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
  • C21C7/10—Handling in a vacuum
• • 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

Definitions

• the composition of the efiluent gases issuing from the decarburization vessel are monitored to provide an indication of the undesirable oxidation of certain metallic values within the bath being decarburized, such as chromium.
• the method includes maximizing the flow and thus, the conversion of oxidizer to carbon monoxide and minimizing the fiow of the diluent gas in order to promote economy of operation.
• the concept contemplates increase in the how of diluent gas upon the observance of excessive oxidation of metallics to promote the maximum oxidation in the carbon within the bath.
• This invention relates to the process control of molten alloy steel decarburization wherein a mixture of oxidizing gas and diluent gas is used to decarburize the molten alloy steel.
• the object of the process control in the decarburization of these molten alloy steels, such as stainless steels, is to preserve certain metallic values within the bath, such as chromium. The unnecessary loss of such values incerases the cost of the decarburization process.
• the process control method discussed herein generally relates to the decarburization of alloy steels and stainless steels using a mixture of argon and oxygen as suggested in the US. Pats. Nos.
• the method described herein requires only a determination of the composition and fiow rate of the input gases.
• These input gases are conventionally high-purity 3,666,439 Patented May 30, 1972 gases of known chemical analysis.
• simple flow rate measurements of the input gas such as argon and oxygen streams are all that are necessary and these may be accomplished by using commercially available gas-flow metering equipment.
• the method disclosed herein further requires analysis, such as by sampling, of the off-gases to determine the specific contents of the various components such as argon, oxygen, carbon monoxide, carbon dioxide, etc.
• a method of initiating and continuing dynamically balanced decarburization of molten metals such as an alloy steel in a vessel wherein gaseous oxidizing material with a gaseous diluent is introduced into the vessel at controlled rates to react with the carbon contained within the molten metal by which the carbon is removed therefrom at a measured rate including the steps of: introducing a nominal flow of diluent gas into the vessel, preferably by submerged blowing, allowing the flow to stabilize, introducing a nominal flow of oxidizing material into the vessel, allowing the reaction to stabilize, measuring the composition of the efiluent gas exiting the vessel, measuring the flow rate of the input gases to determine the rate of carbon removal from said steel, measuring the oxidizer conversion elliciency for said flow rate, alternately increasing oxidizer flow rate in predetermined amounts, and measuring oxidizer conversion eificiency to maximize the oxidizer flow rate at a conversion efiiciency above a predetermined value.
• Preferred embodiments include the steps of alternately reducing the diluent gas flow rate in predetermined amounts and measuring gas conversion flow rate after the oxidizer flow rate has been maximized, thereby minimizing the diluent gas flow rate so long as the conversion gas efficiency remains above the predetermined desirable value. It is also desirable to include control functions for the situation wherein the oxidizer conversion efficiency falls below the predetermined values, which the are steps of increasing the diluent gas [flow rate a predetermined amount for the given oxidizer flow rate, reducing the partial pressure of the carbon oxidizer medium, promoting the removal of carbon from the bath, thereby establishing the decarburization process at an efficiency such that the desirable metallic values are not oxidized from the bath.
• the invention is used in the decarburization of alloy steels to avoid appreciable losses of metallic values such as chromium.
• the procedure for blowing oxidizers such as oxygen into these alloy steels as they are decarburized must be responsive to the variations that occur in any given heat. It is a further objec tive of this invention to initiate the carbon oxidizer reaction without undue oxidation of metallics. It is to be noted that the initiation of this reaction will entail the use of less than optimum gas mixtures.
• the total flow of the injected gases i.e., oxidizer and diluent, as well as the concentration of the oxidizer may be increased to the optimum value.
• the method hereby provided achieves this optimum value, i.e., a selected efficiency, without the appreciable loss of these metallic values.
• the method may be practiced at atmospheric and subatmospheric, e.g., vacuum, pressure conditions.
• Measuring apparatus necessary to provide the control of the reaction is not complex. The only requirements are for the measurement of the flow rate of the injected gases and for sampling the off-gases from the reaction. It is desirable that the off-gases are sampled from a location within the vessel so that there is less chance of sampling entrained air within the vessel.
• the decarburization process is started with a small flow of diluent gas such as argon of commercial purity or a mixture of oxidizer and diluent gas such as argon and oxygen, the mixture generally containing between -10% of oxygen.
