Cross-Re'ference to Related Applications

This application is a continuation-in-part of applica¬ tion U.S.S.N. 748,167, filed June 24, 1985 which, in turn, is a continuation of U.S.S.N. 578,533, filed February 9, 1984.

Background of the Invention

This invention relates to a powder metallurgy process for forming steel articles. Mo-re particularly, this inven¬ tion relates to a method for producing steel rods and wire using mill scale, iron ore, or taconite as the metallic iron source.

Large amounts of particulate waste in the form of scale is produced at steel-making facilities. The scale is a coating of oxide formed at high temperatures -during rolling or forging operations. Thus, steel ingots, billets and other semifinished forms are reheated in furnaces and con¬ verted into shapes such as sheets, bars and structural forms in continuous rolling mills, which force hot oxidized parti¬ cles, known as scale, from the billets or other shapes dur¬ ing the operation. Mill scale is largely particulate iron oxide. Previously, such material has been discarded, not¬ withstanding the large amounts of iron available for recy¬ cle. More recently, processes have been devised for placing the mill scale in a form which will permit recycle of the

ill scale in the form of briquettes containing iron adapted for use as feedstock to steel-making furnaces. Such pro¬ cesses are described for ^' example in U.S. Patent No. 4,369,062 and U.S. Patent No. 3,870,507.

Powder metallurgy processes have been used to produce steel articles for more than 45 years. For example, U.S. Patent No. 2,152,006 to Welch discloses a process which compacts and shapes, in a mold, mixtures of metallic powders and binders under a pressure of the order of 2,000 pounds to the square inch and upward. The shaped and coherent body is then removed from the mold, packed in finely divided alundum or equivalent refractory material, within a suitable tube or boat βkf carbon or other refractory; and so packed, is placed into a sintering furnace. After sintering at 1500-2000°F, the compacted mixture is shaped by forging, rolling, _ die-pressing or the like. The shaped article is then heated to a higher temperature just below the melting point of iron whereupon the component metal powders combine to form the intended alloy while retaining their shape. Upon cooling, a usefully shaped metal article is produced. Unfortunately, however, metal articles produced by the Welch process lack the density (and therefore the strength) of articles produced from conventional rolled steel formed from poured steel ingots.

Summary of the Invention

In accordance with the present invention, mill scale or iron ore i finely divided form is reduced and used to form a particulate admixture along with manganese and carbon. Thereafter, it is admixed with a binder so as .to bond the particulate admixture together. The bonded admixture is then formed into a coherent mass by compressing the bonded admixture. The coherent mass is then sintered .in a non-oxidizing atmosphere to provide structural integrity an indefinite shelf- life. The sintered mass is then heated i a non-oxidizing atmosphere to a temperature just below th

fusion temperature of iron to produce a homogenous stee alloyed article. The article is then worked, at elevate temperature, to increase its density and decrease its grai size.

Preferably, the density of the coherent mass afte compression is at least about 85% of the density of a rolle steel bar produced by conventional methods. Thus, voids an other internal defects which might be detrimental to qualit are controlled and the compressed bonded admixture will b subject to a maximum additional shrinkage rate of only abou 15% upon further treatment.

Surprisingly, it has been discovered that steel arti¬ cles can be produced using the above-described powder metal¬ lurgy technique from mill scale, a material formerly recy¬ cled in blast furnaces, from iron ore or from taconite, in combination with manganese and carbon. The mill scale con¬ tains iron of sufficient purity to enable it to be used in this manner to produce a steel product. Thus, mill scale does not have contaminants such as silicon, phosphorus, sulfur, calcium, silicon or the like which would prevent its utilization in the present powder metallurgy process. Like¬ wise, iron ore and taconite can be processed to the same de¬ gree of purity and substituted for mill scale in the process of this invention.

Brief Description of the Figures

Fig. 1 is a side view in cross-section of a mold which can be used in accordance with this invention to compress an admixture of particulates and binder;

Figs. 2-7 are micrographs of a steel bar produced in accordance with this invention;

Fig. 8 is a graph showing the results of tensil, yield- and elongation tests performed on a steel bar produced in accordance with this invention.

