In recent years, general industrial machine parts such as pressed parts require high toughness and high fatigue limit that can withstand repetitive loading when used in harsh environments. However, porosity inevitably occurs in cast steel products and greatly affects the fatigue limit. In this report, the relationship between the evaluation of fatigue characteristics of cast steel materials used for large cast steel products and porosity was investigated. The results of the investigation using test specimens with different carbon concentrations and mass effects showed that increasing carbon concentration increased tensile strength, increased the solidification temperature range (Θ_L－Θ_S), and increased the √area of the defect at the failure initiation point in the principal stress direction, and that the fatigue limit decreased as √area increased.

  Although there was a significant difference in the cooling rate of the solidification zone for 1ton ingots compared to 50kg ingots, the maximum √area of the fracture initiation point remained the same and the fatigue limits were equivalent.

  The fatigue life prediction equation was obtained from the relationship between the initial stress intensity factor K_max and fatigue life N_f/√area. The lower boundary stress intensity factor range ΔK_th predicted for the smooth material in 1ton ingots and the value obtained from the fatigue crack growth test were equivalent.

  The relation between the supercooled solidification macro- and microstructures of castings and cooling rate during solidification obtained by casting simulation was investigated. In the simulation of pure Al castings under the critical solid fraction of 0.99, the formation of significantly high cooling rate zones corresponding to fine crystal grain structures was observed. In the cross-sectional observation of flake graphite cast iron castings and their simulation under critical solid fractions ranging from 0.6 to 0.99, the correspondence between D-type graphite structures and higher cooling rate regions was also observed. In addition, there was a case suggesting that high cooling rate zones under the critical solid fraction of 0.99 may correspond to the generation of inverse chill in spheroidal graphite cast iron castings. The critical solid fraction when the release of most latent heat completes corresponds to the timing when considerable solutes are discharged from the region to the surrounding unsolidified area during non-equilibrium solidification. This suggests that the cooling rate distribution under critical solid fractions near 1 such as 0.99 reflects the ease of constitutional supercooling due to the liquid concentration in the vicinity of the solid-liquid interface, which causes the supercooled solidification macro- and microstructure such as fine crystal grains, the D-type graphite structure, and inverse chill.

  The scope of the research in Japan was Journal of Japan Foundry Engineering Society, Journal of Japan Institute of Light Metals, Japan Die Casting Conference proceedings, Die and Mold Technology, and Sokeizai, and overseas was Die Casting Congress (NADCA) and China foundry. The review period covered the four years from 2019 to 2022 in Japan, and the three years from 2019 to 2021 overseas. The main topics are Die Casting molds and injection sleeves, melting and molten metal processing, Aluminum alloys for die casting, manufacturing, CAE, new processes and defect suppression technology, and other alloys (Magnesium alloys, Copper alloys, Iron alloys).