Plated steel sheet excellent in strength, formability and surface quality and method for producing the same

Abstract

The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to a galvanized-based steel sheet having high strength and high formability suitable for use as an automotive outer panel material and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet that can achieve vehicle lightweighting and has excellent strength, formability, and surface quality, and a manufacturing method thereof. More specifically, the present invention relates to a galvanized-based steel sheet having high strength and high formability suitable for use as an automotive outer panel material and a manufacturing method thereof. Background Art

[0002] As an automotive outer panel material, cold-rolled steel sheets processed by stamping or the like are used, and generally high formability is required. In recent years, from the viewpoint of preventing global warming, as a carbon dioxide emission control strategy, new improvement targets for automotive fuel efficiency have been set, and it is necessary to improve the fuel efficiency of automobiles. For example, preferential tax systems for low fuel efficiency automobiles have been introduced. In terms of improving the fuel efficiency of automobiles, lightweighting of the vehicle body is an effective means. From the viewpoint of this lightweighting, thinning of the steel sheet for the vehicle body is required. However, in recent years, the number of automobile companies attempting to achieve lightweighting and process shortening and cost reduction by using a thicker thickness but high strength to omit reinforcing members is increasing.

[0003] As a steel sheet for an automotive body that meets the above-described high strength requirements of the steel sheet and is pressed into a complex shape, a galvanized-based high-tensile steel sheet having excellent surface appearance and excellent stamping formability is required.

[0004] In addition, in high-tensile steel sheets for automobiles, in order to increase the strength of the steel sheet, solid solution strengthening elements such as Si, Mn, and P are contained in the steel.

[0005] In particular, P is added to the steel to increase the strength of the steel sheet, but P is a very easily segregated element. The P segregated on the surface of the slab extends in the length direction of the steel sheet through hot rolling and cold rolling, and thus segregation of P is formed on the surface of the coil. Since alloying is delayed during plating in this P segregation, this becomes a cause of linear defects in alloyed hot-dip galvanized steel sheets. To address these problems, as a manufacturing method of an alloyed hot-dip galvanized steel sheet using a steel sheet having a P content of 0.03% or more as a base material, in order to eliminate the non-uniformity of the steel sheet surface, a method of grinding the steel sheet surface according to the grinding amount of P in the steel and performing an alloying treatment in an induction heating type alloying furnace has been proposed (Patent Document 1).

[0006] In this prior art, in order to prevent linear defects in alloyed hot-dip galvanized steel sheets, for example, when using a super low-carbon steel sheet added with Ti having a P content of 0.03% or more, a flame cleaning treatment of 3 mm or more is performed on the surface in the continuous casting step, and in the steel sheet step before plating, the surface is ground by 5 μm or more. Therefore, shape defects after plating are prevented to ensure surface quality, but these are recognized as causes of reduced actual yield.

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

[0008] Technical Problem to be Solved

[0009] According to one aspect of the present invention, there is provided a plated steel sheet having high practical yield, excellent surface appearance, high strength, and high formability, and a method for manufacturing the same.

[0010] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand additional technical problems of the present invention from the entire text of the specification of the present invention.

[0011] Technical Solution

[0012] One aspect of the present invention provides a plated steel sheet, which includes:

[0013] a base steel; and

[0014] a plating layer formed on the base steel,

[0015] wherein, by weight%, the base steel contains: C: 0.003 - 0.009%, Si: 0.05% or less, Mn: 0.4 - 1.0%, P: 0.04 - 0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.03 - 0.08%, Ti: 0.005 - 0.03%, Nb: 0.02 - 0.045%, Cu: 0.06 - 0.15%, B: 0.003% or less, the balance being Fe and other inevitable impurities, and the plated steel sheet satisfies the following relational expression 1.

[0016] [Relational Expression 1]

[0017] 1.6 ≤ Mt / Ms ≤ 6

[0018] (In the relational expression 1, Mt represents the average weight% content of Mo at a position 0.1 μm from the surface of the base steel in the thickness direction on the base steel side. Ms represents the average weight% content of Mo at a position t / 4 from the surface of the base steel in the thickness direction on the base steel side. t represents the total average thickness of the base steel and is in the range of 0.8 - 1 mm.)

[0019] In addition, another aspect of the present invention provides a method for manufacturing a plated steel sheet, which includes the following steps:

[0020] Continuously casting steel, the steel comprising, by weight%, C: 0.003-0.009%, Si: less than 0.05%, Mn: 0.4-1.0%, P: 0.04-0.09%, S: less than 0.01%, N: less than 0.005%, S.Al: less than 0.1%, Mo: 0.03-0.08%, Ti: 0.005-0.03%, Nb: 0.02-0.045%, Cu: 0.06-0.15%, B: less than 0.003%, the balance of Fe and other inevitable impurities;

[0021] reheating the slab obtained by continuous casting;

[0022] hot rolling the reheated slab to provide a hot rolled steel sheet;

[0023] Coiling the hot rolled steel plate;

[0024] cold rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet;

[0025] annealing the cold-rolled steel sheet; and

[0026] The annealed cold rolled steel sheet is immersed in a zinc-based plating bath for hot dip galvanizing.

[0027] Wherein, the continuous casting satisfies the following relational expression 3.

[0028] [Equation 3]

[0029] 1≤Vc≤5-[P] / [Mo]

[0030] (In the relational expression 2, Vc represents the average casting speed during continuous casting, and the unit is m / min. [P] represents the average weight % content of P in the slab, and [Mo] represents the average weight % content of Mo in the slab.)

[0031] Beneficial Effects

[0032] According to one aspect of the present invention, a plated steel sheet having high strength and high formability while ensuring a high yield rate and having an excellent surface appearance and a method for manufacturing the same can be provided.

[0033] Various beneficial advantages and effects of the present invention are not limited to the above contents and can be more easily understood in the process of describing specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a photograph of the surface of the plated steel sheet produced in Inventive Example 1 of the present invention observed with a scanning electron microscope (SEM).

[0035] Figure 2 It is a photograph of the surface of the plated steel sheet manufactured in Comparative Example 4 of the present invention observed with a scanning electron microscope (SEM).

