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ISIJ International Vol. 62 (2022), No. 9

ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559
Publisher: The Iron and Steel Institute of Japan

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ISIJ International Vol. 62 (2022), No. 9

Effect of the Partial Equilibrium State at the Zirconia Surface in a Cr/Cr2O3-Type Zirconia Oxygen Sensor on Continuous Measurement of the Oxygen Content in Molten Steel

Naoya Saeki, Masashi Nakamoto, Toshihiro Tanaka

pp. 1751-1759

Abstract

Zirconia oxygen sensors are used in the steel-refining process to instantly measure the oxygen concentration in molten steel. It is desirable to develop sensors that are capable of continuous and long-time measurements, but such measurements are difficult with conventional sensors. In this study, the factors that hinder long-time measurement by a single zirconia oxygen sensor with a mixture of Cr and Cr2O3 as the reference electrode were investigated. Continuous measurements of a molten iron containing carbon and aluminum for deoxidization were carried out with the sensor. The electromotive force between the reference electrode (positive) and sample electrode (negative) of the sensor decreased from a positive value to zero with time. This was because reduction of chromium oxide at the surface of the zirconia solid electrolyte was promoted owing to oxygen diffusion from the inside to the outside of the zirconia tube and the equilibrium in the reference electrode was disturbed during the measurements. In addition, the reference electrode at the surface partially and temporarily melted owing to diffusion of the components from the electrolyte to the reference electrode. Additionally, a test to extend the lifetime of the sensor was performed with application of a direct electrical current to the sensor. The electromotive force with application of a current was maintained at a higher value than that without application of a current owing to promotion of re-oxidation at the zirconia surface. However, it was found that a high current can cause over-oxidation and dissolution of the reference electrode at the surface.

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ISIJ International Vol.62(2022), No.12

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Effect of the Partial Equilibrium State at the Zirconia Surface in a Cr/Cr2O3-Type Zirconia Oxygen Sensor on Continuous Measurement of the Oxygen Content in Molten Steel

Boron-Iron Separation and Boron Enrichment from Boron-Bearing Iron Concentrate at Low-Temperature Enhanced by Supergravity

Yu Li, Jintao Gao, Xi Lan, Zhancheng Guo

pp. 1760-1767

Abstract

Boron-bearing iron concentrate, as the tailings of boron concentrate otained from ludwigite, still contains more than 5 wt.% of B2O3, which can hard to be utilized. In this study, low-temperature separation compared with high-temperature melting separation of boron and iron from boron-bearing iron concentrate, and the migration, transformation, separation and enrichment behaviors of boron in both processes were studied. High-temperature melting separation of iron and slag could be accomplished at 1823 K for 60 min, where the boron mainly distributed in a form of glass phase in the slag with a B2O3 content of 22.69 wt.%, while 0.35 wt.% of [B] was melted into the liquid iron. In contrast, iron and slag were efficiently separated at a low temperature of 1573 K for 10 min enhanced by supergravity, almost all of boron was enriched into suanite phase in the slag with a significantly higher B2O3 content of 35.61 wt.% and a high recovery ratio of 99.37%, and the content of B was decreased to 0.15 wt.% in the iron. Compared with high-temperature melting separation, low-temperature separation greatly improved the enrichment of boron in slag and avoided the melting of boron into iron.

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Boron-Iron Separation and Boron Enrichment from Boron-Bearing Iron Concentrate at Low-Temperature Enhanced by Supergravity

Quantification of Resistance and Pressure Drop at High Temperature for Various Suction Pressures During Iron Ore Sintering

Tejbir Singh, Subhasish Mitra, Damien O’dea, Lydia Knuefing, Tom Honeyands

pp. 1768-1776

Abstract

Permeability and air flow rate in the bed during iron ore sintering is crucial to control the sinter quality and productivity, due to its influence on the flame front speed. In the present work, the permeability of the bed during sintering of an iron ore blend was analysed utilising simultaneously measured pressure and temperature at multiple points in the bed. Three different zones were categorized, i.e., humidified (green) bed, region of maximum resistance (high-temperature zone) based on temperature (>100°C), and sintered bed. The sintering experiments were conducted in a lab-scale milli-pot (diameter 53 mm, height 500 mm) fitted with six taps and transient variations of pressure-drop in different zones were analysed. The resistance was quantified for the three zones by correlating pressure gradient in each zone (ΔP/L) with the specific kinetic energy of air (v2) during sintering. The high temperature zone was further divided into two sub-zones i.e. pre-melt reaction zone (PMRZ) and granule melting zone. The PMRZ was responsible for the highest pressure gradient in the bed during sintering due to the changes in bed structure and the much greater gas volumetric flow rate due to the high gas temperature. The PMRZ included various physico-chemical processes and it was observed that the resistance in the PMRZ was high at a high suction value across the bed. High productivity was observed for a high suction pressure. The sinter yield was influenced by holding time over 1200°C and an exponential growth function was proposed to estimate the sinter yield.

