ISIJ International, Vol. 37 (1997). No. 1, pp. 74-79

Eff ect of Manganeseon Aging in LowCarbon Sheet Steels

SamKyu CHANGand Jai HyunKWAKTechnical Research Laboratories. POSCO,I Koedong-Dong.Pohang, 790-600 Korea.

(Received on July 22. l996, accepted in final form on Novemberl2. 1996)

For the investigation of the effect of manganeseon aging in the low carbon steels the manganesecontent

wasvaried from 0.02 to 0.25 wt'/• in the O.02wt'/*C steel. The low manganesecontaining steel reveals loweraging index than the high manganesesteel. The aging index decreases with a decrease of manganesecontent and an increase of coiling temperature, because (1 )the amountof solute manganeseplaying a role

in obstructing the movementof solute carbon into the interface of carbide-ferrite, resulting in the hindranceof the carbide growth, is not sufficient and (2) the coarse carbide formed in the hot bandwith higher coiling

temperature is crushed into small fragments during cold rolling, resulting in the development of numerousmicro-voids at the fragments, which provide the numeroussites for the precipitation of solute carbon.

KEYWORDS:aging index; carbide; precipitation; Snoekpeak; solute.

1. Introduction

In annealing of the low carbon steel the solute carbonis required to precipitate as carbides in order to preventthe steel from aging in use. The carbide precipitated

during annealing appears in two waysof which the oneis to precipitate at the ferrite grain boundary duringquench-aging and the other is to precipitate both inside

and at the ferrite grain boundary during overaging.

These carbides show difference in size depending onthe manganesecontent, and have beenknownto appearmuchfiner in the higher manganesecontaining steel thanthe lower manganesesteel.i)

Manganeseatom in the low carbon steel gives aninfluence in recovery and recrystallization by interacting

with the interstitials such as carbon and nitrogen.

However, Lagerberg2) reported that manganeseatomdoes not interact with carbon atom, and Fast3) reported

from the work of the internal friction of Fe-MnCalloy

that manganesedoes not infiuence in the precipitation

of carbon. Tsunoyama4)also reported that there wasnoinfluence of manganeseon the precipitation of carbonin the 0.2 wto/o Mncontaining steel. In opposition to theseworkers,2~4) Era et al.s) reported that manganesegives

an influence on the carbide precipitation in the 0.5 wtolo

containing steel, and Abe et a!.6) suggested from theelectric resistance measurementof Fe-Mn-Calloy duringaging that the substitutional manganeseatom and theinterstitial carbon form aMnCdipole which is dissolved

during the aging treatment resulting in the precipitation

of carbide. Manganeseatoms play a role in obstructingthe movementof carbon atoms, resulting in re~traint ofcarbide formation. Sofar the argumenton the interaction

betweencarbon and manganesehas not been ended.

2. Experimental Procedure

In the low carbon steel with 0.02wto/o carbon man-ganese content wasvaried from 0.25 to O.02wto/o in or-der to investigate its effect on aging and the behaviourof solute atoms. Thechemical compositions are given in

Table I and boron was added to compareto the othersteels concerning scavenging the solute carbon andnitrogen and spheroidizing the carbide precipitates.

Thevacuummelted ingots were hot-rolled to 200mmthick bars. As seen in Fig. l, these bars were precisely

hot-rolled to 3.2mmthick bands which were then cold-rolled to 0.8mmthick sheets, according to the rolling

schedule. The cold rolled sheets were continuouslyannealed in a given heat treatment cycle in the infrared

image furnace.

The annealed sheets were 7.50/0 Prestrained, aged at

100'C for an hour and tensile-tested in order to measurethe aging index. The internal friction test wasperformedto measure the amountof solute carbon and nitrogenafter chemical polishing to remove the surface residual

stress. During heating from 243 up to 393Kat the rateof I K/min, the internal friction was measured in the

constant amplitude methodwith the frequency of IHzusing the low frequency internal friction tester (ULVACIFM1500M).

Table 1. Chemical composition of steels. (wto/~)

Steel C Mn P S sol Al N B *K

A 0.024 0.25 0.009 0.008 0.046 0.0024 0.240

B 0024 0.lO O.OIO 0.008

C 0,02 1 0,02 0.01 1 0.008

D 0.020 0,ll 0.010 0.007

0.047 0.0028 - 0.0900.043 0.0019 - 0.0070.038 0.0021 0.005 0.090

*K (amount of Mnsolute) = Mn(wt"/o) - (55/32)S(wt"/.)

