A novel solid-state joining method called Cold Spot Joining (CSJ) has been successfully developed. In this joining concept, the material near the joining interface is plastically deformed under high pressure to form a joining interface, resulting in the fragmentation of oxide films at the joining interface and the formation of strong interface. Medium carbon steel sheets were CS-joined under various process conditions. The joining temperature can be varied by the applied pressure during CSJ. Microstructural observations and hardness distribution indicated that the appropriate pressurization resulted in joining temperatures below the A₁ point and suppressed the formation of the brittle martensitic structure. By providing appropriate applied pressure, sound S45C spot-welded joints were successfully produced, showing plug failure of the base metal in both tensile shear and cross-tension tests. Further investigation into the mechanism of interface formation reveals that the oxide film at the interface is fragmented and expelled. At the same time, dynamic recrystallization occurs at the interface and extremely fine new grains with dispersed fine cementite are formed at the interface to achieve the sound joining with sufficient strength.

In this study, to elucidate the agglomeration mechanism of various inclusions in molten steel based on their interfacial chemical interactions, the agglomeration forces exerted between the solid-phase oxides of MgO, MgAl₂O₄, ZrO₂, SiO₂, and TiO₂ in molten steel, in addition to those between the reference material Al₂O₃ have been measured directly. We experimentally verified for the first time that the agglomeration force due to the cavity bridge force in molten steel acts in a relatively stable manner between all solid-phase oxides that are difficult to wet with molten steel. Furthermore, this force decreased with increasing O concentration in molten steel, which is attributed to the interfacial activation effect caused by the adsorption of oxygen at the interface between the oxides and molten steel. The agglomeration properties of various oxide inclusions in the deoxidized molten steel were further evaluated from the perspectives of both agglomeration force and thermodynamics. Quantitative analysis indicated easy agglomeration of oxide inclusions in the order MgO ＜ TiO₂ ＜ SiO₂ ＜ MgAl₂O₄ ＜ ZrO₂ ＜ Al₂O₃. A comparative evaluation of the agglomeration and external forces acting on the oxide inclusions in molten steel suggests that any oxide inclusion in the deoxidized state forms cavity bridges and agglomerates and retains that state under intense molten steel flow. However, these agglomerated inclusions may separate again under a molten steel flow at a high O concentration. The extent of separation depends primarily on the type of oxide used.

Cold tool steels contain relatively larger amount of C and Cr to utilize carbide for increase of hardness and wear-resistance. However, in the large-scale ingots, coarsening of carbides caused by microsegregation would degrade steel properties. Also, micro-porosity generated during ingot casting affects the risk of cracking and the occurrence of unclosed defects in the subsequent forging process. Therefore, it is important to predict and control its occurrence. In order to investigate the effect of solidification conditions on the formation of carbides and micro-porosity of 8Cr type tool steel (Fe-1C-8Cr-2mass%Mo), unidirectional solidification experiment with different solidification conditions were conducted. As a result, primary carbides in the interdendritic zone were formed and estimated to be M₇C₃ and M₆C. The distribution coefficients were analyzed using random sampling method. The solid fraction, at which dendrite growth terminated and carbides began to crystallize, changed to higher solid phase fraction as the pull-down speed decreased. G/√R was calculated using the temperature gradient G and cooling rate R, and the relationship between the micro-porosity area fraction was investigated. The results showed that porosity occurrence was minimal in the range of 1.6 to 7.0 (K-s)^1/2/mm. In the larger range of G/√R, the porosity formation is considered to be caused by insufficient supply of liquid phase due to higher solid phase ratio and lower permeability to the final solidification zone when the eutectic phases start to crystallize.

