Impact toughness and microstructure relationship in niobium- and vanadium-microalloyed steels proces

Materials Science and Engineering A437(2006)

436–445

Impact toughness and microstructure relationship in niobium-and vanadium-microalloyed steels processed with varied cooling

rates to similar yield strength

S.Shanmugam a,R.D.K.Misra a,?,T.Mannering b,D.Panda b,S.G.Jansto c

a Center for Structural and Functional Materials and Department of Chemical Engineering,

University of Louisiana at Lafayette,LA70504-4130,USA

b Nucor-Yamato Steel,P.O.Box1228,5929East State Highway18,Blytheville,AR72316,USA

c Reference Metals,1000Ol

d Pond Road,Bridgeville,PA15017,USA

Received12July2006;accepted3August2006

Abstract

We describe here the relationship between microstructure and impact toughness behavior as a function of cooling rate for industrially processed Nb-and V-microalloyed steels of almost similar yield strength(～60ksi).Both Nb-and V-microalloyed steels exhibited increase in toughness with increase in cooling rates during processing.However,Nb-microalloyed steels were characterized by relatively higher toughness than the V-microalloyed steels under identical processing conditions.The microstructure of Nb-and V-microalloyed steels processed at conventional cooling rate,primarily consisted of polygonal ferrite–pearlite microconstituents,while Nb-microalloyed steels besides polygonal ferrite and pearlite contained signi?cant fraction of degenerated pearlite.The microstructure of Nb-and V-microalloyed steels processed at relatively higher cooling rate contained degenerated pearlite and lath-type(acicular)ferrite in addition to the primary ferrite–pearlite constituents.The fraction of degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels.In both Nb-and V-microalloyed steels the precipitation characteristics were similar with precipitation occurring at grain boundaries,dislocations,and in the ferrite matrix.Fine-scale (～5–10nm)precipitation was observed in the ferrite matrix of both the steels.The selected area diffraction(SAD)pattern analysis revealed that these?ne precipitates were MC type of niobium and vanadium carbides in the respective steels and followed Baker–Nutting orientation relationship with the ferrite matrix.The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels is attributed to higher fraction of degenerated pearlite in the steel.

?2006Elsevier B.V.All rights reserved.

Keywords:Microalloyed steels;Precipitation;Degenerated pearlite

1.Introduction

Currently,there is a strong interest to study thermomechan-ical processing and accelerated cooling of high strength low alloy steels to derive mechanical property bene?ts,notably strength–toughness combination[1–8].It is known that cooling rate after the last reduction and coiling temperature has signi?-cant effect on the ultimate microstructure of steels.Accelerated cooling is one of the methods to strengthen the steels with mini-?Corresponding author at:Center for Structural and Functional Materials and Department of Chemical Engineering,University of Louisiana at Lafayette,P.O. Box44130,Lafayette,LA70504-4130,USA.Tel.:+13374826430;

fax:+13374821220.

E-mail address:dmisra@https://www.360docs.net/doc/6618885612.html,(R.D.K.Misra).mal alloying content and to obtain good toughness.It also facil-itates the development of high strength steels with low carbon and manganese content,and consequently improved weldability [2].The lowering of ferrite transformation temperature intro-duced by higher cooling rate promotes ferrite nucleation rate at the austenite grain boundaries and in the grain interior.The enhanced nucleation density restricts grain growth because of impingement of mutual grains,resulting in ferrite grain re?ne-ment[2].Additionally,the volume fraction of non-equilibrium phases increases with higher cooling rates[1–6].Thus,accel-erated cooling after controlled rolling produces a?ne-grained microstructure and promotes the formation of low transforma-tion temperature products,such as degenerated pearlite,bainitic ferrite,acicular ferrite or bainite and martensite–austenite constituent in microalloyed steels[1,2,5,6].In general,the

S.Shanmugam et al./Materials Science and Engineering A437(2006)436–445437

strength of C–Mn steels is enhanced because of replacement of pearlite phase by lath-like bainite using high cooling rate[1].

