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
  • C21C7/0685—Decarburising of stainless steel

Definitions

• the present practice in high-chromium steel melting comprises several phases including (1) meltdown, (2) decarburization, (3) reduction of alloying elements from the slag formed during decarburization and (4) finishing or adjusting the final steel to specifications.
• Pure oxygen blowing is commonly employed to decarburize chromium-containing steels. During decarburization a substantial portion of chromium as well as iron is oxidized into the slag in an eiiort to remove carbon to a low level without necessitating maintenance of temperatures in excess of the refractory limit.
• the steel maker utilizes a lower temperature than required to prevent severe metallic oxidation and decarburizes the molten steel to the desired carbon content and accepts the oxidation of chromium and iron to the slag as an attendant evil.
• reducing agents such as ferro-alloys containing silicon are added to the melt to reduce the chromium and iron oxides from the slag and return the same to the steel melt. Additional carbon may be introduced into the melt from carbon electrodes during electric furnace operations; therefore, allowance must be made for this condition during decarburization.
• the process accomplishing the above-mentioned objects comprises introducing gaseous oxygen and at least one inert gas selected from the group consisting of argon, xenon, helium, krypton and neon into a molten stainless steel inelt containing from 3 to 30 percent chromium to cause decarburization of the melt by reaction between oxygen and carbon and removal of carbon oxide.
• the ratio or" oxygen to inert gas is decreased during and in relation to decarburization in a controlled programmed manner whereby chromium and iron oxidation are cssentially minimized at temperatures utilized in highchromium steel production.
• the maximum permissible oxygen concentration in the gas introduced into the melt is governed by the following variables; namely, chromium content, carbon content and temperature. It has been found that maintenance of a critical oxygen volume percent of the total selected inert gas-oxygen volume introduced into the melt will allow decarburization at various temperatures with minimum oxidation of chromium and iron from the steel. In addition, the oxygen volume percent in the oxygen-inert gas mixture can be critically adjusted to a value determined by the final desired and predetermined carbon and chromium content for given temperatures at the finishing point.
• Alternative (3) will be most eflicient and advantageous from the standpoint of decarburization time but continuous adjustment of the ratio of oxygen volume percent to inert gas volume percent as carbon continuously decreases will be more diilicult.
• the first alternative would appear to be easier to maintain in production and the time for the total decarburization process would not be substantially greater than alternative (3).
• Alternative (2) will allow decarburization to predetermined levels at predetermined chromium contents but will require the relatively longer time for decarburization.
• Alternative (4) in which final decarburization is achieved by the iniection of inert gas alone makes it possible to obtain exceptionally low carbon levels of 0.01 percent and lower providing that preliminary decarburization with oxygenargon mixtures has been carried far enough. Another the melt at a given instant of time.
• the critical oxygen volume percent is dictated by the following relationship which applies for steel containing 3 to 30 percent chromium. It should be noted that a portion of this relationship is shown in the Hilty et al. article on p. 119. This portion of the relationship is utilized in conjunction with modifications which give the following relationship to indicate the maximum critical oxygen volume percent to be maintained during each of the several alternatives in programmed decarburization of stainless steels.
• Oxygen volume percent is noted by percent This is the maximum volume percent of oxygen in relation to total volume of oxygen plus inert gas introduced into
• percent Cr and percent C weight percentages of chromium and carbon desired at the end of a given decarburization step
• T temperature in degrees Kelvin at the end of the step
• Z represents a parameter, not recognized by the prior art, which has a value in the range of about 1.1 to 20.0.
• the recognition and evaluation of the parameter Z comprise further improvements over prior art because knowledge of'its value permits the calculation of the required volume percent oxygen in the inert gasoxygen mixture in order to obtain predetermined final carbon and final chromium contents at melt temperatures of practical interest.
• the value for Z may vary with varying decarburization practice such as dififering blowing rates and injection devices; however, we have found that this value is usually in the range between about 2 and for inert gas-oxygen injection as disclosed herein.
• the percent Cr in the above relationship is desirably maintained at a high level from the beginning of decarburization up to the achievement of the final carbon level, thereby minimizing chromium oxidation.
• stepwise decarburization to successively lower carbon contents down to the desired and predetermined carbon content by removing carbon in increments,
• the oxygen is maintained at a value equal to or less than the value dictated by the above relationship where percent Cr is the desired predetermined chromium content in weight percent at the end of each step, T is the temperature of the melt in K. at the end of each step and percent C is the lower percent. carbon by weight to be obtained during each stepwise reduction in carbon but at all times being not less than the final predetermined and desired carbon content in the finished product.
