Cold-rolled steel sheet and plated steel sheet having excellent surface quality, and method for manufacturing same

Abstract

With an aim of providing a steel suitable as an automotive outer panel material, an aspect of the present invention provides a cold-rolled steel sheet and a plated steel sheet having excellent surface quality, and a methods for manufacturing same.

Description

Cold-rolled steel sheets, galvanized steel sheets with excellent surface quality, and methods for manufacturing the same

[0001] The present invention relates to cold-rolled steel sheets and galvanized steel sheets having excellent surface quality, and methods for manufacturing the same. More specifically, the present invention relates to cold-rolled steel sheets and galvanized steel sheets having excellent surface quality suitable as automotive exterior panel materials, and methods for manufacturing the same.

[0002] Materials produced by press-forming cold-rolled galvanized steel sheets are primarily used as exterior panels for automobiles. Since these exterior panels are press-formed into various shapes, the cold-rolled galvanized steel sheets used for press processing are required to possess excellent surface quality and formability. To improve the workability of automotive steel sheets, there is a so-called IF steel (Interstitial Free Steel) that enhances formability by adding Ti or Nb, either individually or in combination, to ultra-low carbon cold-rolled steel sheets. This process precipitates solid solution elements such as C, N, and S in the form of carbides and nitrides, thereby increasing elongation and plastic deformation ratios. Consequently, conventional methods limit aging phenomena caused by solid solution elements by achieving high purity during the steelmaking stage and adding carbonitride-forming elements, such as Ti, which can fix solid solution elements, to precipitate them.

[0003] Generally, the manufacturing technology of IF steel is ND / <111> To develop the texture, the grain size of the hot-rolled structure is refined through a rapid cooling facility immediately after finishing rolling, resulting in excellent workability in the deformation of the deep drawing mode. However, ultra-low carbon steels with Ti, Nb alone or in combination have been continuously problematic, such as causing poor paintability due to surface curvature caused by non-uniformity of the surface texture, as behaviors such as recrystallization temperature vary depending on the size and fraction of (Ti,Nb)C composite carbides.

[0004] [Prior Art Literature]

[0005] (Patent Document 1) Japanese Published Patent Application No. 1992-280943

[0006] According to one embodiment of the present invention, in providing steel suitable as an exterior panel material for automobiles, a cold-rolled steel sheet with excellent surface quality, a galvanized steel sheet, and a method for manufacturing the same may be provided.

[0007] 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 contents of this specification.

[0008] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight percent, C: 0.0010~0.0030%, Si: 0.030% or less, Mn: 0.06%~0.15%, Al: 0.001~0.100%, Ti: 0.020%~0.080%, Nb: 0.005~0.017%, P: 0.030% or less, and S: 0.0100% or less, and the remainder being Fe and unavoidable impurities, and having a microstructure having a ferrite matrix structure, the area fraction of the R-cube texture of the steel sheet surface layer is 10.0 area% or less, and the density function (ODF) of the three-dimensional crystal orientation (ND plane) of the steel sheet surface layer (ND plane) {Φ1,Φ,Φ2} is such that when Φ is 0°, Φ1 is 0°, and Φ2 is 45° The strength of ODF{0°,0°,45°} is 2.00 or less, and the strength of ODF{30°,55°,45°} when Φ is 55°, Φ1 is 30°, and Φ2 is 45° may be 5.00 or more.

[0009] The area fraction of the ferrite described above is 95 area% or more, and the average grain size of the ferrite described above may be 18 to 25 μm.

[0010] The above-described cold-rolled steel sheet may contain (Ti,Nb)C precipitates within the above-described matrix structure.

[0011] The above-described cold-rolled steel sheet may have an area fraction of 50% or more of γ-fiber structure in the surface layer of the steel sheet.

[0012] A plated steel sheet according to another embodiment of the present invention may include a zinc-based plating layer provided on at least one surface of the cold-rolled steel sheet described above.

[0013] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention comprises the steps of: reheating a steel slab comprising, in weight percent, C: 0.0010~0.0030%, Si: 0.030% or less, Mn: 0.06%~0.15%, Al: 0.001~0.100%, Ti: 0.020%~0.080%, Nb: 0.005~0.017%, P: 0.030% or less, and S: 0.0100% or less, with the remainder being Fe and unavoidable impurities; finishing hot-rolling the steel slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; and pickling the hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet. The above cold-rolled steel sheet may include a step of annealing, and in the step of obtaining the cold-rolled steel sheet, the reduction rate of the last stand during cold rolling may be 2.5 to 16.0%.

