Precipitation of Nb in ferrite after austenite conditioning I. Gutierrez, A. Iza-Mendia, A. Altuna, B. Pereda CEIT and Tecnun (University of Navarra), Manuel de Lardizábal 15, 20018 Donostia-San Sebastián Spain
the other co-authors Amaia Iza-Mendia Alazne Altuna Beatriz Pereda
CEIT is a non profit Research Centre working in partnership with the University of Navarra Donostia-San Sebastian www. ceit .es www. tecnun .es
INTRODUCTION Strip hot rolling mill Reheating Hot rolling Run-out Coiling table The hot rolling mill transforms as-cast steel into finished or semifinished products it can be considered as a tool for getting tailored combinations of mechanical properties
INTRODUCTION Strip hot rolling mill Reheating Hot rolling Run-out Coiling table The main role of Nb microalloying is the control of the austenite microstructure in the hot rolling mill Nb: Ferrite grain • Austenite conditioning - Solute drag refinement - Precipitation
INTRODUCTION Strip hot rolling mill Reheating Hot rolling Run-out Coiling table Over the last 30 years most of the research on Nb microalloyed steels has concentrated on the hot rolling mill
INTRODUCTION S Vervynckt et al. International Materials Review, 2012 Higher requirements in terms of mechanical property at minimum cost Need for process and additions optimisation
INTRODUCTION Reheating 1400 1200 1000 Temperature 800 600 Nb solubility in 400 austenite 200 0 0 50 100 150 200 250 300
INTRODUCTION Reheating Hot rolling 1400 1200 1000 Temperature 800 600 400 Nb in solution 200 0 0 50 100 150 200 250 300
INTRODUCTION Reheating Hot rolling 1400 1200 1000 Temperature 800 600 400 Nb in solution 200 0 0 50 100 150 200 250 300
INTRODUCTION Reheating Hot rolling Run-out 1400 table 1200 1000 Temperature Strain induced precipitation 800 600 400 Nb in solution 200 0 0 50 100 150 200 250 300
INTRODUCTION Reheating Hot rolling Run-out 1400 table 1200 1000 Temperature 800 Coiling 600 400 Nb in solution 200 0 0 50 100 150 200 250 300
INTRODUCTION Strip hot rolling mill Reheating Hot rolling Run-out Coiling table Focus: extra strengthening associated with Nb above that expected from grain refinement
INTRODUCTION • Interphase precipitation Nb in solution • Homogeneous precipitation after austenite • Cluster formation conditioning • Hardenability (transformation dislocations) Complex interactions, depending on Nb free and cooling strategies
INTRODUCTION “ There is still a certain disagreement with regard to the morphology of the precipitates” “There is no definite answer to the question whether precipitation of Nb carbides significantly contribute to the strength or not” Steel Reseach, 2004
OBJECTIVES • Study the potential precipitation of Nb in ferrite during coiling. • Estimate the precipitation strengthening
EXPERIMENTAL C Si Mn Al Nb N 0.06 0.35 1.00 0.047 0.056 0.006 Plane strain compression + simulated coiling Modelling assisted design of the thermomechanical sequences
EXPERIMENTAL C Si Mn Al Nb N 0.06 0.35 1.00 0.047 0.056 0.006 Mechanical testing & microstructural characterisation
EXPERIMENTAL Sequence S1 : 1400 1250ºC, 15 min 1100ºC, 1s-1, =0.3; + 20s holding 1200 1000 Temperature 800 600 Recrystallized 400 750ºC Coiling & 300ºC Nb in solution 200 0 0 50 100 150 200 250 300
EXPERIMENTAL Sequence S2 : 1400 1250ºC, 15 min 1100ºC, 1s-1, =0.3; + 20s holding 1200 1000ºC, 1s-1, =0.3 1000 Temperature 800 600 Strained Coiling 400 750ºC & Some strain induced 300ºC 200 precipitation of Nb 0 0 50 100 150 200 250 300
