US1924153A - Stainless steel - Google Patents

Description

Patented Aug. 29, 1933 UNITED'STATES PATENT OFFICE STAINLESS STEEL Fred J. Crolius and Rudolph w.

burgh, Pa., assignors to Stainless Stuler, Pitts- Steel Corporation, a Corporation of Delaware c5 Claims.

The manufacture of low carbon stainless or non-rusting steels is diflicult, expensive and un-= certain in its product. The present invention has particularly to do with the manufacture of such steels or irons both austenitic and ferritic in a way to produce a substantially uniform product of the required characteristics by a relatively cheap, sure, economical and rapid procedure. The product generally is of a character which is readily, cheaply and economically machined, shaped, formed and worked and in which the loss or waste is reduced to a small proportion of' the total product.

The term, low carbon steel, is here employed to indicate steels having of 1% less. So-called stainless steels usually fall within this classification and the present invention is principally directed toward the manufacture of 'such steels although in some aspects it isv applicable to steels having larger proportions of carbon.

In addition to iron being predominant, the constituents of stainless steel generally include chromium and frequently nickel or manganese or other metals or combinations of some or all of them, and in general the present invention makes no'deviation from the usual chemical composition or constituents of the finished product; although zirconium may be present, but generally not as an alloy.

The element zirconium has been used to some extent in the manufacture of steel. It has entered into and become one of the elements in alloy steels. been employed in such alloys and chemical analyses have indicated various proportions of zirconium in the product. In order. to effectually employ zirconium, it has been found necessary to procure it in certain conditions for introduction into the steel. A chief source of zirconium is the Brazilian ore known as baddeleyite which consists chiefly or largely of zirconium oxide., In

some instances this and the other Brazilian ores contain as high as 98% zirconium oxide. Heretofore, it has been found necessary to treat these ores by expensive and difficult processes obtaining zirconium or zirconium compounds other than the oxide since it has been found diflicult if not impossible to employ the zirconium oxide as found in these ores in metallurgical processes. This preliminary treatment has added materially to the expense of available zirconium and so has very materially limited the useof zirconium in the arts. One phase ofthe present invention is the .employment in the manufacture of steel of the of carbon or,

Various quantities of zirconium havenative zirconium ores, including baddeleyite, containing principally zirconium oxide. Zirconium oxide when employed in accordance with the present invention has a very important beneficial effect in the manufacture of stainless steel.

In order to fully explain the invention, a typical procedure will be described. The description will be given in such terms as will be readily understandable by one versed in the art and it is to be understood that the invention is not limited to the specific details here set out. .Variations may be made in the times, temperatures, quantities of materials, character of materials and for some of the materials, substitutes may be employed or they may be entirely omitted. The

same is true with respect to steps and manipulation. 1

The invention is generally applicable to stainless steels, the composition of which may vary widely in content of chromium and of nickel or 75 content the following procedure may be followed. 5

It may be found economical to employ two furnaces, one an open hearth furnacev or Bessemer converter, the other may preferably be a high frequency electric furnace which may be of relatively small capacity.

For an open hearth furnace there should be selected one which will produce low carbon steel carefully controlled as to silicon, sulphur, carbon, and phosphorous. It is preferred to employ such a steel averaging a trace of silicon, sulphur .03,

carbon .08, phosphorous .03 and manganese 35%.

The high frequency electric furnace may have a lining of chrome, magnesia; magnesite, or preferably zirconia.

If desired, steel from a Bessemer converter may be used or ordinary steel scrap or stainless steel scrap and the like may be melted in the electric furnace.

