Steel for welded structure and producing method thereof

1. A steel for a welded structure, comprising the following composition: by mass %,

C at a C content [C] of 0.010 to 0.065%;

Si at a Si content [Si] of 0.05 to 0.20%;

Mn at a Mn content [Mn] of 1.52 to 2.70%;

Ni at a Ni content [Ni] of 0.69% to 1.50%;

Ti at a Ti content [Ti] of 0.005 to 0.015%;

O at an O content [O] of 0.0010 to 0.0045%;

N at a N content [N] of 0.002 to 0.006%;

Mg at a Mg content [Mg] of 0.0003 to 0.003%;

Ca at a Ca content [Ca] of 0.0003 to 0.003%; and

a balance of Fe and unavoidable impurities,

wherein, a P content [P] is limited to 0.008% or less,

a S content [S] is limited to 0.005% or less,

an Al content [Al] is limited to 0.004% or less,

a Nb content [Nb] is limited to 0.001% or less,

a Cu content [Cu] is limited to 0.03% or less,

a V content [V] is limited to 0.020% or less,

a Mo content [Mo] is limited to 0.05% or less, and

when a steel component parameter P CTOD is defined by a following equation (1),

P CTOD =[C]+[V]/3+[Cu]/22+[Ni]/67

and a steel component hardness parameter CeqH is defined by a following equation (2),

CeqH=[C]+[Si]/4.16+[Mn]/14.9+[Cu]/12.9+[Ni]/105+1.12[Nb]+[V]/1.82

the P CTOD is 0.065% or less, and the CeqH is 0.196% or less.

2. The steel for welded structure according to claim 1 , wherein a total amount of Mg and Ca is 0.0006 to 0.0030% by mass %.

3. A producing method of a steel for welded structure, comprising:

continuously casting steel to manufacture a slab satisfying the composition according to claim 1 or 2 ; and

heating the slab to a temperature of 950° C. to 1100° C. and then subjecting the slab to a thermo-mechanical control process.