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Analysis of the Surface Defects in a Hot-Rolled Low-Carbon C–Mn Steel Plate
Article in Journal of Failure Analysis and Prevention · June 2017
DOI: 10.1007/s11668-017-0281-8
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J Fail. Anal. and Preven.
DOI 10.1007/s11668-017-0281-8
TECHNICAL ARTICLE—PEER-REVIEWED
Analysis of the Surface Defects in a Hot-Rolled Low-Carbon
C–Mn Steel Plate
P. P. Sarkar . S. K. Dhua . S. K. Thakur . S. Rath
Submitted: 20 January 2017 / in revised form: 6 April 2017
ASM International 2017
Abstract In the present study, a microstructural investigation was conducted on surface defects occurring in a
28 mm thick low-carbon C–Mn steel plate with ferrite–
pearlite microstructure. The plate contained transverse
‘‘scraped-out’’-like defect at the top surface edge and a
continuous longitudinal ‘‘V-groove’’-like defect throughout
the length of the plate in the bottom surface. Detailed
microstructural analyses showed formation of several small
as well as long shallow unidirectional unbranched cracks
with oxide entrapments in the defect region at the top
surface associated with partial decarburization and internal
oxidation confirming its genesis at the casting stage. On the
other hand, extensively branched ‘‘stag deer horn’’ crack
heavily filled with compact FeO oxide scale originated
from the bottom surface defect with no microstructural
abnormality confirmed that the groove existed before the
hot rolling operation and the cracks formed during the
rolling under differential loading.
Keywords Surface defect Low-carbon steel plate Hot rolling Oxide scale Crack
Introduction
Considerable advancement in process technology accompanied by continuously growing customer demands has
revolutionized the market dynamics of rolled steel flat
products in the last 10–20 years. Consequently, a wider
product range with improved quality and defect-free
P. P. Sarkar (&) S. K. Dhua S. K. Thakur S. Rath
R&D Centre for Iron and Steel, Steel Authority of India Limited,
Ranchi, Jharkhand 834 002, India
e-mail: ppsarkar@sail-rdcis.com
surface finish of hot-rolled flat products has become the
major focus of steel producers. Despite significant progress
in manufacturing engineering and process optimization
[1–4], the incidence of surface defects in rolled steel
products could not be completely eradicated. Therefore,
challenges to the steel manufacturers are to minimize the
occurrence of these undesirable surface defects and to
control them within the acceptable limit so as to make the
product suitable for end use.
Surface quality problems can result from multiple
sources pertaining to unfavorable alloy chemistry [5],
irregular casting practices [6] and improper processing [7].
Evolution of these defects in rolled steel products may
occur during initial steelmaking stages [8–11], or they may
develop during the subsequent rolling operations [12–15].
Frequently occurring defects in slabs in terms of their
genesis and morphology have been discussed in the literature. Some of these commonly observed defects, if not
severe, can be eliminated by scarfing treatment before hot
rolling of the slabs. On the other hand, the improper
downstream processing associated with reheating and hot
rolling of the continuously cast slabs may also contribute
toward surface defects [16]. Irrespective of the formation
stages, the presence of surface defects not only reduces the
aesthetic appeal to the customer but also significantly
affects the yield of the steel plates which in turn decreases
the mill productivity and increases the labor cost and
energy consumption [17]. Accordingly, for retaining market competitiveness, a systematic metallurgical analysis is
necessary to unravel the genesis of these surface defects.
In the present study, microstructural examination of a
28-mm-thick defective hot-rolled C–Mn steel plate sample
was conducted. The defect morphology did not match
typical defect appearances depicted in consolidated atlas of
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J Fail. Anal. and Preven.
Sample Preparation
common defects observed in hot-rolled steel plates [18].
The steel plant producing these plates was also incurring
huge a loss, by almost 30% rejection of the plates, due to
these defects. Considering the importance of the work and
the distinctive nature of the problem, the authors undertook
this study, and an effort is made toward a comprehensive
metallurgical investigation covering visual inspection, light
microscopy and scanning electron microscopy in order to
identify the root cause of these defects and their possible
remedial measures.
