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
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/56—Manufacture of steel by other methods
  • C21C5/562—Manufacture of steel by other methods starting from scrap
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• This invention relates to the refining of steels, including all grades of plain carbon steels, low and high alloy steels, heat-resisting steels, and stainless steels. More particularly, this invention relates to a new and improved process for the fumeless refining of steels.
• An essential step of refining in all steel-making processes is the removal of carbon by oxidation, together with other oxidisable elements, notably silicon, manganese, and phosphorus. Hydrogen and nitrogen are also removed by the flushing of the carbon boil.
• the amount of carbon to be removed and the rate of its removal governs the speed of the process, and also the final hydrogen and nitrogen contents, and accordingly it is usual to arrange for at least 0.20% of carbon to be removed from ferritic grades of steel, and 0.1% to 0.25% from austenitic stainless steels.
• the extent of refining is such as to leave the steel with the required carbon content, but it is also known to refine a charge initially having a carbon content equal to or less than that required in the refined steel, when the steel after refining inevitably has less than the required amount of carbon. In this case, it is normal to recarburize the refined steel to give it its required carbon content.
• Oxygen-lancing therefore, meets all the requirements of the steel-makers, but it has one serious disadvantage; the inevitable result of oxygen-lancing is that large volumes of dense, brown iron-oxide fumes are emitted when the reaction is proceeding and expensive fume-cleaning equipment must be provided to prevent pollution of the surrounding atmosphere.
• a principal object of the present invention is to provide an economic and efiicient process for the rapid removal of carbon and other oxidisable elements from a molten ferrous bath without the undue emission of the dense, brown iron oxide fumes commonly associated with oxygen-lancing.
• a further object is to provide such a process in which the required reduction in hydrogen and nitrogen contents will be assured.
• a process for refining a steel comprises injecting into a molten ferrous bath a particulate oxide of a metal desired in the finished steel by means of a carrier gas containing free or bound oxygen in such quantities and at a rate sufficient to react with carbon to effect a rapid carbon boil, with evolution of carbon monoxide which flushes hydrogen and nitrogen from the bath, the volume of evolved carbon monoxide in relation to the volume of oxygen available in the carrier gas under conditions prevailing in the bath being such as to give rise to a protective gaseous atmosphere above the bath which is non-oxidising for elemental iron, whereby production of brown iron oxide fume resulting from oxidation of elemental iron is substantially inhibited.
• the carrier gas is one having an oxidising potential not greater than oxygen-enriched air containing about 30% to about 35% by volume, and it is further preferable that the carrier gas be utilised at a ratio of less than about 3 cubic feet of gas per pound of metal oxide injected, so that the volume of carbon monoxide produced exceeds the volume of oxygen injected to substantially inhibit the oxidation of elemental iron by the gas and the production of brown iron oxide fumes.
• the ratio of carrier gas to oxide injected may be about 1 cubic foot of air per pound of oxide.
• the present invention has unexpectedly found that when the volume of carrier gas is kept near the minimum required to inject the oxide that a satisfactory carbon boil and a sufficient flushing of hydrogen and nitrogen is achieved without any or little production of obnoxious, brown iron oxide fumes.
• the invention recognises for the first time that the volume of carbon monoxide resulting from the oxidation of carbon can be utilised to bring about the substantial inhibition of the production of dense iron oxide fume in a refining process that has a rate of carbon elimination comparable with that of oxygen-lancing.
• the carrier gas contains less than about 30% to 35% by volume of oxygen and the volume of the carrier gas such as air, and is preferably substantially no more than that required conveniently to bring about injection of the particulate metal oxide, e.g., less than about 3 cubic feet per pound of oxide
• the volume of carbon monoxide produced by the reaction exceeds the volume of oxygen injected and forms a blanket across the surface of the melt, the non-oxidising nature of which is not disturbed by the relatively small volume of gaseous oxygen from the carrier gas which also escapes from the melt.
• the volume of carrier gas should preferably at most be no more than about 3 cubic feet of gas per pound of metal oxide. When conditions permit, however, even smaller volumes such as 2 cubic feet and preferably one cubic foot of gas per pound of particulate oxide can be employed.
• the carrier gas employed for the purpose of this invention is one which functions essentially as a carrier for the added metal oxide, the oxide being the principal agent for converting carbon in the bath into carbon monoxide.
• the carrier gas may participate to a small extent in the oxidising process without departing from the process of the present invention and, therefore, may be active as far as oxidation reactions are concerned by introducing some oxygen into the molten bath. Accordingly, carbon dioxide, air, or even oxygen-enriched air may be used as the carrier gas provided the oxygen content of the gas is maintained below about 30% by volume and the volume of gas to oxide is maintained within the limits described above. Gases containing higher amounts of oxygen even at minimum ratios of gas to oxide produce objectionable iron oxide fume.
