Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/068—Decarburising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/068—Decarburising
  • C21C7/0685—Decarburising of stainless steel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/28—Manufacture of steel in the converter
  • C21C5/30—Regulating or controlling the blowing

Definitions

• the invention relates to a method for decarburizing a molten steel for the production of high-chromium steels while blowing in oxygen, in which the decarburization rate is measured continuously and the amount of oxygen to be blown in is adjusted as a function of the measured values, the decarburization rate being determined from the CO 2 and CO 2 - Content in the exhaust gas and the exhaust gas flow is determined.
• the proportion of the diluent gas and the amount of gas injected into the melt are changed in a predetermined manner.
• the parameters entered in the model i.e. in a computer program, are compared with actual measured variables, and by comparing the predetermined target values and the determined actual variables, the decarburization process is controlled in such a way that the actual course of the process matches the process simulated in the computer to such an extent corresponds as possible.
• this computer-controlled decarburization processes should be able to precisely control the decarburization process.
• this method is suitable for decarburizing molten steel, due to the model used, this method is not suitable for precisely determining the point in time at which the transition point from the decarburization reaction to metal oxidation is reached.
• control variables are calculated with the aid of a computer on the basis of measured or predetermined values: the duration of the Al-Si oxidation phase at the beginning of the decarburization process, the duration of a main decarburization phase immediately following the Al-Si oxidation phase until the transition point is reached from the decarburization reaction to metal oxidation, the decarburization rate in the main decarburization phase, the decarburization rate in turn being determined from the CO and CO 2 content in the exhaust gas and the exhaust gas flow.
• the process is carried out in such a way that the amount of oxygen blown in immediately after the Al-Si oxidation phase to that amount of oxygen accelerated until the calculated decarburization rate is reached.
• the decarburization rate is then kept substantially constant during the main decarburization phase by changing the amount of oxygen blown in.
• the amount of oxygen blown in is continuously reduced in such a way that the decarburization rate decreases continuously with a predetermined time constant.
• the method according to the invention for the production of high-chromium steels takes advantage of the knowledge that there is a critical decarburization state in the course of the process, i.e. a transition point from the decarburization reaction to metal oxidation, which can be calculated with sufficient accuracy beforehand using a special model, and the optimal process control depends is the timely detection of this state, after which the metal oxidation, especially the chromium oxidation, is favored in the melt to the disadvantage of the decarburization reaction.
• a concrete embodiment of the model for determining the critical decarburization state which makes it possible to determine the duration of the Al-Si oxidation phase ⁇ tAl-Si, the duration of the main decarburization phase ⁇ tkr and the decarburization rate in the main decarburization phase, is given by equations (1) to ( 5).
• This model assumes that there is an almost constant decarburization rate during the main decarburization phase which, after reaching the transition point from the decarburization reaction to metal oxidation, passes into the immediately subsequent post-critical phase.
• the Oxygen inflow multiplied by the efficiency of the oxygen lance in the main decarburization phase constant.
• a very low Cr burn-off is achieved in that, as the decarburization rate decreases, the oxygen supply is reduced continuously with the time constant ⁇ kr calculated using equations (1) to (5).
• the control is very easy to implement by blowing in oxygen with the aid of adjustable gas flow control means.
• the amount of oxygen blown in is adjusted to a predetermined flow rate for the duration of the Al-Si oxidation phase, so that the foaming of the slag does not exceed a certain thickness.
• FIG. 2 shows the oxygen balance of the decarburization kinetics according to FIG. 1.
• Fig. 1 shows schematically the decarburization kinetics of the underlying model.
• the decarburization rate is plotted on the y-axis and the carbon content of the melt on the x-axis.
• the main decarburization phase as can be seen in FIG. 1, is characterized by a constant decarburization rate which, after reaching the critical transition point from the decarburization reaction to metal oxidation, continuously passes into the post-critical phase. From this point of view, the critical transition point belongs to both the main decarburization phase and the post-critical phase. Accordingly, the different kinetics of the decarburization reaction that apply to these two phases are the same, ie. H.:
