High-strength galvanized steel plate excellent in image distinctness and producing method thereof

Abstract

To provide a high strength galvanized steel plate excellent in image distinctness and a producing method thereof.SOLUTION: The galvanized steel sheet of the present invention contains, by mass %: 0.003 to 0.005% of C; 0.05% or less of Si; 0.4 to 1.0% of Mn; 0.04 to 0.06% of P; 0.01% or less of S; 0.005% or less of N; 0.1% or less of S, Al; 0.05 to 0.08% of Mo; 0.005 to 0.03% of Ti; 0.02 to 0.035% of Nb; 0.06 to 0.1% of Cu; 0.0015% or less of B; and a balance of Fe and inevitable impurities. The steel sheet satisfies specific relationships among C, Ti, and Nb, and has a fine microstructure in which an area fraction of ferrite is 95% or more, an average grain size of the ferrite crystals is 15 μm or less, ultrafine grains of 5 μm or less account for 7 to 10% of an area of 1 mm×1 mm, and ΔWsa (defined as surface undulation of the steel sheet after 5% deformation - surface undulation of the steel sheet before deformation) is 0.1 or less.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] For parts of painted steel for external panels of automobiles such as hoods and doors, producers apply strict requirements. One of these requirements relates to the painted appearance of the painted parts. An external panel having a very good painted appearance, that is, a panel having a surface like a mirror that reflects light without distortion and has a clear reflected image, is 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. It is already known to those skilled in the art that larger surface structures, for example, 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. Moreover, recently, automobile manufacturers are aiming to save energy and reduce costs by omitting the intermediate coating in the painting process, so the surface waviness of automobiles has become even more important. Surface waviness is present even after crimping or forming is applied. The 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. whereas larger structures are grasped as so-called surface waviness. It is already known to those skilled in the art that larger surface structures, for example, 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.

[0003] It is already known to those skilled in the art that larger surface structures, for example, 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. Moreover, recently, automobile manufacturers are aiming to save energy and reduce costs by omitting the intermediate coating in the painting process, so the surface waviness of automobiles has become even more important. Surface waviness is present even after crimping or forming is applied. Moreover, recently, automobile manufacturers are aiming to save energy and reduce costs by omitting the intermediate coating in the painting process, so the surface waviness of automobiles has become even more important. Surface waviness is present even after crimping or forming is applied. In the painting process, due to the omission of the intermediate coating for energy saving and cost reduction, the surface waviness of automobiles has become even more important. Surface waviness is present even after crimping or forming is applied. after crimping or forming is applied. It is important to recognize what should be measured.

[0004] The surface waviness of the formed part is the result of an increase in the waviness of the non-deformed, for example, flat part and the waviness introduced by the forming stage, which is known to those skilled in the art. 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] The present invention aims to provide a high-strength hot-dip galvanized steel sheet with excellent formability and a method for producing the same by adding P, Nb, and Ti to an extra-low carbon steel applied to the 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 content. The problems of the present invention can be understood from the entire content of this specification, and those having ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the additional problems of the present invention.

Means for Solving the Problems

[0008] One aspect of the present invention is, by mass%, C: 0.003 to 0.005%, Si: 0.05% or less , Mn: 0.4 - 1.0%, P: 0.04 - 0.06%, S: below 0.01%, N: 0. 005% or less, S.Al: 0.1% or less, Mo: 0.05 - 0.08%, Ti: 0.00 5 - 0.03%, Nb: 0.02 - 0.035%, Cu: 0.06 - 0.1%, B: 0. 0015% or less, the balance being Fe and inevitable impurities, and the steel sheet satisfies the following relational expression 1 for C, Ti and Nb wherein the fine structure of the alloy has a ferrite area fraction of 95% or more and the average size of the crystal grains of the ferrite is 15 μm or less, and the ultrafine grains of 5 μm or less have a proportion of 7 - 10% within an area of 1 mm × 1 mm, and ΔWsa defined by the following relational expression 2 is 0.1 or less, relating to a high - strength hot - dip galvanized steel sheet with excellent surface mapping

[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 above hot - dip galvanized steel sheet may have a tensile strength of 390 - 430 MPa and an elongation of 32% or more

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

[0013] For the alloying heat-treated hot-dip galvanized steel sheet, temper rolling with a reduction of 0.6 to 1.2% can be performed using a skin pass roll having a roughness (R a) 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, so it 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, parts that have not been applied so far, such as side outers, and as a result, the weight reduction of the automobile body can be further achieved.

