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

CASR-seminar 19.12.2013

Materials engineering laboratory/ Jaakko Hannula

CONTENT

• Introduction

• Purpose of the study/ experimental

• Results

• Summary

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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)

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

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COMPOSITIONS

Comp. C Si Mn P S Cr Ni Mo Ti Nb Cu Al B O2 N2

DQP 0.236 0.73 1.7 0.011 0.0015 1.2 0.018 0.11 0.0022 0.0048 0.013 0.65 0.0003 0.0012 0.0018

DQP+0.02 Nb 0.236 0.73 1.7 0.011 0.0015 1.2 0.0087 0.097 0.002 0.032 0.014 0.66 0.0003 0.0012 0.0018

DQP+0.05 Nb 0.232 0.73 1.7 0.011 0.0015 1.1 0.0088 0.1 0.0016 0.058 0.014 0.64 0.0003 0.0012 0.0018

w-%

DQP = Direct Qunched and Partitioned

• high carbon, silicon and aluminium contents stabilize austenite

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QUENCH & PARTITIONING PROCESS

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• 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

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025

050

075

010

0012

5015

0017

5020

0025

0030

0035

0040

0043

5055

2057

6059

4062

4065

4067

8073

200

100

200

300

400

500

600

700

800

900

1000

1100

1200

Heating 1Heating 2

Time [s]

Tem

pera

ture

[°C

]

EXPERIMENTAL HEATING PROCEDURE

Hot furnace

~10 °C/min ~700 °C

~1000 °C

~1150 °C

~5 °C/min

~1200 °C

~1.6 °C/min

Waterquenching

30 min

30 min

30 min

30 min

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AUSTENITE GRAIN SIZE: DQP

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AUSTENITE GRAIN SIZE: DQP

Test temperature: 1050 °C

DQP+0.05 Nb DQP+0.02Nb DQP

25 mm

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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 thecoarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature (2) Coarse γ grains are formed bythe 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.

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Normal grain growth

Abnormal grain growth

ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR

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• 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

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DQP+0.05 Nb

850 °C

ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR

EFFECT OF PRIOR HEAT TREATMENT

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• 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 °C 650 °C (90 min) + 1050 °C

DQP+0.02Nb

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

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EFFECT OF PRIOR HEAT TREATMENT

As cast After 650 °C for 90 min

EFFECT OF STARTING AUSTENITE GRAIN SIZE TO FINAL GRAIN SIZE

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Sellars: Dr=1.1*D0^0.67*ε^-0.67

D0 [γm] ε Dr [γm]50 0.2 44

44 0.2 4141 0.2 3939 0.2 38

D0 [γm] ε Dr [γm]3000 0.2 691

691 0.2 258258 0.2 134134 0.2 86

• Grain sizes after hot rolling• 4 passes above Tnr - 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

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Thank you!

Questions?

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