CA1043571A - Method of producing low nitrogen steel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In bottom blown processes for the production of low carbon steels (i.e. less than 0.2% C), inert gas flushing employed at an intermediate point of the blow, provides more rapid and more efficient removal of N2. Thus, inert gas flushing is initiated at a point when the carbon content of the melt is in excess of 0.3%, and preferably in excess of 0.5%. When N2 is removed to a desired extent, full scale oxygen blowing is resumed to achieve the final degree of decarburization.

Description

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ThiS inventioll relates to the refining of pig iron, by ~ny of the bottom-blown pneumatic steelmaking processes, eg.
3essemer, SIP, Q-BOP. More particularly, this invention is directed o a method for achieving more rapid and more e~ficient removal o~ nt

2~ when such processes are employed ~or decreasing the carbon conte ; 15 f a steel melt to a level below 0.2~, and generally below 0.1~. l In the most widely employed pneumatic steelmaking process,¦
i he Basic Oxygen Process, oxygen is blown from above, through a allce, so as to pierce through 'che overlying slag layer and pcnetra'~l nto the iron melt. When it is desired to remove gaseous impurities , uch as nitrogen, ~rom a BOP refi~ed steel melt; the steel is teemed ~rom the BOP ~urnace into a ladle and inert gas ~lushing is employed . ~or periods ranging from about 10 to 25 minutes. By contrast : lvherewith, in the above noted bottom-blown processes, the oxygen is . ~lown rrom a point below the top surrace Or the melt) through tuyere i ~ocatcd in the bottom or in the sides o~ the converter. With respec~
lo SIP or Q-BOP, a protective gasj generally a hy~rocarbon, is , _ .. . . . . , .. , _ _ . _ _ _ . _ _ _ . .

employed to encase or surround the oxygen stream in order to decrease the inordinately high wear which would occur at the tuyeres and the converter entry area (i.e. the bottom in the Q-BOP). One of the significant advantages of the bottom-blown processes over the BOP, is their adaptability in per-mitting inert-gas purging to be carried out in the steelmaking vessel itself. Additionally, the Q-BOP in particular, provides more effective and efficient degassing in a shorter period of time, as a result of the comparatively higher gas-flow la rates which may be employed. With respect to the efficiency of such purging, a theoretical minimum amount of inert gas (generally argon) is, of course, required for the removal of a desired amount of N2 (see, for example, Kollman and Preusch, Proceedings of the Electric Furnace Conference of AIME, 1961, pp . 23-42). However, in actual practice many times more argon is found to be required, because many of the reactions involved are controlled by mass transfer in the liquid phase and do not go to completion in a practical time period.
In view, thereof, it is a principal object of this invention to provide a purging procedure, in which both the amount of inert gas employed and the time required for effect-ing the removal of a desired degrée of N2, may significantly be decreased.
Other objects and advantages of this invention will become more apparent from the following description when read in conjunction with the appended claims and the drawing, in which The Figure is a graph illustrating the marked difference in nitrogen removal rate between two steels of 3a differing carbon contents.

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Gaseous impurities, such as N2, are normally re-moved by purging the steel with argon; such removal being effected by the lowering of the N2 partial pressure as a result of the diluting effect of the argon. As noted above, such purging is`conventionally accomplished only after the steel melt has been decarburized to the desired extent. It has now been found that the rate of nitrogen removal, at high oxygen activities- (for exampLe S00 or even 300 ppm oxygen) is controlled by a slow chemical reaction on the sur-face of the molten iron. Thus, at such high oxygen activities, the iron surface is essentially covered by a layer of adsorbed oxygen which seriously retards the rate of N2 removal. It has also been determined that the relative importance of this retardation effect decreases with oxygen activity, so that at low oxygen activitiés (for example less than lO0 ppm oxygen) the overall rate of N2 lS controlled either by liquid phase mass transfer or by the saturation'of the inert gas to the equilibrium N2 pressure. In the latter instance (low oxygen activity) liquid phase mass transfer is the dominant control n at relatively high nitrogen levels (of the order of 0.01% N2)iwhile saturation control preva~ls at ver~ l~w n~trogen level~ (~0.002% N2), In view of these findincJs as to the retardation effect o~
adsorbecl oxygen, it may readily b~ understood why the oxygen blow itself, is not very efficient in effecting the removal of N2 from the steel melt.
Since oxygen activity is inversely proportional to carbon activity, it may be seen that the efficiency of inert gas purging may significantly be enhanced by effecting such purging at comparatively high carbon contents, i.e. in excess of 0.3~, and more preferably in excess of 0.5% carbon. The 3a significant benefits resulting from purging at such higher carbon levels is illustratc~d by the two curves of the figure.

