Grease composition

A grease composition for use in lubricating a mechanical part having a steel portion to be lubricated, the portion of which performs a rolling motion and a rolling and sliding motion, consisting of a base oil, a thickener and an additive,

• wherein the thickener is a diurea compound represented by formula (1):
• R1-NHCONH—R2-NHCONH—R3 (1)
• wherein R2 is a bivalent aromatic hydrocarbon group having 6 to 15 carbon atoms;
• and one of R1 or R3 represents a saturated alkyl group having 18 carbon atoms and the other represents a cyclohexyl group, the molar ratio of the cyclohexyl group to the alkyl group being 20:80 to 95:5, and
• wherein the additive consists of:
• at least one compound selected from the group consisting of polyethylene wax having a weight-average molecular weight of 18,000 to 20,000 and a dropping point of 100 to 150° C., and
• 0 to 20 mass % of other additives, with respect to the total mass of the grease composition, selected from the group consisting of:
• an antioxidant selected from the group consisting of phenyl a-naphthylamine, alkylphenyl a-naphthylamine, alkyldiphenylamine, 2,6-di-tert-butyl-p-cresol, pentaerythrityltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate;
• an inorganic passivator;
• a rust preventive;
• a metallic corrosion inhibitor;
• an oiliness improver;
• a phosphorus-containing, sulfur-containing or organic metal-containing antiwear agent or extreme-pressure agent selected from the group consisting of tricresyl phosphate, tri-2-ethylhexylphosphate;
• dibenzyl disulfide, polysulfide;
• triphenylphosphorothionate;
• Mo, Sb or Bi salts of dialkyldithiophosphoric acid, Mo, Zn, Sb, Ni, Cu, or Bi salts of dialkyldithiocarbamic acid;
• ash-free dithiocarbamate, and ash-free dithiophosphate carbamate;
• and
• a solid lubricant selected from the group consisting of oxidized metal salts, metal salts of carbonate and molybdenum disulfide.