origin of copious recrystallization in cold rolled interstitial free (if) steels

www.steel-research.de

FULL

PAPER
Origin of Copious Recrystallization in Cold Rolled
Interstitial Free (IF) Steels

Md Zakaria Quadir� and Paul Richard Munroe

Copious recrystallization occurs during the very early stages of annealing in some deformedgrains in 85% cold rolled IF steel. This event happens to certain regions in g oriented grainswhile the a oriented grains remain unrecrystallized. The orientations of the newly forminggrains are near either {111}<123> or {554}<225>, and as a result the intensity at theseorientations are high in the recrystallization texture. An investigation by scanning electronmicroscope (SEM) and electron backscatter diffraction (EBSD) shows that such copiousrecrystallization event originates from the frequent presence of high crystallographicgradients, at the locations where two set shear bands intersect.

1. Introduction

Interstitial free (IF) steels are widely used in automotive

body panels for having excellent deep drawing ability. This

class of steels exhibits small sensitivity to processing

conditions, which brings numerous technical and com-

mercial advantages during processing of these steels.[1–3] It

is not understood why IF steels are so robust, but the

principal difference between IF steels with other low

carbon steels is with their chemistry, i.e., IF steels contain

strong carbide forming elements (Ti, Nb, etc.) to combine

with any residual carbon atoms from solid solution, and

distribute in the form of complex precipitates in the

matrix and at grain boundaries.[4]

In early studies, the drawability of BCC metals was

explained by the ratio of {111}/{100} peaks, measured by

X-ray line scans. Later, after the arrival of three dimen-

sional rotating capabilities for X-ray holders it was shown

that a particular crystallographic texture, having {111}

plane parallel to the rolling plane (known as g fiber),

enhances drawability. In IF steels, this texture can be

produced by continuous annealing after�75% cold rolling,

which typically contains the g fiber with another orienta-

tion fiber (known as the a fiber). The a fiber grains have

<110> directions parallel to the rolling direction (RD) of

the sheets. The rolling and annealing behavior of IF steel

remains identical at all rolling temperatures in the ferrite

phase.[5,6]

[�] Dr. M. Z. Quadir, P. R. MunroeElectron Microscope Unit and School of Materials Science andEngineering, The University of New South Wales, Sydney, NSW 2052,AustraliaEmail: [email protected]

DOI: 10.1002/srin.201300066

1320 steel research int. 84 (2013) No. 12

From numerous studies it has been found that during

annealing, recrystallization starts in the g oriented grains

and the orientations of the first recrystallized grains are

also g oriented. The a deformed grains rarely recrystallize

at rolling reduction of �80% or less, and are consumed in

the latter stages of recrystallization by the growth of the g

oriented recrystallized grains.[1] Much detailed research

has been done, especially on the g deformed micro-

structures, to study the sub-structural elements, which can

be potential nuclei at the early stage of recrystallization.

In many reports different microstructural features were

proposed as the origin of g nucleation, namely shear

bands,[2,3] deformation bands,[7,8] and the immediate

vicinity of grain boundaries.[9,10] Evidences of these

nucleation types have been demonstrated by optical

microscopy, scanning electron microscope (SEM), elec-

tron backscatter diffraction (EBSD), and transmission

electron microscopy (TEM) investigations. In addition to

these observations, clusters of recrystallized grains also

have been found at the very early stages of recrystalliza-

tion, e.g., when only 5–10% area fraction of the whole

microstructure was occupied by recrystallized grains. In

numerous reports, these clusters have been observed in

partial recrystallization conditions of various low carbon

and micro-alloyed steels.[1–3,5–10] This article shows the

origin of clustered recrystallizations and their orientation

relationship with the matrix.

