Plated steel sheet and method for manufacturing same

Abstract

Provided, according to exemplary embodiments of the present invention, may be a plated steel sheet having excellent appearance characteristics and a method for manufacturing same. Additionally, the present invention may provide a plated steel sheet having excellent plating adhesion and a method for manufacturing same.

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 to the point where they are applied to and formed into products and used. Due to these external environments, oxidation and corrosion may occur, leading to a deterioration in the quality of the 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, if plating adhesion is poor, problems such as plating peeling may occur, which can reduce the lifespan of the galvanized steel sheet. Furthermore, if the appearance of the plating layer is poor, product reliability may be lowered.

[0004] [Prior Art Literature]

[0005] [Patent Literature]

[0006] (Patent Document 1) Korean Published Patent Application No. 10-2022-0088220.

[0007] The problem that the technical concept of the present invention aims to solve is to provide a plated steel sheet with excellent appearance characteristics and a method for manufacturing the same.

[0008] Additionally, the present invention aims to provide a plated steel sheet with excellent plating adhesion and a method for manufacturing the same.

[0009] 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.

[0010] According to exemplary embodiments for solving the problem of the present invention, a plated steel sheet is provided. The plated steel sheet comprises a base steel sheet; and a zinc plating layer provided on the surface of the base steel sheet, wherein the zinc plating layer has an average number of precipitates with a diameter of 1 μm or less observed within a measurement area where the grain boundaries of three spangles are in contact with each other when the point is positioned in a square measurement area with a side length of 20 μm on the surface of the zinc plating layer.

[0011] The above precipitates may have an average distance of 5㎛ or less to the grain boundary of the nearest spangle.

[0012] When the point where the grain boundaries of three spangles meet is positioned in a square measurement area with a side length of 20 μm on the surface of the zinc plating layer, the average number of precipitates with a diameter of 1 μm or less observed within the measurement area may be 10 or more.

[0013] The above-mentioned steel sheet may include an internal oxide layer having a thickness in the thickness direction from the interface with the zinc plating layer to a depth of 2 to 15 μm.

[0014] The above internal oxide layer may contain zinc in an area percentage of 5% or more and less than 70%.

[0015] The above precipitate may be an intermetallic compound containing Fe and Al.

[0016] According to other exemplary embodiments, a method for manufacturing a plated steel sheet is provided. The method for manufacturing a plated steel sheet comprises the step of coiling a hot-rolled steel sheet at a temperature exceeding 580°C;

[0017] The method comprises the steps of: pickling the hot-rolled steel sheet using a 3-20% by weight acid solution at 30-90°C for 15-160 seconds; cold rolling the pickled hot-rolled steel sheet to provide a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a dew point temperature of -60 to +30°C; and forming a plating layer on the surface of the annealed cold-rolled steel sheet.

[0018] The above method for manufacturing the plated steel sheet can satisfy the following relationship 1.

[0019] [Relationship 1]

[0020] 0.3<0.01*S*exp((T-580) / 120)<2.3

[0021] In the above equation 1, S is the holding time (sec) of the pickling step, and T is the coiling temperature (°C).

[0022] The above annealing step can be performed in a temperature range of 700 to 900°C.

[0023] The above hot-rolled steel sheet may be obtained by hot-rolling a steel slab containing, in weight percent, manganese (Mn): 1~5% and silicon (Si): 0.3~3%.

[0024] According to exemplary embodiments of the present invention, a plated steel sheet with excellent appearance characteristics and a method for manufacturing the same can be provided.

[0025] Additionally, the present invention can provide a plated steel sheet with excellent plating adhesion and a method for manufacturing the same.

[0026] 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.

[0027] Figure 1 is an example of an SEM image of a cross-section of a plated steel sheet set to a magnification of approximately 3000x.

[0028] Figure 2 is an example image of the region of Figure 1 analyzed by EPMA.

[0029] Figure 3 is an example image of the area for calculating the zinc fraction of the internal oxide layer.

[0030] Figure 4 is an example drawing to explain the calculation of the number of precipitates around the surface spangle.

