WASHINGTON (Reuters) – The Federal Reserve on Tuesday took fresh steps to lower borrowing costs amid a softening economic recovery, announcing it would use proceeds from its maturing mortgage bonds to buy more government debt. The decision to reinvest proceeds from the more than $1.3 trillion in mortgage-related debt the Fed holds, an effort to keep market-set borrowing costs down, represents a significant policy shift. Just a few months ago, the central bank had been avidly debating an exit strategy from the extraordinary stimulus delivered during the financial crisis. "To help support the economic recovery in a context of price...