Investors can’t get enough of Apple Inc.

The iPhone maker sold $6.5 billion in bonds on Monday, including a round of 30-year debt that will pay 3.5% annually. That is even lower than Apple’s $17 billion bond sale in April 2013, when a 30-year bond yielded about 3.9%.

Apple’s deal is the largest U.S. high-grade corporate-bond sale so far this year, according to S&P Capital IQ LCD. Apple tapped the debt market less than a week after reporting a 38% jump in its latest quarterly profit.

The strong investor demand for the debt sale, which also included notes maturing in five to 10 years, underscores the shifting dynamics in the debt markets amid deepening concerns about the pace of global growth.

The decline of rich-country government-bond yields over the past year has caught many investors by surprise, after Wall Street broadly predicted interest rates would rise as the U.S. economy picked up steam.