US2069758A - Ferrous metal and process for producing same - Google Patents

Description

Feb. 9, 1937. A. HAYES ET AL FERROUS METAL AND PROCESS FOR PRODUCING SAME Filed March 14, 1932 2 Sheets-Sheet. 1

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INVENTORJ A TTORNEYf.

Feb. 9, 1937. A. HAYES El AL 2,069,753

FERROUS METAL AND PROCESS FOR PRODUCING SAME Filed March 14, 1932 2 Sheets-Sheet 2 2;; COLD'QOLLED 196 150 276 com 120M130 beam. l

A TTORNEYJ Fatented Feb. 9, 193'? TABS FATE

Anson Hayes and Ralph 0. Grifiis, Middletown, Ohio, assignors to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio Application March 14, 1932, Serial No. 598,690

13 Claims.

Our invention relates to a novel ferrous prod not which is characterized by practical freedom from aging, blue-brittleness, and by permanence of condition when treated to avoid stretcherstraining. Furthermore, our metal is not materially decreased in ductility by percentages of .cold reduction which will materially decrease the ductility of ordinary mild steel and commercial iron.

Metallurgists and those concerned with the shaping or forming of iron and steel, particularly in sheet form, are familiar with the detrimental effects of aging, blue-brittleness and stretcherstraining, although it is only in recent years that more thorough attention has been given to this subject and the problems involved.

One common characteristic in steel and iron such as is used for deep drawing purposes is that at a certain degree of strain the metal upon drawing does not deform uniformly throughout, which results in elongated irregular depressions where the deformation has been localized, a defect commonly called stretcherstraining. Ordinarily dead soft steel or iron is given a cold rolling or roller leveling treatment previous to being formed or drawn, in order to impart a strain to the metal which prevents stretcher-straining.

The difficulty with this practice is that if the usual steel or iron is given this strain or "temper", as it is called, to-prepare it for drawing, it must be drawn quite promptly or the freedom from stretcher-strain imparted by the cold rolling is lost, because the definite yield point reappears in the metal after it has aged. After a cold rolling of even such a slight degree as to avoid stretcher-strain, such metal progressively increases in hardness and decreases in ductility at atmospheric temperatures, as well as redeveloping a definite yield point as above noted.

Steels and irons lose their desirable working qualities to a marked degree by the process of aging, so much so that the term aging in the industry means the increase of yield point and tensile strength, development of hardness and loss of ductility due to lapse of time.

Aging in mild steel or iron, no matter if annealed or not annealed, develops after a preliminary strain is imparted to the metal.

Aging seems to be present in metals which have also the characteristic known as bluebrittleness. This characteristic is that of increasing markedly in ultimate strength and hardness and decreasing markedly in ductility through a range of temperature characteristic of each metal, and usually above 300 F. and below 900 F.

Chemically, we have directed our attention to providing a steel or iron which is alloyed with a small amount of metal of high avidity for gaseous elements such as oxygen. so that a sufficient residuum of an unoxidized degasifier substance will be present in the final analysis of the iron or steel. In ,this connection it will be evident that with degasifiers which are most avid for oxygen the minimum requirement is less. With the usual deoxidizers aluminum has the most stable oxide, titanium next, silicon next and manganese last and the f minimums for the several deoxidizers in metallic form in the ingot will run substantially "as follows: aluminum .025%, titanium 05%, silicon 25%; fmanganese 4.%. The amount of residual degasifler will be enough, at least, to assure complete deoxidation of the metal. Apparently it results from this composition' that after working and upon heating and slow cooling of the resulting steel or iron body, that the oxygenand any other deleterious elements remaining either in combination with the degaslfier, or with the ferrite, will not establish theunstable relationship with the ferrite, which we believe is a, cause of aging.

We believe that we are the first to produce commercially an iron or steel sheet which does not develop these characteristics. v In considering the production of a. non-aging sheet 'of iron or steel the heating .and cooling down and the effect of the working incident to ordinary rolling imparted to the steel or iron to bring it to commercial shape mustbe taken into consideration. We have found that the proper chemical analysis is not the only criterion in sheets which have been produced in 'a commercial manner, but that we are required in addition to proper analysis, to give a subsequent heat treatment to the sheet product to set up the desired condition after the rolling. Subsequent cold work thereafter will not cause the sheet to age.

