US2677625A

US2677625A - Flat rolled ferrous metal for vitreous enameling

Info

Publication number: US2677625A
Application number: US211187A
Authority: US
Inventors: Joseph C Eckel
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1951-02-15
Filing date: 1951-02-15
Publication date: 1954-05-04
Anticipated expiration: 1971-05-04

Description

Patented May 4, 1954 FLAT ROLLED FERROUS .METAL FOR "VITREOUS ENAMELING Joseph C. 'Eckel, Pittsburgh, Pa., assignor "to United States Steel Corporation, a corporation of New Jersey N Drawing. .ApplicationFebi-uary 15, 1951,

Serial No. 211,187

4 Claims. 1

This invention relates to the preparation of flat rolled ferrous metal for vitreous enameling and, particularly, to the preparation of flat rolled ferrous metal for vitreous enameling which is free from surface defects caused by gas evolution therefrom during the enameling process.

One of the oldest and most vexing problems incident to the manufacture of enamelware formed by fiat rolled ferrous metal coated with vitreous enamel is an unsightly surface condition caused by the presence of black or brown spots or specking. It results either in rejection of the ware or necessitates expensive reoperations, such as stripping, spraying and firing, after which the Ware may still be rejected.

Those skilled in the art have for years directed their attention to solving the problem incident to such surface defects. While at times fully satisfactory enamelware has been produced, it Was never possible to assure uniformly satisfactory" results. It is common for these surface defects to occur in the enameling of one lot of sheets while another lot made to exactly the same specifications is free from them. At times, certain sheets of a lot may show the defect, andothers from the same lot do not exhibit any traces of it. In other words, there has been an uncontrollable and unaccountable lack .of uniformity in results, and the best efforts of those most highly skilled in the art have heretofore failed to provide a complete solution to the problem.

I have discovered that such surface defects can be eliminated by agglomerating carbides present in the metal into concrete masses of the largest possible dimensions and then dispersing theseagglomerated carbides, with simultaneous degasification of the metal, prior to the enameling.

I agglomerat the carbides in the metal by critically straining and high temperature box annealing, and I disperse the agglomerated carhides and degasify the metal prior to enameling by a normalizing treatment, which I shall explain,

The box annealing treatment of the flat rolled ferrou material is effected by heating the stock in th range of 1225 to 1400 FL, preferably in a narrower range of substantially 1225 to 1315 F. The metal is maintained at temperature during the annealing treatment for at least about six hours, best results ordinarily being obtained by keeping the metal within this temperature range for a time approaching twenty hours.

Heatin in the temperature range required for the agglomeration of carbides is preferably followed by slow cooling from this temperature range to a temperature at least as low asabout preferably in the narrower range of to F. per hour.

Before box annealing, I impart to the hot rolled stock an amount of cold reduction which is regulated to critically strain the material. While this amount of cold reduction may vary somewhat, dependent, on composition and prior treatment, it is less than 15% and ordinarily in the range of 5 to 8%. In certain cases the metal may be cold reduced both before and after agglomeration of carbides. For example, before the agg1omeration of carbides, the metal may be cold reduced in the preferred narrow range of 5 to 8%, and after box annealin the metal may b further cold reduced to a greater extent. The total cold reduction either in one or two rollings may, in some cases, be as much as The dispersion of agglomerated carbides and degasifying of metal is preferably accomplished by heating the metal in the range of substantially *1600 to 1850 F., preferably about 1750 F., metal temperature, and cooling it in air.

While my invention may be practiced using any flat rolled ferrous metal suitable for vitreous "enameling, I obtain superior results when the -carbon and manganese contents are maintained desirably-low. For example, a ferrous metal of the following typical analysis:

Per cent Carbon .03 Manganese .07 Phosphorus .015 Sulphur .05 f'Silico'n .015

when treated as herein described produces excellent results.

In reducing my invention to practice, I hot roll blooms of enameling iron to substantially 0.080 age, coil the strip hot, cold work reduce the coiled strip to critically strain it, box anneal the critically strained strip at 1225 to 1315" FL, using a 12-hour soak, and normalize annealed stock at 1725 to 1775 F., preferably in a neutral or a reducing atmosphere.

The process involved, while not perfectly understood at present, depends on gas evolution control. Enameling irons always contain some oxygen. During heating at the enameling temperature, this oxygen combines with carbon present in the metal, and the resulting gas escapes through the viscous layer of fused enamel in the shap of bubbles which collapse on reaching enamel surface, frequently extending from the metal to the surface of the enamel. Bursting of the bubbles exposes the metallic base to the oxidizing effect of furnace atmosphere, leading to the formation of the above-mentioned surface defects.

The present invention avoids gas generation in firing by eliminating oxygen from the carboncontaining base prior to enamel firing operation.

