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 process and this was followed by different post cooling methods. The highest strength was obtained at a faster cooling rate due to the highly dislocated acicular ferrite structure with the fine precipitation of microalloying carbides and carbonitrides. At a slow cooling rate, the strength fell with an increase in ductility due to the larger volume fraction of the less dislocated polygonal ferrite structure. The strength remained almost unchanged with a further decrease in the middle cooling rate due to the formation of a predominantly polygonal ferrite microstructure. At the slower post cooling rates, a high impact toughness value was obtained at the ambient temperature and at -40°C testing temperature; this was due to the fine grained polygonal ferrite microstructure. At all post cooling methods, the change in the impact toughness value at the ambient temperature and at the temperature of 40°C was found to be negligible due to the ultra-low carbon content of the steel.