On the processing of Fe–C–Mn–Nb steels to produce plates for pipelines with sour gas resistance

Several steel plates in the as-hot rolled plus cooled condition were studied, in order to evaluate the impact of the steelmaking route and the controlled thermomechanical processing plus the cooling media, on the resulting microstructure and mechanical properties of steel plates, which will be processed to produce large diameter steel pipes with sour gas resistance. The steelmaking route to produce the slabs involved the use of 100% sponge iron which was feed into an electric arc furnace, vacuum degassed, ladle treated and continuously casted. After soaking, a controlled thermomechanical processing schedule was applied to steel slabs followed by air or accelerated cooling of plates. Most of the resulting steel plates cooled in air showed a banded structure, which sometimes presented a central segregation region. The worst plates with a central segregation region showed intermetallics compounds in it. After modifications of the steelmaking route and to the controlled thermomechanical/cooling schedule, a steel plate with a ferritic microstructure plus 0.5 vol% of bainite was obtained. This microstructure together with the resulting mechanical properties, fulfilled the API grade 5L X-70 properties, required in modern steel pipes demanded by the oil industry.