Cold-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof and, more specifically, to a cold-rolled steel sheet having both excellent strength and formability, and a manufacturing method thereof.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet having excellent strength and formability and a method for manufacturing the same.

[0002] To address the global climate crisis, automakers are demanding that steelmakers gradually supply automotive steel sheets with reduced CO2 emissions, aiming to achieve carbon neutrality by 2050. For this reason, steelmakers are currently developing automotive steel sheets with reduced CO2 emissions through recycling of iron scrap, using existing blast furnace-converter furnace or electric arc furnace steelmaking processes.

[0003] When using iron scrap as a raw material, it is difficult to remove residual elements (Tramp Elements) such as Cu, Cr, and Ni contained in the iron scrap during refining, so they end up being included in the steel after refining. These residual elements have the problem of lowering the physical properties of the steel or deteriorating the surface quality. Therefore, until now, high-quality thin plate products such as automotive steel sheets have been manufactured using molten iron as the main raw material and using a general blast furnace-converter process to greatly reduce the C and N in the steel while extremely controlling the content of residual elements.

[0004] Meanwhile, a technology for manufacturing automobile steel sheets with excellent processability from electric furnace steel containing residual elements using an electric furnace steelmaking method was proposed as follows.

[0005] Patent Documents 1 and 2 present technologies for manufacturing cold-rolled steel sheets with excellent workability. Specifically, Patent Documents 1 and 2 disclose cold-rolled steel sheets with excellent cold workability despite containing a large amount of residual elements.

[0006] However, the above technologies limit the incorporation of Cu and Ni as residual elements, as the formability deteriorates due to a decrease in the Lankford value (r-value) according to the residual elements of the steel sheet.

[0007] In this way, although a technology for manufacturing automotive steel sheets containing residual elements has been proposed, automotive steel sheets with improved strength and formability despite containing residual elements have not yet been developed.

[0008] (Patent Document 1) Japanese Patent Publication No. 1995-118795

[0009] (Patent Document 2) Japanese Patent Publication No. 1998-025541

[0010] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0011] According to one embodiment of the present invention, in manufacturing a steel plate using iron scrap raw material, it is intended to provide a cold rolled steel plate having excellent strength and formability and a manufacturing method thereof.

[0012] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0013] According to one embodiment of the present invention, it includes, in wt%, C: 0.0500% or less, Si: 0.005 to 0.080%, Mn: 0.500% or less, Al: 0.0800% or less, P: 0.0800% or less, S: 0.0500% or less, N: 0.0300% or less, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, Cu: 1.000% or less, Ni: 1.000% or less, Cr: 1.000% or less, and includes at least one of Sb and Sn at 0.500% or less, and the remainder includes Fe and other unavoidable impurities.

[0014] The T value defined in the following relational expression 1 is 0.90 to 1.20,

[0015] A cold-rolled steel sheet having a tensile strength of 330 MPa or more can be provided.

[0016] [Relationship 1]

[0017] T = R / (0.4([Cu]+[Cr]+[Ni])+1.6)

[0018] (In the formula, R is the r-value, and [Cu], [Cr], and [Ni] are the weight percent of each element.)

[0019] The above cold rolled steel sheet may further include, in weight %, Mo: 1% or less and B: 0.02% or less.

[0020] The above cold rolled steel sheet may have Cu+Cr+Ni of 0.7% or less in weight %.

[0021] The above cold rolled steel sheet may have an A value of 0.40 to 1.40 as defined in the following relational expression 2.

[0022] [Relationship 2]

[0023] A = [Mn] / 10([Ti]+[Nb])

[0024] (In the formula, [Mn], [Ti] and [Nb] are the weight percent of each element.)

[0025] The microstructure of the above cold-rolled steel sheet may include, in terms of area %, 95% or more of ferrite and the remainder of the structure.

[0026] The above cold rolled steel sheet may have an average ferrite grain size of 13.0 μm or less.

[0027] The above cold rolled steel sheet may have a tensile strength of 330 to 420 MPa and an elongation of 32% or more.

[0028] The above cold rolled steel sheet may have an r-value of 1.60 or higher.

