US3236630A - Oxygen steelmaking - Google Patents

Description

Feb. 22, 1966 R. c. STEPHAN 3,235,630

OXYGEN STEELMAKING Filed Oct. 23. 1961 STOP VALVE PER CENT OXYGEN AND COMBUST/ELE-F Q N -k m l/VVE/VTOP.

\ RA YMO/VD c. STEPHA/V 0 2 4 s 810/2 /4 I6 is 2022 MM TIME OF OXYGEN TREA TMEN 7, M/NU 75$ 4 r I om ey United States Patent 3,236,630 OXYGEN STEELMAKING Raymond C. Stephan, Gary, Ind., assignor to United States Steel Corporation, a corporation of Delaware Filed Oct. 23, 1961, Ser. No. 146,784 4 Claims. (Cl. 75-60) This application relates to the production of steel by blowing with oxygen and more particularly to a method and apparatus for determining the end point in producing steel by surface blowing with oxygen.

A rapid and efficient manner of refining molten pig iron to produce steel involves the application of gaseous oxygen to the surface thereof. Due to the rapidity of the reactions or progress of the so-called blow, it is difiicult however to determine the desired end point so that the oxygen application can be stopped when the content or level of carbon of the molten bath is at the desired level. Controls based on determination of flame characteristics heretofore used in conventional Bessemer operations are not satisfactory. If the oxygen application is continued too long, both the carbon and manganese are reduced below the desired level so that replacement thereof is necessary, whereby the efficiency of the process is lowered. Moreover, such overblowing, i.e. applying oxygen for too long a period results in lower yields due to excessive oxidization of the iron and a slag very high in FeO. On the other hand, if the heat is underblown, i.e. oxygen not applied for sufficient period, the carbon and other impurities will not be reduced to the desired level. It is also desirable in this process to add fiux additions at a properly controlled time.

It is accordingly an object of this invention to determine rapidly and efficiently the correct time for stopping oxygen application in oxygen steelmaking processes, i.e. to determine the end point of the blow.

It is another object of this invention to determine the proper time for making flux additions in oxygen steelmaking processes.

The foregoing and further objects will be apparent from the following specification when read in conjunction with the attached drawing, wherein:

FIGURE 1 is a schematic elevation, partly in section; and

FIGURE 2 is a graph.

Referring more particularly to the drawing, the numeral 2 designates an open-mouthed converter or vessel suitable for practicing the teachings of my invention. The shape or size of the converter may vary within wide limits. Refining of the bath of molten ferrous metal 4 therein beneath a layer of slag 6 is effected by a stream or jet of commercially pure gaseous oxygen 8 having an oxygen content of at least 80% issuing from a retractable water-cooled lance 10 connected to a suitable source of gaseous oxygen under pressure. Cooling water may be circulated through openings 12 and 12a. Fumes and gases resulting from the refining reactions are collected by a hood 14 connected to a spray chamber 16 in which a plurality of water spray nozzles 18 are mounted. Below the spray chamber and connected thereto is a Venturi scrubber 20 connected by a suitable main 22 to a separator tank 24, having a drain for removing water and solids therefrom. Off-gas from the converter is drawn through the hood 14, chamber 16, scrubber 20, main 22 and separator 24 by a suitable exhaust fan 28 and exhausted to the atmosphere through a stack 30. A main 32 connects the separator, fan, and stack. An electro- 3,230,630 Patented Feb. 22, 1966 static precipitator may be used instead of the Venturi scrubber and separator tank if desired.

In accordance with the teachings of my invention, a conventional continuous gas analyzer 40 is adapted to receive and continuously analyze off-gases from the converter 2. The off-gases may be conveniently directed to the analyzer by a tube 42 connected to the main 32. The analyzer analyzes continuously for volume percent oxygen and volume percent combustibles (e.g. carbon monoxide). Moisture in the gas may be substantially reduced before delivery to the analyzer by a trap 44.

In operating the above-described equipment in accordance with theprocess of my invention, the first step is to charge the converter 2 with scrap metal. The amount of scrap may be from 5 to 35% of the weight of the total charge, depending to some extent on the chemical composition of the hot metal, the temperature of the hot metal, and the temperature of the furnace. Before charging, the vessel may be tilted to one side by conventional means. The desired amount of scrap metal is then charged by any suitable means, such as a portable chute suspended from an overhead crane. Following the scrap addition, the hot metal is charged, the mouth of the furnace still being desirably tilted from under the hood. Water may be started flowing into the spray chamber and Venturi scrubber before charging the furnace through inlet pipes 18a and 20a, and the exhaust fan likewise started at such time.

After charging, the vessel is returned to a vertical position and the oxygen lance 10 is lowered to a position just above the molten metal of the bath, and the oxygen addition or blowing is begun. At such time, the exhaustgas analyzer may suitably be started. Shortly after the blowing is started, flux additions (e.g. roll scale, lime) may be gradually made through a conventional chute.

