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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Advance Publication

  • Identifying Factors for Cu Contained in Carbon Steel Produced in Japan

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    DOI:10.2355/tetsutohagane.TETSU-2018-009

    The steel industry has been concerned with contamination by tramp elements during repeated recycling of carbon steel. Increase of Cu content has never been observed at least by monitoring EAF steel bars from the late 1980s. However, the increase may happen in future. To surely avoid the increase which leads to ineffective recycling, mechanisms of Cu mixing in carbon steel should be understood. The factors for Cu in carbon steel produced in Japan were identified. We distinguished two sources of Cu in obsolete steel scrap: Cu alloyed in carbon steel which has been contaminated by previous recycling and Cu contained in materials beside carbon steel. We found that, by dynamic material flow analysis, the Cu content derived from the former source has gradually increased because of increasing shares of bar and section which have a relatively high Cu content, which leaded to 0.05% increase in Cu content during three decades. On the other hand, Cu derived from the latter has become smaller from the late 1990s. One of the reasons was thought as increase of exporting scrap-mixed metal (often termed “zappin scrap”) from around the year. In the near future, it is predicted that the substantial part of the export will be rapidly reduced by amendment of relevant regulations. We estimate that Cu content in steel bars will become 0.49% on the average, if scrap-mixed metal is domestically recycled in a commercial way. This result underlines the necessity of improving the separation of Cu materials from carbon steel scrap.
  • Causes of Pinhole Generation on Solidification of Low Carbon S-Pb Free-cutting Steel Cast Continuously

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    DOI:10.2355/tetsutohagane.TETSU-2017-098

    In this study, in order to prevent occurrence of surface cracks caused by pinholes occurring in production of low-carbon S-Pb free-cutting steel using continuous casting, on the causes of pinholes generation in this steel, the generating behavior of bubbles in front of solid/liquid interface and engulfment behavior of bubbles by solidifying shell has been theoretically examined from the viewpoint of transport phenomena, thermodynamics, and interfacial science, and the cause of pinhole generation on solidification has been clarified.Since the oxygen concentration of molten steel is high for undeoxidization to ensure machinability, the CO partial pressure at solid/liquid interface is significantly high due to solute enrichment on solidification. And additionally it is presumed that bubbles tend to be generated at the interface because the total gas pressure increases due to evaporation of lead. Furthermore, it was estimated that this steels are extremely high in sulfur concentration, and the surface tension at solid/liquid interface greatly decreases due to the enrichment at the interface, which also promotes the generation of bubbles on solidification. The CO partial pressure of the total gas pressure accounted for about 90%, and it was estimated that decreasing the CO partial pressure is most important in suppressing bubble generation by proper control of carbon and oxygen content of molten steel and application of stirring of molten steel.Furthermore, it has been clarified that detachment of the bubbles from the solid/liquid interface is suppressed by the interfacial tension gradient caused by the concentration gradient of sulfur and/or oxygen.
  • Influence of Transformation Pseudoelasticity and Accumulated Plastic Strain on Low Cycle Fatigue Characteristics of Fe-30Mn-4Si-2Al Alloy

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    DOI:10.2355/tetsutohagane.TETSU-2017-086

    Fe-30Mn-4Si-2Al alloy (mass%) was reported to show excellent low cycle fatigue properties. We investigated fatigue characteristics of the Fe-30Mn-4Si-2Al alloy as a function of accumulative plastic strains, comparing with the low cycle fatigue test results of Fe-28Mn-6Si-Cr-0.5Nb C alloy and SUS304 steel. The obtained results are shown below. The fatigue life of Fe-30Mn-4Si-2Al alloy is the longest in all the strain ranges as compared with Fe-28Mn-6Si-5Cr-0.5NbC alloy and the SUS 304 steel. In particular, it has a long life in test of high strain amplitude. The εpa - Nf characteristics of Fe-30Mn-4Si-2Al alloy show a straight relationship (εpa = Cp/NfKp). The result that the Manson-Coffin rule holds was obtained. In addition, Cp = 5.62, Kp = 0.72, which is an extremely high value. The fatigue damage value D obtained from the Manson-Coffin equation of Fe-30Mn-4Si-2A alloy was almost 1, similar to Fe-28Mn-6Si-5Cr-0.5NbC alloy or SUS 304 steel. However, the relationship between the accumulative plastic strain λp and fatigue life N is much higher than the limit λp of Fe-28Mn-6Si-5Cr-0.5NbC alloy obtained in the previous report. In particular, the results of εta = 2.0% and 1.4% were 20 times the limit λp. It was found that the excellent low cycle fatigue life of Fe-30Mn-4Si-2Al alloy is caused by the much slower accumulation of plastic strain and the extremely high values of Cp and Kp. The above results show that the repetitive motion of partial dislocation progresses slowly as ε martensite repeats normal and reverse transformation and the developmental process of repeated deformed tissue and fatigue crack propagates in a zigzag along γ/ε interface. As a result, it agrees with previously reported fact that crack growth is suppressed.
  • Effect of Cr Content on Softening during Aging in High Cr Ferritic Heat Resistant Steel

