Study on high temperature mechanical properties of niobium microalloyed cold heading steel

In recent years, high-strength micro-alloyed cold heading steel has developed rapidly, mainly by adding Nb, V, Ti and other micro-alloying elements to the steel for grain refinement and precipitation strengthening, it is widely used in automobiles, bridges, construction machinery and other fields due to its excellent strength and toughness, fatigue resistance and corrosion resistance. However, the common quality defects in the process of continuous casting and hot rolling of such steels - transverse cracks on the surface and corners, are still one of the technical difficulties in the production of the world’s iron and steel enterprises. Therefore, it is of great scientific and engineering significance to deeply explore the evolution law of high temperature mechanical properties of niobium microalloyed cold heading steel, to analyze the common generation mechanism of surface transverse cracks and to propose general improvement measures. In this work, the high temperature thermoplastic properties of niobium microalloyed cold heading steel was studied by using Gleeble-3500 thermal simulator, combined with optical microscope (OM) and scanning electron microscope (SEM), the tensile fracture morphology and nearby microstructure were observed, and its high temperature plasticity mechanism was analyzed. The results show that the niobium microalloyed cold heading steel has two brittle temperature ranges: the first brittle zone is from the melting point to 1200°C, the temperature range of the third brittle zone is below 700°C, and the optimum plasticity zone is 700-1200°C. At 650°C, the fracture mode is ductile fracture and cleavage fracture; at 800°C, it is a typical cleavage-brittle fracture type; at 950°C, the fracture mode is dimple fracture, with good plasticity; at 1100°C, the particles become finer and the plastic deformation is more uniform. In order to avoid the occurrence of cracks in the continuous casting billet, the straightening temperature of the continuous casting billet should be higher than 700°C, it should be heated slowly in the early stage of heating, and the hot rolling temperature should be controlled at 700-1100°C.