A lean alloy is defined as a low alloy steel that minimizes the content of the alloying elements, while maintaining the characteristics of the sintered alloy. The purpose of this study is to determine the change in microstructure and mechanical properties due to the addition of silicon or tin in Fe-Mo-P, Fe-Mn-P, and Fe-Mo-Mn-P alloys. Silicon- or tin-added F-Mo-P, Fe-Mn-P, and Fe-Mo-Mn-P master alloys were compacted at 700 MPa and subsequently sintered under a H2-N2 atmosphere at 1120oC. The sintered density of three alloy systems decreases under the same compacting pressure due to dimensional expansion with increasing Si content. As the diffusion rate in the Fe- P-Mo system is higher than that in the Fe-P-Mn system, the decrease in the sintered density is the largest in the Fe-PMn system. The sintered density of Sn added alloys does not change with the increasing Sn content due to the effect of non-dimensional changes. However, the effect of Si addition on the transverse rupture strengthening enhancement is stronger than that of Sn addition in these lean alloys.

The addition of a large amount of alloying elements reduces the compactibility and increases the compacting pressure, thereby shortening the life of the compacting die and increasing the process cost of commercial PM steel. In this study, the characteristic changes of Fe-Mo-P, Fe-Mn-P, and Fe-Mo-Mn-P alloys are investigated according to the Si contents to replace the expensive elements, such as Ni. All compacts with different Si contents are fabricated with the same green densities of 7.0 and 7.2 g/cm3. The transverse rupture strength (TRS) and sintered density are measured using the specimens obtained through the sintering process. The sintered density tends to decrease, whereas the TRS increases as the Si content increases. The TRS of the sintered specimen compacted with 7.2 g/cm3 is twice as high as that compacted with 7.0 g/cm3.

The optimum route to fabricate nano-sized Fe-50 wt% Co and hydrogen-reduction behavior of calcined Fe-/Conitrate was investigated. The powder mixture of metal oxides was prepared by solution mixing and calcination of Fe-/Co-nitrate. A DTA-TG and microstructural analysis revealed that the nitrates mixture by the calcination at 300˚C for 2 h was changed to Fe-oxide/Co3O4 composite powders with an average particle size of 100 nm. The reduction behavior of the calcined powders was analyzed by DTA-TG in a hydrogen atmosphere. The composite powders of Fe-oxide and Co3O4 changed to a Fe-Co phase with an average particle size of 40 nm in the temperature range of 260-420˚C. In the TG analysis, a two-step reduction process relating to the presence of Fe3O4 and a CoO phase as the intermediate phase was observed. The hydrogen-reduction kinetics of the Fe-oxide/Co3O4 composite powders was evaluated by the amount of peak shift with heating rates in TG. The activation energies for the reduction, estimated by the slope of the Kissinger plot, were 96 kJ/mol in the peak temperature range of 231-297˚C and 83 kJ/mol of 290-390˚C, respectively. The reported activation energy of 70.4-94.4 kJ/mol for the reduction of Fe- and Co-oxides is in reasonable agreement with the measured value in this study.