MATERIALS SCIENCE & ENGINEERING

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Materials Science and Engineering A 438-440 (2006) 300-305

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Microstructures and stability of retained austenite in TRIP steels

X.D. Wang^a'*, B.X. Huang^a, Y.H. Rong^a, L. Wang^ab

" School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China ^h Baosteel Research and Development Technology Center, Shanghai 201900, China

Received 27 June 2005; received in revised form 13 January 2006; accepted 11 February 2006

Abstract

Transformation induced plasticity (TRIP) steels exhibit a combination of high strength and ductility due to their multi-phase microstructure, including ferrite, bainite and retained austenite which transforms to martensite under the external stress. The characterization of microstructures is necessary for understanding the relationship between microstructure and property. In the present work, an effort to determine the volume fraction of various phases was made for the conventional TRIP steel containing silicon. The microstructures in the TRIP steel were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, especially, an effective method was developed to identify multi-phase microstructures by atomic force microscopy based on the height difference. Furthermore, the stability of retained austenite determining TRIP effect was evaluated by electrical resistance tests and tensile tests. The results show that retained austenite does not generate martensitic transformation at —80 °C and exhibits a good thermodynamic stability, and the transition temperature from stress-induced martensitic transformation to strain-induced martensitic transformation is determined as about —5 °C, and thus strain-induced martensitic transformation over —5 °C (somewhat lower than room temperature) is favorable for the application of TRIP steels in the automobile industry. © 2006 Elsevier B.V. All rights reserved.

Keywords: TRIP steel; Microstructure; Retained austenite; M° temperature; Mechanical properties

1. Introduction

Low-carbon multi-phase sheet steels, which were developed for automotive applications, have attracted a growing interest in recent years due to their high strength and ductility combination. These excellent mechanical properties mainly arise from a com­bination of a ferrite matrix, for ductility, bainite, for strength, and retained austenite, for uniform elongation produced by marten-site transformation from austenite under the influence of external tensile stress [1-3]. The multi-phase microstructures are formed after an intercritical annealing followed by austempering at the bainite transformation temperature. During the above heat treat­ment, the austenite phase is stabilized by the enrichment of carbon due to the presence of alloying elements such as sili­con or aluminum. From this viewpoint, the volume fraction, the distribution of various phases, especially the stability of retained austenite is of great importance to the mechanical properties of transformation induced plasticity (TRIP) steels. The stability of retained austenite includes thermodynamic and mechanical

* Corresponding author. Tel: [phone]. E-mail address: [e-mail] (X.D. Wang).

stabilities. The M_s temperature is used to characterize the ther­modynamic stability of retained austenite against transformation under cooling, whereas the M" temperature is used to evalu­ate mechanical stability against stress-assisted transformation. At the Mf temperature, the stress reaches the yield stress for slip in the austenite phase [4-6]. In the present paper, a devel­oped method, measuring the height of various phases by means of atomic-force microscopy (AFM) after etching, is built eas­ily and fast to identify various phases in a TRIP steel, and the validity of the method is verified by comprehensive analysis by use of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, the volume fraction of various phases is determined by combina­tion of thermodynamic calculation, metallographic examination and X-ray diffraction (XRD). The stability of retained austenite is further investigated by means of the measurement of M_s and M° temperature, and the different mechanical behaviors in the TRIP steel are explained.

2. Experimental procedure

The chemical composition of the TRIP steel investigated in the present work is listed in Table 1, together with Aci, AC3

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Table 1 Chemical composition of the TRIP steel (mass%)
C Si Mn P	S	Al	N	Aci (°C)	Ac3 (°C)	T50 (°C)
0.11 1.19 1.67 0.013	0.006	0.038	0.003	683	861	803

temperatures and 750 temperature standing for the volume frac­tions of ferrite and austenite in the intercritical temperature are 50%, respectively, which is calculated by thermo-calc software system. It was suggested that slow cooling from an intercritical annealing temperature near AC3 to a temperature close to Aci before rapid cooling could lead to a better balance of strength and ductility [7]. Therefore, the intercritical annealing, as shown a marked "1" line in Fig. 1, was carried out at 800 °C for 70 s, then slow cooling to 690 °C with a cooling rate of 10 s~', isother-mally held at 400 °C for 4 min followed by water quenching.