59

-4- Description of the Preferred Embodiments

In accordance with the' present invention, mill scale, as received from a steel mill is first freed of tramp steel particles and non-metallics by screening. Next, the mill scale is ground to the desired finely divided particulate size so as to permit its successful incorporation into the particulate admixture hereinafter described. The mill scale should be ground as required to provide an average particle diameter of below about 50 microns, preferably between abou 35 and about 45 microns, with about 40 microns being espe¬ cially preferred. Any suitable means for grinding the mil scale can be utilized including ball-milling, rod-milling o hammer-milling.

The mill scale may be determined to be at the desire particle diameter by examining it on a comparator, by pas¬ sage through a suitable mesh screen or by comparing parti¬ cles under a microscope.

Alternatively, either iron ore or taconite, both o which contain Fe ₂ 0-, silica and other contaminants such as CaO, sulphur and phosphorus can be used by grinding the or or taconite to the same size as above-described for mil scale, for example, 40- microns. Next, lime is added - t obtain the proper ratio of lime to silica, namely, a rati of 2 on a molecular basis as determined by the formula

(CaO-3P ₂ 0 ₅ ) .

SiO ₂

The mixture is fired, for example, in a rotary lime kiln a a temperature of about 500°F (260°C). This produces a mag netic ore (Fe^O.) known as "magnetite", which can be crushe and the ^Fe 3°4 ^is magnetically separated from . th non-metallics using a conventional magnetic separator. Th resulting Fe-O. can be substituted for mill scale.

Next, mill scale or magnetite having the desired aver¬ age particle _. diameter is passed to a reducing zone, whic may be a reducing furnace where the mill scale or magnetite is subjected to a temperature in the range of between about 1000° (537°C) and about 1500°F (815°C) , preferably between about 1150° (621°C) and about 1250°F (676°C) , with a temperature of about 1200 ^β F (649°C) being especially pre¬ ferred. This results in reduced mill scale or magnetite free of oxygen and in the form of substantially pure elemental iron.

The reduction furnace may be any suitable design. Such furnaces are known to the art. For example, U.S. 3,941,359 to Shinville et al discloses a reduction furnace for re¬ duction of mill scale, the disclosure of which is hereby incorporated by reference.

The substantially pure metallic iron, is blended with manganese and carbon in a mixing zone in dry form, for exam¬ ple, in a ball mill. Both the manganese and the carbon are in a reduced state to avoid the presence of oxides. The manganese may be used in any suitable particle size. For example, manganese having an average particle diameter of below about 50 microns, preferably between about 35 and about 45 microns, with about 40 microns (about -235 mesh) ^~ being especially preferred. Preferably, the elemental iron and the manganese have the same particle size. Likewise, the carbon, which may be carbide or similar carbonaceous material may be employed having an average particle diameter of below about 50 microns, preferably between about 35 and about 45 microns, with about 40 microns being especially preferred. The carbon should be of. the same particle size as the iron and manganese.

A suitable particulate admixture comprises about 98 weight percent metallic iron; from about .035 to about 2.00 weight percent manganese; a.nd from about 0.04 to about 2.00 weight percent carbon, all in reduced form. For example, a suitable mixture can include those used in A.I.S.I. Nos. C 1008 to C 1021 comprising iron with the remainder being carbon and manganese as follows:

A. I. S.I. No. ^' C Mn.

C 1008 0.10 max. 0.25/Q.50

C 1010 0.08/0.13 0.30/0.60

C 1012 0.10/0-.15 0.30/0.60

C 1015 0.13/0.18 0.30/0.60

C 1016 0.13/0.18 0.60/0.90

C 1017 0.15/0.20 0.30/0.60

C 1018 0.15/0.20 0.60/0.90

C 1019 0.15/0.20 0.70/1.00

C 1020 0.18/0.23 0.30/0.60

C 1021 0.18/0.23 0.60/0.90

Preferably, the particulate admixture comprises metal¬ lic iron, manganese and carbon. ^' However, additional alloy-' ing ingredients may be included, such as chromium, nickel, molybdenum, vanadium, lead, sulfur, aluminum or the like in a reduced state. If such reduced metals are utilized, they should be used in amounts as desired to obtain the strength, ductility, machinability and the like required in the par¬ ticular article being produced..