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    DOI:10.2355/tetsutohagane.TETSU-2017-102

    In order to clarify the effect of Cr content on creep strength in high Cr ferritic steels for turbine rotors, a series of experiments is carried out using the steels in the tampered state and after simple aging at 650°C. In the tempered state, the Vickers hardness of 10.5 Cr steel is higher than that of 9 Cr steel, but it decreases remarkably in 10.5 Cr steel and becomes almost the same value in both steels at after aging for 6700 h. In the tempered state, 10.5 Cr steel shows large number density and small average particle diameter of the fine Laves phase compared with 9 Cr steel, and this condition continues even after aging for 1000 h. However, M23C6 carbide and the Laves phase grow with increasing aging time and their average particle sizes in 10.5 Cr steel become greater than those in 9 Cr steel after aging for over several thousand hours. In 10.5 Cr steel, the amounts of carbonitrides such as NbX, Cr2X, and VX decrease after 3000 to 6000 h due to precipitation of the Z phase, whereas in 9 Cr steel, carbonitrides are still present beyond 6000 h due to retardation of the Z phase precipitation.
  • Evaluation of Coke Degradation Effect on Flow Characteristics in Packed Bed Using 3D Scanning for Rotational Mechanical Strength Test and Solid-liquid-gas Three-phase Dynamic Model Analysis

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    DOI:10.2355/tetsutohagane.TETSU-2018-005

    A 3D scanning technique was applied for understanding coke shapes obtained by a rotational strength test, and a numerical dynamic analysis based on the multi-sphere type discrete element method was carried out to clarify the influence of coke degradation on the packed structure. We constructed a trickle flow simulation of molten slag via the smoothed particle hydrodynamics model, and the liquid-gas permeability characteristics exhibited by the coke shapes in the lower part of the blast furnace are discussed accordingly. Coke diameter decreased due to collisions between particles, via the progress of surface- and volume-destruction, and that the particles subsequently became sphere-like in shape. Static holdup of molten slag showed a decreasing tendency with the coke degradation progress, as the void shape and holdup site became spatially uniform as sphericity increased. In the case of packed bed formed by the initial low sphericity or large-sized cokes, the size of the air gap was maintained, although the flow path was non-uniform. Therefore, even if the large amount of holdup did not block the gaseous main flow, in the case of lower coke strength, the sphericity increased due to the deformation progress, and void uniformity could be retained. However, the existence of many narrow void regions remarkably decreased the gas permeability.
  • Friction Stir Welding of Medium Carbon Steel with Laser-Preheating

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    DOI:10.2355/tetsutohagane.TETSU-2018-001

    Friction Stir Welding (FSW) has expanded to many metallic materials with higher melting points or much higher strength than the aluminum alloys. If the tool travels too quickly along the welding seam during the welding process or if the melting point of the workpiece is high, the frictional heat generated between the tool and the workpiece may not be sufficient to cause material flow. Insufficient heat input results in the formation of groove or tunnel-shaped defects in the stir zone and also severe wear or breaking of the FSW tool. To solve these problems, a higher heat input is required to soften the materials. Therefore, several preheating methods have been adopted to increase the heat input. In this study, a fiber laser was used as the preheating source during the FSW. In this experiment, the effect of the laser-preheating on the defect formation and tool rotational torque during the FSW was investigated. Additionally, a difference in the material flow during the conventional FSW and laser-preheating FSW was observed by two pairs of x-ray transmission real-time imaging systems. As a result, it was found that the laser preheating reduced the defect formation and the tool rotational torque during the FSW. Furthermore, laser beam irradiation on the retreating side (RS) was the most effective in reducing the defect formation. On the other hand, the irradiation on the advancing side (AS) was the most effective in reducing the tool rotational torque.
  • Structural Changes in Galvannealed Coating during Hot-stamping Heating

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    DOI:10.2355/tetsutohagane.TETSU-2017-100

