High-strength galvanized steel sheet with excellent formability and method for producing the same

Abstract

The present invention provides a high-strength galvanized steel sheet with excellent formability and a method for manufacturing the same. The high-strength galvanized steel sheet of the present invention comprises, by mass%, C: 0.005-0.009%, Si: 0.05% or less, Mn: 0.3-0.8%, P: 0.06-0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.01-0.03%, Nb: 0.03-0.045%, C u: 0.06-0.1%, B: 0.0015% or less, the balance Fe and unavoidable impurities, and C, Ti and Nb satisfy equation 1, ferrite in the alloy microstructure is more than 95% by area fraction, the average grain size of the ferrite is less than 15 μm, ultrafine grains of less than 6 μm have a proportion of 5-10% in an area of ​​1 mm × 1 mm, and the surface nanohardness value is 1-1.5 GPa.

Description

Technical Field

[0001] The present invention relates to the manufacture of an ultra-high-strength ultra-low-carbon steel plated steel sheet having excellent formability and used for lightweighting automobiles, and more particularly to a high-strength zinc-based plated steel sheet preferably usable as an automobile exterior panel material and a method for manufacturing the same. Background Art

[0002] Cold-rolled steel sheets processed by stamping and the like are used as outer panel materials for automobiles, and generally require high formability. In addition, from the perspective of preventing global warming, new targets for improving automobile fuel costs are being set as carbon dioxide emission control measures, and a tax system for low-fuel-efficiency automobiles is being introduced, and it is necessary to improve the fuel efficiency of automobiles. In terms of improving the fuel efficiency of automobiles, lightweighting of automobile bodies is an effective means, and from the perspective of such lightweighting, thinning of steel sheets for automobile bodies is required. In addition, from the perspective of ensuring the safety of automobile bodies, high-strength steel sheets for automobile bodies are required. As a steel sheet for automobile bodies that meets the requirements of thinness and high strength of steel sheets as described above and is stamped into a complex shape, a zinc-based plated high-tensile steel sheet with excellent surface appearance and stamping formability is required.

[0003] To improve the formability of automotive steel sheets, so-called IF steels (interstitial-free steels) are available. IF steels are made by adding Ti or Nb, either singly or in combination, to ultra-low carbon cold-rolled steel sheets, causing dissolved elements such as C, N, and S to precipitate as carbides and nitrides, thereby increasing elongation and plastic strain ratio, thereby improving formability. Therefore, in the prior art, high cleanliness is achieved during the steelmaking process, and aging phenomena caused by dissolved elements are limited by adding carbonitride-forming elements such as titanium to precipitate dissolved elements. Furthermore, in high-tensile steel sheets, solid solution strengthening elements such as Si, Mn, and P are included in the steel to increase the strength of the steel sheet.

[0004] In particular, phosphorus (P) is added to steel to improve the strength of steel sheets. However, P is an element that segregates easily. P segregating on the slab surface is elongated along the length of the steel sheet during hot and cold rolling, forming a P-rich layer on the coil surface. Alloying is delayed during plating in this P-rich layer, causing linear defects in alloyed hot-dip galvanized steel sheets. To address this issue, a method for manufacturing alloyed hot-dip galvanized steel sheets using steel sheets with a P content of 0.03% or more as a substrate has been proposed, in which the surface of the steel sheet is abraded by an amount corresponding to the amount of P removed, followed by alloying in an induction heating alloying furnace (Patent Document 1).

[0005] In these prior art methods, to prevent linear defects in alloyed hot-dip galvanized steel sheets, for example, when using ultra-low carbon Ti-added steel sheets with a phosphorus content of 0.03% or more, the surface is flame-finished to a depth of 3 mm or more during the continuous casting process, and the surface is ground to a depth of 5 μm or more during the steel sheeting process before plating. This prevents the occurrence of shape defects after plating, thereby ensuring surface quality. However, this reduces the actual yield. Therefore, there is a need for a method that can achieve excellent surface appearance while ensuring a good yield and producing high formability and high strength.

[0006] [Prior art literature]

[0007] (Patent Document 1) Japanese Patent Publication No. 2004-169160 Summary of the Invention

[0008] Technical problems to be solved

[0009] An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet and a method for producing the same, wherein the steel sheet has excellent formability and distinctness of image when P, Nb, and Ti are added to ultra-low carbon steel used for automobile exterior panels requiring formability to control grain size distribution.

