I am looking to make robot parts out of steel. Parts should be bendable with the help of science. Answer should deliver permanent deformable steel parts to facilitate fast, acrobatic movements for an assassin-type robot requiring agility in combat.
You want a mix of steel parts and Shape-memory alloys.
These alloys have the unique property of returning to a previous shape when heated and/or cooled. In this case, you'd want to have two-way memory alloys that have one shape when hot and another when cold. That allows for bending and straightening the limbs of the android by heating and cooling.
The main problem I see is that cooling especially is hard to do in a split second, which is probably what you need for agile assassin droids. There are some decent ways to increase heat transfer but none at the level you want, so I would "invent" a similar alloy that changes shape when exposed to an electric current. It's not implausable that the increased energy of the electrons cause the molecular bonds to change like heat does for known shape-memory alloys and allows for much quicker and controlled movement.
I don't expect it would actually work to infuse steel with this and have the whole thing deform into many shapes, but the alloys are pretty strong by themselves, so they could form most of the android's body, with some additional steel parts for armoring/weapons. Of course, your scientists might have figured that one out too...
Why not just make flexible parts out of steel?
You don't need to add anything to steel to make it flexible, intelligently designed steel parts can be flexible.
Just because something is made out of steel doesn't mean it's rigid. If you cast a massive steel thing like an engine block - sure its basically static, but you can make thin steel parts that can flex. Flexible steel is all around you:
This steel wire is very flexible;
By interspersing thin parts of steel together your robot arm could flex and morph in any way you need.