The purpose of this study is to investigate the mechanism of the premature failure of bearing steels in rolling contact fatigue (RCF), with a particular focus on the process of the white etching cracks (WECs). A two-roller type rolling contact fatigue test was carried out using a carburized SAE5120 steel, which successfully provides systematic sequences leading to the WECs under a contact pressure of 2700 MPa with 3.0×10⁷ RCF cycles. The process of WECs consisted of crack initiation at prior austenite grain boundaries, crack propagation accompanied by WECs formation, and crack propagation without WECs. The initial stage of the RCF test resulted in the formation of acicular structures, which were caused by {110}<111> slip driven by cyclic shear stress. However, these acicular structures were found to be unnecessary for crack initiation or the formation of white etching area (WEA). Instead, it was observed that crack initiation occurred at the boundaries of the prior austenite grains. After the crack initiation, the WEA was formed around the cracks, indicating that rubbing of the crack surfaces leads to WEA formation. Stress analysis revealed that a mode-I crack was formed due to cyclic compressive stress applied by RCF. Furthermore, it was found that the crack initiation was suppressed with low amount of hydrogen content. This suggests that hydrogen accelerate the crack initiation at prior austenite grain boundary.

The aim of this project was to learn roles of titanium oxide (TiO₂), an impurity contained in iron sand, in the products resulting from traditional iron making processes, tatara operations. For this purpose, iron sand was collected using two different mineral processing methods from four different locations in the Chugoku area of Japan, and these samples were used to run small-scale tatara experiments. Iron sand collected with traditional gravity separation method contained 8 to 12% TiO₂, while iron sand collected with modern magnetic separation method contained less than 5% TiO₂. When gravity-separated iron sand was used in a tatara under strong reducing conditions, zuku (cast iron) flowed out of the tatara. In contrast, magnetically collected iron sand failed to produce zuku, but did produce raw steel at the bottom of the furnace. Further, even magnetically isolated iron sand could produce zuku when it was supplemented with ilmenite, a titanium-iron oxide containing mineral. The results show that TiO₂ plays a key role in producing cast iron in tatara operations, and the fact that Akome iron sand is known to produce cast iron as it contains higher levels of TiO₂. In contrast, Masa iron sand which is known to produce steel (tamahagane) contains much less TiO₂ and hence is not suitable to produce cast iron. These observations agree with historical descriptions stating that pre-modern tatara operators knew to add iron sand from a specific locality (which is rich in TiO₂) to Masa-type iron sand to produce cast iron.

Martensitic steels of Fe-0.1%C-2%Mn-1.6%Mo alloy and Fe-0.1%C-2%Mn-0.2%V alloy were subjected to tempering at 873 K to investigate hydrogen trapping of Mo carbides and V carbides. We carried out the detail analysis of the alloy carbides by atomic-resolution scanning transmission electron microscopy and atom probe tomography, and the evaluation of hydrogen trapping energy by ab initio calculation. The hydrogen content of the Mo added steel tempered for 1.8 ks increases from that of the quenched Mo added steel and the hydrogen content monotonically decreases as the tempering time increases. The hydrogen content of the V added steels increases during the tempering to 7.2 ks and then keeps almost constant. Plate-shaped B1-type Mo carbide with a chemical composition of MoC_0.50 is precipitated in the Mo added steel tempered for 3.6 ks. Needle-shaped HCP Mo₂C is precipitated and the B1-type Mo carbide decreases in the Mo added steel tempered for 14.4 ks. Plate-shaped B1-type V carbides with a chemical composition of VC_0.75 is precipitated in the V added steel tempered for 14.4 ks. We found the positive correlation between the hydrogen content and the product of the interface area and the carbon vacancy fraction of B1-type alloy carbide. The hydrogen trapping energy of the carbon vacancy at the interface between BCC-Fe and B1-type Mo carbide is higher than that of the interstitial sites in BCC-Fe. These results suggest that the main trapping site in the tempered Mo added steel is the carbon vacancy at the interface of B1-type MoC_0.50, not HCP Mo₂C.