C 1997 ISIJ 74

ISIJ International. Vol. 37 (1997), No, 1

3. Results

3.1. Carbide Precipitation in Hot BandThe ferrite grain size increases with increasing coiling

temperature, as shownin Fig. 2revealing an exampleofthe ferrite grain size according to coiling temperature in

the hot bands of the steel D.As seen in Fig. 3, carbide precipitates are coarsely

agglomerated with an increase in the amountof man-ganesecontent. The steel Awith 0.25 wto/o Mncoiled at

600'C shows that carbides precipitated like a necklacealong the grain boundary. The carbide clusters inside agrain are seen as a trace of fine pearlite colonies. At the

I'~~r7~ ' -'~~~r _:i:h~~l-~~;;~~:jJh~~' "~~\ ;')'~

* ^,

/'*'(b_)_--'~; \ .~t"':~"s~

(D

:S

(U

(D

~E(D

F

1200ocx lhr

1OOO'C

3pass, 3.2mm(t)

900'C

25-30 •C/s 600650'C x lhr700

Furnace500'C Caoling

aooacx 30s

,.5 eCls

6700c150c/s

_45'c/s__.wr...._j

o L350oc

Cold Rolling 400 C 180seeto 0.8mm(t)

Air cooling

Time

Hot Rol]ingCold

Rol]ing

ContinuousAnnealing

~t•:~!,,/

::i~S~ ~

_'

~**"~ ~*:~

~~*:S~~'*'~*~~ '

)'.~* ~'

Frg. l. Schematic diagram ofcontinuous annealing.

hot and cold rolling andFig. 2. Optical micrographs showing ferritegrainswithcoiling

temperature of (a) 600, (b) 650 and (c) 700'C in hotbands of the steel D,

Fig. 3. Optical micrographs showing carbides in hot bands with various coiling temperatures,

75 C 1997 ISIJ

ISIJ International. Vol. 37 (1997), No. 1

samecoiling temperature the steel Cwith O.02wto/o Mnreveals a similar carbide morphology to that of the steel

A obtained from coiling at 650'C and over. However,at 700'C the steels A, B and C do not show muchdifference in the carbide morphology. In the steel Dcontaining 0.1 wt"loMn and 0.005wto/.B, the carbidemorphologyis hardly affected by coiling temperature but

appears in fine particles regardless of coiling tempera-ture. This is because the BNparticles finely precipitatedin y phase block the diffusion of carbon into the grainboundary.

Figure 4shows the measurementof the meansize ofcarbides in Fig. 3. The steels A, Band Cshowthat thecarbides gradually agglomerate as increasing coiling

temperature. The dependenceof the agglomeration ofcarbides on the coiling temperature is significant in thehigh Mncontaining steel Aanddecreases with a decreasein the Mncontent. The steel D, on the other hand, hasthe meandiameter of I ,

1-1.25 ~mregardless of coiling

temperature.Fromthe above results, the lower manganesecontent

makesit easier for carbides to agglomerate coarsely atthe grain boundary.

~1:r~2

~~o::~oN-d5

o:g~3co

O8=

3.0

2.5

2,0

1.5

1,o

0.5

+ steel AO Steel Be steel Cl Steel D

600 650 700

Hot Coiling Temp.('C )Fig. 4. Influence of coiling temperature on the meancarbide

size in hot bands

3.2. Aging Index and Solute Elements

The carbon content of the test steels is more than0.02wt'/* which exceeds the maximumsolubility of oc

phase. In continuous annealing having a very shortheating time, it is not easy to stabilize the solute carboneffectively and therefore the aging phenomenonis un-avoidable. Figure 5 reveals the aging index as a func-tion of coiling temperature, which was obtained fromannealing at 800'C followed by overaging treatmentat 400 and 350'C. The aging index increases as the

manganesecontent increases and coiling temperaturedecreases. This meansthat as the aging is closely related

to the solute atoms, the manganesecontent gives aninfluence in the solid solubility of carbon in such lowcarbon steels.

Figure 6 illustrates the internal friction test results

which are well in according with the aging index. Theinternal friction (Q~.1.) js relatively high in the manganesecontaining steel Acomparedto the low manganesesteels.

