the effect of controlled thermo mechanical processing on the properties of a high strength steel

in this paper, an ultra low carbon high strength low alloy grade steel was subjected to a two-step forging processand this was followed by different post cooling methods. the highest strength was obtained at a faster coolingrate due to the highly dislocated acicular ferrite structure with the fine precipitation of microalloying carbides andcarbonitrides. at a slow cooling rate, the strength fell with an increase in ductility due to the larger volume fractionof the less dislocated polygonal ferrite structure. the strength remained almost unchanged with a further decreasein the middle cooling rate due to the formation of a predominantly polygonal ferrite microstructure. at the slowerpost cooling rates, a high impact toughness value was obtained at the ambient temperature and at -40°c testingtemperature; this was due to the fine grained polygonal ferrite microstructure. at all post cooling methods, thechange in the impact toughness value at the ambient temperature and at the temperature of 40°c was found to benegligible due to the ultra-low carbon content of the steel.