Patent Publication Number: US-2021189533-A1

Title: Austenitic steel material having excellent hot workability and manufacturing method therefor

Description:
CROSS REFERENCE 
     This patent application is a continuation of U.S. patent application Ser. No. 16/061,196, filed on Jun. 11, 2018, which is the U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/KR2016/015121, filed on Dec. 23, 2016, which claims the benefit of Korean Patent Application No. 10-2015-0184757, filed on Dec. 23, 2015 and Korean Patent Application No. 10-2016-0176294, filed on Dec. 22, 2016, the entire contents of each are hereby incorporated by reference 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a non-magnetic steel material having high hot workability and a method for manufacturing the non-magnetic steel material. 
     BACKGROUND ART 
     Transformer structures include a case and a lock plate, and steel materials used for such transformer structures are required to have high non-magnetic characteristics. 
     Recently, steel materials having high non-magnetic characteristics in which austenite is stabilized by adding large amounts of manganese (Mn) and carbon (C) while entirely excluding chromium (Cr) and nickel (Ni) have been developed. Austenite is a paramagnetic substance having low magnetic permeability and is more non-magnetic than ferrite. 
     High manganese (Mn) steel materials having austenite in which carbon (C) is contained in large amounts are suitable for use as non-magnetic steel materials due to high stability of austenite. 
     However, if elements such as aluminum (Al) or phosphorus (P), remaining in manufacturing processes of high manganese steel materials, are included in austenite in large amounts, the crack sensitivity of the steel materials increases at high temperatures. This is due to low hot ductility and internal grain boundary oxidation at high temperatures. High crack sensitivity has a large influence on the surface quality of steel materials at room temperature. 
     Therefore, it is necessary to develop a non-magnetic steel material having low crack sensitivity and high surface qualities. 
     DISCLOSURE 
     Technical Problem 
     An aspect of the present disclosure may provide a non-magnetic steel material having high hot workability, low hot crack sensitivity, and high surface qualities. 
     Another aspect of the present disclosure may provide a method for manufacturing a non-magnetic steel material having high hot workability, low hot crack sensitivity, and high surface qualities. 
     Technical Solution 
     According to an aspect of the present disclosure, a non-magnetic steel material having high hot workability may include manganese (Mn): 15 wt % to 27 wt %, carbon (C): 0.1 wt % to 1.1 wt %, silicon (Si): 0.05 wt % to 0.50 wt %, phosphorus (P): 0.03 wt % or less (excluding 0%), sulfur (S): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.050 wt % or less (excluding 0%), chromium (Cr): 5 wt % or less (including 0%), boron (B): 0.01 wt % or less (including 0%), nitrogen (N): 0.1 wt % or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities, wherein the non-magnetic steel material has a composition index of sensitivity expressed by Formula 1 below within a range of 3.4 or less, 
       −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr≤3.4  [Formula 1]
 
     where each of [P], [Al], [B], and [Cr] is a weight percent (wt %) of a corresponding element, 
     wherein the non-magnetic steel material has a microstructure including austenite in an area fraction of 95% or greater. 
     The austenite may have an average grain size of 10 μm or greater. 
     According to another aspect of the present disclosure, a method for manufacturing a non-magnetic steel material having high hot workability may include: 
     preparing a slab, the slab including manganese (Mn): 15 wt % to 27 wt %, carbon (C): 0.1 wt % to 1.1 wt %, silicon (Si): 0.05 wt % to 0.50 wt %, phosphorus (P): 0.03 wt % or less (excluding 0%), sulfur (S): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.050 wt % or less (excluding 0%), chromium (Cr): 5 wt % or less (including 0%), boron (B): 0.01 wt % or less (including 0%), nitrogen (N): 0.1 wt % or less (excluding 0%), and a balance of iron (Fe) and inevitable impurities, the slab having a composition index of sensitivity expressed by Formula 1 below within a range of 3.4 or less, 
       −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr≤3.4  [Formula 1]
 
     where each of [P], [Al], [B], and [Cr] is a weight percent (wt %) of a corresponding element; 
     reheating the slab to a temperature within a range of 1050° C. to 1250° C.; 
     hot rolling the reheated slab to obtain a hot-rolled steel material; and 
     cooling the hot-rolled steel material. 
     Advantageous Effects 
     Embodiments of the present disclosure may provide a non-magnetic steel material having uniform austenite, good non-magnetic characteristics, and high surface qualities owing to low crack sensitivity, and a method for manufacturing the non-magnetic steel material. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating surface quality scores for measuring crack sensitivity, a score of 1 indicating a state having no surface crack, a score of 1.5 indicating a state having fine defects, and a score of 2 indicating a state in which cracks propagate and large cracks are present. 
         FIG. 2  is a schematic example view illustrating crack sensitivity measurement portions for crack sensitivity evaluation. 
         FIG. 3  is a graph illustrating a relationship between crack sensitivity and a composition index of sensitivity. 
     
