet during the process of transferring the steel sheet to the cold rolling facility after pickling. Therefore, additional oxidation of the steel sheet can be prevented by minimizing the exposure time of the steel sheet between the pickling process and the cold rolling process. While there are no specific limitations on performing cold rolling immediately after pickling, as a non-limiting example, cold rolling can be performed within 0 to 300 seconds after pickling. To this end, the pickling process and the cold rolling process may be configured as a continuous process in which a hot-rolled steel sheet strip travels between an unwinder roll and a winder roll. The time for performing cold rolling after pickling can be calculated from the moment the steel sheet, having completed pickling, is withdrawn from the pickling facility and comes into contact with ambient air. The fact that the above time is 0 sec means that the pickling facility and the cold rolling facility are configured as continuous facilities, so that the steel sheet that has completed pickling is not exposed to ambient air and is immediately fed into the cold rolling facility.

[0132] The continuous annealing step can be performed on the cold-rolled steel sheet at a temperature of 700 to 900°C and a dew point temperature of -20 to +30°C. If the continuous annealing temperature is below 700°C, it may be difficult to form austenite, making it difficult to secure final strength. If the continuous annealing temperature exceeds 900°C, excessive energy costs may be required to reach and maintain the temperature. In this regard, the continuous annealing temperature can be performed in the range of 700 to 900°C. More specifically, the continuous annealing temperature can be performed in the range of 750 to 850°C.

[0133] If the dew point temperature during continuous annealing is below -20°C, the partial pressure of oxygen in the atmosphere is insufficient, making it difficult for oxygen to penetrate into the steel sheet and thus making it difficult to reduce B oxides in the extreme surface layer. Consequently, plating delamination may occur later due to B oxides at the interface between the Zn-Mg-Al plating layer and the substrate steel sheet. If the dew point temperature exceeds 30°C, there is a risk of Fe oxidation within the substrate steel sheet. In this regard, the dew point temperature can be set within the range of -20 to 30°C. More specifically, the dew point temperature can be set within the range of -10 to 30°C. Even more specifically, the dew point temperature can be set within the range of -5 to 30°C. Even more specifically, the dew point temperature can be set within the range of -10 to 25°C. Even more specifically, the dew point temperature can be set within the range of -5 to 25°C.

[0134] Furthermore, the continuous annealing process is not particularly limited, and conditions commonly used in the relevant technical field may be applied without restriction.

[0135] Optionally, according to exemplary embodiments, the heating and holding time of the continuous annealing step may be 100 to 300 seconds. If the holding time of the continuous annealing step is excessively short, the control of the internal oxide layer may not be sufficient. If the holding time of the continuous annealing step is excessively long, the alloy elements dispersed from the surface and capable of diffusing into the surface layer may have already completed sufficient reactions, making it difficult to expect additional effects. Above all, the process time may be excessively extended, leading to an excessive increase in process costs. Therefore, the heating and holding time of the continuous annealing step may be 100 to 300 seconds. More specifically, the heating and holding time of the continuous annealing step may be 100 to 230 seconds.

[0136] The step of obtaining a plated steel sheet can be performed by immersing a continuously annealed base steel sheet in a plating bath and then cooling it at a cooling rate of 3 to 30°C / s. If the cooling rate is less than 3°C / s, plating bath components may remain on the surface of the base steel sheet in an unsolidified state for a long period. In this case, flow patterns may occur on the plated surface, so a cooling rate of 3°C / s or higher is desirable. If the cooling rate exceeds 30°C / s, the plating layer solidifies rapidly, and Al components may not be sufficiently dispersed toward the surface of the plating layer. In this case, an excessive amount remains at the interface between the plating layer and the base steel sheet, which may degrade the LME properties of the plated steel sheet. Additionally, when cooling rapidly, oxidation marks may form on the surface of the plating layer due to the supply of an excessive cooling flow rate. In this regard, the cooling rate after plating may be 3 to 30°C / s. More specifically, the cooling rate after plating may be 6 to 30°C / s. More specifically, the cooling rate after plating may be 3 to 20.5°C / s. More specifically, the cooling rate after plating can be 6 to 20.5℃ / s.