Carbon present in solid solution in alpha iron is first coagulated into large masses of cementite b annealing under the A1 point. Since cementit coagulates at the grain boundaries and the size of its masses is a function of grain size, the application of critical strain by appropriate amount of cold work disclosed above greatly improves the efiectiveness of the process. Heating coagulated cementite above the critical point transforms it into an austenitic grain having a eutectoid concentration of carbon assuring in this grain an instantaneous consumption of oxygen, which escapes as CO in. the atmosphere. At the time of the enamel-firing operation, there is no oxygen remaining to come in contact with carbon and cause the surface defects.

When flat rolled ferrous metal is prepared for vitreous enameling by my process, uniformly satisfactory results are assured virtually 00% of the time. Flat rolled ferrous metal prepared by m process can be enameled in conventional manner by applying the ordinary blue undercoat and white or light colored finish coat, or by employing a single coat of white or light colored enamel. In either case, reoperations due to the surface defects are virtually eliminated.

This application is a continuation-impart of my copending application Serial No. 710,948, filed November 19, 1946, now abandoned.

I claim:

1. The method of eliminating oxygen from lowcarbon undeoxidized steel enameling stock and thereby prevent reboiling during firing, comprising imparting a critical strain thereto by cold rolling to insure maximum grain growth when annealed, annealing said stock at a temperature below its A1 point to cause grain growth therein and to coagulate the carbon in said stock into large masses of cementite along the large grain ioundaries and then normalizing said stock to transform said cementite into austenite grains having a eutectoid carbon concentration that combines with oxygen, said combined oxygen being then dispersed from the stock as carbon monoxide.

The method of eliminating oxygen from lowcarbon undeoxidized steel enameling stock and thereby prevent reboiling during firing, comprising imparting a critical strain thereto by cold 4 reducing it between 5 and 8% to insure maximum grain growth when annealed, annealing said stock at a temperature below its A1 and between 1225 and 1400" F. point to cause grain growth therein and to coagulate the carbon in said stock into large masses of cementite along the large grain boundaries and then normalizing said stock at a temperature between 1600 and 1850 F. to transform said cementite into austenite grains having a eutectoid carbon concentration that combines with oxygen, said combined oxygen being then dispersed from the stock.

3. The method of preventing reboiling of enameling stock during firing of enamel coatings thereon, comprising forming enameling stock of lowcarbon undeoxidized steel, imparting a critical strain thereto by cold rolling to insure maximum grain growth when annealed, annealing said stock at a temperature below its A1 point to cause grain growth therein and to coagulate the carbon in said stock into large masses of cementite alon the large grain boundaries, normalizing said stock to transform said cementite into austenite grains having a eutectoid carbon concentration that combines with oxygen, said combined oxygen being then dispersed from the stock as'carbon monoxide to prevent reboiling during firing, placing a coat of enamel frit thereon and then firing it.

4. The method of preventing reboiling of enameling stock during firing of enamel coatings thereon, comprising forming enameling stock of low-carbon undeoxidized steel, imparting a critical strain thereto by cold reducing it between 5 and 8% to insure maximum grain growth when annealed, annealing said stock at a temperature below its A1 and between 1225 and 1400" F. point to cause grain growth therein and to coagulate the carbon in said stock into large masses of cementite along the large grain boundaries, normalizing said stock at a temperature between 1602) and 1850" F. to transform said cementite into austenite grains having a eutectoid carbon concentration that combines with oxygen, said combined oxygen eing then dispersed from the stock to prevent reboiling during firing, placing a coat of enamel frit thereon and then firing it.

References Cited in the file of this patent UNITED STATES PATENTS N umber Name Date ,581,269 Kelley Apr. 20, 1926 1,996,568 Butts Apr, 2, 1935 2,065,392 Porter Dec. 22, 1936 2,303,343 Engel Dec. 1, 1942 2,455,331 Eckel et a1 NOV. 30, 1948 2,532,640 Pfeiffer Dec. 5, 1950

Claims

4. THE METHOD OF PREVENTING REBOILING OF ENAMELING STOCK DURING FIRING OF ENAMEL COATINGS THEREON, COMPRISING FORMING ENAMELING STOCK OF LOW-CARBON UNDEOXIDIZED STEEL, IMPARTING A CRITICAL STRAIN THERETO BY COLD REDUCING IT BETWEEN 5 AND 8% TO INSURE MAXIMUM GRAIN GROWTH WHEN ANNEALED, ANNEALING SAID STOCK AT A TEMPERATURE BELOW ITS A1 AND BETWEEN 1225 AND 1400* F. POINT TO CAUSE GRAIN GROWTH THEREIN AND TO COAGULATE THE CARBON IN SAID STOCK INTO LARGE MASSES OF CEMENTITE ALONG THE LARGE GRAIN BOUNDARIES, NORMALIZING SAID STOCK AT A TEMPERATURE BETWEEN 1600 AND 1850* F. TO TRANSFORM SAID CEMENTITE INTO AUSTENITE GRAINS HAVING A EUTECTOID CARBON CONCENTRATION THAT COMBINES WITH OXYGEN, SAID COMBINED OXYGEN BEING THEN DISPERSED FROM THE STOCK TO PREVENT REBOILING DURING FIRING, PLACING A COAT OF ENAMEL FRIT THEREON AND THEN FIRING IT.