RECENT DEVELOPMENTS OF OIL AND GAS TRANSMISSION PIPELINE STEELS; MICROSTRUCTURE, MECHANICAL PROPERTIES AND SOUR GAS RESISTANCE

摘要

In modern oil and gas transmission pipeline steelstechnology, a suitable microstructure is an important factor forimprovement of strength, toughness and sour gas resistance.Therefore, thermo-mechanically controlled rolling processeshave been developed and their microstructures have beenchanged from ferrite-pearlite to acicular ferrite. Moreover inthe recent years extensive attempts have been made toimprove pipeline steels properties, which include:ⅰ) Ultra fine-grained steels, which are produced byoptimized usage of dynamic recrystallization andstrain-induced transformation with about 1μm equi-axed ferrite grain size.ⅱ) Ultra low carbon steels with less than 0.025 wt%carbon and significant amount of Mo and Nb microalloying elements.ⅲ) Ultra fine acicular ferrite steels, which are produced by application of more accurate controlled thermo mechanical processes and accelerated cooling. ⅳ) Ultra high strength X100 and X120 grade steels,which are produced by thermo-mechanically controlled processes and heavy accelerated cooling. The former is without special technological changes and mainly consist of low carbon upper bainitic microstructure while the latter needs more technological developments with very little amount of boron and mainly consists of lower bainitic microstructure.This paper gives an overview of these new pipeline steels in viewpoint of microstructure, mechanical properties and sour gas resistance. The studies show that ultra fine acicular ferrite is the best alternative microstructure for nowadays ordinary pipeline steels, but because of numerous advantages of ultra high strength pipelines steels which finally reduce the cost of pipeline projects, the trend of the investigations is focused on further development of these steels. Moreover, acicular ferrite microstructure which is generally accepted by pipeline engineers and it is just in doubt because of its differences with acicular ferrite microstructure of weld metal and numerous offered definitions, is completely described.