[0137] The composition of the plating bath is not particularly limited. According to exemplary embodiments, the Zn-Mg-Al plating bath may comprise, in weight percent, magnesium (Mg): 0.1 to 10%, aluminum (Al): 0.1 to 20%, the remainder being Zn and unavoidable impurities. For a description of the composition of such a plating bath, refer to the description of the plating layer described above.

[0138] Furthermore, the plating process is not particularly limited, and conditions commonly used in the relevant technical field may be applied without restriction.

[0139] [Example]

[0140] The present invention will be explained in more detail below 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 rights of the present invention.

[0141] Steel slabs as shown in Table 1 below were prepared and hot-rolled, then pickled under the conditions shown in Table 2 below, and subsequently cold-rolled. Afterward, the cold-rolled steel sheets were continuously annealed and plated under the conditions shown in Table 2 below to produce plated steel sheets. During continuous annealing, the gas atmosphere was controlled to a reducing atmosphere consisting of approximately 5 to 10 volume% hydrogen and the remainder nitrogen, and the plating bath temperature was controlled to approximately 470°C. In addition, the entry temperature of the substrate steel sheet into the plating bath was 475°C. Furthermore, the composition of the plating layer of the plated steel sheet was identical to the composition of the plating bath.

[0142] Steel Grade No. Alloy Composition (Wet%) Csi Mn Al Cr Bn Nt i Relationship 2 Relationship 3A0.120.0220.0300.0020.0010.00430.0010.10.0684B0.080.121.700.030.340.0010.0130.00510.0211.31.15C0.120.51.80.030.020.00010.020.00480.022501.23D0.080.011.80.030.20.0010.020.00390.0190.11.43E0.070.021.90.030.20.0030.020.00540.0240.061.31[Relationship 2][Si] / (100*[B])[Equation 3][Ti] / 3.4[N]

[0143] Classification Steel Grade No. Pickling Passage Average Speed ​​(mpm) Continuous Annealing Plating Bath Alloy Composition (Weight%) Cooling Rate (°C / s) Dew Point Temperature (°C) Temperature (°C) Time (s) AlMgZn Comparative Example 1 A200-547741951.420.9597.6311.2 Comparative Example 2 A188-508122011.571.0297.416.5 Comparative Example 3 A22537991671.721.2397.059.7 Comparative Example 4 B231-428001881.991.2196.816.3 Comparative Example 5 B152- 517671591.391.0297.598.8 Invention Example 1B15577921721.861.3696.7612.5 Invention Example 2B188118151951.721.0597.239.3 Invention Example 3B14427712001.451.0997.4615.7 Comparative Example 6C211-557951741.911.2296.8711.5 Comparative Example 7C22558102051.811.1597.047 .1 Comparative Example 8 D241-377642172.251.3696.397.8 Comparative Example 9 D193-488041911.671.0597.2813.4 Inventive Example 3 D16558171981.751.1397.1210.7 Inventive Example 4 D17727922331.871.2596.888.4 Inventive Example 5 D195118042441.941.2996.7714.2 Comparative Example 10 E217-5277519 61.74 1.2896.9819.3 Comparative Example 11 E200-467982152.421.5596.0322.1 Inventive Example 6 E179-28021881.83 1.2396.949.8 Inventive Example 7 E18677962282.01 1.3396.6615.4 Inventive Example 8 E16414811207 1.81 1.2296.9719.7 Comparative Example 12 E33228042212.151.4196.4410.2

[0144] Table 3 shows the results of measuring the values ​​according to Equation 1, the average thickness of the internal oxide layer, the tensile strength, and the LME maximum crack length for each steel grade.

