ISIJ International, Vol. 37 (1997), No. 3, pp. 232-239

Evaluation

contalnlng

of Susceptibility

Steels by Tensile

to Surtace

Test

Hot Shortness in Ou-

Seok-JongSEO.Kentaro ASAKURA1)and Koji SHIBATA1)

Graduate School. Department of Materials Science. The University of Tokyo. Hongo. Bunkyo-ku, Tokyo, 113Japan.

1)Department of Materials Science. The University of Tokyo. Hongo, Bunkyo-ku. Tokyo. 113Japan.

(Received on August /. 1996, accepted in final form on November12. 1996)

The feasibility of a newmethodfor evaluating the susceptibility of steels to surface hot shortness due to

Cu and Sn was examined. Steels used are IF steels containing 0.1-0.2•1•Cu and 0.1•/•C-0.5'1•Cu steels.

Chemical compositions of the IF steels were controlled with the purpose of being as similar as possible to

those of steels of which the susceptibility waspreviously examinedby a visual method in a practical rolling

process at Nippon Steel Corporation. Tensile tests were carried out at 11OO'Cafter heating a specimen in

air or Ar gas. The total elongation and maximumload were measuredfrom a load-elongation curve. Bydividing the difference between the total elongations or maximumloads obtained in air and Ar gas by thetotal elongation or maximumload in Ar gas. the parameters Ee or Ep were calculated. By considering the

reduction in specimen diameter due to oxidation, the Ep corrected. Ep', was calculated. The Ee, Ep andEp' of the IF steels corresponded to the degree of surface cracks observed in the practical rolling process.Penetration of Cu-enriched phase into grain boundaries tends to occur under thermal and external stresses.

Surface cracks stop growing along the depth direction at the earlier stage of deformation whenthe amountof Cu-enriched phase is the smaller at steel/scale interface. Tensile tests using specimens having artificial

round cracks clarified that the Eeand Ep' increased with an increase in the crack depth and that, in contrast.

the Ep' increased slightly and the Ee decreased with an increase in the numberof cracks. The parameterEe and Ep' showeda maximumvalue at a strain rate of around 3x I0-2s~1, and the difference in their

values amongthe steels is largest around this strain rate. Therefore it is possible to knowsensitively theeffects of any factors like alloying or impurity elements on surface hot shortness at around this strain rate

although this strain rate is muchslower than those of hot workings.

KEYWORDS:copper; tin; scrap iron; oxidation; hot shortness; surface crack; IF steels; carbon steels, tensile

test; strain rate; temperature; artificial crack.

1. Introduction

Copper tends to enter steels through usage of scrapiron and be enriched at steel/scale interface by prefer-

ential oxidation of Fe, Ieading to liquid embrittlement orsurface cracking during hot working. This phenomenonis well knownas surface hot shortness.1) Hot bend tests

are generally usedl,2) in order to evaluate the sus-ceptibility of steels to such shortness in laboratories.

In such bend tests, the severity of surface cracking is

evaluated by visual comparison with a standard scale orby measuring characteristics of surface cracks by their

number, width and depth. The evaluation by visual

comparison, however, is not quantitative and measuringsuch crack characteristics is very laborious. Therefore,it is necessary to establish a newquantitative and easier

evaluation method. Kajitani et al.3) stopped tensile tests

at several pre-determined strains, and measured the

length and the width of surface cracks. Their methodis

useful to knowthe growth behavior of the surface cracks

and effects of impurity and alloying elements on the

severity of surface cracking. In their method, however,

C 1997 ISIJ 232

measuring length andwidth of surface cracks is also verylaborious. Reduction of area obtained by tensile test is

often used to evaluate the ductility of steels at elevated

temperatures. But generally speaking, the reduction of

area is not suitable for the evaluation of surface hotshortness because it yields around 100 o/o in usual casesexcept whentoo muchCu is enriched at the steel/scale

interface.

