Void Closure in a Large Cross Section Bars Hot Rolled from a Low Alloy Steel Ingot Casting

Abstract

The void closure phenomenon of a large size rolled bar made from an as-cast low alloy steel ingot of grade AISI 4130 has been examined. The study involved FEM based simulation of hot rolling of a four metric ton ingot casting to a final product of 320 mm round corner square cross section with a reduction ratio of 4.3. The rolling process was varied at two process conditions. The process variables considered included initial cooling time of the ingot from soaking pit before rolling, rpm, reduction and pass schedule. The defects in the product at the two rolling conditions were experimentally evaluated using ultrasonic technique during hot rolling and compared with void fraction in the simulation. Higher initial time of cooling of the ingot before hot rolling, lower rpm, higher reduction and lesser number of tilts gave products with least core defect which justified simulation results. The study brought out at every stage the load and torque distribution, effective strain, stress triaxiality factor, void volume fraction and void healing hydrostatic integration parameter in the model which had enabled void closure.