The reducibility and mechanical properties of iron ore sinter in blast furnace is critical to effective plant operation. The reduction reaction of sinters progresses heterogeneously owing to microstructures with various mineral phases and pore networks. The reduction process was investigated by semi-microbeam synchrotron X-ray multimodal analysis. Heterogeneous chemical state evolution of Fe and trigger sites of crack formation were visualized using two-dimensional Fe K-edge X-ray absorption near-edge structure analysis and were discussed based on reduction gas transfer. The elemental composition map and X-ray diffraction microanalysis were also combined to reveal the microprocesses during the reduction, such as calcium ferrite decomposition and crystal grain growth.

As-quenched martensite in carbon steels needs to be tempered to restore ductility and toughness for practical applications. During tempering of martensite, microstructural evolutions induced by a series of reactions relevant to carbon diffusion is known to occur. In this study, multi-aspect characterization using advanced techniques such as in-situ neutron diffraction, transmission electron microscopy and three-dimensional atom probe tomography, was performed to investigate the changes in tetragonality, physical properties, microstructure and solute carbon content of high-carbon martensite, with an aim to clarify its low-temperature tempering behaviors. A Fe-0.78 mass%C binary alloy was austenitized and quenched to prepare the as-quenched martensite, followed by tempering via continuous heating at different rates. It was found that various reactions occurred sequentially during tempering, starting from the structure modulation generated by carbon clustering in the 0th stage, then followed by the precipitation of metastable η-carbide preferentially on dislocations in the 1st stage, towards the later decomposition of retained austenite, and precipitation of χ-carbide and cementite in the 2nd and 3rd stages, respectively. After analyzing the experimental results, a compressive residual stress with elastic anisotropy was confirmed in the retained austenite until the temperature range of its decomposition. In addition, the tetragonality and solute carbon content of martensite were found to be continuously decreased especially in the temperature range of the 1st stage. Compared with the tetragonality change of martensite during continuous heating, the lattice volume expansion induced by carbon was found to be more effective to accurately estimate the solute carbon content of tempered martensite.

Friction stir welding (FSW) was applied to a 10 mm-thick plate for the Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy. A sound FSW joint was obtained successfully without macro-defects such as groove-like defects and tunnel holes. However, small pores with diameters of 1–5 μm were formed owing to the wear of the FSW tool during the FSW. The decrease in the heat input suppressed the tool wear. Consequently, the distribution of small pores was limited to the border of the stir zone at the advancing side under smaller heat input conditions. The stir zone of the FSW specimen produced at 125 rpm showed a higher tensile strength of 759 MPa owing to the grain refinement and the high elongation of 50% compared with the base metal. In addition, the stir zone exhibited a remarkable fatigue life of 9,723 cycles. This was higher than that of the base metal (8,908 cycles). Grain refinement occurred by discontinuous dynamic recrystallization (DDRX) via high-angle boundary bulging and direct nucleation in the high-dislocation area. The increase in the heat input suppressed the DDRX owing to the promotion of dynamic recovery.

Hydrogen absorption characteristics and mechanical properties in hydrogen environment of cementite were evaluated by low-temperature thermal desorption analysis and in-situ microbending tests during cathodic hydrogen charging using bulk cementite plates obtained through a vacuum carburizing process. In the low-temperature thermal desorption analysis, no hydrogen desorption was identified up to 1073 K. In the microbending test, notched microcantilevers experienced cleavage fracture in an elastic deformation range in air. The cathodic hydrogen charging increased fracture load (i.e., fracture toughness) with appearance of plasticity while it did not change the fracture surface morphology and Young's modulus. In the present hydrogen charging conditions, the hydrogen atoms are present only near the specimen surface because of high hydrogen migration energy in the cementite. It seems that no hydrogen desorption is detected because the hydrogen atoms are absent in most regions of the specimens. The invariance of the Young's modulus and the fracture surface morphology can be explained by the same reason. On the other hand, it is considered that the fracture toughness is improved because the hydrogen atoms charged near notch bottom of the microcantilever enhance dislocation nucleation and glide, and cause blunting of the notch during the bending.

Surface microstructures were investigated in pure iron and Fe-1mass%M (M = Mn, Cr, Al, Si) alloys gaseous-nitrided at 1123 K and quenching to reveal the alloying effects on surface hardening by nitrogen (N) martensite. Thicker hardened layers with higher hardness than pure iron were obtained in the Mn-added alloys whereas the additions of Si and Al lead to increase the surface hardness with reduction of the hardened layer thickness. On the other hand, adding Cr decreases both the hardness and thickness of the hardened layer. No precipitation of alloy nitride is observed in austenite nor internal ferrite region in Mn-added alloy. Meanwhile, CrN(B1) and AlN(wurtzite) particles are dispersed in ferrite and austenite regions in the Cr- and Al-added alloys, respectively. Unlike those alloys, (austenite+ α-Si₃N₄) lamellar structure is formed in the Si-added alloy followed by martensite transformation of the high temperature austenite during quenching. Phase diagrams of Fe-M-N systems can consistently describe those alloying effects on the microstructure evolution.

