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Materials Science and Engineering A 527 (2010) 4341–4346

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

nfluence of thermomechanical processing and heat treatments on tensile andharpy impact properties of B and Cu bearing high-strength low-alloy steels

youngchul Hwang ∗, Chang Gil Leeerrous Alloys Group, Korea Institute of Materials Science, 531 Changwondaero, Changwon 641-831, Republic of Korea

r t i c l e i n f o

rticle history:eceived 2 February 2010eceived in revised form 16 March 2010ccepted 30 March 2010

a b s t r a c t

High-strength low-alloy steels containing different B and Cu contents were fabricated by thermomechan-ical processing and heat treatments such as accelerated cooling, intercritical annealing, and tempering,and then tensile and Charpy impact tests were conducted on them in order to investigate the mechan-ical properties in terms of yield strength, deformability, and toughness. The test results revealed thatthe B-added acceleratedly cooled specimens composed mostly of lath martensite had relatively high

eywords:igh-strength low-alloy steelshermomechanical processingeat treatmentensile propertiesharpy impact properties

yield strength, poor deformability and low-temperature toughness in contrast to the B-free accelerat-edly cooled specimens consisting mainly of granular bainite and degenerate upper bainite. When theB-added acceleratedly cooled specimens were tempered, the yield strength increased to a certain degreeand the low-temperature toughness was remarkably improved, but the deformability severely deterio-rated. However, the B-added intercritically annealed specimens provided an attractive balance of yieldstrength, deformability, and low-temperature toughness due to the dual-phase microstructure of fine

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ferrite and martensite wi

. Introduction

With ever increasing environmental requirements, recycling ofteel scrap has been received greater attention since considerablemounts of commercial steels are produced from steel scraps con-aining residual elements, e.g. copper. In order to reduce alloyingost and to enhance the recyclability of steel scraps, extensivefforts have been exerted over the past several decades [1–8].resently, it is well established that a small amount of B improvesfficiently the hardenability of steel because it delays the nucle-tion of ferrite on the transformation from austenite to ferrite dueo its segregation at austenite grain boundary [2–5]. Furthermore,he addition of Cu may contribute to solid solution strengtheningnd precipitation strengthening by very fine Cu particles, althoughxcess Cu causes brittleness on hot working [5–8]. Therefore, theevelopment of B and Cu bearing high-strength low-alloy (HSLA)teels with better mechanical properties is very attractive from theconomical and environmental points of view.

On the other hand, HSLA steels used for bridge, building, pres-

ure vessel, offshore and pipeline structures have steadily requiredo have higher strength as well as enhanced low-temperatureoughness and improved weldability. In recent years, there haseen a growing demand for sufficient deformability in steel struc-

∗ Corresponding author. Tel.: +82 55 280 3437; fax: +82 55 280 3599.E-mail address: entropy0@kims.re.kr (B. Hwang).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.03.106

h type morphology.© 2010 Elsevier B.V. All rights reserved.

tures subjected to the progressive and abrupt displacementsresulting from ground movement, earthquake, and load itself[9,10]. Yield to tensile strength ratio, uniform elongation, andwork hardening exponent are considered as important criterionsto evaluate deformability. Since an improvement in deformabilityis often accompanied by the deterioration of strength and tough-ness [11–14], thermomechanical processing and heat treatmenttechniques can be used as one of the most promising methods toachieve a good balance of mechanical properties such as strength,toughness, and deformability.

In the present study, HSLA steels with different B and Cu con-tents were fabricated by varying thermomechanical processing andheat treatments, and then their microstructures were examinedusing scanning and transmission electron microscopes. Tensile andCharpy impact tests were conducted on them in order to investigatethe influence of thermomechanical processing and heat treatmentson microstructure and mechanical properties in terms of yieldstrength, deformability, and toughness.

2. Experimental

2.1. Materials

Six steel ingots with 100 mm in thickness containing differ-ent B and Cu contents, as given in Table 1, were prepared bymelting in vacuum-induction furnace. They were homogenized at1150 ◦C for 2 h and controlled rolled to a final thickness of 15 mm.

4342 B. Hwang, C.G. Lee / Materials Science and E

Table 1Chemical composition of steels investigated in this study (wt.%).