This present paper examines the feasibility of a newmethod for evaluating the susceptibility of steels tosurface hot shortness using total elongation and maxi-

mumload obtained by hot tensile tests.

2. Experimental Procedure

2.1. Steels

Steels used in this work are classified into two groups;Steels A-Care interstitial free (IF) steels and Steels D-Gare 0,lo/oC-O.5010Cu steels. The IF steels were melted

as 30kgf ingots in the laboratory of Nippon Steel Cor-poration, and the latter as 50kgf ingots in the labora-

tory of KawasakiSteel Corporation. Table I shows the

ISIJ International. Vol. 37 (1997), No. 3

Table 1. Chemical compositions of the steels tested in masso/o.

No. C Mn PSi S Cu Ni Cr Sn OAI Ti N

A 0.0015

B 0.001 7

C O.0024

D 0.096

E O. IOO

F 0.099

G O. I05

ool 0,1 o 0,009 0,01 o0,01 0,10 ool I ool 10,01 o12 oolo oolo

o39 o.50 0,002 o002

0,01 0,50 o020 o002

0,38 0,50 o021 o002

0,01 0,50 o002 o002

0,086

0,1 17

o. i75

0,48

0,47

o,53

0,48

0,038 0,058 0,008

0,025 0,056 o027

0,080 o151 o029

0,01 ool

0,01 0.01

ool 0,01

0.01 0,01

o033 o053 13

o.035 0.051 17

o035 0,052 21

0,001 0,001 2135

oool 0,001 28 i8

0,009 0,003 40 18

0,001 0.00i 38 37

* N,O:ppm, Fe:bal,

chemical compositions of these steels in masso/o. In Steel

B. 0.03 o/o Sn is alloyed to the base composition of Steel

A. Comparedwith Steel B, contents of Cu, Ni and Crwere increased in Steel C. Chemical compositions ofthese IF steels werecontrolled to be as similar as possible

to those of steels of which susceptibility to surface hotshortness waspreviously examinedby the visual methodin the practical hot and cold rolling processes at NipponSteel Corporation.4) Onthe other hand, Steels D to Gwere used in the experiments in which effects of strain

rate on surface hot shortness were examined. In Steels

D, Eand F, 0.40/0 Si, O.020/0 Pand 0.40/0Si+0.020/0P

were addedrespectively to the base composition of Steel

G.

2.2. Tensile Test at Elevated Temperatures

Specimens for tensile tests were machined from hotrolled plates. Geometries of the specimensare shownin

Fig. l. The specimens which were shorter were sub-

jected to tensile tests at higher strain rates of above8.3 x l0~1 sl. Tensile tests at slower strain rates wereperformed using a mortar drive type machineequippedwith an electric resistance furnace, while an electro-

hydraulically actuated type machine equipped with ahigh frequency induction heater wasused in the tests at

the strain rate of 5.0 x 10 s~ l. Tensile tests were carried

out in Ar gas at 1100'C after heating the specimen in

air or Ar gas. The purity of Ar gas used was four nine.

The three parameters were calculated by using the fol-

lowing Eqs. (1) to (3) as well as the relationships be-

tween these parameters and susceptibility to surface hotshortness were examined.

M12XI .7515R

6

25 30 25

80

(a)

e(Ar) - e(air)Ee(olo)= xIOO ................(1)

e(Ar)

P(Ar) - P(air)Ep(olo)= x 100 ...............(2)

P(Ar)

P(Ar) - P'(air)Ep'(olo)= x 100 ..............(3)

P(Ar)

wheree(Ar) ande(air) are the total elongations measuredin Ar gas and air, and P(Ar) and P(air) the maximumloads measuredin Ar gas and air, respectively. P'(air) is

the maximumload calculated by considering the reduc-

tion in diameter of the specimenby oxidation using the

233

M8X1.25

18 10

1

14 10 18

70

(b)

Fig, l, Geometries of tensile specimens for (a) Iower strain

rates and (b) higher strain rates.

results of thermogravimetry. In this calculation, the fol-

10wing assumptions were used.(1) Scale is composedof only FeO~(Wtistite).