In order to check the volume fraction of ferrite phase calcu­lated by thermo-calc software, a heat treatment was designed as follows. A sample was treated by intercritical annealing at 800 °C for 70 s, then slow cooling to 690 °C followed by water quenching to room temperature, as shown a marked "2" line in Fig. 1, and the volume fraction of ferrite in the sample was deter­mined by quantitative metallographic examination. The volume fraction of retained austenite in undeformed specimen or ten­sile specimen was measured with XRD. The specimens for OM and SEM were prepared according to the traditional procedure, that is, mechanical grinding and polishing up with 1 |jim dia­mond paste, followed by etching with 2% nital for 15 s. Three specimens for AFM were prepared. One of them was prepared as, same as that of OM, the others were polished up with 1 |jim diamond paste, then were deformed up to 3% strain, of which one specimen was further etched with 2% nital for 15 s. Both deformed and undeformed specimens were observed by TEM.

The thermodynamic stability of retained austenite was deter­mined by measuring the M_s temperature using electrical resis­tance test. In order to characterize the mechanical stability of retained austenite, the Aff temperature of the TRIP steel was determined by measuring its yield strength. The measurement method described by Barbe et al. [6], Berrahmoune et al. [8] was conducted, and tensile tests were carried out at a strain rate of 5 x 10~³ in the rolling direction in temperature range from 20 to-40°C.

3. Results and discussion

3.1. Determination of volume fractions of various phases

Microstructure observed with SEM on specimen treated as a marked "2" line in Fig. 1 is shown in Fig. 2. The volume frac­tion of ferrite (fp) is determined by quantitative metallographic examination as about 56.1%, which is consistent with thermo­dynamic calculation. By XRD analysis, the volume fraction of retained austenite (/Xr) in undeformed samples is determined as about 16%, and about 7% (/Xr')^m deformed samples. Therefore, the volume fraction of bainite (/b) can be calculated, namely /b = 1 —fp —fht = 27.9%, while the volume fraction of marten-site (f\i) in deformed samples due to martensitic transformation from austenite can be determined as 9% (/m =/at —/at')- It indi­cates that about 7% of retained austenite does not transform to martensite during tensile process.

3.2. Phase identification by AFM

OM or SEM observation on specimens etched by nital cannot be used to distinguish bainite, retained austenite and marten-site by their contrast, while ferrite matrix is clearly observable, as black contrast in Fig. 2, and white contrast in Fig. 3. This is because the etching solution of nital attacks ferrite grains and grain boundaries while leaving austenite and martensite

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Phase

Fig. 3. Optical micrographs of the TRIP steel.

intact [9], which results in different height of various phases. The image contrast in OM is different from that in SEM since their imaging principles are different. In recent years, although a modified color etching method [9,10] was used to distinguish bainite from ferrite and tempering martensite from austenite in TRIP steels, this technique is somewhat complicate since the strain-induced martensite must undergo tempering treatment.

A developed method was presented in this paper, in which the height of various phases in undeformed s amples after etching was firstly measured by means of AFM with reference to, the height of retained austenite due to not suffering etching, then the relative height of martensite with retained austenite in deformed sample was determined, finally the height of various phases of the deformed sample was measured. Let the height of retained austenite be zero, and then the height of martensite is positive while the height of the ferrite or bainite is negative.

Fig. 4 is an AFM image of various phases in an undeformed sample. In Fig. 4(a), the retained austenite was located in the cross of three lines, as reference object for measuring the height of various phases. The relative height of various phases is shown in AFM image of section analysis (Fig. 4(b)), such as the phase with the relative height —201.7 nm is ferrite, —97.6 nm is bainite and +1.5 nm is austenite. Fig. 5(a and b) are, respectively, three-dimensional and two-dimensional images of martensite surface relief of a deformed sample, and its section analysis is demon­strated in Fig. 5(c). The height of martensite surface relief is about 50.3nm.The statistical results of the relative height and the surface roughness of various phases are respectively listed in Tables 2 and 3 for undeformed and deformed samples. The statistical results indicate that the increasing sequence of abso­lute height of various phases is ferrite, bainite, austenite and martensite, and the increasing sequence of surface roughness

Table 2

Statistical results of the surface roughness and height difference in the unde­formed sample

Fig. 4. AFM image of various phases in an undeformed sample: (a) two-dimensional image and (b) section analysis.

of various phases is austenite, martensite, bainite and ferrite. Therefore, the multi-phase microstructures in TRIP steel can be easily identified by the above AFM method.

3.3. Stability of retained austenite

The stability of retained austenite includes thermody-namic and mechanical stabilities. Fig. 6 depicts the variation

Table 3

Statistical analysis of the surface roughness and height difference in the deformed

stenite

Retained austenite

Martensite

Bainite Ferrite

Mean height difference	+58.5	0	-79.7	-189.3
(nm)
Surface roughness	7	5	10	18
(nm)

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2<K>nni

(a)

(c)

Fig. 5. AFM image of martensite surface relief on polished surface after deformation: (a) three-dimensional image, (b) two-dimensional image and (c) section analysis.