Majority of the bainitic microstructures obtained in low carbon-microalloyed steels are described as bainitic ferrite or acicular ferrite and granular bainite.Sometimes a mixed microstructure is also obtained in HSLA low carbon steels at high cooling rate[1–8].As the cooling rate increases,the phase transformation temperature,Ar3,decreases and transfor-mation occurs at a rate such that it passes through the two-phase region quickly and dynamic precipitation of ferrite is mini-mum[3].Also,the deformation enhances the transformation driving force and accelerates the continuous cooling transfor-mation,shifting the pearlite transformation curve of CCT dia-gram to the right.When the cooling rate is increased to the extent that the ferrite transformation temperature is surpassed,a fully bainitic microstructure is obtained[8].Accelerated cooling re?nes bainitic microstructure and is attributed to the increase in the driving force for the nucleation rate of ferrite subunits as a consequence of reduction in the bainite transformation start temperature[6,7].A decrease in the bainite transformation start temperature increases the difference in the free energy between austenite and bainite that favoring the formation of bainite.The growth of?ne bainitic ferrite laths is controlled by nucleation rate of subunits,which in turn strongly depends on the driving force(undercooling)for the phase transformation reaction[6,7].

Considering that currently,there is increased demand for high strength structural beams with superior toughness,we are pur-suing microalloying approach to process structural beams with high strength–toughness combination.The present paper is an effort in this direction,where the focus is to study the effect of cooling rate on the mechanical behavior and relate to microstruc-tural features.

2.Steel composition and experimental methods

The chemical composition range of Nb-and V-microalloyed steels is presented in Table1.The composition range meets the ASTM speci?cation A992.The niobium content required to obtain the desired yield strength of55–60ksi was approx-imately one third of the vanadium content.The processing conditions were similar for both Nb-and V-microalloyed steels with no intentional differences.The processing parameters are not described here due to proprietary reasons.A representative beam size is W24×103;the designated size means that the nominal depth of beam is24in.,when the beam is lying in the Table1

Chemical composition range of Nb-and V-microalloyed steels

Elements Nb-microalloyed steel(wt.%)V-microalloyed steel(wt.%) C0.030–0.1000.030–0.100

Mn0.500–1.5000.500–1.500

V0.0010.020–0.050

Nb0.020–0.0500.001

Si0.15–0.250.15–0.25

P0.010–0.0200.010–0.020h-position with the web horizontal,the width is close to24in. The103refers to nominal weight in lbs/ft.

Mean grain size was determined by the linear intercept method.Intercept lengths were determined and then converted to nominal grain size using standard tables.Tensile tests were done according to ASTM E8and ASTM A370speci?cations and Charpy v-notch impact test was carried out according to ASTM E23and ASTM A673standards.

Small coupons were cut from the beams and mounted for metallographic examination.Standard grinding and polishing techniques were employed,and specimens were etched with 2%nital.Light microscopy and scanning electron microscopy (JEOL6300F)imaging techniques were used to obtain low mag-ni?cation images that revealed the overall microstructure.The amount of different microstructural constituents was estimated with conventional point-counting techniques in association with a square point grid as described below.The metallographic mea-surements were made on at least20?elds-of-view in order to obtain representative data for stereological analysis.

Stereological analysis was adopted as one of the approach to understand the underlying differences in toughness of steels, since our recent work indicated that small differences in tough-ness of steels with similar yield strength can be explained in terms of stereological parameters,notably,contiguity ratio[9]. The parameters determined for stereological analysis were vol-ume fraction,mean intercept length,and contiguity ratio.

The volume fraction of ferrite V V

?

was calculated using the systematic point count method.Ferrite grain size and pearlite colony size was estimated in terms of mean intercept length (ˉL?)determined by the following expression[9]:

ˉL?=V V?×L T

N?(1)

where V V

?

is the volume fraction of ferrite phase,N?the number of ferrite grains intercepted by the test lines and L T is the line length of the test lines.

The ferrite contiguity ratio C?was calculated by Eq.(2)[9]:

C?=2(S V)?–?

(S V)?–P+2(S V)?–?

=2(P L)?–?

(P L)?–P+2(P L)?–?

(2)

where(P L)?–?is the number of point intersections per unit length of the test line with ferrite–ferrite,(P L)?–P the num-ber of point intersections per unit length of the test line with ferrite–pearlite boundaries,(S V)?–?the surface area per unit vol-ume of ferrite–ferrite boundaries and(S V)?–P is the surface area per unit volume of ferrite–pearlite boundaries.

Contiguity is de?ned as the fraction of the total interface area of phase that is shared by grains of the same phase.Thus, ferrite contiguity ratio is indicative of the amount of ferrite that is continuous,i.e.,it tells how much ferrite is adjacent to the ferrite. It is calculated by?nding the ratio of number of ferrite–ferrite grain boundaries to the total number of grain boundaries in the microstructure(Eq.(2)).From Eq.(2),contiguity ratio will vary from0to1.

