Low thermal expansion steel with enhanced low-temperature impact toughness and manufacturing method therefor

Abstract

An embodiment of the present disclosure may provide a low thermal expansion steel having improved low-temperature impact toughness including, in percent by weight (wt%), more than 0% and 0.04% or less of carbon (C), 0.1% or more and 0.5% or less of silicon (Si), 0.1% or more and 0.4% or less of manganese (Mn), 0.003% or less of phosphorus (P), 0.003% or less of sulfur (S), more than 0% and 0.5% or less of chromium (Cr), 34% or more and 38% or less of nickel (Ni), more than 0% and 2.0% or less of cobalt (Co), the remainder of iron (Fe) and inevitable impurities, and a value of Formula (1) below may be 112 or less.         Formula (1):     15Cr + 3Ni + 3Co In Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

Description

[Technical Field]

[0001] The present disclosure relates to a low thermal expansion steel having improved low-temperature impact toughness and a method for manufacturing the same.[Background Art]

[0002] Recently, the growing demand for transportation and storage of liquefied natural gas has led to a significant increase in the need for materials for containers that may store liquids at extremely low temperatures.

[0003] As the brittleness of materials increases in an extremely low temperature environment, cracks may easily occur in the storage container material, resulting in leakage.

[0004] Meanwhile, cracks in the material may be caused by stress due to thermal expansion of the material as the temperature changes, or by the decrease in impact toughness of the material in the extremely low temperature environment.

[0005] Accordingly, the development of materials with low thermal expansion and excellent low-temperature impact toughness is required to prevent cracks from occurring even in an extremely low temperature environment.[Prior Art]

[0006] (Patent Document 1) Korean Patent Publication 10-1995-0032674 A (Dec. 22, 1995)[Disclosure][Technical Problem]

[0007] The present disclosure provides a low thermal expansion steel having a low thermal expansion coefficient value and excellent low-temperature impact toughness by optimizing steel components and controlling a manufacturing method, and a method for manufacturing the same.[Technical Solution]

[0008] According to an embodiment of the present disclosure, a low thermal expansion steel having improved low-temperature impact toughness includes, in percent by weight (wt%), more than 0% and 0.04% or less of C, 0.1% or more and 0.5% or less of Si, 0.1% or more and 0.4% or less of Mn, 0.003% or less of P, 0.003% or less of S, more than 0% and 0.5% or less of Cr, 34% or more and 38% or less of Ni, more than 0% and 2.0% or less of Co, the remainder of Fe and inevitable impurities, and a value of Formula (1) below may be 112 or less.         Formula (1):     15Cr + 3Ni + 3Co

[0009] In Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

[0010] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a value of Formula (2) below of 56 or more.         Formula (2):     3Cr + 2Ni - 50Mn - 2Co

[0011] In Formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements. The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a thermal expansion coefficient from room temperature to 100°C of 1.0 x 10-6 / °C or less.

[0012] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have an impact toughness of 140J / cm 2< or more at -196°C.

[0013] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a room temperature impact toughness of 300J / cm 2< or more.

[0014] According to an embodiment of the present disclosure, a method for manufacturing a low thermal expansion steel having improved low-temperature impact toughness includes: manufacturing a slab including, in percent by weight (wt%), more than 0% and 0.04% or less of C, 0.1% or more and 0.5% or less of Si, 0.1% or more and 0.4% or less of Mn, 0.003% or less of P, 0.003% or less of S, more than 0% and 0.5% or less of Cr, 34% or more and 38% or less of Ni, more than 0% and 2.0% or less of Co, the remainder of Fe and inevitable impurities; and hot rolling the slab at 1200 to 1350°C, and hot annealing at 800 to 1000°C to manufacture a hot-rolled steel sheet, wherein the slab may have a value of Formula (1) below of 112 or less.         Formula (1):     15Cr + 3Ni + 3Co

[0015] In Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

[0016] The slab may have a value of Formula (2) below of 56 or more.         Formula (2):     3Cr + 2Ni - 50Mn - 2Co

[0017] In Formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements.

