Study On The Effect Of Ca And Mg Treatments On The Impact Toughness Of The Coarse Grain Heat Affected Zone Of High-rise Building Steel

With the popularization of steel structure,high-rise building steel has been applied more and more widely,and its adaptability to large heat input(?100 k J/cm)has been paid more attention.The inclusion particles of Ca and Mg produced by oxide metallurgy can pin the austenite boundary of hea affected zone(HAZ)and induce the nucleation of intragranular acicular ferrite(IAF),and improve the toughness of coarse grain heat affected zone(CGHAZ).Therefore,the high-rise building steel suitable for large heat input can be obtained.In this paper,thermodynamic calculations are used to analyze the precipitation order of inclusion particles;Combining metallographic microscope(OM),transmission electron microscope(TEM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD)techniques and other analytical methods to study the effects of Ca and Mg treatment on the microstructure,inclusion morphology,composition,the impact of size and impact performance,reveals the mechanism of IAF nucleation in CGHAZ induced by Ca and Mg treatment.Observed and analyzed the microstructure of the thermal simulation impact sample,and it is found: under the condition of 100 k J/cm line energy,the microstructure of tested steels CGHAZ is intragranular acicular ferrite and block ferrite.Among them,the IAF content of the Mg-treated steel sample is up to 82.9%,and the inclusion particle size is 1.3 (?)m,distribution uniformly,0 ?impact energy is 128 J,has the best impact performance;Under the condition of 200 k J/cm line energy,the microstructure of tested steels CGHAZ is still intragranular acicular ferrite,block ferrite and martensite.Among them,the IAF content of the Mg-treated steel sample is up to 53.2%,and the inclusion particle size is 1.1 (?)m,0 ? impact energy is 105 J,has the best impact performance.Comparing different line energy,it is found that the lower the line energy,the larger the size of the inclusion particles generated by the sample CGHAZ,the higher the IAF content,and the higher the impact energy.Under the same welding heat input,the comparison of Ca-treated steel and Mg-treated steel found: the impact properties of Mg-treated steel was significantly higher than that of Ca-treated steel.The main reason is that the precipitation ability of Mg inclusion particles was stronger than that of Ca inclusion particles.In the welding course,Mg inclusion particles have more significant pinning effect on CGHAZ microstructure;At the same time,increasing the content of acicular ferrite,extending the crack propagation path,increasing the energy absorbed during fracture,and improving the toughness of the material.