Solid–liquid interfacial energy of steel during solidification was measured predicted from the both experimental techniques of unidirectional solidification and thermal analysis applying the dendrite growth model and heterogeneous nucleation model. Solid–liquid interfacial energy changed depending on primary phase during solidification, i.e., that of primary δ phase was larger than that of γ phase. When the primary phase was the same, solid–liquid interfacial energy increased with increasing carbon content. Primary dendrite arm spacing changed depending on solid–liquid interfacial energy. A trace amount of bismuth which had the effect of a decrease in the solid–liquid interfacial energy of steel during solidification decreased primary and secondary dendrite arm spacing, significantly.
The dissolution of δ-ferrite in continuously cast slabs of SUS304 has been studied during heat treatment in the temperature range of 1373 to 1473K. The dissolution behavior can be expressed by Kolmogorov–Johnson–Mehl–Avrami equation. The dissolution rate is affected by annealing temperature and secondary dendrite arm spacing. Moreover, the numerical methodology for multi-phase field method has been performed in Fe–Cr–Ni alloy. It is thought that dissolution behavior of δ-ferrite during heat treatment can be also predicted by this method.
Surface defects formed on Si-containing steels in hot-rolling process depend largely on the character of the scale composed of FeO and Fe2SiO4 (FeO+Fe2SiO4 oxide scale) during de-scaling. The formation of FeO+Fe2SiO4 oxide scale is influence by internal oxide precipitates. In order to elucidate quantitatively the internal oxidation, it is required to analyze the distributions of number, radius and volume of internal oxide precipitates in internal oxidation zone (IOZ). Internal oxidation of Fe–1 mass%Si alloy was examined at 1473K in Ar–17.6%H2–12.2%H2O where an outer scale of FeO does not form. The growth rate of IOZ was obeyed the parabolic rate law, indicating that the rate-determining step was the diffusion of oxygen through alloy. SiO2 precipitated from the internal oxidation front to 0.23±0.02 of normalized thickness of IOZ, and Fe2SiO4 precipitated from the position to the gas/alloy interface, depending on the chemical potential distribution of oxygen. Fe2SiO4 is formed by the reaction between SiO2, Fe and dissolved oxygen in IOZ. The number of precipitates in unit volume is reciprocally proportional to depth in IOZ, suggesting that the grain growth of oxide obviously affected the distribution. The radius of the precipitates was proportional to the cubic root of depth so that the volume of the precipitate was proportional to depth. These results elucidated that the volume fraction of precipitates in IOZ was constant.
In microalloyed steel, industrial candidate for application to parts of automobiles, we investigated strengthening mechanism by vanadium carbide (VC) precipitation in the pearlite. The specimens, containing 0.1–0.65% carbon (C) and 0–0.4% vanadium (V), were quenched from 1200 to 650–550°C and kept at this temperature. The more V and the lower transformation temperature, the higher strength can be obtained. The maximum 0.2% proof strength is determined to be 1450 MPa for 0.65%C–0.4%V steel and 1200 MPa for 0.65%C–0.2%V steel. VC is found to precipitate at interphase boundaries between the austenite and the ferrite in the pearlite. When the transformation temperature is lowered, the VC size becomes finer and the precipitation density increases. These results suggest that the strengthening mechanism by VC is the Orowan-type. The amount of precipitation strengthening by VC was calculated by Ashby–Orowan equation. Both the calculated and the experimental results were determined to have good correlation. Strengthening mechanism by VC and that by pearlite lamellar would be competitive, however the amount of precipitation strengthening by VC in pearlite was close to that in ferrite.
The elementary factors which determine the fatigue strength of the carburized steel were studied by investigating the fatigue crack propagation behavior and the change of residual stress during the fatigue test. The fatigue strength for finite life of the carburized steel is determined by the propagation property of the crack within 0.3 mm depth from the surface. The fatigue limit is determined by the crack arrest property of the crack 0.1–0.2 mm depth from the surface. The compressive residual stress increases during fatigue process. The absolute value of maximum compressive residual stress increased during fatigue process is large value such as about 50% of value of stress amplitude and depth from the surface of maximum compressive residual stress is almost same with non-propagation crack length observed of fatigue limit specimen. Therefore, it is considered that compressive residual stress is dominant factor for crack arrest property of the crack 0.1–0.2 mm depth from the surface. The larger the stress amplitude is, the larger the amount of increase of compressive stress during fatigue process is. The reason for increase of compressive stress during fatigue process is that stress/strain induced transformation from retained austenite to martensite causes volume extension.
Inhomogeneous deformation in DP steel has been investigated by SEM-EBSD method and high-precision markers drawn by electron beam lithography. Particular emphasis is laid on the effects of initial hardness of martensite on the development of inhomogeneity in ferrite during tensile deformation. The hardness of each phase in DP steels has been examined using a Berkovich-type indenter. Difference of the hardness between ferrite and martensite in the specimen tempered at 423K was larger than that tempered at 673K. Changes of local orientation obtained by SEM-EBSD method inside the ferrite in the steel tempered at 423K was more noticeable than that tempered at 673K. In addition, we employed a high-precision marking method using electron beam lithography in order to measure the local displacement. Not only the equivalent plastic strain in ferrite but also the one in martensite were obtained by using the method due to tensile deformation. It is clarified that the hardness of martensite effects the development of deformation inhomogeneity in both of ferrite and martensite. On the basis of results, the influence of the characteristic of deformation inhomogeneity on the mechanical property of the DP steels is discussed.