Load hold at peak stress can drastically reduce the fatigue life of Ti-6Al-4V alloys, a phenomenon known as dwell-fatigue life debit. Unlike normal fatigue, dwell-fatigue exhibits a characteristic three-stage strain accumulation curve: an initial rapid strain increase (primary), followed by a steady strain-increase (secondary), and finally an accelerated stage leading to failure (tertiary). In the current study, flat specimens made out of three distinct microstructures of Ti-6Al-4V alloys were subjected to uniaxial fatigue load with 120-second peak stress hold and 0.1 load ratio with real-time acoustic emission (AE) measurement. The microstructures, namely, equiaxed, bimodal, and lamellar, were tailored through different heat treatment procedures. Bayesian statistical analysis of AE data enabled both the detection of stage transition points using Markov chain Monte Carlo (MCMC) simulation and the selection of strain accumulation models through replica MCMC method. The results demonstrated that two change-points detected showed strong correlation with the transitions in the creep-like three-stage curve of strain accumulation. The strain accumulation behavior was successfully predicted using AE parameters. Furthermore, average frequency proved crucial in monitoring the beginning of dwell-fatigue process.

Nitriding is a beneficial surface hardening heat treatment to reduce CO₂ emissions and shows a potential partially replace carburized steels adopted for sliding parts such as bearings and gears. Both the bending fatigue strength and pitting fatigue strength under high contact pressure were evaluated for nitrided JIS-SCM440H materials using two types of controlled-nitridings, i.e. γ' and ε, and conventional gas nitriding. Not only the thickness of the compound layer but also its phase structure such as γ' and ε significantly affected the 10⁷ cycles bending fatigue strength, where the γ'-nitrided specimen was the highest. On the other hand, almost no effects of the thickness and phase structure on the 10⁷ cycles pitting fatigue strength were detected in the specimens. Compared to the carburized specimen, the γ'-nitrided specimen exhibited higher 10⁷ cycles fatigue strength in bending and pitting, but poor fatigue strength in the lower cycles range. The shape and size of the pores in the compound layer were altered during roller-pitching tests, although there was almost no reduction in the thickness of the compound layer. The pore size was reduced by pore closure, especially at higher contact pressures. Pore closure was more pronounced in the γ'-nitrided specimens than in the ε-nitrided specimens at lower contact pressures. The 10⁷ cycles pitting fatigue strength was improved after the pore closure, resulting from fewer defects.

Titanium nitride (TiN) is detrimental to the mechanical properties of steel products and should be properly controlled. For this purpose, the precipitation and evolution of nitride inclusions during the unidirectional solidification of Fe-18Cr-2Mo alloy simulating stainless steel SUS444 were studied using thermodynamic calculations and experimental methods. The relationship between inclusion evolution and solid fraction (f_s), solidification rate, and especially dendritic microstructure was analyzed. (Ti,Nb)N was revealed to be the major inclusion type in all samples. Inclusion number density increased and peaked at intermediate f_s and then decreased towards unity. Cooling rate had limited influence on number density and size variation in the range between 1.25 and 20 K/min. Nitride average composition was found to evolve from Ti_0.75Nb_0.25N to TiN throughout the solidification process.

Changes in the initiation behavior of delayed fracture cracks have been investigated based on quantitative cross-sectional observation near the fracture surface in the previously reported sustained tensile-loading test during hydrogen charging of ultra-high strength steel sheets containing a small amount of retained austenite. The morphology of the delayed fracture surface depends on the applied stress and pre-deformation. With increasing applied stress, the number of delayed fracture cracks decreased in the martensite in the cross section immediately below the fracture surface, whereas the number at the prior austenite grain boundary tends to increase. Upon pre-deformation sufficient to transform from retained austenite to martensite, the number of cracks in the martensite increased at each applied stress in spite of the short time to delayed fracture. Even without fracture at low applied stress (200 MPa), short cracks exist in the martensite, but no long cracks are observed. The long cracks in the martensite increases with increasing applied stress or pre-deformation. The long cracks presumably result from connection of short cracks in the martensite. The long cracks probably enhance transgranular fracture. The results of the present study indicate that applied stress or pre-deformation affects delayed fracture behavior by changing the crack initiation and propagation behavior. With increasing applied stress in the elastic deformation region, the crack suppression effect of bainitic ferrite is likely to weaken, resulting in more transgranular fracture. In addition, the preferential crack initiation in deformation-induced martensite formed by plastic deformation probably shortens the time to delayed fracture.

An efficient scheme to reduce greenhouse gases (GHGs) emissions more than coal replacement with general biomass was proposed. Currently, rice straw is plowed into paddy fields and emits methane, which is one of the major causes of the greenhouse effect. In this scheme, the straw is used as biomass charcoal for steelmaking.

Focusing on rice paddies and steel mills as GHGs sources, methane from straw in paddies and CO₂ from coal at steel mills are currently emitted. The use of the straw in steelworks reduces both emissions, therefore it can be a more efficient GHGs reduction scheme than the use of biomass in general.

In order to evaluate the proposed scheme, methane emissions from rice straw were calculated using the regression formula of methane emission flux estimated by DeNitrification-DeComposition-Rice model (DNDC-Rice model). The methane emissions for the current amount of straw plowed into paddies and for the case where no straw was plowed were calculated, and the difference was evaluated as the methane reductions.

