Nb含量对V-Ti-N工程结构用钢连续冷却转变规律及组织性能的影响

Effect of Nb content on continuous cooling transformation rule,microstructure and properties of V-Ti-N structural steel

利用淬火膨胀仪DIL-805研究了Nb-V-Ti-N微合金钢的连续冷却转变规律,并探讨了Nb含量对钢组织和性能的影响。结果表明,当冷速由0.1℃/s增大至30℃/s时,发生过冷奥氏体向铁素体、珠光体、贝氏体和马氏体转变,冷速范围分别为0.1~20、0.1~5、1~30和10~30℃/s,且铁素体、珠光体和贝氏体的转变温度随冷速的增大而降低,而马氏体转变温度升高;当Nb含量由0.05%(质量分数)增加到0.10%时,奥氏体中Nb的固溶量增加,CCT曲线略向下移动,Ac1和Ac3温度均有所提高。试验钢的显微硬度随冷速的增加不断增大,且当冷速为1~20℃/s时,0.10%Nb试验钢的显微硬度高于0.05%Nb试验钢。这主要取决于钢中硬化相的占比、组织的细化程度及碳氮化物的析出。此外,高温奥氏体区析出的亚微米级富Nb碳氮化物不仅能够钉扎晶界而且可作为异质形核核心诱导晶内铁素体的形成,二者共同作用细化组织,提升强韧性。

Continuous cooling transformation curves of the Nb-V-Ti-N microalloyed steels were conducted on a DIL-805 dilatometer,and the effect of Nb content on the microstructure and properties was investigated.The results show that when the cooling rate increases from 0.1℃/s to 30℃/s,the undercooled austenite successively transforms into ferrite,pearlite,bainite and martensite,and the corresponding cooling rate ranges are 0.1-20,0.1-5,1-30 and 10-30℃/s,respectively.The transformation temperatures of ferrite,pearlite and bainite decrease with the increase of cooling rate,while that of martensite increases.As the Nb content increases from 0.05%to 0.10%,the solid solution of Nb in austenite increases,the CCT curves are slightly downward,and both Ac_1 and Ac_3 temperatures increase.With the increase of cooling rate,the microhardness is enhanced,and the corresponding value in the range of 1-20℃/s for the tested steel containing 0.10%Nb is higher than that for the steel with 0.05%Nb,which is largely related to the percentage of hardening phases,the degree of microstructure refinement and the precipitation of carbonitrides.In addition,the submicron Nb-rich carbonitrides precipitated at the high temperature region of austenite can not only pin grain boundaries but also act as heterogeneous nucleation sites to induce the formation of intragranular ferrite,which effectively refine the microstructure and consequently improve the strength and toughness.