Effect of Mg Content on the Microstructure and Toughness of Heat-Affected Zone of Steel Plate after High Heat Input Weld
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INTRODUCTION
IT has already been reported that microstructure of heat-affected zone (HAZ) formed after high heat input welding process of steel plate is usually composed of grain boundary ferrite (GBF), Widmansta¨tten ferrite (WF), ferrite side plate (FSP), upper bainite (Bu), intragranular acicular ferrite (IAF), and polygonal ferrite (PF). These microstructures can be classified into two types: one is the brittle microstructures, namely GBF, WF, FSP, and Bu, and the other is the ductile microstructures, i.e., IAF and PF.[1–8] The most effective method to improve the toughness of HAZ of steel plate after high heat input welding has been termed as oxide metallurgy[9] or inclusion engineering,[4] which is to make use of oxide particles or other kinds of inclusions and precipitate particles as nucleation sites of IAF, or for pinning the growth of former austenite grain in HAZ of steel plate. The non-metallic inclusions thus play a key role for inducing the formation of IAF and the HAZ toughness can be effectively increased with increasing the proportion of IAF in the microstructure of HAZ.[4,10,11] HAZ toughness can be further improved by the use of strong deoxidizer, such as Mg, in the secondary refining process of steel plate. Chang et al.[12] and Kim et al.[13] have studied the effects of Mg addition as deoxidizer on the variation of inclusions and microstructures during LONG-YUN XU, Doctoral Candidate, and WAN-LIN WANG, Professor, are with the School of Metallurgy and Environment, Central South University, Changsha, 410083, P. R. China. JIAN YANG, Professor, and RUI-ZHI WANG and YU-NAN WANG, Researchers, are with Steelmaking Research Department, Research Institute, Baosteel Group Corporation, Shanghai 201900, P. R. China. Contact e-mail: [email protected] Manuscript submitted January 21, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the solidification of steel after the molten steel was deoxidized by Mn-Si-Ti. It was indicated that Mg addition is beneficial for the heterogeneous nucleation of IAF. Chai et al.[14] suggested that Mg (about 0.002 mass pct) was able to refine the Ti-based inclusions by the formation of Mg-Ti-O inclusions in molten steel. It was found that complex deoxidation of Ti followed by strong deoxidant Mg is one of the most promising methods to precisely control the composition, size, and number of deoxidation particles.[15] Wen et al.[16] reported that the percentage of IAF in steel can be greatly increased with the addition of Mg. Zhu et al.[17] also found that the toughness of HAZ was improved significantly by adding 0.005 mass pct Mg to the Ti-bearing low-carbon steel, since the strong pinning effect was imparted by a great number of pinning particles during the high heat input welding processing. Yang et al. have obtained the excellent HAZ toughness after high heat input welding process of steel plate with Mg deoxidation from both laboratorial studies[6] and industrial scale tests.[7] The microstructure in HAZ of low-carbon steel transforms two times (a fi c fi a) during the thermal cycle of
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