US2298483A - Process of producing pig iron - Google Patents

Description

Oct. 13, 1942.

C. R. HQLZWORTH PROCESS OF PRODUCING PIG IRON Filed July 25, 1940 2 Sheets-Sheet 1 Fig. 5. Regular Pig Iron UHFHI'IHI'MIOO Diam.

g Iran 100 Dium.

Fig. 6. Rl-gular Pig lrnn l'lulml -l00 Diam.

, 2. Graphilizml Pig Imn IIh'hml H1O Dian].

Oct. 13, 1942. c. R. HOLZWORTH 2,298,483

PROCESS OF PRODUCING PIG IRON Filed July 25, 1940 2 Sheets-Sheet 2 Fig. 3. Graphilizml Pig Imn Fig. 7. lh'gular Pig lrun L'm'll'lu-d 500 [)iam. m-lvhml 500 Hiam.

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Patented Oct. 13, 1942 UNITED STATE s PATENT OFFICE raocass or rno ncme PIG IRON Charles B. Holzworth, North Tonawanda, N. Y.

Application July 25,- 1940, Serial No. 347,369 Claims. (ems-131) This invention relates 'to methods for producing pig iron in the blast furnace.

As is well known, the bulk of commercial pig iron is produced from the hematite and limonite iron ores. This is because these ore deposits are larger, are more plentiful, and are more accessible, and also because this type of ore is easier and more economical to reduce or smelt in the blast furnace. Commercial pig iron produced from these ores usually contains from 3.0% to 4.25% in total carbon and in excess of 1.0% of silicon. The total carbon in the pig is composed of combined carbon and free carbon. The combined-carbon content is usually between 35% and .80% and is for the most part in the form of cementite or iron carbide (FeaC). The free carbon content is in the form of separated collections of graphite distributed in the iron matrix, and which precipitates out of liquid solution while cooling to solidus and to atmosphere temperatures. As a rule, castings made from pig are rendered softer or more machinable in proportion to the amount of free carbon present and also in proportion to the degree of uniformity of distribution of the graphite collections. Within certain limits, the silicon in the pig iron has the effect of reducing the amount of combined car-' bon present, and consequently, the chill in the casting made therefrom, and it also has a graphitizing eifect, but silicon also has the eilfect of reducing the total carbon and the solubility of carbon in pig iron. These effects vary in proportion to the amount of silicon present. Hence, the lower the percentage of silicon present in the pig iron the finer and more uniform is the grain, but with an increased hardness. v

Generally speaking, the major fault or disadvantage experienced with prior art pig iron, is that inthe castings made therefrom the'free carbon or graphite is flake-like or vein-like in structure, and that the graphite formations are not as uniformly distributed in the iron matrix, as is desired, and also that the graphite has the tendency to form occasionally in relatively large concretions or lumps. Because of this tendency to have a relatively non-uniform distribution'of the graphitic carbon, the castings made from. the conventional prior art pig iron are irregular in hardness and consequently are relatively low in machinability. The relatively long flake-like or vein-like. graphiteformations of the typical prior art pig iron tend to form cleavage'planes in the matrix of the pig, and in the casting made therefrom, which results in a relatively weak structure and one having relatively low tensile and trans-. verse strength. Also, because of the vein-like One of the principal objects of this invention is to provide a new and improved method for proa distinctly superior product; which pig iron or product has a uniform and relatively close or tight grain, and is also a heavy scrap carrier in the foundry, and which pig iron produces sound castings with a minimum of shrinkage and chill, and castings having increased tensile and transverse strength, and of improved machinability,

Further objects are to provide an improved method for producing a distinctly superior pig iron from the typical and plentiful hematite and limonite iron 'ores and at little or no greater cost than conventional pig iron of the prior art.

Still further objects will be apparent to those skilled in this art from an understanding of the following description and the accompanying drawings, in which drawings- Figures 1, 2, 3, and 4 are photomicrographs showing enlargements of the grain structure of the same sample of pig iron, which sample was produced in accordancewith the present invention. Fig.1 shows the sample as unetched and at 100 magnifications; Fig. 2 shows the. sample at 100 magnifications after being etched with a 2% nital solution; Fig. 3 shows the sample as unetched at 500 magnifications; and Fig. 4 shows the sample at 1000 magnifications after being etched with a 2% nital solution.

