A study has been made on the desulfurization by the use of CaO-CaF₂ flux and CaO-Na₂CO₃ flux with mechanical stirring. The followings were found: 1) The desulfurization rate was correlated with the mixing ratios of CaF₂ and Na₂CO₃ in the fluxes and had local maximum. 2) Although aggregation of flux was observed in the case of the high mixing ratio of CaF₂, the reaction area seemed to be constant throughout the reaction. This is because aggregation was almost completed at the very beginning of the experimental time. 3) It is possible to estimate the rate of desulfurization by using effective liquid phase ratio, in which the liquid phase used as cross-linking agent is excluded from the total liquid phase ratio.

Experimental and numerical investigations were carried out to understand cavity formation on a water and a low-melting-point metal bath induced by gas blowing from a top lance set in the near field of the bath surface. A cavity formation behavior was classified by the critical gas velocity of the droplet formation. The depth of the cavity formed in the near field of the gas jet was reasonably predicted by numerical simulation. An empirical equation was newly proposed for the cavity depth as a function of a modified Froude number.

In recent years, in order to improve the fuel efficiency of automobiles by reducing their weight with maintaining strength, the thinner thickness and higher strength steel sheets tends to be used as their construction materials. For stable and accurate production of these sheets, it is very important for these to be flattened at hot strip rolling process. Therefore, to realize accurate AFC (Automatic Flatness Control), a new shape meter which employed LED dot pattern projection method was developed. This is consist of LED dot pattern projector which can project the staggered periodic dot pattern, formed of 1200 power LED chips, on the rolled strip and area camera which captures the image of projected pattern. Then, instantaneous strip flatness is measured by analyzing the pattern pitch correlative with inclination angle. The shape meter was installed at the hot strip finishing mill exit and evaluated its measurement accuracy and stability. As a result, its inclination angle measurement error was within 0.45 degrees (two sigma) by comparing with the set angle of standard target and the measured flatness of rolling strip was consistent with the visually observed one. Its measurement success rate per entire coil was exceeded above 98.5%. These results indicated that the developed shape meter would be able to apply to the AFC. In addition, applying the measured flatness to the AFC of the work roll bender and leveling, it was confirmed that the strip flatness was improved in a short time.

A novel and efficient simulation technique for the purpose of optimization of vacuum-carburizing process was proposed. This method consists of three steps: calculation of gas convection and diffusion, calculation of only gas diffusion, and calculation of carbon diffusion in steel. The first step provides the gas convection velocity that is employed in the second step. Adsorption rate of carbon on the steel surface is obtained in the second step, and carbon concentration in the steel is calculated in the third step based on the adsorption rate of carbon. Experiments were conducted to verify the proposed method in both laboratory- and industrial-scale reactors. Comparison of the computational predictions to the experimental data revealed that the proposed technique enabled accurate prediction of the adsorption rate of carbon on the steel surface at various temperature conditions, the amount of carburized carbon at each operating time, and the profile of carbon concentration in the steel; in other words, the carburized depth. In addition, the calculation of the industrial-scale reactor, whose simulation model consisted of approximately seven million computational meshes, was completed within about two days. Therefore, the proposed simulation technique could be used to control and optimize the process in industrial vacuum-carburizing reactors.

The microstructure of the bonding interface was investigated for a roll bonded 2-ply clad sheet of 16Cr-stainless steel and aluminum. Structural changes with heat treatment performed after roll-bonding were also investigated. At the bonding interface of as-rolled specimen, intermediate layer with a thickness of about 20 nm is formed. This intermediate layer is uniform without breakage, but has an uneven interfacial surface, consisting of a mixture of various oxides. Since there is no direct contact between the base metal at this bonding interface and atomic diffusion beyond the intermediate layer is not observed, it is considered that the clad sheet is bonded via this intermediate layer. When the heat treatment is performed at a temperature of 300 to 500 degree of centigrade after rolling, the bonding strength increased because the internal structure of the intermediate layer is reconstituted into a uniform layer with the same thickness of about 20 nm consisting of mainly Al amorphous oxides and α-Fe separated into islands inside the intermediate layer. At this time, atomic diffusion beyond the intermediate layer does not occur, presumably owing to the role of the intermediate layer as a diffusion barrier. Furthermore, when the heat treatment temperature rises to 550 degree of centigrade or higher, intermetallic compounds of θ-FeAl₃ and η-Fe₂Al₅ are formed at a thickness of around 10 μm at the joining interface resulting in the interfacial fracture between these intermetallic compounds and the aluminum base metal.

