Advancements in the Development, Industrial Production, and Applications of Ultra-high Nitrogen Low Alloy Steel

The solubility of nitrogen in each phase during the cooling transformation of ultra-high nitrogen low alloy steel was calculated using Thermo-Calc software and validated through laboratory experiments. The nitrogen absorption rate in molten steel is directly proportional to both the nitrogen concentration and the contact area between nitrogen and the molten steel, with a significantly higher absorption rate under high pressure compared to low pressure. Numerical simulations of nitrogen reaction rates closely align with industrial test results. Nitrogen is soluble in austenite, acting as a temporary reservoir that facilitates nitrogen transfer from molten steel and δ ferrite to precipitates. Experimental findings indicate that nitrogen pores with diameters ≤ 500 μm can be effectively closed through rolling. Furthermore, reducing the Al and Nb content in steel reduces grain boundary brittleness, preventing crack formation associated with ultra-high nitrogen levels. The increase in nitrogen content leads to a higher amount of precipitated phases, reduced particle size, and an increased nitrogen mass fraction in the precipitates. This refinement in microstructure enhances the mechanical properties of ultra-high nitrogen low alloy steel, increasing its yield strength by up to 100 MPa. The steel exhibits excellent plastic toughness, with no cracks observed after cold bending or spot welding, demonstrating its superior structural integrity and processability.