Kinetic and microstructural study of aluminium nitride precipitation in a low carbon aluminium-kille

Kinetic and microstructural study of aluminium nitride precipitation

in a low carbon aluminium-killed steel

V.Massardier a,*,V.Gue ′taz b ,J.Merlin a ,M.Soler b

Groupe d’Etudes de Me ′tallurgie Physique et de Physique des Mate ′riaux,UMR CNRS 5510,INSA Lyon,Ba ?t.Blaise Pascal,20A v e.A.Einstein,F-69621Villeurbanne Cedex,France

Arcelor,Direction Recherche De ′v eloppement et Me ′tallurgie,17a v enue des Tilleuls,57191Florange Cedex,France

Recei v ed 3September 2002;recei v ed in re v ised form 22January 2003

Abstract

The precipitation kinetics of free nitrogen present in a low carbon aluminium-killed steel was quantitati v ely followed at

temperatures ranging from 600to 7008C using a methodology based on a succession of thermoelectric power (TEP)measurements.These were compared with the precipitation kinetics of aluminium nitrides determined by con v entional TEP measurements.This comparison showed a difference between the two types of kinetics.In particular,it was obser v ed that after the complete elimination of nitrogen from the solid solution (SS),the microstructure of the steel keeps e v ol v ing.Transmission electron microscopy (TEM)obser v ations,combined with chemical X-ray energy dispersi v e spectroscopy (EDS),allowed to attribute this result to a phase transition between metastable (Al,Cr)N precipitates of cubic structure and equilibrium AlN nitrides of hexagonal structure.The presence of chromium in the initial precipitates was interpreted by a preferential segregation of this element on the former grain or sub-grain boundaries of austenite during or after hot-rolling.Then,during the precipitation of the nitrides,clusters of segregated chromium atoms could ser v e as nucleation sites for the (Al,Cr)N nitrides.#2003Else v ier Science B.V.All rights reserved.

Keywords:Precipitation kinetics;Annealing;Nitrides;Low carbon steel;Thermoelectric power

1.Introduction

During the manufacture of steel sheets containing aluminium,the free nitrogen present in the steel may precipitate to form aluminium nitrides,the structure of which can be either a hexagonal wurtzite structure with a 00.311nm and c 00.4978nm [1](this structure corresponds to the stable form of aluminium nitride)or a metastable cubic structure of NaCl type with a ranging between 0.4045and 0.417nm [2á5].This precipitation may occur either during coiling following hot-rolling or during the recrystallisation annealing performed after cold-rolling.As this precipitation is belie v ed to play an important role for the recrystallisa-tion texture of the steel sheet obtained after the

annealing [6á11]and for its drawability,it is necessary to control this precipitation.

Furthermore,a recent work [12]was performed on a low-carbon low-aluminium steel (56)10(3wt.%C,11)10(3wt.%N,14)10(3wt.%Al,12)10(3wt.%Cr and about 300)10(3wt.%Mn)ha v ing a sub-stoichiometric [Al]/[N]atomic ratio and containing chromium and manganese in addition to aluminium.It was noted that the nitrides formed during an annealing performed at 6008C were not the expected aluminium nitrides of cubic or hexagonal structure but rather complex nitrides containing alumimium,chromium and/or manganese.They were called nitrides of (Al,Cr)N type or of (Al,Cr,Mn)N type,depending on their chemical composition.These nitrides were found to ha v e a cubic structure with a lattice parameter a 00.41290.005nm,a plate-like morphology and a Bain orientation relationship with the iron matrix.Moreo v er,in the same steel,a complete elimination of nitrogen from the solid solution (SS)was obser v ed [13]in spite of

*Corresponding author.Tel.:'33-4-72-436144;fax:'33-4-72-437930.E-mail address:v eronique.massardier@insa-lyon.fr (V.Massardier).Materials Science and Engineering A355(2003)299á

310

www.else v https://www.360docs.net/doc/0d9030461.html,/locate/msea

its sub-stoichiometry in aluminium.This was interpreted by the fact that the lack of aluminium to totally consume the free nitrogen is compensated by the presence of Cr and Mn in the steel.

