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

Tetsu-to-Hagané Vol. 108 (2022), No. 5

  • Development of Cold Rolling Oil for Tinplate with High Emulsion Stability Index

    pp. 287-294

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    DOI:10.2355/tetsutohagane.TETSU-2021-099

    In the previous study, a cationic rolling oil containing cationic and non-ionic emulsifiers was developed and was used for sheet gauge rolling in a tandem mill.In this study, the previously developed cationic rolling oil was further improved and applied to tinplate rolling, and it generated higher friction and pressure in the roll-bite than in sheet gauge rolling.The improved oil was formulated with a special agent to enhance its lubricity during high-speed rolling, and its emulsion stability was also improved by adjusting the emulsifier. The emulsification stabilities and plate-out properties of conventional non-ionic, the previously developed cationic, and the improved cationic rolling oils were investigated on a laboratory scale. In the emulsification stability test, the average particle size of the improved cationic oil solution was found to be reduced to less than half that of the previously developed cationic oil. In addition, the amount of plate-out film of the improved oil was three times higher than that of the conventional non-ionic rolling oil at 170°C. The plate-out properties on high-temperature specimens at 200°C with and without the special agent were also verified.Furthermore, the effectiveness of the improved oil was verified in an actual mill. The maximum rolling speed of improved oil was faster than that of the current rolling oil with nonionic emulsifier. Based on this result, the improved oil having better lubricity was confirmed.In conclusion, the improved oil can be used for tinplate production to maintain stable coolant with a high level of ESI.
  • Influence of Acicular Ferrite Microstructure on Toughness of Ti-Rare Earth Metal (REM)-Zr Killed Steel

    pp. 295-305

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    DOI:10.2355/tetsutohagane.TETSU-2021-127

    Novel oxide particles composed of Ti, rare-earth metal and Zr was found by the present authors to promote the formation of acicular ferrite (AF), but their effect on toughness has not yet been clarified. Therefore, influence of AF formation on toughness of low carbon steel has been investigated using Ti-Rare earth metal-Zr-killed (TRZ) steel and Al-killed (AL) steels in this study. Dominant microstructure of the AL steel is bainite forming block/packet structure elongated from prior austenite grain boundaries. In contrast, a large amount of AF which grows from intragranular inclusions is formed in the TRZ steel. Toughness of the TRZ steel is better than the AL steel. In addition, at higher austenitizing temperature, AF formation is promoted in the TRZ steel, leading to lowering ductile-brittle transition temperature. Cracks in Charpy impact specimens propagate along {001}α plane and deflect at Bain unit boundary across which {001}α planes are largely misoriented, which means the effective grain size for crack propagation is Bain unit size. Bain unit size is smaller in the TRZ steel than in the AL steel since bainite structure is subdivided by pre-formed AFs belonging to different Bain groups from bainite. Martensite-austenite constituent (MA) in AF is less-elongated in comparison to those in bainite and thus the amount of elongated MA in the TRZ steel is less than in the AL steel. These results clarify that AF formation in the TRZ steel improves toughness due to the refinement of Bain unit size and reduction of the amount of elongated MA.
  • Control of Core-shell Type Second Phase Formed via Interrupted Quenching and Intercritical Annealing in a Medium Manganese Steel

    pp. 306-315

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    DOI:10.2355/tetsutohagane.TETSU-2021-123

    Medium manganese steel (Fe-5.0%Mn-1.2%Si-0.10%C alloy) was subjected to interrupted quenching from the austenite single-phase region to a temperature between Ms and Mf followed by intercritical annealing in the ferrite and austenite dual-phase region at 923 K. As a result, a core-shell type second phase, which consisted of a fresh martensite core surrounded by a film-like retained austenite shell, was formed. The mechanism and kinetics of reversion for the interrupted-quenched specimens were analyzed with DICTRA simulation and TEM observation. With regard to the effect of the core-shell type second phase on mechanical properties, it was inferred that the fresh martensite core functioned as a hard second phase and enhanced work hardening by stress partitioning similar to DP steel, while the film-like retained austenite contributed to improved ductility due to the TRIP effect. As the interrupted quenching temperature decreased, the volume fraction of the fresh martensite core decreased, while the stability of the retained austenite shell increased. This showed potential for controlling the strength and ductility balance of medium manganese steel. A possible beneficial effect of the core-shell type second phase on the ductile fracture behavior was also discussed in terms of stress/strain relaxation at the interfaces between hard martensite and ferrite matrix.
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    1. Effect of Stretch-forming on Hydrogen Diffusion Behavior in High-strength Steel Sheet Tetsu-to-Hagané Vol.108(2022), No.5
  • Effect of Stretch-forming on Hydrogen Diffusion Behavior in High-strength Steel Sheet

    pp. 316-324

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    DOI:10.2355/tetsutohagane.TETSU-2021-096

    The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated utilizing a newly developed hydrogen visualization technique using an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. The hydrogen permeation through the steel sheet could be monitored from the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated from the changes in the brightness of the Ir complex. This hydrogen visualization technique was also applied to a steel sheet stretch-formed using a hemisphere punch to simulate press-forming of automotive structural parts. The hydrogen breakthrough time around the top of the specimen was long and decreased with increasing the distance from the top. According to the plastic strain distribution of the specimen calculated by the finite element method, the hydrogen breakthrough time increased with increasing plastic strain. It was considered that the introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, resulting in the increase of hydrogen breakthrough time.
  • Effect of Neodymium Content on Creep Properties of 9Cr-3Co-3W-Nd-B Steel

    pp. 325-333

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    DOI:10.2355/tetsutohagane.TETSU-2021-103

    The effects of the neodymium content on the creep properties and metallographic structure of 9Cr-3Co-3W-Nd-B steel were investigated. Neodymium had a mild effect on the creep rupture strength at contents up to 0.056 mass%. This suggested that the effects of neodymium compounds and solid-dissolved neodymium were minimal in the microstructures and uniformly creep-deformed parts after normalizing and tempering heat treatment. On the other hand, the reduction of area after creep rupture was improved by the addition of neodymium. Creep rupture occurred at the prior austenite grain boundaries on steel without neodymium. Therefore, neodymium conclusively adhered to the segregated sulfur at the prior austenite grain boundaries to suppress the formation of creep cracks.
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