High-strength galvannealed steel sheet with excellent powdering resistance and manufacturing method thereof

Abstract

According to one aspect of the present invention, it is possible to provide a galvannealed steel sheet having high strength properties and excellent powdering resistance, and a method for producing the same.

Description

[Technical field]

[0001] The present invention relates to a galvannealed steel sheet and a manufacturing method thereof, and more particularly to a galvannealed steel sheet having high strength characteristics and excellent powdering resistance and suitable for use as an automobile exterior panel material, and a manufacturing method thereof. [Background technology]

[0002] Galvannealed steel sheets (GA steel sheets) are obtained by heating galvanized steel sheets (GI steel sheets) to diffuse the Fe in the base steel sheet into the coating layer and alloy the Fe and Zn. GA steel sheets have excellent strength, weldability, and corrosion resistance after painting, so they are primarily used for automobile frame components (components that absorb energy during a collision, etc.).

[0003] In recent years, there have been attempts to use GA steel sheets for automotive exterior panels in response to the demand for higher strength in automobile bodies. However, the reality is that GA steel sheets have not been used extensively for automotive exterior panel materials. This is because GA steel sheets have a problem with so-called powdering, in which the coating layer peels off into powder during forming, and there are technical difficulties that must be resolved before GA steel sheets can be used for automotive exterior panel materials that are formed into complex shapes.

[0004] As a method for improving the powdering resistance of GA steel sheets, a method has been proposed in which the iron concentration in the Fe-Zn alloy plating layer is lowered to reduce the brittle gamma (γ) phase. In addition, as in Patent Document 1, a technique has been proposed for improving powdering resistance and flaking resistance by adjusting the amounts of zeta (ζ), delta (δ) and gamma (γ) phases in the plating layer, suppressing the formation of the gamma (γ) phase at the interface between the base steel and the plating layer, and controlling the surface roughness to be low. However, even with such a method, it is not possible to sufficiently ensure powdering resistance, and it is the reality that it is not possible to provide a GA steel sheet that has strength properties that can meet the recent trend of high tensile strength, can be formed into complex shapes while being lightweight, and is suitable for use as an automobile exterior panel material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Registration No. 2695259 Summary of the Invention [Problem to be solved by the invention]

[0006] According to one aspect of the present invention, it is possible to provide a galvannealed steel sheet having high strength characteristics and excellent powdering resistance, and a manufacturing method thereof.

[0007] The object of the present invention is not limited to the above-mentioned contents, and a person skilled 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]

[0008] A galvannealed steel sheet according to one aspect of the present invention includes a base iron; and a galvannealed layer provided on at least one surface of the base iron; the base iron has, by weight percent, carbon (C): 0.003 to 0.009%, silicon (Si): 0.05% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.09%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.02% or less, and a galvannealed steel sheet according to one aspect of the present invention includes: a base iron; and a galvannealed steel layer provided on at least one surface of the base iron. The base iron has, by weight percent, carbon (C): 0.003 to 0.009%, silicon (Si): 0.05% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.09%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.02% or less, and a galvannealed steel sheet according to one aspect of the present invention. 0.1% or less, molybdenum (Mo): 0.05 to 0.08%, titanium (Ti): 0.005 to 0.03%, niobium (Nb): 0.02 to 0.045%, copper (Cu): 0.06 to 0.1%, boron (B): 0.0015% or less, the remainder being Fe and other unavoidable impurities, satisfying the following relational formula 1, and the average thickness of the gamma (γ) phase present at the interface between the base iron and the alloyed hot-dip galvanized layer can be 0.20 μm or less. [Equation 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 In the above Relational Formula 1, [Ti], [Mo] and [Si] respectively represent the contents (wt%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base iron.

[0009] The area ratio of the delta (δ) phase on the surface of the galvannealed layer can be 80 to 100%.

[0010] The galvannealed layer may contain, by weight %, 0.05 to 0.3% aluminum (Al), 10 to 20% iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities.

