STEEL SHEET AND MANUFACTURING METHOD THEREOF

Abstract

 The present invention relates to a steel sheet suitable for various uses including use in automobile parts and, more specifically, to a steel sheet and a manufacturing method thereof.

Description

Technical Field

[0001] The present disclosure relates to a steel sheet suitable for various applications including automobile parts, and more particularly, to a steel sheet and a method for manufacturing the same.

Background Art

[0002] In order to address the global climate crisis, automobile manufacturers are gradually requiring steelmakers to supply automobile steel sheets manufactured with reduced CO₂ emissions, with the goal of achieving carbon neutrality by 2050. For this reason, recently, steelmakers have been developing automobile steel sheets manufactured with reduced CO₂ emissions through recycling of steel scrap using existing blast furnace-basic oxygen furnace steelmaking methods or electric furnace steelmaking methods.

[0003] When steel scrap is used as a raw material, residual elements (tramp elements), such as Cu and Ni, contained in the steel scrap are difficult to remove during refining, and are therefore contained in the steel after refining. Since these residual elements may degrade the properties of the steel or deteriorate surface quality, high-grade thin sheet products represented by automobile steel sheets have so far been manufactured by a method in which hot metal is used as a main raw material, C and N in the steel are greatly reduced in a general blast furnace-basic oxygen furnace process, and the content of residual elements is strictly controlled.

[0004] Meanwhile, technologies for manufacturing automobile steel sheets having excellent formability from electric furnace steel containing residual elements using an electric furnace steelmaking method have been proposed as follows.

[0005] Patent Documents 1 and 2 disclose technologies for manufacturing cold-rolled steel sheets having excellent formability. Specifically, the steel sheets disclosed therein are cold-rolled steel sheets having excellent cold formability despite containing a large amount of residual elements. However, the above technologies do not consider at all an object of improving inferior plating adhesion of the steel sheet or means for achieving the object.

[0006] As described above, although technologies for manufacturing automobile steel sheets containing residual elements have been proposed, an automobile steel sheet having both improved formability and improved plating adhesion despite containing residual elements has not yet been developed.

[0007] (Patent Document 1) Japanese Patent Laid-Open Publication No. 1995-118795

[0008] (Patent Document 2) Japanese Patent Laid-Open Publication No. 1998-025541

Summary of Invention

Technical Problem

[0009] An aspect of the present disclosure is to provide a steel sheet having improved formability and plating adhesion despite containing residual elements when manufacturing a steel sheet for automobiles by recycling steel scrap, and a method for manufacturing the same.

[0010] The object of the present disclosure is not limited to the above description. Those skilled in the art to which the present disclosure pertains will have no difficulty in understanding additional objects of the present disclosure from the overall content of the present specification.

Solution to Problem

[0011] According to an aspect of the present disclosure, a steel sheet contains, by wt%, greater than 0% to 0.0700% of carbon (C), greater than 0% to 0.800% of silicon (Si), greater than 0% to 1.000% of manganese (Mn), greater than 0% to 0.500% of aluminum (Al), 0 to 0.080% of phosphorus (P), 0 to 0.0500% of sulfur (S), 0 to 0.0300% of nitrogen (N), greater than 0% to 1.000% of copper (Cu), greater than 0% to 1.000% of chromium (Cr), greater than 0% to 1.000% of molybdenum (Mo), greater than 0% to 0.5000% of tin (Sn), one or more selected from the following groups (i) to (vi), and the balance of Fe and other inevitable impurities.

(i) 0 to 1.000% of nickel (Ni)

(ii) one or more selected from 0 to 0.500% of titanium (Ti), 0 to 0.500% of niobium (Nb), 0 to 0.500% of vanadium (V), and 0 to 0.0200% of boron (B)

(iii) one or more selected from 0 to 0.050% of magnesium (Mg), 0 to 0.050% of calcium (Ca), and 0 to 0.050% of rare earth elements (REM) excluding yttrium (Y)

(iv) one or more selected from 0 to 0.50% of tungsten (W) and 0 to 0.50% of zirconium (Zr)

(v) one or more selected from 0 to 0.5000% of antimony (Sb) and 0 to 0.5000% of cobalt (Co)

(vi) one or more selected from 0 to 0.200% of yttrium (Y) and 0 to 0.200% of hafnium (Hf).

[0012] In an embodiment in the present disclosure, a steel sheet in which A/B satisfies 0.15 to 30.00 may be provided, where A represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a surface layer portion, and B represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a central portion.

[0013] In an embodiment in the present disclosure, a steel sheet in which a microstructure is a single ferrite phase may be provided, and an average grain size of the ferrite may be 6 to 50 µm.

[0014] In an embodiment in the present disclosure, a steel sheet in which a value (I/II) obtained by dividing an average grain size (I) of a cold-rolled steel sheet by an average grain size (II) of a hot-rolled steel sheet satisfies 0.60 to 0.95 may be provided.

[0015] The steel sheet of the present disclosure may have high strength and excellent formability, and may also have excellent plating adhesion when subjected to a subsequent plating process.

[0016] In an embodiment in the present disclosure, the steel sheet may have a product (TS×El) of tensile strength (TS) and elongation (El) of 4.0 to 30.0 GPa·%, and a product (r-value×El) of Lankford value (r-value) and elongation of 20 to 200%.

[0017] In an embodiment in the present disclosure, the steel sheet may include a plating layer formed on at least one surface thereof, and the plating layer may have a powdering peeling width of less than 7 mm.

[0018] According to another aspect of the present disclosure, a method for manufacturing a steel sheet includes: preparing a steel slab; heating the steel slab in a temperature range of 900 to 1,300°C; finish-rolling the heated steel slab in an austenite region at a temperature equal to or higher than an Ar3 transformation point or in a ferrite region at a temperature equal to or lower than the Ar3 transformation point to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 700°C or higher; cooling the coiled hot-rolled steel sheet at a rate of 10.0°C/min or less to a temperature range of 250 to 350°C; cold-rolling the hot-rolled steel sheet after the cooling at a cold reduction ratio of 50% or more to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at 600°C or higher for 10 seconds or longer.

[0019] In an embodiment in the present disclosure, the steel slab may have the above-described alloy composition.

[0020] In an embodiment in the present disclosure, the steel slab may be obtained by continuously casting molten steel obtained through a steelmaking process in an electric furnace.

[0021] In an embodiment in the present disclosure, the annealing may be performed by heating the cold-rolled steel sheet obtained by cold rolling to an annealing temperature at a heating rate of 2 to 60°C/s.

[0022] In an embodiment in the present disclosure, the method may further include plating the cold-rolled steel sheet after the annealing, and the plating may be hot-dip plating or electroplating.

[0023] In an embodiment in the present disclosure, the hot-dip plating may be hot-dip galvanizing performed in a temperature range of 440 to 520°C, and after the hot-dip galvanizing, alloying heat treatment may be additionally performed.

[0024] According to still another aspect of the present disclosure, there is provided a steel sheet manufactured by the above-described manufacturing method, wherein a value obtained by dividing an average grain size (I) of the cold-rolled steel sheet after the annealing by an average grain size (II) of the hot-rolled steel sheet may satisfy 0.60 to 0.95.