In this context,the aim of the work presented in this paper was to characterise,from a kinetic and micro-structural point of v iew,the precipitation of the nitrides formed in a low carbon steel with an excess of aluminium with regard to the stoichiometric[Al]/[N] atomic ratio.For the kinetic study,a specific methodol-ogy based on thermoelectric power(TEP)measurements performed on the steel was de v eloped in order to be able to follow the e v olution of the nitrogen content in SS during different isothermal treatments.For the micro-structural study Transmission electron microscopy (TEM)obser v ations and energy dispersi v e spectroscopy (EDS)analyses were conducted on the studied steel. 2.Material

The material in v estigated in this work is a low carbon aluminium killed steel.Its chemical composition,ex-pressed in wt.%,is:25)10(3C,5)10(3N,58)10(3 Al,39)10(3Cr and200)10(3Mn.The present study was conducted on a steel sheet which was reheated at a high temperature(12308C),hot-rolled in the austenitic domain and then coiled at5858C.Due to these conditions,it is expected that most of the aluminium and nitrogen present in the steel is in SS in the initial state.The precipitation of the nitrides was studied during isothermal treatments between600and7008C and performed under v acuum,so as to a v oid oxidation and decarburation.

3.Experimental procedure

The microstructural changes of the steel during isothermal treatments were in v estigated essentially with TEP measurements.Howe v er,in order to identify clearly the precipitates formed during the different treatments,TEM obser v ations combined with chemical analyses were also conducted.

3.1.Thermoelectric power(TEP)measurements

Like other physical techniques(such as electrical resisti v ity,for example),TEP can be used to follow the microstructural changes which may occur during the isothermal treatments of metallic materials.The princi-ple is to measure,at room temperature,the TEP of the studied material after different treatment times at the temperature of interest(T p).In these conditions,the TEP v ariations measured between the initial state (before any treatment at T p)and the state obtained after a time t at T p are representati v e of all the microstructural phenomena which may take place at T p(precipitation and/or dissolution of precipitates).In order to deduce quantitati v e information from these TEP v ariations,it is thus necessary to know all the elements,the concentration of which can e v ol v e in solution during the treatment.In the study of the precipitation of nitrides in aluminium-killed steels, such TEP v ariations can only be used to e v aluate quantitati v ely the e v olution of the content of precipi-tated nitrogen,when the following conditions are satisfied:(i)all the nitrogen is in SS in the initial state, (ii)the composition of the nitrides formed during the treatment is known,(iii)the nitrides ha v e no effect on the TEP and(i v)no other phenomena(such as the dissolution of carbides)occur simultaneously.

Owing to the difficulty of checking all these assump-tions,it is not easy to e v aluate directly the content of precipitated nitrogen.This is why it appeared interesting to de v elop a specific methodology based on TEP measurements and so determine quantitati v ely the content of free nitrogen present in a low carbon aluminium-killed steel.

3.1.1.The thermoelectric power test

The principle of this technique[14]is to measure the v oltage(D V)arising from the Seebeck effect between two junctions of the sample with pure iron blocks,the temperature of which are T and T'D T.

The apparatus used in this work gi v es the relati v e TEP(S0D V/D T)with respect to a reference state with a precision of90.002m V K(1.The v alue of S is affected at different le v els by the defects present in the lattice of the iron matrix and is the result of v arious contributions:S0D S SS'D S d'D S pp,where D S SS,D S d and D S pp are due to the elements in solid solution(SS), to dislocations(d)and to precipitates(pp),respecti v ely. The contribution of the elements in SS on the TEP of pure iron is gi v en by the GorteráNordheim law[15], which can be expressed as follows:D S SS0a K i[i]SS,if the concentration of the elements is low(B10(1at.%).

[i]SS represents the content of the element i in SS and K i is a coefficient linking the content of this element in SS with its contribution to the modification of the TEP of pure iron.Table1gi v es the effect of different elements in SS on the TEP of pure iron.The abo v e mentioned elements(except Cr)ha v e a negati v e effect on the TEP of pure iron,when they are in SS.Thus,their precipita-Table1

Values of the coef?cients K i(expressed in m V per(K(wt.%))of different elements in SS in pure iron

K C K N K Al K Mn K Cr

(45(24(30(3'3

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tion should lead to a TEP increase.With regard to the precipitates,their effect can be neglected,except if they are small and coherent.In this case,the main problem is that the effect of the precipitates is not known and is difficult to determine https://www.360docs.net/doc/0d9030461.html,stly,the dis-locations tend to decrease the TEP of pure iron.3.1.2.No v el TEP methodology

