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

• Background and Motivation
• AHSS Implementation – Current State
• 3rd Generation Steel Nomenclature and Qualification
• Application Philosophy
• Identified Applications

• 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:
• 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
• 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)