US2368955A - Treatment of manganese steel - Google Patents

Description

Patented Feb. 6, 1945 TREATMENT OF MANGANESE STEEL .Carl W. Weesner, Warren, Ohio, and Wallace' Bruce Leffingwell, Sharon, Pa., assignors to Sharon Steel Corporation, Sharon, Pa., a corporation of Pennsylvania No Drawing.- Application May 18, 1942,

Serial No. 443,452

8 Claims. 7

The invention relates to a treatment of manganese steel, and more particularly to the treatment of high manganese, high carbon steel to eliminate surface brittleness, toincrease its ductility, and to improve its appearanceand surface characteristics.

Difiiculties are encountered in the manufacture of cold rolled sheets and strips of the thinner gauges from high manganese or "Hadfield steel, that is, steel containing from 10% to 20% manganese and from 1.00% to 1.50% carbon; andit is of great importance that these difficulties be avoided or eliminated, because large quantities of such high manganese strips and sheets are de-.

sired for the manufacture of helmets for the armed forces.

Helmet steel must be soft, ductile and have deep drawing qualities, in order that helmets may be properly formed or drawn to the desired shape from high manganese sheets or strips. The sheets or strips may be as thin as .044 to .033 inch; and cold rolling operations must be performed to roll steel sheets and strips of such gauges. High manganese steel work hardens very quickly and can ordinarily only be cold rolled to a maximum of about 25% reduction. A heat treatment is then required after cold rolling to soften the same, and to enable further cold rolling. H

The heat treatment of high manganese, high carbon steel' to soften the same, or to restore ductility, or to permit further cold rolling after one cold rolling operation, consists in heating the steel to a-temperature-above the upper critical point, approximately 1800 F. to 1900" F and preferably to about 1850 F. in order to dissolve brittle carbides and produce austenite; and in then quickly cooling the same, preferably by quenching in water.

This is because ductility in high manganese, high carbon steel depends upon the formation of gamma iron or austenite; and also upon the retention of gamma iron or austenite at room temperatures, obtained by a quick water quench. Also, the rapid cooling, as by the water quench, retains the carbon in solid solution in the austenite at room temperatures.

Although heat treatment is necessary to soften the steel and to restoreductility following a cold rolling operation, unfortunately steels of the Hadfield types are easily oxidized and decarburized at the surfaces when heated to the required temperature. Many bad surface conditions result from the oxidation or scaling; and the decarburization of the surface'skin of the both, in varying proportions.

metal sheets or strips, apparently causes surface embrittlement.

The decarburization or lowering of the carbon content of the surface skin of the-metal is substantial and the carbon content at thisregion ,may become as low as .50% carbon, or less.

When Hadfield steel is thus decarburized it is unable to completely retain austenite at room temperatures, and on cooling all or part of the austenite apparently decomposes and forms epsilon iron or epsilon iron and alpha iron, or These decarbwrized surface layers containing epsilon iron and alpha iron are brittle and are formed with a myriad of small cracks; and the material though generally soft, has a ductility lower than desired.

When the metal is subjected to deep drawing operations, the small cracks develop and increase in size in the brittle surface layer, giving a characteristic' granulated or frosty appearance. Thus the heat treatment which is intended to soften the steel and-render it more ductile, on

-, the contrary causes surface embrittlement and s cordance with strip practice.

lowered ductility.

These undesirable surface characteristics are present in high manganese sheet steel made in and stripsheets after hot rolling ordinarily require a plurality of cold rolling operations, each followed by a heat treating operation, to reduce the strip or stripsheet thickness down to say .044 to .033 inch.

The formation of the brittle surface layers which are believed to be due to the presence, particularly, of epsilon iron therein,- is cumulativewith repeated heat treatments following cold rolling, operations, so that the undesirable characteristics increase in magnitude, the thinner is the ultimate product, in making high manganese strip or stripsheet steel in accordance .with strip practice. I 1

These difficulties, as stated, result because of the excessive oxidation and decarburization .of

- the surface of the material during the heat treating operations.

These results are contrary to the normal proper- The ultimate result is that the material has low ductility and an unsatisfactory surface and poor appearance because of the for.

ties of austenitic manganese steel, which are those of austenite; namely low elastic limit, but great hardness and toughness and wearing power combined with much ductility.

