Steel sheet and method for manfuacturing same

Abstract

The present invention relates to a material used for vehicle interior panels, reinforcement materials, etc., and relates to a high-yield-ratio and high-strength steel sheet and a method for manufacturing same.

Description

Technical Field

[0001] The present disclosure relates to a material used for vehicle interior panels, reinforcement materials, etc., and relates to a high-yield-ratio and high-strength steel sheet and a method for manufacturing the same.Background Art

[0002] In order to improve automobile fuel efficiency, automobile bodies have been recently made lighter, and for this purpose, the application of high-strength steel sheets capable of reducing a sheet thickness has increased in automobile components. Additionally, high-strength steel sheets are widely used in automobile bodies to ensure passenger safety, and in order to improve the collision performance of automobile bodies, the yield strength of a steel material increases to efficiently absorb collision energy even with low deformation. For this purpose, steel sheets having a high yield ratio are required.

[0003] In order to apply a high-strength steel sheet to automobile bodies, excellent workability is also required, and as a steel material that combines such strength and workability, dual phase steel (hereinafter referred to as DP steel) having a composite structure composed of ferrite as a main phase and hard phase as a second phase is representative. However, DP steel has the problem of low yield ratio because the DP steel uses soft ferrite as a main phase and a hard structure such as bainite, martensite or tempered martensite as a second phase. Accordingly, there are some limitations in the application of DP steel to absorb collision energy while suppressing deformation as an automobile component.

[0004] On the other hand, Patent Document 1 proposed a steel sheet having a structure that prevents recrystallization of ferrite and is comprised of unrecrystallized ferrite and a hard second phase. However, if excessive unrecrystallized ferrite exists, the strength and yield ratio increase, but there may be a problem that the formability is insufficient because the elongation is low.

[0005] To address this problem, Patent Documents 2 to 4 present steel sheets having a structure composed of ferrite and pearlite by refining crystal grains, precipitation strengthening, or reducing the amount of solid solution C in ferrite, and steel sheets achieving both high strength and improved elongation flangeability. However, all of the proposed steel materials have a tensile strength of 500 MPa or less, and it was difficult to achieve high strength exceeding 500 MPa.

[0006] Meanwhile, Patent Documents 5 to 7 present steel sheets having improved elongation flangeability by utilizing unrecrystallized ferrite and having an intermediate hardness between soft ferrite and a hard second phase. However, these patent documents include at least 5% of unrecrystallized ferrite, and there may be a problem that the material is not uniform for each length of coils when manufacturing the coil, in addition to the deviation of the material in each direction.

[0007] That is, while a steel material composed of recrystallized structure has almost no change in material even by the cooling process for coil manufacturing, in the case of steel containing unrecrystallized structure, there is a problem that the unrecrystallized fraction varies depending on the cooling conditions during cooling, resulting in a large deviation of mechanical properties for each location. (Patent Document 1) Japanese Patent Laid-open Publication No. 1978-005018 (Patent Document 2) Japanese Patent Application Publication No. 2007-138261 (Patent Document 3) Japanese Patent Laid-open Publication No. 2007-107099 (Patent Document 4) Japanese Patent Laid-open Publication No. 2001-152288 (Patent Document 5) Japanese Patent Laid-open Publication No. 2008-106351 (Patent Document 6) Japanese Patent Laid-open Publication No. 2008-106352 (Patent Document 7) Japanese Patent Laid-open Publication No. 2008-156680 Summary of InventionTechnical Problem

[0008] An aspect of the present disclosure is to provide a high-strength steel sheet having excellent impact resistance due to a small strength deviation for each length of the steel sheet and a high yield strength, and a method for manufacturing the same.

[0009] The aspects of the present disclosure are not limited to the above-described contents. Additional aspects of the present disclosure are described in overall contents of the specification, and those skilled in the art to which the present disclosure pertains will have no difficulty in understanding the additional aspects of the present disclosure from the contents described in the specification of the present disclosure.Solution to Problem

[0010] An embodiment of the present disclosure relates to a steel sheet, comprising: by wt%, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities,

[0011] wherein a deviation of mechanical properties according to a length of the steel sheet is 60 MPa or less based on a yield strength.

[0012] The microstructure of the steel sheet may comprise, in area%, 80 to 95% of ferrite, and a balance of pearlite and other inevitable structures, and unrecrystallized ferrite, among the ferrite, may be 20 to 50%.

[0013] An aspect ratio of the ferrite may be 5 to 15.

[0014] A total amount of Ti and Nb may be 0.1% or less.

[0015] The steel sheet may satisfy the following relational expression 1. X 222 / X 200 + X 110 + X 112 ≤ 2 (each X-ray diffraction integrated intensity ratio of plane {222}, plane {110}, plane {200}, and plane {112}, parallel to a plane in a depth position of 1 / 4 of a thickness of a cold-rolled steel sheet).

[0016] The steel sheet may further comprise a hot-dip galvanized layer on a surface thereof.

[0017] Another embodiment of the present disclosure relates to a method for manufacturing a steel sheet, comprising: heating a steel slab at a temperature of 1100 to 1250°C, the steel slab comprising, by wt%, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities,; hot-rolling the heated steel slab at a temperature of 880°C or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature 500 to 600°C and coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 45 to 70%; continuously annealing the cold-rolled steel sheet in a temperature within a temperature range of 770 to 820°C, and before coiling the hot-rolled steel sheet, heating a top position of the hot-rolled steel sheet in a range of a coiling temperature of (T) + 30°C to T + 100°C, and heating an end position of the hot-rolled steel sheet in a range of a coiling temperature of T + 80°C to T + 150°C.

