The effect of Nb content on the mechanical properties and solidification structure of stabilized ferritic stainless cast steel with a basic composition of 18%Cr-0.5%Cu- (0.35～1.1%) Nb-0.035% (C+N) was investigated. The tensile test results for specimens containing 0.35～0.45%Nb showed a tensile strength of 370～380 MPa, 0.2% proof stress of 270～280 MPa and elongation of 7～10%. However, as the Nb content increased to 1.1%, the 0.2% proof stress increased to 360 MPa and the elongation decreased significantly to 1%. In all specimens, Nb (C, N) particles were dispersed in the ferrite matrix, and the particle size became larger with increasing Nb content. Some continuous film-like Nb (C, N) appeared at the grain boundaries in the 0.75%Nb sample and increased in the 1.1%Nb sample. These difference in Nb (C, N) distribution should affect the mechanical properties.

  The solidification process of the sample containing 0.45%Nb, which was considered as optimal Nb content, was investigated. The results show that δ-ferrite grows as dendrites in the solidification temperature range from 1768 K to 1713 K. The analysis with Scheil's equation predicted that δ+Nb (C, N) eutectic starts crystalizing at fraction solid of 0.98, but that the δ+Nb (C, N) eutectic does not appear taking into consideration the diffusion of Nb in the solid. The results of the latter analysis considering diffusion in the solid phase were consistent with the solidification structure of the 0.45%Nb sample.

  The effects of trace elements on the soundness and electrical conductivity of pure copper castings were investigated by electrical conductivity measurement, several relevant analyses and thermodynamic calculations. Although the generation of water vapor during solidification thermodynamically decreases with the oxygen content of the melt, the results of thermodynamic calculations and past experiments indicated that the mushy zone during solidification under oxygen content ranges of approximately 0.02-0.1% is wider than that under the other oxygen content range. These suggest that the poor casting soundness under medium oxygen content ranges is caused by the generated bubbles getting trapped easier in the wider mushy zone. In the vicinity of the as-cast surface, the distribution of the hydrogen content of more than the saturation solubility was observed. This suggests that the rapid cooling by the mold or the chill in the vicinity of the as-cast surface easily leads to supersaturation, and that the generation of surface pinholes and fissure defects is caused by the nucleation of bubbles due to hydrogen contents higher than the supersaturatable content at the liquid-solid interface. In addition, the variation of the electrical conductivity in the absence of Cu-Cu₂O eutectic could be explained based on the ridge regression (multiple liner regression) of the electrical resistivity in respect to the contents of the main trace elements. The standardized regression coefficients indicated that the major factors influencing the decrease in the electrical conductivity are contained phosphorus, iron, and sulfur. On the other hand, the electrical resistivity under the presence of Cu-Cu₂O eutectic was independent of the contents of the trace elements. These and past results suggest that the dominant factor influencing the electrical conductivity under the presence of Cu-Cu₂O eutectic is the casting soundness.