HIGH STRENGTH STEEL SHEET HAVING HIGH YIELD RATIO, AND MANUFACTURING METHOD THEREFOR

TECHNICAL FIELD

[0001]The present disclosure relates to a material used for an automobile interior panel, a reinforcing material, or the like, and relates to a high-strength steel sheet having a high yield ratio, and a manufacturing method therefor.

BACKGROUND ART

[0002]In order to improve automobile fuel efficiency, the weight of automobile bodies has recently been reduced, and to this end, the application of a high-strength steel sheet that can reduce a plate thickness is increasing in automobile parts. In addition, a high-strength steel sheet is widely used in automobile bodies to ensure passenger safety, and to improve the collision performance of automobile bodies, a yield strength of a steel material is increased to efficiently absorb collision energy even with low deformation. To this end, a steel sheet having a high yield ratio is required.

[0003]In order to apply a high-strength steel sheet to automobile bodies, excellent processability is also required, and a representative steel material achieving both strength and processability is dual phase (DP) steel, which has a composite structure comprised of ferrite as a main phase and a hard structure as a second phase. However, DP steel comprises soft ferrite as a main phase, and a hard structure such as bainite, martensite, and tempered martensite as a second phase, so that DP steel has the problem of a low yield ratio. Therefore, the application of DP steel for absorbing the collision energy has limitations to some extent while suppressing deformation as an automobile part.

[0004]Meanwhile, Patent Document 1 has proposed a steel sheet having a structure comprised of unrecrystallized ferrite and a hard second phase, which prevents recrystallization of ferrite. 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]For the problems, Patent Documents 2 to 4 have proposed a steel sheet having a structure comprised of ferrite and pearlite by grain refinement, precipitation strengthening, or reduction of an amount of C dissolved in ferrite, and a steel sheet achieving both high strength and improved elongation flangeability. However, all of the proposed steel materials had a tensile strength of 500 MPa or less, and it was difficult to achieve high strength exceeding 500 MPa.

SUMMARY OF INVENTION

Technical Problem

[0014]An aspect of the present disclosure is to provide a steel sheet having a high yield ratio and high strength, as well as having excellent formability, and a method for manufacturing the same.

[0015]An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

[0019]The total content of Ti and Nb may be 0.1% or less.

[0020]The cold-rolled steel sheet may satisfy the following Relational Expression 1.

$\begin{array}{cc}X\left(222\right)/\left[X\left(200\right)+X\left(110\right)+X(112\right]\le 2& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

where X-ray diffraction integral intensity ratio each of {222} plane, {110} plane, {200} plane, and {112} plane parallel to a plane at a depth position of ¼ of a thickness of the cold-rolled steel sheet

[0021]The steel sheet may further include a hot-dip galvanized layer on the surface.

[0022]The steel sheet may have a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more.

[0023]The steel sheet may have a product of yield strength and elongation of 8600 or more.

[0029]The method for manufacturing the steel sheet may satisfy the conditions of the following [Relational Expression 2] and [Relational Expression 3].

$\begin{array}{cc}2566+2.1\star 0.192\star \mathrm{CR}-1.79\star \mathrm{SS}-5.64\star \mathrm{LS}\ge 520& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$$\begin{array}{cc}2438+1.9*0.192\star \mathrm{CR}-1.79*\mathrm{SS}-5.64\star LS\le 700& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$

where CR is a cold reduction ratio (%), SS is an annealing temperature (° C.), and LS is a line speed (mpm) during continuously annealing.

[0030]The method for manufacturing the steel sheet may further include an operation of hot-dip galvanizing the continuously annealed steel sheet.

Advantageous Effects of Invention

[0031]As set forth above, the steel sheet of the present disclosure has high strength and high yield ratio, so when used as an interior panel, a reinforcing material, or the like, the resistance upon collision (collision resistance characteristics) increases, which is advantageous in securing passenger safety. In addition, according to the present disclosure, a steel sheet having excellent formability may be provided.

[0032]The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood through descriptions of specific embodiments of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

[0033]FIG. 1 is a photograph illustrating a microstructure of Inventive Steel 1 among Examples.

