Effects of Aluminium on the Microstructure and Mechanical Properties ...

ISIJ International, Vol. 55 (2015), No. 3, pp. 662–669 ISIJ International, Vol. 55 (2015), No. 3

Effects of Aluminium on the Microstructure and Mechanical Properties in 0.2C–5Mn Steels under Different Heat Treatment Conditions Haifeng XU,1,2)* Wenquan CAO,2) Han DONG2) and Jian LI1) 1) Center for Fuel Cell Innovation, State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074 P. R. China. 2) Special Steel Institute, Central Iron and Steel Research Institute, Beijing, 100081 P. R. China. (Received on July 25, 2014; accepted on November 8, 2014)

The effects of aluminium on mcrostructure and mechanical properties in medium manganese steels were investigated in this study. It is found that addition of aluminium don’t significantly change the microstructure evolution during heat treatment process, aluminium can retard the precipitation of carbides in the following annealing, resulting in higher carbon content in several austenite. Though aluminium addition leads to a certain decline of tensile strength, the yield strength and elongation are neither decreasing significantly, especially the former is improved after heat treatment. Furthermore, the Al-bearing medium manganese steels can achieve optimal product of strength and ductility with austenization at 675°C and short time (5 min) annealing, the excellent mechanical properties are strength ~880 MPa, total elongation ~39.3% and product of tensile strength to total elongation (Rm*AT) ~34.6 GPa%. KEY WORDS: microstructure; mechanical properties; Al-bearing medium manganese steel; carbon content.

1.

strength high up to 1–1.5 GPa and total elongation high up to 30–40%, which was processed by austenite reverted transformation annealing (ART-annealing) with relative long time to develop ultrafine austenite and ferrite duplex structure with austenite fraction about 30–40%.13–17) It is well known that retained austenite fraction and stability are beneficial to excellent mechanical properties of medium manganese steels, moreover chemical composition and heat treatment have significant effects on the retained austenite. Thus it is necessary to investigate the effect of alloying element on microstructure characteristic and mechanical properties under appropriate heat treatment condition.

Introduction

In recent years, the demand for high strength steels with excellent ductility has been increasing in various industries and automobile manufacturing in particular, aimed at not only improving the productivity, safety and comfort levels, but also reducing the weight of automobile body to decrease fuel consumption and exhaust emission as well.1–4) TWIP and TRIP steels have the combination of excellent ductility and high strength, which is due to metastable austenite5,6) and ultrafine grain size through Hall-Petch law in severe plastic deformed materials,7,8) to increase the volume fraction of retained austenite and refine the grain size into the submicron region in high strength low alloyed steel (HSLA) may be a promising way to develop the new type steel with both ultrahigh strength and super ductility.9) Recently, Merwin reported the significant results about the medium manganese steels (0.1C–5Mn, 0.0095C–5.8Mn and 0.099C–7.09Mn), which had combination of high strength 800–1 000 MPa and total elongation 30–40%,10) and H. L. Yi, et al. also reported the ferrite/austenite duplex C–Mn steels (named δ–TRIP steel), which had ultimate strength 900 MPa and total elongation 28%.11,12) Meanwhile, Central Iron and Steel Research Institute (CISRI) developed a new type C–Mn steels with medium manganese content (9% > Mn > 3%, wt%) were reported to be capable of producing substantially improved mechanical properties with

2.

In the study, Fe–0.2C–5Mn (TG5) and Fe–0.2C–5Mn– 1.5Al (TG5Al) steels with main alloying elements of 0.2% C, 4.72% Mn, 1.5% Al as presented in Table 1, and other trace elements were designed and prepared by high frequency induction furnace in a vacuum atmosphere. The ingots were homogenized at 1 250°C for 2 hours, forged in between 1 200–850°C into bars with diameter of 16 mm, Table 1.

