Ferritic stainless steel having improved corrosion resistance and magnetic properties, and manufacturing method therefor

Abstract

Disclosed in the present specification is a ferritic stainless steel having improved corrosion resistance and magnetic properties improved by having an optimized manufacturing process and alloy components; and a manufacturing method therefor. The ferritic stainless steel having improved corrosion resistance and magnetic properties, according to one embodiment of the present invention, may comprise, by wt %, 0.0005-0.035% of C, 0.005-0.05% of N, 0.1-2.0% of Si, 0.1-0.5% of Mn, 16.0-20.0% of Cr, Mo in an amount of more than 0% and less than or equal to 0.5%, Nb in an amount of more than 0% and less than or equal to 0.5%, 0.005-0.30% of Ti, and the balance of Fe and impurities.

Description

TECHNICAL FIELD

[0001] The disclosure relates to a ferritic stainless steel having improved corrosion resistance and magnetic properties and method of manufacturing the same, and more particularly, to a ferritic stainless steel with corrosion resistance and magnetic properties improved by optimizing an alloy composition and manufacturing processes and a method of manufacturing the ferritic stainless steel.BACKGROUND ART

[0002] With the recent development in such technical areas as smartphones, semi-autonomous driving vehicles, etc., various electronic devices are being used, which leads to a rapid increase in the use of electromagnetic waves. This causes an increase in interference by electromagnetic waves between the electronic devices. The interference by electromagnetic waves may cause malfunction of the device or make it difficult for the device to be precisely controlled. To prevent malfunction of the electronic device caused by the interference by electromagnetic waves, critical devices need to be covered by a material that may shield magnetic fields.

[0003] Meanwhile, as ferritic stainless steel has corrosion resistance as well as comparatively high magnetic permeability, the ferritic stainless steel may be used for various purposes requiring both the corrosion resistance and a shielding function. However, there have thus far been not many technologies that may satisfy both high corrosion resistance and high magnetic permeability.

[0004] In patent document 1, disclosed is a ferrite stainless steel with improved corrosion resistance through content control between Si, Ti, Nb and Al. However, it fails to secure high magnetic permeability to be used for the electronic device.PRIOR ART LITERATURE

[0005] Patent Document 1: Korean patent publication No. 10-2302386 (published on Nov. 23, 2020)DISCLOSURETechnical Problem

[0006] To solve the aforementioned problem, the disclosure aims to provide a ferritic stainless steel with corrosion resistance and magnetic permeability improved by optimizing an alloy composition and manufacturing processes and a method of manufacturing the ferritic stainless steel.Solution to Problem

[0007] According to an embodiment of the disclosure, a ferritic stainless steel having improved corrosion resistance and magnetic properties includes, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities, wherein a value of equation 1 below is at least 20:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1

[0008] In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.

[0009] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may have at least 1000 of maximum magnetic permeability value in a 50 Hz frequency band.

[0010] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may have a pitting potential of at least 200 mV.

[0011] Furthermore, in an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may have a surface grain diameter of at least 30 μm.

[0012] According to an embodiment of the disclosure, a method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties includes manufacturing a slab including, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities, wherein a value of equation 1 below is at least 20; reheating the slab at 1100 to 1300° C.; manufacturing a hot-rolled steel sheet by hot rolling and hot annealing the reheated slab; and manufacturing a cold-rolled steel sheet by cold rolling, cold annealing and pickling the hot-rolled steel sheet:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / Mn.Equation⁢ 1

[0013] In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.

[0014] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may satisfy the value of equation 2 below being at least 50:[hot⁢ annealing⁢ temperature⁢ (°⁢ C.)*hot⁢ annealing⁢ time⁢ (min)+
1.1*(cold⁢ annealing⁢ temperature⁡(°⁢ C.)*
cold⁢ annealing⁢ time⁢ (min))] / cold⁢ rolling⁢ reduction⁢ rate⁢ (%).Equation⁢ 2

[0015] In an embodiment of the disclosure, in the method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties, the hot annealing may be performed at 950 to 1150° C. for 1.5 to 2.5 minutes.

[0016] In an embodiment of the disclosure, in the method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties, the cold annealing may be performed at 1000 to 1200° C. for 1 to 2 minutes.

