Rikio Chijiiwa

AP-FIM study on the effect of Mo addition on microstructure in Ti-Nb steel

Abstract AP-FIM technique was applied successfully to analyze the strengthening mechanism of a newly developed Ti-Nb-Mo-bearing fire-resistant steel, which maintained high yield strength at elevated temperatures. FIM observations of Ti-Nb-Mo and Ti-Nb steels before and after aging at 873 K for 0.9 ks made it clear that Nb(C, N) precipitated in ferrite matrix of Ti-Nb-Mo steel during aging was finer in size (2.3 nm) and greater in number than that of Ti-Nb steel. In the as-hot-rolled condition, corresponding to the situation before aging of those steels, a similar tendency was recognized with Nb(C, N), being coarse in size and small in number. These facts suggest that precipitation hardening of Nb(C, N) at 873 K and room temperature in Ti-Nb-Mo steel is higher than that in Ti-Nb steel. AP analyses of Mo-bearing steels, Ti-Mo steel and Ti-Nb-Mo steel, before and after aging, clearly yielded us two important facts. One was that almost all Mo atoms were in solution in ferrite matrix of both of the Mo-bearing steels, which suggested Mo solid-solution hardening was equal for both of the Mo-bearing steels. The other was that Mo atoms strongly segregated at the Nb(C, N)/ferrite matrix interface in Ti-Nb-Mo steel, which probably suppressed the diffusion of Nb atoms into Nb(C, N) from ferrite matrix. This directly results in fine precipitates in Ti-Nb-Mo steel.

Advanced Titanium Oxide Steel With Excellent HAZ Toughness for Offshore Structures

The feature of titanium oxide steel (Ti-O steel) is that heat affected zone (HAZ) toughness is improved due to the refinement of HAZ microstructure through the formation of intragranular ferrite (IGF). This desirable microstructure, IGF, forms radially from titanium oxide particles. Recently, it has been clarified that manganese in Ti-O steel is an indispensable element for the formation of IGF. Therefore, manganese effects on Ti-O steel have been basically studied in this work, and then a new effect has been found. In Ti-O steel, manganese has the effect of suppressing the formation of ferrite side plates (FSP), which are undesirable due to their coarseness. Consequently, HAZ microstructure of Ti-O steel with high manganese content is so refined that HAZ toughness is remarkably improved. Based on the manganese effects, steel plates with excellent HAZ toughness for offshore structures have been developed and commercially mass-produced. The welded joints exhibit excellent toughness.Copyright