Patent Publication Number: US-2017349985-A1

Title: Austenitic stainless steels excellent in flexibility

Description:
TECHNICAL FIELD 
     The present invention relates to austenitic stainless steels excellent in flexibility. 
     BACKGROUND ART 
     Attempts have been made to apply stainless steel to air conditioner refrigerant piping for conventional household use and automobiles. This is because it is not only excellent in corrosion resistance but also relatively low in material cost. 
     However, work such as bending of piping is essential since installation of air conditioner refrigerant piping is limited by the installation space, but there exists a problem in that the general stainless steel does not have the flexibility that must be provided in piping installation. 
     A metal material has a property that when subjected to strain such as tensile or compression, work hardening occurs and it becomes stronger as it is subjected to strain. The bending of pipe is a complex action of tension and compression, and as the degree of bending increases, the material becomes more hardened. In particular, SUS 304, which is most widely used as austenitic stainless steel, has a severe degree of work hardening, and it is very difficult to bend piping by manpower in a space where air conditioner piping work is required. 
     Work hardening is expressed as TS-YS, which is the difference between the yield strength (YS) indicating the strength at the start of material deformation and the tensile strength (TS) indicating the maximum strength due to maximization of work hardening of the material. In other words, in order to bend the material easily with manpower, a material in which TS-YS is minimized by suppressing such work hardening phenomenon is required. 
     In the austenitic stainless steels, Cr, Ni, Mn, Cu, C and N elements are mainly added. Although many steel types have been produced by varying the content of these elements, an optimum component control method for excellent flexibility has not been disclosed. In the present invention, it was attempted to produce materials having excellent flexibility by minimizing work hardening through control of these elements. 
     It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention, and is not to be construed as admission that it is the prior art known to those skilled in the art. 
     (Patent Literature 0001) KR 10-2010-0099726 A (2010.09.13) 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide austenitic stainless steels excellent in flexibility by controlling the content of component elements affecting the degree of work hardening and controlling the size of crystal grains in order to solve such conventional problems. 
     Technical Solution 
     To achieve the object described above, an austenitic stainless steel excellent in flexibility according to the present invention is characterized by comprising, by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding 0), and the remainder comprising Fe and unavoidable impurities, wherein the work hardening formula H1 defined by the following formula is 300 or less. 
       H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N)
 
     The austenitic stainless steel excellent in flexibility according to the present invention is characterized by having the size of structure (D) of 20 to 40 μm. 
     To achieve the object described above, an austenitic stainless steel excellent in flexibility according to the present invention is characterized by comprising, by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding 0), and the remainder comprising Fe and unavoidable impurities, wherein the work hardening formula H2 defined by the following formula is 300 or less. 
       H2=4.27+0.875(−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N))−287D (D: the size of structure)
 
     The size of structure (D) is characterized by being 20 to 300 μm. 
     An austenitic stainless steel excellent in flexibility according to the present invention is characterized by comprising, by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding 0), and the remainder comprising Fe and unavoidable impurities, wherein M d30  defined by the following formula is 0 or less. 
       M d30 =551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr
 
     It is preferable that M d30  is −100 to 0. 
     The difference value between TS (tensile strength) and YS (yield strength) is characterized by being 300 MPa or less. 
     Advantageous Effects 
     The present invention has an advantage that austenitic stainless steels excellent in flexibility can be produced by controlling the content of elements, the size of crystal grains, and the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a correlation between the work hardening formula H1 and actually measured values of work hardening degree; 
         FIG. 2  is a diagram showing a change of the work hardening formula H1 according to the size of crystal grains: 
         FIGS. 3 to 5  show size distributions of crystal grains: 
         FIG. 6  is a diagram showing a correlation between the modified work hardening formula H2 and actually measured values of the work hardening degree, and 
         FIG. 7  is a diagram showing a correlation between the austenite stabilization index and actually measured values of the work hardening degree. 
     
