Patent Publication Number: US-2023151470-A1

Title: Non-magnetic austenitic stainless steel

Description:
TECHNICAL FIELD 
     The present disclosure relates to a non-magnetic austenitic stainless steel, and more particularly, to a non-magnetic austenitic stainless steel applicable as a material for various electronic devices. 
     BACKGROUND ART 
     As smart devices with various functions have been used in recent years, there have been increasing demands for steel materials having reduced magnetic properties to prevent loss of electric power and malfunction. 300 series stainless steels including an austenite phase as a main structure generally have non-magnetic properties and thus they have been widely used as materials for electronic devices. 
     However, common STS304 or STS316 austenitic stainless steels have a δ-ferrite fraction of 1 to 5% during a steelmaking/continuous casting process. The δ-ferrite formed as described above has a structures inducing magnetism, causing a problem in that a final product has magnetic properties. Therefore, these common STS304 and STS316 austenitic stainless steels could not have non-magnetic properties due to the δ-ferrite. 
     The δ-ferrite may be decomposed by heat treatment in a temperature range of 1,300 to 1,400° C. However, δ-ferrite may remain in the structure without being completely removed during rolling and annealing processes and thus magnetic properties may be enhanced by the remaining ferrite, failing to obtain non-magnetic properties. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure provides a non-magnetic austenitic stainless steel applicable as a material for various electronic devices to solve the above-described problems. 
     Technical Solution 
     One aspect of the present disclosure provides a non-magnetic austenitic stainless steel including, in percent by weight (wt %), 0.01 to 0.1% of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 0.5 to 3.5% of manganese (Mn), 16 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 3% or less of molybdenum (Mo), 0.01 to 0.3% of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities, wherein a value of Expression (1) below is a negative value. 
       3*(Cr+Mo)+5*Si−65*(C+N)−2*(Ni+Mn)−28  (1)
 
     In Expression (1), Cr, Mo, Si, C, N, Ni, and Mn denote contents (wt %) of the alloy elements, respectively. 
     The non-magnetic austenitic stainless steel may further include, in percent by weight (wt %), 2.5% or less of copper (Cu). 
     Another aspect of the present disclosure provides a non-magnetic austenitic stainless steel including, in percent by weight (wt %), 0.01 to 0.1% of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 0.5 to 3.5% of manganese (Mn), 16 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 3% or less of molybdenum (Mo), 0.01 to 0.3% of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities, wherein a value of Expression (2) below is 70 or more. 
       Σ A   5   /ΣA× 100  (2)
 
     In Expression (2), ΣA 5  is a sum of areas of ferrite grains having an area of 5 μm 2  or less, and ΣA is a sum of areas of all ferrite grains. 
     The non-magnetic austenitic stainless steel may further include, in percent by weight (wt %), 2.5% or less of copper (Cu). 
     In each of the non-magnetic austenitic stainless steels, a magnetic permeability may be 1.02 or less when a thickness is 1 mm or less. 
     Advantageous Effects 
     According to the present disclosure, a non-magnetic austenitic stainless steel applicable as a material for various electronic devices may be provided by lowering a fraction of a ferrite phase that induces magnetism. 
     According to the present disclosure, the fraction of the ferrite phase may be lowered by inhibiting formation of ferrite via adjustment of alloy components or by accelerating decomposition of ferrite by controlling a microstructure thereof. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a graph illustrating changes in ferrite fractions according to values of Expression (1) of Table 1. 
         FIG.  2    is a graph illustrating changes in magnetic permeability according to values of Expression (2) of Table 2. 
     
    
    
     BEST MODE 
     A non-magnetic austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.01 to 0.1% of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 0.5 to 3.5% of manganese (Mn), 16 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 3% or less of molybdenum (Mo), 0.01 to 0.3% of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities, wherein a value of Expression (1) below is a negative value. 
       3*(Cr+Mo)+5*Si−65*(C+N)−2*(Ni+Mn)−28  (1)
 