Advantageous Effects of Invention

[0025] According to the present disclosure, a steel sheet for automobiles having improved formability and plating adhesion despite containing residual elements may be provided.

[0026] The steel sheet of the present disclosure is manufactured by recycling steel scrap, and thus has an excellent effect of reducing CO₂ emissions when applied as a material for automobile bodies and the like.

[0027] Various and beneficial advantages and effects of the present disclosure are not limited to those described above, and may be more readily understood in the course of describing specific embodiments in the present disclosure.

Best Mode for Invention

[0028] The terminology used herein is merely for referring to particular embodiments and is not intended to limit the present disclosure. Singular forms used herein include plural forms unless phrases clearly indicate a contrary meaning.

[0029] The term "comprising" used in the specification specifies specific properties, regions, integers, steps, operations, elements, and/or components, and does not exclude the presence or addition of other specific properties, regions, integers, steps, operations, elements, components, and/or groups.

[0030] Unless defined otherwise, all terms, including technical terms and scientific terms, used herein have the same meanings as commonly understood by those skilled in the art to which the present disclosure pertains. Terms defined in a generally used dictionary are additionally interpreted as having meanings consistent with related technical documents and the contents disclosed herein, and are not interpreted as having ideal or overly formal meanings unless otherwise defined.

[0031] Hereinafter, a steel sheet according to an aspect of the present disclosure, particularly a steel sheet having excellent formability and plating adhesion, will be described in detail. In the present disclosure, it should be noted that the contents of respective elements are expressed in wt% unless otherwise specified. In addition, ratios of grains or structures are based on area unless otherwise specifically indicated.

[0032] Hereinafter, in the description of the steel sheet, unless otherwise specifically stated, the steel sheet refers to a cold-rolled steel sheet.

[0033] A steel sheet according to an aspect of the present disclosure may contain, by wt%, greater than 0% to 0.0700% of carbon (C), greater than 0% to 0.800% of silicon (Si), greater than 0% to 1.000% of manganese (Mn), greater than 0% to 0.500% of aluminum (Al), 0 to 0.080% of phosphorus (P), 0 to 0.0500% of sulfur (S), 0 to 0.0300% of nitrogen (N), greater than 0% to 1.000% of copper (Cu), greater than 0% to 1.000% of chromium (Cr), greater than 0% to 1.000% of molybdenum (Mo), and greater than 0% to 0.5000% of tin (Sn).

[0034] Hereinafter, the reason for limiting the alloy composition of the steel sheet provided by the present disclosure as described above will be described in detail.

Carbon (C): greater than 0% to 0.0700%

[0035] In an embodiment in the present disclosure, when a content of C exceeds 0.0700%, it is difficult to achieve the elongation and r-value targeted in the present disclosure. Therefore, C may be contained in an amount of 0.0700% or less. In another embodiment in the present disclosure, C may be 0.0650% or less or 0.0600% or less.

[0036] Meanwhile, C is an essential element in manufacturing steel, and the content of C is greater than 0%. However, in an embodiment in the present disclosure, when the content of C is less than 0.0003%, the effect achieved by reducing C is hardly obtained, and rather, steelmaking costs are increased, which is not preferable. Therefore, C may be contained in an amount of 0.0003% or more. In another embodiment in the present disclosure, C may be contained in an amount of 0.0010% or more or 0.0015% or more.

Silicon (Si): greater than 0% to 0.800%

[0037] Silicon (Si) is an element having an effect of improving strength through solid-solution strengthening, and is useful for strengthening ferrite and homogenizing the structure, thereby improving formability. In addition, Si is an element necessary for deoxidation during steelmaking.

[0038] In an embodiment in the present disclosure, when a content of Si exceeds 0.800%, plating defects such as unplated portions may be caused during plating, and weldability of the steel sheet may be impaired. Accordingly, Si may be contained in an amount of 0.800% or less. In another embodiment in the present disclosure, Si may be 0.700% or less.

[0039] Meanwhile, in an embodiment in the present disclosure, Si may be contained in an amount greater than 0%, and the content of Si may be limited to 0.001% or more in consideration of manufacturing costs.

Manganese (Mn): greater than 0% to 1.000%

[0040] Manganese (Mn) is an element useful for simultaneously improving the strength and ductility of steel.

[0041] In an embodiment in the present disclosure, when a content of Mn exceeds 1.000%, formability deteriorates, which is not preferable. In another embodiment in the present disclosure, Mn may be 0.900% or less or 0.400% or less.

[0042] Meanwhile, in an embodiment in the present disclosure, Mn may be contained in an amount greater than 0%, and the content of Mn may be 0.001% or more in consideration of manufacturing costs.

Aluminum (Al): greater than 0% to 0.500%

[0043] Aluminum (Al) is an element that combines with oxygen (O) in steel and performs deoxidation. In addition, similarly to Si, Al is an element that strengthens ferrite, homogenizes the structure, and improves formability.

[0044] In an embodiment in the present disclosure, when a content of Al exceeds 0.500%, formability rather deteriorates. Therefore, Al may be contained in an amount of 0.500% or less. In another embodiment in the present disclosure, Al may be contained in an amount of 0.400% or less.

[0045] Meanwhile, in an embodiment in the present disclosure, Al may be contained in an amount greater than 0%. However, when the content of Al is controlled to less than 0.001%, manufacturing costs may increase. In consideration thereof, the content of Al may be limited to 0.001% or more.

Phosphorus (P): 0.080% or less

[0046] Phosphorus (P) is an element added to improve the strength of steel. In an embodiment in the present disclosure, when a content of P exceeds 0.080%, the formability of the steel may be impaired and brittleness may occur. Accordingly, P may be contained in an amount of 0.080% or less. According to another embodiment in the present disclosure, P may be contained in an amount of 0.070% or less.

[0047] Meanwhile, in an embodiment in the present disclosure, the content of P may be 0%, but may be limited to 0.001% or more in consideration of manufacturing costs.

Sulfur (S): 0.0500% or less

[0048] Sulfur (S) is an element inevitably added during a steel manufacturing process, and impairs the ductility of steel by combining with Mn in the steel to form MnS inclusions. Accordingly, in an embodiment in the present disclosure, a content of S may be limited to 0.0500% or less. In another embodiment in the present disclosure, S may be contained in an amount of 0.0400% or less.

[0049] Meanwhile, in an embodiment in the present disclosure, the content of S may be 0%, but since S may be inevitably contained in steel, the content of S may be greater than 0%. In an embodiment in the present disclosure, when the content of S is controlled to less than 0.0010%, manufacturing costs may significantly increase. Accordingly, S may be contained in an amount of 0.0010% or more.

Nitrogen (N): 0.0300% or less

[0050] Nitrogen (N) is an element inevitably added during a steel manufacturing process, and is an element that causes cracking of a slab by forming nitrides during continuous casting. Accordingly, in an embodiment in the present disclosure, N may be contained in an amount of 0.0300% or less.

[0051] Meanwhile, in an embodiment in the present disclosure, the content of N may be 0%, but since N may be inevitably contained in steel, the content of N may be greater than 0%. In an embodiment in the present disclosure, when the content of N is controlled to less than 0.0010%, manufacturing costs may significantly increase. Accordingly, N may be contained in an amount of 0.0010% or more.