[0031] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concepts of terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0032] In the following descriptions with reference to the drawings, identical or corresponding components are assigned the same reference numerals, and redundant descriptions thereof will be omitted.

[0033] In the following embodiments, the terms first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0034] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0035] 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.

[0036] In the present invention, when indicating the concentration (content) of each element, it refers to weight percent unless specifically otherwise defined. Unless otherwise defined, the concentration and concentration profile mentioned in the present invention refer to the concentration and concentration profile measured using GDS, i.e., a glow discharge optical emission spectrometer.

[0037] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] [Plated Steel Sheet]

[0043] The galvanized steel sheet comprises a base steel sheet and a zinc plating layer provided on the surface of the base steel sheet.

[0044] According to exemplary embodiments, the appearance quality of a plated steel sheet can be improved by controlling surface precipitates of the zinc plating layer. In particular, the appearance quality of the plated steel sheet can be improved by configuring it so that gloss is generated from the plated steel sheet itself, even without applying a separate gloss agent. However, the present invention does not completely exclude additional treatment of the surface of the plated steel sheet.

[0045] According to exemplary embodiments, when the point where the grain boundaries of three spangles meet is positioned in a square measurement area with sides of 20 μm, the average number of precipitates with a diameter of 1 μm or less observed within the measurement area may be five or more. In this way, as a large number of fine precipitates are formed adjacent to the spangle grain boundaries, diffuse reflection of light is reduced, thereby imparting gloss to the surface of the plating layer. More preferably, in terms of improving the appearance quality of the plating layer, the average number of precipitates may be 10 or more. Since a larger number of observed precipitates can increase surface gloss, a larger average number of precipitates is desirable, and there is no particular upper limit on the average number of precipitates. As a non-limiting example, the average number of precipitates may be 150 or fewer.

[0046] When calculating the average number of the above precipitates, the point where the spangle grain boundaries meet can be located at the center of the measurement area. In this case, the point of contact of the spangle grain boundaries does not necessarily have to be located at the exact center of the measurement area. As one example, the point of contact of the spangle grain boundaries may be located on an imaginary line connecting 10 μm points on two parallel sides of the measurement area. As another example, the point of contact of the spangle grain boundaries may be located in a circular area with a diameter of approximately 7.5 μm relative to the center of the measurement area. As yet another example, the point of contact of the spangle grain boundaries may be located on an imaginary line connecting 10 μm points on two parallel sides of the measurement area, but within a circular area with a diameter of approximately 7.5 μm relative to the center of the measurement area. Additionally, when multiple measurement areas are selected, each measurement area may not overlap with one another.

[0047] According to exemplary embodiments, the average distance of the precipitates to the grain boundary of the nearest spangle may be 5 μm or less. As the precipitates are positioned adjacent to the grain boundary of the spangle in this manner, the surface gloss of the plating layer can be further increased. More preferably, in terms of improving the appearance quality of the plated steel sheet, the average distance of the precipitates to the grain boundary of the nearest spangle may be 4 μm or less. The distance from the precipitate to the grain boundary of the nearest spangle refers to the shortest distance from the center of the precipitate to the grain boundary of the nearest spangle.

[0048] According to exemplary embodiments, the precipitates may include iron (Fe) and aluminum (Al). The iron may originate from the base steel sheet. The aluminum may originate from the base steel sheet or be contained within the zinc plating layer itself. In an electron probe microanalysis (EPMA) analysis performed on the aforementioned measurement area, particles composed of iron and aluminum may be identified as precipitates.

[0049] As a non-limiting example, the precipitates may contain alloying elements derived from the base steel sheet as impurities. The alloying elements derived from the base steel sheet may be manganese, chromium, and any one of their alloying elements.

[0050] If the above-described surface characteristics are present, the structure or composition of the base steel sheet and the zinc plating layer of the plated steel sheet are not particularly limited. However, below, exemplary embodiments preferred in terms of the strength of the plated steel sheet will be described.