The finely divided particles of iron, manganese, car¬ bon, and any additional alloying ingredients are admixed in a mixing tank to achieve a substantially homogenous admixture. Next, a binding agent, preferably of a substantially hydrocarbonaceous nature is added to the admixed particulates in the mixing zone. Any suitable hydrocarbonaceous material can be utilized including clear paraffin, coal tars, pitches, petroleum residue pitches or petroleum reforming bottoms in amount sufficient to bind the iron manganese carbon and/or alloys together. No hydrocarbons which will produce ash when burned should be used. Sufficient binder is added to "wet" the particulate mass, and the appropriate amount can be easily determined experimentally. Preferably, zinc stearate is the binder used, and in an amount of from about 1.0 weight percent to about 4.0 weight percent, and preferably about 2.0 weight percent of the total admixture." Zinc stearate is preferred because it fumes off during the sintering process leaving no residue.

Next, the particulate admixture containing the binde is formed into a coherent mass by compressing the admixtur at ambient conditions in a, mold to form a bar or othe shaped product as desired. Preferably, the admixture i subjected to molding pressures such that the resulting com pressed admixture will possess a density of. from about 85 t about 100% and more preferably from about 90 to about 100 of the density of conventional rolled steel. Sinc conventional rolled steel has a density of ^" 0.2833 lbs/in (7.83 g/cm ³ ) , the resulting compressed admixture wil preferably have a density of from about 0.2408 lbs/in ³ (6.6 g/cm ³ ) to about 0.2833 lbs/in ³ (7.83 g/cm ³ ), and mor preferably from about 0.2550 lbs/in ³ (7.05 g/cm ³ ) to abou 0.2833 lbs/in ³ (7.83 g/cm ³ ). Achieving such a density prio to sintering is an important step over the prior art for number of reasons. First, the resulting green strength o the greatly compressed admixture provides the bar with th ability to be handled without fear of fracturing. Othe methods, which use significantly less pressure, produce bar which crumble easily and cannot be handled on a productio basis. Second, bars produced under great compression con tain fewer undesirable voids and do not readily oxidize a do those of the prior art. This allows bars made in accor dance with this invention to be directly inserted into th sintering or alloying furnace. Bars produced under les compression must be protected from oxidation by packing in suitable boat or tube in a finally divided alundum. Third, and perhaps most important, bars of this invention whic have undergone sintering and alloying can be swaged, rolled, drawn or otherwise worked without a great deal of shrinkage, whereas bars having much lower densities can not be furthe worked without significant shrinkage after sintering an alloying.. Thus, the alloyed bars of this invention can b further worked to produce an end-product bar having density equal to that of conventional rolled steel bars. The end-product ^• bar can then be cold-worked to form th desired fabricated article. Cold-working of other . powe metallurgy bars, which have not been worked ^" in accordanc

with this invention, is difficult, if not impossible due their much lower densities.

It should be understood, however, that the foregoi improvements resulting from imparting a density of at lea about 0.2408 lbs/in ³ (6.67 g/cm ³ ) to the compress admixture are not the only improvement provided by t process of this invention when compared to prior powd metallurgical processes.

