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Title Modified Austempering Effects on Fe-0.6c-1.5Si-0.8Mn Steel

Author(s) Tomita, Yoshiyuki; Kijima, Futoshi; Mokuo, Takafumi; Morioka, Kojiro

Editor(s)

CitationBulletin of Osaka Prefecture University. Series. A, Engineering and natur

al sciences. 1998, 46(2), p.125-131

Issue Date 1998-03-31

URL http://hdl.handle.net/10466/7777

Rights

Bulletin of Osaka Prefecture University ' 'Series A, VoL 46, No. 2, 1997, pp.125-131 ,

Modified Austempering Effects on Fe-O.6C-1.5Si,-O.8Mn Steel

. Yoshiyuki ToMITA*, Futoshi KIJIMA** Takafumi MoKuo** tt ' and Kojiro MORIOKA."' tt (Received November l8, 1997)

Modified austemper effects on Fe-O.6C-1.5Si-O.8Mn steel have been studied with the aim of

developing a Fe-C-Si-Mn steel for environmental conscious material applications. The microstructure of the modified austempered･ steel consisted of triple phases of carbide-free

upper bainite, light-tempered martensite and retained austenite. This .microstructure was produced by conventional austenitization at 1173. K followed by interrupted quenching at ei-･

ther 533 or 493 K and then austernpering at 673 K for a required time followed by subSe-

qu'ent water quenching and 473 K tempering (designated 533K IQ Aus, 493K IQ Aus, respectively). Compared to the conventional austemper, (Con Aus), the 533K IQ Aus treat- ment dramatically improved the total elongation (TE) as well as its product of the TE with

the ultimate tehsile strength, (a.)(a.XTE); however, this treatment slightly decreased the

Charpy impact energy in the temperature range of 293 to 373 K. Compared to the quenched and ternpered (QT) steel, the 533K IQ Aus steel had significantly developed tensile and Charpy irnpact properties. The resuits are described and discussed.

'

1. Introduction

As environmental requirements become stricter,

there has been increased demand for environmental

conscious (ECO) materials. A series of Fe-C-Si-Mn

steels are of potential interest as possible ECO ma-

terials. Thetefore, the development of various heat-

treating techniques for improving the mechanical

properties is critical to the commercialization of

these steels. In an. attempt to improve the mechani-

cal properties of the steels, significant attention has

been paid to isothermal transformation in the

bainitic temperature region (austemper). High sili-

con levels in the steel encourage the retention of duc-

tile high carbon austenitic regions in preference to

the formation of brittle interlath cementite film

s')u`). However, improved mechanical properties de-

pend on the premise that the retained austenite

should have an optimum stability and sensitivity to

deformation-induced transformation (DIT) and hence,

effectively cause transformation-induced plasticity

(TRIP) of the retained austenite during plastic

deformation5)-"). Therefore, in our laboratory, the

* Department of Metallurgy and Materials Science, College of Engineering.** Graduate Student, Department of Metallurgy and Materials Science, College of Engineering

***Technology Research Institute of Osaka Prefecture

modified austemper has been studied whereby the re-

tained austenite containing in the steel can have an

optimum stability and effectively cause the TRIP

during the plastic deformat'ion, leading to improved

mechanical properties of the steel.

Modified austemper coupled with interrupted

quenching instead of conventional austemper appears

to be a potential method of improving the mechani-

cal properties o,f steels. Application of the modified

austemper to steel containing high silicon levels

should have a significant advantage since martensite

partially appeared prior to bainitic transformation

which resulted not only in increased strength'but

also in a decreased martensite starting temperature

(M, temperature) for the residual austenite'2)-'`). It

is an additional advantage of the partially trans-

formed martensite that it is partitioning the prior

austenite grains of the matrix. Therefore, the modi-

fied austemper may result not only in increased sta-

bility of the retained austenite but also in a

refinement of the bainitic matrix, leading to in-

creased ductility at higher strength levels.

In the present work, modified austempering effects

on Fe-O.6C-1.5Si-O.8Mn steel have been studied with

the aim of developing a Fe-C-Si-Mn steel for ECO

material applications.