Transmission electron microscopy was carried out on thin

438S.Shanmugam et al./Materials Science and Engineering A 437(2006)436–445

to ～100?m in thickness.Three-millimeter discs were punched from the wafers and electropolished using a solution of 10%perchloric acid in acetic acid electrolyte.Foils were examined with a JEOL FEG TEM/STEM operated at 200kV .3.Results and discussion 3.1.Tensile and impact behavior

Tensile properties of Nb-and V-microalloyed steels are listed in Table 2for conventionally/normally cooled beams.Both the steels exhibited similar yield strength,tensile strength,and per-cent elongation.Almost similar values were obtained at inter-mediate and high cooling rates.However,there was variation in toughness of the two steels as schematically depicted in Fig.1.

Table 2

Representative room temperature tensile properties of Nb-and V-microalloyed steels Properties

Nb-microalloyed steel V-microalloyed steel Yield strength (ksi)57–6058–61Tensile strength (ksi)72–7475–76%Elongation

23–26

23–25

Fig.1shows the variation in impact toughness of Nb-and V-microalloyed steels as a function of cooling rate.It may be noted that both the steels generally experienced improvement in toughness with increase in cooling rate.However,the toughness improvement appeared to be greater for the Nb-microalloyed steel as compared to the V-microalloyed steel.3.2.Microstructures of Nb-and V-microalloyed steels Representative scanning electron micrographs of Nb-and V-microalloyed steels are presented in Figs.2and 3.The low-and high-magni?cation micrographs of Nb-microalloyed steels processed at conventional (low)and high cooling rates are presented in Fig.2a,b and c,d,respectively.Similarly,the micrographs of V-microalloyed steels processed at conventional and high cooling rates are presented in Fig.3a,b and c,d,respectively.The primary microstructural constituents of Nb-and V-microalloyed steels processed at conventional and high cooling rates were polygonal ferrite,pearlite,and degenerated pearlite.It may,however,be noted that the fraction of degen-erated pearlite was high for the steels subjected to relatively high cooling rate (Fig.1b and Table 3),and at a given cooling rate,the Nb-microalloyed steel contained signi?cantly higher amount of degenerated pearlite (Fig.2c and d)as compared to the V-microalloyed steels (Fig.3c and d).The average fer-rite grain size of both the steels processed at conventional and high cooling rates was similar (～26–29?m).The quantitative metallographic data for Nb-and V-microalloyed steels are sum-marized in Table 3.

The microstructures of Nb-and V-microalloyed steels pro-cessed at conventional (normal)cooling rate are presented in Figs.4and 5.The general microstructure and the dislocation substructure in ferrite of Nb-microalloyed steels are presented in Fig.4a and b,respectively.The representative low magni?cation TEM micrographs show large polygonal ferrite grains with high dislocation density.There were some grains that were virtually free of dislocations.Fig.5a and b shows the general microstruc-ture and dislocation density in ferrite of V-microalloyed steels.Representative bright ?eld TEM micrographs of Nb-and V-microalloyed steels subjected to relatively high cooling rate are presented in Figs.6and 7.Two types of ferrite morpholo-gies (polygonal ferrite and lath-type ferrite)were observed in both the steels.The microstructure of Nb-microalloyed steels showing regions of polygonal ferrite,lath-type ferrite grains and degenerated pearlite are presented in Fig.6.Similarly,the microstructures of V-microalloyed steels that contain polygonal ferrite,lath-type ferrite grain structure and degenerated pearlite are presented in Fig.7.At higher cooling rates,it is anticipated that austenite transforms to ?ne ferrite crystals in the interme-diate temperature range as compared to the conventional ferrite structure.In Figs.6and 7,the ferrite grains in groups of parallel laths are termed as acicular ferrite or bainitic ferrite [10].3.2.1.Degenerate pearlite

Degenerated pearlite is formed by nucleation of cementite

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439

Fig.2.Representative low-and high-magni?cation scanning electron micrographs of Nb-microalloyed steel processed at(a and b)conventional and(c and d)high cooling rates.The micrographs(b)and(d)show degenerated pearlite.

temperature between normal pearlite and upper bainite[11].

