Nanocrystalline Advanced High-Strength Steel

Produced by Cold Rolling and Annealing

D. C. Van Aken and D. M. Field

Materials Science and Engineering

Missouri S&T

3

Center launched July 1,

2013

•12 industrial members

•$750,000 current research

•7 funded research projects

−Refractory technology (2)

−Deformation processing (1)

−Casting technology (1)

−Steel alloy development (1)

−Foundry technology

3rd 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

3rd Generation Property Space

• Survey work to

determine chemistry

& properties

• Batch Annealed

material promising

route to 3rd Gen

AHSS

• All alloys produced

and tested are shown

What is Two-Stage TRIP?

= γ-austenite = ε-martensite = α-martensite

Two-Stage Grain Refinement

X

Y

X

Y

Athermal transformation

• ε-martensite segments the austenite

• 6 variants of a-martensite segments the ε-martensite

Carbon’s Effect Upon ε- MS

• 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

• Prefer Mse >Msa Koyama et al., Mater. Sci. Eng. A

vol. 528, (2011) 7310-7316.

Thermodynamic Modeling

• 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

• n=4 for e-martensite

• Reported SFE are RT calculated

Thermodynamic Model for MSε Temperature

Composition limits in wt.% of the 90 alloys from literature

S&T

MSε Map for FeMnSiAl Alloys

• Carbon and nitrogen fixed at 0.07 wt.% and 0.017 wt.%

• n=4 in SFE calculation

Modeling of α-Martensite

• Investigation of 39 alloys

− Dual-TRIP alloys shown as black points

• Driving force relationship to Msα

− Two distinct behaviors

≥ 12 wt% Mn

< 12 wt% Mn

• Normalization with modulus collapses data to single trendline

Composition limits in wt% of the 39 alloys

C Si Mn Al N Cr Ni

Min 0 0 0 0 0 0 0

Max 0.3 3.09 15.7 2.44 0.614 17 15.1

Ave 0.08 1.8 8.2 1.3 0.126 8.9 5.8

Thermodynamic Model for MSa Temperature

S&T

MSa Map for FeMnSiAl Alloys

• Carbon and nitrogen fixed at 0.07% and 0.017%

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

Vo

lum

e P

erc

en

t

Percent Elongation

γ-austenite

α-martensite

ε-martensite

Two Stage TRIP: Interrupted Tensile Test

• 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

Stage

I

Stage

II

Stage

I

Stage

II

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

Processing of Ingots

• Normalization − 2 hrs at 1100 oC (2010 oF) − Air cooled

• Mill castings • Hot rolling

− 950 oC (1740 oF) − 85% reduction − Air cooled after final pass

− 790-760 oC (1450-1400 oF) • Batch annealing

− Cold roll 2-3 times % Elongation − Anneal 600 oC (1110 oF) − 20 hours − Air cool

• Tensile testing ASTM-E8

Hot Band vs. Cold Worked & Batch Annealed

• 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

Hot Band Batch

Annealed

The Aluminum Effect

• 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

Hot Band Batch Anneal

Decreasing

Al

Processing and Properties

• Hot band properties

− Not all alloys within

target window

− 1000 – 1500 MPa

− 30 – 20 %elongation

• Batch annealing (BA)

− All alloys meet or

exceed target

properties

Grain Size Determination: EBSD-OIM

• EBSD scan performed at 15,000 x magnification

− 20.0 kV accelerating voltage

− 0.025 μm step size

• Phase map (bottom)

• Nano-sized grains measured

− g – 130 nm

− e - 87 nm

− a - 120 nm

γ - GREEN

ε - RED

α - BLUE

Grain Size Dependence

• 1/D dependence on

yield strength

• Some alloys have higher

than anticipated

strengths

Alloy Yield Strength

(MPa)

γ-austenite

(μm)

e-martensite

(μm)

α-ferrite

(μm)

Recrystallized

(%)

Substructure

(%)

Deformed

(%)

-2.2 SFE 615 0.347 0.206 0.287 86 12 2

-1.8 SFE 789 0.130 0.087 0.120 72 23 5

-0.2 SFE 837 0.185 0.231 0.181 43 42 15

5.0 SFE 595 0.513 0.378 0.241 85 13 2

7.8 SFE 850 0.247 N/A 0.211 71 17 12

13.3 SFE 855 0.280 0.151 0.255 66 27 7

[32]

[33]

Grain Substructuring

• Recrystallization measured according to angular misorientation

• Deformed (red)

− Grains with internal angular misorientation >7o

• Substructured (yellow)

− Grains with internal angular misorientation <7o

• Recrystallized (blue)

− Grains with internal angular misorientation <1o

• Yield Strength expected to increase with:

− Sub-cell formation (yellow)

− Greater degree cold work (red)

5.0 SFE

-0.2 SFE

Measured Degree of Recrystallization After BA

Alloy Recrystallized

(%)

Substructure

(%)

Deformed

(%)

-2.2 SFE 86 12 2

-1.8 SFE 72 23 5

-0.2 SFE 43 42 15

5.0 SFE 85 13 2

7.8 SFE 71 17 12

13.3 SFE 66 27 7

γ - GREEN

ε - RED

α - BLUE

Microstructural Effects

• Previous relationship established on HB condition

• Lattice parameter change combined with Vf of phases present

• New alloys fit with previous HB dual-TRIP alloys

− Ar-stirred alloys have greater elongation for equivalent volume change

Phase Prediction

• Combination of Msε and Msα to predict phases

• Retained is similar to Koistinen-Marburger relationship

Summary

• Cold working and batch annealing leads to higher

yield strengths and 3rd 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

2016 Heat Campaign: Casting Design

• Casting Design − Casting weight: 163 lbs

− Pouring time: 14 seconds

− Gating ratio: 1:2:2

− “step-down” runner to

trap slag

− 25 lb finished ingot for

hot rolling

− addition of Nb to alloys

− hole expansion testing

Acknowledgements

• Kent D. Peaslee Steel Manufacturing Research Center

• Industrial mentoring committee

− Eric Gallo, Weiping Sun (NUCOR)

− Todd Link (US Steel)

− Luis Garza (AK Steel)

− Narayan Pottore, Bernard Chuwulebe (ArcelorMittal)

• Previous 3rd Gen AHSS graduate students

− Meghan McGrath Ph.D

− Scott Pisarik M.S

− Krista Limmer Ph.D