By Deepa Seetharaman

March 15 (Reuters) - Ford Motor Co announced an expanded and extended revolving credit facility on Thursday, as the No. 2 U.S. automaker tries to bolster its balance sheet and ultimately reach an investment-grade credit rating.

Ford's revolving credit was enlarged to $9.3 billion from $8.9 billion. Of that figure, $9 billion now matures on Nov. 30, 2015, two years later than before. The other $300 million will mature in 2013 as previously planned.

The credit "represents an important source of committed liquidity and financial flexibility for Ford," Ford Treasurer Neil Schloss said. The offer was "significantly oversubscribed" by banks, Schloss said.

"This is an insurance policy that you hope you never have to use," Schloss said.

The revolver is a part of the more than $23 billion Ford borrowed in late 2006 to support its turnaround, secured by assets including the Blue Oval logo. Initially, the line of credit was $11.5 billion.

Ford has since repaid nearly all of the $23 billion.

The collateral pledged to lenders under the revolver would be released when Ford attained investment grade ratings from at least two of the three major ratings agencies.

Moody's Corp's Moody's Investors Service, McGraw-Hill Cos Inc's Standard and Poor's Ratings Service and Fitch Ratings, a venture of Fimalac SA and Hearst Corp, all rate Ford at one notch below investment grade.

Banks participating in the deal understand that this revolving credit would shift to an unsecured facility "in the near future" once Ford reached investment grade, Schloss said.

The company is implementing a "One Ford" plan, to simplify and unify product development and supplies.

In 2011, Ford's U.S. market share was 16.8 percent, behind General Motors Co, which had 19.1 percent.

In afternoon trading, Ford shares were up less than 1 percent to $12.94.

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