In this work, the effect of the slag phase on dephosphorization in BOF (basic oxygen furnace) is explored by industrial practical experiments. The research results show that the final dephosphorization rate is related to the phase of slag rather than basicity and T.Fe content in slag. The correlation between the dephosphorization ability of slag and the proportion of liquid phase (phase B) and phosphorus-containing solid solution phase (phase A) in the slag is much higher than that of RO (MgO·FeO, MnO·FeO) phase (phase C). The phosphorous partition fitting by using the minimum second-order multiplication proposed in this work is compared with the equations suggested by Healy and Ogawa. The comparison result shows that the dephosphorization ability of slag can be well predicted by analyzing the proportion of liquid phase and phosphorus-containing solid solution phase in slag, and the mechanism of the influence of the slag phase on dephosphorization is discussed.

Deformation-induced martensitic transformation (DIMT) during tensile or compressive deformations of the bainitic steels with various carbon content (0.15 %C, 0.25 %C, 0.62 %C) was studied. The initial microstructure before the deformation tests was prepared by the austempering at 400 °C to obtain bainitic structure consisting of bainitic ferrite and retained austenite. The volume fraction of the retained austenite was larger in the bainitic steel with the larger carbon content. In all of the bainitic steels, the tensile deformation exhibited larger work hardening than the compression. This difference indicates the suppression of the DIMT at the compression, and actually the measurements of electron back scattering diffraction (EBSD) confirmed the less reduction of retained austenite at the compression of all the bainitic steels. Additionally, the steel with the highest carbon content was examined by in situ neutron diffraction and clarified the difference similar to that obtained by the EBSD measurement. The regression of the relation between the fraction of austenite and applied strain with the conventional empirical equation revealed that the kinetic of DIMT is strongly dependent with the stress polarity, but not significantly changed by the carbon content. The mechanism of the DIMT dependence of the stress polarity was discussed with the deformation texture and the crystallographic orientation dependence of DIMT.

Steel parts fractured during actual use were investigated, and the mechanism of fracture for some of them was attributed to hydrogen embrittlement. This article describes a summary of the observation results fractured by hydrogen embrittlement. Fracture occurred under tensile stress within the elastic limit including residual stress. Most of the parts had a martensitic microstructure, and intergranular fracture surfaces were always observed in martensitic steels at least on part of the fracture surface. This observation indicates the importance of the role of hydrogen on the prior austenite grain boundary in hydrogen embrittlement.

A CFD-PBM coupled mathematical model was proposed to describe the bubble breakup and coalescence behavior in gas-stirred ladle based on the Eulerian-Eulerian approach. The effect of turbulence shear collision, bubble floating velocity difference collision and turbulence random collision on the bubble coalescence behavior were considered; the sensitivity of three typical breakup model including Luo model, Lehr model and Laakkonen model on bubble size distribution were considered. The result showed that the turbulence random collision and bubble floating velocity difference collision were the main mechanism to affect bubble coalescence behavior, while the effect of turbulence shear collision can be ignored. The bubble size distribution predicted by Lehr model was different with other two models and the measured data, because it highly predicted the bubble breakup frequency in current system. The effect of Luo model and Laakkonen model on bubble size distribution were so small that the bubble size distribution is mainly dominated by bubble coalescence behavior. Bubble-induced turbulence would affect the bubble breakup and coalescence behavior, it would promote the bubble coalescence behavior, the volume fraction of large bubbles increased significantly in this work. The size distribution predicted by the present model agree well with the measured data in the water model.

In this paper, Softening & Melting experiments with different charge ratios are carried out and compared with typical slag phase properties. The relationship between the charge ratio and the softening & melting properties is non-linear correlation. The liquid phase generation process of the slag corresponds to the softening zone and the viscosity change process of the slag phase after the liquid phase is fully generated corresponds to the melting zone. In the softening interval the properties of FeO-CaO-SiO₂ system play a dominant role, while in the melting interval the properties of FeO-CaO-Al₂O₃-SiO₂ system play a dominant role. This provides ideas for the study of the limiting links in the soft fusion zone of complex systems.

To investigate mechanism of hydrogen-induced intergranular fractures, low energy positron beams with different energies were applied to hydrogen-induced intergranular fracture surfaces of 80Ni-20Cr alloy, and depth distributions of the lattice defects were evaluated by Doppler broadening spectroscopy and positron annihilation lifetime one. At least at depths between 0.1 μm and 1 μm from the fracture surface, a large number of the lattice defects were homogeneously distributed. Both the dislocation density and monovacancy-equivalent vacancy-type defect one were around ten-times as large as inside the grains. On the other hand, the absolute value of the monovacancy-equivalent vacancy-type defect density was about 9 appm, and obviously not large enough to cause strength reduction and fractures. It was suggested that stress concentration and disordered structures formation at and on the grain boundaries due to hydrogen-enhanced dislocation nucleation, and reduction in the grain boundary cohesive energy due to the trapped hydrogen atoms jointly cause the intergranular fractures.

