Thermomechanical Processing of a Nb-Microalloyed Steel in a Controlled-Forging Treatment

Abstract

Ferrite grain size is one of the most important microstructural parameters in steels which can be appropriately adjusted to cause a significant strengthening effect. Thermomechanical processing is an effective method for ferrite grain refinement in microalloyed steels. Transformation of deformed austenite with a pancaked grain structure to a relatively fine ferrite phase is an important phenomenon occurring during the thermomechanical processing of microalloyed steels. The final microstructure of steels can be optimized by controlling three critical processing parameters, i.e. i) applied strain (constant strain rate), ii) deformation temperature, and iii) cooling rates following the hot deformation stage. In the present study, a new approach (called controlled-forging treatment) consisting of hot deformation of steel at the austenitic temperature range using an upset forging stage was developed for the ferrite grain refinement in a Nb-microalloyed steel. The investigated steel was subjected to a thermomechanical treatment including reheating, hot deformation for two different strain levels, namely 30 and 50% reductions of height, in a single pass hot-forging stage at temperatures below the TNR(no-recrystallization) and above the TR3(austenite to ferrite transformation) temperatures followed by cooling to room temperature using three different cooling rates. The experimental results obtained from this proposed treatment were more or less similar to those already obtained for the case of controlled-rolling process on Nb-microalloyed steel sheets.