2. Experimental Procedure

A Ti-stabilized IF steel having a 50 � 20 mm grain size was

homogeneously cold rolled by 10% incremental reduction

to a total of 85% reduction in thickness and then annealed

in a preheated (at 700 ˚C) air-circulated furnace for up to

complete recrystallizations. The rolling and annealing

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


FULL

PAPER

texture was measured by {110}, {200}, and {211} crystallo-

graphic reflection of Co X-ray radiation. The data was used

to generate pole figures and orientation distribution

functions (ODF). The sample was cut through the mid

regions to expose the surfaces containing the RD-ND

(normal direction) cross-section. SEM based channeling

contrast (CC) imaging and EBSD study was conducted

of the deformation and partial recrystallization conditions

to find the origin of recrystallized grains, formed in

clusters.

3. Results and Discussions

Figure 1a is taken from literature to show the principal

rolling texture components in a <200> pole figure of a

moderately deformed steel sample.[1] By comparison,

Figure 1b shows that the rolling texture of the current

sample comprises the a ({hkl}<110>) and g ({111}<uvw>)

fibre components. The texture after complete recrystalli-

zation in shown in Figure 1c, whereby the a fiber is

eliminated and the intensities at the {111}<123>

and {554}<225> orientations of g fiber became

enhanced to the maximum �11� random (R). Among

them {554}<225> orientations does not ideally lie on the

g fiber, but is located within a short (�8˚) TD rotated

angular distance. The deformation microstructures of

Figure 1. (a) A <200> pole figure showing the importantorientation components in the rolling texture of steel.[1] Thepositions of {554}<225> and {111}<123> orientation componentsare superimposed as ~ and & symbols. (b) Rolling and (c)recrystallization texture in <200> pole figure.


the a and g fiber grains are also distinctive.[2,3] The a

orientated grains are smooth and have gradual variations

ofmisorientation, and do not participate in the early stages

of recrystallization. In contrast, the g oriented grains are

highly fragmented by the presence of microbands,[11,12]

shear bands,[13,14] and deformations bands,[7,8] and

have frequent change in orientation. It is pertinent to

mention that it has been found from two and three

dimensional EBSD investigations that microband

boundaries comprise low misorientation angle and

therefore do not directly participate in recrystallization

events.[15–17] Whereas, shear bands create either high

and low angle boundaries with the matrix orienta-

tions[13,14] and deformation bands always generate high

angle boundaries.[7,8]

Extensive microstructural investigations show that

during annealing of the current sample recrystallization

primarily occurs at the deformation banding of g

orientated grains[5–8] and the orientations of the newly

recrystallized grains also remain close to g fiber orienta-

tions. These events are easy to detect using SEM CC

imaging and EBSD. The complete recrystallization textures

also have g fiber texture (Figure 1c). Therefore the

recrystallization event has been explained by “oriented

nucleation” theory,[17] which is different from the

“orientated growth” theory, in which the recrystallization

texture is governed by selective growth of certain texture

components. In the early recrystallization stages, some of

the deformed g fiber grains contain regions of profuse

recrystallization, such as the examples shown in Figure 2a

and b. These two images were taken when the overall

recrystallization was only �8% (measured by line

intercept method of a large montage covering many

deformed hot-band grains), but in these regions the

Figure 2. (a)–(b) SEMchanneling contrast images showing profusenucleation of recrystallization at the intersections of two sets ofshear bands.

steel research int. 84 (2013) No. 12 1321

Figure 3. a) SEM channeling contrast image showing twointersecting sets of shear band and b) the associated <200> polefigure showing a large spread in orientations.


FULL

PAPER
recrystallized grains almost covered the deformation
structures. In Figure 2a �18 mm thick hot band grain

reached to completely recrystallization. Shear banding was

a dominant deformation feature in this sample, and it

should be noted here that, at this partial recrystallization

stage, single set shear bands remain unrecrystallized.

As for example, the grain in the bottom of this contains

single set shear bands (arrowed) without recrystallization.

However two set shear banded regions disappeared from

the microstructure and were replaced by clusters of

recrystallized grains. Therefore, it is logical to assume

that profuse nucleation events take place in two set shear

banded regions.

