600 MPa级Nb-Ti微合金化高成形性元宝梁用钢的强化机制

Strengthening Mechanism of 600 MPa Grade Nb-Ti Microalloyed High Formability Crossbeam Steel

采用OM、SEM和TEM等方法,对600 MPa级Nb-Ti微合金化高成形性元宝梁用钢的组织与力学性能进行了测试表征,并分析了强化机制。结果表明,终轧温度对实验用钢的组织与力学性能有显著影响,随着终轧温度的降低,钢中铁素体晶粒尺寸逐渐减小,位错密度逐渐增加,析出物尺寸逐渐减小、数量逐渐增多、Nb/Ti原子比逐渐增大,屈服强度与抗拉强度均呈现出单调上升的规律,而延伸率存在一个最佳温度,终轧温度为840℃时具有最优的力学性能,其屈服强度与抗拉强分别达到了541与615 MPa,延伸率为31.0%,-60℃冲击功为117 J。(Nb,Ti)C在奥氏体中析出的Nr T与PTT曲线表明,在实验温度范围内,均匀形核与位错线形核的形核率随温度的降低而提高,形核孕育时间随温度的降低而缩短,这与观察到的析出物尺寸随着终轧温度的降低而减小、析出物的数量随着终轧温度的降低而增多的规律相符。细晶强化与位错强化是实验用钢主要强化方式,细晶强化占总屈服强度的46%~48%,位错强化占总屈服强度的18%~25%,析出强化对屈服强度的贡献较小,约2%左右。

Automobile beam steel with high strength is the development trend of the automotive in- dustry, which will help to conserve resources and protect the environment. The crossbeam as an impor- tant part of the frame, its strength also affects the overall strength of the frame, which will affect the safety performance of vehicles. At present, the crossbeam steel of heavy-duty vehicles is mainly Q235B and Q345C. About 3% of the crossbeams occur cracking problems when heavy-duty vehicles drive about 10000 km or about half a year or so. The strength increasing will cause the cracking in the forming pro- cess. Therefore, it is of great significance to develop a high strength crossbeam steel with high fatigue and high formability. In this work, the microstructure, properties and strengthening mechanism of 600 MPa grade Nb-Ti microalloyed high formability crossbeam steel were investigated by OM, SEM and TEM. The results show that the finish rolling temperature has a considerable influence on the microstruc- ture and mechanical strength, with the decreasing of finish rolling temperature, the ferrite grain size and the size of precipitates decreases gradually; dislocation density, the number of precipitates and the ratio of Nb/Ti increase gradually; both the yield strength and tensile strength increase monotonously, while the elongation has an optimum temperature. The optimal mechanical properties were obtained when the fin- ish rolling temperature is 840℃, and the yield strength, tensile strength, elongation and impact energy at -60 ℃ reached 541 MPa, 615 MPa, 31.0% and 117 J, respectively. The Nr＇l- and PTT curves of （Nb, Ti） C precipitated in the austenite showed that the nucleation rate of uniform nucleation and dislocation lin- ear nucleation increases and the nucleation time is shortened with the decrease of temperature in the ex- perimental temperature range, which is consistent with the observation result that the size of precipitates decreases with the reduction of the finish rolling temperature, while the number of precipitates increases. The grain refinement strengthening and dislocation strengthening are the main strengthening modes of the steel, the grain refinement strengthening accounts for 46%-48% of the yield strength, the dislocation strengthening accounts for 18%-25%, while the precipitation strengthening only contributes 2% of the yield strength.