Activist investor Ancora Holdings is preparing to wage a proxy battle at U.S. Steel and wants the company to turn the page from its failed marriage with Nippon Steel, according to people familiar with the matter.

The asset management firm Ancora is challenging Nippon Steel's proposed acquisition of US Steel following its purchase of a 0.18% stake in the American company. Ancora has accused the US Steel board and CEO David Burritt of pushing the $15 billion deal with Nippon Steel due to a potential $100 million windfall they'll receive if the deal closes.

Activist investor Ancora has nominated nine candidates to U.S. Steel's board of directors, as it looks to oust company CEO David Burritt and push the American steelmaker to back out of a $14.9 billion merger deal with Japan's Nippon Steel.