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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 60 (2020), No. 8

  • Iron Powder-based Metal Matrix for Diamond Cutting Tools: A Review

    pp. 1571-1576

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    DOI:10.2355/isijinternational.ISIJINT-2019-662

    New material technology, advancement in manufacturing technology and development of alternate materials are key enablers for today’s process and product development. Although Diamond cutting tools has a long history started back in 1862, this industry has gone through a tremendous change in last 50 years with the development of synthetic diamond and continues exploration of alternate metal matrix materials (alternate to cobalt). For each specific application, powder for metal matrix needs careful selection based on its chemical composition, size, shape and thermal stability. Cobalt powder was the most commonly used metal matrix powder. However, fluctuating price and health concerns associated with cobalt powder has compelled the industry to search for more economical and sustainable alternative. In this context Iron and Iron based alloy powders turned out to be promising metal matrix powder alternatives for use in diamond cutting tools catering cutting, drilling, grinding, etc applications.This paper gives an overview on the diamond cutting tools history, evolution, trends and developments with special focus on the usages of iron powder as an alternate matrix material for diamond cutting tools.
  • Evolution of TiN and Oxide Inclusions in Ti-containing Fe-25Ni-15Cr Alloy during Electroslag Remelting

    pp. 1577-1585

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    DOI:10.2355/isijinternational.ISIJINT-2019-603

    The present study was undertaken to investigate the evolution of inclusions in a Ti-containing Fe-25mass%Ni-15mass%Cr alloy during electroslag remelting (ESR). The effect of slag composition on the inclusions in alloy was studied. The inclusions in both consumable electrode and remelted ingots are mainly 1 to 3 µm in size. The inclusions in consumable electrode are TiN, Al2O3–Ti2O3, Al2O3–Ti2O3 with a surrounded TiN layer. The inclusions in liquid metal pool and remelted ingots are TiN, MgO–Al2O3–Ti2O3 inclusion surrounded by TiN, MgO·Al2O3 inclusions, MgO·Al2O3 inclusions with an outer Ti2O3-rich layer. Increasing TiO2 content in slag has no influence on the types of inclusions in remelted ingots. The original TiN inclusions in consumable electrode cannot be dissociated at the electrode tip during the ESR process. TiN inclusions in remelted ingots mainly generated in liquid metal pool during ESR, and the TiN inclusions formed during the solidification of liquid alloy takes up a small amount fraction. Part of Al2O3–Ti2O3 inclusions in consumable electrode were removed through absorbing them into molten slag, and the remaining Al2O3–Ti2O3 inclusions in the liquid alloy reacted with Mg dissolved from ESR slag to form MgO–Al2O3–Ti2O3 inclusions which served as the nucleation sites for TiN inclusion formation. MgO·Al2O3 inclusions in the remelted ingots precipitated in the liquid metal pool during ESR process. The generation of MgO·Al2O3 inclusions with an outer Ti2O3-rich layer originated from the reaction between soluble titanium in liquid alloy and MgO·Al2O3 inclusion to form an outer Ti2O3-rich layer on unreacted MgO·Al2O3 inclusion core.
  • Neural Network Modelling on Contact Angles of Liquid Metals and Oxide Ceramics

    pp. 1586-1595

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    DOI:10.2355/isijinternational.ISIJINT-2019-640

    A neural network model was developed in this paper to predict the contact angles of 21 metals and 14 solid oxides. 15 factors were used in the neural network model to distinguish different metal and oxide categories and experimental conditions. With 1120 contact angle values as the learning data, the neural network model was successfully developed. It can properly reproduce the experimental data on contact angles of molten metals and solid oxides under various conditions. Specifically, only three predictions among the total 1155 predictions were over 20% deviation from the experimental data. All the predictions on the 35 test data are within 20% deviation from the experimental values. Factors such as oxygen partial pressure and surface tension of molten metal were found to be important for a good model prediction. With the developed model, contact angle values of Fe and CeO2 were predicted.
  • Effect of B2O3 on Structure of CaO–Al2O3–SiO2–TiO2–B2O3 Glassy Systems

    pp. 1596-1601

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    DOI:10.2355/isijinternational.ISIJINT-2019-679

    Titanium-bearing blast furnace slag is an important metallurgical waste, but presently, it is difficult for it to be effectively utilized. B2O3 is an important modifier to greatly promote the mass diffusion of crystallization of Ti-enriched phases in molten slags. To furtherly understand the effect of B2O3 on the structure of Ti-bearing slag, CaO–Al2O3–SiO2–TiO2–B2O3 glasses of various B2O3 were investigated by combining Raman, FT-IR, and X-ray photoelectron spectroscopy. The results showed that BO3 was the dominant structure, which decreased slightly as the B2O3 content increased, while BO4 increased. Three coordination forms (TiO4, TiO5, and TiO6) of the Ti-structure were discovered in the prepared Ti-bearing glasses. The percentage of TiO4 gradually increased and became the main structural unit as the increased B2O3. The increase of BO4 and TiO4 leaded to an increased amount of network connection units, such as SiO4, to increase the degree of polymerization of the prepared Ti-bearing glasses.
  • Thermodynamic Study on CaO-SiO2-Al2O3-5%MgO-10%Ce2O3 Slag System: Liquide Lines and Distribution Behavior of Cerium at 1873 K

    pp. 1602-1609

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    DOI:10.2355/isijinternational.ISIJINT-2019-683