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Quantification of Resistance and Pressure Drop at High Temperature for Various Suction Pressures During Iron Ore Sintering

Effect of Top Gas Recycling Technology on the Softening, Melting and Dropping Behavior of V–Ti Bearing Burden in COREX Process

Chen Yin, Shengfu Zhang, Yan Guo, Liangying Wen, Chenguang Bai

pp. 1777-1784

Abstract

In COREX process, the top gas recycling (TGR) is an effective recovery and utilization technology for the high-quality gas by-produced from the melter gasifier, which can reduce its fuel consumption and emissions, and is of great significance to low carbon ironmaking. However, low concentration N2 is contained in the top gas, it is unfavorable to the smelting of V–Ti bearing burden because the high temperature solid solution Ti(C, N) is formed in the semi-coke bed and deteriorates the burden permeability of the melter gasifier. The effect of N2 concentration (0–10 vol%) on the softening, melting and dropping behavior of V–Ti bearing burden is investigated under simulated COREX conditions. Results show that the increasing N2 concentration will promote the formation of Ti(C, N), increase the slag viscosity and its wettability to coke, greatly increase the burden dropping temperature, which reduces the slag dropping rate from 68.2% to 18.8%. In addition, slag foaming is caused and the gas permeability of V–Ti bearing burden is deteriorated. The vanadium contents of dropping iron and residual iron are decreased from 0.051 wt% and 0.166 wt% to 0.043 wt% and 0.161 wt%, when the N2 concentration in reducing gas changes from 0 vol% to 10 vol%. If the pressure drop needs to be controlled below 20 kPa, the N2 concentration in the reducing gas should not exceed 5 vol%, that is, the top gas injection volume should be less than 60%.

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Effect of Top Gas Recycling Technology on the Softening, Melting and Dropping Behavior of V–Ti Bearing Burden in COREX Process

Effect of 2CaO·SiO2 Addition on Reaction Behavior of Iron Ore Sinters

Miyuki Hayashi, Daochang Zhou, Yuji Iwami, Takahide Higuchi, Takashi Watanabe, Rie Endo, Masahiro Susa

pp. 1785-1791

Abstract

The effect of 2CaO·SiO2 (C2S) addition on the melt formation temperatures and on the microstructures of iron ore sinters has been elucidated from the perspective of the formation of silico-ferrite of calcium and aluminum (SFCA) in bonding phases as SFCA phases improve the strength of iron ore sinter. The samples used in this study are categorized into two: “C + S” and “C2S”. “C + S” samples contain CaO and SiO2 as CaO and SiO2 components, while “C2S” samples contain intermediate compounds of CaO and SiO2, i.e., C2S. Both samples have an identical chemical composition. The mixed raw materials were subjected to differential thermal analysis (DTA) and sintering experiment. DTA reveals that the melt formation temperatures are 1182°C and 1216°C for the “C + S” and “C2S” samples, respectively, which are sufficiently lower than the normal sintering temperatures. Sintering experiment reveals that the larger amount of SFCA exist in the “C + S” sample than in the “C2S” sample. This may be associated with the fact that some SiO2 gangue minerals and reagents remain unreacted for the “C + S” sample, causing the CaO/SiO2 ratio of the slag melt to be larger for the “C + S” sample than for the “C2S” sample.

