Study On Phase Transformation Law And Mechanical Property Improvement Of GY4 Armor Steel

With the deeper knowledge of ultra fine grain steel, the process of refinement has became one of the main methods to increase mechanical properties of steel availably nowadays. The phase transformation in refining process is controlled by thermo-mechanical control process, so TMCP has turned into the focus which people concerned.Armor steel is a special kind of steels. High strength, high toughness, high hardness and well welding performance are the requirements to it. So it represents the most advanced technology to produce steel and the last direction of development of steel. Refinement grain is hoped by method of TMCP under microstress. This paper selects different austenizing temperature and cooling velocities to find the regularity of phase transformation in armor steel with the Differential Scanning Calorimeter and the High-precision Differential Dilatometer. Then conventional heat treatment process is optimized according to previous study. And regularity of microstructure evolution is studied with different temperature being loaded microstress and subsequent different cooling velocities. The following conclusions can be found according to experiment.1. The content of pro-eutectoid ferrite decreases with increase of cooling velocity. The precipitation of pro-eutectoid ferrite is restrained completely, when the cooling velocity gets to 100 K/min. In other words, the critical cooling velocity of ferrite is lower than 100 K/min.2. The present of banded structure is not related to cooling velocity, but is related to austenizing temperature and holding time. So high austenizing temperature can slake banded structure.3. Hardness of armor steel is boosted after it austenized at temperature 20℃higher than Ac3 in the second quenched and tempered process,4. Carbides arrange in ferrite laths crosswise and parallelly after the steel is tempered.5. No pro-eutectoid ferrite appears when steel is loaded in 800℃. Cooling velocity after loading can restrains pro-eutectoid ferrite. The ferrite grain gets finer with increasing cooling velocity.6. The grain of armor steel can be refined by choosing different loading temperature and cooling velocity.