The development of ultrafine grained microstructures under severe plastic deformation by burnishing process was investigated using spherical cementite-ferrite (SA) steel and pearlite (P) steel of AISI 52100. Microstructures were analyzed using FE-SEM, FE-TEM and EBSD observations. In the SA steel, equiaxed ultrafine ferrite grains were formed at the burnished surface where the equivalent strain was about 3.9. These ultrafine grains were formed by continuous dynamic recrystallization because they consisted of high angle grain boundaries. On the other hand, in the P steel, the initial lamellar structure was maintained even at the equivalent strain about 4.3, and ferrite grains with a large aspect ratio were formed. These ferrite grains were considered to be non-recrystallized grains because the KAM value within these grains was high. In addition, many dislocation contrasts in the same direction were observed within a ferrite grain by FE-TEM observation. These results suggested that active dislocation slip system in these ferrite grains is limited by lamellar structure. As the strain increased by repeated burnishing process, these ferrite grains of P steel became coarse and the KAM value within these grains decreased. In addition, several dislocation contrasts in multiple directions were observed within a ferrite grain. It can be concluded that the limitation of active dislocation slip system in these ferrite grains were relaxed, and dynamic recovery was occurred in these ferrite grains.

The microstructure and corresponding tensile properties were examined in quenching and partitioning (Q&P) processed low carbon steels, depending on the silicon content ranging from 0.1–2.0 wt.%. The silicon content and process temperature generated a highly interactive influence on the evolution of final microstructure, including the fraction of constituent phases and their characteristics such as solute carbon content in each phase. The yield strength was nearly unchanged or slightly decreased even with the silicon addition for a given Q&P condition. The change of yield strength showed a reasonable correlation with the loss of solute carbon in martensite or bainite caused by the carbide precipitation and the carbon partitioning into austenite, which depended on the silicon content. High partitioning temperature enhanced the yield strength for a given silicon content and quenching condition, because of the tempering effect on the martensite matrix. Although the fraction and stability of retained austenite were still critical for improving ductility, the intrinsic properties of the martensite matrix, such as the occurrence of tempered martensite embrittlement, governed the ductility of Q&P steels in situations where the role of retained austenite was limited due to low fraction or poor mechanical stability.

In the steelmaking industry in recent years, requirements such as mixed production of various types of products and short delivery times have led to a demand for efficient operation methods that allow for efficient production while tolerating the risk of disruptions to stable operations. However, the steelmaking process is a complex supply chain, and even a localized change in one element can cause the entire system to fail.

In this study, we model the steelmaking sub-process using Uppaal SMC, a statistical model checking tool, and analyze evaluation of operational rules and the effects of uncertain events and the place pre- and post- each process through simulation and statistical analysis. The modeling of the steelmaking sub-process consists of five processes (Cold rolling, Continuous Annealing, Inspection, Temper rolling, and storage) and their pre- and post-process storages. The model also includes upper and reprocessing processes that carry works to and from the sub-processes, and other processes where works are carried out of the sub-processes.

Using the model, we conducted simulations and statistical analyses of evaluation of operational rules and the effects of uncertain events on the entire process, including the pre-and post-process yards, equipment failure probabilities, and quality inspection pass rates. Through some numerical simulation results, the effectiveness and the potential, e.g. for clarifying the effect of the operational rules, of the proposed model are investigated.

