Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed by Accumulative Roll Bonding Process

Materials Transactions, Vol. 43, No. 9 (22) pp. 232 to 2325 c 22 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed by Accumulative Roll Bonding Process Seong-Hee Lee 1, Yoshihiro Saito 2,, Kyung-Tae Park 3 and Dong Hyuk Shin 1 1 Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Kyunggi-Do, Korea 2 Department of Materials Science and Engineering, Osaka University, Suita 565-871, Japan 3 Division of Advanced Materials Science and Engineering, Hanbat National University, Taejon 35-719, Korea An ultra low carbon IF steel was severely strained by accumulative roll-bonding (ARB) process and subsequently annealed for.5 h at various temperatures ranging from 673 to 173 K for strengthening by grain refinement. The ARB process was carried out up to 5 cycles (an apparent equivalent strain of 4.) at ambient temperature without lubrication. The as-arbed specimen exhibited a dislocation cell structure with relatively high dislocation density, rather than a well-defined ultrafine grained structure. However, the subsequent annealing at 673 or 773 K resulted in the formation of ultrafine grains in the structure. This indicates that the heating (recovery) is necessary for the formation of ultrafine grains in the present IF steel severely deformed by ARB process. The tensile strength of the as-arb processed IF steel increased with strain, reached a maximum of 813 MPa, which is about 3 times higher than the initial value. The elongation dropped largely from 6 to below 1% at the 1st cycle, but it hardly changed from the 2nd cycle even if the ARB cycle increased. The strength decreased gradually and the elongation increased with annealing temperature. Changes in the mechanical properties with the annealing temperatures corresponded well with the changes in the microstructures. (Received June 6, 22; Accepted July 15, 22) Keywords: accumulative roll-bonding, ultra low carbon IF steel, ultrafine grain, mechanical properties 1. Introduction Emeritus Professor, Osaka University. In recent years, ultrafine grained (UFG) materials have received considerable attention of many researchers for their potential improvements in mechanical properties. UFG materials have been processed in the porosity-free bulk state using severe plastic deformation techniques, including equal channel angular pressing (ECAP), 1, 2) high-pressure torsion (HPT) 3) and accumulative roll-bonding (ARB), 4) etc. These techniques have a great advantage that the grain refinement and high strengthening can be attained without any addition of alloying elements or ceramic particles. Among such techniques, the ARB process is mostly appropriate for fabrication of sheet-shaped materials because it can be carried out readily by the conventional rolling system. It has, to date, been demonstrated that the remarkable strengthening by grain refinement via the ARB process could be achieved in various kinds of aluminum alloys and steels. 4 1) However, the ARB process has not been yet utilized for practical use. In this regards, low ductility of the ARB processed materials must be one of the obstacles for practical use. Heat treatment after the ARB process would become one method for improvement of the ductility. Ito et al. has reported that not only the strength but also the ductility could be improved for 11 commercial pure aluminum through annealing after the ARB. 9) This may provide a possibility for practical utility of the ARB process for aluminum alloys. However, only few studies have been reported for annealing of ARB processed ferrous materials. 5, 9, 1) Ultra low carbon interstitial free steel (IF steel) is used widely for automotive industry because of its excellent deep drawability. Application of the ARB process to the IF steel would result in increase in the strength, and thereby weightsaving of the automotive. Tsuji et al. have applied the ARB process to IF steel at 773 K and succeeded in ultra grain refinement and high strengthening. 5) However, the studies on the ARB of the IF steels at ambient temperature and subsequently annealing have not been reported yet. This study is aimed at investigating systematically the microstructure and mechanical properties of the IF steel processed by the ARB at ambient temperature and subsequently annealed. 2. Experimental Procedures An ultra low carbon IF steel sheet of which the chemical composition is shown in Table 1 was used for this study. Figure 1 shows an orientation map determined by electron back-scatter diffraction (EBSD) measurement of as-received IF steel. The as-received sheet showed a recrystallized structure with the average grain diameter of 27 µm. The schematic illustration of the ARB process is shown in Fig. 2. In the present ARB process, two sheets of 1 mm thick, 25 mm wide and 25 mm long were stacked up and reduced to about 1 mm by one-pass cold rolling. The two sheets were bonded during the rolling. The bonded sheet was cut in halflength, stacked up again and rolled to the thickness of 1 mm. To ensure sound bonding during rolling, the surfaces of the two sheets to be bonded were degreased and scratch-brushed by a stainless-wire bevel brush beforehand. The details of the ARB process were described in the previous studies. 