In a continuous casting process, the clogging of the immersion nozzle with inclusions occurs as a result of the adhesion, agglomeration, and coalescence of inclusions. The most effective way to understand the behavior of inclusions, i.e. oxide particles, in a liquid steel is the in-situ observation. To our knowledge, however, there is no research on the in-situ observation of the behavior of oxide particles in a liquid steel. In the present work, therefore, the in-situ observation method for sintering interface between Al₂O₃ particle / single crystalline Al₂O₃ plate by laser microscope through single crystalline Al₂O₃ plate is proposed. First, the in-situ observation of sintering interface between Al₂O₃ particle and single crystalline Al₂O₃ in Ar atmosphere is conducted to verify the observation method for the interface through single crystalline Al₂O₃ plate. Then, the in-situ observation of sintering interface between Al₂O₃ particle and single crystalline Al₂O₃ in a liquid Ag is challenged. The observations for sintering interface between Al₂O₃ particle and single crystalline Al₂O₃ plate in Ar gas atmosphere and a liquid Ag are achieved by our proposed method. It is verified that the growth of sintering interface in liquid Ag is faster than that in an Ar gas atmosphere. This finding indicates that the non-wetting by liquid Ag of alumina particles promotes the growth of sintering interface in liquid Ag.

During the cold rolling of flat steel products, lubrication is critical to achieve stable rolling. However, the oil film thickness distribution in the roll bite and its effect on friction between the work roll and strip have not been clarified thus far. This study aimed to elucidate the relationship between the oil film thickness distribution and friction by focusing on the rolling oil viscosity and steel grades, because they significantly affect the friction between the work roll and strip. Rolling oil was prepared with quantum dots as the fluorescent additive and used in rolling experiments to identify its distribution. Furthermore, cold rolling experiments were conducted using two types of oil with different viscosities and three different steel grades, namely low-carbon steel (LCS), high-strength steel (HSS), and advanced high-strength steel (AHSS) with tensile strengths of 270, 590, and 1180 MPa. Subsequently, the oil film thickness distribution on the steel strip surface was visualized using quantum dots by fluorescence microscopy. It was demonstrated that the higher the tensile strength of the steel or the higher the oil viscosity, the wider the rolling oil distribution on the strip surface. Numerical analyses revealed that the rolling oil distribution on the steel sheet surface was more widespread for AHSS and HSS than for LCS. The high surface pressure between the roll and steel plate may have increased the oil leaching area by increasing the oil viscosity. These findings demonstrate that rolling oil permeation from the oil-pits reduces the friction between the work rolls and strip.

In the hard cooling process of steel, heat flux and temperature fluctuations are so large due to the wetting that it is difficult to measure unsteady cooling phenomena. In this study, we experimentally analyzed the detailed heat transfer behavior of continuous casting secondary cooling of a moving system using the improved IHCP (Inverse heat conduction problem) analysis with the Laplace transform technique developed in the previous paper. The test piece was a SUS304 rotor with a thickness of 10 mm and an outer diameter of 136 mm, which was heated to 880°C, rotated at a peripheral speed of 0.9 to 8.0 m/min, and cooled from above with a flat air mist spray. Thermocouples were installed at two points 1.5 mm and 3.5 mm from the surface. As a result of the analysis, under the conditions of the film boiling region, the surface heat flux q_w could be expressed as q_w / q_w,peak = (W / W_peak)^0.57 using the spray water flux W. However, after the start of wetting, cooling continued even on the downstream side in the moving direction where the water droplets did not collide directly. In the nucleate boiling region and the film boiling region, the average heat flux when passing through the spray did not change due to the change in casting speed. However, the wetting start temperature became higher as the casting speed becomes slower.

Changing mold width of continuous casting takes long shut down time. Therefore in order to produce various width slabs in roughing mills, sizing press has been installed in most hot strip mills. Installation of sizing press can improve total productivity of continuous casting and hot strip mills.

After sizing press and horizontal rolling, it is known that width around head and tail portions of slab are narrower than that of middle. These narrow portions, which are called “width drop”, cause yield loss. Therefore some technologies have been developed to prevent width drop.

