The decarburization behavior of Fe–Cr–Ni alloys containing small amounts of carbon was investigated under static high-temperature hydrogen exposure, covering a wide compositional range from ferritic and austenitic stainless steels to Ni-based alloys. Under hydrogen exposure at 800 °C, a clear compositional dependence was observed: decarburization was negligible in the ferritic alloy, whereas the austenitic alloys exhibited noticeable decarburization with a distinct dependence on Ni content. This behavior was rationalized by considering both carbon diffusivity and the formation of an oxide barrier, whose protective effect weakens with increasing Ni content. The results provide fundamental insights into the thermodynamic and kinetic factors governing decarburization in Fe-Cr-Ni alloys, offering a guideline for material selection in hydrogen-containing high-temperature environments.

Based on the process characteristics and production data of converter steelmaking, this study proposes an endpoint phosphorus content prediction model using K-means clustering and Bayesian-optimized Stacking ensemble learning. Firstly, the K-means clustering method was applied to analyze converter smelting data, grouping samples with similar process characteristics into homogeneous sub-clusters. Then, within each sub-cluster, a Stacking ensemble model was established with Random Forest (RF), Support Vector Machine (SVM), Extreme Gradient Boosting (XGB), Gradient Boosting Machine (GBM), and Light Gradient Boosting Machine (LGBM) as base learners, and linear regression as the meta-learner. To validate the model's performance, the comparative experiments were conducted between a single Stacking model and individual base models after K-means clustering, with all models optimized via Bayesian hyperparameter tuning. Experimental results demonstrate that the Stacking model enhanced by K-means clustering and Bayesian optimization achieves the highest prediction accuracy. When prediction errors were constrained within ±0.005% and ±0.003%, the hit rates for endpoint phosphorus content reached 94.84% and 84.06%, respectively. This model provides accurate prediction of converter endpoint phosphorus content, offering valuable technical guidance for phosphorus control in actual production.

The effect of Si solid solution on the dislocation behaviour of body-centred cubic (BCC) steels was studied by performing neutron diffraction line profile analysis (LPA) on Fe–4wt%Si (4%Si steel) and interstitial-free (IF) steels strained to 11.7% nominal strain. Dislocation parameters were quantitatively determined: density (ρ), character (q), arrangement parameter (M^*) and crystallite size (D). Plastic deformation substantially increased dislocation density in both steels, with the increment in the 4%Si steel being more than twice that in the IF steel. In the 4%Si steel, plastic deformation also remarkably increased the screw dislocation fraction. This behaviour can be attributed to cross-slip suppression by Si, which confines the screw dislocations to their original slip planes and reduces their annihilation probability. This leads to the formation of a planar dislocation substructure characterised by spatially isolated screw dislocations, which fragment the crystal into relatively small coherent domains and high M^* values. By contrast, the active cross-slip in the IF steel promotes dislocation annihilation and develops a cell substructure with interacting dislocations concentrated in the cell walls, resulting in relatively large coherent domains and low M^* values. Overall, neutron diffraction LPA reveals that Si addition alters dislocation cross-slip behaviour and the resulting substructural development in BCC steels.

The variation of inclusions composition in a high sulfur steel after the addition of gadolinium (Gd) were investigated to understand the influence of Gd on tensile and impact properties of the steel. With increasing the total gadolinium (T.Gd) content, the evolution path of inclusions was MnS/Al-Gd-O/Gd-O-S/(Gd-O-S)-(Gd-S). After the forging of the steel, long strip MnS inclusions exhibited the aggregated distribution, and spherical Gd-bearing inclusions were uniformly distributed in the steel. The proportion of inclusions with aspect ratio in the range of 1 ~ 3 increased from 25.7 % to 98.2 % when increasing T.Gd addition from 0 ppm to 462 ppm. After Gd addition in the steel, the tensile property of the steel was modified by transforming long strip sulfide inclusions into spherical ones and reducing the number density of inclusions. With increasing the T.Gd content from 0 ppm to 462 ppm, the tensile strength of samples increased from 402 MPa to 443 MPa. The grain size reached 19.4 μm and 13.0 μm in the steel with T.Gd contents of 150 ppm and 462 ppm, respectively. The impact property of the steel was influenced by inclusions size and grain size. The addition of gadolinium in the steel reduced the ductile-brittle transition temperature (DBTT) and improved the impact toughness of the steel at low temperature. The impact energy of the steel with 0 ppm and 462 ppm T.Gd at -60 ^oC were 8.6 J and 101.3 J, respectively. The steel sample with 462 ppm T.Gd containing exhibited the best comprehensive properties.

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.

Phosphorus (P) is an essential element not only for human life but also for crop growth and for the production of functional materials. On the other hand, P is classified as a depleted element. Therefore, developing methods to recover P from secondary P resources is important. This study aimed to selectively recover of P from steelmaking slag by combining chlorination, steam reforming, and carbon reduction. First, detailed conditions were investigated to prepare Fe-free chlorinated residue by chlorination of the slag. Subsequently, the chlorinated residue was dechlorinated via steam reforming, and then selective volatilization of P were investigated by carbon reduction of the dechlorinated residue. Although chlorination of slag up to 1000°C enabled selective removal of Fe, selective separation of P was difficult during carbon reduction of this chlorinated residue. This was considered due to the residual chlorine in the chlorinated residue. Therefore, de-chlorination of the chlorinated residue via steam treatment was investigated. The results showed that heating at 1000°C could almost completely remove chlorine from the solid phase. Subsequent carbon reduction of this steam-modified chlorinated residue allowed only P to volatilize. These findings demonstrate that combining chlorination, steam reforming, and carbon reduction processes on steelmaking slag enables the selective, highly efficient volatilization, separation, and recovery of P alone.

With the growing demand for carbon capture and storage (CCS) technologies to achieve a carbon-neutral society, the low-temperature toughness of casing materials used in CO₂ injection wells has become increasingly important. This study investigates the effects of phosphorus (P) segregation at grain boundaries and prior-austenite grain size on the toughness of tempered martensitic steel API 5CT L80 Type1. Steels with different P contents and grain sizes were fabricated and evaluated through Charpy impact testing, fractography of Charpy fracture surfaces, and Auger electron spectroscopy. To comprehensively evaluate toughness-controlling factors, the Hyodo model based on the Brechet-Louchet framework was employed to calculate intergranular fracture stress. This calculation incorporated not only yield strength and grain size but also grain boundary cohesion energy estimated from the measured segregation levels of P and C. The results revealed that higher intergranular fracture stress correlates with lower ductile-to-brittle transition temperature (vTrs), demonstrating that intergranular fracture stress is an effective indicator of toughness for CCS applications. This approach enabled the combined effects of grain size and grain boundary segregation to be integrated into a single parameter.

