High-strength plated steel sheet with excellent formability and surface quality and its manufacturing method

Abstract

According to one aspect of the present invention, it is possible to provide a plated steel sheet that has high strength characteristics, excellent formability, and excellent surface quality, and a method for manufacturing the same.

Description

[Technical field]

[0001] The present invention relates to a plated steel sheet and a manufacturing method thereof, and more particularly to a plated steel sheet which has high strength characteristics while also being excellent in formability and surface quality and can be preferably used as an automobile exterior panel material, and a manufacturing method thereof. [Background technology]

[0002] Materials obtained by pressing cold-rolled plated steel sheets are mainly used as exterior plate materials for automobiles. Since exterior plate materials for automobiles are pressed into various shapes, the cold-rolled plated steel sheets provided for press processing are required to have excellent formability. In addition, new automobile fuel efficiency improvement targets have been set as a regulatory measure against carbon dioxide emissions, and preferential tax systems have been introduced for fuel-efficient automobiles, so that there is a demand for improving automobile fuel efficiency by reducing the weight of automobile bodies. Slimming down steel sheets for automobile bodies is evaluated as the most effective means for reducing the weight of automobile bodies. On the other hand, from the viewpoint of ensuring the safety of automobile bodies, not only slimming down steel sheets for automobile bodies but also high strength is required. High strength properties and formability are difficult to achieve at the same time, and various attempts to achieve both properties are being made in the industry.

[0003] As an example, so-called IF steel (Interstitial Free Steel) has been developed by adding titanium (Ti) or niobium (Nb) alone or in combination to ultra-low carbon cold rolled steel sheet to precipitate solid solution elements such as solid solution carbon (C), nitrogen (N), and sulfur (S) in the form of carbides and nitrides, thereby increasing the elongation rate and plastic deformation ratio, and thereby improving formability.In addition, as an attempt to improve the strength of steel sheet, active research is being conducted on methods of increasing the strength of steel by adding solid solution strengthening elements such as silicon (Si), manganese (Mn), and phosphorus (P) to steel.

[0004] Phosphorus (P) is an element that can economically and effectively increase the strength of steel, but it also deteriorates the surface quality of plated steel sheets. Therefore, there are various technical difficulties in applying the method of increasing strength through the addition of phosphorus (P) to steel sheets for automotive exterior panels. Phosphorus (P) is an element that is highly susceptible to segregation, and phosphorus (P) segregated on the surface of a slab is stretched in the length direction of the steel sheet by hot rolling and cold rolling, forming a phosphorus (P) concentrated layer on the coil surface, and the phosphorus (P) concentrated layer remaining on the steel sheet surface may induce linear defects on the surface of the plated steel sheet. Therefore, although the addition of phosphorus (P) is the most economical and effective technology for increasing the strength of steel sheets, it is not actually applied to plated steel sheets for automotive exterior panels because it causes deterioration of the surface quality of the plated steel sheet.

[0005] Patent Document 1, which relates to a manufacturing method of a galvannealed steel sheet using a steel sheet having a phosphorus (P) content of 0.03% or more as a substrate, proposes a method of forming a coating layer after grinding the surface of the steel sheet according to the phosphorus (P) content in the steel sheet in order to eliminate unevenness in the steel sheet surface. Grinding the surface of the steel sheet in this way is somewhat useful for ensuring the surface quality of the coated steel sheet, but since the surface grinding work is necessary, it inevitably reduces productivity and is accompanied by the disadvantage that the yield of the steel sheet is reduced due to grinding.

[0006] Therefore, there is an urgent need for research and development of a plated steel sheet and a manufacturing method thereof that not only has high strength characteristics but also has excellent formability and surface quality, and is capable of effectively preventing a decrease in yield and productivity. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-169160 A Summary of the Invention [Problem to be solved by the invention]

[0008] According to one aspect of the present invention, it is possible to provide a plated steel sheet having high strength characteristics, excellent formability, and excellent surface quality, and a method for producing the same.

[0009] According to one aspect of the present invention, it is possible to provide a plated steel sheet and a manufacturing method thereof that can effectively prevent a decrease in recovery rate and productivity.

