Grain growth, precipitate state and microstructure evolution in an Nb-alloyed PHFP (AFP) steel

Transcription

1 Grain growth, precipitate state and microstructure evolution in an Nb-alloyed PHFP (AFP) steel Master s Thesis Presentation by Mamta Sharma, M. Sc. Steel Institute, RWTH Aachen Supervisors: Univ. Prof. Dr.-Ing. W. Bleck Dr.-Ing. Ulrich Prahl Dipl. Ing. Konstantin Schacht

2 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

3 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

4 Motivation Impetus for development of microalloyed steels Increasing energy costs; Risk of distortion, quench cracking and hardness variations; Growing competition from cast components. Case Hardening Steels 13% C Steels (untempered) 37% PHFP (AFP) Steels [PROZENTS ATZ] Quenched and Tempered Steels 3% Quenched and Tempered Steels Microalloyed Steels Advantages Shorter process, hence less sources of variations; Saving energy and Production time; Mechanical properties between castings and QT steels. Current Status A major share of forging steels AFP / PHFP; Increasing applications, esp. in automotive industry Crankshaft, Connecting rod, Wheel hub, Steering knuckle; Further advancement of microalloying concept; Optimization of complete forging process cycle.

5 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

6 Process Overview and Scope of the work Aim: To simulate all the steps of forging cycle on a lab-scale, and optimize process parameters further, Apply and test the new process parameters to an industrial forging cycle.

7 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

8 Material Steel grade: AFP (Ausscheidungshärtende Ferritisch-Perlitische Stähle or PHFP: Precipitation Hardened Ferritic- Pearlitic steels). Alloying element, wt% C Si Mn P S Cr Mo Ni Al Nb Ti V N(ppm) PHFP Medium C content: To get sufficient pearlite fraction, Microalloying additions: Grain refinement + Precipitation hardening, Nitrogen: Promote formation of stable microalloy nitrides and/or carbo-nitrides.

9 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

10 Experimental Equipment and tests Hot flow behavior-single hit hot compression test Induction coil Experimental Chamber Distance measuring system Laser Equipment: Deformation dilatometer, DIL A/D 805; Heating: Inductive; Atmosphere: Helium; Punches for deformation: Quartz; Sample Geometry: cylindrical (l/d = 9/5, mm). Significance of the single hit hot compression test Necessary to estimate the forming load; Information on material behavior-physical phenomena (RX matrix); Aids in selection of optimum process conditions. Significance of the double hit and stress relaxation tests Necessary for determination of static recrystallization kinetics (softening) between the forging steps 5 mm 9mm

11 Parameters for SH hot compression tests Two types of process cycles for steel grade AFP4: i. Short cycle - without reheating ii. Long cycle with reheating Two different heating rates: i. 4.5 Cs -1 ii. 106 Cs -1 Temperature range: 1000 to 1200 C Strain rate range: 0.01 to 10 s -1 Short cycle Long cycle Material Heating rate ( Cs -1 ) Temperature ( C) Strain rate (s -1 ) ; ; 1; 10 PHFP ; 1050; 1100; 1150; ; 0.1; 1; 10

12 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

13 Results and Discussion Microalloy Precipitate State- Transmission Electron Microscopy Mainly Nb(C,(N)) with traces of Ti and V; Few Ti,V(C,N); Fine initial precipate state is observed upto 1100 C for short cycle Almost complete dissolution for austenitization at 1250 C/ 5 min. `Long cycle! No. of Min. Max. Mean Test Condition particles dia. dia. dia. analyzed (nm) (nm) (nm) Short cycle; 1050 C; 10s Short cycle; 1100 C; 10s Short cycle; 1200 C; 10s

14 Results and Discussion Long Short SC DRX single peak type, Fine austenite grain. LC RV type flow behavior, Coarse γ + non-drx regions only SRX. SC fine microalloying precipitates + in solution Zener drag LC Nb in solution strong solute drag to hinder DRX Non-RX-region DRX γ = 12 µm 1000 C; 10 s -1 DRX γ = 27 µm 1200 C; 10 s -1 RV+ SRX γ = 23 µm 1000 C; 10 s -1 RV + SRX γ = 40 µm 1200 C; 10 s -1

15 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

16 Learnings from the laboratory-scale experiments The most important result of the lab-scale tests- Short process cycles, employing higher heating rates, can result in: extremely fine RX austenite fine precipitate state, improved mechanical properties, shorten the process times considerably, more economical. Industrially applicable??

