BMW AG has bought a 15 per cent stake in SGL Carbon SE, becoming the third company in nine months to invest in the world\'s largest maker of carbon and graphite products, strategic materials for the car industry. A BMW statement about the transaction didn\'t rule out buying more shares. Competitor Volkswagen AG said it bought an 8.2 per cent stake in SGL in March, prompting speculation the carmakers may fight for control of SGL. The German company started making brake discs for cars including Porsche AG more than a decade ago and now makes carbon-fibre reinforced plastics for the industry. BMW\'s biggest shareholder, German billionaire Susanne Klatten, controls almost 29 per cent of SGL. \"It appears the strategic importance of future lightweight materials is too significant in order to allow a major competitor potentially taking control,\" said Arndt Ellinghorst, an analyst with Credit Suisse in London. SGL produces electrodes and cathodes for the metals industry as well as furnace linings, and is developing carbon fibres and composite materials for the automotive and aerospace industries. Still, the company\'s sales to automotive customers amounted to less than five per cent of total last year, a spokesman said. \"We are a materials specialist, not an automotive supplier,\" he said. SGL CEO Robert Koehler said he expects to generate \"substantial\" revenue with automotive and aviation industry clients starting 2013. The company expects sales to automotive customers will likely be between ten and 15 per cent of total revenue in coming years. The company started a joint venture with BMW in 2009 to make carbon fibres and fabrics for the automotive industry. Such material is key for carmakers to build lighter and more fuel-efficient cars, as safe as models built with more steel. BMW didn\'t disclose financial details of the transaction and said it\'s not seeking a seat on SGL\'s supervisory board.