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Automotive body structure light-weighting for internal combustion engine vehicles is constrained by simultaneous and increasingly challenging vehicle cost, fuel economy and passenger safety standards. Mass optimization via materials selection in internal combustion engine vehicles, therefore, is ultimately dependent on the normalized cost of mass reduction solutions and the associated implications on passenger safety and vehicle performance metrics. These constraints have resulted in development and implementation of increasingly high specific-strength solutions for metallic components in the body structure and chassis. In contrast, mass optimization in battery electric vehicles is subject to alternative performance metrics to fuel efficiency, although considerations for vehicle safety and cost naturally remain directionally similar.

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.

A very low carbon (<0.06 wt%), lean alloyed chemistry approach is utilized for the development of high strength dual phase (DP) steels with enhanced global and local formability though EAF-LMF-CSP route. Optimized process conditions including clean steel practices, choice of suitable casting powder, hot rolling and continuous anneal set points resulted in excellent mechanical properties and formability characteristics of the subject DP steel. The enhanced balance of strength and formability (hole expansion ratio values of 26-35% for thicknesses of 0.80-1.40mm) is attributed to the optimization of the microstructure through refinement, homogeneity, isotropy and balancing of microconstituent mechanical response. In this contribution, production strategy and formability characterization of DP steels with tensile strengths of 780 MPa and above relevant to automotive body structure applications are discussed.

Grey cast iron alloys for brake drum and brake disc applications are being developed with niobium additions and a range of equivalent carbon for commercial, passenger vehicle, and performance applications. The benefit of niobium in cast iron is based on the contribution of strength by matrix refinement for a given carbon equivalence that may permit the direct improvement of wear improvement or allow for an increase in carbon equivalence for a given strength. Proper carbon equivalency and pearlite stabilization contribute to an improved pearlite structure with an optimized distribution of graphite. These structures, when refined with niobium, demonstrate increased service life and reduced wear relative to their niobium-free equivalents as measured by lab dynamometer testing and by on-vehicle testing in passenger bus fleets.

Tensile behavior of advanced high strength steel (AHSS) grades with strengths up to 980 MPa has been extensively studied. However, limited data is found in literature on the tensile behavior of steels with tensile strengths of the order of 1180 MPa, especially at nominal strain rates up to 500/s. This paper examines tensile flow behavior to fracture of four different 1180 MPa grade steels at strain rates of 0.005/s, 0.5/s, 5/s, 50/s and 500/s using an experimental methodology that combines a servo-hydraulic tester and high speed digital image correlation. Even though the strength increase with the strain rate is consistent between the four different materials, the total elongation increase with the strain rate varies widely. Some insights as to why this occurs from examination of the steel microstructure and variation of retained austenite with strain are offered.

Adiabatic heating during plastic straining can slow the diffusionless shear transformation of austenite to martensite in steels that exhibit transformation induced plasticity (TRIP). However, the extent to which the transformation is affected over a strain rate range of relevance to automotive stamping and vehicle impact events is unclear for most third-generation advanced high strength TRIP steels. In this study, an 1180MPa minimum tensile strength TRIP steel with carbide-free bainite is evaluated by measuring the variation of retained austenite volume fraction (RAVF) in fractured tensile specimens with position and strain. This requires a combination of servo-hydraulic load frame instrumented with high speed stereo digital image correlation for measurement of strains and ex-situ synchrotron x-ray diffraction for determination of RAVF in fractured tensile specimens.

Retained austenite stability to both mechanically induced transformation and athermal transformation is of great importance to the fabrication and in-vehicle performance of automotive advanced high strength steels. Selected cold-rolled advanced high strength steels containing retained austenite with minimum tensile strengths of 980 MPa and 1180 MPa were pre-strained to pre-determined levels under uniaxial tension in the rolling direction and subsequently cooled to temperatures as low as 77 K. Room temperature uniaxial tensile results of pre-strained and cooled steels indicate that retained austenite is stable to athermal transformation to martensite at all tested temperatures and pre-strain levels. To evaluate the combined effects of temperature and pre-strain on impact behavior, stacked Charpy impact testing was conducted on the same 980 MPa minimum tensile strength steel following similar pre-straining in uniaxial tension.

Impact toughness (or resistance to fracture) is a key material property for press hardened steel used in construction of the safety-critical elements of automotive body structures. Prior austenite grain size, as primarily controlled by the incoming microstructure and austenitization process, is a key microstructural feature that influences the impact toughness of press hardened steel. In this paper, a special Charpy V-notch impact test is developed to quantify the impact toughness of press hardened steel sheets with various prior austenite grain sizes, by stacking a number of thin sheets via mechanical riveting. Both the ductile-to-brittle transition temperature and upper shelf energy are analyzed in an effort to establish a correlation between impact toughness and prior austenite grain size. Within tested conditions, impact performance shows only a slight decrease as the prior austenitic grain size increases from 18 to 38 microns.