Cold-rolled steel sheet and method for manufacturing the same

Abstract

This invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet with both excellent strength and formability, and a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet with both excellent strength and formability, and a method for manufacturing the same. Background Technology

[0002] To address the global climate crisis and aim for carbon neutrality by 2050, automakers are requesting steel manufacturers to supply automotive steel sheets with reduced carbon dioxide emissions in phases. For this reason, steel manufacturers are currently developing automotive steel sheets that reduce carbon dioxide emissions through scrap recycling, utilizing existing blast furnace-converter or electric arc furnace steelmaking methods.

[0003] When using scrap steel as raw material, it is difficult to remove residual elements such as Cu, Cr, and Ni from the scrap steel during refining, and therefore these elements remain in the steel after refining. These residual elements can reduce the physical properties of the steel or deteriorate its surface quality. Therefore, to date, high-grade thin sheet products, such as automotive steel sheets, are manufactured using molten iron as the main raw material in a typical blast furnace-converter process, by significantly reducing the C and N content in the steel while controlling the content of residual elements to extremely low levels.

[0004] In addition, the following technology was proposed: using electric arc furnace steelmaking to manufacture automotive steel sheets with excellent workability from electric arc furnace steel containing residual elements.

[0005] Patent Document 1 and Patent Document 2 disclose techniques for manufacturing cold-rolled steel sheets with excellent workability. Specifically, Patent Document 1 and Patent Document 2 disclose cold-rolled steel sheets that exhibit excellent cold workability even when containing a large amount of residual elements.

[0006] However, the aforementioned technology reduces the Rankford value (r value) and worsens the formability due to the residual elements in the steel plate, thus limiting the incorporation of Cu and Ni as residual elements.

[0007] As mentioned above, although technologies for manufacturing automotive steel sheets containing residual elements have been proposed, automotive steel sheets with improved strength and formability even with residual elements remain undeveloped.

[0008] (Patent Document 1) Japanese Patent Publication No. 1995-118795 (Patent Document 2) Japanese Patent Publication No. 1998-025541 Summary of the Invention

[0009] (a) Technical problems to be solved According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same are intended to be provided.

[0010] According to one embodiment of the present invention, it is intended to provide a cold-rolled steel sheet with excellent strength and formability when using scrap steel raw materials to manufacture steel sheets, and a method thereof.

[0011] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art will have no difficulty understanding the additional technical problems addressed by this invention from the overall content of this specification.

[0012] (II) Technical Solution According to one embodiment of the present invention, a cold-rolled steel sheet can be provided, comprising, by weight percent: C: less than 0.0500%, Si: 0.005-0.080%, Mn: less than 0.500%, Al: less than 0.0800%, P: less than 0.0800%, S: less than 0.0500%, N: less than 0.0300%, Ti: 0.001-0.050%, Nb: 0.001-0.050%, Cu: less than 1.000%, Ni: less than 1.000%, Cr: less than 1.000%, and containing 0.500% or more of one or more of Sb and Sn, and containing the balance Fe and other unavoidable impurities, wherein the T value defined in Equation 1 below is 0.90 to 1.20, and the tensile strength of the cold-rolled steel sheet is 330 MPa or more.

[0013] [Relation 1] T = R / (0.4([Cu] + [Cr] + [Ni]) + 1.6) (In the formula, R is the r-value, and [Cu], [Cr], and [Ni] are the weight percentages of each element.) The cold-rolled steel sheet may further contain less than 1% Mo and less than 0.02% B by weight.

[0014] In the cold-rolled steel sheet, the Cu+Cr+Ni content, by weight%, can be less than 0.7%.

[0015] The value of A defined in Equation 2 below for the cold-rolled steel sheet can be from 0.40 to 1.40.

[0016] [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) (In the formula, [Mn], [Ti], and [Nb] are the weight percentages of each element.) The microstructure of the cold-rolled steel sheet can contain more than 95% ferrite and the remainder in terms of area percentage.

[0017] The average ferrite grain size of the cold-rolled steel sheet can be below 13.0 μm.

[0018] The tensile strength of the cold-rolled steel sheet can be 330-420 MPa, and the elongation can be 32% or more.

