Cold-rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor, and relates to: a bake-hardenable cold-rolled steel sheet suitable for use in various applications including automobile parts; and a manufacturing method therefor.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a bake-hardened cold-rolled steel sheet suitable for use in various applications including automotive parts and a method for manufacturing the same.

[0002] In response to the global climate crisis, automakers are requesting steel companies to supply automotive steel sheets with reduced CO2 emissions in stages, with the goal of achieving carbon neutrality by 2050. For this reason, steel companies have recently been developing automotive steel sheets with reduced CO2 emissions through the recycling of steel scrap, using the existing blast furnace-converter steelmaking method or the electric furnace steelmaking method.

[0003] When iron scrap is used 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 are inevitably included in the steel. These residual elements are known to degrade the physical properties of the steel or deteriorate the surface quality. For this reason, high-grade thin sheet products, such as automotive steel sheets, have been manufactured using molten iron as the main raw material in a general blast furnace-converter process to significantly reduce C and N in the steel and extremely control the content of residual elements.

[0004] Meanwhile, there is a continuous demand for reducing thickness by increasing the strength of steel sheets to achieve lightweighting for improved fuel efficiency. Bake-hardened cold-rolled steel is known to be the most suitable material for exterior panels due to these characteristics. Bake hardening is a phenomenon in which solid solution carbon and nitrogen, activated during paint baking, adhere to dislocations generated during pressing, thereby increasing yield strength. Steel with excellent bake-hardenability is easy to form before painting and baking, and the resulting product exhibits improved dent resistance, allowing for the simultaneous realization of both formability and high strength. However, due to the solid solution elements within the steel, bake-hardened steel is susceptible to aging degradation, such as yield point elongation, when maintained at room temperature for extended periods. Therefore, it is required to possess a certain level of room-temperature aging resistance to guarantee durability over a specific duration. Thanks to the rapid advancements in steelmaking technology in recent years, it has become possible to control the appropriate amount of dissolved elements in steel. Furthermore, by adding strong carbonitride-forming elements such as Ti or Nb, it is possible to manufacture bake-hardened cold-rolled steel sheets with excellent formability. The use of these bake-hardened cold-rolled steel sheets is continuously increasing, particularly in applications requiring dent resistance, such as automotive body panels.

[0005] Representative technologies for manufacturing bake-hardened cold-rolled steel sheets with high bake-hardenability or low aging index using some of the residual elements include Patent Documents 1 and 2.

[0006] Patent Document 1 describes a technology that increases the Bake Hardening (BH) value by utilizing Sn among the residual elements. Patent Document 2 describes a technology that suppresses the Aging Index (AI) by utilizing Cr. However, Patent Documents 1 and 2 do not mention the influence of other residual elements on BH or AI, as well as the influence on the degree of surface hardening, when scrap is added. Furthermore, they do not mention the influence on BH or the degree of surface hardening when the bake hardening temperature is lowered.

[0007] [Prior Art Literature]

[0008] (Patent Document 1) Japanese Patent Publication No. 1993-093502

[0009] (Patent Document 2) Japanese Patent Publication No. 2003-121334

[0010] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0011] A preferred aspect of the present invention is to provide a cold-rolled steel sheet having good dent resistance and excellent bake hardening properties even at low temperatures, and a method for manufacturing the same.

[0012] One embodiment of the present invention comprises, in weight%, C: 0.0010~0.0050%, Si: 0.0010~0.090%, Mn: 0.070~1.0%, Al: 0.0010~0.060%, P: 0.010~0.040%, S: 0.010% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.00050~0.030%, Nb: 0.00150~0.0150%, B: 0.00010~0.020%, Cu: 0.0010~1.0%, Ni: 0.0010~1.0%, Cr: 0.0010~1.20%, Mo: 0.0010~1.0%, and the remainder being Fe and other A cold-rolled steel sheet is provided that is composed of unavoidable impurities, satisfies the following equation 1, has a microstructure containing 5% or less (including 0%) of cementite and the remainder being ferrite in area %, and has a total average content of Cu, Cr, and Ni in a region from the surface to 0.3 μm in the thickness direction of 0.050 to 1.20 wt%.

[0013] [Relation 1] 0.050 ≤ (Cu+Ni) / Cr ≤ 0.50

[0014] The above cold-rolled steel sheet may additionally contain 0.00050~0.50% of one or more of Sb and Sn.

[0015] The above cold-rolled steel sheet can satisfy the following relationship 2.

[0016] [Equation 2] 0.060 ≤ (α+β) / γ ≤ 0.60

[0017] (However, in the above Equation 2, α represents the average content of Cu in the region up to 0.3 μm in the thickness direction from the surface, β represents the average content of Ni in the region up to 0.3 μm in the thickness direction from the surface, and γ represents the average content of Cr in the region up to 0.3 μm in the thickness direction from the surface.)

[0018] The above cold-rolled steel sheet may have a yield strength (YS): 210~280MPa, tensile strength (TS): 300~380MPa, elongation (El): 38.0~50.0%, and a Lankford value (r): 1.30~1.80.

[0019] The above cold-rolled steel sheet may have an aging index (AI) of 0 to 5.0%.

[0020] The above cold-rolled steel sheet may have a lower bake hardening value (lower BH) of 30 MPa or more at 140~170℃ and an upper bake hardening value (upper BH) of 30 MPa or more at 120~170℃.

[0021] The above cold-rolled steel sheet may have a surface hardness of 120 to 160 Hv.

[0022] The above cold-rolled steel sheet may have at least one surface treatment layer formed on at least one surface, such as a zinc or zinc-based plating layer, a tin plating layer, or an enamel treatment layer.

[0023] Another embodiment of the present invention comprises, in weight percent, C: 0.0010~0.0050%, Si: 0.0010~0.090%, Mn: 0.070~1.0%, Al: 0.0010~0.060%, P: 0.010~0.040%, S: 0.010% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.00050~0.030%, Nb: 0.00150~0.0150%, B: 0.00010~0.020%, Cu: 0.0010~1.0%, Ni: 0.0010~1.0%, Cr: 0.0010~1.20%, Mo: 0.0010~1.0%, and the remainder being Fe and other A method for manufacturing a cold-rolled steel sheet is provided, comprising: a step of preparing a slab composed of unavoidable impurities and satisfying the following equation 1; a step of heating the slab to a slab heating temperature (HT) of 900 to 1300°C at a slab heating rate (HR) of 3 to 15°C / min, and then heating for a slab holding time (HS) of 10 to 90 minutes; a step of finishing hot-rolling the heated slab to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and a step of annealing the cold-rolled steel sheet; wherein, when heating the slab, the method satisfies the following equation 3.