[0036] Best Mode for Carrying Out the Invention

[0037] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art of the present technology.

[0038] As described above, in the prior art, in order to prevent linear defects of the plated steel sheet and ensure the surface quality, it is necessary to perform a flame cleaning treatment on the surface in the continuous casting step and a grinding treatment on the surface in the steel sheet step before plating. Therefore, this has become a reason for the reduction in the actual yield.

[0039] Therefore, as a result of preliminary research by the present inventors to solve the problems of the prior art, it was confirmed that by adding an appropriate amount of titanium (Ti) and / or niobium (Nb), etc., which are strong carbide and nitride forming elements, to the steel, formability can be ensured by minimizing solid solution elements such as carbon (C), nitrogen (N), and sulfur (S). In addition, the present inventors confirmed that by appropriately adding phosphorus (P), molybdenum (Mo), etc. to the steel while adding the above components, a plated steel sheet for automotive outer panels with a tensile strength of 390 MPa or more, excellent surface quality, high strength, and high formability can be manufactured, and thus the present invention was completed.

[0040] Hereinafter, a plated steel sheet with excellent surface quality, high strength, and high formability, which can preferably be used as a material for automotive outer panels of the present invention, and a method for manufacturing the same will be described. At this time, as a plated steel sheet for automotive outer panels, it is necessary to satisfy stamping formability such as high tensile strength and deep drawability.

[0041] In the descaled rolled steel sheet as the base material (base steel) of the plated steel sheet related to this embodiment, in order to improve workability, a high-tensile steel sheet is used. The high-tensile steel sheet is based on ultra-low carbon steel as the basic cost, and appropriately adds strengthening elements such as Mn and P.

[0042] That is, the plated steel sheet according to one aspect of the present invention includes: a base steel; and a plating layer formed on the base steel. At this time, by weight %, the base steel contains: C: 0.003 - 0.009%, Si: 0.05% or less, Mn: 0.4 - 1.0%, P: 0.04 - 0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.03 - 0.08%, Ti: 0.005 - 0.03%, Nb: 0.02 - 0.045%, Cu: 0.06 - 0.15%, B: 0.003% or less, and the balance of Fe and other inevitable impurities. Hereinafter, the reasons for adding the basic component elements of the base steel and the reasons for limiting the contents will be specifically described.

[0043] Carbon (C): 0.003 - 0.009%

[0044] C is an interstitial solid solution element and has a great influence on the formation of the texture of the steel sheet during cold rolling and annealing. When the amount of solid solution carbon in the steel increases, the growth of grains with a {111}γ - fiber texture favorable for drawing is inhibited, and the growth of grains with {110} and {100} textures is promoted, resulting in a decrease in the drawability of the annealed sheet. Furthermore, when the content of C exceeds 0.009%, the contents of Ti and Nb required for precipitating it as carbides increase, which is not only disadvantageous in terms of economy but also forms pearlite, etc., leading to a possible decrease in formability. Therefore, the content of C is preferably limited to 0.009% or less. In addition, when the content of C is less than 0.003%, there may be a problem of insufficient strength, so the content of C is preferably limited to 0.003% or more. Additionally, more preferably, the lower limit of the content of C can be 0.0038%, or the upper limit of the content of C can be 0.008%.

[0045] Silicon (Si): 0.05% or less (except 0%)

[0046] Si is an element that helps to increase strength through solid solution strengthening. In order to exert the effect of increasing strength through such solid solution strengthening, Si must be added, so the Si content in the base steel is limited to more than 0%. However, when the Si content exceeds 0.05%, surface scale defects are caused, so there is a problem of deterioration of the plating surface characteristics. Therefore, in the present invention, the content of Si is controlled within 0.05%. Additionally, the lower limit of the content of Si can be more preferably 0.024%, or the upper limit of the content of Si can be 0.042%.

[0047] Manganese (Mn): 0.4 - 1.0%

[0048] Mn is a solid-solution strengthening element, which not only helps to improve 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 may occur. On the other hand, when the content of Mn exceeds 1.0%, surface problems caused by oxides may occur. Therefore, the content of Mn is preferably limited to 0.4 - 1.0%. Additionally, more preferably, the lower limit of the content of Mn can be 0.48%, or the upper limit of the content of Mn can be 0.67%.

[0049] Phosphorus (P): 0.04 - 0.09%

[0050] P is the element with the most excellent solid-solution effect, which does not significantly impair drawability and ensures the strength of the steel most effectively. When the content of P is less than 0.04%, the desired strength cannot be ensured. On the other hand, when the content of P exceeds 0.09%, secondary brittleness and surface stripe defects may occur due to P segregation. Therefore, the content of P is preferably limited to 0.04 - 0.09%. Additionally, more preferably, the lower limit of the content of P can be 0.048%, or the upper limit of the content of P can be 0.089%.

[0051] Molybdenum (Mo): 0.03 - 0.08%

[0052] Mo is an element with a high affinity for phosphorus (P) and plays a role in suppressing P segregation. In order to ensure high strength in ultra-low carbon steel, it is inevitable to use P. By appropriately adding Mo, it can to some extent help to improve the surface defects caused by P segregation. When the content of Mo is less than 0.03%, the desired surface improvement effect is not significant. In addition, when the content of Mo exceeds 0.08%, due to its high price, the cost competitiveness decreases. Therefore, the content of Mo is preferably limited to 0.03 - 0.08%. Additionally, more preferably, the lower limit of the content of Mo can be 0.05%, or the upper limit of the content of Mo can be 0.078%.

[0053] Sulfur (S): 0.01% or less (except 0%); Nitrogen (N): 0.005% or less (except 0%)

[0054] S and N are impurities present in the steel and are inevitably added. Therefore, in the base steel, the contents of S and N independently exceed 0%. However, in order to ensure excellent welding characteristics, the contents of S and N are preferably controlled as low as possible. Therefore, the content of S is controlled below 0.01%, and the content of N is controlled below 0.005%. Additionally, more preferably, the lower limit of the content of S can be 0.0015%, or the upper limit of the content of S can be 0.0034%. Furthermore, more preferably, the lower limit of the content of N can be 0.0008%, or the upper limit of the content of N can be 0.004%.