(2) Atomic ratio n ofWustite is 1.14.s)

(3) Si and Mnform Si02 and MnO,respectively.

In order to examine the progress of the surface crack-ing during deformation, the depth to the normal direc-

tion to the specimensurface and width of surface cracks

were measured with an optical microscope after the

specimenswere deformed to strains of 10 and 30 o/o at astrain rate of 2.8 > lO~ i s~ I after heated in air at 1100'Cfor 30min.

To clarify the effects of the depth and the numberofsurface cracks on the parameters EeandEp', tensile tests

were performed at elevated temperature in Ar gas usingspecimenshaving artificial cracks of 0.3, 0.6 and 0.9 mmdepths and 0.2mmwidth. Oneto five artificial cracks

weremachinedby electron discharging at equal intervals

as shownin Fig. 2. Froma mechanics viewpoint, these

cracks should be referred to as notches. In this paper,

C 1997 ISIJ

ISIJ International, Vol. 37 (1 997), No, 3

OIR ~":.2mmd= 0.3mm, 0.6mm, 0.9mm

Fig. 2. Schematic geometry ofa tensile specimenhaving arti-

ficial cracks.

however, these will be referred to as cracks becausethese

were machinedartificially in order to simulate the sur-face cracks induced by Cu-enriched liquid phase.

2.3. Observation of Oxidation by Thermogravimetry

Changeof the specimenweight with time wasmeasur-ed in air during heating at 20'C/min and holding at

l 100'C with a gravity-type balance. Specimengeometrywasa cylinder with a diameter of 3mmand a height of3.5 mm.Temperatureandspecimenweight wererecorded

every three seconds with a personal computer.

2.4. Optical Microscopy and EPMASpecimens of lOx lOx lOmmwere oxidized in air

at I 100"C for 30min and air-cooled. Then they weremountedin epoxy resin andpolished mechanically. Afterchemically etched in a 2o/o nital, they were subjected tooptical microscopy and EPMA(Electron Probe Micro-analysis). In optical microscopy. Cu-enriched phasenearsteel/scale interface of the specimen was observed. In

EPMA,concentration behavior of Cu and Sn in the

steel/scale interface region wasexamined.

3. Results and Discussion

3.1. Measurementof the Parameters Ee, Epand Ep'

Figures 3and 4show the total elongation and maxi-

mumload of Steels A-C tested at 1100'C and at 2.8 x10~1s~ l, respectively. Total elongation and maximum10ad in air decrease in the order of Steels A, Band Cwhile those obtained in Ar gas increase in the sameorder.

Figure 5shows the change in specimenweight duringheating at 20'C/min andholding at I 100'C, Upto about30min holding, which is the heating condition in tensile

tests, oxidation rates of the three steels are similar.

Figure 6showsthe parameters Ee, Epand Ep', whichderived from the data of Figs. 3, 4and 5using Eqs. (1),

(2) and (3), respectively. The three values for Steel Aaresmallest, and those for Steel Care largest.

3.2. Comparisonof the Ee, Epand Ep' with the Resultsof the Inspection of Surface Cracks in a PracticalProcessing Line

As mention~d in Sec. 2.1, susceptibility to surface hotshortness of three steels were previously examinedby thevisual method in the practical hot and cold rolling

processes by Yamadaet al. at Nippon Steel Corpora-tion.4) Thechemical composition of the steels which theyexaminedare very similar to those of the present Steels

A, Band C. Therefore, in the present paper, their steels

will be referred to as Steel A', B' and C', respectively.

C 1997 ISIJ 234

~~OO

co

CTS

~)

Oa)

cEf

+'OH

Fig. 3.

'~o)~e1:,cTs

EE'~

~

Fig. 4.

~~EE~~~~~

Fig. 5.