[0051] The base steel sheet may be a steel sheet containing an excess of manganese (Mn) and silicon (Si). According to exemplary embodiments, the base steel sheet may contain, in weight percent, 1 to 5% manganese and 0.3 to 3% silicon. Manganese and silicon can improve the strength and corrosion resistance of the steel sheet. However, manganese and silicon may form an excess of oxides on the surface of the base steel sheet and impair plating properties. According to exemplary embodiments, by forming an internal oxide layer on the base steel sheet, the formation of an excessive oxide by manganese and silicon on the surface of the base steel sheet can be minimized. As a result, incomplete plating of the base steel sheet containing an excess of manganese and silicon can be prevented, and plating adhesion can be improved.

[0052] 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 zinc plating layer to a depth of 2 to 15 μm. The internal oxide layer may include silicon oxides formed during the hot rolling process and some manganese oxides. In this way, plating properties can be improved by suppressing the formation of manganese and silicon oxides on the surface of the substrate steel sheet.

[0053] According to exemplary embodiments, the internal oxide layer may contain zinc (Zn) in an area percentage of 5% or more and less than 70%. This allows for minimizing plating defects and improving plating adhesion. If the zinc content of the internal oxide layer is less than 5%, spot plating defects may occur. If the zinc content of the internal oxide layer is 70% or more, excessive Zn may penetrate into the grain boundaries of the internal oxide layer, and a portion of the internal oxide layer may separate from the substrate steel sheet, which may result in protruding defects.

[0054] The alloy composition of the base steel sheet is not particularly limited as long as it includes the aforementioned manganese and silicon content. However, considering that in the case of high-strength steel sheets containing a large amount of alloy components, severe non-plating and reduced plating adhesion may occur, as a non-limiting example, the base steel sheet may include, in weight percent, manganese: 1-5% and silicon: 0.3-3%, carbon: 0.05-1%, aluminum: 0.005-3.0%, phosphorus: 0.04% or less (excluding 0%), sulfur: 0.015% or less (excluding 0%), chromium: 1% or less (including 0%), and the remainder being Fe and unavoidable impurities. Additionally, it may further include zinc that has penetrated into the internal oxide layer in an amount of 1% or less. However, as described above, it should be noted that the present invention is not necessarily limited thereto.

[0055] The zinc plating layer is a plating layer based on zinc, and other compositions are not particularly limited. According to exemplary embodiments, the zinc plating layer may be any one of a zinc-based, a zinc-magnesium alloy-based, or a zinc-magnesium-aluminum alloy. As long as the zinc plating layer has the surface characteristics described above, its composition is not particularly limited.

[0056] According to exemplary embodiments, the zinc plating layer may comprise, in weight percent, magnesium: 0 to 10%, aluminum: 0 to 20%, the remainder being zinc, and unavoidable impurities. Magnesium may be added to the plating layer to further improve the corrosion resistance of the plated steel sheet. However, if the magnesium content becomes excessively high, an excessive amount of dross in the form of magnesium oxide (MgO) may be generated in the plating bath. Aluminum is a component added to further improve the corrosion resistance of the plated steel sheet, particularly to further improve corrosion resistance against acids. Additionally, it can suppress the generation of dross caused by the addition of magnesium. However, if the aluminum content is excessively high, it may raise the melting point of the plating bath and increase corrosion in alkaline environments.

[0057] According to exemplary embodiments, the zinc plating layer may further comprise any one alloying element selected from the following groups (a) to (h). Each group comprises alloying elements having similar effects, and only one alloying element may be selected from each group.

[0058] The elements of each of the following groups are not essential for achieving the objectives of the present invention, so there is no lower limit on their content. Therefore, the lower limit of the content of each element may be 0%, even if not specifically mentioned below.

[0059] (a) At least one of Si: 0.5% or less, Ni: 0.5% or less

[0060] 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.

[0061] (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

[0062] 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.

[0063] (c) Ti: 0.1% or less

[0064] 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.

[0065] (d) W: 0.5% or less

[0066] 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.