Accordingly, the production, of high manganese, high carbon steel in accordance with strip I practice has been quite limited.

An object of the present invention is therefore to eliminate or remove the brittle surface layers from cold rolled, heat treated, high manganese steel so'as to remove the epsilon or epsilon and alpha iron contained therein, and leave manganese steel which is substantially entirely austenitic in character, so that the normal properties of austenitic manganese steel may be obtained.

A further object of the present invention is to 'provide a commercially usable and satisfactory method of cold rolling high manganese steel in accordance with strip practice to gauges as thin as .044 to .033 inch in thickness.

It would be expected that the undesirable decarburized surface layers could be removed by. continuing the pickling operation used for removing the scale after heat treatment, until asumcient amount of the surface metal is eaten away or dissolved. However, the ordinary pickling acids, such as sulphuric or hydrochloric acids or. combinationsof the same, cannot be used on high manganese steel for metal removal without leaving an etched or pitted or sharp crag-like surface, in the same way that these acids an also nitric acid attack almost all other steels if used for metal removal.

However, we have discovered that cold rolled heat treated high manganese steel strips, sheets and the like may be treated in a dilute solution of nitric acid, approximately up to 5% nitric acid by weight in water, at a temperature of 140 to 200 F. for from one to approximately six min-' utes depending upon the amount of surface metal to be removed and upon the concentration and temperature conditions, for removing the scale formed during the heat treating operation, and

for removing the undesirable decarburized metal surface layers which are extremely detrimental to strength and duciility.

We have further discovered that certain other unexpected results occur when high manganese high carbon steel strips and sheets are so treated to remove scale and the brittle decarburized surface layers. Thus, we have found that seams, scratches, scale patterns, and rough or undesirable surface defects or imperfections which may be present on the surface of the. metal due to previous rolling or due to the oxidation and scaling of the metal during heat treatment, are partially or totally removed, and the surface is greatly improved with a bright and lustrous appearance. Y

Thus, the resulting surface of the metal'is bright in color, is .free from sharp projections or indentations which might tend to injure ductility,

and it has a greater usefulness, aesthetic appearance and corrosion resistance by virtue of the removal of undesirable surface conditions detrimental to strength and ductility. V

. Moreover. the resulting surface, when viewed with a ma nifying glass is minutely ebbled. which is a desirable characteristic. as a "pebbled surface retains die lubricantswhich maybe-used during forming or drawing operations and provides an excellent surface for the retention or adherence of paint or other coatings which may be applied to the finished product.

It is therefore a further object of the present invention to provide a method of pickling high manganese steel with an acid for the removal of scale and brittle surface metal without etching or pitting the metal.

It is also an object of the present invention to provide a new treatment of. high manganese steel utilizing a weak and relatively inexpensive acid for scale and brittle surface metal removal.

Likewise, it is an object of the present invention to provide a method of acid pickling high manganese steel, which not only removes scale and brittle surface metal, but which also partially or totally removes seams, scratches, scale patterns and surface defects or imperfections, leaving a bright or lustrous, smooth, minutely pebbled surface.

Furthermore, it is an object of the present invention to provide a new treatment of high manganese steel by which the surface appearance, toughness, strength, ductility and drawing qualities of the steel are improved.

Also it is an object of the present invention to provide anew method of making cold rolled, light gauge, high manganese strip or stripsheet steel more quickly and more cheaply than it has been produced previously, by enabling the satisfactory commercial production thereof in accordance with strip practice.

And finally, it is an object of the present invention to provide a cold rolled, high manganese,

strip and sheet 'steel product having a bright, lustrous, relatively smooth, pebbled surface, free of brittleness and having high ductility."

These and other objects may be obtained, the

stated results achieved, and the described diiiiculties overcome by the methods, steps, products, treatments, and discoveries whichcomprise the present invention, the nature of whichis set forth in the following general statement, and a preferred. embodiment of which is set forth in the following description, and which are particularly and distinctlypointed out and set .forth in the appended claims forming part hereof.

