IM465 – Conformação Plástica dos Metais – 2º semestre de 2011 – Segundo Artigo

Exercício de caráter estritamente individual. Envie-o para meu e-mail até 07/10/2011

O artigo a seguir apresenta a influência da adição de cobre e fósforo como elementos de liga

de um aço bainítico de baixo-carbono

Responda as seguintes questões considerando o conteúdo do artigo e das aulas da

disciplina:

1) Segundo os autores, quais os objetivos da adição de cobre e fósforo nesse aço?

Como esses elementos beneficiam ou prejudicam as propriedades mecânicas em

função das porcentagens adicionadas?

2) Qual foi o procedimento experimental para a obtenção das chapas e como elas

foram analisadas?

3) Os autores conseguiram demonstrar as modificações de propriedades previstas com

a adição dos elementos de liga?

4) Quais foram os mecanismos de amaciamento observados?

5) Como os átomos de cobre e fósforo afetaram os mecanismos de amaciamento,

particularmente a ocorrência de recuperação estática?

6) Associe o formato, o tamanho e a orientação das partículas de Nb(C,N) aos

mecanismos de endurecimento do aço.

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Materials Science and Engineering A 528 (2011) 6401– 6406

Contents lists available at ScienceDirect

Materials Science and Engineering A

jo ur n al hom epage: www.elsev ier .com/ locate /msea

echanical properties and hot-rolled microstructures of a low carbon bainiticteel with Cu–P alloying

.F. Cuia,∗, S.X. Zhanga,b, Y. Jiangc, J. Dongb, C.M. Liua

Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, ChinaTechnology Center of Laiwu Iron and Steel (Group) Co. Ltd., Laiwu 271104, ChinaSchool of Chemical Engineering, University of Queensland, Brisbane 4072, Australia

r t i c l e i n f o

rticle history:eceived 2 November 2010eceived in revised form 11 March 2011ccepted 9 May 2011vailable online 17 May 2011

eywords:teel

a b s t r a c t

A low carbon bainitic steel with Cu–P alloying was developed. The new steel aims to meet the demandof high strength, high toughness and resistance to chloride ion corrosion for the components used inthe environment of sea water and oceanic atmosphere. Mechanical properties of the steel were testedand strengthening and toughening mechanisms were analyzed by comparing hot-rolled microstructuresof the low carbon bainitic steels with and without Cu–P alloying. The results show that Cu–P alloyingprovided strong solution strengthening with weak effect on ductility. The toughness loss caused by Cu–Palloying could be balanced by increasing the amount of martensite/remained austenite (M/A island)

echanical characterizationainiteusteniterecipitationislocations

at lower finishing temperature. The static recovery process during rolling interval was delayed by theinteraction of phosphorous, copper atoms with dislocations, which was favorable to the formation ofbainitic plates. Super-fine Nb(C, N) particles precipitated on dislocations had coherency with bainiteferrite at 830 ◦C finishing temperature. Raising finishing temperature to 880 ◦C, Nb(C, N) particles wereprone to coarsening and losing coherency. It was also found that no accurate lattice match relationshipamong retained austenite, martensite and bainite in granular bainitic microstructure.

. Introduction

Traditional weathering steels represented by 09CuPCrNi steelave been widely used in the area of buildings, bridges and vehi-les. However, the mechanical properties and corrosion resistancef the weathering steels have no special advantages in Cl− con-aining environment such as sea water or oceanic atmosphere1,2]. The main reason is that the carbides in steel act as role of

icro-cathodes which accelerates corrosion of ferrite matrix [3,4].lso massive pearlite is extremely adverse to the fracture tough-ess of the steel. Recently, an ultra low carbon bainitic (ULCB)teel was developed based on ultra low carbon component andranular bainitic microstructure. ULCB steel has higher tensiletrength, impact toughness and equivalent corrosion resistance inomparison with 09CuPCrNi steel [5,6]. It will be a potential new-ype weathering steel. Nevertheless, the corrosion resistance ofLCB steel in Cl− containing environment still needs to be further

mproved. It has been found that copper and phosphorus ions as

atalysts accelerate the air-oxidation of Fe2+ at low pH, leading tohe self-repair of the protective rust film [7]. Phosphorus also pro-

otes the formation of protective amorphous oxide [8,9]. Thus,

∗ Corresponding author. Tel.: +86 24 83681679; fax: +86 24 23906316.E-mail address: wenfangcui@yahoo.com.cn (W.F. Cui).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.05.013

© 2011 Elsevier B.V. All rights reserved.

ULCB steel with Cu–P alloying is expected to have combinationproperties of high strength, high toughness and good resistanceto oceanic atmosphere corrosion.