BEST MODE FOR INVENTION

[0034]The terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. In addition, the singular forms used herein include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto.

[0035]The term “comprising” as used in the specification means specifying a configuration, and does not exclude the presence or addition of other configurations.

[0036]Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as would be commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the present disclosure.

[0037]In a microstructure of a cold-rolled steel sheet, it is not a common technical idea to leave a portion of ferrite in an unrecrystallized state. Unrecrystallized ferrite is ferrite that has been elongated in a rolling direction by cold rolling, and means that recrystallization has not been completed and dislocations within particles have been restored. If such an unrecrystallized ferrite structure exists in steel, a material of the cold-rolled steel sheet may have a large deviation in width and length directions, and even by a slight difference in a fraction of unrecrystallized ferrite, the material may appear very uneven. Therefore, it was recognized that the best method was to minimize the unrecrystallized structure as much as possible.

[0038]When a significant amount of titanium (Ti) and niobium (Nb) is added to steel containing 0.05 wt % or more of carbon (C) to secure strength, it is very difficult to obtain a complete recrystallized structure through an annealing process. Ti and Nb form carbides such as TiC and NbC in a temperature increase process in a cooling process and an annealing process in a hot rolling operation, which are known as elements that hinder recrystallization and suppress grain growth.

[0039]Therefore, in order to obtain a recrystallized structure, it is necessary to control an annealing temperature to be very high or a special process to suppress precipitation of Tic, Nbc, or the like is required. The special process herein is a process which makes the time for TiC, NbC, or the like, to precipitate through very rapid quenching or very rapid temperature-increase to be insufficient. However, the process is a process which is impossible to achieve during normal operation, and special equipment is required to achieve it. Meanwhile, in order to control the annealing temperature to be a high level and prevent recrystallization inhibition by TiC, NbC, or the like, it is necessary to maintain a temperature of almost 900° C. or higher. Such high-temperature annealing may cause problems such as coil deviation, increased manufacturing costs, or the like. Even if the annealing temperature is increased to 900° C. or higher, it is difficult to secure the high yield ratio required in the present disclosure because a decrease in yield strength occurs due to material softening caused by the formation of recrystallized structures.

[0040]Accordingly, the present inventors have conducted extensive 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. As a result thereof, the present inventors have derived a method that can secure the yield ratio described above using unrecrystallized ferrite while having excellent formability of an elongation of 10% or more, thereby completing the present invention.

[0041]Hereinafter, an embodiment of a steel sheet according to the present disclosure will be described in detail. First, the alloy composition of the above steel plate is described in detail. Hereinafter, unless specifically stated otherwise in the present disclosure, the content of the alloy composition is based on weight %.

[0042]The steel sheet includes 0.05 to 0.12% of carbon (C), 1.0 to 1.8% of manganese (Mn), 0.6% or less (excluding 0%) of silicon (Si), 0.03% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur(S), 0.01% or less (excluding 0%) of nitrogen (N), 0.01 to 0.08% of aluminum (sol.Al), 0.02 to 0.06% of titanium (Ti), 0.02 to 0.06% of niobium (Nb), and 0.005% or less (excluding 0%) of boron (B).

Carbon (C): 0.05 to 0.12 wt %

[0043]Carbon (C) is an element which contributes to an increase in strength and formation of pearlite, and is added in an appropriate amount to secure the target strength. In addition, C is an essential element for forming precipitates with Ti, Nb, or the like in a ferrite phase to impart strength to the steel sheet. When a content of C is less than 0.05%, it is difficult to secure the strength required by the steel of the present disclosure, and when the content of C exceeds 0.12%, it deterioration causes in formability or weldability, so it is effective that the content of C is 0.05 to 0.12%.

Manganese (Mn): 1.0 to 1.8%

[0044]Manganese (Mn) is an element which lowers an Ar3 transformation temperature, which is a temperature at which Ac1 and α-γ transformations are completed and becomes an austenite singe phase. That is, when a content Mn is low, an annealing temperature needs to be increased to promote transformation, which makes: it difficult to secure an appropriate fraction of unrecrystallized ferrite required in the present disclosure. In addition, Mn is an element which contributes to solid solution strengthening, together with Si, and is also effective for increasing strength. From this perspective, it is effective that the content of Mn is 1.0% or more. Meanwhile, when the content of Mn exceeds 1.8%, hardenability increases, so bainite and martensite are easily formed, lowering a yield ratio, so it is effective that the content of Mn does not exceed 1.8%.

