Creep deformation and precipitation behavior of 9Cr lMo V Nb steel with excess nitrogen introduced by solution nitriding were investigated. Precipitation of Cr2N phase was confirmed in addition to M23C6 and MX phases in the tempered microstructure. The creep strength of the steel was significantly reduced by solution nitriding, while the creep rupture elongation was increased. To characterize the complex precipitation behavior of the nitrogen-added steel, a machine learning-based clustering method of the multidimensional scatter diagram of the X-ray intensity of the alloying elements in each pixel of a STEM-EDS map was developed. Reduced number density of precipitates and enhanced coarsening kinetics of both Cr2N and MX were proposed as the mechanism of weakening caused by excess nitrogen.
Effect of cyclic stress changes on creep rupture strength was investigated at 600 degrees C for MGQ and MGS heat of ASME P91 steel. There was no large difference of creep rupture strength among the heats. However, the creep rupture ductility of MGS was lower than that of MGQ in the long-term. The initial creep stress and stress after stress reduction was 120 MPa and 24 MPa to 84 MPa, respectively. The time interval of stress reduction was 6 days for each test. For MGQ heat, no effect of stress reduction to 84 MPa on creep rupture strength was observed. The stress reduction to 60 MPa slightly increased time to rupture as compared to creep test under constant stress. A small amount of decrease in time to rupture was confirmed in case of stress reduction to 36 MPa. Consequently, there was no tendency of effect of stress reduction on creep rupture strength for both heats. The cyclic stress change did not affect the martensitic lath structure and precipitates distribution after creep rupture.
Nitriding is a beneficial surface hardening heat treatment to reduce CO2 emissions and shows a potential partially replace carburized steels adopted for sliding parts such as bearings and gears. Both the bending fatigue strength and pitting fatigue strength under high contact pressure were evaluated for nitrided JIS-SCM440H materials using two types of controlled-nitridings, i.e. y' and s, and conventional gas nitriding. Not only the thickness of the compound layer but also its phase structure such as gamma ' and epsilon significantly affected the 10(7) cycles bending fatigue strength, where the y'-nitrided material was the highest. On the other hand, almost no effects of the thickness and phase structure on the 107 cycles pitting fatigue strength were detected in the materials. Compared to the carburized material, the gamma '-nitrided material exhibited higher 10(7) cycles fatigue strength in bending and pitting, but poor fatigue strength in the lower cycles range. The shape and size of the pores in the compound layer were altered during roller-pitching tests, although there was almost no reduction in the thickness of the compound layer. The pore size was reduced by pore closure, especially at higher contact pressures. Pore closure was more pronounced in the y'-nitrided materials than in the epsilon-nitrided materials at lower contact pressures. The 10(7) cycles pitting fatigue strength was improved after the pore closure, resulting from fewer defects.
The microstructures developed by inhomogeneous plastic flow are known to play a crucial role in the cracking in sheet metal bending. Observations on the cross-section perpendicular to the bending axis show that cracks propagate along banded microstructures, known as shear bands, which have already developed almost diagonally to the depth direction. However, the relationship between the shear-band formation and the initial microstructures has not been clarified for dual-phase (DP) steel with hard martensite and soft ferrite. In this study, the evolution of shear bands under three-point bending was investigated for a coarse grained DP steel. It was shown that shear bands initially develop in ferrite at the boundaries with martensite. They are originally planar and tend to stretch diagonally to the depth direction. When further extension of a shear band is obstructed by hard martensite, it changes direction to circumvent it. Furthermore, these shear bands already develop at the early stages of bending, whereas the formation of surface grooves and voids requires larger bending angles, and their shapes and positions are strongly affected by the shear bands. It is, thus, suggested that the shear-band development is essential to the formation of grooves and voids, which eventually leads to cracking.
In the peritectic solidification, the same solute has been ejected toward the liquid during the growth of primary and secondary phases and these two phases grow interacting with each other. In this study, the unidirectional solidification experiments by using binary Ag-Sn alloy have been performed to find the layered structure solidification, in which primary phase and secondary phase alternately grow in hyper-peritectic Ag-Sn alloy. In order to clarify the mechanism of the aforementioned solidification phenomena, the relationship between fractional solid and Sn concentration distribution obtained by FE-EPMA was examined by using the random sampling methods. Through some theoretical analysis, it has been estimated that solute convection due to the density difference between bulk and inter dendritic liquid could occur and the growth and decline of primary phase could be repeated periodically in the unidirectional solidification. Based on these results, a mechanism for the occurrence of the layered structure solidification induced by solute convection was proposed. Furthermore, it was confirmed that the layered structure solidification also occurs in hypo-peritectic under conditions that promote convection, revealing that the presence or absence of convection directly influences the occurrence of the layered structure solidification.
