Nanocrystalline Advanced High- - Strength Steel Produced by Cold Rolling and Annealing
- 2018-11-29 13:22:00
- LUQIMENG Original
- 2740
3 rd Generation Advanced High-Strength Steel
? NSF / DOE funded work on Nano Acicular Duplex Steels
? Meghan McGrath Ph.D. (2012)
? Krista Limmer Ph.D. (2014)
? PSMRC funded work on Two-stage TRIP AHSS
? Scott Pisarik M.S. (2014)
? Dan Field Ph.D. (2018)
? Industrial partners: Nucor, AK Steel, USS and ArcelorMittal
? Survey work to determine chemistry & properties
? Batch Annealed material promising route to 3 rd Gen AHSS
? All alloys produced and tested are shown
Two-Stage Grain Refinement
Athermal transformation
? ε-martensite segments the austenite
? 6 variants of a-martensite segments the ε-martensite
? Increased C content decreases the martensite start temperature for ε- martensite
? Carbon content should be kept below 0.2 wt% to ensure TRIP effect at room temperature
? Gibbs free energy for the γ→ε phase transformation can be expressed using a regular solution model
? Stacking fault energy SFE describes ε-martensite stability relative to γ-austenite
? Stage I - austenite TRIPs to e-martensite ? low work hardening rate, but e segments the austenite to smaller volumes
? Stage II – e TRIPs to a-martensite with high work hardening rate
Foundry Casting Practice
? Argon cover during melting
? Calcium wire additions to modify and remove sulfides
? In-situ chemical sampling and adjustments
? Hereaus Electronite Celox oxygen probe
? Verichek Foundry-Master UV Arc spectrometer
? LECO TC 500 Nitrogen/Oxygen analyzer
? LECO CS 6000 carbon/sulfur analyzer
? Ladle with teapot dam and lip pour
? Hot band properties related to degree of recrystallization
? Two stage TRIP in all compositions after batch annealing
? High yield strengths after cold working and batch annealing
? Effect of Al on recrystallization
? Change in hot band tensile response based on Al content
? Batch annealed material regains dual-TRIP
? Al potentially increases ease of dynamic recrystallization in hot band steels
? Recrystallization measured according to angular misorientation
? Deformed (red)
? Grains with internal angular misorientation >7 o
? Substructured (yellow)
? Grains with internal angular misorientation <7 o
? Recrystallized (blue)
? Grains with internal angular misorientation <1 o
? Yield Strength expected to increase
with:
? Sub-cell formation (yellow)
? Greater degree cold work (red)
? Previous relationship established on HB condition
? Lattice parameter change combined with V f of phases present
? New alloys fit with previous
HB dual-TRIP alloys
? Ar-stirred alloys have greater elongation for equivalent volume change
? Combination of Ms ε and Ms α to predict phases
? Retained ? is similar to Koistinen-Marburger relationship
Summary
? Cold working and batch annealing leads to higher yield strengths and 3 rd generation property goals
? new models to predict martensitic reactions
? retained austenite related to difference in a and e martensite start temperatures
? model relating transformable products to elongation to failure
? 2016 heat campaign to scale up the casting
