fabrication and microstructural control of nano-structured bulk steels: a review

Fabrication and Microstructural Control of Nano-structured BulkSteels: A Review

Linxiu Du • Shengjie Yao • Jun Hu • Huifang Lan • Hui Xie • Guodong Wang

Received: 4 April 2014 / Revised: 9 May 2014

� The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014

Abstract There exist strong interests of developing nano-grained steels because of the outstanding properties including

high strength/weight ratio, wear resistance, excellent toughness, and favorable cellular activity. This article reviews the

main fabrication process and microstructural control of nano-structured steels over the last decades. Severe plastic

deformation is considered as an effective route of obtaining the nano-grained microstructures. The process of cold-rolling

and annealing of martensitic steel is a viable method to obtain bulk nano-structured low carbon steel, while the final

thickness of the cold-rolling plate is limited. According to the theoretical results of the thermal simulation studies, a novel

alloy design combined with the rapid transformation and rolling process is proposed to successfully fabricate nano-grained

high strength bulk steel. The refinement mechanisms are expected to be taking advantage of increase in the transformation

nucleation sites and inhibiting the grain coarsening. Moreover, corresponding mechanical properties are summarized.

KEY WORDS: Nano-structured steel; SPD; Cold-rolling and annealing martensite; RTRP; Transformation

mechanism; Mechanical properties

1 Introduction

Grain size is a key microstructural factor affecting physical

and mechanical properties of the materials [1, 2]. Grain

refinement can markedly increase the strength as well as

improve the toughness of the metals [3–5], and it is also

reported that some metals with nano-grained/ultrafine-

grained structures have positive effects on the biochemical

response to surrounding media [6]. Therefore, grain

refinement has been attracting considerable interest from

metal engineering scientists [7–16].

Fabrication of nano-structured bulk steels is one of the

highlights in the nanomaterials research area because it is

believed that the nano-structured bulk steels have very

good prospects in the application of structure manufactur-

ing due to their high strength and toughness. Many efforts

have been made to fabricate the bulk nano-structured

steels, including SPD [17, 18], low carbon steel with

martensite cold-rolling and annealing [19, 20], and RTRP.

[21–23]. The history of SPD can be traced back to 1950s,

and since then many new SPD methods have been devel-

oped to obtain nano-grained and ultrafine-grained micro-

structures in the metals. SPD is becoming one of the most

actively developing areas in nanomaterials especially in

recent two decades. Some SPD methods such as ball

milling, sliding wear, ultrasonic shot peening, etc., can be

applied in the fabrication of nano-grained structures in the

metal surface, and the other methods such as accumulative

roll-bonding (ARB) and equal channel angular pressing

Available online at http://link.springer.com/journal/40195

L. Du (&) � J. Hu � H. Lan � H. Xie � G. Wang

State Key Laboratory of Rolling and Automation, Northeastern

University, Shenyang 110819, China

e-mail: [email protected]

S. Yao

School of Materials Science and Engineering, Harbin Institute of

Technology at Weihai, Weihai 264209, China

123

Acta Metall. Sin. (Engl. Lett.)

DOI 10.1007/s40195-014-0077-8

http://link.springer.com/journal/40195

(ECAP) can be used to produce bulk nano-structured metal

materials. Although some important progresses have been

achieved, the uptake by industry has been sluggish, which

is because large amount plastic energy and special proce-

dures are needed, and the throughput is lower [1, 24].

Annealing the cold-rolled martensite is another route for

the fabrication of nano-structured steels. This method can

be applied in low carbon and stainless steels, and the

advantage of this method is that it does not need large

amount of strain. So it is promising in the application of

nano-structured cold-rolled steel sheets.

