HOT DEFORMATION EFFECTS ON AUSTENITE DECOMPOSITION

Transcription

1 OUTLINE---Contents HOT DEFORMATION EFFECTS ON AUSTENITE DECOMPOSITION 3-7 Parameters, techniques 8-17 Ferrite, TTT, acceleration Nucleation and growth Composition: low C tool steels H. J. McQueen, E. V. Konopleva, V.M. Khlestov* Concordia University, Montreal, QC, Canada H3G 1M8 * Priazovsky State Technical University, Mariupol, Ukraine Industrial, CCT Intercritical rolling Bainite, TTT, CCT Summary, bibliography 1 2 HOT DEFORMATION EFFECTS ON AUSTENITE DECOMPOSITION Significant parameters: Steel character Thermomechanical processing Transformation conditions to ferrite/pearlite, bainite Composition ---C content ---Eutectoid ---Alloy content ---Tool steels Preheat Temperature ---Homogeneity ---Grain size ---Stable particles Steel character Structure at rolling ---Stable austenite ---Intercritical ---Alloy carbides 3 4

2 Thermomechanical Processing Transformation conditions Rolling to fixed final dimensions for measurements Temperature ---Preheat ---Multistage ---Cooling rate ---Finishing Strain ---Single stage ---Multistage Time ---Interpass ---Delay after working ---Cooling rate to transformation 5 Rates Isothermal magnetometry ---Fraction vs Time ---TTT curves Continuous cooling ---Dilatometer/thermal analysis ---TTT curves Microscopy of product ---Phase, grain sizes ---Uniformity ---Nucleation sites and rates at various fractions transformed ---Growth rate Ferrite Pearlite Bainite Martensite Alloy carbides 6 γ α + PEARLITE TRANSFORMATION Hot Deformed Stable Austenite Isothermal curves to shorter times Time - Temperature - Transformation (T-T-T) diagram - Nose shifted - to shorter times - to higher temperature Continuous Cooling Transformation (CCT) diagram - Start line shifted upwards along cooling curves - Finish line shifted by half as much Balance magnetometer: 3) MAGNET POLES; 1) the specimen; 5), 9) balance beam and load cell; 4) bath of liquid tin; 6), 8) thermocouple; 7) control furnace; 10) potentiometer. (b) Experimental multistage rolling equipment: 1) specimen; 2) thermocouples; 3) and 6) furnaces; 4) wire to control specimen; 5) reversing rolls. Conflicting influences in CCT as cooling rate reduces: -shift from less undercooling -shift from longer time for substructure degradation Accompanied by grain refinement [240] 7 8

3 Ferrite isothermal transformation Ferrite, TTT Ferrite-pearlite transformation curves of 09Mn2VNb steel. Deformation at C. ε=0 (1); 25% (2). Transformation at 600 o C(a), 650 o C(b), 700 o C(c) Polymorphic transformation curves of 06Cr4Mn2 at C. ε=0 (1); 25% (2); 40% (3); 60% (4). Deformation at 1000 (a), 900 (b), 700 o C (c). Pearlite transformation curves of 35CrNi5Si (a) and 40CrNiMo (b) steels. Deformation at 1000 (2); 800 (3); 600 (4) and 500 o C (5). ε=0 (1); 50% (2-5). TTT diagrams: 70Cr3 (a); Heating:1000 o C, 35Cr3 (b) steels,5 min; Deformation at 800 o C. ε=0 (1); 25% (2); 50% (3). [240,335] 9 [240,335] 10 Ferrite, TTT, acceleration PARAMETER CONTROL RAISING ACCELERATION DEFORMATION OF STABLE AUSTENITE γ α + PEARLITE Greater strain ε, higher έ Lower deformation T D, (stable austenite) Finer γ grain size (lower preheat) Lower hardenability PREVENTION OF DEGRADATION Fewer passes at fixed ε Shorter intervals between passes Shorter delay to transform T T Austenite deformation to reductions of (1) 0%, (2) 20%, (3) 40%, (4) 60% Speeds up austenite decomposition in 0.06C-4.2 Cr-2.2Mn steel as shown in (a) isothermal transformation Acceleration removed by complete SRX (b) TTT diagram and (c, d) acceleration relative to zero strain [300,335,412] 11 12

4 Ferrite acceleration Thermocycle, grain size [240,300] Relative acceleration of 5% (a) and 50% (b) ferrite formation In deformed austenite of 17MnSi 10CrSiNiCu 09Mn2VNb steels 13 [279,412] Austenite grain size distribution 08Cr2Mn2V steel after Conventional heating and Thermocycling: o C 14 Thermocycle, TTT SRX, TTT, micros TTT-diagram of 08Cr2Mn2V steel. Conventional heating and thermocycling. [279,412] 15 [240,335] 16

