Electron-beam welded joint, steel for electron-beam welding, and method of manufacturing the same

1. An electron-beam welded joint having a welded metal and obtained by applying an electron beam to a steel, the steel having a composition including, by mass %:

C: 0.049% to 0.10%;

Si: 0.08% to 0.30%;

Mn: 1.5% to 2.5%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0010% to 0.0035%;

Nb: 0% to 0.020%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

Ni: 0% to 0.50%;

B: 0% to 0.0030%;

S: limited to not more than 0.010%;

P: limited to not more than 0.015%; and

Al: limited to not more than 0.004%,

with a balance including iron and inevitable impurities, wherein

an index value CeEBB obtained by substituting the composition of the steel into following Formula 1 falls in the range of 0.42% to 0.65%,

a number of oxides having an equivalent circle diameter of not less than 1.0 μm is not more than 20 pieces/mm 2 at a thickness center portion in cross section along a thickness direction of the steel,

a number of oxides containing Ti of not less than 10% and having an equivalent circle diameter of not less than 0.05 μm and less than 0.5 μm falls in the range of 3.9×10 3 to 1×10 5 pieces/mm 2 in the thickness center portion,

the welded metal has a composition including, by mass %:

C: 0.049% to 0.10%;

Si: 0.08% to 0.30%;

Mn: 1.2% to 2.4%;

Ni: 1.0% to 2.3%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0004% to 0.0020%;

Nb: 0% to 0.020%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

B: 0% to 0.0030%;

S: limited to not more than 0.010%;

P: limited to not more than 0.015%; and

Al: limited to not more than 0.004%,

with a balance including iron and inevitable impurities,

an index value CeEBW obtained by substituting the composition of the welded metal into following Formula 2 falls in the range of 0.56 to 0.73%,

CeEBB=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 1)

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements contained in the composition of the steel,

CeEBW=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 2)

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements contained in the composition of the welded metal, and

a yield point of the steel is 315 MPa to 380 MPa.

2. The electron-beam welded joint according to claim 1 , wherein

a C/CeEBB, which is a ratio of the amount of C by mass % in the steel relative to the index value CeEBB, falls in the range of 0.02 to 0.15.

3. The electron-beam welded joint according to claim 1 , wherein

the steel has a thickness in the range of 45 to 150 mm.

4. The electron-beam welded joint according to claim 1 , satisfying

0.8≤δ BM /δ WM ≤1.25, and

0.5≤δ HAZ /δ WM ≤1.1, where

δ WM is a CTOD value of the welded metal,

δ HAZ is a CTOD value of a weld heat-affected zone, and

δ BM is a CTOD value of the steel.

5. The electron-beam welded joint according to claim 1 , wherein

the amount of O is 0.0010% to 0.0025% in the steel.

6. The electron-beam welded joint according to claim 1 , wherein

the amount of Nb is 0% to 0.003% in the steel.

7. The electron-beam welded joint according to claim 1 , wherein

the amount of O is 0.0004% to 0.0018% in the welded metal.

8. A steel for electron-beam welding having a composition including by mass %:

C: 0.049% to 0.10%;

Si: 0.08% to 0.30%;

Mn: 1.5% to 2.5%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0010% to 0.0035%;

Nb: 0% to 0.020%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

Ni: 0% to 0.50%;

B: 0% to 0.0030%;

S: limited to not more than 0.010% or less;

P: limited to not more than 0.015% or less; and

Al: limited to not more than 0.004%or less;

with a balance including iron and inevitable impurities, wherein

an index value CeEBB obtained by substituting the composition of the steel into following Formula 1 falls in the range of 0.42 to 0.65%,

a number of oxides having an equivalent circle diameter of not less than 1.0 μm is not more than 20 pieces/mm 2 at a thickness center portion in cross section along a thickness direction of the steel,

a number of oxides containing Ti of not less than 10% and having an equivalent circle diameter of not less than 0.05 μm and less than 0.5 μm falls in the range of 3.9×10 3 to 1×10 5 pieces/mm 2 at the thickness center portion,

CeEBB=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 1)

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements in the composition of the steel, and

a yield point of the steel is 315 MPa to 380 MPa.

9. The steel for electron-beam welding according to claim 8 , wherein

a C/CeEBB, which is a ratio of the amount of C by mass % in the steel relative to the CeEBB, falls in the range of 0.02 to 0.15.

10. The steel for electron-beam welding according to claim 8 , wherein

the thickness of the steel falls in the range of 45 to 150 mm.

11. The steel for electron-beam welding according to claim 8 , wherein

the amount of O is 0.0010% to 0.0025%.

12. The steel for electron-beam welding according to claim 8 , wherein

the amount of Nb is 0% to 0.003%.

13. An electron-beam welded joint having a welded metal and obtained by applying an electron beam to a steel, the steel having a composition including, by mass %:

C: 0.049% to 0.10%;

Si: 0.03% to 0.30%;

Mn: 1.5% to 2.5%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0010% to 0.0035%;

Nb: 0% to 0.003%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

Ni: 0% to 0.50%;

B: 0% to 0.0030%;

S: limited to not more than 0.010%;

P: limited to not more than 0.015%; and

Al: limited to not more than 0.004%,

with a balance including iron and inevitable impurities, wherein

an index value CeEBB obtained by substituting the composition of the steel into following Formula 1 falls in the range of 0.42% to 0.65%,

a number of oxides having an equivalent circle diameter of not less than 1.0 μm is not more than 20 pieces/mm 2 at a thickness center portion in cross section along a thickness direction of the steel,

a number of oxides containing Ti of not less than 10% and having an equivalent circle diameter of not less than 0.05 μm and less than 0.5 μm falls in the range of 3.9×10 3 to 1×10 5 pieces/mm 2 in the thickness center portion,

the welded metal has a composition including, by mass %:

C: 0.049% to 0.10%;

Si: 0.03% to 0.30%;

Mn: 1.2% to 2.4%;

Ni: 1.0% to 2.3%;

Ti: 0.005% to 0.015%

N: 0.0020% to 0.0060%;

O: 0.0004% to 0.0020%;

Nb: 0% to 0.003%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

B: 0% to 0.0030%;

S: limited to not more than 0.010%;

P: limited to not more than 0.015%; and

Al: limited to not more than 0.004%,

with a balance including iron and inevitable impurities,

an index value CeEBW obtained by substituting the composition of the welded metal into following Formula 2 falls in the range of 0.56 to 0.73%,

CeEBB=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 1)

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements contained in the composition of the steel,

CeEBW=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 2)

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements contained in the composition of the welded metal, and

a yield point of the steel is 315 MPa to 380 MPa.

14. The electron-beam welded joint according to claim 13 , wherein

a C/CeEBB, which is a ratio of the amount of C by mass % in the steel relative to the index value CeEBB, falls in the range of 0.02 to 0.15.

15. The electron-beam welded joint according to claim 13 , wherein

the steel has a thickness in the range of 45 to 150 mm.

16. The electron-beam welded joint according to claim 13 , satisfying

0.8≤δ BM /δ WM ≤1.25, and

0.5≤δ HAZ /δ WM ≤1.1, where

δ WM is a CTOD value of the welded metal,

δ HAZ is a CTOD value of a weld heat-affected zone, and

δ BM is a CTOD value of the steel.

17. The electron-beam welded joint according to claim 13 , wherein

the amount of O is 0.0010% to 0.0025% in the steel.

18. The electron-beam welded joint according to claim 13 , wherein

the amount of Si is 0.08% to 0.30% in the steel.

19. The electron-beam welded joint according to claim 13 , wherein

the amount of O is 0.0004% to 0.0018% in the welded metal.

20. A steel for electron-beam welding having a composition including by mass %:

C: 0.049% to 0.10%;

Si: 0.03% to 0.30%;

Mn: 1.5% to 2.5%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0010% to 0.0035%;

Nb: 0% to 0.003%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

Ni: 0% to 0.50%;

B: 0% to 0.0030%;

S: limited to not more than 0.010% or less;

P: limited to not more than 0.015% or less; and

Al: limited to not more than 0.004% or less;

with a balance including iron and inevitable impurities, wherein

an index value CeEBB obtained by substituting the composition of the steel into following Formula 1 falls in the range of 0.42 to 0.65%,

a number of oxides having an equivalent circle diameter of not less than 1.0 μm is not more than 20 pieces/mm 2 at a thickness center portion in cross section along a thickness direction of the steel,

a number of oxides containing Ti of not less than 10% and having an equivalent circle diameter of not less than 0.05 μm and less than 0.5 μm falls in the range of 3.9×10 3 to 1×10 5 pieces/mm 2 at the thickness center portion,

CeEBB=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 1),

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements in the composition of the steel, and

a yield point of the steel is 315 MPa to 380 MPa.

21. The steel for electron-beam welding according to claim 20 , wherein

a C/CeEBB, which is a ratio of the amount of C by mass % in the steel relative to the CeEBB, falls in the range of 0.02 to 0.15.

22. The steel for electron-beam welding according to claim 20 , wherein the thickness of the steel falls in the range of 45 to 150 mm.

23. The steel for electron-beam welding according to claim 20 , wherein the amount of O is 0.0010% to 0.0025%.

24. The steel for electron-beam welding according to claim 20 , wherein

the amount of Si is 0.08% to 0.30%.

25. A method of manufacturing a steel for electron-beam welding having a composition including by mass %:

C: 0.02% to 0.10%;

Si: 0.03% to 0.30%;

Mn: 1.5% to 2.5%;

Ti: 0.005% to 0.015%;

N: 0.0020% to 0.0060%;

O: 0.0010% to 0.0035%;

Nb: 0% to 0.020%;

V: 0% to 0.030%;

Cr: 0% to 0.50%;

Mo: 0% to 0.50%;

Cu: 0% to 0.25%;

Ni: 0% to 0.50%;

B: 0% to 0.0030%;

S: limited to not more than 0.010%;

P: limited to not more than 0.015%; and

Al: limited to not more than 0.004%,

with a balance including iron and inevitable impurities, wherein

an index value CeEBB obtained by substituting the composition of the steel into following Formula 1 falls in the range of 0.42 to 0.65%,

a number of oxides having an equivalent circle diameter of not less than 1.0 μm is not more than 20 pieces/mm 2 at a thickness center portion in cross section along a thickness direction of the steel, and

a number of oxides containing Ti of not less than 10% and having an equivalent circle diameter of not less than 0.05 μm and less than 0.5 μm falls in the range of 3.9×10 3 to 1×10 5 pieces/mm 2 at the thickness center portion,

CeEBB=C+¼Mn+ 1/15Cu+ 1/15Ni+⅕Cr+⅕Mo+⅕V  (Formula 1),

where C, Mn, Cu, Ni, Cr, Mo, and V represent mass % of elements in the composition of the steel, the method including:

after a steel is subjected to casting, cooling the steel in a manner such that a cooling rate in a temperature range of 1300 to 1100° C. is not less than 9° C./min; and

after casting, heating the steel to a temperature range of 950 to 1150° C., and then, applying a thermo-mechanical treatment.

26. The method according to claim 25 , wherein a C/CeEBB, which is a ratio of the amount of C by mass % in the steel relative to the CeEBB, falls in the range of 0.02 to 0.15.