Fig.1shows schematically the different steps of the methodology de v eloped to determine the content of free nitrogen present in an aluminium-killed steel treated for a gi v en time at a temperature T p chosen between 600and 7008C,where free nitrogen may precipitate with aluminium to form nitrides.After each step of the procedure,the relati v e TEP of the sample with respect to that of pure iron is measured.Before any treatment,the steel is in its initial state and is characterised by a gi v en amount of alloying elements in SS.Its TEP is S 0.In state 1,obtained after a time t at the temperature of treatment (T p )and characterised by a TEP v alue noted S {T p }t ,the content in alloying elements in SS is different from that in the initial state,due to the precipitation of nitrogen with one or se v eral other elements and due to the possible dissolution of carbides.State 2is reached after a 1min treatment at 6008C followed by a water-quench.The role of this treatment is to obtain a reference state with regard to the solubility of carbon,i.e.the amount of carbon in SS is the same in all the samples,regardless of their temperature of treatment (T p ).The TEP of this state is noted S {SS}t .In state 3,obtained after a 3h treatment at 2708C,the carbon has almost completely disappeared from the SS due to precipitation of carbides.In fact,a small amount of carbon corresponding to the solubility limit of carbon at 2708C remains in SS after this treatment;this carbon will be called ‘residual carbon’.The content of nitrogen in SS is not modified during this treatment,since it is not possible to form iron nitrides at 2708C in a steel,when the nitrogen content is lower than 10)10(3wt.%[16]:this is the case of the in v estigated steel.Moreo v er,the formation of other nitrides (such as AlN)during a 3h treatment at 2708C is unlikely,as the diffusion of substitutional elements in iron is v ery slow at this low temperature.These results were confirmed by internal friction measurements:no change in the height of the nitrogen peaks was detected during the treatment at 2708C.The TEP of this state is S {V}t and the difference between the TEP of states 2and 3,D S V 0S {V}t (S {SS}t ,corresponds to the amount of carbon which precipitated during the treatment at 2708C,[C pre ],such that D S V 0KC.[C pre ].Whate v er the treatment time at T p ,this quantity must be the same,since all the samples were treated for 1min at 6008C before the treatment at 2708C.

Lastly,the difference D S {V}0S {V}t (S {V}0(where S {V}0and S {V}t are the TEP v alues of state 3for a specimen treated during a time t 00and a time t at T p )is expected to represent all the physical phenomena which can take place during the treatment at T p ,except those concerning carbon.Indeed,in state 3the con-tribution of carbon to the v alue of S {V}0and S {V}t is assumed to be exactly the same,whate v er the treatment time t at T p .As a consequence,the difference D S {V}is independent of the effect of carbon,which is eliminated by subtraction.Thus,D S {V}should reflect the precipita-tion of free nitrogen with other alloying elements.In these conditions,the e v olution of D S {V}as a function of time at T p allows us to follow the precipitation kinetics of the nitrides.This type of TEP measurement has already been used pre v iously [17,18]to follow the aluminium nitride precipitation in different steels.How-e v er,as was mentioned abo v e,to e v aluate the precipi-tated nitrogen from these TEP measurements,se v eral assumptions need to be v erified.This is why two additional states (4and 5)were included to obtain the content of free nitrogen of the studied steel treated for a gi v en time at T p .State 4is obtained after a 80%cold-rolling of the samples,while state 5is reached after

Fig.1.Experimental procedure used to e v aluate the amount of free nitrogen in the studied steel based on successi v e TEP measurements.

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treatment of30min at1208C.Between states3and4,a high dislocation density is introduced into the samples. The corresponding TEP v alue is S{o}.Lastly,between states4and5,during the treatment at1208C,all the interstitial elements present in SS(free nitrogen and residual carbon)are expected to diffuse towards the dislocations.Then,the TEP v ariation measured between states4and5,D S a0S{A}(S{o},represents the amount of interstitial elements which left the SS to segregate to the dislocations and it is directly proportional to the content of residual carbon and nitrogen in SS in state4. Consequently,D S a is the sum of two terms and can be written as D S a0D S a(N)'D S a(C),where:

(D S a(N)0KN[N]SS,t corresponds to the contribu-

tion of the free nitrogen,the concentration of which ([N]SS,t)decreases with the increase in the treatment time at T p.

(D S a(C)0KC[C residual]is the contribution of the

residual carbon,the concentration of which([C resi-dual

])does not depend on the treatment time at T p. From internal friction measurements,it was estab-lished that it is possible to completely eliminate nitrogen from the SS of the studied steel,since no peaks due to nitrogen were detected in the internal friction spectra measured after a prolonged treatment at T p(this is illustrated in the spectra of Fig.5obtained after a treatment at7008C).Thus,after elimination of nitro-gen from the SS,D S a(N)00and D S a0D S a(C)00.067 m V K(1,a v alue which corresponds to about15ppm of residual carbon in SS.From D S a(C),it is possible to e v aluate D S a(N)and subsequently,the v alue of[N]SS,t after different times at T p.Thus,the e v olution of D S a as a function of time is representati v e for the changing amount of free nitrogen in the steel.