About 185 pounds of steel is introduced into the high frequency electric furnace and current tumed on, and the mass brought to a temperature of about 2700 F. or 2800 F. at which temperature it will be molten. There is then added about -100 pounds of low'carbon ferro-chrome preferably containing about 33% iron and 65% From this description one chrome (if desired for any reason an equivalent amount of high carbon ferro-chrome may be used plus chrome ore and lime). To this molten mass may be added about 25 pounds of nickel and 3 pounds or more of manganese, either ferro-manganese or metallic, both of which will immediately melt in the mass which will have been kept at about 2700 F. or 2800 F. The slag which rises to the surface is skimmed 011. There is then added to the bath about '7 or 8 pounds of baddeleyite mixed with about 2 /2 pounds of lime. Preferably, the baddeleyite will have been previously ground or comminuted to a fine powder and in this state thoroughly mixed with the lime and the mixture is sprinkled and distributed over the surface of the turbulent metal and carried down by the internal circulation of the metal and thoroughly mixed with the mass.

The circulation and distribution of the powdered zirconium-oxide may be aided and insured by mechanical manipulation or stirring if such becomes necessary or desirable.

In order to get the proper distribution and prevent the baddeleyite from collecting or adhering to the walls of the furnace thereby allowing little or no effect on the metal bath, it is highly desirable that it be finely ground and introduced as described by sprinkling over the surface of the turbulent metal bath.

The specific order in which the materials are added is not essential to the operation although the procedure here indicated is satisfactory and may be desirable. The zirconium or the ferrochrome and the other ingredients, of course, might be preheated to any desired extent, even to the point of melting, before entering into the composition, but this is not essential.

After the baddeleyite is added to the batch, the electric current may be shut off and the small amount of slag which collects and forms on the top of the bath is removed. The current is again turned on and the metal brought to a boil and about 3 pounds of ferro-titanium may be plunged into the molten metal in such a way as to be sure that the ferro-titanium has gotten well below the surface. The metal may then be shocked by turning off the current and suddenly turning into the furnace the full current. This shocking may be repeated or the full urrent may be left on for a short period.

While the comminuted addeleyite has been described as mixed with line and the mixture added to the batch this is not essential but seems desirable. A lime slag may be formed and skimmed 01f and the baddeleyite added or baddeleyite alone might be used. In the present invention, when zirconium oxide is employed, there is no frothing or heavy effervescence produced and the batch is never wild. The slag is not abundant.

While the use of the high frequency or coreless induction electric furnace may not be an absolute essential, it is highly desirable. The construction and operation issuch that there is caused a very pronounced agitation and stirring of the metal which seems to have a beneficial eifect on the alloying process and also seems to produce an improved, fine grained and more homogeneous product. The agitation or circulation produces a turbulence in the metal which materially aids in the distribution and efficiency of the zirconium ore. It is thus possible to produce a thoroughly incorporated alloy of predetermined composition and characteristics.

An open hearth or electric arc furnace may be used in which event the baddeleyite ore will be mixed with the bath mechanically.

Ordinarily ingots of stainless steel must go through .an expensive and wasteful treatment before they can be rolled. The surface is rough and there are present gas inclusions which may cause what are termed seams and pits in the art and in order to prepare the ingots for the blooming mill operation it is essential that they be ground,chipped and/ or planed to remove these defects which otherwise would persist in the rolled sheets. The cost of the resulting material is thus increased not only by the extra labor which is necessary for preparing the ingot but also by the fact that a material percentage, often 10% to 15%, of the metal in the ingot is cut out and lost. Ingots prepared in accordance with the present invention may be passed immediately to the blooming mill without such preliminary treatment. Of course cold shots or scabs caused by uneven chilling of the surface of the ingot in the mold may be present as in other ingots but this is not a defect which persists in or mars the finished product and need not be corrected.

The usual ingot requires a soaking pit temperature of from 2100 F. to 2500 F. The ingot produced in accordance with the present invention requires a soaking pit temperature ofapproximately 1950 F; and rolls freely through the blooming passes with a much less rapid cooling than usually occurs with high chromium ,steels, and then enters the sheet bar mill at near 1400 F. at which temperature it may be .rolled to a sheet bar. Material produced in accordance with the present invention flows in the mill more readily and better with the same draft than usual stainless steels. There is a substantially uniform consistent flow of the metal. The crop at this point may not exceed 10% whereas with other steels it may be as high as 25%.

With material produced in accordance with the present invention no sheet bar pickling is generally necessary but it may be resorted to if desired.

