- Email: [email protected]
Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing
Accepted Manuscript Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing
Sheida Nikkhah, Hamed Mirzadeh, Mehran Zamani PII:
S0254-0584(19)30253-6
DOI:
10.1016/j.matchemphys.2019.03.053
Reference:
MAC 21492
To appear in:
Materials Chemistry and Physics
Received Date:
21 December 2018
Accepted Date:
16 March 2019
Please cite this article as: Sheida Nikkhah, Hamed Mirzadeh, Mehran Zamani, Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing, Materials Chemistry and Physics (2019), doi: 10.1016/j.matchemphys. 2019.03.053
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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT
Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing Sheida Nikkhah, Hamed Mirzadeh 1, Mehran Zamani
Abstract The simultaneous effects of cold rolling reduction and intercritical annealing time on the microstructure and tensile properties of St12 steel were studied. It was revealed that by increasing the rolling reduction, the ferrite grain size of the dual phase (DP) microstructure is refined and a chain-like morphology of martensite develops, which results in improvement of work-hardening capacity and the tensile properties. Such a microstructure can be obtained via applying an optimum holding time at the intercritical annealing temperature. Beyond that optimum, grain coarsening occurs with the resulting deterioration of tensile properties. It was also shown that by consideration of quenched sheet or by applying low reductions in thickness, secondary recrystallization or abnormal grain growth (AGG) takes place during intercritical annealing, which is characterized by small grains and some large ferrite grains containing martensite islands. This study clarified that applying high reduction in thickness for grain refinement and also controlling the holding time in the two-phase austenite-ferrite region are the essential prerequisites to achieve the desired tensile strength, ductility, and toughness.
Keywords: Dual phase steels; Cold rolling reduction; Intercritical annealing time; Mechanical properties; Strain hardening rate.
1
Corresponding author. Tel.: +982182084080; Fax: +982188006076. E-mail address: [email protected] (H. Mirzadeh).
Affiliation of all authors: School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
1
ACCEPTED MANUSCRIPT 1. Introduction Dual-phase (DP) steels are a major group of modern low-carbon steels with duplex microstructure comprising of ferrite plus 10-40% martensite [1] (or other phases such as bainite [2]). This microstructure allows achieving high ultimate tensile strengths with maintaining high ductility, where these attributes are important for industrial applications of commercial FCC [3,4] and BCC [5,6] steels. Flat-rolled steels with low carbon (<0.10 wt.% C) and manganese (<0.4 wt.% Mn) contents have been recently used to process high-formability DP steels [7]. However, the obtained UTS values were extremely low (~ 400 MPa [7]) and it is highly desirable to improve their microstructure to reach the lower bound of 500 MPa, which needs microstructural modifications as shown by Nouroozi et al. [8], careful controlling of the intercritical annealing conditions as elaborated by Najafkhani et al. [9], or implementation of secondary hardening mechanisms as demonstrated by Zamani et al [10]. The properties of DP steel can be tailored via altering the volume fraction [11], Size [12], distribution [13], and morphology [14] of martensite phase and also ferrite grain size [15]. On the one hand, altering the initial microstructure is quite advantageous, where this can be easily achieved via intercritical annealing (IA) of ferritic-pearlitic microstructures [16], intermediate quenching (IQ) to produce a martensitic structure before intercritical annealing [17], or step quenching (SQ) from the austenitic region to the intercritical annealing temperature [18]. The intermediate quenching has been found as a viable method to enhance the properties of DP steel [19]. On the other hand, grain refinement is known to be one of the best methods for improving the strength-ductility balance of DP steels [20]. This can be realized by grain refinement of the pre-IA microstructure via thermomechanical processing [21] or severe plastic deformation [22], thermal cycling of DP microstructure [23], ultrafast heating to the austenite plus ferrite region [24], and cold-rolling before IA [8]. Applying cold deformation on a martensitic microstructure before intercritical annealing might induce the formation of recrystallized ferrite during heating to the intercritical region or at the initial stages of IA, which results in a fine DP microstructure with chain-like morphology of martensite with high workhardening capacity [14]. 2
ACCEPTED MANUSCRIPT The IA conditions are also important. By increasing the IA temperature, the strength level enhances but the ductility declines [25]. Regarding the IA time, the adjustment of martensite fraction [26], the effect of alloying elements [27], grain growth behavior [9], and the partitioning of Mn [18] have been studied so far. Besides these excellent works, the simultaneous effects of cold reduction and IA time have not been evaluated so far. It was revealed by Nouroozi et al. [8] and Najafkhani et al. [9] that the IA time should be carefully controlled to obtain a fine DP structure with desirable attributes. By applying different cold reductions, these aspects might become much more critical. As a result, this work is dedicated to evaluate these effects to improve the tensile properties of a high-formability 0.035C-0.27Mn-0.03Si (wt%) steel. 2. Experimental details The 0.035C-0.27Mn-0.035Si (wt%) St12 steel sheet after full annealing at 1150 °C for 15 min followed by furnace cooling was considered as the as-received sheet. It was then austenitized at 1150 °C for 15 min followed by water quenching / air cooling to develop essentially martensitic / normalized microstructures, respectively. The as-received, quenched, and normalized sheets were subjected to intercritical annealing at 800 °C for 1 and 5 min to obtain different sheets with ~ 20 vol% martensite as shown in Figure 1. Moreover, cold rolling reductions of 25, 40, and 80% were applied on the water quenched sheet followed by intercritical annealing at 800 °C for 1 and 15 min as shown in Figure 1. The details of the applied processing routes are shown in Table 1.
Figure 1: Processing routes used in this work.
3
ACCEPTED MANUSCRIPT Table 1: Details of the processing routes used in this work. IA represents intercritical annealing.
Pre-IA
Pre-IA cold rolling
microstructure
reduction (%)
IA5A
As-Received
IA5N
Sample
IA temperature (°C)
IA time (min)
0
800
5
Normalized
0
800
5
IQ1
Water Quenched
0
800
1
IQ5
Water Quenched
0
800
5
25CR-IQ1
Water Quenched
25
800
1
40CR-IQ1
Water Quenched
40
800
1
80CR-IQ1
Water Quenched
80
800
1
25CR-IQ15
Water Quenched
25
800
15
40CR-IQ15
Water Quenched
40
800
15
80CR-IQ15
Water Quenched
80
800
15
After etching by the LePera’s and Nital solutions, optical microscopy and FE-SEM (FEI Nova NanoSEM 450) were used for microstructural investigations. The ASTM E8 specimens (with the gauge length of 25 mm, width of 6 mm, and thickness of 1.66, 1.25, 0.95, and 0.45 respectively for cold rolling reductions of 0, 25, 40, and 80%) were subjected to room temperature tensile test under strain rate of 0.001 s-1. These tests were repeated one to insure the reproducibility of the results. The work-hardening rate (θ, via the central difference approach [28-30]) and the modulus of toughness (UT, via estimation of the area under the stressstrain curve according to the trapezoidal rule [31,32]) were obtained based on the following equations:
d / d i { i 1 i 1} /{ i 1 i 1}
(1)
U T (ei ei 1 )( S i S i 1 ) / 2
(2)
where e, ε, S, and σ are engineering strain, true strain, engineering stress, and true stress, respectively. 4
ACCEPTED MANUSCRIPT 3. Results Figure 2 shows the obtained microstructures and tensile stress-strain curves of IQ1, IQ5, IA5A, and IA5N. It can be seen in Figure 2a that the as-received sheet has a coarse ferrite-pearlite microstructure with low strength, high ductility, and yield-point elongation at the beginning, where the latter has been related to the Cottrell atmospheres of interstitial atoms around dislocations [33,34].
