mechanical properties of directly air cooled copper added microalloyed steels (dual phase)

7272019 Mechanical Properties of Directly Air Cooled Copper Added Microalloyed Steels (Dual Phase)

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P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C

o m m u n i c a t i o n s L t d

Mechanical properties of directly air cooledcopper added microalloyed steels

S K Ghosh1 A Haldar 2 and P P Chattopadhyay1

An attempt has been made to develop dual phase like microstructures in directly air cooled

15 wt-Cu added TindashB microalloyed steels The chosen compositions have allowed to avoid

pearlite formation during air cooling and yielded continuous work hardening behaviour with

attractive combination of strength and ductility Aging treatment has effectively improved the

ductility without deterioration of strength Differential JaoultndashCrussard analysis of the tensile

results has explained contributions of the constituent phases in work hardening behaviour of the

investigated steels

Keywords Dual phase Directly air cooled Work hardening JaoultndashCrussard analysis

Introduction

Formable structural steels warrant continuous yielding

behaviour and hence the absence of pearlite in the

microstructure1 Strengthndashductility combination in the

formable structural steels such as dual phase (DP)

steels is achieved through continuous work hardening

of the microstructures contributed by sequential plastic

deformation of the soft ferrite and hard martensite2 The

presence of martensite at the ferrite grain boundary aswell as strain induced transformation of austenite to

martensite significantly influences the work hardening of

ferrite3 Earlier it has been demonstrated that work

hardening behaviour of DP steels is essentially non-

linear and cannot be described completely by Hollomon

equation4 Different stages of work hardening in DP

steels have been successfully described by carrying out

differential and modified Jaoult5 ndashCrussard6 analyses In

a recent work the ranges of strain for three stages of

deformation are identified on the basis of Jaoultndash

Crussard analysis and the work hardening exponent

values at the third stage are determined to compare the

roles of martensite at different strain rates in the chosen

steels7 In DP and transformation induced plasticity

(TRIP) aided steels alloy partitioning during intercritical

annealing improves the hardenability of austenite and

suppresses the pearlite formation89 Suitable microal-

loying addition such as Ti B etc improves the

hardenability to the extent that bainite may form in

low carbon steels even under air cooling condition10

The addition of Cu in high strength low alloy (HSLA)

steels is also known to retard austenite (c)Rferrite (a)

transformation11 In Cu bearing steels Ni is generally

added for minimising hot shortness which additionally

improves the hardenability of austenite12

Ni also

retards Cu precipitation during air cooling of the hotrolled steels particularly at higher temperature dueto segregation at the precipitatematrix interfaces13

Therefore finer precipitates form heterogeneouslyat lower temperature and effectively contribute tostrengthening14

The present study aims to utilise the benefits of Cu

addition and microalloy with Ti and B in low carbonsteel to form the microstructures containing soft ferrite

hard bainite andor martensite along with fine Cuprecipitates by direct air cooling (DAC) of hot rolledsteels To understand the genesis of the strengthndash ductility combination and contributions of the micro-

structural constituents therein the work hardeningbehaviour of the steels has been examined by carryingout differential JaoultndashCrussard analysis vis a vis theHollomon plot

Experimental

In the present study the alloys are prepared by judiciousaddition of Ti B Cu and Ni in low carbon grade steelscraps Melts were prepared in a laboratory scale

induction melting furnace (5 kg crucible capacity)Table 1 presents the compositions of the steels obtainedafter spectroscopic analysis using an optical emissionspectrometer (SPECTROLABndashM8)

The cast ingots after homogenisation at 1200uC for

120 min were forged into bars of 1256125 mmsection The forged bars were soaked at 1200uC andhot rolled with finish rolling temperatures (FRT) of 750uC down to a thickness of 6 mm in three passesAfter completion of rolling samples were cooled in air

To study the microstructures the samples were etched

with Vilellarsquos reagent (composition 1 g picric acid 5 mLhydrochloric acid and 95 mL ethyl alcohol)15

Microstructures of the etched samples were examinedusing a SEM (Model JEOL JSM-5510) operated at

20 kV Transmission electron microscopy of selectedsamples was carried out using a transmission electronmicroscope (PHILIPS CMndash200 with EDAX) at an

1Department of Metallurgy and Materials Engineering Bengal Engineeringand Science University Shibpur Howrah 711 103 India2RampD Division Tata Steel Limited Jamshedpur 831 001 India