Through the bar heating furnace, the roughing mill, the sheet and pair furnaces and the finishing mill, including the doubling operation, practices customary in rolling low carbon sheets will be found suitable, with cooling rates approximately the same as plain carbon steels.

Leaving the hot finishing mill as approximately 24 gauge sheet the, shear loss may average approximately 15%, with strip it may go down to 5%. Sheets or strips made according to the present invention can be pickled as easily and deep drawing, it may be necessary or desirable to resort to annealing or normalizing withor without subsequent cold roll passes. The workability of the material produced in" accordance with the present invention andits improvement over known materials is strikingly shown by the fact that material .30 inches thickv may be reduced to .24 inches in one pass in the cold rolls 3 without annealing. The theory and reasons involved in the present invention are not entirely understood and may not be definitely explained.

' The theories presented may not properly ex- 1 knowledged, and efforts have been made to make them more workable or more machinable. Generally, this has been attempted by adding to the steel batch materials, such as sulfides for instance, to improve the machinability of the resulting steel. Such additions, however, although they may accomplish the purpose for which they are added, have other and undesirable effects upon the steel. For instance, the. transverse strength or other characteristics of the steel may be changed for the worse by such additions. No means have been found for materially improving the adaptability of stainless steels to cold rolling, deep drawing, and the like operations.

It is a purpose of the present invention to produce a stainless steel such, for instance, as one of the well known 18-8-type having substantially all of the desirable and beneficial characteristics of the commercial stainless steels, and in addition thereto, being readily workable and 'machinable after hot rolling without subsequent treatment.

It is not possible in the present state of the art to state with certainty all of the factors which contribute toward making steels more machinable and workable or less machinable and workable. Without limiting the invention there is here stated what is believed may be an explanation of the results produced by the present invention.

plain the phenomena, but are suggested as a possible explanation.

Steels made according to the present inven-' tion may be of the austenitic or of the ferritic type. Because of the great commercial im portance of austenitic steels the characteristics of such steels will be described as they relate to the present invention.

. In austenitic stainless steels the atoms of the metal are arranged in the primary crystals according to the face centered cubic crystal pattern that is typical of gamma iron. This structural modification or phase known generaly as austenitic is itself relatively soft and flows easily I under deforming forces. Austenitic steels would,

if there were no other factors operating, present machining difiiculties for this reason alone. This characteristic is overcome in the present invention by the addition or production in the steel o1)a fine dispersion of segregated inclusions which, scattered all through the metal, tend to impede plastic deformation of the metal thus stiifening the metal and making it more cuttable or more machinable.

In the present invention because of the peculiar use of zirconium ore and the characteristic turbulence in the metal bath, a portion of the finer particles of the ore may be left in the steel to assist in stiffening the metal and so rendering it more cuttable or machinable.

In addition to the above effect which results from the austenitic structure of the metal there is another effect which is more important in affecting machinability and workability. This effect is known as "work hardening. The austenitic or gamma structure of the 188 steels is not stable at room temperature or at temperatures well above room temperatures. The

unstable austenitic or gamma structuretends to revert to the more stable ferritic or alpha structure in which the metal atoms are arranged in the primary crystals according to a body centered crystal pattern. Ferrite or ferritic steels do not fiow easily and are for this reason more machinable. Such steels constitute the bulk of the steels of commerce. The austenitic metal of the l88 steels reverts to ferrite as a consequence of mechanical working or plastic deformation, or plastic fiow. All of the metal body does not change to ferrite. The ferrite, in general, forms only along paths of metal fiow and usually as a thin layer of ferrite between adjacent blocks of austenite which have moved relatively to each other.

The total amount of ferrite formed depends upon the amount of deformation that has occurred.

The ferrite is normally harder than austenite,

but it is rendered much harder than it would be otherwise by virtue of the fact that its grain size is very small. Its presence along paths of flow in plastic deformation tends to prevent further flow along these paths and thus materially hardens the austenite.

This work hardening, though in cases quite eat, would not in itself account for all of the machining diificulties of stainless steels, nor all of the workability difiiculties.

