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.