In recent years, the fabrication of nano-grained bulk steel

through rapid transformation and rolling deformation has

made some attractive progresses [21–23]. There are two key

procedures in this method: first, the austenite was refined to

less than 2–3 lm, and second, the decomposition of the

ultrafine austenite was controlled by employing heavy

deformation during cooling process. The features of this

method are that there is no need for large plastic-working

energy and special procedures, and the microstructures and

properties of nano-structured steels can also be controlled by

austenite decomposition during cooling process.

All above-mentioned methods can be regarded as the ‘‘top

down’’ approach involving grain refinement through

‘‘breaking down’’ the microstructure of the bulk steel to the

submicron scale. In general, these methods can be classified

into three categories: SPD, martensite cold-rolling plus

annealing, and RTRP [1, 24]. In this paper, the authors will

give a brief review on ECAP, ARB, and multi-directional

forging (MDF), which are three typical SPD methods. Then,

two research works about martensite cold-rolling and

annealing for low carbon steel and stainless steels will be

introduced. Finally, the authors will introduce the procedures

of RTRP and the mechanical properties of nano-grained

steels fabricated by RTRP, and the theoretical aspects about

microstructural control during RTRP will also be introduced.

2 Fabrication of Nano-structured Bulk Steels by Severe

Plastic Deformation

There are many SPD methods for grain refinement of

metals, and all the SPD methods have a unique feature that

high strain is imposed at relatively low temperatures

(usually less than 0.4Tm) without any significant change in

the overall dimensions of the work piece. The ECAP, ARB,

and MDF are three typical methods which have advantage

in fabrication of nano-structured bulk steels [1, 2, 24].

2.1 Equal-Channel Angular Pressing (ECAP)

ECAP was developed by Segal et al.[25] in Soviet Union in

the mid-1970s. At present, it is the most popular and

developed SPD processing technique. Figure 1 is the

principle schematic diagram of ECAP [24]. The die using

in the ECAP has a channel that is bent at an abrupt angle,

during which a rod-shaped billet is pressed through the die

and a shear strain is introduced when the billet passes

through the point of intersection of the two parts of the

channel. The equivalent strain, eeq, introduced per pass in

ECAP with an angle of 90� between the channels amounts

to 1.15. Because the cross-sectional dimensions of the

billet remain unchanged during ECAP, the exceptionally

high strains can be obtained when the pressings are repe-

ated, and this accumulated shear strain will ultimately lead

to a UFG structure consisting of homogeneous and equi-

axed grains with grain boundaries having high angles of

misorientation [26, 27]. The typical TEM micrographs of

low carbon steel with nano-grained ferritic microstructure

subjected to ECAP and annealing are depicted in Fig. 2

[17, 24].

Obviously, the conventional ECAP is not cost efficient,

so many modifications for conventional ECAP have been

designed to increase production efficiency and the grain

refinement effect. These modifications include the incor-

poration of a backpressure, the development of continuous

processing by ECAP, and others [24].

2.2 Accumulative Roll Bonding (ARB)

The technique of ARB was introduced by Saito et al. [28],

a process which was supposed to overcome the limitation

of the ECAP, namely the low productivity. The greatest

Fig. 1 The principle schematic diagram of ECAP [24]

L. Du et al.: Acta Metall. Sin. (Engl. Lett.)

123

advantage of this method is that it makes use of a con-

ventional rolling facility. As illustrated in Fig. 3 [24], a

metal sheet is rolled to one-half of the thickness, and the

rolled sheet is cut into two halves and then the two halves

sheets are stacked together. The stacked sheets are then

rolled again to one-half thickness. To achieve good bond-

ing during the rolling operation, the two contact faces are

degreased and wire brushed before placing them in contact.

Thus, a series of rolling, cutting, brushing, and stacking

operations are repeated so that ultimately a large strain is

accumulated in the sheet.

The ARB was successfully applied to a wide range of

materials, including Al–Mg alloy and interstitial-free steel

[29]. It has not been reported that ARB can be applied to

other kind of steels, for example, HSLA steels and alloy

steels. In practice, the UFG structure produced by ARB is

not three-dimensionally equiaxed but rather there is a

pancake-like structure that is elongated in the lateral

direction [24].

