Abstract
Continuous cooling transformation diagrams and microstructures of coarse grain heat-affected zone (CGHAZ) of V-N-Ti and Nb-V-Ti microalloyed steels at different cooling time form 800°C to 500°C (t8/5) were comparatively investigated. With the increase of t8/5, the microstructure of V-N-Ti steel changes from martensite and lath bainite to ferrite, pearlite, lath bainite and granular bainite, however, lath bainite and granular bainite are the main microstructures of Nb-V-Ti steel, and hardness of the CGHAZ of both steels are decreased, but the CGHAZ of Nb-V-Ti steel has a higher hardness than that of V-N-Ti steel. Necklace-like M-A constituents along prior austenite grain boundaries are observed in CGHAZ of Nb-V-Ti steel at all of the t8/5 which could deteriorate the impact toughness. As the simulated HAZ continuous cooling transformation diagrams (SHCCT) are determined, the average transformation start temperature of V-N-Ti steel is nearly 100°C higher than Nb-V-Ti steel, the reason of which is (Ti, V)N particles did not dissolve during the welding thermal circle, and have an effect of pinning on prior austenite grain boundaries and induced the formation of ferrite at prior austenite grain boundaries.
Published Version
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