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Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing
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Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing A.S. Hamada, A.P. Kisko, P. Sahu, L.P. Karjalainen
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)00058-1 http://dx.doi.org/10.1016/j.msea.2015.01.042 MSA31980
To appear in:
Materials Science & Engineering A
Received date: 21 November 2014 Revised date: 15 January 2015 Accepted date: 20 January 2015 Cite this article as: A.S. Hamada, A.P. Kisko, P. Sahu, L.P. Karjalainen, Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing, Materials Science & Engineering A, http: //dx.doi.org/10.1016/j.msea.2015.01.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing A.S. Hamada1,2*, A.P. Kisko1 , P. Sahu3 and L.P. Karjalainen1 1
Centre for Advanced Steels Research, Box 4200, University of Oulu, 90014 Oulu, Finland, Metallurgical and Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez University, Box 43721, Suez, Egypt 3 Department of Physics, Jadavpur University, Kolkata 700 032, India 2
Abstract Controlled martensitic reversion annealing were applied to a heavily cold-worked metastable austenitic low-Ni Cr-Mn austenitic stainless steel (Type 201) to obtain different ultrafine austenite grain sizes to enhance the mechanical properties, which were then compared with the conventional coarse-grained steel. Characterization of the deformed and reversion annealed microstructures was performed by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and light and transmission electron microscopy (TEM). The steel with a reverted grain size ~ 1.5 µm due to annealing at 800 °C for 10s showed significant improvements in the mechanical properties with yield stress ~ 800 MPa and tensile strength ~ 1100 MPa, while the corresponding properties of its coarse grained counterpart were ~ 450 MPa, ~ 900 MPa, respectively. However, the fracture elongation of the reversion annealed steel was ~ 50% as compared to ~ 70% in the coarse grained steel. A further advantage is that the anisotropy of mechanical properties present in work-hardened steels also disappear during reversion annealing.
Keywords: Cr-Mn austenitic stainless steel, reversion treatment, ultrafine grain size, mechanical properties.
*corresponding author: Tel/Fax.: +20 62 3 360 252
E-mail address: [email protected]
1
1. Introduction
The presence of expensive Ni in steel as an alloying element significantly raises the overall cost of the steel grade. Based on metallurgical concepts, the alloying of Mn, Cu, C and N can be
used in lieu of Ni to stabilize the austenitic structure with an acceptable compromise between the production cost and the properties of the steel. Furthermore, nitrogen has a strong, generally beneficial effect on strain hardening behaviour and mechanical properties of these steels. Several austenitic stainless steel grades based on the system Fe-CrMn-N (series 2xx) with low or without Ni content have been developed as an alternative to the expensive austenitic Cr-Ni series 3xx and finding widespread applications in automotive industry, kitchen utilities and even for cryogenic applications at very low temperatures [1,2]. It is well known that the austenitic stainless steels originally having low yield strength (YS) is unsuitable for structural applications, unless the YS are increased by cold rolling [3]. And as the strength increases, the structures also become lighter and therefore there is a growing interest to develop high-strength steels for automotive applications demanding a good combination of high strength and ductility [4].
Furthermore, the minimum targeted YS for an anti-intrusion and industry application should be 600–700 MPa [5]. Recently, Saha et al. [6] succeeded to create nano-structured austenitic high Mn steel with the grain size of 400 nm and the YS of 702 MPa, applying very high cold rolling reduction of 92% with recrystallization at a low temperature to enhance the nanostructure.
The disadvantage of strengthening by cold rolling is the introduction of anisotropy in mechanical properties and the strength being different in tension and compression, as well as in different directions relative to the longitudinal direction [7]. Therefore, the strengthening methods providing isotropic mechanical properties are looked for and grain refinement that can be
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obtained through the reversion annealing of a metastable austenitic stainless steel to transform the strain-induced martensite back to austenite is an efficient route to obtain the envisaged properties. The reversion of deformation-induced martensite to austenite enables a marked grain refinement in the bulk scale of the steel. Recently, the reversion treatment has been applied in a laboratory scale to various metastable austenitic stainless steel grades such as 301 and 301LN Cr-Ni [8], CrMn low Ni [9] and also to Ni-free austenitic stainless steels [10].
We previously [11] investigated the effect of temperature and strain rate on the strain hardening behaviour and the strength-ductility combination of a Type 201L steel with a conventional grain size of 15-20 µm during the strain-induced martensite transformation (the TRIP effect) and the occurrence of mechanical twinning. Kisko et al. [9,12] have subsequently investigated the influence of the reversion treatment on grain size and mechanical properties. Here, we present a comparison of the mechanical properties of reversion annealed 201 steel with that of a coarse grained counterpart to demonstrate the significant advantages of the grain size refinement. Finally, the influence of the reversion annealed grain structures of nano/ultrafine size on the mechanical properties is determined related to the deformation mechanism.
2. Experimental Procedures 2.1. Test Material
The experimental materials were sheets of Type 201 (EN 1.4372) low-Ni Cr-Mn austenitic stainless steel, supplied by Outokumpu Stainless Oy, Tornio Works (Tornio, Finland), with the nominal chemical composition shown in Table I. The initial structures of the sheets are fully austenitic with a coarse-grained (CG) structure (≈ 18 µm) as measured using the linear-intercept method (ASTM E112).