[0018] The manufacturing method satisfies a condition of the following [Relational Expression 2]. K ≤ 10 K = 621 × C + 222 × Ti + 1183 × Nb − 0.694 × X − 0.726 × Y where, if K < 0, this is treated as K = 0, and here, [C], [Ti], and [Nb] represent wt.% addition amounts of C, Ti, and Nb, and further, X represents a temperature increase (°C) as compared to a coiling temperature in a top position of the hot-rolled steel sheet, and Y represents a temperature increase (°C) as compared to a coiling temperature in an end position of the hot-rolled steel sheet.

[0019] The manufacturing method satisfies conditions of the following [Relational Expression 3] and [Relational Expression 4]. 2566 + 2.1 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≥ 520 2438 + 1.9 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≤ 700 where CR refers to a cold reduction ratio (%), SS refers to an annealing temperature (°C), and LS refers to a line speed (mpm) during continuous annealing.

[0020] The method for manufacturing a steel sheet may comprise: hot-dip galvanizing the continuously annealed steel sheet.Advantageous Effects of Invention

[0021] A steel sheet of the present disclosure has high strength and high yield ratio, and thus, when the steel sheet is used as an inner plate, reinforcing material, or the like, the resistance (crash resistance characteristics) in the event of a collision increases, which is advantageous for securing passenger safety. Additionally, according to the present disclosure, a steel sheet having excellent formability may be provided.

[0022] The various advantageous advantages and effects of the present disclosure are not limited to the above-described contents, and will be more easily understood in the process of explaining specific embodiments of the present disclosure.Brief Description of Drawings

[0023] FIG. 1 is, for example, a graph illustrating a temperature change of a top position (inner part of coil) and the end position (outer part of coil) of a hot-rolled steel sheet during coiling after hot rolling. FIG. 2 illustrates a difference in yield strength for each length of a hot-rolled coil of Inventive Steel 1 and Inventive Steel 2 in an embodiment of the present disposed. FIG. 3 illustrates a microstructure of each coil length of Inventive Steel 1 and Comparative Steel 1 finally manufactured in the embodiment of the present disclosure. Best Mode for Invention

[0024] The terms used in the present specification are intended to describe the present disclosure and are not intended to limit the present disclosure. In addition, singular forms used in this specification include plural forms unless a relevant definition thereof indicates the opposite meaning.

[0025] The meaning of "comprise or include" used in the specification specifies the configuration and does not exclude the existence or addition of other configurations.

[0026] Unless otherwise defined, all terms including technical terms and scientific terms used in the present specification have the same meaning as those generally understood by those skilled in the art to which the present disclosure belongs. Pre-defined terms are interpreted to have meanings consistent with the relevant technical literature and the currently disclosed content.

[0027] In a microstructure of a cold-rolled steel sheet, it may be considered that it is not a common idea to leave some of the ferrite in an unrecrystallized state. Unrecrystallized ferrite is ferrite elongated in a rolling direction by cold rolling, which denotes that recrystallization is not complete and the dislocations within the grains are restored. When such unrecrystallized ferrite structure exists in steel, a material of the cold-rolled steel sheet has a large deviation in width and length directions, and even a slight difference in the unrecrystallized fraction may make the material significantly uneven. Accordingly, it is recognized that the best method is to minimize the unrecrystallized structure as much as possible.

[0028] When a significant amount of titanium (Ti) and niobium (Nb) is added to steel to which carbon (C) of 0.05 wt% or more is added to secure strength, it may significantly difficult to obtain a complete recrystallized structure through the annealing process. The Ti and Nb form carbides such as TiC and NbC during a cooling process and the temperature increase process in an annealing process in a hot rolling operation, and are known as elements hindering recrystallization and inhibiting grain growth.

[0029] Accordingly, in order to obtain a recrystallized structure, the annealing temperature should be controlled significantly high, or a special process is required to suppress the precipitation of TiC, NbC, and the like. Here, the special process is to make the time for TiC, NbC, etc. to precipitate insufficient through significantly rapid cooling or significantly rapid temperature increase. However, this is a process that is impossible to achieve in the normal operation process, and special equipment is required to achieve the same. On the other hand, in order to control the annealing temperature to be high and prevent the suppression of recrystallization by TiC, NbC, and the like, it is required to maintain a temperature of almost 900°C or higher. Such high-temperature annealing may cause problems such as coil warping and increased manufacturing costs. Even when the annealing temperature is increased to 900°C or higher, the yield strength decreases due to material softening caused by the formation of recrystallized structures, so that it is impossible to secure a high yield ratio required in the present disclosure.

[0030] Accordingly, the inventors of the present disclosure have conducted in-depth research to manufacture a steel sheet having a high yield ratio, preferably a yield strength of 460 to 600 MPa, a tensile strength of 520 to 700 MPa, and a yield ratio of 0.8 to 0.9. Accordingly, it may be possible to derive a method that may secure an excellent yield ratio by using unrecrystallized ferrite and have excellent formability with an elongation of 10% or more.

[0031] However, at the same time, when manufacturing a steel sheet including fine precipitates such as TiC, NbC, and the like, and also containing unrecrystallized ferrite, it has discovered that there is a problem that the material changes significantly depending on the unrecrystallized ferrite fraction. This was due to a difference in microstructure, and it was recognized that the difference in structure was sensitively dependent on a coiling process after hot rolling during the manufacturing process.