     In Expression (1), Cr, Mo, Si, C, N, Ni, and Mn denote contents (wt %) of the elements, respectively). 
     Mode of the Invention 
     Preferred embodiments of the present disclosure will now be described. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
     The terms used herein are merely used to describe particular embodiments. Thus, an expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In addition, it is to be understood that the terms such as “including” or “having,” etc., are intended to indicate the existence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, steps, functions, components, or combinations thereof may exist or may be added. 
     Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In addition, the terms “about”, “substantially”, etc. used throughout the specification mean that when a natural manufacturing and substance allowable error are suggested, such an allowable error corresponds a value or is similar to the value, and such values are intended for the sake of clear understanding of the present invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention. 
     A non-magnetic austenitic stainless steel according to an embodiment of the present disclosure may include, in percent by weight (wt %), 0.01 to 0.1% of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 0.5 to 3.5% of manganese (Mn), 16 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 3% or less of molybdenum (Mo), 0.01 to 0.3% of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities. The non-magnetic austenitic stainless steel may further include 2.5% or less of copper (Cu). 
     Hereinafter, reasons for numerical limitations on the contents of alloy components in the embodiment of the present disclosure will be described. Hereinafter, the unit is wt % unless otherwise stated. 
     Carbon (C): 0.01 to 0.1 wt % 
     C is an element with powerful effects on stabilizing an austenite phase and inhibiting an increase in magnetism during a solidification process. In the present disclosure, C may be added in an amount of 0.01 wt % or more for the effects on stabilizing the austenite phase. However, when the C content is excessive, C binds to Cr to form a carbide and a Cr content is locally lowered around grain boundaries, resulting in deterioration of corrosion resistance. Therefore, an upper limit of the C content may be set to 0.1 wt % in the present disclosure to obtain sufficient corrosion resistance. 
     Silicon (Si): 1.5 wt % or Less (Excluding 0) 
     Si is an element enhancing corrosion resistance. However, Si is an element stabilizing a ferrite phase that induces magnetism. An excess of Si may promote precipitation of intermetallic compounds such as delta (δ) phase, thereby deteriorating mechanical properties and corrosion resistance. Therefore, an upper limit of the Si content may be set to 1.5 wt % in the present disclosure. 
     Manganese (Mn): 0.5 to 3.5 wt % 
     Mn is an element stabilizing an austenite phase like C and Ni and is effective on enhancing non-magnetic properties. Accordingly, Mn may be added in an amount of 0.5 wt % or more in the present disclosure. However, an excess of Mn may form inclusions such as MnS, thereby deteriorating corrosion resistance and surface gloss. Therefore, an upper limit of the Mn content may be set to 3.5 wt % in the present disclosure. 
     Chromium (Cr): 16 to 22 wt % 
     Cr is a representative element effective on enhancing corrosion resistance of stainless steels and may be added in an amount of 16 wt % in the present disclosure to obtain sufficient corrosion resistance. However, Cr is an element stabilizing a ferrite phase that induces magnetism. In addition, when the Cr content is excessive, a large amount of Ni needs to be added to obtain non-magnetic properties, so that manufacturing costs increase and formation of 6-phase is promoted, resulting in deterioration of mechanical properties and corrosion resistance. Therefore, an upper limit of the Cr content may be set to 22 wt %. 
     Nickel (Ni): 7 to 15 wt % 
     Ni is an element with the most powerful effects on stabilizing an austenite phase and may be added in an amount of 7 wt % or more in the present disclosure to obtain non-magnetic properties. However, as the Ni content increases, costs for raw materials increase, and thus an upper limit of the Ni content may be set to 15 wt %. 
     Molybdenum (Mo): 3 wt % or Less 
     Mo is an element enhancing corrosion resistance. However, Mo is an element stabilizing a ferrite phase and an excess of Mo promotes formation of a a-phase, thereby deteriorating mechanical properties and corrosion resistance. Therefore, an upper limit of the Mo content may be set to 3 wt %. 
     Nitrogen (N): 0.01 to 0.3 wt % 
     N is an austenite phase-stabilizing element and may be added in an amount of 0.01 wt % or more in the present disclosure to obtain non-magnetic properties. However, an excess of N may deteriorate hot processibility of steels and surface quality. Therefore, an upper limit of the N content may be set to 0.3 wt %. 
     The non-magnetic austenitic stainless steel according to an embodiment of the present disclosure may further, optionally include 2.5 wt % or less of copper (Cu). In the following descriptions, the reason for numerical limitations on the Cu content will be described in detail. 
     Copper (Cu): 2.5 wt % or Less 
     Cu is an austenite phase-stabilizing element and may be used to replace expensive Ni. However, an excess of Cu may deteriorate hot processibility by forming a phase having a low melting point, thereby deteriorating surface quality. Therefore, an upper limit of the Cu content may be set to 2.5 wt % or less. 
     The remaining component of the austenitic stainless steel of the present disclosure is iron (Fe). However, in common manufacturing processes, undesired impurities from raw materials or manufacturing environments may be inevitably mixed therewith, and this cannot be excluded. Such impurities are well-known to those of ordinary skill in the art, and thus, specific descriptions thereof will not be given in the present disclosure. 
     In general, STS 304 or 316 stainless steels are formed of an austenite phase as a main structure and have a microstructure in which a ferrite phase formed during a steelmaking/continuous casting process remains. The austenite phase having a face-centered cubic structure is not magnetic but the ferrite phase having a body-centered cubic structure is magnetic. That is, depending on the fraction of the remaining ferrite phase, it may be difficult to obtain non-magnetic properties desired by the present disclosure. Accordingly, the fraction of the ferrite phase that induces magnetism should be decreased as low as possible to obtain non-magnetic properties. Hereinafter, technical methods for obtaining non-magnetic properties desired by the present disclosure will be described in detail. 
     Control of Alloy Components 
     A composition of alloy components significantly affects a fraction of an initially formed ferrite phase. For example, austenite phase-stabilizing elements such as Ni, Mn, C, and N reduce the fraction of the ferrite phase and some elements such as Cr and Mo increase the fraction of the ferrite phase. The present inventors have derived Expression (1) below used to control the fraction of the ferrite phase in consideration of the above-described properties. 
       3*(Cr+Mo)+5*Si−65*(C+N)−2*(Ni+Mn)−28  (1)
 
