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Microstructure Evolution in Fine-Grained Microalloyed Steels

K.R. Lotteya and M. Militzerb

The Centre for Metallurgical Process Engineering

The University of British Columbia

Vancouver, BC Canada V6T [email protected], [email protected]

Keywords: Microstructure, Fine-grained steel, Continuous cooling, Ferrite, Transformation,Modeling Abstract. There is an increasing emphasis to develop novel hot-rolled high strength steels with

fine and ultra fine grain sizes for structural and other applications. Traditionally the concept of

microalloying has been employed to refine microstructures thereby obtaining increased strength

levels. For example, employing an alloying strategy with Nb, Ti and Mo is promising to attain

yield strength levels of 700MPa and beyond. In the present study, the transformation behaviour is

investigated for a HSLA steel containing 0.05wt%C-1.65wt%Mn-0.20wt%Mo-0.07wt%Nb-

0.02wt%Ti. The ferrite formation from work-hardened austenite has been studied for simulated

run-out table cooling conditions employing a Gleeble 3500 thermomechanical simulator equipped

with a dilatometer. The effects of cooling rate and initial austenite microstructure, i.e. austenite

grain size and degree of work hardening, on the austenite decomposition kinetics and resulting

ferrite grain size have been quantified. Based on the experimental results, a phenomenological

transformation and ferrite grain size model is proposed for run-out table cooling conditions. The

transformation model includes submodels for transformation start and ferrite growth. The latter is

described using a Johnson-Mehl-Avrami-Kolmogorov approach. The degree of work hardening isincorporated by introducing an effective austenite grain size as a function of the strain applied

under no-recrystallization condition. The ferrite grain size can be predicted as a function of the

transformation start temperature. Increasing both cooling rate and amount of work hardening can

optimize ferrite grain refinement. In the present steel, ferrite grain sizes of as low as 2µm have

been obtained in this way. The results observed for the present steel are compared to the

transformation behaviour in previously studied Nb-Ti HSLA steels of similar strength levels.

Introduction

There is an increasing demand to develop novel hot-rolled high strength steels with superior

properties such as improved strength and toughness which has resulted in numerous investigationsof steel chemistry and thermomechanical processing. For example, the pipeline industry requires

linepipe grades with increased strength levels, i.e. X100 and X120 [1]. One approach in this regard

has been to develop novel microalloyed steels with refined microstructures that lead to increased

strength levels. In a hot mill the final microstructure and, thus, the properties of the hot-rolled steel

are a function of the austenite decomposition and precipitation that occur during cooling and

coiling. Accelerated cooling on the run-out table in combination with finish rolling under no-

recrystallization conditions is a key processing step for developing the desired resulting fine and

ultra fine grained ferrite microstructure [2].

To design and control novel processing routes, microstructural engineering has gained

significant attention in recent years with the development of predictive tools that quantitatively link

the process parameters with the final properties of hot-rolled steel [3][4]. Such process models

Materials Science Forum Vols. 500-501 (2005) pp. 347-354online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 64.76.110.6-26/02/07,18:53:25)

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currently exist for plain carbon and a number of high strength low alloy (HSLA) steels of up to

550MPa minimum yield strength [4].

The present paper extends previous modeling work for the austenite-to-ferrite transformation to

a microalloyed low-carbon steel with a minimum yield strength of 620MPa. The required model

parameters are obtained from experimental transformation studies for cooling conditions similar to

those found on the run-out table of a hot-strip mill.

Experimental

Methods. The steel investigated was supplied by IPSCO Inc as hot-rolled transfer bar material.

The chemical composition of the steel is given in Table 1. The A e3 temperature of this steel

chemistry is 821oC as obtained by Thermo-Calc software version N utilizing the Fe2000 database.

Table 1 - Steel Composition [wt%]

C Mn Al Si Cu Mo Ni Nb Ti S P

0.05 1.65 0.027 0.025 0.29 0.196 0.16 0.071 0.021 0.004 0.01

In order to establish the parameters required for the overall transformation model, laboratory

experiments were carried out on a Gleeble 3500 thermomechanical simulator equipped with a

dilatometer. Details of the test procedures are described in a previous paper [5]. First, reheat tests

were performed in order to establish suitable austenitization conditions for subsequent

transformation tests. The selected austenitization conditions and the resulting austenite grain sizes,

d γ , are summarized in Table 2 where d γ is reported as the equivalent volumetric grain size which is

required for modeling purposes. To obtain the equivalent volumetric grain size the measured

equivalent area diameter was multiplied by 1.2 [6].

