Heterogeneous microstructure design by pre-manipulating ferrite recrystallization in a cold rolled medium Mn steel

The medium Mn steel (MMS), which is regarded as the most representative candidate of the third generation advanced high strength steels, has received widely attention during the last several decades with respect to the exceptional advantage of low cost and excellent strength-ductility properties. In this study, a microstructural strategy of developing heterogeneous microstructures in a cold rolled MMS is presented. By pre-manipulating occurrence of the ferrite recrystallization, both the lamellar-shaped and granular-shaped ultra-fine retained austenite can be obtained after the two-step intercritical annealing process. Various amounts of recrystallized ferrite and difform martensite can be obtained by adjusting the pre-annealing temperature, which can effectively contribute to producing the two types of heterogeneous retained austenite, i.e., lamellar and granular in the following annealing process. The heterogeneous-structured retained austenite enables an excellent strength–ductility combination and reduced Lüders strain in the cold-rolled MMS.


Introduction
Over the past decades, significant efforts have been made to develop advanced high strength steels (AHSSs) towards the target of satisfying lightweight, energy efficiency and structural safety of automobiles [1].Among various candidates, the medium Mn steel (MMS) with 3-12 wt% Mn content is becoming the most potent of the third generation of AHSSs due to its excellent strength and ductility combination [2][3][4].The MMS is essentially one typical transformation induced plasticity (TRIP) assisted steel, so that sufficient retained austenite with suitable stability is mandatorily demanded to afford the good mechanical properties [5].For the MMS, the retained austenite is commonly produced through austenite reverse transformation (ART) annealing [6].However, very long annealing duration is usually required due to the slow Mn diffusions, which leads to low production efficient and limits the practical application in industry.Of course, the ART process might be accelerated by introducing deformation dislocations into the MMS by cold-rolling, such that the austenite reversion and ferrite recrystallization can be easily triggered to produce ultra-fine duplex microstructures with recrystallized ferrite and metastable austenite, which can lead to the comprehensive mechanical properties of high strength and good elongation [7].However, typical discontinuous yielding behavior has to be expected derived from the ultra-fine equiaxed duplex microstructure during the tensile deformation.The formation and propagation of Lüders band can deteriorate the surface quality of the cold-worked steel product, which is not anticipated for the commercialization of the cold-rolled MMS [8].
Several methods have been proposed to reduce Lüders strain in cold-rolled MMSs, e.g., by pre-heat treatment [9], by designing bimodal grain size microstructures [10], by preserving deformed structures [11], by reducing deformation reduction [12], by introducing pre-strain [13] and by forming heterogeneous microstructures [14].Actually, transformations during the intercritical annealing process of the cold-rolled MMS are intrinsically related, including ferrite recrystallization, austenite reversion transformation and alloy element partitioning, which bring a large variety in diversified microstructures tailoring.In this study, a two-step annealing strategy was used to design heterogeneous microstructures in a 0.15C-4.7Mncold-rolled MMS.The effectiveness of the recrystallization control via adjusting the pre-annealing temperature was analyzed.The formation of heterogeneous microstructures and their influence on the stability of retained austenite and mechanical properties were discussed.