Silicon (Si): 0.6% or Less (Excluding 0%)

[0045]Silicon (Si) is a deoxidizing element and a solid-solution strengthening element, which is effective in increasing strength. However, when a content of Si exceeds 0.6%, Ac1 becomes too high and an annealing temperature needs to be increased, which makes it difficult to secure unrecrystallized ferrite due to promotion of transformation. Therefore, it is effective to control the content of Si to 0.6% or less. In addition, in the case that Si is added excessively, a problem such as reduced plating adhesion, or the like due to oxides during hot-dip galvanizing may occur. However, 0% is excluded considering the amount which is inevitably added during manufacturing.

Phosphorus (P): 0.03% or Less (Excluding 0%)

[0046]Phosphorus (P) is an impurity and is segregated at grain boundaries, which causes a decrease in the toughness or deterioration in weldability of the steel sheet. In addition, since an alloying reaction is very slow during hot-dip galvanizing, productivity may be reduced, so it is effective that a content of P is 0.03% or less.

Sulfur(S): 0.01% or Less (Excluding 0%)

[0047]Sulfur(S) is an impurity which is inevitably included in steel, and it is preferable that a content of S is controlled to be as low as possible. Therefore, the content of S in steel is excluded from 0% considering the case in which it is inevitably included. In particular, since S in steel increases the possibility of causing red-hot embrittlement, it is effective that the content of S is controlled to be 0.01% or less.

Nitrogen (N): 0.01% or Less (Excluding 0%)

[0048]Nitrogen (N) is an impurity which is inevitably included in steel, so it is important that a content of N is controlled to be as low as possible. Therefore, the content of N in steel is excluded from 0% (i.e., more than 0%) considering the case in which it is inevitably included. However, since there may be a problem that the cost of refining steel increases rapidly when the content of N in steel is controlled to be very low, the content of N is controlled to be 0.01% or less, which is a range that operating conditions may be performed.

Acid-Soluble Aluminum (Sol.Al): 0.01 to 0.08%

[0049]Acid-soluble aluminum (sol.Al) is an element added for grain refinement and deoxidation. When a content of sol.Al is less than 0.01%, aluminum-killed steel may not be manufactured in a normal stable state. On the other hand, when the content of sol.Al exceeds 0.08%, it is advantageous for increasing strength due to a grain refinement effect, but the possibility of surface defects in a plated steel sheet increases due to the excessive formation of inclusions during a steelmaking operation. In addition, since there is a problem which causes a rapid increase in manufacturing costs, it is preferable to control the content of sol.Al to be in the range of 0.01 to 0.08%.

Titanium (Ti): 0.02 to 0.06% and Niobium (Nb): 0.02 to 0.06%

[0050]Ti and Nb are elements which promote retention of unrecrystallized ferrite, by suppressing recrystallization of ferrite during an annealing process for the transformed ferrite generated by cold rolling. To obtain such an effect, each of Nb and Ti is preferably added at least 0.02%. However, when Nb and Ti are excessively added, an amount of unrecrystallized ferrite may increase due to the formation of carbides such as TiC, NbC, or the like, and a yield strength and yield ratio may be excessively increased, and the manufacturing costs may be increased due to the excessive addition of alloying elements. Furthermore, it is more effective to control the total content of Ti and Nb (Ti+Nb) to 0.1% or less.

Boron (B): 0.005% or Less (Excluding 0%)

[0051]Boron (B) is an element which improves hardenability, increases strength, and suppresses the of formation nucleation at grain boundaries. When the content of B exceeds 0.005 wt %, not only will the effect become excessive, but it will also cause an increase in manufacturing costs, so it is preferable to control the content of B to 0.005 wt % or less.

