Grain growth behaviour of N IOBIUM- alloyed direct quenched steels during slab reheating

1. Graingrowthbehaviour of NIOBIUM-alloyeddirectquenchedsteelsduringslabreheating CASR-seminar 19.12.2013 Materials engineering laboratory/ Jaakko Hannula

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

3. PROject • Thisstudy is part of the 2-year projectcalled ”OptimumUse of Niobium in ModernDirectQuenchedSteels” • Started 1.10.2012, ending 30.09.2014 • Fundedby Ruukki Oy (Originalfunder CBMM)

4. Purpose of the study • Effect of Nb on graingrowthbehaviour of castmaterial • 4 differenttemperatures: 1050°C, 1100°C, 1150°C, 1200°C • 3 differentcompositions • Slowheatingrate • Austenitegrainsizemeasurements

5. compositions DQP = DirectQunched and Partitioned w-% • highcarbon, silicon and aluminium contents stabilize austenite

6. Quench & Partitioning Process • 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. SlabreheatingBeforehotrolling

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

9. Austenite grain size: DQP

10. Austenite grain size: DQP Test temperature: 1050 °C 25 mm DQP+0.05 Nb DQP+0.02Nb DQP

11. Abnormal α TO γ transformationbehaviour • The same coarse austenite grains as those before austenitizing emerge whenmartensiticor bainiticsteel with coarse grains is reheated to an austenite region at a slow reheating rate. • It can be also called as γ grainmemory • (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.

12. Abnormal α TO γ transformationbehaviour Normal grain growth Abnormal grain growth

13. Abnormal α TO γ transformation behaviour 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

14. Abnormal α TO γ transformation behaviour DQP+0.05 Nb 850 °C

15. Effect of priorheattreatment • Priorheattreatment at 650 °C for 90 minuteswereused to transform the retainedaustenite to ferrite and carbides • Formedcarbides (cementite) act as a new nucleationsites for new grains

16. Effect of priorheattreatment DQP+0.02Nb 1050 °C 650 °C (90 min) + 1050 °C * ResultsfromXRD-measurements

17. Effect of priorheattreatment As cast After 650 °C for 90 min

18. Effect of startingaustenitegrainsize to finalgrainsize • Grainsizesafterhotrolling • 4 passesaboveTnr - temperature

19. summary • Abnormallycoarseaustenitegrainsweredetected in DQP-compositions, whenNbwasused as an alloyingelement • Abnormal α to γ transformation-phenomenonwasdetected • Nbpreventscementiteprecipitation 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

20. Thankyou! Questions?