Machining difliculties are further aggravated in stainless steels by the fact that in machining the work hardened portion of the metal is formed beneath the edge of the cutting tool. If this locally work hardened metal were held firmly in place, and if the cutting tool were properly selected and ground, it 'could be cut by the cutting tool. If, however, the work hardening is quite great, the tool does not cut it but instead drags it through the softer metal beneath. Such action does take place. It'is said that the steel balls up" on the point of the cutting tool. As a result a clean cut is not made but instead a rough cut, the metal after cutting exhibiting a rough, uneven surface. Similar action occurs in polishing but toa different degree. If the hardening is great enough there will be no out at all.

The mere transformation of austenite to ferrite, even under the point of the cutting tool, is

insuflicient to give the great hardness needed to explain the roughness of the cut actually observed in machining usual stainless steels.

It is found, however, that there is still another factor contributing to this hardness interfering with machinability and workabilty. Austenite, or the austenitic or gamma phase of steel has the ability to hold relatively large amounts of carbon in solution. Ferrite does not have this ability, being saturated when as little as .04% of carbon is contained in it.

Austenite in usual stainless steels of the l8-8 type contains much more than .04% carbon. The ferrite that is formed in working, particularly under the point of the cutting tool is supersaturated in carbon and the excess carbon tends to cold forming processes. Here the ferrite formed along the paths of fiow is very much harder because of the presence of the carbon and the work hardening is much greater for a givefi deformation than it would be if no carbon were present.

The steels made according to the present invention whether austenitic or ferritic exhibit the peculiar property that they do not precipitate carbides after cold deformation, or precipitate carbon as carbides to a substantially negligible extent and less than usual stainless steels as indicated by microscopic examination, and as shown by another type of test described below.

The precipitation of carbon, when it occurs in steels, to form fine particles of carbides in the mass. of the ferrite formed from the austenite, depends to some extent on the temperature at which the metal is worked. The precipitation is considerable at room temperature but is greater at higher temperature.

The greatest effect on mechanical properties is reached when the metal is heated to temperatures of around 1000 F. to 1500 F. where there is a sharp drop in ductility due to the embrittlement of the metal by the carbides. This embrittlement becomes evident when an attempt is made to deform the metals within this temperature range and the embrittlement itself may be caused also by the act of deforming at this temperature. An additional effect of heating or heating and working within this temperature range is to make the metal more susceptible to inter-crystalline corrosion.

Other austenitic stainless steels when ruptured at this temperature, as in a tensile testing machine, show marked evidence, in the nature of the deformation of the test piece, and in the character of the final fracture, of the effect of very rapid work hardening. The austenitic steels of the present invention do not show this peculiarity. The character of the deformation, and the character of the break neither indicate rapid work hardening, and the sharp decrease in ductility noted in the usual austenitic steels does not occur with the steels of the present invention as would be expected if there were material precipitation of carbides.

The usual stainless steels as made, in addition to carrying in solid solution carbon, also carry in solid solution oxygen and nitrogen in appreciable quantities. This atomic dispersion of oxygen and nitrogen probably tends to aid the precipitation of carbon with consequent formation of carbides which as indicated above, produce undesirable effects in the steel. Steels made in accordance with the present invention apparently contain substantially no oxygen or nitrogen in solution, since the oxygen or nitrogen which might otherwise be present seems to have combined with the zirconium forming probably zirconium oxides and nitrides which appear as very fine particles more or less evenly or generally distributed throughout themass and within the grain structure. These particles are sometimes referred toas dirt or contained impurities and as so distributed seem to improve the machinability and workability of the steel. The presence of the zirconium in the batch in this respect'seems to have the dual effect of preventing the formation of the precipitated carbides and also of forming nitrides and oxides whose presence, as they appear,

characteristic of the metal. These segregated themselves improve the non-metallic inclusions make for machinability.

Localized ferrite formation with its accompanying carbide precipitation is caused by working and machining ordinary austenitic stainless steels. The steels made according to the process of the present invention do not exhibit this phenomenon of carbide precipitation or at least exhibit it to a much less degree, so that they may be cold worked or machined much more readily than other stainless steels and a fine finish may be obtained with much less expenditure of labor than with other stainless steels.

