High-strength zinc-based plated steel sheet excellent in mapping property and method for producing the same

Abstract

A high-strength zinc-based plated steel sheet with excellent image clarity and a method for producing the same are provided. The high-strength galvanized steel sheet of the present invention contains, in mass%, C: 0.003 to 0.005%, Si: 0.05% or less, Mn: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.035%, Cu: 0.06 to 0.1%, B: 0.00 A steel plate containing 15% or less of C, Ti, and Nb, the balance being Fe and unavoidable impurities, and in which C, Ti, and Nb satisfy relational expression 1, and the microstructure of the alloy is such that, in terms of area fraction, ferrite accounts for 95% or more, the average grain size of the ferrite is 15 μm or less, and ultrafine grains of 5 μm or less account for 7 to 10% within an area of ​​1 mm × 1 mm, and ΔWsa defined by relational expression 2 is 0.1 or less.

Description

Technical Field

[0001] The present invention relates to a high-strength extra-low carbon steel plated steel sheet excellent in formability and for use in the weight reduction of automobiles, and more particularly to a high-strength zinc-based plated steel sheet preferably applicable as an outer panel material for automobiles and a method for producing the same.

Background Art

[0002] Automobile manufacturers apply strict requirements to parts made of painted steel for external panels such as hoods and doors. One of these requirements relates to the painted appearance of the painted parts. External panels with a very good painted appearance, i.e., panels that reflect light without distortion and have a mirror-like surface with clear reflected images, are highly evaluated. The painted appearance is affected not only by the quality of the paint but also by the surface of the (coated) substrate. This surface is composed of structures in a plane of various sizes and scales. Smaller structures are grasped as surface roughness, while larger structures are grasped as so-called surface waviness.

[0003] It is already known to those skilled in the art that larger surface structures, such as surface waviness, are conducted through different coating layers. Therefore, the waviness of the (coated) substrate surface still remains on the surface of the external coating layer for some time. Also, recently, automobile manufacturers are emphasizing the surface waviness of automobiles even more because they are trying to save energy and reduce costs by omitting the intermediate coating in the painting process. It is important to recognize that surface waviness should be measured after crimping or forming is applied.

[0004] It is known to those skilled in the art that the surface waviness of the formed part is the result of the increased surface waviness of the non-deformed, for example, flat part and the waviness introduced by the forming stage. The difference between the waviness of the formed part and the waviness of the non-deformed part is indicated by delta waviness, for example, ΔWsa.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet excellent in imageability and a method for producing the same by adding P, Nb, and Ti to an extra-low carbon steel applied to an outer panel of an automobile that requires formability to control the grain size distribution.

[0007] On the other hand, the problems of the present invention are not limited to the above-described contents. The problems of the present invention can be understood from the entire contents of this specification, and there is no difficulty in understanding the additional problems of the present invention for those having ordinary knowledge in the technical field to which the present invention belongs.

Means for Solving the Problems

[0008] One aspect of the present invention is a steel sheet containing, by mass, C: 0.003 to 0.005%, Si: 0.05% or less, Mn: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.035%, Cu: 0.06 to 0.1%, B: 0.0015% or less, with the balance being Fe and unavoidable impurities, wherein C, Ti, and Nb satisfy the following relational expression 1. The microstructure of the alloy has a ferrite area fraction of 95% or more, the average size of the ferrite crystal grains is 15 μm or less, the ultrafine grains of 5 μm or less have a ratio of 7 to 10% within an area of 1 mm × 1 mm, and ΔWsa defined by the following relational expression 2 is 0.1 or less. The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent surface imageability.

[0009] [Relational Expression 1] 0.03 ≤ [(Nb(48 / 93)) + (Ti(93 / 48)) + (C(12 / 48))] ≤ 0.04

[0010] [Relational Expression 2] ΔWsa = Surface undulation of the steel sheet after 5% deformation - Surface undulation of the steel sheet before deformation

[0011] The hot-dip galvanized steel sheet may have a tensile strength of 390 to 430 MPa and an elongation of 32% or more.

[0012] Another aspect of the present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent surface imageability, including a step of heating a steel slab satisfying the above composition components to 1100 to 1300°C, a step of hot-rolling the heated steel slab so that the finish rolling temperature is 920 to 970°C, and then winding it at a temperature of 600 to 650°C to produce a hot-rolled steel sheet, a step of obtaining a cold-rolled steel sheet by cold-rolling the wound hot-rolled steel sheet with a reduction ratio of 70 to 83% after pickling, a step of annealing the cold-rolled steel sheet within a temperature range of 760 to 830°C, and then performing hot-dip galvanizing, and a step of subjecting the hot-dip galvanized steel sheet to an alloying heat treatment within a temperature range of 500 to 560°C.

