Controlled cooling conveyor system is known, as the starting point for quality processing of wire rod products, obtaining good mechanical properties. This process guarantees excellent performance with high production rates. In order to process several steel grades and different diameters, it is necessary to determine, the different austenitization temperatures and cooling rates that will control austenitic grain growth and, consequently, the pearlite interlamellar spacing. In this work, dilatometric analysis was performed to determine critical temperatures for an hypoeutectoid steel during heating and controlled continuous cooling, similar to the industry conditions for heat treatment. Metallographic analysis and microhardness test validated the results obtained during the dilatometry tests. The critical temperatures will be employed to understand the effect of the thermal conditions in controlled cooling process and adjust the conditions for better results in the austenitic grain size.

A physical model was developed to estimate the thermal history in hypoeutectoid steel under continuous cooling and forced convection conditions. The thermal histories were acquired at different cooling rates to emulate the forced-convection conditions in a controlled cooling conveyor and compared in every cooling condition with the microstructural evolution of the pearlite. Through Scanning Electron Microscopy (SEM) the pearlite interlamellar spacing was determined, as well as Vickers hardness and tensile tests, validated the effect of the cooling rate on microstructural parameters such as transformation temperature and pearlite interlamellar spacing. It was found that air velocity increased the undercooling rate and decreased the pearlite interlamellar spacing.

A hypoeutectoid steel was austenitized at 840 °C for one hour and cooled at two rates. Examination by optical and scanning electron microscopy showed a change in the pearlite microstructure. Cooling in air as compared to furnace cooling reduced the pearlite interlamellar spacing and increased the hardness. The slower cooling resulted in a lower tensile strength, higher tensile elongation, and different fracture appearance.