Materials Science & Engineering A 673 (2016) 557–561

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Short communication

Control of retained austenite morphology through double bainitictransformation

Kyeong-Won Kim a,b, Kyung Il Kim c, Chang-Hoon Lee a,n, Jun-Yun Kang a, Tae-Ho Lee a,Kyung-Mox Cho b, Kyu Hwan Oh c

a Korea Institute of Materials Science, Changwon 51508, Republic of Koreab Pusan National University, Busan 46241, Republic of Koreac Seoul National University, Seoul 08826, Republic of Korea

a r t i c l e i n f o

Article history:Received 5 April 2016Received in revised form18 July 2016Accepted 19 July 2016Available online 28 July 2016

Keywords:TRIPRetained austeniteMorphologyTensile propertiesSemi in-situ EBSD

x.doi.org/10.1016/j.msea.2016.07.08393/& 2016 Elsevier B.V. All rights reserved.

esponding author.ail address: lee1626@kims.re.kr (C.-H. Lee).

a b s t r a c t

The feasibility controlling the morphology of retained austenite in TRIP steel was investigated utilizingdouble bainitic transformation which is two-step isothermal heat treatment. The change in morphologyof the retained austenite from a blocky to film type improved the tensile properties owing to highermechanical stability of film-like retained austenite.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Transformation induced plasticity (TRIP) - assisted steels withretained austenite have drawn a great deal of attention for auto-motive sheets due to their excellent combination of strength andelongation [1–5]. In recent years, many researchers have at-tempted to develop enhanced TRIP steels with high manganesecontent by increasing the fraction of retained austenite [6–10]. Theeffect of characteristics of the retained austenite on the mechan-ical stability against plastic deformation in TRIP steel has been animportant concern. It has been generally reported that film-likeretained austenites are more stable than blocky retained auste-nites due to higher carbon enrichment and a morphology effect.Furthermore, it has also been argued that film-like retained aus-tenite is too stable for TRIP to occur [3,11–17]. Furthermore, acouple of approaches involving multi-step isothermal heat treat-ment to control the morphology of retained austenite have beenintroduced to improve the mechanical properties. Hase et al. [18]showed that tensile properties and fracture toughness were im-proved as a result of eliminating blocky austenites and inducingnanostructured plates of bainitic ferrite by adopting a two-stageheat treatment. Wang et al. [19] reported a significant increase of

strength and charpy impact toughness without sacrificing ductilityby inducing blocky retained austenites to finer bainite throughtwo-step heat treatment. These works reported improved me-chanical properties using high or medium carbon and high alloy-ing steels, both of which were heat treated at quite low tem-perature in a range of 200–350 °C to acquire nanostructured bai-nitic ferrite and film-like retained austenite.

In this study, the feasibility controlling the morphology of re-tained austenites and thereby improve mechanical properties by atwo-step isothermal heat treatment that can be applied to com-mercial products was explored with TRIP assisted steel containingsimple chemical composition such as C, Mn, and Si. In addition, theeffect of the morphology of the retained austenite on the me-chanical stability against plastic deformation was directly in-vestigated using small tensile specimens customized to a de-formation stage for on-site EBSD (Electron-BackScattered Diffrac-tion) analysis.

2. Experimental

The chemical composition of the examined steel was Fe–0.30C–1.5Mn–1.5Si (in wt%), which was similar to that of simple C–Mn–Sisteels reported in previous studies [11,20]. The steel was preparedusing a vacuum induction melting furnace and a 30 kg ingot was

Fig. 1. Schematic diagrams representing heat treatments and microstructural evolutions: (a) single-step heat treatment and (b) two-step heat treatment.

Table 1Heat treatment conditions of single-step and two-step heat treatments.