The hydrogen embrittlement (HE) resistance of a fine-grained equiatomic CoCrFeNi high-entropy alloy (HEA) is investigated via tensile testing under electrochemical H charging. The HE behavior is compared with that of HEA specimens charged with 100 MPa of H gas. The fine-grained HEA shows ˃ 40% elongation with a tensile strength of ~800 MPa under electrochemical H charging. Meanwhile, H gas-charged specimens with a uniform distribution of H show deformation twin-related intergranular cracks, whose initiation length decreases owing to grain refinement. Such small cracks, which feature blunted tips, do not significantly affect the fracture of the specimens. The electrochemically H-charged specimens exhibit numerous surface cracks because of their higher surface H content compared with that of the H gas-charged specimens. Nevertheless, similar to the case of the H gas-charged specimens, most of the cracks do not propagate significantly. In conclusion, fine-grained HEA exhibits remarkable resistance to H-related crack growth.

The hydrogen desorption behaviors of the SUS304 and SUS316L austenitic stainless steels during deformation at ambient temperature were investigated using a tensile test machine in a vacuum chamber equipped with a mass spectrometer. Obvious hydrogen desorption was detected only in the SUS304 steel, which exhibited a distinct martensitic transformation. Because the hydrogen desorption rate in SUS304 decreased when deformation stopped, a significant factor causing transformation-induced hydrogen desorption was an increase in martensite fraction during plastic deformation. Furthermore, hydrogen promoted both martensitic transformations to ε and to α′, which assists the hydrogen desorption. These results indicate the presence of synergistic interactions between the hydrogen uptake/diffusion and martensitic transformation. In contrast, SUS316L steel showed no martensitic transformation and exhibited hydrogen-assisted deformation twinning. No significant increase in hydrogen desorption was observed during plastic deformation. This result indicates that deformation twinning has no effect on hydrogen diffusion/desorption.

Ensuring the safe and steady operation of a blast furnace hinges on accurate predictions of silicon content. However, these predictions often fall short, proving unreliable and imprecise, particularly in unstable furnace conditions and in the face of substantial data swings. The wide variances and low reliability of silicon concentration predictions make them unsuitable as a reference for daily blast furnace maintenance and adjustments. To counter this engineering challenge, we introduce a novel silicon content prediction technique: the Gated Recurrent Unit Network Quantile Regression (GRUQR). This method amalgamates the Gated Recurrent Unit Network with quantile regression to refine the silicon content prediction model. Our approach first leverages GRUQR to anticipate the silicon content in molten iron across various quantiles. Subsequently, we scrutinize the patterns of silicon content changes and identify the optimal quantile for silicon content under different furnace conditions. We also discuss the reliability of our silicon concentration forecasts and present confidence intervals at various levels. To validate the effectiveness of the proposed GRUQR method, we employ real-world data from a Chinese blast furnace ironmaking process. These prediction results serve as a reliable reference for furnace operators, enabling them to determine the furnace temperature under challenging conditions.

In this paper, the cause of bending fracture of high-strength fasteners after electroplating and the effect of T.Ca, T.O, and T.S contents in Al-bearing high carbon steel on the type, size, and shape of inclusions were studied. The fracture surface of fastener exhibited stepwise cracks and a lot of intergranular facets associated with ductile tearing, which were typical of hydrogen embrittlement. The agminated elongated MnS inclusions in the center of fasteners could aggravate the susceptibility of hydrogen embrittlement. Thermodynamic calculation showed that the ideal modification of MnS inclusions needed to meet the conditions that the inclusions in molten steel before solidification should be C₁₂A₇, and the mass ratio of CaS/MnS at the end of solidification was 23.7. Based on the above conditions, the quantitative relationships between the contents of T.Ca, T.O, and T.S were determined. Among them, the T.O content in steel increased with the increase of S content. Therefore, to suppress the formation of type B-stringer shaped inclusions and improve the cleanliness of steel, the sulfur content in steel should not be high.

In order to make better use of sinter return ore and iron oxide scale to achieve effective pre-dephosphorization of molten iron, the effects of final slag morphology, mineral phase structure and polymerization degree on pre-dephosphorization were studied by theoretical analysis, XRD, SEM-EDS, Raman and FTIR spectra. The results show that when the proportion of sinter return ore is less than 20%, the impact on dephosphorization is relatively small. The structural analysis of dephosphorization final slag shows that the increase of sinter return ore will lead to the decrease of phosphorus content in phosphorus-rich phase and the increase of RO phase and iron-rich phase in slag. O^2- destroys P-O-P bond more than Si-O-Si bond. When the phosphorus entering the slag decreases, the content of Q⁰(Si) structure decreases, and silicon tends to exist in the form of higher polymerization degree. With the increase of the proportion of sinter return ore, the structure of [FeO₆]^9- in slag increases, which is not conducive to the migration of phosphorus. When the oxidant is only sinter return ore, the proportion of Q¹(Si) and P-O-Si structure in slag increases obviously, and the evolution of silicate structure is the main reason for the change of polymerization degree of slag. This study can provide theoretical reference and technical basis for the effective utilization of sinter return ore and the reduction of production cost in iron and steel enterprises.

This paper presents an overview of our recent works on the effects of hydrogen on γ-ε martensitic transformations in steels. The study first discusses how hydrogen impacts these transformations. While hydrogen suppresses thermally-induced γ–ε martensitic transformation, it increases the fraction and number density of deformation-induced ε-martensite and decreases its thickness. Secondly, we discuss the effects of γ–ε martensitic transformations on hydrogen kinetics. The study also highlights the significance of low hydrogen diffusivity in the hexagonal-close-packed (HCP) lattice of pure iron, demonstrating the effectiveness of ε-martensite in resisting hydrogen. Moreover, the characteristic behavior of the HCP phase-related diffusionless transformation from a hydride is discussed. We believe that this overview will assist in developing hydrogen-resistant steels and in exploring new microstructural control concepts using hydrogen.