140

120

1oO

80

60

oF~inl~solid: in air

goe

,

oOo

e,

8

8e

A CBSteelS

Total elongation of Steels A, Band C .Tensile tests

performed at 1100'C at a strain rate of 2.8 x lO~ I s~ lin Ar gas after heating for 30min in Ar gas and air.

200

180

160

140

120

r6~~~TIT~Fl~~~J

sond: in air

Oo

O

ee

Oo

e8

g

e,

A B CSteelS

Maximumload of Steels A, Band C. Tensile testsperformed at I 100'C at a strain rate of 2.8 x lO I s~ 1in Ar gas after heating for 30 min in Ar gas a~rd air.

I t

cB t

A

0.8

0.6

0.4

0.2

o

~

,

50 1oo 150 200 2.r

0.8

250

Time(min)

Changein specimenweight during heating at 20'C/minand holding at 1100'C for 2h.

ISIJ International. Vol. 37 (1997), No. 3

50

_ 40~~OO

UJ~ 30

aLUJ 20UJO

10

o

I

O

I

e

A

l

o

JL

A BSteelS

Fig. 6. Comparison of Ee, Ep and Ep'

and C.

c

amongSteels A, B

Ea

~_o

oQoca

tCO~-oJ:,l

O

Fig. 7.

50

40

30

20

10

O

~- cropped bar

=~Pickled plate

-P skin-passed sheet

A C'' B'

SteelS

Maximumdepths of surface cracks measuredYamadaet a/.4) on the practical processing line.

by

scale

50~!m

Fig. 8. Results of EPMAfor Steels A air-cooled after heated at I 100'C for Ih in air.

(a) SEM,(b) Fe, (c) Cu, (d) Sn

According to the report by Yamadaet al., the degrees

of surface cracks of Steels B' and C' were not allowedin all inspections of cropped bars, pickled plates andskin-passed sheets, while for Steel A' the results of all

inspections were allowed. The maximumdepth of sur-face cracks measuredby Yamadaet al. is plotted in Fig.

7. ComparingFigs. 6and 7, it is generally knownthat

the large and small values of Ee, Ep and Ep' of eachSteel A, Bor Ccorrespond to the maximumdepth of

surface cracks of each corresponding Steel A', B' or C'.

That is to say, the Ee, Ep and Ep' represent the degree

of the penetration of surface cracks observed in the

practical rolling process. In the following sections, the

235

relationship between the concentration of Cu due topreferential oxidation of Fe and surface cracks will beexamined.

3.3. Cu-enriched Phaseand Its Penetration into GrainBoundaries

In Steel A. Cu-enriched phase at steel/scale interface

andconcentration of Sncould not be observed by opticalmicroscopy and EPMA.In Steels Band C, however,concentrations of Cu, Ni and Snwere observed at steel/

scale interface. The results of EPMAare shownin Figs.

8and 9 for Steels A and Cair-cooled after heated at

l 100'C for Ih in air. Figure 10 represents the results

C 1997 IS]J

ISIJ International, Vol. 37 (1997), No. 3

c)

Fig. 9. Results of EPMAfor Steels Cair-cooled after heated at I 100"C for i hin air

(a) SEM,(b) Fe, (c) Cu, (d) Sn

~~!'~~~:S_cale

c)

40~!m

Fig, lO. Result of EPMAfor Steel Cfurnace-cooled after heated at I 100'C for Ihin air

(a) SEM,(b) Fe, (c) Cu, (d) Sn

of EPMAfor Steel C furnace-cooled after heated at

1100'C for Ih in air. Penetratlon of Cu-enriched liquidphase into austenitic grain boundaries of the steel can

be observed, but rarely in specimensair-cooled. This fact

suggests that the penetration is hard to occur beforecooling although several previous papers has presumed

C 1997 ISIJ 236

ISIJ International, Vol. 37 (1 997), No. 3

the penetration to occur before cooling.1'6)

The reason for the cooling rate dependenceof thepenetration behavior has remained unclarified, but it canbe presumedthat in the case of air cooling, spontaneousbreak downof the scale tends to occur at the early stageof cooling and tensile stress due to the difference oftherm~l shrinkage betweensteel and scale does not workat the steel surface. It is knownthat linear coefficient ofthermal expansion of Fe is larger by about 25 o/. thanthat of Fe0.6) Onthe other hand, in the case of furnace-cooling the break downof scale is presumedto be delayedand the thermal tensile stress at the steel surface assists

the penetration of Cu-enriched liquid phase into grainboundaries.

