s3 (2)

8222019 s3 (2)

httpslidepdfcomreaderfulls3-2 110

P u b l i s h e d b y M a n e y P u b l i s h i n g ( c

) I O M C

o m m u n i c a t i o n s L t d

Precipitation in V bearing microalloyed steelcontaining low concentrations of Ti and Nb

S Shanmugam12 M Tanniru12 R D K Misra12 D Panda3 and S Jansto4

The paper describes the precipitation behaviour in a thermomechanically processed V bearing

microalloyed steel containing small additions of Ti and Nb (0007ndash0008 wt-) using analytical

transmission electron microscopy An intriguing aspect is the significant precipitation of titanium

and niobium at these low concentrations contributing to strength A high density of

multimicroalloyed precipitates of (VNbTi)(CN) are observed instead of simple TiN TiC and

NbC precipitates They are characterised as cuboidal (45ndash70 nm) spherical (20ndash45 nm)

irregular (20ndash45 nm) and fine (10ndash20 nm) Estimation of solubility products of carbides andnitrides of V Nb and Ti implies that the precipitation of titanium occurs primarily in austenite

Interphase precipitation of niobium occurs during austenite to ferrite transformation while

complete precipitation of vanadium takes place in the austenitendashferrite region close to completion

of transformation Substoichiometric concentrations of Ti and Nb the presence of nitrogen and

the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell

(triplexduplex) precipitates with highly stable nitrides ((TiNbV)N) in the core and carbides

((TiNbV)C) in the shell The qualitative stochiometric ratios of triplex and duplex carbonitrides

were Ti053Nb035V012 and Ti06V04 Nb051V049 and Ti064Nb036 Extensive precipitation of fine

carbides on dislocation substructures and sub-boundaries occurred They were generallycharacterised as vanadium carbide precipitates with ordered cubic L12 structure and exhibited a

BakerndashNutting orientation relationship with the ferrite matrix M4C3 types of carbides were also

observed similar to the steel having high concentrations of Ti and Nb

Keywords Microalloyed steels Precipitation Electron microscopy

IntroductionIn general microalloying elements V Nb and Ti are

added to steels to refine grain size and obtain pre-cipitation strengthening Fine grain size is obtained

through undissolved precipitates precipitates in austenite

or precipitates formed at subsolvus temperature1

Furthermore precipitation strengthening is achieved

from fine scale precipitates that are precipitated at theaustenitendashferrite interphase or precipitated after trans-

formation in ferrite12 The aforementioned requirements

necessitate an understanding of the factors that control

the dissolution of microalloy carbides and nitrides and

also their mutual solubility and precipitation behaviour

The majority of microalloy carbides and nitridesexhibit similar levels of solubility in austenite with the

exception of titanium nitride and vanadium carbide the

former is stable and the latter is soluble in austenite in

comparison to other microalloy carbides and nitrides1

Stable particles provide nucleation sites for othermicroalloy carbides and nitrides with consequent

1Center for Structural and Functional Materials University of Louisiana atLafayette Lafayette LA 70504ndash4130 USA2Department of Chemical Engineering University of Louisiana atLafayette Lafayette LA 70504ndash4130 USA3Nucor-Yamato Steel PO Box 1228 5929 East State Highway 18Blytheville AR 72316 USA4Reference Metals 1000 Old Pond Road Bridgeville PA 15017 USA

Corresponding author email dmisralouisianaedu

1 Equilibrium volume fraction of microalloying elements

precipitated as function of temperature

szlig 2005 Institute of Materials Minerals and Mining Published by Maney on behalf of the InstituteReceived 5 January 2005 accepted 9 March 2004DOI 101179174328405X47564 Materials Science and Technology 2005 VO L 21 NO 8 88 3

8222019 s3 (2)



) I O M C


formation of duplex and triplex precipitates because of

their mutual solubility3

It was previously shown in Fendash TindashNbndashCndashN steel that titanium carbide is the equili-brium carbide at high temperatures and niobium carbidein the low temperature transformation range1 However

during continuous cooling the formation of compoundsdepends on the intraparticle diffusion of the compo-nents There is considerable evidence to suggest that

homogenistion does not occur in multimicroalloyed steeland heavily cored precipitates containing a core of titanium nitride and shell of niobium or vanadiumcarbide are formed1

