某公司投标南非一燃煤电站项目,业主为私人业主,该项目未要求提供投标保证金和投标保函。在经过投标人资格预审、递交标书、业主评标后,业主最终向该公司签发了中标通知函。在收到中标函后,该公司认为该项目风险太大,拒绝签订合同协议书。此时,签订合同是否是国际工程合同生效的必要条件?拒签合同的承包商应当承担什么责任? A company bidding for a coal-fired power project in South Africa, the owner is a private owner, the project did not require bid bond and bid bond. After the prequalification of bidders, bid submission, bid evaluation after the owners, the owner eventually issued a successful bid notice to the company. After receiving the letter of acceptance, the company considered the project risky and refused to sign the contractual agreement. At this point, the signing of the contract is the necessary condition for the entry into force of the international engineering contract? Contractor refused to sign the contract should bear what responsibility?