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Warm Caliber Rolling (WCR), a thermomechanical operation carried out in the ferritic domain in the range of temperatures 500-700 °C, can lead to a strong grain refinement through mechanisms as dynamic recrystallization or recovery induced by accumulation of strain. Under proper conditions, it is possible to obtain ultrafine grained microstructures, which can significantly improve the mechanical properties, especially for those engineer applications – as special steel bars – where high toughness levels are required in combination with high strength.
In this work, thermomechanical cycles with temperatures 600-700 °C and high strain have been designed for a high carbon steel with C=0.80%, by means of preliminary Gleeble tests. WCR experiments were then performed on a pilot scale. As a key requirement for the formation of ultrafine microstructure is very high strain, multi-pass caliber rolling has been used to impart heavy deformations avoiding excessive rolling forces.
Starting from common lamellar pearlite typical of high-C steel, microstructures consisting of ultrafine ferrite matrix (~1 m) and finely dispersed spheroidized cementite (~ 0.5 m) have been achieved under the optimized process conditions.
The tensile strength of the bars subjected to WCR results in general lower than that of the conventionally hot rolled materials, while a significant improvement (from 30 to 60 J) is found in terms of impact toughness. In addition, an increase of fatigue limit – evaluated by means of tension-compression fatigue tests – of about 10 %, has been found with respect to the reference steel.