[0019] The r-value of the cold-rolled steel sheet can be 1.60 or higher.

[0020] According to one embodiment of the present invention, a method for manufacturing cold-rolled steel sheet can be provided, comprising the following steps: reheating a steel billet, wherein the steel billet, by weight percent, comprises C: less than 0.0500%, Si: 0.005-0.080%, Mn: less than 0.500%, Al: less than 0.0800%, P: less than 0.0800%, S: less than 0.0500%, N: less than 0.0300%, and Ti: 0.001-0.0%. The steel billet contains 50% Nb, 0.001-0.050% Cu, less than 1.000% Ni, less than 1.000% Cr, and contains more than 0.500% of one or more of Sb and Sn, and contains the balance Fe and other unavoidable impurities; the reheated steel billet is hot-rolled; the hot-rolled steel sheet is coiled; the coiled steel sheet is cold-rolled; and the cold-rolled steel sheet is annealed.

[0021] The billet may further contain less than 1% Mo and less than 0.02% B by weight.

[0022] In the steel billet, the Cu+Cr+Ni content, by weight%, can be less than 0.7%.

[0023] The value of A for the steel billet, as defined in Equation 2 below, can be between 0.40 and 1.40.

[0024] [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) (In the formula, [Mn], [Ti], and [Nb] are the weight percentages of each element.) The reheating step is carried out in a temperature range of 900-1300℃. The hot rolling step is carried out at a finishing rolling temperature of 880°C or higher. The winding step is performed at a temperature range of 500-800℃. The cold rolling step is carried out with a cold rolling reduction rate of 50-90%. The annealing step is performed at a temperature above 600°C for more than 10 seconds.

[0025] (III) Beneficial Effects According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0026] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent strength and formability, and a method for manufacturing the same, can be provided.

[0027] According to one embodiment of the present invention, a high-strength, high-formability cold-rolled steel sheet that can be used for various applications, including automotive parts, using scrap steel raw materials, and a method thereof can be provided.

[0028] The various and beneficial advantages and effects of this invention are not limited to those described above, and can be more easily understood in the process of explaining the specific embodiments of this invention. Attached Figure Description

[0029] Figure 1 It is a graph that represents the sum of the contents of Cu, Cr, and Ni as r-values. Best practice

[0030] The preferred embodiments of the present invention will be described below. Various modifications may be made to the specific embodiments of the present invention, and the scope of the present invention should not be construed as limited to the specific embodiments described below. These specific embodiments are provided to illustrate the present invention in more detail to those skilled in the art.

[0031] The present invention will now be described in detail.

[0032] The steel composition of the present invention will be described in detail below.

[0033] In this invention, unless otherwise specified, the percentage of each element content is based on weight.

[0034] According to one embodiment of the present invention, the cold-rolled steel sheet may contain, by weight percent: C: less than 0.0500%, Si: 0.005-0.080%, Mn: less than 0.500%, Al: less than 0.0800%, P: less than 0.0800%, S: less than 0.0500%, N: less than 0.0300%, Ti: 0.001-0.050%, Nb: 0.001-0.050%, Cu: less than 1.000%, Ni: less than 1.000%, Cr: less than 1.000%, and contains more than 0.500% of one or more of Sb and Sn.

[0035] Carbon (C): below 0.0500% Carbon (C) is an important element in this invention. When the carbon (C) content exceeds 0.0500%, it may be difficult to ensure the required elongation and r-value of the steel sheet. According to one embodiment of the invention, the carbon (C) content can be below 0.0400%. Furthermore, there is no particular limitation on the lower limit of the carbon (C) content, but when the carbon (C) content is less than 0.0003%, the effect of reducing carbon (C) is almost negligible, which may raise concerns about increased manufacturing costs. According to one embodiment of the invention, the carbon (C) content can be above 0.0003%.

