Metallurgical Mechanism And Process Of Single Snorkel Refining Furnace Making The Stainless Steel

The development of new technologies for stainless steel smelting has always been a research topic of concern to stainless steel metallurgists.Single Snorkel Refining Furnace(SSRF),referred to as "single-snorkel furnace",is an original vacuum-furnace refining device in China.Long-term industrial batch tests have proved that this furnace has the technical advantages of high refining efficiency,low production cost and simple equipment in the smelting of electrical steel,bearing steel and other varieties.Applying the technical advantages of the SSRF to the smelting of stainless steel is a new research and exploration.Carrying out research in this area has important theoretical significance and practical application value for the development of new stainless steel smelting technologies in China.This paper takes the smelting of stainless steel in a SSRF as the research background,and conducts in-depth research around the key metallurgical mechanisms and processes of the smelting process.Through physical and numerical simulations,the evolution behavior of long-distance bubbles floating up was clarified,and the flow field structure of the molten steel area inside the SSRF was analyzed;the optimal control principle of the furnace structure was proposed;the snorkel offset and dual-stirring can effectively improve the fluidity of molten steel around the snorkel,and the best control method for eccentricity and dual-brick layout is proposed.Established a vacuum chamber steel-slag cold state simulation device,clarified the flow characteristics and circulation mechanism of the top slag layer,and further combined the 25-ton industrial SSRF to design the flow field and furnace structure,and the industrial test and smelting effect evaluation of smelting 304 stainless steel have been completed.The mathematical model of the process of smelting stainless steel was established,and the key points of control of the process of stainless-steel smelting were put forward.The main findings are as follows:(1)The gas bubbles blown from ladle bottom will continue to grow during the long-distance floating in the molten steel.After entering the vacuum chamber,and then expand at an accelerated rate after entering the vacuum chamber.The diameter of the bubbles when they overflow the vacuum liquid surface reaches 12.5 times the initial diameter,and the speed is correspondingly increased to 3.5 times the initial speed,effectively expanding the gas-liquid surface active area in the vacuum chamber.Under the action of long-distance bubble stirring,the molten steel flow field in the whole area is composed of 8 characteristic areas.Analysis of the flow field confirms that the flow of molten steel at the bottom of the ladle is mainly driven by the impact of downward flow,while the flow of molten steel on the periphery of the snorkel is driven by the overflow of upward stream.(2)The change of furnace parameters(Inner Diameter of Snorkel-IDS,Blowing Position-BP and Snorkel Insertion Depth-SID)will change the circulation efficiency of the SSRF and the uniformity of the molten steel flow inside and outside the snorkel;the research aims to ensure the circulation efficiency and improve the flow uniformity as the control objective of the molten steel flow field optimization,and proposes the optimization control method of three furnace parameters.Within the capacity range of 25 to 130 tons,the optimal control range for the dimensionless furnace parameters is summarized.The IDS is 0.41?0.48,the SID is 0.135-0.170,and the BP is 0.5;the flow field analysis also confirms that the weak liquidity of the molten steel outside the snorkel is the limiting link that affects the uniformity of the flow field of the centrally symmetric SSRF.(3)Compared with the symmetrical position of the center,the positive deviation of the snorkel can improve the flow strength of the peripheral molten steel and shorten the mixing time of the molten pool.It is concluded that the optimal control range of the dimensionless value of the snorkel eccentricity is 0.2?0.3.In the eccentric SSRF,dual bricks are used for argon blowing and stirring,which can greatly increase the flow strength of the peripheral molten steel.Compared with single brick,the average flow rate of the molten steel around the snorkel is increased 40%,the flow rate difference percentage between the inside and outside of the snorkel is reduced from 54%to less than 10%;the angle between dual bricks is controlled to 180°,and the argon blowing ratio is controlled to the range of 1/7?1/5,which can achieve the best stirring effect.(4)The simulation study of slag entrapment in the vacuum chamber shows that the strong bubble active zone of the vacuum chamber can shred the top slag layer into a large number of fine slag droplets,and entrap them in the molten steel,effectively increasing the contact area between steel-slag;Under the action of the circulating molten steel,most of the slag droplets can circulate between the ladle and the snorkel,forming a long-term infiltration contact with the molten steel;this"large area+long time" flow contact between steel slags improves the reaction efficiency between the two.(5)Based on the actual 25-ton ladle,the key structural parameters of the industrial SSRF were designed,and the industrial test of smelting stainless steel was carried out.The smelting results of 18 furnaces of 304 stainless steel show that the 25-ton eccentric SSRF designed according to the model shows a good application effect in the smelting.The carbon content in the steel can be reduced to 110ppm at the minimum,and the average yield of Cr during the reduction period is 97%;The fluctuation range of the molten steel composition before and after the hollow is small,and the fluctuation of the content of the main elements is less than 5%,and there is no skull and vacuum splash during the smelting process.(6)Based on the established mathematical model of SSRF smelting stainless steel process,the composition and temperature of the molten steel during the smelting process can be dynamically predicted and calculated.Model studies show that the internal decarburization rate of molten steel during the oxygen blowing period is the largest,reaching an average of 113.5 ppm/min,accounting for more than 50%of the total rate;the surface decarburization rate of the vacuum liquid surface in the initial stage of the VCD stage accounts for 70%,while the later stage of VCD,the decarburization mainly relies on the reduction of chromium oxide;the oxygen blowing process of‘dynamic vacuum+dynamic oxygen supply'can effectively increase the decarburization rate of molten steel and reduce the burning loss of the Cr.