Review Microstructure–Mechanical Properties Correlation in

1. Introduction

Thermo-mechanical processing is an important step inthe manufacturing sequence which is used not only toachieve the required shape but also to impart the desiredmicrostructural changes. Among the different strengtheningmechanisms, grain refinement is the only method to im-prove both strength and toughness simultaneously. There-fore, refinement of the grain size has been a key objectiveof the thermo-mechanical processing research. In the con-text of steel making, Thermo-Mechanical Controlled Pro-cessing (TMCP) has evolved as a grain refinement tech-nique that can be utilized to obtain a simultaneous improve-ment in performance and productivity. Therefore, ultra-fine grained steels with relatively simple compositions,strengthened primarily by grain refinement, have great po-tential for replacing conventional high strength low alloyedsteels. The attractive benefits of such an approach are (i) toavoid expensive alloying additions thus facilitating recycla-bility; (ii) to avoid additional heat treatments like quench-ing and tempering to enhance the strength thus reducingproduct cost as well as making the process environmentallygreen, (iii) to improve weldability owing to lower requiredcarbon contents/alloying additions and (iv) to obtain highstrain rate superplasticity at moderate temperatures for sec-ondary processing.

The microstructure of conventional carbon steels is madeof ferrite grains and pearlite colonies, which in turn con-sists of alternate lamellar of cementite (Fe3C) and ferrite.There are essentially two potential routes to achieve grainrefinement in these steels. The first one is transformational

grain refinement,1–3) wherein the austenite→ferrite transfor-mation is explored to obtain refined ferrite grains from aprior austenite grain structure. Here, the main purpose ofplastic deformation is to increase the density of ferrite nu-cleation sites by the refinement and flattening of austenitegrains. The second method is recrystallization grain refine-ment,4,5) wherein the recrystallization of ferrite phase is uti-lized for refining crystal grains from heavily deformed fer-rite. This may be accomplished by the dynamic recrystal-lization occurring at warm working temperatures or by sub-sequent static recrystallization process. In the later case,UFG structure is obtained by large strain deformation offerrite phase at low temperatures and high strain rates i.e.high Z (Zener–Hollomon parameter) conditions. Ultrafinegrained steels in bulk form with microstructures rangingfrom several nanometers to less than a micron have beenfabricated by severe plastic deformation techniques such asmechanical milling with rolling,6,7) equal channel angularpressing (ECAP),8,9) accumulative roll bonding (ARB),10,11)

multiple compression12,13) and multi axial and multi stagedeformation through warm caliber rolling.14–16)

In order to evolve an ultrafine-grained structure, largeplastic strain deformation exceeding a true strain in therange of 1 and 2 is generally necessary. The present authorshave conclusively demonstrated17,18) that a sub micron fer-rite structure can be formed by single pass warm deforma-tion of ferrite phase when the strain exceeds approximately3. However, considering the capacity limitations of practi-cal rolling and processing equipment during manufacturing,this type of heavy single pass deformation is not realisticwith large bulk materials, suggesting that study of a multi-

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ISIJ International, Vol. 48 (2008), No. 8, pp. 1088–1095

Microstructure–Mechanical Properties Correlation in UltrafineGrained Steels Processed by Large Strain Warm Deformation

S. V. S. NARAYANA MURTY and Shiro TORIZUKA

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047 Japan.

(Received on January 21, 2008; accepted on May 12, 2008 )

Ultrafine-grained steels with grain size of about 1 mm offer the prospect of high strength coupled withhigh toughness among conventional steel compositions and are attracting the attention of researchersworldwide. Application of these ultrafine grained steels to the fabrication of potential engineering structuresdemand extensive study of their mechanical properties and reasons for the improvement in order to get de-tailed insight into their behaviour under operating conditions. While there are many studies on the develop-ment of ultrafine grained microstructures per se, fewer studies were reported on the more important as-pect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructuralrefinement at bulk level indeed improves the properties offering the prospect of a realistic replacement ofthe existing conventional steels in the near future. This review article attempts to present a comprehensivepicture on the microstructure-mechanical properties correlation of ultrafine grained steels fabricated by largestrain warm deformation and reasons behind the improved properties from a micromechanics point of view.

