材料期刊网

在Gleeble 1500D热模拟试验机上进行单轴压缩实验, 研究了低合金高碳钢连续冷却至珠光体转变孕育期变形时的组织演变过程, 并探讨了所得组织的超塑性. 结果表明: 组织演变过程包括珠光体相变、渗碳体球化和铁素体再结晶3个阶段, 最终形成微米级(约1 μm)铁素体等轴晶粒+亚微米及纳米级渗碳体颗粒的复相组织; 利用速率突变法测得该复相组织在700 ℃, 1×10^-4-2×10^-4 s^-1应变速率下m值(应变速率敏感性指数)可达0.40; 随预变形量增大, 所得组织在700 ℃各应变速率下的流变抗力降低.

There were some reports on superplasticity of ultra-high carbon steels in the last several decades, mainly referring to the superplasticity of fine-equiaxial double-phase microstructure (fine ferrite + cementite particles). However, in order to get the fine--equiaxial double--phase microstructure, very complicated pre-treatment was needed. An exploration to obtain superplastic microstructure through simple uniaxial compression at pearlite transformation incubation temperature was conducted using Gleeble 1500D in this paper. The microstructural evolution processes of the steel during deformation included (1) pearlite transformation, (2) cementite spheroidization and (3) ferrite recrystallization. The (2) and (3) processes start before the finish of pearlite transformation. Two micro-processes of cementite spheroidization were shown in the experiments. One is that the cementite lamellae were dissolved and broken. This process results in the formation of relatively coarse cementite particles (100-200 nm). Another is that finer cementite particles (10-30 nm) reprecipitated in the ferrite during ferrite recrystallization. Deformation during pearlite incubation period can accelerate pearlite transformation and cementite spheroidization. The above processes lead to form fine double-phase microstructure with sub--micrometer and nanometer cementite particles distributed uniformly in fine ferrite (about 1 μm). Samples with the fine double--phase microstructure show the m value of 0.40 in the strain rate range of 1×10^-4-2×10^-4 s^-1 at 700 ℃. The flow stress under different strain rates reduces with the increase of the prestrain. For example, under the strain rate of 1×10^-4 s^-1 at 700 ℃, the flow stress of the samples with prestrain of 1.2 is only 70 MPa, much lower than 120 MPa of the samples without prestrain. The dispersed cementite particles can prevent ferrite grains from growing during deformation process at high temperature, as a result, the stability of the fine-equiaxial double-phase microstructure is ensured, which is the microstructural condition realizing superplasticity.