5 SRX, micrographs MICROSTRUCTUAL CAUSES OF ACCELERATION γ α + PEARLITE Raising stored energy of austenite Deformation band boundaries - intersections with grain boundaries GB - attain misorientations like GB as ε Deformation substructure density - greater density as T D, ε, έ - reduced density from SRV or SRX during holding periods Grain boundaries strongly serrated due to substructure Nucleation primarily at GB - only 10% recognizably inside grains - finer grain size enhances - low C content (~ 0.04) enhances. [240] Nucleation, growth γ α + PEARLITE ACCELERATION Hot Deformed Stable Austenite Greater for -higher strain -less undercooling (x 5 to 40) Nucleation rate raised - x 10 3 for first 5% fraction (~ 2s) - x 5 at 50% transformed (~ 10s) Growth rate raised - x 1.2 at first 5% fraction (~ 2s) - x 5 at 50% transformed (~ 10s) Grains refined from enhanced nucleation [240,332,369] Effect of deformation at 820 o C on Nucleation (a, b) and Growth (c, d) rates of ferrite at 700 o C of 09Mn2VNb steel. ε=0 (1); 25% (2)

6 Nucleation, micro Nucleation, 2 steels Effects of deformation on Ferrite nucleation (a,b) and Growth (c, d) rates at 700 o C in 10CrSiNiCu 650 o C in 06Cr4Mn2 steels. Ferrite grains in 07Cr2Mn2B steel at 680 o C. x800. Deformation at 800 o C. ε=0 10% ferrite (a); ε=50%: 0.5% ferrite (b); ε=5% ferrite (c). [240] 21 [300,412] 22 Nucleation, micros Nucleation table Volume fraction of ferrite (y) formed during rolling in the intercritical temperature range [332,369] Nucleation of ferrite grains at early stages of γ α transformation in the deformed Steels x400; a) 07Mn2MoNb, 68% red. at 760oC; c) 09Mn2VNb, 68% red. at 770oC. Ferrite precipitates at the stage of 20-25% transformation in deformed steels x400; b) 07Mn2MoNb, 68% red. at 740 o C; d) 09Mn2VNb, 68% red. at 740 o C. Arrow shows ferrite nucleation at twin boundaries. 23 [332,369] 24

7 Nucleation, completion PARAMETER CONTROL RAISING ACCELERATION COMPOSITION, MORPHOLOGY γ α + PEARLITE P - Grain size smaller: - lower preheat T P, shorter time, - thermo-cycling - Reduced γ homogeneity: lower preheat T P - heterogenizing hold at reduced T H - (M2 versus A2 tool steel) - Carbon: peak at 0.04C, little influence above 0.09C Stable carbides: more, finer (M2 tool steel) - No influence of low alloying additions (for hardenability) Ferrite nucleation at austenite grain boundaries of 09Mn2VNb steel during rolling at 740 o C followed by quenching (a) or cooling at 0.8 o C/s (b). Austenitization at 1150 o C, rough rooling at 1000 o C in 3 passes and finish rolling at 740 o C in 5 passes to 20% reduction. - Hypo-, hyper-, eutectoid steels [300] Table: Designation and composition Acceleration vs C-content Enhancement of austenite transformation rate relative to zero strain by 20% reduction at 820 o C (much more so by 40%) Strongly dependent on C content below 0.8% Similar manner for a variety of low alloy steels [240] 27 [300,412] 28

8 Acceleration vs T 3 Tool steels, isothermal Effect of 25% reduction at o C on Relative acceleration of austenite transformation in the pearlite range in Steels of different compositions. Band of acceleration for 5% (1) and 50% (2) transformation. In M2(0.8C-4Cr-5Mo-7W-2V) tool steel, rolling to 50% total reduction in the range o C accelerates the austenite transformation Heating near 1150 o C slows it compared to annealing at 880 o C. Straining before transformation raises the final hardness by 10 to 20%. More hardening at higher T as a result of precipitation of fine carbides. [300] 29 [376,388] 30 3 Tool steels, TTT INDUSTRIAL EFFECTS γ α + P acceleration - advantages of quicker initial cooling upon finishing - reduced duration of cooling - higher permitted coiling T - associated with grain refinement Continuous Cooling Transformation (CCT) diagram - Start line shifted upwards along cooling curves - Finish line shifted by half as much Pearlite range TTT diagram of M2 ( S- beginning, F- end of transformation). Effects of changes in pre-heating T and from rolling under both T. Rolling schedule with 1150 o C preheat causes a net retardation in rate by a factor of 2. Straining increases the rate of transformation by a factor of 3 at 880oC. Conflicting influences in CCT as cooling rate reduces: -shift from less undercooling -shift from longer time for substructure degradation [376,388] 31 32

