A Novel Approach for Controlling the Band Formation in Medium Mn Steels
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Formation of the microstructural ferrite/pearlite bands in medium Mn steels is an undesirable phenomenon commonly addressed through fast cooling treatments. In this study, a novel approach using the cyclic partial phase transformation concept is applied successfully to prevent microstructural band formation in a micro-chemically banded Fe-C-Mn-Si steel. The effectiveness of the new approach is assessed using the ASTM E1268-01 standard. The cyclic intercritical treatments lead to formation of isotropic microstructures even for cooling rates far below the critical one determined in conventional continuous cooling. In contrast, isothermal intercritical experiments have no effect on the critical cooling rate to suppress microstructural band formation. The origin of the suppression of band formation either by means of fast cooling or a cyclic partial phase transformation is investigated in detail. Theoretical modeling and microstructural observations confirm that band formation is suppressed only if the intercritical annealing treatment leads to partial reversion of the austenite-ferrite interfaces. The resulting interfacial Mn enrichment is responsible for suppression of the band formation upon final cooling at low cooling rates.
The formation of ferrite/pearlite microstructural bands in hot-rolled low-alloy steels has been one of the main challenges in the development of medium Mn steels for low-weight automotive applications.[1, 2, 3] The presence of the ferrite/pearlite microstructural banding leads to an undesirable reduction of the mechanical properties perpendicular to the rolling direction.[4, 5, 6, 7, 8, 9, 10] The band formation is the result of significant partitioning of alloying elements during solidification which generally cannot be undone sufficiently during the solid-state homogenization treatments preceding the hot rolling and subsequent thermomechanical processing steps. Not surprisingly, the topic of band formation and its relation to the imposed thermal trajectory when cooling down from the hot rolling temperature has been researched for more than 50 years.[11, 12, 13, 14, 15, 16]
In micro-chemically banded Fe-C-Mn-Si steels, the spatial variation of substitutional alloying element concentrations locally reduces (in regions with high levels of austenite forming elements such as Mn) or raises (in regions with high levels of ferrite forming elements such as Si) the A3 transition temperature. Upon cooling from the austenite, the ferrite starts to nucleate in regions with a high A3 transition temperature as the required undercooling for ferrite nucleation is reached there first. With the formation of the early ferrite, C is injected into the remaining neighboring austenite with a higher Mn content, reducing the local A3 transition temperature even more. Upon further cooling, the composition in these remaining austenitic regions reaches eutectoid levels and the austenite decomposition continues with pearlite formation in a banded arrangement. Hence, formation of the ferrite/pearlite microstructural bands is a matter of balancing nucleation and growth during austenite decomposition.[5,18] In a standard approach, formation of microstructural bands can be suppressed via fast cooling after hot rolling. The severe undercooling from austenitization temperature results in simultaneous nucleation of ferrite in both rich and poor Mn-containing regions. Although fast cooling prevents band formation, but it leads to formation of ferrite with a fine average grain size and a higher hardness and yield strength which is not always desirable.
Recently, the ‘Cyclic Partial Phase Transformation’ (CPPT) approach has been proposed as an scientifically interesting intercritical annealing route to investigate the effect of alloying elements on interface migration kinetics.[19,20] In a CPPT experiment, the temperature is cycled between two temperatures inside the austenite-ferrite two-phase regime such that both austenite and ferrite phases are present at all times. The CPPT experiments demonstrate a clear retardation of the final ferrite growth as a result of local Mn enrichment due to the back and forth migration of the austenite-ferrite interfaces more or less following the same trajectory. In a more recent approach, the cyclic treatments are successfully used as a novel route to stabilize austenite in a medium Mn steel down to room temperature.
The present work introduces a new approach for controlled suppression of ferrite/pearlite band formation in a medium Mn steel using the CPPT approach and making use of the controlled Mn partitioning at moving austenite-ferrite interfaces. The approach led to a considerable lowering of the minimum cooling rate required to prevent band formation. The unique observation of perfectly aligned pearlite rims around ferrite grains is a clear evidence of the impact of local Mn enrichment at halted interfaces.