[0014] The temperature in the reheating step described above is 1050~1200℃, the finishing rolling temperature during hot rolling in the step of obtaining the hot-rolled steel sheet described above is 890℃~950℃, the coiling temperature in the coiling step described above is 690~750℃, and the annealing temperature in the annealing step described above may be 780~860℃.

[0015] The manufacturing method of the cold-rolled steel sheet described above may have a cumulative reduction rate of 75.0 to 83.0% during cold rolling.

[0016] The above-described method for manufacturing cold-rolled steel sheets may utilize a 5-stand mill during cold rolling, and the reduction rates at each stand may be 30.0~40.0%, 25.0~37.0%, 25.0~32.0%, 15.0~24.0%, and 2.5~16.0%, respectively.

[0017] A method for manufacturing a galvanized steel sheet according to another embodiment of the present invention may include the step of preparing a cold-rolled steel sheet obtained according to the method for manufacturing a cold-rolled steel sheet described above; and the step of hot-dip galvanizing the cold-rolled steel sheet to obtain a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the base steel sheet.

[0018] According to the present invention, a plated steel sheet with excellent surface quality and a cold-rolled steel sheet suitable for obtaining such a plated steel sheet can be provided.

[0019] Thus, the present invention can be suitably applied to automobile exterior panel materials.

[0020] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0021] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0022] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0023] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0024] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0025] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. 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.

[0026] The inventors of the present invention have conducted in-depth research on methods to achieve uniformity of the surface of the substrate (base steel plate) applied to the part, as this becomes increasingly important in the manufacture of automotive exterior panel parts having a painted appearance.

[0027] As a result, recognizing the importance of securing uniform and fine grains on the surface of the substrate (base steel plate), we sought to optimize the content of specific elements in the alloy composition and the microstructure.

[0028] In particular, the inventors confirmed that by using ultra-low carbon steel as a base component to ensure high formability and controlling the pancake structure generated during cold rolling, they can provide steel with improved surface quality suitable for use as an automotive body panel material, and thus completed the present invention.

[0029] The present invention will be described in detail below.

[0030] A cold-rolled steel sheet according to one embodiment of the present invention may comprise, in weight percent, C: 0.0010~0.0030%, Si: 0.001~0.030%, Mn: 0.06%~0.15%, Al: 0.001~0.100%, Ti: 0.020%~0.080%, Nb: 0.005~0.017%, P: 0.030% or less, and S: 0.0100% or less, and the remainder being Fe and unavoidable impurities.

[0031] The content of each ingredient and the reason for its limitation are explained below.

[0032] C: 0.0010~0.0030%

[0033] Carbon (C) is an interstitial solid solution element that significantly influences the formation of the texture of steel sheets during cold rolling and annealing processes. When the amount of dissolved carbon in the steel increases, the growth of grains with a {111} gamma (γ)-fiber texture, which is favorable for drawing, is suppressed, while the growth of grains with {110} and {100} textures is promoted, thereby reducing the drawability of the annealed cold-rolled steel sheet. Furthermore, if the content of C exceeds 0.0030%, there is a problem in that fine Ti(Nb)C precipitates are distributed in large quantities within the steel, causing a rapid decrease in drawability. Accordingly, according to one embodiment of the present invention, the C may be included in an amount of 0.0030% or less. Meanwhile, according to another embodiment of the present invention, the C may be included in an amount of 0.0005% or more or 0.0010% or more.

[0034] Si: 0.030% or less

[0035] Silicon (Si) is an element that contributes to the increase in strength through solid solution strengthening, but since the present invention does not require the intentional addition of Si to ensure physical properties, the present invention may not contain Si at all. However, as an example, Si may be added in an amount of 0.001% or more. In one embodiment of the present invention, if the content of Si exceeds 0.030%, it causes scale defects on the surface of the steel sheet and impairs the plating surface characteristics; therefore, the content may be limited to 0.030% or less. According to another embodiment, Si may be included in an amount of 0.0250% or less.

[0036] Mn:0.06%~0.15%

[0037] Manganese (Mn) is a solid solution strengthening element that not only contributes to increasing the strength of steel but also has the effect of suppressing brittleness caused by S by precipitating S present in the steel as MnS. In one embodiment of the present invention, if the content of Mn is less than 0.06%, the intended strength cannot be secured, whereas if the content exceeds 0.15%, there is a risk of surface defects occurring due to oxides. Therefore, in one embodiment of the present invention, the Mn may be included in an amount of 0.06 to 0.15%, and according to another embodiment, it may be included in an amount of 0.07% or more or 0.10% or more. According to yet another embodiment, the Mn may be included in an amount of 0.14% or less.