     In Expression (1), Cr, Mo, Si, C, N, Ni, and Mn denote contents (wt %) of the alloy elements, respectively. 
     According to the present disclosure, when a value of Expression (1) is a negative value, a fraction of the initially formed ferrite phase may be 0%. 
     Microstructure Control 
     Meanwhile, the ferrite phase remaining during a steelmaking/continuous casting process may be decomposed by heat treatment performed later. The present inventors have found that decomposition of the ferrite phase may be accelerated during a heat treatment process by controlling a microstructure even when the ferrite phase remains by a positive value of Expression (1) and thus a steel has magnetic properties. The acceleration of decomposition of the ferrite phase is related to size distribution of the remaining ferrite phase, and Expression (2) was derived by analysis. 
       Σ A   5   /ΣA× 100  (2)
 
     In Expression (2) above, ΣA 5  is a sum of areas of ferrite grains having an area of 5 μm 2  or less, and ΣA is a sum of areas of all ferrite grains. That is, Expression (2) means a percentage of the sum of areas of fine ferrite grains having an area of 5 μm 2  or less per the sum of areas of all ferrite grains. 
     According to an embodiment of the present disclosure, the composition may be controlled such that a value of Expression (2) is 70 or more. According to the present disclosure, decomposition of the ferrite phase may be accelerated during the heat treatment process by controlling the sum of the areas of the fine ferrite grains at a high level as described above. As a result, a magnetic permeability may be 1.02 or less after heat treatment, particularly, a magnetic permeability may be 1.02 or less when a thickness is 1 mm or less. 
     The size distribution of the ferrite phase may be controlled using various processes as long as the value of Expression (2) is 70 or more. For example, the size distribution may be controlled by a forging or rolling process and controlled by adjusting a reduction ratio and the number of rolling in various ways. However, the above-described examples are made only for illustrative purposes, and the present disclosure is not limited thereby. 
     According to the present disclosure, the fraction of the ferrite phase may be controlled to be as low as possible by adjusting the alloy components, controlling the microstructure, or controlling both the alloy components and the microstructure as described above. Therefore, the present disclosure may provide a non-magnetic austenitic stainless steel applied as a material for various electronic devices. 
     Hereinafter, the present disclosure will be described in more detail with reference to the following examples. It should be noted, however, that the following examples are intended to illustrate the present disclosure in more detail and not to limit the scope of the present disclosure. The scope of the present disclosure may be determined by the matters described in the claims and the matters reasonably deduced therefrom. 
     EXAMPLES 
     Slabs having chemical compositions of alloy components shown in Table 1 below were prepared to a thickness of 200 mm by a continuous casting. Then, the cast slabs were reheated at a temperature of 1,250° C. for 2 hours. Subsequently, the reheated slabs were hot-rolled to a thickness of 6 mm and hot-annealed at a temperature of 1,150° C. 
     Values of Expression (1) shown in Table 1 are values obtained by substituting the content (wt %) of each alloy element of Table 1 into Expression (1) below. 
       3*(Cr+Mo)+5*Si−65*(C+N)−2*(Ni+Mn)−28  (1)
 
