Documenting Ferrite Nucleation Behavior Differences in the Heat-Affected Zones of EH36 Shipbuilding Steels with Mg and Z
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ing to traditional views, non-metallic inclusions are difficult to remove thoroughly and have been demonstrated to be detrimental to steel properties, such as toughness, fatigue, and strength.[1,2] However, with improved understanding of inclusions, some inclusions, such as Mg oxides and Zr oxides, have been found to be able to act as potent nucleation sites for the formation of acicular ferrite (AF) structures in low-carbon low-alloy steels.[3–7] The AF-dominated microstructure is one of the most attractive microstructures combining high strength and excellent toughness because of the chaotic arrangement of laths and fine-grained interlocking microstructural features.[8–11] Xu et al.[7] demonstrated that Mg-containing inclusions could effectively facilitate the nucleation of AF
XIAODONG ZOU is with the School of Metallurgy, Northeastern University, Shenyang 110819, P.R. China and also the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. JINCHENG SUN and CONG WANG are with the School of Metallurgy, Northeastern University. Contact e-mail: [email protected] HIROYUKI MATSUURA is with the Department of Materials Engineering, The University of Tokyo. Manuscript submitted April 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and improve the toughness of the heat-affected zone (HAZ) in EH36 shipbuilding steel. In our previous studies, the evolution behavior of inclusions in EH36 shipbuilding steel with Mg addition under different heat inputs was investigated, and ferrite laths were found to prefer to nucleate on the surface of Al-Mg-Ti-O-Mn-S complex inclusions to form AF instead of on grain boundaries under the lower heat input of 120 kJ/ cm.[12,13] Guo et al.[14] reported that the impact toughness of HAZ was improved by the addition of Zr into a high-strength low-alloy pipeline steel, which was attributable to MnS precipitation on the surface of ZrO2 inclusions and thus promoted the formation of AF. However, which types of inclusions are more effective for the formation of AF remains ambiguous. An in situ high-temperature confocal scanning laser microscope (CSLM) was utilized for real-time dynamic observations of the phase transformation, particularly the formation of AF on various inclusions.[15–17] Zhang et al.[15] investigated the effect of Ti addition on the evolution of AF in the HAZ of C-Mn steel and directly observed potent and inactive inclusions with respect to the nucleation and growth of AF by CSLM. Pak et al.[18] examined the surface effect on displacive transformation with CSLM and indicated that during the observation the free surface allowed displacive transformation to occur at a higher temperature with a reduced free energy change. It is noted that CSLM can only provide partial visual information and evidence. To further understand the crystallography of microstructural features, the electron backscatter diffraction (EBSD) technique, which has been frequently used to investigate the final microstructural characteristics,[19,20] is required. Prevailing demand dictates th
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