Study of Heat Transfer Involved in Controlled Cooling of Steel Wire Rod

The heat transfer involved in the controlled water cooling of wire rods passing through water boxes at the rod mill production line at North Star Steel Texas (NSST) is modeled and analyzed in this thesis. The water boxes are situated between the final finishing mill and the Stelmor deck. This cooling stage in the wire rod production process is very crucial and important as the wire rod has to be delivered at the laying head at a proper temperature so as to enable proper austenitizing. The laying head is a unit which coils the wire rod into loops and places them on to the Stelmor conveyor deck The Stelmor conveyor deck consists of a series of fans placed under the conveyor. These high efficiency fans provide uniform air cooling at controlled rates. Austenite to pearlite transformation takes place in the Stelmor deck and hence extreme care should be taken to control the inlet temperature at the deck. It is hence necessary to control the parameters that affect the cooling of the wire rod in the water boxes. For a given operating condition, these parameters set a temperature variation from the laying head to the entry of the Stehnor conveyor deck. If the temperature difference is too great then the solid state transformation in the rod may happen prior to the entry of the Stelmor conveyor deck. This would cause the formation of coarse pearlite which will decrease the mechanical properties of the final product. Very high cooling rates may induce the formation of metastable phases like martensite, hence rendering the rod useless for further operations. Good control of the parameters in the water cooling zone will ensure that austenitic transformation occurs under controlled conditions of the Stelmor conveyor deck, making the steel more responsive to the subsequent patenting operations (Morales et al. 1989). This research considers the rod properties and cooling parameters, termed as Key Input Variables, which control the temperature of the rod surface at the laying head. A numerical model has been developed to estimate the rod surface temperature at the laying head. The model clearly indicates the affect of each key input variable on the laying head temperature. The model has been tested for its accuracy using the data from a similar investigation.