Modelling and Simulation of Microalloyed Austenite During Multipass - PowerPoint PPT Presentation

Modelling and Simulation of Microalloyed Austenite During Multipass Deformation E.J. Palmiere Department of Materials Science & Engineering, The University of Sheffield Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK Charles Hatchett Symposium – 16 July 2014

Modelling Strategies KNOWLEDGE + DATA- Accuracy BASED HYBRID white + grey/black DATA-BASED modules BLACK BOX e.g. ANNs PROBABILISTIC GREY BOX e.g. CA, MC-P KNOWLEDGE-BASED GREY BOX KNOWLEDGE-BASED semi-empirical GREY BOX KNOWLEDGE-BASED equations of state e.g.  FE WHITE BOX constitutive equations Physical Insight Charles Hatchett Symposium – 16 July 2014

Introduction • Strain path changes non-linearly during TMP of metals. • Microstructure models were developed based on total strain, particularly at mid-thickness, and did not take the strain path effect into account. Prediction Measured 100 80 % Recrystallised 60 40 20 0 0 0.2 0.4 0.6 0.8 1 Centre Surface Through Thickness Comparison of the strain path angle evolution through the roll gap, calculated using 2D FE, Al-1%Mn alloy slab subjected to a 50% reduction. A.J. McLaren and C.M. Sellars, Mat. Sci. Tech , 1992, 8, 1090-1094 Charles Hatchett Symposium – 16 July 2014

Materials • Two Fe-30 Ni model alloys with different Nb concentrations • Focusing on alloy B (Nb-bearing) • Large equiaxed grains (> 200 μm ) with annealing twins 2  m Composition (wt%) C Si Mn S P Ni Fe Ti N Nb 0.08 0.25 1.75 0.013 0.010 30.9 66.8 0.003 0.004 0 A 0.09 0.27 1.73 0.016 0.010 29.8 67.8 0.003 0.004 0.09 B Charles Hatchett Symposium – 16 July 2014

Why Model Alloys? • Enable for the development of microstructural models of austenite conditioning (i.e., grain size, substructure, composition, etc.), and its role on subsequent transformation behaviour • Need for a non- transforming C- Mn microalloyed steel analogue Charles Hatchett Symposium – 16 July 2014

Model Alloy Comparison 300 Fe-30Ni-Nb 250 C-Mn-Nb Equivalent Stress (MPa) 200 150 100 Temperature: 900°C 50 ° C Strain Rate: 10 sec-1 0 0.0 0.1 0.2 0.3 0.4 0.5 Equivalent Strain Charles Hatchett Symposium – 16 July 2014

Method – Simulated Schedule • Three different torsion schedules to study the effect of strain path changes With the following passes: [100F] = No strain path change, 0.1 Forward - Strain path changes [83F-17R] = 0.083 Forward – 0.017 Reverse [75F-25R] = 0.075 Forward – 0.025 Reverse Each pass has a total strain of 0.1 Charles Hatchett Symposium – 16 July 2014

D.R. Barraclough, H.J. Whittaker, K.D. Nair and C.M. Sellars, J. Testing & Evaluation , 1973, 1, 220- 226 Charles Hatchett Symposium – 16 July 2014

Method – Simulated Schedule Temperature (° C) Hold at 1250°C Deformation passes of 0.1 strain (Nb in solution) Heat up 20s Quench Time 14 passes between 1100°C and 800°C, each pass has a total strain of 0.1 and strain rate of 1 s -1 and a gap of 20 seconds between passes Charles Hatchett Symposium – 16 July 2014

Simulated Schedule Results Charles Hatchett Symposium – 16 July 2014

Simulated Schedule Results Charles Hatchett Symposium – 16 July 2014

Simulated Schedule Results Charles Hatchett Symposium – 16 July 2014

Recrystallisation-Stop Temperature • Hot rolling schedules occur over many passes at different temperatures • Rolling schedules designed to avoid CCR Rolling passes occurring in partial recrystallisation region (T 5% <T<T 95% ) • Hence, determining T 5% is important E.J. Palmiere, C.I. Garcia and A.J. DeArdo , Metallurgical Transactions A , 1996, 27, 951-960 Charles Hatchett Symposium – 16 July 2014

Flow Stress Behaviour Recovery & Work Hardening T 5% Recrystallisation, Recovery & Work Hardening Increasing amount of effective strain per pass and/or decreasing solute supersaturation Charles Hatchett Symposium – 16 July 2014

Flow Stress Behaviour (without Nb) A-100F A-83F-17R A-75F-25R 960°C 910°C 940°C T 5% increases DEFORMATION TEMPERATURE • Increasing amounts of reversal (with same total strain) reduces the effective strain • Increasing amounts of reversal reduces driving force T 95% for recrystallisation • Reducing amount of recrystallisation T 5% • Increasing T 5% ε pas ε pas STRAIN s s Charles Hatchett Symposium – 16 July 2014

