Carbon fiber strand and process for producing the same

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. This carbon fiber strand has a strand tensile strength of 5,650 MPa or higher, strand tensile modulus of 300 GPa or higher, and strand width of 5.5 mm or larger. No strand crack is observed in an examination by a strand crack evaluation method.