Figures 5, 6, 7, and 8 are photomicrograps showing enlargements of grain structure of the same'sample of a conventional pig iron produced in accordance with known standard methods of the prior art. Fig. 5 shows the sample of conventional pig as unetched at 100 magnifications and should be compared with Fig. 1 Fig. 6 shows the sample of conventional pig at 100 magnifications after being etched with a 2% nital solution and this figure should be compared with Fig.

2; Fig. 7 shows the sample of conventional pig iron as unetched at 500 magnifications and this figure should be compared with Fig. 3; and Fig. 8

shows the sample of conventional pig iron at 1000 magnifications after being etched with a 2% nital solution, and this figure could be compared with Fig. 4.

In the drawings, the dark portions are the graphite particles, and the light portions are the iron constituents, such as separately formed ferrite and cemcntite, and" pearlite, which is the pearl-like striated structure and is a combination of ferrite and cementite. From an examination of Figs. 5, 6, 7. and 8, it willbe noted that the generalcharacter of thegraphite shown therein is clearly flake-like or vein-like in its formation and also that there is a distinct tendency for the graphite to form in relatively large concretions or lumps which, as stated above, re-

sults in defects in castings made therefrom. However, the graphite formation in Figs. 1, 2, 3, and 4 which is the pig iron of the present invention, is decidedly different from that appearing in Figs. 5, 6, '7, and 8. In Figs. 1, 2, 3, and 4 the raphite is clearly broken up into relatively small particles; i. e., the graphite is in a finely divided form, as distinguished from the relatively large concretions and the vein-like or flake-like formations of the ordinary or conventional pig iron appearing in Figs. 5, 6, 7, and 8. Furthermore, it will be apparent that the finely divided graphite of Figs. 1, 2, 3, and 4 is evenly dispersed throughout the iron matrix, and the graphite of Figs. 5, 6, 7, and 8 is not so evenly dispersed. It is clearly apparent from a comparison of Fig. 4 with Fig. 8, that the pearlite laminations of the iron producedin accordance with the present invention is much finer than the pearlite laminations of ordinary iron produced in accordance with the usual or conventional methods of the prior art. The analysis of the iron of Figs. 1, 2, 3, and 4 is as follows: silicon, 2.90%; total carbon, 4.07%; combined carbon .49%; titanium, .41%; Brinell, 179. The analysis of the iron of Figs. 5, 6, 7, and 8 is as follows: silicon, 3.00%; total carbon, 3.86%; combined carbon, .41%; Brinell, 146.

The preponderance of finely divided graphite and the relatively even dispersion of the graphite in Figs. 1, 2, 3, and 4 are beneficial results flowing from my improved method. One of the principal factors causing these beneficial results is the absorption of titanium in a highly reactive and effective form in the molten iron while-in the blast furnace and the retention of titanium in a reactive and effective form in the solid pig iron.

The improvedmethod of this invention may be carried out in any well known form of blast furnace, and because of this it is thought unnecessary for an illustration of such a furnace to be included herein. It will be understood, however, that the conventional blast furnace includes the usual tall, cylindrical stack lined with refractory fire-brick and provided with a charging opening or throat at its top and with sepaabove explained, these ores are used because they are more accessible and are easier and more economical to reduce or smelt in the blast furnace. Such ores intheir natural state contain only a trace or substantially no titanium; that is, the titanium content is rarely over .1%.

In accordance with the present invention, a titanium-bearing material is also added to the blast furnace burden. It is to be noted that in so far as the broader aspects of this invention are concerned, the-titanium-bearing material may be any material suitable for the purpose. As an example, this material may be a titanium are such as rutile or ilmenite; or, it may be a waste product from the operation of the electric furnace engaged in reducing bauxite in the production of aluminum. This waste product is in the form of a sludge and usually contains from 2% to 5% titanium (Ti), 50% iron (Fe), and 7% silicon (Si). A typical analysis for ilmenite is: 50% iron oxide (Fezoa); 45% titanium oxide (T102); 3% silica (SiOz); etc.

