Study on the improving effect of Nb-V microalloying on the hydrogen induced delayed fracture property of 22MnB5 press hardened steel

Press-hardened steel (PHS) is widely used in manufacturing automobile parts and is sensitive to hydrogen embrittlement (HE). A novel 0.04Nb-0.04 V (wt%) PHS (22MnB5NbV) was developed based on 22MnB5. The constant-strain bending test shows the duration of no-cracking for 22MnB5NbV in 0.5 mol/L HCl is > 300 h even though the pre-bending stress is 1500–1600 MPa, and that for 22MnB5 is 12 h. The prior austenite grain size and martensite lath width in 22MnB5NbV decrease from 13.4 to 8.2 μm and from 0.35 to 0.15 μm due to Nb-V microalloying, respectively, resulting in the hydrogen trapped in reversible hydrogen traps increases by 2.09 times according to thermal desorption spectrometry analysis. The leaching experiment presents that 88% Nb and 25% V exist in the form of precipitates rather than in solution in 22MnB5NbV. The nano-precipitates (< 10 nm) are mainly (Nb, Ti, V)C, in which Nb-bearing precipitates account for the largest proportion and supply the heterogeneous nucleation sites for V-bearing precipitates, this is confirmed by atom probe tomography (APT). APT confirms that the hydrogen is mainly trapped at the interface of Fe matrix/carbide. The dispersed nano-carbide and refined microstructure resulting from the Nb-V microalloying are the key factors improving the HE of 22MnB5NbV.