[0018] The method may further include cold rolling the hot-rolled steel sheet at a reduction ratio of 50% or more, and cold annealing at 800 to 950°C to manufacture a cold-rolled steel sheet.[Advantageous Effects]

[0019] According to an embodiment of the present disclosure, by controlling alloying components and a manufacturing method, a low thermal expansion steel having a low thermal expansion coefficient value and excellent low-temperature impact toughness, and a method for manufacturing the same may be provided.[Mode for Invention]

[0020] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the spirit of the present disclosure to those skilled in the art to which the present disclosure pertains, and are not limited to those shown herein, but may be embodied in other forms. The drawings may omit figures not pertinent to the description in order to clarify the present disclosure, and the sizes of configurations may be exaggerated for the purpose of illustration.

[0021] Throughout the specification, when a part "includes" a certain component, this means that the part may further include other components, rather than excluding other components, unless specifically stated otherwise.

[0022] It is to be understood that the singular forms are intended to include the plural forms as well, unless the context clearly dictates otherwise.

[0023] Hereinafter, reasons for numerical limitations on the contents of alloying components in the embodiments of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is percent by weight (wt%).

[0024] According to an embodiment, a low thermal expansion steel having improved low-temperature impact toughness may include, in percent by weight (wt%), more than 0% and 0.04% or less of C, 0.1% or more and 0.5% or less of Si, 0.1% or more and 0.4% or less of Mn, 0.003% or less of P, 0.003% or less of S, more than 0% and 0.5% or less of Cr, 34% or more and 38% or less of Ni, more than 0% and 2.0% or less of Co, the remainder of Fe and inevitable impurities.

[0025] The content of C may be more than 0% and 0.04% or less.

[0026] C is an element effective for increasing strength, and thus it is preferable to add an appropriate amount for utilization as a thin sheet material. However, because C may deteriorate thermal expansion by formation of carbides, it is advantageous to manage the content of C low. Meanwhile, managing the C content excessively low may cause an increase in process costs. Accordingly, the content of C may be controlled to more than 0% and 0.04% or less to secure price competitiveness while maintaining low thermal expansion properties. Preferably, the content of C may be 0.003% or more and 0.037% or less.

[0027] The content of Si may be 0.1% or more and 0.5% or less.

[0028] Si is an element effective for reducing O content and controlling inclusions by facilitating deoxidation during alloy refining. However, because Si addition increases thermal expansion, conventionally the Si content has been controlled to 0.1% or less. According to the disclosure, the content of Si may be controlled to 0.1% or more and 0.5% or less so that deoxidation is easy while suppressing thermal expansion. Preferably, the content of Si may be 0.11% or more and 0.42% or less.

[0029] The content of Mn may be 0.1% or more and 0.4% or less.

[0030] Mn is an element effective for solid solution strengthening and improving hot workability. In particular, Mn may be utilized as a deoxidizer together with Si during alloy refining. However, conventionally the Mn content has been controlled to 0.1% or less to suppress thermal expansion. According to the disclosure, the content of Mn may be controlled to 0.1% or more and 0.4% or less so that thermal expansion may be suppressed even though the Mn content increases, by adjusting Cr, Ni, and Co components. Preferably, the content of Mn may be 0.12% or more and 0.34% or less.

[0031] The content of P may be 0.003% or less or more than 0% and 0.003% or less.

[0032] P is an impurity inevitably contained in steel, and is an element that causes grain boundary corrosion or impairs hot workability. Accordingly, it is advantageous to manage the content of P low. However, controlling the content of P excessively low may cause an increase in process costs. Considering the above, the content of P may be 0.003% or less. Preferably, the content of P may be 0.001% or more and 0.003% or less.

[0033] The content of S may be 0.003% or less or more than 0% and 0.003% or less.

[0034] S is an impurity inevitably contained in steel, and is segregated in grain boundaries and impairs hot workability. In particular, because S may cause shape defects during welding or generate cracks, it is advantageous to manage the content low. Considering the above, an upper limit of the S content may be limited to 0.003% or less. Preferably, the S content may be 0.0004% or more and 0.0022% or less.

