US2181693A - Steel treatment - Google Patents

Description

Nov. 28, 1939. s, EPSTEIN AL 2.181,693

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Patented Nov. 28, 1939 i UNITED STATES PATENT OFFICE 2,181,693 STEEL TREATMENT Samuel Epstein, Bethlehem, and Hugo C. Larson, Johnstown, Pa., assignors to Bethlehem Steel Company, a corporation of Pennsylvania Application September 8, 1937, Serial No.162,848 12 Claims. (01. 75-56) This invention relates to controlling the rim- T avoid a wi d or str n y falling steel on ming action of steel so that the solidified ingot the one hand and a rising steel on the other, a

has a thick-skinned butt which will not crack certain amount of deoxidizer has to be used in and tear during rolling and hence give billets the ladle of low carbon steel in order to leave with good surface. the right amount of active oxygen in the steel. 5

Rimming steel gives off large quantities of The correct amount of deoxidizer can be judged gases (Co, N2, H C0 mainl arbo mehexfrom the composition of the bath and slag in the ide, while it is poured into the ingot mold and f a U u y appreciably less d er is during solidification. This gas evolution causes added to the ladle than is likely to Produce a 1i) the ingot to have a spongy structure without e Steel in the meld- If a Steel which falls any central shrinkage cavity or pipe. To elimip y in the ds u t t falling t nd n y hate pipe and undue sponginess and to get a can e v r ome by adding the p -p amount tough-skinned t t t same t requires of the deoxidizer aluminum in the succeeding close control of the rimming action. melds- Fairly f y, owever, the corre t If the steel as poured contains too much reihE ladle addition nnot be judged acactive oxygen and thus tends to give off too euretely n the Steel rises in the mold. mmuch carbon monoxide it acts wild in the ming Steel Containing ut P t ea m o pouring t meld n up rephhy t bon or higher invariably tends to rise in the mold the molten metal which contains large bubbles even without the addition of y deoxidizer in 30 of t evolving a Then as th gas Suddenly the ladle. In these instances it is advantageous escapes the height of the metal in the mold sinks. to add some substance to e dl me d wh ch sharply d th m requires further fllling has an effect exactly the opposite of a deoxidizer Such an ingot may sink again before solidifyand hence causes the steel to hold steady or ing so that a pipe is formed (sheet or strip rolled fell Slightly in the l instead of risingfrom such steel may also have the defect called The Obvious e p on or the formation of 25 lamination) or it may rise. In the latter case the Pencil-like b ow es a the butts i the enlong pencil-like blowholes close to the surface trepment of gases which a v d dur ng s of the ingot and perpendicular to it are formed lidification- In h upper half of the ingot t at the lower portion or butt. The butt of such c s can escape to the outer air e readily an i ot gives poor surface i rolling because of the smaller ferrostatic head of molten 30 We have found by experience that, if the steel metal. At the lower half Of the ingot OI butt as poured contains the proper amount of reacthe greater ferrostatic head Of the molten metal ti xygen t form, t proper amount of 0 above can prevent escape unless the evolved gas t metal n t m more Steadi1y After has sufiicient pressure to overcome the ferrofilling the metal holds its level and the surface Static Pressure If as solidification takes place, 35 quickly assumes an active boiling appearance, the gases evolved at the IOWGI part Of the ingot Soon the characteristic crust or rim begins to are entrapped, the ingot naturally S S n the form from the sides inward, from which the term moldrimming steel is derived. The crust or solidified Ev n ly it is not enough for ga to be iven top of the ingot is either at the same level as ff arly n t p u period- The eases must 40 that to which the molten steel was poured, or be evolved precisely during the time when solidia inch or two lower, s h an ingot h no pipe fication is proceeding so that the gases evolved and fewer of the long pencil-like blowholes apat the surfaces of the solidifying crystals in the pear at the butt so that it gives a better surface change from the liquid to the solid state, can be in rolling. swept out and can exert enough pressure to es- 45 On the other hand, if the steel as poured conp tains too little reactive oxygen, too little CO is We have found that a simple and effective way evolved. The metal fills the mold steadily and to produce such a gas evolution is to add a strong retains its level but the boiling action may be oxidizing substance to the steel, of the class of somewhat more sluggish. Then the ingot slowly strong oxidizers like potassium chlorate, sodium 50 begins to rise in the mold, the amount of rise nitrate, potassium permanganate, and potassium varying from an inch or two to as much as eight dichromate. Presumably the oxygen in the or ten inches. Such an ingot has a thin-skinned strong oxidizer immediately forms FeO on combutt with large pencil-like blowholes which cause me in Contact w the m t Steel- This P60 poor surface in rolling. along with the Eco already present remains in 55 solution in the molten steel; but FeO is rejected by the solidified steel during solidification. As soon as the molten steel is enriched by this rejected FeO the latter reacts with the carbon in the molten steel according to the equation FeO+C=CO+Fe. Thus more CO is evolved at the surface of the solidifying crystals and this helps increase the gas pressure to above what it would be if the strong oxidizer were not added. The gases evolved can thus escape.

