冷却方式对Nb-Ti微合金钢组织和性能及沉淀行为的影响

两阶段控制轧制后,采用不同的冷却路径进行冷却,研究冷却路径对Nb-Ti微合金钢组织和性能及沉淀行为的影响.结果表明,超快冷+空冷冷却路径可获得细晶组织,晶粒平均尺寸约为7.76μm,屈服强度高达425 MPa,抗拉强度高达500 MPa.超快冷+炉冷试样中存在细小的沉淀粒子,沉淀粒子尺寸主要集中在2—7 nm,而超快冷+空冷试样中只存在少量球形沉淀粒子,轧后直接空冷可获得相间沉淀粒子.不同冷却路径获得的热轧板在700℃下退火300 s后,沉淀粒子发生明显的粗化;退火处理后,超快冷+炉冷试样的晶粒平均尺寸减小为6.47μm,相对于退火前,其屈服强度和抗拉强度分别增加50和30 MPa、强度的增加主要源于细晶强化.对于含0.03%Nb(质量分数)的Nb-Ti微合金钢,由于沉淀粒子的体积分数有限,因此细晶强化效果远高于沉淀强化效果,强度的变化与晶粒尺寸的变化具有很好的对应性.另外,加工硬化指数与晶粒尺寸密切相关.随着晶粒平均尺寸的增加使加工硬化指数增加.

EFFECTS OF COOLING PROCESS ON MICROSTRUCTURE,MECHANICAL PROPERTIES AND PRECIPITATION BEHAVIORS OF NIOBIUM-TITANIUM MICRO-ALLOYED STEEL

The tested steels were cooled to room temperature using different cooling paths after two-stage rolling,and effects of cooling paths on microstructure,mechanical properties and precipitation behaviors of Nb-Ti micro-alloyed steels were investigated.The results show the hot rolled plates with fine grain were produced at the cooling path of ultra fast cooling + air cooling, and the average grain size,lower yield strength and ultimate tensile strength are about 7.76μm, 425 MPa and 500 MPa,respectively.The fine precipitation particles ranging from 2 nm to 7 nm were observed in the samples cooling with ultra fast cooling + furnace cooling,but are only a few globular precipitates in the samples cooling with ultra fast cooling + air cooling.The inter-phase precipitation was observed in samples cooling with air cooling after finish rolling.These plates with different cooling paths were annealed at 700℃for 300 s.The precipitation particles were obviously coarsened during annealing.It can be found that the average grain size of the samples with cooling path of ultra fast cooling + furnace cooling is 6.47μm and the increments of lower yield strength and ultimate tensile strength are about 50 and 30 MPa,respectively.The strength increment mainly depends on fine grain strengthening.For niobium-titanium micro-alloyed steels containing 0.03%Nb(mass fraction),because the volume fraction of precipitates is limited,grain boundaries strengthening is higher than precipitation hardening,making changes of strength be in good agreement with that of grain size.In addition,the strain hardening exponent is mainly related to average grain size,and strain hardening exponent increases with average grain size increasing.