Study On Formation Mechanism And Control Of Surface Micro-cracks In Sulfur-containing Bloom

In recent years,China's automobile industry and machinery manufacturing industry have developed vigorously,and the demand for structural steel has increased significantly.In response to the call for high strength and light weight clearly required in the "Made in China 2025" plan,microalloying elements are added to steel to increase the strength of steel.However,this greatly increases the difficulty of machining.Therefore,in order to meet the processing performance of steel,steel enterprises add an appropriate amount of sulfur to steel to improve cutting and processing performance,Unfortunately,surface cracks are easy to occur in the production of sulfur-containing steel,which seriously affects the production rhythm,increases the cost,endangers the steel quality,and increases the safety risk.Aiming at the problem of surface crack prone to medium carbon sulfur-containing bloom,combined with the production practice of two steel plants,the conditions of crack initiation and propagation are studied in this paper.Several key factors of crack initiation are described,including the properties of mold flux,high temperature thermoplastic,inclusions,macro/microstructure and process equipment conditions.The purpose is to provide theoretical and practical guidance for the surface micro-cracks control of sulfur-containing bloom.Based on the two-dimensional systematic anatomy of the surface micro-cracks of medium carbon sulfur-containing bloom,the main factors affecting the crack formation are determined.It is mainly manifested in poor lubrication of mold flux,uneven heat transfer in mold,shallow immersion depth of nozzle,poor inclusion control,weak secondary cooling and so on.On this basis,the three-dimensional spatial distribution and trend of surface cracks are systematically characterized by means of 3D laser confocal microscope and industrial CT.The internal oxidation behavior of surface crack is studied through the high temperature oxidation experiment system of prefabricated crack,the internal oxidation mechanism of interface oxide layer and oxide dot layer near the surface crack is clarified.The formation temperature of surface crack can be determined quantitatively by analyzing the thickness of oxide dot layer oxidized at different temperatures for different times and combined with bloom surface temperature model.Combined with laboratory hot state test and thermodynamic calculation,the influence law of sulfur in medium carbon steel on proeutectoid ferrite and high temperature thermoplastic of steel is studied.It is clear that MnS inclusion is the key factor affecting microstructure and thermoplastic.Furthermore,the influence law of sulfur in medium carbon steel on surface crack is clarified,which provides a theoretical basis for crack control.Based on the above analysis,the control ideas of surface micro-cracks of medium carbon sulfur-containing bloom are put forward as follows:adjusting the performance of mold flux,optimizing the vibration of mold,increasing the immersion depth of nozzle,optimizing the control of inclusions and strengthening local secondary cooling.Finally,the surface micro-cracks of medium carbon sulfur-containing bloom can be effectively controlled.