[0055] Aluminum (Al): 0.1% or less (except 0%)

[0056] Al precipitates AlN, thus contributing to improving the drawability and ductility of the steel. In order to exert the effect of improving such drawability and ductility, in the base steel, the content of Al exceeds 0%. However, when the content of the Al exceeds 0.1%, there is a problem that excessive Al inclusions are formed during the steelmaking operation, resulting in defects inside the steel plate. Therefore, the content of the Al is preferably controlled below 0.1%. Additionally, more preferably, the lower limit of the content of the Al can be 0.025%, or the upper limit of the content of the Al can be 0.08%.

[0057] Titanium (Ti): 0.005 - 0.03%

[0058] Ti is an element that reacts with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides, thus contributing to significantly improving the drawability of the steel plate. When the content of the Ti is less than 0.005%, the carbonitrides cannot be sufficiently precipitated, resulting in poor drawability. On the other hand, when the content of the Ti exceeds 0.03%, it is difficult to control inclusions during the steelmaking operation, and inclusion defects may occur. Therefore, the content of the Ti is preferably limited to 0.005 - 0.03%. Additionally, more preferably, the lower limit of the content of the Ti can be 0.007%, or the upper limit of the content of the Ti can be 0.012%.

[0059] Niobium (Nb): 0.02 - 0.045%

[0060] Nb is the most effective element that can expand the unrecrystallized region of the austenite region to a high temperature due to the solute drag and precipitation pinning effects during hot rolling, and very fine grains can be prepared through the rolling and cooling processes. When the content of the Nb 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 negligible. On the other hand, when the content of the Nb exceeds 0.045%, there is a problem of difficult hot rolling with the increase in high-temperature strength. Therefore, the content of the Nb is preferably limited to 0.02 - 0.045%. Additionally, more preferably, the lower limit of the content of the Nb can be 0.028%, or the upper limit of the content of the Nb can be 0.044%.

[0061] Boron (B): 0.003% or less (except 0%)

[0062] B is an element added to prevent secondary processing brittleness caused by the addition of P in steel. To achieve the effect of preventing the above-mentioned secondary processing brittleness, the content of B in the base steel exceeds 0%. However, when the content of B exceeds 0.003%, the ductility of the steel plate decreases. Therefore, the content of B is preferably limited to 0.003% or less. Additionally, more preferably, the lower limit of the content of B can be 0.0004%, or the upper limit of the content of B can be 0.0015%.

[0063] Copper (Cu): 0.04 - 0.15%

[0064] Cu is an element added to ensure strength, and Cu is an element that is difficult to remove when adjusting the composition of steel during steelmaking. Therefore, to ensure strength, it is preferable to add 0.04% or more of Cu. However, when the content of Cu exceeds 0.15%, it causes grain boundary embrittlement or cost increase. Therefore, the content of Cu is limited to 0.15% or less. Additionally, more preferably, the lower limit of the content of Cu can be 0.069%, and the upper limit of the content of Cu can be 0.10%.

[0065] In addition, the balance consists of Fe and inevitable impurities. Except for the above composition, the addition of effective components is not excluded. Additionally, the inevitable impurities may include all impurities that are undesirably mixed in the manufacturing process of ordinary galvanized steel sheets. Those skilled in the art can easily understand its meaning, so no special limitation is imposed on it.

[0066] By satisfying the above composition system, the present invention can effectively provide a high-strength, extra-low-carbon alloyed hot-dip galvanized steel sheet for automotive outer panels with excellent formability.

[0067] That is, the present invention is directed to an alloyed hot-dip galvanized steel sheet, which is based on an extra-low-carbon steel as the base component to improve formability, and uses a high-tensile steel sheet containing P as a strengthening element as the base material (base steel). By appropriately adjusting the composition and degree of alloying, the present invention can effectively prevent linear defects caused by segregation due to the inclusion of the above-mentioned P. Therefore, it is possible to effectively provide an alloyed hot-dip galvanized steel sheet that can be used as a material for automotive outer panels and has excellent surface appearance, and a manufacturing method thereof.

[0068] In addition, according to one aspect of the present invention, the base steel corresponds to an extra-low-carbon steel with a C content of 0.01% or less. Therefore, the base steel can have a fine structure based on ferrite. At this time, the fine structure based on ferrite may include other structures that are inevitably formed.

[0069] Specifically, according to one aspect of the present invention, in terms of area fraction, the microstructure of the base steel may contain more than 95% ferrite, and in addition, a small amount of pearlite or the like may remain. Alternatively, more preferably, in terms of area fraction, the microstructure of the base steel may contain more than 99% ferrite, and may contain 1% or less pearlite. Most preferably, the microstructure of the base steel may be a single-phase ferrite (including 100% ferrite).

[0070] By satisfying the above microstructure characteristics, excellent formability can be ensured. That is, in the base steel, when the content of microstructures such as pearlite exceeds 5%, problems such as poor formability may occur.

[0071] According to one aspect of the present invention, although not particularly limited, in the base steel, the average grain size of the ferrite may be in the range of 5 - 15 μm (i.e., 5 μm or more and 15 μm or less). When the average grain size of the ferrite is less than 5 μm, due to excessive strength, problems may occur in that the elongation rate cannot be sufficiently ensured. In addition, when the average grain size of the ferrite exceeds 15 μm, problems may occur in that the desired strength cannot be ensured. Further, although not particularly limited, in terms of further improving the above effects, preferably, the lower limit of the average grain size of the ferrite may be 7 μm, or the upper limit of the average grain size of the ferrite may be 10 μm.

[0072] Among them, the average grain size of the ferrite refers to the average value of the equivalent circle diameter of the grains measured based on the cross-section in the thickness direction of the plated steel sheet (i.e., the direction perpendicular to the rolling direction). More specifically, assuming that spherical particles are drawn with the longest length passing through the interior of the grains as the particle diameter, it represents the average value of the measured particle diameter values.

