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Materials Science and Engineering A 476 (2008) 126–131

Influence of different heat treatment parameters on microstructureand mechanical properties of C–Mn strapping quality steels

S.N. Prasad a,∗, A. Saxena a, M.M.S. Sodhi b, P.N. Tripathi b

a Research and Development Centre for Iron and Steel, Steel Authority of India Limited, Ranchi 834002, Indiab Bokaro Steel Plant, Steel Authority of India Limited, Bokaro Steel City 827001, India

Received 28 December 2006; received in revised form 18 April 2007; accepted 19 April 2007

bstract

Strap steel produced at Bokaro Steel Plant is a plain C–Mn (nominal chemistry C: 0.35%, Mn: 1.35% and Si: 0.25%) cold rolled steel that isustenitised at 930 ◦C and austempered in a lead bath at 450 ◦C. The microstructure consists of ferrite, pearlite and bainite also to achieve ultimateensile strength (UTS) 900 MPa min and elongation 8% min on 150 mm gauge length. In order to achieve such a microstructure consistently,nfluence of chemistry and different heat treatment parameters on strapping quality steels have been studied. It has been found in the laboratoryhat increase in Mn (from 1.35 to 1.55%) and Si (from 0.25 to 0.45%) in a 0.32% C strap steel results in 100% bainite in the microstructurefter austenitisation and subsequent air cooling. The austempering of the modified strap steel (0.4% C, 1.65% Mn and 0.5% Si) at Bokaro Steellant results in excessively high UTS (1040–1100 MPa) without affecting the elongation value (8–10%, 150 mm GL). The microstructure of the

ustempered straps of modified chemistry consists of cent percent bainite. The austenitisation and subsequent air cooling without austemperingf the modified strap steel result in a microstructure consisting of ferrite, pearlite and bainite. The UTS (867–875 MPa) achieved marginally fallshort of the specified values (900 MPa min) while % elongation values (8–10%, 150 mm GL) conform to the IRS: P-41 specification. 2007 Elsevier B.V. All rights reserved.

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eywords: Strap steel; Austempering; Microstructure; Mechanical properties

. Introduction

Strap steel is produced through BOF-CC-HSM-CRM-slittingoute at BSL. The straps cut from the cold rolled sheet areubsequently austenitised at 930 ◦C and austempered passinghrough a lead bath maintained at 450 ◦C. The microstructureonsists of ferrite–pearlite with some amount of bainite whichs needed for high strength (900 MPa) with elongation ≥8%150 mm GL). Bainitic transformation takes place at tempera-ures between those of ferrite–pearlite reaction and martensiteeaction. Therefore, it is possible to produce a range of strengths

xtending between the highest strength in the ferrite pearlitetructures and the strength of martensite by varying the temper-ture at which austenite transforms to ferrite [1–3]. Production

Abbreviations: �, ferrite; M/A, martensite/austenite; UTS, ultimate tensiletrength; YS, yield strength; SEM, scanning electron microscopy; GL, gaugeength; IRS, Indian Railways Specification; BOF, basic oxygen furnace∗ Corresponding author. Tel.: +91 9431701971/6512411132x2294;

ax: +91 6512411064.E-mail address: snprasad1002@rediffmail.com (S.N. Prasad).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.04.071

f bainitic structure by air cooling eliminates economic disad-antage of quenching and subsequent tempering/austempering.t also prevents quench cracking and minimizes distortion dur-ng heat treatment. Use of lower carbon content or lowerarbon equivalent in bainitic steel is required for achievingood weldability and formability. For achieving properties withinimum of variation over wide range of section size, the

ainitic transformation C curve with a flat top is required.he highest possible bainitic hardenability coupled with theinimum martensitic hardenability is needed to obtain the max-

mum impact properties consistent with the strength [4] levelschieved.