[0032] Specifically, as illustrated in FIG. 1, in a process of coiling a hot-rolled steel sheet after hot rolling, a top position (from head to head+50m of coil length) is disposed on an inner sphere of a coil and becomes an inner part of coil, and an end position (from tail to tail+100m of coil length) is disposed on an outer sphere of the coil and becomes an outer part of coil. In this case, as illustrated in FIG. 1, the inner part of coil and the outer part of coil have different cooling behaviors. That is, as compared to the inner part of coil, the outer part of coil is exposed to external air and is cooled faster. This causes low-temperature transformation to occur in a shorter period of time, which causes a problem in which the strength increases relatively. To solve this problem, there is a method of lowering a coiling temperature to lower a temperature deviation of an entire coil, but even with this method, there is a temperature difference between the inner part of coil and the outer part of coil, which causes a difference in the material due to the difference in the structure for each location.

[0033] Accordingly, the inventors of the present disclosure have researched a method that may fundamentally solve this problem and have completed the present disclosure.

[0034] Hereinafter, an embodiment of the steel sheet of the present disclosure will be described in detail. First, the alloy composition of the steel sheet will be described in detail. Hereinafter, unless otherwise specifically stated in the present disclosure, the content of the alloy composition is based on wt%.

[0035] The steel sheet includes carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, and boron (B): 0.005% or less (excluding 0%).Carbon (C): 0.05 to 0.12%

[0036] Carbon (C) is an element contributing to the increase in strength and the formation of pearlite, and is added in an appropriate amount to secure the target strength. Carbon (C) is also an essential element for forming precipitates with Ti, Nb, and the like, in a ferrite phase to impart strength to the steel sheet. When carbon (C) is less than 0.05%, it is difficult to secure the strength required by the steel of the present disclosure, and when carbon (C) exceeds 0.12%, this causes deterioration in formability or weldability, so that it is effective that the content of C is 0.05 to 0.12%. The content of C may be 0.050 to 0.120%.Manganese (Mn): 1.0 to 1.8%

[0037] Manganese (Mn) is an element lowering the Ac3 transformation temperature, which is a temperature at which Ac1 and α-γ transformations are completed and austenite becomes a single phase. In other words, when the content of Mn is low, an annealing temperature needs to be increased to promote transformation, which may make it difficult to secure an appropriate fraction of unrecrystallized ferrite required in the present disclosure. Additionally, Mn is an element contributing to solid solution strengthening together with Si and is also effective for increasing strength. From this point of view, it is effective that the content of Mn is 1.0% or more. On the other hand, when the Mn content exceeds 1.8%, the hardenability increases, so that bainite and martensite are easily formed, lowering the yield ratio, so that it is effective that the content thereof does not exceed 1.8%. The content of Mn may be 1.00 to 1.80%.Silicon (Si): 0.6% or less (excluding 0%)

[0038] Silicon (Si) is a deoxidizing element and a solid solution strengthening element that is effective in increasing strength. However, when the content of Si exceeds 0.6%, Ac1 significantly increases, so that the annealing temperature needs to be increased, which makes it difficult to secure unrecrystallized ferrite by promoting transformation. Accordingly, it is effective to manage the content of silicon (Si) to 0.6% or less. Additionally, when Si is added excessively, problems such as reduced plating adhesion due to oxides during hot-dip galvanizing may occur. However, considering the amount inevitably added during manufacturing, 0% is excluded. The content of Si may be 0.60% or less.Phosphorus (P): 0.03% or less (excluding 0%)

[0039] Phosphorus (P) is an impurity and segregates at grain boundaries, causing a decrease in the toughness of the steel sheet or a deterioration in weldability. Additionally, since an alloying reaction is significantly slow during hot-dip galvanizing, productivity is reduced, so that it is effective to manage the content of P to 0.03% or less.Sulfur (S): 0.01% or less (excluding 0%)

[0040] Sulfur (S) is an impurity that is inevitably included in steel, and it is preferable to manage the content thereof to be as low as possible. Accordingly, considering a case in which the content of S in steel is inevitably included, 0% is excluded. Specifically, since S in steel increases the possibility of causing red-hot embrittlement, it is effective to manage a content thereof to 0.01% or less.Nitrogen (N): 0.01% or less (excluding 0%)

[0041] Nitrogen (N) is an impurity that is inevitably included in steel, and it is important to manage the content of N as low as possible. Accordingly, in consideration of cases in which the content of N in steel is inevitably included, 0% is excluded (i.e., exceeding 0%). However, there is a problem that the costs of refining steel increases sharply in order to manage the content of N in steel significantly low, so that the content of N is managed to 0.01% or less, which is within a range in which operating conditions are possible. The content of N may be 0.010% or less.Acid-soluble aluminum (sol.Al): 0.01 to 0.08%

[0042] Acid-soluble aluminum (sol.Al) is an element added for grain refinement and deoxidation. When the content of sol.Al is less than 0.01%, Al-killed steel may not be manufactured in a normal stable state. On the other hand, when the sol.Al content exceeds 0.08%, it is advantageous for increasing strength due to the grain refinement effect, but the possibility of excessive formation of inclusions during steelmaking and surface defects of the plated steel sheet increases. Additionally, there may be a problem of causing a rapid increase in manufacturing costs, so that it is preferable to manage the content of sol.Al to 0.01 to 0.08%. The content of sol.Al may be 0.010 to 0.080%.Titanium (Ti): 0.02 to 0.06% and niobium (Nb): 0.02 to 0.06%

[0043] Ti and Nb are elements promoting the residue of unrecrystallized ferrite by suppressing recrystallization of ferrite during an annealing process for deformed ferrite generated by cold rolling. To obtain this effect, it is preferable to add Nb and Ti in the content of at least 0.02%, respectively. It is more preferable to add Nb and Ti in the content of 0.020% or more. However, when Nb and Ti are added excessively, the amount of unrecrystallized ferrite increases due to the generation of carbides such as TiC and NbC, and yield strength and yield ratio may excessively increase, and the excessive addition of alloying elements may cause an increase in manufacturing costs. Furthermore, it is more effective to manage the combined amount of Ti and Nb (Ti+Nb) to 0.1% or less.Boron (B): 0.005% or less (excluding 0%)

[0044] Boron (B) is an element improving hardenability, increasing strength, and suppressing nucleation at grain boundaries. When the content of B exceeds 0.005 wt%, an effect thereof becomes excessive and this also causes an increase in manufacturing costs, so that it is preferable to control the content of B to be 0.005 wt% or less.

