Cold-rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet and a manufacturing method therefor, wherein the cold-rolled steel sheet is suitable for use as a material for components requiring highly complex forming, such as outer panels, e.g., vehicle side outers and the like. A preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent formability and excellent material anisotropy so as to be uniformly deformed in all directions, and a manufacturing method therefor.

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 suitable for use as a material for parts subject to very complex forming, such as outer panels for automobile side outers, and a method for manufacturing the same.

[0002] As the design of automobile parts becomes more complex due to the recent increase in the size of automobiles and the emergence of electric vehicles, the demand for improved formability of automobile exterior materials is continuously increasing.

[0003] Ultra-low-carbon cold-rolled steel sheets are primarily used in areas requiring deep-drawing, such as automobile body panels. Consequently, elongation (El) and the Lankford value (r) are highly important. A higher r-value (plastic strain ratio) or elongation mitigates thickness reduction during press forming and increases the fracture limit, allowing for greater processing, thereby preventing fractures even when machining complex parts. Furthermore, the r-value and elongation anisotropy (Δr, ΔEl) are also crucial for component processing. A large Δr leads to significant earing during drawing, making large-scale deep drawing difficult. Furthermore, a high ΔEl increases the likelihood of defects such as wrinkles or processing cracks during press forming due to insufficient elongation in certain directions.

[0004] In general, to secure high deep drawability, IF steel is used in which carbide-forming elements such as Ti or Nb are added to remove dissolved carbon. When the dissolved carbon is removed, many ferrite grains in the {111} orientation are generated during the annealing process after cold rolling, thereby increasing the elongation and r-value. In addition, by coarsening the grains through cold rolling and high-temperature annealing after high-temperature coiling of the IF steel with the addition of Ti or Nb, the elongation and r-value can be increased by promoting the occurrence of recrystallized {111} texture. However, although the above technologies are representative technologies for improving the formability of steel, the development of technologies for securing better formability is necessary to satisfy the complex and diverse demands of recent automobile manufacturers.

[0005] To solve the above-mentioned issues, various methods have been proposed, such as improving the deep drawability of cold-rolled steel sheets by using grain refinement of hot-rolled steel sheets.

[0006] Patent Document 1 proposes a method of hot rolling at a finishing temperature equal to or higher than the Ar3 transformation point and equal to or lower than the Ar3 transformation point + 50°C, and at a final reduction ratio of 30% or more, and starting cooling immediately after hot rolling so that the average cooling rate for 3 seconds from the start is 60°C / s or more, and in particular, the average cooling rate for 1 second from the start is 80°C / s or more.

[0007] Patent Document 2 proposes a method of rolling steel containing more than 0.050% Ti and more than 0.0003% B for more than 5 passes in a dynamic recrystallization temperature range, then performing hot rolling at a finishing rolling temperature of (Ar3 point -20℃) or higher, and then starting rapid cooling treatment within 0.2 seconds after completion of hot rolling to coil at a coiling temperature of 600℃ or lower.

[0008] Patent Document 3 proposes a method of manufacturing a cold-rolled steel sheet by performing hot rolling to complete rolling at (Ar3 point -30℃) or higher, cooling to 750℃ at an average cooling rate of 400℃ / sec or higher within 0.5 seconds after completion of the hot rolling, coiling in a temperature range of 400℃ or higher but lower than 640℃ to obtain a hot-rolled steel sheet, and then pickling and cold rolling. In addition, it is disclosed that the cold-rolled steel sheet obtained by this method has the characteristics of TS-bar (average TS value by direction) of 335 MPa or less, r-bar of 1.70 or more, and |Δr| of 0.46 or less.

[0009] However, in Patent Document 1, a very high final reduction ratio of 30% or more is required during hot rolling to refine the grain size of the hot-rolled steel sheet. However, in this case, the rolling load is too high to be applied to existing equipment, and the resulting hot-rolled steel sheet is prone to having shape defects.

[0010] According to Patent Document 2, to stably refine the grains of hot-rolled steel sheets within the dynamic recrystallization temperature range, rolling must be performed in the range from the lower limit of dynamic recrystallization temperature to (the lower limit of dynamic recrystallization temperature + 80°C) for at least five passes and a total reduction ratio of at least 80%. However, this method requires very precise rolling temperature management and pass schedule control, making it very difficult to apply in actual production. Furthermore, the low coiling temperature of 600°C or less can lead to problems such as reduced ductility as the resulting grains become refined.

[0011] Patent Document 3 discloses that by limiting the time required for cooling to 750°C after hot rolling to within 0.5 seconds, a stable high r-value and low |Δr| value can be secured. However, Patent Document 3 has a drawback in that although it can secure an excellent r-value, the elongation is only at the level of 43-48%. In other words, the processing conditions of recent automobile parts have become more complex, and as a result, it is not able to satisfy the reality that stricter deep drawability is required for automobile body panels such as side outers. Therefore, it is necessary to set target values ​​of TS-bar, r-bar, |Δr|, and |ΔTS| higher than the levels intended to be obtained in Patent Document 3, and to attempt to achieve them.

[0012] Meanwhile, the formability of steel sheets is generally evaluated through elongation and r value, and Δr is used as an index to evaluate anisotropy. However, since the in-plane anisotropy that occurs when press-forming steel sheets appears in various patterns, it is difficult to respond to wrinkle defects caused by differences in strength and elongation in each direction in addition to the earing property evaluated by Δr with the above index. In addition, steel sheets are sometimes cut into blanks depending on the size of the part and press-formed in the longitudinal direction of the coil, but in many cases, the blanks are rotated in a direction perpendicular to the coil length and press-formed. In other words, even if the steel sheet is press-formed in various directions, there should be no problems during processing, and to this end, it is necessary to review improvements in not only Δr but also ΔTS, ΔEl, etc.

