US1974571A - Descaling process - Google Patents

Description

Patented Sept. 25, 1934 1.9 4.511 UNITED STATES PATENT OFFICE.

1,974,571 DESCALING vrnoi'nlss George C, Kiefer, Springdale, Pa... assignor to Allegheny Steel Company, a corporation of Pennsylvania No Drawing.

Application March as, 1932. Serial No. 601,889

- v 8 Claims. (Cl.148-8) the course of their manufacture. For the purpose of this application, I will hereafter designate such articles, sheets, without, however, any intention of limiting the invention to the removal of scale from sheets only.

The alloys with which this invention has particularly to do may be grouped within a classcontaining from 10% to chromium and from 25% to 5% nickel with the remainder principally iron, but which may contain small additions of vanadium; molybdenum, tungsten, copper, etc.-

The most widely 'used of the alloys within such roup is that alloy which contains in addition to iron, about 18% chromium and about 8% nickel.

The removal of scale acquired in rolling, heat treatments and other works operations is a very necessary and important part of the manufacture of sheets from such alloys and presents a highly complex problem one which is much more dithcult of solution than is usually encountered in the descaling of other alloys met with inthe steelmaking art.

sheets from such alloys, the scale formed thereon has a tendency to jump leaving a bare metal od of treating sheets formed from alloys of the "above group in such a manner as to completely remove the scale'or'oxide coatings thereon without over-pickling anyportions of such sheets.

' While'the removal of scale from sheets: made from such alloys without over-pickling or etch-j ing some portions of the. same is very difficult with the-pickling acids or solutions ordinarily employed for this purpose such scale removal without overpickling or etching is made possiblesolution as set forth in my copending application Serial No. 424,143 filed January 28, 1930, for a Method of treating chromium-nickel alloy steels which wlll'not over-etch the sheets, 'no matter how long the same are left in will entirely remove the scale or coating formed during the heat treatment provided the heat treatment is carried out in a certain manner. 7 g g I have discovered that the heat treatment and the pickling of articles made from alloys within the above group go hand-in-hand and that the type of heat treatment or manner of carrying out the same is of utmost importance in the results to be obtained by subsequent pickling in my pickling solution or solutions, which may con-v sist of any suitable or preferred composition such as a solution preferably heated, andcontaining about 25% commercial nitric acid and about 2% hydrofluoric acid (48%) by volume.

While the atmosphere of the furnace withinwhich the heat treatment is carried out has an efiect upon the results of the subsequent pickling, I find that this is not so important as the manner in which the sheets are heated, that is, the manner in which the sheets are brought to an- 4 i healing temperature. During-certain stages in themanufacture of I find that satisfactory results can be obtained in a furnace having a reducing atmosphere or in a mufilefurnace, the atmosphere of which 0on tains but little oxygen. ,7

I find that in order to obtain satisfactory results in pickling, it is necessary to, so to speak, plunge the sheets intoa zone raised to the proper annealing temperature. By this I mean that it is necessary to move them into such zone rapidly and that it is necessary to have the volumetric capacity of such zone such that the sheets, as theyenter the zone, are immediately raised to the anwhere continuousconveyors are employed.

by the proper solution or solutions provided such sheets are first heat treated in .a. certain manner.

In order to render such sheets relatively soft and easy to work, it is necessary toraise the same toannealing temperature and then to quickly cool the same. This heating and cooling which I term a heat treatment as now carried on, produces a scale or coating whichis only removed with great. difliculty with the ordinary acids or solutions now commonly used. I

I have-discovered that a pickling solution either used as a single step solution or as a two step I find that it is necessary to heat treat the sheets, singly in order that they may become uniformly heated on both sides.

I find that the niost satisfactory manner of so heating the sheets is to convey them rapidly into the annealing'zone and there deposit them on support members raised to and maintained at annealing. temperature.

' I find that it is only necessary to keep the sheets within this zone for a short time. I find that a mullie furnace having a door at one end only or doors at opposite ends through which the sheets may be entered and removed serves the purpose admirably. As soon as the sheets are heated to the proper temperature, they'can be removed to the air and thus cooled. Such cooling I term quick orrapid cooling, but if more rapid cooling is deemed necessary, the sheets can be either plunged into a bath ormoved into a cold zone.

The scale formed during hot rolling is the scale which jumps, leaving the sheets in a mottled condition, that is, with areas from which the scale has jumped and areas having a tightly adhering scale, and this tightly adhering scale must be heat treated before it can be removed by my pickling solution or solutions.

The sheets after the last hot rolling operation are quickly heated to annealing temperature and then quickly cooled after which they are subjected to my pickling solution or solutions.