- To the burden thus far described, which is composed of a hematite iron ore and a titanium-bearing material, there is added definite proportions of fluxing material such as limestone, and also, if there is not enough silica present in the iron ore itself to produce a pig containing in excess of 1% silicon, as stated previously, and'to prorate openings at difierent levels at its bottom for tapping slag and molten iron, respectively. Approximately the lower eight feet of the furnace is called the hearth or crucible, and above that extends the widening portion, called the bosh, which extends upward to that portion of the furnace having the greatest diameter. The stack extends from the bosh to the throat. As is well known, the blast furnace charge is'introduced into the'throat or top of the stack in alternate layers of material; viz., ore, coke, limestone, etc., and the preheated air for combustion of the coke is introduced into thefurnace through the tuyeres at the top of the hearth. The combustion zon is at and above the tuyeres, and the molten slag layer is below the tuyeres and floats on the molten iron which collects in the furnace bottom.

In accordance with one aspect of the present invention, the improved method includes the charging of the blast furnace with a burden containing as its main source of iron, a hematite or limonite iron ore. A typical example of such an ore is a Lake Superior hematite analyzing approximately 73.50% iron cxide (F8203) 8% silica (SiOz), 2.5% alumina (A1203), and other normal constituents such as phosphorus and manganese, usually not exceeding a few tenths of 1%. As

duce the desired normal, or ordinary, content of silica in the slag, there is added an additional silica-bearing material, such as silica rock. This burden is charged into the blast furnace in the usual manner and along with sumcient amounts of carbon in the form of coke. Air for supporting combustion'of the coke is introduced at relatively high temperatures through the tuyres to effect eflicient reduction or smelting of the mate rials charged and to efiect the production of molten iron, containing titamium, silicon, etc.

In accordance with the present invention, the blast furnace is so operated that the temperature reached in the reduction zone and in the combustion zone is at least high enough to reduce the oxide of titanium, if the titanium .is present as an oxide in the charge, or to melt the titanium if it is present in the charge as some other compound of titanium. The theoretical reduction temperature of titanium oxide (TiOz) is on the order of 2962 F. and the theoretical temperature of fusion of pure titanium (Ti) is on the order of 3280" F., and above. The exact form of the titanium in the molten iron thus produced cannot be stated with absolute certainty. It may be in the form of elemental titanium (Ti) or in the form of some reactive and effective compound of titanium, for example, titanium carbide (TIC). I believe the titanium is produced in the form of titanium carbide (TiC), because incandescent carbon is present in the reducing atmosphere of the high temperature combustion zone. It seems unlikely that a nitride or some other reactive ,form would be produced under these conditions.

desired quantity and character of slag, as explained hereinafter. It is essential for this purpose that the air blast temperature be in excess of 1500 F. to 1600 F., or even higher if practical slag volume I with the equipment available.

After the molten iron containing titaniumis formed, as just'e'xplained, the next concern. is

. normal slag volume and keeping the basicity of to increase its temperature to a point where the titanium in a reactive and effective form is completely dissolved in the molten iron.v This is done by a selecting and proportioning the materials of the charge that the slag-forming constituents thereof will be of such a quantity and quahty that a larger-than-normal slag volume will be maintained in the furnace. The term larger-than-normal slag volume as used herein, is intended to mean that the volume of slag maintained in the furnace in-accordance with the present invention is greater than that which is usually maintained at the present time in a blast furnace when operating to reduce iron ore in the conventional manner. At the present time, the raw mateirals are beneficiated and the slag volumes are usually below 1000 pounds for each ton of iron. Therefore, the volume of slag maintained in accordance with this invention should be sufficient to produce over 1000 pounds for each ton of iron produced. Preferably, the.

should be between 1000 and 1500 pounds per ton of iron, and better still between 1300 and 1500 pounds per ton of iron produced.

