Effect of Nb-Ti multi-microalloying on the hydrogen trapping efficiency and hydrogen embrittlement susceptibility of hot-stamped boron steel

Multi-microalloying technique has been widely used to improve strength and ductility in steels, however, the effect of multi-microalloying on the hydrogen diffusion and hydrogen embrittlement (HE) behavior of steels is still not completely understood. This study evaluates the effect of Nb-Ti multi-microalloying on the hydrogen trapping efficiency and HE of hot-stamped steel by a combination of hydrogen permeation test, slow strain rate tensile (SSRT) test and quantitative analysis. The microstructural examination and hydrogen permeation tests showed that with increasing Nb + Ti content, more nanosized (Nb, Ti) C precipitates were formed, and the martensite grain boundaries areas increased, thus increasing the irreversible and reversible hydrogen trap sites and decreasing the hydrogen diffusion coefficient in the hot-stamped steel. A quantitative analysis of the hydrogen taps demonstrated that the number of hydrogen traps induced by the (Nb, Ti) C precipitates was larger than that of the grain boundaries and dislocations in the Nb-Ti bearing steels. In addition, the SSRT tests showed that the HE susceptibility of the test steel decreased with increasing Nb + Ti content. This was because the additional hydrogen traps generated by the Nb-Ti addition hindered localized hydrogen accumulation at prior austenite grain boundaries, the cracking resistance was improved by a lower Sigma 3 boundary fraction, and the pinning role of the (Nb, Ti) C precipitates on the movable dislocation could inhibit H-dislocation interaction.