KEY WORDS: ultrafine grained steel; strength; toughness; reduction in area; warm rolling.

Review

pass deformation process is indispensable for industrialproduction of ultrafine grained steels.

One of the obstacles for a systematic study of the me-chanical properties of ultrafine grained materials is the dif-ficulty in obtaining large bulky samples for the mechanicaltests as per standard procedures containing uniform mi-crostructure throughout and across the cross section. Itshould be noted that most of the submicron microstructuresobtained by the severe plastic deformation techniques con-sist of large quantities of low-angle dislocation boundaries,and the so called grain dimensions measured and reportedrefer, in fact, to the thickness of stretched microbands,which is not the same as average grain diameter. Severeplastic deformation produces submicron structures that aretypically more elongated due to the intense deformation im-posed. Around 40% of the boundaries are of the low-angledislocation type (having misorientations �15°), which areless beneficial for the overall mechanical response. Theselow-angle boundaries often appear in TEM as dense dislo-cation walls, rather than as sharp boundaries, which couldmigrate more easily. It is difficult for these cells to be trans-formed into discrete grains surrounded by high-angle grainboundaries without an annealing treatment. In order tomore quantitatively evaluate the microstructure of ultrafinegrained steels, it has become customary to report not onlythe average cell or grain sizes and the corresponding grainsize distributions, but also the fraction of high-angle grainboundaries obtained from the various processing strate-gies.19) Through multi-pass warm caliber-rolling, the pres-ent authors fabricated ultrafine grained steel bars that werelarge enough to machine sufficient test specimens as perstandard test methods containing uniform microstructurethroughout.

Shin et al.20,21) have studied the ultrafine grained mi-crostructures in low carbon steels by ECAP and evaluatedtheir mechanical properties. Storojeva et al.22) have studiedheavy warm deformation of medium carbon steel resulting

in the ultrafine ferrite grains. Song et al.23) have reportedthe large strain warm deformation of 0.2%C–Mn steel lead-ing to the evolution of an ultrafine grained ferrite mi-crostructure and studied their mechanical properties. How-ever, all these studies were conducted on laboratory scalespecimens and full scale impact properties of the resultantultrafine grained microstructures were not reported. How-ever, Ohmori et al.24) and Hanamura et al.25) have carriedout the warm multi-pass caliber rolling of 0.15C steel atvarious temperatures in the range of 500–650°C subjectedto a nominal strain of about 3 and evaluated the strengthand full size Charpy impact properties of the caliber rolledbar.

In order to bring out the correlation between microstruc-ture and mechanical properties of ultrafine grained steels, itis attempted to first discuss the evolution of microstructureand grain boundary characteristics as a function of strainfollowed by interpretation of mechanical properties ob-tained in bulk specimens in terms of microstructure.

2. Evolution of Ultrafine Grained Microstructure as aFunction of Accumulated Strain

Figure 1 shows the scanning electron micrographs26) ofthe cross sections of multipass warm caliber rolled rodsafter different stages of rolling corresponding to various cu-mulative strains (viz. 0.7, 1.5, 2.4, 3.0 and 3.8) as well asundeformed microstructure (e�0) for a steel of composi-tion equivalent to SM490 (composition in wt% C - 0.16,Si - 0.29, Mn - 1.53, P - 0.011, S - 0.0012, Al - 0.029, N -0.0016 and Fe - balance) with an initial ferrite+pearlite mi-crostructure. Square bar with a width of 80 mm were pro-duced by hot forging and heat treated at 870°C for 2 h fol-lowed by air cooling in order to obtain a normalizedferrite�pearlite microstructure. This bar was then subjectedto multi pass isothermal caliber rolling at 500°C. The cal-iber rolling procedure followed is described in detail else-