    
    
     BEST MODE 
     Embodiments of the present disclosure will now be described in detail. 
     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 disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. 
     It will be further understood that the terms “comprises” and/or “comprising” used herein specify the presence of stated features or elements, but do not preclude the presence or addition of one or more other features or elements. 
     Hereinafter, a non-magnetic steel material of the present disclosure having high hot workability will be described in detail. 
     According to an aspect of the present disclosure, the non-magnetic steel material having high hot workability includes: manganese (Mn): 15 wt % to 27 wt %, carbon (C): 0.1 wt % to 1.1 wt %, silicon (Si): 0.05 wt % to 0.50 wt %, phosphorus (P): 0.03 wt % or less (excluding 0%), sulfur (S): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.050 wt % or less (excluding 0%), chromium (Cr): 5 wt % or less (including 0%), boron (B): 0.01 wt % or less (including 0%), nitrogen (N): 0.1 wt % or less (excluding 0%), and the balance of iron (Fe) and inevitable impurities, wherein the non-magnetic steel material has a composition index of sensitivity expressed by Formula 1 below within the range of 3.4 or less and has a microstructure having austenite in an area fraction of 95% or greater. 
       −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr≤3.4  [Formula 1]
 
     (where each of [P], [Al], [B], and [Cr] is the weight percent (wt %) of the corresponding element) 
     First, the alloying elements and the contents of the alloying elements of the steel material will be described. 
     Manganese (Mn): 15 wt % to 27 wt % 
     Preferably, the content of manganese (Mn) is adjusted to be within the range of 15 wt % to 27 wt %. 
     Manganese (Mn) is an element stabilizing austenite. Manganese (Mn) may be added in an amount of 15 wt % or greater to stabilize austenite at very low temperatures. 
     If the content of manganese (Mn) is less than 15 wt %, ε-martensite being a metastable phase may be formed in a steel material having a low content of carbon (C), and may be easily transformed into α′-martensite at a very low temperature by strain induced transformation. Thus, the toughness of the steel material may decrease. 
     Furthermore, in the case of a steel material having a content of carbon (C) increased to guarantee toughness, physical properties of the steel material may markedly deteriorate because of precipitation of carbides. 
     If the content of manganese (Mn) is greater than 27 wt %, manufacturing costs increase, and thus the economic feasibility of the steel material may decrease. 
     More preferably, the content of manganese (Mn) may be within the range of 15 wt % to 25 wt %, and even more preferably within the range of 17 wt % to 25 wt %. 
     Carbon (C): 0.1 wt % to 1.1 wt % 
     Preferably, the content of carbon (C) is adjusted to be within the range of 0.1 wt % to 1.1 wt %. 
     Carbon (C) is an element stabilizing austenite and increasing the strength of the steel material. 
     Carbon (C) may decrease transformation points Ms and Md at which austenite transforms into ε-martensite or α′-martensite during a cooling or processing process. 
     If the content of carbon (C) is less than 0.1 wt %, the stability of austenite is insufficient to obtain stabile austenite at very low temperatures, and austenite may be easily transformed into ε-martensite or α′-martensite by external stress through strain induced transformation, thereby decreasing the toughness and strength of the steel material. 
     If the content of carbon (C) is greater than 1.1 wt %, the toughness of the steel material may markedly decrease because of precipitation of carbides, and the strength of the steel material may excessively increase to result in a decrease in the workability of the steel material. 
     More preferably, the content of carbon (C) may be within the range of 0.1 wt % to 1.0 wt %, and even more preferably within the range of 0.1 wt % to 0.8 wt %. 
     Silicon (Si): 0.05 wt % to 0.5 wt % 
     Like aluminum (Al), silicon (Si) is an element inevitably added in very small amounts as a deoxidizer. If the content of silicon (Si) is excessive, oxides are formed along grain boundaries which may decrease high-temperature ductility and may decrease surface quality by causing cracks. However, costs may be excessively incurred to decrease the content of silicon (Si) in steel, and thus it may be preferable that the lower limit of the content of silicon (Si) be set to be 0.05%. Silicon (Si) is more oxidable than aluminum (Al), and thus if the content of silicon (Si) is greater than 0.5%, oxides may be formed which cause cracks decreasing surface quality. Therefore, it may be preferable that the content of silicon (Si) be adjusted to be within the range of 0.05 wt % to 0.5%. 
