Ni-added steel plate and method of manufacturing the same

1. A Ni-added steel plate comprising, by mass %:

C: 0.04% to 0.10%;

Si: 0.02% to 0.12%;

Mn: 0.3% to 1.0%;

Ni: more than 7.5% to 10.0% or less;

Al: 0.01% to 0.08%;

T.O: 0.0001% to 0.0030%;

P: limited to 0.0100% or less;

S: limited to 0.0035% or less;

N: limited to 0.0070% or less; and

the balance of Fe and unavoidable impurities,

wherein a Ni segregation ratio at an area of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of austenite, an austenite unevenness index, and an average equivalent circle diameter of the austenite measured in a deep cooled sample of the Ni-added steel plate is 0.5% or more, 3.0 or less, and 1 μm or less, respectively.

2. The Ni-added steel plate according to claim 1 , further comprising, by mass %, at least one of:

Cr: 1.5% or less;

Mo: 0.4% or less;

Cu: 1.0% or less;

Nb: 0.05% or less;

Ti: 0.05% or less;

V: 0.05% or less;

B: 0.05% or less;

Ca: 0.0040% or less;

Mg: 0.0040% or less; and

REM: 0.0040% or less.

3. The Ni-added steel plate according to claim 1 or 2 ,

wherein the plate thickness is 4.5 mm to 80 mm.

4. A method of manufacturing a Ni-added steel plate comprising:

performing a first thermomechanical treatment with respect to a steel including, by mass %,

C: 0.04% to 0.10%;

Si: 0.02% to 0.12%;

Mn: 0.3% to 1.0%;

Ni: more than 7.5% to 10.0% or less;

Al: 0.01% to 0.08%;

T.O: 0.0001% to 0.0030%;

P: limited to 0.0100% or less;

S: limited to 0.0035% or less;

N: limited to 0.0070% or less; and

the balance of Fe and unavoidable impurities, wherein the steel is held at a heating temperature of 1250° C. or higher and 1380° C. or lower for 8 hours or longer and 50 hours or shorter and thereafter is cooled by an air cooling to 300° C. or lower;

performing a second thermomechanical treatment with respect to the steel, wherein the steel is heated to 900° C. or higher and 1270° C. or lower, is subjected to a hot rolling at a rolling reduction ratio of 2.0 or more and 40 or less while a temperature at one pass before a final pass is controlled to 660° C. or higher and 900° C. or lower and thereafter is cooled to 300° C. or lower immediately; and

performing a third thermomechanical treatment with respect to the steel, wherein the steel is heated to 500° C. or higher and 650° C. or lower and thereafter is cooled.

5. The method of manufacturing the Ni-added steel plate according to claim 4 ,

wherein the steel further comprises, by mass %, at least one of

Cr: 1.5% or less;

Mo: 0.4% or less;

Cu: 1.0% or less;

Nb: 0.05% or less;

Ti: 0.05% or less;

V: 0.05% or less;

B: 0.05% or less;

Ca: 0.0040% or less;

Mg: 0.0040% or less; and

REM: 0.0040% or less.

6. The method of manufacturing the Ni-added steel plate according to claim 4 or 5 , wherein, in the first thermomechanical treatment, before the air cooling, the steel is subjected to a hot rolling at a rolling reduction ratio of 1.2 or more to 40 or less while a temperature at one pass before a final pass is controlled to 800° C. or higher and 1200° C. or lower.

7. The method of manufacturing the Ni-added steel plate according to claim 4 or 5 , wherein, in the second thermomechanical treatment, the steel is cooled to 300° C. or lower immediately after the hot rolling and is reheated to 780° C. or higher and 900° C. or lower.

8. The method of manufacturing the Ni-added steel plate according to claim 4 or 5 , wherein, in the first thermomechanical treatment, before the air cooling, the steel is subjected to the hot rolling at the rolling reduction ratio of 1.2 or more and 40 or less while the temperature at one pass before the final pass is controlled to 800° C. or higher and 1200° C. or lower, and in the second thermomechanical treatment, the steel is cooled to 300° C. or lower immediately after the hot rolling and is reheated to 780° C. or higher and 900° C. or lower.