| Bearing steel is a kind of important special steel, and its metallurgical qualityseriously affects the stability of mechanical equipments. Chemical composition,cleanliness (inclusions in steel) and microstructure uniformity must be strictlycontrolled to obtain better quabity in production. Especially, composition, morphology,content, size and size distribution of inclusions are important factors affecting thefatigue life of steel.The proportion of A-type inclusions is high, B-type inclusions can’t be controlledstably, and the control of D-type inclusions did not reach optimal level in highcleanliness of GCr15bearing steel. Samples of industrial production of high-qualityGCr15bearing steel steel in different smelting process accurately are selected in thisstudy. Modern analytical techniques, combined with metallurgical thermodynamicscalculation models and software are used to analyze comprehensively the formationand evolution laws of inclusions in the process of refining and casting, and therelationship of composition, morphology, content, size and size distribution ofinclusions with operation conditions in refining and casting were got. Specific resultsare showed as follows:1) The main inclusions were found in refining process are A-type inclusions,B-type inclusions, D-type inclusions, T-type inclusions, and inclusions which B-typeinclusions or D-type inclusions are covered with MnS, TiN, Ti(C, N) or CaS on thesurface. At the end of refining, the proportion of large size inclusions and the numberof inclusions in per square microns both decreased. The details of the two groups ofspecimens are showed respectively as follows: the first group decreased from5.08%to0.51%, from372/mm2to258/mm2, and the second group decreased from1.61%to1.28%, from392/mm2to164/mm2. During LF refining, the size of inclusions aremainly concentrated in1~3μm, and during RH refining, the size of inclusions aremainly concentrated in less than1μm. In slab, the average size of inclusions and the number of inclusions in per square microns were further decreased. More inclusionsin the head and tail of the slab were found.2) Single inclusions had evoluted to become complex increasingly step by stepduring the refining process. The compostions of inclusions is summarized as follows:Complex oxide inclusions were formed according to the route by Al2O3'MgO·Al2O3'(CaO-MgO-A l2O3-(CaS)). Complex oxide-sulphide inclusions were formedaccording to the route by Al2O3'(Al2O3-MnS)'(Al2O3-MnS-Ti(C,N)).3) Methods used to control incluions in the preduction process were proceded asfollows. Adjusting the composition of refining slag to MgO≈5%、SiO2≈10%、Al2O3≈20%、CaO≈50%in oder to increase the area of low melting point, whichmakes more inclusions form as low melting point inclusions as far as possible.Control accurate composition of liquid steel by providing exact deoxidizing alloymaterial. The content of Alsis in the rangeof0.02%~0.04%, Mg and S content doesn’texceed0.0004%and0.003%respectively.Improving furnace lining and preventingsteel from exposing to air can control the source of impurity element such as Mg andN. Factors such as old ladle age and too small free space which will lead to thenumber of inclusions and the size of inclusions increase should be strictly controlled.. |
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