| The 30CrNi3MoV steel is a microalloyed Cr-Ni-Mo ultra-high strength steel by vanadium addition. The phase transformation behavior from the undercooled austenite in continuously-cooled 30CrNi3MoV steel was systematically studied by means of high-resolution dilatometric measurements and microstructural analysis. The formation mechanism of granular bainite, incompleteness phenomenon of bainitic transformation, effect of thermo-mechanical treatment on bainite transformation and the characteristics of martensitic transformation in 30CrNi3MoV steel were investigated, the conclusions were as follows:(1) The transformation behavior of 30CrNi3MoV steel in the process of continuous cooling from the austenite was systematically investigated. All the possible transformations in the experimental steel cooled at 1 ~ 2000℃/min were clarified, and the corresponding CCT diagram was constructed. It shows that the transformation products from the austenite of 30CrNi3MoV steel evolves as"granular bainite→low bainite plus a small amount of upper bainite→lath martensite and acicular martensite"with the increase of the applied cooling rate from 1 to 2000℃/min. In the CCT diagram, no pearlite transformation was detected in the explored 30CrNi3MoV steel except for forming bainite, martensite and a little pre-eutectoid ferrite. The critical cooling rate for the martensite transformation is in the range of 20～25℃/min.(2) Two kinds of granular bainites (named as Bg1 and Bg2, respectively) were formed in the investigated 30CrNi3MoV steel with slow cooling from the high-temperature austenite field at rates from 1 to 5 oC/min. All secondary particles existing in the Bg1 tend to be irregular, which results from the formation and growth of ferrite in an equiaxed way from carbon-poor austenite areas. Granules in the Bg2 are parallel to each other at some preferred orientations, and the massive matrix form by merging of the ferritic laths. Incomplete bainite transformation phenomenon occurs generally in the continuously-cooled 30CrNi3MoV steel, leading to the formation of small carbon-rich retained austenite region after the completion of bainite transformation.(3) The deformation of austenite in the un-recrystallizing stage has remarkable influence on the characteristics of bainitic transformation in the explored 30CrNi3MoV steel. The Bs temperature of the 30CrNi3MoV steel with deformation in austenite increased greatly due to the additional transformation driving force from the high storaged energy in deformed austenite. The size of bainite laths in the 30CrNi3MoV steel decreased remarkably with the deformation in austenite, and their distribution exhibit more orientations.(4) The martensitic transformation in the explored 30CrNi3MoV steel was investigated through kinetic analysis and the microstructure observation in this part. There are two kinds of martensite with different morphology in the quenched 30CrNi3MoV steel, that is, lath martensite and plate martensite. Redistribution of carbon atoms occurred in the process of martensite transformation in the 30CrNi3MoV steel, which results in the detachment between the formation of lath martensite and plate martensite on the kinetic curve of martensite transformation.(5) The size of the austenite grains begin to coarsen when the austenization temperature exceeding 1000℃. The Ms temperature of the 30CrNi3MoV steel increases with the increasing austenization temperature when lower than 950℃, but begins to decrease below that. The great increase of austenization holding time at 900℃will not lead to the obvious growth of the austenite grains. The Ms temperature of the 30CrNi3MoV steel increases remarkably with the extension of holding time at austenzation temperature, but do not increase homogeneously with the increasing grain size of the austenite obtained by holding different time.
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