Duplex stainless steel and method of manufacturing duplex stainless steel
A duplex stainless steel with good low-temperature toughness is provided. The duplex stainless steel has a chemical composition of, in mass %: up to 0.03% C; 0.1 to 0.8% Si; up to 2.3% Mn; up to 0.040% P; up to 0.010% S; up to 0.040% sol. Al; 3 to 7% Ni; 20 to 28% Cr; 0.5 to 2.0% Mo; more than 2.0% and not more than 4.0% Cu; 0.02 to 0.5% Co; 0.1 to 0.35% N; up to 0.010% O; and other elements, the steel having a microstructure including an austenite phase and a ferrite phase, the ferrite phase having an area percentage of 30 to 60%, the steel satisfying the following Formula, (1); 0.70×Ni L ≤Ni H (1), where Ni H and Ni L are obtained by using an electron-beam microanalyzer to measure Ni content and, in a distribution of Ni content, determining two maximum frequencies, and treating the one with a higher Ni content as Ni H and treating the one with a lower Ni content as Ni L .
1. A seamless steel pipe made of a duplex stainless steel having a chemical composition of, in mass %:
up to 0.03% C;
0.1 to 0.8% Si;
up to 1.9% Mn;
up to 0.040% P;
up to 0.010% S;
0.005 to 0.040% sol. Al;
4.21 to 7% Ni;
20 to 28% Cr;
0.5 to 2.0% Mo;
more than 2.0% and not more than 4.0% Cu;
0.02 to 0.5% Co;
0.1 to 0.35% N;
up to 0.010% O;
0 to 1.5% V;
0 to 0.02% Ca;
0 to 0.02% Mg;
0 to 0.02% B;
0 to 0.2% REM; and
the balance being Fe and impurities,
the duplex stainless steel having a microstructure including an austenite phase and a ferrite phase,
the ferrite phase having an area percentage of 30 to 60%,
the duplex stainless steel satisfying the following Formula, (1):
0.70×Ni L ≤Ni H (1),
where Ni H and Ni L are obtained by the following steps:
(a) measuring Ni content for each of 250,000 points arranged grid-wise on a specimen of the duplex stainless steel and separated by 0.6 μm in an area of 300×300 μm 2 using an electron-beam microanalyzer;
(b) creating a histogram showing a frequency distribution of the Ni contents measured for the 250,000 points with a class width of 0.05% by mass; and
(c) determining a first maximal value of frequency and a second maximal value of frequency appearing in the histogram, the first maximal value having a higher Ni content and the second maximal value having a lower Ni content, and defining the first maximal value as Ni H and a second maximal value as Ni L ,
wherein the following Formula, (2), is satisfied:
{1−(AE −60 /AE −20 )}×100≤8 (2),
where AE −60 and AE −20 indicate values of absorbed energy of the steel at 60° C. and −20° C., respectively, and
a shear fracture percentage of the steel at −20° C. is 100%.
2. The seamless steel pipe made of the duplex stainless steel according to claim 1 , wherein the chemical composition includes, in mass %,
0.01 to 1.5% V.
3. The seamless steel pipe made of the duplex stainless steel according to claim 1 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
0.0001 to 0.02% Ca;
0.001 to 0.02% Mg;
0.0001 to 0.02% B; and
0.0005 to 0.2% REM.
4. The seamless steel pipe made of the duplex stainless steel according to claim 2 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
0.0001 to 0.02% Ca;
0.001 to 0.02% Mg;
0.0001 to 0.02% B; and
0.0005 to 0.2% REM.
5. The seamless steel pipe made of the duplex stainless steel according to claim 1 wherein (Ni H /Ni L ) is not less than 0.8.