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 Ac1 and Ac3 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.
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