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Effect of Rolling Reduction on Microstructure
and Mechanical Properties of Plain Low Carbon
Steel
Article in Key Engineering Materials · July 2016
DOI: 10.4028/www.scientific.net/KEM.701.187
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Key Engineering Materials
ISSN: 1662-9795, Vol. 701, pp 187-194
doi:10.4028/www.scientific.net/KEM.701.187
© 2016 Trans Tech Publications, Switzerland
Submitted: 2015-10-21
Revised: 2016-02-02
Accepted: 2016-03-29
Effect of Rolling Reduction on Microstructure and Mechanical
Properties of Plain Low Carbon Steel
PHOUMIPHON NORDALA1,3,a, RADZALI OTHMAN2,b,
AHMAD BADRI ISMAIL1,c,∗
1
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains
Malaysia, 14300 Nibong Tebal Pulau Pinang, Malaysia
2
Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, 76100 Durian
Tunggal, Melaka, Malaysia
3
Department of Mechanical Engineering, Faculty of Engineering, National University of Laos, LaoThai Friendship Road, Sisatanak District, Vientiane Capital, Laos
a,
[email protected], [email protected], c,∗[email protected]
Keywords: Plain low carbon steel, Dual-phase steel, Cold working, Annealing, Ultrafine grains
Abstract. In the present study, the effect of cold-rolling for the amount of reduction in thickness
ranging from 25% to 75% on microstructure and mechanical properties of plain low carbon steel
processed from dual-phase ferrite-martensite starting microstructure was studied. As the coldrolling, the microstructure elongated to rolling direction and more compressed with increasing the
rolling reduction and strength also increased. After annealing at warm temperature 500°C, the
ultrafine grained was obtained in the 75% rolling reduction. Moreover, it was exhibited excellent
strength of 82% and hardness of 66.1% higher than as-received condition with adequate uniform
elongation 9.6%.
Introduction
One of the most major factors in the microstructural control of structural metallic materials is
grain refinement [1]. It has been reported by previous work that ultrafine-grained (UFG) steels with
grain sizes smaller than a few micrometers have super mechanical properties such as high strength,
enhanced superplasticity [1-3]. In order to obtain the UFG microstructure, the severe plastic
deformation (SPD) process such as equal channel angular pressing (ECAP) [4,5], accumulative rollbonding (ARB) [6], and high pressure torsion (HPT) [7], are sometimes used. However, these
processes also have the disadvantages as that they are difficult to use in the mass production and
large dimension for steels. Moreover, they need the special equipment and large amount of plastic
working energy [8,9].
Currently, dual-phase (DP) steels are materials of increasing commercial interest for automotive
application due to these materials have a combination of special mechanical properties, such as
continue yielding behavior, superior strength-ductility and excellent formability. The microstructure
of DP steel consists of hard martensite islands dispersed in a soft and ductile ferrite matrix [10,11],
but the main challenge in producing DP steels is to achieve grain refinement at the same time
making them cost effective. Although researchers have been investigated the grain finement to
improve the mechanical properties of dual-phase steels, but they have largely concentrated on using
higher carbon content (>0.15 wt. %) [12-14] and high alloy [15,16] steels which pose problems
with weldability.
Therefore, the present work is to study the effect of rolling reduction of plain low carbon steel
(0.06 %C). The key of the process is to start from DP ferrite-martensite microstructure.
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Science and Engineering of Materials II
Materials and Experimental Procedures
The chemical composition of the commercial plain low carbon steel used in the present
investigation is shown in Table 1. The microstructure of the as-received hot rolled sheet sample
consists of mainly ferrite with small amount of pearlite as shown in Fig. 1.