Figure 2: Microstructures and tensile stress-strain curves of IQ1, IQ5, IA5A, and IA5N samples. Note that (a), (b), (c), and (e) are optical micrographs and (d) is an SEM image.
5
ACCEPTED MANUSCRIPT After intercritical annealing (IA5A), a nearly similar microstructure comprising of ferrite and martensite develops (Figure 2b) and the tensile strength enhances considerably with the disappearance of the yieldpoint elongation, where the latter has been related to the presence of free dislocations in ferrite that are induced by the transformation of austenite to martensite [35,36]. Intercritical annealing of the normalized and quenched sheets results in grain refinement of the DP structure as can be seen in Figure 2c (IA5N) and Figure 2e (IQ5). However, these modifications have a marginal effect on the tensile stress-strain curves. The microstructure of IQ1 is shown in Figure 2d. It can be seen that 1 min at the IA temperature is not enough to develop the equilibrium microstructure and this can be better realized from the tensile curve that shows the residue of the fading yield-point elongation. Anyway, the tensile strengths of these samples are almost the same. The microstructures of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 are shown in Figure 3. All samples show a duplex ferritic-martensitic microstructure, and by increasing the prior cold rolling reduction, the ferrite grain size is considerably refined from 22 µm for IQ1 to 8.5 µm for 80CR-IQ1. In contrary to 40CR-IQ1 and 80CR-IQ1, the 25CR-IQ1 sample has very large grains along with the finer grains of expected size, where the presence of martensite islands inside ferrite grains is evident. This aspect will be revisited in the discussion section. Tensile stress-strain curves of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 are shown in Figure 4. It can be seen that the application of prior cold rolling results in a considerable enhancement of tensile strength, especially for the 40CR-IQ1 and 80CR-IQ1 samples that show high strength with maintaining good total elongation. Figure 4 also summarizes the values obtained for modulus of toughness (UT) based on Equation 2, which reveals that UT decreases for the 25CR-IQ1 but increases rapidly by increasing the prior cold rolling reduction. These findings will be revisited in the discussion section.
6
ACCEPTED MANUSCRIPT
Figure 3: SEM Images of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 samples.
7
ACCEPTED MANUSCRIPT
Figure 4: Tensile stress-strain curves of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 samples.
Figure 5 shows the microstructures of the samples that are intercritically annealed for 15 min (IQ15, 25CRIQ15, 40CR-IQ15, and 80CR-IQ15). It can be seen that the ferrite grains coarsen considerably at annealing time of 15 min in all samples (summarized in Table 2), especially in the case of 0 and 25% cold reductions. The tensile stress-strain curves of cold rolled samples (25CR-IQ15, 40CR-IQ15, and 80CR-IQ15) are shown in Figure 6. It can be seen that the tensile properties of these samples are inferior compared with the corresponding ones for holding time of 1 min. Table 2: Average ferrite grain size of different DP samples. Sample
IQ1
25CR-IQ1
40CR-IQ1
80CR-IQ1
25CR-IQ15
40CR-IQ15
80CR-IQ15
Grain Size (µm)
22 ± 1.3
14.3 ± 0.9
11.9 ± 1
8.5 ± 0.7
63.3 ± 2.1
20.1 ± 1.1
10.5 ± 1
8
ACCEPTED MANUSCRIPT
Figure 5: Microstructures of IQ15, IQ30, 25CR-IQ15, 40CR-IQ15, and 80CR-IQ15 samples. Note that (a), (b), (c), (d) and (e) are optical micrographs and (f) is an SEM image.
9
ACCEPTED MANUSCRIPT
Figure 6: Tensile stress-strain curves of 25CR-IQ15, 40CR-IQ15, and 80CR-IQ15 samples.
4. Discussion In the previous section, it was revealed that by increasing the cold rolling reduction, the grain size after IA will be considerably refined. At zero and low reduction of 25%, large ferrite grains containing martensite particles were observed. It was also seen that annealing beyond the optimum holding time (1 min in the present work) results in grain coarsening and deterioration of tensile properties. Figure 7 shows some of the pre-IA microstructures. The normalized microstructure (Figure 7a) is finer than the as-received one (Figure 2a), which resulted in a finer DP microstructure (IA5N, Figure 2c) compared with IA5A (Figure 2b). The quenched sample shows a completely different microstructure, i.e. the typical lath martensite (Figure 7b), which provides more nucleation sites for austenite (or martensite after quenching) during IA (Figure 2d). This resulted in a finer DP microstructure of IQ5 (Figure 2e). In fact, IA5N and IQ5 have been processed respectively based on intercritical annealing (IA) and intermediate quenching (IQ), where the advantages of IQ route has been discussed in details by Das and Chattopadhyay [17] and Ahmad et al. [37]. 10
ACCEPTED MANUSCRIPT As shown in Figure 7c, cold reduction of 25% has not considerably changed the microstructure of the quenched sample, and hence, it is logical to expect that this sample shows similar behavior during intercritical annealing when compared with the undeformed sample. Figure 3b and Figure 5c show that for the case low reduction of 25%, at IA time of 1 min (25CR-IQ1) and 15 min (25CR-IQ15), some large grains (with martensite islands inside them) along with fine grains are available. This phenomenon also happened in the case of quenched sample (Zero reduction) at IA time of 15 min (IQ15: Figure 5a) and 30 min (IQ30: Figure 5b). The presence of the martensite islands inside ferrite grains indicates the occurrence of abnormal grain growth (AGG) [38]. Figure 8 shows the progress of AGG at two magnifications. It can be seen that some martensite islands can pin the grain boundaries but some others do not, which results in AGG. A similar AGG behavior has been shown in the case of magnesium alloys containing intermetallic phases by Pourbahari et al. [39], where after grain growth, intermetallic particles remained in the interior of the abnormally grown grains. This work is among the first reports on the AGG behavior during intercritical annealing.
Figure 7: SEM images representing the pre-IA microstructures.
11
ACCEPTED MANUSCRIPT
Figure 8: SEM images of 25CR-IQ15 sample at two magnifications.