Material applied in low-temperature liquefied gas storage tanks is required to have sufficient toughness. In recent years, high Mn austenitic steel has attracted attention for use in this application. In this study, the basic deformation characteristics and toughness of high Mn steel containing about 25 Mn were compared with those of 9 % Ni steel in order to investigate the applicability of high Mn steel to LNG tanks. The high Mn steel showed larger uniform elongation than the 9 % Ni steel due to higher strain-hardening, but elongation after the maximum load was significantly smaller. The plastic flow stress of the high Mn steel increased with decreasing temperature and showed temperature dependence similar to that of 0.2 % proof stress in the 9 % Ni steel. The Charpy absorbed energy of the high Mn steel was about half that of the 9 % Ni steel, with an average value of 86 J at 77 K. Cleavage fracture surfaces were not observed in the fracture surfaces obtained at any temperature, indicating the micro -void coalescence type of fracture. The characteristics of ductile damage in the high Mn steel were discussed based on observation of micro-voids.
The final solidification position in continuous casting is a very important index for productivity and product quality, and continuous measurement is required. An apparatus to measure solidification completion position by electromagnetic ultrasonic method was developed. High sensitivity electromagnetic ultrasonic sensor using digital signal processing and Halbach array, and calibration method using longitudinal and transverse waves of ultrasonic wave enabled high sensitivity and continuous measurement of final solidification position. As a result of the application to the operation improvement, the following effects were obtained: productivity improvement by the improvement of casting speed, and improvement of center segregation in the steel plate for pipeline.
In the point of view of reducing coke production cost and future resource depletion, it is necessary to produce high-strength coke from low-rank coal. It is reported that high strength coke can be obtained by pulverizing, compacting, and carbonizing lowrank coal, non-or-slightly-caking coal. In this study, we research the effects of coal size and coal charging density on coke strength and coke density, and discuss the mechanism for the change of coke properties. Coal of 1.0 mm or less to 0.1 mm or less was compacted to 0.8 g/cm(3) to 1.1 g/cm(3), carbonized at 900 degrees C, and coke strength and coke density were measured. As a result, it was found that coke strength significantly increased by pulverizing to 0.1 mm or less and increasing the coal charging density. The effects of coal particle size and coal charging density on coke properties are examined. When the grain size of coal becomes finer, swelling is suppressed, and large pores and connecting pores of coke are reduced. As the coal charging density increased, the coke density increased due to the shortening of the distance between coal particles.
Sulfur is one of the five ubiquitous elements of steel, and the presence of sulfur reduces the performance of steel. Therefore, the sulfur content in steel must be strictly controlled. This paper focuses on the gravimetric method after separation of iron (JIS G 1215-1) specified in the Japanese Industrial Standards (JIS) as an absolute analysis method for sulfur content in steel. The precipitation formation process of BaSO4 and the rinse process of the formed precipitate had a major influence on the recovery. In the formation process of BaSO4, it was confirmed that the precipitation was almost completely formed under the conditions specified in JIS G 1215-1. However, the coexistence of manganese ions (Mn2+) significantly reduced the precipitation recovery. Ethylenediaminetetraacetic acid (EDTA) was effective for masking Mn2+. In JIS G 1215-1, the BaSO4 formed is rinsed in two steps: first, barium chloride solution (BaCl2) is used to remove foreign substances, followed by hot water to remove the BaCl2. Mn2+ not only inhibited the precipitation of BaSO4 but also reduced the recovery during rinsing with hot water. Sulfur recovery in the entire JIS G 1215-1 process exceeded 100 % regardless of the addition of EDTA. This indicates loss of sulfur during the precipitation process much less contributed to the recovery of sulfur in the total process of JIS G 1215-1.
Copper bearing steel is hard to recycle because it shows severe surface cracking in hot working due to the formation of copper liquid phase on the interface between scale and steel, i.e. hot shortness. Suppression of the hot shortness is a critical issue to attain sustainable society. In order to clarify the mechanism of the surface cracking in hot rolling, in this study, 0.30%Cu with/without 0.15%Ni bearing square steel bars with controlled oxide scale were hot rolled in a laboratory. The height was reduced 5%, 25% or 40% at 1273 K, 1373 K or 1473 K. After the hot rolling at 1273 K, few cracks were found on both upper and side surface where no copper liquid phase appear. After the hot rolling at 1373 K or 1473 K, surface cracks shallower than 50.tm were found on upper surface, while surface cracks deeper than 100.tm were found on side surface of the rolled bars. Depth and width of cracks increased with an increase in height reduction. On the upper surface, cracks were suppressed by temperature drop due to heat transfer to the cold roll. The copper liquid phase on the interface between scale and steel was transformed to solid. On the other hand, edge cracks were formed on side surface (free surface).