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2.2 Reversion annealing processing
For the reversion treatment, the steel sheets were cold rolled in a laboratory rolling mill to 60% reduction and subsequently annealed on a Gleeble 1500 thermomechanical simulator. In the controlled reversion annealing treatment, the specimens were heated to the reversion temperature between 700 and 1000°C, at a rate of 200°C/s for different soaking times and cooled at a rate 200 °C/s down to 400°C, followed by final air cooling to room temperature to obtain different highly refined grain sizes. The reversion heat treatment is presented schematically in Fig. 1
2.3 Mechanical and microstructural characterization The mechanical properties were measured by uniaxial tensile tests at a constant strain rate 5×10-4 s-1 using a Zwick Z 100 tensile machine. The gauge length was short, 15 mm, in the instance of reversion treated samples. Three samples were tensile tested for each annealing condition. Tensile samples used for annealing treatment were cut from the cold rolled sheets in the rolling direction.
Fig. 1. Reversion annealing treatment cycle.
Table 1. Chemical composition of the studied steel [wt-%].
Microstructures were studied by an optical microscope and a field emission gun scanning electron microscope (FEG-SEM Carl Zeiss Ultra plus) with an electron back scatter diffraction (EBSD) unit. An acceleration voltage of 20 kV and a step size of 0.3 µm were used during the EBSD scans. Before the EBSD examination, the steel surfaces were mechanically polished down to 1 µm by using a diamond suspension, and finally chemically polished by using a 0.05 µm colloidal suspension of silica for about 10 min.
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The structural features after interrupted tensile strains were also examined in a transmission electron microscope (TEM) operated at 120 kV. Thin foils were prepared by twin jet electropolishing of 3 mm disks, punched from the specimens, using a solution of 10% perchloric acid in acetic acid electrolyte.
X-ray diffraction (XRD) data acquisition was carried out using Cu Kα radiation in a powder diffractometer (Siemens-D500) equipped with a secondary beam monochromator. The step scan mode, with a preset holding time of 5–10 s at each 0.01° step in 2θ, was used to improve the counting statistics and yield data suitable for stable refinement.
A Ferritescope (Helmut Fisher FMP 30) instrument was also used to measure the fraction of the ferromagnetic α′-martensite phase. The readings obtained by the Ferritescope instrument were multiplied by a correction factor 1.7 for α′-martensite fractions [13].
3. Results and Discussion 3.1 Microstructures and grain sizes
After the 60% cold rolling reduction, the martensite content measured by a Ferritescope was 30%. Hence, the cold-rolled microstructure comprises of a major amount of deformed austenite with significant strain-induced α′-martensite. During reversion annealing even at low temperatures of 700-900 °C, the martensite tends to revert back to austenite with a ultrafine grain size either through martensitic shear or diffusional reversion mechanisms [14]. However, the refinement of deformed retained austenite grains requires the occurrence of static recrystallization, which takes place at slightly higher temperatures and more slowly, as shown by Kisko et al. [12,15].
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It is well established that with increasing the cold rolling deformation beyond 50 pct thickness reduction, the slip bands and dislocation densities significantly increase. Consequently, the indexing of the back-scattered pattern for the cold rolled structures is hindered. Therefore, the mapping of the crystal orientation of the grain structure after cold rolling cannot be shown by EBSD. Sahu et al. [16] observed that with increasing the cold rolling reduction of an austenitic high Mn steel to 50 pct, the grain structure cannot be indexed by EBSD as a result of increasing slip bands and high dislocation density.
Fig. 2 presents the evolution of the microstructures in reversion annealing at different temperatures for various durations. In Fig. 2a, the microstructure at 800 °C for 1s consists of a mixture of fine reverted austenite grains with the submicron grain size of about 500 nm together with large areas of recovered retained austenite, which did not transform to martensite during cold rolling and has not recrystallized yet. Finer retained austenite are formed through the activation of static recrystallization at longer annealing times, as shown in Fig. 2b where the average grain size of the new austenite at 800°C/10 s is 1.5 µm. With increasing the annealing temperature to 900 °C, the grain size structure tends to homogenize, Fig. 2c. However, a relatively coarse-grained structure with the uniform grain size of about 5 µm is induced at high annealing temperature of 1000°C for 1 s, as shown in Fig. 2d.
Fig. 2. Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction.
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3.2 Mechanical Tensile Properties The results of the uniaxial tensile tests in the range of -80°C to 200 °C and two different strain rates conducted on the coarse-grained (15 µm) steel are shown in Fig. 3 [11]. At room temperature, the yield stress is about 450 MPa and the tensile strength 900 MPa. Two deformation mechanisms are activated over the deformation range of temperature: the formation of strain-induced martensite (the TRIP effect) and mechanical twinning (the TWIP effect). The strain rate has a significant influence on the ductility (elongation value) of the steel due to adiabatic heating taking place at the higher strain (10-2) rate affecting the TRIP effect, in particular [11]. At lower strain rate (10-4), the elongation is about 70% at room temperature in absence of any significant adiabatic heating.
Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
XRD patterns of the coarse grained 201 steel after tensile straining in the temperature range: -80 °C and 150 °C are shown in Fig. 4. It is seen that at subzero temperatures, straining induces very high degree of martensitic transformation, which gradually decrease with increasing straining temperature until RT. The Rietveld analysis yields that at RT, there is a significant amount of α′martensite (≈26 %) and also a very small amount of ε-hcp martensite (≈ 1%). However, at -80 °C, the amount of α′- martensite increases to a value of 96%. At temperatures above RT, the XRD patterns presented in Fig. 4b reveal that the TRIP effect is significantly suppressed due to increase in stacking fault energy (SFE) of austenite at higher straining temperatures. For instance, the amount of α′- martensite decreases to (≈ 3%) accompanied by the trace mount of ε-hcp martensite (≈ 1%) at 50°C. The detailed results of the XRD structural and microstructural analysis have been reported in a separate paper [17].