First of all, samples with a size of 5 mm in thickness, 25 mm in width and 100 mm in length
were cut from the plate and then the sample was austenitized at 1000°C for 30 min, followed by
water quenching to get DP ferrite-martensite microstructure. The quenched sheet was cold-rolled to
a reduction of 25%, 50% and 75% in multi-pass at room temperature using a laboratory rolling mill
(roll diameter: 80 mm, speed: 20 rpm/minute). The cold-rolled sheet was annealed at various
temperature ranging from 300°C to 700°C for 30 min before air cooled. The process is shown in
Fig. 2.
Table 1: Chemical composition of investigated steel (wt%).
C
Mn
P
S
Fe
0.06
0.14
0.01
0.01
Bal.
Fig. 1. Microstructure of as-received specimen.
The microstructures of specimens at each stage of the process were observed by optical
microscopy (OM) and Scanning electron microscopy (SEM). The SEM observations were
conducted in a ZEISS SUPRA 35 PV equipment. All the microstructural observations were carried
out from the transverse direction (TD) of the sheets. The specimen surfaces for the OM and SEM
observation were revealed by 2% nital solution. Microhardness tests were carried out with a load of
100g for 10 seconds using Vickers Microhardness Tester (Model: LM 2448 AT). The mechanical
properties of the specimens with a size of 10 mm in gauge length and 5 mm in gauge width were
examined by an INSTRON 5982 digital testing machine with a tensile velocity of 5 mm/min at
room temperature.
Results and Discussion
Fig. 3 shows the Optical microstructures of the as-quenched specimen (a), and 25% (b),
50% (c), and 75% (d), cold-rolled of the specimens. As can be seen in the Fig. 3a, the DP ferritemartensite is obtained after water quenching, which is the starting microstructure of the present
work. In Fig. 3b, the ferrite and martensite are so large that by 25% rolling reduction they become
only closer and there are no significant differences between this microstructure and as-quenched DP
ferrite-martensite (Fig. 3a). In Fig. 3c, the microstructures are elongated to the rolling reduction and
ferrite-martensite are more compressed together but they are still distinguishable. But in Fig. 3d,
ferrite-martensite are no longer recognizable and microstructures are totally smashed. Just by 75%
cold rolling of ferrite-martensite microstructure, the formation of nano/ultrafine grains can be
Key Engineering Materials Vol. 701
189
expected after annealing. After cold rolling, the strengthening of steel occurs because of dislocation
movements and dislocation generation within the crystal microstructure of the steel. Zhao et al. [17]
reported that there is increased in the stored energy of the steel after cold rolling due to the high
dislocation density and this provides the driving force for the ferrite recrystallization during
annealing process.
Fig. 2. Schematic representation of the dual-phase process used in this study.
Fig. 3. Optical microstructures of the as-quenched (a) and cold-rolled to a reduction of 25% (b),
50% (c) and 75% (d).
Fig. 4 illustrates microstructures of the specimens cold-rolled to various reductions and annealed
at 500°C for 30 min where significant changes in mechanical properties are observed. It can be seen
in Fig. 4c, 75% cold-rolled and annealed at 500°C, the microstructure changed to equiaxed ultrafine
ferrite gains with mean grain size of about 300 nm are formed. Additionally, nano-carbide particles
dispersed within the ultrafine grain ferrite matrix, because the starting microstructure was a
supersaturated solid solution carbon (martensite). It is anticipated that the carbides play an
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Science and Engineering of Materials II
important role in the microstructural change in annealing and in turn they extremely affect the
mechanical properties. The similar microstructure has been observed by Hosseini et al. [18]. In Fig.
4(a and b), as 25% and 50% cold-rolled, respectively, and annealed at 500°C indicates only carbide
in microstructure. The size and number of carbide become smaller and lesser with decreasing the
rolling reduction. There is no formation of ultrafine ferrite grain. This is possibly due to rolling
reduction only 25% and 50% were not enough to obtain ultrafine ferrite grain like shows in Fig. 4c.
Fig. 4. SEM microstructure of the DP steel annealed at 500°C for 30 min after cold-rolled to (a)
25%, (b) 50% and (c) 75% reduction.
