ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Volume 63, Issue 2
Displaying 1-24 of 24 articles from this issue
Publication Data
Ironmaking
Review Article
  • HongXing Zhao, FengShan Zhou, XinCheng Bao, SiHan Zhou, ZhongJing Wei, ...
    Article type: Review
    2023 Volume 63 Issue 2 Pages 205-215
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 10, 2022
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    Humic acid is inexpensive, and has a wide range of sources and a strong adsorption force with the surface of iron ore, and the size of its adsorption force is affected by some factors such as humic acid concentration, pH of the solution, and metal cations. The modified humic acid pellet binder (MHA) with strong adhesion and high viscosity was obtained through the separation and purification of solid wastes such as lignite and weathered coal and chemical modification treatment and successfully applied in the production process of iron ore pellets. After industrial tests, MHA can significantly improve the strength of green and dry pellets, less residue after high-temperature roasting, less metallurgical pollution, high strength of fired pellets, and can partially or completely replace bentonite.

Fundamentals of High Temperature Processes
Regular Article
  • Dmitry Chebykin, Hans-Peter Heller, Mariia Ilatovskaia, Klaus Schulz, ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 216-227
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: November 09, 2022
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    The thermal conductivity of two commercial mold fluxes has been investigated using the transient hot-wire method in the temperature range of 298 K to 1573 K. Experiments were conducted on granules, sintered granules, molten fluxes and glassy samples under ambient atmosphere. Characteristic temperatures of the mold fluxes were investigated using a hot stage microscope. Results are discussed in the context of the characteristic temperatures, the particle size of granules, the bulk density and the structure of the mold fluxes. The temperature dependence of the thermal conductivity for granules of mold fluxes is positive in the temperature range of 298 to 873 K (λ = 0.058 + 1.245 × 10−4 T). The further temperature increase causes the rapid increase of the thermal conductivity above the temperature, at which the granules start to sinter Tdeformation. The thermal conductivity of the glassy sample is around two times lower than the thermal conductivity of the devitrified sample. The effect of the glass transition temperature on the thermal conductivity was shown. The thermal conductivity rapidly increases above the glass transition temperature in the glass transition region as the temperature rises and decreases above the softening temperature Tsoftening. The further temperature increase caused the crystallization of the sample. The crystallization of the sample was observed at TTg + 100 K. Moreover, results show the dependence of the thermal conductivity of molten mold fluxes on the basicity (CaO/SiO2) at 1573 K.

  • Isiri Upeksha Nagasinghe, Takeshi Saito, Takato Takemura, Ken Kawamoto ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 228-234
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 18, 2022
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    Acidic soils can induce several negative impacts, especially in agricultural fields. To address these problems, lime is often applied to increase the pH value of acidic soils. Calcium carbonate is the most common and conventional agricultural lime; however, it is a natural and scarce resource. To promote a recycling-based society, alternative neutralizers with lower costs that use alkaline waste and by-products are essential. Therefore, we investigated the effectiveness and applicability of three types of autoclaved lightweight aerated concrete, recycled concrete, steel slag as basic oxygen furnace slag, and fly ash (mainly particles less than 0.106 mm and 0.106–2 mm in size), as alternative neutralizers for three representative acidic soils through laboratory neutralization experiments. The neutralization performance was evaluated by measuring the additive weight percentage of each neutralizer required to convert each acidic soil to neutral soil (pH 7). For neutralizers with two particle sizes, the finer fraction clearly showed lower additive weight percentages indicating higher neutralization performance. Among the six tested alkaline waste and by-products, the steel slag exhibited the highest neutralization performance. In particular, finer fraction steel slag exhibited a high neutralization performance, similar to that of the conventionally used calcium carbonate. This result suggests that fine steel slag (particle size < 0.106 mm) is the most promising and suitable alternative neutralizer.

