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.

Semi-coke is a product of low-temperature pyrolysis by low-rank coal, with a composition similar to that of anthracite for pulverized coal injection (PCI). Herein, we investigated the differences in grindability and combustibility between semi-coke and anthracite, analyzed the compositional and microstructural characteristics related to the performance of semi-coke, and assessed the impacts on grinding efficiency and blast furnace operation after replacing anthracite for injection with two types of semi-coke. Semi-coke is rich in high-hardness quartz that is tightly bound to the carbon matrix, making the semi-coke particles very hard, with a high Hardgrove grindability index (HGI) and high abrasion index. The addition of semi-coke reduced the grinding efficiency of the mill and afforded large-sized milled particles. The developed pore structure of semi-coke can enhance kinetic diffusion, and semi-coke is less ordered than coal, thereby providing more reactive sites for combustion reactions. These two reasons cause the ignition temperature of semi-coke to be significantly lower than that of coal. The addition of semi-coke increased the PCI ratio, decreased the fuel ratio, improved the permeability of the blast furnace, decreased the sulfur content in pig iron and carbon content in blast furnace dust. The difference in grinding productivity between semi-coke and coal widens as grinding time increases, suggesting that the HGI method may overestimate the actual grindability of semi-coke. The feasibility of reducing the grinding energy by optimizing particle sizes of semi-coke and improving the grindability of semi-coke by using selected pyrolytic coal and adjusting the pyrolysis temperature was proposed.

In response to the frequent problem of inconsistent quality of billet castings and their rolled products from each strand by a five-strand tundish, the flow field in tundish is optimized by presenting new flow control devices and conducting isothermal physical modelling along with numerical simulation. The results show that the dead volume fraction of the optimized case A6 is reduced from 27.74% to 19%, the stagnation time is prolonged from 12 s to 35 s, and the flow dynamic consistency for each strand is improved as well. In the subsequent industry production tests, the temperature difference of molten steel at the outlet of each strand is reduced to 1~5 °C. The maximum difference of the as-cast equiaxed crystal rate among five strands is reduced from 5.67% to 2.7%, and the consistency of carbon segregation index is also improved with a basically identical appearance through the billet cross section. The maximum differences in oxygen and nitrogen contents for the rolled products of all strands are 2.7 ppm and 5.7 ppm respectively, which are lower than 5.0 ppm and 13.8 ppm before tundish optimization. The yield strength of rolled products is stabilized with much less divergence as compared to the products with the original tundish. Thus, it is believed that the reasonable flow field optimization to a multi-strand tundish not only will have a well-known positive impact on its tranditional metallurgical effect, but also may bring out an approaching identical steel quality from the same caster as we expected.

With the continued exploitation and utilization of high-grade rare earth ores, it is increasingly important to extract rare earths from separated slag containing low-grade rare earth. The X-ray powder diffraction, scanning electron microscopy, electron probe micro-analyzer and confocal laser scanning microscopy were used to explore the influence of TiO₂ and cooling rate on the crystallization of CaO-SiO₂-TiO₂-10 wt% P₂O₅-8 wt% Nb₂O₅-5 wt% CeO₂-5 wt% CaF₂ slag system. In this study, the britholite was precipitated selectively as the Ce-enriched phase. When TiO₂ was added at less than 12 wt%, the britholite was promoted to crystallize meanwhile the Ca₂Nb₂O₇ was suppressed. However, CaTiSiO₅ inhibited the growth of britholite when the TiO₂ content exceeded 15 wt%. The non-isothermal crystallization kinetics had also been investigated for the TiO₂ content and cooling rate varied from 0-18 wt% and 10-40°C/min, respectively. The continuous cooling transformation diagram and the relative crystallinity of the primary crystals were also constructed. Based on the observation and measurement of crystallization process, the modified Avrami model was applied to determine the crystallization mode of britholite with 9 wt% TiO₂ addition_. It was constant nucleation rate and one-dimensional growth with diffusion controlled. Considering the nucleation and growth of crystals, 20–30°C/min was preferred to be the reasonable parameter during cooling stage.