    The liquidus of CaO-SiO2-Al2O3-MgO (5mass%)-Ce2O3 (10mass%) system at 1873 K was determined under the reducing atmosphere by adopting the equilibrium and quench method. Furthermore, the thermodynamic activities of Ce2O3 in the melts and the cerium distribution ratio between slag and liquid tin were measured at 1873 K by chemical equilibrium method. By using the measured activity data, an empirical formula was proposed to estimate the activity coefficient of Ce2O3. From the formula, it was found that Al2O3 behaved like basic oxide in this system. In addition, for the melts with the same Ce2O3 contents, the activity coefficient of Ce2O3 increase gradually with the increase of the basicity of the melts. This implies that the increase of the basicity is favorable to the enrichment and precipitation of Ce-containing mineral phases. Finally, an empirical formula was introduced to express the relationship between the cerium distribution ratio and the composition of melts. It was found that the cerium distribution ratio decreased gradually with the increase of the basicity of the melts, which means that decrease of the basicity is beneficial to the recovery of cerium from liquid metal phase.
  • MnS Precipitation Behavior in MnO–SiO2 Inclusion in Fe–Mn–Si–O–S Alloy System at Solid-Liquid Coexistence Temperature

    pp. 1610-1616

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    DOI:10.2355/isijinternational.ISIJINT-2019-777

    With the emerging significance of creating an acicular ferrite microstructure to provide an optimum set of properties in steel, MnS precipitation behavior on a MnO–SiO2 inclusion at the solid-liquid coexistence temperature was experimentally investigated and thermodynamically elucidated in this study. Using a direct method of forming inclusions, alloy samples with varying sulfur concentrations [Fe-1.1Mn-0.10Si-0.05O-S (initial mass%); 0.005 to 0.031 initial mass% S] were prepared by holding at the solid-liquid coexistence temperature for 1 hour.In samples with less than 0.011 mass% sulfur, the formation of a MnO–SiO2 inclusion with a SiO2-rich precipitate was observed. Formation of SiO2 was described as a consequence of silicon enrichment in the liquid phase, which, under appropriate thermodynamic conditions, homogeneously precipitated and later on coalesced with the primary MnO–SiO2 phase. On the other hand, in samples with more than 0.022 mass% sulfur, heterogeneous precipitation of MnS along the boundary of the primary MnO–SiO2 inclusion and the alloy matrix was observed. Also, the SiO2-rich phase was found to disappear with increasing sulfur addition. Since the likelihood of heterogeneous nucleation is higher than homogeneous nucleation, it was assumed that MnS precipitation on the surface of the primary MnO–SiO2 prevented the secondary SiO2-rich inclusion from coalescing with the existing MnO–SiO2 inclusion. This was also further validated for solute enrichment in the liquid phase, wherein MnS precipitation temperature was found to shift to higher temperatures in alloys with higher sulfur content.
  • Sulfide Capacities of Solid Oxides in Calcium-Aluminate Systems

    pp. 1617-1623

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    DOI:10.2355/isijinternational.ISIJINT-2019-793

    To clarify the mechanism of CaS formation on the oxide inclusion of the CaO–Al2O3 system, the sulfide capacities of solid oxides was measured in the present study. The results show that the sulfide capacity of (12CaO·7Al2O3; C12A7) was much larger than of other compounds, and increased with temperature. The value for C12A7 was larger than that measured for the liquid oxide of the same composition. Furthermore, the diffusion behavior of sulfur in solid steel to the inclusion of the CaO–Al2O3 system, was investigated using a diffusion couple. After heating at 1473 K for 72 h, in the case of C12A7, the intensity of sulfur in the oxide was high, but the formation of CaS was not detected. This suggests that the formation of CaS was suppressed around the C12A7 particles by the diffusion of sulfur in the solid oxide.
  • Development of Analytical Method of Predicting Fissures Formation for Carbonization Process of Coke

    pp. 1624-1632

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    DOI:10.2355/isijinternational.ISIJINT-2019-457

    The development of a numerical method for fissure formation during the carbonization of coke is needed because the formed fissures determine the coke’s particle size. In this study, we proposed and developed a novel numerical method that can represent initiation, extension, and branching of multiple cracks. Fundamental tests (i.e., bending of a beam, stress analysis of a center crack plate, and crack propagation in a plate under tensile loading) were performed using the proposed method. The test results showed that the numerical accuracy of stress analysis and the fracture analysis using the proposed method were high and were comparable to other standard numerical methods. Furthermore, we applied the proposed method for the coupled analysis of heat conduction and thermal stress and performed numerical simulations for the formation of fissures in coke during carbonization. The numerical simulation results showed that major fissures extended in a linear direction, perpendicular to the direction of an oven wall. And the numerical results reflected qualitative features of an actual coke-making process using a chamber oven. Therefore, the proposed method could be used to reproduce the formation of fissures in coke during the carbonization process.
  • Flue Gas Circulating Sintering Based on Biomass Fuel on Reduction of NOx and SO2 Emission

    pp. 1633-1640

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    DOI:10.2355/isijinternational.ISIJINT-2019-505