In a multimicroalloying approach the objective is toderive the combined benefit from individual elementsto obtain the desired properties Thus it is important tostudy the effect of a microalloying element on theprecipitation behaviour of other elements and theircontribution to the overall strength and toughness Ingeneral a multimicroalloying approach results in the

precipitation of coarse carbonitrides having low densityand these do not significantly contribute to strength incomparison to the fine carbides that interact with thedislocations4 In previous studies we examined themicrostructural aspects of strengthndashtoughness combina-tions in high strength hot rolled microalloyed steelsusing VndashNb5 NbndashTi567 and VndashNbndashTi3 alloy designapproach containing almost similar carbon and manga-nese content The primary concern of the studydescribed here is to obtain an insight into the precipita-tion behaviour in a V microalloyed steel containingminute additions of Nb and Ti The precipitates were

characterised by transmission electron microscopy interms of size morphology chemistry and crystal-lography The precipitation behaviour is discussed interms of the thermodynamic stability of precipitates andis compared with VndashNbndashTi steel containing highconcentrations of V Nb and Ti3 processed undersimilar conditions

Table 1 Empirical equations used for calculation of solubility product nitrides and carbides1

Type of precipitate Precipitate Solubility product Temperature range uC phase Reference

Nitrides VN log [V][N] 5 34 ndash330Tz 0

12( Mn) 870ndash1480 Austenite

1

NbN log [Nb][N] 5 28ndash8500T 870ndash1480 Austenite 1

TiN log [Ti][N] 5 382ndash15020T 870ndash1480 Austenite 1

log [Ti][N] 5 59ndash16586T 1480 Liquid Steel 1

Carbides VC log [V][C] 5 672ndash9500T 870ndash1480 Austenite 1

log [V][C] 5 805ndash12265T 480ndash815 Ferrite 1

NbC log [Nb][C] 5 342ndash7900T 870ndash1480 Austenite 1

log [Nb][C] 5 543ndash10960T 480ndash815 Ferrite 1

TiC log [Ti][C] 5 533ndash10475T 870ndash1480 Austenite 1

log [Ti][C] 5 44ndash9575T 480ndash815 Ferrite 1

2 (a ) TEM micrograph of a representative ferrite region

containing precipitates of different size and morphol-

ogy (b ) SAD pattern for the precipitate identified with

arrows in (a ) showing [012] and [1 macr 13] zones of ferrite

and [1 macr 11] and [001] zones corresponding to M4C3 pre-

cipitates and the ordered vanadium carbide (L12 unit

cell) respectively

3 TEM micrograph illustrating corendashshell structure of

coarse duplextriplex carbonitrides that precipitated in

rows and randomly Inset shows coarse precipitates of

cuboidal and irregular morphology

Shanmugam et al Precipitation in V bearing microalloyed steel containing Ti and Nb

88 4 Materials Science and Technology 2005 VOL 21 NO 8

8222019 s3 (2)



) I O M C


MaterialThe nominal chemical composition of the investigatedV microalloyed steel was Fendash0086Cndash135Mnndash 0044Vndash0007Nbndash0008Ti wt- In general the ther-momechanical processing schedule adopted to producestructural beams involved a series of successive rough-ing and finishing reductions similar to the previouslydescribed work on VndashNbndashTi steel3 To obtain theaimed yield strength of 500 MPa roughening andfinishing temperatures and percentage strain in eachof the deformation steps was carefully controlled andis not discussed here for proprietary reasons

ExperimentalTransmission electron microscopy (TEM) studieswere carried out on thin foils and carbon

extraction replicas The thin foils were prepared

by cutting thin wafers from the steel samples and

grinding to y100 mm in thickness Discs of diameter 3 mm were punched from the wafers and

electropolished using a solution of 10 perchloric

acid in acetic acid electrolyte Carbon extraction

replicas were prepared metallographically by grind-

ing and polishing to 1 mm The surface was etched

with 2 nital and carbon was evaporated ontothe etched surface Finally the surface was scoredinto y2 mm squares and the sample etched in 2

nital with a stainless steel cathode to remove the

replica The extracted carbon replicas were then

rinsed with a wateralcohol mixture and placed on

a copper grid and dried Foils and replicaswere examined by TEM operated at 100 to

