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

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 30×1022/m3,
• granular bainitic ferrite αB and/or quasi-polygonal ferrite αq 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.