It is said that width drop results from difference of width spread in horizontal rolling depending on distribution of dog bone profile along rolling direction of slab. However some experimental and simulated data in some papers indicated width shrinkage with horizontal rolling of slabs whose cross sectional shape was dog bone profile. Although it is presumed that width drop is influenced by not only width spread but also width shrinkage, there are no papers which describe mechanism of width shrinkage clearly.

In this paper, experiment and FE analysis of horizontal rolling with slabs which have dog bone profile were executed in order to investigate mechanism of width shrinkage. As the result of FE analysis, width shrinkage is influenced by area reduction ratio between width edge part and width center part. Furthermore longitudinal velocity distribution along width direction around head and tail portions cause width shrinkage during horizontal rolling.

The aim of this paper is to predict the deformation-induced martensitic transformation of the retained austenite in steels under various deformations, including loading-path changes, by using mesoscopic finite element analyses (FEAs). First, a TRIP steel was subjected to monotonic uniaxial tension and compression, as well as a couple of two-stage loadings to investigate the effect of loading direction and loading-path on transformation behavior experimentally. In monotonic loading, tension induced transformation at higher rate than compression did. Whereas, in two-stage loadings, the transformation progress was suspended immediately after the start of secondary loading. As the secondary tension proceeded, the transformation resumed and gradually accelerated toward the transformation rate for monotonic tension. These experimental results were analyzed by FEAs with a two-dimensional image of microstructure. The transformation rates under monotonic loading are well predicted by the simulation. It is also suggested that the difference in the transformation rate between tension and compression is mainly due to the volumetric expansion associated with martensitic transformation, and that the transformation behavior of the untransformed austenite is dominated by the distribution of the hard transformed martensite. In addition, the prediction of the transformation rate in secondary tension after pre-compression required the consideration of back stress in the austenite. The reproducibility of the transformation behavior just after the onset of secondary deformation was improved by the hypothesis that the equivalent value of back stress tensor at the transformation needs to exceed its maximum value in the past.

The residual stresses at a circular punched end face in tempered martensitic high-strength steel sheets were investigated using triaxial stress analysis via X-ray diffraction. The maximum principal stress and its direction were calculated from the measured nine stress components. The relationship between the directions of the maximum principal stress and hydrogen cracks was verified by generating hydrogen cracks on the punched end face in the same specimen using cathodic hydrogen charging. The direction of the cracks was perpendicular to that of the maximum principal stress. This result indicates that hydrogen embrittlement at the sheared end face is caused by the maximum principal stress. Moreover, the distribution of the residual stresses toward the thickness direction and the relationship between residual stresses and tensile strength of the specimens were investigated. The maximum principal stress on the punch side was lower than that on the dice side. Unlike the maximum principal stresses, the normal stresses did not increase monotonically with the tensile strength of the specimens. Therefore, it was concluded that investigating the maximum principal stress at any area between the dice side and a line located midway from the end face and dice side is crucial for considering the hydrogen embrittlement criteria.

Reduction behavior of various multi-component calcium ferrites at 900°C were investigated by using in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy. Intermediate components were determined by XRD, and change in X-ray absorption spectra of Fe and Ca K-edges were analyzed to determine reaction rate constants. SFCA-I (Ca₃(Ca,Fe)(Fe,Al)₁₆O₂₈) and SFCA (Ca₂(Fe,Ca)₆(Fe,Al,Si)₆O₂₀) consist of layered structure of spinel and pyroxene. Early stage of reduction reaction, diffraction peaks of spinel structure were observed which indicating SFCA-I and SFCA decomposed into these units at the first step of the reduction reactions. The spinel was reduced sequentially into FeO_x then Fe. Intermediate component, Ca₂(Fe,Al)₂O₅ originated in pyroxene module was hard to reduce and reaction was controlled by decomposition of this phase. Reduction of SFCA-I started later than SFCA (with 5.7 mol% Al₂O₃) under hydrogen gas reduction condition at 900°C. SFCA with a high aluminum content indicated lower reducibility than that with a low one.

