Ni-added steel plate and method of manufacturing the same

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

C: 0.03% to 0.10%;

Si: 0.02% to 0.40%;

Mn: 0.3% to 1.2%;

Ni: 5.0% to 7.5%;

Cr: 0.4% to 1.5%;

Mo: 0.02% to 0.4%;

Al: 0.01% to 0.08%;

T.O: 0.0001% to 0.0050%;

P: limited to 0.0100% or less;

S: limited to 0.0035% or less;

N: limited to 0.0070% or less; and

the balance consisting of iron and unavoidable impurities,

wherein a Ni segregation ratio based on mass % at a position of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of an austenite after a deep cooling is 2% or more, an austenite unevenness index after the deep cooling is 5.0 or less, and an average equivalent circle diameter of the austenite after the deep cooling is 1 μm or less,

wherein the austenite unevenness index after the deep cooling is a value obtained by dividing a maximum area fraction by a minimum area fraction, in which, among data which are evaluated such that an evaluation of an area fraction of the austenite is carried out with each viewing areas thereof being defined as a 5×5 μm area and is continuously carried out in the thickness direction with being centered on the position of ¼ of the plate thickness away from the plate surface in the thickness direction, an average of the data of 5 largest area fractions of the austenite is defined to be the maximum area fraction and an average of the data of 5 smallest area fractions of the austenite is defined to be the minimum area fraction.

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

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 2 ,

wherein the Ni by mass % is 5.3% to 7.3%.

4. The Ni-added steel plate according to claim 2 ,

wherein the plate thickness is 4.5 mm to 80 mm.

5. The Ni-added steel plate according to claim 1 ,

wherein the Ni by mass % is 5.3% to 7.3%.

6. The Ni-added steel plate according to claim 1 ,

wherein the plate thickness is 4.5 mm to 80 mm.

7. A method of manufacturing a Ni-added steel plate according to claim 1 , comprising:

a first thermal processing treatment in which a slab containing, by mass %,

C: 0.03% to 0.10%;

Si: 0.02% to 0.40%;

Mn: 0.3% to 1.2%;

Ni: 5.0% to 7.5%;

Cr: 0.4% to 1.5%;

Mo: 0.02% to 0.4%;

Al: 0.01% to 0.08%;

T.O: 0.0001% to 0.0050%;

P: limited to 0.0100% or less;

S: limited to 0.0035% or less;

N: limited to 0.0070% or less; and

the balance consisting of iron and unavoidable impurities is held at a heating temperature of 1250° C. to 1380° C. for 8 hours to 50 hours, and thereafter an air-cooling to 300° C. or lower is performed;

a second thermal processing treatment in which the slab is heated to 900° C. to 1270° C., a hot rolling is performed by a rolling reduction of 2.0 to 40 with controlling a temperature before a final pass to 660° C. to 900° C., and immediately, a cooling is performed;

a third thermal processing treatment in which the slab is heated to 600° C. to 750° C., and thereafter, a cooling is performed; and

a fourth thermal processing treatment in which the slab is heated to 500° C. to 650° C., and thereafter, a cooling is performed.

8. The method of manufacturing the Ni-added steel plate according to claim 7 ,

wherein the slab further contains, by mass %, at least one of

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.

9. The method of manufacturing the Ni-added steel plate according to claim 8 ,

wherein, in the first thermal processing treatment, before the air cooling, a hot rolling is performed by a rolling reduction of 1.2 to 40 with controlling a temperature before a final pass to 800° C. to 1200° C.

10. The method of manufacturing the Ni-added steel plate according to claim 8 ,

wherein, in the second thermal processing treatment, after the hot rolling and the cooling, a reheating to 780° C. to 900° C. is performed.

11. The method of manufacturing the Ni-added steel plate according to claim 8 ,

wherein, in the first thermal processing treatment, before the air cooling, a hot rolling is performed by a rolling reduction of 1.2 to 40 with controlling a temperature before a final pass to 800° C. to 1200° C., and, in the second thermal processing treatment, after the hot rolling and the cooling, a reheating to 780° C. to 900° C. is performed.

12. The method of manufacturing the Ni-added steel plate according to claim 7 ,

wherein, in the first thermal processing treatment, before the air cooling, a hot rolling is performed by a rolling reduction of 1.2 to 40 with controlling a temperature before a final pass to 800° C. to 1200° C.

13. The method of manufacturing the Ni-added steel plate according to claim 7 ,

wherein, in the second thermal processing treatment, after the hot rolling and the cooling, a reheating to 780° C. to 900° C. is performed.

14. The method of manufacturing the Ni-added steel plate according to claim 7 ,

wherein, in the first thermal processing treatment, before the air cooling, a hot rolling is performed by a rolling reduction of 1.2 to 40 with controlling a temperature before a final pass to 800° C. to 1200° C., and, in the second thermal processing treatment, after the hot rolling and the cooling, a reheating to 780° C. to 900° C. is performed.