The nature of the present improvements may be stated in general terms as preferably including, in the treatment of high manganese, high carbon steel, which has been heat treated to soften the same and to restore ductility, the step of sub- .iecting the same to the action of a bath of a dilute aqueous solution of nitric acid, say up to 5% nitric acid by weight in water, at a temperature of about 140" to 200 F. for a suflicient' time to remove scale on the surfaces thereof and to remove brittle surface metal layers thereon resulting from heat treatment, so as to provide a bright, ductile, product free of brittle surface layers, scratches, scale patterns, and roughness.

The manufacture of high manganese steel strips will be described. in detail as an embodi- -ment'of the way in which the method of the into say .079 inch in thickness.

maximum allowable reduction, which is approxi- I mately at 25% reduction" for this type of steel,

The strip is then softened by heating it up to 1800 to 1900 F., preferably about 1850 F., and is then quenched in water. This treatment may be termed austenitizing and dissolves brittle carbides, retains carbon in solid solution, and renders the steel as fully austenitic as is possible.

The steel may then be pickled in any usual way for scale removal, or may besubjected to the special pickling treatment of the present invention to be later described in detail.

The steel may then be cold rolled to maximum allowable reduction, which is approximately 25% reduction, down to say .058 inch in thickness, after which it is again heat treated and quenched. in the manner just described, followed by a usual pickling operation to remove scale, or by the should be replenished with small additions of nitric acid.

When the concentration of dissolved metal in may be made to the bath if desired.

special pickling operation of the present invention.

Thesteel may then be again cold rolled to maximum allowable reduction, say to .044 inch in thickness, which is one gauge of high manganese steel desired for helmet stock, after which it is heat treated. as stated, and if no more cold rolling is to be done, the cold rolled heat treated strip steel must then, in accordance with the present invention, be subjected to the special pickling operation of the present invention.

The special pickling operation may be used for any one of the pickling operations following any cold rolling step. The steel may be further cold rolled to lighter gauges such as .033 inch thick stock. In any event, the last pickling operation following the last cold rolling step must be the special pickling operation-now to be described, in order to obtain the benefits of the discoveries and results of the present invention.

As previously stated, in addition to forming scale, the result of heat treatment utilized after each cold rolling operation. is to decarburize the surface layers of the steel, which apparently results in the formation of ensilon and alpha iron or both in the surface layers upon Quenching.

These undesirable surface layers are removed after heat treatment by the special pickling operation of the invention which may be carried out by passing the strip through and subjecting it to the action of a dilute solution of up to about which the metal has been subjected, and t e ma-- terial will have to remain in the nitric acid bath longer for removing the undesirable surface layers.

If the acid concentration is too weak, the attack is too slow and a longer time .of treatment in the bath will be required. If the acid concentration is too high, the attack is too vigorous and excessive fumes are formed. An increasein the temperature of the bath increases the rate of attack; and again, excessive fumes result when the temperature is too high.

The pickling and metal removal act on of a fresh solution increases when the amount of dissolved iron is slightly increased; butthereafte when the iron content of the bath is considerably increased. the action is sluggish and the bath The action of the bath can be speeded up, if

desired, by using the bath as an electrolyte, and

by using the metal as an anode and passing electric current through the solution to electrolytically treat the metal for removing scale and brittle surface metal layers. Likewise, the electrolytic treatment may be carried out by using the strip as a bi-polar electrode, in accordance with the method set forth in the L. H. Wilson Patent No. 2,197,653.

If the bath is operated as an electrolyte the temperature may be maintained at about 60 F. to 150 preferably. about 120 F. for the required length of time. With a comparatively small current density, a slightly used bath will dissolve the metal electrolytically at 120 F. at the be loosened by passing the strip through a dilute solution of sulphuric acid or hydrochloric acid in water, or through any other usual cleaning solution, prior to the scrubbing operation. After the scrubbing operation, the strip is finally washed in water and dried by hot air blasts or other means, and oiled if desired. A

- When high manganese, high carbon steeltrips are made as outlined above, without using the special pickling operation of the present invention, the finished material is generally soft, well annealed, and free from carbides: but the surfaces are poor, the ductility is lower than desired, and a granulated or "frosty condition occurs when the material is drawn.

