Ford produced most of his own steel and the exact alloys were proprietary.
One that was used for many parts including springs was the now obsolete designation SAE 6000 series. This is a chrome vanadium alloy. It is still used in modern torsion bar applications because of it's very high fatigue resistance.
A lot of different formulations of steel from medium carbon to high carbon or chromium silicon related steels will make a decent spring. The way it's tempered is what makes it spring steel. It's alloy is what makes it last.