[0036] Silicon (Si): 0.005-0.080% Silicon (Si) is an element that enhances strength through solid solution strengthening, strengthens ferrite, homogenizes the microstructure, and improves processability. Furthermore, it is required for deoxidation during steelmaking. When the silicon (Si) content exceeds 0.080%, plating defects such as incomplete plating may occur during the plating process, and the weldability of the steel sheet may be reduced. According to one embodiment of the invention, the silicon (Si) content can be 0.070% or less. On the other hand, when the silicon (Si) content is less than 0.005%, the effect of reducing silicon (Si) is almost negligible, potentially leading to concerns about increased manufacturing costs.

[0037] Manganese (Mn): 0.001-0.500% Manganese (Mn) is a useful element for simultaneously improving strength and ductility. In particular, manganese is a known solid solution strengthening element that causes dissolved sulfur (S) in steel to precipitate as MnS, thereby preventing hot brittleness caused by dissolved S. When the manganese (Mn) content exceeds 0.500%, workability may decrease. According to one embodiment of the invention, the manganese (Mn) content can be 0.400% or less. On the other hand, when the manganese (Mn) content is less than 0.001%, the effect of reducing manganese (Mn) is almost negligible, potentially leading to concerns about increased manufacturing costs. According to one embodiment of the invention, the manganese (Mn) content can be 0.010% or more. According to one embodiment of the invention, the manganese (Mn) content can be 0.100% or more. According to one embodiment of the invention, the manganese (Mn) content can be 0.150% or more.

[0038] Aluminum (Al): Below 0.0800% Aluminum (Al) is an element that combines with oxygen in steel to act as a deoxidizer. Furthermore, like silicon (Si), it is an element that strengthens ferrite, homogenizes the microstructure, and improves workability. When the aluminum (Al) content exceeds 0.0800%, workability may decrease. According to one embodiment of the invention, the aluminum (Al) content can be 0.0600% or less. There is no particular limitation on the lower limit of the aluminum (Al) content, but when the aluminum (Al) content is less than 0.0010%, the effect of reducing aluminum (Al) is almost negligible, which may raise concerns about increased manufacturing costs. According to one embodiment of the invention, the aluminum (Al) content can be 0.0010% or more.

[0039] Phosphorus (P): below 0.0800% Phosphorus (P) is an element used to increase strength. When the phosphorus (P) content exceeds 0.0800%, processability and brittleness may decrease. According to one embodiment of the invention, the phosphorus (P) content can be 0.0700% or less. Furthermore, there is no particular limitation on the lower limit of the phosphorus (P) content, but when the phosphorus (P) content is less than 0.0010%, the effect of reducing phosphorus (P) is almost negligible, which may raise concerns about increased manufacturing costs. According to one embodiment of the invention, the phosphorus (P) content can be 0.0010% or more.

[0040] Sulfur (S): below 0.0500% Sulfur (S) is present as an impurity and forms MnS in steel plates, which is an element that degrades ductility. According to one embodiment of the invention, the upper limit of the sulfur (S) content can be limited to 0.0500%. According to another embodiment of the invention, the sulfur (S) content can be 0.0400% or less. Furthermore, there is no particular limitation on the lower limit of the sulfur (S) content, but when the sulfur (S) content is less than 0.0010%, the effect of reducing sulfur (S) is almost negligible, which may raise concerns about increased manufacturing costs. According to one embodiment of the invention, the sulfur (S) content can be 0.0010% or more.

[0041] Nitrogen (N): below 0.0300% Nitrogen (N) is present as an impurity and forms nitrides during continuous casting, contributing to cracking in steel billets. According to one embodiment of the invention, the upper limit of the nitrogen (N) content can be limited to 0.0300%. According to another embodiment of the invention, the nitrogen (N) content can be 0.0200% or less. Furthermore, there is no particular limitation on the lower limit of the nitrogen (N) content, but when the nitrogen (N) content is less than 0.0010%, the effect of reducing nitrogen (N) is almost negligible, potentially leading to concerns about increased manufacturing costs. According to one embodiment of the invention, the nitrogen (N) content can be 0.0010% or more.

