Austenitic stainless steel and manufacturing method thereof

Abstract

Disclosed are aa austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a manufacturing method thereof. The austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, wherein an average grain size (d) of a central portion in a thickness direction is 5 μm or less, and a martensite area fraction measured in the bent portion after a 180° bending test is 10% or less.

1 . An austenitic stainless steel comprising, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities,
 wherein an average grain size (d) of a central portion in a thickness direction is 5 μm or less, and a martensite area fraction measured in a bent portion after a 180° bending test is 10% or less.   
     
     
         2 . The austenitic stainless steel according to  claim 1 , wherein the bent portion has a center line average height Ra of 0.5 μm or less and a ten point average roughness Rz of 3 μm or less as surface roughness. 
     
     
         3 . The austenitic stainless steel according to  claim 1 , wherein a pitting potential value measured by a 3.5% NaCl solution at 30° C. is 250 mV or more. 
     
     
         4 . A method of manufacturing an austenitic stainless steel, the method comprising:
 hot rolling a slab including, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities;   cold rolling the hot-rolled steel sheet at room temperature with a reduction ratio of 40% or more; and   cold annealing the cold-rolled steel sheet to satisfy a Ω value represented by Equation (1) below to at least −10 but not more than 10:   
       
         
           
             
               
                 
                   
                     Ω 
                     = 
                     
                       406 
                       - 
                       
                         2127 
                         * 
                         
                           [ 
                           C 
                           ] 
                         
                       
                       - 
                       
                         26.2 
                         * 
                         
                           [ 
                           Mn 
                           ] 
                         
                       
                       - 
                       
                         31.5 
                         * 
                         
                           [ 
                           Ni 
                           ] 
                         
                       
                       - 
                       
                         127 
                         * 
                         
                           [ 
                           N 
                           ] 
                         
                       
                       - 
                       
                         48.2 
                         [ 
                         Nb 
                         ] 
                       
                       - 
                       
                         0.108 
                         * 
                         Temp 
                       
                     
                   
                 
                 
                   
                     Equation 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         (wherein in Equation (1), [C], [Mn], [Ni], [N], and [Nb] represent weight percentages (wt %) of respective elements and Temp refers to cold annealing temperature (° C.)) 
       
     
     
         5 . The method according to  claim 4 , wherein the cold rolling is performed after the hot rolling without performing hot annealing.