The cold-rolling reduction(state4)and the final thermal treatment(state5)were chosen to ensure that(i) the deformation induces enough dislocations to trap all the interstitial elements in SS and(ii)the time and the temperature of the final treatment allow a complete diffusion of nitrogen and carbon towards the disloca-tions.

3.2.Internal friction measurements

To complete the TEP measurements,a few internal friction experiments were also carried out with a torsion pendulum at1Hz employing a heating rate of50K h(1 between230and350K.In steels,the damping Q(1is due to the Snoek effect.The dissipation of energy is due to the rearrangement of interstitial elements in the b.c.c. unit cell.The temperature position of the resulting peaks allows us to determine the nature of the interstitial element in SS(N and/or C).Furthermore,the height of the peak of each element is representati v e of the concentration of the element in SS.In the case of the FeáMnáCáN steels,their internal friction spectra are characterised by one peak from carbon in SS and by three peaks from nitrogen.The total amount of nitrogen in SS is gi v en by the sum of the contribution of the three peaks.Since the relati v e weight of the three peaks is not the same and is not known accurately,the quantitati v e e v aluation of the content of nitrogen by internal friction is difficult.

3.3.TEM obser v ations

In order to identify the nitrides formed during the thermal treatments,con v entional and high resolution transmission electron microscopy(CTEM and HRTEM)obser v ations were performed on carbon extraction replicas and on thin foils,prepared using standard techniques and con v entional electropolishing. The obser v ations were conducted in a field-emission gun JEOL2010F microscope operated at200kV which is equipped with a slow-scan CCD camera and with an energy dispersi v e X-ray analyser ha v ing an ultra-thin window and a Link Isis analytical system.

4.Results of the kinetic study based on TEP measurements

The precipitation kinetics of the nitrides or of the free nitrogen were followed at three temperatures:T p0 6008C,T p06508C and T p07008C.

4.1.Precipitation kinetics of the nitrides followed by measurement of the difference D S{V}

As was explained abo v e,the e v olution of the differ-ence D S{V}as a function of time should be representa-ti v e of the precipitation of the nitrides during the treatment at T p

Fig.2.E v olution of D S{V}as a function of the treatment time t at T p0600,650and7008C.

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Fig.2shows the e v olution of the difference D S {V}at the three temperatures of interest.The kinetics is characterised by an increase followed by a final stabi-lisation at the end of the precipitation and is shifted towards low annealing times due to the thermal acti v a-tion as the temperature is increased.These changes can be attributed to the precipitation of nitrogen together with other elements and may be the result of two effects:(i)an effect of the SS due to the decrease in the amount in alloying elements in SS,which is expected to lead to a TEP increase and (ii)an effect due to the precipitates themsel v es,which is not known.

When the same type of TEP measurement was used in the past [17],a final stabilisation of the TEP kinetics was ne v er obser v ed after long treatment times.These obser v ations can be attributed to the conditions of treatment at the temperature T p ,which can lead to a modification of the chemical composition of the samples during the treatment (for example,by introducing elements such as oxygen).This is supported by the fact that the same type of beha v iour was obser v ed in this work,when the treatment was performed in a controlled atmosphere (nitrogen '2.5%hydrogen)instead of in a v acuum,as can be seen in Fig.3.

4.2.Precipitation kinetics of the free nitrogen followed by measurement of the difference D S a

The methodology presented to determine the content of free nitrogen ([N]SS,t )from the quantity D S a was applied to the studied steel.In order to v alidate this methodology,a preliminary work [19]was conducted:this work showed that the results of the quantification of the free nitrogen in the steel deduced from the measure-ments of the quantity D S a are in perfect agreement with those obtained by hot-hydrogen extraction and by electrochemical dissolution followed by mineralisation.Fig.4shows [N]SS,t deduced from the measurements of the quantity D S a at the three temperatures of interest.The three kinetics are characterised by an initial

decrease followed then by a stabilisation,when the free nitrogen has completely disappeared from the SS and are shifted towards low annealing times due to the thermal acti v ation of the precipitation reaction as the temperature is increased.Furthermore,these cur v es show that in the initial state not all the nitrogen present in the steel (about 50ppm)is in SS.About 10ppm has already precipitated before any treatment at T p .

