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

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/mm2 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 3.9×103 to 1×105 pieces/mm2 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.

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/mm2 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 3.9×103 to 1×105 pieces/mm2 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.

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/mm2 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 3.9×103 to 1×105 pieces/mm2 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.

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/mm2 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 3.9×103 to 1×105 pieces/mm2 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.