[0067] (e) Cu: 2.0% or less

[0068] 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.

[0069] (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

[0070] 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.

[0071] (g) At least one of B: 0.1% or less, P: 0.1% or less

[0072] 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.

[0073] (h) At least one of Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less

[0074] 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.

[0075] The zinc plating layer may contain the remaining Zn in addition to the composition described above, and may further contain other unavoidable impurities in addition to the elements described above. Since such unavoidable impurities may be unintentionally incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary steel manufacturing field, all details thereof are not specifically mentioned in this specification.

[0076] [Method for manufacturing galvanized steel sheets]

[0077] A method for manufacturing a plated steel sheet according to exemplary embodiments comprises the steps of: winding; pickling; providing a cold-rolled steel sheet; annealing; and forming a plating layer.

[0078] The coiling step may be a step of coiling a hot-rolled steel sheet at a temperature exceeding 580°C. The hot-rolled steel sheet may be obtained by hot-rolling a steel slab. At this time, the composition of the steel slab is not particularly limited, but may contain an excess amount of manganese and silicon in terms of strength and corrosion resistance. As a non-limiting example, the steel slab may be a steel material containing, in weight percent, manganese (Mn): 1 to 5% and silicon (Si): 0.3 to 3%. More specifically, the steel slab may contain, in weight percent, manganese: 1 to 5% and silicon: 0.3 to 3%, carbon: 0.05 to 1%, aluminum: 0.005 to 3.0%, phosphorus: 0.04% or less (excluding 0%), sulfur: 0.015% or less (excluding 0%), the remainder being Fe and unavoidable impurities, but the present invention is not necessarily limited thereto.

[0079] The hot rolling conditions of the steel slab are not particularly limited. As a non-limiting example, hot rolling can be performed by heating the steel slab to 1000 to 1300°C and then performing finish rolling in a temperature range of 700 to 900°C.

[0080] If the coiling temperature is 580°C or lower, an internal oxide layer may not be sufficiently formed on the surface layer of the hot-rolled steel sheet. In particular, if manganese and silicon are excessively included, plating performance may be inferior, so controlling the coiling temperature is important. Therefore, in terms of forming an internal oxide layer, it is more preferable to coil the hot-rolled slab at a temperature of 600°C or higher.

[0081] According to exemplary embodiments, the depth of the internal oxide layer formed on the surface layer of the hot-rolled steel sheet during the coiling stage may be 2 to 15 μm.

[0082] The pickling step can be performed on hot-rolled steel sheets using a 3-20% by weight acid solution at 30-90°C for 15-160 seconds. This allows for the removal of scraps in the form of iron oxides formed on the surface of the hot-rolled steel sheets. However, during the pickling process, internal oxides formed on the surface layer of the hot-rolled steel sheets may be etched away, potentially removing the internal oxide layer. Therefore, it is important to appropriately control the conditions of the pickling step to leave the internal oxide layer of the hot-rolled steel sheets intact.

[0083] If the temperature of the acid solution is below 30°C, the pickling efficiency may decrease. If the temperature of the acid solution exceeds 90°C, internal oxides of the hot-rolled steel sheet may be excessively removed, and the surface of the hot-rolled steel sheet may be damaged. In this regard, the temperature of the acid solution may be 40 to 85°C.

[0084] If the acid concentration of the acid solution is less than 3% by weight, the pickling efficiency may be low. If the acid concentration of the acid solution exceeds 20% by weight, internal oxides of the hot-rolled steel sheet may be excessively removed, and process wastewater treatment may be difficult. In this regard, the acid concentration may be 8% to 20% by weight.

[0085] In order to properly retain the internal oxide layer of the hot-rolled steel sheet, it is necessary to set the holding time of the pickling step appropriately. If the holding time of the pickling step is less than 15 seconds, the pickling efficiency may be low. If the holding time of the pickling step exceeds 160 seconds, the internal oxides of the hot-rolled steel sheet may be excessively removed. In this regard, the holding time of the pickling step may be 20 to 150 seconds.

