High-strength galvanized steel sheet with excellent distinctness of image and method for producing the same

Abstract

The present invention provides a high-strength galvanized steel sheet with excellent distinctness of image and a method for manufacturing the same. The high-strength galvanized steel sheet of the present invention comprises, by mass%, C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02-0.035%, Cu: 0.01% or less, Cu: 0.01% or less, S: 0.01% or less, N: 0.01% or less, S.Al: 0.01% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02% or less, Cu: 0.01% or less, Cu: 0.01% or less, S.Al: 0.01% or less, Mo: 0.01% or less, Ti: 0.01% or less, Nb ... : 0.06-0.1%, B: 0.0015% or less, the balance being Fe and unavoidable impurities, and C, Ti and Nb satisfying Relationship 1, ferrite in the alloy microstructure is 95% or more by area fraction, the average grain size of the ferrite is 15 μm or less, ultrafine grains of 5 μm or less have a proportion of 7-10% in an area of ​​1 mm×1 mm, and ΔWsa defined by Relationship 2 is 0.1 or less.

Description

Technical Field

[0001] The present invention relates to a high-strength ultra-low carbon steel plated steel sheet having excellent distinctness of image and used for lightweighting automobiles and a method for manufacturing the same. More specifically, the present invention relates to a high-strength galvanized steel sheet preferably used as an automobile exterior panel material and a method for manufacturing the same. Background Art

[0002] Manufacturers have stringent requirements for painted steel parts for exterior panels of cars, such as hoods and doors. One of these requirements concerns the painted appearance of the painted parts. Exterior panels with a very good painted appearance, i.e. panels that reflect light without distortion and thus have a mirror-like surface that produces a sharp reflected image, are highly valued. The painted appearance is influenced by the quality of the paint and the surface of the (coated) substrate. This surface consists of in-plane structures of various sizes and scales. Smaller structures are identified by surface roughness, while larger structures are identified by so-called surface waviness.

[0003] Those skilled in the art are aware that larger surface structures, such as surface waviness, are transmitted through the various paint layers. Therefore, a certain degree of waviness from the (coated) substrate surface remains on the surface of the outer paint layer. Furthermore, in recent years, automotive manufacturers have sought to save energy and reduce costs by omitting midcoats in the painting process, making automotive surface waviness increasingly important. It is important to recognize that surface waviness needs to be measured after application, pressing, or forming.

[0004] As known to those skilled in the art, the surface waviness of a formed portion is the result of the surface waviness of an undeformed portion, e.g., a flat portion, and the increase in waviness introduced by the forming step. The difference between the waviness of the formed portion and the waviness of the undeformed portion is represented by delta waviness, e.g., ΔWsa.

[0005] (Prior art literature)

[0006] (Patent Document 1) Korean Patent Application No. 2013-0160369 Summary of the Invention

[0007] Technical problems to be solved

[0008] An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having excellent distinctness of image by controlling the grain size distribution by adding P, Nb, and Ti to ultra-low carbon steel used for automobile exterior panels requiring formability, and a method for producing the same.

[0009] 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.

[0010] Technical Solution

[0011] One aspect of the present invention relates to a high-strength hot-dip galvanized steel sheet with excellent surface distinctness of image, wherein the steel sheet comprises, in mass %, C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02-0 0.035%, Cu: 0.06-0.1%, B: 0.0015% or less, the balance being Fe and unavoidable impurities, and C, Ti and Nb satisfying the following Relationship 1, ferrite in the alloy fine structure accounts for 95% or more by area fraction, the average grain size of the ferrite is 15 μm or less, ultrafine grains of 5 μm or less have a proportion of 7-10% in an area of ​​1 mm×1 mm, and ΔWsa defined by the following Relationship 2 is 0.1 or less.

[0012] [Equation 1]

[0013] 0.03≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.04

[0014] [Equation 2]

[0015] △Wsa=5%Surface waviness of the steel plate after deformation-Surface waviness of the steel plate before deformation

[0016] The hot-dip galvanized steel sheet may have a tensile strength of 390-430 MPa and an elongation of more than 32%.

[0017] In addition, another aspect of the present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent surface distinctness of image, the method comprising the following process:

[0018] The steel billet having the above-mentioned composition is heated to 1100-1300°C;

[0019] 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;

[0020] 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;

[0021] 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

[0022] The hot-dip galvanized steel sheet is subjected to alloying heat treatment in the temperature range of 500-560°C.

[0023] 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.

[0024] Beneficial effects

[0025] The hot-dip galvanized steel sheet of the present invention, having the above-described composition, exhibits excellent distinctness of image 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.