Hardness tests made with various penetrations or depths to show the relative hardness at various depths, indicated that the brittle surface layer, is not necessarily due to a hard constituent such as martenite. v

Samples were reannealed to determine whether or not the ductility could be improved by changes in the heat treating practice; but these tests in dicated that changes in the heattreatmentdid I not improve ductility or liminatethe brittle or frosty surface conditions. These and other'tests did'showrlaow'ever, that excessive oxidation of the surface and' surface embrittlement go hand in hand; and analysis.

of the .rnetal at various depths indicated that the carbon content of the surface metal was matea Samples having decarburized surfaces were again carburized-to materially increase the carbon content of the surface layers and were then heat treated so that the carbon content ofthe surface layers was approximately that of the core, somewhere between 1% and 1.5% carbon. The result was exceptionally good ductility and a disappearance of the frosty surface conditions, indicating that the brittle surface is produced by depletion of the surface metal carbon. However, it is not feasible in the production of high manganese, high carbon steel strips to eliminate surface embrittlement by a carburizing step following the .final cold'rolling. and heat treatment.

Decarburization alone does not explain the exthe removal of the brittle surface layers due to the precipitation of alpha or epsilon iron or both,

istence of surface embrittlement found in heat treated Hadfieldf steel. The phenomenon is apparently due to some radical structural change, which occurs coincident with the lowering or depletion of the carbon content. As previously stated, when the carbon is materially lowered, austenite is not completely retained upon.cooling, but some of it decomposes into alpha iron or epsilon iron, or both, at approximately 400 F. These alpha and epsilon phases, especially the latter, must be expected to be brittle, and the brittle surface layer probably consists of three phasesalpha, gamma and epsilon irons; which is apparently the structural change occurring with a depletion of the carbon in the surface found that the surface metal and core metal responds differently to different etchants, and careful etching technique reveals that the decar burized surface layer contains considerable Wid- --manstatte structure.

As to the'heat treating cycle, the [best results will be obtained and the surface embrittlement maintained at a by avoiding oxidizing atmospheres, by annealing at the lowest temperature and fastest speed required to dissolve the carbides, and by quenching immediately in water. Oxidizing atmospheres and violently agitated atmosphere tend to increase scaling and surface embrittlement, and the best atmosphere is a quiet atmosphere of reducing gas.

Hadfield" steels must be heated to above the upper critical point, 1800" F. to, 1900 F. in order to dissolve carbides and produce austenite, but higher temperatures than absolutely necessary increase caling-and surface embrittlement. In general two minutes heat treatment at about 1850 F., or between 1850 F. and 1900 F., one minute, coming to temperature and a one min! ute soak, is suflicient. Increasing lifetime in the furnace vproducesheavier scale and increases surface embrittlement and frost.

in the decarburized surface metal, the brittle layers being extremely detrimental to strength and ductility; and because of the formation of a smooth lustrous surface of minutely pebbled (contour.

' The final product is fully austenitic in character with the related characteristics of austenitic manganese steel fully developed entirely throughout the metal of the strip and the improved surface is a superior base for die lubricants in forming operations, and for paints or other coatings applied to the finished product. This surface is much more smooth and ductile than the usual pickled surfaces which contain many sharp projections and irregularities detrimental to strength and ductility. Actual tests have shown that the treatment increases the ductility of the material as much as 10% or more.

We therefore term the special pickling treatment of the present invention a pickle polishing strip steel or strip sheets in accordance with strip practice, the present invention is also applicable tothe manufacture of other high manganese steel products such as sheetsteel in accordance with sheet practice, wherein the pickle polishing operation would be carried out following the final cold rolling and heat treatment operations,

of which there may be only one ortwo in sheet practice, to remove the brittle surface layers .and provide a finished product having the same characteristics as those discussed herein in connection with the manufacture of strips.

Quenching in water is better than air cooling.

The result of the special pickling treatment of I the present invention followingvheat treatment, is to increase the toughness, strength and duetility of the metal because of the removal of scale,

seams, scratches and other surface imperfections detrimental to strength andductility; because-of 75 Accordingly, when the term strips is used herein and in .the'appended claims, the term is intended to include the treatment of hot or cold rolled high manganese steel strips, strip sheets,

sheets, plates, bars, rounds, wire, forgings orcastings and the like, in order to remove scale, seams, scratches, brittle surface layers and other surface imperfections, to provide a surface much improved over that produced by usual pickling methods.