In order to compare the e v olution of the nitrogen content in SS obtained through TEP and internal friction measurements during an isothermal treatment at 7008C,internal friction experiments were performed on the studied steel,treated for different times at 7008C and then for 3h at 2708C (in order to eliminate as much carbon as possible from the SS).The correspond-ing spectra are shown in Fig.5.As was shown by Merlin et al.[13],the peaks present on these spectra are due only to the free nitrogen.Indeed,the residual carbon present in the steel after the treatment at 2708C (about 15ppm)is not v isible on these spectra.In Fig.5,the height of the peaks gradually decreases as the treatment time at 7008C increases due to the precipitation of nitrogen with other elements to form nitrides.Moreo v er,the spectra indicate that the three populations of nitrogen atoms tend to disappear from SS at the same rate during the precipitation.Consequently,the max-imum of the internal friction spectra obser v ed around 307K can be considered as being representati v e of the amount of nitrogen in SS.

In order to correlate the results deduced from the TEP measurements with those obtained from the internal friction measurements,the maxima of the internal friction spectra of Fig.5(noted d max )are shown in Fig.6as a function of the corresponding v alue of D S a .Additional points obtained for the low carbon low aluminium steel studied by Merlin et al.[13]are also shown in Fig.6.In all cases,a linear relationship is obser v ed between D S a (N)and d max ,indicating a good correlation between the relati v e v ariations of

the

Fig.3.E v olution of D S {V}as a function of the treatment time t at T p 07008C under different conditions of

treatment.

Fig.4.Amount of free nitrogen as a function of the treatment time t at T p 0600,650and 7008C.

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amount of free nitrogen deduced from the TEP and the internal friction.

Howe v er,when the v alue of d max reaches zero,D S a is different from zero.This result can be attributed to the residual carbon which is not detected by internal friction but which is detected by TEP as was shown by La v aire [20].

https://www.360docs.net/doc/0d9030461.html,parison between the e v olution of D S {V }and of D S a

In order to compare the precipitation kinetics of the nitrides deduced of D S {V}with those of the free nitrogen deduced of D S a ,the kinetics were normalised.Fig.7represents the normalised v ariation of D S {V}and that of the amount of precipitated nitrogen deduced from the measurements of [N]SS,t at T p 07008C.The compar-ison between these cur v es shows that the two types of kinetics are slightly shifted and that the shift increases when the ageing time at T p is increased.In particular,the precipitation of nitrogen is more rapid than the v ariation of D S {V}.This indicates that the v ariations of D S {V}are not only representati v e of the decrease in the

nitrogen content in SS and in the corresponding amount of aluminium but also take into account other metal-lurgical phenomena.These phenomena could be an e v olution of the chemical composition of the nitrides (which can lead to a v ariation of the content of alloying elements in SS)and/or of their crystallographic structure (which can modify the intrinsic effect of the precipi-tates).

4.4.Analysis of the precipitation kinetics

The precipitation kinetics was analysed using the Johnson áMehl áA v rami áKolmogoro v approach,which reads as:Y 01(exp(((k )t )n ),where Y is the pre-cipitated fraction at a gi v en temperature,k 0A )exp((E A /RT )is a rate constant,n is a coefficient characterising the precipitation mode and E A is the acti v ation energy associated with the phenomena occur-ring between 600and 7008C.In this work,the precipitated fraction Y was assessed using either the normalised v alues of the quantity D S {V}or the normal-ised v alues of the amount of precipitated

nitrogen

Fig.5.Internal friction spectra d 0f (T )of the studied steel treated for different times t at T p 07008C and then for 3h at 2708C (frequency 1

Hz).

Fig. 6.Correlation between the maximum of the internal friction spectra (d max )and the v alue of D S a

Fig.7.E v olution of the normalised v ariations of D S {V}and of the amount of precipitated nitrogen deduced of the measurements of [N]SS,t at T p 07008C.

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deduced from D S a.The normalised cur v es of Fig.7are thus representati v e for Y as a function of time at T p0 7008C.

Table2gi v es n and the acti v ation energy deduced from the analysis of the normalised v ariations of D S{V} and of the quantity of precipitated nitrogen.Two different v alues were obtained for n.The v alue of n deduced from the v ariations of D S{V}(of the order of2) has probably no physical meaning,since these v ariations take into account se v eral metallurgical phenomena.In contrast,the v alue of n deduced from the e v olution of D S a(3.3)should ha v e a physical meaning,since this e v olution is only representati v e of the decrease in the content of free nitrogen.This v alue is in good agreement with that obtained by Lankreijer et al.[21](3.5),who interpreted this v alue by a growth controlled by diffu-sion.

