Facing the problems of damaged bridge piers with box sections due to the rupture of the corner welds, cold press-bent steel structure was considered as a possible solution. However, strain aging induced by cold-forming may lead to decrease in toughness and elongation, as well as increase in hardness and strength. This paper focuses on the adverse effects of strain aging on the test pieces made of steel SM520C, which were evaluated through Vicker’s hardness test and Charpy impact tests. The test pieces were taken from different locations of the cold press-bent corner parts, and the Charpy impact test was conducted after one month, six months and one year correspondingly. The hardness of the material increased due to the effect of strain aging, especially at both edges at the maximum curvature of the corner part. Small effect of strain aging on the Charpy absorbed energy was found under 0 °C, while larger reduction ratio was obtained under a lower temperature −60 °C. In the future, steel materials with high toughness are suggested to be studied in order to widen the usage of cold-formed structures.