[0045] In addition, the steel sheet comprises a balance of Fe and inevitable impurities. The addition of effective components other than the composition is not excluded. That is, the inevitable impurities may all be comprised when they may be unintentionally mixed in the manufacturing process of a normal cold-rolled steel sheet (and a plated steel sheet). Since those skilled in the art may easily understand a meaning thereof, the balance is not specifically limited here.

[0046] In the steel sheet of the present disclosure, a deviation of mechanical properties according to the length may be 60 MPa or less based on the yield strength. Specifically, the deviation of the yield strength for a head part, a middle part, and a tail part of the steel sheet in a longitudinal direction may be 60 MPa or less. The steel sheet refers to a final steel sheet, unlike the hot-rolled steel sheet during the manufacturing process. The head part and the tail part generally refer to portions up to 30 m from an end of the steel sheet (coil), and the remainder may be referred to as the middle part.

[0047] For reference, the top position (inner part of coil) of the hot-rolled steel sheet becomes the tail part after cold rolling and annealing, and the end position (outer part of coil) of the hot-rolled steel sheet becomes the head part after cold rolling and annealing. This is because the outer part of the hot-rolled steel sheet is first released and then cold-rolled and annealed.

[0048] Next, the microstructure of the steel sheet will be described in detail. The microstructure of the steel sheet includes, in area%, 80 to 95% of ferrite, and a balance of pearlite and other inevitable structures. In this case, the inevitable structure is not particularly limited, but may include cementite and carbide.

[0049] Meanwhile, it is effective that the unrecrystallized ferrite, among the ferrite, is 20 to 50% in terms of area%. The area% of the unrecrystallized ferrite refers to a fraction with respect to an entire microstructure.

[0050] Here, the fraction of the unrecrystallized ferrite may be determined by interpreting crystal orientation measurement data of electron back scattering diffraction (EBSD) using a Kernel Average Misorientation method (KAM method). Since the KAM method may quantitatively express a crystal misorientation with respect to adjacent pixels (measurement points), in the present disclosure, particles having an average crystal misorientation of 1° or less from adjacent measurement points are defined as unrecrystallized ferrite. In order to secure sufficient yield strength and yield ratio, it is effective that the area% of the unrecrystallized ferrite is 20 to 50%. When the unrecrystallized ferrite is less than 20%, sufficient yield strength and yield ratio may not be obtained, and when the unrecrystallized ferrite exceeds 50%, the yield strength and yield ratio become excessive due to the high unrecrystallized structure, and the aspect ratio of the crystal grains increases. Accordingly, it is effective that the unrecrystallized ferrite is 20 to 50%.

[0051] It is effective that the total ferrite fraction including the unrecrystallized ferrite is 80 to 95%.

[0052] Meanwhile, the steel sheet of the present disclosure comprises pearlite and inevitable structures in addition to ferrite.

[0053] It is effective that the microstructure of the steel sheet, especially the aspect ratio (A / R) of the ferrite grains is 5 to 15.

[0054] Here, the grain aspect ratio was obtained by etching the microstructure with 5% nital etching solution, observing the microstructure at 500x with a scanning electron microscope (SEM), and interpreting and processing an image thereof using an image analyzer program to obtain a major axis length and a minor axis length of the grains. The aspect ratio was obtained as the major axis length of the grain / the minor axis length of the ellipse. An average of the aspect ratios of each ferrite obtained by this technique is defined as the grain aspect ratio.

[0055] When A / R exceeds 15, this denotes that elongated grains in a rolling direction are significantly large, which denotes that the cold-rolled structure is barely recovered or recrystallized. Such excessive elongated grain formation causes an excessive increase in the yield strength, exceeding the yield strength and yield ratio required by the present disclosure. However, when A / R is less than 5, this denotes that recrystallization has progressed significantly, which denotes that the yield strength is insufficient and the yield ratio is lower than the level required by the steel of the present disclosure due to the softening of the steel.

[0056] It is effective that a texture of the steel sheet satisfies the condition of the following relational expression 1. X 222 / X 200 + X 110 + X 112 ≤ 2

[0057] (Each X-ray diffraction integral intensity ratio of plane {222}, plane {110}, plane {200}, and plane {112}, parallel to a plane in a depth position of 1 / 4 of a thickness of a cold-rolled steel sheet)

[0058] Here, the X-ray diffraction integral intensity ratio is the relative intensity based on the X-ray diffraction integral intensity of a non-directional standard sample. The X-ray diffraction may use an X-ray diffraction device widely used in the technical field to which the present disclosure belongs, such as an energy dispersive type. When an X-ray diffraction integral intensity ratio calculated in the relational expression 1 exceeds 2, this denotes that the fraction of (222) texture, i.e., recrystallized texture, increases, which denotes that the unrecrystallized ferrite fraction in steel is not formed within an appropriate range and an aspect ratio thereof may not be secured.

[0059] Meanwhile, the steel sheet of the present disclosure may include a plating layer for improving corrosion resistance. In the present disclosure, the plating layer is not particularly limited, and any type or method of plating performed in the technical field to which the present disclosure belongs is sufficient. A preferred example is a hot-dip galvanized layer.

[0060] Next, an embodiment of a method for manufacturing the steel sheet of the present disclosure will be described in detail. However, this does not denote that the steel sheet of the present disclosure should be manufactured by the following manufacturing method.