The evolution of microhardness value is shown in Table 2. There was a low microhardness value
in the as-received condition. The microhardness increases as the specimen quenched by water
because of the martensite is the one of common strengthening phases in steel. In general, the
microhardness increases due to the refinement of the primary phases after rapid cooling. It is well
known that quenching by water produces a supersaturated solid solution and vacancies increase
with carbon content in water quenched samples. Therefore, high hardness corresponds with high
resistance to slip and dislocation [19]. Additionally, Kurdjumo and Khachaturyan [20] stated that
the carbon dependence of hardness is attributed to carbon atoms trapped in the octahedral interstitial
sites of martensite crystal microstructure in martensite. Thus, the increase in the microhardness with
water quenched steel can be explained by the increasing relative volume of martensite after
quenching. The microhardness is increased substantially after 25% cold-rolled and increase with
increasing the rolling reduction. On the other hand, as increasing the amount of cold-rolling resulted
increasing of dislocation density (Fig. 3d). Tewary et al. [21] reported that the improvement of
microhardness after cold deformation is due to strain hardening, achieving by dislocationdislocation interaction as well as the interaction of dislocation with twin boundaries. As the
specimen annealed at various temperature, microhardness slightly decrease with increasing the
annealing temperature. It is because of the reduction of dislocation density [2].
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191
Table 2: Evolution of microhardness (HV) value of as-received, as-quenched, as-cold-rolled and asannealed specimen.
Annealing Temperature
Initial
Microhardness
300˚C
400˚C
500˚C
600˚C
700˚C
As received
95.3
-
-
-
-
-
As quenched
144.5
-
-
-
-
-
25% Cold-rolled
161.6
148.2
140.6
137.1
110.4
98.7
50% Cold-rolled
177.0
156.4
148.3
141.8
126.2
105.7
75% Cold-rolled
181.1
167.4
162.7
158.3
135.8
111.3
Specimen
Fig. 5 displays the mechanical properties at each stage of the process. In Fig. 5a illustrates the
stress-strain curves of the as-received with ferrite-pearlite microstructure, the as-quenched with DP
ferrite-martensite microstructure and the cold-rolled to various reductions. It can be observed that
the as-received specimen indicates typical stress-strain curve of plain low carbon steel with ferritepearlite microstructure (Fig. 1); it has yield point elongation and show clear yield point. After
quenching, the strength raises up to 526.4 MPa which is 43.7% higher than as-received specimen.
The as-quenched specimen shows no yield-drop. The uniform elongation decreases from the asreceived state. The curves of all the cold-rolled specimens exhibit their high strength followed by
necking and fracture. As a result, the uniform elongation is limited. Such stress-strain curves are
similar to those of the result that has been also observed as-SPD processed materials having single
phase [22]. The strength increases, but elongation decrease with an increasing rolling reduction, and
the strength of the 75% cold-rolled specimen reaches to 780 MPa, which is 112.9% higher than asreceived specimen. In Fig. 5b exhibits the stress-strain curves of the specimen cold-rolled to a
various reductions and then subsequently annealed at 500°C for 30 min before air cooling. First, the
yielding behavior is discussed. The yield drop phenomenon was observed in all specimens coldrolled arrange from 25% to 75% and annealed at 500°C for 30 min, which had the ultra-ferrite grain
size. This is similar results with the previous studies by Okitsu et al. [23]. They stated that the yield
drop phenomenon was observed due to the ferrite grain size was smaller than 1 µm and the yielding
behavior was slightly different if the grain size was larger than 1 µm. On the other hand, Meyers et
al. [24] described that the phenomenon of a sharp yield drop and the appearance of an upper and
lower yield point in the tensile stress-strain curve due to the effect of interstitial solute carbon atoms
in the low carbon steel, and is called the Cottrell atmosphere, responsible for the locking-in of
dislocation. When a stress is applied to the specimen in a tensile test, it must exceed a certain
critical value to unlock the dislocations. The stress essential to move the dislocation is less than the