Another aspect of this invention provides improv steel alloy articles made from powder metallurgy technique which have properties similar or superior to steel articl made from either conventional methods or other, differe powder metallurgy methods. This is because the powd metallurgy process of this invention provides for worki the resulting alloyed" article at elevated temperature increase its density and decrease its grain size, there increasing its strength and ductility. Preferably, the a loyed article is worked by swaging, rolling, drawing "otherwise until its density is from about 0.2408 lbs/i (6.67 g/cm ³ ) to about 0.2833 lbs/in ³ (7.83 g/cm ³ ), mo preferably from about 0.2550 lbs/in ³ (7.05 g/cm ³ ) to abo 0.2833 lbs/in ³ (7.83 g/cm ³ ), and most preferably a densi equal to that of conventional rolled steel, i.e., abo 0.2833 lbs in ³ (7.83 g/cm ³ ). Even when the density of t alloyed bar is 0.2833 lbs/in ³ (7.83 g/cm ³ ) prior to worki at elevated temperature, the working step is still ve advantageous because it reduces the grain size, which enlarged due to heating during the alloying step.

As discussed above, the compressed admixture preferab has a density of at least about 85% of the density conventional rolled steel. This number is not an empiric value below which the process will, fail, but rather a comm sense value below which the process becomes increasing impractical. Simply stated, the greater the density of t compressed admixture, the lesser the shrinkage resulti from working the resulting alloyed article. Obviously, follows that the lesser the shrinkage, the larger t resulting bar, and since the economics of scale favor larg

T/US87/00975

-9- end-product bars, it follows that greater density is desir in the initial compressed admixture. Furthermore, low densities would mean that much larger molds would required, and sintering and alloying furnaces would eith have to be larger, or they would process much less steel terms of the end-product bar. Thus, for numerous practic reasons, bars of greater density are preferred.

Referring. ow to Figure 1, a mold is shown which can used in accordance with the present invention. The wa thickness E must be sufficient to withstand from about 17 about 40 tons of pressure per square inch (from about 234 about 552 megapascals) . The necessary wall thickness can determined using the formula:

E=PD/(2S) wherein, P=pressure

. D=outside diameter

S=tensile strength of steel used to make the mold.

The inside configuration of the mold can be circula square, rectangular, etc. One end of the mold G is clos by a removable base R. The other end of the mold H is ope The inside wall I tapers inwardly from the open end H, for taper length of L, to point J after which the inside d mensions of the mold remains constant. Thus, the insi dimension of the mold will vary, for a. taper length of from B at the open end H to B* at point J. The degree tapes -C, which is drawn larger than scale, is on the ord of a few thousandths of an inch. The mold is taper because otherwise it would be nearly impossible to remo the compressed admixture from the mold once it has achiev the desired density.

In practice, the admixture is inserted into t open-end H of the mold. A plunger having a transver cross-sectional dimension slightly less than B' . is th inserted in the open end and from about 20 to about 40 to per square inch (from about 276 to about 552 megapascal

and preferably from about 30 to about 40 tons per square inch (from about 414 to about -552 megapascals) of pressure. or more is exerted on the admixture by means of the plunger.

The compression is repeated by withdrawing the plunger, adding additional admixture, and repeating the compression until a compressed admixture of desired length and density is produced. The removable ^' base R is then removed from closed end G and the compressed admixture is pushed out of the mold through the tapered, open end H.

The degree of taper C and length of taper L needed to facilitate instant release of the molded powder can be de¬ termined experimentally based upon the dimensions of the article produced in the mold. It has been found that a mold having a square transverse cross-section which will produce a bar l ¹ ' x 1*' x 24' * requires a degree of taper C of about 0.001 inch, and a length of taper L of about 4 inches.

- Preferably, the inside surface of the mold is covered by a non-stick surface such as polytetrafluoroethylene, etc.

It has been found that "DuPont Teflon Wet and Dry Lubricant", which is a product of E. I. DuPont de Nemours &

Co., Inc., provides satisfactory results.

While the pressure which is exerted on the admixture will generally be in the range of from about 20 to about 40 tons per square inch (from about 276 to about 552 megapascals) , the particular amount of pressure needed will depend upon the desired density. In practice, the density can be quickly calculated by, for example, weighing an amount of admixture which is to be compacted into a bar of known dimensions. Thus, the ^' pressure needed will be that required to compress the total weight of admixture into a pre-determined size bar.

