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
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 3
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
3 rd Generation Property Space Survey work to determine chemistry & properties Batch Annealed material promising route to 3 rd Gen AHSS All alloys produced and tested are shown
What is Two-Stage TRIP? = γ-austenite = ε-martensite = α-martensite
Two-Stage Grain Refinement Y Y X X Athermal transformation ε-martensite segments the austenite 6 variants of a-martensite segments the ε-martensite
Carbon s Effect Upon ε- M S 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 Koyama et al., Mater. Sci. Eng. A vol. 528, (2011) 7310-7316. Prefer Ms e >Ms a
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 M S ε Temperature S&T Composition limits in wt.% of the 90 alloys from literature
M S ε 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 Ms α Two distinct behaviors 12 wt% Mn < 12 wt% Mn Normalization with modulus collapses data to single trendline
Thermodynamic Model for M S a Temperature S&T 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
M S a Map for FeMnSiAl Alloys Carbon and nitrogen fixed at 0.07% and 0.017%
Volume Percent Two Stage TRIP: Interrupted Tensile Test 100 Stage I Stage II 80 Stage I Stage II 60 40 γ-austenite α-martensite ε-martensite 20 Stage I - austenite TRIPs to e-martensite 0 0 5 10 15 20 25 30 35 40 Percent Elongation 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
Processing of Ingots Normalization 2 hrs at 1100 o C (2010 o F) Air cooled Mill castings Hot rolling 950 o C (1740 o F) 85% reduction Air cooled after final pass 790-760 o C (1450-1400 o F) Batch annealing Cold roll 2-3 times % Elongation Anneal 600 o C (1110 o F) 20 hours Air cool Tensile testing ASTM-E8
Hot Band vs. Cold Worked & Batch Annealed Hot Band 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
The Aluminum Effect Hot Band Batch Anneal Decreasing Al 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
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 [33] [32] Alloy Yield Strength γ-austenite e-martensite α-ferrite Recrystallized Substructure Deformed (MPa) (μm) (μm) (μm) (%) (%) (%) -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
Grain Substructuring 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) 5.0 SFE -0.2 SFE
Measured Degree of Recrystallization After BA γ - GREEN ε - RED α - BLUE 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
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 Microstructural Effects
Phase Prediction 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
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 3 rd Gen AHSS graduate students Meghan McGrath Ph.D Scott Pisarik M.S Krista Limmer Ph.D