200 kV

a bright field b dark field

4 TEM micrograph of precipitates together with EDX analysis of precipitates identified in (a ) and (b )


Materials Science and Technology 2005 VO L 21 NO 8 88 5

8222019 s3 (2)



) I O M C


Results and discussionSolubility product of carbides and nitrides of VNb and Ti in V steelThe solubility product of carbides and nitrides of V Nband Ti was calculated using the thermodynamicempirical equations1 presented in Table 1 The equa-tions predict the solubility product in the temperaturerange 600ndash1300uC The solubility product is calculated

for VC NbC TiC VN NbN and TiN precipitatescorresponding to the chemical composition of theinvestigated steel Figure 1 presents the temperature

dependence of solubility products of carbides and

nitrides of microalloying elements in austenite and

ferrite The plot defines equilibrium concentration of

V Nb and Ti precipitated as interphase precipitates orin the ferrite matrix after transformation The filled and

unfilled data symbols represent the data for equilibrium

5 TEM micrograph obtained from replica showing precipitates of different morphology irregular (i) cuboidal (ii) (iii)

and spherical (iv) precipitates



8222019 s3 (2)



) I O M C


concentration of microalloying elements that is pre-cipitated as carbides nitrides and carbonitrides in thepreviously studied Fendash0042Vndash0016Nbndash0017Ti steel3

and Fendash0044Vndash0008Nbndash0007Ti steel examined hereFigure 1 is divided into two parts based on theapproximate austenite to ferrite transformation tem-perature determined from empirical equations8 Figure 1

shows that titanium precipitation occurs predominantlyin the austenite range niobium precipitates in austeniteand in the austenitendashferrite transformation range whilevanadium precipitates during the later stages of auste-nite to ferrite transformation The trend is similar forsteels with significant3 and small concentration of Ti andNb The primary difference between them is that in steelcontaining appreciable concentration of Ti and Nb theprecipitation starts at higher temperature comparedsteels containing very small concentrations of Nb andTi The solubility product data obtained for carbidesand nitrides of titanium niobium and vanadium clearlyimplies that even at very low concentrations of titaniumand niobium precipitation occurs in a manner similar tothat in steel containing significant percentages of Ti andNb3 except for the difference in the volume fraction of the precipitates

In general nitrides are less soluble in ferrite than inaustenite and it is well known that among the threenitrides titanium nitride is most stable in austenite andin liquid steel In Figure 1 titanium precipitates largelyin austenite The presence of data points in the austeniteregion indicates that the small amount of titanium isexpected to be precipitated as carbides as interphaseprecipitates during transformation Similar behaviour is

expected for niobium Considering that niobium nitrideis less soluble in ferrite than in austenite1 the data pointsin the austenite region imply that niobium is predomi-nantly involved in precipitation as carbides or carboni-trides in the austenite region

Vanadium nitride and carbides are soluble in auste-nite but the solubility of vanadium carbide in austeniteis considerably higher than any other microalloy carbideor nitride1 Figure 1 shows s high equilibrium volumefraction of vanadium in Fendash0042Vndash0016Nbndash0017Ti3

and Fendash0044Vndash0008Nbndash0007Ti steels in the austenitendash ferrite transformation temperature range

Microstructure and precipitation behaviour The steel was characterised by a ferritendashpearlite micro-structure The general microstructural features and theprecipitation density in the ferrite are shown in the TEMmicrograph of Fig 2a It is interesting to note that theprecipitates occur in Fendash0044Vndash0008Nbndash0007Ti withvarious size range (10 to 70 nm) and different morphol-ogies similar to Fendash0042Vndash0016Nbndash0017Ti3 steel in

spite of the difference in concentrations The differencesare in the volume fraction of the precipitates Thecorresponding selected area diffraction (SAD) pattern is

presented in Fig 2b The analysis indicated the presence

of multimicroalloying carbides (M4C3 where M 5 V Nband Ti) together with an ordered vanadium carbidephase with cubic L12 unit cell The orientation relation-

ships between ferrite and the precipitates were [012]a

[111]M4C3 [012]

a[001]VC [113]

a[111]M4C3

[113]a[001]VC

The detailed diffraction and EDX analysis of precipi-

tates are given below

Coarse carbonitride precipitates

A representative TEM micrograph showing the coarse

precipitates in Fendash0044Vndash0008Nbndash0007Ti is pre-sented in Fig 3 The micrograph shows four different