Experimental
A schematic illustration of the as-received defective plate
sample is shown in Fig. 1 indicating orientations of the planes
with respect to the rolling direction. The sample was sectioned in short transverse (S-T) orientation, and small-sized
specimens (20 mm 9 20 mm 9 10 mm) were cut from both
top and bottom surface. The samples obtained from the top
edge and bottom surface are designated as sample 1 and
sample 2, respectively. The metallographic specimens were
prepared by conventional mechanical grinding followed by
cloth polishing using 1- and 0.6-lm Al2O3 suspension. Polished samples were then etched with 2% nital solution.
Material and Processing
Optical and Scanning Electron Microscopy
The material used in the present investigation was received
from an integrated steel plant in the form of a 28-mm-thick
plate belonging to IS 2062 E250 B0 grade. Chemical
composition of the steel plate is given in Table 1. The
table also includes the nominal composition of this grade of
steel. This variety of steel plate is equivalent to EN 10025
S275 grade and generally produced in cut to length size
between 6.5 and 10.0 m with a width of 1500 mm and
thickness ranging between 12 and 30 mm. The original
slab was manufactured by continuous casting process with
thickness of 220 mm. Prior to hot rolling, the steel slab was
soaked at a temperature of 1523 K (1250 C) for 3.5 hours
and subsequently rolled down to 28 mm in 16 passes in a
one-stand 4-Hi plate mill per the rolling schedule given in
Table 2. The mechanical properties of the plate were yield
strength (YS): 291 MPa, tensile strength (UTS): 443 MPa,
percent elongation (%EL): 28 and Charpy impact toughness: 66 J at 0 C. These plates find its application for
producing general structures.
Metallographic examinations were carried out at various
magnifications under Olympus make inverted-type GX 71
Rolling
direction
Top surface
edge defect
L-T Plane
L-S Plane
S-T Plane
Bottom surface
longitudinal defect
Fig. 1 Schematic illustration of the surface defects and sample
orientation for metallographic analysis
Table 1 Chemical composition of the as-received defective steel plate sample
Element, wt.%
Material
C
Mn
P
S
Steel plate sample
0.16
1.20
0.022
Specified (IS 2062 E250 B0 grade)
0.22 max
1.5 max
0.045 max
Si
Al
Fe
0.02
0.257
0.045
Bal.
0.045 max
0.40 max
…
Bal.
Table 2 Rolling schedule of the steel plate
Pass no.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Roll gap, mm 220 205
190
170
150
130
115
100
85
70
58
52
45
40
35
31
28
15
20
20
20
15
15
15
15
12
8
7
5
5
4
3
Draft, mm
…
15
% Reduction
…
6.82 7.32 10.53 11.76 13.33 11.54 13.04 15.00 17.65 17.14 13.79 13.46 11.11 12.50 11.43 9.68
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J Fail. Anal. and Preven.
model optical microscope in unetched as well as etched
conditions to examine the crack profiles and microstructural phases, respectively.
Scanning electron microscopy (SEM) of both as-polished (unetched) and nital-etched samples was performed
with a Carl Zeiss, UK make EVO MA 10 model scanning
electron microscope. Energy-dispersive spectroscopy
(EDS) was carried out for microanalysis of the entrapment
present in the steel samples. The applied voltage and probe
current used for SEM observation were 20 keV and
80*100 lA, respectively.
Results and Discussion
Visual Inspection
The as-received sample was carefully examined, and the
macrographs taken are shown in Fig. 2a–b. The plate
Fig. 2 Macrographs of as-received steel plate sample showing
defects in (a) edge of the top surface (sample 1) and (b) bottom
surface (sample 2), respectively
sample contained a transverse ‘‘scraped-out’’-like defect at
the edge. The defect was intermittent and irregularly
shaped. In the bottom surface, a continuous longitudinal
‘‘V-groove’’-like defect near the edge of the plate was
observed. No mark of other damage was found on the top
and bottom surfaces of the sample.
Microstructural Analyses
Light microscopy of the as-polished defective steel plate
(sample 1) revealed a number of thin, short and long
transverse fissures, or cracks (Fig. 3) at the edge of the top
surface intruding from the plate surface to the interior.