Total iron contents in iron ores have been accurately determined by JIS M 8212, in which iron ions in digested solutions of iron ores are reduced to divalent prior to redox titration. It is necessary for the iron reduction process that no reducing chemicals other than iron(II) in the decomposition solutions must not remain after the reduction with titanium(III). However, the redox reactions concerning the chemical species present in the decomposition solution has not been completely elucidated at the present time. In this paper, the redox reactions that occurred in the decomposition solution during the iron reduction in JIS M 8212 were studied by potentiometry and spectrophotometry under nitrogen atmosphere. The redox reaction of tin(II)/(IV) was very slow, causing significant effects on identifying the end point of the indicator for the iron reduction. The copper chloro-complexes were reduced with titanium(III) at a potential higher than that of indigo carmine used as a redox indicator, so that the reduced copper(I) gave a positive error to the potassium dichromate titration. The pentavalent vanadium was reduced with titanium (III) to form a complex with titanium, which also interfered with the potassium dichromate titration positively. To avoid these interferences, titanium(III) chloride was stoichiometrically added to the reaction mixture after addition of tin(II) chloride under nitrogen atmosphere so as to reduce only iron to divalent prior to the following redox titration. Combination of the proposed protocol with the potassium dichromate titration could successfully determine the iron content of certified reference materials of iron ores.

A method to automatically search brittle fracture initiation point is proposed. The method calculates flow paths from each edge of an image to opposite side along flow potential. The potential is derived from the direction of river-pattern and tear ridge. The most concentrated position of flow paths is determined as a fracture initiation point. The method achieves 99% precision and high speed analysis for low magnification images of 206 thick plate samples. Furthermore, discriminating ductile fracture by machine learning and excluding noise potential obtained from it, and considering local feature of river-pattern that spread radially near a fracture initiation point, the method precision is improved for high magnification images. Using fractal feature of brittle fracture and repeatedly applying the method, a fracture initiation point is determined within the size of a single grain level.

Toward a better understanding of the hydrogen embrittlement characteristics in nickel-based superalloy 718, tensile tests were performed under hydrogen pre-charged states (internal hydrogen) as well as in hydrogen gas environment (external hydrogen) at various temperatures ranging from −196 to 300°C. Under the internal hydrogen conditions, hydrogen-induced loss of ductility was maximized at around 25°C, while it was recovered with increasing/decreasing test temperature and almost fully mitigated particularly at −196°C. On the other hand, under the external hydrogen conditions, deleterious impact of hydrogen on the ductility monotonically increased with temperature elevation. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analyses on post-mortem samples revealed that the microstructural initiation sites of hydrogen-induced micro-cracks in internal hydrogen states were annealing twin boundaries or crystallographic slip planes (i.e., {111} planes) at −40~300°C wherein the loss of ductility was substantial, albeit intergranular fracture prevailed at −196°C, accompanying minimum embrittlement effect. Meanwhile, in the case of external hydrogen states, the fracture modes were transitioned from intergranular to slip plane cracking with increasing temperature in response to the augmentation of embrittlement magnitude. The rationales of these multiple hydrogen-related failure modes and their roles on macroscale material performance are discussed on the basis of hitherto-known, unique deformation mechanisms driving the plasticity in this alloy in addition to the hydrogen diffusion rate/pathways which are strongly dependent on temperature.