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Fig.4: XRD patterns of the coarse grained 201steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
The tensile properties for the reversion annealed Type 201 steel are shown as a function of annealing temperature and duration in Fig. 5. It can be seen that the yield stress (Rp0.2) and tensile strength (Rm) increase with decreasing reversion annealing temperature, especially in the regime below 900°C. For example, the yield strength of 800 MPa and tensile strength of 1100 MPa are obtained in annealing at 800°C for 10s having grain size 1.5 µm. Then, the fracture elongation is high, about 50% as measured for a short gauge length of 15 mm. Hence, the yield strength is almost doubled by the reversion treatment compared to that of the conventional steel with the grain size of 15-18 µm. A further advantage is that the anisotropy in mechanical properties, present in work-hardened grades, disappears in annealing [15]. As observed in the present study, the yield stress is strongly influenced by the grain size and significant improvement could be achieved by the grain size is refinement from 10-20 µm to 1-2 µm during reversion treatment, and the strengthening mechanisms have been discussed in detail in previous papers [9,12,15].
Fig. 5.
The mechanical properties of Type 201 steel as reversion annealed at different
temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
It is noteworthy that besides grain refinement, the TRIP and TWIP effects also significantly contribute to the tensile strength and ductility of the steel through the formation of strain-induced α′-martensite and mechanical twinning during tensile straining, respectively. The stability of austenite is dependent on its grain size and the stability increases with decreasing the grain size down to a few micrometers [9]. In agreement, the fraction of α′-martensite was about 28%, 12% and 3% after tensile straining to fracture and the corresponding austenite grain sizes were 15 µm,
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7 µm and 2 µm, respectively. The corresponding elongation values decreased slightly with the refined grain size, but the relationship between the intensity of the TRIP effect and the ductility is not straightforward.
The activated deformation mechanisms during tensile straining have been examined in using a TEM after straining to certain strains. Fig. 6 shows bright field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10s.
Fig. 6. TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
It is generally accepted that Type 201 has a very low SFE and that, in such steel, numerous stacking faults are created due to the movement of a 6 112 Shockley partial dislocations on the close-packed {111} planes of austenite [18]. The microstructure in Fig. 6a reveals the presence of numerous wide intersecting stacking faults within the untransformed austenite, which are arranged in primary and conjugate slip systems. This observation is in agreement to that of a low SFE close packed alloy and confirms the activation of multiple slips at the onset of straining.
Microscopic shear bands are seen after tensile straining to 2% engineering strain, Fig. 6b. The origin of these shear bands is unclear. They can be considered a consequence of overlapping stacking faults or as a consequence of planar slip due to the low-SFE of the material [19,20]. As the tensile straining is continued to 10%, the dark areas seen in the bright-field image in Fig. 6c are presumably α′-martensite, containing a high density of dislocations. Numerous micro shear bands (µSBs) can be seen in the microstructure. The higher magnification microstructure in Fig. 6d distinctly reveals the presence of α′-martensite within the shear bands. Similarly, Talonen et al. [21] observed a number of shear bands and shear band intersections resulting in α′-martensite
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nucleation.
The TEM microstructures of the tensile strained 201 steel after reversion annealed at 1000 °C are shown in Fig. 7. Density of wide stacking faults along with Planar-dislocation arrangements can be seen in the deformed microstructures, see Figs 7a-b. As the deformation proceeds to 10 pct strain, high density of shear bands are formed. Consequently, martensitic regions are induced and bounded by dislocation walls and forests, as shown in Fig. 7c. Another strain-induced microstructural feature has been observed during the deformation proceeding of the reversion annealed steel.
Numerous parallel twins of few tens of a nanometer thick with a small
interdistance can be seen on the related microstructure, as indicated in Fig. 7d.
Fig. 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
The target strength-ductility range (the brown band) set by the European Commission for automotive steels is shown in Fig. 8 [22]. By inserting the data for conventional stainless steels [23] and that for Type 201 steel obtained after various reversion treatments, it becomes apparent that the combinations of tensile strength-ductility achieved in the reversion treated condition are excellent exceeding those of conventional annealed or work-hardened stainless steels and even the target band.
Fig. 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [20]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for work-hardened grades from Ref. [23].
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Conclusions The most relevant results of the present work concerning the deformation processes and the mechanical properties of the reversion annealed 201 steel can be briefly summarized as follows:
1. Reversion annealing of a heavily cold-rolled metastable 201 austenitic stainless steel is an efficient way to refine the austenite grain size to the order of a micrometer. The grain refinement significantly improves its mechanical properties as the yield and tensile strengths are remarkably improved (the yield stress is readily doubled), with marginal drop of the elongation, which makes them superior to the conventional annealed or work-hardened stainless steel. In addition, the anisotropy in the mechanical properties disappears. 2. Formation of the strain-induced martensite (TRIP) effect is more pronounced in the coarse grained structure. However, the nano/submicron austenite grains structure reveals superior mechanical properties with a small amount of strain-induced martensitic transformation.
Acknowledgements The financial support from the Finnish Funding Agency for Technology and Innovation (Tekes) in the Light and Efficient Solutions program (project SPR1) of the Finnish Metals and Engineering Competence Cluster (FIMECC Ltd.) is gratefully acknowledged.
