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以抗拉强度700和780 MPa级新型热轧纳米析出强化钢为研究对象,通过裂纹尖端张开位移法实验评价其断裂韧性,探讨显微组织类型、大角度晶界、位错密度及纳米尺寸析出物对断裂韧性的影响机理.结果表明,实验温度为室温、-10和-30℃时,700 MPa级钢的条件启裂值δ_(Q0.2BL)和δ_(0.2)均大于780 MPa级钢,700 MPa,级钢的断裂韧性优于780MPa级钢.700 MPa级钢与780 MPa级钢的显微组织差异主要包含4个方面:(1)700 MPa级钢的显微组织以铁素体为主,而780 MPa级钢的显微组织以贝氏体铁素体为主;(2)700 MPa级钢中的碳化物形态为颗粒状或短棒状,而780 MPa级钢中的碳化物以长条状为主;(3)780 MPa级钢的位错密度显著高于700 MPa级钢;(4)700和780 MPa级钢中的大角度晶界比例分别为85.6%和76.8%.因此,提高铁素体体积分数和大角度晶界比例、细化碳化物尺寸及降低位错密度可有效提高钢板的断裂韧性;700和780 MPa级钢显微组织中粗大析出物(Nb,Ti)CN及晶界析出物会使钢板韧性恶化,铁素体或贝氏体基体上半共格析出的纳米尺度(Nb,Ti)C对韧性损害较小.
The new hot-rolled nano-precipitation strengthened steels with tensile strengths of 700 and 780 MPa were selected as the research object. The fracture toughness was evaluated by crack tip open displacement test. The effects of microstructure type, large angle grain boundary, dislocation density and nano-size The results show that the crack initiation values δ_ (Q0.2BL) and δ_ (0.2) of 700 MPa steels are both higher than 780 MPa at room temperature, -10 and -30 ℃ The fracture toughness of 700 MPa steel is better than that of 780 MPa steel.The microstructure difference between 700 MPa steel and 780 MPa steel mainly includes four aspects: (1) The microstructure of 700 MPa steel is composed of ferrite While the microstructure of 780 MPa grade steel is bainitic ferrite. (2) The carbide morphology of 700 MPa grade steel is granular or short rod shape, while the carbonization of 780 MPa grade steel (3) The dislocation density of 780 MPa grade steel is significantly higher than that of 700 MPa grade steel; (4) The large angle grain boundaries in 700 and 780 MPa grade steels are 85.6% and 76.8% . Therefore, increasing the volume fraction of ferrite and the ratio of large-angle grain boundaries, refining the carbide size and reducing the dislocation density can effectively improve the fracture toughness of the steel plate; 700 and 7 The coarse precipitates (Nb, Ti) CN and grain boundary precipitates in the 80 MPa grade microstructure will deteriorate the ductility of the steel sheet, and the semi-coherent precipitation of nanosize (Nb, Ti) C on the ferrite or bainite matrix Less damage to toughness.