[0013] For the above alloyed and heat-treated hot-dip galvanized steel sheet, temper rolling can be performed at 0.6 to 1.2% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm.

Advantages of the Invention

[0014] The hot-dip galvanized steel sheet of the present invention having the above-described configuration has excellent imageability and high strength, and thus can be stably used as a steel sheet for the outer panel of an automobile. Therefore, the applicable range of high-strength cold-rolled steel sheets containing P to the automobile body can be expanded to, for example, side outers, etc., which have not been applied so far, and as a result, the weight of the automobile body can be further reduced.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, the present invention will be described.

[0016] As a result of intensive research to solve the above-described problems of the prior art, the present inventors added titanium (Ti) and / or niobium (Nb), which are strong carbonitride-forming elements in steel, to minimize solid solution elements such as carbon (C), nitrogen (N), sulfur (S), etc., and added P, Mo, etc., and confirmed that it is possible to manufacture a high-formability and high-strength steel sheet for the outer panel of a vehicle having a tensile strength of 390 MPa or more and excellent surface quality, and thus completed the present invention. Generally, as a steel sheet for the outer panel of an automobile, it is necessary to satisfy press formability such as deep drawability along with high tensile strength. Therefore, as the base material of the alloyed hot-dip galvanized steel sheet of the present invention, a high-tensile steel sheet containing extra-low carbon steel as a basic component and adding strengthening elements such as Mn and P is used to improve workability.

[0017] Therefore, the high-strength hot-dip galvanized steel sheet with excellent surface imageability according to the present invention provided from such a perspective contains, in mass %, C: 0.003 to 0.005%, Si: 0.05% or less, Mn: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.035%, Cu: 0.06 to 0.1%, B: 0.0015% or less, with the balance being Fe and unavoidable impurities, and is a steel sheet in which C, Ti, and Nb satisfy the above relational expression 1. The microstructure of the alloy has, in area fraction, ferrite of 95% or more, the average size of the crystal grains of the above ferrite is 15 μm or less, ultrafine grains of 5 μm or less have a ratio of 7 to 10% within an area of 1 mm × 1 mm, and ΔWsa defined by the relational expression 2 is 0.1 or less.

[0018] First, the reasons for restricting the alloy components and their contents of the cold-rolled steel sheet that forms the base of the hot-dip galvanized steel sheet of the present invention will be described. Here, “%” means “% by weight” unless otherwise specified.

[0019] · Carbon (C): 0.003 to 0.005% C is an interstitial solid solution element and has a great influence on the formation of the aggregate structure of the steel sheet during cold rolling and annealing. For this purpose, at least 0.003% or more needs to be added. However, when the amount of dissolved carbon in the steel increases, the growth of crystal grains having a {111} gamma (γ)-fiber aggregate structure favorable for drawing is suppressed, and the growth of crystal grains having a {110} and {100} aggregate structure is promoted, resulting in a decrease in the drawability of the annealed sheet. Furthermore, when the content of the above C exceeds 0.005%, the contents of Ti and Nb required to precipitate this as carbides increase, which is not only disadvantageous in terms of economy but also may generate pearlite or the like and reduce the formability. Therefore, in the present invention, it is preferable to limit the content of the above C to the range of 0.003 to 0.005%.

[0020] · Silicon (Si): 0.05% or less Lower Si is an element that contributes to the increase in strength by solid solution strengthening. Since the above Si content exceeding 0.05% may induce surface scale defects and deteriorate the surface characteristics of plating, in the present invention, it is preferable to control the above Si content to 0.05% or less.

[0021] · Manganese (Mn): 0.4 - 1.0% Mn is a solid solution strengthening element, which not only contributes to the increase in strength but also plays a role in precipitating S in steel as MnS. When the content of the above Mn is less than 0.4%, there is a concern about a decrease in strength. On the other hand, when it exceeds 1.0%, surface problems due to oxides may occur. Therefore, it is preferable to limit the content of the above Mn to 0.4 - 1.0%.