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Fig. 1. Scanning electron micrographs of the cross section of (a) undeformed and (b)–(f) caliber rolled specimens for theinitial ferrite�pearlite microstructure taken at the center with different cumulative strains (b) 0.7, (c) 1.5, (d) 2.4,(e) 3.0 and (f) 3.8. On the figures, arrow ‘1’ points the subgrain boundaries, ‘2’ points the ultrafine grains intro-duced and ‘3’ point to the cluster of spheroidized cementite particles.26)

where.14) All microstructural observations were conductedat the center of the rolled bar. This is to ensure clear corre-lation of the microstructure and mechanical properties ofthe specimens, which were taken from the center of therolled bars. Figure 1(a) reveals the initial microstructure ofthe material showing ferrite grains and pearlite colonies.Figures 1(b)–1(f) show the microstructures for the caliberrolled specimens subjected to various cumulative strains. Ata strain of 0.7 (Fig. 1(b)), faintly etched boundaries insideferrite grains can be noticed along with disintegration andpartial spheroidization of cementite at few locations. At thisstage no new grain formation is observed anywhere in thecross section of the bar. The formation of a large number oflow angle boundaries is attributed to the multidirectionaldeformation which promotes rapid formation of intersect-ing sub-boundaries compared to uniaxial compressive de-formation.17,18) After the second deformation stage (e�1.5),the average ferrite grain size decreases (Fig. 1(c)) with theformation of clearly etched (indicating that they are highangled ones) small equiaxed ferrite grains. No morepearlite colonies could be observed and spheroidization ofcementite was complete at this stage. At the end of third de-formation stage, (e�2.4), more and more clearly etchednew ferrite grains were observed (Fig. 1(d)). However, thecementite was still in the form of colonies of spheroidizedparticles. After the fourth stage of deformation, (e�3.0),the microstructure has fine equiaxed ferrite grains (Fig.1(e)). After a cumulative strain of 3.8, the microstructure isessentially similar to that of fourth stage; however, thespheroidized cementite particles are uniformly distributedthroughout the ferrite phase. Large quantities of fine ce-mentite particles are beneficial for the formation of a fineferritic grain structure. They inhibit grain boundary migra-tion due to Zener pinning. This effect stabilizes the ultrafine

grains against grain coarsening, and is thought to inhibitprimary recrystallization. The presence of such fine parti-cles results in an increase in the effective recrystallizationtemperature, widening the temperature windows for corre-sponding warm rolling.22)

3. Evolution of Grain Boundary Characteristics as aFunction of Accumulated Strain

In order to understand the evolution of grain boundarycharacteristics as a function of cumulative strain, the speci-mens deformed to various strains in Fig. 1 are subjected toelectron back scattered diffraction (EBSD).27) Care wastaken to perform the microstructural and EBSD analyses atthe same location. However, due to the difference in thesurface conditions required for these two techniques viz.etched surface for scanning electron microscopy and elec-tropolished surface for EBSD analysis, there is no exactcorrelation between the two photographs in Fig. 1 and Fig.2. Figure 2 shows the boundary map of the cross section ofthe caliber rolled rods after different stages of rolling corre-sponding to various cumulative strains (viz. 0.7, 1.5, 2.4,3.0 and 3.8) as well as undeformed specimen (e�0). In thepresent study, high angle boundaries (q�15°) are repre-sented in red, medium angle boundaries (15°�q�5°) indark blue and low angle boundaries (5°�q�1.5°) in lightblue colors, where q is the misorientation. All microstruc-tural observations were carried out at the center of therolled bar. Figure 2(a) reveals the boundary map of the un-deformed specimen consisting of clear ferrite grains. It maybe noted that the second phase pearlite (ferrite�cementite)can not be clearly observed during the EBSD analysis dueto the difficulty in obtaining the Kikuchi pattern for cemen-tite phase. Therefore, only the refinement of ferrite grains is


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Fig. 2. Boundary maps of the cross section of caliber rolled specimens to various cumulative strains for the initialferrite�pearlite microstructure. The red line represents the high angle grain boundaries of q�15°, dark blue andthe light blue lines represent low angle grain boundaries of 15°�s�5° and 5°�s�1.5°, respectively.27)