[0052]The steel sheet of the present disclosure includes the remaining iron (Fe) and inevitable impurities. The addition of effective components other than the compositions is not excluded. That is, the impurities may all be included if the impurities can be unintentionally mixed in during the manufacturing process of normal cold-rolled steel sheets (and plated steel sheets). Since a person skilled in the relevant technical field can easily understand the meaning, it is not specifically limited herein.

[0053]Next, a microstructure of the steel sheet is described in detail. The microstructure of the steel sheet includes by area %, 80 to 99% of ferrite, and the remainder includes pearlite and other unavoidable structures. In this case, the unavoidable structure is not specifically limited, but may be cementite, carbide, or the like. More specifically, 80 to 95% of ferrite may be included.

[0054]Meanwhile, it is effective that the unrecrystallized ferrite is 20 to 50%, by area %, among the ferrite. The area % of unrecrystallized ferrite refers to a fraction of the entire microstructure.

[0055]Here, the fraction of unrecrystallized ferrite may be determined by interpreting crystal orientation measurement data from electron back scattering diffraction (EBSD) using a Kernel Average Misorientation method (KAM method). Since the KAM method may quantitatively represent a difference in crystal orientation with adjacent pixels (measurement points), in the present disclosure, particles having a difference in average crystal orientation with adjacent measurement points of within 1° are defined as unrecrystallized ferrite. In order to secure a sufficient yield strength and yield ratio, it is effective that the area % of unrecrystallized ferrite is 20 to 50%. When unrecrystallized ferrite is less than 20%, a sufficient yield strength and yield ratio may not be obtained, and when unrecrystallized ferrite exceeds 50%, the yield strength and yield ratio become excessive due to a high unrecrystallized structure, and an aspect ratio of the grains increases. Therefore, it is effective that unrecrystallized ferrite is 20 to 350%.

[0056]It is effective when the fraction of entire ferrite including the unrecrystallized ferrite is 80 to 99%. More specifically, it may be more effective when the total fraction of ferrite is 80 to 95%.

[0057]Meanwhile, the steel sheet of the present disclosure includes pearlite and unavoidable structures in addition to ferrite.

[0058]It is effective that a microstructure of the steel sheet, in particular, an aspect ratio (A/R) of ferrite grains is 5 to 15.

[0059]Here, the aspect ratio of grains was observed at 500× magnification using a scanning electron microscope (SEM) after etching the microstructure with a 5% Nital etching solution and major and minor axis lengths of crystal grains were obtained by image analysis processing using the Image Analyzer program. The aspect ratio was obtained by using the major axis length of the crystal grain and the minor axis length of the ellipse. An average aspect ratio of each ferrite obtained by this technique was defined as an aspect ratio of grains.

[0060]When the A/R exceeds 15, it means that grains in a rolling direction is very large, which means that a cold-rolled structure is hardly recovered or recrystallized. The excessive formation of such grains results in an excessive increase in the yield strength, which exceeds the yield strength and yield ratio required in the present disclosure. However, when the A/R is less than 5, it means that recrystallization has performed significantly, which means that the yield strength is insufficient and the yield ratio is lower than a level required for the steel of the present disclosure due to softening of the steel.

[0061]It is effective that a texture of the steel sheet satisfies the condition of the following Relational Expression 1.

$\begin{array}{cc}X\left(222\right)/\left[X\left(200\right)+X\left(110\right)+X(112\right]\le 2& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

where X-ray diffraction integrated intensity ratio each of {222} plane, {110} plane, {200} plane, and {112} plane, parallel to a plane at a depth position of ¼ of a thickness of the cold-rolled steel sheet.

[0062]Here, the X-ray diffraction integrated intensity ratio is a relative intensity when based on X-ray diffraction integrated intensity of a non-directional standard sample. X-ray diffraction may be performed using an X-ray diffraction device widely used in the technical field to which the present disclosure belongs, such as an energy dispersive type. When the X-ray diffraction integrated intensity ratio calculated in Relational Expression 1 above exceeds 2, it means that a fraction of a (222) texture, i.e., a recrystallized texture, increases, which means that the fraction of unrecrystallized ferrite in steel is not formed within an appropriate range and the aspect ratio may not be secured.