[0022] · Phosphorus (P): 0.04 - 0.06% P has the best solid solution effect and is the most effective element for ensuring the strength of steel without significantly impairing the drawability. When the content of the above P is less than 0.04%, it is impossible to ensure the target strength. On the other hand, when it exceeds 0.06%, secondary brittleness and surface linear defects due to P segregation may occur. Therefore, it is preferable to limit the content of the above P to the range of 0.04 - 0.06%.

[0023] · Molybdenum (Mo): 0.05 - 0.08% Mo is an element with a high affinity for P (phosphorus) and plays a role in suppressing P segregation. In order to ensure high strength in extra-low carbon steel, P must be inevitably utilized. By adding an appropriate amount of Mo, it can partly contribute to the improvement of surface defects due to P segregation. When the content of the above Mo is less than 0.05%, there is no significant effect on the target surface improvement. When it exceeds 0.08%, the price increases and the cost competitiveness decreases. Therefore, it is preferable to limit the content of the above Mo to the range of 0.05 - 0.08%.

[0024] · Sulfur (S): 0.01% or less, Nitrogen (N): 0.005% or less S and N are impurities present in steel, which are inevitably added. However, in order to ensure excellent welding properties, it is preferable to control their contents as low as possible. In the present invention, the content of S is preferably controlled to 0.01% or less, and the content of N is preferably controlled to 0.005% or less.

[0025] · Aluminum (Al): 0.1% or more Lower Al precipitates AlN and contributes to the improvement of the drawability and ductility of steel. However, when the content of Al exceeds 0.1%, there is a problem that internal defects of the steel sheet occur due to excessive formation of Al inclusions during steelmaking operations. Therefore, the content of Al is preferably controlled to 0.1% or less.

[0026] · Titanium (Ti): 0.005 - 0.03% Ti is an element that greatly contributes to the improvement of the drawability of the steel sheet by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides. When the Ti content is less than 0.005%, carbonitrides cannot be sufficiently precipitated, resulting in poor drawability. On the other hand, when it exceeds 0.03%, it becomes difficult to manage inclusions during steelmaking operations, and inclusion property defects may occur. Therefore, the content of Ti is preferably limited to the range of 0.005 - 0.03%.

[0027] · Niobium (Nb): 0.02 - 0.035% Nb is the most effective element that can create very fine grains during the processes of rolling and cooling because the unrecrystallized region in the austenite phase extends to a high-temperature region due to the solute drag and the pinning effect of precipitates during hot rolling. When the Nb content is less than 0.02%, the range of the unrecrystallized temperature region of austenite in the steel becomes narrow, and the effect of grain size refinement is slight. On the other hand, when it exceeds 0.035%, there is a problem that the high-temperature strength increases, resulting in difficulties in hot rolling. Therefore, the content of Nb is preferably limited to the range of 0.02 - 0.035%.

[0028] · Boron (B): 0.0015 Less than (excluding 0%) B is an element added to prevent secondary processing brittleness caused by the addition of P in steel. However, if its content exceeds 0.00 15 %, it will be accompanied by a decrease in the ductility of the steel sheet. Therefore, it is preferable to limit the content of B to 0.00 15 % or less.

[0029] · Copper (Cu): 0.04 - 0.1% Cu is an element that is difficult to remove when adjusting the composition of steel by steelmaking. It is contained in trace amounts (for example, 0.04% or more). However, if it exceeds 0.1%, patterns are likely to occur in the hot-dip galvanized steel sheet, and it will also lead to grain boundary embrittlement and cost increase. Therefore, it is preferable to limit it to the range of 0.04 - 0.1%.

[0030] · Relational Expression 1 In the present invention, it is required to control the contents of C, Ti, and Nb so that the value defined by the following Relational Expression 1 satisfies 0.03 - 0.04. In the present invention, the reason for setting such a Relational Expression 1 is to make good use of the solute drag in the solid solution state and the pinning effect in the precipitation state of Ti and Nb, because the refinement and homogenization of the grain size have the greatest influence on the imageability after coating.

[0031] If the value defined by the following Relational Expression 1 is less than 0.03, the grain size will not be sufficiently refined, the surface deformation amount after deformation will not be constant, and excellent imageability cannot be obtained. On the other hand, if it exceeds 0.04, the addition amount of Nb etc. will relatively increase, which is disadvantageous in terms of cost, and the strength will be higher than expected, so there is a problem in ensuring the elongation rate.