Sulfur is one of the five ubiquitous elements of steel, and the presence of sulfur reduces the performance of steel. Therefore, the sulfur content in steel must be strictly controlled. This paper focuses on the gravimetric method after separation of iron (JIS G 1215-1) specified in the Japanese Industrial Standards (JIS) as an absolute analysis method for sulfur content in steel. The precipitation formation process of BaSO₄ and the rinse process of the formed precipitate had a major influence on the recovery. In the formation process of BaSO₄, it was confirmed that the precipitation was almost completely formed under the conditions specified in JIS G 1215-1. However, the coexistence of manganese ions (Mn²⁺) significantly reduced the precipitation recovery. Ethylenediaminetetraacetic acid (EDTA) was effective for masking Mn²⁺. In JIS G 1215-1, the BaSO₄ formed is rinsed in two steps: first, barium chloride solution (BaCl₂) is used to remove foreign substances, followed by hot water to remove the BaCl₂. Mn²⁺ not only inhibited the precipitation of BaSO₄ but also reduced the recovery during rinsing with hot water. Sulfur recovery in the entire JIS G 1215-1 process exceeded 100 % regardless of the addition of EDTA. This indicates loss of sulfur during the precipitation process much less contributed to the recovery of sulfur in the total process of JIS G 1215-1.

The use of hydrogen (H₂) to partially replace coal or coke in the blast furnace (BF) is a promising solution to decrease CO₂ emissions. Iron ore sinter is the main ferrous burden for the BF, and its reducibility varies depending on its mineralogy and the reducing gas composition. Due to the change in the gas composition with H₂ injection in the BF, the evaluation of reducibility under simulated BF gas compositions is necessary.

In this study, the reducibility of sinter was studied through isothermal reduction of sinter analogues under realistic blast furnace gas compositions containing both H₂ and H₂O. The simulated BF gas compositions for the conventional and H₂-enriched BF contain N₂-CO-CO₂-H₂-H₂O, while the previously simplified gas compositions contained only N₂ with CO or H₂. Sinter analogues were produced under different cooling rates, with a slow cooling rate generating a higher SFCA and lower magnetite content.

The reduction degree for simulated BF gas compositions was lower than the simplified gas, with the final reduction degree following this order: H₂-N₂ > CO-N₂ > H₂-enriched BF > conventional BF. Acceleration of reduction was found for the H₂-enriched BF case compared to the conventional BF. The rate-limiting step was analysed based on the shrinking core model and microstructure observation.

The variation in sinter mineralogy in this study did not affect the reducibility significantly. Sinters with significant differences in SFCA and magnetite contents showed similar reducibility under the same gas composition, suggesting the porosity of the sinter may become dominant in controlling its reducibility.

Metallic iron (MFe) catalyzes the reduction of NO by CO in iron ore sintering, though the underlying mechanism remains insufficiently understood. This study proposes an innovative decoupling strategy that separates the coupled CO-NO reaction into distinct Fe–NO and FeO_y-CO processes. Thermodynamic analyses of the Fe-C-O and Fe-N-O systems were performed using FactSage software, supplemented by fixed-bed experiments to identify key factors influencing the MFe-catalyzed CO-NO reaction. Kinetics were evaluated based on an equivalent particle model. The results demonstrate that MFe serve as the most effective catalyst for the CO-NO reaction. The MFe-NO reaction is self-terminating and exhibits a stage-dependent control mechanism: it is initially dominated by chemical reaction kinetics and later transitions to gas diffusion control. Significant diffusion resistance within the product layer is evidenced by an activation energy of 71.93 kJ/mol, which is 11.33 times that of the surface reaction activation energy of 6.35 kJ/mol. This indicates that the catalytic efficiency is limited by the dense surface structure of MFe. The addition of 3% MFe resulted in a 14.2% reduction in total NO emissions, offering a practical approach for achieving cleaner sintering operations.

The use of general-purpose austenitic stainless steel is being considered to reduce the costs of hydrogen gas piping materials. In austenitic stainless steels, strain-induced martensite forms during plastic deformation, raising concerns about the hydrogen embrittlement resistance, particularly in the welded areas. Evaluating the hydrogen embrittlement requires visualizing the hydrogen distribution within the microstructure and clarifying the hydrogen permeation behavior in specific phases, such as the strain-induced martensite phase. In this study, electron backscatter diffraction microstructure analysis and scanning Kelvin probe force microscopy hydrogen permeation monitoring of welded specimens made with SUS304 base metal and SUS308 welding wire were performed, focusing on the weld zone in which strain-induced martensite was introduced through cold working. The relationship between the microstructure and hydrogen permeation behavior was then investigated. The results confirmed hydrogen permeation from the δ-ferrite and martensite phases in the microstructure around the weld zone, suggesting that the internal crystal structure influences the permeation behavior. Furthermore, ε-martensite present within the martensite phase was found to exhibit a lower hydrogen permeation rate than α′-martensite. These results indicate that material design to suppress the formation of α′-martensite generated by plastic deformation is crucial for reducing the hydrogen embrittlement risk.

Blast furnace charging critically regulates gas flow distribution, directly impacting operational efficiency. Understanding burden distribution under various charging systems via simulation is essential for optimizing control strategies. This study employs the CFD-DEM coupling approach to explore gas-solid interactions during charging, integrating SOLIDWORKS for geometric modeling, EDEM for DEM simulations, and FLUENT for CFD analysis. The coupled model investigates gas flow effects on particle trajectories, inter-particle dynamics, and ore-coke mixing. Results reveal that gas flow significantly enlarges the burden landing radius, exhibiting a positive correlation with gas velocity. Post-gas coupling, both coke and ore experience reduced velocity variations after exiting the chute. Surface accumulation angles for coke increase, with outer angles rising more prominently than inner angles; ore accumulation follows similar trends. Additionally, gas flow broadens the burden platform width. Notably, gas flow suppresses ore-coke mixing, substantially reducing ore particle infiltration into the coke layer. These findings clarify the mechanism by which gas flow reshapes burden distribution, offering insights for real-time charging assessment and process optimization. The CFD-DEM framework demonstrates robust capability in simulating complex gas-solid behaviors, providing a theoretical foundation for enhancing blast furnace efficiency through precise charging control.