EXPERIMENTAL S1 and S2 Reference tests 1400 1250ºC, 15 min 1100ºC, 1s-1, =0.3; + 20s holding 1200 1100ºC, 1s-1, =0.3 1000 Temperature 800 870ºC; 1h 600 400 Rex or Strained Coiling & 200 (650ºC) full Nb precipitation 0 0 50 100 150 200 250 300
PRECIPITATION IN AUSTENITE: REFERENCE TEST 870ºC Nb precipiation model predictions B. López et al., 2006
TEM ANALYSIS Reference tests S1-R S2-R
MICROSTRUCTURE S1 Coiling T 750ºC 0.06%C-0.056%Nb 600ºC T coiling <600ºC Bainitic 500ºC microstructures
TENSILE PROPERTIES
TENSILE PROPERTIES + FERRITE GRAIN SIZE
CONTRIBUTIONS TO YIELD STRENGTH solid solution dislocations 1 / 2 k d y 0 ss d ppt lattice grain size precipitates friction 54 32 Mn 83 Si 5544 ( N C ) 17 . 4 d 1 / 2 yF free free Pickering, 1993
CONTRIBUTIONS TO YIELD STRENGTH 1 / 2 54 32 Mn 83 Si 5544 ( N C ) 17 . 4 d yF free free C free equilibrium N free was determined by modelling yNb y yF exp erimental
EXTRA STRENGTHENING DUE TO Nb 0.06%C-0.056%Nb
EXTRA STRENGTHENING DUE TO Nb 0.06%C-0.056%Nb Full precipitation in
EXTRA STRENGTHENING DUE TO Nb 0.06%C-0.056%Nb Reducing the 0.13%C-0.02%Nb Nb content
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE Coiling at : 750ºC 600ºC S1 IQ-EBSD y Nb (MPa) 90 150
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE Coiling at : 750ºC 600ºC S1 IQ-EBSD y Nb (MPa) 90 150
EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE Coiling at : 750ºC 600ºC S1 IQ-EBSD y Nb (MPa) 90 150
TEM ANALYSIS Thin foil Coiling at 750ºC GB Precipitation at the transformation front
CCT CURVES
CCT + INTERPHASE PTT CURVES T. Sakuma and R.W.K. Honeycombe: 1984
CCT + INTERPHASE PTT + COILING CURVES Coiling at 750ºC
TEM ANALYSIS Coiling at 750ºC • Nb-rich precipitates : • in few regular parallel rows (spacing 1300 nm) • in some segments of rows • irregularly distributed Discontinuous interphase precipitation • AlN nucleated at dislocations
CCT + INTERPHASE PTT + COILING CURVES Coiling at 600ºC
TEM ANALYSIS Thin foil Coiling at 600ºC Homogeneous precipitation in ferrite
TEM ANALYSIS Carbon extraction replica Coiling at 600ºC Cu Cu Nb Fe Al Cu Nb 0 2 4 6 8 10 12 14 16 From the Cu-grid
PRECIPITATION STRENGTHENING ASHBY-OROWAN'S EQUATION f d v ( MPa ) 10 . 8 ln ppt fv: volume fraction 4 d 6 . 125 10 d: average precipitate diameter in m
PRECIPITATION STRENGTHENING Coiling at 650ºC
STABILITY Sequence S1 : 1400 1250ºC, 15 min 1100ºC, 1s-1, =0.3; + 20s holding 1200 1000 Temperature 800 600 600ºC; 24 or 48h 400 Long holding at Coiling 200 temperature 0 0 50 100 150 200 250 300
TENSILE S1 + holding at 600ºC The tensile properties are not affected by a prolonged holding at 600ºC
TEM ANALYSIS S1 + 48h holding at 600ºC Baker-Nutting orientation relationship
CONCLUSIONS • Coiling at 600ºC produces in this steel an homogeneous, extremely fine and stable general precipitation of NbC in ferrite, leading to maximum strengthening: • proportional to the Nb left in solution in austenite; • in agreement with the predictions from Ashby- Orowan’s equation. • Considering homogeneous precipitation of NbC in ferrite as a possible strengthening mechanism opens the way to a better design of the composition and thermomechanical sequences for an improved use of Nb additions.
ACKNOWLEDGEMENTS • To the Institute of Materials, Minerals and Mining (IOM3) • To Companhia Brasileira de Metalurgia e Mineração (CBMM) • To Beta Technology • To Naila Croft, Ben Mico and Georgia Gomes Bemfica.
Modelling to Optimise the Processing of Niobium Steels 50
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