Additionally, the CO₂ emissions associated with coal use were considered and it was found that the proposed scheme could reduce GHGs equivalent to 2.6 to 3.8 tons of coal per ton of coal replaced when using Global Warming Potential over 100 years (GWP-100), or 5.7 to 9.3 tons with GWP-20.

GHGs reduction for partial straw plowing was also evaluated by changing the ratio between the amount of straw plowed and used as charcoal substitute, and it was found that it remained constant regardless of the ratio.

Creep deformation and precipitation behavior of 9Cr-1Mo-V-Nb steel with excess nitrogen introduced by solution nitriding were investigated. Precipitation of Cr₂N phase was confirmed in addition to M₂₃C₆ and MX phases in the tempered microstructure. The creep strength of the steel was significantly reduced by solution nitriding, while the creep rupture elongation was increased. To characterize the complex precipitation behavior of the nitrogen-added steel, a machine learning-based clustering method of the multidimensional scatter diagram of the X-ray intensity of the alloying elements in each pixel of a STEM-EDS map was developed. Reduced number density of precipitates and enhanced coarsening kinetics of both Cr₂N and MX were proposed as the mechanism of weakening caused by excess nitrogen.

New fracture process model of cleavage fracture initiated from a micro-crack, which is nucleated at hard phase in steels, was proposed. In addition, the equation to calculate the stress required for micro-crack propagation into the ferrite matrix was developed based on the Brechet-Louchet model. This equation can reproduce not only ferrite size dependence of cleavage fracture stress 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.

The hydrogen embrittlement of steel can be mitigated by reducing the amount of solid-solution hydrogen that enters from the external environment. In this study, first-principles calculations were performed for various additive elements to identify which elements X effectively reduce hydrogen penetration from the material surface. We used substitutional (Si, Mn, P, S, Cu, Ni, Cr, Ti, Ge, Co, V, Zr, Nb, Mo, Sn, Sb, Ta, and W) and interstitial (C, B, N, and O) solid-solute atoms as additive elements to estimate the potential energy barrier for hydrogen penetration in Fe–X–H systems. Results showed that many additive elements are more effective in inhibiting hydrogen penetration than an Fe–H system and that the penetration pathways tend to be similar in each family of the periodic table. Moreover, it is suggested that the hydrogen penetration suppression effect of the additive elements is related to the first ionization energy in the case of the substitutional solid-solute atoms and to the change in the lattice constant in the case of the interstitial solid-solute atoms.

This paper presents a numerical investigation on the steel/slag interface movement behaviors in a two-strand slab tundish under varying gas curtain conditions of the flow, mass and heat transfer and its effect on the inclusion removal. The results show that the mathematical model accurately simulates turbulence and multiphase flow behaviors in the tundish, which were validated by the water model experiments. It has been found that the gas curtain can effectively enhance molten steel residence time, reduces dead volume, and facilitates removal of small particle inclusions. The metallurgical effect of the tundish initially improves with increasing blowing rate of the gas curtain, reaching its peak at 0.4 m³/h, beyond which it diminishes. The relationship between the slag eye area and blowing rate in the tundish is described by the equation y=-865.7x²+3002.8x-593.2 (0.25≤x≤0.55 m³/h). As the liquid level of tundish decreases, bubble rise paths become more vertical and narrower, driving reflux of liquid steel towards slag layer, thereby reducing steel/slag interface fluctuations and slag eye area. As the tundish liquid level decreases from 892 mm to 392 mm, the fluctuation height of the steel/slag interface reduces from 28.83 mm to 25 mm, and the molten steel exposure area from 394.5 cm² to 0.

This study develops a finite element-based numerical model to analyze the swelling behavior of coal particles during pyrolysis. The model incorporates volatile matter release, gas diffusion, viscoelastic deformation of the coal matrix, and internal bubble pressure to reproduce realistic deformation. Simulations on single coal particles revealed non-uniform swelling, with expansion concentrated in the central region where the matrix is thinner and softer, and suppressed near the particle edges due to higher stiffness. To evaluate the influence of coal properties, the model was applied to various coal types. The results showed clear differences in swelling behavior depending on volatile content and rheological properties. Coals with higher plasticity and volatile yield exhibited earlier and more pronounced expansion, while non-caking coals showed limited swelling and high internal pressure due to their high viscosity. This indicates that both volatile generation characteristics and the temperature-dependent mechanical properties of the coal matrix play key roles in determining swelling behavior. The proposed model provides a useful tool for quantitatively evaluating deformation mechanisms in coal particles and may contribute to the optimization of pore structure in coke manufacturing for ironmaking applications.

