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.

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 ΔKth 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 ΔKth 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 ΔKth value for a long crack in the initial propagation stage just after fatigue crack initiation was discussed.

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. Fullsize Image

The solid waste generated in the steel industry are predominantly recycled via the agglomeration making route. In recent times, the amount of fines percentage has increased in sinter making, thereby affecting the bed permeability which eventually affects sinter quality. Hence, sinter bed permeability is to be monitored continuously. Currently the conventional infrared based velocity measurement is being used for this purpose. The permeability is being calculated based on the flow velocity of flue gases. The current paper is focused on the development of low cost novel vibration sensor for flow measurement to monitor permeability. The measurements are showing with good accuracy (96%) in comparison with the conventional technique. The design and development along with the results are discussed in detail in this paper. Fullsize Image

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. Fullsize Image

Dislocations are often introduced in Ni-based superalloys to impart sufficient strength at both room temperature and high temperatures prior to their use in automobile exhaust gaskets. However, the interaction between the representative γ′ (Ni3(Al, Ti))-phase precipitates and dislocations in high temperature remains unclear. Therefore, this study examined the effect of cold rolling on age-hardening behavior and microstructure evolution, focusing on the formation of γ′-phase Ni3Ti during aging at 700°C for up to 400 h after 60% cold rolling of solution-treated specimens. During the early stage of aging, at 0.03 h, the hardness rapidly increased from 401 HV to 496 HV. Age-hardening continued until 3 h and reached its peak of 536 HV, followed by gradual decrease with aging time. 3D atom probe investigation revealed that the γ′-phase was confirmed after 0.3 h of aging. However, the composition-modulated structure speculated to be caused by spinodal decomposition was observed in the 0.03 h aged specimen. The change in strength with aging time was considered by calculating the contribution of each strengthening mechanism. In the initial stage of aging (0–3 h), dislocation and solid-solution strengthening dominated along with spinodal strengthening. Strengthening by spinodal decomposition in the 0.03 h aged specimen is presumptively accelerated by the introduced dislocations, which is followed by further precipitation strengthening caused by γ′-phase precipitates. In the later stage of aging (3–400 h), precipitation strengthening became dominant and reached its peak at 20 h aging, while dislocation strengthening decreased with aging time. Fullsize Image

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(σnor) 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 σnor or MTR size, the larger was the dwell debit. Therefore, the MTR size was considered the dominant factor determining the dwell fatigue life.

The closure of large voids, whose thickness-to-void-height ratio exceeds 0.2, in S10C steel plates during hot rolling was investigated to determine whether hydrostatic integration (Q-value) can be used to predict the closing behavior of large voids. The steel plates with an open void along the rolling (RD), transverse (TD), and normal (ND) directions were hot rolled at 1000 and 1300°C with a target rolling reduction of 10% at each pass until 40% total target reduction. It was found that the effect of temperature on the closing behavior was negligibly small. RD and TD voids were almost entirely closed at a reduction of 40%, whereas ND voids could not be closed. The width of RD void was almost linearly decreased with reduction increase. TD void were closed at a lower reduction ratio than RD void. The thickness above and below the void was compressed after rolling in RD void but less reduced in TD void, which is presumable reason of the earlier closure of TD void. The FE analysis clarified that the void volume over initial volume (V/V0) of the voids could be expressed as a function of the Q-value in the case of RD and TD voids. However, the closure behavior of the ND void cannot be expressed by the Q-value. These results indicate that the Q-value can be used to predict the closure of large voids in the RD and TD during rolling, although it cannot be used if the void shape is elongated in the compression direction. Fullsize Image

A magnetic field can suppress liquid metal motion and this function is used as an electromagnetic brake in a continuous casting process in the steel industry. Thus, the magnetic field has the potential to reduce macro-segregation, and the electromagnetic braking effect should be clarified under the uniform and gradient magnetic field conditions because the magnetic field must non-uniformly distribute in a large size casting machine. This was experimentally examined in this study. A liquid tin was flowed in the packed bed filled with copper balls or alumina balls as a model of the liquid-solid coexisting phase. As the results, the friction factor for the alumina packed bed and the copper packed bed agreed with that calculated by the Ergun equation under the no-magnetic field condition. By imposing the magnetic field, the friction factor increased, especially for the copper packed bed. That is, the electromagnetic braking effect in the case of the copper packed bed was stronger than that in the case of the alumina packed bed. This is similar to the Hartmann flow theory. The electromagnetic braking effect under the uniform magnetic field condition and that under the gradient magnetic field condition were similar in the case of the alumina packed bed while the former was larger than the latter in the case of the copper packed bed. The oxidation of the copper balls in the packed bed may affect the electromagnetic braking effect. Fullsize Image

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.