[0073] In addition, the plated steel sheet according to the present invention preferably satisfies the following relational expression 1.

[0074] [Relational Expression 1]

[0075] 1.6 ≤ Mt / Ms ≤ 6

[0076] (In the relational expression 1, Mt represents the average weight % content of Mo at a position 0.1 μm from the surface of the base steel in the thickness direction on the base steel side. Ms represents the average weight % content of Mo at a position t / 4 from the surface of the base steel in the thickness direction on the base steel side. t represents the total average thickness of the base steel, and is in the range of 0.8 - 1 mm.)

[0077] When the Mt / Ms value is less than 1.6, there may be a problem of linear defects occurring on the surface. When the Mt / Ms value exceeds 6, there may be a problem that the desired strength and elongation cannot be ensured. Additionally, although not particularly limited, in terms of further improving the above effects, preferably, the lower limit of the Mt / Ms value can be 1.9, or the upper limit of the Mt / Ms value can be 4.9.

[0078] That is, as a result of repeated studies by the present inventors, it has been found that by controlling the ratio of the Mo content in the outermost surface portion of the base steel (at a position 0.1 μm from the surface of the base steel toward the center side of the base steel in the thickness direction) to the Mo content in the surface portion of the base steel (at a position t / 4 from the surface of the base steel toward the center side of the base steel in the thickness direction) within an appropriate range, a plated steel sheet having excellent surface quality and high strength suitable for use as an automotive outer panel material can be obtained.

[0079] Therefore, by satisfying the above relational expression 1, the application range of the high-strength cold-rolled steel sheet containing P for automotive bodies can be expanded to a range that has not been applied so far (for example, for side outer panels, etc.), and ultimately, further lightweighting of automotive bodies can be achieved.

[0080] In addition, according to one aspect of the present invention, the coating layer can be a galvanized layer or a zinc alloy coating layer. Although not particularly limited, by weight%, the coating layer can have a composition containing 8 - 13% of Fe, the balance being Zn and other unavoidable impurities, and by satisfying this condition, excellent coating properties such as powdering can be ensured. At this time, the content of Fe is the mass percentage of Fe in the coating layer relative to the entire coating layer, and refers to the average mass% content of Fe. A method for measuring the content of Fe in the coating layer can be, for example, to dissolve the alloyed hot-dip galvanized layer using hydrochloric acid with an inhibitor and measure it by inductively coupled plasma (ICP) emission spectrometry. When the content of Fe in the coating layer is less than 8%, the Fe-Zn alloy phase is not sufficiently formed, and a soft η-Zn phase remains on the coating surface layer, which may have an adverse effect on workability and adhesiveness. In addition, when the content of Fe in the coating layer exceeds 13%, an excessive amount of hard and brittle Fe-Zn alloy phases (for example, Γ phase or Γ1 phase) are thickly formed at the interface between the coating layer and the base steel, increasing brittleness at the interface between the coating layer and the base steel, and thus problems may occur.

[0081] In addition, although not particularly limited, according to one aspect of the present invention, the plated steel sheet may satisfy the following relational expression 2. The present inventors further found that ensuring the relationship between the surface appearance and the contents of P and Mo has a close influence. That is, by satisfying the relational expression 2, a plated steel sheet with excellent surface appearance can be effectively provided.

[0082] [Relational expression 2]

[0083] [P] / [Mo] ≤ 1.162

[0084] (In the relational expression 2, the [P] represents the average weight % content of P in the base steel, and the [Mo] represents the average weight % content of Mo in the base steel.)

[0085] According to one aspect of the present invention, the tensile strength of the plated steel sheet may be 390 MPa or more, preferably in the range of 390 - 490 MPa. Since the tensile strength of the plated steel sheet satisfies 390 MPa or more (or 690 MPa or more and 490 MPa or less), the weight reduction of automobiles using high-tensile steel can be achieved. Additionally, more preferably, the lower limit of the tensile strength of the plated steel sheet may be 400 MPa, or the upper limit of the tensile strength of the plated steel sheet may be 480 Mpa.

[0086] In addition, according to one aspect of the present invention, the yield strength of the plated steel sheet may be 205 MPa or more, more preferably in the range of 205 - 350 MPa. When the yield strength of the plated steel sheet is too low, surface defects such as dents may occur when used for vehicle outer panels, and when the yield strength is too high, problems such as poor formability may occur.

[0087] According to one aspect of the present invention, the elongation of the plated steel sheet may be 28% or more, more preferably in the range of 28 - 43%, and most preferably in the range of 28 - 38%. Since the elongation of the plated steel sheet satisfies the above range, excellent formability and workability can be ensured.

[0088] Next, the method for manufacturing the plated steel sheet will be described in detail. However, the plated steel sheet of the present invention does not necessarily have to be manufactured by the following manufacturing method.

[0089] Another aspect of the present invention provides a method for manufacturing a plated steel sheet, which includes the following steps: continuously casting steel, by weight%, the steel contains: C: 0.003 - 0.009%, Si: 0.05% or less, Mn: 0.4 - 1.0%, P: 0.04 - 0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.03 - 0.08%, Ti: 0.005 - 0.03%, Nb: 0.02 - 0.045%, Cu: 0.06 - 0.15%, B: 0.003% or less, the balance being Fe and other inevitable impurities; reheating the slab obtained by continuous casting; hot rolling the reheated slab to provide a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and immersing the annealed cold-rolled steel sheet in a zinc-based plating bath for hot-dip galvanizing.

[0090] In addition, the present inventors conducted further research and confirmed that in order to provide a plated steel sheet having high strength and high formability while ensuring high actual yield and excellent surface appearance, it is an important factor to appropriately control the continuous casting speed according to the enrichment degree of P and Mo in the slab during continuous casting.

[0091] That is, according to one aspect of the present invention, in the method for manufacturing a plated steel sheet, it is preferable to satisfy the following relational expression 3 during continuous casting. At this time, the following relational expression 3 is an empirically obtained value, so no special limitation is imposed on the unit, as long as the units of Vc (meters per minute), [P], and [Mo] (weight%) are satisfied.