A schematic diagram illustrating the formation mechanism of degenerated pearlite is presented in Fig.8.Since the advanc-ing ferrite/austenite is enriched with carbon by partitioning process,it is believed that cementite nucleation takes place at ferrite/austenite interface boundary.Similar to lamellar pearlite, degenerated pearlite is also formed by diffusion process and con-sidering its morphology,the difference is attributed to the insuf-?cient carbon diffusion to develop continuous lamellae[12]. It is reported that the interface between ferrite and cementite in degenerated pearlite is wider than the conventional pearlite, thus the ferrite grain boundary area of the controlled-rolled steels that contains degenerated pearlite is higher as compared to the conventionally processed steel[13].A typical bright?eld TEM micrograph of degenerated pearlite formed in Nb-microalloyed steel is presented in Fig.9a and the corresponding

selected

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Table3

Microstructural features of Nb-and V-microalloyed steels

Properties Nb-microalloyed steels V-microalloyed steels

Conventional cooling rate Higher cooling rate Conventional cooling rate Higher cooling rate Ferrite

Area fraction(%)82.4±3.182.8±0.0283±2.582.8±3.8 Mean intercept length(ˉL?)(?m)27±1.527±2.828.9±325.9±1 Contiguity ratio(C?)0.740.760.770.74

Pearlite

Area fraction(%)9.6±3.1 1.2±1.917±2.38.2±2.9 Mean intercept length(ˉL?)(?m)10.3±0.710.2±0.78.4±0.98.9±1 Degenerated pearlite

Area fraction(%)8±0.916±2–9±

1

Fig.4.Bright?eld TEM micrographs of Nb-microalloyed steels processed at conventional(or normal)cooling rate showing(a)polygonal ferrite structure and(b) dislocation substructure in ferrite.

area diffraction(SAD)pattern is shown in Fig.9b.The SAD

pattern analysis suggested that the cementite platelets exhibit

[112]?//[122]Fe

3C orientation relationship with ferrite matrix,

which is close to‘Pitsch’orientation relationship that is com-monly observed in lamellar pearlite.Degenerated pearlite is a microstructural constituent believed to promote toughness in steel[13].3.3.Precipitation in Nb-and V-microalloyed steels

Fig.10a and b shows grain boundary precipitation and pre-cipitation on dislocations in ferrite region of Nb-microalloyed steels,while Fig.11a shows precipitation in ferrite matrix together with the corresponding selected area diffraction(SAD) pattern in Fig.11b.The SAD pattern analysis indicated

that

S.Shanmugam et al./Materials Science and Engineering A437(2006)436–445441

Fig.6.Bright?eld transmission electron micrographs of Nb-microalloyed steels processed at relatively high cooling rate showing(a)polygonal ferrite structure(b) lath-type(acicular)ferrite structure and(c)degenerated pearlite.

Fig.7.Bright?eld transmission electron micrographs of V-microalloyed steels processed at relatively high cooling rate showing(a)polygonal ferrite structure,(b) lath-type(acicular)ferrite structure and(c)degenerated pearlite.

Fig.8.Schematic diagram illustrating the formation mechanism of degenerated pearlite.

Table4

Precipitate characteristics of Nb-and V-microalloyed steels

Properties Nb-microalloyed steels V-microalloyed steels

Conventional cooling rate Higher cooling rate Conventional cooling rate Higher cooling rate Mean particle size(d)(nm) 5.25±3.57.5±4.510.3±48±4.3

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Fig.9.Bright?eld TEM micrographs of Nb-microalloyed steels showing(a)degenerated pearlite and(b)SAD pattern analysis for degenerated pearlite shown in (a).

Fig.10.Bright?eld TEM micrographs of Nb-microalloyed steels showing(a)grain boundary precipitation and(b)precipitation on dislocations.

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443

Fig.12.Bright?eld TEM micrographs of V-microalloyed steels showing(a)grain boundary precipitation and(b)precipitation on dislocations.

the?ne precipitates were MC type of cubic niobium carbides and the precipitates exhibited[100]?//[110]NbC Baker–Nutting orientation relationship with the ferrite matrix.Grain boundary precipitation and precipitation on dislocations in ferrite region of V-microalloyed steels was also observed and is presented in Fig.12.Fig.13a shows precipitation in ferrite matrix of V-microalloyed steels and the corresponding selected area diffrac-tion(SAD)pattern is shown in Fig.13b.In a manner similar to Nb-microalloyed steel the SAD pattern analysis indicated that the?ne precipitates were MC type of cubic vanadium carbides and the precipitates exhibited a cube–cube[001]?//[001]VC Baker–Nutting orientation relationship with the ferrite matrix. The characteristics of precipitates in terms of mean particle size,mean inter-particle distance and particle density in ferrite matrix of both Nb-and V-microalloyed steels are summarized in Table4.

