Lean duplex stainless steel and method for producing the same

Abstract

Provided are lean duplex stainless steel having a dual-phase structure of an austenite phase and a ferrite phase, and a method for producing the lean duplex stainless steel. The lean duplex stainless steel, as a ferrite-austenite stainless steel, has the preferred stacking fault energy (SFE) value of the austenite phase, expressed by the formula 2 below, of 19-37 and critical strain value range, within which the strain-induced martensite phases occurs, of 0.1−0.25. Formula 2: SFE=25.7+1.59×Ni/[K(Ni)−K(Ni)×V(γ)+V(γ)]+0.795×Cu/[K(Cu)−K(Cu)×V(γ)+V(γ)]−0.85×Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]+0.001×(Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]) 2 +38.2×(N/[K(N)−K(N)×V(γ)+V(γ)]) 0.5 −2.8×Si/[K(Si)−K(Si)×V(γ)+V(γ)]−1.34×Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]+0.06×(Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]) 2 . Ni, Cu, Cr, N, Si and Mn indicate the overall content (wt. %) of the respective constituent element, and K(x) is the distribution index of respective constituent element (x) and is expressed by the formula 3 below, and V(γ) is the component ratio of austenite (in the 0.45-0.75 range). Formula 3: K(x)=[amount of element x in ferrite phase]/[amount of element x in austenite phase]

1 . A ferritic-austenitic lean duplex stainless steel wherein the stacking fault energy (SFE) value of the austenite phase represented by the following formula 2 is 19 to 37 and the range of the value of the critical strain for strain induced martensite formation is 0.1 to 0.25:
   SFE=25.7+1.59×Ni/[K(Ni)−K(Ni)×V(γ)+V(γ)]+0.795×Cu/[K(Cu)−K(Cu)×V(γ)+V(γ)]−0.85×Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]+0.001×(Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]) 2 +38.2×(N/[K(N)−K(N)×V(γ)+V(γ)]) 0.5 −2.8×Si/[K(Si)−K(Si)×V(γ)+V(γ)]−1.34×Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]+0.06×(Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]) 2   Formula 2
   wherein Ni, Cu, Cr, N, Si and Mn refer to the overall content (wt. %) of respective constituent element respectively, and K(x) is represented by the following Formula 3 as the distribution coefficient of respective constituent element (x), and V(γ) is the fraction of the austenite phase (the range of 0.45 to 0.75):
   K( x )=[content of element  x  in ferrite phase]/[content of element  x  in austenite phase]   Formula 3.
 
   
     
     
         2 . The lean duplex stainless steel according to  claim 1 , wherein regarding the K(x), K(Cr)=1.16, K(Ni)=0.57, K(Mn)=0.73, and K(Cu)=0.64, and K(N) and K(Si) have the following values depending on the content (wt. %) of N and Si:
 when N is 0.2 to 0.32%, K(N)=0.15;   when N<0.2%, K(N)=0.25;   when Si≦1.5%, K(Si)=2.76-0.96×Si; and   when Si>1.5%, K(Si)=1.4.   
     
     
         3 . The lean duplex stainless steel according to  claim 1 , wherein the elongation of the stainless steel is 45% or more. 
     
     
         4 . The lean duplex stainless steel according to  claim 1 , wherein the stainless steel includes, by weight, C: 0.08% or less (excluding 0%), Si: 0.2 to 3.0%, Mn: 2 to 4%, Cr: 18 to 24%, Ni: 0.2 to 2.5%, Cu: 0.2 to 2.5%, balance Fe and the other unavoidable impurities. 
     
     
         5 . The lean duplex stainless steel according to  claim 4 , wherein the stainless steel further includes, by weight, at least one of W: 0.1 to 1.0% and Mo: 0.1 to 1.0%. 
     
     
         6 . The lean duplex stainless steel according to  claim 4 , wherein the stainless steel further includes, by weight, at least one of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.15%. 
     