[0024] [Relation 1] 0.050 ≤ (Cu+Ni) / Cr ≤ 0.50

[0025] [Relationship 3] 500 ≤ X = (HT+HS) / [{(Cu+Ni) / Cr}×HR] ≤ 2000

[0026] The above slab may additionally contain 0.00050~0.50% of one or more of Sb and Sn.

[0027] The above finishing hot rolling can be performed at 650 to 1050°C.

[0028] The above winding can be performed at 400~800℃.

[0029] The above cold rolling can be performed with a cold reduction rate of 50 to 90%.

[0030] The above annealing can be performed at 600 to 950°C for 10 to 60 seconds.

[0031] After the above annealing, at least one surface treatment among zinc or zinc-based plating, tin plating, and enamel treatment may be additionally performed on at least one surface of the cold-rolled steel sheet.

[0032] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0033] According to a preferred aspect of the present invention, a cold-rolled steel sheet having good dent resistance and excellent bake hardening properties even at low temperatures, and a method for manufacturing the same can be provided.

[0034] Figure 1 is a profile of the surface layer Cu content of Comparative Examples 1 to 3 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0035] Figure 2 is a profile of the surface Cr content of Comparative Examples 1 to 3 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0036] Figure 3 is a profile of the surface layer Ni content of Comparative Examples 1 to 3 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0037] Figure 4 is a profile of the surface layer Cu content of Invention Examples 1 to 4 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0038] FIG. 5 is a profile of the surface layer Cr content of Invention Examples 1 to 4 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0039] FIG. 6 is a profile of the surface layer Ni content of Invention Examples 1 to 4 measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0040] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0041] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0042] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0043] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0044] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0045] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0046] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0047] C: 0.0010~0.0050%

[0048] C is an important interstitial solid solution element in the present invention. Since it is dissolved within the steel sheet during the cold rolling and annealing processes and interacts (locks) with dislocations formed by temper rolling to exhibit bake hardening ability, bake hardening ability is basically improved as the C content increases. If the C content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the C content exceeds 0.0050%, it may result in aging defects that cause a defect called orange peel on the surface during part forming. In other words, it is difficult to secure the formability required by the steel sheet, namely elongation and Lankford value (r), and room temperature aging resistance is significantly reduced, limiting its application to parts. Therefore, the C content may have a range of 0.0010% to 0.0050%. It is more advantageous for the lower limit of the C content to be 0.00130%. It is more advantageous for the upper limit of the C content to be 0.0040%.

[0049] Si: 0.0010~0.090%

[0050] Si is an element that enhances strength through solid solution strengthening, strengthens ferrite, homogenizes the microstructure, and improves workability. It is also an element necessary for deoxidation during steelmaking. If the Si content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.090%, it is difficult to completely remove Si oxides from the surface layer even if pickling is performed before cold rolling. Even if all Si oxides are removed by an acid solution, Si easily becomes concentrated again in the surface layer, causing plating defects such as incomplete plating in the plating process along with P, Mn, etc. Therefore, the Si content may have a range of 0.0010% to 0.090%. It is more advantageous for the lower limit of the Si content to be 0.0050%. It is more advantageous for the upper limit of the Si content to be 0.080%.

[0051] Mn: 0.070~1.0%

[0052] 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 brittleness caused by dissolved sulfur by precipitating dissolved sulfur (S) in steel as MnS. If the Mn content is less than 0.070%, it may be difficult to sufficiently obtain the aforementioned effects. Meanwhile, as an oxide-forming element, Mn forms Mn oxides starting from the hot rolling stage. Although most of these oxides are removed during the pickling process after hot rolling, some segregate in the surface layer of the steel sheet and react with other oxide-forming elements to cause surface defects during the plating process. If the Mn content exceeds 1.0%, excessive Mm concentrations occur in the surface layer, and these concentrations act as a cause of defects in the plating surface layer along with oxidizing elements such as Si and P. Additionally, problems such as reduced formability may occur along with an excessive increase in strength. Therefore, the Mn content may be in the range of 0.070 to 1.0%. It is more advantageous for the lower limit of the above Mn content to be 0.10%. It is more advantageous for the upper limit of the above Mn content to be 0.90%.

[0053] Al: 0.0010~0.060%

[0054] Al is an element that combines with oxygen in steel to perform deoxidation. Additionally, similar to Si, it is an element that strengthens ferrite, homogenizes the microstructure, and improves workability. If the Al content is less than 0.0010%, it is difficult to manufacture aluminum killed steel in a normal, stable state. If the Al content exceeds 0.060%, it is advantageous for increasing strength due to the grain refinement effect, but excessive inclusions are formed during steelmaking / continuous casting operations, increasing the likelihood of surface defects in plated steel sheets. Furthermore, it may be difficult to secure BH at low temperatures. Therefore, the Al content may have a range of 0.0010% to 0.060%. It is more advantageous for the lower limit of the Al content to be 0.0050%. It is more advantageous for the upper limit of the Al content to be 0.0550%.

[0055] P: 0.010~0.040%

[0056] P is an element used to increase strength in ultra-low carbon steel, as it exhibits the most excellent solid solution strengthening effect without significantly impairing drawability. In particular, P easily segregates at grain boundaries, inhibiting grain growth during annealing and thereby refining the grains, which helps improve room-temperature aging resistance. If the P content is less than 0.010%, it may be difficult to secure the desired strength. If the P content exceeds 0.040%, P segregation in the surface layer causes linear defects in the surface layer after hot-dip galvanizing. Furthermore, high P content delays the alloying of hot-dip galvanizing, requiring a higher alloying temperature; consequently, this leads to an increase in brittle Fe-Zn intermetallic compounds within the plating layer, resulting in poor powderability. Additionally, excessive P concentrations are present in the surface layer starting from the hot rolling stage, and the alloying of the P-enriched region is delayed during the hot-dip galvanizing alloying process after annealing, causing linear surface defects. Accordingly, the content of P may have a range of 0.010 to 0.040%. It is more advantageous for the lower limit of the P content to be 0.0150%. It is more advantageous for the upper limit of the P content to be 0.0550%.