[0010] The object of the present invention is not limited to the above-mentioned contents. A person having ordinary skill in the art will have no difficulty in understanding further object of the present invention from the entire contents of this specification. [Means for solving the problem]

[0011] A plated steel sheet according to one embodiment of the present invention includes a base iron; and a plating layer provided on at least one surface of the base iron; the base iron has, by weight percent, carbon (C): 0.002 to 0.01%, silicon (Si): 0.1% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.1%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.1% or less, titanium (Ti): 0. The base iron contains ferrite as a matrix structure, and the base iron can satisfy the range of E defined by the following relational formula 2, which is 0 to 60. [Equation 1] YM = [P] * (1 + 0.5 * [Mo] + 0.1 * [Mn]) In the above Relational Formula 1, [P], [Mo] and [Mn] respectively represent the contents (wt%) of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the base iron. [Equation 2] E=1250*YM-5*X In the above relational expression 2, YM is calculated by the above relational expression 1, and X means the average crystal grain size (μm) of the ferrite.

[0012] In the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface portion of the base iron, the intensity of ODF {0°, 0°, 45°} can be 2.0 or less, and the intensity of ODF {30°, 55°, 45°} can be 5 or more and 9 or less.

[0013] The fraction of the ferrite may be 95 area % or more, and the average crystal grain size of the ferrite may be 15 μm or less.

[0014] The plating layer may be any one selected from a zinc-based plating layer, an aluminum-based plating layer, an alloyed zinc-based plating layer, and an alloyed aluminum-based plating layer.

[0015] The plated steel sheet may have a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more.

[0016] A method for producing a plated steel sheet according to one embodiment of the present invention includes the steps of heating a slab containing, by weight, 0.002 to 0.01% carbon (C), 0.1% or less silicon (Si), 0.4 to 1.0% manganese (Mn), 0.04 to 0.1% phosphorus (P), 0.01% or less sulfur (S), 0.005% or less nitrogen (N), 0.1% or less aluminum (S.Al), 0.005 to 0.03% titanium (Ti), 0.01 to 0.05% niobium (Nb), 0.06 to 0.1% copper (Cu), 0.0015% or less boron (B), and molybdenum (Mo) in a range in which YM defined by the following Relational Formula 1 is 0.03 to 0.1, with the remainder being Fe and other inevitable impurities, at 1100 to 1300°C; f) to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 600 to 650°C; pickling the hot-rolled steel sheet and then cold-rolling it at a rolling reduction of 70 to 83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 750 to 830°C; immersing the annealed cold-rolled steel sheet in a coating bath to obtain a coated steel sheet; and optionally alloying the coated steel sheet at a temperature range of 480 to 610°C. [Equation 1] YM = [P] * (1 + 0.5 * [Mo] + 0.1 * [Mn]) In the above Relational Formula 1, [P], [Mo] and [Mn] respectively represent the contents (wt%) of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the slab. [Equation 3] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] In the above Relational Formula 3, [P], [Ti], and [Nb] respectively represent the contents (wt%) of phosphorus (P), titanium (Ti), and niobium (Nb) contained in the slab.

[0017] The plating bath may be any one selected from a zinc-based plating bath and an aluminum-based plating bath.

[0018] The method may further include temper rolling the plated steel sheet or the alloyed plated steel sheet at a rolling reduction of 0.4 to 1.2%.

[0019] The above solutions to the problems are not intended to be exhaustive, and the various features of the present invention and their associated advantages and effects can be understood in more detail with reference to the following specific embodiments and examples. Effect of the Invention

[0020] According to one aspect of the present invention, it is possible to provide a plated steel sheet and a manufacturing method thereof which not only have favorable physical properties as an automobile exterior panel material, but also have excellent formability and surface quality while maintaining high strength characteristics, and therefore can ensure a high recovery rate.

[0021] The effects of the present invention are not limited to the above-mentioned matters, but can be interpreted as including matters that a person of ordinary skill in the art can reasonably infer from the matters described in this specification. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention relates to a high-strength plated steel sheet having excellent formability and a manufacturing method thereof, and preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

[0023] As a result of intensive research to solve the above-mentioned problems of the conventional art, the inventors of the present invention have found that by adding titanium (Ti) and / or niobium (Nb), which are strong carbonitride forming elements, to steel, solid solution elements such as carbon (C), nitrogen (N) and sulfur (S) can be minimized to ensure formability, while phosphorus (P) can be added to ensure high strength properties, and the amount of molybdenum (Mo) added can be controlled within an optimal range to effectively ensure the surface quality of the plated steel sheet, thereby completing the present invention.

[0024] Hereinafter, a plated steel sheet according to one embodiment of the present invention will be described in more detail.