  • Junwoo Park, Eunju Kim, In-kook Suh, Joonho Lee
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 235-243
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: November 08, 2022
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    Silico-ferrite of calcium and aluminum (SFCA) is the primary bonding phase of iron ore sinter, the world’s most popular artificial feed material for ironmaking. However, fundamental questions about its crystal structure and the atomic occupancy at each site remain unanswered. To date, the quantitative phase analysis (QPA) of SFCA has mostly been conducted using two-dimensional information and only provided information regarding phase fractions. In contrast, Rietveld analysis uses bulk data and provides lattice information in addition to phase fraction information. This study investigates the effects of basicity and Al2O3 concentration on the crystal structure and atomic site occupancy of SFCA through Rietveld analysis of the X-ray diffraction patterns. Raman spectroscopy and micro-Vickers hardness tests are used to verify the analytical results. Changes in the chemical composition affect the atomic occupancies at sites Si1 (Si–Al), Ca2 (Ca–Fe), Ca3 (Ca–Fe), Fe4 (Fe–Al), Fe5 (Fe–Al), and Fe7 (Fe–Al). With increasing basicity or Al2O3 content, the microhardness increases linearly, which can be attributed to the modification of atomic site occupancies. The crystalline structure obtained in this study is essential for developing a thermodynamic model of SFCA that can be used to predict its phase stability. This information can then be used to design a novel high-performance iron ore sinter.

  • Kensuke Takabayashi, Iori Shigematsu, Yoshihiko Higuchi
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 244-252
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 31, 2022
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    In the steelmaking process, it is necessary to decrease impurities in steel to meet the increasing demand for high-grade products. Top blowing and blasting of powder reagent are desirable for the purpose and the deeper penetration of particles into the bath is important for efficient refining. In the present work, CFD calculation with VOF (Volume of Fluid) method and dynamic mesh was executed to study the reported penetration and residual bubble behavior of polypropylene sphere (diameter of 9.6 mm) with a static contact angle of 87° and 143° and an entry solid sphere velocity of 0.63, 0.89, and 1.53 m/s in the water model experiments. Calculated results showed the numerical analysis could evaluate the formation and breakup of air column behind the sphere and the generation of consequent residual bubble on the sphere. Good wettability and high entry speed promoted the deeper penetration of the sphere. Calculated dynamic contact angle on the basis of Kistler’s model indicated that the difference between static and dynamic contact angles was within 13.7° in the present conditions and the discrepancy could not wield a substantial influence on the result of CFD calculation. The adoption of base and refined mesh without parallel zone around the sphere could not give a good agreement with the experimental results. On the other hand, the use of layer mesh was appropriate for reproducing the penetration depth and residual bubble volume observed in the experiments.

Ironmaking
Regular Article
  • Yong Deng, Ran Liu, Kexin Jiao, Hongxiu Ma, Ziyu Guo, Sai Meng, Mingbo ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 253-260
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 18, 2022
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    The iron-carbon interface plays an important role in hearth, the dissection investigation of BFs was carried out to clarify the iron-carbon interface phenomenon and reaction behavior. The results show that the iron-coke interface can be divided into two types: partial coverage and overall coverage. The molten iron penetration from the interface to the center of coke is found. The hot face of residual carbon brick has obvious embrittlement layers, the pulverization phenomenon of carbon brick is observed, cracks appear in the carbon brick near the iron-carbon brick interface. Many holes in coke are filled with iron and slag, the filling rate increases as the coke moves towards the lower part of hearth. The graphitization degree of coke increases through dissolution precipitation and carbide transformation mechanism, the high graphitization of coke caused by carburization at iron-coke interface is the essential cause of coke deterioration. The flexural strength of carbon brick decreases under the alternate process of penetration and dissolution, effect of Zn. The critical temperature difference for crack generation decreases from 286°C to 208°C after the reaction, which makes it easier for the carbon brick to produce cracks. These cracks produce brittle embrittlement layers, which accelerates the erosion of carbon brick. The carburization of coke and the dissolution carburization of carbon brick are carried out simultaneously in hearth, there is a competitive carburization between iron-coke interface and iron-carbon brick interface. The methods of controlling competitive carburization are put forward to delay the erosion of carbon brick.

