Materials Transactions, Vol. 44, No. 8 (2003) pp. 1630 to 1635 #2003 The Japan Institute of Metals EXPRESS REGULAR ARTICLE
Spheroidizatio Spheroid ization n of Low Carbon Steel Processed by Equal Channel Angular Pressing Dong Hyuk Shin 1 , Soo Yeon Han 1 , Kyung-Tae Park 2 , Yong-Seog Kim 3 and Young-Nam Paik 4 ;
*
1
Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-Do 425-791, Korea Department of Advanced Materials Science and Engineering, Taejon National University of Technology, Taejeon 305-719, Korea 3 Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea 4 College of Mechanical Engineering and Industrial System Engineering, Kyung Hee University, Yongin 449-701, Korea 2
Spheroidization behavior of cementite in a low carbon steel processed by the equal channel angular pressing technique was investigated. The effects of annealing temperature and time, and accumulated strain on the morphology of cementite and mechanical properties were studied. The results indica indicated ted that the application application of the severe plastic deformation deformation can improve the kinetics of spheroi spheroidizati dization on significantly. significantly. In this study, the enhanced spheroidization spheroidization kinetics was discuss discussed ed in terms of carbon dissolution dissolution from cementi cementites tes and defects induced in cement cementites ites by the severe plastic deformation. deformation. In additi addition, on, the softeni softening ng of the steel after the spheroi spheroidizat dization ion treatment was evaluat evaluated. ed. (Received April 1, 2003; Accepted June 19, 2003) Keywords: equal channel angular pressing, spheroidization, spheroidization, low carbon steel, pearlite, pearlite, cement cementite ite
1.
Introd Int roduct uction ion
Spheroidization treatment of cementites in steels is used to incr in crea ease se th thei eirr fo forma rmabil bility ity du duri ring ng wi wire re dr draw awin ing g of hi high gh strengt stre ngth h wir wiree rod rodss and heading heading of hig high h str streng ength th bol bolts ts at ambientt tempe ambien temperatures ratures.. A significant research effort has been dire di rect cted ed to in inve vesti stiga gate te th thee me mech chan anism ismss an and d ki kine netic ticss of 1,2) spheroidization process. Based on the investigations, the spheroidization treatment is conducted either by intercritical 3,4) or by sub subcrit critica icall tre treatm atment ent.. The intercr intercritical itical treatme treatment nt refers to a heat treatment treatment scheme that consists of heating to a temper tem peratur aturee reg region ion bet betwee ween n the upp upper er and low lower er crit critica icall temperatures (between A1 and A3 ) prior to the sphero spheroidizin idizing g treatment below the lower critical temperature. The subcritical treatment, on the other hand, denotes the heating of the steel below the lower critical temperature (A1 ). Although the kinetics of cementite spheroidization depends on the chemical compositions of steel and heat treating schedule, time required for the spheroidization usually takes from 10 to 24 hours. Thus the spheroidization treatment is by far the most time consuming steps in manufacturing the wire rod and bolt. Drivin Dri ving g for force ce for the cem cement entite ite sph sphero eroidi idizat zation ion is the redu re duct ctio ion n in su surf rfac acee fr free ee en ener ergy gy of th thee sy syst stem em.. Th Thee spheroidizat sphero idization ion proce process, ss, i.e. transformatio transformation n of cemen cementites tites from lamellar morphology to spherical shapes, decreases the interface area between cementite and ferrite phases, reducing surfac sur facee fre freee ene energy rgy of the sys system. tem. The ene energy rgy red reduct uction ion associated with the spheroidization, in general, is not large enough to drive the process at a high rate, making the process timee co tim cons nsum uming ing.. On Onee of th thee eff effec ectiv tivee me meth thod odss no note ted d to increase the spheroidization speed is to introduce defects in 5) the cementite via severe plastic deformation. Hono et al. showed that the cementites in a near eutectic steel spheroidizes more readily after a severe drawing. Effective strain appl ap plie ied d to th thee sa samp mple le du duri ring ng th thee wi wire re dr draw awin ing g wa wass approximately 1.0. This increased speed of spheroidization has been att attrib ributed uted to the dissolutio dissolution n of car carbon bon in fer ferrite rite matrix mat rix and the intr introdu oducti ction on of def defect ectss suc such h dis disloc locatio ations, ns, *
Graduated Student, Hanyang University.
