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

Definitions

• This invention relates to a method of decarburization, deoxidation and alloy addition in molten iron-base melts promoted by the addition of an inert gas to said molten iron-base melt.
• the present processes for making steel generally proceed in four phases; namely, (1) meltdown (2) decarburization with air or pure oxygen whereby a substantial portion of slag is formed containing alloying element oxidation products from the iron-base melt, (3) reduction of alloying elements from the slag by the addition of metallic reducing agents, and (4) finishing or deoxidation of the final product.
• the present processes find it desirable to add suflicient oxygen to the melt to force the reaction to the right by greatly increasing the iron oxide content of the melt; This causes a substantial portion of the alloying elements in the steel to oxidize and pass to the slag.
• the metal oxides are subsequently reduced by addition of reducing agents such as silicon, manganese or chromium but the addition of reducing agents also cause further introduction'of carbon; therefore, the previously adjusted carbon content is upset.
• reducing agents such as silicon, manganese or chromium
• the diffusion rate could possibly be augmented by severe agitation of the molten metal charge; however, this calls for additional costly equipment installation.
• the presence of slag on the surface of the melt further reduces the effectiveneses of the above-described gas-removal principle by reducing the effective surface area available.
• the inert-gas consumption for the above process is very high, possibly reaching the degree where the cost of the inert gas becomes economically prohibitive.
• An exemplary process accomplishing the above-mentioned objects comprises as a step in the production of steel from a molten iron base melt introducing at least one inert gas selected from the group consisting of argon, krypton, xenon, helium and neon into said molten ironbase melt to decrease the effective pressure of carbon oxide in the melt and thereby permit greater extent of reaction betwen carbon and oxygen in the melt.
• inert gas is defined as a gas which in addition to being chemically inert will not alloy with iron-base melts in appreciable portions when the oxygen content in the melt is low.
• the inert gas should be injected in the form of small single bubbles or a dispersion of small bubbles at least several inches below the melt level. In the preferred embodiment of this invention, the bubbles should not exceed about 3 to 5 millimeters in diameter. This gives minimum inert gas consumption. Larger bubble sizes can also be used if inert gas consumption is not of major importance. Small bubbles provide an extremely large metal surface area per unit amount of gas introduced into the melt. For example, one standard cubic foot of inert gas will be exposed to about 4000 square feet of molten metal surface if the bubble size is about 3 mm.
• the effective surface area can bechosen essentially at will or as required for a particular metal treatment by choosing the proper bubble size.
• the effective surface area in the prior art processes is essentially limited by the furnace or ladle employed. This is not the case with the present invention.
• the mass transfer rate is dependent on the relative partial pressures of the components to be transferred and also on the equilibrium partial pressures of a given component for the dissolved and gaseous state.
• the essentially pure inert gas as employed in the practice of this invention, has an initial partial pressure of carbon monoxide equal to about zero, and, in this manner, provides a large driving force for the rapid removal of the dissolved carbon monoxide.
• the residence time of a bubble in the melt is of the order of about 1 second per foot of melt depth, hence the inert gas bubbles essentially get saturated with carbon monoxide during the bubbling process and are rapidly removed from the melt.
• every bubble gets the benefit of being introduced at essentially zero carbon monoxide partial pressure within the bubble, and therefore deoxidation of the melt can proceed to a much greater extent.
• inert gas is defined to mean a gas selected from the group consisting of helium, krypton, neon, argon, and xenon. We have found that the rate at which nitrogen is dissolved by molten iron and its alloys is influenced markedly by the oxygen content of the metal.
• an inert gas is not utilized.
• an inert gas is utilized as taught in the present process, the effective pressure of carbon oxides is reduced thereby allowing the carbonoxygen reaction to proceed further.
• a deficiency of carbon can be corrected by adding a carbonaceous reducing agent to the melt from some external source.
• a fluidized stream of carbonaceous reducing agent' may be added in the inert gas stream itself.
• a deficiency of oxygen can be supplemented by several different methods of adding oxygen.
• an oxidic metal compound may be added to the melt either fluidized in the inert gas stream or otherwise.