  • Xuheng Chen, Wei Wang, Daiwei Yang, Heng Zheng, Bin Wang
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 261-270
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    The mineral phase composition and structure of sintered ore are closely related to its quality. Analyzing the characteristics of the phase structure of sintered ores and studying the influence of sintering conditions are important to optimize their quality. In this study, the mineralization characteristics of ore during the sintering process were studied through interrupted quenching experiments. The results clarified the evolution of the structure of the sintered ore phases during the heating and cooling and its mechanism. In this study, the effects of the cooling rate on the mineral composition and structure of sintered ore were also studied. The results showed that the structure of the mineral phase changed during the iron ore sintering process as follows: granular → molten → dissolution texture. During the cooling process, as the temperature decreased, the magnetite and liquid phase contents gradually decreased, and the silico-ferrite of calcium and aluminum (SFCA) and hematite contents increased; however, the porosity did not change to a great extent. The formation of SFCA during the cooling stage consumed hematite and magnetite, and SFCA preferentially formed and grew on magnetite. As the cooling rate of the sinter was increased, the magnetite and silicate contents increased, and the hematite and SFCA contents decreased. Meanwhile, the structure of SFCA changed from block-like to columnar and acicular, and the aspect ratio increased.

  • Arijit Chakrabarty, A. Bhanuchander Raju, Sameeran Pani, Ujjal Ghosh, ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 271-281
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    Good permeability of the blast furnace bed is of paramount importance for stable operations of the same and to achieve it, controlled burden distribution is the key. While coke is primarily used to maintain permeability of the furnace bed (with the ferrous burden particles being considerably smaller), global calls for reducing CO2 emissions substantially are pushing blast furnaces to consume lower amounts of the same. Considering this, other burden distribution methods must be explored to maximize permeability of the burden layer. To do so, efforts are made to control pellet dumping in the burden layer in such a way that it occupies more volume in the mid-radial portions away from the walls. Pellets being spherical in shape and having narrower size distributions than sinter offer more inter particle porosity. Discrete element method (DEM) simulations are used to study the effect of the delay in pellet loading time in the ferrous burden (composed of pellets and sinter) on the positioning in the hopper. This positioning is found to subsequently affect the hopper output constitution, which then affects the radial constitution of the ferrous burden layer. Delaying the pellet loading on the ferrous burden is found to delay emptying of the same from the hopper, resulting in more pellets occupying the mid-radial region of ferrous burden layer in the blast furnace. Based on the observations from simulations, trials are taken in blast furnace and relevant parameters are monitored to capture the effect of such a charging practice on operations.

  • Raymond James Longbottom, Joe Perkins, Graham O’brien, Brian Joseph Mo ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 282-293
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: November 04, 2022
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    The effects of coke mineralogy on coke reactivity under conditions representative of the blast furnace were studied. Coke samples were reacted under coke reactivity index (CRI) like conditions (1100°C, 100% CO2) and at higher temperatures and atmospheres designed to replicate conditions lower in the blast furnace.

    The effects of the minerals on reactivity changed as the temperature increased and the atmosphere was modified. At the lower temperatures investigated (1100–1350°C) with CO2 present in the gas, gasification of the coke by CO2 dominated. The effects of minerals in the coke on gasification by CO2 under these conditions were similar to their reported effects on reactivity in the CRI test. At the highest temperature investigated, (1600°C), with no CO2, the mineral-carbon reactions dominated. The main reaction was the reduction of the silica in the coke. These results show that when coke-gasification is dominant, CRI data can be related to conditions beyond the temperature and gas environment the data were obtained, to the higher temperatures and less oxidising conditions of the blast furnace. At higher temperatures, mineral-carbon reactions are dominant, and more data in addition to that of the CRI, may be required to understand coke behaviour in the blast furnace.

    IMDC and RMDC within the coke were identified with the aid of the CGA technique, and the changes in the carbon structures within the coke studied using Raman spectroscopy. The carbon structures within the coke became more graphitic at 1600°C, with the change in RMDC greater than that in IMDC.

