Mechanism of TiN Fracture During the Tensile Process of NM500 Wear-Resistant Steel

Low-alloy high-strength martensitic wear-resistant steel has been widely used in the field of construction machinery due to its low cost and excellent mechanical properties. Microalloying elements, especially Ti, B and other elements, have been widely used to improve the performance of low carbon steel. However, addition of Ti will cause micron-sized Ti precipitates in the continuous casting process, causing cleavage fracture. Therefore, it is necessary to study the micron-sized TiN to reduce its influence on the toughness of the material. SEM, EDS, TEM and EBSD methods were combined with thermodynamic theory to study the precipitation rule of micron-sized TiN in NM500 wear-resistant steel, the fracture mechanism and the influence of matrix on the fracture mechanism. The results show that the tensile fracture mechanism of NM500 steel is mixed mode. There are two fracture morphology of micronsized TiN on fracture surface: TiN is on the fracture surface, being on the tear ridge; TiN is at the bottom of a deep dimple. The Ti element in the steel precipitates at high temperature and forms a large number of micron-sized TIN. There are three kinds of fracture mechanisms in TIN when subjected to tensile stress: A single crack appears in TIN initiates and spreads to the matrix; A single crack appears in TiN initiates but stops at the matrix; A plurality of cracks are generated in the TiN, and the crack stops at the base, with the TiN shape being preserved intact. There are high strain zones and micron-sized TiN in NM500 steel, and the prior austenite grains are coarse. When the TiN cracks, the matrix has a poor ability to arrest the cracks, then the crack can extend on the substrate easily. When a plurality of TiN dusters are formed, the cracks are connected into one piece to be a weak band, leading to a poor plasticity to the steel.