铌微合金化极低屈服点钢的组织与性能

Microstructure and properties of Nb microalloyed ultra low yield point steel

通过超低碳加铌的成分设计和轧后软化热处理工艺成功制备出100 MPa级极低屈服点钢,研究了其组织与性能,并阐述了其软化机理。结果表明,软化热处理前后,试验钢组织均为单一的多边形铁素体,铁素体晶粒和Nb(C,N)析出相随软化热处理温度升高而粗化。特别地,950℃软化热处理时重新奥氏体化后的相变铁素体晶粒尺寸超过90μm。固溶和沉淀强化增量之和与屈服强度随软化热处理温度的变化曲线有很好的对应关系,屈服强度随软化热处理温度升高整体呈降低趋势,主要原因是沉淀强化和细晶强化增量减小,但碳、氮原子重新回溶引起屈服强度回升。试验钢经850℃软化热处理获得最佳综合力学性能,950℃软化热处理后的强塑性很好,但因晶粒粗大导致韧性极低。

The 100 MPa ultra low yield point steel was successfully prepared through the composition design of ultra-low carbon plus Nb and the post-rolling softening heat treatment,the microstructure and properties were researched,and the softening mechanism was discussed. The results showed that before and after softening heat treatment,the microstructure of test steel was single polygonal ferrite,ferrite grains and precipitated phase Nb(C,N)coarsen as the softening heat treatment temperature increases. In particular,the transformed ferrite grain size after re-austenitization at 950 ℃ heat treatment exceeded 90 μm. The sum of the solid solution and precipitation strengthening increments corresponded well with the variation curve of the yield strength at different softening heat treatment temperatures. The main reason for the yield strength decreases as the temperature of softening heat treatment increases was that precipitation strengthening and fine-grained strengthening increments both decreased,but C and N atoms re-dissolved to cause a rise in yield strength. The best comprehensive mechanical properties of test steel were obtained through softening heat treatment at 850 ℃,strength and plasticity of test steel both well after 950 ℃ softening heat treatment,but coarse grains resulted in low toughness.