High-strength plated steel sheet with excellent formability and surface quality and method for producing the same

Abstract

According to one aspect of the present invention, a plated steel sheet having high strength characteristics and excellent formability and surface quality and a method for manufacturing the same can be provided.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly to a plated steel sheet and a method for manufacturing the same, wherein the plated steel sheet has high strength characteristics and excellent formability and surface quality, and thus can be preferably used as an automobile outer panel material. Background Art

[0002] Materials obtained by stamping cold-rolled plated steel sheets are mainly used as outer panel materials for automobiles. Automobile outer panel materials are stamped into various shapes, so the cold-rolled plated steel sheets provided for stamping need to have excellent formability. In addition, as a measure to control carbon dioxide emissions, new automobile fuel efficiency improvement targets are set, and preferential tax systems are introduced for low-fuel-efficiency automobiles. It is necessary to improve the fuel efficiency of automobiles by lightweighting automobile bodies, and thinning of automobile body steel sheets is regarded as the most effective method for lightweighting automobile bodies. In addition, from the perspective of ensuring the safety of automobile bodies, not only thinning of automobile body steel sheets is required, but also high strength of automobile body steel sheets is required. High strength characteristics and formability are physical properties that are difficult to achieve at the same time, and the industry has made various attempts to achieve a balance between these two physical properties.

[0003] As an example, so-called IF steel (Interstitial Free Steel) has been developed. This steel is made by adding titanium (Ti) or niobium (Nb) singly or in combination to ultra-low carbon cold-rolled steel sheets. This allows dissolved elements such as carbon (C), nitrogen (N), and sulfur (S) to precipitate as carbides and nitrides, thereby increasing elongation and plastic strain ratio, thereby improving formability. Furthermore, as an attempt to increase the strength of steel sheets, research is actively underway to increase the strength of steel by incorporating solid solution strengthening elements such as silicon (Si), manganese (Mn), and phosphorus (P).

[0004] Phosphorus (P) is a component that can economically and effectively achieve high strength of steel, but it is also a component that deteriorates the surface quality of coated steel sheets. Therefore, there are various technical difficulties when applying the method of achieving high strength by adding phosphorus (P) to steel sheets for automobile outer panel materials. Phosphorus (P) is an element that is very easy to segregate. The phosphorus (P) segregated on the surface of the slab is stretched in the longitudinal direction of the steel sheet by hot rolling and cold rolling, forming a phosphorus (P) enriched layer on the surface of the coil, and due to the phosphorus (P) enriched layer remaining on the surface of the steel sheet, linear defects may be caused on the surface of the coated steel sheet. Therefore, although adding phosphorus (P) is the most economical and effective technology to achieve high strength of steel sheets, it causes the surface quality of the coated steel sheet to decrease, and therefore cannot be applied to coated steel sheets for automobile outer panel materials.

[0005] Patent Document 1 discloses a method for producing alloyed hot-dip galvanized steel sheet using a steel sheet having a phosphorus (P) content of 0.03% or greater as a substrate. To eliminate surface unevenness in the steel sheet, the method proposes a method for forming a coating by polishing the steel sheet surface in accordance with the phosphorus (P) content. While polishing the steel sheet surface in this manner helps to maintain the surface quality of the plated steel sheet to some extent, the necessary polishing operation inevitably reduces productivity and also results in a lower yield rate for the steel sheet.

[0006] Therefore, there is an urgent need to research and develop a plated steel sheet and a method for manufacturing the same, which has high strength characteristics, excellent formability and surface quality, and can effectively prevent the reduction of yield rate and productivity.

[0007] (Prior art literature)

[0008] (Patent Document) Japanese Patent Publication No. 2004-169160 Summary of the Invention

[0009] Technical problems to be solved

[0010] According to one aspect of the present invention, a plated steel sheet having high strength characteristics and excellent formability and surface quality and a method for manufacturing the same can be provided.

[0011] According to one aspect of the present invention, a plated steel sheet and a method for manufacturing the same can be provided, which can effectively prevent a decrease in yield and productivity.

[0012] The technical problems to be solved by the present invention are not limited to the above contents. A person skilled in the art can easily understand further technical problems of the present invention from the entire contents of this specification.