Steel* C Si Mn Ni + Cr + Mo Nb + V + Ti Cu B

0Cu 0.07 0.25 1.9 0.6–0.8 0.09–0.11 0 00.5Cu 0.51.5Cu 1.50Cu 0 0.001

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0.5Cu–B 0.51.5Cu–B 1.5

* N < 30, P < 100, S < 30 (parts per million).

hey were expected to have an overall grain refinement effect byolling under the rolling reduction ratio more than 50 pct in theon-recrystallization region of austenite [11–13]. Both B-free and-added specimens were acceleratedly cooled to ambient temper-ture below 300 ◦C with a cooling rate of at least 20 ◦C/s followedy air cooling. After the accelerated cooling, the B-added acceler-tedly cooled specimens were intercritically annealed at 780 ◦C for0 min or tempered at temperatures between 400 and 700 ◦C for0 min, followed by water quenching. For convenience, the accel-ratedly cooled, intercritically annealed, and tempered specimensre referred to as ‘AC’, ‘IA’, and ‘T’ specimens, respectively, as shownn Fig. 1.

.2. Microstructural characterization

Longitudinal-short transverse planes of the heat-treated speci-ens were polished and etched by a 2 pct nital solution, and then

heir microstructures were observed in a field-emission scanninglectron microscope (FE-SEM, model: JSM-7001F, JEOL). A moreetailed metallographic examination was performed on selectedpecimens using a transmission electron microscope (TEM, model:EM-2100F, JEOL). Thin foils for TEM observations were prepared intwin-jet electrolytic polishing apparatus using a solution contain-

ng 5 vol. pct perchloric acid and 95 vol. pct methanol, and examinedt 200 kV. The selected area diffraction patterns were analyzedsing Desktop Microsoft V2.2 software (Lucuna Laboratory).

.3. Mechanical testing

Sub-size round tensile test specimens with a gage diameter of.3 mm and a gage length of 25.4 mm were prepared in transverseirection, and tested at room temperature and a crosshead speedf 5 mm/min using a 10-ton universal testing machine (Model:nstron 5882, Instron Co.) in accordance with ASTM E8-04.

Fig. 1. Schematic illustration of thermomech

ngineering A 527 (2010) 4341–4346

For Charpy impact tests, full-size standard Charpy V-notch spec-imens with a 10 mm × 10 mm × 55 mm size were machined in thetransverse-longitudinal orientation according to ASTM E23-02, andwere tested at room temperature and low temperature (approxi-mately −20 ◦C) using an impact tester with 100 J capacity.

3. Results and discussion

3.1. Microstructure

Fig. 2 shows SEM micrographs of the B-free and B-addedacceleratedly cooled, intercritically annealed, and tempered spec-imens. The B-free acceleratedly cooled specimens consist mainlyof granular bainite and degenerate upper bainite, while the B-added acceleratedly cooled specimens are composed mostly oflath martensite with a small amount of lower bainite (Fig. 2aand b). This indicates that the addition of B promotes the forma-tion of relatively hard low-temperature transformation productsby improving hardenability [11–13]. The B-added intercriticallyannealed specimens show dual-phase structure of ferrite andmartensite with lath type morphology because the structure exist-ing prior to annealing was martensite (Fig. 2c). When the B-addedacceleratedly cooled specimens are reheated to the two-phase(� + �) region, austenite nucleates at the prior martensite lathboundary and grows along the lath boundary, thereby formingfibrous fine martensites. On the other hand, the B-added tem-pered specimens have tempered martensitic structure where themartensitic laths are usually recovered, and dislocation tangles aresubstantially eliminated due to the movement of dislocations afterthe B-added acceleratedly cooled specimens were tempered at dif-ferent temperatures [7,8] (Fig. 2d).

Fig. 3a and b is bright-field TEM micrographs showing typicalgranular bainite and lath martensite in the B-free and B-addedacceleratedly cooled specimens, respectively. The granular bainite,as the name implies, consists of equiaxed bainitic ferrites with dis-persed martensite or martensite-austenite (MA) constituents. Thebainitic ferrite grains are coarse with an average grain size of over2 �m because they are transformed at relatively high temperaturescompared to the other austenite transformation products [13,15].The degenerate upper bainite is composed of packets of paral-

lel ferrite laths where continuous interlath metallic phases suchas MA constituents are present [13,16]. The lath martensitic haswell-developed cementite precipitates as auto-tempering occurredduring air cooling after accelerated cooling (Fig. 3b). Fig. 3c and dexhibits bright-field TEM micrographs and selected area diffrac-

anical processing and heat treatments.

