US3754895A - Process for decarburization of steels - Google Patents

Process for decarburization of steels Download PDF

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Publication number
US3754895A
US3754895A US00110324A US3754895DA US3754895A US 3754895 A US3754895 A US 3754895A US 00110324 A US00110324 A US 00110324A US 3754895D A US3754895D A US 3754895DA US 3754895 A US3754895 A US 3754895A
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carbon
gas
decarburization
oxygen
melt
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US00110324A
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S Ramachandran
B Igwe
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Allegheny Ludlum Corp
Pittsburgh National Bank
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Allegheny Ludlum Industries Inc
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Assigned to PITTSBURGH NATIONAL BANK reassignment PITTSBURGH NATIONAL BANK ASSIGNMENT OF ASSIGNORS INTEREST. RECORDED ON REEL 4855 FRAME 0400 Assignors: PITTSBURGH NATIONAL BANK
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    • 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

Definitions

  • ABSTRACT Described herein is an improved process for decarburization of straight carbon as well as alloy-bearing steels abandoned v without the loss through oxidation of valuable metallic U 8 Cl 75/60 75/59 alloying elements such as chromium, manganese.
  • thermodynamic equilibrium existing during decarburization of chromium-bearing steel baths has been studied and the contents of carbon and chromium that exist in equilibrium at a given melt temperature 3 have been established.
  • chromium steel decarburization are the works of I-Iilty (The Relation Between Chromium and Carbon in Chromium Steel Refining, Trans. AIME, Vol. I85, (1949), p. 9l and of I-Iilty, Rassbach and Crafts (Observations of Stainless Steel Melting Practices, Journal of the Iron and Steel Institutes (London), Vol. 180, (1955), p. l 16).
  • a compromise alloy steel making practice thus involves the following steps: meltdown, decarburization by oxygen injection or by charging an oxygenbearing alloy at a tolerable temperature (such decarburization being accompanied by alloy oxidation into the slag), reduction of the alloying elements from the slag back into the metal, and finally finishing which entails the adjustment of the melt chemistry to conform with specifications.
  • the recovery of the alloy values from the slag is never totally effective and usually between and percent of the charged oxidizable alloy elements are lost in the slag.
  • U.S. Pat. No. 3,003,865 (issued to John B. Bridges, Oct. 10, 1961) discloses a process whereby dry gaseous material selected from the group consisting of air, mixtures of air and oxygen, and mixtures of oxygen and an inert gas, is blown through molten steel for purposes of decarburization.
  • Oxygen content of the gas mixture is specified in the range 15 to percent, and sizable reductions in alloy oxidation of chromium-containing melts are achieved, but complete prevention of metallic loss is not realized.
  • T is the temperature of the melt in degrees Kelvin
  • Z is an empirical constant whose value 0 depends on the adopted decarburization practice.
  • the process of the present invention provides improved decarburization by accounting for the kinetics of the carbon-oxygen reaction, and oxygen consumption efficiency (herein defined as the degree to which the supplied oxygen preferentially oxidizes melt carbon).
  • the systems kinetics are taken into consideration in the development of a quantitative relationship for the tolerable oxygen content of the decarburizing gas required to attain given decarburization without excessive chromium loss.
  • the equilibriumcarbon'concentration is recognized to exist at the gasmetal reaction interface only and as a consequence the bulk carbon concentration in the melt differs from the equilibrium value previously used.
  • the decarburization reaction in chromium-bearing steels occurs'at the gas-metal interface.
  • a thin liquid film separates the bulk metal from the gas phase, and the diffusion rate of either oxygen or carbon across this film determines the rate of carbon-oxygen reaction.
  • the reaction occurs instantaneously as soon as carbon and oxygen are in contact.
  • the rate of decarburization is determined largely by the rate of oxygen supply, and since carbon is available in relatively large amounts at the gasmetal interface, it is preferentially oxidized and nosignificant metallic alloy oxidation occurs.
  • the decarburization rate is determined by the rate of carbon diffusion across the liquid film. If oxygen is supplied in amounts exceeding that required to combine with the carbon arriving gas-metal interface, only a fraction of the oxygen reacts with carbon, the balance combining with valuable alloy elements resulting in undesirable metallic oxidation.
  • the decarburization rate is a function of the gas-metal surface area available at any stage of decarburization.
  • the reaction rate depends on the amount of surface and the rate'at which fresh surface is exposed to the reactant gas.
  • a preferred means of optimizing the gas-metal contact area is by injecting the gas by sub-surface means into the volume of melt and thus generating therein a large number of fine bubbles. The size of these bubbles relates to the dimension of the injecting orifice.
  • the mean bubble size formed at an orifice is a function of the orifice Reynolds number, N where:
  • oxidizing gas is introduced at a mean equivalent bubble diameter D in accordance with the following equations:
  • P is the pressure of the bubbles (atmospheres).
  • %O is the volume percentage of oxygen in the bubbles.- As indicated previously, the equilibrium carbon concentration is- 2C 0 (3) 2C0 (g) C CO,(g) 2CO(g) it should be noted that a given volume of steam or-carbon dioxide reacts with only half as much melt carbon as the same volume of oxygen. Consequently, in a mixture of oxygen, steam, and carbon dioxide, %Ox is represented by:
  • equation (7) becomes:
  • the bubble pressure P is the sum of the ambient pressure in the vessel, the ferrostatic head, and the surface tension pressure of the bubble.
  • the present invention assumes that ear bon alone is removed by the injected oxidizing gases.
  • other elements such as silicon are present in the melt and are preferentially oxidized to low equilibrium level prior to the commencement of carbon removal.
  • the gas injection schedule when silicon is present in amounts greater thanthe equilibrium level, should be based on the consideration that at the beginning of the blow, only silicon in part is oxidized into the slag and in part escapes as the volatile oxid'e until equilibrium is attained.
  • the amount of oxygen or oxygen equivalent required for this phase of the process can be computed, assuming chemical stoichiometry, from the reaction:
  • the measurements of total gas and oxygen flow rates are made with the usual volume flow measuring devices such as flow meters and orifice plates.
  • the gas composition is determined with conventional gas analyzing techniques, for example mass spectrometers, etc.
  • the desired carbon should be that specified for the heat.
  • the desired carbon would be some intermediate value that is less than the initial carbon concentration. To ap proach continuous operation, decreasing carbon concentrations may be programmed.
  • the decarburization reaction for a melt containing, in addition to carbon and chromium, the alloying elements Mn, Cu, Mo, and Ni, can be represented as:
  • injected gas composition is varied to yield a lower Pco value.
  • Steps (b) (h) are repeated.
  • the bulk carbon determined in (h) now becomes the initial carbon.
  • the above sequence illustrates a single step decarburization process.
  • a smoothly continuous carbon reduction and gas blow schedule can be generated.
  • the gas ratio reductions can be achieved automatically in this case by a properly designed fiow rate control device.
  • the melt was made in a 500 lb. Basic Oxygen Furv nace.
  • the decarburization gas was supplied via a topsubmerged water-cooled lance whose two orifices (of diameter one-sixteenth inch each) were machined from Type 304 stainless steel. Each orifice was inclined at 10 to the lance axis in order to promote bubble dispersion in the bath. A refractory insulation surrounded the orifices, whose tips were immersed about 2 inches below the melt surface.
  • the gas supply program developed was as follows:
  • the attached figure shows the theoretical and experimental decarburization paths for this heat.
  • the bath temperature during the blow ranged from 2990 F at the start of blow to 3100 F after 4 minutes, and had fallen to 2960 at the end of the process.
  • the melt was decarburized to 0.073 percent carbon.
  • a method of decarburizing molten steel which comprises measuring the carbon content of said steel,
  • C is the desired carbon content
  • Ceq is the equilibrium carbon concentration
  • T is the temperature of the melt in degrees Rankin
  • H is the bubble rise height in inches
  • P is the pressure of the bubbles in atmospheres
  • D is the meanequivalent bubble diameter; controlling the size of said bubbles of oxidizing gas and the point of introduction thereof into said steel melt in accordance with the foregoing equation to provide said necessary decarburization rate.
  • said oxidizing gas includes a diluent gas from the group consisting of argon, helium, nitrogen, and carbon monoxide.

