Effects Of Alloying Element Phase Partitioning On The Eutectoid Transformation And Properties Of Pearlitic Steels

High carbon pearlitic steel wire is the highest metal structure material in industrial production.Due to its excellent strength and plasticity,high carbon pearlitic steel wire is widely used in the national economic fields such as long-span bridge cables,automobile tires and highrise buildings.Micro-alloying is one of the important methods to develop ultra-high strength pearlitic steel wire.Mn and Si elements are the most common addition elements.The addition of Mn and Si elements affects the eutectoid transformation process and microstructure characteristics of pearlite,which is of great significance to the final microstructure and mechanical properties control of pearlitic steel.Research has found that Mn and Si elements have obvious phase partitioning tendency and there is a strong interaction between Mn and Si elements.The phase partitioning behavior of Mn and Si elements not only affects the pearlite transformation kinetics and microstructure,but also has a significant influence on the performance of pearlitic steels,such as working hardening and thermal stability.Therefore,in this paper,the phase partitioning behavior of alloying elements in Fe-C-MnSi quaternary alloy system and its corresponding influence on microstructure and properties have been analyzed.By revealing the dynamic process and physical nature of alloying element phase partitioning behavior,and exploring the interaction mechanism among alloying elements,this paper tries to provide a theoretical basis for the composition design of high-performance pearlitic steels.It is found that in Fe-C-Mn-Si quaternary pearlite steel,the pearlite colony size and lamellar integrity are mainly controlled by elements diffusion.When the prior austenite grain size(PAGS)is small or the transition temperature is high,the concentration distribution required for pearlite growth can be quickly completed due to the fast atom diffusion rate,resulting in the complete straight pearlite lamella and low fragmentation ratio;While when the PAGS is large or the transition temperature is low,it is difficult to form the required concentration distribution near the transition interface due to the low atom diffusion rate,and the pearlite lamella are fragmented with the hindered pearlite growth.For samples with small PAGS,the pearlite growth is controlled by the interface diffusion of Mn and Si elements,while for samples with large PAGS,the pearlite growth is controlled by the volume diffusion of C atoms.Mn element can significantly delay the eutectoid transformation of pearlite and slow down the transformation rate.While Si element can promote Mn element to partition to the cementite phase,increase the stability of cementite,reduce the spheroidization rate of cementite layer in cold drawing pearlite wires during annealing process,and slow down the coarsening rate of cementite particles.In the initial stage of eutectoid transformation,an obvious Mn depleted region is formed at the ferrite side of dual-phase interface,due to the delayed diffusion of Mn atoms in ferrite layer during the diffusion to cementite layer,and the enrichment region of Si elements is formed at the same side.The enrichment of Si elements is found at the same side and can disappear immediately as the transformation proceeded.The first-principles calculation results show that when Mn atom is substituted in the center of cementite lattice,the electron gain and loss of surrounding Fe and C atoms can be increased,improving the chemical bonding ability of cementite phase;when Si atom is substituted in the center of ferrite lattice,the 3p orbital of Si atom is hybridized with the 3d orbital of surrounding Fe atoms to form covalent bonds,and the chemical bond in ferrite lattice can be enhanced.Hence,the cohesive energy and formation energy of pearlite are the lowest when Mn and Si atoms are substituted in the center of cementite and ferrite lattice,respectively,that is,the chemical bonding ability and stability are the highest.With the increase of Si element content,more Mn element is partitioned to cementite phase,while Si element is partitioned to ferrite phase.There exists an "exclusive" interaction between Mn and Si elements near the dual-phase interface during the phase partitioning process.The first-principles calculation results illustrate that when Mn and Si atoms are enriched at cementite side and ferrite side of 4c interface,respectively,the chemical bond ability and stability of pearlite can be enhanced.Moreover,in this condition,a high degree of hybridization among Fe,Mn,Si,and C atoms can be formed at the dual-phase interface.Mn/Si atoms covalently bond with the surrounding Fe/C atoms,and the strong repulsion between C and Si atoms results in the phenomenon of indirect "repulsion" between metal Mn and nonmetal Si atoms.