Table 2 - Heat Treatment Schedules* and Austenite Grain Sizes

Holding Temperature [oC] Holding Time [s] d γ [µm]

950 120 14

1050 120 32

1150 120 53

*A heating rate of 5°C/s to reach the holding temperature was employed in all schedules.

Double-hit axisymmetric compression tests were conducted in another series of tests in order to

establish conditions that introduce retained strain in the initial austenite microstructure, i.e.

pancaked austenite, which reflect the microstructure at the exit of the finishing mill. In order toobtain pancaked austenite the compression tests were carried out at a deformation temperature of

850oC where for a holding time of 15s and the largest prestain of 0.5, the softening remained below

20%. A softening of 20% or lower is assumed to be indicative that just recovery but no

recrystallization has occurred [7].

Austenite decomposition was investigated during continuous cooling with and without prior

deformation of austenite. Continuous cooling transformation (CCT) tests were conducted to

dilatometrically quantify the austenite decomposition kinetics as a function of initial austenite

microstructure and cooling rate, ϕ , which was calculated at the Ae3 temperature. Microstructures

were revealed using standard metallographic procedures.

Results. Figure 1 gives examples of microstructures obtained in the CCT tests with d γ =14µm.

Comparing Figure 1a) with 1c) and 1b) with 1d), respectively, shows that a refinement in themicrostructure was observed as a result of accelerated cooling. In case of tests without

Microalloying for New Steel Processes and Applications348

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deformation, see Figures 1a) and 1c), a transition from polygonal ferrite-pearlite to non-polygonal

transformation products, such as bainite, was observed with accelerated cooling. The effect of

retained strain on the final microstructures is to promote the formation of polygonal ferrite and to

refine the ferrite microstructure, see e.g. Figures 1c) and 1d). The effects of pancaking austenite in

combination with accelerated cooling on the final microstructure are clearly seen by comparing

Figures 1a) and 1d) where a significant refinement in the ferrite microstructure is evident. Themeasured polygonal ferrite grain sizes at various cooling rates and strain levels are shown in Figure

2. Significant ferrite grain refinement of approximately 2µm was achieved with the application of

accelerated cooling and deformation, i.e. the finest ferrite grain size of 2.1µm was attained with the

largest strain of 0.5 and accelerated cooling at 88oC/s while a ferrite grain size of 4µm was

measured from slow cooling at 1oC/s with no retained strain. Similar trends were also observed for

the larger austenite grain size of 32 and 53µm. These tendencies are similar to those observed in

previously studied HSLA steels [8].

While both increase of cooling rate and increase of retained strain leads to grain refinement in

the final microstructure their effect on transformation temperatures are opposite. Increasing the

cooling rate decreases transformation temperatures while increasing the retained strain increases

transformation temperatures. For example, considering the transformation start temperature, T S , in

CCT tests for d γ =53µm, the following was observed: For transformation from work-hardened

austenite (ε =0.5) T S decreases by 65°C when increasing the cooling rate from 1 to 78°C/s whereas

for accelerated cooling conditions (~60°C/s) T S increases by 40°C when the retained strain is

increased from 0 to 0.5.

Figure 1 − Effect of retained strain, ε , and cooling rate on the final microstructure resulting from an

austenite microstructure with d γ =14µm.

εεεε = 0 εεεε = 0.5

a 1oC/s

20 µµµµm

d) 88oC/s

20 µµµµm

c 60oC/s

20 µµµµm

b) 1oC/s

20 µµµµm

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Cooling Rate,oC/s

0 20 40 60 80 100

F e r r i t e G r a i n S i

z e , µ m

1

2

3

4

5ε=0ε=0.25ε=0.5

Figure 2 − Ferrite grain size as a function of cooling rate for an austenite grain size of 14µm andvarious levels of retained strain.

Model

In order to describe the austenite decomposition during continuous cooling a sequential

transformation model is proposed similar to that previously developed for other HSLA steels. This

approach consists of sub-models which predict the transformation start temperature, ferrite growth

kinetics and ferrite grain size. The effect of retained strain, ε , on the austenite decomposition was

incorporated in the model by means of an effective austenite grain size defined as follows [9],

)exp( ε γ −= d d eff (1)

The transformation start temperature, T S , is predicted with a model that combines corner

nucleation of ferrite with early growth, which was originally proposed for plain carbon steels [10].

