GRAIN GROWTH BEHAVIOUR OF NIOBIUM-ALLOYED DIRECT QUENCHED STEELS DURING SLAB REHEATING

Transcription

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

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

3 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)

4 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

5 5 COMPOSITIONS DQP = Direct Qunched and Partitioned w-% Comp. C Si Mn P S Cr Ni Mo Ti Nb Cu Al B O2 N2 DQP DQP+0.02 Nb DQP+0.05 Nb high carbon, silicon and aluminium contents stabilize austenite

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 Temperature [ C] 7 SLAB REHEATING BEFORE HOT ROLLING Heating 1 Heating 2 Time [s]

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

9 AUSTENITE GRAIN SIZE: DQP 9

10 10 AUSTENITE GRAIN SIZE: DQP Test temperature: 1050 C DQP+0.05 Nb DQP+0.02Nb DQP 25 mm

11 ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR 11 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.

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

13 ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR 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

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

15 15 EFFECT OF PRIOR HEAT TREATMENT 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 16 DQP+0.02Nb 1050 C 650 C (90 min) C Composition Vol. Fraction of retained austenite [%] As cast 650 C, 90 min DQP 7.14* DQP+0.02 Nb 6.78* DQP+0.05 Nb 6.77* 2.54* * Results from XRD-measurements

17 EFFECT OF PRIOR HEAT TREATMENT As cast After 650 C for 90 min

18 EFFECT OF STARTING AUSTENITE GRAIN SIZE TO FINAL GRAIN SIZE 18 Grain sizes after hot rolling 4 passes above T nr - temperature D 0 [γm] ε D r [γm] D 0 [γm] ε D r [γm] Sellars: D r =1.1*D 0^0.67*ε^-0.67

19 19 SUMMARY Abnormally coarse austenite grains were detected in DQPcompositions, 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

20 20 Thank you! Questions?