The yield in the iron ore sintering process is managed in daily operations as an important indicator of manufacturing costs and CO2 emissions. Currently, general yield operation actions are "feedback" based on yield results, and do not prevent yield loss. In order to prevent yield decline by "feed-forward" type action targeting yield changes, yield prediction based on operating conditions is essential. In this study, we assumed that yield is synonymous with the strength of the discharged sintered cake, and attempted to apply the porous material strength estimation formula to the sintered cake. The sintered cake was divided into a cake pore part lost by crushing and a sintered ore part as a matrix portion containing pores that remain after crushing, and the formulation of cake porosity and sintered ore strength was studied. Using the amount of melt (melting ratio) generated in liquid-phase sintering and its penetration length into the ore as key factors, the estimation formulas for cake porosity and sintered ore strength were built. In addition, experiments using diffusion pairs were carried out to verify the hypothesis regarding the penetration distance. A pot test were carried out to verify the constructed model, and confirmed a good correlation between the calculated yield by bed temperature measured and the blending components, and the actual measured yield.
Steelmaking slag contains a considerable amount of phosphorus, which is widely used in human society. Phosphorus recovered from steelmaking slag will provide a new resource, and reusing the steelmaking slag, from which phosphorus has been removed, in the steel manufacturing process reduces the total slag volume. In this study, phosphorus was separated from phosphorus -concentrated slag, which was produced through the oxidation of high-phosphorus hot metal using a small amount of steelmaking slag. The leachate of the phosphorus-concentrated slag was obtained by agitating the slag in citric acid at pH=3 using a mill pot containing mill balls. To separate phosphorus from the leachate by the precipitation of calcium phosphate, the pH of the leachate was increased by adding NaOH solution to increase the pH from 3 to 11, and by adding Ca(OH)(2) solution or Ca(OH)(2) powder to increase the pH from 3 to 11. The recovery ratio of phosphorus was over 75% for these methods. The P2O5 content in the recovered precipitates was over 30 mass%, which is higher than that in natural phosphorus ores. It is calculated that 50% of phosphorus in hot metal could be recovered as precipitates, and when the leaching residue was recycled for hot metal dephosphorization, the amount of slag emissions was reduced by 34% compared to the current hot metal dephosphorization operation. It is suggested that phosphorus can be recovered from steelmaking slag and slag emissions can be decreased through slag treatment processes such as slag reduction, dephosphorization, acid leaching, and precipitation.
Two orientations ({110}<001> and {110}<112>) evolve as secondary grains in heavily cold rolled reduction of 91.5% in grain-oriented silicon steel. We investigated the secondary recrystallization mechanism of these two grains by temperature gradient batch annealing method. This method induces the continuous growth of secondary grains along the temperature gradient direction. Consequently, selective growth behavior can be easily evaluated from macrostructure. Furthermore, the orientation relationships between secondary recrystallized grains and primary recrystallized grains at the interface of them can be investigated by interrupting the temperature gradient batch annealing process during the secondary recrystallization. It was clarified that secondary grains which have higher frequency of CSL (Coincidence Site Lattice) boundaries grow more preferentially and the effective CSL boundaries tolerance angle was 10 degrees from precise CSL orientations. Both {110}<001> and {110}<112> grains statistically had a high frequency of effective CSL boundaries (more than 14.5%) and CSL boundaries corresponding to each orientation of secondary grains disappeared preferentially at growing fronts of each secondary grain. It can be deduced that CSL boundaries dominate the selective growth behavior of {110}<001> and {110}<112> grains, which have two or more neighboring CSL boundaries to the matrix and thus successively grow as secondary grains. CSL boundaries are supposed to have lower grain boundary energy and higher mobility. Therefore, CSL boundaries suffer lower pinning forces from inhibitors and start to migrate from higher inhibition level (lower temperature). From these results, CSL boundaries play a dominant role in the secondary recrystallization of heavily cold rolled grain-oriented silicon steel.
Steel slag, a by-product of the steelmaking process, contains free-lime which can expand upon hydration. This expansion characteristic necessitates pre-aging when used as a subbase material. Various countries regulate the expansion characteristics of steel slag through different test methods and standards. However, the temperature conditions of these tests and their correlation to actual environments are not clearly defined. This study aimed to evaluate the temperature dependence of the immersion expansion ratio of steel slag for road construction through expansion tests. The results identified the rate-limiting step in the hydration process and calculated the reaction rate constant. The study compared the immersion expansion test method with actual environments to quantitatively assess the degree of acceleration in the tests. It was found that the hydration of steel slag follows a pseudo -first-order surface reaction. By analyzing the temperature changes in the water tank, the study estimated the continuous immersion expansion test period corresponding to the JIS method. The 80 degrees C continuous immersion expansion test period of 4.1-4.6 days was equivalent to the 10-day JIS immersion expansion test. The JIS immersion expansion standard was found to correspond to a legal service life of 10 years or more.