[0013] [Prior Art Literature]

[0014] [Patent Document]

[0015] (Patent Document 1) Japanese Patent Application Laid-Open No. 1993-112831

[0016] (Patent Document 2) Japanese Patent Application Laid-Open No. 2000-239786

[0017] (Patent Document 3) Japanese Patent Application Laid-Open No. 2010-077513

[0018] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0019] A preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent material anisotropy and enabling uniform deformation in all directions, and a method for manufacturing the same.

[0020] One embodiment of the present invention provides a cold-rolled steel sheet comprising, in wt%, carbon (C): 0.0030% or less (excluding 0%), manganese (Mn): 0.20% or less (excluding 0%), silicon (Si): 0.10% or less (excluding 0%), phosphorus (P): 0.0150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), nitrogen (N): 0.0030% or less (excluding 0%), acid-soluble aluminum (sol.Al): 0.010 to 0.050%, titanium (Ti): 0.030 to 0.070%, niobium (Nb): 0.0010 to 0.010%, the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1 and satisfying the following relational expression 2.

[0021] [Relationship 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32) > 2.0

[0022] [Relationship 2] [X(222)+X(554)] / [X(200)+X(110)+X(112)] > 4.0

[0023] (However, in the above relational expression 1, Ti, Nb, C, N and S represent the content (weight %) of each alloying component, and in the above relational expression 2, X(222), X(554), X(200), X(110) and X(112) represent the X-ray diffraction integrated intensity ratios of the {222} plane, {554} plane, {200} plane, {110} plane and {200} plane parallel to the plane at a position of 1 / 4t (t: steel plate thickness) of the alloyed hot-dip galvanized steel sheet, respectively.)

[0024] The above cold rolled steel sheet may have an average grain size of 25 to 40 μm.

[0025] The standard deviation of the grain size of the above cold-rolled steel sheet may be 12 or less.

[0026] The above cold rolled steel sheet may have a microstructure composed of a ferrite main structure and other inevitable impurity structures.

[0027] The above cold rolled steel sheet may have a yield strength (YS): 170 MPa or less, a tensile strength (TS): 290 MPa or less, an elongation (El): 48% or more, a tensile strength deviation (ΔTS): 10 MPa or less, and an elongation deviation (ΔEl): 2% or less.

[0028] The above cold rolled steel sheet may have an average Rank Ford value (r-bar): 1.9 or more, and a Rank Ford value deviation (Δr): 0.6 or less.

[0029] Another embodiment of the present invention comprises a step of heating a slab at 1100 to 1250°C, which comprises, in wt%, carbon (C): 0.0030% or less (excluding 0%), manganese (Mn): 0.20% or less (excluding 0%), silicon (Si): 0.10% or less (excluding 0%), phosphorus (P): 0.0150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), nitrogen (N): 0.0030% or less (excluding 0%), acid-soluble aluminum (sol.Al): 0.010 to 0.050%, titanium (Ti): 0.030 to 0.070%, niobium (Nb): 0.0010 to 0.010%, the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1; A method for manufacturing a cold rolled steel sheet is provided, comprising: a step of finishing hot rolling the heated slab at 900 to 980°C to obtain a hot rolled steel sheet; a step of cooling the hot rolled steel sheet to 700 to 780°C and then coiling it; a step of cold rolling the coiled hot rolled steel sheet at a cold reduction ratio (CR) satisfying the following equations 3 and 4 to obtain a cold rolled steel sheet; and a step of continuously annealing the cold rolled steel sheet at 840 to 870°C.

[0030] [Relationship 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32) > 2.0

[0031] [Relationship 3] CR > CR1 = 95-4(0.004SS+7Ti+15Nb)

[0032] [Relationship 4] CR < CR2 = 76+4(0.004SS+7Ti+15Nb)

[0033] (However, in the above relational expression 1, Ti, Nb, C, N, and S represent the content (weight %) of each alloying component, and in the above relational expressions 3 and 4, SS represents the annealing temperature, and Ti and Nb represent the content (weight %) of each alloying component.)

[0034] After the above continuous annealing step, a step of immersing the continuously annealed steel sheet in a molten zinc plating bath at 440 to 500°C to obtain a molten zinc-coated steel sheet may be additionally included.

[0035] After the step of obtaining the above-mentioned hot-dip galvanized steel sheet, a step of alloying the above-mentioned hot-dip galvanized steel sheet at 450 to 540°C to obtain an alloyed hot-dip galvanized steel sheet may be additionally included.

[0036] After the step of obtaining the above-mentioned alloyed galvanized steel sheet, a step of subjecting the above-mentioned alloyed galvanized steel sheet to temper rolling at a reduction ratio of 0.20 to 1.0% may be additionally included.

[0037] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0038] According to a preferred aspect of the present invention, a cold-rolled steel sheet having excellent material anisotropy and capable of uniform deformation in all directions and a method for manufacturing the same can be provided.

[0039] Figure 1 illustrates cold rolling reduction conditions according to one embodiment of the present invention.

[0040] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition will be described. The alloy composition described below refers to weight percent unless otherwise specified.