    In order to reduce the emission concentration of sintered flue gas pollutants, the source control technology and process control technology of iron ore sintering were adopted to reduce the emission concentration of flue gas pollutants. In this paper, the experiments of flue gas circulating sintering pot with charcoal instead of partial coke was carried out. The effects of O2 content, temperature, SO2 content and NOx content in circulating flue gas on sinter properties and emission concentration of flue gas pollutants were studied. The experimental results show that the suitable oxygen content in flue gas was 14.9%, and the maximum suitable concentration of SO2 was 500 ppm based on charcoal instead of 40% coke circulating flue gas sintering. The NOx had little effect on the sintering performance index. Under the conditions of flue gas temperature of 200°C and CO content of 0.85%, the content of charcoal instead of coke was increased from 40% to 50%. The emission reduction of sintering flue gas pollutants SO2 and NOx was more than 42.6% and 65.8%, respectively. The comprehensive utilization of sintering flue gas waste heat and the cooperative control of sintering flue gas pollutants were realized.
  • Influence of Ore Assimilation and Pore Formation during Sintering on Reduction Behavior of Sintered Ores

    pp. 1641-1648

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    DOI:10.2355/isijinternational.ISIJINT-2019-615

    Sintered ores with superior high-temperature reducibility can enhance indirect reduction at the shaft and permeability at the cohesive zone through improving the softening-melting behavior, resulting in stable operation of the blast furnace at low coke rates. As the depletion of high-grade ore deposits limits control of the chemical composition of sintered ores, small pores less than 10 µm in diameter in the microstructure of sintered ores were focused on to increase the high-temperature reducibility. Sintering conditions for increasing small pores in constant raw material conditions were examined. Furthermore, considering the heterogeneous structure of the sintered ores, reduction behaviors of relict ores and assimilated structures were estimated individually. The contribution of each structure to small pore formation during sintering and their influence on high-temperature reducibility was discussed.Sintering with a sharp temperature profile led to many small pores in the sintered ores by increasing the amount of relict ores with small pores even in constant raw material conditions. Both, for relict ores and assimilated structures, low-temperature reducibility was determined by the total porosity including large pores, whereas the Al2O3 content in gangue minerals, the <10 µm pore volume fraction, and the amount of gangue mineral influenced high-temperature reducibility. Assimilated structures involving granular hematite contained many small pores, compared with other types of assimilated structures. Results of plant trials for two different methods to increase small pores, in relict ores and in assimilated structures, revealed their potential for improving the high-temperature reducibility of sintered ores without controlling the chemical composition.
  • Forming Behavior of Fine Particulate Matters during Iron Ore Sintering Process

    pp. 1649-1654

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    DOI:10.2355/isijinternational.ISIJINT-2019-637

    Iron and steelmaking process, especially the iron ore sintering process is regarded as a major fixed generation source of PM2.5. This study aims to obtain fundamental knowledge on the grain size and chemical composition of PM2.5 formed during sintering process. A laboratory-scale sintering simulator was used to simulate the iron ore sintering process, and a part of the outlet gas was introduced to a cascade impactor to collect the PM2.5 in particle size stepwise. Concentration of PM in the outlet gas became highest when the admixing ratio of CaO was set at 20 mass%, and it appears that there are two different formation patterns of PM in different size. Obtained PM with the particles size from 0.25 to 2.5 µm were composed of hematite, calciumferrite and some gangue materials. The liquidus temperature of CaO (20%) - Fe2O3 is lower than the maximum sintering bed temperature and therefore iron oxides in the melt tend to be reduced at coke surface promote to form CO bubbles. Then the bubbles move to the melt surface and burst forming many fine droplets. PM with the particles size less than 0.25 µm contains sulfates of Na, K and Ca in addition to hematite and calciumferrite. When coke containing a certain sulfur was replaced by graphite, the major components of the formed PM (−0.25 µm) was chlorides of Na and K.
  • Phase Composition and Properties Distribution of Residual Iron in a Dissected Blast Furnace Hearth

    pp. 1655-1661

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    DOI:10.2355/isijinternational.ISIJINT-2019-712

