Ductility of Microalloyed Steels during Hot Deformation

Abstract:

The ductility behaviour experienced by steels for linepipe (LP) applications in a temperature range generally from 700°C to 1200°C is a widely studied subject in steel research, especially for its implication on cracking during continuous casting and rolling. Hot tensile / torsion tests on as-cast products, conducted until fracture, are normally used to characterise the hot deformability behaviour. Depending on the industrial hot deformation process within which the steel aptitude is being investigated, other types of tests can be more adequate. With the aim to characterize the hot deformability behaviour in terms of the damages each steel presents at moderate strain levels (i.e. far from the onset of necking), a special device for interrupted hot tensile tests, followed by immediate quenching (i.e. to “freeze” the microstructure) was developed. Various industrial steels with different starting microstructures (ad hoc in-lab heat treatments performed before testing) were tested by this method, and subsequent metallurgical investigations of the strained samples were carried out to identify, for each case, the damage mechanism and the microstructure features having the major influence on ductility loss. As a result, it was found that (i) microstructural damages at moderate strain levels can be much better described throughout interrupted hot tensile tests, (ii) different compositions and starting microstructures within the industrial LP scenario lead always to microstructural damages at relatively high deformation temperatures (e.g. 950°C) and moderate strains (e.g. 0.1 to 0.2, very far from the onset of necking), (iii) the common mechanism by which LP steels start voiding is the grain boundary sliding and (iv) the intergranular voids, once formed, grow longer in coarser microstructures.