[0041] Carbon (C): 0.0030% or less (excluding 0%)

[0042] Carbon (C) is an interstitial solid solution element that reduces ductility and deep drawability. Normally, in order to secure excellent formability, it is necessary to completely remove solid solution C using carbonitride forming elements such as Ti and Nb. However, if the C content exceeds 0.0030%, the content of Ti or Nb added to remove solid solution C in the steel increases excessively, which may result in an increase in manufacturing cost and a deterioration in plating quality. Meanwhile, in the present invention, it is preferable that the C content be as low as possible, so there is no need to specifically specify the lower limit thereof. However, since an excessive decrease in the C content may result in a significant increase in refining cost, the C content may be 0.00050% or more. The C content is more advantageously 0.00280% or less, and even more advantageously 0.00250% or less.

[0043] Manganese (Mn): 0.20% or less (excluding 0%)

[0044] Manganese (Mn) generally contributes to increasing the strength of steel through solid solution strengthening, and also prevents the red-hot embrittlement of steel caused by the formation of FeS by fixing S as MnS. However, in ultra-low carbon steel with added Ti, since Ti fixes S to form Ti-based sulfides, Mn remains as solid solution Mn, which increases the strength of the steel and is highly likely to cause a deterioration in ductility. Therefore, in the present invention, the upper limit of the Mn content is limited to 0.20%. Meanwhile, in the present invention, in order to secure the desired ductility and deep drawability, the solid solution elements in the steel should be reduced as much as possible, and therefore, it is advantageous to lower the Mn content as much as possible. Therefore, there is no need to specifically specify the lower limit of the Mn content, and 0% is excluded in consideration of the amount inevitably added during the manufacturing process. The Mn content is more advantageously 0.180% or less, and even more advantageously 0.150% or less.

[0045] Silicon (Si): 0.10% or less (excluding 0%)

[0046] Silicon (Si) is a representative oxidizing element and exists in the form of oxide on the surface from the hot rolling stage, forming Si concentration on the surface. If the Si content exceeds 0.10%, it is not easy to completely remove the surface oxide even if pickling is performed before cold rolling, and even if all Si oxide is removed by an acid solution, the Si concentration layer formed on the surface causes surface defects to be generated after hot-dip galvanizing. Therefore, in the present invention, it is preferable to control the Si content to 0.10% or less. Meanwhile, in the present invention, it is advantageous to lower the Si content as much as possible. Therefore, there is no need to specifically specify the lower limit of the Si content, and 0% is excluded in consideration of the amount inevitably added in the manufacturing process. The Si content is more advantageously 0.080% or less, and even more advantageously 0.050% or less.

[0047] P: 0.0150% (excluding 0%)

[0048] Phosphorus (P) is generally the element with the best solid solution strengthening effect in ultra-low carbon steel, and is effective in securing the strength of steel without significantly impairing drawability. In particular, P is easily segregated at grain boundaries, inhibiting grain growth during annealing and thus playing a role in refining the grains. However, in the present invention, it is preferable to lower the P content as much as possible in order to obtain grains having an average size of 25㎛ or more. If the P content exceeds 0.0150%, the average grain size desired in the present invention cannot be secured, and tensile properties such as elongation may deteriorate. Meanwhile, it is advantageous in the present invention to lower the P content as much as possible. Therefore, there is no need to specifically specify the lower limit of the P content, and 0% is excluded in consideration of the amount inevitably added in the manufacturing process. It is more advantageous for the P content to be 0.0130% or less.

[0049] Sulfur (S): 0.010% or less (excluding 0%)

[0050] Sulfur (S) is an impurity that is inevitably included in steel, and it is desirable to control its content as low as possible. In particular, since S increases the possibility of causing red-hot embrittlement when the content exceeds 0.010%, it is desirable that the content of S be in the range of 0.010% or less. Meanwhile, in the present invention, there is no need to specifically specify the lower limit of the S content, and 0% is excluded in consideration of the amount inevitably added during the manufacturing process. It is more advantageous for the S content to be 0.0080% or less.

[0051] Nitrogen (N): 0.0030% or less (excluding 0%)

[0052] Nitrogen (N) is an impurity that is inevitably included in steel, and it is desirable to control its content as low as possible. However, there is a problem that the cost of steel refining increases rapidly when the N content is controlled very low, so the upper limit is limited to 0.0030% or less. Meanwhile, in the present invention, there is no need to specifically specify the lower limit of the N content, and 0% is excluded in consideration of the amount inevitably added during the manufacturing process. The N content is more advantageously 0.00250% or less, and even more advantageously 0.0020% or less.

[0053] Acid-soluble aluminum (sol.Al): 0.010~0.050%

[0054] Acid-soluble aluminum (sol.Al) is an element added for grain refinement and deoxidation. If the sol.Al content is less than 0.010%, aluminum-killed steel cannot be manufactured in a normal, stable state. On the other hand, if the sol.Al content exceeds 0.050%, excessive inclusions may be formed during steelmaking / casting operations, which may significantly reduce ductility. Therefore, in the present invention, the sol.Al content is preferably in the range of 0.010 to 0.050%. The lower limit of the sol.Al content is more advantageously 0.0150%, and the upper limit of the sol.Al content is more advantageously 0.020%. The upper limit of the sol.Al content is more advantageously 0.0450%.