[0032] [Relational Expression 1] 0.03 ≤ [(Nb(48 / 93)) + (Ti(93 / 48)) + (C(12 / 48))] ≤ 0.04

[0033] In addition, the balance is Fe and unavoidable impurities. The addition of effective components other than the above composition is not excluded.

[0034] Since the hot-dip galvanized steel sheet of the present invention has a very low carbon steel substrate with a C content of 0.005% or less, the microstructure consists of a single-phase ferrite structure. However, since the above single-phase ferrite structure may inevitably contain other generated structures, the microstructure of the alloy of the present invention has a ferrite area fraction of 95% or more, and a small amount of pearlite or the like may remain as a residual component.

[0035] Also, it is preferable that the average grain size of the fine crystal grains of the cold-rolled steel sheet, which is the substrate of the hot-dip galvanized steel sheet of the present invention, is 15 μm or less. If the above average grain size exceeds 15 μm, there is a problem that it is difficult to ensure the desired mapping property due to the variation in surface deformation during forming. More preferably, the average crystal grain size of the microstructure of the substrate is controlled to be less than 10 μm.

[0036] Furthermore, it is preferable that the cold-rolled steel sheet, which is the substrate of the present invention, has ultra-fine grains of 5 μm or less at a ratio of 7 to 10% within an area of 1 mm × 1 mm. By having such a ratio, a hot-dip galvanized steel sheet excellent in surface mapping property with ΔWsa defined by the following relational expression 2 being 0.1 or less can be obtained. If the above ratio is less than 7%, the grain size becomes relatively large, and the amount of surface deformation after forming (after 5% deformation) increases, so that the desired mapping property cannot be ensured. If it exceeds 10%, the strength becomes too high, and there is a problem in ensuring an elongation rate of 32% or more.

[0037] [Relational Expression 2] ΔWsa = Surface undulation of the steel sheet after 5% deformation - Surface undulation of the steel sheet before deformation

[0038] Next, a method for manufacturing a high-strength hot-dip galvanized steel sheet excellent in surface mapping property of the present invention will be described.

[0039] The manufacturing method of the high-strength hot-dip galvanized steel sheet of the present invention includes a step of heating a steel slab satisfying the above composition components to 1100 - 1300°C, a step of hot-rolling the heated steel slab so that the finish rolling temperature is 920 - 970°C, and then winding it at a temperature of 600 - 650°C to produce a hot-rolled steel sheet, a step of obtaining a cold-rolled steel sheet by cold-rolling the wound hot-rolled steel sheet after pickling with a reduction rate of 70 - 83%, a step of annealing the cold-rolled steel sheet within a temperature range of 760 - 830°C, and then performing hot-dip galvanizing, and a step of subjecting the hot-dip galvanized steel sheet to an alloying heat treatment within a temperature range of 500 - 560°C.

[0040] First, in the present invention, a steel slab having the above composition components is heated in a temperature range of 1100 - 1300°C. If the heating temperature is less than 1100°C, production problems may occur due to the rolling load in the FM section. If it exceeds 1300°C, surface scale defects may occur.

[0041] Next, in the present invention, the heated steel slab is hot-rolled so that the finish rolling temperature is 920 - 970°C, and then wound at a temperature of 600 - 650°C to produce a hot-rolled steel sheet.

[0042] In the present invention, it is preferable to limit the finish rolling temperature to 920 - 970°C. If the finish rolling temperature is less than 920°C, problems such as the generation of coarse grains on the surface and non-uniform material quality may occur. If it exceeds 970°C, the grain size may not become sufficiently fine, and ultimately, problems such as insufficient material quality may occur.

[0043] In the present invention, it is preferable to control the coiling temperature in the range of 600 to 650°C. If the coiling temperature is less than 600°C, precipitates such as Ti(Nb)C are not formed, and the amount of solid-solved Ti and Nb increases. During heating in the annealing process, they may precipitate finely as TiC or Ti(Nb)C, or exist in a solid-solved state of Ti and Nb, which may affect the suppression of recrystallization and crystal grain growth, and thus there may be problems in ensuring the intended strength and elongation. If it exceeds 630°C, there may be a problem that the surface deteriorates due to the formation of secondary scale.

[0044] Then, in the present invention, after passing through a pickling process for removing the surface scale of the coiled hot-rolled steel sheet, cold rolling is performed at a reduction ratio of 70 to 83% to produce a cold-rolled steel sheet. If the cold rolling reduction ratio is less than 70%, there is a problem that the {111} texture does not grow sufficiently, resulting in poor formability. On the other hand, if it exceeds 83%, the load on the rolling rolls during on-site production is extremely large, and the shape deteriorates, so there is a problem. Therefore, it is preferable to limit the reduction ratio to 70 to 83%, and more preferably to 74 to 80%.

