Nb对奥氏体热变形后等温回复的影响

采用热模拟实验研究了不同Nb含量的低C高Mn钢在800—900℃变形后奥氏体的回复特征,同时借助Fe-40%Ni-0.1%Nb(质量分数)合金揭示了回复过程中的位错演化及析出行为,建立了位错滑移及溶质拖曳机制的等温回复动力学模型,据此计算拟合了应力松弛曲线上回复实验数据,计算结果与理论分析及实验结果相符.实验及模拟结果表明,Nb溶质拖曳及析出均减慢回复过程,提高变形积累;与Nb溶质拖曳相比,析出能够更有效地延缓回复软化;Nb溶质拖曳通过升高回复激活自由能U0及减小激活长度来实现回复过程的延缓,提高溶质Nb含量,将升高Uo和减小激活长度.对于含Nb低C高Mn微合金钢,在道次间隔短的多道次热连轧精轧阶段,变形积累主要依靠Nb溶质拖曳作用,而对于轧制节奏较慢的中厚板精轧,轧制变形的积累依靠Nb溶质拖曳与析出的共同作用.

EFFECTS OF Nb ON RECOVERY OF HOT-DEFORMED AUSTENITE

Solute and precipitates of Nb can effectively affect stastic recrystallization and recoverry of austnite in steels during hot rolling process.However,more research is concerned about the role of Nb precipitation on the strain accumulation in finish rolling process,the solute drag effect of Nb is neglected comparing with precipitates.In this paper,the stress-relaxation curves of the low C high Mn steels with different Nb content were investigated by thermal simulation test,the evolution of dislocation and its interaction with Nb solute and precipitate during recovery process of deformed autentie in a Fe-40%Ni-0.1%Nb（mass fraction） modle steel was also studied by transmission electron microscopy（TEM）.Thereby,a theoretical model about recovery of deformed austenite was developed according to the slip of dislocations and the solute drag.The values calculated by the model are consistent with the experimental results and the metallurgic principles.It is shown that both solute and precipitation of Nb can slow down the recovery and enhance the strain accumulation.The Nb solute drag can increase the activation free energy of the recovery U₀ and decrease the activation length.It is believed that for Nb micro-alloyed steels with low C and high Mn,the strain accumulation during finish rolling process would be relied on the Nb solute drag effect in hot-strip mill,and both solute drag and precipitation pin effects in heavy plate mill.