Research On Impact Toughness Of Boron-Nickel Added High Strength Low Alloy Steel

High strength low alloy steel (HSLA Steel) is a type of structural steel which has small amounts of alloy elements and superior strength. Underlying the condition of process technology using controlled rolling and controlled cooling, through precipitation strength and fine-grain strength and so on, its properties of strength and toughness has increased significantly. The HSLA steel are widely used in many aspects as engineering structure, such as construction, vehicle, bridges, ships, pressure vessel, offshore drilling platforms and so on, performing remarkable economic and social benefits. With continuous increasing requirements about application, materials researchers must improve original steels constantly or develop new kind of steel. In the course of using, low temperature brittleness is the focus of such steel, hoping to obtain excellent low temperature impact toughness, lower ductile-brittle transition temperature and fine processing performance. Micro-alloying, a new technology of metallurgy, is an effective method to improve use characteristic of HSLA steel. Through adding niobium, vanadium, titanium, boron and so on as elements of making carbide, nitride and carbonitride, then we obtain good comprehensive mechanical properties.The main research object in the paper is combination added boron-nickel Q345E steel. According national standard and quality requirements, we design composition as well as determine the production process. Several material experimental instruments are used to analyze microstructure, mechanical properties and impact toughness, inclusions, including metalloscope, material testing machine, impact tester, scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS) and so on.Through all kinds of experimental methods above, the conclusions are observed as following:The longitudinal impact toughness was significantly higher than the transverse impact toughness, and the main reason for this is related to different distortion of both directions. We will get "fibrous tissue" at longitudinal has big deformation, but it is not obvious of metal streamline at transverse direction, which has small deformation, then it turns up anisotropic, lending the difference of impact toughness. Also the dispersion of longitudinal impact toughness is higher than that of the transverse, and larger volatility. Inclusions in the steel are oxide inclusions (iron oxide, alumina, Calcium oxide) and Sulfide inclusions, those sizes are generally small.With lower temperature, a growing influence on low-temperature brittleness becomes strong, leading higher ductile-brittle transition temperature.