'

'

126 Yoshiyuki ToMITA, Futoshi KIJIMA

' t/ 2. Experimental Procedure

The steel used was Fe-O.60%C-1.53%Si-O.78%Mn-

O.O05%S-O.024%P, which had been electric furnace

air-melted and vacuum degassed. The Ms tempera-

ture was 543 K. The steel was received as 10 mm

thick,hot-rolled plate stock. Test steels were ma-

chined from the plates. Each was fully annealed at

1173 K.

Schematic' illustrations of the heat treatment

schedules in this study are shown in Fig. 1. All the

test steels were austenitized in a tube furnace. The

interrupted quenching and austemper were performed

in neutral salt baths that had a thermal capacity

sufficient to avoid any appreciable temperature

change during the operation. The tempering was

done in an oil or neutral bath.

Tensile properties were determined with an Instron

machine at ambient temperature (293 K) at a con-

stant strain rate of 6.70×10-`ls. Smooth tensile

specimens with a gage length of 12.5 mm and a gage

section of 2.0 mmX4.0 mm and notch tensile speci-

mens with 2 mm V notches in both the shorter sides

and a net section of 2.0 mmX4.0 mm under the

notches were used.

The microstructure was categorized using optical

microscopy, scanning electron microscopy (SEM) and

X-ray diffraction (XRD). The volume fraction of

tempered martensite that appeared from the inter-

1173K(900s) 1173K(900s)

673K(10s)

(conAus) W'Q W'QkQrl) W'C'1173K 900s)

6i,il,Ii/('E/X3i,isi;.30ZS' 473K(72Ks)..

(533K IQAus & 493K IQAus)

Fig. 1 Schematics mustrations of heat treatment schedules.

Takafumi MoKuo and Kojiro MoRIoKA

' rupted quenching treatment was determined by point

countingi5). Errors in the point counting data were

± 2.5%. The retained austenite content was meas- ur'ed by XRD using Miller's technique'6) of rotating

and tilting the sample surface about an incident

beam of MoKa (using a Zr filter). The combination

of peaks chosen for the analysis included (200) a,

(211) a, (200) 7, (220) 7 and (311) 7. The change in

the retained austenite content during plastic defor-

mation was measured using tensile specimens with a

gauge length of 12.5 mm and a gauge section of 2.e

mmX10.0 mm. The change in the retained austenite

conte'nt was evaluated by the retained austenite con-

tent ratio, (RACR%). This ratio is the retained

austenite content of the gauge length to that of the

head of the tensile specimens after uniaxial plastic

deformation.

The carbon content in the retained austenite was

calculated from the following equation based on the

lattice parameter a (nm) determined from (311) 7

by XRDi7) where . C(mass%)= (10a-3.555) 10.044 (1)

3. Results and discussion

3.1 Microstructural analysis and characterization

Figures 2 and 3 show micrographs of various heat-

treated steels. TEM observations revealed that auste-

mper at 673 K produced a carbide-free upper bainite

W.Q. : Water quench

W.C. : Water cool

A.C. : Air cool

823K(7.2Ks)

ModCfied Austempering EIIZfects

whose individual ferrite was separated by a "thin

film" of austenite CFig. 2). SEM observations re-

vealed that the modified austempered steels (533K IQ

Aus and 493K IQ Aus)consisted of triple phases of

bainite, retained austenite and tempered martensite

which appeared in acicular form and partitioned the

prior austenite grains (Figs. 3(b) and (c)), while

the conventionally austemperd (Con Aus) steel pro-

duced a mixed structure of carbide-free upper bainite

and retained austenite (Fig. 3(a)). Table 1 shows

the microconstituents of the various heat-treated

steels which were determined by SEM and XRD.

533K IQ Aus produced triple phases of bainite, 26

vol.% retained austenite and 10 vol.% tempered

martensite and 493K IQ Aus, the mixed structure of

bainite, 22 vol.% retained austenite and 20 vol.%

tempered martensite. The Con Aus steel had the

mixed structure of bainite and 32 vol.% retained

austemte.

on Fe-O.6C-1.5Si-O.8Mn Steel 127

3. 2 Mechanical properties

Table 1 shows the tensile properties of the various

heat-treated steels and Fig. 4 shows the relationship

among the ultimate tensile strength (a.), total

elongation (TE) and formability (a.XTE). Com-

".pt-

40

35

30

25

20

15

10

5

Fig

... -.. 30000

i.. 25000:-.

'･. 20000 L.

]50oo L...

1oooo -....

x. e.

