Intragranular Ferrite in Medium Carbon Vanadium-containing Wheel Steels and Its Effect on Fracture Toughness
YAO Sancheng1,2,*, ZHAO Hai1,2, LIU Xuehua1,2, JIANG Bo1,2, ZOU Qiang1,2, XU Kang2,3
1 Technology Center, Ma’anshan Iron and Steel Co., Ltd., Ma’anshan 243003, Anhui, China 2 Anhui Technology Innovation Center of Rail Transit Key Components, Ma’anshan 243003, Anhui, China 3 Technology Center, Baowu Group Masteel Rail Transit Materials Technology Co., Ltd., Ma’anshan 243003, Anhui, China
Abstract: The effect of low undercooling pretreatment in austenite on microstructure, second phase and mechanical properties of a medium carbon vanadium-containing wheel steel was investigated by optical microscope, scanning electron microscope, transmission electron microscope, tensile test and fracture toughness test. The results show that the final main microstructure of the wheel steel undercooled within 860 — 800 ℃ after it has been fully austenitized at 935 ℃ is slightly varied. The pearlite interlamellar spacing is (131±7) nm, and the volume fraction of pro-eutectoid ferrite is (10.6±0.8)%. However, the number of nano-scale precipitates is directly related to the undercooling temperature, which affects the strength contribution of precipitation strengthening. In the range of 860—840 ℃, the undercooling pretreatment does not reduce the strength obviously, but it increases the average and minimum fracture toughness values by 10.9% — 14.2% and 22.1%— 31.8%, respectively. The strength sacrifice increases with the further decrease of undercooling temperature, so the matching between strength and toughness is unba-lanced. Due to the low undercooling pretreatment, the solid-solved vanadium is pre-precipitated in the form of V(C, N) second phase with a large size and semicoherent with the matrix in the original austenite grain. This phase induces the nucleation of intragranular ferrite (IGF) in the subsequent cooling phase transformation stage. The lower the undercooling temperature, the greater the number of IGFs, though the average IGF size slightly changes. The presence of IGF increases phase interface of the microstructure, and the resistance to crack propagation is further increased, thus resulting in higher fracture toughness at the macro level. The essence of the low undercooling pretreatment in austenite is to change the weighted contribution of the vanadium in precipitation strengthening, cooperating with the control of ferrite morphology distribution, which provides a new way to improve the matching between strength and toughness of medium carbon vanadium-containing wheel steels.
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2023, Vol. 37 
