Duplex Stainless Steel and Method of Manufacturing Duplex Stainless Steel

A duplex stainless steel having 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;
• 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×NiL≤NiH (1),
• where NiH and NiL are obtained by using an electron-beam microanalyzer to measure Ni content at points separated by 0.6 μm in an area of 300×300 μm2 and, in a distribution of Ni content with a class width of 0.05 mass %, determining two maximum frequencies, and treating the one with a higher Ni content as NiH and treating the one with a lower Ni content as NiL.

A method of manufacturing a duplex stainless steel, comprising the steps of:

• preparing a material having 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;
• 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;
• hot-working the material;
• and
• subjecting the hot-worked material to solution treatment at a temperature of 960 to 1045° C.