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Achieving Ultimate Properties in Steel

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3D reconstructed image
of crack tip dislocations.


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Transmission electron micrograph
of high nitrogen nanobainite steel


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Orientation image map
of multi-phase high strength steel


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Die surface morphology
controlled by shot-treatment:
Sliding property (a) good, (b) poor

Steel is the most important of all infrastructure materials. To enhance the quality of steel, we are working on microstructure control via alloy design and heat treatment, clarification of strength mechanisms based on dislocation theory, and deformation and subsequent fracture behavior in various kinds of steels. In addition, we work closely with steel companies to deepen knowledge of various phenomena and to develop new materials.

(1) Microstructure control for improved strength-ductility balance in high strength steel sheets
We are working on the development of new steel sheets with high strength and sufficient workability by applying various strengthening mechanisms, including grain refinement, precipitation of nano-sized particles, dispersion of retained austenite, and more. In addition, we aim to establish authoritative microstructure control guidelines through clarification of the principles of property evolution using the most advanced analysis equipment available.

(2) Increased strength through high nitrogen concentrations in steel and titanium alloys
High-strength austenitic and martensitic steels containing large amounts of nitrogen are produced by solution nitriding, and their mechanical properties and corrosion resistance are examined. We also developed a continuous solution nitriding furnace, which enables continuous production of high-nitrogen steel wire. This technique has been applied to titanium alloys as well. The developed high-nitrogen titanium alloys are superior to conventional titanium alloys in strength-ductility balance.

(3) Elucidating the fundamental mechanism behind the ductile-to-brittle transition.
BCC structured metals such as ferritic steel lose ductility at low temperatures, resulting in brittle fracture. This is referred to as a ductile-to-brittle transition. In order to understand the mechanism behind the ductile-to-brittle transition, which is essential to creating increased reliability in structural metals, we perform fracture tests from a macroscopic point of view and 3D analysis of lattice defects using advanced electron microscopy.

(4) Enhanced galling properties achieved by surface reforming
It is essential to increase the service life of dies in order to press-form ultra-high tensile strength steels in manufacturing. We aim to enhance the service life and galling properties of dies by making an uneven surface and applying solid lubricants to the surface. New steels for automobiles will be produced by innovations in the press forming process as well as by enhancing the properties of materials. The study on galling properties is instrumental to the manufacture of ultra-high tensile steels.

Structural Materials Lab.,
Department of Materials Science and Engineering,
Graduate School of Engineering, Kyushu University
Professor Setsuo Takaki
Professor Zenji Horita
Professor Kenji Kaneko
Professor Masashi Mukaida
Associate Professor Toshihiro Tsuchiyama
Associate Professor Yoshimine Kato
Associate Professor Shinji Munetoh
Associate Professor Ryo Teranishi
Associate Professor Yukio Sato
Assistant Professor Makoto Arita
Assistant Professor Yoshifumi Ikoma
Assistant Professor Yoshimi Hisatsune
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