[0038] Al: 0.001~0.100%

[0039] Aluminum (Al) combines with N present in steel to precipitate as AlN, and this precipitate contributes to improving the drawability and ductility of the steel. To this end, the present invention may include the Al in an amount of 0.001% or more. On the other hand, in one embodiment of the present invention, if the Al content exceeds 0.100%, there is a risk that internal defects in the steel sheet may occur due to the excessive formation of Al inclusions during steelmaking operations. Therefore, in one embodiment of the present invention, the Al may be included in an amount of 0.100% or less, and as another example, the Al may be included in an amount of 0.070% or less or 0.050% or less.

[0040] Ti:0.020%~0.080%

[0041] Ti is an element that significantly contributes to improving the drawability of steel sheets by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides. If the Ti content is less than 0.020%, carbonitrides may not precipitate sufficiently, resulting in inferior drawability. On the other hand, if it exceeds 0.080%, it is difficult to manage inclusions during steelmaking operations, which may lead to defects in inclusion properties; therefore, it is desirable to limit the Ti content to 0.020% to 0.080%. As another example, the Ti may be included in an amount of 0.020% to 0.070%.

[0042] Nb: 0.005~0.017%,

[0043] Niobium (Nb) is an element that improves drawability in the rolling direction and 45° direction by facilitating the formation of a texture during annealing through the precipitation of coarse (Ti,Nb)C complex carbides from dissolved carbon during hot rolling. When the content of Nb is less than 0.005%, most of the dissolved carbon in the steel precipitates as TiC, and the amount of precipitated coarse (Ti,Nb)C complex carbides is small, resulting in inferior drawability. On the other hand, when the content of Nb exceeds 0.017%, most of the dissolved carbon in the steel precipitates as NbC, and not only is the amount of precipitated (Ti,Nb)C complex carbides small, but there is also a problem of material deterioration caused by an increase in the recrystallization temperature. In this case, if very fine TiC or NbC precipitates of several nanometers are mainly precipitated at the grain boundaries, the development of gamma (γ) fibers, which are advantageous for workability during annealing recrystallization, is inhibited; therefore, it is desirable to precipitate coarser (Ti,Nb)C of at least 20 nm. Accordingly, it is desirable to limit the content of Nb to 0.005 to 0.017%. As another example, the Nb may be included in an amount of 0.006 to 0.015%.

[0044] P: 0.030% or less

[0045] Phosphorus (P) is an element that has the best solid solution effect and is effective in securing strength without significantly impairing the drawability of the steel. Since there is no problem in achieving the purpose of the present invention even if the above-mentioned P is not included at all in the steel, the present invention does not separately limit the lower limit of the above-mentioned P. However, as an example, the above-mentioned P may be included at 0.001% or more. In one embodiment of the present invention, if the P content exceeds 0.030%, there is a risk that the strength may become excessively high, or that secondary brittleness and surface streak defects may occur due to P segregation.

[0046] S: 0.0100% or less and N: 0.005% or less

[0047] Sulfur (S) and nitrogen (N) are impurities present in steel and are elements that are inevitably added during the steel manufacturing process. In terms of ensuring the weldability of steel, it is advantageous to control the content of these elements to be as low as possible. In one embodiment of the present invention, the S may be 0.0100% or less, and the N may be 0.005% or less. However, considering the level of inevitability during the steel manufacturing process, the content of S and N may each exceed 0%.

[0048] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. Since these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification. As an example, Cr, Cu, Ni, Sn, Mo, etc., may be included in the steel as impurities.

[0049] In one embodiment of the present invention, the cold-rolled steel sheet of the present invention may have the aforementioned alloy composition containing 0.0030% or less of carbon. Accordingly, in one embodiment of the present invention, the cold-rolled steel sheet of the present invention may have a matrix structure of ferrite as a microstructure. More specifically, the ferrite may be included in an area fraction of 95% or more. As a non-limiting example, the ferrite may be included in an area fraction of 100% or less.

[0050] The average grain size of the ferrite may be 18 to 25 μm. In this case, the grain size may refer to the longest diameter in one direction, such as the equivalent diameter.