types of morphology namely cuboidal (y45ndash70 nm)spherical (y20ndash45 nm) irregular (y20ndash45 nm) and

fineneedle like (20 nm) It may be seen that theprecipitates occur in rows and randomly in ferrite Alsocoarse precipitates exhibit a corendashshell structure In the

previous study on Fendash0042Vndash0016Nbndash0017Ti steel3

EDX analysis indicated a titanium rich core and a shellcontaining niobium or vanadium3 The formation of

equilibrium compounds during continuous coolingmainly depends on the intraparticle diffusion of thecomponents and homogenisation of metal atoms in a

mixed carbonitride1 There is considerable evidence to

suggest that homogenisation does not occur particularlyin the time frame of thermomechancial processing andheavily cored particles are formed with titanium nitrideas core and niobium or vanadium carbide as shell13

Figure 4a and 4b show bright and dark field TEMmicrographs respectively of high density irregularprecipitates in the size range 20ndash45 nm EDX spectra

(Fig 4a (i ii) and 4b (i ii) obtained from the markedirregular shaped particle indicate that these precipitates

Table 2 Characteristics of different precipitates

Spectral ratios

Type Morphology Size range Ti Nb V Precipitates

Triplex Fineneedle like 10ndash20 nm Ti048 Nb032 V02 VC TiC NbC (TiNbV)CSpherical irregular 20ndash45 nm Ti055 Nb035 V01 (TiNbV)N TiNbV)C TiNbV)CNCuboidal 45ndash70 nm Ti057 Nb038 V005 (TiNbV)N

Duplex Fineneedle like 10ndash20 nm Ti06 V04 VC TiC (TiV)CSpherical irregular 20ndash45 nm Nb051 V049 (NbV)C (NbV)CN NCuboidal 45ndash70 nm Ti064 Nb036 (NbTi)N

6 Size distribution of various precipitates in the microal-

loyed steel



8222019 s3 (2)



) I O M C


are rich in titanium vanadium and niobium Other EDXspectra (Fig 4b (i) and 4b(ii)) taken from fine particlespresent in the foil specimen indicates that the precipitatesare rich in vanadium Figure 5 shows a TEM micrographobtained from the extraction replica together with theEDX analysis of various precipitates The EDX spectra

obtained from coarse irregular cuboidal rectangularand spherical precipitates (marked (i) (ii) (iii) and (iv))indicate that they were all rich in titanium niobium andvanadium except the rectangular particle (marked (iii))which indicated the presence of titanium only It isapparent from the micrographs obtained for the thin foil

7 (a ) Dark field TEM micrograph showing fine vanadium carbide precipitates (circled) obtained using diffracted spot

indexed (210)VC

and (120)VC (b

) Superlattice diffraction pattern obtained for micrgraph in part (a

) showing two paral-lel zones [001]

aof ferrite and [001]VC ordered vanadium carbide with L12 cubic unit cell



8222019 s3 (2)



) I O M C


and replica that the majority of the larger precipitates aretriplex in nature

The spectral ratios obtained according to the size and

morphologies of the different precipitates are presented

in the Table 2 It may be seen from Table 2 that for eachmorphology the size range of precipitates is large

About 70 triplex and 40 duplex particles were analysed

from the different regions of the steel to measure theparticle size and the corresponding intensity counts from

EDX analysis The overall precipitate size distribution is

presented in Fig 6

The size difference between the various coarse

precipitates is likely to be a consequence of variationsin nucleation time9 The nucleation temperature of

carbides is another factor that influences the size and

chemical composition of precipitates10 Table 2 sum-

marises a qualitative stoichiometry based on the averagespectral ratios obtained from triplex and duplex pre-

cipitates The general stoichiometry of triplex and duplex

carbidescarbonitride precipitates was (Ti053Nb0

35V0

12)and (Ti06V04) (Nb051V049) and (Ti064Nb036) respec-

tively The stoichiometric ratios of the precipitates seems

to favored by the size range and morphology A detaileddiscussion on the triplexduplex precipitates and their

implications on the precipitation behaviour of VndashNbndashTi

steel was presented in an earlier study3

Fine carbide precipitates

Apart from the coarse precipitates high concentrationsof fine precipitates were also observed They were