studied here due to the above limitation. Spheroidization ofpearlite in to fine cementite particles and its distribution inthe ferrite matrix as a function of strain was discussed inFig. 1. Figures 2(b)–2(f) show the boundary maps for thecaliber rolled specimens subjected to various cumulativestrains. At a strain of 0.7 (Fig. 2(b)), generation of a largenumber of low angle (sub) boundaries can be noted in theboundary map. At this stage no new grain formation is ob-served anywhere in the cross section of the bar. After thesecond deformation stage (e�1.5), the average ferrite grainsize decreases (Fig. 2(c)) with the formation of smallequiaxed ferrite grains surrounded by high angle bound-aries. At the end of third deformation stage, (e�2.4), moreand more grains surrounded by high angle boundaries werenoticed (Fig. 2(d)). After the fourth stage of deformation,(e�3.0), the microstructure has fine equiaxed ferrite grains(Fig. 2(e)). After a cumulative strain of 3.8, the microstruc-ture is essentially similar to that of fourth stage with cumu-lative strain of 3.0 (Fig. 2(e)); however, the complete crosssectional area is filled with ultrafine ferrite grains. Fromthese figures it can be seen that the total grain boundarylength per unit area increases monotonically as the cumula-tive strain increases with the formation of ultrafine ferritegrains.

4. Mechanical Properties of Ultrafine Grained Steels

4.1. Strength

One of the key differences between the earlier28,29) andpresent works probing in to the effect of grain size on themechanical properties of materials is the difference in themicrostructure of the materials under comparison at differ-ent length scales. It may be noted that in early investiga-tions,28,29) different grain sizes were produced by coldrolling and subsequent annealing at different temperatures,which offered the advantage of altering only ferrite grainsize. While the conventional coarse grained microstructureconsists of ferrite and pearlite, ultrafine microstructure iscomposed of ultrafine ferrite grains and nanosized spher-oidized cementite particles. This microstructural differencewill have significant influence on the mechanical propertiesof ultrafine grained steels as can be seen in the followingsections.

Figure 3(a) shows the nominal stress–strain curves ofundeformed (e�0) as well as caliber rolled specimens sub-jected to different cumulative strains viz. 0.7, 1.5, 2.4, 3.0and 3.8 for which microstructural observations were dis-cussed in Fig. 1. Tensile tests were conducted using roundbar specimens with a parallel section length of 24.5 mm anda diameter of f3.5 mm taken from the center of the crosssection in the rolling direction (longitudinal direction ofbars). It may be noted that the undeformed microstructure(e�0) is characterized by lower strength and large strain tofailure. On the other hand, for the warm caliber rolled spec-imens to different strains, as the cumulative strain in-creases, the yield and ultimate tensile strengths increasemonotonically. It can be noted from the Fig. 3(a), that thelower yield strength has doubled in the caliber rolled speci-men even with a cumulative strain of 0.7 compared to theundeformed specimen. Reduction in the elongation to fail-ure for the caliber rolled specimens compared to the unde-

formed specimen may be noticed. However, there is almostno change among the caliber rolled specimens with differ-ent cumulative strains.

Figures 4(a) and 4(b) show the summary of the variationof tensile strength (TS), lower yield strength (LYS), uni-form elongation, total elongation and reduction in area atroom temperature as a function of cumulative strain for themultipass warm rolled specimens. It may be noted fromthese figures that there is a sharp increase in the TS andLYS up to a strain of 0.7 and there after they increase grad-ually. On the other hand, the uniform elongation and per-centage reduction in area sharply decrease up to a strain of


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Fig. 3. Nominal stress–strain curves of undeformed specimen(e�0) as well as the caliber rolled specimens to variouscumulative plastic strains (e�0.7–3.8) for initial ferrite�

pearlite microstructure.26)

Fig. 4. Correlation of mechanical properties to grain boundarycharacteristics as a function of cumulative strain for theinitial ferrite�pearlite microstructure.27)

0.7 and thereafter remain almost unchanged. The tendencyin the mechanical property variation is in close correlationwith the work hardening by rapid generation of low angle(dislocation) boundaries (LAB) up to a strain of 0.7 (asshown in Fig. 2), and there after gradual generation of ultrafine grains surrounded by new high angle grain boundaries(HAGB) with increase in strain up to 3.8. In other words,work hardening (due to LAB) rapidly increases the strengthof the material. On the other hand, there is a gradual in-crease in strength due to the generation of ultrafine grainssurrounded with HAGB. This explains the sharp increase inTS and LYS up to a strain of 0.7 and gradual increase there-after. Decrease in uniform elongation can be explained bythe strain hardening rate; when the strain hardening rate issame, decrease in uniform elongation with increase in TS isexpected.30)