Zinc coated steel is widely utilized for home appliance, construction, automobile, and so on. Chromate-free chemical conversion coating, which is set on zinc coated layer, can inhibit white rust generated by zinc corrosion. When zinc coated layer and chemical conversion coating formed above the layer are strictly damaged, steel can be exposed on corrosive environment. The effect of phosphate acid included in chemical conversion coating, which is generally used as corrosion inhibitor of zinc, on corrosion protection at planes is well known. However, information on parts, where steel is exposed, is not significantly clear.

In this paper, in order to improve corrosion protection of zinc coated steel where steel is exposed, the influence of phosphate acid was investigated on corrosion performance at scratches of zinc coated steel by use of salt spray test. It was clarified that addition of phosphate acid into chemical conversion coating reduced corrosion of zinc coating layer at scratches and steel exposed on corrosive environment was covered with a compound composed of phosphate and zinc. It was considered that phosphate and zinc moved from chemical conversion coating and coating layer near scratches during the salt spray test and the compound protected zinc coated layer from corrosion factor such as salt, water and oxygen. In addition, it was clarified that the larger elution amount of phosphate from chemical conversion coating is, the superior corrosion protection at scratches is.

To elucidate the effects of electrolysis conditions on the formation of electrodeposited invar Fe-Ni alloys with low thermal expansion, Fe-Ni electrodeposition was performed at 10-5000 A·m^–2 and 5×10⁵ C·m^–2 in agitated solution containing NiSO₄, NiCl₂, H₃BO₃, FeSO₄, C₇H₄NNaO₃S and C₃H₄O₄ at 40-60°C. Ni content in deposits significantly decreased in the low current density region with increasing current density, reached a minimum, and then increased due to reaching at diffusion limiting current density for Fe deposition as the current density increased further. With increasing the concentration of FeSO₄ in solution, Ni content in deposits decreased at lower current density region, reached a constant at lower current density, and then began to increase at higher current density. As a result, the current density range where Ni content in deposits was minimum and constant became wider with higher concentration of FeSO₄. With decreasing pH in solution, since the partial polarization curve for H₂ evolution and total polarization curve sifted to higher current density region, Ni content in deposits-current density curve shifted to higher current density region. With stirring the solution, Ni content in deposits reached a minimum as the current density increased, and then began to increase at higher current density than without stirring due to increase in diffusion limiting current density for Fe deposition. The change of the composition in deposits by electrolysis condition can be explained by the changes of the total polarization curve for Fe-Ni alloy deposition and the partial polarization curves for Fe and Ni depositions and H₂ evolution.

A micro-capillary technique was applied to a Devanathan-Stachurski electrochemical cell for local measurement of hydrogen permeation into a steel sheet. An electrolyte-flowing design for the hydrogen entry side of the Devanathan-Stachurski cell successfully allowed the detection of hydrogen permeation response on hydrogen exit side electrode in a micro-capillary cell with a diameter of 250 µm. Phase shift of the detected permeation current from a sinusoidal perturbation of the electrolyte flow rate in the hydrogen entry cell was strongly dependent on the metallographic structure of the steel sheet. A local structure, in which two single grains form grain boundaries, led to hydrogen permeation more frequently than did a local structure of single grains. The results suggested that the diffusion coefficient of the boundaries was at least two-times larger than that of the grains.

In high carbon steel, TTT nose temperature rises and upper baninte becomes easy to be formed with quantity of Si addition. Generation of upper bainite is reduced by boron addition. In this study, the influence of boron addition on isothermal transformation behavior in Si-added high carbon steel was clarified. By boron addition, lamellar spacing and growth rate of pearlite doesn’t change, but the nucleation of pealite is reduced. But nucleation of pearlite is promoted when Fe₂₃(C,B)₆ precipitates. In the Si-added high carbon steel, upper bainite is often formed with the generated ferrite on prior austenite grain boundary. It is inferred that boron reduces ferrite generation in grain boundary which causes upper bainite formation. It is confirmed that effective existence state of boron is grain boundary segregation.

Effect of Nb addition less than 0.05 mass% on the quench and tempering behavior of spheroidized eutectoid steel, which has been usually applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability with brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared to those of Nb free steel. 3) While the effect of Nb addition on the hardness change during low temperature tempering was hardly observed, not only the impact toughness but also the fatigue durability were improved with 0.01 mass% Nb addition. 4) APT (Atom Probe Tomography) analyses indicated that the precipitation of carbon in solution proceeded directly to the ε and/or θ carbides with carbon contents of higher than 25 at% by Nb addition without going through a clustering process up to 10~15at% during low temperature tempering. 5) In spite of the same content of P, the average bulk concentration of P in the martensite phase markedly increased with the addition of Nb up to 0.05 mass%. 6) Regarding the optimum content of 0.01 mass% Nb on the various mechanical properties under the low temperature tempering of martensite, it is considered that they are dominated by the sum of the positive effect for promoting carbide precipitation during low temperature tempering with Nb addition and the negative effect for deteriorating the toughness with increasing bulk concenteration of P in the martensitic phase with addition of Nb higher than 0.02 mass%.