[0042] Titanium (Ti): 0.001-0.050% Titanium (Ti) is an important element in the formation of precipitates in steel sheets. It can be included to improve the strength and impact toughness of the steel sheet. The titanium (Ti) precipitates dissolved C and N in the steel as carbides or nitrides, and is an element that can prevent processability degradation caused by dissolved C and N. To ensure the above effects, the present invention may contain more than 0.001% titanium (Ti). When the titanium (Ti) content is less than 0.001%, the effect of adding titanium (Ti) may be almost negligible. Furthermore, according to one embodiment of the present invention, the upper limit of the titanium (Ti) content can be limited to 0.050%. According to another embodiment of the present invention, the titanium (Ti) content can be less than 0.040%.

[0043] Niobium (Nb): 0.001-0.050% Niobium (Nb) is an important element in the formation of precipitates in steel sheets. It can be included to improve the strength and impact toughness of the steel sheet. Niobium (Nb) precipitates dissolved C and N in the steel as carbides or nitrides, and is an element that can prevent the deterioration of workability caused by dissolved C and N. To ensure the above effects, the present invention may contain more than 0.001% niobium (Nb). When the niobium (Nb) content is less than 0.001%, the effect of adding niobium (Nb) may be almost negligible. Furthermore, according to one embodiment of the present invention, the upper limit of the niobium (Nb) content can be limited to 0.050%. According to another embodiment of the present invention, the niobium (Nb) content can be less than 0.040%.

[0044] Copper (Cu): below 1.000%, Nickel (Ni): below 1.000% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, the enrichment of copper (Cu) and nickel (Ni) on the surface of the steel plate prevents the intrusion of hydrogen moving into the steel plate, thus inhibiting hydrogen-induced delayed fracture. Additionally, when the content of copper (Cu) and nickel (Ni) is 1.000% or less, it can suppress the deterioration of workability. According to one embodiment of the invention, it can be 0.900% or less. Furthermore, there is no particular limitation on the lower limit of the content of copper (Cu) and nickel (Ni), but when their content is less than 0.010%, the effect of adding copper (Cu) and nickel (Ni) may be almost negligible. According to one embodiment of the invention, the content of copper (Cu) and nickel (Ni) can be 0.010% or more.

[0045] Chromium (Cr): below 1.000% Chromium (Cr) is an element that, like Mn, inhibits austenite decomposition during alloying treatment and stabilizes austenite. Furthermore, when the chromium (Cr) content is 1.000% or less, it can suppress the deterioration of workability. According to one embodiment of the invention, the chromium (Cr) content can be 0.900% or less. Additionally, there is no particular limitation on the lower limit of the chromium (Cr) content, but when the chromium (Cr) content is less than 0.010%, the effect of adding chromium (Cr) may be almost negligible. According to one embodiment of the invention, it can contain 0.010% or more of chromium (Cr).

[0046] One or more of antimony (Sb) and tin (Sn): less than 0.500% Antimony (Sb) and tin (Sn) are elements that improve the wettability and adhesion of steel sheets during plating. When the sum of the contents of one or more of antimony (Sb) and tin (Sn) exceeds 0.500%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working. Furthermore, when the sum of the contents of antimony (Sb) and tin (Sn) is less than 0.500%, the deterioration of processability can be suppressed. According to one embodiment of the invention, it can be less than 0.400%. Additionally, there is no particular limitation on the lower limit of the antimony (Sb) and tin (Sn) content, but when the content is less than 0.0005%, the effect of adding antimony (Sb) and tin (Sn) may be almost negligible. According to one embodiment of the invention, one or more of antimony (Sb) and tin (Sn) can be 0.0005% or more.

[0047] The steel of this invention, in addition to the above-described components, may contain other iron (Fe) and unavoidable impurities. These unavoidable impurities may be unintentionally introduced during ordinary manufacturing processes and therefore cannot be eliminated. These impurities are known to those skilled in the art of steel manufacturing, and therefore their contents are not specifically mentioned in this specification.

[0048] According to one embodiment of the invention, the cold-rolled steel sheet may further contain less than 1% Mo and less than 0.02% B by weight.

