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.

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.

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.

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.

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.

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.

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 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 21×10⁻⁵ MPa⁻¹. Thereafter, the coefficient of bainite was determined as 11×10⁻⁵ 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 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 µm 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 µm 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.

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.

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.

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.