[0061] In order to manufacture the steel sheet of the present disclosure, a steel slab satisfying the above-described composition may be manufactured through a process of reheating, hot rolling, coiling, cold rolling, and annealing. Each process will be described in detail below.Reheating: 1100 to 1250°C

[0062] It is effective to reheat the steel slab having the above-described composition in a temperature within a temperature range of 1100 to 1250°C. When a reheating temperature is lower than 1100°C, slab inclusions, or the like, are not sufficiently re-dissolved, which may cause deviation of mechanical properties and surface defects after hot rolling. In contrast, when the slab reheating temperature exceeds 1250°C, the problem of strength reduction due to excessive growth of austenite grains may occur.Hot Rolling: 880°C or higher

[0063] The reheated steel slab is hot-rolled at a temperature of 880°C or higher to manufacture a hot-rolled steel sheet. When a temperature of the hot rolling is lower than 880°C, ferrite transformation occurs during rolling, and an elongated structure is generated, and thus, since problems such as anisotropic deterioration and deterioration of cold rolling properties may occur, it is effective to perform the hot rolling at 880°C or higher.Coiling: 500 to 600°C

[0064] It is effective to coil the hot-rolled steel sheet in a temperature within a temperature range of 500 to 600°C. When the coiling temperature is lower than 500°C, a shape of the steel sheet becomes poor and a transformation structure such as acicular ferrite is generated, which may cause excessive strength increase and ductility decrease of the steel sheet after annealing. However, when the coiling temperature exceeds 600°C, coarse ferrite grains are formed, and coarse carbides and nitrides are likely to be formed, which may deteriorate the steel material. In addition, problems such as heat buckling due to high-temperature coiling occur, resulting in problems such as deterioration of cold rolling properties.

[0065] In order to minimize a deviation of mechanical properties of the manufactured steel sheet in the present disclosure, before coiling the hot-rolled steel sheet obtained through the hot rolling, a top position of the hot-rolled steel sheet (a portion head to head+50m of coil length) may be heated in a range of coiling temperature (T) + 30°C to T + 100°C, and an end position of the hot-rolled steel sheet (a portion from tail to tail+100m of coil length) may be heated to T + 80°C to T + 150°C.

[0066] Additionally, the manufacturing method may satisfy the following relational expression 2. K ≤ 10 K = 621 × C + 222 × Ti + 1183 × Nb − 0.694 × X − 0.726 × Y

[0067] However, if K < 0, this is treated as K = 0.

[0068] Here, C], [Ti], and [Nb] represent wt.% addition amounts of C, Ti, and Nb. In addition, X represents a temperature increase (°C) as compared to the coiling temperature in the top position of the hot-rolled steel sheet, and Y represents a temperature increase (°C) as compared to the coiling temperature at the end position of the hot-rolled steel sheet.

[0069] After coiling the hot-rolled steel sheet, as a result of analyzing a cooling rate and phase transformation behavior in each location, in order to prevent the formation of low-temperature transformation phases such as bainite at the cooling speeds of the top position and the end position, coiling should be performed under the above-described temperature conditions so that ferrite transformation may be sufficiently completed during cooling. When the temperature increase is managed to be lower than the conditions, there is not enough time for ferrite to be sufficiently formed, so that low-temperature transformation phases such as bainite may be generated to excessively increase the strength, and thus a strength deviation therefrom may increase. When the temperature increase is significantly high, the equipment burden may excessively increase.

[0070] High-strength steel materials having a high yield ratio including a certain amount of unrecrystallized ferrite are significantly likely to have deviation of mechanical properties for each length of coils depending on the added components and heat treatment conditions, which is correlated with the unrecrystallized ferrite fraction in the steel. Accordingly, in the present disclosure, In a hot-rolled coiling operation that has the greatest effect on the deviation of mechanical properties for each length of coils in addition to the addition amounts of [C], [Ti], and [Nb] affecting the unrecrystallized ferrite fraction, the relational equation 2 may be satisfied in consideration of the heating of the top position and the end position of the hot-rolled steel sheet (heat-rolled coil).

[0071] When the conditions are satisfied, the deviation of mechanical properties of the steel sheet (coil) in the longitudinal direction after final annealing is significantly small within 60 MPa based on the yield strength, and a steel sheet having a high yield ratio and excellent formability may be obtained.Cold Rolling: Cold Reduction Ratio 45 to 70%

[0072] The hot-rolled steel sheet after coiling and pickling is cold-rolled to manufacture a cold-rolled steel sheet. It is effective to perform the cold rolling at a reduction ratio (cold reduction ratio) of 45 to 70%. When the cold reduction ratio is less than 45%, recrystallization driving force is significantly low, and excessive unrecrystallized ferrite is formed, which may make it difficult to secure the strength required in the present disclosure. On the other hand, when the cold reduction ratio exceeds 70%, the recrystallization driving force becomes significantly high, so that ferrite recrystallization is easily performed even at a low annealing temperature, which may make it difficult to manufacture high-strength steel having a yield ratio of 0.8 to 0.9.Continuous annealing: 770 to 820°C

[0073] It is effective to continuously anneal the cold-rolled steel sheet at a temperature within a temperature range of 770 to 820°C. When the annealing temperature is less than 770°C, the unrecrystallized ferrite fraction is excessively formed, so that the yield strength is high and the ductility deteriorates. On the other hand, when the annealing temperature exceeds 820°C, the unrecrystallized ferrite fraction is significantly low, which may make it difficult to secure the high yield ratio required in the present disclosure.

