摘要
金属相变时伴随相变潜热的释放。金属材料相变潜热将影响材料本身的组织转变,而组织转变过程又影响相变潜热的释放量,两者相互影响。以精确描述奥氏体相变过程为目的,对轧后采取空冷工艺条件下的奥氏体相变过程进行研究,深入的分析研究奥氏体相变过程中相变潜热对组织转变的影响。
在对圆钢轧后空冷状态下的表面温降过程的基础上,在实验室采用埋偶法直接测定40Cr和低铌钢试样心部温降规律。其目的是为了排除轧件心部对表面的反热影响更精确反映相变潜热的影响。通过实验得到了冷却过程中的心部温降曲线、心部的冷却速率,判断了金属相变开始与结束温度,并与40Cr钢的CCT曲线上相变点进行了对比。
以实际生产中热轧圆钢后的空冷过程为计算条件,以40Cr为研究对象,考虑了热传导、辐射、对流等热损失过程,以及相变过程相变潜热的释放等,利用有限单元法建立了圆钢40Cr瞬态温度场模型。计算了整个空冷过程中轧件横断面的温度分布和冷却速度分布,得到了圆钢在考虑相变潜热和未考虑相变潜热两种情况内部温度随时间变化的规律,为分析考虑相变潜热的组织模拟与预报的奠定了基础。
采用反算法对相变热焓进行计算并与综合热分析仪(DSC)实验结果相对比,并且对恒速冷却试件与实际空冷试样进行金相组织观察对比。以温度场模拟为基础上,对是否考虑相变潜热两种情况计算相变过程,得到了相变潜热对各相的体积分数影响。
Latent heat was released in metal phase transformation. Latent heat could influence structural transformation of metallic materials, which could also impact the burst size of latent heat. The impacts between latent heat and structural transformation were mutual. Austenite phase transformation in the air-cooling process after rolling was studied to describe austenite phase transformation exactly. The impacts of latent heat in austenite phase transformation were analyzed in more depth.
The law of temperature dropping in 40Cr and low-Nb cores was determined directly by thermocouple in laboratory on the basis of researching the round steel surface state of temperature dropping by air cooling after rolling. The purpose of experiments was ruling out the impacts of anti-heat in surface by core, reflecting the influences of latent heat more accurately. A number of parameters were got by experiments, including curves of temperature dropping and cooling rate in core, temperatures of beginning and ending in metal phase transformation which were contrasted with transformation points in CCT of 40Cr.
The round steel 40Cr transient temperature field model was established by finite element method. The design condition of the model was air cooling process after round steel hot rolling in actual products. The model used 40Cr as Research Object, considering calorific losses caused by Heat conduction, radiation, convection and so on, the release of latent heat in phase transformations etc. The temperature distribution and cooling rate of rolling cross-section in the whole cooling were calculated, receiving rules of internal temperature changes over time in two conditions with latent heat and no latent heat of round steel, which laid the foundation for latent heat analysis.
Phase transition enthalpies were calculated by anti-algorithm. The result was compared to experimental results calculated with a comprehensive thermal analysis instrument (DSC), and the metallurgical structure in test pieces cooling in constant-speed was examined. Impacts of volume fraction in items caused by latent heat in the conditions of latent heat and no latent heat were calculated on the basis of temperature field simulation.
引文
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