[0035] The content of Cr may be more than 0% and 0.5% or less.

[0036] In the ordinary stainless steel manufacturing process for low thermal expansion steel, Cr may be introduced while manufacturing other steel grades, so removing Cr is not easy. However, Cr is an element that increases a thermal expansion coefficient, and thus it is preferable to control Cr as low as possible. In the disclosure, by adjusting the Ni content, the content of Cr may be controlled to more than 0% and 0.5% or less so that thermal expansion may be suppressed even though the Cr content increases.

[0037] The content of Ni may be 34% or more and 38% or less.

[0038] Ni is an essential element for securing low thermal expansion properties by reducing thermal expansion. Generally, to secure low thermal expansion properties, elements other than Ni and Fe are managed at extremely low contents, leading to an increase in process costs. In the disclosure, the Ni content is controlled to 34 to 38%, and Cr and Co components are adjusted to lower the thermal expansion coefficient. Preferably, the content of Ni may be controlled to 34.6% or more and 37.1% or less.

[0039] The content of Co may be more than 0% and 2.0% or less.

[0040] In general, Co is known as a component that reduces the thermal expansion coefficient when added at 4% or less, and increases the thermal expansion coefficient when added at more than 4%. Also, when the content of Co increases, cryogenic impact toughness may decrease. In the disclosure, the Co content is controlled to more than 0% and 2.0% or less, and the Cr and Ni contents are adjusted to lower the thermal expansion coefficient. Preferably, the content of Co may be more than 0% and 0.5% or less, and more preferably, the content of Co may be 0.01% or more and 0.5% or less.

[0041] The remaining component of the disclosure is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.

[0042] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a value of Formula (1) below of 112 or less.         Formula (1):     15Cr + 3Ni + 3Co

[0043] In the Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

[0044] The disclosure aims to suppress thermal expansion by lowering the thermal expansion coefficient by optimizing alloying components. Accordingly, according to the disclosure, Formula (1) is derived by combining alloying components that may affect the thermal expansion coefficient. In the case where the value of Formula (1) exceeds 112, the thermal expansion coefficient from room temperature to 100°C may exceed 1.0 x 10-6 / °C. That is, in the case where the value of Formula (1) exceeds 112, an ability to suppress thermal expansion may be inferior.

[0045] The value of the Formula (1) may specifically be 102 to 112, more specifically 105 to 112, and even more specifically 108 to 112. Within the above range, the low thermal expansion steel having improved low-temperature impact toughness according to an embodiment of the present disclosure may be more advantageous for achieving both properties of a low thermal expansion coefficient value and excellent low-temperature impact toughness.

[0046] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a value of Formula (2) below of 56 or more.         Formula (2):     3Cr + 2Ni - 50Mn - 2Co

[0047] In the Formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements.

[0048] The disclosure aims to improve low-temperature impact toughness by adjusting the composition. Accordingly, according to the disclosure, Formula (2) is derived by combining alloying components that may affect low-temperature impact toughness. In the case where the value of Formula (2) is less than 56, the impact toughness at -196°C may be less than 140J / cm 2< . That is, in the case where the value of Formula (2) is less than 56, the low-temperature impact toughness may be inferior.

[0049] The value of the Formula (2) may specifically be 56 to 72.5 or less, 56 to 70, more specifically 56 to 65, and even more specifically 58 to 60. Within the above range, the low thermal expansion steel having improved low-temperature impact toughness according to an embodiment of the present disclosure may be more advantageous for achieving both properties of a low thermal expansion coefficient value and excellent low-temperature impact toughness. Also, even more excellent impact toughness properties may be realized by further enhancing the balance between the room temperature impact toughness value and the -196°C impact toughness value.

[0050] The low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have a thermal expansion coefficient from room temperature to 100°C of 1.0 x 10 -6< / °C or less by adjusting the alloy composition and manufacturing method. That is, the low thermal expansion steel having improved low-temperature impact toughness according to an embodiment has a low thermal expansion due to temperature changes, and thus the low thermal expansion steel may be used as a steel for storing low-temperature liquids.