[0011] The base steel may contain 95% or more by area of ​​ferrite, and the average crystal grain size of the ferrite may be 15 μm or less.

[0012] The galvannealed steel sheet has a tensile strength of 390 MPa or more and can have an elongation ratio of 28% or more.

[0013] A method for producing a galvannealed steel sheet according to one aspect of the present invention comprises the steps of: (a) preparing a galvannealed steel sheet having a thickness of 0.003 to 0.009 mm; (b) preparing a galvannealed steel sheet having a thickness of 0.05 to 1.0 mm; (c) preparing a galvannealed steel sheet having a thickness of 0.05 to 1.0 mm; (d) preparing a galvannealed steel sheet having a thickness of 0.05 to 1.0 mm; (e) preparing a galvannealed steel sheet having a thickness of 0.05 to 1.0 mm; (f) preparing a galvannealed steel sheet having a thickness of 0.05 to 1.0 mm; The method includes the steps of: preparing a cold-rolled steel sheet containing 0.045%, copper (Cu): 0.06-0.1%, boron (B): 0.0015% or less, the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1; immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at an intake temperature of 490-500°C based on the surface temperature of the cold-rolled steel sheet to provide a plated steel sheet having a hot-dip galvanized layer formed thereon; and heating the plated steel sheet to a temperature range of 500-560°C to perform an alloying treatment. [Equation 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 In the above Relational Formula 1, [Ti], [Mo] and [Si] respectively represent the contents (wt%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the above cold-rolled steel sheet.

[0014] The above-mentioned hot-dip galvanizing bath may contain, by weight percent, 0.05 to 0.5% aluminum (Al), the remainder zinc (Zn), and other unavoidable impurities.

[0015] The step of preparing the cold-rolled steel sheet may include the steps of: producing a slab having a predetermined alloy composition by continuous casting; heating the slab at 1100 to 1300°C; hot rolling the heated slab at a finish rolling temperature of 920 to 970°C to obtain a hot-rolled steel sheet; coiling the hot 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; and annealing the cold-rolled steel sheet at a temperature range of 760 to 820°C.

[0016] The method for producing the galvannealed steel sheet may further include a step of temper rolling the galvannealed steel sheet at a rolling reduction of 0.6 to 1.2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 1.6 μm.

[0017] 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 benefits can be more fully understood with reference to the following specific examples. Effect of the Invention

[0018] According to one aspect of the present invention, there is provided a galvannealed steel sheet having high strength characteristics and excellent powdering resistance, and having physical properties suitable for use as an automobile exterior panel material, and a method for producing the same.

[0019] 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. [Brief description of the drawings]

[0020] [Figure 1] 1 is a photograph of a cross section of a test piece 1 observed using an SEM. [Diagram 2] 1 is a photograph of a cross section of a test piece 14 observed using an SEM. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention relates to a high-strength galvannealed steel sheet having excellent powdering resistance 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 ways, 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 further explain the present invention in detail to those skilled in the art to which the present invention pertains.

[0022] Hereinafter, a galvannealed steel sheet according to one aspect of the present invention will be described in more detail.

[0023] A galvannealed steel sheet according to one aspect of the present invention includes a base iron; and a galvannealed layer provided on at least one surface of the base iron; the base iron has, by weight percent, carbon (C): 0.003 to 0.009%, silicon (Si): 0.05% or less, manganese (Mn): 0.4 to 1.0%, phosphorus (P): 0.04 to 0.09%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0. 0.1% or less, molybdenum (Mo): 0.05 to 0.08%, titanium (Ti): 0.005 to 0.03%, niobium (Nb): 0.02 to 0.045%, copper (Cu): 0.06 to 0.1%, boron (B): 0.0015% or less, the remainder being Fe and other unavoidable impurities, satisfying the following relational formula 1, and the thickness of the gamma (γ) phase present at the interface between the base iron and the alloyed hot-dip galvanized layer can be 0.20 μm or less. [Equation 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 In the above Relational Formula 1, [Ti], [Mo] and [Si] respectively represent the contents (wt%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base iron.