    Dissection research is the most meaningful way to understand the internal conditions of blast furnace. Because it can obtain samples from hearth after a blast furnace was shut down. In this paper, a 2200 m3 commercial blast furnace was shut down with residual iron remained inside. And then core drilling method was used to obtain a horizontal sample of residual iron, which was located 1610 mm below the centerline of taphole and had a length of 1840 mm. A variety of techniques, such as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Optical Microscope, X-ray diffraction (XRD) and Raman spectroscopy were applied to analysis the microscopic morphology, phase composition and properties distribution of residual iron samples at different position. The results show that complex phases appear near the cold side, including Fe, C, Ti3N2, KAlSi2O6 and Al2O3. And distribution of properties along the core sample from inside to cold side like density, degree of graphitization and thermal diffusion also have a big change. These results are considered to be related to the function of cooling system.
  • Effect of Large Amount of Co-injected Gaseous Reducing Agent on Combustibility of Pulverized Coal Analyzed with Non-Contact Measurement

    pp. 1662-1668

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    DOI:10.2355/isijinternational.ISIJINT-2019-735

    Nowadays, reduction of amounts of CO2 from ironmaking process is important from the aspect of prevention of the global warming. COURSE50 is Japanese national project which aims to reduce the amount of CO2 emission from ironmaking process by 30% by 2050. In COURSE50, we try to reduce iron oxide with H2 to decrease amounts of carbon use, by injecting large amount of gas containing H2 and pulverized coal (PC) from tuyere. In that case, PC combustibility can be different from that in general blast furnace condition, due to high co-injected reducing gas ratio. Though a large number of researches about PC combustion around tuyere of blast furnace has been carried out, the effect of large amounts of co-injected reducing gas on PC combustibility was hardly investigated. To evaluate that, we conducted experiments with two experimental furnaces equipped with various non-contact measurement apparatus and found that;1) The larger amounts of co-injected reducing gas were, the faster O2 and CO2 consumption, and CO and H2 generation in the raceway.2) The amounts of co-injected reducing gas should be optimized for higher PC combustibility.3) Co-injected reducing gas activated PC combustion by raising PC temperature, and that resulted in acceleration of PC.4) Trade-off relationship between rapid heating effect and O2 consumption of co-injected reducing gas could determine the optimum amounts of reducing gas.Consequently, we elucidated how we could co-inject reducing gas with PC as reducing agents without deteriorating PC combustibility.
  • Physico-chemical Properties of Mill Scale Iron Powders

    pp. 1669-1674

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    DOI:10.2355/isijinternational.ISIJINT-2019-737

    Powdered form of iron typically < 250 microns is phrased as iron powder. Commercial manufacturing of iron powder is majorly restricted to atomization, carbonyl, electrolytic and reduction routes. Powders from the later three techniques generally cater to special iron powders applicable to non-powder metallurgy segments such as food fortification, chemical reagents, water purification, etc. In this work, mill scale iron powders are synthesized by thermo-chemical reduction of mill scale, a steel industry by product, and their properties are analysed and compared with commercial iron powders such as carbonyl, electrolytic and reduced. Chemical and physical characterizations such as Optical Microscopy, Scanning Electron Microscopy and X-ray Diffraction of the powders are performed. Obtained results reveal that, mill scale iron powders MIP and MIP45 possess good product properties. Especially, MIP45 grade exhibited finer particle size of D50< 30 microns and BET surface area of 0.63 m2/g along with Fe (T) > 98%, true density-7.55 g/cc, apparent density 2.67–2.83 g/cc, packed bulk density of 3.44 g/cc and good flowability. This product of mill scale with highest apparent density and good surface area is expected to qualify to new segment of applications along with other commercial iron powders.
  • Prediction of Plunging Depth Induced by Top Lance Gas Blowing onto a Low-melting-point Metal Bath

    pp. 1675-1683

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    DOI:10.2355/isijinternational.ISIJINT-2019-790

    Experimental and numerical investigations were carried out with a water bath and a low-melting-point metal bath to understand the cavity formation behavior induced by gas blowing from a top lance set in the near field of the bath surface. Cavity formation behavior was classified by the critical gas velocity for droplet formation. The depth of the cavity formed in the near field of the gas jet was reasonably predicted by numerical simulation. A new empirical equation for the cavity depth as a function of a modified Froude number was proposed.
  • Effects of Nitrogen Gas Pressure on the Solidification Parameters and As-cast Microstructure Revolution during Pressurized Electroslag Remelting AISI 304 Stainless Steel

    pp. 1684-1692

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    DOI:10.2355/isijinternational.ISIJINT-2019-203

    Three AISI 304 stainless steel electrodes were remelted using the lab-scale pressurized electroslag remelting furnace under different nitrogen gas pressure conditions. The solidification parameters and microstructure evolution have been investigated with the sulfur print method method, color metallography and EPMA. The results showed that the pool depth, SDAS and mushy zone width firstly increased and then decreased with the increase of gas pressure from 0.1 to 1.2 MPa. With an approximately equal melting rate, the variation of solidification parameters is dependent on the competition between the heat transfer rate at the slag/pool interface and the ingot/mould interface, because increasing the nitrogen gas pressure could simultaneously increase the two heat transfer rates. Under the current pressure range, the solidification mode and microsegregation during solidification are not affected by the variation of gas pressure. In addition, the variation of nitrogen gas pressure could simultaneously change the nitrogen content and cooling rate in ingots. Both the nitrogen content and cooling rate could affect the content and composition of residual ferrite. However, under the current experiment conditions, the variation of nitrogen content plays a more important role in the content of residual ferrite than the cooling rate, because nitrogen is a strong austenite former element and the cooling rate has no wide variation.
  • Effect of Direct Powder Additions on the Solidification Structure and Microsegregation of 42CrMo4 Steel

    pp. 1693-1702

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    DOI:10.2355/isijinternational.ISIJINT-2019-787