Copper (Cu): greater than 0% to 1.000%

[0052] Copper (Cu) is an element that stabilizes austenite and suppresses corrosion. In addition, Cu is an element useful for suppressing hydrogen delayed fracture by being enriched on a surface of a steel sheet and preventing hydrogen from penetrating into the steel.

[0053] In an embodiment in the present disclosure, when a content of Cu is excessive, formability may deteriorate, and thus the content of Cu may be limited to 1.000% or less. In another embodiment in the present disclosure, Cu may be contained in an amount of 0.900% or less.

[0054] Meanwhile, in an embodiment in the present disclosure, Cu may be contained in an amount greater than 0%, and may be contained in an amount of 0.001% or more in order to sufficiently obtain the effect of Cu.

Chromium (Cr): greater than 0% to 1.000%

[0055] Chromium (Cr) is an element that suppresses decomposition of austenite and stabilizes austenite during alloying of a manufactured steel sheet.

[0056] In an embodiment in the present disclosure, when a content of Cr is excessive, formability may deteriorate, and thus the content of Cr may be limited to 1.000% or less in consideration thereof. In another embodiment in the present disclosure, Cr may be contained in an amount of 0.900% or less.

[0057] Meanwhile, in an embodiment in the present disclosure, Cr may be contained in an amount greater than 0%, and is preferably contained in an amount of 0.001% or more in order to sufficiently obtain the effect of Cr.

Molybdenum (Mo): greater than 0% to 1.000%

[0058] Molybdenum (Mo), similarly to Cr described above, is an element that suppresses decomposition of austenite and stabilizes austenite during alloying of a manufactured steel sheet.

[0059] In an embodiment in the present disclosure, when a content of Mo is excessive, formability may deteriorate, and thus the content of Mo may be limited to 1.000% or less in consideration thereof. According to another embodiment in the present disclosure, Mo may be contained in an amount of 0.900% or less.

[0060] Meanwhile, in an embodiment in the present disclosure, Mo may be contained in an amount greater than 0%, and may be contained in an amount of 0.001% or more in order to sufficiently obtain the effect of Mo.

Tin (Sn): greater than 0% to 0.5000%

[0061] Tin (Sn) is an element that improves plating wettability and plating adhesion of steel. In an embodiment in the present disclosure, when a content of tin in steel exceeds 0.5000%, brittleness of the steel may increase, thereby causing cracks during hot working or cold working. According to another embodiment in the present disclosure, Sn may be contained in an amount of 0.4000% or less.

[0062] Meanwhile, in an embodiment in the present disclosure, Sn may be contained in an amount greater than 0%, and may be contained in an amount of 0.0005% or more in order to sufficiently obtain the effect of Sn.

[0063] The steel sheet of the present disclosure may further contain, in addition to the above-described alloy composition, one or more selected from groups (i) to (vi) described below. Since all of the elements described below may be optionally added, it is noted that the content thereof may be 0%. Meanwhile, the elements of groups (i) to (vi) may be referred to as residual elements.

(i) 0 to 1.000% of nickel (Ni)

(ii) one or more selected from 0 to 0.500% of titanium (Ti), 0 to 0.500% of niobium (Nb), 0 to 0.500% of vanadium (V), and 0 to 0.0200% of boron (B)

(iii) one or more selected from 0 to 0.050% of magnesium (Mg), 0 to 0.050% of calcium (Ca), and 0 to 0.050% of rare earth elements (REM) excluding yttrium (Y)

(iv) one or more selected from 0 to 0.50% of tungsten (W) and 0 to 0.50% of zirconium (Zr)

(v) one or more selected from 0 to 0.5000% of antimony (Sb) and 0 to 0.5000% of cobalt (Co)

(vi) one or more selected from 0 to 0.200% of yttrium (Y) and 0 to 0.200% of hafnium (Hf)

(i) 0 to 1.000% of nickel (Ni)

[0064] Nickel (Ni) is an element that stabilizes austenite and suppresses corrosion, and is also useful for suppressing hydrogen delayed fracture by being enriched on a surface of a steel sheet and preventing hydrogen from penetrating into the steel.

[0065] In an embodiment in the present disclosure, when a content of Ni is excessive, formability may deteriorate, and manufacturing costs may increase because Ni is an expensive element. In consideration thereof, the content of Ni may be limited to 1.000% or less. In another embodiment in the present disclosure, Ni may be contained in an amount of 0.900% or less.

[0066] Meanwhile, in an embodiment in the present disclosure, even when Ni is not intentionally added, the object of the present disclosure may be achieved without difficulty, and therefore, the content of Ni may be 0%. However, when Ni is added, Ni may be contained in an amount of 0.001% or more in order to obtain the above-described effects.

(ii) one or more selected from 0 to 0.500% of titanium (Ti), 0 to 0.500% of niobium (Nb), 0 to 0.500% of vanadium (V), and 0 to 0.0200% of boron (B)

[0067] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates in steel, and the formation of the precipitates may improve the strength and impact toughness of the steel. In particular, Ti, Nb, and V may prevent deterioration in formability caused by solid-solution C and solid-solution N by precipitating solid-solution C and solid-solution N in the steel as carbides, nitrides, and the like.

[0068] In an embodiment in the present disclosure, when the elements are additionally added, each of the elements may be added in an amount of 0.001% or more, and the above-described effects may be obtained therefrom. However, when the content of each element exceeds 0.500%, the addition effect may be saturated and manufacturing costs may increase. Therefore, an upper limit of each element may be limited to 0.500%. According to another embodiment in the present disclosure, when the elements are added, each of the elements may be contained in an amount of 0.400% or less.

[0069] Boron (B) is an element that improves hardenability of steel, increases strength, and suppresses nucleation at grain boundaries.

[0070] In an embodiment in the present disclosure, when a content of B exceeds 0.0200%, deep drawability of the steel may deteriorate. Therefore, when B is added, the content of B may be limited to 0.0200% or less. According to another embodiment, B may be contained in an amount of 0.0100% or less. Meanwhile, in an embodiment in the present disclosure, when B is additionally added, B may be contained in an amount of 0.0001% or more in order to obtain the effect thereof. According to another embodiment, when B is added, B may be contained in an amount of 0.0005% or more or 0.0010% or more.

(iii) one or more selected from 0 to 0.050% of magnesium (Mg), 0 to 0.050% of calcium (Ca), and 0 to 0.050% of rare earth elements (REM) excluding yttrium (Y)

[0071] Magnesium (Mg), calcium (Ca), and rare earth elements (REM) excluding yttrium (Y) are elements that improve ductility of steel by spheroidizing sulfides in the steel. In an embodiment in the present disclosure, when each of the above-described elements is present in an amount exceeding 0.050%, the above-described effect may be saturated, and manufacturing costs may increase. Therefore, the contents thereof may each be limited to 0.050% or less.

[0072] In general, rare earth elements (REM) refer to a total of 17 metallic elements, such as scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce) of the like. According to an embodiment in the present disclosure, since the content of yttrium (Y) is separately limited below, it is noted that, in an embodiment in the present disclosure, REM refers to 16 elements excluding Y.

