Steel wire cord quality is based on tensile strength, elongation, and stiffness. It is manufactured from steel rod with high carbon content; and while the steel wires used have different configurations, all are brass-coated strands twisted together into cords. If the wire is used in a multi-ply tire rather than a belted tire, the fatigue performance will be important. If used in belted tires, then stiffness is of primary concern.
http://www.maxxis.com/AutomobileLight-Truck/How-a-Tire-is-Made.aspx#FabricSteel
http://www.maxxis.com/AutomobileLight-Truck/How-a-Tire-is-Made.aspx#FabricSteel