Accelerated cooling of steel rebars establishment of technological and design parameters of the cooling unit by modelling and experimentation

Abstract

The aim of this work is to develop a computer mathematical model that could be used to predict the design parameters of an accelerated cooling unit. These parameters include length and diameter of cooling tube, size, number and type of nozzles and amount of water needed. Such units are encorporated at the end of or before the last rolling mill of re-inforcing bars for the production of high strength steel re-bars. The production of high strength steel rebars from ordinary 0.2% C - 0.8% Mn by thermo-mechanical treating has been widely employed recently. The control of the cooling rates through the water flow (cross sectional area of tube and amount of water) and cooling time (length of unit which varies according to the rolling speed) affects the process greatly. The bars that leave the stand at about1000°C, are cooled in a long tube with high pressure water. The outer surface cools to about 200-400°Cforming a martensftic layer, while the inner areas remain hot (1000°C). Then both surface and core temperatures equalise at a certain temperature which greatly affects the strength of the cooled rebars. It is of great importance to be able to predict this temperature and previously mentioned design parameters to be able to build the cooling unit required to achieve certain strength levels. The results of the model have been used to develop a cooling unit at El-Ahlya National Company. The performance of the unit was verified experimentally, where several experiments were carried out for different bar diameters, in the range 12-16 mm, and various cooling conditions leading to different bar equalising temperatures.

Strength values in the range from 430 to 1500 MPa were obtained by changing cooling conditions. The obtained mechanical properties after cooling were compared with the predicted equalising temperatures. Also, the microstructures of the cooled bars were compared with those predicted by the cooling curves obtained from the mathematical model results. The results obtained from the model predicted to great proximfty the experimentally obtained results.