9 Ferrite, CCT, micro Ferrite, CCT, micro Combined isothermal (T-T-T) and continuous cooling (C-C-T) diagram (a) for 0.055C-1.9Mn-0.11V-0.28Mo steel the accelerating effects of austenite rolling Micrographs for similarly treated C-1.75Mn-0.06Nb steel (b) ferrite for 1 o C/S and (c) ferrite + some bainite for 10 o C/s. [240] 33 [280,412] 34 Intercritical, multistage Intercritical, micros (a) Continuous torsion tests: o C, 1s -1, 02MnNb (5 min.,1150 o C ) plus Multistage rolling simulation: passes 20% reduction, intervals 120s. At 800 o C, pass curves follow continuous hence DRX Above 900 o C, static softening lowers the pass curves below the continuous peak (b) intercritical multistage rolling simulation at 760 o C for 02MnNb; at 730 o C for 06MnMoV. Steel 09Mn2VNb Top.ε=0 bottom rolled 740 o C.ε=53%red Left cooled 0.5C/s right 3.5C/s [240,249] 35 [249,332] 36

10 Intercritical, fraction Intercritical, micros As a result of rolling, Nb-V steel to 67% red in 5 passes at various T. The dependence of fraction transformed to ferrite. [249] Nb-V steel after furnace cooled. Effect of rolling reduction at 720 o C on hardness. 37 Microstructures (x320), of the Nb-V steel quenched after rolling to 67% red. At different temperatures: (a) 770 o C; (b) 750 o C and (c) 730 o C. White ferrite appears in a matrix of martensite formed from the austenite remaining after rolling. [240,332,369] 38 γ BAINITE (B) MIXED RESPONSE Banite, isothermal trans., TTT Hot Deformed Stable Austenite ** Transformation T T > 400 C. Accelerated - more as T D, ε, έ - less as T T falls - much less than γ α + P - increases more in later stages as T T - greater overall B fraction as T T Transformation T T < 400 C Retarded - more as T D, ε, έ - more as T T falls - less overall B fraction ** Deformation of unstable γ near 500 C - raises bainite start T - accelerates transformation Effect of deformation at 800 o C on the bainite transformation of 50CrNiMo steel. Transformation at 300 (a), 400 (b) and 480 o C (c) ; TTT-diagram (d). 39 [240,279,300] 40

11 Bainite, micro Bainite, TTT 30CrMnSiNi2 (a) and 40CrNiMo (b) steels CCT ( solid lines) and TTT (dashed lines) diagrams. 1 undeformed austenite; 2 austenite deformed to 25% at 900 o C [240] 41 [300] 42 Bainite formation PARAMETER CONTROL TO RAISE - RATE AT LOW T T - CHANGE OVER T Trans Hot Deformed Stable Austenite γ BAINITE (B) Bainite formation at 450 o C in 08CrMn2V (a), 30Cr3 (b) and 30CrMnSiNi2 (c) steels Effect of low T 850 o C High T 1100 o C heating and o C Step heating Higher preheat T P - larger grains Lower strain higher T D - delay for more SRX stepped cooling for segregation of C and alloying Less carbide formers: Cr. Mo, W, Si less C: T Trans (more C destabilization at high T T ) 43 44