Hydrogen-enriched blast furnace (BF) operation is currently being assessed to mitigate greenhouse gas emissions while the steelmaking industry transitions to low carbon emission technologies. Increasing the usage of lump ore in the BF also presents opportunity to decrease carbon emissions, as it can be directly charged to the furnace without agglomeration. Use of lump ore in modern blast furnace operations is facilitated by high temperature interactions with sinter. With more emphasis on hydrogen enrichment in BF operations, the behaviour of lump and sinter mixed burdens must be characterised under new conditions. In this study, 15% hydrogen is added to the standard gas conditions of a Softening and Melting (S&M) apparatus (replacing nitrogen). Analysis of auxiliary reactions such as the Boudouard Reaction and the Water-Gas Shift Reaction is presented and their impact on burden reduction and performance assessed. Results indicate that with the inclusion of hydrogen, the performance of sinter burden deteriorates, while lump burden shows significant improvement. Interaction between sinter and lump still occurred with the inclusion of hydrogen in the gas, and the mixed burden behaviour of 20% lump and 80% sinter fell between that of the individual burdens. From interrupted experiments, it is noted at high degrees of reduction, the lump burden forms a solid metallic layer which maintains its interparticle voidage at high temperatures, supressing exudation of liquid slag.

Thermodynamic data of chromium oxide activities in molten slags are important for improvement of the reduction treatment process of chromium oxides in stainless steel slags. The chemical equilibrium experiments were conducted in the present work to measure the activity coefficients of chromium oxides in the CaO-CaF₂-MgO-Al₂O₃-SiO₂-MnO-CrO_x slags at 1823 K (1550°C) under oxygen partial pressure of 2.57×10^-11 atm with different chromium oxides contents. It is found that no solid phase appears in the slag when the initial CrO_1.5 mole fraction of the slag is no more than 0.00390 (0.5 mass%), which could ensure the accuracy of the chromium valence analysis. The activity coefficient of CrO keeps almost constant with the increase of mole fraction of total chromium oxides, which indicates that CrO obeys Henry's law under the present conditions. The activity coefficient of CrO_1.5 firstly levels off and then rises sharply when the mole fraction of total chromium oxides reaches 0.00198, beyond this point, activity coefficient of CrO_1.5 keeps relatively constant again. Through the Raman spectroscopy analysis, the sharp increase of the activity coefficient of CrO_1.5 is assumed to be caused by the transformation of CrO_1.5 from network former to network modifier.

High-strength low-to-medium carbon martensitic steel is increasingly used in the automobile industry. This study investigated the room temperature aging behavior of as-quenched autotempered Fe-C lath martensitic steels (C: 0.07-0.77 mass%) using kinetic analysis of hardness change and interrupted atom probe (AP) analysis to clarify the dominating factor of hardening. Age-hardening at 23 °C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content of less than 0.25 mass%. The AP and kinetic analyses of hardness evolution indicated that the growth of carbon clusters at dislocations dominates the hardening of martensite. The maximum hardness increment in lath martensite increased with initial excess solute carbon C_sol in the matrix, but the increment in unit C_sol was smaller than that in carbon-supersaturated ferrite. The smaller hardness increase in martensite may indicate the concurrent softening due to the relaxation of the residual lattice strain in martensite by carbon clustering. Interrupted AP analysis of the prolonged aging over two years indicated that the transformation from carbon clusters to iron carbides occurs via an in-situ transformation of the clusters. The microscopic heterogeneity in carbon distribution in the order of martensite blocks and the gradual decrease in excess C_sol during room temperature aging were also confirmed by AP analysis. The persistence of the heterogeneity and excess solute carbon in the martensite matrix after aging and tempering is also discussed.

i>In-situ neutron diffraction measurements were performed on Fe-33Ni-0.004C alloy (33Ni alloy) and Fe-27Ni-0.5C alloy (27Ni-0.5C alloy) during cooling from room temperature to the cryogenic temperature (4 K) to evaluate changes in the lattice constants of austenite and martensite, and changes in the tetragonality of martensite due to thermally induced martensitic transformation. As the martensitic transformation progressed, the lattice constants of austenite in both alloys deviated to smaller values than those predicted considering the thermal shrinkage, accompanied by an increase in the full width at half maximum of austenite. The fresh martensite formed in both alloys had a body-centered tetragonal (BCT) structure, regardless of the carbon content. The tetragonality of martensite decreased with progressive martensitic transformation during cooling in the 33Ni alloy, but was almost constant in the 27Ni-0.5C alloy. This suggests that carbon is necessary to maintain the tetragonality of martensite during cooling. The tetragonality of martensite in the 27Ni-0.5C alloy decreased during room temperature aging because of carbon mobility.

