Faculty Profiles - TAKEMOTO Yoshito

Papers - TAKEMOTO Yoshito

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Alloy Composition Dependency of Plastic Deformation Behavior in Biaxial Compressions of Ti-Nb Alloys Reviewed

Ichiro Shimizu, Kazuki Hisada, Shinichi Ishikawa, Yoshito Takemoto, Naoya Tada

INTERNATIONAL CONFERENCE ON EXPERIMENTAL MECHANICS 2014 9302 2015

Language：English Publishing type：Research paper (international conference proceedings) Publisher：SPIE-INT SOC OPTICAL ENGINEERING

Crystal structure of titanium alloy changes from alpha (hexagonal close-packed) to beta (body centered cubic) with increase of beta stabilizer content. This change of structure strongly influences on the plastic deformation behavior of titanium alloys, because it not only induces changes of slip systems but also activates martensitic transformation and deformation twinning. However, most of past studies on titanium alloys have been focused on the development of specific functionalities induced by alloy designing, and few research works have been reported on metal workability under multi-axial stress conditions, which is key factor to apply titanium alloys for engineering products. In this study, uniaxial and biaxial compression tests of titanium-niobium alloys with various niobium contents have been performed to clarify the influence of beta stabilizer content on the plastic behavior under compressive stress conditions. The titaniumniobium alloys were solution treated and then quenched from beta region to obtain metastable structures. The resultant stress-strain relations together with microscopic observations of texture revealed that the influence of niobium contents on the predominant plastic deformation mechanisms and thus on the hardening phenomena. The equi-plastic work contours obtained by uniaxial and biaxial compression tests also implied the crystal structure dependency of anisotropic hardening, which was evaluated quantitatively by means of Hill's anisotropic yield criterion. The results will provide information on the versatile constitutive relations of titanium alloys containing beta stabilizer elements, that is important to prove the performance of products manufactured by compressive metal working processes such as forging and extrusion.

DOI： 10.1117/12.2077324

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Influence of Microstructures on Hydrogen Embrittlement Susceptibility of Hot Stamped Ultrahigh Strength Components Reviewed

Masahiro Matsumoto, Yoshito Takemoto, Takehide Senuma

HOT SHEET METAL FORMING OF HIGH-PERFORMANCE STEEL CHS2 (5TH INTERNATIONAL CONFERENCE) ( 5 ) 55 - 63 2015

Language：English Publishing type：Research paper (international conference proceedings) Publisher：VERLAG WISSENSCHAFTLICHE SCRIPTEN

One of large hindrances against the increase in the strength of hot stamped components is an increasing risk of the occurrence of delayed fracture. In this study, the influence of microstructures on the hydrogen embrittlement susceptibility of hot stamped ultra-high strength components has been investigated. The following results were obtained.
The grain refinement of martensite reduces the hydrogen embrittlement susceptibility. The bake hardening (BH) treatment through which the lattice stress is released and fine ferrous carbides are precipitated suppresses the delayed fracture significantly. A presence of small amount of ferrite in the martensite microstructure hardly decreases the resistance to delayed fracture. A proper presence of nano-scale precipitates delays the occurrence of the delayed fracture. The precipitation control which creates the proper precipitates trapping hydrogen effectively and suppressing the delayed fracture will be discussed in the paper.
The obtained results indicated that hot stamped components with a tensile strength of more than 2000 MPa without great risk against the delayed fracture can be produced by a sophisticated microstructural control.

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ホットスタンピングの成形性ならびに生産性向上技術 Reviewed

瀬沼武秀,竹元嘉利

鉄と鋼 100 ( 12 ) 1481 - 1489 2014.12

Influence of Predeformation on Tempering Structure of Ti-5Al-2Fe-3 Mo Alloy Reviewed

Yoshito Takemoto, Keita Wada, Kenta Asakura, Takehide Senuma, Tomonori Kunieda, Hideki Fujii