[0086] In this way, by controlling the coiling and pickling conditions, the internal oxide layer of the hot-rolled steel sheet can be retained. Consequently, when the plating layer is subsequently formed, components of the base steel sheet diffuse into the plating layer, forming fine precipitates around the spangle grain boundaries on the surface of the plating layer. As a result, the surface of the plating layer can exhibit gloss and excellent surface quality, while the retention of the internal oxide layer can lead to superior plating adhesion.

[0087] According to exemplary embodiments, a method for manufacturing a plated steel sheet can satisfy the following relationship 1.

[0088] [Relationship 1]

[0089] 0.3<0.01*S*exp((T-580) / 120)<2.3

[0090] In the above equation 1, S is the holding time (sec) of the pickling step, and T is the coiling temperature (°C).

[0091] The coiling temperature directly affects the formation of internal oxides, and the holding time of the pickling step must be appropriately controlled to preserve the internal oxides of the hot-rolled steel sheet. Therefore, by controlling the coiling temperature and the holding time of the pickling step to satisfy the aforementioned Equation 1, the internal oxides within the hot-rolled steel sheet can be preserved, thereby securing a hot-rolled steel sheet with an internal oxide layer. Consequently, when forming the plating layer, a plating layer of excellent appearance quality can be secured, and plating adhesion can be further improved. Since the above Equation 1 is an empirically obtained value, units may not need to be defined separately, and it is sufficient to satisfy only the units of each variable.

[0092] The step of providing cold-rolled steel sheets can be performed by cold-rolling pickled hot-rolled steel sheets, and the cold-rolling conditions are not particularly limited. As a non-limiting example, cold-rolling can be performed with a reduction rate of 10 to 70%.

[0093] The annealing step can be performed on cold-rolled steel sheets at a dew point temperature of -60 to +30℃.

[0094] If the dew point during the annealing step exceeds +30°C, excessive decarburization occurs, and as the internal oxide layer becomes thicker, the strength of the plated steel sheet may be excessively weakened. Furthermore, oxides may form from the surface of the cold-rolled steel sheet to the grain boundaries of the fine recrystallized structure, inhibiting crystal growth and potentially leading to the formation of irregular fine grains. Therefore, the dew point temperature can be controlled to +30°C or lower. Additionally, according to exemplary embodiments, the internal oxide can be retained by controlling the coiling temperature and pickling conditions. Consequently, even without raising the dew point during annealing, a sufficient internal oxide layer can be maintained, thereby improving plating quality. In this regard, the dew point temperature can be controlled to -10°C or lower. More specifically, the dew point temperature can be controlled to -20°C or lower. While the lower limit of the dew point temperature is not specifically restricted, it can be controlled to -60°C or higher. Therefore, the dew point temperature can be -60 to +30℃, more specifically -60 to -10℃, and even more specifically -60 to -20℃.

[0095] According to exemplary embodiments, the annealing step can be performed in a temperature range of 700 to 900°C.

[0096] The step of forming a plating layer can be performed by forming a plating layer on the surface of an annealed cold-rolled steel sheet. The method of forming the plating layer is not particularly limited, but can be performed by immersing the annealed cold-rolled steel sheet in a plating bath having a zinc plating layer composition of any one of the exemplary embodiments described above. As a non-limiting example, the temperature at which the steel sheet enters the plating bath may be 420 to 530°C.

[0097] Optionally, after forming the plating layer, an additional alloying step may be included. The conditions of the alloying step are not particularly limited, but may be performed in a temperature range of 460 to 580°C.

[0098] [Example]

[0099] 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.

[0100] A steel slab as shown in Table 1 below was prepared, and coiling and pickling were performed under the conditions shown in Table 2 below. Afterwards, cold rolling was performed with a reduction rate of 60%, and annealing heat treatment was carried out at a dew point temperature of -40℃. Subsequently, a galvanized steel sheet was manufactured by immersing it in a galvanizing bath containing approximately 0.2% Al.

