Is it permissible to pursue a doctoral research project that addresses "The Mechanical and Chemical Properties of High-Strength Steel for Use in Automobiles and Marine Vehicles," knowing that steel may be used in military industries?

There is no objection to conducting research, for the default rule concerning things is permissibility. The possibility that the research might be utilized for something unlawful does not necessitate prohibiting it, as many permissible things can be used for both permissible and unlawful purposes. It is permissible to manufacture and sell them. The prohibition only arises when a permissible item is provided to someone who is known to intend to use it for unlawful purposes.