A

"' i

oA.e

Con Aus533K IQ Aus493K IQ AusQTo XTE u

'i

500

.4

Table

1000 l500 2000 a u,MPa

Relationship among ultimate tensile stress (u.), total elongation (TE) and

a.×TE

1. Tensile properties of various heat

treated stee]s

Fig. 2 TEM micrograph of Con Aus steel･ 7 u,: o.2%offset and BF indicate retained austenite and TE: total carbide-free upper bainitic ferrite.

//1ss11/if1111//ee,ee,,//,.,/metsmp1'kee'ee'.'.ee-pt.ee$,ees,ee/twge･eees

Fig. 3 SEM micrographs of (a) Con Aus, (b) 533K IQ Aus and (c)

tempered martensite.

Designation ay a. TE a.×rrE

ofsteels (MPa) (MPa) (yo6o) (MPa)

ConAussteel 878 1104 14.3 15787

533KIQAus 904 1195 30.5 36448

493KIQAus 1095 1250 21.5 26875

QT 908 1098 14.8 16250

yield stress;elongation

(c),,die

,st･

of.: ultirriate

ue

tensile

wtee,

stress;

493K

wwk

IQ Aus

paewue"'fieg""i"'

steels. TM shows

･2-OA111

128 Yoshiyuki ToMITA, Futoshi KIJIMA

pared to the Con Aus and QT, 533K IQ Aus signifi-

cantly increased the TE as well as the a. and hence,

dramatically improved a.XTE. Compared to the

Con Aus and QT, 493K IQ Aus increased both the TE

and a.XTE at a highera. Ievel. However, compared

to the 533K IQ Aus, this treatment increaseda, but

decreased the TE and thus decreased the a.XTE

value.

Figure 5 shows the effect of temperature on the

(×104)

E)sop8ex-a.Eitts=o

14

12

10

8

6

4

2

o

-o- Con Aus-d -533IQ Aus-A -493IQ Aus'.--QT

o

o

/- / o7 /!.'lx

4･i /1

/J.-Af

Fig.

100 150 200 250 300 350 Temperture,K

5 Relationship between Charpy energy and Temperature for heat treated steels.

400

lmpact

varlous

Takafumi MoKuo and Kojiro MoRloKA

Charpy impact energy for various heaVtreated

steels. Compared to the Con Aus, the 533K IQ Aus

slightly decreased the Charpy impact energy at and

above 293 K. However, compared to the QT, this

treatment improved Charpy impact energy at the

same temperature levels. Compare to the 533K IQ

Aus, the 493K IQ Aus significantly decreased the

Charpy impact energy. Fig. 6 shows the load versus

displacement curves for the Charpy impact test

(tested at 373 K) of the various heat treated steels.

As can be seen, both the crack initiation (before

arrow in Fig. 6) and propagation energies (after

arrow in Fig. 6) of the Con Aus steel were greater

than those of the 533K IQ Aus steel. Particularly

noteworthy is the fact that the crack propagation

energy of the Con Aus steel was much greater than

that of the 533K IQ Aus steels.

3. 3 True stress and work-hardening rate versus

true strain analysis

In order to clarify the significantly improved TE

of the 533KIQ Aus steel, the true stress, a, and the

ztfoo-

IOOOO

8000

6000

4000

2000

o

-2000

-F

-;

'CVE'

------t-

--:-----

l-

･r

.:

-)

:-

:.

o 4.0 8.0 J.2 l.8 Displacement(mm)

Con Aus

2.0

.

.-

.

.-

' -Li-- tl--------s---------

..-ll-------

.

.

.

.t--'

. ------

-...

.I.J

. .---- t------i-----------t- ------l---..

L-..

' ------

.

.

.

.

.

.. ---- if-----------.----.---------- .--

. ------- .

' .

:CVE -.

.-

t--- Ji----r-T------J-- .Jl---t-- .--

-

. ------

.

.'-

'

.

.-.l- --- -ny .

.

.

.'.

+'t..''

. ------

zV-No-

Ztfco

o-

lOOOO

80(ro

60or)

4000

2000

o

-20oo o

Fig.

4.0 8.0 1.2 1.8 2.0 Displacement(mm)

493K IQ Aus

6 Load versus displacernent curves (tested at 373 K). CVE shows

maximum load.