• the variables dictating the volume percent oxygen are as follows: the percent Cr is the final predetermined chromium content desired in the decarburized steel; T is the final temperature in degrees Kelvin during decarburization; and percent C is the final desired carbon content in the decarburized steel.
• the volume percent oxygen introduced is maintained constant and equal to the above dictated value.
• the volume percent oxygen is maintained essentially equal to the value dictated by the above relationship where percent Cr is the desired and predetermined chromium content after decarburization, T is the temperature during decarburization, and percent C is a constantly decreasing value as decarburization proceeds.
• percent Cr is the desired and predetermined chromium content after decarburization
• T is the temperature during decarburization
• percent C is a constantly decreasing value as decarburization proceeds.
• the inert gas injection step is used at the conclusion of the injection programs as disclosed by the other alternatives, said programs being concluded in this case preferably at a carbon level within about 0.04 percent or less of the desired final carbon content.
• the volume of inert gas required per ton of metal in this final step of alternative (4) depends on the absolute carbon level and also on the amount of carbon removed in this particular step. Since usually no more than about 0.08 percent oxygen exists dissolved in the melt after an oxygen-inert gas blow, stoichiometrically about 0.06 percent carbon can be eliminated without refractory damage by a subsequent inert gas blow. In actual practice the percent carbon eliminated may be somewhat less.
• oxygen volume per-- cent may not be substantially greater than the value resulting from the relationship under the stated conditions or substantial losses in chromium will result due to oxidation of the same and passage of chromium oxide to the slag phase.
• the preferred method entails maintaining the temperature substantially constant or allowing it to increase during decarburization within a range of from 1700 to 2300 degrees Kelvin. Variance within the stated range can be tolenated Without substantial oxidation of chromium if the volume percent of oxygen being introduced into the melt is adjusted to compensate for the change in temperature. Again, the final temperature must be taken into account in the above relationship in arriving at the maximum permissible oxygen volume percent which is not to be exceeded for minimized chromium loss. For example, if a 20 percent chromium stainless steel is treated at about 1700 C.
• Slight decreases in temperature during the decarburization process may be anticipated and compensation can be made accordingly by programming the volume percent oxygen to correspond to the decreased temperature or by maintaining the oxygen volume percent low enough below the maximum volume percent for the higher temperature so as not to expose the melt to a ratio of oxygen to inert gas which will cause substantial oxidation of chromium for certain anticipated temperature decreases during decarburization of the steel.
• a temperature de crease will be undesirable and can be prevented by use of suificiently high blowing rates.
• the time required for decarburization is effected by several variables including temperature and the rate of introduction of gases. Generally higher temperatures and higher blowing rates are conducive to shorter decarburization periods.
• the 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.
• the bubbles should not exceed 3 to 5 millimeters in diameter. This gives minimum gas consumption. It gas consumption is not of major importance, larger bubble sizes can be employed.
• 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 efiective surface area can be chosen essentially at will or as needed by a particular metal treatment by choosing the proper bubble size.
• 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.
• inert gases are not defined simply by the partial pressure laws.
• inert gas is defined to mean a gas selected from the group consisting of helium, neon, argon, krypton, and xenon.
• Nitrogen in the steel is usually undesirable except in special cases, since it promotes blow holes, metal rising, and the like.
• any suitable means such as ceramic tubes, conduits, nozzles, tuyeres, and the like can be employed.
• a relatively non-consumable material of construction is desired, since in this manner, the introduction of undesirable elements into the melt can be avoided.
• the injetion device should cause the gas mixture to bubble thron the steel bath rather than to pass over its surface.
• Injection devices with internal diameters of up to 0.5 inch may be employed.
• a stepwise decarburization process may be conducted on molten stainless steel to remove carbon to about 0.04 percent at a final melt temperature of about 1700 C. when the stainless steel bath initially contains about 15 percent chromium and about 0.15 percent carbon. The carbon is reduced to 0.08 percent in the initial step utilizing about 50 volume percent oxygen and then the volume percent of oxygen is adjusted to about 35 percent to obtain the final desired carbon content of 0.04 percent in the final decarburization step.
• stepwise decarburization from about 0.50 percent carbon to 0.04 percent carbon in molten stainless steel containing about 20 percent chromium at a temperature of decarburization of about l600 may be conducted in three steps as follows: (1) reduce the carbon content from 0.50 percent carbon to 0.26 percent carbon by introducing an oxygen-inert gas mixture containing about 50 volume percent oxygen into the melt; (Z) adjust the volume percent of oxygen to about 30 volume percent and reduce the carbon content to about 0.11 percent; then (3) reduce the carbon content to the final desired carbon content of 0.04 by introducing a volume percent of oxygen of about 18 percent. Substantially no chromium will be lost in the above process.