The dimensional accuracy of power train parts depends on the distortion caused by oil quenching. The parts are manufactured from steels for machine structural use, and the steel transforms from austenite to bainite and martensite during the quenching. For accurately predicting the distortion, it is important to know the accurate transformation plasticity coefficients of both bainite and martensite transformations. Transformation plasticity is a phenomenon caused by the stress due to transformation expansion, resulting in large plastic deformation of the steel. In this paper, a method was proposed to determine both the coefficients of bainite and martensite transformations by fitting the distortions in numerical analyses of the quenching process to oil quenching experiments. In this method, assuming that the transformation plasticity of both bainite and martensite transformations does not interact, two steel sheets with different thicknesses were used. In the previous study, by using this method for a thin sheet of Cr-Mo steel (consisting of martensite), the coefficient of martensite was determined as 21x10^-5 MPa^-1. Thereafter, the coefficient of bainite was determined as 11x10^-5 MPa^-1 by using a thick sheet (consisting of bainite and martensite). An experiment using a heat treatment oil with different cooling capacity was conducted to check the accuracy of the obtained coefficients. The numerical analysis results of cooling curve and microstructure were verified by comparing with the experimental results. The predicted deflection using the obtained coefficients was in good agreement with the experimental result. Thus, this method enabled to accurately predict the distortion caused by quenching.

The contribution of dislocation-slip stability and carbide precipitation morphology to the hydrogen embrittlement (HE) property of tempered martensitic steels with low and high silicon contents (L-Si and H-Si) and oil-quenched martensitic steel (As-OQ), was evaluated by conducting slow strain rate tests. The order of dislocation-slip stability was the H-Si specimen > L-Si specimen > As-OQ specimen. The H-Si and As-OQ specimens had finely dispersed carbides inside prior austenite (γ) grains, whereas the L-Si specimen had coarsely dispersed carbides inside prior γ grains and on the boundaries. Notched specimens were charged with hydrogen in a range of low (0.19–0.31 ppm), medium (1.04–1.49 ppm), and high (2.17–2.33 ppm) hydrogen contents. The H-Si specimen had the highest HE property under the three hydrogen charging conditions. With the low and medium hydrogen charging conditions, the HE property of the L-Si specimen was higher than that of the As-OQ specimen, whereas their HE properties markedly declined to a similar level under the high hydrogen charging condition. The HE property of the L-Si specimen with increased dislocation-slip stability by applying stress relaxation was equivalent to that of the L-Si specimen under the high hydrogen charging condition. These results revealed that increasing dislocation-slip stability improved the HE property in the range of low to medium hydrogen charging. Under the high hydrogen charging condition, dislocation-slip stability did not contribute to improving the HE property, but it was found that the carbide precipitation morphology, particularly coarse carbides precipitated on prior γ grain boundaries, influenced the HE property.

In the ferro-coke production process, the molded briquettes are carbonized as they are to become ferro-coke, and the quality of ferro-coke is greatly reduced when the briquettes deteriorate during transportation to the shaft furnace. Therefore, briquette strength is one of the most important characteristics in ferro-coke production.

The briquette strength is expressed by binders such as coal tar pitch and asphalt pitch, and when the binder is adsorbed on the pores of coal particles, the binder contributing to the adhesion between the particles decreases, and the briquette strength decreases. Therefore, the binder penetration phenomenon in the pores is important to produce high strength briquettes for ferro-coke.

Methylene blue adsorption test of coal was carried out to evaluate the binder penetration characteristics. 10 kinds of coal were used, and it was clarified that the amount of methylene blue adsorbed was affected by the coal pore volume and the wettability of coal. Next, the effect of the binder penetration characteristics on the strength of the ferro-coke briquette was investigated. As a result, it was found that the strength of the briquette decreased when a brand with high methylene blue adsorption was used. Based on the above results, we developed a technology that can control briquette strength by adjusting the amount of methylene blue adsorbed and the amount of binder. With this new technology, high-strength briquettes can be produced with the minimum amount of binder, and ferro-coke quality can be ensured at the same time as low-cost operation.

It is known that the size distribution of inclusions in steels have a significant effect on material properties. The solidification characteristics and TiC formation behavior of alloy 800H were evaluated both by experiment and simulation in this work. The relationship between dendrite arm spacing and the cooling rate was estimated. TiC particles were observed at the interdendritic region. The size distribution of TiC particles was affected by solidification cooling rate. Solidification analysis using the MPF (Multi-Phase Field) method revealed that TiC formation begins at a solid fraction of 0.79, and solidification accelerates due to TiC formation. It was thought that TiC particles generated in the latter part of solidification aggregate and coalesce without engulfment by the solidified shell. The size distribution of TiC particles was also affected by heat treatment after solidification.