[0024] The steel composition contained in the base steel of the present invention will be described in more detail below. Unless otherwise specified, % representing the content of each element is based on weight.

[0025] Carbon (C): 0.003~0.009% Carbon (C) is an interstitial solid solution element, and is an element that 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 deep drawing, is suppressed, and the growth of crystal grains having {110} and {100} textures is promoted, resulting in a decrease in the deep 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 in terms of economy, but also may cause the formation of pearlite and the like, resulting in a decrease in formability. Therefore, in the present invention, the upper limit of the carbon (C) content can be limited to 0.009%. The preferred upper limit of the carbon (C) content can be 0.008%, and the more preferred upper limit of the carbon (C) content can be 0.007%. 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 0.003%. The preferred lower limit of the carbon (C) content is 0.004%.

[0026] Silicon (Si): 0.05% 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 properties, the present invention can limit the upper limit for the silicon (Si) content to 0.05%. A preferred upper limit for the silicon (Si) content can be 0.04%.

[0027] 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 may cause a decrease 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 0.85%.

[0028] Phosphorus (P): 0.04-0.09% Phosphorus (P) has the best solid solution effect and is the most effective element for ensuring the strength of steel without significantly impairing deep drawability. In the present invention, 0.04% or more of phosphorus (P) can be added to achieve this effect. A preferable phosphorus (P) content is more than 0.04%, and more preferably 0.045% or more. On the other hand, when phosphorus (P) is added excessively, secondary embrittlement and surface linear defects due to segregation of phosphorus (P) are a concern, so the present invention can limit the upper limit of the phosphorus (P) content to 0.09%. A preferable upper limit of the phosphorus (P) content can be 0.085%.

[0029] 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, and 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.

[0030] Aluminum (S.Al): 0.1% or less Aluminum (Al) is a component that contributes to improving the deep drawability and ductility of steel by precipitating AlN, and the present invention can add aluminum (Al) to ensure this effect. A preferred aluminum (Al) content is greater than 0%, and a more preferred aluminum (Al) content is 0.01% or more. On the other hand, if aluminum (Al) is added in excess, aluminum (Al) inclusions may be excessively formed during steelmaking operations, which may cause defects inside the steel sheet, so the present invention can limit the upper limit of the aluminum (Al) content to 0.1%.

[0031] Molybdenum (Mo): 0.05~0.08% The inventors of the present invention have conducted specific research into a method for preventing degradation of surface quality due to grain boundary segregation of phosphorus (P) while increasing strength by adding phosphorus (P). As a result, it has been 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, thereby effectively preventing grain boundary segregation of phosphorus (P). Therefore, the present invention may contain 0.05% or more of molybdenum (Mo) to achieve such an effect. A more preferable lower limit of the molybdenum (Mo) content may be 0.055%. However, if molybdenum (Mo) is added excessively, the effect of forming the MoP compound is saturated, while the cost competitiveness is greatly reduced, so the present invention may limit the upper limit of the molybdenum (Mo) content to 0.08%. A preferable upper limit of the molybdenum (Mo) content may be 0.075%.

[0032] Titanium (Ti): 0.005 to 0.03% Titanium (Ti) is an element that contributes greatly to improving the deep drawability of steel sheets by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate titanium (Ti)-based carbonitrides. 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%.

[0033] Niobium (Nb): 0.02-0.04% 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 at high temperatures. In the present invention, 0.02% or more of niobium (Nb) can be added to achieve such grain refinement. The preferred lower limit of the niobium (Nb) content can be 0.025%. On the other hand, if niobium (Nb) is added excessively, the high temperature strength increases and the load of hot rolling can become excessive, so the present invention can limit the upper limit of the niobium (Nb) content to 0.04%. The preferred upper limit of the niobium (Nb) content can be 0.035%.