[0029] According to one embodiment of the present invention, there is provided a step of reheating a steel slab, which comprises, in wt%, C: 0.0500% or less, Si: 0.005 to 0.080%, Mn: 0.500% or less, Al: 0.0800% or less, P: 0.0800% or less, S: 0.0500% or less, N: 0.0300% or less, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, Cu: 1.000% or less, Ni: 1.000% or less, Cr: 1.000% or less, and at least one of Sb and Sn at 0.500% or less, with the remainder being Fe and other unavoidable impurities;

[0030] A step of hot rolling the above reheated steel slab;

[0031] A step of coiling the hot-rolled steel sheet;

[0032] A step of cold rolling the above-mentioned rolled steel plate; and

[0033] A method for manufacturing a cold-rolled steel sheet can be provided, including a step of annealing the cold-rolled steel sheet.

[0034] The above steel slab may further contain, in weight %, Mo: 1% or less and B: 0.02% or less.

[0035] The above steel slab may have Cu+Cr+Ni in wt% or less.

[0036] The above steel slab may have an A value of 0.40 to 1.40 as defined in the following relational expression 2.

[0037] [Relationship 2]

[0038] A = [Mn] / 10([Ti]+[Nb])

[0039] (In the formula, [Mn], [Ti] and [Nb] are the weight percent of each element.)

[0040] The above reheating step is performed at a temperature range of 900 to 1300°C,

[0041] The above hot rolling step is performed at a finishing temperature of 880℃ or higher.

[0042] The above-mentioned winding step is performed at a temperature range of 500 to 800°C,

[0043] The above cold rolling step is performed at a cold rolling ratio of 50 to 90%.

[0044] The above annealing step can be performed for 10 seconds or longer at a temperature range of 600°C or higher.

[0045] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0046] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent strength and formability and a method for manufacturing the same can be provided.

[0047] According to one embodiment of the present invention, a high-strength, high-formability cold-rolled steel sheet and a manufacturing method thereof can be provided using iron scrap raw material and used for various purposes including automobile parts.

[0048] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0049] Figure 1 is a graph showing the r-value according to the sum of Cu, Cr, and Ni contents.

[0050] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0051] Hereinafter, the present invention will be described in detail.

[0052] Below, the steel composition of the present invention is described in detail.

[0053] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0054] A cold-rolled steel sheet according to one embodiment of the present invention contains, in wt%, C: 0.0500% or less, Si: 0.005 to 0.080%, Mn: 0.500% or less, Al: 0.0800% or less, P: 0.0800% or less, S: 0.0500% or less, N: 0.0300% or less, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, Cu: 1.000% or less, Ni: 1.000% or less, Cr: 1.000% or less, and may contain 0.500% or less of one or more of Sb and Sn.

[0055] Carbon (C): 0.0500% or less

[0056] Carbon (C) is an important element in the present invention. If its content exceeds 0.0500%, it may be difficult to secure the required elongation and r-value of the steel sheet. According to one embodiment of the present invention, it may be 0.0400% or less. Meanwhile, the lower limit of the content is not particularly limited, but if it is included below 0.0003%, the effect of reducing carbon (C) is almost nonexistent, and there may be concerns about an increase in steelmaking costs. According to one embodiment of the present invention, the carbon (C) content may be 0.0003% or more.

[0057] Silicon (Si): 0.005–0.080%

[0058] Silicon (Si) is an element that enhances strength through solid solution strengthening, strengthens ferrite, homogenizes the structure, and improves workability. It is also an element necessary for deoxidation during steelmaking. If the silicon (Si) content exceeds 0.080%, it may cause plating defects, such as underplating, during the plating process and reduce the weldability of the steel sheet. According to one embodiment of the present invention, the content may be 0.070% or less. On the other hand, if the content is less than 0.005%, there is little effect in reducing silicon (Si), and there may be concerns about increased manufacturing costs.

[0059] Manganese (Mn): 0.001~0.500%

[0060] Manganese (Mn) is a useful element for increasing both strength and ductility. In particular, it is known as a solid solution strengthening element that prevents red-hot embrittlement caused by solid solution S by precipitating S dissolved in steel as MnS. If the manganese (Mn) content exceeds 0.500%, workability may deteriorate. According to one embodiment of the present invention, it may be 0.400% or less. On the other hand, if the content is less than 0.001%, the effect of reducing manganese (Mn) is almost nonexistent, and there may be concerns about an increase in manufacturing costs. According to one embodiment of the present invention, manganese (Mn) may be 0.010% or more. According to one embodiment of the present invention, manganese (Mn) may be 0.100% or more. According to one embodiment of the present invention, manganese (Mn) may be 0.150% or more.