     Chromium (Cr): 5 wt % or Less (Including 0%) 
     If chromium (Cr) is added to the steel material in an appropriate amount, chromium (Cr) stabilizes austenite and thus improves the low-temperature impact toughness of the steel material. In addition, chromium (Cr) dissolves in austenite and thus increases the strength of the steel material. Furthermore, chromium (Cr) improves the corrosion resistance of the steel material. However, chromium (Cr) is a carbide forming element. Particularly, chromium (Cr) leads to the formation of carbides along grain boundaries of austenite and thus decreases low-temperature impact toughness. Therefore, the content of chromium (Cr) may be determined by considering a relationship with carbon (C) and other elements, and since chromium (Cr) is an expensive element, it may be preferable that the content of chromium (Cr) be adjusted to be 5 wt % or less. 
     More preferably, the content of chromium (Cr) may be within the range of 0 wt % to 4 wt %, and even more preferably within the range of 0.001 wt % to 4 wt %. 
     Boron (B): 0.01 wt % or Less (Including 0%) 
     Preferably, the content of boron (B) may be adjusted to be within the range of 0.01 wt % or less. 
     Boron (B) is an element strengthening austenite grain boundaries. 
     Even a small amount of boron (B) may strengthen austenite grain boundaries and may thus decrease the crack sensitivity of the steel material at high temperatures. To improve surface quality by austenite grain boundary strengthening, the content of boron (B) may preferably be 0.0005 wt % or greater. 
     However, if the content of boron (B) is greater than 0.01%, segregation may occur along austenite grain boundaries, and thus the crack sensitivity of the steel material may increase at high temperatures, thereby decreasing the surface quality of the steel material. 
     Aluminum (Al): 0.050 wt % or Less (Excluding 0%) 
     Preferably, the content of aluminum (Al) may be adjusted to be within the range of 0.05 wt % or less (excluding 0%). 
     Aluminum (Al) is added as a deoxidizer. Aluminum (Al) may form precipitate by reacting with carbon (C) or nitrogen (N) and may thus decrease hot workability. Thus, the content of aluminum (Al) may preferably be adjusted to be 0.05 wt % or less (excluding 0%). More preferably, the content of aluminum (Al) may be within the range of 0.005 wt % to 0.05 wt %. 
     Sulfur (S): 0.01 wt % or Less (Excluding 0%) 
     The content of sulfur (S) may be adjusted to be 0.01% or less for controlling the amounts of inclusions. If the content of sulfur (S) is greater than 0.01%, hot embrittlement may occur. 
     Phosphorus (P): 0.03 wt % or Less (Excluding 0%) 
     Phosphorus (P) easily segregates and leads to cracks during a casting process. To prevent this, the content of phosphorus (P) is adjusted to be 0.03% or less. If the content of phosphorus (P) is greater than 0.03%, castability may decrease, and thus the upper limit of the content of phosphorus (P) is set to be 0.03%. 
     Nitrogen (N): 0.1 wt % or Less (Excluding 0%) 
     Like carbon (C), nitrogen (N) is an element stabilizing austenite and improving toughness. In addition, like carbon (C), nitrogen (N) is very effective in improving strength by the effect of solid solution strengthening or the formation of precipitate. However, if the content of nitrogen (N) is greater than 0.1%, physical properties or surface quality of the steel material deteriorate because of coarsening of carbonitrides coarsen, and thus it may be preferable that the upper limit of the content of nitrogen (N) be set to be 0.1 wt %. More preferably, the content of nitrogen (N) may be within the range of 0.001 wt % to 0.06 wt %, and even more preferably within the range of 0.005 wt % to 0.03 wt %. 
     In the present disclosure, the steel material includes the balance of iron (Fe) and inevitable impurities. 
     Impurities of raw materials or manufacturing environments may be inevitably included in the steel material, and such impurities may not be removed from the steel material. 
     Such impurities are well-known to those of ordinary skill in the steel manufacturing industry, and thus descriptions thereof will not be given in the present disclosure. 
     According to the aspect of the present disclosure, the non-magnetic austenitic steel material having high hot workability has a composition index of sensitivity expressed by Formula 1 below within the range of 3.4 or less. 
       −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr≤3.4  [Formula 1]
 