Rolling experiments were conducted to investigate strip warpage behavior after rolling where the strip was inclined before a roll-bite with the roller guide. Direction of the strip warpage was explained by the shape factor Γ (contact arc length / mean thickness). The relationship between inlet inclination and outlet warpage of the strip was divided into four typical categories, by the shape factor Γ; 1) Γ < 1.0; a positive relation, 2) 1.0 < Γ < 1.9; a negative relation, 3) 1.9 < Γ < 3.1; a positive relation and 4) 3.1 < Γ; a negative relation. In other words, the sign of the warpage changes at three times with increasing the shape factor Γ. These experimental results were predicted by the two-dimensional rigid plastic steady-state finite element analyses. The analyses numerically showed that shear bands are initiated at the entrance of the roll-bite with the shear stress field, and that the intensity and configuration of the shear bands determine strip warpage behavior.

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.

Confusion exists in the literature as to whether the crystal structures are cubic or tetragonal in lath martensites of Fe–C alloys and low-alloy steels. Steels with a range of carbon contents have been quenched and examined by synchrotron x-ray diffraction. The presence of dislocations and residual local strains complicates the analysis since peak splitting of tetragonal lines is obscured by the broadening. Asymmetry of the 200,020/002 lines has been examined and synthesised using model peak functions. A new approach has been to study the 222 peaks which are unique (not split) for both cubic and tetragonal crystals. For low carbon steels (<~0.2%C) the structures are fully or almost completely cubic. Above about 0.7%C the martensite has tetragonal symmetry. Intermediate, medium carbon, steels consist of mixtures of cubic and tetragonal structures.

It is argued that martensite is always tetragonal at the moment when it forms from austenite but it can subsequently decompose into cubic by auto-tempering during the quench. Kinetics of decomposition have been measured using higher carbon steels and these are extrapolated to the situation of auto-tempering. A model is developed whereby the creation of tetragonal martensite according to the Koistinen-Marburger relationship is modified by its rate of decomposition. The final structures are predicted to depend on both the martensite start temperature (Ms) and the cooling rate.

The effect of nitrogen introduced by solution nitriding on microstructure and mechanical properties of modified 9Cr-1Mo (Gr. 91) steel at room temperature was investigated. The nitrogen concentration at the sample surface was 0.164 wt% and nitrogen diffused at least 5000 mm after solution nitriding heat treatment at 1200ºC for 48 hours. The martensite with a small amount of MX carbonitride with cF8 structure and retained austenite was formed on 100 mm from the sample surface. The Cr₂N phase with hP9 structure containing V, Nb and Mo and Cr₂₃C₆ phase with cF116 structure precipitated by tempering. Solute nitrogen improved the hardness, yield strength, ultimate tensile strength and uniform elongation. However, the nitride formation did not contribute to the improvement of hardness and decreased yield strength and ultimate tensile strength. It suggests that the contribution of solid solution strengthening by Cr, V, Nb and Mo is larger than that of precipitation strengthening by nitride at room temperature.

A slab yard within a steel factory comprises multiple Last-In, First-Out (LIFO) buffers, typically managed by a crane operator in a dynamic environment. The efficacy of decision-making in controlling the slab yard hinges on the operator’s cognitive grasp of the task. Therefore, it is crucial to evaluate various cognitive frameworks to enhance and stabilize performance, bolster resilience, and adequately support the operator. This study presents a framework integrating feature variables that convey information about the due dates of slabs, intended for use by and provision to the operator. Subsequently, it employs a combination of behavioural and computational methodologies to assess this framework, utilizing a serious game model of the task for testing purposes. The findings confirm that the effective representation for conveying due date information depends on yard congestion, with the provision of such information potentially backfiring when the yard is crowded. This observation holds true for both computational experiments using a reinforcement learning agent and behavioural experiments using human subjects. Moreover, the consistency of results across both experiments suggests that a reinforcement learning agent could be valuable for formulating plausible hypotheses regarding the suitable cognitive framework for individuals tasked with this responsibility.

