At $55 in cash, the deal is a 40% premium to the closing price of US Steel shares on Friday. Shares of X are up 27% in premarket trading in New York, around $50 a share. 

Nippon believes the acquisition of US Steel "will enhance its world-leading manufacturing and technology capabilities and enable it to expand the geographic areas." The US is currently the world's fourth top producer of crude steel after China, India, and Japan. 

Nippon added that the acquisition will allow it "to capitalize on the growing demand for high-grade steel, automotive and electrical steel, and provide excellent products and services." 

The transaction is expected to close in the second or third quarter of next year and is also subject to approval by US Steel's shareholders. 

Citi is acting as a financial advisor to Nippon. Barclays Capital Inc., Goldman Sachs & Co. LLC, and Evercore are the financial advisors for US Steel. 

Nippon's buyout deal comes just months after US Steel rejected an offer from rival Cleveland-Cliffs Inc. for $7.25 billion.