Nb和Nb-Ti系微合金耐候钢的连续冷却转变与形变热模拟

Continuous cooling transformation and thermal-mechanical simulation of Nb and Nb-Ti microalloy weathering steels

在Gleeble-3500热模拟试验机上对Nb和Nb-Ti微合金耐候钢进行连续冷却转变与形变热模拟试验,观察比较两类钢显微组织、析出相与力学性能;利用TEM观察第二相粒子析出物状态。结果表明:随着冷却速度的提高,组织将由珠光体向贝氏体到马氏体转变。当冷却速度达到5℃/s时Nb微合金钢就发生马氏体转变;而Nb-Ti微合金钢在10℃/s才获得马氏体组织,即马氏体转变延迟。形变热模拟试验中,Nb-Ti微合金钢获得的铁素体更加细小,提高了材料的低温韧性;在TEM观察下,Nb C和(Nb、Ti)C粒子在晶内与晶界上是随机分布的,其中Nb C粒子尺寸大约20 nm,体积分数约为0.6%,(Nb、Ti)C粒子尺寸大约75 nm,体积分数约为1.33%;Nb-Ti微合金钢的硬度比Nb微合金钢的硬度值更高,说明粒子体积分数比粒子尺寸对硬度的贡献更大。

The continuous cooling transformation and thermal-mechanical simulation of Nb and Nb-Ti microalloying steels were conducted on a Gleeble-3500 thermal simulator,and the microstructure,precipitation and mechanical properties of the two steels were observed and compared.The secondary phase particle precipitates were observed by TEM.The results show that the microstructure changes from pearlite to bainite to martensite as the cooling rate increases.The martensite transformation occurs in the Nb microalloy steel when the cooling rate reaches 5℃/s,while that occurs in the Nb-Ti microalloy steel at 10℃/s,i.e.,the martensite transformation is delayed.In the thermalmechanical simulation test,the ferrite size obtained in the Nb-Ti steel is smaller and the low temperature toughness of the material is improved.Under TEM observation,Nb C and(Nb,Ti)C particles are randomly distributed within the grains and at grain boundaries,where the Nb C particle size is 20 nm,the volume fraction is 0.6%,and the(Nb,Ti)C particle size is 75 nm with the volume fraction being1.33%.The hardness of the Nb-Ti microalloying steel is higher than that of the Nb microalloying steel,which indicates that the volume fraction contributes more to hardness than the particle size.