[0061] Aluminum (Al): 0.0800% or less

[0062] Aluminum (Al) is an element that combines with oxygen in steel and acts as a deoxidizer. It is also an element that, like Si, strengthens ferrite, homogenizes the structure, and improves workability. If the aluminum (Al) content exceeds 0.0800%, workability may deteriorate. According to one embodiment of the present invention, it may be 0.0600% or less. The lower limit of the aluminum (Al) content is not particularly limited, but if the content is less than 0.0010%, the effect of reducing aluminum (Al) is minimal, and there may be concerns about an increase in manufacturing costs. According to one embodiment of the present invention, it may be 0.0010% or more.

[0063] Phosphorus (P): 0.0800% or less

[0064] Phosphorus (P) is an element used to increase strength. If its content exceeds 0.0800%, processability and brittleness may deteriorate. According to one embodiment of the present invention, it may be 0.0700% or less. Meanwhile, the lower limit of the phosphorus (P) content is not specifically limited, but if it is less than 0.0010%, the effect of reducing phosphorus (P) is almost nonexistent, and there may be concerns about an increase in manufacturing costs. According to one embodiment of the present invention, it may be 0.0010% or more.

[0065] Sulfur (S): 0.0500% or less

[0066] Sulfur (S) is an element that is contained as an impurity and forms MnS in the steel sheet, thereby deteriorating ductility. According to one embodiment of the present invention, the upper limit of the sulfur (S) content may be limited to 0.0500%. According to one embodiment of the present invention, it may be 0.0400% or less. Meanwhile, the lower limit of the sulfur (S) content is not particularly limited, but if it is less than 0.0010%, the effect of reducing sulfur (S) is almost nonexistent, and there may be concerns about an increase in manufacturing costs. According to one embodiment of the present invention, it may be 0.0010% or more.

[0067] Nitrogen (N): 0.0300% or less

[0068] Nitrogen (N) is an element that is contained as an impurity and forms nitrides during continuous casting, causing cracks in slabs. According to one embodiment of the present invention, the upper limit of the nitrogen (N) content may be limited to 0.0300%. According to one embodiment of the present invention, it may be 0.0200% or less. Meanwhile, the lower limit of the nitrogen (N) content is not particularly limited, but if it is less than 0.0010%, the effect of nitrogen (N) reduction is almost nonexistent and there may be concerns about an increase in manufacturing costs. According to one embodiment of the present invention, it may be 0.0010% or more.

[0069] Titanium (Ti): 0.001~0.050%

[0070] Titanium (Ti) is an important element that forms precipitates in steel sheets. It can be included to improve the strength and impact toughness of the steel sheet. The titanium (Ti) is an element that can prevent deterioration of workability caused by solid solution C and N in steel by precipitating them as carbides or nitrides. In order to secure the above-described effect, the present invention may include 0.001% or more of titanium (Ti). If the content is less than 0.001%, the effect of adding titanium (Ti) may be almost insignificant. Meanwhile, according to one embodiment of the present invention, the upper limit of the titanium (Ti) content may be limited to 0.050%. According to one embodiment of the present invention, it may be 0.040% or less.

[0071] Niobium (Nb): 0.001~0.050%

[0072] Niobium (Nb) is an important element that forms precipitates in steel sheets. It can be included to improve the strength and impact toughness of the steel sheet. The niobium (Nb) is an element that can prevent deterioration of workability caused by solid solution C and N in steel by precipitating them as carbides or nitrides. In order to secure the above-described effect, the present invention may include 0.001% or more. If the content is less than 0.001%, the effect of adding niobium (Nb) may be almost insignificant. Meanwhile, according to one embodiment of the present invention, the upper limit of the niobium (Nb) content may be limited to 0.050%. According to one embodiment of the present invention, it may be 0.040% or less.

[0073] Copper (Cu): 1.000% or less, Nickel (Ni): 1.000% or less

[0074] Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, the copper (Cu) and nickel (Ni) are concentrated on the surface of the steel sheet, thereby preventing the intrusion of hydrogen moving into the steel sheet, thereby suppressing hydrogen-delayed fracture. In addition, when the content of copper (Cu) and nickel (Ni) is 1.000% or less, deterioration of workability can be suppressed. According to one embodiment of the present invention, it may be 0.900% or less. Meanwhile, the lower limit of the content of each of copper (Cu) and nickel (Ni) is not particularly limited, but when the content is less than 0.010%, the effect of adding copper (Cu) and nickel (Ni) may be almost insignificant. According to one embodiment of the present invention, copper (Cu) and nickel (Ni) may each be included in an amount of 0.010% or more.