2.3 Multi-directional Forging (MDF)

MDF is also termed as bi-directional large strain defor-

mation [1, 2]. Compared to the rolling with the deformation

limited to single direction, MDF can be used to impart

large and multi-axial plastic strains in steels. Moreover,

considering the compression resistance, MDF conducts at

elevated temperature within the 0.1Tm - 0.4Tm, where Tm

is the melting temperature. The mechanisms of severe

plastic deformation (SPD) (large and multi-axial strain)

and thermo-mechanical control process (phase transfor-

mation) can be simultaneously exploited. Therefore, the

MDF was capable of fabricating large-size products. The

deformation of hard and brittle metals can be realized by

optimizing the parameters of temperature and reduction.

The ultrafine-grained/nanograined steel has been success-

fully manufactured by MDF technique [18].

3 Fabrication of Nano-structured Bulk Steels

by Cold-Rolling and Annealing

3.1 The Cold-Rolling of Low Carbon Steel

with Martensite and Annealing

Tsuji et al. [19, 20] initiated the research of producing

nano-structured bulk steels by cold-rolling and annealing

of martensite with plain low carbon steel. Since no SPD

and special equipment are needed, this method is consid-

ered to be applicable in mass production of nano-structured

bulk steels. After 50% cold-rolling reduction, lamellar

dislocation cells with thickness of 60 nm are formed. In the

following 450–550 �C annealing, the multiphased

Fig. 2 TEM micrographs of low carbon steel subjected to ECAP and annealing: a ferrite phase of as-ECAP; b ferrite phase annealed for 1 h at

450 �C; c ferrite phase annealed for 1 h at 510 �C [17]

Fig. 3 The principle schematic diagram of ARB [24]


123

microstructure composed mainly of equiaxed ferrite grains

with size of *180 nm and nanosized carbides was formed,

as shown in Fig. 4.

It is noted that only a strain of 0.8 can produce nano-

sized microstructure, while strain over 4 is necessary for

SPD method [30, 31]. This may be attributed to the starting

microstructure. Lath martensite has a hierarchical multi-

scale microstructure including packets, blocks, and laths

[32, 33], as shown in Fig. 5. The packets and blocks,

having high angle boundaries, are considered to refine the

prior austenite grain during martensite transformation.

Furthermore, dislocation cells are generated from the high

dislocated laths after cold rolling, the misorientation of

which becomes larger with increasing strain, so that the

microstructure would be further refined. In addition, con-

tinuous recrystallization [34] is another important factor in

turning the dislocation cells boundaries into true clear grain

boundaries, resulting in real nanosized ferrite grains.

Figure 6 shows the typical tensile stress–strain curves of

the cold-rolled and annealed steel given by Ueji et al. [20].

It can be found that work hardening was limited until the

annealing temperature reached 550 �C. An optimal

strength-ductility balance of 870 MPa tensile strength and

9% uniform elongation was obtained in the nano-structured

steels. They considered the satisfying strength-ductility

balance was attributed to the nanosized carbides, which

help enhance work hardening. However, it is noted that a

remarkable decrease of strength (*300 MPa) occurred as

the annealing temperature increased to 560 �C. Moreover,

remarkable grain growth occurred for the 600 �C anneal-

ing, which implied the inferior thermal stability of the

nanosized microstructure.

In view of this, Lan et al. [35] fabricated the nano-

structured bulk steels with this method using a microal-

loyed steel. The result showed that grain coarsening tem-

perature was increased to 650 �C by introducing a number

of nanosized microalloyed carbides. The reason was

expected to be that the tiny carbides effectively pinned the

dislocation and grain boundary movement, retarding the

progress of continuous recrystallization, and thus increased

the thermal stability of nanosized microstructure.