[0045] Subsequently, annealing is performed on the cold-rolled steel sheet produced as described above, followed by hot-dip galvanizing or alloyed hot-dip galvanizing.

[0046] When annealing the cold-rolled steel sheet, annealing should be performed at a temperature above the recrystallization temperature within the temperature range of 760 to 830°C. By annealing at a temperature above the recrystallization temperature, the deformation generated by rolling can be removed, and the steel sheet can be softened to improve workability.

[0047] The annealed cold-rolled steel sheet is directly hot-dip galvanized in a continuous hot-dip galvanizing line.

[0048] Then, in the present invention, an alloying heat treatment can be performed on the above-produced hot-dip galvanized steel sheet. The alloying heat treatment is carried out within the range of 500 to 560°C after hot-dip galvanizing. If the alloying heat treatment temperature is less than 500°C, alloying will not proceed sufficiently. On the other hand, if it exceeds 560°C, excessive alloying will occur and the plating layer will become brittle, which may induce problems such as peeling of the plating due to processing such as pressing.

[0049] At this time, in the present invention, if necessary, temper rolling with a roughness (Ra) of 1.0 to 1.6 μm can be performed on the above alloying heat-treated hot-dip galvanized steel sheet at a temper rolling rate of 0.6 to 1.2%.

[0050] Hereinafter, the present invention will be described in detail with reference to examples.

Examples

[0051] A 250-mm-thick steel slab having the alloy composition described in Table 1 below was reheated to 1250°C, and then hot rolling, cold rolling, continuous annealing, and alloying hot-dip galvanizing were performed under the conditions shown in Table 2 below to produce a hot-dip galvanized steel sheet.

[0052] Then, for each of the produced hot-dip galvanized steel sheets, the tensile properties, r value (ranking Ford value), which is an index of deep drawing process, grain size, and distribution ratio were measured, and ΔWsa was examined. The measurement method will be described below.

[0053] As the tensile test, YS, TS, and T-El were measured. Here, YS, TS, and T-El mean yield strength, tensile strength, and elongation at break, respectively, and the tensile test was performed using a test piece taken based on JIS No. 5 standard. Such measurement results were considered qualified when the tensile strength was 390 to 430 MPa and the elongation was 32% or more.

[0054] On the one hand, for the evaluation of the r-value, which is an index of deep drawing process, JIS No. 5 tensile test specimens were taken in three directions parallel to the rolling direction, at 45°, and perpendicular to the rolling direction from an alloyed hot-dip galvanized steel sheet, and the r-values of each test specimen were measured. For example, for the measurement of the r-value, at the time when a tensile deformation of about 15% was performed in the above tensile test, the change value of the sheet thickness and the change value of the sheet width were measured, and the ratio of the change value of the sheet width to the sheet thickness was obtained. And, when the r-value parallel to the rolling direction is r0, the r-value at 45° is r 45 , and the r-value perpendicular to the rolling direction is r 90 , the r-values in each direction were calculated by the following mathematical formula A. Also, in this example, when the r-value is 1.2 or more, it was regarded as qualified.

[0055] [Mathematical formula A] A = r0 + 2 * r 45 + r 90 / 4

[0056] And, the grain size and its distribution were evaluated using TSL OIM analysis software by EBSD measurement.

[0057] Also, for the evaluation of Wsa after deformation, between the blankholder and the die, a 225 mm × 225 mm blank was crimped with a crimping machine having a force of a 75 mm diameter hollow punch and a blankholder so that any material movement of the substrate was completely suppressed to produce a cup. For the deformation of the cup, it is preferable that the drawing depth of the punch is about 17 - 18 mm so that the thickness deformation rate of the bottom is about 5% + / - 0.2%. According to Table 3 below, it can be seen that in order to increase the possibility for ΔWsa ≤ 0.1, the grain size of the material should be 15 μm or less.

[0058]

Table 1

[0059]

Table 2

[0060]

Table 3

[0061] As shown in Tables 1 - 3 above, it can be confirmed that Invention Examples 1 - 6, which satisfy not only the composition components of the steel but also the manufacturing process conditions of the plated steel sheet within the scope of the present invention, exhibit excellent tensile properties, r - values, ratios of ultrafine grains, and ΔWsa.