[0075] Chromium (Cr): 1.000% or less

[0076] Chromium (Cr) is an element that suppresses austenite decomposition during alloying treatment and stabilizes austenite, similar to manganese. In addition, when the content of chromium (Cr) is 1.000% or less, deterioration of workability can be suppressed. According to one embodiment of the present invention, it may be 0.900% or less. Meanwhile, the lower limit of the content of chromium (Cr) is not particularly limited, but when the content is less than 0.010%, the effect of adding chromium (Cr) may be almost insignificant. According to one embodiment of the present invention, chromium (Cr) may be included in an amount of 0.010% or more.

[0077] At least one of antimony (Sb) and tin (Sn): 0.500% or less

[0078] Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets. If the sum of the contents of at least one of antimony (Sb) and tin (Sn) exceeds 0.500%, the brittleness of the steel sheet increases, which may cause cracks to occur during hot working or cold working. In addition, if the sum of the contents of antimony (Sb) and tin (Sn) is 0.500% or less, the deterioration of workability can be suppressed. According to one embodiment of the present invention, it may be 0.400% or less. Meanwhile, the lower limit of the contents of antimony (Sb) and tin (Sn) is not particularly limited, but if the contents are less than 0.0005%, the effect of adding antimony (Sb) and tin (Sn) may be almost insignificant. According to one embodiment of the present invention, at least one of antimony (Sb) and tin (Sn) may be 0.0005% or more.

[0079] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0080] A cold rolled steel sheet according to one embodiment of the present invention may further include, in weight %, Mo: 1% or less and B: 0.02% or less.

[0081] Molybdenum (Mo): 1.0% or less

[0082] Molybdenum (Mo) is an element that suppresses austenite decomposition during alloying treatment and stabilizes austenite, similar to manganese. In addition, when the content of molybdenum (Mo) is 1.0% or less, deterioration of workability can be suppressed. According to one embodiment of the present invention, it may be 0.9% or less. Meanwhile, the lower limit of the content of molybdenum (Mo) is not particularly limited, but when the content is less than 0.01%, the effect of adding molybdenum (Mo) may be almost insignificant. According to one embodiment of the present invention, the content of molybdenum (Mo) may be 0.01% or more.

[0083] Boron (B): 0.02% or less

[0084] Boron (B) is an element that improves hardenability, thereby increasing strength and inhibiting nucleation at grain boundaries. If the boron (B) content exceeds 0.02%, the deep drawability of the steel sheet may deteriorate. According to one embodiment of the present invention, it may be 0.01% or less. Meanwhile, the lower limit of the boron (B) content is not particularly limited, but if the content is less than 0.0001%, the effect of adding boron (B) may be minimal. According to one embodiment of the present invention, the boron (B) content may be 0.0001% or more.

[0085] A cold rolled steel sheet according to one embodiment of the present invention may have Cu+Cr+Ni in wt% or less.

[0086] According to one embodiment of the present invention, when manufacturing a steel plate, by using iron scrap as a material, the steel plate may contain Cu, Cr, and Ni, and the sum of their contents may be included at 0.7% or less. The lower limit of the sum of the Cu, Cr, and Ni contents is not particularly limited, but may be included at 0.003% or more.

[0087] A cold rolled steel sheet according to one embodiment of the present invention may have an A value defined in the following relational expression 2 of 0.40 to 1.40.

[0088] [Relationship 2]

[0089] A = [Mn] / 10([Ti]+[Nb])

[0090] (In the formula, [Mn], [Ti] and [Nb] are the weight percent of each element.)

[0091] According to one embodiment of the present invention, even when Cu, Cr, and Ni are included in large amounts, formability can be secured by more strictly controlling the alloy composition through the above relational expression 2.

[0092] If the A value defined in the above relational expression 2 is less than 0.40 or exceeds 1.40, the desired strength and formability cannot be properly secured.

[0093] Below, the steel microstructure of the present invention is described in detail.

[0094] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0095] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention may include, in terms of area %, 95% or more of ferrite and the remainder of the structure.