Conventionally, it has been known that the product yield of the upper part of the sintering layer is extremely low, because of the heat loss caused by transferring heat toward the space above sintering layer, and of the large amount of unburned carbon in upper sintering layer.As a countermeasure, REMO-tec (Re-ignition Method for Optimization of Total Energy Consumption) has been developed. Here, REMO-tec, is the sintering technique of re-igniting sintering packed bed at certain intervals after first ignition. This method has an effect on improving sinter yield with maintaining high sinter reducibility. This effect leads to improving sinter reducibility without decreasing sinter yield by decreasing control of coke breeze content in sinter mixture.This paper focuses on coke combustion efficiency as combustion ratio of carbon in coke breeze for considering improvement of sinter yield through sinter pot test. Here, carbon combustion ratio is defined as proportion of actual heat generation at combustion to ideal heat generation as complete combustion (C+O2→CO2) of all carbon in coke. And it can be calculated based on component analyses of exhaust gas.As the result, it was confirmed as shown bellows.1) By re- ignition, the unburned coke remaining in the upper layer of the sinter packed bed was burned, which has a role of extending keeping time over 1200°C especially in the upper layer of sinter packed bed.2) Due to the effect of 1), the increasing amount of heat supply at “REMO-tec” case was equivalent to the same as the experimental case of increasing coke breeze content, at which increasing heat amount at blending coke breeze content was four times larger of the heat amount at re-ignition. (For a 430 mm layer pot test)3) In addition, since the re-ignition heat is donated to the upper layer (surface layer), the amount of heat consumption in the upper layer of the sinter packed bed increases and the amount of heat consumption in the lower layer decreases compared to the case of increasing coke breeze content, which results in decrease of the difference between heat consumption in upper layer and that in lower layer.In addition, these effects have been also confirmed at the commercial sinter plant. Fullsize Image

The 0.2C-2Si (mass%) steels with the addition of 0–4 mass% Cr were prepared by hot rolling followed by subsequent annealing for normalization. The steels were subjected to friction stir welding (FSW) conducted above A3 temperature. For all the steels, sound FSWed joints were obtained. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials showed a relatively good balance of strength and ductility when the Cr content is over 3 mass% presumably owing to the relatively fine microstructures of ferrite and martensite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zone of the 0.2C-2Si-4Cr joint with fully martensitic structure exhibited the surprisingly high tensile strength of 1720 MPa compared with that of the conventional martensitic steel of 0.2 mass%C together with the excellent balance of ductility. This is assumed to be caused by the refinement of block size in the fresh lath martensite and/or the formation of ausformed martensite induced by the dynamically recrystallized fine austenite grains by FSW of the Cr added steels. Fullsize Image

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.

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. Fullsize Image

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.

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.

For this article, the bottom regiments distribution is designed to investigate the instantaneous kinetic energy of molten pool with the help of numerical simulation. The results indicate that the time required for kinetic energy to reach the stable state is affected by the angle among regiments. When the blowing gas flowrate is 720 Nm3·h−1, the shorter equilibrium time is 66 s with the angle of 15°. Meanwhile, the higher kinetic energy of 2915 J has been discovered as the outer regiments kept at a pitch circle diameter ratio of 0.5, which reflects the preferable internal dynamic conditions of liquid steel. While the molten pool can get enough stirring force, and the outer cluster should not be arranged closely to the furnace wall to avoid unnecessary energy dissipation. Besides, the weak stirring zones inside the bath can be reduced by the elevation of blowing gas intensity, and the proportions of the proper scheme are 0.84, 0.74, 0.69, and 0.67 under four diverse operation conditions, respectively. Fullsize Image

It is expected that phosphorus-concentrated slag produced by using the pyro-reaction between high phosphorus hot metal, which was prepared by the reduction of usual steelmaking slag with a small amount of hot metal, and oxidizing slag can become an important phosphorus resource instead of phosphate rock. For separating phosphorus from the phosphorus-concentrated slag, the citric acid leaching method has been reported to be available. In this study, the effects of cooling condition and oxidation treatment of phosphorus-concentrated slag on the dissolution behaviors of phosphorus and other elements were discussed. Compared to slow-cooled slag, CO2 blowing-quenched slag did not affect dissolution of phosphorus but decreased dissolution of iron. Oxidation treatment increased the dissolution ratio of phosphorus by about twice and decreased that of iron by one-sixth. It was confirmed by the microscopic analysis of the mineral phases in these slags that the concentration of SiO2 and FeO solid-solved in 3CaO·P2O5 (C3P) phase increased by slow cooling, and they precipitated as fine Fe2O3–SiO2 compound in C3P phase by oxidation. Furthermore, the dissolution behavior of SiO2 and FeO solid-solved in C3P phase was investigated. Experimental results showed that the C3P–2CaO·SiO2 dissolved easily and the C3P–3FeO·P2O5 dissolved hardly. From those experimental findings, it was presumed that the change in dissolution behavior due to slag cooling conditions and oxidation treatment were attributed to the concentration of SiO2 and FeO in C3P phase. Fullsize Image