The above results suggest that Nb-and V-microalloyed steels experienced strain induced precipitation at grain boundaries,and dislocations,while the?ne precipitates in ferrite formed during cooling.The precipitation of microalloying elements occurs dur-ing various stages of thermomechanical processing of steels.At soaking temperatures,the microalloying elements,Nb and V,are taken into solution depending on the limitation imposed by the solubility product.For carbide and nitride forming elements,the solubility in austenite at any given temperature depends on C and N content of the steel.When the temperature is lowered during cooling,supersaturation of these solute elements increases and precipitation begins at favorable kinetic conditions.Deforma-tion of austenite introduces large amount of lattice defects such as dislocations and vacancies that assist the diffusional process that control the precipitation kinetics.As a result,strain induced precipitation occurs at the prior austenite grain boundaries or defects.In summary,the Nb-and V-microalloyed steels exhib-ited similar precipitation behavior in ferrite and the size range was from～5to10nm(Table4).It is reported that the effec-tive size range for precipitation hardening is～5–20nm[9,14]. These?ne precipitates exhibited Baker–Nutting orientation rela-tionship(Figs.11b and13b)with the ferrite matrix of Nb-

and

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436–445

Fig.14.Schematic illustration of deformation of cementite in lamellar pearlite and degenerated pearlite.

V-microalloyed steels,con?rming that the precipitation occurred in ferrite.

3.4.Toughness behavior of Nb-and V-microalloyed steels

In the present case the microstructural parameters that are likely to in?uence toughness are ferrite grain size,degenerated pearlite,and acicular ferrite.A ?ner grain size and higher conti-guity ratio is expected to impart higher toughness.From Table 3,it may be noted that the ferrite grain size and ferrite contiguity ratio are similar for both the steels processed at conventional and relatively high cooling rates.However,there are signi?cant differences in the area fraction of degenerated pearlite for the two steels (Fig.1b and Table 3).A schematic diagram illus-trating the nature of cementite plates present in the lamellar pearlite and degenerated pearlite and its effect on plastic defor-mation is presented in Fig.14a and b.It is reported that the cementite thickness in degenerated pearlite is ?ner as compared to the conventional pearlite (Fig.14b),and hence the volume fraction of cementite and ferrite are different in the former as compared to the latter [15].Coarse pearlite deforms inhomo-geneously (Fig.14a)with strain localized in narrow slip bands,where as ?ne degenerated pearlite is expected to experience uni-form strain distribution during deformation.It is also shown that the steels containing degenerated pearlite with speci?c cementite thickness exhibit maximum ductility [15].It is also reported that though cementite is characterized by hard and brittle in nature,it can endure large strain when the cementite platelets are thinner [16].Thus,at present,we believe that the increase in the tough-ness of Nb-and V-microalloyed steels at relatively high cooling rate is a consequence of higher fraction of degenerated pearlite and the presence of lath-type ferrite.While,the higher toughness of Nb-microalloyed steels in relation to V-microalloyed steels at conventional or normal cooling rate is a consequence of higher fraction of degenerated pearlite,with ferrite grain size being similar for the two steels.

The above data is further supported by the recent observations nantly bainitic ferrite with increase in cooling rate is responsi-ble for the strength–toughness combination of Nb-microalloyed steels at high cooling rate.4.Conclusions

1.At conventional cooling rates employed in the mill,the microstructure of Nb-and V-microalloyed steels primarily contained polygonal ferrite–pearlite,while Nb-microalloyed steels contained signi?cant fraction of degenerated pearlite.The Nb-and V-microalloyed steels processed at relatively higher cooling rate compared to conventional or normal cooling rate contained degenerated pearlite and lath-type (acicular)ferrite in addition to the primary ferrite–pearlite constituents.The fraction of degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels.

2.Both Nb-and V-microalloyed steels exhibited similar precip-itation characteristics with precipitation at grain boundaries,dislocations and in the ferrite matrix.Fine-scale (～5–10nm)precipitation occurred in the ferrite matrix of both the steels.The SAD pattern analysis revealed that these ?ne precipitates were MC type niobium and vanadium carbides in the respec-tive steels and obeyed Baker–Nutting orientation relationship with the ferrite matrix.

3.The Nb-and V-microalloyed steels experienced improve-ment in toughness with increase in cooling rates during processing.However,Nb-microalloyed steels seem to exhibit relatively higher toughness than the V-microalloyed steels during processing at conventional and high cooling rates.The increase in toughness of Nb-microalloyed steels is attributed to its higher fraction of degenerated pearlite in the steel.References

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