[0074] In order to secure a certain amount of unrecrystallized tissue, it is important to appropriately manage operating factors controlling the recrystallization driving force in the present disclosure, so that it is effective to satisfy the conditions of [Relational Expression 2] and [Relational Expression 3] below. 2566 + 2.1 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≥ 520 2438 + 1.9 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≤ 700

[0075] Here, CR is the cold reduction ratio (%), SS is the annealing temperature (°C), and LS is the line speed (mpm) during continuous annealing.

[0076] The [Relational Expression 3] and [Relational Expression 4] are operating factors controlling the recrystallization driving force, which include the annealing temperature, the cold reduction ratio, and the line speed during annealing. In the present disclosure, it is effective that the above-mentioned feed speed is 90 to 150 mpm. The line speed is controlled differently depending on the thickness of the steel sheet. That is, in the case of thick materials, the line speed is low, and in the case of thin materials, high-speed work is performed, and it is preferable to manage the cold reduction rate and the annealing temperature together according to these conditions.

[0077] In the present disclosure, subsequent plating may be additionally performed after the continuous annealing.

[0078] The plating may be performed in a manner commonly performed in the technical field to which the present disclosure belongs, and the type and method of plating are not particularly limited. In the present disclosure, the conditions for hot-dip galvanizing are not particularly limited, and hot-dip galvanizing may be performed under common conditions applicable in the same technical field. Through hot-dip galvanizing, the steel sheet according to an embodiment of the present disclosure may include a hot-dip galvanized layer on a surface. As a preferred example, the hot-dip galvanized steel sheet is manufactured by immersing the steel sheet in a hot-dip galvanized plating bath in a temperature within a temperature range of 440 to 500°C. In addition, if necessary, after the hot-dip galvanizing operation, the steel sheet may be subjected to alloying heat treatment, and as an example, the hot-dip galvanized steel sheet may be subjected to alloying heat treatment in a temperature within a temperature range of 460 to 530°C and then cooled to room temperature. Through the alloying heat treatment, the steel sheet may include an alloying hot-dip galvanizing layer on a surface.

[0079] After the hot-dip galvanizing, temper rolling may be performed. The temper rolling may also be performed within the normal range of 0.1 to 1.0%. When the temper rolling elongation is less than 0.1%, it is difficult to control the shape of the sheet. On the other hand, when the temper rolling elongation exceeds 1.0%, in addition to material deterioration due to excessive increase in dislocation density in a surface layer, adverse effects such as sheet fracture may occur due to equipment capacity limitations.Mode for Invention

[0080] Hereinafter, examples of the present disclosure will be described. It will be apparent to those skilled in the art that the embodiments may be variously modified without departing from the scope of the present disclosure. The following examples are provided for the understanding of the present disclosure, and the scope of the rights of the present disclosure should not be limited to the following examples, but should be determined by the claims described below as well as equivalents thereof.(Example)

[0081] A steel slab having the alloy composition (unit: wt%, and a balance being inevitable impurities) described in Table 1 below was hot-rolled and coiled at a reheating temperature of 1200°C, a hot-rolling finishing temperature of 900°C, which is equal to or higher than Ar3 temperature, and a coiling temperature of 560°C.

[0082] Before the coiling, in order to exhibit the heating temperatures of the top position (located in a portion head to head+50m of coil length) and the end position (located in a portion from tail to tail+100m of coil length) of the hot-rolled steel sheet manufactured after hot rolling, increase temperatures as compared to the coiling temperature (CT) are illustrated in Table 2 as X and Y, respectively.

[0083] After coiling the coil, the coil was pickled using hydrochloric acid, and then cold-rolled under the condition of a cold rolling ratio of 60%. After the cold-rolled steel sheet was continuously annealed under the conditions described in Table 2 below, the steel sheet was cooled by furnace cooling, and then, in order to manufacture a hot-dip galvanized steel sheet, the cold-rolled steel sheet was immersed in a hot-dip galvanizing bath maintained at a temperature of approximately 460°C, which is a typical condition, to perform hot-dip galvanizing. Then, for the galvanized steel sheet on which hot-dip galvanizing was completed, a temper rolling rate of 0.5% was applied, as shown in Table 2 below, thereby manufacturing a final hot-dip galvanized steel sheet. Table 1Steel TypeCSiMnPSS.AlTiNbBNTi+Nb10.0850.051.350.020.0060.0210.030.040.0010.0020.0720.0820.061.40.0250.0070.0460.040.030.00080.0020.0730.090.11.450.010.0040.0450.0450.0350.0010.0020.0840.0830.091.40.0250.0040.0430.040.040.00090.0010.0850.0950.091.30.0180.0060.0520.050.030.00090.00350.0860.10.041.70.010.0040.0370.020.040.0010.00280.0670.1050.11.20.0120.0070.0330.030.030.0010.00390.0680.0750.211.50.0110.0030.0350.040.030.00070.0020.0790.0910.081.650.0250.0070.0150.010.010.0030.0040.02100.080.061.750.010.0050.0460.0400.0010.0030.04110.0810.211.550.0090.0030.0560.050.040.00090.0040.09120.0930.351.350.0250.0040.0480.030.030.0010.0020.06130.0950.092.50.0150.0060.0550.040.030.0010.00440.07140.140.121.30.0130.0050.0450.060.0250.00150.00350.09150.0880.151.40.0150.0050.0440.070.080.00210.00340.15 Table 2 Steel TypeManufacturing conditionKDivisionCT(°C)X (°C)Y (°C)SS(°C)LSSPM EI(%)1560501008101000.50Inventive Steel 1103078.0Comparative Steel 12560501008101000.50Inventive Steel 2501053.3Comparative Steel 23560501007901000.50Inventive Steel 307056.5Comparative Steel 34560501008001000.50.4Inventive Steel 4700501000.50.4Comparative Steel 45560501008201000.50Inventive Steel 56560501007801000.56.6Inventive Steel 67560501007901000.50.1Inventive Steel 78560501007901000.50Inventive Steel 89560501008301000.50Comparative Steel 610560501008001000.50Comparative Steel 7560008001000.558.6Comparative Steel 812560501008501000.51.4Comparative Steel 91356010508001000.556.7Comparative Steel 1014560501007201000.50Comparative Steel 1115560501008001000.522.5Comparative Steel 1216560501008001000.557.5Comparative Steel 13