The addition of Ca is a useful method to control the shape of sulfides in steel. In this paper, in order to identify growth behavior of MnS on CaO-MgO-Al₂O₃ oxides in steel, based on two heats of Al-killed Ca-treated resulfurized steel, the characteristics of duplex (Ca,Mn)S inclusions in bars and blooms were observed and analyzed. It is found that there are three types of duplex (Ca,Mn)S inclusions with CaO-MgO-Al₂O₃ core oxides, named as Type-C, Type-MC and Type-M, respectively. In bar, Type-C behaves circular shape, Type-MC behaves spindle shape, and Type-M behaves long strip shape. From Type-C to Type-M, CaO or MgO content in core oxides decreases, Al₂O₃ content in core oxides increases, sizes of core oxides decrease, and the area ratios of wrapping sulfides and core oxides increase. Growth of MnS on CaO-MgO-Al₂O₃ oxides are influenced by the sizes and compositions of oxides. MnS inclusions are easier to grow on CaO-MgO-Al₂O₃ oxides with smaller sizes, lower CaO content, lower MgO content or higher Al₂O₃ content. In order to obtain more spindle-shaped duplex (Ca,Mn)S inclusions, appropriate compositions of core oxides are 5~20% CaO, 5% MgO and 75~90% Al₂O₃, and appropriate sizes of core oxides are 1~3μm.

NdFeB magnets are the most widely used rare earth permanent magnet materials at present. The increasing number of the waste NdFeB magnets and their high rare earth content motivate a search for technologies to allow their cost-effective and environmental-friendly recycling. In this study, removal of oxide inclusions from waste NdFeB magnets by supergravity technology was investigated and the separating conditions were optimized for maximum oxide removal. Under the optimized conditions of G = 800 and t = 15min, the total oxygen of the sample decreased from 410 ppm to 28 ppm, with the oxide removal efficiency of 96.8%. The theoretical time to remove inclusions with different sizes was calculated by Stokes' law, and the experimental phenomena were in good agreement with the calculated ones. The supergravity technology has been demonstrated highly efficient in removing oxide from waste NdFeB magnets for the recycling. A design for an industrial reactor was presented to pave the way for future commercial processing and utilization of waste NdFeB magnets.

Heat transfer model is the basis for control and optimization of continuous casting of steel. In this paper, a three dimensional (3D) real-time heat transfer model has been presented for continuous casting blooms, especially for the initial and final casting stages. To be real-time, an algorithm based on inheritable variable non-uniform grid and variable time steps has been developed, accelerating the 3D heat transfer model by 110~200 times. Meanwhile, the discrete parameters including grid and time step have been optimized, and the influence of central processing units (CPUs) and programming languages has also been studied. Then the relative calculation time, which is the ratio of calculation time to physical time, has been reduced to 0.62 while numerical errors are within 0.74%. Further, to reduce the uncertainty of the model, the machine-dependent parameters appearing in the thermo-physical properties and the boundary conditions have been calibrated with measurements of surface temperature by thermometer and shell-thickness by nail-shooting, then the corresponding optimization problem of minimizing the errors between calculation and measurements has been solved by particle swarm optimization algorithm. After calibration, the model's uncertainty has been obviously reduced. Finally, the calibrated model has been verified by online surface temperature measurement and it shows good agreement as the errors between calculation and measurements are less than ±10°C.

Plasma nitriding was performed to suppress hydrogen uptake and permeation in pure iron. The influence of the phase structure in the nitrogen compound layer on hydrogen uptake and permeation behaviors was examined. The phase structure in the nitrogen compound layer could be controlled by changing the gas composition in N₂-H₂ plasma. The nitrogen compound layers consisting of ε-Fe_2-3N and γ'-Fe₄N with a thickness of 4.9 µm, and of γ'-Fe₄N with a thickness of 2.0 µm were formed by plasma nitriding in this study. During the hydrogen permeation tests, plasma nitriding resulted in small permeation current values compared to the as-polished specimen when it reached the steady state. Especially, hydrogen uptake and permeation for the specimen with the nitrogen compound layer consisting of ε-Fe_2-3N and γ'-Fe₄N were extremely suppressed. Hydrogen uptake and permeation behaviors for the plasma-nitrided pure iron specimens are discussed from the perspective of the catalytic activity for hydrogen evolution and hydrogen diffusivity in the nitrogen compound layers.

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.

