含Nb微合金钢在奥氏体区变形后等温弛豫可得到以贝氏铁素体为主的组织。弛豫时间适中, 析出颗粒基本分布在位错线上并钉扎位错。在随后的650和700℃再加热过程中, 非平衡组织向平衡组织演化. 弛豫60 s的样品热稳定性最高；弛豫1000 s的样品, 组织演化进行得最快, 再加热前的预应变可加速演化过程. 发生的组织演化是以板条内位错多边形化、板条间小角晶界逐渐消失和发生再结晶形成多边形铁素体的次序进行的。再加热等温过程伴随有硬度的起伏, 弛豫样品出现两个硬化峰。显微组织的热稳定性在很大程 度上取决于它的形成过程。
Water cooled treatment after isothermal relaxation of deformed austenite in a Nb--bearing microalloyed steel always induced a bainite ferrite dominated microstructure. Most of strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for proper time. When these samples were reheated and held at 650 or 700℃, the non--equilibrium microstructures tend to evolve into equilibrium ones. The sample relaxed for 60 s displays the highest thermostability, while the microstructure evolution is the quickest in the sample relaxed for 1000 s even though it is the softest before reheating. Pre--strain before reheating accelerates the evolution process. The evolution of microstructures during reheating and holding consisted of polygonization, small angle boundary between laths disappearing and recrystallization induced polygonal ferrite. Hardness measurement showed two peaks in hardness--time curve of each relaxed sample. These results indicated that thermostability of microstructures are mainly determined by their formation history.
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