JOURNAL OF THE JAPAN INSTITUTE OF METALS 78 ( 12 ) 434 - 440 2014.12

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

A near-beta-type Ti-5Al-2Fe-3Mo alloy (Super-TIX (R) 523AFM) was developed as a low-cost and high-strength material for automotive components such as intake valves. Its mechanical properties, such as strength, elongation and Young's modulus, are adjustable via the solution treatment temperature. Furthermore, a unique phenomenon different from the shape memory effect that appears after tempering has recently been discovered in the alloy. In this study, the influence of predeformation on the microstructural evolution attributed to the tempering of the alloy quenched from 900 degrees C (alpha + beta) was investigated to clarify the cause of the unique phenomenon. Although the phase constitution of the quenched specimen was alpha + beta, the effective Mo equivalency (6.5Mo(effeq)) of the phase was lower than the minimum beta composition (10Mo(effeq)) in common beta alloys. Work. induced alpha" martensite plates consisting of single alpha" variant including nanosize beta domains were formed in the beta-phase by cold-rolling. Upon heating the rolled specimen to around 250 degrees C, the plates vanished owing to the alpha"->beta inverse transformation and the hardness decreased. However, when the rolled specimen was heated to 450 degrees C, the hardness markedly increased as a result of the beta ->alpha" transformation, and the plates were formed again. The plates consist of a single alpha" variant, but the alpha" lattice is not extensively continuous and is divided into nanosize domains by boundaries containing many faults. It is considered that the evolution of single-alpha"-variant plates generates a large distortion due to the transformation strain and leads to the unique shape change. Such plates were not formed in undeformed specimen by aging at 450 degrees C, and the beta-phase was transformed into a nanosize domain structure composed of multiple alpha" variants. Consequently, these fine structure in the undeformed specimen led to its higher hardness than that of the rolled specimen.

DOI： 10.2320/jinstmet.J2014035

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V単独，V-Nb複合添加超高強度ホットスタンピング部材の組織と機械的性質 Reviewed

立山祥大, 石尾遼平, 林公平, 須江龍裕, 竹元嘉利, 瀬沼武秀

鉄と鋼 100 ( 9 ) 1114 - 1122 2014.9

Formability and Productivity Improving Technology of Hot Stamping Reviewed

Takehide Senuma, Yoshito Takemoto

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 100 ( 12 ) 1481 - 1489 2014

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

As a method for producing ultrahigh strength automotive steel components with high gauge accuracy the hot stamping technology draws attention and is practiced widely today. For supporting and developing this technology, numerous researches have been actively carried out all over the world. It is a good timing to review this technology on this special issue because further rapid development is to be expected in the next decade. The hot stamping technology consists of a variety of techniques. In this review, the authors focus two items suitable to this special issue, the first one is the formability of hot stamping and the second is the technology for improving its productivity.
The review shows the importance of the temperature control at stamping parts to achieve excellent formability. Through the review on the technology for improving the productivity of hot stamping, the authors propose highly productive hot stamping processes consisting of rapid heating, short time stamping and rapid cooling.

DOI： 10.2355/tetsutohagane.100.1481

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Microstructures and Mechanical Properties of V and/or Nb Bearing Ultrahigh Strength Hot Stamped Steel Components Reviewed

Shota Tateyama, Ryohei Ishio, Kouhei Hayashi, Tatsuhiro Sue, Yoshito Takemoto, Takehide Senuma

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 100 ( 9 ) 1114 - 1122 2014

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

As a method for producing ultrahigh strength automotive steel components with high gauge accuracy, the hot stamping technology draws attention and the amount of hot stamped automotive components has exponentially increased in the last decade. The concerns about the potential danger of embrittlement and delayed fracture, however, limit the strength of conventional hot stamped components to 1500 MPa. If these concerns are eliminated, automotive steel components with higher strength which is demanded by the automotive industry can be further commercialized.
In this study, microstructures and mechanical properties of V and/or Nb bearing ultrahigh strength hot stamped steel components have been investigated. The V addition improves the resistance to delayed fracture by forming VC whose interface with matrix is supposed to act as preferential hydrogen trap site. The Nb addition refines the microstructure, which improves both the toughness and the resistance to delayed fracture. Proper hot stamping conditions for these steels has been discussed to create an ultrafine martensite microstructure and finely dispersed precipitates. The results indicate that 1900 MPa class steel components with good toughness and high resistance to delayed fracture can be produced.

DOI： 10.2355/tetsutohagane.100.1114

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V,Cuの単独ならびに複合添加中炭素鋼の析出硬化挙動 Reviewed

鉄と鋼 99 ( 11 ) 669 - 675 2013.11

Influence of Various Factors on the Refinement of Martensitic Microstructures of Hot Stamped Steel Components Reviewed

Ono Takafumi, Hosomi Tsukasa, Takemoto Yoshito, Senuma Takehide

Tetsu-to-Hagane 99 ( 7 ) 475 - 483 2013.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Iron and Steel Institute of Japan

The toughness and the resistance to delayed fracture are deteriorated with increasing strength of martenstic steels. It has been reported that these properties could be improved by grain refinement of martensite.<br>In this study, the influence of both hot stamping conditions such as heating rate, heating temperature, cooling rate, and material conditions such as hot band microstructure, cold rolling reduction, addition of micro-alloys on the grain refinement of hot stamped ultra-high strength martenstic steel components are investigated. The investigation revealed that low temperature and short time heating, high heating and cooling rates, the decrease in the coiling temperature of hot bands, the increase in cold rolling reduction and addition of micro-alloys are preferable for grain refinement of martensite. The mechanism of the grain refinement was discussed from the viewpoints of the increase in the nucleation rate of austenite during the α→γ transformation and the suppression of the austenite grain growth.