[0063]A steel sheet satisfying the alloy composition and microstructure may have a yield strength of 460 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.8 to 0.9. At the same time, the steel sheet may have an elongation of 10% or more. More specifically, the yield strength of the steel sheet may be 460 to 600 MPa. The tensile strength of the steel sheet may be 520 to 700 MPa.

[0064]More specifically, the steel sheet may have a product of yield strength and elongation of 8500 or more. More specifically, the product of yield strength and elongation of 8900 or more. Typically, as the yield strength increases, the elongation may decrease. However, according to an embodiment of the present disclosure, since the product of the yield strength and the elongation is 8500 or more, the yield strength and the elongation may improved simultaneously.

[0065]Meanwhile, the steel sheet of the present disclosure may include a plating layer to improve 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. As a preferred example, the plating layer may be a hot-dip galvanized layer.

[0066]Next, a detailed description will be provided of one embodiment of a method for manufacturing a steel sheet of the present disclosure. However, this does not necessarily mean that the steel sheet of the present disclosure should be manufactured by the following manufacturing method.

[0067]To manufacture the steel sheet of the present disclosure, a steel sheet satisfying the above-described composition may be manufactured through a process of reheating, hot rolling, coiling, cold rolling, and annealing. Hereinafter, each process will be described in detail.

[0068]Reheating: 1100 to 1250° C.

[0069]It is effective to reheat a steel slab having the above-described composition at a temperature within a range of 1100 to 1250° C. When the reheating temperature is lower than 1100° C., slab inclusions, or the like are not sufficiently redissolved, which may cause material deviation and surface defects after hot rolling. In contrast, when the slab reheating temperature exceeds 1250° C., the problem of reduced strength due to excessive growth of austenite grains may occur.

[0070]Hot rolling: 880° C. or higher

[0071]The reheated steel slab is hot rolled at a temperature within a range of 880° C. or higher to manufacture a hot-rolled steel sheet. When the temperature of the hot rolling is lower than 880° C., ferrite transformation occurs during rolling, creating an elongated structure, which may cause problems such as anisotropic deterioration and reduced cold rolling properties. Therefore, it is effective that the hot rolling is performed at a temperature of 880° C. or higher.

[0072]Coiling: 500 to 600° C.

[0073]It is effective to coil the hot-rolled steel sheet at a temperature within a range of 500 to 600° C. When the coiling temperature is lower than 500° C., the shape of the steel sheet may be poor and a transformation structure such as acicular ferrite may be generated, which may cause an increase in excessive strength and a decrease in ductility of the steel sheet after annealing. However, when the coiling temperature exceeds 600° C., coarse ferrite grains may be formed, and coarse carbides and nitrides may be easily formed, which can deteriorate a steel material. In addition, a problem such as buckling due to high-temperature coiling may occur, which may deteriorate cold-rolling properties.

[0074]Cold rolling: Cold reduction ratio 45 to 70%

[0075]The hot-rolled steel sheet obtained after coiling and pickling are 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%, a recrystallization driving force is very low, so that excessive unrecrystallized ferrite is formed, making it difficult to secure the strength required by the present disclosure. On the other hand, when the cold reduction ratio exceeds 70%, the recrystallization driving force becomes too high, so that ferrite recrystallization is easily performed even at a low annealing temperature, making it difficult to manufacture high-strength steel having a yield ratio of 0.8 to 0.9.

[0076]Continuously Annealing: 770 to 820° C.

[0077]It is effective to continuously anneal the cold-rolled steel sheet at a temperature within a range of 770 to 820° C. When the annealing temperature is lower than 770° C., a fraction of unrecrystallized ferrite is excessively formed, resulting in high yield strength and deteriorated ductility. On the other hand, when the annealing temperature exceeds 820° C. and the fraction of unrecrystallized ferrite is too small, it is difficult to secure the high yield ratio required in the present disclosure.

[0078]In order to secure a certain amount of unrecrystallized structure, it is important to appropriately manage operating factors controlling the recrystallization driving force. Therefore, it is effective to satisfy the conditions of the following [Relational Expression 2] and [Relational Expression 3].