3.4. GrowthBehavior of Surface Cracks

During hot deformation, it is conceived that the Cu-enriched liquid phase penetrates into austenitic grain

boundaries and induce surface cracks under appliedstress. In Figs. 11 and 12, depths of surface cracks areplotted as a function of their widths for Steels A andCdeformed by 10 and 300/. at I 100'C, respectively. InSteel A, both the depth and width are small even after

30 ~/o deformation. In Steel C, cracks over 0.3 mmdepth

are observed even in the specimendeformedby 10 ~/.. Inthis steel, proceeding of deformation up to 30 o/o in-

creases the crack width significantly but does not increase

the depth noticeably. That is to say, surface cracks in

this steel cease the growth along the depth direction

before lO"/* deformation and thereafter cracks grow byincreasing their width. Similar growth behavior of sur-face cracks has beenobserved in a 0.20/0C-1 .OoloCu steel

by Kajitani et al.3) Theyattribute the stop of the growthalong the depth direction to the lack of Cu-enrichedliquid phase at the tip of the crack. In the observationby Kajitani et al., surface cracks increase their depth upto around 20 "/o elongation whereas in the present worksurface cracks stop increasing their depth before 100/*

deformation. This discrepancy is attributable to the

E:~

~::

c:L

o1:)

~:o~O

difference in the amountof Cu-enriched liquid phase atsteel/scale interface becauseCucontent of the steel usedby them is higher by twice than that used in the presentwork.

3.5. Effects of Depth andNumberof the Surface Crackson the Eeand Ep'

Figures 13 and 14 exhibit the results of tensile tests

performed in Ar gas using specimens having artificial

cracks in order to clarify the effects of the depth and thenumberof surface cracks on the parameters, Eeand Ep'.

Total elongation decreases with an increase in the crackdepth and with a decrease in the number of cracks.

Maximumload decreases with an increase in the crackdepth and shows a slight decrease with an increase in

the numberof cracks.

Figure 15 exhibits the effects of the depth and the

Numberof Crack : (a) 59.5 cm~1,

(b) 71 ,2cm~1

E~c~CLo1:,

~ocU

O

400

300

200

1oO

O

ooO

o

8o

oooo

oo~o

~o

o

8

o

(a)

~;(b~:ooo

G~o (~)eoo~~

o

(b)

ooo o

oo

Numberof Crack : (a) 39.7 cm~1, (b) 36.8cm~1

400

300

200

1oo

(a)

oo

(b)

oo(~)

O 50 IoO 150 O 50 IoO 150

Fig. Il.

Fig. 12.

50 100 150 O 50 100

Crack width (/A m)150

~~OO

~C:O

5::!~!:

O)=O~515~OH

Crack depths of surface cracks of Steels Cas a func-tion of their width.(a) 100/, deformation, (b) 300/* deformation

140

120

1oo

80

60

40

20

o

e

numberof artificial cracke :O l:3A:1 ~:5IF

lJL

IF

O 50 100 150 O 50 100 150

Crack width (,t m)

Crack depths of surface cracks of Steels Aas a func-tion of their width.(a) lO"/, deformation, (b) 30'/. deformation

Fig. 13.

O 0.2 0,4 0.6 0.8 iDepth Of artificial crack(mm)

Effects of depth and numberof artificial cracks ontotal elongation.

mark shows the result for the specimen having 5artificial cracks of which depths were 0.6 mmexceptthat at center, 0.9 mm.