DOI： 10.2355/tetsutohagane.99.475

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Other Link： https://jlc.jst.go.jp/DN/JALC/10020767937?from=CiNii

Hydrogen Embrittlement Properties of Ultra High-Strength Low Alloy TRIP-Aided Steels with Hot Forging Process Reviewed

Proc. of the 2nd. Int. Sympo. on Automobile Steel 169 - 172 2013.5

ホットスタンピング部材の生産性向上ならびに高機能化のための組織制御 Reviewed

塑性と加工 54 137 - 142 2013.2

Development of 2000MPa Class Hot Stamped Steel Components with Good Toughness and High Resistance against Delayed Fracture Reviewed

Akitaka Tokizawa, Kousuke Yamamoto, Yoshito Takemoto, Takehide Senuma

HOT SHEET METAL FORMING OF HIGH-PERFORMANCE STEEL CHS2 (4TH INTERNATIONAL CONFERENCE) ( 4 ) 473 - 479 2013

Language：English Publishing type：Research paper (international conference proceedings) Publisher：VERLAG WISSENSCHAFTLICHE SCRIPTEN

Although higher strength of hot stamped automotive components is demanded, the strength higher than 2000 MPa is not realized because of the deterioration of toughness and resistance against delayed fracture.
In this study, we investigated a proper microstructural control to achieve a good strength-toughness balance with high resistance against delayed fracture. As a result, we have determined the condition for producing 2000 MPa class hot stamped components with the toughness and anti-delayed fracture property which are nearly equal to or better than those of conventional 1500 MPa class components. The microstructure of the hot stamped components consists of ultrafine grained martensite matrix with finely dispersed ferrous carbides and alloy precipitates. The grain refinement of martensite implies to improve toughness and the existence of fine ferrous carbides and alloy precipitates indicate to increase the sites trapping hydrogen and to raise the resistance against delayed fracture.

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Precipitation Hardening Behavior of V and/or Cu Bearing Middle Carbon Steels Reviewed

Yuuki Murase, Naoyuki Iwasa, Yoshito Takemoto, Takehide Senuma

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 99 ( 11 ) 669 - 675 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

In this study, the precipitation hardening behavior of middle carbon steels with single and multiple additions of 0.3mass%V and 2mass%Cu has been investigated. The precipitation treatment was carried out isothermally at 550 degrees C, 600 degrees C and 650 degrees C.
In the 0.3mass%V bearing steel, precipitation hardening was observed at 600 degrees C and 650 degrees C while no significant increase in hardness was observed at 550 C. When 2mass% Cu was added to the 0.3mass%V bearing steel, the hardness attributed to the precipitation hardening of Cu was added to that attributed to the precipitation hardening of VC in the cases of 600 degrees C and 650 degrees C. In the case of 550 degrees C, the V addition into the Cu bearing steel markedly increased the hardness although the precipitation hardening of VC was hardly observed in the 0.3mass%V bearing steel. In the paper, the different precipitation hardening behavior of the three steels is discussed in detail in consideration of the observed precipitation behavior.

DOI： 10.2355/tetsutohagane.99.669

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Peculiar Transformation Behavior and Microstructure Evolution with Tempering of Ti-4Fe-7Al Alloy Reviewed