Of the strong oxidizers sodium nitrate is the cheapest. It was found, however, that on adding it to the molds of rimming steel during pouring a rather loud explosion would occasionally occur. The tendency to explode was completely eliminated by mixing the sodium nitrate with finely powdered F6203. Our practice has been to fuse the sodium nitrate and add the powdered F8203 stirring up the mixture and casting it into lb. cakes. These cakes are very strong and are conveniently handled on the pouring platform. For example, the proportions may be 60 parts sodium nitrate to 40 parts F6203 by weight. This has been satisfactory but a lesser or greater proportion of F9203 may also be used. The sodium nitrate may run from 35% to while the ferric oxide may run from 65% to 10%. The cast cakes of the oxidizing agent, of definite and moderate weight, are of considerable practical advantage in enabling the operator to add amount of the re-agent. It is obvious that the amount of-re-agent necessary will vary with the difierent castings, and the same cakes of definite weight enable the operator to add the requisite amount in any particular instance. Cakes of the mxiture can also be made without fusing but by mixing dry, adding a suitable binder and molding under pressure in a die.

For low carbon rimming steel about 2 to 16 oz. per ton of the 60:40 NaNO3 Fe203 mix are required depending upon the degree of deoxidation of the steel. ladle or mold, preferably the latter. For higher carbon rimming steel the amount of the mix required may be two to three times that for lower carbon steels. In using proper amounts of the mix not enough nitrogen isabsorbed from the sodium nitrate to harm the steel in any way. The i is conferred by adding the mix greatest benefit to a rising steel. However, our work indicates that even in the case of a falling steel the addition of the mix gives an ingot with a somewhat thicker and sounder skin at the butt.

Figs. 1a and 127 show photographs of cross-sections through the butts about 12 inches from the bottom of two 22". x 25" rimming steel ingots from the same heat. In A of ingot #3 no addition was made to the mold; in B of ingot # 8, 1%.; lbs. of sodium nitrate was added to the 4-ton' ingot. Note the large pencil-like blowholes near the surface in the untreated ingot and the nuch smaller blowholes further removed from the surface in the treated ingot.

Figs. 2a and 2b show the contours of the tops of the same ingots. Ingot #3 with no mold addi tion rose about 3 inches, where as ingot #8 with an addition of 1 lbs. of sodium nitrate'fell about 3 inches.

Figs. 3a and 3b show photographs of cross sections similar to those of Figs. 1a and 1b of 2 ingots from another heat. Ingot #2 untreated has numerous large pencil-like blowholes near the surface. Ingot #4 treated with 1 lb. of a 3:3:4

sodium nitrate-sodium dichromate-FezOav mix shows no pencil-like blowholes near the surface.

the necessary This amount may be added in the TABLE I.-Summary of surface defects in billets cut from ,the butts of 60 ingots from 6 heats Defective areas-sq. in. per sq. ft. Mold addition N f I t A1" to:

o. 0 ago ga ingots seams scabs cracks hide ls l' gz liing t t 20 1.88 1.8 .08 8.0

0 mm in re. e F8203 mix 40 1.3 1.3 .07 .47

Note that alligator hide which is the most serious defect was almost completely eliminated by the mix. The amount of seams and scabs was also materially reduced.

We appreciate the fact that various substances have previously been added to rimming steel for the purpose of controlling the rimming action.

. Sodium carbonate and calcium fluoride have been used or proposed for use. We have found, however, that the best results in this control can be obtained by using suchsubstances as those mentioned above, namely strongly oxidizing substances. For characterizing the type of material which we have found to be most effective,.they are such substances as are decomposed or dissociated to liberate oxygen at the temperature of molten steel, at the time of casting.

While we have referred tothefuse of F6203 for preventing the explosive action sometimes accompanying the use of strongly oxidizing substances, we do not desire to limit ourselves to the use of the particular substance F6203. Frequently its use is unnecessary. Furthermore, other diluents or interspersing agents may be employed. We prefer to use such a diluent or interspersing agent as an oxide which is not broken down at the'temperature of the molten steel at the time of tapping or casting. Roll scale, for example, may be successfully used.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is;

1. Themethod of manufacturing rimmed steel comprising the step of adding a highly oxidizing agent of the group comprising potassium chlorate, sodium nitrate, potassium permanganate and potassium dichromate and their equivalents .to the molten metal after tapping.

2;. The method of manufacturing rimmed steel comprising the step of adding sodium nitrate tion of steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming steel after tapping of a mixture of an oxidizing substance of the group comprising potassium chlorate, sodium nitrate, potassium permanganate, potassium dichromate and their equivalents and a diluent of weaker oxidizing action than said oxidizing substance.

6..The method of controlling the rimming action of steelv to obtain aningot with a thickskinned butt, comprising the addition to the molten rimming steel after tapping of a mixture of sodium nitrate and a diluent having a weaker oxidizing action than sodium nitrate.

7. The method of controlling the rimming action of steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming-steel after tapping of an oxygen containing substance of the group comprising potassium chlorate, sodium nitrate, potassium permanganate and potassium dichromate and their equivalents.

8. The method of controlling the rimming action of'steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming steel after tapping of sodium nitrate.

9. The method of controlling the rimming action of steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming steel after tapping of a sodium nitrate-iron oxide mixture, the proportions of this mixture varying from about 10 per cent to 65 per cent iron oxide.

10. The method of controlling the rimming a ction of steel to obtain aningot with a thickskinned butt, comprising the addition to the molten rimming steel afte'r tapping of a sodium nitrate-Fe'zOa mixture, the proportions of this mixture varying from about 10 percent to 65 per cent F6203.

11.The method of controlling the rimming action of steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming steel while pouring in the mold of from 2 oz. to 48 oz. per ton of a sodium nitrateiron oxide mixture. r v

12. The method of controllingthe rimming action of steel to obtain an ingot with a thickskinned butt, comprising the addition to the molten rimming steel in the ladle of from 2oz. to 48 oz. per ton of a sodiumnitrate-iron oxide mixture.

SAMUEL EPSTEIN. HUGO C. LARSON.

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