In each steel, Q~*1* decreases with an increase in coiling

temperature, which is the sametendency as the aging in-

dex as seen in Fig. 5. Fromthe Snoekpeakobtained fromthe internal friction test, the individual Snoek peak ofthe solute carbon and nitrogen is calculated from the fol-

lowing equation7); Q~ I =QEi Sech{Hc/R(1/T- l/Tc)} +Q~I Sech{HN/R(1/T- I/TN)} whereRis the gasconstant,Hc and HNare the activation energy of carbon andnitrogen (HC= 19 400cal/mol, HN= 18300cal/mol),8) re-

1,o

~c~

EE:F.~)~~xo1:'

co~,~a)

Steel A

3.6

3.4

3.2

3.0

2.8

2.6

2.4

2.2

+ steel AO Steel Be steel Cl Steel D

o

0.8

0.6

0.4

0.2

o,o

0.7

0,6 Steel B0.5

0.4

0.3

o,2

0.1

0.0

O.6

Steel C0.5

0,4

0,3

0,2

0,1

0.0

Coiling Temp.^ 600 ('C)o 650e 700

600 650 700

Coiling Temp. ('C )Fig. 5. Effect ofcoiling temperature on the aging index after

annealing at 800'C.

o>

ac:o

LL~i

S(D

~;

-20 O 20 40 60 80 1OO 120

Temperature (oc )Fig. 6. Internal friction peaksofspecimensannealed at 800'C

for 30min in the steels A, Band C.

C 1997 ISIJ 76

ISIJ International, Vol,

spectively and Tc and TN are the peak temperature of9,10) re-carbon and nitrogen (Tc=313K, TN=296K),

spectively. Then the Snoek peaks of the solute carbonand nitrogen are illustrated in Table 2and Fig. 7. TheSnoekpeaks of these elements appear higher in the high

manganesecontaining steel than in the low manganesesteel and these are decreased as coiling temperatureincreases in each steel. The steel D reveals no nitrogen

peak because it might be scavengedby boron.

4. Discussion

4.1. Carbide Morphology in Hot BandAs the precipitation site is mostly the ferrite grain

boundary, the grain size of hot band and the density ofthe precipitates pre-existing on the grain boundary are

Steel

A

B

C

o>

acooLLa,co::

Table 2. Internal friction test results

Mncontent(wto/o)

0.8

0.6

0.4

0.2

0,0

0.4

0.2

0,0

Fig. 7.

0.6

0.4

0.2

0.0

0.25

0.lO

0,02

Steel A

Steel C

Steel D

Coiling temp.('C)

C+N

C+N

600650700600650700600650700

\;!c

ili Citll"I11'1Nl'l

~c '

Q~*1*

(x 103)

clf

0.7140.6550.3 l0.6990.636O. 57 l0.5280.5

0.4 17

IF-'

Qcl

(x 103)

0.0760.09

0.043O.0760.0380.0240.0640.048O.031

QNl

(> 103)

ExperimentalCalculated

0.6960.6290.2850.68

0.6390.57

O.4960.4830.4 l

-20 20 40 60 80 1OOOTemperature ('C )

The internal friction curves showing the amountof the

individual solutes C and N calculated from the

experimental Snoekpeaks.

37 (1997). No. 1

important factors in the formation of the carbide mor-phology. In case of low temperature coiling, the solid

solubility of carbon is high and the volume of fine

carbide particles precipitated on the grain boundary is

larger than in high temperature coi]ing because of the

lower diffusion rate. Onthe contrary in high temperaturecoiling, the numberof precipitation sites on the grain

boundary decreases due to large grain size and the lowsolubility of carbon, and the precipitates appear ag-glomerated and coarsened by the help of the sufficient

diffusion of carbon. Therefore, as seen in Figs. 3and 4,

the carbides precipitated in hot band are supposed to

coarsen with an increase in coiling temperature.On the other hand, if fine precipitates exist on the

grain boundary before the formation of carbide, these

can change the shape of carbide. The steel Dcontainingboron showedvery fine carbides and less dependenceofthe carbide morphology on coiling temperature, whichhas been knownas the finely precipitated BNparticles

block the movementof carbon resulting in precipitating

as a carbide agglomerating on the graln boundary.Manganesewasalso found to influence the agglomera-tion of carbides as the lower manganesecontaining steel

showedcarbides to be easily agglomerated at low coiling

temperature. This is because solute manganeseaffects

the precipitation of carbide. AsUshiodaet al. 11)reported,if solute manganeseexists in steel, the growth rate ofcarbide dependson (1) the influence of manganesein the

carbon diffusion in oe phase and (2) the influence of

manganesepartitioning at the interface of oc-carbide in

the movementof the interface. Hillert et al. 12) found that

manganeseatoms were substituted into the iron site

of Fe3Cprecipitated at the equilibrium state in the

manganesecontaining steel. Ushioda et al,ll) reportedthat the above term (1) is negligble from the measure-ment of carbon diffusion rate depending upon the

manganesecontent in c( phasebut the term (2) is possible

from the comparison of the carbide growth rate in

C-Fe-Mnsystem betweenwith and without manganese.Therefore, in the steel containing a large amount of

manganese,even if carbide is precipitated during slowcooling after hot coi]ing, its growth rate will be lowand resulted in the retardation of precipitation andagglomeration of carbide. The present work shows the

sametendency as the Ushioda's results.1 1)