"" ~()4;~5'^~1 For example, an argon flush of 2000 ft3/min., performed for two minutes in a 30-ton heat (whïch had previously been decarburized to a carbon content of 0.05~) was only capable of reducing the initial 0.005% nitrogen content to about 0.004~. By contrast, when the same flushing rate was per-formed on a comparably sized heat prior to the time the carbon content thereof was reduced to 0.5%, the nitrogen content was reduced to nearly 0.001~ for the same two-minute flush.
1~ While it should be understood that the invention is applicable to all bottom-blown steelmaking processes, the procedure for carrylng out the teachings thereof will be described in its specific applicability to the Q-BOP process.
Initial blowing of the pig iron is performed as in conventional Q-BOP practice. That is, a stream of generally commer~ially pure oxygen is introduced into the melt through tuyeres located in or near the converter bottom. The use of oxygen of such purity, would normally result in extremely rapid wear of both the tuyeres and the bottom itself. Therefore, each oxygen stream is surrounded by an encasing or coolant gas to slow down the violent reaction and thereby achieve sub-stantially reduced wear. The ratio of oxygen to encasing gas is desirably held within a critical range so as to per-mit such wear to proceed in a slow and controlled manner.
Thus, during this initial blowing period there are basically two different gas throughput rates which are of concern;
Ro the rate at which 2 is introduced, and Rp the rate at which protective gases (eg. methane) are introduced. As shown in U.S. patent 3,706,549, the disclosure of which is incor-porated herein by reference, Ro ~ Rp. In view thereof, ,1, ~ 4~

1C)4;~5~71 the average total rate RT may be defined as the sum of Ro + Rs, wherein RS iS the rate of introduction of all other gases. For example, RS may equal Rp plUS RA, wherein RA
is the rate of introduction of argon or other inert gas. It should be noted that RT is not necessarily constant, but merely the average total rate of gas introduction. It is necessary, however, that RT be maintained within a re-; quisite throughput range of from about 75 NCF/min per ton of steel to about 160 NCF/min per ton of steel being re-fined. The minimum rate is dictated by the need 4a.

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to maintain suffi¢lent back pre9sure in the tuyeres, ln order to revent molten metal ~rom plugging the tuyere openlngs, While rate~
igher than the above noted maxlmu~ would be deslrable ror shortenir g the length Or the blow (thereby increasing productlon capablllty), it has been ~ound that rates significantly higher than 160 NCF/min per ton result in undesirable splashing and spitting above the onverter, For the initial portion of the blow, RA will generally be zero or negligible, while Ro will be greater than o.8 RT Thereaft~ r, or a steel containing an initial ievel of nitrogen in excess of th~t ~esired in the rinal steel product (l.e ~ 0.002% N, and generally 0.004~ N) the melt will be purged with an inert gas (eg. argon) ~or a time sufflcient to decrease the nitrogen content to the esired level which generally will be less than 80%, and often ess than 50~ of said initial level. Although purging may be initiated at any time before the carbon content of the melt has een reduced to 0,3%, it is preferable that such purging not be nitiated until (i) after the silicon portion of the blow (so as to nsure the achievement of desirablé temperature), but (ii) before he melt has been decarburized to less than 0.5% carbon (to insure esirably low oxygen activity). Purglng is preferably conducted by erminating the oxygen blow and substituting an inert gas therefor;
¦that is Ro will be reduced to zero, while RA ,_ RT. Although less ~esirable, it is not essential, however, that Ro be reduced to zero hus, the lnert gas may contain a small amount of oxygen, since such ~xygen will rapidly be converted to C0; resultlng in an effective ~as retention time in the bath in which the activity of oxygen will evertheless be sufficlently low for the purpose hereof. Therefore, urging within the scope contemplated by this invention may be - 5 Fruehan :1()435~71 conducted wherein the purge gas rate RA is at least 0.8 RT.
As noted above, purging is conducted for a time sufficient to decrease the nitrogen content of the bath to the desired level. Depending both on the amount of nitrogen to be re-moved and the magnitude of RA, times varying from about one-half to two minutes will ordinarily be sufficient. Sub-sequently, decarburization is then resumed by increasing the rate of oxygen introduction so that Ro is again at least 0.8 RT; this resumed oxygen blow continuing until bath ca~bon content is reduced to the desired final level, generally less than 0.1% carbon.

6.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:

1. In a bottom blown process for the refining of molten pig iron wherein a stream of oxygen containing gas is introduced so as to decrease the carbon content in the resulting refined molten steel to a level below 0.2 percent, said gas being introduced at a total rate RT within the requisite throughput range, wherein RT is the sum of RO, the rate at which oxygen is introduced and RS, the rate at which other gases are introduced, and wherein RO is at least 0.8RT, said bath containing an initial level of nitrogen in excess of 0.002%, said initial level being substantially in excess of that desired in the final steel product.
the improvement which comprises a) introducing an inert gas prior to the time the carbon content of the molten bath is decreased to a level of 0.3 percent, so as to achieve a purging rate of inert gas introduction which is at least 0.8RT, said inert gas being introduced at said purging rate for a time suffi-cient to decrease the nitrogen content to a desired level which is less than 80 percent of said initial level;
b) thereafter, increasing the rate of oxygen introduction so that RO is again at least 0.8 RT, said increased rate of oxygen introduction being employed for a time sufficient to decrease the bath carbon content to a final level below 0.2%.

2. The method of claim 1, wherein said inert gas is introduced at said purging rate prior to the time the carbon content of the molten bath is decreased to a level of 0.5 percent.

3. The method of claim 2, wherein said inert gas is introduced at said purging rate for a time sufficient to decrease the nitrogen content to a level which is less than 50 percent of said initial level.

4. The method of claim 3, wherein the bath is decarburized to a final carbon level below 0.1%.

5. The method of claim 4, wherein said initial nitrogen level is equal to or greater than 0.004 percent.

6. The method of claim 5, wherein said purging rate is approximately equal to RT.