2.4x10 "

2.4x10

- 60 -40 -20

Temperature/⁰ C

20

of electrical resistance as a function of temperature dur­ing the thermal cycle from 20 to — 80 °C. In the electrical resistance-temperature curve, there exists no exothermal peak during cooling, which indicates that martensitic transformation does not occur. This experiment demonstrates a good thermo-dynamic stability of the retained austenite. Fig. 7 shows the yield and tensile strength-temperature curve of the TRIP steel in temperatures ranging from 20 to —40 °C. It is worthy to note that the yield strength increases with decreasing temperature

up to —5°C, then decreasing with decreasing temperature till —20 °C, further, it increases with decreasing temperature. This abnormal phenomenon at —5 °C was explained as the transition point (called Mf) from stress-induced martensitic transformation to strain induced martensitic transformation [5]. The relationship between the yield strength and temperature can be reasonably explained by on modulus change of TRIP steel. The modulus of the TRIP steel reflects mean modulus of various phases, and ferrite and bainite have great weight with mean modulus. The TRIP steel has the majority of ferrite and bainite (about 84%) and the minority of retained austenite (about 16%). With the decrease of temperature the mean modulus of the TRIP

Fig. 6. Electrical resistance of the sample at different temperatures.

Fig. 7. Yield strength of the TRIP steel at different deformation temperatures.

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steel increases, which results in the increase of yield strength with decreasing temperature. In the range of temperatures from —5 to — 20 °C, the onset of the martensitic transformation due to tensile stress within the elastic range leads to the local "soft" of modulus in retained austenite, then the drop of yield strength, such effect continues below — 20 °C. However, it is worthy to point out that below —5 °C (Aff), the yield strength rises with decreasing temperature as the same tendency as in the range of temperatures from 20 to —5 °C, and this effect is attributed to the predominant role of ferrite and bainite in the mean modulus, which is different from single austenite phase in Fe-20Ni-0.5C steel [11]. The TRIP steel studied in this work exhibits a good thermodynamic stability and about —5 °C of Aff (somewhat lower than room temperature), and thus this is favorable for the application of TRIP steels in engineering.

The stress-strain relationship at different temperatures was investigated. Fig. 8 shows the stress-strain curves at 0 and —15 °C. In the curve for —15 °C, there is a yield-drop, corresponding to the local "soft" of modulus in retained austenite. From Fig. 8, it can be found that the work hardening rate at —15 °C is greater than that at 0°C, which results in the earlier generation of martensitic transformation due to the decrease of stacking fault energy of austenite with the decrease of temperature [12]. Moreover, the formation of matensite under stress produces more effective "strain hardening" than does the ordinary slip mechanism in fee austenite [11]. From the above analysis, it is suggested that martensitic transformation in TRIP steel has "strain hardening" effect and "uniform elongation" effect due to martensitic transformation causing the redistribution of stress, as a consequence, TRIP steels exhibit greater tensile strength and ductility than many commercial steels currently used in the manufacture of automobiles. The strain-induced martensite in —15 and 0°C deformation was verified as twin-type one by TEM observation, as shown in Fig. 9. The martensitic transformation from retained austenite can relax the accumulated stress in ferrite and bainite around the austenite.

Fig. 9. TEM micrographs of twin-typed martensite of the deformed sample: (a) bright field and (b) diffraction pattern [1 3 l]_m//[l 31],.

4. Conclusions

The microstructure of conventional TRIP steel with Si was investigated by means of OM, SEM, AFM and TEM. The modi­fied method presented in this work exhibits an effective approach to distinguish the multi-phase microstructures. In addition, both thermodynamic and mechanical stability of retained austenite were evaluated by electrical resistance measurement and tensile tests, respectively. Main conclusions can be drawn as follows.

(i) The volume fraction of various phases can be determined by quantitative metallographic examination and XRD. The studied TRIP steel contains 56.1% ferrite, 27.9% bainite and 16% retained austenite while 9% of austenite trans­forms to martensite during tensile stress.

(ii) AFM topographic analysis (height and roughness) makes it possible to identify various phases. The statistical results indicate that the increasing sequence of height of various phases is ferrite, bainite, austenite and martensite.

(iii) The electrical resistance measurement indicates that when the retained austenite is cooled to — 80 °C, martensitic transformation does not occur, which shows that the retained austenite has a good thermodynamic stability, but it is metastable under stress. The Aff temperature is determined as about —5 °C by yield stress-temperature

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curve, moreover, the abnormal drop point (Aff) in yield stress-temperature curve and the different change tendency in stress-strain curve at different temperatures are reason­ably explained respectively based on the mean modulus of TRIP steel and martensitic transformation in TRIP steel.

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