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

Abstract

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.

Keywords

• δ-trip steel
• cooling rate
• phase transformation
• microsegregation
• differential scanning calorimetry