Thermochemical processing of pipeline steels with a reduced Mn content
Lowering the Mn content of hot rolled pipeline steel can economise the steel making process by eliminating costly desulphurisation and at the same time improve product properties, particularly toughness, by reducing the formation of MnS inclusions. Lower Mn contents, however, have significant implications for austenite recrystallization and austenite (γ) to ferrite (α) transformation behaviour. This study compares the recrystallization start (Tnr) and γ to α transformation (Ar3) temperatures of two low Mn steels to those of a commercial steel with a conventional Mn content. Simulations of thermomechanical processing were carried out using a Gleeble 3500 testing machine. The Tnr for the low Mn alloy design were found to be ~25°C higher than for the high Mn steel, while the Ar3 of the low Mn steel were some 50°C higher. The higher Ar3 promotes the formation of coarse ferrite grains, thus reducing the strengthening and toughening benefits of a fine grain size. However, this could largely be overcome by maximising the degree of deformation performed between Tnr and Ar3 and maintaining a combination of high cooling rates and low stop cooling temperatures below Ar3, thus providing a high density of nucleation sites and promoting high nucleation rates. Production trials validated the results from the Gleeble tests.