Effect Of Niobium And Hot Deformation On Microstructure Transformation Of High Carbon Steel During Continuous Cooling

Medium-high carbon pearlitic steels are widely used to manufacture rail steels、 section steels、hard wire、spring steels and so on because of their high strength, high hardness、good wear-resistant quality and fatigue resistance. Microalloy elements can improve the properties of steel without changing its principal properties. In this paper, the research progress relevant to the influence factors on the structure and properties of medium or high carbon steels and niobium micro-alloying and austenite deformation in high carbon steels have been summarized comprehensively. And based on those views, effect of niobium on the structure and properties of medium carbon and high carbon steels has been studied through differert niobium content added into the high carbon steels. Besides, the effect of niobium on the CCT curves of the test steels has been researched by different heating, deformation and cooling in Gleeble1500thermo-mechanical simulator. And the effects of niobium micro-alloying and thermal deformation on structural transformation of the test steels have been investigated.The results show that during the same forging and normalizing conditions, niobium can increase the precipitation of pro-eutectoid ferrite and significantly improve the toughness of high carbon steels with little effect on their strength. When the heating temperature is lower than1150℃, niobium could remarkably hinder the grain coarsening of austenite.The experimental results showed that the addition of niobium can increase the incubation period of pearlite nucleation, and thus depress the start temperature of pearlite transformation. The experimental results indicate that the CCT curve of the steel with niobium moves to right lower compared to the steel without niobium. The addition of niobium to the high carbon steel has three effects:(a) suppressing the austenite grain growth and increase the crystallographic plane, which results in the increase of density of nucleation sites and the nucleation rate;(b) inhibiting the diffusion rate of carbon and decrease the growth rate of pearlite;(c) increasing the phase transformation undercooling and phase transformation speed. The resultant effect is:the transformation time of the high carbon steel with niobium and the niobium free steel is the same, which is manifested as that the width of the average transition temperature range of pearlite transformation has not an obvious change; the transformation time of the high carbon steel containing higher niobium content is extended compared to the steel with less content of niobium, which is presented as that niobium the transition temperature range on CCT curve of test steel containing. At the same cooling rate, Nb can decrease the lamellar spacing of pearlite.Thermal deformation benefits to microstructure refinement of high carbon steels and push CCT diagrams moving to left and upper, and thus increase the phase transition temperature of pearlite. Meanwhile, thermal deformation is beneficial to reduce the formation of low temperature structure. However, it could increase the lamellar spacing of pearlite and improve the precipitation of pro-eutectoid ferrite. Consequently, this would have an noticeable impact on mechanical properties of high carbon steel, such as the decrease of the strength, the increase of plasticity, and the corresponding increase of toughness of high carbon steel containing a little of ferrite. During the thermal deformation process and the subsequent cooling process, the microstructure of the high carbon steel will change a lot in comparison with the niobium free steel, and thus have significant influence on the properties of steel. Therefore, content, austenite deformation and cooling rate are needed to be properly matched in order to obtain high carbon steels with excellent properties.