[0010] In addition, the technical problem of the present invention is not limited to the above content. The technical problem of the present invention can be understood from the entire content of this specification, and those skilled in the art to which the present invention belongs will have no difficulty in understanding the additional technical problems of the present invention.

[0011] Technical Solution

[0012] One aspect of the present invention relates to a high-strength hot-dip galvanized steel sheet with excellent formability, comprising, by mass%, C: 0.005-0.009%, Si: 0.05% or less, Mn: 0.3-0.8%, P: 0.06-0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.01-0.03%, Nb: 0.03% or less. -0.045%, Cu: 0.06-0.1%, B: less than 0.0015%, the balance of Fe and inevitable impurities, C, Ti and Nb satisfy the following relationship 1, in terms of area fraction, ferrite in the alloy microstructure is more than 95%, the average grain size of the ferrite is less than 15μm, in an area of ​​1mm×1mm, ultrafine grains below 6μm have a proportion of 5-10%, and the surface nanohardness value is 1-1.5GPa.

[0013] [Equation 1]

[0014] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065

[0015] The hot-dip galvanized steel sheet may have a tensile strength of 440 MPa or greater, and an r-value of 1.4 or greater.

[0016] In addition, another aspect of the present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability, comprising the following steps:

[0017] The steel billet having the above-mentioned composition is heated to 1100-1300°C;

[0018] Hot rolling the heated steel slab to a finishing temperature of 920-970° C., and then coiling the slab at a temperature of 600-650° C. to produce a hot-rolled steel plate;

[0019] Pickling the coiled hot-rolled steel sheet and then cold-rolling it at a reduction ratio of 70-83% to obtain a cold-rolled steel sheet;

[0020] Annealing the cold-rolled steel sheet at a temperature in the range of 760-830° C. and then hot-dip galvanizing the steel sheet; and

[0021] The hot-dip galvanized steel sheet is subjected to alloying heat treatment in the temperature range of 500-560°C.

[0022] The alloying heat treated hot dip galvanized steel sheet may be temper rolled at 0.6-1.2% using a temper roll having a roughness (Ra) of 1.0-1.6 μm.

[0023] Beneficial effects

[0024] The hot-dip galvanized steel sheet of the present invention, having the above-described composition, exhibits excellent formability and high strength, and can therefore be stably used as a steel sheet for automobile exterior panels. Consequently, the application range of high-strength cold-rolled steel sheets containing phosphorus (P) in automobile bodies can be expanded to previously unused areas, such as side outer panels, thereby further contributing to the lightweighting of automobile bodies. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the correlation between the ratio of ultrafine crystal grains having an average crystal grain size of 6 μm or less and the surface nanohardness in Examples of the present invention.

[0026] Best Practice

[0027] Hereinafter, the present invention will be described.

[0028] As a result of in-depth research to solve the problems of the above-mentioned prior art, the inventors have confirmed that by adding titanium (Ti) and / or niobium (Nb) as strong carbonitride-forming elements to steel, ensuring formability by minimizing solid solution elements such as carbon (C), nitrogen (N), and sulfur (S), and simultaneously adding P and Mo, it is possible to produce a high-formability, high-strength steel sheet for vehicle exterior panels with a tensile strength of 440 MPa or higher and excellent surface quality, thereby completing the present invention. Generally, steel sheets used for automotive exterior panels must have high tensile strength while meeting stamping properties such as deep drawability. Therefore, as the base material of the alloyed hot-dip galvanized steel sheet of the present invention, a high-tensile steel sheet is used that uses ultra-low carbon steel as a basic component and adds Mn, P, etc. as strengthening elements to improve workability.

[0029] Therefore, the high-strength hot-dip galvanized steel sheet with excellent formability of the present invention manufactured from this viewpoint contains, in mass%, C: 0.005-0.009%, Si: 0.05% or less, Mn: 0.3-0.8%, P: 0.06-0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.01-0.03%, Nb: 0.03% or less. -0.045%, Cu: 0.06-0.1%, B: less than 0.0015%, the balance of Fe and inevitable impurities, C, Ti and Nb satisfy the following relationship 1, in terms of area fraction, ferrite in the alloy microstructure is more than 95%, the average grain size of the ferrite is less than 15μm, in an area of ​​1mm×1mm, ultrafine grains below 6μm have a proportion of 5-10%, and the surface nanohardness value is 1-1.5GPa.

[0030] First, the alloy components of the cold-rolled steel sheet constituting the base material of the hot-dip galvanized steel sheet of the present invention and the reasons for limiting their contents are described. In addition, "%" herein means "% by weight" unless otherwise specified.