Steelmaking
Regular Article
  • Lichun Zheng, Baiqiang Yan, Jian Lou, Huabing Li, Zhouhua Jiang
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 294-302
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 18, 2022
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    M2C eutectic carbide favours the mechanical properties of high-speed steels, but is often largely replaced by coarse M6C eutectic carbide in as-cast M42 steel. To deeply understand the formation behavior of M2C and M6C carbides, M2C and M6C eutectic alloys were prepared according to the composition of M2C and M6C eutectic mixtures in M42 steel, and their solidification behavior was investigated. Only one type of eutectic carbide is formed in water-quenched M2C and M6C eutectic alloys, i.e., M2C and M6C, respectively. Both M2C and M6C carbides appear in the alloys cooled at 3°C/min. However, the M2C eutectic alloy was more significantly affected in terms of carbide type by the low cooling rate. According to thermodynamic calculation, M6C carbide in the M2C eutectic alloy is only slightly more stable in thermodynamics above 1210.1°C, below which M2C carbide becomes stable. For the M6C eutectic alloy, however, only M6C eutectic carbide is thermodynamically stable. Furthermore, thermodynamic results reveal that besides raising the content of C and V, reducing the content of Mo can also greatly promote the formation of M2C carbide in M42 steel, which updates the traditional opinion on the influence of Mo element. The results in this work provide the underlying insights needed to promote the formation of M2C carbide in M42 steel by fine-tuning the composition.

  • Xiaoao Li, Nan Wang, Min Chen, Zhiqiang Du
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 303-312
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: November 04, 2022
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    The effects of oxygen, magnesium, and sulfur content in Al-killed tinplate steel on the “liquid zone” of inclusion modification by calcium treatment were clarified through industrial experiments and thermodynamic calculation, and the characteristics of inclusions modification were studied. The results show that the inclusions in molten steel before calcium treatment are mainly Al2O3 inclusions. The inclusions in molten steel are CaO·Al2O3 after calcium treatment with a holding time of 10 min, while the inclusions are mainly 3CaO·Al2O3 with a holding time of 30 min. And 12CaO·7Al2O3 inclusions are observed in molten steel when T.O content increases to 40 ppm after calcium treatment with a holding time of 30 min. As the increase of T.O content from 10 ppm to 40 ppm, the difference between the upper and lower limits of the critical calcium content corresponding to the “liquid zone” increases from 5 ppm to 17 ppm. The increase of T.O content in molten steel will enlarge the “liquid zone” range of inclusion modification by calcium treatment, and increase the critical calcium content. With the increase of magnesium content in molten steel, the liquid phase ratio of inclusions modification by calcium treatment decreases. To obtain the liquid phase ratio of inclusions at least 50% in molten steel, not only the calcium content in steel should be strictly controlled, but also the magnesium content in steel should not be larger than 15 ppm. With the increase of sulfur content in molten steel, the “liquid zone” range of inclusion modification by calcium treatment becomes narrow.

  • Yuxing Liu, Zhi Li, Yoshinao Kobayashi
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 313-318
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: November 08, 2022
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    Thermodynamic properties of chromium oxide in molten slags are essential to optimizing stainless steel refining processes and reduction processes of chromium ores. The present study conducts chemical equilibrium experiments at 1823 K (1550°C) under different oxygen partial pressures in order to investigate chromium oxide activity coefficient in CaO–CaF2–MgO–Al2O3–SiO2–MnO–CrOx slags with low chromium content taking the effect of MgO addition into consideration. An increase in MgO initial content from 0 to 20 mass% increases the activity coefficient of CrOx under the oxygen partial pressures of 6.38×10−12 atm and 1.60×10−10 atm. It is found that lower oxygen partial pressure may reduce solid phase formation. The present study recommends the usage of 29 mass% CaO-5 mass% CaF2-20 mass% MgO-10 mass% Al2O3-29 mass% SiO2-5 mass% MnO-2 mass% Cr2O3 slag to reduce chromium loss at 1823 K (1550°C) under the oxygen partial pressure of 6.38×10−12 atm, where the activity coefficient of CrOx is the highest in the slags without solid phase.

