THE INFLUENCE OF FERRITE ON RETAINED AUSTENITE CHARACTERISTICS IN THERMOMECHANICALLY-PROCESSED LOW-SILICON CONTENT TRIP-ASSISTED STEELS

Transcription

1 THE INFLUENCE OF FERRITE ON RETAINED AUSTENITE CHARACTERISTICS IN THERMOMECHANICALLY-PROCESSED LOW-SILICON CONTENT TRIP-ASSISTED STEELS Seyed Mohammad Kazem HOSSEINI 1, Abbass ZAREI-HANZAKI 2, Steve YUE 3 1 Department of Materials Engineering, Imam Khomeini International University, Qazvin, Iran. P.O. Box: , smk.hosseini@alumni.ut.ac.ir 2 School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, North Karegar Street, Tehran, Iran, zareih@ut.ac.ir 3 Department of Mining, and Materials Engineering, McGill University, University St., H3A 2A7, Montreal, QC, Abstract Canada, steve.yue@mcgill.ca The outstanding balance of strength-ductility exhibited in newly-developed low silicon content TRIP-assisted steels can be achieved through precise controlling of microstructures which are composed of allotrimorphic ferrite, bainitic ferrite, retained austenite, and martensite. Ferrite as the first transformation product of austenite decomposition during cooling stage of any TMP processing has a profound effect on microstructural characteristics of subsequent transformation products and, hence, on resulted mechanical properties. In this study, a typical multi-stage TMP program was simulated in laboratory by using an automated hot-compression machine and the effects of ferrite grain size and its fraction on microstructural characteristics of subsequent austenite transformation products were investigated. Microstructure of samples were characterized by optical and electron microscopy, XRD, and Mössbauer spectroscopy. The results indicated that by increasing the amount of ferrite, retained austenite volume fraction, after a slight plateau, decreases. Ferrite grain refinement has also found to not only increase retained austenite fraction but also changes particle morphology to the desired type of interlath film-like. Keywords: (TRIP-Assisted Steels, Thermomechanical, Ferrite Phase Characteristics, Microstructure) 1. INTRODUCTION Recent studies on low-silicon content multiphase TRIP-assisted steels have shown that superior balance of strength and ductility can be achieved through development of a multiphase microstructure which typically consists of polygonal ferrite as the matrix, ferritic bainite, retained austenite, and martensite [1]. The new composition which contains only 0.6 wt. pct silicon not only benefits from outstanding mechanical properties but also the industrial drawbacks of conventional Si-Mn TRIP steels can be eliminated due to reducing its low silicon content [2-3]. The conventional method of processing low-alloy TRIP-assisted steels is extensive cold rolling of hot-rolled products and then two-stage heat treatment; intercritical annealing followed by bainitic holding. However, thermomechanical processing (TMP), as an alternative cost effective method, eliminates the need for further processing or heat treatment. In any thermomechanical processing, ferrite as the first transformation product of austenite decomposition during cooling stage has a profound effect on microstructural characteristics of subsequent transformation products and hence on resulted mechanical properties. Previous studies on High silicon content Si-Mn TRIP steels have shown that a decrease in ferrite grain size at intermediate ferrite contents leads to the maximum retained austenite in the resulting microstructure [4]. Such effects, however, have not been well-elucidated in the context of low-si content TRIP steels. Therefore, the present study aims at investigating the influence of ferrite phase characteristics on the features of subsequent transformation products in low-si content TRIP-assisted steels.