* Corresponding author: E-mail: [email protected] DOI: http://dx.doi.org/10.2355/isijinternational.55.662

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Experimental Procedure

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Chemical composition (wt.%) and Ac1, Ac3 of the steels in this study. C

Mn

Al

P

S

N

Ac1

Ac3

TG5

0.2

4.72

—

0.008

0.002

0.003

630°C

720°C

TG5Al

0.2

4.72

1.5

0.008

0.002

0.003

660°C

860°C

ISIJ International, Vol. 55 (2015), No. 3

and finally cooled in furnace to room temperature. The critical temperatures of Ac1 and Ac3 of the designed steel were calculated based on the THERMAL-CAL software and were presented in Table 1 and Fig. 1 as well. It can be found that critical temperatures of Al-bearing steels are increasing, which will influence the following intercritical annealing. The forged bars were processed by austenization, quenching and annealing based on the heat treat procedures as described in Table 2 (Austenization temperature (TA), austenization holding time (tA), annealing temperature (TAn) and annealing time (tAn)). Microstructures and austenite fraction of specimens were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Samples for SEM and XRD measurements were mechanically ground and polished, then etched in 2% nital and electrolytically etched in a solution of 10% chromic acid. Volume fraction of retained austenite was estimated by X-ray diffractometry using Co–Kα radiation. The calculations were based on the integrated intensities of (200)α, (211)α, (200)γ, (220)γ and (311)γ diffraction peaks. The volume fraction of retained austenite of each peak, Vi, was calculated from the integrated intensities of ferrite and austenite peaks using the following Eq. (1), and the volume fraction of retained austenite, Vγ, was the average of Vi.17) Vi = 1 / [1 + G( Iα / Iγ )] ........................ (1) where Iγ and Iα are the average integrated intensity obtained at (200)γ, (220)γ, (311)γ diffraction peaks and (200)α, (211)α diffraction peaks. G-value for each peak was presented as follow, 2.5 for I(200)α/I(200)γ, 1.38 for I(200)α/ I(220)γ, 2.02 for I(200)α/I(311)γ, 1.19 for I(211)α/I(200)γ, 0.06 for I(211)α/I(220)γ , 0.96 for I(211)α/I(311)γ . The carbon content of developed austenite was calculated from the integrated intensity of austenite peaks using the following equation18) aγ = 3.556 + 0.0453 XC + 0.00095 X Mn + 0.0056 X Al ....... (2)

Fig. 1.

where XC, XMn (wt.%) are the carbon and manganese concentration of retained austenite, XMn is nearly used 1.5 times of the manganese concentration in steels.19) And XMn = 9% is taken in this calculation because the manganese concentration is about 9% in austenite phase as reported in our previous paper,14) aγ is the lattice parameter, calculated from the Eqs. (3)–(4).20) aγ = λ / (2 × sin θ ) × h2 + k 2 + l 2 ............... (3) dhkl = λ / (2 × sin θ ) .......................... (4) where λ is the X-ray wavelength of cobalt target, θ is the diffraction angle, dhkl is the interplanar spacing, and h, k, l are the Miller indexes of (220)γ and (311)γ. Uniaxial tension test was performed on the dog-bone shaped samples with gauge length of 25 mm and diameter of 5 mm at a strain rate of ~10 − 3/s in an Instron machine (WE-300) at room temperature. Through the tensile test, the mechanical properties of studied steels, i.e., tensile strength (Rm), yield strength (Rp 0.2), total elongation (AT), uniform elongation (AU), reduction of area (Z), and the stress-strain curves were measured. 3.

Results

3.1.

Microstructure Evolution during Different Heat Treatment Conditions The microstructure evolution of TG5/TG5Al steels processed by heat treatment-1 (i.e., austenized at 675°C for 30 minutes, oil quenched and then followed by annealed at 650°C with different time) was examined by SEM as presented in Figs. 2 and 3. From microstructure of as-quenched specimens (Figs. 2, 3(a)), the lamellar typed structure with white and dark laths could be easily distinguished. The annealed structures show that lamellar duplex structure is always remained during annealing process (Figs. 2,

Phase diagram of the designed steels calculated based on the THERMAL-CAL (a) Fe–0.2C–5Mn (TG5) (b) Fe–0.2C–5Mn–1.5Al (TG5Al).

Table 2.　Heat treatment parameters applied in this study. TA and t A

TAn

t An

Heat treatment-1

675°C × 30 minutes

650°C

1 min, 5 min, 10 min, 30 min, 1 hr, 3 hrs and 6 hrs

Heat treatment-2

1 050°C × 30 minutes

650°C

1 min, 5 min, 10 min, 30 min, 1 hr, 3 hrs and 6 hrs

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Fig. 2. Microstructure evolution of TG5 steel austenized at 675°C for 30 minutes, oil quenched and followed by annealing at 650°C for (a) as-quenched, (b) 1 min, (c) 30 min, (d) 6 hrs characterized by SEM.

Fig. 3. Microstructure of TG5Al steels austenitized at 675°C for 30 min, oil quenched and followed by ART-annealing at 650°C for (a) as-quenched, (b) 1 min, (c) 30 min, (d) 6 hrs characterized by SEM.

