鳥塚 史郎

Phosphorus is an element that is highly effective for solid solution strengthening, but its content is kept at minimum in steel making, because an increase in the P content decreases the toughness, especially after quenching and tempering, such as the 50% Charpy absorbed energy transition temperature and the Charpy absorbed energy at low service temperatures. On the other hand, grain refinement improves the toughness of low carbon steels. In this study, an ultrafine grain structure is introduced to low carbon steels to suppress the detrimental effects by the high content of P. The addition of P increases the tensile strength even in an ultra-fine ferrite-cementite microstructure as well as in a ferrite-pearlite microstructure. The 0.1wt. % addition of P decreases the Charpy absorbed energy of the ferrite-pearlite (FLP) and the quench-and-tempered specimens considerably at 233K. On the other hand, the ultra-fine ferrite-cementite specimen with a 0.1wt. % P shows a value that is comparable to that of the F/P specimen without P. The 50% Charpy absorbed energy transition temperature is concurrently improved by the grain refinement.

The plastic strain in a specimen introduced by compression with a pair of anvils is evaluated using a three-dimensional dynamic finite element analysis. The stress-strain relations depending on strain rate and temperature are used in the analysis. The contact condition between the anvil and the specimen is determined from the experimental result from an identically deformed screw set in the specimen. The plastic strain introduced by uni-direction compression is compared with that by bi-direction compression simultaneously or non-simultaneously given from different directions. The non-simultaneous bi-direction deformation is proposed as a method to introduce a large plastic strain widely in the specimen. The bi-direction deformation can be developed as a technique to obtain ultra-fine grain steels for thick plates.

A multi-direction deformation thermo-mechanical treatment simulator was designed and developed to verify the prediction by the FE-anaylsis on the effect of bi-direction deformation which can introduce a large plastic strain widely in a specimen. Using this simulator, both uni-direction and bi-direction deformations were performed for a 0.16%C-0.4%Si-1.4%Mn steel with a prior austenite grain size of 300μm at 1073K at a strain rate of 1/s. The microstructure was composed of grain boundary polygonal ferrite and intragranular Widmanstatten ferrite in the uni-direction deformation specimen. On the other hand, the half thickness of the specimen was composed of fine polygonal ferrite in the bi-direction deformation specimen. This region corresponds well to the region having a plastic strain more than 2. The uniform formation of the fine polygonal ferrite due to the wide introduction of the large plastic strain was verified for the bi-direction deformation.

Ceramic TiB_2-19.0-19.5 wt% (2 mol%Y_2O_3-ZrO_2) composites with 2.5-5.0 wt% SiC were fabricated by hot isostatic pressing (HIP) with and without a glass encapsulation. The effects of SiC addition on the mechanical properties and siterability of TiB_2-20 wt% (2 mol%Y_2O_3-ZrO_2) HIP'ed bodies were investigated. The addition of SiC improved the sinterability of TiB_2-20 % (2 mol%Y_2O_3-ZrO_2) appreciably. Composites of TiB_2-19.0-19.5 wt% (2 mol%Y_2O_3-ZrO_2) with 2.5-5.0 wt% SiC densified up to 96.7% of their theoretical densities by vacuum sintering at 1700℃ for 14.4 ks. These sintered bodies almost fully densified by capsule-free-HIP. On the other hand, TiB_2-20 wt% (2 mol%Y_2O_3-ZrO_2) densified up to 63.7% by the same vacuum sintering. The addition of SiC was also effective in retarding the grain growth of TiB_2 and ZrO_2. The grain sizes of TiB_2 and ZrO_2 of TiB_2-19.0-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC HIP'ed bodies were about 1.5 and 1.2 μm, respectively. They were about 2/3 of those of TiB_2-20 wt% (2 mol%Y_2O_3-ZrO_2) HIP'ed bodies. The three-point bending strengths of TiB_2-19.0-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC glass-encapsulated-HIP'ed bodies were 1240 MPa, which is slightly higher than that of TiB_2-20 wt% (2 mol%Y_2O_3-ZrO_2) HIP'ed bodies. Vickers hardness and fracture toughness were not affected by the addition of 2.5-5.0 wt% SiC. The mechaical properties of TiB_2-(2 mol%Y_2O_3-ZrO_2) HIP'ed bodies were controlled by the monoclinic ZrO_2 ratio in Zr_2 and monoclinic ZrO_2 content in HIP'ed bodies, as we have reported in the previous paper. The addition of SiC affected them only slightly, so that the mechanical properties were also little affected.

A ceramic TiB_2-20 wt% (2 mol%Y_2O_3-ZrO_2) composite has a high three-point bending strength of 1200 MPa. However, this composite lacks sinterability, so that glass encapsulation HIP'ing is required to obtain a full dense sintered compact. However, addition of 2.5-5.0 wt%SiC improved the sinterability drastically. TiB_2-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5 wt%SiC was densified to 96% of its theoretical density by vacuum sintering at 1700℃ for 14.4 ks, and this sintered compact was hot isostatically pressed (HIP'ed) without glass encapsulation. In this work, the microstructure and sintering mechanism of TiB_2-19-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC were investigated. TiB_2-19-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC was densified drastically above 1600℃. TiB_2-1.0-10.0 wt% SiC had the same sinterability as TiB_2-19-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC. Therefore, SiC improved the sinterability of TiB_2, regardless of the presence of ZrO_2. The microstructure of HIP'ed compacts was observed with a SEM and a TEM. Enriched areas of SiC were not observed with the SEM. However, TEM observation showed that thin strip-like grain boundary phases consisting of Ti, Si and Zr exist between grain boundaries of TiB_2 and ZrO_2, and that (Ti, Zr)_5Si_3 exists at the triple points of grain boundaries. These grain boundary phases were formed by the reaction among SiC, Ti, and Zr. These were entirely or partially liquid phase at the sintering temperature of 1700℃. As a result, TiB_2-19-19.5 wt% (2 mol%Y_2O_3-ZrO_2)-2.5-5.0 wt%SiC was densified by liquid phases sintering, which was the main mechanism in which sinterability was improved by the addition of SiC.