500 MPa级高延性方管用钢的开发及加工硬化行为

 为了开发满足二次加工性能要求的500 MPa级高延性方管用钢,采用OM、SEM和TEM等对500 MPa级高延性方管用钢制管前后的组织与性能进行分析,研究了其强化机制与加工硬化机理。结果表明,两种试验钢的组织均由铁素体和少量珠光体组成,低C-低Mn-Nb、Ti微合金化试验钢铁素体晶粒与珠光体球团尺寸更加细小,第二相析出物尺寸稍大,位错密度相似。两种试验钢制管前力学性能相似,低C-低Mn-Nb、Ti微合金化试验钢屈强比较高;制管后低C-低Mn-Nb、Ti微合金化试验钢加工硬化程度显著,屈服强度、抗拉强度分别增加了45与26 MPa,伸长率降低6.0%,高C-高Mn-Nb微合金化试验钢屈服强度、抗拉强度分别增加了22与10 MPa,伸长率降低4.0%。固溶强化与细晶强化是两种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52.9%~61.8%,由固溶强化引起的强度增量占总强度的17.2%~25.3%;析出强化与位错强化对强度的贡献较小。制管后低C-低Mn-Nb、Ti微合金化试验钢位错强化增加显著,达到了82 MPa,明显高于高C-高Mn-Nb微合金化试验钢位错强化的贡献(65 MPa);对于制管用途而言,高C-高Mn-Nb微合金化试验钢制管后综合力学性能更加优异。

Development of 500 MPa grade high ductility square tube steel and its work hardening behavior

In order to develop a 500 MPa grade high ductility square tube steel that meets the requirements of secondary processing,the microstructure,properties,strengthening mechanism and work hardening mechanism of 500 MPa grade high ductility square tube steel were investigated by utilizing optical microscope(OM),scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results show that the microstructures of the two tested steels are composed of ferrite and a small amount of pearlite. Compared with the high C-high Mn-Nb microalloyed test steel,the size of ferrite and pearlite is finer,the size of the second phase precipitates is larger,and the density of dislocation is similar. The mechanical properties of the tested steels are similar before forming,except for the yield ratio of the low C-low Mn-Nb and Ti microalloyed test steel is higher. The low C-low Mn-Nb and Ti microalloyed test steel has a significantly work hardening effect after forming,the yield strength and tensile strength increased by 45 and 26 MPa,respectively,while the elongation decreased by 6.0%. The yield strength and tensile strength of the high C-high Mn-Nb microalloyed test steel increased by 22 MPa and 10 MPa,respectively,and the elongation decreased by 4.0%. The grain refinement strengthening is the most important strengthening modes,the grain refinement strengthening accounts for 52.9%-61.8%,the solid solution strengthening is the second main strengthening mechanism,the solid strengthening accounts for 17.2% to 25.3%,while the precipitation strengthening and dislocation strengthening contributes little to the yield strength. The dislocation strengthening increased to 82 MPa of the low C-low Mn-Nb and Ti microalloyed test steel after forming,which is higher than that of the high C-high Mn-Nb microalloyed test steel. Hence,the high C-high Mn-Nb microalloyed test steel is more suitable for the preparation of high ductility automotive square tube in view of the comprehensive mechanical properties as compared to the other one.