[0145] First, in order to obtain values ​​according to Equation 1, the plating layer of each steel grade was dissolved using a development solution based on chromic acid, and then GDS analysis was performed on the surface where the plating layer was dissolved to obtain A1 and A2 values. When dissolving the plating layer, the time was adjusted according to the amount of plating so that a reaction inhibition layer (Fe-Al alloy phase) remained.

[0146] Tensile strength was measured by taking tensile specimens conforming to the JIS-C standard from each plated steel sheet and performing a tensile test.

[0147] Plating wettability was determined by observing the appearance of the plating material; if there were unplated areas visible to the naked eye, it was judged to be poor, and if no unplated areas were clearly visible to the naked eye, it was judged to be good. In Table 3, "O" was indicated for good results, and "X" was indicated for cases where there was unplating visible to the naked eye.

[0148] The plating adhesion of the plated steel sheet was evaluated by applying an automotive structural adhesive to the surface of the steel sheet, drying it, bending it at 90°, and checking whether the plating layer adhered to the adhesive. The results are shown in Table 3 below. In the plating adhesion item of Table 3, "O" indicates good, "△" indicates spot delamination, and "X" indicates surface delamination or worse.

[0149] The thickness of the internal oxide layer was calculated by confirming the oxide layer using TEM.

[0150] LME resistance was evaluated by measuring the maximum LME crack length of the B-type shoulder section. To this end, spot welding was performed on two stacked galvanized steel plates in accordance with the SEP 1220-2 standard. Subsequently, the current at which expulsion occurs in the galvanized steel plates was identified, and welding was performed four times for each welding current at currents 0.2 kA and 0.5 kA lower than the upper limit current, respectively. Afterward, the cross-sections of the spot welds were subjected to electrical discharge machining (EDM) at 0°, 45°, 90°, and 135° directions. The cross-sections were then epoxy-mounted and polished, after which the presence of cracks was measured using an optical microscope. If no cracks were detected during observation using an optical microscope, it was determined that LME had not occurred; if cracks were observed, the maximum LME crack length was measured using image analysis software.

[0151] Classification Relationship 1 Plating Wetness Plating Adhesion Average Thickness of Internal Oxide Layer (㎛) Tensile Strength (MPa) B-type LME Maximum Crack Length (㎛) Comparative Example 1 1.3XX054111 Comparative Example 2 12.5XX055715 Comparative Example 3 1.9OO0 0.25500 Comparative Example 4 9.5XX065051 Comparative Example 5 7.7XX063239 Invention Example 1 1.900.762821 Invention Example 2 1.400.961517 Invention Example 3 1.600.665719 Comparative Example 6 -0069892 Comparative Example 7 -001.272277 Comparative Example 8 8.2X△06650 Comparative Example 9 10.5XX06330 Invention Example 3 1.500.26270 Invention Example 4 2.200.36620 Invention Example 5 1.300.26450 Comparative Example 10 13.7XX06830 Comparative Example 1 19.9XX06710 Invention Example 6 2.400.26650 Invention Example 7 1.500.36510 Invention Example 8 1.800.36570 Comparative Example 1 22.50△0.36630 [Relationship Formula 1] A1 / A2

[0152] Referring to Tables 1 to 3, it can be seen that the inventive examples satisfying the conditions proposed in the present invention exhibit excellent plating wettability and adhesion, and with outstanding LME resistance, all cracks are within 30 νm. In particular, in the case of Inventive Examples 3 to 8, no LME cracks occurred.

[0153] On the other hand, comparative examples that did not satisfy the conditions proposed in the present invention exhibited problems in one or more of the following items: material, plating wettability and adhesion, and LME resistance. In the case of Comparative Examples 1 and 2, plating adhesion and material could not be secured, while Comparative Example 3 satisfied plating adhesion and LME resistance but lacked strength. Comparative Examples 4 and 5 had inferior plating adhesion and LME cracks exceeding 30 νm. Comparative Examples 6 and 7 were steel grades that did not satisfy Equation 2; due to high Si content and low B content, LME resistance was poor, and although other process conditions were satisfied, sufficient improvement in LME was not achieved. Comparative Examples 8 to 11 satisfied Equation 2 and had excellent LME resistance as almost no Si was added, but plating adhesion was inferior because an internal oxide layer was not sufficiently formed. In the case of Comparative Example 12, the pickling speed prior to cold rolling was excessively fast, resulting in some scale remaining and poor plating adhesion.