237 C 1997 ISIJ

ISIJ International, Vol. 37 (1997), No. 3

~;~~~cu

E=E

.~

:~i

140

180

120

110

1oO

90

80

r~T~:~~,~;~1number_ofart'='ificial

crack

eA

O1 (~

35

el~ L,

~ A

,O 0.2 0.4 0.6 0.8 1Depth of artificial crack(mm)

Effects of depth and numberof artificial cracks onmaximumload.

~> mark shows the result for the specimen having 5artificial cracks of which depths were 0.6 mmexceptthat at center, 0.9mm.

~~OO

C:o

~U~)

Oa)

cV

OH

SOlid : in air, open : in Ar

200

150

1OO

50

Fig. 14.

IF ~ D(+Si)

AA E(+P)I [] F(+Si+P)

e OG(BASE)

IL

10~4 10~3 1O IO IOo IO I02

Strain rate (S~1)

Fig. 16. Effects of strain rate on total elongation of Steels Dto Gat 1100'C.

~~oo

o-LLl

LUo

1oo

solid : in air, open : in ArSOlid : Ee' open: Ep'

80

60

40

20

numberof artificial crack

AA: I l[]: 3 •~: 5

AI

~

OAIO

a~

~

O 0.2 0,4 0.6 0.8 iDepth Of artificial crack(mm)

Fig. 15. Effects ofdepth and numberofcracks on the param-eters Eeand Ep'.

Oand Omarks show the result for the specimenhaving 5artificial cracks of which depths were0,6 mmexcept that at center, O.9 mm.

numberof cracks on the parameters Eeand Ep'. TheEeincreases with an increase in the crack depth and with adecrease in the numberof cracks. In contrast, the Ep'

increases with increases in the crack depth and the num-ber of cracks. That is to say, the larger value of Ep'

suggests the occurrence of the deeper and the larger num-ber of cracks, which meansthe higher susceptibility to

surface defects or surface hot shortness. Therefore, the

parameter Ep' can be said to be a better indicator of the

susceptibility to surface hot shortness than the parameterEe.

Figure 6shows that the values of Ee and Ep increase

with an increase in the Ep' whenCuand Sn are addedalthough the numberof surface cracks increases. Suchachange of the samesense in Ee and Ep' is attributable

to a large increase in the maximumdepth of penetration

'~c,)~:~DcQ

E~.~

~

300

250

200

150

1OO

50

i O'4 10~3 10~2 10~1 i Oo IO I02

Strain rate (S~1)

Frg 17. Effects of strain rate on maximumload of Steels Dto Gat 1100*C.

of Cu-enriched liquid phase by the addition of CuandSn. As for the change of the samesense in Ep and Ep',

it can be explained by the fact that oxidation rates aresimilar amongSteels A, Band Cheated at 1100'C for

30min before tensile tests as shownin Fig. 5.

3.6. Effects of Strain Rate on the ParametersEeandEp'

Figures 16 and 17 show the effects of the strain rate

on the total elongation and maximumload at I 100'Cof Steels DG, respectively. There seems to be a dis-

continuity in total elongations between the strain rates

of 2.8 x l0~1 and 8.3 x l0~1 s~1, whereas such a dis-

continuity cannot be observed in maximumload. Thediscontinuity in total elongation is attributed to the

difference in specimen geometry, area by length ratio.

Theparameters Eeand Ep' are shownin Figs. 18 and19, respectively. Fromthese figures, it is knownthat the

values of these parameters reveal a maximumat a strain

rate of around 3x l0~2 s~1 and that the difference in

their values amongthe steels is largest around this strain

C 1997 ISIJ 238

80

ISIJ Internationai, Vol. 37 (1997). No. 3

60

uJo40

20IF D(+Si)

AE(+P)

I F(+Si+P)

OG(BASE)

~

O10 4 i O'3 10~2 10~1 1Oo 101 i 02

Strain rate (S~1)

Fig. 18. Effects of strain rate on the parameter Ee of Steels

D to G.