[0092] [Relational expression 3]

[0093] 1 ≤ Vc ≤ 5 - [P] / [Mo]

[0094] (In the relational expression 2, the Vc represents the average casting speed during continuous casting, with the unit of meters per minute. The [P] represents the average weight% content of P in the slab, and the [Mo] represents the average weight% content of Mo in the slab.)

[0095] In the present invention, when the average casting speed (Vc) during continuous casting is less than 1 meter per minute, due to the inability to sufficiently control the segregation of P on the surface, surface defects may occur in the final material. In addition, when the average casting speed (Vc) during continuous casting exceeds the value of 5 - [P] / [Mo], surface linear defects are generated, resulting in poor surface quality. This can be explained as follows: as the casting speed increases, the cooling rate of the slab is uneven, resulting in uneven thermal deformation of the slab surface caused by cooling or uneven precipitation amount of NbC precipitated near the surface layer. Therefore, due to the increased sensitivity of surface cracks, the result is poor surface defects.

[0096] In addition, according to one aspect of the present invention, although not particularly limited, during the continuous casting, cooling can be performed with a cooling water amount of 0.80 - 3.0 L / kg (more preferably 0.80 - 1.91 L / kg). During the continuous casting, when the cooling water amount is less than 0.80 L / kg, it is likely to cause uneven cooling rates on the slab surface, and uneven thermal deformation of the slab surface or uneven amounts of Ti-Nb composite precipitates precipitated near the surface layer, resulting in an increased sensitivity to surface cracks, and thus surface defects may occur. In addition, when the cooling water amount exceeds 3.0 L / kg, problems of crack formation on the continuous casting surface may occur due to rapid solidification.

[0097] In addition, according to one aspect of the present invention, the step of reheating the slab can be carried out in the range of 1180 - 1230 °C. When the reheating temperature of the slab is lower than 1180 °C, problems may occur during production due to the rolling load in the FM range. When the reheating temperature of the slab exceeds 1230 °C, surface scale defects may occur.

[0098] In addition, according to one aspect of the present invention, the hot rolling can be carried out until the finish rolling temperature is above Ar3, and more specifically, it can be carried out in the range of 880 - 970 °C. When the hot rolling temperature is lower than 880 °C, cooling occurs below the Ar3 temperature, that is, in the duplex phase region, resulting in the formation of coarse grains in the surface layer and uneven surface grain sizes, and ultimately problems may occur in the distinctness of image. In addition, when the hot rolling temperature exceeds 970 °C, the grain size is not fine enough, and problems of insufficient final material quality may occur. In addition, although not particularly limited, in terms of further enhancing the above effects, the lower limit of the finish rolling temperature is preferably 915 °C, or the upper limit of the finish rolling temperature can be 940 °C.

[0099] In addition, according to one aspect of the present invention, the step of coiling the hot rolled steel sheet can be carried out at 600 - 640 °C. When the coiling temperature is lower than 600 °C, since Ti(Nb)C and other precipitates are not sufficiently formed while the solid solution carbon increases, it affects behaviors such as recrystallization and grain growth during annealing, and thus problems occur in ensuring the required strength and elongation. In addition, when the coiling temperature exceeds 640 °C, problems of surface deterioration may occur due to the formation of secondary scale. In addition, although not particularly limited, in terms of further enhancing the above effects, the lower limit of the coiling temperature is preferably 612 °C, or the upper limit of the coiling temperature can be 635 °C.

[0100] In addition, according to one aspect of the present invention, after a pickling process for removing the surface scale of the hot-rolled steel sheet, a cold-rolled steel sheet can be obtained by cold rolling with a reduction ratio of 70-83%. When the reduction ratio during the cold rolling is less than 70%, the {111} texture cannot grow sufficiently, and there may be a problem of deteriorated formability. On the other hand, when the reduction ratio during the cold rolling exceeds 83%, the roll load is aggravated during on-site manufacturing, resulting in deteriorated shape, so there may be problems. Additionally, although not particularly limited, in terms of further improving the above effects, preferably, the lower limit of the reduction ratio during the cold rolling can be 71%, or the upper limit of the reduction ratio during the cold rolling can be 77%.

[0101] In addition, according to one aspect of the present invention, the step of annealing the cold-rolled steel sheet can be carried out at a temperature above the recrystallization temperature in the range of 740-830°C. By annealing at a temperature above the recrystallization temperature, the deformation caused by rolling is removed and the steel sheet is softened, thereby improving workability. That is, when the annealing temperature is lower than 740°C, due to incomplete ferrite recrystallization, there may be a problem of insufficient elongation caused by ultra-high strength. On the other hand, when the annealing temperature exceeds 830°C, grain growth occurs after recrystallization is completed, so there may be a problem of insufficient strength. Additionally, although not particularly limited, in terms of further improving the above effects, preferably, the lower limit of the annealing temperature can be 742°C, or the upper limit of the annealing temperature can be 810°C.

[0102] According to one aspect of the present invention, after cooling the annealed cold-rolled steel sheet, it may include the step of directly performing hot-dip galvanizing on a continuous hot-dip galvanizing line. That is, it may include the step of immersing the annealed cold-rolled steel sheet in a zinc-based plating bath for hot-dip galvanizing. In the present invention, the zinc-based plating bath refers to a plating bath containing an excessive amount of zinc (i.e., 50 wt% or more and 100% or less). The composition of the zinc-based plating bath is not particularly limited, and it may further contain Al or inevitable impurities, etc., and a plating bath commonly used in the art can be used.

[0103] After that, although not particularly limited, according to one aspect of the present invention, alloying heat treatment can be selectively carried out in the range of 500-560°C after the hot-dip galvanizing. At this time, when the alloying heat treatment temperature is lower than 500°C, alloying cannot be carried out sufficiently, and when the alloying heat treatment temperature exceeds 560°C, alloying is excessive, and due to coating embrittlement, problems such as coating peeling may be caused due to processing such as stamping. Additionally, although not particularly limited, in terms of further improving the above effects, preferably, the lower limit of the alloying heat treatment temperature can be 520°C, or the upper limit of the alloying heat treatment temperature can be 545°C.

