Triple-phase stainless steel and method for producing same

1. A triple-phase stainless steel comprising: in percent (%) by weight of the entire composition, 0.01% or less of carbon (C), 0.5% or less of silicon (Si), 17 to 20% of chromium (Cr), 1.0 to 5.0% of molybdenum (Mo), 0.1 to 0.2% of nickel (Ni), 1.0% or less of manganese (Mn), 0.01 to 0.2% of titanium (Ti), 0.1 to 0.6% of niobium (Nb), 0.1% or less of aluminum (Al), 0.03% or less of phosphorus (P), and 0.005% or less of sulfur (S), and the remainder of iron (Fe) and other inevitable impurities,

wherein the austenite phase, the martensite phase, and the ferrite phase are sequentially formed inward from the surface of the stainless steel,

wherein a content of nitrogen dissolved in the austenite phase is 1.0% by weight or more,

a content of nitrogen dissolved in the martensite phase is from 0.6% by weight or more to less than 1.0% by weight, and

a content of nitrogen dissolved in the ferrite phase is less than 0.6% by weight.

2. The triple-phase stainless steel of claim 1 ,

wherein a ferrite phase formed in a central region;

an austenite phase formed in an outermost region comprising a surface; and

a martensite phase formed between the ferrite phase and the austenite phase.

3. The triple-phase stainless steel of claim 1 ,

wherein a pitting corrosion resistance index of the stainless steel obtained by Equation (1) below is 54 or greater:

PREN=Cr+3.3 Mo+30 N−Mn  Equation (1)

4. The triple-phase stainless steel of claim 1 ,

wherein a content of nitrogen permeating into a surface, which is a value of N in Equation (1) is 1.0% or more.

5. The triple-phase stainless steel of claim 1 ,

wherein the austenite phase has a particle size of 50 μm or less.

6. The triple-phase stainless steel of claim 1 ,

wherein a surface hardness of the stainless steel is a 300 HV or higher.

7. A method of manufacturing the triple-phase stainless steel of claim 1 , the method comprising:

locating a ferritic stainless steel in a furnace chamber in which a temperature is maintained from 900 to 1,280° C.;

forming a nitrogen atmosphere by injecting nitrogen gas (N 2 ) into the furnace chamber; generating generator nitrogen (N) by decomposing the nitrogen gas (N 2 ),

providing 1.0% or more of nitrogen permeating into the steel to phase-transform an outermost region into an austenite phase;

providing 0.6 to 1.0% of nitrogen permeating into the steel to phase-transform an outer region inside the outermost region into a martensite phase; and

providing less than 0.6% of nitrogen permeating into the steel to phase-transform a central are inside the martensite phase to maintain a ferrite phase.