B. Hwang, C.G. Lee / Materials Science and Engineering A 527 (2010) 4341–4346 4343

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ig. 2. SEM micrographs of the (a) 0.5Cu and (b) 0.5Cu–B acceleratedly cooled spe50 ◦C after accelerated cooling.

ion pattern indicating the precipitation of alloy carbides in the-added specimens tempered at 550 ◦C. The result reveals that the-added tempered specimens may be secondarily strengthened by-Cu precipitates and the carbides or carbonitrides of Nb, Ti, V, ando [7,8,11–13].

.2. Tensile behaviors

The tensile stress–strain curves of the specimens investigatedn this study are provided in Fig. 4. According to the yielding andtrain hardening behaviors observed from the stress–strain curves,he specimens show continuous yielding behavior with the excep-

ig. 3. TEM micrographs showing (a) granular bainite and the corresponding selected aricrostructure in the 1.5Cu and 1.5Cu–B acceleratedly cooled specimens, respectively, an

nd the corresponding selected area diffraction pattern (Z = [1 1 1]˛//[1 0 0]MC) indicating

s, (c) 1.5Cu–B specimen annealed at 780 ◦C, and (d) 0Cu–B specimen tempered at

tion of the B-added tempered specimens. It has been known thatthe continuous yielding behavior is attributed to the presence ofmobile dislocations in the ferrite matrix where the dislocationsusually generate during the transformation of austenite to stronglow-temperature transformation products [11–14].

In the B-added tempered specimens, the upper yield pointappears and becomes clear with increasing Cu content (Fig. 4d).

Recently, Yan et al. [7] reported that iron carbide precipitatesformed during tempering acted as strong pinning obstacles fordislocations glide and thus the mobile dislocations density wouldbe strongly reduced, and suggested that the upper yield pointoccurred due to an increase in stress required to drive the dis-

ea diffraction pattern (Z = [1 1 1]˛//[1 1 0]� ), and (b) autotempered lath martensited (c) alloy carbide precipitated inside lath martensite and (d) the bright-field imagematrix and (Nb, Ti)(C,N) in the 0.5Cu–B specimen tempered at 550 ◦C.

4344 B. Hwang, C.G. Lee / Materials Science and Engineering A 527 (2010) 4341–4346

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ig. 4. Room-temperature tensile stress–strain curves of the (a) B-free and (b) B-adnd (d) annealed at 780 ◦C after accelerated cooling.

ocations off the pinning to move. When the dislocations breakway from the pinning precipitates, the mobile dislocations den-ity increase again, thereby decreasing the stress required forliding dislocations. Accordingly, it is reasonable that the upperield point appears in the tensile stress–strain curves of the B-dded tempered specimens. The yield and tensile strengths of allhe specimens primarily increase with Cu content and hence the.5Cu and 1.5Cu–B specimens have the highest strength becausehe addition of Cu enhances hardenability, and thus contributeso solid solution strengthening and transformation strengthen-ng [4,6], although they are differently varied in accordance with

icrostructure (Fig. 2).

.3. Tensile properties

Fig. 5a and b presents combination of yield strength and uniformlongation, and yield strength and yield ratio. The yield strength

ig. 5. Combination of the (a) yield strength and uniform elongation and the (b) yield strnnealing, and tempering, respectively.

cceleratedly cooled specimens, and the B-added specimens (c) tempered at 550 ◦C

tends to be disproportionate to uniform elongation, whereas it isproportional to yield ratio. The B-free acceleratedly cooled speci-mens consisting of granular bainite and degenerate upper bainiteshow the yield strength of 550–750 MPa, uniform elongation of5.0–9.0%, and yield ratio of 0.7–0.8, indicating good deformabil-ity. The yield strength, uniform elongation, and yield ratio of theB-added acceleratedly cooled specimens composed mostly of lathmartensite are 900–1050 MPa, 3.0–4.0%, and 0.8–0.9, respectively.Therefore, the B-free acceleratedly cooled specimens have a loweryield strength than the B-added acceleratedly cooled specimens,but provide a better deformability of high uniform elongationand low yield ratio. In the case of the B-free acceleratedly cooled

specimens, many mobile dislocations are present near interfacesbetween ferrite and MA constituents within granular bainite anddegenerate upper bainite. As a result, they have a low yield ratiobecause the dislocations readily move and thus decrease yieldstrength. Generally, yield ratio, i.e. the ratio of yield strength to

ength and yield ratio. The ‘AC’, ‘IA’, and ‘T’ indicate accelerated cooling, intercritical

B. Hwang, C.G. Lee / Materials Science and Engineering A 527 (2010) 4341–4346 4345

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ig. 6. Combination of the (a) yield strength and Charpy impact energy absorbed atemperature (approximately −20 ◦C). The ‘AC’, ‘IA’, and ‘T’ indicate accelerated cool

ensile strength, and uniform elongation are closely associated withhe safety of structural materials [9–11].