Corresponding author email skghoshmetalbecsacin

szlig 2007 Institute of Materials Minerals and Mining Published by Maney on behalf of the InstituteReceived 8 June 2007 accepted 19 July 2007DOI 101179174328407X239046 Materials Science and Technology 2007 VOL 23 NO 11 1375





operating voltage of 200 kV Energy dispersive Xndashray

spectroscopic analysis (EDS) was conducted to deter-

mine the composition of the precipitates

Room temperature tensile testing was conducted

using the Instronndash4204 testing machine with a crosshead

velocity of 05 mm min21 The test specimen was

prepared as per the ASTM Standard (ASTM

Vol 0301 E8Mndash96) The error in yield strength (YS)and ultimate tensile strength (UTS) measurement was

recorded asiexcl3 and the same for per cent elongation

was iexcl5

Results

Tensile propertiesFigures 1ndash4 present the results obtained from the tensile

testing of the experimental steels Figure 1 shows that

the addition of Ni in 15CundashTindashB steel has improved the

YS values for both the DAC and aged samples For all

the samples a significant improvement of YS values has

been achieved after aging Figure 2 reveals that both

microalloying and aging have generally contributed to

the improvement of the UTS values Figures 3 and 4

show that in DAC samples microalloying of 15Cu steel

has lowered the elongations and the addition of Ni

in 15CundashTindashB steel has simultaneously improved

However for all the three aged steels the elongation

values are comparable It is important to note that

among the three compositions 15CundashNindashTindashB steel has

yielded the best combination of strength and ductility

for both the DAC and aged samples

Work hardening behaviour of directly air cooledsteelsWork hardening behaviour is generally expressed by

Hollomon equation416 that describes the true stress

1 Yield strength values for various investigated steels 2 Ultimate tensile strength values for various investi-

gated steels

3 Ultimate elongation UEL () values for various inves-

tigated steels

Table 1 Chemical composition of investigated steels wt-

Steel identification C Mn Si S P Ti B Cu Ni N

15Cu 004 160 048 0022 0014 ndash ndash 151 ndash 0005115CundashTindashB 004 169 057 0021 0013 0032 00013 154 ndash 0008015CundashNindashTindashB 003 168 053 0020 0013 0032 00012 155 079 00058

Ghosh et al Mechanical properties of directly air cooled copper added microalloyed steels

1376 Materials Science and Technology 2007 VOL 23 NO 11







dislocation and tempering of martensite hardening is

contributed by precipitation of Cu

Generally the JaoultndashCrussard plots of the aged steels

exhibit a faster decrease in the work hardening rate than

the DAC steel at the first stage (stage 1) due to the fact

that the dislocations in ferrite of the air cooled steel are

effectively recovered during aging In contrast with the

Hollomon plots the JaoultndashCrussard plots of the agedsamples have revealed a distinct second stage (stage 2)

hardening (Fig 5a vis a vis Fig 5b2d ) Earlier con-

strained deformation of ferrite was held responsible for

the appearance of a second stage with higher work

hardening in the JaoultndashCrussard plots23 In the present

study prominent appearance of the second stage of work

hardening particularly for the aged steels may reason-

ably be attributed to the finer Cu precipitates which

constrain the deformation of ferrite by pinning down the

dislocations The third stage of work hardening is due to

concomitant plastic deformation of martensite and

dynamic recovery of ferrite under isostrain condition

It may be noted that the influence of aging is not

prominent at the third stage and the DAC and agedsamples exhibit approximately comparable deformation

behaviour

Thus it is apparent that precipitation of Cu by virtue

of hardening of ferrite results in sequential work

hardening characteristic which enhances the strengthndash

ductility combination in multiphase microstructures

5 a Hollomon analysis of ln s (s is in MPa) versus ln e for DAC and aged steels differential JaoultndashCrussard analysis

of ln(ds de) versus ln e for DAC and aged b 15Cu c 15CundashTindashB and d 15CundashTimdashBndashNi steel arrow indicated changes

in slope and numerals represent (n rsquo21) values







6 Image (SEM) of 15Cu alloy in a DAC condition with smooth appearance of islands and b peak aged condition

(500uC 60 min) showing etching effected islands at ferrite boundaries

7 a image (TEM) of 15Cu2Ti2B sample showing ferrite grains with formation of precipitate at dislocation structures

and b EDS plot and chemical composition (inset) from arrowed region in Fig 7a confirms presence of e- Cu

precipitate

8 a image (TEM) of 15CundashTindashB peak aged (500oC 15 min) sample showing fine precipitates (10ndash40 nm) and b EDS plot

along with chemical composition (inset) from region arrowed in Fig 8a


Materials Science and Technology 2007 VOL 23 NO 11 1379





Conclusions

1 Addition of 15 wt-Cu in the Ti2B microalloyedsteel has adequately enhanced the hardenability result-ing in the DP microstructures in directly air cooledcondition

2 Differential JaoultndashCrussard analysis of the tensile

results has demonstrated that Cu precipitation in ferrite

during aging treatment improves the work hardeningbehaviour at the intermediate stage of straining andresulted into attractive strengthndashductility combination

3 The 15Cu2NindashTindashB steel has yielded most attrac-tive strengthndashductility combination in the directly aircooled steels with and without aging

