In recent years, with the active use of Mn-containing high-tensile strength steels as an automotive body material, it has become increasingly difficult to recycle Mn-containing high tensile strength steel scrap as raw material to produce cast iron, due to the fact that Mn is known as an element promoting the chilling tendency of cast iron melt. Therefore, in this study, the effect of Mn on the fatigue limit of flake graphite cast iron was investigated for the purpose of increasing the strength of cast iron and recycling high tensile strength steel. Flake graphite cast irons with different Mn contents (0.5 mass % and 2.0 mass %, the carbon equivalent corresponding to FC200, FC300 and FC400 of JIS specification (G 5501)) were prepared for plane bending fatigue test.

  For melting, pig iron and electrolytic iron were used as iron sources, Fe-Si, Fe-Mn, and Fe-S were used as component adjusting materials, and Ca-Si was used as an inoculant. The melting temperature was 1803 K, the inoculation temperature was 1753 K, and the mold was poured at 1723 K.

  Due to the addition of Mn, the Mn/S ratio increases, resulting in the crystallization of A-Type graphite in high-manganese flake graphite cast iron. With the increase of Mn content, the graphite fraction decreases, the lamellar layer becomes denser and homogeneous, the matrix structure is strengthened, and the tensile strength and fatigue limit of high-manganese flake graphite cast iron are improved. Therefore, it was suggested that high-manganese flake graphite cast iron can be used for thin-walled parts as a cast iron with excellent fatigue strength. In addition, in this thin-walled flake graphite cast iron, Mn acts as a matrix structure strengthening element rather than a chilling promoting element..

  Spheroidal graphite cast iron (SG iron) is produced from pig iron, steel scrap, and return material, and recently, there is a trend to increase Mn content in steel scrap. Since Mn contained in SG iron acts as cementite stabilizing element during eutectic solidification and as pearlite promoting element during eutectoid transformation, it increases strength but decreases machinability and toughness.

  In this study, in order to investigate the effect of Mn mixed from steel scrap, static mechanical properties were investigated by varying the Mn content in SG irons with about 3%Si (3%Si series) and about 4%Si (4%Si series). In the 3%Si series, the tensile strength of SG irons increased as the amount of Mn increased, but when it exceeded 600MPa, the increasing tendency became smaller. In order to increase the tensile strength without reducing the elongation, heat treatment was conducted by normalizing from temperature range in the dual-phase region of ferrite and austenite. The heat-treated SG irons had a fine mixed structure of ferrite and pearlite, and strength was confirmed to improve. It was found that the effect of Mn on the transformation process by normalizing expands the heat treatment temperature range in which the mixed structure can be obtained. In the 4%Si series, when the amount of Mn is low, the tensile strength and elongation had excellent properties. However, it was clarified that when the area fraction of pearlite becomes 10% or more as Mn content increases, the elongation greatly decreases and embrittlement occurs. These results suggest that since embrittlement is improved by conducting ferritized annealing, even if the amount of Mn is large, embrittlement of SG iron will not occur by adopting a ferrite single phase.