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References: [1] J. Charles, “The new 200 series: an alternative answer to Ni. Surcharge? Dream or nightmare?”, fifth European Stainless Steel Science & Market Congress, Seville, Sept. 27-30, 2005, 19−28. [2] H.M. Cobb, Steel Products Manual: Stainless steels, The Iron and Steel Society, Warrendale, PA, USA, 1999. [3] L.P. Karjalainen, T. Taulavuori, M. Sellman, A. Kyröläinen, Steel Res. Int. 79 (2008) 404−412. [4] P.-J. Cunat and T. Pauly, Proc. 4th Eur. Stainless Steel Conf. Science and Market, Paris, France, 2002, 10−18. [5] H-W Yen, M. Huang, C.P. Scott, J-R. Yang, Scripta Mater. 66 (2012) 1018–1023. [6] R. Saha, R. Ueji, N. Tsuji, Scripta Mater. 68 (2013) 813–816. [7] Design Manual for Structural Stainless Steels. 3rd Edition. Euro Inox and the Steel Construction Institute, 2006. [8] M.C. Somani, P. Juntunen, L.P. Karjalainen, R.D.K. Misra, A. Kyröläinen, Metall. Mater. Trans. A 40 (2009) 729−744. [9] A. Kisko, R.D.K. Misra, J. Talonen, L.P. Karjalainen, Mater. Sci. Eng. A 578 (2013) 408−416 [10] P. Behjati, A. Kermanpur, A. Najafizadeh, H. S. Baghbadorani, Mater. Sci. Eng. A 592 (2014) 77−82. [11] A.S. Hamada, L.P. Karjalainen, R.D.K. Misra, J. Talonen, Mater. Sci. Eng. A 559 (2013) 336−344. [12] A. Kisko, A. S. Hamada, L.P. Karjalainen, J. Talonen, Microstructure and Mechanical Properties of Reversion Treated High Mn Austenitic 204Cu and 201Stainless Steels, HMnS2011, Grand Hilton Hotel, Seoul, Paper B-19, May15−18, 2011.
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[13] J. Talonen, P. Aspegren, H. Hänninen, Metall. Mater. Sci. Tech. 20 (2004) 1506−1512. [14] K. Tomimura, S. Takaki, Y. Tokunaga, ISIJ Int. 31 (1991) 1431−1437. [15] A. Kisko, L. Rovatti, I. Miettunen, L.P. Karjalainen, J. Talonen, Microstructure and Anisotropy of mechanical properties in reversion-treated high- Mn type 204Cu and 201 stainless steels, in: Proceedings of the 7th European Stainless Steel Conference Science and Market, Como, Italy, No.81, Sept.21–23, 2011. [16] P. Sahu, A.S. Hamada, T. Sahu, J. Puustinen, T. Oittinen, L.P. Karjalainen, Metall. Mater. Trans. A 43 (2012) 47-55. [17] H. Barman, A.S. Hamada, T. Sahu, B. Mahato, J. Talonen, S.K. Shee, P. Sahu, L.P. Karjalainen, Metall. Mater. Trans. A 45 (2014) 1937−1952. [18] P. Sahu, S.K. Shee, A.S. Hamada, L. Rovatti, T. Sahu, B. Mahato, S.G. Chowdhury , D.A. Porter, L.P. Karjalainen, Acta Mater. 60 (2012) 6907−6919. [19] J.W. Christian, S. Mahajan. Prog. Mater. Sci. 39 (1995) 1−157. [20] L. Bracke, K. Verbeken, L. Kestens, J. Penning, Acta Mater. 57 (2009) 1512−1524. [21] J. Talonen, H. Hänninen, Acta Mater. 55 (2007) 6108−6118. [22] European Steel Technology Platform, From a Strategic Research Agenda to implementation, ISBN 92-79-01283-5, European Communities, 2006. [23] D. Peckner, I.M. Bernstein, Handbook of Stainless Steels, McGraw-Hill Book Company, New York, 1977.
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Figures Captions: Figure 1: Reversion annealing treatment cycle. Figure 2: Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction. Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
Figure 4: XRD patterns of the coarse grained 201steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
Figure 5:
The mechanical properties of Type 201 steel as reversion annealed at different
temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
Figure 6: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
Figure 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
Figure 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [19]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for workhardened grades from Ref. [22].
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Figure 1: Reversion annealing treatment cycle.
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Figure 2: Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction.
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Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
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Figure 4: XRD patterns of the coarse grained 201 steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
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Figure 5: The mechanical properties of Type 201 steel as reversion annealed at different temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
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Figure 6: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
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Figure 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
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Figure 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [19]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for workhardened grades from Ref. [22].
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Table I. Chemical composition of the studied steel [wt-%].