To investigate the impact of deoxidizer addition sequence on the cleanliness of Al-killed and S-containing steel Cf53, adding Al ingots followed by SiCaBa alloys (A-SCB scheme) and adding SiCaBa alloys followed by Al ingots (SCB-A scheme) during converter tapping were conducted in industrial trials. Through systematic sampling and SEM-EDS analysis, the chemical composition of steel and slag, as well as the characteristics (type, size, and quantity) of non-metallic inclusions were compared. Industrial trials reveal the differences in T.O and T.Al contents, along with inclusion quantities during LF refining of the both schemes. The A-SCB scheme produces more Al₂O₃ and MgO-Al₂O₃ inclusions during LF refining, while the SCB-A scheme results in more CaO-MgO-Al₂O₃ inclusions. Both schemes mainly generate CaS-bearing inclusions in casting billets, while the size of inclusions in A-SCB scheme is larger than those in SCB-A scheme. Thermodynamic analysis reveals that adding SiCaBa alloys first acts as a pre-deoxidation step, enhancing alloying effects of subsequent Al ingots addition, thus SCB-A scheme achieves a higher Al yield. Variations in T.Al, T.O content, and inclusion evolution during LF refining are influenced by the composition and oxidation potential of refining slag. In the vacuum degassing treatment and tundish casting, the T.Ca and T.S contents in steel, and molten steel temperature significantly affect the inclusion evolution. Ultimately, altering the deoxidizer addition sequence influences Al yield, but has limited effect on steel cleanliness. This study provides a practical guidance for optimizing deoxidizer addition sequence for Cf53 steel and elucidates the mechanism via thermodynamic analysis.

Following the global consensus on responding to climate change, reducing coal usage has become a critical task given that it is primary contributor to greenhouse gas (GHG) emissions in the steel industry. Given this context, the development of graphics processing units has led to an increasing research interest in applying various deep-learning technologies to enhance process efficiency in steel manufacturing operations. To this end, this study proposes an automatic control logic for supplying coal to rollers during briquette production. The proposed system employs convolutional neural network-long short-term memory for time-series prediction and you look only once for object detection to stabilize coal briquette quality in the FINEX process, replacing the existing manually controlled task in the hot metal production process. Actual process applications were verified by using these models to detect roller currents used as key control factors and briquettes produced below the roller and establish a correlation with compressive strength, which is the main quality indicator of briquettes. Based on these models, an automatic control logic was constructed and tested in actual processes, and the obtained results confirmed a quality improvement effect of ~30 %. The results of this study are expected to contribute to improving coal usage efficiency and reducing GHG emissions in the FINEX process.

In this paper, the oxygen lance of a steel plant is taken as the research object. The jet behavior under the influence of two factors of oxygen nozzle deflection angle and oxygen flow is studied by combining numerical simulation with industrial experiment. The results show that the coalescence behavior of the oxygen jet is obvious, and the oxygen jet basically completes the coalescence at about 0.9m at the outlet of the oxygen lance. When the nozzle deflection angle of oxygen lance is 12°, the surface of liquid steel fluctuates greatly. When the oxygen flow is increased, the impact crater area of scheme 3 is effectively expanded and the stability is increased. Under the three experimental schemes, the maximum exposed area of liquid steel are 7.18m², 7.08m² and 7.42m² respectively. The reflection jet angle(θ) changes the most at low position, from 68.0° to 54.8°, 67.2° to 48.9°, 67.6° to 59.8°, respectively. When the oxygen lance position exceeds 1750mm, the θ changes are less than 50°, the slag diffusion area is the largest, and the liquid steel reaction is dynamically stable. The industrial test shows that the composition of liquid steel and slag fluctuates greatly when 12° oxygen lance is used for blowing. When the nozzle deflection angle of oxygen lance increases to 13° and the oxygen flow increases to 42000Nm³/h, the average oxygen content of liquid steel can be controlled to 573.28ppm, the average total iron content in the slag can be reduced to 16.35%.

Copper is recognized as one of the most detrimental residual elements in steelmaking due to its adverse effects during the thermo-mechanical processing of steel. Selective chlorination volatilization has emerged as a promising method for removing residual copper from liquid steel. In this study, the selective chlorination behavior of copper was systematically investigated using Fe₂O₃-CaCl₂-SiO₂ flux at 1873 K, focusing on optimizing flux composition parameters: Fe₂O₃/Cu molar ratio (n(Fe₂O₃)/n(Cu)), CaCl₂/Cu molar ratio (n(CaCl₂)/n(Cu)), and SiO₂/CaCl₂ molar ratio (n(SiO₂)/n(CaCl₂)). Experimental results demonstrated an ideal copper removal ratio of 39.68 % under optimized conditions, achieved with a flux composition of n(Fe₂O₃)/n(Cu) = 10.0, n(CaCl₂)/n(Cu) = 1.0, and n(SiO₂)/n(CaCl₂) = 1.0. Phase analysis revealed that the primary chlorination volatiles consisted of CuOHCl, Cu₂Cl(OH)₃, and FeCl₂(H₂O)₂, confirming the mechanistic pathway of copper removal. This work substantiates the technical feasibility of selective chlorination for copper elimination from liquid steel and provides a viable strategy for enhancing scrap steel recycling in sustainable metallurgical practices. The proposed method demonstrates significant potential for industrial application in residual element control.

