| The aging behavior of Cu-bearing HSLA steels was studied by using differential scanning calorimetry (DSC), transmission electron microscopy and hardness tester. Two heat evolution peaks were observed during DSC scans over the temperature range of 25∼590℃ at a heating rate of 5℃/min. The peaks appeared in low(241∼319℃ : HSLA-A, 224∼310℃ : HSLA-B) and high temperature (514℃∼ 590℃ : HSLA-A, 451∼558℃ : HSLA-B) are attributed to the formation of coherent Cu-clusters and noncoherent ε-Cu phase, respectively. And at a rapid heating rate over 7.5℃/min, another peak between these two peaks was observed in the temperature range of 300∼370℃. This peak is conjectured to be due to the formation of 9R martensite structures proposed by Othen et al. It will be necessary further research to identify this peak clearly. It was confirmed that as aging proceeds, the coherent bcc Cu-clusters transform to noncoherent fcc ε-Cu phase. In the case of the aging to peak hardness at 300℃ and 400℃, the coherent Cu-clusters contributed to the hardening. As aging time and temperature increase over peak hardness, noncoherent ε-Cu are formed and hardness decreases. The activation energy due to the formation of coherent bcc Cu-clusters in HSLA-B steel is 55㎉/mole. And those due to the formation of nonchorent ε-Cu phase in HSLA-A and HSLA-B steels are 59㎉/mole and 51㎉/mole, respectively. These are similiar to the activation energy(58㎉/mole) for the solute diffusion of Cu in α-Fe. |
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