The acti v ation energy deduced from the e v olutions of D S{V}and D S a can be interpreted as being that associated with the diffusion of aluminium in ferrite, the v alue of which is found to range between200and 250kJ mol(1[22á24].This v alue is not known exactly because it depends on se v eral parameters(temperature, aluminium content of the steel,method of measurement...).The metallurgical phenomena taking place at T p are thus go v erned by the diffusion of aluminium.

5.Results of the microstructural study based on TEM observations coupled with energy dispersive spectroscopy analyses

5.1.Description of the microstructural in v estigations

In order to identify the nitrides leading to the TEP v ariations presented abo v e,TEM obser v ations were conducted on the steel in its initial state and after different treatments.

For the characterisation of the size and morphology of the precipitates,CTEM obser v ations were conducted both on thin foils and on carbon replicas.To determine the chemical composition of the precipitates,nanometric probe analyses were performed on the precipitates present on the replicas.About30precipitates were analysed in each state.From the EDS spectra the mean atomic ratios of the elements present in the different types of precipitate were assessed taking into account an absorption correction.For the analysis of the EDS spectra it was necessary to dissociate the peaks due to the precipitates from those due to their en v iron-ment(Cu and C)or those due to the preparation of the replicas(e.g.silicon,oxygen and iron).The nitrogen content of the nitrides was not assessed quantitati v ely, owing to the fact that the nitrogen and carbon peaks o v erlap,making the quantification of nitrogen impre-cise.In order to identify the structure of the precipitates selected area electron diffraction(SAED)patterns of indi v idual precipitates and/or numerical diffractograms (obtained from the numerical Fast-Fourier Transforms of HRTEM images)were used.

5.2.Characterisation of the microstructure of the steel in its initial state

The TEM obser v ations performed on the steel in its initial state confirmed that after reheating at12308C nitrogen is not completely in SS,since nitrides were obser v ed.Some of them were identified as being titanium nitrides(TiN);others,characterised by a mean length of about30nm,were found to contain aluminium and chromium with a[Al]/[Cr]atomic ratio of2.

5.3.Characterisation of the nitrides formed in the early stages of the precipitation

For treatment times of1h at6008C and3min at 7008C,small nitrides,ha v ing a mean length of the order of592nm,were obser v ed on the replicas of the steel by CTEM.After24h at6008C and16min at 7008C(Fig.8a)these precipitates were still present and their number and size had increased.On the replicas different morphologies(squares or more or less elon-gated rectangles)are clearly v isible.These are compa-tible with a platelet-like morphology.

The CTEM obser v ations of the thin foils of the steel treated for16min at7008C showed the presence of numerous precipitates inside the grains.As in the case of nitrides obser v ed in the low carbon low aluminium steel studied in[12],only the deformation contrast around the precipitates was v isible.From the analysis of the lines of no contrast associated with the precipitates and obtained under v arious conditions,it was shown that the nitrides ha v e a platelet-like morphology and grow on the{100}habit planes of ferrite.

The EDS analyses(Fig.8b)re v ealed the presence of nitrogen,aluminium and also chromium in the nitrides. It was shown that their mean[Al]/[Cr]atomic ratio is equal to491,regardless of the time and temperature of treatment.

Lastly,from the SAED patterns of indi v idual pre-cipitates or from their numerical diffractogram(such as that Fig.9),it was shown that the nitrides ha v e a cubic

Table2

Values of n and E A(JMAK-kinetics)obtained from D S{V}and D S a

n E A(kJ mol(1)

From D S{V} 2.3239910

From D S a 3.3217910

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structure with an https://www.360docs.net/doc/0d9030461.html,ttice.The numerical diffracto-gram of Fig.9and the corresponding scheme are the result of the superposition of the diffraction pattern of ferrite ( 100 Fe -zone axis)and of that of a nitride v iewed along a 110 -zone axis.The analysis of this diffractogram shows that the (001)planes of the nitrides are parallel to the (001)planes of the ferrite.All these results indicate that the nitrides are oriented in the iron matrix according to a Bain orientation relationship:

[110](Al ;Cr)N jj [010]a -Fe and (001)(Al ;Cr)N jj (001)a -Fe

Furthermore,the numerical diffractogram of Fig.9gi v es the lattice parameter of the nitrides after calibra-tion using the (002)spot of ferrite.The lattice parameter a is 0.41290.005nm and is thus v ery similar to that of the cubic structure of AlN (a :0.410á417nm)and of CrN [25]

Fig.8.(a)Nitrides obser v ed on a carbon extraction replica of the steel treated for 16min at 7008C;(b)EDS spectrum of the nitrides present on the

replica.