4.2. Aging Index and Solute ElementsFigure 8 illustrates the relationship between Q~*i.

obtained around at 47'C and the aging index. Theagingindex increases with an increase in Q~*1., which is well

in accordancewith the results obtained by Obaraet al. 13)

and Katoh. 14) Consequently the aging index increases asthe interstitial solutes in steel increase and so the extentof the aging index is related to the amountof the in-

terstitial atoms.

4.3. Effects on Mn and Coiling Temperature on CSolubility

Figure 8shows the internal friction of the hot bandsand the cold rolled and annealed sheets. The amountof interstitial solutes in the hot band increased unex-

77 C 1997 Is[J

ISIJ International, Vol. 37 (1997). No. 1

3.8

+ steel A3.6 o steel B

- 3.4 e steel c~E

E~~) 3.2 +~- o e1)(Dx

3.0 +~o) 28 + o.c_ -

o) e2.6

o2.4

0.4 1,o0.6 0.8

Q~1(x 10s)

Fig. 8. Relation between the aging index and Snoekpeak.

pectedly with an increase in coiling temperature, of whichtendency is apparently seen also in the steel Dcontainingboron. In the steel D, nitrogen is precipitated with boronin austenite region and so the Snoekpeak appears dueto the solute carbon. In the present work it was foundthat the grain size of hot band increases with an increasein coiling temperature. The large grain gives it a longdistance for the solute carbon to diffuse toward the grain

boundary, resulting in the increase of the amountof thesolute carbon undiffused. Any difference in the Snoekpeak according to the amountof manganesein the hotband is not found apparently. Thus the variation of

manganesecontent gives an influence only in the growthrate of carbide as discussed in Sec. 4.1.

Onthe other hand, in the cold rolled and annealedsheet the internal friction showedan apparent differencein the Snoekpeaksaccording to the amountof manganeseexcept the boron containing steel D. The amount ofsolute carbon in the annealed sheet is presumedto comefrom the dissolution of carbide precipitated in hot bandduring annealing. As seen in Fig. 8 the content of in-

terstitial solutes shows a great difference between hotband and annealed sheet, and apart from in the hotband, in the annealed sheet, the Snoekpeak decreaseswith an increase in the coiling temperature, which meansthat the balance of carbides between the dissolution ofcarbides in hot bandduring annealing andreprecipitation

in the overaging treatment is important.In the steels A. Band C, carbides of hot band is so

large that they are hardly dissolved out and give rise tothe formation of micro-voids adjacent to carbides duringcold rolling. These voids offer numeroussites for theprecipitation of carbides during cooling after annealing.

But in the steel D, carbide precipitates are so fine that

the micro-voids are hardly formed at the interface of the

matrix and carbide, and then the sites for the precipita-

tion of carbides are limited, resulting in an increase in

the solute carbon in the annealed sheet such as a boroncontained steel.

In order to ensure the abovediscussion the relationship

between the carbide size of the hot band and the Snoekpeak in the annealed state wasillustrated in Fig. 10 fromwhich the Snoekpeak wasderived as a function of the

C 1997 ISIJ

o

T~a

0.16

0.12

0.08

0.04

0.80

0.60

0.40

+ steel AO Steel BO Steel Cl Steel D

t~~

Hot Rolled Speeimens

l

AnnealedSpecimens

l~~

l

78

600 650 700

Hot Coiling Temp.(oc )Fig. 9. Effect of hot rolling temperature on the Snoek peak

in both hot bands and annealed sheets.

e)o

>

-~a

1,o

0.9

o.8

0.7

0.6

0.5

0.4

l.