(iv) one or more selected from 0 to 0.50% of tungsten (W) and 0 to 0.50% of zirconium (Zr)

[0073] Tungsten (W) and zirconium (Zr) are elements that improve hardenability of steel and increase the strength of the steel. According to an embodiment in the present disclosure, when W and Zr are present excessively, the above-described effects may be saturated, and manufacturing costs may increase. Accordingly, when W and Zr are added, the contents thereof may each be limited to 0.50% or less.

(v) one or more selected from 0 to 0.5000% of antimony (Sb) and 0 to 0.5000% of cobalt (Co)

[0074] Antimony (Sb) and cobalt (Co) are elements that improve plating wettability and plating adhesion of steel. In an embodiment in the present disclosure, when contents of the elements in steel each exceed 0.5000%, brittleness of the steel may increase, thereby causing cracks during hot working or cold working. Therefore, the content of each element may be limited to 0.5000% or less. According to another embodiment in the present disclosure, these elements may each be contained in an amount of 0.4000% or less. Meanwhile, in order to sufficiently obtain the intended effects by these elements, these elements may each be contained in an amount of 0.0005% or more.

(vi) one or more selected from 0 to 0.200% of yttrium (Y) and 0 to 0.200% of hafnium (Hf)

[0075] Yttrium (Y) and hafnium (Hf) are elements that improve corrosion resistance of steel. In an embodiment in the present disclosure, when contents of the elements each exceed 0.200%, ductility of the steel may decrease. Therefore, the content of each element may be limited to 0.200% or less.

[0076] In an embodiment in the present disclosure, the steel sheet may contain the balance of Fe and other inevitable impurities in addition to the above-described components. However, in a typical manufacturing process, unintended impurities may inevitably be introduced from raw materials or the surrounding environment, and thus such impurities cannot be completely excluded. Since these impurities are known to those skilled in the art, not all details thereof are specifically mentioned in the present specification. In addition, the addition of effective components other than the above-described components is not completely excluded.

[0077] According to an embodiment in the present disclosure, contents of some residual elements contained at specific positions in a thickness direction of the steel sheet may be controlled as follows, and therefore, plating adhesion of the steel sheet may be improved.

[0078] Specifically, in an embodiment in the present disclosure, A/B may satisfy 0.15 to 30.00, where A represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a surface layer portion of the steel sheet, and B represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a central portion of the steel sheet.

[0079] In an embodiment in the present disclosure, A/B is significant in that it is used as an indicator representing plating adhesion of the steel sheet. When the value of A/B is less than 0.15 or exceeds 30.00, specific residual elements contained in the steel may not be uniformly distributed.

[0080] Here, the central portion refers to a point t/2 in the thickness direction. In addition, the surface layer portion refers to a region extending from the surface to an inflection point, which is a point at which concentrations change (or rapidly change) when concentration gradients of Cu, Cr, Mo, and Sn are measured in the thickness direction.

[0081] In an embodiment in the present disclosure, when a content of (Cu+Cr+Mo+Sn) at the outermost surface is greater than a content of (Cu+Cr+Mo+Sn) in the central portion, the inflection point may refer to a point corresponding to 99% of the content of (Cu+Cr+Mo+Sn) in the central portion in the measured concentration gradient values. When the content of (Cu+Cr+Mo+Sn) at the outermost surface is less than the content of (Cu+Cr+Mo+Sn) in the central portion, the inflection point may refer to a point corresponding to 1% of the content of (Cu+Cr+Mo+Sn) in the central portion in the measured concentration gradient values. As a nonlimiting example, the concentration gradients of the above-described elements may be measured using FE-SEM or a glow discharge spectrometer (GDS).

[0082] As an example, when the concentration of (Cu+Cr+Mo+Sn) is measured in the thickness direction of the steel sheet, the content of (Cu+Cr+Mo+Sn) at the outermost surface may have the highest value, and the value may tend to linearly decrease from the outermost surface toward the central portion.

[0083] As another example, when the concentration of (Cu+Cr+Mo+Sn) is measured in the thickness direction of the steel sheet, the content of (Cu+Cr+Mo+Sn) at the outermost surface may have the lowest value, and the value may tend to linearly increase from the outermost surface toward the central portion.

[0084] Meanwhile, it is well known in the technical field of the present disclosure that a steel sheet, that is, a cold-rolled steel sheet, is obtained through a series of rolling processes. That is, as will be described below in detail, the cold-rolled steel sheet according to the present disclosure is obtained by subjecting a hot-rolled steel sheet obtained through hot rolling and coiling processes to a cold rolling process (and a subsequent annealing heat treatment process).

[0085] With respect to such hot-rolled steel sheet and cold-rolled steel sheet, in an embodiment in the present disclosure, a ratio between an average grain size of the hot-rolled steel sheet and an average grain size of the cold-rolled steel sheet obtained by subjecting the hot-rolled steel sheet to cold rolling and annealing heat treatment may satisfy the following.

[0086] Specifically, in an embodiment in the present disclosure, a value (I/II) obtained by dividing an average grain size (I) of the cold-rolled steel sheet by an average grain size (II) of the hot-rolled steel sheet may satisfy 0.60 to 0.95.

[0087] As described above, according to the present disclosure, by controlling the grain size of the hot-rolled steel sheet obtained by hot rolling, the grain size of the cold-rolled steel sheet obtained by subsequent cold rolling and annealing heat treatment may be achieved, and thus excellent formability as well as strength may be obtained.

[0088] When the value of I/II is less than 0.60, grains of the hot-rolled steel sheet become excessively fine, and stiffness may significantly increase. On the other hand, when the value exceeds 0.95, grains become coarse, and strength and formability may not be achieved.

[0089] In an embodiment in the present disclosure, the microstructure of the steel sheet may be a single ferrite phase.

[0090] That is, when the microstructure of the steel sheet includes a structure other than ferrite, for example, pearlite, or bainite or martensite formed at a low temperature, the intended formability may not be achieved. However, this is not intended to exclude even an extremely small amount of impurity structure inevitably included due to the process, and such a structure is naturally included in the scope of the present disclosure.

[0091] In an embodiment in the present disclosure, the average grain size of the ferrite may be 6 to 50 µm. When the average grain size of the ferrite is less than 6 µm, stiffness of the steel may increase, and the steel may become brittle. On the other hand, when the size exceeds 50 µm, it may be disadvantageous in achieving strength through grain refinement.

[0092] Meanwhile, it is noted that the grain sizes of the hot-rolled steel sheet and the cold-rolled steel sheet mentioned above refer to the grain size of ferrite.

[0093] The steel sheet according to an embodiment in the present disclosure has not only high strength but also excellent formability.

[0094] Specifically, according to an embodiment in the present disclosure, a product (TS×El) of tensile strength (TS) and elongation (El) of the steel sheet may be 4.0 to 30.0 GPa·%, and a product (r-value×El) of Lankford value (r-value) and elongation of the steel sheet may be 20 to 200%.

[0095] The steel sheet according to an embodiment in the present disclosure may include a plating layer formed on at least one surface.

[0096] In an embodiment in the present disclosure, the plating layer may be a zinc-based plating layer, and the zinc-based plating layer is a plating layer containing zinc (Zn) as a main component, and compositions of plating layers commonly applied in the art may be applied in the same manner. In this case, the zinc-based plating layer may also include an alloyed zinc-based plating layer obtained by alloying.