[0013] Technical Solution

[0014] According to one aspect of the present invention, a plated steel sheet may include: a base iron; and a plating layer provided on at least one side of the base iron, wherein, in terms of weight %, the base iron may contain: 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 ( The base iron may include ferrite as a matrix structure, and E of the base iron may be within a range of 0 to 60 as defined by the following relational expression 2.

[0015] [Equation 1]

[0016] YM=[P]*(1+0.5*[Mo]+0.1*[Mn])

[0017] In the relational formula 1, [P], [Mo], and [Mn] respectively represent the contents (weight %) of phosphorus (P), molybdenum (Mo), and manganese (Mn) contained in the base material iron.

[0018] [Equation 2]

[0019] E=1250*YM-5*X

[0020] In the relational expression 2, YM is calculated from the relational expression 1, and X represents the average grain size (μm) of the ferrite.

[0021] In the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface portion of the substrate 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.

[0022] The fraction of the ferrite may be greater than or equal to 95 area %, and the average grain size of the ferrite may be less than or equal to 15 μm.

[0023] The coating may be any one selected from a zinc-based coating, an aluminum-based coating, an alloyed zinc-based coating or an alloyed aluminum-based coating.

[0024] The plated steel sheet may have a tensile strength of 390 MPa or greater, an elongation of 28% or greater, and an r-value of 1.2 or greater.

[0025] According to one aspect of the present invention, a method for manufacturing a coated steel sheet may include the following steps: heating a slab at 1100-1300° C., wherein the slab comprises, in terms of weight %, 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 equation 1 satisfies the range of 0.03 or more and 0.1 or less, and the balance is Fe and other unavoidable impurities; Finishing rolling temperature (T) that satisfies the following equation 3 f ) hot rolling the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 600-650°C; pickling the hot-rolled steel sheet and then cold rolling it at a reduction ratio of 70-83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 750-830°C; immersing the annealed cold-rolled steel sheet in a plating bath to obtain a plated steel sheet; and selectively alloying the plated steel sheet at a temperature range of 480-610°C.

[0026] [Equation 1]

[0027] YM=[P]*(1+0.5*[Mo]+0.1*[Mn])

[0028] In the relational formula 1, [P], [Mo], and [Mn] represent the contents (weight %) of phosphorus (P), molybdenum (Mo), and manganese (Mn), respectively, contained in the slab.

[0029] [Equation 3]

[0030] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}]

[0031] In the relational expression 3, [P], [Ti], and [Nb] represent the contents (weight %) of phosphorus (P), titanium (Ti), and niobium (Nb), respectively, contained in the slab.

[0032] The plating bath may be any one selected from a zinc-based plating bath or an aluminum-based plating bath.

[0033] The method may further include performing temper rolling on the plated steel sheet or the alloyed plated steel sheet at a reduction ratio of 0.4-1.2%.

[0034] The solutions to the technical problems do not list all the features of the present invention. By referring to the following specific embodiments and examples, the various features of the present invention and its advantages and effects may be understood in more detail.

[0035] Beneficial effects

[0036] According to one aspect of the present invention, a plated steel sheet and a method for manufacturing the same can be provided, wherein the plated steel sheet has high strength characteristics and excellent formability and surface quality, thereby not only having preferred physical properties as an automobile outer panel material but also ensuring a high yield rate.

[0037] The effects of the present invention are not limited to the above-mentioned contents, and can be interpreted as including contents that can be reasonably deduced by those skilled in the art from the contents described in this specification.

[0038] Best Practice

[0039] The present invention relates to a high-strength plated steel sheet with excellent formability and a method for producing the same. Preferred embodiments of the present invention are described below. The embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to those skilled in the art.

[0040] As a result of in-depth research conducted by the inventors to solve the problems of the above-mentioned prior art, they confirmed that by adding titanium (Ti) and / or niobium (Nb) as strong carbonitride-forming elements to steel, the formability is ensured by minimizing the solid solution elements such as carbon (C), nitrogen (N), and sulfur (S), and at the same time, phosphorus (P) is added to ensure high strength characteristics. By controlling the addition amount of molybdenum (Mo) within an optimal range, the surface quality of the plated steel sheet can be effectively ensured, thereby completing the present invention.

