Cold-rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a high yield ratio and high strength cold rolled steel sheet having excellent material uniformity in a width direction, which can be used for automobiles, and a manufacturing method therefor.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a high yield ratio, high strength cold-rolled steel sheet with excellent width-direction material variation and a method for manufacturing the same. More specifically, it relates to a high yield ratio, high strength cold-rolled steel sheet with excellent width-direction material variation that can be used for automobiles and a method for manufacturing the same.

[0002] Recently, as safety regulations for automobile passengers and pedestrians have been strengthened and the installation of safety devices has become mandatory, there is a problem of increasing vehicle body weight, which runs counter to the lightweighting aimed at improving fuel efficiency. Consumer interest in eco-friendly and fuel-efficient hybrid and electric vehicles is growing; however, producing such safe and environmentally friendly cars requires lightweighting of the body structure and ensuring the stability of body materials. Yet, hybrid vehicles incorporate various additional devices, such as electric engines, electric batteries, and secondary fuel storage tanks, in addition to the conventional gasoline engine. Furthermore, the weight of the vehicle body is increasing as driver convenience features are continuously added. Consequently, to achieve vehicle body lightweighting, it is essential to develop materials that are thin yet possess excellent strength, ductility, and bending characteristics. Therefore, to resolve these issues, it is necessary to develop giga-grade steel sheets capable of securing high strength and ductility with a tensile strength of over 980 MPa.

[0003] In the manufacturing of high-strength cold-rolled steel sheets with a tensile strength of 980 MPa or higher, differences in phase transformation may occur due to temperature variations in the width direction of the hot-rolled sheet during ROT cooling and differences in cooling rates between the coil center and edge after coiling. Consequently, the microstructure formed in different parts of the hot-rolled coil varies, which can lead to differences in the material properties of the hot-rolled sheet. These variations in the width direction of the hot-rolled material properties of cold-rolled DP steel result in a decrease in PCM productivity and cause problems such as scratch defects and meandering in subsequent processes due to FH shape defects. Consequently, issues are being raised, such as cutting off the head / tail portion of the hot-rolled material before feeding it into the PCM.

[0004] To resolve these issues, lowering the coiling temperature to 450℃ or lower reduces material variation, but the increase in martensite fraction can cause cold-rolled sheet fracture due to the cold-rolled roll force load. Therefore, considering this, it is necessary to lower the strength of the hot-rolled steel sheet by applying a heat treatment furnace after hot rolling, but there is a problem that process costs increase significantly when applying a heat treatment furnace.

[0005] As such, variations in yield strength, material properties, and microstructure of the hot-rolled steel sheet in the width direction (Edge, Center) can cause variations in the YS material properties and microstructure of the final steel sheet produced after annealing of the cold-rolled material. Such variations can lead to formability issues, such as dimensional defects depending on the width direction, when forming parts using the steel sheet.

[0006] Therefore, research on a method for manufacturing high-strength steel capable of suppressing material variation in the width direction of the aforementioned hot-rolled steel sheet has been ongoing.

[0007] [Prior Art Literature]

[0008] [Patent Literature]

[0009] (Patent Document 1) KR2019-0078259A

[0010] (Patent Document 2) KR10-1561007B1

[0011] According to one embodiment of the present invention, a high yield ratio, high strength cold-rolled steel sheet with excellent material variation in the width direction of the steel sheet can be provided.

[0012] According to another embodiment of the present invention, a method for manufacturing a high yield ratio, high strength cold-rolled steel sheet with excellent material variation in the width direction of the steel sheet can be provided.

[0013] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0014] A low-yield ratio, high-strength cold-rolled steel sheet having excellent material variation in the width direction according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities, wherein C, Si, Mn, Al, Cr, Mo, Ti, Nb, and B satisfy the following Equations 1 and 2, and The microstructure comprises, in area %, less than 10% ferrite, and the remainder bainite and martensite, and the microstructure satisfies the following relationship 3 at all points and can have a yield ratio (YR) of 0.75 or higher.

[0015] [Relationship 1]

[0016] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0017] [Relationship 2]

[0018] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0019] (In the above Equations 1 and 2, each element represents the weight content, and 0 is substituted if not added.)

[0020] [Relationship 3]

[0021] 0.3×F + B + 1.1×M ≥ 95%

[0022] (Here, F, B, and M represent the area fractions of ferrite, bainite, and martensite of the steel sheet, respectively)

[0023] The YS material variation between the edge and center in the width direction of the above cold-rolled steel sheet may be less than 50 MPa. Here, the edge of the steel sheet refers to the area 0 to 10 cm from the edge of the steel sheet's total width, and the center of the steel sheet refers to the area 10 cm from the center of the steel sheet's total width.

[0024] The tensile strength of the above cold-rolled steel sheet may be 980 MPa or higher.

[0025] The above cold-rolled steel sheet may contain, in area %, martensite (M): 5~30% and bainite (B): 70~95%.

[0026] The above martensite may contain 20% or less of tempered martensite area.

[0027] A method for manufacturing a low-yield ratio, high-strength cold-rolled steel sheet with excellent material variation in the width direction according to another embodiment of the present invention comprises, in weight%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities, wherein C, Si, Mn, Al, Cr, Mo, Ti, Nb, and B satisfy the following Equations 1 and 2. A step of heating a slab; a step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the heated steel slab in a temperature range of Ar3+50℃ to 950℃; a step of cooling the manufactured hot-rolled steel sheet to 500~600℃ and then coiling it; a step of cold-rolling the coiled hot-rolled steel sheet with a cold reduction rate of 40~80%; a step of continuously annealing the cold-rolled cold-rolled steel sheet at 840~860℃; a step of slowly cooling the continuously annealed steel sheet to 630~690℃ at a cooling rate of 2~14℃ / s; a step of rapidly cooling the slowly cooled cold-rolled steel sheet to 350~450℃ at a cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and includes a process of cooling the maintained cold-rolled steel sheet to a temperature of Ms~100℃ or lower at an average cooling rate of 3℃ / s or more, or, if necessary, performing hot-dip galvanizing and alloying heat treatment and then cooling to room temperature.

[0028] [Relationship 1]

[0029] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0030] [Relationship 2]

[0031] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0032] (In the above Equations 1 and 2, each element represents the weight content, and 0 is substituted if not added.)

[0033] Less than 1% of temper rolling can be performed on the above-mentioned final cooled cold-rolled steel sheet.