The effect of dislocation density on the σ phase precipitation behavior in KA-SUS304J1HTB was investigated. The specimen was pre-strained by room-temperature uniaxial tensile test and subjected to ageing at 700°C. The number density and area fraction of the σ phase after ageing increased with the amount of pre-strain. On the other hand, the pre-strain had no significant effect on the average σ phase diameter. Electron backscatter diffraction (EBSD) analysis revealed that the geometrically necessary dislocation (GND) density was increased with the amount of pre-strain. Moreover, the GND density of the pre-strained specimens showed little change after the ageing. The EBSD analysis also revealed that the GND density was significantly higher around the grain boundaries compared to the grain interior. Using the GND density around the grain boundaries in the pre-strained specimens, the change in the number density of the σ phase during ageing was calculated based on the classical nucleation theory. The calculated number density of the σ phase showed good agreement with the experimental results. This result implies that the effect of the pre-strain on the number density of the σ phase after ageing can be explained by the change in GND density around the grain boundaries.

Electric arc furnace dust (EAFD) is a hazardous waste generated during the production of steel from electric arc furnaces. it is also known SMD (Steel mill dust).The objective of this work is focusing on the effect of temperature, contact time and carbon content of reducing agent (charcoal) to achieve the maximum reduction percent by the pyrometallurgical treatment for pellets of 100% steel scrap (EAFD) with charcoal in the horizontal tube furnace. Experiments done at different temperatures ranging from 900⁰C to 1300⁰C using different amounts of charcoal as 12.5%,15% ,17.5% and 20% during different contact time (30, 60 and 90 minutes) while maintaining a constant nitrogen gas flow rate about 60 l/h. The experimental results for two parameters (temperature and charcoal percent) show that the best reduction percentage is (62.463%) which achieved at 1200°C with 17.5% charcoal percent at the constant contact time 90 minutes according to the mass loss equation. After studying the effect of contact time and temperature on EAFD reduction, the reduction percent decreased as temperature increase to 1300⁰C with increasing contact time from 60 min to 90 min. The highest reduction percent that achieved at 1300⁰C with 17.5% charcoal during contact time 60 min is 61.969%.

This study investigates the ductile fracture mechanisms in as-quenched (As-Q) and quenched-and-tempered (Q-T) dual-phase steels. Using in-situ tensile testing combined with synchrotron X-ray tomography, we performed four-dimensional quantitative analysis of void evolution to reveal how heat treatment fundamentally alters damage behavior. The high-strength, low-ductility As-Q steel exhibited premature failure. Its significant phase-hardness mismatch between ferrite and martensite phases induced continuous and widespread void nucleation via martensite cracking after reaching the ultimate tensile strength. The subsequent rapid, isotropic growth and coalescence of these numerous voids led to early fracture. In contrast, tempering the Q-T steel reduced the hardness mismatch among phases, yielding superior ductility with lower strength. The Q-T steel, in particular, exhibited a two-stage damage process. Throughout most of the deformation, voids grew anisotropically by elongating along the tensile axis. This stable growth, however, gave way to a drastic change just before fracture, characterized by a rapid proliferation of voids oriented perpendicular to the loading direction. This final phase is attributed to damage in the martensite, suggesting the quantitative and qualitative differences in void evolution between the As-Q and Q-T steels.

This study investigates the strengthening mechanisms in duplex stainless steel (DSS), a dual-phase alloy comprising ferrite (α) and austenite (γ), produced by friction stir welding (FSW) using in situ neutron diffraction during tensile testing. Two welding conditions were applied: a lower peak temperature (FSW300) and a higher peak temperature (FSW600). Electron backscatter diffraction confirmed significant grain refinement in both phases, with γ grains reduced below 1 μm in FSW300. In situ neutron diffraction provided phase-resolved stress data during deformation, enabling direct evaluation of the load-sharing behavior of α and γ. Tensile testing revealed that both FSW conditions increased yield and tensile strength while reducing uniform and total elongation compared with the base metal (BM); however, FSW300 retained greater total elongation than FSW600, attributed to less severe local elongation loss. Neutron diffraction results revealed that γ acted as the harder phase in the BM, whereas α became the harder phase in the FSWed specimens. Phase stress analysis indicated that α is more sensitive to grain refinement strengthening than γ, shifting the dominant contribution to strength and work-hardening from γ in the BM to α in the welded specimens. Although stacking fault formation in γ was more pronounced in the ultrafine-grained microstructures, work-hardening capability of γ decreased, while α showed enhanced texture development and dislocation accumulation. These findings demonstrate that low-temperature FSW improves DSS strength primarily through α refinement, offering insights for designing stronger dual-phase alloy joints.

The objective of this study was to predict the hydrogen content entering steels during immersion in NaCl solutions, using finite element method (FEM) simulations. FEM simulation models were constructed by appropriately defining the boundary conditions for electrochemical reactions, diffusion coefficients of chemical species, and dissolved oxygen concentrations. The simulated polarization curves for carbon steel in various NaCl solutions were in good agreement with experimental results, indicating the high accuracy of the simulation models in representing actual phenomena.

Based on FEM simulations, the cathodic current for the hydrogen evolution reaction (HER) increased as the solution pH decreased. In particular, the HER cathodic current at pH 1 was approximately twice as large as that at pH 2. Considering that the HER cathodic current is correlated with the amount of hydrogen entering the steel, it is predicted that the hydrogen content entering steel at pH 1 is twice that at pH 2. This prediction was well supported by experimental thermal desorption spectroscopy results, suggesting that calculating the HER cathodic current by FEM simulations is an effective approach for predicting hydrogen entry behavior in steels.

To clarify the sticking mechanism and discuss its suppression during hydrogen reduction in the fluidized bed, in-situ observation of the reduction behavior of wüstite using high-temperature microscope and the reduction experiment were carried out under various conditions such as gas composition, temperature, and total pressure.

First, hydrogen reduction leads to the formation of porous structure of wüstite phase. Then, reduction to metallic iron proceeds concentrically, and porous iron forms. A part of metallic iron nuclei grows vertically to the wüstite surface, and iron whisker forms under specific conditions. Under atmospheric pressure conditions, higher temperature and higher hydrogen partial pressure make the surface porous, and iron whisker forms through 100%H₂ reduction at 900°C and 950°C. Increasing total pressure enlarges the formation area of iron whiskers to lower temperature. Furthermore, three-step fluidized bed reduction process was suggested to control the sticking phenomenon of raw materials.