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Effect of 2CaO·SiO2 Addition on Reaction Behavior of Iron Ore Sinters

Effect of Magnetite Concentrate Particle Size on Pellet Oxidation Roasting Process and Compressive Strength

Hao Zhang, Kaikai Bai, Wenguo Liu, Yanbiao Chen, Yaqiang Yuan, Haibin Zuo, Jingsong Wang

pp. 1792-1801

Abstract

During the roasting process of pellets, the particle size of the magnetite concentrates affected its component content, which correlated well with the oxidation roasting process and metallurgical properties of the pellets. In this work, using high-silicon iron ore concentrates as raw material, the effect of high-silicon iron ore concentrates with different particle size on the oxidation behavior of pellets was studied. Coats-Redfern data processing model was carried out to calculate the oxidation kinetics parameters of high-silicon iron ore concentrate. The research results showed that the refinement of high-silicon iron ore concentrates could reduce the SiO2 content to 1.96% and increase the pellet grade to 69.84%. At a heating rate of 15°C/min, the refinement of particle size reduced the activation energy to 45.73 kJ·mol−1, which could greatly improve the degree of oxidation of pellets and increase the consolidation strength to 3213 N. The iron ore concentrates more than 74 µm had lower activation energy at a higher heating rate, whereas an opposite trend was identified for the iron ore concentrates less than 74 µm. Consequently, reducing the particle size range of high-silicon iron ore concentrates can effectively improve the physical and chemical properties and smelting indexes of iron ore concentrates, which indicated the direction of rational utilization of the high-silicon iron ore concentrates.

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Effect of Magnetite Concentrate Particle Size on Pellet Oxidation Roasting Process and Compressive Strength

Microstructure Ti–Fe Phase Separation Mechanism in the Direct Reduction Process of Titanomagnetite with Coal by Microwave Heating

Peng Liu, Siyu Gong, Chao Yuwen, Bingguo Liu, Libo Zhang, Aiyuan Ma

pp. 1802-1815

Abstract

Although microwave heating has been used in the field of metallurgy since the 1980s, there are few reports on using microwave heating to strengthen Ti–Fe phase separation in the reduction process of titanomagnetite. Microstructure Ti–Fe phase separation by microwave heating was found to be beneficial to the reduction process of titanomagnetite through the investigation of formation enthalpy and reduction difficulties. The dielectric properties and energy band structure of Fe3O4, Fe2O3, and TiO2 were determined, the dielectric constant (εr: 20.00) and dielectric loss (εr: 2.82) of Fe3O4 were significantly superior to those of TiO2 (εr: 3.42, εr: 0.04), and the physical thermal stress between Fe3O4 and TiO2 phases in the reduction process was conducive to the microstructure Ti–Fe phase separation process. Besides thermal stress, microstructure Ti–Fe phase separation was mainly due to the ampere traction force generated between local isotropic ring current in adjacent iron atoms. This study revealed the microstructure Ti–Fe phase separation mechanism in the reduction process of titanomagnetite by microwave heating.

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Microstructure Ti–Fe Phase Separation Mechanism in the Direct Reduction Process of Titanomagnetite with Coal by Microwave Heating

Thermodynamic Analysis of the Reduction of Vanadium-titanium Magnetite by Gasification Gas

Weibin Chen, Zhaoqi Dong, Xidong Wang

pp. 1816-1826

Abstract

Vanadium-titanium magnetite (VTM) has a high comprehensive utilization value. The gas-based reduction process combined with the electric furnace melting process has the advantages of a high recovery rate of precious metal elements in VTM, low cost, high-energy efficiency, and low environmental pollution. Gasification gas provided by coal gasification technology is a potential reducing agent. Through FACTSAGE calculations and experiments, the reduction thermodynamic properties of VTM oxidized pellets by gasification gas were systematically studied to illustrate the feasibility of the system before practical experiments. The reaction mechanism of the direct reduction of VTM by gasification gas was proved, and the conversion behavior of iron and titanium in the reduction system was also clarified. This work clearly describes the reduction process of VTM oxidation pellets and clarifies the essence of the direct reduction of gas groups of VTM oxidized pellets.