In the case of 40% (Figure 7d) and 80% (Figure 7e) rolled samples, deformed lath martensitic structures can be seen. In these cases, AGG cannot be seen, and in contrast, fine ferrite grains develop in the microstructure via recrystallization before partial austenitization at the IA temperature and a chain-like martensite morphology around ferrite grains forms as also pointed out by Nakada et al. [14]. Therefore, a minimum reduction in thickness is required to obtain recrystallized ferrite in the DP microstructure with avoiding AGG. This effect has been unraveled for the first time in the present work. Another important aspect of the present work is the enhancement of tensile properties of DP steel by prior cold rolling. Based on Figure 4, it can be realized that the slope of the stress-strain curve is significantly increased by prior cold rolling. This implies the improvement of the work-hardening rate that should be studied based on the true stress and θ curves (Equation 1). These plots are depicted in Figure 9a, which show that by increasing the rolling reduction, the value of θ at each strain increases. Moreover, the intersection of true stress and θ happens at larger strains, which reveals that the uniform elongation increases. 12
ACCEPTED MANUSCRIPT In fact, based on the Considére criterion, plastic instability of strain-rate-insensitive materials occurs when the strain-hardening rate coincides with the flow stress, i.e. d / d [33]. Therefore, the enhancement of work-hardening behavior can postpone necking. However, in the case of IA time of 15 min (for example for 80CR-IQ15 in Figure 9a), θ decreases, which can be ascribed to the coarsening of grains (Figure 9b) and the fading tendency of the chain-like morphology of martensite at ferrite grain boundaries (compare Figure 3e and Figure 5f). These are responsible for the deterioration of tensile properties at IA time of 15 min. Therefore, it can be realized that for obtaining fine-grained DP microstructure with chain-like morphology of martensite, careful controlling the holding time at the IA temperature is quite important. The importance of IA time has been previously pointed out by Nouroozi et al. [8] and Najafkhani et al. [9] and the present work systematically showed the combined effect of rolling reduction and IA time.
Figure 9: (a) Work-hardening rate plots and (b) Dependence of average ferrite grain size of DP steels on cold reduction.
13
ACCEPTED MANUSCRIPT The high work-hardening rate of the 80CR-IQ1 is responsible for the observed good strength-ductility balance, where its UTS reaches the domain of DP600 steels while maintaining a high total elongation of ~ 30% as shown in Figure 10 [40]. This figure reveals that by increasing the rolling reduction, the tensile strength increases toward the domain of advanced steels, and due to grain refinement of the DP microstructure with chain-like morphology of martensite, high ductility can be achieved except the case of 25% reduction (25CR-IQ1) with abnormal grain growth. However, except the case of 80% CR samples, other developed steels fall within the range of conventional steels similar to the previous report on this steel [7]. This reveals that high cold rolling reductions for grain refinement and controlling the IA time are crucial to achieve the desired tensile strength and ductility (Figure 10), and hence, obtaining a high modulus of toughness (Figure 4).
Figure 10: Strength-ductility balance of steels (Based on [40]), where the results of this study are shown on the figure.
5. Conclusions The synergistic effects of cold rolling reduction and intercritical annealing time on the microstructure and tensile properties of a high-formability DP steel were studied. The following conclusions can be drawn form this study: (1) The as-received (annealed) sheet showed a coarse ferrite-pearlite microstructure with low strength, high ductility, and yield-point elongation at the beginning of plastic flow. After intercritical annealing, a nearly 14
ACCEPTED MANUSCRIPT similar microstructure comprising of ferrite and martensite developed and the tensile strength enhanced considerably with the disappearance of the yield-point elongation. While intercritical annealing of the normalized and quenched sheets resulted in grain refinement of the DP structure, these modifications had a marginal effect on the tensile stress-strain curves. (2) It was revealed that by increasing the rolling reduction, the ferrite grain size of the DP microstructure is considerably refined and a chain-like morphology of martensite develops, which results in improvement of work-hardening capacity and the tensile properties. It was shown that such a microstructure can be obtained via applying an optimum holding time at the intercritical annealing temperature. Beyond that optimum, grain coarsening occurs with the resulting deterioration of tensile properties. (3) By consideration of quenched microstructure or by applying low reductions in thickness, abnormal grain growth (AGG) takes place during intercritical annealing, which is characterized by small grains and some large ferrite grains containing martensite islands. Therefore, it was revealed that high cold rolling reductions for grain refinement and also controlling the intercritical annealing time are the essential prerequisite to achieve the desired tensile strength, ductility, and toughness.
References [1] H. Ashrafi, M. Shamanian, R. Emadi, N. Saeidi, Examination of phase transformation kinetics during step quenching of dual phase steels, Materials Chemistry and Physics, 2017, vol. 187, pp. 203-217. [2] L. Schemmann, S. Zaefferer, D. Raabe, F. Friedel, and D. Mattissen, Alloying effects on microstructure formation of dual phase steels, Acta Materialia, 2015, vol. 95, pp. 386-398. [3] J. Li, Y. Cao, B. Gao, Y. Li, Y. Zhu, Superior strength and ductility of 316L stainless steel with heterogeneous lamella structure, Journal of Materials Science, 2018, vol. 53, pp. 10442-10456. [4] J. Li, C. Fang, Y. Liu, Z. Huang, S. Wang, Q. Mao, Y. Li, Deformation mechanisms of 304L stainless steel with heterogeneous lamella structure, Materials Science and Engineering A, 2019, vol. 742, pp. 409-413. [5] S. Sodjit, V. Uthaisangsuk, Microstructure based prediction of strain hardening behavior of dual phase steels, Materials and Design, 2012, vol. 41, pp. 370-379. 15
ACCEPTED MANUSCRIPT [6] N. Saeidi, M. Karimi, M.R. Toroghinejad, Development of a new dual phase steel with laminated microstructural morphology, Materials Chemistry and Physics, 2017, vol. 192, pp. 1-7. [7] M. Maleki, H. Mirzadeh, M. Zamani, Effect of Intercritical Annealing on Mechanical Properties and WorkHardening Response of High Formability Dual Phase Steel, Steel Research International, 2018, vol. 89, pp. 1700412. [8] M. Nouroozi, H. Mirzadeh, and M. Zamani, Effect of microstructural refinement and intercritical annealing time on mechanical properties of high-formability dual phase steel, Materials Science and Engineering A, 2018, vol. 736, pp. 22-26. [9] F. Najafkhani, H. Mirzadeh, M. Zamani, Effect of Intercritical Annealing Conditions on Grain Growth Kinetics of Dual Phase Steel, Metals and Materials International, in press. [10] M. Zamani, H. Mirzadeh, M. Maleki, Enhancement of mechanical properties of low carbon dual phase steel via natural aging, Materials Science and Engineering A, 2018, vol. 734, pp. 178-183. [11] M. Alibeyki, H. Mirzadeh, M. Najafi, and A. Kalhor, Modification of rule of mixtures for estimation of the mechanical properties of dual-phase steels, Journal of Materials Engineering and Performance, 2017, vol. 26, pp. 26832688. [12] H. Seyedrezai, A. K. Pilkey, J. D. Boyd, Effects of martensite particle size and spatial distribution on work hardening behaviour of fine-grained dual-phase steel, Canadian Metallurgical Quarterly, 2018, vol. 57, 2018, pp. 2837. [13] Z. Nasiri, H. Mirzadeh, Enhancement of work-hardening behavior of dual phase steel by heat treatment, Materialwissenschaft und Werkstofftechnik, 2018, vol. 49, pp. 1081-1086. [14] N. Nakada, Y. Arakawa, K.S. Park, T. Tsuchiyama, and S. Takaki, Dual phase structure formed by partial reversion of cold-deformed martensite, Materials Science and Engineering A, 2012, vol. 553, pp. 128-133. [15] M. Calcagnotto, D. Ponge, and D. Raabe, Effect of grain refinement to 1 μm on strength and toughness of dualphase steels, Materials Science and Engineering A, 2010, vol. 527, pp. 7832-7840. [16] N. Shukla, S. Das, S. Maji, S.R. Chowdhury, and B.K. Show, Effect of Pre-intercritical Annealing Treatments on the Microstructure and Mechanical Properties of 0.33% Carbon Dual-Phase Steel, Journal of Materials Engineering and Performance, 2015, vol. 24, pp. 4958-4965.