Steel
C
Si
Mn
Cr
Ni
Mo
Cu
N
201
0.050
0.32
6.74
17.5
3.72
0.05
0.23
0.23
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Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing A.S. Hamada, A.P. Kisko, P. Sahu, L.P. Karjalainen
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)00058-1 http://dx.doi.org/10.1016/j.msea.2015.01.042 MSA31980
To appear in:
Materials Science & Engineering A
Received date: 21 November 2014 Revised date: 15 January 2015 Accepted date: 20 January 2015 Cite this article as: A.S. Hamada, A.P. Kisko, P. Sahu, L.P. Karjalainen, Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing, Materials Science & Engineering A, http: //dx.doi.org/10.1016/j.msea.2015.01.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of mechanical properties of a TRIP-aided austenitic stainless steel by controlled reversion annealing A.S. Hamada1,2*, A.P. Kisko1 , P. Sahu3 and L.P. Karjalainen1 1
Centre for Advanced Steels Research, Box 4200, University of Oulu, 90014 Oulu, Finland, Metallurgical and Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez University, Box 43721, Suez, Egypt 3 Department of Physics, Jadavpur University, Kolkata 700 032, India 2
Abstract Controlled martensitic reversion annealing were applied to a heavily cold-worked metastable austenitic low-Ni Cr-Mn austenitic stainless steel (Type 201) to obtain different ultrafine austenite grain sizes to enhance the mechanical properties, which were then compared with the conventional coarse-grained steel. Characterization of the deformed and reversion annealed microstructures was performed by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and light and transmission electron microscopy (TEM). The steel with a reverted grain size ~ 1.5 µm due to annealing at 800 °C for 10s showed significant improvements in the mechanical properties with yield stress ~ 800 MPa and tensile strength ~ 1100 MPa, while the corresponding properties of its coarse grained counterpart were ~ 450 MPa, ~ 900 MPa, respectively. However, the fracture elongation of the reversion annealed steel was ~ 50% as compared to ~ 70% in the coarse grained steel. A further advantage is that the anisotropy of mechanical properties present in work-hardened steels also disappear during reversion annealing.
Keywords: Cr-Mn austenitic stainless steel, reversion treatment, ultrafine grain size, mechanical properties.
*corresponding author: Tel/Fax.: +20 62 3 360 252
E-mail address: [email protected]
1
1. Introduction
The presence of expensive Ni in steel as an alloying element significantly raises the overall cost of the steel grade. Based on metallurgical concepts, the alloying of Mn, Cu, C and N can be
used in lieu of Ni to stabilize the austenitic structure with an acceptable compromise between the production cost and the properties of the steel. Furthermore, nitrogen has a strong, generally beneficial effect on strain hardening behaviour and mechanical properties of these steels. Several austenitic stainless steel grades based on the system Fe-CrMn-N (series 2xx) with low or without Ni content have been developed as an alternative to the expensive austenitic Cr-Ni series 3xx and finding widespread applications in automotive industry, kitchen utilities and even for cryogenic applications at very low temperatures [1,2]. It is well known that the austenitic stainless steels originally having low yield strength (YS) is unsuitable for structural applications, unless the YS are increased by cold rolling [3]. And as the strength increases, the structures also become lighter and therefore there is a growing interest to develop high-strength steels for automotive applications demanding a good combination of high strength and ductility [4].
Furthermore, the minimum targeted YS for an anti-intrusion and industry application should be 600–700 MPa [5]. Recently, Saha et al. [6] succeeded to create nano-structured austenitic high Mn steel with the grain size of 400 nm and the YS of 702 MPa, applying very high cold rolling reduction of 92% with recrystallization at a low temperature to enhance the nanostructure.
The disadvantage of strengthening by cold rolling is the introduction of anisotropy in mechanical properties and the strength being different in tension and compression, as well as in different directions relative to the longitudinal direction [7]. Therefore, the strengthening methods providing isotropic mechanical properties are looked for and grain refinement that can be
2
obtained through the reversion annealing of a metastable austenitic stainless steel to transform the strain-induced martensite back to austenite is an efficient route to obtain the envisaged properties. The reversion of deformation-induced martensite to austenite enables a marked grain refinement in the bulk scale of the steel. Recently, the reversion treatment has been applied in a laboratory scale to various metastable austenitic stainless steel grades such as 301 and 301LN Cr-Ni [8], CrMn low Ni [9] and also to Ni-free austenitic stainless steels [10].
We previously [11] investigated the effect of temperature and strain rate on the strain hardening behaviour and the strength-ductility combination of a Type 201L steel with a conventional grain size of 15-20 µm during the strain-induced martensite transformation (the TRIP effect) and the occurrence of mechanical twinning. Kisko et al. [9,12] have subsequently investigated the influence of the reversion treatment on grain size and mechanical properties. Here, we present a comparison of the mechanical properties of reversion annealed 201 steel with that of a coarse grained counterpart to demonstrate the significant advantages of the grain size refinement. Finally, the influence of the reversion annealed grain structures of nano/ultrafine size on the mechanical properties is determined related to the deformation mechanism.
2. Experimental Procedures 2.1. Test Material
The experimental materials were sheets of Type 201 (EN 1.4372) low-Ni Cr-Mn austenitic stainless steel, supplied by Outokumpu Stainless Oy, Tornio Works (Tornio, Finland), with the nominal chemical composition shown in Table I. The initial structures of the sheets are fully austenitic with a coarse-grained (CG) structure (≈ 18 µm) as measured using the linear-intercept method (ASTM E112).
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2.2 Reversion annealing processing
For the reversion treatment, the steel sheets were cold rolled in a laboratory rolling mill to 60% reduction and subsequently annealed on a Gleeble 1500 thermomechanical simulator. In the controlled reversion annealing treatment, the specimens were heated to the reversion temperature between 700 and 1000°C, at a rate of 200°C/s for different soaking times and cooled at a rate 200 °C/s down to 400°C, followed by final air cooling to room temperature to obtain different highly refined grain sizes. The reversion heat treatment is presented schematically in Fig. 1
2.3 Mechanical and microstructural characterization The mechanical properties were measured by uniaxial tensile tests at a constant strain rate 5×10-4 s-1 using a Zwick Z 100 tensile machine. The gauge length was short, 15 mm, in the instance of reversion treated samples. Three samples were tensile tested for each annealing condition. Tensile samples used for annealing treatment were cut from the cold rolled sheets in the rolling direction.
