Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 92 (2006), No. 5

  • A New Approach for Interpretation of Strengthening Mechanism of Martensitic Steel through Characterization of Local Deformation Behavior

    pp. 295-310

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    Mechanical characterization in a sub-micron scale by nanoindentation technique is applied to Fe-C base martensitic steels to discuss the strengthening mechanisms. Matrix strengths of block structures were probed to reveal a dependence on a carbon content, tempering temperature and metallurgical process. A grain boundary effect was also evaluated to consider the strengthening factors on the macroscopic strength. It has been found that the grain boundary effect is significantly large on the strength of the Fe-C based martensite due to a fine effective grain of block structure. A locking parameter in the Hall-Petch relation is reduced at tempering temperature above 450°C leading to a "hump" on the temper-softening curve because film-like carbides on grain boundaries are disappeared above 450°C. A microstructure in a conventional quench-tempered process includes high-density carbides on a grain boundary corresponding to a higher locking parameter than that of an ausform-tempered martensite. The lower locking parameter in the ausform-tempered martensite reduces an stress concentration at an interface between the carbide and ferrite phase, which contribute to the improved delayed fracture property of the ausform-tempered martensite.
  • Water Model Experiment on Lead Dispersion during the Refining of Free Cutting Steel

    pp. 311-317

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    Water model experiment has been carried out for understanding the dispersion of lead droplets in a steel bath agitated by offset bottom gas injection. Air, water, and fluorinert are used as the model working fluids for argon gas, molten steel, and molten lead, respectively. The dispersion of fluorinert droplets is significantly dependent on the charge position of fluorinert, gas flow rate, and charge time of fluorinert. An optimum condition for the uniform dispersion is determined.
  • Surface Nonocrystallization of Carburized Steel JIS-SCr420 by Fine Particle Peening

    pp. 318-326

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    The fine particle peening process, which makes particles of several tens microns in diameter impact at high speed, has been adapted industrially to auto motive steel parts or tool steels because the fatigue property is superior to that of a conventional shot peening process. However, microstructural changes obtained from this process have not been fully understood.
    In this study, microstructural change of JIS-SCr420 carburized steel (HV 7.55 GPa) by the fine particle peening process was investigated comparing with cases of a conventional shot peening and of softer steels. In SEM observation of the carburized steel, although plastic flow was observed in the region from the surface to a depth about 2 μm, nanocrystalline structure was not detected. TEM observation revealed that nanocrystalline layer with about 0.5 μm in thickness was produced continuously along the surface. In softer steels which hardness below HV 3.72 GPa in this study, nanocrystalline structure was observed as the folded and layered deformation region, which existed locally and discontinuously along the surface. Morphological difference of these nanocrystalline structures is considered to result from surface deformation modes by particles impact. In softer steels, the convex part in considerable uneven surface formed by the impact is folded and imposed into interior region by the repeated impact and developed to nanocrystalline structure. In carburized steel, owing to absence of the considerable unevenness, throughout surface area deformed during the impact and nanocrystalline structure is developed homogeneously and continuously along the surface. In a conventional shot peening condition, nanocrystalline structures were not formed.
  • Creep Fracture Mechanism Map and Creep Damage of Cr-Mo-V Rotor Steel

    pp. 327-333

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    Specimens of a Cr-M-V rotor steel with creep rupture periods up to 100000 h have been examined metallographically in order to clarify their creep fracture mechanisms and to construct creep fracture mechanism maps for the steel which show the dominant regions of each mechanism. The creep fracture mechanism maps have been drawn taking stress as y-axis and time to rupture as x-axis, and also stress as y-axis and testing temperature as x-axis. The constructed maps include three different creep fracture mechanism fields: transgranular creep fracture, intergranular creep fracture with cavitation and rupture with dynamic recrystallization. The maps indicate that the steel in power plants is being used under the conditions within the field of intergranular creep fracture with cavitation. This means that long time use of the steel under the conditions could cause extensive cavitation and ductility minimum.
  • Forecast for Demand and Supply of Steel Scrap in China

    pp. 334-339

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    Quantitative data on development of steel production in China were collected and examined to estimate the scrap demand in 2010. Taking several influential factors into consideration, it is speculated the amount of scrap increase from 2003 is 15 million tons. This will be covered by in-house scrap, suggesting little increase of scrap import. Moreover, the scrap will used mainly for basic oxygen converter and hence high quality scrap will be requested.
  • Forecast for Demand and Supply of Steel Scrap in Korea

    pp. 340-345

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    1.91 milliont of steel scrap was exported from Japan to Korea in 2003, which accounted for more than 30% of the total steel scrap exported to other countries from Japan. Change in steel scrap demand in Korea in the future will make a great influence on the amount of Japan's steel scrap domestic consumption and export. In this work, quantitative data about the steel production, steel scrap demand and consumption during 1977-2003 in Korea were collected to analyze the amounts of inhouse, industrial and obsolete scrap generation, and total steel accumulation in Korea. Then, the steel scrap demand in Korea in the future was estimated. The total accumulation of steel in Korea was estimated as 380 million t in 2003 and 548 million t in 2010, respectively. The amount of obsolete scrap generation in Korea was 7.1 million t in 1996 and 9.0 million t in 2003, which was about 3.0% and 2.4% of the total steel accumulation in each year. Supposing that the amount of crude steel production, scrap consumption percentages in B.O.F and E.A.F will be stable, the obsolete scrap generation in Korea in 2010 were estimated as 13-17 million t. This significant increase in obsolete scrap generation in Korea could exceed the current amount of the scrap import. So, self-sufficiency of steel scrap could be achieved in around 2010 in Korea.
  • Grinding Sludge Recycling to Reduce Environmental Load

    pp. 346-349

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    Companies like bearing maker which produce precision machinery use many grinding machines in their manufacturing processes. These machines by-produce a lot of grinding sludge, most of which is dumped into landfills now. Being an ISO14001-certified company, it is responsible for establishing of recycling technology and contributing to our recycle-based society. It is however difficult for companies to progress the sludge recycling if additional expense is required. So it is important to establish recycling technology which enables to reduce environmental load together with cost reduction. There are two reasons for difficulty to recycle the grinding sludge. One reason is that it contains a lot of coolant and the other is of its self ignition. Thus separation of the coolant from the sludge economically will be key technology. By using a test machine, it was confirmed that it was possible to separate the coolant from the sludge economically and reuse metal and coolant. It was also confirmed that briquette doesn't have any danger for self ignition after separation.
  • Grinding Sludge Recycling to Reduce Environmental Load

    pp. 350-355

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    Report of "No. 1 Basic investigation for grinding sludge recycling" concluded that it was possible to separate the coolant from the grinding sludge economically and grinding swarf and coolant were able to be reused for cost reduction. Based on that investigation, practical machine has been developed. After one year long run test, the results proved that environmental load reduction and cost reduction have been achieved practically at the same time.

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