Lean duplex stainless steel having improved corrosion resistance and machinability, and manufacturing method therefor

Abstract

A lean duplex stainless steel and a method of manufacturing the same are provided. The lean duplex stainless steel includes, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities, wherein a predicted pitting potential is from 360 to 440 mV. Thus, manufacturing costs may be reduced via adjustment of components of the duplex stainless steel and both of formability and corrosion resistance may be improved by improving corrosion resistance and increasing elongation. Formability may be improved by inhibiting formation of thermal martensite and increasing elongation via adjustment of cooling conditions during coiling and cooling after hot rolling.

1 . A lean duplex stainless steel having improved corrosion resistance and formability comprising, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities, wherein a predicted pitting potential obtained by Equation (1) below is from 360 to 440 mV:
   Pitting potential=−623.2+47.4Cr eq   Equation (1)
   (Cr eq =Cr+1.37Mo+0.75W+1.5Si+2Nb+3Ti+5V+5.5Al).   
     
     
         2 . The lean duplex stainless steel of  claim 1 , further comprising at least one selected from the group consisting of 1.0% or less of molybdenum (Mo) and 1.0% or less of tungsten (W),
 wherein a total content of the molybdenum (Mo) and the tungsten (W) is from 0.15 to 1.0%.   
     
     
         3 . The lean duplex stainless steel of  claim 1 , further comprising at least one selected from the group consisting of 0.05% or less of titanium (Ti), 0.09% or less of niobium (Nb), 0.095% or less of vanadium (V), and 0.19% or less of tin (Sn). 
     
     
         4 . The lean duplex stainless steel of  claim 1 , further comprising at least one selected from the group consisting of 0.19% or less of tin (Sn) and 0.1% of antimony (Sb). 
     
     
         5 . The lean duplex stainless steel of  claim 1 , wherein the stainless steel comprises 40 to 75% of an austenite phase and the remainder of a ferrite phase. 
     
     
         6 . The lean duplex stainless steel of  claim 5 , wherein the stainless steel has a fraction of thermal martensite of 10% or less. 
     
     
         7 . The lean duplex stainless steel of  claim 1 , wherein the stainless steel has a pitting potential of 360 mV or more. 
     
     
         8 . The lean duplex stainless steel of  claim 1 , wherein the stainless steel has a hot rolled elongation of 35% or more. 
     
     
         9 . The lean duplex stainless steel of  claim 1 , wherein the stainless steel has a cold rolled elongation of 40% or more. 
     
     
         10 . A method of manufacturing a lean duplex stainless steel having improved corrosion resistance and formability, the method comprising:
 preparing a lean duplex stainless steel slab comprising, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities; and   hot rolling, hot annealing, coiling, cooling, cold rolling, and cold annealing the slab,   wherein a predicted pitting potential of the stainless steel obtained by Equation (1) below is from 360 to 440 mV:
   Pitting potential=−623.2+47.4Creq  Equation (1)
 
   (Creq=Cr+1.37Mo+0.75W+1.5Si+2Nb+3Ti+5V+5.5Al).   
     
     
         11 . The method of  claim 10 , wherein a coiling temperature and a cooling speed after coiling of a hot annealed steel satisfy Equation (3) below:
     A≤ 690+25*log  B   Equation (3)
   wherein A is coiling temperature (° C.) B is cooling speed after coiling (° C./sec).