[0051] In addition, the low thermal expansion steel having improved low-temperature impact toughness according to an embodiment may have an impact toughness of 140J / cm 2< or more at - 196°C, and a room temperature impact toughness of 300J / cm 2< or more. That is, the low thermal expansion steel having improved low-temperature impact toughness according to an embodiment has low crack formation even in cryogenic environments, and thus the low thermal expansion steel may be used as a steel for supplementing low-temperature liquids.

[0052] Next, a method for manufacturing a low thermal expansion steel having improved low-temperature impact toughness according to another aspect of the disclosure is described.

[0053] The method for manufacturing a low thermal expansion steel having improved low-temperature impact toughness according to an embodiment includes: manufacturing a slab including, in percent by weight (wt%), more than 0% and 0.04% or less of C, 0.1% or more and 0.5% or less of Si, 0.1% or more and 0.4% or less of Mn, 0.003% or less of P, 0.003% or less of S, more than 0% and 0.5% or less of Cr, 34% or more and 38% or less of Ni, more than 0% and 2.0% or less of Co, the remainder of Fe and inevitable impurities; and hot rolling the slab at 1200 to 1350°C, and hot annealing at 800 to 1000°C to manufacture a hot-rolled steel sheet, wherein the slab may have a value of Formula (1) below of 112 or less.         Formula (1):     15Cr + 3Ni + 3Co

[0054] In Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

[0055] In addition, the slab may have a value of Formula (2) below of 56 or more.         Formula (2):     3Cr + 2Ni - 50Mn - 2Co

[0056] In Formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements.

[0057] The reasons for numerical limitations of the component ranges of the respective alloy compositions, Formula (1), and Formula (2) are as described above, and each manufacturing step is described in more detail below.

[0058] After manufacturing the slab satisfying the alloy composition, Formula (1), and Formula (2), a series of hot rolling, hot annealing, cold rolling, and cold annealing processes may be performed.

[0059] First, the slab may be hot-rolled at 1200 to 1350°C, and hot-annealed at 800 to 1000°C to manufacture a hot-rolled steel sheet.

[0060] In the case where the hot rolling temperature is low, it may be difficult to re-dissolve coarse precipitates generated during slab manufacturing. However, in the case where the hot rolling temperature is excessively high, internal crystal grains may become extremely coarse.

[0061] In the case where the hot annealing temperature is low, segregation generated during casting may remain, causing elongation to be inferior. However, in the case where the hot annealing temperature is excessively high, strength may decrease due to crystal grain coarsening.

[0062] Next, if required, the method may further include cold rolling the hot-rolled steel sheet at a reduction ratio of 50% or more, and cold annealing at 800 to 950°C to manufacture a cold-rolled steel sheet.

[0063] In the case where the reduction ratio is less than 50%, recrystallization may decrease during the rolling annealing, causing crystal grains to become coarse.

[0064] In the case where the cold annealing temperature is low, recrystallization may not be sufficient, causing elongation to decrease. However, in the case where the cold annealing temperature is excessively high, crystal grains coarsen, and the depth of oxides formed at grain boundaries deepens, and thus surface quality after pickling may become inferior.

[0065] Hereinafter, the present disclosure is described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present disclosure, and the present disclosure is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present disclosure is determined by matters described in the scope of claims and matters reasonably inferred therefrom.{Embodiments}