Fig. 1. Reversion annealing treatment cycle.
Table 1. Chemical composition of the studied steel [wt-%].
Microstructures were studied by an optical microscope and a field emission gun scanning electron microscope (FEG-SEM Carl Zeiss Ultra plus) with an electron back scatter diffraction (EBSD) unit. An acceleration voltage of 20 kV and a step size of 0.3 µm were used during the EBSD scans. Before the EBSD examination, the steel surfaces were mechanically polished down to 1 µm by using a diamond suspension, and finally chemically polished by using a 0.05 µm colloidal suspension of silica for about 10 min.
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The structural features after interrupted tensile strains were also examined in a transmission electron microscope (TEM) operated at 120 kV. Thin foils were prepared by twin jet electropolishing of 3 mm disks, punched from the specimens, using a solution of 10% perchloric acid in acetic acid electrolyte.
X-ray diffraction (XRD) data acquisition was carried out using Cu Kα radiation in a powder diffractometer (Siemens-D500) equipped with a secondary beam monochromator. The step scan mode, with a preset holding time of 5–10 s at each 0.01° step in 2θ, was used to improve the counting statistics and yield data suitable for stable refinement.
A Ferritescope (Helmut Fisher FMP 30) instrument was also used to measure the fraction of the ferromagnetic α′-martensite phase. The readings obtained by the Ferritescope instrument were multiplied by a correction factor 1.7 for α′-martensite fractions [13].
3. Results and Discussion 3.1 Microstructures and grain sizes
After the 60% cold rolling reduction, the martensite content measured by a Ferritescope was 30%. Hence, the cold-rolled microstructure comprises of a major amount of deformed austenite with significant strain-induced α′-martensite. During reversion annealing even at low temperatures of 700-900 °C, the martensite tends to revert back to austenite with a ultrafine grain size either through martensitic shear or diffusional reversion mechanisms [14]. However, the refinement of deformed retained austenite grains requires the occurrence of static recrystallization, which takes place at slightly higher temperatures and more slowly, as shown by Kisko et al. [12,15].
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It is well established that with increasing the cold rolling deformation beyond 50 pct thickness reduction, the slip bands and dislocation densities significantly increase. Consequently, the indexing of the back-scattered pattern for the cold rolled structures is hindered. Therefore, the mapping of the crystal orientation of the grain structure after cold rolling cannot be shown by EBSD. Sahu et al. [16] observed that with increasing the cold rolling reduction of an austenitic high Mn steel to 50 pct, the grain structure cannot be indexed by EBSD as a result of increasing slip bands and high dislocation density.
Fig. 2 presents the evolution of the microstructures in reversion annealing at different temperatures for various durations. In Fig. 2a, the microstructure at 800 °C for 1s consists of a mixture of fine reverted austenite grains with the submicron grain size of about 500 nm together with large areas of recovered retained austenite, which did not transform to martensite during cold rolling and has not recrystallized yet. Finer retained austenite are formed through the activation of static recrystallization at longer annealing times, as shown in Fig. 2b where the average grain size of the new austenite at 800°C/10 s is 1.5 µm. With increasing the annealing temperature to 900 °C, the grain size structure tends to homogenize, Fig. 2c. However, a relatively coarse-grained structure with the uniform grain size of about 5 µm is induced at high annealing temperature of 1000°C for 1 s, as shown in Fig. 2d.
Fig. 2. Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction.
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3.2 Mechanical Tensile Properties The results of the uniaxial tensile tests in the range of -80°C to 200 °C and two different strain rates conducted on the coarse-grained (15 µm) steel are shown in Fig. 3 [11]. At room temperature, the yield stress is about 450 MPa and the tensile strength 900 MPa. Two deformation mechanisms are activated over the deformation range of temperature: the formation of strain-induced martensite (the TRIP effect) and mechanical twinning (the TWIP effect). The strain rate has a significant influence on the ductility (elongation value) of the steel due to adiabatic heating taking place at the higher strain (10-2) rate affecting the TRIP effect, in particular [11]. At lower strain rate (10-4), the elongation is about 70% at room temperature in absence of any significant adiabatic heating.
Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
XRD patterns of the coarse grained 201 steel after tensile straining in the temperature range: -80 °C and 150 °C are shown in Fig. 4. It is seen that at subzero temperatures, straining induces very high degree of martensitic transformation, which gradually decrease with increasing straining temperature until RT. The Rietveld analysis yields that at RT, there is a significant amount of α′martensite (≈26 %) and also a very small amount of ε-hcp martensite (≈ 1%). However, at -80 °C, the amount of α′- martensite increases to a value of 96%. At temperatures above RT, the XRD patterns presented in Fig. 4b reveal that the TRIP effect is significantly suppressed due to increase in stacking fault energy (SFE) of austenite at higher straining temperatures. For instance, the amount of α′- martensite decreases to (≈ 3%) accompanied by the trace mount of ε-hcp martensite (≈ 1%) at 50°C. The detailed results of the XRD structural and microstructural analysis have been reported in a separate paper [17].
