US2490990A - Method of blowing bessemer steel - Google Patents

Description

Patented Dec. 13, 1949 METHOD OF BLOWING BESSEMER STEEL Harold K. Work, Mount Lebanon, and Richard R. Webster, Beaver, Pa., assignors to Jones & Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application January 23, 1948, Serial No. 4,064

6 Claims. (CI. 75-60) This invention relates to the blowing of Bessemer steel and more particularly to the blowing of such steel in side-blow converters.

An object of our invention is to provide a method of blowing Bessemer steel which affords the advantages of side-blowing but prevents the formation of too fluid slag which ordinarily results from such process. Another object is to lessen the time required in conventional sideblowing. Another object is to provide a method for lowering the carbon content of side-blown Bessemer steel below that obtainable in conventional side-blowing practice.

Bessemer converters may be classified according to the method of blowing into two types, denominated bottom-blown and side-blown converters. In bottom-blown vessels the tuyeres are placed in the vessel bottom and air is blown through the charge from bottom to top. In sideblown converters the tuyeres are placed in the side of the vessel and air is blown through the charge from one side to the other. Bessemer converters may also be classified according to shape as the vertical or upright type and the drum or cylindrical type. The former is usually of circular plan, with tapered bottom and nose; the latter is more or less circular in section, except for the nose, with flat or dished ends. The method of this invention is particularly adapted to side-blown converters of the drum or cylindrical t pe.

It is known that steel produced in side-blown converters is generally lower in nitrogen content than that from bottom-blown vessels. This relatively low nitrogen content is in fact the most significant characteristic of side-blown Bessemer steel and is highly advantageous for many applications, since it has been found by experiment that steels with lower nitrogen contents are less sensitive to the hardening and enbrittling effects of cold work. Cold work is here defined as work performed on the metal below its temperature of recrystallization. However, the process of side-blowing as heretofore carried on produces in the converter an extremely fluid slag (sloppy in the terminology of steel men) which is dimcult to separate from the steel when the later is poured from the converter into the steel ladle. The presence in the ladle of any considerable amount of this slag, which is high in iron oxide, renders the deoxidation of the steel erratic, reduces the efilciency of the manganese addition, and is detrimental to dephosphorization, if this additional step is included. This undesirable nature of the slag is one of the factors which have militated against the general use of the sideblown converter in the steel industry.

A second factor unfavorable to the side-blown converter is the length of time required for a blow, which is appreciably greater than that required by a bottom-blown vessel handling an equal weight of metal.

We have invented a process of side-blowing which largely avoids the slag difiiculties incidental to conventional side-blowing. In addition, our process requires a shorter blowing time than has heretofore been achieved in side-blowmg.

In conventional side-blowing the vessel is tilted so that the tuyere mouths are substantially at the bath surface. It is possible under these conditions to produce blown metal with nitrogen contents in the range .003 %-.004%, or substantially that of the iron charged. However, the slag formed on the bath is high in iron-oxlde-rangng up to 40% or more FeO-and in consequence 1s sloppy, with the undesirable efiects previously mentioned.

We have found that if the tuyere mouths are submerged a short-distance beneath the bath surface the slag produced is lower in iron-oxide and consequently less sloppy. Unfortunately, the nitrogen contents of blows so made are appreciably higher than those of blows made in the conventional manner, and as the depth of immersion increases, results rapidly approach those of bottom-blowing, where nitrogen contents of .015% to .025% are usual. However, by further experimentation, we have discovered that if the blowing process is so conducted that the tuyeres are substantially at the bath surface during a certain portion of the blow and below the bath surface during a portion or portions of the blow, the nitrogen content of the blown metal can be kept at a reasonably low figure while the iron-oxide content of the slag can be held below the level at which a sloppy slag results.

It is recognized that in Bessemer blowing, the silicon in the iron is burned out to a substantial degree before the bulk of the carbon begins to burn. We have discovered that the combustion of silicon is accelerated by blowing with the tuyeres submerged, and the length of the blow appreciably reduced thereby. We have also discovered that, during approximately this same period of refining, blowing with submerged tuyres does not cause any considerable increase in nitrogen pick-up over that incidental to conventional side-blowing, And finally, we have discovered that the slag formed by blowing in this 1 I l l 3 manner while the silicon is being oxidized is relatively low in iron-oxide and not fluid or sloppy. Slag of this type is called dry by Bessemer operators.

After the bulk 01 the silicon is oxidized and the carbon begins to burn at an appreciable rate, we find it necessary to tilt the vessel so that the tuyre mouths lie approximately at the bath surface, if nitrogen pick-up is to be kept low. Surface blowing during the periodof; rapid carbon 3 oxidation tends to produce a thin, sloppy slag high in iron-oxide; however, thismer-ely dilutes the dry slag formed during the period of subsurface blowing, and if the surface blowing period is not too long the final slag is not objectionably thin.

The simplest embodiment, therefore, of the practice of our invention comprises blowing. the.

charge with the tuyere mouths an appreciable distance below the bathsurface during the silicon portion of the blow, then tilting the vessel so that the tuyere mouths are substantially at the bath surface and finishing the blow in this manner. The blown metal so produced generally contains less than 004% to 005% nitrogen, while the slag formed in the vessel generally contains less than about 30% FeO, and so is thick enough to be conveniently skimmed off the blown metal in pouring. The time required for a blow so made is appreciably less than that of;-conventional practice.