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Thermodynamic Analysis of the Reduction of Vanadium-titanium Magnetite by Gasification Gas

In situ Compressive Strength of Iron Coke in High-temperature Carbonization

Wei Lv, Furong Chen, Zhuoliang Liu, Jiwei Bao, Zhenggen Liu, Mansheng Chu, Xuewei Lv

pp. 1827-1835

Abstract

In this study, the in situ compressive strength of iron coke in high-temperature carbonization was investigated. The high-temperature compressive strengths of iron coke briquettes with 20, 30, and 40% iron ore with carbonization at 700, 800, 900, and 1000°C for 3 h were systematically studied. The mass loss and microstructure of the iron coke during carbonization were also investigated. The results showed that the mass loss increased with the iron ore content from 20 to 40%. The liquid phase content of the slag in iron coke was calculated using FactSage software. Results showed an increased of liquid phase slag content with increasing of carbonization temperature and decreasing of Fe3O4 content. Coal coking, iron oxide reduction, pitch volatilization, and CO2 gasification in the carbonization process could significantly change the micromorphology of iron coke and influence its high-temperature compressive strength. At a carbonization temperature of 900°C, the mass loss of iron coke increased from 27.59 to 30.23% and the high-temperature compressive strength decreased from more than 4200 to 1589 N with an increase in the iron ore content from 20 to 40%. The degradation mechanism of the high-temperature compressive strength of iron coke with 30 and 40% iron ore, and increased carbonization temperature from 800 to 1000°C, was discussed.

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In situ Compressive Strength of Iron Coke in High-temperature Carbonization

Effects of Particle Size Distribution of MgO and Carbon on MgO–C Reaction Behaviour

Yuta Hino, Katsunori Takahashi

pp. 1836-1844

Abstract

The effect of the particle size distribution of MgO and carbon on the behaviour of the MgO–C reaction was investigated by heating four kinds of MgO–C sample bricks with different size distributions, and the reaction mechanism was discussed quantitatively using an improved MgO–C reaction model in which the unreacted core model was combined with a statistical distribution function. As a result, it was found that the MgO–C reaction is promoted when the ratio of fine particles of MgO increased and finer particles of carbon were used in the brick. A kinetic model of the MgO–C reaction based on the unreacted core model was developed and improved by combining the model with the statistical distribution function. The improved model considers the effect of the particle size distribution of the refractory materials on the MgO–C reaction rate. The effect of the carbon particle size on the MgO–C reaction can be explained using this model. The effect of the MgO particle size on the reaction behaviour can also be explained by the model under conditions with a relatively large ratio of MgO aggregates. However, when the ratio of fine MgO particles was large, the calculation result was much larger than the experimental result. As a reason for this, it is predicted that the fine particles sinter with each other during heating, resulting in an increase in the apparent particle diameter of the fine MgO.

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Effects of Particle Size Distribution of MgO and Carbon on MgO–C Reaction Behaviour

Optimization of Endpoint Control for Low Carbon and Phosphorus Steel Produced by Converter Single Slag Process

Lv Changhai, Li Jing, Yan Wei, Zhang Hongjun

pp. 1845-1851

Abstract

Through field test and thermodynamic calculation of decarbonization, this paper studies the effect of endpoint control of low-carbon and phosphorus steels produced with single-slag in basic oxygen furnace (BOF) on furnace lining erosion. The study reveals that practicing step control of C% till steel tapping in lieu of the thoroughgoing blowing method can effectively reduce the tapping temperature and FeO% in final slags, thereby mitigating the effect of heat and mass transfer on the compact slag layer of furnace lining and lowering the rate of furnace lining erosion. One key point is to control the endpoint C% somewhere above the critical C% value (Ct) of C–Fe selective oxidation in the final phase of blowing, which can be accomplished effectively by shortening the press-lance time to 40 s–70 s. Another key point is to raise the control range of endpoint carbon C2 to 0.040%–0.050%. The tapping requirement for low-carbon and phosphorus steels can be satisfied through static stirring operation such that the carbon value at steel tapping C3 < 0.038%.

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Optimization of Endpoint Control for Low Carbon and Phosphorus Steel Produced by Converter Single Slag Process

Effect of Cerium and Magnesium Addition on Evolution and Particle Size of Inclusions in Al-killed Molten Steel