16
ACCEPTED MANUSCRIPT [17] D. Das, P.P. Chattopadhyay, Influence of martensite morphology on the work-hardening behavior of high strength ferrite–martensite dual-phase steel, Journal of Materials Science, 2009, vol. 44, pp. 2957-2965. [18] M. Balbi, I. Alvarez-Armas, and A. Armas, Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel, Materials Science and Engineering A, 2018, vol. 733, pp. 1-8. [19] A. Kalhor and H. Mirzadeh, Tailoring the microstructure and mechanical properties of dual phase steel based on the initial microstructure, Steel Research International, 2017, vol. 88, pp. 1600385. [20] H. Mirzadeh, M. Alibeyki, M. Najafi, Unraveling the initial microstructure effects on mechanical properties and work-hardening capacity of dual phase steel, Metallurgical and Materials Transactions A, 2017, vol. 48, pp. 45654573. [21] H. Azizi-Alizamini, M. Militzer, and W.J. Poole, Formation of ultrafine grained dual phase steels through rapid heating, ISIJ International, 2011, vol. 51, pp. 958-964. [22] K.T. Park, Y.K. Lee, D.H. Shin, Fabrication of ultrafine grained ferrite/martensite dual phase steel by severe plastic deformation, ISIJ International, 2005, vol. 45, pp. 750-755. [23] S. Ghaemifar, H. Mirzadeh, Enhanced mechanical properties of dual phase steel by repetitive intercritical annealing, Canadian Metallurgical Quarterly, 2017, vol. 56, pp. 459-463. [24] F. Castro Cerda, C. Goulas, I. Sabirov, S. Papaefthymiou, A. Monsalve, R.H. Petrov, Microstructure, texture and mechanical properties in a low carbon steel after ultrafast heating, Materials Science and Engineering A, 2016, vol. 672, pp. 108-120. [25] R.G. Davies, Influence of martensite composition and content on the properties of dual phase steels, Metallurgical Transactions A, 1978, vol. 9, pp. 671-679. [26] R. Tyagi, S.K. Nath, and S. Ray, Materials Science and Technology, 2004, vol. 20, pp. 645-652, Materials Science and Technology, 2004, vol. 20, pp. 645-652. [27] Y. Mazaheri, N. Saeidi, A. Kermanpur, A. Najafizadeh, Correlation of Mechanical Properties with Fracture Surface Features in a Newly Developed Dual-Phase Steel, Journal of Materials Engineering and Performance, 2015, vol. 24, pp. 1573-1580.
17
ACCEPTED MANUSCRIPT [28] S. Saadatkia, H. Mirzadeh, J.M. Cabrera, Hot deformation behavior, dynamic recrystallization, and physicallybased constitutive modeling of plain carbon steels, Materials Science and Engineering A, 2015, vol. 636, pp. 196-202. [29] B. Pourbahari, H. Mirzadeh, M. Emamy, R. Roumina, Enhanced ductility of a fine-grained Mg–Gd–Al–Zn magnesium alloy by hot extrusion. Advanced Engineering Materials, 2018, vol. 20, pp. 1701171. [30] H. Mirzadeh, J.M. Cabrera, J.M. Prado, A. Najafizadeh, Hot deformation behavior of a medium carbon microalloyed steel, Materials Science and Engineering A, 2011, vol. 528, pp. 3876-3882. [31] R. Zamani, H. Mirzadeh, M. Emamy, Mechanical properties of a hot deformed Al-Mg2Si in-situ composite, Materials Science and Engineering A, 2018, vol. 726, pp. 10-17. [32] B. Pourbahari, H. Mirzadeh, M. Emamy, Toward unraveling the effects of intermetallic compounds on the microstructure and mechanical properties of Mg–Gd–Al–Zn magnesium alloys in the as-cast, homogenized, and extruded conditions, Materials Science and Engineering A, 2017, vol. 680, pp. 39-46. [33] G.E. Dieter, Mechanical Metallurgy, 3rd ed., McGraw-Hill, 1988. [34] M. Alibeyki, H. Mirzadeh, M. Najafi, Fine-grained dual phase steel via intercritical annealing of cold-rolled martensite, Vacuum, 2018, vol. 155, pp. 147-152. [35] G. Krauss, Steels Processing, Structure, and Performance, 2nd edition, ASM International, 2015. [36] S. Ghaemifar, H. Mirzadeh, Refinement of banded structure via thermal cycling and its effects on mechanical properties of dual phase steel, Steel Research International, 2018, vol. 89, pp. 1700531. [37] E. Ahmad, T. Manzoor, M.M.A. Ziai, N. Hussain, Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel, Journal of Materials Engineering and Performance, 2012, vol. 21, pp. 382-387. [38] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Elsevier, 2012. [39] B. Pourbahari, H. Mirzadeh, M. Emamy, Elucidating the effect of intermetallic compounds on the behavior of Mg-Gd-Al-Zn magnesium alloys at elevated temperatures, Journal of Materials Research, 2017, vol. 32, pp. 41864195. [40] H. Hofmann, D. Mattissen, T.W. Schaumann, Advanced cold rolled steels for automotive applications, Steel Research International, 2009, vol. 80, pp. 22-28.
