Enhanced strength and ductility in an ultrafine

Enhanced strength and ductility in an ultrafine-grained Fe-22Mn-0.6C austenitic steel having fully recrystallized structure Y.Z. Tian a*, Y. Bai a, M.C. Chen a, A. Shibata a,b, D. Terada a,b,c, N. Tsuji a,b a

Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan b

c

Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501, Japan

Department of Mechanical Science, Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275-0016, Japan.

Abstract Fe-22wt.%Mn-0.6wt.%C twinning–induced plasticity (TWIP) steel having fully recrystallized ultrafine-grained structure was obtained through a simple thermomechanical process repeating cold-rolling and annealing. The minimum average grain size of 550 nm was successfully obtained. The ultrafine-grained steel exhibited good mechanical properties superior to those previously reported in Fe-Mn-Al-Si TWIP steels and the origin was discussed based on the difference of stacking fault energies. * Corresponding author, E-mail: [email protected]

Materials with high strength and good ductility are attractive for structural applications. However, strength and ductility usually trade off in most metallic materials [1,2]. In previous studies, various kinds of processing procedures have been tried to prepare materials with superior mechanical properties [3-5]. Among these processes, combination of plastic deformation and subsequent annealing is a promising way to realize good strength and good ductility [6,7]. For face-centered cubic (FCC) metals and alloys, it has been known that materials with low stacking fault energies (SFEs) are easy to be refined through plastic deformation [8]. Fully recrystallized and ultrafine-grained structures could be surprisingly fabricated in a 31wt.%Mn-3wt.%Al-3wt.%Si twinning–induced plasticity (TWIP) steel (hereafter Fe-31Mn-3Al-3Si) [9] and a Cu-6.8wt.%Al alloy [10], both with low SFEs, by conventional cold rolling and annealing processes. These materials showed high yield strength and good uniform elongation. Recently, much attention has been paid to TWIP steels due to their excellent mechanical properties [11-14]. TWIP steels were processed by either cold rolling and annealing or equal-channel angular pressing (ECAP) [15,16]. In contrast to the cold worked specimens, the annealed specimens with fully recrystallized or partially recrystallized microstructures could retain excellent ductility and strength [15]. However, the partially recrystallized structure is thermally unstable. Thus it is meaningful to produce fully recrystallized TWIP steels having ultrafine grain sizes. 1

Among various kinds of TWIP steels, a Fe-22wt.%Mn-0.6wt.%C (hereafter Fe-22Mn-0.6C) shows a very large strain hardening capability, which is much higher than that of Fe-31Mn-3Al-3Si [17-19]. In the previous study using the Fe-31Mn-3Al-3Si, 92% cold-rolling and subsequent annealing could result in fully recrystallized nanostructures [9]. The Fe-22Mn-0.6C used in the present study was, however, quickly hardened by cold-rolling and 92% reduction could not be achieved due to the low deformability. In fact, the Fe-22Mn-0.6C TWIP steels were always cold rolled no more than 60% reduction in thickness and then annealed in previous studies, and the mean grain size achievable was larger than 1 m [20-23]. In this study, a thermomechanical process to repeat moderate cold-rolling and annealing was developed for the Fe-22Mn-0.6C steel and grain sizes much smaller than those reported in previous papers [21,22] could be obtained. An Fe-22Mn-0.6C TWIP steel is used in this study, and the chemical composition of the alloy is shown in Table 1. It has been reported that this TWIP steel has a low SFE of about 23 mJm-2 [24]. The starting material is a plate with a thickness of 12 mm and the mean grain size is 21 m. The TWIP steel was firstly cold-rolled to a thickness of 8 mm (33.3% reduction in thickness) and then annealed at 873K (600° C) for 10 min. Then this material was cold-rolled to 5 mm in thickness (37.5% reduction) and annealed at 873K (600° C) for 10 min. The sheet was cold rolled to 3 mm (40% reduction) followed by annealing at 873K (600° C) for 10 min, and finally cold rolled to 1 mm (66.7% reduction and total reduction of 91.7%). The cold-rolled sheet was finally annealed at 823K (550° C) for different periods of time. Microstructures of the cold-rolled sheet were characterized by transmission electron microscopy (TEM) using JEOL 2010 operated at 200 kV. TEM observations were carried out from the transverse direction (TD) of the sheet. Foil specimens were twin-jet electropolished by Struers Tenupole-3 in a solution of 10% HClO4 and 90% C2H5OH with a voltage of 20 V at a low temperature of 253K (-20oC). The annealed specimens were characterized by using a field-emission scanning electron microscope (FE-SEM, FEI (Philips) Siron) equipped with electron backscattering diffraction (EBSD) system at an accelerating voltage of 15 kV. The mean grain size was measured by using the line intercept method on EBSD maps, where all high-angle grain boundaries (HAGBs) and twin boundaries (TBs) were counted. Tensile tests were conducted at an initial strain rate of 8.3 × 10-4 s-1 at room temperature using Shimadzu tensile testing machine. Tensile specimens with a gauge length of 10 mm, width of 5 mm and thickness of 1 mm were cut by an electrical discharge machine. Fig. 1 shows TEM images of the Fe-22Mn-0.6C TWIP steel after the final cold-rolling to 1 mm thickness. Nano lamellae of deformation twins and stacking faults were observed in the TWIP steel having a low SFE. The twins and stacking faults were divided into small pieces by shear bands. Some of such islands were surrounded by white broken lines in Fig. 1a. Fig. 1b displays a microstructure at a higher magnification where different deformation structures were captured. Three selected area diffraction patterns (SADPs) labeled by numbers of 1, 2 and 3 were also inserted in Fig. 1b, which corresponded to three different regions indicated by white dotted circles. The regions 1 and 2 showed banded dislocation substructures and the corresponding SADPs indicated that there were large local misorientations within the regions. In region 3, a lamellar structure was observed, and the SADP indicates that 2

