過剰酸素量を制御したODSマルテンサイト鋼の製造技術開発

Development of manufacturing method of excess oxygen controlled ODS martensitic steel

藤原 優行; 大塚 智史   ; 皆藤 威二 ; 成田 健; 鵜飼 重治  

Fujiwara, Masayuki; Otsuka, Satoshi; Kaito, Takeji; Narita, Takeshi; Ukai, Shigeharu

実用化炉心材料の有力候補材として開発を進めている0DSマルテンサイト鋼において、過剰酸素の増量により酸化物分散粒子径の増加と焼きならし時の未変態粒の現象が生じるとともに、高温強度が低下することが判明している。本検討では目標達成のため、過剰酸素量を0.10wt%以下に低減するための製造条件の検討と過剰酸素が高く焼きならし時に完全化が起きる場合でも、強度向上を図るためのY2O3増加の検討を行った。得られた結果を以下にまとめる。 (1)標準組成材 (0.13C-9Cr-2W-0.2Ti-0.35Y2O3)では、過剰酸素はプレミックス粉末から0.04wt%持ち込まれ、MA処理中に0.040.1程度混入することがわかった。 (2)MA雰囲気ガスに超高純度Arガス(99.9999wt%Ar)を適用すること、アジテータの攪拌用ピン長さを短縮して攪拌エネルギーを低減することにより、過剰酸素量を0.1wt%以下に低減できることがわかった。 (3)過剰酸素量を0.1wt%未満に低減した試作材では、炉冷熱処理後に未変態粒と変態粒の混合組織となり、Mm11と同等の硬さを示した。これらは、Mm11と同等の高温強度を有すると推定される。 (4)過剰酸素量とY2O3添加量を高めたE1、E2では、ほぼ完全化が生じたが、炉冷材および焼き戻し材の硬さは低く、高温強度はあまり期待できない結果となった。

ODS martensitic steel is a prospective candidate cladding matenal for the advanced Fast Breeder Reactor (FBR) core components. A previous result showed that the high temperature strengths were deteriorated by the oxide particle coarsening and the reduction of the residual-alpha grains as excess oxygen (EX.O) increased. In this study, the manufacturing conditions for reducing excess oxygen were examined in order to obtain the target strength. High excess oxygen steels with higher YO addition were also manufactured with the intention of manufacturing the steel which has both fully martensitic microstructure and enough high temperature strength. The derived results can be summarized as follows. (1)It was shown that 0.04wt% excess oxygen was brought from the pre-alloyed raw powders, and 0.04-O.l wt% excess oxygen was mixed in the powders during the Mechanical Alloying(MA). (2)It was shown that excess oxygen could be reduced less than 0.1wt% by applying an ultra high purity Ar gas (99.9999wt%Ar) to MA atmosphere as well as reducing the agitating energy by using pin agitator with shorter length. (3)Residual alpha-grains and transformed alpha-grains were mixed in the furnace-cooled low excess oxygen steels (0.1wt%EX.O). These steels are expected to have the equal high temperature strength to Mm11 because their Vickers hardness is approximately the same Mm11. (3)Full martensitic microstructure was obtained in the higher YO and excess oxygen-added steels (E1,E2). However enough high temperature strength are not expected in these steels because the Vickers hardness of these steels after the furnace-cooling(FC) or normalizing and tempering (NT) are apparently low.