[0034] According to the present invention, a high-strength composite structure steel sheet with excellent material variation and a high yield ratio (YR≥0.75) can be effectively provided, wherein the YS material variation in the width direction of the hot-rolled material is less than 200 MPa, the YS material variation in the width direction of the final annealed material is less than 50 MPa, and the difference in ferrite fraction between the edge portion and the center portion in the width direction of the annealed material is 10% or less.

[0035] Figure 1 is a figure simulating the isothermal phase transformation analysis in the present invention.

[0036] Figure 2 is an SEM image of the edge and center of the hot-rolled material of Invention Example 5 in an embodiment of the present invention.

[0037] Figure 3 is an SEM image of the edge and center of the hot-rolled material of Comparative Example 1 in an embodiment of the present invention.

[0038] Figure 4 is an SEM micrograph of the annealed material of Invention Example 5 in an embodiment of the present invention.

[0039] Figure 5 is an SEM micrograph of the annealed material of Comparative Example 1 in an embodiment of the present invention.

[0040] Preferred embodiments of the present invention will be described below with reference to the attached drawings. 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 drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0043] 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.

[0044] 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.

[0045] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0046] 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.

[0047] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0048] The inventors have confirmed through hot-rolled isothermal phase transformation analysis that it is possible to manufacture a high-strength composite structure steel sheet with excellent material variation (YR≥0.75) and a high yield ratio (YR≥0.75), in which the ferrite+pealite fraction generated during isothermal transformation at 600°C defined by the following Equation 1 is 30% or less, and the bainite fraction generated during isothermal transformation at 450°C defined by the following Equation 2 is 50% or more, without applying the aforementioned heat treatment furnace, such that the YS material variation in the width direction of the hot-rolled material is less than 200 MPa, the YS material variation in the width direction of the final annealed material is less than 50 MPa, and the difference in ferrite fraction between the width direction edge and the center of the coil is 15% or less. Based on this, the inventors propose the present invention.

[0049] [Relationship 1]

[0050] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0051] [Relationship 2]

[0052] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0053] Hereinafter, a high yield ratio, high strength cold-rolled steel sheet capable of suppressing material variation in the width direction of the steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise noted, the content of the alloy composition described below refers to weight percent.

[0054] C: 0.05~0.10%

[0055] Carbon (C) is a very important element added for solid solution strengthening. In addition, carbon contributes to strength improvement by combining with precipitation elements to form fine carbides. If the C content is less than 0.05%, it is very difficult to secure the desired strength. On the other hand, if the C content exceeds 0.10%, the strength increases rapidly due to the excessive formation of martensite during cooling after hot rolling caused by increased hardenability. This can lead to plate fracture under roll force load during cold rolling, and the material properties may be inferior in the final structure due to structural variations caused by the cooling rate in the width direction. Furthermore, weldability is inferior, increasing the likelihood of welding defects occurring during component processing at the customer. Therefore, it is desirable for the C content to be in the range of 0.05 to 0.10%. It is more desirable for the lower limit of the C content to be 0.06%, and more desirable for the upper limit to be 0.09%.

[0056] Si: 0.20~1.0%

[0057] Silicon (Si) is one of the five major elements of steel, and a small amount is naturally added during the manufacturing process. This Si contributes to an increase in strength and suppresses the formation of carbides, preventing carbon from forming carbides during annealing and cooling. Furthermore, by suppressing the formation of pearlite band structures during hot rolling and finely dispersing carbides, it ensures that austenite is evenly dispersed during annealing and that martensite is finely dispersed during final cooling, which is advantageous for securing elongation. If the Si content is less than 0.20%, it may be difficult to sufficiently secure the aforementioned effects. On the other hand, if the Si content exceeds 0.70%, it may cause surface scale defects, degrade the plating surface quality, and reduce chemical treatment performance. Therefore, it is desirable for the Si content to be in the range of 0.20% to 1.0%. It is more preferable for the lower limit of the Si content to be 0.30%, and more preferable for the upper limit to be 0.90%.

[0058] Mn: 2.2~2.8%

[0059] Manganese (Mn) is an element that completely precipitates sulfur in steel as MnS, thereby preventing hot brittleness caused by the formation of FeS and solid solution strengthening the steel. If the Mn content is less than 2.2%, it is difficult to secure the strength targeted in this invention. On the other hand, if the Mn content exceeds 2.7%, there is a high possibility of problems arising regarding weldability and hot rolling performance; at the same time, it may increase hardenability and lead to the excessive formation of martensite, which can result in a decrease in elongation. Furthermore, there is a problem where Mn-bands (regions where concentrated Mn exists in a band-like form) are formed within the microstructure, increasing the risk of processing cracks and plate fracture, and Mn oxides leaching out to the surface during annealing significantly impair plating properties. Therefore, it is desirable for the Mn content to be in the range of 2.2% to 2.8%. It is more preferable for the lower limit of the Mn content to be 2.3%, and more preferable for the upper limit to be 2.7%.

[0060] Phosphorus (P): 0.10% or less

[0061] Phosphorus (P) is the most advantageous element for securing strength without significantly impairing the formability of steel, but when added in excess, the possibility of brittle fracture increases significantly, which increases the likelihood of plate breakage of the slab during hot rolling. In addition, there is a problem in that it acts as an element that impairs the surface characteristics of galvanized steel sheets.

[0062] In the present invention, if the content of P exceeds 0.10%, the aforementioned problems may occur, so the P may be included at 0.10% or less. Meanwhile, considering the level of P that is inevitably added during the steel manufacturing process, 0% of the content may be excluded.

[0063] Sulfur (S): 0.010% or less

[0064] Sulfur (S) is an impurity inevitably added to steel, so it is effective to keep its content as low as possible. In particular, S in steel is highly likely to cause red-hot brittleness.

[0065] In the present invention, if the content of S exceeds 0.010%, the aforementioned problems may occur, so the S may be included in an amount of 0.010% or less. Meanwhile, considering the level of S that is inevitably added during the steel manufacturing process, 0% of the content may be excluded.

[0066] Aluminum (sol.Al): 0.10% or less

[0067] Aluminum (sol.Al) is an element added to steel for grain refinement and deoxidation.

[0068] In the present invention, if the sol.Al content exceeds 0.10%, while it is advantageous for increasing the strength of steel due to the grain refinement effect, excessive inclusions are formed during continuous casting operations, increasing the likelihood of surface defects in galvanized steel sheets. In addition, there is a concern that economic feasibility may be reduced due to increased manufacturing costs.