[0041] Hereinafter, the plated steel sheet according to one aspect of the present invention will be described in more detail.

[0042] A plated steel sheet according to one aspect of the present invention may include: a base iron; and a plating layer provided on at least one side of the base iron, wherein, in terms of weight %, the base iron may contain: 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 ( The base iron may include ferrite as a matrix structure, and E of the base iron may be within a range of 0 to 60 as defined by the following relational expression 2.

[0043] [Equation 1]

[0044] YM=[P]*(1+0.5*[Mo]+0.1*[Mn])

[0045] In the relational formula 1, [P], [Mo], and [Mn] respectively represent the contents (weight %) of phosphorus (P), molybdenum (Mo), and manganese (Mn) contained in the base material iron.

[0046] [Equation 2]

[0047] E=1250*YM-5*X

[0048] In the relational expression 2, YM is calculated from the relational expression 1, and X represents the average grain size (μm) of the ferrite.

[0049] The steel components contained in the base iron of the present invention are described in more detail below. Unless otherwise specified, the percentages representing the content of each element are based on weight.

[0050] Carbon (C): 0.002-0.01%

[0051] Carbon (C) is an interstitial solid solution element and is an element that has a great influence on the formation of the texture of the steel sheet during cold rolling and annealing. When the amount of solid solution carbon in the steel increases, the growth of grains with a {111} γ-fiber texture that is beneficial for drawing is suppressed, and the growth of grains with {110} and {100} textures is promoted, so that the drawability of the annealed sheet decreases. Furthermore, when the carbon (C) content is too high, the content of titanium (Ti) and niobium (Nb) required to precipitate C as carbides increases, which is disadvantageous in terms of economic efficiency. In addition, pearlite and the like are formed, so that the formability may be reduced. Therefore, in the present invention, the upper limit of the carbon (C) content can be limited to 0.01%. The preferred upper limit of the carbon (C) content can be 0.009%, and the more preferred upper limit of the carbon (C) content can be 0.008%. However, if the carbon (C) content is too low, sufficient strength cannot be ensured. Therefore, in the present invention, the lower limit of the carbon (C) content may be limited to 0.002%. The preferred lower limit of the carbon (C) content may be 0.003%.

[0052] Silicon (Si): 0.1% or less

[0053] Silicon (Si) is an element that helps to improve strength through solid solution strengthening. The present invention can add silicon (Si) to exert the effect of improving strength as described above. The present invention does not particularly stipulate the lower limit of the silicon (Si) content, but 0% can be excluded from the lower limit of the silicon (Si) content. The preferred lower limit of the silicon (Si) content can be 0.01%, and the more preferred lower limit of the silicon (Si) content can be 0.02%. However, when the silicon (Si) content is too much, surface oxide scale defects are caused, which may reduce the coating surface properties. In the present invention, the upper limit of the silicon (Si) content can be limited to 0.1%. The preferred upper limit of the silicon (Si) content can be 0.07%, and the more preferred upper limit of the silicon (Si) content can be 0.05%.

[0054] Manganese (Mn): 0.4-1.0%

[0055] Manganese (Mn) is a solid solution strengthening element that not only helps to improve strength, but also plays a role in causing sulfur (S) to precipitate in the form of MnS in steel. The present invention can add more than 0.4% manganese (Mn) to achieve the above-mentioned effects. The preferred lower limit of the manganese (Mn) content can be 0.45%. However, when the manganese (Mn) content is too high, the surface quality may be reduced due to oxides. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 1.0%. The preferred upper limit of the manganese (Mn) content can be 0.9%, and the more preferred upper limit of the manganese (Mn) content can be 0.8%.

[0056] Phosphorus (P): 0.04-0.1%

[0057] Phosphorus (P) has the most excellent solid solution effect and is the most effective element for ensuring the strength of steel without significantly impairing the drawability. In the present invention, in order to achieve the effects as described above, 0.04% or more of phosphorus (P) can be added. The preferred phosphorus (P) content can be 0.042% or more, and more preferably 0.045% or more. On the other hand, when too much phosphorus (P) is added, secondary brittleness and surface linear defects may occur due to the segregation of phosphorus (P). In the present invention, the upper limit of the phosphorus (P) content can be limited to 0.1%. The preferred upper limit of the phosphorus (P) content can be 0.09%, and the more preferred upper limit of the phosphorus (P) content can be 0.08%.