• a gas mixture having an oxygen concentration equal to the maximum permissible oxygen concentration at the final carbon content for a given melt temperature and chromium content may be injected throughout the decarburization. For example, at a melt temperature of 1800 C., a chromium content of 20 percent, and a desired final carbon content of 0.02 percent, a gas mixture containing about 30 percent oxygen is injected. No programming of the oxygen content is necessary in this manner. However, the gas injection period is usually longer as compared to that for programmed injection at approximately the same temperature.
• the present process permits reduction of carbon to a lower level than is shown in the prior art while retaining higher chromium contents at lower temperatures, in addition to enabling an artisan to adjust carbon contents in molten stainless steel melts to predetermined levels for specific chromium contents within the range of about 3 to 30 percent.
• the above embodiment shows achieving a carbon content of 0.04 at a temperature of 1600 C. with a final chromium content of about 20 percent.
• the prior art processes would require a temperature of at least 2000 centigrade to produce the same product with conventional practice. if the prior art processes attempted to produce a 0.04 percent carbon stainless steel at a final temperature of 1600 C.
• a predetermined amount of chromium can be oxidized to produce heat in accordance with the present process by maintaining the oxygen volume percent at a value corresponding to a predetermined chromium content desired in the bath at the finish of the process.
• This final predetermined chromium will necessarily be less than the chromium content before the introduction of oxygen and inert gas, but, by the same token, the present process allows an artisan to control the amount of chromium oxidized in accordance with heat requirements desired during the present process.
• blowing mixture containing about 40 percent oxygen should be injected to arrive at the stated final conditions.
• the proper blowing rate and blowing time to be used in conjunction with this example are a function of a specific furnace size, the desired temperature rise, furnace heat losses, and furnace heat capacity and can be calculated for a specific furnace by those skilled in the art,
• Decarburization using inert gas in the final step of a programmed blow is illustrated by the following example.
• the decarhurizing gas was injected into the melt and induction furnace was employed for melting the charge and maintaining the melt temperature.
• a process for removing carbon from molten steels containing about 3 to 30 percent chromium without substantial loss of chromium comprising adjusting the temperature of said molten steel bath to a range between about 1700 to 2300 degrees Kelvin, introducing into said molten steel containing from 3 to 30 percent chromium a decreasing ratio of volume percents of gaseous oxygen and at least one inert gas selected from the group consisting of argon, xenon, neon, and helium While maintaining said adjusted temperature within said range during said decarburization, said gaseous oxygen caused to react with said carbon to form a volatile carbon oxide to decarburize said molten steel, and said ratio decreasing as carbon is oxidized, the gaseous oxygen volume percent of the total volume of said gaseous oxygen and said selected inert gas in said varying ratio of volume percents of said gaseous oxygen and said selected inert gas being maintained substantially equal to the volume percent of gaseous ox gen resulting at any given carbon content during decarburization from the relationship
• a process in accordance with claim 1 including tne additional step of introducing at least one inert gas into said molten steel after substantial decarburization is accomplished with said oxygendnert gas mixture.
• a process for stepwise removal of carbon to a final predetermined amount in an otherwise finished molten steel containing a predetermined chromium content in the range from 3 to 30 percent chromium without substantial loss of said chromium during decarburization comprising adjusting the temperature of said molten steel to a range between 1700 to 2300 degrees Kelvin, introducing into said molten steel containing from 3 to 30 percent chromium, a stepwise decreasing ratio of volume percent of gaseous oxygen to at least one inert gas selected from the group consisting of argon, xenon, neon and helium, said gaseous oxygen caused to react with said carbon to form a volatile carbon oxide to decarburize said molten steel, and maintaining said adjusted temperature, without substantial decrease, within said range during decarburization and said ratio decreasing stepwise as carbon is oxidized, the gaseous oxygen volume percent of the total volume of said gaseous oxygen and said selected inert gas being decreased stepwise during decarburization to a value less than the volume percent of gas
• Percent C an log T Where percent Cr approximately equals said predetermined chromium content in said finished steel, percent C equals any value above said final predetermined carbon content and equal to the carbon content desired at the end of the next subsequent decarburization step, and T equals the temperature of said steel during said process and is within said adjusted range.
• inert gas is introduced into said References Cited in the file of this patent UNITED STATES PATENTS 1,034,785 reene Aug. 6, 1912 1,034,786 Greene Aug. 6, 1912 1,034,787 Greene Aug. 6, 1912 1,792,967 Clark Feb. 17, 1931 FOREIGN PATENTS 472,397 Great Britain Sept. 12, 1937 OTHER REFERENCES Journal of Iron and Steel Institute, vol. (1955), pp. 97-106, and 1l6l28.

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• Engineering & Computer Science (AREA)
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Cited By (45)

Citations (5)

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  • BE BE610265D patent/BE610265A/xx unknown
• 1960
  • 1960-11-18 US US84956A patent/US3046107A/en not_active Expired - Lifetime

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