Q&P钢残留奥氏体含量的热动力学计算

基于CALPHAD方法建立了Q&P钢的配分扩散模型,并建立了一套特定成分在特定QP工艺下的组织转变计算任务流,通过计算QP钢一次淬火过程的马氏体/残留奥氏体含量和配分过程中残留奥氏体的碳富集量,并结合Thermo-Calc软件内置的基于吉布斯自由能的马氏体相变本构模型,预测稳定保留至室温的残留奥氏体含量。利用该模型计算文献钢种(Fe-0.2C-1.28Mn-0.37Si-0.0018B, wt%)的室温残留奥氏体含量,结果显示计算马氏体转变温度比试验数据高60 ℃,计算室温残留奥氏体含量为4.41%,与试验数据基本吻合,从而验证了该计算模型的半定量性。利用该模型进一步计算分析了碳、锰元素含量和热处理制度对AQT980和AQT1180钢一次残留奥氏体含量的影响规律,计算结果显示碳、锰元素含量的增加可使钢中相变点(A₃、Ms、Mf)温度下降;在固定淬火温度下,钢中的碳含量和锰含量增加可使一次残留奥氏体含量大幅增加;当碳、锰元素含量一定时,一次淬火温度的上升会使一次残奥含量大幅增加。

Thermodynamic calculation of retained austenite content in Q&P steel

Based on CALPHAD method, the diffusion model of Q&P steel during partitioning process was established, and a set of task flow for calculating the microstructure transformation of specific components under specific Q&P process was established. By calculating the martensite/retained austenite content in the primary quenching process of Q&P steel and the carbon enrichment of retained austenite in the partitioning process, combined with the built-in constitutive model of martensite transformation based on Gibbs free energy in Thermo-Calc software, the retained austenite content at room temperature was predicted. The model was used to calculate the retained austenite content at room temperature of steel grades (Fe-0.2C-1.28Mn-0.37Si-0.0018B, wt%) in the literature. The results show that the calculated martensite transformation temperature is 60 ℃ higher than the experimental data, and the calculated retained austenite content at room temperature is 4.41%, which is basically consistent with the experimental data, thus verifying the semi-quantitative nature of the calculation model. This model is used to further calculate and analyze the influence of carbon and manganese content and heat treatment system on the primary retained austenite content of AQT980 and AQT1180 steels. The calculation results show that the increase of carbon and manganese content can reduce the temperature of phase transformation points (A₃, Ms, Mf) in steel. At a fixed quenching temperature, the increase of carbon content and manganese content in the steel can significantly increase the primary retained austenite content. When the contents of carbon and manganese are constant, the increase of primary quenching temperature will significantly increase the primary residual content.