[0069] Therefore, the present invention may include sol.Al in an amount of 0.10% or less. According to another embodiment of the present invention, the lower limit of sol.Al may be set to 0.005% or more, or 0.010% or more.

[0070] Cr: 1.0% or less

[0071] Chromium (Cr) is an element that improves hardenability and increases the strength of steel. However, if the content of Cr exceeds 1.0%, problems of through-corrosion may occur due to the non-uniform formation of Cr oxides in a saltwater atmosphere, and it is also uneconomical to add Cr. Therefore, in the present invention, it is preferable to control the Cr content to a range of 1.0% or less. It is more preferable that the Cr content be 0.90% or less, and even more preferable that it be 0.80% or less. Meanwhile, since the effect of improving hardenability and strength can be obtained even with a small amount in the present invention, the lower limit of Cr is not specifically limited.

[0072] Mo: 0.20% or less

[0073] Molybdenum (Mo) is a carbide-forming element that, when combined with carbide-nitride-forming elements such as Ti, Nb, and V, plays a role in improving yield strength and tensile strength by maintaining the size of precipitates finely. Furthermore, the Mo has the advantage of improving the hardenability of steel, allowing for the fine formation of martensite at grain boundaries, thereby enabling control of the yield ratio. However, since it is an expensive element, there is a disadvantage that manufacturing becomes unfavorable as its content increases; therefore, it is desirable to appropriately control its content. If the Mo content exceeds 0.20%, it leads to a sharp increase in manufacturing costs, resulting in reduced economic feasibility. Moreover, due to excessive grain refinement and solid solution strengthening effects, there is a problem where the ductility of the steel actually decreases. In the present invention, the lower limit of the Mo content is not limited, but preferably, the lower limit of the Mo content is more preferably 0.01%, and more preferably 0.03%. The upper limit of the above Mo content is more preferably 0.18%, and more preferably 0.15%.

[0074] Ti: 0.04% or less

[0075] Titanium (Ti) contributes to securing yield strength and tensile strength as a fine carbide-forming element. Additionally, as a nitride-forming element, Ti precipitates N in steel as TiN, thereby suppressing AlN precipitation and offering the advantage of reducing the risk of cracking during continuous casting. In the present invention, if the Ti content exceeds 0.04%, coarse carbides precipitate, and a decrease in strength and elongation may occur due to a reduction in the dissolved carbon content in the steel, which may also cause nozzle clogging during continuous casting. Therefore, it is desirable for the Ti content to be within a range of 0.04% or less. Although the lower limit of the Ti content in the present invention is not limited, it is preferable to set the lower limit to 0.004%, and it is more preferable to set the upper limit to 0.03%.

[0076] Nb: 0.06% or less

[0077] Niobium (Nb) is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during annealing heat treatment, and contributes to an increase in strength by forming fine carbides. In the present invention, if the Nb content exceeds 0.06%, coarse carbides precipitate, and a decrease in strength and elongation may occur due to a reduction in the amount of dissolved carbon in the steel, and there is a problem of increased manufacturing costs. Therefore, it is desirable for the Nb content to have a range of 0.06% or less. The present invention is not limited to a lower limit of the Nb content, but it is preferable that it be 0.005%, and it is more preferable that the upper limit of the Nb content be 0.05%.

[0078] B: 0.004% or less

[0079] Boron (B) is an element that contributes significantly to securing the hardenability of steel. However, if the content of B exceeds 0.004%, boron carbides are formed at the grain boundaries, providing nucleation sites for ferrite, which may actually worsen the hardenability. Therefore, it is desirable for the content of B to be in the range of 0.004% or less. In the present invention, the lower limit of the B content is not limited but can be 0.0004%, and it is more preferable for the upper limit of the B content to be 0.003%.

[0080] Nitrogen (N): 0.010% or less

[0081] Nitrogen (N) is an impurity that is inevitably added to steel, so it is effective to keep its content as low as possible.

[0082] In one embodiment of the present invention, since there is a concern that the refining cost of steel may increase rapidly when the N content is lowered to the extreme, the content may be limited to a range where operation is possible in consideration of this. As one example, the N content may be limited to 0.010% or less, provided that 0% of the content may be excluded in consideration of the level that is inevitably added during the steel manufacturing process.

[0083] The remaining component of the present invention 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 person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0084] A steel plate according to one embodiment of the present invention may be composed of the aforementioned alloying elements, and the relationship between some of the alloying elements may be limited to specific conditions.

[0085] In one embodiment of the present invention, C, Si, Mn, Al, Cr, Mo, Ti, Nb, and B in the alloy composition satisfy the following Equation 1 and Equation 2.

[0086] [Relationship 1]

[0087] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0088] [Relationship 2]

[0089] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0090] (In Equations 1 and 2, each element represents the weight content, and 0 is substituted if not added.)

[0091] As described above, the inventors confirmed through the analysis of the hot-rolled isothermal phase transformation of FIG. 1 that when the ferrite + pearlite fraction generated during isothermal transformation at 600°C defined by the following equation 1 is 30% or less, and the bainite fraction generated during isothermal transformation at 450°C defined by the following equation 2 is 50% or more, the YS material deviation in the width direction of the hot-rolled material is less than 200 MPa and the YS material deviation in the width direction of the final annealed material is less than 50 MPa, and the difference in ferrite fraction between the edge portion and the center portion of the coil in the width direction is 10% or less, it is possible to manufacture a high-strength composite structure steel sheet with excellent material deviation and a high yield ratio (YR≥0.75) without applying the aforementioned heat treatment furnace.

[0092] If the ferrite + pearlite fraction generated during the 600°C isothermal transformation defined by the above Equation 1 exceeds 30%, a ferrite + pearlite structure is formed in the coil center and a bainite + martensite structure is formed in the coil edge due to the temperature deviation in the width direction of the hot-rolled sheet during ROT cooling and the difference in cooling rates between the coil center and edge after coiling. Consequently, due to this difference in the width direction structure, the width direction YS material deviation exceeds 200 MPa. This width direction hot-rolled material deviation leads to a decrease in PCM productivity and causes problems such as post-process scratch defects and meandering due to FH shape defects, thereby causing issues such as cutting off the head / tail portion of the hot-rolled material before feeding it into the PCM. In addition, if the yield strength material and microstructure variation in the width direction (Edge, Center) of such hot-rolled steel sheets is large, it causes microstructure variation in the final steel sheet manufactured after annealing of the cold-rolled material, resulting in the YS material variation in the width direction of the product exceeding 100 MPa, and such variation may cause formability problems, such as dimensional defects depending on the width direction position, when forming parts using the steel sheets.