[0096] According to one embodiment of the present invention, to ensure formability, the ferrite content may be 95% or more. If the content is less than 95%, there may be a problem of rapid deterioration in formability.

[0097] The remaining structure other than ferrite may include a low-temperature transformation phase, and according to one embodiment of the present invention, the low-temperature transformation phase may include martensite, bainite, etc. According to one embodiment of the present invention, the ferrite may be 100%.

[0098] A cold-rolled steel sheet according to one embodiment of the present invention may have an average ferrite grain size of 13.0 μm or less.

[0099] If the average grain size of ferrite exceeds 13.0 μm, it may be difficult to secure the strength desired in the present invention. According to one embodiment of the present invention, it may be 12.5 μm or less. There is no particular limitation on the lower limit of the average grain size of ferrite, but according to one embodiment of the present invention, it may be 6.5 μm or more.

[0100] A cold rolled steel sheet according to one embodiment of the present invention may have a T value defined in the following relational expression 1 of 0.90 to 1.20.

[0101] [Relationship 1]

[0102] T = R / (0.4([Cu]+[Cr]+[Ni])+1.6)

[0103] (In the formula, R is the r-value, and [Cu], [Cr], and [Ni] are the weight percent of each element.)

[0104] According to one embodiment of the present invention, even when Cu, Cr, and Ni are included in large amounts, the desired r-value can be secured. According to one embodiment of the present invention, as the sum of the Cu, Cr, and Ni contents increases, the r-value can increase.

[0105] The above relational expression 1 can represent the relationship between Cu, Cr, and Ni contents and r-value. According to one embodiment of the present invention, as the sum of the Cu, Cr, and Ni contents increases, the r-value increases, and thus the ratio of these defined in relational expression 1 can be secured within a certain range.

[0106] If the T value defined in the above relational expression 1 is less than 0.90 or exceeds 1.20, the r-value cannot be sufficiently secured while satisfying the desired strength.

[0107] A cold-rolled steel sheet according to one embodiment of the present invention may have a tensile strength of 330 to 420 MPa, an elongation of 32% or more, a product of tensile strength and elongation (TSxEl) of 12.0 to 16.0 GPa·%, and an r-value (Lankford value) of 1.60 or more.

[0108] According to one embodiment of the present invention, the tensile strength may be 340 MPa or more. According to one embodiment of the present invention, the tensile strength may be 410 MPa or less.

[0109] Below, the steel manufacturing method of the present invention is described in detail.

[0110] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by reheating, hot-rolling, coiling, cold-rolling, and annealing a steel slab satisfying the above-described alloy composition.

[0111] Reheating

[0112] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be reheated to a temperature range of 900 to 1300°C.

[0113] If the reheating temperature is lower than 900°C, the rolling load may increase during hot rolling, which may reduce the stability of the hot rolling. According to one embodiment of the present invention, the reheating temperature may be higher than 1100°C. On the other hand, if the reheating temperature exceeds 1300°C, the workability and plating adhesion may be poor, and there is a possibility that the steel may reach its melting point and melt. According to one embodiment of the present invention, the reheating temperature may be lower than 1160°C.

[0114] According to one embodiment of the present invention, the conditions of the solvent process for manufacturing a steel slab or steel ingot are not particularly limited.

[0115] According to one embodiment of the present invention, the steel slab or ingot can be manufactured through an electric furnace or a new blast furnace-converter process, and the raw material used in the electric furnace or blast furnace-converter process can be pig iron together with iron scrap. Here, pig iron refers to molten iron obtained from the blast furnace-converter process, or its cold material (Corrugate), or HBI (Hot Briquette Iron).

[0116] In the case of electric furnaces, desulfurization can be performed through ladle refining after the electric furnace discharge, and desulfurization and subsequent vacuum degassing can be performed. Furthermore, steel obtained from an electric furnace can be adjusted to achieve the desired final alloy composition by adding alloying elements during degassing. Common vacuum degassing methods include the RH and DH methods, but oxygen injection into the degassing tank can also be used. One such method is the oxygen injection method using a top blowing lance.

[0117] According to one embodiment of the present invention, the casting method is not particularly limited, but continuous casting may be used to improve productivity.

[0118] hot rolling

[0119] The above reheated steel slab can be hot rolled at a finishing temperature of 880°C or higher.

