The Influence Of Mg Or Zr Treatment On The Properties Of Submerged Arc Welding Heat Affected Zone Of FH40 Ship Plate Steel

High heat input welding is the important development direction of ship plate steel welding technology,and it is a core problem at present to control the microstructures and properties of welding heat affected zone(HAZ).Oxides metallurgy technology is the key to solve this problem.It plays an important role in improving the oxides metallurgy theory and supporting the development of oxides metallurgy technology to studying on the effects and mechanism of non-metallic inclusions on the microstructures and properties of welding HAZ.The FH40 ship plate steel was selected as experimental steel in this subject.On the basis of experimental steel smelting,welding samples were prepared by forging,rolling and welding.The effects of Mg,Zr treatment alone,Mg-Zr composite treatment,welding energy input on the number,compositions and morphologies of the inclusions,microstructures and mechanical properties in HAZ were investigated,and the microstructures control mechanism of HAZ was discussed.For Mg treatment,the amount of non-metallic inclusions in the steel significantly increased and the size is smaller than that of the base steel.The type of non-metallic inclusions is basically Al-Mg-O(+MnS),which have a strong ability to induce intragranular acicular ferrite(IAF)nucleation and inhibit effectively the formation of coarse grain boundary ferrite(GBF)and Widmanstatten Ferrite(WF)effectively.Experienced a higher heat input welding,the HAZ microstructure didn't show tendency to coarsening,and remained stable impact toughness,better than the base steel.Through Zr treatment,the number density of inclusions significantly increased in steel,and inclusions size refinement effect was also obvious.As the main type of non-metallic inclusions,ZrO2+Al2O3(+MnS),could induce IAF nucleation as well.Compared with the Mgtreated steels,the amount of quasi-polygonal ferrite(QF)of Zr-treated steels is larger in the coarse grained heat affected zone(CGHAZ).Although the grain size of the CGHAZ increases slightly at the higher heat input welding,the impact toughness still remains excellent.It is inferred that the excellent toughness is a result of the QF and IAF interaction together.Mg-Zr composite treatment can also realize the small dispersion distribution of nonmetallic inclusions in steel.Al2O3 can be almost completely transformed into ZrO2+Al-Mg-O at proper Mg-Zr composite treatment.After a higher heat input welding,this kind of composite inclusions can induce IAF nucleation,refine the microstructure of HAZ and exhibit good mechanical properties.It can be explained by the Mn-depletion zone mechanism that the composite inclusions of Al-Mg-O,ZrO2+Al2O3,ZrO2+Al-Mg-O oxides and MnS generated after the Mg,Zr treatment of FH40 ship plate steel can induce IAF nucleation,while the partial inclusions without MnS,such as Al-Mg-O and ZrO2+Al-Mg-O induced IAF nucleation,can be explained by the low misfit mechanism.