2 Experimental Details
In this study, a commercial hot-rolled Fe-0.17C-1.47Mn-1.48Si (all concentrations in mass pct.) steel grade is subjected to different thermal treatments. Solid cylindrical samples for dilatometric experiments with a diameter of 5.0 mm and length of 10.0 mm were wire-cut with the axis of the cylinder parallel to the rolling direction. Qualitative energy dispersive spectroscopy (EDS) across the rolling direction in the samples was done confirming micro-chemical banding of both Mn and Si. All thermal processing was done using a Bähr DIL 805A/D/T Quenching dilatometer. As samples containing micro-chemical bands show different dilatation signals parallel and perpendicular to the rolling direction, the dilatation of the specimens was measured in both the longitudinal and lateral direction. The samples were aligned such that the rolling plane was perpendicular to the direction in which the lateral expansion was measured. All experiments were done in vacuum with a pressure of 5 × 10−4 mbar. Two thermocouples spaced 4 mm apart were spot welded to the samples to ascertain that the thermal gradient along the sample was less than 3 K. All samples were heated with a rate of 10 K/s and fully austenitized at 1273 K (1000 °C) for 300 seconds (5 minutes) and then subjected to either Continuous Cooling (CC) experiments with different cooling rates (CR) from 20 to 0.17 K/s (10 K/min) or to different Cyclic Partial Phase Transformation (CPPT) routes. The samples were cut in the longitudinal direction and polished and etched with a 2 pct Nital etchant and subsequently evaluated using conventional light and scanning electron microscopical methods. The micro-segregation of C, Mn, and Si was determined using a Jeol JXA 8900R microprobe instrument fitted with appropriate wavelength dispersive spectrometers.
As explained above, in order to avoid ferrite/pearlite band formation in a CC experiment, cooling faster than the critical cooling rate of 2 K/s is essential. In the next section, new routes of intercritical annealing using the CPPT approach are explained and the results are presented.
The dilatation behavior for the type H2 CPPT experiment, Figure 5(b), is similar to that for type H1 but linear contraction corresponding to the stagnant stage in austenite decomposition during final cooling continues down to T ≈ 960 K (687 °C) instead of T ≈ 973 K (700 °C) measured in the type H1 route. In type I CPPT (Figure 5(c)), after reaching T1, despite the increase in temperature and the expected decrease in length due to ferrite to austenite transformation, the sample length increases non-linearly for about 15 K. This stage is called ‘inverse transformation’ and is explained by non-equilibrium condition around the austenite-ferrite interface due to incomplete redistribution of C atoms after sudden change in the temperature regime.[19,20] After the inverse transformation stage, the stagnant stage starts and continues during cooling similar to that of type H1 and H2 CPPT routes. Finally, in the isothermal experiment (Figure 5(d)), the austenite to ferrite transformation proceeds isothermally and upon further cooling, the dilatation behavior is analogous to that of the CC curve with no sign of delayed austenite decomposition during final cooling.
The curves, for type H1 CPPT with CR = 0.17 and 0.5 K/s (Figures 9(a) and (b)), are significantly smoothed with a substantial reduction of the relative peak heights, indicative of the more uniform microstructure. The hills in the band formation index indicate that there are still preferential locations for pearlite formation with the typical distance of 50 μm. This indicates that Mn is still non-uniformly distributed along the microstructure, but for the cyclic annealing route, the formation of pearlite in preferential columns is strongly suppressed. The distinct peaks in type H1 with CR = 1 K/s curve in Figure 9(c) are another indication of the anisotropic distribution of phases in microstructure after this treatment. As expected, the curves of band formation index for type H1, H2, and I CPPT routes with CR = 0.17 K/s shown in Figures 9(d) and (e) are flattened, while the curve of band formation index for isothermal experiment in Figure 9(f) clearly shows sharp separate peaks corresponding to diffusely banded and non-uniform microstructure.
The continuous cooling experiments show that in the hot-rolled alloy with micro-segregation of Mn and Si, formation of ferrite/pearlite bands during continuous cooling from austenitic temperature is a virtual function of the applied cooling rate. As shown concisely in Figure 8, cooling with rates equal or higher than 2 K/s is essential to prevent the development of a banded microstructure. This is in agreement with earlier publications showing that even though the presence of micro-chemical segregation of Mn/Si is a prerequisite for band formation, it is the kinetics of the phase transformation, and in particular that of the ferrite, which controls the actual formation of the microstructural bands.[5,18,31] Pearlite grows much faster than the ferrite due to availability of higher driving force at lower transformation temperatures and with a shorter diffusion length as a result of the laminar pearlite structure.[4,5,31] The time available for the growth of the ferrite controls the carbon enrichment of the austenite and this, for non-isothermal conditions, controls the chance of nucleation of either ferrite or pearlite in the Mn-rich and Mn-poor regions. For slow cooling rates, ferrite nucleation starts preferentially in the Mn-poor regions and the time for ferrite formation is long and the C partitions over relatively large distances. Hence, new ferrite nucleation will not occur in the Mn-rich regions assisting continuous pearlite formation in these regions. In general, for differences less than 6 to 8 pct in ferrite nucleation rate, banding does not take place.[5,32,33]
The experimental observations clearly show that the band formation can be suppressed by some of the CPPT treatments (see Figure 4), even for a low cooling rate of 0.17 K/s. The suppression is due to the ferrite formation being halted as a result of the local Mn enrichment at the moving austenite-ferrite interface and the spike of Mn left behind as a result of the cycling process. As the transformation is halted, there is less carbon enrichment and new ferrite formation can take place during the final cooling down. Thus, new ferrite formation can take place in the higher Mn regions and band formation is suppressed.