nonsto nons toic ichi hiom omet etry ry,, as we well ll as ki kink nkss an and d le ledg dges es to th thee 6,7) cementite during the plastic deformation. 8–12) Recently, Shin et al. applied appli ed equal channel angul angular ar (ECA) pressing on low carbon steels for grain refinement. The ECA pressing induces shear deformation in the sample by pas passin sing g thr throug ough h an ang angula ularr cha channe nnell of equ equal al in cro cross ss section. As the cross sectional area of the sample does not change during the ECA pressing, a very high accumulated stra st rain in su such ch as 4– 4–12 12 co coul uld d be imp impar arte ted d to th thee sa samp mple le by repetition of the pressing. Each pass of the ECA pressing induces effective strain approximately 1.0 to the sample. It was noted during the investigation that temperature and time of the cementite spheroidization may be reduced significantly 9,12) in the ECA pressed steels. The spheroidization in ECA pressed presse d sample occurred fully when heated heated at 873 K only for 1 hour. Larger strains imparted to the sample during ECA pressing must have increased the kinetics of the spheroidization. In this study, a more detailed study on the spheroidization of EC ECA A pr pres esse sed d ca carb rbon on st stee eell wa wass co cond nduc ucte ted. d. Th Thee EC ECA A pressed steels processed up to 12 passes were heated for 1 to 72 ho hour urss at te temp mper erat atur ures es ra rang ngin ing g fr from om 72 723 3 to 97 973 3 K. Morphology of the cementites after the heat treatment was observed obser ved using scanning electron microscopy (SEM) and tran tr ansm smiss issio ion n el elec ectr tron on mi micr cros osco copy py (T (TEM EM)) to st stud udy y th thee progress of the spheroidization. In addition, tensile properties of th thee st stee eels ls wi with th th thee sp sphe heroi roidi diza zatio tion n tre treatm atmen entt we were re evaluated. 2.
Experi Exp erimen mental tal Pro Proced cedure ure
For thi thiss exp experim eriment ent,, a low car carbon bon ste steel el (Fe (Fe–0. –0.15% 15%C– C– 0.25%Si–1.1%Mn (mass%)) in a form of bar was used. The samp sa mple le re rece ceiv ived ed wa wass in a no norm rmali alizi zing ng he heat at tr trea eatme tment nt condition condi tion at 1223 K for 30 minute minutess followed by air cooling. The average diameter of grains of the as-received sample was approximately 30 mm. The bar sample was machined into a rod of 18 mm in diameter diameter and 120 mm in length for the ECA pressin pre ssing g exp experim eriment ent.. Pri Prior or to ECA pressing, pressing, the rod was prehea pre heated ted in a fur furnac nacee at 623 K for 10 min minute utess for uniform uniform
Sph eroidizatio n of Low Carbon Steel Proce ssed by Equ al Channel Angular Pres sing
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plunger
f
sample
y
Fig. 1 A schematic illustration of the ECAP die used in the present investigation.
heating and inserted into a die of ECA press as schematically illustrated in Fig. 1. The inner contact angle and the arc of curvature at the out point of contact between the channels of the die were 90 and 20 respectively. The die geometry was designed to yield an effective strain of about 1 per each pass. The die was maintained at the same preheating temperature and its inner surface was coated with graphite lubricant in order to reduce the friction between the sample and die wall. The pressing speed of upper punch was at 2 mm/s. During ECA pressing, the sample was rotated 180 around its longitudinal axis between the passes. This pattern of the 13) pressing is commonly designated as route C. Route C was selected since it restores the shape of the original segment at each even number pass and thereby nearly equiaxed ultrafine grain structure can be obtained. The ECA pressed samples were annealed for spheroidization in a temperature range from 753 to 973 K for 1 to 72 hours. The microstructures of both steels thus produced were examined using an optical microscopy, TEM and SEM. Room temperature tensile properties of the steels were measured with the initial strain rate of 1 33 10 3 s 1 by an Instron machine.
:
3.
Experimental Results and Discussion
Figure 2(a) shows the microstructure of carbon steel used in this study. The pearlite colony distributed uniformly throughout the sample and its area fraction was approximately 15%. The ferrite grains and pearlite colonies were equiaxed in morphology and size of the both phases was 30 mm. A TEM micrograph of the pearlite colony is also shown in Fig. 2(b). The micrograph revealed a well-aligned lamellar morphology cementites in the colony. After the sample was ECA pressed for 4 passes, morphology of the cementites were changed significantly as shown in
Fig. 2 Microstructures of as-received 0.15%C steel: (a) Optical micrograph showing ferrite and pearlite; (b) TEM micrograph showing lamellar structure in pearlite.
