US2438798A - Manufacture of strain-insensitive steel - Google Patents

Description

Patented Mar. 30, 1948 UNITED STATES OFFICE MANUFACTURE OF STRAIN '-INSENSITIVE STEEL No Drawing. Application October 14, 1941,

' Serial No. 414,940

(c1. was) 4 Claims.

This invention relates to the art of making steel, and discloses a means of controlling its strain-sensitivity, or degree of reaction to cold work.

It is well known that practically all physical properties of steel change in some degree under the influence of cold work. Tensile strength is increased slightly; the yield strength is appreciably raised; ductility is reduced; impact strength is reduced; and the hardness is increased, to name only the more common properties. It has also been observed that steels which possess identical physical properties in the unstressed condition do not necessarily react to cold work in the same degree. A steel which exhibits pronounced changes in its physical properties when subjected to increasing amounts of cold work is considered a strain-sensitive steel; and one which exhibits slight changes is considered a strain-insensitive steel.

The relative strain-sensitivity of a given steel is incapable of any permanent artificial modifications after the steel has passed from the molten into the solid state, as far as is now known. The characteristic strain-sensitivity which is typical of a steel until it is remelted is of great importance both in its fabrication and service life. Among the physical properties of a steel which determine its suitability for given fabricating or service conditions, a number are directly related to its characteristic strain-sensitivity. The ability to withstand cold'bending, drawing, hammering, rolling, or punching without undue embrittlement, hardening, or actual breakage is greater for strain-insensitive steels than for those relatively more strain-sensitive. Likewise, the response of a steel to heattreatment appears to be related to its relative strain-sensitivity. This relationship of other physical properties to the strain-sensitivity of a steel makes it of considerable practical importance to be able to control or adjust the strain-sensitivity of a steel during its making.

It is now generally assumed that the element nitrogen is largely responsible for the phenomenon of strain-sensitivity. U. S. Patent No. 2,174,740, H. W. Graham and S. L. Case, discloses that certain nitrides, including iron nitride, when present in steel give rise to the conditionof strain-sensitivity, while the presence of aluminum nitrides has the opposite eiTect and renders the steel strain-insensitive. Graham and Case disclose the method of controlling the strain-sensitivity of steel by varying the relative amounts of the two classes of nitrides, which 2 is done either by adding nitrogen or aluminum, insensitive steels produced by this method are always fully killed, since the aluminum added is a strong deoxidizer.

In the manufacture of rimming or effervescing steels, the amount of aluminum permissible to add is too small to have appreciable effect on strain-sensitivity. However, for many uses it is desirable to be able to produce insensitive rimmed steels. I have discovered a method by which rimmed steels having relatively low strain-sensitivity may be produced. Rimmed steels produced by my method are less strain-sensitive than those made in the ordinary way, although perhaps not as insensitive as aluminum treated steels may be made. My method is also applicable to the mak ing of steels deoxidized only with silicon or manganese. My method is particularly applicable to steels" made by the duplex process; that is, the process wherein the refining is commenced in a Bessemer converter and completed in an open hearth furnace, and makes it possible to produce duplex steels equal in every way to those made from steel scrap in the usual manner.

The process of steel making in the basic open hearth is one of controlled oxidation of the undesirable elements which accompany iron reduced from its ores. These are silicon, carbon, phosphorus, and to a lesser extent sulphur, manganese, and perhaps small amounts of other elements. The oxidation is carried on by supplying the molten metal with iron oxide, which, in turn, oxidizes the elements listed above. The carbon in the bath is oxidized to carbon monoxide, which escapes as a gas. the other elements remain in the bath but tend to rise to the top, being less dense than iron. Except that for carbon, these oxidation reactions are reversible, unless bases are present in quantities sufficient to form with these acid oxides. stable compounds which may be fluxed off. Lime: is the base commonly employed; this lime, to-. gether with the oxides of iron and the oxides of the elements to be removed, forms the slag. To prevent reduction of these oxides from the slag; an excess of lime is carried in conventional practice.

The relative acidity or basicity of a slag is measured by the percentage ratio of its basic constituents to its acid constituents. The principal basic constituent is CaO; the principal acid one is SiO2. The ratio CaO/% SiOz, called in practice the V ratio is a good approximation of relative acidity or basicity. This ratio is sometimes corrected for the effect of P205 and The oxides of all 3 MnO when a more precise determination is re-' quired. In conventional basic open hearth practice, it is customary to aim for a V ratio of about 2.8, in order to make sure of phosphorus removal and removal of a certain amount of sulphur.

