To achieve both quality and productivity in continuous casting at a high dimension, a new electromagnetic flow control system was proposed, in which AC and DC magnetic fields are superimposed. Numerical simulations and experiments at an industrial continuous casting machine were carried out.

(1) A model for predicting bubble defects and mold flux defects by focusing on the steel flow velocity on the wide face and the turbulent kinetic energy on the top surface of the molten steel was developed. The validity of the model was confirmed by comparing it with the actual defect tendency of a continuous casting machine.

(2) Applying an AC magnetic field in the upper stage increases the molten steel flow velocity and suppresses entrapment of bubbles on the wide face, but flux engulfment and entrapment increase, presumably due to the larger random motion of particles caused by the increased turbulent kinetic energy.

(3) When the AC and DC magnetic fields are superimposed in the upper stage, both bubble defects and mold flux defects can be reduced by optimizing the steel flow and turbulent kinetic energy. As a result, surface defects on steel sheets can be roughly halved compared to the conventional flow control method using only DC magnetic field.

(4) The results of this study suggested that it may be possible to control the flow velocity and turbulent kinetic energy by optimizing the AC and DC magnetic field intensities according to the casting conditions, further reducing surface defects caused by bubbles and mold flux.

For the high-performance development of sliding parts, the tribological properties of carbon steel must be improved. In particular, adhesion is an important factor for controlling the tribological properties. However, the dependence of adhesion on crystal orientation has barely been investigated owing to the difficulty in obtaining a single crystal of steel. In this paper, a new analysis method combining electron backscatter diffraction and atomic force microscopy reveals the dependence of adhesion on crystal orientation in carbon steel. The adhesion force was found to increase in the order (100) < (111) < (110). In addition, molecular dynamics simulations can reproduce this trend and clarify the factors governing the dependence of adhesion on crystal orientation at the atomic level. The findings of this study can contribute to controlling the tribological properties of carbon steel and understanding the nanoscale phenomenon of mechanical contact.

An attempt was made to apply the laser powder bed fusion (L-PBF) process to Fe–Ni–Nb ternary alloys as a major composition of heat-resistant Fe-based alloys strengthened by the Fe₂Nb intermetallic phase. This study focused on a composition of Fe–40Ni–15Nb (at%) near a eutectic composition of austenite (γ-Fe) and Fe₂Nb in the Fe–Ni–Nb ternary system for the pre-alloy powder with an average particle size of approximately 17 µm, which was produced by the gas-atomizing process. The Fe–40Ni–15Nb pre-alloy powder exhibited insufficient L-PBF processability for fabricating fully dense centimeter-sized samples, whereas several alloy samples with relative densities above 90% were manufactured. The L-PBF manufactured alloy samples exhibited a representative melt-pool structure in which regions had locally melted and rapidly solidified by scanning laser irradiation in the L-PBF process. The L-PBF sample exhibited a high hardness of approximately 900 HV compared to the slowly solidified alloy sample (about 550 HV). The high hardness could be attributed to the formation of nanoscale γ-Fe/Fe₂Nb eutectic microstructure by the L-PBF process. Intriguingly, the observed melt-pool boundary had several tens of micrometers in width, which significantly varied depending on the applied laser condition. Nanoindentation tests demonstrated the melt-pool boundary region exhibited a relatively lower hardness than inside the melt pools. It was assumed that the unique melt-pool boundary region would consist of a relatively coarsened solidification microstructure formed at the interface of liquid with the solid and a locally coarsened microstructure affected by laser local heating, which can be controlled by manipulating the laser conditions.

Ni films electrodeposited from Watt’s solutions with and without thiourea addition were heat-treated to evaluate the diffusion behavior of hydrogen in the films. Hydrogen permeation measurements were taken using the Devanathan–Stachurski double cell technique. Regardless of whether thiourea was added or not, the crystal grain size of the deposited Ni films increased to several tens microns with heat treatment for 10 min at 800°C. Heat treatment of Ni films deposited with thiourea resulted in the segregation of sulfur at the grain boundaries. The hydrogen permeation rate through the deposited Ni films significantly decreased with increasing grain size due to heat treatment, regardless of whether thiourea was added. The segregation of sulfur at the grain boundaries further reduced the hydrogen permeation rate.