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Next-generation gravitational wave detectors (GWDs) like the Cosmic Explorer and Einstein Telescope require extensive vacuum tubing, necessitating cost-effective materials. This study explores the viability of mild steel as an alternative to austenitic stainless steel for UHV beampipes, focusing on outgassing rates and surface chemistry after low-temperature bakeouts. Mild steels exhibit significantly lower hydrogen outgassing rates, below 10$^{-14}$ mbar l s$^{-1}$ cm$^{-2}$ after bakeouts at 80{\deg}C for 48 hours. While water vapor is the primary residual gas after such low-temperature bakeouts, repeated treatments reduce its outgassing rate and modify surface conditions so that such benefit is preserved after at least six months of exposure to laboratory air. These findings position mild steel as an economical and efficient material for future GWD beampipes.