The ingot chemical analysis of an example of commercial iron productsv that we have produced in accordance with our invention is as follows:-

C. .0, Mn .03 P .005, S .019, Cu .073, nitric acid soluble Ti .067, Tia .019, A1203. .037, metallic A1 .042. For present purposes the titanium content reported in the above analysis has been termed "nitric acid soluble titanium, as determined by the analytical methods propounded in the usual reference books on the'subject, such as The Chemical Analysis of Steel Works Material" by Fred Ibbotson, Longmans Green 8: Company, London, 1920, page 160.

The excess degasifier, titanium, in the analysis above given, is an important feature of our inr vention. We find that under. around .08 Ti in iron will not make it so hard and brittle that it is unsatisfactory for working, In commercial irons, when working with titanium, we find that at least around 02% should be present in combination with iron (not oxygen unless aluminum is entirely absent). In pure irons the residual titanium should probably be more than in manganese bearing steel or iron. Whether the excess degasifier or deoxidizer be titanium or some other degasifying metal, we find that some excess is necessary in order to permit of establishing the desired Stable condition by a proper heat treatment.

In producing this metal the usual open-hearth practice was followed. For this one particular metal we give the following production data.

To around 112,000 pounds of hot blast furnace iron in a basic open-hearth furnace was added around 178,000 pounds of scrap. Limestone (11%) up to around 31,000 pounds was added, and the heat refined in the usual way for making commercially pure iron until the manganese test of 027% was reached. A thousand pounds of silica sand was added a little before the tap. 150 pounds of bar aluminum in sacks was added to the ladle; and 410pounds of shot aluminum, as well as 1220 pounds of ferro-titanium (25% Ti and 7% Al), were fed into the runner between the furnace and the ladle during tapping and 280 pounds added to the ladle during tapping. 500 pounds of a mixture of'burned lime and fiuorspar was added to the ladle.

The ingots were soaked in the usual way and rolled into billets, slabs, bars, rough plate of about .108" thickness and finally into sheets of about .05" thickness by rolling in the usual manner.

The sheets were then given the following heat treatment.

The finished rolled sheets were normalized and then given a final heat treatment consisting in a box anneal, comprising heating to a temperature of 1180 F and holding at this temperature for approximately three hours, when the box was removed from the furnace and allowed to cool in air, which required about three days, at the end of which the box was opened, which corresponds to a cooling rate averaging approximately 10-15 degrees F; per hour. .It should be understood that this is an example without limitation. The cooling rate may be as fast as practicable with contingencies incident to the heat treatment of a large mass of metal as in the present case. so far as the non-aging qualities are concerned, the normalizing above referred to does not seem to be required.

The temperature to which the metal is raised in the final heat treatment, the time of holding at that temperature and the rate of cooling are so coordinated that the dissolved constituents such as oxides precipitated at the higher temperature, plus the constituents which the iron precipitates out as the temperature falls, results at room temperature in no more than a saturated ferrite solution condition, or approximately so. This term no more than a saturated ferrite solution condition is to be considered as a guide in the heat'treatment.. In other words, the heat treatment should be one which on theoretical While a ation. The necessities are to produce the no more than saturated solution above noted.

The temperature and time of holding, and

-material which responds unsatisiactorily to accelerated aging tests, to one which responds satisfactorily. By stabilizing heat treatment", we wish to include any such heat treatment which imparts the desired characteristics to the material that we describe herein. We should state further that the metallurgist should keep in mind the known fact that a holding of steel or iron sheets for a considerable length of time at the higher temperatures introduces the factors of agglomeration of slip interfering particles and grain growth which would bring about a condition that would interfere with our results.

It should be understood that the above is a single example and that we produce many other formulae classifiable as commercial irons and mild steels, all of which have non-aging characteristics.

When heated higher than to around 1500 F. the non-aging qualities of the particular iron material above described did not develop if cooled either as slowly as we were prepared to'do this,

or rapidly. If heated to around 1200 F. and cooled rapidly, the non-aging qualities did not appear. If no residual deoxidizer in unoxidized form is present, the proper heat treatment did not result in non-aging qualities. The practical temperature ranges will vary somewhat for different compositions, but with the data given, the metallurgistcan readily determine the ranges for whatever steel is made.