[0104] In addition, according to one aspect of the present invention, although not particularly limited, it may further include a step of temper rolling the steel sheet subjected to alloying heat treatment by selectively using a temper roll having a roughness (Ra) of 1.0 - 1.6 μm. During the temper rolling, when the roughness (Ra) of the temper roll is less than 1.0 μm, it may be difficult to ensure the distinctness of image (image clarity) with a fine surface after painting. In addition, when the roughness (Ra) of the temper roll exceeds 1.6 μm during the temper rolling, problems may occur in stamping performance.

[0105] In addition, according to one aspect of the present invention, the temper rolling step may be performed at a reduction rate of 0.6 - 1.2%. When the reduction rate during the temper rolling is less than 0.6%, problems such as shape correction may occur, and when the reduction rate during the temper rolling exceeds 1.2%, problems such as excessive yield strength caused by mobile dislocations may occur. Detailed Description of the Invention

[0106] (Examples)

[0107] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention by way of example and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0108] (Experimental Example 1)

[0109] Steel having the alloy composition shown in Table 1 below was continuously cast to meet the average casting speed and cooling water amount shown in Table 2 below, thereby obtaining a steel slab with a thickness of 250 mm. Then, the surface of the slab was subjected to flame cleaning treatment of 2 - 4 mm. The slab was reheated to 1250 °C and then hot-rolled - coiled - cold-rolled under the conditions shown in Table 3 below, thereby obtaining a steel sheet with an average thickness of 0.85 mm. Then, the steel sheet was continuously annealed and immersed in a zinc-based plating bath for alloying hot-dip galvanizing to manufacture a coated steel sheet. Among them, the zinc-based plating bath contains 0.121 - 0.133% of Al, the balance of Zn, and other inevitable impurities by weight %.

[0110] [Table 1]

[0111] Steel Grade C Si P Mo Mn N Al Cu S Ti Nb B Inventive Steel 1 0.004 0.025 0.048 0.05 0.48 0.0013 0.0347 0.08 0.0023 0.01 0.035 0.0008 Inventive Steel 2 0.0038 0.03 0.051 0.053 0.51 0.0017 0.0361 0.07 0.0034 0.008 0.033 0.0004 Inventive Steel 3 0.005 0.031 0.057 0.0576 0.48 0.0015 0.0355 0.08 0.0021 0.007 0.038 0.0006 Inventive Steel 4 0.007 0.034 0.062 0.062 0.51 0.002 0.0345 0.069 0.003 0.009 0.028 0.0007 Inventive Steel 5 0.0064 0.042 0.079 0.068 0.62 0.0013 0.0346 0.078 0.0028 0.012 0.045 0.0006 Inventive Steel 6 0.006 0.032 0.082 0.072 0.61 0.0015 0.0390 0.08 0.0032 0.011 0.041 0.0008 Inventive Steel 7 0.008 0.036 0.089 0.078 0.67 0.0013 0.0410 0.09 0.0021 0.008 0.044 0.0007 Inventive Steel 8 0.0071 0.024 0.056 0.08 0.5 0.0017 0.0290 0.1 0.0015 0.01 0.034 0.0004 Comparative Steel 1 0.014 0.03 0.07 0.015 0.8 0.0018 0.0354 0.13 0.0054 0.03 0.034 0.002 Comparative Steel 2 0.004 0.1 0.09 0.01 0.5 0.0012 0.0389 0.02 0.0022 0.05 0.021 0.001 Comparative Steel 3 0.0028 0.1 0.037 0.005 0.72 0.0018 0.0405 0.05 0.0028 0.02 0.02 0.008 Comparative Steel 4 0.012 0.15 0.045 0.001 0.9 0.002 0.0391 0.5 0.0032 0.045 0.012 0.007 Comparative Steel 5 0.04 0.028 0.07 0.05 0.7 0.0025 0.0361 0.09 0.0028 0.015 0.038 0.0008

[0112] [Table 2]

[0113]

[0114]

[0115] Vc*: Average casting speed during continuous casting

[0116] Wq*: Cooling water volume during continuous casting

[0117] [Table 3]

[0118] Number FDT* CT* Cold Rolling Reduction Ratio Annealing Temperature GA Alloying Temperature Inventive Example 1 915℃ 622℃ 77％ 770℃ 520℃ Inventive Example 2 932℃ 620℃ 77％ 765℃ 525℃ Inventive Example 3 940℃ 618℃ 77％ 780℃ 525℃ Inventive Example 4 930℃ 615℃ 71％ 750℃ 530℃ Inventive Example 5 920℃ 620℃ 71％ 742℃ 530℃ Inventive Example 6 925℃ 635℃ 75％ 800℃ 545℃ Inventive Example 7 930℃ 612℃ 75％ 810℃ 545℃ Inventive Example 8 933℃ 622℃ 71％ 790℃ 520℃ Comparative Example 1 942℃ 710℃ 72％ 810℃ 530℃ Comparative Example 2 923℃ 690℃ 70％ 720℃ 530℃ Comparative Example 3 911℃ 618℃ 70％ 780℃ 530℃ Comparative Example 4 880℃ 620℃ 70％ 780℃ 530℃ Comparative Example 5 930℃ 611℃ 78％ 810℃ 535℃ Comparative Example 6 935℃ 622℃ 77％ 770℃ 520℃

[0119] FDT*: Finish rolling temperature

[0120] CT*: Coiling temperature

[0121] For each of the plated steel sheets obtained from the above Invention Examples 1 to 8 and Comparative Examples 1 to 4, the yield strength (YS), tensile strength (TS), and elongation at break (T-El) were measured and shown in Table 4 below. Specifically, after measuring the width, length of the parallel part, and thickness of the test piece, the test piece was installed on a tensile testing machine, waiting for the test piece to be fractured, and then the yield strength, tensile strength, and elongation at break of the test piece were measured. The yield strength is the ultimate stress when elastic strain occurs, and the value is usually represented by 0.2% offset. The tensile strength represents the value obtained by dividing the maximum load by the cross-sectional area of the circular section. The elongation at break is the amount of deformation of the test piece after fracture in the tensile test, expressed in %. At this time, the case where the tensile strength is 390 MPa or more and the elongation is 28% or more was evaluated as qualified.

