Influence of carbides on the hot working of steel

Carbides have a strong influence on the hot working behavior of steels depending on their size, distribution, solubility and hardness and on the matrix, either austenite or ferrite. Fine carbides pin dislocations slowing dynamic recovery (DRV) and despite the higher dislocation density delay dynamic recrystallization (DRX) by pinning the grain boundaries. Large carbides raise dislocation density in the surrounding region, thus enhancing nucleation of DRX. Fine carbides reduce ductility by slowing DRV and DRX, whereas large carbides cause stress concentrations and cracks. In tool steels, the carbides are partially stable up to quite high temperatures causing considerable increase in strength and activation energy compared to C steels. In HSLA steels, the fine microalloy carbonitrides tend to precipitate at the low end of the hot working range, which raises the strength and the apparent activation energy while lowering the ductility. For steels with an austenitic phase in which the dissolved carbon enhances DRV and ductility, carbides considerably raise the strength and decrease ductility on cooling to the ferritic phase. Pearlitic steels are stronger and less ductile than those in the spheroidized condition; however, they show considerable work softening as the pearlite spheroidizes during hot working. By suitable thermomechanical processing, high carbon steels can be produced with a micro-duplex structure capable of superplastic deformation. In multistage rolling schedules, alloy carbides deter interpass recrystallization and grain refinement at the hot end; fine microalloy carbonitrides delay it at the cool end, resulting in pancaked austenite and fine ferrite grains.