Cold Drawn Steel Tire cord is the strongest metallic reinforcement made. The micro-hardness of the steel filament is similar to that of a quenched steel (Fig. 1). This indicates that cold drawn tire cord filament has strength similar to or higher than quenched swords, knifes and other products with martensitic microstructure resulting in the highest strength achieved in steels by heat treatment. Such a high strength in a cold drawn tire cord filament is due to nano-size of phases present in steel microstructure.

Before drawing, two phases, namely, ferrite, an iron rich ductile phase, and cementite, an intermetallic compound with a high strength (Fig. 2), are present in a form of colonies of plates, so called pearlite (Fig. 3). Thickness of these ferrite and cementite plates randomly orientated with respect to the wire axis is around 60 nm and 10 nm, respectively.

During drawing, ferrite and cementite plates become orientated along the wire axis, and their thickness becomes less than 10 nm and 2 nm, respectively (Fig. 4) showing atomic structure of a drawn steel filament. Such a small thickness of ferrite and cementite plates result in a record high strength of steel filaments. Processing stages of steel cord manufacturing include rough drawing, patenting, brass plating, fine drawing and cabling. In rough drawing, steel rod diameter is reduced from 5.5 mm to around 1.5 mm. Patenting is a special heat treatment used to restore steel ductility before fine drawing during which diameter is further reduced to about 0.2 mm. This deformation is very high resulting in a length increase of a 10 m long wire segment to almost 1 km long filament.

Fig. 1 Scanning electron micro-graph showing micro-hardness indentations in a quenched steel and a drawn filament.

Fig. 2 Scanning electron micro-graph showing ferrite and cementite plates.

Fig. 3 Scanning electron micro-graph showing pearlite colonies in a steel wire before drawing.

Fig. 4 High resolution electron micro-graph showing atomic structure in a drawn filament (Courtesy of Sumitomo Metals Industries).