[0122] In addition, for the evaluation of the r value, which is an index for deep drawing processing, JIS No. 5 tensile test pieces were collected in three directions: the parallel direction, 45° direction, and perpendicular direction of the alloyed hot-dip galvanized steel sheet in the rolling direction, and the r value of each test piece was measured. For example, for the measurement of the r value, the change value of the plate thickness and the change value of the plate width when about 15% of tensile deformation was performed in the above tensile test were measured, and the ratio of the change value of the plate width to the plate thickness was calculated. And, the r value in the direction parallel to the rolling direction was set as r0, the r value in the 45° direction was set as r 45 , and the r value in the perpendicular direction was set as r 90 , when the r value was obtained from Relationship A using the r values in each direction, the case where the r value was 1.2 or more was evaluated as qualified.

[0123] [Relationship A]

[0124] r = r0 + 2 * r 45 + r 90 / 4

[0125] In addition, surface linear defects are in the form of black lines confirmed visually, and the total number of linear defects of each coil can be confirmed in a Surface Defect Detector (SDD), and evaluated as "good" and "bad" according to the following criteria.

[0126] Good: The number of linear defects in the SDD is 100 or less

[0127] Bad: The number of linear defects in the SDD exceeds 100

[0128] In addition, "Mt / Ms" as the Mo ratio obtained from the above relational expression 1 is measured using a Glow Discharge Spectrometer (GDS). Specifically, the coating of a hot-dip galvanized test piece is removed by pickling, and then the content of Mo in the outermost surface layer of the base steel (i.e., at a position 0.1 μm from the surface of the base steel in the thickness direction) is measured using GDS, thereby obtaining the average content (Mt) of Mo in the outermost surface layer. In addition, the base steel is ground from the surface in the thickness direction to t / 4, and the content of Mo in the surface layer of the base steel (i.e., at a position t / 4 from the surface of the base steel in the thickness direction; where t is 0.9 mm) is measured using GDS, thereby obtaining the average content (Ms) of Mo in the surface layer. The enrichment ratio of Mo (i.e., the Mt / Ms value) in the outermost surface layer and the surface layer of the base steel thus obtained is measured and shown in Table 4 below.

[0129] In addition, for the cross-section in the thickness direction (i.e., the direction perpendicular to the rolling direction) of the coated steel sheet, the microstructure is observed through an Optical Microscope (OM), and it is confirmed that the microstructures of the following Invention Examples 1 to 8 and Comparative Examples 1 to 6 contain more than 99% ferrite. At this time, the average grain size of ferrite is measured and shown in Table 4 below.

[0130] [Table 4]

[0131]

[0132] Mt*: The average weight % content of Mo at a position 0.1 μm from the surface of the base steel in the thickness direction on the base steel side

[0133] Ms*: The average weight % content of Mo at a position t / 4 from the surface of the base steel in the thickness direction on the base steel side

[0134] GS* (grain size): Based on the cross-section in the thickness direction of the coated steel sheet, the average grain size (average equivalent circle diameter) of ferrite

[0135] As can be seen from Table 4, in the case of Invention Examples 1 to 8 that satisfy the alloy composition and manufacturing conditions of the present invention, the relational expression 1 is satisfied. For each of the plated steel sheets, the coating layer was dissolved in a hydrochloric acid solution, and then the dissolved liquid was analyzed by a wet analysis (ICP) method to measure the composition of the coating layer. As a result, it was confirmed that, by weight, the coating layer is composed of 8 - 13% Fe, the balance being Zn and other impurities.

[0136] The tensile strength of the plated steel sheets manufactured in the above Invention Examples 1 to 8 is all 390 MPa or more, the yield strength is 205 MPa or more. While the strength characteristics are excellent, the elongation is 28% or more, so the formability is also excellent. In addition, the r value is 1.2 or more. It is confirmed that the deep drawability is excellent and surface linear defects are prevented.

[0137] On the other hand, in the case of Comparative Examples 1 to 6 that do not satisfy one or more of the alloy composition and manufacturing conditions of the present invention, compared with Invention Examples 1 to 8 of the present invention, it is confirmed that one or more of the above characteristics are poor.

[0138] In particular, Figure 1 shows a photograph of observing the surface of the plated steel sheet manufactured in Invention Example 1 with a scanning electron microscope (SEM), and it is confirmed that alloying is carried out uniformly. On the other hand, Figure 2 shows a photograph of observing the surface of the plated steel sheet manufactured in Comparative Example 4 with SEM. In Comparative Example 4, it can be confirmed that alloying is carried out unevenly. This is because in Comparative Example 4 that does not satisfy the relational expression 3 of the present invention, as the segregation range of P increases, the alloying of the segregation range of P on the surface layer is delayed, resulting in uneven alloying. In addition, different from Figure 1 Figure 2 unequal alloying parts such as grooves are found, and black color differences are visually confirmed, so the appearance surface quality is poor.

[0139] (Experimental Example 2)

[0140] For the plated steel sheets obtained from Invention Example 1 and Comparative Example 1 of the above Experimental Example 1, temper rolling was further carried out using a temper roll having a roughness (Ra) under the conditions described in Table 5 below.

[0141] [Table 5]

[0142]

[0143] ​For each of the plated steel sheets obtained from the above-described Invention Examples 1-1 to 1-4 and Comparative Example 1-1, the Mo ratio of Relational Expression 1, the surface roughness (Ra) of the plating layer, the area fraction of ferrite, and the size of ferrite grains were measured and shown in Table 6 below. In addition, for the plated steel sheets obtained from the Invention Examples and Comparative Examples, evaluation was performed in the same manner as in Experimental Example 1 above and shown in Table 6 below.

