US2819956A - Addition agent for and method of treating steel - Google Patents

Description

United States atent G ADDITION AGENT FOR AND METHOD OF TREATING STEEL Application September 15, 1955 5 Serial No. 534,620

3 Claims. (Cl. 75-57) No Drawing.

This invention relates to an addition agent for the treatment of steel and to a method of treating molten steel.

In the production of steel sheets for the manufacture of finished products by pressing, stamping, drawing, etc., it has been customary, wherever possible, to use rimming steel of low carbon and preferably moderately low manganese contend. Such steel possesses low elastic properties, high ductility, good surface, and other desirable qualities.

When, however, deep drawing operations are contemplated, the ordinary rimming steel of commerce is frequently inadequate. Sometimes satisfactory results are achieved by selection; by this I mean either selecting individual heats that are of better quality for the purpose than are other heats made in the same plant, or sometimes taking the bottom portion of each ingot and using this portion for sheets for the more difficult jobs.

An improvement over these practices has resulted from the incorporation of small amounts of vanadium of the general order of about 0.03% to 0.06% in rimming steels which have had previously a partial control of their rimming action or evolution of gases by the addition of limited quantities of aluminum or silico-manganese. The steels of this type in order to retain their good surface must, of course, show on analysis only a very small amount of silicon residual, ordinarily not more, or certainly-not much more, than about 0.02%, so that initial deoxidation by silico-manganese must be done with care. Similarly, aluminum must not be added to such an extent as to interfere with the proper degree of gas liberation or correct rimming. Vanadium rimming steel possesses the same attributes as does ordinary plain carbon rimming steel in respect to ductility and good surface and, additionally, appears to be capable of undergoing considerable deep drawing, regardless of the location in the ingot of the metal ultimately converted into a sheet. Also, when the conditions of the prior deoxidation are just moderately well controlled and the amount of vanadium is correct, differences between successive heats are very considerably smaller than in the case of plain carbon steel. From actual practice, in the-making of large tonnages, it has been shown at these conditions are readily obtained.

There are, however, some jobs of a deep drawing nature that appear to be too severe to be successfully repetitive on a production basis by the use of the vanadium rimming steel. For these applications, steelcompletely killed by aluminum has been employed just as it was employed prior to the development of the vanadium rimming steel. The aluminum killed steel is, however, subject to important defects. The amount of metal recovered, as in the case of any killed steel, is much smaller than for rimmed steel or, in other words, the loss as scrap for remelting is considerably greater. Additionally, reduction of oxides in the steel by the aluminum yields hard aluminarich inclusions that may be either nearly pure alumina or aluminum silicates or other compounds. These appear frequently in great number and in clusters at and near the ice surface and result in rejections in the steel plant on the basis of surface irregularities or during forming by yielding badly distorted or torn deep drawn articles.

I have discovered that the shortcomings of the aluminum killed steels employed in very severe deep drawing applications can be avoided or eliminated by employing calcium and aluminum together in the deoxidation or killing of the steel. Properly proportioned, the calcium and alu-v minum yield a deoxidation product that is a calcium aluminate of moderately low melting point but with extremely high fluidity at steel making temperatures so that it rises readily into that portion of the ingot that is discarded. In other words, alumina-rich inclusions, either individually or in clusters, and either at the surface of a finished sheet or deep-rooted, are no longer found in the finished product.

Minor contamination by silica due to the presence of small amounts of this substance resulting from oxidation of silicon in the addition agent, or by silica and other associated substances introduced by refractories with which the molten steel comes in contact do not detract from the desirable qualities of the calcium aluminate oxidation product and are carried away with it out of the mass of the cooling and solidifying steel. Expressing this fact somewhat diflerently, small amounts of silica coming from the addition agent or the refractories, or titania from either or both of these sources, or alkali oxides from the addition agent or the refractories, or a small amount of unreduced manganese oxide from the steel bath or from the alloy, do not alter the desirable performance of the addition agent which yields an oxidation product preponderately of combined oxides of calcium and aluminum.

