Nanocrystalline Advanced High- - Strength Steel Produced by Cold Rolling and Annealing

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