Two types of nanoclusters were detected using DSC thermal analysis operating at -50 °C with liquid nitrogen. The formation of Cluster (1) was substantially suppressed by the formation of Cluster (2) during step quenching at 100 °C for 0.06 ks. The hardness increase was retarded for a period during two-step aging at 170 °C in the case of natural aging for 604.8 ks. The hardness decrease was determined to be due to the dissolution of nanoclusters in the early stage of two-step aging at 170 °C. On the other hand, hardness was directly increased in the step-quenched samples during multi-step aging at 170 °C. It is noted that Cluster (2) directly transforms into the strengthening phase. The bake hardening response is fairly enhanced by the formation of Cluster (2), which is formed by the step quenching process. It was confirmed that the step-quenched samples had a higher number of precipitates than the naturally-aged ones based on TEM images. The structure of the precipitate was identified to be the β″ phase by analyzing HRTEM image.