| Liquid petroleum gas(LPG)is widely used in industrial and civil fields.In recent years,the global demand for LPG has been increasing,and China is also building a large number of propane dehydrogenation(PDH)projects and LPG intelligent microtube networks using LPG as raw materials.The boiling point of LPG is-42.07 ℃,so both storage and transportation cannot be separated from low-temperature pressure vessels.The mechanical properties of low temperature pressure vessel steels are related to factors such as microalloying elements and heat treatment processes.This paper takes 0.5Ni steel as the research object,and provides theoretical guidance for the application of microalloying elements by calculating and analyzing the thermodynamic and kinetic precipitation behavior of niobium carbide,niobium nitride,and vanadium carbide,vanadium nitride in austenite;On this basis,laboratory hot rolling and post rolling heat treatment were conducted.The effects of heat treatment processes on the microstructure and properties of 0.5Ni steel were studied using optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),transmission electron microscopy(TEM),tensile testing machines,and pendulum impact testing machines.The main research results are as follows:In 0.5Ni steel,the solid solubility of Nb in ferrite has been very small.After the transformation of austenite to ferrite,Nb C and Nb N are basically precipitated.At high temperature,the solubility decreases with the increase of C content;As the temperature increases,the impact increases.N element has a significant effect at lower temperatures,and its solubility decreases with the increase of N content.Increasing the content of Nb,V,and C increases the total solution temperature,and the influence of the content of Nb and V on the total solution temperature is higher than that of C element.Niobium carbonitride is more prone to nucleation on the dislocation line.With the increase of Nb content,the fastest precipitation temperature increases,and the time point of the fastest precipitation presents a trend of first decreasing and then increasing.After deformation is applied,the nose tip temperature of the NrT curve and PTT curve of niobium carbonitride precipitation increases by approximately 25 ℃.The solid solubility of V element in austenite is very high,and VC and VN will precipitate rapidly during the transformation from austenite to ferrite;Vanadium carbonitride is also more prone to nucleation on dislocation lines,but no nasal tip was found on the NrT curve and PTT curve at the austenitic stage.After deformation was applied,the changes in the NrT curve and PTT curve of vanadium carbonitride were not significant and still no nasal tip was found.After different heat treatments,the phase type of 0.5Ni steel has not changed,mainly consisting of ferrite,with a microstructure of pearlite+ferrite,and there are obvious pearlite ferrite bands in the microstructure.With the increase of tempering temperature,the grain size of 0.5Ni steel did not significantly change,and the uniformity of the microstructure improved,but it was found that the content of pearlite decreased;The tensile strength presents a downward trend,and the impact energy presents a trend of increasing first and then decreasing.Under the condition of normalizing at 890 ℃ and tempering at 610 ℃,the impact toughness is the highest.A large number of tear edges are found in the impact fracture morphology,and the fracture type of the material begins to change to a quasi cleavage fracture type,thereby improving the low-temperature toughness of the material. |
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