We know of no mild steel or commercially pure iron, commercial sheet, which can compare in freedom from blue-brittleness to steel and iron sheets made by us according to the above practice.

As a convenient method of indicating the difference between metals which age and do not age, I

and which are or are not blue-brittle, we find the graphic stress-strain curve of the metal beingtested is preferred.

A jagged or uneven course of the stress-strain curve above the yield point when in the bluebrlttle range is a sensitive indication of bluebrittleness, and a smooth or even course of the stress-strain curve above the yield point in the blue-brittle range is an indication of freedom from blue-brittleness. Furthermore, blue-brittle ness is characterized by marked diminution in the elongation and a marked increase in tensile strength determined in the blue-brittle range of temperatures, and freedom from such marked indications show essential freedom from bluebrittleness, which features are also shown on the tress-strain curve, which we believe is true for aging also. Our observations of jagged stressstrain curves of material tested in'the blue-brittle range indicate that that material will age.

In considering the stress-strain curves it should be kept in mind that any iron or steel may show a break in the stress-strain curve at the yield point; and that any iron or steel may show a lack of break in the stress-strain curve at the yield point if the iron or steel previously had been over-strained and then tested Without appreciable lapse of time. An iron or steel subject to aging will show a break in the stress-strain curve at a yield point, however, if over-strained and allowed to rest for a period before testing; but an iron or steel not subject to aging will not again show a break in the stress-strain curve at a definite yield point if over-strained and allowed to rest for a period before testing. This is in accord with the fact above noted, that an iron or steel free from aging will not subsequently show stretcher-straining if first over-strained by an amount in the ranges of approximately 2 2%.

The stress-strain curves thus exemplify what is meant by aging, blue-brittleness and stretcherstraining, out we do not exclude other tests for the description of other manifestations of the aging, blue-embrittling, and stretcher-straining processes.

In the drawings Fig. 1 is a series of stress-strain curves of sheet test pieces. standard shape, .5 inch wide and 2" gauge length, taken at 400 F. illustrating the smoothness of curve of our metal as compared to aging steels.

Fig. 2 is a tensile strength temperature diagram illustrating the freedom from increase of tensile strength with temperature of our metal, as compared to aging steel with similar test pieces.

Fig. 3 is a series of stress-strain curves of similar test pieces taken at room temperature illustrating the non-stretcher-straining feature of our invention.

Fig. 1, curve I, shows the stress-strain diagram for one sample taken from the final product of the above specific example. This curve illustrates the smoothness or lack of irregularity of the curve above the yield point; this material had been correctly treated with degasifier and correctly heat treated, as'specified above.

Fig. 1, curve 2, shows the behavior of some of the same material which had not been properly heat treated. The blue-brittleness features of this material are evident from the irregularity of the curve as well as the decreased elongation with increased tensile strength above the yield point: this material could have been made free from blue-brittleness by proper heat treatment.

Fig. 1, curve 3, illustrates the behavior of material which had not been properly degasified and therefore was not as susceptible to heat treatment for the prevention of blue-brittleness and aging. The irregularity above the yield point again indicates a sensitivity to blue-brittleness.

The blue-brittleness behavior may also be illustrated, as in Fig. 2, which shows in curve 4 the tensile strength of a material susceptible to bluebrittleness and aging, and in curve 5 one which is not. Curve 4 indicates that the tensile strength of the aging material is much higher in the blue-brittle range than at room temperature,'showing a pronounced maximum in this range, whereas in curve 5, which was taken from our new material, there is a decided decrease of the tensile strength in the blue-brittle range. While at 500 F. there appears an insignificant hump in curve 5, the tensile strength remains always below that at room temperature. Thus curve 5 illustrates the superior behavior of our new material.

As the presence of a definite yield point is an indication that the metal will stretcher-strain, we have given in Fig. 3 a series of stress-strain curves, taken at room temperature, to wit, curves 6, 1, and 8, to illustrate'the behavior of our novel material with regard to its practically permanent freedom from stretcher-straining, and similar curves, to wit, curves 9, l0, and l l, contrasting our material with such material that does not have this property, the latter material being a typical commercial mild steel.

Curve 6 is a stress-strain curve of material having a composition and heat treatment according to our invention.