[0097] As described above, a steel sheet including a zinc-based plating layer formed on at least one surface is a zinc-based plated steel sheet, and may be a hot-dip galvanized steel sheet (GI), a galvannealed steel sheet (GA), an electrogalvanized steel sheet (EG), an alloyed electrogalvanized steel sheet, or the like.

[0098] In an embodiment in the present disclosure, the steel sheet including a zinc-based plating layer formed on at least one surface may be the steel sheet according to an embodiment in the present disclosure, and it is noted that the above description may be equally applied to the steel sheet.

[0099] Meanwhile, the steel sheet (cold-rolled steel sheet) according to an embodiment in the present disclosure may have concentration gradients of specific elements, for example, residual elements Cu, Cr, Mo, and Sn, in a thickness direction from a surface. When a plating layer (for example, a zinc-based plating layer) is formed on at least one surface of such a steel sheet, the residual elements may further diffuse into the plating layer due to the influence of a temperature during plating or a temperature during alloying. Accordingly, in the plated steel sheet, the concentration gradients of the residual elements may be realized in the thickness direction of the base material from an interface between the base material, which is the base steel sheet, and the plating layer, or may be realized from an alloy layer formed at an interface portion between the base material and the plating layer.

[0100] That is, the residual elements may have the concentration gradients represented by A/B described above due to differences in diffusion rates of the residual elements diffusing toward a surface side of the steel sheet during a process of manufacturing the cold-rolled steel sheet, and such characteristics of the cold-rolled steel sheet also appear in the plated steel sheet.

[0101] The plated steel sheet according to an embodiment in the present disclosure has excellent powdering properties of the plating layer. Specifically, the plating layer may have a feature in which a powdering peeling width is less than 7 mm.

[0102] Hereinafter, methods for manufacturing a steel sheet (cold-rolled steel sheet) and a plated steel sheet according to still another aspect of the present disclosure will be described in detail.

[0103] First, the steel sheet according to the present disclosure, that is, the cold-rolled steel sheet, may be obtained through a series of processes, for example, processes of [steel slab heating - hot rolling - coiling - cooling - cold rolling - annealing]. Each process step will be described in detail below, and it is noted that the following manufacturing process corresponds to one example for manufacturing the cold-rolled steel sheet and the subsequent plated steel sheet of the present disclosure.

[0104] In an embodiment in the present disclosure, a steel slab or an ingot may be manufactured through an electric furnace or a new blast furnace-basic oxygen furnace process, and raw materials used in the electric furnace or blast furnace-basic oxygen furnace process include pig iron together with steel scrap. Here, pig iron refers to hot metal obtained in a blast furnace-basic oxygen furnace process, a cold material thereof (corrugate), hot briquette iron (HBI), or the like.

[0105] In the case of an electric furnace, after tapping from the electric furnace, desulfurization may be performed by ladle refining, and desulfurization and vacuum degassing treatment in a subsequent process may be performed. In addition, steel obtained from the electric furnace may be adjusted to have the final desired alloy composition by adding alloy components while performing degassing treatment. Here, RH method and DH method are generally used as vacuum degassing treatment methods, but oxygen blowing may also be performed into a degassing vessel. Examples of oxygen blowing include an oxygen blowing method using a top-blowing lance.

[Steel slab heating]

[0106] After a steel slab is prepared, the steel slab may be heated. At this time, the steel slab may be an ingot. In an embodiment in the present disclosure, the steel slab may have the above-described alloy composition, and thus the above description applies to the alloy composition of the steel slab.

[0107] In an embodiment in the present disclosure, when the prepared steel slab or ingot is heated, a heating temperature may be in a range of 900 to 1,300°C. When the heating temperature exceeds 1,300°C, the steel may reach a melting point and melt. On the other hand, when the temperature is lower than 900°C, a rolling load may increase during subsequent hot rolling, thereby reducing hot rolling stability. In another embodiment in the present disclosure, the heating may be performed at 1,160°C or lower.

[Hot rolling]

[0108] The heated steel slab or ingot may be hot-rolled to obtain a hot-rolled steel sheet.

[0109] In an embodiment in the present disclosure, a finishing temperature during hot rolling may be either in an austenite region at a temperature equal to or higher than an Ar3 transformation point or in a ferrite region at a temperature equal to or lower than the Ar3 transformation point. However, when the finishing temperature is too low, a rolling load increases during hot rolling, and thus a lower limit of the temperature may be limited to 750°C or higher.

[Coiling]

[0110] The hot-rolled steel sheet obtained by the hot rolling may be coiled into a coil shape.

[0111] In an embodiment in the present disclosure, the coiling process may be performed at a temperature of 700°C or higher. When the temperature during coiling is lower than 700°C, the intended grain size of the hot-rolled steel sheet obtained after coiling may not be achieved. In another embodiment in the present disclosure, the coiling process may be performed at 720°C or higher.

[0112] Meanwhile, an upper limit of the coiling temperature is not particularly limited. However, in consideration of excessive formation of scale on a surface of the hot-rolled coil, the temperature may be limited to 800°C or lower.

[0113] According to the present disclosure, by performing coiling at the above-described temperature, a relationship between the grain sizes of the hot-rolled steel sheet and the subsequent cold-rolled steel sheet, that is, the range of I/II, may be achieved.

[Cooling]

[0114] The coiled hot-rolled steel sheet may be cooled while being uncoiled.

[0115] In an embodiment in the present disclosure, cooling after coiling may be performed to a temperature range of 250 to 350°C at a cooling rate of 10.0°C/min or less. By performing cooling at such a relatively slow cooling rate, grains in the microstructure may be uniformly obtained.

[0116] In an embodiment in the present disclosure, when the cooling rate during cooling exceeds 10.0°C/min, grains may become coarse. Meanwhile, a lower limit of the cooling rate during cooling is not particularly limited. However, when the cooling rate is excessively low, a load may be applied to cooling equipment, and grains may also become coarse. In consideration thereof, the cooling rate may be limited to 0.2°C/min or more.

[0117] In an embodiment in the present disclosure, when a cooling end temperature during cooling is lower than 250°C, a cooling time may become excessive, thereby reducing productivity. On the other hand, when the cooling end temperature exceeds 350°C, a load may be caused during subsequent cold rolling.

[Cold rolling]

[0118] The cooled uncoiled hot-rolled steel sheet may be cold-rolled to obtain a cold-rolled steel sheet.

[0119] In an embodiment in the present disclosure, cold rolling may be performed at a cold reduction ratio of 50% or more. When the cold reduction ratio is less than 50%, it may be difficult to achieve a target thickness, and shape correction of the steel sheet may be difficult. According to an embodiment in the present disclosure, an upper limit of the cold reduction ratio during cold rolling is not particularly limited. However, when the cold reduction ratio is excessively high, a cold rolling load may increase. In consideration thereof, the cold reduction ratio may be limited to 95% or less.

[Annealing]

[0120] The manufactured cold-rolled steel sheet may be subjected to annealing heat treatment.

[0121] In an embodiment in the present disclosure, the annealing heat treatment may be performed in a general continuous annealing furnace, and may be performed under conditions in which the cold-rolled steel sheet is heated to 600°C or higher and then heat-treated for 10 seconds or longer. When the temperature during the annealing heat treatment is lower than 600°C or the time is shorter than 10 seconds, recrystallization may not sufficiently occur during annealing, and an unrecrystallized structure may be formed. In this case, formability of the steel sheet may be inferior.