[0057] S: 0.010% or less (excluding 0%)

[0058] S is an impurity element, and if the content of S exceeds 0.010%, it forms a large amount of MnS within the steel sheet, thereby degrading ductility. While it is advantageous for S to be excluded from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Accordingly, the content of S may have a range of 0.010% or less (excluding 0%). It is more advantageous for the content of S to be 0.0090% or less. Meanwhile, as an example, the lower limit of the S content may be 0.0010%.

[0059] N: 0.010% or less (excluding 0%)

[0060] N is an impurity element, and if the content of N exceeds 0.010%, it forms nitrides during continuous casting, causing cracks in the slab. While it is advantageous to exclude N from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Accordingly, the content of N may have a range of 0.010% or less (excluding 0%). It is more advantageous for the content of N to be 0.0090% or less, and even more advantageous for it to be 0.0080% or less. Meanwhile, as an example, the lower limit of the N content may be 0.00050%.

[0061] Ti: 0.00050~0.030%

[0062] Ti is an element that affects bake hardenability and room temperature aging resistance by forming TiN and reducing the solid solution element N. If the content of Ti is less than 0.00050%, aging resistance may be inferior. If the content of Ti exceeds 0.030%, bake hardenability may be inferior. Therefore, the content of Ti may have a range of 0.00050% to 0.030%. It is more advantageous for the lower limit of the Ti content to be 0.0010%. It is more advantageous for the upper limit of the Ti content to be 0.0250%.

[0063] Nb: 0.00150~0.0150%

[0064] Nb is an element that affects bake hardenability and room-temperature aging resistance by reducing solid solution carbon through combination with carbon during hot rolling to precipitate NbC. When the Nb content is less than 0.00150%, almost no carbon precipitates as NbC, so most of the carbon in the steel remains as solid solution carbon; while this is advantageous for bake hardenability, it leads to a problem of reduced room-temperature aging resistance, which limits its application in parts. When the Nb content exceeds 0.0150%, most of the carbon in the steel precipitates as NbC, resulting in an absolute shortage of solid solution carbon; therefore, although room-temperature aging resistance may be advantageous, it is difficult to secure bake hardenability. Accordingly, the Nb content may have a range of 0.00150% to 0.0150%. It is more advantageous for the lower limit of the Nb content to be 0.0020%. It is more advantageous for the upper limit of the Nb content to be 0.010%.

[0065] B: 0.00010~0.020%

[0066] B is an element that improves hardenability to increase strength and suppresses the nucleation of grain boundaries. Typically, B has a higher tendency for grain boundary segregation compared to other elements, thereby preventing secondary processing brittleness by suppressing the excessive segregation of P at grain boundaries. Additionally, it increases interaction with dislocations during baking, enabling the securing of bake hardenability. If the content of B is less than 0.00010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of B exceeds 0.020%, deep drawingability may deteriorate. Therefore, the content of B may have a range of 0.00010% to 0.020%. It is more advantageous for the lower limit of the B content to be 0.00050%, and more advantageous for it to be 0.0010%. It is more advantageous for the upper limit of the B content to be 0.0150%.

[0067] Cu: 0.0010~1.0%

[0068] Cu is an element that stabilizes austenite and inhibits corrosion. In addition, the Cu is concentrated in the surface layer of the steel sheet, preventing hydrogen intrusion into the steel sheet and thus inhibiting hydrogen delayed fracture. Furthermore, it improves surface hardness and inhibits the deterioration of workability. If the Cu content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Cu content exceeds 1.0%, localized surface defects may occur in the hot-rolled material. Therefore, the Cu content may have a range of 0.0010% to 1.0%. The lower limit of the Cu content is more advantageous at 0.0030%, more advantageous at 0.0050%, and most advantageous at 0.0070%. The upper limit of the Cu content is more advantageous at 0.90%, more advantageous at 0.70%, and most advantageous at 0.50%.

[0069] Ni: 0.0010~1.0%

[0070] Ni is an element that stabilizes austenite and inhibits corrosion. In addition, the Ni is concentrated in the surface layer of the steel sheet, preventing hydrogen intrusion into the steel sheet and thus inhibiting hydrogen delayed fracture. Furthermore, it improves surface hardness and inhibits the deterioration of workability. If the Ni content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Ni content exceeds 1.0%, there may be disadvantages such as reduced formability and increased manufacturing costs. Therefore, the Ni content may have a range of 0.0010% to 1.0%. The lower limit of the Ni content is more advantageous at 0.0020%, more advantageous at 0.0030%, and most advantageous at 0.0040%. The upper limit of the Ni content is more advantageous at 0.90%, more advantageous at 0.70%, and most advantageous at 0.50%.

[0071] Cr: 0.0010~1.20%

[0072] Cr is an element that inhibits austenite decomposition during alloying treatment and stabilizes austenite similarly to Mn, and plays a role in increasing strength. In addition, it improves surface hardness and inhibits the deterioration of machinability. If the content of Cr is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Cr exceeds 1.20%, there may be disadvantages such as reduced formability and increased manufacturing costs. Therefore, the content of Cr may have a range of 0.0010% to 1.20%. The lower limit of the Cr content is more advantageous at 0.0030%, more advantageous at 0.0040%, and most advantageous at 0.0050%. The upper limit of the Cr content is more advantageous at 1.15%, more advantageous at 1.10%, and most advantageous at 1.05%.

[0073] Mo: 0.0010~1.0%

[0074] Mo is an element that inhibits austenite decomposition during alloying treatment and stabilizes austenite similarly to Mn, and plays a role in increasing strength. In addition, it also inhibits the deterioration of workability. If the content of Mo is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Mo exceeds 1.0%, there may be disadvantages such as reduced formability and increased manufacturing costs. Therefore, the content of Mo may have a range of 0.0010% to 1.0%. The lower limit of the Mo content is more advantageous at 0.0020%, more advantageous at 0.0030%, and most advantageous at 0.0040%. The upper limit of the Mo content is more advantageous at 0.90%, more advantageous at 0.70%, and most advantageous at 0.50%.