• Iron oxide is an example of an oxidic metal addition
• oxidic compounds of chromium, silicon, manganese, and nickel may be added to supply sufilcient oxygen to the melt to supplement the original oxygen content and enable adjustment of the oxygen content'to a level required to completely decarburize the melt.
• the latter alternative presents another facet of the present invention, namely, alloy addition accomplished simultaneously with deoxidation and decarburization to very low levels or in fact to any desired level.
• This three-fold function of the present process can also be applied to accomplish the last three steps in steel making processes without the need or at least greatly reducing the need for adding metallic reducing agents to recover alloying elements from the slag.
• An equilibrium exists between the metals ofthe melt and the corresponding oxides of these metals in the slag over the melt.
• the oxygen is consumed in this reaction, the equilibrium between metal and oxygen in the melt and the corresponding metal oxides in the slag is upset and to reach equilibrium conditions the metal oxide from the slag phase must pass to the melt whereit supplies oxygen for further reaction with carbon. This results in metal addition fromthe slag to the molten metal without the use of reducing agents.
• Another distinct advantage of the present deoxidation and decarburization process is the ability to reduce carbon content to virtually nothing in a virtually completely deoxidized melt without the loss of valuable alloying elements such as chromium, manganese and nickel for example.
• alloying elements such as chromium, manganese and nickel for example.
• a definite relationship exists between carbon, chromium and temperature. Attempts to remove carbon to a low level results in chromium loss by oxidation unless the temperature of the melt is maintained at very high levels during introduction of oxygen.
• the present process permits a steelmaker to reduce carbon contents to lower than four parts per ten thousand with virtually no loss of chromium, for example.
• Molten steel cannot be tdeoxidized to an extremely low oxygen content of, say, about 0.603% while it is being held in a container constructed from or lined with refractory oxide materials. At'certain low oxygen levels, these levels being determined by the particular refractory oxide employed, this refractory oxide will begin to dissolve into themetal. This may. result in both severe refractory damage and oxide inclusions in the metal. is preferred to deoxidize, at least during the last stages of the process, in the presence of a slag which will dissociate into oxygen and metal'before the refractory lining is at tacked. "Suchslag serves the double purpose of protecting the furnace or ladle refractory liningjfroni attack and also of providinga means for the addition of'useful alloys such as chromium and silicon to the molten metal. 7
• the inert gas decarburization and deoxidation process of this invention is particularly applicable for the manufacture of silicon-iron alloys.
• Such alloys are employed in electrical'apparatus and machinery because of their desirable magnetic properties.
• Typical applications of silicon-iron alloys are the manufacture of transformer cores, electric motor armatures, and the like.
• the ideal alloy for such application in order to minimize hysteresis and eddy current losses, is a pure silicon-iron alloy containing essentially no carbon, sulfur, manganese, oxide inclusions or internal stresses.
• the usual processes for the manufacture of silicon-iron alloys meeting such specifications comprise treating a lowcarbon charge in either an electric or an open hearth furnace.
• the main object of the furnace-refining step is to make a very low carbon (about 0.02%) ingot iron.
• This iron is the basic material for the manufacture of the silicon-iron alloys.
• the silicon content of these alloys usually does not exceed about 4 to 4 /2 percent.
• Silicon additions are usually made in the ladle or pouring stream after tapping from the open hearth or electric furnace. A portion of the silicon is oxidized in lowering the ox gen content from that in equilibrium with the carbon to that in equilibrium with the final silicon content of the iron. This leaves carbon in the melt appreciably in excess of the value which would be in equilibrium with the oxygen content of the iron.
• Inert gas decarburization can be beneficially applied at this point.
• the final carbon content is thenreduced to a low level, the exact value depending on the effective pressure or" carbon monoxide in the inert gas bubbles lfiZlV-r ing the melt, and also the oxygen content of the melt. In such a manner oxygen can be removed to levels as low as 0.004 percent.
• Table I is a compilation of the experimental In this experiment, argon was bubbled from the bottom of a lb. crucible containing molten steel covered with fused chrome oxide-containing slag.
• the nozzle used for injecting the gas stream comprised the partially porous bottom of the crucible, through which'gas was forced under pressure.