Yeguang Wang, Chengjun Liu

pp. 1852-1861

Abstract

To investigate the effect of Ce and Mg addition on the evolution of inclusions in Al-killed molten steel, both thermodynamic calculations and laboratory-scale experiments were carried out in the present work. The samples taken at different time after various addition amounts and order of Ce and Mg were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The results show that Al2O3 was firstly modified to MgAl2O4 and then the transformation from MgAl2O4 to CeAlO3 with the addition order of Al → Mg → Ce. Under the addition order of first Ce followed by Mg, CeAlO3 was eventually modified to MgAl2O4 in the case of low Mg contents, and was modified to MgO with high Mg contents. The thermodynamic conditions were performed to discuss the mechanism of inclusions formation and transformation. More importantly, under the condition of Ce addition followed by Mg addition, due to the transformation from large-sized RE-inclusions to disperse MgO-containing ones, the average sizes of inclusions reduced from 3.12 µm and 3.28 µm to 1.99 µm and 1.83 µm, respectively. This evolution behavior of inclusions was firstly observed in-situ by a novel experiment, in which the large-sized CeAlO3 clusters were disaggregated to MgO–Al2O3 particles with smaller sizes. The control strategy of inclusions proposed in this study is expected to improve the casting process and the product quality of Al-killed steel containing rare earth element.

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Effect of Cerium and Magnesium Addition on Evolution and Particle Size of Inclusions in Al-killed Molten Steel

Numerical Simulation of Macrosegregation in a Continuous Casting Mold with Electromagnetic Stirring

Kengo Kihara, Nobuhiro Okada, Shun Saito, Kento Kawashima

pp. 1862-1873

Abstract

Negative segregation beneath the surface of the slab is generated by the flow of molten steel in continuous casting. Negative segregation of austenite stabilizing elements such as Ni and C results in the formation of delta ferrite, which can cause cracks near the slab surface in austenitic stainless steel. In the current study, we developed a numerical simulation model to predict negative segregation in the continuous casting process. The model used an effective distribution coefficient that is dependent on the solidification rate and the liquid velocity in front of a solidifying shell. A comparison of the numerical simulation and the experimental results for the solidification in a crucible with a rotating and cooling pipe indicated the validity of the proposed numerical simulation model. Additionally, the numerical simulation results of continuous casting for austenitic stainless steel showed that the maximum degree of negative segregation occurred near the slab corners when casting at high speed with electromagnetic stirrer. The degree and location of negative segregation in the numerical simulation were comparable to those obtained from EPMA and Spark-OES analysis of slab samples. These results indicated that the proposed simulation model enables accurate prediction of negative segregation beneath the slab surface in continuous casting and is useful for the optimization of continuous casting process. The negative segregation was caused by the molten steel flow in front of a solidifying shell. Consequently, the results from simulation without EMS or at low casting speeds showed that negative segregation was suppressed.

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Numerical Simulation of Macrosegregation in a Continuous Casting Mold with Electromagnetic Stirring

Quantum Optimization with Lagrangian Decomposition for Multiple-process Scheduling in Steel Manufacturing

Kouki Yonaga, Masamichi Miyama, Masayuki Ohzeki, Koji Hirano, Hirokazu Kobayashi, Tetsuaki Kurokawa

pp. 1874-1880

Abstract

Steel manufacturing involves multiple processes, and optimizing the production schedule is essential for improving the quality, cost, and delivery of products. However, the corresponding optimization problems are usually NP-hard and generally intractable even for modern high-performance computers. Recent advances in quantum computers, such as those developed by D-Wave Systems, have opened new possibilities for handling this type of optimization problem. Nevertheless, a major obstacle to the application of currently available quantum computers is the relatively small number of quantum bits, which limits the feasible problem size. To overcome this obstacle, we propose an algorithm for solving optimization problems based on Lagrangian decomposition and coordination. D-Wave quantum computers can explore diverse solutions simultaneously, and we leverage this unique characteristic to obtain the upper bound for Lagrangian decomposition and coordination. We apply the proposed algorithm to a simplified problem of two-process production scheduling. By decomposing the problem into two stages for the processes, the number of steel products that can be handled increases from five to eight (60% increase). The optimal solutions are reached even for the extended case of the eight products. The proposed algorithm is a promising technique for enabling the application of quantum computers to real-world problems by thoroughly exploiting quantum bits.