$\begin{array}{cc}2566+2.1\star 0.192\star \mathrm{CR}-1.79\star \mathrm{SS}-5.64\star \mathrm{LS}\ge 520& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$$\begin{array}{cc}2438+1.9*0.192\star \mathrm{CR}-1.79*\mathrm{SS}-5.64\star LS\le 700& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$

where CR is a cold reduction ratio (%), SS is an annealing temperature (° C.), and LS is a line speed (mpm) during continuous annealing.

[0079][Relational Expression 2] and [Relational Expression 3] above are operating factors controlling the recrystallization driving force, and include an annealing temperature, a cold reduction ratio, and a line speed during annealing. In the present disclosure, the line speed is effectively 90 to 150 mpm. The line speed is controlled differently depending on the thickness of the steel sheet. That is, for a thick material, the operation is performed with a low line speed, and for a thin material, high-speed operation is performed, and it is preferable that the cold reduction rate and annealing temperature are controlled together according to such conditions.

[0080]In the present disclosure, subsequent to the continuously annealing, plating may be additionally performed.

[0081]The plating may be performed in a manner commonly practiced 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 can be performed under normal 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 the surface. As a preferred example, it is immersed in a hot-dip galvanizing bath of 440 to 500° C. to manufacture a hot-dip galvanized steel sheet. In addition, if necessary, after the hot-dip galvanizing operation, the steel sheet may be subjected to an alloying heat treatment. In an embodiment, the hot-dip galvanized steel sheet may be subjected to an alloying heat treatment at a temperature within a 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 galvanized layer on the surface.

[0082]Temper rolling may be performed after the hot-dip galvanizing. Temper rolling may also be performed within a range of 0.1 to 1.0%, which is the normal range. When elongation during temper rolling is less than 0.1%, it is difficult to control the plate shape. On the other hand, when the elongation during temper rolling exceeds 1.0%, adverse effects such as plate fracture may occur due to equipment capacity limitations, as well as material deterioration caused by an increase in excessive dislocation density in the surface layer.

MODE FOR INVENTION

[0083]Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

Example

[0084]A steel slab having an alloy composition (unit: wt %, the remainder being unavoidable impurities) as shown 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 an Ar3 temperature or higher, and a coiling temperature of 560° C.

[0085]After coiling the coil, pickling was performed using hydrochloric acid, and annealed under the cold rolling and continuous annealing conditions described in Table 2 to manufacture a steel sheet.

[0086]In Table 2 above, CR is a cold reduction ratio (%), SS is a continuous annealing temperature (° C.), and LS is a line speed (mpm). Meanwhile, Relational Expressions 2 and 3 are as follows.

$\begin{array}{cc}2566+2.1\star 0.192\star \mathrm{CR}-1.79\star \mathrm{SS}-5.64\star \mathrm{LS}\ge 520& \left[\mathrm{Relational}\mathrm{Expression}2\right]\end{array}$$\begin{array}{cc}2438+1.9*0.192\star \mathrm{CR}-1.79*\mathrm{SS}-5.64\star LS\le 700& \left[\mathrm{Relational}\mathrm{Expression}3\right]\end{array}$

[0087]For the steel sheet manufactured as described above, a tensile test was performed in a rolling direction using a DIN-L standard to measure a yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet, and the results thereof were shown in Table 3 above. In addition, an aspect ratio of ferrite grains and fractions of unrecrystallized and recrystallized ferrite (area %) were measured using a microstructure measured using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) described above. Meanwhile, an X-ray diffraction intensity value for each texture component of the manufactured steel sheet was measured, and an X-ray diffraction intensity ratio was calculated using Relational Expression 1.