Embodiments for Carrying Out the Invention

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

[0016] As a result of intensive research to solve the problems of the above-described conventional technology, 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), and sulfur (S) to ensure formability, and added P, Mo, etc. to obtain a steel sheet for automobiles with a tensile strength of 390 MPa or more and excellent surface quality. It was confirmed that a high formability and high strength steel sheet for outer panels could be manufactured, and the present invention was thus completed. Generally speaking, as a steel sheet for automobile outer panels, with increasing tensile strength, press formability such as deep drawability must be satisfied. Therefore, as the base material of the hot dip galvanized steel sheet of the present invention, in order to improve workability, an extra-low carbon steel is used as the basic component, and strengthening elements such as Mn and P are added to form a high strength steel sheet.

[0017] Therefore, the high strength hot dip galvanized steel sheet with excellent surface imageability of the present invention provided from such a perspective contains, by mass%, C: 0.003 to 0.005%, Si: 0.05% or less, M n: 0.4 to 1.0%, P: 0.04 to 0.06%, S: 0.01% or less, N: 0.00 5% 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 15% or less, with the balance being Fe and unavoidable impurities, and C, Ti, and Nb satisfying the above relational expression 1, and the microstructure of the alloy has, by area fraction, ferrite of 95% or more, the average size of the crystal grains of the above ferrite is 15 μm or less, and ultrafine grains of 5 μm or less have a proportion of 7 to 10% within an area of 1 mm × 1 mm, and ΔWs a defined by 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 forming the base of the hot dip galvanized steel sheet of the present invention will be described. Here, "%", unless otherwise specified, means "% by weight".

[0019] · Carbon (C): 0.003 to 0.005% C is an interstitial solid solution element, which has a great influence on the formation of the microstructure of the steel sheet during the processes of cold rolling and annealing, and for this purpose, an addition of at least 0.003% or more is required. However, when the amount of dissolved carbon in the steel increases, the growth of crystal grains having a {111} gamma (γ)-fiber microstructure, which is advantageous for drawing, is suppressed, and the growth of crystal grains having a {110} and {100} microstructure is promoted, and the drawability of the annealed sheet decreases. Furthermore, when the content of C exceeds 0.0 05%, the contents of Ti and Nb required to precipitate this as carbides increase significantly, which is not only disadvantageous in terms of economy, but also pearlite or the like may be generated, resulting in a decrease in formability. Therefore, in the present invention, it is preferable to limit the content of C to the range of 0.003 to 0.005%.

[0020] · Silicon (Si): 0.05% or less (excluding 0%) Si is an element that contributes to the increase in strength by solid solution strengthening. When the Si content exceeds 0.05%, there is a problem that surface scale defects are induced and the surface characteristics of plating deteriorate. Therefore, in the present invention, it is preferable to control the Si content to 0.05% or less.

[0021] · Manganese (Mn): 0.4 to 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 the steel as MnS. When the content of Mn is less than 0.4%, a decrease in strength is a concern. On the other hand, when it exceeds 1.0%, surface problems due to oxides may occur. Therefore, the content of Mn is preferably limited to 0.4 to 1.0%.

[0022] · Phosphorus (P): 0.04 to 0.06% ​​P is the most excellent in the solution effect and is the most effective element for ensuring the strength of steel without significantly impairing the drawability. When the content of 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 P to the range of 0.04 - 0.06%. When the content of the above-mentioned 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-mentioned 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 ultra-low carbon steel, P must be inevitably utilized. However, 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-mentioned 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-mentioned Mo to the range of 0.05 - 0.08%. When the content of the above-mentioned 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-mentioned 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 and are inevitably added. However, in order to ensure excellent welding characteristics, it is preferable to control their content as low as possible. In the present invention, it is preferable to control the content of the above-mentioned S to 0.01% or less and manage the content of the above-mentioned N to 0.005% or less. In the present invention, it is preferable to control the content of the above-mentioned S to 0.01% or less and manage the content of the above-mentioned N to 0.005% or less.