     In Expression (1), Cr, Mo, Si, C, N, Ni, and Mn denote contents (wt %) of the alloy elements, respectively. 
     Ferrite fractions of Table 1 were derived by measuring ferrite fractions of the hot-rolled, hot-annealed coils using a contact-type ferrite scope. When no value was obtained in a contact state, the ferrite fraction was determined as 0%. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Steel 
                 Alloy composition (wt %) 
                 Expression 
                 Ferrite fraction 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 type 
                 C 
                 Mn 
                 Cr 
                 Ni 
                 Si 
                 Mo 
                 Cu 
                 N 
                 (1) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.022 
                 1.00 
                 21.2 
                 10.0 
                 0.97 
                 0.52 
                 0.21 
                 0.157 
                 8.38 
                 6.3 
               
               
                 2 
                 0.015 
                 0.66 
                 17.7 
                 12.1 
                 0.61 
                 2.07 
                 0.27 
                 0.013 
                 7.02 
                 1.9 
               
               
                 3 
                 0.024 
                 0.67 
                 17.7 
                 12.1 
                 0.67 
                 2.04 
                 0.28 
                 0.020 
                 6.17 
                 1.3 
               
               
                 4 
                 0.030 
                 0.80 
                 21.3 
                 9.3 
                 0.40 
                 0.60 
                 0.80 
                 0.200 
                 4.55 
                 1.2 
               
               
                 5 
                 0.019 
                 1.06 
                 16.1 
                 10.1 
                 0.47 
                 2.04 
                 0.29 
                 0.014 
                 4.43 
                 1.6 
               
               
                 6 
                 0.027 
                 0.92 
                 21.4 
                 9.4 
                 0.39 
                 0.54 
                 0.82 
                 0.207 
                 3.92 
                 0.1 
               
               
                 7 
                 0.041 
                 0.83 
                 20.6 
                 10.9 
                 0.97 
                 0.54 
                 0.21 
                 0.164 
                 3.49 
                 0.4 
               
               
                 8 
                 0.022 
                 0.80 
                 21.3 
                 10.1 
                 0.39 
                 0.60 
                 0.81 
                 0.200 
                 3.42 
                 0.2 
               
               
                 9 
                 0.029 
                 0.97 
                 21.2 
                 9.5 
                 0.37 
                 0.51 
                 0.76 
                 0.209 
                 2.57 
                 0.7 
               
               
                 10 
                 0.026 
                 0.78 
                 21.2 
                 9.3 
                 0.40 
                 0.58 
                 0.84 
                 0.240 
                 1.89 
                 0.3 
               
               
                 11 
                 0.029 
                 0.95 
                 21.2 
                 9.5 
                 0.33 
                 0.55 
                 0.75 
                 0.218 
                 1.95 
                 1.0 
               
               
                 12 
                 0.030 
                 0.80 
                 21.3 
                 10.3 
                 0.40 
                 0.60 
                 0.80 
                 0.220 
                 1.25 
                 0.1 
               
               
                 13 
                 0.030 
                 1.95 
                 21.6 
                 13.7 
                 1.00 
                 0.00 
                 0.99 
                 0.125 
                 0.43 
                 0.2 
               
               
                 14 
                 0.027 
                 0.86 
                 21.4 
                 10.2 
                 0.39 
                 0.58 
                 0.72 
                 0.238 
                 0.55 
                 0.8 
               
               
                 15 
                 0.031 
                 3.07 
                 20.7 
                 10.9 
                 0.97 
                 0.00 
                 2.03 
                 0.133 
                 0.35 
                 0.7 
               
               
                 16 
                 0.032 
                 2.88 
                 20.7 
                 10.0 
                 1.01 
                 0.00 
                 2.00 
                 0.172 
                 0.13 
                 0.4 
               
               
                 17 
                 0.030 
                 2.05 
                 17.1 
                 10.0 
                 1.49 
                 0.50 
                 1.99 
                 0.096 
                 −0.04 
                 0.0 
               
               
                 18 
                 0.029 
                 2.06 
                 17.0 
                 10.0 
                 1.48 
                 0.76 
                 2.00 
                 0.104 
                 −0.09 
                 0.0 
               
