Deformation-Induced Dynamic Ferrite Transformation During Hot-Rolling in Oxide Dispersion-Strengthened Ferritic Steel wi
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INTRODUCTION
OXIDE dispersion-strengthened (ODS) ferritic steels attain better high-temperature creep strength and neutron irradiation resistance than usual ferritic heat-resistant steels. These properties are caused by dispersing nanosized Y2Ti2O7 oxide particles, which act as strong barriers for moving dislocation and as strong sinks at the particle/ferrite matrix interface for radiation-induced vacancies and interstitial atoms.[1–6] These advantages have promoted the extensive development of ODS ferritic steels as candidate fission and fusion structural materials.[4–11] Our previous study has revealed that hot-rolling of 9CrODS steel in the austenitic region enabled the replacement of martensitic block grains with elongated ferrite grains, which led to improved high-temperature tensile strength at 700 C.[12] The strength attained over 500 MPa, which is almost a
SHOKI KASAI, T. YAMASHIRO, and S. ZHANG are with the Graduate School of Engineering, Hokkaido University, Sapporo 0608628, Japan. Contact e-mail: [email protected] S. UKAI, N. OONO, and S. HAYASHI are with the Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan. S. OHTSUKA is with the Japan Atomic Energy Agency, Oarai 311-1393, Japan. H. SAKASEGAWA is with the National Institutes for Quantum and Radiological Science and Technology, Rokkasyo 039-3212, Japan Manuscript submitted date May 25, 2018. Article published online December 6, 2018 590—VOLUME 50A, FEBRUARY 2019
record among ferritic steels; that of non-hot-rolled 9CrODS steels attained about 380 MPa. However, the formation mechanism of the elongated ferrite grains induced by hot-rolling of the 9CrODS steel remains unclear. In the last two decades, thermomechanical controlled processing (TMCP) has been studied to realize grain-ultrarefinement down to 1 lm in steel. Dynamic austenite-to-ferrite transformation induced by hot-rolling and controlled cooling is a typical TMCP method, and it has received significant attention as a dynamic strain-induced transformation of the low Cr steels.[13–16] Mechanical property improvement by means of TMCP has also been implemented for 9 to 12Cr ferrite for fusion application.[17–19] Concerning ODS steels, only one paper other than the authors’[12] has been published, to the best of the authors knowledge; it mentioned that hot-rolling improved fracture toughness, but there is lack of information on microstructure characterization.[20] It has been revealed by the aforementioned studies that austenite-to-ferrite transformation controlled in terms of hot-rolling and cooling refined the grain and modified the carbide precipitate morphology. Our previous result emphasized that transformed ferrite grains are elongated and coarsened by hot-rolling and cooling in ODS ferritic steel; this is opposite to the grain refinement published in the referenced studies. This paper aims to shed light on the mechanism of the
METALLURGICAL AND MATERIALS TRANSACTIONS A
strain-induced ferrite transformation from hot-rolled austenite in 9CrODS steel.
II.
EXPERIMENTAL PROCE
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