[0031] Carbon (C): 0.005-0.009%

[0032] C is an interstitial solid solution element that significantly influences the texture formation of the steel sheet during cold rolling and annealing, necessitating the addition of at least 0.005% C. However, increasing the amount of dissolved carbon in the steel inhibits the growth of grains with a {111} γ-fiber texture, which is beneficial for drawing, and promotes the growth of grains with {110} and {100} textures, thereby reducing the drawability of the annealed sheet. Furthermore, when the C content exceeds 0.009%, the Ti and Nb levels required to precipitate C as carbides increase, which is economically disadvantageous and may also reduce formability due to the formation of pearlite. Therefore, in the present invention, the C content is preferably limited to the range of 0.005-0.009%.

[0033] Silicon (Si): 0.05% or less (except 0%)

[0034] Si is an element that contributes to improving strength through solid solution strengthening. When the Si content exceeds 0.05%, surface scale defects are caused, thereby causing a problem of reduced plating surface properties. Therefore, in the present invention, the Si content is preferably controlled to 0.05% or less.

[0035] Manganese (Mn): 0.3-0.8%

[0036] Mn is a solid solution strengthening element that not only contributes to strength but also precipitates S in the steel as MnS. A Mn content of less than 0.3% may reduce strength, while a Mn content exceeding 0.8% may cause surface problems due to oxides. Therefore, the Mn content is preferably limited to 0.3-0.8%.

[0037] Phosphorus (P): 0.06-0.09%

[0038] Phosphorus (P) has the best solid solution effect and is the most effective element for ensuring steel strength without significantly compromising drawability. When the P content is less than 0.06%, the desired strength cannot be achieved. On the other hand, when the P content exceeds 0.09%, secondary brittleness and surface streak defects caused by P segregation may occur. Therefore, the P content is preferably limited to the range of 0.06-0.09%.

[0039] Molybdenum (Mo): 0.05-0.08%

[0040] Mo has a high affinity for phosphorus (P), suppressing P segregation. Ensuring high strength in ultra-low carbon steel inevitably requires the use of P. Adding an appropriate amount of Mo can partially help improve surface defects caused by P segregation. A Mo content of less than 0.05% does not significantly improve the desired surface quality. However, a Mo content exceeding 0.08% increases the price, reducing cost competitiveness. Therefore, the Mo content is preferably limited to 0.05-0.08%.

[0041] Sulfur (S): 0.01% or less, Nitrogen (N): 0.005% or less

[0042] S and N are impurities present in steel and are inevitably added. To ensure excellent welding properties, it is preferable to control the S and N contents to be as low as possible. In the present invention, the S content is preferably controlled to be less than 0.01%, and the N content is preferably controlled to be less than 0.005%.

[0043] Aluminum (Al): 0.1% or less (except 0%)

[0044] Al contributes to improving the drawability and ductility of steel by precipitating AlN. However, if the Al content exceeds 0.1%, excessive Al inclusions are formed during steelmaking, leading to internal defects in the steel sheet. Therefore, the Al content is preferably kept below 0.1%.

[0045] Titanium (Ti): 0.01-0.03%

[0046] Ti is an element that significantly contributes to improving the drawability of steel sheets by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides. A Ti content of less than 0.01% prevents sufficient precipitation of carbonitrides, resulting in poor drawability. On the other hand, a Ti content exceeding 0.03% makes inclusion control difficult during steelmaking operations, potentially leading to inclusion-related defects. Therefore, the Ti content is preferably limited to the range of 0.01-0.03%.

[0047] Niobium (Nb): 0.03-0.045%

[0048] Niobium (Nb) is an element most effective in forming very fine grains during the rolling and cooling processes, as it expands the unrecrystallized austenite region to high temperatures due to solute drag and precipitate pinning during hot rolling. When the Nb content is less than 0.03%, the unrecrystallized austenite temperature range in the steel narrows, and the grain size reduction effect is insignificant. On the other hand, when the Nb content exceeds 0.045%, the high-temperature strength increases, making hot rolling difficult. Therefore, the Nb content is preferably limited to the range of 0.03-0.045%.

[0049] Boron (B): 0.003% or less (except 0%)

[0050] B is an element added to prevent secondary working embrittlement caused by adding P to steel. However, if the B content exceeds 0.003%, the ductility of the steel sheet decreases. Therefore, the B content is preferably limited to 0.003% or less.