CA Model - Recrystallisation 0.1F-0.1F 0.1F-0.1R 1100°C – 0 sec 1095°C – 5 sec 1090°C – 10 sec 1085°C – 15 sec 1080°C – 20 sec Charles Hatchett Symposium – 16 July 2014

Flow Stress Behaviour (with Nb) • For sample with Nb, increased strain reversal influences two DEFORMATION TEMPERATURE factors: • Reduces driving force for recrystallisation (as with sample A) • Reduces formation of strain induced precipitates (SIPs) • SIPs inhibit grain boundary mobility and recrystallisation T 5% • The fall in T5% shows it is controlled by SIPs for sample B T 5% T 5% falls ε pas ε pas STRAIN s s B-100F B-83F-17R B-75F-25R 990C 978C 963C Charles Hatchett Symposium – 16 July 2014

Precipitation • Precipitates are extremely important to properties of microalloyed steels • Grain refinement • Precipitation hardening • Strain Induced Precipitates form on subgrain boundaries • Size and density important paramaters in how they influence behaviour • Precipitates have been analysed based on extraction replicas 110nm B75F-25R- (thin foil- left, replica- right Charles Hatchett Symposium – 16 July 2014

Precipitation B100 B75F-25R • All large precipitates measured (>100nm) by EDX consist of Ti and Nb • All precipitates measured are FCC with a≈4.5Å consistent of NbC • Density of precipitates and size distributions are significantly different between reverse and non- reversed samples 215nm Charles Hatchett Symposium – 16 July 2014

Small Precipitates Size nm – mean Sample Volume Fraction (stdev) B100 10.6 (3) 0.008 B75 9.5 (4) 0.003 • Similar size values • Volume fraction significantly larger in the sample not reversed: • less regions where they are found • lower density in those regions Charles Hatchett Symposium – 16 July 2014

Precipitation Model y x 100 nm 250 nm z Charles Hatchett Symposium – 16 July 2014

Precipitation Model Charles Hatchett Symposium – 16 July 2014

Precipitation Model 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1 s 1 s 1 s 1 s 1 s 1 s 1 s 1 s 5 s 5 s 5 s 5 s 5 s 5 s 5 s 5 s 15 s 15 s 15 s 15 s 15 s 15 s 15 s 15 s 30 s 30 s 30 s 30 s 30 s 30 s 30 s 30 s 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 50 s 50 s 50 s 50 s 50 s 50 s 50 s 50 s 100 s 100 s 100 s 100 s 100 s 100 s 100 s 100 s 200 s 200 s 200 s 200 s 200 s 200 s 200 s 200 s 500 s 500 s 500 s 500 s 500 s 500 s 500 s 500 s Dependence of 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 z/w z/w z/w z/w z/w z/w z/w z/w concentration of niobium as a function 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 of fractional distance from first generation 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 microbands on time of holding at 950 ° C after single pass 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 deformation leading to C zt /C o C zt /C o C zt /C o C zt /C o C zt /C o C zt /C o C zt /C o C zt /C o w = 720 nm Charles Hatchett Symposium – 16 July 2014

Precipitation Model 1.0 1.0 1.0 1.0 1.0 Nucleation Nucleation Nucleation Nucleation Nucleation 0.8 0.8 0.8 0.8 0.8 Coarsening Coarsening Coarsening Coarsening 0.6 0.6 0.6 0.6 0.6 Change in particle N/N o N/N o N/N o N/N o N/N o density with time on generation one 0.4 0.4 0.4 0.4 0.4 microbands after pass one, on generation two 0.2 0.2 0.2 0.2 0.2 microbands after pass two and on generation                               0.0 0.0 0.0 0.0 0.0 three microbands after 0 0 0 0 0 20 20 20 20 20 40 40 40 40 40 60 60 60 60 60 80 80 80 80 80 100 100 100 100 100 pass three at 950ºC Time (s) Time (s) Time (s) Time (s) Time (s) Charles Hatchett Symposium – 16 July 2014

Precipitation Model Charles Hatchett Symposium – 16 July 2014

Precipitation Model Charles Hatchett Symposium – 16 July 2014

Precipitation Model Charles Hatchett Symposium – 16 July 2014

Precipitation Model Charles Hatchett Symposium – 16 July 2014

Final Microstructure: Sample A without Nb (low magnification) 1000μm 0.72 Plane strain=1.4 strain rate=1 A-100F-0R A-75F-25R Centre strain=0 strain rate=0 Charles Hatchett Symposium – 16 July 2014

Final Microstructure: Sample A without Nb • Grains are elongated towards shear angle A83F-17R- Final • With reversal compared to without: • Larger grain size • Less misorientation in grains A100 - Final A75F-25R- Final torsion direction 500μm Charles Hatchett Symposium – 16 July 2014