As is known, with the decreasing of ferrite grain size,

work hardening may be diminished, which would lead to

Fig. 4 TEM morphology of nano-structured steel produced by 50%

cold rolling and annealing at 500 �C for 30 min [19, 20]

Fig. 5 Microstructural hierarchy of the lath martensite [32]

Fig. 6 Nominal stress–nominal strain curves of specimens corre-

sponding to 50% cold rolling and annealing at various temperatures

[34]


123

fracture almost immediately after the onset of necking. Thus,

it is essential to attain higher work hardening rate in the nano-

structured bulk steels. Lee et al. [36, 37] obtained nano-

structured transformation induced plasticity steel containing

6 wt% Mn by intercritical annealing the cold-rolled mar-

tensite at 640–680 �C. The microstructure was composed of

ferrite grains size by 210–300 nm, and high fraction of

retained austenite ([15 vol%), as shown in Fig. 7. The high

volume fraction of retained austenite was considered to be

attributed to the high stability supplied by the partitioning of

Mn as well as by the refined austenite island size. 1,213 MPa

tensile strength and 9.5% uniform elongation, and espe-

cially, enough work hardening were obtained after 680 �C

annealing. It was indicated that TRIP effect occurred for the

680 �C annealing, which is believed to effectively suppress

the negative effect resulting from the absence of work

hardening in nano-structured steel. This conclusion is con-

sistent with the work by Gibbs et al. [38]. Thus, introducing

metastable retained austenite into the nano-structured steel

seems a promising way to achieve the optimal strength-

ductility balance. Due to the high work hardening rate and

negligible localized deformation, this kind of nano-struc-

tured bulk steel has potentials in forming applications.

3.2 The Cold-Rolling and Annealing of Stainless Steel

with Phase-Reversion-Induced Nanograined/

Ultrafine-Grained Austenite

Compared to carbon mild steels, austenitic stainless steels

enjoy excellent formability, weldability, work hardening

properties, and high energy absorption capabilities [39].

However, the austenitic stainless steels are less suitable

for structural applications due to their relatively low yield

strength of 230–350 MPa [40]. Therefore, ultra refinement

of austenitic grains is indispensable for obtaining the high

strength. The nano-crystalline surface of austenite stain-

less steel exhibited considerably higher resistance to

corrosion, wear, and corrosive wear, compared to those of

regularly grained specimens [41]. Moreover, the austenitic

stainless steels are widely used for biomedical applica-

tions because they are corrosion resistant and have the

biocompatibility [6]. The grain size of austenitic stainless

steels is limited to 20–40 lm by conventional thermal–

mechanical control process (TMCP). Therefore, it is

necessary to employ a novel processing route of devel-

oping nanograined/ultrafine-grained structure in austenitic

stainless steels.

Fig. 7 UFG microstructure and Mn distribution across retained austenite [37]


123

Pajasekhara [40] did systematically job on the ultrafine-

grained structures formed by reversion in metastable au-

stenitic 301LN stainless steels. The Md30 value (35 �C)

above room temperature indicates that stress-induced

martensite will form under deformed at ambient tempera-

ture. The samples from a 1.5-mm thick sheet were sub-

jected to 63% cold reduction. Then the specimens were

annealed at 800–1,000 �C and held for 1–100 s, respec-

tively. The TEM micrographs of cold-rolled sample reveal

that the microstructure primarily consists of martensite

(Fig. 8). Selected area diffraction patterns (SADPs) from

different regions of the specimen indicate the presence of

the lath-type martensite (Fig. 8b) and the dislocation-cell-

type martensite (Fig. 8c). The specimens annealed at

800 �C for 1 and 10 s (Fig. 9) exhibit a mixture of large

equiaxed austenitic grains, small newly nucleated grains of

austenite, and secondary-phase precipitates of CrN. The

specimen annealed at 800 �C for 1 s showed the smallest

average grain size of *0.54 lm.