3(b)–3(d)). With the short time annealing, large amount of carbides precipitation can be found in the matrix. However, with increasing of annealing time, some carbides gradually dissolve and bright/dark lamellar duplex structures become clearer. Moreover, it also can be seen that rod-typed carbides in the dark-laths, but no carbides in the bright-laths. © 2015 ISIJ

In conclusion, the microstructures of TG5 and TG5Al processed by heat treatment-1 consist of austenite (hard-etched phase) and ferrite (easy-etched phase). The microstructure of studied steels processed by heat treatment-2 (i.e., austenized at 1 050°C for 30 minutes, oil quenched and followed by annealing at 650°C with different 664


time) was given in Figs. 4 and 5. As shown in Figs. 4, 5(a), the full martensite structure was obtained after oil quenching, and retained austenite and carbides were hardly found in the matrix. However, it can be seen that large amount of carbides precipitate from the martensite matrix during short time annealing, meanwhile, some bright laths are also

found in the short time annealed structure (Figs. 4, 5(b)–(c)). After long time annealing, only small fraction of carbides precipitation was retained in the dark-laths and lamellar duplex structure gradually became much clearer (Figs. 4, 5(d)). This indicates that the carbides firstly precipitate in the initial stage during annealing and then gradually dissolve

Fig. 4. Microstructure evolution of TG5 steel austenized at 1 050°C for 30 minutes, oil quenched and followed by annealing at 650°C for (a) as-quenched, (b) 1 min, (c) 30 min, (d) 6 hrs characterized by SEM.

Fig. 5. Microstructure TG5Al steels austenitized at 1 050°C for 30 min, oil quenched and followed by ART-annealing at 650°C for (a) as-quenched, (b) 1 min, (c) 30 min, (d) 6 hrs characterized by SEM.

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content of new developed austenite in TG5Al.21,22)

into the new developed austenite phase with increasing of annealing time. As mentioned above, TG5 and TG5Al steels have the similar microstructure evolution under the same heat treatment conditions, the studied steels obtain mastensite structure after oil quenching, then new austenite laths gradually developed between martensitic laths during intercritical annealing, which resulted in the ferrite-austenite lamellar structure.16) However, the microstructures of TG5 and TG5Al still have some differences, i.e., it can be seen that more carbides in TG5 steels than that in TG5Al steels after relatively short time annealing, and it can be also found more bright laths in TG5 steels than that of TG5Al steels after long time annealing, this means that different amount of retained austenite and carbon content in TG5 and TG5Al, which will be further presented in section 3.2.

3.3.

Mechanical Properties of Steels Processed under Different Heat Treatment Conditions Engineering stress-strain curves of the specimens processed under different heat-treatment conditions were shown in Fig. 7. From the Fig. 7, it can be seen that both studied steels continuously yield during tensile process, and flow curves of TG5 steels are characterized by obvious intensive serrations or jerk flow feature, indicating dynamic strain aging and strong localized deformation during tension test (Figs. 7(a)–7(b)), whereas no intensive serrations are found in the flow curve of TG5Al steels (Figs. 7(c)–7(d)). It is considered that the serration is related with the quantity of the coarse carbides. Generally the serration phenomenon is caused by the P–L effect which needs solute carbon atoms (not carbides). From the micrographs, the serration occurred in the structures without coarse carbides as shown in Figs. 2(c), 2(d) and 4(d). Because Al addition retards the diffusion of carbon atoms,23–25) which results in less carbon atoms pinning the dislocation. So the serration feature apparently delays, even completely disappears during tensile deformation. Due to different initial microstructures, the stress strain curves of studied steels have different tendency. In Figs. 7(a), 7(c), it can be found that stress strain curves of the steels austenized at 675°C is hardly influenced by the followed annealing at 650°C. However, in Figs. 7(b), 7(d), the stress strain curves of the steels austenized at 1 050°C vary with annealing time significantly. Figure 8 shows the effect of annealing time on the mechanical properties, i.e., the yield strength (Rp0.2), tensile strength (Rm), total elongation (AT) and uniform elongation (AU). From Fig. 8, it can be found that mechanical properties are nearly independent on the annealing time when TG5/TG5Al steels were austenized at 675°C. When the studied steels are austenized at 1 050°C, the yield strength slightly decreased with increasing of annealing time, and the yield strength of TG5Al is little higher than that of TG5, however, tensile strength of TG5 increases and that of TG5Al slightly decreases with the annealing time (Fig. 8(a)). Furthermore, as shown in Fig. 8(b), the total and uniform elongation both increase with the increasing of annealing time.

