Steel is created by adding carbon to iron. These elements give steel its strength, making it suitable for lifting heavy loads.
The steel is commonly used for the main structure of a racing jack because it is tough and can withstand heavy work loads.
Aluminium
Aluminium is a relatively durable and lightweight metal.
The chassis (internal frame) of some racing jacks are made from aluminium as it lightweight and tough. Also the main structure can be completely made from aluminium.
Most racing jacks are made of both aluminium and steel for added strength and durability. So the racing jack could contain a aluminium chassis and the main structure made from steel.