The l arger-than-n ormalslag volume performs the additional function of a protecting cover for the .molten iron in the crucible. Furthermore, the

slag shouldbe basic in character. For the present purpose the combined or total magnesium oxide and calcium oxide content (MgO-i-CaO) of the slag should exceed 42%, and preferably should be between 44% and 48%. The higher basicity of the slag raises the fusion temperature thereof andthereby results in a higher temperature for a given viscosity of slag. The larger-than-nor'mal slag volume results in extending the time of contact between the molten iron containing titanium and the slag Otherwise, the characteristics of the slag are similar to the generally accepted or standard blast furnace practice in the production of foundry grades of pig iron; wherein the silicon content in the pig iron exceeds 1%, as stated previously herein. As an example, "the silica content in the slag is usually kept between'33% and 38% and the alumina between 11% and 16%. It is noted, however, that in the known and accepted methods of producing foundry grades of pig iron the silica content in the slag is rarely, if ever, permitted to fall below 30% or to be in excess of and a maximum figure for alumina in the slag is considered to be 18%. P

As stated above, an important function per-, formed by the relatively high air blast temperature, the larger-than-normal slag volume, and the basic quality of the slag, superheated in passing through the slag 'layer to a point where the titanium in its'previously explained reactive and effective form, is completely absorbed in the molten iron. The theoretical temperature of absorption of reactive titahium and including its carbides intohomogeheous solution in iron is on the order of 3100 F. The absorption will begin around 2800 and continue until it reaches and passes the 3100 F Heretofore, these temperatures have combustion zone and thereby increase the emciency of reduction.

It is also important to note thatbecause of the larger-than-normal slag volume, its high basicity, and the relativeiyhigh air blast temperature, the molten iron is superheated in passing through the slag layer so that its temperature is raised to a point critical to the absorption of carbon in the molten iron in such a manner that, when the iron cools to the solidus, the free or graphite carbon separated out'will be in finely divided form and uniformly distributed throughout the iron matrix, like that shown in Figs. 1, 2, 3, and 4 and as distinguished from the flakelike and lumpy graphite formations of Figs. 5, 6, '7, and 8. This critical point is on the order of 3000 F.

The effect of superheating the molten iron is to cause the absorption of a larger amount of car- 'bon in the molten iron, and, as stated above, the maximum temperature reached is such that the graphite carbon will be in finely divided form tion in that it acts apparently in the nature of a catalyst and locks or fixes the carbon in the liquid metal, so that when the iron passes through the solidification range the graphitic carbon is retained in a finely divided and evenly dispersed form. The total carbon present in the iron is therefore higher than normal and the graphitic carbon is distributed uniformly and in finesubdivision's throughout the iron matrix.

A further and most beneficial characteristic is that the finely dividedcharacter of the graphitic carbon and its uniform distribution in the pig iron carries through'lto and persists in the castings made therefrom, and while the casting is slightly higher 'inBrinell hardness because of its relatively fine pearlitic structure, it is a more machinable casting having a fine, tight grain and one which has increased tensile and transverse is that the iron is been considered too high to be practical in the normal operation of a blast furnace; but they have been obtained in accordance with the present invention by maintaining the larger-thanstrength. Thus, the superior pig made from the present method is a heavy scrap carrier in the foundry and produces sound castings with a minimum of chill and shrinkage; By the expression heavy scrap carrier" is meant that the same quality of casting can be produced with the pig iron of the present invention by using a higher ratio of scrap to pig than can be done with the pig iron of the prior art.. The titanium present in the iron seems to assist the silicon to carry out its beneficial effect of reducingchill, and it also seems to prevent the normal action of the v silicon from reducing the total carbon in the iron.

A grain structure is provided which is pearlitie in character, with finely divided graphite uni formly distributed throughout, as distinguished from the relatively lower carbon and .the segregatedflake-like and lumpy graphite formations of "the prior art pig.

As an example of the manner in' which the contents of the charge should be proportioned in order to obtain the larger-than-normal slag volume (over 1000 pounds per ton of iron produced) and the high basicity of the slag (in exv cess of 42 which as explained above are large- 1y responsible for the complete absorption of the 'ilmenite, analyzing 50% FezOa, 45% T102, 3% S102 and traces of other elements, a typical ac-- tual charge isas follows:

other constituents of the charge, those skilled in the art will readily understand how a desired burden should be computed to produce a highcarbon, titanium-bearing pig, such as given above. Experience shows that the air blast temperature should exceed 1250 F. and should be as high as practical with the equipment available.

'The following is a table showing a number of actual results obtained at different times by practicing the present invention, as explained above. These results show the bearing which the values of the slag volume and the air blast temperature Pounds Hematite 9,860 Ilmem'te 140 Silica rock or other silica-bearing material- 200 Limestone 3,000 Coke 5,000

The above charge when smelted, produced 5,500 pounds of pig iron and 3,465 pounds of slag.

