Nitrogen in HSLA and dual phase steels
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Nitrogen is universally present in all steels, and although its solubility under normal steelmaking conditions is small it can exert large effects on steel properties. Some of these effects are detrimental, often being associated with various embrittlement phenomena. This has led to liquid steel refining processes designed, amongst other things, to decrease the nitrogen content. However nitrogen also has beneficial effects, and since it is abundant and cheap, different steels containing enhanced nitrogen have been developed, Nitrogen levels obtained in the traditional steelmaking processes are well established. In the present investigation a detailed study of the origin and control of nitrogen in the New Zealand Steel iron and steelmaking process has been studied. The progress of 18 individual heats was followed by collecting samples at successive processing stages, and an attempt was made to correlate the observed nitrogen with recorded process variables. It was found that hot metal from the melters had an average nitrogen content of 0.002% and a further increase in nitrogen content was observed at VRU. However, there was a drop in nitrogen content after oxygen blowing in the KOBM. It is suggested that nitrogen in solution in the liquid steel is absorbed into gas bubbles passing through the steel bath giving a flushing action, consequently carbon monoxide bubbles formed during oxygen lancing will effectively reduce the nitrogen content. There was a significant increase in nitrogen content between the KOBM samples and LTS samples, and a further increase in nitrogen content was observed in the GGM samples. These increases at the LTS and CCM seem to result from the absorption of nitrogen during tapping, transferring the ladle to the LTS and teeming at the CCM where there is little protection of the molten stream from the atmosphere and consequential nitrogen absorption. This investigation examines the effect of thermal treatments on the precipitation of vanadium nitride in high strength, low alloy steels. The thermal cycle of a hot rolling strip mill has been simulated in the laboratory and precipitation of vanadium nitride studied. The solubility of vanadium nitride in high strength, low alloy type steels was determined for temperatures from 900°C - 1250°C, and the ferrite grain size after the simulated thermal cycle determined. Change in yield strength, charpy transition temperature and strain age propensity as a result of vanadium nitride precipitation, have also been determined. The effect of a normalizing heat treatment subsequent to the simulated thermal cycle was also examined. Analysed Ninsol suggests that the precipitation of vanadium nitride is rapid in the high temperature ferrite phase range, and is diffusion controlled. Peak precipitation of vanadium nitride has been shown to occur at a simulated coiling temperature of 700°C for the high strength, low alloy steels examined. Minimum ferrite grain size for the simulated thermal cycle, and for samples subsequently normalized, suggest that vanadium nitride formed in the ferrite phase dominates the subsequent ferrite grain size. The mechanical properties of these high strength, low alloy steels have been shown to be dependent on both grain size and vanadium nitride precipitated during the simulated hot rolling thermal cycle. Nitrogen also has an influence on the microstructure and properties of dual phase steels. Nitrogen increases the hardenability of the austenite phase formed at intercritical annealing temperature by partitioning. Manganese retards the partitioning of nitrogen by forming atom pairs within the iron lattice and providing low energy sites for nitrogen atoms. In this present investigation the partitioning of nitrogen has been investigated for a range of steels with manganese contents up to 1.5%. Significant partitioning of both nitrogen and manganese was shown to occur during the intercritical annealing heat treatment. Partitioning of nitrogen in low manganese steels was shown to be rapid, but with increased managanese content, partitioning of nitrogen retarded. This retardation was proportional to the manganese content. The effect of manganese partitioning on the formation of dual phase microstructure has also been examined.