MAKING QUALITY: QUENCHILOOP TECHNOLOGY

In the thermo-mechanical process the final rolling passes are performed at low temperature by intensively cooling the material entering the finishing sizing stands. This process must be followed by an equalizing section in order to achieve metallurgical properties uniformly across the finished product section. Without the equalizing section, the temperature difference between surface and core can be too high and consequently different microstructures may be generated during the subsequent rolling phase. Siemens has developed a Quenchiloop technology, consisting in a loop with additional water boxes and long equalizing sections between intermediate and finishing mills. From the intermediate mill, the material can go directly to the finishing stands or can be processed in the loop before being rolled in the finishing stands. In such way the Quenchiloop technology allows long equalizing time to the steel grades that are rolled at low temperature and short interstand time to steel grades that require to be rolled as hot as possible. Different process schedules were used, with and without cooling system before the finishing mill, during rolling a medium-carbon ferrite-pearlite steel in order to control the finishing temperature that affects the prior average grain size before transformation. The microstructure was investigated by optical microscopy and hardness was measured on the cross-section of the rolled rods. The final microstructure and its related mechanical properties were then compared with the results obtained by the prediction tool PROMET4.0 developed by the same authors. Quenchiloop technology is a flexible solution that makes the thermo-mechanical process available also for revamping of existing plants, where the space for cooling and equalization section was not provided.