References1 W S Owen Met Technol 1980 7 1ndash13

2 R K Piplani and V Raghavan Steel India 1981 4 (1) 1ndash22

3 P J Jacques J Ladriere and F Delannay Metall Mater Trans

A 2001 32A 275922768

4 J H Hollomon Trans AIME 1945 162 2682290

5 B Jaoult J Mech Phys Solids 1957 5 95ndash114

6 C Crussard Rev Metall (Paris) 1953 10 697ndash710

7 N D Beynon S Oliver T B Jones and G Fourlaris Mater Sci

Technol 2005 21 771ndash778

8 E Navara Proc Int Conf on lsquoHigh strength low alloy steelsrsquo (ed

D P Dunne and T Chandra) 302ndash307 1984 Wollongong

University of Wollongong

9 N R Bandyopadhyay and S Datta ISIJ Int 2004 44 927ndash934

10 X M Wang and X L He ISIJ Int Suppl 2002 42 S38ndashS46

11 A L De Sy Trans Iron Steel Inst Jpn 1974 14 139ndash154

12 G F Vander Voort in lsquoASM Metals hand bookrsquo 10th edn Vol 1

lsquoProperties and selection iron steels and high performance alloysrsquo

389ndash423 1995 Materials Park OH ASM International

13 M E Fine and D Isheim Scr Mater 2005 53 115ndash118

14 M K Banerjee D Ghosh and S Datta ISIJ Int 2001 41 (3)

257ndash261

15 G F Vander Voort lsquoMetallography ndash principles and practicersquo 632

1984 New York McGrawndashHill

16 L F Ramos D K Matlock and G Krauss Metall Trans A

1979 10A 259ndash261

17 T S Byun and I S Kim J Mater Sci 1993 28 2923ndash2932

18 Z Jiang Z Guan and J Lian Mater Sci Eng A 1991 A147 55ndash

65

19 S N Monteiro and R E Reed-Hill Met Trans 1971 2 2947ndash

2949

20 P Ludwik lsquoElement der technolnischen mechanickrsquo 32 1909

Berlin Julius Springer

21 E Girault P Jacques P Harlet K Mols J Van Humbeeck

E Aernoudt and F Delannay Mater Charact 1998 40 111ndash118

22 S Kim and S Lee Metall Mater Trans A 2000 31A 1753ndash1760

23 S Sankaran S Sangal and K A Padmanabhan Mater Sci








operating voltage of 200 kV Energy dispersive Xndashray

spectroscopic analysis (EDS) was conducted to deter-

mine the composition of the precipitates

Room temperature tensile testing was conducted

using the Instronndash4204 testing machine with a crosshead

velocity of 05 mm min21 The test specimen was

prepared as per the ASTM Standard (ASTM

Vol 0301 E8Mndash96) The error in yield strength (YS)and ultimate tensile strength (UTS) measurement was

recorded asiexcl3 and the same for per cent elongation

was iexcl5

Results

Tensile propertiesFigures 1ndash4 present the results obtained from the tensile

testing of the experimental steels Figure 1 shows that

the addition of Ni in 15CundashTindashB steel has improved the

YS values for both the DAC and aged samples For all

the samples a significant improvement of YS values has

been achieved after aging Figure 2 reveals that both

microalloying and aging have generally contributed to

the improvement of the UTS values Figures 3 and 4

show that in DAC samples microalloying of 15Cu steel

has lowered the elongations and the addition of Ni

in 15CundashTindashB steel has simultaneously improved

However for all the three aged steels the elongation

values are comparable It is important to note that

among the three compositions 15CundashNindashTindashB steel has

yielded the best combination of strength and ductility

for both the DAC and aged samples

Work hardening behaviour of directly air cooledsteelsWork hardening behaviour is generally expressed by

Hollomon equation416 that describes the true stress

1 Yield strength values for various investigated steels 2 Ultimate tensile strength values for various investi-

gated steels

3 Ultimate elongation UEL () values for various inves-

tigated steels

Table 1 Chemical composition of investigated steels wt-

Steel identification C Mn Si S P Ti B Cu Ni N

15Cu 004 160 048 0022 0014 ndash ndash 151 ndash 0005115CundashTindashB 004 169 057 0021 0013 0032 00013 154 ndash 0008015CundashNindashTindashB 003 168 053 0020 0013 0032 00012 155 079 00058






























behaviour


















precipitate









Conclusions









A 2001 32A 275922768

















257ndash261




1979 10A 259ndash261



65


2949





































behaviour


















precipitate









Conclusions









A 2001 32A 275922768

















257ndash261




1979 10A 259ndash261



65


2949


















precipitate









Conclusions









A 2001 32A 275922768

















257ndash261




1979 10A 259ndash261



65


2949














Conclusions









A 2001 32A 275922768

















257ndash261




1979 10A 259ndash261



65


2949










mechanical properties of directly air cooled copper added microalloyed steels (dual phase)

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