The reactivity of two set shear banded regions are so

high that it is difficult to capture the partially recrystalli-

zation condition. Nevertheless, an example is shown in

Figure 2b. This image shows many sub-grains bulging

into an equiaxed shape. The original microstructure

in the left part of the grain is already covered by

many recrystallized nuclei. However, traces of deformed

microstructure remains in the right part of the image,

and provides evidence whereby nucleation occur at the

intersections of two sets of shear band (labeled as SB1

and SB2).

The evidence in Figure 2 led to this current investigation

to measure the orientations of the two-set shear banded

regions. One such example is shown in the CCmicrograph

shown in Figure 3a. An EBSD measurement of the entire

area was done and the orientation data is plotted as<200>

pole figure in Figure 3b, to show that there is a large

orientation spread within this deformed hot-band grain.

The overall orientation is centered close to {111}<110>.

Such a large spread was not found to be associated with

single set shear banding,[6] and therefore it is concluded

that two intersecting shear banding creates a larger

orientation spread.

To observe the local variations in orientation across

individual intersecting event, the rectangular area in

Figure 3a is magnified in Figure 4a. EBSD measurements

were conducted in this area and the misorientation

boundaries (>8˚) are plotted in Figure 4b as black lines,

which match well with the trajectories of the shear

bandings shown in Figure 4a. A <200> pole figure (in

Figure 4c) of the line scan (A and B) shows that there are

large orientation differences between the matrix (M) and

shear bands (SB1 and SB2). The matrix orientation (M)

is located at �{111}<110>, and the orientations of

SB1 and SB2 are rotated away by large spreads in

opposite directions, covering a range from �{111}<123>

to �{554}<225> orientations. If this magnitude of spread

persists in the intersecting regions, thenwithin a small area

a large spread is expected. Figure 4d shows the orientation

spread of a (1.5 � 1.5 mm2) area containing an intersection

(of the rectangle marked in Figure 4b) and it is obvious

from the corresponding pole figure (in Figure 4d) that a

large orientation spread is also present there. Such an

extraordinarily large orientation spread within such a


small volume ofmaterial is only found in those areaswhere

two set shear bands intersect. Therefore, these areas

are potent sites for nucleation. In an early investigation

on Fe–50Co–0.4Cr alloy, Buckley[18] showed evidence of

nucleation on shear band intersecting points. At that

time EBSD was not developed and therefore this hypothe-

sis remained untested. In the present sample no sign of

nucleation on a single set shear band was detected. This

lead to speculation that the single set shear bands do not

have enoughmisorientations (with thematrixmaterial) for

nucleation to commence, but when they intersect with a

second set a higher misorientation is possible to generate

and, hence, become potent sites for nucleation. Further-

more, the very fine substructures at the intersecting points

(see Figure 4a) provide those locations with high stored

energy and therefore respond quickly towards recovery

and recrystallization events for obtaining profuse

recrystallization.

Figure 5a shows a very early stage of recrystallization in

a SEM CC image. It is obvious from this image that many

sub-grains have started to bulge out from the intersecting


Figure 4. (a) SEM channeling contrast image and (b) correspond-ing orientation imaging microscopy and (c)–(d) <200> polefigures showing high misorientations in the shear bandingintersecting regions.

Figure 5. SEM channeling contrast image and correspondingorientation imaging microscopy and <200> pole figuresshowing the orientations of recrystallized grains at shear bandintersections.


FULL

PAPER

points of two shear bands, labeled as SB1 and SB2. In

this micrograph, some recrystallized grains have grown

to several microns (in diameter) and therefore can be

considered as successfully recrystallized grains.[17] The

orientation of the entire recrystallized area is shown in

Figure 5c, to be centered on {111}<110> and have large

orientation spreads. The larger grains (>5 mm and

equiaxed) are marked in the EBSD map (Figure 5b) and

their orientations are plotted in Figure 5d, from which

it is clear that these are centered on {111}<123>. By

combining the orientation and microscopic information

of Figure 5 it can be concluded that prolific nucleation

occurs in the regions where the two parallel set shear

bands intersect.