[0058] Sulfur (S): 0.01% or less, Nitrogen (N): 0.005% or less

[0059] Sulfur (S) and nitrogen (N) are components that are inevitably added as impurities to steel. However, to ensure weldability, it is preferable to keep the sulfur (S) and nitrogen (N) 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%). Furthermore, considering the amounts that are inevitably added, the present invention can exclude 0% from the lower limits of the sulfur (S) and nitrogen (N) contents.

[0060] Aluminum (S.Al): 0.1% or less

[0061] Aluminum (Al) is a component that helps improve the drawability and ductility of steel by precipitating AlN. The present invention allows for the addition of aluminum (Al) to ensure these effects. The preferred aluminum (Al) content is greater than 0%, and more preferably, the aluminum (Al) content is greater than 0.01%. On the other hand, excessive addition of aluminum (Al) can lead to excessive aluminum (Al) inclusions during steelmaking, potentially causing internal defects in the steel sheet. Therefore, the present invention limits the upper limit of the aluminum (Al) content to 0.1%.

[0062] Titanium (Ti): 0.005-0.03%

[0063] Titanium (Ti) is an element that reacts with dissolved carbon and dissolved nitrogen during hot rolling to precipitate titanium (Ti)-based carbonitrides, thereby contributing to significantly improving the drawability of steel sheets. The present invention can add more than 0.005% titanium (Ti) to ensure the effects described above. The preferred lower limit of the titanium (Ti) content can be 0.007%. On the other hand, when the titanium (Ti) content is too much, it is difficult to control inclusions during steelmaking operations, and inclusion defects may occur. Therefore, the present invention can limit the upper limit of the titanium (Ti) content to 0.03%. The preferred upper limit of the titanium (Ti) content can be 0.025%.

[0064] Niobium (Nb): 0.01-0.05%

[0065] Niobium (Nb) is an element that contributes to solute drag and precipitate pinning during hot rolling, and is most effective in producing very fine grains through the rolling and cooling processes as the unrecrystallized region of the austenite region expands at high temperatures. The present invention allows the addition of 0.01% or more of niobium (Nb) to achieve the grain refinement described above. The preferred lower limit of the niobium (Nb) content is 0.02%. On the other hand, adding too much niobium (Nb) can increase the hot rolling load due to increased high-temperature strength. Therefore, the present invention limits the upper limit of the niobium (Nb) content to 0.05%. The preferred upper limit of the niobium (Nb) content is 0.045%.

[0066] Copper (Cu): 0.06-0.1%

[0067] Copper (Cu) is a component that helps increase the strength of steel. In the present invention, to achieve the above-mentioned effects, 0.06% or more of copper (Cu) may be added. The preferred lower limit of the copper (Cu) content may be 0.065%, and the more preferred lower limit of the copper (Cu) content may be 0.07%. On the other hand, excessive addition of copper (Cu) can lead to grain boundary embrittlement or increased costs. Therefore, the present invention may limit the upper limit of the copper (Cu) content to 0.1%.

[0068] Boron (B): 0.0015% or less,

[0069] Boron (B) is a component added to prevent secondary processing brittleness caused by the addition of phosphorus (P) to steel. The present invention can add boron (B) to prevent secondary processing brittleness. The preferred lower limit of the boron (B) content can be 0.0002%, and the more preferred lower limit of the boron (B) content can be 0.0004%. However, when the boron (B) content is too high, the ductility of the steel plate is reduced. Therefore, the present invention can limit the upper limit of the boron (B) content to 0.0015%. The preferred upper limit of the boron (B) content can be 0.001%.

[0070] Molybdenum (Mo): YM defined by the following relational formula 1 satisfies the range of 0.03 or more and 0.1 or less.