It is well established that addition of about 0.002% B retardshe polygonal ferrite transformation which nucleates at theustenite grain boundary. In fact, boron being in solution inustenite, segregates to the austenite grain boundaries at whichhe polygonal ferrite nucleates. Boron in the form of oxide or

itride is useless and, therefore, the steels must be aluminiumilled. High nitrogen (>30 ppm) steels are treated with Ti to sup-ress formation of boron nitride [5] which decreases the amountf boron in solution. Boron factor decreases with the increase of

and Engineering A 476 (2008) 126–131 127

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Table 1Chemical compositions (wt.%) of heats of straps steels

Steel C Mn S P Si Al B

1 0.32 1.55 0.026 0.048 0.46 0.04 Trace2 0.36 1.26 0.033 0.029 0.39 0.08 0.00163 0.26 1.41 0.025 0.03 0.52 0.025 0.001445

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S.N. Prasad et al. / Materials Science

arbon in steel and is about unity for C = 0.90% (i.e. about theutectoid carbon content) [6,7].

In addition to the B, Mn, Mo and Cr additions, microalloyingy Nb increases bainitic hardenability [8–11]. The combinedddition of 0.024% Nb with about 0.5% increase over 0.45%f Mn content in Cor-Ten A steel could change completely theerrite–pearlite microstructure to a portion of so called granu-ar bainite on hot rolling [12,13]. Additions of these elementsn the Cor-Ten B steel also have increased bainitic hardenabil-ty and produce bainite–ferrite dual phase steel on hot rolling14]. The structure consists of polygonal ferrite/acicular ferritend the high carbon micro-constituent composed of marten-ite/austenite (M/A) constituent [13–20]. In a recent work atokaro Steel Plant [21], austempering of 0.4% C and 1.35% Mn

trap steel at 450 ◦C in a lead bath after austenitisation at 930 ◦Cas resulted in a microstructure consisting of ferrite–pearliteith some amount of bainite. The austenitisation temperature

930 ◦C) was higher to allow dissolution of all cementite beforehe strap comes out of the austenitisation furnace. The lead bathemperature was optimized to be 450 ◦C for the required prop-rties of the strap. The UTS and % elongation of this steel, soreated, were generally more than 900 MPa and 9% (150 mmL), respectively. However, due to some variations in shoparameters including austenitisation and austempering temper-tures, consistency in properties was not achieved in about 20%f straps produced.

In view of the above, a study was taken up to examine thenfluence of the chemistry (Mn, Si and B) and the heat treatmentarameters on the strap steel in the laboratory and subsequentlyn the steel plant. Based on the laboratory study, the chemistryas modified for heat making in steel plant. The austemper-

ng of the modified steel has resulted in excessively higherroperties than specified in IRS: P-41 specification. Therefore,xperiments on production of simply air-cooled straps afterustenitisation were also carried out which gave encouragingesults. The microstructure and mechanical properties of thetraps produced with different heat treatments are discussed inhis paper.

. Experimental

A batch of three steels (designated as steels 1, 2 and 3 inable 1) with varying amounts of C, Mn and Si were made

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Fig. 1. Schematic diagram of austemp

0.43 1.62 0.003 0.038 0.48 0.032 –0.40 1.35 0.025 0.025 0.2 0.03 –

y melting in 35 kg vacuum induction furnace and cast intongots. The defective portions of the ingots comprising about0% of the height from bottom and 25% from top were dis-arded. The remaining portions of the ingots were soaked at250 ◦C for 2 h and hot rolled to sheets with a final thickness of.4 mm in multipasses (over an approximate temperature rangef 1200–850 ◦C) in an experimental rolling mill. Subsequently,he hot rolled sheets were pickled and cold rolled in multipasseso 1.27 mm thick sheets. Thirty-two millimeter wide straps fromll the three types of steels were cut. They were austenitised inmuffle furnace for 15 min at 930 ◦C and air cooled.

The tensile properties of the heat-treated samples werevaluated using an Instron machine model (1273) at constantross-head speed of 2 mm m−1. Standard specimens of 50 mmauge length were used for tensile tests. Pieces were cut from theeat-treated samples for optical microscopy. They were mechan-cally polished and etched with 2% nital. Optical microstructuresf the steels were observed with a NEOPHOT metallograph.

Having assessed the microstructure and mechanical proper-ies of the above three steels made in the laboratory, an industrialeat (steel 4 in Table 1) was made at Bokaro Steel Plant throughOF-CC-HSM-CR route. Slitting of the cold rolled coil was car-

ied out as usual in 32 mm straps. Heat treatment of the strapsas carried out in the austempering line (Fig. 1) of Bokaro Steellant as per the following parameters:

1) Uncoiling → austenitisation of moving strap at 930 ◦C →austempering of moving strap through lead bath maintainedat 450 ◦C → air cooling → coiling.