characterised as vanadium carbide or M4C3 (where

M 5 V Nb and Ti)Figure 7a shows a dark field TEM micrograph of fine

precipitates (circled) and Fig 7b shows the correspond-

ing composite SAD pattern The diffraction pattern

analysis showed the presence of an ordered cubic L12unit cell of vanadium carbide indicated by a superlattice

diffraction pattern Figure 7a (top and bottom) wasobtained using (two variants) the superlattice diffracted

spots indexed as (120)VC and (210)VC respectively The

above evidence confirms that the diffraction spots are

from the L12 ordered unit cell of carbide precipi-tates1112 Furthermore the lattice parameter calculated

from the diffraction pattern was 04094 nm and is closeto the predicted value of 0415 nm for vanadiumcarbide The small variation in the lattice parameter is

because of mutual solubility of microalloying elements

and a detailed investigation is underway on the effect of mutual solubility of microalloying elements on the

lattice parameter variation of carbonitride precipitates

The orientation relationship between ferrite and vana-

dium carbide was a cubendashcube [001]a

[001]VC Bakerndash Nutting relationship Figure 8a and 8b show bright and

dark field TEM micrographs obtained for another

ferrite region containing fine precipitates The SAD

pattern (Fig 8b) analysis indicated different orientationrelationships namely [012]

a[001]VC and [013]

a[001]VC

between ferrite and vanadium carbide besides the usualBakerndashNutting relationship In the present case vana-

dium carbide also exhibits an ordered phase with L12

cubic unit cell A detailed analysis and discussion on

ordered phase of fine carbide precipitates occurring in amicroalloyed steel with a high percentage of Nb and Ti

was presented in a previous study3

Figure 9a is a bright field TEM micrograph show-ing fine carbide precipitation on dislocations andsub-boundaries (arrowed) The agglomerated fine

8 (a ) Bright field TEM micrograph showing ferrite region

containing fine carbides that are not visible in brightcontrast but are visible in corresponding (b ) dark field

and (c ) SAD pattern for microgaph (a ) showing the

two zones [012]a

and [013]a

of ferrite parallel to [001]VC

zone of vanadium carbide with ordered L12 unit cell



8222019 s3 (2)



) I O M C


precipitates are similar to the carbides present in Figs 7and 8 as finely dispersed precipitates in the ferritematrix The analysis of SAD pattern indicated that thesefine carbides are ordered vanadium carbides with cubicL12 unit cell having a BndashN orientation relationship withthe ferrite matrix Figure 10a b c are bright field TEMmicrographs showing multimicroalloyed carbide (M4C3)precipitates on dislocation substructures subgrain

boundaries and pinning of dislocations The carbideswere characterised as multimicroalloyed carbidesbecause of the typical composite diffraction patterns(shown as inset on each micrograph) that were analysedin detail in a previous study on precipitation behaviourof VndashNbndashTi steel3 All the carbides (MX or M4C3 typewhere M is V or Ti or Nb) have cubic crystal structureand are precipitated in the ferrite matrix It is apparentthat all the superlattice reflections are due to carbides

[TiNbV]C and because of their local ordering acomposite diffraction pattern was obtained The reasonsare (i) the individual microalloying elements form non-stoichiometric cubic carbides of the type MC or M4C3

(where M 5 Ti or V or Nb) with similar d spacing (ii)Nb(CN) and V(CN) are isomorphous with similarlattice parameters and are miscible12 and (iii) niobiumand titanium are interchangeable in NbTiC resulting in

a change in lattice parameter2

Also the lattice para-meters calculated for these complex precipitates did notmatch with the lattice parameter of the respectiveindividual carbide presumably because of miscibilityof the carbonitrides in one another

The strength is derived predominantly from finecarbides that precipitate on sub-boundaries dislocationsubstructures and pinning of dislocations Also finecarbides have higher density per unit volume and

9 (a ) Bright field TEM micrograph showing precipitation of fine carbides on dislocations (arrowed) and subgrain bound-aries (b ) SAD pattern for the image (a ) depicting two zones [001]

aand [013]

aof ferrite parallel to [001]VC zone of

vanadium carbide with ordered L12 unit cell



8222019 s3 (2)