[0055] Titanium (Ti): 0.030~0.070%

[0056] Titanium (Ti) has the function of improving deep drawability and ductility by reducing solid carbon and solid nitrogen by forming carbonitride. If the content of Ti is less than 0.030%, it may be difficult to obtain the properties desired in the present invention. On the other hand, if the content of Ti exceeds 0.070%, problems such as nozzle clogging during steelmaking occur along with an increase in inclusions such as Ti oxide due to excessive Ti. In addition, excessive Ti reduces powdering, thereby deteriorating the plating quality. Therefore, the content of Ti is preferably in the range of 0.030 to 0.070%. The lower limit of the Ti content is more advantageously 0.0350%, and the upper limit of the Ti content is more advantageously 0.040%. The upper limit of the Ti content is more advantageously 0.0650%.

[0057] Niobium (Nb): 0.0010~0.010% (excluding 0%)

[0058] Niobium (Nb) forms carbonitrides, thereby reducing solid solution carbon and solid solution nitrogen, thereby improving deep drawability and ductility. In addition, it has the effect of suppressing recrystallization of austenite, thereby playing a role in refining the grains of the hot-rolled sheet. If the content of Nb is less than 0.0010%, it may be difficult to sufficiently obtain the above-described effect. On the other hand, if the content of Nb exceeds 0.010%, it may be difficult to obtain the average grain size of 25㎛ or more that the present invention seeks to obtain, and thus, there is a high possibility that the problem of low elongation may occur. Therefore, in the present invention, the content of Nb is preferably in the range of 0.0010 to 0.010%. The lower limit of the Nb content is more advantageously 0.00150%, and even more advantageously 0.0020%. The upper limit of the above Nb content is more advantageously set at 0.00950%, and even more advantageously at 0.0090%.

[0059] The remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0060] It is preferable that the cold-rolled steel sheet of the present invention satisfy the above-described alloy composition and also satisfy the following relational expression 1.

[0061] [Relationship 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32) > 2.0

[0062] (However, in the above relational expression 1, Ti, Nb, C, N, and S represent the content (weight%) of each alloy component.)

[0063] The left side of the above relational expression 1 is a parametric expression representing the equivalence ratio of Ti and Nb to C, N, and S, which combine with Ti and Nb to form a compound. The higher the value of the left side, the less solid solution elements there are in the steel, so excellent formability can be expected. In addition, it can also promote the development of a texture that improves deep drawability. If the above relational expression 1 is not satisfied, it may be difficult to sufficiently obtain the above-described effect. Meanwhile, in the present invention, since the smaller the solid solution elements, the more advantageous it is, the upper limit of the left side value of the above relational expression 1 is not particularly limited. The upper limit of the left side value of the above relational expression 1 can be determined by the upper limit of the Ti and Nb contents mentioned above in the present invention.

[0064] It is preferable that the cold rolled steel sheet of the present invention satisfies the following relational expression 2.

[0065] [Relationship 2] [X(222)+X(554)] / [X(200)+X(110)+X(112)] > 4.0

[0066] (However, in the above relational expression 2, X(222), X(554), X(200), X(110), and X(112) mean the X-ray diffraction integrated intensity ratios of the {222} plane, {554} plane, {200} plane, {110} plane, and {200} plane, which are parallel to the plane at a position of 1 / 4t (t: steel plate thickness) of the steel plate, respectively.)

[0067] In terms of formability, it is known that the more Gamma textures, such as {111} and {554} planes parallel to the plate surface of a steel plate having a

[0111] texture, the higher the r value, and the more Alpha textures, such as {100} planes, {110} planes, and {112} planes parallel to the plate surface, the lower the r value. The above relational expression 2 is an expression for improving in-plane anisotropy. According to the present inventors, it was confirmed that in the case of a steel plate in which the X-ray diffraction integrated intensity ratios of the {222} plane, {554} plane, {200} plane, {110} plane and {112} plane parallel to the plane at a position of 1 / 4t (t: steel plate thickness) of the steel plate, i.e., X(222), X(554), X(200), X(110) and X(112), satisfy the condition of the above relational expression 2, an excellent in-plane anisotropy exhibiting Δr of 0.6 or less can be secured. Here, the X-ray diffraction integrated intensity ratio refers to a relative intensity based on the X-ray diffraction integrated intensity of a non-directional standard sample. In addition, the value of the left side of the above relational expression 2 can be obtained using a conventional X-ray diffraction apparatus such as an energy dispersive type.

[0068] The average grain size of the cold-rolled steel sheet of the present invention may be 25 to 40 μm. In addition, the standard deviation of the grain size may be 12 or less. The anisotropy indices ΔTS and ΔEl to be obtained in the present invention are greatly affected by the microstructural characteristics. In addition, in order to secure the mechanical properties to be obtained in the present invention, such as a tensile strength (TS) of 290 MPa or less and an elongation (El) of 48% or more, the average grain size must be increased, and in order to improve the directional anisotropy of strength and elongation, such as a tensile strength deviation (ΔTS) of 10 MPa or less and an elongation deviation (ΔEl) of 2% or less, the grains must be distributed as uniformly as possible. If the average grain size is less than 25 μm or the standard deviation of the grain size exceeds 12, it may be difficult to secure the mechanical properties to be obtained in the present invention. Meanwhile, if the average crystal grain size exceeds 40㎛, a mixed grain structure may occur, which may actually lower the material anisotropy.

