Shigeaki Takajo

Excellent plasticity revealed in high purity Si steels by the addition of Cr

Abstract The plasticity of high Si steels is improved remarkably by adding Cr in high purity steels. The factors which contribute to this phenomenon were investigated. At the normal level of purity, the plasticity of Fe–Si alloys is deteriorated by Cr addition. However, in high purity steels, the plasticity drastically improves when Cr is added. When the influence of microstructure was removed, the improvement of plasticity by adding Cr to high purity Fe–Si alloys was attributed to the suppression of deformation twinning formation. The microstructural improvement was explained by grain refinement and by formation of subgrains during hot rolling. The properties of these steels are expected to be applied to new functional steels.

Fatigue Stremgth of Sintered and Heat Treated Compacts made from Composite-type Alloyed Steel Powder

Compacts made from a composite-type alloyed steel powder with 2% Ni and 1% Mo attained high endurance limits of rolling bending and contact fatigue strengths of 460MPa and 2560MPa, respectively, after case hardening. These endurance limits were much higher than those of a conventional 4%Ni-0.5%Mo-1.5%Cu alloyed steel powder, which were 410MPa and 2430MPa, respectively. When double pressed, double sintered and bright quenched, compacts made from the 2%Ni-1%Mo alloyed steel powder showed endurance limits of rotating bending strength of 390MPa and contact fatigue strength of 2710MPa, both higher than those of the compacts made from the conventional alloyed steel powder, which were 350MPa and 2330MPa, respectively.The results of micro-X-ray analysis indicated the decrease in the amount of austenite phase near the fatigue-fractured surface, The improvement of fatigue strength was therefore attributed to the strain-induced martensitic transformation of Ni-rich austenite.

Developments in Low Alloy Steel Powders for High Strength Sintered Components

The creation of high quality steel powders greatly contributes to the improvements of sintered components. A new family of low alloy steel powders has been developed to meet the requirements of heavy duty structural components mainly for the automobile industry. Composite-type alloyed steel powders containing 2%Cu or 4%Ni1.5%Cu-0.5%Mo consist of iron particles to which alloying elements are finely diffusion-bonded. The powders attain the homogeneity of sintered microstructures while maintaining the compressibility of a pure iron powder. A prealloyed steel powder containing 1.0%Ni-0.3%Cu-0.2%Mo has been designed to achieve high strength after heat-treating high density sintered compacts. The sintered materials remain soft before heat treatment and are effectively densified by repressing.

Magnetic Properties of Sintered Fe-Sn Cores

Sintered iron alloy magnetic cores containing Sn, as well as P and Si for comparison, were prepared and the effect of alloying elements on sintering behaviours and magnetic properties was investigated.