A non-oriented Fe-0.7% Si steel as-cast strip was produced by an industrial twin-roll strip casting production line. The evolution of the microstructure, texture, and properties was characterized along the entire processing route. The microstructure of each cross-section of the industrial cast strip was found to be uniform, with low texture strength and random orientation distribution. After cold rolling, a mixed microstructure formed, consisting of deformation structures and shear bands alternating in the thickness direction. The texture exhibited a mixed texture combining α-fiber texture (<110>//RD) and γ-fiber texture (<111>//ND). During the 850°C annealing process, recrystallized grains nucleated preferentially in shear bands, and α texture and Goss texture began to form. The final annealed sheet texture was composed of Goss texture and γ-fiber texture, with favorable textures constituting the major part. The final sheets exhibited excellent magnetic properties: P_1.5/50 = 4.915 W/kg, B₅₀ = 1.856T in RD direction, and P_1.5/50 = 5.347 W/kg, B₅₀ = 1.765 T in TD direction. Its average yield strength was approximately 278.77 MPa, and the average tensile strength was approximately 505.47 MPa. After aging at 300°C for 10 hours, only a few precipitates formed in the final sheets, and their magnetic properties remained highly stable with little to no change.

This study investigates the enhancement of surface properties of OPTI N+ high-nitrogen stainless steel through a duplex treatment combining ion nitriding and CrSiN coating deposition via DC magnetron sputtering. Ion nitriding produced a hardened layer approximately 50 μm thick with a surface hardness of 1037.1 HV_0.05, primarily composed of Fe₂N, Fe₃N and Fe₄N phases. CrSiN coatings deposited at various temperatures showed that the sample coated at 300°C exhibited the highest hardness (13.07 GPa), optimal H/E ratio (0.055), and superior tribological performance, including the lowest friction coefficient (0.5773), wear loss volume (7.67 × 10⁻⁴ mm³), and specific wear rate (3.23 × 10⁻⁷ mm³·m⁻¹·N⁻¹) under a 5 N load. Electrochemical tests confirmed enhanced corrosion resistance, with the lowest corrosion current density (1.38 × 10⁻⁶ A·cm⁻²) and highest polarization resistance (339.98 Ω·cm²). HRTEM analysis revealed a nanocomposite microstructure consisting of crystalline CrN and amorphous SiN. These results demonstrate that the CrSiN/ion nitriding duplex treatment significantly improves the mechanical, tribological, and corrosion resistance of high-nitrogen stainless steel.

The damage initiation and evolution behaviors of ferrite-martensite dual phase (DP), transformation-induced plasticity (TRIP)-aided dual-phase (TDP), quenched and tempered (QT), and TRIP-aided martensitic (TM) steels during tensile deformation were investigated. Voids were initiated at the phase boundaries and inside the martensite in the DP and TDP steels, whereas fine voids were observed at the prior austenite, packet, and block boundaries in the QT and TM steels. In the DP and TDP steels, the size of the voids remarkably increased with the plastic strain, even though the number of voids increased slightly. By contrast, the QT and TM steels exhibited a drastic increase in the number of voids, whereas a slight increase in the size of the voids was observed. The voids in the TM steel hardly extended as the plastic strain increased unlike those in the QT steel. The extent of voids in the DP and TDP steels might be attributed to stress and plastic strain partitioning between the different phases during tensile deformation. In addition, the promotion of void initiation and suppression of void growth might be attributed to the fine and uniform martensite matrix in the QT and TM steels. The suppression of void initiation in the TDP steel and void growth in the TM steel might be attributed to the stress and plastic strain relaxations at the void initiation site and the vicinity of voids owing to the effective martensitic transformation of retained austenite.

Planar slips have been frequently recognized in nitrogen-added austenitic stainless steels, and often discussed in terms of stacking-fault energy (SFE). On the other hand, nitrogen addition promotes the formation of N-Cr short-range order (SRO), which has been proposed to cause the planar slips. In this study, a chromium-free Fe-N binary austenite with 2.4 mass% of fully solid-solutioned N was fabricated to simplify the relationship among deformation structures, SFE and N-Cr SRO. The Fe-N alloy was found to exhibit wavy slips and possess almost the same SFE as a nitrogen-added austenitic stainless steel with planar slips. TEM results indicate the formation of modulated structures along <001> in the nitrogen-added austenitic stainless steels, which should be attributed to the presence of N-Cr SRO, as reported previously in the literature. These results indicate that the planar slips are primarily caused by N-Cr SRO rather than by SFE.

In this research, to clarify the void closure behavior of large-scale voids elongated in the normal direction, artificial void was introduced to steel plate and hot rolling experiments were conducted at 1000 °C. The void height was systematically changed to investigate the influence on the closure behavior. The closure process was compared with a predicting criteria called Q value. The results show that the closure behavior varies significantly with the increasing of height, the dominant closing direction shifts from the normal direction to the transverse direction, and the critical Q value criteria was not appliable for all cases. To discuss the applicability of the Q value, a representative volume element was built, and finite element analysis under systematically controlled stress state was conducted. The Q value criterion under different stress states has been established.

The utilization of bauxite residue (BR) was experimentally studied through hydrogen reduction, focusing on iron oxide reduction and the formation of leachable alumina phases. Three different types of pellets were made: one from bauxite residue, and the other two via calcite addition with varying calcite amounts. All pellets were isothermally reduced by H₂ gas in a thermogravimetry furnace at constant temperature under fixed hydrogen flow rate and reduction times. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) coupled with Energy dispersive spectroscopy (EDS), were used for the phase and microstructural analysis. The hydrogen reduction rate is influenced by iron-bearing oxides formed during heating. Bauxite residue pellets exhibit a higher initial reduction rate than bauxite residue–calcite pellets as brownmillerite formation occurs in the latter one, which hinders iron oxide phase reducibility. The reduction kinetics are affected by the reduction temperature and calcite quantity added, while dominant phase formation during hydrogen reduction depends on their combined effect. There is no mayenite (Ca₁₂Al₁₄O₃₃) phase formation in reduced BR pellets, while with the addition of calcite, small amount of mayenite phase starts to appear during reduction, which increases with more calcite addition.

This study investigates the effects of copper (Cu) addition on the bending fatigue strength of lath martensite, focusing on dislocation structures, Cu precipitates, and crack initiation sites. Steel specimens with varying Cu contents of 0, 1, 2, and 4 mass% were prepared and tempered at 523 K or 723 K. Fatigue tests revealed that Cu addition significantly enhances fatigue strength, particularly at 723 K, where the 4Cu steel (4 mass% Cu) exhibited superior performance compared to the 0Cu steel (<0.01 mass% Cu). Microstructural analysis by SEM and TEM showed that, after fatigue testing, the Cu precipitates at grain boundaries had undergone plastic deformation, indicating local stress relief at the grain boundaries. This stress relaxation effectively suppressed crack initiation at the grain boundaries and shifted the sites of crack initiation to the grain interior. Intragranular Cu precipitates were found to pin dislocations, delaying crack initiation within grains. It is also speculated that retardation of dislocation alignment during tempering, which is caused by Cu in solid solution and as fine precipitates, may further delay crack initiation, although direct confirmation of this effect is still required. At 523 K, Cu addition did not significantly improve fatigue performance, presumably due to the absence of Cu precipitates at grain boundaries. These findings suggest a dual mechanism of grain boundary strengthening by plastically deformable Cu precipitates and matrix pinning by intragranular Cu precipitates, which would explain the observed improvement in the crack initiation life and fatigue limit of Cu-added lath martensite.

