Influence of Vanadium Content on Bainitic Transformation of a Low-Carbon Boron Steel During Continuous Cooling

doi:10.1007/s40195-015-0249-1

Abstract

Abstract:

The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite transformation behavior is not noticeably altered when the vanadium content is 0.042 and 0.086 wt%, and these steels exhibit full bainitic microstructure even at a cooling rate of 5 °C/s. When vanadium content is increased to 0.18 wt%, ferrite is still present in the microstructure even at a cooling rate of 40 °C/s. Vickers hardness of the steels with 0.042 and 0.086 wt% V is remarkably higher than that of the steel with 0.18 wt% V at a cooling rate higher than 10 °C/s, and the difference is increased with the increase in cooling rate. Moreover, the amount of coarse vanadium precipitates formed in austenite is increased with the increase in vanadium content. The optimum content of vanadium to obtain bainitic microstructure is 0.086 wt% in this experimental low-carbon boron steels.

Key words: Microalloyed, steel, Vanadium, Bainitic, transformation, Boron, Microstructure

Kwang-Su Kim, Lin-Xiu Du, Cai-Ru Gao. Influence of Vanadium Content on Bainitic Transformation of a Low-Carbon Boron Steel During Continuous Cooling[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(6): 692-698.

Figures/Tables 8

Table 1 Chemical compositions of the experimental steels (wt%)

Steel	C	Si	Mn	P	S	Al	V	Ti	N	B	Fe
No. 1	0.12	0.17	1.76	0.003	0.002	0.026	0.042	0.005	0.004	0.0021	Bal.
No. 2	0.12	0.14	1.76	0.002	0.002	0.027	0.086	0.005	0.004	0.0019	Bal.
No. 3	0.11	0.13	1.77	0.003	0.002	0.025	0.18	0.005	0.004	0.0020	Bal.

Fig. 1 Dynamic CCT diagrams of the steels with different V contents: a No. 1 steel, 0.042 wt% V; b No. 2 steel, 0.086 wt% V; c No. 3 steel, 0.18 wt% V

Fig. 2 Optical micrographs of experimental steels with different V contents after continuous cooling to room temperature under different cooling rates: a 0.042 wt% V, 1 °C/s; b 0.042 wt% V, 2 °C/s; c 0.042 wt% V, 5 °C/s; d 0.042 wt% V, 40 °C/s; e 0.086 wt% V, 1 °C/s; f 0.086 wt% V, 2 °C/s; g 0.086 wt% V, 5 °C/s; h 0.086 wt% V, 40 °C/s; i 0.18 wt% V, 1 °C/s; j 0.18 wt% V, 2 °C/s; k 0.18 wt% V, 5 °C/s; l 0.18 wt% V, 40 °C/s

Fig. 3 Volume fraction of ferrite in the experimental steels with different V contents

Fig. 4 SEM micrographs of experimental steels after continuous cooling to room temperature under cooling rates of 10 °C/s: a No. 1 steel composed of granular bainite and lath bainite; b No. 2 steel composed of granular bainite and lath bainite; c No. 3 steel composed of granular bainite and polygonal ferrite

Fig. 5 TEM micrographs and EDX analysis result of precipitates in No. 3 steel cooling at 10 °C/s: a morphology of polygonal ferrite; b morphologies of fine precipitates and coarse precipitates in polygonal ferrite; c EDX analysis result of coarser precipitate

Fig. 6 TEM micrograph and precipitates morphologies of No. 2 steel cooling at 10 °C/s: a morphology of bainite; b morphologies of fine precipitates and coarse precipitates

Fig. 7 Effects of cooling rate and vanadium content on Vickers hardness

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