[0051] If the average grain size of the ferrite exceeds 25㎛, it may be difficult to secure the desired strength. Here, the average grain size of the ferrite refers to the average value of the ferrite grain size over the entire thickness cross-section of the cold-rolled steel sheet. In addition, considering that the average grain size of the ferrite cannot become infinitely small, the average grain size of the ferrite may be 10㎛ or more.

[0052] In addition, as described above, a cold-rolled steel sheet according to one embodiment of the present invention may include (Ti,Nb)C precipitates within the matrix structure. This facilitates the formation of a texture during annealing, thereby improving drawability in the rolling direction and the 45° direction. Since the aforementioned effects can be sufficiently achieved through the aforementioned alloying elements, the present invention does not separately limit the number density, diameter, etc., of the (Ti,Nb)C precipitates.

[0053] In one embodiment of the present invention, the area fraction of the R-cube texture in the surface layer of the steel sheet may be 10.0 area% or less. At this time, the surface layer of the steel sheet may refer to a point up to 100 μm from the surface of the steel sheet in the thickness direction of the steel sheet.

[0054] The above R-cube texture (Rotated-cube texture) represents a {001} texture, and as an example of a method to analyze the area fraction of the texture, it can be analyzed using Electron Back Scattered Diffraction (EBSD).

[0055] The above R-cube texture is the most stable texture formed mainly during cold rolling, and it is characterized by having the lowest plastic anisotropy index (r) value among textures and being energetically stable, so recrystallization does not occur during annealing.

[0056] If such R-cube texture is included in an area of ​​more than 10.0%, clusters of R-cubes may appear on the surface of the steel plate due to rolling, passing, tension, etc., and the curvature of the surface texture may be visually recognized, which may result in a deterioration of surface quality. To prevent this, the present invention may limit the area fraction of the R-cube texture in the surface layer of the steel plate to 10% or less. As another example, the area fraction of the R-cube texture may be 9.0% or less or 8.5% or less.

[0057] In addition, a cold-rolled steel sheet according to one embodiment of the present invention can satisfy the following conditions representing the relationship between the alloy composition and the surface texture.

[0058] In the density function (ODF) {Φ1, Φ, Φ2} of the three-dimensional crystal orientation of the surface layer (ND plane) of the steel plate, the strength of ODF{0°, 0°, 45°} may be 2.0 or less, and the strength of ODF{30°, 55°, 45°} may be 5 or more.

[0059] The intensity of ODF{0°, 0°, 45°} represents the intensity measured in the density function of three-dimensional crystal orientation (ODF) of the surface layer when Φ is 0°, Φ1 is 0°, and Φ2 is 45°, and the intensity of ODF{30°, 55°, 45°} represents the intensity measured in the degree function of three-dimensional crystal orientation (ODF) of the surface layer when Φ is 55°, Φ1 is 30°, and Φ2 is 45°. The intensity value of the density function of crystal orientation (ODF) can be obtained through Electron Backscatter Diffraction (EBSD), and a person skilled in the art can easily measure the intensity value of the density function of crystal orientation (ODF) by utilizing ordinary technical means.

[0060] As a result of repeated research to solve the problem of poor surface formability caused by non-uniformity of the surface structure during processing, the inventor of the present invention discovered that in the density function (ODF){Φ1, Φ, Φ2} of the three-dimensional crystal orientation of the surface layer described above, the strengths of ODF{0°, 0°, 45°} and ODF{30°, 55°, 45°} satisfy specific values, thereby enabling the development of a gamma (γ)-fiber texture, which not only results in excellent surface formability but also allows it to be suitablely used as a steel material for vehicle body panels.

[0061] That is, the cold-rolled steel sheet of the present invention satisfies the above-described conditions and may have an area fraction of 50% or more of γ-fiber structure.

[0062] The above γ-fiber structure refers to a structure in which the {111} planes of the grains are arranged along the ND (Normal Direction) axis perpendicular to the rolling direction.

[0063] Forming the above γ-fiber structure can increase the plastic anisotropy index value, thereby enabling excellent drawing forming. To this end, the present invention may include the above γ-fiber structure in an area of ​​50% or more. As an example, the above γ-fiber structure may be included in an area of ​​95% or less.

[0064] A plated steel sheet according to one aspect of the present invention comprises a plating layer formed on one surface of the above-described cold-rolled steel sheet. In one embodiment of the present invention, the plating layer may be a zinc-based plating layer comprising zinc as a main component, and the composition of a plating layer conventionally applied in the art may be applied to the zinc-based plating layer in the same manner.

