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To research the effect of Mg on the austenite grain growth of the heat-affected zone (HAZ) in low-carbon high-strength steels, two steels with different Mg contents were prepared using a laboratory vacuum. It was observed through optical microscope (OM) that the HAZ austenite grain size decreased from 385 to 86 μ m after Mg treatment. In-situ observation by a confocal scanning laser microscope showed that the HAZ austenite grain size of the steel with Mg treatment could hold fine-grained structure after 1673 K (1400 °C) heating for 300 seconds, which was mainly attributed to the formation of pinning particles after the addition of Mg. Analysis showed that the number of pinning particles was much increased and the mean size was much decreased by the Mg treatment. It also can be found that more particles occurred with the MgO as the nucleating center. The HAZ toughness was improved from 33 to 185 J by refinement of austenite grains in the simulated HAZ due to the Mg treatment. The results presented in this article point to a potential method for improving HAZ toughness of low-carbon high-strength steels.
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