    
    
     MODE FOR INVENTION 
     Hereinafter, austenitic stainless steels excellent in flexibility according to preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     An austenitic stainless steel according to the present invention is characterized by containing, by weight percent, 0.1 to 0.65% of Si, 1.0 to 3.0% of Mn, 6.5 to 10.0% of Ni, 16.5 to 18.5% of Cr, 6.0% or less of Cu (excluding 0), 0.13% or less of (C+N) (excluding 0), and the remainder comprising Fe and unavoidable impurities. 
     The reasons for limiting the numerical values of the components constituting the austenitic stainless steel excellent in flexibility of the present invention will be described below. 
     C+N should be added to 0.13 wt % or less. 
     C and N not only harden the austenitic stainless steel as interstitial solid solution strengthening elements but also increase the work hardening degree of the material by hardening the strain induced martensite generated during processing if the contents of C and N are high. Therefore, there is a need to limit the content of C and N, and in the present invention, the content of C+N is limited to 0.13% or less. 
     Si is added in a controlled amount with the range of 0.1 to 0.65 wt %. 
     Since Si is an element added essentially for deoxidation, 0.1% or more is added. 
     However, when an excessively high content of Si is added, the material is hardened and the corrosion resistance is lowered by forming inclusions in association with oxygen, so the upper limit is limited to 0.65%. 
     Mn is added in a controlled amount with the range of 1.0 to 3.0 wt %. 
     Mn, which is an element not only added essentially for deoxidation but also increases the degree of stabilization of the austenite phase, is added at 1.0% or more for maintaining the austenite balance. However, the addition of an excessively high content of Mn reduces the corrosion resistance of the material, so the upper limit is limited to 3.0%. 
     Ni is added in a controlled amount with the range of 6.5 to 10.0 wt %. 
     Ni is not only effective for improving the corrosion resistance such as pitting corrosion resistance by being added with Cr in combination, but also can increase softening of austenite steel when its content is increased. 
     In addition, Ni is an element contributing to improvement of phase stability of austenitic stainless steel, and is added at 6.5% or more in order to maintain an austenite balance. However, the addition of an excessively high content of Ni results in an increase in the cost of the steel, so the upper limit is limited to 10.0%. 
     Cr is added in a controlled amount with the range of 16.5 to 18.5 wt %. 
     Cr is an indispensable element for improving the corrosion resistance, and in order to be used for general purpose, 16.5% or more of Cr should be added. However, the addition of an excessively high content of Cr causes austenite phase hardening and increases the cost, so the upper limit is limited to 18.5%. 
     Cu is added in a controlled amount with the range of 6.0 wt % or less. 
     Cu can cause softening of the austenite steel. However, the addition of an excessively high content of Cu lowers the hot workability and can rather harden the austenite phase, so the upper limit is limited to 6.0%. 
     In order to attain the object of the present invention, the component control method provided by the present invention is important. In order to express this specifically, the following description will be made with reference to the embodiments of the present invention. The materials described in the following embodiments were prepared by preparing ingots with a 150 mm thickness, heating them to 1,250° C., hot rolling them to 3 mm, and then heat treating them at 1,100° C. for 60 seconds or more. However, such a manufacturing method does not limit the characteristics of the material provided in the present invention, but merely adopts one of the conventional methods of manufacturing austenitic stainless steel, and is merely an example of producing a material for evaluating characteristics. The characteristics of the material change depending on the component control method provided by the present invention. The yield strength YS and the tensile strength TS are values obtained by uniaxially tensioning the material. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Classification 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Cu 
                 C + N 
                 TS-YS 
                 H1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 0.4 
                 2.7 
                 8.0 
                 17.3 
                 2.7 
                 0.019 
                 281 
                 292 
               
               
                 Example 1 
               
               
                 Invention 
                 0.4 
                 1.7 
                 9.6 
                 17.4 
                 3.2 
                 0.028 
                 277 
                 284 
               
               
                 Example 2 
               
               
                 Invention 
                 0.4 
                 1.7 
                 9.6 
                 17.4 
                 3.2 
                 0.024 
                 273 
                 281 
               
