Source: U.S. Department of the Treasury
Release: Corporate Bond Yield Curve
The basic type of selected bond is analogous to conventional Treasury coupon issues: a bond that pays a fixed semiannual nominal coupon denominated in U.S. dollars until maturity, when the principal is returned. The HQM yield curve uses data from a set of high quality corporate bonds rated AAA, AA, or A that accurately represent the high quality corporate bond market.
For more information see https://www.treasury.gov/resource-center/economic-policy/corp-bond-yield/Pages/Corp-Yield-Bond-Curve-Papers.aspx
U.S. Department of the Treasury, 30-Year High Quality Market (HQM) Corporate Bond Par Yield [HQMCB30YRP], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/HQMCB30YRP, September 19, 2018.