Yoshito Takemoto, Kazuya Fujii, Shou Nakastuka, Takehide Senuma

JOURNAL OF THE JAPAN INSTITUTE OF METALS 77 ( 9 ) 368 - 374 2013

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

The transformation behavior and microstructural changes of Ti-4Fe-7Al alloy during tempering were investigated by performing a hardness test and using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Within a brief time of 30 s during tempering at 450 degrees C, the hardness rapidly increased to a maximum value of 600 Hv. The transformation behavior estimated from the increase in hardness indicated "a lower half of C-curve" in the TTT diagram, which suggests that the transformation occurs through certain thermal activation processes. However, it is difficult to assume that the transformation is controlled by the diffusion of substitutional atoms, since the mean diffusion distance of Fe atoms during tempering at 300 degrees C for 1 x 10(3) s, which also maximized the hardness, is much less than the distance of the nearest neighbors in the beta structure. The structure of the alpha '' phase formed by tempering depends on both the temperature and holding time; a high temperature and a long time bring the alpha '' phase close to an hcp structure. As determined by XRD measurement, the sample tempered at 150 degrees C for 60 min was composed of the beta phase. However, the omega and alpha '' phases were also detected in selected area diffraction (SAD) patterns. The sample tempered at 450 degrees C for 60 min consisted of a single alpha '' phase and exhibited a tweed structure in the TEM observation. Although the tweed structure seemed to contain only one alpha ''-variant in the SAD patterns, HR-TEM observation revealed that it was composed of multiple nanoscale variants. It was considered that the marked grain refinement due to the nanoscale variants led to the extreme hardening of the sample and the significant broadening of XRD peaks. When the tempering temperature was increased to 600 degrees C, the a phase and finely dispersed TiFe precipitates were formed after 60 min by the diffusion of atoms, which resulted in the softening of the sample and the sharpening of the XRD peaks.

DOI： 10.2320/jinstmet.J2013027

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Microstructural Control in Hot Stamping Processes for Producing Ultra-high Strength Automotive Steel Components Reviewed

Takehide Senuma, Yoshito Takemoto, Kenichi Shimizu, Tomohiko Hojo

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 98 ( 11 ) 751 - 758 2012.11

Unprecedented Phenomena with Tempering of Ti-4Fe-7Al Alloy Reviewed

Yoshito Takemoto, Masahiro Ochi, Takehide Senuma, Jun Takada, Ichiro Shimizu, Kazuhiro Matsugi

JOURNAL OF THE JAPAN INSTITUTE OF METALS 76 ( 7 ) 449 - 455 2012.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

Unprecedented phenomena were discovered by tempering the Ti-4Fe-7Al alloy quenched from the beta (bcc) field. The alloy became very hard when it was tempered at 450 degrees C for several minutes, and severely rugged surface was generated. The inverse shape recovery phenomenon was also discovered when a quenched specimen that had been bent at room temperature was heated. The tempered microstructure showed almost beta grains and some of the usual martensitic acicular structure areas. However, electron back scattering pattern (EBSP) measurements showed that the beta-like grain was not the bcc structure but was the hcp or orthorhombic structure. X-ray diffraction (XRD) measurements clarified that an orthorhombic alpha '' structure (a = 0.299(5) nm, b = 0.491(3) nm, c=0.465(9) nm) was formed from the beta phase by tempering. Moreover, this a '' structure was confirmed to be a type of martensitic transformation because no concentration distribution was detected in scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis of the microstructure. It was suggested that the essential M-s point of the alloy should be higher than room temperature; however, the martensite transformation could not operate by fast quenching.
The newly-discovered alpha '' martensite is formed without atomic diffusion by heating. When the beta grain transforms into the single alpha ''-variant, a very huge lattice strain is generated, resulting in the severely rugged surface or the inverse shape recovery phenomenon.

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Tensile Properties and Work-hardening Behavior of Fe-3%Al Single Crystal Reviewed

Yoshito Takemoto, Takehide Senuma

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 98 ( 6 ) 216 - 222 2012.6

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

Synopsis: The tensile deformation behavior of single and poly-crystal Fe-3%Al alloys was investigated at room temperature. The work hardening rate (WEIR) of the polycrystalline alloy monotonously decreased with increasing strain and increased with decreasing grain size. The WHR of the single crystal alloy was remarkably lower than that of the polycrystalline alloy, and the single crystal alloy exhibited a peculiar behavior that could be classified into four different types depending on the tensile axes. When the tensile axes were on the line connecting < 102 > and < 101 > in the stereo triangle, the characteristics of deformation of the single crystal alloy were steady and its WHR showed a monotonous decrease similar to that observed in the case of the polycrystalline alloy. For most of the tensile orientations, including < 001 >, the WHR curve changed from concave to convex with a strain increase. The WHR for the < 101 > orientation also showed the maximum and minimum values, i.e., it dropped immediately after reaching the maxima. This behavior was considered ascribable to the transition of the dislocation structure. The deformation for the < 111 > orientation was characterized by the highest yield stress and work softening, which were caused by some intense slip bands with discontinuous stress drops. Lattice rotation with tensile deformation in single crystal alloys was also examined by an electron backscatter diffraction (EBSD) measurement. For determining the rotation direction, the stereo triangle was divided into two areas by a great circle connecting < 113 > and < 102 >.