Specimens Single-step heat treatment Two-step heat treatment

Single-step 400 °C, 300 s –

Two-step 500 °C, 20 s 400 °C, 280 s

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hot rolled to 3 mm thickness. Dilatometry specimens were aus-tenitized at 950 °C for 5 min, followed by isothermal heat treat-ment between bainite (Bs) and martensite (Ms) transformationstart temperatures using a dilatometry machine (Theta, Dilatronic-Ⅲ). Based on the experimental time-temperature transformation(TTT) diagram of this steel, stick specimens were also austenitizedat 950 °C for 5 min and isothermally heat-treated by single-step ortwo-step processes between Bs and Ms temperatures using aContinuous Annealing Line (CAL) simulator (ULVAC, VHC-P616CP).The Bs [21] and Ms [22] were calculated to be 614 °C and 349 °C,respectively. The cooling rate between austenitization and iso-thermal heat treatment was 20 °C/s, which prevents of ferritetransformation during cooling. A schematic representation of theheat treatment cycles of single-step and two-step processes and ofthe behavior of the transformation to bainite during both heattreatments is illustrated in Fig. 1. Single-step and two-step heattreatments were performed at 400 °C for 300 s and at 500 °C for20 s for 50% of transformation to bainite, followed by heat

Fig. 2. Custom-made deformation stage (a) and

treatment at 400 °C for 280 s for completion of bainite transfor-mation, respectively, as shown in Table 1. The specimens werecharacterized using scanning electron microscopy (SEM, JSM-7001 F, JEOL) and the EBSD technique for which a NordlysNanoEBSD detector and AZTEC software from Oxford Instruments wereused. Specimens for SEM were polished and etched using 1% nitalsolution (1 ml HNO3þ100 ml ethyl alcohol), while those for EBSDwere chemo-mechanically polished with colloidal silica withoutapplying etching. To investigate on the mechanical stability ofretained austenites according to their morphology, the progress oftheir transformation with strain increment was directly monitoredin a strain range of 5–25%. Small specimens with dimensions of11.5�28�2 mm were strained in uniaxial tension using a cus-tom-made deformation stage for the SEM in Fig. 2. Furthermore,on a fixed site of a specimen, consecutive EBSD mappings withstrain increments were conducted. Finally, the enrichment ofcarbon in retained austenites was investigated by EPMA (JEOL,JXA-8530F operated at 15 kV and 15 nA beam current).

3. Results and discussion

Fig. 3 shows microstructures and tensile flow curves of thesamples heat treated by single- and two-step processes. The mi-crostructures produced by the single-step heat treatment at 400 °C

dimensions of small tensile specimen (b).

Fig. 3. SEM micrographs of (a) the single-step heat treatment at 400 °C, (b) two-step heat treatment at 500 °C - 400 °C, and (c) engineering stress-strain curve of bothsingle-step and two-step specimens (γB: blocky retained austenite, γF: film-like retained austenite).

Table 2Tensile properties of single-step and two-step specimens.

Specimens YS (MPa) UTS (MPa) Total El. (%) UTS�T. El. (MPa%)

Single-step 611 970 25.0 24,250Two-step 597 948 29.6 28,061

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consist of very fine bainite laths due to the large driving force forbainite transformation resulting from large undercooling and twotypes of retained austenites, i.e. a considerable fraction of blockyretained austenites (γB) and interlath film-like austenites (γF) inFig. 3(a). The two-step heat treatment at 500 °C and the sub-sequent treatment at 400 °C also results in mixed microstructuresof bainite and retained austenites. However, the heat treatmentmakes bainite laths coarser and reduces the amount and size ofblocky retained austenites (Fig. 3(b)), since austenites un-transformed at the end of the first isothermal holding transform tofine bainites and interlath film-like retained austenites (γF2) dur-ing the second isothermal holding. In Fig. 3(c) which shows me-chanical properties of both specimens, the single-step specimenhas slightly higher strength than the two-step specimen, since thebainitic ferrites in the single-step specimen are more refined dueto the transformation at a low temperature. On the other hand,elongation of the two-step specimen increases significantly. Thequantitative measurement results of tensile properties in Fig. 3(c) are summarized in Table 2. Yield and tensile strengths by thetwo-step process are only 10–30 MPa lower than those by thesingle-step heat treatment, while the two-step process showsabout a 5% increase in elongation. As a result, the two-step spe-cimen has superior mechanical balance to the single-step

specimen. It is generally accepted that film-like retained austenitebetween laths has higher mechanical stability against externalplastic deformation, leading to improved ductility. In this study, itis confirmed that film-like retained austenites (γF2) that are ad-ditionally transformed from blocky retained austenites by two-step heat treatment contribute to a significant increase in ductilityand thereby mechanical balance.