     
         7 . A method of manufacturing a ferritic-austenitic lean duplex stainless steel, comprising
 preparing a molten steel; and   treating the molten steel to form the stainless steel wherein the molten steel is treated so that the stacking fault energy (SFE) value of the austenite phase represented by the following formula 2 is 19 to 37 and the range of the value of the critical strain for strain induced martensite formation is 0.1 to 0.25:
   SFE=25.7+1.59×Ni/[K(Ni)−K(Ni)×V(γ)+V(γ)]+0.795×Cu/[K(Cu)−K(Cu)×V(γ)+V(γ)]−0.85×Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]+0.001×(Cr/[K(Cr)−K(Cr)×V(γ)+V(γ)]) 2 +38.2×(N/[K(N)−K(N)×V(γ)+V(γ)]) 0.5 −2.8×Si/[K(Si)−K(Si)×V(γ)+V(γ)]−1.34×Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]+0.06×(Mn/[K(Mn)−K(Mn)×V(γ)+V(γ)]) 2   Formula 2
 
   wherein Ni, Cu, Cr, N, Si and Mn refer to the overall content (wt. %) of respective constituent element respectively, and K(x) is represented by the following Formula 3 as the distribution coefficient of respective constituent element (x), and V(γ) is the fraction of the austenite phase (the range of 0.45 to 0.75):
   K( x )=[content of element  x  in ferrite phase]/[content of element  x  in austenite phase]   Formula 3.
 
   
     
     
         8 . The method of manufacturing the lean duplex stainless steel according to  claim 7 , wherein the process of treating the molten steel to form the stainless steel comprises
 temporarily storing the molten steel in the tundish while maintaining the temperature of the molten steel at the temperature higher than the theoretical solidification temperature by 10 to 50° C.;   primarily cooling the molten steel by injecting the molten steel in the tundish into the mold and passing the molten steel through the mold while maintaining a cooling rate of 500 to 1500° C./min; and   secondarily cooling the molten steel having the solidified shell formed by the primary cooling process while drawing it into a segment and passing through.   
     
     
         9 . The method of manufacturing the lean duplex stainless steel according to  claim 8 , wherein in the secondary cooling process, the cooling water of 0.25 to 0.35 L/Kg is sprayed on the molten steel having the formed solidified shell. 
     
     
         10 . The method of manufacturing the lean duplex stainless steel according to  claim 8 , wherein the method further comprises tertiarily cooling, after the secondary cooling process, by spraying the cooling water of 100 to 125 L/kg·min on the surface of the cast-slab in the range of the surface temperature of the cast-slab being drawn of 1100 to 1200° C. wherein the cooling water is mixed with air such that the ratio of air to cooling water (air/cooling water) is 1.0 to 1.2. 
     
     
         11 . The method of manufacturing the lean duplex stainless steel according to  claim 7 , wherein the process of treating the molten steel to form the stainless steel comprises producing a strip by solidifying the molten steel while passing it between a pair of casting rolls wherein nitrogen, which is contained in the molten steel in the process of producing the strip and exceeds a nitrogen solubility limit, is discharged through the casting roll to the outside of the solidified shell. 
     
     
         12 . The method of manufacturing the lean duplex stainless steel according to  claim 11 , wherein in the process of producing the strip, at least one of a pair of the casting rolls is a casting roll having a gas discharge channel formed in a circumferential direction on the outer peripheral surface. 
     
     
         13 . The method of manufacturing the lean duplex stainless steel according to  claim 12 , wherein the gas discharge channel formed in the casting roll used in the process of producing the strip has a width of 50 to 500 μm and a depth of 50 to 300 μm, a plurality of gas discharge channels is formed in the casting roll, the gap between adjacent gas discharge channels is 100 to 1000 μm and unevennesses of 15 to 25 μm are formed on the surface of the casting roll. 
     
     
         14 . The method of manufacturing the lean duplex stainless steel according to  claim 7 , wherein, in the process of preparing the molten steel, the molten steel comprises, by weight, C: 0.08% or less (excluding 0%), Si: 0.2 to 3.0%, Mn: 2 to 4%, Cr: 18 to 24%, Ni: 0.2 to 2.5%, N: 0.15 to 0.32%, Cu: 0.2 to 2.5%, balance Fe and the other unavoidable impurities. 
     
     
         15 . The method of manufacturing the lean duplex stainless steel according to  claim 14 , wherein, in the process of preparing the molten steel, the molten steel further comprises, by weight, at least one of W: 0.1 to 1.00% and Mo: 0.1 to 1.00%. 
     
     
         16 . The method of manufacturing the lean duplex stainless steel according to  claim 14 , wherein, in the process of preparing the molten steel, the molten steel further comprises at least one of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.15%. 
     
     
         17 . The lean duplex stainless steel according to  claim 2 , wherein the elongation of the stainless steel is 45% or more.