[0034] 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 preferred lower limit of the copper (Cu) content can be 0.065%, and a more preferred lower limit of the copper (Cu) content can be 0.07%. On the other hand, if the amount of copper (Cu) added is excessive, 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%. A preferred upper limit of the copper (Cu) content can be 0.09%.

[0035] Boron (B): 0.0015% or less Boron (B) is a component that prevents secondary work embrittlement caused by the addition of phosphorus (P) in steel, and the present invention can add boron (B) to prevent secondary work embrittlement. A preferred lower limit of the boron (B) content can be 0.0002%, and a more preferred lower limit of the boron (B) content can 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 can limit the upper limit of the boron (B) content to 0.0015%. A preferred upper limit of the boron (B) content can be 0.001%.

[0036] The base steel of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely exclude them. Since these impurities are known to anyone with ordinary knowledge in this technical field, the contents of all of them will not be specifically mentioned in this specification. Furthermore, the addition of additional effective components other than the above-mentioned components is not completely excluded.

[0037] The base steel of the present invention can satisfy the following relational expression 1. [Equation 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 In the above Relational Formula 1, [Ti], [Mo] and [Si] respectively represent the contents (wt%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base iron.

[0038] The inventors of the present invention have conducted specific research into a method for improving the powdering resistance of an alloyed plating layer while increasing the strength by adding phosphorus (P) to a base steel, and have confirmed that when the relative contents of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base steel are controlled within a specific range, the alloying degree of the plating layer can be controlled, thereby effectively preventing the formation of a gamma (γ) phase at the interface between the base steel and the alloyed plating layer. That is, the present invention uses Relation 1 to control the relative contents of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base steel within a certain range, so that the average thickness of the gamma (γ) phase formed at the interface between the alloyed plating layer and the base steel can be limited to 0.20 μm or less, thereby effectively ensuring the powdering resistance of the plating layer.

[0039] The base iron of the present invention may contain ferrite as a matrix structure, and may further contain other fine structures 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. The average crystal grain size of the ferrite may be 15 μm or less.

[0040] A galvannealed steel sheet according to one aspect of the present invention includes a galvannealed layer provided on at least one surface of a base steel, and the galvannealed layer contains, by weight %, 0.05 to 0.3% aluminum (Al), 10 to 20% iron (Fe), the remainder zinc (Zn), and other unavoidable impurities.

[0041] In the galvannealed steel sheet according to one aspect of the present invention, a gamma (γ) phase having a certain thickness is formed at the interface between the base steel and the galvannealed layer, and the preferred average thickness of the gamma (γ) phase can be 0.20 μm or less. The present invention does not particularly specify a lower limit for the thickness of the gamma (γ) phase, but 0 μm can be excluded from the lower limit of the thickness of the gamma (γ) phase in consideration of the amount of the gamma (γ) phase that is inevitably formed. Preferably, the thickness can be 0.02 μm or more. In the galvannealed steel sheet according to one aspect of the present invention, the average thickness of the gamma (γ) phase formed at the interface between the base steel and the galvannealed layer is controlled to a range of 0.20 μm or less, and therefore the powdering resistance of the coating layer can be effectively ensured.

[0042] The area ratio of the delta (δ) phase observed on the surface of the galvannealed steel sheet of the present invention can be 80 to 100%. Since the area ratio of the delta (δ) phase observed on the surface of the coating layer is limited to 80% or more in the present invention, the friction coefficient can be reduced, thereby ensuring excellent flaking resistance.

[0043] The galvannealed steel sheet according to one aspect of the present invention may have a tensile strength of 390 MPa or more and an elongation of 28% or more. In addition, the galvannealed steel sheet according to one aspect of the present invention has not only high strength characteristics and excellent formability but also excellent powdering resistance, and therefore has physical properties suitable for use as an automobile exterior panel material.