Fig. 3. The cementites became wavy (Fig. 3(a)) and severed (Fig. 3(b)) from the severe plastic deformation imposed by the ECA pressing. The difference in morphology of cementites deformed is believed to be caused by the orientation of the cementite in relation to the shear deformation direction of the sample. When the cementite is aligned perpendicular to the shear deformation plane, the thinned and severed morphology might be resulted. On the other hand, if aligned with some angle, they might become wavy as shown in the figure. Both of these morphologies were observed on the cross section of the sample in transverse direction. In a cold drawn wire, the wavy cementite is observed only on the cross section in transverse and the severed one on cross section in 1) longitudinal direction. One of the interesting things to note from the micrographs is that the cementite deformed without fracturing even after such a severe deformation. Previous studies indicated that the cementite fractured due to its lack of slip systems under such 14) a heavy plastic deformation. The compressive stress component of the pressing might have some role in preventing the fracturing of the cementite. In addition, dislocation density at the ferrite in the colony was significantly higher than that in the ferrite grains, as reported in 15) previous studies. The ECA pressed samples were annealed at 753 K for various times for spheroidization (Fig. 4). When the sample was annealed for 1 hour, the morphology of the cementite remained similar to that of the as-ECA pressed sample. As the holding time was increased to 24 hours, the cementite
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D. H. Shin, S. Y. Han, K.-T. Park, Y.-S. Kim and Y.-N. Paik
Fig. 3 TEM micrographs of pearlite in 4 pass steel samples: (a) curled and wavy cementite lamellar plates; (b) severely necked cementite fragments parallel to each other.
Fig. 4
started to spheroidizes in a process like the Reyleigh 16) instability of a liquid jet. The defects such as kinks and ledges on the surface cementite caused by the severe plastic deformation might have caused the instability in the lamellar structure of the cementite. After annealing for 72 hours, most of the cementites became spheroidized with an aspect ratio close to 1 (Fig. 4(c)). The average size of the cementites was approximately 0.06 mm in diameter. The spheroidized cementite size is significantly smaller than that obtained 17) through conventional spheroidization treatment, 0.1–1 mm. Effect of temperature on the cementite spheroidization in ECA pressed steel was investigated by annealing the sample at 783 K or 813 K for 1 hour (Fig. 5). As the temperature is increased, the spheroidization rate was observed to increase. The microstructure of the sample annealed at 783 K for 1 hour appeared to be similar to that annealed 753 K for 24 hours (Fig. 4(b)). When the sample was annealed at 813 K, the cementites became fully spheroidized like the sample annealed 753 K for 72 hours (Fig. 4(c)). The average size of the spheroidized cementites was approximately 0.05 mm. Although the spheroidization occurred mainly at the sites of former colony as noted in Figs. 5(b) and (d), a small fraction of the cementites formed at the boundaries of submicron grains formed by the ECA pressing. The white arrows in Fig. 5(d) indicate such cementites. For these cementites to form, there should be some carbon dissolved over its solubility limit and diffuse along the grain boundaries. This phenomenon is an indirect evidence of carbon dissolution into ferrite 6) matrix by severe deformation as reported by Hono et al. in a carbon steel after severe plastic deformation. During the
TEM micrographs of pearlite in 4 pass steel samples annealed at 753K for various annealing time: (a) 1h; (b) 24 h; (c) 72 h.
Sph eroidizatio n of Low Carbon Steel Pro cessed by Equ al Channel Angular Pres sing
Fig. 5
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TEM and SEM micrographs of pearlite in 4 pass steel samples annealed for 1 h: (a), (b) 783 K; (c), (d) 813 K.
