Abstract: A carbon fiber strand obtained by bundling 20,000-30,000 carbon fibers each having, in the surface thereof, creases which are parallel to the fiber-axis direction. In an examination with a scanning probe microscope, the creases in the carbon fiber surface are apart from each other at a distance of 120-160 nm and have a depth of 12-23 nm, excluding 23 nm. The carbon fibers have an average fiber diameter of 4.5-6.5 nm, specific surface area of 0.9-2.3 m2/g, and density of 1.76 g/cm3 or higher. The carbon strand has a tensile strength of 5,900 MPa or higher and a tensile modulus of 300 GPa or higher. When would on a bobbin at a tension of 9.8 N, the strand on the bobbin has a width of 5.5 mm or larger. When the carbon fiber strand is examined by a strand splitting evaluation method in which the strand is caused to run through three stainless-steel rods while applying a tension of 9.8 N thereto, no strand splitting is observed.

Abstract: A carbon fiber strand which is produced by obtaining a solidified-yarn strand by spinning with a spinneret having 20,000-30,000 spinning holes, passing the strand through an interlacing nozzle having an air blowing pressure of 20-60 kPa to obtain precursor fibers, oxidizing them in heated air having a temperature of 200-280° C. to obtain oxidized fibers, subjecting these oxidized fibers to a first carbonization treatment in an inert-gas atmosphere at a temperature of 300-900° C. in which the fibers are firstly stretched in a stretch ratio of 1.03-1.06 and then secondarily stretched in a stretch ratio of 0.9-1.01, subsequently conducting a second carbonization treatment in an inert-gas atmosphere at 1,360-2,100° C., and then conducting a surface oxidization treatment in an aqueous solution of an inorganic acid salt in a quantity of electricity of 20-100 C per g of the carbon fibers.