4.2. Toughness

While several studies examined tensile properties of ul-trafine grained steels, Charpy impact properties were lesscommonly investigated due to limitations in obtaining bulksample required through the severe plastic deformationprocesses. The impact properties of ultrafine grained IF,low/medium carbon and Nb–V–Ti microalloyed steels havebeen reported by Tsuji et al.,31) Hanamura et al.25) and Songet al.32) Generally, a reduction in the average grain sizecommonly leads to a lower ductile-to-brittle transition tem-perature (DBTT) which can be understood in terms ofcleavage crack initiation and propagation. It is known thatthe grain size is one of the major factors determining thecleavage fracture unit.33) Decreasing the grain size can limitthe propagation of initiated cleavage cracks and raise theCharpy toughness in the transition region.

Figure 4(c) shows the variation of the DBTT and theCharpy impact absorbed energy at two temperatures viz.�40°C (vE�40°C) and �196°C (vE�196°C) as a functionof cumulative strain for the multipass warm rolled speci-mens. Full size 2 mm V-notch Charpy test specimens takenfrom the center of the cross section of warm rolled rod inthe rolling direction (longitudinal direction of bars) wereused. It can be seen that vTrs slightly increases up to astrain of 0.7 and thereafter it decreases sharply to below liq-uid nitrogen temperature. It can be seen from Fig. 4(c) thatvE�40°C decreases initially from undeformed specimen tothat with a strain of 0.7 and thereafter rises monotonicallyup to a strain of 1.5 and thereafter saturates. The Charpyabsorbed energy value for the initial microstructure and forspecimens with cumulative strain up to 1.5 is zero, but in-creases to a value of 100 J for specimens with cumulativestrains beyond 1.5, even at liquid nitrogen temperature.This is attributed to the vTrs value falling below liquid ni-trogen temperature for these specimens. The initial increasein vTrs and decrease in vE�40°C up to a strain of 0.7 is at-tributed to the introduction of LAB indicating that the ma-terial is work hardened which is known to be harmful forCharpy impact properties. This initial deterioration can notbe attributed to the effect of pearlite, since, no significantchange in its morphology is noticed. However, the Charpyimpact properties beyond a strain of 1.5 remain almost con-stant. At this stage two distinct microstructural manifesta-tions take place namely, generation of new ultrafine grains

and spheroidization and dispersion of cementite. The bal-ance of these two manifestations will decide the variationof Charpy impact properties. The generation of new ultrafine grains surrounded with HAGB is responsible for theimprovement of Charpy impact properties beyond a strainof 1.5. This clearly indicates that work hardening is harmfulfor Charpy impact properties while new ultrafine grain for-mation associated with HAGB is beneficial.

4.3. Correlation of Mechanical Properties to GrainBoundary Characteristics

Generally, grain size and dislocation density are used todescribe the relationship between microstructure and me-chanical properties of materials. The microstructure of ma-terial subjected to large strain deformation changes fromwork hardened state to recrystallized state shown in Figs. 1and 2. It is impossible to describe such complicated mi-crostructural change quantitatively with grain size and dis-location density.

Grain boundary length per unit area and grain boundarycharacter are closely related to grain size and dislocationdensity. EBSD can measure grain boundary length per unitarea and grain boundary character quantitatively. It is rele-vant to discuss the relationship between microstructure andmechanical properties with grain boundary length per unitarea and grain boundary character.

While there are many studies on the effect of strain onthe mechanical properties of materials subjected to largestrain deformation, there are no detailed studies on the ef-fect of nature/character of the boundary on the mechanicalproperties. Detailed quantification of the microstructuresproduced by large strain deformation with respect to bothlength fraction and nature of the boundaries and their corre-lation to the mechanical properties in various materials isessential to further corroborate the results presented here.This will further our understanding of the mechanism ofstrengthening of materials by grain refinement.