4 1) In the present study, the ARB process up to 5 cycles (an apparent equivalent strain of 4) was performed at ambient temperature without lubrication. The rolling reduction at the 1st cycle was almost 5%, but it was below 5% from the 2nd cycle because of the elastic deformation of the rolling mill due to the increase in the flow stress of the specimen at higher ARB cycles. Annealing was conducted, for the sample after the ARB

Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed 2321 Table 1 Chemical composition of ultra low carbon IF steel used. (mass %) C Si Mn P S Cr Sol.Al N V Ni Ti Fe.2.1.17.12.8.4.41.27.5.2.72 bal Surface treatment Degreasing Wire brushing Cutting Cold roll bonding Stacking 2 1 Fig. 1 Orientation map determined by EBSD measurement of as-received IF steel. Fig. 2 Schematic illustration showing ARB process. of 5 cycles, for.5 h at various temperatures ranging from 673 to 173 K in an electric furnace. The microstructure was investigated by transmission electron microscopy (TEM). TEM studies were conducted with a HITACHI H-8 microscope operated at 2 kv. Thin diskshaped foils parallel to the rolling plane and the thin rectangular foils perpendicular to the transverse direction were prepared by spark machining and twin-jet polishing for TEM observation. The mechanical properties of the ARB processed and subsequently annealed IF steel were examined at ambient temperature by an Instron-type tensile testing machine. The test pieces were spark-machined so that the tensile direction was parallel to the rolling direction. The gauge length and width were 1 mm and 5 mm, respectively. The initial strain rate was 8.3 1 4 s 1. 3. Results and Discussion 3.1 Microstructure TEM microstructures and the corresponding selected area diffraction (SAD) patterns observed at the plane perpendicular to normal direction (ND plane) of the samples ARBprocessed by 1 cycle, 3 cycles and 5 cycles are shown in Fig. 3. The SAD patterns were taken from the center of the bright field images by using an aperture with a diameter of 1.6 µm. The specimen after 1 cycle (an equivalent strain of.8) exhibits a dislocation cell structure with very high dislocation density [Fig. 3(a)]. The corresponding SAD pattern, viewed from the 11 zone axis, shows a single net pattern, suggesting that the misorientaion between the cells hardly exists. The specimen after 3 cycles (an equivalent strain of 2.4) also exhibits a dislocation cell structure similar to the sample after 1 cycle, however the dislocation density is lower and the cells become more defined than that after 1 cycle [Fig. 3(b)]. The corresponding SAD pattern, viewed from the 123 zone axis, shows a single net pattern indicating that the misorientation between the cells hardly exists as well. The specimen ARBed to 5 cycles (an equivalent strain of 4.) also exhibits a dislocation cell structure similar to that after 1 or 3 cycles [Fig. 3(c)], however, the cells become more defined than that after 3 cycles and the subgrain boundaries of linear morphology are seen in the structure, as indicated by the arrows. The corresponding SAD pattern, viewed from the 111 zone axis, is characterized by slightly diffused spots, suggesting that the misorientaiton between the cells is slightly larger than those after 1 or 3 cycles. The above TEM observation reveals that the ARB of 5 cycles at ambient temperature was not enough to developing a well-defined ultrafine grain structure in the IF steel. By contrast, it was reported that a well-defined ultrafine grain structure was obtained in an aluminum under the same ARB conditions. 4, 7) In addition, the present microstructure is apparently different from the structure (ultrafine grained structure) of IF steel processed by the ARB at 773 K. 5, 1) Tsuji et al. suggested that grain refinement by the ARB is attributed to subdivision of grains to submicrometer scale by severe deformation and the subsequent formation of ultrafine grains with clear boundaries by recovery. 9, 1) Therefore, the ARB has been often done at intermediate temperatures in which the recovery can occur. 4 6, 8, 9) In the case of aluminum alloy, it is possible for UFGs to develop in the as ARB-processed sample even at ambient temperature because the recovery can occur due to temperature rising by plastic deformation. 7) However, in the case of IF steel, the recovery is hard to occur at ambient temperature because the temperature for recovery is much higher compared to aluminum. However, it is worth noting that Azushima et al. have reported the development of UFGs in IF steel processed by ECAP at ambient temperature. 11) According to their report, UFGs with.2 µm wide and.5 µm long developed in the IF steel severely deformed up to an equivalent strain of 11.5 (1 passes), which is about 3 times larger than that in present study. This report suggests that ultra severe plastic deformation may result in developing UFGs at ambient temperature. Accordingly, further studies on se-