• the boil can be initiated and continued at a relatively low bath temperature or the boil enriched and continued at any desired temperature by the maintained application of an external heat source to the furnace;
• part of the oxide can be injected into the slag above the bath to build up slag iron which not only aids carbon removal from the bath but also increases the metal yield, and
• the injection does not give rise to the severe wear of the refractory of the furnace frequently occasioned by 0 lancing or by oxy-fuel burners, neither does it require the consumption of water involved for the cooling of such burners.
• the equipment for dispensing and injecting the oxide is cheap to install and operate.
• it may simply be a hopper having an outlet communicating with the line for the carrier gas with means in the outlet for controlling the flow of powder to the line communicating with a lance for injecting the powder into the bath.
• the powder is controlled by providing means capable of pulsefeeding the powder into the line for the carrier gas, the pulse-feeding may be carried out by providing a valve in the form of a paddle wheel in the bottom of a hopper, the compartments formed between two vanes of the paddle wheel effectively metering the supply of particulate oxide to the gas line.
• two hoppers are provided, secured to a central spindle, one of which is connected to the carrier gas line.
• the hopper When that hopper is empty, the hopper is simply disconnected from the line, the assembly rotated about the spindle to bring the other, full hopper to the position in which it can be connected to the carrier gas line, and while that hopper is emptying, the other hopper is filled.
• the process admits of variation in the initial carbon content of the bath.
• the carbon content of the charge from which the bath is formed is in excess of that required in the refined steel, the injection is continued until carbon has been removed by oxidation to leave an amount equal to that required in the final steel, by which time hydrogen and nitrogen have been appreciably reduced and other undesirable elements such as silicon, manganese, and phosphorus have all been oxidised to the desired level.
• the injection is continued until carbon has been removed by an amount sufiicient to assure removal of hydrogen and nitrogen and oxidation of any other undesirable elements, and the refined steel is then recarburised in known manner, to give it its required carbon content.
• suitable oxides are those of iron, nickel, chromium, molybdenum, copper, cobalt and tungsten.
• iron oxide is used, e.g., in the form of millscale, iron ore, or iron oxide dust fro-m fumecleaning plants; and for stainless steel, oxides of nickel, chromium, molybdenum, copper, cobalt or tungsten, or any combination of these, possibly with an iron oxide also both to promote the oxidation reactions and to provide a .5 cheaper source for at least part of the metal constituents of the steels.
• Particle as used in the specification and claims, is intended to embrace all useful grades from relatively coarse, granular grades (say 5.75 mm. grain size) down to very fine grades.
• the metal oxide cannot be so coarse as to be incapable of being blown through the lance nor can it be so very fine that it would remain entrained in the gas and simply be blown into the furnace atmosphere.
• the following has proved to be an adequate range of gradings:
• Example 1 Carbon steel A 4-ton electric arc furnace was charged with steel scrap and heated to a temperature of 1550 C., a slag being formed on the bath in known manner. The bath was then ready to be oxidised, and powdered millscale with the following grading':
• the bath was sampled immediately before and immediately after the oxide injection and was found to contain (percent by weight):
• This steel was required to have 0.24% CA and accordingly required no recarburisation.
• the bath was then slagged off, de-oxidised, a reducing slag made up, in known manner and finally tapped.
• the rate of carbon removal was such that nitrogen and hydrogen were appreciably reduced as can be seen from the above table and the oxidation proceeded with no visible brown fume from the bath.
• Example 2.Carb0n steel The procedure as outlined in Example 1 was followed and the bath again oxidised by injecting millscale powdered to 5 BSS with compresssed air as the carrier gas, but in this case, 100 lbs/minute of powder was injected for 3 /2 minutes, utilising a total of 375 cubic feet of air at S.T.P. The oxidation proceeded with no visible brown fume from the bath.
• the steel was required to have 0.30% C and accordingly, the steel was recarburised by adding 280 lbs. of pig iron known to contain 4% C.
• Example 3 Austenitic stainless steel The same procedure as outlined in Example 1 was fol lowed, on /2-ton charge in an electric arc furnace, and the bath again oxidised by injecting millscale powdered to 5 BSS (as in Example 1) with compressed air as the carrier gas. The powder was injected at a rate of 20 lbs./ minute for 3 /2 minutes, utilising a total of 105 cubic feet of air at S.T.P. The oxidation proceeded with no visible brown fume from the bath.
