EUROPEAN STEEL TECHNOLOGY AND APPLICATION DAYS 2017 - ESTAD 2017

The article focuses on the hot rolling of large diameter bars (round bars of 500mm) using either continuous casted blooms or ingots. This kind of product is often manufactured by forging an ingot. The forging process is able to applied large deformation and thus to ensure a good metallurgical microstructure (small grain size, no porosities or micro-cracks…). However, this process has two main drawbacks: it has a lack of productivity and the dimensional quality of the forged bars is poor. In comparison hot rolling allows avoiding these drawbacks but the strain applied at each pass is limited by the fact that the bar should enter between the cylinders of the rolling mill without slipping. The hot rolling route has to be determined to avoid the slipping of the bar in the mill and to generate the necessary deformation to refine the material structure. The presented work is dedicated to the numerical modelling and simulation of the rolling of large diameter bars on the Acciaierie Bertoli Safau S.p.A. (ABS) industrial facility called ROTOFORGIA. The objectives of the simulation are twice: - to validate the rolling route according to the bar non-slipping and the geometrical quality conditions - to allow following the microstructure parameters associated with the porosity, the internal cracks , the initial microstructure obtained by the simulation of the continuous casting of the bloom or of ingot casting. In a first step, the rolling process is modelled and simulated with Forge.NxT software. The rolling route is made of fifteen passes. The rolling parameters (roll gap, roll geometry and rotational velocity…) for each pass are given. The friction and heat exchange coefficients are then determined by confronting numerical results with experimental measurements carried out on the industrial rolling mill for a first route. The compared results are the rolling torques and the temperature on the surface of the rolled bar. The numerical model is then tested in the case of a second rolling route involving different process parameters. The validated model is then utilized in order to perform a parametric sensitivity study. The objective of this part is to identify the influence of the process parameters on the shape of the bar, the strain undergone by the material and the non-slipping conditions. The last part presents the first results concerning the numerical study of the evolution of the microstructure through the Yamanaka criterion for the micro cracks and the Niyama criterion for the porosities. The starting point of this study is the numerical simulation result of the continuous casted products. As a conclusion, a numerical model of the rolling of bars of large diameter was implemented and validated. It allows the validation of the rolling route and allows following structure criteria from an initial state (conti-casted blooms or ingot) obtained by numerical simulation.