     (where each of [P], [Al], [B], and [Cr] is the weight percent (wt %) of the corresponding element) 
     If the composition index of sensitivity expressed by Formula 1 is greater than 3.4, cracking may easily occur and propagate, thereby increasing surface defects of products. 
     According to the aspect of the present disclosure, the non-magnetic austenitic steel material having high hot workability has austenite in an area fraction of 95% or greater. 
     Austenite, which is a paramagnetic substance having low magnetic permeability and is more non-magnetic than ferrite, is a key microstructure for guaranteeing non-magnetic characteristics. 
     If the area fraction of austenite is less than 95%, it may be difficult to guarantee non-magnetic characteristics. 
     The average grain size of austenite may be 10 μm or greater. 
     The grain size of austenite obtainable through a manufacturing process of the present disclosure is 10 μm or greater, and since the strength of the steel material may decrease if the grain size markedly increases, it may be preferable that the grain size of austenite be 60 μm or less. 
     According to the aspect of the present disclosure, the non-magnetic steel material having high hot workability may include one or more of precipitates and ε-martensite in an area fraction of 5% or less. 
     If the area fraction of one or more of precipitates and ε-martensite is greater than 5%, the toughness and ductility of the steel material may decrease. 
     Hereinafter, a method for manufacturing a non-magnetic steel material having high hot workability will be described in detail according to the present disclosure. 
     According to another aspect of the present disclosure, the method for manufacturing a non-magnetic steel material having high hot workability includes: 
     preparing a slab, the slab including manganese (Mn): 15 wt % to 27 wt %, carbon (C): 0.1 wt % to 1.1 wt %, silicon (Si): 0.05 wt % to 0.50 wt %, phosphorus (P): 0.03 wt % or less (excluding 0%), sulfur (S): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.050 wt % or less (excluding 0%), chromium (Cr): 5 wt % or less (including 0%), boron (B): 0.01 wt % or less (including 0%), nitrogen (N): 0.1 wt % or less (excluding 0%), and the balance of iron (Fe) and inevitable impurities, the slab having a composition index of sensitivity expressed by Formula 1 below within the range of 3.4 or less, 
       −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr≤3.4  [Formula 1]
 
