Comparison of Dynamic Recrystallization and Conventional Controlled Rolling Schedules by Laboratory Simulation

Abstract

A seamless tube mill rolling process was simulated with the aim of optimizing the ferrite grain refinement by decreasing the finish rolling temperature during the stretch reducing mill (SRM) stage. Tests were performed on one Nb-V and two Ti-V micralloyed steels. For the Ti-V steels, maximum grain refinement is achieved when the SRM exit temperature is just above the Ar3 ; at a cooling rate of 3.5°C/s during transformation, ferrite ASTM grain size numbers of #13 and higher are produced. When the reductions are applied below the no-recrystallization temperature or Tnr, the strains are accumulated at first as a result of the absence of static recrystallization during the short interpass times. At larger strains, dynamic recrystallization is initiated, due to the absence of strain-induced precipitation during the brief unloading intervals. Under these conditions, the rolling loads are lower than when a conventional controlled rolling schedule is employed. In the latter case, the interpass times are sufficiently long for the occurrence of appreciable strain-induced precipitation, which is responsible for the prevention of dynamic recrystallization, and the consequent accumulation of strain and pancaking of the austenite. For the Nb-V steels, when the SRM cooling curve passes through the nose of the CCP (continuous-cooling-precipitation) Ps curve, austenite pancaking takes place. By contrast, when the cooling curve passes above orbe/oivthe CCP curve, the dynamic recrystallization of austenite is initiated, leading to ferrite grain sizes which are generally finer than those produced by the pancaking process. © 1991, The Iron and Steel Institute of Japan. All rights reserved.