Fatigue tests under axial loading were conducted on steel with a shallow hardened layer induced by induction hardening, and in situ X-ray stress measurements were performed to investigate the relaxation of residual stresses during fatigue. The residual stresses were relaxed owing to tensile loading and not compressive loading. The two conditions that bring about this phenomenon are (i) a high peak of tensile residual stress just below the hardened layer, and (ii) the hardened layer coinciding with the compressive residual stress field that prevents the yielding of the compressive residual stress field under compressive loading. In this case, tensile yielding occurred just below the hardened layer under tensile loading, the residual stresses are redistributed, and the compressive residual stress on the material surface is relaxed. The experimental results also showed that the fatigue fracture morphology changed depending on the residual stress relaxation behavior.

In the high purification refining processes, a CaO-containing flux or slag is used commonly for the desulfurization of molten steel. Because the slag generated during the actual process is not always a homogeneous liquid, the quantitative relation between the liquid fraction of the slag and its mass transfer coefficient must be clarified. Therefore, to elucidate the effects of the liquid fraction on the desulfurization rate and the mass transfer coefficient in the slag phase, k_slag, desulfurization experiments were performed on molten steel using a resistance heating furnace with solid CaO-liquid coexisting slag corresponding to the CaO-Al₂O₃ and CaO-CaF₂ systems. The overall mass transfer coefficient and sulfur partition ratio at the slag-metal interface were evaluated based on the regression analysis of the experimental data using the reaction rate equation, and k_slag could be determined. It was found that k_slag decreased slightly with a decrease in the liquid fraction of the slag for both the CaO-Al₂O₃ and CaO-CaF₂ systems. This is because the effective diffusion coefficient decreased with the decrease in the liquid fraction. Two empirical equations for k_slag for the solid-liquid coexisting compositions were formulated; these were based on a previously reported equation for the effective diffusion coefficient and the regression analysis of the experimental data obtained in this study. The threshold value of the liquid fraction between the equations for k_slag was determined to be 0.87. This is possibly because the state of the solid in the slag changes because of the percolation transition.

The effect of Al₂O₃ on the compositional region of silico-ferrite of calcium and aluminum (SFCA) and the liquid phase and the phase equilibria, including SFCA, was investigated in a CaO-SiO₂-Fe₂O₃-5mass%Al₂O₃ system at 1240 ^°C in air. To obtain the desired composition, reagent-grade CaCO₃, SiO₂, Fe₂O₃, and Al₂O₃ powders were weighed, mixed, and equilibrated at 1240 ^°C in air. Each obtained sample was divided into two parts: one was pulverized into a powder and analyzed by XRD, and the other was subjected to microstructural observation and compositional analysis using EPMA. The results revealed that the compositional region of SFCA lies on the CF₃-CA₃-C₄S₃ plane and is C/S = 2.77–7.60 for 5 mass% Al₂O₃. Compared with the SFC composition region for 0 mass% Al₂O₃, the compositional range of SFCA extended in the CF₃-C₄S₃ direction, suggesting that the addition of Al₂O₃ contributes to the stability of SFCA. Furthermore, the liquid-phase region was divided into a ferrite melt with a high Fe₂O₃ concentration and a silicate melt with a high SiO₂ concentration, both of which shifted to the lower Fe₂O₃ side compared to the liquidus isotherm in the CaO-SiO₂-Fe₂O₃ system. Unlike CaO-SiO₂-Fe₂O₃, SFCA-I (SFC-I) was observed in the CaO-SiO₂-Fe₂O₃-5mass%Al₂O₃ system, thus indicating that the addition of Al₂O₃ contributes to the stability of SFCA-I.

