原位观察铌对高碳钢珠光体相变的影响

 为了研究微合金元素铌(Nb)对高碳钢中珠光体相变的影响,在高温激光共聚焦显微镜下原位观察了不含铌和含铌高碳钢连续冷却过程中珠光体动态形核和长大行为。结果表明,在高碳钢中添加铌增加了珠光体形核点的数量,这是因为铌提高珠光体单位面积形核数量。同时,铌元素减慢珠光体长大速率是由于铌显著阻碍珠光体长大,但当铌质量分数超过0.014%后,阻碍珠光体长大速率的效果不再进一步增加。从以上结果可知,在高碳钢中添加铌促进珠光体形核,但是减慢珠光体长大速率。所以,为了更加准确地研究铌元素对珠光体相变的影响,选用不含铌及铌质量分数为0.027%的两种高碳钢,在Gleeble-3500热模拟试验机上进行与高温原位观察试验相同试验工艺的热膨胀试验。通过热膨胀试验发现,铌的添加增大过冷度,导致降低了珠光体相变温度区间,但是铌显著阻碍碳在奥氏体中的扩散系数,所以铌减慢珠光体长大速率。另外,铌减慢连续冷却条件下的珠光体相变动力学,推迟珠光体相变,从而降低珠光体相变速率,表明铌对珠光体长大的阻碍作用强于其对珠光体形核的促进作用。因此,在高碳钢中,铌元素的添加推迟珠光体相变。此外,铌的添加增大过冷度,使含铌高碳钢的珠光体片层细化,提高了含铌高碳钢的硬度,但在铌质量分数超过0.014%后,细化效果不再进一步增强。

In-situ observation for effect of niobium on pearlite transformation in high-carbon steels

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.