In recent years, the gangue component in iron ore deposits has increased owing to the increasing scarcity of high-grade iron ore. Such high levels of gangue causes a loss in strength of the granules used in the sintering process and a decrease in operating efficiency. In this study, the granule strength is compared by applying two types of samples, namely commercial iron ore and the gangue-free iron ore prepared from reagents, to both nuclear ore and fine ore granules. The experimental results confirm that gangue components have little effect on the wet compressive strength. However, the dry compressive strength increases with the addition of gangue in the fine ore. Granules of gangue-free fine ore exhibit significantly lower compressive strength. These results are attributed to the difference in the Coulomb repulsion force caused by the zeta potential of the fine ore.

Ladle bottom-blowing is one of the most important ways to remove the inclusions in the molten steel during steel making process. Many works have been reported to optimize the bottom blowing parameters by numerical simulations. However, it is a challenge to capture the collision between the inclusions and the bubbles during a discrete time step, which may lead to the underestimation of the inclusion removal rate. In this work, a program based on the user-define-function (UDF) in ANSYS FLUENT was developed to solve the problem above. The bottom blowing parameters of the ladle were optimized based on the developed novel model.

The cyclic fatigue, dwell fatigue and room temperature creep properties were evaluated in three types of Ti-6Al-4V forged bar samples having different micro-texture-regions (MTR) and tensile properties in the loading direction. In the S-N curve where the stress(σ_nor) was normalized by 0.2%-proof-stress, the fatigue lives of all samples were almost the same, whereas the dwell fatigue lives were not the same. So the ratio of the cyclic fatigue life to dwell fatigue life (dwell debit) changed to 2–60. In cyclic fatigue the initiation site was a facet of 1–2 α grains, and the fracture surface was typical. In dwell fatigue and creep, on the other hand, facet and dimple regions were confirmed. In addition, the facet region consisted of initiation facets of 1–2 α grains and the propagation facets which were the majority of the facet region. Initiation facets in dwell fatigue occurred earlier than 25% of the life ratio, and the angle between the c-axis of the α grains with the initiation facets and loading direction was 15–55°. The propagation facets were the MTR in which the angle between the c-axis of the α grains and loading direction was 30° or less. The lengths of the facet regions were proportional to the MTR size. In dwell fatigue, the larger the σ_nor or MTR size, the larger was the dwell debit. Therefore, the MTR size was considered the dominant factor determining the dwell fatigue life.

Because the CO gas is usually used as the reduction gas in the blast furnace process, a huge CO₂ gas has been emitted during the ironmaking process. Therefore, H₂ reduction gas has been proposed as a potential alternative to the CO gas for achieving carbon neutrality. However, the diffusion behaviors of CO and H₂ gases inside the iron-oxide particle are markedly different due to the higher gas diffusivity of H₂ gas. The reaction surface is observed in the CO reduction whereas the H₂ reduction has a broadly-reaction area. The conventional reduction analysis models were suitable for use in the CO reduction, as they assumed an exponential gas diffusion behavior through the certain reaction surface inside the particle. However, exponential diffusion is not sufficient to analyze the broad diffusion aspect of H₂ gas. In this study, the H₂-based reduction reactions is applied to the 3D diffusion model, which can accurately analyze the broad H₂ diffusion behavior. The gas components considered were the CO-CO₂-H₂-H₂O-N₂, considering the conditions of the blast furnace. The necessity of the 3D diffusion model was analyzed by comparing the H₂ reduction distributions with those obtained using the shrinking core model. The intra-particle distribution for reducing iron oxide particles, which have pellet and sintered ore shapes, were analyzed in CO-H₂ and CO-CO₂-H₂-H₂O-N₂ gas to clarify the impact of H₂ on reduction behavior. As the results, the presence of H₂ gas affected the effective gas diffusivity of the gas mixture, the reduction rate increased with the H₂ content.

A modified continuum composite model was utilized to analyze the stress–strain behavior of as-quenched steels with a fully martensitic microstructure. The model was employed to express the stress–strain curves obtained by a simple tensile test and those obtained by forward and backward loading using a simple shear test machine. The study confirmed that the model can represent the stress–strain behavior under both forward and backward deformation. In addition, the evolution of local strain distributions during plastic deformation was investigated using a digital image correlation method. The strain distributions and their evolution during deformation were qualitatively represented using this model. The discrepancies between the model calculations and experiments are due to the limitations of the iso-work assumption and the impact of slip deformation on the macroscopic work-hardening behavior of martensitic steels. Highly strain-concentrated regions aligned along the longitudinal direction of the lath and block, known as in-lath-plane slips, may not play an important role in the work-hardening behavior of as-quenched martensitic steels. However, the other slips, namely the out-of-lath slip, may play a significant role in the work hardening of as-quenched martensitic steels.

In order to understand microstructural changes in 9Cr-1Mo-V-Nb weld metal after long term use, microstructure and precipitates distribution before and after aging at 1013K were investigated. In the weld metal, regions with coarse or fine prior austenite grains were observed due to thermal cycle during welding. In the coarse grain region, precipitate particles inferred to M₂₃C₆ were densely located on grain boundaries, however, in the fine grain regions, they were sparsely observed not only on grain boundaries but also inside grains. Post weld heat treatment (1013K/7.7h) followed by aging (1013K/100h) led to ferrite grains formation in the fine grain region. EBSD analysis implied that dislocation density in ferrite grains was low. After the aging, mean diameter of particles became coarser and interparticle spacing became sparser in the fine grain region than in the coarse grain region. On the other hand, dislocation density calculated by hardness in martensite structure was almost no deference between these regions before and after the aging. Therefore, it was suggested that ferrite grains were formed because pinning energy by precipitate particles locally reduced in the fine grain region.