[0122] According to an embodiment in the present disclosure, the upper limits of the temperature and time during the annealing heat treatment are not particularly limited. However, in consideration of the possibility of equipment problems caused by high-temperature annealing and inferior powdering properties, the upper limits may be set to 960°C and 15 minutes, respectively.

[0123] Meanwhile, when the cold-rolled steel sheet is heated to a temperature for annealing heat treatment, heating may be performed at a predetermined heating rate. As an example, heating may be performed at a heating rate of 2 to 60°C/s. When the heating rate during the heating is too slow or excessively fast, the grain characteristics intended in the present disclosure may not be achieved.

[0124] The cold-rolled steel sheet manufactured through the above-described series of processes may have the strength and formability targeted in the present disclosure.

[0125] In particular, the cold-rolled steel sheet according to an embodiment in the present disclosure may have a product (TS×El) of tensile strength (TS) and elongation (El) of 4.0 to 30.0 GPa·%, and a product (r-value×El) of Lankford value (r-value) and elongation of 20 to 200%.

[0126] Meanwhile, according to an aspect of the present disclosure, a plated steel sheet may be obtained by plating a steel sheet, that is, a cold-rolled steel sheet. The cold-rolled steel sheet at this time may be a cold-rolled steel sheet manufactured through a series of processes according to an embodiment in the present disclosure. Accordingly, the plated steel sheet may include a plating layer formed on at least one surface of the above-described cold-rolled steel sheet.

[0127] In an embodiment in the present disclosure, the plating may be a hot-dip plating or electroplating process, and each plating process may be performed under conditions commonly used in the art.

[0128] As an example, the hot-dip plating may be performed by immersing the cold-rolled steel sheet after the annealing heat treatment in a hot-dip galvanizing bath containing zinc as a main component. Since conditions during the hot-dip galvanizing may be conventional conditions, the conditions are not specifically limited in the present specification. However, as one embodiment, GI hot-dip plating may be performed under conventional conditions in a temperature range of 440 to 520°C.

[0129] In addition, according to an embodiment in the present disclosure, a galvannealed steel sheet (GA) may be obtained by selectively subjecting the plated steel sheet including the zinc-based plating layer formed thereon to alloying heat treatment. The alloying heat treatment may also be performed under conventional conditions and therefore is not specifically limited. However, as one example, the alloying heat treatment may be performed in a temperature range of 500 to 560°C.

[0130] In addition, as another example, the electroplating may be performed by placing a base steel sheet (cold-rolled steel sheet) at a negative electrode of a vertical plating cell-type electroplating simulator and then circulating a plating solution containing zinc (Zn), thereby forming a zinc-based plating layer on at least one surface of the base steel sheet (cold-rolled steel sheet).

[0131] Hereinafter, the present disclosure will be described more specifically with reference to Examples. However, it should be noted that the following Examples are only for illustrating the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the matters recited in the claims and matters reasonably inferred therefrom.

Mode for Invention

(Examples)

[0132] Steel slabs having the alloy compositions shown in Table 1 were prepared. The steel slabs were subjected to processes of [reheating - hot rolling - coiling - cooling] under the respective conditions shown in Table 2 to manufacture hot-rolled steel sheets. Thereafter, cold-rolled steel sheets were obtained by subjecting the respective hot-rolled steel sheets to a series of subsequent processes (cold rolling - annealing) shown in Table 2. At this time, the cooling process after coiling was performed to a temperature range of 250 to 350°C.

[0133] The steel slabs were obtained through the following process. First, steel scrap blended with hot metal from an existing blast furnace was charged into an electric furnace to obtain molten steel. Then, the molten steel was transferred to a ladle and subjected to vacuum degassing treatment in an RH degasser (0.1 torr), while alloy components were added to prepare molten steel having the intended composition. Thereafter, the molten steel was manufactured as a slab by continuous casting.

[0134] The microstructure and mechanical properties of the manufactured steel sheets were measured and evaluated. All results are shown in Table 3. At this time, the phase type of the microstructure and the properties were measured and evaluated for the cold-rolled steel sheets, and grain size was measured for both the hot-rolled steel sheets and the cold-rolled steel sheets. The cold-rolled steel sheets at this time corresponded to the cold-rolled steel sheets after annealing.

[0135] First, the microstructure of each cold-rolled steel sheet was observed using a scanning electron microscope (SEM) after a specimen was taken at t/4 in the thickness direction and polished, and then a cross section of the polished specimen was subjected to nital etching. After the nital etching, a structure having no irregularities on the specimen surface was determined to be ferrite, and a structure having a spherical or lamellar structure was determined to be cementite. In addition, in order to confirm the grain size characteristics of ferrite, the average grain size of ferrite was measured using an optical microscope for each specimen taken at t/4 in the thickness direction of the hot-rolled steel sheet and the cold-rolled steel sheet and having a cross section subjected to nital etching.

[0136] In addition, a tensile test was performed to evaluate the properties of each cold-rolled steel sheet. The tensile test was performed using specimens taken according to JIS No. 5 standard in a direction at 90 degrees with respect to the rolling direction of the rolled sheet, and a product (TS×El) of tensile strength (TS) and elongation (El) was calculated. In addition, for the same specimen, the Lankford value (r-value) was measured by a three-point method after 15% tensile pre-strain, and an average value of values in the rolling direction (L direction), a direction perpendicular to the rolling direction (C direction), and a direction at 45 degrees to the rolling direction (D direction) was obtained by the following calculation formula.

[0137] Meanwhile, in order to confirm contents (wt%) of specific elements (Cu, Cr, Mo, and Sn) at specific points in the thickness direction of the cold-rolled steel sheet, a cross section of each specimen was removed by FIB, and then the contents were measured by compositional analysis using FE-SEM. At this time, according to the definition of the surface layer portion, measurements were performed for the content in the surface layer portion and at a central portion (t/2) in the thickness direction, respectively. Measurements were repeated a total of 30 times at random points, respectively, and then the maximum value and the minimum value among the analyzed values of each element were removed, and the content was calculated as an average value.

[0138] In addition, in order to evaluate powdering properties, a 45-degree V-bending test was performed on plated steel sheets obtained by plating the respective cold-rolled steel sheets in a hot-dip galvanizing bath at 440 to 480°C. At this time, the test surface was disposed on the inner side of the bend, bending was performed at 60 degrees using a die having a tip radius of curvature of 1 mm, a tape was attached to the inner side and then removed, and powdering properties were evaluated on a 5-point scale with respect to the plating layer peeled off together with the tape. Specific evaluation criteria were set as follows, and a score of 2 points or higher was evaluated as passing.