The permeability index directly influences the gas-solid equilibrium within blast furnace, affecting the reduction reactions and overall furnace operation. Accurate prediction of permeability index remains challenging due to the system inherent complexity, time delays, and noise interference. This research proposes a hybrid TCN-GRU deep learning framework enhanced by variational mode decomposition (VMD) and error compensation (EC) correction for permeability index. First, a multi-stage feature selection method combining LightGBM and Spearman's rank correlation analysis identifies key predictor variables while addressing time-lag effects. The permeability index series is then decomposed into intrinsic mode functions (IMFs) via VMD to mitigate non-stationarity. Each IMF is modeled using a TCN-GRU architecture that captures multi-scale temporal dependencies through dilated causal convolutions and recurrent gating mechanisms. To further refine results, prediction errors from IMF components are recursively fed back into the model for error compensation. Tested on production data from a steel plant in Southern China, the framework demonstrates exceptional performance in one-hour-ahead permeability index prediction, achieving a RMSE of 0.201, a R² of 0.965, and a remarkable hit rate of 98.039% within ±0.5 error margin. Crucially, it maintains strong multi-step prediction capability, delivering considerable hit rates for two-hour and three-hour predictions. These results underline the model's ability to handle complex blast furnace dynamics, providing a robust tool for proactive process optimization. This approach advances intelligent ironmaking by enabling precise permeability index prediction, supporting energy conservation, and enhancing operational stability in industrial applications.

The third-generation advanced high-strength steels exhibit increased strength and formability due to their chemical composition and precise controlled thermomechanical processing. The chemical composition also affects the properties of the semi-finished steel. Reliable production of these steels requires that the effects of chemical composition on microstructure development be understood during all steps of processing. This study focuses on the as-cast microstructure by characterizing laboratory-cast ingots containing 0.2 mass% C, with varying Mn, Si and Al concentrations. Isothermal heat treatments were also conducted to verify Thermo-Calc predictions of austenitization behavior. The microstructure constituents were compared against Thermo-Calc diffusion simulations. Results show that increasing the Al and Si concentrations increases the A₃ transformation temperature and the amount of ferrite in the as-cast state. Microstructures at room temperature are mainly bainite and ferrite. Steels containing C, Mn, and Si have proeutectoid ferrite and those C, Mn, Si, and Al alloy have primary ferrite. A steel alloyed with only Al has a mixture of primary and proeutectoid ferrite where proeutectoid ferrite regrows from the primary dendrite remnants.

Boron (B) is frequently used as an additive to improve the hardenability of advanced high strength steel. Based on thermodynamical calculations, it has been reported that B in steel reacts with atmospheric N₂ during annealing at a low oxygen potential (low dew point) to form boron nitride (BN). In this study, the effect of BN formation on the steel surface on coatability during hot-dip galvanizing was investigated experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperatures and dew points and then hot-dip galvanized. Annealed specimens were also prepared and analyzed by GD-OES, XPS, SEM-EDX and TEM-EELS to investigate oxide and nitride formation on the steel surface during annealing. As results, coatability was deteriorated as the B content in the steel and the annealing temperature increased and as the dew point decreased. These trends were not correlated with the amount of oxides, but rather, with the amount of BN formation, indicating that BN formation deteriorates coatability. Surface and cross-sectional analyses revealed that BN formed around oxides, covering the steel surface. It was suggested that this leads to deterioration of coatability because most of the steel surface is covered by BN and oxides, which are both known to have low wettability with molten Zn.

In the cold sheet rolling process of steel, the deformation of the steel sheet and friction between the roll and steel sheet lead to the generation of iron fines. When steel is mass produced, the generation of iron fines can cause pollution. When the base oil is hydrolyzed under catalysis by iron fines and heat, hydrolyzed products, such as fatty acids, can react with iron fines to form iron soap, which is the major component of scum. The generation of scum decreases the emulsion stability of the rolling oil coolant, and its accumulation inside the cold rolling mill causes dirt. Therefore, it is important to understand the factors that affect the generation of iron fines.

In this study, experiments were conducted to compare the generation of iron fines under different rolling and lubricant conditions in an experimental rolling mill. The amount of iron fines generated increased with the number of rolling passes at the same reduction in thickness. However, after considering the increase in the rolling distance, the amount of iron fines generated per unit rolling length was similar. The amount of iron fines generated increased as the surface roughness of roll increased, emulsion concentration increased, emulsion temperature decreased and base oil viscosity increased. The results show that the rolling and lubricant conditions can be changed to establish an evaluation method to develop a new rolling oil for achieving low generation of iron fines and good surface cleanliness after cold rolling.

The effects of stress, plastic strain, and hydrogen on hydrogen embrittlement fracture of hole-expanded transformation-induced plasticity-aided martensitic steel were investigated. The hydrogen embrittlement properties were evaluated by means of cathodic hydrogen charging to the hole-expanded specimen. The residual stress and plastic strain distributions in the hole-expanded specimens were analyzed using finite element analysis. The hydrogen content was measured using a thermal desorption spectrometer. Hydrogen embrittlement cracking occurred approximately 3 mm from the hole edge in the radial direction. As the crack propagated, it diverged in the circumferential direction. The fracture morphology primarily consisted of a mixture of intergranular and quasi-cleavage fractures. The tensile stress in the circumferential direction at the position where the hydrogen embrittlement crack was initiated was the highest, and the tensile stress in the radial direction and hydrostatic stress were also high. The hydrogen content in the vicinity of the hole edge of the hole-expanded specimen was the highest owing to the large amount of plastic strain applied by hole punching and hole expanding whereas the hydrogen content at the positions where the hydrogen embrittlement crack was propagated was not very high. Thus, the highest tensile stress in the circumferential direction is the controlling factor in the location of the crack initiation site and the direction of the initial crack and its growth during the initial phase. The high hydrostatic stress that causes hydrogen accumulation could also assist the crack initiation.

