退火工艺对COST-FB2钢组织和硬度的影响

Influence of annealing process on hardness and microstructure of COST-FB2 steel

利用光学显微镜、扫描电镜、透射电镜和硬度测试等方法研究了再结晶退火与热装等温退火工艺对COST-FB2钢显微组织及硬度的影响。结果表明:采用650~700℃再结晶退火,随着退火温度提高,马氏体板条逐渐回复,M23C6型碳化物尺寸增大,硬度下降;750~800℃区间发生再结晶,硬度最低;850℃退火时进入二相区,冷却过程产生二次马氏体,硬度显著回升。延长700℃再结晶退火时间,硬度缓慢下降,16 h后基本保持不变。采用700℃热装等温退火时,8 h后开始发生铁素体转变,硬度明显下降,100 h后铁素转变完全,硬度最低,随后基本保持稳定。热装退火的软化效果优于再结晶退火。

Effect of recrystallization annealing and isothermal annealing processes on hardness and microstructure evolution of COST-FB2 steel was experimentally investigated by using OM, SEM TEM and hardness tester. The results show as follows. For specimens recrystallization annealed at 650-700 ℃,higher temperature results in martensite laths recovery,larger size of M23 C6 carbides and lower hardness value. In addition,specimens recrystallization annealed at 750-800 ℃ shows lowest hardness for reason of recrystallization.Moreover,due to dual phase region quenching and the formation of secondary martensite during cooling process,the hardness increased again for specimen annealed at 850 ℃. Prolonged recrystallization annealing time at 700 ℃ brought out gradually decreased hardness deceases,and the hardness value kept almost constant after annealing for 16 h. Additionally,for specimens isothermal annealed at 700 ℃,ferrite microstructure could be seen in specimen annealed for 8 h,which means the initiation of ferrite transformation. Longer annealing time leads to lower hardness,and the hardness value of this steel kept the same after annealed for 100 h,when full ferrite transformation could be achieved. Compared with recrystallization annealing,isothermal annealing exhibits better softening effect for COST-FB2 steel.