Low ductility is the intrinsic cause of cracking in slab, and cooling rate is a key factor affecting the hot ductility of steel. This article focuses on low carbon V-N steel, and explores the growth characteristics of carbonitride precipitation and microstructure at different cooling rates through thermal simulation tensile experiments. The results show that increasing the cooling rate can improve the ductility in the third brittle zone. The cooling rate increases from 0.5°C/s to 7°C/s, the minimum of the reduction of area increases from 38.1% to 38.9%, 39.8%, 44.9%, and 48.4%, and the third brittle temperature zone shrinks from 725 ~ 860°C to 725 ~ 820°C. Increasing the cooling rate has little effect on austenite grain size, but it will significantly reduce the proeutectoid ferrite formed at austenite grain boundary. With the cooling rate increasing, due to the accelerated transition from austenite to ferrite, the average thickness of the proeutectoid ferrite film decreases from 38μm to 4μm, and no proeutectoid ferrite is formed at some grain boundaries. The cooling rate ≤5°C/s, the carbonitrides are distributed in a chain like manner at austenite grain boundary, and the size is large. However, when the cooling rate is ≥5°C/s, the time for the microstructure to remain in the austenite temperature zone and the high fluidity of the γ/α interface cause some carbonitrides to precipitate in the ferrite matrix, with small and uniform distributions in the original austenite grains.

This study investigates the oxidation behavior of MgO-C refractories containing a combined antioxidant system of Al (2wt%) and B₄C (0~4wt%). Oxidation tests were conducted under air conditions at 1200°C, 1300°C, and 1400°C for 60 and 180 minutes. As the temperature was increased from 1200°C to 1400°C, the non-oxidized area increased by 1.4, 3.3, and 14.1 times, respectively. The weight loss ratio decreased with the B₄C content and the temperature. Above 3wt% of added B₄C, the weight loss ratio remained the same. At 1200°C, the weight loss ratio decreased from 14.2% to 9.9% as the addition of B₄C was increased to 3wt%. At 1300°C, the weight loss ratio decreased from 14.8% to 6.7%. At 1400°C, the weight loss ratio decreased from 16.0% to 1.88%. These findings demonstrate that the effect is significant as the temperature increases. After the experiments, an XRD analysis confirmed the formation of Al₂O₃, B₂O₃, Mg₂B₂O₅, Mg₃B₂O₆, and MgAl₂O₄. Thermodynamics analysis indicates that Mg₃B₂O₆ melts at 1400°C, enabling effective pore sealing. At 1300°C, however, the combined reaction between Al₂O₃ and magnesium borates generates low-melting phase that provides a remarkably strong antioxidation effect even at this lower temperature, making 1300°C the critical point where the Al-B₄C system becomes highly effective. However, at 1200°C, only B₂O₃ is present as a liquid and subsequently forms solid magnesium borates with MgO, offering only a limited improvement in oxidation resistance.

This work deals with the development of new Compacted Graphite Cast Iron Composite Castings as an alternative to Si-Mo ductile iron. The new material is a silicon-molybdenum cast iron (Si-Mo), transformed into a cast composite using the Self-propagating High-temperature Synthesis in Bath (SHSB) method, which synthesizes ceramic carbide particles of metals such as Ti, W, Nb, Mo, Zr. SHSB is used to ensure the formation of thermodynamically stable ceramic phases, in this case, carbide phases. Among the listed metals, titanium was selected for the SHSB synthesis process due to the exceptionally favorable physicochemical properties of titanium carbide and its highly exothermic enthalpy of formation. The described process strengthens the matrix of the material, changing its characteristic operational properties. The resulting composite is designated as Si-Mo TiC. Conventional Si-Mo cast iron is widely applied in the automotive industry, where it is used, for instance, in the production of exhaust manifolds and turbocharger components, where high resistance to thermal shock and excellent heat resistance are required. Transforming this material into a composite material improves many physicochemical parameters. Additionally, titanium desferoidizing effect helps stabilize graphite in the compacted (vermicular) form. Castings of both conventional Si-Mo iron and the TiC-reinforced Si-Mo composite with compacted graphite were produced with varying wall thicknesses. The microstructural characteristics and thermophysical properties, such as thermal conductivity and thermal stability, of the classical and composite materials were compared. The new Si-Mo cast iron reinforced with thermally stable TiC particles has been demonstrated to exhibit excellent structural integrity and thermal stability.

Steelmaking slag contains valuable components. Its high phosphorus content hinders its reutilization in the steelmaking process. This study proposes an efficient phosphate removal process using molten CaCl₂ as the reaction medium, aiming to enable slag recycling and P recovery. Experimental results show that nearly all P-bearing phases in steelmaking slag are leached into molten CaCl₂ under the tested conditions (850 to 1000 °C, slag/CaCl₂ mass ratios 1:10 to 1:2.5). The leaching mechanism involves the reaction of slag's P-bearing phases with CaCl₂ to form intermediates (Ca₂PO₄Cl and Ca₃SiO₄Cl₂), which dissociate into PO₄^3– and other ions in the melt. For other elements: Si(IV) leaches continuously but incompletely; Fe(II) and Mn(II) concentrations in the melt first increase then decrease, attributed to the evaporation of volatile FeCl₂ and MnCl₂; Mg and Al remain nearly unleachable. After leaching, adding carbon to the P-enriched melt achieves complete P(V) removal via carbothermal reduction. This molten CaCl₂-based approach provides a sustainable alternative to conventional technologies, offering valuable insights for the comprehensive utilization of P-bearing solid wastes.

The formation and evolution of plastic strain concentration zones and surface damage introduced by tensile testing of dual-phase steel were investigated by tracking precise markers on the specimen surface using electron beam lithography and a focused ion beam (FIB). Following tensile test at a stress approaching the tensile strength, areas of concentrated plastic deformation and extremely dark surface cracks (surface damage) appeared on the specimen surfaces. Regions of strain concentration with equivalent plastic strain exceeding 0.15 were identified from the displacement of precise markers. These regions appeared within the ferrite, near the ferrite-martensite interface, and within the martensite. Within the ferrite and at the ferrite-martensite boundary, strain concentration due solely to plastic deformation was observed, rather than displacement due to surface damage. The Taylor factor for ferrite grains within these strain concentration areas tended to be lower than those for ferrite grains within the surface damage regions. Furthermore, after we applied the new markers via the FIB and performed additional tensile testing until fracture, new plastic deformation concentration zones appeared within the ferrite, and the Taylor factor of the ferrite was relatively low. The Taylor factor is determined solely by the crystallographic orientation of the ferrite and the tensile deformation conditions. The effects of the ferrite crystallographic orientation on the ductile fracture process were also examined.

