Carbon fiber sizing agent, carbon fiber strand, and fiber-reinforced composite

A sizing agent for carbon fiber used for reinforcing a thermoplastic matrix resin, the sizing agent essentially comprising:

• a polymer component having a glass transition temperature at least 20 deg C. and exhibiting no endothermic peaks indicating an endothermic heat of fusion due to crystalline melting of at least 3 J/g in a determination with a differential scanning calorimeter;
• wherein the weight ratio of the polymer component constitutes from 10 to 100 wt % of the nonvolatile components of the sizing agent;
• wherein the polymer component is at least one component selected from the group consisting of an aromatic polyester resin, aromatic polyester-polyurethane resin and amine-modified aromatic epoxy resin;
• wherein the aromatic polyester-polyurethane resin is a polymer produced by addition polymerization of an aromatic polyester polyol and polyisocyanate;
• wherein the amine-modified aromatic epoxy resin is a reaction product of an aromatic epoxy compound and hydroxyl-group-containing amine compound with the ratio of the hydroxyl-group-containing amine compound ranging from 1.0 to 2.0 mol equivalent to the epoxy groups of the aromatic epoxy compound.