[0025] A plated steel sheet according to one embodiment of the present invention includes a base iron; and a plating layer provided on at least one surface of the base iron; the base iron has, by weight percent, carbon (C): 0.002 to 0.01%, silicon (Si): 0.1% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.1%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.1% or less, titanium (Ti): 0. The base iron contains ferrite as a matrix structure, and the base iron can satisfy the range of E defined by the following relational formula 2, which is 0 to 60. [Equation 1] YM = [P] * (1 + 0.5 * [Mo] + 0.1 * [Mn]) In the above Relational Formula 1, [P], [Mo] and [Mn] respectively mean the contents (wt%) of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the base iron. [Equation 2] E=1250*YM-5*X In the above relational expression 2, YM is calculated by the above relational expression 1, and X means the average crystal grain size (μm) of the ferrite.

[0026] The steel composition contained in the base steel of the present invention will be described in more detail below. Unless otherwise indicated, % indicating the content of each element is based on weight.

[0027] Carbon (C): 0.002~0.01% Carbon (C) is an interstitial solute element and has a large effect on the formation of the texture of the steel sheet during cold rolling and annealing. When the amount of solute carbon in steel is large, the growth of crystal grains having {111} gamma (γ)-fiber texture, which is advantageous for drawing, is suppressed, and the growth of crystal grains having {110} and {100} textures is promoted, resulting in a decrease in the drawability of the annealed sheet. Furthermore, when the carbon (C) content is excessive, the titanium (Ti) and niobium (Nb) contents required for precipitating carbon (C) as carbides become large, which is not only disadvantageous from an economical perspective, but also may cause the formation of pearlite and the like, thereby decreasing the formability. Therefore, the present invention can limit the upper limit of the carbon (C) content to 0.01%. A preferable upper limit of the carbon (C) content can be 0.009%, and a more preferable upper limit of the carbon (C) content can be 0.008%. However, if the carbon (C) content is too small, sufficient strength cannot be ensured, so the present invention limits the lower limit of the carbon (C) content to 0.002%. The preferred lower limit of the carbon (C) content is 0.003%.

[0028] Silicon (Si): 0.1% or less Silicon (Si) is an element that contributes to increasing strength through solid solution strengthening. In the present invention, silicon (Si) can be added to achieve such an effect of increasing strength. The present invention does not particularly specify a lower limit for the silicon (Si) content, but 0% can be excluded from the lower limit. A preferred lower limit for the silicon (Si) content can be 0.01%, and a more preferred lower limit for the silicon (Si) content can be 0.02%. However, since an excessive silicon (Si) content can induce defects in the surface scale and deteriorate the plating surface characteristics, the present invention can limit the upper limit for the silicon (Si) content to 0.1%. A preferred upper limit for the silicon (Si) content can be 0.07%, and a more preferred upper limit for the silicon (Si) content can be 0.05%.

[0029] Manganese (Mn): 0.4-1.0% Manganese (Mn) not only contributes to increasing strength as a solid solution strengthening element, but also plays a role in precipitating sulfur (S) in steel as MnS. In order to achieve this effect, the present invention can add 0.4% or more of manganese (Mn). A preferable lower limit of the manganese (Mn) content can be 0.45%. However, since an excessive manganese (Mn) content can cause a deterioration in surface quality due to oxides, the present invention can limit the upper limit of the manganese (Mn) content to 1.0%. A preferable upper limit of the manganese (Mn) content can be 0.9%, and a more preferable upper limit of the manganese (Mn) content can be 0.8%.

[0030] Phosphorus (P): 0.04-0.1% Phosphorus (P) has the best solid solution effect and is the most effective element for ensuring the strength of steel without significantly impairing the drawability. In the present invention, 0.04% or more of phosphorus (P) can be added to achieve this effect. A preferable phosphorus (P) content can be 0.042% or more, and more preferably 0.045% or more. Meanwhile, when phosphorus (P) is added excessively, secondary embrittlement and surface linear defects due to the segregation of phosphorus (P) are a concern, so the present invention can limit the upper limit of the phosphorus (P) content to 0.1%. A preferable upper limit of the phosphorus (P) content can be 0.09%, and a more preferable upper limit of the phosphorus (P) content can be 0.08%.