[0049] Molybdenum (Mo): below 1.0% Molybdenum (Mo) is an element that, like Mn, inhibits austenite decomposition during alloying processes and stabilizes austenite. Furthermore, when the molybdenum (Mo) content is 1.0% or less, it can suppress the deterioration of workability. According to one embodiment of the invention, the molybdenum (Mo) content can be 0.9% or less. Additionally, there is no particular limitation on the lower limit of the molybdenum (Mo) content, but when the molybdenum (Mo) content is less than 0.01%, the effect of adding molybdenum (Mo) may be almost negligible. According to one embodiment of the invention, the molybdenum (Mo) content can be 0.01% or more.

[0050] Boron (B): less than 0.02% Boron (B) is an element that improves hardenability, thereby increasing strength, and inhibits grain boundary nucleation. When the boron (B) content exceeds 0.02%, it may degrade the deep-drawing properties of the steel sheet. According to one embodiment of the invention, the content can be below 0.01%. Furthermore, there is no particular limitation on the lower limit of the boron (B) content, but when its content is less than 0.0001%, the effect of adding boron (B) may be almost negligible. According to one embodiment of the invention, the boron (B) content can be 0.0001% or more.

[0051] According to one embodiment of the present invention, the cold-rolled steel sheet may contain less than 0.7% Cu+Cr+Ni by weight.

[0052] According to one embodiment of the present invention, when manufacturing steel plates, scrap steel is used as the material, and the steel plates may contain Cu, Cr, and Ni, the sum of which may be less than 0.7%. There is no particular limitation on the lower limit of the sum of the contents of Cu, Cr, and Ni, but it may be greater than 0.003%.

[0053] According to one embodiment of the present invention, the value of A defined in the following relation 2 for cold-rolled steel sheet can be from 0.40 to 1.40.

[0054] [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) (In the formula, [Mn], [Ti], and [Nb] are the weight percentages of each element.) According to one embodiment of the present invention, even when Cu, Cr, and Ni are present in large quantities, the alloy composition can be more strictly controlled through the aforementioned formula 2, thereby ensuring formability.

[0055] When the value of A defined in Equation 2 is less than 0.40 or greater than 1.40, the strength and formability of the target cannot be adequately ensured.

[0056] The steel microstructure of the present invention will be described in detail below.

[0057] In this invention, unless otherwise specified, the percentage of fine tissue is expressed as an area.

[0058] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention may contain more than 95% ferrite and the remainder microstructure by area%.

[0059] According to one embodiment of the present invention, to ensure formability, the content may be 95% or more ferrite. When the ferrite fraction is less than 95%, there may be a problem of a sharp decrease in formability.

[0060] The remaining microstructure besides ferrite may include low-temperature phase transformation products. According to one embodiment of the present invention, the low-temperature phase transformation products may include martensite, bainite, etc. According to one embodiment of the present invention, the fraction of ferrite may be 100%.

[0061] According to one embodiment of the present invention, the average ferrite grain size of the cold-rolled steel sheet can be less than 13.0 μm.

[0062] When the average ferrite grain size exceeds 13.0 μm, it may be difficult to ensure the target strength in this invention. According to one embodiment of the invention, the average ferrite grain size can be 12.5 μm or less. There is no particular limitation on the lower limit of the average ferrite grain size, but according to one embodiment of the invention, the average ferrite grain size can be 6.5 μm or more.

[0063] According to one embodiment of the present invention, the T value defined in the following relation 1 for cold-rolled steel sheet can be from 0.90 to 1.20.

[0064] [Relation 1] T = R / (0.4([Cu] + [Cr] + [Ni]) + 1.6) (In the formula, R is the value of r, and [Cu], [Cr], and [Ni] are the weight percentages of each element.) According to one embodiment of the present invention, the target r value can be ensured even when Cu, Cr, and Ni are present in large quantities. According to one embodiment of the present invention, the more the sum of the contents of Cu, Cr, and Ni increases, the more the r value can increase.

[0065] Equation 1 can represent the relationship between the content of Cu, Cr, and Ni and the value of r. According to one embodiment of the present invention, as the sum of the contents of Cu, Cr, and Ni increases, the value of r increases, so the ratio of them defined in Equation 1 can be ensured to be within a certain range.

[0066] When the T value defined in Equation 1 is less than 0.90 or greater than 1.20, it is impossible to fully ensure the r value while satisfying the target strength.