  • Wenjing Wei, Peter Samuelsson, Anders Tilliander, Dong-Yuan Sheng, Pär ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 319-329
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    A two-dimensional CFD model was developed to simulate the fluid flow and temperature distribution in an AOD nozzle using a mixture of oxygen and nitrogen as the fluid phase, aiming at predicting how the outlet gas properties are influenced by the inlet pressure, inner nozzle length/diameter and process gas composition. The proposed mathematical model assumes a steady, non-isothermal flow condition, using the realizable k-ε turbulence model to describe the gas phase. A mesh sensitivity analysis was performed where predictions were compared to experimental data. The results show that the gas properties are mainly dependent on the inlet pressure, nozzle length/diameter and heating condition but less dependent on the composition of the gas mixture. This fundamental model can be applied to provide a process specified boundary condition for gas blowing when simulating a multiphase flow in a full-scale AOD converter.

  • Naoya Saeki, Masashi Nakamoto, Toshihiro Tanaka
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 330-337
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    A zirconia oxygen sensor is an electrochemical device for rapidly measuring oxygen concentrations in molten metals at high temperatures. It is desirable to make the sensors capable of continuous, long-time measurements. Here, the effect on the electromotive force of a partial equilibrium state on the surface of a zirconia solid electrolyte in a zirconia oxygen sensor was investigated to reveal the factors preventing long-time measurements. The reference electrode was a mixture of W and WO2. Continuous measurements were performed in molten iron containing carbon for deoxidization. The electromotive force between the positive reference electrode and the negative sample electrode of the sensor decreased from a positive value to zero over time. This was because a metallic tungsten layer formed on the surface of the zirconia electrolyte by the reduction of the tungsten oxide via oxygen diffusion through the zirconia tube. In addition, the equilibrium in the reference electrode was disturbed during the measurements. A direct electrical current was applied to the sensor to examine the relationship between the surface and measured values. The electromotive force during the current was maintained at a higher value than that without the current because of induced re-oxidation at the zirconia surface. However, a long-time application could cause over-oxidation and dissolution of the reference electrode surface.

  • Xiaopei Guo, Min Tan, Tao Li, Liying Ju, Jie Dang, Han Guo
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 338-345
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    Silicon deoxidized steel takes a large proportion in addition to aluminum deoxidized steel, since aluminum deoxidation may cause many problems, such as nozzle clogging and low toughness. In this paper, a low aluminum silicon deoxidized steel was prepared in the laboratory. The inclusions in steel mainly include MnS and composite inclusions made of MnS and oxides. The formation of MnS and composite inclusion was discussed based on the thermodynamic calculations. It was clarified that the MnS precipitate during the solidification process of steel. In addition, the formation mechanism of the composite inclusions was illustrated from the perspective of thermodynamics and lattice mismatch. The composite inclusions in steel were reconstructed by Micro-CT and the three-dimensional (3D) morphologies were analyzed in details. This method avoids artifacts and misleading of the two-dimensional observation on the cross sections of the steel samples. Meanwhile, the 3D observation by Micro-CT is an innovation way to characterize the different phases of the inclusion simultaneously, which paved a way for the analysis of the composite inclusions in the steel.

Instrumentation, Control and System Engineering
Regular Article
  • Masato Sugiura, Kohsuke Morioka, Masaki Tsuchiya, Shin Takaya, Ryuji T ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 346-353
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    Radiation thermometry, which enables non-contact temperature measurements of a moving target, plays a crucial role in temperature control during steel production. However, radiation thermometers implemented in manufacturing lines frequently encounter practical problems such as emissivity fluctuation, obstruction in the optical path, and background radiation. Herein, a new approach for solving the obstruction problem arising from the water in hot-rolling processes is presented. When a radiation thermometer detects the radiant flux emitted from a steel surface through unstable flowing water and evaporated water, the apparent transmissivity of these obstacles is unknown and it is extremely difficult to perform precise measurements. Thus, we propose a two-color thermometry technique that use specific wavelengths. A combination of wavelengths unaffected by the presence or absence of water in the optical path was studied both theoretically and experimentally. This two-color method enables simultaneous measurements of the water thickness. A prototype two-color thermometer was designed using detection wavelengths of 1200 nm and 1300 nm. The experimental results indicated that the temperature and apparent water layer thickness of heated steel beneath a water layer can be measured. It was also confirmed that two-color thermometry is useful for the obstruction of foggy steam.