[0031] Sulfur (S): 0.01% or less, Nitrogen (N): 0.005% or less Sulfur (S) and nitrogen (N) are components that are inevitably added as impurities in steel, but in order to ensure welding characteristics, it is preferable to control their contents as low as possible. Therefore, the present invention can limit the sulfur (S) content to 0.01% or less (including 0%), and the nitrogen (N) content to 0.005% or less (including 0%). In addition, taking into account the amounts that are inevitably added, the present invention can exclude 0% from the lower limits of the sulfur (S) and nitrogen (N) contents.

[0032] Aluminum (S.Al): 0.1% or less Aluminum (Al) is a component that contributes to improving the drawability and ductility of steel by precipitating AlN, and the present invention may add aluminum (Al) to ensure this effect. A preferred aluminum (Al) content may be greater than 0%, and a more preferred aluminum (Al) content may be 0.01% or more. Meanwhile, if aluminum (Al) is added in excess, aluminum (Al) inclusions may be excessively formed during steelmaking operations, which may induce internal defects in the steel sheet, so the present invention may limit the upper limit of the aluminum (Al) content to 0.1%.

[0033] Titanium (Ti): 0.005 to 0.03% Titanium (Ti) is an element that greatly contributes to improving the drawability of steel sheet by reacting with dissolved carbon and dissolved nitrogen to precipitate titanium (Ti)-based carbonitrides during hot rolling. In the present invention, titanium (Ti) can be added in an amount of 0.005% or more to ensure such an effect. A preferable lower limit of the titanium (Ti) content can be 0.007%. Meanwhile, if the titanium (Ti) content is excessive, it is difficult to control inclusions during steelmaking operations, and defects due to inclusion properties may occur, so the present invention can limit the upper limit of the titanium (Ti) content to 0.03%. A preferable upper limit of the titanium (Ti) content can be 0.025%.

[0034] Niobium (Nb): 0.01-0.05% Niobium (Nb) contributes to solute drag and precipitate pinning during hot rolling, and is the most effective element for producing very fine grains through the rolling and cooling process when the unrecrystallized region of the austenite region expands to high temperatures. In the present invention, 0.01% or more of niobium (Nb) can be added to achieve such grain refinement. A preferred lower limit of the niobium (Nb) content can be 0.02%. On the other hand, if niobium (Nb) is added excessively, the high temperature strength increases, and the load of hot rolling may become excessive, so the present invention can limit the upper limit of the niobium (Nb) content to 0.05%. A preferred upper limit of the niobium (Nb) content can be 0.045%.

[0035] Copper (Cu): 0.06-0.1% Copper (Cu) is a component that contributes to improving the strength of steel. In the present invention, 0.06% or more of copper (Cu) can be added to achieve this effect. A preferable lower limit of the copper (Cu) content can be 0.065%, and a more preferable lower limit of the copper (Cu) content can be 0.07%. On the other hand, if an excessive amount of copper (Cu) is added, it can lead to grain boundary embrittlement and increased costs, so the present invention can limit the upper limit of the copper (Cu) content to 0.1%.

[0036] Boron (B): 0.0015% or less Boron (B) is a component that prevents secondary processing embrittlement caused by the addition of phosphorus (P) in steel, and the present invention may add boron (B) to prevent secondary processing embrittlement. A preferred lower limit of the boron (B) content may be 0.0002%, and a more preferred lower limit of the boron (B) content may be 0.0004%. However, since an excessive boron (B) content is accompanied by a decrease in the ductility of the steel sheet, the present invention may limit the upper limit of the boron (B) content to 0.0015%. A preferred upper limit of the boron (B) content may be 0.001%.

[0037] Molybdenum (Mo): YM defined by the following formula 1 is in the range of 0.03 to 0.1. The inventors of the present invention have conducted extensive research into a method for preventing the deterioration of surface quality due to grain boundary segregation of phosphorus (P) while increasing strength through the addition of phosphorus (P), and have found that when an appropriate amount of molybdenum (Mo) is added to steel, molybdenum (Mo), which has a high affinity with phosphorus (P), forms a MoP compound, which effectively prevents grain boundary segregation of phosphorus (P). Therefore, the inventors of the present invention have conducted extensive research into the optimal molybdenum (Mo) content that can achieve both high strength properties and surface quality of steel, and have derived that it is possible to achieve both high strength properties and excellent surface quality of steel by controlling the content of molybdenum (Mo) added to steel within an optimal range relative to the contents of phosphorus (P) and manganese (Mn), which are components that contribute to improving the strength of steel but hinder the surface quality of steel. Therefore, the present invention can limit the content of molybdenum (Mo) so that the YM value defined by the following Relational Formula 1 satisfies a specific range. [Equation 1] YM = [P] * (1 + 0.5 * [Mo] + 0.1 * [Mn]) In Relational Formula 1, [P], [Mo], and [Mn] respectively represent the contents (wt%) of phosphorus (P), molybdenum (Mo), and manganese (Mn) contained in the base iron.