3.2. Variation of Austenite Fraction and Carbon Content during Intercritical Annealing Austenite volume fraction and carbon concentration of studied steels were examined by XRD and the results were presented in Fig. 6. From Fig. 6(a), it can be seen that the austenite volume fraction evolutions are different under different austenization temperatures. The austenite fraction of TG5/TG5Al steels with austenization at 675°C both first increases from ~25% to ~30%, then decreases to ~22% with increasing of annealing time, and finally increases to ~30% after 6 hours annealing. Compared to austenization at 675°C, the variation of austenite fraction of TG5/ TG5Al steels austenized at 1 050°C has different evolution tendency, which gradually increases from ~5% to ~38% or from ~3% to ~20% with increase of ART-annealing time. Moreover, the austenite fraction of TG5 steel is higher TG5Al under the same heat treatment condition, this phenomenon is also proven by the microstructure evolution in Figs. 2–5 (section3.1). The carbon content Cγ of new developed austenite, which is calculated based on the XRD data, is also presented in Fig. 6(b). The carbon content Cγ of austenite in TG5 steel ranges from 0.586% to 0.673%, and that in TG5Al ranges from 0.607% to 0.711%, it can be obviously seen that Cγ of austenite in TG5 steels is lower than that in TG5Al. Due to addition of 1.5% Al in studied steels, aluminium prevents the precipitation of carbides during the annealing process, which results in higher carbon

Fig. 6. Variation of (a) initial volume fraction f A and (b) carbon concentration Cγ of retained austenite as a function of annealing time. (Online version in color.)

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Fig. 7.

Engineering stress-strain curves of the samples (a) TG5 austenized at 675°C, (b) TG5 austenized at 1 050°C, (c) TG5Al austenized at 675°C, (d) TG5Al austenized at 1 050°C for 30 minutes, oil quenched and followed by annealing at 650°C for different time. (Online version in color.)

Fig. 8. Mechanical properties of steels processed at different conditions (a) tensile and yield stress (Rm/Rp0.2) as a function of annealing temperature and (b) total and uniform elongation as a function of annealing temperature. (Online version in color.)

4.

during annealing process. However, as the microstructures presented in Figs. 4 and 5 are compared, we can find that lower amount of carbides in TG5Al processed steels during short time annealing, this reason is that aluminium addition can prevent carbides precipitating from the marstensite, which can increase the carbon content of following new developed austenite, this phenomenon is also observed in Fig. 6(b), and with increasing of annealing time at 650°C, the precipitated carbides gradually decomposed and the carbon and/or manganese entered into the new austenite, thus only few carbides can be found in the long time annealed

Discussion

4.1. Variation of Carbides and Austenite with Aluminium Addition during Heat Treatment Process As presented in Figs. 2–5, the microstructure evolution of studied steels was distinctly influenced by different heat treatment conditions (i.e., austenized at 675°C between AC1 and AC3 and austenized at 1 050°C above AC3). In both cases, carbides first precipitated from the marstensite matrix, austenite phase developed between martensite laths and carbides gradually dissolved into new developed austenite 667

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9(b). However, both tensile and ultimate true tensile strength of TG5Al processed with heat treatment-2 have the declining trend, and these properties of TG5Al are lower than that of TG5 with the same heat treatment condition. The similar variation of the yield strength can be found in both studied steels, the yield strength does not change when austenization temperature is 675°C, however, when the studied steels are austenized at 1 050°C, the yield strength significantly decreases with the increase of austenite fraction. The relationship between ductility and austenite fraction is presented in Fig. 9(c), it can be found that both total and uniform elongation increase with austenite fraction. This indicates that the ductility of studied steels is induced by phase transformation from metastable austenite to martensite, i.e., phase transformation induced plasticity (TRIP).26) It is noteworthy that the ductility of studied steels nearly saturates when the austenite fraction increase up to ~30%, this because TRIP effect is not only dependent on austenite fraction but also the stability of austenite.27–29) The low stability of austenite will lead to rapid phase transformation from austenite to martensite at the early stage of deformation, which is not benefit for the improvement of uniform elongation. In contrast, the austenite with high thermal or mechanical stability will not transform into martensite during plastic deformation. Furthermore, when the TG5/ TG5Al steels are austenized at 675°C, the total elongations of TG5/TG5Al steels both are more than 35%, and finally rise up to ~40% with long time annealing. However, when the austenization temperature is 1 050°C, the elongation of