50

40

~ 30LLi

20

IF

10

~D(+Si)

AE(+P)

I F(+Si+P)

OG(BASE)

O _410 i0~3 10~2 10~1 100 10 10

Strain rate (s~1)

Fig. 19. Effects of strain rate on the parameter Ep' of steels

D to G,

rate. At higher strain rates over this value, the values ofthese parameters and the difference in the values of these

parameters amongthe steels decrease. These facts canbe understood if penetration of Cu-enriched liquid phaseinto grain boundaries is suppressed to occur duringtensile tests at higher strain rates. Suchstrain rate effects

showthat it is possible to knowsensitively the effects of

any factors like alloying and impurity elements on sur-face hot shortness by tensile tests at strain rates of around3x l0~2 s~1, although this strain rate is muchslowerthan the strain rates of practical hot rolling.

Decreasing the strain rate from about 3x 10~2s~ I toabout 3x 10~4s~1 seemsto decrease the values of Eeand Ep'. Thedifference in these values amongthe steelsis also decreased. The reason that these facts were ob-served at very low strain rates remains unclarified. It is

necessary to examine the effects of the other factors like

dynamic recrystallization on deformation behavior.

4. Conclusion

The feasibility of a newmethod for evaluating thesusceptibility of steels to surface hot shortness due to Cuwas examined using IF steels containing 0.1-0.2010Cuand 0.1 o/oC-0.5010Cu steels. The following results wereobtained.

(1) The value of parameters Ee, Ep and Ep' cor-responded to the degree of the penetration of surfacecracks observed in the practical rolling process of IFsteels containing Cu.

(2) Penetration of Cu-enriched phase into grainboundaries tends to occur under a thermal or appliedstress.

(3) Surface cracks stop growing along the depthdirection at the early stage of deformation when the

amount of Cu-enriched phase is small at steel/scaleinterface.

(4) By tensile tests using specimen having artificial

round cracks, it was clarified that the value of Ee in-

creases with an increase in the crack depth and with adecrease in the number of cracks. The value of Ep'increases with an increas~ in the crack depth and the

numberof cracks. That is to say, the parameter Ep' is

a better indicator of the susceptibility to surface hotshortness than the parameter Ee.

(5) Theparameters Ee, Epand Ep' revealed a strain

rate dependencyandamaximumat around 3x 10~2s~ I,

The difference in their values amongthe different steels

is largest around this strain rate.(6) Strain rate effects on the values ofthe parameters

showedthat it is possible to sensitively knowthe effects

of any factors like impurity and alloying elements onsurface hot shortness by tensile tests at strain rates ofaround 3x l0~2 s~ 1.

Acknowledgments

Theauthors are grateful to Nippon Steel Corporationand KawasakiSteel Corporation for melting steels. Theyalso wish to thank Mr. S. Sano(under graduate student)for assistance in performing experiments. This study

was partly supported by a Grant-in-Aid for Scientific

Research from the Ministry of Education, Science andCulture of Japan, 07555514.

REFERENCESl) L, Habrakenand J. Lecomte-Beckers: Copper in lron and Steel,

ed. by L. McDonaldSchetky, John Wiley & Sons, NewYork,(1982), 45.

2) A. Nicholson and J. D. Murray: J. Iron Steel Inst., 203 (1965),

l007.

3) T. Kajitani, M, Wakou,N. Tokumitsu, S. Ogibayashi and S.

Mizoguchi: Tetsu-to-Hagan~, 81 (1995), 185.

4) T. Yamada,M. Oda, O. Akisue, H. Gohshi and H. Onishi:

CAMP-ISIJ,4 (1991), 1937.5) N. Birks and G. H. Meier: Introduction to High Temperature

Oxidation of Metals, EdwardArnold (publishers) Ltd., London,(1983).

6) K. Born: Stahl Eisen, 73 (1953), 1268.

239 C 1997 ISIJ