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Quantum Optimization with Lagrangian Decomposition for Multiple-process Scheduling in Steel Manufacturing

Characterization of α-Al2O3 in Structural Isomers of Alumina Formed by Oxidation of Fe–Cr–Al Alloys

Susumu Imashuku, Michihisa Fukumoto, Kano Nakajima, Shigeru Suzuki

pp. 1881-1886

Abstract

To understand the formation of structural isomers of alumina (Al2O3) on the surfaces of Fe–Cr–Al alloys, cathodoluminescence (CL), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were used. The alloys were annealed under oxygen at a low partial pressure to focus on the initial formation of Al2O3. XRD revealed that corundum (α-Al2O3) was predominantly formed on the alloy surfaces after annealing at approximately 1100°C, whereas γ-Al2O3 and θ-Al2O3 were formed at 1000°C. CL peaks around 700 nm, which originated from Cr3+ ions in Al2O3, was drastically increases by rising the annealing temperatures from 1000 to 1100°C because of the increase of the chromium content in the Al2O3 scales that was revealed by XPS. EPMA revealed that the surficial α-Al2O3 had striped patterns, which were similar to those observed using CL imaging, indicating that CL can be conveniently used for imaging α-Al2O3 and analyzing the inhomogeneous distribution of α-Al2O3 on the alloys.

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Characterization of α-Al2O3 in Structural Isomers of Alumina Formed by Oxidation of Fe–Cr–Al Alloys

Acidic Corrosion Behavior of Slag-free Self-shielded Flux-cored Arc Welding Overlay

Dashuang Liu, Ping Wei, Weimin Long, Yucheng Wu, Rui Wang

pp. 1887-1895

Abstract

The corrosion resistance of slag-free self-shielded flux-cored welding overlays with different ferrotitanium (Fe–Ti) additions in 3.5 wt.% NaCl + 0.01 mol/L HCl acidic solution was investigated. The corrosion occurs in the matrix rather than M7(C, B)3, M3(C, B) and TiC in the microstructure of all the welding overlays. The corrosion resistance of hypereutectic welding overlay is decreased when the Fe–Ti addition is increased from 0 wt.% to 12 wt.%. The corrosion resistance of the welding overlay is the lowest since the overlay is a eutectic structure at the Fe–Ti addition of 12 wt.%. The eutectic structure increases the interface of corrosion reaction, which accelerates the corrosion rate and increases the corrosion current. With the Fe–Ti addition increasing from 12 wt.% to 24 wt.%, the corrosion resistance of the welding overlays with hypoeutectic structure is gradually increased, owing to the increase of the proportion of TiC and chromium content in the matrix.

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Acidic Corrosion Behavior of Slag-free Self-shielded Flux-cored Arc Welding Overlay

Analysis of Process Signals of Resistance Spot Welding for DP590 Steel Using Numerical Calculation

Kang Zhou, Gang Wang, Wenxiao Yu, Huan Li, Mikhail Ivanov

pp. 1896-1907

Abstract

Although the finite element (FE) numerical calculation model has been extensively employed in analyzing metal characteristics, very few works use it to analyze features of the main process signals during the resistance spot welding (RSW) process. Hence, its generality was seriously restricted. In this work, a 2D FE model, which coupled thermal, electrical and mechanical fields, was established with reasonable meshing and parameters and boundary conditions. The parent metal used dual phase (DP) 590 steel, which is commonly employed in automobile lightweight manufacturing process. An actual experiment with the same conditions as the numerical calculation was conducted. The dynamic resistance and electrode displacement obtained from numerical calculation and actual experiment were seriously compared and analyzed. Many important characteristics of process signals have been reasonably explained by combining the results obtained from numerical calculation and experiments. The reasonability and accuracy of the FE numerical calculation were not only verified after the welding process but also examined in depth during the intermediate welding process. Hence, the work can effectively enlarge the applications of the FE model and strengthen the correlation between control strategy design, online quality estimation using main process signals and FE numerical calculation during the RSW process.

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Analysis of Process Signals of Resistance Spot Welding for DP590 Steel Using Numerical Calculation

Effects of Carbon Content and Austenite Grain Size on Retained Austenite Fraction in Stir Zone of Friction Stir Welded 6%Ni Carbon Steels

Takuya Miura, Hidetoshi Fujii, Kohsaku Ushioda

pp. 1908-1917

Abstract

Friction stir welding (FSW) was performed under the two welding conditions (rotation speed-traveling speed) of 150 rpm–100 mm/min and 200 rpm–400 mm/min using 6 mass%Ni steels with different carbon contents from 0.14 mass% to 0.63 mass%. The slightly lower peak welding temperature and the higher cooling rate were predicted under the condition of 200 rpm–400 mm/min. The effects of carbon content and prior austenite grain size on retained austenite fraction in the stir zones were evaluated. When carbon content was 0.30 mass% or more, a fine microstructure consisting of lath martensite and retained austenite was formed in the stir zone. Irrespective of welding conditions, the amount of retained austenite increased with the increase of carbon content. More retained austenite was obtained at the stir zone under the 200 rpm–400 mm/min condition, which resulted from rapid cooling and finer prior austenite grains compared with the condition of 150 rpm–100 mm/min. The effect of the prior austenite grain size on the amount of retained austenite was successfully extracted, and the finer austenite grain size was concluded to play an important role to stabilize austenite by analyzing the difference in the martensitic start temperatures predicted based on the chemical composition and the retained austenite fraction based on Koistinen-Marburger equation.