[0069] The present invention aims to reduce material anisotropy in each direction. In-plane anisotropy in automotive steel sheets is one of the most important factors in press processing. No matter how excellent the formability is, a material with high anisotropy increases earing during press processing and causes material loss due to defects such as wrinkles. Previously, Δr was used as an index representing anisotropy, but in the present invention, ΔTS and ΔEl are used together to prevent processing defects due to material anisotropy even in more complex forming modes. The measurement methods for each are as follows.

[0070] Δr = (r0- 2r 45 + r 90 ) / 2

[0071] (However, the above r0, r 45 , r 90 (represents the Lankford value (r value) in the 0°, 45°, and 90° directions during the tensile test, respectively)

[0072] ΔTS = (TS0- 2TS 45 + TS 90 ) / 2

[0073] (However, the above TS0, TS 45 , TS 90 (Represents the tensile strength (TS) in the 0°, 45°, and 90° directions during the tensile test, respectively.)

[0074] ΔEl = (El0- 2El 45 + El 90 ) / 2

[0075] (However, the above El0, El 45 , El 90 (Represents the elongation (El) in the 0°, 45°, and 90° directions during the tensile test, respectively.)

[0076] Meanwhile, in the above tensile test, the 0°, 45°, and 90° directions refer to the 0°, 45°, and 90° directions, respectively, based on the rolling direction.

[0077] Meanwhile, the cold-rolled steel sheet of the present invention may have a microstructure composed of a ferrite main structure and other unavoidable impurity structures. More specifically, since the ferrite is advantageous for securing formability, the microstructure is theoretically preferably composed of 100% ferrite. However, the present invention may include ferrite as the main structure, as well as impurity structures inevitably formed during the manufacturing process. The present invention does not specifically limit the type or fraction of the impurity structures.

[0078] As described above, the cold rolled steel sheet of the present invention may have a yield strength (YS): 170 MPa or less, a tensile strength (TS): 290 MPa or less, an elongation (El): 48% or more, an average Rankford value (r-bar): 1.9 or more, a Rankford value deviation (Δr): 0.6 or less, a tensile strength deviation (ΔTS): 10 MPa or less, and an elongation deviation (ΔEl): 2% or less, thereby ensuring excellent formability and material anisotropy in each direction. Meanwhile, in the present invention, the lower the yield strength, the tensile strength, the Rankford value deviation, the tensile strength deviation, and the elongation deviation, the more advantageous it is, and therefore, the lower limits thereof are not particularly limited, and the higher the elongation and the average Rankford value, the more advantageous it is, and therefore, the upper limits thereof are not particularly limited.

[0079] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0080] First, a slab satisfying the aforementioned alloy composition and relational expression 1 is heated at 1100 to 1250°C. If the heating temperature of the slab is lower than 1100°C, slab inclusions, etc. may not be sufficiently re-dissolved, which may cause material deviation and surface defects after hot rolling. On the other hand, if the heating temperature of the slab exceeds 1250°C, problems such as a decrease in strength or an increase in anisotropy may occur due to excessive growth of austenite grains. Therefore, the heating temperature of the slab may be 1100 to 1250°C. The lower limit of the heating temperature of the slab is more advantageously 1120°C, and the upper limit of the heating temperature of the slab is more advantageously 1150°C. The upper limit of the heating temperature of the slab is more advantageously 1200°C.

[0081] Thereafter, the heated slab is subjected to a finish hot rolling at 900 to 980°C to obtain a hot-rolled steel sheet. If the finish hot rolling temperature is lower than 900°C, ferrite transformation may occur during rolling, creating an elongated structure, which may increase the material anisotropy of the steel sheet. On the other hand, if the finish hot rolling temperature exceeds 980°C, the austenite grain size may become non-uniformly coarse, creating a mixed grain structure, and the surface quality may deteriorate due to high-temperature work. Therefore, the finish hot rolling temperature is preferably 900 to 980°C. It is more advantageous for the lower limit of the finish hot rolling temperature to be 910°C. It is more advantageous for the upper limit of the finish hot rolling temperature to be 970°C, more advantageously 960°C, and most advantageously 950°C.

[0082] Afterwards, the hot-rolled steel sheet is cooled to 700 to 780°C and then coiled. High-temperature coiling is very important in the present invention, as hot-rolled carbides can be coarsened through high-temperature coiling, and these coarse carbides promote grain growth, leading to an increase in elongation and r-value. If the coiling temperature is lower than 700°C, the grain size desired in the present invention cannot be secured, which may cause problems such as reduced ductility. On the other hand, if the coiling temperature exceeds 780°C, the grain size increases, but the excessively high coiling temperature increases oxides of the hot-rolled steel sheet, such as Mn and Si. Therefore, even if a subsequent pickling process is performed, some oxides may remain, or even if the oxides are completely removed, concentrated compounds may form on the surface of the steel sheet, which may cause surface defects during plating. In addition, a deterioration in pickling properties is expected due to a high coiling temperature. Therefore, the coiling temperature is preferably in the range of 700 to 780°C. The lower limit of the above coiling temperature is more advantageously 710°C, more advantageously 720°C, and most advantageously over 750°C.

[0083] Thereafter, the coiled hot-rolled steel sheet is cold-rolled at a cold reduction ratio (CR) that satisfies the following relationships 3 and 4 to obtain a cold-rolled steel sheet. When the cold reduction ratio (CR) is CR1 or less, the grains become unevenly coarse, which increases material anisotropy and thus the conditions of ΔTS of 10 MPa or less and ΔEl of 2% or less that the present invention intends to obtain cannot be satisfied. On the other hand, when the cold reduction ratio (CR) is CR2 or more, the material anisotropy that the present invention intends to obtain can be secured, but the grains become fine due to the excessive cold reduction ratio, and cold-rolling loads such as cracks occurring at the edge of the steel sheet may be induced. Meanwhile, Fig. 1 illustrates the cold reduction ratio conditions suggested by the present invention.

