UPDATE 3-Klatten steals march on VW in carbon fibre race

* BMW secures 15.2 percent stake in SGL Carbon

* BMW investor Klatten already holds about 29 percent stake

* Lightweight construction to play important role in cars

* BMW, Klatten say not acting in concert

* SGL shares up 4.2 percent (Adds further details, more BMW comment)

By Frank Siebelt

FRANKFURT, Nov 18 (Reuters) - Germany's richest woman, Susanne Klatten, tightened her hold on SGL Carbon, putting herself well ahead of Volkswagen in the race for carbon fibre technology.

Carmaker BMW, in which Klatten and her family are a major investor, said on Friday it had secured a 15.2 percent stake in SGL, adding to the 29 percent stake already held by the German heiress.

Shares in SGL Carbon rose more than 7 percent in early trade and were up 4.2 percent at 44.95 euros by 1244 GMT.

The use of carbon fibre reinforced plastics (CFRP), which are 30 percent lighter than aluminium and 50 percent lighter than steel, is one of the hottest trends in the car industry's drive to lower its carbon dioxide emissions.

That has led to a mad dash for carbon fibre specialist SGL Carbon, with Klatten, Volkswagen and engineering company Voith snapping up stakes this year.

"This is a power struggle that's going on without any economic logic," one German analyst said, who asked not to be named, adding there were a number of SGL competitors who could supply carbon fibres just as well.

SGL's stock has gained more than 60 percent so far this year, while Germany's midcap index has declined by more than 5 percent.

VW declined to comment on Friday, while Voith said it was a positive sign that SGL was opening up more to carmakers.

LOGICAL STEP

BMW did not say how much it paid for its stake in SGL, but the current market value of the shares is close to 450 million euros ($608 million).

"Lightweight construction will play an increasingly important role in the automobile industry in the future," BMW finance chief Friedrich Eichiner said in a statement.

"Our stake-holding in the SGL Group is a logical step that will further strengthen our successful cooperation."

In October 2009, SGL struck a cooperation deal with BMW on lightweight carbon parts for its i3 electric vehicle, also known as the MegaCity project, a few months after Klatten emerged as a major shareholder in SGL.

"Just strategically, this is a very good step. But really, BMW is turning up late to the party, it should have done this a year ago," said Daniela Bergdolt, who represents small BMW shareholders for association DSW.

Klatten, whose estimated $15 billion fortune made her the target of a blackmail plot in 2007, this year secured a blocking stake of about 29 percent in SGL, giving her the right to veto strategic decisions.

Klatten holds 12.6 percent in BMW, while her family owns a combined 46.7 percent of the carmaker.

If Klatten and BMW were considered an investor pool, their combined stake would now have risen above 30 percent to about 44 percent, which would trigger a mandatory full takeover offer for SGL under German regulation.

But Klatten's investment vehicle Skion said it was not involved in BMW's decision to take a stake in SGL and said it had no plans for a full takeover of the carbon fibre maker.

A spokesman for BMW also said the carmaker was not acting in concert with Klatten. ($1=0.740 euros) (Additional reporting by Jan Schwarz, Kirsti Knolle and Irene Preisinger; Writing by Maria Sheahan; Editing by Mike Nesbit and Will Waterman)