Big River operates a LEED-certified flex mill in northeast Arkansas that U.S. Steel says is the newest and most advanced flat-rolled mill in North America.
The U.S. Green Building Council's Leadership in Energy and Environmental Design certification ranks the environmental health and efficiency of buildings.
A recent expansion doubled the mill's hot-rolled steel production capacity to 3.3 million tons annually. Big River is based in Osceola, Ark., three hours northeast of Little Rock.
The purchase is expected to immediately add to U.S. Steel's earnings, the Pittsburgh company said. U.S. Steel hopes to close the deal in the first quarter, subject to conditions including antitrust clearance.
U.S. Steel shares at last check rose 9.1% to $18.76.
The company acquired a 49.9% stake in Big River in October 2019 with an option to acquire the rest.
U.S. Steel ended November with about $2.9 billion of liquidity including more than $1.7 billion of cash.
"By completing our top strategic priority, Big River Steel, we expect to strengthen our order book, increase our competitiveness and accelerate further product innovation for our customers," U.S. Steel Chief Executive David B. Burritt said in a statement.
"Longer term, the lower, variable cost structure will increase our efficiency, profitability and cash flow across the business cycle."
Big River produces substrate for U.S. Steel's XG3 high-strength steel and other automotive grades.