New fracture process model of cleavage fracture initiated from cementite crack was proposed. In addition, the equation of propagation of cementite crack into the ferrite grain was developed based on the Brechet-Louchet model. This equation can reproduce not only ferrite size dependence of cleavage fracture stress that the Petch model can reproduce but both of test temperature dependence and strain rate dependence of fracture stress. Furthermore, in exchanging surface energy for grain boundary cohesive energy in the equation, grain boundary fracture stress can be also estimated.

Steel manufacturing often needs to improve its stability margin due to logistics delays, increased manufacturing costs, and sudden production halts. Keeping a certain level of stability margin in steel production is a practical policy to avoid serious deviation from a steady state in operation. This paper proposes a resource-buffered scheduling method based on Critical Chain / Buffer Management (CC/BM) and its fundamental mathematical models. In this study, we consider a planning decision-making chain from raw material blend planning to hot-roll scheduling through cast scheduling in a steel plant. A series of computational experiments demonstrate the proposed method as a decision-making process enhancing "resiliency" in steel-making operations.

Automated Guided Vehicles (AGVs) are utilized in flexible job-shop production systems. In these systems, the most crucial issue is the proper scheduling of jobs. Moreover, AGVs in the factory are constrained to follow predetermined transportation routes, leading to potential spatial interference between AGVs. This paper investigates an analytical model of AGV flow using a multi-agent system and contract net protocol. Our model enables the AGV assignment and job manager agents to collaborate and make decisions based on traffic information feedback. This system allows for continuous monitoring of AGV density and ensures the maintenance of an appropriate distance between AGVs. As a result, it effectively mitigates traffic flow fluctuations, thereby enhancing the efficiency and stability of the AGV flow.

The utility of hyperspectral measurement was assessed as a means of predicting the corrosion risk of steel materials based on surface information. Carbon steels exposed to outdoor conditions in Choshi and Miyakojima were used as the test specimens. Exposure tests were conducted every six months for a duration of two and a half years. Corrosion loss was calculated by comparing the weight of specimens before exposure test and after removing corrosion products from the surface. Hyperspectral measurements were conducted on these specimens, with corrosion products identified through SAM (Spectral Angle Mapper) analysis. α-FeOOH, β-FeOOH, γ-FeOOH, and Fe₃O₄ were employed as reference data for SAM analysis. In both Choshi and Miyakojima test sites, γ-FeOOH was predominantly detected on the specimens after ordinary exposure tests, whereas Fe₃O₄ was prevalent on the specimens exposed to sheltered environments. The correlation between the proportion of each corrosion product identified through SAM analysis and the amount of corrosion change for one year was explored. α-FeOOH exhibited a positive correlation with the amount of corrosion change, whereas the amount of corrosion change tended to decrease with an increase in β-FeOOH fraction.

Super invar alloy, Fe–32%Ni–5%Co, is widely utilized in precision instruments due to its remarkably low thermal expansion coefficient. Additive manufacturing holds promise for fabricating complex-shaped components with this alloy. This study investigated the phase stability and thermal expansion properties of super invar alloy fabricated via Laser Powder Bed Fusion (AM sample), comparing them to those of conventionally cast material (Re-melt sample). Microstructural analysis indicates that the AM sample has a more stable austenitic structure, attributed to minimal micro-segregation. Furthermore, it was observed that the thermal expansion coefficient decreases consistently with higher cooling rates within the temperature range of 400-300 K. As a result, AM sample exhibits lower expansion coefficient and it maintains at lower temperatures.

Using two types of image processing techniques without machine learning, edge extraction processing and keypoint extraction processing, progressively corroded regions under the rust layer from images of corroded steel surfaces was extracted. We found that there was a relatively good correlation between the keypoint strength obtained from the keypoint extraction processing for HSL transformed and histogram flattened corroded surface images and the corrosion depth after rust removal.

