In-situ observation for effect of niobium on pearlite transformation in high-carbon steels
SU Xue1,2, WANG Hou-xin3, ZHU Min4, ZHANG Qi1,2, TIAN Jun-yu2, XU Guang1,2
1. Collaborative Innovation Center of Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China; 2. State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China; 3. CITIC-CBMM Microalloying Technology Center, Beijing 100004, China; 4. Central Research Institute of Baoshan Iron and Steel Co., Ltd., (Qingshan), Wuhan 430081, Hubei, China
Abstract：In order to investigate the effect of niobium (Nb) on pearlite transformation, the dynamic nucleation and growth process of pearlite in high-carbon steels with and without Nb during continuous cooling was observed by in situ observation under laser scanning confocal microscope (LSCM). The results show that Nb addition increases the nucleation density of pearlite, which increases the number of nucleation sites. Besides, the addition of Nb significantly decreases the growth rate of pearlite due to the hindrance in growth of pearlite in high-carbon steels. However, the hindering effect does not further increase when mass percent of Nb exceeds 0.014%. It is known from above results that Nb addition promotes the nucleation but significantly hinders the growth of pearlite in high-carbon steels. Hence, two kinds of high carbon steel without and with mass percent of Nb of 0.027% were selected to perform thermal expansion experiments on Gleeble-3500 thermal simulation test machine in order to study the effect of Nb on pearlite phase transformation more accurately. The thermal expansion experiment shows that addition of Nb increases the degree of undercooling, leading to the reduction of pearlite transformation temperature range, but significantly hinders the diffusion of carbon in austenite, so the pearlite growth rate is decreased. Additionally, Nb reduces pearlite phase transition rate and delays pearlite transformation because it slows down the pearlite transformation kinetics under the continuous cooling condition, indicating that the inhibition effect of Nb on growth is stronger than its promotion effect on nucleation of pearlite. Therefore, the addition of Nb postpones pearlite phase transition in high carbon steel. Moreover, the addition of Nb increases the degree of undercooling, refines pearlite lamellas and improves the hardness of high-carbon steels, but when mass percent of Nb exceeds 0.014%, there is no further refinement effect.
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2022, Vol. 57 