The resulting coherent mass product is then placed in a sintering furnace under a non-oxidizing atmosphere, pref¬ erably a reducing atmosphere of hydrogen, and heated.,to a temperature less than 2600°F (1426 ^β C) ,- preferably from about 1400° (760 ^β C) to- about 1600°F (871 ^β C) , more preferably from about 1450° (778°C) to about 1550 ^β F (843 ^β C) , with. about

1500°F (815°C) being especially preferred. The coheren mass must remain in the sintering furnace for a period o time sufficient to raise the temperature uniforml throughout. Any suitable sintering furnace can be utilize for sintering of the shaped article.

At this stage in the process, the sintered mass may b cooled, for example, to ambient temperature, and is no sufficiently hard and has adequate structural integrity t be stored or handled without fracturing. The bar can the be formed into any desired shaped article, such as wrenches hammers, bolts, nuts, or the like.

If cooled, the resultant sintered mass is reheated in non-oxidizing atmosphere, preferably hydrogen in a furnac to a temperature below the fusion point of the mass, namel 2600°F (1426°C) , at which temperature the carbon an manganese go into solution. If the foregoing cooling ste is omitted, the sintered mass is heated directly to jus below the fusion temperature of the mass. The sintered mas should be heated for a time sufficient to raise th temperature uniformly throughout. Suitable temperature include from about 2300° (1260°C) to about 2600° (1426°C) preferably from about 2350° (1288°C) to about 2499° (1370 ^β C) , with 2400°F (1315°C) being especially preferred At this temperature, a homogenous steel alloyed articl having the desired . ratio of iron to manganese to carbo results.

The resultant alloyed article, unlike other bar produced from powder, can then be immediately ^' passed throug a set of rolls or through a swaging machine, where it i shaped and reduced in size until it ^' has a roun cross-section and. the desired density. Alternatively, th alloyed article can be cooled, for example to room or ambi ent temperature, and later reheated for working, namely, t a temperature that will enable it to be passed through roll or swaged to a round size, if desired. Suitable reheatin temperatures include from about 2100° (1148°C) to abou 2300° (1260°C), preferably from about 2100° (1148°C) t

about 2150 ^β F (1176 ^β C) , with approximately 2100°F (1148 ^β C being especially preferred.

Next, the resultant bar may be drawn through a dra bench and reduced to a size suitable for cold-work produc tion of screws, bolts and nuts or to wire sizes suitable t produce nails, wire cloth,- springs or other wire products Alternatively, the resultant bar can be rolled or forged t produce seamless pierced pipe, or flat rolled bars for pro duction butt-welded pipes and/or electric-welded pipe. Th apparatus for production of such articles is well known t those skilled in this art.

Referring now to figures 2-7, micrographs of a ba produced in accordance with this invention are shown a various stages in the process.

Figure 2 is a micrograph of a cold-pressed bar, un etched at a magnification of 50X. Many black pores ar

^' visible and the cold-pressed bar has a density of abou

85-90% when compared to the density of conventional rolle steel.

Figure 3 is a micrograph of the same bar after heatin at 2250°F for one hour, unetched and at a magnification o 50X. Many black pores can still be seen.

Figure 4 is a micrograph of the longitudinal surface o the same bar after swaging, unetched at a magnification 260X. It is seen that most of the pores have been elimina ed. The gray phase seen in the micrograph is manganes sulfide.

Figure 5 is a micrograph of the transverse surface the same bar after swaging, unetched and at a magnificati of 260X. Again it can be seen that most of the pores ha been eliminated.

Figure 6 is- a micrograph of the the longitudinal su face of the same bar after swaging etched and at a magn fication of 260X. Grain boundries are now visible, and t bar is seen to have ASTM No. 10.grain size, which is commo ly found in conventional rolled steel.

Figure 7 is a micrograph of a transverse surface of t same bar after swaging, etched and at a magnification

260X.

