Thermomechanical rolling relies on processing austenite in the temperature region of nonrecrystallisation and is the most efficient method for achieving grain refinement and thus both, higher strength and toughness. It is the standard means to produce plate for high strength large diameter line pipe in order to fulfil the safety requirements of pipelines. However, there are metallurgical situations or facility limitations, where the necessary processing at temperatures of the metastable austenite is not feasible or advisable. If the amount of solute niobium is increased, retardation of austenite recrystallisation is observed at significantly higher temperatures, thereby allowing the thermomechanical rolling to occur already at higher temperatures. A rather low carbon content and the fixing of nitrogen with titanium, an element with higher affinity for nitrogen than niobium, to avoid niobium carbonitride formation allows the higher niobium content to be easily dissolved during reheating of the slabs. In this paper, such a steel originally designed for high temperature processing (HTP), will be called HTP steel. The HTP concept has been adapted to the available facilities in various companies. The niobium content in this alloy design adds to strength increase not only by facilitating high temperature austenite conditioning but also by promoting the transformation to occur at lower temperature (which results in a higher volume fraction of bainite) and by coherent ferrite precipitates. In order to make optimum use of the niobium addition a low nitrogen content, below 50 ppm, and a stoichiometric addition of titanium is recommended. By increasing the volume fraction of bainite with the above mentioned means, even thick wall sour gas resistant pipes of grade X 80 becomes feasible.
High temperature thermomechanical processing of pipe steel - technical basis and production experience
Thermomechanische Hochtemperaturbehandlung von Rohrstahl - technische Grundlagen und Herstellungserfahrung
2000
15 Seiten, 19 Bilder, 25 Quellen
Aufsatz (Konferenz)
Englisch
The application of thermomechanical processed steel
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