With the aim of developing an iron ore sinter with a better reducibility, the authors researched the co-firing of green pellets and a sinter mix in a packed bed. First, a physical simulation with a simulator for the charging equipment of a sintering machine demonstrated that the green pellets were distributed more strongly to the lowest part of the packed bed because their size is larger than that of the sinter mix. Pot tests based on the simulated pellet distribution proved that it is possible to produce a highly reducible sinter at higher productivity, particularly when a concentrate with a large Blaine surface area is used to produce rigid green pellets. Factors that could reduce product yield were also pointed out, i.e., diffusion of the melt from the sinter mix into the pellets and a larger local heat supply to the sinter mix than is suitable. These factors are thought to hinder the formation of thick and strong bonds in the co-fired sinter bed. A 450 kV class X-ray CT was used to visualize the internal structure of the sinter plugs, and revealed the plug made with the green pellets has higher porosity, which is important for better permeability, but also results in less bond formation, which reduces the yield in a co-fired plug. These findings revealed insights for fully enjoying the advantages of co-firing green pellets and sinter mix, as well as issues to be overcome in order to realize a successful co-firing process.

In this study, a new method of microwave-hydrogen synergistic reduction of vanadium titano-magnetite (VTM) was developed to carry out experimental research. Using the theory of the direct low-temperature reduction process, VTM from the area surrounding Chengde, China was used as raw material and H₂ was used as a reducing agent. The experimental results and the theoretical analysis proved that VTM can be feasibly reduced via microwave-hydrogen synergistic reduction at low temperature. In addition, H₂ reduction of iron titanium oxides was more difficult than that of iron oxides and required a higher reaction temperature. Under microwave heating conditions, increasing the temperature, reduction time, and H₂ proportion improved the metallization rate. When reducing for 40 min at 1100 °C with 60% H₂, the metallization rate reached 92.2%. The reduction product had a porous, sponge-like structure, and it was primarily composed of Fe and Fe_9.64Ti_0.36 phases. This implies that the Fe_9.64Ti_0.36 phase may be the enriched phase of Mg, Ca, and Si. During the synergistic reduction process, the metallic iron that precipitated inside the particles migrated to the outer edge of the particles, and the titanium iron oxides that were difficult to reduce inside were coated with metallic iron.

Fe-Cr-Co alloys are becoming important as a half-hard magnet for their novel applications, including non-contact electromagnetic brakes, because of the controllability of its magnetic hardness depending on the modulated structure formed by spinodal decomposition. However, the experimental optimization of the complicated heat-treatment process to control the microstructure significantly increases the development cost, and microstructure prediction by computational simulation is desired. In this study, we first developed the method of phase-field simulation for spinodal decomposition in Fe-Cr-Co alloy during various heat treatments, including isothermal heat treatment, multistep continuous fast and slow cooling, which allows us to conduct a simulation of spinodal decomposition under conditions close to the condition of practical heat treatment. The simulation results revealed that the morphology of the modulated structure is predominantly determined by the cooling rate and does not change significantly during the subsequent isothermal annealing process, while the difference between the concentrations of the FeCo-rich magnetic phase and the Cr-rich non-magnetic phase increases. Continuous cooling at rates higher than 140 K/h demonstrates the maximum number densities of the ferromagnetic particles of α₁-phase seemingly almost reaching saturation, which is expected to give rise to exhibiting the largest coercive force of the Fe-Cr-Co magnet. Moreover, this method can be extended to other materials for designing a modulated structure to show a desired property.

The treatment of desulfurization ash (DA) by high-temperature can solve the increasingly environmental risk caused by the accumulation desulfurization ash on the one hand, and realize the reuse of Ca and S on the other. However, the understanding of the high-temperature reduction decomposition process of desulfurization ash is still vague. In this study, a multivariate and multiphase reaction mathematical model of the complex system of desulfurization ash, carbon, and gas is established by using the principle of minimum free energy. The modeling results show that the reductive decomposition of DA has four stages, and the decomposition products are different in each stage. This result confirms that the optimal thermodynamic conditions to obtain only CaO as a decomposed product are a temperature greater than 1400 K and a C/S molar ratio of 0.5. Further, the processes of CaO and CaS production are parallel competitive reactions, but are regulated by different factors at different stages. A micropositive pressure equilibrium reaction crucible was designed for laboratory DA decomposition experiments. The correctness of the calculation result of the minimum free energy mathematical model is proved by the high temperature reductive decomposition experiment. When the temperature and C/S molar ratio are 1500 K and 0.5, the DA decomposition rate can reach 100%. The main reaction product is spherical CaO, the minimum S content is approximately 1.5%, and the desulfurization rate can reach approximately 70%. The present strategy is highly promising for application in industrial DA recycling processes.