[0071] The inventors of the present invention conducted in-depth research on methods for achieving high strength by adding phosphorus (P) and preventing phosphorus (P) segregation at grain boundaries, which can lead to reduced surface quality. They discovered that when an appropriate amount of molybdenum (Mo) is added to steel, Mo, which has a high affinity for phosphorus (P), forms a MoP compound, effectively preventing phosphorus (P) segregation at grain boundaries. Therefore, the inventors analyzed various aspects of the optimal molybdenum (Mo) content that achieves a balance between high strength and surface quality in steel. They also derived the contents of phosphorus (P) and manganese (Mn), which contribute to steel strength but hinder surface quality. They concluded that controlling the molybdenum (Mo) content in steel within an optimal range achieves both high strength and excellent surface quality. Therefore, the present invention limits the molybdenum (Mo) content so that the YM value defined by the following equation (1) falls within a specified range.

[0072] [Equation 1]

[0073] YM=[P]*(1+0.5*[Mo]+0.1*[Mn])

[0074] In Relational Formula 1, [P], [Mo], and [Mn] respectively represent the contents (weight %) of phosphorus (P), molybdenum (Mo), and manganese (Mn) contained in the base material iron.

[0075] In the present invention, the lower limit of the YM value defined by Relationship 1 can be set to 0.03 to ensure steel strength. 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 disadvantageous in terms of ensuring surface quality. Therefore, in the present invention, the upper limit of the YM value can be set 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.

[0076] In addition to the above-mentioned components, the iron base material of the present invention may contain a balance of Fe and other unavoidable impurities. However, during typical manufacturing processes, unwanted impurities inevitably enter from the raw materials or the surrounding environment, and therefore cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore, all of them are not specifically mentioned in this specification. Furthermore, the addition of active ingredients other than the above-mentioned components cannot be completely eliminated.

[0077] The base iron of the present invention may comprise ferrite as a matrix structure and may further contain other microstructures such as inevitably formed pearlite as a remainder. The base iron of the present invention comprises ferrite as a matrix structure, thereby effectively ensuring the formability of the plated steel sheet. In the present invention, the ferrite fraction may be 95% by area or greater, and more preferably, 99% by area or greater.

[0078] The E defined by the following Relational Formula 2 of the base material iron of the present invention may satisfy the range of 0 or more and 60 or less.

[0079] [Equation 2]

[0080] E=1250*YM-5*X

[0081] In the above-mentioned Relational Formula 2, YM is calculated from Relational Formula 1, and X represents the average grain size (μm) of ferrite.

[0082] The inventors of the present invention conducted in-depth research on conditions for maximizing formability while achieving both high strength properties and excellent surface quality, and found that by controlling the correlation between the YM value calculated by Relationship 1 and the average grain size of the ferrite structure, both high strength properties and excellent surface quality can be achieved.

[0083] In the present invention, the E value calculated by equation 2 is limited to 0 or greater, thereby effectively ensuring the strength of the plated steel sheet. A more preferred lower limit of the E value may be 10. Furthermore, in the present invention, the E value calculated by equation 2 is limited to 60 or less, thereby effectively ensuring the formability of the plated steel sheet. Furthermore, as a non-limiting example, the average grain size (X) of the ferrite in the present invention may be 5 μm or greater and 15 μm or less.

[0084] In the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface portion of the substrate 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 portion of the substrate iron may represent a position from the surface of the substrate iron to 1 / 6 of the thickness of the substrate iron. The intensity of ODF {0°, 0°, 45°} represents the intensity measured in the three-dimensional crystal orientation density function (ODF) of the surface portion 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 three-dimensional crystal orientation density function (ODF) of the surface portion when Φ is 55°, Φ1 is 30°, and Φ2 is 45°. The density function (ODF) intensity value of the crystal orientation can be obtained by electron back scatter diffraction (EBSD). Those skilled in the art can easily measure the density function (ODF) intensity value of the crystal orientation by using conventional technical methods.

[0085] The inventors of the present invention have conducted repeated studies to solve the problem of poor surface formability caused by uneven surface structure during processing. As a result, they have discovered that in the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the above-mentioned surface portion, as the strength of ODF {0°, 0°, 45°} and ODF {30°, 55°, 45°} meets specific values, the γ-fiber texture can be developed, thereby not only improving the surface formability but also making it suitable for use as a steel for vehicle outer panels.