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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)
US00110324A 1971-01-27 1971-01-27 Process for decarburization of steels Expired - Lifetime US3754895A (en)

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US11032471A 1971-01-27 1971-01-27

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US (1) US3754895A (xx)
AT (1) AT327970B (xx)
AU (1) AU465738B2 (xx)
BE (1) BE778517A (xx)
CA (1) CA959272A (xx)
DE (1) DE2202939A1 (xx)
ES (1) ES399219A1 (xx)
FR (1) FR2123341B1 (xx)
IT (1) IT946864B (xx)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3816720A (en) * 1971-11-01 1974-06-11 Union Carbide Corp Process for the decarburization of molten metal
US3990887A (en) * 1970-02-06 1976-11-09 Nippon Steel Corporation Cold working steel bar and wire rod produced by continuous casting
US6093235A (en) * 1995-10-23 2000-07-25 Mannesmann Ag Process for decarbonising a high-chromium steel melt

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3594155A (en) * 1968-10-30 1971-07-20 Allegheny Ludlum Steel Method for dynamically controlling decarburization of steel

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3990887A (en) * 1970-02-06 1976-11-09 Nippon Steel Corporation Cold working steel bar and wire rod produced by continuous casting
US3816720A (en) * 1971-11-01 1974-06-11 Union Carbide Corp Process for the decarburization of molten metal
US6093235A (en) * 1995-10-23 2000-07-25 Mannesmann Ag Process for decarbonising a high-chromium steel melt

Also Published As

Publication number Publication date
AT327970B (de) 1976-02-25
BE778517A (fr) 1972-07-26
AU3814172A (en) 1973-07-26
FR2123341A1 (xx) 1972-09-08
IT946864B (it) 1973-05-21
AU465738B2 (en) 1975-10-09
DE2202939A1 (de) 1972-08-03
FR2123341B1 (xx) 1976-06-11
CA959272A (en) 1974-12-17
ATA53172A (de) 1975-05-15
ES399219A1 (es) 1974-11-16

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Owner name: ALLEGHENY LUDLUM CORPORATION

Free format text: CHANGE OF NAME;ASSIGNOR:ALLEGHENY LUDLUM STEEL CORPORATION;REEL/FRAME:004779/0642

Effective date: 19860805

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Owner name: PITTSBURGH NATIONAL BANK

Free format text: SECURITY INTEREST;ASSIGNOR:ALLEGHENY LUDLUM CORPORATION;REEL/FRAME:004855/0400

Effective date: 19861226

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Owner name: PITTSBURGH NATIONAL BANK

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST. RECORDED ON REEL 4855 FRAME 0400;ASSIGNOR:PITTSBURGH NATIONAL BANK;REEL/FRAME:005018/0050

Effective date: 19881129