Japan's Nippon Steel has agreed to buy US Steel Corp for $14.1 billion, the companies announced on Monday.

The leading Japanese steelmaker will acquire US Steel in an all-cash transaction at $55 per share, a 40 percent premium above its closing price on December 15, representing an equity value of about $14.1 billion, the companies said in a statement. Nippon will also assume the US firm's debt, taking the total deal to $14.9 billion.