X線小角散乱を用いたばね鋼におけるε炭化物の形態解析

抄録

With the increasing strength requirements for automotive spring steels, hydrogen embrittlement has become a critical issue.

The precipitation of fine ε-carbides has been identified as an effective method in mitigating hydrogen embrittlement. However, precise quantitative evaluation of their size and volume remains essential for effective microstructural control.

In the present study, ε-carbides in spring steel were quantitatively analyzed using small-angle X-ray scattering (SAXS), and the reliability of the results was evaluated. The volume fraction determined by SAXS was compared with the maximum precipitation calculated from solute carbon content, while particle size was assessed based on transmission electron microscopy (TEM) observations.

Specimens of SAE9254 steel were quenched at 950°C and subsequently tempered between 150°C and 450°C. Conventional SAXS analysis, which assumes a single-component fitting, yielded volume fractions that exceeded the calculated maximum. In contrast, the application of a two-component fitting approach, which accounts for scattering contributions from both the martensitic matrix and ε-carbides, produced results consistent with the calculated maximum.

Discrepancies were observed between particle sizes measured by SAXS and TEM, with the divergence increasing at higher tempering temperatures. This discrepancy is primarily attributed to the difficulty in detecting smaller particles using TEM, particularly when the precipitates exhibit a broad size distribution.

These findings confirm the effectiveness of the two-component fitting method and demonstrate enhanced analytical accuracy in the quantitative evaluation of ε-carbides using SAXS.