Plate: Cast Iron Strings: Steel, and copper wound steel in the bass Tuning Pins: Steel Screws and pressure bar: Steel Cabinet screws and hinges: Brass Action Brackets: Cast iron Action parts: Steel Bridge pins: Copper Key weights: Lead Casters: Cast iron
Mostly steel. There's a big frame that all the strings are stretched across. It has to be really strong because all the strings together exert a force of several tons! That frame is a considerable part of the weight of the piano. The strings are made from steel too. There may be some parts made of brass, where machineability is more important than strength or hardness. But my guess is that 95% of the metal in a piano is steel.