annealing treatment, the grain size of ferrite remained close to 0.3 mm in diameter, which might be due to a pinning effect of such grain boundary cementites. As the annealing temperature is further increased to 873 K and 973 K, the spheroidized cementites and ferrite grains were observed to coarsen (Fig. 6). The annealing time was also 1 hour. A SEM micrograph of the sample annealed at 873 K shows that the cementite coarsened to 0 08 mm and the ferrite grains became recrystallized and coarsened to 7 mm in diameter. The coarsening of the cementite was more significant at grain boundaries of ferrite phase. The size was increased to 0.27 mm. As the annealing temperature is increased further to 973 K, the cementite in the ferrite matrix was coarsened more to 0.21 mm and those at grain boundaries were measured to be 0.38 mm. One of the interesting characteristics with sample annealed at 973 K is that the spheroidized cementite was somewhat dispersed throughout the ferrite matrix. In other words, the spheroidization of the cementite was not localized to the former pearlite colony area. Higher annealing temperature might have promoted the diffusion of carbon to ferrite matrix and precipitation of the cementite. Under the two annealing conditions, the ferrite matrix was recrystallized completely. With the annealing conditions in proceeding sections, samples were either polygonized or partially recrystallized. The effect of accumulated strain on the spheroidization behavior was studied by increasing number of ECA passes on the steel. Figure 7 shows TEM micrographs of samples after 8 and 12 passes of ECA pressing at 723 K. The samples were examined without any additional annealing treatment. A :
partial spheroidization of the cementite was noted with the sample after 8 passes and full spheroidization after 12 passes. The increase in the accumulated strain to 12 has reduced the spheroidization temperature significantly down to 723 K. This indicates that the large strain imparted on the sample could reduce the temperature of spheroidization by 300 K. In addition, the spheroidization time was reduced approximately to 12 minutes. Each of the ECA pressing takes about 1 minute. These results shows that the defects such as dislocations as well as carbon dissolution introduced by severe deformation imposed on steels could increase the kinetics of spheroidization significantly. The samples annealed at 873 K and 973 K for 1 hour were tested for their tensile properties along with as-received and as-ECA pressed samples (Fig. 8). The annealed samples were selected since the matrix is completely recrystallized. The yield strength and fracture elongation of the as-received sample was 320 MPa and 30%, respectively. The yield strength of the as-ECA pressed sample was increased to 900 MPa, but the fracture elongation was reduced to 10%, due to work hardening and grain refining effects. Annealing at 873 K reduced the yield strength to 450 MPa and recovered the elongation to 28%. The strength of the sample was higher by 45% and elongation was similar compared with the as-received sample. The grain refining as noted in Fig. 6(a) should have contributed to the increased strength. The annealing treatment at 973 K, on the other hand, reduced the yield strength of the sample lower than that of asreceived sample to 310 MPa. In addition, the fracture elongation was improved to 37%. The spheroidization
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D. H. Shin, S. Y. Han, K.-T. Park, Y.-S. Kim and Y.-N. Paik
Fig. 6 SEM micrographs of microstructural change of ferrite and pearlite regions in 4 pass steel samples annealed at various temperature for 1 h: (a) 873K; (c) 973K.
1000 0.15C steel
As-received As-ECAPed 873K, 1hr annealed 973K, 1hr annealed
800
a P M600 / σ
, s 400 s e r t S 200
0 0
5
10
15
20
25
30
35
40
Strain, % Fig. 8
Stress-strain curves of pressed and annealed steel.
treatment of the sample at 973 K improved the formability of the sample over the as-received sample. This beneficial effect is mainly due to the smaller size of the spheroidized cementites formed in the sample. 4.
Conclusions
The investigation of spheroidization behavior of cementite in ECA pressed steel led to following conclusions:
Fig. 7 TEM micrographs of pearlite in pressed steel samples: (a) 8 passes; (b) 12 passes.
(1) The ECA pressing resulted in wavy and severed cementites in the 0.15%C carbon steel. The cementites, however, were not fractured during the pressing. (2) The spheroidization speed of ECA pressed steel increased with annealing temperature and time. When annealed at 753 K, the spheroidization occurred after 72 hours. When the temperature was increased to 783 K, it took about one hour. Further increase in annealing temperature resulted in coarsening of ferrite grains and cementites. (3) The spheroidization occurred mainly at former pearlite colony sites when the annealing temperature is lower than 873 K. When the temperature was increased to 973 K, the spheroidized cementites formed not only at former pearlite colony sites but also inside ferrite grains. (4) Increase in accumulated strains in the ECA pressed steel was observed to decrease the spheroidization temperature and time. As the steel was pressed up to 12 passes, the spheroidization was observed during the pressing. The pressing temperature was 723 K and total pressing time was approximately 12 minutes. Acknowledgments
This work was sopported by Korea Ministry of Science and Technology through ‘21st Century New Frontier Research and Development Program’ and ‘National Research Laboratory Program’.
Sph eroidizatio n of Low Carbon Steel Proces sed by Equ al Channel Angular Pres sing
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