时效处理对极低碳针状铁素体钢组织和力学性能的影响

EFFECTS OF AGING ON MICROSTRUCTURE AND MECHANICAL PROPERTY OF ULTRALOW CARBON ACICULAR FERRITE STEEL

利用光学显微镜、扫描电镜和透射电镜研究了变温时效对极低碳针状铁素体钢组织和性能的影响规律.结果表明:450—500℃时效处理时,第二相析出较少,组织无明显粗化,强度无明显变化;550—650℃时效时,强度快速上升,主要是ε-Cu和Nb(C,N)第二相粒子的弥散析出所致;650℃以上时效时,发生过时效现象,铁素体板条和第二相粒子粗化,导致强度迅速下降;时效温度达到750℃时进入两柑区,局部富C和合金元素富集区域先奥氏体化,时效后冷却过程中形成了高碳的M/A组织,使钢的韧性迅速恶化.

The effects of aging temperature on the microstructure and mechanical property of ultra-low-carbon acicular ferrite steel was studied by utilizing OM, SEM, TEM, tensile and impact tests. The strength of the specimen is not obviously changed due to little precipitating amount of the second phase and little coarsening of microstructure when aged from 450 to 500 ℃. The rapid increase in strength appears and is attributed to the dispersive precipitations of ε-Cu and Nb（C, N） when aged from 550 to 650 ℃. When aged above 650 ℃, the abrupt decrease in strength appears and is caused by over-aging, i.e., the coalescence of ferrite plates and coarsening of precipitates. When aged at 750 ℃ （in two phase field）, the toughness decreases abruptly, which is related to the formation of martensite/austenite （M/A） with higher carbon content during cooling.