Papers by Author: Kohsaku Ushioda

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Authors: Manabu Kubota, Kohsaku Ushioda, Goro Miyamoto, Tadashi Furuhara
Abstract: The recrystallization behavior of hot-deformed austenite of 0.55% C low alloy steels at 900, 850 and 800°C was investigated by a conventional double-hit compression test and a new method which reconstructs the parent austenite orientation map from an EBSD (electron backscattering diffraction) orientation map of daughter lath martensite. The new method can clearly reconstruct the parent austenite structure at high temperature from the daughter lath martensite structure and we can obtain the information on crystal orientation of the work-hardened austenite. It was revealed that recrystallization of austenite at 800 °C is significantly retarded by the addition of 0.1% V. The strong texture of <110> parallel to the compression direction develops just after the hot-deformation, but this texture becomes weaker as the recrystallization progresses. By applying the reconstruction method, it becomes possible to evaluate various phenomena related to the hot-deformation of austenite
2389
Authors: Kohsaku Ushioda, Sae Nakanishi, Tatsuya Morikawa, Kenji Higashida, Yoshihiro Suwa, Kenichi Murakami
Abstract: Heterogeneous deformation during rolling is a crucial issue for elucidating recrystallization behavior. The progress thus far in our understanding of heterogeneity has been reviewed focusing on grain boundary and shear band. A statistical study on heterogeneous deformation structure using EBSD revealed that heterogeneity along the grain boundary can be classified into three types: 1) relatively flat boundary, 2) irregularly serrated boundary, and 3) boundary associated with fine grains. The fine grains in type 3 seem to be dynamically recovered as a cold-rolled state. Shear band formation is considered to be caused by plastic instability that is accelerated, for instance, by dynamic strain aging. A shear band is revealed to have a feature of recovered fine cells with Goss orientation already embedded in the shear band. The application of the phase-field method is exploited to predict recrystallization behavior and texture evolution during annealing based on the subgrain growth model. In simulation, a bulging mechanism seems to be dominant. Thus, a more rigorous description of the heterogeneous deformation structure is needed in the future
58
Authors: Shigeto Takebayashi, Kohsaku Ushioda, Naoki Yoshinaga, Shigenobu Ogata
Abstract: The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.
1033
Authors: Takaya Suzuki, Y. Ishii, A. Itami, Kohsaku Ushioda, Naoki Yoshinaga, Hiroyasu Tezuka
673
Authors: Hajime Saitoh, Kohsaku Ushioda, Naoki Yoshinaga
Abstract: No clear-cut information is available with regard to the effect of foreign atoms on the solubility limit of C in b.c.c. iron despite many previous studies. Against this backdrop, the influence of substitutional atoms (Mn, Cr, P, Si, Al) on the solubility limit of C in b.c.c. iron in equilibrium with cementite was investigated in low-carbon steels at a temperature of 700°C. In detail, the C solubility limit was determined from internal friction measurements combined with infrared analysis of C using a high-frequency combustion technique. It has been clarified that Mn, Cr, and Al hardly change the C solubility limit, whereas P and Si increase it. The thermodynamical calculation indicates that, under para equilibrium Si increases the C solubility limit and Mn hardly changes it, while under ortho equilibrium Mn and Si decrease it. However, the present experimental condition was verified to be close to ortho form. The discrepancy between the experiments and the calculations seems to come from the fact that: 1) single solute C atoms and the C atoms combined as Substitute-C complex are not distinguished experimentally, and 2) in the regular solution model, the non-uniform distribution of C atoms around alloying atoms is not introduced into the entropy term, which is something that should be studied further in the future.
2348
Authors: Kohsaku Ushioda, Naoki Yoshinaga, H. Saitoh, O. Akisue
375
Authors: Kazuhiko Honda, Kohsaku Ushioda, Wataru Yamada
Abstract: The solidification structure of a hot-dip Zn-11%Al-3%Mg-0.2%Si coating with a Ti addition on a steel substrate was investigated. Steel sheet was coated using a laboratory hot-dip galvanizing simulator. The coating was subsequently characterized via optical and high resolution scanning electron microscopy with EBSD and high temperature X-ray diffractometry. The hot-dip coating consisted of a combination of a Zn/Al/MgZn2 ternary eutectic structure, primary Al phase and MgZn2 phase. TiAl3 acts as a heterogeneous nucleation site for Al, which was shown to have perfect lattice coherency with TiAl3 as epitaxial Al growth from the TiAl3 was found. The growth direction of Al is along <110> and has a random texture, whereas Zn has a rather strong ND//<0001> fiber texture.
2787
Authors: S. Hiwatashi, Tetsuo Hatakeyama, Kohsaku Ushioda, M. Usuda
1585
Authors: Yoshiyuki Ushigami, Yoshihiro Arita, Kohsaku Ushioda
Abstract: It has been observed that grain size of Goss secondary grain has a strong correlation with deviation angle from the exact Goss orientation and sharper Goss grain has larger grain diameter. This orientation selectivity of secondary recrystallization has been investigated with the statistical model of grain growth in which inhibitor and texture are taken into account. The model assumes that sharper Goss grain has a higher frequency of CSL boundaries to the matrix grains and thus has lower statistical grain boundary energy and suffers lower pinning force from the inhibitor. The analysis showed that this model successfully explains orientation selectivity and depicts the effect of inhibitor and texture.
337
Authors: Ken Takata, Kohsaku Ushioda, Masao Kikuchi
Abstract: The precipitation behavior of Mg and Si during storage at RT in Al-Mg-Si alloys pre-aged at 90°C was studied using a tensile test and differential scanning calorimetry (DSC) measurement. Specimens were solutionized at 530°C, water-quenched and then pre-aged for 2, 6 and 12 hours at 90°C during which small precipitates were formed. In the pre-aged alloy, the strengthening rate at RT has two stages. In the initial stage, the yield strength increases slowly with the aging time and in the final stage, it increases rapidly. In the initial stage, the strength in the pre-aged alloy is smaller than that in the non pre-aged alloy, while in the final stage, the strength in the pre-aged alloy is larger than that in the non pre-aged alloy. Furthermore, the period of the initial stage is dependent on the pre-aging period at 90°C. The DSC curves of alloys in the initial stage do not show the presence of clusters, while those in the final stage do. It seems that in the initial stage Mg and Si atoms accumulate around the small precipitates that have been formed in pre-aging at 90°C while in the final stage, the clusters of these atoms are formed.
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