[0093] To resolve these problems, if the coiling temperature is lowered to 450°C or lower, the material variation is reduced. However, if the bainite fraction generated during the 450°C isothermal transformation defined by the above Equation 2 is less than 50%, the martensite fraction increases significantly, causing the strength to rise excessively, which may result in cold-rolled sheet breakage due to the roll force load during cold rolling. Therefore, considering this, it is necessary to apply a heat treatment furnace after hot rolling to lower the strength of the hot-rolled steel sheet, but there is a problem that the process cost increases significantly when applying the heat treatment furnace.

[0094] Next, the microstructure of the cold-rolled steel sheet of the present invention according to one embodiment is described.

[0095] The cold-rolled steel sheet of the present invention comprises, in area %, ferrite: less than 10%, and remainder bainite and martensite. If the ferrite area fraction is 10% or more, the desired strength cannot be secured and a yield ratio of 0.75 or higher cannot be secured.

[0096] Preferably in the present invention, the cold-rolled steel sheet may comprise 5 to 30% martensite and 70 to 95% bainite in area %. If the martensite area fraction is less than 5%, the strength is low and the desired strength cannot be secured, and if it exceeds 30%, the difference in hardness between phases is too high, resulting in inferior bendability and hole expansion properties, and a yield ratio (YR) of 0.75 or higher cannot be secured.

[0097] In addition, the above martensite may contain 20% or less of tempered martensite area.

[0098] And the microstructure of the cold-rolled steel sheet of the present invention may have an R value of 95% or more at all points, defined by the following relationship 3.

[0099] [Relationship 3]

[0100] R: 0.3×F + B + 1.1×M

[0101] In the above equation 3, F, B, and M represent the area fractions of ferrite, bainite, and martensite, respectively.

[0102] In the present invention, in order to obtain a high-strength composite steel sheet with a high yield ratio (YR≥0.75) and excellent material variation, wherein the YS material variation in the width direction of the hot-rolled material is less than 200 MPa, the YS material variation in the width direction of the final annealed material is less than 50 MPa, and the difference in ferrite fraction between the width direction edge and the center of the coil is 10% or less, it is necessary for the R value defined by the above relationship 3 to be 95% or higher at all points of the steel sheet. The above R value is a variable derived based on the research results of the inventors, which analyzed the degree to which each phase in the steel sheet of the present invention affects the physical properties of the steel sheet and found that even if the composition of the microstructure differs due to different thermal histories applied depending on the part of the steel sheet, the yield strength (YS) can be obtained uniformly at all points of the steel sheet. In this way, by controlling the above R value to be 95% or higher at all points of the steel sheet, the yield strength variation of the cold-rolled steel sheet can be controlled to 50 MPa or less regardless of the part. If the material deviation exceeds 50 MPa, it may cause forming deviations such as springback in different parts during forming, which can lead to poor processing of the part.

[0103] In addition, the YS material deviation between the edge and center in the width direction of the cold-rolled steel sheet may be less than 50 MPa. Typically, the YS deviation between the edge and center in the width direction is greatest due to differences in thermal history, but in the steel sheet of the present invention, the YS material deviation can be controlled to 50 MPa or less even at these two points. Here, the edge of the steel sheet refers to the area 0 to 10 cm from the edge of the entire width of the steel sheet, and the center of the steel sheet refers to the area 10 cm from the center of the entire width of the steel sheet. However, the term "steel sheet of the present invention" does not necessarily mean a cold-rolled coil or thin sheet, but includes blanks before processing into parts and processed parts; furthermore, the R value does not necessarily need to be measured at the center and edge that are distinguishable in the coil or thin sheet, but only needs to be measured to see if it is satisfied at all points of the steel sheet.

[0104] And the tensile strength of the above cold-rolled steel sheet may be 980 MPa or higher.

[0105] In addition, the cold-rolled steel sheet of the present invention may have a difference in ferrite fraction between the edge portion in the width direction of the steel sheet and the center portion of 10% or less.

[0106] Meanwhile, a steel plate according to one embodiment of the present invention may be a cold-rolled steel plate, a hot-dip galvanized steel plate having a zinc-based plating layer on at least one surface of the cold-rolled steel plate, or an alloyed hot-dip galvanized steel plate obtained by alloying the hot-dip galvanized steel plate.

[0107] Although not specifically limited, the zinc-based plating layer may be, as one example, a zinc plating layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0108]

[0109] Next, a method for manufacturing a steel plate according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method corresponds to one example for manufacturing a steel plate according to one embodiment of the present invention.

[0110] According to one embodiment of the present invention, a steel plate can be manufactured by undergoing the [heating - hot rolling - cooling - coiling - cold rolling - annealing - cooling] process with respect to a prepared steel slab, and each process step is described in detail below.

[0111] [Heating Steel Slabs]

[0112] After preparing a steel slab according to one embodiment of the present invention, the steel slab may be heated. The heating process of the steel slab is a process to facilitate the hot rolling process described later and to sufficiently obtain the physical properties of the target steel plate. As one example, the steel slab may have the same alloy composition and alloy component relationship formula (Relationship Formula 1 and Relationship Formula 2) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the component relationship formula are replaced by the aforementioned matters.

[0113] In one embodiment of the present invention, it is preferable that the heating of the steel slab be performed in a temperature range of 1050 to 1300°C. If the heating temperature is less than 1050°C, friction between the steel plate and the rolling mill increases, causing a problem in which the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required to raise the temperature increase, but the amount of surface scale also increases, which may lead to material loss.

[0114] Accordingly, in one embodiment of the present invention, the heating process of the steel slab can be performed in a temperature range of 1050 to 1300°C. According to another embodiment of the present invention, the heating process of the steel slab can be performed at 1100°C or higher, and according to yet another embodiment, it can be performed at 1250°C or lower.

[0115] [Hot Rolled]

[0116] A hot-rolled steel sheet can be obtained by hot-rolling the above heated steel slab.

