Passenger vehicles require suspensions for ride comfort. Most today use helical springs, but leaf springs were at one time a major advance over chain or leather strap suspensions, and are still used in some applications today.
Metal springs for vehicles predate the Bessemer process. Making a leaf spring starts with forging a thin, curved piece of metal. Making a superior one calls for a smart cooling schedules (e.g. initial quench, then slow cooling) and a careful choice of materials.
When did they appear in the historical record?