• the energy balance of the melt is such that the instantaneous energy content of the melt is composed of the initial energy content of the primary metal and the stored energy, which is equal to the difference between the energy supply and the energy loss. Furthermore, it is assumed that the target temperature of the melt once reached at the critical point increases only slightly during the further treatment in the post-critical phase. The proposed process control is based on this assumption, in which only a small amount of chromium slagging takes place during the post-critical phase.
• the right side of the energy balance equation (3) has several elements with a positive sign, which record the thermal energy released by the metal burnup (metal oxidation).
• the intensity of the metal erosion is determined for the individual metals by the constant Konst. 1 to const. 7 characterized. These are parameters typical of the melting furnace and the melt.
• the elements of equation (3) provided with a negative sign include the energy losses through the exhaust gas discharge, through the water cooling, through the heat radiation and the energy requirement for the melting of alloys and slags.
• the essential quantity resulting from the solution of the system of equations (1), (2) and (3) is the critical carbon burn-up ⁇ Ckr.
• the critical carbon content ⁇ Ckr that is the carbon content at the transition point of the melt according to FIG. 1, is obtained from the following equation:
• CA is the initial carbon content of the melt.
• the decarburization rate can be calculated taking into account the following equation according to FIG. 1:
• the decarburization process is carried out in such a way that the relevant control variables are calculated at the beginning of decarburization using equations (1) to (5).
• the further process flow is shown schematically in FIG. 2.
• a predetermined oxygen flow and a predetermined inert gas flow (for example argon) are set and passed through the melt.
• the predetermined values lie in a range in which the foaming of the metal slag does not exceed the permissible values.
• the inert gas supply is switched off and the quantity of oxygen supplied is accelerated until the decarburization rate calculated for the main decarburization phase, which is determined from the CO and CO 2 content in the exhaust gas and the exhaust gas flow, is established.
• This decarburization rate is kept essentially constant by regulating the oxygen supply during the main decarburization phase.
• the critical transition point tkr is reached, the amount of oxygen supplied is reduced in proportion to the time with the time constant tkr.
• the special feature of the invention lies in the determination of the metal bath concentrations of the chemical elements, the metal bath temperature at the critical point and the time of its occurrence. At the critical transition point, the chemical-thermodynamic relationships of the chemical reactions taking place in the metal bath are also calculated. With regard to the maximum instantaneous decarburization and the minimum metal slagging, these reaction processes are considered to be optimal.
• the optimal reaction sequence is maintained in the postcritical decarburization phase by using the process variables calculated for the critical transition point on the basis of the model to control the postcritical phase, so that undesired chromium oxidation, oxygen consumption and the consumption of reducing agents, especially silicon, are significantly minimized can. As in the main decarburization phase, the oxygen flow rate is controlled via the decarburization rate.
• the model-based determination of the critical condition also makes it possible to define the optimal input data for the melt.
• the application possibilities of the method basically extend to all processes that take place with the reducing effect of the carbon against the chromium oxidation. These include both vacuum fresh processes (VOD) and AOD converter processes (Argon Oxigen Decarburization) with all technical modifications.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Treatment Of Steel In Its Molten State (AREA)
• Carbon Steel Or Casting Steel Manufacturing (AREA)
• Coating With Molten Metal (AREA)
• Heat Treatment Of Articles (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 1995
  • 1995-10-23 DE DE19540490A patent/DE19540490C1/de not_active Expired - Fee Related
• 1996
  • 1996-10-14 WO PCT/DE1996/001970 patent/WO1997015692A1/fr not_active Application Discontinuation
  • 1996-10-14 BR BR9611224A patent/BR9611224A/pt not_active IP Right Cessation
  • 1996-10-14 RU RU98109904/02A patent/RU2139355C1/ru not_active IP Right Cessation
  • 1996-10-14 EP EP96938964A patent/EP0857222B1/fr not_active Expired - Lifetime
  • 1996-10-14 KR KR1019980701953A patent/KR100275100B1/ko active IP Right Grant
  • 1996-10-14 PL PL96326503A patent/PL186610B1/pl unknown
  • 1996-10-14 CZ CZ981252A patent/CZ125298A3/cs unknown
  • 1996-10-14 CN CN96197803A patent/CN1063493C/zh not_active Expired - Fee Related
  • 1996-10-14 DE DE59604131T patent/DE59604131D1/de not_active Expired - Lifetime
  • 1996-10-14 AT AT96938964T patent/ATE188511T1/de active
  • 1996-10-14 SK SK501-98A patent/SK283186B6/sk unknown
  • 1996-10-14 JP JP51618997A patent/JP3190351B2/ja not_active Expired - Fee Related
  • 1996-10-14 AU AU76197/96A patent/AU701824B2/en not_active Ceased
  • 1996-10-14 US US09/066,483 patent/US6093235A/en not_active Expired - Fee Related
  • 1996-10-14 ES ES96938964T patent/ES2140912T3/es not_active Expired - Lifetime

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