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Fig.4: XRD patterns of the coarse grained 201steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
The tensile properties for the reversion annealed Type 201 steel are shown as a function of annealing temperature and duration in Fig. 5. It can be seen that the yield stress (Rp0.2) and tensile strength (Rm) increase with decreasing reversion annealing temperature, especially in the regime below 900°C. For example, the yield strength of 800 MPa and tensile strength of 1100 MPa are obtained in annealing at 800°C for 10s having grain size 1.5 µm. Then, the fracture elongation is high, about 50% as measured for a short gauge length of 15 mm. Hence, the yield strength is almost doubled by the reversion treatment compared to that of the conventional steel with the grain size of 15-18 µm. A further advantage is that the anisotropy in mechanical properties, present in work-hardened grades, disappears in annealing [15]. As observed in the present study, the yield stress is strongly influenced by the grain size and significant improvement could be achieved by the grain size is refinement from 10-20 µm to 1-2 µm during reversion treatment, and the strengthening mechanisms have been discussed in detail in previous papers [9,12,15].
Fig. 5.
The mechanical properties of Type 201 steel as reversion annealed at different
temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
It is noteworthy that besides grain refinement, the TRIP and TWIP effects also significantly contribute to the tensile strength and ductility of the steel through the formation of strain-induced α′-martensite and mechanical twinning during tensile straining, respectively. The stability of austenite is dependent on its grain size and the stability increases with decreasing the grain size down to a few micrometers [9]. In agreement, the fraction of α′-martensite was about 28%, 12% and 3% after tensile straining to fracture and the corresponding austenite grain sizes were 15 µm,
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7 µm and 2 µm, respectively. The corresponding elongation values decreased slightly with the refined grain size, but the relationship between the intensity of the TRIP effect and the ductility is not straightforward.
The activated deformation mechanisms during tensile straining have been examined in using a TEM after straining to certain strains. Fig. 6 shows bright field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10s.
Fig. 6. TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
It is generally accepted that Type 201 has a very low SFE and that, in such steel, numerous stacking faults are created due to the movement of a 6 112 Shockley partial dislocations on the close-packed {111} planes of austenite [18]. The microstructure in Fig. 6a reveals the presence of numerous wide intersecting stacking faults within the untransformed austenite, which are arranged in primary and conjugate slip systems. This observation is in agreement to that of a low SFE close packed alloy and confirms the activation of multiple slips at the onset of straining.
Microscopic shear bands are seen after tensile straining to 2% engineering strain, Fig. 6b. The origin of these shear bands is unclear. They can be considered a consequence of overlapping stacking faults or as a consequence of planar slip due to the low-SFE of the material [19,20]. As the tensile straining is continued to 10%, the dark areas seen in the bright-field image in Fig. 6c are presumably α′-martensite, containing a high density of dislocations. Numerous micro shear bands (µSBs) can be seen in the microstructure. The higher magnification microstructure in Fig. 6d distinctly reveals the presence of α′-martensite within the shear bands. Similarly, Talonen et al. [21] observed a number of shear bands and shear band intersections resulting in α′-martensite
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nucleation.
The TEM microstructures of the tensile strained 201 steel after reversion annealed at 1000 °C are shown in Fig. 7. Density of wide stacking faults along with Planar-dislocation arrangements can be seen in the deformed microstructures, see Figs 7a-b. As the deformation proceeds to 10 pct strain, high density of shear bands are formed. Consequently, martensitic regions are induced and bounded by dislocation walls and forests, as shown in Fig. 7c. Another strain-induced microstructural feature has been observed during the deformation proceeding of the reversion annealed steel.
Numerous parallel twins of few tens of a nanometer thick with a small
interdistance can be seen on the related microstructure, as indicated in Fig. 7d.
Fig. 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
The target strength-ductility range (the brown band) set by the European Commission for automotive steels is shown in Fig. 8 [22]. By inserting the data for conventional stainless steels [23] and that for Type 201 steel obtained after various reversion treatments, it becomes apparent that the combinations of tensile strength-ductility achieved in the reversion treated condition are excellent exceeding those of conventional annealed or work-hardened stainless steels and even the target band.
Fig. 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [20]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for work-hardened grades from Ref. [23].
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Conclusions The most relevant results of the present work concerning the deformation processes and the mechanical properties of the reversion annealed 201 steel can be briefly summarized as follows:
1. Reversion annealing of a heavily cold-rolled metastable 201 austenitic stainless steel is an efficient way to refine the austenite grain size to the order of a micrometer. The grain refinement significantly improves its mechanical properties as the yield and tensile strengths are remarkably improved (the yield stress is readily doubled), with marginal drop of the elongation, which makes them superior to the conventional annealed or work-hardened stainless steel. In addition, the anisotropy in the mechanical properties disappears. 2. Formation of the strain-induced martensite (TRIP) effect is more pronounced in the coarse grained structure. However, the nano/submicron austenite grains structure reveals superior mechanical properties with a small amount of strain-induced martensitic transformation.
Acknowledgements The financial support from the Finnish Funding Agency for Technology and Innovation (Tekes) in the Light and Efficient Solutions program (project SPR1) of the Finnish Metals and Engineering Competence Cluster (FIMECC Ltd.) is gratefully acknowledged.