[0017] In an embodiment of the disclosure, in the method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties, the cold annealing may be performed with a reduction rate of 60 to 75%.Advantageous Effects

[0018] According to an embodiment of the disclosure, a ferritic stainless steel having corrosion resistance and magnetic properties improved by optimizing an alloy composition and manufacturing processes and a method of controlling the same may be provided.BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a graph representing changes in pitting potential according to equation 1.

[0020] FIG. 2 is a graph representing changes in maximum magnetic permeability according to equation 2.BEST MODE

[0021] According to an embodiment of the disclosure, a ferritic stainless steel having improved corrosion resistance and magnetic properties includes, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities, wherein a value of equation 1 below is at least 20:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1

[0022] In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.Modes

[0023] Reference will now be made in detail to embodiments, which are illustrated in the accompanying drawings. The following embodiments are provided as examples to convey the full spirit of the disclosure to those of ordinary skill in the art to which the embodiments of the disclosure belong. The disclosure is not limited to the embodiments but may be specified in any other forms. In the drawings, unrelated part of the description is not shown to clarify the disclosure, and the size of an element may be a little exaggerated to help understanding.

[0024] Throughout the specification, the term “include (or including)” or “comprise (or comprising)” is inclusive or open-ended and does not exclude additional, unrecited components, elements or method steps, unless otherwise stated.

[0025] It is to be understood that the singular forms “a,”“an,” and “the” include plural references unless the context clearly dictates otherwise.

[0026] A reason for numerical limitation of the content of an alloy composition in an embodiment of the disclosure will now be described. A unit of weight (wt) % will now be used unless otherwise mentioned.

[0027] According to an embodiment of the disclosure, a ferritic stainless steel having improved corrosion resistance and magnetic properties includes, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities.

[0028] The content of C (carbon) may be 0.0005 to 0.035%.

[0029] As impurities increase with an increase of the content of C, the content of C needs to be reduced. However, when the content of C is overly reduced, the refining price becomes expensive. Considering this, at least 0.0005% of C may be added. On the other hand, when the content of C is excessive, the elongation rate decreases and ductile-brittle transition temperature (DBTT) rises, so impact characteristics become worse. Considering this, the upper limit of the content of C may be at least 0.035%.

[0030] The content of N (nitrogen) may be 0.005 to 0.05%.

[0031] When the content of N is too low, TiN crystallization is lowered, leading to a reduction in equiaxed crystal ratio of the slab. Considering this, N may be added in at least 0.005%. However, when the content of N is excessive, the elongation decreases and impact characteristics may become worse. Considering this, the upper limit of the content of N may be at least 0.05%.

[0032] The content of Si (silicon) may be 0.1 to 2.0%.

[0033] Si may increase hardness as a ferrite phase forming element. Considering this, at least 0.1% of Si may be added. However, when the content of Si is excessive, the elongation rate decreases and Si based inclusions increase, so that machinability may be lowered. Considering this, the upper limit of the content of Si may be limited to 2.0%.

[0034] The content of Mn (manganese) may be 0.1 to 0.5%.

[0035] When the content of Mn is too low, fine MnS precipitates may be formed, causing a weak magnetic pull due to grain refinement. Considering this, Mn may be added in at least 0.1%. However, when the content of Mn is excessive, the fraction of the precipitates increases, so the magnetic pull may rather decrease. Considering this, the upper limit of the content of Mn may be limited to 0.5%.

[0036] The content of Cr (chrome) may be 16.0 to 20.0%.

[0037] Cris a ferrite phase stabilizing element along with Si, and not only plays a major role in securing the ferrite phase, but is also an essential element added to improve corrosion resistance. Considering this, Cr may be added in at least 16.0%. However, when the content of Cr content is excessive, it promotes formation of delta (δ) ferrite in the slab, reducing elongation and impact toughness and causing hot rolling sticking defects. Considering this, the upper limit of the content of Cr may be limited to 20.0%.

[0038] The content of Mo (molybdenum) may be more than 0 to 0.5%.

[0039] Mo, along with Cr, is an element that is effective in stabilizing ferrite and improving corrosion resistance. However, Mo may cause worsening of magnetic pull due to grain refinement. Considering this, the upper limit of the content of Mo may be limited to 0.5%.