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

This study investigated the low-temperature fracture toughness of martensite and bainite with various Bain unit sizes. The three-point bending tests revealed that the apparent fracture toughness increased with decreasing the Bain unit size. We also found that even when the carbide size and distribution were almost the same, the apparent fracture toughness of tempered martensite with Bain unit size of 2.5 μm was much higher than that of bainite with Bain unit size of 16.2 μm. The propagation of micro-crack stopped at the Bain unit boundaries when the Bain unit size was small. The additional load was necessary for further propagation of crack which stopped at the Bain unit boundaries, leading to the improvement of fracture toughness. The critical local fracture toughness corresponding to the propagation of crack across the Bain unit boundaries was estimated at 1.04 MPa m^1/2 by finite element simulations. Based on this value, we proposed that the Bain unit boundary whose interval was less than 9.4 μm could become obstacle for the crack propagation after penetrating matrix / carbide boundary.

In this paper, the influence of tempering temperatures on microstructures and tensile properties of a Cr-N alloyed medium Mn martensitic steel was studied. The microstructures formed after the tempering below 400 °C are composed of recovered martensite as the matrix, ultrafine retained austenite (RA) and carbonitrides. The tempering at 100 °C led to the best combination of 2080 MPa ultrahigh ultimate tensile strength (UTS) and 15% total elongation (TE), which is attributed to the prominent strain hardening capacity caused by both the gradually release of internal stress and the pronounced austenite-to-martensite transformation. The tempering at 400 °C resulted in the rapid increase of yield strength (YS) by ~500 MPa due to the relief of internal tensile stress and annihilation of dislocations and the best ductility because it produced the most stable RA grains with the highest C concentration for a sustainable austenite-to-martensite transformation over the large plastic straining. The further increase of temperature to 650 °C caused ferrite formed, which decrease both YS and strain hardening rate, leading to the lowest UTS. Moreover, it was found that higher N content increased YS but had little influence on both UTS and TE because it mainly contributed to enhanced precipitation of carbonitrides. It is then concluded that the strength and ductility of medium Mn martensitic steel could be increased by increasing the strain hardening capacity through tailoring both the internal stress in martensite and the mechanical stability of RA via a proper tempering treatment.

Inhomogeneity in microstructures along the thickness direction in the stir zone of duplex stainless steel (SUS329J4L) welded at rotational rates of 275, 400, and 800 rpm was investigated using the electron backscattered diffraction method. Changes in the volume fractions and average grain sizes of ferrite and austenite along the thickness direction may reflect the temperature gradient along the thickness direction. However, near the top surface, significant grain refinement occurred, presumably owing to the introduction of additional strain by the shoulder. The kernel average misorientation (KAM) values in the stir zone were higher in austenite than in ferrite along all thickness directions, which is inferred to be related to the difference in dynamic recrystallization behavior governed by stacking fault energy, which is lower in the austenite phase than in the ferrite phase. The layer thickness per unit length of the layered structure became smaller than that of the base metal as the rotational rate of friction stir welding (FSW) was reduced to 275 rpm, which implies that new grains nucleated during FSW. Furthermore, some ferrite grains nucleated at the austenite/austenite grain boundaries, satisfying the Kurdjumov-Sachs orientation relationship. FSW is assumed to promote the nucleation of new grains with different phases, probably because of the stirring effect of the elements by FSW. In a duplex structure formed in the stir zone of FSW, a linear relationship between the ferrite and austenite grain sizes was found to hold irrespective of the rotational rate.

The strain distribution and microstructure at the crack nucleation sites in martensitic steel with fine- and coarse-prior austenite grains subjected to tensile deformation were characterized using the digital image correlation method on replica films. Although the tensile properties of the fine- and coarse-prior austenite grain specimens were approximately identical, the total strain was certainly improved in the coarse-prior austenite grain specimen. The crack size increased with the coarsening prior austenite grains, whereas the number of cracks decreased. An inhomogeneous strain was introduced in both the specimens by tensile deformation. The accumulated strain when crack nucleates was approximately the same in both specimens, independent of the prior austenite grain size. In low-strain regions, there were no cracks even though the accumulated strain was comparable to that when crack nucleates in high-strain regions. The strain at the crack nucleation sites was high even before crack nucleation occurred. Cracks primarily nucleated on packet and prior austenite grain boundaries, even in the coarse-prior austenite grain specimen, which confirmed that the prior austenite grain boundary should be a preferential crack nucleation site. It can be concluded that the high local strain and the presence of packet or prior austenite grain boundaries are responsible for crack nucleation in martensitic steel subjected to tensile deformation.

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated using a newly developed hydrogen visualization technique with an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. Hydrogen permeation through the steel sheet could be monitored via the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated based on changes in the brightness of the Ir complex. Additionally, this hydrogen visualization technique was applied to a stretch-formed steel sheet using a hemispherical punch to simulate the press-forming of automotive structural components. The hydrogen breakthrough time around the top of the specimen increased and then decreased as the distance from the top increased. Based on the plastic strain distribution of the specimen calculated using the finite element method, the hydrogen breakthrough time increased with the plastic strain. The introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, thus increasing the hydrogen breakthrough time.