[0117] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing a finishing hot rolling in a temperature range of Ar3+50℃ to 950℃ during the hot rolling process. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature below Ar3+50℃, two-phase rolling of ferrite + austenite is performed, which may cause material non-uniformity. On the other hand, if the temperature exceeds 950℃, material non-uniformity may be caused by the formation of abnormal coarse grains due to high-temperature rolling, and as a result, there is a problem of coil warping occurring during subsequent cooling.

[0118] [Record]

[0119] The above-mentioned manufactured hot-rolled steel sheet can be wound.

[0120] In one embodiment of the present invention, the coiling process can be performed in a temperature range of 500 to 600°C. If the coiling temperature is below 500°C, the strength of the hot-rolled steel sheet becomes excessively high, which may cause a rolling load during subsequent cold rolling. In addition, excessive costs and time are required to cool the hot-rolled steel sheet to the coiling temperature, which causes an increase in process costs. On the other hand, if the temperature exceeds 600°C, excessive scale may form on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and cause a deterioration in plating properties.

[0121] The hot-rolled steel sheet wound as described above may have an edge microstructure containing bainite and martensite, and a center microstructure containing ferrite, bainite, and martensite.

[0122] [Cold Rolled]

[0123] The above-mentioned coiled hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet.

[0124] In one embodiment of the present invention, cold rolling can be performed with a cold reduction rate (total reduction rate) of 40 to 80%. If the cold reduction rate during cold rolling is less than 40%, it becomes difficult to secure the target thickness, and it becomes difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction rate exceeds 80%, there is a high possibility of cracks occurring at the edge of the steel sheet, and there is a problem of generating a load during cold rolling.

[0125] [Continuous Annealing]

[0126] The cold-rolled steel sheet manufactured above can be subjected to continuous annealing treatment. According to one embodiment of the present invention, the continuous annealing treatment can be performed in a continuous alloying molten plating furnace.

[0127] In one embodiment of the present invention, the continuous annealing treatment can be performed in a temperature range of 840 to 860°C. That is, by performing the continuous annealing treatment of a cold-rolled steel sheet in the austenite single-phase region, sufficient recrystallization of the structure can be ensured, and while forming an austenite single phase, carbon within the austenite can be evenly distributed.

[0128] In the present invention, if the continuous annealing temperature is less than 840°C, not only is recrystallization not sufficiently achieved, but the austenite phase is also not sufficiently formed, making it impossible to achieve the target microstructure phase composition after the continuous annealing treatment. On the other hand, if the temperature exceeds 860°C, productivity decreases, and the austenite grain size becomes excessively coarse, making it impossible to obtain the strength-enhancing effect due to grain refinement. Furthermore, due to the formation of coarse martensite, ductility is compromised, and the desired yield ratio (YR) of 0.75 cannot be secured. In addition, the surface concentration of elements such as Si, Mn, and B, which impede the wettability of hot-dip galvanizing in the alloy composition, becomes excessive, which may degrade the plating surface quality.

[0129] Therefore, in the present invention, the continuous annealing treatment can be performed in a temperature range of 840 to 860°C.

[0130] [Cooling and Maintenance]

[0131] Cold-rolled steel sheets that have undergone continuous annealing treatment can be cooled in the above two-phase temperature range.

[0132] In one embodiment of the present invention, the cooling may be performed in stages. As an example, the process may involve first cooling the continuously annealed cold-rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 2 to 14°C / s, and then second cooling the first-cooled cold-rolled steel sheet to a temperature range of 350 to 450°C at a cooling rate of 10°C / s or more. At this time, the cooling rate during the second cooling may be faster than the cooling rate during the first cooling.

[0133] In this way, the type and fraction of the microstructure formed can be controlled by performing stepwise cooling up to a specific temperature range according to the cooling rate during the cooling of a cold-rolled steel sheet in which a certain fraction of a ferrite phase is formed along with austenite in the continuous annealing process according to one embodiment of the present invention.

[0134] In the present invention, a ferrite phase can be additionally introduced into the cold-rolled steel sheet by first cooling the continuously annealed cold-rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 2 to 14°C / s.

[0135] If the cooling rate during the first cooling is less than 2℃ / s, the austenite phase formed during the continuous annealing process transforms into an excessive fraction of ferrite, making it impossible to adequately secure bainite and martensite phases in the final microstructure. On the other hand, if the cooling rate exceeds 14℃ / s, the additionally introduced ferrite phase is insufficient, and there is a risk that the ductility of the steel sheet will decrease.

[0136] In addition, if the cooling end temperature during the first cooling process is less than 630°C, the ferrite phase is not sufficiently formed, whereas if the temperature exceeds 690°C, there is a problem that the cooling rate must be increased excessively during the subsequent second cooling process, and there is a concern that the fraction of the bainite phase and martensite phase in the final microstructure may not be sufficient.

[0137] According to one embodiment of the present invention, during the first cooling process of a continuously annealed cold-rolled steel sheet, an area fraction of 20 to 30 percent of a ferrite phase can be additionally formed, and together with this, the remaining austenite phase is dispersed.

[0138] Furthermore, in the present invention, after the first cooling of the cold-rolled steel sheet subjected to continuous annealing, a second cooling can be performed at a cooling rate of 10°C / s or more up to a temperature range of 350 to 450°C, and by maintaining the cold-rolled steel sheet in such a cooled state, a bainite phase can be introduced.

[0139] If the cooling rate during the above secondary cooling is less than 10℃ / s, pearlite may be generated during the cooling process, and the bainite phase may not be sufficiently formed. Meanwhile, the upper limit of the cooling rate during the above secondary cooling is not specifically limited, and a person skilled in the art may select it appropriately by considering the specifications of the cooling equipment. As an example, it may be performed at 100℃ / s or less.

[0140] In addition, if the cooling end temperature during the second cooling is less than 350°C, the martensite phase is excessively formed, and the martensite phase is tempered during the subsequent holding process, making it impossible to secure the intended low yield ratio and high ductility. On the other hand, if the temperature exceeds 450°C, the bainite phase is not sufficiently formed, and thus the effect of reducing the difference in hardness between phases due to the bainite phase cannot be obtained.

[0141] In the present invention, the holding time of the secondarily cooled cold-rolled steel sheet can be performed for 100 seconds or more. If the holding time is less than 100 seconds, bainite cannot be obtained in a sufficient fraction. The upper limit of the holding time is not specifically limited and can be set as the time at which the bainite phase is formed in a fraction intended by a person skilled in the art.

[0142] In the present invention, the holding process performed after the second cooling can be performed within the temperature range where the second cooling is completed, so the temperature range is not specifically limited. However, as one example, it can be performed at 400℃ or lower.

