Thick-walled high-strength hot rolled steel sheet with excellent low-temperature toughness and method of producing same

A method of producing a thick-walled high-strength hot rolled steel sheet comprising:

• heating a steel material containing, on a mass percent basis,
• 0.02%-0.08% C, 0.01%-0.50% Si,
• 0.5%-1.8% Mn, 0.025% or less P,
• 0.005% or less S, 0.005%-0.10% Al,
• 0.01%-0.10% Nb, 0.001%-0.05% Ti,
• the balance being Fe, and incidental impurities, C, Ti, and Nb being contained to satisfy expression (1):
• (Ti+(Nb/2))/C<4 (1);
• performing hot rolling including rough rolling and finish rolling;
• performing accelerated cooling at an average cooling rate of 10° C./s or more at a middle position of the steel sheet in the thickness direction to a cooling stop temperature of BFS or lower at a middle position of the steel sheet in the thickness direction, wherein BFS is defined by expression (2):
• BFS (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR (2);
• and
• performing coiling at a coiling temperature of BFS0 or lower at the middle position of the steel sheet in the thickness direction, the BFS0 being defined by expression (3):
• BFS0 (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni (3)
• where in expressions (1), (2), and (3), C, Ti, Nb, Mn, Cr, Mo, Cu, and Ni each represent a proportion (percent by mass) thereof, and CR represents a cooling rate (° C./s) at the middle position of the steel sheet in the thickness direction.