Refining Control of Top- and Bottom-blowing Converter by Manipulating Bottom-blown Gas Flow Rate* By Hei-ichiro ISO,** Yujiro UEDA,*** Toru YOSHIDA,*** Shouichi OSADA,**** Shujiro ETO***** and Keiji ARIMA*** Synopsis top- and bottom-blowing converter of which the flow The effectof gas bottom-blowingcondition on the refining characteristics rate of bottom-blown gas could widely be changed of a top- and bottom-blowing converter in comparison with that of top- was developed.5,6~ As a results, it has become pos- blowing condition has been studied, on the basis of the amount of accu- sible to produce a wide variety of steel from low car- mulated oxygenin the converter(Os), which representsthe change in oxida- bon steel to high carbon steel by the top- and bottom- tion/reduction reactions betweenhot metal and slag. blowing converter. It is possible to express quantitatively the relationships between hard blow/soft blow by the manipulation of top-blowingjets and strong agita- Incidentally, from the experience in actual opera- tion/weak agitation by that of bottom-blowngas. The manipulation of tions of top- and bottom-blowing converters, it has the flow rate of bottom-blowngas permits to control the blowing reaction revealed that in order to obtain the economic effects, more effectivelythan that of the top-blowingjet. i.e., improvement of steel yield by a reduction of iron In connectionwith the abovefact, dynamic control of blowing reaction content in slag (hereafter called (T.Fe)) and a reduc- in the top- and bottom-blowingconverter has been done, with the flow rate tion of ferroalloy consumption by an increase in man- of bottom-blowngas as a manipulated variable and the variation of Os ganese content of hot metal, it is not always necessary as a controlled variable. By the application of the multistep optimum to maintain such a flow rate of bottom-blown gas that control theory to the blowing reaction model, the stability of blowing opera- the former researchers considered to be necessary. tion and metallurgical characteristics, i.e., the ranges of changes in (T.Fe) Therefore, such a new model that the dynamic of slag, phosphorus and manganese concentration of molten steel at blow effects of bottom-blown gas on the reactions can be end, were narrowedappreciably. evaluated accurately should be established. Key words: oxygenconverter; top- and bottom-blowingconverter; refin- In the light of blowing control technology, too, ing reaction; agitation energy; bottom blowing; computorcontrol; dynamic neither a blowing control model which is able to ex- control. press the characteristics of a top- and bottom-blowing converter accurately nor an effective method which is able to controll blowing operation of a top- and bot- I. Introduction tom-blowing converter had been reported, though Recent advances in oxygen bottom-blowing con- various blowing control methods had been developed verter processes (Q; BOP etc.) revealed insufficiency of for top-blowing converters. As the blowing control bath agitation in the conventional top-blowing con- models for top- and bottom-blowing converters, there verter. This led to the development of a top- and are only a few examples adapted the static blowing bottom-blowing converter process which aims at in- control model which simply corrects the change in creasing bath agitation by bottom-blown gas as well blowing condition of bottom-blown gas or the dy- as top-blown gas. namic blowing control model originally developed for In order to determine the agitation characteristic top-blowing converters.7~ Thus, any appreciable re- of the bottom-blown gas in bottom-blowing or top- sults obtained by the use of the model which is able and bottom-blowing converter, the physical mixing to make a dynamicall estimation, for example, the ef- condition of liquid has been studied by the use of such fect of the change in flow rate of bottom-blown gas on chemical engineering parameters as agitation energy the refining reactions have not been reported. In and time required for uniform mixing based mainly this respect, a model which is able to evaluate quan- on the water model experiment.i-3) From the results titatively the dynamic effects of bottom-blown gas is obtained, the required ratio between top-blown gas also required. and bottom-blown gas has been determined. How- In order to explain the refining characteristics of ever, in the case of the top- and bottom-blowing con- bottom-blowing gas in a top- and bottom-blowing verter developed in an early stage, the flow rate of converter, the authors utilized the information of ex- bottom-blown gas could hardly be changed, and de- haust gas to calculate the amount of oxygen accumu- phosphorization was inferior to that of top-blowing lated in the converter (hereafter called (Os)) which converters.4~ From