[0075] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0076] The cold-rolled steel sheet of the present invention may additionally contain 0.00050~0.50% of one or more of Sb and Sn.

[0077] At least one of Sb and Sn: 0.00050~0.50%

[0078] Sb and Sn are elements that improve the plating wettability and plating adhesion of steel sheets. In addition, they also suppress the deterioration of workability. It may be difficult to sufficiently obtain the aforementioned effects when the content of one or more of Sb and Sn is 0.00050%. If the content of one or more of Sb and Sn exceeds 0.50%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working. Therefore, the content of one or more of Sb and Sn may have a range of 0.00050% to 0.50%. The lower limit of the content of one or more of Sb and Sn is more advantageous at 0.0010%, more advantageous at 0.0020%, and most advantageous at 0.0030%. The upper limit of the content of one or more of the above Sb and Sn is more advantageous at 0.40%, more advantageous at 0.30%, and most advantageous at 0.20%.

[0079] The cold-rolled steel sheet of the present invention satisfies the aforementioned alloy composition and simultaneously satisfies the following Equation 1. Cu, Cr, and Ni are elements included as impurities (Tramp Elements) when iron scrap is used as a raw material. If the following (Cu+Ni) / Cr value is less than 0.050, bake hardenability decreases, which may affect the lowering of the bake hardenability temperature. If the following (Cu+Ni) / Cr value exceeds 0.50, aging resistance or formability may decrease. Therefore, the following (Cu+Ni) / Cr value may have a range of 0.050 to 0.50. The lower limit of the following (Cu+Ni) / Cr value is more advantageous at 0.055, more advantageous at 0.060, and most advantageous at 0.065. The upper limit of the (Cu+Ni) / Cr value below is more favorable at 0.45, more favorable at 0.40, and most favorable at 0.35.

[0080] [Relation 1] 0.050 ≤ (Cu+Ni) / Cr ≤ 0.50

[0081] The cold-rolled steel sheet of the present invention may contain cementite and the remainder ferrite in an area percentage of 5% or less (including 0%). In the present invention, low strength and high formability can be secured by including the ferrite. However, cementite may inevitably be formed during the manufacturing process, and if its fraction exceeds 5.0%, ductility and formability may be reduced. Of course, since it is advantageous to exclude the cementite as much as possible, its fraction may be 0%. It is more advantageous for the fraction of cementite to be 4.0% or less, more advantageous to be 2.50% or less, and most advantageous to be 1.0% or less.

[0082] The cold-rolled steel sheet of the present invention may have a total average content of Cu, Cr, and Ni in a region extending 0.3 μm from the surface in the thickness direction of 0.3 μm, which is 0.050 to 1.20 wt%. If the total average content of Cu, Cr, and Ni in the region extending 0.3 μm from the surface in the thickness direction of 0.3 μm is less than 0.050 wt%, it is difficult to secure sufficient surface hardness, which may result in reduced dent resistance. If the total average content of Cu, Cr, and Ni in the region extending 0.3 μm from the surface in the thickness direction of 0.3 μm exceeds 1.20 wt%, surface defects may occur during the manufacturing process or processing into parts. The lower limit of the total average content of Cu, Cr, and Ni in the region extending 0.3 μm from the surface in the thickness direction of 0.3 μm is more advantageous at 0.1 wt%, more advantageous at 0.125 wt%, and most advantageous at 0.15 wt%. The upper limit of the sum of the average contents of Cu, Cr, and Ni within the region up to 0.3 μm in the thickness direction from the surface is more advantageous at 1.185 wt%, more advantageous at 1.165 wt%, and most advantageous at 1.15 wt%. Meanwhile, although not specifically limited, in the present invention, the region up to 0.3 μm in the thickness direction from the surface can be defined as the surface layer.

[0083] The cold-rolled steel sheet of the present invention may satisfy the following relationship 2. If the value of (α+β) / γ below is less than 0.060, surface defects may occur during the manufacturing process, and the target bake hardenability may not be secured. If the value of (α+β) / γ below exceeds 0.60, surface defects may occur during the manufacturing process or during processing into parts. Therefore, the value of (α+β) / γ below may have a range of 0.060 to 0.60. The lower limit of the value of (α+β) / γ below is more advantageous at 0.065, more advantageous at 0.070, and most advantageous at 0.075. The upper limit of the value of (α+β) / γ below is more advantageous at 0.55, more advantageous at 0.50, and most advantageous at 0.45.

[0084] [Equation 2] 0.060 ≤ (α+β) / γ ≤ 0.60

[0085] (However, in the above Equation 2, α represents the average content of Cu in the region up to 0.3 μm in the thickness direction from the surface, β represents the average content of Ni in the region up to 0.3 μm in the thickness direction from the surface, and γ represents the average content of Cr in the region up to 0.3 μm in the thickness direction from the surface.)

[0086] The cold-rolled steel sheet of the present invention may have a yield strength (YS): 210~280MPa, a tensile strength (TS): 300~380MPa, an elongation (El): 38.0~50.0%, and a Lankford value (r): 1.30~1.80.

[0087] The cold-rolled steel sheet of the present invention may have an aging index value (AI) of 0 to 5.0%.

[0088] The cold-rolled steel sheet of the present invention may have a lower bake hardening value (lower BH) of 30 MPa or more at 140 to 170°C and an upper bake hardening value (upper BH) of 30 MPa or more at 120 to 170°C. In the present invention, the upper limit of the lower bake hardening value (lower BH) at 140 to 170°C is not specifically limited, but as an example, it may be 90 MPa. In the present invention, the upper limit of the upper bake hardening value (upper BH) at 120 to 170°C is not specifically limited, but as an example, it may be 90 MPa.

[0089] The cold-rolled steel sheet of the present invention may have a surface hardness of 120 to 160 Hv.

[0090] As described above, the cold-rolled steel sheet of the present invention can secure good bake hardness as well as excellent aging resistance and dent resistance, and can be used for various purposes including automotive parts. In particular, it can secure a good level of bake hardness value not only at 170°C, which is the standard temperature for measuring bake hardness values, but also at a low temperature of 120°C.

[0091] Although not specifically limited, the cold-rolled steel sheet of the present invention may have a surface treatment layer formed on at least one surface. The present invention does not specifically limit the type of the surface treatment layer, but as an example, it may be one or more of a zinc or zinc-based plating layer, a tin plating layer, and an enamel treatment layer.