, The austenitic steel made according to the present invention, like other steels, when worked, of course, is converted from gamma iron to alpha iron and the iron thus becomes harder. The distribution of the particles of oxides and nitrides is more or less uniform so that the additional hardness imparted by them is generally distributed. There is thus produced a steel which is hard in a uniform manner which as is well known lends itself to machinability or workability.

There may be present also some relatively large nitride crystals which seems to have no undesirable effect.

When the zirconium oxide is added to the bath it is possible that the oxygen of some of. it is replaced by carbon present in the metal since at the elevated temperature zirconium has a greater aifim'ty for carbon than for oxygen. At the high heat of the bath the zirconium carbide formed,

or some of it, may be broken down and the freed zirconium will associate itself with the nitrogen present in the metal thus extracting the nitrogen and forming nitrides while a portion of the zirconium may associate itself with oxygen present' in solution in the molten alloy. Some of the zirconium may combine with sulphur in the mass. The carbon freed may form carbon dioxide and escape and some of it may remain to form carbides with the metals present.

In ordinary stainless steels containing chromium, the carbides formed are usually chromium carbides (CriC) containing four atoms of chrothan /2% in the final metal product renders the steel too hard to machine. In the steel of the present invention the zirconium is not present as an alloy constituent and, therefore, the percentage of zirconium appearing in the analysis is not necessarily determinative of whether the effects of the present invention are produced.

Physical phenomena displayd by the metal of the present invention which have been discovered may be cited as follows:

The final product is finely surfaced and without scams; it is closely and finely grained, is readily machinable and workable, takes and retains a very high polish producing a mirror-like reflecting surface which persists as a non-rusting, non-corrosive stainless steel.

Ingots, badly poured, have been hot rolled without chipping or surfacing, and then the hot rolled strip product without annealing, has been satisfactorily cold-rolled down to 24 gauge, with excellent surfaces.

Sheet bar, showing Brinell hardness 4'77 or slightly below the hardness of the rolls them- 5 selves, has been doubled and rolled to 24 gauge sheets, no seams showing.

One steel made by this process may show the following compositions on analysis:

Mn Cr Ni 81 S P Zr the balance being iron,

Typical physical properties of this metal cast in a 300 pound ingot and subsequently forged and rolled to a' 1 inch round bar, but not otherwise treated are:

Yield point-45000 pounds per square inch. Tensile strength-47000 pounds per square inch. Percent elongation in two inches-50.5. Reduction of area-69.2%.

Other steels of the present invention showed:

0 Mn Cr Y Ni Si Zr None 02 This steel containing .15 carbon may precipitate carbides, but in other respects has the advantageous qualities herein described.

Austenitic steels made according to the present invention may contain about the following proportions: 4% to 25% chromium, 0% to 0.1% car- .bon, 0% to 18% nickel, 0% to 18% manganese and not over .30%' silicon; both nickel and manganese are not essential but when either or both are present their total will preferably amount to at least 6%. This may be referred to as the group of nickel and manganese and when that term is used in the claims .it contemplates nickel alone or manganese alone or both nickel and manganese;

Ferritic steels made according to the present invention may contain about the following proportions: 4% to 27% chromium, 0% to 0.1% carbon, 0% to 3% nickel, .25% to 3% manganese and not over .30% silicon.

Steels of the ferritic type made according to the present invention have been tested for work-' ability and machinability and have been observed to work or machine with greater ease than other steels of a similar composition.

Ferritic stainless steels generally, for instance stainless steels containing from .08% to .10% carbon and 13% to 14% of chromium but no nickel, manganese or other important alloying constituent, exhibit the characteristics of rapid -"work hardeningand must be annealed frequently in drawing, forging or rolling operations. This effect is due probably to the precipitation of carbides within the ferritic grains. r n The steels made according to the present invention exhibit much less work hardening and consequently can be rolled or drawn to a much greater extent before re-annealing becomes necessary for further working and this is so with steels made in accordance with the present invention containing chrome and whether or not they contain also nickel or manganese or the like. Y The matter .distributedthrough the metal of this invention which has been referred to here in as dirt, which includes probably zirconium oxides and nitrides and the like, seems not to be soluble in either the ferritic or austenitic metal and therefore their beneficial effect persists and they cause no loss of desirable properties.

