Effect of warm-rolling on the formation of microstructure and microtexture of the constituent phases in a duplex steel

PINAKI Prasad Bhattacharjeea

, MOHAMMED Zaid Ahmedb

Department of Materials Science and Engineering

Indian Institute of Technology Hyderabad

Ordinance Factory Estate Yeddumailaram 502205

Andhra Pradesh India

apinakib@iith.ac.in, MS11P008@iith.ac.inb

Keywords: cold-rolling, warm-rolling, duplex stainless steels, microstructure, texture

Abstract. The effect of warm-rolling on the evolution of microstructure and microtexture was

investigated in a duplex steel. For this purpose annealed duplex steel plates were cold and warm-

rolled up to 90% reduction in thickness at room temperature and at 625°C, respectively. The

austenite volume fraction decreased consistently during cold-rolling indicating that austenite was

not stable during cold-rolling. In contrast, austenite was found to be very stable during warm-rolling

at 625°C. Development of an ultrafine lamellar deformation structure with alternate arrangement of

the ferrite and austenite bands could be observed during warm-rolling. A strong pure metal or

copper type texture was observed in the austenite in the warm-rolled material in contrast to brass

texture developed during cold-rolling. Development of RD (RD//<110>) fiber and ND-fiber

(ND//<111>) was observed in ferrite during both cold and warm-rolling. However, the strength of

the RD-fiber was much higher as compared to the ND-fiber in ferrite in cold-rolled DSS as

compared to the ferrite in warm-rolled DSS.

Introduction

Duplex stainless steels (DSS) are two phase steels consisting of ferrite and austenite having

nearly equal volume fraction of the two phases. The usage of these steels has been mostly limited to

chemical industries due to their excellent resistance against corrosion coupled with good

mechanical strength. In order to further expand the usage of the DSS steels for other structural

applications proper understanding of the deformation behavior is required. Although the evolution

of microstructure and texture during cold-rolling in duplex steels have been investigated previously

[1-4], in the present work we study the development of microstructure and microtexture during

warm-rolling of a DSS and compare and contrast those with cold-rolled material in order to

highlight the differences.

Experimental

The as-cast DSS steel having the composition shown in Table 1 in the form of a tapered cylinder

with top diameter 72mm, bottom diameter 62mm and height 290mm was initially

thermomechanically processed through a series of steps including progressive hot forging or

cogging at 1200 ̊C followed by hot-rolling at 1125 ̊C into a final thickness of 10.5mm and final

homogenization annealing at 1175 ̊C for 2 hours. The homogenization annealed material was used as

the starting material for further cold-rolling at room temperature and warm-rolling at 625°C up to

90% reduction in thickness.

The microstructure and texture of the warm-rolled samples were characterized by electron back

scatter diffraction (EBSD) system ((Oxford Instruments, UK) attached to a scanning electron

microscope (Make: Carl Zeiss, Germany, Model: SUPRA-40). The acquired EBSD data was

analyzed using the TSL-OIMTM

software. Samples for EBSD measurements were prepared by

mechanical polishing followed by electro-polishing.

Materials Science Forum Vols. 783-786 (2014) pp 2555-2560Online available since 2014/May/23 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.783-786.2555

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 152.2.176.242, University of North Carolina at Chapel Hill, Chapel Hill, USA-30/11/14,13:57:47)

Table 1: Chemical composition of the experimental DSS

Element C S Cr Mn Ni P Si Mo Fe

Weight% 0.08 0.001 24.18 0.14 10.5 0.017 0.45 3.11 balance

Results and Discussion

Figure 1 shows the microstructure and texture of the starting homogenization annealed material.

The phase map (Fig.1(a)) shows very similar volume fraction of austenite (green) and ferrite (red),

~53% and 47%, respectively. The microstructure also show slightly elongated morphology along

the prior hot-rolling direction. Annealing twins (twin boundaries are shown by white lines) are

clearly observed inside the austenite grains. The Φ2=45° section of the ODF of ferrite reveals the

presence stronger ND-fiber as compared to the RD-fiber and ND-rotated cube component at

(Φ1,Φ,Φ2=30°,0°,45°). On the other hand the (111) pole figure of the austenite (Fig.1(c)) shows

retained deformation texture components after the homogenization annealing treatment.

Fig.1: (a) Phase map of the starting homogenization annealed DSS; (b) and (c) are the (111) pole

figures of the ferrite and austenite, respectively.

Figure 2 shows the evolution of microstructure and texture during cold rolling of the DSS. The

evolution of key microstructural parameters are summarized in Table 2. The phase maps (Fig.2(a)-

(c)) shows gradual development of a lamellar deformation structure with increasing deformation.