[0092] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0093] First, a slab satisfying the aforementioned alloy composition and Equation 1 is prepared. The present invention does not specifically limit the process for preparing the slab, and any process commonly used in the relevant technical field may be utilized. For example, the slab may be manufactured through an electric furnace or a new blast furnace-converter process. As raw materials used in the electric furnace or the new blast furnace-converter process, only iron scrap may be used, or if the desired alloy composition cannot be obtained using only iron scrap, iron scrap and pig iron may be used together. Here, the pig iron refers to molten iron obtained from the existing blast furnace-converter process, its corrugate, or HBI (Hot Briquette Iron), etc.

[0094] When using an electric furnace, desulfurization treatment can be performed by ladle refining after tapping from the electric furnace, and then vacuum degassing treatment can be performed. During the above degassing treatment, an alloy can be added to control the process so that the desired alloy composition is achieved. While the RH method and DH method are generally used for the above vacuum degassing treatment, oxygen injection into the degassing tank can be performed in parallel. An example of the above oxygen injection method is the oxygen injection method using a top-blowing lance.

[0095] When using a casting method, a continuous casting method may be used to improve productivity.

[0096] Afterward, the above slab is heated to a slab heating temperature (HT) of 900 to 1300°C at a slab heating rate (HR) of 3 to 15°C / min, and then heated for a slab holding time (HS) of 10 to 90 minutes. If the above slab heating rate (HR) is less than 3°C / min, there may be a disadvantage in that productivity is reduced and a large amount of scale is generated due to prolonged heat exposure. If the above slab heating rate (HR) exceeds 15°C / min, there may be a disadvantage in that the slab temperature varies by location. The lower limit of the above slab heating rate (HR) is more advantageous at 3.5°C / min, more advantageous at 4.0°C / min, and most advantageous at 4.5°C / min. The upper limit of the above slab heating rate (HR) is more advantageous at 14.5°C / min, more advantageous at 14.0°C / min, and most advantageous at 13.5°C / min. If the above slab heating temperature (HT) is less than 900°C, the rolling load increases during hot rolling, which may reduce hot rolling stability. If the above slab heating temperature (HT) exceeds 1300°C, the workability and plating adhesion of the finally obtained steel sheet may decrease. The lower limit of the above slab heating temperature (HT) is more advantageous at 1000°C, more advantageous at 1050°C, and most advantageous at 1100°C. The upper limit of the above slab heating temperature (HT) is more advantageous at 1290°C, more advantageous at 1280°C, and most advantageous at 1270°C. If the above slab holding time (HS) is less than 10 minutes, the slab may not be sufficiently heated to the target temperature. If the above slab holding time (HS) exceeds 90 minutes, there may be disadvantages such as reduced productivity and the formation of a large amount of scale due to prolonged heat exposure. The lower limit of the above slab holding time (HS) is more advantageous at 15 minutes, more advantageous at 20 minutes, and most advantageous at 25 minutes. The upper limit of the above slab holding time (HS) is more advantageous at 85 minutes, more advantageous at 80 minutes, and most advantageous at 75 minutes.

[0097] When heating the above slab, the following Equation 3 can be satisfied. If the value of X below is less than 500, Cu, Ni, and Cr may not be concentrated in an appropriate amount in the surface layer, resulting in failure to secure the target aging resistance or a decrease in formability. If the value of X above exceeds 2000, Cu, Ni, and Cr may not be concentrated in an appropriate amount in the surface layer, resulting in surface defects during the manufacturing process or failure to secure the target bake hardening properties. The lower limit of the above X value is more advantageous at 530, more advantageous at 560, and most advantageous at 600. The upper limit of the above X value is more advantageous at 1970, more advantageous at 1940, and most advantageous at 1900.

[0098] [Relationship 3] 500 ≤ X = (HT+HS) / [{(Cu+Ni) / Cr}×HR] ≤ 2000

[0099] Subsequently, the heated slab is finished hot-rolled to obtain a hot-rolled steel sheet. The finish hot-rolling can be performed in either the austenite region above the Ar3 transformation point or in the two-phase region, which is a mixed state of austenite and ferrite below the Ar3 transformation point. As an example, the finish hot-rolling can be performed at 650 to 1050°C. If the finish hot-rolling temperature is below 650°C, there may be a disadvantage in that the rolling load increases excessively during hot-rolling. If the finish hot-rolling temperature exceeds 1050°C, thermal shock may be applied to the rolling rolls. The lower limit of the finish hot-rolling temperature is more advantageous at 700°C, more advantageous at 750°C, and most advantageous at 800°C. The upper limit of the above finishing hot rolling temperature is more advantageous at 1025℃, more advantageous at 1000℃, and most advantageous at 975℃.

[0100] Subsequently, the hot-rolled steel sheet is coiled. The coiling may be performed at 400 to 800°C. If the coiling temperature is below 400°C, strength and formability may decrease. If the coiling temperature exceeds 800°C, an excessively thick scale may form on the surface of the hot-rolled coil. It is more advantageous for the lower limit of the coiling temperature to be 450°C, and more advantageous for it to be 500°C. It is more advantageous for the upper limit of the coiling temperature to be 750°C.

[0101] Subsequently, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling may be performed with a cold reduction rate of 50% to 90%. If the cold reduction rate is less than 50%, the workability of the steel sheet may decrease. If the cold reduction rate exceeds 90%, the ductility may decrease. The lower limit of the cold reduction rate is more advantageous at 55%, more advantageous at 60%, and most advantageous at 65%. The upper limit of the cold reduction rate is more advantageous at 88%, more advantageous at 86%, and most advantageous at 84%.

[0102] Subsequently, the cold-rolled steel sheet is annealed. In the present invention, the annealing method is not specifically limited, and as an example, phase annealing or continuous annealing may be used. The annealing may be performed at 600 to 950°C for 10 to 60 seconds. If the annealing temperature is less than 600°C or the annealing time is less than 10 seconds, it may be difficult to secure excellent workability because recrystallization is not completed. If the annealing temperature exceeds 950°C or the annealing time exceeds 60 seconds, there may be disadvantages such as surface defects occurring or the microstructure becoming coarse. The lower limit of the annealing temperature is more advantageous at 650°C, more advantageous at 700°C, and most advantageous at 780°C. The upper limit of the annealing temperature is more advantageous at 935°C, more advantageous at 915°C, and most advantageous at 900°C. The lower limit of the above annealing time is more advantageous at 15 seconds, more advantageous at 20 seconds, and most advantageous at 25 seconds. The upper limit of the above annealing time is more advantageous at 55 seconds, more advantageous at 50 seconds, and most advantageous at 45 seconds.

