Study On Hydrogen Diffusion Behavior In Hot Rolled Steels For Enameling

In this thesis, the hydrogen permeation properties of three kinds of hot rolled steels for enameling have been investigated by the electrochemical method to solve fish-scaling problem of enamelled steel sheets. The microstructures, inclusions and precipitates in these steel sheets were observed by means of optical microscope, scanning electron microscope?SEM? and transmission electron microscope?TEM?. Based on the results above, the effect of the quantity and size of precipitates on hydrogen permeation characteristics and fish-scaling resistance was further studied.At 25 °C, the hydrogen diffusion coefficients of steels 1, 2 and 3 are 0.530×10-6 cm2/s, 1.039×10-6 cm2/s and 2.599×10-6 cm2/s, respectively. The hydrogen breakthrough times in steel 1, 2 and 3 with a thickness of 1 mm are 20.55 min, 10.48 min and 4.19 min, respectively. The critical hydrogen concentrations for causing hydrogen blister are 10.523 wppm, 9.004 wppm and 4.869 wppm, respectively.The non-metallic inclusions in the three kinds of steels are spherical oxides with total levels of 12, 11.5 and 11, respectively. No appreciable difference in microstructures was observed in these sheets, and all the steels are composed of ferrite and a very small amount of pearlite. Therefore, the inclusions and microstructures in three steels are not the main reasons for the different hydrogen diffusion coefficient.The precipitates in the hot rolled steels for enameling are of MnS, TiS, Ti₄C₂S₂ and TiC. According to particle size, the precipitates can be classified into two kinds: the first one with 300¹100 nm includes MnS, TiS and Ti₄C₂S₂ precipitates and the second one with a small size less than 35 nm is TiC precipitates. A large number of precipitates in the hot rolled steel for enameling can act as irreversible traps that strongly influence the hydrogen diffusion behavior. The hydrogen trapping effect increases with the decrease of particle size and the increase of particle amount, leading to the decrease of hydrogen diffusion coefficient. Hence, steel 1 has a best fish-scaling resistance, and steel 3 has the worst fish-scaling resistance.