1.Optical General Microstructure

1.1. Microstructures of Air-Cooled Steels

Figures 1(a) to (c) show the optical microstructures of granular bainite (GB) and lath or plate-like bainite (PB) obtained in the steels processed at different FRTs. Granular bainite is usually distinguished as small (<10 µm) grains with grey contrast. Presence of prior austenite grain boundaries (PAGB) is also apparent which is normally considered as the nucleating site for bainitic ferrite (BF) penetrating towards grain interior. Similar type of microstructural observations have also been reported earlier in a low carbon (0.06 wt %) microalloyed steel by Chen et al. [1]. Ghosh et al.[2] reported a similar type of observation in Ti, Nb microalloyed HSLA steel.The measured average grain size of pan-caked austenite grains lies in the range of 20-30 µm at 850C FRT. The grain size is found to decrease with lowering of FRT which is in good agreement to the achievable grain refinement with the lowering of FRT.

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Fig.1. Optical micrographs of air cooled specimens processed at (a) 850°C FRT, (b) 800ºC FRT and (c) 750ºC FRT.

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1.2. Microstructures of Water-Quenched Steels

Figs. 2(a) to (c) show the optical microstructures of water quenched steels processed at different FRTs. The microstructural features exhibit mixture of lower bainite and lath martensitic structure in prior pan-caked austenite grains (15-20 m) which are clearly demarcated by the contrast developed by the bright martensitic lath and shaded bainitic lath. It is perceived that lath structure becomes gradually finer with the lowering of FRT which is in good agreement with the results reported earlier [48, 50 of project].

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Fig. 2. Optical micrograph of water quenched specimens processed at (a) 850°C FRT, (b) 800ºC FRT and c) 750ºC FRT.

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Fig. 2. Optical micrograph of water quenched specimens processed at (a) 850°C FRT, (b) 800ºC FRT and c) 750ºC FRT.

2. TEM microstructure

2.1. Microstructures of Air-Cooled Steels

Fig. 3 shows the bright field (BF) electron image of the air cooled steel processed at different FRTs. Primarily bainitic lath along with the fine microalloying precipitates (< 35-40 nm) inside the lath is seen in Fig. 3(a) whereas Fig. 3(b) shows the needle like carbide particles as the characteristic features of bainitic lath. Fig. 3(c) demonstrates plate like lower bainite consisting of plate like ferrite with carbide inside the bainitic ferrite lath. The average width of bainitic lath is obtained as about 400 nm (Fig. 3(c)). The high density dislocation networks and dislocation tangles inside the bainitic lath can also be seen in Fig. 3(d) which is additional contributor to ultra high strength in the present steel. The abundant planar array of dislocations, dislocation node–precipitate interaction as seen in Fig. 3(d) are the other significant parameters contributing to strengthening of the present steel which is in good conformity to the results reported earlier [3].

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Fig. 3. Bright field TEM images of air cooled samples processed at different FRTs showing (a) fine microalloying precipitates inside bainitc lath, (b) needle like carbide inside the lath, (c) plate like lower bainite inside bainitc ferrite lath.

2.2. Microstructures of Water-Quenched Steels

The bright field (BF) electron image of the water quenched steel processed at 750 C FRT is depicted in Fig. 4. Mixture of lower bainite and lath martensitic structure along with dislocations and precipitation of microalloying carbide and carbonitride particles are characteristic microstructural features of the water quenched steel which can be seen in Fig. 4(a). The average width of lath martensite has been found to be about 150-200 nm. A fine precipitate in the range of 35-40 nm inside the lath is shown in Fig. 4(b). Both the refinement of lath structure and microalloying precipitates is readily attributed to lower FRT. The dark interlath region of the lath martensite and its corresponding dark field (DF) image along with the SAD pattern at the inset are shown in Fig. 4(c) and (d) respectively. The dark field image shows the interlath region of white contrast which can be conjectured as the film of retained austenite (ɣR) elongated towards the direction of deformation.

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Fig. 4. Bright field TEM images of water quenched samples processed at 750C FRT showing (a) Mixture of lower bainite and lath martensite along with microalloying precipitates, (b) fine precipitate inside

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martensite lath, (c) dark interlath region of lath martensite and (d) corresponding DF image of the interlath region showing film of retained austenite showing the SAD pattern at the inset.

3. Microalloying precipitates

3.1. Microalloying precipitates of Air-Cooled Steels

Fig.5(a) to (f) show the TEM BF micrograph dark precipitate particles within the bainitic ferite of air cooled steel obtained at different FRTs and EDS spectra from the precipitate particles.Fig. 5(e) and (f) show the TEM BF micrograph of dark coarse precipitate particles within the bainitic ferrite of air cooled steel obtained at 750C FRT and the corresponding EDS spectra from the precipitate particles. The corresponding EDS spectra from these coarse particles reveal that the particles are rich in Ti and Nb indicating formation of (Ti, Nb)C precipitates within bainitc ferrite lath and Ti and Nb along with C and N resulting in the indication of the formation of (Ti, Nb)CN particle within the aforesaid lath.

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Fig.5. TEM BF micrograph showing dark precipitate particles within the bainitic ferite of air cooled steel obtained at different FRTs and EDS spectra from the precipitate particles.

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3.2. Microalloying precipitates of Water-Quenched Steels

Fig.6(a) to(f) show TEM BF micrograph of fine precipitate particles within the lath martensite of water quenched steel obtained at differrent FRTs and the corresponding EDS spectra from the precipitate particles. The fine precipitate inside lath martensite is a typical (Ti, Nb)C particle and Ti, Nb along with C and N resulting in the indication of the formation of (Ti, Nb)CN particle within the lath as obtained from the signal of EDS spectrum.

During rolling, the deformation induced abundant dislocation arrays and dislocation nodes could act as preferred nucleation sites for these precipitates. It is worth mentioning that nano-sized fine precipitate has pronounced effect in grain refinement by the pinning action to the grain boundary migration; whereas the relatively large precipitate is less effective in this regard. So, grain refinement and dislocation-precipitate interactions are basically two phenomena which contribute significantly to ultra high strength properties in the present steel.

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Fig. 6. TEM BF micrograph showing the dark precipitate particles within the lath martensite of water quenched steel obtained at 750C FRT and EDS spectra from the precipitate particles.

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References

1.Chen Jun, Tang Shuai, Liu Zhen-Yu, Wang Guo-Dong, Microstructural characteristics with various cooling paths and the mechanism of embrittlement and toughening in low-carbon high performance bridge steel, Materials Science & Engineering A 559 (2013) 241–249.

2. A. Ghosh, S. Das, S. Chatterjee, B. Mishra, and P. RamachandraRao: Mater. Sci. Eng., 2003, vol. A348, pp. 299–308.

3. Queyreau Sylvain, Monnet Ghiath, Devincre Benoit, Orowan strengthening and forest hardening superposition examined by dislocation dynamics simulations, Acta Materialia 58 (2010) 5586–5595.