マルテンサイト系ODS鋼の組織及び高温強度におよぼすWの影響評価

Effects of tungsten on microstructure and high-temperature strength of oxide dispersion strengthened (ODS) martensitic steel

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

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

9CrODSマルテンサイト鋼において、Wはマトリックスの固溶強化元素であり、酸化物分散強化との相乗効果により高温強度を高めるが、添加量を多くした場合フェライト相の生成を増加させ、また高温照射下で金属間化合物(ラーベス相)の析出を促進して延性低下をもたらす。このためW添加量は必要最少量が望ましい。本研究では9CrODSマルテンサイト鋼の組織及び高温強度におよぼすW量の影響を検討した。得られた結果は以下の通りである。(1)9CrODSマルテンサイト鋼において、延性,靭性に悪影響をおよぼすラーベス相はW添加量が2mass%を超えると生成する。したがって現行仕様の目標値であるW=2.0mass%はおおむね適切と考える。(2)W増量による硬さ,引張強度の上昇は、W固溶強化と未変態相増加の両方が関与している。この未変態相はフェライト生成元素であるWの添加により同時に形成されるものであり、973K引張強さにおける強度向上への両者の寄与量はほぼ同等であった。(3)上記の知見に基づき、強度向上と同時にラーベス相生成による脆化を促進するWの添加量を極力減らし、それによる固溶強化低下分を未変態相生成量の増加で補うような成分設計について検討する余地は残されていると考える。

In 9Cr ODS martensitic steel, tungsten(W) is a solid solution strengthening element, whose addition increases high-temperature strength by the combined effect with oxide dispersion strengthening. However, its excessive addition results in the increase of ferrite phase causing precipitation of intermetallic compound (Laves phase) under high temperature irradiation condition and thus ductility degradation. The amount of W addition therefore should be as low as possible. In this report, the effects of W on microstructure and high temperature mechanical properties of 9Cr ODS martensitic steels were examined for obtaining insights into optimum W concentration in terms of high-temperature strength and ductility. The results obtained are as follows: (1)In the 9CrODS martensitic steel, addition of W exceeding 2mass% is shown to cause precipitation of Laves phase which degrades the ductility and fracture toughness. It can be said that the current specification of W concentration, i.e. 2mass%W, is appropriate. (2)Hardness and tensile strength is shown to increase with W concentration. This increase is caused by the increase of solid solution strengthening and residual-alpha ferrite. The retainment of residual-alpha ferrite is enhanced by the addition of W (ferrite former element). The improvement of tensile strength at 973K provided by the solid solution strengthening is shown to be equivalent to that provided by the retainment of residual-alpha ferrite. (3)It would be open task to explorer an improved alloy design concept, i.e. decrease of W as low as possible and increase of residual-alpha ferrite. The degradation of high-temperature strength by decreasing W addition can be made up by the increasing fraction of residual-alpha phase that is provided by reduction of austenite former elements and increasing addition of ferrite former elements.