This study systematically investigates the optical texture evolution of laboratory heat-treated coke under varying temperatures (1100–1500°C) and atmospheres (N₂/CO₂), with findings validated through comparative analysis of coke specimens (including feed coke and tuyere coke) from industrial blast furnaces (BFs). The effects of heat-treatment temperature, gasification reaction, radial position of lump coke, alkali loads, particle size degradation, and effective volumes of BFs on optical texture were quantitatively analyzed. The results demonstrated that under the N₂ atmosphere, the proportion of isotropic textures increased from 32.5% to 41.2% with increasing temperature, while anisotropic textures decreased. Under the CO₂ atmosphere, the Optical Texture Index (OTI) also decreased from 144.0% to 124.8%. This indicates a tendency of coke optical texture to transform from anisotropy to isotropy at high temperatures. Notable differences exist in the radial distribution of the optical texture of tuyere coke. Specifically, the optical texture in the outer edge regions of the lump coke is more significantly influenced by the conditions inside the blast furnaces BFs. Smaller particle sizes in tuyere coke correlated with higher alkali metal content and lower anisotropy. Furthermore, a declining trend in the OTI of tuyere coke was observed with an increasing effective volume of BFs, decreasing from 146% to 109%, suggesting a potential correlation between effective volume of BFs and the OTI of coke. These findings provide critical insights into the evolution of coke optical texture and the degradation mechanisms of coke in high-temperature zones of BFs.

A heat transfer model developed under conditions analogous to industrial online quenching demonstrates enhanced relevance and applicability to real-world manufacturing processes. This research explores the heat transfer characteristics of a single nozzle jet impinging on a steel plate closely resembling industrial environments, primarily focusing on the wetted area, heat flux density, and temperature drop at different widths and depths in the wetted zone. Key experimental parameters include the initial temperature of the steel plate (T₀), jet impingement velocity (v_j), and the moving speed (v_m) of the plate. The T₀ ranges from 750°C to 450°C, v_j varies between 2 and 7 m/s; and v_m ranges from 0.07 to 0.28 m/s. Heat transfer characteristics within the wetted region strongly correlate with T₀, v_j and v_m, with the impact point exhibiting the highest sensitivity. Furthermore, the temperature drop along the depth is influenced not only by these parameters but also by the depth itself. Mathematical expressions are proposed to predict the peak heat flux density and temperature drop based on external parameters (T₀, v_j, v_m). This study deepens the understanding of heat transfer dynamics in moving jet cooling and provides a comprehensive three-dimensional perspective on jet impingement heat transfer.

Although the instrumented Taylor impact (ITI) test has been extended to measure the unknown mechanical behavior of pure aluminum at ultra-high-speed impact, it remains uncertain whether the important assumption that internal force distributes linearly in certain regions holds for other materials. Hence, in this work, pure iron, whose deformation behavior at ultra-high-speed impact remains unknown due to the speed limitations of existing methods, is introduced into the ITI test to verify the assumption and attempt to evaluate its mechanical behavior. The assumption is verified using the digital image correlation (DIC) method. Unlike the pure aluminum specimen, the assumption holds for a relatively longer time period in the case of pure iron. According to the experimental results, a previously uncaptured high frequency component in the impact force wave is newly discovered using a polyvinylidene fluoride (PVDF) film instead of a strain gauge. Hence, the impact force wave should be measured using a PVDF film rather than a strain gauge. The shock wave is observed for the first time in the ITI test using a pure iron specimen. Additionally, a previously unreported opposite pulse in the impact force wave was newly observed in the ITI test, implying that the magnetic properties of the material might have changed.

Deoxidation is a key process in refining liquid steel and enhancing its cleanliness. O removal from the melt is typically achieved by adding elements with a high affinity for O, such as Al, Si, or by adjusting a specific Mn/Si ratio in the melt. In the production of high-Si steels, precise knowledge of the equilibrium between the liquid metal and solid silica (SiO₂) becomes increasingly important. In the present study, a CALPHAD-based thermodynamic model for Si-O deoxidation in liquid Fe was developed, covering the full composition range from pure liquid Fe to pure liquid Si, including the O saturation limit. The Gibbs energy of the ternary Fe-Si-O liquid phase was formulated using the Modified Quasichemical Model (MQM) in the pair approximation, which accounts for strong interactions among Fe, Si, and O. Comparisons with experimental data demonstrated an excellent agreement in predicting the liquid/SiO₂(s) equilibrium within the temperature range of 1550 - 1650 °C. Modeling the deoxidation equilibria in this ternary system can aid in refining the model description in binary Fe-Si liquid. The model predicted a pronounced "deep minimum" in O solubility. It was in agreement with an experimental approach reported by Shibaev et al. (https://doi.org/10.2355/isijinternational.45.1243) and with another thermodynamic approach by Cho and Kang (https://doi.org/10.1007/s12613-023-2766-7). The present assessment can be incorporated into CALPHAD databases for applications in steelmaking and liquid metal processing.