The slag volume, when proportioned, amounts to 1,414 pounds of slag per ton of iron. The pig iron produced amounted to 5,120 pounds of pure iron (Fe), as pig iron contains 93% Fe and 7% total silicon, carbon, manganese, phosphorus, ti-

tanium, etc. The titanium content in the pig amounted to .48%. The basicity of the slag approximated 46%, combined MgO and CaO and, the silica approximated 34% and the alumina 14%. f I

A second example of a blast furnace charge computed in accordance with the present invention and using thesame hematite ore but using the aforementioned by-product sludgeas the titanium-bearing material which analyzes 2-5% Ti, about 50% Fe, and 7% Si, is as follows:

' Pounds Hematite -e 8,400 Sludge 1,600 Silica ro 112 Limestone 3,000 Coke 5,000

invention, it has been determined that upon smelting a charge like the examples given above, approximately 75% of the titanium present in the charge passes into the pig iron, the remaining being in. the slag. The amount of titanium in the pig iron is, of course, a function of the amount of titanium in the charge and that percentage of titanium in the charge which is absorbed in'the iron matrix. Experience shows that the percentage of titanium present in the charge which is absorbed in the iron, varies with the value of the slag volume maintained in the furnace, the basicity of the slag, and the air blast temperature; i. e., as the slag volume, slag basicity and air blast temperature increase the amount'of titanium in the-pig increases. As the amount of slag is a function of the amount of slag-forming constituents of the charge (such as gangue in the ore, silica-rock, limestone, etc.);

and, as the basicity of the slag is determined by the quantity of limestone in proportion to the have on the amount of the carbon and titanium constituents in the pig iron, and also show the influence of the titanium present on the amount' of total carbon present in the pig iron. The silicon in the pig is also included so as to indicate its influence when comparing the several results.

Analysis oi pig Slag Air blast Cast No.- volume oi temperpig nture Si Tztal Tl per ton F. Percent Percent Percml l, 030 1, 300 3. 02 3. 30 1, 030 l, 300 3. 03 3. 35 l, 200 1, 300 3. 00 3. 35 1, 200 1, 500 3. 00 4. 00 35 l, 200 1, 600 8. 28 3. 79 30 1, 400 l, 500 3.10 3. 95 45 l, 400 l, 550 3. 08 3. 98 45 1, 500 1, 600 3. 11 3. 96 48 l, 500 1, 600 2. 83 4.10 42 1, 500 l, 600 2. 26 4. l8 35 It has been found that pig iron produced in accordance with the above described process and containing in excess of .25% Ti, possesses distinctly superior qualities; and, when made into castings, produces good and sound castings, having a line, tight grain, a minimum of shrinkage and chill, increased tensile and transverse strength, and improved machinability. An examination of the pig shows a distinct preponderance of finely divided graphite and pearlite, uniformly distributed throughout. These superior qualities of the pig are carried through into the casting and thereby render this pig iron a heavy scrap carrier in the foundry. All other factors remaining the same, the above-explained superior qualities are present to a degree proportional to the amount of Ti present in the pig. An upper practical limit for the titanium content of the pig iron is believed to be on the order of 1.0%. A practical optimum range for the titanium content is between 30% and .50%, with the most desirable range between .40% and .50%. It will be appreciated that the desired titanium content of the pig will depend upon the amount of scrap to be used with the pig in the cupola. The higher the percentage of scrap, the higher the titanium content iii th pig should be. It has been found in practicing the present invention, that castings made from 100% direct metal from the blast furnace, and without any scrap, produce a 50% increase in tensile strength as the following will -A further actual comparable result in a foundry, and illustrating the superior qualities of the pig iron produced in accordance with the present invention over the prior art pig, is as These results show an increase in combined carbon in the casting when pig iron produced in accordance with the present invention is used, and they show a higher Brinell due to the pearlitic grain structure. These results also show that the machinability of the casting is improved even though the Brinell is higher, and that the tensile and transverse strength is materially increased.