17 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

18 Industrial test forging at an Industrial Partner Process cycles used for industrial forging tests: Forced air + Quasi-isothermal cooling Air cooling Selection of samples for microstructure Characterization (Core, Pos. 2)

19 Industrial test forging at an Industrial Partner Influence of process cycles on microstructure 1250 C / Conventional cycle (long) 1100 C / New process cycle (short) Iso Air Long cycle: Little influence of cooling cycle, relatively coarse F/P; Short cycle: Much refined final microstructure, owing to fine RXed austenite + Isothermal cooling homogeneous F/P.

20 Industrial test forging at an Industrial Partner Influence of process cycles on microstructure (Contd.) Considerable fraction of fine ppt. even at 1100 C - TEM Investigations!! Ferrite fraction (%) Cooling Long cycle C Short cycle C Iso Air Increase in ferrite fraction; However, greater refinement, by ~6 times!

21 Outline Motivation Process overview and scope of the work Material Experimental Equipments and tests Results and Discussion Learnings from the laboratory-scale experiments Industrial test forging at an Industrial partner Conclusions

22 Conclusions Nb alloying combined with a lower forging temperature and higher heating rate has been used to favor DRX and freeze the dynamically recrystallized microstructure. This leads to a remarkably fine microstructure Short process cycle is thus a promising prospect to make the process faster and economical, while ensuring better mechanical properties than the conventional cycle!!

23 Thank you for your attention Vielen Dank für Ihre Aufmerksamkeit The thesis was conducted within the framework of the project Nb- Schmiedesimulation. Financial support and organisation of the AiF and IMU is gratefully acknowledged!

24 Vielen Dank für Ihre Aufmerksamkeit

25 Material Chemical Composition of AFP steels analyzed Alloying element, wt% AFP1 AFP2 AFP4 AFP5 AFP6 AFP7 C Si Mn P S Cr Mo Ni Al Co Cu Nb Ti V W N(ppm) Fe Medium C content 64 to 211 ppm N AFP4- central- high Nb and V content

26 Results and Discussion Hot flow curves-afp4-hr 4.5 Cs -1 Temp. ( C) Short cycle and 4.5 Cs RX RX RX 1000 RX RX RX Strain rate (s -1 ) Temp. ( C) Long cycle and 4.5 Cs RX RX RV 1000 RV RV RV Strain rate (s -1 ) As T and/or strain rate, peak flow stress Peak stress values are nearly same for short and long cycles (+-%), Steady state stress is lower for short cycle. However, physical phenomena are different!!

27 Results and Discussion SRX kinetics by DH; 4.5 vs 106 Cs -1

28 Results and Discussion SRX kinetics-dh test-afp4-hr 106 Cs-1 Short cycle Long cycle γ size = µm (1050 C; 0.5 s -1 ) γ size = µm (1050 C; 0.5 s -1 ) Austenite grain boundaries nucleation sites for RX, Fine grain size Larger grain boundary area, SRX kinetics ~ f(surface area of grain) ~ f(d 2 ) C SRX kinetics is ~ 5 times faster for short cycle as compared with long one.

29 Results and Discussion SRX kinetics-dh test-afp4-hr 106 Cs-1 (Contd.) Short cycle Long cycle γ size = µm (1200 C; 0.5 s -1 ) Approximate estimation of SRX kinetics based on grain size: At 1050 C, (( )/( ))= 3.4 times At 1200 C, (( )/( ))= 1.4 times γ size = µm (1200 C; 0.5 s -1 ) 1200 C SRX kinetics is ~2 times faster for short cycle than for long cycle;

30 Results and Discussion Hot flow behavior of other AFP steels Temp. ( C) All AFP Steels 1200 RX RX RX-RV 1000 RX RX-RV RV Strain rate (s -1 )

31 Results and Discussion SRX kinetics- All AFP steels Time for SRX Steel grade (s) AFP AFP AFP AFP AFP AFP Flow behavior ~ Precipitate state and austenite grain size. AFP5: Fastest SRX, t SRX = 9.30 s, low Nb and Al, Low overall alloy content AFP4, 6 and 7: sluggish SRX or longer t SRX, higher Nb content, high overall alloy content. Industrial relevance: Similar range of forces to deform med. C AFP steels Hence same forging equipment SRX not likely between passes, unless intentionally desired