Steels made according to the present invention may have silicon content approximately 50% lower than ordinary stainless steels. This lowered silicon content may be an important factor in the observed properties of steels made according to the present invention. In general the silicon content will not exceed 30% which is especially desirable for deep drawing steels.

From machinability observation covering tests on steels of the present invention as compared with four difierent austenitic steels representative of other stainless steels of' similar composition, it has been found that the steels of the present invention can be machined withgreater ease withincreased feeds and speeds and with ordinary tool bits of high speed tool steel ground only to slightly keener cutting edges than for the ordinary run of alloy steels. In drilling or cutting the steels of the present invention there was not observed the work hardeningeffect that is present in the usual 18-8 steels when drilled or cut. Comparative tests showed it possible to drill the steels of the present invention approximately three times as fast as it was possible to drill the most machinable one of the other austenitic steel used in the test.

The final finish of the cut or machined part obtained on the steels of the present invention was superior to that obtained on the other steels 110 used in the test.

In general the austenitic steels of the present invention are about as machinable as ordinary' commercial low carbon steels of the non-stainless variety, as for instance, American ingot iron, 115 and compare favorably also with the machinability of Monel metal.

The steels show, on examination, under magnification of 1500 to 2600 diameters, particles of non-metallic inclusions that in some cases have diameters as great as .005 millimeters but in addition to these there are found a very great many small particles that range in size from particles too small for clear resolution to .0003 millimeters diameter.

While the larger particles probably contribute toward better machinability, it is considered that the finer dispersion has a greater influence in improving machinabilty. I

It is not to be inferred that all of the particles of this finer dispersion necessarily come from the interaction of the zirconium and oxygen in the steel. As far as the present invention is concemed, part of the non-metallic particles might be-the original finely powdered zirconium ore, originally added to the batch.

Although in most stainless steels after working evidences of precipitated carbides appear under the microscope at magnifications of 250 or even 100 diameters, in the metal of the present invention substantially no evidence of precipitated carbides can be observed by microscopic inspection at 500 diameters magnification and this seems to indicate that there may be substantially no carbide precipitate or a very much reduced amount. The apparent absence of precipitated carbides is consonant with the fact that there is no typical loss 'ofductility in the metal of the present invention when deformed, at high temperatures between 1000" F. and 1500 1?. Likeceedingly high temperatures and the metals a1- loyed should be acted upon rapidly. It is preferable that the melt be finished and poured not in hours but in minutes and when chromium is present in the mass several hundred degrees of superheat may be profitably imposed upon the normal melting temperatures of the bath. During the superheating period which may last 2 or 3 minutes but preferably not more than 5 minutes in the molten mass violent stirring should be made to take place. For the most certain and economical commercial operations it is desirable that the lower cost melting furnaces such as the open hearth or Bessemer should be employed for "manufacturing the matrix mass of low carbon steel which in its heated molten condition may be drawn or tapped directly into a separate finishing furnace where the metals to be alloyed may be melted or introduced and assimilated and where preferably the desired superheat may be given the bath, the entire time consumed being reduced to a minimum, being less than an hour and possibly as little as 20 to 40 minutes.

The second alloying and superheating furnace may preferably be an open high frequency elec-- trical furnace with its built-in crucible or hearth of chrome, magnesia, magnesite or preferably zirconium. The high frequency electric furnace sometimes referred to as a coreless induction furnace may be so operated by appropriate control of the current input as to cause the molten mass to be readily and quickly heated and maintained at a controlled superheating temperature.