[0065] The cold-rolled steel sheet or galvanized steel sheet of the present invention described above may be characterized by high strength and elongation. Specifically, the cold-rolled steel sheet or galvanized steel sheet of the present invention may have physical properties such as a yield strength of 140 MPa or more, a tensile strength of 270 to 340 MPa, and an elongation of 33% or more, thus having the effect of being suitablely applicable as an exterior panel material for automobiles.

[0066] In addition, the galvanized steel sheet according to one embodiment of the present invention has a suitably developed surface texture and uniform surface grains, thereby having excellent surface quality.

[0067] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method corresponds to one example for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0068] According to one embodiment of the present invention, a cold-rolled steel sheet can be manufactured by passing a heated steel slab through the processes of [reheating - hot rolling - coiling - cold rolling - annealing], and each process step is described in detail below.

[0069] [Reheating of steel slabs]

[0070] A method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may reheat a steel slab after preparing the steel slab. Since the steel slab may have the same alloy composition as the cold-rolled steel sheet according to one embodiment of the present invention, the description of the alloy elements is replaced by the aforementioned details.

[0071] The slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0072] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0073] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in the present invention may be included in the molten steel within permissible limits.

[0074] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0075] The reheating process of the steel slab described above is a process for facilitating the hot rolling process described later.

[0076] In one embodiment of the present invention, the reheating of the steel slab can be performed in a temperature range of 1050 to 1200°C. If the heating temperature is less than 1050°C, there is a risk of rolling load occurring in the subsequent hot rolling process, whereas if the temperature exceeds 1200°C, there is a risk of surface scale defects occurring.

[0077] [Hot Rolled]

[0078] A hot-rolled steel sheet can be obtained by hot-rolling the above steel slab.

[0079] In one embodiment of the present invention, finishing hot rolling can be performed at a temperature of Ar3 or higher during the hot rolling process, and if the temperature is lower than Ar3, abnormal rolling is performed, which may cause microstructural non-uniformity.

[0080] In one embodiment of the present invention, in order to secure fine grains on the surface of the steel sheet during finishing hot rolling, finishing hot rolling can be performed in a temperature range of 890 to 950°C.

[0081] [Record]

[0082] The hot-rolled steel sheet obtained by the above hot rolling can be wound.

[0083] In one embodiment of the present invention, the coiling can be performed in a temperature range of 690 to 750°C. If the temperature during the coiling is below 690°C, a large amount of dissolved Nb, etc., is present in the steel, which has an adverse effect on recrystallization and grain growth inhibition during the subsequent annealing process. On the other hand, if the temperature exceeds 750°C, secondary scale is formed and there is a risk that the surface of the steel sheet will be degraded.

[0084] [Cold Rolled]

[0085] A cold-rolled steel sheet can be obtained by cold-rolling the above-mentioned coiled hot-rolled steel sheet while uncoiling it. At this time, the reduction rate can be controlled to obtain an intended thickness.

[0086] In one embodiment of the present invention, the cold rolling can be controlled to a cumulative reduction rate of 75.0% to 83.0%. If the reduction rate during cold rolling is less than 75.0%, the {111} texture may not grow sufficiently, and there is a risk that the formability will be inferior; on the other hand, if it exceeds 83.0%, the load on the rolling roll becomes very severe, and the shape will be inferior. According to another embodiment of the present invention, the cumulative reduction rate during cold rolling may be 79.0% to 83.0%.

[0087] In addition to the above, the present invention may use a cold rolling mill with a roll diameter of 380 to 550 mm to control the pancake structure of cold rolling and thereby influence the recrystallization behavior during annealing to secure a uniform structure. Furthermore, as an example, the diameters of the upper and lower rolling mills in the cold rolling mill may be the same or different from each other.

[0088] In addition, to simultaneously secure the texture distribution intended by the present invention and the strength values ​​of ODF{0°, 0°, 45°} and ODF{30°, 55°, 45°}, the present invention may use a multi-stand mill having two or more stands.

[0089] In particular, the inventors found that in order to achieve the above-mentioned objective, it is necessary to control the reduction rate of the last stand during cold rolling.

[0090] That is, the method for manufacturing a cold-rolled steel sheet according to one example of the present invention can have a reduction rate of 2.5 to 16.0% at the last stand.

[0091] If the reduction rate of the last stand is less than 2.5%, the {111} texture may not develop sufficiently, or the disappearance of the R-cube texture may be suppressed, and its area fraction may increase. On the other hand, if the reduction rate of the last stand exceeds 16.0%, the shape may deteriorate due to the load on the rolling rolls. Although not necessarily limited to this, the last stand rolling of such a cold rolling mill may use EDT (Electro Discharge Texturing), SBT (Shot Blast Texturing), etc.