[0044] Hereinafter, a method for producing a galvannealed steel sheet according to one aspect of the present invention will be described in more detail.

[0045] A method for producing a galvannealed steel sheet according to one aspect of the present invention includes the steps of: preparing a cold-rolled steel sheet having a predetermined alloy composition; immersing the cold-rolled steel sheet in a galvanizing bath at a drawing temperature of 490 to 500°C based on the surface temperature of the cold-rolled steel sheet to provide a plated steel sheet having a galvanized layer formed thereon; and heating the plated steel sheet in a temperature range of 500 to 560°C to perform an alloying treatment.

[0046] The step of preparing the cold-rolled steel sheet may include the steps of: producing a slab having a predetermined alloy composition by continuous casting; heating the slab at 1100 to 1300°C; hot rolling the heated slab at a finish rolling temperature of 920 to 970°C to obtain a hot-rolled steel sheet; coiling the hot 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; and annealing the cold-rolled steel sheet at a temperature range of 760 to 820°C.

[0047] Slab manufacturing and heating A steel slab having a predetermined composition is produced by continuous casting. 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 replaces the description of the alloy composition of the base iron described above.

[0048] The prepared slab can be heated in the temperature range of 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.

[0049] Hot rolling The heated slab can be hot-rolled to provide a hot-rolled steel sheet. The hot rolling can be performed at a finish rolling temperature in the range of Ar3 or more. More specifically, the finish rolling temperature can be in the range of 900 to 970 ° C. If the hot rolling temperature is too low, the crystal grain size of the final structure becomes too fine and the desired level of formability cannot be ensured, so the lower limit of the finish rolling temperature can be limited to 900 ° C. On the other hand, if the hot rolling temperature is too high, the crystal grain size of the final structure becomes too coarse, not only making it impossible to ensure the desired strength, but also inducing a decrease in the surface quality of the final product, so the present invention can limit the upper limit of the finish rolling temperature to 970 ° C.

[0050] 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 behaviors such as recrystallization and particle growth during annealing are affected, making it difficult to ensure 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 generation may cause a decrease in surface quality, so the present invention can limit the upper limit of the coiling temperature to 650°C.

[0051] Pickling and cold rolling After uncoiling the coiled hot-rolled steel sheet, pickling can be performed to remove surface scale, and cold rolling can be performed at a rolling reduction of 70 to 83% after pickling 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 decrease, so the present invention can limit the lower limit of the cold rolling reduction to 70%. A more preferable lower limit of the rolling reduction can be 74%. On the other hand, if the rolling reduction during cold rolling is excessive, excessive load on the rolling rolls may cause shape defects, so the present invention can limit the upper limit of the cold rolling reduction to 83%. A more preferable upper limit of the rolling reduction can be 80%.

[0052] 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 a certain level, the deformation caused by rolling is not sufficiently removed, ferrite recrystallization is not completed, and the elongation rate may decrease. Therefore, the present invention limits the lower limit of the annealing temperature to 760°C. On the other hand, if the annealing temperature is excessively high, grain growth may occur after recrystallization is completed, resulting in a decrease in strength or a decrease in surface quality, so the present invention limits the upper limit of the annealing temperature to 820°C.

[0053] Plating and selective alloying The annealed cold-rolled steel sheet can be immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer. The hot-dip galvanizing bath used in the present invention can contain, by weight percent, 0.05 to 0.5% aluminum (Al), the remainder zinc (Zn) and other unavoidable impurities. Since the drawing temperature during immersion in the galvanizing bath has a significant effect on the degree of formation of the gamma (γ) phase, the present invention can limit the drawing temperature during immersion in the galvanizing bath to a range of 490 to 500°C based on the surface temperature of the cold-rolled steel sheet.