[0086] Therefore, the present invention limits the strength of the ODF {0°, 0°, 45°} in the three-dimensional crystal orientation density function (ODF) {Φ1, Φ, Φ2} of the surface portion of the substrate iron to less than 2.0, and limits the strength of the ODF {30°, 55°, 45°} to greater than 5 and less than 9, thereby effectively ensuring surface formability.

[0087] A coated steel sheet according to one aspect of the present invention includes a coating formed on one side of an iron substrate. The coating of the present invention may be a hot-dip coating or an alloy coating. As non-limiting examples, the coating of the present invention may be any one selected from the group consisting of a zinc-based coating, an aluminum-based coating, an alloyed zinc-based coating, and an alloyed aluminum-based coating.

[0088] A coated steel sheet according to one aspect of the present invention can exhibit a tensile strength of 390 MPa or greater, an elongation of 28% or greater, and an r-value of 1.2 or greater. Furthermore, the coated steel sheet according to one aspect of the present invention can effectively suppress the occurrence of surface linear defects. Thus, the present invention can provide a coated steel sheet that exhibits high strength, excellent formability, and surface quality, while also possessing preferred physical properties for automotive exterior panel materials.

[0089] Hereinafter, a method for producing a plated steel sheet according to one aspect of the present invention will be described in more detail.

[0090] The method for manufacturing a coated steel sheet according to one aspect of the present invention comprises the following steps: heating a slab having a predetermined composition at 1100-1300° C.; setting a finishing rolling temperature (T f ) hot rolling the heated slab to obtain a hot-rolled steel sheet; after pickling the hot-rolled steel sheet, cold rolling it at a reduction ratio of 70-83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 750-830°C; immersing the annealed cold-rolled steel sheet in a plating bath to obtain a plated steel sheet; and selectively alloying the plated steel sheet at a temperature range of 480-610°C.

[0091] [Equation 3]

[0092] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}]

[0093] In the relational expression 3, [P], [Ti], and [Nb] represent the contents (weight %) of phosphorus (P), titanium (Ti), and niobium (Nb), respectively, contained in the slab.

[0094] Slab preparation and heating

[0095] A steel slab having a predetermined composition is prepared. The steel slab of the present invention has an alloy composition corresponding to the alloy composition of the aforementioned base iron, and therefore the description of the alloy composition of the steel slab will be replaced by the description of the alloy composition of the aforementioned base iron.

[0096] The prepared slab can be heated at 1100-1300°C. If the slab heating temperature is too low, excessive rolling load 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.

[0097] Hot Rolling

[0098] Hot rolled steel sheets can be provided by hot rolling a heated slab. The inventors of the present invention have conducted in-depth research on process conditions that can achieve high strength characteristics while ensuring excellent formability and surface quality. As a result, they have confirmed that hot rolling should be carried out within a range of a finishing temperature of Ar3 or higher, and that the finishing temperature should be strictly controlled according to the relative contents of phosphorus (P), titanium (Ti), and niobium (Nb) contained in the slab, thereby obtaining a hot rolled steel sheet with a surface quality that is consistent with the finishing temperature (T f ) is related to the following relation 3.

[0099] [Equation 3]

[0100] 920-300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}] <T f (℃)<920+300*[3*[P]-{(31 / 48)*[Ti]+(31 / 93)*[Nb]}]

[0101] In the relational expression 3, [P], [Ti], and [Nb] represent the contents (weight %) of phosphorus (P), titanium (Ti), and niobium (Nb), respectively, contained in the slab.

[0102] When the finishing rolling temperature (T f ), the microstructure of the final steel sheet deviates from the desired range of the present invention, so the formability may be deteriorated. In addition, when the finishing rolling temperature (T f ), the microstructure of the final steel plate is not within the desired range of the present invention, so the strength characteristics or surface quality may be deteriorated. Therefore, the present invention can be carried out at a finishing rolling temperature (T f ) to provide hot-rolled steel sheets.

