显微组织对新型热轧纳米析出强化钢断裂韧性的影响

EFFECT OF MICROSTRUCTURE ON FRACTURE TOUGHNESS OF NEW TYPE HOT-ROLLED NANO-SCALE PRECIPITATION STRENGTHENING STEEL

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 fracture toughness of new type hot-rolled nano scale precipitation strengthening steels （tensile strength of 700 MPa grade and 780 MPa grade） were evaluated by crack tip opening displacement （CTOD） experiments, and the influence mechanisms of microstructure, high angle grain boundaries, dislocation density and nano scale precipitation on fracture toughness were discussed. The results indicated, when experimental temperature were room temperature, -10 and -30 1, the 5Q0.2BL value of 700 MPa grade carriage strip were 0.468, 0.333 and 0.248 mm, and the 5o.2 value of 700 MPa grade carriage strip were 0.298, 0.234 and 0.215 mm, respectively. However, the 5Q0.2BL value of 780 MPa grade crossbeam strip were 0.311, 0.290 and 0.247 mm, and the 50.2 value of 780 MPa grade crossbeam strip were 0.212, 0.212 and 0.198 mm, respectively. Therefore, the fracture toughness of 700 MPa grade steel was better than 780 MPa grade steel. The differences of microstructure between 700 MPa grade steel and 780 MPa grade steel mainly included four aspects： （1） the microstructure of 700 MPa grade steel was mainly ferrite, while the microstructure of 780 MPa grade steel was mainly bainitic ferrite; （2） the carbide shape of 700 MPa grade steel was granular or short rod, and 780 MPa grade steel was strip carbide; （3） the dislocation density of 780 MPa grade steel was significantly higher than 700 MPa grade steel; （4） the proportion of large-angle grain boundaries of 700 MPa grade steel and 780 MPa grade steel were 85.6% and 76.8%, respectively. Therefore, improving the volume fraction of ferrite and the proportion of high angle grain boundaries, refining carbide size and reducing dislocation density could effectively improve the fracture toughness of steels. Coarse precipitation （Nb, Ti）CN and grain boundary precipitation in microstructure deteriorated fracture toughness of steel, and semi-coherent precipitates nano scale （Nb, Ti）C on ferrite or bainite matrix have less damaging effect on fracture toughness.