IOooO

80oo

6000

40oo

2000

o

-2000 o

--------------s-----.-

... I..ovE-i,--N' "'

-:-

-l ;-

10ooO

8000

6000

40co

20oo

o

-2000 o

4.0 8.0 1.2 1,8 Displacement(mm)

533K IQ Aus

2.0

ztfas

o-

-- ---it---t-----l]--------{-----p---l-

- -- '' l'`''''''-i--･-･---f-･-･-･-･-:-･

.･･---:･･･-･･{[}VE ･･･÷-･･-･-"-

;---- :-+----!--;-

forCharpy

varlous v-notch

4.0 8.0 1.2 1.8 2.0 Displacement(mm)

QT

heat treated steels in Charpy impact

impact energy. Arrows indicate CVEtest

at

'

ModijCied Austempering ELI[fects

'

work-hardening 'rate, da/dE, of the 533K IQ and

Con Aus steels during tensile deformation were plot-

ted as a function of the true strain,E (Fig. 7). The

d'a/dE value in 'the a-e curve was evaluated by

means of a graphic solution using a half-silvered

mirror. Compared with the Con Aus steel, the 533K

IQ Aus steel had a higher work-hardening rate and a

smaller portion of the decrease in the work-

hardening rate. It is well known that stable plastic

flow will continue until the true stress exceeds the

rate at which the material undergoes a work-

hardening conditioni8). Bgcause the work-hardening

rate is higher and the proporti6n of decrease in the

work hardening is smaller, the condition at which it

begins, that is, da/d aE=a, is shifted to the

higher strain side and, consequently, the ductility in-

creases'9). The results in Fig. 7 strongly suggest that

the improvement in the TE of the 533K IQ Aus steel

is due to TRIP that effectively occurred during plas-

'tic deformation.

20

15

IO

5

o

7

on Fe-O.6C-1.5Si-O,8Mn Steel

£ o..o

.O.Vs.

bb v$oy ts-- -m aOo== ･-H=-o R s i

Fig.

v

I111･l

t

s

s

---･--

da'ld E

`-aH

Con Aus

8 1oo ec- U < co ot O- '--co 80 : g e 7o 8 .g g oo･ th : v.. so -:.

M 4o

Fig. 8

Aus steel

whereas,

at very

hence the

firmed

ing in

the TRIP

3. 5

pered

bon

leading

carbon

Aus and

usmgcan be

tained

changed

thermal

zero

9 shows

Table 2.

129

533K IQ Aus･-･-/-

--

.

ssx

Ns

qNss

o

N

.:

O O.OS O.1 'O.15 O.2 O.25 O.3

True strain, E

Relationship among true stress (a), work hardening rate (daldE) and true

strain (e) for Con Aus and 533K IQ Aus steels.

'3. 4 Deformation induced transformati6n of retained

austenite during plastic deformation

To develop TRIP, retained austenite must exhibit

an optimum mechanical stability and effectively

transform into martensite during plastic deforma-

tion. If the retained austenite undergoes DIT at very

low strains or is very resistance to DIT, the TRIP

effect is minimal. Therefore, DIT of the retained

austenite containing in the steels was investigated

using XRD. Fig. 8 shows the RACR plottgd as a

function of Efor both the Con Aus and 533K IQ Aus

steels. As can be seen, the RACR of the 533K IQ

ss- .

N.- s

- Con Aus .- -O- - 533K IQ Aus

NO-------'O-. b

t. 0 O.05 O.1 'O.15 02 ' ' True strain,E

Relationship between change in re- tained austenite content ratio (RACR)

and true strain (e) for Con Aus and

533K IQ Aus steels. tt t , gradually decreased with an increase in E,

the RACR of the Con Aus steel decreased

low strain in the plastic deformation and

TRIP effect is minimal. These results con-

the fact that the retained austenite contain-

the 533K IQ Aus steel can effectively cause

effect.

' Contribution of tempered martensite to TRIP

Howeverj a question arises about how the tem-

martensite contributes to TRIP. A previous

investigation suggested that an increase in the car-

content pf the retained austenite enhanced the

mechanical stability of the retained austenite, these

to increased elongation5)'i')'20). The average

content in the retained austenite in the Con

533K IQ Aus steels was then.determined

XRD. These results are shown in Table 2. As

seen, the average carbon content in the reL

austenite of the 533K IQ Aus steel was not

eompared with the Con Aus steel. So, the

stability of the retained'austenite at sub-

temperature was examined for both steels. Fig.

the RACR for the Con Aus and 533K IQ

Lattice pararneters and average ca'rbon con-

tent in retained' austenite of Con Aus and 533K IQ Aus steels(lattice parameters meas-

ured by X-ray diffraction, average carbon content calculated by C(mass%)= (10a-3.555)

jo.044). . 'Designationof

steels

Latticeparameter

a,nm

Averagecarbbncontent

C,mass%

ConAus533KIAus

O.3626

O.3627

1.61

1.63

130 Yoshiyuki ToMITA, Futoshi K!JIMA

*5"

E

1{M)

&}

oo

ut)

20

o

Fig.