[0051] Copper (Cu): 0.04-0.1%

[0052] Cu is an element that is difficult to remove when adjusting the steel composition through steelmaking, and is contained in trace amounts (for example, more than 0.04%). However, when the Cu content exceeds 0.1%, it is easy to produce marks in the hot-dip galvanized steel sheet, and it also causes grain boundary embrittlement or increased costs. Therefore, the Cu content is preferably limited to the range of 0.04-0.1%.

[0053] Relationship 1

[0054] In the present invention, the contents of C, Ti, and Nb need to be controlled so that the value defined by the following equation 1 satisfies 0.05 to 0.065. The reason for setting this equation 1 in the present invention is that the elements most effective for grain refinement are Ti and Nb, and these two elements affect recrystallization behavior in a solid solution state and / or as precipitates combined with C. Therefore, in order to achieve the purpose pursued by the present invention, it is important to control the contents of C, Ti, and Nb.

[0055] When the value defined in the following relational formula 1 is less than 0.05, the desired strength cannot be ensured due to insufficient refinement of the grain size, or surface streak defects may occur due to the yield point phenomenon while the solid solution C increases. However, when the value defined in the following relational formula 1 exceeds 0.065, the addition amount of Ti and Nb elements increases relatively and there is a problem of being uncompetitive in terms of cost.

[0056] [Equation 1]

[0057] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065

[0058] Furthermore, the balance of Fe and inevitable impurities are contained, and the addition of effective components other than the above composition is not excluded.

[0059] The present invention is a hot-dip galvanized steel sheet having an ultra-low carbon steel base material with a C content of 0.009% or less. Therefore, the microstructure consists of a single-phase ferrite structure. However, this single-phase ferrite structure may also contain other structures that inevitably form. Therefore, the microstructure of the alloy of the present invention comprises 95% or more of ferrite by area fraction, and a small amount of pearlite and the like may remain as residual components.

[0060] Furthermore, the average grain size of the microstructure grains of the cold-rolled steel sheet serving as the base material of the hot-dip galvanized steel sheet of the present invention is preferably 15 μm or less. If the average grain size exceeds 15 μm or less, the strength desired by the present invention cannot be sufficiently ensured.

[0061] Furthermore, in the cold-rolled steel sheet of the present invention, the proportion of ultrafine grains of 6 μm or less in an area of ​​1 mm x 1 mm is preferably 5-10%. This proportion allows for a hot-dip galvanized steel sheet with excellent formability. If the proportion is less than 5%, the desired strength cannot be achieved. If the proportion exceeds 10%, the strength increases excessively while the elongation decreases, leading to a problem of poor formability.

[0062] Furthermore, in order to ensure the distinctness of image on the surface, the surface nanohardness of the cold-rolled steel sheet of the present invention is preferably controlled within the range of 1 to 1.5 GPa.

[0063] Next, the method of producing a high-strength hot-dip galvanized steel sheet having excellent formability according to the present invention will be described.

[0064] The method for manufacturing a high-strength hot-dip galvanized steel sheet of the present invention comprises the following processes: heating a steel billet meeting the above-mentioned composition to 1100-1300° C.; hot-rolling the heated steel billet to a finishing temperature of 920-970° C., and then coiling at a temperature of 600-650° C. to manufacture a hot-rolled steel sheet; pickling the coiled 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 in a temperature range of 760-830° C. and then hot-dip galvanizing it; and performing an alloying heat treatment on the hot-dip galvanized steel sheet in a temperature range of 500-560° C.

[0065] First, in the present invention, a steel slab having the above-described composition is heated within a temperature range of 1100-1300° C. If the heating temperature is lower than 1100° C., problems may arise in production due to the rolling load in the FM range. If the heating temperature exceeds 1300° C., surface scale defects may occur.

[0066] Next, in the present invention, the heated steel slab is hot rolled to a finishing temperature of 920-970° C., and then coiled at a temperature of 600-650° C. to produce a hot-rolled steel sheet.

[0067] In the present invention, the finishing rolling temperature is preferably limited to 920-970°C. When the finishing rolling temperature is lower than 920°C, coarse grains are formed on the surface, which may cause uneven material quality. When the finishing rolling temperature exceeds 970°C, the grain size is not fine enough, which may eventually cause insufficient material quality.