[0154] Although the invention has been described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. A base steel sheet comprising, in wt%, C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, and the remainder being Fe and unavoidable impurities; and It includes a plating layer disposed on the surface of the above-mentioned base steel plate, and Tensile strength of 590 MPa or higher, A plated steel sheet in which the above-mentioned base steel sheet satisfies the following relationship 1. [Relationship 1] A1 / A2<4 (In the above Equation 1, A1 is the maximum value of element B analyzed by GDS in the region from the surface of the base steel plate to the center of thickness up to 100 nm, and A2 is the average value of element B analyzed by GDS in the region from the surface of the base steel plate to the center of thickness up to 100 nm.) 2. In Paragraph 1, The above-mentioned base steel plate is a plated steel plate that further satisfies the following relationship 2. [Relationship 2] 0 ≤ [Si] / (100*[B]) ≤ 10 (In the above Equation 2, [B] and [Si] represent the content (weight%) of B and Si, respectively.) 3. In Paragraph 1, The above-mentioned base steel sheet is a plated steel sheet further comprising, in weight%, titanium (Ti): 0.005~0.04%.

4. In Paragraph 3, The above-mentioned base steel plate is a plated steel plate that further satisfies the following relationship 3. [Relationship 3] [Ti] / 3.4[N] > 1 (In the above Equation 2, [Ti] and [N] represent the content (weight%) of Ti and N, respectively.) 5. In Paragraph 1, The above plating layer comprises, in weight percent, magnesium (Mg): 0.1~2.5%, aluminum (Al): 0.1~3.0%, the remainder being Zn and unavoidable impurities, in a plated steel sheet.

6. In Paragraph 5, The above plating layer is a plated steel sheet further comprising one or more of the following groups (a) to (h). (a) At least one of Si: 0.5% or less, Ni: 0.5% or less (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 (c) Ti: 0.1% or less (d) W: 0.5% or less (e) Cu: 2.0% or less (f) At least one of Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less (g) At least one of B: 0.1% or less, P: 0.1% or less (h) At least one of Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less 7. In Paragraph 1, The above-mentioned galvanized steel sheet is a galvanized steel sheet having an LME maximum crack length of 30 μm or less.

8. A step of preparing a hot-rolled steel sheet by hot-rolling a steel slab containing, in weight%, C: 0.04~0.20%, Mn: 0.8~3.0%, Si: 0.3% or less (including 0%), Cr: 0.8% or less (excluding 0%), Al: 0.01~0.8%, P: 0.10% or less, S: 0.020% or less, Nb 0.001~0.04%, B: 0.0003~0.010%, N: 0.01% or less, and the remainder being Fe and unavoidable impurities; A step of pickling the above hot-rolled steel sheet at an average through-speed of 80 to 300 mpm; A step of providing a cold-rolled steel sheet by cold-rolling the above-mentioned pickled hot-rolled steel sheet at a reduction rate of 20 to 80%; A step of continuously annealing the above cold-rolled steel sheet at a temperature of 700 to 900°C to a dew point temperature of -20 to +30°C; and A method for manufacturing a plated steel sheet comprising the step of immersing a continuously annealed cold-rolled steel sheet in a plating bath, and then cooling it at a cooling rate of 3 to 30°C / s to obtain a plated steel sheet.

9. In Paragraph 8, A method for manufacturing a plated steel sheet in which the holding time of the above continuous annealing step is 100 to 300 s.

10. In Paragraph 8, A method for manufacturing a plated steel sheet, wherein the plating bath comprises, in weight percent, magnesium (Mg): 0.1~2.5%, aluminum (Al): 0.1~3.0%, the remainder being Zn and unavoidable impurities.