Continuous energy developments around the world provide both an opportunity and challenge to develop linepipe microalloyed steels with superior properties and performance for the oil and gas industry. To fulfill these requirements a fundamental understanding of the composition-processing-microstructure-property relationship is needed. The objective of the present work was to define the optimal microstructure to achieve the required strength-toughness properties.

Different thermodynamic software packages and mean flow stress (MFS) models were used to study the effect of the composition and thermomechanical processing (TMP) conditions on the final microstructure.

The validation stages were performed by laboratory studies and industrial scale trials. Impact tests were used to evaluate the toughness. Advanced characterization techniques; electron optics (SEM, HRTEM) and Electron-Backscattered Diffraction (EBSD) microstructural analysis were carried out to quantify the microstructure, texture, precipitation and the evaluation of crack propagation paths.

The X-70 microalloyed steel exhibited an excellent package of strength-toughness properties. These properties were obtained by a combination of fine bainitic ferrite, fine precipitations and small volume fraction of fine MA islands. The fine bainitic ferrite was the result of the microstructural conditioning of the austenite prior to the transformation and cooling conditions. It provided excellent resistance to crack propagation.

This content is only available via PDF.
You can access this article if you purchase or spend a download.