In this study, the free energy of iron-carbon binary BCT martensite was calculated using the first-principles calculation and the cluster expansion and variational method. The free-energy curves of BCT martensite show the possibility of promoting the clustering of carbon atoms in the tempering process, because there is two-phase separation associated with the formation of metastable BCT-Fe₂C ordered structure. This BCT-Fe₂C structure was found to have many crystallographic similarities to η-carbide (Fe₂C). Then, the energy barrier required for the transition from the BCT-Fe₂C ordered structure to η-carbide was calculated by means of the G-SSNEB method. The obtained activation energy was sufficiently small, suggesting that η-carbide may be formed through the BCT-Fe₂C ordered structure. According to these findings, it was suggested that η-carbide in the low-temperature annealing process of BCT martensite may precipitate through a two-step process in which BCT-Fe₂C ordered structure is formed by the two-phase separating tendency in BCT martensite, and the ordered structure transitions to η-carbide over the energy barrier.

The mechanisms of the static strain aging phenomenon of polycrystalline ferritic steels were investigated focusing on segregation of solute C atoms to dislocations and grain boundaries, precipitation of carbides and structural changes of dislocations and grain boundaries, using partially C-stabilized Nb-bearing ULC steel sheets and IF steel sheets with solute C content of 0–4 mass ppm and ferrite grain sizes from 9.5 μm to 183 μm with aging temperatures from 70ºC to 600ºC. The partially C-stabilized steel sheets exhibited two definite hardening stages. The first hardening appeared in the every partially C-stabilized steel sheet accompanied by an increase in the grain-interior strength, σ₀, which saturated at around 30 MPa. During the first hardening stage, linear and strong segregation of solute C atoms was developed, which was observed by 3DAP, and their concentration was in good agreement with Cottrell’s formula. The second hardening significantly appeared in fine-grained steels, reaching 90 MPa and accompanied by a large increase in the critical grain boundary strength, τ_c. The apparent activation energy of the second hardening was 135 kJ/mol. Neither carbide precipitation nor obvious segregation of carbon atoms to grain boundaries was detected during the aging period. Increase in τ_c was exhibited in a quite wide range of aging temperatures from 170ºC to 600ºC and τ_c reached the value close to that of the recrystallized condition. Local recovery or rearrangement of Fe atoms in deformed grain boundaries is proposed as a possible mechanism of the second hardening.

Powder blasting is often operated in refining process in order to achieve lower sulfur content in molten steel with high desulfurization efficiency. In this study, effects of wettability of a particle on penetration and flotation behavior were examined by water model experiment. A polypropylene particle was blasted onto water surface with Ar gas through a single-hole nozzle, and particle behavior during penetration into water to flotation to water surface was recorded by a high-speed camera. Wettability between the particle and water was changed by applying repellent or hydrophilic material on the particle. According to penetration of the particle, an air column was generated and a residual bubble was remained on the particle after rupture of the air column. The repellent particle floated to the water surface in a short time because maximum penetration depth was small and diameter of a residual bubble was large. On the other hand, the detention time of the hydrophilic particle became longer than the repellent particle because maximum penetration depth was relatively large and the residual bubble was separated from the particle. The reason that wettability between the particle and water affects penetration and floatation behavior is that adhering position of the air column on the particle changed. In the case of repellent particle, the position changed to penetrating direction of the particle. Therefore, the force caused by surface tension of water increases, and the residual air column on the particle after rupture of the air column becomes large.

Electrical resistivity of low-carbon martensitic steels was measured to estimate the carbon concentration in solid solution. Since electrical resistivity is influenced not only by solute carbon but also by substitutional elements, lattice defects and second phase, the effects of these factors need to be subtracted from total electrical resistivity, in order to obtain the accurate solute carbon concentration by this method. As a result, the effects of dislocations and grain boundaries were much smaller than that of solute elements, being only 1 - 2% of the measured electrical resistivity, in martensitic steel. On the other hand, substitutional elements and retained austenite were found to be significantly effective. By subtracting these effects from the measured value, the change in electrical resistivity due to solute carbon, Δρ_sol.C, could be formulated as a function of the carbon concentration in solid solution of martensite, C_sol, as follows:

Δρ_sol.C [mΩmm] = 0.25 × C_sol [mass%]

The estimated solute carbon concentration was confirmed to correspond to the directly measured value by atom probe tomography.