中碳含钒车轮钢中的晶内铁素体及其对断裂韧性的影响

Intragranular Ferrite in Medium Carbon Vanadium-containing Wheel Steels and Its Effect on Fracture Toughness

采用光学显微镜、扫描电镜、透射电镜、常温拉伸及断裂韧性试验等研究了奥氏体区“低过冷”预处理对中碳含钒车轮钢的显微组织、第二相及力学性能的影响。结果表明,在935℃充分奥氏体化后,经860~800℃过冷处理,对车轮钢最终的主体组织组成影响不大,珠光体片间距为(131±7)nm,先共析铁素体体积分数为(10.6±0.8)%;但纳米级析出相的数目与过冷温度直接相关,进而影响析出强化的强度贡献。经860~840℃过冷处理,车轮钢的强度无明显降低,而断裂韧性的均值和最小值分别相对提高10.9%~14.2%和22.1%~31.8%,获得了较好的强韧匹配;过冷温度的进一步降低,强度牺牲增大,强韧匹配出现失衡。“低过冷”预处理使原奥氏体晶内析出尺寸较大、与基体呈半共格关系的V(C,N)第二相,并在后续冷却相变阶段诱导晶内铁素体(IGF)形核,且过冷温度越低,IGF数量越多,但IGF尺寸变化不大。IGF的形成增加了显微组织内的相界面,裂纹扩展阻力进一步提高,宏观表现为更高的断裂韧性。奥氏体区“低过冷”的本质是改变了钒在析出强化方面的权重贡献,协同控制铁素体的形态分布,为改善中碳含钒车轮钢的强韧匹配提供了新途径。

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.