Influences of Martensite Morphology and Precipitation on Bendability in Press-Hardened Steels

Performance evaluation of martensitic press-hardened steels by VDA 238-100 three-point bend testing has become commonplace. Significant influences on bending performance exist from both surface considerations related to both decarburization and substrate-coating interaction and base martensitic steel considerations such as structural heterogeneity, i.e., banding, prior austenite grain size, titanium nitride (TiN) dispersion, mobile hydrogen, and the extent of martensite tempering as result auto-tempering upon quenching or paint baking during vehicle manufacturing. Deconvolution of such effects is challenging in practice, but it is increasingly accepted that surface considerations play an outsized role in bending performance. For specified surface conditions, however, the base steel microstructure can greatly influence bending performance and associated crash ductility to meet safety and mass-efficiency targets. This study reports and elucidates the positive effect of niobium microalloying on bendability of the base PHS alloy through combined structural refinement, microalloy carbide precipitation, and modifications to tempered martensite morphology. Prior austenite grain size refinement due to microalloying results in both incremental strengthening and a reduced solute carbon (C) content in the austenite matrix prior to in-die quenching due to both enhanced segregation of C to austenite grain boundaries. Martensite-start temperature is thusly increased, and a greater degree of auto-tempering results in as-quenched steels. Additionally, the reduction of C content in the as-quenched martensite reduces driving force for oriented transition carbide precipitation during tempering. Measured bend angles are accordingly improved, and reduced strength of martensite, as a result of reduced matrix C, is compensated by the fine dispersion of microalloy carbonitrides. Alloy design and process recommendations are offered based on observed mechanisms.