But the outgoing president – who made the rebuilding of the US manufacturing base a major goal of his administration – announced on Friday that he was blocking the acquisition, saying it threatened national security.
The announcement drew sharp criticism from both companies and Tokyo.
The companies said the body reviewing the deal, the Committee on Foreign Investment in the United States (CFIUS) “failed to conduct a good faith, national security-focused regulatory review process”.
The steelmakers are asking the courts to order a new review of the proposed merger.
On Monday Japanese Prime Minister Shigeru Ishiba urged Washington to dispel concerns that Biden’s block of the deal could impact future investments.
The transaction was a prominent element of the 2024 presidential campaign, given US Steel’s ties to Pennsylvania, a politically critical battleground state and cradle of the American steel industry.
In an interview with Nikkei business daily, Nippon Steel vice chairman Takahiro Mori said his company “will not back down after being treated unreasonably. We will fight tooth and nail”.
The CFIUS review process “lacked integrity”, he said, adding that “the decision was left to the president (Biden) without any substantive consultation” with the companies.