As the above results show that the outlined desirable characteristics present in the pig iron are carried through to castings made therefrom, 7 it is evident that when titanium is produced in v a blast furnace and added to the molten iron by the improved method and process described above, such titanium is retained in the solid pig iron in a highly reactive and effective form. The

identity of its precise form and an explanation of exactly how it is retained in the solid are not critical to a thorough understanding of my improved method by those skilled in this art. I believe that the titanium which is produced and added to the molten iron in the blast furnace in the manner previously explained, is titanium carbide (TiC) and that it is retained in solid solution in the solid pig iron. Th reason for my conclusion is that titanium and its several probable compounds, are very hard and consequently, if the titanium were suspended in the solid iron as a separate phase, hard spots would be present, which is not true of iron produced by my method.

What I claim and desire to secure by Letters Patent of the United States is:

1. The method of producing in a blast furnace a foundry grade pig iron of improved, refined and controlled graphitic structure containing in excess of 25% Ti in a reactive and effective form and in excess of 1% Si, which method comprises charging th furnace with an iron ore containing in its natural state substantially no titanium,

and constituting the principal source of iron, to-

gether with limestone, a titanium-bearing mate- 2. The method of producing in a blast furnace a foundry grade pig iron of improved, refined and controlled graphitic structure containing in excess of .25% Ti in a reactive and effective form and in excess of 1% Si, which method comprises charging the furnace with an iron ore containing in its natural state substantially no titanium, together with limestone, a titanium-bearing material in sufilcient quantity to produce the aforementioned titanium content in the pig iron, said iron ore and said titanium-bearing material constituting the principal source of iron, and with sufiicient coke to reduce the oxides present in the charge; regulating the proportions of the constituents of the charge to maintain a slag volume in the furnace suflicient to produce from 1300 to 1500 pounds of slag for each ton of iron produced, and to produce a basic slag containing in excess of 42% combined CaO and MgO; and

smelting such charge using air temperatures in excess of 1250 F.

3. The method of producing in a blast furnace a foundry grade pig iron of improved, refined and controlled graphitic structure containing in excess of 25% Ti in a reactive and effective form and in excess of 1% Si, which method comprises charging the furnace with an iron ore containing in its natural state substantially no titanium, together with limestone, a titanium-bearing material in sufiicient quantity to produce the aforementioned titanium content in the pig iron, said iron ore and said titanium-bearing material constituting the principal source of iron, and with sufiicient coke to reduce the oxides present in the charge; regulating the proportions of the constituents of the charge to maintain a slag volume in the furnace sufficient to produce from 1000 to 1500 pounds of slag for each tone of iron produced, and to produce a basic slag containing 4. The method of producing in a blast furnace a foundry grade pig iron of improved, refined and controlled graphitic structure containing in excess of .25% Ti in a reactive and effective form and in excess of 1% Si, which method comprises charging the furnace with an iron ore containing in its natural state substantially no titanium, together with limestone, a titanium-bearing material in sufilcient quantity to produce the aforementioned titanium content in the pig iron, said iron ore and said titanium-bearing material constituting the principal source of iron, and with suflicient coke to reduce the oxides present in the charge; regulating the proportions of the constituents of the charge to maintain a slag volume in the furnace sufiicient to produce from 1300 to 1500 pounds of slag for each ton of iron produced, and to produce a basic slag containing between 44% and 48% combined CaOand MgO; and smelting such charge using air temperatures in excess of 1250 F.

5. The method of producing in a blast furnace a foundry grade pig iron of improved, refined a o 1 controlled graphitic structure containing in excess of 25% Ti in a reactive and effective form and in excess of 1% Si, which method comprises charging the furnace with an iron ore containing in its natural state substantially no titanium, together with limestone, a titanium-bearing material in sufficient quantity to produce the aforementioned titanium content in the pig iron, said iron ore and said titanium-bearing material constituting the principal source of iron, and withv duced, and to produce a basic slag containing in excess of 42% combined C20 and MgO; and

suflicient coke to reducethe oxides present in, the charge; regulating the proportions of the constitsmelting such charge using air temperatures in uents of the charge to maintain a slag volume excess of 1250 F.

in the furnace sufficient to produce from 1000 to 5 1500 pounds of slag for each tone of iron. pro- CHARLES R. HOLZWORTH.

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