고 Al 및 고 Mn강 내 개재물 생성거동에 관한 열역학적 연구

Abstract

Recent development of the high-Al and high-Mn containing steels such as TRIP (Transformation Induced Plasticity) and TWIP (TWining Induced Plasticity) steels placed great challenges to steelmaking and casting processes because nitride and oxide inclusions can be easily formed in the liquid steels. In order to control the formation of nitride and oxide inclusions in these steel grades during the steelmaking process, the thermodynamics of nitride and oxide formation in liquid Fe-Mn-Al alloy should be known. In the present study, the thermodynamics of AlN formation in high Mn-Al containing steel melts was investigated by the metal-nitride-gas equilibration technique. The effects of Al and Mn on the N solubility and the AlN solubility product in liquid iron containing up to 1.8 mass%Al and 26 mass%Mn were measured in the temperature range from 1773 to 1873 K. Using the Wagner’s formalism, the experimental results were thermodynamically analyzed to determine the interaction parameters among Al, Mn and N and the equilibrium constant for the AlN formation in liquid iron. Thermodynamic analysis was carried out to estimate the critical N content for the onset of AlN formation in the high Al-Mn steels of commercial grade as a function of melt temperature. In order to check the validity of the parameters determined in the present study, the critical Al and N contents for AlN formation in these alloy steels were measured during cooling, and compared with the predicted AlN stability diagram for a typical TWIP steel. The Al deoxidation equilibrium in high Al-Mn containing steel melts was also studied. The Al deoxidation experiments for Fe and Fe-Mn alloy melts were carried out to measure the Al-O relation and identify the stable oxide phases at high Al-Mn containing steels. However, the relation cannot be described by the conventional random mixing model such as the Wagner’s formalism because of the extremely strong interactions between O and alloying elements. Therefore, the Modified Quasichemical Model (MQM) in pair approximation was used to explain the thermodynamics of liquid solution in the Fe-Mn-Al-O system. For the optimization of the liquid Fe-Mn-Al-O system, its sub-systems of the Fe-Al, Fe-O, Al-O, Mn-O, and Fe-Mn binary and the Fe-Al-O, Fe-Mn-Al, Fe-Mn-O, and Mn-Al-O ternary systems were optimized using the available thermodynamic data and the critical experiments for the O solubility in Al alloy melts, the equilibrium relation between Al and O in Mn-Al-O melts. Using the model and the optimized parameters, the Al deoxidation curves for Fe-Al-O melts and Fe-Mn-Al-O melts, and the inclusion diagram for the Fe-Mn-Al-O system were predicted over the whole composition range as a function of temperature for the first time. The present database can be used as a part of larger thermodynamic database for the steelmaking process of the high Al-Mn alloyed liquid steels.