When blowing has progressed for 3 to 10 minutes, the silicon boil gradually slows down and the carbon boil begins. The readings on the exhaust-gas analyzer indicate this point in the process clearly. As shown in FIGURE 2, the oxygen content of the exhaust gases falls steadily during the silicon boil, i.e. the first 10 to 12 minutes of the blow and reaches a low value (below 2%) at the commencement of the carbon boil at about 10 minutes. During the carbon boil, the oxygen content of the exhaust gases remains low, and the combustibles content of the exhaust gases rises to between 5 and 20%, generally not more than 12%. Toward the end of the carbon boil, i.e. at about 18 minutes, the combustibles content of the exhaust gases falls off. When it reaches a suitable value (preferably about 1%), roll scale and fluorspar are added. The presence of combustibles in the exhaust gas during the carbon boil is attributed to the formation of carbon monoxide in the furnace. Thus, the low combustibles analysis achieved toward the end of the carbon boil indicates that the bath has subsided sutficiently to permit roll scale and fluorspar to be added without causing objectionable slopping.

As soon as the carbon content of the charge is reduced to a low level, the rate of carbon boil is reduced and the oxygen content of the exhaust gas begins to rise. It has been found that by stopping blowing as soon as the oxygen content reaches a certain value, steel containing a certain proportion of carbon can be produced. For example, I have found that if the oxygen is shut off when the oxygen content of the exhaust gas is about 18%, steel containing approximately about 0.05% carbon can be produced.

A specific example of producing a heat of steel by my preferred method is as follows:

Operation: Time Charge 1000 lbs. of scrap to furnace 1:09 Pouring 15,150 lbs. hot metal into furnace and position furnace under blowing hood 1 :33 Lower lance and start oxygen flow of 625 c.f.rn. 134% Add 200 lbs. roll scale and 1400 lbs.

burnt lime 1:34 /2 to 1:44 1% combustible in oif-gas. Add 50 lbs. roll scale and 40 lbs. fiuorspar 1 :54 18% O in off-gas. Shut off flow and raise lance 155% Start tapping furnace into ladle and add 90 lbs. medium carbon ferromanganese 1:56

Pig iron analysis: Percent Carbon 3.82 Manganese .28 Phosphorus .094 Sulphur .032 Silicon 1.25

Steel analysis:

Carbon .25 Manganese .28 Phosphorus .007 Sulphur .030 Silicon .008

The following table gives the 0 content of oil-gas of a number of heats at which 0 application was stopped and the resulting ladle carbon contents.

In these cases, ferromanganese was added to the ladle so that the resulting carbon content is about .02 to .03% higher than it would have been if no ferromanganese additions were made.

Table Oxygen content, percent: Ladle carbon, percent 17.0 0.07 18.0 0.08 18.0 0.08 18.0 0.08 17.0 0.07 18.0 0.07 18.0 0.07 18.0 0.07

It is obvious that the carbon content can be controlled within the desired range of .05 to 2.0% by suitably controlling the point at which 0 additions are stopped in accordance with the teachings of this invention.

A significant benefit from the use of the invention is the addition of flux forming materials such as roll scale and fiuorspar in the latter stages of the blow when the combustible content of the off-gas drops to a predetermined level. This overcomes the difficulty of when to add the final flux additions to bring the slag to the proper fluidity when blowing ceases. If flux is added too soon, the slag becomes foamy and may slop out of the vessel. Another consequence is that sometimes the analysis of the product steel differs markedly from the analysis expected. If flux is added too late, blowing must be continued until a manageable slag is obtained.

While I have shown and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. A method of producing steel having a carbon content of about .05% by blowing oxygen against the top surface only of a bath of molten ferrous metal comprising analyzing the off-gas from the refining reaction for oxygen and terminating the application of oxygen to the bath when the oxygen content of the off-gas rises to about 18%.

2. A method of producing steel having a predetermined carbon content by blowing a jet of gaseous oxygen against the top surface only of a bath of molten metal under a slag layer comprising analyzing the off-gas from the refining reaction for oxygen and combustibles, adding fiuorspar to the bath when the combustible content of the oif-gas falls a predetermined amount below its high point of about 16% and terminating the application of oxygen when oxygen content of the oif-gas rises a predetermined amount above its low point of about 1%.

3. A method of producing steel having a carbon content of about .05% by blowing a jet of gaseous oxygen against the top surface only of a bath of molten metal under a slag layer comprising analyzing the ofi-gas from the refining reaction for oxygen and combustibles, adding fiuorspar to the slag when the combustible content of the oft-gas drops below about 1% and terminating the application of oxygen to the bath when the oxygen content rises to about 18%.

4. A method of producing steel having a predetermined carbon content by blowing a jet of gaseous oxygen against the top surface only of a bath of molten metal under a slag layer comprising analyzing the off-gas from the refining reaction for oxygen and combustibles and adding fiuorspar to the bath when the combustible content of the off-gas drops below its high point.

References Cited by the Examiner UNITED STATES PATENTS 2,207,309 7/1940 Work 60 2,750,280 6/1956 Perrin et al. 75--55 2,800,631 7/1957 Suess et a1 7560 2,831,467 4/1958 Guczky.

2,883,279 4/1959 Graef et al 7559 2,936,230 5/1960 Larsen 7552 BENJAMIN HENKIN, Primary Examiner.

RAY K. WINDHAM, WINSTON A. DOUGLAS,

Exqminers,

Claims (1)

1. A METHOD OF PRODUCING STEEL HAVING A CARBON CONTENT OF ABOUT .05% BY BLOWING OXYGEN AGAINST THE TOP SURFACE ONLY OF A BATH OF MOLTEN FERROUS COMPRISING ANALYZING THE OFF-GAS FROM THE REFINING REACTION FOR OXYGEN AND

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