Mn12Ni2MoTi(Al)钢中Laves析出相的演变规律

The Evolution of Laves Precipitates in Mn12Ni2MoTi(Al)Steel

采用X射线衍射仪(XRD)、扫描电镜(SEM)、透射电镜(TEM)、硬度测试和拉伸实验,对冷轧变形量为65%和80%的Mn12Ni2MoTi(Al)钢经745℃不同时间退火后的微观组织和力学性能进行表征,研究了Laves相的形核析出和粗化动力学特征,评估了Laves相的演变对晶粒细化的影响.结果表明:细小的六角结构(Fe,Mn)2(Mo,Ti)型Laves相在退火5min时已经大量析出;随退火时间的延长,Laves相颗粒不断长大,其体积分数在1 h内快速升高然后缓慢增加,8 h左右达到最大值,之后保持不变.Laves相优先在晶界析出,并钉扎晶界.Laves相对晶界的钉扎力随退火时间的延长先增加后降低,在退火4 h时达到最大值.另外,Laves相随着冷轧预变形量增加,其粗化速率增加,在相同退火条件下其对晶界钉扎力减小.尽管退火时间的延长或者预变形增大使得Laves相钉扎力减小,但依然能有效地钉扎晶界,阻碍晶粒粗化,这极大地提高了实验钢的热稳定性.对65%冷轧变形材料而言,8 h退火后使再结晶晶粒仍保持在亚微米级别.这种具有亚微米级晶粒的Mn12Ni2MoTi(Al)双相钢同时具有高屈服强度和良好延伸率,其强度和塑性均远高于同材质淬火马氏体钢.

Mn12Ni2MoTi(Al)steels were fabricated by cold rolling with total reductions of65%and80%,respectively,and subsequently annealed at745℃for different times.The microstructures and mechanical properties of samples were investigated using X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),and hardness and tensile tests.The nucleation and coarsening kinetics of Laves phase as well as its influence on grain refinement were also examined.The results show that the fine(Fe,Mn)2(Mo,Ti)Laves phases with hexagonal structure have precipitated during annealing for5min.With the increase of annealing time,the Laves particles gradually grow.The volume fraction of Laves particles increases rapidly in the first hour of annealing,and then slowly increases to the maximum value at8h.Afterwards,the volume fraction of Laves particles almost keeps unchanged.The Laves phases firstly precipitate at grain boundary,and then pin the grain boundary.The pinning pressure of Laves phase particles first increases and then decreases with the annealing time.When the annealing time is4h,the pinning pressure reaches the maximum value.Besides,the larger pre-deformation of cold rolled samples results in the higher coarsening rate of Laves phases.At the same annealing condition,the larger pre-deformation of samples leads to the smaller pinning pressure of Laves phases.Although the long-time annealing or large pre-deformation makes the pinning pressure provided by Laves phase particles to be small,Laves phase particles still can effectively pin the grain boundaries,which greatly improve the thermal stability of the recrystallization grain.For the65%CR sample,the recrystallization grain still remains at sub-micron level after annealing for8h.The Mn12Ni2MoTi(Al)duplex steel with submicron recrystallization grain has both high yield strength and good elongation,which is much higher than that of quenched martensitic samples.