    Inoculation and its link with the solidification structure is a relatively new field for low alloy steels. In this study, a cold crucible setup is used to realize direct particle inoculation of 50 g steel ingots. Eight different inoculants powders (oxides, nitrides and carbides) were tried with a 0.3 mass% level addition. Solidification structure sizes and morphologies, presence of inoculant particles and microsegregation have been characterized for all the samples. The best grain refinements were obtained for Si3N4, TiN and CeO2 additions whereas the lowest microsegregation intensities are achieved for Si3N4, HfC and W2C additions. The properties of the inoculants – misfits, solubility products and terminal velocities – are used to discuss those changes. The grain refinement could be linked to the misfit in good agreement with the literature. Other morphological changes (secondary arm spacing and dendricity) were attributed to the presence of inert particles staying inside the liquid during the solidification. Last but not the least, the flattest microsegregation profiles were possibly due to inoculant dissolution leading to a change in the MnS precipitation sequence.
  • An Adaptive Selection of Filter Parameters: Defect Detection in Steel Image Using Wavelet Reconstruction Method

    pp. 1703-1713

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    DOI:10.2355/isijinternational.ISIJINT-2019-464

    We proposed a scheme for adaptively selecting filter parameters for detecting defects in various image textures. To implement the proposed scheme on a target steel image, we used wavelet reconstruction method. The adaptive parameter-selecting scheme was presented by analyzing the textures in an image and obtaining the appropriate parameters from a pretrained neural network by inputting these texture features. Experiments were conducted to detect corner cracks in the images of a steel billet, and the proposed scheme was compared with a conventional wavelet reconstruction method. The experimental results showed that our proposed scheme was effective in detecting defects in various textures of the target images.
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  • Multiscale Analysis of MnS Inclusion Distributions in High Strength Steel

    pp. 1714-1723

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    DOI:10.2355/isijinternational.ISIJINT-2019-739

    In the present study, manganese sulfide (MnS) inclusions in the high-strength steel were observed by mainly three observation methods (optical microscope, ultrasonic test and serial sectioning) to characterize the size, location and shape distributions across multiple length scales. For the inclusion size, ultrasonic C-scan imaging and three-dimensional internal structure observation with serial sectioning were used to measure the distributions of the square root of the projected area of the inclusion. The obtained size distributions were combined by setting the threshold of ultrasonic amplitude. The validity of the amplitude threshold was verified by observing several inclusions with X-ray CT. The spatial distributions of inclusions were also obtained by the three observation methods, and analyzed on the basis of the coefficient of variation of the mean near-neighbor distance of inclusions (COVd). The results of analyzing COVd in both 2D and 3D spaces revealed that the inclusions in this material were arranged in clusters. For the inclusion shape, the three-dimensional geometries of inclusions were reconstructed from the images obtained by the serial sectioning method, and simplified to ellipsoid by principal component analysis. From the above results, the distributions of inclusion size, aspect ratio and direction (angle between rolling direction and major axis) were successfully obtained. The inclusion distributions were applied to fatigue prediction model, and the fatigue crack initiation life and total fatigue life of the high-strength steel were calculated. The calculation results showed that the multiscale analysis of inclusions would be useful for fatigue life prediction.
  • Sample Temperature Effect on Steel Measurement Using SP-LIBS and Collinear Long-short DP-LIBS

    pp. 1724-1731

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    DOI:10.2355/isijinternational.ISIJINT-2019-740

    Laser induced breakdown spectroscopy (LIBS) has been investigated as a potential multi-element quantitative analysis tool for the quality control of on-line steel production. This research investigated influence of sample temperature on steel sample measurement using collinear long-short dual-pulse LIBS (long-short DP-LIBS) and single-pulse LIBS (SP-LIBS). The standard steel sample has been uniformly heated in a muffle furnace from 20°C to 700°C. The experimental results show that sample temperature has significantly effect on measurement result using SP-LIBS. However, long-short DP-LIBS can effectively reduce the sample temperature effect on measurement result. The detection characteristics of long-short DP-LIBS and SP-LIBS were compared using the intensity ratio of I Mn 404.136 nm/I Fe 400.524 nm and I Fe 402.187 nm/I Fe 400.524 nm under different delay time and different sample temperature conditions. The signal intensity and plasma temperature can be maintained higher and more stable for a period of time and at different sample temperature by long-short DP-LIBS with smaller error bar compared with that of SP-LIBS, which indicated long-short DP-LIBS has better measurement repeatability than SP-LIBS. The plasma temperature correction method was applied to compare the detection features of long-short DP-LIBS and SP-LIBS. The signal stability of long-short DP-LIBS measurement was improved significantly at different sample temperature with plasma temperature correction. These results demonstrated that the effect of sample temperature can be reduced using long-short DP-LIBS method to improve the on-line detection capability for steel measurement in complex environment.
  • Sticking in Hot Rolled Sheet of Ferritic Stainless Steel

    pp. 1732-1736

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    DOI:10.2355/isijinternational.ISIJINT-2019-595