12 Summary, TTT, CCT ENHANCED TRANSFORMATION OF AUSTENITE IN THERMO MECHANICAL PROCESSING (TMP) OF STEEL E.V. Konopleva, H.J. McQueen and V.M. Khlestov Schematic of deformation effects on TTT-(a, b) and CCT-(c) diagrams. (1) ε=0; (2) deformation at T>Ac3; (3) holding after deformation 20s In the TMP of steels, austenite transformation develops selected microstructures with advantageous properties that are dependent on the prior hot deformation as well as the cooling conditions. This paper surveys the alteration in formation of ferrite, pearlite, alloy carbides and bainite as a result of laboratory hot rolling that has been conducted to produce specimens of fixed size to facilitate measurements isothermally by magnetometry, or in continuous cooling by dilatometry or thermal analysis. The behavior is dependent on composition, preheat temperature T (grain size, homogeneity), relation to eutectoid, alloy carbide retention and finish rolling T. The effects of strain, temperature, number of passes and time, either between them or after for cooling, on acceleration of austenite decomposition are exhibited in fraction transformed or in T-T-T diagrams both isothermal and continuous cooling. Optical microscopy determined the locations of nucleation and growth, for a wide range of steels, for various TMP and for different fractions transformed. The research included very low carbon, HSLA, plain carbon, low alloy, eutectoid, hypereutectoid and tool steels. In general after hot forming, ferrite/pearlite is accelerated, refined and enhanced in fraction, whereas lower bainite is retarded. [240,335] Hot Deformation AUSTENITE DECOMPOSITION 240 EFFECTS OF HOT DEFORMATION ON AUSTENITE DECOMPOSITION IN ALLOY STEELS, K E.V. Konopleva, V.M. Khlestov and H.J. McQueen, Keynote Lecture Intnl. Symp., Vancouver, Phase Transformation During Thermal/Mechanical Processing of Steel, E.B. Hawbolt and S.Yue eds., Met. Soc. CIM, 1995, pp HOT WORKING AND TRANSFORMATION TO FERRITE OF V-Mo, Nb and Nb-V STEELS, V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Can.Metal. Quart., 1996, 35, pp KINETICS OF BAINITE TRANSFORMATION IN HOT-WORKED AND ULTRAFINE K AUSTENITE OF ALLOY STEELS, E.V. Konopleva and H.J. McQueen, Hot Workablility of Steels and light Alloys-Composites, H. J. McQueen, E.V. Konopleva and N.D. Ryan (eds.), Met. Soc. CIM, Montreal, 1996, pp EFFECT OF HOT WORKING ON TRANSFORMATION OF C-Mn AND HSLA STEELS. H.J. McQueen, E.V. Konopleva, and V.M.Khlestov, ibid 279, pp EFFECTS OF HOT DEFORMATION ON THE AUSTENITE TRANSFORMATION KINETICS AND STRUCTURE IN C-Mn AND Mo-Nb HSLA STEELS, V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Mat. Sci. Tech., , pp EFFECTS OF HOT DEFORMATION ON THE AUSTENITE TRANSFORMATION KINETICS AND STRUCTURE IN C-Mn AND Mo-Nb HSLA STEELS, V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Mat. Sci. Tech., , pp KINETICS OF AUSTENITE TRANSFORMATION DURING THERMOMECHANICAL R PROCESSES (TMP), V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Can. Metal. Quart., 1998, 37, pp Hot Deformation AUSTENITE DECOMPOSITION 332 EFFECT OF HOT DEFORMATION ON FERRITE NUCLEATION IN LOW-CARBON STEELS, E.V. Konopleva, H.J. McQueen and V.M. Khlestov, Metal Welding, Applications and Thermomechanical processing, J.P. Boillot, M. Evrard, A. Galbois, M.V. Krishnadev, eds., Met. Soc. CIM, Montreal, (1999), pp INFLUENCE OF THERMAL-MECHANICAL PRE-TREATMENTS ON TRANSFORMATION OF AUSTENITE, H.J. McQueen, E.V. Konopleva and V.M. Khlestov, Steel For Fabricated Structures, R.I. Asfahani, R.L. Bodnor, eds., ASM Int., Metals Park, OH, (1999), pp FERRITE-PEARLITE TRANSFORMATION IN HOT WORKED AUSTENITE, E.V. Konopleva, H.J. McQueen and V.M. Khlestov, Thermo-mechanical Processing of Steel, D.J. Naylor et al. eds., Inst. Materiasl London (2000) vol. 1 pp EFFECT OF DEFORMATION IN CONTROLLED ROLLING ON FERRITE NUCLEATION, V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Can. Metal. Quart., 40, (2001) EFFECTS OF DEFORMATION AND HEATING TEMPERATURE ON THE AUSTENITE TRANSFORMATION TO PEARLITE IN HIGH ALLOY TOOL STEELS, V.M. Khlestov, E.V. Konopleva and H.J. McQueen, Mat. Sci. Tech 18 (2002), HOT WORKING AND EFFECTS ON AUSTENITE TRANSFORMATION TO PEARLITE IN M2 TOOL STEEL, H.J. McQueen, E.V. Konopleva, C.A.C Imbert, and V.M. Khlestov, Thermomechanical Processing: Mechanics, Microstructure, & Control, E.J. Palmiere et al. eds. University of Sheffield, (2003) pp GRAIN REFINEMENT AND AUGMENTED RATE IN TRANSFORMATION OF HOT WORKED AUSTENITE, H.J. McQueen, E.V.Konopleva & V.M.Khlestov, Ultra-Fine Structured Steels, E.Essadiqi and J. Thomson eds., Met. Soc. CIM, Montreal (2004), pp