[0144] Furthermore, in order to evaluate the distinctness of image, which is a measure of the appearance after painting, cupping was performed at a height of 17 mm, and the waviness (Wsa) was measured in the rolling direction starting from the center portion. When measuring Wsa, in a stamping machine having a hollow punch with a diameter of 75 mm and the force of a blank holder, a blank of 225 mm × 225 mm was stamped to completely suppress any movement of substances on the substrate between the blank holder and the die, thereby manufacturing a cup. For the deformation of the cup, the punch drawing depth was set to 17 - 18 mm, and the thickness deformation rate at the bottom was adjusted to 5% ± 0.2%. When the Wsa value measured by the above method is 0.35 μm or less, it was evaluated as "good", and other cases were evaluated as "bad", and shown in Table 6 below.

[0145] [Table 6]

[0146]

[0147] As can be seen from Table 6 above, in the case of Invention Examples 1-1 to 1-4 that satisfy the alloy composition and manufacturing conditions of the present invention, it was confirmed that the tensile strength was 390 MPa or more, the yield strength was 205 MPa or more, the strength characteristics were excellent, and at the same time, the elongation was 28% or more and the formability was also excellent. In addition, it was confirmed that the r value was 1.2 or more, the deep drawability was excellent, and surface linear defects were prevented.

[0148] In particular, in the above Invention Examples, in the case of Invention Examples 1-1 and 1-2 where the roughness (Ra) of the temper rolling roll was in the range of 1.0 - 1.60 μm and the reduction rate during temper rolling satisfied the range of 0.60 - 1.20%, compared with Invention Examples 1-3 and 1-4, it was confirmed that the surface after painting was beautiful, so the distinctness of image was excellent, the stamping property was excellent, and the yield strength also satisfied the range of 205 - 340 MPa.

[0149] On the other hand, in the case of Comparative Example 1-1 that does not satisfy the alloy composition of the present invention, it was confirmed that the tensile strength was too high and the elongation was low, resulting in poor formability, the r value was less than 1.2, the deep drawability was poor, and surface linear defects occurred.

Claims

1. A plated steel sheet, comprising: a base steel; and a plating layer formed on the base steel, wherein, by weight %, the base steel contains: C: 0.003 - 0.009%, Si: 0.05% or less, Mn: 0.4 - 1.0%, P: 0.04 - 0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.03 - 0.08%, Ti: 0.005 - 0.03%, Nb: 0.02 - 0.045%, Cu: 0.06 - 0.15%, B: 0.003% or less, the balance being Fe and other inevitable impurities, the plated steel sheet satisfies the following relational expression 1: 1.6 ≤ Mt / Ms ≤ 6 In the relational expression 1, Mt represents the average weight % content of Mo at a position 0.1 μm from the surface of the base steel in the thickness direction on the base steel side; Ms represents the average weight % content of Mo at a position t / 4 from the surface of the base steel in the thickness direction on the base steel side; t represents the total average thickness of the base steel and is in the range of 0.8 - 1 mm.

2. The plated steel sheet according to claim 1, wherein, By area fraction, the microstructure of the base steel contains 99% or more ferrite, and the balance is pearlite.

3. The coated steel sheet according to claim 2, wherein, The average grain size of the ferrite is 5 - 15 μm.

4. The coated steel sheet according to claim 1, wherein, The tensile strength of the plated steel sheet is 390 MPa or more, and the elongation is 28 - 43%.

5. The plated steel sheet according to claim 4, wherein, The yield strength of the plated steel sheet is 205 MPa or more.

6. The coated steel sheet according to claim 1, wherein, By weight %, the plating layer contains 8 - 13% Fe, the balance being Zn and other inevitable impurities.

7. The plated steel sheet according to claim 1, wherein, The plated steel sheet satisfies the following relational expression 2: [P] / [Mo] ≤ 1.162 In the relational expression 2, [P] represents the average weight % content of P in the base steel, and [Mo] represents the average weight % content of Mo in the base steel.

8. A method for manufacturing a plated steel sheet, comprising the following steps: Continuous casting of steel, wherein, by weight %, the steel contains: C: 0.003 - 0.009%, Si: 0.05% or less, Mn: 0.4 - 1.0%, P: 0.04 - 0.09%, S: 0.01% or less, N: 0.005% or less, S.Al: 0.1% or less, Mo: 0.03 - 0.08%, Ti: 0.005 - 0.03%, Nb: 0.02 - 0.045%, Cu: 0.06 - 0.15%, B: 0.003% or less, the balance being Fe and other inevitable impurities; Reheating the slab obtained by continuous casting; Hot rolling the reheated slab to provide a hot rolled steel sheet; Coiling the hot rolled steel sheet; Cold rolling the hot rolled steel sheet to provide a cold rolled steel sheet; Annealing the cold rolled steel sheet; and Dipping the annealed cold rolled steel sheet into a zinc - based plating bath for hot - dip galvanizing, wherein the continuous casting satisfies the following relational expression 3: 1 ≤ Vc ≤ 5 - [P] / [Mo] In the relation 3, Vc represents the average casting speed during continuous casting, with the unit of m / min, [P] represents the average weight % content of P in the slab, and [Mo] represents the average weight % content of Mo in the slab.

9. The method for manufacturing a plated steel sheet according to claim 8, wherein, The temperature for reheating the slab is 1180 - 1230 °C. The temperature for hot rolling is 880 - 970 °C. The temperature for coiling the hot-rolled steel sheet is 600 - 640 °C. The reduction ratio during cold rolling is 70 - 83%. The temperature for annealing the cold-rolled steel sheet is 740 - 830 °C.

10. The method for manufacturing a plated steel sheet according to claim 8, wherein, During the continuous casting, cooling is carried out with a cooling water amount of 0.8 - 3.0 L / kg.

11. The method for manufacturing a plated steel sheet according to claim 8, wherein, After the hot-dip galvanizing step, it further includes a step of alloying heat treatment at 500 - 560 °C.

12. The method for manufacturing a plated steel sheet according to claim 11, wherein, The method further includes a step of skin pass rolling the steel sheet after alloying heat treatment with a skin pass roll having a roughness Ra of 1.0 - 1.6 μm.

13. The method for manufacturing a plated steel sheet according to claim 12, wherein, The skin pass rolling step is carried out with a reduction ratio of 0.6 - 1.2%.

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