High strength steel pipe for line pipe superior in low temperature toughness and high strength steel plate for line pipe and methods of production of the same

1. A high strength steel pipe for line pipe superior in low temperature toughness characterized by comprising a steel plate shaped into a tube with seam portions of the steel plate welded by one layer each at the inner side and outer side, wherein a base metal of the steel pipe has a chemical composition containing, by mass %,

C: 0.020 to 0.080%,

Si: 0.01 to 0.50%,

Mo: 0.01 to 0.09%,

Al: 0.0005 to 0.030%, and

Nb: 0.013 to 0.030%

in a range of

C+0.25Si+0.1Mo+Al+Nb: 0.100% or less

further containing,

Mn: 1.50 to 2.50%,

Ti: 0.003 to 0.030%, and

B: 0.0001 to 0.0030%, and

limiting

P: 0.020% or less and

S: 0.0030% or less,

with the balance of Fe and unavoidable impurities, a mixture of austenite and martensite present along prior austenite grain boundaries of reheated parts of a heat affected zone having a width of 10 μm or less and a length of 50 μm or less and wherein the weld metal has a chemical composition containing, by mass %,

C: 0.010 to 0.100%,

Si: 0.01 to 0.50%,

Mn: 1.00 to 2.00%,

Ni: 1.30 to 3.20%,

Al: 0.0005 to 0.100%,

Ti: 0.003 to 0.050%, and

O: 0.0001 to 0.0500%

further containing

a total of one or more of Cr, Mo, and V: 1.00 to 2.50%,

limiting

P: 0.020% or less and

S: 0.0100% or less

with the balance of Fe and unavoidable impurities.

2. A high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 characterized in that the base metal has a tensile strength in the circumferential direction of 900 MPa or more.

3. A high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 characterized in that the weld metal has a chemical composition further containing, by mass %, B: 0.0001 to 0.0050%.

4. A high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 characterized in that the base metal has a chemical composition further containing, by mass %, one or both of

Cu: 0.05 to 1.50% and

Ni: 0.05 to 5.00%.

5. A high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 characterized in that the base metal has a chemical composition further containing, by mass %, one or more of

Cr: 0.02 to 1.50%,

W: 0.01 to 2.00%,

V: 0.010 to 0.100%,

Zr: 0.0001 to 0.0500%, and

Ta: 0.0001 to 0.0500%.

6. A high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 characterized in that the base metal has a chemical composition further containing, by mass %, one or more of

Mg: 0.0001 to 0.0100%,

Ca: 0.0001 to 0.0050%,

REM: 0.0001 to 0.0050%,

Y: 0.0001 to 0.0050%,

Hf: 0.0001 to 0.0050%, and

Re: 0.0001 to 0.0050%.

7. A method of production of the high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 1 , said method of production characterized by melting and casting steel comprising the chemical composition as set forth in claim 1 into a steel slab, reheating the steel slab to 1000° C. or more, hot rolling by a reduction ratio in a nonrecrystallization temperature region of 3 or more to form a steel plate, water cooling, stopping water cooling at 500° C. or lower, shaping the steel plate into a tube, welding the seam potions to produce a pipe, and then enlarging the pipe.

8. A method of production of high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 7 characterized by shaping the steel plate into a tube by a UO process, welding the seam portions from the inner side and outer side by submerged arc welding to produce the pipe, then enlarging the pipe.

9. A method of production of a high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 8 wherein a welding wire used for the submerged arc welding has a chemical composition containing, by mass %,

C: 0.01 to 0.12%,

Si: 0.05 to 0.50%,

Mn: 1.00 to 2.50%, and

Ni: 2.00 to 8.50%

further containing one or more of Cr, Mo, and V in a range of

Cr+Mo+V: 1.00 to 5.00%

with a balance of Fe and unavoidable impurities.

10. A method of production of high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 9 , the chemical composition of the welding wire further containing, by mass %, B: 0.0001 to 0.0050%.

11. A method of production of high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 7 further comprising heat treating the weld zone and heat affected zone after welding and before pipe expansion.

12. A method of production of high strength steel pipe for line pipe superior in low temperature toughness as set forth in claim 11 wherein the heating temperature of heat treating the weld zone and heat affected zone is 200 to 500° C.