500 MPa级Nb Ti微合金化方矩形管用钢的强化机制

采用光学显微镜和透射电子显微镜等对500 MPa级Nb Ti微合金化方矩形管用钢的组织与性能进行了分析，研究了其强化机制。结果表明，终轧温度和卷取温度对试验钢的组织和力学性能有显著影响，在研究的温度范围内，终轧温度和卷取温度的降低均有利于获得更加细小的铁素体晶粒与细小弥散的第二相析出物；当卷取温度不变时，随着终轧温度的下降，屈服强度、抗拉强度和断后伸长率均升高；当终轧温度不变时，随着卷取温度的逐渐下降，屈服强度和抗拉强度呈现出先上升后下降的规律，而断后伸长率呈现出单调上升的规律；试验钢在终轧温度为840 ℃和卷取温度为570 ℃时可获得最优的综合力学性能，其屈服强度和抗拉强度分别为537和578 MPa，断后伸长率为33.5%；细晶强化是试验钢最主要的强化机制，由晶粒细化引起的强度增量占总强度的49%～51%，由固溶强化引起的强度增量次之，占总强度的23%～27%，由析出强化引起的强度增量较小，仅占总强度的3.8%～8.2%。

Strengthening mechanism of 500 MPa grade Nb Ti #br# microalloyed square pipe steel

The microstructure, properties and strengthening mechanism of 500 MPa grade Nb Ti microalloyed square pipe steel were investigated by utilizing the optical microscope (OM) and transmission electron microscopy (TEM). The results showed that the finish rolling temperature (FDT) and the coiling temperature (CT) had considerable influences on the microstructure and mechanical strength. The decrease of the FDT and CT was benefitted to obtain the finer ferrite grain size and finer dispersion of the second phase precipitate in the studied temperature range. The yield strength, tensile strength and elongation rate were increased with the decrease of the FDT when the CT was constant. The yield strength and tensile strength displayed the tendency of declining at the beginning and rising up in late, and the elongation rate was monotonically increased as the CT decreased gradually. The optimal mechanical properties were obtained when the FDT and CT were 840 and 570 ℃, respectively, and the yield strength, tensile strength and elongation rate were 537 MPa, 578 MPa and 33.5%. The grain refinement strengthening was the most important strengthening mode, which accounted for 49% to 51% of the yield strength, and the solid solution strengthening was the second main strengthening mode, which accounted for 23% to 27%, while the precipitation strengthening was only accounted for 3.8% to 8.2% of the yield strength.