Effect of Grain Size on Thermophysical Properties in Twinning-Induced Plasticity Steel

This study investigated the thermophysical properties of TWIP steel with respect to grain size. The coefficient of thermal expansion (β) of TWIP steel was approximately 22.4 × 10^-6 °C^-1, and this value was hardly affected by the grain size. Therefore the density of TWIP steel was also unaffected by grain size within the tested range. The β in TWIP steel was higher than that of plain carbon steels (13-15 × 10^-6 °C^-1) such as interstitial free (IF) steel and low-carbon steel, and stainless steels (18-21 × 10^-6 °C^-1) such as X10NiCrMoTiB1515 steel and 18Cr-9Ni-2.95Cu-0.58Nb-0.1C steel. The specific heat capacity (c_p) increased with temperature because the major factor affecting c_p is the lattice vibrations. As the temperature increases, atomic vibrations become more active, allowing the material to store more thermal energy. Meanwhile, c_p slightly increased with increasing grain size since grain boundaries can suppress lattice vibrations and reduce the material's ability to store thermal energy. The thermal conductivity (k) in TWIP steel gradually increased with temperature, consistent with the behavior observed in other high-alloy metals. k slightly increased with grain size, especially at lower temperatures, due to the increased grain boundary scattering of free electrons and phonons. This trend aligns with the Kapitza resistance model. While TWIP steel with refined grains exhibited higher yield and tensile strengths, this came with a decrease in total elongation and k. Thus, optimizing grain size to enhance both mechanical and thermal properties presents a challenge. The k in TWIP steel was substantially lower compared with that of plain carbon steels such as AISI 4340 steel, especially at low temperatures, due to its higher alloy content. At room temperature, the k of TWIP steels and plain carbon steels were approximately 13 W/m°C and 45 W/m°C, respectively. However, in higher temperature ranges where face centered cubic structures are predominant, the difference in k of the two steels became less pronounced. At 800 °C, for example, TWIP and plain carbon steels exhibited k values of approximately 24 W/m°C and 29 W/m°C, respectively.