    In hot rolling of ferritic stainless steels, prevention of sticking to rolls is very important from the viewpoint of productivity. However, the formation mechanism of the sticking has not been clarified sufficiently. Therefore, in this work, rolling experiments were carried out using a tribo-simulator. The results clarified the following points: Sticking occurs more easily on ferritic stainless steel than on high strength steel. On ferritic stainless steel, a work roll sticks with the hot-rolled sheet at the entrance of roll-bite, and the work roll then moves forward on the hot-rolled sheet. Therefore, it is thought that the surface layer of the hot-rolled sheet is fractured by large frictional shear stress, and the work roll stuck with the fractured layer advances further on the sheet, forming defects with an accumulated fractured surface layer. Lubrication with oil is effective for prevention of sticking between the work roll and the hot-rolled sheet.
  • Effects of Thermal Shields on Temperature of a Hot Steel Strip in Rolling Process

    pp. 1737-1742

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    DOI:10.2355/isijinternational.ISIJINT-2019-687

    The temperature of the hot steel strip between roughing and finishing mills is important parameter to manufacture the high-quality products. It has great influences on mechanical properties and manufacturing costs of the steel products. The hot steel strip between roughing and finishing mills has considerable heat loss and temperature drop due to the exposure to cold environment and air. The thermal shield is required to reduce the heat loss and temperature drop of the hot steel strip. Thus, the model prediction for the temperature of the hot steel strip is developed to design the thermal shield and identify its effects. The experiment for the hot steel strip sample is performed to validate the model prediction. The results of the experiment and model prediction have the good agreement for different emissivity and temperature of the thermal shields. In addition, the temperature variation and distribution of the hot steel strip between roughing and finishing mills is predicted for different geometries of the thermal shields. The thermal shield with low emissivity and high temperature reduces the temperature drop of the hot steel strip for practical purposes.
  • Transient Bottom Jet Impingement Cooling of Steel

    pp. 1743-1751

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    DOI:10.2355/isijinternational.ISIJINT-2019-691

    Accelerated run-out table cooling has become a key technology that determines microstructure and resulting mechanical properties of thermo-mechanically controlled processed (TMCP) steels. The present study quantifies the heat transfer mechanisms during bottom jet cooling of a stationary steel plate with systematic pilot-scale experiments. The emphasis of the study is to quantify the effect of process parameters, i.e. jet impingement velocity, water temperature and nozzle orientation, on heat extraction rates. Experimental results are described and quantitatively analyzed, adding to the database for run-out table cooling.
  • Tracking the Welding Line in Lap Welding Using Pattern Matching

    pp. 1752-1757

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    DOI:10.2355/isijinternational.ISIJINT-2019-319

    Lap welding is employed to join thin plate materials in car assemblies. The quality of welding depends on its conditions and seam tracking in the welding line. An arc sensor was employed by oscillating the welding torch in a thick weld. Since the welding torch was not oscillated in the welding of thin plate materials, the arc sensor could not be applied to lap welding. To trace the welding line, the weld pool in pulsed metal active gas (MAG) welding under the shielding gas of Ar 80% and CO2 20% was pictured using a CMOS camera. The weld pool was observed in the rear to the welding direction. The features of the weld pool were investigated to recognize the welding line. Since the arc affected the brightness of the weld pool, the timing of the shutter of the CMOS camera was synchronized with the current waveform to take clear weld pool images. The welding line was detected using an image processing method. In this paper, a banalization method and a pattern matching method was investigated. The pattern matching method is useful than the binarization method. A digital controller was designed to trace the welding line. The validity of the proposed method was verified by carrying out a tracking experiment.
  • Crystallographic Orientation Control of 316L Austenitic Stainless Steel via Selective Laser Melting

    pp. 1758-1764

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    DOI:10.2355/isijinternational.ISIJINT-2019-744

    In recent years, additive manufacturing has attracted attention as a technology that enables control of the crystallographic texture of metallic materials. We achieved successful control of the crystallographic texture of 316L austenitic stainless steel using selective laser melting (SLM). Three distinguished textures were achieved by changing the laser scan speed, namely: the single crystalline-like texture with {001} orientation in the build direction, the crystallographic lamellar texture in which two kinds of grains with {011} and {001} orientations in the build direction are alternately stacked, and polycrystalline with relatively random orientation. The melt pool shape and the solidification behavior (thermal gradient and migration velocity of solid/liquid interface) in a melt pool could be important controlling factors for the evolution of the crystallographic texture under the SLM process.
  • Segregation Mechanism of Al-based Oxides on Surface of Zn-0.2mass%Al Hot-dip Galvanized Steel Sheets

    pp. 1765-1773

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    DOI:10.2355/isijinternational.ISIJINT-2019-749