these are deformation twins and stacking faults. It can be concluded that the cold-rolled Fe-22Mn-0.6C TWIP steel has a nanocrystalline and complicated deformation structure with high dislocation density as well as deformation twins and stacking faults. Fig. 2 shows EBSD maps of the Fe-22Mn-0.6C TWIP steel after final annealing at 823K (550° C) for 1 min, 2 min and 4 min, respectively. Three maps in the upper row (a,b,c) show inverse pole figure (IPF) maps, while the lower maps (d,e,f) are grain boundary maps where HAGBs, low-angle grain boundaries (LAGBs) and TBs were drawn by black lines, green lines and red lines, respectively. After annealing at 823K (550° C) for 1 min, it was found that the specimen was only partially recrystallized and most of the regions were still covered by deformed microstructures where sound Kikuchi-lines could not be obtained due to the fine deformed structures with high dislocation density so that ‘mosaic’ colors were observed in Fig. 2a. When the annealing period was increased to 2 min, most of the regions were filled with equiaxed recrystallized grains, as shown in Figs. 2b and 2e, but still recrystallization was not completed. After annealing for 4 min, the steel was fully recrystallized and showed an equiaxed grain structure with low dislocation density, as depicted in Figs. 2c and 2f. The mean grain size measured was 550 nm. That is, a fully recrystallized ultrafine-grained could be obtained in the present Fe-22Mn-0.6C TWIP steel. This is the first report showing an ultrafine-grained Fe-22Mn-0.6C TWIP steel with fully recrystallized and submicrometer-sized grains, and the mean grain size (550 nm) is the minimum among various studies in the Fe-22Mn-0.6C TWIP steel. It should be emphasized that the structure was fabricated by simple combinations of conventional cold-rolling and annealing. Fig. 3 shows engineering stress-strain curves of the as-received, cold-rolled and annealed Fe-22Mn-0.6C TWIP steel. The coarse-grained starting material (grain size of 21 m) showed a low yield strength (0.2% proof stress) of 317 MPa, but the uniform elongation was very large (78%), which was due to the large strain-hardening capability of this TWIP steel [17]. After the final cold-rolling process (as-rolled), it was found that the yield strength and tensile strength of the TWIP steel increased to 1810 MPa and 2120 MPa, respectively, but the uniform elongation decreased to only 2.7%. This is attributed to the limited strain hardening capability in the fine and complicated deformation microstructure with highly stored dislocations (Fig. 1) where it is difficult to introduce more dislocations. In that case, the uniform elongation is limited because of the early plastic instability [7]. After annealing at 823K (550° C) for 1 min, it was found that the yield strength and tensile strength remained 1080 MPa and 1434 MPa, respectively, while the uniform elongation significantly recovered to 28%. It is noteworthy that such a good balance of strength and ductility was achieved in the partially recrystallized state shown in Figs. 2a and 2d. When the steel was furthermore annealed for 2 min at 823K (550° C), the yield strength decreased to 840 MPa but the uniform elongation significantly increased to 45%. Although the steel was still not fully recrystallized at this stage, as shown in Figs. 2b and 2e, it showed large strain hardening after yielding and the tensile strength reached to 1299 MPa. The steel was fully recrystallized after annealing at 823K (550° C) for 4 min (Figs. 2c and 2f), which exhibited the yield strength of 793 MPa, tensile strength of 1247 MPa and uniform elongation of 47%. Among various TWIP steels, Fe-22Mn-0.6C and Fe-31Mn-3Al-3Si are typical ones having different SFEs and they have been systematically studied. Mechanical 3