To obtain the nanocrystalline structure, Eskandari et al.

[39] used a repetitive thermo-mechanical process.

Decreasing supersaturating strain with reduced rolling

temperature was concluded. It was also proposed that the

volume fraction of SIM increases with increasing reduction

(or strain) and strain rate. Consequently, the nanocrystal-

line austenitic structure was obtained.

4 Fabrication of Nano-structured Bulk Steels by Rapid

Transformation and Rolling Process (RTRP)

4.1 Fabrication Process of Nano-structured Bulk Steels

by RTRP

4.1.1 Thermal Simulation Experiments

Yokota et al. [42] thermodynamically analyzed the possibility

of fabricating nano-sized microstructure in transformation.

Fig. 8 TEM images of cold-rolled AISI 301LN SS a, SADP of lath-type martensite b, SADP of dislocation-cell type martensite c


123

The theoretical derivation represents that ferrite grains trans-

formed from austenite in appropriate grain size could be

refined to below 100 nm with improved driving force. How-

ever, that great driving force is impractical to realize. But if the

austenite grain size was reduced to close to nanoscale, ferrite

grains should be nanocrystallized with applicable

undercooling.

Therefore, some tentative thermal simulation experi-

ments were implemented in a Nb-bearing steel [43]. The

austenite grain size was refined to 1–2 lm through heavy

warm deformation and cycle-quenching. Under common

continuous cooling conditions, the grain size of ferrite

transformed from the ultrafine austenite will be near to or

larger than the original austenite grain size (Fig. 10a), but if

the heavy deformation was applied below Ar3 during cooling,

the uniform and equiaxed ferrites with submicro size can be

obtained (Fig. 10b). Further characterizations in TEM

microstructure and SAD (Fig. 11) indicate that high angle

grain boundaries locate in the ultrafine ferrite grains.

4.1.2 Warm Rolling Bulk Nanograined Steel Strip

According to the results of thermal simulation experiments,

a novel V–N–Cr microalloyed steel was designed. The V–

N microalloying concept [44, 45] was adopted to exploit

VN precipitates in austenite for the intragranular nucleation

of ferrite, given the small lattice mismatch between VN

(lattice parameter is 0.4139 nm) and ferrite (lattice

parameter is 0.2865 nm) for (100)VN//(100)a planes, facil-

itating ferrite nucleation. The addition of Cr to the exper-

imental steel stabilizes austenite and avoids decomposition

of austenite prior to warm-rolling.

The two-step cycle quenching was conducted on the

50 mm thick slab for refining the prior austenite grain size

[46]. The 50 mm thick slab was heated to 900 �C and held

for 300 s for complete austenization followed by water-

cooling to 550 �C at a cooling rate of 35 �C/s. The slab was

warm-rolled to plate thickness of 5.5 mm in ten passes

using a U450 mm rolling mill, and finally air-cooled to

Fig. 9 TEM images of cold-rolled AISI 301LN SS samples annealed at 800 �C for 1 s a and 10 s b

Fig. 10 Microstructures of the samples with ultrafine-grained austenite after cooling from 900 �C to room temperature (RT) with different ways:

a cooling rate 10 �C/s, ferrite ? pearlite; b cooling to 700 �C, 75% deformation and air-cooling to RT


123

room temperature. During finish rolling, the temperature

raised from 550 to 580 �C. Another plate was water-

quenched to room temperature after warm-rolling. The

image of nano-grained bulk experimental steel plates with

the dimensions of 5.5 mm in thickness and 65 mm in width

is depicted in the Fig. 12.

In a manner similar to warm-rolled and air-cooled steel,

the polygonal ferrite grains of warm-rolled and water-

quenched steel are 300–400 nm (Figs. 13a, 14a), and the

grain boundaries are well-developed. This suggests that the

dynamic transformation and dynamic recrystallization

(DRX) occurred during warm-rolling. However, few ferrite

grains of *200 nm thick are elongated and martensite is

present (Fig. 14b). Compared to warm-rolled and air-

cooled experimental steel, there is lower fraction of coarse

(V, Cr, Fe)(C, N) precipitates in the warm-rolled and

water-quenched experimental steel.