[0092] In addition, the reduction rate of each stand in the multi-stage rolling mill can be applied in various ways within a range that does not impair the effect of the aforementioned cold rolling.

[0093] As a non-limiting example, the present invention may use a 5-stand mill as the multi-stage rolling mill, and the reduction rates at each stand may be 30.0~40.0%, 25.0~37.0%, 25.0~32.0%, 15.0~24.0%, and 2.5~16.0%, respectively.

[0094] In one embodiment of the present invention, prior to performing the cold rolling, a pickling process may be additionally performed for the purpose of removing surface scale from the coiled hot-rolled steel sheet. The pickling process may be performed under normal conditions, and such conditions are not specifically limited.

[0095] [Sodun]

[0096] Next, the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention can anneale the cold-rolled steel sheet obtained as described above.

[0097] In one embodiment of the present invention, the annealing treatment may be performed at a temperature above the recrystallization temperature to remove deformation caused by rolling and to soften the material, thereby improving processability.

[0098] In one embodiment of the present invention, the annealing treatment may be performed in a temperature range of 780 to 860°C. When the annealing treatment is performed, if the temperature is less than 780°C, sufficient recrystallization driving force cannot be secured, whereas if the temperature exceeds 860°C, there is a risk that the crystal grains will coarsen.

[0099] Hereinafter, a method for manufacturing a galvanized steel sheet according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method corresponds to one example for manufacturing a galvanized steel sheet according to one embodiment of the present invention.

[0100] A method for manufacturing a galvanized steel sheet according to one embodiment of the present invention may include the step of preparing a cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet described above; and the step of hot-dip galvanizing the cold-rolled steel sheet to obtain a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the base steel sheet. Each process step is described in detail below.

[0101] [Preparation of the grated slate]

[0102] First, a base steel sheet is prepared for performing a galvanizing process according to one embodiment of the present invention. In one embodiment of the present invention, the base steel sheet may be a cold-rolled steel sheet, and the cold-rolled steel sheet may be a cold-rolled steel sheet according to one embodiment of the present invention. That is, since it may be the aforementioned cold-rolled steel sheet, it should be noted that the content of the base steel sheet is replaced with the content of the aforementioned cold-rolled steel sheet.

[0103] [Hot-dip galvanizing]

[0104] By performing hot-dip galvanizing on the above-prepared base steel sheet, i.e., cold-rolled steel sheet, in a continuous hot-dip galvanizing line, a galvanized steel sheet with a zinc-based plating layer formed on at least one surface of the above-prepared cold-rolled steel sheet can be obtained.

[0105] In one embodiment of the present invention, hot-dip galvanizing can be performed by immersing a steel sheet substrate in a plating bath having zinc as the main component, and the components within the plating bath are not particularly limited. In other words, it is noted that it can be performed according to the usual conditions applied to manufacturing a galvanized steel sheet by a hot-dip galvanizing process.

[0106] In one embodiment of the present invention, the molten zinc plating may be performed in a temperature range of 430 to 490°C. The temperature range refers to the temperature of the plating bath.

[0107] The present invention will be described in detail below through examples. However, it should be noted that the examples described below 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.

[0108] (Example)

[0109] A steel slab with a thickness of 250 mm and having the alloy composition shown in Table 1 below was prepared, heated to 1050–1200°C, and then subjected to hot rolling, coiling, cold rolling, and annealing treatments under the conditions shown in Table 2 below to produce a cold-rolled steel sheet. Each cold-rolled steel sheet was immersed in a molten zinc plating bath at 455±5°C to perform plating, thereby obtaining a galvanized steel sheet.

[0110] The mechanical properties and microstructure of the galvanized steel sheet manufactured according to the above were evaluated, and the results are shown in Table 3 below.

[0111] The tensile properties of yield strength (YS), tensile strength (TS), and elongation (T-El) were measured using a universal tensile testing machine in accordance with ISO 6892 after taking test specimens in accordance with JIS No. 5.