[0040] The content of Nb (niobium) may be more than 0 to 0.5%.

[0041] When the content of Nb is excessive, Nb based precipitates may overly increase so that the grain size may not grow sufficiently. Hence, when the Nb content is excessive, the magnetic permeability may decrease. Considering this, the upper limit of the content of Nb may be limited to 0.5% or less.

[0042] The content of Ti (titanium) may be 0.005 to 0.30%.

[0043] Ti is an effective element to enhance strength by causing precipitation. Considering this, Ti may be at least 0.005%. However, when the content of Ti is excessive, it may cause a decrease in magnetic permeability due to grain refinement. Considering this, the upper limit of the content of Ti may be limited to 0.30%.

[0044] The remaining ingredient is iron (Fe) in the disclosure. However, unintended impurities may be inevitably mixed in from raw materials or surroundings in the normal manufacturing process, so they may not be excluded. These impurities may be known to anyone skilled in the ordinary manufacturing process, so not all of them are specifically mentioned in this specification.

[0045] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may satisfy the value of equation 1 below being at least 20:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1

[0046] In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.

[0047] FIG. 1 is a graph representing changes in pitting potential according to equation 1.

[0048] Referring to FIG. 1, it may be understood that the larger the value of equation 1, the higher the pitting potential. Especially, in order to maintain the pitting potential for securing high corrosion resistance to be at least 200 mV, ranges of the ally ingredients may be controlled for the value of equation 1 to be at least 20.

[0049] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may have at least 1000 of maximum magnetic permeability value in a 50 Hz frequency band.

[0050] As will be described later, the maximum magnetic permeability value may be secured to at least 1000 by controlling hot annealing temperature, hot annealing time, cold rolling reduction rate, cold annealing temperature and cold annealing time, which are main process factors that affect magnetic properties of the stainless steel.

[0051] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may have a surface grain diameter of at least 30 μm.

[0052] As the surface grain diameter becomes smaller, the magnetic properties may deteriorate. Hence, in the disclosure, the magnetic properties are improved by optimizing the contents of Mn, Mo, Nb, Ti, etc., and thus controlling the surface grain diameter to at least 30 μm.

[0053] A method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties according to another aspect of the disclosure will now be described.

[0054] In an embodiment of the disclosure, a method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties includes manufacturing a slab including, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities, wherein a value of equation 1 below is at least 20; reheating the slab at 1100 to 1300° C.; manufacturing a hot-rolled steel sheet by hot rolling and hot annealing the reheated slab; and manufacturing a cold-rolled steel sheet by cold rolling, cold annealing and pickling the hot-rolled steel sheet:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1

[0055] In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.

[0056] The reason of numerical limitation of equation 1 and the ingredient range of each alloy composition is as described above, and each manufacturing step will now be described in more detail.

[0057] A slab that satisfies the alloy composition and equation 1 may be manufactured first, and may then undergo a series of reheating, hot rolling, hot annealing, cold annealing and pickling processes.

[0058] The manufactured slab may first be reheated at 1100 to 1300° C.

[0059] When the reheating temperature is low, it is difficult to re-decompose coarse precipitates produced during slab casting. Considering this, the reheating temperature may be at least 1100° C. However, when the reheating temperature is too high, internal grains may become too coarse. Considering this, the upper limit of the reheating temperature may be limited to 1300° C.

[0060] In an embodiment of the disclosure, the ferritic stainless steel having improved corrosion resistance and magnetic properties may satisfy the value of equation 2 below being at least 50:[hot⁢ annealing⁢ temperature⁢ (°⁢ C.)*hot⁢ annealing⁢ time⁢ (min)+
1.1*(cold⁢ annealing⁢ temperature⁡(°⁢ C.)*
cold⁢ annealing⁢ time⁢ (min))] / cold⁢ rolling⁢ reduction⁢ rate⁢ (%).Equation⁢ 2

[0061] FIG. 2 is a graph representing changes in maximum magnetic permeability according to equation 2.

[0062] Referring to FIG. 2, it may be understood that the larger the value of equation 2, the higher the maximum magnetic permeability. Hence, the magnetic properties of stainless steel may be improved by controlling the value of equation 2 including hot annealing temperature, hot annealing time, cold rolling reduction rate, cold annealing temperature and cold annealing time, which are main process factors that affect the magnetic properties, to at least 50.

