Research On The Quenching And Tempering Process For The Boron-Nickel Treated Q345E H-beams

High Strength Low Alloy Steel is a type of low-carbon low-alloy structural steel, which, as a result of grain boundary hardening and precipitation hardening with micro alloying, exhibits high strength, high impact toughness, good weld-ability, and great resistance to corrosion. HSLA steel is widely used in structures that are designed to handle large amounts of stress or need a good strength-to-weight ratio, such as offshore drilling platforms, bridges and large ships. However, most hot-rolled HSLA steels can not provide high impact toughness at low temperature due to their relatively high fracture appearance transition temperature (FATT), and thus do not perform well in extremely cold regions. Heat treatment is often associated with increasing the strength of material, especially steels, since they respond well to change of temperature. Our research group has used heat treatment on a novel HSLA steel to increase its impact toughness, specifically the low-temperature-impact-toughness. Therefore, the improvement significantly extends the scope of applications of the steel.In our research, we have discovered the relationship between heat treatment process and the properties of the niobium added HSLA steel. Eventually, with an optimized quenching-tempering schedule, we have produced a new type of steel with a tensile strength at about 500Mpa, elongation at about 25%. The overall performance of the steel, especially the low-temperature-impact-toughness has significantly improved, as the FATT of the steel is below -70℃, and impact toughness is about 100J at -90℃. Along with the help of compound effect of boron (10ppm) and Nickel (0.5%), the new type of steel is able to replace 3.5Ni steel in certain circumstances since it is much more cost efficient.In this paper, we first built up a CCT (continuous cooling transformation) diagram using thermal simulation technology. Based on the analysis of the diagram, we are able to determine the phase transition temperature of the steel. After a quenching test and a hardness test, the depth of hardening was measured. By varying parameters of quenching and tempering, we studied how quenching temperature and time of maintaining, and tempering temperature influenced the microstructure of the steel. So an optimum quenching and tempering process control was obtained. Hardness tests, tensile tests and impact toughness tests at a series of low temperatures were performed afterwards to further examine the result. Several characterization techniques were used to analyze the microstructures, phase composition, and distribution of second phase particles of the sample, including SEM, TEM, XRD and photoelectron spectroscopy. The results indicate that complicated ingredient carbides generated along grain boundaries after tempering considerably inhibit grow of grains during recrystallization. Those fine distributive carbides do not damage the toughness of the steel. However, they have prevented fractures from spreading. Consequently, the toughness of the steel is enormously increased.