2) Uncoiling → austenitisation of moving strap at 930 ◦C →air cooling of strap passing through open windows ofexhaust hoods over the lead bath → coiling (faster air cool-ing without austempering).

ering unit at Bokaro Steel Plant.

128 S.N. Prasad et al. / Materials Science and Engineering A 476 (2008) 126–131

Table 2Mechanical properties of cold rolled sheet austenitised and air cooled in thelaboratory

Steel YS (MPa) UTS (MPa) % El (50 mm GL)

1 644 801 14.34681 850 15.5

2 524 719 11.52601 722 15.2

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3) Uncoiling → austenitisation of moving strap at 930 ◦C →air cooling of strap passing through closed windowsof exhaust hoods over the lead bath → coiling (withoutaustempering).

After the above heat treatments, tensile testing of the strapsas carried out using specimens of 150 mm gauge length. As

bove, samples for metallography were prepared and etched with% nital. Optical microscopy and scanning electron microscopyere carried out to study the different micro-constituents present

n the steel.

. Results and discussion

The yield strength (YS), ultimate tensile strength (UTS) andelongation of the three laboratory steels (1–3, Table 1) are

resented in Table 2. The data presented are for the two samplesested in the case of each steel. The YS (644, 681 MPa) and UTS801, 850 MPa) of steel 1 are the highest, though there is wideariation in the results of two samples tested. The % elongationor all the steels is about 15% except one sample of steel 2 whichs 11.5%.

The optical micrographs of steels 1–3 (Table 1) are shown inig. 2a–c, respectively. Steel 1 shows a microstructure consistingf cent percent upper bainite (Fig. 2a). The microstructures ofteels 2 and 3 consist of mostly ferrite and pearlite (Fig. 2b and, respectively).

The mechanical properties achieved in the plant trials of mod-fied strap steel 4 (Table 1) are given in Table 3. The strapustenitised at 930 ◦C, austempered at 450 ◦C in lead bath andubsequently air cooled (treatment 1 in Table 3) has very highTS (1040–1100 MPa) without any sacrifice in the elongation

9–10%). On the other hand, when the same strap after austeni-ising at the same temperature (930 ◦C) was simply air cooledwithout austempering) passing through holes made in open win-ows exhaust hoods over the lead bath (treatment 2 in Table 3),he UTS and % elongation achieved were 867–875 MPa and–11%, respectively. In the next experiment, the same strapith the same austenitisation temperature (930 ◦C) when passed

hrough closed window hoods (i.e. without ventilation, lesserooling rate) resulted in slightly lower UTS (845–850 MPa) with

–11% elongation level. The UTS and elongation achieved inhe conventional strap steel no. 5 in Table 1 are in the range of70–970 MPa and 8–10%, respectively, when all the processingarameters are closely maintained.

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ig. 2. Microstructures of laboratory steels showing: (a) completely upper bai-ite in steel 1, (b) mostly ferrite–pearlite in steel 2 and (c) mostly ferrite–pearliten steel 3.

The optical micrographs of plant trial samples are given inig. 3. Fig. 3a shows very uniformly distributed carbides inustempered-modified steel (treatment 1) and it appears to beent percent bainite. Fig. 3b also shows bainite along with someerrite grains in the strap austenitised and air cooled with openindows of exhaust hoods (treatment 2). Fig. 3c shows similaricrostructure in austenitised and air-cooled sample with closedindows of exhaust hoods (treatment 3) but the ferrite grains are

elatively more in this case. Fig. 4 shows banded microstructuren air-cooled straps produced through closed windows of exhaustoods. The SEM micrographs of the straps produced with threeifferent types of treatments are presented in Fig. 5. Fig. 5a

S.N. Prasad et al. / Materials Science and Engineering A 476 (2008) 126–131 129

Table 3Mechanical properties of straps heat treated in austempering line, R&R shop, BSL