[0112] Meanwhile, to confirm the grain distribution of the surface layer of the cold-rolled steel sheet used as the substrate for the galvanized steel sheet, the Orientation Imaging Microscopy (ODF) intensity values ​​of the surface layer were determined using OIM (Orientation Imaging Microscopy) analysis software from Te×SEM Laboratories (TSL) through Electron Backcattered Diffraction (EBSD) analysis on the annealed cold-rolled steel sheet. Pretreatment is very important for specimens used for EBSD observation. When pretreatment is performed through mechanical polishing, a significant depth of the surface layer is removed, which can lead to errors in determining the actual microstructure of the surface layer. Therefore, mechanical polishing was minimized, and EBSD specimens were prepared and observed using electrolytic polishing to minimize the removal of the surface layer depth.

[0113] In addition, for each EBSD specimen, the area fractions of the R-cube texture and the γ-fiber texture were calculated using Electron Back Scattered Diffraction (EBSD).

[0114] In addition, when the cross-sectional microstructure of each cold-rolled steel sheet was examined using an optical microscope, ferrite and (Ti,Nb)C precipitates with an area fraction of over 99% were observed in all steel grades.

[0115] The average effective grain size of the ferrite was measured using EBSD based on a misorientation angle of 15° or more.

[0116] The surface quality of cold-rolled steel sheets and galvanized steel sheets was classified into grades 1 to 4 according to the following criteria by preparing 600X600 specimens and applying a 3% deformation using a mini-press, and a surface grade of up to grade 2 was accepted as a good sheet.

[0117] Surface Grade Assessment

[0118] Grade 1: No wavy or wrinkled defects

[0119] Grade 2: Wavy and wrinkle-like defects are detected, but appear small and fine.

[0120] Grade 3: Wavy and wrinkled patterns are observed with the naked eye.

[0121] Grade 4: Waves and wrinkles are thick and distinct

[0122] Steel Type Alloy Composition (Wet%) CSI Mn PS Sol Al Ti Nb Invention Steel 10.00 190.00 30.09 0.01 00.00 380.03 50.02 10.010 Invention Steel 20.00 150.01 20.1 00.01 30.00 540.02 90.02 40.012 Invention Steel 30.00 220.01 00.1 30.00 90.00 240.02 40.0650.006Invention 40.00110.0240.070.0070.00290.0420.0190.013Comparison 10.00180.0150.120.0140.00310.0280.0340.018Comparison 20.00230.0080.140.0200.00350.0420.0100.035

[0123] Steel Grade Classification Finish Hot Rolling Temperature (°C) Coiling Temperature (°C) Cold Rolling Reduction Rate (%) by Stand Cold Reduction Rate (%) Annealing Temperature (°C) No.1 No.2 No.3 No.4 No.5 Inventive Steel 1 Inventive Example 19 30 715 38.3 34.6 30.8 20.6 13.2 79.5 832 Comparative Example 19 15 720 38.3 34.6 30.8 20.6 13.2 79.5 775 Inventive Steel 2 Inventive Example 28 9 5 70 5 32.8 27.8 27.3 23.4 4.5 83.0 835 Comparative Example 29 20 69 9 38.2 36.4 33.3 25.31.6 76.2 815 Inventive Steel 3 Inventive Example 391871232.827.526.522.37.681.2858 Invention Example 4 Invention Example 4935720343330161575.6856 Comparative Example 392972538.336.433.125.51.480.5822 Comparative Example 1 Comparative Example 489972138.235.531.928.10.976.5846 Comparative Example 2 Comparative Example 5911718353027.523.45.282.0833

[0124] Steel Classification grain size (㎛) YP (Mpa) TS (Mpa) EL. (%) {0°, 0°, 45°} Strength {30°, 55°, 45°} Strength γ-fiber fraction (%) Rotate cube fraction (%) Surface quality grade Inventive Steel 1 Inventive Example 1 21 165 29 34 70.5 28.9 65 41 Comparative Example 1 18 180 315 35 4.8 7.8 54 14 4 Inventive Steel 2 Inventive Example 2 23 155 300 42 1.6 85.3 51 7.9 2 Comparative Example 2 25 162 31 14 14.3 5.7 42 13 3 Inventive Steel 3 Inventive Example 3 22 159 298 43 1.5 25.6 25 28 2 Inventive Steel 4 Inventive Example 423161321390.97.23613.51 Comparative Example 319175293456.96.945164 Comparative Strong 1 Comparative Example 420182326353.454.2241133 Comparative Strong 2 Comparative Example 515235345311.84.3344112

[0125] As shown in Tables 1 to 3 above, Invention Examples 1 to 4, which satisfy all the alloy composition and manufacturing conditions proposed in the present invention, had an area fraction of Rotat cubes on the surface of the base steel sheet (cold-rolled steel sheet) of 10 area% or less. As a result, Invention Examples 1 to 4 had a yield strength of 140 MPa or more, a tensile strength of 270 to 340 MPa, and an elongation of 33% or more, and all had good material quality with surface quality within grade 2.