[0066] With respect to the various alloying component ranges shown in Table 1 below, a slab was manufactured in a vacuum induction melting furnace. The manufactured slab was hot-rolled at 1250°C, and hot-annealed at 900°C to manufacture hot-rolled steel sheet. The hot-rolled steel sheet was cold-rolled at a 60% reduction ratio, and cold-annealed at 900°C to manufacture specimens. [Table 1]Alloy elements (wt%)CSiMnPSCrNiCoExample10.0280.170.150.00100.00050.534.70.10Example20.0350.110.260.00120.00050.037.10.12Example30.0330.200.320.00100.00040.236.00.30Example40.0290.150.340.00170.00080.136.70.10Example50.0210.180.270.00230.00040.136.20.50Example60.0270.200.250.00140.00050.335.70.02Example70.0240.370.190.00130.00050.036.00.01Example80.0030.320.260.00180.00120.036.10.01Example90.0370.190.120.00140.00110.534.60.20Example100.0180.230.260.00140.00200.036.50.02Example110.0260.420.250.00150.00220.036.10.01Example120.0310.200.290.00300.00190.135.60.01Comparative Example10.0170.180.280.00310.00080.235.10.98Comparative Example20.0270.170.270.00140.00040.134.71.02Comparative Example30.0300.200.350.00260.00050.234.21.90Comparative Example40.0290.190.270.00150.00060.134.01.90Comparative Example50.0260.190.260.00160.00070.136.02.10 Comparative Example60.0220.160.310.00140.00060.236.51.48Comparative Example70.0200.360.280.00270.00040.136.20.95Comparative Example80.0390.200.250.00300.00050.336.30.03Comparative Example90.0020.230.300.00140.00050.336.50.01Comparative Example100.0270.410.240.00150.00100.536.00.04Comparative Example110.0260.370.270.00140.00040.9 35.70.12Comparative Example120.0360.200.100.0031 0.00041.0 36.10.01

[0067] Table 2 below shows the value of Formula (1), value of Formula (2), thermal expansion coefficient, room temperature impact toughness, and -196°C impact toughness. The value of Formula (1) was calculated by Formula (1) below.         Formula (1):     15Cr + 3Ni + 3Co

[0068] In Formula (1), Cr, Ni, and Co represent the content (wt%) of the respective elements.

[0069] The value of Formula (2) was calculated by Formula (2) below.         Formula (2):     3Cr + 2Ni - 50Mn - 2Co

[0070] In Formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements.

[0071] The thermal expansion coefficient was measured using a dilatometer. First, a change in length of the specimen according to temperature change was measured when heating from room temperature to 120°C at a rate of 1°C / s. Thereafter, a linear thermal expansion coefficient under the condition of 25 to 100°C was calculated through Formula (3) below. α m = 1 L 0 Δ L Δ T

[0072] In Formula (3), α m is a thermal expansion coefficient, L 0 is an initial length, △L is the change in length, and ΔT is the change in temperature.

[0073] Room temperature impact toughness and -196°C impact toughness were measured at low temperatures of 25°C and -196°C using an impact tester from Zwick Roell. Meanwhile, the - 196°C impact toughness evaluation was performed by immersing the specimen in liquid nitrogen for 5 minutes. [Table 2]Formul(1)Formula (2)Thermal Expansion Coefficient (x10 -6< / °C)Room temperature impact toughness (J / cm 2< , Room temperature)-196°C impact toughness (J / cm 2< , - 196°C)Example1111.963.21.0306172Example2111.761.01.0314164Example3111.956.00.8310155Example4111.956.50.9309168Example5111.658.20.9319157Example6111.759.80.9312161Example7108.062.50.8308168Example8108.359.20.9302166Example9111.964.30.8303179Example10109.660.00.9310157Example11108.359.70.9309156Example12108.357.00.8304152Comparative Example1111.254.8 0.7298 119 Comparative Example2108.754.2 0.8304123 Comparative Example3111.347.7 0.6297 100 Comparative Example4109.251.0 0.8311105 Comparative Example5115.8 55.1 1.5 307135 Comparative Example6116.9 55.1 1.5 306130 Comparative Example7113.0 56.81.1 308140Comparative Example8113.5 60.91.1 307183Comparative Example9114.0 58.91.2 307188Comparative Example10115.6 61.41.3 309191Comparative Example11121.0 60.41.8 303199Comparative Example12123.3 70.21.8 306194