However, while the microstructure after 90% cold-rolling (Fig.2(c)) clearly shows continuous

lamellar phase bands of ferrite extended parallel to the RD the austenite phase bands appear to be

fragmented. The austenite fraction after 40% reduction in thickness is 45% which decreases to

38% and 25% upon further cold rolling to 70% and 90% reduction in thickness, respectively

(Table 2). The consistent decrease of austenite with increasing deformation indicates the strain

induced transformation of austenite to martensite. Both the phases are refined with increasing

deformation which is evident from the increasing HAGB fraction and decreasing HAGB spacing

(Table 2).

100µm

2556 THERMEC 2013

The Φ2=0° section of the ODF of ferrite in 40% cold-rolled DSS (Fig.2(d)) shows the presence

of ND-fiber. However, the intensity along the ND-fiber is not continuous and the intensity maxima

is located at 111 121 location. The ND-fiber is having a volume fraction of ~12%. RD-fiber

components are rather weak in this deformed condition which is consistent with very small volume

fraction ~3%. Presence of ND-rotated cube (Φ1,Φ,Φ2=30°,0°,45°) is indicated by intensities

around this location. However, the overall texture is rather weak having maximum ODF intensity

of 4.59. Up on further cold rolling to 70% reduction in thickness (Fig.2(e)) the texture is

strengthened (maximum ODF intensity 14.3). Weak intensities around the ND-fiber but much

stronger intensities along the RD-fiber could be observed which indicates that the RD-fiber is

strengthened with increasing deformation. This is supported by a significantly higher volume

fraction of RD-fiber components (~22%) than the ND-fiber components (~5%). The ND-rotated

cube component continues to persist

Fig.2: (a)-(c) Are the phase maps of 40%, 70% and 90% cold-rolled DSS, respectively; (d)-(e) are

the Φ2=45° section of the ODFs of ferrite in DSS cold-rolled to 40%, 70% and 90% reduction in

thickness, respectively; (g)-(i) are the (111) pole figures of austenite in DSS cold-rolled to 40%,

70% and 90% reduction in thickness, respectively. The intensities of the contour lines are same as

in Fig.1.

Materials Science Forum Vols. 783-786 2557

Table 2: Evolution of key microstructural parameters during cold-rolling

Parameter

Reduction (%)

40 70 90

A F A F A F

Phase

Fraction

(%)

45 55 38 62 25 75

HAGB

fraction

(%)

16 9.3 20 10 22 24

HAGB

spacing

(µm)

2.6 3 2.4 2.7 0.9 0.75

A = austenite, F = ferrite

even at this deformation level. The maximum ODF intensity is 14.3. Following 90% thickness

reduction (Fig.2(f)) the texture is significantly strengthened (maximum ODF intensity of 24.40).

Nevertheless, much stronger RD-fiber as compared to the ND-fiber is developed which is easily

understood from the intensity distribution along the two fibers in the Φ2=0° section of the ODF

(Fig.2(f)) . This is also corroborated by much higher volume fraction of the RD-fiber (~67%) as

compared to the ND-fiber (~12%) in this deformed condition.

The (111) pole figure of the austenite in 40% cold-rolled DSS (Fig.2(g)) shows a pure metal

type texture very similar to the starting homogenized material. However, after 90% cold rolling the

pole figure (Fig.2(i)) shows the development of a predominantly brass or alloy type texture in the

austenite. The pole figure in the 70% cold-rolled condition (Fig.2(h)) can be best described by an

intermediate texture between pure metal and alloy type. The transition of texture from pure metal

type to brass or alloy type texture is a characteristic of low stacking fault energy materials which is

observed in single phase austenitic steels and other low SFE material such as Ni-Co alloy [5, 6].

This transition in texture is attributed to deformation twinning which becomes an important

deformation mechanism in low SFE materials in addition to dislocation slip.

Figure 3 shows the microstructure and texture of the DSS after 90% warm-rolling at 625°C. The

phase map (Fig.3(a)) shows a very well-developed ultrafine lamellar microstructure with alternate

arrangement of the two phase bands. Each of these phase bands are further sub-divided by HAGBs

(shown by black lines in Fig.3(a))). The HAGB spacing of austenite and ferrite is ~300 nm and

400 nm, respectively which are much lower than the corresponding values in the 90% cold-rolled

material. This clearly indicates that warm-rolling is much more effective in achieving ultrafine

grain size as compared to cold-rolling. The volume fraction of the austenite is ~50% which is very

similar to that in the starting homogenized material. This clearly shows that austenite is completely

stable during warm-rolling in contrast to the cold-rolling process.