The yield strength, tensile strength, and elongation to

fracture of warm-rolled and air-cooled experimental steel

are 885 MPa, 920 MPa, and 19.8%, respectively. They are

745 MPa, 935 MPa, and 19.5% for the warm-rolled and

water-quenched experimental steel. The tensile properties

of ferrite-martensite microstructure with nanoscale features

are impressive. The warm-rolled and water-quenched

experimental steel exhibits enhanced work hardening

ability because of the existence of martensite islands [47].

4.2 Three Theoretical Aspects About Microstructural

Control During RTRP

4.2.1 The Deformation Behavior of Ultrafine Austenite

Deformation behaviors of austenite grains in steels are

always investigated in the size range of much more than

10 lm and three typical mechanisms (work hardening,

dynamic recovery, and recrystallization) have been con-

cluded until now [48–51]. However, as austenite grain size

decreases to smaller than 10 lm, the amount of austenite

grain boundary increases remarkably. Therefore, the

deformation mechanism of ultra-fine austenite grains may

be different from the coarse austenite grains. Taking these

important possibilities into consideration, the ultra-fine

austenite grains were first fabricated, and then their cor-

responding deformation behavior was systematically

studied.

Some efforts had been made in order to illustrate the

coordinating mechanism happening in the deformation of

austenite grains in size of 1–3 lm (Fig. 15) [22]. Different

kinds of true strain-true stress curves are clearly identified

at both 900 and 950 �C in single pass compression at a

strain rate of 10 s-1 (Fig. 16). In this condition, DRX can

hardly occur at 900 �C, especially with such a high strain

rate. Therefore, it is deduced that the softening phenome-

non in our experiments could be mostly recognized as a

result of grain boundary sliding.

Further evidence, as shown in Fig. 17, presents coars-

ening austenite grains that still keep equiaxial existing

Fig. 11 TEM image showing the microstructure a and corresponding selected area diffraction pattern b of the sample deformed 75% at 700 �C

and air-cooling to RT after cycle-quenching for three times

Fig. 12 Photo of nano-grained bulk experimental steel plates with

the dimensions of 5.5 mm in thickness and 65 mm in width


123

partially in the samples, and means grain boundary sliding

and/or grain rotation definitely occurs during the process of

high-speed deformation. There is no other mechanism that

can make grains grow without changing their shape, which

is more involved in the researches about superplastic

deformation. Theoretically, superplasticity can be achieved

in any kind of materials if special preconditions are

selected. In most cases, sample with equiaxial ultra-fine

grains is apt to obtain superplasticity with T C 0.5Tm and

low strain rate ( _e B 10-3 s-1) [52]. Therefore, austenite

grain boundary sliding could be mainly in connection with

the relatively higher deformation temperatures (T & 0.6Tm

in our research) and such small initial grain size (1–3 lm).

Deformation behaviors of austenite grains with the size

C10 lm are mainly dominated by the dislocation move-

ment including work hardening, dynamic recovery, and

recrystallization, which are the theoretical basis of TMCP

of the steels. It is observed that the deformation resistance

of *2 lm austenite is reduced by taking the advantage of

the new mechanisms of grain boundary sliding and/or grain

Fig. 13 SEM micrograph of the warm-rolled and air-cooled experimental steel, showing the polygonal ferrite with dispersive precipitates a and

EDS analysis of precipitates b

Fig. 14 SEM micrographs of the warm-rolled and water-quenched experimental steel: a fully transformation and completely recrystallization;

b partly transformation and inadequately recrystallization

Fig. 15 Ultrafine austenite obtained after four-time repetitive treat-

ment of reheating and quenching


123

rotation. Therefore, it is expected that the hard and brittle

metals are capable to be processed at low deformation

temperature and high strain rate if the grain size is ultra-

fine. The microstructural evolution of the steels with ultra-

fine austenite during hot deformation is different from that

with coarse austenite grain (C10 lm), so in order to make a

better use of TMCP, the steel with ultra-fine austenite

should be further investigated.