Curve 1 represents this material after it has received a 2% cold reduction.

Curve 8 represents the same material as curve I, however, after artificial aging for 24 hours at 212 F.

It should be noted that no definite yield point has reappeared, which we take as an indication of the practically permanent freedom of our material from stretcher-straining, if stored at room temperature.

Curve 9 is a stress-strain curve of material which does not have a composition according to our invention and did not respond to our heat treatment.

Curve l0 represents this material after it has received a 2% cold reduction.

Curve l I represents the same material as curve Ill, however, after artificial aging for 24 hours at 212 F. Since a definite yield point has reappeared, this is an indication that the cold reduction did not result in a practically permanent elimination of stretcher-straining in this metal. Hence, in spite of the treatment for nonstretcher-straining, such metal will again stretcher-strain after a comparatively short lapse of time.

This curve 8 illustrates the superior behavior of our metal with regard to practically permanent freedom from stretcher-straining after it is once sufficiently cold worked.

The favorable and unusual behavior of our material with respect to aging is shown in the. accompanying table of tests applied to the same material as above described. This table shows the absence of change in our material in the Rockwell hardness, the yield point, the tensile strength, and the elongation upon aging after cold working to an extent of 2% reduction in thickness. The differences which are to be found in the numerical representations in no case would be classified as a commercial difference, and indeed in no case is there as much difference as could not be ascribed to permissible allowances in accuracy of such tests.

. Physical Test Aging treatment mummy Rockwell Fresh 48.87 hardness Aged 6 days at 2l2 F i 41). B scale. Aged days at room temperature 47. 50 Yield point Fresh 27, 1100 pounds AgedGdaysatZlZ" F 2x737 per sq. in. Aged 30 days at room temperature. 2%, 887 T e n s i l e Fresh 43. ,J/ g f l Aged 6 days at 2l2 F 44. 250 p u Aged 30 days at room temperature 44. 02? per sq. ln. Elongation Fresh 40.12 percent in Aged 6 days M212 F. 39.1141 2 inches. {Aged 30 days at room temperature 41.02

We have found no material decrease in duetility as measured by elongation of this material with cold rolling up to around two to twoandone-half percent reduction in thickness. Inmild Ill) iii

steel a two percent cold rolling reduction will normally decrease the elongation by around ten percent of the initial elongation. This is very valuable property since it increases greatly the degree of temper cold rolling which can be given to avoid -stretcher-straining in preparing the materal for deep drawing.

With reference to the materials whose stressstrain curves are given in Fig. 3, the result of 2% cold reduction of the metal in curve 6, was to inthe ductility from an original figure of 39.87% elongation to 40.62% elongation, whereas the metal of curve 9 was decreased in ductility by the 2% cold reduction from 41.20% elongation to 37.85% elongation.

Data as to a typical mild steel product according to our invention are given as follows:

Composition of sheet Percent Carbon 0.042

Manganese 0.47 Phosphorus 0.011 Sulphur 0.014 Silicon 0.073

Titanium 0.058 Aluminum 0.037 A1203 0.019 TiOi ..a. 0.012

This material was processed in the usual manner involving normalizing afterrolling, and then rendered non-blue-brittle and non-aging by heating to 1100 F. and holding for 1 hour and cooling to room temperature 'in four days. Freedom from stretcher-straining was also produced in this metal by giving it subsequently a cold rolling pass of 0.3% reduction.

It should be noted in closing that the degasifiers so far investigated by us result in fixation of nitrogen as well as oxygen, so that the absence of nitrogen from unstable combination with iron may have some bearing on the results that we have obtained. It is for this reason that we refer to degasifying instead of merely deoxidizing as a feature of our invention.

We include zirconium'when. we speak of titanium in our specification and claims, because the two elements are generally active in a similar manner.

By using in the appended claims the term a degasiiier or a deoxidizer we do not wish to limit ourselves to one degasifying or deoxidizlng element. as a combination of two or more elements may be used, e. g., aluminum and titanium, or manganese, aluminum and titanium, 'or combinations of other elements.

From a general point of view. it would be in accordance with our invention to use a degaslfier which is so effective in stabilizing the gaseous constituents of the metal as to make it unnecessary to add the stabilizing thermal treatment. None of the deoxidizers that we have used commercially or experimented with has exhibited such a property.