The model assumes that early growth of corner ferrite nucleated at T N is controlled by carbon

diffusion in austenite. The radius of the growing ferrite grain, R f , can be calculated by [10],

f

o

c

f

Rcc

cc D

dt

dT

dT

dR 1

α γ

γ

−

−= (2)

assuming steady-state growth conditions along the grain boundaries where Dc is the diffusivity of

carbon in austenite, co is the average carbon concentration and cγ and cα are the equilibrium carbon

concentrations in austenite and ferrite, respectively. Orthoequilibrium is assumed in the

transformation start model and the required thermodynamic data are obtained from Thermo-Calc.

Nucleation site saturation at the austenite grain boundaries is achieved when the carbon enrichment

at the austenite grain boundaries reaches a critical level, c*, above which ferrite nucleation is

inhibited. This can be written as follows,

2

*eff

o

o

f

d

cc

cc R

−

−≥

γ

(3)


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and is suggested to coincide with measurable transformation start. Using equations (2) and (3) T S

can be predicted for any cooling path. For the present steel the model parameter T N was estimated

to be 787oC from the experimentally observed transformation start at a cooling rate of 1

oC/s and

d γ =14µm. The second model parameter c* can be represented as follows,

( ) o N cT T c ))0002.0exp(5.25.2(* 7.1−−+= (4)

Model predictions and observed transformation start temperatures for various strain levels and

austenite grain sizes are shown in Figure 3. The experimental T S is defined as the temperature

where 5% fraction transformed is observed. As seen in Figure 3, the proposed model provides an

accurate description of the observed transformation start temperature for the investigated cooling

conditions and initial austenite microstructures.

ϕdeff

2,

oCs

-1µm

2

101 102 103 104 105

U n d e r c o o l i n g ( A

e 3 - T s ) ,

o C

0

50

100

150

200

250

300

14 µm32 µm53 µm

ε 0 0.25 0.5

Model

Figure 3 – Undercooling for transformation start as a function of cooling rate , ϕ , and effective

austenite grain size, d eff .

The subsequent ferrite growth is modeled by employing the Johnson-Mehl-Avrami-Kolmogorov

(JMAK) approach, which is currently one of the most widely used transformation models in run-

out table process models [4], and adopting the additivity rule. In order to model the ferrite growth,

any non-polygonal and secondary transformation products and the associated kinetics were

excluded from the analysis. Thus, the analysis was applied to transformation kinetics that had a

minimum polygonal ferrite fraction of 85%, as determined from microstructural observations. Inthis approach, the normalized polygonal ferrite fraction transformed, X=Y/F eq, is introduced where

Y is the total fraction transformed and F eq is the orthoequilibrium polygonal ferrite fraction at each

temperature increment. In the JMAK model the change of X can be represented with respect to

time as follows,

[ ] nn

n X X nbdt

dX 11

)1ln()1(−

−−−= (5)

where the criterion for additivity is satisfied if the parameter b is a function of temperature and the

Avrami exponent, n, is a constant for any given initial austenite microstructure. The analysis of thepresent experimental data suggests that n=0.85 can be taken independent of the initial austenite

microstructure such that b is a function of temperature and initial austenite microstructure. For run-


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out table cooling conditions, b can be expressed as a function of undercooling below the Ae3

temperature and the initial effective austenite grain size in the format as previously proposed [4],

i.e.,

( )( )meff

Ae

d

bT T bb 231exp +−= (6)

where b1 , b2 and m are constants. For the present steel, m=2.2, b1=0.04K-1

and b2=−29.7 have been

determined. As shown in Figure 4 the proposed JMAK model provides an adequate description of

the ferrite formation during continuous cooling.

Temperature,oC

540 560 580 600 620 640 660 680

F e r r i t e F r a c t i o n

0.0

0.2

0.4

0.6

0.8

1.0

dγ = 14µm; ε = 0

dγ = 14µm; ε = 0.5

Model

dγ = 32µm; ε = 0

Figure 4 − Application of the JMAK model incorporating the effective austenite grain size for

various strain conditions and austenite grain sizes at a cooling rate of 25oC/s.

