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An epoxy resin composition suitably used for a prepreg which can complete curing in a short time even at a low temperature and secure a sufficient usable period under preservation at room temperature, in comparison with conventional epoxy resin compositions. An epoxy resin composition comprising at

An epoxy resin composition suitably used for a prepreg which can complete curing in a short time even at a low temperature and secure a sufficient usable period under preservation at room temperature, in comparison with conventional epoxy resin compositions. An epoxy resin composition comprising at least one of an epoxy resin, an amine compound having at least one sulfur atom in the molecule thereof, and a reaction product of an epoxy resin and an amine compound having at least one sulfur atom in the molecule thereof, and an amine compound having at least one sulfur atom in the molecule thereof, and a urea compound and a dicyandiamide, wherein each of the contents of the sulfur atom and the urea compound in the epoxy resin composition is respectively 0.2 to 7% by mass and 1 to 15% by mass.

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1. A prepreg obtained from an epoxy resin composition as a matrix resin impregnated in a reinforced fiber, wherein the epoxy resin composition comprises a sulfur-containing thermosetting component;(C) a urea compound, which is a solid substance having an average particle diameter of no greater than

1. A prepreg obtained from an epoxy resin composition as a matrix resin impregnated in a reinforced fiber, wherein the epoxy resin composition comprises a sulfur-containing thermosetting component;(C) a urea compound, which is a solid substance having an average particle diameter of no greater than 150 μm, and(D) a dicyandiamide; wherein:the sulfur containing thermosetting component is selected from the group consisting of: a blend of (A) an epoxy resin excluding glycidylamine type epoxy resins and (B-1) an amine compound having at least one sulfur atom in its molecule; a reaction product (B-2) of (A) and (B-1); and a blend of (B-2) and one of (A) and (B-1);the content of the urea compound in the epoxy resin composition is 1 to 15% by mass;the prepreg has a gel time at 130° C. of no more than 200 seconds;the prepreg has a usable period of at least 21 days;the epoxy resin composition does not contain a polyvinylacetal resin; and the epoxy resin composition has a sulfur atom content in the range of 1.17 to 7% by mass. 2. The prepreg according to claim 1, wherein the matrix resin content is equal to or less than 33%. 3. The prepreg according to claim 1, wherein the reinforced fiber is a carbon fiber. 4. The prepreg according to claim 3, wherein the reinforced fiber is a woven carbon fiber. 5. The prepreg according to claim 4, wherein a ratio (W/t) of weight per unit area (W g/m2) and thickness (t mm) of the woven carbon fiber is in a range of 700 to 1700. 6. The prepreg according to claim 4, wherein a cover factor of the woven carbon fiber is in a range of 90 to 100%. 7. The prepreg according to claim 1, wherein the sulfur-containing thermosetting component is selected from the group consisting of (B-2) and a blend of (B-2) and one of (A) and (B-1). 8. The prepreg according to claim 1, wherein the content of component (C) from 3 to 12% by mass. 9. The prepreg according to claim 1, wherein the content of component (D) from 0.1 to 10% by mass. 10. The prepreg according to claim 1, wherein component (D) has an average particle diameter of no greater than 150 μm. 11. The prepreg according to claim 1, wherein the urea compound has no halogen in its molecule. 12. The prepreg according to claim 1, wherein the prepeg is obtained by: mixing (A), (B-1), (C) and (D); and impregnating the resulting mixture in the reinforced fiber while heating. 13. The prepreg according to claim 12, wherein the resin composition consists of (A), (B-1), (C), and (D). 14. The prepreg according to claim 1, wherein the prepeg is obtained by: mixing (A) and (B-1); heating the resulting mixture to form (B-2) and to completely consume at least one of (A) and (B-1) mixing (C) and (D) with (B-2) or a resulting blend of (B-2) and one of (A) and (B-1); and impregnating the resulting mixture in the reinforced fiber while heating. 15. The prepreg according to claim 14, wherein the sulfur-containing thermosetting component is a blend of (B-2) and (A), and the prepeg is obtained by mixing (A) and (B-1); heating the resulting mixture to form (B-2) and to completely consume (B-1); mixing (C) and (D) with the resulting blend of (B-2) and (A); and impregnating the resulting mixture in the reinforced fiber while heating. 16. The prepreg according to claim 15, wherein the resin composition consists of (A), (B-2), (C), and (D). 17. The prepreg according to claim 1, wherein the prepreg has a gel time at 130° C. of no more than 170 seconds.