Steels are widely used as a structural material for infrastructure, such as bridges. However, some of bridges built in Japan, have passed more than 50 years, since construction. Thus, these bridges will need to be reconstruction, because atmospheric corrosion of these materials will occur in this condition. However, there are numerous bridges in Japan, and reconstruction of these bridges would need much cost. Therefore, it is necessary to establish the technique for selection of the bridges with severe damage by atmospheric corrosion. To establish a new non-destructive and non-contact techniques for identifying the sites of atmospheric corrosion occurred in steel, used as structural materials for bridges, the relation between corrosion site and color of corrosion product, surface morphology or distribution of surface potential of the pure iron and steel after corrosion test were evaluated.

Pure iron specimens after wet-dry cycling test with NaCl solution to simulate the atmospheric corrosion, and steel after exposure test, were observed, in this study. From these results, there is a good matching between the site of corrosion and color of corrosion product and distribution of surface potential for pure iron after wet-dry cycling test. On the other hand, there is a good relationship between the site of corrosion and distribution of surface potential for steel after exposure test.

Boron (B) is frequently used as additives to improve the hardenability of advanced high strength steel. It has been reported that B in steel reacts with atmospheric N₂ during annealing at low oxygen potential (low dew point) to form boron nitride (BN) by the thermodynamical calculation. In this study, the effect of BN formation on the steel surface on the coatability during hot-dip galvanizing was investigated, experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperature and dew point, and then hot-dip galvanized. The annealed specimens were also prepared and analyzed with GD-OES, XPS, SEM-EDX and TEM-EELS to investigate the oxide and nitride formation on the steel surface during annealing. As results, coatability deteriorated as the amount of B in steel and the annealing temperature increase, and as the dew point decrease. These trends were not correlated with the amount of oxide but the amount of BN formation, suggesting that BN formation deteriorated the coatability. The surface and cross-sectional analysis revealed that BN formed around the oxide to cover the steel surface. This would lead the deterioration of the coatability because most of the steel surface was covered with BN as well as oxide, which are known to have low wettability with molten Zn.

Appropriate maintenance and maintenance of infrastructures that has been used for a long time is required because there is the concern that safety will deteriorate due to atmospheric corrosion. However, the cost of maintenance and management is also increasing, and there is required to save labor and improve efficiency of maintenance and management. Therefore, the purpose of this study was to easily estimate the rust composition by the image processing of image of steel surface corroded by the outdoor exposure test. The outdoor exposure test was conducted in Choshi City and those conditions were open exposure and shelter exposure for 2.5 years. The　compositions of those corrosion products measured by X-ray diffraction. Exposed test pieces were photographed RAW with the digital camera. Those photographs were developed and trimmed. The obtained images were converted from RGB images to La*b* and LCh images. The histograms of hue were fitted to the Gaussian function to determine the peak position and the spread of the histogram. As a result, it was indicated that the peak position shown in the histogram of hue shifted to the low-angle side due to the increase in FeFe₂O₄. In addition, it was indicated that the composition ratio of α-FeOOH, β-FeOOH, γ-FeOOH and FeFe₂O₄ can be estimated by the shape, spread and peak position of the histogram of hue. As shown in the graphical abstract, the composition ratio of the corrosion products measured by RIR method and estimated by image processing was in good agreement.

Conventionally, it has been known that the product yield of the upper part of the sintering layer is extremely low, because of the heat loss caused by transferring heat toward the space above sintering layer, and of the large amount of unburned carbon in upper sintering layer.

As a countermeasure, REMO-tec (Re-ignition Method for Optimization of Total Energy Consumption) has been developed. Here, REMO-tec, is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method has an effect on improving sinter yield with maintaining high sinter reducibility. This effect leads to improving sinter reducibility without decreasing sinter yield by decreasing control of coke breeze content in sinter mixture.