18
ACCEPTED MANUSCRIPT
Research Highlights
DP steel with fine ferrite grains and chain-like martensite was developed
Effects of cold rolling reduction and intercritical annealing time was studied
AGG at low rolling reductions resulted in poor strength-ductility balance
Results were discussed based on work-hardening behavior and tensile toughness
Sheida Nikkhah, Hamed Mirzadeh, Mehran Zamani PII:
S0254-0584(19)30253-6
DOI:
10.1016/j.matchemphys.2019.03.053
Reference:
MAC 21492
To appear in:
Materials Chemistry and Physics
Received Date:
21 December 2018
Accepted Date:
16 March 2019
Please cite this article as: Sheida Nikkhah, Hamed Mirzadeh, Mehran Zamani, Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing, Materials Chemistry and Physics (2019), doi: 10.1016/j.matchemphys. 2019.03.053
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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT
Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing Sheida Nikkhah, Hamed Mirzadeh 1, Mehran Zamani
Abstract The simultaneous effects of cold rolling reduction and intercritical annealing time on the microstructure and tensile properties of St12 steel were studied. It was revealed that by increasing the rolling reduction, the ferrite grain size of the dual phase (DP) microstructure is refined and a chain-like morphology of martensite develops, which results in improvement of work-hardening capacity and the tensile properties. Such a microstructure can be obtained via applying an optimum holding time at the intercritical annealing temperature. Beyond that optimum, grain coarsening occurs with the resulting deterioration of tensile properties. It was also shown that by consideration of quenched sheet or by applying low reductions in thickness, secondary recrystallization or abnormal grain growth (AGG) takes place during intercritical annealing, which is characterized by small grains and some large ferrite grains containing martensite islands. This study clarified that applying high reduction in thickness for grain refinement and also controlling the holding time in the two-phase austenite-ferrite region are the essential prerequisites to achieve the desired tensile strength, ductility, and toughness.
Keywords: Dual phase steels; Cold rolling reduction; Intercritical annealing time; Mechanical properties; Strain hardening rate.
1
Corresponding author. Tel.: +982182084080; Fax: +982188006076. E-mail address: [email protected] (H. Mirzadeh).
Affiliation of all authors: School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
1
ACCEPTED MANUSCRIPT 1. Introduction Dual-phase (DP) steels are a major group of modern low-carbon steels with duplex microstructure comprising of ferrite plus 10-40% martensite [1] (or other phases such as bainite [2]). This microstructure allows achieving high ultimate tensile strengths with maintaining high ductility, where these attributes are important for industrial applications of commercial FCC [3,4] and BCC [5,6] steels. Flat-rolled steels with low carbon (<0.10 wt.% C) and manganese (<0.4 wt.% Mn) contents have been recently used to process high-formability DP steels [7]. However, the obtained UTS values were extremely low (~ 400 MPa [7]) and it is highly desirable to improve their microstructure to reach the lower bound of 500 MPa, which needs microstructural modifications as shown by Nouroozi et al. [8], careful controlling of the intercritical annealing conditions as elaborated by Najafkhani et al. [9], or implementation of secondary hardening mechanisms as demonstrated by Zamani et al [10]. The properties of DP steel can be tailored via altering the volume fraction [11], Size [12], distribution [13], and morphology [14] of martensite phase and also ferrite grain size [15]. On the one hand, altering the initial microstructure is quite advantageous, where this can be easily achieved via intercritical annealing (IA) of ferritic-pearlitic microstructures [16], intermediate quenching (IQ) to produce a martensitic structure before intercritical annealing [17], or step quenching (SQ) from the austenitic region to the intercritical annealing temperature [18]. The intermediate quenching has been found as a viable method to enhance the properties of DP steel [19]. On the other hand, grain refinement is known to be one of the best methods for improving the strength-ductility balance of DP steels [20]. This can be realized by grain refinement of the pre-IA microstructure via thermomechanical processing [21] or severe plastic deformation [22], thermal cycling of DP microstructure [23], ultrafast heating to the austenite plus ferrite region [24], and cold-rolling before IA [8]. Applying cold deformation on a martensitic microstructure before intercritical annealing might induce the formation of recrystallized ferrite during heating to the intercritical region or at the initial stages of IA, which results in a fine DP microstructure with chain-like morphology of martensite with high workhardening capacity [14]. 2
ACCEPTED MANUSCRIPT The IA conditions are also important. By increasing the IA temperature, the strength level enhances but the ductility declines [25]. Regarding the IA time, the adjustment of martensite fraction [26], the effect of alloying elements [27], grain growth behavior [9], and the partitioning of Mn [18] have been studied so far. Besides these excellent works, the simultaneous effects of cold reduction and IA time have not been evaluated so far. It was revealed by Nouroozi et al. [8] and Najafkhani et al. [9] that the IA time should be carefully controlled to obtain a fine DP structure with desirable attributes. By applying different cold reductions, these aspects might become much more critical. As a result, this work is dedicated to evaluate these effects to improve the tensile properties of a high-formability 0.035C-0.27Mn-0.03Si (wt%) steel. 2. Experimental details The 0.035C-0.27Mn-0.035Si (wt%) St12 steel sheet after full annealing at 1150 °C for 15 min followed by furnace cooling was considered as the as-received sheet. It was then austenitized at 1150 °C for 15 min followed by water quenching / air cooling to develop essentially martensitic / normalized microstructures, respectively. The as-received, quenched, and normalized sheets were subjected to intercritical annealing at 800 °C for 1 and 5 min to obtain different sheets with ~ 20 vol% martensite as shown in Figure 1. Moreover, cold rolling reductions of 25, 40, and 80% were applied on the water quenched sheet followed by intercritical annealing at 800 °C for 1 and 15 min as shown in Figure 1. The details of the applied processing routes are shown in Table 1.
Figure 1: Processing routes used in this work.
3
ACCEPTED MANUSCRIPT Table 1: Details of the processing routes used in this work. IA represents intercritical annealing.
Pre-IA
Pre-IA cold rolling
microstructure
reduction (%)
IA5A
As-Received
IA5N
Sample
IA temperature (°C)
IA time (min)
0
800
5
Normalized
0
800
5
IQ1
Water Quenched
0
800
1
IQ5
Water Quenched
0
800
5
25CR-IQ1
Water Quenched
25
800
1
40CR-IQ1
Water Quenched
40
800
1
80CR-IQ1
Water Quenched
80
800
1
25CR-IQ15
Water Quenched
25
800
15
40CR-IQ15
Water Quenched
40
800
15
80CR-IQ15
Water Quenched
80
800
15
After etching by the LePera’s and Nital solutions, optical microscopy and FE-SEM (FEI Nova NanoSEM 450) were used for microstructural investigations. The ASTM E8 specimens (with the gauge length of 25 mm, width of 6 mm, and thickness of 1.66, 1.25, 0.95, and 0.45 respectively for cold rolling reductions of 0, 25, 40, and 80%) were subjected to room temperature tensile test under strain rate of 0.001 s-1. These tests were repeated one to insure the reproducibility of the results. The work-hardening rate (θ, via the central difference approach [28-30]) and the modulus of toughness (UT, via estimation of the area under the stressstrain curve according to the trapezoidal rule [31,32]) were obtained based on the following equations:
d / d i { i 1 i 1} /{ i 1 i 1}
(1)
U T (ei ei 1 )( S i S i 1 ) / 2
(2)
where e, ε, S, and σ are engineering strain, true strain, engineering stress, and true stress, respectively. 4
ACCEPTED MANUSCRIPT 3. Results Figure 2 shows the obtained microstructures and tensile stress-strain curves of IQ1, IQ5, IA5A, and IA5N. It can be seen in Figure 2a that the as-received sheet has a coarse ferrite-pearlite microstructure with low strength, high ductility, and yield-point elongation at the beginning, where the latter has been related to the Cottrell atmospheres of interstitial atoms around dislocations [33,34].