Creep testing is time-consuming and costly, leading institutions to limit the number of tests conducted to the minimum necessary for their specific objectives. By pooling data from each institution, it is anticipated that predictive models can be developed for a wide range of materials, including welded joints and degraded materials exposed to service conditions. However, the data obtained by each institution is often highly confidential, making it challenging to share with others. Federated learning, a type of privacy-preserving computation technology, allows for learning while keeping data confidential. Utilizing this approach, it is possible to develop creep life prediction models by leveraging data from various institutions. In this paper, we constructed global deep neural network models for predicting the creep rupture life of heat-resistant ferritic steels in collaboration with eight institutions using the federated learning system we developed for this purpose. Each institution built a local model using only its own data for comparison. While these local models demonstrated good predictive accuracy for their respective datasets, their predictive performance declined when applied to data from other institutions. In contrast, the global model constructed using federated learning showed reasonably good predictive performance across all institutions. The distance between each institution's data was defined in the space of explanatory variables, with the NIMS data, which had the largest dataset, serving as the reference point. The global model maintained high predictive accuracy regardless of the distance from the NIMS data, whereas the predictive accuracy of the NIMS local model significantly decreased as the distance increased.
Plasma nitriding of JIS SKD61 tool steel was performed by open-air type atmospheric-pressure plasma jet. The results of our experiments show that the surface hardness of tool steel work pieces was increased by more than two times that of the core material after within 30 min of treatment time.
New fracture process model of cleavage fracture initiated from cementite crack was proposed. In addition, the equation of propagation of cementite crack into the ferrite grain was developed based on the Brechet-Louchet model. This equation can reproduce not only ferrite size dependence of cleavage fracture stress that the Petch model can reproduce but both of test temperature dependence and strain rate dependence of fracture stress. Furthermore, in exchanging surface energy for grain boundary cohesive energy in the equation, grain boundary fracture stress can be also estimated.
Boron (B) is frequently used as additives to improve the hardenability of advanced high strength steel. It has been reported that B in steel reacts with atmospheric N2 during annealing at low oxygen potential (low dew point) to form boron nitride (BN) by the thermodynamical calculation. In this study, the effect of BN formation on the steel surface on the coatability during hot-dip galvanizing was investigated, experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperature and dew point, and then hot-dip galvanized. The annealed specimens were also prepared and analyzed with GD-OES, XPS, SEM-EDX and TEM-EELS to investigate the oxide and nitride formation on the steel surface during annealing. As results, coatability deteriorated as the amount of B in steel and the annealing temperature increase, and as the dew point decrease. These trends were not correlated with the amount of oxide but the amount of BN formation, suggesting that BN formation deteriorated the coatability. The surface and cross-sectional analysis revealed that BN formed around the oxide to cover the steel surface. This would lead the deterioration of the coatability because most of the steel surface was covered with BN as well as oxide, which are known to have low wettability with molten Zn.
Casting experiments of Al-10 wt.%Cu alloy were carried out using an impreved Satou mold (iST mold). The mold was a rectangular parallelepiped (inner dimensions 30 mm(T) x 50 mm(W) x 140 mm(H)), with a porous alumina plate on the wide side of the mold and a chill set at a height of 70 to 80 mm from the bottom. Four metal materials (stainless, steel, brass, and copper) with different thermal conductivities were used for the chill. To investigate the effect of bridging on the formation of macrosegregation, X-ray CT analysis of the macrosegregation distribution and morphology, observation of micro- and macro-structures, and analysis of temperature and solid fraction distribution were performed for samples obtained under each condition. Bridging formed near the chill under all conditions, and channels consisting of positive segregation and cavities were formed below it. The volume fraction of positive segregation decreased as the thermal conductivity of the chill material increased. In the samples using stainless and copper as chill materials, the volume fractions of positive segregation were 73.8% and 11.7%, respectively. Consequently, we confirmed that the bridging-formed conditions have a significant effect on the formation of macrosegregation.
As a countermeasure for deterioration of raw materials' quality, improvement of sinter productivity is needed. To increase sinter productivity, steam heating granulation technologies have been developed. Heated granules reduce water condensation at wetting zone of sintering bed. It improves permeability of sinter bed and sinter productivity. At first, sintering properties and heat conductive efficiency were investigated at laboratory steam injection apparatus. As a result of the laboratory test, it was confirmed that the moisture condensation at the raw material bed was suppressed by steam injection, permeability was improved, and the sinter productivity was improved. Second, on the actual plant test, the rise of raw material's temperature by steam heating was verified. On that test, granules were heated by 20 degrees C. In terms of difference of heat conductive efficiency between laboratory and actual plant test, heat and material balance were examined to establish the heat transfer model at actual process. According to the heat analysis of actual plant test, indirect heat transfer via lining of drum mixer also contributed to heat granules.
