Per: fELIPE BASTOS (cbmm / cEIT), jose maria rodriguez-ibabe (CeIT AND tECNUN), bEATRIZ LOPEZ (CEIT AND TECNUN), beatriz pereda (CEIT AND TECNUN)
Abstract:
The modeling of austenite evolution during the hot rolling of steels is covered in the literature for a wide range of chemical compositions, although mainly for carbon contents below 0.20%. In addition, most of the investigations on this topic are oriented for processes that are characteristic of flat products. In these cases, hot rolling is well-defined in two zones: a roughing stage, where deformation temperatures are usually above 950ºC, and a finishing stage, that is usually performed at temperatures below this range.
Compared to those processes, the hot rolling of bars for structural applications presents considerable particularities. First, in bar rolling, together with dimension changes, the cross sectional shape is modified through each pass. This results in dimensional change through width that cannot be neglected as in flats, leading to a more complex state of deformation. Second, as structural bar rolling is strongly driven by high production output, the process is performed at high speeds and the rolling temperatures remain practically above 950ºC during the whole process.
Considering the deformation parameters, these particularities result in (i) strain values that are considerably higher than those applied in flat rolling, usually above 0.45, (ii) much shorter interpass times, usually of fractions of seconds during the last stages of the rolling, and (iii) much higher strain rates, as great as 1000s-1 depending of final diameter. Regarding the thermal profile, rolling is performed with an overall variation of only 100ºC, with final temperatures usually above 950ºC. Consequently, the rolling window has less room for modifications.
All these particularities affect the austenite microstructure evolution. On the one hand, it is well-known that the application of high strains at high temperatures can activate dynamic recrystallization. Therefore, post-dynamic softening mechanisms can play an important role in microstructure evolution. On the other hand, if the steel is microalloyed with elements such as Nb, strain-induced precipitation can take place during rolling and can interact with these dynamic and post-dynamic softening mechanisms.
The aim of this work has been to develop an austenite microstructural evolution prediction program, MicroSim-Bars, which takes into account all the above mentioned singularities. In the program, the input information is the composition and data from industrial schedule. This is converted to nominal deformation conditions following the approach proposed by Lee [1 ]. Different types of torsion tests have been performed in the laboratory to develop austenite microstructural equations suitable for the steel compositions and deformation conditions usually employed in bar hot rolling, as well as to understand the interaction between the individual microstructural evolution mechanisms at these conditions. The equations have been implemented in the program, and both torsion and industrial microstructural data have been used to validate the model.
1) Y. Lee, S. Choi, P.D. Hodgson, Analytical model of pass-by-pass strain in rod (or bar) rolling and its applications to prediction of austenite grain size, Materials Science and Engineering A336 (2002) 177–189