COREX shaft furnace adopts screws and corresponding guiding flow insert for burden discharge. To understand and then control burden flow in the furnace, it is essential to study the influence of discharge conditions and other factors on granular flow. Common CFD method cannot predict the orientation and size of stagnant zone formed by discharge in lower part of the furnace and thus is not easy to reasonably examine the effect of distribution of discharge conditions. Although the influence of discharge conditions could be studied by DEM, numerical studies of COREX shaft furnace by the method had obvious deviations in predicting descent velocity and resident time of burden in the furnace due to limitation of computational cost. In this paper, appropriate boundary conditions were used to describe lower screw discharger and guiding flow insert in moving bed, and a new CFD method based on novel potential flow model was used to numerically study 3D granular flow field including stagnant zone in a physical simulation bed of COREX shaft furnace. With the movement times of granular materials in bed reasonably predicted, the influences of not only screw discharger and guiding flow insert but also shaft angle of furnace on granular flow were investigated by the method in detail. Present work may provide a more reliable numerical basis for studying and then controlling burden flow in COREX shaft furnace.

Material applied in low-temperature liquefied gas storage tanks is required to have sufficient toughness. In recent years, high Mn austenitic steel has attracted attention for use in this application. In this study, the basic deformation characteristics and toughness of high Mn steel containing about 25% Mn were compared with those of 9% Ni steel in order to investigate the applicability of high Mn steel to LNG tanks. The high Mn steel showed larger uniform elongation than the 9% Ni steel due to higher strain-hardening, but elongation after the maximum load was significantly smaller. The plastic flow stress of the high Mn steel increased with decreasing temperature and showed temperature dependence similar to that of 0.2% proof stress in the 9% Ni steel. The Charpy absorbed energy of the high Mn steel was about half that of the 9% Ni steel, with an average value of 86 J at 77 K. Cleavage fracture surfaces were not observed in the fracture surfaces obtained at any temperature, indicating the micro-void coalescence type of fracture. The characteristics of ductile damage in the high Mn steel were discussed based on observation of micro-voids.

This research aimed to investigate the influence of Fe-based addition on the solution-loss kinetics of coke in hydrogen-enriched blast furnace. The solution-loss reactions of base coke (BC) and Fe-based coke (BC+Fe) in the CO₂ + 20%H₂O atmosphere across the temperature 1000-1200 °C were carried out by a homemade coke reactivity measurement device with continuous water inflow. The kinetics of the Boudouard reaction (C + CO₂ = 2CO) and the water-gas reaction (C + H₂O = CO + H₂) were assessed by monitoring the outlet gas composition (CO and H₂) to quantitatively evaluate the catalytic influence of Fe₂O₃ on the solution-loss reaction. The results indicate that the solution-loss rates of BC+Fe coke are more those of BC coke, and the solution-loss ratios of BC+Fe coke are 10.5-26.8% for the Boudouard reaction and 12.1-42.2% for the water-gas reaction higher than those of BC coke. Furthermore, Fe₂O₃ lowers the apparent activation energy (E_a) of the Boudouard reaction by 4.2% and that of the water-gas reaction by 7.8%, which shows that the catalytic effect of Fe₂O₃ is stronger for the water-gas reaction than for the Boudouard reaction. SEM analysis shows that the BC+Fe coke has a more varied pore structure and wider range of pore sizes on the surface. XRD analysis indicates that Fe₂O₃ reacts with Si and Al species in the minerals to form Fe-based silicates and aluminosilicates, which could contribute to the catalytic effect of the coke solution-loss reaction.

This study has analyzed the growth and removal mechanisms of Al₂O₃, MgO, MgAl₂O₄, ZrO₂, SiO₂, and Ti₃O₅ inclusions in molten steel formed through the addition of various deoxidizing elements by dividing them into single inclusions and cluster inclusions resulting from the agglomeration of these inclusions with a focus on kinetics. Additionally, we have evaluated the maximum particle diameter of cluster inclusions from both thermodynamic and agglomeration force perspectives to examine the agglomeration properties and mechanisms of various inclusions. The growth mechanism of various single inclusions, measuring several micrometers in diameter and suspended in molten steel, is governed by Ostwald ripening with collision agglomeration due to Brownian motion and turbulent stirring. Contrarily, cluster inclusions with diameters of 10 µm or more float in molten steel agglomerate with suspended single inclusions. Depending on the inclusion type, they also agglomerate with other clusters along their floating paths, growing larger and undergoing floating separation. Furthermore, the agglomeration strengths of various inclusions in molten steel follows the order MgO < Ti₃O₅, SiO₂ < MgAl₂O₄ < ZrO₂ < Al₂O₃. The kinetic mechanism of agglomeration growth is explained in a unified manner by the interparticle interactions of agglomeration force driven by cavity bridge forces.

Pearlitic steels achieve an exceptional balance of strength and ductility through the lamellar stacking of ferrite and cementite. While this synergy enhances mechanical performance, cementite also serves as a preferential site for crack initiation, making its thickness and the extent of deformation localization caused by dislocation pile-ups critical factors in the plasticity–fracture transition. In this study, molecular dynamics simulations were performed to clarify how cementite thickness and dislocation pile-ups govern deformation and fracture. The results reveal that thinner cementite or smaller pile-ups promote dislocation emission across the interface, whereas thicker cementite and larger pile-ups facilitate crack initiation within cementite. Comparison with a conventional continuum model showed qualitative agreement but also highlighted nanoscale effects—such as core relaxation of penetrated dislocations in cementite—that are beyond continuum descriptions. These findings provide atomistic insights into the mechanisms controlling the plasticity–fracture transition in pearlitic microstructures.

A hydrogen visualization technique based on Prussian blue (PB), a fully reversible hydrogenochromic material, was developed for detecting hydrogen distribution in a pure Fe sheet. A PB layer was electrochemically formed on a Pd-coated Fe sheet. The Pd intermediate layer acted as a catalyst for the reduction of PB by hydrogen atoms diffusing through the Fe sheet. During hydrogen charging, the PB layer exhibited a color change from deep blue to light blue, effectively visualizing the hydrogen distribution in the Fe sheet. The color gradually recovered in air via oxidation by O₂ in air, and the color restoration was significantly accelerated by heating at 70°C. The PB layer exhibited excellent reversibility of color change over multiple hydrogen charging and heating cycles. This study highlights the potential of PB as a reusable sensor for the in situ monitoring of hydrogen distribution in metals.