Having thus described our invention, what we.

claim as new and desire to secure by Letters Patcnt is:

l. A non-aging wrought ferrous body containing Percent Mn up to about v .04 Ti .O-.10

balance principally commercial iron, said body being characterized by a stable condition of its Percent balance principally mild steel, said body being characterized by a stable condition of its ferritesoluble constituents whereby aging produces substantially no change of the physical properties.

3. A process of producing non-aging ferrous metal wrought bodies comprising the following stepsz-Making a heat of commercial iron or mild steel, treating said heat with sufilcient titanium so as to leave from .02-10% of residual titanium in metallic form in the metal, working said metal into the desired form and subjecting the resulting bodies to a heat treatment consisting of heating them to a. temperature below but in the neighborhood of the Al point and slow cooling, whereby said titanium bearing ferrous bodies are brought to a stable condition 01 their ferrite-soluble constituents so that aging produces substantially no change of the physical properties.

4. A deep drawing sheet of commercial iron containing Percent Mn .01-.04 Ti .05-.10

balance principally iron, said sheet being substantially permanently devoid of a sharply de- 5. 'Adeep drawing sheet of iron or mild steel containing Per cent balanced principally iron, said sheet being characterized by its substantial permanent absence of a sharp yield point.

6. A process of producing ferrous metal sheets for deep drawing purposes, comprising the following stepsz-Making a heat of iron or mild steel, treating said heat with sufficient scavenging metal of the group consisting of titanium and zirconium so as to leave from .02 to 10% of residual scavenging metal in metallic form in the metal, forming said metal into sheets, subjecting the sheets to a heat treatment consisting of heating the sheets to a point and for a length of time which will bring about a stable normal precipitation of solute therein but not as high as l500 F, cooling slowly enough to avoid supersaturation and finally imparting a cold rolling treatment of the nature employed for removing the yield point.

7. A process of rendering iron or mild steel free from biuebrittleness and free from strain-aging. in which is included freedom from the re-development of a definite yield point upon aging after the yield point has once been destroyed therein, which process comprises adding to ferrous material when in a molten state a degasifying element in a quantity in excess of the amount required to combine completely with the gaseous elements of the iron or steel, forming an ingot therefrom, working said metal, thereafter heating said metal to a predetermined temperature below the A: point and preferably below but in the neighborhood of the A1 point, then cooling said metal to room temperature at a predetermined rate varying from 10 to 15 degrees F. per hour on the average, whereby to produce in the metal a no more than saturated ferrite condition, and subjecting I the metal to a cold straining to eliminate its defisuch that the sheet is characterized by no more than a saturated ferrite condition, said metal being further characterized by freedom from blue brittleness, freedom from a definite yield point, and freedom from tendency upon aging to redevelop a definite yield point.

9. An iron or mild steel sheet or strip which is the product of the process set forth in claim 7.

10. That process of producing iron or mild steel substantially free from blue brittleness, having no definite yield point, and free from the tendency to redevelop a definite yield point upon aging, which comprises adding to ferrous metal when in a molten state a'degasifying element in a quantity slightly in excess of the amount required completely to degasify the said metal, forming an ingot therefrom, working the resultant ingot.

thereafter heating said metal to a temperature below the A; point and preferably in the neighborhood of the A1 point and then coolingsaid body sufficiently slowly to establish freedom from the unstable condition in the ferrite which gives rise to overstraining aging, and finally subjecting the body to cold straining to eliminate its definite yield point as shown by a stress strain diagram.

11. An iron or mild steel sheet or strip which is the product of the process set forth in claim 10. 12. A process of producing iron or mild steel substantially free from blue brittleness and free a from the tendency to re-develop a definite yield point after the yield point has once been destroyed therein, which comprises adding to ferrous metal when in a molten state a degasifying element in a quantity slightly in'excess of the amount required completely to degasify the said metal, forming an ingot therefrom, working the resultant body, thereafter heating said metal to a temperature below the A: point and preferably in the neighborhood of the A1 point, and then cooling said body sufficiently slowly to establish freedom from the unstable condition in the ferrite which gives rise to overstrain aging.

13. An iron or mild steel sheet or strip which is the product of the process set forth in claim 12.

ANSON HAYES. RALPH O. GRHFIS.

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