The Φ2=0° section of the ODF of ferrite in 90% warm-rolled DSS (Fig.3(b)) shows the presence

of both ND and RD fibers. The volume fraction of the ND and RD fiber components are ~28% and

38%, respectively. This is in stark contrast to the texture of ferrite in 90% cold-rolled DSS where a

much stronger RD-fiber is observed to develop as compared to the ND-fiber. It may be noticed that

the relative strength of the two fibers may be related to the choice of slip systems in BCC materials

2558 THERMEC 2013

[7]. The observed differences in the present case might be due to the difference in the preference of

the slip system during cold-rolling at room temperature and at warm-rolling at elevated

temperature.

The (111) pole figure of the austenite in 90% warm-rolled DSS (Fig.3(c)) shows typical pure

metal or copper type texture in sharp contrast to a predominantly alloy or brass type texture in

austenite of 90% cold-rolled DSS (Fig.2(i)). The observed differences between the textures of cold

and warm-rolled DSS may be attributed to dislocation slip being preferred over deformation

twinning due to the increase in SFE with increasing temperature [8].

Fig.3: (a) Phase map of 90% warm-rolled DSS; (b) and (c) are the Φ2=45° section of the ODF of

ferrite and (c) (111) pole figure of the austenite. The intensities of the contour lines are same as in

Fig.1.

Summary

The evolution of microstructure and texture in the two constituent phases in duplex steel during

cold and warm-rolling has been compared. The austenite is very stable during warm-rolling

whereas during cold-rolling strain induced phase transformation is indicated. Much stronger RD-

fiber than ND-fiber is observed in ferrite of cold-rolled DSS as compared to the warm-rolled

condition. This is attributed to the preference in the choice of slip system during cold and warm-

rolling. A texture transition from pure metal or copper type to predominantly alloy or brass type

texture with increasing deformation is observed during cold-rolling in sharp contrast to warm-

rolling where a strong pure metal type texture is observed even at the highest deformation level.

This is attributed to deformation slip being preferred over deformation twinning at warm-rolling

temperature due to the increase in stacking fault energy of austenite with increasing temperature.

Acknowledgement

The authors would like to acknowledge Council of Scientific and Industrial Research (CSIR),

India. The authors would like to gratefully acknowledge Dr. G. Malakondaiah, former Director,

DMRL, Hyderabad for his kind permission to prepare the starting alloy used in the present research

work using the facilities at DMRL, Hyderabad and Dr. Raghu and Mr. Satheesh, scientists at

DMRL, Hyderabad for their kind support in preparing the experimental alloy.

Materials Science Forum Vols. 783-786 2559

References

[1] J. Keichel, J. Foct, G. Gottstein, Deformation, and annealing behavior of nitrogen alloyed

duplex stainless steels. Part II: Annealing. ISIJ Int, 43(2003) 1788-1794.

[2] J. Keichel, J., J. Foct, and G. Gottstein, Deformation and annealing behavior of nitrogen

alloyed duplex stainless steels. Part I: Rolling. ISIJ Int, 43(2003) 1781-1787.

[3] A. Belyakov, R. Kaibyshev, Y. Kimura, K. Tsuzaki, Recrystallization Mechanisms in Severely

Deformed Dual-Phase Stainless Steel. Materials Science Forum, 638-642 (2010) 1905-1910.

[4] A. Belyakov, Y. Kimura, K. Tsuzaki, Microstructure evolution in dual-phase stainless steel

during severe deformation. Acta Mater, 54 (2006) 2521-2532.

[5] T. Leffers, R.K. Ray, The brass-type texture and its deviation from the copper-type texture.

Progress in Materials Science, 54 (2009) 351-396.

[6] R.K. Ray, Rolling textures of pure nickel, nickel-iron and nickel-cobalt alloys. Acta Metall

Mater, 43 (1995) 3861-3872.

[7] J.L. Raphanel, P. Van Houtte, Simulation of the rolling textures of b.c.c. metals by means of

the relaxed taylor theory. 33 (1985) Acta Metall 1481-1488.

[8] P.C. J. Gallagher, The influence of alloying, temperature, and related effects on the stacking

fault energy. Metall Trans, 1 (1970) 2429-2461.

2560 THERMEC 2013

THERMEC 2013 10.4028/www.scientific.net/MSF.783-786 Effect of Warm-Rolling on the Formation of Microstructure and Microtexture of the Constituent

Phases in a Duplex Steel 10.4028/www.scientific.net/MSF.783-786.2555

DOI References

[1] J. Keichel, J. Foct, G. Gottstein, Deformation, and annealing behavior of nitrogen alloyed duplex stainless

steels. Part II: Annealing. ISIJ Int, 43(2003) 1788-1794.

http://dx.doi.org/10.2355/isijinternational.43.1788 [2] J. Keichel, J., J. Foct, and G. Gottstein, Deformation and annealing behavior of nitrogen alloyed duplex

stainless steels. Part I: Rolling. ISIJ Int, 43(2003) 1781-1787.

http://dx.doi.org/10.2355/isijinternational.43.1781