These micrographs indicate that the product produc has all of the characteristics of steel rods and bars roll on conventional mills. In fact, the product is even super or in many respects, because there is an absence non-metallic stringers of silicon and aluminum, and becau the grain structure is more uniform than steel which h been produced from poured steel ingots. This will be great advantage to heat treaters because the chemistry the product is uniform and not subject to segregation as poured steel ingots. Furthermore, the range of carbo manganese, and other alloying ingredients will be unifo throughout the bar, thus making it easier to arrive at given hardness. The bars of the present invention will al save a great deal of wasted material because there will fewer failures do to the uneven chemistry ranges present steel from poured ingots. Additionally, quality will superior because there will not be seams which are found rolled steel, and therefore, fewer defects in the finish product.

Referring now to Figure 8, the results of tensi strength, yield strength, and total elongation tests a shown graphically. The sample with which the tests we performed was a one inch gauge length bar having a 3/16 in diameter reduced section. The yield strength of the samp is 45,000 lbs/in ³ ; the tensile strength is 5'4,000 lbs/in ² and the total elongation is- 23%. These tests show th steel bars produced in accordance with this inventi possess ductility equivalent to that found in rolled ste bars produced by conventional means.

The following examples illustrate the present inventi and are not intended to limit the invention, but rather, a presented for purposes of illustration. The percentages a by weight unless.otherwise specified.

Example 1

Mill scale from steel mill operation is screened an placed in a ball mill to reduce the particle size to a average of 40 microns. Next the mill scale is passed to reduction furnace where it is reduced in a hydrogen atmo sphere for 30 minutes at a temperature of about 1200° (649 ^β C) . After cooling, the reduced mill scale is passed t a mixer to which is added manganese and carbon each bein about 40 microns in particle size in amounts to provide th required percentages to meet ASTM, AISI or AP specifications. For example, A.I.S.I., C 1010 is produce containing 0.08 to 0.013 carbon, 0.30 to 0.60 manganese with the remainder being reduced mill scale. The resultan particulate mass is thoroughly admixed and clear liqui paraffin is added.

The resultant admixture is removed from the mixer an pressed in a closed die to form a bar. The resulting bar i sintered at. a temperature of about 1500°F (815 ^β C) under hydrogen atmosphere.

The resultant bar is then heated in a hydrogen atmo sphere to about 2400 ^C F (1315°C) , cooled, reheated to 2100° (1148°C) and passed to a swaging machine to form a rod. Th rod is then drawn on a draw bench to produce a wire produc having good tensile strength.

Example 2

The procedure of Example ^' 1 is repeated by substitutin for mill scale F^O _* obtained from iron ore containin Fe-O-, silicon and other contaminants, by grinding the iro ore to an average particle size of about 40 microns - admixing it with lime, firing in a rotary kiln at 1500° (815 ^β C) , crushing and magnetically separating the resulti Fe-O. particles. The ^* resulting wire product has go tensile strength _*

Example 3

Mill scale, iron pellets or taconite pellets ar crushed to -235 mesh. The iron powder is analyzed to deter mine, silicon content, which is compensated by addition o calcium to for tri-calcium silicate crystals. Th non-metallics such as sulfur, silica, calcium an phosphorous and any tramp guage are treated and dissolve with a suitable acid. The iron and acid mixture is passe through a filter press which effectively separates the iro from the impurities leaving 99%+ pure iron powder. Th powder is then washed, dried, and roasted at about 1500 ^β F (815°C) in a rotary kiln or stationary continuous furnace where it is reduced. The iron powder is then recrushed. The reduced iron powder is magnetically separated, and mixed with carbon, manganese, other desired alloying components, and a binder which is preferably zinc stearate, thus forming a particulate admixture. The particulate admixture is then pressed into a bar under a pressure of about 20 tons per square inch (276. megapascals) , or a amount of pressure sufficient to form a coherent mass having a density of at least about 0.2408 lbs/in ³ (6.67 g/cm ³ ). The bar is then sintered at 1700 ^β F (927°C) for two hours and then heated to about 2300°F (1260°C) . The resultant steel alloyed bar, while still hot, is swaged, rolled, or drawn to size to form the product bar. The product bar can then be cold-worked into the final product.

Although the invention has been describee! in consider¬ able detail with particular reference to certain preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore, and as defined in the appended claims.