After the scale has been removed and the sheets scrubbed with plain'water, they are cold rolled to approximate gauge and then annealed and pickled. This annealing is of the same type, that is, the sheets are plunged into the annealing zone, quickly cooled and then immersed in a bath or baths of my pickling solution or solutions; thenthey are ready for final finishing operations such as polishing, or grinding and polishing. The annealing of the cold rolled sheets is preferably carried on in a muiiie furnace.

The annealing of the hot rolled sheets is preferably carried on in a normalizing furnace in which a reducing atmosphere is maintained and in which the sheets are quickly heated to an annealing temperature and thereafter quickly cooled.

If it were not necessary to heat treat such alloy sheets after hot rolling, the scale on such sheets might be removed by my pickling solution because the heating of the sheets in the sheet furnace for the final stage of hot rolling places the scale in condition tobe readily removed by such pickling solution.

After hot rolling, the sheets must be heat treated, and pickled and if thereafter cold rolled they must again be heat treated and pickled after the cold rolling before they are suitable for commercial purposes.

The temperature to which I preferably raise the sheets during heat treatment is between 1850 and 1950 F. after which the sheets must be quickly cooled in order to render them ductile.

This treatment, in the case of hot rolled sheets from which-a portion of the scale has jumped, places such sheets in condition to have complete scale removal by means of my pickling solution, and, in the case of cold rolled sheets, the heat treatment or heat treatments plus the cooling step forms a scale which is readily removed by my pickling solution without etching the surface of the sheets, no matter how long the same are left in the solution. It.,is impossible to overpickle the sheets, but unless the sheets have been quickly heated to annealing temperature in a reducing or non-oxidizing atmosphere, the

scale thereon cannot be readily removed by means of my pickling solution. The heat treatment, therefore, in my opinion, forms an important and integral part of my scale removing process.

The above is intended more in an illustrative than in a limitative manner and I do not limit myself thereto except by the subjoined claims.

- My present application is in part a continuation of "my copending application Serial No. 424,143, filed January 28, 1930 for a Method of treating chromium-nickel alloy steels.

'iron alloy sheets or the like comprising rapidly heating such sheets to annealing temperature,

annealing them, rapidly cooling them and. then subjecting them to a pickling bath consisting only of nitric and hydrofluoric acids.

2. A process for descaling chromium nickel iron alloy sheets or. the like comprising rapidly heating such sheets to annealing temperature, annealing them, rapidly cooling them and then subjecting them to a pickling bath consisting of about 25% commercial nitric acid and about 2% hydrofluoric acid (48%) 3. A process for descaling chromium nickel iron alloy sheets or the like comprising rapidly heating such sheets to annealing temperature in an atmosphere of sub-normal oxygen content as compared with the oxygen content in the air, annealing them, rapidly cooling them and then subjecting them to a pickling bath consisting of about 25% commercial nitric acid and about 2% hydrofluoric acid (48%) 4. That step in the process of descaling chromium nickel iron alloy sheets and other chromium nickel iron alloy articles comprising subjecting the same to a heat treatment which includes rapidly heating the same to 1850-1950 F. in an atmosphere. containing less oxygen'than is contained in air and then rapidly cooling the same preliminary to subjecting such sheets and articles to a pickling bath containing nitric and hydrofluoric acids but in which the nitric acid is the predominating acid and thereafter pickling such sheets and articles in such bath.

5. That step in the process of descaling chromium nickel iron alloy sheets and other chromium nickel iron alloy articles comprising subjecting the same to a heat treatment which includes rapidly heating the same to 1850-1950" F. in an atmosphere containing less oxygen than is contained in air and then rapidly cooling the same preliminary to subjecting such sheets and articles to a pickling bath containing nitric and hydrofluoric acids but in which the nitric acid is the predominating acid and thereafter pickling such sheets and articles in such bath, said chromium nickel iron alloy containing about 18% chromium and about 8% nickel.

6. In descaling chromium nickel iron alloy products, the new combination of steps comprising rapidly annealing such alloys in an atmosphere relatively poor in oxygen as compared to ordinary air at 1850-4950 F. and then pickling in a nitric acid-hydrofluoric acid bath in which nitric acid is the predominating acid.

'7. The method of treating sheet-like material made from an iron chromium nickel alloy having a chromium content of from about 10% to about 30% and a nickel content of from about 25% toabout 5%,'comprising rapidly raising the temperature of the material to from about 1850 -to about 1950 F., then rapidly cooling the material, then subjecting the material to a pickling procedure in a bath containing about 25% commercial nitric acid and about 2% hydrofluoric acid (48%).

8. The method of treating sheet-like material idly'cooling the material and then subjecting the material to a pickling procedure in a bath containing about 25% commercial nitric acid and about2% hydrofluoric acid .8%).

GEORGE C. KIEF'ER.