Phosphorus is an effective strengthening element in low car-bon low alloying steel [10]. In some cases phosphorus behavesa good balance of yield strength and ductility in ultra-fine lowcarbon steels [9] because the strong interaction between solutephosphorus atoms and dislocations increases working hardeningrate. But in other cases phosphorus is a harmful element becauseexcessive phosphorous content in steels causes cold embrittlementphenomenon after quenching or tempering [11–14]. Up to date, thedifferent roles of phosphorus in different steels have been attractingresearcher’s attention [15,16].

The present paper investigated the effects of Cu–P alloyingon the mechanical properties and microstructure of low carbonbainitic steel. The hot-rolled microstructures at different finishingtemperatures were observed and the mechanisms of strengthen-ing and toughening of the steel were analyzed. For comparison, themechanical properties and microstructure of low carbon bainiticsteel without Cu–P alloying was also studied.

2. Experimental procedures

The chemical compositions (wt.%) of low carbon bainitic steelwith Cu–P alloying (LCB-CuP steel) and the comparative steel

6402 W.F. Cui et al. / Materials Science and Engineering A 528 (2011) 6401– 6406

Table 1The chemical compositions of tested steels (wt.%).

Steel C Mn Si Nb Ni P Cu B S Fe

LCB-CuP 0.07 1.43 0.27 0.035 0.19 0.052 0.26 0.0035 0.0025 BalLCB 0.06 1.51 0.25 0.04 – 0.008 – 0.0035 0.003 Bal

Table 2Mechanical properties of the steels at different finishing temperatures.

Steel/finishing temp. Akv (J) Rp0.2 (MPa) Rm (MPa) A (%) Z (%) Akv (J)

Room temp. −40 ◦C

LCB/830 ◦C 515.5 658.1 28.4 72.7 279 140◦

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LCB/880 C 517.2 688.7

LCB-CuP/830 ◦C 578.2 727.6

LCB-CuP/880 ◦C 566.6 741.5

ithout Cu–P alloying (LCB steel) are shown in Table 1. The twoteels were prepared by melting in vacuum induction furnace. Theorged slabs were austenitized at 1200 ◦C for 2 h and then hot-olled into plates by TMCP process. Rolling temperatures startedt 1100 ◦C and finished at 830 ◦C and 880 ◦C, respectively. Afterolling finished, the steel plates were immediately cooled by spray-ng water in order to avoid the appearance of ferrite and pearlitehase. The microstructures of steel plates were observed by Olym-us GX71 optical microscope and Philips CM200 transmissionlectron microscope (TEM). The �3 mm thin foils for TEM obser-ation were ground mechanically to 50 �m thickness and thenlectrolytically polished at −20 ◦C in a solution containing perchlo-ic (5 vol.%) + ethanol (95 vol.%). The tensile specimens were takenrom the plates along rolling direction and machined into 50 mmauge length and 12 mm gauge width. The tensile tests were per-ormed at constant cross-head speed of 1.8 mm/min. Charpy impactests were carried out at room temperature and −40 ◦C, respec-ively.

. Results and analysis

.1. Mechanical properties and microstructures of rolled-plates

Table 2 lists the mechanical properties and impact toughnessf the two experimental steels. LCB-CuP steel has the higher yieldtrength, higher ultimate strength and the same ductility at 880 ◦Cnd 830 ◦C finishing temperatures in comparison with LCB steel.his proves that phosphorus and copper resulted in strong solu-

ion strengthening as well as more uniform plastic deformation.lthough the impact toughness of LCB-CuP steel was lower thanCB steel, but when finishing temperature decreased to 830 ◦C, thempact toughness of LCB-CuP steel still reached 112J at −40 ◦C. The

Fig. 1. Metallographs of hot-rolled steel pl

31.3 70.7 205 9530.4 70.2 223 11229.8 66.6 161 66

value can completely meet the engineering demand of structuralsteels. The result indicates that the toughness loss of LCB-CuP steelcan be covered by reducing finishing temperature.

It is known that phosphorus in steel increases the sensibil-ity of cold embrittlement because of segregation phenomenonat grain boundary. However, in present investigation, the specialcomponent design and the particular microstructural morphologyof LCB-CuP steel balanced the detrimental effect of high phos-phorus content on the low temperature toughness. Firstly, traceboron atoms in the steel preferentially occupied the positionsat prior austenite grain boundary through a competition system,which led to that the segregation tendency of phosphorus atoms atgrain boundary was weakened [17]. Secondly, Boron element effec-tively delayed ferrite transformation and promoted the formationof granular bainitic microstructure under moderate cooling rate,as seen in Fig. 1. Large numbers of martensite/retained austenitephase (M/A small islands) randomly distributed on bainite ferriteand acted as roles of complex phase strengthening and toughen-ing. The volume fraction of M/A islands in LCB-CuP steel looked tobe more than LCB steel. This is because phosphorus decreased thestarting temperature of bainite transformation by inhibiting carbondiffusion, which caused the increase of amount of untransformedaustenite [9].

