Study On Technologies Of Preventing The20MnSi/30MnSi Steel Bar From Cracking

Fushun New steel limited liability company has produced30MnSi plain bars,20MnSi twisted steel for many years, however, the phenomenon that30MnSi plain bars,20MnSi twisted steel crack often occurs, which produced a lot of rolling scrap and affected production costs. In this thesis, by the inclusions observation, microstructure observation, SEM and EDS, the author studied the crack parts of steel and drew the following conclusions:(1) Observing the microstructure of30MnSi plain bars near the crack, the authors found that most of the grains in the surface cracks are larger than the matrix grains. The crack surface occurs decarburization phenomenon, which denotes the cracks produce from casting blank;(2) Observing the20MnSi steel microstructure near the crack, a large number of inclusion, needle holes and bubbles are in it. By SEM analysis, O, Si, Mn, Al and other elements are in the crack, especially the O, that is to say much more oxide inclusions are in it.(3) By EDS analysis, inclusions with high levels of Na element conclude that steel is involved in the casting powder;(4) Poor quality in the casting bank and serious delamination in the casting bank with crack affect product quality seriously.According to Fushun New Steel production situation, this paper proposes a series of measures. It includes that we according to the silicon content in molten iron to determine the amount of each material, specification for argon blowing, using premelted slag in ladle, tundish stamped and strengthen baking, use of protective casting technology etc.During the stage of smelting, with the application of low-S hot metal, the final content of [O] in the BOF decreased from330to180,the accuracy rate of the end determination increased by20.8%through the silicon-smelting, and after blowing rate decreased apparently.Because of the change of argon blowing system, we found that the oxygen content in liquid steel of30MnSi and20MnSi are respectively reduced from33×10-6and22×10-6to18×10-6, the casting blank’s quality has also been promoted.As a result of the improvement of the process and the adoption of the advanced technology, the slab quality of20MnSi and30MnSi is improved considerably. The average frequency of steel cracking and slab impurity is reduced from2.4to0.7monthly for20MnSi and30MnSi. In a word, it achieves remarkable results and it is worth of generalizing.