Steel cord having layer-twisted structure of helicoidal filaments for reinforcing rubber product

A steel cord having a layer-twisted structure for reinforcing a rubber product, comprising:

• a core structure formed by 1 to 3 steel filaments each having a diameter of from 0.15 mm to 0.25 mm, said core structure having a cord axis;
• and
• a surrounding structure formed by at least 6 steel filaments each having a diameter of from 0.15 mm to 0.25 mm, said steel filaments being helicoidally twisted around said steel filaments of said core structure in the same direction and at the same twist pitch as the direction and twist pitch of said steel filaments of said core structure, said filaments in said surrounding structure forming at least one layer, each layer forming a circumscribed circle defined by the distance between said cord axis and the center of each filament forming said layer, wherein each of said filaments in said surrounding structure has a helicoidal structure having a diameter with a shaping ratio of 80%-90% as measured when being unraveled, the shaping ratio being defined as the ratio of the diameter of said helicoidal structure of each filament to the diameter of the circumscribed circle of the layer including said filament, said shaping ratio being such that the degree of movement of each steel filament in said at least one layer from said cord axis is (-0.5454d+0.1454).times.10.sup.3 .mu.m or less, wherein d is a diameter in millimeters of said filament, as measured when said steel cord is bent from a straight state to a state in which a radius of curvature of said steel cord is d/(17.times.10.sup.-3).