Figure 4(d) shows the variation of the length fraction(boundary length per unit area of 1 mm2) of high, mediumand low angle boundaries as a function of cumulativestrain. A close examination of the variation of length frac-tions of various types of boundaries with strain and theircorrelation to mechanical properties yields interesting ob-servations. With increasing cumulative strain, the totalgrain boundary length increases monotonically. On theother hand, the length fraction of grains surrounded by highangle boundaries decreases rapidly up to a strain of 0.7 witha simultaneous increase in the length fraction of low angleboundaries. Thereafter, the length fraction of high angleboundaries gradually increases and that of low angleboundaries gradually decreases as a function of strain up toa strain of 3.8. TS and YS increases monotonically withstrain. This means that TS and YS increase monotonicallywith total grain boundary length. On the other hand, DBTTand vE once deteriorate and improve with strain. Thischange of toughness is very similar to the change of lowand high angle grain boundary fraction. Based on this dis-tinction, it can be clearly seen that low angle grain bound-aries are helpful in improving the TS and LYS while theyare harmful for Charpy impact properties. The TS and LYSare affected by the total grain boundary length. On the


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other hand, Charpy impact properties are affected by lengthfraction of high, medium and low angle grain boundaries.

4.4. Ductility

Grain refinement down to submicron size deteriorates theuniform elongation in the tensile tests of the steels and in-creases their yield strength considerably.16,34–36) As a resultof the decrease in strain-hardening, the tensile strength be-comes very close to the yield strength of the ultrafinegrained structures. Such loss of ductility has been argued tobe an inherent feature of the ultrafine-grained steels, whichcan be regarded as the “Achilles’ heel” that severely re-stricts their potential applications.37)

The plastic instability condition in tensile tests is ex-pressed by the formula s�ds /de , where s is the flowstress and e is the true strain. Since the flow stress s in-creases monotonically with grain refinement, a largerstrain-hardening rate ds /de is required to avoid the plasticinstability and to improve the uniform elongation. Ashby etal.38–40) showed that the strain-hardening rate of crystals de-pends on the dispersion of hard second phase particles andis proportional to a dispersion parameter ( f /d)1/2, where f isthe volume fraction of the second phase and d is the meandiameter of the particles. The concept of a strain-hardeningdesign using second phases has been proposed to improvethe strength–ductility balance of the ultrafine-grainedsteels.41)

Figure 5(a) shows the nominal stress–strain curves of ul-trafine grained steels with carbon contents in the range of0.02–0.45%.42) The chemical composition of the steels re-ported here corresponds to 0.3Si–1.5Mn–0.01P–0.002Swith varying carbon contents viz. 0.05, 0.15, 0.30, 0.45%(wt%). The steel with 0.02C has a slightly different chemi-

cal composition with 0.3Si–0.2Mn–0.01P–0.002S (wt%).When carbon content is 0.02%, lower yield strength is700 MPa and the total elongation is as small as 5%. No uni-form elongation appears in the stress–strain curve. This in-dicates that immediately after yielding, plastic instabilityoccurs. For steel with carbon content of 0.05%, yieldstrength increases to 830 MPa and total elongation drasti-cally increases to 14%. Further, uniform elongation of 3%appears in this stress–strain curve which is attributed tostrain hardening by cementite particles. When carbon con-tent is 0.15%, strain hardening is evident. Strain hardeningbecomes more apparent when the carbon content is 0.3%.Ultimate tensile strength is clearly observed in steels withcarbon content of 0.3 and 0.45%. The tensile strength of1 000 MPa and total elongation of 20% is obtained whenthe carbon content is 0.45%. It can be seen that the failurestress also increases with an increase in carbon content mo-notonously. The volume fraction of cementite is virtuallyzero in steel with carbon content of 0.02%, resulting in nouniform elongation. The presence of cementite is clearlyvery effective in increasing uniform elongation. Therefore,it is evident that ultrafine grained steels get more strainhardened as the carbon content increases.

Figure 5(b) shows the true stress–true strain curves aswell as work hardening rate as a function of true strainwhich was obtained from the room temperature tensile testresults presented in Fig. 5(a). The effect of carbon content(in other words the volume fraction of cementite) on thetrue stress is clear from the comparison of data shown inFig. 5(b) for various compositions. An increase in the car-bon content causes an increase in the true stress. The strainhardening rate, on the other hand drastically increases withcarbon content from 0.02C to 0.05C and thereafter steadilyup to 0.3C.