[0068] Furthermore, in the present invention, the coiling temperature is preferably controlled within the range of 600-650°C. When the coiling temperature is below 600°C, precipitates such as Ti(Nb)C are not formed, and the amount of dissolved Ti and Nb increases. During heating in the annealing process, these precipitates finely as TiC and Ti(Nb)C, or Ti and Nb exist in a solid solution state, inhibiting recrystallization and grain growth. This can lead to problems in achieving the desired strength and elongation. On the other hand, when the coiling temperature exceeds 630°C, secondary oxide scale formation can lead to surface degradation.

[0069] Furthermore, in the present invention, after undergoing a pickling process to remove surface oxide scale from the coiled hot-rolled steel sheet, the sheet is cold-rolled at a reduction ratio of 70-83% to produce the cold-rolled steel sheet. If the cold-rolling reduction ratio is less than 70%, the {111} texture fails to fully develop, resulting in poor formability. On the other hand, if the cold-rolling reduction ratio exceeds 83%, the roll load during on-site manufacturing becomes excessive, leading to poor shape and a corresponding problem. Therefore, the reduction ratio is preferably limited to 70-83%, and more preferably to 74-80%.

[0070] Subsequently, the cold-rolled steel sheet manufactured as described above is subjected to an annealing process and hot-dip galvanizing or alloyed hot-dip galvanizing.

[0071] When annealing cold-rolled steel sheets, annealing is performed at a temperature above the recrystallization temperature within the temperature range of 760-830°C. Annealing at a temperature above the recrystallization temperature removes deformation caused by rolling and softens the steel, thereby improving workability.

[0072] The annealed cold-rolled steel sheet can be directly hot-dip galvanized in a continuous hot-dip galvanizing line.

[0073] Furthermore, in the present invention, the hot-dip galvanized steel sheet can be subjected to an alloying heat treatment. The alloying heat treatment is performed at a temperature between 500°C and 560°C after hot-dip galvanizing. If the alloying heat treatment temperature is lower than 500°C, sufficient alloying cannot be achieved. On the other hand, if the alloying heat treatment temperature exceeds 560°C, excessive alloying occurs, resulting in embrittlement of the coating, which may cause problems such as coating peeling during processing such as stamping.

[0074] At this time, in the present invention, as needed, the hot-dip galvanized steel sheet subjected to the alloying heat treatment may be subjected to 0.6-1.2% temper rolling using a temper rolling roll having a roughness (Ra) of 1.0-1.6 μm. DETAILED DESCRIPTION

[0075] Hereinafter, the present invention will be described in detail with reference to examples.

[0076] (Example)

[0077] A 250 mm thick steel slab having the alloy composition described in Table 1 below was reheated to 1250° C. and then subjected to hot rolling, cold rolling, continuous annealing, and alloying hot-dip galvanizing under the conditions shown in Table 2 below to produce a hot-dip galvanized steel sheet.

[0078] Furthermore, for each produced hot-dip galvanized steel sheet, tensile properties, r-value (Lankford value) as an indicator of deep drawing, grain size and distribution ratio, and surface nanohardness were measured. The measurement methods are described below.

[0079] YS, TS, and T-E1 were measured through a tensile test. YS, TS, and T-E1 represent yield strength, tensile strength, and elongation at break, respectively. Test specimens prepared according to JIS 5 standards were used for the tensile test. A tensile strength of 440 MPa or greater was considered acceptable.

[0080] In addition, to evaluate the r value, which is an indicator of deep drawing, JIS No. 5 tensile test pieces are taken from the alloyed hot-dip galvanized steel sheet in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction, and the r value of each test piece is measured. For example, to measure the r value, the change in plate thickness and the change in plate width when a tensile deformation of about 15% is performed in the above-mentioned tensile test are measured, and the ratio of the change in plate width to the change in plate thickness is calculated. In addition, the r value in the direction parallel to the rolling direction is referred to as r0, and the r value in the direction 45° is referred to as r 45 , the vertical r value is set to r 90When , the r value in each direction is calculated by mathematical formula A.

[0081] [Mathematical formula A]

[0082] A=r0+2*r 45 +r 90 / 4

[0083] And, for particle size and its distribution, by EBSD measurement and utilize TSL OIM analysis software to evaluate.In addition, surface nanohardness is the value of indentation measurement with a depth of 500nm after surface electropolishing pretreatment.By observing 5 positions in total, average value is preferably 1-1.5GPa.

[0084] [Table 1]

[0085]

[0086]

[0087] *In Table 1, the contents of Al and N in all steel grades are in the range of 0.02% and 0.0005%, respectively, and the balance is Fe and unavoidable impurities.

