Effect Of Zr On Microstructure And Toughness Of HAZ In HSLA Steels

High-strength low-alloy(HSLA)steels show excellent comprehensive properties such as weldability,good toughness as well as high strength,which have been widely applied in engineering construction equipment.Austenite grain coarsening and embrittlement in coarse-grained heat-affected zone are the main problems of high strength steels.Acicular ferrite is recognized as a desirable microstructural constituent for refining the prior austenite grain and improving the toughness of coarse-grained heat-affected zone.In this study,effect of Zr on microstructure and toughness in HAZ of HSLA steels were systematically researched by means of electronic microscopes,electron backscattered diffraction analysis,crack tip open displacement and first-principles ccalculation.The results are listed below point by point:(1)Different level addition of zirconium resulted in different inclusions in the type and size.The elongated manganese sulfide(Mn S)strips,the small equiaxed complex oxygen sulfide inclusions,and the big equiaxed oxide inclusions were developed in the 0.005% Zr,0.013% Zr,and 0.054% Zr steels,respectively.It was also found that the complex inclusions Zr O2+Mn S and Zr O2 promoted the formation of ferrite grains except Mn S.(2)Zr manifested a significant effect on the grain refinement of the simulated CGHAZ.(3)There is optimum concentration of Zr.The 0.013 Zr steel had much better low-temperature impact toughness compared with 0.005 Zr and 0.054 Zr steel.(4)Nanosized Zr-Ti compound oxysulfides were formed in HAZ,which enhance the nucleation of acicular ferrite and improve the toughness in HAZ.(5)The first principle calculation results show that: Mn diffusion into Zr O2 by means of cation vacancy;S diffusion into Zr O2 by means of anion vacancy;or Mn S precipitate on the pre-formed Zr O2.All of this could result in the formation of Mn depletion zone around the oxides,which could enhance the nucleation of acicular ferrite and improve the toughness.