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Effects of Carbon Content and Austenite Grain Size on Retained Austenite Fraction in Stir Zone of Friction Stir Welded 6%Ni Carbon Steels

Synergistic Effect of Brightener and Solution Temperature on the Electrodeposition Behavior of Zn–Ni Alloy from Alkaline Zincate Solution

Sung Hwa Bae, Satoshi Oue, Yu-ki Taninouchi, Injoon Son, Hiroaki Nakano

pp. 1918-1929

Abstract

Zn–Ni alloys were electrodeposited on a Cu electrode at 10–5000 A·m−2, 5 × 104 C·m−2, 293 K, 313 K, and 333 K in unagitated zincate solutions containing the reaction product of epichlorohydrin and imidazole (IME) as a brightener. The synergistic effect of IEM and solution temperature on the deposition behavior of Zn–Ni alloys was investigated. The transition current density at which the deposition behavior shifted from normal to anomalous one decreased with IME at 293 K, but did not change regardless of IME addition at 313 K and 333 K. The suppression effect of IME on the Zn and Ni depositions during alloy deposition was observed at 293 K while at 313 K and 333 K, the suppression effect decreased on the Zn deposition but was maintained on the Ni deposition. Therefore, Ni content in deposits significantly decreased with IME as the temperature increased. The current efficiency of Zn deposition significantly decreased with IME at 293 K, with a small degree of decrease at 313 K and 333 K. The C content in deposits was the highest at 293 K and decreased with increasing solution temperature, indicating that the adsorption ability of IME on the cathode decreases with the increasing temperature. As a result, the suppression effect of IME on the Zn deposition decreases with the increasing temperature. The gloss of deposited films was the highest at 293 K, attributed to the IME adsorption ability being large at 293 K and deposited films with fine crystals becoming smooth.

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Synergistic Effect of Brightener and Solution Temperature on the Electrodeposition Behavior of Zn–Ni Alloy from Alkaline Zincate Solution

Visualization of Coupling Current and Immersion Potential of Iron Surface Corroding Beneath the Ice in the Temperature Cycling under the Freezing Point

Kazuhisa Azumi, Tatsuki Nishioka

pp. 1930-1936

Abstract

A test specimen, comprising 100 iron wires of 1 mmφ diameter arranged in a 10 × 10 matrix, was exposed to temperature cycling between 0°C and −20°C. NaCl solution (0.1 wt%) was dropped on the surface to form an ice droplet, and the coupling current of each iron electrode against the other 99 electrodes was measured sequentially to obtain a coupling current map. The average coupling current of 100 electrodes varied with temperature cycling. The coupling current increased at a relative humidity > 65% in the absence of an ice droplet, which was similar to atmospheric corrosion at a room temperature higher than the freezing point. When an ice droplet exists, the coupling current increased with increasing temperature and did not depend on relative humidity. This behavior was interpreted as the formation of a thin solution layer of concentrated NaCl solution at the interface between the electrode surface and ice due to the exclusion of NaCl from the growing ice crystal of pure water. From the coupling current map, the inner area of the iron electrodes beneath the ice droplet tended to be a cathode, whereas the outer and surrounding area tended to be an anode. An open circuit potential map was also measured using a quasi-Ag/AgCl electrode placed on the specimen surface. The potential of the inner area was less noble than the outer and surrounding areas and shifted in a less noble direction with temperature. The ice droplet shrank during the temperature cycling and left rust on the surface.