[0143] [Final Cooling]

[0144] The cold-rolled steel sheet, which has undergone the above stepwise (first and second) cooling and holding processes, can be finally cooled.

[0145] In one embodiment of the present invention, the final cooling of the cold-rolled steel sheet, which has undergone a stepwise cooling and holding process, can be carried out at a cooling rate of 3℃ / s or more to a temperature of Ms-100℃ or lower, and a martensite phase can be introduced during this process.

[0146] If the cooling rate during the final cooling is less than 3℃ / s or the cooling end temperature exceeds Ms-100℃, the martensite phase cannot be secured to the intended level. In one embodiment of the present invention, the upper limit of the cooling rate during the final cooling and the lower limit of the cooling end temperature are not specifically limited, but in terms of forming a certain fraction of the martensite phase, the cooling can be performed at a cooling rate of 50℃ / s or less, as an example, and as another example, the final cooling can be performed to room temperature.

[0147] According to one embodiment of the present invention, the austenite remaining after transforming into ferrite and bainite phases during the stepwise cooling and holding process can be transformed into a martensite phase during the final cooling process. At this time, the austenite phase that transforms into a martensite phase during the final cooling process is uniformly and finely distributed within the steel sheet due to the dispersion effect during the previous stepwise cooling and holding process, so the martensite phase formed during the final cooling process can also be formed with a fine and uniform distribution.

[0148] In one embodiment of the present invention, a plated steel sheet can be obtained by plating a cold-rolled steel sheet, and as one example, hot-dip galvanizing can be performed on a cold-rolled steel sheet that has undergone a stepwise cooling and holding process prior to final cooling.

[0149] [Hot-dip galvanizing]

[0150] A hot-rolled zinc-plated steel sheet can be manufactured by immersing a cold-rolled steel sheet according to one embodiment of the present invention, that is, a cold-rolled steel sheet that has undergone a stepwise cooling and holding process, in a hot-rolled zinc-plated bath.

[0151] In one embodiment of the present invention, the molten zinc plating process may be performed under normal conditions, but as an example, it may be performed in a temperature range of 430 to 490°C. In addition, the composition of the molten zinc-based plating bath during the molten zinc plating is not particularly limited and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0152] [Alloying Heat Treatment]

[0153] In addition, if necessary, an alloyed hot-dip galvanized steel sheet can be obtained by alloying a hot-dip galvanized steel sheet produced by hot-dip galvanizing according to one embodiment of the present invention.

[0154] In one embodiment of the present invention, the alloying heat treatment process conditions are not particularly limited and normal conditions are acceptable. As one example, the alloying heat treatment process can be performed in a temperature range of 480 to 600°C.

[0155] [Tough Rolling]

[0156] Furthermore, if necessary, a temper rolling process may be further performed, and the above temper rolling process may be performed not only on the cold-rolled steel sheet that has undergone final cooling, but also on the molten zinc-plated steel sheet or alloyed molten zinc-plated steel sheet that has undergone final cooling.

[0157] According to one embodiment of the present invention, bake hardenability can be further improved by forming a large amount of dislocations within the steel through a temper rolling process, and as an example, it can be performed with a reduction rate of less than 1% (excluding 0%). When the reduction rate during the temper rolling is 1% or more, it is advantageous in terms of dislocation formation, but adverse effects such as plate breakage may occur due to the limitations of equipment capacity.

[0158] 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.

[0159] (Example)

[0160] Steel slabs having the alloy composition system of Table 1 below were heated at a temperature of 1050–1250°C, and each heated slab was finished hot-rolled at Ar3+50°C–950°C to produce hot-rolled steel sheets. Subsequently, each hot-rolled steel sheet was pickled under normal conditions, coiled in the temperature range of 500–600°C as shown in Table 2 below, and then cold-rolled with a cold reduction rate of 40–80% to produce cold-rolled steel sheets. Meanwhile, to verify the presence or absence of material variation in the width direction of the pickled hot-rolled steel sheets, the yield strength and tensile strength at the edge and center of the hot-rolled steel sheets were measured, respectively, and the results are shown in Table 2 below. In this experiment, the yield strength and tensile strength at the edge and center of the hot-rolled steel sheets were measured in the L direction according to JIS standards on specimens taken from the edge and center of the manufactured hot-rolled steel sheets. As described above, the steel plate edge sample was taken from a point 0 to 10 cm from the edge of the steel plate width, and the steel plate center sample was taken from a point 10 cm from the center of the steel plate width.

[0161] Afterwards, each cold-rolled steel sheet was subjected to continuous annealing treatment under the conditions shown in Table 2 below, followed by stepwise cooling (1st - 2nd cooling) and a process of holding for 100 seconds or more.

[0162] After that, the final cold-rolled steel sheet was manufactured by cooling to room temperature at a cooling rate of 5~10℃ / s and then temper rolling with a reduction rate of 0.2%.

[0163] Steel composition (wt%) relationship formula 1 2 Remarks CSIMnPSAlCrMoTiNbBN10.060.42.30.0110.0040.0350.850.130.020.050.00250.004459 Invention Steel20.090.42.40.0120.0020.030.10.150.020.030.00010.004258930.100.22.50.0110.0080.030.40.100.020.02-0.005299140.080.72.70.0120.0020.030.10.050.0350.050.00250.003218750 .080.52.60.0150.0040.030.30.050.0250.020.00250.002127060.151.22.40.0140.0070.03--0.020.020.00200.0057758Comparison70.131.02.20.0160.0050.030.70.100.02-0.00100.006514880.141.52.40.0140.0020.03---0.020.00200.004724390.101.12.80.0110.0080.03-----0.0034580

[0164] Steel Type Coiling Temperature (°C) Hot-rolled Material Cold-rolled Properties Annealing Temperature (°C) Primary Cooling Secondary Cooling Remarks Edges Center Deviation Termination Temperature (°C) Cooling Rate (°C) Termination Temperature (°C) Cooling Rate (°C / s)YS(MPa)YS(MPa)dYS(MPa)152078869197Good 850650540020Invention Example 12500850702148Good 8436801038025Invention Example 23600751601150Good 860640836015Invention Example 34510774643131Good 845630341030Invention Example 45580761650111Good 840660445033Invention Example 56580757464293Inferior 860650545020Comparative Example 17550786512274 Inferior 850670740030 Comparative Example 28450840564276 Inferior 841640839025 Comparative Example 39630709466243 Inferior 845680437032 Comparative Example 41680744490254 Inferior 845650442025 Comparative Example 514001021723298 Inferior ----- Comparative Example 6152078869197 Good 820650340025 Comparative Example 7152078869197 Good 850770147020 Comparative Example 8

[0165] For each cold-rolled steel sheet manufactured according to the above, the microstructure was measured and the mechanical properties were evaluated, and the results are shown in Tables 3 and 4.