As mentioned, the oxidation of undesired elements is accomplished by FeO supplied to the bath. This iron oxide may be furnished by the oxidation of the scrap by the'fiame'dmi nemelt- I down, or by additions to the furnace of iron ore or roll scale. After refining of the metal is completed, it is necessary to deoxidize'iigif a 'kilied steel is to be made; if the steelito be rimmed, enough iron oxide must be present to insure rimming but not enough to cause the steel tubecome wild. In either case, therefore, the iron oxide at tap must be kept within appropriate limits. Because of this, it is common practice to restrict additions to the bath of oxidizing agents, such asiron ore, particularly in the latter'stages of the refining; in thebeliefthat such additions raise the FeO content of the metal. refining of steel is an oxidizingprocess, .2. ourtailment of ozqngensupply resultsnaturally in a decrease of the rate of oxidation and a lengthening of the time necessary for refining.

I have discovered that if the customary limitations on slag basicity and iron oxide additions are disregarded, a superior steel can be produced in less time. My invention consists e ssential ly .of adjusting conditions of charge and furnace operation to effect an intense an'd rapid Oxidation of carbon and other metalloids in the charge, throughout the refining period, under a. slag having a V-ratio less thanabouttZB. Rapid carbon oxidation is convenientlyobtained "by adding large quantities of ore or otheri'ron oxide to the bath.

Rimmed steel so produced is considerably less.

subject to embrittling; and hardening, by .cold work than similar steel produced by conventional practice. p

I believe that the e fectiveness of my practice lies in its ability to remove dissolved nitrogen from the molten bath. Itisfmy theorythat in steels containing no aluminum nitride or similar stable nitride the strain-sensitivity is more or Since the higher than that desired in the finished steel,

and then to charge iron ore or roll scale in large quantities. Alternatively the ore or roll scale may. be made apart of the metallic. charge. To counteract the cooling efiect of these additions andtoipromote oxidation, the heat input to the I furnace should be raised to the maximum degree allowable. I have found that under favorable conditions an increase in heat input of 25% over' the heat input customarily used may increase the rate of oxidation of carbon as much as three-- fold.

Where the furnace chargeis composed partly or principally of Bessemerized metal, as in the duplex process of steel making, a rapid oxidation of carbon with consequent nitrogen'removalmay be promoted'by charging alternate ladles of hi h carbon and low carbon Bessemerized 'rnetal' r nitrogen in the. bath is probablypresent asa .dis-

solved nitride, which will not of itself separate out to any considerable extent. The oxidation, of carbon by iron oxide results in the forrnationin the bath ofthe. gas carbon monoxideaccording to the reaction:

oxidation of carbon under-aslag-havingav ratio less than about 2.3; When these conditions are lowcarbon Bessemerized metal and molten iron. The intimate mechanical mixing effected by pouring in successivel'adles of full blownBe'ssemerized'metaliand. molten pig iron or hi h carbon Bessemerized metal, together with the reac.- tion between the FeO of the full bIownBe'sse- 'merized metal and" the carbon of. the pig ironior high carbon Bessemerized metal, result in a'rapid carbon elimination and an appreciable n trogen removal'in a short'time. Iron ore or. roll scale,

as above, may .be introduced along with the charge or after charging. In the casepfjalow carbon Bessemerized metal charge, t'he am'ounts 'of iron ore-or scale will not be as largeais required for. ascrap .charge, because of the considerable amount of; FeO-introduced with the Bessemerized metal. In thiscase; as in'the preceding, the heat input to "thefurnace should be increased to ymaximum value.

Since'the carbon content of the bath is. highest at melt-down, the-most rapid carbonelimination is-obtainable during the early part of the'heat.

Ifind'thati-i the chargeis adjustedsothatihe 'carbon on nriel-t -down of the metallic charge is about 1.25%;to 1.80%, and'the heat Worked'un'der the-proper slag: by additions of oreiorro'll scale, at -maximum heat input; so thatthe carbonat the time known tojfurnace, men as half lime upg -L-e when the limestone charge is about "half melted-is'about .80 or less, the greater part/of thenitrogen content of the charge willibe eliminated. The rate of carbondrop durihgthis period may-be as high as 189% per'horln and forbes't results should notbe allowed ;to drop below about .40% per hour. 1 Some furthernitrogen reduction maygbeobtained, if an active oxida 'tion-of' carbon isrmaintained up to the .deoxida tionperiod; 7 f r During theperiod of: extremely rapid: carbon oxidation ,theFV ratioof the slagfshould not exceed about ;1.8-1,and for best resultssh'ould fbe kept-somewhat below this value, If additionspf ore are employed to s bply e io the bath'. silica ofJithe ore wiiliincrease the acidity of the slagandghelp to maintain; the VP ratiQ hearthe Pr me value;jh t if l sca e is use.d; it may be necessary to add sand. The "V-ratio of the slag should be maintained below about 2.3 throughout the refining period, which may be defined as extending from the time the metallic charge has become molten until the carbon has been oxidized to substantially the desired final value. It is, of course, necessary to finish the heat under a basic slag, if phosphorus or sulphur are to be removed, and for this purpose limestone additions may be made as required, after the bulk of the carbon has been oxidized. I find it beneficial, however, to increase slag basicity only to the minimum necessary to prevent phosphorus reversion-that is, to a V ratio not more than about 2.6, and lower, if permissible. It should be understood that slag conditions conducive to phosphorus and sulphur removal are unfavorable to nitrogen elimination.

