NM500耐磨钢拉伸过程中TiN的破碎机制

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

采用SEM、EDS、TEM和EBSD技术结合热力学理论计算研究了NM500耐磨钢中微米级TiN的析出规律、破碎机制,以及基体对破碎机制的影响。结果表明,NM500钢拉伸断裂机制为混合模式,断口面微米级TiN存在2种破碎形貌:TiN处于断口表面,自身处于撕裂脊上;TiN处于深韧窝底部。钢中Ti元素在高温液态析出,形成大量微米级TiN颗粒,在受拉应力作用时出现3种破碎机制:TiN内单条裂纹萌生并扩展至基体,TiN内单条裂纹萌生但在基体处止裂,TiN内萌生多条裂纹并在基体处止裂。NM500耐磨钢中存在高应变区与微米级TiN,且原奥氏体晶粒粗大,TiN上产生裂纹后,基体止裂能力较差,从而使裂纹极易在基体上延伸。当存在多个TiN团簇时,裂纹连成一片形成薄弱带,从而使钢的塑性变差。

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 clusters are formed,the cracks are connected into one piece to be a weak band,leading to a poor plasticity to the steel.