[0038] In the present invention, in order to ensure the strength of the steel, the lower limit of the YM value defined by Relation 1 can be limited to 0.03. A preferred lower limit of the YM value can be 0.035, and a more preferred lower limit of the YM value can be 0.04. A larger YM value is advantageous in terms of ensuring strength, but is disadvantageous in terms of ensuring surface quality, so the present invention can limit the upper limit of the YM value to 0.1. A preferred upper limit of the YM value can be 0.095, and a more preferred upper limit of the YM value can be 0.09.

[0039] The base steel of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and therefore, it is not possible to completely exclude them. Since these impurities are known to anyone having ordinary knowledge in this technical field, the contents of all of them will not be specifically mentioned in this specification. In addition, the addition of additional effective components other than the above-mentioned components is not completely excluded.

[0040] The base iron of the present invention may contain ferrite as a matrix structure, and may further contain other microstructures such as pearlite, which are inevitably formed, as a remaining structure. Since the base iron of the present invention contains ferrite as a matrix structure, the formability of the plated steel sheet can be effectively ensured. In the present invention, the fraction of ferrite may be 95 area % or more, and more preferably the fraction of ferrite may be 99 area % or more.

[0041] In the base steel of the present invention, E defined by the following relational expression 2 can be in the range of 0 or more and 60 or less. [Equation 2] E=1250*YM-5*X In the above Relational Formula 2, YM is calculated by Relational Formula 1, and X means the average crystal grain size (μm) of ferrite.

[0042] The inventors of the present invention have conducted extensive research into conditions for maximizing formability under conditions that achieve both high strength properties and excellent surface quality, and have found that by controlling the correlation between the YM value calculated by Relation 1 and the average crystal grain size of the ferritic structure, it is possible to achieve both high strength properties and excellent surface quality.

[0043] Since the present invention limits the E value calculated by Relational Formula 2 to 0 or more, the strength of the plated steel sheet can be effectively ensured, and a more preferable lower limit of the E value can be 10. Furthermore, since the present invention limits the E value calculated by Relational Formula 2 to 60 or less, the formability of the plated steel sheet can be effectively ensured. Furthermore, as a non-limiting example, the average crystal grain size (X) of the ferrite of the present invention can be 5 μm or more and 15 μm or less.

[0044] In the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface part of the base iron, the intensity of ODF {0°, 0°, 45°} may be 2.0 or less, and the intensity of ODF {30°, 55°, 45°} may be 5 or more and 9 or less. The surface part of the base iron may mean from the surface of the base iron to a point of 1 / 6 of the thickness of the base iron. The intensity of ODF {0°, 0°, 45°} refers to the intensity measured by the three-dimensional crystal orientation density function (ODF) of the surface part when Φ is 0°, Φ1 is 0°, and Φ2 is 45°, and the intensity of ODF {30°, 55°, 45°} refers to the intensity measured by the three-dimensional crystal orientation density function (ODF) of the surface part when Φ is 55°, Φ1 is 30°, and Φ2 is 45°. The intensity value of the crystal orientation density function (ODF) can be obtained through EBSD (Electron Backscatter Diffraction), and a person skilled in the art can easily measure the intensity value of the crystal orientation density function (ODF) using conventional technical means.

[0045] The inventors of the present invention have conducted extensive research to solve the problem of poor surface formability due to non-uniform structure in the surface portion during processing, and have discovered that when the intensities of ODF{0°,0°,45°} and ODF{30°,55°,45°} in the three-dimensional crystal orientation density function (ODF) {Φ1,Φ,Φ2} of the above-mentioned surface portion satisfy specific values, it is possible to develop a gamma (γ)-fiber texture, which not only provides excellent surface formability but also makes the steel suitable for use as a steel material for vehicle exterior panels.

[0046] Therefore, the present invention limits the intensity of the ODF {0°, 0°, 45°} in the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface portion of the base steel to 2.0 or less, and limits the intensity of the ODF {30°, 55°, 45°} to 5 or more and 9 or less, thereby effectively ensuring surface formability.