Experimental
The nominal composition of the MMS used in the present study was Fe-4.7Mn-0.15C(wt.%).The equilibrium transformation temperatures of Ae1 and Ae3 were calculated to be 502 °C and 731 °C, respectively, using the Thermo-Calc software.The as-received MMS was cast using a 50 kg vacuum induction melting, then homogenized at 1200 °C for 10 h and rolled between 1050 °C and 900 °C to produce the hot-rolled plate in 3 mm thickness.After soft annealing at 500 °C for 1 h, the plate was cold-rolled to the final thickness of 1.5 mm.The microstructure of cold rolled sheet mainly contains deformed martensite with cementite precipitations, as shown in figure 1a.Here, a strategy of two-step intercritical annealing processing was adopted.The cold-rolled MMS was pre-annealed firstly at 680, 700 and 720 °C for 10 min, respectively, such that different scenarios of ferrite recrystallization did take place to produce various microstructures with different recrystallized degrees.Then a conventional ART annealing processing of 650 °C holding for 30 min was subsequently performed using the pre-manipulated recrystallized microstructures as the precursors.The samples after two-step annealing are referred as P680-ART, P700-ART and P720-ART.For comparisons, the conventional ART annealing during which the cold-rolled MMS was intercritically annealed at 650 °C for 1 h was also considered (hereafter referred as C-ART).The microstructures of the annealed samples were characterized by MERLIN Compact field emission scanning electron microscope (SEM) equipped with an electron backscatter diffraction detector (EBSD) operated at 20 kV.Samples for EBSD were prepared by mechanical polishing finished with fine polishing using SiO2 suspension.A Talos F200X mode field emission transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDXS) was used to characterize the fine microstructures and distributions of alloy elements.The TEM foils were prepared by twin-jet electro-polishing with a solution containing 5% perchloric and 95% ethanol at the temperature of about −30 °C and a voltage of 30 V. Uniaxial tensile tests were carried out on a Zwick/Roell Z150 testing machine at a constant rate of 2.4 mm /min.The tensile samples were prepared along the rolling direction with a gauge length of 25 mm and a width of 4 mm.

Results and Discussion
Figure 1 shows the microstructures of the cold-rolled MMS produced by the two-step annealing using the pre-annealed microstructure at 700 °C and by conventional ART annealing.It can be seen that, an ultra-fine duplex microstructure of granular-shaped ferrite (αG) and austenite (γG) is produced by the conventional ART processing.The heavily deformed martensite matrix has been completely recrystallized.In the P700-ART sample, however, typical heterogeneous microstructures consisting of the granular-shaped αG and γG, as well as ultra-fine lamellar-shaped ferrite (αL) and austenite (γL) have been obtained.The volume fraction of retained austenite of P700-ART sample is measured to be about 36.2 %, which is higher than 33.2 % of the C-ART sample.It suggests that the ART process has been significantly accelerated by introducing the pre-annealing process.(d-f) and microstructure schematics (g-i) of the cold-rolled 0.15C-4.7MnMMS pre-annealed at 680 °C (a, d, g), 700 °C (b, e, h), and 720 °C (c, f, i).The gray and black lines of IPF images in a-c represent boundaries with misorientations of 2° < θ < 15° and θ > 15°, respectively.The green, blue, and black lines of band contrast images in d-f represent boundaries with misorientations of 2° < θ < 5°, 5° < θ < 15°, and θ > 15°, respectively.

Ferrite recrystallization manipulation during the pre-annealing
Figure 2 shows the microstructures of the cold rolled MMS pre-annealed at different intercritical temperatures.Since the pre-annealing temperature is relative high, the recrystallization of ferrite is easy to be triggered.It can be seen that after annealing at 680 °C, 700 °C and 720 °C, ferrite recrystallization and austenite formation have taken place simultaneously, forming a fine dual phase structure composed of equiaxed recrystallized ferrite and reversed austenite.However, since the pre-annealing temperature adopted is higher than the optimal temperature of conventional ART treatment for the 0.15C-4.7Mnsteel, the concentration of C and Mn in the formed austenite is low.Therefore, the reversed austenite formed in the pre-annealing will transform into martensite during cooling due to the low thermal stability.The microstructure of pre-annealed sample at 680 °C is composed of 60% recrystallized ferrite and 40% martensite, and the microstructure is maintained as an equiaxed structure with uniform distribution, as shown in figure 2d.When the pre-annealing temperature is increased to 700 °C, the phase transformation of austenite is promoted obviously.Although the volume fraction of recrystallized ferrite decreases, the grain size maintains fine equiaxed at the level of 1~2 μm, which indicates that the coarsening of recrystallized ferrite grains in the formed duplex structure is effectively inhibited.At the same time, some austenite grains tend to merge and coarsen, forming the reversed austenite with various grain sizes (figure 2e).It can be seen that the large size austenite grains are transformed into fresh martensite during cooling.When the pre-annealing temperature rises to 720 °C, the volume fraction of recrystallized ferrite is further reduced to about 10%, and large lath martensite is formed from the coarsened austenite grains, as shown in figure 2f.There are abundant large angle grain boundaries and subgrain boundaries in the new martensite grains.In addition, stress is introduced while the martensite transformation is taking place.The existence of these substructures and local stored energy will greatly promote the subsequent austenite transformation during the ART annealing.In a word, the volume fraction control of recrystallized ferrite can be achieved and fresh martensite structures with diversified proportions and sizes are significantly obtained, which provides a space for the control of retained austenite in subsequent ART annealing process.