) I O M C


impede dislocation movements in comparison to coarseprecipitates4

Thermodynamic implications of small additionsof Ti (0008) and Nb (0007) on theprecipitation behaviour of V steelIn general for each of the microalloying elements (V

Nb Ti) the nitrides are likely to be more stable than thecarbides in austenite The solubility in liquid steel is notreported for the majority of carbides and nitrides exceptthat the solubility product for titanium nitride in liquidsteel is about 100 times larger than that in the austenitephase1ndash4 However in general the solubility of nitride isinfluenced by the steel composition and depends onwhether the microalloy concentration is substoichio-metric or superstoichiometric with respect to nitrogen1

In the event that the microalloy concentration issubstoichiometric with respect to nitrogen then thesolubility of the microalloy nitrides is extremely low As

a result only a small concentration of the particularmicroalloying element may be present in solution Thisreduces the extent of carbide formation of the particularmicroalloying element to low levels (only a smallfraction will be carbide in the carbonitride precipitate)On the other hand if the microalloying element issuperstoichiometric with respect to nitrogen it results information of the highly stable microalloy nitridecompletely depleting the nitrogen from the solutionand leaving significant amount of microalloying elementin solution for interphase precipitation or precipitationafter transformation as microalloy carbides at low

temperatures on cooling

1

In the present study the V steel contains0008 wt-Ti 0007 wt-Nb and 0009 wt-N Bothtitanium and niobium are individually substoichiometricwith nitrogen while vanadium is superstoichiometricwith nitrogen Thermodynamically titanium and nio-bium form the least soluble (highly stable) nitridesleaving small concentrations of titanium and niobium insolution for subsequent precipitation On the otherhand vanadium forms highly soluble nitride leavingsignificant amount of vanadium in solution forinterphase or post-transformation precipitation Thisexplains the formation of highly stable cuboidal nitride

precipitates observed in the present steel Secondly sinceTiN NbN and VN exhibit extensive mutual solubilitybecause of their similar cubic crystal structure andsimilar lattice parameter1 and because they are com-pletely miscible113 the multimicroalloying nitridesinitially precipitate at higher temperatures as core andthe multimicroalloy carbides nucleate and grow on thenitrides as shell (Ti Nb V are completely interchange-able in their lattice with each other213) This explains theEDX spectra analysis of corendashshell or duplextriplexprecipitates with different morphologies (Fig 3) whichindicate the presence of all three microalloying elements

in a single precipitateAnother observation is the presence of fine carbides

on dislocation substructures and sub-boundaries Theywere mainly characterised as vanadium carbides bydiffraction analysis (Figs 2 7ndash9) The extensive carbideprecipitation is because of the significant amount of vanadium in the solid solution for precipitation duringcooling as discussed above On the other hand diffrac-tion pattern analysis of precipitates in Figs 2 and 10

10 (a ) Bright field TEM micrograph showing extensive

fine scale precipitation of carbides on dislocation

substructures and a row of coarse carbonitridesInset shows SAD pattern for ferrite and multimicroal-

loyed carbides ((VNbTi)C) (b ) TEM micrograph

showing multimicroalloyed carbides in the insert (c )

Bright field TEM micrograph showing pinning of dis-

locations by fine carbides and composite SAD pat-

tern from multimicroalloyed carbides and parallel

[111]a and [011]a zones of ferrite in the inset



8222019 s3 (2)



) I O M C


indicated the presence of multimicroalloying carbidesM4C3 (where M 5 V Nb Ti) It is interesting to observethese M4C3 carbides for two primary reasons the totalconcentration of Ti and Nb in the steel is small andthermodynamically Ti and Nb are substoichiometricwith nitrogen and are expected to form highly stablenitrides leaving minute levels of Ti and Nb in solution

The later is supported by the solubility productcalculations (002 wt-V at 845uC 0004wt-Nb at930uC 00052 wt-Ti at 1200uC) The concentration of the elements here represents the amount of the particularmicroalloying element present in solution at therespective temperature In the given temperature rangethe carbides start precipitating together with nitrides of each element according to the empirical equations(Table 2) for the solubility product Thus the precipita-tion of M4C3 is related to (i) the effective carbideforming ability of Ti and Nb even at small concentra-tion (ii) extensive mutual solubility of microalloyingcarbonitrides and (iii) interchangeability of microalloy-

ing elements in the lattice2

Conclusions1 Significant precipitation of coarse multimicroal-

loyed carbonitrides (triplex and duplex type) of Ti Nband V occurs in a V steel containing small concentra-tions of Nb and Ti (0008 wt-Nb 0007 wt-Ti) Theprecipitates are characterised by cuboidal (y45ndash70 nm)spherical (y20ndash45 nm) irregular (y20ndash45 nm) andfine (10ndash20 nm) morphology