[0103] Meanwhile, after the annealing, surface treatment may be performed on at least one surface of the cold-rolled steel sheet to improve chemical treatment properties, weldability, press formability, corrosion resistance, etc. In the present invention, the surface treatment method is not specifically limited, but as an example, it may be one or more of zinc or zinc-based plating, tin plating, and enamel treatment.

[0104] Subsequently, the annealed cold-rolled steel sheet is subjected to temper rolling. The temper rolling may be performed with an elongation of 0.50 to 2.0%. If the temper rolling elongation is less than 0.50%, sufficient dislocations are not formed, which is disadvantageous in terms of sheet shape, and there is a risk of plating surface defects occurring. If the temper rolling elongation exceeds 2.0%, material deterioration due to an excessive increase in dislocation density in the surface layer may occur, along with adverse effects such as sheet breakage due to equipment capacity limitations. The lower limit of the temper rolling elongation is more advantageous at 0.60%, more advantageous at 0.70%, and most advantageous at 0.80%. The upper limit of the temper rolling elongation is more advantageous at 1.90%, more advantageous at 1.80%, and most advantageous at 1.70%.

[0105] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0106] (Example)

[0107] After obtaining molten steel in an electric furnace using iron scrap and conventional blast furnace molten iron, RH degassing treatment (0.1 torr) was performed simultaneously with the addition of alloying elements, and continuous casting was carried out to prepare a slab having the alloy composition listed in Tables 1 and 2 below. The above slab was heated, hot-rolled, coiled, cold-rolled, annealed, and temper-rolled under the manufacturing conditions listed in Tables 3 and 4 below to produce a cold-rolled steel sheet.

[0108] The microstructure, surface layer enrichment, and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 5 to 7 below.

[0109] The types and fractions of the microstructure were measured using an SEM at 1000x magnification after taking a specimen from the cold-rolled steel sheet manufactured above and Nital etching the cross-section of the specimen.

[0110] At this time, the structure without irregularities was identified as ferrite, and the structure having a spherical or lamellar structure was identified as cementite.

[0111] Surface layer enrichment was measured on the surface layer of the above-manufactured cold-rolled steel sheet using a GDMS (Glow Discharge Mass Spectrometry) analysis device.

[0112] Among the mechanical properties, tensile strength (TS), yield strength (YS), and elongation (El) were measured by taking specimens based on the 0° direction relative to the rolling direction of the cold-rolled steel sheet manufactured above and performing a tensile test in accordance with JIS No. 5.

[0113] The Lankford value (r) among the mechanical properties was calculated by measuring the values ​​for the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the 45° angle to the rolling direction (D direction) using the 3-point method after 15% tensile pre-deformation of the above-mentioned cold-rolled steel sheet, and then using the following formula. In this invention, the magnitude of the r value is r D < r L < r C It appeared in order.

[0114] Rankford value (r) = (r L + 2r D + r C ) / 4

[0115] Among the mechanical properties, surface hardness was measured using a Vickers hardness tester, and the average value was calculated after measuring at five random locations. At this time, an indenter was positioned at a point w / 4 (where w represents the width in mm) in the width direction of each steel plate surface, and a load was applied to take the measurement. A pyramid-shaped diamond material was used as the indenter, and the applied load was 500g.

[0116] Among the mechanical properties, the bake hardening value (BH) and the aging index value (AI) were measured using JIS standards (JIS G 3135 and JIS G 3313, respectively), and the aging index value (AI) represents the yield point elongation (YP_El) × 10. Meanwhile, the bake hardening value (BH) was measured not only after 2% elongation and heat treatment at 170°C for 20 minutes according to the above JIS standards, but also after heat treatment at 140°C and 120°C for 20 minutes. The above bake hardening value (BH) represents the difference between the tensile test results obtained after the elongation and heat treatment and the tensile test results with only deformation applied. When comparing the difference in yield strength, the result with only deformation applied shows a yield value without upper or lower yield points, whereas the yield strength after heat treatment shows upper and lower yield points. At this time, the difference between the yield strength value of the specimen with only deformation and the lower yield point of the specimen after heat treatment is called lower BH, and the difference between the yield strength value of the specimen with only deformation and the upper yield point of the specimen after heat treatment is called upper BH. The tensile test at this time was conducted under the same conditions as the tensile test described above.

[0117] Steel Grade No. Alloy Composition (Wt%) CSiMnPSAlTiNA 0.00200.0200.150.020.0050.010.0010.0042 B 0.00190.0230.150.020.0050.010.0010.0039 C 0.00150.0220.150.020.0050.010.0010.0046 D 0.00140.0220.150.020.0050.010.0010.0044 E 0.00200.0300.140.020.0050.01 0.0010.0039F0.00200.0300.140.020.0050.010.0010.0046G0.00200.0300.140.020.0050.010.0010.0053XA0.00200.0200.1 50.020.0080.0440.0010.0015

[0118] Steel Grade No. Alloy Composition (Wt%) One or more of Cu, Cr, Ni, Nb, B, Mo, Sb, and Sn 1A0.0680.5370.0270.0030.00020.00450.0040.18B0.0670.9800.02 70.0030.00020.00440.0040.10C0.1020.5480.0390.0030.00020.00 560.0070.26D0.1060.9980.0390.0030.00030.00540.0070.15E0.10 90.5840.0440.0030.00030.00560.0100.26F0.1100.5850.0440.0030 .00030.00540.0200.26H0.1070.5800.0440.0030.00020.00530.0100.26G0.1140.7100.0450.0030.00030.00580.0100.22XA0.0090.0070.0050.0030.00020.0020.0022.00XB0.0690.0370.0270.0030.00020.00430.0042.59XC0.1030.0530.0390.0030.00020.00530.0072.68 1: (Cu+Ni) / Cr