If, by the time the carbon content of the bath is oxidized to the desired value, the nitrogen content of the bath has not been reduced to the value desired, it is, of course, possible to recarburize the bath by any convenient means-i. e. by adclitions of molten pig iron, high carbon Bessemerized iron, coal, etc.and again adjust conditions to produce a rapid carbon removal under a slag having a V-ratio less than about 2.3.

When the heat has been worked as described to the desired nitrogen content, it may be finished, cast into ingots, and rolled, etc., in the usual way.

I do not find that large additions of iron oxide bearing constituents, as required by the practice of my invention, increase appreciably the FeO content of the bath, nor do I find it necessary or desirable to slow down the rate of carbon drop near the end of the heat, any more than is required by the exigencies of phosphorus and sulphur removal. I do find that a heat of steel may be made by the practice of my invention in less time than required by conventional practice, and yet possess superior cold working properties.

The invention is not limited to the preferred procedures but may be otherwise embodied or practiced within the scope of the following claims:

I claim:

1. The method of making steel having a reduced strain-sensitivity, which comprises forming a basic open hearth heat containing iron oxide and limestone and having about 1.25% to 1.80% carbon at melt-down of the metallic charge, and oxidizing the carbon intensely and rapidly under a, slag the percentage CaO to SiOz ratio of which is maintained under 2.3 so that the carbon is not over about .80% when the limestone charge is about half melted, whereby nitrogen is reduced to a low value.

2. The method of making steel having a reduced strain-sensitivity, which comprises forming a basic open hearth heat containing iron oxide and limestone and having about 1.25% to 1.80% carbon at melt-down of the metallic charge, and oxidizing the carbon intensely and rapidly under a slag the percentage CaO to SiOz ratio of which is maintained at not over about 1.8 so that the carbon is not over about when the limestone charge is about half melted, whereby nitrogen is reduced to a low value.

3. The method of making steel having a reduced strain-sensitivity which comprises forming a basic open hearth heat containing iron oxide and limestone and having about 1.25% to 1.80% carbon at melt-down of the metallic charge, and oxidizing the carbon under a slag the percentage CaO to SiOz ratio of which is maintained under 2.3 the rate of oxidation of the carbon during the period from melt-down of the metallic charge to the time when the limestone charge is about half melted being at least 40% per hour, whereby nitrogen is reduced to a low value.

4. The method of making steel having a reduced strain-sensitivity, which comprises forming a basic open hearth heat containing iron oxide and limestone and having about 1.25% to 1.80% carbon at melt-down of the metallic charge, and oxidizing the carbon under a slag the percentage CaO to SiOa ratio of which is maintained at not over about 1.8, the rate of oxi dation of the carbon during the period from meltdown of the metallic charge to the time when the limestone charge is about half melted being at least .40% per hour, whereby nitrogen is reduced to a low value.

HENRY M. BANTA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,628,281 Schueler May 10, 1927 1,970,067 Wallman et a1 Aug. 14, 1934 2,046,811 Cook July 7, 1936 2,181,468 Witherspoon Nov. 28, 1939 OTHER REFERENCES Stal, vol. 9, No. 2, 1939, pages 13 to 17.

Transactions of the American Institute of Mining and Metallurgical Engineers, vol. 73, 1926, pages 1079 to 1088.

The Iron Age, March 14, 1940, pages 27 to 30. Basic Open Hearth Slag Control, by Washburn and Philbrook.

Physical Chemistry of Steel Making, Bulletin 68, page 35 published in 1934 by Mining and Metallurgical Advisory Board, Pittsburgh, Pa.

Iron and Steel, page 2'75; Boylston, published in 1928 by John Wiley and Sons, New York.

Open Hearth Steel Making, page 75, by Cook, published in 1937 by The American Society for Metals, Cleveland, Ohio.

Metals and Alloys, April 1941, page 421.