               
                 19 
                 0.031 
                 2.03 
                 18.8 
                 10.0 
                 0.96 
                 0.00 
                 2.01 
                 0.114 
                 −0.29 
                 0.0 
               
               
                 20 
                 0.020 
                 2.02 
                 17.0 
                 9.1 
                 1.48 
                 0.50 
                 1.99 
                 0.140 
                 −0.74 
                 0.0 
               
               
                 21 
                 0.025 
                 2.00 
                 18.0 
                 8.0 
                 0.99 
                 0.00 
                 1.98 
                 0.156 
                 −0.82 
                 0.0 
               
               
                 22 
                 0.032 
                 1.96 
                 19.9 
                 9.0 
                 1.01 
                 0.00 
                 2.01 
                 0.209 
                 −0.84 
                 0.0 
               
               
                 23 
                 0.025 
                 0.86 
                 21.2 
                 9.4 
                 0.42 
                 0.54 
                 0.79 
                 0.280 
                 −1.03 
                 0.0 
               
               
                 24 
                 0.025 
                 0.96 
                 20.4 
                 12.4 
                 0.97 
                 0.20 
                 0.30 
                 0.179 
                 −1.33 
                 0.0 
               
               
                 25 
                 0.031 
                 2.00 
                 20.3 
                 10.9 
                 0.99 
                 0.00 
                 0.99 
                 0.180 
                 −1.67 
                 0.0 
               
               
                 26 
                 0.023 
                 1.27 
                 17.3 
                 14.4 
                 0.45 
                 2.55 
                 0.00 
                 0.048 
                 −2.16 
                 0.0 
               
               
                 27 
                 0.024 
                 1.31 
                 17.3 
                 14.6 
                 0.47 
                 2.54 
                 0.20 
                 0.049 
                 −2.70 
                 0.0 
               
               
                 28 
                 0.033 
                 1.98 
                 17.9 
                 7.8 
                 1.01 
                 0.00 
                 2.00 
                 0.197 
                 −3.76 
                 0.0 
               
               
                 29 
                 0.050 
                 1.02 
                 20.3 
                 12.1 
                 0.93 
                 0.00 
                 0.00 
                 0.200 
                 −4.94 
                 0.0 
               
               
                 30 
                 0.097 
                 0.98 
                 20.5 
                 12.2 
                 0.98 
                 0.00 
                 0.00 
                 0.210 
                 −7.92 
                 0.0 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, because Steel Types 17 to 30 satisfy the alloy composition range limited by the present disclosure and values of Expression (1) thereof are negative values, ferrite fractions thereof were 0%. On the contrary, because values of Expression (1) of Steel Types 1 to 16 are positive values, ferrite remains after heat treatment although the alloy compositions of Steel Types 1 to 16 are within the range provided in the present disclosure. 
       FIG.  1    is a graph illustrating changes in ferrite fractions according to values of Expression (1) of Table 1. Referring to  FIG.  1   , it may be confirmed that the ferrite fraction tends to increase at a point where the value of Expression (1) is changed from 0 to a positive value. That is, as a result of controlling the value of Expression (1) to be a negative value, it may be visually confirmed that the ferrite fraction tends to be 0% based on  FIG.  1   . 
     Based on the above-described results, the ferrite fraction may be controlled to be 0% by adjusting the value of Expression (1) to be a negative value. As a result, target non-magnetic properties may be obtained. 
     Meanwhile, even in the case of Steel Types 1 to 16 having ferrite fractions exceeding 0.0%, a magnetic permeability may be controlled at a low level by accelerating decomposition of ferrite by controlling the microstructure. Evaluation results of Table 2 below were obtained from Steel Types 1 to 16 in which the ferrite phase remains since the ferrite fraction exceeded 0.0%. The hot-rolled coils of Steel Types 1 to 16 having a thickness of 6 mm were cold-rolled to a thickness of 1 mm or less and annealed. 
     Values of Expression (2) shown in Table 2 were derived by image analysis using an optical microscope after the cold rolling. 
     Ferrite fractions of Table 2 were derived by measuring ferrite fractions of the cold-rolled, cold-annealed coils using a contact-type ferrite scope. When no value was obtained in a contact state, the ferrite fraction was determined as 0%. 
     Magnetic permeabilities (μ) shown in Table 2 were measured using a Ferromaster that is a contact-type magnetic permeability meter. Steel Types 1 to 16 were cold-rolled to a thickness of 1 mm or less by applying various reduction ratios. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Steel 
                 Thickness 
                 Expression 
                 Ferrite fraction 
                 Magnetic permeability 
               
