Development Of Self-shielded Flux Cored Wire For Ultra-low Carbon Bainite Steel

Ultra-low carbon bainite（ULCB）steel,whose carbon content is generally less than 0.05wt%,is widely used in oil and gas pipeline,shipbuilding,marine facilities and other fields,due to the high strength,good toughness and weldability.With the growth of the demand for high-grade pipeline steel,the research of ultra-low carbon bainitic steel has made a great progress in China,but the correspondingly high-quality welding materials are relatively scarce.The development of the self-shielded flux cored wire for ULCB steel has a great application prospect.In this study,a self-shielded flux cored wire for ULCB steel,with Mn-Ni-Mo-Ti-B alloy system and low alkaline BaF;slag system,was developed by orthogonal test,which was based on the matching principle between weld zone and base metal and the slag theory of self-shielded flux cored wire.The microstructure of the slag and welded joint were characterized by optical microscope（OM）,scanning electron microscope（SEM）and X-ray diffractometer（XRD）.And the processing properties of wire and the mechanical properties of joint were tested.All the results meet the standard requirements.The processing properties show that with increase of the content of Al-Mg alloy and BaF:,the welding fume increases.With increase of CaCO₃ and BaF₂ content,the splash increases.With increase of the content of TiO2 and BaF₂,the slag is easer to be removed.Because there are more complex oxides and fluorides to form,such as BaTiO₃ and BaMg₃Al₂Si₂O₁₀F₂,and the microstructure of the slag is rod-like and block-like.However,if the content of BaF₂ is more than 30wt%,a sharp drop of slag viscosity will lead to terrible oxidation of the weld metal and slag detachability.With increase of CaCO₃ content,the slag detacliability get worse.Because Ca2SiO4 and the spinel in slag increase,but BaMg₃Al₂Si₂O₁₀F₂ and BaTiO₃ in slag decrease,which results in the formation of glassy slag.Finally,the slag detachability get worse due to the weaker internal binding force and the stronger adhesion between the slag and weld metal.The microstructure and mechanical properties show that the microstructure of the weld zone with the best mechanical properties is granular bainite（GB）,acicular ferrite（AF） and a small amount of polygonal ferrite（PF）,and the fine acicular ferrite intersperses in the granular bainite,which can effectively hinder crack propagation,so that the tensile strength and impact energy can be improved at the same time.More than one ferrite laths or plates are nucleated on one Al₂O₃ inclusion.The crystallographic orientation of acicular ferrite is not completely random.With increase of the Mn equivalent in the weld,the tensile strength of the joint increases and then tends to be constant.With increase of the Al content in the weld,impact energy presents a tendency to rise first and then decrease.