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as another potential reinforcing agent for the development of new hydrogen-compatible steels with acceptable cost performance. Fatigue crack growth (FCG) behavior of three eutectoid steels with different microstructure sizes (i.e., ferrite/cementite interlamellar spacing, colony and block sizes) and strength levels was investigated in a 90 MPa hydrogen gas, an essential evaluation when attempting to perform a defect tolerant design of the components used for high-pressure gases.

The pearlitic steels clearly exhibited the acceleration of their FCG rate in hydrogen gas up to a hundred times that in air, wherein its magnitude was greater in the material with finer microstructure and concomitant higher strength. The delamination of ferrite/cementite lamellae, which were inclined largely from the loading axis, was determined to be the primary cause of such HA-FCG in pearlitic steels. Nevertheless, the extent of FCG acceleration was minor with respect to martensitic steels. The fact was ascribed to the barrier role of the cementite platelets oriented nearly perpendicularly to the crack as well as to the geometrical retardation effects arising from the crack deflection and blanching. Ultimately, pearlite was superior to martensite from the perspective of HA-FCG resistance; besides, the superiority was more substantial as the loading rate became slower.

Recycling phosphorus from phosphorus-containing wastes and byproducts is a promising secondary phosphorus resource. Steelmaking slag is generated in large quantities as a byproduct of the iron and steelmaking industry, and contains phosphorus that could be a possible secondary phosphorus resource. Phosphorus can be extracted from slag into aqueous solution; however, the phosphorus concentration is generally lower than other elements, causing a problem for phosphorus recovery. In this study, bipolar membrane electrodialysis was applied to separate and recover phosphorus from solution. The reaction and performance of phosphorus recovery were investigated. The effect of initial phosphorus concentration, volume ratio, and electric potential on recovery process was determined. Ion adsorption on an anion-exchange membrane during the initial stages and ion competition between H₂PO₄^- and OH^- in the later stages are the main factors affecting the extraction efficiency. In this study, the maximum concentration ratio achieved was 2.46, and the minimum phosphorus concentration in treated solution was 3.64 mg/L, which is under the Japanese effluent standard. In general, bipolar membrane electrodialysis has good potential for concentrating and recovering phosphorus from aqueous solution.

This paper is the first of a series of papers on the roughness characteristics of the burden surface in the blast furnace. The measurement method of the burden surface roughness texture is described, and the overall roughness characteristics of the burden surface are studied from a statistical point of view. This study focuses on two typical granularized burden materials, coke and sintered ore, which present four kinds of burden particles under two individual particle sizes. Simulated cold-state burden belts proportional to the practical burden radial sectors were stacked in a pilot plant. An RGBD camera was used to measure the simulated burden belt surfaces to obtain the surface texture details. Four corresponding digital elevation models of the burden belts were obtained through data processing. The root mean squared height, skewness, kurtosis, and spatial autocorrelation function are selected as statistical indexes. The obtained digital elevation models were counted. The results show that all four kinds of burden surfaces are Isotropic-Rough-Surface. In addition, the height distributions of the rough burden surface are close to the Gaussian distribution. Also, the spatial autocorrelation functions of the coke and large-sized sintered ore burden surfaces are close to the Gaussian function form. And lastly, the spatial autocorrelation function of the small sintered ore burden surface is close to the exponential function form.

Aluminum cations are generally present in four-fold (^[4]Al³⁺) or five-fold coordination (^[5]Al³⁺) in aluminosilicate slags, where the concentration of ^[5]Al³⁺ varies depending on the type of charge compensator, for example, Mg²⁺ and Ca²⁺. Although it has been reported that the amount of ^[5]Al³⁺ species increases with the replacement of CaO with MgO in the CaO–MgO–SiO₂–Al₂O₃ system, the detailed mechanism underlying the change in the local structure near the aluminum cations remains unclear. Because the residual negative charge on the bridging oxygen between ^[4]Si⁴⁺ and ^[5]Al³⁺ (^[4]Si⁴⁺–O_BO–^[5]Al³⁺) is larger than that of ^[4]Si⁴⁺–O_BO–^[4]Al³⁺, it is essential to understand the positive charge contributions of alkaline-earth cations to compensate for these negative charges on the bridging oxygens. In the present study, the valence of a single chemical bond near Mg²⁺ and Ca²⁺ cations in the chosen aluminosilicate glasses was determined using a simple empirical model, which enabled calculation of the bond valence from the observed interatomic distance of near alkaline-earth cations by synchrotron X-ray total scattering. Magnesium cations had a larger average bond valence (+0.39) than calcium cations (+0.31). The difference in the positive charge contribution from Mg²⁺ and Ca²⁺ should explain the variation in the coordination number of aluminum cations.