[0084] In the Table 2, CT represents a coiling temperature after hot rolling, X represents a temperature increase of the top position of the hot-rolled steel sheet, and Y represents a temperature increase of the end position. SS represents a continuous annealing temperature (°C), LS represents the line speed (mpm). SPM El represents a temper rolling ratio.

[0085] Meanwhile, K is derived from the following relational expression 2. K ≤ 10 K = 621 × C + 222 × Ti + 1183 × Nb − 0.694 × X − 0.726 × Y

[0086] However, if K < 0, this is treated as K = 0.

[0087] Here, [C], [Ti], and [Nb] represent wt.% addition amounts of C, Ti, and Nb. In addition, X represents the temperature increase (°C) as compared to the coiling temperature in the top position of the hot-rolled steel sheet, and Y represents a temperature increase (°C) as compared to the coiling temperature in the end position of the hot-rolled steel sheet.

[0088] For the steel sheet manufactured as described above, a tensile test was performed in a rolling direction using the DIN-L standard, and the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet were measured 5 times or more, and average results thereof are shown in Table 3 below. In order to confirm the deviation of mechanical properties for each length of coils, the yield strengths of the head part, the middle part, and the tail part of the annealed steel sheet coil were measured, and the remaining tensile strength and elongation were selected as values of the middle part of the coil. The yield ratio (YS) was also calculated using a ratio of the yield strength and tensile strength for the middle part of the length of the coil. Meanwhile, the aspect ratio was measured using the microstructure measured using the SEM and EBSD described above. Table 3:Steel TypeMechanical PropertiesAspect ratioDivisionYS-Head (MPa)YS-Middle (MPa)YS-Tail (MPa)TS-Middle (MPa)EL-Middle (%)YR-MiddleYS Deviation1532505563595210.85587.9Inventive Steel 1603510646605200.841368.1Comparative Steel 12511493539613200.80468.2Inventive Steel 2521503614621190.811118.2Comparative Steal 23543515573625180.82585.8Inventive Steel 3625510578611200.831155.7Comparative Steel 34502485544605190.80597.5Inventive Steel 4420380436525250.72562.5Comparative Steel 45534495551585210.85566.6Inventive Steel 56533525558655170.80598.2Inventive Steel 67533511568635180.805710.1Inventive Steel 78544523565645170.814212.1Inventive Steel 89366345376455300.76312.5Comparative Steel 510375380389545270.7093.8Comparative Steel 6466375491569220.661164.1Comparative Steel 711406365425565280.65602.5Comparative Steel 812610535650615200.871154. 8Comparative Steel 91371069071178560.885421Comparative Steel 101475170075882550.855825Comearative Steel 111567664068572680.884519Comparative Steel 12

[0089] As may be seen in Table 3 above, Inventive examples 1 to 8 satisfying the conditions of the present disclosure satisfy the mechanical properties presented in the steel of the present disclosure because the inventive examples have a yield strength of 485 to 523 MPa, a tensile strength of 585 to 655 MPa, an elongation of 17 to 21%, and a yield ratio (YR) of 0.80 to 0.85, based on the middle part within the length of the coil, and may have a deviation of mechanical properties of 42 to 59 MPa in the head part, the middle part, and the tail part within the length of the coil, and a yield strength deviation for each length is within 60 MPa. In addition, in the case of a steel material having excellent properties like the steel of the present disclosure, an aspect ratio thereof is 6.2 to 12.1, which sufficiently satisfies the condition of aspect ratio of 5 to 15 presented in the steel of the present disclosure. Meanwhile, in terms of a value K of the relational expression 2, the inventive steels were within 0 to 8.8, which satisfied the condition of K ≤ 10 presented in the inventive steel.

[0090] FIG. 2 illustrates a difference in yield strength for each length of hot-rolled coils of Inventive Steel 1 and Inventive Steel 2. As a result of performing an operation under the condition of a target coiling temperature of (T) + 50°C for the top position and a target coiling temperature of (T) + 100°C for the end position during hot-rolling coiling presented in the inventive steel, a yield strength deviation for each length of the inventive steel was within 15 MPa, which was greatly excellent.

[0091] Comparative steels 1 to 3 and 9 are cases in which temperatures of the top position and the end position of the hot-rolled coil was low during hot-rolling coiling. As a result, the value K was significantly high, and a yield strength deviation for each length of the annealed coil was 100 MPa or more, which is significantly high.

[0092] FIG. 3 illustrates a microstructure of the manufactured steel plate coil of Inventive Steel 1 and Comparative Steel 1. Inventive steel 1, which increases the temperature of the top position by 50°C and the end position by 100°C during hot-rolling coiling and in which the value K of the relational expression 2 satisfies the conditions presented in the steel of the present disclosure, has almost no difference in the microstructure for each longitudinal direction, but in the case of Comparative Steel 1, which increases the temperature of the top position by 10°C and the end position by 20°C during hot-rolling coiling, it may be seen that a head part and a tail part of the annealed coil have even more unrecrystallized structures than the middle part, and shows an uneven structure distribution for each length of the coil.

