Comparative Study On Microstructure And Toughness Of Coarse Grain Zone In Critical Reheating Of V - N And Nb - V Microalloyed Normalized Steel Plate

High-strength low-alloy steel is extensively used in large engineering components, such as ships, bridges and buildings, for the reason that producing high strength and good toughness, after being added micro-alloying elements. High-strength low-alloy steel achieves the requirement of engineering components, however, it produces poor toughness under welding conditions and can’t afford to satisfy the demand for weldability. So it is of necessity to study the reason for giving rise to this phenomena.This paper takes V-N and Nb-V normalized steel for experimental objects, the weld heat affected zone (HAZ) double pass thermal simulation were performed on Gleeble-3800. It is proposed that in the double pass thermal simulation process, intercritically reheated coarse grained heat affected zone (ICCG HAZ) producing the poorest toughness. After experiencing normalized, V-N and Nb-V steel microstructure both transform to ferrite and perlite, and their mechanical properties appear similarly, as well. However, when they are subjected to thermal simulated process, it generates great disparity between V-N and Nb-V steel for the microstructure and mechanical properties. This paper explains the reason why there has a significant difference after being performed on simulation. It is reported that niobium is added to enhance the strength of HSLA, however, under welding condition, niobium has a detrimental influence on the HAZ toughness and promotes formation of a coarse structure of bainite with blocky martensite-austenite constituent (M-A). Intensive analysis and research has carried out to compare the discrepancies between V-N and Nb-V steels, by means of analyzing microstructure, M-A constituents, secondary microcracks, effective grain size, high grain boundary, and second phases and so on. And mainly compared the mechanical properties and microstructures change during thermal simulation at 790℃. The result reveals that V-N microstructure contains high proportions of bainite and polygonal ferrite, the latter can efficiently deflect or even arrest the propagations of cleavage microcracks and meet the demand. Nb-V microstructure, by contrast, is comprised of bainite and nearly connected blocky M-A constituents, which contribute to the poorest toughness on weldability.