3.2. TEM observation of the hot-rolled microstructures

3.2.1. Microstructures at 830 ◦C finishing temperatureBy TEM observation, the details of hot-rolled microstructure can

be examined clearly. Fig. 2 is TEM images of hot-rolled microstruc-tures of LCB-CuP steel at 830 ◦C finishing temperature. Bainiticplates with 0.4–1.0 �m width and 3–6 �m length were frequentlyobserved (Fig. 2(a)). M/A film in between bainitic plates and ran-

ates: (a) LCB steel; (b) LCB-CuP steel.

W.F. Cui et al. / Materials Science and Engineering A 528 (2011) 6401– 6406 6403

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ig. 2. Hot-rolled microstructures of LCB-CuP steel at 830 ◦C finishing temperatureark field image of Nb(C, N) precipitations at dislocations; (d) SAD patterns of Nb(C

omly distributed massive M/A islands are indicated by blackrrows. Fig. 2(b) displays a strong strain contrast at plate interfaces.he image reflects that bainitic plates were formed by shear trans-ormation mechanism. The shear stress produced by one bainitic

ig. 3. Hot-rolled microstructures of LCB steel at 830 ◦C finishing temperature. (a) Subgrg = (0 1̄ 1) operation reflection; (c) slim and massive M/A islands; (d) SAD patterns of M/

inite plates and M/A film; (b) strong strain contrast at bainitic plates interface; (c)rticle and bainitic ferrite.

plate transformation agitated another bainitic plate transforma-tion. The asynchronous transformation of bainitic plates causedhigher orientation angle difference between adjacent plates. Actu-ally, the bainitic plate interfaces with lattice misorientation and

ains and high-density dislocations; (b) Nb(CN) precipitations at dislocations underA island and bainite ferrite as indicated by white circle in (c).

6404 W.F. Cui et al. / Materials Science and Engineering A 528 (2011) 6401– 6406

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ig. 4. Morphologies of dislocations and precipitations at 880 ◦C finishing temperatne twins in retained austenite film, (d) SAD patterns of big Nb(C, N) particle and b

/A film at interface increased the impact absorption work byeflecting the direction of crack propagation [18]. In addition tohis, the massive M/A islands also increased the crack propaga-ion energy because the stress concentration at the crack tip waseakened due to the “stress induction phase transformation” effect

19–21]. Therefore, both bainitic plates and M/A islands contributedo toughening of the steel.

It was found that accumulated deformation induced a lotf super-fine Nb(C, N) precipitations on dislocations. Fig. 2(c)s a dark field image of Nb(C, N) particles. These spheri-al or spheroid particles were only 5–15 nm long and hadhe coherent orientation relationship with bainite ferrite:0 1 1̄)B//(0 2 2̄)Nb(CN), [0 1 1]B//[1̄ 1 1]Nb(CN). Apparently, dislo-ations were the best sites for Nb(C, N) precipitations. At theame time, the precipitations produced strong pinning effect toislocation movement, which stabilized dislocation structure andtrengthened bainite matrix.

Contrast to LCB-CuP steel, many substructures were observedn LCB steel besides bainitic plates (Fig. 3(a)). Since spraying waterooling was used immediately after the last rolling pass, there waso enough time to bring about recovery process during cooling.he formation of subgrains resulted from static recovery or staticecrystallization between rolling pass at the stage of finish rolling.he high-density dislocations within subgrains were produced byhe subsequent rolling deformation. Since the size of subgrain wasoo small (ranging from 0.5 to 1.5 �m) to form bainitic plates withinubgrains. Thus, we seldom observed bainitic plates in the area ofubstructures. Fig. 3(b) and (c) are TEM images of dislocation struc-ure, Nb(C, N) precipitation and M/A islands in LCB steel. They had

imilar features to LCB-CuP steel. But one important phenomenons that no accurate lattice match relationships were found amongetained austenite, martensite and bainite. Fig. 3(d) is selected areaiffraction (SAD) patterns of M/A island and bainite ferrite and their

LCB-CuP steel. (a) and (b) Straight and parallel dislocations; (c) bainitic plates and ferrite in (a).

corresponding analyzing results. It is seen that (0 1̄ 1)M and (011̄)BFcrystal plane deviates 1–2◦ relative to (1̄ 1 1̄)� plane as electronbeam directions exist [0 1 1]� //[1̄ 1 1]M//[0 0 1]BF, The results arenot in agreement with K–S or G–T relationship. It is possible thatthe existence of high-density dislocations distorted the lattice ofaustenite or bainite ferrite phase, resulting in measurement errorof crystal plane direction by using SAD technology. Nevertheless, itis indeed reported that there is the orientation relationships deviat-ing classical theory in some alloys even measured by using Kikuchipattern [22]. Further investigation is needed to clarify accurate ori-entation relationship between different phases in granular bainiticmicrostructure.