[0084] [Relationship 3] CR > CR1 = 95-4(0.004SS+7Ti+15Nb)

[0085] [Relationship 4] CR < CR2 = 76+4(0.004SS+7Ti+15Nb)

[0086] (However, in the above equations 3 and 4, SS represents the annealing temperature, and Ti and Nb represent the content (weight%) of each alloy component.)

[0087] Thereafter, the cold-rolled steel sheet is continuously annealed at 840 to 870°C. In the present invention, in order to satisfy an El of 48% or more, an r-bar of 1.9 or more, and excellent material anisotropy, annealing at a high temperature is necessary. When the annealing temperature is less than 840°C, recrystallization of the steel is complete, but the structure generated by the cold reduction ratio according to Equations 3 and 4 is not sufficiently recrystallized after nucleation, making it difficult to secure an average grain size of 25 μm or more. On the other hand, when the annealing temperature exceeds 870°C, the possibility of equipment trouble occurring in the field may greatly increase due to the excessively high annealing temperature. In addition, the grains may become excessively coarse, which may cause a mixed grain structure and lower the material anisotropy. In addition, the increase in surface oxides may cause surface defects to occur in the alloyed hot-dip galvanized layer during subsequent alloyed hot-dip galvanizing. Therefore, the continuous annealing temperature may be 840 to 870°C.

[0088] After the continuous annealing step, a step of immersing the continuously annealed steel sheet in a molten zinc plating bath at 440 to 500°C to obtain a molten zinc-plated steel sheet may be additionally included. If the temperature of the molten zinc plating bath is lower than 440°C, sufficient alloying may not be achieved due to reduced fluidity of the plating bath. If the temperature of the molten zinc plating bath exceeds 500°C, excessive alloying may result in reduced powdering properties and plating defects such as dross, thereby deteriorating the plating quality. Therefore, the temperature of the molten zinc plating bath may be 440 to 500°C. It is more advantageous that the lower limit of the temperature of the molten zinc plating bath is 450°C. It is more advantageous that the upper limit of the temperature of the molten zinc plating bath is 490°C.

[0089] After the step of obtaining the above-described hot-dip galvanized steel sheet, a step of alloying the hot-dip galvanized steel sheet at 450 to 540°C to obtain an alloyed hot-dip galvanized steel sheet may be additionally included. If the alloying temperature is lower than 450°C, an unplated area may occur across the entire width of the steel sheet. On the other hand, if the alloying temperature exceeds 540°C, a large amount of brittle Fe-Zn intermetallic compounds (Γ) are formed due to excessive alloying, resulting in poor powdering characteristics. Therefore, the alloying temperature may be 450 to 540°C. The lower limit of the alloying temperature is more advantageously 460°C. The upper limit of the alloying temperature is more advantageously 530°C, and 520°C is further advantageous.

[0090] After the step of obtaining the above-described alloyed hot-dip galvanized steel sheet, a step of performing temper rolling on the above-described alloyed hot-dip galvanized steel sheet at a reduction ratio of 0.20 to 1.0% may be additionally included. If the reduction ratio during the temper rolling is less than 0.2%, it is disadvantageous in terms of plate shape control, and there is a risk of the occurrence of plating surface defects such as dross. On the other hand, if the reduction ratio during the temper rolling exceeds 1.0%, the yield strength may increase, the elongation may decrease slightly, and the material may deteriorate. Therefore, the reduction ratio during the temper rolling may be 0.20 to 1.0%. It is more advantageous when the lower limit of the reduction ratio during the temper rolling is 0.30%. It is more advantageous when the upper limit of the reduction ratio during the temper rolling is 0.80%, 0.60% is further advantageous, and 0.50% is most advantageous.

[0091] 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 further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0092] (Example)

[0093] A slab having the alloy composition described in Table 1 below was heated at 1200°C and then finish-hot rolled under the conditions described in Table 2 below to manufacture a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was cooled to 750°C, coiled, and cold-rolled under the cold reduction ratio conditions described in Table 2 below to manufacture a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was continuously annealed under the conditions described in Table 2 below. The microstructure, X-ray diffraction integrated intensity ratio (Equation 2), and mechanical properties of the cold-rolled steel sheet thus manufactured were measured, and the results are shown in Table 3 below. Thereafter, the steel sheet was immersed in a hot-dip galvanizing bath at 460°C to obtain a hot-dip galvanized steel sheet, and then alloyed at 520°C to manufacture an alloyed hot-dip galvanized steel sheet. Thereafter, the alloyed hot-dip galvanized steel sheet was subjected to temper rolling at a reduction ratio of 0.4%. The powdering properties of the alloyed hot-dip galvanized steel sheet manufactured in this manner were measured, and the results are shown in Table 3 below.

[0094] The type and fraction of microstructure were measured by photographing an arbitrary point at 200x magnification using SEM (Scanning Electron Microscopy) at a 1 / 4 thickness location of the cold-rolled steel plate, and the microstructure of all steel types was 100% ferrite.

[0095] The average grain size and standard deviation of the grain size of the microstructure were measured by photographing an arbitrary point at 200x magnification using SEM (Scanning Electron Microscopy) at a 1 / 4 thickness location of the cold-rolled steel sheet, and then analyzing the crystal orientation using an EBSD (Electron Back Scattering Diffraction) analysis device included in the SEM device.