Fatigue crack initiation and subsequent crack propagation behaviour in as-quenched low-carbon low-alloy steel were examined using a rotating-bending fatigue test and electron backscatter diffraction analysis to clarify the relationship between the fatigue limit and the microstructural heterogeneity of martensite. The as-quenched low-carbon low-alloy steel exhibited a low fatigue limit relative to its ultimate tensile strengths. The fatigue fracture was originated from slip deformation due to dislocation glide in the matrix. Furthermore, a tensile test revealed a low elastic limit in the steel, which can be explained by the movement of high-density mobile dislocations introduced during the transformation. These findings suggest that the low fatigue limit of as-quenched low-carbon low-alloy steel is due to its low elastic limit. Fatigue cracks initiated at prior austenite grain boundaries (PAGBs), at packet boundaries, and parallel to the block boundaries. These crack initiations were triggered by the preferential activation of slip systems parallel to the habit plane in the coarse martensite, which was nucleated at the PAGBs in the early stage of transformation and satisfied the Kurdjumov–Sachs orientation relationship (K–S OR), with not only its own parent austenite grain but also the adjacent austenite grain (i.e. the double K–S OR). Additionally, the initiated cracks were arrested at the fatigue limit. This is probably due to plasticity-induced crack closure stemming from the significant plastic deformation of the early transformed coarse martensite.

We employed the high-resolution digital image correlation study to investigate the plastic deformation of low-carbon lath martensite. The strain localization bands were mainly categorized into two types: boundary slips and intra-block deformation. Misorientation angle and inclination angle with respect to loading direction primarily determines the slip activation at boundaries. The competitive relationship between the activation of in-lath-plane and out-of-lath-plane slip systems follows the Schmid effect. The in-lath-plane slip systems were only activated in blocks with high in-lath Schmid factor (SF) value. The out-of-lath-plane slip systems were activated only when its SF is much higher than the maximum in-lath SF, offsetting the effect of the higher critical resolved shear stress (CRSS) for in-lath-plane slip systems. Moreover, the block morphology also affects the slip activation behaviour: in-lath-plane slip systems in columnar blocks were preferentially activated due to both crystallographic dynamics and strain accommodation. In contrast, out-of-plane slip systems were only observed in equiaxed blocks where the boundary effect is weakened.

In an extensive flow of studies on material fracture, fracture mechanics has successfully established engineering standards for the safety evaluation of structural components. Vital difficulties in theories have been the management of the crack-tip stress singularity and the existence of incipient crack(s). Plasticity complicates fracture theories, and understanding the microscopic process of fracture is crucial for material design. This paper aims to shed light on the role of plasticity throughout the entire fracture process, remarking mostly brittle-like fracture, both in theory and experiment. Lattice deterioration due to plastic deformation increases potential energy, a key concept in deriving fracture criteria. Studies demonstrating the maturing of strain-induced lattice defects, primarily vacancy clustering, are reviewed to play a crucial role, operating as void source in fracture as a precursor to crack initiation.

Strain localization due to microstructural inhomogeneities are remarked to characterize the material's susceptibility to fracture. The extent of strain localization, coupled with external and local stresses, provides favorable fracture paths through crack nucleation and extension, as exhibited in fracture surface morphology. However, a single type of morphology does not specify a fracture event, and its continuous transition during crack extension suggests operation of an essentially common mechanism between seemingly different morphologies.

Lattice defects generated during plastic deformation persist into later stages, and environmental variations alter dislocation configurations, generating vacancies. As a method to assess the intrinsic material's susceptibility, detecting the progress of lattice deterioration in response to cyclic stressing is proposed.

Building on our previously established trap-free lattice transport framework for Ni–X binary model alloys, we extend the analysis to multicomponent Fe–Cr–Ni austenite and systematically evaluate how Ni, Mn, and N influence lattice expansion, mechanical strength, hydrogen diffusivity, and hydrogen solubility under 100 MPa hydrogen within a temperature range of 160–270°C. Mn showed the most pronounced austenite lattice expansion, whereas N provided the strongest solid-solution strengthening. Ni increased hydrogen diffusivity as measured by the desorption method, while Mn and N had little effect. Across Fe–Cr–Ni and previously studied Ni–X systems, variations in diffusivity are primarily governed by changes in activation energy, with the pre-exponential factor remaining nearly constant across alloys within each system. Hydrogen solubility, assessed by thermal desorption analysis (TDA) after 100 MPa exposure, showed that Mn markedly increased solubility, whereas Ni slightly decreased it and N had a negligible effect. These alloying effects on solubility are also explained by changes in activation energy, consistent with a narrowly distributed pre-exponential factor. TDA spectra computed from the measured temperature dependence of diffusivity reproduced the experimental peaks for almost all alloys, indicating trap-free lattice diffusion; a shoulder observed for the Mn-rich composition suggests ordering-induced trapping during heating. The results highlight the distinct roles of the alloying elements: Ni accelerates hydrogen transport, Mn increases uptake, and N enhances strength with minimal impact on transport, thereby providing composition–property guidelines for hydrogen management in austenitic stainless steels.

Tensile test results obtained from millimetre- and micrometre-scale specimens were correlated using crystal plasticity analysis to examine the microstructural factors dominating the mechanical properties of the as-built maraging steel produced by the laser powder bed fusion (LPBF) method for repairing die-casting tools. Micrometre-scale tensile tests revealed that the mechanical properties of single prior austenite grain (PAG) structures are dominated by the deformation of the coarse block according to Schmid's law rather than by habit-plane-orientation-dependent slip deformation. This is owing to the low aspect ratio of the lath structure in the maraging steel produced by the LPBF method. In the millimetre-scale tensile specimens consisting of multiple PAGs in the gauge section, anisotropy of the ultimate tensile strength and elongation-to-failure was not observed, which was attributed to the high energy density in LPBF process. It was revealed that the specimen with the loading direction (LD) parallel to the build direction exhibited earlier work softening than the specimen with the LD perpendicular to the build direction, regardless of the energy density. This anisotropy was examined using crystal plasticity analysis with material parameters obtained through the fitting analysis of micrometre-scale specimens. The analysis results indicated that the anisotropic work-hardening behaviour related to the build direction was due to differences in the overall Schmid factor and the degree of lattice rotation, both of which stemmed from the texture. As the anisotropic mechanical properties observed in this study were insignificant, the application of the high-energy-density LPBF method to maraging steel is useful for repairing mechanical components.