2 2. MATERIALA AND EXPERIMENTAL PROCEDURES A cast ingot of steel with a composition listed in Table 1 was hot-rolled to a final thickness of 14 mm following a classical rolling procedure at CANMET, Ottawa. All hot-deformation programs were carried out on a computerized Materials Testing System, adopted for compression test. Basically, the equipment is composed of an MTS automated testing machine coupled with a radiant furnace and a tool geometry that allows for quenching of specimens at any time. The TMP scheme, which is typical of a discontinuous cooling path (i.e. multiple cooling stages), is represented schematically by Fig. 1. Those parameters, which were kept constant throughout the deformation or cooling program, have been optimized elsewhere [3,5]. Critical transformation temperatures of samples were measured using continuous cooling compression method [4]. Table 1 Chemical composition and Critical Transformation Temperatures of steel under investigation C Mn Si Ni Mo Al P,S N, ppm < Figure 1 Schematic diagram of heat and deformation schedule Microstructures were studied by using optical and scanning electron microscopy. Specimens were electrolytically polished and then were prepared using a particular tint-etching method, adopted from LePera s technique [3]. After tint etching, the martensite / Retained Austenite (M/A) phases appear white and easily became distinguishable in black and white mode pictures. Furthermore, a developed tint etching method based on sodium metabisulfates reagent was employed to reveal the retained austenite characteristics (for instance, dispersion and overall morphology) and existence of any pearlite packets within microstructure [3]. Following this method, retained austenite appears white, polygonal ferrite and bainite orange to brown color, pearlite as dispersed black particles, and martensite as blue. For detailed observation at higher magnification, specimens were examined by scanning electron microscopy. The RA measurement was accomplished by Mössbauer spectroscopy, using backscatter method at a working voltage of 7 kv. The carbon content of retained austenite was measured by X-ray diffraction using the lattice parameter extrapolation method and the following empirical equation [6]: a %C (1) 0 3. RESUTLTS AND DISCUSSION 3.1. Ferrite Volume Fraction To investigate the effect of ferrite volume fraction V ) on retained austenite stability and resulted mechanical properties, compression samples with an identical prior austenite grain size were subjected to deformation at 1050⁰C and then cooled down to α+γ region and held at this temperature for various times to ( PF

3 obtain 10, 40, 53, and 58 percent polygonal ferrite (Fig. 2). All samples were quenched into a salt bath at 450⁰C, held for 60s and subsequently cooled down to room temperature. The primary effect of increasing V PF is carbon enrichment of remaining austenite due to diffusion of carbon in the austenite ahead of the advancing ferrite/austenite interface, thereby austenite stability is increased during subsequent quenching to room temperature. If it is assumed that all carbon concentrates in austenite, the upper bounds of carbon in the remaining austenite in samples containing 10, 40, 53, and 58% ferrite would equal to 0.11, 0.16, 0.19, and 0.21, respectively. On the other hand, substitutional alloying elements such as Mn, Ni and Si may also partition depending on the kinetics of transformation. In a ternary Fe-C-X system, at para-equilibrium condition (PE), where a local equilibrium can be achieved only for carbon, other elements are supposed to be unaffected by the passage of the interface to make the concentration ratios (Fe/X) at the interface equivalent to that of overall composition. In contrast, partitioning (LE) is anticipated for X where local equilibrium is preserved at the moving interface for all elements. The extent of partitioning, however, depends on transformation time, temperature and undercooling so that at lower undercooling conditions partial local equilibrium (PLE) is envisaged whereas negligible partitioning (NPLE) for X is expected at higher undercooling condition [8]. It was reported that a kinetic transition in ferrite growth from PE to NPLE condition possibly occurs during isothermal treatment provided that the transformation Figure 2. Variation of volume fraction of ferrite as a function of intercritical annealing time and prior austenite grain size Figure 3. Variation of volume fraction of microconstituent as a function of ferrite fraction temperature was low enough to permit a faster growth of ferrite at early stage. Guo et al. [9] reported that three growth stages were recognized in the ferrite transformation in a quaternary Fe-0.04C-3Mn-1.9Si alloy, which included a first rapid partitionless growth (i.e. PE), a second stage of no bulk partition, but enrichment of Mn and Si at the α/γ boundary accompanying a considerable drop of growth rate, and a third slow partitioned growth stage. Capdevila et al. [8] also reported a similar result in Fe-0.37C-1.8Mn steel. As an approximation, the kinetic transition can be assumed to happen where there is a slop change in the curve of ferrite volume fraction as a function of isothermal holding time [8]. As shown in Fig. 2, ferrite volume fraction in samples with prior austenite grain size of 52µm depict a slop change at 10 min holding time. Therefore, prolonging holding time over this limit resulted in establishing local partitioning condition for substitutional elements such as Mn and Ni into austenite which further increases the stability of remaining austenite. Although carbon and Mn partitioning increases stability of remaining austenite, prolonging holding time over 10 min (i.e. 40% ferrite) resulted in a decrease in volume fraction of retained austenite ( V RA ) as shown in Fig. 3. This was attributed to the change of retained austenite morphology from interlath film type to isolated