Sample number/treatment number Treatment/IRS: P-41 specification UTS (MPa) % El (150 mm GL)

1 Austenitisation: 930 ◦C, austempering: 450 ◦C in lead bath 1040–1100 9–102 Austenitisation: 930 ◦C air cooling with open windows of exhaust

hoods on the way of moving strap867–875 10–11

3 Austenitisation: 930 ◦C air cooling with closed windows of exhaust 845–850 9–11

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hoods on the way of moving strapConventional steelIRS: P-41 specification

hows almost cent percent non-lamellar carbides in austempered

traps (sample 1 in Table 3). Fig. 5b and c shows microstructuresonsisting of lamellar as well as non-lamellar carbides indicat-ng presence of pearlite and bainite in addition to ferrite in their-cooled strap (without austempering, sample 2 in Table 3)

ig. 3. Microstructures of steel 4 (steel plant heat) showing: (a) mostly bainite asresult of treatment 1, (b) ferrite grains in bainite/pearlite as a result of treatmentand (c) slightly more and bigger ferrite grains in bainite/pearlite as a result of

reatment 3.

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roduced with open windows of exhaust hoods. Fig. 5d showsimilar SEM micrograph consisting of ferrite, pearlite and bai-ite observed in the air-cooled strap produced closed windowsxhaust hoods (sample 3 in Table 3).

The microstructure of steel 1 appears to be completely bainiteFig. 2a) where as steels 2 and 3 very clearly show polygonalerrite and pearlite grains in Fig. 2b and c, respectively. Whilexamining the chemistry of the three laboratory steels, steel 1ontains maximum amount of Mn (1.52%), the Si content is.46% where as C content of 0.32% is less than steel 2 (0.36%)ut more than steel 3 (0.26%). Thus, it appears that Mn hasignificant contribution in increasing bainitic hardenability ofteel 1 which has resulted completely in bainitic transformationfter 930 ◦C austenitisation and subsequent air cooling. On thether hand, steel 2 having 0.36% C, 1.26% Mn, 0.39% Si anddditionally 16 ppm of B, which is well known for increasingardenability, could produce only polygonal ferrite pearlite. Theddition of 14 ppm of B in steel 3 with 0.26% C, 1.41% Mnnd 0.52% Si behaved in a manner similar to steel 2 leading toredominantly polygonal ferrite and pearlite microstructure.

While examining the mechanical properties of the threexperimental steels (Table 1), the UTS of steel 1 is the high-st of all (801, 850 MPa) with an elongation level of ∼15%50 mm GL). This is quite obvious because the microstructuref this steel consists of cent percent bainite. The UTS of steelis about 720 MPa. Lower UTS of this steel is attributed to

he lower Mn content (1.26%) of this steel, though it has gotdditionally 16 ppm of B. Thus, Mn appears to be very effec-ive for increasing bainitic hardenability of the steel. Due to

ig. 4. Banded microstructure of strap produced on air cooling passing throughlosed windows of exhaust hoods (treatment 3).

130 S.N. Prasad et al. / Materials Science and Engineering A 476 (2008) 126–131

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ig. 5. SEM micrographs of steel 4 (steel plant heat) showing: (a) non-lamellar carbides in bainite/pearlite formed as a result of treatment 2 and (d) non-lamell

ower C (0.26%) and lower Mn (1.41%) even with 14 ppmf B, steel 3 has got the lowest UTS (about 651 MPa). Theicrostructure is also ferrite–pearlite which results in lowerTS. The ductility (% elongation) of all the steels is similar

∼15%). Therefore, the microstructure of steel 1 is very use-ul to achieve a high strength–ductility combination. Generallyn ferrite–pearlite steel, when the UTS increases, the ductility% elongation) decreases. Therefore, the kind of microstructureeveloped in steel 1, simply after austenitisation and air cool-ng in the laboratory, is desirable for the strap steel producedt BSL, Bokaro, as it has got the highest strength without anyacrifice in the ductility. However, as per the IRS: P-41 speci-cation the required UTS is 900 MPa with 8% elongation (GL50 mm). Thus, though the % El (15%, 50 mm GL) is enough,he UTS falls short of specified value in IRS: P-41 specification.o compensate for the lower strength, the steel was redesignedith little higher Mn and Si (for solid solution hardening). Theodified strap steel 4 (Table 1) was processed through BOF-C-HSM-CRM-slitting route and subsequently different heat