[0063] In the meantime, the hot annealing may be performed at 950 to 1150° C. for 1.5 to 2.5 minutes.

[0064] When the hot annealing temperature is low or the performing time is short, grains do not grow sufficiently, which may adversely affect the magnetic properties. On the other hand, when the hot annealing temperature is too high or the performing time is too long, it may lower the strength due to grain coarsening. Considering this, the hot annealing may be performed at 950 to 1150° C. for 1.5 to 2.5 minutes, and more desirably, at 1000 to 1100° C. for 2 to 2.5 minutes.

[0065] The cold annealing may be performed at 1000 to 1200° C. for 1 to 2 minutes.

[0066] When the cold annealing temperature is low or the performing time is short, grains do not grow sufficiently and the elongation may decrease. On the other hand, when the cold annealing temperature is too high or the performing time is too long, it may lower the strength due to grain coarsening. Considering this, the cold annealing may be performed at 1000 to 1200° C. for 1 to 2 minutes, and more desirably, at 1100 to 1200° C. for 1.5 to 2 minutes.

[0067] In the meantime, the cold annealing may be performed with a reduction rate of 60 to 75%.

[0068] When the reduction rate decreases, the grain diameter grows, which is favorable for magnetic properties, but when the reduction rate is too small, it may unfavorable for machinability. Hence, it is desirable to perform the cold annealing with a reduction rate of 60 to 75%.

[0069] Embodiments of the disclosure will now be described in more detail. The embodiments may be merely for illustration, and the disclosure is not limited thereto. The scope of the disclosure is defined by the claims and their equivalents.Embodiment

[0070] A slab was manufactured in a melting furnace with various alloy ingredient ranges shown in table 1 below. A cold-rolled steel sheet was manufactured by reheating the manufactured slab at 1250° C. and then applying the hot annealing temperature, the hot annealing time, the cold annealing temperature, the cold annealing time and the cold rolling reduction rate shown in table 2 below.TABLE 1alloy ingredientssectionCSiMnCrMoNNbTiEmbodiment 10.00520.150.3017.720.4530.00690.0070.278Embodiment 20.00870.380.2717.870.0120.00990.4190.175Comparative0.03400.300.5016.120.0150.04330.0060.009example 1Comparative0.00680.180.2316.230.0110.00960.0000.178example 2Comparative0.01400.600.2515.900.0100.00900.0000.256example 3Comparative0.00860.170.3017.330.0070.00720.0050.314example 4Comparative0.00660.390.2117.850.0090.01000.4500.205example 5Comparative0.00801.920.3016.220.0070.00880.0000.220example 6Comparative0.00780.350.2517.780.010.010.0050.08example 7TABLE 2hothotcoldcoldcoldannealingannealingannealingannealingrollingtemperaturetimetemperaturetimereductionsection(° C.)(min)(° C.)(min)rate (%)Embodiment 19891.9110311.6762Embodiment 210441.8710051.2264Comparative6001.717611.1374example 1Comparative10441.7610100.8978example 2Comparative9882.169860.9281example 3Comparative9932.4010251.3171example 4Comparative5912.778701.4775example 5Comparative6801.749600.7784example 6Comparative6602.809551.4870example 7In table 3 below: shown are values of equation 1, values of equation 2, pitting potential and maximum magnetic permeability. The value of equation 1 is shown by calculating the following equation 1:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1In equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements.

[0073] The value of equation 2 is shown by calculating the following equation 2:[hot⁢ annealing⁢ temperature⁢ (°⁢ C.)*hot⁢ annealing⁢ time⁢ (min)+
1.1*(cold⁢ annealing⁢ temperature⁡(°⁢ C.)*
cold⁢ annealing⁢ time⁢ (min))] / cold⁢ rolling⁢ reduction⁢ rate⁢ (%).Equation⁢ 2

[0074] The pitting potential was measured using a potentiostat device. In this case, when the stainless steel was immersed in a NaCl solution and a voltage of 20 mV / min was applied thereto, the pitting potential at which the current reached 100 μA was measured. The NaCl solution had a temperature of 30° C. and a concentration of 3.5%. In the meantime, the higher the pitting potential value, the better the corrosion resistance.