[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 390 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 of the present invention with excellent surface distinctness of image produced from this viewpoint contains, by mass%, C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02% or less, and Cr: 0.01% or less. -0.035%, Cu: 0.06-0.1%, B: less than 0.0015%, the balance Fe and inevitable impurities, and C, Ti and Nb satisfy the relationship 1, in terms of area fraction, ferrite in the alloy fine structure is more than 95%, the average grain size of the ferrite is less than 15μm, in an area of ​​1mm×1mm, ultrafine grains of 5μm or less have a proportion of 7-10%, and ΔWsa defined by the relationship 2 is less than 0.1.

[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): 0.003-0.005%

[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 0.003% 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 0.005%, 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 0.003-0.005%.

[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): 0.4-1.0%

[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 0.4% may reduce strength, while a Mn content exceeding 1.0% may cause surface problems due to oxides. Therefore, the Mn content is preferably limited to 0.4-1.0%.

[0037] Phosphorus (P): 0.04-0.06%

[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 0.04%, the desired strength cannot be achieved. On the other hand, when the P content exceeds 0.06%, secondary brittleness and surface streak defects caused by P segregation may occur. Therefore, the P content is preferably limited to the range of 0.04-0.06%.

[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): 0.005-0.03%

[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 0.005% 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 0.005-0.03%.

[0047] Niobium (Nb): 0.02-0.035%

[0048] Niobium (Nb) is an element most effective in forming very fine grains during rolling and cooling, as the unrecrystallized austenite region expands to high temperatures due to solute drag and precipitate pinning during hot rolling. When the Nb content is less than 0.02%, 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 0.035%, the high-temperature strength increases, making hot rolling difficult. Therefore, the Nb content is preferably limited to the range of 0.02-0.035%.

[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 values ​​defined by the following equation 1 satisfy 0.03 to 0.04. The reason for setting this equation 1 in the present invention is that particle size refinement and uniformity, which effectively utilize the solute drag of Ti and Nb in their solid solution state and the pinning effect in their precipitated state, have the greatest impact on distinctness of image after coating.

[0055] When the value defined in the following relational expression 1 is less than 0.03, the particle size cannot be made sufficiently fine, and the surface deformation amount after deformation is not constant, and excellent distinctness of image cannot be obtained. On the other hand, when the value defined in the following relational expression 1 exceeds 0.04, the amount of Nb and the like added increases relatively, which is disadvantageous in terms of cost, and the strength becomes higher than expected, which causes problems in ensuring elongation.

[0056] [Equation 1]

[0057] 0.03≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.04

[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 0.005% 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 of the cold-rolled steel sheet used as the substrate for 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, uneven surface deformation during molding can make it difficult to achieve the desired distinctness of image. More preferably, the average grain size of the substrate's microstructure is controlled to be less than 10 μm.

[0061] Furthermore, in the cold-rolled steel sheet of the present invention, the proportion of ultrafine grains of 5 μm or less in an area of ​​1 mm x 1 mm is preferably 7-10%. This proportion allows for a hot-dip galvanized steel sheet with excellent surface distinctness of image (DVSA) of 0.1 or less, as defined by the following equation 2. If the proportion is less than 7%, the grain size becomes relatively large, and the amount of surface deformation after forming (after 5% deformation) increases, making it impossible to ensure the desired distinctness of image. If the proportion exceeds 10%, the strength becomes excessively high, resulting in problems in ensuring an elongation of 32% or more.

[0062] [Equation 2]

[0063] △Wsa=5%Surface waviness of the steel plate after deformation-Surface waviness of the steel plate before deformation

[0064] Next, the method of producing a high-strength hot-dip galvanized steel sheet having excellent surface distinctness of image according to the present invention will be described.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Furthermore, in the present invention, the coiling temperature is preferably controlled within the range of 600-650°C. When the coiling temperature is lower than 600°C, precipitates such as Ti(Nb)C are not formed, and the solid-solution Ti and Nb increase. During heating in the annealing process, they finely precipitate as TiC and Ti(Nb)C, or exist in a solid-solution state of Ti and Nb, which inhibits recrystallization and grain growth. This may cause problems in ensuring the desired strength and elongation. When the coiling temperature exceeds 630°C, secondary oxide scale may form, resulting in surface degradation.

[0070] 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%.

[0071] 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.

[0072] 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.

[0073] The annealed cold-rolled steel sheet can be directly hot-dip galvanized in a continuous hot-dip galvanizing line.

[0074] 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.

