Toward development of efficient process for titanium alloys

Car weight can be reduced more by utilizing commercially pure titanium(CPTi) and titanium(Ti) alloys. This provides a good opportunity for titanium industries to develop a more efficient mass production process,i.e.,continuous casting and hot direct rolling process(CC-HDR). This process enables mass production having homogeneous quality with low price as was realized in the steel industries. In this paper, we discuss the possibility of the introduction of CC-HDR process, based on the studies of hot workability and thermomechanical treatment. The characteristics of strength and ductility at elevated temperature were examined for most of CP Ti as well as Ti alloys. The Gleeble machine was quite useful tool to characterize the hot workability of these titanium alloys as has been found previously for steels. Embrittlement in Ti alloys exists in the temperature range between Tβ and Tβ-200K in α, near α,and α+β alloys, which is related to the transformation from β phase into α phase and the difference of the strength between two phases. This embrittlement can be suppressed by getting fine grain size and rapid cooling from β region, but it is intrinsic in nature. On the other hand, hot ductility in the CPTi, near βand βalloys shows more than 60% in reduction of area for a wide range of temperature ranging from T_m to 700K although the strength becomes higher below 1200K compared with carbon steels. Thus, it appears possible to apply continuous casting such as a strip casting or straight type cast strand CC and HDR in the temperature range above 1200K to get sound products without any cracking problems.