[0047] A plated steel sheet according to an embodiment of the present invention includes a plating layer formed on one surface of a base steel. The plating layer of the present invention may be a hot-dip plating layer or an alloyed plating layer, and as a non-limiting example, the plating layer of the present invention may be any one selected from a zinc-based plating layer, an aluminum-based plating layer, an alloyed zinc-based plating layer, and an alloyed aluminum-based plating layer.

[0048] The plated steel sheet according to one aspect of the present invention may have a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more. Furthermore, the plated steel sheet according to one aspect of the present invention may effectively suppress the occurrence of surface linear defects. Thus, the present invention can provide a plated steel sheet that has high strength characteristics, excellent formability, and excellent surface quality, and thus has physical properties suitable for use as an automobile exterior plate material.

[0049] Hereinafter, a method for producing a plated steel sheet according to one aspect of the present invention will be described in more detail.

[0050] A method for producing a plated steel sheet according to one embodiment of the present invention includes the steps of heating a slab having a predetermined composition at 1100 to 1300°C; f ) to obtain a hot-rolled steel sheet; pickling the hot-rolled steel sheet and then cold rolling it at a rolling reduction of 70 to 83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 750 to 830°C; immersing the annealed cold-rolled steel sheet in a coating bath to obtain a coated steel sheet; and optionally alloying the coated steel sheet at a temperature range of 480 to 610°C. [Equation 3] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] In the above Relational Formula 3, [P], [Ti] and [Nb] respectively represent the contents (wt%) of phosphorus (P), titanium (Ti) and niobium (Nb) contained in the slab.

[0051] Slab preparation and heating A steel slab having a predetermined composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the base iron described above, the description of the alloy composition of the steel slab is substituted for the description of the alloy composition of the base iron described above.

[0052] The prepared slab can be heated at 1100-1300°C. If the slab heating temperature is too low, excessive rolling loads may occur during hot rolling, so the lower limit of the slab heating temperature can be limited to 1100°C. If the slab heating temperature is too high, surface scale defects may become a problem, so the upper limit of the slab heating temperature can be limited to 1300°C.

[0053] Hot rolling The heated slab can be hot-rolled to provide a hot-rolled steel sheet. The inventors of the present invention have conducted extensive research into process conditions that can ensure excellent formability and surface quality while providing high strength properties. As a result, the inventors have confirmed that hot rolling is performed at a finish rolling temperature of Ar3 or more, but that the finish rolling temperature must be strictly controlled depending on the relative contents of phosphorus (P), titanium (Ti), and niobium (Nb) contained in the steel slab. f ) the following equation 3 was derived: [Equation 3] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] In the above Relational Formula 3, [P], [Ti], and [Nb] respectively represent the contents (wt%) of phosphorus (P), titanium (Ti), and niobium (Nb) contained in the slab.

[0054] The finish rolling temperature (T f ), the microstructure of the final steel sheet may deviate from the range intended by the present invention, and formability may be poor. f ), the microstructure of the final steel sheet may deviate from the range targeted by the present invention, and the strength characteristics or surface quality may be deteriorated. Therefore, the present invention provides a finish rolling temperature (T f ) to provide a hot-rolled steel sheet.

[0055] Winding The hot-rolled steel sheet obtained by hot rolling can be coiled in a temperature range of 600 to 650°C. If the coiling temperature is excessively low, precipitates such as Ti(Nb)C are not sufficiently generated, the amount of dissolved carbon increases, and this affects behavior such as recrystallization and grain growth during annealing, which may cause problems in ensuring the desired strength and elongation. Therefore, the present invention can limit the lower limit of the coiling temperature to 600°C. On the other hand, if the coiling temperature is excessively high, secondary scale may be generated, which may deteriorate the surface quality, so the present invention can limit the upper limit of the coiling temperature to 650°C.

[0056] Pickling and cold rolling After uncoiling the coiled hot-rolled steel sheet, pickling may be performed to remove surface scale, and after pickling, cold rolling may be performed at a rolling reduction of 70 to 83% to obtain a cold-rolled steel sheet. If the rolling reduction of the cold rolling is less than a certain level, the {111} texture may not grow sufficiently, and formability may be poor. Therefore, the present invention may limit the lower limit of the cold rolling reduction to 70%. A more preferable lower limit of the rolling reduction may be 71%. On the other hand, if the rolling reduction is excessive during cold rolling, excessive load on the rolling rolls may cause shape defects, so the present invention may limit the upper limit of the cold rolling reduction to 83%. A more preferable upper limit of the rolling reduction may be 80%.