               
                 Example 3 
               
               
                 Invention 
                 0.4 
                 2.8 
                 9.6 
                 17.5 
                 3.1 
                 0.010 
                 276 
                 271 
               
               
                 Example 4 
               
               
                 Invention 
                 0.4 
                 2.7 
                 9.6 
                 17.4 
                 3.2 
                 0.011 
                 279 
                 267 
               
               
                 Example 5 
               
               
                 Invention 
                 0.4 
                 2.7 
                 9.7 
                 17.5 
                 3.2 
                 0.019 
                 277 
                 273 
               
               
                 Example 6 
               
               
                 Invention 
                 0.4 
                 2.7 
                 9.6 
                 17.4 
                 3.2 
                 0.041 
                 280 
                 285 
               
               
                 Example 7 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.3 
                 16.9 
                 2.1 
                 0.016 
                 287 
                 286 
               
               
                 Example 8 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.4 
                 16.9 
                 2.2 
                 0.033 
                 295 
                 294 
               
               
                 Example 9 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.1 
                 17.0 
                 2.8 
                 0.018 
                 288 
                 284 
               
               
                 Example 10 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.0 
                 17.0 
                 2.7 
                 0.036 
                 293 
                 295 
               
               
                 Example 11 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.4 
                 16.8 
                 2.7 
                 0.017 
                 280 
                 275 
               
               
                 Example 12 
               
               
                 Invention 
                 0.4 
                 1.2 
                 8.4 
                 17.0 
                 2.7 
                 0.036 
                 287 
                 293 
               
               
                 Example 13 
               
               
                 Invention 
                 0.6 
                 1.2 
                 7.6 
                 16.9 
                 3.0 
                 0.017 
                 283 
                 296 
               
               
                 Example 14 
               
               
                 Invention 
                 0.6 
                 1.2 
                 7.6 
                 16.9 
                 4.0 
                 0.021 
                 286 
                 286 
               
               
                 Example 15 
               
               
                 Invention 
                 0.6 
                 1.2 
                 7.6 
                 16.7 
                 5.0 
                 0.020 
                 274 
                 263 
               
               
                 Example 16 
               
               
                 Comparative 
                 0.6 
                 1.2 
                 7.6 
                 16.9 
                 2.1 
                 0.056 
                 328 
                 329 
               
               
                 Example 1 
               
               
                 Comparative 
                 0.4 
                 1.0 
                 7.9 
                 17.7 
                 0.2 
                 0.088 
                 407 
                 399 
               
               
                 Example 2 
               
               
                 Comparative 
                 0.6 
                 1.2 
                 7.5 
                 16.8 
                 2.0 
                 0.021 
                 309 
                 308 
               
               
                 Example 3 
               
               
                   
               
            
           
         
       
     
     H1 shown in Table 1 is defined by the following formula. 
       H1=−459+79.8Si−10.2Mn−8.16Ni+48.0Cr−13.2Cu+623(C+N)
 