[0067] According to one embodiment of the present invention, the cold-rolled steel sheet has a tensile strength of 330-420 MPa, an elongation of 32% or more, and the product of tensile strength and elongation (TS×E1) is 12.0-16.0 GPa·%, with an r value (Lunkford value) of 1.60 or more.

[0068] According to one embodiment of the present invention, the tensile strength can be 340 MPa or higher. According to another embodiment of the present invention, the tensile strength can be 410 MPa or lower.

[0069] The steel manufacturing method of the present invention will be described in detail below.

[0070] According to one embodiment of the present invention, cold-rolled steel sheet can be manufactured by reheating, hot rolling, coiling, cold rolling and annealing a steel billet that satisfies the above alloy composition.

[0071] Reheating A steel billet with an alloy composition that meets one embodiment of the present invention can be reheated in a temperature range of 900-1300°C.

[0072] When the reheating temperature is below 900°C, the rolling load increases during hot rolling, which may reduce hot rolling stability. According to one embodiment of the invention, the reheating temperature can be above 1100°C. On the other hand, when the reheating temperature exceeds 1300°C, the workability and coating adhesion may deteriorate, and there is a possibility that the steel may melt at its melting point. According to one embodiment of the invention, the reheating temperature can be below 1160°C.

[0073] According to one embodiment of the present invention, the conditions for the smelting process for manufacturing steel billets or ingots are not particularly limited.

[0074] According to one embodiment of the present invention, the steel billet or ingot can be manufactured by an electric furnace or a new blast furnace-converter process, wherein the raw materials used in the electric furnace or blast furnace-converter process can be pig iron used together with scrap steel. Here, pig iron refers to molten iron or its corrugate or hot briquette iron (HBI) obtained in the blast furnace-converter process.

[0075] In the case of an electric arc furnace (EAF), steel can undergo desulfurization through ladle refining after tapping, and can also undergo vacuum degassing for subsequent processes. Furthermore, alloying elements can be added to the steel obtained from the EAF during degassing to adjust it to the desired final alloy composition. Vacuum degassing methods generally include the RH (reverse halogenation) method and the DH (desulfurization) method, but oxygen injection can be performed concurrently within the degassing tank. Regarding oxygen injection, there is a method using a top-blown oxygen lance.

[0076] According to one embodiment of the present invention, the casting method is not particularly limited, but continuous casting can be used to improve productivity.

[0077] Hot rolling The reheated steel billet can be hot rolled at a finishing rolling temperature of 880°C or higher.

[0078] During hot rolling, if the finishing temperature is below 880°C, rolling in the two-phase region may cause uneven microstructure. According to one embodiment of the present invention, the conditions for the hot rolling process are not particularly limited, and common conditions applicable in the same technical field can be applied.

[0079] Collect The hot-rolled steel sheet can be coiled up at a temperature range of 500-800℃.

[0080] To improve the strength and formability of the steel sheet, the winding temperature can be limited to 500°C or higher. According to one embodiment of the invention, the winding temperature can be 600°C or higher. On the other hand, when the winding temperature exceeds 800°C, it may cause the formation of an excessively thick oxide scale on the surface of the hot-rolled coil.

[0081] Cold rolling The coiled steel sheet can be cold rolled at a cold rolling reduction rate of 50-90%.

[0082] To improve the workability of the steel sheet, the cold rolling reduction rate can be 50% or higher. When the cold rolling reduction rate is less than 50%, it may be difficult to ensure the target thickness and to perform shape correction of the steel sheet. In this invention, there is no particular upper limit to the cold rolling reduction rate, but excessive reduction may cause cold rolling load. Therefore, considering this, it can be limited to 90% or lower.

[0083] annealing The cold-rolled steel sheet can be annealed at a temperature range of 600°C or higher for more than 10 seconds.

[0084] According to one embodiment of the present invention, the annealing process conditions are only the usual conditions, and can be carried out by box annealing or continuous annealing.