[0093] Comparative Steel 4 is a case in which the coiling temperature is 700°C, which is significantly high. The components and the temperature conditions of top position and the end position during hot-rolling coiling satisfy the conditions presented in the steel of the present disclosure, so that deviation of mechanical properties of a annealed plate for each length of the coil is low, but due to the high coiling temperature, the yield strength is significantly low, and the properties of the high yield ratio steel presented in the steel of the present disclosure may not be satisfied.

[0094] Comparative Steels 5 to 7 are cases in which the contents of Ti and Nb are lower than those proposed by the steel of the present disclosure, and specifically, Comparative Steel 7 is a case in which the contents of Ti and Nb are low and the temperatures of the top position and the end position of the coil are not increased during hot-rolling. Due to the low contents of Ti and Nb, recrystallization occurred easily during annealing due to the lack of precipitates in steel, and as a result, the yield strength of the annealed plate was low and an aspect ratio value thereof did not satisfy the conditions of the present disclosure. Specifically, Comparative Steel 7 in which the temperature conditions during coiling were not properly controlled, also had a significantly high yield strength deviation of the annealed plate.

[0095] Comparative steel 8 is a case in which the composition and temperature conditions during hot-rolling satisfy the value K proposed by the steel of the present disclosure, and the deviation of mechanical properties for each length of the annealed plate was low, but an annealing temperature was 850°C, which was significantly high. A ferrite recrystallization fraction increased due to high-temperature annealing, and the yield strength and yield ratio were low, so that the results of the aspect ratio and the like did not satisfy the conditions of the steel of the present disclosure.

[0096] Comparative steel 10 is a case in which the annealing temperature is 720°C, and an annealing operation is performed at a significantly low temperature. Although other conditions may satisfy the standard of the steel of the present disclosure, the annealing temperature may be significantly low, so that there may not be sufficient time to secure the ferrite recrystallization fraction required by the steel of the present disclosure, and excessive increases in the yield strength and the yield ratio cause problems such as deterioration of an elongation.

[0097] Comparative Steel 11 is a case in which the content of carbon is 0.14%, which deviates from a range of the composition of the steel of the present disclosure. Although other conditions satisfy the standard of the steel of the present disclosure, the excessive content of carbon increases the amount of carbides in the steel, which causes problems such as an increase in a yield ratio and deterioration of an elongation. Additionally, excessive addition of carbon causes deterioration of weldability.

[0098] Comparative Steel 12 is a case in which the contents of Ti and Nb are 0.07% and 0.08%, respectively, which exceed the standard of the steel of the present disclosure. The increase in such carbon nitride forming elements causes excessive TiC and NbC precipitation, which causes problems such as an increased yield ratio due to delayed recrystallization.

Claims

1. A steel sheet, comprising: by wt%, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si):

0. 6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities, wherein deviation of mechanical properties according to a length of the steel sheet is 60 MPa or less based on a yield strength.

2. The steel sheet of claim 1, wherein the microstructure of the steel sheet comprises, in area%, 80 to 95% of ferrite, and a balance of pearlite and other inevitable structures, and unrecrystallized ferrite, among the ferrite, is 20 to 50%.

3. The steel sheet of claim 2, wherein an aspect ratio of the ferrite is 5 to 15.

4. The steel sheet of claim 1, wherein a total amount of Ti and Nb is 0.1% or less.

5. The steel sheet of claim 1, wherein the steel sheet satisfies the following relational expression 1, X 222 / X 200 + X 110 + X 112 ≤ 2 each X-ray diffraction integrated intensity ratio of plane {222}, plane {110}, plane {200}, and plane {112}, parallel to a plane in a depth position of 1 / 4 of a thickness of a cold-rolled steel sheet.

6. The steel sheet of claim 1, wherein the steel sheet further comprises a hot-dip galvanized layer on a surface thereof.

7. A method for manufacturing a steel sheet, comprising: heating a steel slab at a temperature of 1100 to 1250°C, the steel slab including, by wt%, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 to 0.06%, niobium (Nb): 0.02 to 0.06%, boron (B): 0.005% or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities,; hot-rolling the heated steel slab at a temperature of 880°C or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature 500 to 600°C and coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 45 to 70%; continuously annealing the cold-rolled steel sheet in a temperature within a temperature range of 770 to 820°C, and before coiling the hot-rolled steel sheet, heating a top position of the hot-rolled steel sheet in a range of a coiling temperature of (T) + 30°C to T + 100°C, and heating an end position of the hot-rolled steel sheet in a range of a coiling temperature of T + 80°C to T + 150°C.

8. The method for manufacturing a steel sheet of claim 7, wherein the manufacturing method satisfies a condition of the following [Relational Expression 2], K ≤ 10 K = 621 × C + 222 × Ti + 1183 × Nb − 0.694 × X − 0.726 × Y where, if K < 0, this is treated as K = 0, and here, [C], [Ti], and [Nb] represent wt.% addition amounts of C, Ti, and Nb, and furthermore, X represents a temperature increase (°C) as compared to a coiling temperature in a top position of the hot-rolled steel sheet, and Y represents a temperature increase (°C) as compared to a coiling temperature in an end position of the hot-rolled steel sheet.

9. The method for manufacturing a steel sheet of claim 7, wherein the manufacturing method satisfies conditions of the following [Relational Expression 3] and [Relational Expression 4], 2566 + 2.1 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≥ 520 2438 + 1.9 * 0.192 * CR − 1.79 * SS − 5.64 * LS ≤ 700 where CR refers to a cold reduction ratio (%), SS refers to an annealing temperature (°C), and LS refers to a line speed (mpm) during continuous annealing.

10. The method for manufacturing a steel sheet of claim 7, further comprising: hot-dip galvanizing the continuously annealed steel sheet.

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