4.2.2 Stability of the Ultrafine Austenite

After the accomplishment of refining austenitic grains, it

is necessary to understand the growth kinetics and

thermodynamics mechanism in samples with ultrafine

grain size. The reason lies in that controlling the prior

austenite grains is a key point for the decomposition

characteristic of austenite and the effect on microstruc-

ture-property relationship. In fact, most of the studies

about the austenite grain growth were focused on the

grain size more than 10 lm [53–58], thus we concen-

trated on discovering the theoretical differences when

austenite grain size came to smaller than 10 lm, even

within the submicron scale.

Austenite grain size ranged from *2 lm to more than

100 lm were successfully prepared in a Nb–V–Ti steel,

and the isothermal growth kinetics was systematically

investigated [21, 59]. Figure 18 illustrates the logarithmic

plot for isothermal growth behaviors of ultra-fine austenite

0.0 0.3 0.6 0.90

50

100

150

200

250

300

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300d

γ=13μm

dγ=81μm

dγ=100 μm

dγ=40 μm

dγ=9 μm d

γ=5 μm

Tru

e st

ress

(M

Pa)

True strain

dγ=2μm

(a)

dγ=100 μm

dγ=80μm

dγ=40μm

Tru

e st

ress

(M

Pa)

True strain

dγ=13μm

(b)

Fig. 16 True strain versus stress curves of samples deformed at 900 �C a and 950 �C b with different austenite grain size ( _e = 10 s-1)

Fig. 17 Morphology of austenite grains evolving with deformation at 900 �C: a e = 0.2; b e = 0.4


123

grains, extracted by D ¼ Ktn, where D, K, t, and n are mean

grain diameter, rate constant, holding time, and time

exponent, respectively, which is widely used for predicting

the growth behavior of austenite grains. Apparently, curve

slopes change a lot with variation of holding temperature,

but it comes to be a constant when the temperature is

higher than 1000 �C, which is deduced about 0.28. Then,

according to the classic model of isothermal grain growth,

D1/n - D01/n = Kt, where D0 is the grain size at t = 0 s,

and Arrhenius curve, K = A exp (-Q/RT), where A is a

constant, R and T are the gas constant and temperature, the

model of growth kinetics for ultrafine-grained austenite can

be deduced as: D1/0.28 - D01/0.28 = [1.79 9 1030 exp

(-6.932 9 105/RT)]/t. The much higher activation energy

(Q) that was calculated to be Q = 693.2 kJ/mol is probably

attributed to the intense drag force of solute atoms, such as

V and Nb. Reasonable agreement between calculations and

experimental measurements is also shown in Fig. 19.

According to Figs. 18 and 19, ultrafine austenite grains

obtained in Nb–V–Ti steel have relatively high thermal

stability at the temperatures below 1000 �C, but the driving

force of grain coarsening can be significantly improved

with the increase in strain energy. So it is important to

suppress the grain coarsening through boundary sliding or

grain rotation in order to take maximum advantages of

refining austenite for the transformation during cooling

stage.

4.2.3 The Effects of Ferrite Deformation on the Austenite

Transformation During Heating Process

Most researches about the method of austenite refinement

are cyclic heat treatment, such as repetitive reheating and

quenching [60]. However, it may decrease the production

efficiency for industrial manufacturing. We paid more

attention on simplifying the fabrication of refining aus-

tenite grains in recent years [23], and the results demon-

strate that deformation has great effect on the austenite

transformation thermodynamics and kinetics during heat-

ing process.