During peritectic steel continuous casting, mold flux properties and flux film structures play significant roles in controlling slab quality. In this study, mold fluxes and flux films for casting peritectic steel slab were obtained using mineral raw materials such as quartz, wollastonite, fluorite, soda ash and others. The effects of mineral raw materials on mold flux properties and flux film structures were investigated through the measurement of melting point, viscosity, crystallization temperature, critical cooling rate, crystallization ratio and crystalline phase content. The results indicated that with increasing the quartz addition (16 to 24 mass%) and the wollastonite addition (11 to 19 mass%) in mineral raw materials, the melting point, viscosity and wollastonite content of flux film increased, while the crystallization temperature, critical cooling rate, crystallization ratio and cuspidine content of flux film decreased. The melting point, viscosity and wollastonite content of flux film reduced with increasing the fluorite addition (8 to 16 mass%) and soda ash addition (10 to 18 mass%) in mineral raw materials. Furthermore, compared with soda ash, the fluorite predominantly enhanced the crystallization temperature, critical cooling rate, crystallization ratio, cuspidine content of flux film. In addition, it was showed that the heat transfer performance and the slab quality might be primarily attribute to the crystallization ratio and cuspidine content of flux film. These results provided a theoretical foundation for optimizing the mold flux of the peritectic steel and were vital to improving the slab quality.

The quasi-particle structure significantly affects the combustion heat and mass transfer process. To investigate the influence of the solid fuel existence state in iron ore sintering on the combustion behavior and CO and NO emission characteristics, four kinds of quasi-particles with different structures were prepared by using pure analytical reagents and coke for non-isothermal combustion experiments and CO and NO emission intensity detection. The following results were obtained. The S-type adhesive layer increases the diffusion resistance of the gas phase, resulting in the prolonged residence time of CO and NO in quasi-particles. The strong exothermic reduction reaction of CO-NO was promoted, the combustion efficiency was significantly improved, and the CO and NO emission concentrations were greatly reduced. The P-type had fines cluster structure, and the gas diffusion channel was easily blocked by the formed high-temperature liquid phase. The gas phase diffusion resistance was increased, the gas phase reaction between CO and O₂ and NO was promoted, the CO and NO generation rates were reduced, the heat release was increased. The all-around performance of the S'-type and the C-type was the worst. The gas phase products CO and NO generated by combustion were easy to escape quickly with the external airflow, and the emission concentration was higher.

The fatigue crack propagation properties of newly-developed SM490 grade steels were investigated in comparison with a conventional steel of the same grade. The fatigue crack propagation rate of the developed steel in Region II of the da/dN-ΔK relationship was suppressed to about 1/2 that of the conventional steel, and its ΔK_th value was more than twice as large as in the conventional steel. However, fatigue crack resistance for long crack propagation does not necessarily improve the fatigue life in a condition of increasing ΔK from a small defect, which is usually detected in practical fatigue damage in actual structures in service. The developed steels were subjected to surface crack propagation tests using specimens with artificial small defects to examine their potential under more practical conditions. The fatigue life of the developed steel was about three times longer than that of the conventional steel. A detailed analysis of the surface crack propagation revealed crack propagation below ΔK_th only in the developed steels, which suggested the so-called "short crack regime" in a fatigue crack. The crack propagation from a surface defect that deviated from long crack behavior was convincingly explained by the corrected threshold using the R-curve model of a short crack proposed in the previous literature. Based on the experimental fatigue life improvement and its analytical estimation of the propagation resistance in the short crack regime, the effect of the ΔK_th value for a long crack in the initial propagation stage just after fatigue crack initiation was discussed.

The strain distribution in the 9mass%Ni steel introduced by tensile deformation at cryogenic temperature was visualized using a digital image correlation method, and the relationship between the strain distribution and the microstructure of the steel was systematically investigated. Based on the obtained results, the factors that influence the strain distribution were discussed. In the present 9mass%Ni steel, regions consisting of tempered- and fresh-martensite and austenite (TFMA) were embedded in the tempered martensite matrix. The volume fraction of the retained austenite varied along the normal direction of the hot-rolled plates, indicating that Ni segregation occurred during the manufacturing process. As the tensile stress increased, the total elongation remained constant with decreasing temperature. Strain was introduced inhomogeneously via tensile deformation at 77 K. The high- and low-strain regions tended to be distributed in a unit of the block, indicating that the deformability differed among the blocks. The average strain in the block (ε_block) was strongly correlated with the Schmid factor of the slip system on the habit plane (SF_habit) and area fraction of TFMA in a block (A_MA). The least absolute shrinkage and selection operator regression revealed that the contributions of SF_habit and A_MA to ε_block were nearly equal. Therefore, the deformability of the block in the 9mass%Ni steel is dominated by SF_habit and A_TFMA.

The hearth liquid level is essential for optimizing the tapping strategies. Considering that the tapping strategy is crucial for the stable operation and longer campaign life of a blast furnace, a hearth liquid level monitoring (LLM) system is developed by using the strain gauge method together with finite element analysis. Since 2020, the real-time LLM system has been successfully implemented at Taiwan's China Steel Corporation (CSC) on three blast furnaces (BFs): two (BF3, BF4) have the spray-cooled hearths with four tapholes, and the other (BF2) has a stave-cooled hearth with two tapholes.