• Argon bubbling was continued for 70 minutes, at an argon flow rate of about 4 standard cubic feet/hour.
• the average bubble size was about 4 mm. in diameter, and the average bubble residence time Within the melt was about 1 second/foot of melt depth.
• Table II The experimental results are compiled in Table II.
• Oxygen injection started at 150 lb./sq. in. pressure.
• EXAMPLE III This experiment was performed by introducing a fine spray of argon bubbles at the bottom of a 50-lb. crucible. The molten metal charge was covered with a high-silica slag. The no'zzle'used for injecting the gas stream comprised the partially porous bottom of the crucible, through which gas was forced under pressure. Argon bubbling was continued for 82 minutes at an argon fiow rate of about 4 s.c.f./hour. The average bubble size was about 4 millimeters in diameter, and the average bubble residence time within the melt was about 1 second/foot of melt depth. The experimental results are compiled in Table III.
• EXAMPLE IV A 5-lb. molten iron charge containing 0.04% oxygen was placed in a crucible. No slag was employed. To this charge 1.4 grams of carbon were added. After the carbon addition, argon was bubbled through the molten charge for 1 minute at a rate of about 15 sci/hour. The bubbling was done by means of a -inch orifice in a magnesia tube. The final analysis of the melt showed 0.01% oxygen.
• This experiment illustrates effective deoxidation of a molten iron charge by the addition of carbon to the melt and subsequent bubbling. with an inert gas.
• a 2000-lb. molten iron charge in a furnace contains 0.2 0% carbon by analysis.
• a fluidized stream of Cr O in argon is introduced into the molten iron charge through a bottom nozzle having a 0.5-inch inside diameter.
• About 6.33 lb. of C1 O are introduced into the melt in this manner.
• About 225 standard cubic feet of argon are used.
• About 25.6 cubic feet of carbon monoxide are produced and the carbon content of the steel is reduced to about 0.10%.
• the chromium content of the melt is increased by about 0.216 weight percent.
• additional carbon as carbon particles or a carbonaceous gas may also be introduced into the melt for the purpose of removing the dissolved oxygen.
• ()nly enough carbon is introduced in the inert gas stream to react with the oxygen present. The carbon content of the molten metal charge remains essentially unaltered during this operation.
• both carbon and an alloying element or an alloying element oxide may be introduced into the melt via the inert gas stream.
• control of the severity of both the decarburization and deoxidation reactions may be achieved.
• denitriding can be accomplished in conjunction with decarburization, deoxidation and alloy addition.
• denitriding has been accomplished to a limited extent in the prior art by flushing molten melts with inert gases
• denitriding has not been accomplished in the prior art during decarburization, deoxidation and/ or alloy addition by introducing inert gases into molten iron-base melts.
• the above four adjustments can be made in a single step.
• denitriding can be carried out alone by the introduction of inert gases into molten irons as shown below.
• Inert gas injection according to this invention can also be combined with a vacuum deoxidizing process for very ell'ective nitrogen removal.
• a vacuum deoxidizing process the relatively high surface area required is produced by the gases released from the melt.
• oxygen and carbon concentrationin the melt become low, the available surface area is drastically reduced and degassing practically ceases before nitrogen'removal issubstantia ly completed.
• the process cannot then be accelerated significantly either by producing a higher vacuum or by pro longing'thetime of treatment.
• the vacuum degassing may be carried out until an oxygen concentration in the melt of'about 0.01%
• inert gas injection is then carried out until the desired nitrogen level in the melt is attained. Simultaneous additional deoxidation is also achieved in this manner. As an alternate mode of operation, inert gas injection may be carried out simultaneously with the vacuum degassing step.
• EXAMPLE VI A melt of non-aging mild steel having an approximate composition of about 0.10% C, 0.03% and 0.006% N; is degassed in a vacuum vessel at a pressure of about 1 mm. of mercury. The degassing by vacuum is continued until the melt contains about 0.01% oxygen. The vacuum is interrupted and argon is then injected into the melt near the bottom of the molten metal container in the form of small bubbles at a rate of about 300 s.c.f./ton of steel by means of a suitable ceramic injection device. The final nitrogen content of the melt is less than about 0.001%.