[0103] Rolling

[0104] The hot-rolled steel sheet obtained by hot rolling can be coiled within a temperature range of 600-650°C. When the coiling temperature is too low, precipitates such as Ti(Nb)C cannot be fully formed, the amount of dissolved carbon increases, and behaviors such as recrystallization and grain growth are affected during annealing, which may cause problems in ensuring the target strength and elongation. Therefore, in the present invention, the lower limit of the coiling temperature can be limited to 600°C. On the other hand, when the coiling temperature is too high, the surface quality may deteriorate due to the formation of secondary oxide scale, so in the present invention, the upper limit of the coiling temperature can be limited to 650°C.

[0105] Pickling and cold rolling

[0106] After the coiled hot-rolled steel sheet is uncoiled, it can be pickled to remove surface oxide scale. After pickling, it can be cold rolled at a reduction rate of 70-83% to obtain a cold-rolled steel sheet. When the reduction rate of cold rolling is less than a certain level, the {111} texture does not grow sufficiently, so the formability may deteriorate. In the present invention, the lower limit of the cold rolling reduction rate can be limited to 70%. A more preferred lower limit of the reduction rate can be 71%. On the other hand, when the reduction rate during cold rolling is too high, poor shape may occur due to excessive roll load. Therefore, in the present invention, the upper limit of the cold rolling reduction rate can be limited to 83%. A more preferred upper limit of the reduction rate can be 80%.

[0107] annealing

[0108] The cold-rolled steel sheet obtained by cold rolling can be heated to a temperature above the recrystallization temperature for annealing. When the annealing temperature is below a certain level, the deformation caused by rolling cannot be fully removed, and the ferrite cannot complete recrystallization, so the elongation may deteriorate. Therefore, in the present invention, the lower limit of the annealing temperature can be limited to 750°C. On the other hand, when the annealing temperature is too high, after the recrystallization is completed, the grain growth may occur, resulting in the possibility of deterioration of strength or surface quality. In the present invention, the upper limit of the annealing temperature can be limited to 830°C.

[0109] Plating and selective alloying

[0110] The annealed cold-rolled steel sheet can be immersed in a plating bath to form a coating. The plating bath used in the present invention can be a plating bath commonly used in the production of coated steel sheets for automotive exterior panels, preferably a zinc-based plating bath or an aluminum-based plating bath. After immersion in the plating bath, alloying can be performed selectively, preferably at a temperature in the range of 480-610°C.

[0111] The plated steel sheet manufactured by the above manufacturing method can have a tensile strength of 390 MPa or more, an elongation of 28% or more, and an r-value of 1.2 or more, and can also have excellent surface quality. DETAILED DESCRIPTION

[0112] The following describes the coated steel sheet and its manufacturing method of the present invention in more detail through specific examples. It should be noted that the following examples are provided solely for understanding the present invention and are not intended to define the scope of the present invention. This is because the scope of the present invention is determined by the claims and any reasonable derivation thereof.

[0113] (Example)

[0114] A steel billet having a thickness of 250 mm and having the alloy composition described in Table 1 below was heated to 1250° C., and then a plated steel sheet having an average thickness of 0.85 mm was produced by applying the process conditions of Table 2. At this time, a zinc (Zn) plating bath containing 0.13% aluminum (Al) was used as the plating bath. After each test piece was cut in the thickness direction, the fraction of the microstructure and the average grain size were measured using an optical microscope (OM), and the results are recorded in Table 3. In addition, after each test piece was electrolytically polished, the density function (ODF) of the three-dimensional crystal orientation of the substrate iron surface portion of each test piece (from the substrate iron surface to 1 / 6 of the thickness of the substrate iron) was measured using electron backscatter diffraction (EBSD), and the results are recorded together in Table 3.

[0115] Each specimen was subjected to a tensile test using a tensile testing machine, and the tensile strength (TS), yield point (YP), and elongation at break (T-El) were measured and reported in Table 4. Specifically, after measuring the width, length, and thickness of the specimen, the specimen was mounted in the tensile testing machine and allowed to stand until the specimen broke. The tensile strength, yield point, and elongation at break were then measured. The yield point is the ultimate stress at which elastic strain occurs, typically expressed as a 0.2% offset. The tensile strength is the maximum load divided by the circular cross-section. The elongation at break is expressed in % as the deformation of the specimen after breaking during the tensile test.

