Carbon fiber, manufacturing method thereof, and carbon fiber composite material

A method of producing a carbon fiber, the method comprising:

• a first oxidation process in which a polyacrylonitrile-based carbon fiber-precursor fiber is subjected to a oxidation treatment over a period of 8 to 25 minutes until the resulting fiber has a peak intensity ratio of the infrared spectrum at 1,453 cm−1 to that at 1,370 cm−1 ranging from 0.98 to 1.10,
• a second oxidation process in which the fiber obtained in the first oxidation process is subjected to a oxidation treatment over a period of 5 to 14 minutes until the resulting fiber has a peak intensity ratio of the infrared spectrum at 1,453 cm−1 to that at 1,370 cm−1 ranging from 0.70 to 0.75 and a peak intensity ratio of the infrared spectrum at 1,254 cm−1 to that at 1,370 cm′ ranging from 0.50 to 0.65,
• a pre-carbonization step in which the fiber obtained in the second oxidation process is pre-carbonized in an inert atmosphere at a maximum temperature of 500 to 1,200° C. while being stretched at a draw ratio ranging from 1.16 to 1.25,
• a carbonization step in which the fiber obtained in the pre-carbonization step is carbonized in an inert atmosphere at a maximum temperature of 1,000 to 1,500° C. under conditions where the duration X of the treatment at the maximum temperature ranges from 20 to 60 seconds and the temperature rising rate Y ranges from 0.40 to 1.1° C./sec, and
• a step in which the fiber obtained in the carbonization step is modified by an electrolytic surface treatment to obtain the carbon fiber.