This paper focuses on coke combustion efficiency as combustion ratio of carbon in coke breeze for considering improvement of sinter yield through sinter pot test. Here, carbon combustion ratio is defined as proportion of actual heat generation at combustion to ideal heat generation as complete combustion (C+O₂→CO₂) of all carbon in coke. And it can be calculated based on component analyses of exhaust gas.

As the result, it was confirmed as shown bellows.

1) By re- ignition, the unburned coke remaining in the upper layer of the sinter packed bed was burned, which has a role of extending keeping time over 1200℃ especially in the upper layer of sinter packed bed.

2) Due to the effect of 1), the increasing amount of heat supply at “REMO-tec” case was equivalent to the same as the experimental case of increasing coke breeze content, at which increasing heat amount at blending coke breeze content was four times larger of the heat amount at re-ignition. (For a 430 mm layer pot test)

3) In addition, since the re-ignition heat is donated to the upper layer (surface layer), the amount of heat consumption in the upper layer of the sinter packed bed increases and the amount of heat consumption in the lower layer decreases compared to the case of increasing coke breeze content, which results in decrease of the difference between heat consumption in upper layer and that in lower layer.

In addition, these effects have been also confirmed at the commercial sinter plant.

This paper describes an evaluation method for corrosion rate of carbon steel covered with rust layer by admittance analysis in electrochemical impedance spectroscopy. The polarization resistance, R_p, is often estimated from the impedance spectrum of carbon steel to determine the corrosion rate. The R_p can be determined from the impedance spectrum when the low frequency impedance is converted to the real axis on the Nyquist diagram. Because the impedance spectrum of carbon steel covered with rust layer often describes a part of loop in the low frequency range, it is difficult to determine the R_p by extrapolating the low frequency impedance to the real axis. In the present study, an admittance analysis was employed to determine the R_p from the admittance spectrum of carbon steel covered with rust layer. The admittance is plotted as the reciprocal of impedance on the complex plane. In this case, the R_p can be determined from the admittance spectrum when the low frequency admittance is converged to the real axis. The admittance spectrum of carbon steel with rust layer indicated that the low frequency admittance was converted to the real axis, namely, the R_p could be determined from the admittance spectrum. The corrosion rate of carbon steel with rust layer could be estimated from the R_p by admittance analysis, demonstrating that the value was corelated to that estimated from corrosion loss. The impedance and admittance simulations were performed using an equivalent circuit to discuss the time constant observed in the low frequency range.

Corrosion products formed on steel in various environments were identified with image analysis, Raman spectroscopy, and X-ray diffraction (XRD). The results of Raman spectroscopy showed that γ-FeOOH exhibits bright yellow color and β-FeOOH exhibits gray color. Main corrosion products composition formed on the steels by XRD analysis was γ-FeOOH. Using proper threshold of HSV color space, makes it possible to identify corrosion products formed on steel. Comparison of corrosiveness of exposed environments and formed corrosion products, α-FeOOH is preferentially formed in mild corrosive environments, while β-FeOOH was formed in severe corrosive environments.

Analysis of the corrosion distribution and composition of corrosion products on steel surfaces using supervised machine learning of optical microscopic images was investigated. The accuracy of the artificial intelligence in evaluating the composition of iron compound reference samples was affected by the illumination intensity and surface roughness during image capture. The evaluation accuracy was high for compounds with a wide distribution of R value such as Fe₂O₃ and FeOOH, but low for compounds with a narrow distribution such as Fe₃O₄. The results of wet-dry cycling tests on weathering steel with NaCl particles on the surface showed that the transition of corrosion products during the corrosion progress can be analyzed from optical microscope images.

