Roughing of thick, coarse-grained high-temperature-processing steels processed via compact strip production

Abstract

The roughing conditions required to avoid local heterogeneous regions in the final microstructure of high temperature processing (HTP) Nb linepipe steels have been investigated for quasi-compact strip production (CSP) conditions. The influence of strain sequence, rolling temperatures and Mn content on recrystallisation and carbonitride precipitation before and after roughing were studied using laboratory simulation, mathematical modelling and transmission electron microscopy. The roughing conditions necessary to avoid the formation of local heterogeneous regions have been established for HTP steels with an initial grain size of 850 m. Low Mn Nb–Ti steels experience more sluggish recrystallisation kinetics and are more vulnerable to forming local heterogeneous regions during roughing. To prevent local heterogeneous regions if finishing commences at 900°C, sufficient effective strain is necessary to produce a bulk softened fraction of at least 0.55 in each of the first two roughing passes. If finishing commences at 1 000°C, slow air cooling from roughing provides additional time for recrystallisation to go to completion and so prevent the occurrence of local heterogeneous regions. Maintaining the strand temperature as high as possible prior to the commencement of roughing encourages recrystallisation. A roughing start temperature of 1 100°C, as opposed to 1 075°C, significantly reduces the risk of forming local heterogeneous regions. For the conditions tested, no correlation between softening fraction and carbonitride precipitate characteristics was found. Thus, the influence of Nb on austenite recrystallisation is expected to be due to either solute drag or solute clustering.