Specimens with different microstructures (bainite, as-quenched martensite, and tempered martensite) were fabricated using a Fe-0.4C-1.5Si-2Mn steel sheet, and the pitting corrosion resistances of these microstructures were compared. Retained austenite was barely detected in the X-ray diffraction analysis. The Vickers hardness values of the microstructures were ordered as (high) as-quenched martensite > tempered martensite ≈ bainite in the 325°C-austempered specimen > bainite in the 425°C-austempered specimen (low). The pitting corrosion resistance of each microstructure was evaluated by potentiodynamic polarization in boric-borate buffer solutions containing NaCl (pH 8.0) under naturally aerated conditions. The pitting corrosion resistances of the microstructures were ordered as (high) as-quenched martensite > bainite in the 325°C-austempered specimen > tempered martensite > bainite in the 425°C-austempered specimen (low). The lower active dissolution rates of the microstructures were determined to provide superior pitting corrosion resistance.

The reverse transformation behavior during heating in Fe-10%Mn-0.1%C (mass%) martensitic alloy consisting of α'-martensite, ε-martensite and retained austenite was investigated using the in-situ neutron diffraction. When the temperature was elevated with a heating rate of 10 K/s, the ε→γ reverse transformation occurred first at the temperature range of 535–712 K, where Fe and Mn hardly diffused. In the temperature range where the ε→γ reverse transformation occurred, the full width at half maximum of the 200γ peak increased, indicating that the austenite reversed from ε-martensite contains high-density dislocations. In addition, the transformation temperature hardly depends on the heating rate and the crystal orientation of the reversed austenite was identical to that of the prior austenite (austenite memory), which suggests that the ε→γ reverse transformation would proceed through the displacive mechanism. After completion of the ε→γ transformation, the α'→γ reverse transformation occurred at the temperature range of 842–950 K. When the heating rate is low (<10 K/s), the reverse transformation start temperature significantly depends on the heating rate. It could be because the diffusional reverse transformation accompanying the repartitioning of Mn occurs. On the other hand, a higher heating rate (≥10 K/s) resulted in the disappearance of the heating rate dependence. This was probably due to the change in the transformation mechanism to the massive-type transformation, which is diffusional transformation without repartitioning of Mn.

The dependence of crack initiation sites and main factors causing hydrogen embrittlement fracture on carbide precipitation states has been investigated for tempered martensitic steels with the same tensile strength of 1450 MPa. Notched specimens charged with hydrogen were stressed until just before fracture and subsequently unloaded. The crack initiation site exhibited intergranular (IG) fracture at 21 μm ahead of the notch tip as observed by scanning electron microscopy (SEM) for 0.28% Si specimens with plate-like carbide precipitates on prior austenite (γ) grain boundaries. This crack initiation site corresponded to the vicinity of the maximum principal stress position as analyzed by a finite element method (FEM). The initiation site corresponded to the triple junction of prior γ grain boundaries as analyzed by electron backscattered diffraction (EBSD). In contrast, the crack initiation site exhibited quasi-cleavage (QC) fracture at the notch tip for 1.88% Si specimens with fine and thin carbide particles in the grains. This crack initiation site corresponded to the maximum equivalent plastic strain site obtained by FEM. Additionally, the crack initiated on the inside of prior γ grain boundaries and propagated along the {011} slip plane with higher kernel average misorientation (KAM) values as analyzed by EBSD. These findings indicate that differences in carbide precipitation states changed the crack initiation sites and fracture morphologies involved in hydrogen embrittlement depending on mechanical factors such as stress and strain and microstructural factors.

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550 ^oC for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550 ^oC, resulting in enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

Many vacancy-type defects (vacancy, vacancy clusters, and hydrogen-vacancy complexes) are generated in metals by plastic deformation in hydrogen environments. In this study, we use extensive molecular dynamics calculations based on a highly accurate interatomic potential to examine how vacancy-type defects affect the mobilities of edge dislocations in α-iron at a temperature range of 300–500 K and a dislocation speed ????d range of 0.1–10 m/s. Under all conditions, the edge dislocation absorbs the vacancies along the slip plane and causes them to migrate with the edge dislocation. Although the necessary shear stress to glide edge dislocation in α-iron containing vacancy increases with dislocation speed, the effect is small compared to the hydrogen effects. The dislocation absorbs the hydrogen-vacancy complex along the slip plane and causes the hydrogen and the jog to migrate with the edge dislocation at low dislocation velocity regimes (????d ≤ 0.1 m/s). Therefore, the hydrogen-vacancy complex exerts a continuous drag effect on the dislocation. At higher dislocation speeds (???????? ≥ 1 m/s), hydrogen does not migrate with the dislocation, resulting in the formation of isolated hydrogen detached from the dislocation and diffused into the material; only vacancy is absorbed. When multiple hydrogen-vacancy complexes are arranged along the slip plane, the dislocation absorbs them if they interact with dislocation at different points rather than at a single point to avoid the formation of a large jog at the colliding segment, and the required shear stress increases as the hydrogen atoms in the dislocation core increase.