Effect of Magnesium on the Austenite Grain Growth of the Heat-Affected Zone in Low-Carbon High-Strength Steels
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IN recent years, some high efficiency methods such as vertical electro gas arc welding (EGW) were used in welding structure steels, where the welding heat input could sometimes range from 500 to 1000 kJ/cm[1] and the steel near the fusion line was heated to a temperature of 1673 K (1400 °C) or higher for a considerable time. The high temperatures can lead to significant austenite grain coarsening, and the combination of a coarse austenite grain size and rapid cooling promotes brittle microstructures, which contain high proportions of ferrite side plates and bainite. The balance of high strength and good toughness in structure steels can be upset by the thermal cycles experienced during welding, producing poor toughness in the heat-affected zone (HAZ). So, there is great demand for the steel in the HAZ with high strength and good toughness after high heat input welding. Oxide metallurgy has a beneficial effect on the toughness of the HAZ. The second-phase particles were added into the steels. The micron-size second-phase particles promote intragranular nucleation of acicular ferrite, and the nanosize second-phase particles (including TiN, Ti2O3, CaS, MnS, CaO, ZrO2, MgS, and MgO) act as pinning particles during austenite grain growth and the control of intragranular microstructure in HAZ. Most research mainly focuses on the influence of Ti addition on the properties and microstructure of the intercritically reheated coarse-grained HAZ in KAI ZHU, Engineer, and ZHENGUO YANG, Professor, are with the Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China. Contact e-mail: 081030008@ fudan.edu.cn Manuscript submitted July 26, 2010. Article published online March 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
low-carbon microalloyed steels. Ti2O3, compared with other titanium oxides, was believed to be the most effective nucleant of intragranular ferrite in Ti-bearing low carbon steels.[2–5] Ti2O3 was also very effective for the nucleation of acicular ferrite in the steel-making process.[6–8] Some researchers reported that TiO particles were the most effective nucleation sites responsible for the formation of acicular ferrite.[9,10] The influence of Zr and Mn addition on the properties and microstructure of the intercritically reheated coarse-grained HAZ in low-carbon microalloyed steels was also studied. It was reported that the particles of ZrN, ZrO2, and MnS were effective in inhibiting austenite grain growth.[11–17] Furthermore, it was also reported that MgO was effective in inhibiting austenite grain growth in the HAZ of low-carbon steels.[18–20] However, the research only covers the relation of the toughness of HAZ and the concentration of the second-phase particles, while the relation of the microstructure evolution and the concentration of the second-phase particles is seldom studied. The objective of this study was to assess the effects of magnesium additions on inhibiting HAZ austenite grain growth of low-carbon microalloyed steels by the Gleeble HAZ simulation and confocal s
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