The strength at 600 °C in fire-resistant steel exhibits a significant dependence on strain rate, likely attributed to a shift in deformation mechanism from slip deformation to grain boundary sliding (GBS) with increasing temperature. In this study, GBS was observed using a grid method, and the strain distribution introduced by a high-temperature tensile test in a carbon steel was visualized. The characteristics of the strain distribution in the grains composing the grain boundary where GBS occurred were discussed. Although the test temperature was 500 °C, high-temperature tensile tests with strain rates of 10^-3 and 10^-5 s^-1 orders were successfully conducted. Strain rate dependence of strength was comparably small at 500 °C. The discontinuous slide of the grid line at the grain boundary was observed after the tensile tests at 500 °C, regardless of strain rate, indicating that GBS occurred. The strain was distributed inhomogeneously during the high-temperature tensile tests. The grain boundary generating GBS lay 45 ° from the tensile direction and consisted of a pair of high- and low-strain grains. The m' value, which represents the ease of slip transfer between adjacent grains, was low at the grain boundaries generating GBS. This suggests that GBS was induced by pile-up dislocations at the grain boundary.

The detection of hydrogen that permeates the steel specimen is crucial in the study of the mechanism of hydrogen embrittlement. However, the applicable analytical methods have been limited. In this study, we developed an experimental system of photoelectron yield spectroscopy (PYS) measurements to detect hydrogen in steel. The PYS measurement can evaluate the work function of a specimen under atmospheric conditions by detecting photoelectrons emitted from its surface. The measurement system was applied to monitor the work function of the nickel-plated surface on the hydrogen detection side of iron during a hydrogen permeation test. The initial value of the work function was 4.72 eV, indicating the presence of nickel hydroxide on the detection side. It was found that the work function of the detection side decreased by 0.35 eV after the introduction of hydrogen. In addition, the shape of the photoelectron yield spectrum changed, indicating a mixed phase of the nickel hydroxide and other substances with a smaller work function. It demonstrated that the PYS can detect the hydrogen at the specimen surface and can be a powerful tool in the study of hydrogen permeation.

The intergranular crack growth in an Fe-25Cr-1.1N austenitic steel (in wt.%) was examined by in situ scanning electron microscopy and three-dimensional tomographic reconstruction based on Xe-focused-ion beam serial sectioning. The intergranular crack growth exhibited discontinuity, crack deflection/branching along {111}, and crack tip blunting. These features could be interpreted by considering the effects of planar dislocation slip that causes stress concentration at grain boundaries and Lomer-Cottrell sessile dislocations. The models explaining the intergranular cracking and associated crack deflection were proposed based on an assumption of intense planar slip and no cross slip until near-fracture, which was observed by in situ electron channeling contrast imaging under mechanical loading in the present study. In this context, because crack tip deformation is significantly constrained in the specimen interior (plain strain condition), the dislocation-driven intergranular crack growth occurred preferentially in the specimen interior, and subsequently, surface crack propagation occurs in a ligament portion. After blunting of the main crack tip, the coalescence of the main crack and planar-slip-induced brittle crack allows further crack growth.

By using finite element method for crystal plasticity, we investigated the accumulation behavior of dislocation and atomic vacancies introduced by non-uniform deformation in pure iron polycrystals. Dislocation density was calculated from the increment of plastic shear strain and spatial gradient of the slip systems for SS and GN dislocation densities. Vacancy density was calculated from the edge component of SS dislocation density and the incremental plastic shear strain by expanding the theory of Essmann and Mughrabi, in which atomic vacancies are released by the annihilation of edge dislocations, for each slip system. The cyclic loading analysis was performed under strain-controlled with 10 cycles between a tensile process up to 0.5 % nominal strain and a compressive process down to 0 %. For comparison, a monotonic loading analysis was also performed. The macroscopic mechanical responses were significantly different under the two conditions, and the work hardening rate under cyclic loading was less than half that under monotonic loading. The localization of plastic strain was more pronounced in the cyclic loading deformation than in the monotonic one. The low work hardening rate for cyclic loading deformation was attributed to the low accumulation of GN dislocations due to the relaxation of the plastic shear strain gradient caused by the load reversal. The average vacancy density was twice higher for monotonic loading deformation than for cyclic loading deformation. On the other hand, the maximum value of vacancy density was almost the same in both conditions, indicating that the cyclic loading deformation was more localized.

This study investigates the effect of Ca addition on the brittle-to-ductile transition (BDT) and grain boundary decohesion by S segregation in as-quenched low-carbon fully martensitic steel. Temperature dependence of the impact absorbed energy was examined in two kinds of steels with different Ca content (Ca-added steel and Ca-free steel). The BDT temperature of the fully martensitic steel was significantly decreased with the Ca addition. The temperature dependence of the 0.2% proof stress was measured to discuss the decrease in the BDT temperature based on shielding theory. The temperature dependence of 0.2% proof stress was compatible between the two steels, indicating that Ca addition did not affect the dislocation mobility regardless of the Ca content. Observations of brittle fracture surface revealed that intergranular fracture was prominent in the Ca-free steel, whereas it was suppressed in the Ca-added steel. Auger electron spectroscopy further revealed that S was segregated at prior austenite grain boundaries in the Ca-free steel. These results suggest that the improvement in low-temperature toughness in the Ca-added steel is attributed to the increase in surface energy for intergranular fracture, resulting from the suppression of S segregation by Ca addition.

The strain distribution developed during high-cycle fatigue bending tests exceeding 10⁶ cycles was visualized in 780 MPa high-strength steels with various microstructures using the digital image correlation (DIC) method for the secondary electron images of the specimen surface. The tensile stress was approximately identical among the steels with ferrite (F), bainite (B), and ferrite + pearlite (FP) microstructures. Microcracks were detected in the B and FP steels after the fatigue bending test, whereas no cracks were present in the F steel even after 10⁶ cycles. The strain distribution developed in the high-cycle fatigue bending test was successfully visualized using the DIC method for the first time. The strain was inhomogeneously distributed for up to 10² cycles even under an applied stress less than the yield stress. The average strain along the loading direction was approximately zero regardless of the number of cycles for all specimens. The standard deviation calculated from the strain histogram continuously increased with an increasing number of cycles for all steels. This suggests that strain gradually accumulated during the fatigue test. Microcracks tended to nucleate in high-strain regions. The plane of the microcrack was the slip plane, and its Schmid factor was high. Therefore, the microcracks were generated through intrusion and extrusion mechanisms with local slip deformation at the specimen surface.

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.

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.