Ultrahigh-strength welded joint and production method thereof

1. An ultrahigh-strength welded joint with a tensile strength of 1138 MPa or more and an absorbed energy in a 2 mm V-notch Charpy impact test at −40° C. (vE-40) of 27 J/cm 2 or more, comprising a steel plate having a plate thickness of 4 mm to 12 mm and including a weld metal with an almost full martensite structure, wherein, in a cross-section of the welded joint in a direction perpendicular to a welding direction, a cross-sectional shape of the weld metal has a width W 1 at a surface of the steel plate and a width W 2 at a position which is separated from the surface by three-quarters of the plate thickness, the width W 1 being 2.0 mm to 7.0 mm and the width W 2 being 0.5 mm to 2.4 mm, and

wherein the weld metal comprises, by mass %,

C: over 0.09% to 0.24%;

Si: 0.2% to 1.0%;

Mn: 0.5% to 2.5%;

P: 0.02% or less;

S: 0.02% or less;

Al: 0.004% to 0.08%;

Ti: 0.005% to 0.15%;

O: 0.005% to 0.05%; and

Ni: 1.0% to 9%,

wherein a carbon equivalent (Ceq) as defined by equation (1) is 0.40% to 1.00%, a value Y as defined by equation (2) is 0.07% to 0.20%, and a balance of the weld metal is constituted by unavoidable impurities and Fe:

Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14  (1)

Y=([Si]+[Mn])/40+[Al]+[Ti]  (2), and

wherein in equations (1) and (2), elements with “[ ]” represent contents (mass %) of respective elements.

2. The ultrahigh-strength welded joint according to claim 1 , wherein the weld metal further comprises, by mass %, one or more of:

Cr: 0.1% to 2.0%;

Mo: 0.1% to 1.5%;

V: 0.005% to 0.2%;

Nb: 0.005% to 0.1%;

Cu: 0.01% to 1.0%; and

B: 0.0005% to 0.01%.

3. An ultrahigh-strength welded joint with a tensile strength of 1100 MPa or more and an absorbed energy in a 2 mm V-notch Charpy impact test at −40° C. (vE-40) of 27 J/cm 2 or more, comprising a steel plate having a plate thickness of 4 mm to 12 mm and including a weld metal with an almost full martensite structure, wherein, in a cross-section of the welded joint in a direction perpendicular to a welding direction, a cross-sectional shape of the weld metal has a width W 1 at a surface of the steel plate and a width W 2 at a position which is separated from the surface by three-quarters of the plate thickness, the width W 1 being 2.0 mm to 7.0 mm and the width W 2 being 0.5 mm to 2.4 mm, and

wherein the weld metal comprises, by mass %,

C: over 0.09% to 0.24%;

Si: 0.2% to 1.0%;

Mn: 0.5% to 2.5%;

P: 0.02% or less;

S: 0.02% or less;

Al: 0.004% to 0.08%;

Ti: 0.005% to 0.15%;

O: 0.005% to 0.05%; and

Ni: 1.0% to 9%,

wherein a carbon equivalent (Ceq) as defined by equation (1) is 0.40% to 1.00%, a value Y as defined by equation (2) is 0.07% to 0.20%, and a balance of the weld metal is constituted by unavoidable impurities and Fe:

Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14  (1)

Y=([Si]+[Mn])/40+[Al]+[Ti]  (2), and

wherein in equations (1) and (2), elements with “[ ]” represent contents (mass %) of respective elements.

4. The ultrahigh-strength welded joint according to claim 3 , wherein the weld metal further comprises, by mass %, one or more of:

Cr: 0.1% to 2.0%;

Mo: 0.1% to 1.5%;

V: 0.005% to 0.2%;

Nb: 0.005% to 0.1%;

Cu: 0.01% to 1.0%; and

B: 0.0005% to 0.01%.

5. The ultrahigh-strength welded joint according to claim 1 or 3 , wherein the welded joint is obtained by means of single-pass welding using laser hybrid welding.