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