* JFE, U.S. Steel to jointly bid for unit - sources
* Arcelor, Nippon also join forces to bid - sources
* Deadline for final bids is Feb 28 - sources
FRANKFURT, Feb 10 (Reuters) - Bidders competing for ThyssenKrupp AG's loss-making U.S. and Brazilian steel mills are teaming up in an effort to make the investment more digestible, according to people familiar with the transaction.
Germany's biggest steelmaker ThyssenKrupp said in May 2012 that it was considering options for the mills which lost about one billion euros in the financial year ended September 2012, but deemed initial bids, which sources said were in the region of 3-4 billion euros, as too low.
Japan's JFE Steel Corp and U.S. Steel are planning to hand in a joint bid by a February 28 deadline for final bids, two people familiar with the process said.
"The bidding process is running according to plan," a spokesman for ThyssenKrupp said, without confirming the deadline for bids. "We are optimistic to find a solution by the end of the fiscal year."
JFE and U.S. Steel were not immediately available for comment.
Europe's ArcelorMittal and Japan's Nippon Steel have also joined forces in the auction according to sources familiar with the process, though have not confirmed their plan, while Brazil's Cia Siderúrgica Nacional (CSN) has secured financial support from state development bank BNDES .
ThyssenKrupp heavily wrote down the value of its Steel Americas unit to 3.9 billion euros ($5.22 billion) in December. It had wanted to carve out new markets with two mills in Brazil and the U.S. but they were hit by cost overruns, poor project management and weaker than expected demand.
Brazil's Vale, the world's No.2 miner, owns a 27 percent stake in Thyssen's Brazilian plant CSA, but has said it is not interested to buying Thyssen's stake.
Goldman Sachs and Morgan Stanley are advising Thyssen on the sale.
A downturn in the European steel market has prompted the group to shift investment to higher margin products like elevators, plant components and submarines.