• the table shows clearly the reduction by oxidation of the oxidisable elements.
• the Cr content of the steel has been reduced to a level too low for the required final analysis of the steel, and accordingly, additions of reducing agents, such as ferro-silicon are made to the slag whereby Cr is released from the slag back to the bath. After sampling, the Cr content was found to have increased to 14%.
• the bath was then slagged off and a reducing slag made up and finally tapped. If it had been the case that the other alloying elements needed to be increased, suitable additions of such elements could be made after the reducing slag had been made up.
• Example 55% chrome alloy steel The same procedure as outlined in Example 1 was followed, using a mixture of 50% chromite sand (crushed chrome ore) and 50% millscale, both graded in similar manner to the millscale of Example 1. The oxidation proceeded with no visible brown fume given off from the bath.
• the bath was sampled and found to contain 0.75% carbon and 6.18% chromium. After oxidation, the bath contained 0.65% carbon and 5.80% chromium, well within the 5% to 6% chro- 8 Examples 8 and 9 Examples 8 and 9 are carried out to illustrate the limit required on the amount of oxygen that should be in the carrier gas to prevent the production of brown fume.
• the chromite sand has, 5 i
• 5 i In each fzxample Powdered ⁇ ninscale is into therefore, provided a cheap source of chromium for the reslstor furnace usuig comgressed an as the h d h if I if t th d b carrier gas.
• the air is then enriched with oxygen up to f an as e i 0 8 re uc y the point when brown oxide fume is first observed and non of the chrommm, making unnecessary the addition 10 the pfircentage of oxygen in the carrier gas at that point of expensive chromium at the reducing stage.
• i e o d d i th following table:
• a ti rod furnace was l f 2 1th stleal at S.T.P. of compressed air were used as a carrier gas 30 Zg i: g i ffi i i .f 3% at a pressure of 50 p.s.i.
• the oxidation proceeded with mm on e a now anner a no visible brown fume from the bath was t ..n ready to be oxidised and powdered millscale,
• the bath was sampled immediately before and immegenerally 9 the giadmg referred to m prevlou? examdiately after injection and was found to contain (percent g .mlected mm the .bath for (one) mmute. at b Weight): s./n 11nute by a lance mmersed in the bath, using y carbon dioxide as the carr1er gas.
• the total volume of carbon dioxide was 50 cubic feet when converted to S.T.P.
• Particle Size Percent y The invention in its broader aspects is not limited to 33904590 the specific details shown and described and departures may be made from such details without departing from 106041025 the principles of the invention and without sacrificing its 0325-0318 chief advantages. (1018-0015 11-6 What We claim 1.
• said oxide being in- Percent 5 jected by means of a carrier gas and at a rate sufiicient to Sample Percent C C oxidize the carbon in the bath and effect a rapid carbon
• said carrier 513/ rm'nslafter melt-out Finish 11139011011 1 gas being used in an amount suflicient to inject the deg gfif g gfggfi fi .17 7 sired rate of oxide but having an oxidizing potential not greater than oxygen-enriched air containing about 35%
• the steel was then tapped in the usual manner and cast oxygen by volume so that the carrier gas will not sub ⁇ into ingots.
• the carrier gas is selected from the group consisting of air, oxygen-enriched air containing up to 35% oxygen by volume, carbon dioxide, or mixtures thereof.
• iron oxide is in the form of millscale, iron ore, or iron oxide dust.
• a process for the refining of carbon steel in which the production of brown iron oxide fume during decarburization is substantially reduced comprising injecting into a molten ferrous bath containing carbon, particulate iron oxide suspended in air as a carrier gas for the oxide, said oxide being injected at a rate sufiicient to oxidize the carbon in the bath and effect a rapid carbon boil with the evolution of carbon monoxide and the flushing of hydrogen and nitrogen from the bath, and said carrier air being used at a ratio of less than about three 3) cubic feet of air per pound of iron oxide injected, so that the air will not substantially influence the rate of oxidation of carbon from the bath and so that the iron oxide will be the principal oxidizing agent for the carbon, whereby the evolving carbon monoxide will create a non-iron oxidizing atmosphere above the bath and substantially reduce the production of brown iron oxide fumes during the decarburizing.

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

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Cited By (3)

• 1971
  • 1971-01-11 US US00105701A patent/US3749567A/en not_active Expired - Lifetime
  • 1971-09-16 FR FR7134156A patent/FR2106591A1/fr not_active Withdrawn
  • 1971-09-17 BE BE772760A patent/BE772760A/xx unknown
  • 1971-09-17 ES ES395185A patent/ES395185A1/es not_active Expired

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