• diluent gas such as argon of commercial purity or a mixture of oxidizer and diluent gas such as argon and oxygen
• the gases are supplied from a tuyere already submerged in the bath in which there is a small flow of gas trickling through the tuyere to keep it from plugging
• the diluent gas flow is increased to a small predetermined value.
• the gas flow is started and then the lance is submerged into the bath.
• the oxygen flow in the input gas is increased by a small amount to begin the decarburization reaction.
• the amount is empirically determined to all several (i.e., 4 or more) of these increase steps prior to reaching the maximum flow rate. The amount of the increase may be lessened when approaching the predetermined maximum efliciency so that an overshoot situation may be avoided.
• the oif-gas composition is monitored continuously at short time intervals to seconds). Concurrent with the off-gas composition monitoring, the oxygen conversion efficiency is calculated from the composition of the off-gas and the flow rate of the input gases as is taught in my Pat. No. 3,594,155 previously mentioned.
• the oxidizer flow is continually increased in this step-wise manner, allowing 20-30 seconds between each for stabilizing of the system and measurement of the oil-gases.
• the rate of carbon drop for the decarburization process is also calculated.
• the decarburization is proceeding satisfactorily, i.e., all of the oxygen being introduced to the system is being converted either to carbon dioxide, carbon monoxide, and all of the oxygen is being devoted to the decarburization of the steel and not to the reduction in the other metallic values within the bath, such as the formation of chromium oxide, etc., so long as the efficiency continues to be above this predetermined level.
• One object of the process is to maximize the amount of oxidizer flow.
• the amount of oxidizer flow is increased in the small empirically determined amounts as previously mentioned with concurrent measurements of ofi-gas compositions and calculations of oxygen conversion efficiency.
• the increasing of flow rate and the computation of conversion efi'iciency are alternately continued until the amount of oxygen or oxidizer being flowed into the system is at the supply maximum or the oxygen conversion efliciency reaches the minimum preselected value.
• the next step is to minimize the diluent gas flow.
• the efiect of this is to reduce the cost associated with the diluent gas which is unnecessary to the process.
• the reduction in diluent gas e.g., argon, is efiected in a similar manner to that of the increase of oxygen, i.e., alternately reducing flow rate While monitoring off-gas composition and the input-gas flow rates and computing the oxygen conversion efiiciency of the process. If, in the measurements, the calculations reveal that the supposed maximum oxygen fiow is less than the actual oxygen flow determined from the measurements, then errors are indicated in the system which must be resolved to insure accuracy of the measurements and minimization of loss of metallic values.
• the oxidizer conversion eificiency is less than those familiar with the art will recognize that metallic oxidation is occurring. It may be advantageous in the decarburization process to allow some metallic oxidation. In such cases, the gas conversion efficiency may be a value less than 100% but above that value which would indicate the maximum metallic oxidations to be permitted. In that event, so long as the process is continuing with a gas conversion efiiciency above this minimal value, the process is considered under control. In those instances where the actual efiiciency is less than the desired or predetermined efiiciency, it is necessary to reduce the oxygen flow to a value equal to that of the existing oxygen flow multiplied by the actual efficiency of the operation and that value divided by the desired efiiciency.
• This calculation will provide a value for oxygen input to establish a gas conversion efiiciency consistent with the minimum predetermined value. At this point it is desirable to increase the diluent flow rate a predetermined amount.
• diluent gas so long as it is not a carboncontaining gas such as carbon monoxide, will promote the decarburization process. This is true because the increased diluent gas serves to lower the partial pressure for carbon monoxide within the process and thus lower the end point carbon value that may be reached in the decarburization process.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Treatment Of Steel In Its Molten State (AREA)

Applications Claiming Priority (1)

Publications (1)

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ID=21774309

Family Applications (1)

Country Status (8)

Cited By (7)

Families Citing this family (3)

• 1970
  • 1970-03-02 US US15914A patent/US3666439A/en not_active Expired - Lifetime
• 1971
  • 1971-02-25 CA CA106233A patent/CA923710A/en not_active Expired
  • 1971-03-01 DE DE19712109676 patent/DE2109676A1/de active Pending
  • 1971-03-01 BE BE763595A patent/BE763595A/fr unknown
  • 1971-03-01 AT AT171171A patent/AT323775B/de active
  • 1971-03-02 FR FR7107152A patent/FR2081633B1/fr not_active Expired
  • 1971-03-02 ES ES388791A patent/ES388791A1/es not_active Expired
  • 1971-04-19 GB GB23629/71A patent/GB1300344A/en not_active Expired

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