this reason, top- and bottom- represents the changes in conditions of oxidation and blowing was applied almost exclusively to converters reduction between hot metal and slag in the con- producing low carbon steels. Then, a new type of verter. * Manuscript received on August 3, 1987; accepted in the final form on December 11, 1987. © 1988 ISIJ ** Formerly Sakai Works, Nippon Steel Corporation. Now at R & D Laboratories-III, Nippon Steel Corporation, Edamitsu, Ya- hatahigashi-ku, Kitakyushu 805. Sakai Works, Nippon Steel Corporation, Chikko-Yawata-cho, Sakai 590. **** New Materials Project Bureau, Nippon Steel Corporation, Otemachi, Chiyoda-ku, Tokyo 100. ***** Electronics & Information Systems Division, Nippon Steel Corporation, Otemachi, Chiyoda-ku, Tokyo 100. 372 ) Research Article Transactions ISIJ, Vol. 28, 1988 (373) This paper describes the characteristics of hot metal ers,8~ has been modified so as to calculate Os, which agitation by bottom-blown gas in a top- and bottom- is a dynamic parameter of refining reactions in the blowing converter analyzed by Os as a dynamic pa- top- and bottom-blowing converter. Actually, Os is rameter of the reactions between hot metal and slag, calculated as follows. First, the change in oxygen and the method of dynamic control of the blowing balance of the converter is calculated from Eq. (1). reactions in top- and bottom-blowing converter with Then, it is integrated, and the amount of oxygen Os as a means of evaluation, and with bottom-blown which reacts with silicon in the hot metal to form gas as a means of dynamic control. silicon dioxide, and is not directly given any effects on the subsequent oxidation and reduction in the con- II. Experimental Method verter as in Eq. (2), is subtracted from it. 1. Top- and Bottom-blowingConverter dOs = {FTo2+FBco2+~ (aZ+a2+l/211)Wi} The experiments were done with a 170-t top- and - (1/2F o+Fco bottom-blowing converter at Sakai Works (hereafter 2) ..............................(1) called LD-CB5~). This converter uses C02, N2, or Ar Os = (dOs)dt-icWHM[Si]HM.....................(2) as bottom-blown gas as shown in Fig, 1. A maxi- mum gas pressure is 25 kg/cm2. Figure 2 shows the t here, Fc = F00 +2 • (21/79FN2-Fo2) design of the bottom-blowing nozzle. The nozzle Fco, = FC02-2. (21/79FN,-Fo made of refractory has multiple small-diameter pipes ,) with a common header. The most remarkable fea- Fco = Fex(t-z)Cco(t) ture of this nozzle is that the flow rate of bottom- Fco, = Fex(t-z)Cco2(t) blown gas can widely be changed freely in a short FN2- Fex(t-z)CN2(t) time during blowing. 2. Amount of OxygenAccumulated in ConverterCalculated where, dOs : Change in oxygen balance of convert- from Informationof Exhaust Gas er (Nm3/h) Figure 3 shows the system to calculate the amount Os : Amount of oxygen accumulated in of oxygen accumulated in the converter on the basis converter (Nm3) of the information of exhaust gas. The dynamic con- FT 02: Flow rate of top-blown oxygen (Nm3/ trol system originally used for top-blowing convert- h) Fig. 1. Outline of top- and bottom-blowing converter (LD-CB). Fig. 2, Design of bottom-blowing nozzle. Fig. 3. Measuring system of top- and bottom- blowing converter. 2 (374) Transactions ISIJ, Vol. 28, 1988 FB002 : Flow rate of bottom-blown carbon The experimental method is schematically shown dioxide (Nm3/h) in Fig. 4. The chemical compositions of metal and a2: Oxygen contained in flux and coolant slag needed for the subsequent experiments were de- of i except for oxygen combined with termined by emission spectroscopic analysis and fluo- carbon (Nm3/kg) rescent X-ray analysis, respectively. 13i• . Carbon dioxide contained in flux and coolant of i (Nm3/kg) III. Results of Experiment rz : Hydrogen contained in flux and cool- Figure 5 shows the experimental results, with each ant of i (Nm3/kg) of the manipulated variables on the horizontal axis WI : Charged ratio of flux and coolant of i and the change in dOs on the vertical axis. Each (kg/h) manipulated variable shows a good correlation with i : Type of flux and coolant the change in dOs : the lance height shows a positive Fco : Flow rate of carbon monoxide in ex- correlation, and the flow rates of top-blown oxygen haust gas (Nm3/h) and bottom-blown C02 each show a negative correla- Fco2: Flow rate of carbon dioxide in ex- tion. It should be noted, however, that in actual haust gas (Nm3/h) blowing operation the variable range of lance height FN2: Flow rate of nitrogen in exhaust gas is limited in a certain range and that in the range (Nm3/h) adopted in the present experiment the lance height Foe : Flow rate of oxygen in exhaust gas has less remarkable effect on dOs than that of the flow (Nm3/h) rate of top-blown oxygen or bottom-blown C02 which Fc : Flow rate of carbon monoxide gen- can be varied more widely.