2322 S.-H. Lee, Y. Saito, K.-T. Park and D. H. Shin Fig. 3 TEM micrographs and the corresponding SAD patterns observed from ND plane of ultra low carbon IF steel processed by ARB at ambient temperature. After 1 cycle (a), 3 cycles (b) and 5 cycles (c). 3.2 Mechanical properties Nominal stress-strain curves of the ARBed IF steel are shown in Fig. 5. The as-received one shows relatively low strength and large elongation. The tensile strength increases with increasing the number of ARB cycles and it reaches a maximum of about 8 MPa at the 5th cycle. On the other hand, the total elongation dropped largely at the 1st cycle and it hardly changes with the number of ARB cycles after the 2nd cycle. The uniform elongation is also very low, which is a common characteristic of severely deformed materials. The mechanical properties of the ARBed IF steels are plotted against the equivalent strain in Fig. 6. For comparison, the results of the IF steel ARBed at 773 K 5) and the samples ECAPed at ambient temperature 11) are plotted together in Fig. 6. The tensile strength increases rapidly with the equivatures below 873 K [Fig. 4(d)]. That is, it shows a structure similar to conventional recrystallized structure, i.e. equiaxed shape and dislocation-free in the grain interior. However, the grain size is almost equal to that annealed at 823 K, indicating that most of dislocations in the grain interior were annihilated by the recovery without grain growth. Figures 4(d) (h) shows the grain growth sequence with annealing temperature above 873 K. As shown in Fig. 4(f), abnormal grain growth occurred at 898 K. vere plastic deformation at ambient temperature are needed to clarify the mechanism for development of UFGs. Changes in TEM microstructure observed at ND plane of the specimen ARBed by 5 cycles with annealing temperature are shown in Fig. 4. As shown in Fig. 4(a), the ultrafine grains with clear grain boundaries were formed partially after annealing at 673 K, as indicated by the arrows. The corresponding SAD pattern, viewed from the 111 zone axis, shows the appearance of a diffused ring pattern and many extra spots, indicating the presence of the high-angle boundaries in the resultant structure. This microstructure is apparently different from that of as-arb processed one shown in Fig. 3(c). In addition, lots of dislocations are seen inside the ultrafine grains formed, suggesting that their formation mechanism is different from that of the conventional recrystallization. The sample annealed at 773 K also shows an ultrafine grained structure, however the fraction of the ultrafine grains is higher and the grain size is larger than that of the sample annealed at 673 K. Annealing at 823 K results in further grain growth and the relatively well-defined ultrafine grains covers almost all regions, as shown in Fig. 4(c). However, the dislocation density in the grains still remains high and the grains are elongated along a certain direction, supposedly the rolling direction. The specimen annealed at 873 K exhibits quite different structure, compared to the samples annealed at temper-

Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed 2323 alent strain at the initial stage, primarily due to strain hardening at any cases. At large equivalent strain regime, a gradual increase of the tensile strength was resulted. It is noted that the present ARB process shows the largest strength increment among them. The elongation dropped largely at strain of about 1., however, above strain of 1., it hardly changes with strain at any processes. ECAPed one shows larger elongation than the ARBed sample at any equivalent strain. In general, strengthening during the ARB process is caused by not only strain hardening but also grain refinement. However, the present experimental results reveals that, for the IF steel ARBed at ambient temperature, the strain hardening was primarily responsible for the strengthening because the recovery did not occur so actively and the grain refining was not observed. Therefore, the difference in the strength of the ARBed IF steel between the present and Tsuji et al. s studies would be primarily due to the different strain hardening rate associated with ARB processing temperature. Figure 7 shows the changes in nominal stress-strain curves with the annealing temperatures. The tensile strength decreased and the total elongation increased gradually with increasing the annealing temperature up to 898 K. Annealing at 923 K caused drastic