Formation of the heterogeneous microstructures
The final microstructures of two-step annealed cold-rolled MMS are shown in figure 3.For comparison, the microstructure of C-ART sample is also discussed.Based on the precursors via different pre-annealing, the sample is subjected to ART treatment at a lower critical temperature of 650 °C.Consequently, the thermal stability of the retained austenite is improved, which makes it easy to remain at room temperature.As is seen in figure 3e, both ferrite and retained austenite grains of P680-ART sample inherit the fine and equiaxial characteristics of the pre-annealed samples.The austenite transformation is mainly taking place in the equiaxed martensite formed in the pre-annealing.The grain size of the recrystallized ferrite is obviously smaller than that of C-ART sample and the austenite grains are also equiaxed with an average grain size of about 500 nm, which means that no obvious grain growth occurs.The grain refinement will significantly reduce the diffusion distance of C element, so that the austenite reverse transformation kinetics can be accelerated significantly.
As mentioned above, the volume fraction and grain size of recrystallized ferrite and fresh martensite can be controlled according to the pre-annealing temperature.Unlike the fine martensite of the P680-ART sample, coarsened martensite with sub-structures can also be formed when increasing the pre-annealing temperature, which gradually acts as major nucleation sites for austenite formation.In addition to equiaxed ferrite grains, two kinds of fine austenite grains with heterogeneous morphology are simultaneously obtained in the P700-ART sample: lamellar (γL) and equiaxed (γG).The lamellar-shaped austenite is mainly formed along the martensite lath interface inside the large size of fresh martensite generated in the pre-treatment, while the equiaxed austenite is mainly from the small size of martensite.Therefore, in this case, heterogeneous structural austenite composed of two different morphologies are formed, as shown in figure 3f.For the P720-ART sample, large scale coarsening occurs in the reversed austenite, and abundant grain boundaries and substructures are formed during cooling.In the subsequent annealing, the formation of austenite mainly occurs at the interface of martensite lath, which attributes to the generation of uniform lamellar-shaped austenite (figure 3g).Generally, by pre-annealing, two morphologies of small size martensite and large size martensite are formed in the microstructure.After the second step of ART treatment, martensite with relatively small size transform into equiaxed retained austenite (γG), while larger martensite colonies transform into lamellar retained austenite (γL) and lamellar ferrite (αL), resulting in the formation of ultra-fine heterogeneous microstructures.Figure 4 shows the duplex microstructures and associated Mn distributions developed by the pre-annealing at 700 °C and after the two-step ART treatment.It can be seen that Mn partitioning has occurred significantly from the ferrite matrix to the intercritical austenite during pre-annealing.The increased Mn diffusivity due to the high annealing temperature may accelerate the Mn partition between the two phases, introducing evident chemical heterogeneity in the formed duplex microstructures, as shown in figure 4b.During the subsequent ART annealing, the Mn-enriched fresh martensite colonies may act as active Mn reservoirs and provide preferred routes for austenite formation due to the higher chemical driving force and intensive tangled dislocations.Apparently, the Mn enrichment will be inherited by the newly formed heterogeneous austenite in the second ART process.For equiaxed austenite, although they are formed following the same mechanism of austenite reversion as that in the pre-annealing process, their transformation kinetics are accelerated due to the prior Mn enrichment.Additionally, continuously Mn partition from ferrite permits the equiaxed austenite stabilization to room temperature.Another type of fine lamellar austenite is formed in the interior of relatively coarse martensite colonies.An increased Mn content can be obtained not only from the inheritance of chemical heterogeneity from prior fresh martensite, but also due to further formation of the adjacent lamellar ferrite through a diffusive austenite reverse transformation (figure 4e).Thus, the lamellar austenite with nanoscale will gain higher stability as the lamellar ferrite inhibit austenite coarsening.These two types of retained austenite show in wider chemical concentrations and morphological features, which present heterogeneous TRIP effects during tensile testing compared to that containing more homogeneous austenite in the C-ART sample.

