Austenitic stainless steel

1. An austenitic stainless steel having a chemical composition comprising, by mass %:

C: 0.01 to 0.15%;

Si: 2.0% or less;

Mn: 3.0% or less;

Cr: 10.0 to 20.0%;

Ni: 5.0 to 13.0%;

N: 0.01 to 0.30%;

Nb: 0 to 0.5%;

Ti: 0 to 0.5%;

V: 0 to 0.5%, and

the balance: Fe and impurities, wherein

a tensile strength is 1200 MPa or more,

an average grain size is 10.0 μm or less,

a difference in value of an average lattice constant d Ave. of an austenite phase between a surface portion and a center portion is 0.010 Å or more, the average lattice constant d Ave. being defined by a following formula (i), where the surface portion is a zone of 10 μm from an outermost surface of the steel,

a martensite exists in the center portion, and

a value of a diffraction peak integrated intensity ratio r at a surface is 95% or more, the diffraction peak integrated intensity ratio r being defined by a following formula (ii):

d Ave. ={d γ(111) ×I γ(111) +d γ(200) ×I γ(200) +d γ(220) ×I γ(220) +d γ(311) ×I γ(311) }/{I γ(111) +I γ(200) +I γ(220) +I γ(311)}   (i)

where d γ(hkl) : lattice constant (Å) that is calculated from a Bragg angle of an X-ray diffraction peak on an (hkl) plane of the austenite phase,

I γ(hkl) : integrated intensity (cps·deg) of the X-ray diffraction peak on the (hkl) plane of the austenite phase,

r= 100×Σ I γ /ΣI ALL   (ii)

where ΣI γ : sum of integrated intensities (cps·deg) at X-ray diffraction peaks of all austenite phases,

ΣI ALL : sum of integrated intensities (cps·deg) at all X-ray diffraction peaks.

2. The austenitic stainless steel according to claim 1 , wherein the difference in value of an average lattice constant d Ave. of an austenite phase between a surface portion and a center portion is 0.030 Å or more, the average lattice constant d Ave. being defined by the formula (i).

3. The austenitic stainless steel according to claim 2 , wherein the chemical composition further containing one or more elements selected from, by mass %:

Nb: 0.01 to 0.5%;

Ti: 0.01 to 0.5%; and

V: 0.01 to 0.5%.

4. The austenitic stainless steel according to claim 1 , wherein the chemical composition further containing one or more elements selected from, by mass %:

Nb: 0.01 to 0.5%;

Ti: 0.01 to 0.5%; and

V: 0.01 to 0.5%.