Under certain conditions a modification of this simple two-stage practice maybe desirable. We have observed that in conventional side-blowing the carbon content of the blown metal, before ladle additions, averages .06% to 07%. Theaddition of manganesein theusual form of 80% ferro-manganese'which generally contains 6% or 7% carbon, raises thef'blown-metal carbon content, of coursa so that it is difiicult, if not impossible to make side blown Bessemer steel of normal manganese content- 3092 and above-- with the low carbon desired. This difficulty is not avoided by the simple embodiment of our invention so far described. However, we have found that if the surface -blowin'g stage of our abovedescribed practice is discontinued before the blows end, and the vesseltilted-again so that the tuyere mouths are submerged a short distance below the batn'surraee, in which'lp'osition the blowing is finished, the carbon content of the blown metal will average no more than about 04%. Anextra point or two of nitrogen is picked up in this'se'c'ond sub-surface blowing but even so the blown metal nitrogen content seldom exceeds .006% to '.007-%. The steel resulting hasbeen found to have excellent cold-forming and work;- ing properties, as compared with conventional bottom-blown Bessemer steel.

The practice of our invention, modified as de scribed above, comprisesblowing the charge with the tuyre mouths gajsubs'tantial distance below the bath surface during the'period of 'pronounce d silicon oxidation, blowing with thetuyere rnouths substantially at the bath surface during the major portion of the period of pronounced carbon oxidation, and finallyjfinishin'g' the blow: with the tuyeres againsubnierged below the bath surface. The slag produced by such practice is thick or dry enough to be skimmed oil the blown-metal by the usual methods. The time required for a blow is appreciably less thantha t of conventional side-blowing, and the steel produced is; lower in carbon content and only slightly higher in nitro gen than conventional side-blown steel Having presented-the essentialfeatures of our invention we now describe certain phases of the process in more detail. The duration of the first or sub-surface stage of our process corresponds to the time of the silicon blow in which the bulk of the silicon in the iron is oxidized. This period can be determined approximately by visual observation of the conver ter fiame, which is short, transparent and colorless until appreciable amounts of carbon begin to burn. It then becomes more intense, brighter, opaque and yell lowish in color. More precise control is afforded by the photocell method of H. K. Work, Patent No. 2,207,309, whichwe prefer to use. When the converter flame displays in full the above described characteristics of the carbon blow, the vessel may be tilted so that the tuyere months are approximately at the bath surface, and the blowcontinued until completion in this way.

In the three-stage embodiment of our invention, we prefer to terminate the first sub-surface stage of the blow as described above. The second, or surface blowing stage, may continue'until the carbon appears to be oxidized to the limit of normal side-blowing,- which is indicated by the drop of the fiame familiar to Bessemeroperators and others skilled in the art of Bessemer steelmakingt At this pointthe vessel is again tilted so that the tuyeramouths are submerged, and the blow continuedfor a short-period of time. We have observed thatwhen thevessel is tilted at this point a reaction of some violence takes place in the vessel, which in itselfcauses an appreciable reduction the carbon" content. If minimum blowing time is desired, however, the surface blowing stage is-terminatedseveral minutes before the normal completion Qf'the blow and the last 20% to 25% of the blowing time, which may amountto; 4 or 5 minutes, is given over to subsurface blowing.- This practice-res sults in lowest blowing time, lowest carbon content and least troublesome slag, with nitrogen contents only slightly higher than those of the two stageor first-mentioned embodiment of our three-stage practice.

The depth of tuyre-mouthsbelow the bath surface is not criticalinthe first or sub-surface blowin stage of; eitherembodiment of our invention; In the second sub-surface blowing stage, however, we;find' itdesirable to submerge the tuyeres notmore than 2 to-3 inches, in order to minimize nitrogenlpick-ury.

The invention is not limited to the preferred embodiment but may beotherwise embodied or I practiced withinthe scope of-the followingclaims.

We claimi 1. The process of converting molten iron to steel in aside-blown Bessemerconverter, which comprises blowingair into--the iron bath, substantially the entire blastbeingadmitted below the bath surface during-an earl-y part of the blow and substantially at the bath surface during a later part of the blow.

2; The process of -converting molten iron to steel in a side-blown Bessemerconverter, which comprises blowin air into-the iron bath, substantially the entire blast being admitted below the bath surfaoe duringthe siliconblow and subt n lly. t th ba h sena e i a h carbon blow.

3. The processor; converting molten iron to steel in aside-,blownhessemer converter, which comprises blowing air into the iron bath, substantially the entire blast admitted below the bath snrface frorn substantially the comme wear awe ew. ntilh c on e iron is burning vigorously; and substantially at 5 the bath surface for at least a portion of the remainder of the blow.

4. The process of convertin molten iron to steel in a side-blown Bessemer converter, which comprises blowing air into the iron bath, substantially the entire blast being admitted below the bath surface during the silicon blow, thereafter substantially at the bath surface during the carbon blow, and thereafter below the bath surface for a period not exceeding about onefourth of the total blowing time.

5. The process of converting molten iron to steel in a side-blown Bessemer converter, which comprises blowing air into the iron bath, substantially the entire blast being admitted below the bath surface during the silicon blow, thereafter substantially at the bath surface during the carbon blow until the drop of the flame, and thereafter below the bath surface.

6. The process of converting molten iron to steel in a. side-blown Bessemer converter, which comprises blowing air into the iron bath, substantially the entire blast being admitted below REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 277,929 Reese May 22, 1883 553,875 Sherk et al. Feb. 4, 1896 1,312,474 Fisk Oct. 5, 1919 FOREIGN PATENTS Number Country Date 13,275 Great Britain of 1885