    It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55–57 g/m2 including 0.19–0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.
  • Effect of Retained Austenite on Sub-surface Initiated Spalling during Rolling Contact Fatigue in Carburized SAE4320 Steel

    pp. 1774-1783

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    DOI:10.2355/isijinternational.ISIJINT-2019-715

    The effect of retained austenite (γR) on the rolling contact fatigue (RCF) properties of carburized SAE4320 steel was carefully investigated. We prepared specimens comprising four volume fractions of γR from 6% to 39% by controlling the subzero heat treatment. The effect of γR on the RCF was investigated using hardness measurements, X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy. The RCF test revealed that the sub-surface initiated spalling life was prolonged as the volume fraction of γR increased. In the area at the depth z0 where the orthogonal shear stress was maximum, the majority of the γR was transformed to martensite, thus resulting in a significant increase in the Vickers hardness. The result of SEM observation showed that the region initially comprising γR exhibited a high resistance to RCF. Moreover, the TEM analysis revealed that the initial γR region changed into a mixture of very fine hard martensite and some unchanged γR during RCF. This suggests that the transformation of γR into fine hard martensite during RCF contributed to the improvement of the RCF life.
  • Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

    pp. 1784-1795

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    DOI:10.2355/isijinternational.ISIJINT-2019-723

    The partitioning of solute elements during intercritical annealing and the effects of partitioning on ferrite transformation during slow cooling after intercritical annealing in a 0.17% C–1.5% Si–1.7% Mn (mass%) steel were investigated by a new FE-EPMA (field emission electron probe microanalysis) technique. This new technique enables highly accurate measurement of the C distribution. During the intercritical annealing, C and Mn concentrated into austenite, while Si concentrated into ferrite. The distribution of Mn in austenite was inhomogeneous, and austenite with small Mn content was transformed into ferrite during slow cooling. This ferrite transformation proceeded in the NPLE (negligible partitioning local equilibrium) mode. Two kinds of ferrite were produced due to slow cooling, one being intercritically-annealed ferrite, and the other transformed ferrite. The transformed ferrite had larger Mn content than the intercritically-annealed ferrite. Furthermore, the transformed ferrite was classified into the ferrite grown epitaxially from the intercritically-annealed ferrite and that nucleated in the austenite with relatively small Mn content. Prior microstructure and distribution of solute elements before cooling are determined by the intercritical annealing conditions, and then control the ferrite transformation. Precise control of the ferrite transformation is effective for stable production of cold-rolled high strength steel with composite microstructure.
  • Effect of Boron Addition for on Time Temperature Transformation Behavior in Si Added High Carbon Steels

    pp. 1796-1802

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    DOI:10.2355/isijinternational.ISIJINT-2019-725

    In high carbon steel, TTT nose temperature rises and upper baninte becomes easy to be formed with quantity of Si addition. Generation of upper bainite is reduced by boron addition. In this study, the influence of boron addition on isothermal transformation behavior in Si-added high carbon steel was clarified. By boron addition, lamellar spacing and growth rate of pearlite doesn’t change, but the nucleation of pealite is reduced. But nucleation of pearlite is promoted when Fe23(C,B)6 precipitates. In the Si-added high carbon steel, upper bainite is often formed with the generated ferrite on prior austenite grain boundary. It is inferred that boron reduces ferrite generation in grain boundary which causes upper bainite formation. It is confirmed that effective existence state of boron is grain boundary segregation.
  • Manipulating Dislocations Using Electric Field to Repair Embrittlement Damage

    pp. 1803-1809

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    DOI:10.2355/isijinternational.ISIJINT-2019-583

    Dislocation defects induced by neutron irradiation can degrade the mechanical properties of reactor pressure vessel steel; although the properties can be restored by annealing, this treatment is energy- and time-intensive. A fast and energy-efficient method of decreasing the dislocation density is urgently needed. In this study, electric pulse treatment was applied to damaged A508-3 steel to remove the dislocation defects quickly. The pulsed electric current reduced the dislocation density in a brittle sample and improved the impact toughness. After electric pulse treatment, the ductile–brittle transition temperature was 11.8°C lower. Calculations revealed that the higher temperature around the dislocations and the electron wind force under the electric field decreased the activation energy of dislocation motion, causing the dislocations to move more easily. The dislocation defects can be annihilated as they move, decreasing the dislocation density.
  • Lattice Strain and Strength Evaluation on V Microalloyed Pearlite Steel

    pp. 1810-1818

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    DOI:10.2355/isijinternational.ISIJINT-2019-708