Currently, steel products are manufactured by continuous casting or large-sized ingot casting, and macrosegregation that occurs during the manufacturing process significantly impacts product quality in terms of cracks and deterioration of mechanical properties. To clarify the principle of casting defects, such as shrinkage porosity and macrosegregation due to the formation of bridging of the columnar dendrite of the medium-carbon steel, in this experiment, a laboratory-scale local-chilled mold in which the middle part was forcedly cooled was designed to cause bridging, shrinkage porosities, and macrosegregation. Enough risers were also designed to simulate realistic gravity casting as much as possible. The solidification structure morphology was observed, concentration analysis of alloying elements was performed, and the effect of bridging on macrosegregation was investigated. Solidification proceeded preferentially from the chill plate, and the bridging was formed successfully at a high casting temperature. The high casting temperature condition could cause bridging, but large shrinkage porosities would be formed as well. On the contrary, the lower casting temperature condition could increase the grain density and form shrinkage porosities that are smaller in size but larger in number and more dispersed, compared with the case cast with no chill plate mold. Due to the formation of bridging, macrosegregation was formed, and the difference between positive and negative segregation was increased from the longitudinal center of the sample.

In the industrial steel manufacturing process, such as continuous casting and ingot casting, macrosegregation occurs due to the effect of bridging and contraction flow during the middle and end periods of solidification. Since the macrosegregation results in a nonuniform structure and eventually cracks, there has been a demand for technological development that does not cause macrosegregation in the casting process. In this study, model experiments using the medium-carbon steel cast with a laboratory-scale local-chilled mold at different superheating have been carried out to investigate the relationship between solidified structure and macrosegregation occurred by local bridging during casting. The morphology of shrinkage porosities and dendrite structures was observed. The concentrations of the alloying elements were analyzed for macrosegregation by an electron probe micro-analyzer. Chill plates successfully formed the columnar dendrite bridging area and the columnar dendrite shell in the sample with high superheating. During solidification, the negative pressure of the region below the bridging increased, and the concentrated contraction flow flowed into the bottom of the large shrinkage porosity. Finally, V segregation was formed in the bridging area, large shrinkage porosities remained below the bridging area, and point-like or band-like positive macrosegregation occurred in the interdendritic region between columnar dendrites and equiaxed dendrites below the bridging. In comparison, a lower casting temperature increased the grain density and formed shrinkage porosities that were smaller in size but larger in number and more dispersed.

This study explores the underlying mechanism between secondary refining efficiency, gas flow rate, and slag properties. The secondary refining efficiency is directly affected by the slag-metal interface area. Traditionally, the slag-metal interface has been limited to the liquid-liquid interface of the ladle cross-section and does not include the interface area between the entrained slag droplets and metal. To investigate the interface area with different slags and metals under various bottom blow rates, a physical model of a single-nozzle gas-stirred ladle was established using oil to simulate slag and water to simulate metal. The roles of relevant variables that affect the volume of entrained oil, the diameter of entrained droplets, and interface area were studied, as well as oil viscosity, interfacial tension, and oil thickness. Experimental data were collected using colorants and image processing techniques. Based on these findings, the increase in gas flow rate and oil layer thickness increased the volume of entrained oil and interface area, while the increase in oil viscosity and interfacial tension decreased these parameters. When the gas flow rate increased, the mean diameter of droplets first increased and then decreased. However, the specific surface area of droplets revealed the opposite trend. Furthermore, the mean diameter and specific surface area increased and decreased with increasing oil-layer thickness.

