US1785786A - Method of rolling and annealing sheet metal - Google Patents

Description

Patented Dec. 23, 1930 PATENT OFFICE ALEXANDER PA'IEBSON, OF BUFFALO, NEW YORK METHOD OF ROLLING AND ANNEALING SHEET METAL Ho Drawing. Application filed March 26, 1928, Serial No. 264,960. Renewed Kay 14, 1880.

An object of the invention is to enable the production of fine grained sheets without requiring the annealing temperature of the metal to be increased beyond a value slightly I greater than the critical point, that is to say, the point at which the grains begin to grow, and without requiring any special features in the annealing practice, thereby effecting important economies in time and practice.

The theory underlying the various effects obtained in connection. with the working or rolling and annealing of steel is so comprehensive as to render it diflicult to set forth a short and accurate statement of any particular phase of this field. In order, however, that the essence of this invention may be more clearly set forth and readily understood, it is believed that a very general discussion. of the theory here involved will be of assistance.

In considering the mechanical treatment of steel, or of any metal, the first distinction to be made is that of hot and cold working. These terms, hot and cold, as they are commonly used, are more or less indefinite, but

as ap illied to the mechanical working of steel, t ey are separated by a sharp and welldefined' line, in the case of any particular class of steel, at least. This line is described by the temperature at which ra id crystalline or grain growth occurs. apid grain growth, being a physical phenomena similar to fusing or boiling, be 'ns at a definite temperature for any sing e'substance, if other conditions are constant. This definite temperature is the critical oint above referred to. Thus, in the manu acture of sheet mill products, while the reduction of the sheets on the hot millsis commonly referred to as the hot rolling operation, it is, nevertheless, a cold'working of the steel inasmuch as thehot rolling operation is performed upon sheets having a temperature somewhat below the critical point.

It is well known in this art that the hot rolling of steel causes the grains of the metal to be flattened and elongated so as to give the metal a striated fibrous structure. As a consequence of this working and resultant distortion of the grains, hot rolled metal is hard and not very ductile. To obtain ductile metal, it is necessary, therefore, to anneal the metal, that is to say, to heat the metal to a temperature which is high enough to relieve the stresses and bring about a reformation of the grains or in other words modify the striated structure. The temperature above which this change will take place is the above mentioned critical temperature or point. In other words to modify the striated structure it is necessary to-heat the metal to a temperature above the critical point.

As previously stated, however, the grain growth phenomenon occurs when the metal is heated above the critical point and within \a certain range herein termed the growth range. This range is in the neighborhood of 1292 F. to 1700 F. (700 G.-930 C.) and embraces the usual annealing temperatures, it being noted that grain growth proceeds with particular rapidity in box annealin operations. Grain growth is objectionable in respect to many uses of the meta'l inasmuch as metal having large grains is liable to score or to be of leaden characteristic or to flake under drawing stress.

The extent to which the grains will grow is directly proportional, among other factors, to the actual temperature of the metal up to a certain temperature beyond the critical point which temperature is, as revious- 1y stated, approximately 1700 F. 930 C.) and constitutes the up er limit of the growth range and beyond which the grains grow smaller. The grain owth is also roportional to the length 0 time the meta is subjected to. a temperature within the owth range. For instance, if the metal is eld at a certain temperature within the owth ran c, it may be completel annealed fore the imit of grain growth dr that particular temperature is reached. In such cases, it is apparent that if the metal, when completely annealed, is immediately cooled, to a temperature below the critical point, the grain growth will be stopped at t at stage resulting in a metal having a smaller grain and which is, therefore, a higher grade product. With this in mind it will be readily seen that the smaller the grains of the metal before annealing the less will be their size as a re- 'sult of growth during the annealing process.

The prior practice with regard to the annealing of metal has been to take the metal directly from the last hot reducing mill and place it in the annealing furnace or oven, the temperature of the metal being substantially above 800 F. (427 C.) .The efforts to overcome the objectionable grain growth in such practice have been directed to variations in the annealing process.

- .The present invention difiers from the prior practice in that the variations of the procedure are introduced in thereducing mill operation instead of the annealing operation. Thus instead of effecting the last hot reductionwhen the metal has a temperature over 800 F. (427 0.), I efiect such reduction.

while the metal has a temperature of between 600 F. (316 C.) and 800 F. (427 0.). When the metal as delivered from the last hot stand has a temperature over 800 F. (427 C.) I cool the metal to a temperature ranging from 800 F. (427 0.) to 600 F. (310 C.) and while it is at such reduced temperature effect the last reduction. This range of temperature is known in the art as the blue short zone and when the metal is reduced in this zone, its granular structure is broken down more completely, by reduction, than if reduced at a temperature above or below this zone. metal in the blue short zone gives the finest possible granular structure. Having thus obtained the finest grain possible, the metal may be box-annealed in the ordinary manner indicated heretofore and still have a comparatively fine granular structure since the annealing will be completed within the growth range but at a temperature but slightly in excess of the critical point (for example with .10 carbon steel at a temperature of about 1450 F. (788 C.)) and in a period which, within the growth range, affords but little time for the grain growth whereby the sheets are completely annealed well before the grains reach or even approach the grain size developed in the growth range of sheets which'are taken directly from the last hot stand at a temperature well above the blue short zone.

The granular structure may be made more uniform by insuring a uniform blue short temperature throughout the metal before effecting the last hot reduction. To this end I prefer to pass the metal through an oven in which the desired blue short tem erature is maintained whereby the metal is rought to such temperature before receiving the final reduction.

The final reduction or pass is preferably In other words, reduction of the effected on the same stand in which the metal received the previous pass inasmuch-as the colder finished passes tend to keep down the iemperature of the rolls to a more uniform eat.

While the invention has been described in connection-with hot mill operation, it is to be understood that this is merely for the purpose of promoting a clearer idea of the essence of the invention and not intended as a limitation. For instance, it may be desirable to process cold rolled metal in this manner, that is to say to heat the metal up to the blue short zone and there effect the final reduction and subsequent annealing.

I claim as my invention 1. The method of treating sheet metal consistingin reducing the metal while at a temperature within the blue short zone and then annealing it at a temperature within the growth range.

2. The method of treating sheet metal consisting in partially reducing the metal while at a temperature above the blue short zone, finally reducing the metal while at a temperature within the blue-short zone and then annealing it at a temperature within the growth range.

3. The method of manufacturing sheet metal consisting in treating the'metal from the final. hot roll to give it a uniform temperature within the blue short zone, reducing the metal at such temperature, and then annealing it at a temperature within the growth range.

In testimony whereof I hereby affix my signature.

ALEXANDER PATERSON.