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The optimal aging temperatures of 400 - 550 °C for high strength steels falls into the dangerous temperature range of temper-embrittlement. To obtain a good low-temperature toughness, temperatures above 600 °C are usually selected for aging treatments, resulting in a huge loss of strength. In this work, the effects of aging treatments at 500 and 550 °C on the impact performance of a Cu precipitation-strengthened steel at a low temperature of -80 °C were systematically investigated. The solute segregation at lath boundaries is found to be the main factor controlling the low-temperature toughness. Excellent impact performance of ∼180 J at -80 °C along with a high yield strength of ∼1050 MPa and a total elongation of 19% can be obtained by controlling the segregation of solute elements, specifically Mo and Mn at the lath boundaries. The evolutions of matrix and precipitates during aging treatments were investigated. The strengthening and toughening mechanisms are also critically discussed.