| In recent years,the number of large-span,wide-width and heavy-duty steel structure bridges has increased.In order to improve welding efficiency,advanced welding techniques with large heat inputs,including double submerged-arc welding,multi-wire gas welding,etc.,have been extensively applied in the fields of bridges.However,the existing weathering bridge steels of Cu,Cr and Ni system are incapable of such a large heat input welding,due to the coarse grain heat affected zone（CGHAZ）microstructure is severely coarsened and the corresponding toughness is significantly reduced under large heat inputs conditions.Low carbon V-Ti-N microalloying technologies can regulate CGHAZ precipitated particles and hard phase M/A constituents via N addition,and promote the formation of high density heterogeneous nucleated ferrite and dispersion-refined M/A constituents,which provides a novel idea for the development of weathering bridge steels suitable large heat inputs.For this attempt,we carefully prepared low carbon V-Ti-N microalloying weathering bridge steels with varied N content.The GleebleTM system was used to conduct experiments on CGHAZ large heat input welding thermal simulation,series heat inputs CGHAZ welding thermal simulation,and thermo-mechanical control process（TMCP）optimization.The microstructure was observed using characterization techniques such as scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and electron back-scattering diffraction（EBSD）,and combined with the results of mechanical properties of the impact and tensile tests,the influence of base metal microstructure and their CGHAZ microstructure on mechanical properties was comprehensively analyzed.The effects of N content on the continuous cooling transformation behaviors and weldability of V-Ti-N microalloying weathering bridge steel were studied.The CGHAZ SHCCT curves of weathering bridge steel with different N contents were constructed.Under various T8/5,phase transition temperature Ar3 increased with the increase in N content.With the increase in T8/5,the regions of bainite ferrite decreased while the regions of ferrite transformation increased.The weldability test results showed that the welding crack sensitivity and the hardening tendency of heat affected zone of the test steels with varied N contents were lower.The effects of N contents on the CGHAZ microstructure and impact toughness were studied at Ej=80 kJ/cm.The CGHAZ particles and M/A constituents were significantly regulated with the increase in N content from 0.0030 wt.%to 0.0075 wt.%;the enhanced pinning effects of nano-sized particles on prior austenite grain boundaries（PAGBs）resulted in a decrease in the size of prior austenite grains（PAGs）,the increase in the mass fraction of V in submicron-sized（Ti,V）（C,N）particles reduced their lattice disregistry with ferrite,the corresponding content of CGHAZ heteronucleated ferrite increased significantly.Meanwhile,the M/A constituents became dispersed,refined,and their hardness decreased via N addition.All these factors increased the crack initiation energy and propagation energy with the increase in N content.The effects of welding heat inputs on the CGHAZ microstructure and mechanical properties were studied under 0.0075 wt.%N condition.The results showed that with the Ejincreased from 15 kJ/cm to 35 kJ/cm and 55-75 kJ/cm,the low-temperature toughness of CGHAZ first decreased and then increased;mainly due to the increase in Ej from 15 kJ/cm to 35 kJ/cm,the density of high-angle grain boundaries（HAGBs）that hinder crack propagation decreased due to the coarsening of PAGs size and packet structures;while the Ej further increased to 55-75 kJ/cm,a large number of heteronucleated ferrite increased the density of HAGBs.In addition,the increase in Ej elevated the grain size of CGHAZ microstructure with low-angle grain boundaries（LAGBs）,and the corresponding CGHAZ strength monotonically decreased.The mean equivalent diameter of grains defined by the misorientation tolerance angles ranging from 2-6°had the strongest contribution to YS due to their higher fitting coefficient of the Hall-Petch relationship.The effects of key TMCP processes parameters on the microstructure and mechanical properties of test steel with 0.0075 wt.%N were studied,and the comprehensive regulatory effects and mechanisms of the main rolling process on the high strength toughness and low yield ratio of the test steel were clarified.Reducing the final rolling temperature was beneficial to obtain high strength and toughness,but led to the increase in yield ratio.Reducing the starting cooling temperature and increasing the final cooling temperature would reduce the strength and toughness,but were beneficial to obtain a low yield ratio.In order to ensure that the test steel has good strength and toughness and low yield to strength ratio,the final rolling temperature,starting temperature and final cooling temperature should be controlled at 820-860℃,740-800℃and 500-600℃,respectively. |