"•• I +l ol

+ steel AO Steel Be steel CI Steel D

oO

e

o

0.30.5 1.o I .5 2.0 2.5 3,0

MeanCarbide Size (um)

Fig. lO. Relation between the Snoekpeak of annealed sheets

and the meancarbide size of hot bands.

meancarbide size by the non-lineal regression method,expressed as Q~*1*=0.667x d-2+0.361, where d is the

meancarbide diameter (ktm) of the hot band. Theresults

show the ninety three percents reliability regardless of

manganesecontent.As seen in Fig. 7, the internal friction was classified

into the nitrogen and carbon peaks. From the carbonpeak the solute carbon content wascalculated from the

equations derived by manyworkersl5 ~ 17) and revealed

as a function of carbide size in Fig. 11 in which the meancarbide size of the hot band and solute carbon content

are in inverse proportion as though the calculated solute

carbon content showsdifferent values amongthe equa-tions.

As discussed above, the reasons why the carbide size

of the hot band is inversely proportional to the solute

carbon content after annealing are basedon the following

two possibilities; (1) in rapid and short time heating like

ISIJ International, Vol. 37 (1997). No. 1

E~CL

O~1:7

oCU

~5

O

20

18

16

14

12

10

8

6

4

2

e

+

~0,~Oo

oe

,+ +ooo

O Aoki

+ Furusawa

e saitoh

~0~~t~~o

1.9 2.72.30.7 1.1 1.5

MeanCarbide Size (um)Fig. Il. Relation betweenthe calculated solute Cin annealed

sheets and the meancarbide size of hot bands.

continuous annealing, the coarse carbides in the hot bandare not dissolved completely and the remained carbides

provide the nucleation sites for precipitation duringcooling and overaging process after annealing, or (2) as

seen in Figs. 3and 4, the carbides becomecoarser with

an increase in coiling temperature, and these carbides

are broken downduring cold rolling, resulting in de-

veloping numerousmicro-voids at the interface betweenthe carbide and matrix and providing sites for carbideprecipitation during the cooling andoveraging treatment.

However, on the other hand, as solute manganesehinders the growth of carbide precipitates, the lower

manganesecontaining steel shows lower aging index.

Consequently the aging index is shownlow in the lower

manganeseand higher coiling temperature.

5. Conclusions

The low manganesecontaining steel reveals the lower

aging index than the high manganesesteel. The agingindex decreases with a decrease of manganesecontentand an increase of coiling temperature, because (1) the

amountof solute manganeseplaying a role in obstructingthe movementof solute carbon into the interface ofcarbide-ferrite, resulting in the hidrance of the carbidegrowth, is not sufficient and (2) the coarse carbide formedin the hot bandwith higher coiling temperature is crushedsmall fragments during cold rolling, resulting in the

developmentof numerousmicro-voids at the fragments,which provide the sites for the precipitation of solute

carbon.

l)

2)

3)

4)

5)

6)

7)

8)

9)

lO)

l l)

l2)

13)

l4)

15)

16)

l7)

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J. D. Past: The lron and Coa! Trades Rev., 176 (1950), 6.

K. Tsunoyama:KawasakiStee/ Tech. Rep., 12 (1973), 288.

H. Era, J. H. Chung and M. Shimizu: Low-Carbon Steel

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H, Abc. T. Suzuki and S. Okada: T,'ans. Jpn. Inst. Met., 25 (1984),

215.

T. Ototani, Y. Kataura and T. Hukuda: Tetsu-to-Hagan~, 57(1971), 566.

G. J. Couperand R. Kennedy: J. Iron Stee/ Inst., 205 (1967), 642.

H. Suto. T. Imai and T. Uno: J. Jpn. Inst. Met., 34 (1970), 171.

Y. Inokuchi: J. Jpn. Inst. Met., 39 (1975), 1039.

K. Ushioda, K. Koyamaand M. Takahashi: Tetsu-to-Hagan~,

76 (1990), 134.

M, Hillert, T. Wadaand H. Wada:J. Iron Stee! Inst., 205 (1967),

539.

T. Obara, K. Sakata and T. Irie: Proc, on Metallurgy ofContinuous Annealed Sheet Steel, ed, by B. L. Bramfitt and P.

L. Mangonon,AIME, Dallas, (1982), 83.

H. Katoh: Proc. on Technology of Continuously AnnealedCold-Rolled Sheet Steel, ed. by R. Pradhan, AIME, Detroit,

(1984), 37.

K. Aoki. S. Sekino and T. Fujishima: Tetsu-to-Hagan~, 48 (1962),

156.

K. Furusawaand K. Tanaka: J, Jpn. Inst. Met., 33 (1969), 985.

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79 C 1997 ISIJ