     (where each of [P], [Al], [B], and [Cr] is the weight percent (wt %) of the corresponding element); 
     reheating the slab to a temperature within a range of 1050° C. to 1250° C.; 
     hot rolling the reheated slab to obtain a hot-rolled steel material; and 
     cooling the hot-rolled steel material. 
     Reheating of Slab 
     A slab is reheated in a heating furnace to a temperature of 1050° C. to 1250° C. for a hot rolling process. 
     If the reheating temperature is too low, that is, lower than 1050° C., the load acting on a rolling mill may be markedly increased, and alloying elements may not be sufficiently dissolved in the slab. Conversely, if the reheating temperature is too high, grains may excessively grow to cause a strength decrease, and the reheating temperature may be higher than the temperature of the solidus curve of the slab to cause poor rollability. Therefore, it may be preferable that the upper limit of the reheating temperature be 1250° C. 
     Hot Rolling 
     A hot rolling process is performed on the reheated slab to obtain a hot-rolled steel material. 
     The hot rolling process may include a rough rolling process and a finish rolling process. 
     Preferably, the temperature of the hot finish rolling process may be adjusted to be within the range of 800° C. to 1050° C. If the hot rolling temperature is less than 800° C., a great rolling load may be applied, and if the hot rolling temperature is greater than 1050° C., an intended degree of strength may not be obtained because of coarse grains. Thus, it may be preferable that the upper limit of the hot rolling temperature be set to be 1050° C. 
     Cooling 
     The hot-rolled steel material obtained through the hot rolling process is cooled. 
     After the finish rolling process, the hot-rolled steel material may be cooled at a sufficiently high cooling rate to suppress the formation of carbides along grain boundaries. If the cooling rate is less than 10° C./s, the formation of carbides may not be sufficiently suppressed, and thus carbides may precipitate along grain boundaries during cooling. This may cause problems such as premature fracture, a ductility decrease, and a wear resistance decrease. Therefore, the cooling rate may be adjusted to be as high as possible, and the upper limit of the cooling rate may not be limited to a particular value as long as the cooling rate is within an accelerated cooling rate range. However, since it is generally difficult to increase the cooling rate of accelerated cooling to be greater than 100° C./s, it may be preferable that the upper limit of the cooling rate of the cooling process be set to be 100° C./s. 
     When the hot-rolled steel material is cooled, a cooling stop temperature may preferably be set to be 600° C. or less. Although the steel material is cooled at a high cooling rate, if the cooling of the steel material is stopped at a high temperature, carbides may be formed and grown in the steel material. 
     MODE FOR INVENTION 
     Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples should be considered in a descriptive sense only and not for purpose of limitation. The scope of the present invention is defined by the appended claims, and modifications and variations reasonably made therefrom. 
     Examples 
     Slabs satisfying compositions shown in Table 1 below were reheated to 1200° C. and were hot rolled under the hot finish rolling conditions shown in Table 1 below to manufacture hot-rolled steel materials having a thickness of 12 mm. Then, the hot-rolled steel materials were cooled to 300° C. at a cooling rate of 20° C./s 
     The grain size, yield strength, tensile strength, elongation, and crack sensitivity of the hot-rolled steel sheets (steel materials) manufactured as described above were measured, and results thereof are shown in Table 2 below. 
     The crack sensitivity is a reference for checking the hot workability of the steel materials, and as shown in  FIG. 2 , the surface quality of a lateral edge, a leading edge, and an upper surface of each of the steel materials were measured to evaluate the crack sensitivity. The degree of sensitivity of each measurement portion was scored according to references shown in  FIG. 1 , and the product of scores of the three portions was shown as sensitivity in Table 2 below. In Table 2 below, if the sensitivity is 3 or less, it is determined as having good surface quality. 
     In addition, Table 2 below shows a composition index of sensitivity which is −0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr. 
     In addition, a relationship between the sensitivity and the composition index of sensitivity which is 0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr, shown in Table 2, is illustrated in  FIG. 3 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Finish 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 rolling 
               
            
           
           
               
               
               
            
               
                   
                 Composition (wt %) 
                 temperature 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Mn 
                 Si 
                 P 
                 S 
                 N 
                 Al 
                 B 
                 Cr 
                 (° C.) 
               
               
                   
               
               
                 *E1 
                 0.42 
                 20.3 
                 0.21 
                 0.016 
                 0.004 
                 0.015  
                 0.028 
                 — 
                 — 
                 870 
               
               
                 E2 
                 0.46 
                 25.0 
                 0.29 
                 0.016 
                 0.004 
                 0.020  
                 0.026 
                 0.0042 
                 3.93  
                 891 
               
               
                 E3 
                 0.40 
                 19.9 
                 0.17 
                 0.016 
                 0.003 
                 0.018  
                 0.025 
                 0.0023 
                 2.05  
                 930 
               
               
                 E4 
                 0.39 
                 21.6 
                 0.19 
                 0.017 
                 0.007 
                 0.019  
                 0.025 
                 0.0045 
                 2.06  
                 905 
               
               
                 E5 
                 0.40 
                 25.0 
                 0.22 
                 0.016 
                 0.004 
                 0.021  
                 0.026 
                 — 
                 — 
                 885 
               
               
                 E6 
                 0.40 
                 22.1 
                 0.21 
                 0.016 
                 0.004 
                 0.016  
                 0.021 
                 0.0030 
                 — 
                 940 
               
               
                 E7 
                 0.39 
                 19.6 
                 0.18 
                 0.018 
                 0.009 
                 0.018  
                 0.022 
                 0.0038 
                 2.03  
                 938 
               
               
                 E8 
                 1.10 
                 17.9 
                 0.21 
                 0.018 
                 0.004 
                 0.018  
                 0.028 
                 0.0040 
                 2.70  
                 937 
               