When the B-added acceleratedly cooled specimens are tem-ered, the yield strength increases up to 1200 MPa, but the uniformlongation decreases and the yield ratio largely increases, as com-ared to the B-added acceleratedly cooled specimens, because ofhe formation of tempered martensite and of the precipitation oflloy carbides or carbonitrides. Since a minimum uniform elonga-ion of 5.0% and a maximum yield ratio of 0.93 are regarded as apecification for strain-based design of pipeline [5], the B-addedempered specimens are not suitable for achieving high deforma-ility.

On the other hand, the yield strength of the B-added inter-ritically annealed specimens is intermediate between that of the-free and B-added acceleratedly cooled specimens, and the uni-

orm elongation is from 5.0 to 6.5%. The B-added intercriticallynnealed specimens also have a relatively low yield ratio for theame level of yield strength, indicating a better deformability. Con-equently, they have a good balance of yield strength, uniformlongation, and yield ratio, which is possibly due to the dual-phaseicrostructure of fine ferrite and martensite with lath type mor-

hology. Dual-phase microstructure has been generally known toave a high deformability because it offers superior strain capacityue to the initial yielding in soft ferrite phase and the subsequenttrain hardening during load transfer to hard phase [11–14].

.4. Charpy impact properties

Fig. 6a and b shows combination of yield strength and Charpympact energy absorbed at room and low temperatures. The B-freecceleratedly cooled specimens consisting of granular bainite andegenerate upper bainite have impact energy of 100–200 J, whilehe B-added acceleratedly cooled specimens composed mostly ofath martensite have an impact energy of 50–100 J. The impactnergy of the B-added acceleratedly cooled specimens is lowerhan that of the B-free acceleratedly cooled specimens irrespec-ive of test temperature because relatively hard low-temperatureransformation phases have lower impact energy [13,16]. In the B-dded tempered specimens, the impact energy absorbed at roomnd low temperatures significantly increases up to about 250 Jithout sacrificing the yield strength, but their deformability is

ery poor, as compared to the B-added acceleratedly cooled spec-mens. On the other hand, both yield strength and Charpy impact

nergy of the B-added intercritically annealed specimens havingual-phase structure composed of ferrite and martensite are inter-ediate between those of the B-free and B-added acceleratedly

ooled specimens, indicating an impact energy of approximately00 J.

temperature and the (b) yield strength and Charpy impact energy absorbed at lowtercritical annealing, and tempering, respectively.

To conclude, the B and Cu bearing HSLA specimens investigatedin this study had a wide variety of mechanical properties of yieldstrength, uniform elongation, yield ratio, and low-temperaturetoughness, which were affected by microstructure depending onthermomechanical processing and heat treatments. These findingsrepresent that the dual-phase microstructure of fine ferrite andmartensite with lath type morphology is suitable for providingan attractive balance of yield strength, deformability, and low-temperature toughness.

4. Conclusions

The conclusions of this work are the following:

1. The B-added acceleratedly cooled specimens composed mostlyof lath martensite showed relatively high yield strength, butprovided a poor deformability of low uniform elongation andhigh yield ratio, as compared to the B-free acceleratedly cooledspecimens, because the addition of B promoted the formationof relatively hard low-temperature transformation products byimproving hardenability.

2. Since a number of mobile dislocations were formed nearinterfaces between bainitic ferrite and martensite–austeniteconstituent within the granular bainite and degenerate upperbainite and thus contribute to decreasing yield strength, theB-free acceleratedly cooled specimens mainly consisting of gran-ular bainite and degenerate upper bainite showed the low yieldratio, indicating a better deformability.

3. When the B-added acceleratedly cooled specimens were tem-pered, the Charpy impact energy was remarkably improved andupper yield point appeared. However, tempering at differenttemperatures decreased the uniform elongation and increasedthe yield ratio, thereby leading to the deterioration of deforma-bility.

4. Among the specimens investigated in this study, the B-addedintercritically annealed specimens showed an attractive balanceof yield strength and deformability as well as Charpy impactenergy due to the dual-phase microstructure of fine ferrite andmartensite with lath type morphology.

Acknowledgements

This work was financially supported by the Ministry of Knowl-edge Economy, Korea. The authors would like to thank Mr. HongDong Kim of Korea Institute of Materials Science for his help with

heat treatments and tensile tests.

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