[0126] On the other hand, Comparative Example 1 had a low annealing temperature, so it was not possible to secure a sufficient recrystallized structure, and thus the surface quality was poor.

[0127] Comparative Examples 2 and 3 had a cold reduction rate of the last stand in the cold rolling stage that fell short of the range of the present invention, and as a result, the surface grains of the base steel sheet (cold-rolled steel sheet) were non-uniform, leading to inferior development of the texture of the surface layer, and consequently, it was difficult to secure good surface quality.

[0128] Comparative Example 4 had an excessive Nb content in the steel and a low cold reduction rate of the last stand in the cold rolling stage, so the surface quality was not good.

[0129] Finally, Comparative Example 5 had an excessive amount of Nb in the steel, so the grain size was refined, and as a result, the tensile strength exceeded the range of the present invention and the elongation was poor.

Claims

In weight percent, it comprises C: 0.0010~0.0030%, Si: 0.030% or less, Mn: 0.06%~0.15%, Al: 0.001~0.100%, Ti: 0.020%~0.080%, Nb: 0.005~0.017%, P: 0.030% or less, and S: 0.0100% or less, and the remainder consists of Fe and unavoidable impurities, In terms of microstructure, it has a ferrite matrix structure, The area fraction of the R-cube texture in the surface layer of the steel plate is 10.0 area% or less, and A cold-rolled steel sheet having a density function (ODF) of three-dimensional crystal orientations (ND plane) of the steel sheet surface layer (Φ1,Φ,Φ2), wherein the strength of the ODF{0°,0°,45°} when Φ is 0°, Φ1 is 0°, and Φ2 is 45° is 2.00 or less, and the strength of the ODF{30°,55°,45°} when Φ is 55°, Φ1 is 30°, and Φ2 is 45° is 5.00 or more. In paragraph 1, The area fraction of the above ferrite is 95 area% or more, and A cold-rolled steel sheet having an average grain size of 18 to 25 μm of the ferrite. In paragraph 1, Cold-rolled steel sheet containing (Ti,Nb)C precipitates within the above-mentioned base structure. In paragraph 1, Cold-rolled steel sheet having an area fraction of γ-fiber structure in the surface layer of the steel sheet of 50 area% or more. In any one of paragraphs 1 through 4, A plated steel sheet comprising a zinc-based plating layer provided on at least one surface of the above-mentioned cold-rolled steel sheet. A step of reheating a steel slab comprising, in weight%, C: 0.0010~0.0030%, Si: 0.030% or less, Mn: 0.06%~0.15%, Al: 0.001~0.100%, Ti: 0.020%~0.080%, Nb: 0.005~0.017%, P: 0.030% or less, and N: 0.005% or less, with the remainder being Fe and unavoidable impurities; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above steel slab; Step of winding the above hot-rolled steel sheet; A step of obtaining a cold-rolled steel sheet by pickling the above hot-rolled steel sheet and then cold-rolling it; The above cold-rolled steel sheet includes the step of annealing, A method for manufacturing a cold-rolled steel sheet, wherein the reduction rate of the last stand during cold rolling in the step of obtaining the cold-rolled steel sheet is 2.5 to 16.0%. In paragraph 6, The temperature in the above reheating step is 1050~1200℃, and In the step of obtaining the above hot-rolled steel sheet, the finishing rolling temperature during hot rolling is 890℃~950℃, and The winding temperature in the above winding step is 690~750℃, and A method for manufacturing a cold-rolled steel sheet, wherein the annealing temperature in the above-mentioned annealing step is 780 to 860℃. In paragraph 6, A method for manufacturing cold-rolled steel sheets, wherein the cumulative reduction rate during cold rolling is 75.0~83.0%. In paragraph 6, A 5-stand mill is used for cold rolling, and A method for manufacturing cold-rolled steel sheets, wherein the reduction rates at each stand are 30.0~40.0%, 25.0~37.0%, 25.0~32.0%, 15.0~24.0%, and 2.5~16.0%, respectively. A step of preparing a cold-rolled steel sheet obtained according to a method for manufacturing a cold-rolled steel sheet according to any one of claims 6 to 9; and A method for manufacturing a galvanized steel sheet, comprising the step of hot-dip galvanizing the above cold-rolled steel sheet to obtain a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the above base steel sheet.