[0085] When the annealing temperature is below 600°C or the annealing time is less than 10 seconds, recrystallization cannot proceed sufficiently, and excellent processability may not be guaranteed afterwards. According to one embodiment of the invention, the annealing can be carried out at 800°C or above. In this invention, there is no particular upper limit on the temperature and time during annealing, but considering concerns about equipment failure and deterioration of powderiness caused by high-temperature annealing, 960°C and 15 minutes can be set as upper limits, respectively.

[0086] According to one embodiment of the present invention, after annealing, a surface treatment process can be performed as needed. The surface treatment process may include zinc plating, tin plating, enamel plating, etc.

[0087] Furthermore, according to one embodiment of the present invention, special treatments can be performed as needed to improve chemical processing properties, weldability, stamping properties, corrosion resistance, etc.

[0088] The present invention will now be described in more detail through embodiments. However, it should be noted that the embodiments described below are merely illustrative examples to provide a more detailed explanation and are not intended to limit the scope of the invention. Detailed Implementation

[0089] After preparing steel billets with the alloy compositions shown in Table 1, cold-rolled steel sheets were manufactured according to the conditions shown in Table 2. The steel billets were obtained through the following process: First, scrap steel combined with molten iron from an existing blast furnace was fed into an electric furnace to obtain molten steel. This molten steel was then transferred to a ladle and subjected to vacuum degassing in an RH degassing unit (0.1 torr), while alloying components were added to produce molten steel with the intended composition. Subsequently, the molten steel was continuously cast into steel billets.

[0090] [Table 1] [Table 2] Table 3 below shows the results of the measurement and evaluation of the microstructure and physical properties of the manufactured cold-rolled steel sheets.

[0091] The microstructure of the steel plate was obtained by collecting a sample from the surface of the steel plate at a position one-quarter of its thickness, grinding it, and then etching the cross-section of the ground sample with nitric acid and alcohol, followed by observation using a scanning electron microscope (SEM). After etching with nitric acid and alcohol, the sample surface with no unevenness was identified as ferrite, while the microstructure with spherical or layered structures was identified as cementite. Furthermore, the ferrite grain size was measured as the average grain size of the sample using an optical microscope. Additionally, both the inventive example and the comparative example contain more than 95% ferrite.

[0092] The texture evaluation of the steel sheet was performed using an electron backscattered diffraction (EBSD) measuring device. A cross-section of the cold-rolled steel sheet was prepared as a test piece, and the sample was observed and scanned along the C-direction. The measured data were rotated in the plan view direction. Measurements parallel to the

[001] direction, when viewed from the

[001] direction, were taken as having a positive impact on formability. <111> The volume fraction of grains in the directional direction. The {111} plane is conducive to deformation, therefore a higher volume fraction can improve formability. That is, it can be said that... <111> ∥

[001] The higher the volume fraction, the better the formability.

[0093] In addition, in order to evaluate the physical properties of each steel plate, a tensile test was carried out. The tensile test was evaluated using test pieces collected according to JIS No. 5 standard with the 0° direction relative to the rolling direction of the rolled sheet as a reference, and the product of the tensile strength (TS) and the elongation rate (El) (TS×El) was calculated. In addition, for the same test piece, the Lankford (r) value was measured by the three-point method after 15% tensile pre-deformation, and the average value in the rolling direction (L direction), the perpendicular direction to the rolling direction (C direction), and the 45-degree direction to the rolling direction (D direction) was obtained by the following calculation formula. In the present invention, the r value is expressed in the order of rL < rD < rC.

[0094] r value = (rL + 2rD + rC) / 4 [Table 3] [Relationship 1] T = R / (0.4([Cu] + [Cr] + [Ni]) + 1.6) (In the formula, R is the r value, and [Cu], [Cr], and [Ni] are the weight percentages of each element.) As shown in Table 3 above, in the case of the invention examples that satisfy the alloy composition and manufacturing conditions of the present invention, the microstructural characteristics proposed in the present invention are satisfied, and the physical properties targeted in the present invention can also be ensured.

[0095] Furthermore, Figure 1 is a graph showing the r value according to the sum of the Cu, Cr, and Ni contents. As Figure 1 shown, it can be confirmed that in the invention examples according to an embodiment of the present invention, as the sum of the Cu, Cr, and Ni contents increases, the volume fraction of <111> ∥

[001] increases, and from this, it can be confirmed that as the content of the elements increases, the r value increases.