reatments were carried out at Bokaro Steel Plant.The high UTS (1040–1100 MPa) (Table 3) with adequate

uctility (9–10% elongation) is attributed to the bainiticicrostructure (Figs. 3a and 5a) achieved in the austempered

trap with modified chemistry (steel 4). However, as per theRS: P-41 specification, the requirements of UTS and % elon-ation are only 900 MPa and 8% (150 mm GL), respectively.herefore, having achieved such over strength after austemper-

ng, treatment 2 (air cooling) of the straps was carried out which

esulted in UTS 867–875 MPa and elongation 9–11%. The low-ring of UTS is quite expected as transformation takes place atrange of temperature during air cooling. Before it comes to BS

emperature some amount of ferrite and pearlite is also expected.

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es of bainite as a result of treatment 1, (b and c) non-lamellar as well as lamellarlamellar carbides in bainite formed as a result of treatment 3.

The optical micrograph for treatment 2 (Fig. 3b) shows fer-ite grains and the SEM micrograph (Fig. 5b) shows pearliteamellae. Similarly, in case of treatment 3 (slow air cooling),he lesser UTS (845–850 MPa) is attributed to relatively moreerrite grains which are also slightly bigger in size (Fig. 3c).hus, the modified strap steel has resulted in excessively highTS (1040–1100 MPa) after austempering and slightly lesserTS (867–875 MPa) than specification (900 MPa) simply after

ustenitisation and air cooling.Banded microstructure (Fig. 4) has been observed at few

ocations in the case of treatment 3. This is due to slow cool-ng rate in this case. During early solidification stage, dendritictructure leads to rejection of Mn in the interdendritic regions22,23]. Due to its low diffusivity in austenite phase, Mnemains segregated on subsequent solidification of interden-ritic region and Mn rich regions spread out across the platehickness due to heavy deformation in the hot rolling [24].

hen such steel is austenitised and air cooled, Mn depletedegions (with higher Ar3 temperature) transform to ferrite beforeransformation of Mn rich areas (with lower Ar3 temperature).he carbon atoms rejected from Mn depleted austenite tend

o diffuse and segregate to the Mn rich area where transfor-ation occurs at lower temperature [25]. Subsequently, these

egregated carbon atoms form pearlite/cementite in the formf banded structure when the steel is cooled slowly belowr1.In view of the above studies, it appears that the conventional

ustempering process of manufacturing straps may be replaced

y a much simpler and economical process of austenitisation andir cooling of straps. It may require one more modification inhemistry keeping in mind load of the cold rolling mill. Forcedir cooling of straps may also be helpful.

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S.N. Prasad et al. / Materials Science

. Conclusions

The following conclusions may be drawn based on the presenttudies on the manufacture of strap steel at BSL.

1) Increase in Mn (from 1.35 to 1.55%) and Si (from 0.2 to0.45%) in a 0.32% C strap steel results in formation of 100%upper bainite in the microstructure after austenitisation andsubsequent air cooling.

2) Austempering of the modified strap steel (0.4% C, 1.65%Mn and 0.5% Si) results in excessively high UTS(1040–1100 MPa) without affecting the elongation value(8–10%, 150 mm GL).

3) The microstructure of the austempered straps of modifiedchemistry consists of cent percent bainite.

4) Austenitisation and air cooling of the modified strap steelresult in a microstructure consisting of ferrite, pearlite andbainite. The UTS (867–875 MPa) achieved marginally fallsshort of the specified values (900 MPa min), while % elon-gation values (8–10%, 150 mm GL) conform to the IRS:P-41 specification.

cknowledgements

The taskforce expresses thanks to Mr. A.K. Singh DGMR&R) for his continuous help during the execution of theroject. Thanks are also due to Dr. Vinod Kumar, Sr. ManagerSP), for his help during cold rolling at DMRL Hyderabad. Theaskforce also acknowledges gratefully the encouragement andupport provided by managements of BSL and RDCIS.

eferences

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Engineering A 476 (2008) 126–131 131

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