The strengthening mechanism of ausforming in martensitic steels is believed to be due to the inheritance of dislocations in austenite by the subsequently transformed martensite. However, no studies to date have quantified the dislocation density before and after ausforming. In this study, the dislocation densities of Fe-5%Mn-C alloys were analyzed, and the relationship between hardening by ausforming and dislocation accumulation, as well as the effect of carbon on this relationship, were investigated. The hardness of ausformed martensite increased with the ausforming reduction in austenite, and the strengthening effect of ausforming increased with the addition of carbon. Similarly, the dislocation density of ausformed martensite increased with the ausforming reduction in austenite, and the dislocation accumulation by ausforming increased with the addition of carbon. Because the hardness of the ausformed martensite follows the Bailey–Hirsch relationship, the strengthening mechanism owing to ausforming could be explained by dislocation strengthening. To understand the dislocation accumulation process during ausforming, the dislocation density of austenite immediately after ausforming was measured by in-situ heating neutron diffraction. Consequently, the dislocation density of the ausformed austenite was not dependent on the carbon content, indicating that dislocations are not inherited in carbon-free steels. By contrast, in steels with sufficient carbon content, not only are dislocations inherited but additional dislocations are introduced during martensitic transformation.

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.

Microtensile tests combined with crystal plasticity finite element simulations were performed on a single-packet structure of a nano-bainite steel. The steel microstructure consisted of bainitic ferrite and retained austenite, and its constituents were formed according to the Nishiyama–Wassermann crystallographic orientation relationship. The single-packet specimens exhibited plastic anisotropy in their yielding behavior, similar to those of lath martensite and upper bainite. The deformation-induced transformed martensite variants demonstrated high kernel average misorientation values owing to the introduction of a large number of dislocations, and these crystallographic orientations were similar to those of the adjacent bainite variants, in which the slip system with the highest Schmid factor was nearly parallel to the transformation system. A numerical analysis incorporating a crystal plasticity constitutive model that accounted for the martensitic transformation closely represented experimental stress–strain responses, demonstrating the effectiveness of the proposed method. When applied to polycrystalline structures, multiaxial stress distributions promoted martensitic transformations, leading to significant strain hardening. Assuming a high austenite strength in the analysis model, the strength increased owing to the bainitic matrix, whereas strain hardening was limited because of the suppressed martensitic transformation. In contrast, assuming low-strength austenite in the model, the yield stress was slightly reduced while the martensitic transformation was enhanced, which resulted in pronounced strain hardening and a high tensile strength. These findings suggest that the incorporation of low-strength austenite within a high-strength matrix can optimize the balance between strength and ductility.

The metastable Fe₅₀Mn₃₀Cr₁₀Co₁₀ (at. %) high-entropy alloy (HEA) integrates the principles of HEA and transformation-induced plasticity (TRIP), resulting in an exceptional combination of tensile strength and ductility. However, the inherently low yield strength restricts its application as a structural material. In this study, 50% warm rolling at 573 K was employed as a straightforward and cost-effective thermomechanical processing strategy to fabricate metastable Fe₅₀Mn₃₀Cr₁₀Co₁₀ HEA with enhanced yield strength while retaining large ductility. The mechanical responses and deformation behaviors of the metastable HEA were examined using room temperature tensile tests, along with postmortem X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) analyses. Compared to the fully recrystallized counterparts, the warm-rolled specimens exhibited a threefold increase in yield strength while retaining substantial uniform elongation. The warm rolling treatment increased the mechanical stability of the austenitic phase against stress-assisted γ-ε martensitic transformation, shifting the yielding mechanism from martensitic transformation to dislocation slip, thereby enhancing the yield strength. Moreover, warm rolling completely suppressed the athermal γ-ε martensitic transformation even at 77 K, while the strain-induced γ-ε martensitic transformation remained pronounced at room temperature, contributing to the alloy's high strength and large ductility.

Three-way servovalve controlled cylinder (TSCC) is the main method used in hydraulic automatic gauge control (HAGC) system of rolling mill, which generally only foucus on the screw-down speed of HAGC cylinder under load resistance. However, with the development of plan view pattern control and variable gauge rolling, the lifting-up speed of HAGC cylinder is required to be as high as the screw-down speed. At present, high frequency response and large flow servovalves or even double servovalves in parallel are usually used to achieve high lifting-up speed, resulting in a substantial increase in equipment cost. In this paper, the nonlinear mathematical models of the TSCC and four-way servovalve controlled cylinder (FSCC) are firstly established, and the steady-state speed equations of the two methods are derived. Then, the steady-state speed of the two methods with different HAGC cylinder sizes under different load conditions is compared and analyzed, and it is proved that FSCC has obvious advantages in lifting-up speed. Finally, simulation experiments of variable gauge rolling on a 1050mm cold-rolling mill are carried out. In the rolling process of transition zone where the roll gap increases, the FSCC has higher control precision and wider dynamic adjustment ability, which is more conducive to improving rolling speed, production efficiency and shape quality.

The effects and mechanisms of various alloying elements on the characteristics of Ni-based alloys have not yet been systematically investigated, despite the widespread application of such alloys in diverse domains. To address this gap, in this study, we investigated the effects of the substitutional alloying elements, specifically Fe, Cr, Mo, and Mn, on the lattice expansion, mechanical properties, hydrogen diffusivities, and solubilities of Ni. These elements led to both austenite lattice expansion and strengthening. We measured hydrogen diffusivities under high-pressure hydrogen environments (100 MPa) and desorption at constant temperatures. Notably, all the examined alloying elements reduced the hydrogen diffusivity of Ni in the order: Mn < Mo ≈ Fe < Cr. The effects of alloying cannot be simply explained by lattice expansion or strengthening but are attributed to increased activation energy for hydrogen diffusivity. We also assessed the hydrogen solubility through thermal desorption analysis (TDA) after exposure to high-pressure hydrogen (100 MPa). Except for Fe, alloying elements increased hydrogen solubility in the order: Cr < Mo ≈ Mn. These effects are attributed to changes in the activation energy of hydrogen solubility. Additionally, TDA spectra for almost all the alloys, simulated based on the temperature dependence of hydrogen diffusivity, indicated that hydrogen diffusion through the face-centered cubic lattice remained unaffected by hydrogen trapping.