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Visualization of Coupling Current and Immersion Potential of Iron Surface Corroding Beneath the Ice in the Temperature Cycling under the Freezing Point

Evaluation of Corrosion Progress Using Multichannel Iron Wire Electrodes Embedded in a Cement Test Block

Kazuhisa Azumi, Shu Saito, Kou Ishida

pp. 1937-1945

Abstract

Eight thin iron wires were embedded in a cement test block to evaluate corrosion behavior during cement carbonization by CO2 absorption. The gases supplied sequentially to the chamber containing the cement test block were air, CO2, and then air. Electrical resistance measurement of the thin wires revealed that the iron wires remained passivated during the supply of air and uncorroded during the supply of CO2 to promote carbonization. The iron wires started corroding from the surface when the air was resupplied after carbonization due to the presence of O2 in the air as an oxidizing agent. A set of two-electrode impedances between two selected iron wires was measured as a function of depth from the surface. The impedance of all electrodes dropped considerably several hours after the CO2 supply due to the difference in the immersion potential of the iron wires as a function of depth, that is, the pH transition from alkaline to neutral. Consequently, the iron wires were polarized when electrically connected to each other during carbonization. After carbonization, each iron wire showed an impedance response corresponding to its corrosion condition, which depended on its depth.

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Evaluation of Corrosion Progress Using Multichannel Iron Wire Electrodes Embedded in a Cement Test Block

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

Ryunosuke Minami, Yoshihiro Terada

pp. 1946-1951

Abstract

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostwald ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.

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Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

Computer-aided High-efficiency Detection of Fracture Initiation Sites in Steel

Miho Muramatsu, Zhi-Lei Wang, Toshio Ogawa, Yoshitaka Adachi, Tetsuya Namegawa, Kyohei Ishikawa, Hiroyuki Shirahata, Masaaki Fujioka

pp. 1952-1956

Abstract

Finding the fracture initiation sites is critical for understanding the fracture mechanism and thus improving the products’ quality, whereas some hard-to-detected features are easily missed in the human-effort-based characterizations with human eye and involve lots of labor force. In this study, computer-aided detection of fracture initiation sites is proposed to augment human expertise to efficiently find the fracture initiation sites, and thus to reduce the labor cost. With a deep-learning you only look once object detector, the fracture initiation sites of steel were successfully detected in this study. Furthermore, based on the trained detection model, an easy-to-use application for detecting initiation sites has been further developed, exhibiting great potential for high-efficiency detection of fracture initiation sites.

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Computer-aided High-efficiency Detection of Fracture Initiation Sites in Steel

Effect of Si and Al Additions to Steel on Machinability in Gear Cutting

Toshiharu Aiso, Takashi Matsumura

pp. 1957-1966

Abstract

The influence of Si and Al additions to 0.55 mass% C steel on machinability is discussed in cutting with a fly tool of TiAlN coated high speed steel, as performed in gear cutting. Three model steels are prepared with controlling nearly the same hardness to study the effects of the alloying elements: one reference steel with C as the only alloying element (Base steel), and two steels alloyed also with 1 mass% Si or Al. The cutting tests are performed to obtain the cutting forces, observe the cutting chips and analyze the damage on the rake faces of the tools. The orthogonal cutting data in the cutting force simulation are identified to minimize the discrepancies between the measured and the simulated forces for the tested steels. When cutting the Base steel, few adhered materials form and the coated thin layer is worn mainly by abrasion. When cutting the Si alloyed steel, the coating surface is covered by adhered layers containing Si–O, Fe2SiO4 and FeO, which contribute to the lowest friction and protect the coated thin layer from wear. When cutting the Al alloyed steel, an Al2O3 layer forms on the coating. The Al2O3 layer induces high friction, large cutting forces and cutting heat, resulting in the rapid substrate softening and coating fracture.

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Effect of Si and Al Additions to Steel on Machinability in Gear Cutting

Lamb-Dip Laser-Induced Fluorescence Spectroscopy in a Supersonic Plasma Jet for Isotope Identification

Akira Kuwahara

pp. 1967-1969

Abstract

Isotope analysis is challenging because spectral resolution is reduced in high-temperature plasma. In this study, a method involving Lamb-dip laser-induced fluorescence spectroscopy combined with a supersonic plasma jet was developed for the detection of strontium (Sr) isotopes. This newly developed system, with a counter-propagating laser configuration to provide an incident laser beam intensity of 61.8 mW mm−2, selectively detected the Lamb-dip of 88Sr using a spectral frequency width of 35 MHz.

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Lamb-Dip Laser-Induced Fluorescence Spectroscopy in a Supersonic Plasma Jet for Isotope Identification

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