[0116] In addition, as for the r value, which is an indicator of deep drawing processing, JIS No. 5 tensile test pieces were taken from three directions: parallel to the rolling direction, 45° to the rolling direction, and perpendicular to the rolling direction. The r value of each test piece was measured and evaluated. The r value can be obtained by measuring the change in plate thickness and plate width when a tensile deformation of about 15% is performed in the above-mentioned tensile test, and using the ratio of the change in plate width to the change in plate thickness. In addition, the r value parallel to the rolling direction is set as r0, and the r value in the direction at 45° to the rolling direction is set as r 45 , the r value in the direction perpendicular to the rolling direction is set to r 90 When , the r value is calculated using the relational expression 4 using the r value in each direction.

[0117] [Equation 4]

[0118] r=r0+2*r 45 +r 90 / 4

[0119] Surface linear defects are visually confirmed as black lines. The number of total linear defects per coil can be confirmed using a surface defect detector (SDD). Surface linear defects are evaluated as "good" or "bad" based on the following criteria:

[0120] Good: The number of linear defects in the SDD is 100 or less

[0121] Bad: The number of linear defects in the SDD exceeds 100

[0122] [Table 1]

[0123]

[0124] [Table 2]

[0125]

[0126] [Table 3]

[0127]

[0128] [Table 4]

[0129]

[0130] From Tables 1 to 4, it can be seen that the test pieces that meet the alloy composition, microstructure and process conditions of the present invention have 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 properties. On the other hand, it can be seen that the test pieces that do not meet any one of the alloy composition, microstructure and process conditions of the present invention cannot simultaneously ensure 390 MPa.

[0131] Tensile strength above 28%, elongation above 28%, r value above 1.2 and excellent surface properties.

[0132] Therefore, according to one aspect of the present invention, a plated steel sheet having high strength characteristics and excellent formability and surface quality and a method for manufacturing the same can be provided.

[0133] While the present invention has been described in detail above by way of the embodiments, embodiments in different forms are also possible. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A coated steel sheet comprising: A substrate iron; and a plating layer provided on at least one side of the substrate iron, wherein, in terms of weight %, the substrate iron comprises: 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 equation 1 satisfies the range of 0.03 or more and 0.1 or less, and the balance is Fe and other inevitable impurities, the base iron contains ferrite as a matrix structure, and E of the base iron defined by the following equation 2 satisfies the range of 0 or more and 60 or less, [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 of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the above base material iron, wherein the unit of the contents is weight %. [Equation 2] E = 1250 * YM - 5 * X In the relational expression 2, YM is calculated by the relational expression 1, X represents the average grain size of the ferrite, and the unit of the average grain size is μm.

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

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

4. The plated steel sheet according to claim 1, wherein The coating is any one selected from zinc-based coating, aluminum-based coating, alloyed zinc-based coating or alloyed aluminum-based coating.

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

6. A method for manufacturing a plated steel sheet, comprising the following steps: The slab is heated at 1100-1300° C., and contains, in terms of weight%, 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 relationship 1 satisfies the range of 0.03 or more and 0.1 or less, and the balance is Fe and other inevitable impurities; The finishing rolling temperature T is to satisfy the following equation 3: f hot rolling the heated slab to obtain a hot-rolled steel plate; Coiling the hot-rolled steel plate at a temperature of 600-650° C. Pickling the hot-rolled steel sheet, and then cold-rolling it at a reduction ratio of 70-83% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature ranging from 750° C. to 830° C. immersing the annealed cold-rolled steel sheet in a plating bath to obtain a plated steel sheet; and The plated steel sheet is selectively alloyed at a temperature of 480-610°C. [Equation 1] YM = [P] * (1 + 0.5 * [Mo] + 0.1 * [Mn]) In the above equation 1, [P], [Mo] and [Mn] represent the contents of phosphorus (P), molybdenum (Mo) and manganese (Mn) contained in the slab, respectively, wherein the unit of the contents is weight %. [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 relational expression 3, [P], [Ti], and [Nb] represent the contents of phosphorus (P), titanium (Ti), and niobium (Nb) contained in the slab, respectively, wherein the unit of the contents is weight %.

7. The method for producing a plated steel sheet according to claim 6, wherein: The plating bath is selected from either a zinc-based plating bath or an aluminum-based plating bath.

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

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