[0120] During hot rolling, if the finishing temperature is below 880°C, abnormal rolling may occur, potentially causing uneven texture. According to one embodiment of the present invention, there are no particular limitations on the conditions during the hot rolling process, and typical conditions applicable in the same technical field may be applied.

[0121] Winding

[0122] The above hot-rolled steel plate can be coiled at a temperature range of 500 to 800°C.

[0123] To enhance the high strength and formability of the steel plate, the coiling temperature may be limited to 500°C or higher. According to one embodiment of the present invention, the coiling temperature may be 600°C or higher. On the other hand, if the coiling temperature exceeds 800°C, it may cause the problem of excessively thick scales being formed on the surface of the hot-rolled coil.

[0124] cold rolling

[0125] The above-mentioned rolled steel plate can be cold rolled at a cold rolling ratio of 50 to 90%.

[0126] To improve the workability of the steel plate, the cold rolling ratio may be 50% or more. If the cold rolling ratio is less than 50%, it may be difficult to secure the target thickness and correct the shape of the steel plate. In the present invention, the upper limit of the cold rolling ratio is not particularly limited. However, if it is excessively high, cold rolling load may be induced, and therefore, taking this into consideration, it may be limited to 90% or less.

[0127] Sodun

[0128] The above cold-rolled steel sheet can be annealed at a temperature range of 600°C or higher for 10 seconds or longer.

[0129] According to one embodiment of the present invention, the annealing process conditions may be conventional conditions, and may be performed using a box-type annealing method or a continuous annealing method.

[0130] If the above-mentioned annealing temperature is less than 600°C or the time is less than 10 seconds, recrystallization may not be sufficiently performed, making it difficult to secure excellent processability thereafter. According to one embodiment of the present invention, the annealing may be performed at 800°C or higher. In the present invention, the upper limits of the temperature and time during the annealing are not particularly limited, but considering the risk of equipment trouble due to high-temperature annealing and the inferiority of powdering properties, the upper limits may be limited to 960°C and 15 minutes, respectively.

[0131] According to one embodiment of the present invention, after annealing, a surface treatment process may be additionally performed as needed. The surface treatment process may include zinc plating, tin plating, enamel plating, etc.

[0132] In addition, according to one embodiment of the present invention, special treatment may be performed to improve the Mars processability, weldability, press formability, corrosion resistance, etc., as needed.

[0133] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0134] After preparing a steel slab having the alloy composition shown in Table 1 below, a cold-rolled steel sheet was manufactured using the steel slab according to the conditions shown in Table 2 below. The steel slab was obtained through the following process. First, iron scrap mixed with molten iron from an existing blast furnace was introduced into an electric furnace to obtain molten steel, and then the molten steel was transferred to a ladle and vacuum degassing was performed in an RH degassing furnace (0.1 torr) while simultaneously adding alloying components to manufacture molten steel having the desired composition. Thereafter, the molten steel was manufactured into a slab by continuous casting.

[0135] Steel alloy composition (weight %)Cu+Cr+NiRelationship 2CSiMnPSAlTiNCuCrNiNbSn+SbAValueA0.00180.0200.1930.03900.00870.06400.0100.00140.0750.0390.0290.0180.0190.1430.69B0.00190.0200.1930.03800.00840.04500.0100.00150.1030.0540.0410.0180.0210.1980.69C0.00200.0190.1910.03800.00850.04700.00 90.00130.1400.0720.0530.0190.0200.2650.68D0.00150.0190.1970.03 900.00820.04300.0100.00130.1800.0880.0670.0190.0220.3350.68E0.0 0150.0190.1930.03800.00880.03600.0100.00150.2200.1040.0780.018 0.0200.4020.69F0.00130.0180.1930.03800.00830.04800.0090.00140.3 800.1720.1300.0180.0210.6820.71G0.00160.0280.1600.01900.00800. 03900.0010.00070.0750.0390.0290.0050.00120.1432.67H0.00130.028 0.1500.01900.00800.03900.0010.00090.1030.0540.0410.0040.00130. 1983.00I0.00150.0270.1500.02000.00800.03700.0010.00100.1400.072 0.0530.0050.00110.2652.50J0.00150.0290.1600.01900.00800.03900. 0010.00090.1800.0880.0670.0050.00120.3352.67K0.00140.0300.1500. 01900.00800.03400.0010.00100.2200.1040.0780.0050.00110.4022.50 L0.00140.0300.1500.01900.00800.03800.0010.00100.3800.1720.1300.0050.00130.6822.50M0.00220.0010.0690.01400.00470.02140.0270.0027 0.1130.0710.0700.0090.0030.1430.19N0.00160.0040.0580.00940.00430. 04000.0270.00170.1360.1050.0810.0070.0040.1980.17O0.00280.0050.06 40.01500.00440.03000.0260.00280.1310.0680.0680.0090.0090.2650.18.