[0074] Referring to Table 2, Examples 1 to 12 satisfied the alloying components, value of Formula (1), value of Formula (2), and manufacturing method of the disclosure. Accordingly, Examples 1 to 12 satisfied a thermal expansion coefficient from room temperature to 100°C of 1.0 x 10 -6< / °C or less, an impact toughness of 140J / cm 2< or more at -196°C, and a room temperature impact toughness of 300J / cm 2< or more. That is, Examples 1 to 12 had low thermal expansion and excellent low-temperature impact toughness. However, Comparative Examples 1 to 6 did not satisfy the value of Formula (2) of 56 or more. Accordingly, Comparative Examples 1 to 6 did not satisfy the impact toughness of 140J / cm 2< or more at -196°C. That is, Comparative Examples 1 to 6 had inferior low-temperature impact toughness.

[0075] Comparative Examples 5 to 12 did not satisfy the value of Formula (1) of 112 or less. Accordingly, Comparative Examples 5 to 12 did not satisfy the thermal expansion coefficient from room temperature to 100°C of 1.0 x 10 -6< / °C or less. That is, Comparative Examples 5 to 12 exhibited relatively large thermal expansion.

[0076] According to an embodiment of the disclosure, by controlling alloying components and a manufacturing method, a low thermal expansion steel having a low thermal expansion coefficient value and excellent low-temperature impact toughness, and a method for manufacturing the same may be provided.

Claims

1. A low thermal expansion steel having improved low-temperature impact toughness, comprising, in percent by weight (wt%), more than 0% and 0.04% or less of carbon (C), 0.1% or more and 0.5% or less of silicon (Si), 0.1% or more and 0.4% or less of manganese (Mn), 0.003% or less of phosphorus (P), 0.003% or less of sulfur (S), more than 0% and 0.5% or less of chromium (Cr), 34% or more and 38% or less of nickel (Ni), more than 0% and 2.0% or less of cobalt (Co), the remainder of iron (Fe) and inevitable impurities, and wherein a value of Formula (1) below is 112 or less,         Formula (1):     15Cr + 3Ni + 3Co (wherein Cr, Ni, and Co represent the content (wt%) of the respective elements).

2. The low thermal expansion steel of claim 1, wherein a value of Formula (2) below is 56 or more,         Formula (2):     3Cr + 2Ni - 50Mn - 2Co (wherein Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements).

3. The low thermal expansion steel of claim 1, wherein a thermal expansion coefficient from room temperature to 100°C is 1.0 x 10-6 / °C or less.

4. The low thermal expansion steel of claim 1, wherein an impact toughness is 140J / cm2 or more at -196°C.

5. The low thermal expansion steel of claim 1, wherein a room temperature impact toughness is 300J / cm2 or more.

6. A method for manufacturing a low thermal expansion steel having improved low-temperature impact toughness, the method comprising: manufacturing a slab including, in percent by weight (wt%), more than 0% and 0.04% or less of carbon (C), 0.1% or more and 0.5% or less of silicon (Si), 0.1% or more and 0.4% or less of manganese (Mn), 0.003% or less of phosphorus (P), 0.003% or less of sulfur (S), more than 0% and 0.5% or less of chromium (Cr), 34% or more and 38% or less of nickel (Ni), more than 0% and 2.0% or less of cobalt (Co), the remainder of iron (Fe) and inevitable impurities; and hot rolling the slab at 1200 to 1350°C, and hot annealing at 800 to 1000°C to manufacture a hot-rolled steel sheet, wherein the slab has a value of Formula (1) below of 112 or less,         Formula (1):     15Cr + 3Ni + 3Co (wherein Cr, Ni, and Co represent the content (wt%) of the respective elements).

7. The method of claim 6, wherein the slab has a value of Formula (2) below of 56 or more,         Formula (2):     3Cr + 2Ni - 50Mn - 2Co (wherein Cr, Ni, Mn, and Co represent the content (wt%) of the respective elements).

8. The method of claim 6, further comprising: cold rolling the hot-rolled steel sheet at a reduction ratio of 50% or more, and cold annealing at 800 to 950°C to manufacture a cold-rolled steel sheet.

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