Considering only the calcium and aluminum contained in the addition agent, they should be present in the following proportions: 51 to 64% Ca, 36 to 49% Al, and may be introduced in any convenient form. Certain metallic impurities may be present in the addition agent up to definite, critical and permissible limits. Additionally, because of its contribution to improvements in manufacture and also to the functioning in use, there is present in the addition agent silicon to the extent of about 0.3% to 3.0% inasmuch as this limited quantity appears to have a desirable effect in insuring that the total of the oxides of calcium and aluminum combine to form the desired'end product. No harm appears to result from the presence of small amounts of titanium oxide or the oxides of manganese or the alkali oxides so that in the addition agent, titanium may be present up to about 1% maximum, manganese up to about 6% maximum and the alkali metals up to about 0.5% maximum. It has been determined that these maxima are quite critical; above the stated level, alkali metals will tend seriously to attack ladle liningsand titanium, through its oxide, will tend to raise the melting point of the resultant slags quite substantially. Above the level given for manganese, this metal would be added to the steel to an extent too great to satisfy some steel specifications. Iron mayor may not be present but, ifpres'ent, the total of iron and manganese should not exceed" 20%. Thus, the addition agent comprises a limited amount of silicon, a low maximum of titanium, a low maximum of manganese, a low maximum of the alkali metal and up to a total amount of iron and manganese of 20%, the remainder consisting of calcium and aluminum in the proportions indicated above, namely, in the range of ratios of calcium to aluminum of 1.00:1 to 1.75:1.

Within the critical limits herein described, an alloy contributing the desired properties has the following composition:

The metals' cannot satisfactorily be added individually in view of the low specific gravity of calcium and the resultant difiiculty of introducing all of it beneath the surface of the molten steel effectively and without great loss. It can be done, of course, by means of a projecting device suchas a gun or by a plunger to force or hold the reagent in the liquid steel, but with larger masses of steel these measures are inconvenient.

Accordingly, the metals can be added in the form of an alloy which should have the above stated proportions. However, since this alloy also is highly reactive, an improved method comprises having an alloy somewhat richer incalcium and lower in aluminum (which is more readily prepared) enclosed in an aluminum capsule which would thensupply some of the aluminum required for achieving the desired rangeof oxide composition; the total addition would then have the two metals in the above preferred proportion. Iron may be. added, either in the capsule or as the: cover for an open end capsule, solely in order to add weight. Or, if preferred, an iron capsule may be used, enclosing the metals in the desired proportion, either in the form of a crushed alloy or as the individual metals or as a combination thereof, When the individual metals are enclosed in such a device, they should be so comminuted that the reaction products can readily combine under ordinary practice of steel plant use.

Another suitable means of introducing these deoxidizing elements into steel baths is that of containing the metals in a tube which may be closed at one or both ends and which can be immersed in the steel bath to a predetermined depth, correspondingto the'volume of oxygen contained in themetal bath.

Thenew means which I have developed for deoxidizing steels to produce combined oxides having a low melting point which will separate effectively from the metal during its cooling and not be retained in the ingot after solidification, avoids the-addition of any substantial amount of silicon to the steel during deoxidation, so that at most a very minor residual silicon can result. The afiinity of calcium and aluminum for oxygen is greater than that of silicon, which is oxidized only after the major portion of the other two metals have been bound to oxygen. Silicon has a strong tendency to alloy with the steel. However, with the percentage of silicon in the addition agent being limited as above described, even if none were oxidized, the amount introduced will be well within permissible limits. Since silicon may be introduced into the steel from other sources, if the addition agent contains more than 3% silicon, there is danger that the silicon content of the steel may be raised above the level that would permit producing suitable deep-drawing steel. At the maximum silicon content of 3%, if perchance conditions are such that all of the silicon is oxidized, the silica will not have a detrimental effect on the results obtained in the use of the addition agent, in that the product of the fully oxidized metals will not have a higher melting point. The presence of the specified amount of silicon is desired, as it appears that small amounts of silica have the eflfect of priming the fusion of the slag.