[0136] Specimen number Steel material Heating Hot rolling Coiling Cold rolling Annealing Temperature (℃) Finishing temperature (℃) Temperature (℃) Rolling rate (%) Temperature (℃) Time (sec)

[0137] Table 3 below shows the microstructure and physical properties measured and evaluated for the manufactured cold-rolled steel sheet.

[0138] The microstructure of the steel plate was determined by taking a specimen at a point 1 / 4 of the thickness from the surface of the steel plate, polishing it, and observing the cross-section of the polished specimen using a scanning electron microscope (SEM) after nital etching. After the nital etching, the structure without irregularities on the surface of the specimen was determined to be ferrite, and the structure with a spherical or lamellar structure was determined to be cementite. In addition, the average grain size of the ferrite was measured for the specimen using an optical microscope. Meanwhile, both the inventive examples and the comparative examples contained ferrite at a content of 95% or more.

[0139] The texture evaluation of the steel plate was performed by preparing a cross-section of a cold-rolled steel plate as a specimen using an EBSD (Electron Back Scattered Diffraction) measuring device, observing it in the C direction, and scanning the photograph. The measured data were rotated in the plan view direction and observed in the

[0001] direction, and the data were parallel to the

[0001] direction, which has a positive effect on formability. <111> The volume fraction of grains in the direction was measured. The {111} planes are advantageous for deformation, so a higher volume fraction can improve formability. That is, <111> ∥

[0001] The higher the volume fraction, the better the formability.

[0140] In addition, a tensile test was conducted to evaluate the physical properties of each steel plate. The tensile test was evaluated using specimens collected according to the JIS No. 5 standard with respect to the 0° direction with respect to the rolling direction of the rolled plate as the standard, and the product of tensile strength (TS) and elongation (El) (TSХEl) was calculated. In addition, the Lankford value (r-value) for the same specimen was measured by the three-point method after 15% tensile pre-strain, and the average values ​​in the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the direction 45 degrees to the rolling direction (D direction) were obtained by the following calculation formula. In the present invention, the r-value appeared in the order of rL < rD < rC.

[0141] r-value = (rL + 2rD + rC) / 4

[0142] Specimen number, steel grade, microstructure, property relationship equation 1, type, ferrite, grain size (μm), texture, fraction (%) <111> ∥

[0001] TS(MPa)El(%)TS*El(GPa·%)r-valuer-value*El(%)T Value 1A8.840.637635.213.31.6156.70.97 Invention Example 12B8.644.739034.613.51.6256.00.96 Invention Example 23C8.946.338633.312.91.6454.60.96 Invention Example 34D8.553.538932.612.71.6955.00.97 Invention Example 45E8.753.539332.212.61.7255.30.98 Invention Example 56F8.955.640232.112.91.8258.30.97 Invention Example 67A10.94 8.336438.113.91.7265.51.04Invention Example 78B10.351.237437.013.81.7564.71.04Invention Example 89C9.954.237538.014.31.7566.61.03Invention Example 910D10.856.337837.114.01.8075.21.04Invention Example 1011E9.857.938136.814.01.8467.61.04Invention Example 1112F10.958.039036.114.11.9470.11.04Invention Example 1213A11. 750.735842.815.31.8378.41.10 Invention Example 1314B12.251.836340.514.71.8675.31.11 Invention Example 1415C11.956.936742.015.41.8778.61.10 Invention Example 1516D12.560.737041.915.51.9179.91.10 Invention Example 1617E11.861.337340.715.21.9679.91.11 Invention Example 1718F11.561.538139.315.02.0580.51.09 Invention Example 1819G13.250.630547.814.61.6779.81.01Comparative Example 120H14.644.031045.814.21.5870.50.94Comparative Example 221I13.838.331444.914.11.4062.80.82Comparative Example 322J13.338.231846.214.71.3863.70.80Comparative Example 423K14.237.632345.014.51.3259.30.75Comparative Example 524L14.436.432545.214.71.2657.00.67Comparative Example 625M16.255.631645.614.41.8082.11.06Comparative Example 726N16.152.332045.014.41.7277.40.99Comparative Example 827O16.248.231245.014.01.7076.51.00Comparative Example 9.

