Abstract: Through analyzing acicular ferrite nucleating mechanism in the weld metal of the high strength low-alloy structural steel(HSLA), a submerged arc welding agglomerated flux with the CaF₂-MgO-Al₂O₃-MnO-TiO₂-B₂O₃ fluorine was developed by using alkali flux system.The results indicate that the existence of MnO in the flux benefits the transition of Mn to weld metal, and brings down the γ→αtransformation temperature, thus high temperature ferrite production is held down, but the acicular ferrite increases in the weld metal. Because the Pcm of weld metal is higher, the austenite grain is thinner as-welded in air cooling condition.However, a excessive content of MnO in the flux will lead to the excessive Mn in the weld metal and a too low γ→α transform temperature.Thereby, the austenite grain boundaries cannot be decorated by allotriomorphic ferrite, thus the austenite grain boundary increases, which make more bainite nucleation site being produced. It is disadvantageous to the intragranularly nucleated acicular ferrite.In addition, the rareearth element has some contributions to enhance the acicular ferrite content in the weld metal.