Effects of the Addition of Nb and V on the Microstructural Evolution and Hydrogen Embrittlement Resistance of High Strength Martensitic Steels

Abstract

Hydrogen embrittlement can easily occur in high strength martensitic steel, manifesting itself as a sudden failure or fracture without warning and greatly threatening the safety of automotive applications. Optimizing the composition of the alloy can be performed by matching heat treatment processing methods and controlling the precipitation amounts to form hydrogen traps. In doing so, the hydrogen embrittlement susceptibility of steel can be effectively delayed, reducing the risk of hydrogen-induced delayed cracking. In this study, four kinds of 1500 MPa strength grade martensitic steel were selected for testing and supplemented with different loadings of Nb and V, respectively. Their grains, phases, and precipitations were compared by optical microscopy (OM), electron backscattered diffraction (ESBD), and transmission electron microscopy (TEM) analyses. After the addition of Nb and V, the microstructure was refined, the residual austenite content increased, and the hydrogen embrittlement resistance was significantly improved.

Characterization of Nanosized Carbide Precipitates in Multiple Microalloyed Press Hardening Steels

“…However, during manufacturing of PHS components, the quenched steel is only exposed to low-temperature tempering, during paint baking, at around 170 • C. This condition does not enable microalloy precipitation. For triple microalloyed PHS, it was argued that a higher particle density would provide additional grain refinement and enhance hydrogen trapping [7,18]. However, the results reported so far indicate that a triple microalloyed TiNbV PHS has negligible advantage over dual microalloyed TiNb PHS.…”

“…Recently, some PHS alloy developments proposed incorporating vanadium microalloying, either as dual Ti-V steel or as triple microalloyed Ti-Nb-V steel. For dual Ti-V steels, rather high vanadium additions, up to 0.2 mass percent, were reported [8,17], whereas in triple Ti-Nb-V, all three microalloying elements were typically added in the same range of 0.02-0.04% [7,18]. Higher vanadium additions are common in quenched and tempered martensitic steels.…”

“…In fact, only 0.05% out of the actual 0.2% vanadium added to these 30MnB5 and 35MnB5 steels was finally represented in VC particles. The systematic comparison of multiple microalloying concepts in 22MnB5 by Liu et al [18] indicated that single microalloying with titanium does not provide coarsening resistance. Dual TiV microalloying results in moderate coarsening resistance, whereas NbTi microalloying shows strong coarsening resistance.…”

Characterization of Retained Austenite in Advanced High-Strength Steel

“…In fact, the interface of the RA and martensite matrix is a strong hydrogen trap. Therefore, an appropriate amount of RA can make good hydrogen traps, which improves the hydrogen embrittlement (HE) resistance of the ultra-highstrength martensitic steels [13][14][15].…”

Hydrogen Embrittlement and Microstructure Characterization of 1500 MPa Martensitic Steel

“…The addition of Nb (with or without V) leads to the increase of residual austenite content and the existence of a large number of fine Nb–V–Ti nanoprecipitates in the matrix, which results in several benign hydrogen traps, which greatly improves the HE resistance of the material. [ 7 ] Motomichi et al believed that for austenitic stainless steel with a variable pressure between 800 and 1000 MPa, the combined action of reduced Mn element and grain refinement almost achieved complete HE resistance. [ 8 ] Yang Chen et al deduce that the cracks usually occurred at the original austenite GB and lath martensite interface, and the crack direction was perpendicular to the lath martensite orientation, and the transgranular fracture occurred at the previous austenite grain.…”