increase in the total elongation, accompanying with large decrease of the strength. This corresponds well with the grain growth at 923 K, as shown in Fig. 4(g). This indicates that it is necessary to anneal at temperatures above 923 K for obtaining enough uniform elongation. The softening curves of the as-arb processed IF steel against annealing temperatures is plotted in Fig. 8. The specimen annealed at 673 K almost retains the strength level of as-arb processed one. However, annealing at temperatures above 673 K resulted in gradual decreasing in the strength. As a result, the strength largely dropped to about 3 MPa after annealing at 923 K. This softening behavior is very different from the normal one showing the drastic strength drop at a certain annealing temperature. On the other hand, the total elongation increased gradually with the annealing temperature up to 898 K and it increased drastically above 923 K. This behavior corresponds well with the gradual change in microstructure with annealing temperature. 4. Conclusions An ultra low carbon IF steel was severely deformed to an equivalent strain of about 4. by the ARB process at ambient temperature and subsequently annealed at various temperatures. (1) ARBed IF steel exhibited a dislocation cell structure with relatively high dislocation density and did not develop ultrafine grains even in the specimen severely strained to a strain of 4. The subsequent annealing at 673 or 773 K resulted in developing ultrafine grains in the structure. This indicates that the recovery is necessary for the formation of ultrafine grains in the IF steel severely deformed by ARB pro-

2324 S.-H. Lee, Y. Saito, K.-T. Park and D. H. Shin Fig. 4 TEM micrographs and the corresponding SAD patterns observed from ND plane of ultra low carbon IF steel annealed for.5 h at various temperatures after ARB of 5 cycles. Annealed at 673 K (a), 773 K (b), 823 K (c) and 873 K (d) respectively. Annealed at 898 K (e), 898 K (f), 923 K (g) and 973 K (h). 1 1 7 Nominal Stress, s / MPa 8 6 4 2 4c 2c 1c 5c 3c as received 1 2 3 4 5 6 7 Nominal Strain, e (%) Fig. 5 Nominal stress-nominal strain curves of ultra low carbon IF steel processed by ARB at ambient temperature. cess at ambient temperature. (2) The tensile strength of the ARBed IF steel increased with strain and it reached a maximum of 813 MPa at strain of 4, which is about 3 times higher than the initial value. The elongation dropped largely from 6 to below 1% at the 1st Tensile Strength, B / MPa 8 6 4 2 tensile strength ARB at RT (this study) ARB at 773K (Tsuji et al.) ECAP at RT ( Azushima et al.) elongation 1 2 3 4 5 6 Equivalent Strain, cycle. But, it hardly changed with strain from the 2nd cycle. The annealing resulted in the gradual decrease in the strength 6 5 4 3 2 1 Elongation, e (%) Fig. 6 Mechanical properties of the ARBed IF steel plotted against the equivalent strain during ARB process at ambient temperature. The results are compared to those of the other processes.

Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed 2325 1 1 5 Nominal Stress,s /MPa 8 6 4 2 As ARBed 673K 773K 823K 873K 893K 923K 973k 173K 1 2 3 4 5 6 7 8 Nominal Strain, e (%) Fig. 7 Nominal stress-nominal strain curves of ultra low carbon IF steel processed by ARB of 5 cycles at ambient temperature and subsequently annealed at various temperatures. Tensile Strength, B / MPa 8 4 tensile strength 6 3 4 2 2 elongation 1 2 4 6 8 1 12 Annealing Temperature,T / K Elongation, e (%) Fig. 8 Softening curves of ultra low carbon IF steel processed by ARB of 5 cycles plotted against annealing temperatures. and the large increase in the elongation. REFERENCES 1) R. Z. Valiev, N. A. Krasilnikov and N. K. Tsenev: Mater. Sci. Eng. A137 (1991) 35 4. 2) D. H. Shin, Y. S. KIM and E. J. Lavernia: Acta Mater. 49 (21) 2387 2393. 3) Z. Horita, D. J. Smith, M. Furukawa, M. Nemoto, R. Z. Valiev and T. G. Langdon: J. Mater. Res. 11 (1996) 188 189. 4) Y. Saito, N. Tsuji, H. Utsunomiya, T. Sakai and R. G. Hong: Scr. Mater. 39 (1998) 1221 1227. 5) N. Tsuji, Y. Saito, H. Utsunomiya and S. Tanigawa: Scr. Mater. 4 (1999) 795 8. 6) Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai: Acta. Mater. 47 (1999) 579 583. 7) S. H. Lee, Y. Saito, T. Sakai and H. Utsunomiya: Mater. Sci. Eng. A325 (22) 228 235. 8) Y. Ito, N. Tsuji, Y. Saito, H. Utsunomiya and T. Sakai: J. Japan. Inst. Metals 64 (2) 429 437. 9) N. Tsuji, Y. Saito, Y. Ito, H. Utsunomiya and T. Sakai: Ultrafine Grained Materials, Edited by R. S. Mishra, S. L. Semiatin, C. Suryanarayana, N. N. Thadhani and T. C. Lowe (The Minerals, Metals & Materials Society, 2) pp. 27 218. 1) N. Tsuji, Y. Ito, Y. Koizumi, Y. Minamino and Y. Saito: Ultrafine Grained Materials 2, Edited by Y. T. Zhu, T. G. Langdon, R. S. Mishra, S. L. Semiatin, M. J. Saran and T. C. Lowe (The Minerals, Metals & Materials Society, 22) pp. 389 397. 11) A. Azushima and K. Aoki: Mater. Sci. Eng. A, in press.