[0096] On the other hand, Comparative Examples 1 to Comparative Examples 9 are examples that do not satisfy the alloy composition conditions proposed in the present invention. Therefore, the physical properties targeted in the present invention cannot be ensured. In addition, it can be confirmed that as the sum of the Cu, Cr, and Ni contents increases, the volume fraction of <111> ∥

[001] decreases, and from this, it is confirmed that as the content of the elements increases, the r value decreases.

[0097] The present invention has been described in detail above through embodiments, but embodiments in different forms are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the embodiments.

Claims

1. A cold-rolled steel sheet, by weight percent, comprising: C: less than 0.0500%, Si: 0.005-0.080%, Mn: less than 0.500%, Al: less than 0.0800%, P: less than 0.0800%, S: less than 0.0500%, N: less than 0.0300%, Ti: 0.001-0.050%, Nb: 0.001-0.050%, Cu: less than 1.000%, Ni: less than 1.000%, Cr: less than 1.000%, and containing at least 0.500% of one or more of Sb and Sn, and the balance being Fe and other unavoidable impurities. In relation 1 below, the value of T is defined as ranging from 0.90 to 1.

20. The tensile strength of the cold-rolled steel sheet is above 330 MPa. [Relation 1] T = R / (0.4([Cu] + [Cr] + [Ni]) + 1.6) In the formula, R is the value of r, and [Cu], [Cr] and [Ni] are the weights of each element.

2. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises, by weight percent: Mo: less than 1%, B: less than 0.02%.

3. The cold-rolled steel sheet according to claim 1, wherein, In the cold-rolled steel sheet, the Cu+Cr+Ni content is less than 0.7% by weight.

4. The cold-rolled steel sheet according to claim 1, wherein, The value of A, as defined in Equation 2 below, for the cold-rolled steel sheet is between 0.40 and 1.

40. [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) In the formula, [Mn], [Ti], and [Nb] are the weights of the elements.

5. The cold-rolled steel sheet according to claim 1, wherein, The microstructure of the cold-rolled steel sheet contains more than 95% ferrite and the remainder by area percentage.

6. The cold-rolled steel sheet according to claim 1, wherein, The average ferrite grain size of the cold-rolled steel sheet is below 13.0 μm.

7. The cold-rolled steel sheet according to claim 1, wherein, The tensile strength of the cold-rolled steel sheet is 330-420 MPa, and the elongation is above 32%.

8. The cold-rolled steel sheet according to claim 1, wherein, The r-value of the cold-rolled steel sheet is 1.60 or higher.

9. A method for manufacturing cold-rolled steel sheet, comprising the following steps: The steel billet is reheated, and by weight, the steel billet contains less than 0.0500% C, less than 0.005-0.080% Si, less than 0.500% Mn, less than 0.0800% Al, less than 0.0800% P, less than 0.0800% S, less than 0.0500% N, less than 0.0300% Ti, less than 0.001-0.050% Nb, less than 0.001-0.050% Cu, less than 1.000% Ni, less than 1.000% Cr, and less than 1.000%, and contains more than 0.500% of one or more of Sb and Sn, and contains the balance Fe and other unavoidable impurities; The reheated steel billet is then hot-rolled; The hot-rolled steel sheet is then coiled up; The coiled steel sheet is then cold-rolled. as well as The cold-rolled steel sheet is then annealed.

10. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The billet further comprises, by weight percent, less than 1% Mo and less than 0.02% B.

11. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, In the steel billet, the Cu+Cr+Ni content is less than 0.7% by weight.

12. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The value of A, as defined in Equation 2, for the steel billet is between 0.40 and 1.

40. [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) In the formula, [Mn], [Ti], and [Nb] are the weights of the elements.

13. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The reheating step is carried out in a temperature range of 900-1300℃. The hot rolling step is carried out at a finishing rolling temperature of 880°C or higher. The winding step is performed at a temperature range of 500-800℃. The cold rolling step is carried out with a cold rolling reduction rate of 50-90%. The annealing step is performed at a temperature above 600°C for more than 10 seconds.

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