Basic Research On Dynamics Of Vacuum Reaction And Process Optimization In RH

RH is an important refining way, which plays an important role in producing clean steel especially deep drawing steel with ultra low carbon. Combining with RH production process, decarburization velocity of vacuum and technology level of carbon-oxygen balance in RH was measured, and series of basic researches were done on this basis. The work mainly includes increasing circulation flow rate of reaction layer, enlarging reaction interface of decarburization by optimizing geometric parameters of RH vacuum chamber and snorkel. An optimized practical and efficient RH equipment was finally obtained, which could be used to improve refining level of RH and production efficiency.The research in this paper was done by industry experiment conducted on the spot firstly, then study and discussion were carried out to optimized ways,which include changing shape of snorkel、enlarging vacuum chamber and installing weir in vacuum bottom, through cold model simulation experiments, numerical simulation, model and thermodynamics calculation analysis. At last optimized ways of decarburization process parameters were put forward. Based on comprehensive consideration, a new optimization scheme of RH equipment was put forward. The research result has reference meaning to industry and lays a good foundation for application. The main research work is as follows:(1) Study on dynamics of carbon-oxygen reaction on field testThe process parameters of producing ultra-low carbon steel ([C]<0.002%)during RH refining were tracked and analyzed by industrial experiments. At first the rate constant for decarburization, Kc, were measured and evaluated. The process of vacuum-pumping stage, blowing oxygen decarburization stage, natural decarburization stage and interface decarburization stage was discussed and analyzed. In order to produce ultra-low carbon steel with carbon content less than 0.002% and to control oxygen content, the optimized process parameters of RH were determined:vacuum pumping pressure should be 54KPa before treatment, controlling [C] content 0.05～ 0.06%, reducing [O] content to 0.03～0.04%, increasing pumping speed, controlling thickness of carbon oxygen reaction layer to 200～400mm; the [C]-[O] content ratio should be 2～2.5 before oxygen was blown, while the beginning vacuum level stayed between 12～15KPa and the intensity of oxygen blowing was over 0.012 Nm3/(t. min); [C] content was 0.015～0.02% and [C]-[O]content ratio should be 0.6～0.75 at endpoint of blowing oxygen; there were more than 15 minutes for natural decarburization of decarburization stage and the intensity of argon blowing was 0.015Nm3/(t. min);in the interface decarburization stage, the area of reaction interface and the flow rate of reaction layer should be increased. After being discussed, the approach of developing new techniques to produce clean steel of ultra low carbon on the high efficiency was to optimize RH equipment by increasing circulation flow rate, volumetric mass transfer coefficient and steel flow of interface reaction layer, to promote carbon oxygen reaction close to gas-phase reaction equilibrium.(2) Experimental study on increasing circulation flow rate of RH· The circulation flow rate of the RH with oval-shaped snorkel and round-shaped snorkel increased with gas flow. The circulation flow rate of the RH with oval-shaped snorkel was larger than RH with round-shaped snorkel, and had stronger superiority with gas flow increasement. Compared with gas flow rate per unit area, the circulation flow rate of RH with oval-shaped snorkel was increase by 50% than round-shaped RH when the rate was 6.5Nm3/(m2.min). It was calculated that circulation flow rate can increase by about 68%, when cross section of snorkel increased 70%.· This paper put forward the relationship between gas flow, cross section of snorkel and circulation flow rate:the circulation flow rate increased with gas flow when gas flow was smaller than critical gas flow, and change of cross section had no effect. The circulation flow rate increased with the increasement of gas flow and cross section, when gas flow was located between critical flow and saturated flow. If gas flow was larger than saturated flow, the circulation flow rate decreased with rise of gas flow.· The formula of circulation flow rate with different cross sections was obtained by experiment: The formula has a higher calculate reliability than common formula while gas flow was small.· When the area of cross section increased 30,50 and 70% than round-shaped snorkel, the max Kc will advanced from 0.19mm-1 to 0.206mm-1,0.214mm-1 and 0.224min-1 respectively.(3) Experimental study on increasing volumetric mass transfer coefficient ak of RH· Steel surface of vacuum increased by about 66%. The circulation flow rate of RH with larger vacuum was smaller than common RH. When the gas flow rate was advanced, the circulation flow rate and mixing time were close to common RH.· The steel flow field of vacuum changed because of enlarging vacuum chamber:the path of surround flow of upper steel in vacuum was prolonged, and circulation flow rate was enlarged; There was a new vortex near the riser snorkel, the peak near upper snorkel grew larger, and small peak was found above the downcomer snorkel; The surface velocity of steel increased while the lower part velocity decreased; a great deal of bulbs travelling side was generated because of shear flow; the residence time increased due to changing of the flow field.· Because static reaction surface of liquid steel in RH with enlarger vacuum chamber rose about 66%,dynamic reaction interface increased by 81% because of vortex, peak, a great number of bulbs moved along lateral wall make the reaction ak of bulb decarburization increase by 34%,and steel flow rate of interface layer(<30mm) of RH increased by 15～25%. Ak of RH with enlarged vacuum chamber increased by more than 40%. The rate constant for decarburization, Kc, were enlarged. It can be estimated that the max Kc was increased from 0.189min-1 to 0.21mm-1.(4) Experimental study on improving steel flow rate of reaction layer of RH●Experimental study of RH with a dam was done. Because the dam hinders a part of steel flow, the circulation flow rate was decreased. The circulation flow rate was 87～91% of common RH when the gas flow was higher than 3.1Nm3/h.●It can be obtained from water model experiment, the proportion of piston steel increased from 38% of common RH to 74% of RH with dam. The proportion of backmixing steel reduced less than 20%, and no change of lag steel proportion. The steel flow in interface layer (<30mm) of viston steel and lag steel was 3.2 times of common RH. By simulation calculating, it can be concluded that steel flow rate of interface layer (<30mm)of RH with dam was 38～46% of all steel flow, while the flow rate of common RH was only 19～25%.●The decarburization rate of steel was risen by installing dam in chamber, especially in natural decarburization and interface decarburization stage. It was calculated that Kc could increase from 0.12min-1 and 0.08min-1 of common RH to 0.175min-1 and 0.1 min-1 respectively by changing the circulation flow rate and the steel flow of reaction layer.Because of increase of the reaction layer flow, the thickness decreased to 39mm, and the apparent carbon-oxygen equilibrium was reduced from 1.1×10-4 to 6.2×10-5.(5) Experimental study of comprehensive optimizing model of RHA optimizing scheme of RH equipment was put forward based on previous research results:sectional area of snorkel and chamber increased separately by 70% and 40%,and a dam was installed (width is 100mm and height is 200mm).The velocity of steel passed dam was promoted according to experiment and calculation, and the main fluid change direction just above downcomer snorkel then flow into it which improve the recycle efficiency. It can be concluded that circulation flow rate increased by 23% in same normal flow condition through the water model experiment. By numerical calculating, it can be concluded that the max circulation flow rate increased by 58%. Due to the enlargement of vacuum inner diameter, the new vortex and peak enlarged the area of reaction surface and ak of optimized RH rose 56%. The amount of steel entered into decarburization reaction layer(30mm) reached 1.9～2.5 times higher than common RH. The rate constant for decarburization enlarged. The maximum Kc of 5# heat could reach 0.31min-1.The Kc can increase from 0.12 min-1 and 0.08mm-1 to 0.19mm-1 and 0.12mm-1 respectively during natural decarburization and interface decarburization stage.