[0025] · Aluminum (Al): 0.1% or less (excluding 0%) Al precipitates AlN and contributes to the improvement of the drawability and ductility of steel. However, when the content of the above-mentioned Al exceeds 0.1%, internal defects of the steel sheet due to excessive formation of Al inclusions may occur during the steelmaking operation. ​ Since there is a problem of occurrence, it is preferable to control the above Al content to 0.1% or less. Yes.

[0026] · Titanium (Ti): 0.005 - 0.03% Ti reacts with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides, which greatly contributes to improving the drawability of the steel sheet. When the above Ti content is less than 0.005 %, carbonitrides cannot be sufficiently precipitated, resulting in poor drawability. On the other hand, when it exceeds 0.0 3%, it becomes difficult to manage inclusions during steelmaking operations, and there may be problems of inclusion property defects. Therefore, it is preferable to limit the above Ti content to the range of 0.005 - 0.03%. Yes.

[0027] · Niobium (Nb): 0.02 - 0.035% Nb is due to the solute drag and the pinning effect of precipitates during hot rolling. Since the unrecrystallized region in the austenite region spreads to the high-temperature region, it is the most effective element that can create very fine grains during the processes of rolling and cooling. When the above 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 becomes high, resulting in difficulties in hot rolling. Therefore, it is preferable to limit the above Nb content to the range of 0.02 - 0.035%. Yes.

[0028] · Boron (B): 0.003% or less (excluding 0%) B is an element added to prevent secondary processing brittleness caused by the addition of P in the steel. However, when its content exceeds 0.003%, it is accompanied by a decrease in the ductility of the steel sheet. Therefore, the content of the above B It is preferably limited to 0.003% or less.

[0029] · Copper (Cu): 0.04 - 0.1% Cu is an element that is difficult to remove when adjusting the steel composition by steelmaking. It is contained in a trace amount (for example, 0.04% or more), but when it exceeds 0.1%, patterns are likely to occur in the hot-dip galvanized steel sheet. Furthermore, it leads to grain boundary embrittlement and cost increase. Therefore, it is preferably limited 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, and to minimize the influence of grain size refinement and homogenization on the mapping property after coating. If the value defined by the following Relational Expression 1 is less than 0.03, the grain size will not be sufficiently fine, the surface deformation amount after deformation will not be constant, and excellent mapping property cannot be obtained. On the other hand,

[0031] if it exceeds 0.04, the addition amount of Nb etc. will relatively increase, which is disadvantageous in terms of cost, and moreover, since the strength becomes higher than expected, 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] The present invention relates to a hot-dip galvanized steel sheet having a substrate of extra-low carbon steel with a C content of 0.005% or less. Therefore, 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, the average grain size of the fine crystal grains of the microstructure of the cold-rolled steel sheet, which is the substrate of the hot-dip galvanized steel sheet of the present invention, is preferably 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 variations 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, the cold-rolled steel sheet, which is the substrate of the present invention, preferably has a ratio of ultrafine grains of 5 μm or less of 7 to 10% within an area of 1 mm × 1 mm. By having such a ratio, a hot-dip galvanized steel sheet with excellent surface mapping property, in which ΔWsa defined by the following relational expression 2 is 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 the desired mapping property cannot be ensured. If it exceeds 10%, there is a problem in ensuring an elongation rate of 32% or more because the strength becomes too high.

[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 with excellent surface imageability according to the present invention will be described. This will be done.

[0039] The method for manufacturing a high-strength hot-dip galvanized steel sheet according to the present invention includes a step of heating a steel slab satisfying the above composition components to 1 100 to 1300 °C, hot rolling the heated steel slab so that the finish rolling temperature is 92 0 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 alloying heat treatment within a temperature range of 500 to 560 °C. This includes.