Forming Processing and Thermomechanical Treatment
Regular Article
  • Hyung-Won Park, Kyunghyun Kim, Hyeon-Woo Park, Akira Yanagida, Jun Yan ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 354-365
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 31, 2022
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    This study aims to formulate a constitutive equation to accurately determine the flow stress of superalloy 718 for effective production of gas turbine disks. A hot-compression test with superalloy 718 at temperatures ranging from 900 to 1000°C, a reduction rate of 67%, and strain rates of 0.1, 1, and 10 s−1, respectively, was conducted to analyze the flow stress in the dynamic precipitation region. An accurate flow stress curve was obtained for each strain rate and initial temperature. The flow curves obtained at a deformation temperature of 900°C and strain rates of 0.1 and 1 s−1, represent a combination of work-hardening and dynamic recovery. Dynamic recrystallization (DRX) behavior was observed under other deformation conditions. At a deformation temperature of 950°C and each strain rate, the strain at the onset of DRX (εc) decreases, and DRX tends to occur rapidly. In addition, the steady-state stress at a strain rate of 1 s−1 was greater than that at a higher strain rate of 10 s−1. The lowest steady-state stress among all the experimental conditions was observed at a strain rate of 10 s−1. This may be attributed to the role of nucleation sites, precipitation hardening caused by dynamically precipitated γ” phases at approximately 950°C and a strain rate of 1 s−1, and dynamic softening effects due to significant heat generated by deformation at a strain rate of 10 s−1. A new constitutive equation for the generalized flow curve of superalloy 718 was obtained by considering these metallurgical phenomena.

Transformations and Microstructures
Regular Article
  • Guangming Cao, Wenchao Shan, Chenyang Wang, Zhongxiang Zhou, Zhenyu Li ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 366-374
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    The phase transformation of the oxide scale that forms on hot-rolled steel strips during cooling is one of the most important phenomena, which determines the surface quality of the strips, because the properties of the oxide scale, such as spallation resistance, crack initiation, and propagation, and pickling behavior, are strongly dependent on the oxide scale microstructure. To obtain steel strips with high surface quality, it is crucial to understand the microstructural development of the oxide scale. In this paper, a thermogravimetric analyzer was used to study the isothermal structure transformation behavior of FeO formed at high temperature in an inert gas (Ar) at 300–550°C for 1000–15000 s. Meanwhile, the isothermal dynamic model of FeO eutectoid transformation was established based on the JMAK equation combined with experimental data. The experimental results show that the oxide scale structure of the sample after pre-oxidation is composed of Fe2O3, Fe3O4, and FeO. At 300°C and 550°C, only the pro-eutectoid structure is formed in the oxide scale, but no eutectoid structure is formed. When the isothermal temperature is in the range of 350–500°C, both pro-eutectoid structure and eutectoid structure are formed in the oxide scale. With the extension of isothermal time, the eutectoid reaction continues until the complete decomposition of FeO. In addition, the FeO eutectoid transformation isothermal dynamics model was used to predict the eutectoid structure content of FeO eutectoid decomposition under different isothermal conditions, and the experimental data were compared with the predicted results, which verified the high prediction accuracy of the model.

