High-strength steel is used in high-rise buildings and automobiles. In recent years, high-strength composite steel materials have been developed that are easy to process by distributing the easily deformable γ-phase in a hard steel structure. However, high-strength steel is prone to hydrogen embrittlement, which is deterioration caused by hydrogen that penetrates the steel, and that is a serious problem in realizing a carbon-neutral hydrogen society. In particular, the composite material tends to deteriorate easily due to hydrogen concentrating on the surface of the γ-phase.
 Because hydrogen is trapped in various defects with steel, in addition to the γ-phase surface, it is important to determine the amount of hydrogen concentrated at the γ-phase surface. To achieve this, steel samples are gradually heated, and the amount of hydrogen released at different temperatures from various defects and desorbed from the surface is measured as a function of temperature, yielding a thermal desorption spectrum from which hydrogen trapping states associated with defects are estimated. Fig.1 shows the estimated hydrogen trapping states in the steel containing the γ-phase from the numerical simulations of thermal desorption spectra. The temperature and height of the desorption peak are related to the hydrogen trapping state of the defects. It shows that when there is little hydrogen content, the surface and interior of the γ-phase do not trap hydrogen, however, when there is more hydrogen, they do, and that the interior of the γ-phase releases hydrogen at relatively low temperature, making it less capable of trapping hydrogen than the surface. These results suggest that while the γ-phase improves the properties of high-strength steels, the effects of hydrogen on its interior and surface on embrittlement must be closely examined.

This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) Grant Number JP19K05069, and carried out in collaboration with Nippon Steel Corporation and Sophia University.