| abstract |
[Problem] To provide an ultrafine fiber having high strength, excellent yarn production stability and productivity, and excellent quality that can be used in various fields such as clothing and industrial materials. An ultrafine fiber having an average single yarn fiber diameter of 10 to 2500 nm and a tensile strength of 4.5 cN / dtex or more, which is obtained by eluting and removing sea components of a sea-island type composite fiber. Is. The composite mass ratio, the sea-island melt viscosity ratio, the spinning speed in melt spinning, the draw ratio in preheat roller stretching, the residual elongation, etc. are specified. [Selection figure] None |