Effect of boron on bainitic transformation kinetics after ausforming in low carbon steels

doi:10.1016/j.jmst.2017.05.006

Abstract

Abstract:

The addition of boron (B) is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.

Key words: Ausforming, Boron, Bainitic transformation, Dislocations, Cottrell atmosphere

He Binbin, Xu Wei, Huang Mingxin. Effect of boron on bainitic transformation kinetics after ausforming in low carbon steels[J]. J. Mater. Sci. Technol., 2017, 33(12): 1494-1503.

Figures/Tables 13

Table 1 Chemical compositions of investigated steels (wt%).

Steel	C	Mn	Cr	Ti	B	V	Mo	P	Si	Al	Nb	Fe
Base	0.2	1.6	2	0.001	0.0002	0.002	0.0015	0.0012	0.0038	0.013	0.0034	Bal.
B steel	0.2	1.6	2	0.023	0.0041	0.002	0.0017	0.0012	0.0041	0.005	0.0029	Bal.

Table 2 Ac1Ac3, Ms and prior austenite grain size of the investigated steels.

Steel	A_c1 (°C)	A_c3 (°C)	M_s (°C)	Grain size (μm)
Base	762	795	391	12 ± 2
B steel	765	805	367	11 ± 3

Fig. 1. (a) TTT diagram of Base steel and B steel. The incubation time was based on the 5% transformed volume fraction. The best deformation temperature for both two steels is 550 °C; (b) a schematic illustration of temperature program employed in the present study. Red line represents the deformation at 550 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Dilatational curves of bainitic transformation at 450 °C after different amounts of plastic deformation at 550 °C for (a) Base steel and (b) B steel; (c) engineering stress-strain curves for Base steel and B steel.

Fig. 3. Bainitic transformation kinetics at 450 °C after different amounts of deformation based on the aspects of (a) incubation time, (b) transformation velocity and (c) transformed volume fraction for Base steel and B steel.

Fig. 4. SEM images of transformed products after different amounts of deformation for Base steel: (a) 0%; (b) 2%; (c) 10%; (d) 25%; (e) magnified view of dashed rectangle region in (c); (f) magnified view of dashed rectangle region in (d) (B: bainite; M/A: martensite/austenite; white arrows in (b) mark the bainitic sheaf structure; white arrows in (e) and (f) mark the small bainite which is surrounded by M/A islands).

Fig. 5. SEM images of transformed products after different amounts of deformation for B steel: (a) 0%; (b) 2%; (c) 10%; (d) 25%; (e) magnified view of dashed rectangle region in (c); (f) magnified view of dashed rectangle region in (d) (White arrows in (d) and (f) mark the small bainites which are surrounded by M/A islands).

Fig. 6. Volume fraction of bainite obtained from SEM images. The errors bar represents the standard deviation from calculation using different SEM images.

Fig. 7. EBSD all Euler orientation maps of transformed product with deformation amounts of 0% (a, c) and 25% (b, d) for Base steel (a, b) and B steel (c, d) (Black lines represent high angle boundaries (? > 15°)).

Fig. 8. TEM bright field images of transformed product of Base steel after deformation of 2%: (a) formation of bainite and martensite in prior austenite grain. White dashed lines mark the PAGB; (b) magnified view of red dashed rectangle in (a), showing granular morphology of bainite with distribution of cementite precipitation; (c) magnified view of white solid rectangle in (a), showing lath morphology of martensite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. TEM micrographs of dislocations in bainite of B steel after different amounts of deformation: (a) 2%; (b) 10%; (c)-(f) 25%; (d) magnified view of dashed rectangle in (c); (e) weak beam condition of the same interested area; (f) bright field image of dislocation at the prior austenite grain interior. Note that the scales are different. White dashed lines mark the prior austenite grain boundaries.

Fig. 10. TEM micrographs of precipitations in bainite of B steel after deformation of 2% (a) and 25% (b)-(d). (a, b) Precipitations at the prior austenite grain boundary (PAGB); (c) precipitation at the prior austenite grain interior; (d) magnified view of dashed rectangle in (c), the white dashed lines mark the PAGB; (e) EDX spectrum of precipitation in (d).

Fig. 11. Distribution of GND density in transformed product of B steel after different amounts of deformation: (a) 0%; (b) 25%.

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