[0057] Annealing The cold-rolled steel sheet obtained by cold rolling can be annealed by heating to a temperature equal to or higher than the recrystallization temperature. If the annealing temperature is below the certified level, the deformation caused by rolling is not sufficiently removed and ferrite recrystallization is not completed, which may result in deterioration of elongation. Therefore, the present invention may limit the lower limit of the annealing temperature to 750°C. On the other hand, if the annealing temperature is excessively high, grain growth may occur after recrystallization is completed, which may result in deterioration of strength or surface quality, so the present invention may limit the upper limit of the annealing temperature to 830°C.

[0058] Plating and selective alloying The annealed cold-rolled steel sheet can be immersed in a plating bath to form a plating layer. The plating bath used in the plating of the present invention can be a plating bath commonly used in the manufacture of plated steel sheets for automobile exterior panels, and can be preferably any one selected from a zinc-based plating bath or an aluminum-based plating bath. After immersion in the plating bath, selective alloying can be performed, and the preferred alloying temperature can be in the range of 480 to 610°C.

[0059] The plated steel sheet produced by the above-mentioned production method not only has a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more, but also has excellent surface quality. EXAMPLES

[0060] Hereinafter, the plated steel sheet and the manufacturing method thereof according to the present invention will be described in more detail with reference to specific examples. It should be noted that the following examples are provided for the purpose of understanding the present invention and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0061] (Example) A 250 mm thick steel slab having the alloy composition shown in Table 1 below was heated to 1250°C, and plated steel sheets with an average thickness of 0.85 mm were manufactured using the process conditions shown in Table 2. A zinc (Zn) plating bath containing 0.13% aluminum (Al) was used as the plating bath. Each specimen was cut in the thickness direction, and the microstructure fraction and average crystal grain size were measured using an optical microscope (OM), and the results are shown in Table 3. In addition, each specimen was electrolytically polished, and the three-dimensional crystal orientation density function (ODF) of the surface of the raw steel of each specimen (from the surface of the raw steel to 1 / 6 of the thickness of the raw steel) was measured using electron backscatter diffraction (EBSD), and the results are also shown in Table 3.

[0062] A tensile test was performed on each test piece using a tensile tester, and the tensile strength (TS), yield point (YP), and elongation at break (T-El) were measured and listed in Table 4. Specifically, after measuring the width, length, and thickness of the test piece, the test piece was attached to the tensile tester and waited until the test piece broke, and then the tensile strength, yield point, and elongation at break of the test piece were measured. The yield point is the limit stress when elastic deformation occurs, and its value is usually indicated with a 0.2% offset, the tensile strength means the value obtained by dividing the maximum load by the original cross section, and the elongation at break is the deformation amount of the test piece after breaking from the tensile test, expressed in %,.

[0063] Meanwhile, the r-value, an index of deep drawing, was evaluated by taking JIS No. 5 tensile test pieces from each test piece in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction, and measuring the r-value of each test piece. The r-value can be calculated by measuring the change in thickness and width at the point where a tensile deformation of about 15% has been achieved in the above-mentioned tensile test, and using the ratio of the change in width to the thickness. The r-value parallel to the rolling direction is then taken as r0, and the r-value in the 45° direction is taken as r 45 , the r value in the perpendicular direction is r 90 Then, the r-value was calculated using relational expression 4 using the r-values ​​in each direction. [Equation 4] r=r0+2*r 45 +r 90 / 4

[0064] Surface linear defects are visually confirmed as black lines, and the total number of linear defects per coil can be confirmed on a surface defect detector (SDD). Surface linear defects are rated as "good" or "bad" according to the following criteria. Good: 100 or fewer linear defects on the SDD Defects: More than 100 linear defects on the SDD

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] [Table 4]

[0069] From Tables 1 to 4, it can be seen that the test pieces satisfying the alloy composition, microstructure and process conditions of the present invention not only have a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more, but also have excellent surface characteristics. On the other hand, it can be seen that the test pieces not satisfying any one or more of the alloy composition, microstructure and process conditions of the present invention cannot simultaneously secure a tensile strength of 390 MPa or more, an elongation of 28% or more, an r-value of 1.2 or more, and excellent surface characteristics.

