US1989884A - Method of annealing and cleaning ferrous articles - Google Patents

Description

Patented Feb. s, 1935 METHOD OF ANNEAIJNG AND CLEANING FERROUS ARTICLES Gustav A. Reinhardt, Youngstown, Ohio, assignorto The Youngstown Sheet and Tube Company, Youngstown, Ohio, a corporation of Ohio No Drawing.

3 Claims.

My invention relates to the treatment of ferrous articles in the course of their manufacture and more particularly to an improved method for simultaneously cleaning and annealing ferrous 5 sheets and plates prior to their being coated with zinc or tin although my invention may also advanta'geously be employed for substantially similar purposes in the manufacture of other ferrous articles or shapes and at other stages in the course of manufacture.

In the preparation of iron and steel sheets, plates and other ferrous articles for galvanizing or tinning, ithas heretofore been customary to first anneal the articles in a furnace by either of the following well-known processes, box annealing, open annealing or normalizing, and thereafter pickle them by introducing them into an acid solution, the annealing being efiected for the purpose of obtaining the desired ductility and grain structure and the pickling to remove scale and other foreign matter from the surface of the metal so as to enable a satisfactory coating to be subsequently applied thereto. My invention is directed to a method by which these two operations may be combined and the annealing and cleaning accomplished by a single step or operation and as it is particularly useful when employed for treating iron and steel plates and sheets just prior to galvanizing or tinning, it will herein be more particularly described with reference thereto.

When in the ordinary course of ferrous sheet manufacture the sheet has been rolled to the desired thickness, its grain structure is such that a rearrangement thereof is usually desired and is ordinarily effected by box annealing or other equivalent process preparatory to pickling by immersion in an acid bath. At this stage in accordance with my improved method and instead of annealing them in a furnace, I introduce the sheets into a basic bath comprising molten sodium carbonate maintained at a temperature above its melting point, 852 C. After the sheets have been immersed in this bath a short time, for example, with ordinary sheets such as are used for tin plate, about fifteen seconds, they are removed from the bath, cooled, then washed in or sprayed with water and allowed to dry. After treatment in the manner described the sheets are found to be rearranged and refined as to grain structure similarly to the rearrangement eifected by furnace annealing and are, additionally, free of scale and other surface impurities, which, if present, would interfere with subsequent tinning or galvanizing.

Application May 11, 1932, Serial No. 610,708

- In accordance with my method when a lower temperature bath is desired a small quantity of sodium hydroxide may be added to the sodium carbonate, thus somewhat reducing the melting point of the latter and permitting it to be maintained in a fluid state at a relatively low temperature.

The sheets upon their removal from the molten bath, when treated in accordance with the method just described, carry a thin coating of sodium ferrate, resulting from the reaction of sodium carbonate with the iron oxide scale usually present before the treatment together'with an outer film of the sodium carbonate, which prevents oxidation of the sheets during the subsequent cooling. Both of these compounds, being soluble in water, are readily removed from the sheets by washing so that the sheets are thereafter perfectly clean, well descaled, brighter than an ordinary pickled sheet and more rust-resistant. Moreover, whereas acid pickling usually results in absorption of hydrogen by the sheets with resultant defects in the subsequent coating when the sheets are galvanized or tinned, sheets treated by my method are substantially free of hydrogen and therefore-may be coated with zinc or tin without the blistering frequently encountered in the case of pickled sheets and which is thought to be due to expansion of this occluded hydrogen under the influence of the heat of the galvanizing or tinning operation.

It will, of course, be appreciated that since molten sodium carbonate causes rapid corrosion of certain metals, it is necessary to use for holding the bath a pot made of some material which is not readily attacked thereby and I have found nickel and certain alloys thereof particularly useful for this purpose, since they are generally substantially inert to the action of the compounds I employ, even when in a molten state.

For the most successful and satisfactory practice of the invention, the articles being treated should be heated slightly above the thermal critical point or temperature of the metal of about 900 C.; in the case of low carbon steel, such as is commonly used for the manufacture of sheets and plates, this result is readily accomplished by the use of a bath of soda ash or sodium carbonate as above described since the melting point thereof, namely 852 C. is very slightly below the thermal critical point or temperature of such steel. Moreover, an important advantage of the invention resides in the fact that while the article is in the bath it is subjected to a uniform temperature at all points so that the granular strucwill subsequently have substantially uniform characteristics as to grain refinement, while as less metal is removed from the surface than by the usual pickling operation, a considerable saving of metal in the aggregate is accomplished.

while I am aware it has heretofore been suggested that in the treatment of sheets prior to the galvanizing or tinning it might be desirable to subject them to the action of a bath consisting of a molten mixture of sodium chloride, sodium carbonate and sodium cyanide, this practice would be highly dangerous and thus impractical for use on a commercial scale because highly poisonous fumes are given off by such a bath due to the presence of the cyanide, which result in serious or even fatal poisoning of operatives in the vicinity of the bath, whereas when the bath of my invention is employed, no poisonous fumes or gases of any character are given off and the safety of the operatives is thus unaffected.

By my invention not only is the annealing accomplished substantially instantaneously, as distinguished from the several hours heretofore required for box annealing, but, additionally, the necessity for producing the relatively expensive pickling machines heretofore used is avoided and the costs involved in acid pickling of the sheets substantially eliminated. It is thus evident that by the use of my method very substantial savings in ultimate manufacturing costs may be effected, the operations corresponding to annealing and pickling in the methods heretofore used being accomplished relatively rapidly with the aid of apparatus of very substantially less cost both initially and for maintenance than that theretofore used to effect corresponding results. Furthermore, the output of a plant may be appreciably increased by the elimination of pickling losses, it being well known in the industry that the iron dissolved from sheets of the character under consideration by the ordinary acid pickling ture throughout the entire extent of the article bath constitutes a relatively large percentage of their total weight, whereas when my invention is employed, substantially no loss of ferrous metal whatever is occasioned.

While I have herein described my invention with some particularity as it may be utilized in the manufacture of sheets preparatory to coating them with protective materials, it will be understood that it may be equally well employed for the treatment of other kinds of sheets and plates as well as other ferrous shapes and manufactured articles generally and that changes and modifications in the details thereof will readily occur to those skilled in the art and may be made if desired without departing from the spirit and scope of the invention as defined in the appended claims.

Having thus described my invention, I claim and desire to protect by Letters Patent of the United States;

1. A method of annealing ferrous articles which comprises the steps of immersing the articles in molten sodium carbonate, maintaining the articles submerged in the bath for a predetermined time less than one minute, and thereafter removing the articles and allowing them to cool.

2. A method of annealing a ferrous sheet prior to coating it with metal which comprises the steps of dipping the sheet in a molten bath of sodium carbonate of slightly higher temperature than the critical temperature of the sheet metal, maintaining the sheet in the bath for approximately fifteen seconds, thereafter removing and cooling the sheet, and finally washing it to thereby remove a surface coating resulting from the bath and adhering to the sheet.

3. In a method of simultaneously cleaning and annealing a ferrous sheet, the step of immersing the sheet in a bath of molten sodium carbonate mixed with molten sodium hydroxide for a period of time not exceeding one minute.

GUSTAV A. REINHARDT.