Opportunities and Challenges for 3rd Generation Advanced High-Strength Steels in Automotive Body Structures

? Remaining Challenges and Next Steps

? The need for increased AHSS usage is required to maintain steel as the material-of-choice for light-weighting pending finalization of 2025 EPA/NHTSA mandates.

? Meanwhile, mixed material strategies for lightweight body structure design are strong competitors for steel intensive structures.

? Optimize Applications of Materials
– Right Material in the Right Application
? Mixed Material Strategy
– Advanced and Ultra High-Strength Steels
– Aluminum Sheet, Castings and Extrusions
– Balance Above Dictated by Cost/Performance Targets

? GM issued a worldwide technical specification, using strength and ductility based nomenclature, for  retained austenite bearing (TRIP) steels. The types of steels covered are:
? TRIP
? TBF (TRIP-Aided Bainitic Ferrite)
? CFB (Carbide Free Bainitic TRIP)
? Q&P (Quench and Partition TRIP)
? Medium Manganese TRIP/ TRIP-TWIP Ultimate tensile strength levels range from 690 MPa to 1180 MPa for 3rd generation TRIP steels. Significantly less for medium manganese steels.

? To assist with global grade definition and material homologation, GM has the issued a worldwide technical specification for retained austenite bearing steels.

? Nearly a decade of development between the global steel and automotive industries has resulted in production applications of third generation AHSS
? The first application on a GM vehicle → 2016 Chevy Sail (Quench and
Partition, 980 MPa Min. Tensile Strength – 15% Min. Total Elongation @ ASTM
E8) – GMW17627
? Increased global applications require greater region-by-region availability
? Selected grades identified for global development
? Qualification in-process for select grades at 980 MPa and 1180 MPa strength levels.
? Multiple additional applications are under consideration.

Material Characterization - Microstructure
? Material Performance
? Uniaxial Tensile Property Statistics (Multiple Heats)
? Mill Capabilities (Gauge-Width)
? High Rate Tensile Behavior
? Temperature Variability of Tensile Properties
? Fracture Toughness (Stacked Charpy V-Notch)
? Bendability
? FLD
? Tension-Compression
? Hole Expansion Ratio / True Strain at Fracture
? H Embrittlement Sensitivity (GMW17058 and SEP1970)

? Direct materials substitution for light-weighting
? Enhanced strength/elongation at given gauge for energy absorption
(enhanced crashworthiness)
? Cost reduction via PHS replacement
? Part consolidation enabled by enhanced formability
? Geometric optimization due to enhanced formability
? Potential applications identified for A-, B-Pillar, Roof Rail, Roof Bow and
underbody reinforcements.

The good news...
? Global supply of 3rd generation TRIP steels is increasing, and supply and technical risk tolerance appears increasingly “manageable.”
? TRIP/TBF/Q&P display equivalency in tested metrics (via Pre-Qualification Testing)

Ongoing Challenges…
? Liquid Metal Embrittlement of Spot Welded Zn-Coated TRIP Steels
? High Strain Rate Behaviors - as an Example of Fundamental (Inherent) Performance Analysis Challenges of Medium Manganese Steels
? Incorporation of UHSS and AHSS Fracture Phenomena in Design and CAE Best Practices
? Spot Welding and Spot Weld Strength
? Hydrogen Cracking and Low Yield Strengths (Medium Manganese Steels)