the present invention is directed to tfs sheets having a plating of metallic chromium ranging from 50 to 200 mg per square meters on each sheet surface and a coating of hydrated chromium oxides ranging from 5 to 30 mg per square meters on the metallic chromium plating surface . tfs sheets generally have a metallic chromium plating of 50 to 200 mg / m 2 because thinner platings of less than 50 mg / m 2 have poor corrosion resistance . thicker platings exceeding 200 mg / m 2 do not provide an additional improvement in corrosion resistance . tfs sheets having a coating of hydrated chromium oxides of 5 to 30 mg / m 2 ( calculated as metallic chromium ) because thinner coatings of less than 5 mg / m 2 do not provide the necessary paint adhesion . thicker coatings exceeding 30 mg / m 2 have a poor appearance and are prone to cracking during subsequent processing and thus impractical . the most desirable range is 8 - 25 mg / m 2 . the chromium plating bath and electrolytic chromate bath used in tfs manufacture are basically aqueous solutions of chromic acid , chromates and / or dichromates , to which a variety of assistants are added . most of these assistants contain one or more anions such as sulfate and fluoride anions , and such anions are codeposited in a substantial proportion in hydrated chromium oxide coatings formed on the tfs surface . particularly , the sulfate codeposited in the coating is detrimental because it can be dissolved out during a retorting treatment of bonded tfs cans to give rise to paint film - tfs interfacial separation as described earlier . the inventors carried out a basic experiment in order to find adequate conditions for the electrolytic chromate treatment to assure that the resulting tfs sheets show consistently excellent retorting resistance with respect to paint adhesion . steel sheets were electrolytically degreased , rinsed and pickled with sulfuric acid in a conventional manner before they were cathodically treated in a chromium plating bath having a composition of 100 to 200 g of cro 3 , 5 to 8 g of na 2 sif 6 and 0 . 5 to 1 g of h 2 so 4 per liter of the bath . the chromium plated steel sheets were successively subjected to reverse electrolysis in the same bath while they were set as an anode . the steel sheets were rinsed again with water . the steel sheets were further subjected to an electrolytic chromate treatment in aqueous chromate solutions prepared from reagent grade chromate ( cro 3 ) with or without adding h 2 so 4 thereto while they were set as a cathode . in order to examine the paint adhesion to the thus obtained tfs sheets during retorting treatment , a retorting test was carried out on the tfs sheets having paint applied thereon by the following procedure . a tfs sheet was coated on one surface with an epoxy - phenol resin paint in an amount of 60 mg / dm 2 and baked at 210 ° c . for 12 minutes . the sheet was then coated on the other surface with the same paint in an amount of 25 mg / dm 2 and baked under the same conditions as above . the double - coated sheet was cut to pieces of 70 mm wide by 60 mm long . two pieces were bonded with an adhesive along their edges . fig1 a shows a specimen consisting of two bonded pieces 2 and fig1 b is an enlarged view of the bonded portion of the specimen . as best shown in fig1 b , one piece 2 was partially overlaid on another piece 2 over an overlapping distance of 8 mm between their mating longitudinal edges while an adhesive nylon film 10 of 100 μm thick was sandwiched between a thick paint build - up 6 of one piece 2 and a thin paint build - up 8 of the other piece 2 . it should be understood that the thick and thin paint build - ups 6 and 8 were formed on the opposed surfaces of a sheet by applying a phenolepoxy resin paint to 60 and 25 mg / dm 2 , respectively , as described above . using a hot press , the sandwich of the adhesive film between the partially overlapped pieces was presssure bonded by preheating it at 200 ° c . for 120 seconds and further heating at 200 ° c . under a pressure of 3 kg / cm 2 for 30 seconds . ten specimens were prepared in this manner . as shown in fig1 a , each specimen consisting of adhesive bonded two pieces was bent to substantially the same curvature as a can barrel before it was press fitted between the corners of an angle 4 having a bottom length of 70 mm . these test assemblies were kept for 150 and 300 minutes in a retort at 125 °- 130 ° c . and 1 . 6 - 1 . 7 kg / cm 2 . after the test assemblies were taken out of the retort , the specimens were examined for bond failure . the number of separated specimens in a set of 10 specimens is the index representative of retorting resistance of a paint - coated tfs sheet . the test results are shown in fig2 by plotting the number of separated specimens in relation to the concentration of so 4 -- in gram / liter in the chromate solutions used in the electrolytic chromate treatment . as evident from fig2 retorting resistance is suddenly aggravated when the so 4 -- concentration exceeds 0 . 