UPDATE 1-Brazil's CSN interested in Thyssen unit-paper

* CSN's Steinbruch "interested" in details of deal-paper

* CSA is venture between ThyssenKrupp, Brazil's Vale

* Thyssenkrupp wants to exit CSA as costs and losses soar

* Brazil's government looking to keep unit locally owned

SAO PAULO, June 17 (Reuters) - CSN , Brazil's second-largest producer of flat steel products, will analyze ThyssenKrupp's joint venture in local slab-making mill CSA if the German steelmaker formally puts its stake up for sale, a local newspaper reported in its online edition.

"When the details are made available (from CSA to interested parties), which has not yet occurred, we will surely be interested in studying them," CSN chief executive officer Benjamin Steinbruch said on the sidelines of the Rio+20 conference in Rio de Janeiro on Saturday, according to newspaper O Estado de S. Paulo's website.

A spokesman for CSN in Sao Paulo confirmed the comments but declined to elaborate.

The giant CSA mill, 73 percent owned by ThyssenKrupp with the remainder held by Brazil mining giant Vale, is the largest foreign investment in Brazil in the last decade at an estimated cost of $9 billion.

ThyssenKrupp is looking to sell its stake in the mill as it struggles with delays and cost overruns that led it to report a loss last year. The mill has been operating for more than two years but has fallen in value to about $5 billion, in the opinion of some analysts.

On Friday, Reuters reported that Brazil's government wants to prevent ThyssenKrupp from selling its stake to another foreign company, and has contacted local steel groups to assess their appetite for a stake in CSA.