Effect Of Rare Earth On The Formation Of Acicular Ferrite In C-Mn Steel

Application of rare earth (RE) in steel has always been attracting a lot of interest since the 1960s. RE can play the role of deoxidation, desulfurization and inclusion modification in steel, form a large number of high melting point, dispersive and tiny inclusions in steel. Some of the RE inclusions can induce intragranular acicular ferrite (AF) nucleation, refine the microstructure and improve the strength and toughness of steel. However, the study on the formation of intragranular acicular ferrite in steel treated by RE is still scarce at present. Focused on the RE oxides metallurgy technology, the changes of inclusion and microstructure in steel and effects of steel composition and heat treatment process on the formation of acicular in the steel containing RE have been investigated systematicall in this paper. Moreover, the influence of RE treatment on microstructure transformation and performance of the welding heat affected zone in C-Mn steel has been also explored. This study can clarify the formation condition of intragranular acicular ferrite in RE treated C-Mn steel, and provide theoretical and technical support to expand the application of RE in steel.The study on the influence of RE addition on inclusion and microstructure in C-Mn steel showed that under present experimental condition, the calculation of FactSage thermodynamics software and the detections of scanning electron microscope (SEM) and transmission electron microscope (TEM) indicated that the types of inclusion are RE2O2S, MnS and small amount of aluminosilicate after 0.017 wt% RE added into C-Mn steel, which fine distributed in the steel. The microstructure of water-quenched sample from 1100℃ transformed from martensite to intragranular acicular ferrite. RE metals complex addition is better than single addition for intragranular ferrite formation, and the best mass ratio of La and Ce for combined treatment is 3/1 in C-Mn steel. The size of inclusion nucleus in favor of intragranular ferrite nucleation is mainly concentrated in 1 μm～4μm, which is primarily formed in liquid steel. The quantity of the inclusion nucleus with the size of 1μm～4μm, is maximum at 5 min after RE added into C-Mn steel.The coordinate effects of C, Mn and Al content in steel and rare earth inclusion on the formation of acicular ferrite were system explored. It is found that the influence of elements C, Mn on inclusion composition, quantity and size is little. Under the experimental condition, there would be a lot of intragranular acicular ferrite formed in C-Mn steel when RE treatment coordinated with C content about 0.10 wt%-0.18 wt%. and Mn content about 0.75 wt%～1.31 wt%. Al is a kind of strong deoxidizer, the calculation of FactSage software and the detections of SEM found that deoxidation product of Al can convert the RE inclusion into REAlO3 and RE2S3 when Al content is more than 0.004 wt% in RE treated steel. With the increase of Al content, the beginning temperature of the austenite to ferrite transformation increase, the inclusion quantity is fewer, which is detrimental to the nucleation of intragranular acicular ferrite.The nucleation mechanism analysis of intragranular acicular ferrite, induced by RE inclusion found that the most effective pure RE inclusion to induce intragranular acicular ferrite nucleation is RE2O2S, as the lattice misfit between this inclusion and a-Fe is the lowest. When patches of MnS precipitated on RE2O2S forming a complex inclusion in C-Mn steel, due to the combine effect of low misfit and Mn depleted zone, the ability of RE2O2S+MnS complex inclusion to induce intragranular acicular ferrite nucleation is stronger than that of pure RE2O2S.The investigation of intragranular acicular ferrite formation after heat treatment in C-Mn steel treated by RE showed that the best heat treatment condition for intragranular acicular ferrite formation is austenitizing under 1100 "C, in 20 min-30 min. The optimal size range of prior austenitic grain for intragranular acicular ferrite nucleation is about 150 μm, and the best cooling rate range is about 2℃/s-8℃/s. The optimum austenitizing temperature, holding times and cooling rate for intragranular acicular ferrite formation is similar in different RE metals treated C-Mn steel. When Al content increased to 0.027 wt%, the optimum cooling rate for intragranular acicular ferrite formation decreased to about 2℃/s-5℃/s. In addition, RE treatment can effectively inhibit austenite grain growth, even though incubated 40 min under 1100℃, the austenite grain growth is not obvious in C-Mn steel, the size was about 150μm.The welding thermal cycle simulation was conducted on Gleeble 1500 thermal simulator to analyze the microstructural evolution and mechanical properties variation in the heating affected zone (HAZ) of the RE treated steel plate with thickness of 15 mm. In the RE treated C-Mn steel, the microstructure is mainly composed of bainite at the welding heat input of 25 kJ/cm. With the increase of welding heat input, AF content in HAZ increases when welding heat input is in the range of 50 kJ/cm-100 kJ/cm. The hardness of HAZ decreases with heat input increasing from 25 kJ/cm to 100 kJ/cm. When welding heat input energy is about 100 kJ/cm, there were so much intragranular acicular ferrite formed that room-temperature impact toughness of the heat affected zone in rare earth treated steel is nearly same to that of the base material, significantly improved the room-temperature impact toughness of weld heat affected zone.There is little difference in microstructure. hardness and impact toughness for C-Mn steels with different types of RE addition. When heat input is 75 kJ/cm and 100 kJ/cm, toughness of HAZ in complex RE treated steel is better than that in single RE treated steel. The continuous cooling transformation curve of weld heat affected zone in C-Mn steel treated by RE showed that the At8/5 range to obtain intragranular acicular ferrite is about 40 s-600 s. It is difficult to get intragranular acicular ferrite in HAZ for Al killed C-Mn steel even if treated by RE. The range of Atg/5 for intragranular acicular ferrite formation is narrow, about 100 s-300 s. When the welding heat input energy is 75 kJ/cm and 100 kJ/cm, it is easy to form thick grain boundary ferrite and bainite/widmanstatten in HAZ.