[0101] Steel gradeMn(wt%)Si(wt%)C(wt%)Al(wt%)P(wt%S(wt%)Cr(wt%)A2.00.70.10.030.010.0010.3B2.61.50.180.050.010.0010.5C 2.60.10.130.030.010.0010.7D2.21.50.20.030.010.0010E2.310.10.030.010.0010F2.30.50.070.030.010.0010.8*Remaining Fe

[0102] Classification Steel Type Coiling Temperature (°C) Pickling Time (sec) Relationship 1 Example 1 E610300.39 Example 2 F630300.46 Example 3 F6301001.50 Example 4 F6301502.28 Comparative Example 1 A530300.20 Comparative Example 2 B520300.18 Comparative Example 3 C600300.35 Comparative Example 4 D580300.3 Comparative Example 5 F630100.15 Comparative Example 6 F6301902.88 *[Relationship 1]: 0.3<0.01 *S*exp((T-580) / 120)<2.3 S is the holding time (sec) of the pickling step, and T is the coiling temperature (°C).

[0103] Subsequently, the depth (thickness) of the internal oxide layer, the zinc fraction of the internal oxide layer, the number of precipitates around the surface spangles, and the plating quality of each steel grade were measured and evaluated, and are shown in Table 3 below. The depth of the internal oxide layer was measured by using Scanning Transmission Electron Microscopy (STEM) to determine the depth of internal oxidation of silicon.

[0104] The zinc fraction of the internal oxide layer can be determined using a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA).

[0105] Figure 1 is an example of an SEM image of a cross-section of a plated steel sheet set to a magnification of approximately 3000x.

[0106] Figure 2 is an example image of the region of Figure 1 analyzed by EPMA.

[0107] Figure 3 is an example image of the area for calculating the zinc fraction of the internal oxide layer.

[0108] Referring to FIGS. 1 to 3, the interface (①) between the substrate steel sheet and the plating layer is first marked in the BSE (Back Scattered Electron) image captured by SEM. Additionally, the point of maximum depth of the internal oxide layer (②) is marked (see FIG. 1). Subsequently, the point of maximum depth of the internal oxide layer (②) is marked at the same location in the Zn map image of EPMA (see FIG. 2). Next, the area of ​​the bright region (zinc detection region) relative to the entire area from that location to the interface between the substrate steel sheet and the plating layer (the entire area of ​​the internal oxide layer) is calculated and used as the zinc fraction of the internal oxide layer. At this time, the zinc detection region generated from the plating layer at the interface is excluded from the calculated area (see FIG. 3).

[0109] As described above, the number of precipitates around the surface spangles was determined by placing the point where three spangles meet in a square measurement area with sides of approximately 20 μm, and then measuring the average number of precipitates with a diameter of 1 μm or less observed within the measurement area. At this time, particles composed of iron and aluminum were identified as precipitates through EPMA analysis of the measurement area.

[0110] Figure 4 is an example drawing to explain the calculation of the number of precipitates around the surface spangle.

[0111] From the left side of Fig. 4, an SEM image of the measurement area on the surface of a plated steel sheet, an EPMA component map targeting Fe, and an EPMA component map targeting Al are shown. As shown in Fig. 4, after positioning the point where the grain boundaries of three spangles meet in a predetermined measurement area (indicated by a square), the average number of precipitates identified through EPMA and SEM can be measured.

[0112] Among the plating quality, whether it is unplated was evaluated by observing the surface of the plated steel sheet with the naked eye and an optical microscope to see the areas where the base steel sheet was exposed on the surface without the zinc plating layer covering it.

[0113] The adhesion of the plating layer was evaluated using the Sealer Bending Test. The Sealer Bending Test is a method in which an automotive structural sealer is adhered to the surface of a plated material and cured to produce a sample; the sample is then bent to forcibly separate the sealer from the sample, and if even a small portion of the plating layer peels off from the separated sealer surface, it is considered that plating layer delamination has occurred, which is evaluated as poor adhesion.

[0114] The plating appearance was evaluated as excellent if a gloss appeared on the surface of the plated steel sheet when observed with the naked eye.