Fig.9.(a) 100 Fe áHRTEM image of a nitride of (Al,Cr)N type and associated numerical diffractogram:the precipitate is v iewed with its habit plane parallel to the electron beam.(b)Scheme gi v ing the crystallographic relations between the nitrides and the iron matrix.

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306

5.4.Characterisation of the nitrides formed at the end of the precipitation

For treatment times of30min at7008C and of24h at6008C,a second type of nitride ha v ing a mean size of the order of2095nm was detected by CTEM.In these two states a coexistence of these new precipitates with nitrides of the(Al,Cr)N type was noted.Furthermore,it was shown that nitrides of the(Al,Cr)N type are progressi v ely replaced by the second type of nitrides as the treatment time is increased(1or4h at7008C and1 week at6008C).These nitrides are v isible in Fig.10(a and b)after a treatment of4h at7008C.They contain only nitrogen and aluminium,as can be seen in the EDS spectrum of Fig.10c.Thus,they are AlN precipitates of a hexagonal wurtzite structure[1].Fig.10d shows a typical diffraction pattern obtained from a precipitate v iewed along a 0001 AlN-zone axis.No orientation relationship was obser v ed between these nitrides and the iron matrix.

6.Discussion

The main results deduced from the kinetic and microstructural studies presented abo v e are summarised in Table3.The kinetic study showed a difference between the kinetics controlling D S{V}and D S a(or [N]SS,t).The v ariations of D S{V}as a function of time are less rapid than those of D S a.This was explained by the fact that the v ariations of D S{V}take into account se v eral metallurgical phenomena,which leads to a difference in the time necessary to reach the final stabilisation of the TEP kinetics.This difference indi-cates that the end of the precipitation of the free nitrogen in the steel does not correspond to the end

of Fig.10.CTEM obser v ations of the nitrides formed in the steel treated for4h at7008C:(a)carbon replica;(b)thin foil;(c)EDS spectrum of the nitrides present on a replica;(d)typical SAED pattern of a nitride.

V.Massardier et al./Materials Science and Engineering A355(2003)299á310307

the microstructural changes of the steel.The two kinetics are thus complementary.

The microstructural study pro v ided e v idence for the precipitation of two types of nitride:(Al,Cr)N and AlN. The nitrides of(Al,Cr)N type were already obser v ed in a low-carbon low-aluminium steel with a sub-stoichio-metric[Al]/[N]atomic ratio[12].For this steel,the presence of chromium and/or manganese in the nitrides was explained by its low aluminium content,i.e.the lack of aluminium is compensated by chromium and/or manganese.The present work shows that it is possible to form(Al,Cr)N nitrides in a steel with an excess of aluminium o v er nitrogen.In this case,the aluminium content of the steel is high enough to allow the complete elimination of nitrogen from SS.Ne v ertheless,the presence of chromium was detected in the first nitrides formed during the precipitation.

With regard to the AlN precipitates,the TEM obser v ations showed that their formation is obtained after precipitation of the metastable(Al,Cr)N nitrides. As the(Al,Cr)N nitrides are semi-coherent and ha v e a coherency plane with iron(such that the(Al,Cr)N cube de v elops on the four diagonals of the(001)planes of iron),their nucleation and growth is promoted in the early stages of the precipitation.Then,as the treatment time is increased,hexagonal AlN nitrides appear.First, they coexist with the(Al,Cr)N nitrides and then,they gradually replace the initial nitrides,indicating that a phase transformation occurs between the(Al,Cr)N and AlN nitrides.The mechanism by which the metastable (Al,Cr)N nitrides are replaced by the equilibrium AlN nitrides was not clearly identified.Howe v er,two me-chanisms can be considered:(i)the AlN precipitates form after the dissolution of the intermediate(Al,Cr)N nitrides or(ii)the(Al,Cr)N nitrides transform into AlN precipitates by local atomic migration,chromium being rejected into the matrix.

From all these obser v ations,the difference between the e v olution of D S{V}and of D S a(or[N]SS,t)can be explained.This difference can be attributed to the(Al, Cr)N0AlN transformation obser v ed for the long treatment times.This transformation is not reflected in the e v olution of D S a and occurs after the complete elimination of free nitrogen from the SS.In contrast, this transformation leads to v ariations of D S{V},which can be attributed to the e v olution of the intrinsic effect of the nitrides and/or of their chemical composition during the transformation.Concerning the intrinsic effect of the precipitates,its influence on the v ariations of D S{V}cannot be assessed.In contrast,the effect of the changing chemical composition of the nitrides is ex-pected to lead to a TEP increase,due to the replacement of chromium atoms by aluminium atoms in the nitrides. Indeed,chromium in SS is assumed to ha v e a positi v e effect on the TEP of iron,while aluminium in SS has a negati v e effect.