Peeling width of less than 2 mm: 5 points

Peeling width of 2 mm or more and less than 3 mm: 4 points

Peeling width of 3 mm or more and less than 5 mm: 3 points

Peeling width of 5 mm or more and less than 7 mm: 2 points

Peeling width of 7 mm or more: 1 point

[Table 1]

Steel type	Alloy composition (wt%)
	C	Si	Mn	P	S	Al	N	Cu	Cr	Mo	Sn	Ti	Others
A	0.0008	0.002	0.052	0.003	0.0013	0.024	0.0014	0.800	0.660	0.830	0.350	0.011	Hf 0.004
B	0.0014	0.005	0.093	0.006	0.0050	0.027	0.0035	0.620	0.860	0.430	0.190	0.032	Nb 0.062
													Y 0.005
C	0.0028	0.014	0.160	0.011	0.0080	0.030	0.0056	0.450	0.380	0.420	0.120	0.050	W 0.02
D	0.0037	0.025	0.201	0.023	0.0150	0.035	0.0060	0.370	0.240	0.310	0.060	0.054	Mg 0.003
E	0.0155	0.029	0.258	0.035	0.0170	0.038	0.0087	0.280	0.220	0.200	0.040	0.079	Ni 0.23
													Zr 0.02
F	0.0280	0.027	0.294	0.046	0.0250	0.034	0.0095	0.130	0.100	0.120	0.020	0.093	B 0.0023
													Sb 0.002
G	0.0506	0.034	0.368	0.061	0.0340	0.030	0.0166	0.003	0.002	0.001	0.001	0.144	Ca 0.004
													REM 0.002
XA	0.0755	0.024	0.240	0.026	0.0230	0.029	0.0044	0.180	0.210	0.170	0.020	0.040	-
XB	0.0021	0.832	0.248	0.029	0.0260	0.026	0.0056	0.230	0.230	0.200	0.023	0.052	-
XC	0.0023	0.020	1.054	0.020	0.0310	0.031	0.0065	0.220	0.240	0.150	0.021	0.058	-
XD	0.0021	0.025	0.235	0.017	0.0320	0.528	0.0069	0.210	0.190	0.180	0.015	0.063	-
XE	0.0020	0.022	0.227	0.021	0.0290	0.033	0.0345	0.190	0.200	0.170	0.017	0.297	-
XF	0.0023	0.026	0.213	0.026	0.0250	0.030	0.0054	1.070	0.230	0.220	0.021	0.049	-
XG	0.0021	0.024	0.322	0.022	0.0260	0.034	0.0053	0.230	1.030	0.280	0.019	0.045	-
Xh	0.0020	0.023	0.276	0.028	0.0200	0.028	0.0048	0.280	0.230	1.180	0.023	0.042	-
XI	0.0025	0.025	0.241	0.023	0.0240	0.026	0.0043	0.310	0.250	0.190	0.517	0.037	-
In Table 1, REM means REM excluding Y.

[Table 2]

Steel type	Reheating temperat ure (°C)	Finish rolling temperature (°C)	Coiling tempera ture (°C)	Cooling after coiling (°C/min)	Cold reducti on ratio (%)	Heatin g rate (°C/s)	Annealing temperatu re (°C)	Anneal ing time (s)	I/II	Speci men No.
A	1160	880	780	0.5	76	46	940	40	0.96	1
B	1160	880	750	0.6	76	39	820	40	0.58	2
A	1270	930	830	0.4	76	48	940	600	0.93	3
B	1050	780	400	0.5	76	37	750	20	0.55	4
A	1160	880	750	0.5	76	57	880	40	0.75	5
B	1160	880	750	0.2	76	32	880	40	0.79	6
B	1310	950	750	0.6	76	25	880	40	0.80	7
B	880	620	480	0.4	76	27	880	40	0.73	8
B	1270	930	830	0.5	76	32	880	40	0.61	9
B	1160	880	680	0.8	76	33	880	40	0.87	10
B	1160	880	750	0.7	47	30	880	40	0.50	11
B	1160	880	750	0.9	76	35	570	40	0.31	12
B	1160	880	750	1.0	76	33	980	40	0.97	13
B	1160	880	750	0.9	76	25	880	5	0.34	14
B	1160	880	750	0.9	76	24	880	1000	0.97	15
B	1160	880	700	0.1	76	17	860	40	0.98	16
B	1160	880	750	10.8	76	22	860	40	0.56	17
B	1160	880	700	0.7	76	1	880	40	0.97	18
B	1160	880	750	0.5	76	63	880	40	0.54	19
C	1160	880	750	0.6	76	24	860	40	0.80	20
D	1160	880	720	1.0	76	19	880	40	0.91	21
E	1050	880	780	1.2	76	10	880	40	0.63	22
F	1250	880	750	2.8	76	6	880	40	0.73	23
G	1160	880	750	5.5	76	2	880	40	0.75	24
XA	1160	880	750	0.9	76	16	880	40	0.45	25
XB	1160	880	750	0.6	76	23	880	40	0.57	26
XC	1160	880	750	0.7	76	25	880	40	0.46	27
XD	1160	880	750	0.6	76	22	880	40	0.47	28
XE	1160	880	750	0.8	76	24	880	40	0.56	29
XF	1160	880	750	0.4	76	23	880	40	0.59	30
XG	1160	880	750	0.6	76	19	880	40	0.56	31
XH	1160	880	750	0.5	76	17	880	40	0.57	32
XI	1160	880	750	0.7	76	20	880	40	0.58	33
In Table 2, I/II means a value (I/II) obtained by dividing an average grain size (I) of the cold-rolled steel sheet by an average grain size (II) of the hot-rolled steel sheet.

[Table 3]

Specimen No.	A/B	Average grain size of cold-rolled steel sheet (µm)	TS×El (GPa%)	r-value×El (%)	Powdering properties
1	0.52	48	27	195	4
2	0.54	25	16	94	4
3	31.34	53	33	216	1
4	0.53	5	3.2	16	4
5	0.17	38	28	197	5
6	0.33	32	22	160	5
7	0.37	16	3.7	18	4
8	0.52	8	3.4	15	4
9	33.20	34	25	158	1
10	0.49	13	3.7	17	4
11	0.57	16	5.2	18	3
12	0.13	9	3.6	13	1
13	0.38	36	26	176	3
14	0.12	11	3.2	14	1
15	0.25	35	24	168	3
16	0.40	42	28	179	2
17	0.39	13	3.1	16	4
18	0.42	34	23	148	2
19	0.33	15	3.6	18	4
20	0.87	26	25	138	4
21	2.56	21	19	90	4
22	8.68	17	13	62	4
23	17.50	13	6.2	30	4
24	28.80	7	4.4	29	3
25	16.20	5	2.7	13	2
26	26.20	8	3.5	14	2
27	23.40	6	3.8	16	3
28	22.80	8	2.3	11	3
29	20.50	5	2.7	13	3
30	21.70	10	3.5	17	2
31	25.10	13	3.3	15	2
32	16.80	12	2.6	14	2
33	18.70	14	2.5	16	2
In Table 3, A/B means a value (A/B) obtained by dividing an average content of (Cu+Cr+Mo+Sn) in a surface layer portion (wt%, A) by an average content of (Cu+Cr+Mo+Sn) in a central portion (wt%, B).

[0139] As shown in Tables 1 to 3, in the case of the specimens satisfying both the alloy composition and manufacturing conditions proposed in the present disclosure, the microstructure was formed with the intended grains, and strength, formability, and the like were all excellent.

[0140] On the other hand, in the case of the specimens not satisfying one or more of the alloy composition and manufacturing conditions of the present disclosure, the grains of the microstructure were not formed as intended, or specific elements were non-uniformly distributed, and thus at least one property, such as strength, formability, and plating adhesion, was inferior.