For the further improvement of the press formability of steel sheets, it is important to clarify the relationship between macro mechanical properties and microstructure under multi-axial deformation state. The objective of this work is to develop the experimental system of in-situ observation and analysis for biaxial tensile deformation using electron back scatter diffraction patterns (EBSD) with scanning electron microscope (SEM). The appropriate shape of cruciform specimen for the system was examined first by using finite element analysis, and the biaxial tensile test system in vacuum SEM chamber was developed. In-situ observation of microstructure during equibiaxial tensile deformation was then conducted using the developed system and the proposed cruciform specimen. The material used in this study was an interstitial-free steel. It was validated by the comparison with the results obtained by the Marciniak type macro test that the developed system realized equibiaxial tensile deformation. Finally, some information obtained from SEM and EBSD analysis was illustrated. It was found for example that the grains with {001} plane orientations deformed easily and might cause the surface roughness.

2 types of dissimilar welded joints (DWJ), Mod.9Cr-1Mo steel / Alloy 600 / SUS304, which were differential heat input during welding to the ferrite steels were manufactured in this study. Welding methods for Mod.9Cr-Mo steel were used Plasma Arc Welding (PAW) and GTAW, respectively. Constitutions of the heat affected zone (HAZ) formed in Mod.9Cr-1Mo steel were different in PAW and GTAW. The coarse grain HAZ (CGHAZ) was formed in Mod.9Cr-1Mo steel adjacent to Alloy 600 in the dissimilar welded joint by PAW (PAW_DWJ). In contrast, the fine grain HAZ (FGHAZ) was formed in Mod.9Cr-1Mo steel adjacent to Alloy 600 in the dissimilar welded joint by GTAW (GTAW_DWJ). Specimens were sampled from Mod.9Cr-1Mo steel / Alloy 600 part. Creep tests were conducted at 550°C. In creep tests, a GTAW_DWJ failed at the interface, the PAW_DWJs failed at Mod.9Cr-1Mo steel part i.e. not at the interface. Therefore, the Speckle Image Correlation Analysis (SPICA) and the Finite Element Method (FEM) in specimens were carried out to analyze the deformation behavior of the HAZ to focus on mechanical factors of the interface failure mechanisms. SPICA revealed that the concentration of strain in the PAW_DWJ was issued the width of CGHAZ away from interface. The analysis combined with FEM results suggests that slow creep strain rate of CGHAZ formed adjacent to Alloy 600, the difference in creep strain rate between Mod.9Cr-1Mo steel and Alloy 600 was able to be mitigated. As a result, a noticeable shear stress causing interface failure wasn’t issued in the PAW_DWJ.

We constructed an automatic in situ fatigue observation system to monitor small fatigue crack behavior at the microstructural level. Applying a digital image correlation (DIC) technique in combination enabled us to continuously and automatically track and record microscopic deformation behavior. To verify the effectiveness of this system, we applied it to small fatigue crack evaluation in heat-treated low-carbon steel. The results demonstrated the feasibility, using our developed system, of automatically tracking and recording microstructurally small fatigue crack initiation and early growth behavior. By utilizing DIC analysis, we also succeeded in visualizing microscopic deformations such as inhomogeneous strain concentration by microstructures causing fatigue cracks and small fatigue crack opening-closing behavior. Although early stage fatigue crack growth rate accelerated compared to long crack data, it was consistent with long crack data by using ΔK_eff calculated from crack open stress measured by DIC.

The effects of the crosshead speed, hydrogen content and temperature on fracture strength and fracture surface morphology were investigated using a tempered martensitic steel containing 1.67 mass % of Si (H-Si) and one containing 0.21 mass% of Si (L-Si). When L-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from quasi-cleavage (QC) to intergranular-like (IG-like) to intergranular (IG) at room temperature. In contrast, when H-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from QC to IG-like at room temperature. This transition in the fracture surface morphology can be explained by the magnitude relationship between intergranular and transgranular strengths under hydrogen charging. At a temperature of –196 °C, hydrogen did not lower the fracture strength nor did it change the fracture surface morphology. Hence, hydrogen embrittlement at room temperature was presumably caused by hydrogen accumulation and lattice defect formation during stress application as well as by hydrogen trapped before stress was applied. Fracture strength decreased and converged to a constant value (lower critical stress) with decreasing crosshead speed. The crosshead speed for obtaining lower critical stress decreased as the fracture surface changed from IG to IG-like to QC. Therefore, the crosshead speed for obtaining lower critical stress should not be treated as a constant but should be determined experimentally for each type of fracture surface.