Furthermore, it should be noted that calcium has a very great affinity for sulphur, and aluminum has a great aflinity for nitrogen. While the deoxidizing alloy of my invention will ordinarily be added to steel only after substantial desulphurization has taken place (if any is required), any small residual amountsof sulphur that they still remain will be at least in part bound to the calcium.

The nitrogen content of the steel depends to a large extent on manufacturing methods employed in its production. However, nitrogen rarely exceeds 0.01% and aluminum will combine with this nitrogen, thus being productive of a steel of customary ductility.

While it can, therefore, be expected that small'amounts of calcium and aluminum will be removed from the alloy to bind sulphur and nitrogen, the extent to which. this will occur is not large and will not afiect the results'hereinlie fore described.

The oxidation products of the addition agent of this invention are strongly basic in character, the oxidation products of an addition agent containing 61% calcium and 39% aluminum having a basicity factor of 0.45 (that is, /a of the percentage of alumina divided by /3 of the percentage of alumina plus the percentage of lime). By contrast, the oxidation products of the customary'deoxidizers (such as, aluminum, ferro-siliconor calcium silicide) are strongly acid in character, namely, witha: factor greater than 1.00. Thus, this feature distinguishes the new addition agent from other deoxidation agents.

Essentially, the addition agent of my invention permits simultaneous or nearly simultaneous deoxidation ofsteel by two metals, calcium and. aluminum, and is particularly applicable to steels requiring very low silicon contentsi The oxides of these metals readily combine to form low melting calcium aluminates which have astrong tendency. to aggregate and, by virtue of these two factors, tend: to rise out of the melt and go into: the slag- If the agent or alloy should. be" used with silicon containingsteels and if. some of the soft calcium aluminates should-accidentally be retained in the metal, they will do no damage-to tools" used in machining. Thus, the invention may also-be ap:- plied in the manufacture of killed steels for bars-,xfotgings, casings, etc.

The invention is. not limited to the preferred-embodh ment but may be otherwise embodied 'orpracticedwithin' the scope of the. following claims.

I. claim:

1. An addition agent for the treatment of steel, contain-; ing. 0.3 to 3.0% silicon, up to 1% titanium, .up to 6% manganese, up to 0.5% alkali metals and upto a totah amount of iron and manganese of 20% the remainder con sisting, of calcium and aluminum in therange. of ratios of calcium tov aluminum of 1.00:1 to 1.75:1.

2. The method of treating.v molten steel,.whiclrcorn prises adding, thereto in anamount sufiicient to. steel, an addition agent in which the principal.- compos nents are calcium and aluminum, the calcium being..between 51 and 64% and the aluminum being, between36 and.49% of the total of the calcium and aluminum-..

3. The method. of treating molten steel, whichco'm prises adding thereto in an amount suflicient-to kill: the steel, an addition agent accordingto claim 1.

References Cited in the file of this patent UNITED STATES- PATENTS 875,668 Me-slans Dec. 31, 1907" 1,348,458 Strasser Aug. 3, 1920 1,471,401 Koppers Oct. 23, 1923? 2,154,613 Guthrie Apr. 18-, 1939

Claims (1)

1. AN ADDITION AGENT FOR THE TREATMENT OF STEEL, CONTAINING 0.3 TO 3.0% SILICON, UP TO 1% TITANIUM, UP TO 6% MANGANESE, UP TO 0.5% ALKALI METALS AND UP TO A TOTAL AMOUNT OF IRON AND MANGANESE OF 20%, THE REMAINDER CONSISTING OF CALCIUM AND ALUMINUM IN THE RANGE OF RATIOS OF CALCIUM TO ALUMINUM OF 1.00:1 TO 1.75:1.