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

[0041] Next, in the present invention, the heated steel slab is hot-rolled so that the finish rolling temperature is 920 to 970 °C and then wound at a temperature of 600 to 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 to 970 °C. If the finish rolling temperature is less than 920 °C, there may be a problem that coarse grains are generated on the surface and the material becomes non-uniform. If it exceeds 970 °C, the grain size may not become sufficiently fine, and ultimately, there may be a problem of insufficient material. There is a possibility of occurrence.

[0043] In the present invention, the coiling temperature is preferably controlled within a range of 600 to 650° C. If the coiling temperature is less than 600°C, precipitates such as Ti(Nb)C will be generated. The amount of dissolved Ti and Nb increases, and TiC and Ti(Nb)C are formed minutely during heating in the annealing process. Fine precipitates or Ti and Nb exist in solid solution and have the effect of suppressing recrystallization and grain growth. Therefore, there may be a problem in securing the strength and elongation that the invention aims to achieve. If the temperature exceeds 30°C, problems may occur in which the surface deteriorates due to the formation of secondary scale. There is.

[0044] In the present invention, the coiled hot-rolled steel sheet is pickled to remove surface scale. After the process, cold rolling is performed at a rolling reduction of 70 to 83% to produce cold-rolled steel sheets. If the rolling reduction is less than 70%, the {111} texture does not develop sufficiently, resulting in poor formability. On the other hand, if it exceeds 83%, the load on the rolling rolls during on-site manufacturing will be very high. This is problematic because the shape becomes poor and the rolling reduction is therefore limited to 70-83%. It is preferable to limit the range to 74 to 80%.

[0045] Next, the cold-rolled steel sheet manufactured as described above is subjected to an annealing process, and then to hot-dip galvanizing or alloying. The metal is then plated with hot-dip galvanizing.

[0046] When annealing cold-rolled steel sheets, the temperature should be within the range of 760 to 830°C and be equal to or higher than the recrystallization temperature. Annealing at a temperature above the recrystallization temperature can remove the residual lattice defects that occur during rolling. Deformation is eliminated and the material is softened, improving processability.

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

[0048] And 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 does not proceed sufficiently. On the other hand, if it exceeds 560 °C, excessive alloying proceeds and the plating layer becomes 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 skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm is performed on the above alloying heat-treated hot-dip galvanized steel sheet at 0.6 to 1 .2%.

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

Examples

[0051] A steel slab with a thickness of 250 mm 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, the r-value (ranking value), which is an index of deep drawing processability, the grain size, and the distribution ratio were measured, and ΔW sa was examined. The measurement method will be described below.

[0053] As tensile tests, 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 test pieces taken based on JIS No. 5 standard. For such measurement results, when the tensile strength is 390 - 430 MPa and the elongation rate is 32% or more, it is considered qualified.

[0054] On the other hand, for the evaluation of the r value, which is an index of deep drawing process, JIS No. 5 tensile test pieces were taken in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction, from the galvanized - alloyed steel sheet, and the r value of each test piece was measured. For example, for the measurement of the r value, at the time of about 15% tensile deformation in the above - mentioned tensile test, the change value of the plate thickness and the change value of the plate width were measured, and the ratio of the change value of the plate width to the plate thickness was obtained. And when the r value parallel to the rolling direction is r0, the r value in the 45° direction is r 45 and the r value in the perpendicular 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 is considered qualified.

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

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

[0057] Also, for the evaluation of Wsa after deformation, between the blankholder and the die, in order to completely suppress any movement of substances on the substrate, a hollow punch with a diameter of 75 mm and the force of the blankholder were used A cup was produced by crimping a 225 mm × 225 mm blank with a crimping machine . The deformation of the cup is such that the thickness deformation rate of the bottom is about 5% ± 0.2%. It is preferable to set the punching depth to about 17 - 18 mm so that . According to Table 3 below, in order to increase the possibility for ΔWsa ≦ 0.1 , the grain size of the material should be 15 μm or less . It can be seen

[0058]

Table 1

[0059]

Table 2

[0060]