As concisely shown in Figure 8, the values of AI very close to 1 for type H1, H2, and I CPPT routes with CR = 0.17 K/s indicate that a CPPT route such as that of type H in which the austenite-ferrite interface is forced to reverse its propagation direction, provides the conditions required for suppression of band formation.
Ferrite grains at a lower Mn regions,
Pearlite grains in high Mn regions,
Pearlite rims in low Mn regions,
Pearlite islands in low Mn regions, and
Ferrite grains in high Mn regions.
In a fully banded structure, the first two components dominate the microstructure. Formation of the last three components can be linked to an interrupted band formation process as a result of the CPPT treatment. Figure 14(b) shows a ferrite grain in a low Mn region with the so-called ‘pearlite rims’ perfectly aligned and decorating the ferrite grain and a ‘pearlitic island’ surrounded by different ferrite grains. The cementite lamella in the pearlite rim are more or less parallel all along the curved rim and indicates that the pearlite rim has a growth orientation relationship with the enclosed ferrite grain. As shown in Figure 13(b), with cyclic experiment, a bump of residual Mn forms opposing the migrating ferrite interface. If the size of remaining austenite is big enough, no saturation of carbon occurs in the austenite. When the ferrite front reaches this locally enriched Mn, the interface is halted until further cooling provides sufficient driving force for the transformation, and the ferritic interface continues its growth in the form of pearlite. The thickness of ~ 3 μm of pearlite rims in Figure 14(b) is in good agreement with the width of residual Mn bump in Figure 13(b) predicted by the LE model.
In contrast, the pearlite islands consist of different growth orientations of cementite indicating that it has formed through several nucleation events. The pearlitic islands are small austenitic regions surrounded by growing ferrite fronts. As shown in Figure 13(c) by modeling, the incomplete ferrite transformation during cyclic experiment provides time for full carbon rejection from ferritic zones into some of the remaining austenite. During final cooling, as a result of high carbon content, these zones reach eutectoid composition and transform into isolated pearlitic islands.
The ferritic zones inside high Mn regions are the result of austenite back transformation during the cyclic experiments. Adjacent to ferrite grains formed in low Mn regions, new low carbon content austenite forms. During final cooling, the low carbon austenite region transforms into ferrite and prevents formation of continuous pearlite band in high Mn regions.
In this study, the effect of cooling rate on ferrite/pearlite band formation in a medium Mn alloy during continuous cooling experiments starting from the fully austenitic state is explored systematically. The experiments showed that for linear cooling band formation could only be suppressed by cooling at rates in excess of 2 K/s. However, when a cyclic partial phase transformation treatment in the intercritical domain was included in the final cooling down from the austenitic state, band formation was suppressed even at cooling rates as low as 0.17 K/s. Simple isothermal annealing in the intercritical region was found to have no effect on band formation tendency.
Suppression of band formation during continuous cooling will only occur when the ferrite nucleation is promoted over the ferrite formation by the application of a higher cooling rate.
In case of an interim cyclic partial phase transformation in the intercritical region, the growth of the initially formed ferrite is significantly retarded such that new ferrite formation is made possible even in the case of a low cooling rate.
Controlled local enrichment of Mn at reversing austenite-ferrite interfaces during cyclic transformations is responsible for the suppression of ferrite growth leading to the prevention of ferrite/pearlite band formation even when a low cooling rate is applied.
The cyclic partial phase transformation can lead to the formation of pearlite rims not encountered in conventionally cooled material.
This research was funded by ArcelorMittal, France. The authors acknowledge Dr. Didier Huin from ArcelorMittal for continuous support. They also thank Mr. C. Kwakernaak and Dr. W. G. Sloof, both at the Delft University of Technology, for the electron probe microanalysis measurements.
- 18.F.W. Y. Zhang, Y, H. Liu, X. Ruan, G. Li, L. Bai, Y.-L. Zhang, H.-Y. Liu, X.-J. Ruan, G.-Z. Li, L.-G. Bai, and F.-M. Wang: Beijing Keji Daxue Xuebao/Journal Univ. Sci. Technol. Beijing, 2009, vol. 31, pp. 199–206.Google Scholar
- 19.H. Chen and S. van der Zwaag: Metall. Mater. Trans. A, 2016, pp. 1–10.Google Scholar
- 28.ASTM E1268-01(2016): 2016.Google Scholar
- 30.The Math Works Inc: 2007.Google Scholar
- 36.H. Chen and S. van der Zwaag: Philos. Mag. Lett. https://doi.org/10.1080/09500839.2011.634840.
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