[0070] Therefore, according to one aspect of the present invention, it is possible to provide a plated steel sheet having high strength characteristics, excellent formability, and excellent surface quality, and a manufacturing method thereof.

[0071] Although the present invention has been described in detail with reference to the embodiments, other embodiments may be possible and therefore the spirit and scope of the following claims should not be limited to the embodiments.

Claims

1. A base steel; and a plating layer provided on at least one surface of the base steel; The base iron is composed of, by weight percent, carbon (C): 0.002 to 0.01%, silicon (Si): 0.1% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.1%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.1% or less, titanium (Ti): 0.005 to 0.03%, niobium (Nb): 0.01 to 0.05%, copper (Cu): 0.06 to 0.1%, boron (B): 0.0015% or less, molybdenum (Mo): in a range where YM defined by the following relational expression 1 is 0.03 to 0.1, with the remainder being Fe and other unavoidable impurities. The base steel contains ferrite as a matrix structure, The base steel is a plated steel sheet in which E defined by the following relational expression 2 is in the range of 0 to 60. [Relationship 1] YM=[P]*(1+0.5*[Mo]+0.1*[Mn]) In the above Relational Formula 1, [P], [Mo] and [Mn] respectively represent the contents (wt%) of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the base iron. [Relationship 2] E=1250*YM-5*X In the above-mentioned Relational Formula 2, YM is calculated by the above-mentioned Relational Formula 1, and X means the average crystal grain size (μm) of the ferrite.

2. 2. The plated steel sheet according to claim 1, wherein in a three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of a surface portion of the base steel, an intensity of ODF {0°, 0°, 45°} is 2.0 or less, and an intensity of ODF {30°, 55°, 45°} is 5 or more and 9 or less.

3. The plated steel sheet according to claim 1 , wherein a fraction of the ferrite is 95 area % or more, and an average crystal grain size of the ferrite is 15 μm or less.

4. The plated steel sheet according to claim 1 , wherein the plating layer is any one selected from the group consisting of a zinc-based plating layer, an aluminum-based plating layer, an alloyed zinc-based plating layer, and an alloyed aluminum-based plating layer.

5. The plated steel sheet according to claim 1 , wherein the plated steel sheet has a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more.

6. a step of heating a slab including, in weight percent, carbon (C): 0.002-0.01%, silicon (Si): 0.1% or less, manganese (Mn): 0.4-1.0%, phosphorus (P): 0.04-0.1%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.1% or less, titanium (Ti): 0.005-0.03%, niobium (Nb): 0.01-0.05%, copper (Cu): 0.06-0.1%, boron (B): 0.0015% or less, molybdenum (Mo): YM defined by the following relational expression 1 is in the range of 0.03 to 0.1, the remainder being Fe and other inevitable impurities, at 1100 to 1300° C.; The finishing rolling temperature (T ｆ ) hot rolling the heated slab to obtain a hot rolled steel sheet; coiling the hot-rolled steel sheet at a temperature in the range of 600 to 650°C; pickling the hot-rolled steel sheet and then cold-rolling it at a rolling reduction of 70 to 83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature in the range of 750 to 830°C; immersing the annealed cold-rolled steel sheet in a plating bath to obtain a plated steel sheet; and The method for producing a plated steel sheet according to claim 1, further comprising the step of selectively alloying the plated steel sheet at a temperature in the range of 480 to 610°C. [Relationship 1] YM=[P]*(1+0.5*[Mo]+0.1*[Mn]) In the above Relational Formula 1, [P], [Mo] and [Mn] respectively represent the contents (wt%) of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the slab. [Relationship 3] 920300000000000000000000000000000000000000000000, SUGYS(3193)SIGN THIS ｆ (40200300000000000000000000000000000000000 / 0 48000000000000000000000000000 In the above-mentioned relational expression 3, [P], [Ti] and [Nb] respectively represent the contents (wt%) of phosphorus (P), titanium (Ti) and niobium (Nb) contained in the slab.

7. The method for producing a plated steel sheet according to claim 6, wherein the plating bath is any one selected from a zinc-based plating bath and an aluminum-based plating bath.

8. The method for producing a plated steel sheet according to claim 6, further comprising the step of temper rolling the plated steel sheet or the alloyed plated steel sheet at a rolling reduction of 0.4 to 1.2%.

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