    Hot-stamping process is widely applied to coated steel sheets including galvannealed steel sheets (GA) to produce high-strength automobile components. In this study, we investigated the structural changes in galvannealed coating during hot-stamping heating. It was revealed that the original δ1 phase decomposed to the Γ phase and Zn liquid above the peritectic temperature of the δ1 phase, then the Γ phase further decomposed to the Fe-Zn solid-solution and Zn liquid above the peritectic temperature of the Γ phase, and finally the coating changed to a single phase of the Fe-Zn solid-solution. The series of structural changes in GA coating can be explained with the Fe-Zn binary phase diagram.
  • Suppression Effect of Fretting Wear in Railway Axle Journal Bearings by Means of Hard-film Coatings

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    DOI:10.2355/tetsutohagane.TETSU-2017-088

    Axle journal bearings composed of an inner ring, a backing ring, etc. are used in railway applications. The bearing often show fretting wear due to slight relative slip between the inner ring and the backing ring. In this work, the authors have investigated the effects of hard films coated on the backing ring on the fretting wear. Rotational tests using full-scale railway axle bearings with backing rings of side faces which are coated with either of three hard films (titanium nitride (TiN), chromium nitride (CrN) and diamond-like carbon (DLC)) were conducted. It was found that the fretting wear with the TiN film is the worst as a result of the decohesion of whole TiN film, and the fretting wear of the backing ring coated with the CrN film is slight, although the film was not able to follow the deformation of the base material. It was also found that the fretting wear of the backing ring coated with the DLC film which is the thinnest and the hardest among the three films is as slight as that of the backing ring coated with the CrN film, though some of the DLC film left. Accordingly, it is supposed that the fretting wear of the backing ring can be suppressed, if the film coating has a lower friction coefficient, higher hardness and higher bonding strength with the base material.
  • Effect of Natural Gas Injection Point on Combustion and Gasification Efficiency of Pulverized Coal under Blast Furnace Condition

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    DOI:10.2355/tetsutohagane.TETSU-2017-087

    The reduction of CO2 emission from the ironmaking process is important issue from the view of environmental problems typified by global warming in recent years. Low RAR (reducing agent rate) operation of the blast furnace is one of effective measures for reducing CO2 emission. Injection of HRA (hydrogenous reducing agents) from the tuyere (where is the lower part of blast furnace) is also effective measure. In this study, the influence of HRA injection point on combustion and gasification efficiency of pulverized coal (PC) in the case of simultaneous injection of HRA and PC from double-channel lance was examined by small scale combustion furnace and three-dimensional numerical simulation for improvement permeability in blast furnace. Combustion experimental conditions were in three cases, case1: injected HRA from outer side and PC from inner side of double-channel lance, case2: injected HRA from inner side and PC from outer side of double-channel lance and case3: injected HRA and PC premixed. As a result, the combustion and gasification efficiency was increase in the order of case3, case2 and case1. The rate of combustion and gasification of PC was investigated in case1. Not only the oxidation reaction was also accelerated CO2 and H2O gasification reaction in the case of simultaneous injection HRA and PC. A three-dimensional numerical simulation of the experimental furnace was conducted, we confirmed the increase of combustion temperature, the acceleration of oxygen consumption and gasification reaction as with the experimental results in the case of simultaneous injection HRA and PC.
  • Reduction of Contact Resistance on Titanium Sheet Surfaces by Formation of Titanium Carbide and Nitride, and Its Stability in Sulfuric Acid Aqueous Solution

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    DOI:10.2355/tetsutohagane.TETSU-2017-085

    Separators for solid polymer fuel cells must have a low contact resistance with the carbon paper and stability in a corrosive environment of sulfuric acid in the cell. The titanium surface is highly resistant to corrosion thanks to a passive film but has high contact resistance.In this study, titanium carbide or nitride as the electrical conductor was formed on the surface by annealing commercially pure titanium sheet. The contact resistances of these sheets were evaluated before and after a sulfuric acid aqueous solution exposure test, “pH4 at 80°C for 4 days”, briefly simulating the operating environment. In addition, the same evaluation test was conducted with a surface with TiC formed dipped in nitric acid to enhance the stability in a sulfuric acid solution.The initial contact resistance falls below 10 mΩ·cm2 by formation of TiC and TiN, Ti2N on sheet surface. However, the contact resistance rises to 100 or above after the exposure test because a large amount of TiO2 precipitates. This is probably because TiC and TiN are dissolved by sulfuric acid, generating TiO2.By contrast, dipping in nitric acid hardly raises the contact resistance from less than 10 even after the exposure test. It is considered from the results of surface analyses that Ti ion generated by partial dissolution of TiC is turned into TiO2 by the oxidizability of nitric acid, changing the surface structure covering TiC. It is considered that the newly formed TiO2 film enhanced stability in a sulfuric environment.

Article Access Ranking

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