[0141] As described above, the present disclosure has technical significance in that excellent formability and plating adhesion may be simultaneously achieved by controlling grain size characteristics and content ratios of specific elements at respective positions in the thickness direction.

Claims

1. A steel sheet comprising, by wt%, greater than 0% to 0.0700% of carbon (C), greater than 0% to 0.800% of silicon (Si), greater than 0% to 1.000% of manganese (Mn), greater than 0% to 0.500% of aluminum (Al), 0 to 0.080% of phosphorus (P), 0 to 0.0500% of sulfur (S), 0 to 0.0300% of nitrogen (N), greater than 0% to 1.000% of copper (Cu), greater than 0% to 1.000% of chromium (Cr), greater than 0% to 1.000% of molybdenum (Mo), greater than 0% to 0.5000% of tin (Sn), one or more selected from the following groups (i) to (vi), and the balance of Fe and other inevitable impurities:

(i) 0 to 1.000% of nickel (Ni),

(ii) one or more selected from 0 to 0.500% of titanium (Ti), 0 to 0.500% of niobium (Nb), 0 to 0.500% of vanadium (V), and 0 to 0.0200% of boron (B),

(iii) one or more selected from 0 to 0.050% of magnesium (Mg), 0 to 0.050% of calcium (Ca), and 0 to 0.050% of rare earth elements (REM) excluding yttrium (Y),

(iv) one or more selected from 0 to 0.50% of tungsten (W) and 0 to 0.50% of zirconium (Zr),

(v) one or more selected from 0 to 0.5000% of antimony (Sb) and 0 to 0.5000% of cobalt (Co), and

(vi) one or more selected from 0 to 0.200% of yttrium (Y) and 0 to 0.200% of hafnium (Hf),

wherein A/B satisfies 0.15 to 30.00, where A represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a surface layer portion, and B represents an average content (wt%) of (Cu+Cr+Mo+Sn) in a central portion,

the central portion refers to a point t/2 in a thickness direction, the surface layer portion refers to a region extending from a surface to an inflection point, which is a point at which concentrations change when concentration gradients of Cu, Cr, Mo, and Sn are measured in the thickness direction, and the inflection point refers to a point corresponding to 99% of a content of (Cu+Cr+Mo+Sn) in the central portion in the measured concentration gradient values when a content of (Cu+Cr+Mo+Sn) at the outermost surface is greater than the content of (Cu+Cr+Mo+Sn) in the central portion, or refers to a point corresponding to 1% of the content of (Cu+Cr+Mo+Sn) in the central portion in the measured concentration gradient values when the content of (Cu+Cr+Mo+Sn) at the outermost surface is less than the content of (Cu+Cr+Mo+Sn) in the central portion, and

an average grain size is 6 to 50 µm.

2. The steel sheet of claim 1, wherein the steel sheet is a cold-rolled steel sheet manufactured by subjecting a hot-rolled steel sheet obtained by hot rolling to cold rolling and annealing heat treatment, and
a value (I/II) obtained by dividing an average grain size (I) of the cold-rolled steel sheet by an average grain size (II) of the hot-rolled steel sheet satisfies 0.60 to 0.95.

3. The steel sheet of claim 1, wherein a microstructure of the steel sheet is a single ferrite phase.

4. The steel sheet of claim 1, wherein the steel sheet has a product (TS×El) of tensile strength (TS) and elongation (El) of 4.0 to 30.0 GPa·%.

5. The steel sheet of claim 1, wherein the steel sheet has a product (r-value×El) of Lankford value (r-value) and elongation of 20 to 200%.

6. The steel sheet of claim 1, wherein the steel sheet includes a plating layer formed on at least one surface.

7. The steel sheet of claim 6, wherein the plating layer has a powdering peeling width of less than 7 mm.

8. A method for manufacturing a steel sheet, the method comprising:

preparing a steel slab containing, by wt%, greater than 0% to 0.0700% of carbon (C), greater than 0% to 0.800% of silicon (Si), greater than 0% to 1.000% of manganese (Mn), greater than 0% to 0.500% of aluminum (Al), 0 to 0.080% of phosphorus (P), 0 to 0.0500% of sulfur (S), 0 to 0.0300% of nitrogen (N), greater than 0% to 1.000% of copper (Cu), greater than 0% to 1.000% of chromium (Cr), greater than 0% to 1.000% of molybdenum (Mo), greater than 0% to 0.5000% of tin (Sn), one or more selected from the following groups (i) to (vi), and the balance of Fe and other inevitable impurities:

(i) 0 to 1.000% of nickel (Ni),

(ii) one or more selected from 0 to 0.500% of titanium (Ti), 0 to 0.500% of niobium (Nb), 0 to 0.500% of vanadium (V), and 0 to 0.0200% of boron (B),

(iii) one or more selected from 0 to 0.050% of magnesium (Mg), 0 to 0.050% of calcium (Ca), and 0 to 0.050% of rare earth elements (REM) excluding yttrium (Y),

(iv) one or more selected from 0 to 0.50% of tungsten (W) and 0 to 0.50% of zirconium (Zr),

(v) one or more selected from 0 to 0.5000% of antimony (Sb) and 0 to 0.5000% of cobalt (Co), and

(vi) one or more selected from 0 to 0.200% of yttrium (Y) and 0 to 0.200% of hafnium (Hf);

heating the steel slab in a temperature range of 900 to 1,300°C;

finish-rolling the heated steel slab in an austenite region at a temperature equal to or higher than an Ar3 transformation point or in a ferrite region at a temperature equal to or lower than the Ar3 transformation point to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet at 700°C or higher;

cooling the coiled hot-rolled steel sheet at a rate of 10°C/min or less to a temperature range of 250 to 350°C;

cold-rolling the hot-rolled steel sheet after the cooling at a cold reduction ratio of 50% or more to obtain a cold-rolled steel sheet; and

annealing the cold-rolled steel sheet at 600°C or higher for 10 seconds or longer.

9. The method of claim 8, wherein a value (I/II) obtained by dividing an average grain size (I) of the cold-rolled steel sheet after the annealing by an average grain size (II) of the hot-rolled steel sheet after the coiling satisfies 0.60 to 0.95.

10. The method of claim 8, wherein the steel slab is obtained by continuously casting molten steel obtained through a steelmaking process in an electric furnace.

11. The method of claim 8, wherein the annealing is performed by heating the cold-rolled steel sheet to an annealing temperature at a heating rate of 2 to 60°C/s.

12. The method of claim 8, further comprising plating the cold-rolled steel sheet after the annealing,
wherein the plating is a hot-dip plating or electroplating process.

13. The method of claim 12, wherein the hot-dip plating is hot-dip galvanizing performed in a temperature range of 440 to 520°C.

14. The method of claim 13, further comprising performing alloying heat treatment after the hot-dip galvanizing.

15. A steel sheet manufactured by the method of any one of claims 8 to 14,
wherein a value obtained by dividing an average grain size (I) of the cold-rolled steel sheet after the annealing by an average grain size (II) of the hot-rolled steel sheet satisfies 0.60 to 0.95.

16. The steel sheet of claim 15, wherein the steel sheet includes a plating layer formed on at least one surface of the steel sheet.

17. The steel sheet of claim 16, wherein the plating layer has a powdering peeling width of less than 7 mm.