Table 3

[0061] As shown in Tables 1 - 3 above, not only the composition components of the steel but also the manufacturing process conditions of the galvanized steel sheet satisfy the scope of the present invention. Invention Examples 1 - 6, which satisfy the scope of the invention, can be confirmed to exhibit excellent tensile properties, r - value, ratio of ultrafine grains, and ΔWs a

[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 galvanized steel sheet is outside the scope of the present invention

[0063] Specifically, in Comparative Example 1 and Comparative Example 3, the FDT (Finish Mill Delivery Temperature) in the hot rolling process is operated at a temperature below the Ar3 temperature, and the surface layer As a result of the increase in the grain size, the proportion 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-rolled CT temperature was as high as 700 °C, and due to the coarsening of the grain size, the desired fine grain fraction could not be ensured. And in Comparative Example 4, the annealing temperature was below the recrystallization temperature, sufficient recrystallization did not occur, and the desired strength and elongation could not be ensured.

[0065] Also, not only the composition components of the steel, but also all the manufacturing process conditions of the plated steel sheet deviated from the scope of the present invention in Comparative Examples 5-7. It can be seen that the proportion of ultrafine grains was not satisfied and the ΔWsa value was large, resulting in poor imaging properties. On the other hand, in Comparative Example 8, even when the plated steel sheet was manufactured by the manufacturing process of the plated steel sheet of the present invention in the case where the relational expression 1 deviated from the scope of the present invention in the composition components of the steel, the fraction of the final fine grains was not sufficient, and the target ΔWsa value could not be ensured. As described above, in the detailed description of the present invention, the preferred embodiments of the present invention have been described.

[0066] However, it goes without saying that those with ordinary knowledge in the technical field to which the present invention belongs 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 the scope of the claims described later and equivalents thereof. and equivalents thereof.

[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 with ordinary knowledge in the technical field to which the present invention belongs 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 the scope of the claims described later and equivalents thereof. and equivalents thereof.

Claims

1. A steel sheet comprising, 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: 0.06-0.1%, B: 0.0015% or less, with the remainder being Fe and unavoidable impurities, wherein C, Ti, and Nb satisfy the following relational formula 1. A hot-dip galvanized steel sheet having a microstructure in which, by area fraction, ferrite accounts for 95% or more, with the remainder being pearlite and an inevitably formed structure, the average size of the crystal grains in the microstructure of the steel sheet being 15 μm or less, ultrafine grains of 5 μm or less accounting for 7 to 10 area % within a 1 mm × 1 mm area of ​​the microstructure of the steel sheet, and ΔWsa, defined by the following relational formula 2, being 0.1 μm or less. [Relationship 1] 0.03≦[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≦0.04 (In the above relational formula 1, each component element represents the mass percentage of that element.) [Relationship Equation 2] ΔWsa = Surface waviness of the steel plate after 5% deformation - Surface waviness of the steel plate before deformation

2. The hot-dip galvanized steel sheet according to Claim 1, wherein the tensile strength is 390 to 430 MPa and the elongation is 32% or more.

3. A step of heating a steel slab to 1100 to 1300°C, comprising, 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 remainder being Fe and unavoidable impurities, wherein C, Ti and Nb satisfy the following relational formula 1; The process involves hot-rolling the heated steel slab to a finish rolling temperature of 920 to 970°C, and then winding it up at a temperature of 600 to 650°C to produce a hot-rolled steel sheet. The process of obtaining a cold-rolled steel sheet by pickling the wound hot-rolled steel sheet and then cold-rolling it with a reduction ratio of 70-83%, The process involves annealing the cold-rolled steel sheet to a temperature range of 760 to 830°C, followed by hot-dip galvanizing. A method for manufacturing a hot-dip galvanized steel sheet, comprising the step of alloying the hot-dip galvanized steel sheet at a temperature range of 500 to 560°C. [Relationship 1] 0.03≦[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≦0.04 (In the above relational formula 1, each component element represents the mass percentage of that element.)

4. The method for producing a hot-dip galvanized steel sheet according to Claim 3, characterized in that the hot-dip galvanized steel sheet that has undergone alloying heat treatment is subjected to a 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.