Figure 3a shows light micrograph of a typical short transverse crack at the location of the edge defect on the top
surface (sample 1) at 2009 magnification in unetched
condition. The crack was linear and at an angle nearly 45
to the plate surface. A montage of unetched light micrographs showing propagation of a long, slender and shallow
crack originating from a different location of the edge
defect on the top surface is presented in Fig. 3b. Both the
cracks were unidirectional, and no branching of the cracks
could be observed. It is evident from the micrographs that
the intruding fissures contained dark gray entrapments at
places. A light micrograph in unetched condition of the
defect region of the bottom surface (sample 2) is presented
in Fig. 4 at 1009 magnification showing the origin of an
extensively branched ‘‘stag deer horn’’ crack from the deep
groove defect. Unlike the top surface cracks, the bottom
surface cracks were heavily filled with gray-colored
entrapments.
The microstructure of the steel indicated that the matrix
structure was ferrito–pearlitic (FP) as commonly observed
in C–Mn steel plates. A typical banded ferrite–pearlite
microstructure elongated in the plate rolling direction is
shown in Fig. 5 at 2009 magnification. The banding of the
ferrite–pearlite structure generally occurs due to
microsegregation of alloying elements present in the steel,
which can be eliminated by annealing treatment at a high
austenitization temperature for a long period [17, 19, 20].
No microstructural abnormality could be observed in the
steel sample.
A montage of light micrographs of the long transverse
crack in sample 1 after etching in 2% nital solution is
provided in Fig. 6 at 2009 magnification. Evidence of
partial decarburization at the location of crack origin as
well as along the crack length could be observed very
clearly in the micrographs. The figure also indicates the
presence of non-metallic entrapments (dark gray colored)
within the crack. Figure 7 shows a montage of light
micrographs of the longitudinal crack shown in Fig. 4 in
sample 2 etched in 2% nital solution at 1009 magnification. The figure revealed multiple crack branches filled
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J Fail. Anal. and Preven.
Fig. 3 Optical micrographs of the steel plate specimen in as-polished and unetched condition sectioned from defective region of the top surface
(sample 1) showing transverse cracks of varying lengths at two different locations; (a) short crack and (b) long crack at the plate edge; 9200 mag
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J Fail. Anal. and Preven.
Fig. 4 Optical micrograph of the steel plate specimen in as-polished
and unetched condition sectioned from defective region of the bottom
surface (sample 2) showing deep-cut groove associated with transverse branched crack originating from the groove; 9100 mag
Fig. 5 Microstructure of the steel plate in 2% nital-etched condition
showing ferrite–pearlite structure elongated in the rolling direction;
9200 mag
with massive gray-colored entrapments. No microstructural
abnormality like grain coarsening or deformed grains could
be seen in either side of the bottom surface crack; rather,
the grain structure and distribution of the grain size in the
neighborhood of the defect appeared to be recrystallized
and uniform in nature. Additionally, in contrast to the top
surface edge defect (sample 1), bottom surface longitudinal
defect (sample 2) comprised of two distinguished morphological features having different genesis (Figs. 4, 7),
viz., the ‘V’-groove defect and a multiple branched ‘‘stag
deer horn’’ crack filled with oxide entrapments emanated
from the tip of the ‘V’-groove defect. The formation
mechanisms of these two adjoining defects were different.
The absence of any decarburized layer near the crack origin
or along the crack branches confirmed that the longitudinal
‘V’-shaped groove existed in the slab before the hot rolling
operation in the casting stage itself due to casting deficiencies and the multiple branched crack formed at the tip
of the ‘V’-groove during the rolling under differential
loading.
The scanning electron micrograph of the long crack
observed in sample 1 is shown in Fig. 8a. The enlarged
SEM image of the crack tip, as indicated in the square box
of Fig. 8a, is presented in Fig. 8b. Closer examination of
this region (Fig. 8c) indicated incidence of several fine
spherical particles uniformly dispersed in the steel matrix
on either side of the crack tip. EDS analyses (Fig. 8d)
confirmed these globules to be of iron oxides containing
Mn [17] resulting from internal oxidation. The SEM images of the part of the top and bottom surface crack
entrapment observed in samples 1 and 2 are shown in
Figs. 9a and 10a, respectively. Corresponding EDS spectra
are given in Fig. 9b and 10b, respectively. The elemental
analyses obtained from both the defect spots are provided
in Table 3 which indicate Fe and O with stoichiometric
ratio of FeO (wustite)-type oxide scales. Manifestation of
the scale and internal oxidation associated with the defect
in sample 1 indicated that the cracks were pre-existing at
the surface or subsurface locations in the cast product [21].