[0096] The X-ray diffraction integrated intensity ratio (Formula 2) was measured at a 1 / 4 thickness position of the cold-rolled steel sheet, and the intensity ratio of each component represents the relative ratio of the X-ray diffraction integrated intensity of each component based on the X-ray diffraction integrated intensity of a non-oriented standard sample.

[0097] Among the mechanical properties, yield strength (YS), tensile strength (TS), and elongation (El) were measured by taking tensile test specimens of JIS-5 standard from the cold-rolled steel sheet and performing a tensile test on the tensile test specimens in the rolling direction. The average Rankford value (r-bar) was measured using the tensile test specimens according to the JIS Z2241 standard. Δr, ΔTS, and ΔEl, which indicate material anisotropy, were calculated using the r values, TS, and El in the directions of 0°, 45°, and 90° relative to the rolling direction during the tensile test.

[0098] Powdering properties were evaluated by attaching tape to the surface of a specimen cut from the above-mentioned alloyed hot-dip galvanized steel sheet to a length (L) of 60 mm and a width (W) of 30 mm, performing a 60° bending test on the specimen, removing the tape from the specimen, and measuring the width of the Zn powder attached to the tape. Typically, if the width of the Zn powder is 4 mm or less, the plating properties can be evaluated as good, and if it exceeds 4 mm, it can be evaluated as poor.

[0099] Steel grade No. Alloy composition (weight %) CSiMnPSSol.AlTiNbN formula 1. Invention steel 10.00110.020.100.0110.0060.0210.0500.0030.00202.54. Invention steel 20.00120.020.110.0080.0050.0340.0550.0050.00203.01. Invention steel 30.00120.040.070.0050.0040.0450.0610.0040.00153.96. Invention steel 40.00190.010.060.0090.0040.0430.0580.0060.00103.59. Invention steel 50.00130.020.050.0130 .0060.0420.0550.0050.00152.98Inventive steel 60.00190.030.040.0110.0040.0370.0530.0020.00182.73Inventive steel 70.00180.020.110.0090.0070.0330.0650.0060.00133.07Comparative steel 10.00190.020.150.0350.0020.0330.0630.0060.00203.79Comparative steel 20.00150.500.160.0090.0040.0350.0480.0070.00212 .69Comparative steel 30.00130.010.070.0050.0070.0450.0100.0090.00280.58Comparative steel 40.00110.030.080.0050.0050.0460.0550.0550.00115.32Inventive steel 80.00090.040.0150.0090.0030.0360.0500.0050.00153.97Comparative steel 50.00170.020.150.0100.0040.04800.0050.00210.13Comparative steel 60.00180.010.070.0090. 0050.0460.0600.0020.00651.65Comparative steel70.00110.040.580.0130.0060.0250.0500.0070.00152.89Comparative steel80.00510.030.060.0120.0050.0350.0500.0050.00141.61Comparative steel90.00230.040.170.0120.0050.0440.0900.0040.00114.50Inventive steel90.00160.030.090.0080.0040.0360.0600.0040.00213.17[Formula 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32).

[0100] Classification Steel grade No. Finishing Hot rolling temperature (℃) Cold reduction ratio (CR) (%) CR1CR2 Continuous annealing temperature (℃) Invention example 1 Invention steel 1 9308179.891.2850 Invention example 2 Invention steel 2 9208479.591.5855 Invention example 3 Invention steel 3 9258579.391.7860 Invention example 4 Invention steel 4 9208279.591.5845 Comparative example 1 Invention Example 5: Invention Example 5, Invention Example 5, Invention Example 5, Comparative ... 0Comparative Example 4Comparative Steel 29408579.691.4850Comparative Example 5Comparative Steel 39338580.590.5855Comparative Example 6Comparative Steel 38508580.490.6860Comparative Example 7Inventive Steel 89218276.594.5855Comparative Example 8Comparative Steel 49207579.591.5860Comparative Example 9Comparative Steel 59108281. 289.8845Comparative Example 10Comparative Steel 69158579.691.4850Comparative Example 11Comparative Steel 79259579.491.6860Comparative Example 12Comparative Steel 89108079.691.4855Comparative Example 13Comparative Steel 99168378.592.5860Comparative Example 14Inventive Steel 99258478.792.3900[CR1] 95-4(0.004SS+7Ti+15Nb)[CR2] 76+4(0.004SS+7Ti+15Nb)

[0101] ClassificationAverage grain size(㎛)Grain size standard deviation formula 2YS(MPa)TS(MPa)El(%)r-barΔrΔTSΔElPowdering property(mm)Invention example 12995.3148288482.10.4903.2Invention example 23067.2145285492.20.4513.8Invention example 33386.9151279502.10.3712.6Invention example 43186.6155284512.20.3813.3Comparative example 129133 .9165288481.80.81533.5 Invention Example 53274.9135278492.30.2503.4 Comparative Example 21982.9199295441.70.31533.4 Invention Example 62785.2144289482.20.2402.9 Invention Example 73186.8149279522.10.3613.1 Comparative Example 32262.1199328441.70.3812.1 Comparative Example 4 24 10 3.9 18 8 29 9 4 5 1.6 0.4 4 22 7 Comparative Example 5 22 6 2.1 2 1 1 3 0 5 4 2 1.5 0.4 20 6 3.3 Comparative Example 6 28 15 1.9 2 3 3 1 1 4 0 1.4 0.3 5 5 5 3.3 Comparative Example 7 18 6 5 5 19 9 3 0 5 4 2 20 3 8 13 7 Comparative Example 8 28 9 2.2 1 7 5 2 7 8 4 8 1.6 0.8 22 4 3.5 Comparative Example 9 22 6 1.9 2 4 5 2 8 4 2 1.30.73553.3Comparative Example 101952.1201287441.50.83543.4Comparative Example 111644.6198301421.90.4803.6Comparative Example 122263.2201299431.70.71533.3Comparative Example 132996.9155277492.20.7716.5Comparative Example 1455225.3144268542.30.93545.5[Formula 2] [X(222)+X(554)] / [X(200)+X(110)+X(112)]

