Carbon fiber bundle, prepreg, and fiber-reinforced composite material
An object of the present invention is to provide a carbon fiber which exhibits excellent strength development rate when used in a composite material. The present invention that solves the problems is a carbon fiber which simultaneously satisfies the following formulae (1) and (2): Lc/d ≤3 (1) TS× d×Lc >6.0×10 5 (2) wherein: Lc is an X-ray crystallite size (Å), d is a filament diameter (μm), and TS is a strand tensile strength (MPa).
1. A method for producing a carbon fiber, the method comprising the steps of:
imparting a flame resistance to an acrylonitrile precursor fiber bundle to obtain an oxidized fiber having a fiber density of 1.34 to 1.38 g/cm 3 (oxidization step); and
subjecting the oxidized fiber to carbonization treatment in an inert atmosphere in a first carbonizing furnace, and then further subjecting the resultant fiber to carbonization treatment in an inert atmosphere in a second carbonizing furnace while applying a tension of 160 to 300 mg/dtex to the fiber (carbonization step),
wherein, in the carbonization step, the temperature gradient is 300 to 600° C./min and the integral quantity of heat given to the oxidized fiber is 80 to 105 J·h/g.
2. The method for producing a carbon fiber according to claim 1 , wherein the acrylonitrile precursor fiber bundle has a filament diameter of 10 to 14 μm.