| The cleanliness of horizontal continuous casting billets of 36Mn2 and 37Mn5 used for oil pipe production were studied systematically and in-depth in this paper. T[O] and [N] in the steel, micro-and macro-inclusions in the molten steel from fore-and after-LF refining and from tundish process, in casting billets and pipes as well as their variation regularities were investigated by means of some analysis methods such as oxygen/nitrogen analyzer, metallographic microscope, SEM, EDAX, EPMA, bulk sample electrolysis, tracer analysis, chemical analysis etc. Meanwhile, the types, constituents, shape and origin of the inclusions were determined. The main results are as follows:1) T[O] in horizontal continuous casting billet of 36Mn2V is 39.6×10-6, [N] 60.7×10-6, and the relative content of micro-inclusions 15.81/mm2. While, in 37Mn5 T[O], [N] and the relative content of micro-inclusions are 39.1×10-6, 67×10-6, and 16.02/mm2, respectively. The content of macro-inclusions in 36Mn2V and 37Mn5 billets is 16.0mg/10kg. The data above shows that the cleanliness of 36Mn2V and 37Mn5 horizontal casting billets is not very well.Because of overoxidation of liquid steel at the end point of EAF refining, the [O] in steel is very high and the oxide inclusions formed during the period of deoxidating is much more. And the deoxidation by Si is unfavoured for SiO2 inclusions floating to the surface of molten steel. Otherwise, the relatively low adsorptivity of inclusion of actual LF slag results in a high content of macro-inclusions in steel. The reoxidation of liquid steel, LF slag carry-over and tundish slag entrapment in the casting process and the low inclusion adsorptivity of tundish slag are responsible for the high content of macro-inclusions.2) In the direction of the vertical-section of the casting billet, the content of micro-inclusions is the most in the center place, next lies in the lower of 1/2 radius and top brim, and the lowest is in the bottom brim. In the horizontal section of casting billet, the distribution of microinclusions is centrosymmetric and the content of inclusion is at most in the place of 1/2 radius, and less is in the centre, while, the least lie in the brim of the billet.3) Micro-inclusions in the casting billet contain silicate, silico aluminate, magnesia alumina spinel, sulphide, Al2O3 and SiO2, etc. The origin of micro-inclusions are mainly deoxidation products and reoxidation products, sulphide and the drop-out products from refractory material are in the next place and few come from slag entrapment.4) Macro-inclusions in the casting billet are main silico aluminate, calcium silico aluminate, calcium-magnesium silico aluminate, SiO2, Al2O3, magnesia alumina spinel, etc. Macro-inclusions are mainly from tundish slag entrapment, LF slag carry-over and deoxidaton products in molten steel; little partial are the falling off products from the refractory material.5) LF refining process is advantageous to improve the cleanliness of steel in certain extend, and its effect remains to be improved in future, which is related to the end-point control of EAF, the deoxidation technique of molten steel and the selecting of LF refining slag system etc. The key procedure that decide the the quality of casting billet is tundish process. Because of the special structure of horizontal continuous casting, the ability of removing inclusions is limited, and the inclusions in tundish are hard to be removed. Therefore, the cleanliness level of casting billet is not very high. |
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