The ferrite grain size, d α , is essentially determined at the start of transformation assuming

nucleation site saturation and can be expressed as a function of the transformation start

temperature, i.e. [11],

3 / 1)]exp([ S T E BF d −=α (7)

where F is the final ferrite fraction, E is a constant, B depends on the initial austenitemicrostructure and T S is in Kelvin. The model parameter B can be written as a function of retained

strain and austenite grain size as follows,

( ) βε γ −+= exp Dd C B (8)

where C , D and β are model parameters. Quantifying d α as equivalent area diameter in µm,

C =13.4, D =0.056 µm-1

, β =6.3 and E =10000 K-1

have been found for the present steel where F is

approximated to be 0.9 from microstructural measurements. The model accurately describes the

ferrite grain size for the various strain conditions investigated as illustrated in Figure 5.

For sufficiently large values of ε the second term in equation (8) can be neglected and B becomes a constant, i.e. here B=13.4. Within the experimental error of the grain size measurements


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this convergence is attained for the highest strain level employed in the tests, i.e. ε =0.5. This

observation is similar to that made for other microalloyed low-carbon steels with similar yield

strength levels of 550MPa and higher [4][12]. As discussed by Nakata and Militzer [12] the ferrite

grain size obtained from sufficiently work-hardened austenite in these previously studied steels can

be presented as a function of transformation start temperature using equation (7) with steel

independent parameters, i.e. F =0.95, B=22.3 and E =18100. Even though these parameters aredifferent from those obtained for the present steel it is worthwhile to compare the predicted ferrite

grain sizes for both cases. This comparison is given in Figure 6 where the data of the 780MPa steel

studied in [12] are used to illustrate the behaviour of the previously investigated steels. As can be

seen, the predictions are similar in the fine-grained range which is associated with transformation

start temperatures of 600-650°C. However, for higher transformation start temperatures, the ferrite

grain size increases less rapidly in the present Mo-bearing steel than in those previously

investigated which do not contain Mo. Further studies are required to rationalize this finding.

Transformation Start Temperature,oC

620 640 660 680 700 720 740 760

F e r r i t e G r a i n S i z e , µ m

2.0

2.5

3.0

3.5

4.0

4.5

Model

ε 0 0.25 0.5

Figure 5 – Ferrite grain size as a function of transformation start temperature for d γ =32µm.

Transformation Start Temperature,oC

600 650 700 750 800

F e r r i t e G r a i n S i z

e , µ m

1

2

3

4

5 Present steel780 MPa steel [12]

Figure 6 − Ferrite grain sizes obtained from work-hardened austenite (ε =0.5) in the present steel

and a previously studied 780MPa HSLA steel [12]; lines indicate model predictions.


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Conclusions

The effects of cooling rate and initial austenite microstructure on austenite decomposition and

ferrite grain refinement have been quantified for a microalloyed low-carbon steel of 620MPa

minimum yield stress. Ferrite grain refinement from 4 to 2.1µm can be achieved with a

combination of accelerated cooling and deformation of austenite under no-recrystallization

condition, as observed by increasing the cooling rate from 1 of 88oC/s and the retained strain from

0 to 0.5 for d γ =14µm. Similar trends for ferrite refinement were observed for larger austenite grain

sizes. A previously proposed transformation model, with steel specific parameters, has been

applied which provides an accurate description for predicting the transformation start temperature,

ferrite growth and resulting ferrite grain size under run-out table cooling conditions for the

investigated steel. The fine ferrite grain sizes of approximately 2µm are associated with

transformation start temperatures in the range of 600-650°C which is similar to observations made

in other microalloyed steels of comparable strength levels.

Acknowledgments

Financial support received from the Natural Sciences and Engineering Research Council of Canada

(NSERC) is gratefully acknowledged. Materials were supplied by IPSCO Inc.

References

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[2] M. Umemoto, Z.H. Guo and I. Tamura: Mat. Sci. Techn. Vol. 3 (1987), p. 249

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[4] M. Militzer, B. Hawbolt and R. Meadowcroft: Metall. Mater. Trans. Vol. 31A (2000), p. 1247[5] R. Lottey and M. Militzer: in Ultra-fine Structured Steel (Met. Soc. of CIM, Canada, 2004), p.

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[6] A. Giumelli, M. Militzer and E.B. Hawbolt: ISIJ International, Vol. 39 (1999), p. 271

[7] W.P. Sun and E.B. Hawbolt: ISIJ International, Vol. 37 (1997), p.1000

[8] M. Militzer, R. Pandi and E.B. Hawbolt: in Accelerated Cooling/Direct Quenching of Steels

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[9] S. Lacroix, Y. Brechet, M. Veron, D. Quidort, M. Kandel and T. Iung: in Austenite Formation

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