    The strengthening mechanism of microalloyed vanadium (V) on eutectoid pearlite steel was investigated from the perspectives of nano-precipitation and lattice strain. The 0.2% proof stress of specimens, isothermally transformed at 873 K, increased by about 160–170 MPa with the addition of 0.1% V. However, the interphase precipitation of vanadium carbide (VC), regarded as the principal strengthening factor, was detected neither by transmission electron microscopy nor by 3D atom probe microscopy (3D-AP). A lattice strain in lamellar ferrite, analyzed by broadening of the X-ray diffraction peak, has been experimentally estimated to understand the strengthening mechanisms by V-addition. The lattice strain data of 0.1% V-added pearlite specimens were plotted on the same correlation line as those of the V-free specimens with proof stress. In addition, the elemental map obtained by 3D-AP showed that V atoms concentrate in lamellar cementite rather than ferrite, which could change the cementite lattice parameters and gain ferrite/cementite misfit, causing lattice strain increment. These results revealed that microalloyed V influences not only VC precipitation in lamellar ferrite but also the lattice strain increment in pearlite lamellar. In the case of pearlite steels containing at most 0.1% V, lattice strain was considered the major factor of their yield behaviors. Furthermore, 0.1% V addition did not enhance work-hardening behavior as notably as that estimated by Ashby’s work-hardening theory of dispersion-hardened crystals. Therefore, VC precipitation is not necessary for the V strengthening effect on pearlite steel.
  • Modelling and Crystal Plasticity Analysis for the Mechanical Response of Alloys with Non-uniformly Distributed Secondary Particles

    pp. 1819-1828

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    DOI:10.2355/isijinternational.ISIJINT-2019-754

    The relationship between yield stress and the distribution of microscopic plastic deformation was numerically investigated by using a crystal plasticity finite element method (CP-FEM) in the model where particles were randomly distributed. It was in order to reveal which particle spacing. i.e., the maximum, minimum or average particle spacing, can be taken as the representative length which controls yielding. The critical resolved shear stress for the onset of the slip deformation in any element was defined under the extended equation in the Bailey-Hirsch type model. The model includes the term of the Orowan stress obtained from the local values of the representative length. Each particle spacing was distributed with a standard deviation of approximately 2 to 3 times larger than the average particle spacing. The macroscopic mechanical properties obtained with CP-FEM were in good agreement with those experimentally obtained. The onset of microscopic slip deformation depended on the particle distribution. Plastic deformations started first in the area where the particle size is larger, then the plastic region grows in the areas where the particle spacing is smaller. Slip deformation had occurred in 90% of the matrix phase by the macroscopic yield point. The length factor in the Orowan equation was the average spacing of the particles in the model, which is in good agreement with Foreman and Makin. The CP-FEM indicated that in dispersed hardened alloys, microscopic load transfer occurred between the areas where the large particles spacing and the small one at the yielding.
  • Formation of Monotectic Sulfide in Free-Machining Ferritic Stainless Steels during Solidification

    pp. 1829-1831

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    DOI:10.2355/isijinternational.ISIJINT-2019-792

    The sulfide morphology in free-machining ferritic stainless steel was investigated and the mechanism of monotectic sulfide formation was evaluated. Metastable monotectic sulfide containing oxygen was mainly observed. The decrease in sulfide activity by dissolved oxygen can be considered to contribute to the formation of monotectic sulfide. It is considered that the oxygen content greatly affected the formation of monotectic sulfide because of the generation of oxide inclusions (SiO2–Al2O3–MnO–Cr2O3), which became the nucleation sites for liquid sulfide in the monotectic reaction. Microsegregation of sulfur and oxygen during solidification reduces the interfacial energy between liquid and monotectic sulfide. Nucleation plays an important role in the selection of sulfide morphology.
  • Discussion of the Accuracy of the Multi-Phase-Field Approach to Simulate Grain Growth with Anisotropic Grain Boundary Properties

    pp. 1832-1834

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    DOI:10.2355/isijinternational.ISIJINT-2019-722

    Well-defined benchmark problems based on simple geometries and idealized assumptions are extremely useful, because they offer a precise analytical solution as reference for quantitative validation of alternative numerical simulation approaches. In a recently published paper,1) different phase-field approaches for anisotropic grain growth were validated by application to a tri-crystal benchmark problem. It was concluded that the multi-phase-field (MPF) approach by Steinbach and Pezzola2) can only be applied to predict anisotropic grain growth, if an error of around 10% is accepted. An extended phase-field approach3) with adjusted higher-order energy terms was claimed to allow for prediction with significantly improved accuracy. However, a wide-spread approximate solution to the tri-crystal problem was used as reference to validate the phase-field results. As the mathematical inaccuracy of this approximation widely exceeds the evaluated inaccuracy of the phase-field results, the conclusions of this validation have to be questioned. The present paper provides the accurate analytical solution to the tri-crystal problem and discusses the implications of the approximation on the accuracy evaluation. For a reevaluation, a series of own MPF simulations were performed. Comparison with the derived analytical solution proves the high-accuracy of the MPF2) formulation and demonstrates that additional higher-order energy terms in the free energy functional are not required, but can result in considerable deviation from the targeted sharp interface solution.

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