JPS6352134B

A carbon fiber, a process for production thereof, and composite materials prepared therewith are described, wherein the carbon fiber has a single yarn diameter of from 2 to 6 microns and showing a strand knot strength of 7 kilograms or more when formed into a strand of 0.4+/-0.01 gram per meter, which comprises subjecting a 0.1 to 0.6 denier acrylonitrile-based fiber having a strength of 6 grams per denier or more to a preoxidation treatment wherein the acrylonitrile-based fiber is treated under such tension as to provide a shrinkage of at least 3% until the equilibrium water content reaches 5%, and (b) a second preoxidation treatment wherein the acrylonitrile-based fiber is treated under such tension as to provide a shrinkage of at least 1%, said preoxidation treatment is conducted in an oxidizing atmosphere having a temperature of from 240 DEG C. to 300 DEG C. while maintaining the relation between the preoxidation treatment time (t) (minutes) and average preoxidation treatment temperature (T) ( DEG C.) satisfying the equation (1): (310-T)x(0.8 DIFFERENCE 3)=t (I) and subjecting the thus preoxidized fiber to a carbonization treatment in an inert gas maintained at from 1,000 DEG C. to 1,800 DEG C.