Study On The Effect Of The Austenite Mixed Grain Structure On Grain Boundary Crack In The Surface Of Casting

The coarse original austenite grain on the surface of continuous casting slab is the key factor leading to the grain boundary crack of austenite.The existing crack sensitivity criterion of grain boundary is the coarse original austenite grain critical size model.When the coarse grain size is larger than the critical diameter(1mm in diameter),the crack sensitivity of grain structure is promoted and further the crack of grain boundary is more likely to occur.In the experiment,we found that the crack in austenitic organization is less than the critical grain size and the flawless area in austenitic organization is greater than the critical of austenitic grain size.It's not comprehensive that if only through the thick grain diameter of critical criterion characterization of crack sensitivity.Therefore it is necessary to put forward the crack sensitivity criterion of mixed grain index and make up for the bulky original austenitic grain diameter of critical criterion,based on the uniformity of austenite grain size of quantitative characterization,which is of great significance to improve the quality of casting surface.In this paper,the surface austenite grain structure of continuous casting slab is taken as the research object.The as-cast microstructure characteristics of the original austenite at the surface defects of the slab were analyzed,and the austenite grain size was calculated and the microstructure uniformity was quantitatively characterized.The relationship between grain structure and grain boundary crack sensitivity was established based on the mixed grain index and the critical grain size of 1mm in diameter of coarse grain.The main research results are summarized as follows:(1)The grain structure of the original austenite is not uniform to different degrees in the deep vibration mark,the austenite grain boundary crack and the surface depression of the continuous casting slab;(2)When the diameter of the original austenite grain is greater than the critical grain size of 1mm in diameter,the mixed grain index is larger than 0.5,and the surface grain boundary crack sensitivity is strong.On the contrary,if the mixed crystal index is less than 0.5,the sensitivity of grain boundary crack is small and grain boundary crack is not easy to occur.When the diameter of the original austenite grain is less than the critical grain size of 1mm in diameter,the mixed grain index is greater than 0.7,and the grain boundary crack sensitivity of the slab surface is strong,which is easy to produce grain boundary crack.However,when the mixed grain index is less than 0.7,the grain boundary crack sensitivity is small and grain boundary crack is not easy to occur;(3)The reliability of the criterion was verified by using cold heading steel of 10B21 and SCM440.