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References: [1] J. Charles, “The new 200 series: an alternative answer to Ni. Surcharge? Dream or nightmare?”, fifth European Stainless Steel Science & Market Congress, Seville, Sept. 27-30, 2005, 19−28. [2] H.M. Cobb, Steel Products Manual: Stainless steels, The Iron and Steel Society, Warrendale, PA, USA, 1999. [3] L.P. Karjalainen, T. Taulavuori, M. Sellman, A. Kyröläinen, Steel Res. Int. 79 (2008) 404−412. [4] P.-J. Cunat and T. Pauly, Proc. 4th Eur. Stainless Steel Conf. Science and Market, Paris, France, 2002, 10−18. [5] H-W Yen, M. Huang, C.P. Scott, J-R. Yang, Scripta Mater. 66 (2012) 1018–1023. [6] R. Saha, R. Ueji, N. Tsuji, Scripta Mater. 68 (2013) 813–816. [7] Design Manual for Structural Stainless Steels. 3rd Edition. Euro Inox and the Steel Construction Institute, 2006. [8] M.C. Somani, P. Juntunen, L.P. Karjalainen, R.D.K. Misra, A. Kyröläinen, Metall. Mater. Trans. A 40 (2009) 729−744. [9] A. Kisko, R.D.K. Misra, J. Talonen, L.P. Karjalainen, Mater. Sci. Eng. A 578 (2013) 408−416 [10] P. Behjati, A. Kermanpur, A. Najafizadeh, H. S. Baghbadorani, Mater. Sci. Eng. A 592 (2014) 77−82. [11] A.S. Hamada, L.P. Karjalainen, R.D.K. Misra, J. Talonen, Mater. Sci. Eng. A 559 (2013) 336−344. [12] A. Kisko, A. S. Hamada, L.P. Karjalainen, J. Talonen, Microstructure and Mechanical Properties of Reversion Treated High Mn Austenitic 204Cu and 201Stainless Steels, HMnS2011, Grand Hilton Hotel, Seoul, Paper B-19, May15−18, 2011.
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[13] J. Talonen, P. Aspegren, H. Hänninen, Metall. Mater. Sci. Tech. 20 (2004) 1506−1512. [14] K. Tomimura, S. Takaki, Y. Tokunaga, ISIJ Int. 31 (1991) 1431−1437. [15] A. Kisko, L. Rovatti, I. Miettunen, L.P. Karjalainen, J. Talonen, Microstructure and Anisotropy of mechanical properties in reversion-treated high- Mn type 204Cu and 201 stainless steels, in: Proceedings of the 7th European Stainless Steel Conference Science and Market, Como, Italy, No.81, Sept.21–23, 2011. [16] P. Sahu, A.S. Hamada, T. Sahu, J. Puustinen, T. Oittinen, L.P. Karjalainen, Metall. Mater. Trans. A 43 (2012) 47-55. [17] H. Barman, A.S. Hamada, T. Sahu, B. Mahato, J. Talonen, S.K. Shee, P. Sahu, L.P. Karjalainen, Metall. Mater. Trans. A 45 (2014) 1937−1952. [18] P. Sahu, S.K. Shee, A.S. Hamada, L. Rovatti, T. Sahu, B. Mahato, S.G. Chowdhury , D.A. Porter, L.P. Karjalainen, Acta Mater. 60 (2012) 6907−6919. [19] J.W. Christian, S. Mahajan. Prog. Mater. Sci. 39 (1995) 1−157. [20] L. Bracke, K. Verbeken, L. Kestens, J. Penning, Acta Mater. 57 (2009) 1512−1524. [21] J. Talonen, H. Hänninen, Acta Mater. 55 (2007) 6108−6118. [22] European Steel Technology Platform, From a Strategic Research Agenda to implementation, ISBN 92-79-01283-5, European Communities, 2006. [23] D. Peckner, I.M. Bernstein, Handbook of Stainless Steels, McGraw-Hill Book Company, New York, 1977.
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Figures Captions: Figure 1: Reversion annealing treatment cycle. Figure 2: Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction. Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
Figure 4: XRD patterns of the coarse grained 201steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
Figure 5:
The mechanical properties of Type 201 steel as reversion annealed at different
temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
Figure 6: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
Figure 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
Figure 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [19]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for workhardened grades from Ref. [22].
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Figure 1: Reversion annealing treatment cycle.
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Figure 2: Reversion annealed microstructures at temperatures/durations: SEM-EBSD maps: (a) 800 °C/1 s, (b) 800 °C/10 s, optical microscope images: (c) 900 °C/1 s, and (d) 1000 °C/1 s.
The microstructures are parallel to the rolling direction.
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Figure 3: Mechanical properties of coarse-grained Type 201 steel. YS = yield stress, UTS = tensile strength, A80 = elongation. Data from [11].
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Figure 4: XRD patterns of the coarse grained 201 steel after tensile strained up to fracture at a low strain rate 5×10-4 s-1 and different temperatures. a) Subzero temperatures, b) elevated temperatures (RT- 150 °C)
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Figure 5: The mechanical properties of Type 201 steel as reversion annealed at different temperatures for various durations, (a) yield stress, (b) tensile strength, (c) fracture elongation.
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Figure 6: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 800 °C for 10 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
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Figure 7: TEM bright-field micrographs of the Type 201 steel tensile strained after reversion annealing at 1000 °C for 1 s: (a) and (b) 2% strain, (c) and (d) 10% strain.
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Figure 8: The ESPEP map: New Generation of High Strength Steels for Light-Weight Construction [19]. Comparison of the mechanical properties (tensile strength and elongation) between the reversion annealed Type 201 and some other current steel grades. The data for workhardened grades from Ref. [22].
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Table I. Chemical composition of the studied steel [wt-%].
Steel
C
Si
Mn
Cr
Ni
Mo
Cu
N
201
0.050
0.32
6.74
17.5
3.72
0.05
0.23
0.23
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