[0054] The plated steel sheet on which the hot-dip galvanized layer is formed can be subjected to an alloying treatment by heating to a temperature range of 500 to 560°C. If the alloying temperature is excessively high, there is a risk of excessive formation of gamma (γ) phase, so the present invention can limit the upper limit of the alloying temperature to 560°C. On the other hand, if the alloying temperature is low, sufficient alloying is not achieved, so the present invention can limit the lower limit of the alloying temperature to 500°C.

[0055] Temper rolling In order to ensure the surface quality of the galvanized steel sheet, it is possible to selectively carry out temper rolling. Preferably, the galvannealed steel sheet can be temper rolled at a rolling reduction of 0.6 to 1.2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 1.6 μm.

[0056] The plated steel sheet produced by the above-mentioned production method not only has a tensile strength of 390 MPa or more and an elongation rate of 28% or more, but also has excellent powdering resistance. EXAMPLES

[0057] 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.

[0058] (Example) A steel slab having a thickness of 250 mm and an alloy composition shown in Table 1 below was produced by continuous casting. The steel slab was heated to 1250°C, and then hot-rolled. The hot-rolled steel sheet was wound up to prepare a hot-rolled coil. At this time, the finish rolling temperature and winding temperature of the hot rolling were applied under the conditions shown in Table 2. The hot-rolled coil was uncoiled and pickled, and cold-rolled at a rolling reduction of 70 to 83% to produce a cold-rolled steel sheet having a thickness of 0.6 to 1.0 mm. Each cold-rolled steel sheet was annealed under the conditions shown in Table 2. Thereafter, the cold-rolled steel sheet was immersed in a hot-dip galvanizing bath containing 0.12 wt% Al to form a coating layer, and subsequently, an alloying treatment was performed. The drawing temperature and alloying temperature during immersion in the coating bath were applied under the conditions shown in Table 2.

[0059] [Table 1]

[0060] [Table 2]

[0061] Tensile tests were performed on each test piece to measure the tensile strength, yield strength, and elongation at break. The tensile tests were performed using test pieces taken based on JIS No. 5 standard. The interface region of each test piece was observed using SEM, and the thickness of the gamma (γ) phase was measured at three or more points and the average value was calculated. Test piece 8 was unalloyed due to the low alloying temperature, so it was not possible to distinguish the gamma (γ) phase and the delta (δ) phase.

[0062] Powdering resistance was evaluated by bending each test piece 60°, then bending it again 60° in the opposite direction from the same point, attaching transparent tape to the surface of each test piece, then removing it and measuring the powdering peel width. If the powdering peel width is 6 mm or less, it can be considered to have suitable powdering resistance for automotive exterior panels.

[0063] [Table 3]

[0064] As shown in Tables 1 to 3, the test pieces satisfying the alloy composition and process conditions of the present invention not only have a tensile strength of 390 MPa or more and an elongation rate of 28% or more, but also have excellent powdering resistance. In contrast, it can be seen that the test pieces not satisfying any one or more of the alloy composition or process conditions of the present invention do not satisfy the tensile strength of 390 MPa or more or the elongation rate of 28% or more, or have reduced powdering resistance. Therefore, according to one aspect of the present invention, it is possible to provide a galvannealed steel sheet having physical properties suitable for use as an automobile exterior panel material, and a manufacturing method thereof.

[0065] Although the present invention has been described in detail with reference to the embodiments, other embodiments are possible and the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims

1. A base steel; and a galvannealed layer provided on at least one surface of the base steel; The base iron contains, by mass%, 0.003 to 0.009% carbon (C), 0.05% or less silicon (Si), 0.4 to 1.0% manganese (Mn), 0.04 to 0.09% phosphorus (P), 0.01% or less sulfur (S), 0.005% or less nitrogen (N), 0.1% or less aluminum (S.Al), 0.05 to 0.08% molybdenum (Mo), 0.005 to 0.03% titanium (Ti), 0.02 to 0.045% niobium (Nb), 0.06 to 0.1% copper (Cu), and 0.0015% or less boron (B), with the remainder being Fe and other unavoidable impurities, and satisfies the following relational expression 1: The galvannealed steel sheet has an average thickness of 0.20 μm or less of a gamma (γ) phase present at an interface between the base steel and the galvannealed layer. [Relationship 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 (In the above-mentioned relational formula 1, [Ti], [Mo] and [Si] respectively represent the contents (mass%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the base iron.)