[0166] First, the microstructure of the cold-rolled steel sheet in Table 3 was analyzed by measuring the fractions of ferrite, bainite, and martensite phases using FE-SEM and an image analyzer after Nital corrosion on specimens taken at the 1 / 4 t (t: steel sheet thickness (unit mm)) point in the thickness direction.

[0167] In addition, the ferrite fractions in the edge and center of the annealed structure in Table 4 were measured using an image analyzer after FE-SEM measurements were taken at a thickness of 1 / 4t after Nital corrosion, using samples taken at points 0 to 10 cm from the edge of the steel plate and at a point 10 cm from the center, respectively. The reason for measuring the structure in the center and edge is that these two regions typically exhibit the largest variation in structural composition within the steel plate, and this is based on the inventors' experience that if the value of the relationship R in these two regions is 95% or higher, the R value in other regions is also 95% or higher.

[0168] Meanwhile, to evaluate the tensile properties of each cold-rolled steel sheet in Table 4, specimens were taken from the edge and center of the manufactured cold-rolled steel sheets (taken from a point 0 to 10 cm from the edge of the entire width of the steel sheet and from a point 10 cm from the center) and tested in the L direction according to JIS standards. In addition, the Hole Expansion Ratio (HER) was evaluated by applying the Japanese JSF T001-1996 standard.

[0169] Microstructure of center (Area %) Microstructure of edge (Area %) EAR*CAR* Remarks FBMFBM 128711194599.799.8 Invention Example 125905295397.098.9 Invention Example 23468283889100.098.8 Invention Example 342772168212100.797.0 Invention Example 4547818685999.09 6.7 Invention Example 56 1447394712594.199.7 Comparative Example 17 1652328791392.095.7 Comparative Example 28 1659251089191.393.1 Comparative Example 39 176221988390.294.0 Comparative Example 41 1158311385295.491.1 Comparative Example 5 1---- -- --Comparative Example 613340274852079.666.4 Comparative Example 713046244159081.471.3 Comparative Example 8 - F represents ferrite, B represents bainite, and M represents martensite - EAR* represents the value for the following Equation 3 at the edge of the steel plate - CAR* represents the value for the following Equation 3 at the center of the steel plate - [Equation 3] 0.3×F + B + 1.1×M

[0170] In Table 3, F represents the ferrite, B the bainite, and M the martensite fraction.

[0171] Steel Type Steel Plate Material Steel Plate Structure Remarks Edge Center Deviation Edge Center Deviation YS(MPa)TS(MPa)El(%)HER(%)YRYS(MPa)TS(MPa)El(%)HER(%)YRdYS(MPa)F(%)F(%)dF(%) 1854 106 1957 0.808 28104 8955 0.792 6022 Invention Example 1283 3105 496 10.7981 110399570.7822253 Invention Example 23877107110550.8283310589510.7944044 Invention Example 3484810729550.79824105510530.7824121 Invention Example 45839105510550.8081710389510.7922242 Invention Example 5682310191041 0.81765104311360.735811413Comparative Example 17810102612430.79741103211350.726931613Comparative Example 28857102111470.84772100612380.778531613Comparative Example 39802101112460.7973296914330.76706171 1 Comparative Example 4 184310509540.80748104411390.729511110 Comparative Example 5 1-------------- Comparative Example 6 1776107211380.72694102112360.688229334 Comparative Example 7 1838112510410.74729102511380.7110925305 Comparative Example 8

[0172] As shown in Tables 1 to 4, Inventive Examples 1 to 5, which satisfy both the alloy composition system and manufacturing conditions according to one embodiment of the present invention, have a microstructure formed as intended, and thus exhibit excellent material variation between the edge and center of the steel plate. Furthermore, it can be confirmed that a high yield ratio steel plate with excellent hole expansion properties and a tensile strength of 980 MPa or higher can be manufactured accordingly. On the other hand, Comparative Examples 1 to 8, which do not satisfy at least one of the alloy composition system and manufacturing conditions according to one embodiment of the present invention, did not form a microstructure as intended. That is, the conditions according to Equation 3 were not simultaneously satisfied in both the edge and center of the steel plate microstructure.

[0173]

[0174] In contrast, in Comparative Example 1, the R value of Equation 3 exceeds 95% at the center but falls short at the edge, which is indicated by material variation. As a result, the difference in yield strength is high at 58 MPa. Similarly, in Comparative Example 2, the value at the edge also falls short, resulting in a material variation of 69 MPa. In Comparative Examples 3 to 5 and Comparative Examples 7 and 8, the R value fell short of 95% at any point, and as a result, the material variation was high at 70 MPa or higher.

[0175] Specifically, in Comparative Examples 1 to 4, during the hot rolling stage, the edges form a bainite and martensite structure, while the center forms a ferrite and pearlite band composite structure. In the case of the bainite and martensite at the edges, fine carbides and carbon are evenly distributed in the bainite and martensite laths and grain boundaries. These carbides and carbon in the laths and grain boundaries can become austenite nucleation sites formed during annealing. On the other hand, in the case of the ferrite-pearlite structure at the center, carbides and carbon are mainly distributed in the plate-like cementite within the pearlite, causing nucleation sites to be concentrated in the pearlite region during annealing. Consequently, during annealing, nucleation and growth occur uniformly in all regions of the edges, resulting in a uniform distribution of carbon. During the subsequent cooling process, a structure in which bainite and martensite are uniformly distributed across the entire region is formed. Conversely, in the center, nucleation and growth occur mainly in the pearlite region, causing carbon to be concentrated in this area. As a result, the austenite formed in the pearlite region of the center has high hardenability, which delays the bainite transformation during the first cooling process after annealing, causing the bainite fraction to decrease and the martensite fraction to increase. In the ferrite region, the bainite fraction increases and the martensite fraction decreases, resulting in a highly non-uniform structure in the center. As such, Comparative Examples 1 to 4 not only differ in the structure of the edge portion and the center of the cold-rolled steel sheet, but also have the degree of influence of each structure affecting the material outside the scope of the present invention, so the variation in yield strength in the width direction exceeds 50 MPa.