[0062] On the other hand, Comparative Examples 1 - 4 are cases where, although the composition components of the steel satisfy the scope of the present invention, the manufacturing process of the plated steel sheet is outside the scope of the present invention.

[0063] Specifically, in Comparative Examples 1 and 3, the FDT (Finish Mill Delivery Temperature) in the hot - rolling process was carried out at a temperature below the Ar3 temperature, resulting in a large grain size on the surface layer. As a result, the ratio of fine grains in the final annealing structure was low, and the desired ΔWsa could not be ensured.

[0064] In Comparative Example 2, the hot - rolling CT temperature was as high as 700 °C, and the desired fraction of fine grains could not be ensured due to the coarsening of the grain size. And in Comparative Example 4, the annealing temperature was below the recrystallization temperature, and sufficient recrystallization did not occur, so the desired strength and elongation could not be ensured.

[0065] Also, it can be seen that Comparative Examples 5 - 7, in which not only the composition components of the steel but also the manufacturing process conditions of the plated steel sheet are all outside the scope of the present invention, do not satisfy the ratio of ultrafine grains and have a large ΔWsa value, resulting in poor imaging properties.

[0066] On the other hand, Comparative Example 8 is a case where the relational expression 1 is outside the scope of the present invention in the composition components of steel, and even when a plated steel sheet is manufactured by the manufacturing process of the plated steel sheet of the present invention, the fraction of the final fine grains is not sufficient, and the target ΔWsa value could not be secured.

[0067] As described above, in the detailed description of the present invention, the preferred embodiments of the present invention have been described. However, it goes without saying that those having ordinary knowledge in the technical field to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be determined by being limited to the described embodiments, but should be determined by not only the scope of the claims described later but also equivalents thereof.

Claims

1. By mass%, C: 0.003 to 0.005%, Si: 0.05% or less, Mn: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.035%, Cu: 0.06 to 0.1%, B: 0.0015% or less, consisting of the balance Fe and unavoidable impurities, and a steel sheet in which C, Ti, and Nb satisfy the following relational expression 1: The microstructure of the steel sheet consists of ferrite with an area fraction of 95% or more, residual pearlite, and unavoidably formed structures. The average size of the crystal grains of the microstructure forming the steel sheet is 15 μm or less. Among the microstructures forming the steel sheet, ultrafine grains of 5 μm or less have a ratio of 7 to 10 area% within an area of 1 mm × 1 mm, and ΔWsa defined by the following relational expression 2 is 0.1 μm or less. A hot-dip galvanized steel sheet. [Relational expression 1] 0.03 ≤ [(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))] ≤ 0.04 (In the above relational expression 1, each component element represents the mass% of that element.) [Relational expression 2] ΔWsa = surface undulation of the steel sheet after 5% deformation - surface undulation of the steel sheet before deformation

2. The hot-dip galvanized steel sheet according to claim 1, having a tensile strength of 390 to 430 MPa and an elongation of 32% or more. zinc-plated steel sheet.

3. By mass%, C: 0.003 to 0.005%, Si: 0.05% or less, Mn: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.035%, Cu: 0.06 to 0.1%, B: 0.0015% or less, consisting of the balance Fe and unavoidable impurities, and a step of heating a steel slab in which C, Ti, and Nb satisfy the following relational expression 1 to 1100 to 1300 °C: After hot-rolling the heated steel slab so that the finish rolling temperature is 920 to 970°C, winding it at a temperature of 600 to 650°C to produce a hot-rolled steel sheet; After pickling the wound hot-rolled steel sheet, cold-rolling it at a reduction ratio of 70 to 83% to obtain a cold-rolled steel sheet; After annealing the cold-rolled steel sheet within a temperature range of 760 to 830°C, performing hot-dip galvanizing; A method for manufacturing a hot-dip galvanized steel sheet according to claim 1, comprising alloying heat-treating the hot-dip galvanized steel sheet within a temperature range of 500 to 560°C. [Formula 1] 0.03 ≤ [(Nb(48 / 93)) + (Ti(93 / 48)) + (C(12 / 48))] ≤ 0.04 (In the above formula 1, each component element represents the mass% of the element.)

4. For the alloyed heat-treated hot-dip galvanized steel sheet, performing temper rolling treatment of 0.6 to 1.2% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm. Characterized by The method for manufacturing a hot-dip galvanized steel sheet according to claim 3.

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