4 and blocky types. According to several workers the interlath film-like retained austenite can strongly contribute to the TRIP effect and, hence, changing RA particle morphology to blocky type reduces its stability [9]. Furthermore, prolongation of isothermal holding and carbon enrichment of remaining austenite both can promote pearlite formation. Fig. 4 shows microstructure of a sample which held for 30min in which pearlite can be identified by its lamellar structure. Likewise to the RA content, the maximum volume fraction of martensite ( V M ), which is formed during quenching of samples subsequent to bainitic holding, has been observed in the sample which contains 40% Ferrite. At lower ferrite fractions, the austenite is less stabilized by carbon and alloying elements such as Mn and Ni and thus higher fraction of bainite is anticipated to form during isothermal bainitic holding. Furthermore, the bainite which is transformed from lower carbon-enriched austenite has the characteristics of lower bainite structure which, in turn, provides less space geometrically available for austenite. In contrast, at higher fraction of ferrite, remaining austenite particles will be too stabilized to transform to martensite after quenching to room temperature [4] Ferrite Grain Size Samples with different prior austenite grain sizes have been selected and held in α+γ region for various holding time to obtain microstructures containing an identical volume fraction of ferrite but with different grain size. The effects of different prior austenite grain size on the kinetics of austenite to ferrite transformation and characteristics of transformation products were explained elsewhere [3] and thus will not be reiterated here. However, for the sake of clarity, the previous studies have shown that an increase in prior austenite grain size resulted in a decrease in volume fraction of retained austenite and martensite. Figure 4. Pearlite lamellar structure in the sample held for 30min in intercritical temperature Fig. 5 shows variation of volume fraction of microconstituents as a function of ferrite grain size. The optical and corresponding scanning electron microscopy (SEM) micrographs of samples are also shown in Fig. 6. As it can be seen, an increase in ferrite grain size resulted in: Figure 5. Variation of volume fraction of microconstituent as a function of ferrite grain size A decrease in bainite volume fraction and increase in bainite pocket size An increase of martensite volume fraction A decrease in retained austenite volume fraction and change of RA particles to isolated and blocky types rather than interlath film type.

5 Figure 6. Optical and corresponding scanning electron microscopy (SEM) micrographs of samples with different ferrite grain sizes a1, a2) d PF=15µm, b1 and b2) dpf=21µm and c1 and c2) dpf=32µm Since no microstructural changes (apart from ferrite and prior austenite grain sizes) were observed in the corresponding specimens during the intercritical treatment, one may conclude that the increase in bainitic ferrite volume fraction (VBF) and decrease in their packet size in the specimen which contains finer polygonal ferrite can be attributed to the higher density of grain boundaries which, in turn, accelerates kinetics of bainite reaction thanks to the more preferential sites available for nucleation of ferrite platelets. Consequently, remained austenite were enriched by carbon to a higher extent at the end of bainitic holding and, moreover, film-type interlath type was identified to be the prevailing morphology of retained austenite particles. Presence of martensite is a result of martensitic transformation occurring during the last stage of cooling of samples to room temperature. The transformation indicates insufficient carbon enrichment of austenite and possible carbide precipitation during bainite transformation. Fig. 5 depicts a reduction in martensite volume fraction as ferrite grain size decreases. This is attributed to the lower fraction of remained austenite left at the end of bainitic holding, higher carbon-enrichment of austenite, and higher resistance of matrix against latticeinvariant shear that concurrently proceeds with the transformation in the specimen contains fine ferrite grains. As it can be seen in Fig. 5 the effect of ferrite grain coarsening on reducing of VRA is more pronounced at smaller ferrite grain size. This observation can be rationalized as follows. To achieve an identical volume fraction of ferrite after intercritical annealing, specimens with different prior austenite grain sizes annealed at a given temperature for various times (e.g. 10, 15, and 30min for ferrite grain sizes of 15μm, 21μm and 32μm, respectively). As it was stated in previous section, the paraequilibrium condition is anticipated to be established in the specimen held for only 10 minutes in intercritical region which means that no partitioning of X between austenite and ferrite occurred at any temperature from the Ae3 to the Ms temperature, when X was Si, Mn, Co, Al, Cr, and Cu. As is well known, the partitioning of