Subsequently, during reheating of the slab before rolling,
oxygen present in the reheating furnace must have
ingressed through the crack opening and reacted with the
carbon of the steel resulting in partial decarburization
around the crack. Figure 6 clearly indicates that the entire
exterior surface was not decarburized at all, rather only a
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J Fail. Anal. and Preven.
Fig. 6 Montage of optical micrographs of sample 1 etched in 2% nital solution showing gray-colored entrapment within the long transverse
crack shown in Fig. 3b and partial decarburization along the crack length; 2009 mag
Fig. 7 Montage of optical micrographs of sample 2 etched in 2%
nital solution condition showing gray-colored entrapment within the
crack shown in Fig. 4; 1009 mag
123
layer along the adjoining area of the crack got decarburized. Therefore, deoxidation around the cracks must have
occurred during reheating prior to rolling. These pre-existing cracks in cast slabs usually surface during early
stages of hot deformation of thicker gauge plates. Further,
these casting defects can be linked with the initial stages of
solidification of the steel slab. It is presumed that, during
casting, the solidifying shell at the edge of the steel slab
experienced gross damage by the action of pinch rolls of
the slab caster, producing cracks [22]. On subsequent hot
rolling, these defects were broadened causing the cracks to
extend further from the plate surface to the interior.
On the other hand, the longitudinal groove defect in
sample 2 is believed to have existed prior to hot rolling
from the casting stage itself. During hot rolling operations,
due to differential loading on account of the groove, the
cracks must have developed in the initial rolling stages and
branched during the subsequent rolling passes. The oxide
scales observed within the cracks must have formed during
the cooling process after hot rolling. The absence of any
decarburization around the multiple branched cracks in the
bottom surface confirmed that these cracks must have
formed fresh during the hot rolling process and no time was
J Fail. Anal. and Preven.
Fig. 8 Scanning electron micrograph showing the (a) long transverse crack at the top surface (sample 1); (b) oxide layer in the crack; (c)
incidence of FeO particles at the crack tip; and (d) its corresponding EDS spectrum
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J Fail. Anal. and Preven.
Fig. 9 Scanning electron micrograph of the entrapment in the long
transverse crack at the top surface (sample 1) showing (a) incidence
of FeO and (b) its corresponding EDS spectrum
available for the decarburization reaction to occur. This
constitutes the testimony of their non-existence in the
reheating stage prior to the hot rolling.
Fig. 10 Scanning electron micrograph of the entrapment in longitudinal crack at the bottom surface (sample 2) showing (a) incidence of
FeO and (b) its corresponding EDS spectrum
Table 3 Quantitative analysis data obtained from EDS analysis
(wt.%) of elements present in the transverse crack at the top surface
(sample 1) and longitudinal crack of bottom surface (sample 2)
Element, wt.%
Conclusions
Sample
O
Fe
(1)
Sample 1
19.39
80.61
Sample 2
18.67
81.33
(2)
The defective sample exhibited both short and long
unidirectional transverse cracks in the edge defect
region of the top surface and multiple branched
cracks emanating from the deep grooved longitudinal defect in the bottom surface.
Microstructural examination of the defect regions
revealed partial decarburization and internal oxidation along the length of the top surface edge crack
and at the crack tip, respectively, confirming it to be
a casting defect. On the other hand, absence of any
microstructural abnormality near the crack origin or
123
vicinity of the crack branches of the longitudinal
defect (groove) in the bottom surface of the plate
also confirmed that the groove existed before the hot
rolling operation from the casting stage due to
casting deficiencies and the fresh cracks have
originated from the groove during rolling due to
differential loading.
J Fail. Anal. and Preven.
(3)
The above investigation pinpointed the genesis of
the defects. Based on these findings, the problem
area was identified in the casting stages and necessary corrective measures were taken to minimize the
defect incidences. Presently, the defect occurrences
have been reduced from 30% to 5%.
Acknowledgments The authors are thankful to the Management of
R&D Centre for Iron and Steel (RDCIS), Steel Authority of India
Limited (SAIL), Ranchi, India, for their support and encouragement
in pursuing this work. Heartfelt thanks are due to Mr. J. Guria for his
assistance in optical and scanning electron microscopy job.
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