Effect Of Zr And Ti On Microstructure And Toughness In The Simulated Heat-affected Zone Of High-strength Low-alloy Steels

High strength low alloy?HSLA?steel is widely used in engineering structures because of its good strength,toughness,weldability and other comprehensive properties.The toughness of coarse-grained heat-affected zone?CGHAZ?of HSLA steel decreases sharply when it is constructed by welding.In order to ensure the safety of engineering structures,it is very important to investigate how to improve the toughness of CGHAZ of HSLA steel.In this paper,different content of Zr?0%,0.0124%?and Ti?0.012%,0.061%?were added and the effect of them on microstructure and impact toughness in the simulated CGHAZ of HSLA steel were investigated.Microstructure and second phase particle structure of CGHAZ were quantitatively characterized by optical microscopy?OM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and electron backscatter diffraction?EBSD?.The relationship between microstructures and impact toughness was analyzed based on the results of mechanical properties test.The experimental results are as follows:?1?After the addition of 0.0124%Zr,the inclusions in CGHAZ of HSLA steel evolve from Al-Ti composite oxides to Zr-Al-Ti composite oxides and the precipitates evolve from?Ti,Nb?N to?Al,Ti,Nb?N and?Ti,Nb?N.Because of the substitution reaction between Zr and Al-Ti-O composite oxides,the solubility of Al and Ti in molten steel is increased,and more fine nano-precipitates containing Al are precipitated during cooling.?2?The grains in CGHAZ can be refined by adding Zr under both 20kJ·cm^-11 and100kJ·cm^-1 input welding thermal cycle.Because the nano-scale particles pin the austenite grain growth and refine the CGHAZ grain under 20kJ·cm^-11 input welding thermal cycle;while under the combined effect of pinning effect nano-scale particles and formation of a large number of acicular ferrite,the grain is refined effectively under 100kJ·cm^-1 input welding thermal cycle.?3?Compared with Zr-free steel,the effective grain size is smaller in CGHAZ of Zr-bearing steel,which can effectively hinder the extension of cracks.It shows good toughness in low temperature impact test and typical ductile fracture in impact section.?4?Because of 0.061%Ti addition,the size of?Ti,Nb?N precipitates were increased and pinning force of particles decreased,resulting in a larger austenite grain size in the simulated CGHAZ of 0.061%Ti steel,and part of the Al-Ti-Mn oxides were covered with TiN in 0.061%Ti steel owing to the excessive Ti addition.The ability of inclusion for the nucleation of acicular ferrite decreased,which resulted in a lower fraction of acicular ferrite in the simulated CGHAZ of 0.061%Ti steel.In addition,the temperature difference between B_s and B_f decreases with the increase of Ti content,resulting in the content of M-A component in CGHAZ of 0.061%Ti steel higher than that in 0.012%Ti steel.?5?Compared with 0.012%Ti steel,0.061%Ti steel CGHAZ has poor toughness and cleavage fracture characteristics due to the interaction of coarse austenite grain,large and hard TiN particles,fewer acicular ferrite and more M-A constituent.