HIGH STRENGTH HOT ROLLED STEEL SHEET FOR LINE PIPE USE EXCELLENT IN LOW TEMPERATURE TOUGHNESS AND DUCTILE FRACTURE ARREST PERFORMANCE AND METHOD OF PRODUCTION OF SAME

1 . High strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance containing, by mass %,

C=0.02 to 0.06%,

Si=0.05 to 0.5%,

Mn=1 to 2%,

P≦0.03%,

S≦0.005%,

O=0.0005 to 0.003%,

Al=0.005 to 0.03%,

N=0.0015 to 0.006%,

Nb=0.05 to 0.12%,

Ti=0.005 to 0.02%,

Ca=0.0005 to 0.003% and

N−14/48×Ti≧0% and

Nb−93/14×(N−14/48×Ti)>0.05%,

further containing

V≦0.3% (not including 0%),

Mo≦0.3% (not including 0%), and

Cr≦0.3% (not including 0%), where

0.2%≦V+Mo+Cr≦0.65%, containing

Cu≦0.3% (not including 0%) and

Ni≦0.3% (not including 0%), where

0.1%≦Cu+Ni≦0.5%, and

having a balance of

Fe and unavoidable impurities,

in which said steel sheet,

the microstructure is a continuously cooled transformed structure, in which continuously cooled transformed structure,

precipitates containing Nb have an average size of 1 to 3 nm and are included dispersed at an average density of 3 to 30×10 22 /m 3 ,

granular bainitic ferrite α B and/or quasi-polygonal ferrite α q are included in 50% or more in terms of fraction,

furthermore, precipitates containing Ti nitrides are included,

the precipitates containing Ti nitrides have an average circle equivalent diameter of 0.1 to 3 μm and include complex oxides including Ca, Ti, and Al in 50% or more in terms of number.

2 . High strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance as set forth in claim 1 , further containing, by mass %,

B=0.0002 to 0.003%.

3 . High strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance as set forth in claim 1 , further containing, by mass %,

REM=0.0005 to 0.02%.

4 . A method of production of high strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance comprising preparing molten steel for obtaining hot rolled steel sheet having the ingredients as set forth any one of claims 1 to 3 at which time preparing the molten steel to give a concentration of Siof 0.05 to 0.2% and a concentration of dissolved oxygen of 0.002 to 0.008%, adding to the molten steel Ti in a range giving a final content of 0.005 to 0.3% for deoxidation, then adding Al within 5 minutes to give a final content of 0.005 to 0.02%, furthermore adding Ca to give a final content of 0.0005 to 0.003%, then adding the required amounts of alloy ingredient elements to cause solidification, cooling a resultant cast slab, heating said cast slab to a temperature range of an SRT (° C.) calculated by formula (1) to 1260° C., further holding the slab at said temperature range for 20 minutes or more, then hot rolling by a total reduction rate of a non-recrystallization temperature range of 65% to 85%, ending the rolling in a temperature range of 830° C. to 870° C., then cooling in a temperature range down to 650° C. by a cooling rate of 2° C./sec to 50° C./sec and coiling at 500° C. to 650° C.:

SRT(° C.)=6670/(2.26−log([% Nb]×[% C]))−273   (1)

where [% Nb]and [% C]show the contents (mass %) of Nb and C in the steel material.

5 . A method of production of high strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance as set forth in claim 4 characterized by cooling before rolling in said non-recrystallization temperature range.

6 . A method of production of high strength hot rolled steel sheet for line pipe use excellent in low temperature toughness and ductile fracture arrest performance as set forth in claim 4 characterized by continuously casting said cast slab at which time lightly rolling it while controlling the amount of reduction so as to match solidification shrinkage at a final solidification position of the cast slab.