Figure 2: Microstructures and tensile stress-strain curves of IQ1, IQ5, IA5A, and IA5N samples. Note that (a), (b), (c), and (e) are optical micrographs and (d) is an SEM image.
5
ACCEPTED MANUSCRIPT After intercritical annealing (IA5A), a nearly similar microstructure comprising of ferrite and martensite develops (Figure 2b) and the tensile strength enhances considerably with the disappearance of the yieldpoint elongation, where the latter has been related to the presence of free dislocations in ferrite that are induced by the transformation of austenite to martensite [35,36]. Intercritical annealing of the normalized and quenched sheets results in grain refinement of the DP structure as can be seen in Figure 2c (IA5N) and Figure 2e (IQ5). However, these modifications have a marginal effect on the tensile stress-strain curves. The microstructure of IQ1 is shown in Figure 2d. It can be seen that 1 min at the IA temperature is not enough to develop the equilibrium microstructure and this can be better realized from the tensile curve that shows the residue of the fading yield-point elongation. Anyway, the tensile strengths of these samples are almost the same. The microstructures of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 are shown in Figure 3. All samples show a duplex ferritic-martensitic microstructure, and by increasing the prior cold rolling reduction, the ferrite grain size is considerably refined from 22 µm for IQ1 to 8.5 µm for 80CR-IQ1. In contrary to 40CR-IQ1 and 80CR-IQ1, the 25CR-IQ1 sample has very large grains along with the finer grains of expected size, where the presence of martensite islands inside ferrite grains is evident. This aspect will be revisited in the discussion section. Tensile stress-strain curves of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 are shown in Figure 4. It can be seen that the application of prior cold rolling results in a considerable enhancement of tensile strength, especially for the 40CR-IQ1 and 80CR-IQ1 samples that show high strength with maintaining good total elongation. Figure 4 also summarizes the values obtained for modulus of toughness (UT) based on Equation 2, which reveals that UT decreases for the 25CR-IQ1 but increases rapidly by increasing the prior cold rolling reduction. These findings will be revisited in the discussion section.
6
ACCEPTED MANUSCRIPT
Figure 3: SEM Images of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 samples.
7
ACCEPTED MANUSCRIPT
Figure 4: Tensile stress-strain curves of IQ1, 25CR-IQ1, 40CR-IQ1, and 80CR-IQ1 samples.
Figure 5 shows the microstructures of the samples that are intercritically annealed for 15 min (IQ15, 25CRIQ15, 40CR-IQ15, and 80CR-IQ15). It can be seen that the ferrite grains coarsen considerably at annealing time of 15 min in all samples (summarized in Table 2), especially in the case of 0 and 25% cold reductions. The tensile stress-strain curves of cold rolled samples (25CR-IQ15, 40CR-IQ15, and 80CR-IQ15) are shown in Figure 6. It can be seen that the tensile properties of these samples are inferior compared with the corresponding ones for holding time of 1 min. Table 2: Average ferrite grain size of different DP samples. Sample
IQ1
25CR-IQ1
40CR-IQ1
80CR-IQ1
25CR-IQ15
40CR-IQ15
80CR-IQ15
Grain Size (µm)
22 ± 1.3
14.3 ± 0.9
11.9 ± 1
8.5 ± 0.7
63.3 ± 2.1
20.1 ± 1.1
10.5 ± 1
8
ACCEPTED MANUSCRIPT
Figure 5: Microstructures of IQ15, IQ30, 25CR-IQ15, 40CR-IQ15, and 80CR-IQ15 samples. Note that (a), (b), (c), (d) and (e) are optical micrographs and (f) is an SEM image.
9
ACCEPTED MANUSCRIPT
Figure 6: Tensile stress-strain curves of 25CR-IQ15, 40CR-IQ15, and 80CR-IQ15 samples.
4. Discussion In the previous section, it was revealed that by increasing the cold rolling reduction, the grain size after IA will be considerably refined. At zero and low reduction of 25%, large ferrite grains containing martensite particles were observed. It was also seen that annealing beyond the optimum holding time (1 min in the present work) results in grain coarsening and deterioration of tensile properties. Figure 7 shows some of the pre-IA microstructures. The normalized microstructure (Figure 7a) is finer than the as-received one (Figure 2a), which resulted in a finer DP microstructure (IA5N, Figure 2c) compared with IA5A (Figure 2b). The quenched sample shows a completely different microstructure, i.e. the typical lath martensite (Figure 7b), which provides more nucleation sites for austenite (or martensite after quenching) during IA (Figure 2d). This resulted in a finer DP microstructure of IQ5 (Figure 2e). In fact, IA5N and IQ5 have been processed respectively based on intercritical annealing (IA) and intermediate quenching (IQ), where the advantages of IQ route has been discussed in details by Das and Chattopadhyay [17] and Ahmad et al. [37]. 10
ACCEPTED MANUSCRIPT As shown in Figure 7c, cold reduction of 25% has not considerably changed the microstructure of the quenched sample, and hence, it is logical to expect that this sample shows similar behavior during intercritical annealing when compared with the undeformed sample. Figure 3b and Figure 5c show that for the case low reduction of 25%, at IA time of 1 min (25CR-IQ1) and 15 min (25CR-IQ15), some large grains (with martensite islands inside them) along with fine grains are available. This phenomenon also happened in the case of quenched sample (Zero reduction) at IA time of 15 min (IQ15: Figure 5a) and 30 min (IQ30: Figure 5b). The presence of the martensite islands inside ferrite grains indicates the occurrence of abnormal grain growth (AGG) [38]. Figure 8 shows the progress of AGG at two magnifications. It can be seen that some martensite islands can pin the grain boundaries but some others do not, which results in AGG. A similar AGG behavior has been shown in the case of magnesium alloys containing intermetallic phases by Pourbahari et al. [39], where after grain growth, intermetallic particles remained in the interior of the abnormally grown grains. This work is among the first reports on the AGG behavior during intercritical annealing.
Figure 7: SEM images representing the pre-IA microstructures.
11
ACCEPTED MANUSCRIPT
Figure 8: SEM images of 25CR-IQ15 sample at two magnifications.