6 any alloying element depends on the diffusion characteristics. The latter is a function of temperature, time, concentration gradients, diffusion path and diffusion extent. Prolonging of annealing time can provide sufficient time for alloying elements such as Mn, Ni, and Pt to partition into austenite. Since Mn and Ni are austenite stabilizer, the diffusional (e.g. pearlite) and martensitic transformations are both retarded, leading to a less-pronounced reduction of retained austenite for the specimens with larger grains of ferrite [4]. CONCLUSION The results of this study can be summarized as follows: The maximum volume fraction of retained austenite can be achieved at intermediate ferrite contents, namely 40%. Higher fractions of ferrite which were formed after prolonged intercritical annealing resulted in formation of pearlite which acts as a carbon sink and, hence, decreased both VRA% and CRA. A decrease in ferrite grain size led to an increase in fractions of bainitic ferrite and retained austenite but resulted in a decrease in martensite fraction. It was found that interlath film-type is the dominant morphology of retained austenite particles in a microstructure containing fine ferrite grains. LITERATURE [1] JACQUES, P. et al., Enhancement of the Mechanical Properties of Low-C, Low-Si Steels by Formation of Multiphase Microstructure Containing Retained Austenite, Metall. Mater. Trans. A, 29 (1998), s.2383 [2] HOSSEINI, S.M.K. et al., ANN model for prediction of the effects of composition and process parameters on tensile strength and percent elongation of Si Mn TRIP steels, Mater. Sci. Eng. A., Vol. 374 (2004), s [3] HOSSEINI, S.M.K. et. al., Effect Of Prior Austenite Characteristics on Mechanical Properties of Thermomechanically-Processed Multiphase TRIP Assisted Steels, Mater Sci. Tech., Vol. 11 (2008), s [4] ZAREI-HANZAKI, A. et al., Ferrite Formation Characteristics in Si-Mn TRIP Steels, ISIJ International, Vol. 37 (1997), s [5] HOSSEINI, S.M.K. et. al., Microstructure Development after Coiling Process in Low-Si Content TRIP-Assisted Steels, J. Mech. Eng. Autom., Vol. 3 (2013), s [6] CULLITY, B. D. Elements of X-ray diffraction, 2 nd ed., Addison Wesley Publ. Co., 1978 [7] LUCAS, G.E. et al. The Use of Small-Scale Specimens for Testing of Irradiated Material, ASTM STP 888, ASTM Philadelphia, PA, 1986, s.112 [8] CAPDEVILA, C. et al., Kinetic Transition During Ferrite Growth in Fe-C-Mn Medium Carbon Steels, Metall. Mater. Trans. A, Vol. 42 (2011), s [9] HOSSEINI, S.M.K. et al, The Effect of Intercritical Deformation on Microstructure Development in Thermomechanically-Processed Low-Silicon TRIP-Assisted Steels, Adv. Mater. Res, Vol. 856 (2014), s. 251.