The present paper deals with the development of low carbon (, 0 . 04%) microalloyed steel plates for high grade (> X80, yield strength > 550 MPa) longitudinally welded large diameter pipes for gas transmission. The interest in increasing the grade level of steels for linepipe is to improve transportation efficiency by an increase in operation pressure. In addition, increasing the grade allows thinner wall pipes to be used, reducing costs and improving the rate of field joint welding. A general requirement for linepipe steels is a simultaneous increase in fracture toughness as the strength increases. However, it is remarkably difficult to achieve a combination of high strength and good low temperature toughness. Bainitic microstructures have shown the capability to attain such requirements.
The application of high strength steels like X100 are considered as an economical necessity in order to supply large volumes of gas over long distances in a competitive manner. The development of the grade X100 started more than 10 years ago and has progressed significantly. Therefore the focus of actual development and research work is focused on questions regarding the installation and the service behaviour of such high strength steels. In this context the TAP project has been launched by the ENI group in order to provide answers regarding the economic viability, the technological reliability and the real possibility of constructing high pressure pipelines with this newly developed steel grade. In this paper the manufacturing of line pipe in grade X100 for the TAP project including the two fibre reinforced crack arrestors will be summarized and the production results regarding strength and toughness we presented. Furthermore the analysis of tensile test results regarding the influence of specimen type, anisotropy and the effect of thermal treatment (low temperature ageing) on the stress-strain curve will be discussed.
This paper deals with the development of low carbon NbTiB micro-alloyed high strength low alloy steel for heavy plates with high wall thickness. In the production of heavy plate it is remarkably difficult to achieve a combination of high strength and good low-temperature toughness. Bainitic microstructures have shown the capability to attain such requirements. To achieve a bainitic microstructure even for heavy wall products the formation of bainite can be promoted and supported by the use of small amounts of boron as a micro-alloying element. This industrial research project is based on the addition of small amounts of boron to promote the desired bainitic structure. Mill rolling trials were carried out to determine the optimum process parameters. The results of experimental mill rolling trials on 35 mm plates will be presented in this paper.
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