[0119] Classification Steel Grade No. Slab Heating Rate (HR) (°C / min) Slab Heating Temperature (HT) (°C) Slab Holding Time (HS) (min) X Invention Example 1 A 7.31 200 50 951 Invention Example 2 B 7.31 200 50 1712 Invention Example 3 C 7.31 200 50 659 Invention Example 4 D 7.31 200 50 1142 Invention Example 5 E 7.31 200 50 659 Invention Example 6 F 7.31 200 50 659 Invention Example 7 G 7.31 200 50 778 Invention Example 8 A 6.81 170 40 989 Invention Example 9 B 6.81 17 0401779 Invention Example 10 C6.8117040684 Invention Example 11 D 6.81170401186 Invention Example 12 E 6.8117040684 Invention Example 13 F 6.8117040684 Invention Example 14 G 6.8117040809 Comparative Example 1 X A 7.312005086 Comparative Example 2 X B 7.312005066 Comparative Example 3 X C 7.312005064 Comparative Example 4 X A 6.811704089 Comparative Example 5 X B 6.811704069 Comparative Example 6 X C 6.811704066 X = (HT+HS) / [{(Cu+Ni) / Cr}×HR](wherein the value of X above, (Cu+Ni) / Cr is calculated to 2 significant figures.)

[0120] Classification Steel Grade No. Finish Hot Rolling Temperature (°C) Coiling Temperature (°C) Cold Reduction Rate (%) Annealing Temperature (°C) Annealing Time (Sec) Invention Example 1 A 9155 207880035 Invention Example 2 B 9155 207880035 Invention Example 3 C 9155 207880035 Invention Example 4 D 9155 207880035 Invention Example 5 E 9155 207880035 Invention Example 6 F 9155 207880035 Invention Example 7 G 9155 207880035 Invention Example 8 A 9156 507883035 Invention Example 9 B 9156 50788303 5 Invention Example 10C9156507883035 Invention Example 11D9156507883035 Invention Example 12E9156507883035 Invention Example 13F9156507883035 Invention Example 14G9156507883035 Comparative Example 1XA9155207880035 Comparative Example 2XB9155207880035 Comparative Example 3XC9155207880035 Comparative Example 4XA9156507883035 Comparative Example 5XB9156507883035 Comparative Example 6XC9156507883035

[0121] Classification Ferrite (Area %) Cementite (Area %) Sum of average contents of Cu, Cr, and Ni (weight %) within the region up to 0.3 μm in the thickness direction from the surface αβγ Formula 2 Invention Example 1 10000 0.59 0.085 0.035 0.47 0.26 Invention Example 2 1000 1.04 0.085 0.035 0.92 0.13 Invention Example 3 1000 0.68 0.13 0.055 0.49 0.38 Invention Example 4 1000 1.11 0.13 0.055 0.925 0.20 Invention Example 5 1000 0.72 0.13 0.055 0.53 0.35 Honorary 610000.720.130.0550.530.35 Invention Example 710000.860.150.0550.650.32 Invention Example 810000.590.0850.0350.470.26 Invention Example 910001.040.0850.0350.920.13 Invention Example 1010000.680.130.0550.490.38 Invention Example 1110001.110.130.0550.9250.20 Invention Example 1210000.720.130.0550.530.35 Invention Example 1310000.720.130.0550.530.35 Invention Example 1410000.860.150.0550.650.32 Comparative Example 110000.0220.0200.00210.0 Comparative Example 21 0000.1750.0850.0350.0552.18 Comparative Example 3 10000.2600.130.0550.0752.47 Comparative Example 4 10000.0220.0200.00210.0 Comparative Example 5 10000.1750.0850.0350.0552.18 Comparative Example 6 10000.2600.130.0550.0752.47 [Equation 2] (α+β) / γ (wherein in Equation 2 above, α represents the average content of Cu in the region up to 0.3 μm in the thickness direction from the surface, β represents the average content of Ni in the region up to 0.3 μm in the thickness direction from the surface, and γ represents the average content of Cr in the region up to 0.3 μm in the thickness direction from the surface.)

[0122] Classification TS(MPa) YS(MPa) El(%) r Surface Hardness (Hv) AI(%) Invention Example 1 3 2 4 2 4 1 4 5.2 1.3 5 1 4 0.3 4.9 Invention Example 2 3 3 7 2 6 0 4 5.4 1.3 1 5 2.7 3.8 Invention Example 3 3 3 4 2 5 3 4 5.1 1.3 4 1 4 8.2 4.8 Invention Example 4 3 4 3 2 6 8 4 3.7 1.3 0 1 5 7.9 3.2 Invention Example 534625740.41.32149.61.3 Invention Example 634424942.41.30144.30.6 Invention Example 734424642.11.33143.80.0 Invention Example 833324744.01.32144.25.0 Invention Example 933324344.81.53141.63.4 Invention Example 1032923045 .41.49 133.1 4.9 Invention Example 11 34 22 37 44.3 1.3 11 37.7 3.3 Invention Example 12 32 72 14 44.6 1.46 12 0.6 0.0 Invention Example 13 32 82 25 43.9 1.5 11 31.2 0.0 Invention Example 14 3 34 23 4 44.1 1.3 41 36.3 0.0 Comparative Example 13 0 52 11 5 0.41.5 0 18.20.0 Comparative Example 234126341.21.34154.742.3 Comparative Example 334227140.31.30159.934.7 Comparative Example 429819351.61.52105.30.0 Comparative Example 533624742.51.42144.239.7 Comparative Example 633925942.11.34152.128.0

[0123] Classification 170℃ 140℃ 120℃ Lower BH(MPa) Upper BH(MPa) Lower BH(MPa) Upper BH(MPa) Lower BH(MPa) Upper BH(MPa) Invention Example 1 486937673160 Invention Example 2 315830411731 Invention Example 3 456433542742 Invention Example 4 355530401932 Invention Example 5 495333393035 Invention Example 6 485240513850 Invention Example 7 415232462539 Invention Example 8 547049703856 Invention Example 9 415334542539 Invention Example 10 466546684 154 Invention Example 11 446032492131 Invention Example 12 435034363034 Invention Example 13 454942484146 Invention Example 14 384930452838 Comparative Example 13 2451328916 Comparative Example 26 29054844966 Comparative Example 35 48250814559 Comparative Example 43 24420281219 Comparative Example 56 77855694858 Comparative Example 66 07953604059

[0124] As can be seen from Tables 1 to 7 above, in the case of Invention Examples 1 to 14, the alloy composition and manufacturing conditions of the present invention are satisfied, thereby satisfying the microstructure composition and surface layer enrichment composition proposed by the present invention, and it can be seen that they possess the mechanical properties targeted by the present invention.