[0143] [Relationship 1]

[0144] T = R / (0.4([Cu]+[Cr]+[Ni])+1.6)

[0145] (In the formula, R is the r-value, and [Cu], [Cr], and [Ni] are the weight percent of each element.)

[0146] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0147] Furthermore, Fig. 1 is a graph showing the r-value according to the sum of Cu, Cr, and Ni contents. As shown in Fig. 1, the invention examples according to one embodiment of the present invention show that as the sum of Cu, Cr, and Ni contents increases, <111> ∥

[0001] It can be confirmed that the volume fraction increases, and through this, it can be confirmed that the r-value increases as the content of the above elements increases.

[0148] On the other hand, Comparative Examples 1 to 9 are examples that do not satisfy the alloy composition conditions proposed in the present invention. As a result, the properties desired in the present invention could not be secured. In addition, as the sum of the Cu, Cr, and Ni contents increases, <111> ∥

[0001] It can be confirmed that the volume fraction decreases, and through this, it was confirmed that the r-value decreases as the content of the above elements increases.

[0149] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in wt%, C: 0.0500% or less, Si: 0.005 to 0.080%, Mn: 0.500% or less, Al: 0.0800% or less, P: 0.0800% or less, S: 0.0500% or less, N: 0.0300% or less, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, Cu: 1.000% or less, Ni: 1.000% or less, Cr: 1.000% or less, and contains at least one of Sb and Sn at 0.500% or less, and the remainder contains Fe and other unavoidable impurities. The T value defined in the following relational expression 1 is 0.90 to 1.20, Cold rolled steel sheet with a tensile strength of 330 MPa or more. [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 percent of each element.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing, in weight %, Mo: 1% or less and B: 0.02% or less.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having Cu+Cr+Ni of 0.7% or less in weight%.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an A value of 0.40 to 1.40 as defined in the following relational expression 2. [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) (In the formula, [Mn], [Ti] and [Nb] are the weight % of each element.

5. In claim 1, The microstructure of the above cold rolled steel sheet is a cold rolled steel sheet containing 95% or more of ferrite and the remainder of the structure in area %.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an average ferrite grain size of 13.0 μm or less.

7. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 330 to 420 MPa and an elongation of 32% or more.

8. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with an r-value of 1.60 or higher.

9. A step of reheating a steel slab containing, by weight%, C: 0.0500% or less, Si: 0.005 to 0.080%, Mn: 0.500% or less, Al: 0.0800% or less, P: 0.0800% or less, S: 0.0500% or less, N: 0.0300% or less, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, Cu: 1.000% or less, Ni: 1.000% or less, Cr: 1.000% or less, and containing 0.500% or less of at least one of Sb and Sn, and the remainder being Fe and other unavoidable impurities; A step of hot rolling the above reheated steel slab; A step of coiling the hot-rolled steel plate; A step of cold rolling the above-mentioned rolled steel plate; and A method for manufacturing a cold rolled steel sheet, comprising: a step of annealing the cold rolled steel sheet.

10. In claim 9, A method for manufacturing a cold rolled steel sheet, wherein the steel slab further contains, in weight %, Mo: 1% or less and B: 0.02% or less.

11. In claim 9, The above steel slab is a method for manufacturing a cold rolled steel sheet having a Cu+Cr+Ni content of 0.7% or less in weight%.

12. In claim 9, The above steel slab is a cold rolled steel sheet manufacturing method having an A value of 0.40 to 1.40 as defined in the following relational expression 2. [Relationship 2] A = [Mn] / 10([Ti]+[Nb]) (In the formula, [Mn], [Ti] and [Nb] are the weight % of each element.) 13. In claim 9, The above reheating step is performed at a temperature range of 900 to 1300°C. The above hot rolling step is performed at a finishing temperature of 880℃ or higher. The above-mentioned winding step is performed at a temperature range of 500 to 800°C. The above cold rolling step is performed at a cold rolling ratio of 50 to 90%. A method for manufacturing cold rolled steel sheets, wherein the above annealing step is performed at a temperature range of 600°C or higher for 10 seconds or longer.

Images