2012 Volume 52 Issue 2 Pages 274-280
To ensure safety against delayed fracture caused by hydrogen embrittlement under service environments of steel sheets for auto body parts used after forming and paint baking, it is quite important to understand hydrogen entry into steels under the service environments, which could be influenced by part processing parameters such as forming and the following paint baking conditions. In this study, the influence of temperature and holding time in low-temperature heat treatment after deformation on hydrogen entry into steel sheets was investigated in detail, and the mechanism of the decrement of hydrogen entry due to the heat treatment was discussed using 0.2%C steel, 0.002%C steel and IF steel. The steels were 10–20% cold rolled to introduce plastic strain, then heat-treated at 50–170°C for 0.2–20160 min, followed by immersion in pH 1-HCl for at most 72 h to introduce hydrogen into the steels. Hydrogen entry into the steels during immersion in pH 1-HCl increased with the cold rolling and decreased with the following low-temperature heat treatment, and as the heating temperature and holding time increased, hydrogen entry during immersion in HCl decreased. This indicates hydrogen trapping sites, which were introduced by the cold rolling, decreased due to the low-temperature heat treatment. From a comparison of steels with and without solute carbon atoms, the decrement of hydrogen trapping sites due to the low-temperature heat treatment was attributed to both the decrement of vacancies and occupation of hydrogen trapping sites around dislocations by interstitial solute atoms such as carbon.