[0075] The maximum magnetic permeability was measured using the Single Sheet Test from Brockhaus. It may be estimated that the higher the maximum magnetic permeability the better the magnetic properties.TABLE 3pittingmaximumpotentialmagneticsectionequation 1equation 2(mV)permeabilityEmbodiment 123.9261.012361650Embodiment 222.0651.692191343Comparative17.7126.73178500example 1Comparative18.4136.43156634example 2Comparative19.8038.77189517example 3Comparative19.2654.321861143example 4Comparative23.3340.63229780example 5Comparative23.7123.73223641example 6Comparative20.0448.61205940example 7

[0076] Referring to table 3, as embodiments 1 and 2 satisfied the alloy composition, ingredient ranges, equation 1, manufacturing processes and equation 2 proposed in the disclosure, pitting potential satisfied at least 200 mV and the maximum magnetic permeability satisfied at least 1000. On the other hand, comparative examples 1 to 4 did not satisfy the value of equation 1 being at least 20. Hence, comparative examples 1 to 4 did not satisfy the pitting potential being at least 200 mV. In other words, comparative examples 1 to 4 had deteriorated corrosion resistance.

[0077] Furthermore, comparative examples 1 to 3 and 5 to 7 did not satisfy the value of equation 2 being at least 50. Hence, comparative examples 1 to 3 and 5 to 7 did not satisfy the maximum magnetic permeability being at least 1000. In other words, comparative examples 1 to 3 and 5 to 7 had deteriorated magnetic properties.INDUSTRIAL APPLICABILITY

[0078] According to an embodiment of the disclosure, a ferritic stainless steel having corrosion resistance and magnetic properties improved by optimizing an alloy composition and manufacturing processes and a method of controlling the same may be provided, so the industrial applicability is acknowledged.

Claims

1. A ferritic stainless steel having improved corrosion resistance and magnetic properties, the method comprising:in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities,wherein a value of equation 1 below is at least 20:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1(in equation 1, Cr, Mo, N, Si, Nb, Ti, and Mn refer to wt % of the respective elements).

2. The ferritic stainless steel of claim 1, wherein a maximum magnetic permeability value in the 50 Hz frequency band is at least 1000.

3. The ferritic stainless steel of claim 1, wherein pitting potential is at least 200 mV.

4. The ferritic stainless steel of claim 1, wherein a surface grain diameter is at least 30 μm5. A method of manufacturing a ferritic stainless steel having improved corrosion resistance and magnetic properties, the method comprising:manufacturing a slab including, in percent by weight (wt %), 0.0005 to 0.035% of C, 0.005 to 0.05% of N, 0.1 to 2.0% of Si, 0.1 to 0.5% of Mn, 16.0 to 20.0% of Cr, more than 0 to 0.5% of Mo, more than 0 to 0.5% of Nb, 0.005 to 0.30% of Ti, and the remainder having Fe and impurities, wherein a value of equation 1 below is at least 20;reheating the slab at 1100 to 1300° C.;manufacturing a hot-rolled steel sheet by hot rolling and hot annealing the reheated slab; andmanufacturing a cold-rolled steel sheet by cold rolling, cold annealing and pickling the hot-rolled steel sheet:Cr+(3*Mo+10*N+S⁢i+N⁢b+Ti) / MnEquation⁢ 1(in equation 1, Cr, Mo, N, Si, Nb, Ti and Mn refer to wt % of the respective elements).

6. The method of claim 5, wherein a value of equation 2 below is at least 50:[hot⁢ annealing⁢ temperature⁢ (°⁢ C.)*hot⁢ annealing⁢ time⁢ (min)+
1.1*(cold⁢ annealing⁢ temperature⁡(°⁢ C.)*
cold⁢ annealing⁢ time⁢ (min))] / cold⁢ rolling⁢ reduction⁢ rate⁢ (%).Equation⁢ 27. The method of claim 5, wherein the hot annealing is performed at 950 to 1150° C. for 1.5 to 2.5 minutes.

8. The method of claim 5, wherein the cold annealing is performed at 1000 to 1200° C. for 1 to 2 minutes.

9. The method of claim 5, wherein the cold annealing is performed with a reduction rate of 60 to 75%.

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