While not essential to the present invention, all of the metals to be alloyed may be in a molten state before they are added to each other. It may, therefore, be convenient to melt in an electric furnace the appropriate quantities of manganese and chromium or ferro-chrome and pour into the molten mass, the molten low carbon steel drawn directly from the furnace. It is desirable that the iron or steel employed be produced in a furnace manufacturing low carbon steel carefully controlled as to silicon, sulphur carbon and phosphorous, composition preferably being maintained at approximately a trace of silica, sulphur .03, carbon .08, phosphorous .03 and manganese .35. Appropriate proportions of chromium, nickel, manganese and the likemay be added. As the metals are being alloyed the molten bath may be more or less thoroughly and completely deoxidized and scavenged by injecting under the surface of the metal a few ounces of barium peroxide. While this is not essential, possibly an improved product may be produced by such use of barium peroxide.

Zirconium oxide preferably in the native form of baddeleyite or other ores having a content of added to the bath in quantities approximating 5% of the batch.

In the electric furnace the mixture of metals will remain molten at about 2850" F. Preferably a be rendered highly fluid. The mass may be given 'wise this apparent absence of precipitated carsuperheat of 200 F. or 300 F. may be given by increasing the current input to the furnace to cause the mass to reach a superheat temperature of approximately 3200 F., when the metals will a five-minute boil at this temperature followed by a momentary cooling temperature to approximately 2950 F. when it may be poured into ingot molds.

This application is a continuation in part of our applications Ser. No. 485,294, filed September 29, 1930, and Ser. No. 558,634 filed August 21, 1931.

The terms, stainless steel, non-corrosive 'steel, stainless iron and non-corrosive iron, are used technically and in the trade without very definite lines of demarcation. The present invention relates to materials which may be designated by any of these terms and the use in this application of the term, stainless steel, is intended to be an equivalent of any of the terms indicated above as used in the art. Because these alloys contain chromium they may be referred to as chrome stainless steel.

We claim as our invention:

1. The process of producing stainless steel comprising mixing low carbon steel with chromium and manganese in a high frequency electric furnace and superheating the mass to approximately 3200 F. before pouring the ingot.

2. The process of producing stainless steel comprising mixing low carbon steel with chromium and manganese in an induction furnace, addin baddeleyite thereto, and superheating the-mass before pouring the ingot.

3. The process of producing stainless steel comprising mixing molten low carbon steel with chromium and manganese in a high frequency electric furnace, and adding zirconium oxide to the batch.

4. The process of producing stainless steel comprising mixing molten low carbon steel with chromium and manganese in a high frequency electric furnace and adding baddeleyite to the batch.

5. The process of producing stainless steel comprising mixing low carbon steel with chromium and manganese in a high frequency electric furnace adding zirconium oxide and superheating the patch before pouring the ingot.

6. The process of producing stainless steel comprising mixing low carbon steel with chromium and manganese in a high frequency electric furnace, adding zirconium oxide, superheating the batch and reducing the temperature before pouring the ingot.

'7. The process of producing stainless steel comprising mixing low carbon steel with chromium and manganese in a high frequency electric furnace and superheating the mass to approximately 3200 F. and then cooling to approximately 2950 F. before-pouring the ingot.

8. The method of producing stainless steel containing substantially 18% chromium and 8% nickel which substantially retains its ductility when deformed at temperatures between 1000 F. and 1500 F. comprising mixing zirconium oxide into the molten batch.

9. The method of producing chrome stainless steel containing finely divided particles of zirconium oxide more or less generally distributed 145 within its grains comprising mixing zirconium oxide into the molten batch.

10. The method'of producing chrome stainless steel containing finely divided particles of zirconium nitride more or less generally distributed 150 steel containing less than .30% silicon comprising mixing zirconium oxide into the molten batch.

13 The method of producing chrome stainless steel which on working/retains substantially all of its carbon in solution comprising mixing zirconium oxide into the molten batch.

- 14. The method of producing chrome stainless steel containing less than ,10% carbon which on working retains substantially all of its carbon in solution comprising mixing zirconium oxide into the molten batch.

15.1 The step in the" manufacture of chrome stainless steel comprising intimately mixing with the hot batch of steel finely comminuted zirconium oxide mixed withlime.