--e-- Con Aus

+ 533K iQ Aus

e-------r---.--''

f'

-

'"i

'

t

''

y'tt

'

'

'

''

50 1oo 150 2oo .250 orX) Temperature ,K

9 Relationship between RACR and tem- perature for Con Aus and 533K IQ Aus

steels. '

Aus steels plotted as a function of temperature.

Compared to 'that of the Con Aus steel, the decom-

position temperature of the retained austenite in the

533K IQ Aus steel was lower by about 20 K. The

RACR of the 533K IQ Aus steel was smaller than

that of the Con Aus steel when compared at similar

temperatures. This is attributed to the folloWing

facts: (1) dislocations in the residuai austenite are

anchored by interstitial atoms (i.e., carbon and ni-

trogen) that are coagulated on the dislocations dur-

ing interrupted quenching(Cottrell effect), which

suppresses nucleations and the growth of martensite2');

and (2) the residual austenite is strained and refined

through'the appearance of the martensite. Based

on these results, it was concluded that the contribu-

tion of the tempered martensite to the TRIP results

from the increased mechanical stability of the re

tained austenite by decreasing the Ms temperature.

However, a light-tempering treatment helps to relax

the stress introduced by the austempering treatment,

which increases the. mechanical stability of the re-

tained austenite.

Despite the above results and argument, the

Charpy impact energy of the 533K IQ Aus steel de-

creased somewhat compared to that of the Con Aus

steel. This could.be .due to the fact that the TRIP

effect is minimi･zed ,when the loading speed increased

and then fracture' occurred,by local shear stress

under the sharp notch22); (2) thus, the greater the re

tained austenite, the higher is the crack propagation

energy of the steel because crack blunting and arrest-

ing effects inereased with the retained austenite.

Takafumi MoKuo and Kojiro MoRIoKA

4. Conclusions

1 . A modified austemper (533K IQ Aus), which ih-

volves conventional austenitization at 1173 K fol-

lowed by interrupted quenching at 533 K,and then

austempering at 673 K, subsequent water quenching

and 473 K tempering, produced a triple phase of car-

bide free upper bainite, 29 vol.%. retained austenite

and 10 vol. % ferrite.'

2. Compared to conventional austemper (Con Aus)

and quench and temper (QT), the 533K IQ Aus dra-

matically increased total elongation (TE) and its

product with the ultimate tensile stress (a.) of the

steel (o.XTE); however, this treatment slightly de-

creased the Charpy impact energy in the temperature

range of 293 to 373 K.

3. Compared to the quenched and tempered (QT)

steel, the 533K IQ Aus steel significantly developed

the tensile and Charpy impact properties.

4. The beneficial effect on the tensile properties of

the 533K IQ Aus steel is due to the fact that TRIP

effectively occurs during plastic deformation.

5. The contribution to the TRIP of the tempered

martensite results from the increased mechanical

stability of the retained austenite by decreasing the

Ms temperature.

References

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14) K. R. Kinsman and J. C. Shyne, Actd Metall., 15,

1527(1967). '15) C. A. Clark and P. M. Munro, J. Iron Steel Inst.,

ModCfied Austenzpering Elirects on Fe-O.6C-1.5Si-O.81Lttn Steel 131

200, 395(1962).

16) R. L. Miller,, Trans. ASM, 61, 592(1968).

17) N. Ridley, H. Stuart and L Zwell, Trans. AIME, 245, 1834(1969).

18) G. E. Dieter, in H. Paxton (ed), Ductility, ASM,

Metals Park, OH (1968)1.19) J. M. Rigsbee and P. J. V. Arend, in A. T. Davenport (ed), Formable HSLA and Dual-Phase Steel, TMS-AIME, Warrendale, PA (1979)56.20) Y. Tomita, J. Mater. Sci., 29, 2605(1994).

21) M. Cohen, Trans. ASM, 41, 35(1949).

22) I. Tamura, Metals Sci, 16, 245(1982).

'