Lastly,it is necessary to discuss the presence of chromium in the nitrides formed in the early stages of the precipitation of the studied steel.First,it is important to recall the location of these nitrides in the iron matrix.In the present work the TEM obser v ations were conducted on thin foils ha v ing their surface parallel to the rolling plane and a rather homogeneous distribu-tion of(Al,Cr)N nitrides was obser v ed in the iron matrix.Howe v er,in some cases,when the thin foil was tilted,an alignment of nitrides was detected,as that shown in Fig.11.Michalak et al.[8]ha v e already obser v ed this type of alignment of precipitates on grain or sub-grain boundaries parallel to the rolling plane.

In the present work,the alignment of nitrides was obser v ed on steel sheets which were not cold-rolled before the treatment at T p.This indicates that such alignment results from the thermomechanical history of the steel before cold-rolling.From all these considera-tions,it can be suggested that the(Al,Cr)N nitrides probably precipitate on prior grain or sub-grain bound-aries of austenite formed during hot-rolling.It is possible that a preferential segregation of substitutional

Table3

Synthesis of the main results deduced from the kinetic study a and of the microstructural study

1h24h1week T p06008C

D S{V}24h:stabilisation of the cur v e D S{V}0f(t)

D S a,[N]SS,t8h:end of the precipitation of the free nitrogen

TEM obser v ations'EDS analyses(Al,Cr)N(Al,Cr)N'AlN AlN

3á16min30min1á4h T p07008C

D S{V}30min:stabilisation of the cur v e D S{v}0f(t)

D S a,[N]SS,t10min:end of the precipitation of the free nitrogen

TEM obser v ations'EDS analyses(Al,Cr)N(Al,Cr)N'AlN AlN

a based on the measurement of TEP v ariations:D S

{V}or D S a.

V.Massardier et al./Materials Science and Engineering A355(2003)299á310 308

atoms (such as chromium and/or aluminium)occurred at the grain or sub-grain boundaries of austenite during or after hot-rolling.This hypothesis is supported by the work of Papworth et al.[26]who obser v ed a segregation of chromium atoms at the prior grain boundaries of austenite.During the precipitation of the nitrides at T p ,these boundaries are no longer present in the steel but the clusters of substitutional elements are still present and could ser v e as nucleation sites for the (Al,Cr)N nitrides.Due to the high chemical affinity between nitrogen and chromium or aluminium,nitrogen could diffuse towards these clusters and form (Al,Cr)N nitrides.Finally,the growth of these nitrides could occur by diffusion of nitrogen,aluminium and/or chromium atoms present in SS in the iron matrix towards the initial nitrides located on the former grain or sub-grain boundaries of austenite.

A complementary study of the precipitation of the nitrides formed in the studied steel [27],reheated at 12308C but not hot-rolled before the treatment at T p ,showed the absence of chromium in the nitrides.This supports the fact that segregation of substitutional atoms (in particular,chromium)is associated with the hot-rolling stage.Without hot-rolling,this segregation does not occur and the result is that the nitrides do not contain chromium.Let us note that the mechanism of formation of the (Al,Cr)N nitrides described abo v e can also explain the presence of chromium in the nitrides formed in the steel studied in [12].

7.Summary

The precipitation kinetics of the nitrides formed

during the isothermal annealing of a low carbon aluminium-killed steel were followed using two different

types of TEP measurement and were compared.This comparison showed that the kinetics of precipitation of nitrides is less rapid than that of the free nitrogen content and that the microstructure of the steel con-tinues to e v ol v e after the complete elimination of free nitrogen from SS.

A microstructural study based on TEM obser v ations re v ealed the formation of two types of nitrides during the isothermal treatments.At the early stages of the precipitation,(Al,Cr)N nitrides were detected.They were found to ha v e a cubic structure with a lattice parameter a 00.41290.005nm.Then,it was obser v ed that these metastable nitrides are progressi v ely replaced by equilibrium AlN nitrides of a hexagonal structure,this transformation taking place after the complete elimination of the free nitrogen from SS.

As a result,the changes in the microstructure of the steel detected after elimination of nitrogen from SS could be attributed to the (Al,Cr)N 0AlN transforma-tion.

It was suggested that the presence of chromium in the (Al,Cr)N nitrides is due to a segregation of chromium atoms on the grain or sub-grain boundaries of austenite during or after hot-rolling.Then,during the treatment of precipitation of the nitrides,these clusters of chro-mium atoms could ser v e as nucleation sites for the (Al,Cr)N nitrides.

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