Tensile behaviors of the heterogeneous microstructures
Figure 5 shows the tensile behaviors of cold-rolled MMS under tensile deformation at room temperature.By pre-manipulating ferrite recrystallization, the mechanical properties of the final annealed samples are significantly improved.The product of strength and elongation for two-step annealing samples is increased to a level of 37-41 GPa•%, compared with 32.3 GPa•% for the C-ART sample.More importantly, the Lüders strain of the pre-manipulated samples is significantly reduced.The work hardening behavior of P680-ART sample is similar to that of C-ART sample, mainly because the similar fine equiaxial duplex microstructures [15].However, shorter ART treatment will reduce the mechanical stability of the retained austenite formed.In this case, the TRIP can occur firstly within the austenite with low mechanical stability, which may inhibit the Lüders deformation at a much low strain.Moreover, the deformation induced martensite will continue to participate in the further strengthening during subsequent deformation, so that the tensile strength is significantly improved.The high volume fraction of austenite can also afford a high elongation in sample.Among the heterogenous austenite of different morphology obtained in P700-ART sample, equiaxial austenite has low mechanical stability, while lameller austenite with higher Mn concentration and smaller size has high mechanical stability.The formation of heterogeneous austenite enables the internal TRIP effect to occur continuously and gradually within a wide strain range, which is also the main reason for the annealed sample to obtain high strength with no obvious reduction in plasticity.In addition, the strengthening effect caused by TRIP effect at the initial stage of deformation can coordinate the localization of plastic deformation, thus effectively inhibiting the development of bands [16].The results show that there is no fluctuation of work hardening rate in the P720-ART sample during Lüders deformation in the early deformation stage, indicating that TRIP effect has not been effectively activated at this time, which is mainly caused by the lameller austenite with high mechanical stability.However, the mechanical stability of austenite is lower than that of C-ART sample, and the occurrence of TRIP still reduces the Lüders strain and improves both strength and plasticity.

Conclusions
By manipulating ferrite recrystallization at different pre-annealing temperatures, diversified microstructural precursors consisting of recrystallized ferrite and fresh martensite with different volume fractions and grain sizes can be obtained.By using these microstructures as precursors for subsequent ART processing, ultra-fine heterogeneous microstructures with both lameller-shaped and granular-shaped retained austenite are successfully produced, which may provide diversified mechanical stability to make the TRIP effect occurring continuously during the tensile deformation.Compared with the conventional ART treatment of the cold-rolled MMS, significant enhanced strength and suppressed Lüders strain can be achieved by including ferrite recrystallization manipulated annealing.It suggests a feasible way to adjust the final heterogeneous microstructures with shortened duration time of the ART process and optimize the mechanical properties of the cold rolled medium Mn steel.

Figure 4 .
Figure 4. TEM images (a, d), Mn distribution maps (b, e) and corresponding Mn concentration profiles (c, f) along the corresponding white arrows L1 and L2 of the cold-rolled 0.15C-4.7MnMMS after pre-annealing (a-c) at 700 °C and after two-step annealing (d-f).

Figure 5 .
Figure 5. Tensile curves (a) and work hardening rate curves (b) of the cold-rolled 0.15C-4.7MnMMS after two-step annealing and conventional annealing.