Development of Heavy Gauge Hot Coils with High Strength and Excellent Low Temperature Toughness by Controlled Rolling

Abstract

Steel plate used for line pipe is produced by both plate and hot-strip mills. Controlled rolling is used in both type of mills to obtain a good combination of strength and low temperature toughness. A Iaboratory simulation method for producing coiled-strip and plates is devised, taking account of a great difference in thermal conditions after finish-rolling.
The results in laboratory test reveals that microstructure and precipitation of carbonitride in the simulated hot coils differ from those in the air-cooled plates, and these lead to different controlling practices between hot-strip and plate mills.
For niobium bearing steel, bainitic transformation occurs in hot-coils by rapid cooling on runout table. To avoid the embrittlement caused by a bainitic structure, carbon content should be reduced to the level similar to that of acicular ferrite steel and also the slab-reheating temperature and rough rolling temperature should be lowered.
For vanadium bearing steel, lowering the slab-reheating temperature in the middle position of the length is recommended to prevent the embrittlement caused by vanadium carbonitride precipitation during slow cooling especially in that position after coiling.
The manufacturing trials of 14.3 mm thick heavy gauge coils are carried out, using 0.14%C-Nb-V steel and 0.04%C-1.9%Mn-Nb-Mo-Ca steel. Both steels are found to meet API-X70 strength with excellent low temperature toughness.