(AK Steel takeover rumors article updated for company and analyst's comments and for closing stock prices.)NEW YORK ( TheStreet) -- Shares of AK Steel ( AKS) popped Thursday in frantic trading as takeover rumors appeared to give traders a reason to bid up the name. An item on Briefing.com pointed to Brazilian steel producer Companhia Siderurgica Nacional ( SID), or CSN, as the alleged pursuer. A representative of West Chester, Ohio's AK Steel cited the standard corporate policy of not commenting on "market speculation." AK's shares closed Thursday at $24.37, up $1.56, or 6.8% after earlier moving as high as $24.60. Turnover surpassed 26 million shares, more than triple the daily average. Some industry observers were skeptical that AK, a manufacturer of flat-rolled steel products, could truly be in play. One veteran analyst noted the deep uncertainty surrounding U.S. automobile sales, AK's core customer base. If no one really knows how many cars will be sold this year, the analyst asked rhetorically, how could any potential buyer reasonably value AK enough to make a formal offer? The company was also the subject of takeover speculation back in 2007, when rumors fingered the Japanese industrial conglomerate Mitsui -- which had just acquired the U.S.'s Steel Technologies -- as the potential buyer. Nothing, of course, came of it. Mitsui has been active in the U.S. of late, however. On Tuesday, Nucor ( NUE) announced the formation of a joint venture with Mitsui & Co., the conglomerate's steelmaking group -- a move that may also have sparked Thursday's heated action in AK Steel shares. The Nucor venture, which will take over the assets of Mitsui's Steel Technologies unit, has plans to "invest in various steel and steel related activities," Nucor said.