The present work was conducted to elucidate the influence of inclusionson surface quality of deep cold-drawn battery cups for electrical cars. The obtained results revealed that, to prevent the surface defects of sand-holes in battery cups in deep cold-drawing process, attentions should be paid to steelmaking and casting process for optimal control of inclusions. Because sand-holes were often caused by large Al₂O₃ clusters and CaO-Al₂O₃ particles. Most important, a worthy finding was that these inclusions were not safe when they were over 100 μm and bigger than 27 μm, respectively, which was important for process optimization. By reducing (FeO) contents in ladle slag and tundish covering flux to about 5% or lower, together with optimal fluid flow of molten steel in tundish, such large inclusions can be well decreased to prevent the occurrences of sand-holes in battery cups in industrial production.

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

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.

The distribution and morphology of inclusions in steel have an important effect on the quality of steel. It has been proved that the oxide inclusions can be modified into small and dispersed spinel inclusions by adding proper amount of Mg in steel. The MnS-MgAl₂O₄ composite inclusions are formed with the core of MgAl₂O₄ inclusions during the solidification process of molten steel, which has deforming ability and can improve the properties of materials steel. However, the investigation of the control of the composite inclusions is limited by the lack of understanding structure of the inclusions. In this study, the Mg treated steel samples were prepared by induction furnace in this study. In the experiment, SEM-EDS was used to characterize the samples, and thermodynamic calculations were used to describe the evolution mechanism of inclusions and MnS-MgAl₂O₄ composite inclusions formed in steel samples with different Mg contents. The atomic mismatch calculated between MnS and MgAl₂O₄ proves that they can nucleate effectively. The three-dimensional (3D) morphology of the composite inclusion of MnS-MgAl₂O₄ in steel samples were observed by using the X-ray Micro-CT in the beamline of BL16U2 at Shanghai Synchrotron Radiation Facility (SSRF). It is proved that MnS and MgAl₂O₄ phases exist in the form of co-associated, which is valuable for the control of composite inclusions in steel. The current work provide a powerful method to analyze the detailed structure of the composite inclusions in the steel.

Recrystallization and grain growth during plate rolling are prevented by Nb addition both with the solute drag and the Nb carbide precipitation. Although a fine microstructure is achieved in the base material, welding heat completely changes the microstructure in the heat affected zone (HAZ). In this study, laboratory simulation of the coarse grain HAZ (CGHAZ) thermal cycle of double submerged arc welded linepipe was carried out using low carbon steels containing different Nb contents. Extraction residue analysis of the simulated CGHAZ samples revealed that almost all the Nb remained in solid solution. To clarify the interaction of Nb carbide dissolution and grain growth on overall simulated HAZ microstructure evolution, additional weld HAZ thermal simulations were performed. It was found that Nb carbides remain undissolved at HAZ peak temperatures up to 1200°C and showed significant pinning effect to prevent austenite grain growth. Significant grain growth was seen after continuous fast heating to 1350°C peak temperature, while the higher Nb added steel showed a slower overall austenite grain growth rate, suggesting that grain growth in the HAZ at higher temperature was suppressed by the combined effects of slower coarse Nb carbide dissolution providing some pinning, and the solute drag effect of higher amounts of Nb in solid solution. A pronounced retardation of longer-term isothermal grain growth was identified at 1350°C at higher levels of solute Nb, confirming the influence of Nb solute drag on high temperature resistance to austenite grain coarsening.

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 𝑉d 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 (𝑉d ≤ 0.1 m/s). Therefore, the hydrogen-vacancy complex exerts a continuous drag effect on the dislocation. At higher dislocation speeds (𝑉𝑑 ≥ 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.

The strength, pore structure, and material constants of coke prepared from caking coal (Coke A) and non- or slightly caking coal (Coke C) were experimentally and numerically investigated with a particular focus on those values at high temperatures. Coke A showed higher strength and lower porosity than Coke C. The pore structure imaged by X-ray computed tomography was translated to the finite element mesh with the image-based modeling, and the stress analysis based on the finite element method was performed to calculate the mode value of maximum principal stress at different Young's modulus and Poisson's ratio. Young's modulus of Coke A and Coke C at a constant Poisson's ratio decreased and increased, respectively by heating. When the temperature increased, the compression stress of Coke A increased. The result indicated that the coke strength could be increased by heating because of the decrease in apparent Young's modulus, accompanied by the occurrence of creep.