2 Estimation of the equivalent volume fraction of carbides and nitrides of V Nb and Ti from the solubilityproductndashtemperature relationship indicates that the pre-cipitation of titanium occurs predominantly in the austeniteand to a small extent during austenitendashferrite transfor-mation Niobium precipitation occurs largely duringaustenitendashferrite transformation while the completeprecipitation of vanadium takes place in the ferritendash austenite two phase region at lower temperatures

3 Substoichiometric concentration of Ti and Nbwith nitrogen and the mutual extensive solubility of microalloying carbonitrides explains the formation of corendashshell precipitates with a stable nitride [(VNbTi)N]core and carbide [(VNbTi)C] shell

4 The qualitative stoichiometric ratio of triplexand duplex carbonitrides of Vndash NbndashTi steel wasTi053Nb035V012 and Ti06V04 Nb051V049 andTi064Nb036

5 The steel exhibited extensive fine scale carbideprecipitation on dislocation substructures sub-bound-aries and dislocation pinning of precipitates They were

predominantly characterised as vanadium carbide withordered cubic L12 unit cell and followed BakerndashNuttingorientation relationships with the ferrite matrix

6 M4C3 type carbides precipitate in a manner similarto steel containing higher concentrations of microalloyelements of Ti and Nb

Acknowledgements

The authors gratefully acknowledge financial supportfrom Reference Metals Pittsburgh and the University of

Louisiana at Lafayette

References1 T Gladman lsquoPhysical metallurgy of microalloyed steelsrsquo 1997

London The Institute of Materials

2 M Charleux W J Poole M Militizer and A Deschamps Metall

Trans A 2001 32A 1635ndash1646

3 S Shanmugam M Tanniru R D K Misra D Panda and

S Jansto Mater Sci Technol 2005 21 165ndash177

4 W Saikaly X Bano C Issartel G Rigaut L Charrin and

A Charai Metall Trans A 2001 32A 1939ndash1947

5 R D K Misra G C Weatherly J E Hartmann and A J

Boucek Mater Sci Technol 2001 17 1119ndash1129

6 R D K Misra K K Tenneti G C Weatherly and G Tither

Metall Trans A 2003 34A 2341ndash2351

7 R D K Misra H Nathani J E Hartmann and F SicilianoMater Sci Eng A 2005 A394 339

8 C Ouchi T Shanpe and J Kozazu Trans Iron Steel Inst Jpn

1982 22 214ndash216

9 T N Baker Y Li J A Wilson A J Craven and D N Crowther

Mater Sci Technol 2004 20 720ndash730

10 W-B Lee S-G Hong C-G Park and S-H Park Metall Trans

A 2002 33A 1689ndash1698

11 J W Edington lsquoPractical electron microscopy in material sciencersquo

Vol 3 110ndash111 1975

12 B Fultz and J M Howe lsquoTransmission electron microscopy

and diffractometry of materials 2002 Berlin Heidelberg

Springer-Verlag

13 M J Crooks A J Garratt-Reed J B Vander Sande and W S

Owen Metall Trans A 1981 12A 1999ndash2013


89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C


formation of duplex and triplex precipitates because of

their mutual solubility3

It was previously shown in Fendash TindashNbndashCndashN steel that titanium carbide is the equili-brium carbide at high temperatures and niobium carbidein the low temperature transformation range1 However

during continuous cooling the formation of compoundsdepends on the intraparticle diffusion of the compo-nents There is considerable evidence to suggest that

homogenistion does not occur in multimicroalloyed steeland heavily cored precipitates containing a core of titanium nitride and shell of niobium or vanadiumcarbide are formed1