               
                 *CE1  
                 0.40 
                 22.0 
                 0.19 
                 0.029 
                 0.004 
                 0.018 
                 0.026 
                 — 
                 — 
                 922 
               
               
                 CE2 
                 0.40 
                 22.1 
                 0.18 
                 0.027 
                 0.003 
                 0.017  
                 0.072 
                 0.0037 
                 — 
                 938 
               
               
                 CE3 
                 0.40 
                 22.2 
                 0.20 
                 0.015 
                 0.004 
                 0.017  
                 0.051 
                 — 
                 — 
                 894 
               
               
                 CE4 
                 0.40 
                 22.2 
                 0.20 
                 0.030 
                 0.003 
                 0.017  
                 0.060 
                 — 
                 — 
                 933 
               
               
                 CES 
                 0.40 
                 22.1 
                 0.22 
                 0.030 
                 0.003 
                 0.018  
                 0.059 
                 — 
                 — 
                 885 
               
               
                   
               
               
                 *E: Example, 
               
               
                 **CE: Comparative Example 
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                   
                 Properties 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Surface quality 
                 Grain 
                 Yield 
                 Tensile 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Composition 
                   
                 size 
                 strength 
                 strength 
                 Elongation 
               
               
                 No. 
                 index 
                 Sensitivity 
                 (μm) 
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 *E1 
                 3.21 
                 1.00 
                 28 
                 371.4 
                 977.4 
                 50.9 
               
               
                 E2 
                 1.70 
                 1.00 
                 37 
                 427.1 
                 871.5 
                 59.3 
               
               
                 E3 
                 2.11 
                 1.00 
                 32 
                 350.6 
                 946.0 
                 55.9 
               
               
                 E4 
                 0.39 
                 1.00 
                 33 
                 358.9 
                 905.3 
                 57.1 
               
               
                 E5 
                 2.98 
                 1.50 
                 26 
                 360.5 
                 918.0 
                 27.0 
               
               
                 E6 
                 0.03 
                 1.50 
                 43 
                 329.9 
                 896.6 
                 56.0 
               
               
                 E7 
                 0.64 
                 1.50 
                 29 
                 344.1 
                 933.7 
                 45.9 
               
               
                 E8 
                 1.50 
                 2.25 
                 31 
                 508.3 
                 1003.9 
                 29.5 
               
               
                 **CE1 
                 3.43 
                 3.38 
                 30 
                 342.5 
                 925.9 
                 61.9 
               
               
                 CE2 
                 5.52 
                 3.38 
                 40 
                 325.5 
                 887.0 
                 53.1 
               
               
                 CE3 
                 5.73 
                 8.00 
                 28 
                 356.2 
                 928.7 
                 52.7 
               
               
                 CE4 
                 7.24 
                 8.00 
                 35 
                 339.0 
                 920.0 
                 61.4 
               
               
                 CES 
                 7.13 
                 8.00 
                 33 
                 352.5 
                 899.9 
                 39.2 
               
               
                   
               
               
                 *E: Example, 
               
               
                 **CE: Comparative Example 
               
            
           
         
       
     
     As shown in Tables 1 and 2 above, Examples 1 to 8 had good surface quality because the sensitivity thereof was within the range of 3 or less as proposed in the present disclosure. 
     Comparative Example 1, having a high content of phosphorus (P), had relatively high crack sensitivity, that is, a composition index of 3.43. 
     Comparative Example 2 to which boron (B) was added had a decreased composition index because of a relatively high aluminum (Al) content and thus, decreased crack sensitivity. However, the composition index and crack sensitivity of Comparative Example 2 were outside of the ranges proposed in the present disclosure. 
     Comparative Example 3, having an aluminum (Al) content outside of the range proposed in the present disclosure, had a composition index of 5.73 and a crack sensitivity of 8.00. 
     Comparative Examples 4 and 5 had a relatively high composition index and crack sensitivity because of the addition of phosphorus (P) and aluminum (Al). 
     In addition, as illustrated in  FIG. 3 , when the composition index of sensitivity expressed by 0.451+34.131*P+111.152*Al−799.483*B+0.526*Cr was 3.4 or less, sensitivity of 3 or less was obtained, that is, good surface quality was obtained. 
     While embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.