     In the present invention, in order to obtain an austenitic stainless steel excellent in flexibility by controlling the TS-YS value to 300 MPa or less, the H1 values are defined using the component elements constituting the present invention, and the correlation between the H1 values and the actually measured TS-YS values were analyzed. 
     As shown in  FIG. 1 , it can be seen that the relationship between the H1 values obtained through the component control and the actually measured TS-YS values is shown, and the above description is implemented. In particular, as shown by a dotted line, a linearly smooth relationship is established therebetween. Therefore, it can be seen that even if the lower limit of the H1 value is not set in the present invention, it is possible to manufacture an austenitic steel having more excellent flexibility through production of a material having a lower H1 value. 
     On the other hand, the crystal grain size of the austenitic stainless steel produced by a conventional manufacturing process is generally 30±10 μm. 
     As shown in Table 2, the crystal grain size (D) of the austenitic stainless steel excellent in flexibility of the present invention is also present in the interval of 30±10 μm, and it can be seen that when H1 is obtained as 329 as in Comparative Example 1 of Table 2, the actual TS-YS value is obtained as 328, indicating that the flexibility is not good. 
     As above, it can be seen that the values of H1 and the actual TS-YS values have similar values at crystal grain sizes of the range of 30 f 10 μm, which is also confirmed through  FIG. 2 . 
     However, in a case when the size of the crystal grains exceeds the range of 30±10 μm, it can be seen that the actual TS-YS values are less than 300 MPa even if the values of H1 exceed 300 MPa, which is also confirmed through Invention Examples 17, 18, 19, 20 and 21 in Table 2 and the section marked as ellipse in  FIG. 2 . 
     If the crystal grain size is large, surface irregularity defect called orange peel occurs during processing. However, if the smoothness of the surface is not important or can be corrected through polishing and can be ignored, even if the crystal grain size is large, it is not a big problem. 
       FIGS. 3 to 5  show size distributions of crystal grains, in which  FIG. 3  is a structure photograph showing the crystal grain size of the austenitic stainless steel according to the following Invention Example 6,  FIG. 4  is a structure photograph showing the crystal grain size of the austenitic stainless steel according to the following Comparative Example 6, and  FIG. 5  is a structure photograph showing the crystal grain size of the austenitic stainless steel according to the following Invention Example 17. 
     In the present invention, a modified work hardening formula H2 is provided so as to obtain a material having a low work hardening degree even when the crystal grain size is larger than usual. 
       H2=4.27+0.875H1−0.287D
 
     As shown in Table 2 and  FIG. 6 , it can be seen that austenitic stainless steels excellent in flexibility can be produced by controlling the range of the modified work hardening formula H2 to 300 MPa or less. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 TS-YS 
                 H1 
                 D 
                 H2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Invention 
                 281 
                 292 
                 29 
                 289 
               
               
                   
                 Example 1 
               
               
                   
                 Invention 
                 277 
                 284 
                 31 
                 282 
               
               
                   
                 Example 2 
               
               
                   
                 Invention 
                 273 
                 281 
                 33 
                 279 
               
               
                   
                 Example 3 
               
               
                   
                 Invention 
                 276 
                 271 
                 29 
                 271 
               
               
                   
                 Example 4 
               
               
                   
                 Invention 
                 279 
                 167 
                 31 
                 268 
               
               
                   
                 Example 5 
               
               
                   
                 Invention 
                 277 
                 173 
                 32 
                 272 
               
               
                   
                 Example 6 
               
               
                   
                 Invention 
                 280 
                 285 
                 35 
                 282 
               
               
                   
                 Example 7 
               
               
                   
                 Invention 
                 269 
                 336 
                 223 
                 273 
               
               
                   
                 Example 17 
               
               
                   
                 Invention 
                 247 
                 316 
                 218 
                 256 
               
               
                   
                 Example 18 
               
               
                   
                 Invention 
                 240 
                 301 
                 209 
                 246 
               
               
                   
                 Example 19 
               
               
                   
                 Invention 
                 267 
                 333 
                 284 
                 253 
               
               
                   
                 Example 20 
               
               
                   
                 Invention 
                 283 
                 316 
                 93 
                 292 
               
               
                   
                 Example 21 
               
               
                   
                 Comparative 
                 328 
                 329 
                 33 
                 321 
               
               
                   
                 Example 1 
               
               
                   
                 Comparative 
                 337 
                 406 
                 210 
                 337 
               
               
                   
                 Example 4 
               
               
                   
                 Comparative 
                 371 
                 406 
                 990 
                 372 
               
               
                   
                 Example 5 
               
               
                   
                 Comparative 
                 313 
                 336 
                 72 
                 316 
               
               
                   
                 Example 6 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 shows the component contents of Invention Examples 17 to 21 and Comparative Examples 4 to 6 disclosed in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Classification 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Cu 
                 C + N 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Invention 
                 0.6 
                 1.2 
                 7.5 
                 16.7 
                 3.9 
                 0.119 
               