[0088] [Table 2]

[0089]

[0090] [Table 3]

[0091]

[0092] As shown in Tables 1 to 3, it can be confirmed that Invention Examples 1 to 6, in which the steel composition and the manufacturing process conditions of the plated steel sheets satisfy the scope of the present invention, exhibit excellent tensile properties, r-values, ultrafine grain ratios, and surface nanohardness.

[0093] On the other hand, Comparative Examples 1 to 4 are cases where the composition of the steel satisfies the range of the present invention but the manufacturing process of the plated steel sheet does not fall within the range of the present invention.

[0094] Specifically, in Comparative Examples 1 and 3, the grain size was not sufficiently fine while operating at annealing temperatures as high as 830°C or higher, thus failing to achieve the desired tensile strength and surface nanohardness values. Furthermore, in Comparative Examples 2 and 4, the finish mill delivery temperature (FDT) during hot rolling was below the Ar3 temperature, resulting in increased surface grain size and a low proportion of fine particles in the final annealed structure, thus failing to achieve the desired surface nanohardness.

[0095] Furthermore, it can be seen that Comparative Examples 5 to 7, whose steel composition and plated steel sheet manufacturing process conditions fall outside the scope of the present invention, do not meet the requirements for the ultrafine grain ratio and surface nanohardness, and therefore cannot ensure the desired strength. In particular, it can be seen that the annealing temperature in Comparative Example 6 was too low, and sufficient recrystallization did not occur. Therefore, while the ultrafine grain ratio and strength were met, the elongation and formability r-value were not satisfactory.

[0096] Furthermore, Comparative Example 8 is a case where Relationship Formula 1 in the composition of the steel is not within the scope of the present invention. It can be confirmed that even when the plated steel sheet is manufactured by the process for manufacturing the plated steel sheet of the present invention, a sufficient fraction of fine grains cannot be ensured, and the desired surface nanohardness value cannot be ensured.

[0097] in addition, Figure 1 This is a graph showing the correlation between the ratio of ultrafine crystal grains having an average crystal grain size of 6 μm or less and the surface nanohardness in Examples of the present invention.

[0098] As described above, the preferred embodiments of the present invention are described in the detailed description of the present invention. However, those skilled in the art may make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention is not limited to the described embodiments but is determined by the claims and their equivalents.

Claims

1. A high-strength hot-dip galvanized steel sheet with excellent formability, comprising, in mass%, C: 0.005-0.009%, Si: 0.05% or less, Mn: 0.3-0.8%, P: 0.06-0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.01-0.03%, Nb: 0.03-0.045%, Cu: 0.06-0.1%, B: 0.0015% or less, and the balance being Fe and unavoidable impurities, wherein C, Ti, and Nb satisfy the following relationship 1: The steel plate has a microstructure comprising ferrite accounting for 95% or more by area fraction, an average grain size of 15 μm or less, ultrafine grains of 6 μm or less in an area of ​​1 mm×1 mm in the microstructure of the steel plate accounting for 5-10% of the total, and a surface nanohardness of 1-1.5 GPa. [Equation 1] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.

065.

2. The high-strength hot-dip galvanized steel sheet having excellent formability according to claim 1, wherein The hot-dip galvanized steel sheet has a tensile strength of 440 MPa or greater and an r value of 1.4 or greater.

3. A method for manufacturing the high-strength hot-dip galvanized steel sheet having excellent formability according to claim 1, comprising the following steps: The steel slab is heated to 1100-1300° C., and contains, in terms of mass%, C: 0.005-0.009%, Si: 0.05% or less, Mn: 0.3-0.8%, P: 0.06-0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.01-0.03%, Nb: 0.03-0.045%, Cu: 0.06-0.1%, B: 0.0015% or less, and the balance being Fe and unavoidable impurities, wherein C, Ti, and Nb satisfy the following relationship 1: [Equation 1] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065; Hot rolling the heated steel slab to a finishing temperature of 920-970° C., and then coiling the slab at a temperature of 600-650° C. to produce a hot-rolled steel plate; Pickling the coiled 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 760-830° C. and then hot-dip galvanizing the steel sheet; as well as The hot-dip galvanized steel sheet is subjected to alloying heat treatment in the temperature range of 500-560°C.

4. The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to claim 3, wherein: The hot-dip galvanized steel sheet subjected to alloying heat treatment was subjected to 0.6-1.2% temper rolling using a temper rolling roll having a roughness Ra of 1.0-1.6 μm.

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