Figure 20a shows the austenite grain size versus different

deformation temperatures. The austenite grains were greatly

refined to a submicron scale when the sample was deformed

at 800 �C, whereas, those deformed at other temperatures

revealed much larger grains. Moreover, when the strain rate

is 0.1 s-1, the austenite grain size reaches a minimum value

at the strain of 0.8, but it is followed by a rapid increment as

the strain increases to 1.0 (Fig. 21). Meanwhile, the austenite

grain size gradually decreased with strain increase at a rel-

atively high strain rate of 1 s-1 as illustrated in Fig. 20b. It

probably indicates that the dynamic formation of ultrafine

austenite grains is a time-dependent process including

nucleation of austenite, deformation-induced ferrite trans-

formation, and normal grain growth.

Additionally, the austenite grains obtained in the sam-

ples with initial microstructures of warm-rolled ferrite/

pearlite can obviously differ according to the characteris-

tics of the warm-rolled microstructure. Deformation-

induced intrinsic energy improvement and dispersed,

spheroidizing pearlite will remarkably influence the aus-

tenite transformation during the heating stage, such as the

nucleate rate and nucleation site. According to our previous

work, the ausforming process should be correspondingly

optimized when the characterization of deformed initial

microstructure is considered.

5 Concluding Remarks

This overview summarizes the fundamental theory and

industrial viability of the nano-structured bulk steels

Fig. 18 Logarithmic plot for ultra-fine austenite grains growth

behaviors

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

ln(D

cal (

μm))

ln(Dmea

(μm))

Fig. 19 Reliability of isothermal grain growth model


123

including various fabrication processes and the corre-

sponding microstructural control. The routes of SPD

including ECAP, ARB, and MDF are described as an

effective way of obtaining the nano-grained microstruc-

tures in the small specimens. Compared to SPD limited to

laboratory-scale curiosity, the improved process of cold

rolling and annealing of martensitic steel is demonstrated

to be more significant and viable to fabricate the bulk nano-

structured low carbon steel, while the final thickness of the

cold-rolling plate is limited due to the requirement of total

deformation reduction.

The theoretical result of the thermal simulation studies

demonstrates that the refinement of austenite is the pre-

requisite of obtaining the nano-grained ferrite. The austenite

grain size can be refined to 1–2 lm through heavy warm

deformation and cycle-quenching. The nano-grained steel

(300–400 nm) of 5.5 mm in thickness and 65 mm in width

can be obtained by RTRP on the trial rolling mill combining

the mechanisms of dynamic transformation, intragranular

nucleation ferrite, and dynamic recrystallization. The

deformation behavior of ultrafine austenite, stability of the

ultrafine austenite, and the effect of ferrite deformation on

austenite transformation during heating process are proposed

to be three inspiring theoretical aspects about the micro-

structural control during RTRP. These challenging resear-

ches are deserved to be studied by nano-grained steel

scientists.

Although the theoretical debates are unclear and con-

tinuous, the recent worldwide research results demonstrate

that the fabrication of nano-structured bulk steels is prob-

able. Moreover, the comprehensive mechanical properties

are superior. All these encouraging aspects indicated that

the development of the nano-grained steels in place of

conventional coarse grain counterparts is a promising route

in some important fields.

The future study of the nano-grained steels should be

focused on simplifying the fabrication process, designing

the new chemical composition, controlling the micro-

structural proportion, and uncovering the unique physical

and chemical properties.

Fig. 20 Dependence of austenite grain size on deformation temperature, strain value and strain rate: a austenite grain size versus deformation

temperature; b austenite grain size versus strain and strain rate

Fig. 21 Austenite grains obtained in sample without reheating after deforming with various strains at 800 �C: a e = 0.8, _e = 0.1 s-1; be = 0.9, _e = 0.1 s-1; c e = 1.1, _e = 0.1 s-1


123

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fabrication and microstructural control of nano-structured bulk steels: a review

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