stress required to unlock them; thus the occurrence of a sharp yield drop and the phenomenon of a
lower and upper yield point in the tensile stress-strain curve. Moreover, Brindley et al. [25] reported
that the yield drop phenomenon is well known and commonly observed in tensile deformation of
carbon steel due to the presence of interstitial alloying elements, especially carbon. The yield drop
phenomenon is not observed usually in the tensile deformation of IF steels because they do not
contain solute carbon and nitrogen. Although the mechanism of the yield drop phenomenon in the
ultrafine grained steels has not been fully clarified yet, however, it is well known that all types of
yield drop phenomenon are due to the lack of mobile dislocations and the present results seem to be
attributed to the mobile dislocations as well. In the present study, it is noteworthy that the specimen
75% cold-rolled and annealed at 500°C performs excellent strength which is 82% higher than asreceived specimen. This means that significant strengthening by grain refinement was achieved in
the present ultrafine grained steel (Fig. 4c). As 25% and 50% cold-rolled specimen indicate lower
strength compared to 75% cold-rolled specimen. This is due to rolling reduction were small, and not
enough to obtain ultrafine grained like 75% cold-rolled specimen (Fig. 4c). On the other hand, it
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Science and Engineering of Materials II
can be observed that in the Fig. 5b, the reason for sudden drop in tresses for 75% and 50% at strain
within 10%-20% is because the amount of cold worked applied to the specimen which is 75% and
50% with the reduction of 666.7 MPa and 619.6 MPa, respectively. It is clearly calculated from Fig.
5(a and b) for the amount of reduction in stress is about 19%. It is well known that the strengths
induced by cold works is a function of the amount of cold worked done.
Fig. 5. Stress-strain curves plain low carbon steel (a) cold-rolled to a reduction of 25%, 50% and
75%. The data of the as-received (ferrite-pearlite) and the as-quenched (dual-phase ferritemartensite) specimen are also shown, (b) cold-rolled to a various reduction and subsequently
annealed at 500°C for 30 min.
Recently, it has been reported from the previous work on developing UFG steels. Among of
them, Azizi-Zlizamini et al. [3] reported a considerable uniform elongation of 14%, but the ultimate
tensile strength only 550 MPa, and Furuhara et al. [26] showed a good balance of strength and
elongation, but the high carbon content of these materials seems to be difficult to apply pearlitic
steels to automobile body parts due to poor spot-weldability. However, the present study exhibits
UFG microstructure with excellent tensile strength (666.7 MPa) and adequate uniform elongation
(9.6%).
Conclusion
The effect of the rolling reduction ranging from 25 % to 75% on microstructure and mechanical
properties of the plain low carbon steel process from DP ferrite-martensite microstructure can be
concluded as follows:
1. In the cold-rolled condition, the grain size become more compressed and elongated in the
rolling direction with increasing the rolling reduction.
2. The strength increased, but the elongation decreased with increasing the reduction.
3. After annealing at warm temperature 500°C, the ultrafine grain ferrite was obtained with
nano-carbide particles dispersed within the ultrafine grain ferrite matrix, because the starting
microstructure was a supersaturated solid solution carbon (martensite).
4. Mechanical properties in terms of microhardness and tensile test exhibited that the specimen
75% cold-rolled and subsequently annealed at 500°C performs the best microhardness,
strength and uniform elongation balance which are 158.3 HV, 666.7 MPa and 9.6%,
respectively. On the other hand, the 25% and 50% cold-rolling illustrated low mechanical
properties due to the rolling reduction were small and not enough to obtain UFG like 75%
cold rolling.
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193
Acknowledgement
The authors gratefully acknowledge the financial support from AUN/SEED-Net project under
JICA, Grant No: 304/PBAHAN/6050283/A119 and technical support from Universiti Sains
Malaysia (USM) and Universiti Teknikal Malaysia Melaka (UTeM).
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