[0115] Classification Hot-rolled Internal Oxide Layer Depth (㎛) Plating Internal Oxide Layer Depth (㎛) Zinc (Area %) Average Number of Surface Precipitates Plating Quality Level of Non-plating Adhesion Plating Appearance Example 1 73.5 1215 Good Good Excellent Example 2 156 2258 Good Good Excellent Example 3 156 2872 Good Good Good Excellent Example 4 156 50 100 ≤ Good Good Excellent Comparative Example 1 -000 Non-plating Occurs Good Poor Comparative Example 2 -000 Non-plating Occurs Peeling Occurs Poor Comparative Example 3 -000 Good Good Poor Comparative Example 4 ≤3 1.5 34 Non-plating Occurs Good Poor Comparative Example 5 156 -0 Non-plating Occurs Peeling Occurs Poor Comparative Example 6 150 -0 Non-plating Occurs Peeling Occurs Poor

[0116] Referring to Table 3, the plating quality of Examples 1 to 4, which satisfy the conditions presented in the present invention, was excellent. However, Comparative Examples 1 to 6, which do not satisfy the conditions presented in the present invention, showed incomplete plating and / or plating peeling, and the appearance quality was poor. Although the invention has been described with reference to the above examples, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. Base steel plate; and It includes a zinc plating layer provided on the surface of the above-mentioned steel plate, and The above zinc plating layer is, A plated steel sheet having an average number of precipitates with a diameter of 1 μm or less observed within a measurement area of ​​a square with a side length of 20 μm on the surface of the zinc plating layer, wherein the point where the grain boundaries of three spangles meet is positioned within the measurement area, and the number of precipitates with a diameter of 1 μm or less observed within the measurement area is 5 or more.

2. In Paragraph 1, The above precipitates are plated steel sheets in which the average distance to the grain boundary of the nearest spangle is 5㎛ or less.

3. In Paragraph 1, A plated steel sheet having an average number of precipitates with a diameter of 1 μm or less observed within a measurement area of ​​a square with a side of 20 μm on the surface of the zinc plating layer, wherein the point where the grain boundaries of three spangles meet is positioned within the measurement area, and the number of precipitates with a diameter of 1 μm or less observed within the measurement area is 10 or more.

4. In Paragraph 1, The above-mentioned base steel plate is a plated steel plate comprising an internal oxide layer having a thickness in the thickness direction from the interface with the zinc plating layer to a depth range of 2 to 10 μm.

5. In Paragraph 4, The above internal oxide layer is a plated steel sheet containing zinc in an area percentage of 5% or more and less than 70%.

6. In Paragraph 1, The above-mentioned precipitate is a plated steel sheet that is an intermetallic compound containing Fe and Al.

7. A step of coiling hot-rolled steel sheets at a temperature exceeding 580℃; A step of pickling the above hot-rolled steel plate for 15 to 160 seconds using an acid solution of 3 to 20 weight percent at 30 to 90°C; A step of providing a cold-rolled steel sheet by cold-rolling a pickled hot-rolled steel sheet; A step of annealing the above cold-rolled steel sheet to a dew point temperature of -60 to +30℃; and A method for manufacturing a plated steel sheet comprising the step of forming a plating layer on the surface of an annealed cold-rolled steel sheet.

8. In Paragraph 7, The above method for manufacturing a plated steel sheet is a method for manufacturing a plated steel sheet satisfying the following relationship 1. [Relationship 1] 0.3<0.01*S*exp((T-580) / 120)<2.3 (In the above Equation 1, S is the holding time (sec) of the pickling step, and T is the winding temperature (°C).) 9. In Paragraph 7, A method for manufacturing a plated steel sheet in which the above annealing step is performed in a temperature range of 700 to 900℃.

10. In Paragraph 7, A method for manufacturing a plated steel sheet, wherein the above hot-rolled steel sheet is obtained by hot-rolling a steel slab containing, in weight percent, manganese (Mn): 1~5% and silicon (Si): 0.3~3%.

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