1. GRAIN GROWTH BEHAVIOUR OF NIOBIUM-ALLOYED DIRECT QUENCHED STEELS DURING SLAB REHEATING CASR-seminar 19.12.2013 Materials engineering laboratory/ Jaakko Hannula

2. CONTENT Introduction Purpose of the study/ experimental Results Summary 2

3. PROJECT This study is part of the 2-year project called ”Optimum Use of Niobium in Modern Direct Quenched Steels” Started 1.10.2012, ending 30.09.2014 Funded by Ruukki Oy (Original funder CBMM)

4. PURPOSE OF THE STUDY Effect of Nb on grain growth behaviour of cast material 4 different temperatures: 1050°C, 1100°C, 1150°C, 1200°C 3 different compositions Slow heating rate Austenite grain size measurements 4

5. COMPOSITIONS Comp.CSiMnPSCrNiMoTiNbCuAlBO2N2 DQP0.2360.731.70.0110.00151.20.0180.110.00220.00480.0130.650.00030.00120.0018 DQP+0.02 Nb0.2360.731.70.0110.00151.20.00870.0970.0020.0320.0140.660.00030.00120.0018 DQP+0.05 Nb0.2320.731.70.0110.00151.10.00880.10.00160.0580.0140.640.00030.00120.0018 w-% DQP = Direct Qunched and Partitioned high carbon, silicon and aluminium contents  stabilize austenite 5

6. QUENCH & PARTITIONING PROCESS 6 carbide precipitation and decomposition of austenite are suppressed by appropriate alloying the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase  increasing the stability of the residual austenite  microstructures with martensite/austenite combinations

7. SLAB REHEATING BEFORE HOT ROLLING 7

8. EXPERIMENTAL HEATING PROCEDURE Hot furnace ~10 °C/min ~700 °C ~1000 °C ~1150 °C ~5 °C/min ~1200 °C ~1.6 °C/min Water quenching 30 min 8

9. AUSTENITE GRAIN SIZE: DQP 9

10. Test temperature: 1050 °C DQP+0.05 NbDQP+0.02NbDQP 25 mm 10

11. ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR The same coarse austenite grains as those before austenitizing emerge when martensitic or bainitic steel with coarse grains is reheated to an austenite region at a slow reheating rate. It can be also called as γ grain memory (1) Coarse γ grains and fine globular γ grains that nucleate along the coarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature  (2) Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains that corresponds to retained γ between laths.  (3) Abnormal α to γ transformation is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before reheating α to γ transformation behavior is governed by competition between the nucleation and growth of newly formed γ from the dissolution of cementite and the growth and coalescence of retained γ. 11

12. Normal grain growth Abnormal grain growth ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR 12

13. Nb prevents cementite to precipitate in bainite  carbon can enrich to austenite + Si (0,73 %) and Al (0,65 %) will stabilize formed retained austenite  normal grains can’t nucleate by dissolution of cementite, because there are not any cementite particles  abnormal, very large grains will grow from retained austenite between martensite/bainite laths  very coarse austenite grain structure ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR 13

14. 14 DQP+0.05 Nb 850 °C ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR

15. EFFECT OF PRIOR HEAT TREATMENT 15 Prior heat treatment at 650 °C for 90 minutes were used to transform the retained austenite to ferrite and carbides Formed carbides (cementite) act as a new nucleation sites for new grains

16. EFFECT OF PRIOR HEAT TREATMENT 1050 °C650 °C (90 min) + 1050 °C DQP+0.02Nb CompositionVol. Fraction of retained austenite [%] As cast650°C, 90 min DQP7.14* DQP+0.02 Nb6.78* DQP+0.05 Nb6.77*2.54* * Results from XRD-measurements 16

17. EFFECT OF PRIOR HEAT TREATMENT As castAfter 650 °C for 90 min

18. EFFECT OF STARTING AUSTENITE GRAIN SIZE TO FINAL GRAIN SIZE 18 Sellars: D r =1.1*D 0 ^0.67*ε^-0.67 D 0 [γm]εD r [γm] 500.244 0.241 0.239 0.238 D 0 [γm]εD r [γm] 30000.2691 0.2258 0.2134 0.286 Grain sizes after hot rolling 4 passes above T nr - temperature

19. SUMMARY Abnormally coarse austenite grains were detected in DQP- compositions, when Nb was used as an alloying element Abnormal α to γ transformation-phenomenon was detected Nb prevents cementite precipitation in bainite  no nucleation of new austenite grains  coarse austenite grains can grow This can be suppressed by decreasing the amount of retained austenite or by increasing the amount of cementite precipitates  this was achieved by heat treatment at 650 °C for ~90 minutes 19

20. Thank you! Questions? 20