In order to directly investigate the difference in mechanicalstability according to the morphology of retained austenite withboth single-step and two-step heat treatments, the same area ofsmall tensile specimens strained in a range of0–25% on the de-formation stage in the SEM was observed using EBSD. EBSD resultson the specimens by one-step heat treatment with 0%, 6%, and 14%strain and those by two-step heat treatment with 0%, 8%, 14%, and25% strain are shown in Fig. 4(a)–(c) and Fig. 4(d)–(g), respectively.(Green, red, and black colors represent bainitic ferrite, retainedaustenite, and martensite, respectively. Black pixels are vacanciesof crystallographic information caused by indexing failures ofEBSD, and thus some microstructural boundaries could be in-cluded.). From the comparison of the initial phase maps, i.e. Fig. 4(a) and (d), it could be reconfirmed that the two-step treatmentretained more film-like austenites. At 6–8% strain, most of theblocky retained austenites in both specimens were transformed tomartensites, while interlath film-like retained austenites remaineduntransformed in Fig. 4(b) and (e), respectively. In Fig. 4(c) and(f) at 14% strain, a considerable amount of film-like retained aus-tenites was still untransformed in the two-step specimen (Fig. 4(f)); in contrast to the single-step specimen in which most of theretained austenites, regardless of their morphology, transformedto martensites by strain (Fig. 4(c)). The film-like retained auste-nites in the two-step specimen that survived at 14% strain finally

Fig. 4. EBSD results showing transformation from retained austenites to martensite in single-step specimen at (a) no strain, (b) 6% strain, (c) 14% strain and in two-stepspecimen at (d) no strain, (e) 8% strain, (f) 14% strain, (g) 25% strain. (For interpretation of the references to color in this figure, the reader is referred to the web version of thisarticle.)

Fig. 5. Normalized volume fraction of retained austenites with increasing strain.

Table 3Carbon enrichment of retained austenites in single-step and two-step specimens.

Specimens Carbon concentration in blockyretained austenite

Carbon concentration in film-like retained austenite

Single-step 0.553 wt% 0.841 wt%Two-step 0.993 wt% 1.24 wt%

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transformed to martensites at 25% strain in Fig. 4(g). Blocky re-tained austenites that were transformed at an early stage of de-formation in both specimens would have a limited contribution toductility. In contrast, a more gradual transformation of the film-like retained austenites could be more efficient in terms of pro-longed plasticity. Thus, it can be reasoned that the larger fractionand the higher stability of film-like austenites resulted in in-creased ductility of the two-step specimen.

Fig. 5 indicates the normalized volume fraction of the un-transformed retained austenites, calculated from Fig. 4. The resultsconfirm the improved mechanical stability of the film-like re-tained austenite with a larger volume fraction in the two-stepspecimen. The direct and step-wise observations of the mechan-ical stability of the retained austenite with respect to morphologywere consistent with the tensile properties. It is confirmed thatonly modification of the heat treatment process without addi-tional alloying can improve the mechanical properties of TRIP-ai-ded steels by controlling the morphology of the retained austenite,even in simple C–Mn–Si steels, as shown in this study.

Carbon enrichment was observed by EMPA to investigate the

main factor causing the difference in mechanical stability betweenfilm-like and blocky retained austenites, as shown in Table 3. Film-like retained austenites were highly carbon-enriched compared toblock type retained austenites in both specimens. The enhancedmechanical stability of the film-like retained austenite with two-step heat treatment results from higher enrichment of carbon,which is a strong austenite stabilizer. During the second bainitetransformation at a low temperature, additional transformationfrom untransformed austenites at the first transformation to bai-nites causes the carbon in the austenites to diffuse out into in-terlath film-like retained austenites, leading to improved me-chanical stability. However, further research is necessary on thefactors of retained austenite that affect mechanical stability in TRIPsteels, i.e., the size or aspect ratio of retained austenites.

4. Conclusions

The effects of two-step isothermal heat treatment on tensileproperties and microstructural changes, especially retained aus-tenites, of TRIP assisted steel with a conventional C–Mn–Si com-position were investigated. The tensile properties of TRIP assistedsteel with two-step isothermal heat treatment can be improved byincreasing the fraction of film-like retained austenites and en-dowing higher mechanical stability, which is supported by ob-servation of EBSD using small tensile specimens strained step bystep.

Acknowledgments

The authors are grateful for the financial support for this re-search from the Fundamental Research Program of Korea Instituteof Materials Science (KIMS).

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