10 g / l , although retorting resistance is kept very well at so 4 -- concentrations of lower than 0 . 10 g / l ( inclusive ). fig3 is a diagram in which the amount of sulfer ( s ) codeposited in 1 mg / m 2 of hydrated chromium oxides is plotted in relation to the concentration of so 4 -- in gram / liter in the chromate solutions used in the electrolytic chromate treatment . as evident from fig3 the amount of sulfur codeposited suddenly increases when the so 4 -- concentration exceeds 0 . 10 g / l . in order to make tfs sheets having improved retorting resistance , not only the reverse electrolysis should be effected after the chromium plating so as to reduce the sulfate codeposited with hydrated chromium oxides , but the electrolytic chromate treatment following the reverse electrolysis and rinsing should also be effected in a chromate solution having a limited so 4 -- concentration of 0 . 10 g / l or lower . as the so 4 -- conentration is further lowered below 0 . 10 g / l , the effect thereof on retorting resistance is saturated or leveled and the operating cost increases . for commercial practice , the lower limit of the so 4 -- concentration in the electrolytic chromate solution should be 0 . 01 g / l . the following example is set forth by way of illustration and not by way of limitation . a cold rolled steel sheet designated t4ca having a thickness of 0 . 22 mm was electrolytically degreased in a 5 % homezarine solution at a temperature of 80 ° c . and a current density of 10 ampere / dm 2 , rinsed with water , immersed in a 10 % h 2 so 4 at 40 ° c . for 5 seconds , and rinsed again with water . the sheet was then subjected to the following treatments in sequence : ( c ) electrolytic chromate treatment . steps ( a ) and ( b ) were successively carried out in the same electrolytic bath . cold water rinsing and hot water rinsing were carried out both between steps ( b ) and ( c ) and at the end of step ( c ). conditions used in the respective steps are shown in table 1 . the chromium plating was carried out in two different baths . the anodizing treatment was carried out at an electricity quantity of 1 coulomb / dm 2 except that sample no . 1 was not subjected to reverse electrolysis for comparison purpose . the baths used in the electrolytic chromate treatment contained 60 g / l of cro 3 while the concentration of h 2 so 4 was varied from 0 . 01 g / l to 0 . 20 g / l . the electrolytically treated tfs sheets were determined for paint adhesion under retorting conditions by the same test procedure as used in the above - described basic experiment . the results are also shown in table 1 . table 1__________________________________________________________________________ electrolytic chromate treatment retortingsample chromium reverse cro . sub . 3 h . sub . 2 so . sub . 4 bath electrolytic resistance * no . plating electrolysis ( g / l ) ( g / l ) temp . condition 150 min . 300__________________________________________________________________________ min . 1 comparison no 0 . 01 15 a / dm . sup . 2 10 10 cro . sub . 3 150 g / l 1 . 0 sec . 2 invention na . sub . 2 sif . sub . 6 5 g / l 0 . 01 0 03 comparison h . sub . 2 so . sub . 4 0 . 6 g / l 0 . 20 8 104 &# 34 ; 50 ° c . 0 . 12 1 85 invention 50 a / dm . sup . 2 0 . 10 0 16 &# 34 ; 1 . 4 sec . 5 60 0 . 06 40 ° c . 15 0 07 &# 34 ; a / dm . sup . 2 0 . 02 a / dm . sup . 2 0 08 &# 34 ; cro . sub . 3 150 g / l 0 . 2 sec . 0 . 02 20 sec . 0 0 h . sub . 2 so . sub . 4 0 . 6 g / l9 &# 34 ; 50 ° c . 0 . 10 0 1 50 a / dm . sup . 210 comparison 1 . 4 sec . 0 . 20 7 10__________________________________________________________________________ * retorting resistance of electrolytically treated steel sheets having paint applied to the opposed surfaces , expressed by the number of separated specimens in a set of 10 specimens each consisting of paint builtup tfs sheets bonded with an adhesive film after exposure to a retorting treatment for 150 and 300 minutes . as seen from the data in table 1 , tfs sheets having improved retorting resistance are obtained through a sequence of steps of chromium plating , reverse electrolysis , and electrolytic chromate treatment as long as the chromate solution has a limited so 4 -- concentration of 0 . 01 to 0 . 10 g / l . as demonstrated by the above example , the present invention allows tin - free steel sheets having improved retorting resistance to be produced by carrying out an electrolytic chromate treatment in a chromate solution whose so 4 -- concentration is limited to the range from 0 . 01 to 0 . 10 g per liter of the solution .