Online crown calculations are essential for effective shape control of hot-rolled strip steel, the accuracy of which is often limited by modeling assumptions and the complexity of operating conditions. Model parameters significantly influence calculation accuracy, requiring process engineers to make regular adjustments based on their experience. As customization, small batch sizes, and multiple specifications become mainstream in production and supply modes, reliance on manual experience is increasingly insufficient. This paper aims to enhance the accuracy of online crown calculations for hot-rolled strip steel. Initially, based on the mechanism model incorporating gain coefficients and formula derivation, a relational expression between the gain coefficients for each stand in the tandem rolling mill group and the final crown is established, transforming the discrete issue of correcting influential factors into a unified problem of solving gain coefficients, thus improving correction efficiency. To ensure the reliability of the estimated results, three forms of constructing the constraint equation are compared: no constraint, limiting the proportional crown difference, and proportional exit crown. Utilizing rolling production data, two parameter estimation methods—normal linear regression and robust regression—are explored to determine the gain coefficients. Results indicate that combining the constraint based on proportional exit crown and robust regression will provide more reliable gain coefficients and significantly enhances the accuracy of online crown calculations, improving precision by over 50% compared with the model before correction.

Controlling martensitic transformation behaviors in metastable austenitic steels is key to developing high-performance materials with an optimal balance of strength and ductility. When a bending load is applied during the manufacturing process, tensile and compressive loads are applied simultaneously, which can result in a transformation behavior that differs from that observed under uniaxial tensile loading due to the distributions of stress/strain and stress triaxiality. In the present study, a multimodal approach combining high-resolution X-ray nano-tomography and X-ray diffraction (XRD) was employed to investigate the martensitic transformation behavior of transformation-induced plasticity (TRIP) steel during bending tests. The martensitic transformation of austenite grains was analyzed to evaluate the effects of the grain size, grain shape, crystal orientation, and stress distribution. The transformation behavior was influenced primarily by the stress distribution, whereas the transformation rate was scattered among individual grains due to differences in grain size and shape. These variations were attributed to the effects of local stress concentrations.

Reducing the greenhouse gas emissions from steel production can be done through direct reduction inside a shaft furnace using hydrogen gas as a reductant, generating water as an off gas. The temperature varies along the height of the shaft furnace, and studying the non-isothermal reduction is therefore necessary. In this work, industrial hematite pellets were non-isothermally reduced in a vertical tube furnace. Different gas mixtures containing water and hydrogen were used for reduction. The reduction gas used contained water vapor contents of 5%, 10%, and 20%, respectively, and the remaining gas was hydrogen. The experimental setup was carefully designed for the reductions to be carried out under well-controlled experimental conditions. It was clear that the water present in the reduction gas significantly decreased the reduction rate, especially at the lower temperatures. Moreover, the onset temperature of reduction was increased to around 525 °C when water was present, compared to 450 °C when pure hydrogen was used. Water contents above 5% lead to a low-rate stage at reduction degrees between 0.11 to 0.15. The low-rate stage ended when the wüstite phase became stable, changing the mechanism of reduction, which altered the chemical reaction rate. The reduction rate was less affected by water when the heating rate increased, since an increasing heating rate led to the reduction occurring at a higher temperature. Finally, the present study showed that the kinetics of non-isothermal reduction, using different water vapor contents, are very different from isothermal reduction.

The microstructural evolution and tensile properties of joints fabricated by the newly developed cold spot joining (CSJ) method were investigated using galvannealed DP 780 MPa steel sheet. The novel solid-state joining method called CSJ is proved to make the joining interface plastically deformed under high pressure and appropriate current by expelling Zn-Fe coated layer, resulting in the sound joints with strong interface. Joints exploiting CSJ method were made focusing on the effects of the pressing speed and current level. Microstructural observations of the joints revealed that the lower pressing speed increases the interface temperature. In addition, the increase in the current also increases the interface temperature. The increase in the interface temperature has a positive effect in terms of expelling Zn-Fe coated layer. The positive effect of increasing current is more significant than that of decreasing the pressing speed. The increase in temperature near the interface by increasing current promotes the removal of the Zn-Fe coating layer, resulting in plastic deformation near the joining interface. Appropriate pressure and current settings can facilitate the sound spot joints with enough tensile strength. Both tensile-shear and cross-tension tests have confirmed a plug failure in the base material region.

In this study, the effect of hydrogen on the spot-welded tensile shear strength of transformation-induced plasticity (TRIP)-aided martensitic (TM) steel sheet was investigated. The tensile shear tests were carried out on an Instron-type universal testing machine using the tensile shear specimen which was spot-welded at the lapped portion of 30×30 mm² using specimens with dimensions of width of 30 mm and length of 170 mm at crosshead speeds of 0.5-100 mm/min without and with hydrogen. The results were summarized as follows.

