Influence of martensitic multi-level structure and TiN inclusions on fracture toughness of 20CrMnTi steel

doi:10.13251/j.issn.0254-6051.2025.02.007

Abstract

Abstract: Fracture toughness (K_IC) of 20CrMnTi steel was tested by using three-point bending specimens, and it was found that the value of K_IC decreases with grain coarsening, which contradicts the relationship between the K_IC and the grain size of other low carbon lath martensitic steels. Based on this, the fracture behavior of the 20CrMnTi steel was analyzed by means of OM, EBSD, SEM and thermodynamic model, fracture mechanics model. The results show that the relationship between the multi-level microstructure of the 20CrMnTi steel and K_IC established based on the Hall-Petch equation reveals that the martensitic lath block is the effective grain for fracture toughness. Still, the fracture morphology analysis reveals that the effective grains in the coarse or fine grain states of the 20CrMnTi steel are different. Further analysis reveals the presence of a certain number of irregular TiN inclusions on the fracture, and thermodynamic calculations show that the TiN inclusions are formed in the liquid-phase region with coarse size, which can easily lead to crack initiation. The crack propagation is analyzed with the help of SEM and fracture mechanics calculations, revealing that the crack propagation in the fine grain state of the 20CrMnTi steel is zigzagging and cross-cutting the lath martensite packet, which leads to a better fracture toughness, whereas the crack path in the coarse grain state is straight and shows a poor toughness. Therefore, the formation of TiN inclusions is the key factor leading to the abrupt change in fracture toughness of the 20CrMnTi steel, and the results of this study are of great significance in guiding the preparation of the high-performance 20CrMnTi steel.

Key words: low carbon 20CrMnTi steel, lath martensite, multi-level structure, TiN, fracture toughness

CLC Number:

TG142.1⁺3

Long Shaolei, Zhu Dandan, Luo Xianglan, Yi Yanliang, Yang Ming, Lu Yemao, Liang Yilong. Influence of martensitic multi-level structure and TiN inclusions on fracture toughness of 20CrMnTi steel[J]. Heat Treatment of Metals, 2025, 50(2): 43-51.

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