2. The galvannealed steel sheet according to claim 1, wherein an occupied area ratio of the delta (δ) phase on a surface of the galvannealed layer is 80 to 100%.

3. The galvannealed steel sheet according to claim 1, wherein the galvannealed layer contains, in mass %, 0.05 to 0.3% aluminum (Al) and 10 to 20% iron (Fe), with the remainder being zinc (Zn) and other unavoidable impurities.

4. The base steel contains 95% or more by area of ​​ferrite, The galvannealed steel sheet according to claim 1, wherein the average grain size of the ferrite is 15 µm or less.

5. The galvannealed steel sheet according to claim 1, wherein the galvannealed steel sheet has a tensile strength of 390 MPa or more and an elongation rate of 28% or more.

6. A step of preparing a cold-rolled steel sheet containing, by mass%, carbon (C): 0.003-0.009%, silicon (Si): 0.05% or less, manganese (Mn): 0.4-1.0%, phosphorus (P): 0.04-0.09%, sulfur (S): 0.01% or less, nitrogen (N): 0.005% or less, aluminum (S.Al): 0.1% or less, molybdenum (Mo): 0.05-0.08%, titanium (Ti): 0.005-0.03%, niobium (Nb): 0.02-0.045%, copper (Cu): 0.06-0.1%, and boron (B): 0.0015% or less, with the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1: immersing the cold-rolled steel sheet in a hot-dip galvanizing bath at a drawing temperature of 490 to 500° C. based on the surface temperature of the cold-rolled steel sheet to provide a galvanized steel sheet having a hot-dip galvanized layer formed thereon; and The galvannealed steel sheet is alloyed by heating the galvannealed steel sheet at a temperature in the range of 500 to 560° C. to obtain a galvannealed steel sheet; The method for producing a galvannealed steel sheet, wherein the galvannealed steel sheet has an average thickness of a gamma (γ) phase present at an interface between the cold-rolled steel sheet and the galvannealed layer of 0.20 μm or less. [Relationship 1] 0.08≦[Ti] / {48*([Mo] / 96+[Si] / 28)}≦0.3 (In the above-mentioned relational formula 1, [Ti], [Mo] and [Si] respectively represent the contents (mass%) of titanium (Ti), molybdenum (Mo) and silicon (Si) contained in the cold-rolled steel sheet.)

7. The method for producing a galvannealed steel sheet according to claim 6, wherein the hot-dip galvanizing bath contains, by mass%, 0.05 to 0.5% aluminum (Al), and the remainder is zinc (Zn) and other inevitable impurities.

8. The step of preparing the cold rolled steel sheet includes: producing a slab having a predetermined alloy composition by continuous casting; heating the slab at 1100-1300°C; hot rolling the heated slab at a finish rolling temperature of 920 to 970°C to obtain a hot rolled steel sheet; coiling the hot-rolled steel sheet at a temperature in the range of 600 to 650°C; The hot-rolled steel sheet is pickled and then cold-rolled at a rolling reduction of 70 to 83% to obtain a cold-rolled steel sheet; and The method for producing a galvannealed steel sheet according to claim 6, further comprising the step of annealing the cold-rolled steel sheet at a temperature in the range of 760 to 820°C.

9. The method for producing a galvannealed steel sheet according to claim 6, further comprising the step of temper rolling the galvannealed steel sheet at a reduction ratio of 0.6 to 1.2% using a skin pass roll having a surface roughness (Ra) of 1.0 to 1.6 μm.

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