Effects Of Al Element On Austenite Grain Growth And Continuous Slow Cooling Transformation Of High Carbon Steels

Cold drawn pearlite steel wire is widely used in the production of high-end metal wire such as steel wire for bridge cables,steel cords for automobile tires,and steel wire ropes.Adjusting the alloy composition and improving the heat treatment scheme have an extremely important role in the organization and properties of the wire rod and subsequent cold drawn steel wire.At the same time,the austenite grain size has an important influence on the final properties of the product.Exploring the microstructure parameters such as the original austenite grain size is helpful to understand the relationship between the microstructure and the microstructure of pearlite,To achieve targeted control of the microstructure of structural parts.Therefore,the law of change has received much attention.This paper designed a set of high carbon steel（Al-Free&1.5Al）,the austenite grain size was measured by oxidation method,and the continuous cooling transformation test was carried out by Thermecmaster thermal simulator.After heat treatment,the tested samples were analyzed by OM（optical microscopy）,SEM（scanning electron microscopy）,TEM（transmission electron microscopy）and XRD（X-ray diffraction）,phase diagram calculation et al.We investigated the influence Al element on the austenite grain size,and influence Al element on the continuous slow cooling transformation and microstructure of high carbon steels.The main results are as follows:1.The effects of Al elements on austenite grains were investigated by changing the heating temperature and heating time.The results show that the austenite grain size of both steels increases with increasing temperature or time,but the grain size and growth rate of 1.5Al steel are much smaller than that of Al-Free steel;And when the temperature is above 1000℃,the grain size of Al-Free steel appears to increase rapidly with the increase of temperature,while the 1.5Al steel still maintains a slow growth trend.It is indicated that the Al element has the effect of refining the austenite grains and suppressing austenite grain coarsening in the high temperature stage（above1000℃）of the high carbon steel.2.The addition of 1.5 wt.%Al increased the A_c11 point of the alloy system by 13°C and the A_c33 point by 71°C,which expanded theα+γtwo-phase region of the alloy system.The increase of eutectoid transformation temperature accelerates the diffusion rate of carbon in austenite,increased the driving force and thus the pearlite transformation was promoted.3.The continuous cooling transition process and microstructure of the two steels were studied by designing several different cooling rates（0.1,0.5,1.0,10°C/s）.The phase transition temperature was measured and its continuous cooling transition（CCT）curve was established.The experimental results show that the addition of 1.5 wt.%Al element increases the critical cooling rate of the pearlite transformation of the alloy system and the pearlite transformation at the same cooling rate.4.At the cooling rate of 0.1°C/s,the 1.5 Al steel can achieve a finer pearlite interlamellar spacing（120 nm）and a higher hardness（336 HV1）,while the Al-Free alloy has a larger pearlite interlamellar spacing（150 nm）and lower hardness（318 HV1）.Indicating that the Al element can effectively refine the pearlite interlamellar spacing and increase the hardness of the structure.