3.2.2. Microstructures at 880 ◦C finishing temperatureAt 880 ◦C finishing temperature bainitic plates can still be

observed, but dislocation structures displays some difference in thetwo steels. Straight and parallel dislocation arrangement was seenin LCB-CuP steel (Fig. 4(a) and (b)), while the tangled and zigzaggeddislocation structure in LCB steel (Fig. 5(a) and (b)). Obviously, thecross slip and climb movement of dislocation appeared in LCB steel,and mainly planar slip movement of dislocation in LCB-CuP steel.The result shows that the way of dislocation movement in LCB-CuPsteel was affected by small size solute atoms, e.g. phosphorous andcopper atom. Some investigations revealed that copper atoms areprone to segregate around the dislocations forming ε-Cu precipita-tions [23,24]. Likewise, phosphorous atoms are often captured bydislocation core forming so-called “Cottrell atmosphere”. The inter-action between dislocation kink and phosphorous atoms strongly

affects the property of dislocations [25–27]. At the stage of fin-ish rolling, the diffusion of phosphorus and copper atoms wereaccelerated by high-density dislocations, which promoted to formphosphorus-rich or copper-rich atmospheres. The segregation of

W.F. Cui et al. / Materials Science and Engineering A 528 (2011) 6401– 6406 6405

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ig. 5. Dislocations and M/A islands at 880 ◦C finishing temperature in LCB steel. (a)trip; (d) SAD patterns of retained austenite and bainite ferrite.

hosphorus and copper around dislocations also promoted Nb(C,) precipitations on dislocations. The early precipitated Nb(C, N)articles were prone to coarsen and lose coherency, as seen inig. 4(a). The interactions of phosphorous and copper atoms withislocations and Nb(C, N) precipitations on dislocations remarkably

ncreased the resistance to cross slip and climb of dislocation andhus delayed static recovery process of LCB-CuP steel. This is ben-ficial to the formation of more bainitic plates during cooling afterolling.

Fig. 5(c) displays the internal details of M/A island. Retainedustenite lath and transformed martensite arranged alternatively.t is not clear why retained austenite and martensite arranged inuch a way, but it is presumed that the morphology related to thehort-distance diffusion of carbon. In the process of cooling, thelternatively arranged carbon-rich and carbon-poor areas gradu-lly formed within prior austenite. Then the carbon-rich areas weretabilized to room temperature to become retained austenite andhe carbon-poor areas transformed into martensite. The alternativerrangement way of retained austenite and martensite was con-idered to be good for balancing plastic deformation between hardartensite phase and soft austenite phase and avoiding microvoid

ucleation and propagation in local regions. The SAD pattern of/A island in Fig. 5(d) proved again that retained austenite has no

ccurate lattice match relationship with bainite ferrite.

. Conclusions

In this paper, the mechanical properties and hot-rolledicrostructures of low carbon bainite steel with and without Cu–P

lloying at different finishing temperatures were investigated. Thetrengthening and toughening mechanisms of the steels were clar-fied by analyzing microstructural characteristics. The conclusionsre summarized as follows:

b) Tangled and zigzagged dislocations; (c) M/A island containing retained austenite

(1) Cu–P alloying in low carbon bainitic steel produced obvioussolution strengthening together with the decreased of impacttoughness as compared with the steel without Cu–P alloying.However, the loss of toughness due to Cu–P alloying couldbe recovered by decreasing finishing temperature because themore M/A islands and bainitic plates formed at lower finishingtemperature. Both retained austenite and bainitic plate inter-faces were beneficial to inhibiting crack propagation.

(2) Phosphorus-rich or copper-rich atmospheres formed by inter-action of phosphorous and copper atoms with dislocationsinhibited cross slip and climb of dislocation, and thus delayedstatic recovery process at the stage of finish rolling.

(3) At 830 ◦C finishing temperature, disperse super-fine Nb(C, N)particles precipitated on dislocations produced strong pre-cipitation strengthening because of the complete coherentrelationship of Nb(C, N) particles with bainite ferrite. Butat 880 ◦C finishing temperature Nb(C, N) particles tended tocoarsen and lose coherency, which lead to the weakening ofstrengthening effect.

Acknowledgment

This work was financially supported by Program for ChangjiangScholars and Innovative Research Team in University (IRT0713).

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