Fosun International Ltd , an investment company controlled by Chinese billionaire Guo Guangchang, said it had offered to buy the 80 percent it does not already own in U.S. insurer Ironshore Inc (IRSH.N) for $1.8 billion. The move follows Fosun’s decision to buy a 20 percent stake in the casualty and property insurer for $464 million in August 2014 in a bid to expand its global insurance network.