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Influence of Al Addition on Microstructures and Mechanical Properties of Ti-Fe Alloys Reviewed

Yoshito Takemoto, Masahiro Ochi, Takehide Senuma, Jun Takada, Ichiro Shimizu, Kazuhiro Matsugi

JOURNAL OF THE JAPAN INSTITUTE OF METALS 76 ( 5 ) 332 - 337 2012.5

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

The influence of Al content on the evolution of microstructures and mechanical properties of Ti-xFe-yAl alloys quenched from a beta field was investigated via hardness measurements, tensile tests, scanning electron microscopy (SEM) equipped with electron back scattering pattern (EBSP) analyzer, and conventional transmission electron microscopy (TEM). The hardness of Ti-(3-6)Fe-yAl decreased remarkably on addition of 1% Al. In comparison to binary alloys, the maximum hardness decrease was achieved in Ti-4Fe-1Al. The hardness of Ti-4Fe-yAl decreased with increasing Al content up to around 5%, reaching a minimum, then gradually increased. Though all of the Ti-3Fe-yAl alloys showed the martensite structure, Ti-4Fe-yAl showed a structural change of beta + omega -> beta + omega + martensite -> beta + omega with increasing Al content. Consequently, Ti-4Fe-7Al had the lowest density in the beta-type alloys equivalent to that of Ti-6Al-4V. The ratio of reciprocal distance d(0002 omega)*/d(222 beta)* on selected-area diffraction (SAD) pattern, which is often used as an indicator of {332}< 113 > twin formation, was nearly constant up to 5% Al of Ti-4Fe-yAl, but decreased over 6% Al. The tensile tests of both Ti-4Fe-5Al and Ti-4Fe-7Al showed ductility. The deformation behavior of Ti-4Fe-5Al producing strain-induced martensite resembled that of Ti-8Mo. On the other hand, Ti-4Fe-7Al was deformed by a coarse slip of {112}< 111 > in the same manner as Ti-20Mo, and the SAD pattern analysis revealed that the diffuse reflection of the omega phase in the non-deformed area turned into the spots at the deformation band. It was suggested that Al behaves as a beta stabilizer element when added to the lowest beta alloy and is available for weight saving, while maintaining the beta structure.

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MODEL for PREDICTING RECRYSTALLIZATION BEHAVIOR OF COLD ROLLED EXTRALOW CARBON STEEL SHEETS Reviewed

Takehide Senuma, Yoshito Takemoto

THERMEC 2011, PTS 1-4 706-709 2302 - 2307 2012

Language：English Publishing type：Research paper (international conference proceedings) Publisher：TRANS TECH PUBLICATIONS LTD

Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures.
The authors measured the orientations of a recovered microstructure of a cold rolled extralow carbon steel sheet by SEM-EBSP and classified the deformed grains into several types due to the orientation and its scattering degree.
In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed by applying the model of Humphreys modified to the grain of each type. A good agreement between the experimental and calculated results using the model developed was obtained.

DOI： 10.4028/www.scientific.net/MSF.706-709.2302

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Microstructural Evolution of Carbon Steels in Hot Stamping Processes Reviewed

Kenta Hidaka, Yoshito Takemoto, Takehide Senuma

ISIJ INTERNATIONAL 52 ( 4 ) 688 - 696 2012

Language：English Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

Hot stamping is a very promising method for producing ultrahigh-strength automotive components. The heating and cooling conditions in the conventional hot stamping process are quite limited. To improve the productivity of the process and/or the mechanical properties of the product, optimizing the heating and cooling conditions is an important and urgent research subject.
In this study, the influence of the heating rate, heating temperature, and cooling conditions on the microstructure and mechanical properties of 0.22%C-2.5%Mn steel was investigated. Samples were heated at two different heating rates (10 degrees C/s and 200 degrees C/s) to various temperatures and then either cooled in air or quenched using water sprays. The sample heated at 200 degrees C/s to 800 degrees C and then immediately quenched in water exhibited a very fine martensite microstructure haying a grain size with an average diameter of 4 mu m and a hardness value around 550 Hy. The sample heated above Ac3 and subsequently cooled in air exhibited a full martensite microstructure and a hardness of 450 Hy. A model that predicted the precipitation of cementite during cooling was developed to clarify the cause of the difference in hardness of the martensite in the samples that were quenched in water and cooled in air. The calculations showed that the hardness of samples cooled in air was lower mainly because of the decrease in the solid solution hardening of C owing to the precipitation of cementite below the Ms temperature. Drawing on the results obtained, hot stamping technology is discussed and favorable operational conditions proposed.

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