[0102] As can be seen from Tables 1 to 3 above, examples 1 to 7 of the present invention, which satisfy the conditions proposed by the present invention, exhibit excellent formability and directional material anisotropy. Furthermore, the powdering properties are also at a good level.

[0103] Comparative Examples 1 to 14 do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, and thus do not satisfy the microstructure conditions or X-ray diffraction integrated intensity ratio that the present invention seeks to obtain, and thus the mechanical properties or powdering properties are not at a good level.

Claims

1. Contains, in weight%, carbon (C): 0.0030% or less (excluding 0%), manganese (Mn): 0.20% or less (excluding 0%), silicon (Si): 0.10% or less (excluding 0%), phosphorus (P): 0.0150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), nitrogen (N): 0.0030% or less (excluding 0%), acid-soluble aluminum (sol.Al): 0.010 to 0.050%, titanium (Ti): 0.030 to 0.070%, niobium (Nb): 0.0010 to 0.010%, and the remainder is composed of iron and other unavoidable impurities. Satisfies the following relation 1, Cold rolled steel sheet satisfying the following relationship 2. [Relationship 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32) > 2.0 [Relationship 2] [X(222)+X(554)] / [X(200)+X(110)+X(112)] > 4.0 (However, in the above relational expression 1, Ti, Nb, C, N, and S represent the content (weight %) of each alloying component, and in the above relational expression 2, X(222), X(554), X(200), X(110), and X(112) represent the X-ray diffraction integrated intensity ratios of the {222} plane, {554} plane, {200} plane, {110} plane, and {200} plane, which are parallel to the plane at a position of 1 / 4t (t: steel plate thickness) of the alloyed hot-dip galvanized steel sheet, respectively.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with an average grain size of 25 to 40 μm.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a standard deviation of grain size of 12 or less.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet whose microstructure is composed of a ferrite main structure and other unavoidable impurities.

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength (YS): 170 MPa or less, a tensile strength (TS): 290 MPa or less, an elongation (El): 48% or more, a tensile strength deviation (ΔTS): 10 MPa or less, and an elongation deviation (ΔEl): 2% or less.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an average Rankford value (r-bar): 1.9 or more and a Rankford value deviation (Δr): 0.6 or less.

7. A step of heating a slab, which comprises, by weight%, carbon (C): 0.0030% or less (excluding 0%), manganese (Mn): 0.20% or less (excluding 0%), silicon (Si): 0.10% or less (excluding 0%), phosphorus (P): 0.0150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), nitrogen (N): 0.0030% or less (excluding 0%), acid-soluble aluminum (sol.Al): 0.010 to 0.050%, titanium (Ti): 0.030 to 0.070%, niobium (Nb): 0.0010 to 0.010%, the remainder being iron and other unavoidable impurities, and satisfying the following relationship 1, at 1100 to 1250°C; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab at 900 to 980°C; A step of cooling the above hot-rolled steel plate to 700 to 780°C and then coiling it; A step of cold rolling the above-mentioned hot rolled steel sheet at a cold reduction ratio (CR) satisfying the following relationships 3 and 4 to obtain a cold rolled steel sheet; and A method for manufacturing a cold rolled steel sheet, comprising: a step of continuously annealing the cold rolled steel sheet at 840 to 870°C. [Relationship 1] (Ti / 48+Nb / 93) / (C / 12+N / 14+S / 32) > 2.0 [Relationship 3] CR > CR1 = 95-4(0.004SS+7Ti+15Nb) [Relationship 4] CR < CR2 = 76+4(0.004SS+7Ti+15Nb) (However, in the above relational expression 1, Ti, Nb, C, N, and S represent the content (weight %) of each alloy component, and in the above relational expressions 3 and 4, SS represents the annealing temperature, and Ti and Nb represent the content (weight %) of each alloy component.) 8. In claim 7, A method for manufacturing a cold rolled steel sheet, further comprising, after the above-mentioned continuous annealing step, a step of immersing the continuously annealed steel sheet in a molten zinc-plating bath at 440 to 500°C to obtain a molten zinc-plated steel sheet.

9. In claim 8, A method for manufacturing a cold rolled steel sheet, comprising, after the step of obtaining the above-mentioned hot-dip galvanized steel sheet, an additional step of alloying the above-mentioned hot-dip galvanized steel sheet at 450 to 540°C to obtain an alloyed hot-dip galvanized steel sheet.

10. In claim 9, A method for manufacturing a cold rolled steel sheet, further comprising, after the step of obtaining the above-mentioned alloyed hot-dip galvanized steel sheet, a step of temper rolling the above-mentioned alloyed hot-dip galvanized steel sheet at a reduction ratio of 0.20 to 1.0%.

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