Influence of Carbon Content on the Microstructure and Precipitates of the V-N Microalloyed Steels
The microstructure and precipitates of the V-N microalloyed steels whose carbon content were respectively 0.27% and 0.35%, were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that the microstructures of the test steels are all composed of ferrite and pearlite, the area percent of ferrite decreases from 49% to 40% and the grain size also descends from 8.96μm to 8.61μm with the increase of carbon content. TEM results show that the precipitates in two kinds of steels all include a large amount of 10~20nm dispersion distribution irregular flake VC, the part of fibrous VC that grows toward to intragranular ferrite along the grain boundary in the local area, and only a small amount of spherical VN or V (C, N). When the carbon content increases from 0.27% to 0.35%, the number of the spherical VN or V(C,N) increases obviously and the size of it varies from 20~100nm to 45~105nm, while the number of flake VC and fibrous VC decreases significantly and the length of fibrous VC shortens from several micrometers to nanometer size. Experimental results indicate that most of the spherical VN or V (C, N) firstly appear in austenite, then the flake VN and fibrous VN with precipitation strength effect emerge in ferrite during the following γ→α transformation and cooling process. The increase of carbon content can lead to the increase of driving force that VN or V (C, N) firstly appear in austenite, which results in the significant increase of ferrite nucleation rate and the refinement of microstructure.
Yun-Hae Kim, Prasad Yarlagadda, Xiaodong Zhang and Zhijiu Ai
J. Y. Ma et al., "Influence of Carbon Content on the Microstructure and Precipitates of the V-N Microalloyed Steels", Advanced Materials Research, Vols. 335-336, pp. 645-649, 2011