$\begin{array}{cc}X\left(222\right)/\left[X\left(200\right)+X\left(110\right)+X(112\right]\le 2& \left[\mathrm{Relational}\mathrm{Expression}1\right]\end{array}$

where each X-ray diffraction integrated intensity ratio of {222} plane, {110} plane, {200} plane, and {112} plane parallel to a plane at a depth position of ¼ of a thickness of the cold-rolled steel sheet

[0088]As can be seen in Table 3 above, Inventive steels 1 to 8 satisfying the alloy composition and manufacturing conditions of the present disclosure have a yield ratio of 469 to 545 MPa, a tensile strength of 545 to 655 MPa, an elongation of 18 to 21%, a yield ratio (YR) of 0.82 to 0.86, and a product of the yield strength and elongation of 8900 or more, thereby satisfying the mechanical properties presented in the Inventive Steels. In addition, the Inventive Steel has an aspect ratio of grains of 5.9 to 10.2, which satisfies the condition of the aspect ratio of grains of 5 to 15 presented in the present disclosure, and also has a fraction of unrecrystallized ferrite of 29 to 45%, which satisfies the condition of the fraction of unrecrystallized ferrite of 20 to 50% presented in the present disclosure. An X-ray diffraction ratio of such steel materials is also 0.6 to 1.2, which sufficiently satisfies the conditions presented in the present disclosure.

[0089]FIG. 1 illustrates an SEM photograph illustrating a microstructure of an annealed plate of Inventive Steel 1 among Examples. The microstructure included unrecrystallized ferrite (symbol {circle around (1)}) and recrystallized ferrite (symbol {circle around (2)}), and some pearlite.

[0090]Comparative steels 1 and 3 had cold reduction ratios, much lower than the conditions disclosed in the present disclosure. This caused a lack of ferrite recrystallization during annealing, so a yield strength was very high and a yield ratio exceeded the standard of the present disclosure. In addition, an aspect ratio value of grains was very high due to the lack of recrystallization.

[0091]Comparative steels 2 and 7 correspond to cases in which a cold reduction ratio is very high at 80%, opposite to that of Comparative steel 1. When the cold reduction ratio is high, ferrite recrystallization occurs easily even at a low annealing temperature. This is because the strength decreases due to an increase in the fraction of recrystallization, and the conditions of yield strength and yield ratio required by the present disclosure cannot be satisfied. In addition, the grain aspect ratio and X-ray integrated intensity deviated from the standards of the present disclosure.

[0092]Comparative steel 4 had very low amounts of Ti and Nb added of 0.01%, respectively. Due to the lack of TiC and NbC precipitates, recrystallization was promoted, and the fraction of unrecrystallized ferrite after annealing was reduced, which caused the yield strength and yield ratio to deviate from the conditions of the present disclosure.

[0093]Comparative steels 5 and 8 illustrate cases in which no Ti or Nb was added at all. Due to the lack of precipitates in steel, recrystallization occurred easily during annealing, and as a result thereof, the yield strength of the steel sheet was low, and the grain aspect ratio and X-ray diffraction intensity ratio did not satisfy the conditions of the present disclosure.

[0094]Comparative steel 6 is steel with the same composition as Comparative steel 5, and had a very low cold reduction ratio of 35% under conditions in which precipitates were insufficient, and thus did not satisfy the conditions of the present disclosure even after high-temperature annealing at 860° C.

[0095]The added components in Comparative steel 9 satisfies all the ranges of the steel of the present disclosure, but an annealing temperature was very high at 850° C. As the annealing temperature increases, a fraction of ferrite recrystallization increases, and due thereto, the yield strength and yield ratio were low, and the results, such as the grain aspect ratio, deviated from the conditions of the present disclosure.

[0096]Comparative steel 10 corresponds to a case in which the content of Mn deviates from the scope of the present disclosure and an annealing temperature is 750° C., which is very low. Such a low annealing temperature causes lack of excessive ferrite recrystallization, which causes a problem such as deterioration in processability to 10% or less of elongation due to an excessive increase in yield strength and yield ratio.

[0097]Comparative steel 11 has a carbon content of 0.14%, which deviates from the range of the components of the steel of the present disclosure. Due to the excessive content of carbon, carbides in steel increased, which caused a problem such as an increase in the yield ratio and a deterioration in the elongation. In addition, the addition of excessive content of carbon causes a deterioration in weldability.

[0098]Comparative steel 12 has a Ti content of 0.08%, which deviates from a standard of the present disclosure, and the total content of Ti+Nb also deviates from the standard. This increase in carbonitride forming elements causes excessive TiC and NbC precipitation, causing a problem such as the increase in yield ratio due to delayed recrystallization.