[0176] In the case of Comparative Example 5, the hot rolling coil temperature is too high, so a martensite + bainite structure is formed at the edge, but a ferrite + pearlite band structure is formed at the center, and as a result, the R value at the center is less than 95%, and the yield strength deviation in the width direction also exceeds 50 MPa.

[0177] In Comparative Example 6, the hot rolling coil temperature is too low, so the yield strength of the edge portion exceeds 1000 MPa, and as the Roll Force increases significantly during cold rolling, cracks occur in the edge portion, causing plate breakage, and as a result, subsequent processes cannot be carried out.

[0178] In Comparative Example 7, the annealing temperature was too low at 820℃, so the R value at each location fell significantly below 95%, resulting in a very large variation in material properties. Additionally, the ferrite fraction was high, making it impossible to secure the target yield strength and yield ratio of 0.75 or higher.

[0179] In Comparative Example 8, the first cooling end temperature was too high at 750℃, the cooling rate was too slow at 1℃ / s, and the second cooling end temperature was too high at 470℃; similarly, the R value at each point fell significantly below 95%, resulting in a very large material variation, and the amount of ferrite transformation increased too much during cooling, making it impossible to secure a target yield ratio of 0.75 or higher.

[0180] Meanwhile, Figure 2 shows SEM micrographs of the edge and center of the hot-rolled material of Invention Example 5 in an embodiment of the present invention. The microstructures of both the edge and the center consist of bainite and martensite structures, showing that the two phases are finely and uniformly distributed. Such uniform phase distribution in the hot-rolled steel sheet allows the R value to be 95% or higher at all points of the cold-rolled steel sheet that has undergone an annealing process, thereby reducing material variation and enabling high strength and yield ratio to be obtained.

[0181] Figure 3 is an SEM micrograph of the edge and center of the hot-rolled material of Comparative Example 1 in an embodiment of the present invention. It can be seen that the edge consists of bainite and martensite structures, while the center consists of ferrite and coarse pearlite band structures. As a result, the R value of the edge of the cold-rolled steel sheet did not satisfy the conditions of the present invention, which resulted in an uneven strength distribution and may cause an increase in material variation.

[0182] Figure 4 shows SEM micrographs of the edge and center of the annealed material (cold-rolled steel sheet) of Invention Example 5 in an embodiment of the present invention. Both the edge and the center are composed of a small amount of ferrite, as well as bainite and martensite structures, and the distribution is uniform. As a result, the R values ​​all exceeded 95%, the material variation was low, and the yield strength and yield ratio were high.

[0183] Figure 5 is an SEM micrograph of the edge and center portions of the annealed material (cold-rolled steel sheet) of Comparative Example 1 in an embodiment of the present invention. As can be seen in the photograph, not only are the microstructures between the center and the edge portions different, but the influence on strength also falls outside the scope of the present invention, resulting in a high material variation.

Claims

1. In wt%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities, wherein C, Si, Mn, Al, Cr, Mo, Ti, Nb, and B satisfy the following Equations 1 and 2, and The microstructure, in area %, comprises ferrite: less than 10%, and the remainder being bainite and martensite, The above microstructure satisfies the following relationship 3 at all points and is a cold-rolled steel sheet having a yield ratio (YR) of 0.75 or higher. [Relationship 1] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30 [Relationship 2] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50 (In the above Equations 1 and 2, each element represents the weight content, and 0 is substituted if not added.) [Relationship 3] 0.3×F + B + 1.1×M ≥ 95% (Here, F, B, and M represent the area fractions of ferrite, bainite, and martensite of the steel sheet, respectively) 2. In Paragraph 1, A cold-rolled steel sheet having a YS material deviation of less than 50 MPa between the edge portion and the center portion in the width direction of the above cold-rolled steel sheet. Here, the edge portion of the steel plate refers to the area from the edge to 10 cm from the edge of the steel plate's total width, and the center portion of the steel plate refers to the area from the center to 10 cm from the center of the steel plate's total width.

3. In Paragraph 1, A cold-rolled steel sheet having a tensile strength of 980 MPa or higher.

4. In Paragraph 1, The above cold-rolled steel sheet comprises, in area %, martensite (M): 5~30% and bainite (B): 70~95%.

5. In Paragraph 4, The above-mentioned martensite is a cold-rolled steel sheet containing 20% ​​or less of tempered martensite.

6. In Paragraph 1, A cold-rolled steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on the surface of the above cold-rolled steel sheet.

7. A step of heating a steel slab comprising, in wt%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~0.7%, manganese (Mn): 2.2~2.7%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.003% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities, wherein C, Si, Mn, Al, Cr, Mo, Ti, Nb, and B satisfy equation 2 when related to equation 1 below; A step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab in a temperature range of Ar3+50℃ to 950℃; A step of cooling the above-mentioned manufactured hot-rolled steel sheet to 500~600℃ and then coiling it; A step of cold rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 40~80%; A step of continuously annealing the above cold-rolled steel sheet at 840~860℃; A step of slowly cooling the above continuously annealed steel plate to 630~690℃ at a cooling rate of 2~14℃ / s; A step of rapidly cooling the above-mentioned slow-cooled cold-rolled steel sheet to 350~450℃ at a cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and A method for manufacturing a high yield ratio, high strength cold-rolled steel sheet with excellent width-direction material variation, comprising the step of cooling the above-mentioned maintained cold-rolled steel sheet to a temperature of Ms~100℃ or lower at an average cooling rate of 3℃ / s or more. [Relationship 1] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30 [Relationship 2] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50 (In the above Equations 1 and 2, each element represents the weight content, and 0 is substituted if not added.) 8. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet comprising the step of immersing a cold-rolled steel sheet, maintained for at least 100 seconds after rapid cooling, into a molten zinc-based plating bath to manufacture a molten zinc-based plated steel sheet.

9. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet comprising the step of manufacturing an alloyed hot-dip galvanized steel sheet by alloying the above-mentioned manufactured hot-dip galvanized steel sheet through heat treatment.

10. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet by performing temper rolling of less than 1% on the above-mentioned final cooled cold-rolled steel sheet.

11. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet in which the above-mentioned wound hot-rolled steel sheet has an edge microstructure comprising bainite and martensite and a center microstructure comprising ferrite, bainite, and martensite.

Images