In the case of 40% (Figure 7d) and 80% (Figure 7e) rolled samples, deformed lath martensitic structures can be seen. In these cases, AGG cannot be seen, and in contrast, fine ferrite grains develop in the microstructure via recrystallization before partial austenitization at the IA temperature and a chain-like martensite morphology around ferrite grains forms as also pointed out by Nakada et al. [14]. Therefore, a minimum reduction in thickness is required to obtain recrystallized ferrite in the DP microstructure with avoiding AGG. This effect has been unraveled for the first time in the present work. Another important aspect of the present work is the enhancement of tensile properties of DP steel by prior cold rolling. Based on Figure 4, it can be realized that the slope of the stress-strain curve is significantly increased by prior cold rolling. This implies the improvement of the work-hardening rate that should be studied based on the true stress and θ curves (Equation 1). These plots are depicted in Figure 9a, which show that by increasing the rolling reduction, the value of θ at each strain increases. Moreover, the intersection of true stress and θ happens at larger strains, which reveals that the uniform elongation increases. 12
ACCEPTED MANUSCRIPT In fact, based on the Considére criterion, plastic instability of strain-rate-insensitive materials occurs when the strain-hardening rate coincides with the flow stress, i.e. d / d [33]. Therefore, the enhancement of work-hardening behavior can postpone necking. However, in the case of IA time of 15 min (for example for 80CR-IQ15 in Figure 9a), θ decreases, which can be ascribed to the coarsening of grains (Figure 9b) and the fading tendency of the chain-like morphology of martensite at ferrite grain boundaries (compare Figure 3e and Figure 5f). These are responsible for the deterioration of tensile properties at IA time of 15 min. Therefore, it can be realized that for obtaining fine-grained DP microstructure with chain-like morphology of martensite, careful controlling the holding time at the IA temperature is quite important. The importance of IA time has been previously pointed out by Nouroozi et al. [8] and Najafkhani et al. [9] and the present work systematically showed the combined effect of rolling reduction and IA time.
Figure 9: (a) Work-hardening rate plots and (b) Dependence of average ferrite grain size of DP steels on cold reduction.
13
ACCEPTED MANUSCRIPT The high work-hardening rate of the 80CR-IQ1 is responsible for the observed good strength-ductility balance, where its UTS reaches the domain of DP600 steels while maintaining a high total elongation of ~ 30% as shown in Figure 10 [40]. This figure reveals that by increasing the rolling reduction, the tensile strength increases toward the domain of advanced steels, and due to grain refinement of the DP microstructure with chain-like morphology of martensite, high ductility can be achieved except the case of 25% reduction (25CR-IQ1) with abnormal grain growth. However, except the case of 80% CR samples, other developed steels fall within the range of conventional steels similar to the previous report on this steel [7]. This reveals that high cold rolling reductions for grain refinement and controlling the IA time are crucial to achieve the desired tensile strength and ductility (Figure 10), and hence, obtaining a high modulus of toughness (Figure 4).
Figure 10: Strength-ductility balance of steels (Based on [40]), where the results of this study are shown on the figure.
5. Conclusions The synergistic effects of cold rolling reduction and intercritical annealing time on the microstructure and tensile properties of a high-formability DP steel were studied. The following conclusions can be drawn form this study: (1) The as-received (annealed) sheet showed a coarse ferrite-pearlite microstructure with low strength, high ductility, and yield-point elongation at the beginning of plastic flow. After intercritical annealing, a nearly 14
ACCEPTED MANUSCRIPT similar microstructure comprising of ferrite and martensite developed and the tensile strength enhanced considerably with the disappearance of the yield-point elongation. While intercritical annealing of the normalized and quenched sheets resulted in grain refinement of the DP structure, these modifications had a marginal effect on the tensile stress-strain curves. (2) It was revealed that by increasing the rolling reduction, the ferrite grain size of the DP microstructure is considerably refined and a chain-like morphology of martensite develops, which results in improvement of work-hardening capacity and the tensile properties. It was shown that such a microstructure can be obtained via applying an optimum holding time at the intercritical annealing temperature. Beyond that optimum, grain coarsening occurs with the resulting deterioration of tensile properties. (3) By consideration of quenched microstructure or by applying low reductions in thickness, abnormal grain growth (AGG) takes place during intercritical annealing, which is characterized by small grains and some large ferrite grains containing martensite islands. Therefore, it was revealed that high cold rolling reductions for grain refinement and also controlling the intercritical annealing time are the essential prerequisite to achieve the desired tensile strength, ductility, and toughness.
References [1] H. Ashrafi, M. Shamanian, R. Emadi, N. Saeidi, Examination of phase transformation kinetics during step quenching of dual phase steels, Materials Chemistry and Physics, 2017, vol. 187, pp. 203-217. [2] L. Schemmann, S. Zaefferer, D. Raabe, F. Friedel, and D. Mattissen, Alloying effects on microstructure formation of dual phase steels, Acta Materialia, 2015, vol. 95, pp. 386-398. [3] J. Li, Y. Cao, B. Gao, Y. Li, Y. Zhu, Superior strength and ductility of 316L stainless steel with heterogeneous lamella structure, Journal of Materials Science, 2018, vol. 53, pp. 10442-10456. [4] J. Li, C. Fang, Y. Liu, Z. Huang, S. Wang, Q. Mao, Y. Li, Deformation mechanisms of 304L stainless steel with heterogeneous lamella structure, Materials Science and Engineering A, 2019, vol. 742, pp. 409-413. [5] S. Sodjit, V. Uthaisangsuk, Microstructure based prediction of strain hardening behavior of dual phase steels, Materials and Design, 2012, vol. 41, pp. 370-379. 15
ACCEPTED MANUSCRIPT [6] N. Saeidi, M. Karimi, M.R. Toroghinejad, Development of a new dual phase steel with laminated microstructural morphology, Materials Chemistry and Physics, 2017, vol. 192, pp. 1-7. [7] M. Maleki, H. Mirzadeh, M. Zamani, Effect of Intercritical Annealing on Mechanical Properties and WorkHardening Response of High Formability Dual Phase Steel, Steel Research International, 2018, vol. 89, pp. 1700412. [8] M. Nouroozi, H. Mirzadeh, and M. Zamani, Effect of microstructural refinement and intercritical annealing time on mechanical properties of high-formability dual phase steel, Materials Science and Engineering A, 2018, vol. 736, pp. 22-26. [9] F. Najafkhani, H. Mirzadeh, M. Zamani, Effect of Intercritical Annealing Conditions on Grain Growth Kinetics of Dual Phase Steel, Metals and Materials International, in press. [10] M. Zamani, H. Mirzadeh, M. Maleki, Enhancement of mechanical properties of low carbon dual phase steel via natural aging, Materials Science and Engineering A, 2018, vol. 734, pp. 178-183. [11] M. Alibeyki, H. Mirzadeh, M. Najafi, and A. Kalhor, Modification of rule of mixtures for estimation of the mechanical properties of dual-phase steels, Journal of Materials Engineering and Performance, 2017, vol. 26, pp. 26832688. [12] H. Seyedrezai, A. K. Pilkey, J. D. Boyd, Effects of martensite particle size and spatial distribution on work hardening behaviour of fine-grained dual-phase steel, Canadian Metallurgical Quarterly, 2018, vol. 57, 2018, pp. 2837. [13] Z. Nasiri, H. Mirzadeh, Enhancement of work-hardening behavior of dual phase steel by heat treatment, Materialwissenschaft und Werkstofftechnik, 2018, vol. 49, pp. 1081-1086. [14] N. Nakada, Y. Arakawa, K.S. Park, T. Tsuchiyama, and S. Takaki, Dual phase structure formed by partial reversion of cold-deformed martensite, Materials Science and Engineering A, 2012, vol. 553, pp. 128-133. [15] M. Calcagnotto, D. Ponge, and D. Raabe, Effect of grain refinement to 1 μm on strength and toughness of dualphase steels, Materials Science and Engineering A, 2010, vol. 527, pp. 7832-7840. [16] N. Shukla, S. Das, S. Maji, S.R. Chowdhury, and B.K. Show, Effect of Pre-intercritical Annealing Treatments on the Microstructure and Mechanical Properties of 0.33% Carbon Dual-Phase Steel, Journal of Materials Engineering and Performance, 2015, vol. 24, pp. 4958-4965.