property data of these two kinds of TWIP steels were picked up from previous literatures and plotted together with the present results in Fig. 4 showing strength-ductility balances [9,18,22,25]. In Fig. 4, only the data of fully recrystallized specimens are shown. Fig. 4a demonstrates that the balance of yield strength and uniform elongation is similar between Fe-31Mn-3Al-3Si and Fe-22Mn-0.6C, and the present TWIP steel having a fully recrystallized ultrafine-grained structure shows the highest yield strength in the series, as highlighted in the figure. On the other hand, Fig. 4b indicates that the balances of tensile strength and uniform elongation of two kinds of steels lay on totally different curves and Fe-22Mn-0.6C exhibits much better strength-ductility balance than Fe-31Mn-3Al-3Si, which is presumably attributed to the higher strain-hardening rate in Fe-22Mn-0.6C [17]. It has been reported that the SFE of the Fe-31Mn-3Al-3Si TWIP steel is 40 mJ m-2 [26], which is higher than that of the Fe-22Mn-0.6C TWIP steel (23 mJm-2). Note that a ultrafine-grained Fe-31Mn-3Al-3Si with a grain size of 400 nm was recently prepared by Saha et al. [9] and the yield strength of the material was 702 MPa, which was close to the present fully recrystallized Fe-22Mn-0.6C TWIP steel having a grain size of 550 nm (Fig. 4a). However, the UTS of the ultrafine-grained Fe-31Mn-3Al-3Si was only 843 MPa which was significantly lower than the present ultrafine-grained Fe-22Mn-0.6C which was 1247 MPa (Fig. 4b). The significant difference between the yield strength and the tensile strength clearly proves the higher strain hardening capability in Fe-22Mn-0.6C [17]. Among all the fully recrystallized TWIP steels, it is found that the present TWIP steel having 550 nm grain size exhibits the highest yield strength and UTS while retaining good uniform elongation of 47%. In summary, a fully recrystallized ultrafine-grained structure was realized in an Fe-22Mn-0.6C TWIP steel for the first time through a simple thermomechanical process repeating conventional cold-rolling and annealing. The minimum mean grain size of 550 nm was produced in this TWIP steel, which was also the minimum among previous studies about this steel. The ultrafine-grained steel showed the yield strength of 793 MPa, tensile strength of 1247 MPa and uniform elongation of 47%, which maintained good strength-ductility balance. Y.Z.T. gratefully acknowledges the support of the Japan Society for the Promotion of Science (JSPS) for awarding a postdoctoral fellowship. This work was financially supported by the Grant-in-Aids for Scientific Research on Innovative Area “Bulk Nanostructured Metals” (area No.2201), for Scientific Research (A) (No.24246114) and for Challenging Exploratory Research (No.25600039), and the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Table 1 Chemical composition of the Fe-Mn-C TWIP steel used in the present study. Eleme nt Mass%

C

Si

Mn

P

S

Al

Cr

O

N

Fe

0.5 6

0.0 6

21.6 8