16. The step in the manufacture of chrome stainless steel containing less than 10% carbon comprising intimately mixing with the hot batch of steel highly comminuted zirconium oxide.

17. The step in the manufacture of chrome stainless steel comprising sprinklingon the top steel which substantially retains its .ductility when deformed at temperatures between 1000" F. and 1500 F..comprising zirconium into the molten batch.

26. The method of producing chrome stainless steel which is readily machinable and workable; has the characteristics of steels generally referred to'as stainless steels; which substan- I tially retains its ductility 'when deformed at temperatures between 1000 F. and 1500 F.; is substantially free from inter-crystalline corrosion; and is substantially free from'precipitated carbides, comprising mixing zirconium oxide into. the molten batch. I

27..I'he process of producing chrome stainless steel comprising producing a molten bath of the metals tofbe alloyed in a high frequency electric.

furnace and superheating the mass to approximately 3200 F. and then cooling to approximately 2950" F. before pouring the ingot.

o 28. Austenitic stainless steel containing chro- 'miumabout 4% to 25%, about 6% to'.36% from the group of nickel and manganese, carbon not more than 0.1% and a small amount of zirconium oxide which retains substantially all of its carbon in solution when Worked belowabout 1500" F. 29. Austeniticstainless steel containing chromium about 4% to 25%", about 6% to 36% from at temperatures and intimately mixing with the hot batch of steel the group of nick l and man an se, a n n t 195 highly comminuted zirconium oxide. I

18.-The stepin the manufacture of chrome stainless steel comprising sprinkling onthe top Q and intimately mixing with the hot batch of steel highly comminuted baddeleyite.

' 19. The step in the manufacture of chrome stainless steelcomprising intimately mixing with the hot batch of steel highly comminuted baddel'eyite.

20. The method of steel substantially free from inter-crystalline corrosion comprising mixing zirconium oxide into the molten batch. n v e I 21. The, method of roducing chrome stainless steel having its alpl a iron substantially free from precipitated carbides comprising mixing zirconium oxide into the molten metal before casting. I

22. The, method of improving the workability and machinability of chrome stainless steel comand intimately of steel finely comminuted baddeleyite.- I j I 23; The method of producing chrome stainless steel-comprising producing a molten bath of the metals-to be alloyed in a coreless induction fur-v nace,'removing the slag. prinkling oirthe top or the batch and intimately mixing therewith-flnely comminuted zirconium ,oxid' mixed 1 with lime.

injecting ferro-titanium'into the bath. removing the slag, and: shocking the metal :by suddenly turning the current into the furnace. J

. 24. The method of improving the quality'of chrome stainless steel comprising mixing zir-' qonium oxide "into the. bath to eliminate theoxygen and nitrogen held in solution.-

' 25. The'method .of producing chrome stainless more than 0.1 and a small amount of zirconium oxide which shows substantially no loss of duc- 30. Stainless steel containing chromium about 4% to 27 a small amount of zirconium oxide and not more than .30% of silicon.

3L Stainless steel containing chromium about I 4%jto 27%, and a small amount of zirconium making chrome stainless oxide, which isfree from precipitated carbides sufilcient to interfere with machinability and workability.

32. Stainless steel cbntaining chromium a out a small-amount of zirconium 4% to 27% and oxide which is readily inachinable and work- -able','has the characteristics of steels generally referred to as stainless steels, substantially retains its ductility when tures between about 1000 is substantially free from intercrystalline corrosion.

particles of zirconium oxide more or less gen erally distributed within itsgrains.

34. Stainless steel containing chromium about 4% to 27% and asmall amount of finely divided particles of zirconium nitride more orless gen erally distributed within .its grains.

. 35. Stainless steel containing chromium about 4% to'27% and a small amount of finely divided nitride its grains. I

- FRED J. CROLIUS.

RUDOLPH w. BTULER.

deformed at tempera-1 F. and 1500" F. and 1'25 particles of zirconium oxide and zirconiummore or less generally distributed within I 33. Stainless steel containing chromium about 4% to 27% and a small amount of finely divided