16
ACCEPTED MANUSCRIPT [17] D. Das, P.P. Chattopadhyay, Influence of martensite morphology on the work-hardening behavior of high strength ferrite–martensite dual-phase steel, Journal of Materials Science, 2009, vol. 44, pp. 2957-2965. [18] M. Balbi, I. Alvarez-Armas, and A. Armas, Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel, Materials Science and Engineering A, 2018, vol. 733, pp. 1-8. [19] A. Kalhor and H. Mirzadeh, Tailoring the microstructure and mechanical properties of dual phase steel based on the initial microstructure, Steel Research International, 2017, vol. 88, pp. 1600385. [20] H. Mirzadeh, M. Alibeyki, M. Najafi, Unraveling the initial microstructure effects on mechanical properties and work-hardening capacity of dual phase steel, Metallurgical and Materials Transactions A, 2017, vol. 48, pp. 45654573. [21] H. Azizi-Alizamini, M. Militzer, and W.J. Poole, Formation of ultrafine grained dual phase steels through rapid heating, ISIJ International, 2011, vol. 51, pp. 958-964. [22] K.T. Park, Y.K. Lee, D.H. Shin, Fabrication of ultrafine grained ferrite/martensite dual phase steel by severe plastic deformation, ISIJ International, 2005, vol. 45, pp. 750-755. [23] S. Ghaemifar, H. Mirzadeh, Enhanced mechanical properties of dual phase steel by repetitive intercritical annealing, Canadian Metallurgical Quarterly, 2017, vol. 56, pp. 459-463. [24] F. Castro Cerda, C. Goulas, I. Sabirov, S. Papaefthymiou, A. Monsalve, R.H. Petrov, Microstructure, texture and mechanical properties in a low carbon steel after ultrafast heating, Materials Science and Engineering A, 2016, vol. 672, pp. 108-120. [25] R.G. Davies, Influence of martensite composition and content on the properties of dual phase steels, Metallurgical Transactions A, 1978, vol. 9, pp. 671-679. [26] R. Tyagi, S.K. Nath, and S. Ray, Materials Science and Technology, 2004, vol. 20, pp. 645-652, Materials Science and Technology, 2004, vol. 20, pp. 645-652. [27] Y. Mazaheri, N. Saeidi, A. Kermanpur, A. Najafizadeh, Correlation of Mechanical Properties with Fracture Surface Features in a Newly Developed Dual-Phase Steel, Journal of Materials Engineering and Performance, 2015, vol. 24, pp. 1573-1580.
17
ACCEPTED MANUSCRIPT [28] S. Saadatkia, H. Mirzadeh, J.M. Cabrera, Hot deformation behavior, dynamic recrystallization, and physicallybased constitutive modeling of plain carbon steels, Materials Science and Engineering A, 2015, vol. 636, pp. 196-202. [29] B. Pourbahari, H. Mirzadeh, M. Emamy, R. Roumina, Enhanced ductility of a fine-grained Mg–Gd–Al–Zn magnesium alloy by hot extrusion. Advanced Engineering Materials, 2018, vol. 20, pp. 1701171. [30] H. Mirzadeh, J.M. Cabrera, J.M. Prado, A. Najafizadeh, Hot deformation behavior of a medium carbon microalloyed steel, Materials Science and Engineering A, 2011, vol. 528, pp. 3876-3882. [31] R. Zamani, H. Mirzadeh, M. Emamy, Mechanical properties of a hot deformed Al-Mg2Si in-situ composite, Materials Science and Engineering A, 2018, vol. 726, pp. 10-17. [32] B. Pourbahari, H. Mirzadeh, M. Emamy, Toward unraveling the effects of intermetallic compounds on the microstructure and mechanical properties of Mg–Gd–Al–Zn magnesium alloys in the as-cast, homogenized, and extruded conditions, Materials Science and Engineering A, 2017, vol. 680, pp. 39-46. [33] G.E. Dieter, Mechanical Metallurgy, 3rd ed., McGraw-Hill, 1988. [34] M. Alibeyki, H. Mirzadeh, M. Najafi, Fine-grained dual phase steel via intercritical annealing of cold-rolled martensite, Vacuum, 2018, vol. 155, pp. 147-152. [35] G. Krauss, Steels Processing, Structure, and Performance, 2nd edition, ASM International, 2015. [36] S. Ghaemifar, H. Mirzadeh, Refinement of banded structure via thermal cycling and its effects on mechanical properties of dual phase steel, Steel Research International, 2018, vol. 89, pp. 1700531. [37] E. Ahmad, T. Manzoor, M.M.A. Ziai, N. Hussain, Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel, Journal of Materials Engineering and Performance, 2012, vol. 21, pp. 382-387. [38] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Elsevier, 2012. [39] B. Pourbahari, H. Mirzadeh, M. Emamy, Elucidating the effect of intermetallic compounds on the behavior of Mg-Gd-Al-Zn magnesium alloys at elevated temperatures, Journal of Materials Research, 2017, vol. 32, pp. 41864195. [40] H. Hofmann, D. Mattissen, T.W. Schaumann, Advanced cold rolled steels for automotive applications, Steel Research International, 2009, vol. 80, pp. 22-28.
18
ACCEPTED MANUSCRIPT
Research Highlights
DP steel with fine ferrite grains and chain-like martensite was developed
Effects of cold rolling reduction and intercritical annealing time was studied
AGG at low rolling reductions resulted in poor strength-ductility balance
Results were discussed based on work-hardening behavior and tensile toughness