Processing and Properties of an Alternative Alloying Concept for an Export Subsea Pipeline in Grade X70

The Thermomechanically Controlled Rolling Process (TMCP) is used to produce heavy plates for a wide field of applications due to the excellent combination of mechanical properties, strength and toughness properties as well as excellent weldability. In order to achieve these properties lean alloying concepts with microalloying of Nb, Ti and/or V in combination with temperature controlled rolling are used. The basic metallurgical properties are controlled recrystallisation, α/γ grain refinement, phase transformation and precipitation hardening. This paper deals with the evaluation of an alternative alloying concept for a major plate order for the construction of an export subsea gas pipeline in grade X70. The challenge of this project was focussed on the evaluation of an alloying concept which was able to meet the demands of an offshore grade X70 and in addition to fulfil severe requirements regarding weldability and toughness properties. In addition to the aforementioned requirements the alloying concept had of course to be within the restrictions of the customer specification. Before start of production a suitable process design had to be developed in a laboratory scale as well as large scale trials. Special focus was laid on the influence of rolling parameters on the obtained mechanical properties. Therefore different alloying concepts and the achieved mechanical properties as well as temperature ranges for the rolling process will be reviewed and presented. The finally chosen product and process design lead to excellent results which stayed in a very narrow scatter band throughout the mass production of more than 300,000 tons.