Relationship Between Impact Toughness and Microstructure for the As-Rolled and Simulated HAZ of Low-Carbon Steel Contain
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HIGH heat input welding process is generally used during processing of large scale steel structures such as shipbuilding, large-diameter linepipe and offshore platform to minimize fabrication cost and increase productivity.[1–3] However, under high heat input, coarse grain boundary ferrite (GBF), ferrite side-plates (FSP), and martensite-austenite (M-A) constituents tend to form in the HAZ, which are detrimental to impact toughness.[4–6] The technology of oxide metallurgy is considered to be an effective approach to improve HAZ properties, which focuses on introducing particular inclusions to promote acicular ferrite (AF) transformation.[7] AF microstructure is known to improve steel properties,
XIN WANG, CHAO WANG, JIAN KANG, GUODONG WANG, and GUO YUAN are with the State Key Laboratory of Rolling and Automation, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, P.R. China. Contact e-mail: [email protected], [email protected] DEVESH MISRA is with the Laboratory for Excellence in Advanced Steel Research, University of Texas at El Paso, El Paso, TX 79968-0521. Manuscript submitted November 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
especially toughness. It has been previously observed when Ti and Al content is controlled in the range of 20 to 60 ppm and 5 to 20 ppm, respectively, impact toughness of rolled plate is enhanced in pilot trials.[8] A study by Li et al. has shown that acicular ferrite formation in CGHAZ microstructure was enhanced by Mg deoxidization treatment.[9] The addition of Mg significantly refined the microstructure and improved the strength of steel.[10] Additionally, the increased amount of Zr-containing inclusions promoted the proportion of AF and improved the strength and toughness of weld metals.[11] Moreover, it was shown that the inclusions in Ti-Zr deoxidized steel were primarily composite ZrO2-MnS type with high number density, where intragranular ferrite nucleation was significantly promoted.[12] Among the non-metallic inclusions, Ti and Mn-containing complex oxysulfide particles are described as effective intragranular nucleation sites for AF. Acicular ferrite has good impact toughness, which is mainly because of small effective grain size and interlocked microstructure with high-angle grain boundaries between ferrite plates.[13,14] Recently, the focus has been on Ti-Ca complex deoxidized treatment, and modification of non-metallic inclusions during Ca treatment process was explored.[15] It was found that Ca modified the morphology of solid
Al-Ti inclusions to spherical particles.[16] Moreover, Ca treatment was more effective in Al-Ti deoxidized steel.[17] However, the effect of Ti-Ca oxide particles on transformation behavior and the mechanisms involved continue to be unclear. Thus, we processed low-carbon steel deoxidized by Ti-Ca. The number, size and distribution of inclusions were analyzed. Microstructure evolution and impact properties of HAZ were studied. The AF nucleation mechanism and the interplay between microstructure and toughness was discussed.
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