4.5. Tensile Strength-reduction in Area Balance

While uniform elongation is a measure of ductility of thematerial, reduction in area in tensile tests is also an impor-tant measure of ductility. However, measurement of the re-duction in area requires standard tensile specimen which inturn requires bulk material for the fabrication of the speci-men. Reduction in area is affected by second phase parti-cles and inclusions. Ultrafine grained steels usually consistof ferrite and dispersed cementite particles in contrast tothe conventional ferrite�pearlite steels. These cementiteparticles are formed through the fragmentation of pearliteby the large strain deformation process and get dispersed inthe ultrafine grained ferrite matrix. Therefore, the volumefraction of the cementite as a second phase in ultrafinegrained steel is far less than the volume fraction of pearliteof starting ferrite–pearlite steels. Therefore, ultrafinegrained steels are expected to have higher reduction in area,compared to conventional grain size ferrite�pearlite steels.

Figures 6(a) and 6(b) show the variation of percentageelongation and reduction in area as a function of the vol-ume fraction of cementite. Here, the volume fractions of ce-mentite particles for the steels with the various carbon con-tents were evaluated roughly by the thermodynamic equi-librium calculations. It can be clearly seen from Fig. 6(a),that while the uniform elongation gradually increases as thevolume fraction of cementite increases, the non uniform


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Fig. 5. (a) Nominal stress–strain curves of ultrafine-grained steelwith carbon content in the range of 0.02–0.45wt% and(b) True stress–true strain curves as well as work harden-ing rate as a function of true strain.42)

elongation drastically increases from the specimen with nocementite (0.02C) to specimen with about 0.5 volume per-cent cementite (0.05C). This rapid rise in non uniform elon-gation is followed by a slight decrease with further increasein the volume fraction of cementite. On the other hand, thetotal elongation rises drastically with the introduction of ce-mentite and thereafter increases gradually. It can be seenfrom Fig. 6(b) that the reduction in area slightly decreaseswith increase in the volume fraction of cementite. It may benoted that uniform elongation is enhanced by the presenceand volume fraction of second phase cementite particlesdue to enhanced work hardening. However, second phasedispersion helps in increasing the non uniform elongationat small volume fractions, but slightly decreases at largefractions. On the other hand, reduction in area monotoni-cally decreases with second phase dispersion even at largevolume fractions. The decrease in the volume fraction ofsecond phase due to ultrafine grain refinement from initialpearlite to uniformly dispersed cementite helps in achievingthis superior tensile strength-reduction in area balancethrough effective second phase dispersion strengthening. Inaddition, since the size of second phase also affects the re-duction in area, further research is necessary to understandthe strength-reduction in area balance more precisely.

5. Conclusions

The following are the conclusions on the microstructure–mechanical property correlation of the ultrafine grainedsteels produced by warm multipass caliber rolling:

(1) The microstructure of warm multipass caliberrolled low carbon steel is characterized by work hardenedstate with the presence of large fraction of LAB up to a cu-

mulative strain of 0.7. Beyond a strain of 1.5, new ultrafinegrains surrounded by HAGB are formed with their volumefraction increasing with increasing cumulative strain.

(2) Tensile strength and lower yield strength increasemonotonically as a function of cumulative strain; very rap-idly up to a strain of 0.7 and gradually thereafter up to astrain of 3.8. Uniform elongation sharply decreases up to astrain of 0.7 beyond which it is almost unchanged.

(3) Charpy fracture appearance transition temperature(vTrs) deteriorates up to a strain of 0.7 and there after dras-tically improves. Beyond a cumulative strain larger than 2,vTrs drops below liquid nitrogen temperature.

(4) Tensile strength and lower yield strength are af-fected by the total grain boundary length but not grainboundary character or their fraction; Charpy impact proper-ties are affected by length fraction of high, medium and lowangle grain boundaries. In other words, low angle bound-aries are helpful for improving the TS and LYS while theyare harmful for Charpy impact properties.

(5) Decrease in second phase volume fraction (pearlite)and its effective dispersion in the ferrite matrix as finespheroidized cementite particles along with ultrafine grainrefinement results in superior mechanical properties.

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