When the term "strips of the lighter gauges is used herein and in the appended claims, that term is intended to include strips, stripsheets, sheets and the like, asthin as .044 to .033 inch in thickness, or thinner, and which require forpro "Hadfield type, which generally contains from 10% to 20% manganese and from 1% to 1.5%

carbon, with possible additions of small percent- 1 ages of other alloying elements, such as nickelare'sometimes added to:

.andchromium, which Hadfield" steels.

Thus, the present inmprovements and discoveries provide for the ready and economical manufacture of high manganese steel strips of the lighter gauges having extremely high ductility characteristic of fully austenitic manganese steel, and having a bright color and sheen adapted for forming and bending operations because of the minutely "pebbled texture thereof, the pebbled projections being smooth and rounded so as to appear bright to the naked eye.

Having now described the .features of the invention, and an embodiment of steps by which the invention may be carried out, and the advantages and results attained by the invention, and the new discoveries modem connection with the treatment of high manganese steel; the new and useful methods, steps, treatments, arrangements and products, are set forth in the appended claims.

I claim;

1. In the manufacture of thin gauge, cold rolled, heat treated, high manganese, high carbon,

strip steel and the like; the step of subjecting such steel strip after cold rolling and austenitizing heat treatment to the action of a dilute aqueous solution of up to approximately 5% by weight nitric acid to remove scale and surface layers of brittle metal formed by heat treatment, and to render the steel'fully austenitic throughout.

- carbon, strip steel by heating the same to between 2. In the manufacture of heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating high manganese, high carbon, strip steel by heating the same to between 1800 F. and 1900 F., and then quenching in water; and then pickle polishing the steel in a dilute aqueous solution of up to approximately 5% by weight nitric acid to remove scale and surface metaland to form smooth bright surfaces thereon.

3. The method of treating thin gauge, cold rolled, austenitized, high manganese, high carbon, strip steel and the like, to remove scale, I

brittle surface layers, scratches, scale patterns and surface defects, and to increase the ductility thereof; which includes pickling such steel in a dilute aqueous solution of up to approximately 5% by weight nitric acid to remove scale, and continuing the pickling operation to uniformly dissolve the thin layers of brittle surface metal thereon, and to form smooth, bright, lustrous, minutely pebbled surfaces thereon.

4. In the manufacture of thin gauge, cold rolled, heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating cold rolled, high manganese, high earbon, strip steel by heating the same for up to about 2 minutes to between 1800 F. and 1900 F., and then quenching inwater; and then removing surface metal layers of the strip by pickling in a dilute aqueous solution of up to approximately 5% by weight nitric acid to eliminate decarburized surface metal containing alpha and epsilon iron formed by the heat treatment.

5. In the manufacture of heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating high manganese, high carbon, strip steel by heating the same to between 1800 F. and 1900 F., and then quenching in water; and then pickle polishing the steel in a solution of 1% to 5% by weight of nitric acid in water at a temperature of from F. to 200 F. for 1 to 6 minutes to remove'scale and surface metal.

6. In the manufacture of heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating high manganese; high 1800 F. and 1900 E, and then quenching in water; and then pickle polishing the steel in 'a solution of approximately 5% nitric acid by weight in water at a temperature of about 180 F. for from 1 to 6 minutes to remove scale and surface metal and to form smooth bright surfaces thereon.

'7. In the manufacture of heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating high manganese, high carbon, strip steel by heating the same for upwards of 2 minutes at a temperature of from 1850 F. to 1900" F., and then quenching in water; and then pickle polishing the steel in a dilute aqueous solution of up to 5% nitric acid by weight in water toremove scale and surface metal and to form smooth, bright surfaces thereon.

8. In the manufacture of heat treated, high manganese, high carbon, strip steel and the like; the steps of heat treating high manganese, high carbon, stripsteel by heating the same for about 2 minutes at a temperature of about 1850 F.,

" and then quenching in water; and then pickle polishin thev steel in a-solution of 1% to 5% nitric acid by weight in water at a temperature of aboutv 180 F. for from 1 to 6 minutes to remove scale and surface metal and to form smooth bright surfaces thereon.

CARL W. WEESNER.

W. BRUCE LEFF'INGWELL.