[0075] 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

[0076] Hereinafter, the present invention will be described in detail with reference to examples.

[0077] (Example)

[0078] 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.

[0079] Furthermore, for each of the produced hot-dip galvanized steel sheets, tensile properties, r value (Lankford value) as an indicator of deep drawing, grain size and distribution ratio were measured, and ΔWsa was examined. The measurement methods are described below.

[0080] Tensile testing measures YS, TS, and T-E1. YS, TS, and T-E1 represent yield strength, tensile strength, and elongation at break, respectively. Test specimens prepared according to JIS 5 standards are used for these tests. A tensile strength of 390-430 MPa and an elongation of 32% or greater are considered acceptable.

[0081] 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 90 When , the r value in each direction is calculated by the following mathematical formula A. In this embodiment, the case where the r value is 1.2 or more is considered to be acceptable.

[0082] [Mathematical formula A]

[0083] A=r0+2*r 45 +r 90 / 4

[0084] The particle size and its distribution were measured by EBSD and evaluated using TSL OIM analysis software.

[0085] Furthermore, to evaluate the Wsa after deformation, a 225 mm x 225 mm blank was punched in a press with a 75 mm diameter hollow punch and blank holder to completely suppress any movement of the substrate between the blank holder and the die, thereby producing a cup. For cup deformation, the punch draw depth was preferably approximately 17-18 mm, resulting in a thickness deformation rate of approximately 5% + / - 0.2% at the bottom. Table 3 below indicates that to increase the likelihood of achieving ΔWsa ≤ 0.1, the material's grain size should be 15 μm or less.

[0086] [Table 1]

[0087]

[0088] *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.

[0089] [Table 2]

[0090]

[0091] [Table 3]

[0092]

[0093] As shown in Tables 1 to 3, it can be confirmed that Inventive Examples 1 to 6, in which the steel composition and the plated steel sheet manufacturing process conditions satisfy the scope of the present invention, exhibit excellent tensile properties, r-values, ultrafine grain ratios, and ΔWsa.

[0094] 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.

[0095] Specifically, in Comparative Examples 1 and 3, the hot rolling process was operated at a Finish Mill Delivery Temperature (FDT) below the Ar3 temperature, which increased the grain size of the surface layer and resulted in a low proportion of ultrafine grains in the final annealing structure, thus failing to ensure the desired ΔWsa.

[0096] In Comparative Example 2, since the hot rolling coiling (CT) temperature was as high as 700°C, the grain size coarsened and the desired ultrafine grain fraction could not be ensured. In Comparative Example 4, since the annealing temperature was below the recrystallization temperature, sufficient recrystallization could not be achieved and the desired strength and elongation could not be ensured.

[0097] Furthermore, it is found that Comparative Examples 5 to 7, in which the steel composition and the production process conditions of the plated steel sheets are outside the range of the present invention, do not satisfy the ratio of ultrafine grains and have large ΔWsa values, resulting in poor distinctness of image.

[0098] In addition, in Comparative Example 8, the relationship 1 in the steel composition is not within the scope of the present invention. Even if the plated steel sheet is manufactured by the plated steel sheet manufacturing process of the present invention, the final ultrafine grain fraction is insufficient, and therefore the desired ΔWsa value cannot be ensured.

[0099] 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 having excellent surface distinctness of image, comprising, in mass%, C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02-0.035%, 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 alloy microstructure comprises 95% or more ferrite by area fraction, the average grain size of the ferrite is 15 μm or less, ultrafine grains of 5 μm or less have a proportion of 7-10% in an area of ​​1 mm×1 mm, and ΔWsa defined by the following relational expression 2 is 0.1 or less. [Equation 1] 0.03≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.04 [Equation 2] ΔWsa = 5% surface waviness of the steel plate after deformation - surface waviness of the steel plate before deformation.

2. The high-strength hot-dip galvanized steel sheet having excellent surface distinctness of image according to claim 1, characterized in that: The tensile strength of the steel plate is 390-430 MPa, and the elongation is above 32%.

3. A method for producing a high-strength hot-dip galvanized steel sheet with excellent surface distinctness of image, comprising the following steps: The steel slab is heated to 1100-1300° C., wherein the steel slab comprises, in terms of mass%, C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05-0.08%, Ti: 0.005-0.03%, Nb: 0.02-0.035%, Cu: 0.06-0.1%, B: 0.0015% or less, and the balance being Fe and unavoidable impurities, and C, Ti and Nb satisfy the following relationship 1; 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℃. [Equation 1] 0.03≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.

04.

4. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface distinctness of image 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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