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

CONTENT Introduction Purpose of the study/ experimental Results Summary 2

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

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

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

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

SLAB REHEATING BEFORE HOT ROLLING 7

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

AUSTENITE GRAIN SIZE: DQP 9

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

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

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

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

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

EFFECT OF PRIOR HEAT TREATMENT 1050 °C650 °C (90 min) °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

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

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] D 0 [γm]εD r [γm] Grain sizes after hot rolling 4 passes above T nr - temperature

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

Thank you! Questions? 20