In a multimicroalloying approach the objective is toderive the combined benefit from individual elementsto obtain the desired properties Thus it is important tostudy the effect of a microalloying element on theprecipitation behaviour of other elements and theircontribution to the overall strength and toughness Ingeneral a multimicroalloying approach results in the

precipitation of coarse carbonitrides having low densityand these do not significantly contribute to strength incomparison to the fine carbides that interact with thedislocations4 In previous studies we examined themicrostructural aspects of strengthndashtoughness combina-tions in high strength hot rolled microalloyed steelsusing VndashNb5 NbndashTi567 and VndashNbndashTi3 alloy designapproach containing almost similar carbon and manga-nese content The primary concern of the studydescribed here is to obtain an insight into the precipita-tion behaviour in a V microalloyed steel containingminute additions of Nb and Ti The precipitates were

characterised by transmission electron microscopy interms of size morphology chemistry and crystal-lography The precipitation behaviour is discussed interms of the thermodynamic stability of precipitates andis compared with VndashNbndashTi steel containing highconcentrations of V Nb and Ti3 processed undersimilar conditions

Table 1 Empirical equations used for calculation of solubility product nitrides and carbides1

Type of precipitate Precipitate Solubility product Temperature range uC phase Reference

Nitrides VN log [V][N] 5 34 ndash330Tz 0

12( Mn) 870ndash1480 Austenite

1

NbN log [Nb][N] 5 28ndash8500T 870ndash1480 Austenite 1

TiN log [Ti][N] 5 382ndash15020T 870ndash1480 Austenite 1

log [Ti][N] 5 59ndash16586T 1480 Liquid Steel 1

Carbides VC log [V][C] 5 672ndash9500T 870ndash1480 Austenite 1

log [V][C] 5 805ndash12265T 480ndash815 Ferrite 1

NbC log [Nb][C] 5 342ndash7900T 870ndash1480 Austenite 1

log [Nb][C] 5 543ndash10960T 480ndash815 Ferrite 1

TiC log [Ti][C] 5 533ndash10475T 870ndash1480 Austenite 1

log [Ti][C] 5 44ndash9575T 480ndash815 Ferrite 1

2 (a ) TEM micrograph of a representative ferrite region

containing precipitates of different size and morphol-

ogy (b ) SAD pattern for the precipitate identified with

arrows in (a ) showing [012] and [1 macr 13] zones of ferrite

and [1 macr 11] and [001] zones corresponding to M4C3 pre-

cipitates and the ordered vanadium carbide (L12 unit

cell) respectively

3 TEM micrograph illustrating corendashshell structure of

coarse duplextriplex carbonitrides that precipitated in

rows and randomly Inset shows coarse precipitates of

cuboidal and irregular morphology



8222019 s3 (2)



) I O M C
















200 kV





8222019 s3 (2)



) I O M C













8222019 s3 (2)



) I O M C














[111]M4C3 [012]

a[001]VC [113]

a[111]M4C3

[113]a[001]VC
















Spectral ratios





loyed steel



8222019 s3 (2)



) I O M C





indexed (210)VC

and (120)VC (b






8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C
















200 kV





8222019 s3 (2)



) I O M C













8222019 s3 (2)



) I O M C














[111]M4C3 [012]

a[001]VC [113]

a[111]M4C3

[113]a[001]VC
















Spectral ratios





loyed steel



8222019 s3 (2)



) I O M C





indexed (210)VC

and (120)VC (b






8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C













8222019 s3 (2)



) I O M C














[111]M4C3 [012]

a[001]VC [113]

a[111]M4C3

[113]a[001]VC
















Spectral ratios





loyed steel



8222019 s3 (2)



) I O M C





indexed (210)VC

and (120)VC (b






8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C














[111]M4C3 [012]

a[001]VC [113]

a[111]M4C3

[113]a[001]VC
















Spectral ratios





loyed steel



8222019 s3 (2)



) I O M C





indexed (210)VC

and (120)VC (b






8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C





indexed (210)VC

and (120)VC (b






8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C









presented in Fig 6








35V0



























a[001]VC and [013]

a[001]VC










two zones [012]a

and [013]a





8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C










aand [013]





8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C








1
















8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

8222019 s3 (2)



) I O M C













Acknowledgements

















1982 22 214ndash216




A 2002 33A 1689ndash1698


Vol 3 110ndash111 1975



Springer-Verlag




89 2 i l S i d h l 2005 21 8

Documents

s3 (2)