• EXAMPLE VIII Two Armco iron melts were separately saturated with nitrogen gas at 1 atmosphere pressure in a controlled atmosphere furnace. Melt (1) contained 0.076 Weight percent oxygen and melt (2) contained 0.010 weight percent oxygen.
• Melt deoxidation was achieved by the addition of stoichiometric amounts of carbon to each melt.
• the carbon content of each melt after deoxidation was about 0.02 weight percent.
• argon was introduced into each of melts (1) and (2) for various periods of time.
• Melt (1) showed a nitrogen content of 0.012 weight percent at the end of about 108 minutes of argon introduction and melt (2) showed a nitrogen content of about 0.004 at the end of about 40 minutes of argon introduction.
• a process for treating a molten iron-base melt I and reducing alloy constituents from a molten slag in contact with a molten iron-base melt to cause an increase in the alloy constituents in said molten iron-base melt comprising providing a molten iron-base melt and a molten slag covering said iron-base melt said slag containing at least one oxidic alloying compound containing an element capable of alloying with said molten iron-base melt and introducing at least one inert gas selected from the group consisting of argon, xenon, krypton, neon, and helium into said iron-base melt and introducing a carbonaceous reducing agent into said melt; substantially decreasing the effective pressure of carbon oxide in the melt thereby causing a greater extent of reaction between said carbon and oxygen in said molten iron-base melt to cause reduction of said oxidic form of said alloying element from said slag and increasing the content of said alloy element in said molten iron-base melt.
• sufiicient carbon is introduced into said molten iron-base melt as a fluidized stream of carbonaceous reducing agent in said selected inert gas to'react with the oxygen in said melt.
• a process for treating a molten high-carbon ironbase melt comprising introducing at least one inert gas selected from the group consisting of argon, xenon, krypton, neon and helium into said melt, said selected gas containing fluidized oxidic alloying compounds; substantially decreasing the effective pressure of carbon oxide in said molten iron-base melt thereby causing a greater extent of reaction between said carbon in said molten iron-base melt and said oxygen in said iron base melt; and reducing alloying elements from said introduced oxidic alloying compounds to increase the alloy element content in said iron-base melt.
• a process for concurrent deoxidation, denitriding, decarburization, and alloy addition in molten iron-base melts comprising introducing at least one inert gas selected from the group consisting of argon, xenon, krypton, neon, and helium into said molten iron-base melt and introducing oxidic alloying compounds into said molten iron-base melt and introducing sufiicient carbon into said molten iron-base melt to react with substantially all the oxygen present in said molten iron-base melt, reducing the etfective pressure of carbon oxide in said melt to cause an increased extent of reaction between carbon and oxygen in said melt.
• a non-vacuum process for treating a molten ironbase melt and reducing alloy constituents from a molten slag in contact with a molten iron-base melt to cause an increase in the alloy constituents in said molten ironbase melt comprising, providing a molten iron-base melt and a molten slag covering said iron-base melt, said slag comprising at least one oxidic alloying compound containing an element capable of alloying with said molten iron-base melt; introducing a flow of at least one inert gas selected from the group consisting of argon, xenon, krypton, neon, and helium into said iron-base melt, said melt being in a non-vacuum environment; and introducing a carbonaceous reducing agent into said melt; maintaining said flow at a rate which substantially decreases the eifective pressure of carbon oxide in the melt thereby causing a greater extent of reaction between said carbon and oxygen in said molten iron-base melt to cause reduction of said oxidic
• sufcient carbon is introduced into said molten iron-base melt as a fluidized stream of carbonaceous reducing agent in said selected inert gas to react with the oxygen in said melt.

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• Treatment Of Steel In Its Molten State (AREA)

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Cited By (15)

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• 0
  • BE BE609880D patent/BE609880A/xx unknown
  • CA CA692788A patent/CA692788A/en not_active Expired
  • NL NL270519D patent/NL270519A/xx unknown
• 1960
  • 1960-11-18 US US70121A patent/US3169058A/en not_active Expired - Lifetime
• 1961
  • 1961-10-30 FR FR877472A patent/FR1304799A/fr not_active Expired

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