Ferritic stainless steel and process for producing same

1. A ferritic stainless steel comprising:

a chemical composition consisting of, in mass %,

C: 0.005% to 0.050%,

Si: 0.01% to 1.00%,

Mn: 0.01% to 1.0%,

P: 0.040% or less,

S: 0.010% or less,

Cr: 15.5% to 17.5%,

Ni: 0.01% to 1.0%,

Al: 0.001% to 0.10%, and

N: 0.005% to 0.06%,

and optionally

one or more selected from Cu: 0.01% to 1.0%, Mo: 0.01% to 0.5%, and Co:

0.01% to 0.5%,

and optionally

one or more selected from V: 0.01% to 0.25%, Ti: 0.001% to 0.10%, Nb: 0.001% to 0.05%, Ca: 0.0002% to 0.0020%, Mg: 0.0002% to 0.0050%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%,

with a balance being Fe and incidental impurities;

a microstructure containing ferrite crystal grains which satisfy at least one of a C concentration of 2C C or more and an N concentration of 2C N or more, the ferrite crystal grains having a volume fraction with respect to a whole volume of the microstructure of 5% or more and 50% or less, where C C and C N are respectively C content and N content in the steel in mass %; and

a Vickers hardness of 180 or less.

2. The ferritic stainless steel according to claim 1 , wherein in the chemical composition, Cu content is 0.01% to 0.5%.

3. The ferritic stainless steel according to claim 2 ,

wherein in the chemical composition, Mo content is 0.01% to 0.3%.

4. The ferritic stainless steel according to claim 1 ,

wherein in the chemical composition, Mo content is 0.01% to 0.3%.

5. The ferritic stainless steel according to claim 1 ,

wherein in the chemical composition, C content is 0.005 mass % to 0.030 mass %, Si content is 0.25 mass % or more and less than 0.40 mass %, and Mn content is 0.05 mass % to 0.35 mass %,

the volume fraction of the ferrite crystal grains is 5% or more and 30% or less, and

the ferritic stainless steel further comprises elongation after fracture in a direction orthogonal to a rolling direction is 28% or more, and a ridging height is 2.5 μm or less.

6. The ferritic stainless steel according to claim 1 ,

wherein in the chemical composition, C content is 0.005 mass % to 0.025 mass %, Si content is 0.05 mass % or more and less than 0.25 mass %, Mn content is 0.60 mass % to 0.90 mass %, and N content is 0.005 mass % to 0.025 mass %,

the volume fraction of the ferrite crystal grains is 5% or more and 20% or less, and

the ferritic stainless steel further comprises elongation after fracture in a direction orthogonal to a rolling direction is 30% or more, and a ridging height is 2.5 μm or less.

7. A process for producing the ferritic stainless steel according to claim 1 , the process comprising:

hot rolling a steel slab having the chemical composition according to claim 1 into a hot rolled sheet;

performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 900° C. or more and 1050° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet;

cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and

performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 800° C. or more and less than 900° C. for 5 seconds to 5 minutes.

8. A process for producing the ferritic stainless steel according to claim 5 , the process comprising:

hot rolling a steel slab having the chemical composition according to claim 5 into a hot rolled sheet;

performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 940° C. or more and 1000° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet;

cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and

performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 820° C. or more and less than 880° C. for 5 seconds to 5 minutes.

9. A process for producing the ferritic stainless steel according to claim 6 , the process comprising:

hot rolling a steel slab having the chemical composition according to claim 6 into a hot rolled sheet;

performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 960° C. or more and 1050° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet;

cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and

performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 820° C. or more and less than 880° C. for 5 seconds to 5 minutes.