  • Xin Geng, Xue-ru Tao, Zhou-hua Jiang, Leizhen Peng, Fu-bin Liu, Hua-bi ...
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 375-381
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    Advance online publication: October 18, 2022
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    Microstructure observation and hardness analysis of 9CrMoCoB steel with different N content during long-term aging at 620°C were carried out by Thermo-Calc thermodynamic calculation, scanning electron microscope, transmission electron microscope and hardness test. The results show that the average size of M23C6 phase increases first with the extension of aging time, then stabilizes after aging for 2000 h, and decreases gradually with the increase of N content. The Laves phase precipitated after aging for 1000 h and coarsened continuously with the aging time prolonging. The coarsening rate tended to be flat after 2000 h. The average size of Laves phase decreased gradually with the increase of N content. With the increase of N content, the average diameter and unit number of BN inclusions increase gradually, and the unit number of BN inclusions larger than 2 µm increases obviously when N content is 0.025% and 0.030%. The hardness of 9CrMoCoB steel decreases gradually with the aging time and tends to be stable after aging for 2000 h. With the increase of N content, the hardness and stability of 9CrMoCoB steel first increase and then decrease, and both reach the maximum when N content is 0.020%. Considering comprehensively, in order to give full play to the strengthening effect of B and N in steel and improve the properties and stability of 9CrMoCoB steel, the optimal control range of N content in steel is around 0.020%.

Mechanical Properties
Regular Article
  • Anastasiia Dolzhenko, Andrey Belyakov, Rustam Kaibyshev
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 382-389
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
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    The fracture behavior of a high-strength low-alloy steel subjected to tempforming at 873 K, 923 K, or 973 K was studied by means of impact and bending tests. A decrease in tempforming temperature promoted the grain refinement resulting in the ultrafine grained lamellar-type microstructure with finely dispersed particles that led to significant strengthening along with an increase in the impact toughness. The tempformed steel samples exhibited Charpy V-notch impact energy well above 100 J at temperatures of 183–293 K due to delamination along the rolling plane during bending, which was attributed to high anisotropy of cleavage fracture stress. Delamination owing to easy cleavage crosswise to the impact direction blunted the primary crack and resulted in the zigzag crack propagation, leading to high impact toughness. Depending on tempforming temperature, three types of the delamination behavior were observed in the V-notch specimens upon three-point bending tests at room temperature. Namely, the early, restrained, and late delaminations took place in the samples after tempering at 873 K, 923 K, and 973 K, respectively. On the one hand, a decrease in the test temperature promoted delamination, and on the other the strengthening by lowering tempforming temperature is accompanied by a suppression of ductile fracture. The sample tempformed at 873 K exhibited the highest impact toughness at room temperature, whereas the samples tempformed at 923–973 K were characterized by the higher impact toughness at 183–233 K.

  • Afshin Yousefi, Goroh Itoh, Zoha Ghorani, Shigeru Kuramoto
    Article type: Regular Article
    2023 Volume 63 Issue 2 Pages 390-394
    Published: February 15, 2023
    Released on J-STAGE: February 15, 2023
    JOURNAL OPEN ACCESS FULL-TEXT HTML

    Tensile properties of a JIS SUS329J4L duplex stainless steel with and without hydrogen plasma charging at various strain rates ranging from 1.38 × 10−7 to 1.38 × 10−3 s−1 in ambient atmosphere were investigated. Fracture surface was evaluated by scanning electron microscope (SEM) and the trapping state of hydrogen was assessed by thermal desorption spectroscopy (TDS). It was confirmed that ultimate tensile strength and elongation to failure were decreased with increasing charging time for testing at 1.38 × 10−3 s−1, while the ultimate tensile strength was affected less markedly by hydrogen charging than the elongation to failure. The extent of hydrogen embrittlement took a maximum at a strain rate of 1.38 × 10−6 s−1, where the proportion of smooth area (area without dimples) in relation to the total fracture area reached a maximum. This conformity between the extent of the degradation and the area without dimple was in accord with that reported in the authors’ previous paper using electrolytical hydrogen charging. On the other hand, the strain rate where the extent of degradation became maximum for the current tests was two magnitudes lower than that in the electrolytically charged test piece. It was assumed that the trapping site for plasma-charged hydrogen is more stable than that for electrolytically charged hydrogen. This was confirmed by TDS results that the desorption spectrum of the 24 h plasma-charged specimen had a shoulder at about 200°C and the peak temperature significantly higher than that of the 24 h electrolytically charged specimen.

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