Abstract
Effects of micro-alloying have been examined on hydrogen embrittlement (HE) of various hot stamping (HS) steel sheets. Referring to these researches, formation and function of the alloy carbonitrides were given systematic consideration for analyzing their effects on HE. The state of hydrogen is important issue, and was determined preferably by thermal desorption analysis (TDA) rather than hydrogen permeation measurement. Although the coherent precipitates (carbonitrides) containing Nb, Ti or V in ferritic matrix can trap hydrogen effectively, they have negligible influence on the easy-to-move hydrogen that should be responsible to HE. This might be caused by the decreased amount of the precipitates in the HS process. The alloy carbonitrides, which had remained during austenitizing treatment and is in-coherent to ferritic matrix, restrain grain growth and prevent the degradation of grain boundary (GB) cohesion. Molybdenum (Mo) strengthens GB cohesion, not by forming carbonitrides but by segregating along GB. Both carbon (C) and boron (B) also segregate along GB and would strengthen GB cohesion. These enhanced GB cohesion makes the steels resisting to HE. It is true for steels like high strength spring steel, V alloyed high strength bolt steel and hydrogen gas accumulator steel, that micro-alloying of Nb is the key issue for improving the anti-HE property. The influence of V carbonitride on HE susceptibility appreciates continuous attention, especially in case of high temperature tempering.