[0125] In the case of Comparative Examples 1 and 4, since the (Cu+Ni) / Cr and X values ​​are not satisfied, the surface layer thickening composition proposed by the present invention is not satisfied, and it can be seen that the surface hardness and bake hardening properties are insufficient. In particular, in the case of Comparative Example 4, it can be seen that the tensile strength and yield strength are also at a low level.

[0126] In the case of Comparative Examples 2, 3, 5, and 6, the (Cu+Ni) / Cr and X values ​​are not satisfied, so the surface layer enrichment composition proposed by the present invention is not satisfied, and it can be seen that the aging resistance is insufficient.

[0127] Figure 1 is a profile of the surface layer Cu content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Comparative Examples 1 to 3. Figure 2 is a profile of the surface layer Cr content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Comparative Examples 1 to 3. Figure 3 is a profile of the surface layer Ni content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Comparative Examples 1 to 3. Figure 4 is a profile of the surface layer Cu content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Inventive Examples 1 to 4. Figure 5 is a profile of the surface layer Cr content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Inventive Examples 1 to 4. Figure 6 is a profile of the surface layer Ni content measured using a GDMS (Glow Discharge Mass Spectrometry) analysis device for Inventive Examples 1 to 4. As can be seen from FIGS. 1 to 6, in the case of Inventive Examples 1 to 4, Cu, Cr, and Ni are all concentrated in appropriate amounts in the surface layer of the steel plate, whereas in the case of Comparative Examples 1 to 3, Cu, Cr, and Ni are concentrated at a low level in the surface layer of the steel plate.

Claims

In wt%, it contains C: 0.0010~0.0050%, Si: 0.0010~0.090%, Mn: 0.070~1.0%, Al: 0.0010~0.060%, P: 0.010~0.040%, S: 0.010% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.00050~0.030%, Nb: 0.00150~0.0150%, B: 0.00010~0.020%, Cu: 0.0010~1.0%, Ni: 0.0010~1.0%, Cr: 0.0010~1.20%, Mo: 0.0010~1.0%, and the remainder being Fe and other unavoidable impurities. It is accomplished, Satisfying the following relation 1, and The microstructure contains 5% or less (including 0%) of cementite and the remainder being ferrite in area percent, and Cold-rolled steel sheet having a total average content of Cu, Cr, and Ni of 0.050 to 1.20 wt% in a region from the surface to 0.3 μm in the thickness direction. [Relation 1] 0.050 ≤ (Cu+Ni) / Cr ≤ 0. 50 In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet additionally containing 0.00050~0.50% of one or more of Sb and Sn. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following relationship 2. [Equation 2] 0.060 ≤ (α+β) / γ ≤ 0.60 (However, in the above Equation 2, α represents the average content of Cu in the region up to 0.3 μm in the thickness direction from the surface, β represents the average content of Ni in the region up to 0.3 μm in the thickness direction from the surface, and γ represents the average content of Cr in the region up to 0.3 μm in the thickness direction from the surface.) In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a yield strength (YS): 210~280MPa, tensile strength (TS): 300~380MPa, elongation (El): 38.0~50.0%, and a Lankford value (r): 1.30~1.

80. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having an aging index value (AI) of 0 to 5.0%. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a lower bake hardening value (lower BH) of 30 MPa or more at 140~170℃ and an upper bake hardening value (upper BH) of 30 MPa or more at 120~170℃. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet with a surface hardness of 120 to 160 Hv. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having at least one surface treatment layer formed on at least one surface, selected from a zinc or zinc-based plating layer, a tin plating layer, and an enamel treatment layer. In wt%, it contains C: 0.0010~0.0050%, Si: 0.0010~0.090%, Mn: 0.070~1.0%, Al: 0.0010~0.060%, P: 0.010~0.040%, S: 0.010% or less (excluding 0%), N: 0.010% or less (excluding 0%), Ti: 0.00050~0.030%, Nb: 0.00150~0.0150%, B: 0.00010~0.020%, Cu: 0.0010~1.0%, Ni: 0.0010~1.0%, Cr: 0.0010~1.20%, Mo: 0.0010~1.0%, and the remainder being Fe and other unavoidable impurities. A step of preparing a slab that satisfies the following relationship 1, which is carried out; A step of heating the above slab to a slab heating temperature (HT) of 900 to 1300°C at a slab heating rate (HR) of 3 to 15°C / min, and then heating for a slab holding time (HS) of 10 to 90 minutes; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated slab; Step of winding the above hot-rolled steel sheet; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet; and The step of annealing the above cold-rolled steel sheet; is included, A method for manufacturing a cold-rolled steel sheet that satisfies the following relationship 3 when heating the above slab. [Relation 1] 0.050 ≤ (Cu+Ni) / Cr ≤ 0.50 [Relation 3] 500 ≤ X = (HT+HS) / [{(Cu+Ni) / Cr}×HR] ≤ 2000 In paragraph 8, A method for manufacturing a cold-rolled steel sheet in which the above slab additionally contains 0.00050~0.50% of one or more of Sb and Sn. In Paragraph 9, The above finishing hot rolling is a method for manufacturing cold-rolled steel sheets performed at 650 to 1050℃. In Paragraph 9, The above-mentioned coiling is a method for manufacturing cold-rolled steel sheets performed at 400 to 800°C. In Paragraph 9, The above cold rolling is a method for manufacturing a cold-rolled steel sheet, performed with a cold reduction rate of 50 to 90%. In Paragraph 9, A method for manufacturing cold-rolled steel sheets in which the above annealing is performed at 600 to 950°C for 10 to 60 seconds. In Paragraph 9, A method for manufacturing a cold-rolled steel sheet, wherein after the above-mentioned annealing, at least one surface treatment among zinc or zinc-based plating, tin plating, and enamel treatment is additionally performed on at least one surface of the cold-rolled steel sheet.

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