冷却速率对δ-TRIP钢包晶相变的影响

Effect of cooling rate on the peritectic transformation of δ-TRIP steel

在钢的凝固过程中冷却速率对钢的相变具有不可忽视的影响.本研究采用Thermo-calc热力学软件,模拟计算了含Al 3.52%(质量分数)的δ铁素体相变诱导塑性(δ-TRIP)钢的相转变过程,并分别使用差示扫描量热法(DSC)和Ohnaka微观偏析模型,分析了不同冷却速率对3.52%Alδ-TRIP钢凝固过程中的包晶相变温度,以及溶质元素偏析的影响.结果表明,冷却速率越小,DSC试验所得的相变温度越接近Thermo-calc计算的热力学平衡值.随着冷却速率从10、30增加到50℃·min–1,L→L+δ的转变温度降低,L+δ→L+δ+γ和L+δ+γ→δ+γ的转变温度先降低后升高,前者主要受过冷度的影响,后者主要受元素偏析的影响.冷却速率对C、Mn、S的偏析影响很小,对Si、P、Al的偏析影响较大,并且随着冷却速率的增加,Si、P、Al偏析程度增加.Si和P的偏析会小幅度延缓包晶反应的进程;Al对改变包晶反应进程作用明显,随着冷却速率的增加,包晶反应区域逐渐下移,且下移趋势渐缓.

The phase transformation of carbon steel has always been a research hotspot. Researchers study the phase transformation process of steel in terms of the original structure, chemical composition, and process conditions, and the cooling rate in process conditions has an important influence on the phase transformation of steel. In this study, Thermo-calc thermodynamic software is used to simulate and calculate the phase transformation process of 3.52%Al(mass fraction) delta ferrite transformation-induced plasticity(δ-TRIP) steel, and differential scanning calorimetry(DSC) and the Ohnaka microsegregation model are used to analyze the effect of cooling rate on the peritectic transformation temperature and solute element segregation during solidification of 3.52%Al δ-TRIP steel.The results show that the smaller the cooling rate is, the closer the DSC phase transition temperature is to the thermodynamic equilibrium value calculated by Thermo-calc. Increasing the cooling rate from 10 to 30 to 50 ℃·min^(-1) decreases the phase transition temperature of L→L+δ and first decreases and then increases those of L+δ→L+δ+γ and L+δ+γ→δ+γ. The former temperature is mainly affected by cooling, and the latter temperatures are mainly affected by element segregation. Among the six elements(C, Si, Mn, P, S, and Al) of 3.52%Al δ-TRIP steel, the segregation of S is the most severe. This result is obtained because the partition coefficient k of the S element at the solid –liquid interface is much smaller than those of other solute elements. The rapid S element enrichment at the end of solidification increases the possibility of sulfide precipitation, forms a low melting point liquid film between dendrites, reduces the zero plastic temperature, and increases the solidification brittleness range and crack sensitivity. Therefore, the sulfur content in steel should be strictly controlled during composition smelting. The cooling rate slightly affects C, Mn, and S segregation but greatly affects Si, P, and Al segregation, and the degree of segregation of Si, P, and Al increases with the cooling rate. The segregation of Si, P, and Al delays the peritectic reaction process, the segregation of Si and P slightly delays the peritectic reaction process, and Al segregation clearly delays the peritectic reaction process. With increasing cooling rate, the lower the peritectic reaction area moves, the slower the peritectic reaction process. This study can provide a theoretical basis for the continuous casting process parameters of δ-TRIP steel.