It is to be noted that the number of grains having

microstructures of two interesting shear bands are not

large following 85% deformation. However, that number

increases significantly as the deformation continues to

higher strains. It is also noted that, the orientation of

recrystallized grains from intersecting shear banding are

expected to be close to {111}<123> and {554}<225>, as

shown in Figure 4 and 5. Therefore, high intensities in the

recrystallization texture at those orientations have proba-

bly originated from nucleation on double set shear

banding regions. 85% reduction is considered to be at

the high end of current industrial practices. However, with

the recent trend of heavier deformation for materials

strengthening, rolling reductions up to 98% are commonly

targeted. In such situations this mechanism would be

more important.


4. Conclusion

Single set shear banding in 85% cold rolled IF steels do not

participate in recrystallization. However, certain areas of

grains contain two parallel intersecting sets of shear bands

and high crystallographic rotations. Profuse nucleation

takes place in these locations in the early stages of

recrystallization. These grains grow quickly and appear as

clusters in the deformation microstructures. The orienta-

tions of recrystallization clusters are centered at {554}<

225> and {111}<123>.

Acknowledgments

The authors are pleased to thank Professor Brian John

Duggan of the University of Hong Kong (HKU) for

providing IF steel samples. Part of the work was done in

HKU while M. Z. Quadir was working there.

steel research int. 84 (2013) No. 12 1323


FULL

PAPER
Received: February 11, 2013;
Published online: June 20, 2013

Keywords: IF steels; recrystallization; rolling; shear

banding

References

[1] W. B. Hutchinson, Int. Mater. Rev. 1984, 29, 25.

[2] M. R. Barnett, J. J. Jonas, ISIJ Int. 1997, 37, 697.

[3] M. R. Barnett, J. J. Jonas, ISIJ Int. 1998, 38, 78.

[4] D. O. Wilshynski-Dresler, D. K. Matlock, G. Krauss,

Int. Forum Phys. Metall. IF Steels ISIJ 1994, 13.

[5] M. Z. Quadir, B. J. Duggan, Acta Mater. 2004, 52, 4011.

[6] M. Z. Quadir, B. J. Duggan, Acta Mater. 2006, 54, 4337.

[7] Y. Y. Tse, G. L. Liu, B. J. Duggan, Scr. Mater. 2000,

42, 25.


[8] G. L. Liu, B. J. Duggan, Metall. Mater. Trans. A 2001,

32, 125.

[9] Y. Hayakawa, J. A. Szpunar, Acta Mater. 1997, 5, 3721.

[10] H. Inagaki, Trans. ISIJ 1994, 34, 313.

[11] M. Z. Quadir, N. Mateescu, L. Bassman, W. Xu,

M. Ferry, Scr. Mater. 2007, 57, 977.

[12] Z. Chen, M. Z. Quadir, B. J. Duggan, Philos. Mag.

2006, 23, 3633.

[13] M. Z. Quadir, B. J. Duggan, ISIJ Int. 2006, 46, 1495.

[14] L. Kestens, J. J. Jonas, Metall. Trans. A 1996, 27, 155.

[15] N. Afrin, M. Z. Quadir, L. Bassman, J. H. Driver,

A. Albou, M. Ferry, Scr. Mater. 2011, 64, 221.

[16] N. Afrin, M. Z. Quadir, M. Xu, M. Ferry, Acta Mater.

2012, 60, 6288.

[17] F. J. Humphreys, M. Hatherly, Recrystallization and

Related Annealing Phenomena, 2nd ed., Pergamon

Press, Oxford, UK 2004.

[18] R. A. Buckley, Metal Sci. 1979, 13, 67.


origin of copious recrystallization in cold rolled interstitial free (if) steels

Documents