(1) The ultrahigh-strength TM steel without and with hydrogen charging possessed an excellent tensile shear stress (τ_f) in comparison with the hot-stamped (HS1) steel. This might be attributed to the TRIP effect of the TM steel which exhibits volume fraction of retained austenite of 1.52 vol% and low absorbed hydrogen concentration compared with that of the HS1 steel.

(2) The τ_f decreased with decreasing the deformation speed in the TM and HS1 steels with hydrogen, whereas the τ_f was hardly changed by the crosshead speed in the HS1 steel without hydrogen. The decrease in τ_f at slow strain rate might be caused by the occurrence of hydrogen diffusion to crack initiation site and crack tip to accelerate hydrogen embrittlement crack propagation.

(3) The hydrogen embrittlement crack was initiated at heat affected zone (HAZ) due to the hydrogen diffusion and hydrogen concentration at the HAZ which is softer than its surroundings, so the deformation is concentrated and HAZ becomes the origin of fracture, resulting in the stress concentration.

The warm V-bendabilities and hydrogen embrittlement properties of ultrahigh-strength Quenching and Partitioning (QP)-Transformation-Induced Plasticity (TRIP) steel sheets were investigated to apply the QP-TRIP steel sheets for automotive structural parts manufactured by cold or warm press forming. V-bending tests were carried out at a crosshead speed of 1 mm/min at V-bending temperatures of 25, 100 and 150°C using a hydraulic servo testing machine with a 88-deg. V-punch (punch tip radius R = 2 mm, R/t₀ = 1.7) and a V-dice (dice groove size l = 12 mm, dice shoulder diameter 0.8 mm) using V-bend specimens with dimensions of 5-mm width, 50-mm length and 1.2-mm thickness without and with hydrogen charging. Hydrogen charging was conducted by means of cathodic charging using a 3 wt% NaCl + 3 g/L NH₄SCN solution at a current density of 10 A/m² for 48 h before V-bending. The main results were obtained as follows.

(1) QP-A steel enabled to conduct V-bending at a V-bending temperature T = 25°C although the bending angle after unloading (θ₂) was less than 90-deg.

(2) When V-bending tests were carried out at T = 100°C, QP-B, C, and E steels without hydrogen and QP-B steel with hydrogen charging enabled to conduct V-bending. In addition, QP-B steel was also possible to carry out the V-bending at T = 150°C. These results implied that the V-bending at warm temperatures can improve the V-bendabilities of the QP-TRIP steels.

In the steelmaking industry, the utilization of CO₂ as a resource has become one of the important research projects. The decarburization and desilication mechanism were investigated to the extension range of low carbon condition and in Fe-C-Si ternary system in 1873K. ¹³CO₂ and ¹⁸O₂ dual isotope gases were use to clarify the complicated oxidation process. The results showed that in Fe-1.0 mass%C-0.5 mass%Si molten alloy, CO₂ substitution part of O₂ within 40% slightly reduced the decarburization rates in the advantage area of decarburization, and after reaching the oxidation equilibrium curve, the decarburization rates were basically stable and low even 100% CO₂. However, with introducing CO₂ from 0 to 40%, the desilication rates were dropped a lot. CO₂ replacing part of O₂ was beneficial to reducing the loss of silicon during decarburization process at 1873 K. The critical value of carbon content was around 0.3 mass%, that the main limiting rate step was transformed to the diffusion of carbon in molten under the correspondindg condition of 1.2 to 3.2 Nm³/t·min gas supply intensity in the present experiments. When the gas supply was sufficient, CO₂ participation ratio was decreased and that of O₂ was increased with the carbon content decreasing. When the gas supply was insufficient, both CO₂ and O₂ participation ratios were relatively stable until the carbon transfer limiting the reactions.

Strain-induced lattice defects that form during the incubation stage of hydrogen embrittlement fracture in the plastic region were quantified and their relationship with mechanical properties and fracture morphologies was investigated. Pure iron was subjected to plastic strain by tensile testing at various strain rates and hydrogen charging conditions. After charging tracer hydrogen as a probe for detecting lattice defects under conditions that reached equilibrium, specimens were quickly cooled with liquid nitrogen to prevent hydrogen desorption, and total tracer hydrogen was detected using low-temperature thermal desorption spectroscopy (L-TDS), which is capable of continuously elevating the temperature and subsequently performing measurement from that temperature. Dislocation density was not affected by the strain rate or hydrogen content. However, the vacancy concentration increased in the presence of hydrogen and displayed strain rate dependence even at the same strain level. A comparison of the mechanical properties with/without hydrogen showed that the flow stress with hydrogen increased with a decreasing strain rate compared with that without hydrogen, i.e., dislocation mobility decreased. It was established that strain-induced vacancies, which were excessively generated in the presence of hydrogen and formed complexes with it, were responsible for reducing dislocation mobility. Furthermore, fractures, albeit predominantly quasi-cleavage ones, along the {001} plane, which is the cleavage plane in body centered cubic iron, were present on the fracture surfaces, and their proportion increased with decreasing dislocation mobility. This suggests that vacancy-hydrogen complexes contribute to cleavage fracture by inhibiting dislocation motion.