               
                 type 
                 (mm) 
                 (2) 
                 (%) 
                 (μ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 0.5 
                 45.29 
                 2.4 
                 1.247 
               
               
                   
                 0.3 
                 59.15 
                 1.0 
                 1.063 
               
               
                 2 
                 1.0 
                 60.49 
                 0.8 
                 1.046 
               
               
                   
                 0.8 
                 75.28 
                 0.0 
                 1.018 
               
               
                 3 
                 1.0 
                 62.48 
                 0.6 
                 1.036 
               
               
                   
                 0.8 
                 78.59 
                 0.0 
                 1.016 
               
               
                 4 
                 0.5 
                 64.26 
                 0.7 
                 1.041 
               
               
                   
                 0.3 
                 71.63 
                 0.0 
                 1.012 
               
               
                 5 
                 0.5 
                 62.83 
                 0.7 
                 1.042 
               
               
                   
                 0.3 
                 76.13 
                 0.0 
                 1.017 
               
               
                 6 
                 0.5 
                 81.55 
                 0.0 
                 1.006 
               
               
                   
                 0.2 
                 80.71 
                 0.0 
                 1.004 
               
               
                 7 
                 0.5 
                 73.59 
                 0.0 
                 1.008 
               
               
                   
                 0.3 
                 83.02 
                 0.0 
                 1.003 
               
               
                 8 
                 1.0 
                 75.67 
                 0.0 
                 1.008 
               
               
                   
                 0.8 
                 85.56 
                 0.0 
                 1.003 
               
               
                 9 
                 1.0 
                 72.85 
                 0.0 
                 1.019 
               
               
                   
                 0.8 
                 74.15 
                 0.0 
                 1.009 
               
               
                 10 
                 1.0 
                 79.32 
                 0.0 
                 1.010 
               
               
                   
                 0.3 
                 81.57 
                 0.0 
                 1.005 
               
               
                 11 
                 1.0 
                 56.94 
                 0.3 
                 1.023 
               
               
                   
                 0.5 
                 71.10 
                 0.0 
                 1.012 
               
               
                 12 
                 0.5 
                 76.37 
                 0.0 
                 1.009 
               
               
                   
                 0.3 
                 81.87 
                 0.0 
                 1.005 
               
               
                 13 
                 0.5 
                 83.68 
                 0.0 
                 1.004 
               
               
                   
                 0.3 
                 84.64 
                 0.0 
                 1.003 
               
               
                 14 
                 1.0 
                 66.96 
                 0.3 
                 1.021 
               
               
                   
                 0.2 
                 78.91 
                 0.0 
                 1.006 
               
               
                 15 
                 1.0 
                 68.01 
                 0.4 
                 1.027 
               
               
                   
                 0.2 
                 80.03 
                 0.0 
                 1.006 
               
               
                 16 
                 0.5 
                 72.14 
                 0.0 
                 1.012 
               
               
                   
                 0.3 
                 79.80 
                 0.0 
                 1.006 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, when the microstructure is controlled such that the value of Expression (2) is 70 or more, remaining ferrite is completely decomposed during the annealing process after the rolling process so that the ferrite fraction becomes 0.0%. As a result, magnetic permeabilities of 1.02 or less may be obtained. On the contrary, when the value of Expression (2) is less than 70, remaining ferrite is not completely decomposed during the annealing process after the rolling process, so that the magnetic permeability exceeds 1.02. 
       FIG.  2    is a graph illustrating changes in magnetic permeability according to values of Expression (2) of Table 2. Referring to  FIG.  2   , it may be confirmed that the magnetic permeability tends to be less than 1.02 at a point where the value of Expression (2) is exceeds 70. That is, in the present disclosure, when the value of Expression (2) is controlled to be 70 or more, it may be visually confirmed that the magnetic permeability tends to be 1.02 or less based on  FIG.  2   . 
     Based on the above-described results, even when ferrite remains after the hot rolling and annealing process, desired non-magnetic properties may be obtained by accelerating decomposition of remaining ferrite during the cold annealing process after the cold rolling process by controlling the value of Expression (2) to be 70 or more. 
     While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.