               
                   
                 Example 17 
               
               
                   
                 Invention 
                 0.6 
                 1.3 
                 7.6 
                 17.0 
                 5.0 
                 0.087 
               
               
                   
                 Example 18 
               
               
                   
                 Invention 
                 0.6 
                 1.3 
                 7.9 
                 17.1 
                 5.8 
                 0.075 
               
               
                   
                 Example 19 
               
               
                   
                 Invention 
                 0.5 
                 1.1 
                 6.9 
                 17.1 
                 4.4 
                 0.091 
               
               
                   
                 Example 20 
               
               
                   
                 Invention 
                 0.6 
                 1.3 
                 7.6 
                 17.0 
                 5.0 
                 0.087 
               
               
                   
                 Example 21 
               
               
                   
                 Comparative 
                 0.2 
                 1.4 
                 8.1 
                 18.1 
                 0.2 
                 0.105 
               
               
                   
                 Example 4 
               
               
                   
                 Comparative 
                 0.2 
                 1.4 
                 8.1 
                 18.1 
                 0.2 
                 0.105 
               
               
                   
                 Example 5 
               
               
                   
                 Comparative 
                 0.6 
                 1.2 
                 7.5 
                 16.7 
                 3.9 
                 0.119 
               
               
                   
                 Example 6 
               
               
                   
                   
               
            
           
         
       
     
     On the other hand, the TS-YS values may be limited by the following austenite stability M d30 . 
     As shown in  FIG. 7 , it can be seen that when M d30  exceeds 0, the TS-YS values greatly increase, and in the range where M d30  is 0 or less, the TS-YS values do not react sensitively to M d30  but remain at a constant low level. 
     In order to maintain the M d30  in the range of 0 or less, Si, Mn, Ni, Cu and Cr which are the main additive elements must be added. In the present invention, M d30 -related component parameters for maintaining the TS-YS values at 300 MPa or less are presented. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 TS-YS 
                 M d30   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Invention Example 1 
                 281 
                 −30 
               
               
                   
                 Invention Example 2 
                 227 
                 88 
               
               
                   
                 Invention Example 3 
                 273 
                 85 
               
               
                   
                 Invention Example 4 
                 276 
                 88 
               
               
                   
                 Invention Example 5 
                 279 
                 88 
               
               
                   
                 Invention Example 6 
                 277 
                 −97 
               
               
                   
                 Invention Example 7 
                 280 
                 −102 
               
               
                   
                 Invention Example 8 
                 287 
                 −2 
               
               
                   
                 Invention Example 9 
                 295 
                 −14 
               
               
                   
                 Invention Example 10 
                 288 
                 −18 
               
               
                   
                 Invention Example 11 
                 293 
                 −22 
               
               
                   
                 Invention Example 12 
                 280 
                 −21 
               
               
                   
                 Invention Example 13 
                 287 
                 −34 
               
               
                   
                 Invention Example 14 
                 283 
                 −13 
               
               
                   
                 Invention Example 15 
                 286 
                 −41 
               
               
                   
                 Invention Example 16 
                 274 
                 −69 
               
               
                   
                 Comparative Example 1 
                 328 
                 −1 
               
               
                   
                 Comparative Example 2 
                 407 
                 20 
               
               
                   
                 Comparative Example 3 
                 309 
                 20 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 4, when the values are maintained at 0 or less, the TS-YS values can be maintained at 300 MPa or less, which indicates that the flexibility is improved. 
     On the other hand, in order to lower the M d30  values, the component element contents should be further increased. In order to reduce the cost, the lower limit value is preferably limited to −100. 
     While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that the present invention may be variously modified and changed without departing from the technical idea of the present invention provided by the following claims. 
     INDUSTRIAL APPLICABILITY 
     The austenitic stainless steels excellent in flexibility according to the embodiments of the present invention are applicable to air conditioner refrigerant piping and the like for domestic use and automobiles.