Degradation due to atmospheric corrosion is an important problem for steel structures such as bridges. In order to maintain steel structures safely over a long period of time, there is a need for a low-cost and easy-to-use method to evaluate corrosion degradation. In this study, corrosion morphology under atmospheric corrosion environment was focused on. The relation between the surface appearance and the distribution of corrosion depth of the carbon steel specimen after atmospheric exposure was analyzed, and the prediction of corrosion morphology under rust layers by surface observation of steel specimens was examined. It was found that deeply-corroded areas were possible to be located within the dark brown regions in the rusted specimen after the atmospheric exposure. As the exposure period increased, the correspondence between the dark brown regions in the rusted specimen and deeply-corroded areas became clearer. Since the corrosion progressed more locally as the exposure period increased, it is considered that the surface appearance of the rusted specimen showed traces of the deeply-corroded areas.

A fundamental study on the axial crush performances of HSS (High Strength Steel) was carried out to clarify the effects of microstructure and mechanical properties on crashworthiness. Axial crush tests were performed to evaluate the crush performances of the HSS with different microstructures and mechanical properties and identify the fracture origins. The cracks in the press formed area were observed and the cracks led to the fractures. The high λ (Hole expansion ratio) steel showed excellent crush performances by crack suppression. Crash deformation in the press formed area was simulated by the ORB (Orthogonally Reverse Bending) fracture tests and the crack suppression factors were investigated. Through the ORB fracture test, it was clarified that the reduction of the hardness gaps between phases and the refinement of the hard phases (Fresh martensite) were effective for suppressing cracks in the press formed area. These microstructures were occurred by the Q&P (Quenching & Partitioning) process for increasing λ. Therefore, it was found that the microstructural design for increasing λ also contributed to excellent crush performances.

Daily and seasonal changes in temperature (T) and relative humidity (RH) were monitored using a sensor network system installed in the Hokkaido Centennial Memorial Tower, built more than 50 years ago using weathering steel, to investigate its corrosion condition. Five T-RH sensors were set at the south side wall, inside the south tower, in the semi-open central area, inside the north tower, and on the north side wall on the 4th, 14th, and 24th floors. The T changed as a function of altitude, location in the floor, season, weather, solar radiation, diurnal cycle, distance from the wall, etc. The highest T of the south wall at daytime in the winter season could rise more than 30 °C even if the outer temperature was below 0 °C due to solar radiation causing the repetition of ice or snow melting in the daytime and freezing of water at night. The change in RH and T inside the tower followed a Tomashov-type RH-T curve (high RH at low T in the morning and evening). In winter, however, T and RH distribution, i.e., high-RH (> ca.60%) area below the freezing point and low-RH area with the high-T, caused air transportation inside the tower, condensation (and freezing) in the low-T area, and drying in the high-T area. In the visual inspections, severe corrosion, such as blistering and peeling, has been observed at the bottom of the tower, where snow has accumulated, and rainwater has stayed for a long time, especially at welds and joints.

Controlling the size, number, and composition of secondary inclusions is vital in the production of high-quality steels. In this study, experimental and computational investigation of the relationship between secondary inclusion formation in Fe-36mass%Ni alloy and cooling rate was carried out. Assuming the case of large ingots, solidification experiments using various cooling rates (0.17 to 128 K/min) were employed and the size, number, composition, and distribution of inclusions were analyzed by SEM-EDS automatic inclusion analysis. Like previous studies, inclusion number density increased with increasing cooling rate, while inclusion size decreased with increase of cooling rate. On the contrary, oxide inclusion area fraction was found to have little relationship with the cooling rate and was instead found related with oxygen content of the sample. As a new attempt to investigate the relationship between microsegregation and secondary inclusion formation, a combination of SEM-EDS analysis and EPMA mapping analysis was carried out. By superimposing information of microsegregation and inclusions, it was found that high-Al₂O₃ inclusions formed during the early stage of solidification, whereas low-Al₂O₃ inclusions formed during the later stage of solidification. These findings suggest that Al₂O₃ inclusions formed in the early stage of solidification reacted with the remaining Si-enriched liquid steel and changed into low-Al₂O₃ inclusions. Experimental results were also confirmed by thermodynamic calculations. Present work made it possible to understand deeper the relationship between microsegregation and secondary inclusion formation.