structure. On the basis of initial microstructure after quenching, the amount of retained austenite has different variation in the following annealing process. The austenite fraction of specimen processed by austenization between Ac1 and Ac3 is higher than that of specimen austenized above Ac3. Furthermore, aluminium addition also has obvious effect on the formation and size of retained austenite, the austenite fraction of TG5Al is much lower than that of TG5 during the annealing process (Fig. 6(a)), especially with austenization at 1 050°C, which is also proven by the amount of brightdark laths in Figs. 2–5. Aluminium can promotes the formation of ferrite phase and inhibits the growth of austenite phase after quenching with austenization at 675°C, which will influence the amount of austenite during following annealing. Meanwhile, due to increasing of the critical temperature Ac1 and Ac3 by adding aluminium, the austenite fraction of Al-bearing steels also changes with addition of aluminium. 4.2.

Relationship between Mechanical Properties and Austenite Fraction in the Studied Steels Dependence of mechanical properties on the volume fraction of retained austenite is presented in Fig. 9. From Figs. 9(a), 9(b), it can be seen that the tensile strength of TG5 steels slightly changes with austenite fraction when the amount of austenite is lower than 15%. With increasing of austenite, the tensile strength of TG5 steels obviously increases, especially the ultimate true tensile strength in Fig.

Fig. 9.

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Mechanical properties dependence on the volume fraction of austenite (a) tensile stress and yield stress, (b) ultimate true tensile stress and yield stress, (c) total elongation and uniform elongation, (d) product of tensile strength to total elongation/uniform elongation. (Online version in color.)

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TG5 steel is more excellent than that of TG5Al, especially the uniform elongation given in Fig. 9(c). Figure 9(d) shows the variation of product of tensile strength to elongation as a function of austenite fraction, it can be found that the product linearly increases with the amount of retained austenite, which means TRIP effect is the most important factor of enhancing the mechanical properties. In Fig. 9(d), we also find that the optimal combination strength and ductility of TG5/TG5Al steels are obtained when the specimens are austenitized at 675°C, the product of strength to ductility can be ~35 GPa% or above. Moreover, due to the amount of retained austenite, the product of TG5 steels is superior to that of TG5Al. 5.

steels can also obtain large amount of austenite (~30%) with austenitization at 675°C and annealing in intercritical region, due to phase transformation induced plasticity of retained austenite, high volume fraction of metastable austenite leads to excellent product of strength and ductility, which can satisfy the requirement of automobile manufacture. Acknowledgement This research is supported by the National Natural Science Foundation of China No. 51371057) and National Basic Research Program of China (973 Program, No. 2010CB630803). REFERENCES

Conclusion

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The effect of aluminium addition on the microstructure and mechanical properties of medium manganese steels under different heat treatment conditions were investigated in this study. The results were summarized as follows. (1) The medium manganese steels with addition of aluminium does not nearly change the microstructure evolution. It is found that austenization in the intercritical region (between Ac1 and Ac3) obtaines a duplex structure after oil quenching, which hardly changes in the following annealing. However, high temperature austenization (above Ac3) leads to full martensite structure after oil quenching, which gradually transformes into ultrafine lamellar duplex structure during followed annealing process. The large austenite fraction (~30%) can be also obtained through 5 min annealing when the austenization temperature is 675°C, and aluminium addition retards the carbides precipitation which results in high carbon content in several austenite. Furthermore, the increasing of critical temperatures results in a certain differences of the microstructure and austenite fraction. (2) Effect of aluminium element on the mechanical properties is obvious. Both tensile stress and ultimate true tensile stress decreases with annealing time, especially austenization at 1 050°C. However, the yield stress of TG5Al steels is higher than that of TG5 with the same heat treatment. Moreover, the medium manganese steels with aluminium can also achieve excellent combination of strength and ductility (above 30 GPa%) when the specimens are austenitized at 675°C, oil quenched, and intercritically annealed with different time at 650°C. The best mechanical properties of Al-bearing medium manganese steels: tensile strength is 880 MPa, total elongation is 39.3%, and strengthductility balance is 34.6 GPa%. (3) Through the analysis of the microstructure and mechanical properties, the addition of aluminium in the medium manganese steels results in a certain decline of the tensile strength, however, the total and uniform elongation are improved after annealing, and the Al-bearing studied

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